Estrogen receptor “hijacking” by AhR-agonists and dioxin-like PCB congeners 126 and 77 in fish

Aryl hydrocarbon receptor (AhR) agonists are known to have anti-estrogenic properties. The anti-estrogenicity of AhR agonists are believed to be mediated through crosstalk between the estrogen receptor (ER) and AhR. In salmonid, this crosstalk is bidirectional and can be observed as alterations in gene and protein expression patterns. In the present study, we show that the strong AhR and dioxin-like PCB congener 126 produced estrogenic responses (ER-hijacking) in salmon hepatocytes. ER-hijacking, a phenomenon that was recently described in mammalian system occurs when the AhR-Arnt complex binds directly to the ER and induces transcription of ER controlled genes. Salmon hepatocytes isolated by two-way perfusion techniques were exposed to PCB126 (0.001, 0.01 and 0.05 uM) and the ER agonist nonylphenol (NP) at 5 and 10 uM, singly and also in combination. A second experiment was performed using cultured hepatocytes and 1 uM PCB-77 with sampling at 12, 24, 48 and 72 h after exposure. The third experiment was performed with PCB126 at the above named concentrations in the presence and absence of ICI (ER-antagonist). The mRNA expressions were analyzed by quantitative PCR using gene specific primers of genes involved in the AhR pathway (AhR2alfa, AhR2beta, Arnt, AhR-repressor, CYP1A1, UGT and a proteasome unit) and genes regulated by ER (ERalfa, ERbeta, Zr- protein, Vtg and vigilin). While NP alone induced the expression of genes controlled by ER, anti-NP actions of PCB126 were only observed in the expression patterns of ERa and ERb. Interestingly, the expression levels of ER-isoforms, Zr-protein and Vtg significantly increased in cells treated with PCB126 and 77 alone and these effects were reduced in the presence of ICI in a time-specific manner. Exposure of cells to PCB126 or PCB77 alone, induced AhR mediated transcription of CYP1A1, Ugt and Arnt, and NP inhibited these gene responses. These results strongly indicate that ER-hijacking is part of our recently postulated complex interactions between the ER and AhR in salmon. We believe that these results, especially the estrogenic actions of PCB126 and 77, despite being novel, will assist to further unravelling the mechanisms involved in ER-AhR crosstalk.