Selenoprotein gene expression during selenium-repletion of selenium-deficient rats

Role for Selenium in Metabolic Homeostasis and Human Reproduction

Selenium (Se) is a micronutrient essential for life. Dietary intake of Se within the physiological range is critical for human health and reproductive functions. Selenium levels outside the recommended range have been implicated in infertility and variety of other human diseases. However, presently it is not clear how different dietary Se sources are processed in our bodies, and in which form or how much dietary Se is optimum to maintain metabolic homeostasis and boost reproductive health. This uncertainty leads to imprecision in published dietary guidelines and advice for human daily intake of Se and in some cases generating controversies and even adverse outcomes including mortality. The chief aim for this review is to describe the sources of organic and inorganic Se, the metabolic pathways of selenoproteins synthesis, and the critical role of selenprotenis in the thyroid gland homeostasis and reproductive/fertility functions. Controversies on the use of Se in clinical practice and future directions to address these challenges are also described and discussed herein.

Comparison of analytical methods using enzymatic activity, immunoaffinity and selenium-specific mass spectrometric detection for the quantitation of glutathione peroxidase 1

Glutathione peroxidase 1 (Gpx1), one of the most responsive selenoproteins to the variation of selenium concentration, is often used to evaluate "selenium status" at a cellular or organismal level. The four major types of analytical methodologies to quantify Gpx1 were revisited. They include (i) an enzymatic assay, (ii, iii) polyacrylamide gel electrophoresis (PAGE) with (ii) western blot detection of protein or (iii) inductively coupled plasma mass spectrometry (ICP MS) detection of selenium, and (iv) size-exclusion chromatography with ICP MS detection. Each of the four methods was optimized for the quantification of Gpx1 with maximum sensitivity. The methods based on the enzymatic and immunodetection offer a much higher sensitivity but their accuracy is compromised by the limited selectivity and limited dynamic range. The advantages, drawbacks and sources of error of each technique are critically discussed and the need for the cross-validation of the results using the different techniques to assure the quality assurance of quantitative analysis is emphasized.

Redox status of patients before cardiac surgery

Objectives: Redox regulation plays a crucial role in balancing the cardiovascular system. In this prospective study we aimed to identify currently unknown correlations valuable to cardiovascular research and patient management.

Methods: Blood samples from 500 patients were collected directly before cardiosurgical interventions (Ethics Committee reference number 85/11). Four central redox parameters were determined together with about 30 clinical, anthropometric, and metabolic parameters.

Results: Creatinine levels and pulmonary hypertension were significant predictors of the total antioxidant status (TAOS) in the patients; total glutathione levels were linked to C-peptide, and creatinine, gender, and ventricular arrhythmia influenced nitrate/nitrite levels. Notably, significant interactions were found between medication and redox parameters. Calcium channel blockers (CCBs) were positive predictors of total glutathione levels, whereas angiotensin-converting enzyme inhibitors and CCBs were negative predictors of NOx levels. Age showed the highest correlation with the duration of the intensive care stay, followed by NOx levels, creatinine, TAOS, and C-reactive protein.

Discussion: In this prospective study we determined multiple correlations between redox markers and parameters linked to cardiovascular diseases. The data point towards so far unknown interdependencies, particularly between antihypertensive drugs and redox metabolism. A thorough follow-up to these data has the potential to improve patient management.

Abbreviations: A: absorption; ΔA: absorption difference; ABTS: 2,2′-azino-di(3-ethylbenzothiazoline sulfonate); ACE: angiotensin-converting enzyme; AO: antioxidant; ARB: angiotensin receptor blocker; BMI: body mass index; CAD: coronary artery disease; CCB: calcium channel blocker; CDC: coronary heart diseases; COPD: chronic obstructive pulmonary disease; CRP: C-reactive protein; CVD: cardiovascular diseases; Cu-OOH: cumene hydroperoxide; D: dilution factor; DAN: 2,3-diaminonaphtalene; DMSO: dimethylsulfoxide; DNA: deoxyribonucleic acid; DTNB: 5,5-dithiobis(2-nitrobenzoate); ϵ: extinction coefficient; EDRF: endothelium-derived relaxing factor; fc: final concentration; GPx: glutathione peroxidases; (h)GR: (human) glutathione reductase; GSH: (reduced) glutathione; GSSG: glutathione disulfide; GST: glutathione-S-transferase; Hb: hemoglobin; HDL: high-density lipoprotein; Hk: hematocrit; H₂O₂: hydrogen peroxide; ICS: intensive care stay; LDH: lactate dehydrogenase; LDL: low-density lipoprotein; MI: myocardial infarction; NED: N-(1-naphthyl)-ethylendiamine-dihydrochloride; NOS: nitric oxide synthase; NOx: nitrate/nitrite; NR: nitrate reductase; PBS: phosphate buffered saline; PCA: principle component analysis; PH: pulmonary hypertension; ROS: reactive oxygen species; RNS: reactive nitrogen species; RT: room temperature (25°C); SA: sulfanilamide; SOD: superoxide dismutase; SSA: sulfosalicylic acid; TAC: total antioxidant capacity; TAOS: total antioxidant status; TEAC: trolox equivalent antioxidative capacity; TG: triglycerides; tGSH: total glutathione; TNB-: 2-nitro-5-thiobenzoate; U: unit; UV: ultraviolet; VA: volume activity; Wc: working concentration; WHR: waist-hip ratio.

