Expression of the type II iodothyronine deiodinase in cultured rat astrocytes is selenium-dependent

Thyroid-Disrupting Effects of Exposure to Fipronil and Its Metabolites from Drinking Water Based on Human Thyroid Follicular Epithelial Nthy-ori 3-1 Cell Lines

Fipronil is a broad-spectrum insecticide used for plants and poultry. Owing to its widespread use, fipronil and its metabolites (fipronil sulfone, fipronil desulfinyl, and fipronil sulfide), termed FPM, can be frequently detected in drinking water and food. Fipronil can affect the thyroid function of animals, but the effects of FPM on the human thyroid remain unclear. We employed human thyroid follicular epithelial Nthy-ori 3-1 cells to examine combined cytotoxic responses, thyroid-related functional proteins including the sodium-iodide symporter (NIS), thyroid peroxidase (TPO), deiodinases I-III (DIO I-III), and the nuclear factor erythroid-derived factor 2-related factor 2 (NRF2) pathway induced by FPM of 1-1000-fold concentrations detected in school drinking water collected from a heavily contaminated area of the Huai River Basin. Thyroid-disrupting effects of FPM were evaluated by examining biomarkers of oxidative stress and thyroid function and tetraiodothyronine (T4) levels secreted by Nthy-ori 3-1 cells after FPM treatment. FPM activated the expression of NRF2, HO-1 (heme oxygenase 1), TPO, DIO I, and DIO II but inhibited NIS expression and increased the T4 level of thyrocytes, indicating that FPM can disrupt the function of human thyrocytes through oxidative pathways. Given the adverse impact of low FPM concentrations on human thyrocytes, supportive evidence from rodent studies, and the critical importance of thyroid hormones on development, the effects of FPM on the neurodevelopment and growth of children warrant priority attention.

The Barrier-Enhancing Function of Soluble Yam (Dioscorea opposita Thunb.) Polysaccharides in Rat Intestinal Epithelial Cells, as Affected by the Covalent Se Conjugation

The non-starch yam polysaccharides (YP) are the bioactive substances of edible yam, while Se is an essential nutrient for the human body. Whether a covalent conjugation of Se to YP might cause bioactivity change for the resultant selenylated YP in the intestine is still insufficiently studied, including the critical intestinal barrier function. In this study, two selenylated YP products, namely, YPSe-I and YPSe-II, with corresponding Se contents of 795 and 1480 mg/kg, were obtained by the reaction of YP and Na2SeO3 in the presence of HNO3 and then assessed for their bioactivities to a cell model (i.e., rat intestinal epithelial IEC-6 cells). The results showed that YP, YPSe-I, and YPSe-II at 5–80 μg/mL dosages could promote cell growth with treatment times of 12–24 h. The three samples also could improve barrier integrity via increasing cell monolayer resistance and anti-bacterial activity against E. coli or by reducing paracellular permeability and bacterial translocation. Additionally, the three samples enhanced F-actin distribution and promoted the expression of the three tight junction proteins, namely, zonula occluden-1, occludin, and claudin-1. Meanwhile, the expression levels of ROCK and RhoA, two critical proteins in the ROCK/RhoA singling pathway, were down-regulated by these samples. Collectively, YPSe-I and, especially, YPSe-II were more potent than YP in enhancing the assessed bioactivities. It is thus concluded that this chemical selenylation of YP brought about enhanced activity in the cells to promote barrier integrity, while a higher selenylation extent of the selenylated YP induced much activity enhancement. Collectively, the results highlighted the important role of the non-metal nutrient Se in the modified polysaccharides.

Physicochemical characterization, antioxidant and hypoglycemic activities of selenized polysaccharides from Sargassum pallidum

This study was carried out to study the effects of selenylation on physicochemical and biological properties of polysaccharide (SPP) extracted from Sargassum pallidum. The selenized derivative of SPP (Se-SPP) with the selenium content of 2419 μg/g was synthesized by sodium selenite/dilute nitric acid method. Physicochemical characterization indicated that selenylation modification resulted in some changes in chemical composition, monosaccharide composition, molecular weight and surface morphology of polysaccharides. FT-IR spectroscopy showed that a new absorption peak appeared at 675 cm-1 in Se-SPP probably due to the substitution of selenyl groups. Bioactivity assay showed that Se-SPP exhibited higher scavenging radical activities and ferrous ion chelating activities than native SPP. Compared with SPP and acarbose, Se-SPP showed more significantly inhibitory effect on α-glucosidase activity in a noncompetitive inhibition type. The IC50 values of SPP, Se-SPP and acarbose were determined as 1.579, 0.896 and 2.742 mg/mL, respectively. These results suggest that Se-SPP can be used to develop a new selenium-complementary ingredient in functional foods.

Effect of selenium enrichment on metabolism of tomato (Solanum lycopersicum) fruit during postharvest ripening

BACKGROUND

Selenium (Se) enrichment of plants seems effective in enhancing the health-related properties of produce and in delaying plant senescence and fruit ripening. The current study investigated the effects of Se on tomato fruit ripening. Tomato (Solanum lycopersicum L.) plants were grown in hydroponics with different Se-enriched nutrient solutions. Se, as sodium selenate, was added at a rate of 0 mg L^-1 (control), 1, and 1.5 mg L^-1 .

RESULTS

Selenium was absorbed by roots and translocated to leaves and fruit. Enrichment with Se did not significantly affect the qualitative parameters of fruit at commercial harvest; instead it delayed ripening by affecting specific ripening-related processes (respiration, ethylene production, color evolution) during postharvest. In the current experiment 100 g of tomato hydroponically grown with a 1.5 mg Se L^-1 enriched solution provided a total of 23.7 µg Se. Selenium recommended daily intake is 60 µg for women and 70 µg for men; thus the daily consumption of 100 g of enriched tomato would not lead to Se toxicity but would provide a good Se diet supplementation.

CONCLUSIONS

The cultivation of tomato plants in a Se-enriched solution appeared effective in producing tomato fruit with improved performance during storage and postharvest shelf life, and also with greater potential health-promoting properties. © 2018 Society of Chemical Industry.

Selenium maintains Ca2+ homeostasis in sheep lymphocytes challenged by oxidative stress

Selenium (Se) is an essential element in human and animal diets, based upon a widespread range of beneficial effects that are primarily due to its antioxidant properties. While Se can be associated to anti-cancer and anti-diabetic activities, reproductive efficiency, and enhancement of the immune system, the mechanistic details of the corresponding biological processes are still largely elusive. To avoid deficiencies and increase bioavailability, Se it is generally supplied to livestock through Se-supplemented feeds or forage plants fertilized with inorganic Se. While the relationship between Ca²⁺ and ROS (reactive oxygen species) is well known, only a few studies have addressed the possible involvement of Se in the control of cytosolic Ca²⁺ in oxidative stress. The results on Ca²⁺ homeostasis were obtained adding exogenous Se in the form of SeO₄^2- to sheep lymphomonocytes cultured in vitro. In particular, Se strongly attenuated 1mM H₂O₂-induced alteration of intracellular [Ca²⁺]_C as well as the entry of extracellular Ca²⁺ into the cells with comparable EC50 values for sodium selenate accounting to 1.72 and 2.28 mM, respectively. In an ex vivo trial, it was observed that Ca²⁺ homeostasis can effectively be rescued in sheep lymphomonocytes exposed in vivo to a Se concentration of approximately 1.9 mM, that was achieved by feeding sheep with olive leaves previously sprayed with 500 mg/plant Na-selenate. Thus the results obtained suggest that the mode of action of selenium markedly influenced Ca²⁺-related signaling events. Furthermore, results clearly reveal that the protective effect of Se on Ca²⁺ homeostasis under oxidative challenge can be clearly and effectively achieved through an appropriate dietary regimen obtained also in a circular economy logic using pruning of olive trees treated to reduce tree drought stress.

Grain amino acid composition of barley (Hordeum vulgare L.) cultivars subjected to selenium doses

Background:

Selenium (Se34) is an essential micronutrient for humans and animals and has growth promoting and antioxidative effects at low concentrations.

