Inhibition of rat liver iodothyronine deiodinase. Interaction of aurones with the iodothyronine ligand-binding site

Deiodinases control local cellular and systemic thyroid hormone availability

Iodothyronine deiodinases (DIO) are a family of selenoproteins controlling systemic and local availability of the major thyroid hormone l-thyroxine (T4), a prohormone secreted by the thyroid gland. T4 is activated to the active 3,3'-5-triiodothyronine (T3) by two 5'-deiodinases, DIO1 and DIO2. DIO3, a 5-deiodinase selenoenzyme inactivates both the prohormone T4 and its active form T3. DIOs show species-specific different patterns of temporo-spatial expression, regulation and function and exhibit different mechanisms of reaction and inhibitor sensitivities. The main regulators of DIO expression and function are the thyroid hormone status, several growth factors, cytokines and altered pathophysiological conditions. Selenium (Se) status has a modest impact on DIO expression and translation. DIOs rank high in the priority of selenium supply to various selenoproteins; thus, their function is impaired only during severe selenium deficiency. DIO variants, polymorphisms, SNPs and rare mutations have been identified. Development of DIO isozyme selective drugs is ongoing. A first X-ray structure has been reported for DIO3. This review focusses on the biochemical characteristics and reaction mechanisms, the relationships between DIO selenoproteins and their importance for local and systemic provision of the active hormone T3. Nutritional, pharmacological, and environmental factors and inhibitors, such as endocrine disruptors, impact DIO functions.

Flavonol morin targets host ACE2, IMP-α, PARP-1 and viral proteins of SARS-CoV-2, SARS-CoV and MERS-CoV critical for infection and survival: a computational analysis

A sudden outbreak of a novel coronavirus SARS-CoV-2 in 2019 has now emerged as a pandemic threatening to efface the existence of mankind. In absence of any valid and appropriate vaccines to combat this newly evolved agent, there is need of novel resource molecules for treatment and prophylaxis. To this effect, flavonol morin which is found in fruits, vegetables and various medicinal herbs has been evaluated for its antiviral potential in the present study. PASS analysis of morin versus reference antiviral drugs baricitinib, remdesivir and hydroxychloroquine revealed that morin displayed no violations of Lipinski's rule of five and other druglikeness filters. Morin also displayed no tumorigenic, reproductive or irritant effects and exhibited good absorption and permeation through GI (clogP <5). In principal component analysis, morin appeared closest to baricitinib in 3D space. Morin displayed potent binding to spike glycoprotein, main protease 3CLPro and papain-like protease PLPro of SARS-CoV-2, SARS-CoV and MERS-CoV using molecular docking and significant binding to three viral-specific host proteins viz. human ACE2, importin-α and poly (ADP-ribose) polymerase (PARP)-1, further lending support to its antiviral efficacy. Additionally, morin displayed potent binding to pro-inflammatory cytokines IL-6, 8 and 10 also supporting its anti-inflammatory activity. MD simulation of morin with SARS-CoV-2 3CLPro and PLPro displayed strong stability at 300 K. Both complexes exhibited constant RMSDs of protein side chains and Cα atoms throughout the simulation run time. In conclusion, morin might hold considerable therapeutic potential for the treatment and management of not only COVID-19, but also SARS and MERS if studied further. Communicated by Ramaswamy H. Sarma.

Apigenin-mediated Alterations in Viability and Senescence of SW480 Colorectal Cancer Cells Persist in The Presence of L-thyroxine

INTRODUCTION

Deregulation of Thyroid Hormones (THs) system in Colorectal Cancer (CRC) suggests that these hormones may play roles in CRC pathogenesis. Flavonoids are polyphenolic compounds, which possess potent antitumor activities and interfere, albeit some of them, with all aspects of THs physiology. Whether the antitumor actions of flavonoids are affected by THs is unknown. Therefore, we investigated the effects of apigenin (Api), a well-known flavone, on some tumorigenic properties of SW480 CRC cells in the presence and absence of L-thyroxine (T4).

METHODS

Cell viability was assessed by MTT assay. Flow cytometry and DNA electrophoresis were used to evaluate cell death. Cell senescence was examined by in situ detection of β-galactosidase activity. Protein expression was assessed by antibody array technique.

