[The role of topoisomerase II in mammalian cells response to ionizing radiation. II. Effect of a small concentration of novobiocin on cell cycle in x-rays irradiated HeLa cells].

Flow cytometry was used to study cell cycle recovery in X-irradiated HeLa cells after the action of novobiocin, an inhibitor of topoisomerase II. A prolonged treatment with 0.4 mM novobiocin (30 h) of intact cells did not affect cell cycle progression. Novobiocin treatment of 5 Gy-irradiated cells resulted in a significant decrease of G2-block, if compared with X-irradiated cells. These data, together with our previous results on the effect of 1 mM novobiocin on cell cycle in X-irradiated CHO K1 cells, allow to suggest an involvement of topoisomerase II in cell response to ionizing radiation. But one cannot exclude a concurrent effect of novobiocin with proteins other than topoisomerase II, for example, with protein p34cdc2-kinase, known to be involved in cell cycle regulation.