Hydrogen peroxide degradation and glutathione peroxidase activity in cultures of thyroid cells

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.

Oxidative Stress Correlates with More Aggressive Features in Thyroid Cancer

Oxidative stress (OS) can have an impact in the pathogenesis and in the progression of thyroid cancer. We investigated the levels of reactive oxygen species (ROS) in 50 malignant and benign thyroid lesions and 41 normal tissues, and correlated them with the thyroid differentiation score-TDS and the clinico-pathologic features. NOX4 expression, GPx activity and the genetic pattern of tumors were evaluated. In malignant and benign lesions, ROS generation and NOX4 protein expression were higher than in normal tissues. Follicular (FTCs) and anaplastic/poorly differentiated cancers had increased OS relative to papillary tumors (PTCs). Moreover, OS in FTCs was higher than in follicular adenomas. Mutated PTCs showed increased OS compared with non-mutated PTCs. In malignant tumors, OS was inversely correlated with TDS, and directly correlated with tumor stage and ATA risk. GPx activity was increased in tumors compared with normal tissues, and inversely correlated to OS. In conclusion, our data indicate that thyroid tumors are exposed to higher OS compared with normal tissues, while showing a compensative increased GPx activity. OS correlates with tumor aggressiveness and mutations in the MEK-ERK pathway in PTC. The inverse correlation between OS and TDS suggests that ROS may repress genes involved in thyroid differentiation.

Protective effects of selenium and zinc against potassium dichromate-induced thyroid disruption, oxidative stress, and DNA damage in pregnant Wistar rats

Hexavalent chromium (CrVI) is an environmental pollutant and an endocrine-disrupting metal. Se and Zn are essential trace elements, known to play a crucial role in thyroid homeostasis. However, there is a lack of data reporting thyrotoxicity during gestation. In this study, we investigated the protective effects of selenium and zinc against potassium dichromate-induced thyrotoxicity in pregnant Wistar rats. Thirty pregnant Wistar rats were divided into control and four treated groups receiving subcutaneously (s.c) on the 3rd day of pregnancy, K2Cr2O7 (10 mg/kg, s.c) alone, or in association with Se (0.3 mg/kg, s.c), ZnCl2 (20 mg/kg, s.c), or both of them simultaneously. The hormonal profile, oxidative stress biomarkers, DNA damage, and histological modifications were evaluated. Our main findings showed that K2Cr2O7 promoted hypothyroidism, oxidative stress, genotoxicity, and histological alterations in the thyroid gland. The co-treatment with Se or ZnCl2 has mitigated K2Cr2O7-induced thyrotoxicity in pregnant Wistar rats by exhibiting antioxidant and genoprotective effects. However, the combined co-treatment of both of them was less thyroprotective, and therefore, further investigations on the synergetic interaction of Se and Zn against CrVI toxicity using different doses and exposure routes are required.

Effects of bamboo shoots (Bambusa balcooa) on thyroid hormone synthesizing regulatory elements at cellular and molecular levels in thyrocytes

ETHNOPHARMACOLOGICAL RELEVANCE

Bamboo shoots (BS) are consumed in various forms and used largely in naturopathy for curing ailments since ancient times to present days. It is eaten in South East Asian countries in several indigenous preparations. In north east India, it is consumed predominantly and used as natural cure to treat various diseases. Although known for its beneficial effects, adverse effects including goitrogenic/antithyroidal potential are emerging.

AIM OF THE STUDY

Endemic goiter exists in Manipur, India even after adequate iodine intake for consumption of BS. It is thus important to study the impact of this goitrogenic food on certain thyroid hormone synthesizing regulatory factors at cellular and molecular level in thyrocytes.

MATERIALS AND METHODS

Phytochemical analysis of BS - Bambusa balcooa Roxb (BSBR) extract conducted. IC₅₀ of the extract on thyrocytes in culture was determined. To study the antithyroid effects of this goitrogenic food, activity status of Na⁺-K⁺-ATPase, TPO and Deiodinase, mRNA and protein expressions of NIS, TPO and PAX8 were investigated with and without extra iodine in culture media. Simultaneously ROS generation in terms of H₂O₂ and antioxidant status, NO, LPO were assayed.

RESULTS

Activities of the studied enzymes decreased depending on dose and time with increased H₂O₂, decreased antioxidants followed by increased NO with LPO. DNA damage and LDH also increased while mRNA and protein expression of NIS, TPO and PAX8 were downregulated. Extra iodine ameliorated all such effects partially.

CONCLUSIONS

Bioactive constituents of the extract imbalances oxidative status of thyrocytes impairing action of hormone synthesizing elements at cellular and molecular level.

H2O2 Metabolism in Normal Thyroid Cells and in Thyroid Tumorigenesis: Focus on NADPH Oxidases

Thyroid hormone synthesis requires adequate hydrogen peroxide (H₂O₂) production that is utilized as an oxidative agent during the synthesis of thyroxin (T4) and triiodothyronine (T3). Thyroid H₂O₂ is generated by a member of the family of NADPH oxidase enzymes (NOX-es), termed dual oxidase 2 (DUOX2). NOX/DUOX enzymes produce reactive oxygen species (ROS) as their unique enzymatic activity in a timely and spatially regulated manner and therefore, are important regulators of diverse physiological processes. By contrast, dysfunctional NOX/DUOX-derived ROS production is associated with pathological conditions. Inappropriate DUOX2-generated H₂O₂ production results in thyroid hypofunction in rodent models. Recent studies also indicate that ROS improperly released by NOX4, another member of the NOX family, are involved in thyroid carcinogenesis. This review focuses on the current knowledge concerning the redox regulation of thyroid hormonogenesis and cancer development with a specific emphasis on the NOX and DUOX enzymes in these processes.

