Different competition of thyroxine binding to transthyretin and thyroxine-binding globulin by hydroxy-PCBs, PCDDs and PCDFs

Optimized methods for measuring competitive binding of chemical substances to thyroid hormone distributor proteins transthyretin and thyroxine binding globulin

Transthyretin (TTR) and thyroxine-binding globulin (TBG) are two major thyroid hormone (TH) distributor proteins in human plasma, playing important roles in stabilizing the TH levels in plasma, delivery of TH to target tissues, and trans-barrier transport. Binding of xenobiotics to these distributor proteins can potentially affect all these three important roles of distributor proteins. Therefore, fast and cost-effective experimental methods are required for both TTR and TBG to screen both existing and new chemicals for their potential binding. In the present study, the TTR-binding assay was therefore simplified, optimized and pre-validated, while a new TBG-binding assay was developed based on fluorescence polarization as a readout. Seven model compounds (including positive and negative controls) were tested in the pre-validation study of the optimized TTR-binding assay and in the newly developed TBG-binding assay. The dissociation constants of the natural ligand (thyroxine, T4) and potential competitors were determined and compared between two distributor proteins, showing striking differences for perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA).

Identification of new endocrine disruptive transthyretin ligands in polluted waters using pull-down assay coupled to non-target mass spectrometry

Surface and treated wastewater are contaminated with highly complex mixtures of micropollutants, which may cause numerous adverse effects, often mediated by endocrine disruption. However, there is limited knowledge regarding some important modes of action, such as interference with thyroid hormone (TH) regulation, and the compounds driving these effects. This study describes an effective approach for the identification of compounds with the potential to bind to transthyretin (TTR; protein distributing TH to target tissues), based on their specific separation in a pull-down assay followed by non-target analysis (NTA). The method was optimized with known TTR ligands and applied to complex water samples. The specific separation of TTR ligands provided a substantial reduction of chromatographic features from the original samples. The applied NTA workflow resulted in the identification of 34 structures. Twelve compounds with available standards were quantified in the original extracts and their TH-displacement potency was confirmed. Eleven compounds were discovered as TTR binders for the first time and linear alkylbenzene sulfonates (LAS) were highlighted as contaminants of concern. Pull-down assay combined with NTA proved to be a well-functioning approach for the identification of unknown bioactive compounds in complex mixtures with great application potential across various biological targets and environmental compartments.

Thyroid Carcinoma: A Review for 25 Years of Environmental Risk Factors Studies

Environmental factors are established contributors to thyroid carcinomas. Due to their known ability to cause cancer, exposure to several organic and inorganic chemical toxicants and radiation from nuclear weapons, fallout, or medical radiation poses a threat to global public health. Halogenated substances like organochlorines and pesticides can interfere with thyroid function. Like phthalates and bisphenolates, polychlorinated biphenyls and their metabolites, along with polybrominated diethyl ethers, impact thyroid hormones biosynthesis, transport, binding to target organs, and impair thyroid function. A deeper understanding of environmental exposure is crucial for managing and preventing thyroid cancer. This review aims to investigate the relationship between environmental factors and the development of thyroid cancer.

Parental exposure to propiconazole at environmentally relevant concentrations induces thyroid and metabolism disruption in zebrafish (Danio rerio) offspring: An in vivo, in silico and in vitro study

Propiconazole is used against fungal growth in agriculture and is released into the environment, but is a potential health threat to aquatic organisms. Propiconazole induces a generational effect on zebrafish, although the toxic mechanisms involved have not been described. The aim of this study was to investigate the potential mechanisms of abnormal offspring development after propiconazole exposure in zebrafish parents. Zebrafish were exposed to propiconazole at environmentally realistic concentrations (0.1, 5, and 250 μg/L) for 100 days and their offspring were grown in control solution for further study. Heart rate, hatching rate, and body length of hatched offspring were reduced. An increase in triiodothyronine (T₃) content and the T₃/T₄ (tetraiodothyronine) ratio was observed, indicating disruption of thyroid hormones. Increased protein level of transthyretin (TTR) in vivo was consistent with the in silico molecular docking results and T₄ competitive binding in vitro assay, suggests higher binding affinity between propiconazole and TTR, more than with T₄. Increased expression of genes related to the hypothalamus-pituitary-thyroid (HPT) axis and altered metabolite levels may have affected offspring development. These findings emphasizes that propiconazole, even on indirect exposure, represents health and environmental risk that should not be ignored.

The Association between Blood Concentrations of PCDD/DFs, DL-PCBs and the Risk of Type 2 Diabetes Mellitus and Thyroid Cancer in South Korea

Background and Objectives: Epidemiological studies have inconsistently shown an association between dioxin and risk of type 2 diabetes mellitus (T2DM) and cancer. This study aims to examine the effects of blood concentration of dioxin-like polychlorinated biphenyls (DL-PCBs) and polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/DFs) on T2DM and thyroid cancer. Methods: We conducted a nested case–control study within the Korean cancer prevention study-II (KCPS-II) consisting of 15 thyroid cancer cases, 30 T2DM cases, and 55 controls. A total of 500 samples were used in 100 pooling samples. An average value of a pooled sample was calculated weighted by the blood volume of each sample. Results: The study population included 100 participants from the KCPS-II (median (IQR) baseline age, 54.06 [21.04] years; 48 women). The toxic equivalents of PCDD/DFs showed a significant positive association with T2DM and thyroid cancer, after adjustments for potential confounders (T2DM ORs  =  1.23; 95% CI  =  1.05–1.43; thyroid cancer ORs  =  1.34; 95% CI  =  1.12–1.61). Conclusion: In this study, both T2DM and thyroid cancer were associated with the blood concentrations of PCDD/DFs. The association between PCDD/DFs and T2D was found among women but not among men. Our findings suggest that further biochemical in vivo research and epidemiologic studies are needed to clarify the association between dioxins concentrations and diseases.

Structure-based mimicking of hydroxylated biphenyl congeners (OHPCBs) for human transthyretin, an important enzyme of thyroid hormone system

In humans, transthyretin (hTTR) is a plasma protein act as a transporter of thyroxine (T4) in the blood. Polychlorinated biphenyls (PCBs) are used in coolants, transformers, plasticizers, and pesticide extenders, etc. due to their physical properties, chemical stability, and dielectric properties. Cytochrome P450 can oxidize the PCBs into hydroxylated PCBs (OHPCBs) which can further interact with hTTR results in hepatoxicity, loss of metabolic rate, memory problems, and neurotoxicity. Molecular docking results show that OHPCBs bind at the active site of hTTR with a more binding affinity as compared to T4. Further, molecular dynamics simulation has been done to confirm the stability of hTTR-OHPCBs complexes. Several analysis parameters like RMSD, RMSF, Rg, SASA, hydrogen bonds numbers, PCA, and FEL revealed that binding of OHPCBs with hTTR results in the formation of stable hTTR-OHPCBs complexes. Individual residues decomposition analysis confirms that Lys15, Leu17, Ala108, Ala109, Leu110, Ser117, and Thr119 of hTTR plays a major role in the binding of OHPCBs to form the lower energy hTTR-OHPCBs complexes. Molecular docking and simulations results emphasize that OHPCBs can efficiently bind at the active site of hTTR, which further leads to inhibition of transportation of T4 in human blood.

