The major hypothesis of the proposal is that plasma lipid transfer proteins play an important role in regulating the movement of lipids between plasma lipoproteins or between lipoproteins and cells. The strategy is initially to study in vitro properties of well defined model systems, consisting of purified lipoproteins, enzymes and lipid transfer proteins. Subsequently, more complex experiments will be conducted using plasma or cultured cells. Eventually the findings will be directly applied to studies of human physiology, whenever possible. Two separate lipid transfer proteins, a phospholipid transfer protein (PTP), and a cholesteryl ester transfer protein (CETP), will be purified from human plasma. The role of the PTP in enhancing movement of phospholipids from triglyceride-rich lipoproteins into HDL during lipolysis will be studied using VLDL, HDL and purified bovine milk lipoprotein lipase. The possibility that PTP enhances phospholipid and cholesterol transfer from cultured endothelial cells into HDL and provides substrate (phospholipid and cholesterol) to stimulate plasma lecithin: cholesterol acyltransferase will be investigated. Monoclonal antibodies to PTP will be used to investigate its mass (radioimmunoassay), distribution (immunoblotting and immunoaffinity chromatography) and mechanism of action. The interaction of CETP with lipoprotein lipase will also be explored to determine the mechanism and significance of the recent finding that lipoprotein lipase enhances the ability of CETP to stimulate cholesteryl ester transfer from HDL to VLDL. These studies will help to elucidate the mechanisms of formation of HDL and the role of HDL in cholesterol transport. The latter may be of central importance in the apparent protective effect of HDL in atherosclerotic cardiovascular disease. Further experiments will be conducted with triglyceride-rich lipoproteins, lipases and CETP to see if CETP interacts with lipase to enhance transfer of cholesteryl esters from triglyceride-rich lipoproteins into cultured endothelial cells or macrophages. Macrophages secrete lipase, CETP, by stimulating exchange of cellular triglycerides with VLDL cholesteryl ester, may promote hydrolysis of cellular triglyceride and cytoplasmic storage of lipoprotein-derived cholesteryl esters, inducing foam cell formation. Thus, the interactions of CETP with lipase may also be important in non-receptor dependent mechanisms of entry of cholesterol into cells and in the process of atherogenesis.
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