Steroid‐Modulated Transcription Synergistically Forms <scp>DNA</scp> Double‐Strand Breaks With Topoisomerase <scp>II</scp> Inhibitor

The synergistic effects of drug combinations have emerged as a promising approach for achieving efficient cancer treatment. Through our exploration of drug combinations, we found that medroxyprogesterone acetate (MPA), a steroid, induced a synergistic antitumor effect in combination with the topoisomerase II inhibitor etoposide (ETP). In this study, we investigated the mechanisms underlying this synergistic effect for potential clinical applications. To elucidate the relevant mechanisms, we performed a cell viability assay, cell cycle analysis, DNA repair assays, detection of DNA double-strand breaks (DSBs) and the nuclear localization of topoisomerase II (Top2), and genome-wide detection of DSBs. MPA synergistically increased ETP-induced DSBs, resulting in cell cycle arrest in the G2/M phase. Interestingly, this effect was not due to the inhibition of DSB repair but to a specific increase in the Top2-DNA covalent complex formed by ETP. A genome-wide search for DSB locations revealed that DSB formation was promoted near promoter regions, suggesting the involvement of MPA transcriptional modulation in this mechanism. We also found that various steroids promoted DSB formation when combined with ETP, strongly supporting our synergistic model. Therefore, this synergistic effect is based on an innovative mechanism that differs from conventional strategies targeting the DNA damage response and is expected to contribute toward novel therapeutic options.