Uptake of LDL-Sized Particles Extracted from Human Aortic Lesions by Macrophages in Culture
We have isolated a lipoprotein particle, designated A-LDL, from atherosclerotic lesions in human aortas by a combination of affinity chromatography and gel filtration. This lipoprotein, believed to be derived from plasma LDL that has accumulated in the arterial intima, possesses apo B immunoreactivity and is of the same size as LDL. However, compared to LDL, it is more electronegative, its protein-to-lipid ratio is lower, and its hydrated density is lower and more hetereogeneous. Furthermore, apo B in A-LDL is highly degraded, and A-LDL stimulated cholesterol esterification in mouse peritoneal macrophages (MPM) in a dose-dependent fashion with saturation kinetics. Stimulation of cholesterol esterification and accumulation of cholesteryl esters in MPM over 48 hr was linear and about 18-fold greater than that induced by plasma LDL, suggesting no down-regulation of the uptake mechanism by intracellular cholesterol content. Uptake of A-LDL did not appear to occur via the scavenger receptor on MPM since (1) pretreatment of cells with trypsin resulted in less inhibition of cholesterol esterification by A-LDL than by acetyl-LDL, and (2) cross-competition studies with radiolabeled ligands showed no inhibition of uptake between A-LDL and acetyl-LDL. However, since even an excess of A-LDL failed to inhibit uptake of radiolabeled A-LDL, it is likely that MPM uptake of A-LDL occurs by a low-affinity, high-capacity system. This assumption was confirmed by the fact that the uptake of A-LDL, in contrast to cholesterol esterifìcation, was essentially linear up to a concentration of 400 μg lipoprotein cholesterol/ml. Thus, the intracellular processing of A-LDL appears to be sluggish, since the uptake of A-LDL markedly exceeded the subsequent stimulation of cholesterol esterification. When a 24-hr period without lipoprotein was introduced after an initial incubation of MPM with A-LDL, cholesterol esterifìcation matched A-LDL uptake, indicating that complete processing of A-LDL can occur, albeit sluggishly. These results suggest that the modifications in composition and structure in A-LDL relative to LDL dramatically alter its functional properties, leading to loading of macrophages with cholesteryl esters. If these interactions also occur in vivo, they could, in part, explain the etiology of the fatty streak lesion.
KeywordsCholesteryl Ester Cholesterol Esterification Scavenger Receptor Mouse Peritoneal Macrophage Human Aorta
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