Identification of iopanoic acid as substrate of type 1 deiodinase by a novel nonradioactive iodide-release assay

In Vitro Screening for ToxCast Chemicals Binding to Thyroxine-Binding Globulin

Thyroid hormone (TH) carrier proteins play an important role in distributing TH to target tissue as well as maintaining the balance of free versus bound TH in the blood. Interference with the TH carrier proteins has been identified as a potential mechanism of thyroid system disruption. To address the lack of data regarding chemicals binding to these carrier proteins and displacing TH, a fluorescence-based in vitro screening assay was utilized to screen over 1,400 chemicals from the U.S. EPA's ToxCast phase1_v2, phase 2, and e1k libraries for competitive binding to one of the carrier proteins, thyroxine-binding globulin. Initial screening at a single high concentration of 100 μM identified 714 chemicals that decreased signal of the bound fluorescent ligand by 20% or higher. Of these, 297 produced 50% or greater reduction in fluorescence and were further tested in concentration-response (0.004 to 150 μM) to determine relative potency. Ten chemicals were found to have EC50 values <1 μM, 63 < 10 μM, and 141 chemicals between 10 and 100 μM. Utilization of this assay contributes to expanding the number of in vitro assays available for identifying chemicals with the potential to disrupt TH homeostasis. These results support ranking and prioritization of chemicals to be tested in vivo to aid in the development of a framework for predicting in vivo effects from in vitro high-throughput data.

Screening the ToxCast Chemical Libraries for Binding to Transthyretin

Transthyretin (TTR) is one of the serum binding proteins responsible for transport of thyroid hormones (TH) to target tissue and for maintaining the balance of available TH. Chemical binding to TTR and subsequent displacement of TH has been identified as an end point in screening chemicals for potential disruption of the thyroid system. To address the lack of data regarding chemicals binding to TTR, we optimized an in vitro assay utilizing the fluorescent probe 8-anilino-1-napthalenesulfonic acid (ANSA) and the human protein TTR to screen over 1500 chemicals from the U.S. EPA's ToxCast ph1_v2, ph2, and e1k libraries utilizing a tiered approach. Testing of a single high concentration (target 100 μM) resulted in 888 chemicals with 20% or greater activity based on displacement of ANSA from TTR. Of these, 282 chemicals had activity of 85% or greater and were further tested in 12-point concentration-response with target concentrations ranging from 0.015 to 100 μM. An EC50 was obtained for 276 of these 301 chemicals. To date, this is the largest set of chemicals screened for binding to TTR. Utilization of this assay is a significant contribution toward expanding the suite of in vitro assays used to identify chemicals with the potential to disrupt thyroid hormone homeostasis.

Gene biomarkers for the assessment of thyroid-disrupting activity in zebrafish embryos

Active ingredients of pesticides or biocides and industrial chemicals can negatively affect environmental organisms, potentially endangering populations and ecosystems. European legislation mandates that chemical manufacturers provide data for the environmental risk assessment of substances to obtain registration. Endocrine disruptors, substances that interfere with the hormone system, are not granted marketing authorization due to their adverse effects. Current methods for identifying disruptors targeting the thyroid hormone system are costly and require many amphibians. Consequently, alternative methods compliant with the 3R principle (replacement, reduction, refinement) are essential to prioritize risk assessment using reliable biomarkers at non-protected life stages. Our study focused on detecting robust biomarkers for thyroid-disrupting mechanisms of action (MoA) by analyzing molecular signatures in zebrafish embryos induced by deiodinase inhibitor iopanoic acid and thyroid peroxidase inhibitor methimazole. We exposed freshly fertilized zebrafish eggs to these compounds, measuring lethality, hatching rate, swim bladder size and transcriptomic responses. Both compounds significantly reduced swim bladder size, aligning with prior findings. Transcriptome analysis revealed specific molecular fingerprints consistent with the MoA under investigation. This analysis confirmed regulation directions seen in other studies involving thyroid disruptors and allowed us to identify genes like tg, scl2a11b, guca1d, cthrc1a, si:ch211-226h7.5, soul5, nnt2, cox6a2 and mep1a as biomarker genes for thyroid disrupting MoA in zebrafish embryos as per OECD test guideline 236. Future screening methods based on our findings will enable precise identification of thyroid-related activity in chemicals, promoting the development of environmentally safer substances. Additionally, these biomarkers could potentially be incorporated into legally mandated chronic toxicity tests in fish, potentially replacing amphibian tests for thyroid disruption screening in the future.

Two novel in vitro assays to screen chemicals for their capacity to inhibit thyroid hormone transmembrane transporter proteins OATP1C1 and OAT4

Early brain development depends on adequate transport of thyroid hormones (THs) from the maternal circulation to the fetus. To reach the fetal brain, THs have to cross several physiological barriers, including the placenta, blood-brain-barrier and blood-cerebrospinal fluid-barrier. Transport across these barriers is facilitated by thyroid hormone transmembrane transporters (THTMTs). Some endocrine disrupting chemicals (EDCs) can interfere with the transport of THs by THTMTs. To screen chemicals for their capacity to disrupt THTMT facilitated TH transport, in vitro screening assays are required. In this study, we developed assays for two THTMTs, organic anion transporter polypeptide 1C1 (OATP1C1) and organic anion transporter 4 (OAT4), both known to play a role in the transport of THs across barriers. We used overexpressing cell models for both OATP1C1 and OAT4, which showed an increased uptake of radiolabeled T4 compared to control cell lines. Using these models, we screened various reference and environmental chemicals for their ability to inhibit T4 uptake by OATP1C1 and OAT4. Tetrabromobisphenol A (TBBPA) was identified as an OATP1C1 inhibitor, more potent than any of the reference chemicals tested. Additionally perfluorooctanesulfonic acid (PFOS), perfluoroctanic acid (PFOA), pentachlorophenol and quercetin were identified as OATP1C1 inhibitors in a similar range of potency to the reference chemicals tested. Bromosulfophthalein, TBBPA, PFOA and PFOS were identified as potent OAT4 inhibitors. These results demonstrate that EDCs commonly found in our environment can disrupt TH transport by THTMTs, and contribute to the identification of molecular mechanisms underlying TH system disruption chemicals.

Iopanoic acid alters thyroid hormone-related gene expression, thyroid hormone levels, swim bladder inflation, and swimming performance in Japanese medaka

Disruption of the thyroid hormone system by synthetic chemicals is gaining attention owing to its potential negative effects on organisms. In this study, the effects of the dio-inhibitor iopanoic acid (IOP) on the levels of thyroid hormone and related gene expression, swim bladder inflation, and swimming performance were investigated in Japanese medaka. Iopanoic acid exposure suppressed thyroid-stimulating hormone β (tshβ), tshβ-like, iodotyronin deiodinase 1 (dio1), and dio2 expression, and increased T4 and T3 levels. In addition, IOP exposure inhibited swim bladder inflation, reducing swimming performance. Although adverse outcome pathways of thyroid hormone disruption have been developed using zebrafish, no adverse outcome pathways have been developed using Japanese medaka. This study confirmed that IOP inhibits dio expression (a molecular initiating event), affects T3 and T4 levels (a key event), and reduces swim bladder inflation (a key event) and swimming performance (an adverse outcome) in Japanese medaka.

