Biosensor discovery of thyroxine transport disrupting chemicals

Detrimental consequences of micropolymers associated plasticizers on endocrinal disruption

The prevalence of polymer usage in everyday activities has emerged as a detriment to both human life and the environment. A large number of studies describe severe impacts of micropolymers (MP) and nanopolymers (NP) on various organ systems, including the endocrine system. Additionally, plasticizers utilized as additives have been identified as endocrine-disrupting chemicals (EDCs). MP/NP, along with associated plasticizers, affect principal signalling pathways of endocrine glands such as the pituitary, thyroid, adrenal, and gonads, thereby disrupting hormone function and metabolic processes crucial for maintaining homeostasis, fertility, neural development, and fetal growth. This review delves into the sources, distribution, and effects of micropolymers, nanopolymers, and associated plasticizers acting as EDCs. Furthermore, it provides a detailed review of the mechanisms underlying endocrine disruption in relation to different types of MP/NP.

Association between brominated flame retardants and risk of endocrine-related cancer: A systematic review and meta-analysis

BACKGROUND

The incidence of endocrine-related cancer, which includes tumors in major endocrine glands such as the breast, thyroid, pituitary, and prostate, has been increasing year by year. Various studies have indicated that brominated flame retardants (BFRs) are neurotoxic, endocrine-toxic, reproductive-toxic, and even carcinogenic. However, the epidemiological relationship between BFR exposure and endocrine-related cancer risk remains unclear.

METHODS

We searched the PubMed, Google Scholar, and Web of Science databases for articles evaluating the association between BFR exposure and endocrine-related cancer risk. The odds ratio (OR) and its corresponding 95% confidence interval (95% CI) were used to assess the association. Statistical heterogeneity among studies was assessed with the Q-test and I2 statistics. Begg's test was performed to evaluate the publication bias.

RESULTS

We collected 15 studies, including 6 nested case-control and 9 case-control studies, with 3468 cases and 4187 controls. These studies assessed the risk of breast cancer, thyroid cancer, and endocrine-related cancers in relation to BFR levels. Our findings indicate a significant association between BFR exposure in adipose tissue and an increased risk of breast cancer. However, this association was not observed for thyroid cancer. Generally, BFR exposure appears to elevate the risk of endocrine-related cancers, with a notable increase in risk linked to higher levels of BDE-28, a specific polybrominated diphenyl ether congener.

CONCLUSIONS

In conclusion, although this meta-analysis has several limitations, our results suggest that BFR exposure is a significant risk factor for breast cancer, and low-brominated BDE-28 exposure could significantly increase the risk of endocrine-related cancers. Further research is essential to clarify the potential causal relationships between BFRs and endocrine-related cancers, and their carcinogenic mechanisms.

In vitro screening of per- and polyfluorinated substances (PFAS) for interference with seven thyroid hormone system targets across nine assays

The US Environmental Protection Agency is evaluating the ecological and toxicological effects of per- and polyfluorinated chemicals. A number of perfluorinated chemicals have been shown to impact the thyroid axis in vivo suggesting that the thyroid hormone system is a target of these chemicals. The objective of this study was to evaluate the activity of 136 perfluorinated chemicals at seven key molecular initiating events (MIE) within the thyroid axis using nine in vitro assays. The potential MIE targets investigated are Human Iodothyronine Deiodinase 1 (hDIO1), Human Iodothyronine Deiodinase 2 (hDIO2), Human Iodothyronine Deiodinase 3 (hDIO3), Xenopus Iodothyronine Deiodinase (xDIO3); Human Iodotyrosine Deiodinase (hIYD), Xenopus Iodotyrosine Deiodinase (xIYD), Human Thyroid Peroxidase (hTPO); and the serum binding proteins Human Transthyretin (hTTR) and Human Thyroxine Binding Globulin (hTBG). Of the 136 PFAS chemicals tested, 85 had sufficient activity to produce a half-maximal effect concentration (EC50) in at least one of the nine assays. In general, most of these PFAS chemicals did not have strong potency towards the seven MIEs examined, apart from transthyretin binding, for which several PFAS had potency similar to the respective model inhibitor. These data sets identify potentially active PFAS chemicals to prioritize for further testing in orthogonal in vitro assays and at higher levels of biological organization to evaluate their capacity for altering the thyroid hormone system and causing potential adverse health and ecological effects.

Endocrine Disruptors and Thyroid Health

A wide variety of thyroidal endocrine-disrupting chemicals (EDCs) have been identified. Exposure to known thyroidal EDCs is ubiquitous, and many likely remain unidentified. The sources of exposure include contaminated drinking water, air pollution, pesticides and agricultural chemicals, flame retardants, cleaning supplies, personal care products, food additives and packaging materials, coatings and solvents, and medical products and equipment. EDCs can affect thyroid hormone synthesis, transport, metabolism, and action in a myriad of ways. Understanding the health effects of thyroidal EDCs has been challenging because individuals may have multiple concomitant EDC exposures and many potential EDCs are not yet well characterized. Because of the importance of thyroid hormone for brain development in early life, pregnant women and young infants are particularly vulnerable to the effects of environmental thyroid disruption. The thyroidal effects of some EDCs may be exacerbated in iodine-deficient individuals, those with thyroid autoimmunity, and those with mutations in deiodinase genes. Differential exposures to EDCs may exacerbate health disparities in disadvantaged groups. High-throughput in vitro assays and in silico methods and methods that can detect the effects of relevant EDC mixtures are needed. In addition, optimal methods for detecting the effects of thyroidal EDCs on neurodevelopment need to be developed. Common sense precautions can reduce some thyroidal EDC exposures; however, regulation of manufacturing and drinking water content will ultimately be needed to protect populations.

Thyroid Dysfunction Induced by Fungicide Famoxadone Exposure Contributes to Nonalcoholic Fatty Liver Disease in Male Mice: In Vivo, In Vitro, and In Silico Studies

Thyroid dysfunction has become a serious public health problem, which is considered a trigger of nonalcoholic fatty liver disease (NAFLD). Pesticide exposure could contribute to thyroid dysfunction and NAFLD, but the relationship between these factors remains unclear. In this study, the effects of subchronic famoxadone exposure on thyroid and liver at no observed adverse effect level (NOEL) related concentrations were investigated using in vivo, in vitro, and in silico models. Famoxadone caused hepatic steatosis, lipid metabolism disorder, and liver oxidative stress and induced NAFLD in male mice. The suppression of hepatic fatty acid β-oxidation was the key factor of NAFLD, which was highly associated with hypothalamic-pituitary-thyroid (HPT) axis hormones disorder. Famoxadone disrupted thyroid hormone biosynthesis by causing thyroid follicle aberrations and abnormal HPT axis-related gene expression. In vitro studies confirmed that famoxadone inhibited the transport of thyroxine (T4) into hepatocytes and the conversion of T4 to triiodothyronine (T3). In silico studies verified that famoxadone interfered with the binding of thyroid hormones to proteins mediating thyroid hormone transport, conversion, and activation. This study comprehensively reported the association between NAFLD and thyroid dysfunction caused by famoxadone, providing new perspectives for the health risk evaluation of pesticides with a similar structure in mammals.

Determination of safe levels of persistent organic pollutants in toxicology and epidemiology

We reviewed published manuscripts from toxicology and epidemiology reporting harmful health effects and doses of persistent organic pollutants (POPs), published between 2000 and 2021. We found 42 in vitro, 32 in vivo, and 74 epidemiological studies and abstracted the dose associated with harm in a common Molar unit. We hypothesized that the dose associated with harm would vary between animal and human studies. To test this hypothesis, for each of several POPs, we assessed the significance of variation in the dose associated with a harmful effect [categorized as non-thyroid endocrine (NTE), developmental neurotoxicity (DNT), and Thyroid] with study type (in vitro, in vivo, and Epidemiology) using a linear model after adjustment for basis (lipid weight, wet weight). We created a Calculated Safety Factor (CSF) defined as the toxicology dose divided by epidemiology dose needed to exhibit significant harm. Significant differences were found between study types ranging from <1 to 5.0 orders of magnitude in the dose associated with harm. Our CSFs in lipid weight varied from 12.4 (95% confidence interval (CI) 3.3, 47) for NTE effects in Epidemiology relative to in vivo studies to 6,244 (95% CI 2510, 15530) for DNT effects in Epidemiology relative to in vitro in wet weight representing 12.4 to 6.2 thousand-fold more sensitivity in people relative to animals, and mechanistic models, respectively. In lipid weight, all CSF 95% CI lower bounds across effect categories were less than 6.5. CIs for CSFs ranged from less than one to four orders of magnitude for in vivo, and two to five orders of magnitude for in vitro vs. Epidemiology. A global CSF for all Epidemiology vs. all Toxicology was 104.6 (95% CI 72 to 152), significant at p<0.001.