Methods:

Effects of various sodium selenite (Na2SeO3) doses on grain amino acid content of barley cultivars (Bülbül 89 and Çetin 2000) was investigated using ion exchange liquid chromatography.

Results:

Majority of the amino acids could be altered with Selenium (Se) fertilization. Grain Se content of Bülbül 89 (0.175 mg kg−1) and Çetin 2000 (0.171 mg kg−1) were similar and both displayed an increase in proteinogenic, essential, and sulfur amino acids. The response of cultivars was more pronounced for Se accumulation and amino acid content at mid dose (12.5 mg ha−1). The quantities of proteinogenic, essential and sulfur amino acids increased considerably at that dose. Se induced increase in nitrogen content might cause an increase in some of the proteins of grain and consequently can alter amino acid composition. An obvious increase in the limiting amino acids (lysine and threonine) were prominent in response to Se fertilization.

Conclusion:

Se treatment influence amino acid composition of barley grains; especially improve the quantity of limiting amino acids and consequently nutritional value of the grain.

Selenylation of Polysaccharide from the Sweet Potato and Evaluation of Antioxidant, Antitumor, and Antidiabetic Activities

Interest in sweet potato as a functional food is growing. A polysaccharide (SWP) was isolated from the sweet potato tuber and elucidation of its structure as composed of rhamnose, glucose, and galactose undertaken. To improve its activity, selenylation of this novel polysaccharide (Se-SWP) was undertaken by using microwave synthesis. In vitro evaluation showed that the Se-SWP has excellent antioxidant activity on scavenging free radicals and reducing capacity. In vivo antitumor evaluation showed selenylation polysaccharide could effectively inhibit tumor growth (>50%) and adjust immune factor levels in the mice (IL-2, TNF-α, and VEGF). The antidiabetic potential of Se-SWP was tested in STZ-induced diabetic rats. The results indicated that the Se-SWP treatment significantly reduced the levels of malondialdehyde and other disadvantageous factors that were increased by the STZ treatment. Meanwhile, the Se-SWP treatment caused a significant increase in the activities of enzymatic antioxidants and the levels of nonenzymatic antioxidants in the organs of diabetic rats. All of the activity evaluations indicated that the selenylation method could improve the activity of sweet potato polysaccharide and its efficacy as a potential therapeutic, which will be the focus of further study.

Disruption of type 2 iodothyronine deiodinase activity in cultured human glial cells by polybrominated diphenyl ethers

Polybrominated diphenyl ether (PBDE) flame retardants are endocrine disruptors and suspected neurodevelopmental toxicants. While the direct mechanisms of neurodevelopmental toxicity have not been fully elucidated, it is conceivable that alterations in thyroid hormone levels in the developing brain may contribute to these effects. Cells within the brain locally convert thyroxine (T4) to the biologically active triiodothyronine (T3) through the action of the selenodeiodinase type 2 iodothyronine deiodinase (DIO2). Previous studies have demonstrated that PBDEs can alter hepatic deiodinase activity both in vitro and in vivo; however, the effects of PBDEs on the deiodinase isoforms expressed in the brain are not well understood. Here, we studied the effects of several individual PBDEs and hydroxylated metabolites (OH-BDEs) on DIO2 activity in astrocytes, a specialized glial cell responsible for production of more than 50% of the T3 required by the brain. Primary human astrocytes and H4 glioma cells were exposed to individual PBDEs or OH-BDEs at concentrations up to 5 μM. BDE-99 decreased DIO2 activity by 50% in primary astrocyte cells and by up to 80% in the H4 cells at doses of ≥500 nM. 3-OH-BDE-47, 6-OH-BDE-47, and 5'-OH-BDE-99 also decreased DIO2 activity in cultured H4 glioma cells by 45-80% at doses of approximately 1-5 μM. Multiple mechanisms appear to contribute to the decreased DIO2 activity, including weakened expression of DIO2 mRNA, competitive inhibition of DIO2, and enhanced post-translational degradation of DIO2. We conclude that decreases in DIO2 activity caused by exposure to PBDEs may play a role in the neurodevelopmental deficits caused by these toxicants.

Genetics in endocrinology: genetic variation in deiodinases: a systematic review of potential clinical effects in humans

Iodothyronine deiodinases represent a family of selenoproteins involved in peripheral and local homeostasis of thyroid hormone action. Deiodinases are expressed in multiple organs and thyroid hormone affects numerous biological systems, thus genetic variation in deiodinases may affect multiple clinical endpoints. Interest in clinical effects of genetic variation in deiodinases has clearly increased. We aimed to provide an overview for the role of deiodinase polymorphisms in human physiology and morbidity. In this systematic review, studies evaluating the relationship between deiodinase polymorphisms and clinical parameters in humans were eligible. No restrictions on publication date were imposed. The following databases were searched up to August 2013: Pubmed, EMBASE (OVID-version), Web of Science, COCHRANE Library, CINAHL (EbscoHOST-version), Academic Search Premier (EbscoHOST-version), and ScienceDirect. Deiodinase physiology at molecular and tissue level is described, and finally the role of these polymorphisms in pathophysiological conditions is reviewed. Deiodinase type 1 (D1) polymorphisms particularly show moderate-to-strong relationships with thyroid hormone parameters, IGF1 production, and risk for depression. D2 variants correlate with thyroid hormone levels, insulin resistance, bipolar mood disorder, psychological well-being, mental retardation, hypertension, and risk for osteoarthritis. D3 polymorphisms showed no relationship with inter-individual variation in serum thyroid hormone parameters. One D3 polymorphism was associated with risk for osteoarthritis. Genetic deiodinase profiles only explain a small proportion of inter-individual variations in serum thyroid hormone levels. Evidence suggests a role of genetic deiodinase variants in certain pathophysiological conditions. The value for determination of deiodinase polymorphism in clinical practice needs further investigation.

Production and in vivo antioxidant activity of Zn, Ge, Se-enriched mycelia by Cordyceps sinensis SU-01

Cordyceps sinensis, a traditionally edible and medicinal fungus in China, cannot be artificially solid-cultured. Zinc (Zn), germanium (Ge), and selenium (Se) are the essential trace elements for human body. In this work, C. sinensis SU-01 was cultivated in liquid medium simultaneously containing Zn, Ge, and Se. The bioactive ingredients and in vivo antioxidant activities of Zn, Ge, Se-enriched mycelia (ZGSM) of C. sinensis SU-01 were investigated. Under the determined conditions, the Zn, Ge, and Se contents of ZGSM were 2543.16 ± 158.92, 1873.85 ± 81.82, and 1260.16 ± 107.12 μg/g, respectively. The optimal concentrations of Zn, Ge, and Se had a positive effect on biosynthesis of protein, polysaccharide, cordycepic acid, and amino acids. The activities of glutathione peroxidase (GSH-Px) and superoxide dismutase (SOD) of mice blood were 3.72 ± 0.15 and 28.74 ± 2.53 % higher than that of control, respectively, and the content of malondialdehyde (MDA) was 41.01 ± 3.66 % lower than that of control.

Sodium selenite increases the transcript levels of iodothyronine deiodinases I and II in ovine and bovine fetal thyrocytes in vitro

Selenium is essential for thyroid hormone homeostasis. Selenium is co-translationally incorporated into the protein backbone of 5' deiodinase enzymes, which are responsible for the intra- and extra-thyroidal activation of thyroid hormones. The objective of this study was to evaluate the effects of sodium selenite on the transcript levels of type I (DIO1) and II (DIO2) deiodinases in the primary culture of ovine and bovine fetal thyroid. By culture of fetal thyrocytes in the presence or absence of sodium selenite, and quantification of DIO1 and DIO2 transcripts using real-time reverse transcription polymerase chain reaction (RT-qPCR), we found that sodium selenite is able to increase the abundance of transcripts for DIO1 and DIO2 genes. We also found that cultured thyrocytes in the presence of sodium selenite compared to control cultured thyrocytes release more T3 into the culture medium. This indicates that in the presence of sodium selenite higher levels of DIO1 and DIO2 enzymes are produced, which are able to convert T4 to T3. In conclusion, we have shown that sodium selenite is increasing the abundance of DIO1 and DIO2 transcripts and increasing the production and release of T3 from cultured fetal thyrocytes. This finding emphasizes the role of selenium in transcriptional and expression processes during development and suggests that selenium deficiency during pregnancy in sheep and cattle may lead to the lower levels of DIO1 and DIO2 transcription in fetal thyroid, and thus, lower level of thyroidal T3 release into the fetal serum.

