In vitro thyroperoxidase inhibition assessment by LC-ICP-MS-based L-tyrosine iodination assay: comparison with Amplex Ultrared assay and its modifications.

Disruption of the thyroid hormone (TH) system is connected with diverse adverse health outcomes in wildlife and humans. It is crucial to develop and validate suitable in vitro assays capable of measuring the disruption of the thyroid hormone (TH) system. These assays are also essential to comply with the 3R principles, aiming to replace the ex vivo tests often utilised in the chemical assessment. We compared the two commonly used assays applicable for high throughput screening [Luminol and Amplex UltraRed (AUR)] for the assessment of inhibition of thyroid peroxidase (TPO, a crucial enzyme in TH synthesis) using several cell lines and 21 compounds from different use categories. As the investigated cell lines derived from human and rat thyroid showed low or undetectable TPO expression, we developed a series of novel cell lines overexpressing human TPO protein. The HEK-TPOA7 model was prioritised for further research based on the high and stable TPO gene and protein expression. Notably, the Luminol assay detected significant peroxidase activity and signal inhibition even in Nthy-ori 3-1 and HEK293T cell lines without TPO expression, revealing its lack of specificity. Conversely, the AUR assay was specific to TPO activity. Nevertheless, despite the different specificity, both assays identified similar peroxidation inhibitors. Over half of the tested chemicals with diverse structures and from different use groups caused TPO inhibition, including some widespread environmental contaminants suggesting a potential impact of environmental chemicals on TH synthesis. Furthermore, in silico SeqAPASS analysis confirmed the high similarity of human TPO across mammals and other vertebrate classes, suggesting the applicability of HEK-TPOA7 model findings to other vertebrates.

Abstract. Several alterations of thyroid function parameters have been reported in patients treated with phenylbutazone and we have studied the effect of this drug on the intrathyroidal iodine metabolism. An inhibition of the iodide transport expressed in terms of T/M ratios was observed in bovine thyroid slices incubated with high phenylbutazone concentrations. 10−3m produced 72% inhibition whereas lower concentrations showed no significant difference as compared with controls. Iodotyrosine synthesis was affected by 10−4m and 10−5m phenylbutazone. Formation of iodothyronine synthesis was markedly affected between 10−4m and 10−7m phenylbutazone concentrations. Thyroid peroxidase activity was measured by tyrosine-iodinase, triiodide and guaiacol assays. Soluble, pseudosolubilized and crude peroxidase preparations from bovine glands, as well as the soluble enzyme from human thyroids, have shown inhibition of tyrosine-iodinase activity when incubated with phenylbutazone in concentrations ranging from 10−3m to 10−8m, with a Ki of 4 × 10−6m for bovine thyroid peroxidase and of 6 × 10−6m for human soluble peroxidase. Formation of triiodide was affected between 10−3m and 10−8m phenylbutazone concentrations. Guaiacol peroxidation was scarcely affected by the action of the drug. We have concluded that phenylbutazone affects the intrathyroidal iodine metabolism through the inhibition of thyroid peroxidase in concentrations which are usually present in the sera of patients treated with this drug.

Thiourea, methylmercaptoimidazole, propylthiouracil, and thiouracil are all potent inhibitors of thyroid peroxidase (TPO)-catalyzed iodination. Unlike the cyclic thioureylenes, thiourea at 5 mM has no effect on guaiacol oxidation. If iodide is added to guaiacol assays containing thiourea, enzyme activity is lost. The latter observation may be explained as follows. In the presence of iodide, the iodinating species [TPO.Ioxid], oxidizes thiourea to formamidine disulfide. This product decomposes to cyanamide at neutral pH. We have shown cyanamide to be an inhibitor of the peroxidative and iodinating functions of TPO. Studies in rats demonstrate that doses of thiourea which completely inhibit in vivo protein-bound iodine formation have no irreversible effect on TPO, as measured by guaiacol peroxidation after removal of the thyroids. The major in vivo action of cyanamide is similar to that of thiourea. The data suggest that the primary in vivo and in vitro mode of action of thiourea is the reversible Ioxid-trapping mechanism. The anomalous inhibition of guaiacol peroxidation seen in the presence of thiourea plus iodide derives from the formation of formamide disulfide, followed by its nonenzymic decomposition to cyanamide.

