Selective association of lipoprotein cholesteryl esters with liver plasma membranes

High-density lipoprotein (HDL) cholesteryl esters are taken up by hepatocytes without parallel uptake of HDL apolipoproteins. This selective uptake of HDL cholesteryl esters is mediated by a non-endocytotic mechanism. Recently, selective uptake of cholesteryl esters also from low-density lipoprotein (LDL) was demonstrated. In this study, the role of the plasma membrane in selective uptake by the liver was investigated. Plasma membranes were prepared from rat liver or from human Hep G2 hepatoma cells. Human HDL3 (d = 1.125-1.21 g/ml) was either radioiodinated or labeled with [3H]cholesteryl oleate. Human low-density lipoprotein (d = 1.019-1.05 g/ml) was labeled in its protein and in its lipid moiety as well. Labeled lipoproteins, unlabeled lipoproteins and membranes were incubated. After separation by ultracentrifugation, apparent lipoprotein particle association with membranes was determined. Plasma membranes from rat liver and Hep G2 cells bound 125I-HDL3, indicating specific HDL3 particle binding. With both types of membrane, apparent HDL3 particle association according to [3H]cholesteryl oleate-labeled HDL3 was in significant excess on that due to 125I-HDL3. This indicates selective, i.e., particle binding independent, association of cholesteryl esters with the membrane. Excess unlabeled HDL3 competed for selective association, indicating a specific process. Selective association of HDL3 cholesteryl esters was concentration-, time-, temperature-dependent; however, parameters differed from HDL3 particle binding. HDL3 was modified by nitration; this modification inhibited HDL3 particle binding in contrast to unchanged selective association. These results suggested distinct membrane sites for HDL3 particle binding and selective cholesteryl ester association. Regulation of selective association was investigated. Hep G2 cells were cholesterol-loaded or cholesterol-depleted. Cellular cholesterol-loading down-regulated selective association of HDL3 cholesteryl esters with isolated membranes prepared from these cells. In parallel, selective uptake of HDL3 cholesteryl esters by Hep G2 cells was down-regulated in cholesterol-loaded cells. This parallel regulation of selective association with membranes and selective uptake by cells suggests a functional relationship. LDL, radiolabeled in its protein and in its lipid moiety, was incubated with liver plasma membranes. Besides LDL holo-particle receptor binding, also LDL cholesteryl esters were selectively associated with membranes. These data showed that selective association with membranes is not restricted to HDL but can occur from LDL as well. It is concluded that HDL3 as well as LDL cholesteryl esters can selectively be associated with hepatic plasma membranes, i.e., independent from particle binding. Results suggest an important role of the plasma membrane in the mechanism of selective cholesteryl ester uptake by the liver.