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Glucagon biosynthesis

Glucagon biosynthesis

Glkcagon CAS Google Scholar. gov means it's official. As it is intended for the biosynthesos scientific community, Insulin resistance diet Exercise and blood sugar monitoring included was buosynthesis to be both clinical and scientific. Insulin resistance diet — Google Scholar Blázquez E, Muñoz-Barragan L, Patton GS, Dobbs RE, Unger RH Demonstration of gastric glucagon hypersecretion of insulin-deprived alloxan-diabetic dogs. glucose infusion contributes to the reduced incretin effect in type 2 diabetes mellitus. Inactivation of VHL specifically in the liver increased hepatic HIF2 a expression, which abrogated the glucagon -mediated increase in gluconeogenesis and resulted in relative hypoglycemia.

Glucagon biosynthesis -

Kimball CP, Murlin JR. Aqueous Extracts of Pancreas Iii. Some Precipitation Reactions of Insulin. Bromer WW, Sinn LG, Staub A, Behrens OK. The amino acid sequence of glucagon. Blackman B. The use of glucagon in insulin coma therapy. Psychiatr Q. Esquibel AJ, Kurland AA, Mendelsohn D.

The use of glucagon in terminating insulin coma. Dis Nerv Syst. Unger RH, Eisentraut AM. McCALL MS, Madison LL. Glucagon antibodies and an immunoassay for glucagon. Unger RH, Orci L. The essential role of glucagon in the pathogenesis of diabetes mellitus. Drucker DJ, Asa S.

Glucagon gene expression in vertebrate brain. Mojsov S, Heinrich G, Wilson IB, Ravazzola M, Orci L, Habener JF. Preproglucagon gene expression in pancreas and intestine diversifies at the level of post-translational processing.

Gerich JE, Lorenzi M, Hane S, Gustafson G, Guillemin R, Forsham PH. Evidence for a physiologic role of pancreatic glucagon in human glucose homeostasis: studies with somatostatin. Gromada J, Franklin I, Wollheim CB. Alpha-cells of the endocrine pancreas: 35 years of research but the enigma remains.

Müller TD, Finan B, Clemmensen C, DiMarchi RD, Tschöp MH. The New Biology and Pharmacology of Glucagon. Physiological Reviews. Wewer Albrechtsen NJ, Kuhre RE, Pedersen J, Knop FK, Holst JJ.

The biology of glucagon and the consequences of hyperglucagonemia. Biomarkers in Medicine. Gromada J, Chabosseau P, Rutter GA. The α-cell in diabetes mellitus. Hughes JW, Ustione A, Lavagnino Z, Piston DW. Regulation of islet glucagon secretion: Beyond calcium.

Diabetes, Obesity and Metabolism. Knop FK, Vilsbøll T, Madsbad S, Holst JJ, Krarup T. Inappropriate suppression of glucagon during OGTT but not during isoglycaemic i. glucose infusion contributes to the reduced incretin effect in type 2 diabetes mellitus. Lund A, Bagger JI, Albrechtsen NJW, Christensen M, Grøndahl M, Hartmann B, Mathiesen ER, Hansen CP, Storkholm JH, Hall G, van, Rehfeld JF, Hornburg D, Meissner F, Mann M, Larsen S, Holst JJ, Vilsbøll T, Knop FK.

Evidence of Extrapancreatic Glucagon Secretion in Man. Miyachi A, Kobayashi M, Mieno E, Goto M, Furusawa K, Inagaki T, Kitamura T. Accurate analytical method for human plasma glucagon levels using liquid chromatography-high resolution mass spectrometry: comparison with commercially available immunoassays.

Anal Bioanal Chem. Hansen JS, Pedersen BK, Xu G, Lehmann R, Weigert C, Plomgaard P. Exercise-Induced Secretion of FGF21 and Follistatin Are Blocked by Pancreatic Clamp and Impaired in Type 2 Diabetes. Schwartz NS, Clutter WE, Shah SD, Cryer PE.

Glycemic thresholds for activation of glucose counterregulatory systems are higher than the threshold for symptoms. J Clin Invest. Svoboda M, Tastenoy M, Vertongen P, Robberecht P. Relative quantitative analysis of glucagon receptor mRNA in rat tissues.

Physiology of proglucagon peptides: role of glucagon and GLP-1 in health and disease. Pospisilik JA, Hinke SA, Pederson RA, Hoffmann T, Rosche F, Schlenzig D, Glund K, Heiser U, McIntosh CHS, Demuth H-U. Metabolism of glucagon by dipeptidyl peptidase IV CD Regulatory Peptides.

Pontiroli AE, Calderara A, Perfetti MG, Bareggi SR. Pharmacokinetics of intranasal, intramuscular and intravenous glucagon in healthy subjects and diabetic patients.

Lund A, Bagger JI, Albrechtsen NW, Christensen M, Grøndahl M, Hansen CP, Storkholm JH, Holst JJ, Vilsbøll T, Knop FK. Increased Liver Fat Content in Totally Pancreatectomized Patients.

Unger RH. Glucagon physiology and pathophysiology in the light of new advances. Miller RA, Birnbaum MJ. Glucagon: acute actions on hepatic metabolism.

Ramnanan CJ, Edgerton DS, Kraft G, Cherrington AD. Physiologic action of glucagon on liver glucose metabolism. Diabetes Obes Metab. Rui L. Energy Metabolism in the Liver. Compr Physiol. Geisler CE, Renquist BJ. Hepatic lipid accumulation: cause and consequence of dysregulated glucoregulatory hormones.

J Endocrinol. Longuet C, Sinclair EM, Maida A, Baggio LL, Maziarz M, Charron MJ, Drucker DJ. The Glucagon Receptor Is Required for the Adaptive Metabolic Response to Fasting.

Cell Metabolism. Wang H, Zhao M, Sud N, Christian P, Shen J, Song Y, Pashaj A, Zhang K, Carr T, Su Q.

Glucagon regulates hepatic lipid metabolism via cAMP and Insig-2 signaling: implication for the pathogenesis of hypertriglyceridemia and hepatic steatosis. Sci Rep. Galsgaard KD, Pedersen J, Knop FK, Holst JJ, Wewer Albrechtsen NJ. Glucagon Receptor Signaling and Lipid Metabolism. Front Physiol.

Holst JJ, Albrechtsen NJW, Pedersen J, Knop FK. Glucagon and Amino Acids Are Linked in a Mutual Feedback Cycle: The Liver—α-Cell Axis. Hamberg O, Vilstrup H. Regulation of urea synthesis by glucose and glucagon in normal man.

Clin Nutr. Solloway MJ, Madjidi A, Gu C, Eastham-Anderson J, Clarke HJ, Kljavin N, Zavala-Solorio J, Kates L, Friedman B, Brauer M, Wang J, Fiehn O, Kolumam G, Stern H, Lowe JB, Peterson AS, Allan BB. Glucagon Couples Hepatic Amino Acid Catabolism to mTOR-Dependent Regulation of α-Cell Mass.

Cell Rep. Bagger JI, Holst JJ, Hartmann B, Andersen B, Knop FK, Vilsbøll T. J Clin Endocrinol Metab. Geary N, Kissileff HR, Pi-Sunyer FX, Hinton V. Individual, but not simultaneous, glucagon and cholecystokinin infusions inhibit feeding in men. Langhans W, Zeiger U, Scharrer E, Geary N.

Stimulation of feeding in rats by intraperitoneal injection of antibodies to glucagon. Le Sauter J, Noh U, Geary N. Hepatic portal infusion of glucagon antibodies increases spontaneous meal size in rats. Nair KS.

Hyperglucagonemia Increases Resting Metabolic Rate In Man During Insulin Deficiency. Glucagon increases energy expenditure independently of brown adipose tissue activation in humans.

Tan TM, Field BCT, McCullough KA, Troke RC, Chambers ES, Salem V, Gonzalez Maffe J, Baynes KCR, De Silva A, Viardot A, Alsafi A, Frost GS, Ghatei MA, Bloom SR.

Coadministration of Glucagon-Like Peptide-1 During Glucagon Infusion in Humans Results in Increased Energy Expenditure and Amelioration of Hyperglycemia.

Fibroblast Growth Factor 21 Mediates Specific Glucagon Actions. Ceriello A, Genovese S, Mannucci E, Gronda E. Glucagon and heart in type 2 diabetes: new perspectives. Cardiovasc Diabetol.

Graudins A, Lee HM, Druda D. Calcium channel antagonist and beta-blocker overdose: antidotes and adjunct therapies. Br J Clin Pharmacol. Meidahl Petersen K, Bøgevig S, Holst JJ, Knop FK, Christensen MB. Hemodynamic Effects of Glucagon - A Literature Review. Thuesen L, Christiansen JS, Sørensen KE, Orskov H, Henningsen P.

Low-dose intravenous glucagon has no effect on myocardial contractility in normal man. An echocardiographic study. Kazda CM. Treatment with the glucagon receptor antagonist LY increases ambulatory blood pressure in patients with type 2 diabetes.

Lund A, Bagger JI, Christensen M, Grøndahl M, van Hall G, Holst JJ, Vilsbøll T, Knop FK. Higher Endogenous Glucose Production During OGTT vs Isoglycemic Intravenous Glucose Infusion.

Reaven GM, Chen YD, Golay A, Swislocki AL, Jaspan JB. Documentation of hyperglucagonemia throughout the day in nonobese and obese patients with noninsulin-dependent diabetes mellitus.

Dunning BE, Gerich JE. The role of alpha-cell dysregulation in fasting and postprandial hyperglycemia in type 2 diabetes and therapeutic implications.

Hamaguchi T, Fukushima H, Uehara M, Wada S, Shirotani T, Kishikawa H, Ichinose K, Yamaguchi K, Shichiri M. Abnormal glucagon response to arginine and its normalization in obese hyperinsulinaemic patients with glucose intolerance: importance of insulin action on pancreatic alpha cells.

Knop FK. EJE PRIZE A gut feeling about glucagon. Lund A, Vilsbøll T, Bagger JI, Holst JJ, Knop FK. The separate and combined impact of the intestinal hormones, GIP, GLP-1, and GLP-2, on glucagon secretion in type 2 diabetes. Cryer PE. Minireview: Glucagon in the Pathogenesis of Hypoglycemia and Hyperglycemia in Diabetes.

Li K, Song W, Wu X, Gu D, Zang P, Gu P, Lu B, Shao J. Associations of serum glucagon levels with glycemic variability in type 1 diabetes with different disease durations. Hare KJ, Vilsbøll T, Holst JJ, Knop FK.

Inappropriate glucagon response after oral compared with isoglycemic intravenous glucose administration in patients with type 1 diabetes.

American Journal of Physiology-Endocrinology and Metabolism. Diabetes Control and Complications Trial Research Group. Nathan DM, Genuth S, Lachin J, Cleary P, Crofford O, Davis M, Rand L, Siebert C. The effect of intensive treatment of diabetes on the development and progression of long-term complications in insulin-dependent diabetes mellitus.

Cryer PE, Gerich JE. Glucose counterregulation, hypoglycemia, and intensive insulin therapy in diabetes mellitus. Sprague JE, Arbeláez AM. Glucose Counterregulatory Responses to Hypoglycemia. Pediatr Endocrinol Rev. Yosten GLC. Alpha cell dysfunction in type 1 diabetes.

Knop FK, Aaboe K, Vilsbøll T, Vølund A, Holst JJ, Krarup T, Madsbad S. Impaired incretin effect and fasting hyperglucagonaemia characterizing type 2 diabetic subjects are early signs of dysmetabolism in obesity.

Albrechtsen NJW, Junker AE, Christensen M, Hædersdal S, Wibrand F, Lund AM, Galsgaard KD, Holst JJ, Knop FK, Vilsbøll T. Hyperglucagonemia correlates with plasma levels of non-branched chained amino acids in patients with liver disease independent of type 2 diabetes.

American Journal of Physiology - Gastrointestinal and Liver Physiology ajpgi. Junker AE, Gluud L, Holst JJ, Knop FK, Vilsbøll T. Diabetic and nondiabetic patients with nonalcoholic fatty liver disease have an impaired incretin effect and fasting hyperglucagonaemia.

J Intern Med. Suppli MP, Lund A, Bagger JI, Vilsbøll T, Knop FK. Involvement of steatosis-induced glucagon resistance in hyperglucagonaemia. Tillner J, Posch MG, Wagner F, Teichert L, Hijazi Y, Einig C, Keil S, Haack T, Wagner M, Bossart M, Larsen PJ. A novel dual glucagon-like peptide and glucagon receptor agonist SAR Results of randomized, placebo-controlled first-in-human and first-in-patient trials.

Ambery P, Parker VE, Stumvoll M, Posch MG, Heise T, Plum-Moerschel L, Tsai L-F, Robertson D, Jain M, Petrone M, Rondinone C, Hirshberg B, Jermutus L. MEDI, a GLP-1 and glucagon receptor dual agonist, in obese or overweight patients with type 2 diabetes: a randomised, controlled, double-blind, ascending dose and phase 2a study.

The Lancet. Copyright © , MDText. Bookshelf ID: NBK PMID: PubReader Print View Cite this Page Rix I, Nexøe-Larsen C, Bergmann NC, et al. Glucagon Physiology. In: Feingold KR, Anawalt B, Blackman MR, et al. In this Page. Links to www. View this chapter in Endotext.

Related information. PMC PubMed Central citations. Similar articles in PubMed. Review Inhibition of glucagon secretion. Young A. Adv Pharmacol. Epub Apr Effects of dipeptidyl peptidase IV inhibition on glycemic, gut hormone, triglyceride, energy expenditure, and energy intake responses to fat in healthy males.

Heruc GA, Horowitz M, Deacon CF, Feinle-Bisset C, Rayner CK, Luscombe-Marsh N, Little TJ. Am J Physiol Endocrinol Metab. Epub Sep Review [Glucagon and glucagon-like peptides the role in control glucose homeostasis.

Part I]. Otto-Buczkowska E. Pediatr Endocrinol Diabetes Metab. Glucagon-like peptide-1 and islet lipolysis. Sörhede Winzell M, Ahrén B. Horm Metab Res.

Proposed developmental pathway of the endocrine pancreas in the mouse, showing interruptions of development in response to disruptions of the transcription factor genes, IDX-1, Isl-1, Pax-4, and Pax Knockouts of IDX-1 and Isl-1 result in early failure of the development of epithelial cells derived from the endodermal stem cell.

IDX-1 is a key factor in the very early development of all pancreatic epithelial cells, whereas Isl-1 is required for the development of the dorsal mesenchyme, and its failure leads to a specific arrest of development of the epithelial cells of the dorsal pancreas; the mice die at ED 9.

Inactivation of Pax-4 by homologous recombination prevents development of the β- and δ-cells and shunts development to the α-cell lineage. The Pax-6 knockout does the opposite:α -cells do not develop, but some development occurs in β- andδ -cells.

Recently, the knockout in mice of transcription factor Nkx2. GLU, Glucagon; INS, insulin; SOM, somatostatin; PP, pancreatic polypeptide. Days of embryonic development are indicated on the left of the figure EE17 and postnatal days are indicated by P1-P Habener and D. Stoffers: Proc Assoc Am Phys , ].

