Cis-acting elements are required for selenium regulation of glutathione peroxidase-1 mRNA levels

Antioxidant Effects of Statins by Modulating Nrf2 and Nrf2/HO-1 Signaling in Different Diseases

Statins are competitive inhibitors of hydroxymethylglutaryl-CoA (HMG-CoA) reductase and have been used to treat elevated low-density lipoprotein cholesterol (LDL-C) for almost four decades. Antioxidant and anti-inflammatory properties which are independent of the lipid-lowering effects of statins, i.e., their pleiotropic effects, might be beneficial in the prevention or treatment of many diseases. This review discusses the antioxidant effects of statins achieved by modulating the nuclear factor erythroid 2 related factor 2/ heme oxygenase-1 (Nrf2/HO-1) pathway in different organs and diseases. Nrf2 and other proteins involved in the Nrf2/HO-1 signaling pathway have a crucial role in cellular responses to oxidative stress, which is a risk factor for ASCVD. Statins can significantly increase the DNA-binding activity of Nrf2 and induce the expression of its target genes, such as HO-1 and glutathione peroxidase) GPx, (thus protecting the cells against oxidative stress. Antioxidant and anti-inflammatory properties of statins, which are independent of their lipid-lowering effects, could be partly explained by the modulation of the Nrf2/HO-1 pathway.

Characterization and expression analysis of seven selenoprotein genes in yellow catfish Pelteobagrus fulvidraco to dietary selenium levels

BACKGROUND

Selenium (Se) appears in the selenoproteins in the form of selenocysteine (Sec) and is important for the growth and development of vertebrates. The present study characterized seven selenoproteins, consisting of the GPX1, GPX3, GPX4, SELENOW, SELENOP, TXNRD2 and TXNRD3 cDNAs in various tissues of yellow catfish, explored their regulation to dietary Se addition.

METHODS

3' and 5' RACE PCR were used to clone full-length cDNA sequences of seven selenoprotein genes (GPX1, GPX3, GPX4, SELENOW, SELENOP, TXNRD2 and TXNRD3). Their molecular characterizations were analyzed, including conservative motifs and the SECIS elements. The phylogenetic trees were generated through neighbor-joining (NJ) method with MEGA 6.0 with 1000 bootstrap replications. Quantitative real-time PCR was used to explore their mRNA tissue distribution in the heart, anterior intestine, dorsal muscle, head kidney, gill, liver, brain, spleen and mesenteric fat. Yellow catfish (mixed sex) were fed diets with dietary Se contents at 0.03 (low Se), 0.25 (adequate Se) and 6.39 (high Se) mg Se/kg, respectively, for 12 weeks, and their spleen, kidney, testis and brain were used for the determination of the mRNA levels of the seven selenoproteins.

RESULTS

The seven selenoproteins had similar domains to their corresponding members of other vertebrates. They were widely expressed in nine tissues, including heart, liver, brain, spleen, head kidney, dorsal muscle, mesenteric fat, anterior intestine and gill, but showed tissue-dependent expression patterns. Dietary Se addition affected the expression of the seven genes in spleen, kidney, testis and brain tissues of yellow catfish.

CONCLUSION

Taken together, our study demonstrated the characterization, expression and regulation of seven selenoproteins, which increased our understanding of the biological functions of Se and selenoproteins in fish.

The Interaction between Dietary Selenium Intake and Genetics in Determining Cancer Risk and Outcome

There is considerable interest in the trace element selenium as a possible cancer chemopreventive dietary component, but supplementation trials have not indicated a clear benefit. Selenium is a critical component of selenium-containing proteins, or selenoproteins. Members of this protein family contain selenium in the form of selenocysteine. Selenocysteine is encoded by an in-frame UGA codon recognized as a selenocysteine codon by a regulatory element, the selenocysteine insertion sequence (SECIS), in the 3′-untranslated region of selenoprotein mRNAs. Epidemiological studies have implicated several selenoprotein genes in cancer risk or outcome based on associations between allelic variations and disease risk or mortality. These polymorphisms can be found in or near the SECIS or in the selenoprotein coding sequence. These variations both function to control protein synthesis and impact the efficiency of protein synthesis in response to the levels of available selenium. Thus, an individual’s genetic makeup and nutritional intake of selenium may interact to predispose them to acquiring cancer or affect cancer progression to lethality.

Selenium at the redox interface of the genome, metabolome and exposome

Selenium (Se) is a redox-active environmental mineral that is converted to only a small number of metabolites and required for a relatively small number of mammalian enzymes. Despite this, dietary and environmental Se has extensive impact on every layer of omics space. This highlights a need for global network response structures to provide reference for targeted, hypothesis-driven Se research. In this review, we survey the Se research literature from the perspective of the responsive physical and chemical barrier between an organism (functional genome) and its environment (exposome), which we have previously termed the redox interface. Recent advances in metabolomics allow molecular phenotyping of the integrated genome-metabolome-exposome structure. Use of metabolomics with transcriptomics to map functional network responses to supplemental Se in mice revealed complex network responses linked to dyslipidemia and weight gain. Central metabolic hubs in the network structure in liver were not directly linked to transcripts for selenoproteins but were, instead, linked to transcripts for glucose transport and fatty acid β-oxidation. The experimental results confirm the survey of research literature in showing that Se interacts with the functional genome through a complex network response structure. The results imply that systematic application of data-driven integrated omics methods to models with controlled Se exposure could disentangle health benefits and risks from Se exposures and also serve more broadly as an experimental paradigm for exposome research.

Human selenoprotein P and S variant mRNAs with different numbers of SECIS elements and inferences from mutant mice of the roles of multiple SECIS elements

Dynamic redefinition of the 10 UGAs in human and mouse selenoprotein P (Sepp1) mRNAs to specify selenocysteine instead of termination involves two 3' UTR structural elements (SECIS) and is regulated by selenium availability. In addition to the previously known human Sepp1 mRNA poly(A) addition site just 3' of SECIS 2, two further sites were identified with one resulting in 10-25% of the mRNA lacking SECIS 2. To address function, mutant mice were generated with either SECIS 1 or SECIS 2 deleted or with the first UGA substituted with a serine codon. They were fed on either high or selenium-deficient diets. The mutants had very different effects on the proportions of shorter and longer product Sepp1 protein isoforms isolated from plasma, and on viability. Spatially and functionally distinctive effects of the two SECIS elements on UGA decoding were inferred. We also bioinformatically identify two selenoprotein S mRNAs with different 5' sequences predicted to yield products with different N-termini. These results provide insights into SECIS function and mRNA processing in selenoprotein isoform diversity.