Thyroid hormone and the neuroglia: both source and target

Thyroid hormone plays a crucial role in the development and function of the nervous system. In order to bind to its nuclear receptor and regulate gene transcription thyroxine needs to be activated in the brain. This activation occurs via conversion of thyroxine to T3, which is catalyzed by the type 2 iodothyronine deiodinase (D2) in glial cells, in astrocytes, and tanycytes in the mediobasal hypothalamus. We discuss how thyroid hormone affects glial cell function followed by an overview on the fine-tuned regulation of T3 generation by D2 in different glial subtypes. Recent evidence on the direct paracrine impact of glial D2 on neuronal gene expression underlines the importance of glial-neuronal interaction in thyroid hormone regulation as a major regulatory pathway in the brain in health and disease.

Characteristics of Se-enriched mycelia by Stropharia rugoso-annulata and its antioxidant activities in vivo

Selenium (Se) is an essential trace element for humans and animals. Stropharia rugoso-annulata is a nutritional and functional mushroom containing many kinds of bioactive ingredients. The aims of this study were to investigate the Se-enrichment characteristics of S. rugoso-annulata in submerged culture and evaluate the antioxidant activities of Se-enriched mycelia in vivo in terms of the values of glutathione peroxidase (GSH-Px), superoxide dismutase (SOD), and malondialdehyde (MDA). The optimum parameters of Se-enrichment under the optimal Se concentration (150 microg/mL) in media were as follows: biomass 8.11 +/- 0.25 g/L, Se content in mycelia 4,727.68 +/- 13 microg/g, Se-accumulated rate 24.68 +/- 1.67%, and percentage of organic Se 96.27 +/- 3.26%. The mainly subsistent forms of selenium in Se-enriched mycelia were selenoprotein and selenium-polysaccharide. The contents of total amino acids (TAA) and essential amino acids (EAA) in Se-enriched mycelia were increased by 13.5 +/- 1.09% and 12.8 +/- 0.89%, respectively. It was efficient for Se-enriched mycelia to elevate GSH-Px and SOD activities and decrease MDA content. These results indicated that Se-enriched mycelia of S. rugoso-annulata represent a novel dietary source of bioavailable supplemental selenium.

Thyroid hormone activation in vascular smooth muscle cells is negatively regulated by glucocorticoid

BACKGROUND

Type 2 iodothyronine deiodinase (D2) catalyzes the production of triiodothyronine from thyroxine. D2 is present in rat aorta media, and there is a circadian variation in the D2 expression. In rat aorta media, the D2 activity exhibited the maximal value at 1200 hour and low value between 1800 and 2400 hour. To understand the mechanisms that induce the circadian variation in the D2 expression, we examined the effects of glucocorticoid on the D2 activity and mRNA in rat aorta media and cultured vascular smooth muscle cells (VSMCs).

METHODS

The effects of intrinsic and extrinsic glucocorticoid on the D2 activity and mRNA in rat aorta media were studied using metyrapone, a corticosterone synthesis inhibitor, and dexamethasone (DEX). Further, the effects of DEX on D2 expression were studied using the cultured rat VSMCs.

RESULTS

The trough values of D2 activity and mRNA at 2100 hour were increased by the treatment with metyrapone. On the other hand, the peak values of D2 activity and mRNA were decreased by the treatment with DEX. D2 activity and mRNA in cultured rat VSMCs were increased by the addition of 10(-3) M dibutyryl cyclic adenosine monophosphate [(Bu)(2)cAMP]. The increments were reduced by coincubation with 10(-6) M DEX.

CONCLUSIONS

These results suggest that glucocorticoids might directly suppress the D2 expression in rat VSMCs induced by a cAMP-dependent mechanism.

Rhythmic expression of clock genes in the ependymal cell layer of the third ventricle of rodents is independent of melatonin signaling

Reproductive physiology is regulated by the photoperiod in many mammals. Decoding of the photoperiod involves circadian clock mechanisms, although the molecular basis remains unclear. Recent studies have shown that the ependymal cell layer lining the infundibular recess of the third ventricle (EC) is a key structure for the photoperiodic gonadal response. The EC exhibits daylength-dependent changes in the expression of photoperiodic output genes, including the type 2 deiodinase gene (Dio2 ). Here we investigated whether clock genes (Per1 and Bmal1) and the albumin D-binding protein gene (Dbp) are expressed in the EC of Syrian hamsters, and whether their expression differs under long-day and short-day conditions. Expression of all three genes followed a diurnal rhythm; expression of Per1 and Dbp in the EC peaked around lights-off, and expression of Bmal1 peaked in the early light phase. The amplitude of Per1 and Dbp expression was higher in hamsters kept under long-day conditions than in those kept under short-day conditions. Notably, the expression of these genes was not modified by exogenous melatonin within 25 h after injection, whereas Dio2 expression was inhibited 19 h after injection. Targeted melatonin receptor (MT1, MT2, and both MT1 and MT2) disruption in melatonin-proficient C3H mice did not affect the rhythmic expression of Per1 in the EC. These data show the existence of a molecular clock in the rodent EC. In the hamster, this clock responds to long-term changes in the photoperiod, but is independent of acute melatonin signals. In mice, the EC clock is not affected by deletion of melatonin receptors.

Oxidative stress regulates type 3 deiodinase and type 2 deiodinase in cultured rat astrocytes

Type 2 deiodinase (D2) and type 3 deiodinase (D3) locally achieve the determination of the concentration of T3, which binds to the thyroid hormone receptor with high affinity. D2 converts T4 into T3, and D3 degrades T4 and T3. Neurons take up T3 released by astrocytes, the main cerebral site for the D2 expression. Because oxidative stress is believed to be involved in several neurological disorders, we explored the effects of oxidative stress on D3 and D2 in primary culture of rat astrocytes. H2O2 (250 microm) increased D3 activity with maximal effects around 8 h. Stimulation of D3 activity by H2O2 was synergistic with T4, phorbol ester, and also cAMP. H2O2 (250 microm) did not affect basal D2 activity but inhibited the stimulation of D2 activity by cAMP and factors implicating cAMP-independent pathways in astrocytes, TSH, and phorbol ester. N-Acetyl cysteine and selenium repletion, which respectively increase intracellular glutathione and glutathione peroxidase, inhibited D2 and D3 regulation by H2O2, whereas L-buthionine sulfoximine, which decreases intracellular glutathione, mimicked H2O2 effects. Oxidative stress up-regulated D3 and inhibited cAMP-stimulated D2 by transcriptional mechanisms. A decrease in cAMP by oxidative stress could contribute to the inhibition of cAMP-stimulated D2. Using specific inhibitors of signaling pathways, we show that the ERK pathway was required in D2 and D3 regulation by oxidative stress and that the p38 MAPK pathway was implicated in H2O2-induced D3. We suggest that the expected decrease in T3 might modulate the cellular injury of oxidative stress in some pathological brain conditions.

