Glucagon is a 29 amino acid peptide hormone liberated in the α cells of the islets of Langerhans. For an overview of glucagon action, see the section on the Glucagon receptor
Glucagon-producing α cells represent one of the earliest populations of detectable islet cells in the developing endocrine pancreas. Glucagon is generally viewed as a hormone that opposes the action of insulin in peripheral tissues, predominantly the liver, where the insulin:glucagonratio determines the intricate control of gluconeogenesis and glycogenolysis. The action of glucagon in the liver is complex and involves coordinate regulation of transcription factors and signal transduction networks which converge on regulation of amino acid, lipid and carbohydrate metabolism. For an overview of selected recent insights into the downstream signals activated following engagement of the hepatic glucagon receptor, see CREB regulates hepatic gluconeogenesis through the coactivator PGC-1. Nature. 2001 Sep 13;413(6852):179-83 and CREB controls hepatic lipid metabolism through nuclear hormone receptor PPAR-mgama. Nature. 2003 Nov 13;426(6963):190-3. and Glucagon represses signaling though the mammalian target of rapamycin in rat liver by activating AMP-activated protein kinase. J Biol Chem. 2004 Oct 19 [Epub ahead of print].
Although meals generally suppress glucagon secretion from the normal α cell, subjects with diabetes frequently exhibit disordered control of glucagon secretion leading to excess hepatic glucose production. Given the structural relationships between glucagon, oxyntomodulin, and glicentin, many commercial assays that claim to measure 29 amino acid pancreatic glucagon may exhibit lack of specificity, and occasionally, problems with sensitivty. Hyperglucagonaemia analysed by glucagon sandwich ELISA: nonspecific interference or truly elevated levels? Diabetologia. 2014 Sep;57(9):1919-26 and Specificity and sensitivity of commercially available assays for glucagon and oxyntomodulin measurement in humans Eur J Endocrinol. 2014 Mar 8;170(4):529-38
The tissue-specific liberation of proglucagon is controlled by cell-specific expression of prohormone convertase (PC) enzymes. An essential role for PC2 in the processing of islet proglucagon is revealed by studies of the PC2 knockout mouse. This mouse has mild hypoglycemia, elevated proinsulin, and exhibits a major defect in the processing of proglucagon to mature pancreatic glucagon, and the murine islet α cells secrete proglucagon from atypical secretory granules. See Incomplete processing of proinsulin to insulin accompanied by elevation of Des-31,32 proinsulin intermediates in islets of mice lacking active PC2. J Biol Chem. 1998 Feb 6;273(6):3431-7 and Severe defect in proglucagon processing in islet A-cells of prohormone convertase 2 null mice. J Biol Chem. 2001 Jul 20;276(29):27197-202
A major component of the phenotype exhibited by the PC2 knockout mouse, namely mild hypoglycemia and marked alpha cell hyperplasia, appears attributable to defective levels of circulating glucagon, as glucagonreplacement by osmotic mini-pump corrected hypoglycemia and produced a significant decrease in the number of hyperplastic alpha cells. The islet remodeling was detectable by 11 days, and after 25 days, PC2-/- islets resembled wild type islets. Apoptosis of islet α cells appears to contribute to the remodeling process, implying an important role for a threshold of circulating glucagon in the regulation of both islet α cell proliferation and survival. See Glucagon Replacement via Micro-Osmotic Pump Corrects Hypoglycemia and ?-Cell Hyperplasia in Prohormone Convertase 2 Knockout Mice. Diabetes. 2002 Feb;51(2):398-405
Glucagon is also synthesized in the CNS, where its actions may include regulation of peripheral glucoregulation, yet remain less well understood.
Glucagon release is stimulated by hypoglycemia and inhibited by hyperglycemia, insulin, and somatostatin. The inhibitory effect of somatostatin appears to be mediated via the SST-2 receptor, as revealed through studies using a SST-2-selective receptor antagonist DC-41-33. This compound enhanced glucagon release in studies using isolated rat islets, perifused isolated rat islets, and isolated perfused rat pancreas, as shown in Intra-Islet Somatostatin Regulates Glucagon Release via Type 2 Somatostatin Receptors in Rats. Diabetes. 2003 May;52(5):1176-81
How does the islet a-cell sense and respond to changes in blood glucose and what goes wrong in the face of repeated hypoglycemia?
A major problem in diabetic patients with repeated hypoglycemia is the development of defective counter-regulatory responses that include reduced or absent glucagon responses to hypoglycemia. Hence understanding how and why the autonomic nervous system and islet α cell develop defects in glucagonsecretion leading to hypoglycemia insensitivity is a major challenge in diabetes research.