Aflatoxin B1 Induced Systemic Toxicity in Poultry and Rescue Effects of Selenium and Zinc

Among many challenges, exposure to aflatoxins, particularly aflatoxin B₁ (AFB₁), is one of the major concerns in poultry industry. AFB₁ intoxication results in decreased meat/egg production, hepatotoxicity, nephrotoxicity, disturbance in gastrointestinal tract (GIT) and reproduction, immune suppression, and increased disease susceptibility. Selenium (Se) and zinc (Zn), in dietary supplementation, offer easy, cost-effective, and efficient ways to neutralize the toxic effect of AFB₁. In the current review, we discussed the impact of AFB₁ on poultry industry, its biotransformation, and organ-specific noxious effects, along with the action mechanism of AFB₁-induced toxicity. Moreover, we explained the biological and detoxifying roles of Se and Zn in avian species as well as the protection mechanism of these two trace elements. Ultimately, we discussed the use of Se and Zn supplementation against AFB₁-induced toxicity in poultry birds.

The utilization of selenocysteine-tRNA[Ser]Sec isoforms is regulated in part at the level of translation in vitro

The tRNA for the 21st proteinogenic amino acid, selenocysteine, exists in mammalian cells as 2 isoforms differing by a single 2'-O-methylribosyl moiety at position 34 (Um34). These isoforms contain either 5-methoxycarbonylmethyluridine (mcm⁵U) or 5-methoxycarbonylmethyl-2'-O-methyluridine (mcm⁵Um) at position 34. The accumulation of the mcm⁵Um isoform is tightly correlated with the expression of nonessential "stress response" selenoproteins such as glutathione peroxidase 1 (GPX1). The expression of essential selenoproteins, such as thioredoxin reductase 1 (TXNRD1), is not affected by changes in Sec-tRNA^[Ser]Sec isoform accumulation. In this work we used purified mcm⁵U and mcm⁵Um Sec-tRNA^[Ser]Sec isoforms to analyze possible differences in binding to the selenocysteine-specific elongation factor, EEFSEC, and the translation of GPX1 and TXNRD1in vitro. Our results indicate that no major distinction between mcm⁵U and mcm⁵Um isoforms is made by the translation machinery, but a small consistent increase in GPX1 translation is associated with the mcm⁵Um isoform. These results implicate fundamental differences in translation efficiency in playing a role in regulating selenoprotein expression as a function of isoform accumulation.

A system for coordinated analysis of translational readthrough and nonsense-mediated mRNA decay

The nonsense-mediated mRNA decay (NMD) pathway degrades mRNAs containing premature termination codons, limiting the expression of potentially deleterious truncated proteins. This activity positions the pathway as a regulator of the severity of genetic diseases caused by nonsense mutations. Because many genetic diseases result from nonsense alleles, therapeutics inducing readthrough of premature termination codons and/or inhibition of NMD have been of great interest. Several means of enhancing translational readthrough have been reported to concomitantly inhibit NMD efficiency, but tools for systematic analysis of mammalian NMD inhibition by translational readthrough are lacking. Here, we introduce a system that allows concurrent analysis of translational readthrough and mRNA decay. We use this system to show that diverse readthrough-promoting RNA elements have similar capacities to inhibit NMD. Further, we provide evidence that the level of translational readthrough required for protection from NMD depends on the distance of the suppressed termination codon from the end of the mRNA.

Viral Evasion and Manipulation of Host RNA Quality Control Pathways

Viruses have evolved diverse strategies to maximize the functional and coding capacities of their genetic material. Individual viral RNAs are often used as substrates for both replication and translation and can contain multiple, sometimes overlapping open reading frames. Further, viral RNAs engage in a wide variety of interactions with both host and viral proteins to modify the activities of important cellular factors and direct their own trafficking, packaging, localization, stability, and translation. However, adaptations increasing the information density of small viral genomes can have unintended consequences. In particular, viral RNAs have developed features that mark them as potential targets of host RNA quality control pathways. This minireview focuses on ways in which viral RNAs run afoul of the cellular mRNA quality control and decay machinery, as well as on strategies developed by viruses to circumvent or exploit cellular mRNA surveillance.

The molecular biology of selenocysteine

Selenium is an essential trace element that is incorporated into 25 human proteins as the amino acid selenocysteine (Sec). The incorporation of this amino acid turns out to be a fascinating problem in molecular biology because Sec is encoded by a stop codon, UGA. Layered on top of the canonical translation elongation machinery is a set of factors that exist solely to incorporate this important amino acid. The mechanism by which this process occurs, put into the context of selenoprotein biology, is the focus of this review.

Selenocysteine incorporation: A trump card in the game of mRNA decay

The incorporation of the 21st amino acid, selenocysteine (Sec), occurs on mRNAs that harbor in-frame stop codons because the Sec-tRNA(Sec) recognizes a UGA codon. This sets up an intriguing interplay between translation elongation, translation termination and the complex machinery that marks mRNAs that contain premature termination codons for degradation, leading to nonsense mediated mRNA decay (NMD). In this review we discuss the intricate and complex relationship between this key quality control mechanism and the process of Sec incorporation in mammals.

Selenoproteins: molecular pathways and physiological roles

Selenium is an essential micronutrient with important functions in human health and relevance to several pathophysiological conditions. The biological effects of selenium are largely mediated by selenium-containing proteins (selenoproteins) that are present in all three domains of life. Although selenoproteins represent diverse molecular pathways and biological functions, all these proteins contain at least one selenocysteine (Sec), a selenium-containing amino acid, and most serve oxidoreductase functions. Sec is cotranslationally inserted into nascent polypeptide chains in response to the UGA codon, whose normal function is to terminate translation. To decode UGA as Sec, organisms evolved the Sec insertion machinery that allows incorporation of this amino acid at specific UGA codons in a process requiring a cis-acting Sec insertion sequence (SECIS) element. Although the basic mechanisms of Sec synthesis and insertion into proteins in both prokaryotes and eukaryotes have been studied in great detail, the identity and functions of many selenoproteins remain largely unknown. In the last decade, there has been significant progress in characterizing selenoproteins and selenoproteomes and understanding their physiological functions. We discuss current knowledge about how these unique proteins perform their functions at the molecular level and highlight new insights into the roles that selenoproteins play in human health.

