Glutaredoxin (Grx) is an important regulator of redox signaling and a key enzyme of the glutathione- dependent antioxidant system, which protects cells against oxidative injury. Numerous studies provide evidence that macrophage injury and cell death are major factors in the development of atherosclerotic lesions. Although free radicals and oxidative stress play a central role in atherogenesis, the impact of Grx on macrophage injury and the development and progression of atherosclerotic lesions has not been studied. The mechanisms underlying macrophage injury and cell death in vivo are unclear. Electron microscopy studies indicate that in human atherosclerotic lesions, the predominant mode of cell death is not apoptosis but oncosis, which is associated with an intense inflammatory response and hence with atherosclerotic lesion progression. Recently we demonstrated that in human monocyte-derived macrophages oxidized LDL (OxLDL) promotes cell death in a caspase-independent process resembling oncosis. We went on to show that OxLDL-induced macrophage death involves mitochondrial dysfunction and is mediated by peroxyl radical formation but that ROS formation alone cannot explain OxLDL cytotoxicity. Our preliminary data now demonstrate that OxLDL also promotes the massive depletion of reduced glutathione and to a lesser extent, of glutathione disulfide. This depletive process results in the collapse of the glutathione/glutathione disulfide ratio, a condition that favors the formation of mixed disulfides between reactive protein thiols and glutathione in a reaction referred to as protein-S-glutathionylation. Our studies show that thiol oxidative stress and protein-S-glutathionylation induced by OxLDL promote macrophage death. siRNA-mediated inhibition of Grx, the enzyme responsible for the degluathionylation of proteins, potentiated OxLDL-induced macrophage injury, indicating that Grx is required to protect macrophages from OxLDL-induced cell death. Because macrophage death appears to be a major contributing factor to the development and progression of atherosclerotic lesions, increasing the macrophage's protection against pathophysiological protein-S- glutathionylation likely will not only prevent macrophage death but may also decrease the severity of atherosclerosis. The proposed studies will test this hypothesis and examine possible molecular mechanisms of Grx-mediated protection of macrophages from mitochondrial dysfunction and cell death.
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