Growth state- and cell cycle-dependent fluctuation in the expression of two forms of DNA topoisomerase II and possible specific modification of the higher molecular weight form in the M phase

The fluctuations in the expression of two forms of DNA topoisomerase II, p170 and p180, were examined by immunoblotting during the transition of the growth state and the cell cycle in Swiss 3T3 cells. The level of p170 expression was higher during the exponentially growing phase than in the stationary phase, and the level of p180 did not appear to change when the cells changed the growth state. When quiescent cells were stimulated with serum, the p170 level began to increase at 12 h and reached a maximal level at 24-28 h, corresponding to the G2 phase. In contrast, the level of p180 was almost constant during the cell cycle, and the p180 band became obscure when the number of mitotic cells increased. Immunoblotting of the samples prepared from metaphase-synchronized HeLa cells did not have a p180 band, but did exhibit a band with a higher molecular weight. This band increased and p180 decreased in parallel with the increase in the number of mitotic cells. When the cells exited from the M phase, p180 reappeared and the higher molecular weight band disappeared.