A Halogen Bonding Perspective on Iodothyronine Deiodinase Activity

Iodothyronine deiodinases (Dios) are involved in the regioselective removal of iodine from thyroid hormones (THs). Deiodination is essential to maintain TH homeostasis, and disruption can have detrimental effects. Halogen bonding (XB) to the selenium of the selenocysteine (Sec) residue in the Dio active site has been proposed to contribute to the mechanism for iodine removal. Polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs) are known disruptors of various pathways of the endocrine system. Experimental evidence shows PBDEs and their hydroxylated metabolites (OH-BDEs) can inhibit Dio, while data regarding PCB inhibition are limited. These xenobiotics could inhibit Dio activity by competitively binding to the active site Sec through XB to prevent deiodination. XB interactions calculated using density functional theory (DFT) of THs, PBDEs, and PCBs to a methyl selenolate (MeSe⁻) arrange XB strengths in the order THs > PBDEs > PCBs in agreement with known XB trends. THs have the lowest energy C–X*-type unoccupied orbitals and overlap with the Se lp donor leads to high donor-acceptor energies and the greatest activation of the C–X bond. The higher energy C–Br* and C–Cl* orbitals similarly result in weaker donor-acceptor complexes and less activation of the C–X bond. Comparison of the I···Se interactions for the TH group suggest that a threshold XB strength may be required for dehalogenation. Only highly brominated PBDEs have binding energies in the same range as THs, suggesting that these compounds may inhibit Dio and undergo debromination. While these small models provide insight on the I···Se XB interaction itself, interactions with other active site residues are governed by regioselective preferences observed in Dios.

Effects of PCB exposure on serum thyroid hormone levels in dogs and cats

Polychlorinated biphenyls (PCBs) and their hydroxylated metabolites (OH-PCBs) might disrupt thyroid function. However, there is no clear evidence of PCB exposure disrupting thyroid hormone (TH) homeostasis in dogs and cats. The present study conducted in vivo experiments to evaluate the effects of a mixture of 12 PCB congeners (CB18, 28, 70, 77, 99, 101, 118, 138, 153, 180, 187 and 202, each congener 0.5 mg/kg BW, i.p. administration) on serum TH levels in male dogs (Canis lupus familiaris) and male cats (Felis silvestris catus). In PCB-exposed dogs, the time courses of higher-chlorinated PCBs and L-thyroxine (T4)-like OH-PCBs (4-OH-CB107 and 4-OH-CB202) concentrations were unchanged or tended to increase, whereas those of lower-chlorinated PCBs and OH-PCBs tended to decrease after 24 h. In PCB-exposed cats, concentrations of PCBs increased until 6 h and then remained unchanged. The levels of lower-chlorinated OH-PCBs including 4'-OH-CB18 increased until 96 h and then decreased. In PCB-exposed dogs, free T4 concentrations were higher than those in the control group at 48 and 96 h after PCB administration and positively correlated with the levels of T4-like OH-PCBs, suggesting competitive binding of T4 and T4-like OH-PCBs to a TH transporter, transthyretin. Serum levels of total T4 and total 3,3',5-triiodo-L-thyronine (T3) in PCB-exposed dogs were lower than in the control group at 24 and 48 h and negatively correlated with PCB concentrations, implying that PCB exposure enhanced TH excretion by increasing TH uptake and TH conjugation enzyme activities in the dog liver. In contrast, no obvious changes in TH levels were observed in PCB-exposed cats. This could be explained by the lower levels of T4-like OH-PCBs and lower hepatic conjugation enzyme activities in cats compared with dogs. Different effects on serum TH levels in PCB-exposed dogs and cats are likely to be attributable to species-specific PCB and TH metabolism.

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.

Depressive Symptoms After PCB Exposure: Hypotheses for Underlying Pathomechanisms via the Thyroid and Dopamine System

Polychlorinated biphenyls’ (PCB) exposure has been reported to be associated with depressive symptoms, which is correlated to lower dopamine- (DA) and thyroxine-concentrations (T4). T4 is necessary for DA-synthesis and it binds to transthyretin (TTR) being transported into the brain. PCBs can displace T4 by binding to TTR itself, being transported into the brain and disturbing DA-synthesis, where depressive symptoms might occur. Consequently, the free T4-concentration (fT4) increases when PCBs bind to TTR. The interaction of PCBs with fT4 and its associations with the main DA metabolite, homovanillic acid (HVA), and depressive symptoms were investigated. In total, 116 participants (91.6% men) were investigated, who took part in three annual examinations (t1–t3) of the HELPcB health surveillance program. Blood was collected for measuring PCBs, hydroxy PCBs (OH-PCBs), and fT4 and urine for HVA. Depressive Symptoms were assessed with a standardized questionnaire. Interactions were tested cross-sectionally with multiple hierarchical regressions and longitudinally with mixed effect models. Related to HVA, an interaction was cross-sectionally found for lower-chlorinated PCBs (LPCBs) and dioxin-like PCBs (dlPCBs); longitudinally only for LPCBs. Related to depressive symptoms, the interaction was found for LPCBs, dlPCBs, and OH-PCBs; longitudinally again only for LPCBs. The results give first hints that a physiological process involving the thyroid and DA system is responsible for depressive symptoms after PCB exposure.

Dioxin Exposure Alters Molecular and Morphological Responses to Thyroid Hormone in Xenopus laevis Cultured Cells and Prometamorphic Tadpoles

Amphibian metamorphosis is driven by thyroid hormone (TH). We used prometamorphic tadpoles and a cell line of the African clawed frog (Xenopus laevis) to examine immediate effects of dioxin exposure on TH. Gene expression patterns suggest cross-talk between the thyroid hormone receptor (TR) and aryl hydrocarbon receptor (AHR) signaling pathways. In XLK-WG cells, expression of Cytochrome P450 1A6 (cyp1A6), an AHR target, was induced 1000-fold by 100 nM TCDD (2, 3, 7, 8 tetrachlorodibenzo-p-dioxin). Krüppel-Like Factor 9 (klf9), the first gene induced in a cascade of TH responses tied to metamorphosis, was upregulated over 5-fold by 50 nM triiodothyronine (T3) and 2-fold by dioxin. Co-exposure to T3 and TCDD boosted both responses, further inducing cyp1A6 by 75% and klf9 about 60%. Additional canonical targets of each receptor, including trβa and trβb (TR) and udpgt1a (AHR) responded similarly. Induction of TH targets by TCDD in XLK-WG cells predicts that exposure could speed metamorphosis. We tested this hypothesis in two remodeling events: tail resorption and hind limb growth. Resorption of ex vivo cultured tails was accelerated by 10 nM T3, while a modest increase in resorption by 100 nM TCDD lacked statistical significance. Hind limbs doubled in length over four days following 1 nM T3 treatment, but limb length was unaffected by 100 nM TCDD. TCDD co-exposure reduced the T3 effect by nearly 40%, despite TCDD induction of klf9 in whole tadpoles, alone or with T3. These results suggest that tissue-specific TCDD effects limit or reverse the increased metamorphosis rate predicted by klf9 induction.