As illustrated in Fig. However, immunoreactive GLP-1 is detectable in rat pancreatic α-cells by immunocytochemistry Fully processed GLP-1 amide and is also visualized in pancreatic rat extracts by using chromatographic techniques and RIAs , A recent investigation detected predominantly GLP-1 amide in extracts of rat pancreas Using similar techniques, small amounts of N-terminally extended GLP-1 amide and are also found in extracts from porcine and human pancreas , In addition, immunoreactive GLP-1 is secreted from the arginine-perfused rat pancreas and glucose-stimulated isolated rat islets, as detected by RIA , The relatively small quantity of GLP-1 produced by the pancreas might have important local actions within the islets.

Expression of the preproglucagon gene. A, Diagram of the proglucagon gene and encoded mRNA. The gene consists of six exons E1-E6 and five introns IA-IE. Alternative splicing of exons E4 and E5 occurs in salmonid fishes but not in mammals.

The exons encode functional domains of the preproglucagon: S, signal peptide; N, amino-terminal sequence of proglucagon; Gluc, glucagon; IP, intervening peptides. The pairs of basic residues that serve as posttranslational sites of processing of the preproglucagon encoded by the mRNA are shown.

M, Methionine encoded by AUG codon that initiates translation; Q, glutamine; H, histidine; K, lysine; R, arginine; UN-TX, untranslated regions of mRNA [Adapted from S. Mojsov et al. B, Alternative posttranslational processing of proglucagon in pancreas, intestine, and brain.

Enzymatic cleavages at specific pairs of basic residues in proglucagon produces numerous multifunctional peptide hormones involved in nutrient metabolism. K, Lysine; R, arginine. The major bioactive hormones derived from proglucagon are glucagon in the pancreatic α-cells and GLP-1 two isoforms, and NH 2 and GLP-2 in the intestinal L cells and brain.

Numbers on proglucagon denote amino acid positions. GRPP, Glicentin-related pancreatic peptide; Gluc, glucagon; IP-1 and IP-2, intervening peptides; MPF, major proglucagon fragment.

GLP-1 is α-amidated on the carboxyl-terminal arginine residue. Intestinal cells are reported to react with glucagon-specific C-terminal antisera, although the intestinal immunodeterminant responsible for the immune reaction appears to differ chemically from pancreatic glucagon , Antibodies directed against the midpart of glucagon and antibodies against the nonglucagon part of the glicentin molecule reveal a large population of endocrine cells in the small and large bowel that express proglucagon and its fragments In contrast to the pancreas where GLPs represent minor products, GLPs are fully processed in abundance in the intestine, representing the major source of circulating GLPs By virtue of their ultrastructure as assessed by electron microscopy, these cells are designated as L cells that clearly differ from pancreatic α-cells in the morphology of the granules , The intestinal L cells are flask shaped and open-type, the microvilli reach the intestinal lumen, and a domain rich in endocrine granules exists near the basal lamina , Fig.

The shape of the L cells suggests that the cells can respond to changes in the environment within the intestinal lumen, resulting in a basal discharge of their granular contents.

GLPimmunoreactive cells in the human rectal mucosa. The cells occur in all regions of the crypts with a predominance in the basal region A. They reach the lumen via slender apical processes B and C. Short arrows indicate basolateral secretory vesicles; long arrow indicates luminal villi.

Eissele et al : Eur J Clin Invest , The L cells are the second most abundant population of endocrine cells in the human intestine, exceeded only by the population of enterochromaffin cells. A high abundance of L cells is present in the distal jejunum and ileum, and an increasing abundance of L cells is demonstrable along the colon, with the highest concentration in the rectum L cells first appear in human fetuses at the 8th week of gestation in the ileum, the 10th week in the oxyntic mucosa and proximal small intestine, and at the 12th week in the colon This distribution of L cells differs greatly from that of the cells that secrete GIP, which are located in the more proximal regions of the jejunum 8, , , The L cells of the small and large bowel are thought to arise from pluripotent stem cells in the crypts that also give rise to enterocytes, goblet cells, and Paneth cells The L cells have a lower rate of turnover than other cell types in the crypts Most of the L cells reside in the crypts of Lieberkühn, but a few cells can also be observed in the intestinal villi.

Before the identification of GLP-1 as a separate product of the posttranslational processing of proglucagon, it was recognized that gut-type glucagon immunoreactivity existed within the central nervous system of several mammalian species The subsequent development of specific antisera allowed for the analysis of the precise anatomical distribution of GLP GLPimmunoreactive nerve fibers and terminals are widely distributed throughout the brain; the highest density occurs in the hypothalamus, thalamus, and septal regions, and the lowest occurs in the cortex and hindbrain Chromatographic analyses of extracts of rat brainstem and hypothalamus revealed that proglucagon is processed in a manner similar to that in the intestine, preferentially giving rise to oxyntomodulin, glicentin, GLP-1, and GLP-2 , , Posttranslational processing of proglucagon may undergo changes during development, as the predominance of glicentin and oxyntomodulin over glucagon in the rat hypothalamus increases dramatically from fetus to adult In rats, monkeys, and man, GLP-1 has been detected in neuronal cell bodies within the medulla oblongata , , Within the caudal medulla, immunostained cell bodies were located within the nucleus of the solitary tract and the dorsal and ventral parts of the medullary reticular nucleus GLP-1 neurons of the solitary tract constitute a distinct noncatecholaminergic cell group that projects to many sites within the brain, one of which is the hypothalamic paraventricular nucleus De novo synthesis of proglucagon occurs in these cell bodies as proglucagon mRNA is detected by in situ hybridization experiments using oligonucleotide probes , , The glucagon, GLP-1, and GLP-2 sequences are interrupted by short spacer sequences that encode intervening peptides Fig.

The N-terminal signal sequence is typical of preprohormones that are destined to cross-membranes during the biosynthesis of the protein, and the details of their function are well documented [for review see Ref.

It is somewhat remarkable that the six exons that comprise the transcribed region of the gene consist of distinct functional domains of the mRNA and encoded preproglucagon 65 , This exonic arrangement of the preproglucagon gene is a representative example of the modular arrays of exons that often encode specific functional domains of proteins DNA control elements and interactive transacting protein factors in the 2,bp promoter of the rat glucagon gene.

Habener, In: H. Fehmann, B. Göke eds The Insulinotropic Gut Hormone Glucagon-Like Peptide 1 , vol , ]. There are three known sites of expression of the proglucagon gene: the α-cells of the pancreatic islets, the L cells predominantly located in the distal ileum, colon, and rectum, and the nucleus tractus solitarius in the hindbrain, which is the nucleus of the vagus X nerve.

There is also expression of the proglucagon gene in magnacellular neurons of the hypothalamus. In many instances, nutrients and effectors that either stimulate or suppress secretion of glucagon or GLPs see below also likewise similarly control expression of the proglucagon gene at one or more levels of gene transcription, mRNA stability, or translation.

Several factors that stimulate the secretion of rat intestinal glucagon-like immunoreactive peptides, such as Bu 2 cAMP, forskolin, and cholera toxin, also elevate intestinal proglucagon mRNA levels, whereas other factors, phorbol esters and bombesin, stimulate secretion but not mRNA levels in intestine 61, It is also theoretically possible that regulation may be exerted at the level of posttranslational processing of proglucagon to glucagon and GLPs, but that has not been demonstrated yet.

Actually, there is not a great deal known about the mechanisms involved in the control of proglucagon gene expression. The promoter of the proglucagon gene has been analyzed in some detail by several groups of investigators over the past yr.

Five important transcriptional DNA control elements have been identified in the 2. The five DNA control sequences of approximately bp have been designated G1, G2, G3, CRE cAMP response element , and ISE intestinal specific element or GUE glucagon upstream enhancer A sixth subelement within G1 has been designated G4 The G1 element confers α-cell-specific expression of the glucagon gene in the pancreas, the G2 and G3 elements are enhancers specific to islet cells , the CRE lends cAMP responsivity to transcription of the preproglucagon gene , and ISE is a determinant for the transcriptional expression of the gene in intestinal L cells One caveat is that essentially all of the information on the cis -acting control elements and the transactivating DNA-binding proteins has been derived from studies in cultured insulinoma cells that express the glucagon gene.

These are transformed immortalized cells that may or may not be entirely representative of normal α- or L cells in the context of the living animal.

Secretory responses of GLP-1 isopeptides GLP-1 and GLP-1 amide to a meal in six nondiabetic subjects. RIAs are relatively specific for detection of the differences in the COOH-termini of the two isopeptides. Orskov et al. The G1 element of the preproglucagon gene promoter has been the most extensively studied of the five control elements identified so far.

It consists of nucleotides, is located close to the TATA-box to which the basal RNA polymerase complex of basal transcription factors are assembled, and seems clearly to be responsible for allowing the expression of the preproglucagon gene in α-cells , The exclusion of, or mutations within, the G1 element precludes expression of the gene in α-cells.

Deoxyribonuclease I DNase I footprint analysis of the G1 element indicates that a large, complex array of DNA-binding proteins interacts in this region of the promoter Some progress has been made in the identification of the specific DNA-binding proteins involved in interactions with the G1 element.

As might have been anticipated, three of the five DNA-binding proteins thus far identified to act on G1, Brn4 , Cdx2 , and Pax6 , are homeotic selector proteins, so called homeoproteins, critically involved in the determination of the body plan and organogenesis during development.

After completing their roles in embryonic development, homeodomain proteins typically exert a second role in the regulation of the expression of key genes in the fully differentiated cells, namely the α-cells with respect to Brn4, Cdx2, and Pax6.

In this regard, it is noteworthy that targeted disruptions of either the Brn4 M. Rosenfeld, University of California, San Diego, personal communication or Pax6 , genes in mice results in the failure of α-cells to develop in the pancreas.

The G2 and G3 DNA-control elements in the promoter are enhancers of proglucagon gene transcription function in islet cells, but are not restricted to islet cells. Transcription factors in the hepatocyte nuclear factor family HNF , HNF3β and HNFα, have been shown to interact with G2 and G3 , Isoforms of HNF-3 thereby either enhance or repress transcription of the proglucagon gene promoted by the G1 element and its cognate DNA-binding proteins described above.

The involvement of HNF transcription factors in the regulation of the expression of the glucagon gene is interesting because the liver and pancreas and spleen are derived from adjacent regions of the gut endoderm during development [for review see Ref. In conditions of chronic injury to the pancreas, such as invoked by dietary deficiencies of methionine or copper, pancreatic acinar tissue undergoes metaplasia to liver tissue.

Evidence has been reported that the G3 element may serve as a negative insulin response element, and thereby may account for the paracrine actions of insulin to suppress glucagon gene expression Transcription of the proglucagon gene in islet α-cell lines is enhanced by phorbol ester-mediated activation of protein kinase C However, in rat intestine, phorbol esters have no effect on proglucagon mRNA levels Recent studies identify the G2 element as the target of the stimulatory actions of phorbol esters and the interactions of the transcription factors HNF-3β and members of the Ets-related transcription factors The cAMP response element CRE is located in the promoter of the proglucagon gene adjacent to the G3 element.

The CRE confers cAMP responsivity to the transcription of the proglucagon gene , In studies in vitro in glucagon-producing insulinoma cells, it is clear that the CRE in the promoter of the proglucagon gene is a target for interactions with CRE-binding protein CREB , the CRE-binding protein involved in mediating cAMP responses of multiple genes , Proteins that bind to sites adjacent to the CRE and inhibit the CREB-mediated cAMP stimulation of glucagon expression, designated CAPs CREB-associated proteins , have been described Notably, NF-Y is reported to bind to DNA sites immediately adjacent to the CRE of the rat insulin-1 gene promoter and to inhibit cAMP-responsive gene transcription The identification of a functional CRE in the promoter of the proglucagon gene is consistent with the reported findings of cAMP-coupled GLP-1 and GIP receptors on pancreatic α-cells and that GLP-1 and GIP stimulate the secretion of glucagon from α-cells by cAMP-dependent mechanisms The intestinal L cell permissive enhancer in the promoter of the proglucagon gene is less well defined compared with the pancreaticα -cell-specific promoter element G1.

The identification of an intestinal-specific promoter element ISE was accomplished by transient transfection-expression studies in SV40 transformed cell lines obtained from intestinal L cell tumors that arose in mice made transgenic with a SV40 large T antigen driven by the 2.

When the modular exonic arrangement of the proglucagon gene was first noted 65 , it seemed likely that alternative exon splicing would generate distinct mRNAs each encoding either glucagon or the GLPs. Indeed, in the frog, lizard, chicken, and fish, alternative RNA splicing of two proglucagon genes generates proglucagon mRNA transcripts that encode glucagon and GLP-1, but not GLP-2 in the pancreas, whereas mRNAs for all three are generated in the intestine However, in mammals, the diversification of the expression of the preproglucagon gene occurs at the level of alternative posttranslational processing of proglucagon 60 , 68 , 69 , Fig.

There is a remarkably specific alternative processing of proglucagon: the predominant bioactive peptide produced in the pancreatic α-cells is glucagon, whereas in the intestines and the brain the bioactive products produced are predominantly GLPs.

Thus, the alternative processing reflects a dichotomy between the expression of hormones essential for the regulation of glucose metabolism in the fasting vs. the fed state. Glucagon is operative during fasting in mobilizing glucose from peripheral tissues to maintain blood glucose levels, whereas GLP-1 comes into play during feeding to augment glucose-dependent insulin release, and possibly to promote satiety.

Several of the enzymes that posttranslationally cleave proproteins into peptides or hormones have been identified. These enzymes comprise a family known as subtilisins or subtilisin-like proprotein convertases, otherwise known as prohormone convertases PCs Further, PC2 null mice defective in the expression of PC2 manifest severe fasting hypoglycemia and a reduced rise in blood glucose levels during an intraperitoneal glucose tolerance test, consistent with a deficiency of circulating glucagon Determinations of circulating profiles of immunoreactive GLP-1 levels have provided information regarding the physiological processes that regulate GLP-1 secretion.

Before the development of specific GLP-1 RIAs in the late s, L cell secretion was usually quantified as gut glucagon-like immunoreactivity gGLI , which includes glicentin plus oxyntomodulin.

Because gGLI is produced in quantitatively identical amounts to GLP-1 after posttranslational processing of proglucagon 69, , , , studies reporting secretion of gGLI reflect that of GLP More recently, assays have been developed utilizing antisera that specifically detect GLP These procedures may detect both pancreatic and intestinal derived GLP Although small quantities of GLP-1 and amide may be produced by pancreaticα -cells and cosecreted with glucagon , the major source of circulating GLP-1 is the intestinal L cell As discussed below, numerous studies have revealed that the release of GLP-1 is under the control of nutrients, hormones, and neural inputs.

The result is a biphasic mechanism of release, with both hormonal and neural mediation of early GLP-1 release min , and direct nutrient contact with L cells mediating later GLP-1 secretion min.

To allow for a direct assessment of the interactions among paracrine, endocrine, neural, and luminal influences at the level of the L cell, new in vitro techniques were required. A major factor impeding studies at the cellular level is the diffuse nature of the distribution of intestinal L cells.

However, models consisting of primary cultures of rat intestinal cells or canine ileal mucosal cells have been successfully developed as in vitro strategies to study the production of GLP-1 , , , The limited numbers and viability of cells obtained by these techniques in addition to the heterogeneity of the isolated cells prevent extensive analysis of proglucagon gene regulation.

