Macrophages require both iron and copper to oxidize low-density lipoprotein in Hanks' balanced salt solution

The oxidation of low-density lipoprotein (LDL) may be important in the pathogenesis of atherosclerosis. However, the interactions between cells and metals in promoting LDL oxidation are inadequately understood. A sensitive high-performance liquid chromatography analysis of cholesterol, cholesteryl esters, and their oxidation products was used to identify and accurately measure LDL oxidation achieved in thiol-free Hanks' balanced salt solution (HBSS) at pH 7.4. Mouse peritoneal macrophages inhibited LDL oxidation when incubated in HBSS containing either 10 microM iron or 1 microM copper, but were markedly prooxidant in the presence of both metals. The prooxidant effect of macrophages in the presence of both iron and copper did not require the provision of added disulfides or thiols. Both Fe2+ and macrophages were demonstrated to independently reduce Cu2+ to Cu1+ in HBSS, indicating that the direct reduction of copper by cells or iron may underlie the observed promotion of LDL oxidation by macrophages in this system. We conclude that macrophages can either promote or inhibit metal-mediated LDL oxidation and that externally supplied thiols are not essential to the promotion of LDL oxidation by cells. The presence of both iron and copper may be particularly important for macrophages to promote LDL oxidation in vivo.