Sensitization of pancreatic carcinoma cells for gamma-irradiation-induced apoptosis by XIAP inhibition.

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 gamma-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 gamma-irradiation. Interestingly, inhibition of caspases also reduced the cooperative effect of XIAP targeting and gamma-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 gamma-irradiation-induced apoptosis, whereas they had no effect on gamma-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.