Oxidized low density lipoprotein causes impaired efflux of cholesterol from macrophages : implications for atherogenesis

Abstract

Oxidation of low density lipoproteins (LDL) has been implicated as a causal factor in the pathogenesis of atherosclerosis. Oxidized LDL has been found to exhibit numerous potentially atherogenic properties, including receptor-mediated uptake by macrophages. The present study confirms that oxidized LDL is resistant to intracellular degradation and accumulates in a lysosomal compartment in macrophages. Continued accumulation of lipid by macrophages in the arterial intima results in transformation of these cells into lipid-laden "foam cells", which are one of the earliest signs of atherosclerosis. It is believed that high density lipoprotein (HDL) protects against atherosclerosis by removing excess cholesterol from cells of the artery wall, thereby retarding lipid accumulation by macrophages. We investigated the relative rates of HDL-mediated cholesterol efflux in murine resident peritoneal macrophages that had been loaded with acetylated LDL or oxidized LDL. It was found that acetylated LDL-loaded cells released approximately one-third of their total cholesterol to HDL-containing medium, whereas oxidized LDL-loaded cells released only one-tenth of their total cholesterol. To determine whether the impairment of cholesterol efflux was due to modification of the apolipoprotein B or the lipid component of LDL, cells were loaded with either acetylated LDL that had been oxidized for 5 hours or native LDL that had been modified with oxidized arachidonic acid. It was found that both the oxidized lipid component and the modified apolipoprotein B contributed to impaired efflux of cholesterol. To ascertain the proportion of cholesterol efflux mediated by the apolipoprotein component compared to the lipid component of HDL, we measured the rate of efflux when the cells were treated with trypsinized HDL. Very little efflux occurred when the macrophages were treated with trypsinized HDL, underscoring the importance of intact apolipoprotein in HDL-mediated cholesterol efflux. We also investigated the subcellular distribution of cholesterol in oxidized LDL-loaded cells and acetylated LDL-loaded cells. Our findings suggest that in contrast to acetylated LDL-derived cholesterol, oxidized LDL-derived cholesterol accumulates within lysosomes. The observed impairment of cholesterol efflux in oxidized LDL-loaded macrophages may influence the generation of foam cells in vivo, and should, therefore, be considered one of the more important atherogenic effects of oxidized LDL.