e ability of cells to undergo apoptosis. Thus, experimentally increasing intracellular GSH decreases apoptosis while cells with lower GSH are more susceptible to apoptotic stimuli. Intracellular GSH levels are regulated by three major ways during oxidant injury: by inducing enzymatic synthesis of GSH via upregulation of GCLC, by the action of GR, which rapidly converts GSSG to GSH using NADPH as a substrate, and by cellular transport of GSH. Our data indicate that the extracellular GSH transport mediated by MRP1 in response to oxidative injury may predispose RPE cells to caspase-mediated apoptosis given the known role of MRP1 in GSH and GSSG release. Our study shows that GSSG levels were also increased in MRP1 silenced RPE cells and oxidative injury further increased GSSG by 4 fold. However, MRP1 silencing allows RPE cells to A-83-01 manufacturer maintain their intracellular redox potential by upregulating GR activity which rapidly converts the toxic GSSG to GSH and may enhance cell survival. Similar findings were reported in human aortic endothelial cells where MRP1 inhibition prevented the decline in intracellular GSH, and reduced apoptosis caused by oscillatory shear ” by increasing GR activity. Inhibition of MRP1 increased cellular GSH levels and reduced intracellular ROS and prevented angiotensin-induced apoptosis in endothelial progenitor cells. In addition, in vivo studies show that the rate of apoptosis was significantly reduced in MRP1 KO mice and improved re-endothelialization after carotid artery injury. Thus, multiple mechanisms may be operative in MRP1-inhibited cells that are more resistant to apoptosis. On the other hand, we found that MRP1 overexpressing RPE cells release more GSH under unstressed and stressed conditions, further confirming the role “
19648907“of MRP1 as an effective GSH transporter. ” Because of the increased GSH release, steady state intracellular GSH levels are significantly lower in MRP1 MRP1-Mediated GSH Efflux in RPE Cells overexpressing cells. Our study demonstrated that under milder conditions of oxidative stress RPE cells remain viable and GSH release in MRP1 overexpressing cells was increased without affecting intracellular GSH levels, presumably because GSH biosynthesis was stimulated by a feedback mechanism. However, prolonged treatment with H2O2 significantly increased the percentage of apoptotic cells and caspase activation in MRP1 overexpressing cells compared to control cells. It is well known that treatment with peroxides depletes GSH levels in RPE cells leading to apoptosis. Thus, enhanced GSH release and depletion of intracellular GSH are important for the progression of apoptosis, and this phenomenon is applicable to MRP1 overexpressing cells with prolonged H2O2 exposure where the levels of cellular GSH is reduced by 62% and efflux increased by 1.8 fold. In support, similar results were reported in V79 Chinese hamster cells overexpressing MRP1 which did not show increased resistance to multiple stressors. Similarly, treatment of MRP1 overexpressing BHK-21 cells with either verapamil or its derivative rapidly depleted intracellular GSH content with a strong decrease occurring during the first hour of treatment, followed by apoptosis. The overexpression of MRP1 in HeLa 
cells while contributing to cell death by oxidative stress through enhanced GSH efflux also prevents intracellular GSSG accumulation. Thus the cell death observed in MRP1 overexpressing cells can be attributed to accumulation of ROS from GSH depletion. Howe