Merbarone, a novel DNA topoisomerase II (topo II) inhibitor, differs from teniposide (VM-26) in that it inhibits topo II activities without stabilizing topo II-DNA covalent complexes. Thus, while the cellular effects of VM-26 are the consequences of inhibition of topo II catalytic activities and generation of topo II-mediated DNA damage, those of merbarone may be due to inactivation of topo II catalytic function. To address the issues of mechanisms of cell cycle effects and pharmacological actions of these two topo II inhibitors in mammalian cells, we used synchronized cultures of HeLa cells to study the effects of these drugs on cell cycle processes where topo II is essential (e.g., chromosome separation) or possibly involved (e.g., G2 arrest, DNA replication). We found that both drugs inhibited chromosome separation and cell division without preventing cells from exiting mitosis. Both drugs caused S-phase retardation, G2 arrest, and phase-specific cytotoxicity in that they are more toxic to S, M, and G2 cells than G0/G1 cells. However, merbarone produced the above effects in convergent dosages that were within one to five times its 90% inhibitory cytotoxic concentration, whereas the concentrations of VM-26 to cause quantitatively similar effects were quite divergent. VM-26 is 50–100-fold more efficient in causing G2 arrest than in inhibiting chromosome separation. Furthermore, at concentrations showing similar levels of S-phase suppression, VM-26 caused significant DNA breaks, while merbarone had no such effect. Our data suggest that the effects of merbarone and VM-26 during mitosis are most likely due to inhibition of topo II function. We conclude that while G2 arrest by VM-26 is related to topo II-mediated DNA damage and its sequelae, G2 arrest by merbarone likely results from different mechanisms.
This work was supported in part by Research Grants CA-40570 and CA-30103, by Cancer Center Support (CORE) Grant CA-21765 from the National Cancer Institute, Department of Health and Human Services (Bethesda, MD), and in part by American Lebanese Syrian Associated Charities (Danny Thomas, Founder).