H2O2 induces nuclear transport of the receptor tyrosine kinase c-MET in breast cancer cells via a membrane-bound retrograde trafficking mechanism

Reactive oxygen species (ROS) are cellular by-products produced from metabolism and also anticancer agents, such as ionizing irradiation and chemotherapy drugs. The ROS H₂O₂ has high rates of production in cancer cells because of their rapid proliferation. ROS oxidize DNA, protein, and lipids, causing oxidative stress in cancer cells and making them vulnerable to other stresses. Therefore, cancer cell survival relies on maintaining ROS-induced stress at tolerable levels. Hepatocyte growth factor receptor (c-MET) is a receptor tyrosine kinase overexpressed in malignant cancer types, including breast cancer. Full-length c-MET triggers a signal transduction cascade from the plasma membrane that, through downstream signaling proteins, up-regulates cell proliferation and migration. Recently, c-MET was shown to interact and phosphorylate poly(ADP-ribose) polymerase 1 in the nucleus and to induce poly(ADP-ribose) polymerase inhibitor resistance. However, it remains unclear how c-MET moves from the cell membrane to the nucleus. Here, we demonstrate that H₂O₂ induces retrograde transport of membrane-associated full-length c-MET into the nucleus of human MCF10A and MCF12A or primary breast cancer cells. We further show that knocking down either coatomer protein complex subunit γ1 (COPG1) or Sec61 translocon β subunit (SEC61β) attenuates the accumulation of full-length nuclear c-MET. However, a c-MET kinase inhibitor did not block nuclear c-MET transport. Moreover, nuclear c-MET interacted with KU proteins in breast cancer cells, suggesting a role of full-length nuclear c-MET in ROS-induced DNA damage repair. We conclude that a membrane-bound retrograde vesicle transport mechanism facilitates membrane-to-nucleus transport of c-MET in breast cancer cells.

Neuroprotective effects of the new Na channel blocker rs100642 in global ischemic brain injury

INTRODUCTION

RS100642, a mexiletine analogue, is a novel sodium channel blocker with neuroprotective and antioxidant activities. The protectivity of RS100642, which has been shown against focal cerebral ischemia, was investigated in global cerebral ischemia in this study.

MATERIAL AND METHODS

Global cerebral ischemia was induced for five minutes in adult male Wistar Albino rats via the 4-vessel occlusion method. Intravenous administration of 1 mg/kg RS100642 following reperfusion for 30 min (RS100642 group) was compared with a sham treatment group (ischemia group) and nonischemized group (control) histologically based on morphology and caspase-3 immunohistochemistry, and biochemically based both on measurement of oxidative stress including malondialdehyde (MDA) levels, superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase (CAT) activities and on assessment of apoptosis including caspase-3 and -8 activities and tumor necrosis factor α (TNF-α) levels at the end of 6 h.

RESULTS

While the RS100642 group had significantly lower MDA levels and higher SOD activities than the sham treatment group (p < 0.05), GPx and CAT activities of the RS100642 and sham treatment groups were similar (p > 0.05) and significantly lower than those of the controls (p < 0.05). Necrosis and caspase-3 activity and immunoreactivity in the RS100642 group were significantly lower than those in the sham treatment group (p < 0.05), while there was no significant difference between groups regarding caspase-8 and TNF-α (p > 0.05).

CONCLUSIONS

Na⁺ channel blockade by RS100642 has remarkable neuroprotective effects following global brain ischemia/reperfusion damage. Further research is required to determine the optimum dose and time of administration.

Participation of NADPH 4 oxidase in thyroid regulation

Different factors are involved in thyroid function and proliferation such as thyrotropin (TSH), insulin, growth factors, iodide, etc. TSH and IGF1/insulin increase proliferation rate and stimulate genes involved in thyroid differentiation. In the present study, we analyse the physiological regulation of NOX4 expression by TSH, insulin and iodine, and the role of NOX4 on thyroid genes expression. Differentiated rat thyroid cells (FRTL-5) were incubated in the presence or absence of TSH/insulin and TTF2, PAX8, TPO, NIS, NOX4, TGFβ1, FOXO1/3 mRNA levels were examined by Real Time PCR. We showed that TSH and insulin repress NOX4 expression and appears to be inversely correlated with some thyroid genes. SiRNA targeted knockdown of NOX4 increased mRNA levels of TGFβ1, TPO, PAX8, TTF2, FOXO1 and FOXO3. A PI3K inhibitor (LY294002), increases the expression of NIS, TTF2 and FOXO1/3, however PI3K/AKT pathway does not regulate NOX4 expression. We observed that iodine increased NOX4 expression and knockdown of NOX4 reduced ROS and reversed the inhibitory effect of iodine on NIS, TPO, PAX8 and TTF2 expression. Our findings provide strong evidence that NOX4 could be a novel signaling modulator of TSH/insulin pathway and would have a critical role in the autoregulatory mechanism induced by iodine.

Comparative effects of transforming growth factor beta isoforms on redox metabolism in thyroid cells

INTRODUCTION

Transforming growth factor beta (TGF-β) regulates thyroid function and growth. However, tumoral thyroid cells became resistant to this factor as they undifferentiated. Little is known about the effects of TGF-β isoforms. We compared the role of redox metabolism in the response to TGF-β isoforms between non tumoral and tumoral thyroid cells.

METHODOLOGY AND RESULTS

Differentiated rat thyroid cells (FRTL-5) and human thyroid follicular carcinoma cells (WRO) were treated with the three isoforms of TGF-β. TGF-β isoforms stopped cell cycle at different steps; G1 for FRTL-5 and G2/M for WRO. The three isoforms decreased cell viability and increased ROS accumulation in both cell lines. These effects were more pronounced in FRTL-5 than in WRO, and the isoform β1 was more potent in ROS production than the other two. TGF-β isoforms decreased total glutathione, catalase expression and it activity in both cell lines. Only in FRTL-5 the lipid peroxidation was demonstrated. Moreover, TGF-β1 decreased glutathione peroxidase and mitochondrial superoxide dismutase mRNA expression and increased mitochondrial ROS in FRTL-5, but no in WRO. Pretreatment with selenium increased glutathione peroxidase activity and decreased ROS production in WRO treated with TGF-β isoforms. Furthermore, selenium partially reversed the effect of TGF-β isoforms on cell viability only in WRO cells. The knockdown of endogenous NOX4 significantly reduced the TGF-β1 effect on cell viability in WRO but no in FRTL-5.

