Synergistic interaction between histone deacetylase and topoisomerase II inhibitors is mediated through topoisomerase IIβ

Background: DNA topoisomerase If inhibitors and poisons are among the most efficacious drugs for the treatment of cancer. Sensitivity of cancer cells to the cytotoxic effects of topoisomerase II targeting agents is thought to depend on the expression of the topoisomerase II alpha isoform, and drug resistance is often associated with loss or mutation of topoisomerase II alpha. Histone deacetylase inhibitors (HDACi) are a novel class of compounds that potentiate the antitumor effects of topoisomerase II-targeting agents. Methods: The interaction between HDACi and topoisomerase II-targeting agents in cancer cells was evaluated as a function of topoisomerase Ila and topoisomerase lip expression. Topoisomerase II isoforms were selectively depleted using small interfering RNA and antisense. Drug-induced formation of cleavable complexes involving topoisomerase II alpha and topoisomerase II beta was evaluated by trapped-in-agarose DNA immunostaining and band depletion assays in the presence and absence of HDACi. Results: Preexposure to HDACi increased the cytotoxicity of topoisomerase II poisons. This was associated with a down-regulation of topoisomerase II alpha expression but had no effects on topoisomerase II alpha. In the setting of HDACi-induced chromatin decondensation and topoisomerase Ila depletion, topoisomerase II poison cytotoxicity was mediated through topoisomerase II beta cleavable complex formation. The HDACi-induced sensitization was also observed in cells with target-specific resistance to topoisomerase II poisons. Conclusions: The recruitment of topoisomerase II beta as a target may overcome primary or emergent drug resistance to topoisomerase II-targeting agents and hence may broaden the applicability of this important class of anticancer agents.