At present, there is very little known about the factors which directly regulate α-cell secretion. Insulin is a potent inhibitor of islet glucagon release. Somatostatin and GLP-1 also inhibit glucagon secretion. Glucose suppresses glucagon secretion, but may do so indirectly through insulin or GABA as outlined in Glucagon response to hypoglycemia is improved by insulin-independent restoration of normoglycemia in diabetic rats. Endocrinology. 1996 Aug;137 (8):3193-9 and Insulin, but not glucose lowering corrects the hyperglucagonemia and increased proglucagon messenger ribonucleic acid levels observed in insulinopenic diabetes. Endocrinology. 1998 Nov;139(11) :4540-6 and Glucose-inhibition of glucagon secretion involves activation of GABAA-receptor chloride channels. Nature. 1989 Sep 21;341(6239):233-6.
Interestingly, the incretin hormones GIP and GLP-1 have divergent effects on glucagon secretion, with GIP stimulating but GLP-1 inhibiting glucagon secretion. In isolated islets, the inhibitory effects ofGLP-1 are not associated with augmentation of insulin or somatostatin secretion but involves PKA signaling. GLP-1 receptor expression was detectable but very low in purified isolated a-cells whereas the GIPR and adrenergic receptors were considerably more abundant in a-cells. Only 1% of a-cells were GLP-1R immunopositive. Low concentrations of forskolin also inhibited GLP-1 secretion in murine islets. Activation of Epac2 mimicked the stimulatory effect of forskolin and adrenaline on alpha cell exocytosis and Epac2 was required for the stimulatory effects of adrenaline. GLP-1 also modulated action potential firing in isolated a-cells GLP-1 inhibits and adrenaline stimulates glucagon release by differential modulation of N- and L-type Ca2+ channel-dependent exocytosis Cell Metab. 2010 Jun 9;11(6):543-53
Classical studies of isolated α-cells by Pipeleers and colleagues have demonstrated important differences between α vs. β-cells and their respective responses to varying glucose concentrations. Early classical studies demonstrated how glucose, nutrients, insulin and somatostatin regulate glucagon secretion and signal transduction pathways in purified rat α-cells as outlined in Interplay of nutrients and hormones in the regulation of glucagon release. Endocrinology. 1985 Sep;117(3):817-23.
Administration of SGLT2 inhibitors has been associated with increased glucagon secretion in diabetic human subjects, and enhanced hepatic glucose production Dapagliflozin improves muscle insulin sensitivity but enhances endogenous glucose production J Clin Invest. 2014 Feb;124(2):509-14 and
Metabolic response to sodium-glucose cotransporter 2 inhibition in type 2 diabetic patients J Clin Invest. 2014 Feb;124(2):499-508. Bonner and colleagues demonstrated that rodent and human a cells express a functional SGLT2 protein, which is downregulated in rodents with experimental hyperglycemia. Genetic or pharmacological reduction of SGLT2 expression/activity enhanced islet Gcg expression and glucagon secretion. Hence SGLT2-mediated glucose sensing in a cells may represent an adaptve response to prevent hypoglycemia Inhibition of the glucose transporter SGLT2 with dapagliflozin in pancreatic alpha cells triggers glucagon secretion Nature Medicine (2015) doi:10.1038/nm.3828
There is comparatively little information about the study of glucagon biosynthesis and secretion using human a-cells. The Kieffer lab, in collaboration with Betalogics/Centocor, has reported derivation of a novel population of human islet cells with a highly differentiated a-cell phenotype using human ES cells as starting material. High through put screening enabled identification of multiple small molecules, that were combined with established growth and differentiation factors, to direct differentiation of alpha cells after sequential passage through a 6 stage process, using validated markers for islet and alpha cell differentiation. Stage 6 clusters expressed Pc2, Arx but not pdx-1. Even at stage 5, considerable number of islet cells co-expressed glucagon and insulin whereas by stage 6, a greater enrishment for glucagon-producing cells was achieved. Some cell proliferation was observed within stage 6 cell clusters. Glucagon secretion was stimulated by arginine and KCL and inhibited by somatostatin. Secretion of glucagon from transplanted cells in vivo was regulated in appropriate directions with fasting and feeding. PlasmaGLP-1levels were also elevated in mice receiving the a-cell transplants. Production of Functional Glucagon-Secreting Alpha Cells from Human Embryonic Stem Cells Diabetes. 2010 Oct 22. [Epub ahead of print]
In purified rat β-cells, the intracellular concentration of glucose or 3-O-methyl-D-glucose equilibrates within 2 min with the extracellular levels, and, like in intact islets, the rate of glucose oxidation displays a sigmoidal dose-response curve for glucose. In contrast, even after 5 min of incubation, the apparent distribution space of D-glucose or 3-O-methyl-D-glucose in a-cells remains much lower than the intracellular volume. In a-cells, both the rate of 3-O-methyl-D-glucose uptake and glucose oxidation proceed proportional to the hexose concentration up to 10 mM and reach saturation at higher concentrations, whereas exogenous insulin failed to affect 3-O-methyl-D-glucose or D-glucose uptake and glucose oxidation by purified α-cells See Differences in glucose handling by pancreatic A- and B-cells. J Biol Chem. 1984 Jan 25;259(2):1196-200.
