Selenoprotein P-expression, functions, and roles in mammals

Selenium homeostasis and antioxidant selenoproteins in brain: implications for disorders in the central nervous system

The essential trace element selenium, as selenocysteine, is incorporated into antioxidant selenoproteins such as glutathione peroxidases (GPx), thioredoxin reductases (TrxR) and selenoprotein P (Sepp1). Although comparatively low in selenium content, the brain exhibits high priority for selenium supply and retention under conditions of dietary selenium deficiency. Liver-derived Sepp1 is the major transport protein in plasma to supply the brain with selenium, serving as a "survival factor" for neurons in culture. Sepp1 expression has also been detected within the brain. Presumably, astrocytes secrete Sepp1, which is subsequently taken up by neurons via the apolipoprotein E receptor 2 (ApoER2). Knock-out of Sepp1 or ApoER2 as well as neuron-specific ablation of selenoprotein biosynthesis results in neurological dysfunction in mice. Astrocytes, generally less vulnerable to oxidative stress than neurons, are capable of up-regulating the expression of antioxidant selenoproteins upon brain injury. Occurrence of neurological disorders has been reported occasionally in patients with inadequate nutritional selenium supply or a mutation in the gene encoding selenocysteine synthase, one of the enzymes involved in selenoprotein biosynthesis. In three large trials carried out among elderly persons, a low selenium status was associated with faster decline in cognitive functions and poor performance in tests assessing coordination and motor speed. Future research is required to better understand the role of selenium and selenoproteins in brain diseases including hepatic encephalopathy.

Localization and regulation of pancreatic selenoprotein P

Progressive loss of pancreatic β-cell mass is a crucial feature of type 2 diabetes mellitus. As β-cells express very low amounts of the antioxidant enzymes catalase and glutathione peroxidase (GPx), they appear to be particularly vulnerable to oxidative damage in the pathogenesis of diabetes. Here, we investigated the pancreatic expression pattern and regulation of selenoprotein P (Sepp1), which may serve as an additional antioxidant enzyme inside and outside of cells. Sepp1 was detected in rodent pancreas by immunofluorescence and real-time RT-PCR. Regulation of Sepp1 biosynthesis in INS-1 rat insulinoma cells was investigated by real-time RT-PCR, luciferase gene reporter assay, and immunoblotting. Sepp1 and Gpx1 gene expressions in rat pancreas were 58 and 22% respectively of the liver values. Pancreatic Sepp1 expression was restricted to the endocrine tissue, with Sepp1 being present in the α- and β-cells of mouse islets. In INS-1 insulinoma cells, Sepp1 expression was stimulated by the selenium compound sodium selenate and diminished in the presence of high glucose (16.7 vs 5  mM) concentrations. Sepp1 mRNA stability was also lowered at 16.7  mM glucose. Moreover, Sepp1 mRNA levels were decreased in isolated murine islets cultured in high-glucose (22  mM) medium compared with normal glucose (5.5  mM) medium. Pancreatic Sepp1 expression was elevated upon treatment of mice with the β-cell toxin streptozotocin. This study shows that pancreatic islets express relatively high levels of Sepp1 that may fulfill a function in antioxidant protection of β-cells. Downregulation of Sepp1 expression by high glucose might thus contribute to glucotoxicity in β-cells.

Physiology of blood-brain interfaces in relation to brain disposition of small compounds and macromolecules

The brain develops and functions within a strictly controlled environment resulting from the coordinated action of different cellular interfaces located between the blood and the extracellular fluids of the brain, which include the interstitial fluid and the cerebrospinal fluid (CSF). As a correlate, the delivery of pharmacologically active molecules and especially macromolecules to the brain is challenged by the barrier properties of these interfaces. Blood-brain interfaces comprise both the blood-brain barrier located at the endothelium of the brain microvessels and the blood-CSF barrier located at the epithelium of the choroid plexuses. Although both barriers develop extensive surface areas of exchange between the blood and the neuropil or the CSF, the molecular fluxes across these interfaces are tightly regulated. Cerebral microvessels acquire a barrier phenotype early during cerebral vasculogenesis under the influence of the Wnt/β-catenin pathway, and of recruited pericytes. Later in development, astrocytes also play a role in blood-brain barrier maintenance. The tight choroid plexus epithelium develops very early during embryogenesis. It is specified by various signaling molecules from the embryonic dorsal midline, such as bone morphogenic proteins, and grows under the influence of Sonic hedgehog protein. Tight junctions at each barrier comprise a distinctive set of claudins from the pore-forming and tightening categories that determine their respective paracellular barrier characteristics. Vesicular traffic is limited in the cerebral endothelium and abundant in the choroidal epithelium, yet without evidence of active fluid phase transcytosis. Inorganic ion transport is highly regulated across the barriers. Small organic compounds such as nutrients, micronutrients and hormones are transported into the brain by specific solute carriers. Other bioactive metabolites, lipophilic toxic xenobiotics or pharmacological agents are restrained from accumulating in the brain by several ATP-binding cassette efflux transporters, multispecific solute carriers, and detoxifying enzymes. These various molecular effectors differently distribute between the two barriers. Receptor-mediated endocytotic and transcytotic mechanisms are active in the barriers. They enable brain penetration of selected polypeptides and proteins, or inversely macromolecule efflux as it is the case for immnoglobulins G. An additional mechanism specific to the BCSFB mediates the transport of selected plasma proteins from blood into CSF in the developing brain. All these mechanisms could be explored and manipulated to improve macromolecule delivery to the brain.

Effect of prenatal selenium supplementation on cord blood selenium and lipid profile

BACKGROUND

Selenium is an essential trace element and as a component of selenoproteins it plays a key role as an antioxidant. We aimed to evaluate the effect of selenium supplementation during pregnancy on cord blood selenium content and lipid profile.

METHODS

This trial was performed on 166 eligible women who were randomized to receive 100 μg of selenium, as selenium-yeast (Se group) or a placebo-yeast tablet (placebo group). Umbilical cord blood samples were collected at the time of delivery and selenium concentration and lipid profile were measured.

RESULTS

Triglyceride levels were found to be significantly higher in the Se group than in the placebo group (p=0.01). However, no significant difference in cord blood selenium was observed between the groups nor were there any significant correlations between cord blood selenium and lipid profile parameters.

CONCLUSION

Our findings suggest that selenium supplementation in pregnant women may be associated with an increased cord-blood triglyceride level, although total cholesterol, low-density lipoprotein and high-density lipoprotein cholesterol levels did not change significantly. The clinical significance of the increased cord triglyceride concentration needs to be evaluated.

The biological fate of silver ions following the use of silver-containing wound care products - a review

Ionic silver has a long history as an antimicrobial in human health care. This article is a review of the published literature on how ionic silver may enter the body from exposure to silver-containing wound care products and its eventual metabolic fates, in an assessment of the safety during normal use of these products in wound care. Following the application to breached skin, there appears to be little evidence of localised or systemic toxicity, and this is borne out by the continuous use of silver sulfadiazine formulations for more than 50 years. Consequently, following normal use, the risk of silver ion toxicity locally and systemically is considered to be low or negligible.

