Novel cell-based assay for detection of thyroid receptor beta-interacting environmental contaminants

Tetrabromobisphenol A exerts thyroid disrupting effects but has little overt impact on postnatal brain development and neurobehaviors in mice

Tetrabromobisphenol A (TBBPA) is a widely used brominated flame retardant. There is evidence showing that TBBPA can exert thyroid disrupting effects in mammals, but different results were also reported, along with inconsistent reports regarding its neurotoxicity. Here, we investigated thyroid disrupting effects and neurotoxicity of TBBPA (5, 50, 500 µg/(kg·day)) to male mice following maternal and direct exposure through drinking water, with the anti-thyroid drug propylthiouracil (PTU) as the positive control. On postnatal day (PND) 15, we expectedly observed severe thyroid compensatory hyperplasia and cerebellar developmental retardation in PTU-treated pups. The highest dose of TBBPA also caused thyroid histological alteration but had no effects on cerebellar development in terms of Purkinje cell morphology and the thickness of the internal granular layer and the molecular layer of the cerebellum. During puberty and adulthood, the thyroid morphological alterations became more pronounced in the TBBPA-treated animals, accompanied by decreased serum thyroid hormone levels. Furthermore, the 50 and 500 µg/(kg·day) TBBPA groups showed a significant decrease in the serum level of serotonin, a neurotransmitter associated with anxiety behaviors. Correspondingly, the highest dose group displayed anxiety-like behaviors in the elevated plus-maze test on PND 35, but this neurobehavioral alteration disappeared on PND 56. Moreover, no changes in neurobehavioral parameters tested were found in TBBPA-treated animals at puberty and adulthood. Altogether, all observations show that TBBPA can exert thyroid disrupting effects but has little overt impact on brain development and neurobehaviors in mice, suggesting that thyroid disruption does not necessarily cause overtly adverse neurodevelopmental outcomes.

Update of the scientific opinion on tetrabromobisphenol A (TBBPA) and its derivatives in food

The European Commission asked EFSA to update its 2011 risk assessment on tetrabromobisphenol A (TBBPA) and five derivatives in food. Neurotoxicity and carcinogenicity were considered as the critical effects of TBBPA in rodent studies. The available evidence indicates that the carcinogenicity of TBBPA occurs via non-genotoxic mechanisms. Taking into account the new data, the CONTAM Panel considered it appropriate to set a tolerable daily intake (TDI). Based on decreased interest in social interaction in male mice, a lowest observed adverse effect level (LOAEL) of 0.2 mg/kg body weight (bw) per day was identified and selected as the reference point for the risk characterisation. Applying the default uncertainty factor of 100 for inter- and intraspecies variability, and a factor of 3 to extrapolate from the LOAEL to NOAEL, a TDI for TBBPA of 0.7 μg/kg bw per day was established. Around 2100 analytical results for TBBPA in food were used to estimate dietary exposure for the European population. The most important contributors to the chronic dietary LB exposure to TBBPA were fish and seafood, meat and meat products and milk and dairy products. The exposure estimates to TBBPA were all below the TDI, including those estimated for breastfed and formula-fed infants. Accounting for the uncertainties affecting the assessment, the CONTAM Panel concluded with 90%-95% certainty that the current dietary exposure to TBBPA does not raise a health concern for any of the population groups considered. There were insufficient data on the toxicity of any of the TBBPA derivatives to derive reference points, or to allow a comparison with TBBPA that would support assignment to an assessment group for the purposes of combined risk assessment.

Novel biosensor for high-throughput detection of progesterone receptor-interacting endocrine disruptors

Progesterone receptor (PR)-interacting compounds in the environment are associated with serious health hazards. However, methods for their detection in environmental samples are cumbersome. We report a sensitive activity-based biosensor for rapid and reliable screening of progesterone receptor (PR)-interacting endocrine disrupting chemicals (EDCs). The biosensor is a cell line which expresses nuclear mCherry-NF1 and a green fluorescent protein (GFP)-tagged chimera of glucocorticoid receptor (GR) N terminus fused to the ligand binding domain (LBD) of PR (GFP-GR-PR). As this LBD is shared by the PRA and PRB, the biosensor reports on the activation of both PR isoforms. This GFP-GR-PR chimera is cytoplasmic in the absence of hormone and translocates rapidly to the nucleus in response to PR agonists or antagonists in concentration- and time-dependent manner. In live cells, presence of nuclear NF1 label eliminates cell fixation and nuclear staining resulting in efficient screening. The assay can be used in screens for novel PR ligands and PR-interacting contaminants in environmental samples. A limited screen of river water samples indicated a widespread, low-level contamination with PR-interacting contaminants in all tested samples.