Reduced Dietary Selenium Impairs Vascular Function by Increasing Oxidative Stress in Sprague-Dawley Rat Aortas

This study aimed to determine whether low dietary Se content affects the function and mechanisms mediating the vascular relaxation of rat aortas, and to test the role of oxidative stress in observed differences. Male Sprague Dawley (SD) rats were maintained for 10 weeks on low Se (low-Se group; N = 20) or normal Se content (norm-Se group; N = 20) rat chow. Dose responses to acetylcholine (ACh; 10-9-10-5M) and the response to reduced pO₂ were tested in noradrenaline-precontracted aortic rings in the absence/presence of the nitric oxide synthase (NOS) inhibitor nitro-l-arginine methyl ester (l-NAME), the cyclooxygenase 1 and 2 (COX-1, 2) inhibitor Indomethacin, and the antioxidative agent Tempol in tissue bath. mRNA expression of glutathione peroxidase 1 (GPx1), catalase (CAT), and Cu/Zn superoxide dismutase (SOD) was measured in rat aortas. Oxidative stress (Thiobarbituric Acid Reactive Substances; TBARS), antioxidative plasma capacity (ferric reducing ability of plasma assay; FRAP), and protein levels of GPx1 were measured in plasma and serum samples, respectively. Reduced ACh-induced relaxation (AChIR) (dominantly mediated by NO) in the low-Se group compared to the norm-Se group was restored by Tempol administration. Hypoxia-induced relaxation (HIR) (dominantly mediated by COX-1, 2), TBARS, and FRAP as well as GPx1 serum concentrations were similar between the groups. mRNA GPx1 expression in rat aortas was significantly decreased in the low-Se compared to the norm-Se group. These data suggest that low dietary Se content increases the local oxidative stress level, which subsequently affects the NO-mediated vascular response.

Analysis of Manganese Superoxide Dismutase and Glutathione Peroxidase 1 Gene Polymorphisms in Vitiligo

Vitiligo is a hereditary/acquired progressive pigmentation disorder characterized by discoloration of skin as a result of melanocyte dysfunction. Recent studies have proposed that oxidant/antioxidant status plays an important role in vitiligo pathogenesis because of the toxic effects on melanocytes. In this study, we aimed to investigate possible associations of MnSOD Ala-9Val and GPx1 Pro198Leu polymorphisms with vitiligo with in Turkish population. The study group consists of 57 patients with vitiligo and 69 healthy controls. Genotyping is performed to identify MnSOD Ala-9Val and GPx1 Pro198Leu polymorphisms. The method used for genotyping was based on the PCR amplification and detection of polymorphisms by hybridization probes labeled with fluorescent dyes. Both the genotype and allele frequencies of MnSOD Ala-9Val (p = 0.817 and p = 0.553, respectively) and GPx1 Pro198Leu polymorphisms (p = 0.422 and p = 0.673, respectively) were not significantly different between vitiligo patients and the control group. Although no significant difference was found, this is the first report investigating the possible associations between the MnSOD Ala-9Val and GPx1 Pro198Leu polymorphisms in Turkish population. Further studies with large populations will be able to clarify the association better.

Selenium-enriched foods are more effective at increasing glutathione peroxidase (GPx) activity compared with selenomethionine: a meta-analysis

Selenium may play a beneficial role in multi-factorial illnesses with genetic and environmental linkages via epigenetic regulation in part via glutathione peroxidase (GPx) activity. A meta-analysis was undertaken to quantify the effects of dietary selenium supplementation on the activity of overall GPx activity in different tissues and animal species and to compare the effectiveness of different forms of dietary selenium. GPx activity response was affected by both the dose and form of selenium (p < 0.001). There were differences between tissues on the effects of selenium supplementation on GPx activity (p < 0.001); however, there was no evidence in the data of differences between animal species (p = 0.95). The interactions between dose and tissue, animal species and form were significant (p < 0.001). Tissues particularly sensitive to changes in selenium supply include red blood cells, kidney and muscle. The meta-analysis identified that for animal species selenium-enriched foods were more effective than selenomethionine at increasing GPx activity.