CONCLUSION

TGF-β disrupted the redox balance and increased ROS accumulation in both cell lines. FRTL-5 cells showed reduced antioxidant capacity and had a greater sensitivity to TGF-β isoforms, while WRO cells were more resistant. This observation provides new insights into the potential role of TGF-β in the redox regulation of thyroid cells.

Lipid peroxidation and antioxidants status in human malignant and non-malignant thyroid tumours

Introduction:

Thyroid epithelial cells produce moderate amounts of reactive oxygen species that are physiologically required for thyroid hormone synthesis. Nevertheless, when they are produced in excessive amounts, they may become toxic.

Objective:

The present study is aimed to compare the lipid peroxidation (LPO), antioxidant enzymes – superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and non-protein thiols (reduced glutathione (GSH)) in human thyroid tissues with malignant and non-malignant disorders.

Design and Methods:

The study used human thyroid tissues and blood samples from 157 women (147 diseased and 10 normal). Thyroid hormones, oxidative stress markers and antioxidants were estimated by standard methods.

Results:

LPO significantly increased in most of the papillary thyroid carcinoma (PTC: 82.9%) and follicular thyroid adenoma (FTA: 72.9%) tissues, whilst in a majority of nodular goitre (69.2%) and Hashimoto’s thyroiditis (HT: 73.7%) thyroid tissues, it remained unaltered. GSH increased in PTC (55.3%), remained unaltered in FTA (97.3%) and all other goiter samples studied. SOD increased in PTC (51.1%) and all other malignant thyroid tissues studied. CAT remained unaltered in PTC (95.7%), FTA (97.3%) and all other non-malignant samples (HT, MNG, TMNG) studied. GPx increased in PTC (63.8%), all other malignant thyroid tissues and remained unaltered in many of the FTA (91.9%) tissues and all other non-malignant samples (HT, MNG, TMNG) studied.

Conclusions:

In the case of non-malignant thyroid tumours, the oxidant–antioxidant balance was undisturbed, whilst in malignant tumours the balance was altered, and the change in r value observed in the LPO and SOD pairs between normal and PTC tissues and also in many pairs with multi-nodular goitre (MNG)/toxic MNG tissues may be used as a marker to differentiate/detect different malignant/non-malignant thyroid tumours.

Targeting Nrf2 in healthy and malignant ovarian epithelial cells: Protection versus promotion

Risk factors indicate the importance of oxidative stress during ovarian carcinogenesis. To tolerate oxidative stress, cells activate the transcription factor Nrf2 (Nfe2l2), the master regulator of antioxidant and cytoprotective genes. Indeed, for most cancers, hyperactivity of Nrf2 is observed, and siRNA studies assigned Nrf2 as therapeutic target. However, the cancer-protective role of Nrf2 in healthy cells highlights the requirement for an adequate therapeutic window. We engineered artificial transcription factors to assess the role of Nrf2 in healthy (OSE-C2) and malignant ovarian cells (A2780). Successful NRF2 up- and downregulation correlated with decreased, respectively increased, sensitivity toward oxidative stress. Inhibition of NRF2 reduced the colony forming potential to the same extent in wild-type and BRCA1 knockdown A2780 cells. Only in BRCA1 knockdown A2780 cells, the effect of Nrf2 inhibition could be enhanced when combined with PARP inhibitors. Therefore, we propose that this combination therapy of PARP inhibitors and Nrf2 inhibition can further improve treatment efficacy specifically in BRCA1 mutant cancer cells without acquiring the side-effects associated with previously studied Nrf2 inhibition combinations with either chemotherapy or radiation. Our findings stress the dual role of Nrf2 in carcinogenesis, while offering approaches to exploit Nrf2 as a potent therapeutic target in ovarian cancer.

Multiassay analysis of the toxic potential of hydrogen peroxide on cultured neurons

To clarify discrepancies in the literature on the adverse effects of hydrogen peroxide on neurons, this study investigated the application of this peroxide to cultured cerebellar granule neurons with six assays frequently used to test for viability. Cultured neurons efficiently cleared exogenous H2O2. Although viability was not affected by exposure to 10 µM hydrogen peroxide, an exposure to the peroxide in higher concentrations rapidly lowered, within 15 min, the cellular 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltertrazolium bromide (MTT) reduction capacity to 53% ± 1% (100 µM) and 31% ± 1% (1,000 µM) and the 3-amino-7-dimethylamino-2-methyl-phenazine hydrochloride (neutral red; NR) uptake to 84% ± 6% (100 µM) and 33% ± 1% (1,000 µM) of control cells. The release of glycolytically generated lactate was stopped within 30 min in neurons treated with 1,000 µM peroxide. In contrast, even hours after peroxide application, the cell morphology, the number of propidium iodide-positive cells, and the extracellular activity of the cytosolic enzyme lactate dehydrogenase (LDH) were not significantly altered. The rapid loss in MTT reduction and NR uptake after exposure of neurons to H2O2 for 5 or 15 min correlated well with a strongly compromised MTT reduction and a very high extracellular LDH activity observed after further incubation in peroxide-free medium for a total incubation period of 24 hr. These data demonstrate that cultured neurons do not recover from damage that is inflicted by a short exposure to H2O2 and that the rapid losses in the capacities of neurons for MTT reduction and NR uptake are good predictors of delayed cell damage.

