Thyroid endocrine status of wild European eels (Anguilla anguilla) in the Loire (France). Relationships with organic contaminant body burdens
Résumé
In teleost fish, thyroid function is involved in various critical physiological processes. Given the complexity of the
hypothalamo–pituitary–thyroid (HPT) axis, a large number of genes and proteins can be the potential target of
endocrine-disrupting compounds (EDCs). The aim of this study was to evaluate, in yellow and silver European
eels (Anguilla anguilla), potential effects of EDCs on thyroid status by analyzing the associations between EDC
body burdens and thyroid hormones (THs). In yellow individuals, greater free T3/T4 ratios (FT3/FT4) and
lower plasma FT4 levels were associated with greater concentrations of polychlorinated biphenyls (PCBs) and
polybrominated diphenyl ethers (PBDEs), as highlighted by significant correlations with many congeners. Few
positive relationships with alkylphenols were noticed. In contrast, silver eels usually exhibited less significant
correlations between THs and contaminant loads. Expression of a series of genes involved in the HPT axis was
also investigated in the silver individuals. Concerning mRNA expression in silver females, some main correlations
were noticed: thyroid-stimulating hormone (TSHβ) gene expression was significantly correlated to numerous
PCBs, and hepatic mRNA levels of deiodinase 2 (Deio 2) were negatively correlated to 2-hydroxyfluorene (2-
OHF) and 2-naphtol (2-OHNa). Thyroid receptor (TRα and TRβ) mRNA levels exhibited weak negative correlations with some PBDEs in silver females and males. Hepatic vitellogenin (Vtg) mRNA levels were detected in all silver males but at lower levels than in silver females. In males, Vtg mRNA levels were positively associated to
FT4/TT4. In silver females, strong positive correlations were found between congeners of PCBs, PBDEs and PFAS
suggesting potential estrogenic effects. Overall, the observed results indicate that several organic contaminants,
mainly dl-, ndl-PCBs and PBDEs, could be associated with changes in thyroid homeostasis in these fish, via direct
or indirect interactions with peripheral deiodination, metabolism of T4 and mechanisms involved in TSHβ, Deio 2
and Vtg gene transcription.