Studies using rat islets, isolated purified α-cells, and GABA antagonists have provided evidence that GABA receptor subunits are expressed in α cells and that inhibition of GABA receptor activation is associated with complete failure of glucose to suppress glucagon secretion. See Glucose Inhibition of Glucagon Secretion From Rat alpha-Cells Is Mediated by GABA Released From Neighboring beta-Cells. Diabetes. 2004 Apr;53(4):1038-45
Elegant studies from Wang and colleagues demonstrate that insulin induces activation of GABA(A) receptors in the alpha cells by GABA receptor translocation via an Akt kinase-dependent pathway. This leads to membrane hyperpolarization in the alpha cells and, ultimately, suppression of glucagon secretion. Hence, insulin may directly inhibit glucagon secretion, and indirectly potentiate the inhibitory effects of GABA concomitant released by β-cells-See Intra-islet insulin suppresses glucagon release via GABA-GABA(A) receptor system. Cell Metab. 2006 Jan;3(1):47-58
Furthermore, the α cell secretory response may be modified, in an autoregulatory pathway, by secretion of L-glutamate which in turn triggers the secretion of gamma-aminobutyric acid (GABA) from β-cells. See Metabotropic Glutamate Receptor Type 4 Is Involved in Autoinhibitory Cascade for Glucagon Secretion by alpha-Cells of Islet of Langerhans. Diabetes. 2004 Apr;53(4):998-1006.
Insulin and zinc are potent inhibitors of glucagon secretion from rat α cells, perhaps through modulation of KATP channel activity, whereas the inhibitory effect of GLP-1 on α-cells may be indirect, as outlined in studies of rat α cells in vitro β-Cell Secretory Products Activate a-Cell ATP-Dependent Potassium Channels to Inhibit Glucagon Release. Diabetes. 2005 Jun;54(6):1808-15, and in experiments with mouse α- β-Cell Pdx1 Expression Is Essential for the Glucoregulatory, Proliferative, and Cytoprotective Actions of Glucagon-Like Peptide-1. Diabetes 2005 54: 482-491.
In contrast, similar experiments using isolated murine α cells demonstrate that glucose and insulin but not zinc exerts an inhibitory effect on regulation of glucagon secretion in normal murine islets, dissociated a-cells, or the aTC-1 cell line, as demonstrated in Glucose or Insulin, but not Zinc Ions, Inhibit Glucagon Secretion From Mouse Pancreatic a-Cells. Diabetes. 2005 Jun;54(6):1789-9
The complexity of how glucose controls glucagon secretion from individual α cells vs. islets retaining their normal anatomical relationship was further highlighted by Hutchens and Piston, who revealed a role for ephrin signaling in the paracrine inhibition of tonic glucagon secretion. Ephrine signaling in islet cells was manipulated through synthetic IgG-ephrin ligands. Manipulation of ephrin signaling at low vs. high glucose produced the expected corresponding changes in insulin and glucagon secretion in both murine and human islets. Acute antagonism of the insulin andsomatostatin receptors was used to tease out direct vs. indirect effects of ephrin signaling on glucagon secretion. Loss of EphA4 signaling in KO mice produced enhanced glucagon secretion from αEpha4-/- islets as compared to wild-type islets at both low and high glucose. The authors provide evidence for the complexity of glucose-dependent, ephrin-mediated modulation of both insulin and glucagon secretion, likely through as yet unidentified paracrine mediators EphA4 Receptor Forward Signaling Inhibits Glucagon Secretion from α-cells
A contrasting study using the perfused diabetic Wistar rat pancreas came to opposite conclusions, namely a decrement in the local concentration of zinc, but not insulin, resulted in improvement in the α-cell function. See Zinc, Not Insulin, Regulates the Rat Alpha Cell Response to Hypoglycemia in vivo. Diabetes. 2007 Feb 22; [Epub ahead of print]
The direct effect of glucose on pancreatic α-cells is difficult to study due to the challenge of isolating a pure α-cell population independent of contaminating β-cells. Glucose generally inhibits α-cells in the context of whole islets. A biphasic response of glucose-regulated α-cells, with inhibition of Glucose stimulates glucagon release in single rat alpha-cells by mechanisms that mirror the stimulus-secretion coupling in beta-cells. Endocrinology. 2005 Nov;146(11):4861-70. and in Paradoxical stimulation of glucagon secretion by high glucose concentrations. Diabetes. 2006 Aug;55(8):2318-23. Similarly, whereas raising the glucose concentration to 20 mM significantly increased the amount of Ca2+ entering beta cells, the sugar was without effect on Ca2+ entry into the alpha-cells as described in Glucose stimulates the entry of Ca2+ into the insulin-producing beta cells but not into the glucagon-producing alpha 2 cells. Acta Physiol Scand. 1987 Oct;131(2):230-4. The importance of somatostatin for glucose-mediated suppression of glucagon secretion is illustrated by studies using SST-/- mice, which fail to exhibit glucose-mediated suppression of glucagon secretion in studies using isolated islets Somatostatin secreted by islet cells fulfils multiple roles as a paracrine regulator of islet function. Diabetes November 4, 2008 10.2337/db08-0792
Intriguingly, gastrin has been shown to stimulate glucagon secretion in some but not all experimental models, and the fetal endocrine pancreas contains a large amount of gastrin, following which islet gastrin expression diminishes postnatally. Gastrin, together with EGF receptor ligands, may promote expansion of islet mass through enhanced islet neogenesis. Mice with disruption of the gastrin gene exhibit normal islets and basal levels of pancreatic glucagon content, yet exhibit mild hypoglycemia a defective glucagon secretory response to insulin induced hypoglycemia. See Hypoglycemia, defective islet glucagon secretion, but normal islet mass in mice with a disruption of the gastrin gene Gastroenterology 2003 125:1164-174
An important role for the insulin receptor, and by inference insulin, in the control of a-cell function was demonstrated by Kawamori and colleagues in their analysis of a mouse with a-cell-specific deletion of the insulin receptor. These mice exhibited modest hyperglucagonemia in the basal fed state, and enhanced glucagon secretion in response to arginine or hypoglycemia. These findings imply that basal insulin receptor signaling is important for tonic inhibition of the a-cell. Insulin signaling in alpha cells modulates glucagon secretion in vivo Cell Metab. 2009 Apr;9(4):350-61.
