Indole-3-carbinol selectively uncouples expression and activity of estrogen receptor subtypes in human breast cancer cells.

Estrogen-responsive breast cancer cells, such as MCF7 and T47D cells, express both estrogen receptor (ER)-alpha (ERalpha) and ERbeta. Indole-3-carbinol (I3C) strongly down-regulated ERalpha protein and transcript levels, without altering the level of ERbeta protein, in both cell lines. In cells transfected with the ERalpha promoter linked to a luciferase gene reporter, I3C ablated ERalpha promoter activity. Propyl pyrazole triol (PPT) is a highly selective ERalpha agonist, whereas, 17beta-estradiol activates both ERalpha and ERbeta. I3C treatment inhibited the PPT- and 17beta-estradiol-induced proliferation of breast cancer cells, disrupted the PPT and 17beta-estradiol stimulation of estrogen response element (ERE)-driven reporter plasmid activity as well as of endogenous progesterone receptor transcripts. Using an in vitro ERE binding assay, I3C was shown to inhibit the level of functional ERalpha and stimulated the level of ERE binding ERbeta even though the protein levels of this receptor remained constant. In ERalpha-/ERbeta+ MDA-MB-231 breast cancer cells, I3C treatment stimulated a 6-fold increase in binding of ERbeta to the ERE. I3C also induced ERE- and activator protein 1-driven reporter plasmid activities in the absence of an ER agonist, suggesting that ERbeta is activated in indole-treated cells. Taken together, our results demonstrate that the expression and function of ERalpha and ERbeta can be uncoupled by I3C with a key cellular consequence being a significantly higher ERbeta:ERalpha ratio that is generally highly associated with antiproliferative status of human breast cancer cells.

Estrogen receptor beta inhibits human breast cancer cell proliferation and tumor formation by causing a G2 cell cycle arrest.

Studies indicate that estrogen receptor (ER) alpha mediates breast cancer-promoting effects of estrogens. The role of ERbeta in breast cancer is unknown. Elucidating the role of ERbeta in the pathogenesis of breast cancer is important because many human breast tumors express both ERalpha and ERbeta. We show that adenovirus-mediated expression of ERbeta changes the phenotype of ERalpha-positive MCF-7 cells. Estradiol increases cell proliferation and causes tumor formation of MCF-7 cells expressing only ERalpha. In contrast, introducing ERbeta into MCF-7 cells causes an inhibition of proliferation in vitro and prevents tumor formation in a mouse xenograft model in response to estradiol. ERbeta inhibits proliferation by repressing c-myc, cyclin D1, and cyclin A gene transcription, and increasing the expression of p21(Cip1) and p27(Kip1), which leads to a G(2) cell cycle arrest. These results demonstrate that ERalpha and ERbeta produce opposite effects in MCF-7 cells on cell proliferation and tumor formation. Natural or synthetic ERbeta-selective estrogens may lack breast cancer promoting properties exhibited by estrogens in hormone replacement regimens and may be useful for chemoprevention of breast cancer.