The ATM inhibitor KU55933 sensitizes radioresistant bladder cancer cells with DAB2IP gene defect

Purpose: Our preliminary results showed that differentially expressed in ovarian cancer-2/disabled homolog 2 (DOC-2/DAB2) interactive protein (DAB2IP), a putative tumor suppressor gene, is down-regulated in bladder cancer (BCa) with aggressive phenotypes. In this study, we investigated how DAB2IP knockdown influenced BCa cell response to ionizing radiation (IR) and discussed possible ways to enhance cell radiosensitivity. Methods and materials: The small interfering RNA (siRNA) system was implemented to inhibit endogenous DAB2IP expression in two human BCa cell lines, T24 and 5637. Cell sensitivity to IR alone or combined treatment was measured by a colony formation assay (CFA). Western blot was used to determine the phosphorylation levels of ataxia-telangiectasia mutated (ATM), catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs) and related DNA damage repair (DDR) proteins. Immunofluorescence as well as a flow cytometry assay were employed to detect DNA double-strand break (DSB) repair and cell cycle distribution, respectively. Results: DAB2IP-knockdown of BCa cells (i.e., siDAB2IP) exhibit increased clonogenic survival in response to IR compared with control cells (i.e., siCON) expressing an endogenous level of DAB2IP. The mechanism in siDAB2IP cells could be explained by elevated ATM expression and activation, increased S phase cell distribution as well as faster DSB repair kinetics. 2-morpholin-4-yl-6-thianthren-1-yl-pyran-4-one (KU55933) significantly sensitized siDAB2IP cells to IR due to inhibition of the phosphorylation of ATM and its downstream targets following IR and slower DSB repair kinetics. Conclusions: Loss of DAB2IP expression in BCa cells signifies their radioresistance. KU55933, which suppresses ATM phosphorylation upon irradiation, could be applied in the radiotherapy of BCa patients with a DAB2IP gene defect.