Selenium supplementation in thyroid associated ophthalmopathy: an update

The therapeutic effect of selenium (Se) has already been proven in thyroid disease and thyroid associated ophthalmopathy (TAO). In spite of clear scientific proof of its benefits in TAO, there appears to be no clear agreement among the clinicians regarding its optimum dose, duration of the treatment, efficacy and safety to date. In this review, the author summarises the findings of 135 English language articles published on this subject over the past four decades from 1973 to 2013. The regulation and metabolism of thyroid hormones require a steady supply of Se and recent studies have revealed several possible mechanisms by which Se improves the severity of thyroid disease and TAO. These mechanisms include 1) inhibitory effect of HLA-DR molecule expression on thyrocytes; 2) profound reductions of thyroid stimulating hormone (TSH) receptor antibodies (TSHR-Ab) and TPO antibodies (TPO-Ab); 3) prevention of dysregulation of cell-mediated immunity and B cell function; 4) neutralising reactive oxygen species (ROS) and inhibition of redox control processes required for the activation, differentiation and action of lymphocytes, macrophages, neutrophils, natural killer cells involved in both acute and chronic orbital inflammation in TAO; 5) inhibition of expression of pro-inflammatory cytokines and 6) inhibition of prostaglandin and leukotriene synthesis. An increased oxidative stress has been observed in both acute and chronic phases of thyroid disease with raised tissue concentrations of ROS. The benefits of Se supplementation in individuals with TAO appear to be proportionate to the degree of systemic activity of the thyroid disease. The maximal benefit of Se supplementation is therefore seen in the subjects who are hyperthyroid. Restoration of euthyroidism is one of the main goals in the management of TAO and when anti-thyroid drugs are combined with Se, the patients with Graves' disease (GD) and autoimmune thyroiditis (AIT) achieved euthyroidism faster than those treated with anti-thyroid drugs alone. Se status of normal adult humans can vary widely and Se supplementation may confer benefit only if serum Se levels are insufficient. The author recommends that serum Se levels of patients with TAO to be assessed prior to and during Se supplementation at regular intervals to avoid potential iatrogenic chronic Se overdose.

Selenium supplementation can slow the development of naphthalene cataract

PURPOSE

To evaluate the effect of selenium supplementation on the progress of naphthalene cataract.

MATERIALS AND METHODS

Sprague-Dawley rats were randomly divided into five groups as follows: normal control, naphthalene control and selenium-supplemented groups (Selenium I, II and III, which were orally administrated with selenium at doses of 0.0104 mg/kg, 0.0208 mg/kg and 0.0416 mg/kg, respectively). All the intervention groups were orally administered with 10% naphthalene solution for 5 weeks. The lens density of each group was determined by photography. Moreover, glutathione peroxidase (GPx) activity in the lens, erythrocyte and plasma was investigated. In addition, lens glutathione (GSH), malondialdehyde (MDA) and hydroxyl radical levels were evaluated. Selenium level in aqueous humor was determined using atomic absorption spectrometry.

RESULTS

The maximum, mean and minimum densities of lens opacities were lower in Selenium group II and III than those in naphthalene group. The maximum density of the lens increased more slowly in Selenium group I than that in naphthalene controls. In selenium-supplemented groups, blood and lens GPx activities as well as aqueous humor selenium level increased significantly. Selenium supplementation also significantly ameliorated the decrease in GSH level and increase in MDA and hydroxyl radical levels in the lens of naphthalene-treated rats.

CONCLUSIONS

Selenium supplementation could slow the development of naphthalene cataract possibly by attenuating the oxidative stress in the lens.

Selenoprotein P regulation by the glucocorticoid receptor

Maintenance of the antioxidant activity of selenoproteins is one potential mechanism of the beneficial health effects of selenium. Selenoprotein P is the primary selenium distribution protein of the body as well as the major selenium containing protein in serum. The transcriptional regulation of selenoprotein P is of interest since the extrahepatic expression of this gene has demonstrated differentiation-dependent expression in development as well as under different disease states. SEPP1 displays patterned expression in numerous tissues during development and the loss of SEPP1 expression has been observed in malignancy. In addition, factors that influence inflammatory processes like cytokines and their regulators have been implicated in selenoprotein P transcriptional control. Herein, we identify a retinoid responsive element and describe a mechanism where the glucocorticoid receptor negatively regulates expression of selenoprotein P. Luciferase reporter assays and quantitative PCR were used to measure selenoprotein P transcription in engineered HEK-293 cells. When stimulated with ecdysone analogs, selenoprotein P expression was increased with the use of a fusion transcription factor that contains the glucocorticoid receptor DNA binding domain, an ecdysone ligand-binding domain, and a strong transactivation domain as well as the retinoid X receptor. The native glucocorticoid receptor inhibited selenoprotein P transactivation, and selenoprotein P was further attenuated in the presence of dexamethasone. Our results may provide insight into a potential mechanism by which selenium is redistributed during development, differentiation or under conditions of critical illness, where glucocorticoid levels are typically increased.

Identification of a signature motif for the eIF4a3-SECIS interaction

eIF4a3, a DEAD-box protein family member, is a component of the exon junction complex which assembles on spliced mRNAs. The protein also acts as a transcript-selective translational repressor of selenoprotein synthesis during selenium deficiency. Selenocysteine (Sec) incorporation into selenoproteins requires a Sec Insertion Sequence (SECIS) element in the 3′ untranslated region. During selenium deficiency, eIF4a3 binds SECIS elements from non-essential selenoproteins, preventing Sec insertion. We identified a molecular signature for the eIF4a3-SECIS interaction using RNA gel shifts, surface plasmon resonance and enzymatic foot printing. Our results support a two-site interaction model, where eIF4a3 binds the internal and apical loops of the SECIS. Additionally, the stability of the complex requires uridine in the SECIS core. In terms of protein requirements, the two globular domains of eIF4a3, which are connected by a linker, are both critical for SECIS binding. Compared to full-length eIF4a3, the two domains in trans bind with a lower association rate but notably, the uridine is no longer important for complex stability. These results provide insight into how eIF4a3 discriminates among SECIS elements and represses translation.

