Thyroid Hormone Axis Research Articles

Identification of genes induced by benzophenone-2 in human thyroid follicular FTC-238 cells

Benzophenone-2 (BP2) is used as a UV screen in many cosmetics of daily use, which regularly come in contact with the human skin. It is well known that BP2 is an endocrine-disrupting chemical (EDC) that disrupts the thyroid hormone axis via the inhibition of thyroid peroxidase. However, the molecular mechanism of BP2-induced cellular toxicity remains unclear. We undertook a genome-wide analysis of transcriptional responses of human thyroid follicular carcinoma FTC-238 cells to non-toxic doses of BP2 (1 and 50 μM). Whole human genome microarrays detected a total of 4,495 genes that exhibited altered expression after exposure to both non-toxic doses of BP2 compared with a vehicle control, DMSO. We found that 534 and 4,142 genes showed different expression levels following treatment with the low and high nontoxic doses of BP2, respectively. Thus, low doses of BP2 can induce various changes in gene expression. The BP2 responsive genes were involved in glycolysis, translation, apoptosis, cell proliferation, generation of precursor metabolites and energy, and microtubulebased processes. Commonly expressed genes by 1 μM and 50 μM of BP2 treatments were associated with glycolysis components, including GAPDH, ENO1, PGK1, TROVE2, LDHA, ALDOA, and PKM2. The downregulated GAPDH, and ENO1 are typical glycolytic transcripts in glycolysis. The disruption of glycolysis including nutrients-replete conditions influences on cell cycle arrest or apoptosis. The possible roles of the responsive genes and their related pathways in BP2-induced cellular responses are discussed. This study helps to understand how cells respond to BP2 exposure via transcriptional regulation.

Fate and Developmental Effects of Dietary Uptake of Methylmercury in Silurana tropicalis Tadpoles

Adverse effects of methylmercury (MeHg) exposure during amphibian metamorphosis remain to be fully characterized. Most previous investigations determined effects of short-term exposure to elevated dose rates, without information on mercury (Hg) depuration and degradation pathways. Since metamorphosis is primarily controlled by thyroid hormones (TH), alterations in this process suggest a disruption of the TH endocrine axis. The aim of this research was to (1) characterize patterns of MeHg accumulation and depuration in tadpoles and (2) examine effects of MeHg accumulation on metamorphosis and the TH axis. Silurana tropicalis tadpoles were exposed to environmental levels of dietary MeHg until metamorphic climax. Whole-body MeHg and total Hg (THg) levels were measured, as well as the number of metamorphs, rate of metamorphosis, body size, and whole-body triiodothyronine (T3) levels at metamorphosis. Tadpoles exposed to a higher level of MeHg exhibited increased mortality and size, and reduced metamorphosis. At lower levels of MeHg, body burdens increased rapidly and eventually reached a plateau, whereas no plateau was reached at a higher level of MeHg exposure. T3 levels were not affected. Data indicate that at low and medium levels of exposure, depuration of MeHg may prevent toxicity in tadpoles. However, depuration mechanisms may be insufficient at high doses, producing disruption of metamorphosis and death. Although there were no marked effects of MeHg on whole-body T3 levels, further investigation of other components of the TH axis is warranted.

Danthron Functions as a Retinoic X Receptor Antagonist by Stabilizing Tetramers of the Receptor

Retinoic X receptor (RXR) is a promising target for drug discovery against cancer and metabolic syndromes. Here, we identified a specific RXRα antagonist, danthron, from the traditional Chinese medicine rhubarb. Danthron repressed all tested RXRα-involved response element transcription, including the RXRE, PPRE, FXRE, and LXRE. Results from native PAGE and isothermal titration calorimetry (ITC)-based assays indicated that danthron bound to the tetrameric RXRα-LBD in a specific stoichimetric ratio, and such a binding could influence the corepressor SMRT affinity to the receptor. Additionally, a unique tetrameric structure of the apo-RXRα ligand-binding domain (LBD) was determined, which exhibited a larger tetramer interface and different ligand-binding pocket size compared with the one previously reported. Together with the biochemical and biophysical results, the determined crystal structure of danthron-soaked RXRα-LBD suggested a new mechanism for danthron antagonism to tetrameric RXRα. Moreover, the in vivo efficient improvement of insulin sensitivity by danthron was observed in diet-induced obese (DIO) mice. Thus, our findings were expected to supply new insights into the structural basis of RXRα antagonist for its further potential therapeutic application.