Expression and regulation of mouse selenoprotein P transcript variants differing in non-coding RNA

Selenoprotein P (Sepp1), a glycoprotein rich in selenium, is thought to function in selenium transport throughout the body. The sepp1 gene locus potentially produces three alternative transcripts that differ only in their 5' untranslated regions (5'UTRs) and not in their protein coding regions, as indicated by transcript information in genomic databases. Here we investigated the distribution, relative expression, and biological significance of these transcript variants. We confirmed the expression of Sepp1 transcript variants using PCR and sequencing. Using 5'-RACE, we identified multiple 5'-termini upstream from three different splice donor sites, and a single splice acceptor site for exon 2. We found regional and temporal changes in variant expression in select adult and neonate murine tissue and brain regions. Distribution of variants in heart and kidney varied with stage of development. Notably, the Sepp1b variant was localized specifically to the hippocampus in brain. Targeted silencing of individual variants using RNAi demonstrated the biological importance for all transcript variants in cell viability. Additionally, we determined that the Sepp1b variant is a specific target for the miR-7 microRNA by means of its unique 5'UTR structure. Our results emphasize the importance of non-coding transcript variations as a regulatory means for Sepp1 expression in different tissues and stages of development. The presence of a variant localized in the hippocampus and regulated by a microRNA may have implications for the known deficits in synaptic function caused by genetic deletion of Sepp1.

Selenoprotein P status correlates to cancer-specific mortality in renal cancer patients

Selenium (Se) is an essential trace element for selenoprotein biosynthesis. Selenoproteins have been implicated in cancer risk and tumor development. Selenoprotein P (SePP) serves as the major Se transport protein in blood and as reliable biomarker of Se status in marginally supplied individuals. Among the different malignancies, renal cancer is characterized by a high mortality rate. In this study, we aimed to analyze the Se status in renal cell cancer (RCC) patients and whether it correlates to cancer-specific mortality. To this end, serum samples of RCC patients (n = 41) and controls (n = 21) were retrospectively analyzed. Serum Se and SePP concentrations were measured by X-ray fluorescence and an immunoassay, respectively. Clinical and survival data were compared to serum Se and SePP concentrations as markers of Se status by receiver operating characteristic (ROC) curve and Kaplan-Meier and Cox regression analyses. In our patients, higher tumor grade and tumor stage at diagnosis correlated to lower SePP and Se concentrations. Kaplan-Meier analyses indicated that low Se status at diagnosis (SePP<2.4 mg/l, bottom tertile of patient group) was associated with a poor 5-year survival rate of 20% only. We conclude that SePP and Se concentrations are of prognostic value in RCC and may serve as additional diagnostic biomarkers identifying a Se deficit in kidney cancer patients potentially affecting therapy regimen. As poor Se status was indicative of high mortality odds, we speculate that an adjuvant Se supplementation of Se-deficient RCC patients might be beneficial in order to stabilize their selenoprotein expression hopefully prolonging their survival. However, this assumption needs to be rigorously tested in prospective clinical trials.

Production of selenoprotein P (Sepp1) by hepatocytes is central to selenium homeostasis

BACKGROUND

Sepp1 transports selenium, but its complete role in selenium homeostasis is not known.

RESULTS

Deletion of Sepp1 in hepatocytes increases liver selenium at the expense of other tissues and decreases whole-body selenium by increasing excretion.

CONCLUSION

Sepp1 production by hepatocytes retains selenium in the organism and distributes it from the liver to peripheral tissues.

SIGNIFICANCE

Sepp1 is central to selenium homeostasis. Sepp1 is a widely expressed extracellular protein that in humans and mice contains 10 selenocysteine residues in its primary structure. Extra-hepatic tissues take up plasma Sepp1 for its selenium via apolipoprotein E receptor-2 (apoER2)-mediated endocytosis. The role of Sepp1 in the transport of selenium from liver, a rich source of the element, to peripheral tissues was studied using mice with selective deletion of Sepp1 in hepatocytes (Sepp1(c/c)/alb-cre(+/-) mice). Deletion of Sepp1 in hepatocytes lowered plasma Sepp1 concentration to 10% of that in Sepp1(c/c) mice (controls) and increased urinary selenium excretion, decreasing whole-body and tissue selenium concentrations. Under selenium-deficient conditions, Sepp1(c/c)/alb-cre(+/-) mice accumulated selenium in the liver at the expense of extra-hepatic tissues, severely worsening clinical manifestations of dietary selenium deficiency. These findings are consistent with there being competition for metabolically available hepatocyte selenium between the synthesis of selenoproteins and the synthesis of selenium excretory metabolites. In addition, selenium deficiency down-regulated the mRNA of the most abundant hepatic selenoprotein, glutathione peroxidase-1 (Gpx1), to 15% of the selenium-replete value, while reducing Sepp1 mRNA, the most abundant hepatic selenoprotein mRNA, only to 61%. This strongly suggests that Sepp1 synthesis is favored in the liver over Gpx1 synthesis when selenium supply is limited, directing hepatocyte selenium to peripheral tissues in selenium deficiency. We conclude that production of Sepp1 by hepatocytes is central to selenium homeostasis in the organism because it promotes retention of selenium in the body and effects selenium distribution from the liver to extra-hepatic tissues, especially under selenium-deficient conditions.

SeP, ApoER2 and megalin as necessary factors to maintain Se homeostasis in mammals

Selenoprotein P (SeP) is an extracellular protein containing ten selenium atoms in the form of selenocysteine, secreted mainly from the liver. About 60% of the whole plasma selenium level is present in SeP, which makes it a useful biomarker of selenium nutritional status. The main functions of SeP are transport and storage of selenium in plasma. It is especially an important protein for the brain, testes and kidneys where the supplementation of the proper amount of Se ensures the synthesis of selenoenzymes with antioxidant properties.Recently, it has been found that SeP uptake in kidneys, testes and brain depends on the apolipoprotein receptor 2 (ApoER2) and lipoprotein megalin receptor (Lrp2). Megalin receptor represents a physiological SeP receptor in kidneys, mediating the re-uptake of secreted SeP from the primary urine. The absence of a functional megalin receptor causes a significant reduction of plasma selenium and the SeP levels as a result of Se excretion. ApoER2 is a SeP receptor in the brain and testes which uptakes Se from the extracellular fluid. Deletion of ApoER2 in mice leads to a lowered selenium level in the brain and testes, neurological dysfunction, production of abnormal spermatozoa, infertility and even death when the subjects are fed a low-selenium diet.