Structure-Based Virtual Screening Protocol for in Silico Identification of Potential Thyroid Disrupting Chemicals Targeting Transthyretin

Thyroid disruption by xenobiotics is associated with a broad spectrum of severe adverse outcomes. One possible molecular target of thyroid hormone disrupting chemicals (THDCs) is transthyretin (TTR), a thyroid hormone transporter in vertebrates. To better understand the interactions between TTR and THDCs, we determined the crystallographic structures of human TTR in complex with perfluorooctanesulfonic acid (PFOS), perfluorooctanoic acid (PFOA), and 2,2',4,4'-tetrahydroxybenzophenone (BP2). The molecular interactions between the ligands and TTR were further characterized using molecular dynamics simulations. A structure-based virtual screening (VS) protocol was developed with the intention of providing an efficient tool for the discovery of novel TTR-binders from the Tox21 inventory. Among the 192 predicted binders, 12 representatives were selected, and their TTR binding affinities were studied with isothermal titration calorimetry, of which seven compounds had binding affinities between 0.26 and 100 μM. To elucidate structural details in their binding to TTR, crystal structures were determined of TTR in complex with four of the identified compounds including 2,6-dinitro-p-cresol, bisphenol S, clonixin, and triclopyr. The compounds were found to bind in the TTR hormone binding sites as predicted. Our results show that the developed VS protocol is able to successfully identify potential THDCs, and we suggest that it can be used to propose THDCs for future toxicological evaluations.

A Preliminary Link between Hydroxylated Metabolites of Polychlorinated Biphenyls and Free Thyroxin in Humans

BACKGROUND

Polychlorinated biphenyls (PCBs) and their hydroxylated metabolites (HO-PCBs) interfere with thyroid hormone action both in vitro and in vivo. However, epidemiologic studies on the link between PCB exposure and thyroid function have yielded discordant results, while very few data are available for HO-PCBs.

OBJECTIVES

Our study aimed at investigating the relationship between clinically available markers of thyroid metabolism and serum levels of both PCBs and HO-PCBs.

SUBJECTS AND METHODS

In a group of 180 subjects, thyroid-stimulating hormone (TSH) and free thyroxin (fT4), 29 PCBs (expressed both in lipid weight and in wet weight) and 18 HO-PCBs were measured in serum.

RESULTS

In regression models, adjusted for gender, age, current smoking behavior, BMI and total lipid levels, serum levels of 3HO-PCB118 and 3HO-PCB180, and PCB95(lw), PCB99(lw) and PCB149(lw) were independent, significant predictors of fT4. A stepwise, multiple regression with gender, age, current smoking behavior, BMI and total lipid levels and all five previously identified significant compounds retained age, BMI, PCB95(lw), PCB99(lw) and 3HO-PCB180 as significant predictors of fT4. TSH levels were not predicted by serum levels of any of the PCBs or HO-PCBs.

CONCLUSIONS

Our study indicates that in vivo, circulating fT4 levels can be linked to serum levels of several PCBs and hydroxylated PCB metabolites.

Serum dioxin concentrations and thyroid hormone levels in the Seveso Women's Health Study

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a persistent environmental contaminant. Although experimental evidence suggests that TCDD alters thyroid hormone levels in rodents, human data are inconsistent. In 1976, a trichlorophenol plant exploded in Seveso, Italy. Women living in highly exposed areas were followed through the Seveso Women's Health Study. TCDD concentrations were measured in 1976 (n = 981) and 1996 (n = 260), and levels of total thyroxine, free thyroxine, free triiodothyronine, and thyroid-stimulating hormone were measured in 1996 (n = 909) and 2008 (n = 724). We used conditional multiple linear regression and marginal structural models with inverse-probability-of-treatment weights to evaluate associations and causal effects. TCDD concentration in 1976 was inversely associated with total thyroxine level in 1996 but not in 2008. Associations were stronger among women who had been exposed before menarche. Among these women, associations between total thyroxine and concurrent 1996 TCDD were slightly weaker than those with 1976 TCDD. A model including both 1976 and 1996 measurements strengthened the relationship between 1976 TCDD and total thyroxine but drove the association with 1996 TCDD to the null. TCDD exposure was not associated with levels of other thyroid hormones. TCDD exposure, particularly exposure before menarche, may have enduring impacts on women's total thyroxine levels. Initial exposure appears to be more influential than remaining body burden.

Wild fish from the Bay of Quinte Area of Concern contain elevated tissue concentrations of PCBs and exhibit evidence of endocrine-related health effects

The Bay of Quinte (BOQ) is an Area of Concern listed under the Great Lakes Water Quality Agreement. The presence of dioxins and dioxin-like PCBs in fish in the BOQ AOC has led to restrictions on fish consumption by humans, which is a beneficial use impairment. Adult yellow perch (Perca flavescens) and brown bullhead (Ameiurus nebulosus) were sampled from Trenton, Belleville, and Deseronto (reference site) in the BOQ. A suite of hormone assays and various measures of exposure and/or sublethal health effects were used to assess the health status of fish of both species and sex. Condition factor, hepatosomatic index, ethoxyresorufin-O-deethylase activity, circulating steroid and thyroid hormones, thyroid activation, oocyte size distribution, spermatogenic cell stages, and plasma vitellogenin were among the endpoints that were significantly (p < 0.05) affected by location. Many of those effects corresponded with significantly (p < 0.05) greater tissue concentrations of polychlorinated biphenyls (PCBs) at Belleville and Trenton. Hepatic extracts from brown bullhead sampled from Trenton had significantly (p < 0.05) greater binding activity to the androgen receptor and sex steroid binding protein. Taken together, these data and preliminary data from a concomitant study suggest that PCBs are likely being hydroxylated in vivo, resulting in enhanced bioactivity at endocrine receptors and measurable health responses. The present study supports the growing body of evidence that PCBs and their metabolites can affect fish thyroid and steroid hormone systems.

Environmental factors affecting thyroid function of wild sea bass (Dicentrarchuslabrax) from European coasts

Thyroid functional status of wild fish in relation with the contamination of their environment deserves further investigation. We here applied a multi-level approach of thyroid function assessment in 87 wild sea bass collected near several estuaries: namely the Scheldt, the Seine, the Loire, the Charente and the Gironde. Thyroxine (T(4)) and triiodothyronine (T(3)) concentrations in muscle were analyzed by radioimmunoassay. The activity of hepatic enzymes involved in extrathyroidal pathways of thyroid hormone metabolism, viz. deiodination, glucuronidation and sulfatation were analyzed. Last, follicle diameter and epithelial cell heights were measured. We observed changes that are predicted to lead to an increased conversion of T(4)-T(3) and lowered thyroid hormone excretion. The changes in the metabolic pathways of thyroid hormones can be interpreted as a pathway to maintain thyroid hormone homeostasis. From all compounds tested, the higher chlorinated PCBs seemed to be the most implicated in this perturbation.

PCBs and OH-PCBs in polar bear mother-cub pairs: a comparative study based on plasma levels in 1998 and 2008

The aim of this study was to examine the plasma concentrations and prevalence of polychlorinated biphenyls (PCBs) and hydroxylated PCB-metabolites (OH-PCBs) in polar bear (Ursus maritimus) mothers (n=26) and their 4 months old cubs-of-the-year (n=38) from Svalbard to gain insight into the mother-cub transfer, biotransformation and to evaluate the health risk associated with the exposure to these contaminants. As samplings were performed in 1997/1998 and 2008, we further investigated the differences in levels and pattern of PCBs between the two sampling years. The plasma concentrations of Σ(21)PCBs (1997/1998: 5710 ± 3090 ng/g lipid weight [lw], 2008: 2560 ± 1500 ng/g lw) and Σ(6)OH-PCBs (1997/1998: 228 ± 60 ng/g wet weight [ww], 2008: 80 ± 38 ng/g ww) in mothers were significantly lower in 2008 compared to in 1997/1998. In cubs, the plasma concentrations of Σ(21)PCBs (1997/1998: 14680 ± 5350 ng/g lw, 2008: 6070 ± 2590 ng/g lw) and Σ(6)OH-PCBs (1997/1998: 98 ± 23 ng/g ww, 2008: 49 ± 21 ng/g ww) were also significantly lower in 2008 than in 1997/1998. Σ(21)PCBs in cubs was 2.7 ± 0.7 times higher than in their mothers. This is due to a significant maternal transfer of these contaminants. In contrast, Σ(6)OH-PCBs in cubs were approximately 0.53 ± 0.16 times the concentration in their mothers. This indicates a lower maternal transfer of OH-PCBs compared to PCBs. The majority of the metabolite/precursor-ratios were lower in cubs compared to mothers. This may indicate that cubs have a lower endogenous capacity to biotransform PCBs to OH-PCBs than polar bear mothers. Exposure to PCBs and OH-PCBs is a potential health risk for polar bears, and the levels of PCBs and OH-PCBs in cubs from 2008 were still above levels associated with health effects in humans and wildlife.

