Macrophage Foam Cell Formation with Native Low Density Lipoprotein

Abstract

This investigation has elucidated a mechanism for development of macrophage foam cells when macrophages are incubated with native low density lipoprotein (LDL). LDL is believed to be the main source of cholesterol that accumulates in monocyte-derived macrophages within atherosclerotic plaques, but native LDL has not previously been shown to cause substantial cholesterol accumulation when incubated with macrophages. We have found that activation of human monocyte-derived macrophages with phorbol 12-myristate 13-acetate (PMA) stimulates LDL uptake and degradation and acyl-CoA:cholesterol acyltransferase-mediated esterification of LDL-derived cholesterol, resulting in massive macrophage cholesterol accumulation that could exceed 400 nmol/mg of cell protein. Cholesterol accumulation showed a biphasic linear LDL concentration dependence with LDL levels as high as 4 mg/ml, similar to LDL levels in artery intima. Protein kinase C mediated the PMA-stimulated macrophage uptake of LDL because the protein kinase C inhibitors, Gö6983 and GF109203X, inhibited cholesterol accumulation. LDL receptors did not mediate macrophage cholesterol accumulation because accumulation occurred with reductively methylated LDL and in the presence of an anti-LDL receptor-blocking monoclonal antibody. LDL-induced cholesterol accumulation was not inhibited by antioxidants, was not accompanied by increased LDL binding to macrophages, did not depend on the apoB component of LDL, and was not down-regulated by prior cholesterol enrichment of macrophages. We have shown that the mechanism of LDL uptake by macrophages was PMA-stimulated endocytosis of LDL taken up as part of the bulk phase fluid (i.e. fluid phase endocytosis). The amount of LDL taken up with the bulk phase fluid was measured with [(3)H]sucrose and accounted for a minimum of 83% of the LDL cholesterol delivery and accumulation in PMA-activated macrophages. This novel mechanism of macrophage cholesterol accumulation shows that modification of LDL is not necessary for foam cell formation to occur. In addition, the findings direct attention to macrophage fluid phase endocytosis as a relevant pathway to target for modulating macrophage cholesterol accumulation in atherosclerosis.