Endocrine disrupting chemicals: interference of thyroid hormone binding to transthyretins and to thyroid hormone receptors

Serum thyroid hormone concentrations and thyroid histomorphology as biomarkers in bowhead whales (Balaena mysticetus)

Serum thyroid hormone (TH) concentrations have been used alone or with other measurements to assess health status or effects of toxicant exposure in marine mammals. Histological sections from thyroid glands of the bowhead whale ( Balaena mysticetus L., 1758) were examined in conjunction with serological TH analyses. Serum was assayed for total and free triiodothyronine and total and free thyroxine via radioimmunoassay. Histomorphology of thyroid tissue was assessed by light microscopy and the utilization of an epithelial-follicular index (EFI). Age, sex, or season did not significantly affect serum TH levels. However, TH concentrations in pregnant or lactating females were found to be significantly lower than in the other sex and reproductive groups investigated. The EFI and epithelial height (EH) were greater in spring subadult and adult whales compared with those that were landed in the fall. No correlation was found between serum TH concentrations and serum, blubber, or liver levels of select polychlorinated biphenyl metabolites and organochlorine congeners examined. Low variability in concentrations of the serum THs across age, season, and sex and reproductive groups supports the existence of strong homeostatic mechanisms for maintaining TH concentrations in these presumably healthy animals. Departures from these ranges may indicate a disturbance in these regulatory mechanisms and may be a useful indication of toxicity or other health disorders.

Thyroid hormone receptor isoform selectivity of thyroid hormone disrupting compounds quantified with an in vitro reporter gene assay

Some compounds, including brominated diphenyl ethers (BDEs), can interfere with thyroid hormone (TH) receptor (TR)-mediated TH-signalling. In this study, the TR isoform selectivity of some TH disrupting compounds was investigated with TRα/β specific reporter gene assays. For this purpose, the effects of compounds on 3,3',5-triiodothyronine (T(3))-induced TRα- or TRβ-activation were tested in green monkey kidney fibroblast (CV-1) cells transiently transfected with Xenopus TRs and a luciferase reporter gene. The T(3)-like BDE-OH and diiodobiphenyl (DIB) increased T(3)-induced TRα-activation, but not T(3)-induced TRβ-activation. BDE28 (100nM) did not act via TRα, but almost tripled T(3)-induced TRβ-activation relative to T(3) at its EC(50). BDE206 (100nM) was antagonistic on both TRs with a maximum repression -54% relative to T(3) at its EC(50). Contrary to previous results obtained with the T-screen, HBCD was inactive. The present study illustrates the importance of testing potential TH disrupting compounds in model systems that enable independent characterization of effects on both T(3)-induced TRs.

Endocrine disrupters with (anti)estrogenic and (anti)androgenic modes of action affecting reproductive biology of Xenopus laevis: I. Effects on sex steroid levels and biomarker expression

Adult Xenopus laevis were exposed in vivo to ethinylestradiol, tamoxifen, methyldihydrotestosterone and flutamide as (anti)estrogenic and (anti)androgenic compounds, respectively, for four weeks at a concentration of 10(-8) M and to Lambro river water, a polluted river from Italy. Effects of the treatments were analysed by mRNA expression of retinol-binding protein (RBP), transferrin (TF), transthyretin (TTR) and vitellogenin (VTG) in the liver of male and female X. laevis, to analyse the potential of these genes to detect endocrine disrupting compounds (EDC) with different modes of action. In addition, plasma VTG and sex steroid levels, estradiol-17beta (E(2)) and testosterone (T), were analysed. Sex steroids were depressed by ethinylestradiol in both sexes whereas tamoxifen increased E(2) in females. The induction of VTG protein plasma levels was more pronounced at the protein level compared to hepatic VTG mRNA expression in response to estrogenic treatment but VTG mRNA expression detected both, estrogenic and antiestrogenic EDC. The mRNA expression of TF was decreased by estrogenic and increased by antiestrogenic treatment while TTR mRNA expression was down-regulated and RBP mRNA up-regulated by estrogenic exposure. The other treatments did not affect the mRNA expression of the examined genes.

