Radioresistance markedly impairs the efficacy of cancer radiotherapy and involves antiapoptotic signal transduction pathways that prevent radiationinduced cell death. The aggressive cancer cell phenotype is the result of a variety of genetic and epigenetic alterations leading to deregulation of intracellular signaling pathways, including an impaired ability of the cancer cell to undergo apoptosis. Primary or acquired resistance of hormonerefractory prostate cancer to current treatment protocols has been associated with apoptosis resistance in cancer cells and is linked to therapy failures. Androgen SDMA ablation therapy produces only temporary responses because of the development of androgen independence.Failure to respond to radiation represents a critical problem in radiotherapy of hormonerefractory human prostate cancer.This study aims to investigate the radiosensitizing potential of a novel smallmolecule IAP inhibitor both in vitro and in vivo in androgenindependent prostate cancer and the possible mechanism of radiosensitization.More importantly, the success of this strategy will pave the way to develop the smallmolecule IAP inhibitors as an entirely new class of anticancer therapy for radiosensitizing human prostate cancer.XIAP effectively inhibits both intrinsic and extrinsic apoptosis pathways by binding and inhibiting both initiator and effector caspases, whose activity is crucial for the execution of apoptosis. Because effector caspase activity is both necessary and sufficient for irrevocable programmed cell death, XIAP functions as a gatekeeper to this final stage of the process.XIAP is widely expressed in cancer cell lines and tumor tissues and a high level of XIAP makes cancer cells apoptosisresistant to a wide variety of therapeutic agents. Most components of the major cell death regulatory pathways have been implicated in radiationinduced cell death. Accumulating evidences show that XIAP and cIAP, two IAP members that are mostly studied for antiapoptosis and cell survival signaling, play a crucial role in chemoresistance or radioresistance. Luciferase stably transfected DU cells. All types of cell lines were routinely cultured in highglucose DMEM supplemented with fetal bovine serum in a CO humidified incubator at jC.For nonreducing and nondenaturing SDSPAGE, samples were mixed with loading buffer and directly loaded on a gel without boiling.After electrophoresis, the proteins were electrotransferred to nitrocellulose membranes, probed with the relevant primary antibody followed by horseradish peroxidaseconjugated secondary antibody. In brief, DU cells were seeded into sixwell plates and treated with SH or negative control SH with or without pretreatment with the pancaspase inhibitor zVAD. Xray irradiation was done immediately after drug addition.Proteolytic release of AFC was monitored atkex nm andkem nm using a microplate reader. Enhancement ratio was calculated from mean inactivation dose in control group divided by that in treated group.Twentyfour hours after incubation, cells were harvested and processed for further detection.A, early apoptotic cell populations after treatment.After incubation at jC for h, released AFC was monitored by a microplate reader. The vehicle control and radiationonly groups received the same amount of DMSO solvent.Tumor size and body SDMA weight were measured twice a week.Tumor volume was calculated using the formula: V a b, where a andbrepresented both the long and the vertical short diameter of the tumor.The proteasome inhibitor MG was used as a positive control.

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