Nonsense-mediated decay factors are involved in the regulation of selenoprotein mRNA levels during selenium deficiency

Selenoproteins contain the unique amino acid selenocysteine (Sec), which is encoded by the triplet UGA. Since UGA also serves as a stop codon, it has been postulated that selenoprotein mRNAs are targeted for degradation by the nonsense-mediated mRNA decay pathway (NMD). Several reports have observed a hierarchy of selenoprotein mRNA expression when selenium (Se) is limiting, whereby the abundance of certain transcripts decline while others do not. We sought to investigate the role of NMD in this hierarchical response that selenoprotein mRNAs exhibit to environmental Se status. Selenoprotein mRNAs were categorized as being predicted sensitive or resistant to NMD based on the requirements held by the current model. About half of the selenoprotein transcriptome was predicted to be sensitive to NMD and showed significant changes in mRNA abundance in response to cellular Se status. The other half that was predicted to be resistant to NMD did not respond to Se status. RNA immunoprecipitation with essential NMD factor UPF1 revealed that the mRNAs that were the most sensitive to Se status were also the most enriched on UPF1 during Se deficiency. Furthermore, depletion of SMG1, the kinase responsible for UPF1 phosphorylation and NMD activation, abrogated the decline in transcript abundance of Se-responsive transcripts. Lastly, mRNA decay rates of Se-responsive transcripts were altered upon the addition of Se to resemble the slower decay rates of nonresponsive transcripts. Taken together, these results present novel evidence in support of a crucial role for the NMD pathway in regulating selenoprotein mRNA levels when Se is limiting.

Translational redefinition of UGA codons is regulated by selenium availability

Incorporation of selenium into ~25 mammalian selenoproteins occurs by translational recoding whereby in-frame UGA codons are redefined to encode the selenium containing amino acid, selenocysteine (Sec). Here we applied ribosome profiling to examine the effect of dietary selenium levels on the translational mechanisms controlling selenoprotein synthesis in mouse liver. Dietary selenium levels were shown to control gene-specific selenoprotein expression primarily at the translation level by differential regulation of UGA redefinition and Sec incorporation efficiency, although effects on translation initiation and mRNA abundance were also observed. Direct evidence is presented that increasing dietary selenium causes a vast increase in ribosome density downstream of UGA-Sec codons for a subset of selenoprotein mRNAs and that the selenium-dependent effects on Sec incorporation efficiency are mediated in part by the degree of Sec-tRNA([Ser]Sec) Um34 methylation. Furthermore, we find evidence for translation in the 5'-UTRs for a subset of selenoproteins and for ribosome pausing near the UGA-Sec codon in those mRNAs encoding the selenoproteins most affected by selenium availability. These data illustrate how dietary levels of the trace element selenium can alter the readout of the genetic code to affect the expression of an entire class of proteins.

A luciferase reporter assay to investigate the differential selenium-dependent stability of selenoprotein mRNAs

The mechanisms regulating the differential selenium (Se)-dependent stability of selenoprotein mRNAs are partially characterized. To further study the Se-dependent regulation of selenoproteins, we developed a novel chemiluminescent reporter to monitor the steady-state mRNA level of an artificial selenoprotein. Our reporter is a fusion of the Renilla luciferase gene and of the β-globin gene, but contains features required for incorporation of selenocysteine (SEC), namely, a UGA-SEC codon and a 3' untranslated region RNA stem loop called a SEC incorporation sequence (SECIS). At various levels of Se, the activity of reporters containing GPX1 or GPX4 SECIS elements is proportional to the steady-state mRNA level of the reporter construct and reflects the level of the corresponding endogenous mRNA. In a reporter containing a UGA codon and a functional GPX1 SECIS, Se-dependent nonsense-mediated decay (NMD) occurred in the cytoplasm, as opposed to the more typical nuclear location. To validate the reporter system, we used genetic and pharmacologic approaches to inhibit or promote NMD. Modulation of UPF1 by siRNA, overexpression, or by inhibition of SMG1 altered NMD in this system. Our reporter is derived from a Renilla luciferase reporter gene fused to an intron containing B-globin gene and is subject to degradation by NMD when a stop codon is inserted before the second intron.

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.

Selenium regulation of the selenoprotein and nonselenoprotein transcriptomes in rodents

This review discusses progress in understanding the hierarchy of selenoprotein expression at the transcriptome level from selenium (Se) deficiency to Se toxicity. Microarray studies of the full selenoproteome have found that 5 of 24 rodent selenoprotein mRNA decrease to <40% of Se adequate levels in Se deficient liver but that the majority of selenoprotein mRNA are not regulated by Se deficiency. These differences match with the hierarchy of selenoprotein expression, helping to explain these differences and also showing that selenoprotein transcripts can be used as molecular biomarkers for assessing Se status. The similarity of the response curves for regulated selenoproteins suggests one underlying mechanism is responsible for the downregulation of selenoprotein mRNA in Se deficiency, but the heterogeneity of the UGA position in regulated and nonregulated selenoprotein transcripts now indicates that current nonsense mediated decay models cannot explain which transcripts are susceptible to mRNA decay. Microarray studies on the full liver transcriptome in rats found only <10 transcripts/treatment were significantly down- or upregulated by Se deficiency or by supernutritional Se up to 2.0 μg Se/g diet (20× requirement), suggesting that cancer prevention associated with supernutritional Se may not be mediated by transcriptional changes. Toxic dietary Se at 50× requirement (5 μg Se/g diet), however, significantly altered ∼4% of the transcriptome, suggesting number of transcriptional changes itself as a biomarker of Se toxicity. Finally, panels of Se regulated selenoprotein plus nonselenoprotein transcripts predict Se status from deficient to toxic better than conventional biomarkers, illustrating potential roles for molecular biomarkers in nutrition.

Rescue of non-sense mutated p53 tumor suppressor gene by aminoglycosides

Mutation-based treatments are a new development in genetic medicine, in which the nature of the mutation dictates the therapeutic strategy. Interest has recently focused on diseases caused by premature termination codons (PTCs). Drugs inducing the readthrough of these PTCs restore the production of a full-length protein. In this study, we explored the possibility of using aminoglycoside antibiotics to induce the production of a full-length functional p53 protein from a gene carrying a PTC. We identified a human cancer cell line containing a PTC, for which high levels of readthrough were obtained in the presence of aminoglycosides. Using these cells, we demonstrated that aminoglycoside treatment stabilized the mutant mRNA, which would otherwise have been degraded by non-sense-mediated decay, resulting in the production of a functional full-length p53 protein. Finally, we showed that aminoglycoside treatment decreased the viability of cancer cells specifically in the presence of nonsense-mutated p53 gene. These results open possibilities of developing promising treatments of cancers linked with non-sense mutations in tumor suppressor genes. They show that molecules designed to induce stop-codon readthrough can be used to inhibit tumor growth and offer a rational basis for developing new personalized strategies that could diversify the existing arsenal of cancer therapies.

