The role of ATM in radio-resistant PCa cells caused by DAB2IP knockdown was investigated in this study. The response of two PCa cell lines, PC3 shVector and PC3 shDAB2IP to γ-rays (0, 2, 4, 6, 8 Gy) alone or combined with 10 μmol/L KU55933, an ATM inhibitor, were measured by a colony formation assay. Quantitative real-time polymerase chain reaction (qRT-PCR) and western blot were used to determine the amount of ATM mRNA and ATM phosphorylation, respectively. The results showed that DAB2IP-knockdown cells (PC3 shDAB2IP) exhibited increased clonogenic survival in response to IR compared with control cells (PC3 shVector) expressing an endogenous level of DAB2IP. KU55933 significantly sensitized PC3 shDAB2IP cells to IR due to inhibition the ATM mRNA synthesis and ATM phosphorylation. It suggests that elevated ATM expression and activation observed in DAB2IP deficient cells could be a key event in resulting in resistance to IR, and ATM inhibitor could be applied to improve the radio-sensitivity of PCa patients with DAB2IP genetic defects.Cited