Development of a Zebrafish Embryo-Based Test System for Thyroid Hormone System Disruption: 3Rs in Ecotoxicological Research

There is increasing concern regarding pollutants disrupting the vertebrate thyroid hormone (TH) system, which is crucial for development. Thus, identification of TH system-disrupting chemicals (THSDCs) is an important requirement in the Organisation for Economic Co-operation and Development (OECD) testing framework. The current OECD approach uses different model organisms for different endocrine modalities, leading to a high number of animal tests. Alternative models compatible with the 3Rs (replacement, reduction, refinement) principle are required. Zebrafish embryos, not protected by current European Union animal welfare legislation, represent a promising model. Studies show that zebrafish swim bladder inflation and eye development are affected by THSDCs, and the respective adverse outcome pathways (AOPs) have been established. The present study compared effects of four THSDCs with distinct molecular modes of action: Propylthiouracil (PTU), potassium perchlorate, iopanoic acid, and the TH triiodothyronine (T3) were tested with a protocol based on the OECD fish embryo toxicity test (FET). Effects were analyzed according to the AOP concept from molecular over morphological to behavioral levels: Analysis of thyroid- and eye-related gene expression revealed significant effects after PTU and T3 exposure. All substances caused changes in thyroid follicle morphology of a transgenic zebrafish line expressing fluorescence in thyrocytes. Impaired eye development and swimming activity were observed in all treatments, supporting the hypothesis that THSDCs cause adverse population-relevant changes. Findings thus confirm that the FET can be amended by TH system-related endpoints into an integrated protocol comprising molecular, morphological, and behavioral endpoints for environmental risk assessment of potential endocrine disruptors, which is compatible with the 3Rs principle. Environ Toxicol Chem 2024;00:1-18. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Screening for endocrine disrupting chemicals inhibiting monocarboxylate 8 (MCT8) transporter facilitated thyroid hormone transport using a modified nonradioactive assay

Early neurodevelopmental processes are strictly dependent on spatial and temporally modulated of thyroid hormone (TH) availability and action. Thyroid hormone transmembrane transporters (THTMT) are critical for regulating the local concentrations of TH, namely thyroxine (T4) and 3,5,3'-tri-iodothyronine (T3), in the brain. Monocarboxylate transporter 8 (MCT8) is one of the most prominent THTMT. Genetically induced deficiencies in expression, function or localization of MCT8 are associated with irreversible and severe neurodevelopmental adversities. Due to the importance of MCT8 in brain development, studies addressing chemical interferences of MCT8 facilitated T3 uptake are a crucial step to identify TH system disrupting chemicals with this specific mode of action. Recently a non-radioactive in vitro assay has been developed to rapidly screen for endocrine disrupting chemicals (EDCs) acting upon MCT8 mediated transport. This study explored the use of an UV-light digestion step as an alternative for the original ammonium persulfate (APS) digestion step. The non-radioactive TH uptake assay, with the incorporated UV-light digestion step of TH, was then used to screen a set of 31 reference chemicals and environmentally relevant substances to detect inhibition of MCT8-depending T3 uptake. This alternative assay identified three novel MCT8 inhibitors: methylmercury, bisphenol-AF and bisphenol-Z and confirmed previously known MCT8 inhibitors.

Low thyroxine serves as an upstream regulator of ecophysiological adaptations in Ansell's mole-rats

Introduction

About 10% of all rodent species have evolved a subterranean way of life, although life in subterranean burrows is associated with harsh environmental conditions that would be lethal to most animals living above ground. Two key adaptations for survival in subterranean habitats are low resting metabolic rate (RMR) and core body temperature (Tb). However, the upstream regulation of these traits was unknown thus far. Previously, we have reported exceptionally low concentrations of the thyroid hormone (TH) thyroxine (T4), and peculiarities in TH regulating mechanisms in two African mole-rat species, the naked mole-rat and the Ansell's mole-rat.

Methods

In the present study, we treated Ansell's mole-rats with T4 for four weeks and analyzed treatment effects on the tissue and whole organism level with focus on metabolism and thermoregulation.

Results

We found RMR to be upregulated by T4 treatment but not to the extent that was expected based on serum T4 concentrations. Our data point towards an extraordinary capability of Ansell's mole-rats to effectively downregulate TH signaling at tissue level despite very high serum TH concentrations, which most likely explains the observed effects on RMR. On the other hand, body weight was decreased in T4-treated animals and Tb was upregulated by T4 treatment. Moreover, we found indications of the hypothalamus-pituitary-adrenal axis potentially influencing the treatment effects.

Conclusion

Taken together, we provide the first experimental evidence that the low serum T4 concentrations of Ansell's mole-rats serve as an upstream regulator of low RMR and Tb. Thus, our study contributes to a better understanding of the ecophysiological evolution of the subterranean lifestyle in African mole-rats.

Assessing utility of thyroid in vitro screening assays through comparisons to observed impacts in vivo

To better understand endocrine disruption, the U.S. Environmental Protection Agency's (USEPA) Endocrine Disruptor Screening Program (EDSP) utilizes a two-tiered approach to investigate the potential of a chemical to interact with the estrogen, androgen, or thyroid systems. As in vivo testing lacks the throughput to address data gaps on endocrine bioactivity for thousands of chemicals, in vitro high-throughput screening (HTS) methods are being developed to screen larger chemical libraries. The primary objective of this work was to investigate for how many of the 52 chemicals with weight-of-evidence (WoE) determinations from EDSP Tier 1 screening there are available in vitro HTS data supporting a thyroid impact. HTS data from the USEPA ToxCast program and the EDSP WoE were collected for this analysis. Considering the complexity of endocrine disruption and interpreting HTS data, concordance between in vitro activity and in vivo effects ranges from 58 to 78%. Based on this evaluation, we conclude that the current suite of HTS assays is beneficial for prioritizing chemicals for further inquiry; however, without a more detailed analysis, one cannot conclude whether HTS results are the primary mode-of-action. Furthermore, development of in vitro assays for additional thyroid-relevant molecular initiating events is required to effectively predict in vivo thyroid impacts.

Combining In Vitro and In Silico New Approach Methods to Investigate Type 3 Iodothyronine Deiodinase Chemical Inhibition Across Species

New approach methodologies (NAMs) are being developed to reduce and replace vertebrate animal testing in support of ecotoxicology and risk assessment. The US Environmental Protection Agency's Sequence Alignment to Predict Across Species Susceptibility (SeqAPASS) bioinformatic tool was used to evaluate amino acid sequence conservation of the type 3 iodothyronine deiodinase (DIO3) enzyme across species to demonstrate NAM applications for understanding effects of chemical interactions with a specific protein target. Existing literature was used to identify critical amino acids for thyroid hormone binding and interaction with a reducing cofactor. The SeqAPASS tool identifies whether known critical amino acids involved in ligand binding are exact, partial, or not matches across species compared with a template species based on molecular weight and side chain classification. This evaluation guided the design of variant proteins representing critical amino acid substitutions found in various species. Site-directed mutagenesis of the wild-type (WT) human DIO3 gene sequence was used to create six variant proteins expressed in cell culture, which were then tested in vitro for chemical inhibition. Significant differences in in vitro median inhibitory concentration results were observed among variants for potential competitive inhibitors. A molecular model representing the WT human DIO3 was constructed using Molecular Operating Environment (MOE) software and mutated in silico to create the six variants. The MOE Site Finder tool identified the proposed catalytic and cofactor sites and potential alternative binding sites. Virtual docking did not provide affinity scores with sufficient resolution to rank the potency of the chemical inhibitors. Chemical characteristics, function and location of substituted amino acids, and complexities of the protein target are important considerations in developing NAMs to evaluate chemical susceptibility across species. Environ Toxicol Chem 2023;42:1032-1048. © 2023 University of Wisconsin-Madison. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

Comparative analysis of thyroid hormone systems in rodents with subterranean lifestyle

African mole-rats are subterranean rodents inhabiting underground burrows. This habitat entails risks of overheating, hypoxia, and scarce food availability. Consequently, many subterranean species have evolved low basal metabolism and low body temperature, but the regulation of these traits at the molecular level were unknown. Measurements of serum thyroid hormone (TH) concentrations in African mole-rats have revealed a unique TH phenotype, which deviates from the typical mammalian pattern. Since THs are major regulators of metabolic rate and body temperature, we further characterised the TH system of two African mole-rat species, the naked mole-rat (Heterocephalus glaber) and the Ansell's mole-rat (Fukomys anselli) at the molecular level in a comparative approach involving the house mouse (Mus musculus) as a well-studied laboratory model in TH research. Most intriguingly, both mole-rat species had low iodide levels in the thyroid and naked mole-rats showed signs of thyroid gland hyperplasia. However, contrary to expectations, we found several species-specific differences in the TH systems of both mole-rat species, although ultimately resulting in similar serum TH concentrations. These findings indicate a possible convergent adaptation. Thus, our study adds to our knowledge for understanding adaptations to the subterranean habitat.

