Effect of ionizing radiation and topoisomerase II inhibitors on DNA synthesis in mammalian cells

We have used a novel, quantitative approach to study the effect of γ‐radiation and topoisomerase‐II inhibitors on the initiation of DNA synthesis in eukaryotic cells. We found out that mild γ‐irradiation caused an almost immediate decrease in the rate of initiation of genomic DNA replication and stimulated DNA repair. This held true for two different cell lines, Ehrlich ascites tumor cells and Friend transformed erythroid cells, although the effect of γ‐radiation on Friend cells was more pronounced. At the same time, the synthesis of mitochondrial DNA was not affected by the irradiation. The effect of topoisomerase‐II inhibitors on DNA initiation closely paralleled that of γ irradiation, but did not stimulate repair. The fact that γ‐radiation and topoisomerase‐II inhibitors, two types of agents that differ so profoundly, have practically the same effect on DNA synthesis speaks strongly in favour of the idea that eykaryotic cells have a general mechanism for coping with any disturbances in DNA integrity and chromatin structure. This mechanism is probably similar to the SOS‐mechanism of prokaryotic cells and includes, as an early step, a slowdown of the initiation of replicative DNA synthesis.