EFFLUX OF NEWLY SYNTHESIZED CHOLESTEROL AND BIOSYNTHETIC STEROL INTERMEDIATES FROM CELLS - DEPENDENCE ON ACCEPTOR TYPE AND ON ENRICHMENT OFCELLS WITH CHOLESTEROL

Previous studies suggest that during sterol synthesis in cells, choles terol and precusor sterols are transported to the plasma membrane and that this transport is stimulated by the binding of high density lipop rotein (HDL) to its putative cell surface receptor, leading to enhance d sterol efflux. Little is known about the identities of synthesized s terols subject to efflux or whether efflux of cholesterol and precurso r sterols are stimulated equally by HDL. To address these issues, cell s were incubated with [H-3]acetate or [H-3]mevalonate and sterol accep ters, and then the labeled sterols in cells and efflux media were anal yzed by high pressure liquid chromatography methods that resolved chol esterol and precursor sterols. In non-hepatic cells (Chinese hamster o vary (CHO), fibroblasts, and smooth muscle), cholesterol and multiple precursor sterols accumulated. In CHO cells, the major products were c holesterol and des-mosterol, which together constituted 50% of labeled nonsaponifiable lipids. When media contained human HDL(3) (1 mg of pr otein/ml), the molar efflux of synthesized desmosterol was four times that of cholesterol, and the 8-h efflux of those sterols, each normali zed to its own production, averaged 48 and 16%, respectively. When med ia contained egg phosphatidylcholine vesicles (1 mg/ml), the efflux of these sterols averaged 18 and 2.4%, respectively. Thus, with both acc epters, desmosterol was the major synthesized sterol released from cel ls, and its efflux was substantially greater than that of synthesized cholesterol. High relative efflux of desmosterol (or a desmosterol-lik e sterol) occurred in all cell types and in both cholesterol-enriched and unenriched cells. These results demonstrated qualitatively similar efflux of synthesized sterols in the presence of HDL(3) and phospholi pid vesicles, arguing against an absolute requirement for accepters th at interact with the HDL receptor. To probe for possible quantitative differences in the capabilities of these two accepters, the ratios of (efflux to HDL(3))/(efflux to phosphatidylcholine vesicles) were calcu lated for synthesized cholesterol and desmosterol, plasma membrane cho lesterol, and lysosomal cholesterol. In comparison to plasma membrane cholesterol, there was little or no HDL selectivity for lysosomal chol esterol or synthesized desmosterol, whereas there was a 2-3-fold selec tivity for synthesized cholesterol, suggesting that the ability of HDL to enhance the efflux of synthesized sterols is a modest quantitative effect and confined to cholesterol.