Hypophosphorylation of topoisomerase II in etoposide (VP-16)-resistant human leukemia K562 cells associated with reduced levels of β(II) protein kinase C

We selected and characterized a 30-fold etoposide (VP-16)resistant subline of K562 human leukemia cells (KNP.5) that exhibits quantitative and qualitative changes in topoisomerase II, including hypophosphorylation of this drug target. The initial rate of topoisomerase II phosphorylation was reduced 3-fold in K/VP.5 compared with K562 cells, but the rate of dephosphorylation was similar. Analysis of potential topoisomerase II protein kinases revealed a 3-fold reduction in the level of the β(II) protein kinase C (PKC) in K/VP.5 cells, whereas levels of α- and ε PKC, casein kinase II, p42(map) kinase, and p34(cdc2) kinase were comparable for both cell lines. The PKC activator, bryostatin 1, together with K562 nuclear extracts potentiated VP-16-induced topoisomerase II/DNA covalent complex formation in nuclei isolated from K/VP.5 cells but not from K562 cells. Bryostatin 1 effects were blocked by the PKC inhibitor 7-O-methyl-hydroxy-staurosporine. Bryostatin 1 also up-regulated topoisomerase II phosphorylation and potentiated VP-16 activity in intact K/VP.5 cells but had no enhancing effect in K562 cells. 4β-Phorbol-12,13-dibutyrate and 12-O-tetradecanoylphorbol-13- acetate did not potentiate VP-16-induced topoisomerase II/DNA complex formation in intact cells or in isolated K/VP.5 nuclei. Together, our results indicate that β(II), PKC plays a role in modulating the VP-16-induced DNA binding activity of topoisomerase II in resistant K/VP.5 cells through a mechanism linked to phosphorylation of topoisomerase II.