Disruption of the thyroid system by diethylstilbestrol and ioxynil in the sea bream (Sparus aurata)

Evaluation of thyroid-disrupting effects of bisphenol F and bisphenol S on zebrafish (Danio rerio) using anti-transthyretin monoclonal antibody-based immunoassays

The thyroid disrupting chemicals (TDCs) have raised great concerns due to their adverse impacts on thyroid hormones (THs). In this study, we investigated the thyroid-disrupting effects of bisphenol F (BPF) and bisphenol S (BPS), two major BPA substitutes, on adult zebrafish (Danio rerio). Firstly, anti-transthyretin (TTR) monoclonal antibody (anti-TTR mAb) was prepared and used to establish an indirect ELISA, which had a working range of 15.6∼1000 ng/mL of a detection limit of 6.1 ng/mL. The immunoassays based on anti-TTR mAb showed that exposure to BPF (10 and 100 μg/L) and BPS (100 μg/L) significantly elevated the levels of TTR protein in the plasma, liver, and brain tissues. Moreover, immunofluorescence showed that 100 μg/L BPF and BPS induced the production of TTR protein in liver and brain tissues. In addition, BPF and BPS increased THs levels and damaged thyroid tissue structure in adult female zebrafish. Especially, 100 μg/L BPF significantly increased T4 and T3 levels by 2.05 and 1.14 times, and induced pathological changes of thyroid follicles. The changes in the expression levels of genes involved in the hypothalamus-pituitary-thyroid (HPT) axis further illustrated that BPF and BPS had significant adverse effects on THs homeostasis and thyroid function in zebrafish. Therefore, TTR immunoassays could be used for the evaluation of thyroid-disrupting effects in fish and BPF exhibited greater disruption than BPS.

Ioxynil and diethylstilbestrol impair cardiac performance and shell growth in the mussel Mytilus coruscus

The herbicide ioxynil (IOX) and the synthetic estrogen diethylstilbestrol (DES) are environmentally relevant contaminants that act as endocrine disruptors (EDCs) and have recently been shown to be cardiovascular disruptors in vertebrates. Mussels, Mytilus coruscus, were exposed to low doses of IOX (0.37, 0.037 and 0.0037 mg/L) and DES (0.27, 0.027 and 0.0027 mg/L) via the water and the effect monitored by generating whole animal transcriptomes and measuring cardiac performance and shell growth. One day after IOX (0.37 and 0.037 mg/L) and DES (0.27 and 0.027 mg/L) exposure heart rate frequency was decreased in both groups and 0.27 mg/L DES significantly reduced heart rate frequency with increasing time of exposure (P < 0.05) and no acclimatization occurred. The functional effects were coupled to significant differential expression of genes of the serotonergic synapse pathway and cardiac-related genes at 0.027 mg/L DES, which suggests that impaired heart function may be due to interference with neuroendocrine regulation and direct cardiac effect genes. Multiple genes related to detoxifying xenobiotic substances were up regulated and genes related to immune function were down regulated in the DES group (vs. control), indicating that detoxification processes were enhanced, and the immune response was depressed. In contrast, IOX had a minor disrupting effect at a molecular level. Of note was a significant suppression (P < 0.05) by DES of shell growth in juveniles and lower doses (< 0.0027 mg/L) had a more severe effect. The shell growth depression in 0.0027 mg/L DES-treated juveniles was not accompanied by abundant differential gene expression, suggesting that the effect of 0.0027 mg/L DES on shell growth may be direct. The results obtained in the present study reveal for the first time that IOX and DES may act as neuroendocrine disrupters with a broad spectrum of effects on cardiac performance and shell growth, and that DES exposure had a much more pronounced effect than IOX in a marine bivalve.

Oral cortisol and dexamethasone intake: Differential physiology and transcriptional responses in the marine juvenile Sparus aurata

This study approached the long-term oral administration of cortisol (F) and dexamethasone (DEX), two synthetic glucocorticoids, compared to a control group (CT) in the juveniles of a marine teleost, the gilthead seabream (Sparus aurata). Physiologically, DEX treatment impaired growth, which appears to be linked to carbohydrate allocation in muscle and liver, hepatic triglycerides depletion, and reduced hematocrit. Hypophyseal gh mRNA expression was 2-fold higher in DEX than in CT or F groups. Similarly, hypothalamic trh and hypophyseal pomcb followed this pattern. Plasma cortisol levels were significantly lower in DEX than in CT, while F presented intermediate levels. In the posterior intestine, measured short circuit-current (Isc) was more anion absorptive in CT and F compared to the DEX group, whereas Isc remained unaffected in the anterior intestine. The derived transepithelial electric resistance (TEER) significantly differed between intestinal regions in the DEX group. These results provide new insights to understand better potential targeted biomarkers indicative of the differential glucocorticoid or mineralocorticoid-receptors activation in fish.

Ioxynil and diethylstilbestrol increase the risks of cardiovascular and thyroid dysfunction in zebrafish

Endocrine disruption results from exposure to chemicals that alter the function of the endocrine system in animals. Chronic 60 days of exposure to a low dose (0.1 μM) of ioxynil (IOX) or diethylstilbestrol (DES) via food was used to determine the effects of these chemicals on the physiology of the heart and thyroid follicles in juvenile zebrafish. Immunofluorescence analysis and subsequent 3D morphometric analysis of the zebrafish heart revealed that chronic exposure to IOX induced ventricle deformation and significant volume increase (p < 0.001). DES exposure caused a change in ventricle morphology, but volume was unaffected. Alongside, it was found that DES exposure upregulated endothelial related genes (angptl1b, mhc1lia, mybpc2a, ptgir, notch1b and vwf) involved in vascular homeostasis. Both IOX and DES exposure caused a change in thyroid follicle morphology. Notably, in IOX exposed juveniles, thyroid follicle hypertrophy was observed; and in DES-exposed fish, an enlarged thyroid field was present. In summary, chronic exposure of juvenile zebrafish to IOX and DES affected the heart and the thyroid. Given that both chemicals are able to change the morphology of the thyroid it indicates that they behave as endocrine disruptive chemicals (EDCs). Heart function dynamically changes thyroid morphology, and function and hence it is likely that the observed cardiac effects of IOX and DES are the source of altered thyroid status in these fish.

Exposure to N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) affects the growth and development of zebrafish embryos/larvae

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is used as a ubiquitous rubber antioxidant worldwide and has been shown to be potentially toxic to aquatic organisms. In this study, zebrafish embryos were exposed to 6PPD for five days starting at two hours post-fertilization at concentrations of 0, 0.0022, 0.022, and 0.22 mg/L to investigate its effects on embryonic development, the growth hormone/insulin-like growth factor (GH/IGF) axis, and the hypothalamic-pituitary-thyroid (HPT) axis. The results showed that the 96 h LC50 of 6PPD was 2.2 mg/L. 6PPD exposure decreased hatchability, lowered autonomous movement, reduced body length in zebrafish embryos and caused deformities. The hormones levels and the expression of genes related to GH/IGF and HPT axis were altered after exposure to 6PPD in zebrafish larvae. These results indicated that the GH/IGF and HPT axis was disturbed. Moreover, treatment of 6PPD produced oxidative stress in zebrafish embryos. Overall, the present study thus demonstrated that exposure to 0.22 mg/L 6PPD caused developmental toxicity and disrupted the GH/IGF and HPT axis of zebrafish, which could be responsible for developmental impairment and growth inhibition.

Phenanthrene disrupting effects on the thyroid system of Arabian seabream, Acanthopagrus arabicus: In situ and in vivo study

Phenanthrene, a polycyclic aromatic hydrocarbon (PAH), is one of the endocrine disrupting chemicals (EDCs). The present study aimed to evaluate the effects of phenanthrene on histophysiology of thyroid in Arabian seabream (Acanthopagrus arabicus). In this regards, different concentrations of phenanthrene (2, 20 and 40 pg/gbw) were injected to Acanthopagrus arabicus and changes in thyroid tissue structure and the serum levels of triiodothyronine (T3) and Thyroxine (T4) were assessed. The experiment lasted 21 days. Alterations in thyroid tissue structure and T3 and T4 serum levels also were assessed in Acanthopagrus arabicus caught from different stations of the Persian Gulf (Jafari, Samail, Arvand, Zangi, Bahrakan). In addition, the concentration of phenanthrene was measured in the fish muscle and sediment samples from the stations. Phenanthrene concentration reached the maximum level in the muscle of all injected fish after 4 days and then decreased by the end of the experiment. The highest and lowest concentrations of phenanthrene were recorded in the fish muscle and sediment samples collected from Jafari and Bahrakan, respectively. The levels of T3 and T4 decreased dose dependently in phenanthrene-injected fish up to day 7 and then increased by the end of the experiment. The serum level of T3 and T4 in fish collected from different stations was as follows: Jafari<Samail<Arvand<Zangi≤Bahrakan. Some tissue changes observed in fish included decrease in the thickness of thyroid follicle epithelium and increase in follicle diameter. In conclusion, according to the results, phenanthrene significantly affected thyroid function in fish.