Levels and patterns of hydroxylated polychlorinated biphenyls (OH-PCBs) and their associations with thyroid hormones in hooded seal (Cystophora cristata) mother-pup pairs

Blood (plasma/serum) samples from 14 adult female and their pups (1-4 days old) captured in the West Ice, east of Greenland were analysed for concentrations of total and free thyroxine and triiodothyronine (TT4, FT4, TT3, FT3), and hydroxylated polychlorinated biphenyls (OH-PCBs). The levels of all thyroid hormones (THs) were significantly higher in pups than in mothers. Sum OH-PCB levels (ΣOH-PCBs: 4-OH-CB107, 3'-OH-CB138, 4-OH-CB146, 4'-OH-CB172, 4-OH-CB187) were significantly higher in mothers (3.98 ± 1.55 pmol/g; 1.40 ± 0.54 ng/g wet weight) as compared to pups (1.95 ± 0.78 pmol/g; 0.68 ± 0.28 ng/g wet weight). Plasma levels of TT4 and FT4 in mothers increased as a function of pup age, as did levels of individual OH-PCBs in both mothers and pups. The pattern of OH-PCBs in the pups was similar to their mothers. We suggest that OH-PCBs found in pups are transferred from their mothers during gestation and that the transfer also continues after parturition via milk. Principal component analysis (PCA) showed that in pups, 4-OH-CB107 and 3'-OH-CB138 were negatively associated with FT4:FT3 and TT3:FT3 ratios, respectively. These relationships were confirmed by partial correlation analysis correcting for pup age. PCA suggested that 4'-OH-CB172 and 4-OH-CB187 were negatively associated with TT3 in mothers. However, this was not confirmed by correlation tests. Although statistical relationships should be interpreted with caution, the study indicates that young developing seals are more sensitive compared to adults with respect to TH-related effects of OH-PCBs.

Anaemia, hypothyroidism and immune suppression associated with polychlorinated biphenyl exposure in bottlenose dolphins (Tursiops truncatus)

Polychlorinated biphenyls (PCBs), persistent chemicals widely used for industrial purposes, have been banned in most parts of the world for decades. Owing to their bioaccumulative nature, PCBs are still found in high concentrations in marine mammals, particularly those that occupy upper trophic positions. While PCB-related health effects have been well-documented in some mammals, studies among dolphins and whales are limited. We conducted health evaluations of bottlenose dolphins (Tursiops truncatus) near a site on the Georgia, United States coast heavily contaminated by Aroclor 1268, an uncommon PCB mixture primarily comprised of octa- through deca-chlorobiphenyl congeners. A high proportion (26%) of sampled dolphins suffered anaemia, a finding previously reported from primate laboratory studies using high doses of a more common PCB mixture, Aroclor 1254. In addition, the dolphins showed reduced thyroid hormone levels and total thyroxine, free thyroxine and triiodothyronine negatively correlated with PCB concentration measured in blubber (p = 0.039, < 0.001, 0.009, respectively). Similarly, T-lymphocyte proliferation and indices of innate immunity decreased with blubber PCB concentration, suggesting an increased susceptibility to infectious disease. Other persistent contaminants such as DDT which could potentially confound results were similar in the Georgia dolphins when compared with previously sampled reference sites, and therefore probably did not contribute to the observed correlations. Our results clearly demonstrate that dolphins are vulnerable to PCB-related toxic effects, at least partially mediated through the endocrine system. The severity of the effects suggests that the PCB mixture to which the Georgia dolphins were exposed has substantial toxic potential and further studies are warranted to elucidate mechanisms and potential impacts on other top-level predators, including humans, who regularly consume fish from the same marine waters.

Alterations in thyroid hormone status in Greenland sledge dogs exposed to whale blubber contaminated with organohalogen compounds

As a model of high trophic level carnivores, sledge dogs were fed from 2 to 18 months of age with minke whale blubber containing organohalogen compounds (OHC) corresponding to 128 μg PCB/day. Controls were fed uncontaminated porcine fat. Thyroid hormone levels were assessed in 7 exposed and 7 control sister bitches (sampled at age 6-18 months) and 4 exposed and 4 control pups, fed the same diet as their mothers (sampled age 3-12 months). Lower free and total T3 and T4 were seen in exposed vs. control bitches beyond 10 months of age, and total T3 was lower through 3-12 months of age in exposed pups. A negative correlation with thyroid gland weight was significant for ΣDDT, as was a positive association with total T3 for dieldrin. This study therefore supports observational data that OHCs may adversely affect thyroid functions, and it suggests that OHC exposure duration of 10 months or more may be required for current OHC contamination levels to result in detectable adverse effects on thyroid hormone dynamics.

Do Polybrominated Diphenyl Ethers (PBDEs) Increase the Risk of Thyroid Cancer?

An increased incidence of thyroid cancer has been reported in many parts of the world including the United States during the past several decades. Recently emerging evidence has demonstrated that polyhalogenated aromatic hydrocarbons (PHAHs), particularly polybrominated diphenyl ethers (PBDEs), alter thyroid hormone homeostasis and cause thyroid dysfunction. However, few studies have been conducted to test whether exposure to PBDEs and other PHAHs increases the risk of thyroid cancer. Here, we hypothesize that elevated exposure to PHAHs, particularly PBDEs, increases the risk of thyroid cancer and may explain part of the increase in incidence of thyroid cancer during the past several decades. In addition, genetic and epigenetic variations in metabolic pathway genes may alter the expression and function of metabolic enzymes which are involved in the metabolism of endogenous thyroid hormones and the detoxification of PBDEs and other PHAHs. Such variation may result in different individual susceptibilities to PBDEs and other PHAHs and the subsequent development of thyroid cancer. The investigation of this hypothesis will lead to an improved understanding of the role of PBDEs and other PHAHs in thyroid tumorigenesis and may provide a real means to prevent this deadly disease.

