Ribotoxic stress sensitizes glioblastoma cells to death receptor-induced apoptosis: Requirements for c-Jun NH₂-terminal kinase and Bim

A prominent feature of glioblastoma is its resistance to death receptor-mediated apoptosis. In this study, we explored the possibility of modulating death receptor-induced cell death with the c-Jun-NH ₂-terminal kinase (JNK) activator anisomycin. Anisomycin activates JNK by inactivating the ribosome and inducing "ribotoxic stress." We found that anisomycin and death receptor ligand anti-Fas antibody CH-11 or tumor necrosis factor - related apoptosis-inducing ligand (TRAIL) synergistically induce apoptosis in multiple human glioblastoma cell lines. For example, in U87 cells, anisomycin reduced the IC₅₀ of CH-11 by more than 20-fold (from 500 to 25 ng/mL). Cell viability in response to anisomycin, CH-11, and their combination was 79%, 91%, and 28% (P < 0.001), respectively. Anisomycin and TRAIL were found to be similarly synergistic in glioblastoma cells maintained as tumor xenografts. The potentiation of death receptor - dependent cell death by anisomycin was specific because emetine, another ribosome inhibitor that does not induce ribotoxic stress or activate JNK, did not have a similar effect. Synergistic cell death was predominantly apoptotic involving both extrinsic and intrinsic pathways. Expression of Fas, FasL, FLIP, and Fas-associated death domain (FADD) was not changed following treatment with anisomycin + CH-11. JNK was activated 10- to 22-fold by anisomycin + CH-11 in U87 cells. Inhibiting JNK activation with pharmacologic inhibitors of JNKK and JNK or with dominant negative mitogen-activated protein kinase (MAPK) kinase kinase 2 (MEKK2) significantly prevented cell death induced by the combination of anisomycin + CH-11. We further found that anisomycin + CH-11 up-regulated the proapoptotic protein Bim by ∼ 14-fold. Simultaneously inhibiting Bim expression and JNK activation additively desensitized U87 cells to anisomycin + CH-11. These findings show that anisomycin-induced ribotoxic stress sensitizes glioblastoma cells to death receptor-induced apoptosis via a specific mechanism requiring both JNK activation and Bim induction.