title: Increased γ-H2A.X intensity in response to chronic medium-dose-rate γ-ray irradiation.
abstract: BACKGROUND
  The molecular mechanisms of DNA repair following chronic medium-dose-rate (MDR) γ-ray-induced damage remain largely unknown.
  
  
  RESULTS
  We used a cell function imager to quantitatively measure the fluorescence intensity of γ-H2A.X foci in MDR (0.015 Gy/h and 0.06 Gy/h) or high-dose-rate (HDR) (54 Gy/h) γ-ray irradiated embryonic fibroblasts derived from DNA-dependent protein kinase mutated mice (scid/scid mouse embryonic fibroblasts (scid/scid MEFs)). The obtained results are as follows: (1) Automatic measurement of the intensity of radiation-induced γ-H2A.X foci by the cell function imager provides more accurate results compared to manual counting of γ-H2A.X foci. (2) In high-dose-rate (HDR) irradiation, γ-H2A.X foci with high fluorescence intensity were observed at 1 h after irradiation in both scid/scid and wild-type MEFs. These foci were gradually reduced through de-phosphorylation at 24 h or 72 h after irradiation. Furthermore, the fluorescence intensity at 24 h increased to a significantly greater extent in scid/scid MEFs than in wild-type MEFs in the G(1) phase, although no significant difference was observed in G(2)/M-phase MEFs, suggesting that DNA-PKcs might be associated with non-homologous-end-joining-dependent DNA repair in the G(1) phase following HDR γ-ray irradiation. (3) The intensity of γ-H2A.X foci for continuous MDR (0.06 Gy/h and 0.015 Gy/h) irradiation increased significantly and in a dose-dependent fashion. Furthermore, unlike HDR-irradiated scid/scid MEFs, the intensity of γ-H2A.X foci in MDR-irradiated scid/scid MEFs showed no significant increase in the G(1) phase at 24 h, indicating that DNA repair systems using proteins other than DNA-PKcs might induce cell functioning that are subjected to MDR γ-ray irradiation.
  
  
  CONCLUSIONS
  Our results indicate that the mechanism of phosphorylation or de-phosphorylation of γ-H2A.X foci induced by chronic MDR γ-ray irradiation might be different from those induced by HDR γ-ray irradiation.
authors.affiliation: Department of Radiobiology, Institute for Environmental Sciences, Rokkasho, Kamikita, Aomori, Japan. sugihara@ies.or.jp
authors.forename: Takashi
authors.initials: T
authors.lastname: Sugihara
authors.forename: Hayato
authors.initials: H
authors.lastname: Murano
authors.forename: Kimio
authors.initials: K
authors.lastname: Tanaka
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D004260:DNA Repair / Q000528:radiation effects; D005720:Gamma Rays; D005838:Genotype; D006657:Histones / Q000378:metabolism*; D007150:Immunohistochemistry; D051379:Mice; D010766:Phosphorylation / Q000528:radiation effects; D017354:Point Mutation
chemical_list: D006657:Histones; C475961:gamma-H2AX protein, mouse