Rapid increase in the activity of DNA topoisomerase I, but not topoisomerase II, in HL-60 promyelocytic leukemia cells treated with a phorbol diester

There is significant evidence to suggest that protein kinase C and DNA topoisomerases are functionally linked in signal transduction pathways. Much of this is based on the observation that phosphorylation of topoisomerase II by protein kinase C may lead to its activation in vitro and that inhibitors of topoisomerase II block phorbol diester-induced differentiation in HL-60 cells. In the present study, the activities of the DNA topoisomerases I and II have been quantitated to examine their regulation in phorbol diester-treated HL-60 cells undergoing differentiation. The activity of topoisomerase I increased rapidly after treatment with phorbol myristate acetate (PMA); it increased maximally (150% of control activity) at 3 hr post-treatment and remained elevated for at least 24 hr. Conversely, from the onset of exposure to PMA through 12 hr, there was no measurable alteration in topoisomerase II activity in PMA-treated cells. Moreover, there was a measurable decrease in topoisomerase II activity at the later time points, a result that occurred concomitantly with the loss of proliferative potential in differentiating HL-60 cells. Similar results were obtained when the activities of both enzymes were measured in nuclear extracts. The apparent increase in topoisomerase I activity was not due to an increase in the mass of the enzyme after PMA treatment, as measured by both western blotting and by the formation of camptothecin-dependent, topoisomerase I-DNA complexes. Taken together, these data suggest that the activities of the topoisomerases I and II may have been regulated independently in PMA-treated HL-60 cells, that the activity of topoisomerase II was not increased under conditions in which protein kinase C was activated in vivo, and that an increase in the activity of topoisomerase I may have had a role in the mechanism through which HL-60 cells underwent monocytic maturation in response to phorbol diesters.