Mutagenesis Advance Access originally published online on January 24, 2006
Mutagenesis 2006 21(1):83-90; doi:10.1093/mutage/gel002
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The repair of gamma-radiation-induced DNA damage is inhibited by microcystin-LR, the PP1 and PP2A phosphatase inhibitor
1Department of Radiobiology and Immunology, Institute of Biology, Swi
tokrzyska Academy, Kielce, Poland, 2Department of Plant Physiology and Development, Faculty of Biotechnology, Jagiellonian University, Kraków, Poland, 3Department of Biological Sciences, Wright State University, Dayton, OH, USA, 4Department of Radiobiology and Health Protection, Institute of Nuclear Chemistry and Technology, Warsaw, Poland and 5Department of Medical Physics, Swietokrzyskie Oncology Center, Kielce, Poland
The genotoxic activity of microcystin-LR (MC-LR) is a matter of debate. MC-LR is known to be a phosphatase inhibitor and it may be expected that it is involved in the regulation of the activity of DNA-dependent protein kinase (DNA-PK), the key enzyme involved in the repair of radiation-induced DNA damage. We studied the effect of MC-LR on the repair capacity of radiation-induced DNA damage in human lymphocytes and human glioblastoma cell lines MO59J and MO59K. A dose of 0.5 µg/ml of MC-LR was chosen because it induced very little early apoptosis which gives no false positive results in the comet assay. Human lymphocytes in G0-phase of the cell cycle were pre-treated with MC-LR for 3 h and irradiated with 2 Gy of gamma radiation. The kinetics of DNA repair was assessed by the comet assay. In addition the frequencies of chromosomal aberrations were analysed. The pre-treatment with MC-LR inhibited the repair of radiation-induced damage and lead to enhanced frequencies of chromosomal aberrations including dicentric chromosomes. The results of a split-dose experiment, where cells were exposed to two 1.5 Gy doses of radiation separated by 3 h with or without MC-LR, confirmed that the toxin increased the frequency of dicentric chromosomes. We also determined the effect of MC-LR and ionizing radiation on the frequency of 
-H2AX foci. The pre-treatment with MC-LR resulted in reduced numbers of -H2AX foci in irradiated cells. In order to elucidate the impact of MC-LR on DNA-PK we examined the kinetics of DNA repair in human glioblastoma MO59J and MO59K cells. Both cell lines were exposed to 10 Gy of X-rays and DNA repair was analysed by the comet assay. A strong inhibitory effect was observed in the MO59K but not in the MO59J cells. These results indicate that DNA-PK might be involved in DNA repair inhibition by MC-LR.
* To whom correspondence should be addressed at: Department of Radiobiology and Immunology, Institute of Biology, Swietokrzyska Academy, ul. Swietokrzyska 15, 25-406 Kielce, Poland. Tel: +4841 349 62 98; Fax: +4841 349 62 92; Email: alankoff{at}pu.kielce.pl