Diabetes has long been viewed as a bihormonal disorder, with glucagon excess, in the setting of insulin deficiency or insulin resistance, contributing significantly to the development of hyperglycemia. Accordingly strategies for reducing glucagon secretion (amylin, GLP-1), or inhibiting glucagon action, may be useful for the treatment of excess glucose production in patients with diabetes. Intriguingly, reduction of glucagon action may also have unexpected benefits on the ischemic heart. Although long known to increase heart rate and ventricular contractility, some existing literature suggests that glucagon agonism may potentially be detrimental for the ischemic heart.
Ali and Ussher demonstrated that acute glucagon administration to mice with LAD coronary artery ligation markedly impaired ventricular function, increased cardiomyocyte apoptosis, and significantly reduced survival. Glucagon also impaired recovery of ventricular developed pressure in the isolated WT mouse heart ex vivo. The adverse actions of glucagon were mediated through p38 MAPK, as they were complateley abrogated by SB203580. To determine the consequences of reducing glucagon action selectively in cardiomyocytes (CM), they studied a newly generated mouse with conditional CM-selective inactivation of the Gcgr. Remarkably, GcgrCM-/- mice exhibited a cardioprotective phenotype in response to coronary arterly ligation, with reduced infarct size and increased survival. Glucagon administration increased the abundance of long chain acylcarnitines in the ischemic mouse heart whereas disruption of CM Gcgr signaling was accompanied by an acylcarnitine profile reflecting reduced fatty acid oxidation. These findings highlight the importance of understanding the cardiovascular consequences of Gcgr signaling for efforts directed at manipulating glucagon action for the treatment of metabolic disorders. Cardiomyocyte glucagon receptor signaling modulates outcomes in mice with experimental myocardial infarction Molecular Metabolism doi:10.1016/j.molmet.2014.11.005
Shah and colleagues examined the importance of the ambient insulin concentration for development of glucagon-mediated hyperglycemia in human subjects following a prandial glucose load. The authors find that glucagon excess in the presence of relative insulin deficiency clearly contributes to impaired suppression of glucose production and hyperglycemia Impact of lack of suppression of glucagon on glucose tolerance in humans. Am J Physiol. 1999 Aug;277(2 Pt 1):E283-90.
Studies in patients with type 2 diabetes suggests that lack of glucagon suppression contributes to postprandial hyperglycemia in part via accelerated glycogenolysis. Analysis of blood glucose in the presence or absence of somatostatin-induced glucagon suppression during an OGTT revealed a significant increase in glucose in subjects with higher glucagon levels. See Lack of suppression of glucagon contributes to postprandial hyperglycemia in subjects with type 2 diabetes mellitus. J Clin Endocrinol Metab. 2000 Nov;85(11):4053-9
In this regard, development of structural and functional glucagon receptor antagonists represents a potential approach to decrease hepatic glucose production and lower blood glucose in patients with diabetes. The fungal bisanthroquinone skyrin, isolated from Talaromyces wortmannin, inhibits glucagon-stimulated cAMP formation and glucose output from rat and human hepatocytes. Skyrin does not appear to interfere with glucagon receptor binding in CHO cells or hepatocytes. Whether skyrin will also reduce blood glucose in normal or diabetic rodents in vivo remains unclear. See Effects of skyrin, a receptor-selective glucagon antagonist, in rat and human hepatocytes. Diabetes. 2000 Dec;49(12):2079-86
Complete elimination of signaling through the glucagon receptor in the mouse has been reported. Remarkably, glucagon receptor knockout mice are viable, and do exhibit mild hypoglycemia and improved glucose tolerance with elevated levels of circulating glucagon. See Glycemic control in mice with targeted disruption of the glucagon receptor gene. Biochem Biophys Res Commun. 2002 Jan 18;290(2):839-43
Moreover, 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
Gcgr-/- mice also exhibit resistance to diet-induced obesity, elevated levels of GLP-1, reduced gastric emptying, and resistance to streptozotocin-induced diabetes, as outlined in Glucagon receptor knockout mice are resistant to diet-induced obesity and streptozotocin-mediated beta cell loss and hyperglycaemia. Diabetologia. 2007 Jan;50(1):142-50. Epub 2006 Nov 28
Similar studies targeting transient partial reduction of the Gcgr in a variety of different rodent models have been reported using glucagon receptor antisense oligonucleotides (ASOs). Treatment of ob/ob and db/db mice, and ZDF rats, with twice weekly glucagon receptor ASOs produced striking and long lasting improvements in glycemia, together with reduced levels of triglycerides, and a decrease in plasma insulin. Consistent with findings in the Gcgr-/- mouse, rodents treated with Gcgr ASOs exhibited a-cell hyperplasia, markedly increased levels of circulating glucagon, and significantly (10-15-fold) increased levels of circulating GLP-1, together with increased levels of islet GLP-1. Hence, targeting the glucagon receptor and disrupting normal Gcgr signaling unmasks a compensatory increase in islet α cell activity accompanied by a shift towards islet GLP-1 production, with therapeutic benefits for the treatment of experimental diabetes. See Hepatic and glucagon-like peptide-1mediated reversal of diabetes by glucagon receptor antisense oligonucleotide inhibitors J. Clin. Invest. 113:1571-1581 (2004). and Reduction in glucagon receptor expression by an antisense oligonucleotide ameliorates diabetic syndrome in db/db mice Diabetes. 2004 Feb;53(2):410-7.
