Roles of DNA-dependent protein kinase and ATM in cell-cycle-dependent radiation sensitivity in human cells

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Purpose : The roles of DNA-dependent protein kinase (DNA-PK) and ATM in the cell-cycle-dependent radiosensitivity in human cells were investigated. Methods and materials : A DNA-PK activity-deficient human glioblastoma cell line M059J, ataxia telangiectasia cell lines AT3BISV and AT5BIVA, and control cell lines were used. Wortmannin inhibited DNA-PK and ATM activities. Cells were synchronized by hydroxyurea. Progression through the cell cycle was analysed by flow cytometry. Results : M059J exhibited hyper-radiosensitivity throughout the cell cycle, with extreme hyper-radiosensitivity in G to early S-phase compared with the control cell line M059K. AT3BISV and AT5BIVA exhibited hyper-radiosensitivity throughout the cell cycle but showed a similar pattern of cell-cycle-dependent radiosensitivity to that observed in LM217 or HeLa cells. In AT3BISV and AT5BIVA, radiosensitization by wortmannin was observed throughout the cell cycle and was most prominent in G 1 to early S-phase. Wortmannin did not sensitize M059J to ionizing radiation in any cell-cycle phase. DNA-PK activities were not different throughout the cell cycle. Conclusion : The results suggest that (1) non-homologous endjoining plays a dominant role in G 1 to early S-phase and a minor role in late S to G 2 -phase in repairing DNA double-strand breaks, (2) the role of ATM in repairing double-strand breaks may be almost cell-cycle-independent and (3) the dominant role of non-homologous end-joining during G 1 to early S-phase is not due to cell-cycle-dependent fluctuations in DNA-PK activity.