Inhibition of H2O2 production by iodoaldehydes in cultured dog thyroid cells

2-iodohexadecanal induces autophagy during goiter involution

BACKGROUND

Iodine plays an important role in thyroid physiology and biochemistry. The thyroid is capable of producing different iodolipids such as 2-iodohexadecanal (2-IHDA). Data from different laboratories have shown that 2-IHDA inhibits several thyroid parameters and it has been postulated as intermediary on the action of iodide function.

OBJECTIVE

To explore different mechanisms involved during the involution of the hyperplastic thyroid gland of Wistar rats towards normality induced by 2-IHDA.

METHODS

Goiter was induced by the administration of MMI for 10 days, then the treatment was discontinued and Wistar rats were injected with 2-IHDA or KI.

RESULTS

During involution, 2-IHDA treatment reduced PCNA expression compared to spontaneous involution. KI treatment caused an increase of Caspase-3 activity and TUNEL-positive cells. In contrast, 2-IHDA failed to alter this value but induced an increase of LC3B expression. KI but not 2-IHDA led to an increase in peroxides levels, catalase and glutathione peroxidase activity.

CONCLUSIONS

We demonstrated that 2-IHDA, in contrast to iodide, did not lead to an increase in oxidative stress or apoptosis induction, indicating that the involution triggered by 2-IHDA in Wistar rats, is primarily due to the inhibition of cell proliferation and the induction of autophagy.

Selenium bioavailability modulates the sensitivity of thyroid cells to iodide excess

INTRODUCTION

Iodide is an essential micronutrient for the synthesis of thyroid hormones and its imbalance is involved in the origin of different thyroid pathological processes. Selenium (Se) is another essential trace element that contributes to thyroid preservation through the control of the redox homeostasis. Different studies have demonstrated that sodium-iodide-symporter (NIS) is downregulated in the presence of iodide excess and Se supplementation reverses this effect. We also demonstrated that NOX4-derived ROS are involved in NIS repression induced by iodide excess. The aim of this study was to investigate how Se bioavailability is decisive in the sensitivity to iodide excess on a differentiated rat thyroid cell line (FRTL-5).

RESULTS

We demonstrated that siRNA-mediated silencing of Nox4 suppressed AKT phosphorylation induced by iodide excess. Iodide increases TGF-β1 mRNA expression, AKT phosphorylation, ROS levels and decreases GPX1 and TXRND1 mRNAs expression while Se reversed these effects. Furthermore, iodide induced Nrf2 transcriptional activity only in Se-supplemented cultures, suggesting that Se positively influences Nrf2 activation and selenoenzyme response in FRTL-5. Se, also inhibited NF-κB phosphorylation induced by iodide excess. In addition, we found that iodide excess decreased total phosphatase activity and PTP1B and PTEN mRNA expression. Se supply restored only PTEN mRNA expression. Finally, we studied the 2-α-iodohexadecanal (2-IHD) effects since it has been proposed as intermediary of iodide action on thyroid autoregulation. 2-IHD stimulated PI3K/AKT activity and reduced NIS expression by a ROS-independent mechanism. Also, we found that 2-IHD increased TGF-β1 mRNA and TGF-β inhibitor (SB431542) reverses the 2-IHD inhibitory effect on NIS mRNA expression, suggesting that TGF-β1 signaling pathway could be involved. Although Se reduced 2-IHD-induced TGFB1 levels, it could not reverse its inhibitory effect on NIS expression.

CONCLUSION

Our study suggests that Se bioavailability may improve the expression of antioxidant genes through the activation of Nrf2, interfere in PI3K/AKT signaling and NIS expression by redox modulation.

The Effect of Long-Term Inorganic Iodine on Intrathyroidal Iodothyronine Content and Gene Expression in Mice with Graves' Hyperthyroidism