S-adenosylmethionine-dependent protein methylation is required for expression of selenoprotein P and gluconeogenic enzymes in HepG2 human hepatocytes

Cellular methylation processes enable expression of gluconeogenic enzymes and metabolism of the nutrient selenium. Selenium status has been proposed to relate to type II diabetes risk, and plasma levels of selenoprotein P (SEPP1) have been positively correlated with insulin resistance. Increased expression of gluconeogenic enzymes glucose-6-phosphatase (G6PC) and phosphoenolpyruvate carboxykinase 1 (PCK1) has negative consequences for blood glucose management in type II diabetics. Transcriptional regulation of SEPP1 is directed by the same transcription factors that control the expression of G6PC and PCK1, and these factors are activated by methylation of arginine residues. We sought to determine whether expression of SEPP1 and the aforementioned glucoconeogenic enzymes are regulated by protein methylation, the levels of which are reliant upon adequate S-adenosylmethionine (SAM) and inhibited by S-adenosylhomocysteine (SAH). We treated a human hepatocyte cell line, HepG2, with inhibitors of adenosylhomocysteine hydrolase (AHCY) known to increase concentration of SAH before analysis of G6PC, PCK1, and SEPP1 expression. Increasing SAH decreased 1) the SAM/SAH ratio, 2) protein-arginine methylation, and 3) expression of SEPP1, G6PC, and PCK1 transcripts. Furthermore, hormone-dependent induction of gluconeogenic enzymes was reduced by inhibition of protein methylation. When protein-arginine methyltransferase 1 expression was reduced by siRNA treatment, G6PC expression was inhibited. These findings demonstrate that hepatocellular SAM-dependent protein methylation is required for both SEPP1 and gluconeogenic enzyme expression and that inhibition of protein arginine methylation might provide a route to therapeutic interventions in type II diabetes.

Disruption of the selenocysteine lyase-mediated selenium recycling pathway leads to metabolic syndrome in mice

Selenium (Se) is an essential trace element used for biosynthesis of selenoproteins and is acquired either through diet or cellular recycling mechanisms. Selenocysteine lyase (Scly) is the enzyme that supplies Se for selenoprotein biosynthesis via decomposition of the amino acid selenocysteine (Sec). Knockout (KO) of Scly in a mouse affected hepatic glucose and lipid homeostasis. Mice lacking Scly and raised on an Se-adequate diet exhibit hyperinsulinemia, hyperleptinemia, glucose intolerance, and hepatic steatosis, with increased hepatic oxidative stress, but maintain selenoprotein levels and circulating Se status. Insulin challenge of Scly KO mice results in attenuated Akt phosphorylation but does not decrease phosphorylation levels of AMP kinase alpha (AMPKα). Upon dietary Se restriction, Scly KO animals develop several characteristics of metabolic syndrome, such as obesity, fatty liver, and hypercholesterolemia, with aggravated hyperleptinemia, hyperinsulinemia, and glucose intolerance. Hepatic glutathione peroxidase 1 (GPx1) and selenoprotein S (SelS) production and circulating selenoprotein P (Sepp1) levels are significantly diminished. Scly disruption increases the levels of insulin-signaling inhibitor PTP1B. Our results suggest a dependence of glucose and lipid homeostasis on Scly activity. These findings connect Se and energy metabolism and demonstrate for the first time a unique physiological role of Scly in an animal model.

Long isoform mouse selenoprotein P (Sepp1) supplies rat myoblast L8 cells with selenium via endocytosis mediated by heparin binding properties and apolipoprotein E receptor-2 (ApoER2)

In vivo studies have shown that selenium is supplied to testis and brain by apoER2-mediated endocytosis of Sepp1. Although cultured cell lines have been shown to utilize selenium from Sepp1 added to the medium, the mechanism of uptake and utilization has not been characterized. Rat L8 myoblast cells were studied. They took up mouse Sepp1 from the medium and used its selenium to increase their glutathione peroxidase (Gpx) activity. L8 cells did not utilize selenium from Gpx3, the other plasma selenoprotein. Neither did they utilize it from Sepp1(Δ240-361), the isoform of Sepp1 that lacks the selenium-rich C-terminal domain. To identify Sepp1 receptors, a solubilized membrane fraction was passed over a Sepp1 column. The receptors apoER2 and Lrp1 were identified in the eluate by mass spectrometry. siRNA experiments showed that knockdown of apoER2, but not of Lrp1, inhibited (75)Se uptake from (75)Se-labeled Sepp1. The addition of protamine to the medium or treatment of the cells with chlorate also inhibited (75)Se uptake. Blockage of lysosome acidification did not inhibit uptake of Sepp1 but did prevent its digestion and thereby utilization of its selenium. These results indicate that L8 cells take up Sepp1 by an apoER2-mediated mechanism requiring binding to heparin sulfate proteoglycans. The presence of at least part of the selenium-rich C-terminal domain of Sepp1 is required for uptake. RT-PCR showed that mouse tissues express apoER2 in varying amounts. It is postulated that apoER2-mediated uptake of long isoform Sepp1 is responsible for selenium distribution to tissues throughout the body.

Absence of selenoprotein P but not selenocysteine lyase results in severe neurological dysfunction

Dietary selenium restriction in mammals causes bodily selenium to be preferentially retained in the brain relative to other organs. Almost all the known selenoproteins are found in brain, where expression is facilitated by selenocysteine (Sec)-laden selenoprotein P. The brain also expresses selenocysteine lyase (Scly), an enzyme that putatively salvages Sec and recycles the selenium for selenoprotein translation. We compared mice with a genetic deletion of Scly to selenoprotein P (Sepp1) knockout mice for similarity of neurological impairments and whether dietary selenium modulates these parameters. We report that Scly knockout mice do not display neurological dysfunction comparable to Sepp1 knockout mice. Feeding a low-selenium diet to Scly knockout mice revealed a mild spatial learning deficit without disrupting motor coordination. Additionally, we report that the neurological phenotype caused by the absence of Sepp1 is exacerbated in male vs. female mice. These findings indicate that Sec recycling via Scly becomes limiting under selenium deficiency and suggest the presence of a complementary mechanism for processing Sec. Our studies illuminate the interaction between Sepp1 and Scly in the distribution and turnover of body and brain selenium and emphasize the consideration of sex differences when studying selenium and selenoproteins in vertebrate biology.

Ferric His93Gly myoglobin cavity mutant and its complexes with thioether and selenolate as heme protein models

The composition of ferric exogenous ligand-free His93Gly sperm whale myoglobin (H93G Mb) at neutral pH has been determined by examination of the spectral properties of the protein over the pH range from 3.0 to 10.5. An apparent pKa value of ~6.6 has been observed for the conversion of a postulated six-coordinate bis-water-bound coordination structure at pH 5.0 to a five-coordinate hydroxide-bound form at pH 10.5. Starting from the exogenous ligand-free ferric H93G protein, ferric mono- and bis-thioether (tetrahydrothiophene, THT)-ligated adducts have been prepared and characterized by UV-visible (UV-vis) absorption and magnetic circular dichroism (MCD) spectroscopy. The mon-THT ferric H93G Mb species has hydroxide as the sixth ligand. The bis-THT derivative is a model for the low-spin ferric heme binding site of native bis-Met-ligated bacterioferritin or streptococcal heme-associated protein (Shp). A novel THT-bound ferryl H93G Mb moiety has been partially formed. The high-spin five-coordinate ferric H93G(selenolate) Mb complex has been prepared using benzeneselenol and characterized by UV-vis and MCD spectroscopy as a model for Se-Cys-ligated ferric cytochrome P450. The results described herein further demonstrate the versatility of the H93G cavity mutant for modeling the coordination structures of novel heme iron protein active sites.

