Cellular oxidation of low density lipoprotein is caused by thiol production in media containing transition metal ions

Abstract

The oxidation of low density lipoprotein (LDL) may be important in atherosclerosis. LDL can be oxidized by cultured cells, including macrophages and endothelial cells. This cellular oxidation is dependent on transition metal ions in the medium. We now report that LDL oxidation by endothelial cells and macrophages is caused by cell-dependent appearance of thiol in the medium ("thiol production"). Thiol appeared in medium when cells were incubated under standard serum-free culture conditions. L-Cystine in the medium was required for thiol production and also for LDL oxidation. Cell-dependent appearance of thiol was inhibited by glutamate (which blocks cystine uptake) and by diethylmaleate (which reacts with thiols). Both compounds also blocked cellular LDL oxidation, even though neither compound had antioxidant activity. Finally, we designed an enzymatic system, based on glutathione reductase, that mimicked cellular thiol production. This enzymatic system caused LDL oxidation, and showed the same dependency for transition metal ions as did cellular LDL oxidation. We conclude that in media containing transition metal ions, cellular oxidation of LDL can be explained by the cell-dependent appearance of thiol in the medium. A very similar mechanism was proposed in 1987 by Heinecke et al. (J. Biol. Chem. 262: 10098-10103). Under other conditions, however, cellular oxidation of LDL may occur by other mechanisms.