Juxtaposition of intensive agriculture, vulnerable aquifers, and mixed chemical/microbial exposures in private-well tapwater in northeast Iowa

In the United States and globally, contaminant exposure in unregulated private-well point-of-use tapwater (TW) is a recognized public-health data gap and an obstacle to both risk-management and homeowner decision making. To help address the lack of data on broad contaminant exposures in private-well TW from hydrologically-vulnerable (alluvial, karst) aquifers in agriculturally-intensive landscapes, samples were collected in 2018-2019 from 47 northeast Iowa farms and analyzed for 35 inorganics, 437 unique organics, 5 in vitro bioassays, and 11 microbial assays. Twenty-six inorganics and 51 organics, dominated by pesticides and related transformation products (35 herbicide-, 5 insecticide-, and 2 fungicide-related), were observed in TW. Heterotrophic bacteria detections were near ubiquitous (94 % of the samples), with detection of total coliform bacteria in 28 % of the samples and growth on at least one putative-pathogen selective media across all TW samples. Health-based hazard index screening levels were exceeded frequently in private-well TW and attributed primarily to inorganics (nitrate, uranium). Results support incorporation of residential treatment systems to protect against contaminant exposure and the need for increased monitoring of rural private-well homes. Continued assessment of unmonitored and unregulated private-supply TW is needed to model contaminant exposures and human-health risks.

Endocrine disruptors of sex hormone activities

Sex hormones, such as androgens, estrogens and progestins are naturally occurring compounds that tightly regulate endocrine systems in a variety of living organisms. Uncontrolled environmental exposure to these hormones or their biological and synthetic mimetics has been widely documented. Furthermore, water contaminants penetrate soil to affect flora, fauna and ultimately humans. Because endocrine systems evolved to respond to very small changes in hormone levels, the low levels found in the environment cannot be ignored. The combined actions of sex hormones with glucocorticoids and other nuclear receptors disruptors creates additional level of complexity including the newly described "dynamic assisted loading" mechanism. We reviewed the extensive literature pertaining to world-wide detection of these disruptors and created a detailed Table on the development and current status of methods used for their analysis.

Endocrine disrupting chemicals differentially alter intranuclear dynamics and transcriptional activation of estrogen receptor-α

Transcription is a highly regulated sequence of stochastic processes utilizing many regulators, including nuclear receptors (NR) that respond to stimuli. Endocrine disrupting chemicals (EDCs) in the environment can compete with natural ligands for nuclear receptors to alter transcription. As nuclear dynamics can be tightly linked to transcription, it is important to determine how EDCs affect NR mobility. We use an EPA-assembled set of 45 estrogen receptor-α (ERα) ligands and EDCs in our engineered PRL-Array model to characterize their effect upon transcription using fluorescence in situ hybridization and fluorescence recovery after photobleaching (FRAP). We identified 36 compounds that target ERα-GFP to a transcriptionally active, visible locus. Using a novel method for multi-region FRAP analysis we find a strong negative correlation between ERα mobility and inverse agonists. Our findings indicate that ERα mobility is not solely tied to transcription but affected highly by the chemical class binding the receptor.

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Development of a new algorithm for multi-foci FRAP analysis

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ERα agonists can be segregated into fast-moving and slow-moving receptor groups

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ERα inverse agonists receptor mobility inversely correlates with transcription

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Steroidal compounds result in a slower moving receptor than other classes

Mapping multiple endocrine disrupting activities in Virginia rivers using effect-based assays

Water sources are frequently contaminated with natural and anthropogenic substances having known or suspected endocrine disrupting activities; however, these activities are not routinely measured and monitored. Phenotypic bioassays are a promising new approach for detection and quantitation of endocrine disrupting chemicals (EDCs). We developed cell lines expressing fluorescent chimeric constructs capable of detecting environmental contaminants which interact with multiple nuclear receptors. Using these assays, we tested water samples collected in the summers of 2016, 2017 and 2018 from two major Virginia rivers. Samples were concentrated 200× and screened for contaminants interacting with the androgen (AR), glucocorticoid (GR), aryl hydrocarbon (AhR) and thyroid receptors. Among 45 tested sites, over 70% had AR activity and 60% had AhR activity. Many sites were also positive for GR and TRβ activation (22% and 42%, respectively). Multiple sites were positive for more than one type of contaminants, indicating presence of complex mixtures. These activities may negatively impact river ecosystems and consequently human health.

