Topoisomerase activities and levels in irradiated Chinese hamster AA8 cells and in its radiosensitive mutant EM9

Purpose: To investigate possible variations in topoisomerase (topo) I and II activities and levels after X-ray treatment in the radiation repair proficient AA8 Chinese hamster cell line for comparison with the radiation sensitive mutant EM9. Materials and methods: AA8 and EM9 cells were irradiated with 5Gy of X-rays and the activities of topoisomerases I and II in nuclear extracts were studied. Immunological detection of both topoisomerases was carried out in order to detect any changes in the expression of these enzymes as a consequence of irradiation. Results: Topoisomerase activities and levels in irradiated EM9 cells were the same as in control non-irradiated cells. In fact, both topo I and topo II activities clearly increased shortly after irradiation in the parental AA8 cells, with a more rapid increase for topo I than for topo II. In the AA8 cells, an increased level of topo I detectable immunologically was only observed at a later time (1h) after irradiation, while no similar change was detectable for topo II. Conclusions: While this hypothesis needs further testing, an attractive idea is that DNA topoisomerases might be involved in the cellular response to radiation damage, either through a direct participation in repair mechanisms or indirectly.