Oxidation of low-density lipoprotein and macrophage derived foam cells

Copper-oxidized LDL has many of the characteristics of the modified LDL generated in the artery wall during the initial stages of atherosclerosis. It is not, however, a chemically defined species but shows significant variations in both its chemical composition and behaviour in biological systems depending upon the extent to which the peroxidation reaction has occurred (Fig. 1). Taking care to define the extent of LDL modification we have used this form of oxidized LDL to investigate the effects on the macrophage of this potentially toxic particle. This cell, in contrast to endothelial cells, appears to be particularly well adapted to detoxify lipid peroxidation products since it possesses glutathione peroxidases capable of metabolizing oxidized LDL and responds to oxidized LDL by increasing its GSH content. Acetylated LDL had little or no effect on GSH levels showing that lipid loading per se or recognition by the macrophage scavenger receptor is not sufficient to induce the synthesis of this antioxidant. We have confirmed the observation that oxidized LDL does not activate expression of the gene for TNF and raise the possibility that PGE2 produced by the cells and possibly during the oxidation of LDL may be the mediator suppressing the synthesis of this cytokine. Our results support the hypothesis that the lipid-laden macrophage does not contribute to an inflammatory response in the artery wall and imply a protective role for the macrophage in scavenging oxidized LDL.