Perfluoroalkyl substances exposure and thyroid hormones in humans: epidemiological observations and implications

Thyroid hormones play crucial roles in normal neurodevelopment of fetus and child. Many chemicals can affect control and homeostasis of thyroid hormones, and eventually lead to various adverse health effects including neurodevelopmental disorders. Perfluoroalkyl substances (PFASs) are among the thyroid disrupting chemicals that can be encountered among general human population. Due to their unique physicochemical characteristics, PFASs have been used as surfactants and surface coating materials in many applications. Therefore, PFASs have been frequently detected in humans and environment worldwide. In cross-sectional studies using nationally representative general human populations of United States, several PFASs have shown significant associations with thyroid hormones. Moreover, among pregnant women and their infants, not only major PFASs such as perfluorooctane sulfonic acid and perfluorooctanoic acid, but also those with shorter or longer carbon chains showed significant associations with thyroid hormones. Often demographic characteristics such as sex, age, and disease status appear to influence the associations between PFASs exposure and thyroid hormones. In general, major PFASs showed hypothyroidism effects among pregnant women and infants. As 8 carbon based PFASs have been phased out, those with shorter or longer carbon chains have been used in growing amount as replacement. However, only limited information is available for their occurrences and toxicity among humans. Further investigations on these substituting PFASs are required. In addition, efforts are warranted to identify sources of and mitigate exposure to these thyroid disrupting chemicals especially during pregnancy and early stages of life.

Estradiol and triiodothyronine differentially modulate reproductive and thyroidal genes in male goldfish

While the reproductive and thyroidal systems are extensively studied in fish, they are largely studied in isolation from one another, but there is evidence supporting cross-regulation between these two systems. To better understand hormone action and the potential cross-regulation between estrogen and thyroid hormones, we examined gene expression changes in estrogen receptor (ER) and thyroid receptor (TR) subtypes and key enzymes responsible for the local synthesis and availability of estrogen and thyroid hormones (aromatase B and deiodinase, respectively) in sexually regressed, adult, male goldfish in response to 3 days waterborne exposures to 17β-estradiol (E2; 1 nM), triiodothyronine (T3; 20 and 100 nM), and co-treatments thereof. Treatments with E2 alone did not effect ER subtype transcripts in the liver, telencephalon, or testis; however, in the testis, 1 nM T3 decreased ERα and ERβ1 and co-treatments of T3 and E2 decreased ERβ1 levels. TRα-1 and TRβ transcripts were not auto-regulated by T3 or cross-regulated by E2. Although deiodinase type I levels were also unaffected, deiodinase type II decreased in response to T3 treatments. Liver deiodinase type III transcripts increased in response to T3 treatments, while E2 exhibited antagonistic effects on this T3-mediated induction. These results provide novel evidence of cross-talk between the reproductive and thyroid endocrine axes in a model teleost.

Thyroid disruption in walleye (Sander vitreus) exposed to environmental contaminants: cloning and use of iodothyronine deiodinases as molecular biomarkers

Thyroid hormones play a role in the initiation of ovarian maturation in fish. Thus, reports of delayed sexual maturation in female walleye (Sander vitreus) exposed to contaminants in the Ottawa River suggest the presence of endocrine disrupting chemicals. The objectives of this study were to assess the effects of environmental contaminants in the Ottawa River on thyroid hormones of immature walleye and to develop a molecular biomarker of thyroid status. Walleye were sampled in the Ottawa River at Deep River (reference site), at Rivière Blanche (downstream from the Ottawa and Gatineau municipal wastewater treatment plants outflows), and at Plaisance (downstream from a pulp and paper mill). Plasma thyroid hormone levels were measured by radioimmunoassay. Walleye at Plaisance had two-fold elevated levels of thyroxine (T(4)) and 1.5-fold elevated levels triiodothyronine (T(3)), whereas the molar ratio of T(3):T(4) was reduced by over 50% compared to Deep River. Plasma T(3) levels were also elevated by approximately 1.5-fold at Rivière Blanche. Three iodothyronine deiodinases, a family of enzymes responsible for converting the prohormone T(4) to biologically active T(3), as well as for inactivating these two hormones, were partially cloned in walleye. A real-time PCR assay of deiodinase expression indicated that hepatic mRNA levels of type I and type III deiodinase were not modified between sites, whereas they were increased for type II deiodinase at Rivière Blanche as compared to the other sites. The response of this novel molecular transcript indicates a divergence with that expected based on the effects of experimentally induced hyperthyroidism on fish deiodinase expression; additional endpoints are therefore necessary to interpret changes in thyroid hormones levels in fish exposed to environmental contaminants.

A cDNA for a putative type III deiodinase in the trout (Oncorhynchus mykiss): influence of holding conditions and thyroid hormone treatment on its hepatic expression

A putative rainbow trout type III deiodinase (D3) cDNA was amplified by PCR, using primers to evolutionarily conserved sequences. The RACE-derived complete cDNA was then identified by sequence comparison to that in tilapia and other vertebrates. The cDNA coded for a predicted 31,500 kDa protein of 278 amino acids, with a hydrophobic trans-membrane segment and with 80% similarity to tilapia D3 and 39% similarity to rainbow trout type II deiodinase (D2). It also showed a selenocysteine codon at position 141 and a putative SECIS element in the 3' untranslated end. In the liver, a second form of D3 was found that differed only at this 3' untranslated region; the coding region was identical in both forms. The D3 mRNA, measured by RT-PCR using primers located within the common, translated portion of the cDNA, was expressed in the brain and, depending on thyroidal status, in liver and kidney. Holding trout for 7 days in static water as opposed to flowing water caused increased plasma T4 levels, decreased hepatic D2 mRNA levels and T4 outer-ring deiodination (ORD) activity and increased D3 mRNA levels and T3 inner-ring (IRD) activity. Trout held in flowing water and fed T3 for 7 days showed increased plasma T3 levels and hepatic D3 mRNA levels and T3 IRD activity but decreased D2 mRNA levels and T4ORD activity. Trout held in static water and exposed to ambient T4 for 7 days showed increased plasma T4 levels and hepatic T3IRD activity but with no significant change in D2 or D3 mRNA levels. We conclude that hepatic D3 mRNA levels and T3IRD activity are enhanced and D2 mRNA levels and T4ORD activity are suppressed by adverse holding conditions or T3 treatment suggesting that the putative D3 cDNA and D2 cDNA represent respectively the genes determining T3IRD and T4ORD activities. However, there were changes in the ratios of mRNA levels to enzyme activity, raising the potential for post-transcriptional regulation and showing that mRNA levels alone may be unreliable indices of deiodinase activity. Post-transcriptional regulation of D3 enzyme activity may be influenced by the observed alternative 3' and 5' untranslated regions of the D3 mRNA.