Transciptional and proteomic profiling of liver and metabolomic analysis of Gcgr-/- plasma demonstrated multiple abnormalities in genes and proteins regulating gluconeogenesis, amino acid catabolism, and lipid synthesis and oxidation. Heaptic genes regulating glycolysis, and fatty avid syntheiss and oxidation were increased in Gcgr-/- mice, and plasma levels of most amino acids were also significantly increased Polyomic Profiling Reveals Significant Hepatic Metabolic Alterations in Glucagon-Receptor (GCGR) Knockout Mice: Implications on Anti-Glucagon Therapies for Diabetes. BMC Genomics. 2011 Jun 1;12(1):281.
Plasma levels of FGF21 were reported to be 25-fold higher in Gcgr-/- mice; FGF21 was detected by immunocytochemistry in islet α and b cells, as well as in liver and adipose tissue and FGF21 and FGFR3 mRNA levels were higher win liver from Gcgr-/- mice. Acute administration of FGF21 antisera significantly increased glycemic excursion in STZ-treated Gcgr-/- mice. Fibroblast Growth Factor 21 (FGF21) and Glucagon Like-Peptide 1 Contribute to Diabetes Resistance in Glucagon Receptor Deficient Mice Diabetes , doi:10.2337/db13-0710
Consistent with the changes in genes regulating heptic lipid synthesis and oxidation in Gcgr-/- mice outlined above, 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. The Gcgr is also required for the salutary effects of exercise on reduction of fatty liver in high fat fed mice Hepatic Glucagon Action Is Essential for Exercise-Induced Reversal of Mouse Fatty Liver.Diabetes. 2011 Sep 1. [Epub ahead of print]
Ali and colleagues examined the importance of increased GLP-1 action for the improved glucose tolerance in Gcgr-/- mice. Remarkably, Gcgr-/-:Glp1r-/- mice exhibit substantial reversal of the improvements in ambient, and unexpectedly, even deterioration of fasting glycemia, demonstrating the large contribution of increased GLP-1 action to the phenotype of Gcgr-/- mice. Surprisingly, although gastric emptying was normalized following disruption of GLP-1 action, oral glucose tolerance remained susbtantially improved despite genetic removal of both Gcgr and GLP-1 receptors. Similarly, even marked reduction of Gcgr expression in Glp1r-/-:Gipr-/- mice still resulted in preservation of improved oral glucose tolerance despite deterioration of IP glucose tolerance. Remarkably, disruption of the normal incretin axis in mice leads to upregulation of the islet CCKAR and enhanced sensitivity to CCK both in vitro and in vivo. Similarly islet GPR119 expression is dynamically upregulated under the same cicrcumstances and incretin receptor knockout mice exhibit substantially increased sensitivity to GPR119 agonists. Hence, the murine b-cell exhibits considerable plasticity and is capable of upregulating non-classical incretin receptors to preserve the intestinal-islet incretin axis in vivo Dual elimination of the glucagon and GLP-1 receptors in mice reveals plasticity in the incretin axis J Clin Invest. 2011 doi:10.1172/JCI43615.