Differential responses to selenomethionine supplementation by sex and genotype in healthy adults

A year-long intervention trial was conducted to characterise the responses of multiple biomarkers of Se status in healthy American adults to supplemental selenomethionine (SeMet) and to identify factors affecting those responses. A total of 261 men and women were randomised to four doses of Se (0, 50, 100 or 200 μg/d as L-SeMet) for 12 months. Responses of several biomarkers of Se status (plasma Se, serum selenoprotein P (SEPP1), plasma glutathione peroxidase activity (GPX3), buccal cell Se, urinary Se) were determined relative to genotype of four selenoproteins (GPX1, GPX3, SEPP1, selenoprotein 15), dietary Se intake and parameters of single-carbon metabolism. Results showed that supplemental SeMet did not affect GPX3 activity or SEPP1 concentration, but produced significant, dose-dependent increases in the Se contents of plasma, urine and buccal cells, each of which plateaued by 9-12 months and was linearly related to effective Se dose (μg/d per kg0·75). The increase in urinary Se excretion was greater for women than men, and for individuals of the GPX1 679 T/T genotype than for those of the GPX1 679 C/C genotype. It is concluded that the most responsive Se-biomarkers in this non-deficient cohort were those related to body Se pools: plasma, buccal cell and urinary Se concentrations. Changes in plasma Se resulted from increases in its non-specific component and were affected by both sex and GPX1 genotype. In a cohort of relatively high Se status, the Se intake (as SeMet) required to support plasma Se concentration at a target level (Se(pl-target)) is: Se(in) = [(Se(pl - target) - Se(pl))/(18.2ng d kg⁰.⁷⁵/ml per mu g)] .

The role of selenium in inflammation and immunity: from molecular mechanisms to therapeutic opportunities

Dietary selenium (]Se), mainly through its incorporation into selenoproteins, plays an important role in inflammation and immunity. Adequate levels of Se are important for initiating immunity, but they are also involved in regulating excessive immune responses and chronic inflammation. Evidence has emerged regarding roles for individual selenoproteins in regulating inflammation and immunity, and this has provided important insight into mechanisms by which Se influences these processes. Se deficiency has long been recognized to negatively impact immune cells during activation, differentiation, and proliferation. This is related to increased oxidative stress, but additional functions such as protein folding and calcium flux may also be impaired in immune cells under Se deficient conditions. Supplementing diets with above-adequate levels of Se can also impinge on immune cell function, with some types of inflammation and immunity particularly affected and sexually dimorphic effects of Se levels in some cases. In this comprehensive article, the roles of Se and individual selenoproteins in regulating immune cell signaling and function are discussed. Particular emphasis is given to how Se and selenoproteins are linked to redox signaling, oxidative burst, calcium flux, and the subsequent effector functions of immune cells. Data obtained from cell culture and animal models are reviewed and compared with those involving human physiology and pathophysiology, including the effects of Se levels on inflammatory or immune-related diseases including anti-viral immunity, autoimmunity, sepsis, allergic asthma, and chronic inflammatory disorders. Finally, the benefits and potential adverse effects of intervention with Se supplementation for various inflammatory or immune disorders are discussed.

Selenium and human health

Selenium is incorporated into selenoproteins that have a wide range of pleiotropic effects, ranging from antioxidant and anti-inflammatory effects to the production of active thyroid hormone. In the past 10 years, the discovery of disease-associated polymorphisms in selenoprotein genes has drawn attention to the relevance of selenoproteins to health. Low selenium status has been associated with increased risk of mortality, poor immune function, and cognitive decline. Higher selenium status or selenium supplementation has antiviral effects, is essential for successful male and female reproduction, and reduces the risk of autoimmune thyroid disease. Prospective studies have generally shown some benefit of higher selenium status on the risk of prostate, lung, colorectal, and bladder cancers, but findings from trials have been mixed, which probably emphasises the fact that supplementation will confer benefit only if intake of a nutrient is inadequate. Supplementation of people who already have adequate intake with additional selenium might increase their risk of type-2 diabetes. The crucial factor that needs to be emphasised with regard to the health effects of selenium is the inextricable U-shaped link with status; whereas additional selenium intake may benefit people with low status, those with adequate-to-high status might be affected adversely and should not take selenium supplements.

Understanding selenoprotein function and regulation through the use of rodent models

Selenium (Se) is an essential micronutrient. Its biological functions are associated with selenoproteins, which contain this trace element in the form of the 21st amino acid, selenocysteine. Genetic defects in selenocysteine insertion into proteins are associated with severe health issues. The consequences of selenoprotein deficiency are more variable, with several selenoproteins being essential, and several showing no clear phenotypes. Much of these functional studies benefited from the use of rodent models and diets employing variable levels of Se. This review summarizes the data obtained with these models, focusing on mouse models with targeted expression of individual selenoproteins and removal of individual, subsets or all selenoproteins in a systemic or organ-specific manner. This article is part of a Special Issue entitled: Cell Biology of Metals.

Impact of intensive physical activity on selenium status

Various biomarkers were used to assess selenium (Se) status during 3 months of basic military training in a group of recruits. Samples of whole blood and plasma or serum were taken from a group of conscripts three times: at the beginning (n = 15), in the middle immediately after a severely stressful physical activity (n = 15) and at the end of military training (n = 13). Selenium was determined in diet samples, blood, plasma and plasma protein fractions as selenoprotein P (SelP) and glutathione peroxidase (eGPx). Selenium was determined by hydride generation-atomic fluorescence spectrometry and fractionation of proteins performed by affinity chromatography. Total serum glutathione, erythrocyte and serum GPx activity were followed as well. The average Se intake was calculated according to meal consumption questionnaires and Se determined in composite diet samples, giving an assessed intake of 48 ± 10 μg/day. At all three samplings, the average blood Se concentration was within the framework of adequate supply (87 ± 12, 94 ± 15 and 80 ± 13 ng/g). Plasma Se was between 70 and 80 ng/g (71 ± 10, 79 ± 9 and 76 ± 10 ng/g), which is believed to enable the full expression of plasma GPx. The average shares of plasma Se proteins were 61 ± 6%, 58 ± 6% and 54 ± 9% for SelP and 27 ± 4%, 34 ± 7% and 29 ± 5% for GPx. Although the observed tendency of the increases of serum and erythrocyte GPx activities at the second and third samplings with respect to the first was statistically insignificant, it is still indicative of some protection against oxidative stress, while the decreasing SelP levels during training suggest a slowly decreasing biologically active selenium pool.

