Cell biology of H2O2 generation in the thyroid: investigation of the control of dual oxidases (DUOX) activity in intact ex vivo thyroid tissue and cell lines

Ameliorative effect of ferulic acid on thyroid dysfunction against propyl-thiouracil induced hypothyroid rats

PURPOSE

Hypothyroidism is an endocrine disorder characterised by decreased T3, T4 and increased TSH levels. This study aims to examine the potential effects of Ferulic acid (FA) on rats with hypothyroidism induced by propylthiouracil through the estimation of biochemical parameters and histopathological studies.

METHODS

Twenty-five female wistar rats were allocated into five groups: Control group [1% CMC, p.o.], Disease group [PTU-50 mg/kg, p.o.], [Levothyroxine (LT4) group - 20 µg/kg, p.o. + PTU-50 mg/kg, p.o.], [FA -25 mg/kg, p.o. + PTU-50 mg/kg, p.o.] and [FA 50 mg/kg, p.o. + PTU-50 mg/kg, p.o.]. On 15th day blood was collected and serum was separated for estimation of biochemical parameters, liver and kidney homogenate was utilised for the estimation of oxidative stress markers and the thyroid gland was dissected to examine histological features.

RESULTS

PTU administration for 14 days showed a substantial decline in T3 and T4 and increases in TSH levels. PTU-administered rats significantly increased TC, TG and LDL levels, and decreased HDL levels. AST, ALT, urea, creatinine, and IL-6 were determined and these levels were significantly altered in PTU-induced hypothyroid group. In hypothyroid rats MDA, NO, GSH and SOD levels were significantly altered. However, treatment with FA for 14 days attenuated PTU-induced alterations. Furthermore, FA improves the histological changes of the thyroid gland.

CONCLUSION

In conclusion, FA treatment showed a protective effect against hypothyroidism by stimulating the thyroid hormones through the activation of thyroid peroxidase enzyme and improving thyroid function. In addition, FA diminished the increase in lipids, liver and kidney markers, oxidative stress and inflammation.

The association between the composite dietary antioxidant index and thyroid functionality among adults in the USA: NHANES 2007-2012

Objective

Composite Dietary Antioxidant Index (CDAI) values serve as a summary of an individual's combined dietary antioxidant intake. Although specific antioxidants are known to reduce thyroid damage from oxidative stress, the relationship between the CDAI and thyroid function remains uncertain. The purpose of this study was thus to investigate this relationship in greater detail while focusing on a representative American adult population.

Methods

A total of 6,860 subjects from the 2007-2012 NHANES cohort were included in this study. Associations between CDAI values and thyroid function were evaluated with weighted linear regression models and smoothed curve fitting. Subgroup analyses were also performed.

Results

The weighted mean (SD) values for variables analyzed in this study included a CDAI of 0.13 (0.06), serum free T4 (FT4) levels of 0.80 (0.01) ng/dL, and serum total T4 (TT4) levels of 7.80 (0.03) ug/dL. Lower CDAI values were found to be associated with higher levels of FT4 and TT4 using both unadjusted and adjusted models that accounted for relevant confounders (adjusted model, FT4 β = -0.003, p = 0.005; TT4 β = -0.035, p < 0.001). This negative correlation persisted when CDAI was categorized into quartiles (FT4, p for trend = 0.014; TT4, p for trend = 0.003).

Conclusion

These findings suggest that a diet rich in antioxidants, as reflected by higher CDAI scores, is associated with significant decreases in levels of free and total T4. Further analyses will be necessary to better clarify the underlying mechanisms behind these observations.

Involvement of toxic metals and PCBs mixture in the thyroid and male reproductive toxicity: In silico toxicogenomic data mining

Toxicological research is mostly limited to considering the effects of a single substance, even though the real exposure of people is reflected in their daily exposure to many different chemical substances in low-doses. This in silico toxicogenomic study aims to provide evidence for the selected environmental (organo)metals (lead, cadmium, methyl mercury) + polychlorinated biphenyls mixture involvement in the possible alteration of thyroid, and male reproductive system function, and furthermore to predict the possible toxic mechanisms of the environmental cocktail. The Comparative Toxicogenomic Database, GeneMANIA online software, and ToppGene Suite portal were used as the main tools for toxicogenomic data mining and gene ontology analysis. The results show that 35 annotated common genes between selected chemicals and endocrine system diseases can interact on the co-expression level. Our study highlighted the disruption of the cytokines, the cell's response to oxidative stress, and the influence of the transcription factors as the potential core of toxicological mechanisms of the discussed mixture's effects. The connected toxicological effects of the tested mixture were abnormal sperm cells, a disrupted level of testosterone, and thyroid hormones. The core mechanisms of these effects were inflammation, oxidative stress, disruption of androgen receptor signaling, and the alteration of the FOXO3-Keap-1/NRF2-HMOX1-NQO1 pathway signaling most likely controlled by the co-expression of overlapped genes among used chemicals. This in silico research can be used as a potential core for the determination of biomarkers that can be monitored in future further in vitro and in vivo experiments.

Tumor Microbial Communities and Thyroid Cancer Development-The Protective Role of Antioxidant Nutrients: Application Strategies and Future Directions

Thyroid cancer (TC), the most frequent malignancy of the endocrine system, has recorded an increasing incidence in the last decades. The etiology of TC remains at least partly unknown and, among modifiable risk factors, the gut microbiota and dietary nutrients (vitamins, essential microelements, polyphenols, probiotics) have been recognized to not only influence thyroid function, but exert critical effects on TC development and progression. Recent discoveries on the existence of tumor microbiota also in the TC microenvironment provide further evidence for the essential role of tumor microorganisms in TC etiology and severity, as well as acting as prognostic markers and as a potential target of adjuvant care in the treatment of TC patients. Therefore, in this review, we summarize current knowledge on the relationship of the tumor microbiome with the clinical tumor characteristics and TC progression, also illustrating the molecular mechanisms underlying this association, and how antioxidant nutrients may be used as a novel strategy to both control gut health and reduce the risk for TC. Furthermore, we discuss how new technologies might be exploited for the development of new foods with high nutritional values, antioxidant capability, and even attractiveness to the individual in terms of sensory and emotional features.

