Co-planar 3,3′,4,4′5-pentachlorinated biphenyl and non-co-planar 2,2′,4,6,6′-pentachlorinated biphenyl differentially induce recruitment of oestrogen receptor α to aryl hydrocarbon receptor target genes

In the present study we examined the ability of 3,3',4,4'5-pentachlorinated biphenyl [PCB126 (polychlorinated biphenyl 126)], a prototypical AHR (aryl hydrocarbon receptor) agonist, and 2,2',4,6,6'-PCB (PCB 104), which does not activate AHR, to induce the recruitment of ER alpha (oestrogen receptor alpha) to CYP1A1 (cytochrome P4501A1 gene) and CYP1B1 promoters in T-47D human breast cancer cells and other cell lines. PCB 126 treatment strongly induced CYP1A1 and CYP1B1 mRNA expression that was unaffected by co-treatment with E2 (17 beta-oestradiol). PCB 104 failed to induce changes in either CYP1A1 or CYP1B1 expression levels. ChIP (chromatin immunoprecipitation) assays show that PCB 126, but not PCB 104, increased the promoter occupancy by ERa to CYP1A1 and CYP1B1 promoters. Co-treatment with PCB126 + E2 significantly enhanced the promoter occupancy of ERa at CYP1A1, whereas co-treatment with PCB126 + 4-hydroxytanioxifen orICI182,780 did not. Competitive binding studies revealed that neither PCB126 nor PCB104 bound to ERa. HEK-293 cells (human embryonic kidney-293 cells) stably transfected with ERa showed significantly higher PCB126-induced CYP1A1 expression compared with empty vector controls, whereas no increase was observed in cells stably transfected with ERa lacking its N-terminal AF1 (activation function-1) domain (ER alpha Delta AF1). Despite no increase in AHR-mediated gene expression, ChIP assays revealed that ER alpha Delta AF1 was present at CYP1A1 and CYP1B1 promoters. HC11 mouse mammary cells stably expressing shRNA (small-hairpin RNA) against ERa showed an 8-fold reduction in PCB 126-dependent Cyp1a1 expression. Our results provide further evidence that AHR agonists induce ERa promoter occupancy at AHR target genes through indirect activation of ERa, and support a role for ERa in AHR transactivation.