Abstract #2666: Timosaponin A3 is a steroidal saponin from Anemarrhena asphodeloides that has a selective cytotoxic activity towards cancer cells
Authors: Sylvia Fong, Frank King, Xiaoyue Zhao, Isaac Cohen, Emma Shtivelman
Journal: Cancer Research (2009)

Botanical medicine is one of the most popular complementary medical approaches, and herbal therapies are frequently sought and used by breast cancer patients. However, the molecular mechanisms through which certain herbal extracts exert growth inhibitory activity on breast cancer cells remain largely unknown. BN108, aqueous extract of Anemarrhena asphodeloides Bunge, induces cell death selectively in breast cancer lines and tumor cells of various origins but not in normal mammary epithelial cells and fibroblasts. Breast cancer cells sensitive to BN108 undergo apoptotic death, confirmed by Annexin V staining, caspase activation, cleavage of PARP and DNA fragmentation. In particular, caspases 4 and 9, whose activation is observed during endoplasmic reticulum (ER) stress induced apoptosis, are proteolytically activated. Inhibition of caspase 4 partially protects breast cancer cells from death induced by BN108. Expression array analysis of cells treated with BN108 shows induction of expression of several known pro-apoptotic and anti-proliferative genes such as REDD1, p21CIP, cyclin G2, stratifin and more. None are affected in normal mammary cells. BN108 induces rapid inactivation of AKT and mTOR kinases in breast cancer but not in non-transformed cells. The well-defined targets of mTORC1, S6kinase, S6 ribosomal protein and 4eBP1 are inactivated in BN108 treated cells. Expression array analysis also shows the induction of numerous genes encoding enzymes within the cholesterol synthesis pathway by BN108. They are induced to various degrees in all cell lines examined. However, the changes in total cholesterol levels are relatively minor in treated cells. In addition, BN108 also induces pattern of gene expression consistent with ER stress. We have identified timosaponin A3 (TspA3) as an active compound from BN108 that is responsible for the selective cytotoxicity for the whole extract. TspA3 is a steroidal saponin whose activity against cancer cells remained unexplored until now. Treatment with purified TspA3 at concentrations similar to those in the BN108 extract induces apoptosis in breast cancer cells but not in normal cells. TspA3 and BN108 induce largely overlapping transcriptional changes in cells. Similar to BN108, TspA3 inactivates major signaling pathways for growth and survival selectively in cancer cells (Akt and mTORC) and induces expression of proteins involved in cholesterol biosynthesis pathway and ER stress response. In conclusion, a component of BN108 extract, TspA3 is selectively cytotoxic for cancer versus normal cells. The selective cytotoxic properties of TspA3 could be related to the inhibition of major oncogenic pathways and induction of ER stress. Future studies will be aimed at understanding the relationship between the effect of TspA3 on these pathways and induction of apoptosis, which may give rise to a unique pathway for targeting tumor cells.  Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 2666.