Increased cytotoxicity of an unusual DNA topoisomerase II inhibitor compound C-1305 toward HeLa cells with downregulated PARP-1 activity results from re-activation of the p53 pathway and modulation of mitotic checkpoints.

Our previous studies have shown that murine fibroblast cells, in which PARP-1 gene was inactivated by gene disruption, are extremely sensitive to triazoloacridone compound C-1305, an inhibitor of DNA topoisomerase II with unusual properties. Here, we show that pharmacological inhibition of PARP-1 activity by its inhibitor compound NU1025, sensitizes human cervical carcinoma HeLa cells to compound C-1305 compared to treatment with drug alone. Cytotoxic effect of drug/NU1025 of other topoisomerase II inhibitors varied depending on the dose of PARP-1 inhibitor. Increased cytotoxicity of topoisomerase II inhibitor/NU1025 combinations was attributable to the re-activation of the p53 pathway in drug-treated HeLa cells. This lead to a more stringent cell cycle checkpoint control during G2 and M and enhanced cell death by mitotic catastrophe induced by drug/NU1025 combinations. Interestingly, treatment of HeLa cells with NU1025 alone also increased p53 expression. This effect is, at least in part, related to the inhibition of proteasome activity by drug treatments. Together, our results show that concomitant inhibition of topoisomerase II and PARP-1 leads to the synergistic cytotoxic effect toward tumor cells that may be important for combination therapies with NU1025 and topoisomerase II inhibitors. We also confirmed our earlier work and show the important role of PARP-1 activity in the maintenance of the G2 arrest induced by DNA damaging drugs. Finally, based on our studies we propose that NU1025 and possibly other inhibitors of PARP-1 may be used as non-genotoxic agents to activate p53 in tumor cells with non-functional p53 pathways.