Inhibition of intestinal bile acid absorption has been shown to reduce plasma cholesterol. Although, bile acid sequestrants are clinically proven to lower cholesterol, poor patient compliance necessitates the development of better therapeutic modalities to directly inhibit bile acid absorption. Apical Sodium Dependent Bile Acid Transporter (ASBT) absorbs majority of the bile acids in the ileum and is essential for maintaining bile acid pool in the enterohepatic circulation. Therefore, ASBT inhibition represents an attractive therapeutic means for lowering plasma cholesterol. In this regard, recent studies from our laboratory demonstrated that ASBT is inhibited by signaling intermediates including protein tyrosine phosphatases (PTPases) via membrane recycling events. Also beneficial dietary components such as green tea catechin, (-)-epigallocatechin-3-gallate, EGCG, inhibits ASBT function in a lipid-raft dependent manner. We hypothesized that phosphorylation/dephosphorylation processes, lipid raft-dependent mechanisms and membrane trafficking events play critical roles in the inhibition of ASBT function. Hence, a comprehensive understanding of these inhibitory pathways is crucial to exploit their utilization as an effective therapy for hypercholesterolemia associated with diabetes mellitus. Our preliminary studies showed that ASBT function and expression are upregulated in rat model of streptozotocin (STZ)-induced diabetes mellitus. This in vivo model of diabetes mellitus will provide an exceptional tool to investigate the underlying mechanisms of ASBT upregulation in diabetes mellitus as well as determining the impact of ASBT inhibition on associated hypercholesterolemia. Our studies are designed to systematically delineate the cellular and molecular mechanisms inhibiting ASBT utilizing in vitro models and to examine their dysregulation in vivo utilizing diabetes mellitus rat model.
In Specific Aim 1, we will investigate the regulation of ASBT function and phosphorylation by protein phosphatases (PPase) in cell culture models.
In Specific Aim 2, our studies will focus on elucidating the inhibitory mechanisms of ASBT function by membrane recycling events and lipid rafts as well as delineating the molecular basis for EGCG-mediated inhibition. Studies designed for Specific Aim 3 will focus on investigating mechanisms underlying ASBT upregulation in rat model of STZ-induced diabetes mellitus and determine the efficacy of the beneficial dietary compound EGCG and specific ASBT inhibitors (developed by Biotechnology Company Albireo) in lowering the levels of plasma cholesterol. Our proposed studies are critical for providing novel insights into the regulation of ASBT under normal and pathophysiological conditions and may provide better strategies for the management of hypercholesterolemia associated with several disorders.
An increase in intestinal bile acid absorption has been implicated in the elevated plasma cholesterol levels and the increased risk of cardiovascular disorders in patients with diabetes mellitus. The major protein responsible for bile acid absorption is ileal ASBT. Therefore, investigating the mechanisms of ASBT inhibition is crucial to improve current therapy of hypercholesterolemia especially in diabetic patients, who do not efficiently respond to available therapeutic modalities. The proposed studies will focus on elucidating the mechanisms involved in the inhibition of ASBT function and evaluating their impact on hypercholesterolemia in rat model of diabetes mellitus. These studies have direct relevance to public health because of the high incidence of diabetes mellitus, and are of a great importance for designing better therapeutic modalities for the management of cholesterol-related disorders in the future.
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