Deiodinases control local cellular and systemic thyroid hormone availability

Iodothyronine deiodinases (DIO) are a family of selenoproteins controlling systemic and local availability of the major thyroid hormone l-thyroxine (T4), a prohormone secreted by the thyroid gland. T4 is activated to the active 3,3'-5-triiodothyronine (T3) by two 5'-deiodinases, DIO1 and DIO2. DIO3, a 5-deiodinase selenoenzyme inactivates both the prohormone T4 and its active form T3. DIOs show species-specific different patterns of temporo-spatial expression, regulation and function and exhibit different mechanisms of reaction and inhibitor sensitivities. The main regulators of DIO expression and function are the thyroid hormone status, several growth factors, cytokines and altered pathophysiological conditions. Selenium (Se) status has a modest impact on DIO expression and translation. DIOs rank high in the priority of selenium supply to various selenoproteins; thus, their function is impaired only during severe selenium deficiency. DIO variants, polymorphisms, SNPs and rare mutations have been identified. Development of DIO isozyme selective drugs is ongoing. A first X-ray structure has been reported for DIO3. This review focusses on the biochemical characteristics and reaction mechanisms, the relationships between DIO selenoproteins and their importance for local and systemic provision of the active hormone T3. Nutritional, pharmacological, and environmental factors and inhibitors, such as endocrine disruptors, impact DIO functions.

AOP Report: Thyroperoxidase Inhibition Leading to Altered Visual Function in Fish Via Altered Retinal Layer Structure

Thyroid hormones (THs) are involved in the regulation of many important physiological and developmental processes, including vertebrate eye development. Thyroid hormone system-disrupting chemicals (THSDCs) may have severe consequences, because proper functioning of the visual system is a key factor for survival in wildlife. However, the sequence of events leading from TH system disruption (THSD) to altered eye development in fish has not yet been fully described. The development of this adverse outcome pathway (AOP) was based on an intensive literature review of studies that focused on THSD and impacts on eye development, mainly in fish. In total, approximately 120 studies (up to the end of 2021) were used in the development of this AOP linking inhibition of the key enzyme for TH synthesis, thyroperoxidase (TPO), to effects on retinal layer structure and visual function in fish (AOP-Wiki, AOP 363). In a weight-of-evidence evaluation, the confidence levels were overall moderate, with ample studies showing the link between reduced TH levels and altered retinal layer structure. However, some uncertainties about the underlying mechanism(s) remain. Although the current weight-of-evidence evaluation is based on fish, the AOP is plausibly applicable to other vertebrate classes. Through the re-use of several building blocks, this AOP is connected to the AOPs leading from TPO and deiodinase inhibition to impaired swim bladder inflation in fish (AOPs 155-159), together forming an AOP network describing THSD in fish. This AOP network addresses the lack of thyroid-related endpoints in existing fish test guidelines for the evaluation of THSDCs. Environ Toxicol Chem 2022;41:2632-2648. © 2022 SETAC.

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.

Tentative Application of a Streamlined Protocol to Determine Organ-Specific Regulations of Deiodinase 1 and Dehalogenase Activities as Readouts of the Hypothalamus-Pituitary-Thyroid-Periphery-Axis

In animal studies, both in basic science and in toxicological assessment of potential endocrine disruptors, the state of the thyroid hormone (TH) axis is often described and defined exclusively by the concentrations of circulating THs and TSH. Although it is known that the local, organ-specific effects of THs are also substantially regulated by local mechanisms such as TH transmembrane transport and metabolism of TH by deiodinases, such endpoint parameters of the axis are rarely assessed in these experiments. Currently developed in vitro assays utilize the Sandell-Kolthoff reaction, a photometric method of iodide determination, to test the effect of chemicals on iodotyrosine and iodothyronine deiodinases. Furthermore, this technology offers the possibility to determine the iodine content of various sample types (e.g., urine, ex vivo tissue) in a simple way. Here, we measured deiodinase type 1 and iodotyrosine dehalogenase activity by means of the Sandell-Kolthoff reaction in ex vivo samples of hypo- and hyperthyroid mice of two age groups (young; 3 months and old; 20 months). In thyroid, liver and kidney, organ-specific regulation patterns emerged across both age groups, which, based on this pilot study, may serve as a starting point for a deeper characterization of the TH system in relevant studies in the future and support the development of Integrated Approach for Testing and Assessment (IATA).

Characterization of the mechanistic linkages between iodothyronine deiodinase inhibition and impaired thyroid-mediated growth and development in Xenopus laevis using iopanoic acid

Iodothyronine deiodinases (DIO) are key enzymes that influence tissue-specific thyroid hormone levels during thyroid-mediated amphibian metamorphosis. Within the larger context of evaluating chemicals for thyroid system disrupting potential, chemical activity toward DIOs is being evaluated using high-throughput in vitro screening assays as part of U.S. EPA's ToxCast program. However, existing data gaps preclude any inferences between in vitro chemical inhibition of DIOs and in vivo outcomes relevant to ecological risk assessment. This study aimed to generate targeted data in a laboratory model species (Xenopus laevis) using a model DIO inhibitor, iopanoic acid (IOP), to characterize linkages between in vitro potency, in vivo biochemical responses, and adverse organismal outcomes. In vitro potency of IOP toward DIOs was evaluated using previously developed in vitro screening assays, which showed concentration-dependent inhibition of human DIO1 (IC50: 97 µM) and DIO2 (IC50: 231 µM) but did not inhibit human or X. laevis DIO3 under the assay conditions. In vivo exposure of larval X. laevis to 0, 2.6, 5.3 and 10.5 µM IOP caused thyroid-related biochemical profiles in the thyroid gland and plasma consistent with hyperthyroxinemia but resulted in delayed metamorphosis and significantly reduced growth in the highest two exposure concentrations. Independent evaluations of dio gene expression ontogeny, together with existing literature, supported interpretation of IOP-mediated effects resulting in a proposed adverse outcome pathway for DIO2 inhibition leading to altered amphibian metamorphosis. This study highlights the types of mechanistic data needed to move toward predicting in vivo outcomes of regulatory concern from in vitro bioactivity data.

The thyroid hormone converting enzyme human deiodinase 1 is inhibited by gold ions from inorganic salts, organic substances, and by small-size nanoparticles

The selenocysteine-containing enzyme class deiodinases (DIO) consists of three isoforms. DIOs play a role in regulation of thyroid hormone (TH) signaling through the removal of iodide from TH leading to TH that interact with the hypothalamic-pituitary-thyroid (HPT) axis with differing potency. Some gold-containing organic substances are known to inhibit many selenoenzymes, including DIOs. It is, however, unclear whether the Au-containing substances or the Au ions are causing the inhibition. In this study, five organic and inorganic gold substances as well as three gold nanoparticles (AuNPs) were tested for their potential to inhibit DIO1. The enzyme activity was tested using human liver microsomes as an enzyme source and reverse T3 as a substrate; iodide release was measured by the Sandell-Kolthoff method. The three organic gold substances aurothioglucose, auranofin and sodium aurothiomalate inhibited DIO1 with IC₅₀ between 0.38 and 0.75 μM while their structural analogues lacking the gold ion did not. Likewise, the two tested gold salts, Au(I) and Au(III) chloride, showed a concentration-dependent inhibition of the DIO1 with IC₅₀ values of 0.95 and 0.57 μM. Further, AuNPs of different sizes (100, 30 and 5 nm diameter) were tested with only the 5 nm AuNPs leading to inhibition with an IC₅₀ of 8 × 10¹⁴ AuNP/L. This inhibition was not caused by the Au ions released by the AuNP into the incubation media. The exact mechanism of inhibition of DIO1 by 5 nm AuNPs should be further examined. In conclusion, the microsomal DIO1 assay demonstrated the inhibition of DIO1 by gold ions originating from different gold-containing substances, but not by Au released from AuNPs; rather DIO1 is inhibited by 5 nm, but not larger, AuNPs.

