Topoisomerase activity in irradiated mammalian cells

The role of topoisomerase enzymes in the response of HeLa S3 cells to ionizing radiation was investigated. Exposure of cells to 100 Gy of X-radiation had no detectable effect either on the total cellular topoisomerase activity as measured by the relaxation of supercoiled plasmid DNA by cell sonicates or on the total cellular topoisomerase II activity as measured by plasmid DNA catenation. Total topoisomerase II activity remained constant for up to 90 min after cell irradiation. The effect of 2 drugs (caffeine and novobiocin) which inhibit topoisomerase II activity on the HeLa cell response to radiation was determined. Both drugs were found to inhibit topoisomerase II in vitro and to inhibit the recovery of nucleoid sedimentation in irradiated cells in vivo to the same extent. Topoisomerase II was inhibited by 50% by exposure to 10 mM caffeine and 0.79 mM novobiocin. At low concentrations neither drug affected the induction frequency, nor the rejoining rate, of DNA double-strand breaks. Caffeine (5 mM) inhibited the short-term recovery of cells from radiation while novobiocin (0.79 mM) had no detectable effect on the capacity of cells to recover from radiation exposure. The results indicate that topoisomerase II is not required for DNA double-strand break rejoining though it could be required for the recovery of DNA coiling in the irradiated cell. If topoisomerase II is involved at all in cell recovery from irradiation, this role does not apparently involve an ATP-dependent enzyme activity.