Antithyroid effects of propylthiouracil and sulfamonomethoxine in rats and monkeys

Scintigraphic demonstrations of a retrosternal goiter

Metribuzin-induced non-adverse liver changes result in rodent-specific non-adverse thyroid effects via uridine 5′-diphospho-glucuronosyltransferase (UDPGT, UGT) modulation

Annona squamosa and Tetracapidium conophorum leaf extracts have been documented to possess anti-hyperthyroidal activity and are used together in ethnomedicine; however, their effective combination dose ratios have not been explored. This study investigated the combined therapeutic effects of Annona squamosa and Tetracapidium conophorum extracts in in vitro and in vivo models of hyperthyroidism. In vitro studies showed concentration-dependent inhibition of thyroid peroxidase (TPO) enzyme activity by both extracts, with Tetracapidium conophorum exhibiting a lower EC50 (224.05 μg/ml) than that of Annona squamosa (437.24 μg/ml). combination of half their EC50 produced 51% TPO inhibition and a combination index (CI) of 1.0, indicating additivity, while 250 μg/ml Annona squamosa combined with 65 μg/ml Tetracapidium conophorum resulted in 57% inhibition and a CI of 0.8, suggesting synergism. In an in vivo hyperthyroidism model induced by L-thyroxine, both extracts dose-dependently increased thyroid-stimulating hormone (TSH) levels. The combination dose ratios of 30 mg/kg Annona squamosa + 90 mg/kg Tetracapidium conophorum and 25 mg/kg Annona squamosa + 105 mg/kg Tetracapidium conophorum produced a CI of 0.7, indicating synergism. The combination treatment also significantly reduced elevated T3 and T4 levels, liver enzymes, and lipid peroxidation while improving kidney function markers. Annona squamosa and Tetracapidium conophorum extracts exhibited both additive and synergistic effects at different dose ratios in hyperthyroidism models, highlighting their potential to enhance antithyroid efficacy through strategic combinations.

Thyroid peroxidase (TPO) plays an important role in thyroid hormone biosynthesis, as it catalyzes all of the essential steps in iodide organification. TPO activity can be detected using the guaiacol assay; however, this assay is complex and very time-consuming. Therefore, we focused on devising a simplified method using microarrays to detect changes in TPO activity, which is a target for disruption of the thyroid hormone axis. These experiments have systematically assessed the potential utility of transcriptomic end points as enhancements to the guaiacol assay. Previously, we demonstrated that benzophenone-2, benzophenone, perfluorooctane sulfonate, bisphenol A bis ether, and vinclozolin decreased TPO activity, and that dibutyl phthalate, carbaryl, dibenzo(a,h)anthracene, benzo(a)pyrene, and methylmercury increased TPO activity. In this work, we used human oligonucleotide chips to examine changes in the gene expression profile of FTC-238 human follicular thyroid carcinoma cells expressing human recombinant TPO, after exposure of the cells to TPO activity-disrupting agents. We identified 362 classifiers that could predict the effect of the toxicants on TPO activity with about 70% accuracy. These classifiers are potential markers for predicting the effects of chemicals on thyroid hormone production.

Thyroid Peroxidase (TPO), a dimeric, membrane-bound glycoprotein found in the thyroid follicular lumen, catalyses the biosynthesis of thyroxine, and is also a major autoantigen in autoimmune thyroid disease (AITD). TPO molecules undergo posttranslational modifications such as glycosylation, heme incorporation, dimer formation, and proteolytic processing in the N-terminal region. Due to propeptide deletion, in human thyroid cells the TPO protein loses 108 amino acids in the N-terminus to yield mature enzyme. During the course of studies to obtain homogenous preparations of recombinant TPO for structural studies, we investigated the role of the propeptide in TPO structure, function and trafficking. We engineered recombinant human TPO truncated at the N-terminal residue (TPO-pro) and subsequently expressed in Chinese hamster ovary (CHO) cells. Then we characterized TPO-pro and compared its properties with wild-type recombinant TPO (TPOwt) based upon its subcellular localisation, immunological and biochemical properties such as glycosylation and enzymatic activity. Molecular modeling and dynamics simulation of TPO-pro comprising a dimer of MPO-like domains were performed in order to investigate the impact of propeptide removal. Using immunodetection and densitometric analysis we showed that deletion of the propeptide did not influence expression level of TPO-pro in comparison with TPOwt. Upon SDS-PAGE under non-reducing conditions, we observed dimer formation for both TPOwt and TPO-pro. Furthermore, we tested by flow cytometry the reactivity of TPO constructs with anti-TPO antibodies and confirmed that the propeptide deletion did not disturb TPO transport to the membrane of CHO cells. This finding was further supported by immunocytochemistry. ELISA and flow cytometry analysis revealed that the conformation of epitopes recognized by a panel of human anti-TPO antibodies was not considerably affected in TPO-pro. Measurement of the Amplex Ultra Red oxidation of cells expressing TPOwt and TPO-pro confirmed faithful enzymatic activity of extracellular truncated TPO. Study of the carbohydrate content showed that TPO-pro is effectively N-glycosylated. Molecular modelling and dynamics simulations were consistent with our findings. In conclusion, the propeptide is not essential for the proper folding and transport of TPO produced in CHO cells. The successful expression in a membrane-anchored, active form that is insensitive to intramolecular proteolysis is a key advance for purification of substantial quantities of homogenous preparation of TPO for structural and crystallization studies.

