Phorbol Ester Effects on Topoisomerase II Activity and Gene Expression in HL-60 Human Leukemia Cells with Different Proclivities toward Monocytoid Differentiation

We examined the effects of phorbol ester treatment on topoisomerase H-mediated events in two human leukemia cell lines with different proclivities toward phorbol ester-induced monocytoid differentiation. HL-60 is the parent line that will terminally differentiate; 1E3 is derived line that will not terminally differentiate. Within 24 h of phorbol ester treatment, etoposide-induced, topoisomerase H-mediated DNA cleavage declined 10-fold, whereas 4′-(9-acridinylamino)-methanesulfon-m-anisidide-induced DNA cleavage declined 3-fold in HL-60. In phorbol-treated 1E3, etoposide-induced DNA cleavage declined only 2-fold, whereas 4′-(9-acridinylamino)methanesulfon-/n-anisidide-induced cleavage was barely affected. There was a 2-to 3-fold decline in topoisomerase II activity within the nuclear extracts from phorbol-treated HL-60 cells but not from phorbol-treated 1E3 cells. Immunoblotting experiments with anti-topoisomerase II antibodies indicated that phorbol treatment pro duced a structural change in the immunoreactive topoisomerase II in HL-60 nuclear extracts but produced no change in 1E3 topoisomerase II. Phorbol ester treatment also produced a decline in the level of topoisom erase II gene expression in HL-60 but not in 1E3 cells. By contrast, the cytotoxicity of etoposide in both lines was decreased following phorbol treatment. Thus, phorbols may uncouple the mechanisms linking druginduced, topoisomerase II-DNA cleavable complex stabilization with drug-induced cytotoxicity, particularly in 1E3.