Linking Transthyretin-Binding Chemicals and Free Thyroid Hormones: In Vitro to In Vivo Extrapolation Based on a Competitive Binding Model

Large numbers of pollutants competitively inhibit the binding between thyroid hormones and transthyretin (TTR) in vitro. However, the impact of this unintended binding on free thyroid hormones in vivo has not yet been characterized. Herein, we established a quantitative in vitro to in vivo extrapolation (QIVIVE) method based on a competitive binding model to quantify the effect of TTR-binding chemicals on free thyroid hormones in human blood. Twenty-five TTR-binding chemicals including 6 hydroxyl polybromodiphenyl ethers (OH-PDBEs), 6 hydroxyl polychlorobiphenyls (OH-PCBs), 4 halogenphenols, 5 per- and polyfluorinated substances (PFASs), and 4 phenols were selected for investigation. Incorporating the in vitro binding parameters and human exposure data, the QIVIVE model could well predict the in vivo effect on free thyroid hormones. Co-exposure to twenty-five typical TTR-binding chemicals resulted in median increases of 0.080 and 0.060% in circulating levels of free thyroxine (FT4) and free triiodothyronine (FT3) in the general population. Individuals with occupational exposure to TTR-binding chemicals suffered 1.88-32.2% increases in free thyroid hormone levels. This study provides a quantitative tool to evaluate the thyroid-disrupting risks of TTR-binding chemicals and proposes a new framework for assessing the in vivo effects of chemical exposures on endogenous molecules.

An Analysis of the Structural Relationship between Thyroid Hormone-Signaling Disruption and Polybrominated Diphenyl Ethers: Potential Implications for Male Infertility

Polybrominated diphenyl ethers (PBDEs) are a common class of anthropogenic organobromine chemicals with fire-retardant properties and are extensively used in consumer products, such as electrical and electronic equipment, furniture, textiles, and foams. Due to their extensive use, PBDEs have wide eco-chemical dissemination and tend to bioaccumulate in wildlife and humans with many potential adverse health effects in humans, such as neurodevelopmental deficits, cancer, thyroid hormone disruption, dysfunction of reproductive system, and infertility. Many PBDEs have been listed as chemicals of international concern under the Stockholm Convention on Persistent Organic Pollutants. In this study, the aim was to investigate the structural interactions of PBDEs against thyroid hormone receptor (TRα) with potential implications in reproductive function. Structural binding of four PBDEs, i.e., BDE-28, BDE-100, BDE-153 and BDE-154 was investigated against the ligand binding pocket of TRα using Schrodinger's induced fit docking, followed by molecular interaction analysis and the binding energy estimation. The results indicated the stable and tight binding of all four PDBE ligands and similarity in the binding interaction pattern to that of TRα native ligand, triiodothyronine (T3). The estimated binding energy value for BDE-153 was the highest among four PBDEs and was more than that of T3. This was followed by BDE-154, which is approximately the same as that of TRα native ligand, T3. Furthermore, the value estimated for BDE-28 was the lowest; however, the binding energy value for BDE-100 was more than BDE-28 and close to that of TRα native ligand, T3. In conclusion, the results of our study suggested the thyroid signaling disruption potential of indicated ligands according to their binding energy order, which can possibly lead to disruption of reproductive function and infertility.

Endocrine Disruptor Chemicals and Children's Health

We are all exposed to endocrine-disrupting chemicals (EDCs) starting from embryonic life. The fetus and child set up crucial developmental processes allowing adaptation to the environment throughout life: they are extremely sensitive to very low doses of hormones and EDCs because they are developing organisms. Considering the developmental origin of well-being and diseases, every adult organism expresses consequences of the environment in which it developed. The molecular mechanisms through which the main EDCs manifest their effects and their potential association with endocrine disorders, such as diabetes, obesity, thyroid disease and alteration of adrenal hormones, will be reviewed here. Despite 40 years having passed since the first study on EDCs, little is yet known about them; therefore, our purpose is to take stock of the situation to establish a starting point for further studies. Since there is plenty of evidence showing that exposure to EDCs may adversely impact the health of adults and children through altered endocrine function-suggesting their link to endocrinopathies-it is essential in this context to bear in mind what is already known about endocrine disruptors and to deepen our knowledge to establish rules of conduct aimed at limiting exposure to EDCs' negative effects. Considering that during the COVID-19 pandemic an increase in endocrine disruptor effects has been reported, it will also be useful to address this new phenomenon for better understanding its basis and limiting its consequences.

Binding of common organic UV-filters to the thyroid hormone transport protein transthyretin using in vitro and in silico studies: Potential implications in health

Several anthropogenic contaminants have been identified as competing with the thyroid hormone thyroxine (T₄) for binding to transport proteins as transthyretin (TTR). This binding can potentially create toxicity mechanisms posing a threat to human health. Many organic UV filters (UVFs) and paraben preservatives (PBs), widely used in personal care products, are chemicals of emerging concern due to their adverse effects as potential thyroid-disrupting compounds. Recently, organic UVFs have been found in paired maternal and fetal samples and PBs have been detected in placenta, which opens the possibility of the involvement of TTR in the transfer of these chemicals across physiological barriers. We aimed to investigate a discrete set of organic UVFs and PBs to identify novel TTR binders. The binding affinities of target UVFs towards TTR were evaluated using in vitro T₄ competitive binding assays. The ligand-TTR affinities were determined by isothermal titration calorimetry (ITC) and compared with known TTR ligands. In parallel, computational studies were used to predict the 3-D structures of the binding modes of these chemicals to TTR. Some organic UVFs, compounds 2,2',4,4'-tetrahydroxybenzophenone (BP2, Kd = 0.43 μM); 2,4-dihydroxybenzophenone (BP1, Kd = 0.60 μM); 4,4'-dihydroxybenzophenone (4DHB, Kd = 0.83 μM), and 4-hydroxybenzophenone (4HB, Kd = 0.93 μM), were found to display a high affinity to TTR, being BP2 the strongest TTR binder (ΔH = -14.93 Kcal/mol). Finally, a correlation was found between the experimental ITC data and the TTR-ligand docking scores obtained by computational studies. The approach integrating in vitro assays and in silico methods constituted a useful tool to find TTR binders among common organic UVFs. Further studies on the involvement of the transporter protein TTR in assisting the transplacental transfer of these chemicals across physiological barriers and the long-term consequences of prenatal exposure to them should be pursued.

Endocrine disrupting chemicals: effects on pituitary, thyroid and adrenal glands

BACKGROUND

In recent years, scientific research has increasingly focused on Endocrine Disrupting Chemicals (EDCs) and demonstrated their relevant role in the functional impairment of endocrine glands. This induced regulatory authorities to ban some of these compounds and to carefully investigate others in order to prevent EDCs-related conditions. As a result, we witnessed a growing awareness and interest on this topic.

AIMS

This paper aims to summarize current evidence regarding the detrimental effects of EDCs on pivotal endocrine glands like pituitary, thyroid and adrenal ones. Particularly, we directed our attention on the known and the hypothesized mechanisms of endocrine dysfunction brought by EDCs. We also gave a glimpse on recent findings from pioneering studies that could in the future shed a light on the pathophysiology of well-known, but poorly understood, endocrine diseases like hormone-producing adenomas.

