Cleavage/Religation and Sensitivity to the Antineoplastic Drugs Protein Kinase C: Effects on Enzyme-mediated DNA Phosphorylation of Topoisomerase II by Casein Kinase II and Updated Version

The effects of serine phosphorylation on the DNA cleavage/religation equilibrium of topoisomerase II and the sensitivity of the enzyme to antineoplastic drugs were characterized. Both casein kinase II and protein kinase C »ere used for these studies. Each kinase incorporated a maxi mum of ~ 1.4 phosphate molecules per homodimer of topoisomerase II. When the enzyme was incubated with both kinases simultaneously, phosphate incorporation increased to ~2.6 molecules/homodimer. In the absence of antineoplastic drugs, phosphorylation had only a slight effect on the DNA cleavage/religation equilibrium of topoisomerase II. Flowever, in the presence of etoposide or 4'-(9-acridinylamino)methanesulfon-m-anisidide, phosphorylation attenuated the ability of drugs to stabilize enzyme-DNA cleavage complexes. Levels of drug-induced DNA cleavage products decreased ~33% following phosphorylation of topoi somerase II by casein kinase II, ~17% following modification by protein kinase C, and ~50% following simultaneous phosphorylation of the enzyme by both kinases. This latter 50% reduction in DNA cleavage products correlated with an ~2-fold increase in the apparent first order rate constant for DNA religation mediated by simultaneously modified topoisomerase II. These results strongly suggest that the sensitivity of topoisomerase II toward antineoplastic drugs can be modulated by alter ing the phosphorylation state of the enzyme.