Pilot study of global endocrine disrupting activity in Iowa public drinking water utilities using cell-based assays

Some anthropogenic substances in drinking water are known or suspected endocrine disrupting compounds (EDCs), but EDCs are not routinely measured. We conducted a pilot study of 10 public drinking water utilities in Iowa, where common contaminants (e.g., pesticides) are suspected EDCs. Raw (untreated) and finished (treated) drinking water samples were collected in spring and fall and concentrated using solid phase extraction. We assessed multiple endocrine disrupting activities using novel mammalian cell-based assays that express nuclear steroid receptors (aryl hydrocarbon [AhR], androgenic [AR], thyroid [TR], estrogenic [ER] and glucocorticoid [GR]). We quantified each receptor's activation relative to negative controls and compared activity by season and utility/sample characteristics. Among 62 samples, 69% had AhR, 52% AR, 3% TR, 2% ER, and 0% GR activity. AhR and AR activities were detected more frequently in spring (p =0 .002 and < 0.001, respectively). AR activity was more common in samples of raw water (p =0 .02) and from surface water utilities (p =0 .05), especially in fall (p =0 .03). Multivariable analyses suggested spring season, surface water, and nitrate and disinfection byproduct concentrations as determinants of bioactivity. Our results demonstrate that AR and AhR activities are commonly found in Iowa drinking water, and that their detection varies by season and utility/sample characteristics. Screening EDCs with cell-based bioassays holds promise for characterizing population exposure to diverse EDCs mixtures.

Reconnaissance of Surface Water Estrogenicity and the Prevalence of Intersex in Smallmouth Bass (Micropterus Dolomieu) Inhabiting New Jersey

The observation of testicular oocytes in male fishes has been utilized as a biomarker of estrogenic endocrine disruption. A reconnaissance project led in the Northeastern United States (US) during the period of 2008–2010 identified a high prevalence of intersex smallmouth bass on or near US Fish & Wildlife Service National Wildlife Refuges that included the observation of 100% prevalence in smallmouth bass males collected from the Wallkill River, NJ, USA. To better assess the prevalence of intersex smallmouth bass across the state of New Jersey, a tiered reconnaissance approach was initiated during the fall of 2016. Surface water samples were collected from 101 (85 river, 16 lake/reservoir) sites across the state at base-flow conditions for estrogenicity bioassay screening. Detectable estrogenicity was observed at 90% of the sites and 64% were above the US Environmental Protection Agency trigger level of 1 ng/L. Median surface water estrogenicity was 1.8 ng/L and a maximum of 6.9 ng/L E2Eq_BLYES was observed. Adult smallmouth bass were collected from nine sites, pre-spawn during the spring of 2017. Intersex was identified in fish at all sites, and the composite intersex prevalence was 93.8%. Prevalence across sites ranged from 70.6% to 100%. In addition to intersex, there was detectable plasma vitellogenin in males at all sites. Total estrogenicity in surface water was determined at these fish collection sites, and notable change over time was observed. Correlation analysis indicated significant positive correlations between land use (altered land; urban + agriculture) and surface water estrogenicity. There were no clear associations between land use and organismal metrics of estrogenic endocrine disruption (intersex or vitellogenin). This work establishes a baseline prevalence of intersex in male smallmouth bass in the state of New Jersey at a limited number of locations and identifies a number of waterbodies with estrogenic activity above an effects-based threshold.

Molecular Functions of Thyroid Hormone Signaling in Regulation of Cancer Progression and Anti-Apoptosis