A role for the Per-arnt-sim (PAS) domain containing protein kinase in the control of glucagon secretion has been proposed by Xavier and colleagues. PASK mRNA was increased by glucose in cultures of rodent or human islets and found to be expressed in both a and b-cells. Pask-/- mice exhibited fasting hyperglycemia and increased levels of plasma glucagon and Pask-/- islets contained less insulin and secreted more glucagon at 10 mM glucose compared to Pask+/+ islets. Reduction of Pask expression in aTC cells using siRNA increased glucagon gene expression and glucagon secretion, but did not inhibit the inhibitory effect of insulin on glucagon secretion. Conversely, overexpression ofPask in a-TC cells and human islets inhibited glucagon secretion See Per-arnt-sim (PAS) domain-containing protein kinase is downregulated in human islets in type 2 diabetes and regulates glucagon secretion Diabetologia. 2010 Dec 23. [Epub ahead of print]
The a-cell, under some circumstances, may also synthesize and secrete GLP-1, which in turn, is a robust inhibitor of glucagon secretion-See GLP-1 and the Alpha Cell
Glucagon secretion, a-cell function and inflammation
Historical evidence points to a link between inflammatory stimuli, and increased plasma levels of glucagon Effect of inflammatory and noninflammatory stress on plasma ketone bodies and free fatty acids and on glucagon and insulin in peripheral and portal blood Inflammation. 1979 Jul;3(3):289-94. Indeed intravenous administration of E. Coli to dogs produced rapid increases in plasma levels of pancreatic glucagon and larger molecular forms of enteroglucagon were detected within hours, with comparatively greater increases seen in levels of gut-derived glucagon, with elevated levels persisiting for days Changes of plasma gastrointestinal glucagon concentrations following lethal infusions of E. coli Circ Shock. 1986;19(3):301-8. Similarly, cecal ligation and puncture leads to hyperglucagonemia in rodents Cecal ligation and puncture with total parenteral nutrition: a clinically relevant model of the metabolic, hormonal, and inflammatory dysfunction associated with critical illness J Surg Res. 2004 Oct;121(2):178-86. The sepsis-induced hyperglucagonemia has been linked to increased hepatic glucose production Importance of hyperglucagonemia in eliciting the sepsis-induced increase in glucose production Circ Shock. 1989 Nov;29(3):181-91, providing a possible explanation for a role for hyperglucagonemia in the metabolic response to sepsis. Intravenous administration of lipopolysaccharide rapidly increases plasma levels of TNF-α and Il-6, and glucagon in normal healthy volunteers Metabolic and physiologic effects of an endotoxin challenge in healthy obese subjects Clin Physiol Funct Imaging. 2011 Sep;31(5):371-5, and LPS similarly increased circulating levels of GLP-1 in mice Lipopolysaccharides-mediated increase in glucose-stimulated insulin secretion: involvement of the GLP-1 pathway Diabetes. 2014 Feb;63(2):471-82. IL-6 appears to increase GLP-1 levels through direct effects on L cells and islet a-cells in mice Interleukin-6 enhances insulin secretion by increasing glucagon-like peptide-1 secretion from L cells and alpha cells Nat Med. 2011 Oct 30;17(11):1481-9. Similarly, Chow and colleagues also demonstrated that IL-6 directly stimulates glucagon secretion from rodent and human islets, and levels of islet IL-6 are elevated in rodent models of experimental inflammation. Mice with a pancreatic a-cell knockout of glycoprotein 130, a key component of the IL-6 signal transduction pathway, were resistant to experimental islet inflammation and exhibited improved glycemia and reduced glucagon responses after HFD/STZ administration Glycoprotein 130 Receptor Signaling Mediates α-Cell Dysfunction in a Rodent Model of Type 2 Diabetes Diabetes. 2014 Sep;63(9):2984-95. Intriguingly, Barnes and colleauges invoked an additional role for CNS mechanisms, in addition to direct effects on islets, in the IL-6-dependent induction of glucagon secretion in response to hypoglycemia or LPS Interleukin-6 Amplifies Glucagon Secretion: Coordinated Control via the Brain and Pancreas Am J Physiol Endocrinol Metab. 2014 Sep 9. pii: ajpendo.00343.2014
The mechanisms regulating degradation and clearance of glucagon remain incompletely understood. The enzyme neutralendopeptidase 24.11 has been shown to regulate the levels of glucagon in pigs. Studies using candoxatril, a selective NEP inhibitor, demonstrated that levels of both endogenous and exogenously infused glucagon are increased following candoxatril administration as shown in Neutral endopeptidase 24.11 is important for the degradation of both endogenous and exogenous glucagon in anesthetized pigs. Am J Physiol Endocrinol Metab. 2004 Sep;287(3):E431-8. and Characterisation of the processing by human neutral endopeptidase 24.11 of GLP-1(7-36) amide and comparison of the substrate specificity of the enzyme for other glucagon-like peptides. Regul Pept. 1995 Aug 22;58(3):149-56