Differential regulation of the production of reactive oxygen species in Th1 cytokine-treated thyroid cells

BACKGROUND

Th1 cytokines exert pleiotropic effects in Hashimoto's thyroiditis. Previous studies reported a downregulation of thyroperoxidase and dual oxidase (DUOX) protein and mRNA expression in thyroid cells treated with Th1 cytokines. Although this effect is partially mediated by intracellular reactive oxygen species (ROS) and reactive nitrogen species, the nature and the source of the ROS involved are currently unknown. The aim of this study was to examine further the nature and source of the ROS produced in response to Th1 cytokines.

METHODS

Two rat thyroid cell lines (PCCL3 and FRTL-5) and human thyrocytes were incubated with Th1 cytokines (interleukin [IL]-1α and interferon-γ) in the presence or absence of the Th2 cytokine IL-4, the nitric oxide synthase inhibitor N-nitroso-L-arginine methyl ester (L-NAME), or the synthetic antioxidant N-acetylcysteine. The nature and source of the intracellular and extracellular ROS produced were determined.

RESULTS

A rapid increase in intracellular ROS was observed in cells incubated with Th1 cytokines. This increase was not caused by extracellular hydrogen peroxide (H2O2) produced by DUOX because both DUOX expression and extracellular H2O2 synthesis were decreased by Th1 cytokines. Confocal colocalization experiments showed that the Th1 cytokine-triggered ROS were not produced from mitochondria. Electron paramagnetic resonance investigations of PCCL3 cells indicated that the highly reactive hydroxyl radical was not involved in the response to Th1 cytokines. NOX2 mRNA expression was significantly increased in PCCL3 cells incubated with Th1 cytokines, as was the expression of the protein in the thyroid of Hashimoto's thyroiditis patients. NOX4 expression was by contrast unaffected. These results suggest that at least superoxide could be produced after exposure of thyroid cells to Th1 cytokines. The effects of L-NAME and IL-4, both of which partially or totally reverse Th1 cytokine-induced effects, on ROS release were also analyzed. L-NAME and IL-4 significantly reduced the Th1 cytokine-induced surge of intracellular ROS in PCCL3 and human thyroid cells.

CONCLUSION

The data presented here reinforce the idea that ROS, other than extracellular H2O2 produced by DUOX, are released from NOX2 after exposure of thyroid cells to Th1 cytokines. ROS/reactive nitrogen species act as important, but as further explained, not exclusive intracellular mediators of Th1 cytokine-induced effects in thyroid cells.

Inhibition of lactoperoxidase-catalyzed oxidation by imidazole-based thiones and selones: a mechanistic study

Herein, we describe the synthesis and biomimetic activity of a series of N,N-disubstituted thiones and selones that contain an imidazole pharmacophore. The N,N-disubstituted thiones do not show any inhibitory activity towards LPO-catalyzed oxidation reactions, but their corresponding N,N-disubstituted selones exhibit inhibitory activity towards LPO-catalyzed oxidation reactions. Substituents on the N atom of the imidazole ring appear to have a significant effect on the inhibition of LPO-catalyzed oxidation and iodination reactions. Selones 16, 17, and 19, which contain methyl, ethyl, and benzyl substituents, exhibit similar inhibition activities towards LPO-catalyzed oxidation reactions with IC50 values of 24.4, 22.5, and 22.5 μM, respectively. However, their activities are almost three-fold lower than that of the commonly used anti-thyroid drug methimazole (MMI). In contrast, selone 21, which contains a N-CH2CH2OH substituent, exhibits high inhibitory activity, with an IC50 value of 7.2 μM, which is similar to that of MMI. The inhibitory activity of these selones towards LPO-catalyzed oxidation/iodination reactions is due to their ability to decrease the concentrations of the co-substrates (H2O2 and I2), either by catalytically reducing H2O2 (anti-oxidant activity) or by forming stable charge-transfer complexes with oxidized iodide species. The inhibition of LPO-catalyzed oxidation/iodination reactions by N,N-disubstituted selones can be reversed by increasing the concentration of H2O2. Interestingly, all of the N,N-disubstituted selones exhibit high anti-oxidant activities and their glutathione peroxidase (GPx)-like activity is 4-12-fold higher than that of the well-known GPx-mimic ebselen. These experimental and theoretical studies suggest that the selones exist as zwitterions, in which the imidazole ring contains a positive charge and the selenium atom carries a large negative charge. Therefore, the selenium moieties of these selones possess highly nucleophilic character. The (77)Se NMR chemical shifts for the selones show large upfield shift, thus confirming the zwitterionic structure in solution.

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.

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.

Selenium and thyroid autoimmunity

The trace element selenium (Se) occurs in the form of the amino acid selenocysteine in selenoproteins. Selenoproteins exerts multiple physiological effects in human health, many of which are related with regulation of reduction-oxidation processes. In fact, the selenoenzyme families of glutathione peroxidase (GPx) and thioredoxin reductase (TRx) display the ability to act as antioxidants, protecting cells from oxidative damage. Furthermore, another class of selenoproteins are the iodothyronine deiodinase enzymes (DIO), which catalyze the conversion of thyroxine (T4) in triiodothyronine (T3), then exerting a fine tuned control on thyroid hormones metabolism. Several studies have investigated the potential positive effects of Se supplementation in thyroid diseases, characterized by increased levels of hydrogen peroxide and free radicals, like autoimmune chronic thyroiditis. These studies have supplied evidences indicating that Se supplementation, maximizing the antioxidant enzymes activity, may reduce the thyroid inflammatory status. Then, it may be postulated that Se could play a therapeutical role in thyroid autoimmune diseases. Despite the fact that recent studies seem to be concordant about Se beneficial effects in decreasing thyroid peroxidase antibodies (TPOAb) titers and ameliorating the ultrasound echogenicity pattern, several doubts have to be still clarified, before advising Se supplementation in chronic autoimmune thyroiditis.

Functional consequences of dual oxidase-thyroperoxidase interaction at the plasma membrane

CONTEXT

Thyroperoxidase (TPO) and dual oxidase (DUOX) are present at the apical membrane of thyrocytes, where TPO catalyzes thyroid hormone biosynthesis in the presence of H2O2 produced by DUOX. Both enzymes are colocalized and associated, but the consequences of this interaction remain obscure.