Bilateral Shifts in Deuterium Supply Similarly Change Physiology of the Pituitary–Thyroid Axis, but Differentially Influence Na+/I− Symporter Production

Deuterium, a stable isotope of hydrogen, is abundant in organisms. It is known to produce various biological effects. However, its impact in thyroid hormone synthesis and secretion is poorly studied. The aim of this investigation was to evaluate the dynamics of thyroid hormones and pituitary thyroid-stimulating hormone secretion during bilateral shifts in deuterium supply and assess a possible role of the Na+/I− symporter (NIS), the main iodide transporter, in altered thyroid function. The experiment was performed on adult male Wistar rats, which consumed deuterium-depleted ([D] = 10 ppm) and deuterium-enriched ([D] = 500,000 ppm) water for 21 days. The assessment of total thyroxine and triiodothyronine and their free fractions, as well as thyroid-stimulating hormone in blood serum, revealed the rapid response of the thyroid gland to shifts in the deuterium/protium balance. The present investigation shows that the bilateral changes in the deuterium body content similarly modulate thyroid hormone production and functional activity of the pituitary gland, but the responses of the thyroid and pituitary glands differ. The response of the thyroid cells was to increase the synthesis of the hormones and the pituitary thyrotropes, in order to reduce the production of the thyroid-stimulating hormone. The evaluation of NIS serum levels found a gradual increase in the rats that consumed deuterium-enriched water and no differences in the group exposed to deuterium depletion. NIS levels in both groups did not correlate with thyroid hormones and pituitary thyroid-stimulating hormone production. The data obtained show that thyroid gland has a higher sensitivity to shifts in the deuterium body content than the hypothalamic–pituitary complex, which responded later but similarly in the case of deuteration or deuterium depletion. It indicates a different sensitivity of the endocrine glands to alterations in deuterium content. It suggests that thyroid hormone production rate may depend on deuterium blood/tissue and cytosol/organelle gradients, which possibly disturb the secretory process independently of the NIS.

Disulfiram/Cu Kills and Sensitizes BRAF-Mutant Thyroid Cancer Cells to BRAF Kinase Inhibitor by ROS-Dependently Relieving Feedback Activation of MAPK/ERK and PI3K/AKT Pathways

BRAF^V600E, the most common genetic alteration, has become a major therapeutic target in thyroid cancer. Vemurafenib (PLX4032), a specific inhibitor of BRAF^V600E kinase, exhibits antitumor activity in patients with BRAF^V600E-mutated thyroid cancer. However, the clinical benefit of PLX4032 is often limited by short-term response and acquired resistance via heterogeneous feedback mechanisms. Disulfiram (DSF), an alcohol-aversion drug, shows potent antitumor efficacy in a copper (Cu)-dependent way. However, its antitumor activity in thyroid cancer and its effect on cellular response to BRAF kinase inhibitors remain unclear. Antitumor effects of DSF/Cu on BRAF^V600E-mutated thyroid cancer cells and its effect on the response of these cells to BRAF kinase inhibitor PLX4032 were systematically assessed by a series of in vitro and in vivo functional experiments. The molecular mechanism underlying the sensitizing effect of DSF/Cu on PLX4032 was explored by Western blot and flow cytometry assays. DSF/Cu exhibited stronger inhibitory effects on the proliferation and colony formation of BRAF^V600E-mutated thyroid cancer cells than DSF treatment alone. Further studies revealed that DSF/Cu killed thyroid cancer cells by ROS-dependent suppression of MAPK/ERK and PI3K/AKT signaling pathways. Our data also showed that DSF/Cu strikingly increased the response of BRAF^V600E-mutated thyroid cancer cells to PLX4032. Mechanistically, DSF/Cu sensitizes BRAF-mutant thyroid cancer cells to PLX4032 by inhibiting HER3 and AKT in an ROS-dependent way and subsequently relieving feedback activation of MAPK/ERK and PI3K/AKT pathways. This study not only implies potential clinical use of DSF/Cu in cancer therapy but also provides a new therapeutic strategy for BRAF^V600E-mutated thyroid cancers.

The protective role of nutritional antioxidants against oxidative stress in thyroid disorders

An imbalance between pro-oxidative and antioxidative cellular mechanisms is oxidative stress (OxS) which may be systemic or organ-specific. Although OxS is a consequence of normal body and organ physiology, severely impaired oxidative homeostasis results in DNA hydroxylation, protein denaturation, lipid peroxidation, and apoptosis, ultimately compromising cells' function and viability. The thyroid gland is an organ that exhibits both oxidative and antioxidative processes. In terms of OxS severity, the thyroid gland's response could be physiological (i.e. hormone production and secretion) or pathological (i.e. development of diseases, such as goitre, thyroid cancer, or thyroiditis). Protective nutritional antioxidants may benefit defensive antioxidative systems in resolving pro-oxidative dominance and redox imbalance, preventing or delaying chronic thyroid diseases. This review provides information on nutritional antioxidants and their protective roles against impaired redox homeostasis in various thyroid pathologies. We also review novel findings related to the connection between the thyroid gland and gut microbiome and analyze the effects of probiotics with antioxidant properties on thyroid diseases.

Emerging Role of Oxidative Stress on EGFR and OGG1-BER Cross-Regulation: Implications in Thyroid Physiopathology

Thyroid diseases have a complex and multifactorial aetiology. Despite the numerous studies on the signals referable to the malignant transition, the molecular mechanisms concerning the role of oxidative stress remain elusive. Based on its strong oxidative power, H2O2 could be responsible for the high level of oxidative DNA damage observed in cancerous thyroid tissue and hyperactivation of mitogen-activated protein kinase (MAPK) and PI3K/Akt, which mediate ErbB signaling. Increased levels of 8-oxoG DNA adducts have been detected in the early stages of thyroid cancer. These DNA lesions are efficiently recognized and removed by the base excision repair (BER) pathway initiated by 8-oxoG glycosylase1 (OGG1). This study investigated the relationships between the EGFR and OGG1-BER pathways and their mutual regulation following oxidative stress stimulus by H2O2 in human thyrocytes. We clarified the modulation of ErbB receptors and their downstream pathways (PI3K/Akt and MAPK/ERK) under oxidative stress (from H2O2) at the level of gene and protein expression, according to the mechanism defined in a human non-pathological cell system, Nthy-ori 3-1. Later, on the basis of the results obtained by gene expression cluster analysis in normal cells, we assessed the dysregulation of the relationships in a model of papillary thyroid cancer with RET/PTC rearrangement (TPC-1). Our observations demonstrated that a H2O2 stress may induce a physiological cross-regulation between ErbB and OGG1-BER pathways in normal thyroid cells (while this is dysregulated in the TPC-1 cells). Gene expression data also delineated that MUTYH gene could play a physiological role in crosstalk between ErbB and BER pathways and this function is instead lost in cancer cells. Overall, our data on OGG1 protein expression suggest that it was physiologically regulated in response to oxidative modulation of ErbB, and that these might be dysregulated in the signaling pathway involving AKT in the progression of thyroid malignancies with RET/PTC rearrangements.

Sex Bias in Differentiated Thyroid Cancer

Differentiated thyroid cancers are more frequent in women than in men. These different frequencies may depend on differences in patient's behavior and in thyroid investigations. However, an impact on sexual hormones is likely, although this has been insufficiently elucidated. Estrogens may increase the production of mutagenic molecules in the thyroid cell and favor the proliferation and invasion of tumoral cells by regulating both the thyrocyte enzymatic machinery and the inflammatory process associated with tumor growth. On the other hand, the worse prognosis of thyroid cancer associated with the male gender is poorly explained.