Glucagon has also been shown to be a pharmacological substrate for DPP-4 in vitro Metabolism of glucagon by dipeptidyl peptidase IV (CD26). Regul Pept. 2001 Jan 12;96(3):133-41 and Dipeptidyl peptidase IV (DPIV/CD26) degradation of glucagon. Characterization of glucagon degradation products and DPIV-resistant analogs. J Biol Chem. 2000 Feb 11;275(6):3827-34. however whether DPP-4 regulates physiological levels of endogenous glucagon remains unclear.
As but one example, levels of intact glucagon are not significantly changed in pigs subjected to treatment with a DPP-4 inhibitor, with the kidney functioning as a major determinant for glucagon elimination Differential regional metabolism of glucagon in anesthetized pigs. Am J Physiol Endocrinol Metab. 2003 Sep;285(3):E552-60.
Glucagon has been shown to increase cAMP in the CNS and the Gcgr has been localized to multiple regions of the brain. Preclinical studies also provide evidence that glucagon may be a satiety factor that also controls body weight through regulation of energy expenditure. Mighiu and colleagues demonstrated that central infusion of glucagon increased c-Fos and pCREB expression, inhibited hepatic glucose production (HGP) and improved glucose tolerance in rats (and mice) whereas inhibition of glucagon action with a monoclonal antibody or receptor antagonist abrogated these effects. These actions were mimicked by central MBH PKA activation and required an intact hepatic branch of the vagus nerve, and suggest that brain glucagon signaling acts as a brake to diminish hepatic glucagon action. The central actions of glucagon to inhibit HGP were lost after 3 days of high fat feeding in rats. Hypothalamic glucagon signaling inhibits hepatic glucose production Nat Med. 2013 May 19. doi: 10.1038/nm.3115
The actions of glucagon to reduce body weight, increase energy expenditure, and decrease adipose tissue expansion requires FGF-21 in mice. Acute Gcgr activation increased plasma FGF-21 levels and hepatic FGF-21 mRNA transcripts. Gcgr activation lowered plasma cholesterol on WT but not FGF-21-/- mice, without changes in hepatic lipid levels. Acute glucagon administration also increased plasma FGF-21 levels sin human subjects Fibroblast Growth Factor 21 Mediates Specific Glucagon Actions. Diabetes. 2013 Jan 10
The acute anorectic actions of glucagon injected into the rat CNS have been studied in lean and obese rats. Central icv injection of glucagon into the arcuate nucleus (but not the VMH) decreased food intake acutely (but the effect waned by 6 hrs) and reduced levels of CaMKKβ and its downstream targets pAMPK and pACC. Hence glucagon acts in the hypothalamus to transiently inhibit feeding via a PKA/CaMKKβ/AMPK-dependent pathway, specifically in the ARC. icv glucagon rapidly and transiently increased p-CREB and decreased calcium/calmodulin-dependent protein kinase kinase beta (CaMKKβ) expression in the hypothalamus. The acute anorectic actions of icv glucagon were blocked by a PKA inhibitor. Conversely, icv injection of the glucagon receptor antagonist des-His1 (Glu9) glucagon amide transiently increased food intake, as did genetic reduction of Gcgr expression using a lentivirus injected into the ARC. This satiety effect of glucagon involved downregulation of AgRP gene expression, yet was lost in rats with diet-induced obesity, attributed to CaMKKβ which seemed to mediate the obesity-induced hypothalamic resistance. Conversely reduction of Gcgr expression centrally in obese rats did not increase food intake. Blocking the actions of using a CaMKKβ-DN adenovirus restoted the anorectic action of icv glucagon in DIO rats. Hypothalamic CaMKKβ mediates glucagon anorectic effect and its diet-induced resistance Molecular Metabolism http://dx.doi.org/10.1016/j.molmet.2015.09.014
Lapierre and colleagues studied the role of CNS glucagon signaling in the effects of high vs. low protein diets and hepatic glucose production in rats. They report that high protein diets increases plasma glucagon levels, which in turns acts in the dorsal vagal complex to lower hepatic glucose production via a Gcgr-PKA-ERK KATP channel mechanism. The authors elaborae a mechanism whereby elevated plasma glucagon appears to preferentially lower glucose production/glycemia via the CNS, using clamped conditions in rats to elucidate the mechanisms. Glucagon signalling in the dorsal vagal complex is sufficient and necessary for high-protein feeding to regulate glucose homeostasis in vivo. EMBO Rep. 2015 Oct;16(10):1299-307.
