Topoisomerase II (Topo2) inhibitors in combination with cisplatin represent a common treatment modality used for glioma patients. The main mechanism of their action involves induction of DNA double-strand breaks (DSBs). DSBs are repaired via the homology-dependent DNA repair (HRR) and non-homologous end-joining (NHEJ). Inhibition of the NHEJ or HRR pathway sensitizes cancer cells to the treatment. In this work, we investigated the effect of three Topo2 inhibitors-etoposide, NK314, or HU-331 in combination with cisplatin in the U-87 human glioblastoma cell line. Etoposide as well as NK314 inhibited Topo2 activity by stabilizing Topo2-DNA cleavable complexes whereas HU-331 inhibited the ATPase activity of Topo2 using a noncompetitive mechanism. To increase the effectiveness of the treatment, we combined cisplatin and Topo2 inhibitor treatment with DSB repair inhibitors (DRIs). The cells were sensitized with NHEJ inhibitor, NU7441, or the novel HRR inhibitor, YU238259, prior to drug treatment. All of the investigated Topo2 inhibitors in combination with cisplatin efficiently killed the U-87 cells. The most cytotoxic effect was observed for the cisplatin + HU331 treatment scheme and this effect was significantly increased when a DRI pretreatment was used; however, we did not observed DSBs. Therefore, the molecular mechanism of cytotoxicity caused by the cisplatin + HU331 treatment scheme is yet to be evaluated. We observed a concentration-dependent change in DSB levels and accumulation at the G2/M checkpoint and S-phase in glioma cells incubated with NK314/cisplatin and etoposide/cisplatin. In conclusion, in combination with cisplatin, HU331 is the most potent Topo2 inhibitor of human glioblastoma cells.
Keywords: DNA double strand breaks; HRR; Human glioblastoma; NHEJ; Topo2 inhibitor.