2541 Background: TRAP1 has been recently described as a component of a mitochondrial pathway selectively up- regulated in tumor cells which antagonizes the proapoptotic activity of cyclophilin D and is responsible for maintenance of mitochondria integrity, thus favoring cell survival. Interestingly, novel TRAP1 antagonists cause sudden collapse of mitochondrial function and selective tumor cell death, suggesting that this pathway may represent a novel molecular target to improve anticancer therapy. Furthermore, we recently observed that TRAP1 is significantly up-regulated in osteosarcoma cells adapted to growth in mild oxidizing conditions and this correlates with a phenotype more resistant to H2O2-induced DNA damage and apoptosis. This evidence prompted us to investigate the role of TRAP1 as being responsible for multi-drug resistance in human colorectal cancer. Methods and Results: The evaluation of TRAP1 expression by immunoblot and quantitative RT-PCR analysis in a series of 26 human colorectal carcinomas allowed us to observe up-regulation in 17/26 tumors. Accordingly, TRAP1 protein levels were increased in HT-29 colorectal carcinoma cells resistant to 5-fluorouracil, oxaliplatin and irinotecan. Interestingly, HT-29 colorectal carcinoma and Saos-2 osteosarcoma cells transfected with TRAP1 gene exhibited a phenotype resistant to 5-fluorouracil-, oxaliplatin- and irinotecan-induced apoptosis, evaluated by MTT incorporation analysis and annexin V staining. Consistently, the transfection of a dominant negative N-terminal deletion mutant of TRAP1 as well as the inhibition of TRAP1 activity by the TRAP1 ATPase antagonist, shepherdin, increased the sensitivity to apoptosis induced by 5-fluorouracil, oxaliplatin and irinotecan in wild type HT-29 colorectal carcinoma cells and in HT-29 cells resistant to the single agents. Conclusions: These results suggest that the increased expression of TRAP1 in human colorectal carcinomas could be part of a pro-survival pathway responsible for multidrug resistance. Thus, targeting TRAP1 may represent a novel strategy to improve the efficacy of anticancer agents in colorectal cancer. No significant financial relationships to disclose.