Regulation of DNA topoisomerases during cellular differentiation.

DNA topoisomerase II has been shown to be a nuclear marker for cell proliferation and a therapeutic target in cancer chemotherapy. In order to study the regulation of DNA topoisomerases during cellular differentiation, MELC were induced to differentiate by treatment with 5 mM HMBA. At day five, approximately 95% of MELC had reproducibly undergone differentiation. The level of topoisomerase II, as measured by immunoblotting with anti-topoisomerase II antisera, showed a parallel decrease to approximately 5-10% of the control level by day five. Activity measurements showed a similar decrease during the time course of MELC differentiation. Immunofluorescence studies at day five showed that only about 5% of the MELC had strong nuclear immunofluorescence. These results indicate that the level and activity of DNA topoisomerase II are significantly lower in differentiated versus undifferentiated cells. We also observed that the level and activity of topoisomerase II dropped twofold as cells grew to high cell densities in the absence of HMBA. In contrast, the topoisomerase I levels in MELC remained relatively constant throughout growth and differentiation.