Protein kinase C phosphorylates topoisomerase II: Topoisomerase activation and its possible role in phorbol ester-induced differentiation of HL-60 cells

DNA topoisomerase II from Drosophila was phosphorylated effectively by protein kinase C. With a K(m) of about 100 nM, the reaction was rapid, occurring at 4°C as well as at 30°C and requiring as little as 0.6 ng of the protein kinase per 170 ng of topoisomerase. About 0.85 mole of phosphate could be incorporated per mole of topoisomerase II, with phosphoserine as the only amino acid produced. The reaction was dependent on Ca²⁺ and phosphatidylserine and was stimulated by phorbol esters. Calmodulin-dependent protein kinase II, but not cyclic AMP-dependent protein kinase, was also able to phosphorylate the topoisomerase. Phosphorylation of topoisomerase II by protein kinase C resulted in appreciable activation of the topoisomerase, suggesting that it may represent a possible target for the regulation of nuclear events by protein kinase C. This possibility is supported by the finding that the phorbol ester-induced differentiation of HL-60 cells was blocked by the topoisomerase II inhibitors novobiocin and 4'-(9-acridinylamino)methanesulfon-m-anisidid(m-AMSA), but not by the inactive analog o-AMSA.