The role of apolipoproteins of HDL in the selective uptake of cholesteryl linoleyl ether by cultured rat and bovine adrenal cells

Abstract

Rat adrenal cells in culture were used to study the uptake of cholesteryl linoleyl ether ([ 3H]cholesteryl linoleyl ether), a nonhydrolyzable analog of cholesteryl ester. When ([ 3H]cholesteryl linoleyl ether was added in the form of liposomes, its uptake was enhanced by adrenocorticotropin (ACTH) and by addition of milk lipoprotein lipase and interfered by heparin. When the adrenal cells were incubated with homologous [ 3H]cholesteryl linoleyl ether-HDL, ACTH treatment also resulted in an increase in ([ 3H]cholesteryl linoleyl ether uptake. The uptake of [ 3H]cholesteryl linoleyl ether was in excess of the uptake and metabolism of 125I-labeled HDL protein and was not sensitive to heparin. Unlabeled HDL or delipidated HDL reduced very markedly the uptake of [ 3H]cholesteryl linoleyl ether, while addition of phosphatidylcholine liposomes had little effect. Attempts were made to deplete and enrich the adrenal cells in cholesterol and, while depletion resulted in a decrease in [ 3H]cholesteryl linoleyl ether-HDL uptake, enrichment of cells with cholesterol had no effect. Among the individual apolipoproteins tested, apolipoprotein A-I and the C apolipoproteins reduced [ 3H]cholesteryl linoleyl ether uptake, while apolipoprotein E was not effective. Since the labeled ligand studied was a lipid, these effects could not be due to an exchange of apolipoproteins, but indicated competition for binding sites. Preferential uptake of human [ 3H]cholesteryl linoleyl ether-HDL 3 by bovine adrenal cells was found when compared to the uptake and metabolism of 125I-labeled HDL. The present results suggest that the preferential uptake of HDL cholesteryl ester (as studied with [ 3H]cholesteryl linoleyl ether) requires an interaction between the apolipoproteins of HDL and cell surface components.