Genetic variation in GPX1 is associated with GPX1 activity in a comprehensive analysis of genetic variations in selenoenzyme genes and their activity and oxidative stress in humans

Previous studies suggest some effects of selenium on risk of several chronic diseases, which may be mediated through a small number of selenoenzymes with antioxidant properties. In this cross-sectional analysis of 195 participants from the Seattle Barrett's Esophagus Study who were free of esophageal cancer at the time of blood draw, we examined whether the number of the minor alleles in 26 tagging single nucleotide polymorphisms (SNP) of five selenoenzyme genes [i.e., glutathione peroxidase 1-4 (GPX1-4) and selenoprotein P (SEPP1)] was associated with activity of GPX1 in white blood cells and GPX3 in plasma, and concentrations of SEPP1 and markers of oxidative stress [malondialdehyde (MDA) and protein carbonyl content] in plasma. At the gene level, associations were observed between overall variation in GPX1 and GPX1 activity (P = 0.02) as well as between overall variation in GPX2 and SEPP1 concentrations (P = 0.03). By individual SNP, two variants in GPX1 (rs8179164 and rs1987628) showed a suggestive association with GPX1 activity (P = 0.10 and 0.08, respectively) and two GPX2 variants (rs4902346 and rs2071566) were associated with SEPP1 concentration (P = 0.004 and 0.002, respectively). Furthermore, two SNP in the SEPP1 gene (rs230813 and rs230819) were associated with MDA concentrations (P = 0.03 and 0.02, respectively). Overall, our study supports the hypothesis that common genetic variants in selenoenzymes affect their activity.

Deletion of selenoprotein P results in impaired function of parvalbumin interneurons and alterations in fear learning and sensorimotor gating

One of the primary lines of defense against oxidative stress is the selenoprotein family, a class of proteins that contain selenium in the form of the 21st amino acid, selenocysteine. Within this class of proteins, selenoprotein P (Sepp1) is unique, as it contains multiple selenocysteine residues and is postulated to act in selenium transport. Recent findings have demonstrated that neuronal selenoprotein synthesis is required for the development of parvalbumin (PV)-interneurons, a class of GABAergic neurons involved in the synchronization of neural activity. To investigate the potential influence of Sepp1 on PV-interneurons, we first mapped the distribution of the Sepp1 receptor, ApoER2, and parvalbumin in the mouse brain. Our results indicate that ApoER2 is highly expressed on PV-interneurons in multiple brain regions. Next, to determine whether PV-interneuron populations are affected by Sepp1 deletion, we performed stereology on several brain regions in which we observed ApoER2 expression on PV-interneurons, comparing wild-type and Sepp1(-/-) mice. We observed reduced numbers of PV-interneurons in the inferior colliculus of Sepp1(-/-) mice, which corresponded with a regional increase in oxidative stress. Finally, as impaired PV-interneuron function has been implicated in several neuropsychiatric conditions, we performed multiple behavioral tests on Sepp1(-/-) mice. Our behavioral results indicate that Sepp1(-/-) mice have impairments in contextual fear extinction, latent inhibition, and sensorimotor gating. In sum, these findings demonstrate the important supporting role of Sepp1 on ApoER2-expressing PV-interneurons.

Selenoproteins in bladder cancer

Selenoproteins with genetically encoded selenium (Se) are very important in response to oxidative stress, redox balance and regulation of various metabolic and developmental processes. Although increased circulating Se has been associated with 33% risk reduction of bladder cancer, there are little data on selenoprotein expression at the protein and genetic level from both human and animal studies. Data from the Mammalian Gene Collection (MGC) Project clearly showed that highest mRNA expression in human urinary epithelium for TRXR1 (thioredoxin reductase 1), GPX1 (glutathione peroxidase 1), SEP15 (15 kDa selenoprotein), SELT (selenoprotein T) and SEPW1 (selenoprotein W1). Although bladder tumor has been characterized by increased Se, GPX and TRXR activity, circulating Se and GPX was interestingly decreased in these cancer patients. As such, selenoprotein expression in urinary epithelium may be involved in bladder cancer (development, progression and recurrence) and may play a significant role in chemotherapeutic intervention. Despite these findings, the role of selenoproteins in bladder cancer has rarely been investigated and the significance of selenoproteins in normal and malignant uroepithelium remains poorly understood.

High-resolution imaging of selenium in kidneys: a localized selenium pool associated with glutathione peroxidase 3

AIM

Recent advances in quantitative methods and sensitive imaging techniques of trace elements provide opportunities to uncover and explain their biological roles. In particular, the distribution of selenium in tissues and cells under both physiological and pathological conditions remains unknown. In this work, we applied high-resolution synchrotron X-ray fluorescence microscopy (XFM) to map selenium distribution in mouse liver and kidney.

RESULTS

Liver showed a uniform selenium distribution that was dependent on selenocysteine tRNA([Ser]Sec) and dietary selenium. In contrast, kidney selenium had both uniformly distributed and highly localized components, the latter visualized as thin circular structures surrounding proximal tubules. Other parts of the kidney, such as glomeruli and distal tubules, only manifested the uniformly distributed selenium pattern that co-localized with sulfur. We found that proximal tubule selenium localized to the basement membrane. It was preserved in Selenoprotein P knockout mice, but was completely eliminated in glutathione peroxidase 3 (GPx3) knockout mice, indicating that this selenium represented GPx3. We further imaged kidneys of another model organism, the naked mole rat, which showed a diminished uniformly distributed selenium pool, but preserved the circular proximal tubule signal.

INNOVATION

We applied XFM to image selenium in mammalian tissues and identified a highly localized pool of this trace element at the basement membrane of kidneys that was associated with GPx3.

CONCLUSION

XFM allowed us to define and explain the tissue topography of selenium in mammalian kidneys at submicron resolution.

Biochemical discrimination between selenium and sulfur 1: a single residue provides selenium specificity to human selenocysteine lyase

Selenium and sulfur are two closely related basic elements utilized in nature for a vast array of biochemical reactions. While toxic at higher concentrations, selenium is an essential trace element incorporated into selenoproteins as selenocysteine (Sec), the selenium analogue of cysteine (Cys). Sec lyases (SCLs) and Cys desulfurases (CDs) catalyze the removal of selenium or sulfur from Sec or Cys and generally act on both substrates. In contrast, human SCL (hSCL) is specific for Sec although the only difference between Sec and Cys is the identity of a single atom. The chemical basis of this selenium-over-sulfur discrimination is not understood. Here we describe the X-ray crystal structure of hSCL and identify Asp146 as the key residue that provides the Sec specificity. A D146K variant resulted in loss of Sec specificity and appearance of CD activity. A dynamic active site segment also provides the structural prerequisites for direct product delivery of selenide produced by Sec cleavage, thus avoiding release of reactive selenide species into the cell. We thus here define a molecular determinant for enzymatic specificity discrimination between a single selenium versus sulfur atom, elements with very similar chemical properties. Our findings thus provide molecular insights into a key level of control in human selenium and selenoprotein turnover and metabolism.

