Effects of a combined treatment with tamoxifen and estrogen receptor β agonists on human breast cancer cell lines.

Coexpression of estrogen receptors (ER) α and β is present in about half of all breast cancer cases. Whereas ERα is a well-established target for endocrine therapy with the selective estrogen receptor modulator tamoxifen, the applicability of ERβ as target in breast cancer therapy is unclear. In this study, we examined the effects of two synthetic ERβ agonists alone and in combination with tamoxifen on ERα/β-positive breast cancer cells. We treated MCF-7 and T-47D breast cancer cells with the ERβ agonists ERB-041 and WAY-200070 and measured the effects on cell growth. In addition, transcriptome analyses were performed by means of Affymetrix GeneChip arrays. When given alone, ERβ agonists ERB-041 and WAY-200070 did not affect the growth of MCF-7 or T-47D cells. In contrast, addition of these drugs to tamoxifen increased its growth-inhibitory effect on both cell lines. This effect was more pronounced under serum-free conditions, but was also observed in the presence of serum in T-47D cells. Transcriptome analyses revealed a set of genes regulated after addition of ERβ agonists including S100A8 and CD177. The observed enhanced growth-inhibitory effects of a combination of tamoxifen and ERβ agonists in vitro encourage further studies to test its possible use in the clinical setting.