Role of the Renin-Angiotensin System in Glucose Homeostasis

项目总结/文摘

PROJECT SUMMARY/ABSTRACT In treating type 2 diabetes (T2D), there is a paucity of medications simultaneously targeting deficits in both β-cell function and mass. Angiotensin(1-7), a metabolite of the renin-angiotensin system, may fill this gap; however, its underlying mechanisms of action are incompletely understood. Our data reveal that the insulinotropic action of angiotensin(1-7) is dependent on its hydrolysis to the dipeptide, angiotensin(1-2), the latter also conveying pro-survival and proliferative effects in β cells. Angiotensin(1-2) activates G-protein- coupled receptor family C group 6 member A (GPRC6A), which we show is expressed in islet a cells. Further, angiotensin(1-2) increases a cell-derived glucagon-like peptide-1 (GLP-1) release, suggesting it acts via GPRC6A on the a cell to promote insulin secretion in a paracrine fashion. Indeed, in GLP-1 receptor deficient islets, we find that angiotensin(1-2) fails to potentiate insulin secretion; however, its ability to enhance β-cell survival and proliferation is retained. The latter suggests angiotensin(1-2)’s action is, in part, GLP-1 receptor- independent. Based on these novel data, we hypothesize angiotensin(1-2) enhances β-cell function via intra- islet paracrine signaling, and promotes β-cell survival/proliferation via a novel GLP-1-independent mechanism. The following specific aims address this hypothesis, with the goal of improving treatment options in T2D: Specific Aim 1. To determine the mechanism by which angiotensin(1-2) increases islet-derived GLP-1 and insulin secretion. Mice (in vivo) and islets (in vitro) with diphtheria toxin-induced a-cell destruction or a cell-specific GPRC6A knockout will be used to determine whether angiotensin(1-2)-mediated increases in insulin release require a cells or a-cell GPRC6A, respectively. We will probe mechanisms for increased GLP-1 release, and confirm key findings in human islets with and without GLP-1 receptor or GPRC6A blockade. Specific Aim 2. To identify signaling pathways/proteins mediating the β-cell survival and proliferative effects of Ang(1-2) in human islets. The ability of angiotensin(1-2) to inhibit apoptosis, reduce dedifferentiation and enhance proliferation of β cells will be examined in human islets under non-diabetic and diabetic conditions. The contribution of mechanisms independent of GLP-1 or GPRC6A will be determined. Effectors of angiotensin(1-2) action will be identified using non-biased phosphoproteomics, then loss-/gain-of- function studies in islets will serve as a preliminary screen for effectors that could be targeted therapeutically. Specific Aim 3. To determine whether Ang(1-2) improves human islet function/survival in vivo, and whether hydrolysis of Ang(1-7) is required for its anti-diabetic effects. We will utilize a human islet transplant model to determine the ability of angiotensin(1-2) to improve β-cell function/survival and glycemia in diabetic mice. Further, we will test whether the insulinotropic action of angiotensin(1-7) requires its hydrolysis, as is the case in vitro. The latter is highly significant for clinical management of T2D because it will inform on the utility of hydrolysis-resistant angiotensin(1-7)-based medications currently in development.