Decrease in reactive amino groups during oxidation or endothelial cell modification of LDL. Correlation with changes in receptor-mediated catabolism

Abstract

The monocyte/macrophage appears to be the percursor of many of the lipid-laden cells in atherosclerotic lesions, but the mechanism by which these cells accumulate cholesterol to become foam cells remains unclear. We have previously reported that cultured endothelial cells can modify low density lipoprotein (LDL) in a manner that leads to rapid uptake by the acetyl LDL receptor of macrophages. This modification involves free radical-induced peroxidation of LDL and is accompanied by many changes in the physicochemical properties of LDL including increased electrophoretic mobility, increased density, decreased content of esterified cholesterol, hydrolysis of phosphatidylcholine, and fragmentation of apolipoprotein B. Under conditions highly favorable to oxidation, a similar modification can occur even in the absence of cells. In the present studies, oxidation of LDL simply by exposure to 5 μM Cu++ resulted in a modification that was indistinguishable from that produced by endothelial cells. Moreover, it was demonstrated that LDL oxidation by either method is accompanied by a marked decrease in amino group reactivity, comparable to that seen with the chemical modifications of LDL that lead to recognition by the acetyl LDL receptor. Inhibitors of proteolytic enzymes did not reduced fragmentation of apolipoprotein B during oxidation. The rate of catabolism of intravenously injected oxidized LDL in guinea pigs was very rapid, and over 80% of the degradation occurred in the liver. These studies demonstrate that all of the changes associated with endothelial cell modification of LDL can be attributed to oxidation. The cells can, however, promote oxidation under conditions where it would otherwise occur very slowly. Modifiction of LDL by endothelial cells or 5 μM Cu++ results in a marked decrease in LDL amino group reactivity that correlates with accelerated LDL clearance via the acetyl LDL receptor and decreased clearance by the classical LDL receptor in cultured cells and in vivo.