CONCLUSIONS

Although intriguing, studies on endocrine dysfunctions brought by EDCs are challenging, in particular when investigating long-term effects of EDCs on humans. However, undoubtedly, it represents a new intriguing field of science research.

Mechanisms of Male Reproductive Toxicity of Polybrominated Diphenyl Ethers

Polybrominated diphenyl ethers (PBDE) are a group of flame retardants used in a variety of artificial materials. Despite being phased out in most industrial countries, they remain in the environment and human tissues due to their persistence, lipophilicity, and bioaccumulation. Populational and experimental studies demonstrate the male reproductive toxicity of PBDEs including increased incidence of genital malformations (hypospadias and cryptorchidism), altered weight of testes and other reproductive tissues, altered testes histology and transcriptome, decreased sperm production and sperm quality, altered epigenetic regulation of developmental genes in spermatozoa, and altered secretion of reproductive hormones. A broad range of mechanistic hypotheses of PBDE reproductive toxicity has been suggested. Among these hypotheses, oxidative stress, the disruption of estrogenic signaling, and mitochondria disruption are affected by PBDE concentrations much higher than concentrations found in human tissues, making them unlikely links between exposures and adverse reproductive outcomes in the general population. Robust evidence suggests that at environmentally relevant doses, PBDEs and their metabolites may affect male reproductive health via mechanisms including AR antagonism and the disruption of a complex network of metabolic signaling.

Impact of Chemical Endocrine Disruptors and Hormone Modulators on the Endocrine System

There is growing concern regarding the health and safety issues of endocrine-disrupting chemicals (EDCs). Long-term exposure to EDCs has alarming adverse health effects through both hormone-direct and hormone-indirect pathways. Non-chemical agents, including physical agents such as artificial light, radiation, temperature, and stress exposure, are currently poorly investigated, even though they can seriously affect the endocrine system, by modulation of hormonal action. Several mechanisms have been suggested to explain the interference of EDCs with hormonal activity. However, difficulty in quantifying the exposure, low standardization of studies, and the presence of confounding factors do not allow the establishment of a causal relationship between endocrine disorders and exposure to specific toxic agents. In this review, we focus on recent findings on the effects of EDCs and hormone system modulators on the endocrine system, including the thyroid, parathyroid glands, adrenal steroidogenesis, beta-cell function, and male and female reproductive function.

Towards regulation of Endocrine Disrupting chemicals (EDCs) in water resources using bioassays - A guide to developing a testing strategy

Endocrine disrupting chemicals (EDCs) are found in every environmental medium and are chemically diverse. Their presence in water resources can negatively impact the health of both human and wildlife. Currently, there are no mandatory screening mandates or regulations for EDC levels in complex water samples globally. Bioassays, which allow quantifying in vivo or in vitro biological effects of chemicals are used commonly to assess acute toxicity in water. The existing OECD framework to identify single-compound EDCs offers a set of bioassays that are validated for the Estrogen-, Androgen-, and Thyroid hormones, and for Steroidogenesis pathways (EATS). In this review, we discussed bioassays that could be potentially used to screen EDCs in water resources, including in vivo and in vitro bioassays using invertebrates, fish, amphibians, and/or mammalians species. Strengths and weaknesses of samples preparation for complex water samples are discussed. We also review how to calculate the Effect-Based Trigger values, which could serve as thresholds to determine if a given water sample poses a risk based on existing quality standards. This work aims to assist governments and regulatory agencies in developing a testing strategy towards regulation of EDCs in water resources worldwide. The main recommendations include 1) opting for internationally validated cell reporter in vitro bioassays to reduce animal use & cost; 2) testing for cell viability (a critical parameter) when using in vitro bioassays; and 3) evaluating the recovery of the water sample preparation method selected. This review also highlights future research avenues for the EDC screening revolution (e.g., 3D tissue culture, transgenic animals, OMICs, and Adverse Outcome Pathways (AOPs)).

First report on hydroxylated and methoxylated polybrominated diphenyl ethers in terrestrial environment from the Arctic and Antarctica

Terrestrial plants, which account for the world's largest biomass and constitute the basis of most food webs, take up, transform, and accumulate organic chemical contaminants from the ambient environment. In this study, we determined the concentrations and congener profiles of polybrominated diphenyl ethers (PBDEs) and hydroxylated and methoxylated polybrominated diphenyl ethers (OH-PBDEs and MeO-PBDEs) in surface soil and vegetation samples collected from the Arctic (Svalbard) and Antarctica (King George Island) during the Chinese Scientific Research Expeditions. The concentrations of total PBDEs (∑PBDEs) in soil and vegetation samples collected from the Arctic (5.6-270 pg/g dry weight) were higher than those from Antarctica (2.3-33 pg/g dw), whereas the concentrations of ∑MeO-PBDEs and ∑OH-PBDEs were lower in Arctic terrestrial samples (n.d.-0.75 and 0.0008-1.1 ng/g dw, respectively) than in samples from Antarctica (0.007-4.0 and 0.034-25 ng/g dw, respectively). Long-range atmospheric transport and human activities were potential sources of PBDEs in polar regions, whereas the dominance of ortho-substituted MeO-PBDE and OH-PBDE congeners in terrestrial matrices indicated the importance of natural sources. To the best of our knowledge, this study represents the first report on the levels and behaviors of MeO-PBDEs and OH-PBDEs in terrestrial environment of polar regions.

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

SYSTEMATIC REVIEW REGISTRATION

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

Structural binding perspectives of common plasticizers and a flame retardant, BDE-153, against thyroxine-binding globulin: potential for endocrine disruption

The human exposure to diverse endocrine-disrupting chemicals (EDCs) has increased dramatically over several decades with very adverse health effects. Plasticizers and flame retardants constitute important classes of EDCs interfering in endocrine physiology including the thyroid function. Thyroxine (T4) is an important hormone regulating metabolism and playing key roles in developmental processes. In this study, six phthalate and nonphthalate plasticizers and one flame retardant (BDE-153) were subjected to structural binding against thyroxine-binding globulin (TBG). The aim was to understand their potential role in thyroid dysfunction using structural binding approach. The structural study was performed using Schrodinger's induced fit docking, followed by binding energy estimations of ligands and the molecular interaction analysis between the ligands and the amino acid residues in the TBG ligand-binding pocket. The results indicated that all the compounds packed tightly into the TBG ligand-binding pocket with similar binding pattern to that of TBG native ligand, T4. A high majority of TBG interacting amino acid residues for ligands showed commonality with native ligand, T4. The estimated binding energy values were highest for BDE-153 followed by nonphthalate plasticizer, DINCH, with values comparable with native ligand, T4. The estimated binding energy values of other plasticizers DEHP, DEHT, DEHA, ATBC, and TOTM were less than DINCH. In conclusion, the tight docking conformations, amino acid interactions, and binding energy values of the most of the indicated ligands were comparable with TBG native ligand, T4, suggesting their potential for thyroid dysfunction. The results revealed highest potential thyroid disruptive action for BDE-153 and DINCH.

Human transthyretin binding affinity of halogenated thiophenols and halogenated phenols: An in vitro and in silico study

Serious harmful effects have been reported for thiophenols, which are widely used industrial materials. To date, little information is available on whether such chemicals can elicit endocrine-related detrimental effects. Herein the potential binding affinity and underlying mechanism of action between human transthyretin (hTTR) and seven halogenated-thiophenols were examined experimentally and computationally. Experimental results indicated that the halogenated-thiophenols, except for pentafluorothiophenol, were powerful hTTR binders. The differentiated hTTR binding affinity of halogenated-thiophenols and halogenated-phenols were observed. The hTTR binding affinity of mono- and di-halo-thiophenols was higher than that of corresponding phenols; while the opposite relationship was observed for tri- and penta-halo-thiophenols and phenols. Our results also confirmed that the binding interactions were influenced by the degree of ligand dissociation. Molecular modeling results implied that the dominant noncovalent interactions in the molecular recognition processes between hTTR and halogenated-thiophenols were ionic pair, hydrogen bonds and hydrophobic interactions. Finally, a model with acceptable predictive ability was developed, which can be used to computationally predict the potential hTTR binding affinity of other halogenated-thiophenols and phenols. Taken together, our results highlighted that more research is needed to determine their potential endocrine-related harmful effects and appropriate management actions should be taken to promote their sustainable use.

