Hydrogen peroxide signaling is required for glucocorticoid-induced apoptosis in lymphoma cells

Glucocorticoid-induced apoptosis is exploited clinically for the treatment of hematologic malignancies. Determining the required molecular events for glucocorticoid-induced apoptosis will identify resistance mechanisms and suggest strategies for overcoming resistance. In this study, we found that glucocorticoid treatment of WEHI7.2 murine thymic lymphoma cells increased the steady-state [H ₂O ₂] and oxidized the intracellular redox environment before cytochrome c release. Removal of glucocorticoids after the H ₂O ₂ increase resulted in a 30% clonogenicity; treatment with PEG-CAT increased clonogenicity to 65%. Human leukemia cell lines also showed increased H ₂O ₂ in response to glucocorticoids and attenuated apoptosis after PEG-CAT treatment. WEHI7.2 cells that overexpress catalase (CAT2, CAT38) or were selected for resistance to H ₂O ₂ (200R) removed enough of the H ₂O ₂ generated by glucocorticoids to prevent oxidation of the intracellular redox environment. CAT2, CAT38, and 200R cells showed a 90-100% clonogenicity. The resistant cells maintained pERK survival signaling in response to glucocorticoids, whereas the sensitive cells did not. Treating the resistant cells with a MEK inhibitor sensitized them to glucocorticoids. These data indicate that: (1) an increase in H ₂O ₂ is necessary for glucocorticoid-induced apoptosis in lymphoid cells, (2) increased H ₂O ₂ removal causes glucocorticoid resistance, and (3) MEK inhibition can sensitize oxidative stress-resistant cells to glucocorticoids.