Background: The main molecular mechanism underlying acute suppression of iodine organification in normal thyroids after an excessive iodine load, that is, the Wolff-Chaikoff effect, is assumed to be suppression of iodine oxidation and iodothyronine synthesis. However, the mechanism underlying chronic antithyroid action of inorganic iodine in Graves' disease is not fully understood. Using a mouse model of Graves' hyperthyroidism, we examined changes in iodothyronine content and gene expression profiles in the thyroid glands after inorganic iodine loading. Materials and Methods: Graves' hyperthyroidism was induced and maintained in BALB/c mice by repeated immunizations of recombinant adenovirus expressing the human thyrotropin (TSH) receptor A-subunit. Hyperthyroid mice were left untreated (GD-C; n = 8) or treated with inorganic iodine for 12 weeks (GD-NaI; n = 8). We used unimmunized BALB/c mice as a control group (n = 10). In each mouse, serum thyroxine (T4) levels were measured with enzyme-linked immunosorbent assay (ELISA) at 4-week intervals. The intrathyroidal iodothyronine content and gene expression levels were, respectively, evaluated by mass spectrometry and RNA sequencing (RNA-seq) at the end of the experimental period. Results: Serum T4 levels in the GD-C group remained higher than in the control group, whereas those in the GD-NaI group declined to normal levels during the experimental period. Intrathyroidal triiodothyronine (T3), reverse T3 (rT3), and T4 contents in the GD-C group were higher than the control group, and rT3 and T4 were further increased in the GD-NaI group. The observed alterations in iodothyronine levels in the thyroid and sera may be explained by altered expression levels of genes for iodothyronine biosynthetic molecules, their transporter, and deiodinases. Conclusion: In this mouse model of hyperthyroidism, higher intrathyroidal accumulation of T4 and reduced gene expression data of iodothyronine transporters in the GD-NaI group suggest that chronic antithyroid action of iodine in Graves' disease involves suppression of hormone secretion.

Intrathyroidal feedforward and feedback network regulating thyroid hormone synthesis and secretion

Thyroid hormones (THs), including T4 and T3, are produced and released by the thyroid gland under the stimulation of thyroid-stimulating hormone (TSH). The homeostasis of THs is regulated via the coordination of the hypothalamic-pituitary-thyroid axis, plasma binding proteins, and local metabolism in tissues. TH synthesis and secretion in the thyrocytes-containing thyroid follicles are exquisitely regulated by an elaborate molecular network comprising enzymes, transporters, signal transduction machineries, and transcription factors. In this article, we synthesized the relevant literature, organized and dissected the complex intrathyroidal regulatory network into structures amenable to functional interpretation and systems-level modeling. Multiple intertwined feedforward and feedback motifs were identified and described, centering around the transcriptional and posttranslational regulations involved in TH synthesis and secretion, including those underpinning the Wolff-Chaikoff and Plummer effects and thyroglobulin-mediated feedback regulation. A more thorough characterization of the intrathyroidal network from a systems biology perspective, including its topology, constituent network motifs, and nonlinear quantitative properties, can help us to better understand and predict the thyroidal dynamics in response to physiological signals, therapeutic interventions, and environmental disruptions.

S1P and plasmalogen derived fatty aldehydes in cellular signaling and functions

Long-chain fatty aldehydes are present in low concentrations in mammalian cells and serve as intermediates in the interconversion between fatty acids and fatty alcohols. The long-chain fatty aldehydes are generated by enzymatic hydrolysis of 1-alkyl-, and 1-alkenyl-glycerophospholipids by alkylglycerol monooxygenase, plasmalogenase or lysoplasmalogenase while hydrolysis of sphingosine-1-phosphate (S1P) by S1P lyase generates trans ∆2-hexadecenal (∆2-HDE). Additionally, 2-chloro-, and 2-bromo- fatty aldehydes are produced from plasmalogens or lysoplasmalogens by hypochlorous, and hypobromous acid generated by activated neutrophils and eosinophils, respectively while 2-iodofatty aldehydes are produced by excess iodine in thyroid glands. The 2-halofatty aldehydes and ∆2-HDE activated JNK signaling, BAX, cytoskeletal reorganization and apoptosis in mammalian cells. Further, 2-chloro- and 2-bromo-fatty aldehydes formed GSH and protein adducts while ∆2-HDE formed adducts with GSH, deoxyguanosine in DNA and proteins such as HDAC1 in vitro. ∆2-HDE also modulated HDAC activity and stimulated H3 and H4 histone acetylation in vitro with lung epithelial cell nuclear preparations. The α-halo fatty aldehydes elicited endothelial dysfunction, cellular toxicity and tissue damage. Taken together, these investigations suggest a new role for long-chain fatty aldehydes as signaling lipids, ability to form adducts with GSH, proteins such as HDACs and regulate cellular functions.

