Molecular Basis of Cellular Cholesterol Efflux and Regulation of Plasma HDL Level

Abstract

HDL plays a central role in cholesterol transport from peripheral tissues to the liver for its conversion to bile acids, but what is known for this system is limited. We discovered helical apolipoproteins in their free form directly to interact with the cells to generate new HDL with cellular phospholipid and cholesterol. This reaction is one of the major pathways for cellular cholesterol efflux in addition to the classical mechanism by its diffusion. Three major topics are presented in this session: 1) The cellular interaction cite with apolipoprotein is required for the reaction. The cite is proteolysis susceptible, cell specific, and induced in mouse monocytic leukemia cells RAW264 by cAMP which is inhibited by the reagents for the various stage of protein synthesis. These results are all independent of the nonspecific diffusion-mediated cellular cholesterol efflux. 2) The apolipoprotein-cell interaction to generate HDL is a major source of plasma HDL. We found that probucol strongly reduces plasma HDL blocks, the apolipoprotein-cell interaction and inhibits the generation of HDL in vitro. When mice are given probucol, HDL was severely reduced without change in CETP, HDL fractional clearance rate and apoA-I synthesis, and the cells from these mice lacked the interaction with apolipoproteins. Thus, being consistent with the recent finding with Tangier disease, this reaction was shown to be the major source of plasma HDL and a major regulator for plasma HDL level. 3) Specific intracellular cholesterol transport is required for its incorporation into the HDL generated by this reaction. Protein kinase C inhibitors selectively down regulated this process, resulting in generation of cholesterol poor HDL. Human monocytic leukemia cells THP1 generate cholesterol and sphingomyelin poor HDL with apolipoprotein, but generate the cholesterol rich HDL after the differentiation by PMA accompanied by induction of caveolin-1 expression. The antisense DNA of caveolin-1 reduced the protein expression and incorporation of cholesterol into the HDL. Thus, caveorin-1 is shown to be responsible for the specific intracellular cholesterol trafficking for the apolipoprotein-mediated HDL generation.