Reduction in Glucagon Receptor Expression by an Antisense Oligonucleotide Ameliorates Diabetic Syndrome in db/db Mice

Abstract

Excess glucagon levels contribute to the hyperglycemia associated with type 2 diabetes. Reducing glucagon receptor expression may thus ameliorate the consequences of hyperglucagonemia and improve blood glucose control in diabetic patients. This study describes the antidiabetic effects of a specific glucagon receptor antisense oligonucleotide (GR-ASO) in db/db mice. The ability of GR-ASOs to inhibit glucagon receptor mRNA expression was demonstrated in primary mouse hepatocytes by quantitative real-time RT-PCR. Intraperitoneal administration of GR-ASO at a dosage of 25 mg/kg twice a week in db/db mice for 3 weeks resulted in 1) decreased glucagon receptor mRNA expression in liver; 2) decreased glucagon-stimulated cAMP production in hepatocytes isolated from GR-ASO-treated db/db mice; 3) significantly reduced blood levels of glucose, triglyceride, and free fatty acids; 4) improved glucose tolerance; and 5) a diminished hyperglycemic response to glucagon challenge. Neither lean nor db/db mice treated with GR-ASO exhibited hypoglycemia. Suppression of GR expression was also associated with increased ( approximately 10-fold) levels of plasma glucagon. No changes were observed in pancreatic islet cytoarchitecture, islet size, or alpha-cell number. However, alpha-cell glucagon levels were increased significantly. Our studies support the concept that antagonism of glucagon receptors could be an effective approach for controlling blood glucose in diabetes.

Function and expression of somatostatin receptors of the endocrine pancreas

“…Hyperglucagonemia further exacerbates hyperglycemia Page 7 of 29 A c c e p t e d M a n u s c r i p t 7 through stimulation of hepatic glucose production (Ohneda et al, 1978;Dinneen et al, 1995;Shah et al, 2000;Unger and Orci 1975;Unger 1975). The immunoneutralization of the endogenous glucagon, administration of glucagon receptor antagonists, reduction of glucagon receptor expression or the deletion of glucagon receptors leads to a reduction of hyperglycemia in diabetic animal models (Brand et al, 1994;Liang et al, 2004;Gelling et al, 2003). Given the potential for SST to inhibit glucagon secretion it has been considered as adjunct therapy with insulin to treat diabetes (de Laszlo et al, 1999;Petersen and Sullivan 2001;Madsen et al, 2002).…”

The regulation of pre- and post-maturational plasticity of mammalian islet cell mass

“…In humans, an increase in alpha cell relative area has been reported in insulin-resistant patients [75]; however, the mechanism(s) underlying this apparent increase in alpha cell mass remains unclear. Disruptions in glucagon signalling induced in animal models by decreasing glucagon receptor (GR) expression [29,76] or modulating proteins in the receptor signalling pathway [77] determine alpha cell hyperplasia. Studies on these disruptions suggest that, similar to the observations on beta cell hyperplasia in insulin resistance, an increase in alpha cell proliferation and hormone production occurs in states of glucagon resistance.…”

Ectopic expression of glucagon receptor in skeletal muscles improves glucose homeostasis in a mouse model of diabetes

“…Given the much greater muscle mass compared with that of liver and kidneys, the ectopic production of GCGR in the muscle thus provides effective 'decoy receptors'. However, unlike the robust hyperglucagonaemia observed in the Gcgr −/− mice [10] or the mice receiving therapeutic intervention by GCGR antagonistic strategies [13,15], Mck/Gcgr mice displayed a relatively mild increase in glucagon levels. Furthermore, elevated glucagon levels were associated with increased insulin levels in Mck/Gcgr mice, allowing the maintenance of an appropriate glucagonto-insulin ratio, which is critical in maintaining glycaemic stability, particularly in extremes of glucose influx or efflux [6,30].…”

“…Studies in humans using GCGR antagonists have shown a significant reduction in hepatic glucose production and the prevention of hyperglycaemia following glucagon infusion [12]. Consistently, the inhibition of GCGR by ASOs significantly reduced the circulating levels of triacylgycerol, NEFA and glucose in db/db mice [13]. Furthermore, the use of GCGR neutralising antibody improved glycaemic control in obese or diabetic mouse models [14,15].…”