s associated with GSH efflux in the retina is scarce. Differences in mRNA expression of MRPs in different RPE cell lines was reported. However, the role of efflux transporters, particularly MRP1 in GSH regulation in RPE cells under unstressed and stressed conditions has not been studied so far. MRP1-Mediated GSH Efflux in RPE Cells a-Crystallins have been found in many INK1117 non-lenticular tissues including the retina. aA and aB crystallin both serve a cell protection function and a chaperone function. In lens epithelial cells, a-crystallins are anti-apoptotic against UVA-irradiation and tumor necrosis factor-a stimulation. a-Crystallins also function as chaperones by preventing aggregation and pathologic protein misfolding. Overexpression of either human HSP27 or aB crystallin resulted in increased total GSH levels and decreased basal levels of intracellular reactive oxygen species . Our laboratory has investigated the role of a-crystallins in RPE cell physiology and their regulation by oxidative stress. Lack of a-crystallins rendered RPE cells more susceptible to apoptosis caused by oxidative stress. Overexpression of aA or aB crystallin had similar degrees of protection in lenticular as well as non-lenticular cells. We showed that RPE cells lacking either aA or aB crystallin are equally susceptible to H2O2induced oxidant insult. Recently, we discovered that aB crystallin is secreted from RPE cells in exosomes, and exogenous aB crystallin protected RPE cells from oxidative stress-induced apoptosis. The link between the protective function of a-crystallin and cellular antioxidant status is not well understood. Both GSH and redoxins are major factors with critical redox functions in RPE cells. GSH levels are elevated in a-crystallin overexpressing human lens epithelial cells. However, the nature and mechanism of GSH participation in the a-crystallin-mediated antiapoptotic function of RPE cells has not been studied. Here, we investigated the 23416332” relationship between GSH, redoxins and the antiapoptotic function of 
a- crystallins in RPE. For the first time, we provide evidence that MRP1 plays a key role in maintaining cellular thiol homeostasis by regulating GSH efflux in RPE. although higher ” than controls, was less than that of the mitochondrial fraction. Overall, these results suggest the significance of GSH and its biosynthetic enzymes in protection against oxidant stress in ARPE-19 cells overexpressing a-crystallins. Thiol status of the retina is compromised in a-crystallin KO retina Retinas from a-crystallin KO mice are highly susceptible to cobalt chloride-induced oxidative stress. We studied the changes in thiol status in mouse retina lacking aA-or aB crystallin. Total GSH levels in the neural retina and choroid/ RPE complex of the a-crystallin KO and WT controls were determined. Under unstressed conditions, GSH levels were about 50% lower in aA crystallin and aB crystallin KO RPE/choroid complex while corresponding neural retina showed a 30% and 50% decrease in GSH, respectively. No significant changes in the levels of the catalytic or modifier subunits of GCL in aA and aB crystallin KO mice were found either at the mRNA or the protein level of samples from the posterior eye cups. Additionally, we determined the effect of a-crystallin KO on the expression of thioredoxins and glutaredoxins in the retina and in the RPE cells. Trx1, Trx2, Grx1 and Grx2 mRNAs were significantly downregulated in a-crystallin KO retina. Similarly, Trx1, Trx2, and
