DNA damage, c-myc suppression and apoptosis induced by the novel topoisomerase II inhibitor, salvicine, in human breast cancer MCF-7 cells

Salvicine, a diterpenoid quinone compound, possesses potent in vitro and in vivo antitumor activity. Salvicine is a novel non-intercalative topoisomerase II poison. In this study salvicine induced evident DNA damage, which was further characterized as double-strand breaks mainly in MCF-7 human breast cancer cells. The degree of damage was highly correlated with growth inhibition of MCF-7. Using a PCR-stop assay we demonstrated that this damage was selective. Preferential damage occurred in the p2 promoter region, but not the 3'-end of the protooncogene c-myc. The expression of oncogenes, such as c-myc and c-jun, was additionally investigated. Salvicine induced a dose-dependent decrease in c-myc gene transcription, concomitant with an increase in c-jun expression. Furthermore, reverse-transcription PCR and Western blotting data revealed that salvicine failed to stimulate the mRNA and protein levels of p53 and its downstream targets p21 and bax. The phosphorylation degree of serine 15 of p53, which is thought to be an active form of p53 in response to cellular DNA damage, remained in a steady state. In view of these results, we propose that the downregulation of c-myc resulting from selective damage plays a role in apoptosis signaling. Moreover, salvicine-induced apoptosis in MCF-7 subsequent to DNA damage seems to be mediated through a p53-independent pathway.