A protease pathway for the repair of topoisomerase II-DNA covalent complexes

Despite rapid advances in the field of DNA repair, little is known about the repair of protein-DNA adducts. Previous studies have demonstrated that topoisomerase II (TopII)-DNA adducts (TopII-DNA covalent complexes) are rapidly degraded by the proteasome. It has been hypothesized that proteasomal degradation of TopII-DNA covalent adducts exposes TopII-concealed DNA double-strand breaks (DSBs)for repair. To test this hypothesis, the anticancer drug, VP-16 ( etoposide), was employed to induce TopII-DNA covalent complexes in mammalian cells, and the involvement of proteasome in processing TopII-DNA covalent complexes into DSBs was investigated. Consistent with the hypothesis, VP-16-induced DSBs as monitored by neutral comet assay, as well as DNA damage signals ( e. g. gamma-H2AX) were significantly reduced in the presence of the proteasome inhibitor, MG132. Using both top2 beta knock-out mouse embryonic fibroblasts and Top2 beta small interfering RNA knockdown PC12 cells, as well as postmitotic neurons in which TopII alpha was absent, we showed that VP-16-induced DNA damage signals were attenuated upon proteasome inhibition, suggesting the involvement of proteasome in the repair/processing of both TopII alpha-DNA and TopII beta-DNA adducts. By contrast, hydrogen peroxide-induced gamma-H2AX was unaffected upon proteasome inhibition, suggesting a specific requirement of the proteasome pathway in the processing of TopII-DNA covalent complexes into DNA damage.