Glucagon also acutely increases energy expenditure in humans, through incompletely delineated mechanisms. Glucagon infusion did not increase neck termperature of FDG uptake in classical BAT of 11 healthy male volunteers but did produce a 15% increase in energy expenditure. Infusion of glucagon 50 ng/kg/min resulted in plasma glucagon levels peaking at 370 ± 87 pmol/L however glucagon did not augment cold exposure-induced increases in EE. A non-significant rise in FGF-21 levels, mean increase of 238 ± 197 pg/ml, was reported in these same subjects, norepinephrine levels did not change, and heart rate increased modestly. Glucagon Increases Energy Expenditure Independently of Brown Adipose Tissue Activation in Humans Diabetes Obes Metab. 2015 Oct 5. doi: 10.1111/dom.12585.
The key biological actions of glucagon converge on regulation of glucose homeostasis through enhanced synthesis and mobilization of glucose in the liver. Glucagon receptors are also expressed on human islet b cells and in some experiments, may contribute to the regulation of glucose-stimulated insulin secretion as shown in Diabetologia 2000 Aug;43(8):1012-9. The relative importance of glucagon and GLP-1 for augmentation of the insulin secretory response to high glucose (20 mM) was also examined in the perfused rat pancreas preparation, wherein neither the GLP-1 receptor antagonist exendin(9-39) or the glucagonreceptor antagonist [des-His1-des-Phe6,Glu9] glucagon-NH2 inhibited the insulin secretory response to hyperglycemia. Similarly, augmenting endogenous glucagon secretion using isoproterenol had no effect on glucose-induced insulin secretion. Hence the precise physiological actions of glucagon receptors on islet b -cells remains a subject for ongoing investigation. See Assessment of the Role of Interstitial Glucagon in the Acute Glucose Secretory Responsiveness of In Situ Pancreatic beta-Cells. Diabetes. 2002 Mar;51(3):669-75
The metabolic effects of transient administration of native glucagon and GLP-1, infused alone or together for 45 minutes were assessed in overweight or obese healthy non-diabetic human volunteers. Peak levels of glucagon and GLP-1 achieved during the infusion were ~260 and 103 pmol/L, respectively. Both glucagon and GLP-1 reduced levels of non-esterified free fatty acids, and increased insulin levels. Plasma glucose levels rose with glucagon infusion and the glucagon-stimulated increase in plasma glucose was attenuated by co-administration of GLP-1. Resting energy expenditure increased modestly with glucagon alone, with no detectable change in core temperature, and co-administration of GLP-1 had no further effect on energy expenditure but did significantly reduce plasma levels of total and acyl ghrelin. There is currently no available information on whether the acute metabolic effects of a glucagon-GLP-1 co-agonist would be sustained with chronic administration in humans. Coadministration of Glucagon-Like Peptide-1 During Glucagon Infusion in Man Results in Increased Energy Expenditure and Amelioration of Hyperglycemia Diabetes. 2012 Dec 17. [Epub ahead of print]
Glucagon action in the liver
Fasting hyperglucagonemia is an early defect in the pathogensis of type 2 diabetes. Analysis of islet hormones in young obese adloescent subjects demonstrated significantly increased levels of fasting glucagon, particularly in obese individuals with insulin resistance or IGT. Glucagon secretion was appropriately suppressed by glucose or insulin in these subjects Basal alpha-cell up-regulation in obese insulin-resistant adolescents J Clin Endocrinol Metab. 2011 Jan;96(1):91-7
Glucagon generally functions as a counter-regulatory hormone, opposing the actions of insulin, and maintaining the levels of blood glucose, particularly in patients with hypoglycemia. In patients with diabetes, excess glucagon secretion plays a primary role in the metabolic perturbations associated with diabetes, such as hyperglycemia. Although the molecular control of α cell function leading to excess glucagon secretion is not well understood, individuals with a single amino acid polymorphism, Glu23Lys in the KIR6.2 potassium channel exhibit higher levels of glucose during an OGTT and defective glucose-mediated suppression of glucagon secretion in vivo. See The Prevalent Glu23Lys Polymorphism in the Potassium Inward Rectifier 6.2 (KIR6.2) Gene Is Associated With Impaired Glucagon Suppression in Response to Hyperglycemia. Diabetes. 2002 Sep;51 (9): 2854-60.
