Although glucagon-mediated signaling effects and biological actions have been studied for years in different cell types and tissues, a major advance in our understanding of glucagon action was the cloning of the glucagon receptor cDNA, as initially reported in Expression cloning and signaling properties of the rat glucagon receptor. Science. 1993 Mar 12;259(5101):1614-6. The cloned receptor encoded a 485 amino acid protein with a predicted molecular weight of 54,962 daltons, and exhibits ~ 42% aa identity with the GLP-1 receptor.
Numerous studies have subsequently shown that the cloned Gcgr signals through both adenylate cyclase- and also mediates an increase in intracellular calcium, consistent with earlier descriptions of dual glucagon signaling pathways in hepatocytes Activation of two signal-transduction systems in hepatocytes by glucagon. Nature. 1986 Sep 4-10;323(6083):68-71
High affinity glucagon binding sites have been identified in liver, kidney, intestinal smooth muscle, brain and adipose tissue. Glucagon receptors are also expressed on islet β cells, where they are coupled to stimulation of insulin secretion. Nevertheless, the Ec50 for glucagon-stimulated cAMP of insulin secretion in purified rat islet cells is ~ 1 nM, considerably higher than the 10-50 pM required for stimulation by GLP-1 and GIP, respectively Expression and functional activity of glucagon, glucagon-like peptide I, and glucose-dependent insulinotropic peptide receptors in rat pancreatic islet cells. Diabetes. 1996 Feb;45(2):257-61. Experiments using glucagon and GLP-1R antagonists suggest that glucagon activates β cell signaling via both the glucagon and GLP-1 receptors Dual glucagon recognition by pancreatic beta-cells via glucagon and glucagon-like peptide 1 receptors. Diabetes. 1998 Jan;47(1):66-72
A few patients with germline (presumed loss of function) mutations in the human GCGR gene have been described who presented with Glucagon Cell Adenomatosis (GCA), with multiple pancreatic endocrine glucagon+ adenomas and/or hyperplasia of islet a cells. Sipos et al Glucagon cell hyperplasia and neoplasia with and without glucagon receptor mutations J Clin Endocrinol Metab. 2015 Feb 19:jc20144405 reported a series of six cases, and exon sequencing of the GCGR revealed germline mutations in 3 subjects. Intriguingly, the proliferation rate within the tumors, as assessed by Ki67 staining, was not elevated and was <1%. GCA patients with GCGR mutations were younger, had larger tumors (>5 mm) and a greater degree of a cell hyperplasia. Genetic analyses were negative for MEN1 and VHL in all 6 patients. These findings are consistent with the similar cases of inactivating GCGR mutations presenting with a 'pancreatic tumor' reported by Zhou and colleagues, reviewed in Pancreatic α-cell hyperplasia: facts and myths. J Clin Endocrinol Metab. 2014 Mar;99(3):748-56
Disruption of glucagon receptor activity has also been described in the Gcgr-/- mouse. Gcgr-/- mice exhibit a number of unexpected and striking phenotypes, including a significant increase in total pancreatic weight, marked islet α cell hyperplasia, extremely large elevations in circulating levels of circulating glucagon and GLP-1, and mild reproductive abnormalities. For a detailed overview of the phenotype, see Lower blood glucose, hyperglucagonemia, and pancreatic alpha cell hyperplasia in glucagon receptor knockout mice. Proc Natl Acad Sci U S A. 2003 Feb 4;100(3):1438-43
Furthermore, these mice appear to exhibit multiple defects in development of islet cell phenotypes, including loss of control of cell replication, implying a complex role for glucagon in islet development. See Ablation of the glucagon receptor gene increases fetal lethality, and produces alterations in islet development and maturation. Endocrinology. 2006 Apr 20; [Epub ahead of print]
Studies of the Gcgr-/- mouse have revealed important insights into the role of the Gcgr in hepatocytes beyond control of hepatic glucose production. Sinclair and colleagues demonstrated that the Gcgr is essential for hepatocyte survival by demonstrating that hepatic Gcgr signaling is coupled to cell survival/anti-apoptotic pathways in the liver. Gcgr-/- hepatocytes exhibit increased susceptibility to apoptotic injury as outlined in Glucagon Receptor Signaling Is Essential for Control of Murine Hepatocyte Survival. Gastroenterology. 2008 Aug 3. [Epub ahead of print]
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.
