| Reduced glutathione (GSH) is the major regulator of the cellular thiol redox status, and plays a pivotal role in a number of biochemical pathways and cellular functions. Recent studies indicate that GSH is involved in the activation and progression of apoptosis, although its precise roles are not well defined. Interestingly, GSH is exported at relatively high rates from cells undergoing apoptosis; however, the physiological significance and mechanism behind this release are unknown. The present studies addressed these questions by examining death receptor apoptosis in three human cell lines. The cells were analyzed for apoptotic characteristics, GSH release, and for the presence of GSH transporters, namely the multidrug resistance-associated proteins (MRPs). In HepG2 and Jurkat T cells, GSH was released early during apoptosis. Organic anion transport inhibitors prevented GSH release during apoptosis and delayed cell death, indicating that GSH release occurs through a specific transport mechanism. These same compounds are also known to inhibit the MRP transport proteins, which are expressed by these cell lines, suggesting that MRP proteins are involved in apoptotic GSH release. Interestingly, the organic anion-transport inhibitors also prevented phosphatidylserine externalization during apoptosis in the Jurkat T cells. In Raji cells, a cell line deficient in phosphatidylserine externalization, GSH release was not observed during death receptor apoptosis, yet these cells displayed other characteristics of apoptosis. Taken together, these results indicate that GSH release is a specific transport event, one that may involve the MRP proteins, and that this GSH release allows the activation of additional events in apoptosis. The results also indicate that phosphatidylserine externalization and GSH release are interdependent, and may be occurring through a similar mechanism. |