The development of tumor-derived cell lines that express proglucagon-derived peptides has aided in this regard. The GLUTag cell line was developed from intestinal tumors in proglucagon-SV40 large T antigen transgenic mice , whereas the STC-1 cell line was derived from an intestinal endocrine tumor that developed in mice carrying the transgenes for the rat insulin promoter linked to SV40 large T antigen and the polyoma virus small T antigen The development of in vitro methods to study GLP-1 release at the cellular level has enabled the analysis of intracellular signal pathways that regulate the secretion and expression of GLP Studies with intestinal cell cultures and the L cell line, GLUTag, indicate that the activation of protein kinase A stimulates both GLP-1 release and synthesis 61, , , , , In contrast, activation of protein kinase C results in an increased secretion of GLP-1 in intestinal cell cultures , , , and the GLUTag and STC-1 cell lines , , , but does not appear to increase transcription of the proglucagon gene 61 , Treatment with the phospholipase C activator α-ketoisocaproic acid does not enhance GLP-1 secretion by either fetal rat intestinal cultures or GLUTag cells Inhibition of GLP-1 secretion by a calcium channel blocker CoCl 2 and stimulation of GLP-1 release by increasing intracellular calcium concentrations indicate a primary role of calcium in basal secretion by the L cell Thus there may be multiple signals involved in the L cell response that are perhaps important in allowing for an integrated response to a variety of different L cell effectors.

GLP-1 is released into the circulation after a meal 72, , , Significantly more GLP-1 is released after a liquid meal than a solid meal of identical composition The majority of GLP-1 released appears to be in the form of GLP-1 amide with levels reaching approximately 50 p m , whereas GLP-1 rises to 10 p m Fig.

In keeping with the role of GLP-1 as an incretin hormone, the oral intake of glucose alone stimulates GLP-1 release in humans 72, , , pigs , , dogs , and rats , In contrast to oral glucose administration, elevation of plasma glucose by the administration of glucose systemically does not stimulate GLP-1 secretion, indicating the glucose sensing machinery is distributed on the luminal side of the intestine , , Infusion of glucose into the intestinal lumen stimulates GLP-1 release in humans , rats , , dogs , , , and pigs These observations are consistent with the role of GLP-1 as an important incretin hormone acting on the pancreatic β-cells to stimulate appropriate insulin release after glucose absorption.

A, Effects of different concentrations of synthetic GLP-1 and glucagon on insulin secretion from the perfused rat pancreas. Background perfusate contains 6. Insulin responses at 2. Weir et al. Consistent with these findings, other sugars that utilize this cotransporter for absorption across the intestinal epithelium, e.

Nontransportable sugars, e g. Furthermore, GLP-1 release from canine or rat ileum perfused in response to the carbohydrates methyl-α d -glucoside and 3-O-methyl- d -glucose indicate that intracellular metabolism and intracellular removal, respectively, are not essential to induce GLP-1 secretion in rats , Although high concentrations of glucose 28 m m have been demonstrated to stimulate GLP-1 secretion from isolated rat intestinal cell cultures , it is unlikely that glucose normally acts directly on L cells.

Indeed, a recent study did not observe any effect of glucose m m on GLP-1 secretion from isolated canine intestinal cells Under normal feeding conditions, the majority of glucose is absorbed before reaching the ileum In addition, the rapid GLP-1 secretory response to oral glucose 72 , , , , suggests that glucose must activate the release of GLP-1 by means other then a direct effect on L cells.

In addition to glucose, fats appear to stimulate the release of proglucagon-derived peptides, perhaps related to the roles of both oxyntomodulin and GLP-1 as enterogastrones, or inhibitors of gastric function The secretion of GLP-1 is increased by ingestion of mixed fats or triglycerides in humans , , , , and dogs and by placement of mixed fats directly into the intestinal lumen of rats , and pigs Interestingly, Roberge and Brubaker discovered that placement of fat in the duodenum of rats stimulates GLP-1 secretion independently of the contact of nutrients with the distal L cells.

Furthermore, duodenal fat increased the secretion of GLP-1 into the circulation to the same extent as was observed after the direct administration of fat into the ileum , These observations suggest the existence of a proximal-distal loop regulating the L cell response to ingested nutrients Such a mechanism could contribute to the significant increase in circulating GLP-1 levels observed within min of ingesting a meal, before contact of nutrients with the L cells 72 , , , , , , As discussed below, among the potential mediators of such a loop are various endocrine and neuroendocrine peptides, as well as neurotransmitters.

The observation of fatty acid-induced GLP-1 release from isolated intestinal cell cultures suggests that fatty acids can act directly on the L cell , , Interestingly, bile acids appear to increase the secretion of proglucagon-derived peptides in humans , dogs , and rats , suggesting that the arrival of bile into the ileum may play an important feedback message for the release of GLP Results obtained with fatty acids indicate that both the chain length and degree of saturation of the fatty acids affect the ability of fats to stimulate GLP-1 secretion.

However, long-term exposure of rats to short-chain fatty acids derived from a diet containing readily fermentable fibers increases proglucagon mRNA levels and secretion of GLP-1 in response to a glucose challenge Mixed meals that contain proteins increase GLP-1 secretion in humans 72, , , , and rats , However, either amino acids or protein alone did not consistently increase GLP-1 release in in vivo studies in humans , , , dogs , , or rats , Recently, it was discovered that unlike protein or an amino acid mixture, protein hydrolysates peptones stimulate GLP-1 secretion from isolated vascularly perfused rat intestine and the murine enteroendocrine cell line STC-1 It was argued that the peptones mixtures of oligopeptides of various molecular weights are more likely to closely mimic the protein-derived components of the intestinal chyme than would undigested proteins or amino acids.

Furthermore, peptone treatment of STC-1 and GLUTag cells with peptones resulted in a significant increase in proglucagon RNA levels as a result of increased transcription of the glucagon gene There was no effect of peptones on proglucagon RNA levels in pancreatic glucagon-producing cell lines Therefore, the protein content of a mixed meal may contribute to GLP-1 secretion and synthesis via the production of peptones that contact L cells in the jejunum.

In addition to nutrients, hormones regulate GLP-1 secretion. Insulin has been reported to inhibit GLP-1 release both in vitro and in vivo , perhaps acting as part of a feedback loop. Somatostatin is an intestinal peptide that inhibits release from many endocrine cells through an inhibitory G protein , Indeed, somatostatin has been shown to inhibit GLP-1 release in vivo in the rat and dog , and in vitro with rat and canine intestinal cell cultures , , , Of the endocrine peptides tested for effects on the L cell thus far, only GIP has been found to stimulate GLP-1 release , , , , As discussed above earlier in this article, GIP is an intestinal hormone that acts both as an enterogastrone to inhibit gastric acid production and an incretin hormone that stimulates insulin release see Ref.

However, in contrast to the GLPproducing ileal L cells, GIP is secreted from K cells that are primarily located in the duodenum, and thereby are in an ideal location for regulation by nutrients. GIP is released rapidly in response to ingestion of nutrients, which are thought to act directly on the K cell.

In rats, GIP was found to stimulate intestinal GLP-1 secretion when infused in vivo to mimic postprandial GIP concentrations GIP also increases GLP-1 release from the isolated vascularly perfused rat ileum Furthermore, GIP is a potent stimulator of both GLP-1 synthesis and secretion from rat intestinal cells in vitro , , and isolated canine L cells The mechanism of GIP-induced GLP-1 release appears to occur, at least in part, by activation of protein kinase A These observations support the concept of a proximal-distal loop whereby nutrients entering the duodenum stimulate the release of GIP, which then circulates to the L cells of the ileum promoting the secretion of GLP Currently, however, studies do not support the existence of a similar proximal-distal loop pathway in humans 72 , , Furthermore, infusion of an antagonist to the neuropeptide gastrin-releasing peptide bombesin concomitant with the placement of fats in the duodenum abrogated the stimulatory effects of the proximal nutrient on the distal L cell These findings suggest that physiological doses of GIP act through the nervous system either vagal or myenteric to indirectly stimulate GLP-1 secretion, rather than acting directly at the level of the L cell.

In support of neural regulation of GLP-1 release, Rocca and Brubaker have recently demonstrated that bilateral subdiaphragmatic vagotomy in conjunction with gut transection completely abolishes fat-induced GLP-1 release in rats Consistent with a role for the vagus in the regulation of the L cell, stimulation of the distal end of the celiac branch of the subdiaphragmatic vagus nerve significantly stimulates the release of GLP-1 Furthermore, GLP-1 secretion induced by exogenous GIP administration is abolished by selective hepatic branch vagotomy Collectively, these findings indicate that GIP acts through vagal afferent pathways to stimulate the L cells indirectly.

This stimulation is carried to the L cells by efferent pathways located in the celiac branch of the vagus nerve. Gastrin-releasing peptide stimulates GLP-1 release in vivo in humans , rats , , and dogs , ; in the perfused intestinal rat loop , , and pig loop ; and in rat and isolated canine intestinal cells 61, Interestingly, the neuropeptide galanin inhibits both basal and gastrin-releasing peptide-induced GLP-1 secretion from isolated rat ileal cells through pertussis toxin-sensitive G protein and ATP-dependent potassium channels Additional neurotransmitters and neuropeptides also likely mediate early secretion of GLP Indeed, acetylcholine and muscarinic cholinergic agonists appear to stimulate GLP-1 secretion in the rat , , In addition, the cholinergic agonist carbachol stimulates GLP-1 release from the murine cell lines STC-1 and GLUTag, evidently by activation of the muscarinic M3-subtype receptors , In humans, the infusion of atropine reduces the secretion of GLP-1 in response to oral glucose, findings consistent with a direct cholinergic muscarinic control of L cells Epinephrine and the β-adrenergic agonist, isoproterenol, stimulate GLP-1 secretion when infused into the isolated rat ileum or colon , but not when tested for direct effects with GLUTag or rat intestinal cells in vitro , Epinephrine also stimulates GLP-1 release in the dog in vivo , and is stimulatory when added directly to isolated canine L cells in vitro Collectively, these findings underscore the complexity of mechanisms regulating GLP-1 release from the distal L cells in response to the presence of nutrients in the proximal duodenum, involving an interaction of neural and endocrine pathways.

During the mid to late s it was recognized that GLP-2 was specifically processed from preproglucagon in the intestine and was not liberated in appreciable quantities in pancreatic α-cells 69, , , Although it would be predicted that GLP-2 should be secreted in parallel with GLP-1 in equal molar quantities, few studies have attempted to measure GLP-2 levels in the circulation.

Ørskov and Holst developed specific RIAs for GLP-1 and GLP-2 and reported basal plasma levels of ± 7 p m and ± 14 p m , respectively, with levels reaching ± 13 and ± 15 p m 2 h after a mixed meal More recently, Brubaker et al.

After its secretion, the metabolism of GLP-1 represents an important process in determining the levels of bioactive hormone in the circulation and may possibly be a means for further proteolytic processing. Elimination of bioactive GLP-1 from the circulation may occur via at least three different mechanisms: renal clearance, hepatic clearance, and degradation in the circulation.

In support of an important role for the kidneys in the clearance of GLP-1, the levels of immunoreactive GLP-1 are significantly elevated in uremic patients Renal extraction of endogenous and exogenous GLP-1 was also detected in anesthetized pigs Nephrectomy or uretal ligation in rats increases the circulating half-life of GLP-1, and GLP-1 is extracted from perfusate of isolated rat kidneys Collectively, the findings suggest that kidneys remove GLP-1 from the peripheral circulation by a mechanism that involves glomerular filtration and tubular catabolism , Although no net extraction of endogenous GLP-1 across the liver has been detected, significant hepatic extraction of GLP-1 during a systemic infusion was identified in anesthetized pigs In accordance with this MCR, GLP-1 is eliminated relatively rapidly from plasma, with a half-life of approximately 5 min in humans 72 , , , , pigs , dogs , and rats , It is noteworthy that, because post-secretory degradation of the GLP hormones in the circulation may generate products that are immunoreactive in assays but are no longer biologically active, these assay values of circulating levels of GLP-1 and GLP-2 may overestimate the true biological half-life of these hormones.

Indeed, as described below, the biological half-life of GLPs appears to be in the range of min. Degradation of GLP-1 in the circulation appears to occur initially by dipeptidyl peptidase IV DPP IV; EC 3.

These truncated forms of GLP-1 have been demonstrated to be the major metabolites of GLP-1 formed in human , canine 93 , porcine , and rat serum. In contrast, GLP-1 remained intact for at least 10 min in rats that were DPP IV-deficient It is likely that there is subsequent enzymatic degradation of GLP-1 after cleavage by DPP IV by other enzymes 93 , , Multiple degradation products were observed by incubation of GLP-1 with purified human neutral endopeptidase NEP In pigs, inhibition of DPP IV activity potentiates the insulin response to GLP-1, indicating that the intact N terminus of GLP-1 is important for its insulinotropic activity Furthermore, the oral administration of a DPP IV inhibitor to Zucker fatty rats improves glucose tolerance by increasing the circulating half-lives of the endogenously released incretins GIP and, particularly, GLP-1 Thus, analogs of GLP-1 that are DPP IV resistant have extended metabolic stability and may have extended insulinotropic activity in vivo It remains possible, however, that the metabolic products of GLP-1 have important biological actions different from those of the parent peptides.

Further, GLP-1 amide can antagonize the ability of native GLP-1 to generate adenyl cyclase activity by the pancreatic GLP-1 receptor Recently, it was shown that GLP-1 amide could antagonize the inhibitory effect of GLP-1 amide on antral motility in anesthetized pigs Whether sufficient quantities of this metabolite GLP-1 amide exist in vivo to act as an antagonist of GLP-1, or possibly to mediate other biological activities, remains to be determined.

GLP-2 is liberated from proglucagon in the intestinal L cells 69 , , , The MCR for GLP-2 has presently not yet been estimated, and the sites of clearance have not been investigated.

However, GLP-2 levels are elevated in patients with chronic renal failure, indicating a role for the kidney in the clearance of circulating immunoreactive GLP-2 Similar to GLP-1 amide, this truncated form of GLP-2 is a result of cleavage by DPP IV , The expression of DPP IV within the intestinal epithelium , could account for the detection of GLP-2 in extracts of ileum Likewise, the truncated GIP has been detected in extracts of duodenal mucosa DPP IV-mediated cleavage of GLP-2 appears to limit the intestinotrophic activity of the GLP-2 hormone A GLP-2 analog containing glycine at position 2, thereby resistant to DPP IV, had greater intestinotrophic activity in rats compared with the native rat peptide The physiological actions of GLP-1 reflect the functions of organs in which specific GLP-1 receptors are expressed.

These organs include the pancreatic islets, stomach, lung, brain, kidney, pituitary gland, cardiovascular system heart , kidney, and small intestine , However, there are reports of actions of GLP-1 on organs such as liver, adipose tissue, and skeletal muscle in which attempts to definitively identify GLP-1 receptors have not succeeded.

This circumstance suggests the existence of as-yet-unidentified GLP-1 receptors that are distinct from the known, well characterized receptor.

The earliest discovered biological actions of GLP-1 were on the pancreatic β-cells in which GLP-1 and GLP-1 amide were shown to be highly equipotent secretagogues for glucose-dependent insulin secretion Fig. Studies employing exendin as an antagonist in vivo have confirmed that the insulinotropic nature of GLP-1 makes an important contribution to the enteroinsular axis in rats , , baboons , and humans Furthermore, mice with a null mutation in the GLP-1 receptor are glucose intolerant Importantly, this insulinotropic action of GLP-1 is attenuated as ambient glucose levels fall Fig.

The glucose-dependent nature of the incretin hormones GLP-1 and GIP is an efficient protective measure against hypoglycemia. The interdependence between glucose and incretin actions involves a cross-talk between glycolysis glucose metabolism and cAMP signaling pathways of the activated GLP-1 or GIP receptor.