Upf1 senses 3'UTR length to potentiate mRNA decay

The selective degradation of mRNAs by the nonsense-mediated decay pathway is a quality control process with important consequences for human disease. From initial studies using RNA hairpin-tagged mRNAs for purification of messenger ribonucleoproteins assembled on transcripts with HIV-1 3' untranslated region (3'UTR) sequences, we uncover a two-step mechanism for Upf1-dependent degradation of mRNAs with long 3'UTRs. We demonstrate that Upf1 associates with mRNAs in a 3'UTR length-dependent manner and is highly enriched on transcripts containing 3'UTRs known to elicit NMD. Surprisingly, Upf1 recruitment and subsequent RNA decay can be antagonized by retroviral RNA elements that promote translational readthrough. By modulating the efficiency of translation termination, recognition of long 3'UTRs by Upf1 is uncoupled from the initiation of decay. We propose a model for 3'UTR length surveillance in which equilibrium binding of Upf1 to mRNAs precedes a kinetically distinct commitment to RNA decay.

Redox control in mammalian embryo development

The development of an embryo constitutes a complex choreography of regulatory events that underlies precise temporal and spatial control. Throughout this process the embryo encounters ever changing environments, which challenge its metabolism. Oxygen is required for embryogenesis but it also poses a potential hazard via formation of reactive oxygen and reactive nitrogen species (ROS/RNS). These metabolites are capable of modifying macromolecules (lipids, proteins, nucleic acids) and altering their biological functions. On one hand, such modifications may have deleterious consequences and must be counteracted by antioxidant defense systems. On the other hand, ROS/RNS function as essential signal transducers regulating the cellular phenotype. In this context the combined maternal/embryonic redox homeostasis is of major importance and dysregulations in the equilibrium of pro- and antioxidative processes retard embryo development, leading to organ malformation and embryo lethality. Silencing the in vivo expression of pro- and antioxidative enzymes provided deeper insights into the role of the embryonic redox equilibrium. Moreover, novel mechanisms linking the cellular redox homeostasis to gene expression regulation have recently been discovered (oxygen sensing DNA demethylases and protein phosphatases, redox-sensitive microRNAs and transcription factors, moonlighting enzymes of the cellular redox homeostasis) and their contribution to embryo development is critically reviewed.

Threading the needle: getting selenocysteine into proteins

The co-translational incorporation of selenocysteine (Sec) requires that UGA be recognized as a sense rather than a nonsense codon. This is accomplished by the concerted action of a Sec insertion sequence (SECIS) element, SECIS binding protein 2, and a ternary complex of the Sec specific elongation factor, Sec-tRNA(Sec), and GTP. The mechanism by which they alter the canonical protein synthesis reaction has been elusive. Here we present an overview of the mechanistic perspective on Sec incorporation, highlighting recent advances in the field.

mRNA transcripts as molecular biomarkers in medicine and nutrition

In medicine, mRNA transcripts are being developed as molecular biomarkers for the diagnosis and treatment of a number of diseases. These biomarkers offer early and more accurate prediction and diagnosis of disease and disease progression, and ability to identify individuals at risk. Use of microarrays also offers opportunity to identify orthogonal (uncorrelated) biomarkers not known to be linked with conventional biomarkers. Investigators are increasingly using blood as a surrogate tissue for biopsy and analysis; total RNA isolated from whole blood is predominantly from erythroid cells, and whole blood mRNA shares more than 80% of the transcriptome with major tissues. Thus blood mRNA biomarkers for individualized disease prediction and diagnosis are an exciting area in medicine; mRNA biomarkers in nutrition have potential application that parallels these opportunities. Assessment of selenium (Se) status and requirements is one area where tissue mRNA levels have been used successfully. Selenoprotein-H and selenoprotein-W as well as glutathione peroxidase-1 (Gpx1) mRNAs are highly down-regulated in Se deficiency in rat liver, and the minimum dietary Se requirement is 0.06-0.07 microg Se/g based on these biomarkers, similar to requirements determined using conventional biomarkers. Blood Gpx1 mRNA can also be used to determine Se requirements in rats, showing that blood mRNA has potential for assessment of nutrient status. Future research is needed to develop mRNA biomarker panels for all nutrients that will discriminate between deficient, marginal, adequate and supernutritional individuals and populations, and differentiate between individuals who will benefit vs. be adversely affected by nutrient supplementation.

Mouse models targeting selenocysteine tRNA expression for elucidating the role of selenoproteins in health and development

Selenium (Se) deficiency has been known for many years to be associated with disease, impaired growth and a variety of other metabolic disorders in mammals. Only recently has the major role that Se-containing proteins, designated selenoproteins, play in many aspects of health and development begun to emerge. Se is incorporated into protein by way of the Se-containing amino acid, selenocysteine (Sec). The synthesis of selenoproteins is dependent on Sec tRNA for insertion of Sec, the 21^st amino acid in the genetic code, into protein. We have taken advantage of this dependency to modulate the expression of Sec tRNA that in turn modulates the expression of selenoproteins by generating transgenic, conditional knockout, transgenic/standard knockout and transgenic/conditional knockout mouse models, all of which involve the Sec tRNA gene, to elucidate the intracellular roles of this protein class.

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.

Selenium status highly regulates selenoprotein mRNA levels for only a subset of the selenoproteins in the selenoproteome

Gpx (glutathione peroxidase)-1 enzyme activity and mRNA levels decrease dramatically in Se (selenium) deficiency, whereas other selenoproteins are less affected by Se deficiency. This hierarchy of Se regulation is not understood, but the position of the UGA selenocysteine codon is thought to play a major role in making selenoprotein mRNAs susceptible to nonsense-mediated decay. Thus in the present paper we studied the complete selenoproteome in the mouse to uncover additional selenoprotein mRNAs that are highly regulated by Se status. Mice were fed on Se-deficient, Se-marginal and Se-adequate diets (0, 0.05 and 0.2 microg of Se/g respectively) for 35 days, and selenoprotein mRNA levels in liver and kidney were determined using microarray analysis and quantitative real-time PCR analysis. Se-deficient mice had liver Se concentrations and liver Gpx1 and thioredoxin reductase activities that were 4, 3 and 3% respectively of the levels in Se-adequate mice, indicating that the mice were Se deficient. mRNAs for Selh (selenoprotein H) and Sepw1 (selenoprotein W) as well as Gpx1 were decreased by Se deficiency to <40% of Se-adequate levels. Five and two additional mRNAs were moderately down-regulated in Sedeficient liver and kidney respectively. Importantly, nine selenoprotein mRNAs in liver and fifteen selenoprotein mRNAs in the kidney were not significantly regulated by Se deficiency, clearly demonstrating that Se regulation of selenoprotein mRNAs is not a general phenomenon. The similarity of the response to Se deficiency suggests that there is one underlying mechanism responsible. Importantly, the position of the UGA codon did not predict susceptibility to Se regulation, clearly indicating that additional features are involved in causing selenoprotein mRNAs to be sensitive to Se status.

