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 I I activities and levels after X-ray treatment in the radiation repair profi cient AA8 Chinese hamster cell line for comparison with the radiation sensi tive mutant EM9. Materials and methods: AA8 and EM9 cells were irradiated with 5 Gy of X-ray s and the activities of topoisomerases I and II in nuclear extracts were st udied. Immunological detection of both topoisomerases was carried out in or der to detect any changes in the expression of these enzymes as a consequen ce of irradiation. Results: Topoisomerase activities and levels in irradiated EM9 cells were t he same as in control non-irradiated cells. In fact, both topo I and topo I I activities clearly increased shortly after irradiation in the parental AA 8 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 ob served at a later time (Ih) after irradiation, while no similar change was detectable for topo II. Conclusions: While this hypothesis needs Further testing, an attractive ide a is that DNA topoisomerases might be involved in the cellular response to radiation damage, either through a direct participation in repair mechanism s or indirectly.