Effect of triclosan, triclocarban, 2,2′,4,4′-tetrabromodiphenyl ether, and bisphenol A on the iodide uptake, thyroid peroxidase activity, and expression of genes involved in thyroid hormone synthesis

Disruption of the thyroid hormone system during development can impair brain development and cause irreversible damage. Some thyroid hormone system disruptors act by inhibiting the thyroperoxidase (TPO) enzyme, which is key to thyroid hormone synthesis. For the potent TPO-inhibiting drug propylthiouracil (PTU) this has been shown to result in thyroid hormone system disruption and altered brain development in animal studies. However, an outstanding question is which chemicals beside PTU can cause similar effects on brain development and to what degree thyroid hormone insufficiency must be induced to be able to measure adverse effects in rats and their offspring. To start answering these questions, we performed a perinatal exposure study in pregnant rats with two TPO-inhibitors: the drug methimazole (MMI) and the triazole herbicide amitrole. The study involved maternal exposure from gestational day 7 through to postnatal day 22, to MMI (8 and 16 mg/kg body weight/day) or amitrole (25 and 50 mg/kg body weight/day). Both MMI and amitrole reduced serum T4 concentrations in a dose-dependent manner in dams and offspring, with a strong activation of the hypothalamic-pituitary-thyroid axis. This reduction in serum T4 led to decreased thyroid hormone-mediated gene expression in the offspring’s brains and caused adverse effects on brain function, seen as hyperactivity and decreased habituation in preweaning pups. These dose-dependent effects induced by MMI and amitrole are largely the same as those observed with PTU. This demonstrates that potent TPO-inhibitors can induce effects on brain development in rats and that these effects are driven by T4 deficiency. This knowledge will aid the identification of TPO-inhibiting thyroid hormone system disruptors in a regulatory context and can serve as a starting point in search of more sensitive markers of developmental thyroid hormone system disruption.

The cDNAs coding for human full-length and soluble thyroid peroxidase (TPO) were constructed, cloned into a baculovirus transfer vector and used for infection of Spodoptera frugiperda (Sf9) cells. The soluble TPO lacking 87 amino acids of the C-terminal transmembrane and intracisternal domains was designed as a fusion protein with a histidine-hexapeptide as an affinity ligand at its C-terminus. Whereas the recombinant full-length TPO was expressed mainly in an insoluble form in Sf9 cells, the recombinant soluble TPO was almost completely secreted into the culture medium. Both the full-length and the soluble TPO were purified by conventional methods and by a specific affinity chromatography using metal chelating matrix respectively, and tested for their autoantigenicity towards anti-TPO autoantibodies. The ELISA established with the purified recombinant soluble TPO as antigen demonstrated its specificity, practicability and reproducibility in screening of anti-TPO autoantibodies in sera of autoimmune thyroid patients. High correlation (r = 0.89, n = 175) was obtained between the soluble TPO and natural TPO prepared from human thyroid glands. Pathological sera (n = 200) were positively assayed with a significance of 91%.

In a panel of 13 mouse monoclonal antibodies generated against native (nondenatured) human thyroid peroxidase (TPO), only 1 (monoclonal antibody 47) recognized TPO protein fragments expressed in a human TPO cDNA sublibrary. Determination of the nucleotide sequences of 18 clones recognized by monoclonal antibody 47 localized its epitope to 9 amino acids (residues 713-721) in the human TPO protein. On Western blot analysis, only TPO monoclonal antibody 47 recognized the 933-amino acid TPO molecule after denaturation and reduction of the latter, supporting the concept that the major part of the epitope is represented by a continuous portion of the TPO sequence. The binding of TPO monoclonal antibody 47 to native TPO is inhibited by immunoglobulin G in the serum of patients with autoimmune thyroid disease. The epitope for monoclonal antibody 47 defined in the present study is, therefore, part of or in the vicinity of an epitope for autoimmune thyroid disease-associated TPO antibodies.