RESULTS

While T4 had minimal effects, Api reduced cell growth and senescence by induction of apoptosis. Expression of anti-apoptotic and pro-apoptotic proteins were differentially affected by Api and T4. Survivin, HSP60 and HTRA were the most expressed proteins by the cells. Almost all Api-induced effects persisted in the presence of T4.

CONCLUSION

These data suggest that Api may inhibit CRC cell growth and progression through induction of apoptosis rather than cell necrosis or senescence. In addition, they suggest that T4 has minimal effects on CRC cell growth, and is not able to antagonize the anti-growth effects of Api. Regardless of the treatments, cells expressed high levels of survivin, HSP60 and HTRA, indicating that these proteins may play central roles in SW480 CRC cell immortality.

Insights from zebrafish deficiency models to understand the impact of local thyroid hormone regulator action on early development

Thyroid hormones (THs) stimulate and coordinate a wide range of processes to ensure normal development, mainly by binding of the most active TH 3,5,3'-triiodothyronine (T₃) to nuclear receptors resulting in changes in gene transcription. Local TH action is monitored at three distinct levels by different types of regulators: transmembrane transporters (TH influx and efflux), deiodinases (TH activation and inactivation) and nuclear receptors (TH signalling). Since TH regulators are strongly conserved among vertebrate species, the externally and rapidly developing zebrafish (Danio rerio) has become one of the favourite models to study their role in TH-dependent development. Most regulators are expressed in zebrafish from early stages in development in a dynamic and tissue-specific pattern. Transient or permanent disruption of a given regulator severely perturbs development of multiple organs. These zebrafish deficiency models help to explain why, next to overall hypo-/hyperthyroidism, inactivating mutations in the genes encoding TH regulators such as MCT8 and THRA/B have irreversible adverse effects on human development. Zebrafish are also increasingly used as a high-throughput model to assess the toxicity of various xenobiotics and their impact on development. While adverse effects on TH metabolism and gene expression have been shown, information on direct interaction with TH regulators is scarce, albeit essential to fully understand their mechanism of action. For the future, the combination of novel gene silencing tools, fluorescent reporter lines and (single-cell) transcriptomics holds promise for new zebrafish models to further elucidate the role of each TH regulator in vertebrate development.

Screening the ToxCast Phase 1, Phase 2, and e1k Chemical Libraries for Inhibitors of Iodothyronine Deiodinases

Deiodinase enzymes play an essential role in converting thyroid hormones between active and inactive forms by deiodinating the pro-hormone thyroxine (T4) to the active hormone triiodothyronine (T3) and modifying T4 and T3 to inactive forms. Chemical inhibition of deiodinase activity has been identified as an important endpoint to include in screening chemicals for thyroid hormone disruption. To address the lack of data regarding chemicals that inhibit the deiodinase enzymes, we developed robust in vitro assays that utilized human deiodinase types 1, 2, and 3 and screened over 1800 unique chemicals from the U.S. EPA's ToxCast phase 1_v2, phase 2, and e1k libraries. Initial testing at a single concentration identified 411 putative deiodinase inhibitors that produced inhibition of 20% or greater in at least 1 of the 3 deiodinase assays, including chemicals that have not previously been shown to inhibit deiodinases. Of these, 228 chemicals produced enzyme inhibition of 50% or greater; these chemicals were further tested in concentration-response to determine relative potency. Comparisons across these deiodinase assays identified 81 chemicals that produced selective inhibition, with 50% inhibition or greater of only 1 of the deiodinases. This set of 3 deiodinase inhibition assays provides a significant contribution toward expanding the limited number of in vitro assays used to identify chemicals with the potential to interfere with thyroid hormone homeostasis. In addition, these results set the groundwork for development and evaluation of structure-activity relationships for deiodinase inhibition, and inform targeted selection of chemicals for further testing to identify adverse outcomes of deiodinase inhibition.

Structural aspects of thyroid hormone binding to proteins and competitive interactions with natural and synthetic compounds

Thyroid hormones and their metabolites constitute a vast class of related iodothyronine compounds that contribute to the regulation of metabolic activity and cell differentiation. They are in turn transported, transformed and recognized as signaling molecules through binding to a variety of proteins from a wide range of evolutionary unrelated protein families, which renders these proteins and their iodothyronine binding sites an example for extensive convergent evolution. In this review, we will briefly summarize what is known about iodothyronine binding sites in proteins, the modes of protein/iodothyronine interaction, and the ligand conformations. We will then discuss physiological and synthetic compounds, including popular drugs and food components, that can interfere with iodothyronine binding and recognition by these proteins. The discussion also includes compounds persisting in the environment and acting as endocrine disrupting chemicals.