DUOX Defects and Their Roles in Congenital Hypothyroidism

Extracellular hydrogen peroxide is required for thyroperoxidase-mediated thyroid hormone synthesis in the follicular lumen of the thyroid gland. Among the NADPH oxidases, dual oxidases, DUOX1 and DUOX2, constitute a distinct subfamily initially identified as thyroid oxidases, based on their level of expression in the thyroid. Despite their high sequence similarity, the two isoforms present distinct regulations, tissue expression, and catalytic functions. Inactivating mutations in many of the genes involved in thyroid hormone synthesis cause thyroid dyshormonogenesis associated with iodide organification defect. This chapter provides an overview of the genetic alterations in DUOX2 and its maturation factor, DUOXA2, causing inherited severe hypothyroidism that clearly demonstrate the physiological implication of this oxidase in thyroid hormonogenesis. Mutations in the DUOX2 gene have been described in permanent but also in transient forms of congenital hypothyroidism. Moreover, accumulating evidence demonstrates that the high phenotypic variability associated with altered DUOX2 function is not directly related to the number of inactivated DUOX2 alleles, suggesting the existence of other pathophysiological factors. The presence of two DUOX isoforms and their corresponding maturation factors in the same organ could certainly constitute an efficient redundant mechanism to maintain sufficient H₂O₂ supply for iodide organification. Many of the reported DUOX2 missense variants have not been functionally characterized, their clinical impact in the observed phenotype remaining unresolved, especially in mild transient congenital hypothyroidism. DUOX2 function should be carefully evaluated using an in vitro assay wherein (1) DUOXA2 is co-expressed, (2) H₂O₂ production is activated, (3) and DUOX2 membrane expression is precisely analyzed.

RELATIONSHIP BETWEEN THE EFFECTIVENESS OF INORGANIC IODINE AND THE SEVERITY OF GRAVES THYROTOXICOSIS: A RETROSPECTIVE STUDY

OBJECTIVE

Inorganic iodine is often used to treat patients with Graves thyrotoxicosis who do not tolerate thionamides due to adverse effects. However, predictors of continued inorganic iodine efficacy have not been fully elucidated. This study aimed to investigate the factors affecting the continued efficacy of potassium iodide (KI) in patients with Graves thyrotoxicosis.

METHODS

In this study, among 1,197 patients with Graves disease who were initially treated with thionamides, we retrospectively studied 24 consecutive Japanese patients whose treatment was changed to KI alone due to the adverse effects of thionamides. We divided these patients into 2 groups: patients who had maintained euthyroid function for at least 180 days (nonrecurrence group, n = 11), and patients who had not maintained euthyroid function for 180 days (recurrence group, n = 13).

RESULTS

Free triiodothyronine (FT3) and free thyroxine (FT4) levels on the day of changing from thionamides to KI were statistically higher in the recurrence group than in the nonrecurrence group (FT3, 9.3 [range, 5.2-11.6] vs. 3.7 [3.3-4.8] pg/mL, P = .02 and FT4, 3.6 [1.8-4.5] vs. 1.4 [1.2-1.9] ng/dL, P = .02). FT4 levels on the day of drug change were significantly higher in the recurrence group, even after adjusting for thionamide or KI dose. In the recurrence group, the duration of KI effect was inversed correlated with FT3 and FT4 levels on the day of drug change.

CONCLUSION

Continued efficacy of KI after thionamides might be inversely correlated with thyrotoxicosis severity on the day of drug change.

ABBREVIATIONS

ANOVA = analysis of variance eTV = estimated thyroid volume FT3 = free triiodothyronine FT4 = free thyroxine IQR = interquartile range KI = potassium iodide MMI = thiamazole PTU = propylthiouracil RAIT = radioactive iodine therapy TRAb = TSH receptor antibody TSH = thyroid stimulating hormone.

Effects of 2-iodohexadecanal in the physiology of thyroid cells

Iodide has direct effects on thyroid function. Several iodinated lipids are biosynthesized by the thyroid and they were postulated as intermediaries in the action of iodide. Among them, 2-iodohexadecanal (2-IHDA) has been identified and proposed to play a role in thyroid autoregulation. The aim of this study was to compare the effect of iodide and 2-IHDA on thyroid cell physiology. For this purpose, FRTL-5 thyroid cells were incubated with the two compounds during 24 or 48 h and several thyroid parameters were evaluated such as: iodide uptake, intracellular calcium and H₂O₂ levels. To further explore the molecular mechanism involved in 2-IHDA action, transcript and protein levels of genes involved in thyroid hormone biosynthesis, as well as the transcriptional expression of these genes were evaluated in the presence of iodide and 2-IHDA. The results obtained indicate that 2-IHDA reproduces the action of excess iodide on the "Wolff-Chaikoff" effect as well as on thyroid specific genes transcription supporting its role in thyroid autoregulation.

Inhibitory effects of 2-iodohexadecanal on FRTL-5 thyroid cells proliferation

Although thyroid gland function is mainly under the control of pituitary TSH, other factors, such as iodine, play a role in this process. The thyroid is capable of producing different iodolipids such as 6-iodo-deltalactone and 2-iodohexadecanal (2-IHDA). It was shown that these iodolipids mimic some of the inhibitory effects of excess iodide on several thyroid parameters.

OBJECTIVES

To study the effect of 2-IHDA on cell proliferation and apoptosis in FRTL-5 cells.

