3,3[prime]-diindolylmethane (DIM) is an acid-catalyzed dimer of idole-3-carbinol (I3C), a phytochemical found in cruciferous vegetables that include broccoli, Brussels sprouts and cabbage. DIM is an aryl hydrocarbon receptor (AhR) ligand and a potential anticancer agent, namely for the treatment of breast cancer. It is also advertised as a compound that regulates sex hormone homeostasis.

Here we make use of RNA expression assays coupled to Chromatin Immunoprecipitation (ChIP) in breast cancer cell lines to study the effect of DIM on estrogen signaling. We further make use of growth assays, as well as fluorescence-activated cell sorting (FACS) assays, to monitor cell growth.

In this study, we report that 'physiologically obtainable' concentrations of DIM (10 muM) activate the estrogen receptor alpha (ERalpha) signaling pathway in the human breast cancer cell lines MCF7 and T47D, in a 17beta-estradiol (E2)-independent manner. Accordingly, we observe induction of ERalpha target genes such as GREB1 and TFF1, and an increase in cellular proliferation after treatment with 10 muM DIM in the absence of E2. By using an ERalpha specific inhibitor (ICI 182 780), we confirm that the transcriptional and proliferative effects of DIM treatment are mediated by ERalpha. We further show that the protein kinase A signaling pathway participates in DIM-mediated activation of ERalpha. In contrast, higher concentrations of DIM (e.g. 50 muM) have an opposite and expected effect on cells, which is to inhibit proliferation.

We document an unexpected effect of DIM on cell proliferation, which is to stimulate growth by inducing the ERalpha signaling pathway. Importantly, this proliferative effect of DIM happens with potentially physiological concentrations that can be provided by the diet or by taking caplet supplements.