Thyroid-adrenergic interactions: physiological and clinical implications

The sympathoadrenal system, including the sympathetic nervous system and the adrenal medulla, interacts with thyroid hormone (TH) at various levels. Both systems are evolutionary old and regulate independent functions, playing probably independent roles in poikilothermic species. With the advent of homeothermy, TH acquired a new role, which is to stimulate thermogenic mechanisms and synergize with the sympathoadrenal system to produce heat and maintain body temperature. An important part of this new function is mediated through coordinated and, most of the time, synergistic interactions with the sympathoadrenal system. Catecholamines can in turn activate TH in a tissue-specific manner, most notably in brown adipose tissue. Such interactions are of great adaptive value in cold adaptation and in states needing high-energy output. Conversely, in states of emergency where energy demand should be reduced, such as disease and starvation, both systems are turned down. In pathological states, where one of the systems is fixed at a high or a low level, coordination is lost with disruption of the physiology and development of symptoms. Exaggerated responses to catecholamines dominate the manifestations of thyrotoxicosis, while hypothyroidism is characterized by a narrowing of adaptive responses (e.g., thermogenic, cardiovascular, and lipolytic). Finally, emerging results suggest the possibility that disrupted interactions between the two systems contribute to explain metabolic variability, for example, fuel efficiency, energy expenditure, and lipolytic responses.

Hypoxia stabilizes type 2 deiodinase activity in rat astrocytes

T(4) activation into T(3) is catalyzed by type 2 deiodinase (D2) in the brain. The rapid induction of D2 in astrocytes by transient brain ischemia has prompted us to explore the effects of hypoxia on D2 in cultures of astrocytes. Hypoxia (2.5% O(2)) of cultured astrocytes increased D2 activity, alone or in association with agents stimulating the cAMP pathway. Hypoxia had no effect on D2 mRNA accumulation. Cycloheximide did not block the effect of hypoxia on D2 activity and D2 half-life was enhanced under hypoxia demonstrating a posttranslational action of hypoxia. Furthermore, the D2 activity increase by hypoxia was not additive with the increase promoted by the proteasome inhibitor carbobenzoxy-L-leucyl-L-leucyl-L-leucinal (MG132). This strongly suggests that hypoxia leads to stabilization of D2 by slowing its degradation by the proteasome pathway. Hypoxia, in contrast to MG132, did not block the T(4)-induced D2 inactivation. A contribution of prolyl hydroxylase to the hypoxia effects on D2 was also suggested on the basis of increased D2 activity after addition of different prolyl hydroxylase inhibitors (cobalt chloride, desferrioxamine, dimethyloxalylglycine, dimethylsuccinate). Specific inhibitors of ERK, p38 MAPK, or phosphatidylinositol 3-kinase pathways were without any effect on hypoxia-increased D2 activity, eliminating their role in the effects of hypoxia. Interestingly, diphenyleneiodonium, an inhibitor of nicotinamide adenine dinucleotide phosphate oxidase inhibited the hypoxia-increased D2 indicating a role for some reactive oxygen species in the mechanism of D2 increase. Further studies are required to clarify the precise molecular mechanisms involved in the D2 stabilization by hypoxia.

Thyroid Hormones Reorganize the Cytoskeleton of Glial Cells Through Gfap Phosphorylation and Rhoa-Dependent Mechanisms

Thyroid hormones (3,5,3'-triiodo-L: -thyronine, T3; 3,5,3',5'-L: -tetraiodothyronine, T4; TH) play crucial roles in the growth and differentiation of the central nervous system. In this study, we investigated the actions of TH on proliferation, viability, cell morphology, in vitro phosphorylation of glial fibrillary acidic protein (GFAP) and actin reorganization in C6 glioma cells. We first observe that long-term exposure to TH stimulates cell proliferation without induce cell death. We also demonstrate that after 3, 6, 12, 18, and 24 h treatment with TH, C6 cells and cortical astrocytes show a process-bearing shape. Furthermore, immunocytochemistry with anti-actin and anti-GFAP antibodies reveals that TH induces reorganization of actin and GFAP cytoskeleton. We also observe an increased in vitro 32P incorporation into GFAP recovered into the high-salt Triton insoluble cytoskeletal fraction after 3 and 24 h exposure to 5 x 10(-8) and 10(-6) M T3, and only after 24 h exposure to 10(-9) M T4. These results show a T3 action on the phosphorylating system associated to GFAP and suggest a T3-independent effect of T4 on this cytoskeletal protein. In addition, C6 cells and astrocytes treated with lysophosphatidic acid, an upstream activator of the RhoA GTPase pathway, totally prevented the morphological alterations induced by TH, indicating that this effect could be mediated by the RhoA signaling pathway. Considering that IF network can be regulated by phosphorylation leading to reorganization of IF filamentous structure and that alterations of the microfilament organization may have important implications in glial functions, the effects of TH on glial cell cytoskeleton could be implicated in essential neural events such as brain development.

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.

Thyroid hormone deiodinases in the central and peripheral nervous system

Thyroid hormones play a critical role in development and functioning of the nervous system. Deiodinases (type 2 [D2] and type 3 [D3]) contribute to the control of thyroid hormone action in the nervous system by regulating the local concentrations of triiodothyronine (T(3)), the main active thyroid hormone. Most brain T(3) is indeed locally formed by deiodination of thyroxine (T(4)). This reaction is catalyzed by D2 expressed in astrocytes throughout the brain and in tanycytes in the mediobasal hypothalamus. D3, which inactivates both T(4) and T(3), is mainly expressed in neurons also throughout the brain, with high expression in hippocampus and pyriform cortex. The regulation of deiodinases by many factors in addition to the thyroid hormones indicate that their role is not limited to mitigate the fluctuations in plasma T(4) and T(3). In contrast to the brain, deiodinases are not expressed in the adult peripheral nerve. Nerve lesions induce D2 in peripheral nerve sheaths and D3 in the endoneurial compartment containing Schwann cells. On the basis of available data summarized in this review, D2 and D3 clearly contribute to determine T(3) concentrations depending on the area of the nervous system, the state of development, and the pathophysiologic conditions.

Induction of type 2 iodothyronine deiodinase in astrocytes after transient focal cerebral ischemia in the rat

This study investigated the expression of deiodinases of thyroid hormones in the rat brain after transient occlusion of the middle cerebral artery. The activity of type 2 deiodinase (D2), which catalyzes the deiodination of thyroxine into the more active thyroid hormone 3,5,3'-triiodothyronine, was strongly increased by cerebral ischemia at 6 and 24 hours in the striatum and at 24 hours in the cerebral cortex. The activity of type 3 deiodinase, which catalyzes the inactivation of thyroid hormones, was not affected by ischemia. In situ hybridization showed, as soon as 6 hours, an upregulation of the expression of D2 mRNA in the ipsilateral striatum, which disappeared at 24 hours. In the ipsilateral cortex, the induction of D2 mRNA started at 6 hours, was increased at 24 hours and finally declined at 72 hours. These results were confirmed by reverse transcription-PCR for D2 mRNA in the striatum and cerebral cortex. The upregulation of D2 mRNA after ischemia was mainly localized in astrocytic cell bodies. These results show that D2 is rapidly induced in astrocytes after ischemic stroke. Future work will include the exploration of the role of the upregulation of this enzyme, responsible for local 3,5,3'-triiodothyronine production as a neuroprotective mechanism in the brain.

Expression of type 2 iodothyronine deiodinase in human osteoblast is stimulated by thyrotropin

Thyroid hormones play important roles in bone growth, development, and turnover. To exert its biological activity, T(4) needs to be converted to T(3) by iodothyronine deiodinase. In human thyroid gland as well as rat brown adipose tissue, type 2 iodothyronine deiodinase (D2) expression is regulated by a TSH receptor-cAMP-mediated mechanism. TSH receptor knockout mice demonstrated the direct effects of TSH on bone via TSH receptors found on osteoblast and osteoclast precursors. In the present study we investigated the possible expression and function of iodothyronine deiodinase and TSH receptors in human osteoblast-like osteosarcoma (SaOS-2) cells and normal human osteoblast (NHOst) cells. Iodothyronine deiodinase activity was detected in SaOS-2 cells and NHOst cells, and all of the characteristics of deiodinating activity were compatible with those of D2. Northern analysis demonstrated D2 mRNA expression in SaOS-2 cells and NHOst cells. D2 mRNA levels as well as D2 activities were rapidly increased by dibutyryl cAMP or forskolin in SaOS-2 cells and NHOst cells. TSH receptor mRNA was demonstrated in SaOS-2 cells and NHOst cells, and D2 mRNA and D2 activity were stimulated by TSH in both cells. In addition, all T(3) receptor isoforms were detected by RT-PCR in SaOS-2 cells and NHOst cells. The present results indicate the expression of functional TSH receptors and D2 in human osteoblasts and suggest previously unrecognized roles of TSH receptors and local T(3) production by D2 in the pathophysiology of human osteoblasts.