Use of Endocrine Hormones for Treating Depression

Disturbances in some endocrine hormones have been implicated in the pathophysiology of depression. Some of these hormones (and drugs that affect hormone function) have been used as therapeutic agents for the treatment of depression, especially adrenal, thyroid, and gonadal axis hormones. Open-label and controlled studies of various drugs that directly suppress or inhibit adrenal axis function have shown some benefit for the treatment of major depression, including treatment-resistant depression. Thyroid hormone augmentation is effective for nonresponders to antidepressant agents, although it has not been studied extensively. Estrogen may improve mild mood symptoms in perimenopausal women but may not be effective alone for major depression. Evidence of the antidepressant effects of testosterone in men is inconsistent, with mixed results from controlled studies. The adrenal steroid hormone dehydroepiandrosterone has an important role in mood regulation and may have significant antidepressant effects.

Stress in special groups

Stress occurs when homeostasis is threatened or perceived to be so. The stress system with its central and peripheral effectors regulate the brain's cognitive, reward and fear systems, wake-sleep centers as well as the growth, reproductive and thyroid hormone axes, and influence the gastrointestinal, cardiorespiratory, metabolic and immune systems. Malfunction of the stress system might impair growth, development, behavior and metabolism, which potentially lead to various acute and chronic disorders. The speech will be focused: a) On linguistics, where stress is the relative emphasis that may be given to certain syllables in a word and stress manifests itself in the speech stream are highly language dependent. b) Gender is an important biological determinant of vulnerability to psychosocial stress. Research studies focused on the sex differences in stress responses revealed individual differences in stress reactivity in addition to genetic, socio-cultural, hormonal and developmental factors. Women appear more physiologically reactive to social rejection challenges, but men react more to achievement challenges. Women's greater reactivity to rejection stress may contribute to the increased rates of affective disorders in women. Another study specifically indicates that women with the BDNF Val/Met genotype and men with the Val/Val may be particularly vulnerable to social stress mediated by brain stress system activity (Psychoneuroendocrinology, 2009). c) Poor self-concept, impatience, poor consequential thinking, inability to set realistic goals and to prioritize, poor resource management, poor health habits, over-dependence, inflexibility are some of personality-related stressors. d) Stress responses associated with possible implications for psychopathology during childhood, adolescents and older adults. Prenatal stress and maternal exposure to exogenous glucocorticoids can lead to permanent modification of hypothalamo-pituitary-adrenal function and stress-related behaviour. Childhood stress and trauma have been related to adult psychopathology in different psychiatric disorders. e) Some transitions and dynamics can lead families to excessive stress. There are many stresses in healthy and dysfunctional families. Research revealed that secure-base interactions between parents and adolescents facilitate physiological regulation of stress, especially for adolescents with externalising symptomatology. f) Finally, the effects of stress reactivity may lead to physical symptoms and disorders (e.g. hypertension, appetite and gastrointestinal problems, skeletal muscles increased tension, smooth muscles increased contraction, visceral obesity, hyperlipidemia, hyperglycemia, cardiovascular disease, type II diabetes, suppression of immune system). There is also a link between stress and fertility. Women with infertility report equivalent levels of anxiety and depression as women with cancer, HIV status, and heart disease. Men also suffer fertility related stress, though they frequently keep their anxiety to themselves for fear of adding to their partner's burden.