Adaptive dysfunction of selenoproteins from the perspective of the triage theory: why modest selenium deficiency may increase risk of diseases of aging

The triage theory proposes that modest deficiency of any vitamin or mineral (V/M) could increase age-related diseases. V/M-dependent proteins required for short-term survival and/or reproduction (i.e., "essential") are predicted to be protected on V/M deficiency over other "nonessential" V/M-dependent proteins needed only for long-term health. The result is accumulation of insidious damage, increasing disease risk. We successfully tested the theory against published evidence on vitamin K. Here, we review about half of the 25 known mammalian selenoproteins; all of those with mouse knockout or human mutant phenotypes that could be used as criteria for a classification of essential or nonessential. Five selenoproteins (Gpx4, Txnrd1, Txnrd2, Dio3, and Sepp1) were classified as essential and 7 (Gpx1, Gpx 2, Gpx 3, Dio1, Dio2, Msrb1, and SelN) nonessential. On modest selenium (Se) deficiency, nonessential selenoprotein activities and concentrations are preferentially lost, with one exception (Dio1 in the thyroid, which we predict is conditionally essential). Mechanisms include the requirement of a special form of tRNA sensitive to Se deficiency for translation of nonessential selenoprotein mRNAs except Dio1. The same set of age-related diseases and conditions, including cancer, heart disease, and immune dysfunction, are prospectively associated with modest Se deficiency and also with genetic dysfunction of nonessential selenoproteins, suggesting that Se deficiency could be a causal factor, a possibility strengthened by mechanistic evidence. Modest Se deficiency is common in many parts of the world; optimal intake could prevent future disease.

Nucleolin binds to a subset of selenoprotein mRNAs and regulates their expression

Selenium, an essential trace element, is incorporated into selenoproteins as selenocysteine (Sec), the 21st amino acid. In order to synthesize selenoproteins, a translational reprogramming event must occur since Sec is encoded by the UGA stop codon. In mammals, the recoding of UGA as Sec depends on the selenocysteine insertion sequence (SECIS) element, a stem-loop structure in the 3' untranslated region of the transcript. The SECIS acts as a platform for RNA-binding proteins, which mediate or regulate the recoding mechanism. Using UV crosslinking, we identified a 110 kDa protein, which binds with high affinity to SECIS elements from a subset of selenoprotein mRNAs. The crosslinking activity was purified by RNA affinity chromatography and identified as nucleolin by mass spectrometry analysis. In vitro binding assays showed that purified nucleolin discriminates among SECIS elements in the absence of other factors. Based on siRNA experiments, nucleolin is required for the optimal expression of certain selenoproteins. There was a good correlation between the affinity of nucleolin for a SECIS and its effect on selenoprotein expression. As selenoprotein transcript levels and localization did not change in siRNA-treated cells, our results suggest that nucleolin selectively enhances the expression of a subset of selenoproteins at the translational level.

Selenium and thyroid

Inadequate supply of the essential trace element selenium (Se) has been associated with predisposition for, or manifestation of, various human diseases such as Keshan and Kashin-Beck disease, cancer, impaired immune function, neurodegenerative and age-related disorders and disturbances of the thyroid hormone axis. Se deficiency in combination with inadequate iodine contributes to the pathogenesis of myxedematous cretinism. The recent identification of various distinct selenocysteine-containing proteins, encoded by 25 human genes, provides information on the molecular and biochemical basis of beneficial and possible adverse effects of this trace element. The thyroid gland is among the human tissues with the highest Se content per mass unit similar to other endocrine organs and the brain. Selenoproteins involved in cellular antioxidative defence systems and redox control, such as the glutathione peroxidase (GPx) and the thioredoxin reductase (TxnRd) family, are involved in protection of the thyroid gland from excess hydrogen peroxide and reactive oxygen species produced by the follicles for biosynthesis of thyroid hormones. In addition, the three key enzymes involved in activation and inactivation of thyroid hormones, the iodothyronine deiodinases (DIO1,2,3), are selenoproteins with development, cell- and pathology-related expression patterns. While nutritional Se supply is normally sufficient for adequate expression of functional Dio enzymes with exception of long-term parenteral nutrition and certain diseases impairing gastrointestinal absorption of Se compounds, the nutritional Se supply for the protection of the thyroid gland and synthesis of some more abundant selenoproteins of the GPx and the TrxR family might be limiting their proper expression under (patho-)physiological conditions.

Eukaryotic initiation factor 4a3 is a selenium-regulated RNA-binding protein that selectively inhibits selenocysteine incorporation

The synthesis of selenoproteins requires the translational recoding of the UGA stop codon as selenocysteine. During selenium deficiency, there is a hierarchy of selenoprotein expression, with certain selenoproteins synthesized at the expense of others. The mechanism by which the limiting selenocysteine incorporation machinery is preferentially utilized to maintain the expression of essential selenoproteins has not been elucidated. Here we demonstrate that eukaryotic initiation factor 4a3 (eIF4a3) is involved in the translational control of a subset of selenoproteins. The interaction of eIF4a3 with the selenoprotein mRNA prevents the binding of SECIS binding protein 2, which is required for selenocysteine insertion, thereby inhibiting the synthesis of the selenoprotein. Furthermore, the expression of eIF4a3 is regulated in response to selenium. Based on knockdown and overexpression studies, eIF4a3 is necessary and sufficient to mediate selective translational repression in cells. Our results support a model in which eIF4a3 links selenium status with differential selenoprotein expression.