Developmental neurotoxicity of polybrominated diphenyl ether (PBDE) flame retardants

Polybrominated diphenyl ethers (PBDEs) are a class of flame retardants used in a variety of consumer products. In the past 25 years, PBDEs have become ubiquitous environmental contaminants. They have been detected in soil, air, sediments, birds, marine species, fish, house dust, and human tissues, blood and breast milk. Diet and house dust appear to be the major sources of PBDE exposure in the general population, though occupational exposure can also occur. Levels of PBDEs in human tissues are particularly high in North America, compared to Asian and European countries, and have been increasing in the past 30 years. Concentrations of PBDEs are particularly high in breast milk, resulting in high exposure of infants. In addition, for toddlers, dust has been estimated to account for a large percentage of exposure. PBDEs can also cross the placenta, as they have been detected in fetal blood and liver. Tetra-, penta- and hexaBDEs are most commonly present in human tissues. The current greatest concern for potential adverse effects of PBDEs relates to their developmental neurotoxicity. Pre- or postnatal exposure of mice or rats to various PBDEs has been shown to cause long-lasting changes in spontaneous motor activity, mostly characterized as hyperactivity or decreased habituation, and to disrupt performance in learning and memory tests. While a reduction in circulating thyroid hormone (T(4)) may contribute to the developmental neurotoxicity of PBDEs, direct effects on the developing brain have also been reported. Among these, PBDEs have been shown to affect signal transduction pathways and to cause oxidative stress. Levels of PBDEs causing developmental neurotoxicity in animals are not much dissimilar from levels found in highly exposed infants and toddlers.

Thyroid hormone status of newborns in relation to in utero exposure to PCBs and hydroxylated PCB metabolites

The associations between in utero exposure to polychlorinated biphenyls (PCBs) or hydroxylated PCB metabolites (OH-PCBs), and free thyroxin (fT4) or thyroid-stimulating hormone (TSH) status in the newborn were investigated as a pilot study of a large-scale epidemiologic study on in utero PCB or OH-PCB exposure and thyroid function of the newborns. Umbilical cord tissue was used as the media for the biological monitoring of PCBs/OH-PCBs exposure in utero. For the measurement of fT4 and TSH, a heel-prick blood sample spotted on filter paper, which is called Guthrie card, is collected from each neonate at day 4-6 postpartum for this study when the mass screening sampling was performed. We showed that the concentration of total OH-PCBs and one of the OH-PCB congeners (OH-PCB 187) was related significantly to higher fT4 level of newborns. On the other hand, the concentration of total PCBs and PCB congeners (PCB 118, 138, 153, and 180) showed no relationship with fT4 and TSH level of the newborns. The results obtained in this pilot study indicated the possibility that in utero OH-PCBs exposure affects thyroid hormone status of newborns.

The menace of endocrine disruptors on thyroid hormone physiology and their impact on intrauterine development

The delivery of the appropriate thyroid hormones quantity to target tissues in euthyroidism is the result of unopposed synthesis, transport, metabolism, and excretion of these hormones. Thyroid hormones homeostasis depends on the maintenance of the circulating 'free' thyroid hormone reserves and on the development of a dynamic balance between the 'free' hormones reserves and those of the 'bound' hormones with the transport proteins. Disturbance of this hormone system, which is in constant interaction with other hormone systems, leads to an adaptational counter-response targeting to re-establish a new homeostatic equilibrium. An excessive disturbance is likely to result, however, in hypo- or hyper- thyroid clinical states. Endocrine disruptors are chemical substances forming part of 'natural' contaminating agents found in most ecosystems. There is abundant evidence that several key components of the thyroid hormones homeostasis are susceptible to the action of endocrine disruptors. These chemicals include some chlorinated organic compounds, polycyclic aromatic hydrocarbons, herbicides, and pharmaceutical agents. Intrauterine exposure to endocrine disruptors that either mimic or antagonize thyroid hormones can produce permanent developmental disorders in the structure and functioning of the brain, leading to behavioral changes. Steroid receptors are important determinants of the consequences of endocrine disruptors. Their interaction with thyroid hormones complicates the effect of endocrine disruptors. The aim of this review is to present the effect of endocrine disruptors on thyroid hormones physiology and their potential impact on intrauterine development.

Spreeta-based biosensor assays for endocrine disruptors

The construction and performance of an automated low-cost Spreeta-based prototype biosensor system for the detection of endocrine disrupting chemicals (EDCs) is described. The system consists primarily of a Spreeta miniature liquid sensor incorporated into an aluminum flow cell holder, dedicated to support a Biacore chip frame, in combination with a simple pressurized air-driven fluid system. During the optimization, a monoclonal antibody (MAb)-based immunoassay for the estrogenic compound bisphenol A (BPA) was used as a model. After the optimization two thyroxine transport protein inhibition assays for thyroid endocrine disruptors were implemented. The average noise of the system for 1 min of baseline was 1.1 microRIU (refractive index units) and it could be operated in the range of 18-22 degrees C with a minimum baseline drift (5-10 microRIU/100 min). Optimum signal to noise ratio (S/N R) was obtained using a flow cell height of 100 microm and a flow rate of 180 microl/min. The sensitivity of the Spreeta-based biosensor inhibition assays implemented (50% inhibition concentration (IC50) of 30.2 nM for BPA using MAb12 and 12.3 and 11.6 nM for thyroxine (T4) using thyroxine-binding globulin (TBG) and recombinant transthyretin (rTTR), respectively) was comparable to the sensitivity previously obtained using a Biacore 3000 (IC50 of 39.9 nM for BPA and 8.6 and 13.7 nM, respectively, for T4). The results indicate that the alternative prototype system can be used in combination with ready-to-use biosensor chip surfaces and it is potentially a useful tool for the bioeffect-related screening of EDCs.

Exposure to PCB 77 induces tissue-dependent changes in iodothyronine deiodinase activity patterns in the embryonic chicken

PCB 77 is a dioxin-like PCB that has been shown to reduce circulating thyroid hormone (TH) levels. This may be an important factor contributing to its neurotoxicity, since THs are essential for normal brain development. In this study, we investigated the changes in TH activating and inactivating iodothyronine deiodinase (D) activities in liver, telencephalon and cerebellum of chicken embryos during the final stages of embryonic development and hatching. We combined these results with measurements of plasma TH levels and intracellular TH availability in the tissues mentioned above, to find out whether D activity was a factor contributing to the PCB 77-induced decrease in peripheral TH levels and/or whether it was capable of reducing the adverse effects on brain via compensatory mechanisms. PCB 77 reduced both T(4) and T(3) levels in plasma and brain. Its effect on hepatic D1 and D3 activity was limited and rebuts a causative role of hepatic Ds in the drop of plasma TH levels. In cerebellum, D2 increased and D3 decreased, indicating a compensatory mechanism in this brain part, mainly during the stages of pipping and hatching. The changes in telencephalon occurred at the earlier stages and included an increase in both D2 and D3 activity.

T-screen to quantify functional potentiating, antagonistic and thyroid hormone-like activities of poly halogenated aromatic hydrocarbons (PHAHs)

The present study investigates chemical thyroid hormone disruption at the level of thyroid hormone receptor (TR) functioning. To this end the (ant)agonistic action of a series of xenobiotics was tested in the newly developed T-screen. This assay makes use of a GH3 rat pituitary cell line, that specifically proliferates when exposed to 3,3',5-triiodo-L-thyronine (T3). The growth stimulatory effect is mediated via T3-receptors. (Ant)agonistic and potentiating action of compounds was studied in absence and presence of T3 at its EC50 level (0.25 nM). The compounds tested included the specific TR-antagonist amiodarone, as well as a series of brominated diphenyl ethers (BDEs), including specifically synthesized BDEs with a structural resemblance to 3,5-diiodo-L-thyronine (T2), T3 and T4 (3,3',5,5'-tetraiodo-L-thyronine). The results obtained reveal that only BDE206 and amiodarone are specific antagonists. Interestingly some compounds which did not respond in the T-screen in absence of T3, potentiated effects when tested in combination with T3. This points at possibilities for disruption at the TR in vivo, where exposure generally occurs in presence of T3. Altogether the results of the present study show that the newly developed T-screen can be used as a valuable tool for identification and quantification of compounds active in disturbing thyroid hormone homeostasis at the level of TR-functioning.