Melanoma onset after estrogen, thyroid, and growth hormone replacement therapy

Background: Acute sun exposure is the main risk factor for the development of melanoma, especially if associated with a large number of benign melanocytic nevi. Although epidemiologic studies have investigated the effects of exogenous triggers, particularly hormones, our understanding of melanoma is still inadequate. Objective: The aim of this study was to report a case of melanoma that developed after hormonal therapy. Case summary: We report a case of a 26-year-old white woman (weight, 48 kg; Fitzpatrick skin phototype IV; no previous pregnancy) who was referred to the Department of Dermatology, Casa Sollievo della Sofferenza Hospital-IRCCS, San Giovanni Rotondo, Italy, with a malignant melanoma on the left thigh. At the age of 18 years (year 2000), the patient presented with amenorrhea, but no therapy was initiated until 2004. At this time, insufficiency of the gonadal, thyroid, and growth hormone (GH) axes was diagnosed without evidence of hypothalamic-pituitary anatomic damage or of congenital or acquired causes. The patient had an inadequate level of GH (base: 0.8 g/mL; peak: 1.0 ng/mL) during an insulin tolerance test, low levels of thyroid hormones, and a blunted response of luteinizing hormone (base: 0.2 mIU/mL; peak: 10 mIU/mL) and follicle-stimulating hormone (base: 2.6 mIU/mL; peak: 18.5 mIU/mL) to a gonadotrophinreleasing hormone stimulation test. Consequently, replacement therapy with ethinyl estradiol (20 μg) plus progestin (75 μg) (once daily for 21 days/month), levothyroxine (25 μg once daily), and recombinant human GH (0.8 mg SC once daily) was initiated. GH replacement therapy was discontinued after 2 years (June 2006), and thyroid and estrogen replacement therapy were discontinued after 4 years (February 2008). The patient reported first noticing the pigmented lesion 8 months after GH withdrawal, during treatment with the estrogen/progestin combination. Conclusion: We report a case of melanoma onset in a patient who had received hormonal substitutive therapy, where the role of GH therapy alone or in combination with other hormones could not be ruled out.

Selenium and thyroid

Inadequate supply of the essential trace element selenium (Se) has been associated with predisposition for, or manifestation of, various human diseases such as Keshan and Kashin-Beck disease, cancer, impaired immune function, neurodegenerative and age-related disorders and disturbances of the thyroid hormone axis. Se deficiency in combination with inadequate iodine contributes to the pathogenesis of myxedematous cretinism. The recent identification of various distinct selenocysteine-containing proteins, encoded by 25 human genes, provides information on the molecular and biochemical basis of beneficial and possible adverse effects of this trace element. The thyroid gland is among the human tissues with the highest Se content per mass unit similar to other endocrine organs and the brain. Selenoproteins involved in cellular antioxidative defence systems and redox control, such as the glutathione peroxidase (GPx) and the thioredoxin reductase (TxnRd) family, are involved in protection of the thyroid gland from excess hydrogen peroxide and reactive oxygen species produced by the follicles for biosynthesis of thyroid hormones. In addition, the three key enzymes involved in activation and inactivation of thyroid hormones, the iodothyronine deiodinases (DIO1,2,3), are selenoproteins with development, cell- and pathology-related expression patterns. While nutritional Se supply is normally sufficient for adequate expression of functional Dio enzymes with exception of long-term parenteral nutrition and certain diseases impairing gastrointestinal absorption of Se compounds, the nutritional Se supply for the protection of the thyroid gland and synthesis of some more abundant selenoproteins of the GPx and the TrxR family might be limiting their proper expression under (patho-)physiological conditions.

TSH-Lowering Effect of Metformin in Type 2 Diabetic Patients

OBJECTIVETo assess the interplay between metformin treatment and thyroid function in type 2 diabetic patients.RESEARCH DESIGN AND METHODSThe acute and long-term effects of metformin on thyroid axis hormones were assessed in diabetic patients with primary hypothyroidism who were either untreated or treated with levothyroxine (L-T4), as well as in diabetic patients with normal thyroid function.RESULTSNo acute changes were found in 11 patients with treated hypothyroidism. After 1 year of metformin administration, a significant thyrotropin (TSH) decrease (P < 0.001) was observed in diabetic subjects with hypothyroidism who were either treated (n = 29; from 2.37 ± 1.17 to 1.41 ± 1.21 mIU/l) or untreated (n = 18; 4.5 ± 0.37 vs. 2.93 ± 1.48) with L-T4, but not in 54 euthyroid subjects. No significant change in free T4 (FT4) was observed in any group.CONCLUSIONSMetformin administration influences TSH without change of FT4 in patients with type 2 diabetes and concomitant hypothyroidism. The need for reevaluation of thyroid function in these patients within 6–12 months after starting metformin is indicated.

