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- The goitrogenic efficiency of thioamides in a marine teleost, sea bream (Sparus auratus)Publication . Campinho, Marco António; Morgado, Isabel; Pinto, Patricia IS; Silva, Nádia; Power, DeborahStudies on the role of thyroid hormones (THs) in teleost fish physiology have deployed the synthetic goitrogens, methimazol (MMI), propilthiouracil (PTU) and thiourea (TU) that are used to treat human hyperthyroidism. However, the action of the goitrogens, MMI, PTU and TU at different levels of the hypothalamic–pituitary–thyroid (HPT) axis in teleosts is largely unknown. The central importance of the hypothalamus and pituitary in a number of endocrine regulated systems and the cross-talk that occurs between different endocrine axes makes it pertinent to characterize the effects of MMI, PTU and TU, on several endpoints of the thyroid system. The marine teleost, sea bream (Sparus auratus) was exposed to MMI, PTU and TU (1 mg/kg wet weight per day), via the diet for 21 days. Radioimmunoassays (RIA) of plasma THs and ELISA of the TH carrier transthyretin (TTR) revealed that MMI was the only chemical that significantly reduced plasma TH levels (p < 0.05), although both MMI and PTU significantly (p < 0.05) reduced plasma levels of circulating TTR (p < 0.05). Histological analysis of the thyroid tissue revealed modifications in thyrocyte activity that explain the reduced circulating levels of THs. MMI also significantly (p < 0.05) up-regulated transcript abundance of liver deiodinase 1 and 2 while significantly (p < 0.05) decreasing TRb expression in the pituitary, all hallmarks of HPT axis action of goitrogens in vertebrates. The results indicate that in the sea bream MMI is the most effective goitrogen followed by PTU and that TU (1 mg/kg wet weight for 21 days) failed to have a goitrogenic effect. The study highlights the non-uniform effect of goitrogens on the thyroid axis of sea bream and provides the basis for future studies of thyroid disrupting pollutants.
- Regulation of transthyretin by thyroid hormones in WshPublication . Morgado, Isabel; Santos, C. R. A.; Jacinto, R.; Power, DeborahTransthyretin (TTR) is a thyroid hormone-binding protein (THBP) which in its tetrameric form transports thyroid hormones (THs), thyroxine (T4) and triiodothyronine (T3) in the blood of vertebrates. The principal site of production of TTR is the liver but in the sea bream TTR mRNA is also present in the heart, intestine and brain. The regulation of TTR is unstudied in Wsh and the normal circulating level of this THBP is unknown. The aim of the present study was to establish factors which regulate TTR production in Wsh. As a Wrst step a number of tools were generated; sea bream recombinant TTR (sbrTTR) and speciWc sbrTTR antisera which were used to establish an ELISA (enzyme-linked immunosorbent assay) for measuring TTR plasma levels. Subsequently, an experiment was conducted to determine the inXuence of THs on TTR production. Circulating physiological levels of TTR in sea bream determined by ELISA are approximately 3.8 gml¡1. Administration of T3 and T4 to sea bream signiWcantly increased (p< 0.001 and p<0.005, respectively) the concentration of circulating TTR (V11.5 gml¡1) in relation to control Wsh, but did not change gene transcription in the liver. Methimazol (MMI) an antithyroid agent, failed to signiWcantly reduce circulating THs below control levels but signiWcantly increased (p < 0.005) plasma TTR levels (approximately 10.8 gml¡1) and decreased (p< 0.05) transcription in the liver. Future studies will aim to elucidate in more detail these regulatory pathways.
- Disruption of thyroid hormone binding to sea bream recombinant transthyretin by ioxinyl and polybrominated diphenyl ethersPublication . Morgado, Isabel; Hamers, Timo; Van der Ven, Leo; Power, DeborahA number of chemicals released into the environment share structural similarity to the thyroid hormones (THs), thyroxine (T4) and triiodothyronine (T3) and it is thought that they may interfere with the thyroid axis and behave as endocrine disruptors (EDs). One of the ways by which such environmental contaminants may disrupt the TH axis is by binding to TH transporter proteins. Transthyretin (TTR) is one of the thyroid hormone binding proteins responsible for TH transport in the blood. TTR forms a stable tetramer that binds both T4 and T3 and in fish it is principally synthesized in the liver but is also produced by the brain and intestine. In the present study, we investigate the ability of some chemicals arising from pharmaceutical, industrial or agricultural production and classified as EDs, to compete with [I125]-T3 for sea bream recombinant TTR (sbrTTR). Ioxinyl, a common herbicide and several polybrominated diphenyl ethers were strong inhibitors of [I125]-T3 binding to TTR and some showed even greater affinity than the natural ligand T3. The TTR competitive binding assay developed offers a quick and effective tool for preliminary risk assessment of chemicals which may disrupt the thyroid axis in teleost fish inhabiting vulnerable aquatic environments.