Evidence for the involvement of DNA-dependent protein kinase in the phenomena of low dose hyper-radiosensitivity and increased radioresistance

PURPOSE: To investigate the role of DNA-dependent protein kinase (DNA-PK) in the phenomena of low dose hyper-radiosensitivity (HRS) and increased radioresistance (IRR) using the genetically related M059 cell lines of disparate PRKDC status. MATERIALS AND METHODS: Clonogenic survival was measured for the three cell lines following low doses of X-irradiation using a flowactivated cell sorting (FACS) plating technique. The presence of PRKDC, G22p1 and Xrcc5 proteins was determined by Western blotting and a kinase assay used to measure DNA-PK complex activity. RESULTS: The survival responses for the three cell lines over the 0-0.3Gy dose range were comparable, but differences in radiosensitivity were evident at doses >0.4Gy. M059K and M059J/Fus1 cells (both PRKDC competent) exhibited marked HRS/IRR responses, albeit to different extents. M059J cells (PRKDC incompetent) were extremely radiosensitive exhibiting a linear survival curve with no evidence of IRR. The presence of IRR was coincident with the presence of PRKDC protein and functional DNA-PK activity. CONCLUSIONS: HRS is a response that is independent of DNA-PK activity. In contrast, IRR showed a dependence on the presence of PRKDC protein and functional DNA-PK activity. These data support a role for DNA-PK activity in the IRR response.