title: 1-Methylxanthine enhances the radiosensitivity of tumor cells.
abstract: OBJECTIVE
  To determine the efficacy of a caffeine derivative 1-methylxanthine (1-MTX) in increasing radiosensitivity of cancer cells and elucidate the underlying mechanisms in vitro.
  
  
  METHODS
  RKO human colorectal cancer cells carrying wild type protein 53 kDa (p53) were incubated with 3 mM 1-MTX for 30 min, exposed to 4 Gy ionizing radiation, and further incubated with 1-MTX for three days. The clonogenic cell death was determined, and the cell cycle distribution and apoptosis were studied with flow cytometry at different times after irradiation. The DNA double strand break (DNA DSB) was examined using phosphorylated Histone2A (gamma-H2AX) foci formation, and the expression/activity of checkpoint 2 kinase (Chk2), cell division cycle 25 (Cdc25) phosphatase and cyclin B1/Cdc2 kinase were also investigated using western blotting and in vitro kinase assays.
  
  
  RESULTS
  The treatment with 3 mM 1-MTX increased the radiation-induced clonogenic and apoptotic cell death. The radiation-induced phosphorylation of Chk2 and Cdc25c and the radiation-induced increase in the cyclin B1/Cdc2 kinas activity were little affected by 1-MTX. The radiation-induced G2/M arrest was only slightly shortened and the expression of radiation-induced gamma-H2AX was markedly prolonged by 1-MTX.
  
  
  CONCLUSIONS
  1-MTX significantly increased the radiosensitivity of RKO human colorectal cancer cells carrying wild type p53 mainly by inhibiting the repair of radiation-induced DNA DSB without causing significant alteration in radiation-induced G2/M arrest. Such a radiosensitization occurred at 1-MTX concentrations almost non-toxic to the target tumor cells.
authors.affiliation: Department of Microbiology, Center for Advanced Medical Education by BK21 Project, College of Medicine, Inha University, Inchon, Korea.
authors.forename: Hyewon
authors.initials: H
authors.lastname: Youn
authors.forename: Yeon
authors.initials: Y
authors.lastname: Hee Kook
authors.forename: Eun-Taex
authors.initials: ET
authors.lastname: Oh
authors.forename: Seong-Yun
authors.initials: SY
authors.lastname: Jeong
authors.forename: Chulhee
authors.initials: C
authors.lastname: Kim
authors.forename: Eun
authors.initials: E
authors.lastname: Kyung Choi
authors.forename: Byung
authors.initials: B
authors.lastname: Uk Lim
authors.forename: Heon Joo
authors.initials: HJ
authors.lastname: Park
mesh_terms: D017209:Apoptosis / Q000528:radiation effects; D045744:Cell Line, Tumor; D004249:DNA Damage; D016195:G2 Phase / Q000528:radiation effects; D006657:Histones / Q000032:analysis; D006801:Humans; D009369:Neoplasms / Q000532:radiotherapy*; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology*; D016159:Tumor Suppressor Protein p53 / Q000032:analysis; D014970:Xanthines / Q000494:pharmacology*; D020687:cdc25 Phosphatases / Q000502:physiology
chemical_list: C468783:H2AX protein, human; D006657:Histones; D011838:Radiation-Sensitizing Agents; D016159:Tumor Suppressor Protein p53; D014970:Xanthines; C053085:1-methylxanthine; C517009:CDC25C protein, human; D020687:cdc25 Phosphatases