New insights into the structure and mechanism of iodothyronine deiodinases

Iodothyronine deiodinases are a family of enzymes that remove specific iodine atoms from one of the two aromatic rings in thyroid hormones (THs). They thereby fine-tune local TH concentrations and cellular TH signaling. Deiodinases catalyze a remarkable biochemical reaction, i.e., the reductive elimination of a halogenide from an aromatic ring. In metazoans, deiodinases depend on the rare amino acid selenocysteine. The recent solution of the first experimental structure of a deiodinase catalytic domain allowed for a reappraisal of the many mechanistic and mutagenesis data that had been accumulated over more than 30 years. Hence, the structure generates new impetus for research directed at understanding catalytic mechanism, substrate specificity, and regulation of deiodinases. This review will focus on structural and mechanistic aspects of iodothyronine deiodinases and briefly compare these enzymes with dehalogenases, which catalyze related reactions. A general mechanism for the selenium-dependent deiodinase reaction will be described, which integrates the mouse deiodinase 3 crystal structure and biochemical studies. We will summarize further, sometimes isoform-specific molecular features of deiodinase catalysis and regulation, and we will then discuss available compounds for modulating deiodinase activity for therapeutic purposes.

The diversity of mechanisms influenced by transthyretin in neurobiology: development, disease and endocrine disruption

Transthyretin (TTR) is a protein that binds and distributes thyroid hormones (THs). TTR synthesised in the liver is secreted into the bloodstream and distributes THs around the body, whereas TTR synthesised in the choroid plexus is involved in movement of thyroxine from the blood into the cerebrospinal fluid and the distribution of THs in the brain. This is important because an adequate amount of TH is required for normal development of the brain. Nevertheless, there has been heated debate on the role of TTR synthesised by the choroid plexus during the past 20 years. We present both sides of the debate and how they can be reconciled by the discovery of TH transporters. New roles for TTR have been suggested, including the promotion of neuroregeneration, protection against neurodegeneration, and involvement in schizophrenia, behaviour, memory and learning. Recently, TTR synthesis was revealed in neurones and peripheral Schwann cells. Thus, the synthesis of TTR in the central nervous system (CNS) is more extensive than previously considered and bolsters the hypothesis that TTR may play wide roles in neurobiological function. Given the high conservation of TTR structure, function and tissue specificity and timing of gene expression, this implies that TTR has a fundamental role, during development and in the adult, across vertebrates. An alarming number of 'unnatural' chemicals can bind to TTR, thus potentially interfering with its functions in the brain. One role of TTR is delivery of THs throughout the CNS. Reduced TH availability during brain development results in a reduced IQ. The combination of the newly discovered sites of TTR synthesis in the CNS, the increasing number of neurological diseases being associated with TTR, the newly discovered functions of TTR and the awareness of the chemicals that can interfere with TTR biology render this a timely review on TTR in neurobiology.

Facile, Protection-Free, One-Pot Synthesis of Aureusidin

A new, reliable, and convenient protection-free one-pot method for the synthesis of aureusidin (1) is described. The present synthetic approach involves the condensation of 4,6-dihydroxybenzofuranone with 3,4-dihydroxybenzaldehyde in the presence of concentrated HCl to afford aureusidin (1) in good yield with high purity. This procedure offers a short and simple route for the preparation of aureusidin (1), a bioactive natural product from several vegetal species, as well as for synthesis of other aurones.

Inhibition of thyroid hormone sulfotransferase activity by brominated flame retardants and halogenated phenolics