Selenoprotein gene expression in thyroid and pituitary of young pigs is not affected by dietary selenium deficiency or excess

Expression and function of selenoproteins in endocrine tissues remain unclear, largely due to limited sample availability. Pigs have a greater metabolic similarity and tissue size than rodents as a model of humans for that purpose. We conducted 2 experiments: 1) we cloned 5 novel porcine selenoprotein genes; and 2) we compared the effects of dietary selenium (Se) on mRNA levels of 12 selenoproteins, activities of 4 antioxidant enzymes, and Se concentrations in testis, thyroid, and pituitary with those in liver of pigs. In Experiment 1, porcine Gpx2, Sephs2, Sep15, Sepn1, and Sepp1 were cloned and demonstrated 84-94% of coding sequence homology to human genes. In Experiment 2, weanling male pigs (n = 30) were fed a Se-deficient (0.02 mg Se/kg) diet added with 0, 0.3, or 3.0 mg Se/kg as Se-enriched yeast for 8 wk. Although dietary Se resulted in dose-dependent increases (P < 0.05) in Se concentrations and GPX activities in all 4 tissues, it did not affect the mRNA levels of any selenoprotein gene in thyroid or pituitary. Testis mRNA levels of Txnrd1 and Sep15 were decreased (P < 0.05) by increasing dietary Se from 0.3 to 3.0 mg/kg. Comparatively, expressions of Gpx2, Gpx4, Dio3, and Sep15 were high in pituitary and Dio1, Sepp1, Sephs2, and Gpx1 were high in liver. In conclusion, the mRNA abundances of the 12 selenoprotein genes in thyroid and pituitary of young pigs were resistant to dietary Se deficiency or excess.

Comparison of glutathione peroxidase 1 and iodothyronine deiodinase 1 mRNA expression in murine liver after feeding selenite or selenized yeast

The experiment was conducted to compare the effect of different selenium sources on the expression of glutathione peroxidase 1 (GPx1) and iodothyronine deiodinase 1 (Dio1) mRNA in mice by quantitative real-time PCR. A total of 60 male Kunming mice at average body weight of 20 g were allotted to three groups in a randomized complete block design, namely two treatments and one control. Mice in Group 1 were fed a basal diet as control, while mice in Groups 2 and 3 were fed the basal diet supplemented with 0.1mg/kg selenium as sodium selenite or selenized yeast, respectively. Whole feeding experiment lasted for 30 d. At the end of the feeding trial, liver mRNA levels of GPx1 and Dio1 were determined by quantitative real-time PCR, as well as growth performance, body composition, blood and GPx activity were determined. The results showed that no significant differences in overall growth performance and body composition, including body weight, body length, heart weight, kidney weight and liver weight, were found between the experimental groups (P>0.05). Blood GPx activity increased in all of the selenium supplemented groups compared with control group (P<0.01). However, blood GPx activity in selenized yeast group was higher than that in sodium selenite group (P<0.05). Liver mRNA levels of GPx1 and Dio1 also increased in the two selenium supplemented groups compared with the control group (P<0.05), while there was no significant difference between the sodium selenite and selenized yeast groups (P>0.05). In conclusion, selenium increased the mRNA expression of GPx1 and Dio1 genes in murine liver, and there was no significant difference between the organic or inorganic form of selenium used.

Importance of molar ratios in selenium-dependent protection against methylmercury toxicity

The influence of dietary selenium (Se) on mercury (Hg) toxicity was studied in weanling male Long Evans rats. Rats were fed AIN-93G-based low-Se torula yeast diets or diets augmented with sodium selenite to attain adequate- or rich-Se levels (0.1, 1.0 or 15 micromol/kg, respectively) These diets were prepared with no added methylmercury (MeHg) or with moderate- or high-MeHg (0.2, 10 or 60 micromol/kg, respectively). Health and weights were monitored weekly. By the end of the 9-week study, MeHg toxicity had impaired growth of rats fed high-MeHg, low-Se diets by approximately 24% (p < 0.05) compared to the controls. Growth of rats fed high-MeHg, adequate-Se diets was impaired by approximately 8% (p < 0.05) relative to their control group, but rats fed high-MeHg, rich-Se diets did not show any growth impairment. Low-MeHg exposure did not affect rat growth at any dietary Se level. Concentrations of Hg in hair and blood reflected dietary MeHg exposure, but Hg toxicity was more directly related to the Hg to Se ratios. Results support the hypothesis that Hg-dependent sequestration of Se is a primary mechanism of Hg toxicity. Therefore, Hg to Se molar ratios provide a more reliable and comprehensive criteria for evaluating risks associated with MeHg exposure.

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.