The Influence of Oxidative Stress on Thyroid Diseases

Thyroid diseases, including neoplasms, autoimmune diseases and thyroid dysfunctions, are becoming a serious social problem with rapidly increasing prevalence. The latter is increasingly linked to oxidative stress. There are many methods for determining the biomarkers of oxidative stress, making it possible to evaluate the oxidative profile in patients with thyroid diseases compared to the healthy population. This opens up a new perspective for investigating the role of elevated parameters of oxidative stress and damage in people with thyroid diseases, especially of neoplastic nature. An imbalance between oxidants and antioxidants is observed at different stages and in different types of thyroid diseases. The organ, which is part of the endocrine system, uses free radicals (reactive oxygen species, ROS) to produce hormones. Thyroid cells release enzymes that catalyse ROS generation; therefore, a key role is played by the internal defence system and non-enzymatic antioxidants that counteract excess ROS not utilised to produce thyroid hormones, acting as a buffer to neutralise free radicals and ensure whole-body homeostasis. An excess of free radicals causes structural cell damage, undermining genomic stability. Looking at the negative effects of ROS accumulation, oxidative stress appears to be implicated in both the initiation and progression of carcinogenesis. The aim of this review is to investigate the oxidation background of thyroid diseases and to summarise the links between redox imbalance and thyroid dysfunction and disease.

Role of Dual Oxidases in Ventilator-induced Lung Injury

Positive-pressure ventilation results in ventilator-induced lung injury, and few therapeutic modalities have been successful at limiting the degree of injury to the lungs. Understanding the primary drivers of ventilator-induced lung injury will aid in the development of specific treatments to ameliorate the progression of this syndrome. There are conflicting data for the role of neutrophils in acute respiratory distress syndrome pathogenesis. Here, we specifically examined the importance of neutrophils as a primary driver of ventilator-induced lung injury in a mouse model known to have impaired ability to recruit neutrophils in previous models of inflammation. We exposed Duoxa^+/+ and Duoxa^-/- mice to low- or high-tidal volume ventilation with or without positive end-expiratory pressure (PEEP) and recruitment maneuvers for 4 hours. Absolute neutrophils in BAL fluid were significantly reduced in Duoxa^-/- mice compared with Duoxa^+/+ mice (6.7 cells/μl; 16.4 cells/μl; P = 0.003), consistent with our hypothesis that neutrophil translocation across the capillary endothelium is reduced in the absence of DUOX1 or DUOX2 in response to ventilator-induced lung injury. Reduced lung neutrophilia was not associated with a reduction in overall lung injury in this study, suggesting that neutrophils do not play an important role in early features of acute lung injury. Surprisingly, Duoxa^-/- mice exhibited significant hypoxemia, as measured by the arterial oxygen tension/fraction of inspired oxygen ratio and arterial oxygen content, which was out of proportion with that seen in the Duoxa^+/+ mice (141, 257, P = 0.012). These findings suggest a role for dual oxidases to limit physiologic impairment during early ventilator-induced lung injury.

Impact of Antioxidant Natural Compounds on the Thyroid Gland and Implication of the Keap1/Nrf2 Signaling Pathway

BACKGROUND

Natural compounds with potential antioxidant properties have been used in the form of food supplements or extracts with the intent to prevent or treat various diseases. Many of these compounds can activate the cytoprotective Nrf2 pathway. Besides, some of them are known to impact the thyroid gland, often with potential side-effects, but in other instances, with potential utility in the treatment of thyroid disorders.

OBJECTIVE

In view of recent data regarding the multiple roles of Nrf2 in the thyroid, this review summarizes the current bibliography on natural compounds that can have an effect on thyroid gland physiology and pathophysiology, and it discusses the potential implication of the Nrf2 system in the respective mechanisms.

METHODS & RESULTS

Literature searches for articles from 1950 to 2018 were performed in PubMed and Google Scholar using relevant keywords about phytochemicals, Nrf2 and thyroid. Natural substances were categorized into phenolic compounds, sulfur-containing compounds, quinones, terpenoids, or under the general category of plant extracts. For individual compounds in each category, respective data were summarized, as derived from in vitro (cell lines), preclinical (animal models) and clinical studies. The main emerging themes were as follows: phenolic compounds often showed potential to affect the production of thyroid hormones; sulfur-containing compounds impacted the pathogenesis of goiter and the proliferation of thyroid cancer cells; while quinones and terpenoids modified Nrf2 signaling in thyroid cell lines.

CONCLUSION

Natural compounds that modify the activity of the Nrf2 pathway should be evaluated carefully, not only for their potential to be used as therapeutic agents for thyroid disorders, but also for their thyroidal safety when used for the prevention and treatment of non-thyroidal diseases.

Class III PI3K Vps34 Controls Thyroid Hormone Production by Regulating Thyroglobulin Iodination, Lysosomal Proteolysis, and Tissue Homeostasis

Background: The production of thyroid hormones [triiodothyronine (T3), thyroxine (T4)] depends on the organization of the thyroid in follicles, which are lined by a monolayer of thyrocytes with strict apicobasal polarity. This polarization supports vectorial transport of thyroglobulin (Tg) for storage into, and recapture from, the colloid. It also allows selective addressing of channels, transporters, ion pumps, and enzymes to their appropriate basolateral [Na⁺/I^- symporter (NIS), SLC26A7, and Na⁺/K⁺-ATPase] or apical membrane domain (anoctamin, SLC26A4, DUOX2, DUOXA2, and thyroperoxidase). How these actors of T3/T4 synthesis reach their final destination remains poorly understood. The PI 3-kinase isoform Vps34/PIK3C3 is now recognized as a main component in the general control of vesicular trafficking and of cell homeostasis through the regulation of endosomal trafficking and autophagy. We recently reported that conditional Vps34 inactivation in proximal tubular cells in the kidney prevents normal addressing of apical membrane proteins and causes abortive macroautophagy. Methods:Vps34 was inactivated using a Pax8-driven Cre recombinase system. The impact of Vps34 inactivation in thyrocytes was analyzed by histological, immunolocalization, and messenger RNA expression profiling. Thyroid hormone synthesis was assayed by ¹²⁵I injection and plasma analysis. Results:Vps34 conditional knockout (Vps34^cKO) mice were born at the expected Mendelian ratio and showed normal growth until postnatal day 14 (P14), then stopped growing and died at ∼1 month of age. We therefore analyzed thyroid Vps34^cKO at P14. We found that loss of Vps34 in thyrocytes causes (i) disorganization of thyroid parenchyma, with abnormal thyrocyte and follicular shape and reduced PAS⁺ colloidal spaces; (ii) severe noncompensated hypothyroidism with extremely low T4 levels (0.75 ± 0.62 μg/dL) and huge thyrotropin plasma levels (19,300 ± 10,500 mU/L); (iii) impaired ¹²⁵I organification at comparable uptake and frequent occurrence of follicles with luminal Tg but nondetectable T4-bearing Tg; (iv) intense signal in thyrocytes for the lysosomal membrane marker, LAMP-1, as well as Tg and the autophagy marker, p62, indicating defective lysosomal proteolysis; and (v) presence of macrophages in the colloidal space. Conclusions: We conclude that Vps34 is crucial for thyroid hormonogenesis, at least by controlling epithelial organization, Tg iodination as well as proteolytic T3/T4 excision in lysosomes.