RESULTS

FRTL-5 cells were grown in the presence of TSH and treated with increasing concentrations of KI and 2-IHDA (0.5, 5, 10 and 33 µM) for 24, 48 and 72 h. Whereas KI inhibited cell proliferation only at 33 µM after 72 h of treatment, 2-IHDA inhibited in a time and concentration dependent manner. Analysis of cell cycle by flow cytometric DNA analysis revealed an accumulation of cells in G1 phase induced by 2-IHDA. The expression of cyclin A, cyclin D1 and cyclin D3 were reduced after treatment with 2-IHDA whereas CDK4 and CDK6 proteins were not modified. 2-IHDA induced a dynamic change in cytoplasmic to nuclear accumulation of p21 and p27 causing these proteins to be accumulated mostly in the nucleus. We also observed evidence of a pro-apoptotic effect of 2-IHDA at highest concentrations. No significant effect of KI was observed.

CONCLUSION

These results suggest that the inhibitory effects of 2-IHDA on FRTL-5 thyroid cell proliferation are mediated by cell cycle arrest in G1/S phase and cell death by apoptosis.

6-iodolactone, key mediator of antitumoral properties of iodine

An iodinated derivative of arachidonic acid, 5-hydroxy-6-iodo-8,11,14-eicosatrienoic acid, δ-lactone (6-IL) has been implicated as a possible intermediate in the autoregulation of the thyroid gland by iodine. In addition to antiproliferative and apoptotic effects observed in thyrocytes, this iodolipid could also exert similar actions in cells derived from extrathyroidal tissues like mammary gland, prostate, colon, or the nervous system. In mammary cancer (solid tumors or tumor cell lines), 6-IL has been detected after molecular iodine (I2) supplement, and is a potent activator of peroxisome proliferator-activated receptor type gamma (PPARγ). These observations led us to propose I2 supplement as a novel coadjutant therapy which, by inducing differentiation mechanisms, decreases tumor progression and prevents chemoresistance. Some kinds of tumoral cells, in contrast to normal cells, contain high concentrations of arachidonic acid, making the I2 supplement a potential "magic bullet" that enables local, specific production of 6-IL, which then exerts antineoplastic actions with minimal deleterious effects on normal tissues.

Naturally occurring organoiodines

This review, with 290 references, presents the fascinating area of iodinated natural products over the past hundred years for the first time.

Recent insights into the cell biology of thyroid angiofollicular units

In thyrocytes, cell polarity is of crucial importance for proper thyroid function. Many intrinsic mechanisms of self-regulation control how the key players involved in thyroid hormone (TH) biosynthesis interact in apical microvilli, so that hazardous biochemical processes may occur without detriment to the cell. In some pathological conditions, this enzymatic complex is disrupted, with some components abnormally activated into the cytoplasm, which can lead to further morphological and functional breakdown. When iodine intake is altered, autoregulatory mechanisms outside the thyrocytes are activated. They involve adjacent capillaries that, together with thyrocytes, form the angiofollicular units (AFUs) that can be considered as the functional and morphological units of the thyroid. In response to iodine shortage, a rapid expansion of the microvasculature occurs, which, in addition to nutrients and oxygen, optimizes iodide supply. These changes are triggered by angiogenic signals released from thyrocytes via a reactive oxygen species/hypoxia-inducible factor/vascular endothelial growth factor pathway. When intra- and extrathyrocyte autoregulation fails, other forms of adaptation arise, such as euthyroid goiters. From onset, goiters are morphologically and functionally heterogeneous due to the polyclonal nature of the cells, with nodules distributed around areas of quiescent AFUs containing globules of compact thyroglobulin (Tg) and surrounded by a hypotrophic microvasculature. Upon TSH stimulation, quiescent AFUs are activated with Tg globules undergoing fragmentation into soluble Tg, proteins involved in TH biosynthesis being expressed and the local microvascular network extending. Over time and depending on physiological needs, AFUs may undergo repetitive phases of high, moderate, or low cell and tissue activity, which may ultimately culminate in multinodular goiters.