Cell and Molecular Biology of Transthyretin and Thyroid Hormones

Advances in four areas of transthyretin (TTR) research result in this being a timely review. Developmental studies have revealed that TTR is synthesized in all classes of vertebrates during development. This leads to a new hypothesis on selection pressure for hepatic TTR synthesis during development only, changing the previous hypotheses from "onset" of hepatic TTR synthesis in adulthood to "maintaining" hepatic TTR synthesis into adulthood. Evolutionary studies have revealed the existence of TTR-like proteins (TLPs) in nonvertebrate species and elucidated some of their functions. Consequently, TTR is an excellent model for the study of the evolution of protein structure, function, and localization. Studies of human diseases have demonstrated that TTR in the cerebrospinal fluid can form amyloid, but more recently there has been recognition of the roles of TTR in depression and Alzheimer's disease. Furthermore, amyloid mutations in human TTR that are the normal residues in other species result in cardiac deposition of TTR amyloid in humans. Finally, a revised model for TTR-thyroxine entry into the cerebrospinal fluid via the choroid plexus, based on data from studies in TTR null mice, is presented. This review concentrates on TTR and its thyroid hormone binding, in development and during evolution, and summarizes what is currently known about TLPs and the role of TTR in diseases affecting the brain.

Polychlorinated biphenyls and their hydroxylated metabolites (OH-PCBS) in pregnant women from eastern Slovakia

OBJECTIVE

Our aim in the present study was to characterize and quantify the levels of polychlorinated biphenyls (PCBs) and specific polychlorobiphenylol (OH-PCB) metabolites in maternal sera from women delivering in eastern Slovakia.

DESIGN

During 2002-2004, blood samples were collected from women delivering in two Slovak locations: Michalovce district, where PCBs were formerly manufactured, and Svidnik and Stropkov districts, about 70 km north.

PARTICIPANTS

A total of 762 and 341 pregnant women were sampled from Michalovce and Svidnik/Stropkov, respectively, and OH-PCBs were measured in 131 and 31. EVALUATION/MEASUREMENTS: We analyzed PCBs using gas chromatography (GC)/electron capture detection. OH-PCBs and pentachlorophenol (PCP) were determined as methyl derivatives using GC-electron capture negative ionization/mass spectrometry. We characterized distributions in the full cohort using inverse sampling weights.

RESULTS

The concentrations of both PCBs and OH-PCB metabolites of Michalovce mothers were about two times higher than those of the Svidnik/Stropkov mothers (p < 0.001). The median weighted maternal serum levels of the sum of PCBs (sigmaPCBs) were 5.73 ng/g wet weight (Michalovce) and 2.82 ng/g wet weight (Svidnik/Stropkov). The median sum of OH-PCBs (ZOH-PCBs) was 0.55 ng/g wet weight in Michalovce mothers and 0.32 ng/g wet weight in Svidnik/Stropkov mothers. 4-OH-2,2',3,4',5,5',6-Heptachlorobiphenyl (4-OH-CB187) was a primary metabolite, followed by 4-OH-2,2',3,4',5,5'-hexachlorobiphenyl (4-OH-CB146). Only four PCB congeners-CBs 153, 138, 180, and 170--had higher concentrations than 4-OH-CB187 and 4-OH-CB146 (p < 0.001). The median ratio of the sigmaOH-PCBs to the sigmaPCBs was 0.10.

CONCLUSIONS

Mothers residing in eastern Slovakia are still highly exposed to PCBs, and their body burdens of these pollutants and OH-PCB metabolites may pose a risk for adverse effects on health for themselves and their children.

Thyroid system-disrupting chemicals: interference with thyroid hormone binding to plasma proteins and the cellular thyroid hormone signaling pathway

In vertebrates, thyroid hormones are essential for post-embryonic development, such as establishing the central nervous system in mammals and metamorphosis in amphibians. The present paper summarizes the possible extra-thyroidal processes that environmental chemicals are known to or suspected to target in the thyroid hormone-signaling pathway. We describe how such chemicals interfere with thyroid-hormone-binding protein functions in plasma, thyroid-hormone-uptake system, thyroid-hormone-metabolizing enzymes, and activation or suppression of thyroid-hormone-responsive genes through thyroid-hormone receptors in mammals and amphibian tadpoles. Several organohalogens affect different aspects of the extra-thyroidal thyroid-hormone-signaling pathway but hardly affect thyroid hormone binding to receptors. Rodents and amphibian tadpoles are most sensitive to the effects of environmental chemicals during specific thyroid-hormone-related developmental windows. Possible mechanisms by which environmental chemicals exert multipotent activities beyond one hormone-signaling pathway are discussed.