Many halogenated organic contaminants (HOCs) are considered endocrine disruptors and affect the hypothalamic-pituitary-thyroid axis, often by interfering with circulating levels of thyroid hormones (THs). We investigated one potential mechanism for TH disruption, inhibition of sulfotransferase activity. One of the primary roles of TH sulfation is to support the regulation of biologically active T3 through the formation of inactive THs. We investigated TH sulfotransferase inhibition by 14 hydroxylated polybrominated diphenyl ethers (OH BDEs), BDE 47, triclosan, and fluorinated, chlorinated, brominated, and iodinated analogues of 2,4,6-trihalogenated phenol and bisphenol A (BPA). A new mass spectrometry-based method was also developed to measure the formation rates of 3,3'-T2 sulfate (3,3'-T2S). Using pooled human liver cytosol, we investigated the influence of these HOCs on the sulfation of 3,3'-T2, a major substrate for TH sulfation. For the formation of 3,3'-T2S, the Michaelis constant (Km) was 1070 ± 120 nM and the Vmax was 153 ± 6.6 pmol min(-1) (mg of protein)(-1). All chemicals investigated inhibited sulfotransferase activity with the exception of BDE 47. The 2,4,6-trihalogenated phenols were the most potent inhibitors followed by the OH BDEs and then halogenated BPAs. The IC50 values for the OH BDEs were primarily in the low nanomolar range, which may be environmentally relevant. In silico molecular modeling techniques were also used to simulate the binding of OH BDE to SULT1A1. This study suggests that some HOCs, including antimicrobial chemicals and metabolites of flame retardants, may interfere with TH regulation through inhibition of sulfotransferase activity.

Aurones: Interesting Natural and Synthetic Compounds with Emerging Biological Potential

Aurones [2-benzylidenebenzofuran-3(2H)-ones] are either natural or synthetic compounds, belonging to the flavonoid family. They are isomeric to flavones and provide a bright yellow color to the plants in which they occur. Today, a literature survey indicates that the related flavonoids have been studied not only for their physiological properties and effects on Nature, but also for their therapeutic potential. Aurones are recently attracting the interest of an increasing number of research groups, and, since the last review, some interesting advances have been made in understanding the aurones.

In this review, we report the recent advances made on the synthetic routes towards aurones. We also highlight their activity in different biological areas, as well as applied genetic plant modifications to produce these colored compounds. Their synthesis, structure–activity relationships and the importance of the substitution pattern will also be mentioned. Finally, some aspects regarding the possible development of aurones will be discussed briefly.

Halogenated phenolic contaminants inhibit the in vitro activity of the thyroid-regulating deiodinases in human liver

Halogenated contaminants, particularly brominated flame retardants, disrupt circulating levels of thyroid hormones (THs), potentially affecting growth and development. Disruption may be mediated by impacts on deiodinase (DI) activity, which regulate the levels of active hormones available to bind to nuclear receptors. The goal of this study was to develop a mass spectrometry-based method for measuring the activity of DIs in human liver microsomes and to examine the effect of halogenated phenolic contaminants on DI activity. Thyroxine (T4) and reverse triiodothyronine (rT3) deiodination kinetics were measured by incubating pooled human liver microsomes with T4 or rT3 and monitoring the production of T3, rT3, 3,3'-diiodothyronine, and 3-monoiodothyronine by liquid chromatography tandem mass spectrometry. Using this method, we examined the effects of several halogenated contaminants, including 2,2',4,4',5-pentabromodiphenyl ether (BDE 99), several hydroxylated polybrominated diphenyl ethers (OH-BDEs), tribromophenol, tetrabromobisphenol A, and triclosan, on DI activity. The Michaelis constants (K(M)) of rT3 and T4 deiodination were determined to be 3.2 ± 0.7 and 17.3 ± 2.3μM. The V(max) was 160 ± 5.8 and 2.8 ± 0.10 pmol/min.mg protein, respectively. All studied contaminants inhibited DI activity in a dose-response manner, with the exception of BDE 99 and two OH-BDEs. 5'-Hydroxy 2,2',4,4',5-pentabromodiphenyl ether was found to be the most potent inhibitor of DI activity, and phenolic structures containing iodine were generally more potent inhibitors of DI activity relative to brominated, chlorinated, and fluorinated analogues. This study suggests that some halogenated phenolics, including current use compounds such as plastic monomers, flame retardants, and their metabolites, may disrupt TH homeostasis through the inhibition of DI activity in vivo.