Zebrafish as the toxicant screening model: Transgenic and omics approaches

The production of large amounts of synthetic industrial and biomedical compounds, together with environmental pollutants, poses a risk to our ecosystem and induces negative effects on the health of wildlife and human beings. With the emergence of the global problem of chemical contamination, the adverse biological effects of these chemicals are gaining attention among the scientific communities, industry, governments, and the public. Among these chemicals, endocrine disrupting chemicals (EDCs) are regarded as one of the major global issues that potentially affecting our health. There is an urgent need of understanding the potential hazards of such chemicals. Zebrafish have been widely used in the aquatic toxicology. In this review, we first discuss the strategy of transgenic lines that used in the toxicological studies, followed by summarizing the current omics approaches (transcriptomics, proteomics, metabolomics, and epigenomics) on toxicities of EDCs in this model. We will also discuss the possible transgenerational effects in zebrafish and future prospective of the integrated omics approaches with customized transgenic organism. To conclude, we summarize the current findings in the field, and provide our opinions on future environmental toxicity research in the zebrafish model.

Differential susceptibility to endocrine disruptor-induced epimutagenesis

There is now considerable evidence indicating the potential for endocrine disrupting chemicals to alter the epigenome and for subsets of these epigenomic changes or "epimutations" to be heritably transmitted to offspring in subsequent generations. While there have been many studies indicating how exposure to endocrine disrupting chemicals can disrupt various organs associated with the body's endocrine systems, there is relatively limited information regarding the relative susceptibility of different specific organs, tissues, or cell types to endocrine disrupting chemical-induced epimutagenesis. Here we review available information about different organs, tissues, cell types, and/or cell lines which have been shown to be susceptible to specific endocrine disrupting chemical-induced epimutations. In addition, we discuss possible mechanisms that may be involved, or impacted by this tissue- or cell type-specific, differential susceptibility to different endocrine disrupting chemicals. Finally, we summarize available information indicating that certain periods of development display elevated susceptibility to endocrine disrupting chemical exposure and we describe how this may affect the extent to which germline epimutations can be transmitted inter- or transgenerationally. We conclude that cell type-specific differential susceptibility to endocrine disrupting chemical-induced epimutagenesis is likely to directly impact the extent to, or manner in, which endocrine disrupting chemical exposure initially induces epigenetic changes to DNA methylation and/or histone modifications, and how these endocrine disrupting chemical-induced epimutations can then subsequently impact gene expression, potentially leading to the development of heritable disease states.

Evaluation and mechanistic study of chlordecone-induced thyroid disruption: Based on in vivo, in vitro and in silico assays

The present study aimed to evaluate the thyroid-disrupting potency of chlordecone, and reveal the underlying mechanism. In the in vivo assays, rare minnow embryos were exposed to 0, 0.01, 0.1, 1 and 10 μg·L^-1 chlordecone until sexually mature. The results showed decreased T4 but increased T3 concentrations in plasma, upregulated mRNA levels of thyrotropin-releasing hormone receptor (trhr) and sodium-iodide symporter (nis) in the brain, and transthyretin (ttr), thyroid hormone receptor α (trα) and deiodinase enzymes (dio1 and dio2) in the liver of adult fish. In the in vitro assays, single chlordecone treatments promoted growth hormone (GH) and prolactin (PRL) secretion in GH3 cells. Transcription of thyroid receptor (trβ) was inhibited, but this is not likely responsible for chlordecone-induced GH secretion and altered transcription. When co-treated with T3, chlordecone acted independently of the effect of T3 on GH secretion; chlordecone-induced GH/PRL secretion and mRNA expression were further promoted when co-treated with E2, but inhibited when co-treated with ICI, indicating an important role for estrogen receptors (ERs) in chlordecone-induced changes in GH3 cells. Furthermore, in silico prediction suggested no stable interactions between chlordecone and thyroid hormone-related proteins, as well as a regulatory role for ERs in thyroid systems. Overall, our results indicated that chlordecone may have adverse effects on thyroid systems upon long-term exposure. However, rather of TRs, ERs may be responsible for thyroid disruption following chlordecone exposure.

Expression profiles of types 2 and 3 iodothyronine deiodinase genes in relation to vitellogenesis in a tropical damselfish, Chrysiptera cyanea

Thyroid hormone (TH) is involved in regulating the reproduction of vertebrates. Its physiological action in the target tissues is due to the conversion of TH by iodothyronine deiodinases. In this study, we aimed to clone and characterize type 2 (sdDio2) and type 3 (sdDio3) of the sapphire devil Chrysiptera cyanea, a tropical damselfish that undergoes active reproduction under long-day conditions, and to study the involvement of THs in the ovarian development of this species. When the cDNAs of sdDio2 and sdDio3 were partially cloned, they had deduced amino acid sequences of lengths 271 and 267, respectively, both of which were characterized by one selenocysteine residue. Real-time quantitative PCR (qPCR) revealed that both genes are highly expressed in the whole brain, and sdDio2 and sdDio3 are highly transcribed in the liver and ovary, respectively. In situ hybridization analyses showed positive signals of sdDio2 and sdDio3 transcripts in the hypothalamic area of the brain. Little change in mRNA abundance of sdDio2 and sdDio3 in the brain was observed during the vitellogenic phases. It is assumed that simultaneous activation and inactivation of THs occur in this area because oral administration of triiodothyronine (T3), but not of thyroxine (T4), upregulated mRNA abundance of both genes in the brain. The transcript levels of sdDio2 in the liver and sdDio3 in the ovary increased as vitellogenesis progressed, suggesting that, through the metabolism of THs, sdDio2 and sdDio3 play a role in vitellogenin synthesis in the liver and yolk accumulation/E2 synthesis in the ovary. Taken together, these results suggest that iodothyronine deiodinases act as a driver for vitellogenesis in tropical damselfish by conversion of THs in certain peripheral tissues.

Embryonic exposure to soil samples from a gangue stacking area induces thyroid hormone disruption in zebrafish

The total accumulative stockpiles of gangue from long-term coal mining exceed 1 billion tons and occupy 182 square kilometers, and 50 million tons of additional gangue are generated per year in Shanxi, a major energy province in China. The objective of this study was to examine whether exposure to village soils affected by gangue stacking would disrupt thyroid hormone system homeostasis and eventually affect endocrine system and development, using zebrafish (Danio rerio) as a model organism. The zebrafish embryos were exposed to village soil leachates at 0, 1:9, 1:3 and 1:1 from 1 to 120 h postfertilization (hpf), and the sample caused a dose-dependent increase in the mortality and malformation rate, and decrease in the heart rate, hatching rate and body length of zebrafish larvae. Importantly, the soil leachate alleviated the whole-body triiodothyronine (T3) and thyroxine (T4) levels at higher concentrations, and altered the expression of the hypothalamic-pituitary-thyroid (HPT) axis-regulating genes crh, trh, tshβ, nis, tg, nkx2.1, pax8, hhex, ttr, dio1, dio2, ugt1ab, trα, and trβ and the PAH exposure-related genes ahr2 and cyp1a. These findings highlight the potential risk of thyroid hormone disruption and developmental toxicity from soil samples around coal gangue stacking areas.