The Dual Oxidase Duox2 stabilized with DuoxA2 in an enzymatic complex at the surface of the cell produces extracellular H2O2 able to induce DNA damage in an inducible cellular model

Thyroid hormone synthesis requires H₂O₂, produced by two NADPH oxidases, Duox1 and Duox2. To be fully active at the apical pole of the thyrocytes, these enzymes need additional maturation factors DuoxA1 and DuoxA2. The proteins have been shown to be localized at the cell surface, suggesting that they could form a complex with Duox counterparts. We have generated multiple HEK293 Tet-On3G cell lines that express various combinations of DuoxA upon doxycycline induction, in association with a constitutive expression of the Duox enzyme. We compared Duox specific activity, Duox/DuoxA cell surface interactions and the cellular consequences of sustained H₂O₂ generation. By normalizing H₂O₂ extracellular production by Duox or DuoxA membrane expression, we have demonstrated that the most active enzymatic complex is Duox2/DuoxA2, compared to Duox1/DuoxA1. A direct cell surface interaction was shown between Duox1/2 and both DuoxA1 and DuoxA2 using the Duolink® technology, Duox1/DuoxA1 and Duox2/DuoxA2 membrane complexes being more stable than the unpaired ones. A significant increase in DNA damage was observed in the nuclei of Duox2/DuoxA2 expressing cells after doxycycline induction and stimulation of Duox catalytic activity. The maturation and activity of Duox2 were drastically impaired when expressed with the glycosylation-defective maturation factor DuoxA2, while the impact of the unglycosylated DuoxA1 mutant on Duox1 membrane expression and activity was rather limited. The present data demonstrate for the first time that H₂O₂ produced by the Duox2/DuoxA2 cell surface enzymatic complex could provoke potential mutagenic DNA damage in an inducible cellular model, and highlight the importance of the co-expressed partner in the activity and stability of Duox/DuoxA complexes.

Human exposure to bisphenol AF and diethylhexylphthalate increases susceptibility to develop differentiated thyroid cancer in patients with thyroid nodules

Pollutants represent potential threats to the human health, being ubiquitous in the environment and exerting toxicity even at low doses. This study aims at investigating the role of fifteen multiclass organic pollutants, assumed as markers of environmental pollution, most of which exerting endocrine-disrupting activity, in thyroid cancer development. The increasing incidence of differentiated thyroid cancer (DTC) may be related to the rising production and environmental dissemination of pollutants. Fifty-five patients, twenty-seven with diagnosis of benign thyroid nodules and twenty-eight suffering from differentiated thyroid cancer, were enrolled and the concentration levels of seven bisphenols, two phthalates (i.e. di(2-ethylhexyl) phthalate (DEHP) and its main metabolite, mono-(2-ethyl-hexyl) phthalate) (MEHP)), two chlorobenzenes, (1,4-dichlorobenzene and 1,2,4,5-tetrachlorobenzene), and 3 phenol derivatives (2-chlorophenol, 4- nonylphenol, and triclosan) were determined in their serum by using a validated analytical method based on high performance liquid chromatography with ultraviolet tandem fluorescence detection. A significant relationship was found between malignancy and the detection in the serum of both bisphenol AF and DEHP. Indeed, their presence confers a more than fourteen times higher risk of developing differentiated thyroid cancer. Relationship between these two pollutants and the risk of malignancy was dose-independent and not mediated by higher thyroid stimulating hormone levels. Even if a conclusive evidence cannot still be drawn and larger prospective studies are needed, the exposure to low doses of environmental endocrine-disrupting contaminants can be considered consistent with the development of thyroid cancer.

Factors contributing to the resistance of the thyrocyte to hydrogen peroxide

We studied the mechanism that may explain the relative resistance of thyrocytes to H₂O₂ compared to other cell types. Ability to degrade H₂O₂, glutathione peroxidase (GPx) activity, heme oxygenase-1 (HO-1) expression, cell survival and capacity to repair DNA damage after H₂O₂ exposure or irradiation were measured in human thyrocytes in primary culture and compared to the values obtained in human T-cells and different cell lines. Compared to other cell types, thyrocytes presented a low mortality rate after H₂O₂ exposure, rapidly degraded extracellular H₂O₂ and presented a high basal seleno-dependent GPx activity. Only in thyrocytes, H₂O₂ up-regulated GPx activity and expression of HO-1 mRNA. These effects were not reproduced by irradiation. DNA damage caused by H₂O₂ was more slowly repaired than that caused by irradiation and not repaired at all in T-cells. Our study demonstrates that the thyrocyte has specific protective mechanisms against H₂O₂ and its mutagenic effects.

Expression, Localization, and Regulation of the Sodium Bicarbonate Cotransporter NBCe1 in the Thyroid

BACKGROUND

The intrafollicular space of thyroid follicles is the storage compartment for thyroid hormones. Its pH has been established at around 7.6 at least after thyrotropin (TSH) stimulation. This alkaline intrafollicular pH is thought to be critical for iodide coupling to thyroglobulin and internalization of iodinated thyroglobulin. At least in mice, this alkalinization requires the expression of pendrin (Slc26a4) within the apical membrane, and a lack of pendrin results in acidic follicular lumen pH. Yet, the mechanism importing HCO₃^- into the cytoplasm is unknown. This study investigated whether the rather ubiquitous sodium bicarbonate cotransporter NBCe1 (SLC4A4) might play this role. It also examined which variant was expressed and where it was localized in both rat and human thyroid tissue. Lastly, the dependence of its expression on TSH was studied.

METHODS

Reverse transcription polymerase chain reaction, immunofluorescence, and Western blotting were used to test whether TSH stimulated NBCe1 protein expression in vivo. Subcellular localization of NBCe1 was performed using immunofluorescence in both rat and human thyroid. Cultured thyroid cells were also used to attempt to define how TSH affects NBCe1 expression.

RESULTS

Only transcripts of the NBCe1-B variant were detected in both rat and human thyroid. Of interest, NBCe1-C was not detected in human tissues, not even in the brain. On immunofluorescence microscopy, the immunostaining of NBCe1 mainly appeared in the basolateral membrane upon stimulation with TSH. This TSH induction of basolateral membrane expression of NBCe1 protein was confirmed in vivo in rat thyroid and in vitro on human thyroid slices.