Thyroid hormone system disrupting chemicals

The thyroid hormone system is a main target of endocrine disruptor compounds (EDC) at all levels of its intricately fine-tuned feedback regulation, synthesis, distribution, metabolism and action of the 'prohormone' thyroxine and its active metabolites. Apart from classical antithyroid effects of EDC on the gland, the majority of known and suspected effects occurs at the pre-receptor control of T3 ligand availability to T3 receptors exerting ligand modulated thyroid hormone action. Tissue-, organ- and cell-specific expression and function of thyroid hormone transporters, deiodinases, metabolizing enzymes and T3-receptor forms, all integral components of the system, may mediate adverse EDC effects. Established evidence from nutritional, pharmacological and molecular genetic studies clearly support the functional, biological, and clinical relevance of these targets. Iodine-containing thyroid hormones and the organization of this system are highly conserved during evolution from primitive aquatic life forms, amphibia, birds throughout all vertebrates including humans. Mechanistic studies from various animal experimental models strongly support cause-effect relationships upon EDC exposure, hazards and adverse effects of EDC across various species. Retrospective case-control, cohort and population studies linking EDC exposure with epidemiological data on thyroid hormone-related (dys-)functions provide clear evidence that human development, especially of the fetal and neonatal brain, growth, differentiation and metabolic processes in adult and aging humans are at risk for adverse EDC effects. Considering that more than half of the world population still lives on inadequate iodine supply, the additional ubiquitous exposure to EDC and their mixtures is an additional threat for the essential thyroid hormone system, the health of the human population and their future progenies, animal life forms and our global environment.

Exploring the Biological Activity and Mechanism of Xenoestrogens and Phytoestrogens in Cancers: Emerging Methods and Concepts

Xenoestrogens and phytoestrogens are referred to as "foreign estrogens" that are produced outside of the human body and have been shown to exert estrogen-like activity. Xenoestrogens are synthetic industrial chemicals, whereas phytoestrogens are chemicals present in the plant. Considering that these environmental estrogen mimics potentially promote hormone-related cancers, an understanding of how they interact with estrogenic pathways in human cells is crucial to resolve their possible impacts in cancer. Here, we conducted an extensive literature evaluation on the origins of these chemicals, emerging research techniques, updated molecular mechanisms, and ongoing clinical studies of estrogen mimics in human cancers. In this review, we describe new applications of patient-derived xenograft (PDX) models and single-cell RNA sequencing (scRNA-seq) techniques in shaping the current knowledge. At the molecular and cellular levels, we provide comprehensive and up-to-date insights into the mechanism of xenoestrogens and phytoestrogens in modulating the hallmarks of cancer. At the systemic level, we bring the emerging concept of window of susceptibility (WOS) into focus. WOS is the critical timing during the female lifespan that includes the prenatal, pubertal, pregnancy, and menopausal transition periods, during which the mammary glands are more sensitive to environmental exposures. Lastly, we reviewed 18 clinical trials on the application of phytoestrogens in the prevention or treatment of different cancers, conducted from 2002 to the present, and provide evidence-based perspectives on the clinical applications of phytoestrogens in cancers. Further research with carefully thought-through concepts and advanced methods on environmental estrogens will help to improve understanding for the identification of environmental influences, as well as provide novel mechanisms to guide the development of prevention and therapeutic approaches for human cancers.

Perinatal effects of persistent organic pollutants on thyroid hormone concentration in placenta and breastmilk

Thyroid hormones (TH) are known to play a critical role in regulating many biological processes including growth and development, energy homeostasis, thermogenesis, lipolysis and metabolism of cholesterol. Severe TH deficiency especially during fetal development results in cretinism, but can also lead to an imbalance in metabolism with, among others, an alteration in body weight composition. Over the past two decades, increasing evidence has shown that certain persistent organic pollutants (POP) can interfere with the endocrine system. These POP referred to as "endocrine disrupting chemicals" are widely present in the environment and populations are exposed globally. Moreover, epidemiological studies have shown that a particularly sensitive period is the pre- and postnatal time. Indeed, perinatal exposure to such chemicals could lead to the onset diseases in later life. It is known, that, maternal thyroid hormones are transported by the placenta to the fetus from 6 weeks of gestation and it seems that during the first trimester, and part of the second, the fetus is entirely dependent on maternal TH supply for its development. Interferences in the TH-network as a consequence of the exposure to such pollutants could cause variations in TH concentration. Only small changes in maternal thyroid hormone levels in early stages of pregnancy can influence fetal neurological and cardiovascular development, as well as according to recent studies, have effect on childhood body composition. With this review, we will report the most recent and important studies concerning the association between thyroid hormone concentration and POP levels measured during the perinatal period. We will mostly focus on the data recently reported on placenta and breastmilk as main sources for understanding the potential consequences of exposure. The possible link between exposure to pollutants, TH dysregulation and possible adverse outcome will also be briefly discussed. From our literature search, several studies support the hypothesis that pre- and postnatal exposure to different pollutants might play a role in causing variation in thyroid hormone concentration. However, few research papers have so far studied the relationship linking exposure to pollutants, TH concentration and possible health consequences. Therefore, this review highlights the need for further research in this direction.

40 Years of Research on Polybrominated Diphenyl Ethers (PBDEs)-A Historical Overview and Newest Data of a Promising Anticancer Drug

Polybrominated diphenyl ethers (PBDEs) are a group of molecules with an ambiguous background in literature. PBDEs were first isolated from marine sponges of Dysidea species in 1981 and have been under continuous research to the present day. This article summarizes the two research aspects, (i) the marine compound chemistry research dealing with naturally produced PBDEs and (ii) the environmental toxicology research dealing with synthetically-produced brominated flame-retardant PBDEs. The different bioactivity patterns are set in relation to the structural similarities and dissimilarities between both groups. In addition, this article gives a first structure-activity relationship analysis comparing both groups of PBDEs. Moreover, we provide novel data of a promising anticancer therapeutic PBDE (i.e., 4,5,6-tribromo-2-(2',4'-dibromophenoxy)phenol; termed P01F08). It has been known since 1995 that P01F08 exhibits anticancer activity, but the detailed mechanism remains poorly understood. Only recently, Mayer and colleagues identified a therapeutic window for P01F08, specifically targeting primary malignant cells in a low µM range. To elucidate the mechanistic pathway of cell death induction, we verified and compared its cytotoxicity and apoptosis induction capacity in Ramos and Jurkat lymphoma cells. Moreover, using Jurkat cells overexpressing antiapoptotic Bcl-2, we were able to show that P01F08 induces apoptosis mainly through the intrinsic mitochondrial pathway.

Structural studies on the endocrine-disrupting role of polybrominated diphenyl ethers (PBDEs) in thyroid diseases

Polybrominated diphenyl ethers (PBDEs) are synthetic brominated flame retardants with extensive applications in daily-life consumer products. However, PBDEs have become ubiquitous environmental contaminants due to their leach-out capability. The hazardous human health effects and endocrine-disrupting activity of PBDEs have led many governmental organizations to impose ban on their manufacture, causing their gradual phase out from commercial products. However, PBDEs and their metabolites are still being detected from biological and environmental samples owing to their persistence and bioaccumulation. The PDBE metabolites in these samples are present in concentrations often higher and even with higher toxic potential than parent PBDEs. The two commonly detected environmental PBDE congeners, 2,2',4,4'-tetra-bromodiphenyl ether (BDE-47) and 2,2',4,4',5-penta-bromodiphenyl ether (BDE-99), and their HO- and MeO- metabolites were considered in this study for their potential disrupting activity on thyroid hormone transport. Specifically, the study involved structural binding characterization of BDE-47 and BDE-99 including their two HO- and two MeO- metabolites with thyroxine-binding globulin (TBG), which is the main thyroid hormone transport protein in blood. The results showed that the binding pattern and molecular interactions of above two PBDEs and their metabolites exhibited overall similarity to native ligand, thyroxine in dock score, binding energy, and amino acid interactions with TBG. The BDE-99 and its metabolites were predicted to have stronger binding to TBG than BDE-47 with the metabolite 5-MeO-BDE-99 showing equal binding affinity to that of thyroxine. It is concluded that BDE-47 and BDE-99 and their metabolites have the potential to disrupt thyroid hormone transport and interfere in thyroid function.