Longuet et al. subsequently assessed the Gcgr-dependent signals and tissues responsible for induction of islet α cell hyperplasia in both Gcgr-/- mice and in mice with liver-specific inactivation of the Gcgr. Inactivation of the hepatic Gcgr recapitulated the phenotype of whole body Gcgr-/- mice, including marked islet α cell hyperplasia, improved oral and intraperitoneal glucose tolerance, and reduced fasting glucose. Remarkably transplantation ofGcgr+/+ islets into either Gcgr-/- recipients, or GcgrHep-/- recipients, resulted in proliferation of islet α cells in transplanted +/+ islets underneath the kidney capsule. These findings imply that the liver responds to interruption of the glucagon receptor pathway by initiating one or more signals that promote robust and rapid islet α cell proliferation independent of the normal islet localization and pancreatic location. Liver-Specific Disruption of the Murine Glucagon Receptor Produces α-Cell Hyperplasia: Evidence for a Circulating α-Cell Growth Factor Diabetes published ahead of print November 16, 2012, doi:10.2337/db11-1605
The Gcgr been inactivated in zebrafish, providing a complementary model for studies of alpha cell hyperplasia. Li et al inactivated both zebra fish genes (zebrafish also have two proglucagon genes), individually and together. Notably, the expression patterns of Gcgra and Gcgrb overlap (liver, brain and pancreas), yet are otherwise distinct. The single and double Gcgra/b knockout fish exhibited α cell hyperplasia (modest in the single knockouts, more robust, ~50% increase in α cell number in the DKO fish) and increased α cell proliferation, detectable wthin several days of fetilization. Remarkably, the Gcgra-/-:Gcgrb-/- double KO fish also exhibited increased levels (content)of glucagon yet reduced glucose levels, recapitulating findings in mouse models. Furthermore, the increase in α cell number in the DKO was quite rapid, detectable as early as 4 dpf. Intriguingly, increased expression of both pancreatic and intestinal proglucagon gene expression was detected in DKO zebrafish. Glucagon receptor inactivation leads to α-cell hyperplasia in zebrafish J Endocrinol. 2015 Nov;227(2):93-103
Solloway and colleagues postulated that dysregulated hepatic amino acid metabolism (induced by Gcgr antagonists) leads to increased circulating levels of amino acids, which in turn, promotes marked α cell hyperplasia. These authors demonstrated that the α cell hyperplasia induced by Gcgr antagonists in mice results from replication of pre-existing α cells and could be attenuated by co-administration of rapamycin. Amino acids stimulated α cell proliferation in murine islets ex vivo, however there were no experiments demonstrating that amino acid infusions increased α cell proliferation in mice. Glucagon Couples Hepatic Amino Acid Catabolism to mTOR-Dependent Regulation of α-Cell Mass. Cell Rep. 2015 Jul 21;12(3):495-510
Lee and colleagues studied the consequences of ablating b-cells in Gcgr-/- mass by administration of either STZ or alloxan. These agents produced over 90% destruction of b-cell mass, and further increased a-cell mass in Gcgr-/- mice. Remarkably however, plasma glucose and both IP and oral glucose tolerance was not significantly impaired despite almost complete absence of insulin, in the presence of genetic loss of Gcgr signaling. See Glucagon receptor knockout prevents insulin-deficient type 1 diabetes in mice Diabetes. 2011 Feb;60(2):391-7
Ouhilal reported that Gcgr-/- mice exhibit reduced fetal weight, increased fetal demise at the end of gestation, and extensive abnormalities in the placenta, associated with changes in the expression of genes important for placental growth and function. Timed pregnancies from mating of homozygous genotypes were generated and studied. Fertility appered normal in Gcgr-/- pregnancies, as did the structure and cellular composition of the pituitary. Oocyte number in response to superovaluation was also normal in female Gcgr-/- mice. At E18.5 Gcgr-/- pups were significntly smaller and Gcgr-/- placentas were significantly large relative to genetic controls. The majority of Gcgr+/- placentas were normal however Gcgr-/- placentas were hyperemic, edemetous, and exhibit necrosis of vessels, and other abnormalities. Placental glycogen content was normal in Gcgr-/- mice. The relaive importance of the metabolic abnormalities in Gcgr-/- vs a direct role for the Gcgr in fetal and placental development remains uncertain. Hypoglycemia, hyperglucagonemia and feto-placental defects in glucagon receptor knockout mice: a role for glucagon action in pregnancy maintenance Am J Physiol Endocrinol Metab. 2011 Dec 13
Grigoryan and colleagues demonstrate a significantly increased number of enteroendocrine cells exhibiting both GLP-1 and GIP immunopositivity (LK cells) in the ileum of Gcgr-/- mice. The cells also co-expressed Pdx1. Exogenous administration of exendin-4 did not affect the number of L cells or the crypt plus villus axis in CD1 mice. Immunopositive LK cells did not express classical markers of cell proliferation however chronic labelling with BrDU followed by temporal analysis of L cell populations suggested an increase in L cell progenitor cycling; an expansion of the colon crypt compartment was also detected in Gcgr-/- colon. Administration of exendin(9-39) reduced the rate of L cell proliferation in Gcgr-/- mice, and decreased the length of the transit amplifying zone in Gcgr-/- but not in WT CD1 mouse colon. Nevertheless, analysis of GLP-1R expression by immunohistochemistry failed to detect GLP-1R+ L cells, suggesting an indirect effect of GLP-1 on L cell populations. Regulation of Mouse Intestinal L Cell Progenitors Proliferation by the Glucagon Family of Peptides Endocrinology. 2012 May 8
Does glucagon directly autoregulate α cell activity in the islets? The evidence remains unclear, but it appears that the majority of rat α cells do not express detectable glucagon receptors, with expression detected on 9% of α cells in one study Distribution of glucagon receptors on hormone-specific endocrine cells of rat pancreatic islets. Endocrinology. 1996 Nov;137(11):5119-25.