The obesogen hypothesis: a shift of focus from the periphery to the hypothalamus

The obesogen concept proposes that environmental contaminants may be contributing to the epidemic of obesity and its related pathology, metabolic disorder. The first references to such a notion appeared at the beginning of the current decade, with the hypothesis that the correlation between increasing incidence of obesity and enhanced industrial chemical production was not simply coincidental, but potentially causally related. The next event was the introduction of the term "obesogen" as representing an environmental pollutant that adversely affects various aspects of adipose tissue functions. More recently, the concept was extended to include substances that may modify metabolic balance at the central, hypothalamic level. The actions of two prime candidate obesogens, tributyltin (TBT) and tetrabromobisphenol A (TBBPA), acting at the central level are the main focus of this review. Having discussed the evidence for contaminant accumulation in the environment and in human tissues and the potential mechanisms of action, data are provided showing that these two widespread pollutants modify hypothalamic gene regulations. Our studies are based on maternal exposure and measurement of effects in the progeny, mainly based on in vivo gene reporter assays. Such models are obviously pertinent to testing current hypotheses that propose that early exposure might exert effects on later development and physiological functions. The potential molecular mechanisms involved are discussed, as are the broader physiological consequences of these hypothalamic dysregulations.

Xanthohumol, a prenylated chalcone from hops, modulates hepatic expression of genes involved in thyroid hormone distribution and metabolism

In the present study, we analyzed the influence of xanthohumol (XN) on thyroid hormone (TH) distribution and metabolism in rats. A potent and selective competition of XN for thyroxine (T4) binding to transthyretin (IC(50)=1 microM at 1.7 nM [(125)I]T4) was found in human and rat sera in vitro. Female rats treated orally with XN showed increased hepatic expression of T4-binding globulin and decreased transthyretin and albumin. Thyrotropin levels and hepatic type 1 deiodinase activity were moderately increased. Northern blot analysis revealed diminished expression of liver sulfotransferase (Sult1a1) and uridine-diphosphate glucuronosyltransferase (Ugt1a1) after XN treatment. The transcript levels of constitutive androstane receptor (CAR), known to be involved in regulation of enzymes metabolizing hormones, drugs and xenobiotics, was lower in rats treated with >10 mg XN/kg body weight per day. Immunoblot analysis indicates reduced amounts of CAR protein. The phenobarbital-inducible cytochrome P450 mRNA level was decreased in rats treated with >10 mg XN/kg/day, in agreement with reduced CAR protein. Although only moderate changes in TH serum levels were observed, the XN-dependent altered expression of components involved in TH homeostasis might be important not only for hormone metabolism, but also for hepatic phase I and II elimination of drug metabolites and xenobiotics.

The effects of amitrole on thyroglobulin and iodide uptake in FRTL-5 cells

Thyroid is a frequent target for endocrine effects of pesticides. Thyroglobulin (TG) and iodide uptake are crucial to thyroid hormone synthesis and may be targets of thyroid-disrupting chemicals. In our study, thyroid follicular FRTL-5 cells were treated with amitrole, an inhibitor of the thyroid peroxidase (TPO), and the effects on TG and total iodide uptake were observed. The results showed that 1-100 mg/L amitrole had no marked effects on FRTL-5 cell proliferation and DNA synthesis. However, it significantly increased the transcription of tg gene and inhibited the total iodide uptake. And 10-100 mg/L amitrole significantly decreased TG in the culture medium. The data suggests amitrole may disrupt the expression and secretion of TG and iodide uptake.

Essential molecular determinants for thyroid hormone transport and first structural implications for monocarboxylate transporter 8

Monocarboxylate transporter 8 (MCT8, SLC16A2) is a thyroid hormone (TH) transmembrane transport protein mutated in Allan-Herndon-Dudley syndrome, a severe X-linked psychomotor retardation. The neurological and endocrine phenotypes of patients deficient in MCT8 function underscore the physiological significance of carrier-mediated TH transmembrane transport. MCT8 belongs to the major facilitator superfamily of 12 transmembrane-spanning proteins and mediates energy-independent bidirectional transport of iodothyronines across the plasma membrane. Structural information is lacking for all TH transmembrane transporters. To gain insight into structure-function relations in TH transport, we chose human MCT8 as a paradigm. We systematically performed conventional and liquid chromatography-tandem mass spectrometry-based uptake measurements into MCT8-transfected cells using a large number of compounds structurally related to iodothyronines. We found that human MCT8 is specific for L-iodothyronines and requires at least one iodine atom per aromatic ring. Neither thyronamines, decarboxylated metabolites of iodothyronines, nor triiodothyroacetic acid and tetraiodothyroacetic acid, TH derivatives lacking both chiral center and amino group, are substrates for MCT8. The polyphenolic flavonoids naringenin and F21388, potent competitors for TH binding at transthyretin, did not inhibit T(3) transport, suggesting that MCT8 can discriminate its ligand better than transthyretin. Bioinformatic studies and a first molecular homology model of MCT8 suggested amino acids potentially involved in substrate interaction. Indeed, alanine mutation of either Arg(445) (helix 8) or Asp(498) (helix 10) abrogated T(3) transport activity of MCT8, supporting their predicted role in substrate recognition. The MCT8 model allows us to rationalize potential interactions of amino acids including those mutated in patients with Allan-Herndon-Dudley syndrome.

