Ataxia-telangiectasia and Rad3-related and DNA-dependent protein kinase cooperate in G2 checkpoint activation by the DNA strand-breaking nucleoside analogue 2'-C-cyano-2'-deoxy-1-beta-D-arabino-pentofuranosylcytosine.

2'-C-cyano-2'-deoxy-1-beta-D-arabino-pentofuranosylcytosine (CNDAC), the prodrug (sapacitabine) of which is in clinical trials, has the novel mechanism of action of causing single-strand breaks after incorporating into DNA. Cells respond to this unique lesion by activating the G2 checkpoint, affected by the Chk1-Cdc25C-cyclin-dependent kinase 1/cyclin B pathway. This study aims at defining DNA damage checkpoint sensors that activate this response to CNDAC, particularly focusing on the major phosphatidylinositol 3-kinase-like protein kinase family proteins. First, fibroblasts, deficient in ataxia-telangiectasia mutated (ATM), transfected with empty vector or repleted with ATM, were arrested in G2 by CNDAC to similar extents, suggesting ATM is not required to activate the G2 checkpoint. Second, chromatin associations of RPA70 and RPA32, subunits of the ssDNA-binding protein, and the ataxia-telangiectasia and Rad3-related (ATR) substrate Rad17 and its phosphorylated form were increased on CNDAC exposure, suggesting activation of ATR kinase. The G2 checkpoint was abrogated due to depletion of ATR by small interfering RNA, and impaired in ATR-Seckel cells, indicating participation of ATR in this G2 checkpoint pathway. Third, the G2 checkpoint was more stringent in glioma cells with wild-type DNA-dependent protein kinase catalytic subunit (DNA-PKcs) than those with mutant DNA-PKcs, as shown by mitotic index counting. CNDAC-induced G2 arrest was abrogated by specific DNA-PKcs inhibitors or small interfering RNA knockdown in ML-1 and/or HeLa cells. Finally, two phosphatidylinositol 3-kinase-like protein kinase inhibitors, caffeine and wortmannin, abolished the CNDAC-induced G2 checkpoint in a spectrum of cell lines. Together, our data showed that ATR and DNA-PK cooperate in CNDAC-induced activation of the G2 checkpoint pathway.