CONCLUSIONS

This study demonstrates the expression of the sodium bicarbonate cotransporter NBCe1-B in rat and human thyroid. Additionally, the data suggest that TSH blocks the degradation of NBCe1 protein by trafficking it to the basolateral membrane. Hence, TSH increases NBCe1 half-life without increasing its synthesis.

Carcinogenesis and Reactive Oxygen Species Signaling: Interaction of the NADPH Oxidase NOX1-5 and Superoxide Dismutase 1-3 Signal Transduction Pathways

SIGNIFICANCE

Reduction/oxidation (redox) balance could be defined as an even distribution of reduction and oxidation complementary processes and their reaction end products. There is a consensus that aberrant levels of reactive oxygen species (ROS), commonly observed in cancer, stimulate primary cell immortalization and progression of carcinogenesis. However, the mechanism how different ROS regulate redox balance is not completely understood. Recent Advances: In the current review, we have summarized the main signaling cascades inducing NADPH oxidase NOX1-5 and superoxide dismutase (SOD) 1-3 expression and their connection to cell proliferation, immortalization, transformation, and CD34+ cell differentiation in thyroid, colon, lung, breast, and hematological cancers.

CRITICAL ISSUES

Interestingly, many of the signaling pathways activating redox enzymes or mediating the effect of ROS are common, such as pathways initiated from G protein-coupled receptors and tyrosine kinase receptors involving protein kinase A, phospholipase C, calcium, and small GTPase signaling molecules.

FUTURE DIRECTIONS

The clarification of interaction of signal transduction pathways could explain how cells regulate redox balance and may even provide means to inhibit the accumulation of harmful levels of ROS in human pathologies.

Homozygous loss-of-function mutations in SLC26A7 cause goitrous congenital hypothyroidism

Defects in genes mediating thyroid hormone biosynthesis result in dyshormonogenic congenital hypothyroidism (CH). Here, we report homozygous truncating mutations in SLC26A7 in 6 unrelated families with goitrous CH and show that goitrous hypothyroidism also occurs in Slc26a7-null mice. In both species, the gene is expressed predominantly in the thyroid gland, and loss of function is associated with impaired availability of iodine for thyroid hormone synthesis, partially corrected in mice by iodine supplementation. SLC26A7 is a member of the same transporter family as SLC26A4 (pendrin), an anion exchanger with affinity for iodide and chloride (among others), whose gene mutations cause congenital deafness and dyshormonogenic goiter. However, in contrast to pendrin, SLC26A7 does not mediate cellular iodide efflux and hearing in affected individuals is normal. We delineate a hitherto unrecognized role for SLC26A7 in thyroid hormone biosynthesis, for which the mechanism remains unclear.

Iodine consumption and cognitive performance: Confirmation of adequate consumption

Iodine, a dynamic nutrient present in thyroid hormones, is responsible for regulating thyroid function, supporting a healthy metabolism, and aiding growth and development. Iodine is also essential for brain development during specific time windows influencing neurogenesis, neuronal and glial cell differentiation, myelination, neuronal migration, and synaptogenesis. About 1.5 billion people in 130 countries live in areas at risk of iron deficiencies (IDs). Reduced mental ability due to IDs occurs in almost 300 million people. Ensuring the consumption of minimum recommended daily allowances of iodine remains challenging. The effects of ID disorders range from high mortality of fetuses and children to inhibited mental development (cretinism). Poor socioeconomic development and impaired school performance are also notable. Currently, ID disorders are the single greatest contributor to preventable brain damage in fetuses and infants and arrested psychomotor development in children. Iodized salt may help fulfill iodine requirements. Increases in food salt iodization programs can help overcome ID disorders. Dietary plans can be well adjusted to incorporate iodinated foods. Maternal iodine supplementation for offspring requires adequate attention. Fruits, vegetables, bread, eggs, legumes (beans and peas), nuts, seeds, seafood, lean meats and poultry, and soy products provide small quantities of iodine. Nutrient-dense foods containing essential vitamins and minerals such as iodine may confer positive effects. To some extent, fortified foods and daily dietary supplements can be provided for different nutrients including iodine; otherwise, iodine may be consumed in less than the recommended amounts. This review focuses on aspects of adequate iodine consumption to avoid cognitive impairments.

Biosynthesis of human myeloperoxidase

Members of Chordata peroxidase subfamily [1] expressed in mammals, including myeloperoxidase (MPO), eosinophil peroxidase (EPO), lactoperoxidase (LPO), and thyroid peroxidase (TPO), express conserved motifs around the heme prosthetic group essential for their activity, a calcium-binding site, and at least two covalent bonds linking the heme group to the protein backbone. Although most studies of the biosynthesis of these peroxidases have focused on MPO, many of the features described occur during biosynthesis of other members of the protein subfamily. Whereas MPO biosynthesis includes events typical for proteins generated in the secretory pathway, the importance and consequences of heme insertion are events uniquely associated with peroxidases. This Review summarizes decades of work elucidating specific steps in the biosynthetic pathway of human MPO. Discussion includes cotranslational glycosylation and subsequent modifications of the N-linked carbohydrate sidechains, contributions by molecular chaperones in the endoplasmic reticulum, cleavage of the propeptide from proMPO, and proteolytic processing of protomers and dimerization to yield mature MPO. Parallels between the biosynthesis of MPO and TPO as well as the impact of inherited mutations in the MPO gene on normal biosynthesis will be summarized. Lastly, specific gaps in our knowledge revealed by this review of our current understanding will be highlighted.

A simple short term method to study thyroid disruption using a fetal rat thyroid culture

INTRODUCTION

Thyroid modulation activity has not been investigated for many chemical substances. Due to ethical, practical and financial reasons, in vivo evaluation of a large number of compounds is not feasible. It has been proposed that an in vitro mechanism-based strategy could be more adequate for the identification of thyroid hormone disrupting chemicals. Here we describe a simple and mostly inexpensive, short term culture assay to study thyroid disruption.

METHODS

Fetal thyroids collected from gestation day 20.5 were cultured up to 24h in Hank's saline solution, at 37°C with oxygenation at 0 and 12h. Viability of the cultured explants was evaluated by the MTT assay. Positive (thyroid stimulating hormone, TSH) and negative (6-propyl-2-thiouracil, PTU) modulation of cultured thyroids was assessed with morphometrical analysis of H & E stained gland sections. Thyroxine expression was evaluated by immunohistochemistry.

RESULTS

Viability was shown to increase with time of culture with higher metabolic activity being achieved at 24h as compared to shorter periods of incubation. Follicular epithelial cells exhibited a statistically significant dependence on thyrotropin concentration, although more evident in the inner than in the outer portion of the glands. As expected, TSH induced expression of thyroxin while PTU inhibited it.

