We have previously cloned and characterized three estrogen receptors (ER) in the zebrafish (zfERalpha, zfERbeta1 and zfERbeta2). We have also shown that they are functional in vitro and exhibit a distinct expression pattern, although partially overlapping, in the brain of zebrafish. In this paper, we have shown that the hepatic expression of these zfER genes responds differently to estradiol (E2). In fact, a 48-h direct exposure of zebrafish to E2 resulted in a strong stimulation of zfERalpha gene expression while zfERbeta1 gene expression was markedly reduced and zfERbeta2 remained virtually unchanged. To establish the potential implication of each zfER in the E2 upregulation of the zfERalpha gene, the promoter region of this gene was isolated and characterized. Transfection experiments with promoter-luciferase reporter constructs together with different zfER expression vectors were carried out in different cell contexts. The data showed that in vivo E2 upregulation of the zfERalpha gene requires ERalpha itself and a conserved transcription unit sequence including at least an imperfect estrogen-responsive element (ERE) and an AP-1/ERE half site at the proximal transcription initiation site. Interestingly, although in the presence of E2 zfERalpha was the most potent at inducing the expression of its own gene, the effect of E2 mediated by zfERbeta2 represented 50% of the zfERalpha activity. In contrast, zfERbeta1 was unable to upregulate the zfERalpha gene whereas this receptor form was able to tightly bind E2 and activate a reporter plasmid containing a consensus ERE. Altogether, these results indicated that the two ERbeta forms recently characterized in teleost fish could have partially distinct and not redundant functions.

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