The role of hepatic insulin resistance on SR-BI dependant HDL cholesterol uptake

描述（由申请人提供）：本提案的总体目标是研究受损的胰岛素信号在多大程度上改变HDL介导的胆固醇逆向转运（RCT）。HDL的一个关键的心脏保护功能是通过随机对照试验将多余的胆固醇从血浆和组织输送到肝脏，然后从体内排出。SR-BI是肝脏主要的高密度脂蛋白受体，介导高密度脂蛋白胆固醇酯（HDL- ce）的选择性摄取。与其他循环脂蛋白相比，高密度脂蛋白颗粒携带的胆固醇以SR-BI依赖的方式优先分泌到胆汁中。除了SR-BI在肝内胆固醇转运中的作用外，功能获得和功能丧失的研究都证实SR-BI是肝脏胆固醇代谢的决定因素，包括胆固醇合成、胆道胆固醇分泌率和胆固醇向VLDL的结合。HDL-CE摄取和胆道胆固醇分泌均受到胰岛素信号受损的负面影响，胰岛素信号受损是代谢综合征（MetS）和心血管疾病风险增加的关键因素。我们发表的和初步的数据表明，胰岛素信号通过一种新的翻译后机制调节SR-BI，该机制促进SR-BI在质膜上的表达。我们假设胰岛素抵抗降低了HDL-CE的摄取，并改变了SR-BI依赖的胆固醇向肝脏调节和分泌池的传递，其机制包括SR-BI重新分配到细胞内腔室。特异性Aim 1将确定肝脏胰岛素信号传导和胰岛素抵抗对SR-BI介导的固醇转运的影响

DESCRIPTION (provided by applicant): The overall objective of this proposal is to examine the extent to which impaired insulin signaling alters HDL- mediated reverse cholesterol transport (RCT). A key cardioprotective function of HDL is the delivery of excess cholesterol from plasma and tissues to the liver for elimination from the body via RCT. SR-BI is the major hepatic HDL receptor and mediates the selective uptake of HDL cholesterol esters (HDL-CE). Compared to other circulating lipoproteins, cholesterol carried on HDL particles is preferentially secreted int bile in an SR-BI dependent manner. In addition to SR-BI's role in trans-hepatic sterol transport, both gain and loss-of-function studies have established that SR-BI is a determinant of hepatic cholesterol metabolism including cholesterol synthesis, biliary cholesterol secretion rates and the incorporation of cholesterol into VLDL. Both HDL-CE uptake and biliary cholesterol secretion have been shown to be negatively affected by impaired insulin signaling, a key contributing element to the Metabolic Syndrome (MetS) and the increased risk for cardiovascular disease. Our published and preliminary data indicate that insulin signaling regulates SR-BI by a novel post-translational mechanism that promotes SR-BI expression on the plasma membrane. We hypothesize that insulin resistance reduces HDL-CE uptake and alters SR-BI-dependent delivery of cholesterol to hepatic regulatory and secretory pools through mechanisms that include the redistribution of SR-BI to an intracellular compartment. Specific Aim 1 will determine the effect of hepatic insulin signaling and insulin resistance on SR-BI mediated sterol trafficking to secretory and regulatory hepatic cholesterol pools. Hepatic insulin signaling will be disrupted by selective depletion of insulin receptors or by expression of a dominant negative phosphatidylinositol-3 kinase (AdnilPI3K) and the extent to which hepatic insulin resistance reduces SR-BI mediated HDL-C selective cholesterol ester uptake determined. Studies will also determine how reduced SR-BI function alters steady-state hepatic cholesterol synthesis and secretion in the setting of impaired insulin signaling? Specific Aim 2 will test the hypothesis tha transhepatic (THCE) elimination of HDL cholesterol in bile is reduced in the setting of hepatic insulin resistance and hypertriglyceridemia. This will be accomplished by a) determining whether reduced SR-BI function diminishes THCE of HDL-C, b) determining whether hepatic IR decreases macrophage to feces RCT in an SR-BI dependent manner, and c) examining whether changes in HDL particle composition disrupt THCE of HDL-C and synergize with SR-BI dysfunction to reduce HDL RCT function. Specific Aim 3 will determine the molecular mechanisms by which insulin signaling regulates SR-BI cell surface localization in hepatocytes. The effect of insulin-dependent PI3K signaling on SR- BI subcellular distribution will be examined in cultured hepatocytes, together with the identity of the intracellular compartment(s) in which SR-BI accumulates as a result of impaired insulin signaling. The responsible insulin- dependent signaling pathways will be defined and studies will be performed to identify the determinants on SR- BI necessary for its regulation by insulin signaling.