Biologically Modified LDL Increases the Adhesive Properties of Endothelial Cells

Adhesion of monocytes to the arterial endothelium is an important early event in atherosclerosis. Several lines of evidence have suggested that oxidation of low density lipoprotein (LDL) in the arterial wall may initiate the inflammatory-like process that generally is present in atherosclerotic lesions. In vitro, oxidation of LDL can be obtained both by exposure to divalent ions, such as Cu2+, or by incubation with different cell types, including monocytes and endothelial cells. The present study was designed to investigate the possible influence of oxidized LDL on the adhesive properties of endothelial cells. We report here that Cu(2+)-oxidized LDL is as effective as interleukin 1 beta in stimulating the ability of cultured human endothelial cells to bind U937 monocytic cells. The stimulation was inhibited by cycloheximide, indicating that de novo protein synthesis is required. Biologically modified LDL, obtained by incubation with human peripheral blood monocytes, also enhanced the adhesiveness of endothelial cells. This effect was not due to an increased secretion of interleukin 1 beta from the monocytes exposed to LDL. Treatment of endothelial cells for 24 h with native LDL was also found to increase the adhesion of U937 cells. Exposure of endothelial cells to LDL for 24 h resulted in an oxidative modification of LDL. Furthermore, the antioxidant butylated hydroxytoluene inhibited both the endothelial-dependent oxidation of LDL as well as the increased adhesion of U937 cells, suggesting a coupling between these two processes. The results indicate that LDL, modified by exposure to monocytes or endothelial cells in the arterial wall, may increase the adhesive properties of the endothelium.