Iodine nutrition and toxicity in Atlantic cod (Gadus morhua) larvae

Copepods as feed promote better growth and development in marine fish larvae than rotifers. However, unlike rotifers, copepods contain several minerals such as iodine (I), at potentially toxic levels. Iodine is an essential trace element and both under and over supply of I can inhibit the production of the I containing thyroid hormones. It is unknown whether marine fish larvae require copepod levels of I or if mechanisms are present that prevent I toxicity. In this study, larval Atlantic cod (Gadus morhua) were fed rotifers enriched to intermediate (26 mg I kg^-1 dry weight; MI group) or copepod (129 mg I kg^-1 DW; HI group) I levels and compared to cod larvae fed control rotifers (0.6 mg I kg^-1 DW). Larval I concentrations were increased by 3 (MI) and 7 (HI) fold compared to controls during the rotifer feeding period. No differences in growth were observed, but the HI diet increased thyroid follicle colloid to epithelium ratios, and affected the essential element concentrations of larvae compared to the other groups. The thyroid follicle morphology in the HI larvae is typical of colloid goitre, a condition resulting from excessive I intake, even though whole body I levels were below those found previously in copepod fed cod larvae. This is the first observation of dietary induced I toxicity in fish, and suggests I toxicity may be determined to a greater extent by bioavailability and nutrient interactions than by total body I concentrations in fish larvae. Rotifers with 0.6 mg I kg^-1 DW appeared sufficient to prevent gross signs of I deficiency in cod larvae reared with continuous water exchange, while modelling of cod larvae versus rotifer I levels suggests that optimum I levels in rotifers for cod larvae is 3.5 mg I kg^-1 DW.

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

H2O2 generation by dual oxidase (DUOX) at the apex of thyroid cells is the limiting factor in the oxidation of iodide and the synthesis of thyroid hormones. Its characteristics have been investigated using different in vitro models, from the most physiological thyroid slices to the particulate fraction isolated from transfected DUOX expressing CHO cells. Comparison of the models shows that some positive controls are thyroid specific (TSH) or require the substructure of the in vivo cells (MβCD). Other controls apply to all intact cell models such as the stimulation of the PIP(2) phospholipase C pathway by ATP acting on purinergic receptors, the activation of the Gq protein downstream (NaF), or surrogates of the intracellular signals generated by this cascade (phorbol esters for protein kinase C, Ca(++) ionophore for Ca(++)). Still, other controls, exerted by intracellular Ca(++) or its substitute Mn(++), the intracellular pH, or arachidonate bear directly on the enzyme. Iodide acts at the apical membrane of the cell through an oxidized form, presumably iodohexadecanal. Cooling of the cells to 22°C blocks the activation of the PIP(2) phospholipase C cascade. All these effects are reversible. Their kinetics and concentration-effect characteristics have been defined in the four models. A general scheme of the thyroid signaling pathways regulating this metabolism is proposed. The probes characterized could be applied to other H2O2 producing cells and to pathological material.

Biochemical changes during goiter induction by methylmercaptoimidazol and inhibition by delta-iodolactone in rat

BACKGROUND

We have demonstrated that the administration of delta-iodolactone (i.e., 5-iodo-delta lactone) of arachidonic acid (IL-delta), a mediator in thyroid autoregulation, prevents goiter induction by methylmercaptoimidazol (MMI) in rats. Other studies have shown that transforming growth factor beta-1 (TGF-beta1) mimics some of the actions of excess iodide, but its participation in autoregulation is disputed. The present studies were performed to test the hypotheses that IL-delta decreases thyroid growth by inhibition of cell proliferation and/or by stimulation of apoptosis due to oxidative stress, that TGF-beta is stimulated by an excess of iodide and by IL-delta, and that c-Myc and c-Fos expression are upregulated during goiter induction and downregulated during goiter inhibition.

METHODS

Rats were treated with MMI alone or together with iodide or IL-delta. Thyroid weight, cell number, cell proliferation, apoptosis, and oxidative stress were determined. Proliferating cell nuclear antigen (PCNA), TGF-beta1, TGF-beta3, c-Myc, and c-Fos were measured by Western blot.

RESULTS

MMI caused a progressive increase in thyroid weight accompanied by an increase in cell number, asymmetry of the ploidy histograms, and PCNA, c-Fos, and c-Myc expression. In addition, an early increase of apoptosis was observed. Peroxides as well as glutathione peroxidase and catalase activities were also increased in goitrous animals. The inhibitory action of IL-delta on goiter formation was accompanied by the inhibition of cell proliferation evidenced by a significant decrease in cell number, PCNA expression, and asymmetry of the ploidy histograms. A transient stimulation of apoptosis after 7 days of treatment was also observed. MMI administration stimulated TGF-beta1 but not TGF-beta3 synthesis. IL-delta alone caused a slight increase of TGF-beta3 but not TGF-beta1, whereas potassium iodide (KI) stimulated both isoforms and MMI reversed KI effect on TGF-beta1 expression but not on TGF-beta3.

CONCLUSIONS

The goiter inhibitory action of IL-delta is due to the inhibition of cell proliferation and the transient stimulation of apoptosis. This latter action does not involve oxidative stress. TGF-beta1 does not play a role in the autoregulatory pathway mediated by IL-delta. Iodide stimulates TGF-beta3 without the need of being organified. These results suggest that there may be more than one pathway involved in the autoregulatory mechanism.

