Differences between normal and ras-transformed NIH-3T3 cells in expression of the 170kD and 180kD forms of topoisomerase II.

The activity of topoisomerase II and the cellular content of the 170kD and 180kD forms of the enzyme were studied as functions of transformation and growth state by using normal and ras-transformed NIH-3T3 cells. Total topoisomerase II activity, as measured by the unknotting of P4 DNA, was higher in ras-transformed than in normal cells in similar growth states, and was higher in exponentially growing than in plateau cells for both cell lines. Total topoisomerase II levels, as measured by immunoblotting, showed a similar dependence on transformation and growth state. The relative amounts of the 170kD and 180kD forms of the enzyme varied as a function of transformation and growth state. The proportion of 170kD topoisomerase II was higher in ras-transformed than in untransformed cells and depended much less on growth state in the ras-transformed cells. The topoisomerase II activity in extracts of ras-transformed cells was more sensitive to inhibition by teniposide and merbarone, drugs which selectively inhibit the 170kD form of topoisomerase II. The ras-transformed cells were also more sensitive to the cytotoxic effects of these drugs. An increase in the relative cellular content of 170kD topoisomerase II is characteristic of ras-transformed 3T3 cells, and the levels of this form of the enzyme appear to be less dependent on proliferation state than in untransformed cells. The susceptibility of certain tumors to killing by topoisomerase II-directed drugs may be due to a higher proportion of 170kD enzyme as well as a higher level of total topoisomerase II activity.