OBJECTIVE

The objective of this study was to evaluate the functional consequences of TPO-DUOX interaction at the plasma membrane.

DESIGN

The functional consequences of DUOX-TPO interaction were studied by measuring extracellular H2O2 concentration and TPO activity in a heterologous system. For this purpose, HEK293 cells were transiently transfected with a combination of human TPO with human DUOX1 or DUOX2 in the presence of their respective maturation factors, DUOXA1 or DUOXA2. The effect of human DUOX2 mutants in which cysteine residues in the N-terminal domain were replaced by glycines was also analyzed.

RESULTS

We observed that production of H2O2 decreases both TPO and DUOX activities. We show that TPO presents a catalase-like effect that protects DUOX from inhibition by H2O2. This catalase-like effect depends on the association between both enzymes, which probably occurs through the DUOX peroxidase-like domain because this effect was not observed with human DUOX2 mutants.

CONCLUSION

The DUOX-TPO association at the plasma membrane is relevant for normal enzyme properties. Normally, TPO consumes H2O2 produced by DUOX, decreasing the availability of this substance at the apical membrane of thyrocytes and, in turn, probably decreasing the oxidative damage of macromolecules.

Preconditioning protects the retinal pigment epithelium cells from oxidative stress-induced cell death

PURPOSE

The cytotoxic effects of oxidative stress, which play an important role in ocular diseases, are well known. In this study, we investigated the effect of non-lethal doses of oxidative stress on various cell functions, namely cell viability, cell attachment and cell migration in a widely used retinal pigment epithelium (RPE) cell line (ARPE-19).

METHODS

A single exposure to various concentrations of hydrogen peroxide (H(2)O(2)) was used to establish a dose response for H(2)O(2)-induced cell death. Other cellular responses, such as changes in cell attachment and migration, were monitored after exposure to increasing doses. Finally, the effects of preconditioning cells with increasing non-lethal doses of H(2)O(2), with and without a subsequent exposure to lethal doses of H(2)O(2), were determined.

RESULTS

The optimum dose for inducing cell death in ARPE-19 cells was between 900 and 1000 microm H(2)O(2). Preconditioning the cells with 1, 10 and 50 microm of H(2)O(2) provided a dose-dependent protection against cell death induced by a lethal dose (900-1000 microm) of H(2)O(2). Preconditioning with higher doses caused cells to become more susceptible to the cytotoxic effects of the lethal dose. Although H(2)O(2) increased cell attachment in lower doses, it induced a dose-dependent inhibition of cell attachment to the substrate in higher doses. H(2)O(2) did not affect cell migration in sub-lethal doses.

CONCLUSION

Preconditioning RPE cells with limited exposure to non-lethal oxidative stress confers significant protection against subsequent H(2)O(2)-induced cell death. It also affects cell attachment in a dose-specific manner. This finding may help in understanding the pathogenesis of diseases in which oxidative stress plays an important role and in determining the suitability of certain treatment strategies, in particular RPE transplantation in the treatment of age-related macular degeneration.

The interactions between selenium and iodine deficiencies in man and animals

Up to one billion people live in areas where they may be at risk from I deficiency. Many of the debilitating effects of the deficiency may be irreversible, consequently it is essential to understand the mechanisms whereby lack of I can cause disease through decreased thyroxine and 3, 3',5-triiodothyronine (T3) synthesis. Since Se has an essential role in thyroid hormone metabolism, it has the potential to play a major part in the outcome of I deficiency. These effects of Se derive from two aspects of its biological function. First, three Se-containing deiodinases regulate the synthesis and degradation of the biologically active thyroid hormone, T3. Second, selenoperoxidases and possibly thioredoxin reductase (EC1.6.4.5) protect the thyroid gland from H2O2produced during the synthesis of thyroid hormones. The mechanisms whereby Se deficiency exacerbates the hypothyroidism due to I deficiency have been elucidated in animals. In contrast to these adverse effects, concurrent Se deficiency may also cause changes in deiodinase activities which can protect the brain from low T3concentrations in I deficiency. Animals with Se and I deficiency have changes in serum thyroid hormone concentrations that are similar to those observed in patients with I deficiency disease. However such animal models show no thyroid involution, a feature which is characteristic of myxoedematous cretinism in man. These observations imply that if Se deficiency is involved in the outcome of I deficiency in human populations it is likely that other interacting factors such as goitrogens are also implicated. Nevertheless the protection of the thyroid gland from H2O2and the regulation of tissue T3levels are the functions of Se that are most likely to underlie the interactions of Se and I.

Peroxides and peroxide-degrading enzymes in the thyroid

Iodination of thyroglobulin is the key step of thyroid hormone biosynthesis. It is catalyzed by thyroid peroxidase and occurs within the follicular space at the apical plasma membrane. Hydrogen peroxide produced by thyrocytes as an oxidant for iodide may compromise cellular and genomic integrity of the surrounding cells, unless these are sufficiently protected by peroxidases. Thus, peroxidases play two opposing roles in thyroid biology. Both aspects of peroxide biology in the thyroid are separated in space and time and respond to the different physiological states of the thyrocytes. Redox-protective peroxidases in the thyroid are peroxiredoxins, glutathione peroxidases, and catalase. Glutathione peroxidases are selenoenzymes, whereas selenium-independent peroxiredoxins are functionally linked to the selenoenzymes of the thioredoxin reductase family through their thioredoxin cofactors. Thus, selenium impacts directly and indirectly on protective enzymes in the thyroid, a link that has been supported by animal experiments and clinical observations. In view of this relationship, it is remarkable that rather little is known about selenoprotein expression and their potential functional roles in the thyroid. Moreover, selenium-dependent and -independent peroxidases have rarely been examined in the same studies. Therefore, we review the relevant literature and present expression data of both selenium-dependent and -independent peroxidases in the murine thyroid.

