Evidence that selenium deficiency results in the cytoplasmic decay of GPx1 mRNA dependent on pre-mRNA splicing proteins bound to the mRNA exon-exon junction

Transcriptional Regulation of Selenoprotein F by Heat Shock Factor 1 during Selenium Supplementation and Stress Response

Changes of Selenoprotein F (SELENOF) protein levels have been reported during selenium supplementation, stressful, and pathological conditions. However, the mechanisms of how these external factors regulate SELENOF gene expression are largely unknown. In this study, HEK293T cells were chosen as an in vitro model. The 5′-flanking regions of SELENOF were analyzed for promoter features. Dual-Glo Luciferase assays were used to detect promoter activities. Putative binding sites of Heat Shock Factor 1 (HSF1) were predicted in silico and the associations were further proved by chromatin immunoprecipitation (ChIP) assay. Selenate and tunicamycin (Tm) treatment were used to induce SELENOF up-regulation. The fold changes in SELENOF expression and other relative proteins were analyzed by Q-PCR and western blot. Our results showed that selenate and Tm treatment up-regulated SELENOF at mRNA and protein levels. SELENOF 5′-flanking regions from −818 to −248 were identified as core positive regulatory element regions. Four putative HSF1 binding sites were predicted in regions from −1430 to −248, and six out of seven primers detected positive results in ChIP assay. HSF1 over-expression and heat shock activation increased the promoter activities, and mRNA and protein levels of SELENOF. Over-expression and knockdown of HSF1 showed transcriptional regulation effects on SELENOF during selenate and Tm treatment. In conclusion, HSF1 was discovered as one of the transcription factors that were associated with SELENOF 5′-flanking regions and mediated the up-regulation of SELENOF during selenate and Tm treatment. Our work has provided experimental data for the molecular mechanism of SELENOF gene regulation, as well as uncovered the involvement of HSF1 in selenotranscriptomic for the first time.

Protective Role of Selenium in Immune-Relevant Cytokine and Immunoglobulin Production by Piglet Splenic Lymphocytes Exposed to Deoxynivalenol

Deoxynivalenol (DON) is a mycotoxin that causes immunosuppression, especially in swine. Selenium (Se) is essential for proper functioning of the immune system in animals. However, little is known about the effects of DON and Se on cytokine or immunoglobulin production in piglets. Here, we addressed this gap by examining piglet splenic lymphocyte responses in vitro. Cells were stimulated with concanavalin A, a T cell stimulatory lectin, in the absence or presence of DON (0.1, 0.2, 0.4, and 0.8 μg/mL), Se (Na₂SeO₃, 2 μM), or combinations of Se 2 μM and DON 0.1-0.8 μg/mL for 12, 24, or 48 h. At each time point, supernatants and cells were collected and the expression of cytokine and immunoglobulin protein and mRNA was examined. Compared with control and Se-alone treatments, DON exposure significantly and dose dependently decreased the expression levels of IL-2, IL-4, IL-6, IL-10, IFN-γ, IgG, and IgM mRNA and protein. By contrast, co-treatment with DON + Se significantly increased the mRNA and protein levels of all factors examined, except IL-4 and IL-6, compared with DON treatment alone. The results of this investigation demonstrate that Se has the potential to counteract DON-induced immunosuppression in piglets and is a promising treatment for DON-mediated toxicity.

Selenium-Dependent Antioxidant Enzymes: Actions and Properties of Selenoproteins

Unlike other essential trace elements that interact with proteins in the form of cofactors, selenium (Se) becomes co-translationally incorporated into the polypeptide chain as part of 21st naturally occurring amino acid, selenocysteine (Sec), encoded by the UGA codon. Any protein that includes Sec in its polypeptide chain is defined as selenoprotein. Members of the selenoproteins family exert various functions and their synthesis depends on specific cofactors and on dietary Se. The Se intake in productive animals such as chickens affect nutrient utilization, production performances, antioxidative status and responses of the immune system. Although several functions of selenoproteins are unknown, many disorders are related to alterations in selenoprotein expression or activity. Selenium insufficiency and polymorphisms or mutations in selenoproteins' genes and synthesis cofactors are involved in the pathophysiology of many diseases, including cardiovascular disorders, immune dysfunctions, cancer, muscle and bone disorders, endocrine functions and neurological disorders. Finally, heavy metal poisoning decreases mRNA levels of selenoproteins and increases mRNA levels of inflammatory factors, underlying the antagonistic effect of Se. This review is an update on Se dependent antioxidant enzymes, presenting the current state of the art and is focusing on results obtained mainly in chicken.