The Nutritional Supply of Iodine and Selenium Affects Thyroid Hormone Axis Related Endpoints in Mice

Selenium and iodine are the two central trace elements for the homeostasis of thyroid hormones but additional trace elements such as iron, zinc, and copper are also involved. To compare the primary effects of inadequate intake of selenium and iodine on the thyroid gland, as well as the target organs of thyroid hormones such as liver and kidney, mice were subjected to an eight-week dietary intervention with low versus adequate selenium and iodine supply. Analysis of trace element levels in serum, liver, and kidney demonstrated a successful intervention. Markers of the selenium status were unaffected by the iodine supply. The thyroid gland was able to maintain serum thyroxine levels even under selenium-deficient conditions, despite reduced selenoprotein expression in liver and kidney, including deiodinase type 1. Thyroid hormone target genes responded to the altered selenium and iodine supply, whereas the iron, zinc, and copper homeostasis remained unaffected. There was a notable interaction between thyroid hormones and copper, which requires further clarification. Overall, the effects of an altered selenium and iodine supply were pronounced in thyroid hormone target tissues, but not in the thyroid gland.

Maternal Thyroid Hormone Programs Cardiovascular Functions in the Offspring

Background: Maternal thyroid hormone (TH) plays an essential role for fetal development, especially for the cardiovascular system and its central control. However, the precise consequences of altered TH action during the different periods in pregnancy remain poorly understood. Methods: To address this question, we used mice heterozygous for a mutant thyroid hormone receptor α1 (TRα1) and wild-type controls that were born to wild-type mothers treated with 3,3',5-triiodothyronine (T3) during the first or the second half of pregnancy. We then phenotyped the offspring animals as adults by in vivo measurements and postmortem tissue analyses. Results: Maternal T3 treatment in either half of the pregnancy did not affect postnatal growth development. Serum thyroxine and hypophyseal thyrotropin subunit beta or deiodinase type II expression was also not affected in any group, only TRα1 mutant males exhibited a reduction in serum T3 levels after the treatment. Likewise, hepatic deiodinase type I was not altered, but serum selenium levels were reduced by the maternal treatment in wild-type offspring of both genders. Most interestingly, a significant increase in heart weight was found in adult wild-types born to mothers that received T3 during the first or second half of pregnancy, while TRα1 mutant males were protected from this effect. Moreover, we detected a significant increase in heart rate selectively in male mice that were exposed to elevated maternal T3 in the second half of the pregnancy. Conclusion: Taken together, our findings demonstrate that maternal TH is of particular relevance during the second half of pregnancy for establishing cardiac properties, with specific effects depending on TRα1 or gender. The data advocate routinely monitoring TH levels during pregnancy to avoid adverse cardiac effects in the offspring.

Polypharmacy Is Associated With Amiodarone-Induced Hypothyroidism

Background: Amiodarone is rich in iodine, so in clinical practice amiodarone-induced hypothyroidism (AIH) is a major side effect. This drug is used in patients with arrhythmias, especially atrial fibrillation, the most common sustained arrhythmia. Polypharmacy, which can result in complex drug-drug interactions, occurs in more than 70% of the patients with atrial fibrillation. Therefore, polypharmacy may be involved in the expression of AIH. In this study, we investigated the association between polypharmacy and AIH. Methods: We conducted a retrospective study using data from January 2006 to May 2020 collected from a large, organized database of prescriptions constructed by the Japan Medical Information Research Institute, Inc. (Tokyo, Japan). To investigate the association between number of prescribed drugs with amiodarone and AIH, we divided patients into two groups: polypharmacy (≥ 5 prescribed drugs) and non-polypharmacy (< 5 prescribed drugs). We then performed a sequence symmetry analysis on the two groups: incident thyroxine after incident amiodarone and incident thyroxine before incident amiodarone. Finally, we conducted a case-control study on two further groups: those prescribed thyroxine after incident amiodarone (AIH group; n=555) and those not prescribed thyroxine after incident amiodarone (non-AIH group; n=6,192). Results: Sequence symmetry analysis revealed a significant association between amiodarone and thyroxine in both the polypharmacy and non-polypharmacy groups. The ranges for the adjusted sequence ratio in the two groups were 12.0-16.7 and 7.3-9.0, respectively. The case-control study showed that ≥5 prescribed drugs at the first prescription of amiodarone were found to significantly increase the odds of AIH (odds ratio: 1.48, 95% confidence interval: 1.18-1.84). Conclusion: Polypharmacy was suggested as an independent risk factor for AIH. Careful assessment of the appropriateness of prescription is warranted.

Selenium Deficiency Due to Diet, Pregnancy, Severe Illness, or COVID-19-A Preventable Trigger for Autoimmune Disease

The trace element selenium (Se) is an essential part of the human diet; moreover, increased health risks have been observed with Se deficiency. A sufficiently high Se status is a prerequisite for adequate immune response, and preventable endemic diseases are known from areas with Se deficiency. Biomarkers of Se status decline strongly in pregnancy, severe illness, or COVID-19, reaching critically low concentrations. Notably, these conditions are associated with an increased risk for autoimmune disease (AID). Positive effects on the immune system are observed with Se supplementation in pregnancy, autoimmune thyroid disease, and recovery from severe illness. However, some studies reported null results; the database is small, and randomized trials are sparse. The current need for research on the link between AID and Se deficiency is particularly obvious for rheumatoid arthritis and type 1 diabetes mellitus. Despite these gaps in knowledge, it seems timely to realize that severe Se deficiency may trigger AID in susceptible subjects. Improved dietary choices or supplemental Se are efficient ways to avoid severe Se deficiency, thereby decreasing AID risk and improving disease course. A personalized approach is needed in clinics and during therapy, while population-wide measures should be considered for areas with habitual low Se intake. Finland has been adding Se to its food chain for more than 35 years-a wise and commendable decision, according to today's knowledge. It is unfortunate that the health risks of Se deficiency are often neglected, while possible side effects of Se supplementation are exaggerated, leading to disregard for this safe and promising preventive and adjuvant treatment options. This is especially true in the follow-up situations of pregnancy, severe illness, or COVID-19, where massive Se deficiencies have developed and are associated with AID risk, long-lasting health impairments, and slow recovery.

Xenopus laevis and human type 3 iodothyronine deiodinase enzyme cross-species sensitivity to inhibition by ToxCast chemicals

Deiodinase enzymes are critical for tissue-specific and temporal control of activation or inactivation of thyroid hormones during vertebrate development, including amphibian metamorphosis. We previously screened ToxCast chemicals for inhibitory activity toward human recombinant Type 3 iodothyronine deiodinase enzyme (hDIO3) and subsequently produced Xenopus laevis recombinant dio3 enzyme (Xldio3) with the goals to identify specific chemical inhibitors of Xldio3, to evaluate cross-species sensitivity and explore whether the human assay results are predictive of the amphibian. We identified a subset of 356 chemicals screened against hDIO3 to test against Xldio3, initially at a single concentration (200 μM), and further tested 79 in concentration-response mode. Most chemicals had IC₅₀ values lower for hDIO3 than for Xldio3 and many had steep Hill slopes (a potential indication of non-specific inhibition). However, eight of the most potent chemicals are likely specific inhibitors, with IC₅₀ values of 14 μM or less, Hill slopes near -1 and curves not significantly different between species likely due to conservation of catalytically active amino acids. Controlling for assay conditions, human in vitro screening results can be predictive of activity in the amphibian assay. This study lays the groundwork for future studies using recombinant non-mammalian proteins to test cross-species sensitivity to chemicals. DISCLAIMER: The views expressed in this paper are those of the authors and do not necessarily reflect the views or policies of the U.S. Environmental Protection Agency. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.

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.

Anthropogenic stressors impact fish sensory development and survival via thyroid disruption

Larval metamorphosis and recruitment represent critical life-history transitions for most teleost fishes. While the detrimental effects of anthropogenic stressors on the behavior and survival of recruiting fishes are well-documented, the physiological mechanisms that underpin these patterns remain unclear. Here, we use pharmacological treatments to highlight the role that thyroid hormones (TH) play in sensory development and determining anti-predator responses in metamorphosing convict surgeonfish, Acanthurus triostegus. We then show that high doses of a physical stressor (increased temperature of +3 °C) and a chemical stressor (the pesticide chlorpyrifos at 30 µg L⁻¹) induced similar defects by decreasing fish TH levels and affecting their sensory development. Stressor-exposed fish experienced higher predation; however, their ability to avoid predation improved when they received supplemental TH. Our results highlight that two different anthropogenic stressors can affect critical developmental and ecological transitions via the same physiological pathway. This finding provides a unifying mechanism to explain past results and underlines the profound threat anthropogenic stressors pose to fish communities.