Expression of thyroid hormone receptor isoforms in the oligodendrocyte lineage

Thyroid hormone (T3) regulates brain development and function and in particular ensures normal myelination. Animal models and in vitro systems have been employed to demonstrate the effects of T3, which acts via nuclear hormone receptors. T3 receptors (TRs) are transcription factors that activate or suppress target gene expression, such as myelin basic protein (MBP), in a hormone-dependent or -independent fashion. Two distinct genes, TR alpha and TR beta, encode several receptor isoforms with specific functions. This overview summarizes current knowledge on the cellular expression and the role of these isoforms and also examines the action of T3 on oligodendrocyte lineage cell types at defined developmental stages. Re-expression of TRs and also that of other transcription factors in oligodendrocytes may constitute some of the metabolic changes required for succesfull remyelination in the adult central nervous system after demyelinating lesions.

Trends in selenium biochemistry

The biochemistry of selenium-containing natural products, including selenoproteins, is reviewed up to May 2002. Particular emphasis is placed on the assimilation of selenium from inorganic and organic selenium sources for selenoprotein synthesis, the catalytic role of selenium in enzymes, and medical implications of an unbalanced selenium supply. The review contains 393 references on key discoveries and recent progress.

The mRNA structure has potent regulatory effects on type 2 iodothyronine deiodinase expression

Type 2 deiodinase (D2) is a selenoenzyme catalyzing the activation of T(4) to T(3). D2 activity/mRNA ratios are often low, suggesting that there is significant posttranscriptional regulation. The D2 mRNA in higher vertebrates is more than 6 kb, containing long 5' and 3' untranslated regions (UTRs). The D2 5'UTRs are greater than 600 nucleotides and contain 3-5 short open reading frames. These full-length 5'UTRs reduce the D2 translation efficiency approximately 5-fold. The inhibition by human D2 5'UTR is localized to a region containing the first short open reading frame encoding a tripeptide-MKG. This inhibition was abolished by mutating the AUG start codon and weakened by modification of the essential purine of the Kozak consensus. Deletion of the 3.7-kb 3'UTR of the chicken D2 mRNA increased D2 activity approximately 3.8-fold due to an increase in D2 mRNA half-life. In addition, alternatively spliced D2 mRNA transcripts similar in size to the major 6- to 7-kb D2 mRNAs but not encoding an active enzyme are present in both human and chicken tissues. Our results indicate that a number of factors reduce the D2 protein levels. These mechanisms, together with the short half-life of the protein, ensure limited expression of this key regulator of T(4) activation.

Protection of mice from allergen-induced asthma by selenite: prevention of eosinophil infiltration by inhibition of NF-kappa B activation

The potential anti-inflammatory effect of sodium selenite in a mouse model of asthma was investigated. Selenite was injected into the peritoneum of allergen (ovalbumin)-sensitized mice before allergen challenge. Ovalbumin challenge resulted in activation of the transcription factor NF-kappaB and an increase in the expression of cell adhesion molecules (intercellular adhesion molecule 1, vascular cell adhesion molecule 1, and E-selectin, which are encoded by NF-kappaB-dependent genes) in lung tissue as well as in the recruitment of eosinophils to lung airways. These effects of ovalbumin challenge were all inhibited by pretreatment of mice with selenite. Selenite administration also increased the activity of selenium-dependent glutathione peroxidase in lung tissue. Furthermore, supplementation of A549 human airway epithelial cell cultures with selenite increased glutathione peroxidase activity as well as inhibited both the generation of hydrogen peroxide and the activation of NF-kappaB induced by tumor necrosis factor alpha in these cells. Selenite also reversed in vitro the activation of NF-kappaB induced by this cytokine in intact A549 cells. These results suggest that selenite regulates the activity of NF-kappaB by increasing the activity of glutathione peroxidase, thereby removing potential activators of NF-kappaB, and possibly also by direct oxidation of critical sulfhydryl groups of this transcription factor. These effects of selenite likely underlie its anti-inflammatory action in asthma.

Triiodothyronine is required for the stimulation of type II 5'-deiodinase mRNA in rat brown adipocytes

Type II 5'-iodothyronine deiodinase (D2), produces triiodothyronine (T(3)) and is stimulated by cold exposure via norepinephrine (NE) release in brown adipose tissue. Cultured rat brown adipocytes require T(3) for the adrenergic stimulation of D2 activity. D2 mRNA expression in cultured brown adipocytes is undetectable with the use of basal conditions or NE without T(3). Full D2 expression is achieved using NE + T(3), especially after prolonged T(3) exposure. beta(3)-Adrenergic agonists mimic the NE action, whereas cAMP analogs do not. Prolonged exposure to T(3) alone increases D2 mRNA. High T(3) doses (500 nM) inhibit the adrenergic stimulation of D2 activity while increasing D2 mRNA. The effects obtained with NE + T(3) or T(3) alone are suppressed by actinomycin, but not by cycloheximide, which leads to accumulation of short D2 mRNA transcripts. Prolonged or short exposure to T(3) did not change D2 mRNA half-life, but T(3) seemed to elongate it. In conclusion, T(3) is an absolute requirement for the adrenergic stimulation of D2 mRNA in brown adipocytes. T(3) upregulates D2 mRNA, an effect that might involve stimulation of factors required for transcription or for stabilization of D2 mRNA.

Type 2 deiodinase in the peripheral nervous system: induction in the sciatic nerve after injury

Thyroid hormones are essential for the development and function of the brain and also for the maturation and repair of the peripheral nervous system. In the brain, most of the 3,5,3'-triiodothyronine is locally produced by 5'-deiodination of thyroxine catalyzed by the type 2 deiodinase. The absence of any information about thyroid hormone metabolism in the peripheral nervous system prompted us to study the expression of type 2 deiodinase (mRNA and activity) in the peripheral nervous system. Expression of type 2 deiodinase mRNA was very low in the sciatic nerve of rats until day 5 after birth, then increased from day 10 to 35-45 and gradually decreased afterwards, down to the low basal levels observed in the adult. A lesion of the sciatic nerve in the adult induced an increase in type 2 deiodinase mRNA and activity. After a cryolesion, the stimulation was observed as early as 4 h and mRNA levels increased until 24-48 h, then gradually declined down to basal levels around 28 days, when regeneration and functional recovery were completed. After a permanent transection, up-regulation of type 2 deiodinase persisted in both proximal and distal segments until the end of the experiment (28 days). Transection and cryolesion were also followed by increased type 2 deiodinase mRNA expression in the ipsilateral L4/L6 dorsal root ganglia within 24 h. Both mRNA and activity were found in the peripheral nerve sheaths but not in the internal compartment of the intact or injured nerve. Cultured fibroblasts from the sciatic nerve expressed type 2 deiodinase 4 h after stimulation by 10 microM forskolin, whereas purified Schwann cells did not. The present study provides evidence that the peripheral nervous system has its own system responsible for the local production of 3,5,3'-triiodothyronine, which may play a key role during the regeneration process.