Methimazole increases H2O2 toxicity in human thyroid epithelial cells

Hydrogen peroxide (H(2)O(2)) is necessary for thyroid hormone production and also for intracellular signalling purposes. Owing to its oxidative properties, however, it is harmful to cells, and deregulation of thyroid oxidative state has been implicated in the pathology of thyroid cancer. In this project, we studied the effects of H(2)O(2) on morphology and histochemical indicators of differentiated function (intracellular thyroglobulin), ability to generate NADPH (glucose-6-phosphate dehydrogenase (G6PD) activity) and vitality (apoptosis assay) in human thyroid epithelial cells. We further evaluated whether methimazole, an antithyroid drug reported to have antioxidative properties, could counteract the effects of H(2)O(2). Our data demonstrate tolerance to H(2)O(2) in concentrations less than 0.3mM and harmful effects at higher concentrations. 10mM methimazole sensitizes the cells towards H(2)O(2), possibly due to a dose-dependent inhibition of G6PD. Our data demonstrate the importance of this antioxidative system and point towards a relevant, but seldom recognized, influence of methimazole.

The role of selenium in thyroid autoimmunity and cancer

The essential micronutrient selenium (Se) occurs in the form of the amino acid selenocysteine in selenoproteins which exert various effects, while maintaining the cell reduction-oxidation balance. The discovery that all three deiodinases that convert thyroxine (T4) into triiodothyronine (T3) contain selenocysteine illustrates how the production of the active thyroid hormone is dependent on Se status. The selenoenzyme families of glutathione peroxidases (GPx) and thioredoxin reductases (TRx) possess powerful antioxidant properties and form a complex defense system that protects thyrocytes from oxidative damage. Se supplementation in patients with autoimmune thyroiditis seems to modify the immune response, probably by enhancing plasma GPx activity and decreasing excess levels of hydrogen peroxide. However, the enhancement of immunocompetence may also be the result of the synergistic action of various selenoproteins and not exclusively of GPx. There is evidence supporting considerable oxidative stress in Graves' disease where Se supplementation, because of its free radical scavenging properties, may increase the enzymatic antioxidant activity. TRx has been found significantly elevated in GD revealing its involvement in the pathogenesis of this condition and representing a potential future target for therapeutical intervention. Low Se serum levels have also been associated with increased risk of thyroid cancer and may play a role in carcinogenesis. It is noteworthy, that the Food and Drug Administration has recently determined that there is sufficient evidence to warrant a qualified health claim for Se and cancer. Furthermore, the recent discovery that defects in the SECIS-binding protein 2 (SBP2), which is an indispensable protein for the incorporation of Se into the selenoproteins, result in thyroid dysfunction, together with the recognition of the many roles of selenoprotein P in Se distribution and storage in the human body, reveal not only the indispensability of Se and the selenoproteins as essential factors in thyroid metabolism and pathogenesis, but open up new prospects for enhanced treatment.

Bioinorganic chemistry in thyroid gland: effect of antithyroid drugs on peroxidase-catalyzed oxidation and iodination reactions

Propylthiouracil (PTU) and methimazole (MMI) are the most commonly used antithyroid drugs. The available data suggest that these drugs may block the thyroid hormone synthesis by inhibiting the thyroid peroxidase (TPO) or diverting oxidized iodides away from thyroglobulin. It is also known that PTU inhibits the selenocysteine-containing enzyme ID-1 by reacting with the selenenyl iodide intermediate (E-SeI). In view of the current interest in antithyroid drugs, we have recently carried out biomimetic studies to understand the mechanism by which the antithyroid drugs inhibit the thyroid hormone synthesis and found that the replacement of sulfur with selenium in MMI leads to an interesting compound that may reversibly block the thyroid hormone synthesis. Our recent results on the inhibition of lactoperoxidase (LPO)-catalyzed oxidation and iodination reactions by antithyroid drugs are described.

Selenium, the thyroid, and the endocrine system

Recent identification of new selenocysteine-containing proteins has revealed relationships between the two trace elements selenium (Se) and iodine and the hormone network. Several selenoproteins participate in the protection of thyrocytes from damage by H(2)O(2) produced for thyroid hormone biosynthesis. Iodothyronine deiodinases are selenoproteins contributing to systemic or local thyroid hormone homeostasis. The Se content in endocrine tissues (thyroid, adrenals, pituitary, testes, ovary) is higher than in many other organs. Nutritional Se depletion results in retention, whereas Se repletion is followed by a rapid accumulation of Se in endocrine tissues, reproductive organs, and the brain. Selenoproteins such as thioredoxin reductases constitute the link between the Se metabolism and the regulation of transcription by redox sensitive ligand-modulated nuclear hormone receptors. Hormones and growth factors regulate the expression of selenoproteins and, conversely, Se supply modulates hormone actions. Selenoproteins are involved in bone metabolism as well as functions of the endocrine pancreas and adrenal glands. Furthermore, spermatogenesis depends on adequate Se supply, whereas Se excess may impair ovarian function. Comparative analysis of the genomes of several life forms reveals that higher mammals contain a limited number of identical genes encoding newly detected selenocysteine-containing proteins.