Bioconcentration, Biotransformation, and Thyroid Endocrine Disruption of Decabromodiphenyl Ethane (Dbdpe), A Novel Brominated Flame Retardant, in Zebrafish Larvae

The brominated flame retardant decabromodiphenyl ethane (DBDPE), an alternative to decabrominated diphenyl ether (BDE209), has become a widespread environmental contaminant, but its possible toxic effects to wildlife remain unknown. Using zebrafish as a model, we investigated the bioconcentration and impact of DBDPE on thyroid endocrine function after water-borne exposure, compared to BDE209. Zebrafish embryos were exposed to DBDPE or BDE209 (0, 3, 10, 30, 100, 300 nM) for 6 or 14 days. Chemical analysis revealed that DBDPE and BDE209 were bioconcentrated in zebrafish larvae, with similar magnitudes of accumulated concentrations. Based on screened by chromatograms, at least seven unknown compounds were observed in DBDPE-treated larvae, indicating biotransformation of the chemical. Significant increases in whole body content of triiodothyronine (T3) and thyroxine (T4) were detected in DBDPE-treated larvae, but decreased in BDE209-treated groups. Alterations in gene transcription along the related hypothalamic-pituitary-thyroid (HPT) axis were observed. Furthermore, the binding and transport protein transthyretin (TTR) was significantly increased in DBDPE exposure groups. Histological examination and stereological analysis showed no obvious pathological changes in the thyroid gland. The present study demonstrates for the first time the bioavailability, biotransformation and thyroid endocrine disruption associated with DBDPE exposure in fish. Further studies are warranted to identify the metabolites of DBDPE and to define its environmental risks to aquatic organisms.

Effects of tris(1,3-dichloro-2-propyl) phosphate (TDCPP) and triphenyl phosphate (TPP) on sex-dependent alterations of thyroid hormones in adult zebrafish

Organophosphate flame retardants (OPFRs) have been widely used as alternatives to polybrominated diphenyl ethers for fire prevention. OPFRs are suspected of causing potential thyroid disruption in humans. In fish, their thyroid hormone modulation is reported but the mechanisms of this modulation are less understood. Thyroid-disturbing effects of OPFRs were evaluated using adult zebrafish (Danio rerio) following 14d exposure to tris(1,3-dichloro-2-propyl) phosphate (TDCPP) or triphenyl phosphate (TPP). Plasma concentrations of thyroid hormones were measured and transcriptions of several genes involved in thyroid function were quantified in brain, thyroid, and liver. Exposure to TDCPP or TPP led to significant decreases in plasma triiodothyronine (T3) and thyroxine (T4) concentrations in the male fish, while the increases were observed in the female fish. Exposure to the OPFRs also altered the transcription of regulatory genes and receptors in hypothalamus, pituitary, and thyroid of the fish in sex-dependent manner. In the male fish, transcriptions of corticotropin-releasing hormone (crh) and thyroid-stimulating hormone (tsh) in the brain were significantly up-regulated, probably as a compensation for hypothyroidism, but thyroglobulin (tg) and deiodinase 2 (dio2) were down-regulated in thyroid or liver. In contrast, in the females, transcriptions of crh and tsh genes were significantly down-regulated. These observations show that TDCPP and TPP exposure can lead to sex-dependent disruptions of thyroid hormone balances in the adult zebrafish through alterations of the hypothalamus-pituitary-thyroid (HPT) axis.

Ioxynil and diethylstilbestrol disrupt vascular and heart development in zebrafish

BACKGROUND

Endocrine disruption is one of the consequences of industrialization and chemicals released into the environment have a profound impact on organisms. Waterborne micromolar concentrations of ioxynil (IOX) and diethylstilbestrol (DES) in fish affect the development of the heart, vasculature and thyroid gland.

OBJECTIVES

The present study aimed to determine how IOX and DES disrupt the crosstalk between the developing thyroid gland and cardio-vascular system in zebrafish.

METHODS

Twelve hours post fertilization (hpf) wild type, Tg(fli1:GFP) or Tg(cmalc2:GFPCaaX) zebrafish embryos were exposed to 0.1 μM IOX or DES for 36 h (up until 48 hpf) or 60 h (up until 72 hpf). Embryos were used for vascular endothelial cell sorting, whole-mount immunohistochemistry, tissue selective transcriptomics, selected gene expression analysis by quantitative real-time polymerase chain reaction analysis and determination of heart rate by live imaging.

RESULTS

Exposure of zebrafish embryos to IOX and DES (0.1 μM) increased heart beat frequency and reduced ventricle volume and aorta diameter. The transcriptome of endothelial cells from blood vessels of hypertrophic, dilated and arrhythmogenic right ventricular cardiomyopathy was significantly changed and compound-specific toxic effects were found in IOX and DES exposed embryos. Both DES and IOX directly affected vascular and heart development and this indirectly impaired thyroid gland development in zebrafish. Even though the toxicity end-point of the two chemicals was similar, their action seemed to be via different gene regulatory pathways and physiological mechanisms.

CONCLUSION

IOX and DES directly disrupt cardiovascular development and there is an associated disruption of thyroid tissue that most likely has long term consequences for this endocrine axis.

What can we learn from glucocorticoid administration in fish? Effects of cortisol and dexamethasone on intermediary metabolism of gilthead seabream (Sparus aurata L.)

In aquaculture facilities fish welfare could be compromised due to stressors. Fish deal with stress, inter alia, through the activation of the hypothalamic-pituitary-interrenal endocrine axis and, as a result, corticosteroids are released into the blood. Recent studies have described that corticosteroids actions depend on the specific affinities to their receptors, and the subsequent differentiated responses. Cortisol is the main corticosteroid hormone in teleost fish, being its actions dependent on the intensity and time of exposure to stressors. Short-term effects of corticosteroids are well described, but long-term effects, including changes in the energy management directly affecting growth and survival, are less understood in fish. Here we show the effects of chronic oral administration of cortisol and the synthetic glucocorticoid dexamethasone (DXM) on the intermediary metabolism of the gilthead seabream (Sparus aurata). We described a higher energy expenditure associated to both corticosteroids resulting in lower growth rates of fish. Moreover, the effects of these compounds were tissue-dependant, with differences between both hormones. Thus, cortisol-fed animals accumulated triglycerides in the liver, while DXM treatment led to glycogen storage. Cortisol and DXM stimulated amino acids catabolism and gluconeogenic pathways in muscle and gills, but the effects were significantly enhanced in DXM-fed fish. The described effects highlighted differentiated mechanisms of action associated to both corticosteroids under chronic stress conditions. Further studies should aim at describing those pathways in detail, with special attention to the functionality of glucocorticoid receptor isoforms. The effects described here for S. aurata juveniles, may serve as a basis to assess long-term stress in future comparative studies with other aquaculture species.

Exposure to PFDoA causes disruption of the hypothalamus-pituitary-thyroid axis in zebrafish larvae

Perfluorododecanoic acid (PFDoA), a kind of perfluorinated carboxylic acid (PFCA) with 12 carbon atoms, has an extensive industrial utilization and is widespread in both wildlife and the water environment, and was reported to have the potential to cause a disruption in the thyroid hormone system homeostasis. In this study, zebrafish embryos/larvae were exposed to different concentrations of PFDoA (0, 0.24, 1.2, 6 mg/L) for 96 h post-fertilization (hpf). PFDoA exposure caused obvious growth restriction connected with the reduced thyroid hormones (THs) contents in zebrafish larvae, strengthening the interference effect on the growth of fish larvae. The transcriptional level of genes within the hypothalamic-pituitary-thyroid (HPT) axis was analyzed. The gene expression levels of thyrotropin-releasing hormone (trh) and corticotrophin-releasing hormone (crh) were upregulated upon exposure to 6 mg/L of PFDoA, and iodothyronine deiodinases (dio2) was upregulated in the 1.2 mg/L PFDoA group. The transcription of thyroglobulin (tg) and thyroid receptor (trβ) were significantly downregulated upon exposure to 1.2 mg/L and 6 mg/L of PFDoA. PFDoA could also decrease the levels of sodium/iodide symporter (nis) and transthyretin (ttr) gene expression in a concentration-dependent manner after exposure. A significant decrease in thyroid-stimulating hormoneβ (tshβ), uridinediphosphate-glucuronosyltransferase (ugt1ab) and thyroid receptor (trα) gene expression were observed at 6 mg/L PFDoA exposure. Upregulation and downregulation of iodothyronine deiodinases (dio1) gene expression were observed upon the treatment of 1.2 mg/L and 6 mg/L PFDoA, respectively. All the data demonstrated that gene expression in the HPT axis altered after different PFDoA treatment and the potential mechanisms of the disruption of thyroid status could occur at several steps in the process of synthesis, regulation, and action of thyroid hormones.

Effects of the environmental endocrine disrupting compound benzo[a]pyrene on thyroidal status of abu mullet (Liza abu) during short-term exposure

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The half-life of BaP is 3–4 days and then it metabolized in the liver.

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BaP concentration in the muscle of treated fish reached a maximum level after 4 days.

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Exposure of fish to BaP resulted in a decrease in T3 and T4 plasma levels up to day 4.

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Exposure of fish to BaP resulted in an increase in TSH plasma level up to day 4.

Benzo[a]Pyrene (BaP) is a ubiquitous polycyclic aromatic hydrocarbon (PAH) that has been shown to disrupt the metabolism of thyroid hormone. Then, the present investigation aimed to study the effects of BaP on thyroid function in Liza abu. Fish were injected with 2, 10 and 25 mg/kg-bw of BaP. Samples were taken from blood, thyroid and muscle tissues at days 1, 2, 4, 7, and 14. Blood was evaluated for changes in the plasma levels of TSH, T3 and T4. Also, BaP bioaccumulation in the fish muscle was measured. Thyroid tissues were processed for routine histology. BaP concentration in the muscle of treated fish reached a maximum level after 4 days. Exposure of fish to BaP resulted in a significant decrease in T3 and T4 plasma level and increase in TSH concentration up to day 4. Also some pathological alterations were observed in BaP-exposed fish such as hemorrhage and increased number of large follicles with squamous epithelium. In conclusion, according to the results of the present investigation, short term exposure to sublethal concentrations of BaP significantly affected thyroid function in fish. The results revealed BaP ability to alter thyroid function.

Acute exposure to tris (2-butoxyethyl) phosphate (TBOEP) affects growth and development of embryo-larval zebrafish

Tris (2-butoxyethyl) phosphate (TBOEP), is used as a flame retardant worldwide. It is an additive in materials and can be easily discharged into the surrounding environment. There is evidence linking TBOEP exposure to abnormal development and growth in zebrafish embryos/larvae. Here, using zebrafish embryo as a model, we investigated toxicological effects on developing zebrafish (Danio rerio) caused by TBOEP at concentrations of 0, 20, 200, 1000, 2000μg/L starting from 2h post-fertilization (hpf). Our findings revealed that TBOEP exposure caused developmental toxicity, such as malformation, growth delay and decreased heart rate in zebrafish larvae. Correlation analysis indicated that inhibition of growth was possibly due to down-regulation of expression of genes related to the growth hormone/insulin-like growth factor (GH/IGF) axis. Furthermore, exposure to TBOEP significantly increased thyroxine (T4) and 3,5,3'-triiodothyronine (T3) in whole larvae. In addition, changed expression of genes involved in the hypothalamic-pituitary-thyroid (HPT) axis was observed, indicating that perturbation of HPT axis might be responsible for the developmental damage and growth delay induced by TBOEP. The present study provides a new set of evidence that exposure of embryo-larval zebrafish to TBOEP can cause perturbation of GH/IGF axis and HPT axis, which could result in developmental impairment and growth inhibition.

Effects of the soya isoflavone genistein in early life stages of the Senegalese sole, Solea senegalensis: Thyroid, estrogenic and metabolic biomarkers

This study examines the effects induced by environmentally relevant concentrations of the isoflavone genistein (3mg/L and 10mg/L) during early life stages of the Senegalese sole. Throughout the hypothalamus-pituitary-thyroid (HPT) axis, several neurohormonal regulatory thyroid signalling patterns (thyroglobulin/Tg, thyroid peroxidase/TPO, transthyretin/TTR, thyroid receptors/TRβ, and iodothrynonine deiodinases, Dio2 and Dio3) were analysed. Furthermore, the expression patterns of estrogen receptor ERβ and haemoprotein Cyp1a were also evaluated. In the control larvae, progressive increases of constitutive hormonal signalling pathways have been evidenced from the pre-metamorphosis phase onwards, reaching the highest expression basal levels at the metamorphosis (Tg, TPO, Dio2) and/or during post-metamorphosis (TTR, TRβ, ERβ). When the early larvae were exposed to both genistein concentrations (3mg/L and 10mg/L), a statistically significant down-regulation of TPO, TTR and Tg mRNA levels was clearly detected at the metamorphic stages. In addition, the Dio2 and Dio3 transcript expression levels were also down and up-regulated when exposed to both genistein concentrations. In the larvae exposed to genistein, no statistically significant responses were recorded for the TRβ expression patterns. Nevertheless, the ERβ and Cyp1a transcript levels were up-regulated at the middle metamorphic stage (S2, at 16 dph) in the larvae exposed to high genistein concentrations and, only the ERβ was down-regulated (S1, at 12dph) at the lower doses. Finally, all these pointed out imbalances were only temporarily disrupted by exposure to genistein, since most of the modulated transcriptional signals (i.e. up or down-regulation) were quickly restored to the baseline levels. Additionally, the control and genistein-exposed Senegalese sole specimens showed characteristic ontogenetic patterns and completely suitable for an optimal development, metamorphosis, and growth.

Semicarbazide-induced thyroid disruption in Japanese flounder (Paralichthys olivaceus) and its potential mechanisms

Semicarbazide (SMC) is a carcinogenic and genotoxic substance that has been found in aquatic systems. SMC may also cause thyroid follicular epithelial cell injury in rats, but the thyroid-disrupting properties of SMC and its potential mechanisms remain unclear. In this study, we exposed fertilized eggs of Japanese flounder (Paralichthys olivaceus) to 1, 10, 100, and 1000μg/L SMC for 55 d to assess the impact of SMC exposure on the thyroid system. The number of larvae in each metamorphic stage was counted, the concentrations of whole-body thyroid hormones (THs) 3,5,3'-triiodothyronine (T3) and thyroxine (T4) were measured, and the transcription levels of genes involved in the hypothalamic-pituitary-thyroid (HPT) axis and gamma-aminobutyric acid (GABA) synthesis were quantified. The results showed that 10μg/L SMC significantly increased whole-body T3 levels, and 100 and 1000μg/L SMC markedly enhanced whole-body T4 and T3 levels. Furthermore, 100μg/L SMC exposure shortened the time it took for flounder larvae to complete metamorphosis by 2 d as compared to the control group. Thus, this study demonstrated that SMC exerted thyroid-disrupting effects on Japanese flounder. SMC-mediated stimulation of TH levels was primarily related to transcriptional alterations of pituitary-derived thyroid stimulating hormone β-subunit (tshβ) and hepatic deiodinase (id). In the 10 and 100μg/L SMC exposure groups, the increased TH levels may have resulted from inhibition of TH metabolism caused by down-regulation of id3 mRNA expression, while at 1000μg/L SMC-exposed group, up-regulation of tshβ and id1 transcripts was expected to enhance the synthesis of T4 and the conversion of T4 to T3 and, consequently, result in higher T4 and T3 levels. In addition, 1000μg/L SMC-induced down-regulation in glutamic acid decarboxylase gad65 and gad67 transcription may have also contributed to the increased TH levels. The thyroid-disrupting effects of 10 and 100μg/L SMC indicated that environmentally relevant concentrations of SMC posed potential environmental risks to aquatic organisms. Overall, our results demonstrated for the first time that SMC exhibited thyroid-disrupting properties by affecting the HPT axis and GABA synthesis, providing theoretical support for environmental risk assessment.

Effects of exposure to microcystin-LR at environmentally relevant concentrations on the metabolism of thyroid hormones in adult zebrafish (Danio rerio)

Microcystin-LR (MC-LR) has the potential to disturb thyroid hormone homeostasis. However, the effects of MC-LR at environmentally relevant concentrations on the thyroid system in adult fish are still unclear. In this study, adult zebrafish were exposed to 0, 1, 5, and 25 μg/L MC-LR for 7, 14, 21, and 28 days. Whole-body thyroid hormones (THs) levels and thyroid follicle histology were used to assess thyroid function. The transcription of corticotropin-releasing hormone (crh), thyroid-stimulating hormone (tsh), transthyretin (ttr), thyroid hormone receptors (trs) genes, and the activities of iodothyronine deiodinases (IDs) were investigated to study the process of TH metabolism disruption. No differences in the histopathology of thyroid follicles and unchanged T₄ levels were observed in adult zebrafish. A significant decline in T₃ levels associated with a decrease in ID2 activity in male zebrafish was observed at 21 days exposure. Moreover, the mRNA expression of tsh, ttr and trs appeared to be a dynamic process as expression first decreased and then increased with continued exposure. These results indicated that exposure to MC-LR did not inhibit the production of TH. The decrease in ID2 activity may be an important factor in the decline of T₃ levels. Furthermore, it seems that the fish triggered a compensatory mechanism to maintain TH homeostasis in respond to environmental concentrations of MC-LR which induced TH disruption.

Permanent Deiodinase Type 2 Deficiency Strongly Perturbs Zebrafish Development, Growth, and Fertility

Iodothyronine deiodinases are selenocysteine-containing enzymes that activate or inactivate thyroid hormones (THs). Deiodinase type 2 (Dio2) catalyzes the conversion of the prohormone T4 into the transcriptionally active T3 and is the predominant activating deiodinase in zebrafish. Using zinc finger nucleases, we generated two different dio2(-/-) mutant zebrafish lines to investigate the physiological function of this TH activator. The first line contains a deletion of 9 bp, resulting in an in-frame elimination of three conserved amino acids. The other line is characterized by an insertion of 4 bp, leading to the introduction of a premature stop-codon. Both lines completely lack Dio2 activity, resulting in a strong reduction of T3 abundancy in all tissues tested. Early development is clearly perturbed in these animals, as shown by a diverse set of morphometric parameters, defects in swim bladder inflation, and disturbed locomotor activity tested between 1 and 7 days after fertilization. Permanent Dio2 deficiency also provokes long-term effects because growth and especially fertility are severely hampered. Possible compensatory mechanisms were investigated in adult dio2(-/-) mutants, revealing a down-regulation of the inactivating deiodinase Dio3 and TH receptor transcript levels. As the first nonmammalian model with permanent Dio2 deficiency, these mutant zebrafish lines provide evidence that Dio2 is essential to assure normal development and to obtain a normal adult phenotype.

Tributyltin disrupts feeding and energy metabolism in the goldfish (Carassius auratus)

Tributyltin (TBT) can induce obesogen response. However, little is known about the adverse effects of TBT on food intake and energy metabolism. The present study was designed to investigate the effects of TBT, at environmental concentrations of 2.44 and 24.4 ng/L (1 and 10 ng/L as Sn), on feeding and energy metabolism in goldfish (Carassius auratus). After exposure for 54 d, TBT increased the weight gain and food intake in fish. The patterns of brain neuropeptide genes expression were in line with potential orexigenic effects, with increased expression of neuropeptide Y and apelin, and decreased expression of pro-opiomelanocortin, ghrelin, cocaine and amphetamine-regulated transcript, and corticotropin-releasing factor. Interestingly, the energy metabolism indicators (oxygen consumption, ammonia exertion and swimming activity) and the serum thyroid hormones were all significantly increased at the 2.44 ng/L TBT group in fish. However, no changes of energy metabolism indicators or a decrease of thyroid hormones was found at the 24.4 ng/L TBT group, which indicated a complex disrupting effect on metabolism of TBT. In short, TBT can alter feeding and energy metabolism in fish, which might promote the obesogenic responses.

Waterborne exposure to bisphenol F causes thyroid endocrine disruption in zebrafish larvae

While bisphenol F (BPF) has been frequently detected in various environmental compartments, limited information is available on its effect on thyroid endocrine system. In the present study, zebrafish (Danio rerio) embryos were exposed to 0.2, 2, 20, and 200 μg/L of BPF from 2 h post-fertilization (hpf) to 144 hpf. The whole-body content of thyroid hormones, thyroid-stimulating hormone (TSH), and transcription of genes belonging to the hypothalamic-pituitary-thyroid (HPT) axis were investigated. BPF exposure resulted in alterations of both T3 and T4 contents, increased the ratios of T3/T4, demonstrating thyroid endocrine disruption. Moreover, TSH content was significantly induced in a concentration-dependent manner after exposure to BPF. The increased gene transcription of dio2 might assist to degrade increased T3 contents. Treatment with BPF also significantly increased transcription of genes involved in thyroid hormone regulation (crh) and synthesis (nis and tg) as a compensatory mechanism for the decrease of T4 contents. However, the gene encoding protein involved in TH transport (ttr) was transcriptionally significantly down-regulated after exposure to BPF. Taken together, these results suggest that BPF alters the transcription of genes involved in the HPT axis as well as changes whole-body contents of thyroid hormones and TSH in zebrafish embryos/larvae, thus causing an endocrine disruption of the thyroid system.

The expression of selected genes linked to metabolic homeostasis in obese pansteatitis-suffering Mozambique tilapia, Oreochromis mossambicus (Peters)

The Oreochromis mossambicus (Peters) population inhabiting Lake Loskop, South Africa, is characterized by a high incidence of obesity and pansteatitis. We investigated potential links between the impaired health of Lake Loskop O. mossambicus and the endocrine system by assessing the expression of selected genes associated with the thyroid and adrenal endocrine axes as well as peroxisome proliferator-activated receptor gamma (pparg). Moreover, contaminant-induced thyroid and/or metabolic modulation in Lake Loskop water was evaluated using juvenile O. mossambicus in laboratory exposures. The expression of thyroid hormone receptor alpha (thra) and type 2 deiodinase (dio2) was higher in Lake Loskop O. mossambicus than fish from another population, suggesting a degree of thyroid disruption. The altered gene expression may be a consequence, rather than cause of obesity. Expression of dio2 and pparg was higher in juvenile O. mossambicus exposed to unfiltered compared to filtered lake water, and our data suggest fasting as causative factor. Micro-organism abundance can therefore be a confounding factor in studies applying molecular markers to test for thyroid modulation by environmental waters. Pansteatitis was not a significant source of variance in the expression of any of the genes investigated, suggesting that the disease is not associated with disrupted endocrine signalling.

Waterborne exposure to microcystin-LR causes thyroid hormone metabolism disturbances in juvenile Chinese rare minnow (Gobiocypris rarus)

Microcystin-LR (MC-LR) has the potential to disturb thyroid hormone homeostasis, but little is known about the underlying mechanisms of MC-LR in fish. In the present study, juvenile Chinese rare minnows (Gobiocypris rarus) were exposed to various concentrations of MC-LR (0 µg/L, 50 µg/L, 100 µg/L, and 500 µg/L) for 7 d. The whole-body thyroid hormone content, the histology of thyroid follicle epithelial cells, the activities of hepatic iodothyronine deiodinases, and the transcription of selected genes associated with thyroid hormone synthesis, transport, and metabolism were analyzed. Following exposure to MC-LR, whole-body concentrations of both thyroxine (T4 ) and triiodothyronine (T3 ) were significantly decreased. The levels of messenger RNA for sodium/iodide symporter, transthyretin, thyroid hormone receptor-α, iodothyronine deiodinase2, and iodothyronine deiodinase3 were significantly down-regulated after exposure to 500 µg/L MC-LR. A significant decrease in ID2 activity was also observed in the 500-µg/L MC-LR exposure group. Moreover, hypertrophy of thyroid follicle epithelial cells was observed after exposure to MC-LR. The results indicate that acute MC-LR exposure has the potential to disturb the homeostasis of thyroid hormone metabolism, leading to a hypothyroidism state in the juvenile Chinese rare minnow.

A review on the effects of PBDEs on thyroid and reproduction systems in fish

The objective of this review was to summarize and discuss the effects of Polybrominated diphenyl ethers (PBDEs) on thyroid and reproduction systems in fish. We reviewed the evidences and mechanisms for PBDEs-induced thyroid and reproduction disruption, as well as the cross-talk between the two systems in fish. In thyroid disruption, we mainly paid attention to the effects of PBDEs on hypothalamic-pituitary-thyroid (HPT) axis, thyroid hormones (THs) transport and metabolism, thyroid receptors (TRs) and thyroid follicle histology. In reproduction disruption, we focused on the effects of PBDEs on steroid hormone production, expression of genes involved in steroidogenesis, and gonadal development. Despite that there is an interaction between thyroid and reproductive systems in fish, it is still remains unclear that PBDE-induced reproductive impairments are caused by direct effects on hypothalamic-pituitary-gonadal (HPG) functioning or by indirect action through cross-talk between the two systems. Future studies are needed to explore the relationships between reproductive toxicity and thyroid system disruption after PBDEs exposure.

Deiodinases and thyroid metabolism disruption in teleost fish

Many xenobiotic compounds with endocrine disrupting activity have been described since the late eighties. These compounds are able to interact with natural hormone systems and potentially induce deleterious effects in wildlife, notably piscine species. However, while the characterization of endocrine disruptors with "dioxin-like", estrogenic or androgenic activities is relatively well established, little is known about environmentally relevant pollutants that may act at thyroid system level. Iodothyronine deiodinases, the key enzymes in the activation and inactivation of thyroid hormones, have been suggested as suitable biomarkers for thyroid metabolism disruption. The present article reviews the biotic and abiotic factors that are able to modulate deiodinases in teleosts, a representative model organism for vertebrates. Data show that deiodinases are highly sensitive to several physiological and physical variables, so they should be taken into account to establish natural basal deiodination patterns to further understand responses under chemical exposure. Among xenobiotic compounds, brominated flame retardants are postulated as chemicals of major concern because of their similar structure shared with thyroid hormones. More ambiguous results are shown for the rest of compounds, i.e. polychlorinated biphenyls, perfluorinated chemicals, pesticides, metals and synthetic drugs, in part due to the limited information available. The different mechanisms of action still remain unknown for most of those compounds, although several hypothesis based on observed effects are discussed. Future tasks are also suggested with the aim of moving forward in the full characterization of chemical compounds with thyroid disrupting activity.

Toxicological effects of clofibric acid and diclofenac on plasma thyroid hormones of an Indian major carp, Cirrhinus mrigala during short and long-term exposures

In the present investigation, the toxicity of most commonly detected pharmaceuticals in the aquatic environment namely clofibric acid (CA) and diclofenac (DCF) was investigated in an Indian major carp Cirrhinus mrigala. Fingerlings of C. mrigala were exposed to different concentrations (1, 10 and 100μgL(-1)) of CA and DCF for a period of 96h (short term) and 35 days (long term). The toxic effects of CA and DCF on thyroid hormones (THs) such as thyroid stimulating hormone (TSH), thyroxine (T4) and triiodothyronine (T3) levels were evaluated. During the short and long-term exposure period TSH level was found to be decreased at all concentrations of CA (except at the end of 14(th) day in 1 and 10μgL(-l) and 21(st) day in 1μgL(-l)) whereas in DCF exposed fish TSH level was found to be increased when compared to control groups. T4 level was found to be decreased at 1 and 100μgL(-l) of CA exposure at the end of 96h. However, T4 level was decreased at all concentrations of CA and DCF during long-term (35 days) exposure period. Fish exposed to all concentrations of CA and DCF had lower level of T3 in both the treatments. These results suggest that both CA and DCF drugs induced significant changes (P<0.01 and P<0.05) on thyroid hormonal levels of C. mrigala. The alterations of these hormonal levels can be used as potential biomarkers in monitoring of pharmaceutical drugs in aquatic organisms.

Thyroid endocrine system disruption by pentachlorophenol: an in vitro and in vivo assay

The present study aimed to evaluate the disruption caused to the thyroid endocrine system by pentachlorophenol (PCP) using in vitro and in vivo assays. In the in vitro assay, rat pituitary GH3 cells were exposed to 0, 0.1, 0.3, and 1.0 μM PCP. PCP exposure significantly downregulated basal and triiodothyronine (T3)-induced Dio 1 transcription, indicating the antagonistic activity of PCP in vitro. In the in vivo assay, zebrafish embryos were exposed to 0, 1, 3, and 10 μg/L of PCP until 14 days post-fertilization. PCP exposure resulted in decreased thyroxine (T4) levels, but elevated contents of whole-body T3. PCP exposure significantly upregulated the mRNA expression of genes along hypothalamic-pituitary-thyroid (HPT) axis, including those encoding thyroid-stimulating hormone, sodium/iodide symporter, thyroglobulin, Dio 1 and Dio 2, alpha and beta thyroid hormone receptor, and uridinediphosphate-glucuronosyl-transferase. PCP exposure did not influence the transcription of the transthyretin (TTR) gene. The results indicate that PCP potentially disrupts the thyroid endocrine system both in vitro and in vivo.

Tissue distribution and thyroid hormone effects on mRNA abundance for membrane transporters Mct8, Mct10, and organic anion-transporting polypeptides (Oatps) in a teleost fish

Many of the actions of thyroid hormones (THs) occur via TH binding to intracellular receptors. Although it was long thought that THs diffused passively across plasma membranes, it is now recognized that cellular entry is mediated by a variety of membrane transporter proteins. In this study, we identified cDNAs encoding the TH transporters monocarboxylate transferases 8 (mct8) and 10 (mct10) as well as eight distinct organic anion-transporting polypeptide (oatp) proteins from fathead minnow (Pimephales promelas). Analysis of the tissue distribution of transporter mRNAs revealed that mct8 and mct10 transcripts were both abundant in liver, but also present at lower levels in brain, gonad and other tissues. Transcripts encoding oatp1c1 were highly abundant in brain, liver and gonad, and exhibited significant sex differences in the liver and gonad. Treatment of adult male minnows with 3,5,3'-triiodothyronine (T3) or the goitrogen methimazole altered gene transcript abundance for several transporters. Fish given exogenous T3 had reduced mct8 and oapt1c1 mRNA levels in the liver compared to methimazole-treated fish. In the brain, transcripts for mct8, mct10, oatp2b1, and oatp3a1 were each reduced in abundance in fish with elevated T3. As a whole, these results provide evidence that TH status influences the transcriptional dynamics of mct8, mct10 and several Oatp genes including oatp1c1 in teleost fish.

Waterborne exposure of zebrafish embryos to micromole concentrations of ioxynil and diethylstilbestrol disrupts thyrocyte development

The herbicide ioxynil (IOX) and synthetic estrogen diethylstilbestrol (DES) are common aquatic contaminants with an endocrine disrupting action. In juvenile teleost fish IOX and DES disrupt the hypothalamic-pituitary-thyroid (HPT) axis. To assess how IOX and DES influence the developing HPT axis prior to establishment of central regulation of thyroid hormones, zebrafish embryos were exposed to low concentrations of the chemicals in water. IOX and DES (1 and 0.1 μM) exposure failed to modify hypothalamic development but had a negative effect on thyrocyte development. Specifically, IOX and DES caused a significant (p<0.05) reduction in the size of the thyroid anlagen by decreasing the mRNA expression field of both nk2.1a and thyroglobulin (Tg) genes. Inhibition of thyroid gland development by IOX and DES (0.1 μM) was strongly associated with altered heart morphology. To test if the effect of IOX and DES on the thyroid was a consequence of altered cardiac development a morpholino (MO) against zebrafish cardiac troponin I (zcTnI) was microinjected. The zcTnI morphants had modified heart function, a small thyroid anlagen and a reduction in the mRNA expression of nk2.1a and Tg genes similar to that of zebrafish exposed to IOX (1 and 0.1 μM) and DES (0.1 μM). Collectively the data indicate that IOX and DES alter thyroid development in zebrafish and chemicals that alter heart development and function can have an indirect endocrine disrupting action on the thyroid in teleosts.

Biochemical responses of juvenile European sturgeon, (Huso huso) to a sub-lethal level of copper and cadmium in freshwater and brackish water environments

In Caspian Sea basin, sturgeons spend the larval and juvenile stages in freshwaters of rivers and then, they migrate to brackish waters of the sea where they grow and mature. With regard to the elevation of the metal concentrations in coastal waters and sediments of the Caspian Sea and its adjacent rivers, it is likely that juvenile sturgeon are exposed to sub-lethal levels of metals during seawater entry process. We compared the biochemical responses of juvenile European sturgeon, (Beluga, Huso huso) exposed to a sub-lethal level of copper (Cu, 20 μg/L) and cadmium (Cd, 300 μg/L) in freshwater (FW, 0 ppt) and brackish water (BW, 11 ppt) for seven days. The results showed that the levels of plasma glucose increased significantly in BW and in all metal exposed groups. Also, plasma cortisol concentrations showed significant increases when juveniles were exposed to BW, Cu(FW/BW) and Cd(BW). The activity of liver superoxide dismutase (SOD) decreased significantly in BW compared with FW. Moreover, Cu and Cd exposure enhanced the activity of SOD in BW, while SOD did not show any changes in FW. The levels of tissue and plasma proteins as well as plasma triiodothyronine (T3), thyroxine (T4) and liver Catalase (CAT) activity remained constant when animals were exposed to Cu/Cd in both FW and BW environments. Our data indicate that exposure of juvenile beluga to BW stimulated the general biochemical responses of stress such as cortisol and glucose, while sub-lethal exposure to Cu and Cd caused oxidative stress in BW environment but not in FW.

Thyroid hormone deiodinase type 2 mRNA levels in sea lamprey (Petromyzon marinus) are regulated during metamorphosis and in response to a thyroid challenge

Thyroid hormones (THs) are crucial for normal vertebrate development and are the one obligate morphogen that drives amphibian metamorphosis. However, contrary to other metamorphosing vertebrates, lampreys exhibit a sharp drop in serum TH early in metamorphosis, and anti-thyroid agents such as potassium perchlorate (KClO(4)) induce metamorphosis. The type 2 deiodinase (D2) enzyme is a key regulator of TH availability during amphibian metamorphosis. We set out to determine how D2 may be involved in the regulation of lamprey metamorphosis and thyroid homeostasis. We cloned a 1.8Kb Petromyzon marinus D2 cDNA that includes the entire protein coding region and a selenocysteine (Sec) codon. Northern blotting indicated that the lamprey D2 mRNA is the longest reported to date (>9Kb). Using real-time PCR, we showed that intestinal and hepatic D2 mRNA levels were elevated prior to and during the early stages of metamorphosis and then declined dramatically to low levels that were sustained for the remainder of metamorphosis. These data are consistent with previously reported changes in serum TH levels and deiodinase activity. Treatment of larvae with either TH or KClO(4) significantly affected D2 mRNA levels in the intestine and liver. These D2 mRNA levels during metamorphosis and in response to thyroid challenges suggest that D2 may function in the regulation of TH levels during lamprey metamorphosis and the maintenance of TH homeostasis.

The goitrogenic efficiency of thioamides in a marine teleost, sea bream (Sparus auratus)

Studies 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 (1mg/kg wet weight per day), via the diet for 21days. 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 TRβ 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 (1mg/kg wet weight for 21days) 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.

Species-dependent effects of the phenolic herbicide ioxynil with potential thyroid hormone disrupting activity: modulation of its cellular uptake and activity by interaction with serum thyroid hormone-binding proteins

Ioxynil, a phenolic herbicide, is known to exert thyroid hormone (TH) disrupting activity by interfering with TH-binding to plasma proteins and a step of the cellular TH-signaling pathway in restricted animal species. However, comparative studies are still lacking on the TH disruption. We investigated the interaction of [125I]ioxynil with serum proteins from rainbow trout, bullfrog, chicken, pig, rat, and mouse, using native polyacrylamide gel electrophoresis. Candidate ioxynil-binding proteins, which included lipoproteins, albumin and transthyretin (TTR), differed among the vertebrates tested. Rainbow trout and bullfrog tadpole serum had the lowest binding activity for ioxynil, whereas the eutherian serum had the highest binding activity. The cellular uptake of, and response to, ioxynil were suppressed by rat serum greater than by tadpole serum. The cellular uptake of [125I]ioxynil competed strongly with phenols with a single ring, but not with THs. Our results suggested that ioxynil interferes with TH homeostasis in plasma and with a step of cellular TH-signaling pathway other than TH-uptake system, in a species-specific manner.

Acute toxicity and bioconcentration of pyrimorph in zebrafish, Brachydanio rerio

BACKGROUND

Pyrimorph is a novel fungicide being developed in China that shows high antifungal activity against diseases caused by Phytophthora infestans, Phytophthora capsici, Rhizoctonia solani, Peronophythora litchi and Pseudoperonospora cubensis. Until now, no information on the toxicity of pyrimorph to untargeted organisms has been reported. To assess the potential environmental impacts of pyrimorph in fish, the acute toxicity and bioconcentration of pyrimorph in zebrafish were studied in this paper.

RESULTS

When tested by the semi-static method, the 48, 72 and 96 h median lethal concentration (LC50 ) values of pyrimorph to zebrafish were 24.33, 22.61 and 19.79 mg L(-1) respectively. To study the bioconcentration of pyrimorph in zebrafish, the fish were exposed to sublethal concentrations of pyrimorph (2.00 and 0.25 mg L(-1) ) for 192 h, a modified QuEChERS (Quick, Easy, Cheap, Effective, Rugged and Safe) method was applied to extract pyrimorph from samples and the residues of pyrimorph in the water and fish were determined by high-performance liquid chromatography (HPLC). The highest bioconcentration factor (BCF) of pyrimorph in the fish was 1.07 × 10(2) (144 h) and 23.1 (96 h) after exposure to 2.00 and 0.25 mg L(-1) of pyrimorph respectively.

CONCLUSIONS

The novel fungicide pyrimorph has low toxicity to zebrafish; however, it showed medium BCF to zebrafish. Therefore, more attention should be paid to the residue of pyrimorph in fish, as well as to its long-term ecological effects.

Alterations along the Hypothalamic-Pituitary-Thyroid Axis of the Zebrafish (Danio rerio) after Exposure to Propylthiouracil

In the past, various approaches have been developed to detect adverse effects of pollutants on the thyroid of vertebrates, most of these with special emphasis on the South African clawed frog, Xenopus laevis. Although fish are primarily affected by thyroid-disrupting chemicals, studies into alterations of the thyroid of fish are scarce. Therefore, effects of the reference compound propylthiouracil on histopathology of the thyroid axis were analyzed in a modified early life-stage test with zebrafish (Danio rerio) exposed to propylthiouracil. The test substance induced dose-dependent alterations of thyroidal tissue concomitant with increases in the number of surrounding blood vessels. Despite this massive proliferation of the thyroid, zebrafish were not able to maintain thyroxin concentrations. The pituitary was affected displaying significant alterations in thyroid-stimulating hormone cell counts. Quantitative evaluation of pituitary surface areas revealed a dose-dependent increase of adenohypophyseal tissue. Distinct histopathological effects may contribute to a more easy identification and interpretation of alterations induced by thyroid-disrupting chemicals.

Tissue-specific thyroid hormone regulation of gene transcripts encoding iodothyronine deiodinases and thyroid hormone receptors in striped parrotfish (Scarus iseri)

In fish as in other vertebrates, the diverse functions of thyroid hormones are mediated at the peripheral tissue level through iodothyronine deiodinase (dio) enzymes and thyroid hormone receptor (tr) proteins. In this study, we examined thyroid hormone regulation of mRNAs encoding the three deiodinases dio1, dio2 and dio3 - as well as three thyroid hormone receptors trαA, trαB and trβ - in initial phase striped parrotfish (Scarus iseri). Parrotfish were treated with dissolved phase T(3) (20 nM) or methimazole (3 mM) for 3 days. Treatment with exogenous T(3) elevated circulating T(3), while the methimazole treatment depressed plasma T(4). Experimentally-induced hyperthyroidism increased the relative abundance of transcripts encoding trαA and trβ in the liver and brain, but did not affect trαB mRNA levels in either tissue. In both sexes, methimazole-treated fish exhibited elevated dio2 transcripts in the liver and brain, suggesting enhanced outer-ring deiodination activity in these tissues. Accordingly, systemic hyperthyroidism elevated relative dio3 transcript levels in these same tissues. In the gonad, however, patterns of transcript regulation were distinctly different with elevated T(3) increasing mRNAs encoding dio2 in testicular and ovarian tissues and dio3, trαA and trαB in the testes only. Thyroid hormone status did not affect dio1 transcript abundance in the liver, brain or gonads. Taken as a whole, these results demonstrate that thyroidal status influences relative transcript abundance for dio2 and dio3 in the liver, provide new evidence for similar patterns of dio2 and dio3 mRNA regulation in the brain, and make evident that fish exhibit tr subtype-specific transcript abundance changes to altered thyroid status.

Nitrate-induced goiter in captive whitespotted bamboo sharks Chilosryllium plagiosum

Elasmobranch susceptibility to goiter formation in captive environments has been well documented. Until recently, most public aquariums operated under the belief that the etiology of goiter in elasmobranchs was nutritional and specifically caused by insufficient dietary iodine. Recent studies have demonstrated that high environmental nitrate (NO3-N) inhibits the ability of the thyroid gland to utilize available iodide, resulting in thyroid gland overstimulation by thyroid stimulating hormone and ultimately leading to the development of goiter. The objective of this study was to evaluate the effects of high environmental nitrate concentrations on thyroid function in juvenile whitespotted bamboo sharks Chiloscyllium plagiosum. In July 2008, five juveniles (80-150 g) were exposed to a low-nitrate environment (NO3-N concentration < 1 mg/L of water) and five were exposed to an elevated-nitrate environment (NO3-N = 70 mg/L) for 29 d in a flow-through natural seawater system. Nitrate exposure did not affect growth rates (e.g., weight, length, and condition factor) and did not alter free plasma thyroxine concentrations during the 29-d experimental period. However, histological examination of thyroid glands from sharks exposed to elevated nitrate revealed the development of diffuse hyperplastic goiter. With increasing restrictions on water use, most modern aquaria operate as recirculating systems, which results in higher and more chronic nitrate exposure for captive animals. Goiter is one of the most common health problems in captive elasmobranchs, and this study suggests that nitrate exposure is an important factor in the etiology of this disease.

Effects of metolachlor on transcription of thyroid system-related genes in juvenile and adult Japanese medaka (Oryzias latipes)

Metolachlor (MT) is one of the most important pesticides applied to corn and other crops for controlling broadleaf and grass weeds. However, the effects of MT on the thyroid system in fish remain to be elucidated. In the present experiment, transcription of genes related to the thyroid system, including thyrotropin releasing hormone (Trh), deiodinase 2 (Dio2), thyroid hormone receptor α (Thrα), and thyroid hormone receptor β (Thrβ), were induced by MT in a sex-, developmental stage-, and tissue- specific manner when medaka were exposed to various concentrations of MT for 14 days. The transcriptional levels of the genes were only significantly altered in both juvenile and adult female medaka in response to MT exposure. And the lowest concentrations able to significantly induce transcription of the selected genes were 10 and 100 μg/L in juvenile and adult female medaka, respectively. In adult female medaka, a significant up-regulation of these genes was detected only in the brain, with little or no effect in the liver. Furthermore, MT-induced (100 μg/L) transcription of thyroid system-related genes was enhanced significantly in male juvenile medaka in the presence of estrogen (E2) (50 and 100 ng/L). Moreover, the mRNA levels of Thrα and Thrβ in males increase with the combined treatments of 100 μg/L MT and 100 ng/L E2. Dio2 increased when exposed to 100 μg/L MT and 50 or 100 ng/L E2. The information obtained in the present study suggests that MT has the potential to influence several steps of the hypothalamus-pituitary-thyroid (HPT) axis homeostasis and to disrupt the thyroid system in medaka.

The hypothalamus-pituitary-thyroid axis in teleosts and amphibians: endocrine disruption and its consequences to natural populations

Teleosts and pond-breeding amphibians may be exposed to a wide variety of anthropogenic, waterborne contaminants that affect the hypothalamus-pituitary-thyroid (HPT) axis. Because thyroid hormone is required for their normal development and reproduction, the potential impact of HPT-disrupting contaminants on natural teleost and amphibian populations raises special concern. There is laboratory evidence indicating that persistent organic pollutants, heavy metals, pharmaceutical and personal care products, agricultural chemicals, and aerospace products may alter HPT activity, development, and reproduction in teleosts and amphibians. However, at present there is no evidence to clearly link contaminant-induced HPT alterations to impairments in teleost or amphibian population health in the field. Also, with the exception of perchlorate for which laboratory studies have shown a direct link between HPT disruption and adverse impacts on development and reproductive physiology, little is known about if or how other HPT-disrupting contaminants affect organismal performance. Future field studies should focus on establishing temporal associations between the presence of HPT-disrupting chemicals, the occurrence of HPT alterations, and adverse effects on development and reproduction in natural populations; as well as determining how complex mixtures of HPT contaminants affect organismal and population health.

Perfluorooctane sulfonate increased hepatic expression of OAPT2 and MRP2 in rats

The toxicity of perfluorooctane sulfonate (PFOS), a persistent organic compound, is of great concern. Several studies have reported that PFOS decreases circulating thyroid hormone (TH) concentrations. However, the mechanisms involved remain to be determined. Female rats were exposed to (1) vehicle; (2) PFOS (0.2, 1.0, and 3.0 mg/kg); (3) propylthiouracil (PTU, 10 mg/kg); or (4) PTU (10 mg/kg) + PFOS (3.0 mg/kg) by gavage once a day for 5 consecutive days. Parameters including contents of total T4 (TT4) and total T3 (TT3) in both serum and bile, serum concentrations of transthyretin and thyroglobulin, as well as transcripts of transporters involved in hepatic uptake and efflux of T4 were determined in control and PFOS-exposed groups. TT4 and TT3 were also analyzed in PTU and PTU + PFOS groups in order to reflect the different hormone effects between PFOS, PTU, and PFOS + PTU. Results showed that serum TT4 and TT3 decreased, while bile TT4 and TT3 remained stable following PFOS exposure. Exposure to 3.0 mg/kg of PFOS significantly enhanced hepatic organic anion transporter OATP2 mRNA expression (1.43 times of control). Treatment with PFOS increased hepatic expression of multidrug resistance--associated protein MRP2, approximately 1.80 and 1.69 times of control in 1.0 and 3.0 mg/kg groups, respectively. Spearman's correlation coefficients revealed that MRP2 mRNA expression correlated well with serum TT4 level (r = -0.528, P = 0.012). Serum thyroglobulin and transthyretin levels remained stable. Serum TT3, bile TT4, and bile TT3 were significantly different between PFOS and PTU groups. No significant differences of TT4 and TT3 in both serum and bile were observed between PTU and PTU + PFOS (P > 0.05). In conclusion, PFOS increased hepatic expression of OAPT2, which could possibly enhance hepatic uptake and metabolism of T4 in rats. PFOS-induced TT4 deficiency is mainly due to the extrathyroidal metabolism of T4, which is probably different from the classic goitrogen, PTU.

Evidence for thyroid endocrine disruption in wild fish in San Francisco Bay, California, USA. Relationships to contaminant exposures

It is well documented that many coastal and estuarine environments adjacent to developed and industrialized urban centers, such as the San Francisco Bay Area, are significantly contaminated by anthropogenic chemicals. However, it is not well understood to what extent existing contaminants, many with continuing inflows into the environment, may impact exposed wildlife. This study provided an initial characterization of thyroid endocrine-related effects and their relationship to accumulated contaminants in two indigenous fish species sampled from different San Franicsco Bay Area study sites. Plasma concentrations of thyroxine (T4) were significantly reduced in fish sampled from highly impacted locations such as Oakland Inner Harbor and San Leandro Bay as compared with fish from other locations representing relatively lower human impact, including Bodega Bay, Redwood City and a remote site on Santa Catalina Island. Triiodothyronine (T3) levels also varied significantly by location, with differing T3/T4 ratios in fish from some locations suggestive of altered peripheral deiodinase activity. The changes in thyroid endocrine parameters were significantly correlated with hepatic concentrations of certain environmental contaminants. A large number of polychlorinated biphenyl (PCB) congeners, both co-planar (dioxin-like) and non-co-planar, exhibited significant inverse correlations with T4 levels in the fish, while in contrast, T3 and T3/T4 ratio were positively correlated with PCB exposures. The positive correlation between T3/T4 ratio and PCBs supports the hypothesis that environmental PCBs may alter T4 deiodination or turnover, actions of PCBs reported in laboratory experiments. Some relationships between chlorinated pesticides including DDT and chlordanes, but fewer relationships with PAHs, were also observed. Together, these findings indicate that the thyroid endocrine system is exhibiting alterations associated with different aquatic environments in the San Francisco Bay Area, which are significantly related to current-day exposures of the fish to contaminant chemicals such as PCBs.

Waterborne exposure to PFOS causes disruption of the hypothalamus-pituitary-thyroid axis in zebrafish larvae

Thyroid hormones (THs) play an important role in the normal development and physiological functions in fish. Environmental chemicals may adversely affect thyroid function by disturbing gene transcription. Perfluorooctane sulfonate (PFOS), a persistent compound, is widely distributed in the aquatic environment and wildlife. In the present study, we investigated whether PFOS could disrupt the hypothalamic-pituitary-thyroid (HPT) axis. Zebrafish embryos were exposed to various concentrations of PFOS (0, 100, 200 and 400 microgL(-1)) and gene expression patterns were examined 15d post-fertilization. The expression of several genes in the HPT system, i.e., corticotropin-releasing factor (CRF), thyroid-stimulating hormone (TSH), sodium/iodide symporter (NIS), thyroglobulin (TG), thyroid peroxidase (TPO), transthyretin (TTR), iodothyronine deiodinases (Dio1 and Dio2) and thyroid receptor (TRalpha and TRbeta), was quantitatively measured using real-time PCR. The gene expression levels of CRF and TSH were significantly up-regulated and down-regulated, respectively, upon exposure to 200 and 400 microg L(-1) PFOS. A significant increase in NIS and Dio1 gene expression was observed at 200 microg L(-1) PFOS exposure, while TG gene expression was down-regulated at 200 and 400 microg L(-1) PFOS exposure. TTR gene expression was down-regulated in a concentration-dependent manner. Up-regulation and down-regulation of TRalpha and TRbeta gene expression, respectively, was observed upon exposure to PFOS. The whole body thyroxine (T(4)) content remained unchanged, whereas triiodothyronine (T(3)) levels were significantly increased, which could directly reflect disrupted thyroid hormone status after PFOS exposure. The overall results indicated that PFOS exposure could alter gene expression in the HPT axis and that mechanisms of disruption of thyroid status by PFOS could occur at several steps in the synthesis, regulation, and action of thyroid hormones.