title: Examining the non-homologous repair process following cisplatin and radiation treatments.
abstract: OBJECTIVE
  To investigate the extent of non-homologous end-joining (NHEJ) in the mechanism of cisplatin radiosensitization.
  
  
  METHODS
  Ku80-deficient cells are deficient in the non-homologous DNA double-strand break repair process, while the wild-type MEF cells maintain full mammalian cell repair capabilities. Both cell lines were exposed to clinically applicable doses of cisplatin (1, 3 and 6 microg x ml(-1)) for 1 h immediately before exposure to 250 kV X-rays. Radiation responses were plotted for each cell line and for all doses of cisplatin to observe relative levels of radiosensitization. Split-dose experiments were also performed on each cell line to measure levels of sublethal damage repair.
  
  
  RESULTS
  Radiosensitization was observed in the wild-type cells but not in the Ku80 cells when treated with a combination of 1 microg x ml(-1) cisplatin followed by X-rays, implying that the NHEJ pathway may play a large role in cisplatin radiosensitization. Concurrent administration of this cisplatin dose with radiation produced similar levels of radiosensitization. Conversely, 3 and 6 microg x ml(-1) cisplatin applied immediately before radiation revealed an increasing resistance to radiation in both cell lines--possibly due to resistant subpopulations of cells remaining after subsequent lethal doses of cisplatin. Further experiments revealed that the high concentrations of cisplatin did not alter cell cycle distribution. Finally, split-dose experiments revealed that the NHEJ pathway also plays a significant role in sublethal damage repair.
  
  
  CONCLUSIONS
  The study reveals that clinically applicable doses of cisplatin treatment results in the radiosensitization of mammalian cells due to the inhibition of the operation of NHEJ.
authors.affiliation: Department of Medical Physics, Ottawa Regional Cancer Center, 503 Smyth Road, Ottawa, Ontario K1H 1C4, Canada.
authors.forename: W K
authors.initials: WK
authors.lastname: Myint
authors.forename: Cheng
authors.initials: C
authors.lastname: Ng
authors.forename: G P
authors.initials: GP
authors.lastname: Raaphorst
mesh_terms: D000818:Animals; D034961:Antigens, Nuclear; D000970:Antineoplastic Agents / Q000494:pharmacology*; D002460:Cell Line; D002945:Cisplatin / Q000494:pharmacology*; D004247:DNA / Q000187:drug effects* / Q000528:radiation effects*; D004249:DNA Damage; D004265:DNA Helicases; D004260:DNA Repair / Q000235:genetics; D004268:DNA-Binding Proteins / Q000172:deficiency / Q000235:genetics; D004307:Dose-Response Relationship, Radiation; D000072200:Ku Autoantigen; D051379:Mice; D009154:Mutation; D009687:Nuclear Proteins / Q000172:deficiency / Q000235:genetics; D011836:Radiation Tolerance / Q000187:drug effects / Q000235:genetics
chemical_list: D034961:Antigens, Nuclear; D000970:Antineoplastic Agents; D004268:DNA-Binding Proteins; D009687:Nuclear Proteins; D004247:DNA; D004265:DNA Helicases; C106893:XRCC5 protein, human; C000606509:Xrcc6 protein, human; C000606510:Xrcc6 protein, mouse; D000072200:Ku Autoantigen; D002945:Cisplatin