Nevertheless, a series of elegant studies using isolated mouse and rat α cells demonstrated that glucagon stimulates an exocytotic response from α cells in a PKA-dependent manner. These actions were blocked by a glucagon antagonist des-His1-[glu9]-glucagon-amide but not by the GLP-1 receptor antagonist exendin(9-39). Glucagon receptor mRNA transcripts were detected by RT-PCR in RNA purified from rat α cells. The extent to which glucagon produces an autoregulatory positive or negative effect on glucagon secretion remains unclear. See Glucagon stimulates exocytosis in mouse and rat pancreatic alpha-cells by binding to glucagon receptors. Mol Endocrinol. 2005 Jan;19(1):198-212
Glucagon receptor expression is positively regulated by glucose and negatively regulated by glucagon and agents that increase intracellular cAMP Regulation of glucagon receptor mRNA in cultured primary rat hepatocytes by glucose and cAMP. J Biol Chem. 1995 Jun 30;270(26):15853-7. GLU-R expression is also linked to intra hepatic glucose metabolism, with increased glucose flux associated with induction of, and glycolytic inhibitors leading to a reduction of GLU-R mRNA transcripts, respectively. In vivo and in vitro regulation of hepatic glucagon receptor mRNA concentration by glucose metabolism. J Biol Chem. 1998 Apr 3;273(14):8088-93
As both GLP-1 and GLP-2 promote cell survival in studies using beta cells or intestinal epithelial cells, respectively, the cytoprotective actions of glucagon in hepatocytes was examined in wildtype and Gcgr-/- mice. Glucagon increased the survival of injured hepatocytes in vitro in different experimenatl models of hepatotoxicity, and administration of glucagon protects liver cells in vivo from injury induced by Fas ligand activation. Conversely, genetic elimination of Gcgr signaling resulted in enhanced susceptibility to liver injury following Jo2 administration or following high fat feeding in mice. Hence, a threshold level of Gcgr signaling may be important for liver cell survival Glucagon Receptor Signaling Is Essential for Control of Murine Hepatocyte Survival. Gastroenterology. 2008 Aug 3. [Epub ahead of print]
Human Glucagon Receptor Polymorphisms
Glucagon receptor polymorphisms, principally a Gly to Ser missense mutation in exon 2 at amino acid 40, have been observed with higher frequency in some but not all diabetic populations ( (Analysis of the Gly40Ser polymorphism in the glucagon receptor gene in a German non-insulin-dependent diabetes mellitus population. Clin Chem Lab Med. 1999 Jul;37(7):719-21; Polymorphism of the glucagon receptor gene and non-insulin-dependent diabetes mellitus in the Russian population. Exp Clin Endocrinol Diabetes. 1997;105(4):225-6; A mutation in the glucagon receptor gene (Gly40Ser): heterogeneity in the association with diabetes mellitus. Diabetologia. 1995 Aug;38(8):983-5; Mutation of the glucagon receptor gene and diabetes mellitus in the UK: association or founder effect? Hum Mol Genet. 1995 Sep;4(9):1609-12.; A missense mutation in the glucagon receptor gene is associated with non-insulin-dependent diabetes mellitus. Nat Genet. 1995 Mar;9(3):299-304; Screening for the Gly40Ser mutation in the glucagon receptor gene among patients with type 2 diabetes or essential hypertension in Taiwan. Pancreas. 1999 Mar;18(2):151-5)
Paradoxically, the G40S mutation results in a receptor with reduced sensitivity to glucagon in vitro ((The Gly40Ser mutation in the human glucagon receptor gene associated with NIDDM results in a receptor with reduced sensitivity to glucagon. Diabetes. 1996 Jun;45(6):725-30)(Physiological and genetic characterization of the Gly40Ser mutation in the glucagon receptor gene in the Sardinian population. The Sardinian Diabetes Genetic Study Group. Diabetologia. 1997 Jan;40(1):89-94.)(Glucagon receptor gene mutation in essential hypertension. Nat Genet. 1996 Feb;12(2):122)(Glucagon receptor gene mutation (Gly40Ser) in human essential hypertension: the PEGASE study. Hypertension. 1999 Jul;34(1):15-7; Screening for the Gly40Ser mutation in the glucagon receptor gene among patients with type 2 diabetes or essential hypertension in Taiwan. Pancreas. 1999 Mar;18(2):151-5) (Altered renal sodium handling and hypertension in men carrying the glucagon receptor gene (Gly40Ser) variant. J Mol Med. 2001 Oct;79(10):574-80)