We have shown previously that withaferin A (WA), a constituent of Indian Ayurvedic medicine plant Withania somnifera that has been used safely for thousands of years for treatment of different ailments, inhibits growth of cultured MDA-MB-231 (an estrogen receptor negative cell line with mutant p53) and MCF-7 (an estrogen-responsive cell line with wild-type p53) human breast cancer cells. Moreover, the WA administration significantly inhibits growth of MDA-MB-231 xenografts in female athymic mice without causing any side effects. The WA-mediated inhibition of breast cancer cell growth in culture and xenografts correlates with apoptosis induction. However, the mechanism underlying proapoptotic effect of WA is not fully understood. The present study was designed to study the role of signal transducer and activator of transcription 3 (STAT3), an oncogenic transcription factor activated in many human malignancies including breast cancer, in proapoptotic response to WA using MDA-MB-231 and MCF-7 cells. Growth suppressive and proapoptotic concentrations of WA (2 and 4 μM) decreased constitutive (MDA-MB-231) or interleukin-6 (IL-6)-inducible (MDA-MB-231 and MCF-7) phosphorylation of STAT3 (Tyr705) and its upstream regulator Janus-activated kinase 2 (JAK2; Tyr1007/1008) in MDA-MB-231, which was accompanied by suppression of their protein levels especially at the higher concentration. Exposure of MDA-MB-231 or MCF-7 cells to WA also resulted in suppression of (a) transcriptional activity of STAT3 with or without IL-6 stimulation in both cells; (b) dimerization of STAT3 (MDA-MB-231); and (c) nuclear translocation of Tyr705 phosphorylated STAT3 in both cells. To our surprise, the IL-6-stimulation, either before or after WA treatment, did not have an appreciable effect on WA-mediated apoptosis in MDA-MB-231 or MCF-7 cell line. The IL-6-stimulated activation of STAT3 conferred only a modest protection against WA-mediated suppression of MDA-MB-231 cell invasion. General implication of these findings is that WA can trigger apoptosis and largely inhibit cell migration/invasion of breast cancer cells even after IL-6-induced activation of STAT3, which should be viewed as a therapeutic advantage for this agent. This investigation was supported by the USPHS grant CA142604-01, awarded by the National Cancer Institute.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3686. doi:10.1158/1538-7445.AM2011-3686