Topoisomerase II inhibitors affect entry into mitosis and chromosome condensation in BHK cells.

DNA topoisomerase II (topo II) is required at mitosis in yeast for high chromosome condensation and for chromosome segregation. Recent studies on intact mammalian cells using topo II inhibitors that do not stabilize cleavable complexes also suggest a requirement for topo II for complete chromosome condensation and perhaps also for entry into mitosis. We have investigated the effects of merbarone, ICRF-187, and aclarubicin, three topo II inhibitors that do not stabilize the cleavable complex, on entry into mitosis and on chromosome condensation in BHK and in tsBN2 cells. We have compared their effects with those of etoposide, a topo II inhibitor that stabilizes the cleavable complex. All inhibitors induced a concentration-dependent G2 delay or arrest that could be overcome with fostriecin or okadaic acid or by inactivation of RCC1 in tsBN2 cells. Mitotic chromosomes from cells treated with etoposide were extensively fragmented, whereas mitotic chromosomes from cells treated with merbarone, ICRF-187, or aclarubicin were intact but elongated and tangled. These results provide strong evidence that topo II activity is required in chromosome condensation for final coiling of the chromatids. Our results also indicate that protein phosphatases and RCC1 play a role in G2 delay induced by all inhibitors, whether they do or do not stabilize the cleavable complex.