Human health implications of environmental contaminants in Arctic Canada: A review

The objectives of this paper are to: assess the impact of exposure to current levels of environmental contaminants in the Canadian Arctic on human health; identify the data and knowledge gaps that need to be filled by future human health research and monitoring; examine how these issues have changed since our first assessment [Van Oostdam, J., Gilman, A., Dewailly, E., Usher, P., Wheatley, B., Kuhnlein, H. et al., 1999. Human health implications of environmental contaminants in Arctic Canada: a review. Sci Total Environ 230, 1-82]. The primary exposure pathway for contaminants for various organochlorines (OCs) and toxic metals is through the traditional northern diet. Exposures tend to be higher in the eastern than the western Canadian Arctic. In recent dietary surveys among five Inuit regions, mean intakes by 20- to 40-year-old adults in Baffin, Kivalliq and Inuvialuit communities exceeded the provisional tolerable daily intakes (pTDIs) for the OCs, chlordane and toxaphene. The most recent findings in NWT and Nunavut indicate that almost half of the blood samples from Inuit mothers exceeded the level of concern value of 5 microg/L for PCBs, but none exceeded the action level of 100 microg/L. For Dene/Métis and Caucasians of the Northwest Territories exposure to OCs are mostly below this level of concern. Based on the exceedances of the pTDI and of various blood guidelines, mercury and to a lesser extent lead (from the use of lead shot in hunting game) are also concerns among Arctic peoples. The developing foetus is likely to be more sensitive to the effects of OCs and metals than adults, and is the age groups of greatest risk in the Arctic. Studies of infant development in Nunavik have linked deficits in immune function, an increase in childhood respiratory infections and birth weight to prenatal exposure to OCs. Balancing the risks and benefits of a diet of country foods is very difficult. The nutritional benefits of country food and its contribution to the total diet are substantial. Country food contributes significantly more protein, iron and zinc to the diets of consumers than southern/market foods. The increase in obesity, diabetes and cardiovascular disease has been linked to a shift away from a country food diet and a less active lifestyle. These foods are an integral component of good health among Aboriginal peoples. The social, cultural, spiritual, nutritional and economic benefits of these foods must be considered in concert with the risks of exposure to environmental contaminants through their exposure. Consequently, the contamination of country food raises problems which go far beyond the usual confines of public health and cannot be resolved simply by risk-based health advisories or food substitutions alone. All decisions should involve the community and consider many aspects of socio-cultural stability to arrive at a decision that will be the most protective and least detrimental to the communities.

Environmental xenobiotics and nuclear receptors--interactions, effects and in vitro assessment

A group of intracellular nuclear receptors is a protein superfamily including arylhydrocarbon AhR, estrogen ER, androgen AR, thyroid TR and retinoid receptors RAR/RXR as well as molecules with unknown function known as orphan receptors. These proteins play an important role in a wide range of physiological as well as toxicological processes acting as transcription factors (ligand-dependent signalling macromolecules modulating expression of various genes in a positive or negative manner). A large number of environmental pollutants and other xenobiotics negatively affect signaling pathways, in which nuclear receptors are involved, and these modulations were related to important in vivo toxic effects such as immunosuppression, carcinogenesis, reproduction or developmental toxicity, and embryotoxicity. Presented review summarizes current knowledge on major nuclear receptors (AhR, ER, AR, RAR/RXR, TR) and their relationship to known in vivo toxic effects. Special attention is focused on priority organic environmental contaminants and experimental approaches for determination and studies of specific toxicity mechanisms.

The dioxin-like PCB 77 but not the ortho-substituted PCB 153 interferes with chicken embryo thyroid hormone homeostasis and delays hatching

The effects of the dioxin-like polychlorinated biphenyl (PCB) 77 and the ortho-substituted PCB 153 on thyroid hormone availability were investigated during the last week of embryonic development in chicken. High doses of these PCBs (1microg PCB 77 and 20microg PCB 153) were injected into chicken eggs at day 4 of incubation. Blood and tissue samples were collected from day 14 of incubation until 1 day after hatching. We did not observe influences of PCB 153 on thyroid hormone (TH) levels. Treatment with PCB 77, on the other hand, decreased plasma total T(4) concentrations but increased hepatic T(4) levels at day 14 of incubation. Later in development, at stages near the process of hatching, severe decreases of T(4) and T(3) levels were observed in the PCB 77 group, both in plasma and tissues. PCB 77 severely reduced the TH peak that normally coincides with the stage of internal pipping. This reduction was accompanied by a considerable delay in the moment of hatching. We conclude that the dioxin-like PCB 77, but not the ortho-substituted PCB 153, can decrease TH availability towards the end of embryonic development and hence disturb the process of hatching.

Thyroid hormone-like and estrogenic activity of hydroxylated PCBs in cell culture

The thyroid hormone-disrupting activity of hydroxylated PCBs was examined. 4-Hydroxy-2,2',3,4',5,5'-hexachlorobiphenyl (4-OH-2,2',3,4',5,5'-HxCB), 4-hydroxy-3,3',4',5-tetrachlorobiphenyl (4-OH-3,3',4',5-TCB) and 4,4'-dihydroxy-3,3',5,5'-tetrachlorobiphenyl (4,4'-diOH-3,3',5,5'-TCB), which have been detected as metabolites of PCBs in animals and humans, and six other 4-hydroxylated PCBs markedly inhibited the binding of triiodothyronine (1x10(-10)M) to thyroid hormone receptor (TR) in the concentration range of 1 x 10(-6) to 1 x 10(-4) M. However, 4-hydroxy-2',4',6'-trichlorobiphenyl (4-OH-2',4',6'-TCB), 3-hydroxy-2,2',5,5'-tetrachlorobiphenyl, 4-hydroxy-2,2',5,5'-tetrachlorobiphenyl, 4-hydroxy-2,3,3',4'-tetrachlorobiphenyl, 2,3',5,5'-tetrachlorobiphenyl and 2,3',4',5,5'-pentachlorodiphenyl did not show affinity for TR. The thyroid hormonal activity of PCBs was also examined using rat pituitary cell line GH3 cells, which grow and release growth hormone in a thyroid hormone-dependent manner. 4-OH-2,2',3,4',5,5'-HxCB, 4,4'-diOH-3,3',5,5'-TCB and 4-OH-3,3',4',5-TCB enhanced the proliferation of GH3 cells and stimulated their production of growth hormone in the concentration range of 1 x 10(-7) to 1 x 10(-4) M, while PCBs which had no affinity for thyroid hormone receptor were inactive. In contrast, only 4-OH-2',4',6'-TCB exhibited a significant estrogenic activity using estrogen-responsive reporter assay in MCF-7 cells. However, the 3,5-dichloro substitution of 4-hydroxylated PCBs markedly decreased the estrogenic activity. These results suggest that, at least for the 17 PCB congeners and hydroxylated metabolites tested, a 4-hydroxyl group in PCBs is essential for thyroid hormonal and estrogenic activities, and that 3,5-dichloro substitution favors thyroid hormonal activity, but not estrogenic activity.

