We have shown previously that withaferin A (WA), a C5,C6-epoxy steroidal lactone derived from an Ayurvedic medicine plant (Withania somnifera), not only inhibits growth of human breast cancer cells in vitro and in vivo but also prevents mammary cancer development in a transgenic mouse model without any toxicity. However, the mechanisms underlying cancer preventive effect of WA are not fully understood. Herein, we report that tubulin is a novel target of WA in human breast cancer cells. WA treatment resulted in G2 and mitotic arrest in MCF-7 (estrogen-receptor positive), SK-BR-3 (estrogen-receptor negative), and SUM159 (triple-negative) cells in association with a marked decrease in protein level of β-tubulin. These effects were not observed with the naturally-occurring C6,C7-epoxy analogs of WA (withanone and withanolide A). In addition, a non-tumorigenic normal human mammary epithelial cell line (MCF-10A) was markedly more resistant to mitotic arrest and tubulin downregulation by WA treatment compared with breast cancer cells. Downregulation of tubulin protein in WA-treated cells was due to transcriptional repression as well as proteasomal degradation. Vehicle-treated control breast cancer cells exhibited a normal bi-polar spindle with chromosomes aligned along the metaphase plate. In contrast, WA treatment led to a severe disruption of the normal spindle morphology in breast cancer cells. Unlike breast cancer cells, WA-treated MCF-10A cells exhibited aggregation of tubulin around nucleus. NMR analyses revealed that the A-ring enone in WA, but not in withanone or withanolide A, was highly reactive with cysteamine and rapidly succumbed to irreversible nucleophilic addition. Mass spectrometry demonstrated direct covalent binding of WA at cysteine-303 of β-tubulin in MCF-7 cells. Molecular docking studies indicated that the WA-binding pocket was located on the surface of the β-tubulin and characterized by a hydrophobic floor, a hydrophobic wall, and a charge-balanced hydrophilic entrance. These results provide novel insights into the mechanism of anticancer effect of WA. This work was supported by the grants RO1 CA142604 and P30 CA047904 awarded by the National Cancer Institute.

Citation Format: Shivendra V. Singh, Marie Lue Antony, Joomin Lee, Eun-Ryeong Hahm, Su-Hyeong Kim, Guillermo Romero, Adam I. Marcus, Zhen Yang, Vandana Kumari, Xinhua Ji, Courtney L. Vowell, Peter Wipf, Guy T. Uechi, Nathan A. Yates. Withaferin A downregulates tubulins and covalently binds β-tubulin at cysteine-303 in human breast cancer cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 228. doi:10.1158/1538-7445.AM2014-228