A novel interaction between thyroid hormones and 1,25(OH)(2)D(3) in osteoclast formation

Thyroid hormones enhance osteoclast formation and their excess is an important cause of secondary osteoporosis. 3,5,3' -Triiodo-L-thyronine (T3) induced the mRNA expression of receptor activator of nuclear factor-kappa B ligand (RANKL), which is a key molecule in osteoclast formation, in primary osteoblastic cells (POB). This effect was amplified in the copresence of 1 alpha,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)). Although T3 alone did not induce octeoclasts in coculture of bone marrow cells with POB, T3 enhanced 1,25(OH)(2)D(3)-induced osteoclast formation. Thyroxine (T4) also enhanced 1,25(OH)(2)D(3)-induced osteoclast formation. These data suggested that T4 was locally metabolized to T3 for its action, since T4 is a prohormone with little hormonal activity. The mRNA expression of type-2 iodothyronine deiodinase (D2), which is responsible for maintaining local T3 concentration, was induced by 1,25(OH)(2)D(3) dose- and time-dependently. Our data would facilitate our understanding of the mechanism of osteoclast formation by thyroid hormones and suggest a novel interaction between thyroid hormones and 1,25(OH)(2)D(3).

Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases

The goal of this review is to place the exciting advances that have occurred in our understanding of the molecular biology of the types 1, 2, and 3 (D1, D2, and D3, respectively) iodothyronine deiodinases into a biochemical and physiological context. We review new data regarding the mechanism of selenoprotein synthesis, the molecular and cellular biological properties of the individual deiodinases, including gene structure, mRNA and protein characteristics, tissue distribution, subcellular localization and topology, enzymatic properties, structure-activity relationships, and regulation of synthesis, inactivation, and degradation. These provide the background for a discussion of their role in thyroid physiology in humans and other vertebrates, including evidence that D2 plays a significant role in human plasma T(3) production. We discuss the pathological role of D3 overexpression causing "consumptive hypothyroidism" as well as our current understanding of the pathophysiology of iodothyronine deiodination during illness and amiodarone therapy. Finally, we review the new insights from analysis of mice with targeted disruption of the Dio2 gene and overexpression of D2 in the myocardium.

Type 2 iodothyronine deiodinase expression is upregulated by the protein kinase A-dependent pathway and is downregulated by the protein kinase C-dependent pathway in cultured human thyroid cells

Type 1 and 2 iodothyronine deiodinases (D1 and D2) catalyze thyroxine (T4) activation. In human thyroid, unlike rodents', both D1 and D2 are expressed. We have investigated the effects of thyrotropin (TSH), dibutyryl cyclic adenosine monophosphate [(Bu)2cAMP] (an activator of protein kinase A [PKA]), 12-O-tetradecanoylphorbor 13-actate (TPA) (an activator of protein kinase C [PKC]), T4, and triiodothyronine (T3) on the D2 mRNA levels and activity in cultured human thyroid cells. D2 mRNA levels were increased by TSH and (Bu)2cAMP, and the increment was faster and greater than that of D1 mRNA levels. The increment of the maximum velocity (Vmax) value for D2 by (Bu)2cAMP stimulation was similar to that of D2 mRNA levels, suggesting that (Bu)2cAMP enhances D2 activity mainly at the pretranslational level. Cycloheximide, a protein synthesis inhibitor, partially inhibited the increase of D2 mRNA levels by (BU)2cAMP, suggesting that de novo protein synthesis-dependent pathways are involved. TPA suppressed the D2 mRNA levels in the presence of (Bu)2cAMP. However, T3 and T4 did not significantly change the D2 mRNA levels and activity. In conclusion, D2 expression in human thyroid cells is more rapidly and strongly upregulated by the PKA pathway than D1 expression, and is downregulated by the PKC pathway.

The human type 2 iodothyronine deiodinase is a selenoprotein highly expressed in a mesothelioma cell line

Types 1 and 3 iodothyronine deiodinases are known to be selenocysteine-containing enzymes. Although a putative human type 2 iodothyronine deiodinase (D2) gene (hDio2) encoding a similar selenoprotein has been identified, basal D2 activity is not selenium (Se)-dependent nor has D2 been labeled with (75)Se. A human mesothelioma cell line (MSTO-211H) has recently been shown to have approximately 40-fold higher levels of hDio2 mRNA than mesothelial cells. Mesothelioma cell lysates activate thyroxine (T(4)) to 3,5,3'-triiodothyronine with typical characteristics of D2 such as low K(m) (T(4)), 1.3 nm, resistance to propylthiouracil, and a short half-life ( approximately 30 min). D2 activity is approximately 30-fold higher in Se-supplemented than in Se-depleted medium. An antiserum prepared against a peptide deduced from the Dio2 mRNA sequence precipitates a (75)Se protein of the predicted 31-kDa size from (75)Se-labeled mesothelioma cells. Bromoadenosine 3'5' cyclic monophosphate increases D2 activity and (75)Se-p31 approximately 2.5-fold whereas substrate (T(4)) reduces both D2 activity and (75)Se-p31 approximately 2-3-fold. MG132 or lactacystin (10 microm), inhibitors of the proteasome pathway by which D2 is degraded, increase both D2 activity and (75)Se-p31 3-4-fold and prevent the loss of D2 activity during cycloheximide or substrate (T(4)) exposure. Immunocytochemical studies with affinity-purified anti-hD2 antibody show a Se-dependent increase in immunofluorescence. Thus, human D2 is encoded by hDio2 and is a member of the selenodeiodinase family accounting for its highly catalytic efficiency in T(4) activation.

Expression and regulation of type II iodothyronine deiodinase in human thyroid gland

We have studied the expression of type II iodothyronine deiodinase (DII) in human thyroid tumors and cultured human thyroid cells to elucidate the mechanisms involved in the regulation of DII expression in human thyroid gland. Three cases with hyperfunctioning thyroid adenoma, including a case that showed an activating mutation of G(s)alpha with a constitutive activation of cAMP production in cultured cells, and six cases with papillary thyroid carcinoma were analyzed in the present study. Free T(3) was increased, whereas free T(4) was within the normal range in all patients with hyperfunctioning thyroid adenoma. Thyroid tumor tissue and surrounding nontumor tissue were obtained at the time of surgery, and DII expression was compared between tumor tissue and nontumor tissue in each case. Northern analysis demonstrated the presence of DII messenger RNA (mRNA) approximately 7.5 kb in size in all of the tumor and nontumor tissues. DII mRNA and DII activity in hyperfunctioning thyroid adenoma were significantly increased compared with those in nontumor tissue in each case. In contrast, DII mRNA and DII activity in papillary thyroid carcinoma were decreased compared with those in nontumor tissue in each case. DII mRNA and DII activity in cultured human thyroid cells were significantly stimulated by TSH in a dose-dependent manner. The promoter activity of the human DII gene including the complete cAMP response element, transfected to cultured human thyroid cells, was stimulated by (Bu)(2)cAMP. In summary, these results suggest that DII expression in human thyroid gland is regulated at the transcriptional level through the TSH receptor-G(s)alpha-cAMP regulatory cascade, which may be related to the increase in circulating T(3) level in patients with Graves' disease and hyperfunctioning thyroid adenoma.

Thyrotropin receptors in brown adipose tissue: thyrotropin stimulates type II iodothyronine deiodinase and uncoupling protein-1 in brown adipocytes

It has been demonstrated that TSH receptors are expressed not only in thyroid gland but also in extrathyroidal tissues. Brown adipose tissue of guinea pig has been reported to express TSH receptor messenger RNA (mRNA), but the physiological roles of TSH receptors in brown adipose tissue have not been understood. We studied the expression and function of TSH receptors in rat brown adipose tissue and cultured rat brown adipocytes. Northern analysis demonstrated the expression of TSH receptor mRNA in rat brown adipose tissue and cultured rat brown adipocytes. TSH receptor mRNA in rat brown adipose tissue was decreased by cold exposure of the rat, and its mRNA in cultured rat brown adipocytes was also decreased by incubation with TSH or (Bu)(2)cAMP. TSH increased the intracellular cAMP concentration in cultured rat brown adipocytes in a dose dependent manner. Type II iodothyronine deiodinase mRNA, its activity, and uncoupling protein-1 mRNA in cultured rat brown adipocytes were significantly increased by incubation with TSH in a dose-dependent manner. These results suggest the expression of functional TSH receptors in brown adipose tissue, which may be involved in regulation of the expression of type II iodothyronine deiodinase and uncoupling protein-1.