Hepatic deiodinase activity is dispensable for the maintenance of normal circulating thyroid hormone levels in mice

Thyroid hormone (TH) homeostasis depends on peripheral activation and inactivation of iodothyronines by selenoenzymes of the deiodinase (Dio) family. We genetically inactivated hepatic selenoenzyme expression, including Dio1, in order to determine the contribution of hepatic Dio to circulating TH levels. Serum levels of TSH, total T(4), and total T(3) were not different from controls. We measured Dio1 and Dio2 in kidney, skeletal muscle, heart, brown adipose tissue, and brain, but did not find compensatory up-regulation in these tissues. Finally, we determined expression in the liver of the following T(3) target genes: Spot14, alpha-glycerophosphate dehydrogenase (alphaGPD), and malic enzyme (ME). On the transcript level, both Spot14 and alphaGPD were reduced in Dio-deficient liver to about 60-70% of controls. However, mRNA and activity of ME were significantly increased in the same mice. Together, our results indicate that hepatic Dio1 activity is not absolutely required to sustain the euthyroid state in mice.

New roles for an old selenoenzyme: evidence from glutathione peroxidase-1 null and overexpressing mice

Cellular glutathione peroxidase-1 (GPX1) is the first identified and the most abundant selenoprotein in mammals. Although GPX1 has been widely considered to be a major antioxidant enzyme, there has been no direct evidence for such role in vivo until GPX1 transgenic and null mice became available 10 y ago. Using these new models, we demonstrated that GPX1 protects against oxidative stress mediated by reactive oxygen species (ROS), and the physiologic importance of this protection varies with insult level and body Se status. Full expression of GPX1 is needed, and overexpression of GPX1 is beneficial for Se-adequate mice to defend against severe oxidative stress. This function of GPX1 is associated with attenuating the prooxidant-induced oxidation of NADPH, NADH, lipid, and protein in various tissues. In Se-deficient mice, a minute amount of GPX1 activity (4% of adequate levels) protects against hepatic aponecrosis induced by mild oxidative stress. In contrast, knockout of GPX1 renders mice and their hepatocytes resistant to oxidative stress related to reactive nitrogen species (RNS). More intriguingly, mice overexpressing GPX1 develop insulin resistance and obesity, accompanied by a downregulation of insulin-mediated phosphorylations of insulin receptor and Akt protein. In conclusion, GPX1 seems to play contrasting roles in coping with ROS vs. RNS, and its metabolic functions extend beyond redox regulation.

Selenium and the control of thyroid hormone metabolism

Thyroid hormone synthesis, metabolism and action require adequate availability of the essential trace elements iodine and selenium, which affect homeostasis of thyroid hormone-dependent metabolic pathways. The three selenocysteine-containing iodothyronine deiodinases constitute a novel gene family. Selenium is retained and deiodinase expression is maintained at almost normal levels in the thyroid gland, the brain and several other endocrine tissues during selenium deficiency, thus guaranteeing adequate local and systemic levels of the active thyroid hormone T(3). Due to their low tissue concentrations and their mRNA SECIS elements deiodinases rank high in the cellular and tissue-specific hierarchy of selenium distribution among various selenoproteins. While systemic selenium status and expression of abundant selenoproteins (glutathione peroxidase or selenoprotein P) is already impaired in patients with cancer, disturbed gastrointestinal resorption, unbalanced nutrition or patients requiring intensive care treatment, selenium-dependent deiodinase function might still be adequate. However, disease-associated alterations in proinflammatory cytokines, growth factors, hormones and pharmaceuticals modulate deiodinase isoenzyme expression independent from altered selenium status and might thus pretend causal relationships between systemic selenium status and altered thyroid hormone metabolism. Limited or inadequate supply of both trace elements, iodine and selenium, leads to complex rearrangements of thyroid hormone metabolism enabling adaptation to unfavorable conditions.

Adenosine-Dependent Induction of Glutathione Peroxidase 1 in Human Primary Endothelial Cells and Protection Against Oxidative Stress

Cellular glutathione peroxidase (GPx-1), a selenocysteine-containing enzyme, plays a central role in protecting cells from oxidative injury. GPx-1 is ubiquitously expressed in eukaryotic cells where it reduces hydrogen and lipid peroxides to alcohols. Adenosine, which is released from stressed or injured cells, protects against ischemia/reperfusion injury and apoptosis. In this study, we hypothesize that the cytoprotective effect of adenosine involves an increase in the activity of GPx-1. Treatment of human primary pulmonary artery endothelial cells (HPAECs) with 50 μmol/L adenosine in the presence of 10 μmol/L erytho-9-(2-hydroxy-3-nonyl)adenine (EHNA), an adenosine deaminase inhibitor, for 48 hours increased GPx-1 mRNA levels 2-fold. GPx-1 protein and enzyme activity also increased ≈2-fold after treatment. The induction of GPx-1 expression was found to be a consequence of increased mRNA stability and not an increase in transcription. Bisindolylmaleimide I (BIM), a protein kinase C signaling pathway inhibitor, significantly attenuated the induction of GPx-1 mRNA by ≈36%. The adenosine/EHNA-treated cells were more resistant to hydrogen peroxide stress. Both pharmacological inhibition and siRNA knockdown of GPx-1 attenuated the protective affect of adenosine/EHNA treatment, indicating that the adenosine-induced increase in GPx-1 contributes to an increase in cellular protection against oxidative stress. These data suggest that adenosine may protect the cardiovascular system from ischemia/reperfusion injury, in part, by enhancing the expression of the central intracellular antioxidant enzyme, GPx-1.