Anti-Thyroid Drugs and Thyroid Hormone Synthesis: Effect of Methimazole Derivatives on Peroxidase-Catalyzed Reactions

Syntheses and characterization of the selenium analogue (MSeI) of anti-thyroid drug methimazole and a series of organoselenium compounds bearing N-methylimidazole pharmacophore are described. In contrast to the sulfur compound that exists predominantly in its thione form, the selenium analogue exists in a selenol form, which spontaneously oxidizes in air to produce the corresponding diselenide. The reduction of the diselenide by GSH or NaBH(4) affords the biologically active selenol, which effectively inhibits the lactoperoxidase (LPO) activity in vitro. The monoselenides having N-methylimidazole moiety are found to be much less active than the selenol, suggesting that the presence of a selenol moiety is important for the LPO inhibition. The kinetic and mechanistic studies reveal that MSeI inhibits the LPO activity by reducing the H(2)O(2), providing a novel method to reversibly inhibit the enzyme. Although MSeI strongly inhibits LPO, the enzyme's activity can be completely recovered by increasing the H(2)O(2) concentration. On the other hand, the inhibition by methimazole (MMI), the sulfur analogue, cannot be reversed by increasing the H(2)O(2) concentration, leading to a complete inactivation of the enzyme. The reversible inhibition of LPO by some of the selenium derivatives is correlated with their glutathione peroxidase (GPx) activity, and the high GPx activity of the selenium compounds as compared with their sulfur analogues suggests that the selenium derivatives may protect the thyroid gland from oxidative damage.

Identification of a novel partner of duox: EFP1, a thioredoxin-related protein

H(2)O(2) is a crucial substrate of thyroproxidase (TPO) to iodinate thyroglobulin and synthesize thyroid hormones in thyroid. ThOX proteins (thyroid oxidase) also called Duox are believed to be responsible for H(2)O(2) generation. Duoxs expressed in transfected cells do not generate an active system, nor permit their membrane localization suggesting that other proteins are required to fulfill these functions. In this study, we demonstrate interactions of Duoxs with TPO and with p22(phox) without any effect on Duox activity. By yeast two-hybrid method using EF-hand fragment of dog Duox1 as the bait we have isolated EFP1 (EF-hand binding protein 1), one partner of Duoxs that belongs to the thioredoxin-related protein family. EFP1 shares moderate similarities with other members of thioredoxin-related proteins, but the characteristic active site and the folding structures are well conserved. EFP1 can be co-immunoprecipitated with Duoxs in transfected COS cells as well as in primary cultured human thyrocytes. It interacts also with TPO but not thyroglobulin. Immunofluorescence studies show that EFP1 and Duox proteins are co-localized inside the transfected cells, suggesting that EFP1 is not sufficient to induce either the expression of Duox at the plasma membrane or to permit H(2)O(2) production. EFP1 and Duox mRNA share similar distribution in nine different tissues. These results suggest that EFP1 could be one of the partners in the assembly of the multiprotein complex constituting the thyroid H(2)O(2) generating system but is certainly not sufficient to permit H(2)O(2) generation.

A metabolic model describing the H2O2 elimination by mammalian cells including H2O2 permeation through cytoplasmic and peroxisomal membranes: comparison with experimental data

We have constructed a metabolic model describing the H2O2 elimination by mammalian cells. It comprises three compartments (medium, cytosol, and peroxisome) separated by cytoplasmic and peroxisomal membranes, and H2O2 moves across the membranes with different permeation rate constants. Catalase localizes to peroxisomes, while glutathione peroxidase (GPx) and GSH recycling system (glutathione reductase (GR) and the oxidative pentose phosphate pathway (PPP)) localize to cytosol. The rates of individual enzyme reactions were computed using the experimentally determined activities and rate equations known for mammalian enzymes. Using the model, the concentration dependence of H2O2 elimination rate was obtained by numerical simulation and was compared with experimental data obtained previously with cultured mammalian cells (fibroblasts, human umbilical vein endothelial cells (HUVEC), and PC12 cells). The model was shown to be able to reproduce the data well by assuming appropriate values for the permeability rate constants. The H2O2 permeability coefficients thus estimated for cytoplasmic and peroxisomal membranes were in the same order of magnitude, except that the value for cytoplasmic membrane of PC12 cell was significantly smaller. The results suggest that the membrane permeability is one of the rate-limiting factors in the H2O2 elimination by mammalian cells. Using the model and estimated parameter values, we have examined the rate-limiting enzyme of the metabolic system, as well as the intracellular H2O2 concentration under steady-state and non-steady-state conditions.

Biomimetic Studies on Anti-Thyroid Drugs and Thyroid Hormone Synthesis

The selenium analogues of anti-thyroid drugs exhibit their anti-thyroid action by a mechanism different from that of MMI. The selenium analogue of MMI and related selenium compounds exhibit high GPx activity, providing a novel method for the reversible inhibition of thyroid hormone biosynthesis.

Hydrogen peroxide attenuates store-operated calcium entry and enhances calcium extrusion in thyroid FRTL-5 cells

Redox modulation participates in the regulation of intracellular free calcium concentration ([Ca(2+)](i)) in several cell types. In thyroid cells, including FRTL-5 cells, changes in [Ca(2+)](i) regulate several important functions, including the production of H(2)O(2) (hydrogen peroxide). As H(2)O(2) is of crucial importance for the production of thyroid hormones, we investigated the effects of H(2)O(2) on [Ca(2+)](i) in thyroid FRTL-5 cells. H(2)O(2) itself did not modulate basal [Ca(2+)](i). However, H(2)O(2) attenuated store-operated calcium entry evoked by thapsigargin, both in a sodium-containing buffer and in a sodium-free buffer. The effect of H(2)O(2) was abrogated by the reducing agent beta-mercaptoethanol. H(2)O(2) also attenuated the thapsigargin-evoked entry of barium and manganese. The effect of H(2)O(2) was, at least in part, mediated by activation of protein kinase C (PKC), as H(2)O(2) enhanced the binding of [(3)H]phorbol 12,13-dibutyrate. H(2)O(2) also stimulated the translocation of the isoenzyme PKCepsilon from the cytosolic fraction to the particulate fraction. Furthermore, H(2)O(2) did not attenuate store-operated calcium entry in cells treated with staurosporine or calphostin C, or in cells with down-regulated PKC. H(2)O(2) depolarized the membrane potential in bisoxonol-loaded cells and when patch-clamp in the whole-cell mode was used. The depolarization was attenuated in cells with down-regulated PKC. This depolarization, at least in part, explained the H(2)O(2)-evoked inhibition of calcium entry. In addition, H(2)O(2) enhanced the extrusion of calcium from cells stimulated with thapsigargin and this effect was abolished in cells with down-regulated PKC and after treatment of the cells with the reducing agent beta-mercaptoethanol. In conclusion H(2)O(2) attenuates an increase in [Ca(2+)](i). As H(2)O(2) is produced in thyroid cells in a calcium-dependent manner, our results suggest that H(2)O(2) may participate in the regulation of [Ca(2+)](i) in these cells via a negative-feedback mechanism involving activation of PKC.

