Induction of multinucleated cells after the treatment with Ku55933, a specific ATM inhibitor. A, NCI-H460 and A549 cells were grown on coverslips and treated with 10 μmol/L Ku55933 for 30 min and then exposed to 6 Gy ionizing radiation (IR). After 72 h, cells were fixed, stained with DAPI, and observed under confocal microscopy (X400). Cells with multinuclei were indicated with arrow. The number of multinucleated cells was counted and plotted. At least 100 cells were counted and each column represents the means of three independent experiments. Error bars, SE. B, Western blots after Ku55933 and IR treatment. Cells were pretreated with 10 μmol/L Ku55933 for 30 min, exposed to 6 Gy IR, and harvested after 30 min. Western blot analysis was done with anti-pATM, anti-ATM, anti-pChk2, anti-Chk2 antibodies. β-Actin was used as a loading control. Blots representative of three independent experiments.