A role for PKCζ in potentiation of the topoisomerase II activity and etoposide cytotoxicity by wortmannin

Enhanced cytotoxicity of etoposide by wortmannin, an inhibitor of enzymes holding a phosphatidylinositol 3-kinase domain, was investigated in eight cell lines proficient or deficient for DNA double-strand break repair. Wortmannin stimulated the decatenating activity of topoisomerase II, promoted etoposide-induced accumulation of DNA double-strand breaks, shifted the specificity for cell killing by etoposide from the S to G₁ phase of the cell cycle, and potentiated the cytotoxicity of etoposide through two mechanisms. (a) Sensitization to high, micromolar amounts of etoposide required integrity of the nonhomologous end-joining repair pathway. (b) Wortmannin dramatically increased the susceptibility to low, submicromolar amounts of etoposide in a large fraction of the cell population irrespective of the status of ATM, Ku86, and DNA-PK_cs. It is shown that this process correlates depression of phosphatidylinositol 3-kinase-dependent phosphorylation of the atypical, ζ isoform of protein kinase C (PKCζ). Stable expression of a dominant-negative, kinase-dead mutant of PKCζ in a tumor cell line reproduced the hypersensitivity pattern induced by wortmannin. The results are consistent with up-regulation of the topoisomerase II activity in relation to inactivation of PKCζ and indicate that PKCζ maybe a useful target to improve the efficiency of topoisomerase II poisons at low concentration.