Transcript analysis of the selenoproteome indicates that dietary selenium requirements of rats based on selenium-regulated selenoprotein mRNA levels are uniformly less than those based on glutathione peroxidase activity

Dietary selenium (Se) requirements in rats have been based largely upon glutathione peroxidase-1 (Gpx1) enzyme activity and Gpx1 mRNA levels can also be used to determine Se requirements. The identification of the complete selenoprotein proteome suggests that we might identify additional useful molecular biomarkers for assessment of Se status. To characterize Se regulation of the entire rat selenoproteome, weanling male rats were fed a Se-deficient diet (<0.01 microg Se/g) supplemented with graded levels of Se (0-0.8 microg/g diet) for 28 d, Se status was determined by tissue Se concentration and selenoenzyme activity, and selenoprotein mRNA abundance in liver, kidney, and muscle was determined by quantitative real-time-PCR. Tissue Se and selenoenzyme biomarkers indicated that minimal Se requirements were <or=0.1 microg Se/g diet for most biomarkers. Liver Gpx1 mRNA also decreased to <10% of Se-adequate levels, with a minimum Se requirement at 0.07 microg/g diet. Five selenoprotein mRNA in liver, 4 in kidney, and 2 in muscle decreased to <41% of Se-adequate levels, all with minimum Se requirements at <or=0.07 microg/g diet; the majority of selenoprotein mRNA in each tissue were not significantly regulated by Se status, and 1 selenoprotein, selenophosphate synthetase-2, was upregulated in Se-deficient kidney. Plateau breakpoints for all regulated selenoprotein mRNA were very similar, suggesting that 1 underlying mechanism is in play in Se regulation of selenoprotein mRNA. Lastly, we did not find any selenoprotein mRNA that could be used as biomarkers for super-nutritional/anticarcinogenic levels (up to 0.8 microg Se/g diet) of Se.

Simvastatin activates Keap1/Nrf2 signaling in rat liver

Some of the statins' pleiotropic actions have been attributed to their antioxidant activity. The Nrf2 transcription factor controls the expression of a number of protective genes in response to oxidative stress. In the present study, wistar rats, primary hepatocytes as well as ST2 cells, were employed to explore the potential role of Nrf2 in mediating the reported antioxidant effects of statins. Simvastatin triggered nuclear translocation of Nrf2 in rat liver and in primary rat hepatocytes in a mevalonate-dependent and cholesterol-independent way. In liver, nuclear extracts from simvastatin-treated rats, the DNA-binding activity of Nrf2, was significantly increased and the mRNA of two known targets of Nrf2 (HO-1 and GPX2) was induced. In ST2 cells stably transfected with constructs bearing Nrf2-binding site (antioxidant responsive element), simvastatin enhanced Nrf2-mediated transcriptional activity in a mevalonate-dependent and cholesterol-independent fashion. In conclusion, activation of Keap1/Nrf2 signaling pathway by simvastatin might provide effective protection of the cell from the deleterious effects of oxidative stress.

SBP2 binding affinity is a major determinant in differential selenoprotein mRNA translation and sensitivity to nonsense-mediated decay

Selenoprotein mRNAs are potential targets for degradation via nonsense-mediated decay due to the presence of in-frame UGA codons that can be decoded as either selenocysteine or termination codons. When UGA decoding is inefficient, as occurs when selenium is limiting, termination occurs at these positions. Based on the predicted exon-intron structure, 14 of the 25 human selenoprotein mRNAs are predicted to be sensitive to nonsense-mediated decay. Among these, sensitivity varies widely, resulting in a hierarchy of preservation or degradation of selenoprotein mRNAs and, thus, of selenoprotein synthesis. Potential factors in dictating the hierarchy of selenoprotein synthesis are the Sec insertion sequence RNA-binding proteins, SBP2 and nucleolin. To investigate the mechanistic basis for this hierarchy and the role of these two proteins, we carried out knockdowns of SBP2 expression and assessed the effects on selenoprotein mRNA levels. We also investigated in vivo binding of selenoprotein mRNAs by SBP2 and nucleolin via immunoprecipitation of the proteins and quantitation of bound mRNAs. We report that SBP2 exhibits strong preferential binding to some selenoprotein mRNAs over others, whereas nucleolin exhibits minimal differences in binding. Thus, SBP2 is a major determinant in dictating the hierarchy of selenoprotein synthesis via differential selenoprotein mRNA translation and sensitivity to nonsense-mediated decay.

Selective restoration of the selenoprotein population in a mouse hepatocyte selenoproteinless background with different mutant selenocysteine tRNAs lacking Um34

Novel mouse models were developed in which the hepatic selenoprotein population was targeted for removal by disrupting the selenocysteine (Sec) tRNA([Ser]Sec) gene (trsp), and selenoprotein expression was then restored by introducing wild type or mutant trsp transgenes. The selenoprotein population was partially replaced in liver with mutant transgenes encoding mutations at either position 34 (34T-->A) or 37 (37A-->G) in tRNA([Ser]Sec). The A34 transgene product lacked the highly modified 5-methoxycarbonylmethyl-2'-O-methyluridine, and its mutant base A was converted to I34. The G37 transgene product lacked the highly modified N(6)-isopentenyladenosine. Both mutant tRNAs lacked the 2'-methylribose at position 34 (Um34), and both supported expression of housekeeping selenoproteins (e.g. thioredoxin reductase 1) in liver but not stress-related proteins (e.g. glutathione peroxidase 1). Thus, Um34 is responsible for synthesis of a select group of selenoproteins rather than the entire selenoprotein population. The ICA anticodon in the A34 mutant tRNA decoded Cys codons, UGU and UGC, as well as the Sec codon, UGA. However, metabolic labeling of A34 transgenic mice with (75)Se revealed that selenoproteins incorporated the label from the A34 mutant tRNA, whereas other proteins did not. These results suggest that the A34 mutant tRNA did not randomly insert Sec in place of Cys, but specifically targeted selected selenoproteins. High copy numbers of A34 transgene, but not G37 transgene, were not tolerated in the absence of wild type trsp, further suggesting insertion of Sec in place of Cys in selenoproteins.

The selenoproteome exhibits widely varying, tissue-specific dependence on selenoprotein P for selenium supply

Selenoprotein P (Sel P) is a selenium-rich glycoprotein believed to play a key role in selenium (Se) transport throughout the body. Development of a Sel P knockout mouse model has supported this notion and initial studies have indicated that selenium supply to various tissues is differentially affected by genetic deletion of Sel P. Se in the form of the amino acid, selenocysteine, is incorporated into selenoproteins at UGA codons. Thus, Se availability affects not only selenoprotein levels, but also the turnover of selenoprotein mRNAs via the nonsense-mediated decay pathway. We investigated how genetic deletion of Sel P in mice affected levels of the mRNAs encoding all known members of the murine selenoprotein family, as well as three non-selenoprotein factors involved in their synthesis, selenophosphate synthetase 1 (SPS1), SECIS-binding protein 2 (SBP2) and SECp43. Our findings present a comprehensive description of selenoprotein mRNA expression in the following murine tissues: brain, heart, intestine, kidney, liver, lung, spleen and testes. We also describe how abundance of selenoproteins and selenoprotein-synthesis factors are affected by genetic deletion of Sel P in some of these tissues, providing insight into how the presence of this selenoprotein influences selenoprotein mRNA levels, and thus, the selenoproteome.