Stress and disorders of the stress system

All organisms must maintain a complex dynamic equilibrium, or homeostasis, which is constantly challenged by internal or external adverse forces termed stressors. Stress occurs when homeostasis is threatened or perceived to be so; homeostasis is re-established by various physiological and behavioral adaptive responses. Neuroendocrine hormones have major roles in the regulation of both basal homeostasis and responses to threats, and are involved in the pathogenesis of diseases characterized by dyshomeostasis or cacostasis. The stress response is mediated by the stress system, partly located in the central nervous system and partly in peripheral organs. The central, greatly interconnected effectors of this system include the hypothalamic hormones arginine vasopressin, corticotropin-releasing hormone and pro-opiomelanocortin-derived peptides, and the locus ceruleus and autonomic norepinephrine centers in the brainstem. Targets of these effectors include the executive and/or cognitive, reward and fear systems, the wake-sleep centers of the brain, the growth, reproductive and thyroid hormone axes, and the gastrointestinal, cardiorespiratory, metabolic, and immune systems. Optimal basal activity and responsiveness of the stress system is essential for a sense of well-being, successful performance of tasks, and appropriate social interactions. By contrast, excessive or inadequate basal activity and responsiveness of this system might impair development, growth and body composition, and lead to a host of behavioral and somatic pathological conditions.

Thyroid hormones and seasonal reproductive neuroendocrine interactions

Many animals that breed seasonally measure the day length (photoperiod) and use these measurements as predictive information to prepare themselves for annual breeding. For several decades, thyroid hormones have been known to be involved in this biological process; however, their precise roles remain unknown. Recent molecular analyses have revealed that local thyroid hormone activation in the hypothalamus plays a critical role in the regulation of the neuroendocrine axis involved in seasonal reproduction in both birds and mammals. Furthermore, functional genomics analyses have revealed a novel function of the hormone thyrotropin. This hormone plays a key role in signaling day-length changes to the brain and thus triggers seasonal breeding. This review aims to summarize the currently available knowledge on the interactions between elements of the thyroid hormone axis and the neuroendocrine system involved in seasonal reproduction.

Effects of the synthetic liver X receptor agonist T0901317 on the growth hormone and thyroid hormone axes in male rats

Liver X receptors (LXRs), activated by oxysterols, play an important role in the regulation of lipid and glucose metabolism, which is also markedly dependent on thyroid hormone and growth hormone (GH) status. Here, we investigated how a 1-week exposure to the synthetic LXR agonist T0901317 affected GH secretion and thyroid hormone status in male rats. While the pulse frequency of GH secretion was marginally affected there was a highly significant decrease in the triiodo-L-thyronine/thyroxine (T3/T4) ratio in plasma. This effect was associated with decreased expression of deiodinase 1 (DIO1) and 2 (DIO2) mRNA in the liver and thyroid gland, respectively. Expression of sterol regulatory element binding protein-1c (SREBP-1c), the hallmark of stimulated lipogenesis, was markedly increased in both thyroid and pituitary implying that protracted pharmacological LXR activation may promote lipid accumulation in these endocrine tissues. These findings suggest that attention must be given to pituitary hormone dependent axes when developing therapeutic strategies based on agonism of the LXRs, e.g. for treatment of atherosclerosis.

