Circulating Oxidized Low-density Lipoprotein (LDL) Induces Apoptosis and Stimulates Monocyte Chemoattractant Protein-1 Expression in Cultured Endothelial Cells

Circulating oxidized low-density lipoprotein (LDL) is a biochemical risk marker for cardiovascular disease in humans. Several groups have developed specific enzyme-linked immunosorbent assay systems to measure plasma levels of oxidized LDL. However, most of the studies did not address the issue whether circulating oxidized LDL that they measured could exert actually atherogenic properties on endothelial cells (ECs). In this study, we purified oxidized LDL from human plasma with a modified high performance liquid chromatographic (HPLC) method using a diethylaminoethyl-type anion-exchange gel column and then examined the effects of oxidized LDL on ECs. Our HPLC method separated LDL into three fractions, which were designated LDL-1, -2, and -3 by their elution order. LDL-3, the most strongly retained fraction identified as oxidized LDL not only induced apoptotic cell death, but also stimulated monocyte chemoattractant protein-1 production in cultured human umbilical vein endothelial cells, both of which were blocked by the treatment with an anti-oxidant N-acetylcysteine. The present results demonstrate that oxidized LDL present in human plasma is pro-apoptotic and pro-inflmmatory to ECs, thus suggesting the active participation of plasma oxidized LDL in the process of atherosclerosis. Our HPLC method for measuring plasma levels of oxidized LDL may be useful to evaluate the efficacy of novel therapeutics on atherosclerosis.