Antiestrogenic Effects of 2,3,7,8-Tetrachlorodibenzo-p-dioxin Are Mediated by Direct Transcriptional Interference with the Liganded Estrogen Receptor

Aryl hydrocarbon receptor (AhR) ligands have diverse biological effects including striking antiestrogenic activity. We have investigated at the molecular level the antiestrogenic activity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We show that the previously documented TCDD-mediated decrease in estradiol-inducible gene products such as cathepsin D (cat D) is due to a sharp decline in mRNA accumulation despite any change in estrogen receptor (ER) mRNA levels. The decline in cat D mRNA level is most likely due to a decrease in transcription of the cat D gene since TCDD blocks the ability of ER to transactivate from an estrogen response element. AhR is required for this activity as TCDD is no longer antiestrogenic in a mutant cell line that is deficient in functional AhR. We provide evidence that the loss of transactivation potential by ER in the presence of TCDD is due to a sharp decrease in its ability to bind to an estrogen response element. Reciprocally, estradiol treatment blocked TCDD-induced accumulation of CYP1A1 mRNA and AhR-mediated activation of the CYP1A1 promoter. This is due to the ability of liganded ER to interfere with the binding of AhR to the xenobiotic response element. These results provide a molecular mechanism for the antiestrogenic effects of TCDD and demonstrate the presence of a two-way crosstalk between the intracellular signaling pathways involving estrogens and aryl hydrocarbons.