Prooxidant and antioxidant activities of macrophages in metal-mediated LDL oxidation: The importance of metal sequestration

Murine macrophages incubated in metal-supplemented RPMI could block or promote oxidation of low-density lipoprotein (LDL) depending on the degree of metal supplementation. Only at high concentrations of Cu (1 μmol/L) and Fe (30 μmol/L) were cells prooxidant, leading to an accelerated rate of LDL oxidation over that measured in comparable cell-free media. At lower concentrations of Cu and Fe in RPMI, LDL oxidation in the presence of macrophages was inhibited relative to the cell-free condition. This appeared to be dependent on a stable modification of the culture medium, because preconditioning of media by incubation with macrophages could also decrease their capacity to sustain subsequent cell-free LDL oxidation. This was due, in part, to a removal of metal from the media during preconditioning. However, resupplementation of media with metals did not fully restore oxidative capacity, indicating that other cell-dependent antioxidant modifications occurred. This did not involve significant alterations to the thiol content of the media. This study highlights the complexity of the role that cells such as macrophages have with regards to LDL oxidation in vitro and demonstrate that there are both antioxidative and prooxidative components.