Selenium and its' role in the maintenance of genomic stability

Selenium (Se) is an essential micronutrient for humans, acting as a component of the unusual amino acids, selenocysteine (Se-Cys) and selenomethionine (Se-Met). Where Se levels are low, the cell cannot synthesise selenoproteins, although some selenoproteins and some tissues are prioritised over others. Characterised functions of known selenoproteins, include selenium transport (selenoprotein P), antioxidant/redox properties (glutathione peroxidases (GPxs), thioredoxin reductases and selenoprotein P) and anti-inflammatory properties (selenoprotein S and GPx4). Various forms of Se are consumed as part of a normal diet, or as a dietary supplement. Supplementation of tissue culture media, animal or human diets with moderate levels of certain Se compounds may protect against the formation of DNA adducts, DNA or chromosome breakage, and chromosome gain or loss. Protective effects have also been shown on mitochondrial DNA, and on telomere length and function. Some of the effects of Se compounds on gene expression may relate to modulation of DNA methylation or inhibition of histone deacetylation. Despite a large number of positive effects of selenium and selenoproteins in various model systems, there have now been some human clinical trials that have shown adverse effects of Se supplementation, according to various endpoints. Too much Se is as harmful as too little, with animal models showing a "U"-shaped efficacy curve. Current recommended daily allowances differ among countries, but are generally based on the amount of Se necessary to saturate GPx enzymes. However, increasing evidence suggests that other enzymes may be more important than GPx for Se action, that optimal levels may depend upon the form of Se being ingested, and vary according to genotype. New paradigms, possibly involving nutrigenomic tools, will be necessary to optimise the forms and levels of Se desirable for maximum protection of genomic stability in all humans.

Probing the bioinorganic chemistry of toxic metals in the mammalian bloodstream to advance human health

The etiology of numerous grievous human diseases, including Alzheimer's and Parkinson's Disease is not well understood. Conversely, the concentration toxic metals and metalloids, such as As, Cd, Hg and Pb in human blood of the average population is well established, yet we know strikingly little about the role that they might play in the etiology of disease processes. Establishing functional connections between the chronic exposure of humans to these and other inorganic pollutants and the etiology of certain human diseases is therefore viewed by many as one of the greatest challenges in the post-genomic era. Conceptually, this task requires us to uncover hitherto unknown biomolecular mechanisms which must explain how small doses of a toxic metal/metalloid compound (low μg per day) - or mixtures thereof - may eventually result in a particular human disease. The biological complexity that is inherently associated with mammals, however, makes the discovery of these mechanisms a truly monumental task. Recent findings suggest that a better understanding of the bioinorganic chemistry of inorganic pollutants in the mammalian bloodstream represents a fruitful strategy to unravel relevant biomolecular mechanisms. The adverse effect(s) that toxic metals/metalloid compounds exert on the transport of essential ultratrace elements to internal organs appear particularly pertinent. A brief overview of the effect that arsenite and Hg(2+) exert on the mammalian metabolism of selenium is presented.

Selenium status and blood lipids: the cardiovascular risk in Young Finns study

BACKGROUND

Concern has been recently raised about possible adverse cardio-metabolic effects of high selenium status, such as increased risks of diabetes and hyperlipidaemia. However, most of the evidence comes from selenium-replete populations such as that of the United States.

OBJECTIVES

To examine cross-sectional and longitudinal associations of serum selenium with cardiovascular risk factors in Finland where selenium levels were amongst the lowest in the world until the early 1980s before the implementation of a nationwide selenium fertilization programme.

METHODS

Serum selenium was measured in 1235 young Finns aged 3-18 years at baseline in 1980 (prefertilization) and in a subgroup (N = 262) at the 6-year follow-up (1986, postfertilization). During the 27-year follow-up, serum lipids, blood pressure, body mass index and smoking were assessed five times (1980, 1983, 1986, 2001 and 2007).

RESULTS

Mean (±SD) serum selenium concentrations were 74.3 ± 14.0 ng mL(-1) in 1980 and 106.6 ± 12.5 ng mL(-1) in 1986 (average increase 32.3 ng mL(-1); 95% CI: 30.3 to 34.3, P < 0.0001). In univariate and multivariable cross-sectional models in 1980 and 1986, increased serum selenium levels were consistently associated with increased total, HDL and Low-density lipoprotein (LDL) cholesterol. However, the average longitudinal changes in lipids were -0.20 mmol L(-1) (95% CI: -0.30 to -0.10, P < 0.0001) for total cholesterol, 0.06 mmol L(-1) (95% CI: 0.03 to 0.10, P < 0.0001) for HDL cholesterol, and -0.23 mmol L(-1) (95% CI: -0.31 to -0.14, P < 0.0001) for LDL cholesterol. Selenium measured in 1986 was not associated with lipids assessed in 2001 and 2007.

CONCLUSIONS

Cross-sectional findings from the Young Finns study corroborate positive associations of selenium status with serum lipids. However, longitudinal evidence does not support the causality of this link.

Organic and inorganic selenium: I. Oral bioavailability in ewes

Although the essentiality of dietary Se for sheep has been known for decades, the chemical source and Se dosage for optimal health remain unclear. In the United States, the Food and Drug Administration (FDA) regulates Se supplementation, regardless of the source of Se, at 0.3 mg of Se/kg of diet (as fed), which is equivalent to 0.7 mg of Se/d or 4.9 mg of Se/wk per sheep. The objectives of this study were to evaluate the effects of Se source (inorganic vs. organic) and supplementation rate (FDA vs. supranutritional rates of 14.7 and 24.5 mg of Se/wk) on whole-blood (WB) and serum-Se concentrations. Mature ewes (n = 240) were randomly assigned to 8 treatment groups (n = 30 each) based on Se supplementation rate (4.9, 14.7, and 24.5 mg of Se•wk(-1)•sheep(-1)) and source [Na-selenite, Na-selenate (4.9 mg/wk only), and organic Se-yeast] with a no-Se control group (0 mg of Se/wk). Treatment groups were balanced for healthy and footrot-affected ewes. For 1 yr, ewes were individually dosed once weekly with 0, 4.9, 14.7, or 24.5 mg of Se, quantities equivalent to their summed daily supplementation rates. Serum- and WB-Se concentrations were measured every 3 mo in all ewes; additionally, WB-Se concentrations were measured once monthly in one-half of the ewes receiving 0 or 4.9 mg of Se/wk. Ewes receiving no Se showed a 78.8 and 58.8% decrease (P < 0.001) in WB- (250 to 53 ng/mL) and serum- (97 to 40 ng/mL) Se concentrations, respectively, over the duration of the study. Whole-blood Se decreased primarily during pregnancy (-57%; 258 to 111 ng/mL) and again during peak lactation (-44%; 109 to 61 ng/mL; P < 0.001). At 4.9 mg of Se/wk, Se-yeast (364 ng/mL, final Se concentration) was more effective than Na-selenite (269 ng/mL) at increasing WB-Se concentrations (P < 0.001). Supranutritional Se-yeast dosages increased WB-Se concentrations in a dose-dependent manner (563 ng/mL, 14.7 mg of Se/wk; 748 ng/mL, 24.5 mg of Se/wk; P < 0.001), whereas WB-Se concentrations were not different for the Na-selenite groups (350 ng/mL, 14.7 mg of Se/wk; 363 ng/mL, 24.5 mg of Se/wk) or the 4.9 mg of Se/wk Se-yeast group (364 ng/mL). In summary, the dose range whereby Se supplementation increased blood Se concentrations was more limited for inorganic Na-selenite than for organic Se-yeast. The smallest rate (FDA-recommended quantity) of organic Se supplementation was equally effective as supranutritional rates of Na-selenite supplementation in increasing WB-Se concentrations, demonstrating the greater oral bioavailability of organic Se.