Structure-Directed Screening and Analysis of Thyroid-Disrupting Chemicals Targeting Transthyretin Based on Molecular Recognition and Chromatographic Separation

Exposure to thyroid-disrupting chemicals (TDCs) poses a great threat to human health. However, the screening and analysis of TDCs in environmental samples remain a tough work. In this study, we reported a structure-directed strategy for analyzing TDCs targeting transthyretin (TTR) based on molecular imprinting and chromatographic separation. The imprinted composites were prepared using l-thyroxine (T4) as a template and a tryptophan-like monomer screened from the amino acid library. The imprinted composites exhibited an adsorption capacity of 22.2 μmol g^-1 for T4 and an imprinting factor of 2.1. Chromatographic testing was then conducted among 72 chemicals using the imprinted composites-packed column. High retention factors were observed for chemicals that were structurally similar to T4. The chromatographic results were compared with a data set of 45 chemicals with known activities toward TTR. The results suggested that chemicals can be distinguished as TTR binders and nonbinders by retention factors, with a predictive accuracy of more than 90%. Moreover, the retention factors of chemicals were highly correlated with the reported relative potencies obtained from TTR assays. Thus, screening of TTR-binding chemicals can be realized through this simple chromatographic method. The imprinted composites were applied for target analysis and nontarget analysis of TTR-binding chemicals in dust samples. Three new TTR binders were successfully identified and verified by this method. The combination of molecular imprinting and chromatography opens up a new approach for screening TDCs targeting TTR.

Evaluation of structurally different brominated flame retardants interacting with the transthyretin and their toxicity on HepG2 cells

Brominated flame retardants (BFRs) are found at quantifiable levels in both humans and wildlife and may potentially cause a health risk. For BFRs and their derivatives, limited information regarding the relationship among the structure, binding affinity to the target protein and toxicity is currently available. In the present work, representative BFRs with different hydroxyl- or bromo-substituents, namely 2, 2', 4, 4'-tetrabromodiphenyl ether (BDE-47), 3-hydroxy-2, 2', 4, 4'-tetrabromodiphenyl ether (3-OH-BDE-47) and tetrabromobisphenol A (TBBPA), were selected to investigate the interactions with transthyretin (TTR) by electrospray ionization mass spectrometry (ESI-MS) and cytotoxicity on HepG2 cells. It was noted that BDE-47 had a weak binding affinity to TTR, while 3-OH-BDE-47 and TBBPA had a stronger binding affinity than BDE-47 and thyroxine (T4). Hence, 3-OH-BDE-47 and TBBPA could affect the binding of TTR with its native ligand T₄ by competitive binding to TTR, even at equal concentrations, which might be associated with BFR toxicity of endocrine disruption. Negative cooperativity was found for 3-OH-BDE-47 and TBBPA binding to TTR, similar to T₄ with a well-established negatively cooperative binding mechanism. The tendency of toxic effects on HepG2 cells for these three BFRs was, 3-OH-BDE-47 > TBBPA > BDE-47, and this order was in good agreement with the binding ability explored by ESI-MS experiments and molecular docking simulation. The observations obtained by this study demonstrate that the binding properties of these BFRs to TTR and their cytotoxicity are correlated with structure differentials and functional substituents.

Bisphenols as Environmental Triggers of Thyroid Dysfunction: Clues and Evidence

Bisphenols (BPs), and especially bisphenol A (BPA), are known endocrine disruptors (EDCs), capable of interfering with estrogen and androgen activities, as well as being suspected of other health outcomes. Given the crucial role of thyroid hormones and the increasing incidence of thyroid carcinoma in the last few decades, this review analyzes the effects of BPS on the thyroid, considering original research in vitro, in vivo, and in humans published from January 2000 to October 2019. Both in vitro and in vivo studies reported the ability of BPs to disrupt thyroid function through multiple mechanisms. The antagonism with thyroid receptors (TRs), which affects TR-mediated transcriptional activity, the direct action of BPs on gene expression at the thyroid and the pituitary level, the competitive binding with thyroid transport proteins, and the induction of toxicity in several cell lines are likely the main mechanisms leading to thyroid dysfunction. In humans, results are more contradictory, though some evidence suggests the potential of BPs in increasing the risk of thyroid nodules. A standardized methodology in toxicological studies and prospective epidemiological studies with individual exposure assessments are warranted to evaluate the pathophysiology resulting in the damage and to establish the temporal relationship between markers of exposure and long-term effects.

Toward a better understanding of the effects of endocrine disrupting compounds on health: Human-relevant case studies from sheep models

There are many challenges to overcome in order to properly understand both the exposure to, and effects of, endocrine disruptors (EDs). This is particularly true with respect to fetal life where ED exposures are a major issue requiring toxicokinetic studies of materno-fetal exchange and identification of pathophysiological consequences. The sheep, a large, monotocous, species, is very suitable for in utero fetal catheterization allowing a modelling approach predictive of human fetal exposure. Predicting adverse effects of EDs on human health is frequently impeded by the wide interspecies differences in the regulation of endocrine functions and their effects on biological processes. Because of its similarity to humans as regards gestational and thyroid physiologies and brain ontogeny, the sheep constitutes a highly appropriate model to move one step further on thyroid disruptor hazard assessment. As a grazing animal, the sheep has also proven to be useful in the evaluation of the consequences of chronic environmental exposure to "real-life" complex mixtures at different stages of the reproductive life cycle.

Binding interactions of halo-benzoic acids, halo-benzenesulfonic acids and halo-phenylboronic acids with human transthyretin

The anionic form-dependent binding interaction of halo-phenolic substances with human transthyretin (hTTR) has been observed previously. This indicates that ionizable compounds should be the primary focus in screening potential hTTR disruptors. Here, the potential binding potency of halo-benzoic acids, halo-benzenesulfonic acids/sulfates and halo-phenylboronic acids with hTTR was determined and analyzed by competitive fluorescence displacement assay integrated with computational methods. The laboratorial results indicated that the three test groups of model compounds exhibited a distinct binding affinity to hTTR. All the tested halo-phenylboronic acids, some of the tested halo-benzoic acids and halo-benzenesulfonic acids/sulfates were shown to be inactive with hTTR. Other halo-benzoic acids and halo-benzenesulfonic acids/sulfates were moderate and/or weak hTTR binders. The binding affinity of halo-benzoic acids and halo-benzenesulfonic acids/sulfates with hTTR was similar. The low distribution ability of the model compounds from water to hTTR may be the reason why they exhibited the binding potency observed with hTTR. By introducing other highly hydrophobic compounds, we observed that the binding affinity between compounds and hTTR increased with increasing molecular hydrophobicity. Those results indicated that the highly hydrophobic halo-benzoic acids and halo-benzenesulfonic acids/sulfates may be high-priority hTTR disruptors. Finally, a binary classification model was constructed employing three predictive variables. The sensitivity (S_n), specificity (S_p), predictive accuracy (Q) values of the training set and validation set were >0.83, indicating that the model had good classification performance. Thus, the binary classification model developed here could be used to distinguish whether a given ionizable compound is a potential hTTR binder or not.