Anthropogenic stressors affect many aspects of marine organismal health. Here, the authors expose surgeonfish to temperature and pesticide stressors and show that the stressors, separately and in combination, have adverse effects on thyroid signaling, which disrupts several sensory systems and important predation defenses.

Aging affects sex- and organ-specific trace element profiles in mice

A decline of immune responses and dynamic modulation of the redox status are observed during aging and are influenced by trace elements such as copper, iodine, iron, manganese, selenium, and zinc. So far, analytical studies have focused mainly on single trace elements. Therefore, we aimed to characterize age-specific profiles of several trace elements simultaneously in serum and organs of adult and old mice. This allows for correlating multiple trace element levels and to identify potential patterns of age-dependent alterations. In serum, copper and iodine concentrations were increased and zinc concentration was decreased in old as compared to adult mice. In parallel, decreased copper and elevated iron concentrations were observed in liver. The age-related reduction of hepatic copper levels was associated with reduced expression of copper transporters, whereas the increased hepatic iron concentrations correlated positively with proinflammatory mediators and Nrf2-induced ferritin H levels. Interestingly, the age-dependent inverse regulation of copper and iron was unique for the liver and not observed in any other organ. The physiological importance of alterations in the iron/copper ratio for liver function and the aging process needs to be addressed in further studies.

Aging Is Associated with Low Thyroid State and Organ-Specific Sensitivity to Thyroxine

Background: Serum thyroid state in older adults correlates with extended longevity. We hypothesized that age impacts not only systemic but also organ-specific thyroid state and response to thyroxine (T4). Methods: Young (3 months) and old (23 months) male mice were analyzed at baseline and after acute T4 challenge. Age effects on circulating thyrotropin (TSH) and thyroid hormone (TH) concentrations, transcript expression in the pituitary and thyroid were compared with organ-specific responses characterized by hepatic and cardiac content of TH and TH metabolites and expression of TH-target genes, as well as hepatic deiodinase 1 activity. Results: Circulating TH concentrations and hepatic and cardiac TH content were lower in old versus young mice. After injection with T4, conversion of T4 to triiodothyronine was decreased in old mice while TH transport in liver and heart was not affected. Organ-specific TH response was augmented in old mice in liver but not heart, indicating age- and tissue-specific sensitivity to TH. A compensatory increase of thyroid stimulating hormone subunit beta expression in the pituitary and increased serum TSH concentrations, but reduced expression of thyroid differentiation markers were found in old mice. Conclusions: We suggest that a reduced activity of the aged thyroid is responsible for the systemic low TH state in old mice. Further, divergent TH metabolism and tissue response in liver and heart occur after T4 treatment in an aged organism. These rodent data are in agreement with a much narrower window for T4 substitution in the older adults to avoid overtreatment.

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.

The isoflavones genistein and daidzein increase hepatic concentration of thyroid hormones and affect cholesterol metabolism in middle-aged male rats

We examined whether isoflavones interfere with thyroid homeostasis, increase hepatic thyroid hormone concentrations and affect cholesterol metabolism in middle-aged (MA) male rats. Thirteen-month-old Wistar rats were injected subcutaneously with 35 mg/kg b.w./day of genistein, daidzein or vehicle (controls) for four weeks. Hepatic Dio1 gene expression was up-regulated by 70% (p < 0.001 for both) and Dio1 enzyme activity increased by 64% after genistein (p < 0.001) and 73% after daidzein treatment (p < 0.0001). Hepatic T₃ was 75% higher (p < 0.05 for both), while T₄ increased only after genistein treatment. Serum T₄ concentrations were 31% lower in genistein- and 49% lower in dadzein-treated rats (p < 0.001 for both) compared with controls. Hepatic Cyp7a1 gene expression was up-regulated by 40% after genistein and 32% after daidzein treatment (p < 0.05 for both), in agreement with a 7α-hydroxycholesterol increase of 50% (p < 0.01) and 88% (p < 0.001), respectively. Serum 24- and 27-hydroxycholesterol were 30% lower (p < 0.05 for both), while only 24-hydroxycholesterol was decreased in the liver by 45% after genistein (p < 0.05) and 39% (p < 0.01) after dadzein treatment. Serum concentration of the cholesterol precursor desmosterol was 32% (p < 0.05) lower only after dadzein treatment alone, while both isoflavones elevated this parameter in the liver by 45% (p < 0.01). In conclusion, isoflavones increased T₃ availability in the liver of MA males, despite decreasing serum T₄. Hepatic increase of T₃ possibly contributes to activation of the neutral pathway of cholesterol degradation into bile acids in the liver. While isoflavones obviously have the potential to trigger multiple mechanisms involved in cholesterol metabolism and oxysterol production, they failed to induce any hypocholesterolemic effect.

Screening the ToxCast Phase 1, Phase 2, and e1k Chemical Libraries for Inhibitors of Iodothyronine Deiodinases

Deiodinase enzymes play an essential role in converting thyroid hormones between active and inactive forms by deiodinating the pro-hormone thyroxine (T4) to the active hormone triiodothyronine (T3) and modifying T4 and T3 to inactive forms. Chemical inhibition of deiodinase activity has been identified as an important endpoint to include in screening chemicals for thyroid hormone disruption. To address the lack of data regarding chemicals that inhibit the deiodinase enzymes, we developed robust in vitro assays that utilized human deiodinase types 1, 2, and 3 and screened over 1800 unique chemicals from the U.S. EPA's ToxCast phase 1_v2, phase 2, and e1k libraries. Initial testing at a single concentration identified 411 putative deiodinase inhibitors that produced inhibition of 20% or greater in at least 1 of the 3 deiodinase assays, including chemicals that have not previously been shown to inhibit deiodinases. Of these, 228 chemicals produced enzyme inhibition of 50% or greater; these chemicals were further tested in concentration-response to determine relative potency. Comparisons across these deiodinase assays identified 81 chemicals that produced selective inhibition, with 50% inhibition or greater of only 1 of the deiodinases. This set of 3 deiodinase inhibition assays provides a significant contribution toward expanding the limited number of in vitro assays used to identify chemicals with the potential to interfere with thyroid hormone homeostasis. In addition, these results set the groundwork for development and evaluation of structure-activity relationships for deiodinase inhibition, and inform targeted selection of chemicals for further testing to identify adverse outcomes of deiodinase inhibition.

Screening the ToxCast Phase 1 Chemical Library for Inhibition of Deiodinase Type 1 Activity

Thyroid hormone (TH) homeostasis is dependent upon coordination of multiple key events including iodide uptake, hormone synthesis, metabolism, and elimination, to maintain proper TH signaling. Deiodinase enzymes catalyze iodide release from THs to interconvert THs between active and inactive forms, and are integral to hormone metabolism. The activity of deiodinases has been identified as an important endpoint to include in the context of screening chemicals for TH disruption. To begin to address the potential for chemicals to inhibit these enzymes an adenovirus expression system was used to produce human deiodinase type 1 (DIO1) enzyme, established robust assay parameters for nonradioactive determination of iodide release by the Sandell-Kolthoff method, and employed a 96-well plate format for screening chemical libraries. An initial set of 18 chemicals was used to establish the assay, along with the known DIO1 inhibitor 6-propylthiouracil as a positive control. An additional 292 unique chemicals from the EPA's ToxCast phase 1_v2 chemical library were screened. Chemicals were initially screened at a single high concentration of 200 µM to identify potential DIO1 inhibitors. There were 50 chemicals, or 17% of the TCp1_v2 chemicals tested, that produced >20% inhibition of DIO1 activity. Eighteen of these inhibited DIO1 activity >50% and were further tested in concentration-response mode to determine IC50s. This work presents an initial effort toward identifying chemicals with potential for affecting THs via inhibition of deiodinases and sets the foundation for further testing of large chemical libraries against DIO1 and the other deiodinase enzymes involved in TH function.