6 Iodo-delta-lactone reproduces many but not all the effects of iodide

BACKGROUND

Iodide has direct effects on thyroid function. Several iodinated lipids are biosynthesized by the thyroid and they were postulated as intermediaries in the action of iodide. Among them 6 iodo-delta-lactone (IL-delta) has been identified and proposed to play a role in thyroid autoregulation. The aim of this study was to compare the effect of iodide and IL-delta on several thyroid parameters.

METHODS

Thyroid bovine follicles were incubated with the different compounds during three days.

RESULTS

KI and IL-delta inhibited iodide uptake, total protein and Tg synthesis but only KI had an effect on NIS and Tg mRNAs levels. Both compounds inhibited Na+/K+ ATPase and deoxy-glucose uptake. As PAX 8, FOXE 1 and TITF1 are involved in the regulation of thyroid specific genes their mRNA levels were measured. While iodide inhibited the expression of the first two, the expression of TITF1 was stimulated by iodide and IL-delta had no effect on these parameters.

CONCLUSION

These findings indicate that IL-delta reproduces some but not all the effects of excess iodide. These observations apply for higher micromolar concentrations of iodide while no such effects could be demonstrated at nanomolar iodide concentrations.

Induction of goitrous hypothyroidism by dietary iodide in SJL mice

Prolonged intake of large amounts of iodide has been reported to increase the incidence of goiter and/or hypothyroidism in humans as well as animals prone to spontaneous autoimmune thyroiditis. In the current study, we investigated the role of dietary iodide on the development of hypothyroidism, as well as thyroiditis, in strains of mice that do not develop spontaneous autoimmune thyroiditis. Intake of 0.05% NaI via drinking water for 10 wk induced hypothyroidism in SJL/J mice as indicated by elevated TSH and depressed total T(4) values in serum and formation of colloidal goiter with an inactive flattened thyroid epithelium. Hypothyroidism did not appear to have an autoimmune basis because only focal mononuclear cell infiltrates were found intrathyroidally, and antithyroglobulin antibodies or increased organification of iodide were not detected. These phenomena were not observed in similarly treated CBA/J mice, suggesting polymorphisms in genes controlling events downstream of iodide uptake by thyrocytes. Interestingly, RT-PCR analysis indicated that unlike CBA/J, SJL/J mice could not down-regulate Na/I symporter gene expression during the NaI treatment. No significant temporal or strain differences were observed regarding the expression of thyroglobulin, pendrin, thyroid peroxidase, and DUOX1 and DUOX2 genes after NaI intake. Our results point to the generation of a mouse model for the study of iodine-induced hypothyroidism, which does not seem to have an autoimmune basis.

From the molecular characterization of iodide transporters to the prevention of radioactive iodide exposure

In the event of a nuclear reactor accident, the major public health risk will likely result from the release and dispersion of volatile radio-iodines. Upon body exposure and food ingestion, these radio-iodines are concentrated in the thyroid, resulting in substantial thyroidal irradiation and accordingly causing thyroid cancers. Stable potassium iodide (KI) effectively blocks thyroid iodine uptake and is thus used in iodide prophylaxis for reactor accidents. The efficiency of KI is directly related to the physiological inhibition of the thyroid function in the presence of high plasma iodide concentrations. This regulation is called the Wolff-Chaikoff effect. However, to be fully effective, KI should be administered shortly before or immediately after radioiodine exposure. If KI is provided only several hours after exposure, it will elicit the opposite effect e.g. lead to an increase in the thyroid irradiation dose. To date, clear evaluation of the benefit and the potential toxicity of KI administration remain difficult, and additional data are needed. We outline in this review the molecular characterization of KI-induced regulation of the thyroid function. Significant advances in the knowledge of the iodide transport mechanisms and thyroid physiology have been made. Recently developed molecular tools should help clarify iodide metabolism and the Wolff-Chaikoff effect. The major goals are clarifying the factors which increase thyroid cancer risk after a reactor accident and improving the KI administration protocol. These will ultimately lead to the development of novel strategies to decrease thyroid irradiation after radio-iodine exposure.

[Enzymes involved in thyroid iodide organification]

Thyroid hormone biosynthesis depends on iodide uptake and its incorporation into the acceptor protein thyroglobulin (Tg), a high molecular weight protein secreted into the follicular lumen. The sodium-iodide symporter (NIS) is responsible for thyroid iodide uptake, the first step in thyroid hormonogenesis. Iodide is subsequently transported through the cellular membrane by pendrin (PDS) and then incorporated into Tg. Iodide oxidation and organification occur mainly in the thyrocyte apical surface and these reactions are catalyzed by thyroperoxidase (TPO) in the presence of hydrogen peroxide. Thus, thyroid iodide organification depends on TPO activity, which is modulated by the concentration of substrates (thyroglobulin and iodide) and cofactor (hydrogen peroxide). Hydrogen peroxide generation is catalyzed by the thyroid NADPH oxidase (ThOx), which is present in the apical pole of thyrocytes, is stimulated by thyrotropin and is inhibited by iodide. Hydrogen peroxide generation is the limiting step in thyroid hormone biosynthesis under iodine sufficiency conditions.

