SR-BI and SR-BII Recycling and Selective Lipid Uptake

HDL levels are inversely related to coronary artery disease. The HDL receptor. SR-BI, plays a key role in HDL metabolism, SR-BI binds HDL and mediates the process known as selective lipid uptake, the mechanism of which is not understood. Various studies have indicated that selective uptake by SR-BI in certain cellI occurs at the cell surface by a non-endocytic procell. However, recent studies have reported that In hepatocytel SR-BI mediates the Internalization of Intact HDL particles and suggest that selective lipid uptake Involves HDL internalization and recycling in celli. We have shown that SR-BII, a variant of SR-BI, differs markedly from SR-Blln Its cetlular traflicking and endocytosis of HDL. The central hypothesis of this proposal Is that the functions of SR-BI and SR-Bliin Intracellular cholesterol trafficking are coupled to receptornlgand recycling in cells and that SR-BI and SR-BII follow different internalization and recycling routes and differentially affect cellular chOlesteroltrafficking and regulation. The specific alms are: 1) determine the contribution of HDL internalization and Intracellular recydlng to the selective lipid uptake procell mediated by SR-BI and SR-BII. HDL recycling by each of the two receptors will be quantified in hepatocyte& and other cells, 2) elucidate the mechanisms. responsible for HDL Internalization and recycling by SR-BI and SR-BII. Endocytic and recycling pathway(s) used by SR-BI and SR-BII will be studied and protein motifs In the C-terminal tails of SR-BI and SR-BII that are responsible for receptor targeting will be identified, 3) determine how differences In the Intracellular targeting of HDL by SR-BI and SR-BU influence cellular cholesterol regulation and trafficking.We will determine the effects of selective lipid uptake via SR- 81 and SR-BII on the pool of regulatory sterols in cells and on cholesterol efftux and 4) assess the roles of SR-Bland SR-BfI in HDL metabolism and atherogenesis in vivo using isofonn-speciflC knock-in mice. lsoform-specific knock-in mice wUl be generated and the effects of each receptor on plasma lipoproteins, H