The enzymes important for gluconeogenesis and glucagon action may also be expressed and functional in the kidney and intestine. Mutel and colleagues used mice with liver-specific inactivation of glucose-6-phosphatase to demonstrate that fasting glucose was reasonably maintained via contributions of the kidney and GI tract to gluconeogenesis. Fasting induced the expression of G6pase, PEPCK-c and glutaminase in these mice. Fasting glucagon levels were slightly higher and plasma corticosterone levels increased in L-G6pc-/- mice. Administration of a Gcgr antagonist L-168-049 reduced hepatic Pck1 expression, and reduced levels of G6pc RNA in kidneys and intestine of fasted L-G6pc-/- mice. Conversely, glucagon administration increased p-CREB binding to the G6pc promoter in CHIP assays in intestine and kidney of WT mice. Control of Blood Glucose in the Absence of Hepatic Glucose Production During Prolonged Fasting in Mice: Induction of Renal and Intestinal Gluconeogenesis by Glucagon Diabetes. 2011 Oct 19. [Epub ahead of print
A role for the VHL-HIF2a axis in control of hepatic glucagon action has also been outlined using genetic studies in mice. VHL normally controls degradation of HIF2a, which in turn controls glucagon action in the liver. Inactivation of VHL specifically in the liver increased hepatic HIF2a expression, which abrogated the glucagon-mediated increase in gluconeogenesis and resulted in relative hypoglycemia. Intriguingly, refeeding, a state characterized by reduction of glucagon secretion/action was found to be associated with transient hepatic hypoxia and induction of hepatic HIF2a expression, which in turn reduced glucagon action by repressing CREB signaling, due to enhanced degradtion of cyclic AMP via PDE4 and ERK signaling. HIF2α Is an Essential Molecular Brake for Postprandial Hepatic Glucagon Response Independent of Insulin Signaling Cell Metab. 2016 Feb 3. pii: S1550-4131(16)00038-3. doi: 10.1016/j.cmet.2016.01.00
Dozens of studies have elucidated molecular mechanisms linking glucagon receptor signaling to control of hepatic glucose flux. For example, a combination of biochemical and genetic studies in mice, murine, hepatocytes, and human hepatocytes identified a pathway involving mitochondrial anaplerotic flux from glutamine that increases the contribution of this amino acid to the carbons of glucose generated during gluconeogenesis
Administration ofglucagon pharmacologically leads to a rapid rise in blood glucose, hence injectable glucagon is used as a pharmacological treatment for diabetic patients at risk for significant hypoglycemia. See Glucagon, Hypoglycemia and Counterregulation
Given the central importance of glucagon for blood sugar control, glucagon antagonists are also being developed for possible therapeutic use in patients with diabetes. For an overview, see Glucagon Receptor Antagonists
To review the importance of glucagon receptor signaling, see Glucagon Receptor
Although glucagon-producing tumors in human subjects are rare, they may be associated with clinical manifestations such as mucositis, anemia, weight loss, and necrolytic migratory erythema, in addition to hyperglycemia and rarely, intestinal hyperplasia.
A number of studies have demonstrated that glucagon promotes degradation of fat (known as lipolysis) both in cell preparations and in vivo. Does glucagon promote lipolysis in human adipose tissue? Older studies had been contradictory, with some reports affirming a role for glucagon in human adipocyte lipolysis Human glucagon and vasoactive intestinal polypeptide (VIP) stimulate free fatty acid release from human adipose tissue in vitro, whereas other experiments failed to show significant effects of glucagon on lipolysis in isolated human fat cells Glycerol release from incubated human adipocytes is not affected by gastrointestinal peptides. Int J Obes. 1985;9(1):25-7.
Gary Lewis and colleagues infused glucagon into healthy human subjects and assessed hepatic and intestinal lipoprotein production and clearance using tracer techniques. Subjects were subjected to low and high dose glucagon infusions in the fed state, resulting in glucagon levels of 64.5 vs 183.2 pg/ml, while levels of other endogenous islet hormones were controlled using a pancreatic clamp. Although no significant effects were detected on markers of de novo lipogenesis, high dose glucagon infusion reduced levels of VLDL1 Apo100 fractional catabolic rate and production rate, but no effect on intestinal lipoprotein metabolism was observed. Plasma levels of triglycerides and free fatty acids did not change during acute glucagon infusion. See Effects of acute hyperglucagonemia on hepatic and intestinal lipoprotein production and clearance in healthy humans Diabetes. 2010 Oct 27.
Longuet et al outlined a role for hepatic Gcgr signaling in the regulation of hepatic lipid metabolism, particularly during the fasting state. Administration of glucagon results in reduction of circulating triglycerides, whereas fasting upregulates a hepatic gene expression profile regulating control of lipid oxidation. Although glucagon regulates lipid synthesis, secretion, and oxidation in normal hepatocytes, Gcgr-/- hepatocytes exhibit profound defects in lipid oxidation, and accumlate excessive lipid in the liver during fasting. The actions of glucagon to control lipid oxidation appear to be mediated in part through a PPARa-dependent pathway as described in The Glucagon Receptor Is Required for the Adaptive Metabolic Response to Fasting Cell Metabolism November 2008 Nov;8(5):359-71.
