Macrophage Oxidation of Low Density Lipoprotein Generates a Modified Form Recognized by the Scavenger Receptor

Incubation of low density lipoprotein (LDL) with endothelial cells or smooth muscle cells overnight has resulted in an oxtdative modification of LDL that results in its recognition by macrophages by way of the acetyl LDL receptor. In the present study, we examined whether macrophages themselves can oxidize and modify LDL in a manner similar to that of endothelial cells. Incubation of l-labeled LDL with resident or thioglycollate-elicited macrophages for 24 hours in Hamʼs F-10 medium resulted in the appearance of thiobarbituric acid (TBA) reactive materials and trichloroacetic acid (TCA) soluble radioactivity in the medium. The LDL harvested from these incubations showed increased electrophoretic mobility and was degraded rapidly when added to fresh macrophages as compared to LDL previously incubated in the absence of cells. These macrophage-induced modifications could be prevented if the first incubation was carried out in the presence of the antioxidant butylated hydroxytoluene (BHT) or in Dulbeccoʼs modified Eagleʼs medium (DMEM). The degradation of l-labeled macrophage-modified LDL by macrophages was competitively inhibited by unlabeled acetyl LDL or unlabeled endothelial cell-modified LDL but not by native LDL, indicating that the degradation was mediated by the acetyl LDL receptor.