The role of type I and type II 5′ deiodinases on hexachlorobenzene-induced alteration of the hormonal thyroid status

Treatment of male Wistar rats with hexachlorobenzene (HCB) (1000 mg/kg b.w.) for 3-30 days decreases circulating levels of thyroxine (T4) but does not affect triiodothyronine (T3). Time courses were determined for 5' deiodinase type I (5' D-I) activity in thyroid, liver, and kidney and 5' deiodinase type II (5' D-II) activity in brown adipose tissue (BAT) to test the possibility that increased deiodinase activity might contribute to the maintenance of the serum T3 level. Specific 5' D-I activity was increased in the thyroid at 21 days and thereafter. No significant changes were observed in the liver, however, total 5' D-I activity in this tissue was increased at 30 days of treatment as a consequence of liver weight enhancement. HCB decreased kidney 5' D-I activity after 15 days, and BAT 5' D-II activity after 21 days of treatment. Total body 5' D-I activity was significantly increased by 30 days of HCB-treatment. HCB increased the activity of hepatic T4 uridine diphosphoglucuronosyl transferase (UDPGT) in a time-dependent manner, without changes in T3 UDPGT. We propose that increased T4 to T3 conversion in the thyroid and in the greatly enlarged liver may account for the maintenance of serum T3 concentration in hypothyroxinemic HCB-treated rats.

Background exposure to PCDDs/PCDFs/PCBs and its potential health effects: a review of epidemiologic studies

Here we review epidemiologic studies dealing with the dietary intake and the body burden of polychlorinated dibenzo-p-dioxins (PCDDs)/polychlorinated dibenzo-furans (PCDFs)/ polychlorinated biphenyls (PCBs) in the general population, and potential adverse health effects of these substances, especially on the risk of diabetes mellitus and endometriosis, and on thyroid function and the neurodevelopment of infants. The mean or median intake of dioxin-related compounds among the general populations of various countries is lower than the maximum tolerable daily intake (TDI) set by the WHO in 1998 (4pg TEQ/kg/day). However, there have been few reports on the distribution of intake and the proportion of subjects whose exposure levels exceed the maximum TDL. At present, it remains unclear whether background exposure to dioxin-related compounds is associated with increased risk of diabetes (because of lack of longitudinal studies), endometriosis (because of lack of studies with sufficient statistical power), or altered thyroid function (because of inconsistent results on humans). Consistent results have been reported for the association between exposure to background levels of PCBs/dioxins, especially trans-placental PCBs, and defective neurodevelopment of infants in the U.S. and Europe. Thus, efforts should be made to further decrease the body burden among women of reproductive age by reducing the release of PCDDs/PCDFs/PCBs into the environment.

Importance of the kidneys in metabolism of vitamins A1 and A2 and their fatty acyl esters in mink feeding on fish-based diets and exposed to Aroclor 1242

Two-month-old female mink (Mustela vison) were fed diets based on freshwater smelt (Osmerus eperlanus), Baltic herring (Clupea harengus membras), or North-Atlantic herring (Clupea harengus harengus) for 21 weeks. Half of the smelt-fed mink were exposed to the commercial polychlorinated biphenyl (PCB) preparation Aroclor 1242 (1 mg/day) in the feed. Retinol (vitamin A(1)), 3,4-didehydroretinol (vitamin A(2)), their fatty acyl esters, and vitamin E were studied in the kidneys by reversed-phase high-performance liquid chromatography. Exposure to Aroclor 1242 resulted a significant decrease in the alcoholic and esterified vitamin A(2). Levels of the A(1) analogs did not change due to the PCBs and were the same in mink fed either smelt or Baltic herring. In mink fed very fatty Atlantic herring, the renal levels of vitamin A(1) and E were significantly lower than in the other mink and apparently consumed by lipid peroxidation. The vitamins were located mainly in the cortex, and the fatty acyl esters showed a fatty acid composition that differed from those in liver and plasma. In the kidneys of the smelt-fed mink (control or Aroclor-exposed) the ratio of vitamin A(2) to A(1) was much lower than the ratios in the liver or plasma, suggesting inefficient uptake of A(2) in the kidneys. In the PCB-exposed mink, in which vitamin losses are increased, tissue levels of vitamin A(2) may be more difficult to maintain than levels of vitamin A(1).

Increased [3H]phorbol ester binding in rat cerebellar granule cells and inhibition of 45Ca(2+) buffering in rat cerebellum by hydroxylated polychlorinated biphenyls

Our previous structure-activity relationship (SAR) studies indicated that the effects of polychlorinated biphenyls (PCBs) on neuronal Ca(2+) homeostasis and protein kinase C (PKC) translocation were associated with the extent of coplanarity. Chlorine substitutions at ortho position on the biphenyl, which increase the non-coplanarity, are characteristic of the most active congeners in vitro. In the present study, we investigated the effects of selected hydroxylated PCBs, which are major PCB metabolites identified in mammals, on the same measures where PCBs had differential effects based on structural configuration. These measures include PKC translocation as determined by [3H]phorbol ester ([3H]PDBu) binding in cerebellar granule cells, and Ca(2+) sequestration as determined by 45Ca(2+) uptake by microsomes isolated from adult rat cerebellum. All the selected hydroxy-PCBs with ortho-chlorine substitutions increased [3H]PDBu binding in a concentration-dependent manner and the order of potency as determined by E(50) (concentration that increases control activity by 50%) is 2',4',6'-trichloro-4-biphenylol (32 +/- 4 microM), 2',5'-dichloro-4-biphenylol (70 +/- 9 microM), 2,2',4',5,5'-pentachloro-4-biphenylol (80 +/- 7 microM) and 2,2',5'-trichloro-4-biphenylol (93 +/- 14 microM). All the selected hydroxy-PCBs inhibited microsomal 45Ca(2+) uptake to a different extent. Among the hydroxy-PCBs selected, 2',4',6'-trichloro-4-biphenylol is the most active in increasing [3H]PDBu binding as well as inhibiting microsomal 45Ca(2+) uptake. 3,5-Dichloro-4-biphenylol and 3,4',5-trichloro-4-biphenylol did not increase [3H]PDBu binding, but inhibited microsomal 45Ca(2+) uptake. This effect was not related to ionization of these two hydroxy-PCBs. Hydroxylated PCBs seemed to be as active as parent PCBs in vitro. These studies indicate that PCB metabolites such as hydroxy-PCBs might contribute significantly to the neurotoxic responses of PCBs.

Polychlorinated biphenyl (PCB) exposure in relation to thyroid hormone levels in neonates

Polychlorinated biphenyls (PCBs) are industrially produced environmentally persistent compounds. In developed countries all humans have detectable levels in blood and other tissues. PCBs alter thyroid hormone metabolism in animal experiments, and human data suggest background-level exposure may have similar effects in neonates. We evaluated this possible effect among 160 North Carolina children whose in utero PCB exposure was estimated on the basis of the mother's PCB levels in milk and blood, in 1978-1982 (estimated median PCB level in milk at birth, 1.8 mg/kg lipid). Their umbilical cord sera were thawed in 1998 and assayed for total thyroxine, free thyroxine, and thyroid stimulating hormone. We found that PCB exposure was not strongly related to any of the thyroid measures. For example, for a one unit change in milk PCB concentration (mg/kg lipid), the associated multivariate-adjusted increase in thyroid stimulating hormone level was 7% (95% confidence limits (CL) = -6, 21). Despite the possibility of sample degradation, these data suggest that within the range of background-level exposure in the United States, in utero PCB exposure is only slightly related to serum concentration of total thyroxine, free thyroxine, and thyroid stimulating hormone at birth.