In vitro and in vivo analysis of the thyroid system-disrupting activities of brominated phenolic and phenol compounds in Xenopus laevis

We investigated the effects of the brominated phenolic and phenol compounds, some of which are brominated flame retardants, on the binding of (125)I-3,3',5-L-triiodothyronine ((125)I-T(3)) to purified Xenopus laevis transthyretin (xTTR) and to the ligand-binding domain of X. laevis thyroid hormone receptor beta (xTR LBD), on the induction of a T(3)-responsive reporter gene in a recombinant X. laevis cell line (XL58-TRE-Luc) and on T(3)-induced or spontaneous metamorphosis in X. laevis tadpoles. Of the brominated phenolic and phenol compounds tested, 3,3',5-tribromobisphenol A and 3,3'-dibromobisphenol A were the most potent competitors of (125)I-T(3) binding to xTTR and the xTR LBD, respectively. Structures with a bromine in either ortho positions with respect to the hydroxy group competed more efficiently with T(3) binding to xTTR and the xTR LBD. 3,3',5-Tribromobisphenol A and 3,3',5,5'-tetrabromobisphenol A, at 0.1-1.0 microM, exerted both T(3) agonist and antagonist activities in the T(3)-responsive reporter gene assay. Sera obtained from fetal bovine and bullfrog tadpoles weakened the T(3) agonist and antagonist activities of 3,3',5-tribromobisphenol A, but not the T(3) antagonist activity of o-t-butylphenol, for which xTTR has no significant affinity. The T(3) agonist and antagonist activities of 0.5 microM 3,3',5-tribromobisphenol A were confirmed in the in vivo, short-term gene expression assay in premetamorphic X. laevis tadpoles using endogenous, T(3)-responsive genes as molecular markers. Our results suggest that 3,3',5-tribromobisphenol A affects T(3) binding to xTTR and xTR and that it interferes with the intracellular T(3) signaling pathway.

Detection of thyroid system-disrupting chemicals using in vitro and in vivo screening assays in Xenopus laevis

We developed a thyroid hormone (TH) inducible primary screening assay for the identification and assessment of man-made chemicals that interfere with the TH-signalling pathway within target cells. The assay was developed in a Xenopus laevis cell line that was transduced with a self-inactivating (SIN) lentivirus vector (LV) containing a luciferase gene. The luciferase activation in this cell line was TH-specific: 3,3',5-L-triiodothyronine (T(3)) > 3,3'5-L-triiodothyroacetic acid (Triac) > 3,3',5-D-triiodothyronine (D-T(3)), > L-thyroxine (T(4)) > 3,3',5'-L-triiodothyronine (rT(3)). The application of the ligand-dependent luciferase assay for screening for thyroid system-disrupting chemicals revealed that three phthalates (dicyclohexyl phthalate, n-butylbenzyl phthalate, and di-n-butyl phthalate), two herbicides (ioxynil and pentachlorophenol) and a miticide (dicofol) had 3,3',5-L-triiodothyronine- T(3)- antagonist activity at concentrations ranging from 10(-6) to 10(-5) M. These chemicals also inhibited the expression of the endogenous primary T(3)-response TH nuclear receptor beta (TRbeta) gene. The inhibitory characteristics of these chemicals were similar for both assays performed, although the assay for T(3)-dependent activation of TRbeta gene was more sensitive than the luciferase assay. These results indicate that the luciferase assay was a rapid method with a small intra-assay variation for the primary screening of thyroid system-disrupting chemicals. Of the six chemicals, only n-butylbenzyl phthalate and pentachlorophenol exhibited T(3)-antagonist activity in an in vivo metamorphosis-based assay. It should be noted that chemicals elicited thyroid system-disrupting activity in the luciferase assay did not always interfere with the thyroid system in vivo.