Thyroid hormone activation in human vascular smooth muscle cells: expression of type II iodothyronine deiodinase

Thyroid hormone has been reported to have significant effects on the peripheral vascular system, including relaxation of vascular smooth muscle cells and antiatherosclerotic effects. To exert its biological activity, thyroxine, which is a major secretory product of thyroid gland, needs to be converted to 3,5,3'-triiodothyronine (T(3)) by iodothyronine deiodinase. Type I iodothyronine deiodinase (DI) is widely distributed and maintains circulating T(3) level, whereas type II iodothyronine deiodinase (DII) is present in a limited number of tissues to provide local intracellular T(3). In the present study, we have identified iodothyronine deiodinase in cultured human coronary artery smooth muscle cells (hCASMCs) and human aortic smooth muscle cells (hASMCs). All of the characteristics of the deiodinating activity in hCASMCs and hASMCs were compatible with DII. Northern analysis demonstrated that DII mRNA was expressed in both hCASMCs and hASMCs, and DII mRNA levels as well as DII activities were rapidly increased by dibutyryl-cAMP or forskolin. These data demonstrate, for the first time, the expression of DII in human vascular smooth muscle cells, which is regulated by a cAMP-mediated mechanism. The present results suggest a previously unrecognized role of local T(3) production by DII in the pathophysiology of human vascular smooth muscle cells.

Cloning, expression, and functional characterization of the substrate binding subunit of rat type II iodothyronine 5'-deiodinase

Type II iodothyronine 5'-deiodinase catalyzes the bioactivation of thyroid hormone in the brain. In astrocytes, this approximately 200-kDa, membrane-bound enzyme is composed of at least one p29 subunit, an approximately 60-kDa, cAMP-induced activation protein, and one or more unidentified catalytic subunit(s). Recently, an artificial type II-like selenodeiodinase was engineered by fusing two independent cDNAs together; however, no native type II selenodeiodinase polypeptide is translated in the brain or brown adipose tissue of rats. These data suggest that the native type II 5'-deiodinase in rat brain is unrelated to this artificial selenoprotein. In this report, we describe the cloning of the 29-kDa subunit (p29) of type II 5'-deiodinase from a lambdazapII cDNA library prepared from cAMP-induced astrocytes. The 3.3-kilobase (kb) cDNA encodes an approximately 30-kDa, 277-amino acid long, hydrophobic protein lacking selenocysteine. Northern blot analysis showed that a 3.5-kb p29 mRNA was present in tissues showing type II 5'-deiodinase activity such as brain and cAMP-stimulated astrocytes. Domain-specific, anti-p29 antibodies specifically immunoprecipitated enzyme activity. Overexpression of exogenous p29 or a green fluorescence protein (GFP)-tagged p29 fusion protein led to a >100-fold increase in deiodinating activity in cAMP-stimulated astrocytes, and the increased activity was specifically immunoprecipitated by anti-GFP antibodies. Steady-state reaction kinetics of the enzyme in GFP-tagged p29-expressing astrocytes are identical to those of the native enzyme in brain. Direct injection of replication-deficient Ad5-p29(GFP) virus particles into the cerebral cortex of neonatal rats leads to a approximately 2-fold increase in brain type II 5'-deiodinating activity. These data show 1) that the 3.3-kb p29 cDNA encodes an essential subunit of rat type II iodothyronine 5'-deiodinase and 2) identify the first non-selenocysteine containing subunit of the deiodinase family of enzymes.

Biochemical and molecular biological evidence for the presence of type II iodothyronine deiodinase in mouse mammary gland

In the present study we have obtained several lines of evidence indicating the presence of type II iodothyronine deiodinase (DII) in the mouse mammary gland. 5'-deiodinase activity in the mammary gland has an apparent K(m) value of 4.4 nM for T(4) and is inhibited by aurothioglucose but not by propylthiouracil. These characteristics are similar to those of DII in other tissues. We cloned a 1.4-kb cDNA, which contains the entire mouse DII coding region and has high homology with the rat DII cDNA, from the mammary gland and brain. Northern blot analysis showed the presence of 7.9 kb DII mRNA in the mammary gland and brain. The levels of DII activity and mRNA in lactating gland were significantly lower than those in virgin and pregnant glands, suggesting that DII is regulated at the pretranslational level. In addition, we found the low level of DII enzyme activity and transcript in various other mouse tissues.

Characterization of the 5'-flanking and 5'-untranslated regions of the cyclic adenosine 3',5'-monophosphate-responsive human type 2 iodothyronine deiodinase gene

The type 2 iodothyronine deiodinase (D2) catalyzes T4 activation. In humans, unlike rodents, it is widely expressed, and its action probably contributes to both intracellular and plasma T3 pools. We have isolated the 6.5-kb 5'-flanking region (FR) and the previously uncloned 553 nucleotides (nt) of the 5'-untranslated region (UTR) of hdio2. The 5'-UTR is complex, with three transcription start sites (TSS) (708, 31, and approximately 24 nt 5' to the ATG), an alternatively spliced approximately 300-nt intron in the 5'-UTR, and three short open reading frames 5' to the initiator ATG. The previously reported approximately 7.5-kb D2 messenger RNA (mRNA) is actually an approximately 7-kb doublet that is present in thyroid, pituitary, cardiac and skeletal muscle, and possibly brain, but with only the longer transcript in placenta. A canonical cAMP response element-binding protein-binding site is present at about 90 bp 5' to the most 5'-TSS. It accounts for the robust response of the 6.8-kb hdio2 5'-FR to protein kinase A. Forskolin increases D2 mRNA in human thyroid cells, which may explain the high D2 mRNA in Graves' thyroid and thyroid adenomas. The hdio2 gene structure and Northern blot results suggest that D2 expression is tightly controlled and tissue specific.

The diverse role of selenium within selenoproteins: a review

Selenium functions within mammalian systems primarily in the form of selenoproteins. Selenoproteins contain selenium as selenocysteine and perform a variety of physiological roles. Eleven selenoproteins have been identified: cellular or classical glutathione peroxidase; plasma (or extracellular) glutathione peroxidase; phospholipid hydroperoxide glutathione peroxidase; gastrointestinal glutathione peroxidase; selenoprotein P; types 1, 2, and 3 iodothyronine deiodinase; selenoprotein W; thioredoxin reductase; and selenophosphate synthetase. Of these, cellular and plasma glutathione peroxidase are the functional parameters used for the assessment of selenium status. Glutathione peroxidases catalyze the reduction of peroxides that can cause cellular damage. Thioredoxin reductase provides reducing power for several biochemical processes and defends against oxidative stress. Selenoprotein P appears to play a role in oxidant defense. Selenoprotein W may play a role in oxidant defense and be involved with muscle metabolism. Thyroid deiodinases function in the formation and regulation of active thyroid hormone. Selenophosphate synthetase is an enzyme required for the incorporation of selenocysteine into selenoproteins. In addition, a protein in the sperm mitochondrial capsule, which is vital to the integrity of sperm flagella, may be a unique selenoprotein. Recommended intakes, food sources, and status assessment of selenium, as well as selenium's role in health and disease processes, are reviewed.

