Various inhibitors of DNA topoisomerases diminish repair-specific DNA incision in UV-irradiated human fibroblasts.

A function for topoisomerases I and II in DNA excision repair can be postulated from the organization of the mammalian chromosome, involving nucleosomal structures and matrix-attached DNA loops. To analyse this function we determined UV-induced DNA incision in confluent human fibroblasts in the presence of 16 inhibitors of topoisomerases I and II which belonged to at least five different drug categories, based on their mechanism of action. Dose-response experiments were performed, analysed by linear regression and the concentrations at which DNA-incising activity was reduced to 50% were calculated (K50 values). The majority of these values represent concentrations for which interfering cell toxicity could be excluded. K50 concentrations, which were determined by extrapolating dose-response data, may hit the toxicity range, nevertheless, we deem our K50 scale useful for making biochemical comparisons. With respect to topoisomerase I, camptothecin and topotecan diminished repair-specific DNA incision to a small extent, whereas distamycin, which binds to the minor groove of DNA, caused a stronger effect. With respect to topoisomerase II the results were as follows. (i) The DNA intercalator ethidium bromide decreased DNA-incising activity at rather low concentrations, which indicates marked inhibitory potency. Quinacrine was less effective. (ii) Inhibitors intercalating and binding to the 'cleavable' DNA-topoisomerase complex (m-AMSA, mitoxantrone, doxorubicin and daunorubicin) strongly suppressed reparative DNA incision. (iii) Only small effects were observed using several drugs which act by trapping the 'cleavable' DNA-enzyme complex, namely nalidixic acid and oxolinic acid. In contrast, etoposide and teniposide inhibited post-UV DNA cleavage sizeably. (iv) Merbarone had to be applied at very high concentrations to reduce UV-induced DNA incision. (v) Novobiocin, an inhibitor of the ATPase subunit of topoisomerase II, markedly diminished repair-specific DNA cleavage. A comparison of the K50 values for DNA incision with those for DNA repair synthesis (1) shows that the majority of the investigated drugs inhibited both repair parameters. There were, however, differences in the concentrations required to achieve the 50% inhibition level. The results are best explained by assuming that in UV-irradiated human fibroblasts the 180 kd form of topoisomerase II is a target enzyme for inhibitors which suppressed repair and that this isozyme is involved in steps preceding repair-specific DNA incision.