Selenium Homeostasis and Clustering of Cardiovascular Risk Factors: A Systematic Review

Selenium is a trace element required for a range of cellular functions. It is widely used for the biosynthesis of the unique amino acid selenocysteine [Sec], which is a structural element of selenoproteins. This systematic review focused on the possible relation between selenium and metabolic risk factors. The literature was searched via PubMed, Scopus, ISI Web of Science, and Google Scholar. Searches were not restricted by time or language. Relevant studies were selected in three phases. After an initial quality assessment, two reviewers extracted all the relevant data, whereas the third reviewer checked their extracted data. All evidence came from experimental and laboratory studies. Selenoprotein P is the best indicator for selenium nutritional levels. In addition, high levels of selenium may increase the risk of metabolic syndrome while the lack of sufficient selenium may also promote metabolic syndrome. selenium supplementation in subjects with sufficient serum selenium levels has a contrary effect on blood pressure, LDL, and total cholesterol. According to the bioavailability of different types of selenium supplementation such as selenomethionine, selenite and selenium-yeast, it seems that the best nutritional type of selenium is selenium-yeast. Regarding obtained results of longitudinal studies and randomized controlled trials, selenium supplementation should not be recommended for primary or secondary cardio-metabolic risk prevention in populations with adequate selenium status.

Chemical Speciation of Selenium and Mercury as Determinant of Their Neurotoxicity

The antagonism of mercury toxicity by selenium has been well documented. Mercury is a toxic metal, widespread in the environment. The main target organs (kidneys, lungs, or brain) of mercury vary depending on its chemical forms (inorganic or organic). Selenium is a semimetal essential to mammalian life as part of the amino acid selenocysteine, which is required to the synthesis of the selenoproteins. This chapter has the aim of disclosing the role of selenide or hydrogen selenide (Se^-2 or HSe^-) as central metabolite of selenium and as an important antidote of the electrophilic mercury forms (particularly, Hg²⁺ and MeHg). Emphasis will be centered on the neurotoxicity of electrophile forms of mercury and selenium. The controversial participation of electrophile mercury and selenium forms in the development of some neurodegenerative disease will be briefly presented. The potential pharmacological use of organoseleno compounds (Ebselen and diphenyl diselenide) in the treatment of mercury poisoning will be considered. The central role of thiol (-SH) and selenol (-SeH) groups as the generic targets of electrophile mercury forms and the need of new in silico tools to guide the future biological researches will be commented.

Association of Complement Receptor 2 Gene Polymorphisms with Susceptibility to Osteonecrosis of the Femoral Head in Systemic Lupus Erythematosus

Osteonecrosis of the femoral head (ONFH) is a complex and multifactorial disease that is influenced by a number of genetic factors in addition to environmental factors. Some autoimmune disorders, including systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), and inflammatory bowel disease (IBD), are associated with the development of ONFH. Complement receptor type 2 (CR2) is membrane glycoprotein which binds C3 degradation products generated during complement activation. CR2 has many important functions in normal immunity and is assumed to play a role in the development of autoimmune disease. We investigated whether CR2 gene polymorphisms are associated with risk of ONFH in SLE patients. Eight polymorphisms in the CR2 gene were genotyped using TaqMan™ assays in 150 SLE patients and 50 ONFH in SLE patients (SLE_ONFH). The association analysis of genotyped SNPs and haplotypes was performed with ONFH. It was found that three SNPs, rs3813946 in 5′-UTR (untranslated region), rs311306 in intron 1, and rs17615 in exon 10 (nonsynonymous SNP; G/A, Ser639Asn) of the CR2 gene, were associated with an increased risk of ONFH under recessive model (P values; 0.004~0.016). Haplotypes were also associated with an increased risk (OR; 3.73~) of ONFH in SLE patients. These findings may provide evidences that CR2 contributes to human ONFH susceptibility in Korean SLE patients.

Update on selenoprotein biosynthesis

SIGNIFICANCE

Selenium is an essential trace element that is incorporated in the small but vital family of proteins, namely the selenoproteins, as the selenocysteine amino acid residue. In humans, 25 selenoprotein genes have been characterized. The most remarkable trait of selenoprotein biosynthesis is the cotranslational insertion of selenocysteine by the recoding of a UGA codon, normally decoded as a stop signal.

RECENT ADVANCES

In eukaryotes, a set of dedicated cis- and trans-acting factors have been identified as well as a variety of regulatory mechanisms, factors, or elements that control the selenoprotein expression at the level of the UGA-selenocysteine recoding process, offering a fascinating playground in the field of translational control. It appeared that the central players are two RNA molecules: the selenocysteine insertion sequence (SECIS) element within selenoprotein mRNA and the selenocysteine-tRNA([Ser]Sec); and their interacting partners.

CRITICAL ISSUES

After a couple of decades, despite many advances in the field and the discovery of many essential and regulatory components, the precise mechanism of UGA-selenocysteine recoding remains elusive and more complex than anticipated, with many layers of control. This review offers an update of selenoproteome biosynthesis and regulation in eukaryotes.

