Effect of ceramide on intracellular glutathione determines apoptotic or necrotic cell death of JB6 tumor cells

Selective induction of cell death is a means to remove unwanted cell populations from a tissue or organ. Understanding the signaling events responsible for mediating cell death by cytokines, such as tumor necrosis factor-alpha (TNFα) are key to the development of pharmacologic inducers of this response. Ceramide has been implicated as a secondary messenger for TNFα-induced cell death, but many of the intracellular effects of ceramide are not fully understood. Recent reports suggest that ceramide signaling may involve oxidative stress. To explore the relationship between TNF sensitivity and ceramide signaling, two genetic variants of mouse JB6 RT101 epidermal tumor cells, one resistant and one sensitive to TNFα- induced cytotoxicity, were treated with C₂-ceramide. Treatment with 20 μM ceramide induced apoptosis and this was quickly followed by oncotic necrosis in the TNFα- sensitive JB6 (TNFs) cells. The same concentration of ceramide induced apoptosis, but not oncotic necrosis of the TNFα resistant JB6 (TNFr) cells. The basal level of glutathione was significantly higher in TNFr cells than in TNFs cells. Treatment with 20 μM ceramide decreased cellular glutathione in TNFs cells by 50%, in contrast to an insignificant decrease in the TNFr cells. A significant increase in reactive oxygen was noted in TNFs cells treated with 10 or 20 μM ceramide. Furthermore, pretreatment with the antioxidant N-acetylcysteine or with glutathione monoethylester delayed the onset of ceramide-induced oncotic necrosis, but did not inhibit apoptosis. Our results suggest that the severity of the decrease in glutathione appears to determine whether cells undergo just apoptosis or also oncotic necrosis. They also suggest that ceramide-induced oncotic necrosis is modulated by a decline in cellular glutathione and an elevation of reactive oxygen. These results suggest that a decrease in cellular redox potential determines susceptibility to ceramide-dependent killing pathways.