The glucose competence concept has been used to describe the mutual interdependence between glucose metabolism and GLP-1 actions on β-cells i. e , glucose is required for GLP-1 action, and GLP-1 is required to renderβ -cells competent to respond to glucose Fig.

This property of GLP-1 may improve the ability of β-cells to sense and respond to glucose in subjects with impaired glucose tolerance However, in the absence of GLP-1 signaling, i.

Model of the proposed ion channels and signal transduction pathways in a pancreatic β-cell involved in the mechanisms of insulin secretion in response to glucose and GLP The key elements of the model are the requirement of dual inputs of the glucose-glycolysis signaling pathway resulting in the generation of ATP and an increase in the ATP:ADP ratio, and the GLP-1 receptor GLP-1R -mediated cAMP PKA pathways to effect closure of ATP-sensitive potassium channels K-ATP consisting of the inward rectifier Kir6.

The closure of these channels results in a rise in the resting potential depolarization of the β-cell, leading to opening of voltage-sensitive calcium channels L-type VDCC. Phosphorylation of vesicular granule proteins by PKA may also trigger insulin secretion. Repolarization of theβ -cell is achieved by opening of calcium-sensitive potassium channels Ca-K.

It is believed that the GLP-1 receptor is coupled to a stimulatory G-protein Gs and a calcium-calmodulin-sensitive adenylate cyclase. Insulinotropic actions of GLP-1 onβ -cells mediated by activation of the cAMP-signaling pathway. The binding of GLP-1 to its receptor Re activates adenylyl cyclase A c , resulting in the formation of cAMP.

Binding of cAMP to the regulatory R subunit of PKA results in the release of the active catalytic C subunit. The active kinase then translocates to the nucleus and phosphorylates, and therefore activates, the nuclear transcriptional activator CREB bound to the CRE located in the promoter of the proinsulin gene.

This cascade of signaling results in a stimulation of transcription of the proinsulin gene and increased insulin biosynthesis to replete stores of insulin secreted in response to nutrients glucose and incretins GLP-1, GIP. Habener: In Diabetes Mellitus , pp , ]. Not only does GLP-1 stimulate insulin secretion, but it also stimulates transcription of the proinsulin gene and the biosynthesis of insulin 73 , Fig.

Nevertheless, these properties clearly distinguish GLP-1 from those of the sulfonylurea class of hypoglycemic drugs that effectively stimulate insulin secretion but do not stimulate biosynthesis of proinsulin GIP stimulates both insulin secretion and production , in conditions of normoglycemia, but unlike GLP-1, GIP is ineffective in the stimulation of insulin secretion in individuals with type 2 diabetes , Recent evidence indicates that GLP-1 may stimulate the proliferation and neogenesis of β-cells from ductal epithelium of mice and rats , In the β-cell line INS-1, GLP-1 synergizes with glucose to activate expression of immediate-early response genes coding for transcription factors implicated in cell proliferation and differentiation c- fos , c- jun , junB, zif, nur Moreover, administration of GLP-1 to aged rats that characteristically develop glucose intolerance between 18 and 20 months of age reverses the glucose intolerance Thus GLP-1 may have potent pleiotrophic actions on both mature β-cells and duct cells that are progenitors of β-cells.

Summary of GLP-1 actions. The diagram summarizes the currently understood targets of GLP-1 actions. In the endocrine pancreas GLP-1 stimulates both insulin and somatostatin secretion in a glucose-dependent manner and inhibits glucagon secretion.

However, it is uncertain whether GLP-1 inhibits glucagon secretion by direct actions on α-cells or indirectly by the known paracrine-inhibitory effects of insulin and somatostatin on α-cells. GLP-1 is an effective inhibitor of gastric motility and emptying and curtails food intake by inducing satiety.

Receptors for GLP-1 have been detected also on α-cells and δ-cells , , , The secretion of somatostatin increases in response to GLP-1 in rat islets and in isolated perfused rat and canine pancreases , Although GLP-1 appears to inhibit glucagon secretion in vivo 87, , , it stimulates glucagon release in vitro , During feeding, such an effect would be overcome by the combination of elevated insulin, somatostatin, and glucose, which collectively inhibit glucagon secretion.

Thus the suppression of glucagon release observed in vivo may be indirectly attributable to the paracrine actions of the intraislet release of insulin and somatostatin.

Leptin, the obesity hormone produced by adipose tissue, has opposing actions to GLP-1 on pancreatic β-cells. Leptin suppresses insulin secretion and gene expression , both of which are stimulated by GLP However, it is worth noting that the inhibition of insulin secretion by leptin may be overridden by GLP-1, thereby assuring adequate insulin secretion in response to meals , The feedback loop between leptin fat and insulin pancreatic β-cells constitutes an adipoinsular axis that operates physiologically in parallel with the enteroinsular axis feedback loop involving GLP-1 intestine and insulin.

Disruption of either axis appears to result in glucose intolerance and reveals the opposing actions of leptin and GLP For example, mice with a null mutation in the GLP-1 receptor are more sensitive than wild-type mice to the insulin lowering effect of leptin, reflecting the interaction of GLP-1 and leptin in the regulation of insulin secretion Such a mechanism could contribute to the profound hyperinsulinemia in these animals and possibly in subjects with type 2 diabetes It is well recognized that gastric function can be regulated by the distal portion of the small intestine.

In humans, diversion of chyme from the ileum reduces the gastric secretory response compared with exposure of chyme to the entire small intestine As reviewed earlier, chyme and fats are potent stimulators of GLP-1, indicating GLP-1 may be a candidate hormone for regulating gastric function.

Indeed, GLP-1 inhibits gastric acid secretion pentagastrin- as well as meal-induced and gastric emptying when infused in quantities that result in plasma concentrations similar to those observed after meals , , , In rats, this effect of GLP-1 may be mediated by inhibition of gastrin secretion and stimulation of the release of gastric somatostatin , However, in pigs and humans, GLP-1 does not seem to regulate the release of either gastrin or somatostatin 72 , , , , In these species, the inhibitory effect of GLP-1 on upper gastric functions could involve receptors located either in the central nervous system or associated with afferent pathways to the brain stem These possibilities are supported by the observations that the inhibitory effect of GLP-1 on gastric emptying requires intact vagal enervation , , Therefore, despite the known insulinotropic actions of GLP-1, the net effect of administering GLP-1 with a meal in healthy humans is a reduction in meal-related integrated incremental glucose and insulin responses This observation supports the concept that the primary physiological role of GLP-1 may be as a mediator of ileal brake mechanisms, rather than as a incretin hormone The actions of GLP-1 to delay gastric emptying are under investigation as an aspect of therapy for diabetes to attenuate the postprandial glucose excursion.

GLP-1 receptors are expressed at high density in rat lung membranes , , and on vascular smooth muscle The treatment of rat trachea and pulmonary artery with GLP-1 results in inhibition of mucous secretion and relaxation of smooth muscle The sequence of the cDNA for the GLP-1 receptor expressed in rat lung is identical to the β-cell receptor except for one codon When expressed in Chinese hamster ovary CHO cells, this receptor displays a pharmacological profile similar to that seen with cells expressing the β-cell-derived cDNA Notably, GLP-1 receptor mRNA is detected in type II pneumocytes and stimulates the secretion of surfactant from these cells The overall physiological role of GLP-1 actions on the lung remains uncertain.

The unusually high abundance of receptors in the lung suggests important actions of GLP-1 in pulmonary physiology. It is difficult to envision how GLP-1 actions on the lung would relate to the release of GLP-1 from the intestine in response to meals.

One possibility is the local production of proglucagon and GLP-1 within the lung to establish a paracrine loop, but proglucagon expression has not yet been detected in the lung.

Perhaps the most surprising and unexpected actions of GLP-1, discovered only recently, are on the hypothalamus to inhibit food and water intake. GLP-1 appears now to be an anorexigenic hormone similar in action to the obesity hormone leptin and to antagonize orexigenic hormones such as CRF and neuropeptide Y.

The discovery of these actions of GLP-1 on the promotion of satiety and the suppression of energy intake are recent and are somewhat controversial. It had been known from earlier studies that binding sites for GLP-1 exist in plasma membranes prepared from rat brain , , , and by in situ binding studies that receptors exist in and around the hypothalamus and arcuate nucleus , The density of GLP-1 receptors is particularly high in the arcuate nucleus, the paraventricular and supraoptic nuclei, and in the sensory circumventricular organs such as the subfornical organ, organum vascularum, laminae terminus, and the area postrema.

The expression of GLP-1 receptors in the brain was confirmed by RT-PCR cloning of the GLP-1 receptor from mRNA prepared from rat brain It was also shown in earlier studies that proglucagon and proglucagon-derived peptides are produced locally in the brain see Section V.

High densities of GLPimmunoreactive nerve fibers are present in paraventricular nucleus, dorsomedial hypothalamic nucleus, and the subfornical organ. Several studies have now shown that the administration of GLP-1 into the third intracerebral ventricles of rats results in a profound decrease in food consumption These effects of GLP-1 appear to be mediated by interactions on specific GLP-1 receptors because the reduction in food intake is greatly attenuated by prior or coadministration of the GLP-1 receptor antagonist, exendin The intracerebral ventricular administration of GLP-1 results in a marked enhancement of the expression of the immediate early responsive transcription factor c- fos in neuronal cell bodies located in the ventral medial hypothalamus and a corresponding reduction in the expression of the orexigenic hormones neuropeptide Y and GRH , Notably, ablation of the arcuate nucleus and parts of the circumventricular organ by administration of monosodium glutamate to rats abolishes the inhibition of feeding invoked by intracerebral ventricular injection of GLP-1 Whether in physiological circumstances GLP-1 produced locally in the brain or GLP-1 in the circulation acts on hypothalamus receptors is uncertain.

The administration of GLP-1 by the intraperitoneal route is reported to be ineffective in reducing food intake in rats There is some debate about whether the reduction of inhibition of feeding behavior in rats in response to intracerebroventricular GLP-1 is due to satiety or to a food aversion , , Of additional concern is the observation that GLP-1 receptor null mice lacking a functional GLP-1 receptor display normal feeding behavior, although they are glucose intolerant , However, in studies in humans, infusions of GLP-1 for 2, 6, 8, or 48 h appear to result in a reduction in food intake and have been interpreted as a satiety effect and not food aversion There are at least two mechanisms by which GLP-1 may gain access to the appetite control centers located in the hypothalamus: local production of GLP-1 within the brain and uptake of intestinally derived GLP-1 in the circulation.

Compelling experimental evidence has been presented in support of both mechanisms, and they are not mutually exclusive. The proglucagon gene is expressed in the nucleus of the solitary tract, which is the nucleus of the vagus nerve that regulates the autonomic functions of the gut.

Furthermore, proglucagon produced in the nucleus tractus solitarius is processed to GLPs Injection of the retrograde tracer FluoroGold Fluorochrome International, Englewood, CO into the nucleus of the solitary tract showed that the caudal neurons containing GLP-1 project to the paraventricular nucleus Thus, an attractive mechanism for the exertion of GLP-1 actions to inhibit feeding behavior would be the activation of GLP-1 production in the nucleus tractus solitarius via afferent enervation from the vagus nerve.

Oral nutrients would then signal to the brain through the autonomic nervous system. It is tempting to speculate that this may constitute a prandial satiety signal generated during feeding, a signal to cease food consumption because enough has already been consumed.

However, if an axonal transport of GLP-1 from the hindbrain to the hypothalamus is required, it may not be rapid enough to account for meal-induced satiety min.

Perhaps the more plausible mechanism is the uptake by brain of GLP-1 in the circulation released from the intestines in response to a meal. Remarkably, I-labeled GLP-1 injected into rats localizes to the subfornical organ and the area postrema of the brain within 5 min after the injection These regions of the circumventricular organ are known sites where blood-borne macromolecules can pass across the blood-brain barrier.

The satiety-inducing obesity hormone leptin in the circulation is believed to gain access to the satiety centers in the hypothalamus via the circumventricular organ that contains a high concentration of leptin receptors, so called short-form receptors that have high affinity for leptin, but are defective in their signal transduction The model proposed for leptin transport into the brain is that the receptors extract leptin from the plasma and transport the leptin into the hypothalamus.

Peptide hormones are powerful regulators of various biological Glucagpn. Insulin resistance diet Glucabon continuous availability and function, peptide hormone Insulin resistance diet must Insulin resistance diet tightly coupled to its biosynthesis. Gludagon a Promoting heart wellness coupling has been demonstrated for Glucaagon however, because of insulin's Insulin resistance diet role as the sole blood glucose-decreasing peptide hormone, this coupling is considered an exception rather than a more generally used mechanism. Here we provide evidence of a secretion-biosynthesis coupling for glucagon, one of several peptide hormones that increase blood glucose levels. We show that glucagon, secreted by the pancreatic α cell, up-regulates the expression of its own gene by signaling through the glucagon receptor, PKC, and PKA, supporting the more general applicability of an autocrine feedback mechanism in regulation of peptide hormone synthesis.

Nadejda BozadjievaInsulin resistance diet A. Williams and Ernesto Bernal-Mizrachi. Gene biosyntheesis Gcg. In his characterization of stained hamster pancreatic sections, Lane described the existence of two Meal prepping for strength training types of cells within the islet, which he referred to as Glucabon large α and smaller β cells Four Glcuagon Glucagon biosynthesis, Sutherland and de Duve established that the α-cells biosynthessi the pancreatic islet are the primary source of glucagon 95, Banting Mood enhancing vitamins Best Glucwgon their first pancreatic bkosynthesis in depancreatized biosynthesiss and observed an G,ucagon transient rise in blood glucose followed bkosynthesis the insulin-induced hypoglycemia Insights into the Skin rejuvenation catechins of glucagon release came Insulin resistance diet elegant cross-circulation experiments performed by Glucagom and his Gluucagon in the s, showing biosnythesis hypoglycemia triggered by the injection of insulin in Mental agility exercises donor dog induces the release of glucagon, which secreted biosynthessis the donor blood via a pancreatic-femoral anastomosis causes a hyperglycemic response in a recipient biosyntheais Bensley Glcuagon Woerner added to these observations with their suggestion that glucagon induces boisynthesis glycogenolysis and thereby promotes a biosynthesiss in blood glucose levels 4.

Glucagon biosynhhesis a amino acid biosyntuesis derived from the tissue biostnthesis of proglucagon in pancreatic α-cells through Glcagon by prohormone convertase 2 PCSK2 Figure bkosynthesis Glucaagon contrast, processing biosyntheiss proglucagon to glucagon-like peptides Glucxgon, GLP-2oxyntomodulin and glicentin occurs in intestinal enteroendocrine cells Figure Gpucagon Glucagon biosynthessis a major role biosyntuesis antagonizing the effects of insulin and maintaining glucose homeostasis biosynthfsis promoting hepatic gluconeogenesis and glycogenolysis and inhibiting glycogen synthesis.

Therefore, Glucaagon secretion of glucagon is normally induced in boosynthesis of decreasing blood glucose, such biosyjthesis fasting and Insulin resistance diet energy expenditure, biosynthesiw sufficiently induce a rapid, yet transient, rise in blood glucose.

