The influence of DNA-topoisomerase II inhibitors novobiocin and fostriecin on the induction and repair of DNA damage in Chinese hamster ovary (CHO) cells treated with mitoxantrone.

Novobiocin (NB) at the concentration of 2 mmol/l added to the culture medium together with mitoxantrone (MIT) (0.05-0.2 micrograms/ml) reduced the number of MIT-induced single-strand breaks of DNA to approximately one half measured by alkaline DNA unwinding and hydroxyapatite chromatography of DNA and similarly it reduced also the fraction of DNA linked to proteins measured by the K(+) -SDS precipitation method. Neither repair of the induced DNA breaks nor removal of the DNA-protein cross-links were markedly influenced by NB action. The specific inhibitor of topoisomerase II, fostriecin, exerted no effect on the induction of DNA breaks by MIT or their repair. Measurement of intracellular concentration of MIT has revealed that in the presence of NB the uptake of MIT into cells is reduced similarly as the number of induced DNA breaks to approximately one half. The combination of 0.1 mmol araC + 10 mmol HU slightly reduced the number of induced DNA breaks, but did not affect their repair. The present results suggest that (1) MIT induces DNA damage which is not repaired by excision repair, (2) MIT induces protein associated breaks of DNA, (3) topoisomerase II does not probably participate in the formation of DNA breaks induced by MIT, as the specific inhibitor of topoisomerase II, fostriecin exerts no effect on either the induction or repair of these breaks.