Versatile and expeditious synthesis of aurones via Au I-catalyzed cyclization

Aurones are conveniently formed in a three-step procedure including a goldI-catalyzed cyclization of 2-(1-hydroxyprop-2-ynyl)phenols as a highly regio- and stereoselective key step. A wide diversity of derivatives can be obtained starting from substituted salicylaldehydes. Synthesis of natural 4,6,3',4'-tetramethoxyaurone and structure revision of two natural products (dalmaisione D and 4'-chloroaurone) were achieved.

Endocrine disruptors and the thyroid gland--a combined in vitro and in vivo analysis of potential new biomarkers

BACKGROUND

There is growing evidence that, in addition to the reproductive system, the hypothalamic-pituitary-thyroid axis is a target of endocrine-disrupting compounds (EDCs). However, this is not reflected adequately in current screening and assessment procedures for endocrine activity that to date determine only general parameters of thyroid function.

OBJECTIVE AND METHODS

We used several in vitro and ex vivo assays in an attempt to identify suitable biomarkers for antithyroid action testing a selected panel of putative EDCs.

RESULTS

In vitro we detected stimulation or inhibition of iodide uptake into FRTL-5 rat thyroid cells, inhibition of thyroid hormone binding to transthyretin, agonistic or antagonistic effects in a thyroid hormone receptor-dependent reporter assay, and inhibition of thyroid peroxidase using a novel assay system based on human recombinant thyroperoxidase that might be suitable for routine screening for potential EDCs. In rats, chronic application of several EDCs led to changes in thyroid morphology, alterations of thyrotropin and thyroid hormone serum levels as well as alterations in peripheral thyroid hormone-regulated end points such as malic enzyme and type I 5'-deiodinase activity.

CONCLUSIONS

As the effects of EDCs do not reflect classic mechanisms of hormone-dependent regulation and feedback, we believe multitarget and multimodal actions of EDCs affect the hypothalamic-pituitary-thyroid axis. These complex effects require a diverse approach for screening, evaluation, and risk assessment of potential antithyroid compounds. This approach involves novel in vitro or cell-based screening assays in order to assess thyroid hormone synthesis, transport, metabolism, and action as well as in vivo assays to measure thyroid hormone-regulated tissue-specific and developmental end points in animals.

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.

Genistein and other soya isoflavones are potent ligands for transthyretin in serum and cerebrospinal fluid

Consumption of soya-based nutrients is increasing in modern society because of their potentially protective effects against chronic diseases. Soya products are also heavily advertised as alternative drugs for relief from symptoms of the menopause and for hormone replacement therapy. However, because of their oestrogenic activity, negative effects of isoflavones have been postulated. Therefore, we analysed influences of soya isoflavones, major soya constituents with endocrine activity, on thyroxine (T4) binding to its distribution proteins. Serum binding of (125)I-labelled L-T4 was analysed in the absence or presence of increasing concentrations of soya isoflavones using non-denaturing PAGE for analysis. Complete displacement of [(125)I]T4 binding to transthyretin (TTR) was observed in human serum incubated with genistein at concentrations >10 microM; interference started at >0.1 microM. Glycitein showed decreased and daidzein the lowest displacement potency. [(125)I]T4 was displaced to albumin in rat and to T4-binding globulin in human serum. Soya isoflavones also obstruct [(125)I]T4 binding to TTR in human cerebrospinal fluid (CSF). The inhibitory effect was confirmed in direct binding assays using purified TTR with 50% inhibitory concentration values of 0.07 microM for genistein, 0.2 microM for glycitein and 1.8 microM for daidzein. The present study underlined a potent competition of soya isoflavones for T4 binding to TTR in serum and CSF. Isoflavones might alter free thyroid hormone concentrations resulting in altered tissue availability and metabolism. As a consequence of this interference, one could expect a disturbance in the feedback regulation of hormonal networks, including the pituitary-thyroid-periphery axis during development and in adult organisms.