Association between mixture of persistent organic pollutants and thyroid pathologies in a Belgian population

Previous years, the incidence of autoimmune thyroid diseases has increased worldwide. The presence of many pollutants in the environment suspected to be thyroid disruptors may have contributed to the observed increase. Unfortunately, the results from epidemiological studies assessing the association between pollution and thyroid disorders remain inconsistent, maybe due to a nearly complete neglect of the mixture effect. The blood levels of 12 brominated flame retardants, 3 polychlorinated biphenyls, 16 organochlorine pesticides, 7 perfluoroalkyl substances and 16 phenolic organohalogens were measured in 35 hypothyroid and 44 hyperthyroid volunteers and in 160 individuals from the general population designed as controls. Weighted quantile sum (WQS) regressions were performed to compute indexes representing the mixture of POPs, and we assessed the relations with thyroid disorders. Nineteen pollutants were detected in more than 40% of the individuals and were thus included in the WQS indexes. The WQS index was statistically significantly associated with an increased odds of hypothyroidism (odds ratio (OR) = 98.1; 95% CI: 5.51-1747) with the highest weights attributed to PCB 138 (w = 0.210), 3-OH-CB 180 (w = 0.197), 4-OH-CB 146 (w = 0.188), 4',4-DDE (w = 0.156) while there were no evidence of a relation with increased odds of hyperthyroidism. Given the relative low number of individuals included in the present investigation, standard WQS methodology could not be used, this study should thus be considered as a preliminary, hypothesis-generating study. Nevertheless, these results highlighted the importance of considering the potential effect of chemical mixture when studying endocrine disruptors.

Levels and distribution of polybrominated diphenyl ethers in humans and environmental compartments: a comprehensive review of the last five years of research

Polybrominated diphenyl ethers (PBDEs) are a class of brominated flame retardants (BFRs), present in the environment, animals, and humans. Their levels, distribution, and human exposure have been studied extensively, and over the last decade, various legal measures have been taken to prohibit or minimize their production and use due to the increasing amount of evidence of their harmful effects on human and animal health.Our aim here was to make a comprehensive and up-to-date review of the levels and distribution of PBDEs in the aquatic environment, air, and soil, in indoor dust, and in humans. To fulfill this, we searched through Web of Science for literature data reported in the last five years (2015-2019) on levels of at least six key PBDE congeners in abovementioned matrices. According to our summarized data, significant PBDE mass concentrations/fractions are still being detected in various sample types across the world, which implies that PBDE contamination is an ongoing problem. Secondary sources of PBDEs like contaminated soils and landfills, especially those with electronic and electrical waste (e-waste), represent a particular risk to the future and therefore require a special attention of scientists.

Bisphenols and Thyroid Hormone

In recent decades, attention has been directed toward the effects of bisphenol A (BPA) on human health. BPA has estrogenic activity and is regarded as a representative endocrine disruptor. In addition, mounting evidence indicates that BPA can disrupt thyroid hormone and its action. This review examined human epidemiological studies to investigate the association between BPA exposure and thyroid hormone levels, and analyzed in vivo and in vitro experiments to identify the causal relationship and its mechanism of action. BPA is involved in thyroid hormone action not only as a thyroid hormone receptor antagonist, but also through several other mechanisms. Since the use of bisphenols other than BPA has recently increased, we also reviewed the effects of other bisphenols on thyroid hormone action.

Disruption of thyroid hormone regulated proteins and gene expression by polychlorinated biphenyls, polybrominated diphenyl ethers and new flame retardants in residents of an e-waste region

Polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs) and new flame retardants (NFRs) are known thyroid hormone (TH) disruptors, but their disrupting mechanisms in humans are not completely understood. In this study, we aimed to explore the disrupting mechanisms of the aforementioned chemicals via examining TH-regulated proteins and gene expression in human serum. Adult participants from an e-waste dismantling (exposed group) and a control region (control group) in South China provided blood samples for the research. Some compounds of PCBs, PBDEs, and NFRs showed strong binding affinity to the thyroid-stimulating hormone (TSH), thyroglobulin, thyroxine-binding globulin (TBG), gene expression of TH receptor α (TRα) and β, and iodothyronine deiodinase I (ID1). The highly exposed individuals had lower levels of TBG, TSH, and expression of TRα, but higher expression of ID1 than those of the control group. The disruption of TH-regulated proteins and gene expression suggested the exertion of different and, at times, even contradictory effects on TH disruption. However, no statistically significant difference was found in the TH levels between the exposed and the control group, implying that the TH disruption induced by these chemicals depends on the combined influence of multiple mechanisms. Gene expression appears to be an effective approach for investigations of TH disruption and the potential health effects.

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.

Environmental ototoxicants, a potential new class of chemical stressors

Hearing loss is an injury that can develop over time, and people may not even be aware of it until it becomes a severe disability. Ototoxicants are substances that may damage the inner ear by either affecting the structures in the ear itself or by affecting the nervous system. We have examined the possibility that ototoxicants may present a health hazard in association with environmental exposures, adding to existing knowledge of their proven hazards under medical therapeutic conditions or occupational activities. In addition to the already described human environmental ototoxicants, mainly organochlorines such as polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), dichlorodiphenyltrichloroethane (DDT), dichlorodiphenyldichloroethylene (DDE), hexachlorocyclohexane (HCH) and hexachlorobenzene (HCB), we have examined the ubiquitous chemical stressors phthalates, bisphenol A/S/F/, PFCs, flame retardants (FRs) and cadmium for potential ototoxic properties, both as single substances or as chemical mixtures. Our literature review confirmed that these chemicals may disturb thyroid hormones homeostasis, activate aryl hydrocarbon receptor (AhR), and induce oxidative stress, which in turn may initiate a chain of events resulting in impairment of cochlea and hearing loss. With regard to auditory plasticity, diagnostics of a mixture of effects of ototoxicants, potential interactions of chemical and physical agents with effects on hearing, parallel deterioration of hearing due to chemical exposures and ageing, metabolic diseases or obesity, even using specific methods as brainstem auditory evoked potentials (BAEP) or otoacoustic emissions (OAEs) registration, may be difficult, and establishment of concentration-response relationships problematic. This paper suggests the establishment of a class of environmental oxotoxicants next to the established classes of occupational and drug ototoxicants. This will help to properly manage risks associated with human exposure to chemical stressors with ototoxic properties and adequate regulatory measures.

Characterization of non-radiolabeled Thyroxine (T4) uptake in cryopreserved rat hepatocyte suspensions: Pharmacokinetic implications for PFOA and PFOS chemical exposure

The alteration of thyroxine (T₄) cellular uptake by an environmental chemical can serve as a contributing factor in thyroid hormone (TH) disruption. Herein, we describe a non-radiolabeled (LC-MS/MS) oil-filtration technique designed to characterize the mechanism(s) responsible for T₄ cellular uptake in cryopreserved rat hepatocyte suspensions. The environmental chemicals perfluorooctanoate (PFOA) and perfluorooctane sulfonate (PFOS) were evaluated for their effect on T₄ hepatic uptake. At 37 °C, hepatic assays demonstrated saturable kinetics with increasing T₄ concentrations, while a linear uptake rate consistent with passive diffusion was detected at 4 °C. Carrier-mediated (37-4 °C) transport of T₄ was the predominant hepatic uptake process versus passive diffusion. Cyclosporin A (CsA) chemically inhibited T₄ hepatic uptake, whereas PFOA/PFOS displayed no inhibition of T₄ translocation. Increasing PFOA/PFOS concentration levels with the T₄ serum carrier-protein transthyretin (TTR) present resulted in a dose-response increase in T₄ hepatic uptake rates, correlating with increased T₄ free fraction values. Hepatic assays conducted in the presence of PFOA/PFOS and TTR displayed an enhanced first-order T₄ hepatic uptake rate consistent with carrier-mediated transport. These in vitro findings characterizing increased T₄ hepatic uptake provides mechanistic insight regarding decreased T₄ serum levels (hypothyroxinemia) previously observed within in vivo rodent studies following perfluorinated chemical exposure.

