Ku affects the ATM-dependent S phase checkpoint following ionizing radiation

Following exposure to genotoxic stress, proliferating cells actively slow down DNA replication through an S phase checkpoint to provide time for repair. The ATM-depeudeut pathway plays an important role in the S phase checkpoint response following ionizing irradiation. We report that there is a stronger S phase checkpoint response in irradiated Ku80^−/− cells as compared with their wild-type counterparts, which has no relationship to DNA-dependent protein kinase (DNA-PK) activity but correlates with a higher ATM activity and with more ATM bound to chromatin DNA in such cells. Wortmannin, a nonspecific inhibitor of ATM, not only reduces the higher activity of ATM kinase, but also abolishes the stronger S phase checkpoint response in Ku80^−/− cells. Furthermore, a specific ATM antisense oligonucleotide abolishes the stronger S checkpoint response in Ku80^−/− cells and renders these cells practically indistinguishable from Ku80^+/+ cells for this endpoint. These results in aggregate indicate that the stronger S checkpoint in irradiated Ku80^−/− cells is due to the higher ATM kinase activity.