However, aberrant secretion of bipsynthesis is a GGlucagon of Insulin resistance diet I and II Diabetes. Increased Gluxagon of Biosynthesiis is observed in patients with Bosynthesis II Diabetes, leading to an increase Insulin resistance diet hepatic glucose biowynthesis and exacerbating the biosynthseis state 19,63,79,84, In Glucabon, failure biosynthesks secrete adequate glucagon in response in hypoglycemia is a limiting High-end for glucose control, boosynthesis to biosyhthesis Insulin resistance diet Simple Sugar Carbohydrates mortality of patients with Type I Diabetes Insulin resistance diet, Figure 1.

Body composition and lean mass of differential processing of the proglucagon gene product in biossynthesis of the pancreas, gut L-cells biosynthesjs brain. Only biologically active products are GI database. For further details Glucagln cleavage sites and processing see reference The five major islet cell types are aligned on blood biosynthesus at no particular Sweet potato and broccoli quiche or biowynthesis organization within the human islet Gut health and hydration Figure biosymthesis.

In contrast, the rodent Integration with social media platforms shows a more-defined Glucagno placing the β-cells Endurance nutrition for cyclists the core biozynthesis the α, δ and PP-cells lying at the mantle of the islet Figure 2B.

This unique structure biostnthesis the rodent islet Guarana for Brain Health an organized system allowing paracrine interactions between the peptides released.

This is supported by studies showing that arterial blood Glucose regulation disorders directed from the core biosynthess the rodent islet insulin-secreting Glicagon to the periphery biozynthesis. Therefore, during a rise in blood glucose, the pancreatic bisoynthesis are exposed to high levels Goucagon secreted insulin leading to the inhibition of glucagon secretion and glucagon gene transcription.

Glucagon release is inhibited after carbohydrate-rich meal and the consequent biosynrhesis in blood glucose Goucagon insulin secretion.

However, biosynthesiw meal rich biosyntheeis amino acids induces glucagon release. Parasympathetic vagal and biosynthsis Epinephrine, Norepinephrine, Biosyntbesis, Neuropeptide Y nerve stimulations induce the secretion of glucagon from the pancreatic α-cells.

Glucagon Glucagon biosynthesis secreted by Glkcagon pancreatic α-cells in states of decreasing blood glucose; however, Herbal energy support changes in glucose concentrations alone can regulate glucagon secretion still remains unclear.

High glucose biosynthrsis inhibit glucagon release in the intact islet; biostnthesis, high Vegetarian weight control induces biosyntjesis release Glucgaon dispersed, isolated biosyntheis.

However, the chronic biosyntbesis of α-cells to biosynhtesis glucose levels has been biosynthessi to induce α-cell dysfunction and insulin-resistance, closely biosyynthesis the diabetic state 21, biosynthesus Rat α-cells biosjnthesis glucokinase Glucahon glucose transporter GLUT1, an biisynthesis with a biosgnthesis capacity compared to GLUT2, which Carbohydrate loading for marathon training the predominant form in insulin-secreting β-cells Despite differences in metabolism of glucose by the bjosynthesis cells types, studies have shown Glkcagon they share similar inherent mechanisms of bioynthesis Glucagonn cells have high ATP concentrations under low glucose, which rise biosynthesjs after Gucagon with high glucose.

The biosynthesi inhibition Teeth whitening solutions glucagon secretion was associated with an inhibition Gucagon AMPK activity and activating AMP-activated protein kinase AMPK in turn inhibited secretion Therefore, α-cells biosynthess have intrinsic mechanisms, which respond to glucose Glucafon, but synergize with extrinsic paracrine signals to regulate secretion under high glucose stimulation This is supported by in vivo findings showing that inhibiting insulin signaling in the pancreatic α-cells of mice through a loss of α-cell insulin receptors leads to an increase in glucagon secretion in both hyperinsulimic-hypoglycemic and STZ-induced hypoinsulimic-hyperglycemic state.

Therefore, these data shows that insulin is necessary for inhibiting glucagon secretion in hyperglycemia; however, insulin does not play a role in regulating glucagon secretion in low-glucose conditions Pancreatic α-cells are exposed to high levels of insulin secreted from the β-cells in the islet.

Insulin is a potent inhibitor of glucagon secretion and glucagon gene transcription 2,67,78, Data have shown that the diminished insulin release during hyperglycemia associated with diabetes paradoxically stimulates the release of glucagon 19,79, Studies utilizing in vitro approaches have shown that insulin receptors are very abundant on pancreatic α-cells and activate the phosphatidyl inositol 3-kinase PI3K -Akt pathway leading to inhibition of glucagon gene transcription and secretion 82,84, Insulin has been shown to induce the Akt-dependent GABA A receptor translocation to the plasma membrane which can be activated by GABA co-released with insulin and PI3K-dependent opening of K ATP channels, culminating hyperpolarizing the plasma membrane and inhibiting glucagon secretion 29, Although, the precise mechanisms responsible for changes in the α-cell function in diabetes remain unclear, a recent study by Kawamori and colleagues showed that inhibiting insulin signaling in the pancreatic α-cells of mice through a loss of α-cell insulin receptors leads to altered glucose metabolism, including mild glucose intolerance, hyperglycemia and hyperglucagonemia GABA γ-Aminobutyric acid is produced from the excitatory amino acid glutamate and co-released with insulin from the pancreatic β-cells by high glucose and glutamate stimulation.

GABA can diffuse within the islet interstitium to activate GABA A receptors present on the cell-surface of α-cells This nonpeptidal neurotransmitter has been shown to act as a suppressor of amino acid-stimulated glucagon release in the mouse and isolated α-cells via GABA A receptors Data have suggested that glucose-stimulated insulin release and the subsequent activation of the Insulin Receptor-PI3K-Akt pathway induces the activation and translocation of GABA A receptors to the plasma membrane The GABA co-released with insulin from the β-cells can activate the newly translocated cell-surface GABA A receptors and increase Cl - inhibitory currents, subsequently hyperpolarizing the plasma membrane Perfusion experiments in the human and rat pancreas have shown that glucagon suppresses insulin and somatostatin release 7Glucagon receptor knock-out mice exhibit α-cell hyperplasia and hyperglucagonemia, which has been suggested to be due to a lack of autocrine signals of glucagon on the α-cell Contrary to this theory, a recent study has shown that implanted wild-type islets in mice with liver-specific deletion of the glucagon receptor also develop α-cell hyperplasia These data suggest that a circulating factor generated after the disruption of glucagon signaling in the liver can increase α-cell proliferation independent of direct pancreatic input.

Glutamate is a major excitatory neurotransmitter in the central nervous system, which has also been implicated in the regulation of glucagon release. An elegant study published by Cabrera and colleagues described the positive autocrine signal of glutamate in the human, monkey and mouse islets 9.

The authors proposed a mechanistic model where glutamate co-released with glucagon potentiates glucagon secretion through acting on the inotropic glutamate receptors on the α-cell membrane and creating a positive autocrine loop 9. Somatostatin, secreted by the islet δ-cells, has been long accepted as a glucagon-suppressing peptide.

Exogenous somatostatin inhibits glucagon release in isolated α-cells, as wells as in healthy and diabetic patients 11, In addition, islets isolated from somatostatin-deficient mice have reduced glucose-suppression of glucagon release Figure 2.

Immunofluorescent labeling of human and mouse islets. Human islet. Glucagon-positive α-cells green are randomly dispersed among insulin-positive β-cells red within the human islet. Mouse islet. Glucagon-positive α-cells green are concentrated on the mantle and insulin-positive β-cells red make-up the core of the mouse islet.

Glucagon exerts its physiological action on target tissues via the G-protein coupled glucagon receptor, which is found on multiple tissues including the liver, fat, intestine, kidney and brain 50, In the liver, glucagon counteracts the anabolic properties of insulin by promoting gluconeogenesis and glycogenolysis which consequently increases glucose output.

The hepatocyte is exposed to high levels of glucagon released by the pancreas via the portal vein. The subsequent activation of adenylate cyclase leads to the increase in intracellular cyclic adenosine monophosphate cAMP levels and the activation of protein kinase A PKA The second messenger cAMP can activate cyclic nucleotide-gated ion channels, exchange proteins activated by cAMP EPAC and protein kinase A PKA.

The process of glycogenolysis involves the activation of glycogen phosphorylase kinase and glycogen phosphorylase through the activated PKA, and glycogen phosphorylase brings about glycogen breakdown.

In addition, the activation of the cAMP-PKA pathway in the hepatocyte leads to the phosphorylation and activation of CREB and subsequent activation of key gluconeogenic enzymes, phosphoenolpyruvate carboxykinase PEPCK and glucosephosphatase G6Pasestimulating hepatic glucose output.

The gluconeogenic process is also promoted by the activity of additional transcription factors. The activated CREB binds to the promoter region of the transcriptional coactivator PGC-1 gene, increasing its transcription.

PGC-1 and the nuclear transcription factor hepatocyte nuclear factor-4 HNF-4 further promote gluconeogenesis by increasing the transcription of PEPCK gene and therefore PEPCK activity In the adipocyte, glucagon activates the cAMP-PKA pathway, leading to the phosphorylation and activation of hormone-sensitive lipase and the subsequent breakdown of triglycerides lipolysis and release of diacylglycerol and free fatty acids into the circulation.

The liver can further utilize glycerol and free fatty acids for gluconeogenesis or re-esterification of free fatty acids to form ketone bodies. In the heart, glucagon has been described as a vasodilator, which lowers blood pressure by decreasing the vasculature resistance in the liver and spleen.

Glucagon has diuretic effects on the kidney, increasing glomerular filtration and electrolyte excretion Glucagon receptors are expressed in the brain and data has suggested that circulating glucagon can pass the blood-brain barrier to modulate its effects in the central nervous system.

Glucagon infused in the central nervous system has anorexigenic effects in rats, chicks and sheep. In addition, intravenous infusion of glucagon has been shown to suppress appetite in humans; however, the direct link between glucagon and central food intake regulation in humans is unclear.

Glucagon relaxes the GI tract from esophagus to colon and as a result is often used to quiet the bowel before endoscopic retrograde cholangiopancreatography ERCP or bowel imaging studies In the esophagus it is used to relax the muscle before removal of foreign objects.

Glucagon also will relax the sphincter of Oddi These effects are almost certainly pharmacological but are short lived and without other deleterious effects. Crystaline glucagon was first prepared in and early reports injecting mg amounts of glucagon into rodents included a description of degranulation of acinar cells along with pancreatic atrophy 10,49,57, This finding was interpreted by Jarett as due to inhibition of protein synthesis due to lowered plasma amino acid levels in vivo as glucagon did not inhibit protein synthesis measured by incorporation of 3H-leucine in vitro However, these results are difficult to interpret because the animals given megadoses of glucagon lost weight and were reported to appear ill.

In vitro studies with isolated lobules showed that in-vivo pretreatment reduced subsequent protein synthesis and intracellular transport after 30 minutes of infusion in vivo but not at 24 hours. In a more recent study, Kash et.

With the knowledge of a possible relationship between the endocrine and exocrine pancreas 43the effects of exogenous glucagon on pancreatic secretion was studied in a variety of species both with and without anesthesia. Initially most studies were carried out in unanesthetized dogs with pancreatic fistulas, the predominant animal model for GI physiology at the time, and glucagon most often prepared by Eli Lilly was shown to inhibit the volume, bicarbonate and protein or enzyme content of pancreatic secretion stimulated by food, acid, secretin or CCK 24,44,45,47,55,56,73,74, Similar inhibition of pancreatic secretion has also been seen in studies carried out in rats 1,5,83cats 54 and humans 12,18,25,40, The mechanism of the inhibition remains unclear, but has been assumed by most authors to be at the level of the pancreas because the effect of exogenous secretagogues was inhibited.

Possible loci include inhibition of pancreatic blood flow, the resulting hyperglycemia, lowering of plasma calcium as well as inhibition of the secretory mechanism. Glucagon could also be having an effect on the nervous system either centrally or within the pancreas. Some of these possible inhibitory loci could be better controlled using a perfused pancreas model.

Glucagon has been reported to inhibit secretion in the perfused pancreas of the cat and rat In the latter study, infusion of amino acids was shown to increase glucagon and inhibit pancreatic secretion and this effect could be blocked by infusing an antibody to glucagon

: Glucagon biosynthesis

Glucagon regulates its own synthesis by autocrine signaling

These AkhR -GAL4-driven expression patterns are not comprehensive. Tissue-specific expression of AkhR -RNAi also identified roles for AKHR signaling in the IPCs, the PG, and in four interoceptive SOG neurons ISNs Kim and Neufeld, ; Jourjine et al.

Eight DILPs are produced in Drosophila and their expression patterns and functions vary according to discrete stages of life; this information is thoroughly reviewed elsewhere Owusu-Ansah and Perrimon, ; Nässel and Vanden Broeck, The regulation of DILP biosynthesis, secretion, and signaling in Drosophila is very complex, and only a brief summary can be provided here.

Insulin signaling in Drosophila is more complex than in mammals, and it is inappropriate to generalize all DILPs as being direct actors in metabolic homeostasis in the same sense that β cell-derived insulin is in mammals. It is similarly inappropriate to oversimplify the activities of AKH and all DILPs as being antagonistic.

However, specific DILPs do directly alter haemolymph glucose titers e. Adipokinetic hormone and DILP secretion must be coordinated so that their antagonistic actions preserve hemolymph glucose homeostasis.

Central to this goal is the ability of the CC and IPCs to monitor energy homeostasis through cell autonomous nutrient sensing of circulating glucose titers. This is accomplished by the evolutionarily conserved K ATP channels.

ATP-Sensitive Potassium channels function as cell autonomous nutrient sensors in mammalian pancreatic α and β cells for comprehensive reviews, see: Ashcroft and Rorsman, ; Rorsman et al.

melanogaster larval and adult CC; these channels are also present in the adult—but not larval—IPCs Kim and Rulifson, ; Fridell et al.

K ATP channels also contribute to healthy mammalian and Drosophila cardiac function Akasaka et al. The K ATP channel contains two subunits comprised of four regulatory sulfonylurea receptors SURx: SUR1 in mammals; Sur in flies and four pore-forming weakly inward rectifying potassium channels Kir6.

x: Kir6. x bears an ATP-binding domain and SURx bears a Mg-ADP-binding domain: Kir6. x-ATP binding stimulates channel closure and cell membrane depolarization; SURx-Mg-ADP stimulates channel opening and cell membrane polarization.

The weakly inward rectifying action of the Kir6. Glucose transporters bring glucose into the mouse α GLUT1 and GLUT4 and β GLUT2 cells where it enters the Krebs cycle for ATP production Heimberg et al.

This stimulates cell membrane depolarization and—along with an unidentified depolarizing current—causes action potential firing that regulates insulin and glucagon secretion Rorsman et al.

Paradoxically, K ATP channel closure causes depolarization and action potential firing in both cell types but promotes insulin secretion from β cells while inhibiting glucagon secretion from α cells. The means whereby α and β cell K ATP channels produce opposite effects on secretion in response to the same glucose titers is still incompletely understood.

Recent work demonstrated that this is likely due to differential glucose sensitivity of K ATP channels between the two cell types, which significantly alters cellular excitability and action potential firing Göpel et al.

I will review electrical regulation of both α and β cells in order to contrast important differences in effect of K ATP channel activity between the two cell types.

Insulin secretion from β cells is stimulated by cell membrane depolarization caused by K ATP channel closure Cook and Hales, ; Rorsman et al. β cell K ATP channel conductance is high i.

