Dexamethasone Inhibits TRAIL-Induced Apoptosis through c-FLIP(L) Upregulation and DR5 Downregulation by GSK3β Activation in Cancer Cells

Dexamethasone (DEX), a synthetic glucocorticoid, is commonly used as immunosuppressive and chemotherapeutic agent. This study was undertaken to investigate the effects of DEX on the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in cancer cells. We found that upregulation of c-FLIP(L) and downregulation of death receptor 5 (DR5; receptor for TRAIL ligand) contribute to the anti-apoptotic effect of DEX on TRAIL-induced apoptosis. DEX increased c-FLIP(L) expression at the transcriptional levels through the GSK-3β signaling pathway. The pharmacological inhibitor and catalytic mutant of GSK-3β suppressed DEX-induced upregulation of c-FLIP(L) expression. Furthermore, GSK-3β specific inhibitor markedly abolished DEX-mediated reduction of TRAIL-induced apoptosis in human renal cancer cells (Caki-1 and A498), human lung cancer cells (A549), and human breast cancer cells (MDA-MB361). In addition, DEX decreased protein stability of DR5 via GSK-3β-mediated upregulation of Cbl, an E3 ligase of DR5. Knockdown of Cbl by siRNA markedly inhibited DEX-induced DR5 downregulation. Taken together, these results suggest that DEX inhibits TRAIL-mediated apoptosis via GSK-3β-mediated DR5 downregulation and c-FLIP(L) upregulation in cancer cells.