Recent reports indicate that SAhRMs such as diindolylmethane (DIM) inhibit α-estrogen receptor (ER) expression and estrogen signaling in human breast cancer cells. There is also increasing evidence that SAhRMs demonstrate anti estrogenic and antitumorigenic activity in ER positive human breast adenocarcinoma (MCF-7) cells via cross talk between aromatic hydrocarbon receptor (AhR) and ER pathways. Additional reports suggest that BRCA1 transcriptional activity is enhanced by interactions with AhR and the levels of BRCA1 expression play a role in tamoxifen sensitivity/resistance in breast cancer cells by altering estrogen receptor-coregulator interactions. However, the effects of AhR and its agonists and antagonists, and their modulatory effect on BRCA1 in resistant human breast cancer cells have not been investigated. We have tested the hypothesis that DIM, an AhR agonist may inhibit cell growth in a concentration and time dependant manner against doxorubicin resistant MCF-7 (MCF-7/dox) cells and may enhance the activity of 4-OH tamoxifen (TAM) against both MCF-7 and MCF-7/dox cells by enhancing the expression of BRCA1. Initially we determined the cytotoxic effect of DIM against the two cell lines by utilizing the MTT assay. Both DIM and TAM were equally effective against MCF-7 and MCF-7/dox cells with the IC50 values of approximately 90 μM and 20 μM, respectively at 72 hr. However, a combination of DIM and TAM resulted in an increase in cytotoxcity and statistically significant lowering of the IC50 concentration to 50 μM and 10 μM for each drug, respectively. We also investigated the effects of AhR antagonist, α-naphthoflavone (NF) which had a similar IC 50 value of approximately 112 μM against both these cell lines, whereas a combination of NF and TAM required only half the concentrations of 50 μM and 10 μM to inhibit 50 % of the cell growth of each cell line. Since BRCA1, a tumor suppressor gene, has been implicated in the mechanism of tamoxifen induced growth inhibition, we determined the expression of this protein upon exposure of these cells to DIM. Results from western blot analysis show that there is a significant increase in BRCA1 and AhR protein levels in these cells upon treatment with DIM when compared to protein levels from untreated MCF-7 and MCF-7 /dox cells. Thus activation of BRCA1 by SAhRMs appears to be one of the mechanisms responsible for enhanced growth inhibitory effect of tamoxifen against these cells. These data suggest that either an AhR agonist or an antagonist can be conveniently employed in combination with TAM to increase the activity of TAM against both MCF-7 cells and doxorubicin resistant cells.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5419.