Disruption of thyroid hormone levels by decabrominated diphenyl ethers (BDE-209) in occupational workers from a deca-BDE manufacturing plant

While there is some evidence that exposure to decabrominated diphenyl ethers (BDE-209) affects thyroid function, the results obtained to date have been inconsistent. No studies have been performed on workers in deca-BDE manufacturing who had a high level of exposure to BDE-209 and relatively little exposure to other contaminants. In the present study, the relationship between BDE-209 exposure and thyroid hormone in occupational workers from a deca-BDE manufacturing plant was investigated. The serum and urine levels of polybrominated diphenyl ethers (PBDEs) and serum thyroid hormones were measured in 72 workers recruited from the deca-BDE manufacturing plant. The associations between their thyroid hormone levels and their exposure to BDE-209 were examined using multiple linear regression models. Serum concentrations of BDE-209 ranged from 67.4 to 109,000 ng/g lipid weight (lw), with a median of 3420 ng/g lw, contributing to 93.1% of the total PBDEs. The concentration of BDE-209 in urine was highly correlated with that in the serum (r² = 0.440, p < 0.001), indicating that urine may be a good non-invasive biomonitoring medium of BDE-209 body burden in occupational workers. BDE-209 in the serum was significantly and positively correlated with total thyroxine (tT4, r = 0.270, p = 0.029) and marginally and positively correlated with total triiodothyronine (tT3, r = 0.232, p = 0.061) in all occupational workers after adjusting for gender, age, BMI, and occupational exposure duration. A 10-fold increase in the serum BDE-209 concentration was associated with an increase in tT4 (8.63 nmol/L) [95% confidence interval (CI): 0.930-16.3] and tT3 (0.106 nmol/L) [95% confidence interval (CI): -0.005-0.219], corresponding to the increase of 7.8% in tT4 level and 5.4% in tT3 level. Associations between urine BDE-209 levels and thyroid hormones were similar to the results for the serum levels. These findings offer new evidence for proving the thyroid disrupting effects of BDE-209, impacting the direction of hyperthyroidism.

Metabolic enhancement of 2,3',4,4',5-pentachlorobiphenyl (CB118) using cytochrome P450 monooxygenase isolated from soil bacterium under the presence of perfluorocarboxylic acids (PFCAs) and the structural basis of its metabolism

2,3',4,4',5-Pentachlorobiphenyl (CB118) is one of the most abundant polychlorinated biphenyl (PCB) congeners in the environment, and perfluoroalkyl acids, including perfluorocarboxylic acids (PFCAs), are widely distributed in the environment. Although CB118 and perfluoroalkyl acids are present in all humans and biota, effects in the metabolic fate of CB118 leading to toxicity change are unclear. P450BM3, which is isolated from the soil bacterium Bacillus megaterium, metabolized CB118 to three different hydroxylated pentachlorobiphenyls (M1-M3). M2 was identified as 4'-OH-2,3',4,5,5'-pentachlorobiphenyl. These reactions were promoted by the presence of PFCAs, and perfluorooctanoic acid (PFCA-C8) was the most effective for accelerating these reactions among PFCAs with different carbon chain length. The production rate of M2 was accelerated by 25-times using PFCA-C8. Furthermore, the docking models of P450BM3 with CB118 and PFCAs revealed that the conformational changes of the substrate-binding cavity of P450BM3 after binding of PFCAs to P450BM3 were important for selective production of CB118 metabolites. This study leads to the clarification of the different metabolic fates of PCBs under complex contamination with PFCAs.

Intracellular toxicity exerted by PCBs and role of VBNC bacterial strains in biodegradation

Polychlorinated biphenyls (PCBs) are xenobiotic compounds that persists in the environment for long-term, though its productivity is banned. Abatement of the pollutants have become laborious due to it's recalcitrant nature in the environment leading to toxic effects in humans and other living beings. Biphenyl degrading bacteria co-metabolically degrade low chlorinated PCBs using the active metabolic pathway. bph operon possess different genetic arrangements in gram positive and gram negative bacteria. The binding ability of the genes and the active sites were determined by PCB docking studies. The active site of bphA gene with conserved amino acid residues determines the substrate specificity and biodegradability. Accumulation of toxic intermediates alters cellular behaviour, biomass production and downturn the metabolic activity. Several bacteria in the environment attain unculturable state which is viable and metabolically active but not cultivable (VBNC). Resuscitation-promoting factor (Rpf) and Rpf homologous protein retrieve the culturability of the so far uncultured bacteria. Recovery of this adaptive mechanism against various physical and chemical stressors make a headway in understanding the functionality of both environmental and medically important unculturable bacteria. Thus, this paper review about the general aspects of PCBs, cellular toxicity exerted by PCBs, role of unculturable bacterial strains in biodegradation, genes involved and degradation pathways. It is suggested to extrapolate the research findings on extracellular organic matters produced in culture supernatant of VBNC thus transforming VBNC to culturable state.

Effects of Polybrominated Diphenyl Ethers on Child Cognitive, Behavioral, and Motor Development

Polybrominated Diphenyl Ether (PBDE) flame retardants are environmental chemicals that cross the placenta during pregnancy and have shown evidence of neurotoxicity. As the in utero period is a sensitive developmental window, such exposure may result in adverse childhood outcomes. Associations between in utero PBDE exposure and neurodevelopment are found in animal models and increasingly in human population studies. Here, we review the epidemiological evidence of the association between prenatal exposure to PBDEs and motor, cognitive, and behavioral development in infants and children. Published work suggests a negative association between PBDE concentrations and neurodevelopment despite varying PBDE congeners measured, bio-specimen matrix used, timing of the biological sampling, geographic location of study population, specific developmental tests used, age of children at time of testing, and statistical methodologies. This review includes 16 published studies that measured PBDE exposure in maternal blood during pregnancy or in cord blood at delivery and performed validated motor, cognitive, and/or behavioral testing at one or more time during childhood. We evaluate possible mediation through PBDE-induced perturbations in thyroid function and effect measure modification by child sex. While the majority of studies support an adverse association between PBDEs and neurodevelopment, additional research is required to understand the mechanism of action, possibly through the perturbations in thyroid function either in the pregnant woman or in the child, and the role of biologically relevant effect modifiers such as sex.

Polybrominated diphenyl ether (PBDE) exposures and thyroid hormones in children at age 3 years

BACKGROUND

Polybrominated diphenyl ethers (PBDEs) reduce serum thyroid hormone concentrations in animal studies, but few studies have examined the impact of early-life PBDE exposures on thyroid hormone disruption in childhood.

METHODS

We used data from 162 mother-child pairs from the Health Outcomes and Measures of the Environment Study (2003-2006, Cincinnati, OH). We measured PBDEs in maternal serum at 16 ± 3 weeks gestation and in child serum at 1-3 years. Thyroid hormones were measured in serum at 3 years. We used multiple informant models to investigate associations between prenatal and early-life PBDE exposures and thyroid hormone levels at age 3 years.

RESULTS

Prenatal PBDEs were associated with decreased thyroid stimulating hormone (TSH) levels at age 3 years. A 10-fold increase in prenatal ∑PBDEs (BDE-28, -47, -99, -100, and -153) was associated with a 27.6% decrease (95% CI -40.8%, -11.3%) in TSH. A ten-fold increase in prenatal ∑PBDEs was associated with a 0.25 pg/mL (0.07, 0.43) increase in free triiodothyronine (FT3). Child sex modified associations between prenatal PBDEs and thyroid hormones, with significant decrements in TSH among females and decreased free T4 (FT4) in males. Prenatal ∑PBDEs were not associated with TT4, FT4, or total T3.

CONCLUSIONS

These findings suggest an inverse relationship between prenatal ∑PBDEs and TSH at 3 years. Associations may be sexually dimorphic, with an inverse relationship between prenatal BDE-47 and -99 and TSH in females and null associations among males.