The impact of iron and selenium deficiencies on iodine and thyroid metabolism: biochemistry and relevance to public health

Several minerals and trace elements are essential for normal thyroid hormone metabolism, e.g., iodine, iron, selenium, and zinc. Coexisting deficiencies of these elements can impair thyroid function. Iron deficiency impairs thyroid hormone synthesis by reducing activity of heme-dependent thyroid peroxidase. Iron-deficiency anemia blunts and iron supplementation improves the efficacy of iodine supplementation. Combined selenium and iodine deficiency leads to myxedematous cretinism. The normal thyroid gland retains high selenium concentrations even under conditions of inadequate selenium supply and expresses many of the known selenocysteine-containing proteins. Among these selenoproteins are the glutathione peroxidase, deiodinase, and thioredoxine reductase families of enzymes. Adequate selenium nutrition supports efficient thyroid hormone synthesis and metabolism and protects the thyroid gland from damage by excessive iodide exposure. In regions of combined severe iodine and selenium deficiency, normalization of iodine supply is mandatory before initiation of selenium supplementation in order to prevent hypothyroidism. Selenium deficiency and disturbed thyroid hormone economy may develop under conditions of special dietary regimens such as long-term total parenteral nutrition, phenylketonuria diet, cystic fibrosis, or may be the result of imbalanced nutrition in children, elderly people, or sick patients.

Glutathione peroxidase activity and selenoprotein W levels in different brain regions of selenium-depleted rats(1)

Previous studies in selenium (Se)-depleted sheep and rats showed that selenoprotein W (SeW) levels decreased in all tissues except brain. To further investigate this depletion in different parts of the brain, second generation Se-depleted rats were used. Dams consumed a Se-deficient basal diet during gestation and lactation, and deficient rats were obtained by continuation on the same diet. Control rats were fed a diet with 0.1-mg Se/kg diet after weaning. Glutathione peroxidase (GPX) activities were measured for comparative purposes to SeW levels. GPX activity in muscle, skin, spleen, and testis increased about 4-fold with Se repletion and reached a plateau after 6 or 10 weeks, but GPX activity decreased to almost one tenth of the original activity with continuous Se depletion. In contrast, GPX activities increased, rather than declined, in various brain regions (cortex, cerebellum, and thalamus) with time of feeding the deficient diet. An experiment with first generation rats, however, indicated that GPX activity was significantly lower in these three brain regions from rats fed the deficient diet as compared to rats fed the supplemented diet. SeW levels in skin, spleen, muscle, and testis were undetectable in weanling rats, but became detectable after 6 weeks of Se repletion. In contrast, the expression of SeW in cortex, cerebellum, and thalamus was not significantly affected by Se depletion, but increased SeW levels occurred only in thalamus with Se supplementation. The results with GPX using first and second generation rats suggest that there are "mobile" and "immobile" GPX fractions in the brain.

mRNA stability and selenocysteine insertion sequence efficiency rank gastrointestinal glutathione peroxidase high in the hierarchy of selenoproteins

The recently described gastrointestinal glutathione peroxidase (GI-GPx) is the fourth member of the family of the selenoenzymes glutathione peroxidases (GPx). In contrast to the more uniform distribution of, for example, the classical glutathione peroxidase (cGPx), it is expressed exclusively in the gastrointestinal tract and has, therefore, been suggested to function as a primary barrier against alimentary hydroperoxides. In order to get an idea of its relative importance we investigated its position in the hierarchy of selenoprotein expression. The selenium-dependent expression of GI-GPx was analyzed in comparison with that of other GPx types at the level of mRNA and protein in HepG2 and CaCo-2 cells. Furthermore, the selenocysteine insertion sequence (SECIS) efficiencies of GI-GPx, phospholipid hydroperoxide glutathione peroxidase (PHGPx) and cGPx in response to selenium were determined by a reporter-gene assay in human hepatoma cells and baby hamster kidney cells. GI-GPx mRNA levels increased during selenium deficiency, whereas cGPx mRNA levels decreased and PHGPx mRNA levels remained almost unaffected. In cells grown in selenium-poor media, all GPx-types were low in both activity and immunochemical reactivity. Upon selenium repletion immunoreactive GI-GPx protein reached a plateau after 10 h, whereas cGPx started to be expressed at 24 h and did not reach its maximum level before 3 days. SECIS efficiencies decreased in the order PHGPx > cGPx > GI-GPx. The augmentation of SECIS efficiencies by selenium was highest for cGPx and intermediate for PHGPx, whereas it was marginal for GI-GPx. The high mRNA stability under selenium restriction, the speed of biosynthesis upon selenium repletion and the marginal effect of selenium on the SECIS efficiency indicate that of the GPx isotypes, GI-GPx ranks highest in the hierarchy of selenoproteins and point to a vital role of GI-GPx in the gastrointestinal tract.

