There is interest in understanding the factors that regulate HDL and apoA-I levels in humans as these are inversely correlated with the risk for atherosclerotic vascular disease. Plasma levels of HDL and apoA-I are related to the rate of apoA-I catabolism rather than apoA-I production. Neither the mechanisms underlying accelerated catabolism nor the site(s) of apoA-I removal from plasma have been clearly delineated. This proposal focuses on the role of the HDL receptor, SR-B1, in regulating apoA-I catabolic rate. SR-B1 delivers cholesteryl ester to cells via a process that is fundamentally distinct from receptor-mediated endocytosis. During this process, HDL binds to SR-B1 on the cell surface and transfers cholesteryl ester from the core of the particle to the cell without internalization of the apolipoprotein moiety. Despite the absence of apolipoprotein uptake, increased selective lipid uptake in the liver mediated by SR-B1 is associated with increased catabolism of apoA-I, and at least some of this catabolism occurs in the kidney. Thus, a central hypothesis of this proposal is that SR-B1 interaction with HDL initiates changes in the lipoprotein particle that promotes subsequent catabolism by a mechanism that is independent of SR-B1. SR-B1 binds with high affinity a number of different lipoprotein particles in addition to HDL, including VLDL, LDL, and oxidized LDL. The important metabolic event subsequent to lipoprotein binding by SR-B1 is the selective delivery of CE from the core of particles to cells. We propose that apoA-I is a critical ligand for SR-B1-mediated selective uptake, and that apoA-I conformation on the HDL particle can influence its interaction with SR-B1 to impede or promote receptor binding and/or selective uptake. The efficiency of selective uptake mediated by SR-B1:apoA-I interaction can be modulated by properties of the HDL particle (lipid composition, apolipoprotein content, or the structure of apoA-I itself). These hypotheses will be tested by defining: 1) the sub-population of HDL particles that are the preferred substrate for SR-B1-mediated lipid transfer; 2) the structure and composition of HDL particles after processing by SR-B1 (including the potential for apoA-I proteolysis); and 3) the metabolic fate of HDL particles after SR-B1 processing. Understanding the role of SR-B1 in HDL metabolism could define new areas for potential therapeutic assault.
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