Application of biomarkers for exposure and effect of polyhalogenated aromatic hydrocarbons in naturally exposed European otters (Lutra lutra)

In the serious decline of European otters (Lutra lutra) over the last decades, polychlorinated biphenyls (PCBs) are considered to be one of the major factors. As no experiments can be conducted with otters, an eco-epidemiological study was performed to derive no observed effect concentrations (NOECs) for PCBs in the otter. A strong negative correlation was found between hepatic vitamin A and polychlorinated biphenyl (PCB) concentrations expressed as TCDD-equivalents (TEQs), coinciding with a higher incidence of infectious diseases. The no-effect concentration for vitamin A reduction was 2 ng TEQ/g lipid, 10-fold reduction was already found in animals with 5 ng TEQ/g lipid. The TEQ-levels measured with a reporter gene assay based on chemical-activated luciferase expression (the CALUX assay) correlated well with the TEQ levels calculated based on non- and mono-ortho PCB concentrations. The TEQ levels in blood and liver correlated well when expressed on a lipid basis. In living captive otters blood plasma TEQ levels (either measured based on gas chromatography (GC) or CALUX measurement) were lower than in the feral otters, and positively correlated with plasma total and free thyroid hormone but not with plasma retinol levels. Hepatic vitamin A concentration was found to be a physiologically relevant effect parameter. The NOEC for hepatic vitamin A reduction was translated into TEQ levels in fish and sediment. The CALUX response in 50-500 μl blood plasma proved to be a sensitive non-destructive biomarker for quantification of internal TEQ levels.

Interactions of persistent environmental organohalogens with the thyroid hormone system: mechanisms and possible consequences for animal and human health

Several classes of environmental contaminants have been claimed or suggested to possess endocrine-disrupting potency, which may result in reproductive problems and developmental disorders. In this paper the focus is on the multiple and interactive mechanisms of interference of persistent polyhalogenated aromatic hydrocarbons (PHAHs) and their metabolites with the thyroid hormone system. Evidence suggests that pure congeners or mixtures of PHAHs directly interfere with the thyroid gland; with thyroid hormone metabolizing enzymes, such as uridine-diphosphate-glucuronyl transferases (UGTs), iodothyronine deiodinases (IDs), and sulfotransferases (SULTs) in liver and brain; and with the plasma transport system of thyroid hormones in experimental animals and their offspring. Changes in thyroid hormone levels in conjunction with high PHAH exposure was also observed in captive as well as free ranging wildlife species and in humans. Maternal exposure to PHAHs during pregnancy resulted in a considerable fetal transfer of hydroxylated PHAHs, which are known to compete with thyroxine (T4) for plasma transthyretin (TTR) binding sites, and thus may be transported to the fetus with those carrier proteins that normally mediate the delivery of T4 to the fetus. Concomitant changes in thyroid hormone concentrations in plasma and in brain tissue were observed in fetal and neonatal stages of development, when sufficient thyroid hormone levels are essential for normal brain development. Alterations in structural and functional neurochemical parameters, such as glial fibrillary acidic protein (GFAP), synaptophysin, calcineurin, and serotonergic neurotransmitters, were observed in the same offspring up to postnatal day 90. In addition, some changes in locomotor and cognitive indices of behavior were observed in rat offspring, following in utero and lactational exposure to PHAHs. Alterations in thyroid hormone levels and subtle changes in neurobehavioral performance were also observed in human infants exposed in utero and through lactation to relatively high levels of PHAHs. Overall these studies indicate that persistent PHAHs can disrupt the thyroid hormone system at a multitude of interaction sites, which may have a profound impact on normal brain development in experimental animals, wildlife species, and human infants.

Binding of a 3,3', 4,4'-tetrachlorobiphenyl (CB-77) metabolite to fetal transthyretin and effects on fetal thyroid hormone levels in mice

The present study was conducted in order to study the effect of the PCB congener 3,3', 4,4'-tetrachlorobiphenyl (CB-77) on fetal thyroxin homeostasis in the mouse, and to examine a possible underlying mechanism behind the effect. C57BL mice were treated with 14C-labelled or unlabelled CB-77 (1 or 10 mg/kg body wt.) on day 13 of gestation, and control animals were treated with corn oil. The experiment was terminated at 4 days after exposure. Maternal and fetal plasma and livers, and whole fetuses for homogenate preparation, were collected and analysed for total radioactivity, in vitro binding of 125I-thyroxin to plasma transthyretin (TTR; a thyroxin-transporting protein), and free and total thyroxin (FT4, TT4) levels. Maternal plasma, fetal plasma and homogenates were also analyzed for presence of CB-77 and metabolites. Results showed a dose-dependent uptake of radioactivity in plasma and liver, fetal plasma 14C-levels being about five-times higher in 10 mg/kg dosed animals as after 1 mg/kg. Fetal; plasma levels of total radioactivity were four- to nine-times above maternal levels and corresponded to only one compound, the metabolite 4-OH-3,3', 4',5-tetrachlorobiphenyl (4-OH-tCB). 4-OH-tCB was the major metabolite also in whole fetuses, with only small amounts of the parent compound (approximately 15% of the 4-OH-tCB) and traces (approximately 6%) of two other metabolites, 2-OH-3,3, 4,4'-tetrachlorobiphenyl and 5-OH-3,3', 4,4'-tetrachlorobiphenyl. Polyacrylamide gel electrophoresis confirmed that the 14C-radioactivity in fetal plasma was bound to TTR, and revealed that in vitro binding of 125I-T4 to fetal TTR was reduced to 50% of control values in treated animals (10 mg/kg body wt.). Fetal plasma FT4 and TT4 levels were significantly decreased (64 and 55% of control fetuses) after 10 mg/kg treatment. In conclusion, exposure of pregnant mice to CB-77 results in the accumulation of the metabolite 4-OH-tCB in fetal mouse plasma. The metabolite binds to TTR and is accompanied by a significant decrease in fetal plasma T4 levels. A causative correlation between TTR binding and effects on T4 levels is suggested.

Crystal structure of the transthyretin--retinoic-acid complex

Retinoids are quite insoluble and chemically unstable compounds in the aqueous medium, such that natural retinoids need to be bound to specific retinoid-binding proteins to be protected, solubilized and transported in body fluids. All-trans retinoic acid exhibits a relatively high affinity for thyroxine-binding transthyretin in vitro and this protein is a good candidate for the transport of retinoic acid administered as pharmacological or antitumor agent. To define structural features essential for the recognition by transthyretin of a ligand which is structurally unrelated to thyroxine, we have cocrystallized human transthyretin with retinoic acid and determined its structure at 0.18-nm resolution. The retinoid fits into the two chemically identical thyroxine-binding sites, which are located in the central channel that runs through the tetrameric transthyretin. The cyclohexene ring of the bound retinoid is innermost, occupying the same position of the phenolic ring of the bound 3,3'-diiodo-L-thyronine, whereas the carboxylate group, like the same group of the thyroid hormone, participates in an ionic interaction with the Lys15 side chain at the entrance of the channel. Despite the fact that transthyretin was cocrystallized with all-trans-retinoic acid, the isoprene chain of the bound retinoid has been found in a non-extended conformation. This feature, that allows the carboxylate to orient in a manner suitable for ion-pair association with the Lys15 side chain, is attributable to the conversion of all-trans-retinoic acid into cis-isomers or folded conformers. It is concluded that the presence, in an essentially hydrophobic molecular core of the appropriate size, of a negatively charged group at the correct position is a crucial requirement for ligand-transthyretin recognition. Whereas the binding of the ligand has no remarkable consequences for the protein structure, all-trans-retinoic acid undergoes structural changes such as to interact favorably with residues present in the thyroxine-binding sites, resembling roughly the natural ligand.