Abstract

Ligands of the aryl hydrocarbon receptor (AhR) include the environmental xenobiotic 2,3,7,8 tetrachlorodibenzo(p)dioxin (TCDD), polycyclic aryl hydrocarbons, and the dietary compounds 3, 3'-diindolylmethane (DIM), a condensation product of indol-3-carbinol found in Brassica vegetables, and the phytoalexin resveratrol (RES). The AhR and its cofactors regulate the expression of target genes at pentameric (GCGTG) xenobiotic responsive elements (XRE). Because the activation of cyclooxygenase-2 (COX-2) expression by AhR ligands may contribute to inflammation and tumorigenesis, we investigated the epigenetic regulation of the COX-2 gene by TCDD and the reversal effects of DIM in MCF-7 breast cancer cells. Results of DNA binding and chromatin immunoprecipitation (ChIP) studies documented that the treatment with TCDD induced the association of the AhR to XRE harbored in the COX-2 promoter and control CYP1A1 promoter oligonucleotides. The TCDD-induced binding of the AhR was reduced by small-interfering RNA for the AhR or the cotreatment with synthetic (3-methoxy-4-naphthoflavone) or dietary AhR antagonists (DIM, RES). In time course ChIP studies, TCDD induced the rapid (15 min) occupancy by the AhR, the histone acetyl transferase p300, and acetylated histone H4 (AcH4) at the COX-2 promoter. Conversely, the cotreatment of MCF-7 cells with DIM (10 micromol/L) abrogated the TCDD-induced recruitment of the AhR and AcH4 to the COX-2 promoter and the induction of COX-2 mRNA and protein levels. Taken together, these data suggest that naturally occurring modulators of the AhR such as DIM may be effective agents for dietary strategies against epigenetic activation of COX-2 expression by AhR agonists.