Several physiological processes, including cellular growth, embryonic development, differentiation, metabolism and proliferation, are modulated by genomic and nongenomic actions of thyroid hormones (TH). Several intracellular and extracellular candidate proteins are regulated by THs. 3,3,5-Triiodo-L-thyronine (T₃) can interact with nuclear thyroid hormone receptors (TR) to modulate transcriptional activities via thyroid hormone response elements (TRE) in the regulatory regions of target genes or bind receptor molecules showing no structural homology to TRs, such as the cell surface receptor site on integrin αvβ3. Additionally, L-thyroxine (T₄) binding to integrin αvβ3 is reported to induce gene expression through initiating non-genomic actions, further influencing angiogenesis and cell proliferation. Notably, thyroid hormones not only regulate the physiological processes of normal cells but also stimulate cancer cell proliferation via dysregulation of molecular and signaling pathways. Clinical hypothyroidism is associated with delayed cancer growth. Conversely, hyperthyroidism is correlated with cancer prevalence in various tumor types, including breast, thyroid, lung, brain, liver and colorectal cancer. In specific types of cancer, both nuclear thyroid hormone receptor isoforms and those on the extracellular domain of integrin αvβ3 are high risk factors and considered potential therapeutic targets. In addition, thyroid hormone analogs showing substantial thyromimetic activity, including triiodothyroacetic acid (Triac), an acetic acid metabolite of T₃, and tetraiodothyroacetic acid (Tetrac), a derivative of T₄, have been shown to reduce risk of cancer progression, enhance therapeutic effects and suppress cancer recurrence. Here, we have reviewed recent studies focusing on the roles of THs and TRs in five cancer types and further discussed the potential therapeutic applications and underlying molecular mechanisms of THs.

Limited Chemical Structural Diversity Found to Modulate Thyroid Hormone Receptor in the Tox21 Chemical Library

BACKGROUND

Thyroid hormone receptors (TRs) are critical endocrine receptors that regulate a multitude of processes in adult and developing organisms, and thyroid hormone disruption is of high concern for neurodevelopmental and reproductive toxicities in particular. To date, only a small number of chemical classes have been identified as possible TR modulators, and the receptors appear highly selective with respect to the ligand structural diversity. Thus, the question of whether TRs are an important screening target for protection of human and wildlife health remains.

OBJECTIVE

Our goal was to evaluate the hypothesis that there is limited structural diversity among environmentally relevant chemicals capable of modulating TR activity via the collaborative interagency Tox21 project.

METHODS

We screened the Tox21 chemical library (8,305 unique structures) in a quantitative high-throughput, cell-based reporter gene assay for TR agonist or antagonist activity. Active compounds were further characterized using additional orthogonal assays, including mammalian one-hybrid assays, coactivator recruitment assays, and a high-throughput, fluorescent imaging, nuclear receptor translocation assay.

RESULTS

Known agonist reference chemicals were readily identified in the TR transactivation assay, but only a single novel, direct agonist was found, the pharmaceutical betamipron. Indirect activation of TR through activation of its heterodimer partner, the retinoid-X-receptor (RXR), was also readily detected by confirmation in an RXR agonist assay. Identifying antagonists with high confidence was a challenge with the presence of significant confounding cytotoxicity and other, non-TR-specific mechanisms common to the transactivation assays. Only three pharmaceuticals-mefenamic acid, diclazuril, and risarestat-were confirmed as antagonists.

DISCUSSION

The results support limited structural diversity for direct ligand effects on TR and imply that other potential target sites in the thyroid hormone axis should be a greater priority for bioactivity screening for thyroid axis disruptors. https://doi.org/10.1289/EHP5314.

A comprehensive evaluation of anti-diabetic drugs on nuclear receptor PXR platform

Pregnane & Xenobiotic Receptor (PXR), one of the members of nuclear receptor superfamily, acts as a 'master-regulator' of drug metabolism and disposition machinery (DMD). Activation of PXR enables detoxification and elimination of toxic xenobiotics/endobiotics, and defends our body against chemical insults. On the contrary, PXR activation also imposes a serious concern for drug-drug interactions (DDIs). Such DDIs could either decrease the efficacy or lead to accumulation of co-administered drugs at toxic level. Therefore, it is desirable that during drug development process the small drug molecules are screened on PXR-platform prior to their clinical trial and prevent late stage failures. In view of this, we have selected a group of anti-diabetic drug molecules to examine if the success and potential failure of small molecule modulators can be pre-assessed and judiciously correlated on PXR platform. For this purpose, we have examined the PXR activation potential of the selected anti-diabetic drugs. Subsequent to screening of these anti-diabetic drugs, we elaborated the study further with rosiglitazone and pioglitazone (thiazolidinediones, TZDs) which are oral anti-diabetic formulations and have been in controversy owing to their association with cardiotoxicity and bladder cancer respectively. Our study revealed that some of the selected anti-diabetic drugs possess PXR activation potential, implying that these can up-regulate the expression of CYP3A4, UGT1A1, MDR1 and thereby can be predicted to inflict undesirable consequences.

