Ligand-independent activation of estrogen receptor function by 3,3 '-diindolylmethane in human breast cancer cells

3,3'-Diindolylmethane (DIM), a major in vivo product of acid-catalyzed olig omerization of indole-3-carbinol (I3C), is a promising anticancer agent pre sent in vegetables of the Brassica genus. We investigated the effects of DI M on estrogen-regulated events in human breast cancer cells and found that DIM was a promoter-specific activator of estrogen receptor (ER) function in the absence of 17 beta-estradiol (E-2). DIM weakly inhibited the E-2-induc ed proliferation of ER-containing MCF-7 cells and induced proliferation of these cells in the absence of steroid, by approximately 60% of the E-2 resp onse. DIM had little effect on proliferation of ER-deficient MDA-MB-231 cel ls, suggesting that it is not generally toxic at these concentrations. Alth ough DIM did not bind to the ER in this concentration range, as shown by a competitive ER binding assay, it activated the ER to a DNA-binding species. DIM increased the lever of transcripts for the endogenous pS2 gene and act ivated the estrogen-responsive pERE-vit-CAT and pS2-tk-CAT reporter plasmid s in transiently transfected MCF-7 cells. In contrast, DIM failed to activa te transcription of the simple E-2- and diethylstilbesterol-responsive repo rter construct pATC2. The estrogen antagonist ICI 182780 (7 alpha-[9-[(4,4, 5,5,5-pentafluoropentyl)sulfonyl-]nonyl]-estra-1,3,5(10)-triene-3,17 beta-d iol) was effective against DIM-induced transcriptional activity of the pERE -vit-CAT reporter, which further supports the hypothesis that DIM is acting through the ER. We demonstrated that ligand-independent activation of the ER in MCF-1 cells could be produced following treatment with the D1 dopamin e receptor agonist SKF-82958 [(+/-)6-chloro-7,8-dihydroxy-3-allyl-1-phenyl- 2,3,4,5-tetrahydro-1H-3-benzazepinehydrobromide]. We also demonstrated that : the agonist effects of SKF-82958 and DIM, but not of E-2, could be blocke d by co-treatment with the protein kinase A (PKA) inhibitor H-89 (N-[2-(p-b romocinnamylamino)ethyl]-5-isoquinolinesulfonamide). These results have unc overed a promoter-specific, ligand-independent activation of ER signaling f or DIM that may require activation by PKA, and suggest that this major I3C product may be a selective activator of ER function. (C) 2000 Elsevier Scie nce Inc.