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.

Plasma sex steroid and thyroid hormones profile in male water frogs of the Rana esculenta complex from agricultural and pristine areas

Some chemical compounds used in intensive agriculture have been found to induce estrogenic effects; therefore a histological analysis of the testes and an evaluation of plasma levels of sex steroid, thyroid hormones, and vitellogenin were carried out in adult male water frogs of two coexisting taxa (Rana lessonae and the hemiclonal hybrid Rana esculenta) sampled in agricultural and pristine areas. Differences in seasonal profiles of hormones were found in water frogs living in the agricultural area where the presence of endocrine disrupting compounds was suspected on the basis of a previous study. In R. esculenta, sampled in the pristine area, high androgen levels were found in May; the opposite trend was found for R. esculenta sampled in agricultural areas in which the highest androgen levels were found in September, significantly lower compared with those found in R. esculenta sampled in the pristine area. Low androgen levels were also recorded in R. lessonae males sampled both in pristine and agricultural areas, while the highest levels were found in September. Regarding the trend of estradiol-17beta, an increase of this hormone was found in July both in esculenta and lessonae sampled in the agricultural area, and in the same month an estradiol-17beta peak, even though lower, was also found both in esculenta and lessonae males captured in the pristine area; detectable vitellogenin was found neither in males captured in the agricultural area, nor in those sampled in the pristine one. Moreover, while no significant changes of thyroid hormones were found either in the esculenta or lessonae males sampled in the pristine area, increased T3 and T4 titers were found in July in both esculenta and lessonae captured in the agricultural area. Morphological differences of the testes in males of parental species captured in the agricultural area were also observed. These findings indicate alterations in endocrine and reproductive function in frogs in the agricultural area, that could suggest the presence of endocrine disrupting compounds.

High-Performance Frontal Analysis of the Binding of Thyroxine Enantiomers to Human Serum Albumin Binding of Thyroxine Enantiomers to Human Serum Albumin Kimura

PURPOSE

The aim of this study was to characterize the binding property between thyroxine and human serum albumin (HSA) qualitatively and enantioselectively using high-performance frontal analysis (HPFA).

METHODS

An on-line HPLC system consisting of an HPFA column, an extraction column, and an analytical HPLC column was developed to be used to determine the unbound concentrations of thyroxine enantiomers.

RESULTS

Both enantiomers were bound to human serum albumin at two high-affinity sites with similar affinities. The binding constant (K) and the number of binding sites on an HSA molecule (n) evaluated from Scatchard plot analysis were K = 1.01 x 10(6)m(-1) and n = 1.90 for L: -thyroxine, and K = 9.71 x 10(5) m(-1) and n = 1.97 for D: -thyroxine. The binding sites were identified using phenylbutazone and diazepam as site-specific probes for sites I and II, respectively, and each enantiomer was found to bind to both sites. Incorporation of a chiral HPLC column into the on-line system permitted the investigation of enantiomer-enantiomer interactions, which revealed that both enantiomers competitively bind to the same binding sites without significant allosteric effects.

Evaluation of mechanisms inducing thyroid toxicity and the ability of the enhanced OECD Test Guideline 407 to detect these changes