Thyroid Hormones and Derivatives: Endogenous Thyroid Hormones and Their Targets

More than a century after the discovery of L-Thyroxine, the main thyroid hormone secreted solely by the thyroid gland, several metabolites of this iodinated, tyrosine-derived ancestral hormone have been identified. These are utilized as hormones during development, differentiation, metamorphosis, and regulation of most biochemical reactions in vertebrates and their precursor species. Among those metabolites are the thyromimetically active 3,3',5-Triiodo-L-thyronine (T3) and 3,5-Diiodo-L-thronine, reverse-T3 (3,3',5'-Triiodo-L-thyronine) with still unclear function, the recently re-discovered thyronamines (e.g., 3-Iodo-thyronamine), which exert in part T3-antagonistic functions, the thyroacetic acids (e.g., Tetrac and Triac), as well as various sulfated or glucuronidated metabolites of this panel of iodinated signaling compounds. In the blood most of these hydrophobic metabolites are tightly bound to the serum distributor proteins thyroxine binding globulin (TBG), transthyretin (TTR), albumin or apolipoprotein B100. Cellular import and export of these charged, highly hydrophobic amino acid derivatives requires a number of cell-membrane transporters or facilitators such as MCT8 or MCT10 and members of the OATP and LAT families of transporters. Depending on their structure, the thyroid hormone metabolites exert their cellular action by binding and thus modulating the function of various receptors systems (e.g., ανβ3 integrin receptor and transient receptor potential channels (TRPM8) of the cell membrane), in part linked to intracellular downstream kinase signaling cascades, and several isoforms of membrane-associated, mitochondrial or nuclear thyroid hormone receptors (TR), which are members of the c-erbA family of ligand-modulated transcription factors. Intracellular deiodinase selenoenzymes, which obligatory are membrane integrated enzymes, ornithine decarboxylase and monoamine oxidases control local availability of biologically active thyroid hormone metabolites. Inactivation of thyroid hormone metabolites occurs mainly by deiodination, sulfation or glucuronidation, reactions which favor their renal or fecal elimination.

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.

Structural aspects of thyroid hormone binding to proteins and competitive interactions with natural and synthetic compounds

Thyroid hormones and their metabolites constitute a vast class of related iodothyronine compounds that contribute to the regulation of metabolic activity and cell differentiation. They are in turn transported, transformed and recognized as signaling molecules through binding to a variety of proteins from a wide range of evolutionary unrelated protein families, which renders these proteins and their iodothyronine binding sites an example for extensive convergent evolution. In this review, we will briefly summarize what is known about iodothyronine binding sites in proteins, the modes of protein/iodothyronine interaction, and the ligand conformations. We will then discuss physiological and synthetic compounds, including popular drugs and food components, that can interfere with iodothyronine binding and recognition by these proteins. The discussion also includes compounds persisting in the environment and acting as endocrine disrupting chemicals.

Fish larval recruitment to reefs is a thyroid hormone-mediated metamorphosis sensitive to the pesticide chlorpyrifos

Larval recruitment, the transition of pelagic larvae into reef-associated juveniles, is a critical step for the resilience of marine fish populations but its molecular control is unknown. Here, we investigate whether thyroid-hormones (TH) and their receptors (TR) coordinate the larval recruitment of the coral-reef-fish Acanthurus triostegus. We demonstrate an increase of TH-levels and TR-expressions in pelagic-larvae, followed by a decrease in recruiting juveniles. We generalize these observations in four other coral reef-fish species. Treatments with TH or TR-antagonist, as well as relocation to the open-ocean, disturb A. triostegus larvae transformation and grazing activity. Likewise, chlorpyrifos, a pesticide often encountered in coral-reefs, impairs A. triostegus TH-levels, transformation, and grazing activity, hence diminishing this herbivore’s ability to control the spread of reef-algae. Larval recruitment therefore corresponds to a TH-controlled metamorphosis, sensitive to endocrine disruption. This provides a framework to understand how larval recruitment, critical to reef-ecosystems maintenance, is altered by anthropogenic stressors.

Many animals go through a larval phase before developing into an adult. This transformation is called metamorphosis, and it is regulated by hormones of the thyroid gland in vertebrates. For example, most fish found on coral reefs actually spend the first part of their life as free-swimming larvae out in the ocean. The larvae usually look very different from the juveniles and adults. When these fish become juveniles, the larvae undergo a range of physical and behavioral changes to prepare for their life on the reef. Yet, until now it was not known what hormones control metamorphosis in these fish.

To address this question, Holzer, Besson et al. studied the convict surgeonfish Acanthurus triostegus. This herbivorous coral-reef fish lives in the Indo-Pacific Ocean, and the results showed that thyroid hormones do indeed regulate the metamorphosis of its larvae. This includes changing how the larvae behave and how their adult features develop. Further, Holzer, Besson et al. found that this was also true for four other coral-reef fish, including the lagoon triggerfish and the raccoon butterflyfish. In A. triostegus, thyroid hormones controlled the changes that enabled the juveniles to efficiently graze on algae growing on the reef such as an elongated gut.

When the fish larvae were then exposed to a pesticide called chlorpyrifos, a well-known reef pollutant, their hormone production was disturbed. This in turn affected their grazing behavior and also their metamorphosis. These fish had shortened, underdeveloped guts and could not graze on algae as effectively.

Herbivorous fish such as A. triostegus play a major role in supporting coral reef ecosystems by reducing algal cover and therefore promoting coral recruitment. These new findings show that pollutants from human activities could disturb the metamorphosis of coral-reef fish and, as a consequence, their ability to maintain the reefs. A next step will be to test what other factors can disrupt the hormones in coral-reef fish and thus pose a threat for fish populations and the coral-reef ecosystem.

Effects of age and soybean isoflavones on hepatic cholesterol metabolism and thyroid hormone availability in acyclic female rats

Soy-food and its isoflavones, genistein (G) and daidzein (D), were reported to exert mild cholesterol-lowering effect, but the underlying mechanism is still unclear. In this research, first we studied age-related alterations in hepatic cholesterol metabolism of acyclic middle-aged (MA) female rats. Then we tested if purified isoflavones may prevent or reverse these changes, and whether putative changes in hepatic thyroid hormone availability may be associated with this effect. Serum and hepatic total cholesterol (TChol), bile acid and cholesterol precursors, as well as serum TSH and T4 concentrations, hepatic deiodinase (Dio) 1 enzyme activity and MCT8 protein expression were determined by comparing data obtained for MA with young adult (YA) intact (IC) females. Effects of subcutaneously administered G or D (35mg/kg) to MA rats were evaluated versus vehicle-treated MA females. MA IC females were characterized by: higher (p<0.05) serum TChol, lower (p<0.05) hepatic TChol and its biosynthetic precursors, lower (p<0.05) hepatic 7α-hydroxycholesterol but elevated (p<0.05) 27- and 24-hydroxycholesterol in comparison to YA IC. Both isoflavone treatments decreased (p<0.05) hepatic 27-hydroxycholesterol, G being more effective than D, without affecting any other parameter of Chol metabolism. Only G elevated hepatic Dio1 activity (p<0.05). In conclusion, age-related hypercholesteremia was associated with lower hepatic Chol synthesis and shift from main neutral (lower 7α-hydroxycholesterol) to alternative acidic pathway (higher 27-hydroxycholesterol) of Chol degradation to bile acid. Both isoflavones lowered hepatic 27-hydroxycholesterol, which may be considered beneficial. Only G treatment increased hepatic Dio1 activity, thus indicating local increase in thyroid hormones, obviously insufficient to induce prominent cholesterol-lowering effect.

3-Iodothyronamine Decreases Expression of Genes Involved in Iodide Metabolism in Mouse Thyroids and Inhibits Iodide Uptake in PCCL3 Thyrocytes

BACKGROUND

3-Iodothyronamine (3-T₁AM) is an endogenous decarboxylated thyroid hormone (TH) metabolite. Pharmacological doses of 3-T₁AM decrease heart rate, body temperature, and metabolic rate in rodents-effects that are contrary to classic TH excess. Furthermore, a single dose of 3-T₁AM was shown to suppress the hypothalamic-pituitary-thyroid (HPT) axis in rats. It was hypothesized that 3-T₁AM might play a role in the fine-tuning of TH action and might have a direct regulatory effect on the thyroid gland.

METHODS

This study tested whether repeated 3-T₁AM treatment interfered with thyroid function and the HPT axis in mice. Therefore, male C57BL/6 mice were intraperitoneally injected with 5 mg/kg of 3-T₁AM or vehicle daily for seven days. Additionally, the effects of 3-T₁AM on the differentiated rat thyrocyte cell line PCCL3 were analyzed.