Endocrine active compounds affect thyrotropin and thyroid hormone levels in serum as well as endpoints of thyroid hormone action in liver, heart and kidney

To assess interference with endocrine regulation of the thyroid axis, rats (female, ovariectomised) were treated for 12 weeks with the suspected endocrine active compounds (EAC) or endocrine disrupters (ED) 4-nonylphenol (NP), octyl-methoxycinnamate (OMC) and 4-methylbenzylidene-camphor (4-MBC) as well as 17beta-estradiol (E2) and 5alpha-androstane-3beta,17beta-diol (Adiol) on the background of a soy-free or soy-containing diet, and endpoints relevant for regulation via the thyroid axis were measured. Thyrotropin (TSH) and thyroid hormone (T4, T3) serum levels were altered, but not in a way consistent with known mechanisms of feedback regulation of the thyroid axis. In the liver, malic enzyme (ME) activity was significantly increased by E2 and Adiol, slightly by OMC and MBC and decreased by soy, whereas type I 5'-deiodinase (5'DI) was decreased by all treatments. This may be due rather to the estrogenic effect of the ED, as there is no obvious correlation with T4 or T3 serum levels. None of the substances inhibited thyroid peroxidase (TPO) in vitro, except for NP. In general, several endocrine active compounds disrupt the endocrine feedback regulation of the thyroid axis. However, there was no uniform, obvious pattern in the effects of those ED tested, but each compound elicited its own spectrum of alterations, arguing for multiple targets of interference with the complex network of thyroid hormone action and metabolism.

The total synthesis of an aurone isolated from Uvaria hamiltonii : aurones and flavones as anticancer agents

The naturally occurring aurone 1, isolated from Uvaria hamiltonii, and a series of aurones analogues based structurally on known tubulin binding agents were prepared and evaluated for anticancer activity. Aurone 20 was the most active (IC(50) K562 50 nM) and caused significant G2/M cell-cycle arrest.

Amphibians as a model for the study of endocrine disruptors

Evidence shows that environmental compounds can interfere with the endocrine systems of wildlife and humans. The main sink of such substances, called endocrine disruptors (EDs), which are mainly of anthropogenic origin, is surface water; thus, aquatic vertebrates such as fishes and amphibians are most endangered. Despite numerous reports on EDs in fishes, information about EDs in amphibians is scarce, and this paucity of information is of particular concern in view of the worldwide decline of amphibians. EDs could contribute to changes of amphibian populations via adverse effects on reproduction and the thyroid system. In amphibians, EDs can affect reproduction by (anti)estrogenic and (anti)androgenic modes of action that produce severe effects including abnormal sexual differentiation. ED actions on the thyroid system cause acceleration or retardation of metamorphosis, which may also affect population levels. Our broad knowledge of amphibian biology and endocrinology indicates that amphibians are very suitable models for the study of EDs. In particular, effects of EDs on the thyroid system triggering metamorphosis can be determined easily and most sensitively in amphibians compared to other vertebrates. A new classification of EDs according to their biological modes of action is proposed because EDs have quite heterogeneous chemical structures, which do not allow prediction of their biological effects. Methods and strategies are proposed for identification and risk assessment of EDs, whether as pure test substances or as mixtures from environmental samples. Effects of EDs on the thyroid system of amphibians can be assessed by a single animal model (Xenopus laevis), whereas the various types of reproduction need comparative studies to investigate whether general endocrine principles do exist among several species of anurans and urodeles. Thus, at least one anuran and one urodelean model are needed to determine ED interference with reproduction.

Mechanisms of molecular recognition: crystal structure analysis of human and rat transthyretin inhibitor complexes

