A crucial role for ATR in the regulation of deoxycytidine kinase activity.

Deoxycytidine kinase (dCK) (EC 2.7.1.74) is a key enzyme for salvage of deoxynucleosides and activation of numerous anticancer and antiviral nucleoside analogs. dCK activity is enhanced in response to several genotoxic treatments, which has been correlated with an increase of dCK phosphorylation at Ser-74. ATM was recently identified as the kinase responsible for Ser-74 phosphorylation and dCK activation after ionizing radiation (IR). Here, we investigated the role of ATM and the related kinase ATR in dCK activation induced by other types of DNA damage. Using ATM-deficient cells or the ATM inhibitor KU-60019, we found that ATM was not required for dCK activation caused by UV light, aphidicolin, cladribine, and unexpectedly also IR. On the other hand, the selective ATR inhibitor VE-821 significantly reduced up-regulation of dCK activity induced by these genotoxic agents, though not IR, and also down-regulated basal dCK activity. A role for ATR in the control of dCK activity was confirmed by using ATR siRNA and ATR-Seckel cells. ATR was also found to directly phosphorylate dCK at Ser-74 in vitro. Further studies revealed that ATR, which is also activated in response to IR, although later than ATM, was responsible for IR-induced dCK activation in ATM-deficient cells or in the presence of KU-60019. Overall, our results demonstrate that ATR controls basal dCK activity and dCK activation in response to replication stress and indicate that ATR can activate dCK after IR if ATM is lacking or inhibited.