| Summary: | 博士 === 高雄醫學大學 === 天然藥物研究所 === 96 === Apoptosis is presently one of the most obvious targets for cancer treatment as its relatively common inactivation in tumors, contributes to carcinogenesis, as well as resistance to chemotherapy. In the present study, the anticancer mechanisms of goniothalamin and tubocapsenolide A were investigated at molecular levels in human breast cancer MDA-MB-231 cells, respectively.
Goniothalamin is a styryl-lactone with putative anticancer activities, but its mechanism of action has never been explored before. Goniothalamin treatment of cells significantly induced cell cycle arrest at G2/M phase and apoptosis. By means of cell-cycle synchronization, the G2/M phase cells proved to be the most sensitive fraction to goniothalamin-induced apoptosis. Cells treated with goniothalamin revealed an increase in intracellular reactive oxygen species (ROS) and a decrease in intracellular glutathione (GSH) contents. The disruption of intracellular redox balance caused by goniothalamin was associated an enhancement of Cdc 25C degradation. Furthermore, the antioxidant N-acetylcysteine (NAC) and the GSH synthesis inhibitor DL-buthionine-(S, R)-sulfoximine (BSO), inhibited and enhanced, respectively, the effects of goniothalamin on cell cycle arrest and apoptosis. Taken together, our result demonstrates for the first time that goniothalamin disrupts intracellular redox balance and induces Cdc 25C degradation, which in turn causes cell cycle arrest and cell death maximally at G2/M phase in MDA-MB-231 cells.
Tubocapsenolide A, a novel withanolide-type steroid, has been reported to exhibit potent cytotoxicity against several human cancer cell lines. Treatment of MDA-MB-231 cells with tubocapsenolide A led to cell cycle arrest at G1 phase and apoptosis. The actions of tubocapsenolide A were correlated with proteasome-dependent degradation of Cdk4, cyclin D1, Raf-1, Akt and mutant p53, which are heat shock protein 90 (Hsp90) client proteins. The redox states of cells treated with tubocapsenolide A were interfered by transiently increasing the ROS formation and decreasing the intracellular GSH contents. Nonreducing SDS-PAGE revealed that tubocapsenolide A rapidly and selectively induced thiol oxidation and aggregation of Hsp90 and Hsp70, both in intact cells and in cell-free systems using purified recombinant proteins. Furthermore, tubocapsenolide A inhibited the chaperon activity of Hsp90-Hsp70 complex in the luciferase refolding assay. NAC, a thiol antioxidant, prevented all of tubocapsenolide A-induced effects, including oxidation of heat shock proteins, degradation of Hsp90 client proteins as well as apoptosis. In contrast, non-thiol antioxidants (trolox and vitamin C) were ineffective to prevent Hsp90 inhibition and cell death. Taken together, our results demonstrate that the tubocapsenolide A inhibits the activity of Hsp90-Hsp70 chaperone complex, at least in part, by a direct thiol oxidation, which in turn leads to the destabilization and depletion of Hsp90 client proteins, and thus causes cell cycle arrest and apoptosis in MDA-MB-231 cells. Therefore, tubocapsenolide A can be considered as a new type of inhibitor of Hsp90-Hsp70 chaperone complex, which has the potential to be developed as a novel strategy for cancer treatment.
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