Molecular mechanism of glutathione-mediated protection from oxidized low-density lipoprotein-induced cell injury in human macrophages: Role of glutathione reductase and glutaredoxin

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Macrophage death is a hallmark of advanced atherosclerotic plaque, and oxidized low-density lipoprotein (OxLDL) found in these lesions is believed to contribute to macrophage injury. However, the underlying mechanisms of this phenomenon are only poorly understood. Here we show that in human monocyte-derived macrophages, OxLDL depleted intracellular glutathione (GSH) and inhibited glutathione reductase, resulting in a marked diminution of the glutathione/glutathione disulfide ratio. In the absence of OxLDL, an 80% depletion of intracellular GSH levels did not affect cell viability, but glutathione depletion dramatically increased OxLDL-induced cell death. Conversely, supplementation of intracellular GSH stores with glutathione diethyl ester substantially diminished OxLDL toxicity. OxLDL also promoted protein- S-glutathionylation, which was increased in macrophages pretreated with the glutathione reductase inhibitor BCNU. Knockdown experiments with siRNA directed against glutathione reductase and glutaredoxin showed that both enzymes are essential for the protection of macrophages against OxLDL. Finally, the peroxyl-radical scavenger Trolox did not prevent GSH depletion but completely blocked OxLDL-induced protein- S-glutathionylation and cell death. These data suggest that OxLDL promotes ROS formation and protein- S-glutathionylation by a mechanism independent from its effect on GSH depletion. Neither mechanism was sufficient to induce macrophage injury, but when stimulated concurrently, these pathways promoted the accumulation of protein-glutathione mixed disulfides and cell death.