Over the recent years, considerable progress has been made in elucidating processes that are critical for thyroid hormone (TH) synthesis and action. In this issue of Thyroid Research several German groups dedicated to thyroid research have assembled and discussed novel aspects of thyroid hormone action with the perspective to improve diagnosis and therapy of thyroid disorders by better understanding disturbed thyroid differentiation. The over-arching theme addressed in the contributions of this special issue relates to the main focus of current thyroid research and treatment of thyroid disesase, i.e. “What defines thyroid health?” [1]. Thereby the putative role of seemingly established players in TH synthesis and action are revisited. A report on a patient characterized by a novel germline TSH-receptor mutation links this case of non-autoimmune hyperthyroidism to new impliations in TSH-receptor structure [2]. The study highlights the importance of introducing novel diagnostic approaches to the clinics, because this particular mutation in the TSH-receptor is correlated to altered Gq-signalling pathways, thereby pointing to an important role of Gq signalling in the thyroid It is known only since recently that transport of TH is an active and regulated mechanism. The monocarboxylate transporter 8 (MCT8) is a crucially important transporter of TH and it transports triiodothyronine (T3) specifically well. Mutations in MCT8 are known to cause a severe syndrome of X-linked mental retardation known as Allan-Herndon Dudley syndrome. Based on homology modeling all of the currently reported pathogenic mutations in MCT8 are studied in the article by Kleinau et al. [3]. A focus is set on identification of those MCT8 mutations that are most sensitive for substrate transport with the goal to simulate in silico the potencies of future therapies based on TH transporter-specific inhibitors mimicking TH structurally. Besides MCT8 other transporters have been proposed to play important roles in TH transport (MCT10, OATP1C1, OATP1A2, OATP14, LAT1 and LAT2). The structure-function relationships of primary TH transporters like MCT8 and additional, so-called secondary TH transporters such as those listed above is reviewed by Kinne et al. [4] with special attention on their expression profiles, transport specificities and substrates. Similarities and differences in structure and molecular features of the known and suspected TH transporters are compared between man and mouse, in particular asking their crucial impact on TH actions in the central nervous system. TH actions in the brain and clinical consequences of non-classical, rather novel treatment protocols to relieve symptoms of severe mood and depression disorders is reviewed in the article by Pilhatsch et al. [5]. For a long time, only genomic actions of TH, namely of T3 were recognized by both, thyroid researchers and clinicians. However this picture has to be revised as non-classical, non-genomic TH actions were described which lead to activation of alternative signaling pathways via PI3K and MAPK. The impact and perspectives of these recent findings are summarized by Moller and Brocker-Preus [6] who comment on the outcomes of activation of alternative signaling pathways on transcriptional regulation of TH target cells. Non-classical and classical TH actions must be taken into consideration for better management of thyroid malignancies. Moreover, it is increasingly appreciated that disturbances in TH synthesis occur in thyroid tumorigenesis and that disturbed TH action affects carcinogenesis in general. Of the many novel aspects in thyroid carcinogenesis, it was the recent identification of miRNAs and their intricate involvement in the regulation of transcriptional, translational and epigentic control mechanisms that provides one of the most promising diagnostic and high-potential, future therapeutic tool. The aspects of miRNA relevance for thyroid pathologies are discussed by Braun and Huttelmaier [7]. Likewise, regulation of transcription and translation by RNA binding proteins is addressed in the article by Trojanowicz et al. The adenylate uridylate-rich element binding proteins AUF together with HuR was reported to regulate mRNA stability and could hence participate in thyroid carcinogenesis. RNA binding proteins may harbor multifunctional properties that not only affect thyrocyte differentiation but could also affect TH synthesis [8]. Finally, the characterization of protein transport pathways and the identification of altered trafficking in thyroid carcinoma cells are approached in the experimental study by Tedelind et al. [9] that investigated the cathepsins which are known as thyroglobulin-processing enzymes with important functions in TH biosynthesis. We believe that the papers collected for this special issue of Thyroid Research will have a stimulatory effect not only for thyroid researchers but will be interesting for those from associated disciplines.

The antithyroid microsomal antibodies found in the serum of patients with autoimmune thyroid disease are directed largely, if not entirely, against thyroid peroxidase (TPO). In this study we used a highly purified, well characterized, large tryptic fragment of porcine TPO (hereafter referred to as purified porcine TPO) to examine possible differences among microsomal antibodies in patients with autoimmune thyroid disease. Antibodies against this TPO preparation and also against a synthetic peptide corresponding to residues 780-793 of the deduced sequence of the native enzyme were compared with microsomal antibodies from patients in immunoblot experiments. The antiporcine TPO and antisynthetic peptide antibodies reacted with crude preparations of human TPO. Binding of serum microsomal antibodies to purified porcine TPO was also found. Purified porcine TPO shows two fragments after gel electrophoresis under reducing conditions: a 59K fragment corresponding to the amino end of the molecule, and two approximately 30K fragments corresponding to the carboxyl end. Using an immunoblot procedure with purified porcine TPO as the antigen, we found that at least two epitopes were involved in microsomal antibody production: one associated with the 59K fragment and the other with the approximately 30K fragment(s). The distribution of serum antibodies against these epitopes differed among the patients, indicating that these antibodies comprise a heterogeneous group. Serum from patients with autoimmune thyroid disease significantly inhibited human TPO activity, raising the possibility that microsomal antibodies may contribute to the impaired thyroid function that occurs in some patients with autoimmune thyroid disease.