Relevance of selenoprotein transcripts for selenium status in humans

The most commonly used methods for assessing the selenium (Se) status in humans involve analysis of Se concentration, selenoprotein activity, and concentration in the blood and its compartments. Recently, it has been suggested that the expression of selenoprotein mRNA in circulating blood leukocytes could differently reflect Se status, due to prioritization of specific selenoprotein synthesis in response to dietary Se supply. Whereas the Se levels required for optimization of selenoprotein P level and plasma glutathione peroxidise activity are well known, estimation of Se level that is required for maximal mRNA expression of selenoprotein in humans is the subject of current investigations. Studies on rats suggest that whole blood selenoprotein mRNA level can be used as the relevant molecular biomarker for assessing Se status, and suboptimal Se intake may be sufficient to achieve effective expression. Human studies, however, did not confirm this hypothesis. According to studies on rodents and humans discussed in this review, it appears that suboptimal Se intake may be sufficient to satisfy molecular requirements of Se and it is lower than current recommended dietary intake in humans. The use of selenoprotein transcripts as a molecular biomarker of Se status requires further studies on a large group of healthy individuals with different baseline Se, including data regarding genetic polymorphism of selenoproteins and data regarding potential modifiers of Se metabolism.

Progression of neurodegeneration and morphologic changes in the brains of juvenile mice with selenoprotein P deleted

Selenoprotein P (Sepp1) is an important protein involved in selenium (Se) transport and homeostasis. Severe neurologic dysfunction develops in Sepp1 null mice (Sepp1(-/-)) fed a selenium-deficient diet. Sepp1(-/-) mice fed a selenium-deficient diet have extensive degeneration of the brainstem and thalamus, and even when supplemented with selenium exhibit subtle learning deficits and altered basal synaptic transmission and short-term plasticity in the CA1 region of the hippocampus. The goal of this study was to delineate the regional progression of neurodegeneration in the brain, determine the extent of neuronal cell death, and evaluate neurite structural changes within the hippocampus of Sepp1(-/-) mice. Whole brain serial sections of wild-type and Sepp1(-/-) mice maintained on selenium-deficient or supplemented diets over the course of 12 days from weaning were evaluated with amino cupric silver neurodegeneration stain. The neurodegeneration was present in all regions upon weaning and progressed over 12 days in Sepp1(-/-) mice fed selenium-deficient diet, except in the medial forebrain bundle and somatosensory cortex where the neurodegeneration developed post-weaning. The neurodegeneration was predominantly axonal, however the somatosensory cortex and lateral striatum showed silver-stained neurons. Morphologic analysis of the hippocampus revealed decreased dendritic length and spine density, suggesting that loss of Sepp1 also causes subtle changes in the brain that can contribute to functional deficits. These data illustrate that deletion of Sepp1, and presumably selenium deficiency in the brain, produce both neuronal and axonal degeneration as well as more moderate and potentially reversible neurite changes in the developing brain.

Transcriptomics and functional genetic polymorphisms as biomarkers of micronutrient function: focus on selenium as an exemplar

Micronutrients are essential for optimal human health. However, in some cases, raising intake by supplementation has not proven to be beneficial and there is even some evidence that supplementation may increase disease risk, highlighting the importance of assessing the functional status of micronutrients. Techniques such as gene microarrays and single-nucleotide polymorphism analysis have the potential to examine effects of micronutrient intake on patterns of gene expression and inter-individual variation in micronutrient metabolism. Recent genomic research related to selenium (Se) provides examples illustrating how studies of functional single-nucleotide polymorphism and gene expression patterns can reveal novel biomarkers of micronutrient function. Both in vitro and in vivo experiments show that there are functionally relevant polymorphisms in genes encoding glutathione peroxidases 1, 3 and 4, selenoprotein P, selenoprotein S and the 15 kDa selenoprotein. Disease association studies investigating these gene variants have so far been relatively small but an association of a polymorphism in the selenoprotein S gene with colorectal cancer risk has been replicated in two distinct populations. Future disease association studies should examine effects of multiple variants in combination with nutritional status. Gene microarray studies indicate that changes in Se intake alter expression of components of inflammatory, stress response and translation pathways. Our hypothesis is that Se intake and genetic factors have linked effects on stress response, inflammation and apoptotic pathways. Combining such data in a systems biology approach has the potential to identify both biomarkers of micronutrients status and sub-group populations at particular risk.

Dietary selenium affects host selenoproteome expression by influencing the gut microbiota

Colonization of the gastrointestinal tract and composition of the microbiota may be influenced by components of the diet, including trace elements. To understand how selenium regulates the intestinal microflora, we used high-throughput sequencing to examine the composition of gut microbiota of mice maintained on selenium-deficient, selenium-sufficient, and selenium-enriched diets. The microbiota diversity increased as a result of selenium in the diet. Specific phylotypes showed differential effects of selenium, even within a genus, implying that selenium had unique effects across microbial taxa. Conventionalized germ-free mice subjected to selenium diets gave similar results and showed an increased diversity of the bacterial population in animals fed with higher levels of selenium. Germ-free mice fed selenium diets modified their selenoproteome expression similar to control mice but showed higher levels and activity of glutathione peroxidase 1 and methionine-R-sulfoxide reductase 1 in the liver, suggesting partial sequestration of selenium by the gut microorganisms, limiting its availability for the host. These changes in the selenium status were independent of the levels of other trace elements. The data show that dietary selenium affects both composition of the intestinal microflora and colonization of the gastrointestinal tract, which, in turn, influence the host selenium status and selenoproteome expression.

Selenium in human health and disease

This review covers current knowledge of selenium in the environment, dietary intakes, metabolism and status, functions in the body, thyroid hormone metabolism, antioxidant defense systems and oxidative metabolism, and the immune system. Selenium toxicity and links between deficiency and Keshan disease and Kashin-Beck disease are described. The relationships between selenium intake/status and various health outcomes, in particular gastrointestinal and prostate cancer, cardiovascular disease, diabetes, and male fertility, are reviewed, and recent developments in genetics of selenoproteins are outlined. The rationale behind current dietary reference intakes of selenium is explained, and examples of differences between countries and/or expert bodies are given. Throughout the review, gaps in knowledge and research requirements are identified. More research is needed to improve our understanding of selenium metabolism and requirements for optimal health. Functions of the majority of the selenoproteins await characterization, the mechanism of absorption has yet to be identified, measures of status need to be developed, and effects of genotype on metabolism require further investigation. The relationships between selenium intake/status and health, or risk of disease, are complex but require elucidation to inform clinical practice, to refine dietary recommendations, and to develop effective public health policies.