A Linkage Between Thyroid and Breast Cancer: A Common Etiology?

Breast and thyroid cancers are two malignancies with highest incidence in women. These cancers often occur metachronously. Women with thyroid cancer are at increased risk for subsequent breast cancer; women with breast cancer have an increased incidence of later development of thyroid cancer, suggesting a common etiology. This bidirectional relationship is reported worldwide; however, the underlying reasons for this co-occurrence are unknown. In this review, we summarize the current epidemiologic evidence and putative mechanisms of these metachronous or synchronous cancers. Key potential causative factors are chemotherapy and radiotherapy of the primary tumor, genetic variants linking the two diseases, hormonal signaling both from the thyroid gland and from estrogens, and lifestyle and environmental factors. There is a critical need for additional epidemiologic studies focused on gender and regional incidence together with molecular investigations on common tumorigenic pathways in these endocrine cancers. Understanding the putative mechanisms will aid in the diagnosis and clinical management of both diseases.

Bioactivity of Thyroid Hormone Analogs at Cancer Cells

In the context of genomic thyroid hormone actions in normal (noncancer) cells that involve primary interactions with nuclear thyroid hormone receptors (TRs), L-thyroxine (T4), and 3,3',5'-triiodo-L-thyronine (reverse T3, rT3) have little bioactivity. In terms of TRs, T4 is a prohormone from which the active nuclear ligand, 3,5,3'-triido-L-thyronine (T3), is generated by deiodination. Deaminated T4 and T3 metabolites have different genomic effects: tetraiodothyroacetic acid (tetrac) is a low grade thyromimetic derivative of T4, whereas triiodothyroacetic acid (triac), the acetic acid metabolite of T3, has substantial thyromimetic activity. In cancer cells, the cell surface receptor for thyroid hormone on integrin αvβ3 mediates non-genomic actions of thyroid hormone analogs. The integrin is expressed in large measure by cancer cells and dividing endothelial cells and has a substantially different panel of responses to thyroid hormone analogs. At αvβ3, T4 is a potent proliferative, anti-apoptotic and pro-angiogenic hormone and is the primary ligand. rT3 may also be proliferative at this site. In contrast, tetrac and triac are antagonists of T4 at αvβ3, but also have anticancer properties at this site that are independent of their effects on the binding of T4.

Endocrine Disruption by Mixtures in Topical Consumer Products

Endocrine disruption has been gathering increasing attention in the past 25 years as a possible new threat for health and safety. Exposure to endocrine disruptor has been progressively linked with a growing number of increasing disease in the human population. The mechanics through which endocrine disruptors act are not yet completely clear, however a number of pathways have been identified. A key concern is the cumulative and synergic effects that endocrine disruptors could have when mixed in consumer products. We reviewed the available literature to identify known or potential endocrine disruptors, as well as endocrine active substances that could contribute to cumulative effects, in topical consumer products. The number of endocrine actives used daily in consumer products is staggering and even though most if not all are used in concentrations that are considered to be safe, we believe that the possibility of combined effects in mixtures and non-monotonic dose/response is enough to require further precautions. A combined in vitro approach based on existing, validated OECD test methods is suggested to screen consumer products and mixtures for potential interaction with estrogen and androgen hormone receptors, in order to identify products that could have cumulative effects or support their safety concerning direct endocrine disruption capabilities.

A plurality of molecular targets: The receptor ecosystem for bisphenol-A (BPA)

Bisphenol-A (BPA) is a well-known endocrine disrupting compound (EDC), capable of affecting the normal function and development of the reproductive system, brain, adipose tissue, and more. In spite of these diverse and well characterized effects, there is often comparatively little known about the molecular mechanisms which bring them about. BPA has traditionally been regarded as a primarily estrogenic EDC, and this perspective is often what guides research into the effects of BPA. However, emerging data from in-vitro and in-silico models show that BPA binds with a significant number of hormone receptors, including a number of nuclear and membrane-bound estrogen receptors, androgen receptors, as well as the thyroid hormone receptor, glucocorticoid receptor, and PPARγ. With this increased diversity of receptor targets, it may be possible to explain some of the more puzzling aspects of BPA pharmacology, including its non-monotonic dose-response curve, as well as experimental results which disagree with estrogenic positive controls. This paper reviews the receptors for which BPA has a known interaction, and discusses the implications of taking these receptors into account when studying the disruptive effects of BPA on growth and development.