Effects of selenium supplementation on expression of glutathione peroxidase isoforms in cultured human lung adenocarcinoma cell lines

Selenium is an essential nutrient, a component of several anti-oxidant enzymes, and a possible factor in cancer risk, including lung cancer. We determined the subtoxic range of selenium concentration (as sodium selenite) required to increase and maintain the expression of anti-oxidant selenoproteins gluthathione peroxidases GPX1 and GPX4 at a constant level in cultures of human lung adenocarcinoma cell lines (H460, H1703 and H1944) and in HPL1D, a non-transformed lung epithelial cell line. Selenium dose-dependently increased GPX1 protein expression 1.8-fold in HPL1D cells and approximately 40-fold in H460 and H1944 cancer cells, with maximum effects at 20-40 nM. GPX4 protein was also increased, but more so in HPL1D (five-fold) than in H460 or H1944 cells (two- to three-fold). GPX1 mRNA showed similar patterns but differences of lesser magnitude. GPX1 protein and activity level was not consistently detectable in H1703 cells, with or without Se supplementation; its mRNA was present but very low. GPX4 protein level was also low in H1703 cells, but was markedly increased by selenium supplementation (48-fold). These results confirm a role for selenium in risk of lung cancer and the independent regulation of GPX1 and GPX4. Characterization of individual tumors with regard to GPX1 and GPX4 levels and regulation might be useful for interpretation of clinical studies on effects of selenium in lung cancer risk.

Supramolecular complexes mediate selenocysteine incorporation in vivo

Selenocysteine incorporation in eukaryotes occurs cotranslationally at UGA codons via the interactions of RNA-protein complexes, one comprised of selenocysteyl (Sec)-tRNA([Ser]Sec) and its specific elongation factor, EFsec, and another consisting of the SECIS element and SECIS binding protein, SBP2. Other factors implicated in this pathway include two selenophosphate synthetases, SPS1 and SPS2, ribosomal protein L30, and two factors identified as binding tRNA([Ser]Sec), termed soluble liver antigen/liver protein (SLA/LP) and SECp43. We report that SLA/LP and SPS1 interact in vitro and in vivo and that SECp43 cotransfection increases this interaction and redistributes all three proteins to a predominantly nuclear localization. We further show that SECp43 interacts with the selenocysteyl-tRNA([Ser]Sec)-EFsec complex in vitro, and SECp43 coexpression promotes interaction between EFsec and SBP2 in vivo. Additionally, SECp43 increases selenocysteine incorporation and selenoprotein mRNA levels, the latter presumably due to circumvention of nonsense-mediated decay. Thus, SECp43 emerges as a key player in orchestrating the interactions and localization of the other factors involved in selenoprotein biosynthesis. Finally, our studies delineating the multiple, coordinated protein-nucleic acid interactions between SECp43 and the previously described selenoprotein cotranslational factors resulted in a model of selenocysteine biosynthesis and incorporation dependent upon both cytoplasmic and nuclear supramolecular complexes.

Nuclear assembly of UGA decoding complexes on selenoprotein mRNAs: a mechanism for eluding nonsense-mediated decay?

Recoding of UGA from a stop codon to selenocysteine poses a dilemma for the protein translation machinery. In eukaryotes, two factors that are crucial to this recoding process are the mRNA binding protein of the Sec insertion sequence, SBP2, and the specialized elongation factor, EFsec. We sought to determine the subcellular localization of these selenoprotein synthesis factors in mammalian cells and thus gain insight into how selenoprotein mRNAs might circumvent nonsense-mediated decay. Intriguingly, both EFsec and SBP2 localization differed depending on the cell line but significant colocalization of the two proteins was observed in cells where SBP2 levels were detectable. We identify functional nuclear localization and export signals in both proteins, demonstrate that SBP2 undergoes nucleocytoplasmic shuttling, and provide evidence that SBP2 levels and localization may influence EFsec localization. Our results suggest a mechanism for the nuclear assembly of the selenocysteine incorporation machinery that could allow selenoprotein mRNAs to circumvent nonsense-mediated decay, thus providing new insights into the mechanism of selenoprotein translation.

Unique features of selenocysteine incorporation function within the context of general eukaryotic translational processes

Unlike other essential dietary trace elements, selenium exerts its biological actions through its direct incorporation into selenoproteins, as a part of the 21st amino acid, selenocysteine. Fundamental studies have elucidated the unique structures and putative functions of multiple co-translational factors required for the incorporation of selenocysteine into selenoproteins. The current challenge is to understand how these selenocysteine incorporation factors function within the framework of translation. In eukaryotes, co-ordinating nuclear transcription with cytoplasmic translation of genes is a challenge involving complex spatial and temporal regulation. Selenoproteins utilize the common cellular machinery required for synthesis of non-selenoproteins. This machinery includes the elements involved in transcription, mRNA splicing and transport, and translational processes. Many investigators have emphasized the differences between the expression of selenoproteins and other eukaryotic proteins, whereas this review will attempt to highlight common themes and point out where additional interactions may be discovered.

Selenoprotein synthesis: a unique translational mechanism used by a diverse family of proteins

The purpose of this review is to provide an overview of the unique mechanism by which mammalian selenoprotein synthesis occurs. Selenoprotein synthesis requires translational recoding of the UGA codon from a stop signal to a selenocysteine insertion signal (SECIS). Dedicated factors directly involved in this translation process include specific secondary structure in the mRNA (SECIS), a unique tRNA (Sec-tRNA(Sec)), an RNA binding protein (SBP2), and a specialized elongation factor (EFsec). Regulation of this process is discussed along with physiologic and clinical issues regarding selenoprotein synthesis, including the side effects associated with statin drugs.

A novel eukaryotic selenoprotein in the haptophyte alga Emiliania huxleyi

The diversity of selenoproteins raises the question of why many life forms require selenium. Especially in photosynthetic organisms, the biochemical basis for the requirement for selenium is unclear because there is little information on selenoproteins. We found six selenium-containing proteins in a haptophyte alga, Emiliania huxleyi, which requires selenium for growth. The 27-kDa protein EhSEP2 was isolated, and its cDNA was cloned. The deduced amino acid sequence revealed that EhSEP2 is homologous to protein disulfide isomerase (PDI) and contains a highly conserved thioredoxin domain. The nucleotide sequence contains an in-frame TGA codon encoding selenocysteine at the position corresponding to the cysteine residue in the reaction center of known PDIs. However, no typical selenocysteine insertion sequence was found in the EhSEP2 cDNA. The EhSEP2 mRNA level was related to the abundance of selenium. E. huxleyi possesses a novel PDI-like selenoprotein and may have a novel type of selenocysteine insertion machinery.

