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-kB in g-irradiation-induced apoptosis of glioblastoma cells by showing, for the first time, that NF-kB is critically required for Smac mimetic-mediated radiosensitization. BV6 significantly increases g-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 g-irradiation is highly synergistic (CI < 0.3). Molecular studies show that BV6 stimulates NF-kB activation, which is critical for radiosensitization, because genetic inhibition of NF-kB by overexpression of the dominant-negative superrepressor IkBa-SR significantly decreases BV6-and g-irradiation-induced apoptosis. Also, the BV6-mediated enhancement of g-irradiation-triggered caspase activation, drop of mitochondrial membrane potential, and cytochrome c release is abolished in cells overexpressing IkBa-SR. Similarly, NF-kB inhibition by ectopic expression of a kinase dead mutant of IKKb prevents the BV6-mediated sensitization for g-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 g-irradiation. In conclusion, we identify NF-kB 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. Ó2011 AACR.

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Resistance of pancreatic cancer to current treatments including radiotherapy remains a major challenge in oncology and may be caused by defects in apoptosis programs. Since 'inhibitor of apoptosis proteins' (IAPs) block apoptosis at the core of the apoptotic machinery by inhibiting caspases, therapeutic modulation of IAPs could tackle a key resistance mechanism. Here, we report that targeting X-linked inhibitor of apoptosis (XIAP) by RNA-interference-mediated knockdown or overexpression of second mitochondria-derived activator of caspase significantly enhanced apoptosis and markedly reduced clonogenic growth of pancreatic carcinoma cells upon c-irradiation. Analysis of signaling pathways revealed that antagonizing XIAP increased activation of caspase-2, -3, -8 and -9 and loss of mitochondrial membrane potential upon c-irradiation. Interestingly, inhibition of caspases also reduced the cooperative effect of XIAP targeting and c-irradiation to trigger mitochondrial perturbations, suggesting that XIAP controls a feedback mitochondrial amplification loop by regulating caspase activity. Importantly, our data demonstrate for the first time that small molecule XIAP inhibitors sensitized pancreatic carcinoma cells for c-irradiation-induced apoptosis, whereas they had no effect on c-irradiation-mediated apoptosis of non-malignant fibroblasts indicating some tumor specificity. In conclusion, targeting XIAP, for example by small molecules, is a promising novel approach to enhance radiosensitivity of pancreatic cancer that warrants further investigation.

BackgroundJunctional adhesion molecule-A (JAM-A) is an adhesion molecule whose overexpression on breast tumor tissue has been associated with aggressive cancer phenotypes, including human epidermal growth factor receptor-2 (HER2)-positive disease. Since JAM-A has been described to regulate HER2 expression in breast cancer cells, we hypothesized that JAM-dependent stabilization of HER2 could participate in resistance to HER2-targeted therapies.MethodsUsing breast cancer cell line models resistant to anti-HER2 drugs, we investigated JAM-A expression and the effect of JAM-A silencing on biochemical/functional parameters. We also tested whether altered JAM-A expression/processing underpinned differences between drug-sensitive and -resistant cells and acted as a biomarker of patients who developed resistance to HER2-targeted therapies.ResultsSilencing JAM-A enhanced the anti-proliferative effects of anti-HER2 treatments in trastuzumab- and lapatinib-resistant breast cancer cells and further reduced HER2 protein expression and Akt phosphorylation in drug-treated cells. Increased epidermal growth factor receptor expression observed in drug-resistant models was normalized upon JAM-A silencing. JAM-A was highly expressed in all of a small cohort of HER2-positive patients whose disease recurred following anti-HER2 therapy. High JAM-A expression also correlated with metastatic disease at the time of diagnosis in another patient cohort resistant to trastuzumab therapy. Importantly, cleavage of JAM-A was increased in drug-resistant cell lines in conjunction with increased expression of ADAM-10 and -17 metalloproteases. Pharmacological inhibition or genetic silencing studies suggested a particular role for ADAM-10 in reducing JAM-A cleavage and partially re-sensitizing drug-resistant cells to the anti-proliferative effects of HER2-targeted drugs. Functionally, recombinant cleaved JAM-A enhanced breast cancer cell invasion in vitro and both invasion and proliferation in a semi-in vivo model. Finally, cleaved JAM-A was detectable in the serum of a small cohort of HER2-positive patients and correlated significantly with resistance to HER2-targeted therapy.ConclusionsCollectively, our data suggest a novel model whereby increased expression and cleavage of JAM-A drive tumorigenic behavior and act as a biomarker and potential therapeutic target for resistance to HER2-targeted therapies.Electronic supplementary materialThe online version of this article (10.1186/s13058-018-1064-1) contains supplementary material, which is available to authorized users.