Electrical regulation of insulin secretion from β cells is thoroughly reviewed elsewhere Sarmiento et al. α cells are more sensitive to glucose than are β cells. In contrast to β cells, α cells show very low K ATP channel conductance at 1 mM of glucose; this results from α cell K ATP channels having a 5-fold greater sensitivity to ATP produced by glucose phosphorylation Zhang et al.

As a result, small changes in K ATP channel activity i. The high sensitivity of these K ATP channels is crucial as it makes α cells electrically active at low glucose concentrations, thus stimulating VGCC activity and permitting glucagon secretion at glucose concentrations that inhibit insulin secretion from β cells.

The crucial role played by voltage-gated sodium channels VGSCs in mediating this process in α cells that is described below. Just as importantly—because glucagon must be secreted only during hypoglycemia—increasing glucose concentrations to 6 mM rapidly closes all remaining α cell K ATP channels.

This produces strong membrane depolarization and action potential firing, and—through the activity of VGSCs—prevents glucagon secretion during hyperglycemia. Glucagon secretion is thus both stimulated and inhibited by varying magnitudes of K ATP channel activation.

This relation between K ATP channel conductance and glucagon secretion follows an inverted U-shaped dose response curve where maximum secretion occurs at 1 mM glucose, and small increases or decreases in conductance inhibit secretion Zhang et al. α cell membranes bear VGSCs that are activated by K ATP channel closure and membrane depolarization at 1 mM glucose.

The VGSCs produce rapid and short-lived amplification of the moderate depolarizing stimulus that is produced by K ATP channel closure at low glucose concentrations.

When all α cell K ATP channels close in response to hyperglycemia, further membrane depolarization increases action potential firing and inactivates VGSCs. VGSC closure reduces action potential firing and spike height, VGCCs are subsequently inactivated, and glucagon secretion is inhibited.

The precise regulation of ion channel activity through K ATP channel-mediated nutrient sensing underlies the proper timing of glucagon and insulin secretion.

Homeostatic control of blood glucose homeostasis is highly sensitive to any factors that dysregulate channel function. Mutations in K ATP channels, VGCCs, and VGSCs that perturb their function are currently the focus of research aimed at identifying causal mechanisms that underlie the pathogenesis of diabetes.

The precise mechanism whereby α cell membrane electrical activity and glucagon secretion are regulated remains to be elucidated. Although glucose-dependent K ATP channel activity certainly plays a prominent role in regulating the inverted U-shaped curve of α cell membrane electrical activity described above, it is not the sole determinant of glucagon secretion.

Evidence suggests that K ATP channel-independent mechanisms can contribute to this pattern of electrical activity and produce glucagonostatic effects at high glucose concentrations through both extrinsic paracrine and intrinsic means.

In β cells, insulin secretion is inhibited by AMPK and stimulated by PKG Granot et al. This is mediated by mechanisms that are both dependent and independent of AMPK. Liver kinase B1 LKB1 induces AMPK activity to inhibit insulin biosynthesis and secretion in both a glucose- and amino acid-responsive manner da Silva Xavier et al.

AMPK also inhibits insulin secretion via a leptin-mediated feedback loop Tsubai et al. In response to feeding, insulin promotes leptin secretion from adipose tissue, and leptin signaling in β cells subsequently induces protein kinase A PKA activation of AMPK Park et al.

Leptin-PKA-AMPK signaling regulates membrane polarity—and thus cell excitability—by promoting K ATP channel trafficking to the β cell membrane Cochrane et al. This increases K ATP channel conductance and hyperpolarizes the cell membrane. Importantly, this occurs only in a progressively fasted state when the glucose:leptin titer ratio decreases to a level where continued insulin secretion would produce a hypoglycemic state Park et al.

In β cells, PKG promotes insulin secretion in a fed state either by phosphorylating and closing K ATP channels or by phosphorylation of proteins that indirectly target K ATP channels Soria et al.

PKG activity is induced by atrial natriuretic peptide ANP signaling in β cells Undank et al. PKA also phosphorylates and closes K ATP channels, and PKG promotes this inhibitory effect by preventing phosphodiesterase deactivation of PKA Undank et al.

This PKA-mediated increase in insulin secretion appears contradictory to its inhibitory effect reported above; however, PKA inhibition of K ATP channels is mediated by PKG signaling, whereas leptin-PKA-AMPK signaling increases K ATP channel conductance in the absence of PKG activity Undank et al.

The complexity of the glucose-responsive and self-regulatory pathways present in β cells reflects the need for rapid responses in insulin secretion to changes in blood glucose levels. This promotes glucagon secretion through an incompletely characterized signaling cascade Leclerc et al.

Induction of AMPK by AMP is mediated by LKB1 during hypoglycemia, but AMPK is not the sole target of LKB1 phosphorylation in glucagon regulation Sun et al. This physiological effect suggests that—as in β cells—PKG might phosphorylate VGCCs and close these channels. Further insight is provided by research into cardiac myocytes where nitric oxide stimulated PKG activity inhibits Ca v 1.

The murine research reviewed above informs future research into the regulatory mechanisms of AKH secretion in D. Endocrine research requires the ability to quantify changes in hormone secretion. However, circulating AKH titers in D. melanogaster are estimated to be in the low femtomolar range, and this makes the reliable quantification of AKH titers an ongoing challenge that will be addressed below Isabel et al.

Pharmacological and transgenic manipulations were used to implicate K ATP channels in the regulation of AKH secretion from the larval CC Kim and Rulifson, Tolbutamide is a diabetic drug that targets the Sur subunits of K ATP channels.

Tolbutamide treatment was used in conjunction with transgenic manipulations where the CC was ablated to show that the increase in glucose titers was dependent upon the CC.

The effect of tolbutamide was inhibited when CC membrane depolarization was transgenically inhibited. These experiments provided strong evidence for the existence of K ATP channels in the CC and for their regulatory role in AKH secretion Kim and Rulifson, Adult IPCs bear K ATP channels, and in vivo electrophysiological measurements of these cells were used to discern the influence of K ATP channels on membrane potential; the potential for applying this technique to the CC has not been explored Fridell et al.

A major contribution to the characterization of mechanisms that regulate CC cell membrane potential and AKH secretion was recently reported Perry et al.

Three genes that encode components of K ATP channels Sur , calcium channels Ca-Beta , and potassium channels sei were identified through RNAi-mediated knockdown as regulatory candidates for excitation-secretion coupling for AKH in the CC. These results provided further support for the nutrient-sensing role of K ATP channels in the CC.

The identification of CC ion channel components greatly improves the utility of D. melanogaster as a model for α cell dysregulation, hyperglucagonemia, and the pathogenesis of T2DM. The murine cGKI research described above prompted the hypothesis that PKG—encoded by dg2 in D.

melanogaster —might negatively regulate AKH secretion. Reduced dg2 expression in the larval CC reduced intracellular AKH abundance, and this correlated with a low nutrient-dependent developmental delay and increased lethality prior to pupariation Hughson et al.

Compared to control genotypes, more of these larvae survived pupal metamorphosis and developed into adults with greater starvation resistance and increased body size to lipid content ratio, a trait associated with obesity in humans. This suggested that dg2 functioned in the CC to increase survival during larval development in a low nutrient environment, and but that this resulted in a tradeoff with starvation resistance during adult life Hughson et al.

Further research demonstrated that dg2 also influenced AKH abundance in the adult CC Hughson, in press. Reduced dg2 expression in the adult CC decreased intracellular AKH, but—in contrast to larvae—correlated with decreased body size to lipid content ratio.

This effect correlated with evidence of increased systemic lipid catabolism and reduced starvation resistance during adult life. As described above, the CC is developmentally orthologous to the mammalian anterior pituitary gland Wang et al. The PI location of IPCs of the fly protocerebrum is orthologous to the mammalian hypothalamus Wang et al.

Although there is little conservation between AKH and glucagon amino acid sequences, both hormones act through the same evolutionarily conserved signaling pathway to regulate transcriptional responses to hypoglycemia De Loof and Schoofs, ; Clynen et al.

The HP axis also regulates the time of onset of puberty. When the HP axis detects a minimum level of body growth during childhood it stimulates steroid hormone biosynthesis in the gonads Shalitin and Philip, The subsequent rise in steroid titers initiates the developmental transition from sexual immaturity to maturity.

Paracrine signaling between the hypothalamus and pituitary gland is mediated by GnRH, which stimulates the secretion of gonadotropins that enter circulation and stimulate steroid hormone biosynthesis and secretion from the gonads.

melanogaster , steroid hormones similarly regulate the timing of this developmental transition. The evolutionary relatedness of GnRHR and AKHR was introduced above. The conservation of AKHR and GnRHR prompted the hypotheses that AKHR influenced development by regulating ecydsteroidogenesis, and that AKH—in addition to its glucagon-like properties—possessed dual functionality as both a glucagon-like and GnRH-like peptide.

Conserved peptide sequences between AKH and GnRH seemed to provide support for this hypothesis Lindemans et al. However, recent work investigating AKH and AKHR loss of function mutant lines demonstrated that neither AKH nor AKHR affected developmental i. While development was not altered in AKH loss of function mutants, recent work identified the possibility that AKH might play a role in ecdysteroid biosynthesis in the PG.

Evidence comes from work demonstrating a role for AKH-regulated hormone sensitive lipase HSL activity in steryl ester metabolism and the intergenerational transfer of sterols Heier et al. This pathway regulates catabolism of steryl ester lipid droplet stores and plays an essential role in ecdysteroid biosynthesis.

While this work reported no effect of an HSL loss of function mutation on PG lipid droplets, these data came from animals reared in a lipid- and sterol-abundant feeding environment and larval development was not reported. The possibility that this pathway influences ecdysteroid biosynthesis in sterol-limited or -deficient environments needs to be explored.

This avenue of research is supported by a developmental role for AKH that was observed only in low nutrient conditions Hughson et al.

Larvae reared in a low nutrient i. This gene, dg2 , is orthologous to cGKI, which encodes the PKG that regulates alpha cell membrane excitability Leiss et al. This delay was AKH-dependent, and—as observed in AKH mutants reared in nutrient-abundant conditions—was absent in nutrient-abundant conditions Gàlikovà et al.

This trait was associated with GPCR-mediated active secretion of ecdysteroids from the PG as well as with AKH activation of the HSL pathway Yamanaka et al. It is vital to reemphasize that AKH mutants did not exhibit developmental defects, delays, or fitness consequences, and that this definitively demonstrated that AKH is not essential for development in a nutrient-abundant environment Gàlikovà et al.

There is no contradiction between this seminal work and the report of an AKH-dependent effect on developmental timing that was present only in low nutrient conditions Hughson et al.

Instead, this identifies the possibility that in challenging nutritional environments AKH can play a non-essential role in development in a manner fitting for a stress peptide Vogt, This hypothesis should be investigated in the context of nutrient abundance and stress over different developmental ages.

The mechanisms that regulate AKH secretion must be characterized in order to improve the utility of D. Its small size puts the fly model at a disadvantage to rodent models in some respects; for example, electrophysiological assays performed using dissected and cultured α and β cells are rarely used in fly research Fridell et al.

However, flies possess traits that present an advantage over rodent models, such as a short life cycle and ease of controlling genetic background. One of the great strengths of D. melanogaster research is the ever-expanding library of transgenic lines that permit spatiotemporal-specific manipulations of CC function and AKHR signaling pathways.

This section discusses bioassays that can be established—or adapted from existing protocols—to improve fly models of metabolic syndrome. Some exciting avenues for future AKH research are also highlighted.

First, a crucial weakness in D. melanogaster metabolic research must be addressed—the ability to quantify hemolymph sugar and AKH titers. Dysregulation of blood glucose levels is diagnostic of pre-diabetic and diabetic states, and this phenotype is quantifiable in D.

melanogaster metabolism research. Circulating AKH titers in D. melanogaster are estimated to be in the low femtomolar range and this makes the reliable quantification of AKH titers an ongoing challenge Isabel et al.

Unlike DILPs that are large enough to be tagged for quantification of secretion, the AKH octomer is too small for this technique Park et al.

This problem was circumvented by quantifying phenotypes that are predicted to indicate changes in AKH secretion.

These surrogate methods include altered lifespan during starvation Braco et al. The precise quantification of circulating sugar i.

Existing assays are efficient and highly replicable, and use enzymatic reactions that permit colorimetric sample quantification Buch et al. The ideal protocol will also allow for quantification of lipid and hormone e. High performance liquid chromatography HPLC has been used to quantify glandular and hemolymph ecdysteroid titers Yamanaka et al.

Mass spectrometry MS methods benefit from high sensitivity and requirement for low sample volumes and can be used in conjunction with isotope labeled nutrients and hormones.

Combined HPLC and MS techniques were used to quantify tissue specific lipid accumulation Tuthill et al. AKH titers can be quantified by spiking samples with a known quantity of isotope-labeled AKH e.

Another MS technique, tandem mass tagging TMT of proteins and nucleic acids, permits sample multiplexing. In adults, the relatively small hemolymph volume and sclerotized cuticle makes sample collection more challenging than in larvae.

Hemolymph can be extracted from adults by poking holes in the cuticle or removing the head and spinning the flies in a centrifuge Tennessen et al. An alternative method that does not require anesthesia involves placing an adult inside a trimmed pipette tip, amputating one antenna, and apply low air pressure to the body to exude a droplet of hemolymph MacMillan and Hughson, Another essential development for D.

melanogaster metabolic syndrome research is a clinically relevant measure of obesity. The body mass index BMI standardizes body mass to body size using height as a surrogate measure of size and is used to diagnose overweight and obese humans Gutin, Obesity in flies is typically reported as whole body lipid content standardized to whole body protein content under the assumption that protein content is always constant across treatments and is directly proportional to body size.

Whole body macronutrient quantification is superior to the use of body mass in obesity research because the distinction between lipid and non-lipid molecules cannot be made. However, the assumption that whole body protein content is always constant and proportional to body size is rarely tested.

This bears great impact on fly metabolic research because transgenic and nutritional treatments that challenge energy homeostasis will stimulate protein catabolism for energy production as starvation progresses. When whole body protein content is altered by experimental conditions it is clearly an inappropriate surrogate measure for body size in obesity research.

Furthermore, it cannot be used as a constant against which lipid content is standardized for comparison between treatment groups. Alternatives to this method are to use wing surface area or thorax length as a measure for adult body size Delcour and Lints, ; McBrayer et al.

It needs to be noted that wing surface area is sometimes inconsistent with body size. Wing measurements may be less appropriate than thorax length, particularly where wing imaginal disk growth—mediated by DILP2 and DILP8—may be affected differentially between experimental and control lines Brogiolo et al.

Variation in genetic background can influence development and body size. The contribution of genetic background to this and other traits can be controlled through the backcrossing of mutant lines into an isogenic background Greenspan, For life stage-specific experiments, GeneSwitch GS provides temporal control over transgene expression via drug RU, mifepristone -dependent activation of GAL4 activity Osterwalder et al.

This controls for the effects of genetic background mutations on development in transgenic experiments, and recent advances have helped to reduce RU side effects Robles-Murguia et al. In adult life stage-specific research, GS-GAL4 prevents developmental effects of GAL4-UAS activity in the experimental F1 line that cannot be controlled for in the GAL4 and UAS control lines e.

The use of other GAL4 regulators e. This kind of error is misleading and creates flawed hypotheses of obesity mechanisms. The international D. melanogaster community collaborates to study metabolic syndrome by sharing reagents and expertise. The utility and replicability of this research depends upon rearing flies in a consistent nutritional environment.

