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.
This proposal examines the effects of insulin resistance on the functionality of hepatic SR-BI. SR-BI is the primary liver receptor for HDL and plays a key role in liver uptake and elimination of excess cholesterol delivered from peripheral tissues. This process, called reverse cholesterol transport, underlies a key cardioprotective function of HDL. The completion of this proposal will further our understanding of the impact of obesity and insulin resistance on the cardioprotective effects of HDL and guide current and future therapeutic strategies that target HDL in the treatment of cardiovascular disease.
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