The OECD has developed an "enhanced Test Guideline 407" (TG 407) protocol for detecting endocrine effects during the course of a 28-day testing scheme. This protocol has gone through a validation process with (anti)estrogenic and (anti)androgenic compounds and substances that affect the thyroid (thyroxine and propylthiouracil). This review investigates whether a 28-day testing scheme would show up alterations in the thyroid-related parameters of the "enhanced TG 407" (T3, T4, TSH, thyroid weight and histopathology), irrespective of the mode of action. For each mode of action, a generally accepted reference chemical was selected and an in-depth literature survey was carried out, and the chemical was evaluated for treatment-related changes of thyroid-dependent parameters. The following model chemicals were selected: ion perchlorate, blockage of iodine uptake; propylthiouracil, inhibition of thyroid hormone synthesis; excess of iodine, blockage of thyroid hormone release; pyrazole, thyroid cytotoxicity; minocycline, thyroid pigmentation; amiodarone, inhibition of TSH synthesis; diethylstilbestrol, competition for thyroid hormone binding globulin; selenium-deficient diet, inhibition of thyroxine deiodination; FD&C Red No. 3, inhibition of peripheral 5'-deiodinase; cadmium, lipid peroxidation; phenobarbital, increase in thyroxine conjugation and biliary excretion; temelastine, thyroxine accumulation. Test data for treatments lasting approximately one month were available for most of these model chemicals, and these demonstrated the expected thyroid-related changes. Thus, it can be concluded that a 28-day testing scheme allows for the detection of thyroid-disrupting chemicals. The literature data also were evaluated according to whether preference can be given to any of the thyroid-related parameters (thyroid/pituitary hormones, thyroid weight and histopathology) with regard to dose-related sensitivities. Due to different study designs (such as treatment duration, application mode, dose selection and parameters used), no clear picture emerged. Therefore, consideration should be given to all of these parameters, which should also help to define the mode of action. Overall, this literature review provides support for the contention that the newly developed "enhanced TG 407" test protocol is well suited to the detection of chemicals that affect the thyroid gland.

Low-temperature arrest of the triiodothyronine-dependent transcription in Rana catesbeiana red blood cells

We examined possible molecular mechanisms for the low-temperature arrest of T3-induced Rana catesbeiana metamorphosis. Scatchard plots revealed that the ratios of maximum binding capacity/dissociation constant for high-affinity sites of tadpole serum proteins for T3 at 20 and 28 C was 3.3-4.6 times less than that at 4 C, due to the decrease in maximum binding capacity values. Kinetic studies of T3 uptake into tadpole red blood cells demonstrated that the ratio of maximum uptake rate/Michaelis constant at 23 C was approximately 13 times greater than that at 4 C. The process of intracellular transport of T3 into the nucleus was not arrested at 4 C. The ratio of T3 incorporated into nuclei to that taken up into red blood cells was not significantly different at 4, 20, and 28 C, indicating the absence of temperature-sensitive sites in this process. T3 binding to the T3 receptors alpha and beta were not temperature sensitive at least at 4 and 20 C. Transcription of the tr genes, early primary T3 response genes, was activated by 10 nM T3 at 20 and 28 C but was barely detected at 4 C. These results indicate that the major molecular event causing the low-temperature arrest of amphibian metamorphosis occurs after T3 entry into the nucleus but before or during the transcriptional activation of the tr genes. Plasma proteins binding T3 and the cellular thyroid hormone uptake system on the plasma membrane may contribute to the slowing of the incorporation of T3 into nucleus at 4 C by decreasing the uptake velocity of T3.

The Crystal Structure of Transthyretin in Complex with Diethylstilbestrol

Transthyretin (TTR) is a homotetrameric plasma protein that, in conditions not yet completely understood, may aggregate, forming the fibrillar material associated with TTR amyloidosis. A number of reported experiments indicate that dissociation of the TTR tetramer occurs prior to fibril formation, and therefore, studies aiming at the discovery of compounds that stabilize the protein quaternary structure, thereby acting as amyloid inhibitors, are being performed. The ability of diethylstilbestrol (DES) to act as a competitive inhibitor for the thyroid hormone binding to TTR indicated a possible stabilizing effect of DES upon binding. Here we report the crystallographic study of DES binding to TTR. The structural data reveal two different binding modes, both located in the thyroxine binding channel. In both cases, DES binds deeply in the channel and establishes interactions with the equivalent molecule present in the adjacent binding site. The most remarkable features of DES interaction with TTR are its hydrophobic interactions within the protein halogen binding pockets, where its ethyl groups are snugly fitted, and the hydrogen bonds established at the center of the tetramer with Ser-117. Experiments concerning amyloid formation in vitro suggest that DES is effectively an amyloid inhibitor in acid-mediated fibrillogenesis and may be used for the design of more powerful drugs. The present study gave us further insight in the molecular mechanism by which DES competes with thyroid hormone binding to TTR and highlights key interactions between DES and TTR that oppose amyloid formation.

