The isothiocyanates (ITCs) have long been known to possess chemopreventive activities for a variety of neoplasms including breast cancer, but the molecular mechanism by which ITCs prevent breast cancer development has not been established. In this study, we investigated the effects of benzyl and phenethyl isothiocyanate (BITC and PEITC) on the estrogen-stimulated growth of estrogen receptor α (ERα)-positive breast cancer MCF7 and T-47D cells. BITC and PEITC inhibited estrogen-stimulated cell growth and reduced the expression levels of ERα in MCF7 and T-47D cells in a dose- and time-dependent and reversible manner. In addition, BITC and PEITC also abrogated the transcriptional activity of ERα and hence inhibited estrogen-stimulated expression of the estrogen responsive gene, pS2. These results demonstrated that BITC and PEITC function as potent ERα disruptors to abrogate mitogenic estrogen signaling in ER-positive breast cancer cells, which provides a molecular explanation for the growth inhibitory function of ITCs in breast cancer development, and a rational for further exploration of ITCs as chemopreventive agents for human mammary carcinogenesis.

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