DISCUSSION

GD20.5 fetal thyroids may be cultured up to 24h under relatively simple laboratory conditions during which viability and function of the gland are preserved showing that it is possible to reproduce in vivo response under in vitro conditions. This culture could be a suitable short term assay to study mechanism of thyroid disruption.

Reactive Oxygen Species Link Gonadotropin-Releasing Hormone Receptor Signaling Cascades in the Gonadotrope

Biological rhythms lie at the center of regulatory schemes that control many aspects of living systems. At the cellular level, meaningful responses to external stimuli depend on propagation and quenching of a signal to maintain vigilance for subsequent stimulation or changes that serve to shape and modulate the response. The hypothalamus-pituitary-gonad endocrine axis that controls reproductive development and function relies on control through rhythmic stimulation. Central to this axis is the pulsatile stimulation of the gonadotropes by hypothalamic neurons through episodic release of the neuropeptide gonadotropin-releasing hormone. Alterations in pulsatile stimulation of the gonadotropes result in differential synthesis and secretion of the gonadotropins LH and FSH and changes in the expression of their respective hormone subunit genes. The requirement to amplify signals arising from activation of the gonadotropin-releasing hormone (GnRH) receptor and to rapidly quench the resultant signal to preserve an adaptive response suggests the need for rapid activation and feedback control operating at the level of intracellular signaling. Emerging data suggest that reactive oxygen species (ROS) can fulfill this role in the GnRH receptor signaling through activation of MAP kinase signaling cascades, control of negative feedback, and participation in the secretory process. Results obtained in gonadotrope cell lines or other cell models indicate that ROS can participate in each of these regulatory cascades. We discuss the potential advantage of reactive oxygen signaling for modulating the gonadotrope response to GnRH stimulation and the potential mechanisms for this action. These observations suggest further targets of study for regulation in the gonadotrope.

Overexpression of Interleukin-4 in the Thyroid of Transgenic Mice Upregulates the Expression of Duox1 and the Anion Transporter Pendrin

BACKGROUND

The dual oxidases (Duox) are involved in hydrogen peroxide generation, which is essential for thyroid hormone synthesis, and therefore they are markers of thyroid function. During inflammation, cytokines upregulate DUOX gene expression in the airway and the intestine, suggesting a role for these proteins in innate immunity. It was previously demonstrated that interleukin-4 (IL-4) upregulates DUOX gene expression in thyrocytes. Although the role of IL-4 in autoimmune thyroid diseases has been studied extensively, the effects of IL-4 on thyroid physiology remain largely unknown. Therefore, a new animal model was generated to study the impact of IL-4 on thyroid function.

METHODS

Transgenic (Thyr-IL-4) mice with thyroid-targeted expression of murine IL-4 were generated. Transgene expression was verified at the mRNA and protein level in thyroid tissues and primary cultures. The phenotype of the Thyr-IL-4 animals was characterized by measuring serum thyroxine (T4) and thyrotropin levels and performing thyroid morphometric analysis, immunohistochemistry, whole transcriptome sequencing, quantitative reverse transcription polymerase chain reaction, and ex vivo thyroid function assays.

RESULTS

Thyrocytes from two Thyr-IL-4 mouse lines (#30 and #52) expressed IL-4, which was secreted into the extracellular space. Although 10-month-old transgenic animals had T4 and thyrotropin serum levels in the normal range, they had altered thyroid follicular structure with enlarged follicles composed of elongated thyrocytes containing numerous endocytic vesicles. These follicles were positive for T4 staining the colloid, indicating their capacity to produce thyroid hormones. RNA profiling of Thyr-IL-4 thyroid samples revealed modulation of multiple genes involved in inflammation, while no major leukocyte infiltration could be detected. Upregulated expression of Duox1, Duoxa1, and the pendrin anion exchanger gene (Slc26a4) was detected. In contrast, the iodide symporter gene Slc5a5 was markedly downregulated resulting in impaired iodide uptake and reduced thyroid hormone levels in transgenic thyroid tissue. Hydrogen peroxide production was increased in Thyr-IL-4 thyroid tissue compared with wild-type animals, but no significant oxidative stress could be detected.

CONCLUSIONS

This is the first study to show that ectopic expression of IL-4 in thyroid tissue upregulates Duox1/Duoxa1 and Slc26a4 expression in the thyroid. The present data demonstrate that IL-4 could affect thyroid morphology and function, mainly by downregulating Slc5a5 expression, while maintaining a normal euthyroid phenotype.

Glycolysis-associated enzymes existing in the follicular lumen of the thyroid may interfere with energy metabolism

Synthesis and storage of the thyroid hormone precursor, thyroglobulin (TG), occurs within the follicular lumen of the thyroid and the TG is then absorbed into cells for further processing before release into the blood. However, the mechanism of energy metabolism in the follicular lumen of the thyroid remains unknown. In the present study, the three dimensional structure of thyroid follicles was constructed using a primary culture of swine cells and the follicular protein was identified via matrix-assisted laser desorption/ionization-time of flight mass spectrometry. Three glycolysis-associated enzymes, enolase, pyruvate kinase and phosphoglyceraldehyde dehydrogenase were identified in addition to TG. These results support the hypothesis that anaerobic glycolysis of glucose exists in the follicle and supports energy consumption for hormone synthesis.

Hypothalamus-Pituitary-Thyroid Axis

The hypothalamus-pituitary-thyroid (HPT) axis determines the set point of thyroid hormone (TH) production. Hypothalamic thyrotropin-releasing hormone (TRH) stimulates the synthesis and secretion of pituitary thyrotropin (thyroid-stimulating hormone, TSH), which acts at the thyroid to stimulate all steps of TH biosynthesis and secretion. The THs thyroxine (T4) and triiodothyronine (T3) control the secretion of TRH and TSH by negative feedback to maintain physiological levels of the main hormones of the HPT axis. Reduction of circulating TH levels due to primary thyroid failure results in increased TRH and TSH production, whereas the opposite occurs when circulating THs are in excess. Other neural, humoral, and local factors modulate the HPT axis and, in specific situations, determine alterations in the physiological function of the axis. The roles of THs are vital to nervous system development, linear growth, energetic metabolism, and thermogenesis. THs also regulate the hepatic metabolism of nutrients, fluid balance and the cardiovascular system. In cells, TH actions are mediated mainly by nuclear TH receptors (210), which modify gene expression. T3 is the preferred ligand of THR, whereas T4, the serum concentration of which is 100-fold higher than that of T3, undergoes extra-thyroidal conversion to T3. This conversion is catalyzed by 5'-deiodinases (D1 and D2), which are TH-activating enzymes. T4 can also be inactivated by conversion to reverse T3, which has very low affinity for THR, by 5-deiodinase (D3). The regulation of deiodinases, particularly D2, and TH transporters at the cell membrane control T3 availability, which is fundamental for TH action. © 2016 American Physiological Society. Compr Physiol 6:1387-1428, 2016.

