Pancreatic cancer is one of the leading causes of cancer-related death due to its resistance towards conventional therapies. Therefore, a better understanding of the molecular mechanisms underlying apoptosis resistance of pancreatic cancer is crucial to improve cancer therapy. Here, we identify X-linked inhibitor of apoptosis (XIAP) as a key determinant of apoptosis resistance of pancreatic carcinoma. XIAP was expressed at high levels in primary pancreatic carcinoma samples and the majority of pancreatic carcinoma cell lines in sharp contrast to absent or low XIAP expression in normal pancreatic tissue. Stable downregulation of XIAP by RNA interference (RNAi) strongly sensitized pancreatic carcinoma cells for TRAIL-induced apoptosis and significantly reduced cell viability. Similarly, downregulation of XIAP markedly sensitized pancreatic carcinoma cells for CD95- or gamma-irradiation-induced apoptosis. Importantly, knockdown of XIAP also inhibited clonogenicity of pancreatic cancer cells treated with TRAIL indicating that XIAP promotes clonogenic survival. Analysis of apoptosis signaling pathways revealed that knockdown of XIAP resulted in enhanced cleavage and enzymatic activity of caspase-3, -9, -2 and -8. Interestingly, downregulation of XIAP also promoted cleavage of Bid, the conformational change of Bax, drop of mitochondrial membrane potential and increased cytochrome c release after stimulation with TRAIL in a caspase-dependent manner, indicating that XIAP also controls a mitochondrial feedback amplification loop that is regulated by effector caspases. In support of this notion, inhibition of caspase-3 blocked Bid cleavage, the conformational change of Bax, drop of mitochondrial membrane potential and cytochrome c release in XIAP knockdown cells upon TRAIL treatment. Most importantly, knockdown of XIAP markedly enhanced TRAIL-induced caspase-3 activation and apoptosis and profoundly inhibited tumor growth and invasion in an in vivo model of pancreatic carcinoma. Notably, knockdown of XIAP also abolished the enhanced invasive growth of pancreatic carcinoma in vivo that was observed upon treatment with TRAIL alone. Similar to XIAP knockdown by RNAi, inhibition of XIAP by small molecule antagonists sensitized pancreatic cancer cells to TRAIL-, CD95- or gamma-irradiation-induced apoptosis. By demonstrating that targeting XIAP significantly enhanced death receptor or gamma-irradiation-induced apoptosis and also suppressed tumor growth and invasion of pancreatic carcinoma cells in vivo, our findings have important clinical implications. Thus, targeting XIAP represents a novel, promising strategy to overcome apoptosis resistance in pancreatic cancer that warrants further investigation.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA