Structure–activity relationships of 44 halogenated compounds for iodotyrosine deiodinase-inhibitory activity

Update of the risk assessment of polybrominated diphenyl ethers (PBDEs) in food

The European Commission asked EFSA to update its 2011 risk assessment on polybrominated diphenyl ethers (PBDEs) in food, focusing on 10 congeners: BDE-28, -47, -49, -99, -100, -138, -153, -154, -183 and ‑209. The CONTAM Panel concluded that the neurodevelopmental effects on behaviour and reproductive/developmental effects are the critical effects in rodent studies. For four congeners (BDE-47, -99, -153, -209) the Panel derived Reference Points, i.e. benchmark doses and corresponding lower 95% confidence limits (BMDLs), for endpoint-specific benchmark responses. Since repeated exposure to PBDEs results in accumulation of these chemicals in the body, the Panel estimated the body burden at the BMDL in rodents, and the chronic intake that would lead to the same body burden in humans. For the remaining six congeners no studies were available to identify Reference Points. The Panel concluded that there is scientific basis for inclusion of all 10 congeners in a common assessment group and performed a combined risk assessment. The Panel concluded that the combined margin of exposure (MOET) approach was the most appropriate risk metric and applied a tiered approach to the risk characterisation. Over 84,000 analytical results for the 10 congeners in food were used to estimate the exposure across dietary surveys and age groups of the European population. The most important contributors to the chronic dietary Lower Bound exposure to PBDEs were meat and meat products and fish and seafood. Taking into account the uncertainties affecting the assessment, the Panel concluded that it is likely that current dietary exposure to PBDEs in the European population raises a health concern.

Lighting Up Fluorescence: Precise Recognition of Halogenated Solvents Through Effective Fluorescence Detection Using Chalcone Derivatives as a D-A-D-A-type Fluorescent Chemosensor

In this paper, we report a D-A-D-A-type fluorescence sensor, FX, composed of triphenylamine and pyrazine units as electron donors, pyridine units, and α-β unsaturated carbon-based structures as electron acceptors. FX exhibits typical ICT characteristics. As a dual-emission material, FX undergoes acid-base-induced color changes and displays mechanofluorochromic properties in the solid state. In solution, FX, as an AIE material, shows significant fluorescence enhancement behavior in most halogenated solvents. Notably, it achieves a high quantum yield of 0.672 in a chloroform solution. We utilized this phenomenon to quantitatively detect chloroform through fluorescence titration analysis, with a detection limit of 0.061%. Additionally, we developed a test paper to verify the practical applicability of the sensor for detecting halogenated solvents. The fluorescence enhancement behavior was confirmed through DFT calculations. The results indicate that FX is not only a multifunctional dual-state emission material but also provides valuable references for the fluorescence detection of halogenated solvents.

Cross-species applicability of an adverse outcome pathway network for thyroid hormone system disruption

Thyroid hormone system disrupting compounds are considered potential threats for human and environmental health. Multiple adverse outcome pathways (AOPs) for thyroid hormone system disruption (THSD) are being developed in different taxa. Combining these AOPs results in a cross-species AOP network for THSD which may provide an evidence-based foundation for extrapolating THSD data across vertebrate species and bridging the gap between human and environmental health. This review aimed to advance the description of the taxonomic domain of applicability (tDOA) in the network to improve its utility for cross-species extrapolation. We focused on the molecular initiating events (MIEs) and adverse outcomes (AOs) and evaluated both their plausible domain of applicability (taxa they are likely applicable to) and empirical domain of applicability (where evidence for applicability to various taxa exists) in a THSD context. The evaluation showed that all MIEs in the AOP network are applicable to mammals. With some exceptions, there was evidence of structural conservation across vertebrate taxa and especially for fish and amphibians, and to a lesser extent for birds, empirical evidence was found. Current evidence supports the applicability of impaired neurodevelopment, neurosensory development (eg, vision) and reproduction across vertebrate taxa. The results of this tDOA evaluation are summarized in a conceptual AOP network that helps prioritize (parts of) AOPs for a more detailed evaluation. In conclusion, this review advances the tDOA description of an existing THSD AOP network and serves as a catalog summarizing plausible and empirical evidence on which future cross-species AOP development and tDOA assessment could build.

Sex, menopause, and age differences in the associations of persistent organic pollutants with thyroid hormones, thyroxine-binding globulin, and peripheral deiodinase activity: A cross-sectional study of the general Korean adult population

Persistent organic pollutants (POPs) can disrupt the thyroid hormone system in humans. We assessed the associations of several POPs with serum thyroid hormones (T3 and T4) and thyroid-stimulating hormone, and investigated the modulating effects of sex, menopausal status, and age on these associations, in a subgroup of the adult population (n = 1250) from the Korean National Environmental Health Survey. PCB105 and PCB118 were negatively associated with total T4 in premenopausal females and males aged <50, whereas the associations were insignificant in other groups. PCB180, p,p'-DDE, and p,p'-DDT showed positive associations with total T3 in postmenopausal females; however, among males aged ≥50, PCB118, PCB138, and p,p'-DDE showed negative associations with total T3. The effects of exposure to multiple POPs were examined in multi-factor analyses. Factor 2 comprised PCB52, hexachlorobenzene, and BDE-47 was associated with an increase in free T4 in premenopausal females (β = 0.015, p = 0.024), while Factor 1, which contained most POPs, was associated with a change in total T3 in postmenopausal females (β = 0.032, p = 0.040) and males aged ≥50 (β = -0.039, p = 0.023). Changes in total T4 or total T3 could be explained by differences in thyroxine-binding globulin (TBG) and peripheral deiodinase activity (G_D). Negative associations of TBG with PCB105 in premenopausal females and PCB153 in males aged <50 may mediate the effect of decreasing total T4. PCB180, p,p'-DDE, p,p'-DDT, and Factor 1 were positively associated with G_D, which is consistent with an increased total T3 in postmenopausal females. PCB118 was negatively associated with G_D and total T3 in males aged ≥50. BDE-47 and β-hexachlorocyclohexane were associated with thyroid autoantibodies in premenopausal females and males aged <50. Our observations suggest that the thyroid-disrupting effects of POPs may differ by sex, sex hormonal status, and age, and may be mediated by TBG and G_D.

Cross-species comparison of chemical inhibition of human and Xenopus iodotyrosine deiodinase

The transition to include in vitro-based data in chemical hazard assessment has resulted in the development and implementation of screening assays to cover a diversity of biological pathways, including recently added assays to interrogate chemical disruption of proteins relevant to thyroid signaling pathways. Iodotyrosine deiodinase (IYD), the iodide recycling enzyme, is one such thyroid-relevant endpoint for which a human-based screening assay has recently been developed and used to screen large libraries of chemicals. Presented here is the development of an amphibian IYD inhibition assay and its implementation to conduct a cross-species comparison between chemical inhibition of mammalian and non-mammalian IYD enzyme activity. The successful development of an amphibian IYD inhibition assay was based on demonstration of sufficient IYD enzyme activity in several tissues collected from larval Xenopus laevis. With this new assay, 154 chemicals were tested in concentration-response to provide a basis for comparison of relative chemical potency to results obtained from the human IYD assay. Most chemicals exhibited similar inhibition in both assays, with less than 25% variation in median inhibition for 120 of 154 chemicals and 85% concordance in categorization of "active" (potential IYD inhibitor) versus "inactive". For chemicals that produced 50% or greater inhibition in both assays, rank-order potency was similar, with the majority of the IC50s varying by less than 2-fold (and all within an order of magnitude). Most differences resulted from greater maximum inhibition or higher chemical potency observed with human IYD. This strong cross-species agreement suggests that results from the human-based assay would be conservatively predictive of chemical effects on amphibian IYD.

Towards a science-based testing strategy to identify maternal thyroid hormone imbalance and neurodevelopmental effects in the progeny-part III: how is substance-mediated thyroid hormone imbalance in pregnant/lactating rats or their progeny related to neurodevelopmental effects?

This review investigated which patterns of thyroid- and brain-related effects are seen in rats upon gestational/lactational exposure to 14 substances causing thyroid hormone imbalance by four different modes-of-action (inhibition of thyroid peroxidase, sodium-iodide symporter and deiodinase activities, enhancement of thyroid hormone clearance) or to dietary iodine deficiency. Brain-related parameters included motor activity, cognitive function, acoustic startle response, hearing function, periventricular heterotopia, electrophysiology and brain gene expression. Specific modes-of-action were not related to specific patterns of brain-related effects. Based upon the rat data reviewed, maternal serum thyroid hormone levels do not show a causal relationship with statistically significant neurodevelopmental effects. Offspring serum thyroxine together with offspring serum triiodothyronine and thyroid stimulating hormone appear relevant to predict the likelihood for neurodevelopmental effects. Based upon the collated database, thresholds of ≥60%/≥50% offspring serum thyroxine reduction and ≥20% and statistically significant offspring serum triiodothyronine reduction indicate an increased likelihood for statistically significant neurodevelopmental effects; accuracies: 83% and 67% when excluding electrophysiology (and gene expression). Measurements of brain thyroid hormone levels are likely relevant, too. The extent of substance-mediated thyroid hormone imbalance appears more important than substance mode-of-action to predict neurodevelopmental impairment in rats. Pertinent research needs were identified, e.g. to determine whether the phenomenological offspring thyroid hormone thresholds are relevant for regulatory toxicity testing. The insight from this review shall be used to suggest a tiered testing strategy to determine whether gestational/lactational substance exposure may elicit thyroid hormone imbalance and potentially also neurodevelopmental effects.

The Influence of Triclosan on the Thyroid Hormone System in Humans - A Systematic Review

OBJECTIVES

Triclosan is an antibacterial agent suspected to disrupt the endocrine system. The aim of this study was to investigate the influence of triclosan on the human thyroid system through a systematic literature review of human studies.

METHODS

Eligibility criteria and method of analysis were registered at Prospero (registration number: CRD42019120984) before a systematic search was conducted in Pubmed and Embase in October 2020. Seventeen articles were found eligible for inclusion. Thirteen studies were observational, while four had a triclosan intervention. Participants consisted of pregnant women in eight studies, of men and non-pregnant women in seven studies and of chord samples/newborns/children/adolescents in six studies. The outcomes were peripheral thyroid hormones and thyroid-stimulating hormone (TSH) in blood samples.

RESULTS

Several studies found a negative association between triclosan and triiodothyronine and thyroxine, and a positive association with TSH; however, the opposite associations or no associations were also found. In general, the studies had limited measurement timepoints of thyroid outcomes, and the interventional studies used low concentrations of triclosan. Thus, study design limitations influence the quality of the dataset and it is not yet possible to conclude whether triclosan at current human exposure levels adversely affects the thyroid hormone system.

CONCLUSIONS

Further larger studies with more continuity and more elaborate outcome measurements of thyroid function are needed to clarify whether triclosan, at current exposure levels, affects the human thyroid hormone system.

SYSTEMATIC REVIEW REGISTRATION

http://www.crd.york.ac.uk/PROSPERO/display_record.asp?ID=CRD42019120984, identifier PROSPERO (CRD42019120984).

In vitro screening for chemical inhibition of the iodide recycling enzyme, iodotyrosine deiodinase

The iodide recycling enzyme, iodotyrosine deiodinase (IYD), is a largely unstudied molecular mechanism through which environmental chemicals can potentially cause thyroid disruption. This highly conserved enzyme plays an essential role in maintaining adequate levels of free iodide for thyroid hormone synthesis. Thyroid disruption following in vivo IYD inhibition has been documented in mammalian and amphibian models; however, few chemicals have been tested for IYD inhibition in either in vivo or in vitro assays. Presented here are the development and application of a screening assay to assess susceptibility of IYD to chemical inhibition. With recombinant human IYD enzyme, a 96-well plate in vitro assay was developed and then used to screen over 1800 unique substances from the U.S. EPA ToxCast screening library. Through a tiered screening approach, 194 IYD inhibitors were identified (inhibited IYD enzyme activity by 20% or greater at target concentration of 200 μM). 154 chemicals were further tested in concentration-response (0.032-200 μM) to determine IC₅₀ and rank-order potency. This work broadens the coverage of thyroid-relevant molecular targets for chemical screening, provides the largest set of chemicals tested for IYD inhibition, and aids in prioritizing chemicals for targeted in vivo testing to evaluate thyroid-related adverse outcomes.

Enzymatic assays for the assessment of toxic effects of halogenated organic contaminants in water and food. A review

Halogenated organic compounds are a particular group of contaminants consisting of a large number of substances, and of great concern due to their persistence in the environment, potential for bioaccumulation and toxicity. Some of these compounds have been classified as persistent organic pollutants (POPs) under The Stockholm Convention and many toxicity assessments have been conducted on them previously. In this work we provide an overview of enzymatic assays used in these studies to establish toxic effects and dose-response relationships. Studies in vivo and in vitro have been considered with a particular emphasis on the impact of halogenated compounds on the activity of relevant enzymes to the humans and the environment. Most information available in the literature focuses on chlorinated compounds, but brominated and fluorinated molecules are also the target of increasing numbers of studies. The enzymes identified can be classified as enzymes: i) the activities of which are affected by the presence of halogenated organic compounds, and ii) those involved in their metabolisation/detoxification resulting in increased activities. In both cases the halogen substituent seems to have an important role in the effects observed. Finally, the use of these enzymes in biosensing tools for monitoring of halogenated compounds is described.

Evaluating Iodide Recycling Inhibition as a Novel Molecular Initiating Event for Thyroid Axis Disruption in Amphibians

The enzyme iodotyrosine deiodinase (dehalogenase, IYD) catalyzes iodide recycling and promotes iodide retention in thyroid follicular cells. Loss of function or chemical inhibition of IYD reduces available iodide for thyroid hormone synthesis, which leads to hormone insufficiency in tissues and subsequent negative developmental consequences. IYD activity is especially critical under conditions of lower dietary iodine and in low iodine environments. Our objective was to evaluate the toxicological relevance of IYD inhibition in a model amphibian (Xenopus laevis) used extensively for thyroid disruption research. First, we characterized IYD ontogeny through quantification of IYD mRNA expression. Under normal development, IYD was expressed in thyroid glands, kidneys, liver, and intestines, but minimally in the tail. Then, we evaluated how IYD inhibition affected developing larval X. laevis with an in vivo exposure to a known IYD inhibitor (3-nitro-l-tyrosine, MNT) under iodine-controlled conditions; MNT concentrations were 7.4-200 mg/L, with an additional 'rescue' treatment of 200 mg/L MNT supplemented with iodide. Chemical inhibition of IYD resulted in markedly delayed development, with larvae in the highest MNT concentrations arrested prior to metamorphic climax. This effect was linked to reduced glandular and circulating thyroid hormones, increased thyroidal sodium-iodide symporter gene expression, and follicular cell hypertrophy and hyperplasia. Iodide supplementation negated these effects, effectively rescuing exposed larvae. These results establish toxicological relevance of IYD inhibition in amphibians. Given the highly conserved nature of the IYD protein sequence and scarcity of environmental iodine, IYD should be further investigated as a target for thyroid axis disruption in freshwater organisms.

Relationship between triclosan exposure and thyroid hormones: the Second Korean National Environmental Health Survey (2012-2014)

Background

5-chloro-2-(2,4-dichlorophenoxy)phenol (triclosan) is used as an antiseptic and is a potential endocrine-disrupting chemical that can affect thyroid hormone levels. This study evaluated the relationship between triclosan exposure and thyroid hormones.

Methods

Data from the second Korean National Environmental Health Survey (2012-2014) were analyzed. Triclosan exposure was evaluated using urinary triclosan concentrations and classified into 2 groups: 'below detection (< limit of detection [LOD])' vs. 'detected (≥ LOD).' Multiple linear regression analysis was conducted to determine the relationship between triclosan exposure and the serum thyroid hormone concentrations, adjusting for age, body mass index, urinary creatinine, and smoking status.

Results

When grouped by sex, triclosan exposure was positively associated with the serum thyroid-stimulating hormone (TSH) concentrations in females with marginal significance (β = 0.066, p = 0.058). However, no significant association was identified between triclosan exposure and serum total triiodothyronine and thyroxine in both males and females, and TSH in males.

Conclusions

This study is the first human study to evaluate the relationship between triclosan exposure and serum thyroid hormone concentrations in the Korean population. There was suggestive positive association between triclosan exposure and the serum TSH in females. Further studies need to evaluate the relationship between long-term exposure to low-dose triclosan and thyroid hormones.

Evaluating Chemicals for Thyroid Disruption: Opportunities and Challenges with in Vitro Testing and Adverse Outcome Pathway Approaches

BACKGROUND

Extensive clinical and experimental research documents the potential for chemical disruption of thyroid hormone (TH) signaling through multiple molecular targets. Perturbation of TH signaling can lead to abnormal brain development, cognitive impairments, and other adverse outcomes in humans and wildlife. To increase chemical safety screening efficiency and reduce vertebrate animal testing, in vitro assays that identify chemical interactions with molecular targets of the thyroid system have been developed and implemented.

OBJECTIVES

We present an adverse outcome pathway (AOP) network to link data derived from in vitro assays that measure chemical interactions with thyroid molecular targets to downstream events and adverse outcomes traditionally derived from in vivo testing. We examine the role of new in vitro technologies, in the context of the AOP network, in facilitating consideration of several important regulatory and biological challenges in characterizing chemicals that exert effects through a thyroid mechanism.

DISCUSSION

There is a substantial body of knowledge describing chemical effects on molecular and physiological regulation of TH signaling and associated adverse outcomes. Until recently, few alternative nonanimal assays were available to interrogate chemical effects on TH signaling. With the development of these new tools, screening large libraries of chemicals for interactions with molecular targets of the thyroid is now possible. Measuring early chemical interactions with targets in the thyroid pathway provides a means of linking adverse outcomes, which may be influenced by many biological processes, to a thyroid mechanism. However, the use of in vitro assays beyond chemical screening is complicated by continuing limits in our knowledge of TH signaling in important life stages and tissues, such as during fetal brain development. Nonetheless, the thyroid AOP network provides an ideal tool for defining causal linkages of a chemical exerting thyroid-dependent effects and identifying research needs to quantify these effects in support of regulatory decision making. https://doi.org/10.1289/EHP5297.

State of knowledge on current exposure, fate and potential health effects of contaminants in polar bears from the circumpolar Arctic

The polar bear (Ursus maritimus) is among the Arctic species exposed to the highest concentrations of long-range transported bioaccumulative contaminants, such as halogenated organic compounds and mercury. Contaminant exposure is considered to be one of the largest threats to polar bears after the loss of their Arctic sea ice habitat due to climate change. The aim of this review is to provide a comprehensive summary of current exposure, fate, and potential health effects of contaminants in polar bears from the circumpolar Arctic required by the Circumpolar Action Plan for polar bear conservation. Overall results suggest that legacy persistent organic pollutants (POPs) including polychlorinated biphenyls, chlordanes and perfluorooctane sulfonic acid (PFOS), followed by other perfluoroalkyl compounds (e.g. carboxylic acids, PFCAs) and brominated flame retardants, are still the main compounds in polar bears. Concentrations of several legacy POPs that have been banned for decades in most parts of the world have generally declined in polar bears. Current spatial trends of contaminants vary widely between compounds and recent studies suggest increased concentrations of both POPs and PFCAs in certain subpopulations. Correlative field studies, supported by in vitro studies, suggest that contaminant exposure disrupts circulating levels of thyroid hormones and lipid metabolism, and alters neurochemistry in polar bears. Additionally, field and in vitro studies and risk assessments indicate the potential for adverse impacts to polar bear immune functions from exposure to certain contaminants.

Contaminants in Atlantic walruses in Svalbard Part 2: Relationships with endocrine and immune systems

Marine mammals in the Barents Sea region have among the highest levels of contaminants recorded in the Arctic and the Atlantic walrus (Odobenus rosmarus rosmarus) is one of the most contaminated species within this region. We therefore investigated the relationships bewteen blubber concentrations of lipophilic persistent organic pollutants (POPs) and plasma concentrations of perfluoroalkyl substances (PFASs) and markers of endocrine and immune functions in adult male Atlantic walruses (n = 38) from Svalbard, Norway. To do so, we assessed plasma concentrations of five forms of thyroid hormones and transcript levels of genes related to the endocrine and immune systems as endpoints; transcript levels of seven genes in blubber and 23 genes in blood cells were studied. Results indicated that plasma total thyroxine (TT4) concentrations and ratio of TT4 and reverse triiodothyronine decreased with increasing blubber concentrations of lipophilic POPs. Blood cell transcript levels of genes involved in the function of T and B cells (FC like receptors 2 and 5, cytotoxic T-lymphocyte associated protein 4 and protein tyrosine phosphatase non-receptor type 22) were increased with plasma PFAS concentrations. These results suggest that changes in thyroid and immune systems in adult male walruses are linked to current levels of contaminant exposure.

Cross-sectional associations between urinary triclosan and serum thyroid function biomarker concentrations in women

INTRODUCTION

Exposure to the antimicrobial agent triclosan is ubiquitous. Research in animals shows that triclosan can cause decreases in thyroxine concentrations. However, the potential effects of triclosan on thyroid function in humans are unclear.

OBJECTIVE

To estimate the association between urinary triclosan concentrations and serum thyroid function biomarkers in women seeking assisted reproduction treatment in the Environment and Reproductive Health (EARTH) Study.

METHODS

We conducted a cross-sectional study of 317 women enrolled in the EARTH Study, a prospective preconception cohort that recruits Boston area couples. Using samples collected at study entry, we quantified urinary triclosan and serum thyroid function biomarker concentrations, specifically free and total thyroxine and triiodothyronine, thyroid-stimulating hormone (TSH), and thyroid antibodies. We estimated covariate-adjusted differences in thyroid function biomarkers per 10-fold increase in triclosan using linear regression models. We examined effect modification by body mass index (BMI) and infertility diagnosis.

RESULTS

The median urinary triclosan concentration was 7.8 μg/L (IQR: 3.0-59 μg/L). Each 10-fold increase in triclosan was inversely associated with free triidothyronine (T₃) (β: -0.06 pg/mL; 95% CI: -0.1, -0.01), thyroperoxidase antibody (TPOAb) (-10%; 95% CI: -19, -0.4), and thyroglobulin antibody (TgAb) (-12%; 95% CI: -23,0.9) concentrations. BMI and infertility diagnosis modified the association of triclosan with free T₃ and TPOAb, respectively.

CONCLUSION

Urinary triclosan concentrations were inversely associated with specific serum thyroid function biomarkers in this cohort, suggesting that triclosan may affect thyroid homeostasis and autoimmunity.

Food Additives and Child Health

Our purposes with this policy statement and its accompanying technical report are to review and highlight emerging child health concerns related to the use of colorings, flavorings, and chemicals deliberately added to food during processing (direct food additives) as well as substances in food contact materials, including adhesives, dyes, coatings, paper, paperboard, plastic, and other polymers, which may contaminate food as part of packaging or manufacturing equipment (indirect food additives); to make reasonable recommendations that the pediatrician might be able to adopt into the guidance provided during pediatric visits; and to propose urgently needed reforms to the current regulatory process at the US Food and Drug Administration (FDA) for food additives. Concern regarding food additives has increased in the past 2 decades, in part because of studies in which authors document endocrine disruption and other adverse health effects. In some cases, exposure to these chemicals is disproportionate among minority and low-income populations. Regulation and oversight of many food additives is inadequate because of several key problems in the Federal Food, Drug, and Cosmetic Act. Current requirements for a "generally recognized as safe" (GRAS) designation are insufficient to ensure the safety of food additives and do not contain sufficient protections against conflict of interest. Additionally, the FDA does not have adequate authority to acquire data on chemicals on the market or reassess their safety for human health. These are critical weaknesses in the current regulatory system for food additives. Data about health effects of food additives on infants and children are limited or missing; however, in general, infants and children are more vulnerable to chemical exposures. Substantial improvements to the food additives regulatory system are urgently needed, including greatly strengthening or replacing the "generally recognized as safe" (GRAS) determination process, updating the scientific foundation of the FDA's safety assessment program, retesting all previously approved chemicals, and labeling direct additives with limited or no toxicity data.

Current Knowledge on Endocrine Disrupting Chemicals (EDCs) from Animal Biology to Humans, from Pregnancy to Adulthood: Highlights from a National Italian Meeting

Wildlife has often presented and suggested the effects of endocrine disrupting chemicals (EDCs). Animal studies have given us an important opportunity to understand the mechanisms of action of many chemicals on the endocrine system and on neurodevelopment and behaviour, and to evaluate the effects of doses, time and duration of exposure. Although results are sometimes conflicting because of confounding factors, epidemiological studies in humans suggest effects of EDCs on prenatal growth, thyroid function, glucose metabolism and obesity, puberty, fertility, and on carcinogenesis mainly through epigenetic mechanisms. This manuscript reviews the reports of a multidisciplinary national meeting on this topic.

[Effect of the Metabolic Modification of Environmental Chemicals on Endocrine-disrupting Activity]

　The endocrine-disrupting activities of various environmental chemicals are metabolically activated. For example, diphenyls, styrene oligomers, chalcones, trans-stilbene and 2-nitrofluorene are not estrogens, but after incubation with liver microsomes, their metabolites show estrogenic activities. Thus, these chemicals are estrogenically activated by the cytochrome P450 system. In contrast, the antiandrogenic activity of fenthion, an organophosphorus insecticide, is abolished after metabolism to sulfoxide and sulfone derivatives. Structural requirements of twenty bisphenol A related compounds, as well as various benzophenones, for estrogenic and antiandrogenic activities have been investigated. The estrogenic and antiandrogenic activities of Benzophenone 3, a representative UV absorbant, are activated by oxidative metabolism. Parabens (used as antimicrobial agents) exhibit estrogenic activity, and their potency shows a bell-shaped curve between C1 (methylparaben) and C12 (dodecylparaben) parabens. The AhR ligand activity of indirubin is decreased by metabolism. Polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDE) are activated by hydroxylation to show estrogenic and thyroid hormone-disrupting activities. Halogen adjacent to a hydroxyl group is essential for thyroid hormone-disrupting activity. Tetrabromobisphenol A, tetrachlorobisphenol A and tetramethylbisphenol A also exhibit thyroid hormone-disrupting activity. Amphibian metamorphosis of tadpoles to frogs is affected by hydroxylated PCB, hydroxylated PBDE and bisphenol A derivatives. These chemicals suppress thyroid hormone-dependent metamorphosis, acting as antagonists of thyroid hormone. Thus, metabolic modification can have a dramatic impact on the endocrine-disrupting activities of environmental chemicals.

Associations of early life urinary triclosan concentrations with maternal, neonatal, and child thyroid hormone levels

BACKGROUND

Triclosan, an antimicrobial agent used in some consumer products, reduces endogenous thyroid hormone concentrations in rodents. Despite ubiquitous triclosan exposure and the importance of thyroid hormones for normal fetal development, few human studies have examined the impact of triclosan exposure on maternal, neonatal, or child thyroid hormones.

METHODS

In the HOME Study, a prospective cohort from Cincinnati, OH, we measured urinary triclosan concentrations up to three times in pregnant women between 16weeks and delivery, and up to three times in children between age 1-3years. We quantified serum concentrations of thyroid stimulating hormone and total and free thyroxine and triiodothyronine in mothers at 16-weeks gestation (n=202), neonates at delivery (n=274), and children at age 3years (n=153). We estimated covariate-adjusted differences in thyroid hormones with a 10-fold increase in triclosan using linear regression and multiple informants models.

RESULTS

Triclosan was not associated with thyroid hormones during pregnancy. We observed a few associations of triclosan concentrations with thyroid hormone concentrations in neonates at delivery and children at age 3years. Higher gestational triclosan, particularly around the time of delivery, was associated with lower cord serum total thyroxine (β: 0.3μg/dL; 95% CI: -0.6, -0.0). Childhood triclosan, particularly at age 1year, was positively associated with total thyroxine at age 3years (β: 0.7μg/dL; 95% CI: 0.3, 1.2).

CONCLUSION

Our findings suggest that triclosan exposure may influence some features of neonatal and early child thyroid function. Given the large number of comparisons we made, these findings should be replicated in other cohorts.

Novel thyroid hormone analogues, enzyme inhibitors and mimetics, and their action

Thyroid hormones (THs) play key roles in modulating the overall metabolism of the body, protein synthesis, fat metabolism, neuronal and bone growth, and cardiovascular as well as renal functions. In this review, we discuss on the thyroid hormone synthesis and activation, thyroid hormone receptors (TRs) and mechanism of action, applications of thyroid hormone analogues, particularly the compounds that are selective ligands for TRβ receptors, or enzyme inhibitors for the treatment of thyroidal disorders with a specific focus on thyroid peroxidase and iodothyronine deiodinases. We also discuss on the development of small-molecule deiodinase mimetics and their mechanism of deiodination, as these compounds have the potential to regulate the thyroid hormone levels.

Active Site Binding Is Not Sufficient for Reductive Deiodination by Iodotyrosine Deiodinase

The minimal requirements for substrate recognition and turnover by iodotyrosine deiodinase were examined to learn the basis for its catalytic specificity. This enzyme is crucial for iodide homeostasis and the generation of thyroid hormone in chordates. 2-Iodophenol binds only very weakly to the human enzyme and is dehalogenated with a k_cat/K_m that is more than 4 orders of magnitude lower than that for iodotyrosine. This discrimination likely protects against a futile cycle of iodinating and deiodinating precursors of thyroid hormone biosynthesis. Surprisingly, a very similar catalytic selectivity was expressed by a bacterial homologue from Haliscomenobacter hydrossis. In this example, discrimination was not based on affinity since 4-cyano-2-iodophenol bound to the bacterial deiodinase with a K_d lower than that of iodotyrosine and yet was not detectably deiodinated. Other phenols including 2-iodophenol were deiodinated but only very inefficiently. Crystal structures of the bacterial enzyme with and without bound iodotyrosine are nearly superimposable and quite similar to the corresponding structures of the human enzyme. Likewise, the bacterial enzyme is activated for single electron transfer after binding to the substrate analogue fluorotyrosine as previously observed with the human enzyme. A cocrystal structure of bacterial deiodinase and 2-iodophenol indicates that this ligand stacks on the active site flavin mononucleotide (FMN) in a orientation analogous to that of bound iodotyrosine. However, 2-iodophenol association is not sufficient to activate the FMN chemistry required for catalysis, and thus the bacterial enzyme appears to share a similar specificity for halotyrosines even though their physiological roles are likely very different from those in humans.

Sex-specific changes in thyroid gland function and circulating thyroid hormones in nestling American kestrels (Falco sparverius) following embryonic exposure to polybrominated diphenyl ethers by maternal transfer

High concentrations of polybrominated diphenyl ethers (PBDEs) accumulate in predatory birds. Several PBDE congeners are considered thyroid disruptors; however, avian studies are limited. The authors examined circulating thyroid hormones and thyroid gland function of nestling American kestrels (Falco sparverius) at 17 d to 20 d of age, following embryonic exposure by maternal transfer only to environmentally relevant levels of PBDEs (DE-71 technical mixture). Nestlings were exposed to in ovo sum (Σ) PBDE concentrations of 11 301 ± 95 ng/g wet weight (high exposure), 289 ± 33 ng/g wet weight (low exposure), or 3.0 ± 0.5 ng/g wet weight (controls, background exposure). Statistical comparisons are made to controls of the respective sexes and account for the relatedness of siblings within broods. Circulating concentrations of plasma total thyroxine (TT4 ) and total triiodothyronine (TT3 ) in female nestlings were significantly influenced overall by the exposure to DE-71. Following intramuscular administration of thyroid-stimulating hormone, the temporal response of the thyroid gland in producing and/or releasing TT4 was also significantly affected by the females' exposure to DE-71. The altered availability of T4 for conversion to T3 outside of the gland and/or changes in thyroid-related enzymatic activity may explain the lower TT3 concentrations (baseline, overall) and moderately altered temporal TT3 patterns (p = 0.06) of the treatment females. Controlling for the significant effect on TT3 levels of the delayed hatching of treatment females, baseline TT3 levels were significantly and positively correlated with body mass (10 d, 15 d, 20 d), with PBDE-exposed females generally being smaller and having lower TT3 concentrations. Given that exposure concentrations were environmentally relevant, similar thyroidal changes and associated thyroid-mediated processes relating to growth may also occur in wild female nestlings. Environ Toxicol Chem 2016;35:2084-2091. © 2016 SETAC.

New insights into the structure and mechanism of iodothyronine deiodinases

Iodothyronine deiodinases are a family of enzymes that remove specific iodine atoms from one of the two aromatic rings in thyroid hormones (THs). They thereby fine-tune local TH concentrations and cellular TH signaling. Deiodinases catalyze a remarkable biochemical reaction, i.e., the reductive elimination of a halogenide from an aromatic ring. In metazoans, deiodinases depend on the rare amino acid selenocysteine. The recent solution of the first experimental structure of a deiodinase catalytic domain allowed for a reappraisal of the many mechanistic and mutagenesis data that had been accumulated over more than 30 years. Hence, the structure generates new impetus for research directed at understanding catalytic mechanism, substrate specificity, and regulation of deiodinases. This review will focus on structural and mechanistic aspects of iodothyronine deiodinases and briefly compare these enzymes with dehalogenases, which catalyze related reactions. A general mechanism for the selenium-dependent deiodinase reaction will be described, which integrates the mouse deiodinase 3 crystal structure and biochemical studies. We will summarize further, sometimes isoform-specific molecular features of deiodinase catalysis and regulation, and we will then discuss available compounds for modulating deiodinase activity for therapeutic purposes.

EDC-2: The Endocrine Society's Second Scientific Statement on Endocrine-Disrupting Chemicals

The Endocrine Society's first Scientific Statement in 2009 provided a wake-up call to the scientific community about how environmental endocrine-disrupting chemicals (EDCs) affect health and disease. Five years later, a substantially larger body of literature has solidified our understanding of plausible mechanisms underlying EDC actions and how exposures in animals and humans-especially during development-may lay the foundations for disease later in life. At this point in history, we have much stronger knowledge about how EDCs alter gene-environment interactions via physiological, cellular, molecular, and epigenetic changes, thereby producing effects in exposed individuals as well as their descendants. Causal links between exposure and manifestation of disease are substantiated by experimental animal models and are consistent with correlative epidemiological data in humans. There are several caveats because differences in how experimental animal work is conducted can lead to difficulties in drawing broad conclusions, and we must continue to be cautious about inferring causality in humans. In this second Scientific Statement, we reviewed the literature on a subset of topics for which the translational evidence is strongest: 1) obesity and diabetes; 2) female reproduction; 3) male reproduction; 4) hormone-sensitive cancers in females; 5) prostate; 6) thyroid; and 7) neurodevelopment and neuroendocrine systems. Our inclusion criteria for studies were those conducted predominantly in the past 5 years deemed to be of high quality based on appropriate negative and positive control groups or populations, adequate sample size and experimental design, and mammalian animal studies with exposure levels in a range that was relevant to humans. We also focused on studies using the developmental origins of health and disease model. No report was excluded based on a positive or negative effect of the EDC exposure. The bulk of the results across the board strengthen the evidence for endocrine health-related actions of EDCs. Based on this much more complete understanding of the endocrine principles by which EDCs act, including nonmonotonic dose-responses, low-dose effects, and developmental vulnerability, these findings can be much better translated to human health. Armed with this information, researchers, physicians, and other healthcare providers can guide regulators and policymakers as they make responsible decisions.

Expression of Iodotyrosine Deiodinase in Thyroid and Other Organs in Iodine-Deficient and Iodine-Excess Rats

In mammals, iodothyronine deiodinase and iodotyrosine deiodinase (IYD) are known to catalyze the reductive dehalogenation. IYD is a critical enzyme in maintaining iodine homeostasis. Advances in the study of iodothyronine deiodinase have been published steadily; research on IYD has been slow on its function and regulation. We studied the expression of IYD in thyroid, liver, and kidney in conditions such as iodine deficiency and excess to determine its regulation and role in iodine recycling. Sixty 4-week-old female Wistar rats were randomly divided into two groups, with each group containing three subgroups. The rats were fed with different iodine intake for 3 months. After 3 months, all the rats were sacrificed, and the expression of IYD in thyroid, liver, and kidney of the rats were determined. We found that the expression of thyroidal IYD in 0.3-fold-iodine intake group was significantly higher as compared with the low-iodine feed control group (p < 0.01), whereas the expression in 6-fold-iodine intake group was significantly decreased as compared with normal-iodine feed control group (p < 0.01). However, the variation of IYD expression in thyroid was not similar to liver and kidney. In conclusion, iodine deficiency results in an increased expression of IYD in thyroid, whereas excess iodine decreases the expression of thyroidal IYD. In humans, daily iodine intake of <75 or >500 μg can affect the expression of thyroidal IYD. The safety range of iodine intake is narrow. In addition, further investigations are required to study the expression and regulation of IYD in various organs.

Neurobehavioral deficits, diseases, and associated costs of exposure to endocrine-disrupting chemicals in the European Union

CONTEXT

Epidemiological studies and animal models demonstrate that endocrine-disrupting chemicals (EDCs) contribute to cognitive deficits and neurodevelopmental disabilities.

OBJECTIVE

The objective was to estimate neurodevelopmental disability and associated costs that can be reasonably attributed to EDC exposure in the European Union.

DESIGN

An expert panel applied a weight-of-evidence characterization adapted from the Intergovernmental Panel on Climate Change. Exposure-response relationships and reference levels were evaluated for relevant EDCs, and biomarker data were organized from peer-reviewed studies to represent European exposure and approximate burden of disease. Cost estimation as of 2010 utilized lifetime economic productivity estimates, lifetime cost estimates for autism spectrum disorder, and annual costs for attention-deficit hyperactivity disorder. Setting, Patients and Participants, and Intervention: Cost estimation was carried out from a societal perspective, ie, including direct costs (eg, treatment costs) and indirect costs such as productivity loss.

RESULTS

The panel identified a 70-100% probability that polybrominated diphenyl ether and organophosphate exposures contribute to IQ loss in the European population. Polybrominated diphenyl ether exposures were associated with 873,000 (sensitivity analysis, 148,000 to 2.02 million) lost IQ points and 3290 (sensitivity analysis, 3290 to 8080) cases of intellectual disability, at costs of €9.59 billion (sensitivity analysis, €1.58 billion to €22.4 billion). Organophosphate exposures were associated with 13.0 million (sensitivity analysis, 4.24 million to 17.1 million) lost IQ points and 59 300 (sensitivity analysis, 16,500 to 84,400) cases of intellectual disability, at costs of €146 billion (sensitivity analysis, €46.8 billion to €194 billion). Autism spectrum disorder causation by multiple EDCs was assigned a 20-39% probability, with 316 (sensitivity analysis, 126-631) attributable cases at a cost of €199 million (sensitivity analysis, €79.7 million to €399 million). Attention-deficit hyperactivity disorder causation by multiple EDCs was assigned a 20-69% probability, with 19 300 to 31 200 attributable cases at a cost of €1.21 billion to €2.86 billion.

CONCLUSIONS

EDC exposures in Europe contribute substantially to neurobehavioral deficits and disease, with a high probability of >€150 billion costs/year. These results emphasize the advantages of controlling EDC exposure.