Gambogic acid inhibits the catalytic activity of human topoisomerase IIα by binding to its ATPase domain

This study is intended to characterize the cellular target of gambogic acid (GA), a natural product isolated from the gamboge resin of Garcinia hurburyi tree, which possesses potent in vitro and in vivo antitumor activities. The antiproliferative activity of GA was further confirmed here in a panel of human tumor cells and multidrug-resistant cells. We found that GA significantly inhibited the catalytic activity of topoisomerase (Topo) II and, to a comparatively less extent, of Topo I, without trapping and stabilizing covalent topoisomerase-DNA cleavage complexes. Down-regulation of Topo IIα but not Topo I and Topo IIβ, reduced GA-induced apoptosis and the phosphorylation of c-Jun, and restored cell proliferation upon GA treatment. Moreover, GA antagonized etoposide-induced DNA damage and abrogated the antiproliferative activity of etoposide, whereas it did not affect camptothecin-induced DNA damage. By dissecting the actions of GA on the individual steps of Topo IIα catalytic cycle, we found that GA inhibited DNA cleavage and ATP hydrolysis. Moreover, GA directly bound to the ATPase domain of Topo IIα, and may share common binding sites with ATP. The results reported here show that GA exerts its antiproliferative effect by inhibiting the catalytic activity Topo IIα. They also indicate that GA inhibits Topo IIα-mediated DNA cleavage and modulate the activity of Topo II poisons, which provide rationale for further clinical evaluation of GA. [Mol Cancer Ther 2007;6(9):2429–40]