MECHANISMS OF ENHANCED MACROPHAGE APOE SECRETION BY OXIDIZED LDL

Previous studies have demonstrated that atherosclerotic lesions contai n apoE synthesized primarily by macrophages. As oxidized LDL has been implicated in the development of atherosclerosis, its effect on macrop hage apoE synthesis and secretion was examined. Human monocytic leukem ia cells, THP-1, and human monocyte-derived macrophages were exposed t o 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 incre ase in apoE mRNA and protein as compared to other forms of oxidized LD L, i.e., LDL modified by soybean lipoxygenase (SLO), azoamidinopropane HCl (AAPH), and hypochlorite (HOCl). Consistent with these results, e xperiments 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 exp ression due to Cu-oxidized LDL was shown to be due to cholesterol accu mulation as well as additional factors. HPLC analysis of the various f orms of modified LDL revealed that 7-ketocholesterol was the major oxy sterol present in Cu-oxidized LDL. AAPH-oxidized LDL contained signifi cantly less 7-ketocholesterol than Cu-oxidized LDL and virtually no 7- ketocholesterol was detected in SLO- or HOCl-oxidized LDL. Northern bl ot analysis indicated an increase in apoE mRNA in response to increasi ng concentrations of 7-ketocholesterol. These results elucidate a pote ntial role of oxidized LDL, and specifically 7-ketocholesterol, in the stimulation of macrophage apoE secretion in atherosclerotic lesions.