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)-α (ERα) and ERβ. Indole-3-carbinol (I3C) strongly down-regulated ERα protein and transcript levels, without altering the level of ERβ protein, in both cell lines. In cells transfected with the ERα promoter linked to a luciferase gene reporter, I3C ablated ERα promoter activity. Propyl pyrazole triol (PPT) is a highly selective ERα agonist, whereas, 17β-estradiol activates both ERα and ERβ. I3C treatment inhibited the PPT- and 17β-estradiol-induced proliferation of breast cancer cells, disrupted the PPT and 17β-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 ERα and stimulated the level of ERE binding ERβ even though the protein levels of this receptor remained constant. In ERα−/ERβ+ MDA-MB-231 breast cancer cells, I3C treatment stimulated a 6-fold increase in binding of ERβ to the ERE. I3C also induced ERE- and activator protein 1-driven reporter plasmid activities in the absence of an ER agonist, suggesting that ERβ is activated in indole-treated cells. Taken together, our results demonstrate that the expression and function of ERα and ERβ can be uncoupled by I3C with a key cellular consequence being a significantly higher ERβ:ERα ratio that is generally highly associated with antiproliferative status of human breast cancer cells.