Abstract 408: Macrophage-mediated Low Density Lipoprotein Modification Reduces Foam Cell Formation

Abstract

Macrophage metabolism of LDL-derived cholesterol is thought to be a key process regulating the composition and progression of atherosclerotic lesions, including the generation of cholesterol-loaded macrophage foam cells and the lipid-rich necrotic core. Many mechanisms contribute to macrophage endocytosis of LDL, and we recently reported that selective cholesteryl ester (CE) uptake contributes to LDL-induced macrophage cholesterol accumulation. Selective uptake is expected to result in the net removal of cholesteryl ester from LDL; however, the effect of this and other LDL modifications that may occur during incubation with macrophages is not understood. In the current investigation, we pre-incubated LDL for 1-2 days with macrophages in order to promote macrophage-mediated LDL modification. Surprisingly, when macrophage-conditioned LDL was transferred to fresh macrophages, much less cholesterol accumulation resulted compared to cells incubated with unconditioned LDL, as indicated by cholesterol quantification and by neutral lipid staining with Oil Red-O. This difference was not accounted for by changes in uptake of the modified 125I-LDLs, but instead was associated with a large decrease in selective [3H]CE uptake. In contrast, uptake of the non-hydrolyzable CE analog [3H]cholesteryl ether was not reduced, suggesting that reduced uptake in macrophage-conditioned LDL may be due to changes in the hydrolysis of LDL-derived CE or its subsequent metabolism. Cell-mediated LDL oxidation was considered as a possible mechanism for changes in selective CE uptake. However, agarose gel electrophoretic mobility of LDL and analysis of LDL cholesteryl esters indicated that reduced selective uptake and foam cell formation did not depend on extensive oxidative modifications of LDL. These results indicate the importance of selective CE and suggest that macrophage-mediated LDL modification may reduce foam cell formation in some circumstances.