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