Scavenger receptor class B, type I (SR-BI) facilitates selective uptake of cholesteryl ester (CE) from lipoprotein particles such as high-density lipoprotein (HDL) by a process in which HDL core-CE is taken into cells without a parallel uptake and degradation of the HDL particle itself. This SR-BI-mediated 'selective'pathway represents a major route for the delivery of CEs to the liver and steroidogenic tissues of rodents and humans for the biosynthesis of bile acids and steroid hormones, respectively. Besides facilitating selective CE transport, SR-BI also promotes reverse cholesterol transport (RCT) and exerts atheroprotective actions. Although much is known about these various functions of SR-BI, the cellular mechanisms controlling functional expression of SR-BI are not fully established. The overall goal of this competitive renewal application is to elucidate mechanisms by which certain positive and negative modulators regulate the expression and function of SR-BI in the liver, adrenal and gonads. Specifically, we propose to determine the stimulatory and inhibitory actions of SF-1/LRH-1 and NHERF1/2 and SIK-1 on SR-BI expression, SR-BI mediated selective CE transport and bile acid and steroid synthesis in liver and steroidogenic tissues.
The specific aims of the proposal are to: 1) characterize and establish the roles of LRH-1 and SF-1 in hormone-induced SR-BI expression and function in intact hepatic and steroidogenic tissues and isolated cells;2) determine how NHERF1 and NHERF2 potentially impact SR-BI expression, its cellular localization and SR-BI-mediated selective HDL-CE uptake in vitro and in vivo using hepatic and steroidogenic cells and genetic mouse models;and 3) identify underlying mechanisms by which SIK-1 inhibits SR-BI-dependent selective HDL-CE uptake in vivo and in hepatic and steroidogenic cell model systems in vitro. This proposal utilizes state-of-the-art molecular, cellular and biochemical and biophysical approaches and unique cell and animal models to dissect the molecular events mediating the modulatory actions of nuclear receptors SF-1 and LRH-1, PDZ-domain containing proteins NHERF1/2 and hormone-regulated SIK-1 protein. Successful completion of the proposed studies is likely to identify novel targets that can be exploited for the development of new therapies to treat bile acids and steroid hormone related diseases. On a broader scope, since SR-BI is atheroprotective, the current studies may also aid in the development of new therapeutic and preventive strategies in the clinical management of cardiovascular disease.
Bile acids and steroid hormones are chemicals produced by the liver and adrenal/gonads, respectively, and are essential for a multitude of normal physiological processes. Bile acids play essential roles in facilitating lipid digestion and absorption, and also serve as important regulators of hepatic and intestinal function. Likewise, adrenal steroids influence carbohydrate, lipid, and protein metabolism, and salt and water balance, as well as inflammation, immunity and blood pressure, whereas gonadal steroids are critical for secondary sex characteristics, reproduction and muscle and bone growth. Both bile acids and steroids are synthesized from SR-BI-mediated selectively delivered cholesterol, thus underscoring the relevance of understanding the cellular mechanisms controlling the functional expression of SR-BI.
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