Mechanisms of enhanced macrophage apoE secretion by oxidized LDL

Abstract

Previous studies have demonstrated that atherosclerotic lesions contain apoE synthesized primarily by macrophages. As oxidized LDL has been implicated in the development of atherosclerosis, its effect on macrophage apoE synthesis and secretion was examined. Human monocytic leukemia cells, THP-1, and human monocyte-derived macrophages were exposed to various forms of oxidatively modified LDL for determination of their effect on apoE mRNA and protein levels. Extensively copper oxidized (Cu-oxidized) LDL resulted in a time- and concentration-dependent increase in apoE mRNA and protein as compared to other forms of oxidized LDL, i.e., LDL modified by soybean lipoxygenase (SLO), azoamidinopropane HCl (AAPH), and hypochlorite (HOCl). Consistent with these results, experiments using THP-1 cells transfected with the apoE promoter linked to a luciferase reporter gene indicated that Cu-oxidized LDL was the most potent stimulator of apoE transgene expression. Enhanced apoE expression due to Cu-oxidized LDL was shown to be due to cholesterol accumulation as well as additional factors. HPLC analysis of the various forms of modified LDL revealed that 7-ketocholesterol was the major oxysterol present in Cu-oxidized LDL. AAPH-oxidized LDL contained significantly less 7-ketocholesterol than Cu-oxidized LDL and virtually no 7-ketocholesterol was detected in SLO- or HOCl-oxidized LDL. Northern blot analysis indicated an increase in apoE mRNA in response to increasing concentrations of 7-ketocholesterol. These results elucidate a potential role of oxidized LDL, and specifically 7-ketocholesterol, in the stimulation of macrophage apoE secretion in atherosclerotic lesions.