Influence of a combination of two tetrachlorobiphenyl congeners (PCB 47; PCB 77) on thyroid status, choline acetyltransferase (ChAT) activity, and short- and long-term memory in 30-day-old Sprague-Dawley rats

The important role of thyroid hormones in growth and development, maintenance of body temperature, digestion, cardiac function, and normal brain development can be disrupted by environmental contaminants like polychlorinated biphenyls (PCB). Polychlorinated biphenyls are environmental contaminants that are widespread, persistent, lipophilic, and bioaccumulate through food webs, concentrating in adipose tissue. Placental and lactational PCB exposure of offspring causes metabolic and endocrine disruptions including hypothyroxinemia, spatial learning and memory deficits, neurochemical and neurobehavioral alterations, and reproductive problems. Previous studies in our lab using the individual congeners PCB 47 (2,2',4,4'-tetrachlorobiphenyl, ortho-substituted) and PCB 77 (3,3',4,4'-tetrachlorobiphenyl, non-ortho-substituted) have demonstrated alterations in thyroid hormone levels, alterations in brain choline acetyltransferase (ChAT) activity, and spatial learning deficits. In the present study, pregnant Sprague-Dawley rats were fed a diet with or without a mixture of PCB 47/77 at 1.25 ppm, 12.5 ppm or 25.0 ppm (w/w). Rat pups were swum in the Morris water maze four times a day on days 21-29 in order for the animals to learn the position of a submerged fixed platform. A probe test was run on day 24 (30 min after last swim) for short-term memory, and on day 29 (24 h after the last swim) for long-term memory after removal of the platform. Time spent in the quadrant previously containing the platform was recorded. Rats were decapitated on day 30, serum collected and frozen at -20 degrees. ChAT activity was measured radiometrically in basal forebrain and hippocampus. All PCB-treated animals experienced a depression in both triiodothyronine (T3) and thyroxine (T4). The present study found that all doses of PCB depressed ChAT activity in hippocampus with no significant alteration in the basal forebrain. In PCB-treated animals, short-term memory showed a trend toward improvement and long-term memory toward depression, but these trends were not significant. The consequences likely stem from endocrine disruption, especially with regard to the thyroid.

Competitive interactions of chlorinated phenol compounds with 3,3',5-triiodothyronine binding to transthyretin: detection of possible thyroid-disrupting chemicals in environmental waste water

Chlorinated phenol compounds, such as the chlorinated derivatives of bisphenol A, have been detected in effluents from paper manufacturing plants. We investigated the effects of bisphenol A, nonylphenol, and their seven chlorinated derivatives on 3,3',5-[(125)I]triiodothyronine ([(125)I]T(3)) binding to purified chicken and bullfrog transthyretin (cTTR and bTTR) and to the ligand-binding domains of chicken and bullfrog thyroid hormone receptor beta (cTR LBD and bTR LBD). The concentrations at which the chlorinated derivatives displaced [(125)I]T(3) from TTR were 10-10(3) times less than those of their parent molecules. 2,6-Dichloro-4-nonylphenol and 3,3',5-trichlorobisphenol A were the most potent competitors of T(3) binding to cTTR and to bTTR, respectively. The interactions of the chlorinated derivatives with the cTR and the bTR LBDs were weaker than those of the chlorinated derivatives with cTTR and bTTR. Chlorinated derivatives with a greater degree of chlorination were more efficient competitors of T(3) binding to TTR and TR. A structure-activity relationship between the phenol compounds and TTR (TTR assay) and TR (TR assay) was established. Structures with chlorine in either ortho position or both ortho positions, with respect to the hydroxy group, were more efficient competitors. Chemicals that interacted with bTTR, cTTR, and Japanese quail TTR were detected in water downstream of effluents from paper manufacturing plants using the TTR assay. As some of the chlorinated bisphenols and nonylphenols were potent competitors of T(3) binding to TTRs, the TTR assay could be applied as primary screening for possible thyroid-disrupting chemicals in environmental waste water.