RESULTS

Repeated administration of 3-T₁AM decreased thyroidal mRNA content of the sodium iodide symporter (Nis), thyroglobulin, and pendrin in mice. No interference with the HPT axis was observed, as determined by unaltered pituitary mRNA levels of triiodothyronine-responsive genes, including thyrotropin subunit β. Furthermore, 3-T₁AM treatment did not change transcript levels of hepatic triiodothyronine-responsive genes, such as deiodinase 1. In line with this, serum TH concentrations were not changed after the treatment period of seven days. In concordance with the in vivo findings, 3-T₁AM decreased the thyrotropin-dependent expression of Nis and functional iodide uptake in PCCL3 cells in vitro. Additionally, uptake and metabolism of 3-T₁AM by PCCL3 cells was observed, as well as 3-T₁AM-dependent changes in intracellular Ca²⁺ concentration that might be involved in mediating the reported effects.

CONCLUSIONS

In conclusion, 3-T₁AM application decreased expression of selected TH synthesis genes by acting directly on the thyroid gland, and it might therefore affect TH synthesis without involvement of the HPT axis.

Identifying reference chemicals for thyroid bioactivity screening

Reference chemicals were selected based on thyroid bioactivity in 'Tier 1' screening assays used by the U.S. EPA's Endocrine Disruptor Screening Program. Active reference chemicals had significant effects on thyroid-responsive endpoints in the amphibian metamorphosis assay, and the male and female pubertal rat assays. In the absence of thyroid weight or histopathological effects, additional published studies providing mechanistic data on thyroid activity were required for active chemicals. Inactive reference chemicals had no significant effects on thyroid-responsive endpoints in Tier 1 assays, or in amphibian or rodent studies from several online databases. The 34 reference chemicals (29 active and five inactive) will be useful for performance-based validation of alternative, high throughput screening assays for thyroid bioactivity.

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.

Strong induction of iodothyronine deiodinases by chemotherapeutic selenocompounds

The biological activity of thyroid hormones (TH) is regulated by selenoenzymes of the iodothyronine deiodinase (DIO) family catalysing TH activating and inactivating reactions. Besides TH metabolism, several studies indicate an important role of DIO isoenzymes in tumorigenesis and cancer growth. It is therefore of therapeutic importance to identify modulators of DIO expression. We have synthesized and studied a series of selenocompounds containing a methyl- or benzyl-imidoselenocarbamate backbone. One of these novel compounds had chemotherapeutic activities in a murine xenograft tumour model by an unknown mechanism. Therefore, we tested their effects on DIO expression in vitro. In HepG2 hepatocarcinoma cells, DIO1 activity was strongly (up to 10-fold) increased by the methyl- but not by the corresponding benzyl-imidoselenocarbamates. Steady-state mRNA levels remained unaltered under these conditions indicating a post-transcriptional mode of action. The effects were further characterized in HEK293 cells stably expressing DIO1, DIO2 or DIO3. Even within the artificial genetic context of the expression vectors, all three DIO isoenzymes were up-regulated by the methyl- and to a lesser extent by the benzyl-imidoselenocarbamates. Consistent stimulating effects were observed with methyl-N,N'-di(quinolin-3-ylcarbonyl)-imidoselenocarbamate (EI201), a selenocompound known for its anti-tumour activity. DIO inducing effects were unrelated to the intracellular accumulation of selenium, yet the precise mode of action remains elusive. Collectively, our data highlight that these selenocompounds may constitute interesting pharmacological compounds for modifying DIO expression potentially affecting the balance between cell differentiation and proliferation.

Identification of cromolyn sodium as an anti-fibrotic agent targeting both hepatocytes and hepatic stellate cells

Liver fibrosis and cirrhosis, the late stage of fibrosis, are threatening diseases that lead to liver failure and patient death. Although aberrantly activated hepatic stellate cells (HSCs) are the main cause of disease initiation, the symptoms are primarily related to damaged hepatocytes. Thus, damaged hepatocytes, as well as HSCs, need to be simultaneously considered as therapeutic targets to develop more efficient treatments. Here, we suggest cromolyn sodium as an anti-fibrotic agent to commonly modulate hepatocytes and hepatic stellate cells. The differentially expressed genes from 6 normal and 40 cirrhotic liver tissues which were collected from GEO data were assessed by pharmacokinetic analysis using a connectivity map to identify agents that commonly revert abnormal hepatocytes and HSCs to normal conditions. Based on a series of analyses, a few candidates were selected. Candidates were tested in vitro to determine their anti-fibrotic efficacy on HSCs and hepatocytes. Cromolyn, which was originally developed as a mast cell stabilizer, showed the potential to ameliorate activated HSCs in vitro. The activation and collagen accumulation for HSC cell lines LX2 and HSC-T6 were reduced by 50% after cromolyn treatment at a low concentration without apoptosis. Furthermore, cromolyn treatment compromised the TGF-β-induced epithelial mesenchyme transition and replicative senescence rate of hepatocytes, which are generally associated with fibrogenesis. Taken together, cromolyn may be the basis for an effective cure for fibrosis and cirrhosis because it targets both HSCs and hepatocytes.

Gene-specific regulation of hepatic selenoprotein expression by interleukin-6

Sepsis is a severe inflammatory disease resulting in excessive production of pro-inflammatory cytokines including interleukin-6 (IL-6), causing oxidative stress, tissue damage and organ dysfunction. Health benefits have been observed upon selenium (Se) supplementation in severe sepsis. Selenium is incorporated into selenoproteins implicated in anti-oxidative defence, thyroid hormone metabolism and immunoregulation. Selenium metabolism is controlled by hepatocytes synthesizing and secreting the Se transporter selenoprotein P (SePP). The circulating SePP declines in sepsis causing low serum Se levels. Dysregulation of the hepatic selenoenzyme deiodinase type 1 (DIO1) potentially contributes to the low T3 (thyroid hormone) syndrome observed in severe diseases. We hypothesized that IL-6 affects hepatic selenoprotein biosynthesis directly. Testing human hepatocytes in culture, IL-6 reduced the concentrations of SePP mRNA and secreted SePP in a dose-dependent manner. In parallel, expression of DIO1 declined at the mRNA, protein and enzyme activity level. The effects of IL-6 on glutathione peroxidase (GPX) expression were isozyme-specific; GPX1 remained unaffected, while transcript concentrations of GPX2 increased and those of GPX4 decreased. This pattern of IL-6-dependent effects was mirrored in reporter gene experiments with SePP, DIO1, GPX1, and GPX2 promoter constructs pointing to direct transcriptional effects of IL-6. The redirection of hepatic selenoprotein biosynthesis by IL-6 may represent a central regulatory circuit responsible for the decline of serum Se and low T3 concentrations in sepsis. Accordingly, therapeutic IL-6 targeting may be effective for improving the Se and thyroid hormone status, adjuvant Se supplementation success and survival in sepsis.

Testosterone and estradiol treatments differently affect pituitary-thyroid axis and liver deiodinase 1 activity in orchidectomized middle-aged rats

We previously reported that orchidectomy (Orx) of middle-aged rats (15-16-month-old; MA) slightly affected pituitary-thyroid axis, but decreased liver deiodinase (Dio) type 1 and pituitary Dio2 enzyme activities. At present, we examined the effects of subsequent testosterone-propionate treatment (5mg/kg; Orx+T), and compared the effects of testosterone with the effects of estradiol-dipropionate (0.06mg/kg; Orx+E) treatment. Hormones were subcutaneously administered, daily, for three weeks, while Orx and sham-operated (SO) controls received only the vehicle. The applied dose of T did not alter serum TSH, T4 and T3 concentrations in Orx- MA, though it increased TSH when administrated to Orx young adults (2.5-month-old; Orx-YA). However, pituitaries of Orx-MA+T rats had higher relative intensity of immunofluorescence (RIF) for TSHβ; in their thyroids we found increased volume and height of follicular epithelium, decreased volume of the colloid and higher RIF for T4-bound to thyroglobulin (Tg-T4). Liver Dio1 activity was increased. E-treatment did not affect serum hormone levels, pituitary RIF for TSHβ, or liver Dio1 activity in Orx-MA rats. Thyroids had decreased relative volume and height of follicular epithelium, increased relative volume of the colloid, decreased volume of sodium-iodide symporter-immunopositive epithelium and lower RIF for Tg-T4. Detected changes were statistically significant. In conclusion, androgenization enhanced pituitary TSHβ RIF, thyroid activation and liver Dio1 enzyme activity in Orx-MA, without elevating serum TSH as in Orx-YA rats. Estrogenization induced pituitary enlargement with no effect on pituitary TSHβ RIF, serum TSH or liver Dio1 activity. E also induced alterations in thyroid histology that indicate mild suppression of its functioning, and contributed to thyroid blood vessel enlargement in Orx-MA rats.