Environment and endocrinology: The case of thyroidology

Evidence is accumulating for interference of selected endocrine disrupting chemicals (EDC) with the thyroid axis. EDC disturb thyroid hormone (TH) homeostasis leading to developmental defects, hypothyroidism and altered thyroid growth patterns. A rising incidence of papillary thyroid carcinoma (PTC) in several Western countries cannot be definitely accounted for by improved diagnosis or management of thyroid cancer or improved iodine supply. In recent studies, we and others detected, within the thyroid hormone axis, multiple molecular targets of disruption by EDC, which are used in cosmetics, as pesticides or plasticizers or consumed as plant-derived compounds with the diet or with nutritional supplements. Several of these agents exert adverse effects on thyroid growth and function in animal or in vitro cellular models. Major targets are the sodium iodide symporter (NIS), the hemoprotein thyroperoxidase (TPO), the T4 distributor protein transthyretin (TTR), the deiodinases, TH conjugating enzymes and the TR thyroid hormone receptor family. Still prevailing iodine deficiency in many parts of the world predisposes the thyroid gland to adverse effects of endocrine disrupters especially under phases of vulnerability during development and under adaptive challenges during diseases.

Xenopus tropicalis as a Test System for Developmental and Reproductive Toxicity

The usefulness of Xenopus tropicalis as a model species to investigate endocrine disruption and developmental reproductive toxicity was assessed. In our test system tadpoles were exposed to test substances from shortly after hatching until metamorphosis, including the period of gonadal differentiation. Effects on the sex hormone and thyroid hormone axes were evidenced as skewed sex ratios, malformations of reproductive organs, altered cytochrome (CYP19) (aromatase) activity, and gene expression in gonads and brain, as well as changed thyroid histology and time to metamorphosis. Reproductive toxicity was evaluated at sexual maturity. Male-to-female sex reversal was implied at concentrations as low as 6 pM (1.8 ng/L) ethynylestradiol (EE2), which is comparable to EE2 levels observed in the environment. EE2-exposed males that were not sex reversed had significantly reduced fertility and a reduced amount of spermatozoa in testes compared with control males. This indicates that reproduction in wild frogs might be impaired by estrogenic environmental pollutants. Aromatase activity in brain and testes of adult frogs was not affected by larval EE2 exposure. Preliminary results indicate that exposure to the environmentally relevant pharmaceutical clotrimazole modulated aromatase activity in brain and gonads during sex differentiation, which warrants further investigation. The susceptibility to estrogen-induced sex reversal of X. tropicalis was comparable to that of other frog species and fish. Similarities between the reproductive effects in X. tropicalis and those reported in fish, birds, and mammals after developmental exposure to estrogens make X. tropicalis promising model for research on endocrine disruption and developmental reproductive toxicity.

Feeding and Metabolism in Mice Lacking Pituitary Adenylate Cyclase-Activating Polypeptide

Disruption of the pituitary adenylate cyclase-activating polypeptide (PACAP) gene in mice has demonstrated a role for this highly conserved neuropeptide in the regulation of metabolism and temperature control. Localization of PACAP neurons within hypothalamic nuclei that regulate appetite suggest PACAP may affect feeding and thus energy balance. We used PACAP-null mice to address this question, examining both food intake and energy expenditure. PACAP-null mice were leaner than wild-type littermates due to decreased adiposity and displayed increased insulin sensitivity. The lean phenotype in the PACAP-null mice was completely eliminated if animals were fed a high-fat diet or housed near thermoneutrality (28 C). Further metabolic analyses of PACAP-null mice housed at 21 C indicated that the reduced body weight could not be explained by decreased food intake, increased metabolic rate, or increased locomotor activity. The thyroid hormone axis of PACAP-null mice was affected, because mRNA levels of hypothalamic TRH and brown adipose tissue type 2 deiodinase were reduced in PACAP-null mice housed at room temperature, and brain deiodinase activity was lower in PACAP-null mice after an acute cold challenge compared with wild-type controls. These results demonstrate that PACAP is not required for the regulation of food intake yet is necessary to maintain normal energy homeostasis, likely playing a role in central cold-sensing mechanisms.

