Original contributions from our laboratory have established novel roles for small G-proteins [e.g., Rac1, Cdc42] in glucose-stimulated insulin secretion [GSIS] from normal rodent and human islets. We provided the first evidence to suggest that these G-proteins undergo post-translational lipidation via incorporation of farnesyl or geranylgeranyl groups at their C-termini. Recent studies from our laboratory have demonstrated novel regulatory roles for protein farnesyltransferase [FTase] and geranylgeranyltransferase [GGTase] in -cell function including GSIS. Our efforts have also led to the identification of novel regulatory factors for these G-proteins including GDP-dissociation inhibitors [GDIs] and guanine nucleotide exchange factors [GEFs], which appear to precisely control signaling steps leading to GSIS. During the course of these investigations, we noticed significant defects in FTase/GGTase signaling pathways in in vitro and in vivo models of glucolipotoxicity, endoplasmic reticulum [ER] stress and T2DM. As a logical extension of the previously funded studies, and based on recently accrued exciting preliminary evidence we propose to test the overall hypothesis that glucolipotoxic and ER stress conditions induce defects in the FTase/GGTase signaling cascade leading to sustained activation of small G-proteins [Rac1] and mitochondrial dysregulation and demise of the islet -cell. We will accomplish this goal via experiments described under three Specific Aims.
In Aim 1, we will determine if glucolipotoxic and ER stress conditions promote alterations in the FTase/GGTase signaling pathway leading to the metabolic dysfunction and apoptosis of the islet -cell. In its support, we present preliminary evidence to suggest significant reduction of FTase/GGTase activity and sustained activation of small G-proteins [Rac1] in pancreatic -cells under glucolipotoxic conditions. Studies in Aim 2 will delineate the mechanisms underlying sustained activation of G-proteins and accelerated stress kinase signaling steps [JNK1/2, p38 MAP kinase] in the islet -cell under conditions of glucolipotoxicity and ER stress. Our preliminary evidence also suggests that inhibitors of Tiam1 [NSC23766] and Vav2 [Ehop-016], two known GEFs for Rac1, significantly attenuated glucotoxic effects in islet -cells. Experiments described under Aim 3 will determine if pharmacological intervention and prevention of alterations in FTase/GGTase signaling axis, sustained [constitutive] activation of G-proteins, and abnormalities in downstream metabolic events restore normal -cell function in animal models of diet-induced obesity and T2DM. The proposed studies are innovative and carry significant translational impact as they will provide novel insights into the underlying mechanisms for defective FTase/GGTase signaling events leading to islet dysfunction in T2DM, and the data accrued from these studies will form the basis for the identification of novel targets in the FTase/GGTase and G-protein signaling cascades for development of therapeutics to treat T2DM in humans. Our long-standing expertise in the area of G-protein prenylation in islet function including GSIS gives us a unique opportunity to address these important aspects of islet function in health and in diabetes.
Recent evidence implicates novel roles for small G-proteins in a variety of -cell functions including proliferation and physiological insulin secretion. This class of signaling proteins is modified by specific lipids of the cholesterol biosynthetic pathway, which appear to be critical for islet -cell survival and insulin secretion. We have recently noted significant defects in this lipidation pathway in in vitro and in vivo models of oxidative and endoplasmic reticulum stress that could contribute to -cell dysfunction and demise in T2DM. We propose studies to investigate the underlying mechanisms and test novel pharmacological probes to prevent genesis of these defects in the islet -cell under the duress of oxidative and endoplasmic reticulum stress. We envision that data accrued from our proposed investigations will provide novel insights into the development of therapeutics for the treatment of diabetes.
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