Esterification of oxysterols in human serum: effects on distribution and cellular uptake

Oxysterols, oxidized derivatives of cholesterol, may enter the circulation as contaminants of cholesterol-containing food, arise through peroxidation of lipoproteins, or be generated by enzymatic reactions. They are found in serum associated either with lipoproteins or with albumin. In these studies, 25-hydroxycholesterol (25OHC) was used as a model oxysterol to investigate the effect of esterification on the association of oxysterols with serum components and their delivery to cultured cells. 25OHC added in vitro to fresh human serum was readily esterified during incubation at 37 degrees C, most likely by serum lecithin:cholesterol acyltransferase (LCAT) as it was blocked by known inhibitors of LCAT. The 25OHC-esters formed were identified as monoesters by comparing their elution on high performance liquid chromatography and thin-layer chromatography with that of chemically synthesized 25OHC mono- and diesters. Esterification doubled the percentage of 25OHC associated with lipoproteins, concomitantly decreasing the amount associated with albumin. Whereas unesterified 25OHC readily transferred between isolated lipoproteins, 25OHC-esters remained associated with donor lipoproteins unless human lipoprotein-deficient serum was added. That cholesteryl ester transfer protein (CETP) mediated transfer of 25OHC-esters was demonstrated by the ineffectiveness of rat lipoprotein-deficient serum as well as by the ability of IC-4, an anti-CETP monoclonal antibody, to suppress the transfer. Esterification of 25OHC in serum limited its entry into human erythrocytes and fibroblasts (GM 3468A cells) in vitro. Up-regulation of fibroblast low density lipoprotein (LDL)-receptors enhanced the uptake of esterified 25OHC from medium, but did little to enhance the total uptake of 25OHC. Thus, esterification of oxysterols in serum shifts their distribution away from albumin into LDL and high density lipoprotein (HDL) and limits their availability to cells in culture.