Interactions Between Growth Hormone and the Thyroid Gland - with Special Reference to Biochemical Diagnosis

The diagnosis of pituitary disorders is difficult because several hormone systems are involved. The clinical presentation is often vague and slowly progressing, and clinicians therefore have to rely very much on correct biochemical measurements. This is also associated with difficulties since the pituitary hormones interact, the binding proteins are influenced by the other axes and a variety of other effects, and finally the hormone measurements in serum are not totally adequate Several studies have investigated the interaction between the thyroid and growth hormone axes with very variable results. The present review is focussing on the aspects related to clinical decision making based on biochemical assessments. Because of the strong and sometimes unpredictable interrelations between the hypothalamo-pituitary thyroid and the hypothalamo-GH-IGF axes, and the many pitfalls in the measurement of peripheral hormones and interpretation of stimulation tests, clinicians and clinical biochemists should collaborate closely. Only then can the diagnostic accuracy and the management of patients with both growth hormone and thyroid disorders be improved.

Targeting thyroid hormone receptor-β agonists to the liver reduces cholesterol and triglycerides and improves the therapeutic index

Despite efforts spanning four decades, the therapeutic potential of thyroid hormone receptor (TR) agonists as lipid-lowering and anti-obesity agents remains largely unexplored in humans because of dose-limiting cardiac effects and effects on the thyroid hormone axis (THA), muscle metabolism, and bone turnover. TR agonists selective for the TRbeta isoform exhibit modest cardiac sparing in rodents and primates but are unable to lower lipids without inducing TRbeta-mediated suppression of the THA. Herein, we describe a cytochrome P450-activated prodrug of a phosphonate-containing TR agonist that exhibits increased TR activation in the liver relative to extrahepatic tissues and an improved therapeutic index. Pharmacokinetic studies in rats demonstrated that the prodrug (2R,4S)-4-(3-chlorophenyl)-2-[(3,5-dimethyl-4-(4'-hydroxy-3'-isopropylbenzyl)phenoxy)methyl]-2-oxido-[1,3,2]-dioxaphosphonane (MB07811) undergoes first-pass hepatic extraction and that cleavage of the prodrug generates the negatively charged TR agonist (3,5-dimethyl-4-(4'-hydroxy-3'-isopropylbenzyl)phenoxy)methylphosphonic acid (MB07344), which distributes poorly into most tissues and is rapidly eliminated in the bile. Enhanced liver targeting was further demonstrated by comparing the effects of MB07811 with 3,5,3'-triiodo-l-thyronine (T(3)) and a non-liver-targeted TR agonist, 3,5-dichloro-4-(4-hydroxy-3-isopropylphenoxy)phenylacetic acid (KB-141) on the expression of TR agonist-responsive genes in the liver and six extrahepatic tissues. The pharmacologic effects of liver targeting were evident in the normal rat, where MB07811 exhibited increased cardiac sparing, and in the diet-induced obese mouse, where, unlike KB-141, MB07811 reduced cholesterol and both serum and hepatic triglycerides at doses devoid of effects on body weight, glycemia, and the THA. These results indicate that targeting TR agonists to the liver has the potential to lower both cholesterol and triglyceride levels with an acceptable safety profile.

A synchronized amphibian metamorphosis assay as an improved tool to detect thyroid hormone disturbance by endocrine disruptors and apolar sediment extracts

