Role of regulator of G protein signaling in desensitization of the glucose-dependent insulinotropic peptide receptor.

The glucose-dependent insulinotropic peptide receptor (GIP-R) is a member of the G protein-coupled receptors. Recent studies have indicated that elevated serum GIP concentrations in type II diabetic patients might induce desensitization of the GIP-R, and this mechanism could contribute to impaired insulin secretion. The cellular and molecular mechanisms governing GIP desensitization are unknown. Here, we report the results of studies on a new family of proteins known as regulators of G protein signaling (RGS) that have been shown to mediate the desensitization process of other receptors. GIP-R and RGS1, -2, -3, and -4 complementary DNAs were cotransfected into human embryonic kidney cells (L293). GIP-stimulated cAMP generation in control cells and in those coexpressing RGS1, -3, and -4 displayed a dose-dependent increase 10 min after GIP treatment. In contrast, RGS2 expression inhibited the GIP-induced cAMP response by 50%, a response similar to that of cells desensitized by preincubation with 10(-7) M GIP. In betaTC3 cells, preincubation of GIP attenuated GIP-induced insulin release by 45% at 15 min and by 55% at 30 min. Expression of RGS2 in the betaTC3 cells significantly decreased GIP-stimulated insulin secretion, whereas glucose-induced insulin release was not affected. RGS2 messenger RNA was identified by Northern blot analysis to be expressed endogenously in betaTC3 and L293 cells, and its level was significantly induced by GIP treatment in betaTC3 cells. Moreover, RGS2 bound Gs alpha protein in an in vitro system, suggesting that RGS2 attenuated the Gs-adenylate cyclase signaling pathway. These results suggest a potential role for RGS2 in modulating GIP-mediated insulin secretion in pancreatic islet cells.