Cloning of two human thyroid cDNAs encoding new members of the NADPH oxidase family

Two cDNAs encoding NADPH oxidases and constituting the thyroid H(2)O(2) generating system have been cloned. The strategy of cloning was based on the functional similarities between H(2)O(2) generation in leukocytes and the thyroid, according to the hypothesis that one of the components of the thyroid system would belong to the gp91(Phox)/Mox1 gene family and display sequence similarities with gp91(Phox). Screening at low stringency with a gp91(Phox) probe of cDNA libraries from thyroid cells in primary culture yielded two distinct human cDNA clones harboring open reading frames of 1551 (ThOX1) and 1548 amino acids (ThOX2), respectively. The encoded polypeptides display 83% sequence similarity and are clearly related to gp91(Phox) (53 and 47% similarity). The theoretical molecular mass of 177 kDa is close to the apparent molecular mass of 180 kDa of the native corresponding porcine flavoprotein and the protein(s) detected by Western blot in dog and human thyroid. ThOX1 and ThOX2 display sequence similarities of 53% and 61%, respectively, with a predicted protein of Caenorhabditis elegans over their entire length. They show along their first 500 amino acids a similarity of 43% with thyroperoxidase. The corresponding genes of ThOX1 and ThOX2 are closely linked on chromosome 15q15.3. The dog mRNA expression is thyroid-specific and up-regulated by agents activating the cAMP pathway as is the synthesis of the polypeptides they are coding for. In human thyroid the positive regulation by cAMP is less pronounced. The proteins ThOX1 and ThOX2 accumulate at the apical membrane of thyrocytes and are co-localized with thyroperoxidase.

Stimulation by iodide of H(2)O(2) generation in thyroid slices from several species

The regulation of thyroid metabolism by iodide involves numerous inhibitory effects. However, in unstimulated dog thyroid slices, a small inconstant stimulatory effect of iodide on H(2)O(2) generation is observed. The only other stimulatory effect reported with iodide is on [1-(14)C]glucose oxidation, i.e., on the pentose phosphate pathway. Because we have recently demonstrated that the pentose phosphate pathway is controlled by H(2)O(2) generation, we study here the effect of iodide on basal H(2)O(2) generation in thyroid slices from several species. Our data show that in sheep, pig, bovine, and to a lesser extent dog thyroid, iodide had a stimulatory effect on H(2)O(2) generation. In horse and human thyroid, an inconstant effect was observed. We demonstrate in dogs that the stimulatory effect of iodide is greater in thyroids deprived of iodide, raising the possibility that differences in thyroid iodide pool may account, at least in part, for the differences between the different species studied. This represents the first demonstration of an activation by iodide of a specialized thyroid function. In comparison with conditions in which an inhibitory effect of iodide on H(2)O(2) generation is observed, the stimulating effect was observed for lower concentrations and for a shorter incubation time with iodide. Such a dual control of H(2)O(2) generation by iodide has the physiological interest of promoting an efficient oxidation of iodide when the substrate is provided to a deficient gland and of avoiding excessive oxidation of iodide and thus synthesis of thyroid hormones when it is in excess. The activation of H(2)O(2) generation may also explain the well described toxic effect of acute administration of iodide on iodine-depleted thyroids.

IGF-1 or insulin, and the TSH cyclic AMP cascade separately control dog and human thyroid cell growth and DNA synthesis, and complement each other in inducing mitogenesis

The regular doubling of cell mass, and therefore of cell protein content, is required for repetitive cell divisions. Preliminary observations have shown that in dog thyrocytes insulin induces protein accumulation but not DNA synthesis, while TSH does not increase protein accumulation but triggers DNA synthesis in the presence of insulin. We show here that EGF and phorbol myristate ester complement insulin action in the same way. HGF is the only factor activating both protein accumulation and DNA synthesis. The effects of insulin on protein accumulation and in permitting the TSH effect are reproduced by IGF-1 and are mediated, at least in part by the IGF-1 receptor. The concentration effect curves are similar for both effects. Similar results are obtained in human thyrocytes. They reflect true cell growth, as shown by increases in RNA content and cell size. Carbachol and fetal calf serum also stimulate protein synthesis and accumulation without triggering DNA synthesis, but they are not permissive for the mitogenic effects of TSH or of the general adenylate cyclase activator, forskolin. Moreover the mitogenic effect of TSH greatly decreased in cells deprived of insulin for 2 days although these cells remain hypertrophic. Hypertrophy may therefore be necessary for cell division, but it is not sufficient to permit it. Three different mechanisms can therefore be distinguished in the mitogenic action of TSH: (1) the increase of cell mass (hypertrophy) induced by insulin or IGF-1; (2) the permissive effect of insulin or IGF-1 on the mitogenic effect of TSH which may involve both the increase of cell mass and the induction of specific proteins such as cyclin D3 and (3) the mitogenic effect of the TSH cyclic AMP cascade proper.

Autonomy in endemic goiter

The occurrence of hyperthyroidism in many individuals after introduction of iodine prophylaxis in endemic goiter areas can have dramatic consequences for the affected individuals. It indicates that in such individuals the increase of serum thyroid hormone level in response to iodine supplementation does not exert its normal negative feedback on thyroid activity, ie, that in such individuals some thyroid tissue has become autonomous. In this short review we summarize what is known about the possible mechanisms, cause, diagnosis, and consequences of thyroid autonomy.