NF-κB Is Required for Smac Mimetic-Mediated Sensitization of Glioblastoma Cells for γ-Irradiation-Induced Apoptosis

Evasion of apoptosis contributes to radioresistance of glioblastoma, calling for novel strategies to overcome apoptosis resistance. In this study, we investigated the potential of the small molecule Smac mimetic BV6 to modulate radiosensitivity of glioblastoma cells. Here, we identify a novel proapoptotic function of NF-kappa B in gamma-irradiation-induced apoptosis of glioblastoma cells by showing, for the first time, that NF-kappa B is critically required for Smac mimetic-mediated radiosensitization. BV6 significantly increases gamma-irradiation-triggered apoptosis in several glioblastoma cell lines in a dose- and time-dependent manner. Calculation of combination index (CI) reveals that the interaction of BV6 and gamma-irradiation is highly synergistic (CI < 0.3). Molecular studies show that BV6 stimulates NF-kappa B activation, which is critical for radiosensitization, because genetic inhibition of NF-kappa B by overexpression of the dominant-negative superrepressor I kappa B alpha-SR significantly decreases BV6-and gamma-irradiation-induced apoptosis. Also, the BV6-mediated enhancement of gamma-irradiation-triggered caspase activation, drop of mitochondrial membrane potential, and cytochrome c release is abolished in cells overexpressing I kappa B alpha-SR. Similarly, NF-kappa B inhibition by ectopic expression of a kinase dead mutant of IKK beta prevents the BV6-mediated sensitization for gamma-irradiation. The clinical relevance is underscored by experiments with primary tumor samples showing that BV6 sensitizes primary cultured glioma cells as well as glioblastoma-initiating cancer stem cells derived from surgical specimens for gamma-irradiation. In conclusion, we identify NF-kappa B as a critical mediator of Smac mimetic-conferred radiosensitization of glioblastoma cells. These results have important implications for the development of Smac mimetic-based combination protocols for radiosensitization of glioblastoma. Mol Cancer Ther; 10(10); 1867-75. (C) 2011 AACR.