The TSH-dependent variation of the essential elements iodine, selenium and zinc within human thyroid tissues

Instrumental Neutron Activation Analysis was used in order to measure iodine, selenium and zinc concentration in thyroid samples. A pair of samples of normal and nodular tissue were collected from the thyroid gland from 72 patients selected on the basis of pathological criteria (44 cases of multinodular goiter, 12 of chronic lymphocytic thyroiditis (CLT), 6 of thyroid adenoma (TA) and 12 of thyroid cancer (TC)). The check for tissue homogeneity and sampling error was performed by means of the coefficient of variation (CV%) of the elements in replicate samples of normal and altered tissues. High CV% values (> 15%) for iodine reflected a functional variability in thyroid follicles, while low CV% values (< 10%) for selenium and zinc indicated that the composition of selected tissues was rather homogeneous. The variation of the element's concentration was compared in normal and altered tissues. The mean element concentrations had values close to those already reported in the literature; furthermore, our patients had marginal iodine and selenium deficiency. Both normal and nodular tissues in CLT showed statistically significant lower zinc values as compared with the other thyroid diseases. To evaluate the thyroid function, thyroid stimulating hormone (TSH) and thyroxine (T4) levels were measured in the serum of patients. Two arbitrary serum-TSH threshold levels (TSH < 1.0 and > 4.0 mU/L) were introduced in order to classify, respectively, hyperthyroidism and hypothyroidism, as well as euthyroid conditions (1.0 < TSH < 4.0 mU/L), and each patient was assigned to one of these groups. The influence of TSH in the variation of the concentration of iodine, selenium and zinc in normal and altered human thyroid tissues was significant.

Effect of selenium deficiency on cellular and extracellular glutathione peroxidases: immunochemical detection and mRNA analysis in rat kidney and serum

To determine the effect of selenium (Se) deficiency on expression of glutathione peroxidase (GSH-Px) 1 and 2, we measured GSH-Px activity in rat serum, liver and kidneys, serum immunoreactive GSH-Px 2, and the mRNAs of kidney GSH-Px 1 and 2. We purified rat GSH-Px 2 and raised polyclonal antibodies. Immunoreactive GSH-Px 2 was measured by rocket immunoelectrophoresis. GSH-Px 2 was purified 1470-fold with a specific activity of 250 units/mg. Immunoblotting detected only GSH-Px 2 in rat serum, and much less GSH-Px 2 than GSH-Px 1 in kidney. Immunoblot signal of kidney GSH-Px 1 and 2 decreased progressively in Se deficient rats. Serum GSH-Px activity in Se deficient rats at 1, 2, 3, and 4 weeks declined to 33, 20, 10, and 9% of the control, while the serum level of immunoreactive GSH-Px 2 was 58, 24, 15, and 10% of the control, suggesting the presence of an inactive protein at week 1. GSH-Px activity declined to 4 and 11% of the control in the liver and kidney at 4 weeks. The mRNAs of kidney GSH-Px 1 and 2 showed similar decreases, and were 24 and 23% of the control at 4 weeks. GSH-Px mRNA levels were better preserved than GSH-Px activity, suggesting that GSH-Px expression was regulated at both pre-translational and translational levels.

Liver selenium and testis phospholipid hydroperoxide glutathione peroxidase are associated with growth during selenium repletion of second-generation Se-deficient male rats

We have previously shown that changes in glutathione peroxidase-1 (GPX1; H2O2:oxidoreductase, EC 1.11.1.9), plasma thyroid hormone and glutathione-S-transferase were not associated with changes in growth observed in second-generation (F2) severely Se-deficient rats; we also found that liver phospholipid hydroperoxide glutathione peroxidase (GPX4; EC 1.11.1.12) activity falls in first-generation (F1) Se-deficient rats to 41% of levels in Se-adequate rats. The purposes of this study were to determine the effect of F2 Se deficiency on GPX4 and to detect early changes in Se parameters associated with growth after single, small Se injections. Se-deficient male F2 weanling rats were randomly divided into two groups and fed a Se-deficient crystalline amino acid (0.003 microg Se/g diet; -Se) diet or that diet supplemented for 14 d with 0.2 microg Se/g diet (+Se) as Na2SeO3. Growth of -Se rats was 55% of the rate of +Se rats. Liver Se, GPX1 activity, GPX4 activity and testis GPX4 activity in -Se rats at 14 d were 1, 2, 23 and 13%, respectively, of levels in +Se rats. In a series of experiments, additional F2 male weanling rats were fed the -Se diet for 14 d and then were given an intraperitoneal single saline injection of 0, 1 or 5 microg Se/100 g body weight (BW) as Na2SeO3 and killed 1 or 7 d later. Rats injected with 1 or 5 microg Se/100 g BW grew 36 or 48%, respectively, above the rate of saline-injected rats. Liver Se concentration increased 367% and testis GPX4 activity doubled in rats 1 d after injection of 1 microg Se/100 g BW compared with saline-injected rats; these parameters were further elevated with 5 microg Se/100 g BW injections. Increases in liver Se and testis GPX4 activity were the parameters best associated with improved growth after Se injection, but the molecular role for Se in growth remains unclear.