The influence of selenium-enriched milk proteins and selenium yeast on plasma selenium levels and rectal selenoprotein gene expression in human subjects

Certain forms of dietary Se may have advantages for improving human Se status and regulating the risk for disease, such as cancers, including colorectal cancer (CRC). The present study compared the effects of a Se-enriched milk protein (dairy-Se) with a Se-rich yeast (yeast-Se) on plasma Se levels and rectal selenoprotein gene expression since we reasoned that if these genes were not regulated, there was little potential for regulating the risk for CRC in this organ. A total of twenty-three healthy volunteers with plasma Se in the lower half of the population range were supplemented with dairy-Se (150 μg/d) or yeast-Se (150 μg/d) for 6 weeks, followed by 6 weeks of washout period. Blood was sampled every 2 weeks, and rectal biopsies were obtained before and after Se supplementation and after the washout period. Plasma Se levels and glutathione peroxidase (GPx) activity, and rectal mRNA of selenoprotein P (SeP), cytosolic GPx-1 (GPx-1), gastrointestinal GPx-2 (GPx-2) and thioredoxin reductase-1 (TrxR-1) were measured. Plasma Se levels increased rapidly in both Se groups (P < 0·001); plasma GPx activity was not significantly changed. Rectal SeP mRNA increased at 6 weeks compared with baseline in both Se groups (P < 0·05); only dairy-Se resulted in a sustained elevation of SeP after the washout period (P < 0·05). Rectal GPx-1 and GPx-2 mRNA were higher with dairy-Se (P < 0·05) than with yeast-Se at 6 weeks. In conclusion, three rectal selenoprotein mRNA were differentially regulated by dairy-Se and yeast-Se. Changes in rectal selenoproteins are not predicted by changes in plasma Se; dairy-Se effectively regulates the expression of several rectal selenoproteins of relevance to the risk for CRC.

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.

Reduced utilization of selenium by naked mole rats due to a specific defect in GPx1 expression

Naked mole rat (MR) Heterocephalus glaber is a rodent model of delayed aging because of its unusually long life span (>28 years). It is also not known to develop cancer. In the current work, tissue imaging by x-ray fluorescence microscopy and direct analyses of trace elements revealed low levels of selenium in the MR liver and kidney, whereas MR and mouse brains had similar selenium levels. This effect was not explained by uniform selenium deficiency because methionine sulfoxide reductase activities were similar in mice and MR. However, glutathione peroxidase activity was an order of magnitude lower in MR liver and kidney than in mouse tissues. In addition, metabolic labeling of MR cells with (75)Se revealed a loss of the abundant glutathione peroxidase 1 (GPx1) band, whereas other selenoproteins were preserved. To characterize the MR selenoproteome, we sequenced its liver transcriptome. Gene reconstruction revealed standard selenoprotein sequences except for GPx1, which had an early stop codon, and SelP, which had low selenocysteine content. When expressed in HEK 293 cells, MR GPx1 was present in low levels, and its expression could be rescued neither by removing the early stop codon nor by replacing its SECIS element. In addition, GPx1 mRNA was present in lower levels in MR liver than in mouse liver. To determine if GPx1 deficiency could account for the reduced selenium content, we analyzed GPx1 knock-out mice and found reduced selenium levels in their livers and kidneys. Thus, MR is characterized by the reduced utilization of selenium due to a specific defect in GPx1 expression.

Selenium toxicity but not deficient or super-nutritional selenium status vastly alters the transcriptome in rodents

Background

Protein and mRNA levels for several selenoproteins, such as glutathione peroxidase-1 (Gpx1), are down-regulated dramatically by selenium (Se) deficiency. These levels in rats increase sigmoidally with increasing dietary Se and reach defined plateaus at the Se requirement, making them sensitive biomarkers for Se deficiency. These levels, however, do not further increase with super-nutritional or toxic Se status, making them ineffective for detection of high Se status. Biomarkers for high Se status are needed as super-nutritional Se intakes are associated with beneficial as well as adverse health outcomes. To characterize Se regulation of the transcriptome, we conducted 3 microarray experiments in weanling mice and rats fed Se-deficient diets supplemented with up to 5 μg Se/g diet.

Results

There was no effect of Se status on growth of mice fed 0 to 0.2 μg Se/g diet or rats fed 0 to 2 μg Se/g diet, but rats fed 5 μg Se/g diet showed a 23% decrease in growth and elevated plasma alanine aminotransferase activity, indicating Se toxicity. Rats fed 5 μg Se/g diet had significantly altered expression of 1193 liver transcripts, whereas mice or rats fed ≤ 2 μg Se/g diet had < 10 transcripts significantly altered relative to Se-adequate animals within an experiment. Functional analysis of genes altered by Se toxicity showed enrichment in cell movement/morphogenesis, extracellular matrix, and development/angiogenesis processes. Genes up-regulated by Se deficiency were targets of the stress response transcription factor, Nrf2. Multiple regression analysis of transcripts significantly altered by 2 μg Se/g and Se-deficient diets identified an 11-transcript biomarker panel that accounted for 99% of the variation in liver Se concentration over the full range from 0 to 5 μg Se/g diet.

Conclusion

This study shows that Se toxicity (5 μg Se/g diet) in rats vastly alters the liver transcriptome whereas Se-deficiency or high but non-toxic Se intake elicits relatively few changes. This is the first evidence that a vastly expanded number of transcriptional changes itself can be a biomarker of Se toxicity, and that identified transcripts can be used to develop molecular biomarker panels that accurately predict super-nutritional and toxic Se status.

Trace elements and ageing, a genomic perspective using selenium as an example

Trace elements are key regulators of metabolic and physiological pathways known to be altered during the ageing process and therefore have the capacity to modulate the rate of biological ageing. Optimal intake is required to maintain homeostasis and to increase cell protection. Deficiencies are associated with specific illnesses. However the contribution of commonly observed life-long sub-optimal intakes of trace elements to the development and severity of age-related chronic diseases is less appreciated. Additionally, reduce intake of several trace elements has been shown to be particularly challenging for elderly people. This review will use selenium as an example to illustrate how a trace element can influence ageing and how the Omics technologies could help to study the effect of trace elements on the ageing process. Although transcriptomics and proteomics approaches in animal models have so far enabled us to identify downstream targets of trace elements in pathways related to age-related diseases processes, future approaches of combining nutrigenomics with longevity studies in humans will help us to identify mechanisms whereby trace elements affect the ageing process.