PHORBOL REGULATION OF TOPOISOMERASE-I AND TOPOISOMERASE-III IN HUMAN LEUKEMIA-CELLS - STUDIES IN AN ADDITIONAL CELL-PAIR SENSITIVE OR RESISTANT TO PHORBOL-INDUCED DIFFERENTIATION

We previously reported (Zwelling et al., Cancer Res 50: 7116-7122, 199 0) that etoposide-induced DNA cleavage and mRNA coding for topoisomera se II are reduced in HL-60 cells induced to differentiate by phorbol e ster. Reduction of etoposide-induced cleavage and topoisomerase II mes sage did not occur in the derived cell line 1E3 (which is resistant to phorbol-induced differentiation), implying that topoisomerase II acti vity may be related to the state of cell differentiation. We have exte nded 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 topois omerase II enzyme in whole S cells. However, immunoreactive topoisomer ase II extracted from the nuclei of phorbol-treated S cells was not re duced compared with that from the nuclei of untreated S cells. This su ggests that topoisomerase II contained in nuclear extracts is not alwa ys 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 c onsistent with the potential involvement of topoisomerase II in monocy toid differentiation. Levels of topoisomerase I enzyme and mRNA fell i n both S and PET cells after phorbol treatment; therefore, the genes f or topoisomerases I and II did not appear to be regulated coordinately .