Glutathione regulation of neutral sphingomyelinase in tumor necrosis factor-α-induced cell death

Tumor necrosis factor-α (TNFα)-induced cell death involves a diverse array of mediators and regulators including proteases, reactive oxygen species, the sphingolipid ceramide, and Bcl-2. It is not known, however, if and how these components are connected. We have previously reported that GSH inhibits, in vitro, the neutral magnesium-dependent sphingomyelinase (N- SMase) from Molt-4 leukemia cells. In this study, GSH was found to reversibly inhibit the N-SMase from human mammary carcinoma MCF7 cells. Treatment of MCF7 cells with TNFα induced a marked decrease in the level of cellular GSH, which was accompanied by hydrolysis of sphingomyelin and generation of ceramide. Pretreatment of cells with GSH, GSH-methylester, or N- acetylcysteine, a precursor of GSH biosynthesis, inhibited the TNFα-induced sphingomyelin hydrolysis and ceramide generation as well as cell death. Furthermore, no significant changes in GSH levels were observed in MCF7 cells treated with either bacterial SMase or ceramide, and GSH did not protect cells from death induced by ceramide. Taken together, these results show that GSH depletion occurs upstream of activation of N-SMase in the TNFα signaling pathway. TNFα has been shown to activate at least two groups of caspases involved in the initiation and 'execution' phases of apoptosis. Therefore, additional studies were conducted to determine the relationship of GSH and the death proteases. Evidence is provided to demonstrate that depletion of GSH is dependent on activity of interleukin-1β-converting enzyme-like proteases but is upstream of the site of action of Bcl-2 and of the execution phase caspases. Taken together, these studies demonstrate a critical role for GSH in TNFα action and in connecting major components in the pathways leading to cell death.