The mammalian homolog of the frog type II selenodeiodinase does not encode a functional enzyme in the rat

Type II iodothyronine deiodinase is a short-lived, membrane-bound enzyme found in rat brain, brown adipose tissue, and cAMP-stimulated astrocytes. Recently, a full-length complementary DNA (cDNA) encoding a 30-kDa, type II-like selenodeiodinase was cloned from frog, and a homologous partial cDNA (rBAT 1.1), containing two in-frame selenocysteine codons (UGA), was isolated from rat brown adipose tissue. Importantly, the rBAT 1.1 cDNA was derived from a 7.5-kb messenger RNA (mRNA) and did not encode a functional selenoenzyne unless an enabling selenocysteine insertion sequence was appended to the presumed coding region and this cDNA. In this study we determined whether the native 7.5-kb SeD2 mRNA in rat tissues programmed the synthesis of the native type II deiodinase using specific antibodies that were raised against the C-terminus of full-length, 30-kDa SeD2 protein and against the catalytic core of SeD2. Direct analysis of the translation products programmed by the native SeD2 mRNA in cAMP-stimulated astrocytes was performed using antisense deoxynucleotides and hybrid selection strategies. (Bu)2cAMP-stimulated rat astrocytes expressed both type II deiodinase activity (approximately 2500 U/mg protein) and contained abundant levels of the 7.5-kb SeD2 mRNA. However, no immunoreactive 30-kDa SeD2 protein was identified by Western analysis, immunoprecipitation, or immunocytochemistry, and the specific C-terminus antiserum failed to immunoprecipitate deiodinase activity from (Bu)2cAMP-stimulated astrocytes, brown adipose tissue or brain. Instead, the native 7.5-kb SeD2 mRNA encoded a 15-kDa protein that terminated at the first UGA codon and contained the catalytically inactive, N-terminal 129 amino acids of SeD2. These data show that the native 7.5-kb SeD2 mRNA in stimulated astrocytes does not encode D2.

Mutation of the Secys residue 266 in human type 2 selenodeiodinase alters 75Se incorporation without affecting its biochemical properties

Type 2 deiodinase (D2) is a low Km iodothyronine deiodinase that metabolizes thyroxine (T4) to the active metabolite T3. We have recently shown that the cDNA for the human D2 coding region contains two in-frame selenocysteine (TGA) codons. The 3' TGA is seven codons 5' to a universal stop codon, TAA. The human D2 enzyme, transiently expressed in HEK-293 cells, can be in vivo labeled with 75Se as a doublet of approximately 31 kDa. This doublet is consistent with the possibility that the carboxy-terminal TGA codon can either encode selenocysteine or function as a stop codon. To test this hypothesis we mutagenized the second selenocysteine codon to a cysteine (TGC) or to an unambiguous stop codon (TAA). While the selenium incorporation pattern is different between the wild-type and mutant proteins, the deiodination properties of the enzyme are not affected by mutating the 3'TGA codon. Thus, we conclude that neither this residue nor the remaining seven carboxy-terminal amino acids are critical for the deiodination process.

The trace element selenium and the thyroid gland

Apart from the essential trace element iodine, which is the central constituent of thyroid hormones, a second essential trace element, selenium, is required for appropriate thyroid hormone synthesis, activation and metabolism. The human thyroid gland has the highest selenium content per gram of tissue among all organs. Several selenocysteine-containing proteins respectively enzymes are functionally expressed in the thyroid, mainly in thyrocytes themselves: three forms of glutathione peroxidases (cGPx, pGPx, and PH-GPx), the type I 5-deiodinase, thioredoxin reductase and selenoprotein P. The thyroidal expression of type II 5-deiodinase still is controversial. As thyrocytes produce H2O2 continuously throughout life an effective cell defense system against H2O2 and reactive oxygen intermediates derived thereof is essential for maintenance of normal thyroid function and protection of the gland. In experimental animal models long-term and strong selenium deficiency leads to necrosis and fibrosis after high iodide loads. Combined iodide and selenium deficiency such as in central Zaire is thought to cause the myxedematous form of endemic cretinism. Inadequate selenium supply and prediagnostically low serum selenium levels are significantly correlated with the development of thyroid carcinoma and other tumors. Though selenium supply controls expression and translation of selenocysteine-containing proteins no direct correlation is found between selenium tissue content and expression of various thyroidal selenoproteins, indicating that other regulatory factors contribute to or override selenium-dependent expression control, e.g., in thyroid adenoma, carcinoma or autoimmune disease. As both trace elements, iodine and selenium, were washed out from the upper layers of the soil during and after the ice ages in many regions of the world adequate supply with these essential compounds needs to be provided either by a balanced diet or supplementation.

Pretranslational regulation of rhythmic type II iodothyronine deiodinase expression by beta-adrenergic mechanism in the rat pineal gland

It has been demonstrated that type II iodothyronine deiodinase is present in rat pineal gland, and the deiodinase activity markedly increases during the hours of darkness, primarily through beta-adrenergic mechanism. We have studied the relationship between pineal type II iodothyronine deiodinase messenger RNA (mRNA) and the deiodinase activity to elucidate the mechanisms involved in the nocturnal rise in pineal deiodinase activity. Northern analysis has demonstrated that type II iodothyronine deiodinase mRNA is expressed in rat pineal gland, and the mRNA markedly increases during the hours of darkness. The nocturnal increase in pineal type II iodothyronine deiodinase activity is preceded by the increase in its mRNA. Daytime isoproterenol administration resulted in a rapid increase in pineal type II iodothyronine deiodinase mRNA followed by the increase in deiodinase activity. Propranolol treatment, bilateral superior cervical ganglionectomy, or constant light exposure significantly suppressed the nocturnal rise in type II iodothyronine deiodinase mRNA as well as the deiodinase activity. Moreover, isoproterenol or (Bu)2AMP stimulated type II iodothyronine deiodinase mRNA and the deiodinase activity in cultured rat pineal glands. These results suggest that the rhythmic change in pineal type II iodothyronine deiodinase activity is regulated at least in part at the pretranslational level by a beta-adrenergic mechanism transmitted through superior cervical ganglia.

Induction of type 2 deiodinase activity by cyclic guanosine 3',5'-monophosphate in cultured rat glial cells

We investigated the effects of cyclic guanosine 3',5'-monophosphate (cGMP) on type 2 iodothyronine deiodinase (D2) in cultured rat glial cells. Rat glial cells were cultured in Dulbecco's modified Eagle's medium supplemented with 15% fetal bovine serum. When cells were cultured in the presence of 8-bromo cGMP (8-Br cGMP), an analogue of cGMP, D2 activity was increased in a time- and concentration-dependent manner. Lineweaver-Burk plots revealed that the stimulation of D2 activity by 8-Br cGMP (10(-3) M) was associated with fivefold increase in maximum velocity but without a significant change in Michaelis-Menten constant, suggesting that cGMP increases D2 activity via new enzyme synthesis. Both atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) are well known to increase the intracellular cGMP level via their guanylate cyclase-linked receptors in rat glial cells. In the present study, ANP (10(-6) M) and CNP (10(-6) M) significantly increased the D2 activity in rat glial cells (1.9-fold [ANP] or 2.3-fold [CNP] compared with control activity, respectively). Northern blot analysis demonstrated that D2 mRNA level increased in the presence of 8-Br cGMP (10(-3) M), and reached a plateau (six-fold) after 4 hours of incubation. The increment of D2 mRNA level by 8-Br cGMP was comparable with the increase of the D2 activity by this agent. Our data suggest that cGMP induces rat D2 activity, at least in part, at the pretranslational level, and that ANP and CNP increase D2 activity most likely via their guanylate cyclase-linked receptors in rat glial cells.

Northern analysis of type II iodothyronine deiodinase mRNA in rat Harderian gland

It has been known that type II iodothyronine deiodinase activity is present in rat Harderian gland and the activity is significantly increased by isoproterenol administration. We have performed Northern analyses to study whether the transcript for type II iodothyronine deiodinase is expressed in rat Harderian gland and whether the isoproterenol stimulation of type II iodothyronine deiodinase activity in rat Harderian gland is due to the change in its mRNA level. Northern analyses have demonstrated that type II iodothyronine deiodinase mRNA, approximately 7.5 kb in size, is expressed in rat Harderian gland, and the mRNA levels as well as the deiodinase activities are greater in hypothyroid rats than those in euthyroid rats. Type II iodothyronine deiodinase mRNA levels and the deiodinase activities in Harderian gland were increased by isoproterenol administration, and the increase in the mRNA levels preceded that in the deiodinase activities. These results indicate that 7.5 kb transcript for type II iodothyronine deiodinase is expressed in rat Harderian gland and beta-adrenergic stimulation of type II iodothyronine deiodinase activity is due at least in part to the increase in its mRNA level.