GnRH Regulates Gonadotropin Gene Expression Through NADPH/Dual Oxidase-Derived Reactive Oxygen Species

The appropriate control of synthesis and secretion of the gonadotropin hormones LH and FSH by pituitary gonadotropes is essential for the regulation of reproduction. The hypothalamic neuropeptide GnRH is the central regulator of both processes, coordinating secretion with transcription and translation of the gonadotropin hormone subunit genes. The MAPK family of second messengers is strongly induced in gonadotropes upon GnRH stimulation, and multiple pathways activate these kinases. Intracellular reactive oxygen species participate in signaling cascades that target MAPKs, but also participate in signaling events indicative of cell stress. The NADPH oxidase (NOX)/dual oxidase (DUOX) family is a major enzymatic source of intracellular reactive oxygen, and we show that GnRH stimulation of mouse primary pituitary cells and the LβT2 gonadotrope cell line elevates intracellular reactive oxygen via NOX/DUOX activity. Mouse pituitary and LβT2 cells abundantly express NOX/DUOX and cofactor mRNAs. Pharmacological inhibition of NOX/DUOX activity diminishes GnRH-stimulated activation of MAPKs, immediate-early gene expression, and gonadotropin subunit gene expression. Inhibitor studies implicate the calcium-activated DUOX family as a major, but not exclusive, participant in GnRH signaling. Knockdown of DUOX2 in LβT2 cells reduces GnRH-induced Fshb, but not Lhb mRNA levels, suggesting differential sensitivity to DUOX activity. Finally, GnRH pulse-stimulated FSH and LH secretion are suppressed by inhibition of NOX/DUOX activity. These results indicate that reactive oxygen is a potent signaling intermediate produced in response to GnRH stimulation and further suggest that reactive oxygen derived from other sources may influence the gonadotrope response to GnRH stimulation.

Anoctamin-1/TMEM16A is the major apical iodide channel of the thyrocyte

Iodide is captured by thyrocytes through the Na(+)/I(-) symporter (NIS) before being released into the follicular lumen, where it is oxidized and incorporated into thyroglobulin for the production of thyroid hormones. Several reports point to pendrin as a candidate protein for iodide export from thyroid cells into the follicular lumen. Here, we show that a recently discovered Ca(2+)-activated anion channel, TMEM16A or anoctamin-1 (ANO1), also exports iodide from rat thyroid cell lines and from HEK 293T cells expressing human NIS and ANO1. The Ano1 mRNA is expressed in PCCl3 and FRTL-5 rat thyroid cell lines, and this expression is stimulated by thyrotropin (TSH) in rat in vivo, leading to the accumulation of the ANO1 protein at the apical membrane of thyroid follicles. Moreover, ANO1 properties, i.e., activation by intracellular calcium (i.e., by ionomycin or by ATP), low but positive affinity for pertechnetate, and nonrequirement for chloride, better fit with the iodide release characteristics of PCCl3 and FRTL-5 rat thyroid cell lines than the dissimilar properties of pendrin. Most importantly, iodide release by PCCl3 and FRTL-5 cells is efficiently blocked by T16Ainh-A01, an ANO1-specific inhibitor, and upon ANO1 knockdown by RNA interference. Finally, we show that the T16Ainh-A01 inhibitor efficiently blocks ATP-induced iodide efflux from in vitro-cultured human thyrocytes. In conclusion, our data strongly suggest that ANO1 is responsible for most of the iodide efflux across the apical membrane of thyroid cells.

Roles of DUOX-mediated hydrogen peroxide in metabolism, host defense, and signaling

SIGNIFICANCE

Among the NADPH oxidases, the dual oxidases, DUOX1 and DUOX2, constitute a distinct subfamily initially called thyroid oxidases, based on their high level of expression in thyroid tissue. Genetic alterations causing inherited hypothyroidism clearly demonstrate their physiological implication in thyroid hormonogenesis. However, a growing list of biological functions triggered by DUOX-dependent reactive oxygen species (ROS) in highly differentiated mucosae have recently emerged.

RECENT ADVANCES

A role of DUOX enzymes as ROS providers for lactoperoxidase-mediated killing of invading pathogens has been well established and a role in bacteria chemorepulsion has been proposed. Control of DUOX expression and activity by inflammatory molecules and immune receptor activation consolidates their contributions to innate immune defense of mucosal surfaces. Recent studies conducted in ancestral organisms have identified effectors of DUOX redox signaling involved in wound healing including epithelium regeneration and leukocyte recruitment. Moreover, local generation of hydrogen peroxide (H2O2) by DUOX has also been suggested to constitute a positive feedback loop to promote receptor signaling activation.

CRITICAL ISSUES

A correct balance between H2O2 generation and detoxification mechanisms must be properly maintained to avoid oxidative damages. Overexpression of DUOX genes has been associated with an increasing number of chronic inflammatory diseases. Furthermore, H2O2-mediated DNA damage supports a mutagenic function promoting tumor development.

FUTURE DIRECTIONS

Despite the high sequence similarity shared between DUOX1 and DUOX2, the two isoforms present distinct regulations, tissue expression and catalytic functions. The phenotypic characterization of novel DUOX/DUOXA invalidated animal models will be very useful for defining their medical importance in pathological conditions.

Role of the NADPH Oxidases DUOX and NOX4 in Thyroid Oxidative Stress

Somatic mutations are present at high levels in the rat thyroid gland, indicating that the thyrocyte is under oxidative stress, a state in which cellular oxidant levels are high. The most important class of free radicals, or reactive metabolites, is reactive oxygen species (ROS), such as superoxide anion (O2 (-)), hydroxyl radical (OH) and hydrogen peroxide (H2O2). The main source of ROS in every cell type seems to be mitochondrial respiration; however, recent data support the idea that NADPH:O(2) oxidoreductase flavoproteins or simply NADPH oxidases (NOX) are enzymes specialized in controlled ROS generation at the subcellular level. Several decades ago, high concentrations of H2O2 were detected at the apical surface of thyrocytes, where thyroid hormone biosynthesis takes place. Only in the last decade has the enzymatic source of H2O2 involved in thyroid hormone biosynthesis been well characterized. The cloning of two thyroid genes encoding NADPH oxidases dual oxidases 1 and 2 (DUOX1 and DUOX2) revealed that DUOX2 mutations lead to hereditary hypothyroidism in humans. Recent reports have also described the presence of NOX4 in the thyroid gland and have suggested a pathophysiological role of this member of the NOX family. In the present review, we describe the participation of NADPH oxidases not only in thyroid physiology but also in gland pathophysiology, particularly the involvement of these enzymes in the regulation of thyroid oxidative stress.

