Proliferation-dependent regulation of DNA topoisomerase II in cultured human cells.

The intracellular level of DNA topoisomerase II appears to be reversibly regulated by serum concentration in cultured primary human skin fibroblasts (HSF). Upon serum starvation, the intracellular level of topoisomerase II in HSF, as monitored by immunoblotting with antitopoisomerase II antibodies, gradually decreased to a nondetectable level (less than 10(4) copies/cell) over a period of 72 h. Addition of 10% serum to the starved cells led to a gradual increase of the intracellular topoisomerase II to the original level (approximately 10(6) copies/cell) over a period of 24 h. The intracellular DNA topoisomerase II level in HSF is also sensitive to cell density; minimally a 7-fold decrease was observed when HSF were grown to saturation density in a constant serum concentration. Similarly, the intracellular levels of DNA topoisomerase II in other "nontransformed" cells such as mouse NIH 3T3 and 3T6 cells are also sensitive to both the serum concentration and the cell density. In contrast, topoisomerase II levels in transformed cells such as HeLa cells, L1210 cells, and SV40 T-antigen-transformed COS-1 cells are maintained at high levels (approximately 10(6) copies/cell) and are much less sensitive to growth conditions. The topoisomerase II level in HeLa cells synchronized by a double thymidine block remained relatively constant (less than 2-fold difference) throughout the late G1, S, G2, and M phases of the cell cycle. Our results suggest that the level of DNA topoisomerase II is primarily regulated in the G0-G1 phase of the cell cycle and is elevated to a high level (approximately 10(6) copies/cell) in proliferating cells. In contrast, the intracellular levels of DNA topoisomerase I in these cells were largely unaffected by these growth conditions either in HSF or in HeLa cells.