Effect of iodide on nicotinamide adenine dinucleotide phosphate oxidase activity and Duox2 protein expression in isolated porcine thyroid follicles

Thyroperoxidase requires H(2)O(2) to catalyze the biosynthesis of thyroxine residues on thyroglobulin. Iodide inhibits the generation of H(2)O(2), and consequently the synthesis of thyroid hormones (Wolff-Chaikoff effect). The H(2)O(2) generator is a calcium-dependent nicotinamide adenine dinucleotide phosphate (NADPH) oxidase involving the flavoprotein Duox2. NADPH oxidase activity and Duox2 require cAMP to be expressed in pig thyrocytes. We studied the effect of iodide on NADPH oxidase activity, the DUOX2 gene, and Duox2 protein expression in pig thyroid follicles cultured for 48 h with forskolin or a cAMP analog. Iodide inhibited the cellular release of H(2)O(2) and NADPH oxidase activity, effects prevented by methimazole. Northern blot studies showed that iodide did not reduce DUOX2 mRNA levels but did reduce those of TPO and NIS. Western blot analyses using a Duox2-specific antipeptide showed that Duox2 has two N-glycosylation states, which have oligosaccharide motifs accounting for about 15 kDa and 25 kDa, respectively, of the apparent molecular mass. Cyclic AMP increased the amount of the highly glycosylated form of Duox2, an effect antagonized by iodide in a methimazole-dependent manner. These data suggest that an oxidized form of iodide inhibits the H(2)O(2) generator at a posttranscriptional level by reducing the availability of the mature Duox2 protein.

The sodium/iodide Symporter (NIS): characterization, regulation, and medical significance

The Na(+)/I(-) symporter (NIS) is an integral plasma membrane glycoprotein that mediates active I(-) transport into the thyroid follicular cells, the first step in thyroid hormone biosynthesis. NIS-mediated thyroidal I(-) transport from the bloodstream to the colloid is a vectorial process made possible by the selective targeting of NIS to the basolateral membrane. NIS also mediates active I(-) transport in other tissues, including salivary glands, gastric mucosa, and lactating mammary gland, in which it translocates I(-) into the milk for thyroid hormone biosynthesis by the nursing newborn. NIS provides the basis for the effective diagnostic and therapeutic management of thyroid cancer and its metastases with radioiodide. NIS research has proceeded at an astounding pace after the 1996 isolation of the rat NIS cDNA, comprising the elucidation of NIS secondary structure and topology, biogenesis and posttranslational modifications, transcriptional and posttranscriptional regulation, electrophysiological analysis, isolation of the human NIS cDNA, and determination of the human NIS genomic organization. Clinically related topics include the analysis of congenital I(-) transport defect-causing NIS mutations and the role of NIS in thyroid cancer. NIS has been transduced into various kinds of cancer cells to render them susceptible to destruction with radioiodide. Most dramatically, the discovery of endogenous NIS expression in more than 80% of human breast cancer samples has raised the possibility that radioiodide may be a valuable novel tool in breast cancer diagnosis and treatment.

Moderate doses of iodide in vivo inhibit cell proliferation and the expression of thyroperoxidase and Na+/I- symporter mRNAs in dog thyroid

The function and the growth of adult thyroid gland is controlled by the opposite actions of thyrotropin (TSH) and iodide, the main substrate of the gland. Iodide deprivation leads to stimulation of the thyroid, improving the efficiency of iodide transport for hormone biosynthesis. We have investigated cell proliferation and thyroid specific gene expression 24 and 48 h after administering KI to dogs previously treated with goitrogens and perchlorate. In the hypothyroid dogs T3 and T4 serum levels decreased from 53 +/- 4 to < 30 ng/dl and from 1.6 +/- 0.6 to < 1 microg/dl respectively; TSH concentration increased from 0.16 +/- 0.02 to 2.7 +/- 0.4 ng/ml. After a 24 h moderate KI treatment (300 microg KI/dog of +/- 10 kg) serum T3 concentrations rose higher than the initial normal values, while T4 concentrations increased to reach values equivalent to the normal level. The high TSH concentration did not change significantly. The hyperplasia of the chronically stimulated thyroid resulting from goitrogens/NaClO4 treatment was not modified by this short term treatment with KI. In contrast, KI decreased the weight of the total gland and the level of cell proliferation, as determined by the fraction of cells incorporating BrdU. The effect of acute administration of KI on the expression of four major thyroid genes, the TSH receptor (TSHr), thyroglobulin (Tg), thyroperoxidase (TPO), and Na+/I- symporter (NIS) was analyzed by Northern blot. Tg, TPO and NIS mRNA expressions were up-regulated by chronic stimulation. The expression of the mRNAs of TSHr and Tg did not significantly differ between hyperstimulated and KI-treated dogs while TPO and NIS mRNA expression decreased after a 48 h KI treatment. TPO and NIS are therefore the only of these four genes whose expression is acutely modulated by iodide in vivo. Under TSH stimulation low doses of iodide resulted in: (1) decreased cell proliferation, (2) reestablished synthesis and secretion of thyroid hormones, (3) diminished TPO and NIS mRNA expression. Notably low doses of iodide under the same conditions had no effect on Tg and TSHr mRNA expression.