Intriguingly, low dose glucagon together with GLP-1R activation produces near synergistic efficacy with regard to weight loss, lipolysis, and glucose homeostasis in rodent. Thee action recapitulate, to some extent, the profile of actions described for oxyntomodulin, a peptide that exerts dual Gcgr:GLP-1R agonism. See A new glucagon and GLP-1 co-agonist eliminates obesity in rodents Nature Chemical Biology Published online: 13 July 2009 | doi:10.1038/nchembio.209 and GLP-1/GCGR dual agonism reverses obesity in mice Diabetes ; published ahead of print July 14, 2009, doi:10.2337/db09-0278
Glucagon withdrawal or physiological hyperglucagonemia in vivo did not produce significant changes in palmitate flux, an index of lipolysis, in normal or diabetic human subjects Effects of glucagon on free fatty acid metabolism in humans. J Clin Endocrinol Metab. 1991 Feb;72(2):308-15.
Similar negative findings were reported in the May 2001 issue of the JCEM, wherein 7 healthy male subjects were implanted with indwelling microdialysis catheters in the abdominal wall, and the effects of glucagon infusion on interstitial glycerol, and plasma glycerol and FFAs were examined. No effects on glycerol or free fatty acids were detected with systemic glucagon infusion, with or without exogenous glucose. See Physiological Levels of Glucagon Do Not Influence Lipolysis in Abdominal Adipose Tissue as Assessed by Microdialysis. J Clin Endocrinol Metab. 2001 May 1;86(5):2085-2089. Similar negative results were obtained in a study of lipolysis in normal male subjects with indwelling microdialysis catheters implanted into abdominal adipose tissue Physiological levels of glucagon do not influence lipolysis in abdominal adipose tissue as assessed by microdialysis. J Clin Endocrinol Metab. 2001 May;86(5):2085-9. Hence, the available data does not support an important physiological role for glucagon in lipolysis.
Glucagon has anti-motility effects on the gastrointestinal tract (esophagus, stomach, and small and large intestine) when administered pharmacologically to human subjects. See Glucagon effects on the human small intestine. Dig Dis Sci. 1979 Jul;24(7):501-8 and Glucagon and the colon. Gut. 1975 Dec;16(12):973-8 and A comparison of virtual and conventional colonoscopy for the detection of colorectal polyps. N Engl J Med. 1999 Nov 11;341(20):1496-503. Glucagon may also relax smooth muscle in the gallbladder and ureter, leading to occasional use during radiology studies of the gallbladder and kidney.
Administration of glucagon to human subjects leads to a rapid increase in blood glucose after either iv or intramuscular (IM) injection. Curiously, IM, but not IV glucagon, seems to activate components of the HPA axis (ACTH and cortisol predominantly). The mechanisms underlying this difference are not clear. See Glucagon is an ACTH secretagogue as effective as hCRH after intramuscular administration while it is ineffective when given intravenously in normal subjects. Pituitary. 2000 Nov;3(3):169-73.
Glucagon and Taste
Periodic studies have examined the importance of glucagon for taste sensing. Elson and colleagues reported local co-expression of the proglucagon gene and the Gcgr in subsets of murine lingual taste buds the majority of which also expressed PLCb2 and T1R3, a subunit of the sweet/umami taste receptors. The Gcgr antagonist L-168,048 exhibited reduced taste responsiveness in the presence of sucrose, suggesting a role for glucagon in sweet taste responsivity Glucagon signaling modulates sweet taste responsiveness. FASEB J. 2010 Jun 14. [Epub ahead of print]
Glucagon and the heart
Considerable evidence demonstrates a role for glucagon in the activation of cardiac adenylate cyclase leading to increased cyclic AMP formation.
Several studies, principally from the laboratory of D Bataille, have documented that a proteolytic fragment of 29 amino acid glucagon, glucagon (19-29), is liberated following cleavage of glucagon at the Arg17-Arg18 amino acid doublet. Processing may occur locally in target tissues such as the pancreas, liver or heart, as well as in the circulation Endopeptidase from rat liver membranes, which generates miniglucagon from glucagon. J Biol Chem. 1993 Oct 15;268(29):21748-53. To date, a separate receptor for miniglucagon has not been identified, although various actions have been ascribed to this peptide, including effects in the liver, heart and pancreas, including inhibition of insulin secretion Miniglucagon (glucagon 19-29), a potent and efficient inhibitor of secretagogue-induced insulin release through a Ca2+ pathway. J Biol Chem. 1999 Apr 16;274(16):10869-76. See Synergistic actions of glucagon and miniglucagon on Ca2+ mobilization in cardiac cells. Circ Res. 1996 Jan;78(1):102-9; and Glucagon-(19-29) exerts a biphasic action on the liver plasma membrane Ca2+ pump which is mediated by G proteins. J Biol Chem. 1990 Jun 15;265(17):9876-80.