Antiestrogenicity of beta-naphthoflavone and PAHs in cultured rainbow trout hepatocytes: evidence for a role of the arylhydrocarbon receptor

The aims of the present study were to assess, (1) if polyaromatic hydrocarb ons (PAHs) are able to inhibit estradiol-regulated vitellogenin synthesis i n fish; and (2) if this antiestrogenic activity is mediated through the bin ding of PAHs to the arylhydrocarbon receptor (AhR). Cultured liver cells of rainbow trout, Oncorhynchus mykiss, were co-exposed to PAHs and 17 beta-es tradiol (E2), and the resulting effects on induction of AhR-regulated 7-eth oxyresorufin-O-deethylase (EROD) activity and on E2-regulated vitellogenesi s were investigated. The following test compounds were compared: the PAH 3- methylcholanthrene (3MC), which is a strong EROD inducer, the PAH anthracen e (ANT), which is not an inducer of EROD activity, and the model EROD induc er, beta-naphthoflavone (beta NF). 3MC and beta NF led to significant decre ases of E2-triggered hepatocellular VTG synthesis, whereas ANT exerted no a ntiestrogenic activity. The rank order of the antiestrogenic activity of th e test substances agreed with their EROD-inducing potency suggesting that t heir antiestrogenicity might be mediated through the AhR. Further evidence for this assumption comes from the observation that inhibitors such as alph a-naphthoflavone which interferes with ligand-AhR binding, and 8-methoxypso ralen (8MP), which prevents binding of the occupied AhR to responsive DNA e lements, clearly reduced the antiestrogenic effects of the xenobiotics. Fur thermore, from the comparison of estradiol concentrations in media of liver cells exposed to the CYP 1A-inducing agents and in media of control cells it is unlikely that the observed antiestrogenic effects were caused by an e nhanced E2 catabolism. In conclusion, the results from this study indicate that, (1) AhR-binding PAHs possess ail antiestrogenic activity; and (2) tha t the antiestrogenic activity is mediated through the AhR. (C) 2000 Elsevie r Science B.V. All rights reserved.