An Improved Nonradioactive Screening Method Identifies Genistein and Xanthohumol as Potent Inhibitors of Iodothyronine Deiodinases

BACKGROUND

Deiodinases (DIO1, 2, and 3) are key enzymes in thyroid hormone (TH) activation and inactivation with impact on energy metabolism, development, cell differentiation, and a number of other physiological processes. The three DIO isoenzymes thus constitute sensitive rate-limiting components within the TH axis, prone to dysregulation by endocrine disruptive compounds or disease state. In animal models and cell culture experiments, they serve as readout for local TH status and disarrangement of the hormonal axis. Furthermore, some human diseases are characterized by apparent deiodinase dysregulation (e.g., the low triiodothyronine syndrome in critical illness). Consequently, these enzymes are targets of interest for the development of pharmacological compounds with modulatory activities. Until now, the portfolio of inhibitors for these enzymes is limited. In the clinics, the DIO1-specific inhibitor propylthiouracil is in use for treatment of severe hyperthyroidism. Other well-known inhibitors (e.g., iopanoic acid or aurothioglucose) are nonselective and block all three isoenzymes. Furthermore, DIO3 was shown to be a potential oncogenic gene, which is strongly expressed in some tumors and might, in consequence, protect tumor tissue form differentiation by TH. With respect to its role in tumorigenesis, specific inhibitors of DIO3 as a potential target for anticancer drugs would be highly desirable. To this end, a flexible and convenient assay for high-throughput screening is needed. We recently described a nonradioactive screening assay, utilizing the classic Sandell-Kolthoff reaction as readout for iodide release from the substrate molecules. While we used murine liver as enzyme source, the assay was limited to murine DIO1 activity testing. Here, we describe the use of recombinant proteins as enzyme sources within the assay, expanding its suitability from murine Dio1 to human DIO1, DIO2, and DIO3.

METHODS

As proof-of-concept, deiodination reactions catalyzed by these recombinant enzymes were monitored with various nonradioactive substrates and confirmed by liquid chromatography-tandem mass spectrometry.

RESULTS

The contrast agent and known DIO inhibitor iopanoic acid was characterized as readily accepted substrate by DIO2 and Dio3. In a screening approach using established endocrine disrupting compounds, the natural food ingredient genistein was identified as a further DIO1-specific inhibitor, while xanthohumol turned out to potently block the activity of all three isoenzymes.

CONCLUSIONS

A rapid nonradioactive screening method based on the Sandell-Kolthoff reaction is suitable for identification of environmental, nutritive and pharmacological compounds modulating activities of human deiodinase enzymes.

A Nonradioactive Uptake Assay for Rapid Analysis of Thyroid Hormone Transporter Function

Thyroid hormones (TH) are actively taken up into target cells via TH-transmembrane transporters (THTT). Their activity and expression patterns define a layer of endocrine regulation that is poorly understood. Therefore, THTT are potential targets for interfering agents (endocrine disruptors) as well as for pharmacological interventions. Inactivating mutations have been identified as the underlying cause of heritable diseases (monocarboxylate transporter 8-associated Allan-Herndon-Dudley syndrome) and might also define a class of subclinical TH insensitivity. As a basic tool to solve questions regarding THTT substrate specificity, activation or inactivation by compounds and functional changes from mutations, uptake assays with radiolabeled tracers are standard. Due to the need for radioactive isotopes, this technique is limited to screening of labelled substrates and disadvantageous regarding handling, setup, and regulatory issues. To overcome these hurdles, we developed an uptake assay protocol using nonradioactive ligands. In brief, uptake of nonradioactive iodine-containing substrate molecules was monitored via Sandell-Kolthoff reaction. The novel assay was designed to the common microtiter plate layout. As a prove-of-principle, we measured TH uptake by monocarboxylate transporter 8-transfected MDCK1 cells. Titrations with bromosulphthalein as an example for inhibitor screening setups and a side-by-side comparison with the radioactive method prove this assay to be reliable, sensitive, and convenient. Furthermore, the method was applicable on primary murine astrocytes, which enables high-throughput screening studies on in vitro model systems with physiological transporter regulation. Due to its design, it is applicable for high-throughput screening of modulatory compounds, but it is also a safe, inexpensive and an easily accessible method for functional testing of THTT in basic science.

3,5-Diiodo-L-thyronine (3,5-t2) exerts thyromimetic effects on hypothalamus-pituitary-thyroid axis, body composition, and energy metabolism in male diet-induced obese mice

Effective and safe antiobesity drugs are still needed in face of the obesity pandemic worldwide. Recent interventions in rodents revealed 3,5-diiodo-L-thyronine (3,5-T2) as a metabolically active iodothyronine affecting energy and lipid metabolism without thyromimetic side effects typically associated with T3 administration. Accordingly, 3,5-T2 has been proposed as a potential hypolipidemic agent for treatment of obesity and hepatic steatosis. In contrast to other observations, our experiments revealed dose-dependent thyromimetic effects of 3,5-T2 akin to those of T3 in diet-induced obese male C57BL/6J mice. 3,5-T2 treatment exerted a negative feedback regulation on the hypothalamus-pituitary-thyroid axis, similar to T3. This is demonstrated by decreased expression of genes responsive to thyroid hormones (TH) in pituitary resulting in a suppressed thyroid function with lower T4 and T3 concentrations in serum and liver of 3,5-T2-treated mice. Analyses of hepatic TH target genes involved in lipid metabolism revealed T3-like changes in gene expression and increased type I-deiodinase activity after application of 3,5-T2 (2.5 μg/g body weight). Reduced hepatic triglyceride and serum cholesterol concentrations reflected enhanced lipid metabolism. Desired increased metabolic rate and reduction of different fat depots were, however, compromised by increased food intake preventing significant body weight loss. Moreover, enlarged heart weights indicate potential cardiac side effects of 3,5-T2 beyond hepatic thyromimetic actions. Altogether, the observed thyromimetic effects of 3,5-T2 in several mouse TH target tissues raise concern about indiscriminate administration of 3,5-T2 as powerful natural hormone for the treatment of hyperlipidemia and pandemic obesity.

Hypoxia reduces and redirects selenoprotein biosynthesis

Selenium deficiency constitutes a risk factor for the incidence and negative course of severe diseases including sepsis, stroke, autoimmune diseases or cancer. In this study, hypoxia is identified as a powerful stimulus to redirect selenoprotein biosynthesis causing reduced selenoprotein P expression and diminished selenium export from hepatocytes in favour of increased biosynthesis of the essential protective intracellular phospholipid hydroperoxide glutathione peroxidase GPX4. Specifically, hypoxia decreases transcript concentrations of central factors controlling selenium and selenocysteine metabolism including selenophosphate synthetase-2, phosphoseryl-tRNA(SerSec) kinase and selenocysteine lyase, which are all proven to be rate-limiting enzymes in selenoprotein biosynthesis. These effects are paralleled by a general decline of selenoprotein expression; however, not all selenoproteins are affected to the same extent by hypoxia, and GPX4 constitutes an exception as its expression becomes slightly increased. Supplemental selenium is able to overcome the hypoxia-dependent down regulation of selenoprotein expression in our cell culture model system, supporting the concept of using selenium as an adjuvant treatment option in severe diseases. Although it remains to be tested whether these effects constitute a hepatocyte-specific response, the selenium-dependent decline of selenoprotein P biosynthesis under hypoxic conditions may explain the progressive selenium deficit developing in severe diseases.