Epigenetic regulation of survivin by Bmi1 is cell type specific during corticogenesis and in gliomas

Polycomb group proteins are essential regulators of stem cell function during embryonic development and in adult tissue homeostasis. Bmi1, a key component of the Polycomb Repressive Complex 1, is highly expressed in undifferentiated neural stem cells (NSC) as well as in several human cancers including high-grade gliomas--highly aggressive brain tumors. Using a conditional gene activation approach in mice, we show that overexpression of Bmi1 induces repressive epigenetic regulation of the promoter of Survivin, a well-characterized antiapoptotic protein. This phenomenon is cell type-specific and it leads to apoptotic death of progenitor cells exclusively upon commitment toward a neuronal fate. Moreover, we show that this is triggered by increased oxidative stress-induced DNA damage. In contrast, undifferentiated NSC as well as glioma-initiating cells display an open chromatin configuration at the Survivin promoter and do not undergo apoptotic death. These findings raise the possibility that normal and neoplastic stem cells depend on the same mechanism for surviving the hyperproliferative state induced by increased Bmi1 expression.

Diabetes mellitus increases reactive oxygen species production in the thyroid of male rats

Diabetes mellitus (DM) disrupts the pituitary-thyroid axis and leads to a higher prevalence of thyroid disease. However, the role of reactive oxygen species in DM thyroid disease pathogenesis is unknown. Dual oxidases (DUOX) is responsible for H(2)O(2) production, which is a cosubstrate for thyroperoxidase, but the accumulation of H(2)O(2) also causes cellular deleterious effects. Nicotinamide adenine dinucleotide phosphate oxidase 4 (NOX4) is another member of the nicotinamide adenine dinucleotide phosphate oxidase family expressed in the thyroid. Therefore, we aimed to evaluate the thyroid DUOX activity and expression in DM rats in addition to NOX4 expression. In the thyroids of the DM rats, we found increased H(2)O(2) generation due to higher DUOX protein content and DUOX1, DUOX2, and NOX4 mRNA expressions. In rat thyroid PCCL3 cells, both TSH and insulin decreased DUOX activity and DUOX1 mRNA levels, an effect partially reversed by protein kinase A inhibition. Most antioxidant enzymes remained unchanged or decreased in the thyroid of DM rats, whereas only glutathione peroxidase 3 was increased. DUOX1 and NOX4 expression and H(2)O(2) production were significantly higher in cells cultivated with high glucose, which was reversed by protein kinase C inhibition. We conclude that thyroid reactive oxygen species is elevated in experimental rat DM, which is a consequence of low-serum TSH and insulin but is also related to hyperglycemia per se.

Mechanism-based testing strategy using in vitro approaches for identification of thyroid hormone disrupting chemicals

The thyroid hormone (TH) system is involved in several important physiological processes, including regulation of energy metabolism, growth and differentiation, development and maintenance of brain function, thermo-regulation, osmo-regulation, and axis of regulation of other endocrine systems, sexual behaviour and fertility and cardiovascular function. Therefore, concern about TH disruption (THD) has resulted in strategies being developed to identify THD chemicals (THDCs). Information on potential of chemicals causing THD is typically derived from animal studies. For the majority of chemicals, however, this information is either limited or unavailable. It is also unlikely that animal experiments will be performed for all THD relevant chemicals in the near future for ethical, financial and practical reasons. In addition, typical animal experiments often do not provide information on the mechanism of action of THDC, making it harder to extrapolate results across species. Relevant effects may not be identified in animal studies when the effects are delayed, life stage specific, not assessed by the experimental paradigm (e.g., behaviour) or only occur when an organism has to adapt to environmental factors by modulating TH levels. Therefore, in vitro and in silico alternatives to identify THDC and quantify their potency are needed. THDC have many potential mechanisms of action, including altered hormone production, transport, metabolism, receptor activation and disruption of several feed-back mechanisms. In vitro assays are available for many of these endpoints, and the application of modern '-omics' technologies, applicable for in vivo studies can help to reveal relevant and possibly new endpoints for inclusion in a targeted THDC in vitro test battery. Within the framework of the ASAT initiative (Assuring Safety without Animal Testing), an international group consisting of experts in the areas of thyroid endocrinology, toxicology of endocrine disruption, neurotoxicology, high-throughput screening, computational biology, and regulatory affairs has reviewed the state of science for (1) known mechanisms for THD plus examples of THDC; (2) in vitro THD tests currently available or under development related to these mechanisms; and (3) in silico methods for estimating the blood levels of THDC. Based on this scientific review, the panel has recommended a battery of test methods to be able to classify chemicals as of less or high concern for further hazard and risk assessment for THD. In addition, research gaps and needs are identified to be able to optimize and validate the targeted THD in vitro test battery for a mechanism-based strategy for a decision to opt out or to proceed with further testing for THD.

Regulation of dual oxidase expression and H2O2 production by thyroglobulin

BACKGROUND

Thyroglobulin (Tg) is a macromolecular precursor in thyroid hormone synthesis to which iodine is stably bound. Tg, which is stored in the follicular space, is also a potent negative feedback regulator of follicular function, and this is achieved by suppressing mRNA levels of thyroid-specific genes such as the sodium/iodide symporter (Slc5a5), Tg, and thyroid peroxidase. Dual oxidase 1 (DUOX1) and DUOX2, originally identified in the thyroid, are nicotinamide adenine dinucleotide phosphate (NADPH) oxidases that are necessary to produce the H2O2 required for thyroid hormone biosynthesis. Since follicular Tg regulates the expression of genes that are essential for thyroid hormone synthesis, we hypothesized that Tg might also regulate DUOX expression and H2O2 production.

METHODS

Rat thyroid FRTL-5 cells were treated with Tg, and the mRNA expression of Duox1 and Duox2 and their corresponding maturation factors Duoxa1 and Duoxa2 were evaluated by DNA microarray and real-time PCR. Duox2 promoter activity was examined by luciferase reporter gene assay. Protein levels of DUOX2 were also examined by Western blot analysis. Intracellular H2O2 generation was quantified by a fluorescent dye, 5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate, and acetyl ester (CM-H2DCFDA).

RESULTS

mRNA levels of Duox2 and its activation factor Duoxa2 (but not Duox1 or Duoxa1) were significantly suppressed by Tg in a dose-dependent manner and a time-dependent fashion in rat thyroid FRTL-5 cells. DUOX2 promoter activity was significantly suppressed by Tg in a dose-dependent manner. Protein levels of DUOX2 and H2O2 generation in cells were also reduced by Tg treatment.

CONCLUSIONS

We show that physiological concentrations of Tg suppressed the expression and function of DUOX2 in thyroid cells. These results suggest that Tg is a strong suppressor of the expression and the activity of DUOX2/DUOXA2, thereby regulating iodide organification and hormone synthesis in the thyroid. The evidence supports a reported model in which accumulated Tg in thyroid follicles plays important roles in autoregulating the function of individual follicles, which produces the basis of follicular heterogeneity.