Functional characterization of estrogen receptor subtypes, ERα and ERβ, mediating vitellogenin production in the liver of rainbow trout
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The estrogen-dependent process of vitellogenesis is a key function on oviparous fish reproduction and it has been widely used as an indicator of xenoestrogen exposure. The two estrogen receptor (ER) subtypes, ERalpha and ERbeta, are often co-expressed in the liver of fish. The relative contribution of each ER subtype to modulate vitellogenin production by hepatocytes was studied using selected compounds known to preferentially interact with specific ER subtypes: propyl-pyrazole-triol (PPT) an ERalpha selective agonist, methyl-piperidino-pyrazole (MPP) an ERalpha selective antagonist, and diarylpropionitrile (DPN) an ERbeta selective agonist. First, the relative binding affinity of the test compounds to estradiol for rainbow trout hepatic nuclear ER was determined using a competitive ligand binding assay. All the test ligands achieved complete displacement of specific [(3)H]-estradiol binding from the nuclear ER extract. This indicates that the test ligands have the potential to modify the ER function in the rainbow trout liver. Secondly, the ability of the test compounds to induce or inhibit vitellogenin production by primary cultures of rainbow trout hepatocytes was studied. Estradiol and DPN were the only compounds that induced a dose-dependent increase on vitellogenin synthesis. The lack of vitellogenin induction by PPT indicates that ERalpha could not have a role on this reproductive process whereas the ability of DPN to induce vitellogenin production supports the participation of ERbeta. In addition, this hypothesis is reinforced by the results obtained from MPP plus estradiol. On one hand, the absence of suppressive activity of MPP in the estradiol-induced vitellogenin production does not support the participation of ERalpha. On the other hand, once blocked ERalpha with MPP, the only manifestation of agonist activity of estradiol would be achieved via ERbeta. In conclusion, the present results indicate that vitellogenin production is mainly mediated through ERbeta, implying, furthermore that compounds which only exhibit ERalpha selectivity are not detected by vitellogenin bioassay.
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