Radiosensitization of Human Cancers by Smac Agonists.

Resistance to current treatment regimens such as radiation therapy remains a major concern in oncology and may be caused by defects in apoptosis programs. Since “Inhibitor of apoptosis proteins” (IAPs), which are expressed at high levels in many tumors, block apoptosis at the core of the apoptotic machinery by inhibiting caspases, therapeutic modulation of IAPs could target a key control point in resistance. Here, we report for the first time that full length or mature Smac, an inhibitor of IAPs, significantly enhanced γ-irradiation-induced apoptosis and reduced clonogenic survival in neuroblastoma, glioblastoma or pancreatic carcinoma cells. Notably, Smac had no impact on DNA damage/DNA repair, activation of NF-κB, upregulation of p53 and p21 proteins or cell cycle arrest following γ-irradiation indicating that Smac did not alter the initial damage and/or cellular stress response. Smac enhanced activation of caspase-2, -3, -8 and -9, loss of mitochondrial membrane potential and cytochrome c release upon γ-irradiation. Inhibition of caspases also blocked γ-irradiation-induced mitochondrial perturbations, indicating that Smac facilitated caspase activation, which in turn triggered a mitochondrial amplification loop. Interestingly, mitochondrial perturbations were completely blocked by the broad range caspase inhibitor zVAD.fmk or the relatively selective caspase-2 inhibitor zVDVAD.fmk, whereas caspase-8 or caspase-3 inhibitors only inhibited the increased drop of mitochondrial membrane potential provided by Smac, suggesting that caspase-2 was acting upstream of mitochondria upon γ-irradiation. In conclusion, our findings provide evidence that targeting IAPs, e.g. by Smac agonists, is a promising strategy to enhance radiosensitivity in human cancers.