Structure-activity data show that many pharmacological agents are strong competitive inhibitors for thyroxine (T4) binding to transthyretin (TTR) and that this competition can interfere with their normal pharmacological actions. TTR is a tetrameric serum protein responsible for the transport of 20% of the circulating T4 in man, while in lower vertebrates such as rats it is the only carrier. The sequence of rat TTR is 85% homologous to the human protein. Crystallographic analyses of ligand co-crystal complexes of human and rat TTR have been studied to understand the molecular basis for binding selectivity of competitor binding to TTR. Analysis of TTR crystal complexes with several classes of competitors (hormone metabolites, flavonoids, fluorescent probes, analgesics and cardiac agents) revealed multiple modes of binding with both forward and reverse ligand binding orientations. These ligands also have different binding positions along the length of the channel with the smallest ligands located deeper within the hormone domain. Data for the human TTR complex with the bromoflavone EMD21388 incubated at different times revealed variable binding positions and occupancies dependent upon incubation time. Comparison of the structures of T4 thyroacetic acid in complex with both human and rat TTR revealed forward and reverse binding, but also showed different modes of binding in the rat compared to the human complex. These data highlight the importance of hydrogen bonding with Lys-15 and Ser-117 and provide insight into ligand binding affinity and negative cooperativity.

4-Hydroxy-6-methoxyaurones with high-affinity binding to cytosolic domain of P-glycoprotein

A series of 4-hydroxy-6-methoxyaurones and 4,6-dimethoxyaurones has been synthesised and tested for their binding affinity toward the nucleotide-binding domain of P-glycoprotein, an ABC (ATP-Binding Cassette) transporter which mediates the resistance of cancer cells to chemotherapy. These compounds differ from each other by the nature of the substituent on the aurone B-ring. The binding affinity seems to be linked to the nature of the substituent, as well as to the presence or the absence of a hydroxy group at position 4. The most active compounds were 4'-bromo-4-hydroxy-6-methoxyaurone and 4-hydroxy-4'-iodo-6-methoxyaurone.

Method for the quantitation of iodothyronines in body tissues and fluids using high-performance liquid chromatography

The separation and quantitation of iodotyrosines and iodothyronines [3-monoiodo-L-tyrosine, 3,5-diiodo-L-tyrosine, 3,5-, 3,3' and 3',5'-diiodo-L-tyronines, 3,5,3'-triiodo-L-thyronine (T3), reverse 3,3',5'-triiodo-L-thyronine and 3,3',5,5'-tetraiodo-L-thyronine (T4)] from animal tissues (brain, liver and serum) by a new high-performance liquid chromatographic (HPLC) method is described. Rats were infused with iso-osmotic sodium chloride containing 100 microM phloretin to block deiodination. The tissues were extracted using differential pH values to separate other amines from the amine containing iodothyroid hormones. Aliquots of tissue extracts (25-100 microliters) were reacted overnight with 5-dimethylaminonaphthalene-1-sulfonyl chloride and their iodotyrosine and iodothyronine content determined by HPLC utilizing fluorimetric detection. Resolution of the individual compound peaks was achieved by gradient elution with a 3.0 mM H3PO4 buffer. Greater sensitivity has been achieved (less than 1.0 pmol/g) utilizing fluorescence rather than ultraviolet absorbance for the quantitation of these iodinated compounds. The method is superior also to other methods in that recoveries, based on those of 125I-labelled T4 and T3, were 89-97%.

In vivo effects of flavonoid EMD 21388 on thyroid hormone secretion and metabolism in rats

In vitro, the synthetic flavonoid EMD 21388 appears to be a potent inhibitor of thyroxine (T4) 5'-deiodinase and diminishes binding of T4 to transthyretin. In this study, in vivo effects of long-term administration of EMD 21388 on thyroid hormone production and metabolism were investigated. Intact male rats received EMD 21388 (20 mumol.kg body wt-1.rat-1.day-1) for 14 days. [125I]T4 and 3,5,3'-[131I]triiodotyronine (T3) were infused continuously and intravenously in a double-isotope protocol for the last 10 and 7 days, respectively. EMD 21388 decreased plasma thyroid hormone concentrations, but thyrotropin levels in plasma and pituitary did not change. Plasma clearance rates for T4 and T3 increased. Thyroidal T4 secretion was diminished, but T3 secretion was elevated. Extrathyroidal T3 production by 5'-deiodination was lower. T4 concentrations were markedly lower in all tissues investigated. Total tissue T3 was lower in brown adipose tissue, brain, cerebellum, and pituitary, tissues that express the type II 5'-deiodinase isozyme due to decreased local T3 production. Most tissues showed increased tissue/plasma ratios for T4 and T3. These results indicate that this flavonoid diminished T4 and increased T3 secretion by the thyroid, probably in analogy with other natural flavonoids, by interference with one or several steps between iodide uptake, organification, and hormone synthesis.