Selective Rescue of Selenoprotein Expression in Mice Lacking a Highly Specialized Methyl Group in Selenocysteine tRNA

Selenocysteine (Sec) is the 21st amino acid in the genetic code. Its tRNA is variably methylated on the 2'-O-hydroxyl site of the ribosyl moiety at position 34 (Um34). Herein, we identified a role of Um34 in regulating the expression of some, but not all, selenoproteins. A strain of knock-out transgenic mice was generated, wherein the Sec tRNA gene was replaced with either wild type or mutant Sec tRNA transgenes. The mutant transgene yielded a tRNA that lacked two base modifications, N(6)-isopentenyladenosine at position 37 (i(6)A37) and Um34. Several selenoproteins, including glutathione peroxidases 1 and 3, SelR, and SelT, were not detected in mice rescued with the mutant transgene, whereas other selenoproteins, including thioredoxin reductases 1 and 3 and glutathione peroxidase 4, were expressed in normal or reduced levels. Northern blot analysis suggested that other selenoproteins (e.g. SelW) were also poorly expressed. This novel regulation of protein expression occurred at the level of translation and manifested a tissue-specific pattern. The available data suggest that the Um34 modification has greater influence than the i(6)A37 modification in regulating the expression of various mammalian selenoproteins and Um34 is required for synthesis of several members of this protein class. Many proteins that were poorly rescued appear to be involved in responses to stress, and their expression is also highly dependent on selenium in the diet. Furthermore, their mRNA levels are regulated by selenium and are subject to nonsense-mediated decay. Overall, this study described a novel mechanism of regulation of protein expression by tRNA modification that is in turn regulated by levels of the trace element, selenium.

Selenoprotein mRNA is expressed in blood at levels comparable to major tissues in rats

Liver glutathione peroxidase-1 (GPX1) mRNA is highly regulated by Se status relative to other parameters, but is of limited use for determining Se requirements in humans. To examine the efficacy of using blood for Se status assessment using molecular biology markers, we used a ribonuclease protection assay (RPA) to study mRNA levels in whole blood relative to 16 other rat tissues. Significant amounts of total RNA (>50 microg) were obtained from 1 mL of whole blood. Total RNA from 28-d postweaning Se-adequate (0.2 microg Se/g diet) male rats was analyzed for GPX1, GPX4, GPX3, thioredoxin reductase-1 (TRR1), and selenoprotein-P (SelP). RPA detected significant mRNA expression for at least 1 selenoprotein in all tissues except pancreas. GPX1 mRNA expression using this mix of RPA probes yielded the highest signal for GPX1 relative to the other selenoprotein signals in all tissues except testis; GPX1 expression was 4th highest in blood and similar to the major organs (liver, 1st; heart, 5th; kidney, 6th). Kidney was highest for GPX3, and testes was highest for GPX4, TRR1, and SelP. This study is the first to report the gene expression pattern for a number of selenoproteins and across a comprehensive set of tissues. The mRNA levels for all selenoproteins in blood were comparable to levels in the major organs, and decreases in blood and liver GPX1 mRNA levels in Se deficiency were similar, supporting potential use of whole blood for assessing Se status using molecular biology markers.

Selenium and brain function: a poorly recognized liaison

Molecular biology has recently contributed significantly to the recognition of selenium (Se)2 and Se-dependent enzymes as modulators of brain function. Increased oxidative stress has been proposed as a pathomechanism in neurodegenerative diseases including, among others, Parkinson's disease, stroke, and epilepsy. Glutathione peroxidases (GPx), thioredoxin reductases, and one methionine-sulfoxide-reductase are selenium-dependent enzymes involved in antioxidant defense and intracellular redox regulation and modulation. Selenium depletion in animals is associated with decreased activities of Se-dependent enzymes and leads to enhanced cell loss in models of neurodegenerative disease. Genetic inactivation of cellular GPx increases the sensitivity towards neurotoxins and brain ischemia. Conversely, increased GPx activity as a result of increased Se supply or overexpression ameliorates the outcome in the same models of disease. Genetic inactivation of selenoprotein P leads to a marked reduction of brain Se content, which has not been achieved by dietary Se depletion, and to a movement disorder and spontaneous seizures. Here we review the role of Se for the brain under physiological as well as pathophysiological conditions and highlight recent findings which open new vistas on an old essential trace element.

Mechanism and regulation of selenoprotein synthesis

Selenium is an essential trace element that is incorporated into proteins as selenocysteine (Sec), the twenty-first amino acid. Sec is encoded by a UGA codon in the selenoprotein mRNA. The decoding of UGA as Sec requires the reprogramming of translation because UGA is normally read as a stop codon. The translation of selenoprotein mRNAs requires cis-acting sequences in the mRNA and novel trans-acting factors dedicated to Sec incorporation. Selenoprotein synthesis in vivo is highly selenium-dependent, and there is a hierarchy of selenoprotein expression in mammals when selenium is limiting. This review describes emerging themes from studies on the mechanism, kinetics, and efficiency of Sec insertion in prokaryotes. Recent developments that provide mechanistic insight into how the eukaryotic ribosome distinguishes between UGA/Sec and UGA/stop codons are discussed. The efficiency and regulation of mammalian selenoprotein synthesis are considered in the context of current models for Sec insertion.

Regulation of gene expression by stop codon recoding: selenocysteine

The regulation of gene expression at the translational level not only allows for rapid changes in specific protein levels but also provides an opportunity to alter codon specificity. For the incorporation of selenocysteine (Sec) into protein, the UGA codon is transformed from one that signals translation termination to one specific for Sec. This review provides a look at Sec incorporation from the perspective of the individual steps involved in protein synthesis: initiation, elongation and termination. The roles of the factors known to be required for Sec incorporation are considered in the context of each step in translation including structural modeling of the differences between the standard elongation factor eEF1A and the Sec-specific counterpart, eEFSec.