Glucagon receptor antagonists and Type 2 Diabetes
The concept that antagonism of glucagon receptor action may be coupled to attenuation of experimental hyperglycemia has been examined for more than 2 decades, as clearly outlined in Hyperglycemia of diabetic rats decreased by a glucagon receptor antagonist. Science. 1982 Feb 26;215(4536):1115-6.
Just over twenty years later, a related proof of concept study for blocking glucagon action was carried out in db/db mice using antisense oligonucleotide administration to reduce glucagon receptor expression in the liver. This 3 week experiment demonstrated successful reduction of blood glucose, free fatty acids, and triglycerides, without the development of hypoglycemia, as outlined in Reduction in Glucagon Receptor Expression by an Antisense Oligonucleotide Ameliorates Diabetic Syndrome in db/db Mice. Diabetes. 2004 Feb;53(2):410-417
Immunoneutralization of endogenous glucagon action using acute or chronic administration of monoclonal antibody directed against glucagon produced acute and sustained reduction on plasma glucose, triglycerides, glucose tolerance, HbA1c, and hepatic glucose production in ob/ob mice as described in Immunoneutralization of Endogenous Glucagon Reduces Hepatic Glucose Output and Improves Long-Term Glycemic Control in Diabetic ob/ob Mice. Diabetes. 2006 Oct;55(10):2843-8
A glucagon receptor antagonists which blocked glucagon action in experiments employing cell lines or primary cultures in vitro, or rodent studies in vivo as described in: Design and synthesis of glucagon partial agonists and antagonists. Biochemistry. 1986 Dec 16;25(25):8278-84 and Synthesis of two glucagon antagonists: receptor binding, adenylate cyclase, and effects on blood plasma glucose levels. J Med Chem. 1987 Aug;30(8):1409-15 and Biological activities of des-His1[Glu9]glucagon amide, a glucagon antagonist. Peptides. 1989 Nov-Dec;10(6):1171-7.
The majority of the initial antagonists were peptide-based, whereas more recent efforts have been directed at identification of non-peptide orally available agents, as exemplified in Discovery and structure-activity relationship of the first non-peptide competitive human glucagon receptor antagonists. J Med Chem. 1998 Dec 17;41(26):5150-7
Qureshi and colleagues identified a novel chemical series of glucagon antagonists by screening a chemical library for compounds that blocked binding to membranes isolated from CHO cells expressing the human Gcgr. A prototype antagonist was identified, N-[3-cyano-6-(1, 1-dimethylpropyl)-4, 5, 6, 7-tetrahydro-1-benzothien-2-yl]-2 -ethylbutanamide (Cpd 1), that blocked A novel glucagon receptor antagonist inhibits glucagon-mediated biological effects. Diabetes. 2004 Dec;53(12):3267-73
The concept that glucagon receptor antagonism may be a useful approach to therapy of diabetic patients was tested in preliminary human studies using the Bayer antagonist Bay 27-9955. This compound appeared safe and blocked exogenous glucagon action in short term human studies, however clinical development of this particular compound was not pursued. Effects of a novel glucagon receptor antagonist (Bay 27-9955) on glucagon-stimulated glucose production in humans. Diabetologia. 2001 Nov;44(11):2018-24
Nevertheless, there remains active pharmaceutical industry interest in developing ideal compounds that might block glucagon receptor action in diabetic subjects, as outlined in Glucagon receptor antagonists for the treatment of type II diabetes: current prospects. Curr Opin Investig Drugs. 2002 Nov;3(11):1617-23.
Mu and colleagues assessed the properties of "Compound A" a human Gcgr antagonist in humanized mice that express the human Gcgr. CpdA increased plasma glucagon levels and improved glucose tolerance in hGcgr HFD/STZ mice, either alone, or in combination with sitagliptin. Circulating glucagon and GLP-1 levels were modestly elevated after administration of CpdA, however chronic treatment with CpdA for almost 7 weeks did not induce a-cell hyperplasia in hGcgr mice. Cessation of CpdA administration rapidly reversed the mild hyperglucagonemia Chronic treatment with a glucagon receptor antagonist lowers glucose and moderately raises circulating glucagon and glucagon-like peptide 1 without severe alpha cell hypertrophy in diet-induced obese mice Diabetologia. 2011 Jun 22