Regulation of DNA-dependent protein kinase activity by ionizing radiation-activated Abl kinase is an ATM-dependent process

Ionizing radiation (IR) treatment results in activation of the nonreceptor tyrosine kinase c-Abl because of phosphorylation by ATM. In vitro evidence indicates that DNA-dependent protein kinase (DNA-PK) can also phosphorylate and thus potentially activate Abl kinase activity in response to IR exposu re. To unravel the role of ATM and DNA-PK in the activation of Abl, we assa yed Abl, ATM, and DNA-PK activity in ATM- and DNA-PKcs-deficient cells afte r irradiation. Our results show that despite the presence of higher than no rmal levels of DNA-PK kinase activity, c-Abl fails to become activated afte r IR exposure in ATM-deficient cells. Conversely, normal activation of both ATM and c-Abl occurs in DNA-PKcs- deficient cells, indicating that ATM but not DNA-PK is required for activation of Abl in response to IR treatment. Moreover, activation of Abl kinase activity by IR correlates well with acti vation of ATM activity in all phases of the cell cycle. These results indic ate that ATM is primarily responsible for activation of Abl in response to IR exposure in a cell cycle-independent fashion. Examination of DNA-PK acti vity in response to IR treatment in Abl-deficient cells expressing mutant f orms of Abl or in normal cells exposed to an inhibitor of Abl suggests an i n vivo role for Abl in the down-regulation of DNA-PK activity. Collectively , these results suggest a convergence of the ATM and DNA-PK pathways in the cellular response to IR through c-Abl kinase.