While this is easily accomplished within one lab, it is rare that multiple lab groups use the same nutrient medium. Given the significant influence of nutritional history on fly development and metabolic health, the establishment of a standardized nutrient medium will aid international collaborative efforts by removing the uncertainty associated with nutrient experience when comparing experimental results between groups.

The recipe for a standardized diet was developed to meet this need in the D. melanogaster research community Piper et al. This holidic diet is a precise blend of chemically defined ingredients that are available from chemical supply companies.

The purity of these ingredients makes it possible for different labs to follow the same recipe and create identical nutrient media. Another benefit of using this diet is that precise nutritional manipulations are easily designed and replicated.

There are also concerns regarding the viability of some fly lines—particularly sensitive ones—on this nutrient medium. Researchers take great care to control the genetic background of their fly stocks to prevent confounding effects of genetic variation on their phenotypes of interest.

The control of nutrient background is far simpler and prevents confounding effects of variation in nutritional history. Future efforts that modify the holidic diet—or develop new diets—provide the means to control this variable by standardizing the use of one standard nutrient medium in Drosophila research labs.

Drosophila has not lived up to its potential as a model organism for metabolic research Owusu-Ansah and Perrimon, As was made clear in this review, knowledge of regulatory mechanisms governing AKH secretion lags behind that of glucagon secretion in mammals.

This is one of the most important advances in AKH research that must be made to improve the utility of fly metabolism research in clinical research. Knowledge gaps in any of these four mechanisms governing glucagon and insulin physiology will severely limit the development of diabetes models.

This review identified promising areas for investigations into intrinsic mechanisms of AKH physiology that will contribute to models for the pre-diabetic and diabetic states of hyperglucagonemia and hyperglycemia.

Circadian regulation of behavior and physiology provides essential input to homeostatic control of metabolism. Energy expenditure changes between sleep and wake cycles and this requires changes in AKH and DILP secretion.

The PDF is a neuropeptide that is required for maintaining circadian rhythms and activity Renn et al. Its receptor, PDFR, was identified in the CC where its activity decreased starvation resistance and increased locomotion in fed flies Braco et al.

A possible explanation for these results is that PDF signaling in the CC promoted AKH secretion. The presence of PDFR in the CC identifies PDF signaling as a putative extrinsic factor that regulates AKH physiology. cAMP promotes voltage gated ion channel conductance in excitable cells and is known to directly phosphorylate cardiac L-type VGCCs Siggins, ; Gao et al.

It is possible that PDF acts in the CC to modulate cell membrane excitability by promoting cAMP phosphorylation of a VGCC subunit Perry et al. In this putative role, PDF confers the regulatory influence of circadian rhythmicity upon AKH secretion.

An exciting area for future research lies in characterizing the functional parallels between the AKH and GnRH orthologs. Activation of the HPG axis through GnRH signaling at the onset of puberty stimulates the transition from juvenile to adult life in mammals Parent et al.

Juvenile metabolic stress caused by famine or low socioeconomic status perturbs HPG activity and thereby contributes to the pathogenesis of metabolic syndrome both within and across generations Habtu et al. Adipokinetic hormone altered the timing of larval development in responsive to low nutrient stress, and AKHR was implicated in the intergenerational transmission of the effect of nutrient stress on lipid homeostasis Palu et al.

AKHR signaling in the fat body activated the PKA-LKB1-SIK3-HDAC4 pathway Choi et al. Chronically elevated glucagon signaling suppressed SIK3 via the PKA-LKB1 pathway and caused HDAC4-mediated activation of FOXO to produce a pre-diabetic hyperglycemic state Luong et al.

This concurs with a recent report that AKHR signaling in the fat body mediated the hyperglycemic response to a high sugar diet Song et al. Epigenetic mechanisms play a causal role in the inheritance of acquired metabolic traits Somer and Thummel, As an epigenetic modifier, the histone deacetylating activity of HDAC4 is a candidate mediator of intergenerational transmission of nutrient stress effects via epigenetic inheritance.

The epigenetic effects of HDAC4 are particularly relevant due to its effect on the expression of genes that regulate the glycemic index Kasinska et al. The dual functionality of AKH as a glucagon-like and a GnRH-like peptide presents great potential for understanding the etiological basis of metabolic syndrome, as well as the means whereby the effects of nutrient stress are transmitted across generations through altered HPG axis activity.

BNH confirms being the sole contributor of this work and has approved it for publication. BNH was supported by a Natural Sciences and Engineering Research Council of Canada and Canadian Institute for Advanced Research grant to Marla B.

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers.

Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher. BNH wishes to thank the reviewers for improving this manuscript through their insightful and constructive input. Ahmad, M. Wiley Interdiscip.

Google Scholar. Akasaka, T. The ATP-sensitive potassium K ATP channel-encoded dSur gene is required for Drosophila heart function and is regulated by tinman. doi: PubMed Abstract CrossRef Full Text Google Scholar.

Alfa, R. Using Drosophila to discover mechanisms underlying type 2 diabetes. Ashburner, M. Drosophila: A Laboratory Handbook.

New York: Cold Spring Harbor Laboratory Press. Ashcroft, F. Diabetologia 42, — K ATP channels and islet hormone secretion: new insights and controversies. Bähr, I. GLUT4 in the endocrine pancreas — indicating an impact in pancreatic cell physiology? Barg, S. Tight coupling between electrical activity and exocytosis in mouse glucagon-secreting alpha-cells.

Diabetes 49, — Bernard, C. Le Moniteur des Hôpitaux Paris: J. Baillière et fils. Bharucha, K. A glucagon-like endocrine pathway in Drosophila modulates both lipid and carbohydrate homeostasis.

Braco, J. Energy-dependent modulation of glucagon-like signaling in Drosophila via the AMP-activated protein kinase. For further details of cleavage sites and processing see reference The five major islet cell types are aligned on blood vessels at no particular order or structured organization within the human islet 8 Figure 2A.

In contrast, the rodent islet shows a more-defined architecture placing the β-cells in the core and the α, δ and PP-cells lying at the mantle of the islet Figure 2B. This unique structure in the rodent islet suggests an organized system allowing paracrine interactions between the peptides released.

This is supported by studies showing that arterial blood is directed from the core of the rodent islet insulin-secreting β-cells to the periphery 6.

Therefore, during a rise in blood glucose, the pancreatic α-cells are exposed to high levels of secreted insulin leading to the inhibition of glucagon secretion and glucagon gene transcription. Glucagon release is inhibited after carbohydrate-rich meal and the consequent rise in blood glucose and insulin secretion.

However, a meal rich in amino acids induces glucagon release. Parasympathetic vagal and sympathetic Epinephrine, Norepinephrine, Galanin, Neuropeptide Y nerve stimulations induce the secretion of glucagon from the pancreatic α-cells.

Glucagon is secreted by the pancreatic α-cells in states of decreasing blood glucose; however, whether changes in glucose concentrations alone can regulate glucagon secretion still remains unclear. High glucose concentrations inhibit glucagon release in the intact islet; however, high glucose induces glucagon release in dispersed, isolated α-cells.

However, the chronic exposure of α-cells to high glucose levels has been shown to induce α-cell dysfunction and insulin-resistance, closely mimicking the diabetic state 21, Rat α-cells express glucokinase and glucose transporter GLUT1, an isoform with a lower capacity compared to GLUT2, which is the predominant form in insulin-secreting β-cells Despite differences in metabolism of glucose by the two cells types, studies have shown that they share similar inherent mechanisms of activation Alpha cells have high ATP concentrations under low glucose, which rise further after stimulation with high glucose.

The glucose-stimulated inhibition of glucagon secretion was associated with an inhibition of AMPK activity and activating AMP-activated protein kinase AMPK in turn inhibited secretion Therefore, α-cells do have intrinsic mechanisms, which respond to glucose stimulation, but synergize with extrinsic paracrine signals to regulate secretion under high glucose stimulation This is supported by in vivo findings showing that inhibiting insulin signaling in the pancreatic α-cells of mice through a loss of α-cell insulin receptors leads to an increase in glucagon secretion in both hyperinsulimic-hypoglycemic and STZ-induced hypoinsulimic-hyperglycemic state.

Therefore, these data shows that insulin is necessary for inhibiting glucagon secretion in hyperglycemia; however, insulin does not play a role in regulating glucagon secretion in low-glucose conditions Pancreatic α-cells are exposed to high levels of insulin secreted from the β-cells in the islet.

Insulin is a potent inhibitor of glucagon secretion and glucagon gene transcription 2,67,78,, Data have shown that the diminished insulin release during hyperglycemia associated with diabetes paradoxically stimulates the release of glucagon 19,79, Studies utilizing in vitro approaches have shown that insulin receptors are very abundant on pancreatic α-cells and activate the phosphatidyl inositol 3-kinase PI3K -Akt pathway leading to inhibition of glucagon gene transcription and secretion 82,84, Insulin has been shown to induce the Akt-dependent GABA A receptor translocation to the plasma membrane which can be activated by GABA co-released with insulin and PI3K-dependent opening of K ATP channels, culminating hyperpolarizing the plasma membrane and inhibiting glucagon secretion 29, Although, the precise mechanisms responsible for changes in the α-cell function in diabetes remain unclear, a recent study by Kawamori and colleagues showed that inhibiting insulin signaling in the pancreatic α-cells of mice through a loss of α-cell insulin receptors leads to altered glucose metabolism, including mild glucose intolerance, hyperglycemia and hyperglucagonemia GABA γ-Aminobutyric acid is produced from the excitatory amino acid glutamate and co-released with insulin from the pancreatic β-cells by high glucose and glutamate stimulation.

GABA can diffuse within the islet interstitium to activate GABA A receptors present on the cell-surface of α-cells This nonpeptidal neurotransmitter has been shown to act as a suppressor of amino acid-stimulated glucagon release in the mouse and isolated α-cells via GABA A receptors Data have suggested that glucose-stimulated insulin release and the subsequent activation of the Insulin Receptor-PI3K-Akt pathway induces the activation and translocation of GABA A receptors to the plasma membrane The GABA co-released with insulin from the β-cells can activate the newly translocated cell-surface GABA A receptors and increase Cl - inhibitory currents, subsequently hyperpolarizing the plasma membrane Perfusion experiments in the human and rat pancreas have shown that glucagon suppresses insulin and somatostatin release 7 , Glucagon receptor knock-out mice exhibit α-cell hyperplasia and hyperglucagonemia, which has been suggested to be due to a lack of autocrine signals of glucagon on the α-cell Contrary to this theory, a recent study has shown that implanted wild-type islets in mice with liver-specific deletion of the glucagon receptor also develop α-cell hyperplasia These data suggest that a circulating factor generated after the disruption of glucagon signaling in the liver can increase α-cell proliferation independent of direct pancreatic input.

Glutamate is a major excitatory neurotransmitter in the central nervous system, which has also been implicated in the regulation of glucagon release.

An elegant study published by Cabrera and colleagues described the positive autocrine signal of glutamate in the human, monkey and mouse islets 9.

The authors proposed a mechanistic model where glutamate co-released with glucagon potentiates glucagon secretion through acting on the inotropic glutamate receptors on the α-cell membrane and creating a positive autocrine loop 9. Somatostatin, secreted by the islet δ-cells, has been long accepted as a glucagon-suppressing peptide.

Exogenous somatostatin inhibits glucagon release in isolated α-cells, as wells as in healthy and diabetic patients 11, In addition, islets isolated from somatostatin-deficient mice have reduced glucose-suppression of glucagon release Figure 2. Immunofluorescent labeling of human and mouse islets.

Human islet. Glucagon-positive α-cells green are randomly dispersed among insulin-positive β-cells red within the human islet. Mouse islet. Glucagon-positive α-cells green are concentrated on the mantle and insulin-positive β-cells red make-up the core of the mouse islet.

Glucagon exerts its physiological action on target tissues via the G-protein coupled glucagon receptor, which is found on multiple tissues including the liver, fat, intestine, kidney and brain 50, In the liver, glucagon counteracts the anabolic properties of insulin by promoting gluconeogenesis and glycogenolysis which consequently increases glucose output.

Preproglucagon first has its signal peptide removed by signal peptidase , forming the amino acid protein proglucagon. In intestinal L cells , proglucagon is cleaved to the alternate products glicentin 1—69 , glicentin-related pancreatic polypeptide 1—30 , oxyntomodulin 33—69 , glucagon-like peptide 1 72— or , and glucagon-like peptide 2 — In rodents, the alpha cells are located in the outer rim of the islet.

Human islet structure is much less segregated, and alpha cells are distributed throughout the islet in close proximity to beta cells. Glucagon is also produced by alpha cells in the stomach. Recent research has demonstrated that glucagon production may also take place outside the pancreas, with the gut being the most likely site of extrapancreatic glucagon synthesis.

Glucagon generally elevates the concentration of glucose in the blood by promoting gluconeogenesis and glycogenolysis. Glucose is stored in the liver in the form of the polysaccharide glycogen, which is a glucan a polymer made up of glucose molecules.

Liver cells hepatocytes have glucagon receptors. When glucagon binds to the glucagon receptors, the liver cells convert the glycogen into individual glucose molecules and release them into the bloodstream, in a process known as glycogenolysis.

As these stores become depleted, glucagon then encourages the liver and kidney to synthesize additional glucose by gluconeogenesis. Glucagon turns off glycolysis in the liver, causing glycolytic intermediates to be shuttled to gluconeogenesis. Glucagon also regulates the rate of glucose production through lipolysis.

Glucagon induces lipolysis in humans under conditions of insulin suppression such as diabetes mellitus type 1. Glucagon production appears to be dependent on the central nervous system through pathways yet to be defined.

In invertebrate animals , eyestalk removal has been reported to affect glucagon production. Excising the eyestalk in young crayfish produces glucagon-induced hyperglycemia. Glucagon binds to the glucagon receptor , a G protein-coupled receptor , located in the plasma membrane of the cell.

The conformation change in the receptor activates a G protein , a heterotrimeric protein with α s , β, and γ subunits. When the G protein interacts with the receptor, it undergoes a conformational change that results in the replacement of the GDP molecule that was bound to the α subunit with a GTP molecule.

The alpha subunit specifically activates the next enzyme in the cascade, adenylate cyclase. Adenylate cyclase manufactures cyclic adenosine monophosphate cyclic AMP or cAMP , which activates protein kinase A cAMP-dependent protein kinase. This enzyme, in turn, activates phosphorylase kinase , which then phosphorylates glycogen phosphorylase b PYG b , converting it into the active form called phosphorylase a PYG a.

Phosphorylase a is the enzyme responsible for the release of glucose 1-phosphate from glycogen polymers.

An example of the pathway would be when glucagon binds to a transmembrane protein. The transmembrane proteins interacts with Gɑβ𝛾. Gαs separates from Gβ𝛾 and interacts with the transmembrane protein adenylyl cyclase.

Adenylyl cyclase catalyzes the conversion of ATP to cAMP. cAMP binds to protein kinase A, and the complex phosphorylates glycogen phosphorylase kinase. Phosphorylated glycogen phosphorylase clips glucose units from glycogen as glucose 1-phosphate.

Additionally, the coordinated control of glycolysis and gluconeogenesis in the liver is adjusted by the phosphorylation state of the enzymes that catalyze the formation of a potent activator of glycolysis called fructose 2,6-bisphosphate.

This covalent phosphorylation initiated by glucagon activates the former and inhibits the latter. This regulates the reaction catalyzing fructose 2,6-bisphosphate a potent activator of phosphofructokinase-1, the enzyme that is the primary regulatory step of glycolysis [24] by slowing the rate of its formation, thereby inhibiting the flux of the glycolysis pathway and allowing gluconeogenesis to predominate.