Association between organohalogenated pollutants in cord blood and thyroid function in newborns and mothers from Belgian population

The last decades have seen the increasing prevalence of thyroid disorders. These augmentations could be the consequence of the increasing contamination of the environment by chemicals that may disrupt the thyroid function. Indeed, in vitro studies have shown that many chemicals contaminating our environment and highlighted in human serum, are able to interfere with the thyroid function. Given the crucial importance of thyroid hormones on neurodevelopment in fetus and newborns, the influence of these pollutants on newborn thyroid homeostasis is a major health concern. Unfortunately, the overall evidence for a deleterious influence of environmental pollutants on thyroid remains poorly studied. Therefore, we assessed the contamination by polychlorinated biphenyls (PCBs), organochlorine pesticides and perfluorinated compounds (PFC) in 221 cord blood samples collected in Belgium between 2013 and 2016. Our results showed that compared to previous studies performed on newborns recruited in Belgium during the two last decades, the present pollutant contamination is declining. Multivariate statistical analyses pointed out a decrease of thyroid stimulating hormone (TSH) level in male newborns with detectable level of 4,4'- dichlorodiphenyldichloroethylene (4,4'-DDE) in comparison with those with no detectable level (p = 0.025). We also highlighted a negative association between perfluorononanoic acid (PFNA) concentration and TSH in male newborns (p = 0.018). Logistic regression showed increased odds ratio for presentation of hypothyroid in mother for each one unit augmentation of log natural concentration of PFOA (OR = 2.30, [1.18-4.5]) and PFOS (OR = 2.03 [1.08-3.83]). Our findings showed that the residual contamination by PFCs and organochlorine pollutants in cord blood are correlated with thyroid hormone in the newborns and the risk of hypothyroid in mothers.

Association between serum polybrominated diphenyl ethers, new flame retardants and thyroid hormone levels for school students near a petrochemical complex, South China

As surrogates of polybrominated diphenyl ethers (PBDEs), new flame retardants (NFRs) include a series of chlorinated and brominated flame retardants. Though the NFRs are thought to induce similar thyroid hormone (TH) disrupting effects as PBDEs, few studies have focused on them. Given the increasing levels of NFRs in the environment, more in depth investigation of the potential TH disrupting effects of NFRs is warranted. This research involved a health survey to collect data and examine the associations between PBDEs, NFRs and TH. 174 school students lived near a petrochemical complex in South China participated in the survey, completing questionnaires and providing blood samples. Thirteen congeners of PBDEs, eight species of NFRs, TH and thyroid-stimulating hormone (TSH) were measured. The median levels of ΣPBDE (sum of thirteen congeners of PBDEs) and ΣNFR (sum of eight species of NFRs) for students were 140 and 240 ng g^-1 lipid, respectively. Nonmonotonic relationships were observed between quartile levels of PBDEs, NFRs and corresponding TH. In contrast to ΣPBDE that was positively associated with triidothyrine (T₃) level, ΣNFR was not statistically associated with TH. ΣPBDE + NFR (sum of thirteen congeners of PBDEs and eight species of NFRs) was significantly associated with T₃ level.

Exposure to polybrominated diphenyl ethers (PBDEs) and child behavior: Current findings and future directions

Polybrominated diphenyl ethers (PBDEs) are recognized neurotoxicants, but the extent to which PBDEs influence various domains of behavior in children is not fully understood. As such, we reviewed epidemiologic studies published to date to provide an overview of the current state of knowledge on PBDEs' potential role in behavioral development. We identified 19 epidemiologic studies reporting on associations of prenatal and childhood concentrations of PBDEs with behaviors assessed in children from 1 to 12years, including executive function, attention, externalizing and internalizing behaviors, adaptive skills, and social behaviors/Autism Spectrum Disorder (ASD). While the mechanisms of PBDE neurotoxicity in humans are still not clearly elucidated, findings from this review indicate that PBDE exposure during fetal development is associated with impairments in executive function and poorer attentional control in children. Results from large prospective cohorts demonstrate that prenatal and postnatal PBDE exposure adversely impacts externalizing behavior (e.g., hyperactivity and conduct problems). Additional studies are needed to determine whether PBDEs are associated with internalizing problems, adaptive skills, and social behaviors/ASD in children. Future studies will help better understand the potential neurotoxic effects of PBDE exposures during adolescence, possible sex-dependent effects, and the impact of exposure to BDE-209 and alternative flame retardants. Future studies should also examine chemical mixtures to capture real-world exposures when examining PBDEs and their impact on various behavioral domains in the context of multiple chemical exposures.

Mechanisms of Action Point Towards Combined PBDE/NDL-PCB Risk Assessment

At present, human risk assessment of the structurally similar non-dioxin-like (NDL) PCBs and polybrominated diphenylethers (PBDEs) is done independently for both groups of compounds. There are however obvious similarities between NDL-PCBs and PBDEs with regard to modulation of the intracellular calcium homeostasis (basal calcium levels, voltage-gated calcium channels, calcium uptake, ryanodine receptor) and thyroid hormone (TH) homeostasis (TH levels and transport). which are mechanisms of action related to neurobehavioral effects (spontaneous activity, habituation and learning ability). There also similarities in agonistic interactions with the hepatic nuclear receptors PXR and CAR. Several effects on developmental (reproductive) processes have also been observed, but results were more dispersed and insufficient to compare both groups of compounds. The available mechanistic information is sufficient to warrant a dose addition model for NDL-PCBs and PBDEs, including their hydroxylated metabolites.Although many of the observed effects are similar from a qualitative point of view for both groups, congener or tissue specific differences have also been found. As this is a source of uncertainty in the combined hazard and risk assessment of these compounds, molecular entities involved in the observed mechanisms and adverse outcomes associated with these compounds need to be identified. The systematical generation of (quantitative) structure-activity information for NDL-PCBs and PBDEs on these targets (including potential non-additive effects) will allow a more realistic risk estimation associated with combined exposure to both groups of compounds during early life. Additional validation studies are needed to quantify these uncertainties for risk assessment of NDL-PCBs and PBDEs.

Maternal-fetal transfer rates of PCBs, OCPs, PBDEs, and dioxin-like compounds predicted through quantitative structure-activity relationship modeling

The present study aims to predict the maternal-fetal transfer rates of the polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs), and polybrominated diphenyl ethers (PBDEs), and dioxin-like compounds using a quantitative structure-activity relationship model. The relation between the maternal-fetal transfer rate and the contaminants' physicochemical properties was investigated by multiple linear regression (MLR), partial least square regression (PLS), and random forest regression (RF). The 10-fold cross-validation technique estimated low predictive performances for both MLR and PLS models (R ²_CV = 0.425 ± 0.0964 for MLR and R ²_CV = 0.492 ± 0.115 for PLS) and is in agreement with an external test (R ²_pred = 0.129 for MLR and R ²_pred = 0.123 for PLS). In contrast, the RF model exhibits good predictive performance, estimated through 10-fold cross-validation (R ²_CV = 0.566 ± 0.0885) and an external test set (R ²_pred = 0.519). Molecular weight and polarity were selected in all models as important parameters that may predict the ability of a molecule to cross the placenta to the fetus.

Childhood polybrominated diphenyl ether (PBDE) exposure and executive function in children in the HOME Study

Prenatal exposure to polybrominated diphenyl ethers (PBDEs) have been reported to impair executive function in children, but little is known whether childhood PBDE exposures play a role. Using the Health Outcomes and Measures of the Environment (HOME) Study, a prospective birth cohort in the greater Cincinnati area, we investigated the association between repeated measures of PBDEs during childhood and executive function at 8 years in 208 children and whether effect modification by child sex was present. We used child serum collected at 1, 2, 3, 5, and 8 years to measure PBDEs. The Behavior Rating Inventory of Executive Function was completed by parents to assess executive function at 8 years. We used multiple informant models to examine childhood PBDEs during several exposure windows. Null associations were observed between early childhood PBDEs and executive function. However, we observed significant adverse associations between a 10-fold increase in concurrent concentrations of BDE-28 (β=4.6, 95% CI 0.5, 8.7) and BDE-153 (β=4.8, 95% CI 0.8, 8.8) with behavioral regulation. In addition, PBDEs at 8 years were significantly associated with poorer emotional and impulse control. No associations were noted between childhood PBDEs and metacognition or global executive function. However, child sex significantly modified the associations, with significantly poorer executive function among males with higher concurrent BDE-153, and null associations in females. Our study findings suggest that concurrent PBDE exposures during childhood may be associated with poorer executive function, specifically behavior regulation. Males may also be more sensitive to adverse associations of concurrent PBDEs on executive function.

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.