Catalase-overexpressing thymocytes are resistant to glucocorticoid-induced apoptosis and exhibit increased net tumor growth.

Glucocorticoids are used for the treatment of lymphoid neoplasms, taking advantage of the well-known ability of these compounds to cause apoptosis in lymphoid tissues. Previously, we have shown that dexamethasone, a synthetic glucocorticoid, causes a down-regulation of several antioxidant defense enzymes and proteins, including catalase and thioredoxin, concomitant with the induction of apoptosis in WEHI7.2 mouse thymoma cells. To test whether this down-regulation plays a critical role in the mechanism of steroid-induced apoptosis, WEHI7.2 cells were transfected with rat catalase. Two clones, expressing 1.4-fold and 2.0-fold higher catalase specific activity, respectively, when compared with vectoronly transfectants were selected for further study. An increase to 1.4-fold parental cell catalase activity delayed cell loss after dexamethasone treatment, whereas a 2.0-fold parental catalase activity prevented dexamethasone-induced cell loss for 48 h after treatment. Dexamethasone treatment of the WEHI7.2 cells stimulated a release of cytochrome c into the cytosol. Catalase-overexpressing cells showed a delay or lack of cytochrome c release from the mitochondria, which correlated temporally with the delay or prevention of cell loss in the culture after dexamethasone treatment. A decreased amount of cell death from WEHI7.2 cells overexpressing catalase was also seen in tumor xenografts in severe combined immunodeficient mice when compared with tumors from vector-only transfected cells. Similarly, thioredoxin-overexpressing WEHI7.2 cells, shown previously to be apoptosis resistant, showed decreased cell death in tumor xenografts. This resulted in larger tumors from cells overexpressing these proteins. Cell death in control transfectant tumor xenografts was primarily attributable to apoptosis. In contrast, the cell death we observed in tumors from thioredoxin- or catalase-overexpressing cells had a higher frequency of a nonapoptotic, nonnecrotic type of cell death termed para-apoptosis. These data suggest that: (a) oxidative stress plays a critical role in steroid-induced apoptosis prior to the commitment of the cells to undergo apoptosis; and (b) resistance to oxidative stress can contribute to tumor growth.