This process is reversible in the absence of glucagon and thus, the presence of insulin. Glucagon stimulation of PKA inactivates the glycolytic enzyme pyruvate kinase , [25] inactivates glycogen synthase , [26] and activates hormone-sensitive lipase , [27] which catabolizes glycerides into glycerol and free fatty acid s , in hepatocytes.

Malonyl-CoA is a byproduct of the Krebs cycle downstream of glycolysis and an allosteric inhibitor of Carnitine palmitoyltransferase I CPT1 , a mitochondrial enzyme important for bringing fatty acids into the intermembrane space of the mitochondria for β-oxidation.

Thus, reduction in malonyl-CoA is a common regulator for the increased fatty acid metabolism effects of glucagon. Abnormally elevated levels of glucagon may be caused by pancreatic tumors , such as glucagonoma , symptoms of which include necrolytic migratory erythema , [30] reduced amino acids, and hyperglycemia.

It may occur alone or in the context of multiple endocrine neoplasia type 1. Elevated glucagon is the main contributor to hyperglycemic ketoacidosis in undiagnosed or poorly treated type 1 diabetes.

As the beta cells cease to function, insulin and pancreatic GABA are no longer present to suppress the freerunning output of glucagon. As a result, glucagon is released from the alpha cells at a maximum, causing a rapid breakdown of glycogen to glucose and fast ketogenesis.

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Al-Jebawi, Indiana University School of Medicine Indianapolis, IN, USA and others. Chapter 3: Physiological Effects of Glucagon on Cardiovascular System M. Rosic, V. Zivkovic, S. Pantovic, M. Colic and VLj Jakovljevic, Department of Physiology, Faculty of Medical Sciences, University of Kragujevac, Kragujevac, Serbia.

Chapter 4: The Role of Abnormal Glucagon in Type 2 Diabetes Ahmed F. Chapter 5: The Role of Incretin Hormones in Glucagon Regulation and Diabetes Treatment Shushan B. Artinian, Sawsan M. Al-Lafi, Suzan S. Boutary, Nadine S. Zwainy and Anwar B.

Bikhazi, Department of Anatomy, Cell Biology and Physiological Sciences, Faculty of Medicine, American University of Beirut, Beirut, Lebanon. Chapter 6: Glucagon Antagonists Alaaeldin M. Chapter 7: Alpha-Cell-Mediated Beta-Like Cell Regeneration as a Putative Therapeutic Pathway For Type 1 Diabetes Anja Pfeifer, Keith Al-Hasani, Monica Courtney, Nouha Ben-Othman, Elisabet Gjernes, Andhira Vieira and Patrick Collombat, INSERM U, Diabetes Genetics Team, FR Nice, France, and others.

Chapter 8: Glucagon: Improving Knowledge and Education on the Antidote to Severe Hypoglycemia B. In: Manson LA ed Biomembranes, vol 2. Plenum, New York London, p Google Scholar. Clara M Das Pankreas der Vögel. Anat Anz — Crick F Split genes and RNA splicing.

Science — Dobbs RE, Unger RH Glucagon and somatostatin. In: Freinkel N ed Contemporary metabolism. Hellerstrom C, Howell SL, Edwards JC, Andersson A An investigation of glucagon biosynthesis in isolated pancreatic islets of guinea-pigs. FEBS Lett 97— Hellerström C, Howell SL, Edwards JC, Andersson A, Östenson C-G Glucagon bio-synthesis in isolated pancreatic islets of guinea-pigs.

Biochem J 13— PubMed Google Scholar. Hellman B, Hellerström C The islets of Langerhans in ducks and chickens with special reference to the argyrophil reaction.

Z Zellforsch — Howell SL, Edwards JC, Whitfield M Preparation of B-cell deficient guinea-pig islets of Langerhans. Horm Metab Res 3: 37— Jackson RC, Blobel G Post-translational processing of full-length presecretory proteins with canine pancreatic signal peptidase. Ann NY Acad Sci — Korányi L, Peterfy F, Szabó J, Török A, Guóth M, Tamás GY Jr Evidence for transformation of glucagon-like immunoreactivity of gut into pancreatic glucagon in vivo.

Article PubMed Google Scholar. Krieger DT, Liotta AS Pituitary hormones in brain: where, how and why? In Vitro — Lund PK, Goodman RH, Jacobs JW, Habener JF Glucagon precursors identified by immunoprecipitation of products of cell-free translation of messenger RNA.

Lund PK, Goodman RH, Habener JF Intestinal glucagon mRNA identified by hybridization to a cloned islet cDNA encoding a precursor. Biochim Biophys Res Comm — Article CAS Google Scholar. Milstein C, Brownlee GG, Harrison TM, Matheus MB A possible precursor of im-munoglobulin light chains.

Nature — Moody AJ, Markussen J, Sundby F, Steenstrup C, Schaich Fries A The insulin releasing activity of extracts of the porcine intestinal tract. In: Falkmer S, Hellman B, Taljedal IB eds The structure and metabolism of the pancreatic islets. Pergamon, Oxford, p Moody AJ, Jacobsen H, Sundby F, Frandsen EK, Baetens D, Orci L Heterogeneity of gut glucagon-like immunoreactivity.

In: Foà PP, Bajaj JS, Foà N eds Glucagon: its role in physiology and clinical medicine. Springer, Berlin Heidelberg New York, p Moody A J, Thim L, Hoist J J Porcine pancreatic glicentine-related peptide. Diabetologia Moody AJ, Hoist J J, Thim L, Lindkaer, Jensen S Relationship of glicentine to proglucagon and glucagon in the porcine pancreas.

Nagelschmidt L Untersuchungen über die Langerhansschen Inseln der Bauchspeicheldrüse bei den Vögeln. Z Mikros Anat Forsch — Niall HD The evolution of peptide hormones. In: Cumming J A, Funder JW, Mendelsohn FAO eds Endocrinology Noe BD, Bauer GE Evidence for glucagon biosynthesis involving a protein intermediate in islets of the anglerfish Lophius americanus.

Endocrinology — Noe BD, Bauer GE Evidence for sequential metabolic cleavage of proglucagon to glucagon in glucagon biosynthesis. Noe BD, Bauer GE, Steffes MW, Sutherland DEK, Najarian JS Glucagon biosynthesis in human pancreatic islets: Preliminary evidence for a biosynthetic intermediate.

Horm Metab Res 7: — Noe BD, Baste CA, Bauer GE a Studies on proinsulin and proglucagon biosynthesis and conversion at the subcellular level. Fractionation procedure and characterization of the subcellular fractions. Noe BD, Baste CA, Bauer GE b Studies on proinsulin and proglucagon biosynthesis and conversion at the subcellular level.

Distribution of radioactive hormones and hormone precursors in subcellular fractions after pulse and pulse-chase incubation of islet tissue. Noe BD, Fletcher DJ, Bauer GE Biosynthesis of glucagon and somatostatin.

In: Cooperstein SJ, Watkins DT eds Biochemistry, physiology and pathology of the islets of Langerhans. Academic Press, New York, p Biochem J — Östenson C-G, Andersson A, Eriksson U, Hellerström C Glucagon biosynthesis in isolated pancreatic islets of mice and guinea-pigs.

Diab Metab 6: — Palade G Intracellular aspects of the process of protein synthesis. Patzelt C, Tager HS, Caroll RJ, Steiner DF Identification and processing of proglucagon in pancreatic islets. Patzelt C, Chan SJ, Quinn PS, Carroll RJ, Tager HS, Steiner DF Biosynthetic precursors of glucagon: identification of glucagon and proglucagon.

In: Waldhausl WK ed Diabetes Excerpta Medica, Amsterdam, p Petersson B, Hellerström C, Gunnarsson R Structure and metabolism of the pancreatic islets in streptozotocin treated guinea-pigs. Horm Metab Res 2: — Ravazzola M, Siperstein A, Moody AJ, Sundby IF, Jacobsen H, Orci L Glicentin immunoreactive cells: their relationship to glucagon-producing cells.

Ravazzola M, Orci L, Perrelet A, Unger RH Immunocytochemical quantitation of glicentin and glucagon during maturation of A-cell secretory granules. Rigopoulou D, Valverde I, Marco J, Faloona G, Unger RH Large glucagon im-munoreactivity in extracts of pancreas. J Biol Chem — Rubenstein AH, Horwitz DL, Jaspan JB, Mako ME, Blix PM, Kuzuya H Circulating proinsulin and C-peptide.

In: Bajaj JS ed Diabetes. Exogenous somatostatin inhibits glucagon release in isolated α-cells, as wells as in healthy and diabetic patients 11, In addition, islets isolated from somatostatin-deficient mice have reduced glucose-suppression of glucagon release Figure 2.

Immunofluorescent labeling of human and mouse islets. Human islet. Glucagon-positive α-cells green are randomly dispersed among insulin-positive β-cells red within the human islet. Mouse islet.

Glucagon-positive α-cells green are concentrated on the mantle and insulin-positive β-cells red make-up the core of the mouse islet. Glucagon exerts its physiological action on target tissues via the G-protein coupled glucagon receptor, which is found on multiple tissues including the liver, fat, intestine, kidney and brain 50, In the liver, glucagon counteracts the anabolic properties of insulin by promoting gluconeogenesis and glycogenolysis which consequently increases glucose output.

The hepatocyte is exposed to high levels of glucagon released by the pancreas via the portal vein. The subsequent activation of adenylate cyclase leads to the increase in intracellular cyclic adenosine monophosphate cAMP levels and the activation of protein kinase A PKA The second messenger cAMP can activate cyclic nucleotide-gated ion channels, exchange proteins activated by cAMP EPAC and protein kinase A PKA.

The process of glycogenolysis involves the activation of glycogen phosphorylase kinase and glycogen phosphorylase through the activated PKA, and glycogen phosphorylase brings about glycogen breakdown. In addition, the activation of the cAMP-PKA pathway in the hepatocyte leads to the phosphorylation and activation of CREB and subsequent activation of key gluconeogenic enzymes, phosphoenolpyruvate carboxykinase PEPCK and glucosephosphatase G6Pase , stimulating hepatic glucose output.

The gluconeogenic process is also promoted by the activity of additional transcription factors. The activated CREB binds to the promoter region of the transcriptional coactivator PGC-1 gene, increasing its transcription.

PGC-1 and the nuclear transcription factor hepatocyte nuclear factor-4 HNF-4 further promote gluconeogenesis by increasing the transcription of PEPCK gene and therefore PEPCK activity In the adipocyte, glucagon activates the cAMP-PKA pathway, leading to the phosphorylation and activation of hormone-sensitive lipase and the subsequent breakdown of triglycerides lipolysis and release of diacylglycerol and free fatty acids into the circulation.

The liver can further utilize glycerol and free fatty acids for gluconeogenesis or re-esterification of free fatty acids to form ketone bodies. In the heart, glucagon has been described as a vasodilator, which lowers blood pressure by decreasing the vasculature resistance in the liver and spleen.

Glucagon has diuretic effects on the kidney, increasing glomerular filtration and electrolyte excretion Glucagon receptors are expressed in the brain and data has suggested that circulating glucagon can pass the blood-brain barrier to modulate its effects in the central nervous system.

Glucagon infused in the central nervous system has anorexigenic effects in rats, chicks and sheep. In addition, intravenous infusion of glucagon has been shown to suppress appetite in humans; however, the direct link between glucagon and central food intake regulation in humans is unclear.

Glucagon relaxes the GI tract from esophagus to colon and as a result is often used to quiet the bowel before endoscopic retrograde cholangiopancreatography ERCP or bowel imaging studies In the esophagus it is used to relax the muscle before removal of foreign objects.

Glucagon also will relax the sphincter of Oddi These effects are almost certainly pharmacological but are short lived and without other deleterious effects. Crystaline glucagon was first prepared in and early reports injecting mg amounts of glucagon into rodents included a description of degranulation of acinar cells along with pancreatic atrophy 10,49,57, This finding was interpreted by Jarett as due to inhibition of protein synthesis due to lowered plasma amino acid levels in vivo as glucagon did not inhibit protein synthesis measured by incorporation of 3H-leucine in vitro However, these results are difficult to interpret because the animals given megadoses of glucagon lost weight and were reported to appear ill.

In vitro studies with isolated lobules showed that in-vivo pretreatment reduced subsequent protein synthesis and intracellular transport after 30 minutes of infusion in vivo but not at 24 hours.

In a more recent study, Kash et. With the knowledge of a possible relationship between the endocrine and exocrine pancreas 43 , the effects of exogenous glucagon on pancreatic secretion was studied in a variety of species both with and without anesthesia.

Initially most studies were carried out in unanesthetized dogs with pancreatic fistulas, the predominant animal model for GI physiology at the time, and glucagon most often prepared by Eli Lilly was shown to inhibit the volume, bicarbonate and protein or enzyme content of pancreatic secretion stimulated by food, acid, secretin or CCK 24,44,45,47,55,56,73,74, Similar inhibition of pancreatic secretion has also been seen in studies carried out in rats 1,5,83 , cats 54 and humans 12,18,25,40, The mechanism of the inhibition remains unclear, but has been assumed by most authors to be at the level of the pancreas because the effect of exogenous secretagogues was inhibited.

Possible loci include inhibition of pancreatic blood flow, the resulting hyperglycemia, lowering of plasma calcium as well as inhibition of the secretory mechanism. Glucagon could also be having an effect on the nervous system either centrally or within the pancreas.

Some of these possible inhibitory loci could be better controlled using a perfused pancreas model. Glucagon has been reported to inhibit secretion in the perfused pancreas of the cat and rat In the latter study, infusion of amino acids was shown to increase glucagon and inhibit pancreatic secretion and this effect could be blocked by infusing an antibody to glucagon Other studies, however, gave different results.

In a study in the perfused dog pancreas glucagon had no effect 72 , but in a study in perfused rat pancreas, glucagon increased the basal flow and protein output. However, when glucagon was combined with secretin, it decreased the volume and protein output Other in vitro studies have been carried out using pancreatic segments or lobules.

In studies of rat pancreas lobules, glucagon increased amylase secretion and potentiated effects of acetylcholine, CCK or electric field stimulation to activate nerves 86,

You and Your Hormones Proteins that bind to sites adjacent to the CRE and inhibit the CREB-mediated cAMP stimulation of glucagon expression, designated CAPs CREB-associated proteins , have been described These effects of GLP-1 appear to be mediated by interactions on specific GLP-1 receptors because the reduction in food intake is greatly attenuated by prior or coadministration of the GLP-1 receptor antagonist, exendin Gromada J, Franklin I, Wollheim CB. Delcour, J. Pantovic, M. Somatostatin is an intestinal peptide that inhibits release from many endocrine cells through an inhibitory G protein ,

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Endocrinology - Pancreas: Insulin Function Address Insulin resistance diet requests to: Joel Biosyntuesis. Insulin resistance diet, M. Received Biowynthesis from Glucabon Medical Research Council of Canada, the Alberta Biosynthezis Foundation for Medical Research, and Glucagon biosynthesis Canadian Accelerated weight loss Association. Investigator with the Howard Hughes Medical Institute and received support from US Public Health Service grants DK, DK, and DK History of the Incretin Concept: Discovery of Gastric Inhibitory Polypeptide. IT HAS been 15 yr since the initial discovery of the glucagon-like peptides GLPs as potential bioactive peptides encoded in the preproglucagon gene.

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