We have shown previously that withaferin A (WA), a promising anticancer constituent of Ayurvedic medicine plant Withania somnifera, inhibits growth of MCF-7 and MDA-MB-231 human breast cancer cells in culture and MDA-MB-231 xenografts in vivo by causing apoptosis. However, the mechanism of WA-induced apoptosis is not fully understood. The present study was designed to systematically determine the role of tumor suppressor p53 and estrogen receptor-? (ER-?) in proapoptotic response to WA using MCF-7, T47D, and ER-? overexpressing MDA-MB-231 cells as a model. WA treatment resulted in induction as well as increased S15 phosphorylation of p53 in MCF-7 cells, but RNA interference of this tumor suppressor conferred modest protection at best against WA-induced apoptosis. WA-mediated growth inhibition and apoptosis induction in MCF-7 cells were significantly attenuated in the presence of 17?-estradiol (E2). Exposure of MCF-7 cells to WA resulted in a marked decrease in protein levels of ER-? (but not ER-?) and ER-? regulated gene product pS2, and this effect was markedly attenuated in the presence of E2. WA-mediated down-regulation of ER-? protein expression correlated with a decrease in its nuclear level, suppression of its mRNA level, and inhibition of E2-dependent activation of ERE2e1b-luciferase reporter gene. Ectopic expression of ER-? in the MDA-MB-231 cell line conferred partial but statistically significant protection against WA-mediated apoptosis, but not G2/M phase cell cycle arrest. Collectively, these results indicate that WA functions as an anti-estrogen, and the proapoptotic effect of this promising natural product is partially attenuated by p53 knockdown and E2-ER-?.