Protein Prenyltransferases in Glucose-Stimulated Insulin Secretion

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DESCRIPTION (provided by applicant): Emerging evidence implicates small G-proteins [e.g., Rac1 and Cdc42] in glucose-stimulated insulin secretion [GSIS] in the islet ¿ cell. However, the precise cellular mechanisms underlying glucose-mediated activation of these proteins remain only partially understood. Using generic [e.g., statins], and more specific pro-drug [e.g., allyl or vinyl farnesols and geranylgeraniols] and peptidomimetic [e.g., GGTI-2147] inhibitors of protein prenyltransferases [PPTases], we have recently reported that C-terminal prenylation of islet G- proteins is essential for GSIS. We have also obtained preliminary evidence to indicate defective expression of PPTases, as well as glucose-mediated activation of specific G-proteins in islets from rodent models of type 2 diabetes. Based on these and other preliminary data, we hypothesize that glucose-mediated activation of PPTases is necessary for GSIS from the islet and that deficient activation of PPTases by glucose results in defective GSIS in in vitro and in vivo models of glucotoxicity and type 2 diabetes. We will test our hypotheses in normal rat islets and INS-1 cells by utilizing various biochemical, physiological, and molecular biological approaches. Specific Aim 1 is to demonstrate that glucose-mediated activation of PPTases is necessary for GSIS in isolated ¿ cells. We will determine the regulation, by glucose, of the phosphorylation status and associated catalytic activation of PPTases in isolated ¿ cells. We will further determine the essential nature of PPTase activation in GSIS either via transfection of dominant negative PPTase mutants or via siRNA-mediated depletion of endogenous PPTases. Specific Aim 2 is to demonstrate that specific defects in glucose-mediated activation of PPTases, and associated abnormalities in G-protein prenylation culminates in insulin secretory defects in in vitro and in vivo models of impaired GSIS. First, we will examine the functional status of the PPTase signaling pathway, prenylation of candidate G-proteins and subsequent GSIS in INS-1 cells or normal rat islets chronically exposed to hyperglycemic conditions. To further validate our hypothesis, we will extend these in vitro studies to islets derived from two rodent models of type 2 diabetes, namely the GK rat and the ZDF rat. Herein, we will determine if abnormalities in GSIS in these animal models are due to defects in glucose-induced activation of PPTases and prenylation of specific G-proteins [e.g., Rac1] in these islets. Together, we believe that the proposed studies are the first to determine potential cause vs. effect for the abnormalities in PPTase signaling and GSIS in in vitro and in vivo models of impaired GSIS. Our goals are to examine the physiological control of protein prenylation in ¿ cell stimulus-secretion coupling leading to GSIS, and to determine potential defects in this signaling cascade leading to abnormalities in GSIS in models of glucotoxicity and diabetes.