Amphibian metamorphosis assays are used to evaluate potential effects of endocrine disrupting compounds on the thyroid hormone axis. In this study, Xenopus laevis tadpoles are kept in a solution of 0.2% thiourea (TU) to arrest and synchronise them in their development. The advantage of this synchronized amphibian metamorphosis assays is that synchronised tadpoles are available at any time to start metamorphosis experiments, and experimental groups are much more homogenous at the start of experimental exposure compared with groups selected from an untreated pool of animals. The water volume per animal was kept constant throughout the experimental period to overcome the influence of declining numbers of animals per aquarium due to metamorphosis and mortality on the density dependent development of the remaining tadpoles. Clophen A50 (a technical PCB mixture), the single congener 3,3′,4,4′-tetrachlorobiphenyl (PCB 77) and apolar sediment extracts that were previously tested positive in the T-Screen, an in vitro proliferation assay for thyroid hormone disruption, were tested in the Synchronized Amphibian Metamorphosis Assay. Endpoints studied were mortality, malformations, body weight, and percentage of metamorphosed froglets at the end of the 60-day experimental period, percentage of tadpoles in different developmental stages, and developmental stage-dependent awarded penalty points. Dietary exposure to Clophen A50 (0.2–50 mg/kg food) resulted in a significant increased percentage of tadpoles that did not pass metamorphosis at concentrations higher than 2 mg/kg food. Time until metamorphosis in those animals that were able to metamorphose after the 60-days experimental period was significantly decreased. Dietary exposure to PCB 77, a congener that can be readily metabolised, did not result in significant effects in any exposure group (2–500 μg/kg food). Apolar sediment extracts from two of the three sites that are contaminated with a wide variety of chemicals significantly decreased the percentage of metamorphosed animals and significantly increased the number of tadpoles that remained in early and late metamorphic stages. These effects already occurred when the extracts where diluted more than 1000 times (on an organic carbon base) compared to environmental concentrations. The rank of potency was comparable to results obtained with the T-screen. This suggests the presence of thyroid hormone disrupting compounds in the aquatic environment and possible effects of such compounds on animal development in the wild.

Introduction: Endocrine Disruptors—Exposure Assessment, Novel End Points, and Low-Dose and Mixture Effects

With the aim of discussing new research findings about chemicals able to interfere with the endocrine system, so-called endocrine disruptors, an international workshop was held in Prague, Czech Republic, 10–12 May 2005. The workshop was organized jointly by the EDEN project (Endocrine Disrupters: Exploring Novel Endpoints, Exposure, Low-Dose and Mixture-Effects in Humans, Aquatic Wildlife and Laboratory Animals; http://www.edenresearch.info) and the FIRE project (Risk Assessment of Brominated Flame Retardants as Suspected Endocrine Disrupters for Human and Wildlife Health; http://www.rivm.nl/fire), both large-scale consortia funded by the European Union (EU). The meeting was attended by more than 170 scientists from academia, industry, government agencies, and other organizations. EDEN and FIRE were part of the CREDO cluster (Cluster for Research on Endocrine Disruption), an umbrella organization of more than 60 EU-funded laboratories working on endocrine disruptors. Together with the COMPRENDO (Comparative Research on Endocrine Disruption) and the EURISKED projects (Multi-organic Risk Assessment of Selected Endocrine Disruptors; http://www.eurisked.org), EDEN and FIRE formed the core of the cluster. CREDO served as a platform for cooperation and exchange between its constituent projects. With the completion of almost all its member projects, CREDO has recently ceased to exist, but information about the cluster is still available on online (http://www.credocluster.info). The Prague workshop was the last in a series of meetings organized under the auspices of the CREDO cluster. The themes of previous workshops were risk assessment and the ecologic relevance of chemically induced endocrine disruption in wildlife (Jobling and Tyler 2006). This monograph contains papers presented at the Prague workshop and follows the session structure of the meeting, focusing on four main topics: “Indicators of Human and Wildlife Exposure to EDCs ” reports on new findings relating human exposure to endocrine disruptors in Europe to reproductive disorders, including cryptorchidism, hypospadias, and semen quality. New information about exposure trends for brominated flame retardants in freshwater and marine ecosystems is also given. “Novel models, end points, and biomarkers” summarizes research on the effects of endocrine disruptors on steroid-metabolizing enzymes and gives new data on alkylphenols as endocrine disruptors. Additional articles focus on the consequences of prolonged phytoestrogen exposure for reproductive organs and the outcomes of in utero exposure to endocrine disruptors for testicular dysgenesis, prostate development, and the thyroid hormone axis. “Low-dose effects of endocrine disruptors” gives a synthesis of recent observations in in vitro systems and highlights the importance of statistical power considerations in resolving the low-dose issue. “Mixture effects of endocrine disruptors and their assessment” contains reviews of combination effects, assesses the implications of low-level exposure to multiple chemicals and describes the joint effect of endocrine disruptors in fish and in a developmental toxicity rat model. Taken together, the articles presented in this monograph capture a great deal of the lively debates that took place during the Prague workshop and within the CREDO cluster as a whole. The workshop made considerable progress toward answering key questions that were unresolved a few years ago. It has also stimulated a consensus statement among scientists actively engaged in research in this field, the “Prague Declaration on Endocrine Disruption.” The Prague Declaration, also published as part of this monograph (available as Supplemental Material online at http://www.ehponline.org/docs/2007/10517/suppl.pdf), summarizes issues upon which the majority of scientists can agree, outlines research priorities for the next decade, and highlights steps that can be taken today to prevent health risks to humans and wildlife. It has been signed by more than 200 scientists from all over the world. All contributors to the workshop are thanked cordially. Without their enthusiasm and hard work the meeting and this monograph would not have happened.

