Lipoprotein-proteoglycan complexes from atherosclerotic lesions promote cholesteryl ester accumulation in human monocytes/macrophages.

Abstract

Lipoprotein-proteoglycan complexes from human atherosclerotic lesions were studied to determine their ability to stimulate cholesteryl ester accumulation in human monocytes/macrophages. Complexes containing apolipoprotein (apo) B lipoproteins and proteoglycans were extracted from fatty streaks and fibrous plaque lesions of human aortas by extraction with 0.15 M NaCl. Fractionation of the complex with Bio-Gel A-50m yielded a single fraction from fatty streaks and two fractions from fibrous plaques. The complexes were further purified by anti-apo B affinity chromatography and analyzed for apolipoproteins, lipids, and glycosaminoglycans Apo B was the only apolipoprotein present in the complexes. Although the complexes from fatty streaks and fibrous plaques contained varying proportions of hyaluronic acid, chondroitin 6-sulfate, and dermatan sulfate, heparin was present in only the fibrous plaque complexes. All three lipoprotein-proteoglycan complexes increased the rate of incorporation of [14C]oleate into cholesteryl [14C]oleate and stimulated cholesteryl ester accumulation in monocytes/macrophages. However, the complexes from fibrous plaques were more potent than those from fatty streaks in this regard. Cholesteryl ester synthesis that is mediated by the uptake of the complexes was dose dependent and showed apparent saturation, suggesting that cell surface binding may be required. Chloroquine, a lysosomotropic agent, inhibited cholesteryl ester synthesis that is induced by the complexes, indicating that lysosomal hydrolysis was essential. Cholesteryl ester synthesis that is mediated by the complexes was inhibited 70-79% by polyinosinic acid. Furthermore, excess unlabeled fibrous plaque complexes significantly inhibited the binding and internalization of in vitro 125I-low density lipoprotein (LDL)-proteoglycan complexes and 125I-acetylated-LDL and not 125I-LDL. These results suggest the involvement of the scavenger receptor in the uptake of the complexes. Phagocytosis played a minor role in the metabolism of these ligands because cytochalasin D inhibited cholesteryl ester synthesis, which is mediated by fibrous plaque complexes, by 7.5-25%. Cholesteryl ester synthesis increased linearly over 32 hours in macrophages incubated with the complexes, indicating an apparent lack of downregulation of binding sites. This resulted in the appearance of intracellular oil red O-positive lipid droplets. These studies show for the first time that apo B lipoprotein-proteoglycan complexes isolated from human atherosclerotic lesions can induce cholesteryl ester accumulation in monocytes/macrophages.