Coupled tRNA(Sec)-dependent assembly of the selenocysteine decoding apparatus

SECIS elements recode UGA codons from "stop" to "sense." These RNA secondary structures, present in eukaryotic selenoprotein mRNA 3' untranslated regions, recruit a SECIS binding protein, which recruits a selenocysteine-specific elongation factor-tRNA complex. Elucidation of the assembly of this multicomponent complex is crucial to understanding the mechanism of selenocysteine incorporation. Coprecipitation studies identified the C-terminal 64 amino acids of the elongation factor as sufficient for interaction with the SECIS binding protein. Selenocysteyl-tRNA is required for this interaction; the two factors do not coprecipitate in its absence. Finally, through promoting this interaction, selenocysteyl-tRNA stabilizes the C-terminal domain of the elongation factor. We suggest that the coupling effect is critical to preventing nonproductive decoding attempts and hence forms a basis for effective selenoprotein synthesis.

3'UTRs of glutathione peroxidases differentially affect selenium-dependent mRNA stability and selenocysteine incorporation efficiency

Selenoprotein mRNAs are particular in several aspects. They contain a specific secondary structure in their 3'UTR, called Secis (selenocysteine inserting sequence), which is indispensable for selenocysteine incorporation, and they are degraded under selenium-limiting conditions according to their ranking in the hierarchy of selenoproteins. In the familiy of selenium-dependent glutathione peroxidases (GPx) the ranking is GI-GPx > or = PHGPx > cGPx = pGPx. This phenomenon was studied by mutually combining the coding regions of GI-GPx, PHGPx and cGPx with their 3'UTRs. HepG2 cells were stably transfected with the resulting constructs. Expression of glutathione peroxidases was estimated by activity measurement and Western blotting, the selenium-dependent mRNA stability by real-time PCR. Whereas 3'UTRs from stable PHGPx and GI-GPx could be exchanged without loss of stability, they were not able to stabilize cGPx mRNA. cGPx 3'UTR rendered GI-GPx and PHGPx mRNA unstable. Thus, cGPx mRNA contains selenium-responsive instability elements in both the translated and the untranslated region, which cannot be compensated by one of the stable homologs. Stabilizing efficiency of an individual GPx 3'UTR did not correlate with the efficiency of selenocysteine incorporation. PHGPx 3'UTR was equally effective as cGPx 3'UTR in enhancing GPx activity in all constructs, while GI-GPx 3'UTR showed a markedly lower efficacy. We conclude that different mRNA sequences and/or RNA-binding proteins might regulate mRNA stability and translation of selenoprotein mRNA.

Intracellular trafficking of micronutrients: from gene regulation to nutrient requirements

The intracellular distribution of micronutrients, as well as their uptake, is important for cell function. In some cases the distribution of micronutrients or their related proteins is determined by gene expression mechanisms. The 3' untranslated region (3'UTR) of metallothionein-1 mRNA determines localisation of the mRNA, and in turn intracellular trafficking of the protein product. Using transfected cells we have evidence for the trafficking of metallothionein-1 into the nucleus and for its involvement in protection from oxidative stress and DNA damage. When nutritional supply of Se is limited, selenoprotein expression is altered, but not all selenoproteins are affected equally; the available Se is prioritised for synthesis of particular selenoproteins. The prioritisation involves differences in mRNA translation and stability due to 3'UTR sequences. Potentially, genetic variation in these regulatory mechanisms may affect nutrient requirements. Genetic polymorphisms in the 3'UTR from two selenoprotein genes have been observed; one polymorphism affects selenoprotein synthesis. These examples illustrate how molecular approaches can contribute at several levels to an increased understanding of nutrient metabolism and requirements. First, they provide the tools to investigate regulatory features in genes and their products. Second, understanding these processes can provide model systems to investigate nutrient metabolism at the cellular level. Third, once key features have been identified, the availability of human genome sequence information and single nucleotide polymorphism databases present possibilities to define the extent of genetic variation in genes of nutritional relevance. Ultimately, the functionality of any variations can be defined and subgroups of the population with subtly different nutrient requirements identified.

Down-regulation of GPx1 mRNA and the loss of GPx1 activity causes cellular damage in the liver of selenium-deficient rabbits

The effects of 10 weeks of dietary selenium and/or vitamin E deficiency (< 0.03 mg Se and 1.5 mg vitamin E per kg diet) on body Se and vitamin E stores and on the down-regulation of liver cellular glutathione peroxidase (GPx1) and plasma glutathione peroxidase (GPx3) were examined in growing female New Zealand White rabbits in comparison to Se (+ 0.40 mg Se/kg diet) and/or vitamin E (+ 150 I.U./kg diet) supplemented controls. Additionally plasma lactate dehydrogenase (LDH) activity, liver thiobarbituric acid-reactive substances (TBA-RS) and liver protein carbonyls were measured to assess the development of oxidative stress during an alimentary Se and/or vitamin E deficiency. Significantly decreased concentrations of Se and vitamin E in plasma (Se: - 70%; vitamin E: - 87%) and liver (Se: - 90%; vitamin E: - 95%) indicated an efficacious Se and vitamin E depletion of the rabbits within 10 weeks. GPx1 messenger RNA levels (GPx1 mRNA) in the livers of Se-depleted rabbits were down-regulated to 1/3-1/8 of the Se supplemented controls. GPx1 enzyme activity in the livers of Se-deficient rabbits declined to 10% of the Se-supplied control rabbits. A significantly elevated LDH activity in the blood plasma of Se- and vitamin E-deficient rabbits indicated a general impairment of tissues. Markedly increased TBA-RS concentrations and protein carbonyl contents in the livers of Se- and vitamin E-deficient rabbits gave further evidence for severe oxidative damage of cellular lipids and proteins during an alimentary Se and/or vitamin E deficiency. Both a full expresssion of GPx1 attained by dietary Se supplementation and dietary vitamin E supply effected an almost complete protection against oxidative cellular damage of the liver.

Gene expression profiling of low selenium status in the mouse intestine: transcriptional activation of genes linked to DNA damage, cell cycle control and oxidative stress

The essential trace mineral selenium (Se) has been shown previously to inhibit intestinal, prostate, lung and liver tumor development and associated mortality in both experimental animals and humans. Although Se is likely to be one of the most powerful cancer chemopreventive agents in the human diet, its mechanism of action is unknown. To better understand the biological consequences of alterations in Se status, the gene expression profile associated with low Se status in the intestine of C57Bl/6J mice was analyzed. Mice were fed either a high fat (14%), torula yeast-based, Se-deficient diet (<0.01 mg/kg) or the same diet supplemented with a high level of dietary Se (1 mg/kg, as seleno-L-methionine) for 90 d. Use of high density oligonucleotide arrays representing 6347 genes revealed that low Se status results in a differential gene expression pattern indicative of activation of genes involved in DNA damage, oxidative stress and cell cycle control, and a decrease in the expression of genes involved in detoxification. These results suggest that suboptimal intake of a single trace mineral can have broad effects on gene expression patterns, providing a framework for understanding the multiple beneficial effects of Se in cancer chemoprevention and human health.