Biosynthesis and metabolism of 2-iodohexadecanal in cultured dog thyroid cells

2-Iodohexadecanal (2-IHDA) is a major thyroid iodolipid. It mimics the main regulatory effects of iodide on thyroid metabolism: inhibition of H2O2 production and of adenylyl cyclase. The biosynthesis of 2-IHDA and its metabolism have been investigated in cultured dog thyroid cells maintained in a differentiated state by forskolin. Incubation of these cells with [9,10-3H]hexadecan-1-ol or [9,10-3H]palmitic acid labeled several phospholipids, but [9, 10-3H]hexadecan-1-ol was selectively incorporated into plasmenylethanolamine. In the presence of an exogenous H2O2 generating system (glucose oxidase), iodide induced the production of [9,10-3H]2-IHDA from [9,10-3H]hexadecan-1-ol-labeled cells but not from [9,10-3H]palmitic acid-labeled cells. 2-IHDA was also generated during the lactoperoxidase-catalyzed iodination of brain and heart plasmalogens, and of ethyl hexadec-1-enyl ether, a synthetic vinyl ether-containing compound. Taken together, these results show that thyroid 2-IHDA is derived from plasmenylethanolamine via an attack of reactive iodine on the vinyl ether group. 2-Iodohexadecan-1-ol (2-IHDO) was also detected in these studies; it was formed later than 2-IHDA, and thyroid cells converted exogenous 2-IHDA into 2-IHDO in a time-dependent way. The ratio of 2-IHDO/2-IHDA increased with H2O2 production and decreased as a function of iodide concentration. An aldehyde-reducing activity was detected in subcellular fractions of the horse thyroid. No formation of 2-iodohexadecanoic acid could be detected. Reduction into the biologically inactive 2-IHDO is thus a major metabolic pathway of 2-IHDA in dog thyrocytes.

Inhibition of human thyroid adenylyl cyclase by 2-iodoaldehydes

2-Iodohexadecanal (IHDA), which can be formed upon addition of iodine to the vinyl ether group of plasmalogens, has been identified as a major thyroid iodolipid (Pereira et al. (1990) J. Biol. Chem. 265, 17018-17025). In this study, we have investigated the possibility that it would be a mediator of the inhibitory effect of iodide on thyroid adenylyl cyclase. In human thyroid membranes, IHDA inhibited the adenylyl cyclase activity stimulated by thyrotropin (TSH), GTP-gamma-S or forskolin (FSK), whereas it did not decrease the specific binding of TSH to its receptors. The inhibitory effect on the cyclase reached a maximum after a 1-h-pre-incubation of the membranes with IHDA at 30 degrees C and was poorly reversible. It was also observed following a 4-h incubation with IHDA at 4 degrees C, a condition in which adenylyl cyclase is protected against heat inactivation. IHDA decreased the Vmax of adenylyl cyclase, but had no effect on the Km for ATPMg2-.IHDA also inhibited the FSK-stimulated adenylyl cyclase activity in liver and kidney cortex membranes, but had no effect on the Mg(2+)-ATPase activity of thyroid membranes. The inhibitory effect of IHDA has also been demonstrated in intact cells. As in membranes, IHDA decreased the rise in cAMP induced by TSH in cultured dog thyroid cells and this inhibition was maintained following pretreatment of the cells with pertussis toxin. In order to evaluate the specificity of the IHDA action, various analogs have been synthesized. This study has permitted the identification of two major structural features required for the inhibition of human thyroid adenylyl cyclase; the terminal aldehyde function and an iodine atom at C2, other halogens being ineffective. In conclusion, we have shown that IHDA exerts a direct inhibitory effect at or near adenylyl cyclase; all the properties of this effect characterized so far are identical to those of the adenylyl cyclase inhibition obtained following the exposure of thyroid tissue to iodide.