Phorbol regulation of topoisomerases I and II in human leukemia cells: Studies in an additional cell pair sensitive or resistant to phorbol-induced differentiation

Abstract

We previously reported (Zwelling et al., Cancer Res 50: 7116–7122, 1990) that etoposide induced DNA cleavage and mRNA coding for topoisomerase II are reduced in HL-60 cells induced to differentiate by phorbol ester. Reduction of etoposide-induced cleavage and topoisomerase II message did not occur in the derived cell line 1E3 (which is resistant to phorbol-induced differentiation), implying that topoisomerase II activity may be related to the state of cell differentiation. We have extended these studies using a new phorbol sensitive/resistant cell pair, S (sensitive) and PET (phorbol ester tolerant). Phorbol ester exposure not only reduced etoposide-induced DNA cleavage and topoisomerase II mRNA in S cells but also decreased the amount of immunoreactive topoisomerase II enzyme in whole S cells. However, immunoreactive topoisomerase II extracted from the nuclei of phorbol-treated S cells was not reduced compared with that from the nuclei of untreated S cells. This suggests that topoisomerase II contained in nuclear extracts is not always representative of the total cellular enzyme. Dramatic decreases in the amount, activity, or gene expression of topoisomerase II were not observed after phorbol treatment of the resistant PET cells; this is consistent with the potential involvement of topoisomerase II in monocytoid differentiation. Levels of topoisomerase I enzyme and mRNA fell in both S and PET cells after phorbol treatment; therefore, the genes for topoisomerases I and II did not appear to be regulated coordinately.