DNA topoisomerase II is a nuclear enzyme that regulates DNA topology via transient double-strand breaks in the DNA helix (5, 38, 39). Topoisomerase II is involved in cell proliferation and has been implicated in indispensable cellular processes, such as replication, transcription, recombination, and chromosomal condensation and segregation (38). This enzyme also has an essential function as a structural component of mitotic chromosome and interphase nuclear scaffolds (9, 14). Given the fact that expression of topoisomerase II in proliferating cells is higher than that in quiescent cells, this enzyme is a clinically useful target to elicit cytotoxic effects in proliferating tumor cells. Indeed, a variety of anticancer agents target topoisomerase II (20) and interfere with its catalytic activity by trapping the enzyme in a form that is covalently bound to DNA. These stable enzyme-associated DNA complexes induce DNA damage and cell death. In addition to catalytically inactivating topoisomerase II, previous studies have shown that aberrant expression of topoisomerase II is associated with the induction of apoptosis (28, 37). While forced expression of topoisomerase IIα in cells triggered apoptotic cell death, nuclear localization of the enzyme was required for efficient apoptotic induction. By contrast, another report demonstrated that depletion of topoisomerase IIα conferred induction of apoptosis (1). Taken together, these results suggest that appropriate regulation of topoisomerase IIα expression is essential for cell viability and proliferation. In this regard, deregulated expression of topoisomerase IIα is associated with the commitment of apoptotic cell death; however, the mechanism remains unclear. The protein kinase C (PKC) family of serine/threonine kinases is subdivided into (i) conventional PKCs (PKC α [PKCα], PKCβ, and PKCγ) that are calcium dependent and activated by diacylglycerol (DAG), (ii) novel PKCs (PKCδ, PKCɛ, PKCθ, and PKCμ) that are calcium independent and activated by DAG, and (iii) atypical PKCs (PKCζ and PKCλ) that are calcium independent and not activated by DAG (32). The ubiquitously expressed novel PKC, PKCδ, is tyrosine phosphorylated and activated by c-Abl and Lyn in the response to DNA damage (45, 48). PKCδ interacts with the nuclear DNA-dependent protein kinase catalytic subunit (DNA-PKcs) (2). Phosphorylation of DNA-PKcs by PKCδ inhibits the function of DNA-PKcs to form complexes with DNA and to phosphorylate downstream targets (2). In addition, cells deficient in DNA-PK are resistant to apoptosis induced by overexpression of the PKCδ catalytic domain. Other studies have demonstrated that the nuclear complex of c-Abl and Lyn includes the protein tyrosine phosphatase SHPTP1 (Src homology 2 domain [SH2]-containing protein tyrosine phosphatase 1) (21, 42) and that PKCδ phosphorylates and inactivates SHPTP1 in response to DNA damage (44). In cells that respond to genotoxic stress with apoptosis, PKCδ is cleaved by caspase-3 into a constitutively active catalytic fragment (PKCδCF) (10, 11). The finding that PKCδCF induced nuclear condensation and DNA fragmentation indicates that cleavage of PKCδ contributes to the apoptotic response (15). In this context, a recent study has demonstrated that PKCδ translocated to the nucleus and regulated Rad9 by phosphorylation in the apoptotic response to DNA damage (46). Furthermore, another study showed that cells derived from PKCδ-null transgenic mice were defective in mitochondrion-dependent apoptosis induced by various agents such as UV irradiation and hydrogen peroxide (27). These findings collectively support an essential role for PKCδ in the induction of apoptosis in the genotoxic stress response. The present study demonstrates that PKCδ interacts with topoisomerase IIα. This interaction is required for stabilization and activation of topoisomerase IIα following DNA damage. The results also demonstrate that genotoxic stress-induced topoisomerase IIα activation confers the induction of PKCδ-mediated apoptosis.