ABCs of Cholesterol Regulation in the Insulin Secretory Pathway

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DESCRIPTION (provided by applicant): This proposal addresses a new area in diabetes-related research regarding cholesterol distribution and its regulatory roles in the insulin secretory pathway. Recent studies including those forming the basis of this proposal have shown that deficiencies in pancreatic ¿ cells in the ATP-binding cassette transporters ABCA1 and ABCG1, which have been implicated to promote cellular export of cholesterol to plasma lipoproteins by most cell types, impair systemic glucose tolerance through inhibition of insulin secretion. Moreover, expression of these transporters is reduced in models of type 2 diabetes and also figures in glucose sensitivity in response to anti-diabetic thiazolidinediones. Detailed analysis of ABCG1 in islet cells and islet-derived cell lines has shown unexpectedly that this transporter mostly resides in the membranes of insulin granules; its deficiency leads to enlargement of granules, reduced granule cholesterol content and reduced ability of granules to undergo exocytosis in response to glucose or K+ stimulation. It appears as if granule formation and/or maturation are perturbed. Preliminary findings show that ABCA1 also localizes to granules and that its deficiency causes similar intracellular effects as caused by reduced ABCG1. Strikingly, all changes elicited by ABCG1 deficiency are reversed by addition of exogenous cholesterol, identifying cholesterol as the likely common denominator. Taken together, these observations have led to the working hypothesis that ABCG1, in collaboration with ABCA1 and other lipid regulatory proteins, functions in the formation of insulin granules by promoting assembly of a cholesterol-enriched limiting membrane that is able to support the sorting and export activities of the regulated secretory pathway. To explore this hypothesis, a combination of cell biological and biochemical/biophysical approaches will be used on pancreatic islet derived cells and insulinoma cell lines to pursue four specific aims. First, ABCA1's functions alongside ABCG1 in insulin's secretory pathway will be tested using procedures already successfully applied to ABCG1. Second, lipid composition and protein sorting activities within the secretory pathway will be examined to evaluate the roles of the two ABCs in cholesterol dependent granule formation/maturation. Third, using isolated insulin granule fractions, fluorescent lipid analogs will be employed to assay ABC transporter and related lipid translocation mechanisms that are thought to contribute to insulin granule formation. Fourth, real-time microscopic imaging will be used to identify the level(s) at which ABC deficiency affects insulin granule exocytosis. The proposed studies highlight the insulin granule as a major cholesterol regulatory compartment; they represent the first exploration of intracellular roles of ABCs A1 and G1 that are unrelated to cellular cholesterol efflux; and they complement ongoing investigations by others seeking to understand mechanisms of insulin granule exocytosis and perturbations that might relate to ¿ cell failure in type 2 diabetes.