Disruption of thyroid hormone binding to sea bream recombinant transthyretin by ioxinyl and polybrominated diphenyl ethers

A number of chemicals released into the environment share structural similarity to the thyroid hormones (THs), thyroxine (T 4) and triiodothyronine (T 3) and it is thought that they may interfere with the thyroid axis and behave as endocrine disruptors (EDs). One of the ways by which such environmental contaminants may disrupt the TH axis is by binding to TH transporter proteins. Transthyretin (TTR) is one of the thyroid hormone binding proteins responsible for TH transport in the blood. TTR forms a stable tetramer that binds both T 4 and T 3 and in fish it is principally synthesized in the liver but is also produced by the brain and intestine. In the present study, we investigate the ability of some chemicals arising from pharmaceutical, industrial or agricultural production and classified as EDs, to compete with [I 125]-T 3 for sea bream recombinant TTR (sbrTTR). Ioxinyl, a common herbicide and several polybrominated diphenyl ethers were strong inhibitors of [I 125]-T 3 binding to TTR and some showed even greater affinity than the natural ligand T 3. The TTR competitive binding assay developed offers a quick and effective tool for preliminary risk assessment of chemicals which may disrupt the thyroid axis in teleost fish inhabiting vulnerable aquatic environments.

The Ultraviolet Filter Benzophenone 2 Interferes with the Thyroid Hormone Axis in Rats and Is a Potent in Vitro Inhibitor of Human Recombinant Thyroid Peroxidase

Endocrine disrupting chemicals (EDCs), either plant constituents or contaminants deriving from industrial products, may interfere with the thyroid hormone (TH) axis. Here, we examined whether selected EDCs inhibit the key reactions of TH biosynthesis catalyzed by thyroid peroxidase (TPO). We used a novel in vitro assay based on human recombinant TPO (hrTPO) stably transfected into the human follicular thyroid carcinoma cell line FTC-238. F21388 (synthetic flavonoid), bisphenol A (building block for polycarbonates), and the UV filter benzophenone 2 (BP2) inhibited hrTPO. BP2 is contained in numerous cosmetics of daily use and may be in regular contact with human skin. Half-maximal inhibition in the guaiacol assay occurred at 450 nmol/liter BP2, a concentration 20- and 200-fold lower than those required in case of the TPO-inhibiting antithyroid drugs methimazole and propylthiouracil, respectively. BP2 at 300 nmol/liter combined with the TPO substrate H(2)O(2) (10 mumol/liter) inactivated hrTPO; this was, however, prevented by micromolar amounts of iodide. BP2 did not inhibit iodide uptake into FRTL-5 cells. In BP2-treated rats (333 and 1000 mg/kg body weight), serum total T(4) was significantly decreased and serum thyrotropin was significantly increased. TPO activities in the thyroids of treated animals were unchanged, a finding also described for methimazole and propylthiouracil. Thus, EDCs, most potently BP2, may disturb TH homeostasis by inhibiting or inactivating TPO, effects that are even more pronounced in the absence of iodide. This new challenge for endocrine regulation must be considered in the context of a still prevailing iodide deficiency in many parts of the world.