FUTURE DIRECTIONS

The regulation of selenoproteins in response to a variety of pathophysiological conditions and cellular stressors, including selenium levels, oxidative stress, replicative senescence, or cancer, awaits further detailed investigation. Clearly, the efficiency of UGA-selenocysteine recoding is the limiting stage of selenoprotein synthesis. The sequence of events leading Sec-tRNA([Ser]Sec) delivery to ribosomal A site awaits further analysis, notably at the level of a three-dimensional structure.

Protein Mis-Termination Initiates Genetic Diseases, Cancers, and Restricts Bacterial Genome Expansion

Protein termination is an important cellular process. Protein termination relies on the stop-codons in the mRNA interacting properly with the releasing factors on the ribosome. One third of inherited diseases, including cancers, are associated with the mutation of the stop-codons. Many pathogens and viruses are able to manipulate their stop-codons to express their virulence. The influence of stop-codons is not limited to the primary reading frame of the genes. Stop-codons in the second and third reading frames are referred as premature stop signals (PSC). Stop-codons and PSCs together are collectively referred as stop-signals. The ratios of the stop-signals (referred as translation stop-signals ratio or TSSR) of genetically related bacteria, despite their great differences in gene contents, are much alike. This nearly identical Genomic-TSSR value of genetically related bacteria may suggest that bacterial genome expansion is limited by their unique stop-signals bias. We review the protein termination process and the different types of stop-codon mutation in plants, animals, microbes, and viruses, with special emphasis on the role of PSCs in directing bacterial evolution in their natural environments. Knowing the limit of genomic boundary could facilitate the formulation of new strategies in controlling the spread of diseases and combat antibiotic-resistant bacteria.

Selenium-enriched probiotics improve antioxidant status, immune function, and selenoprotein gene expression of piglets raised under high ambient temperature

This research was conducted to evaluate the effects of selenium-enriched probiotics (SP) on growth performance, antioxidant status, immune function, and selenoprotein gene expression of piglets under natural high ambient temperature in summer. Forty-eight crossbred weanling piglets randomly allocated to four groups were fed for 42 days ad libitum a basal diet without (Con, 0.16 mg Se/kg) and with supplementation of probiotics (P, 0.16 mg Se/kg), sodium selenite (SS, 0.46 mg Se/kg), and SP (0.46 mg Se/kg). From each group, three piglets were randomly selected for blood collection on days 0, 14, 28, and 42 and tissue collection on day 42. The SP improved growth performance of piglets. Both SS and SP increased blood glutathione peroxidase activity and tissue thioredoxin reductase 1 mRNA expression, with SP being higher than SS. All P, SS, and SP supplementation increased the superoxide dismutase activity (40.1, 53.0, and 64.5%), glutathione content (84.6, 104, and 165%), TCR-induced T lymphocyte proliferation (20.8, 26.4, and 50.0%), and IL-2 concentration (24.9, 27.2, and 46.2%) and decreased malondialdehyde content (25.1, 26.3, and 49.3%), respectively. The greatest effects of SP supplementation suggest that SP may serve as a better feed additive than P or SS for piglets under high-temperature environments.

Selenium promotes T-cell response to TCR-stimulation and ConA, but not PHA in primary porcine splenocytes

There is controversy in the literature over whether the selenium (Se) influences cellular immune responses, and the mechanisms possibly underlying these effects are unclear. In this study, the effects of Se on T-cell proliferation and IL-2 production were studied in primary porcine splenocytes. Splenocytes were treated with different mitogens in the presence of 0.5-4 µmol/L sodium selenite. Se significantly promoted T-cell receptor (TCR) or concanavalin A (ConA)-induced T-cell proliferation and IL-2 production but failed to regulate T-cell response to phytohemagglutinin (PHA). In addition, Se significantly increased the levels of cytosolic glutathione peroxidase (GPx1) and thioredoxin reductase 1 (TR1) mRNA, the activity of GPx1 and the concentration of reduced glutathione (GSH) in the unstimulated, or activated splenocytes. These results indicated that Se improved the redox status in all splenocytes, including unstimulated, TCR, ConA and PHA -stimulated, but only TCR and ConA-induced T-cell activation was affected by the redox status. N-acetylcysteine (NAC), a pharmacological antioxidant, increased T-cell proliferation and IL-2 production by TCR and ConA stimulated splenocytes but had no effect on the response to PHA in primary porcine splenocytes confirming that PHA-induced T-cell activation is insensitive to the redox status. We conclude that Se promotes GPx1 and TR1 expression and increases antioxidative capacity in porcine splenocytes, which enhances TCR or ConA -induced T-cell activation but not PHA-induced T-cell activation. The different susceptibilities to Se between the TCR, ConA and PHA -induced T-cell activation may help to explain the controversy in the literature over whether or not Se boosts immune responses.

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.

From selenium to selenoproteins: synthesis, identity, and their role in human health

The requirement of the trace element selenium for life and its beneficial role in human health has been known for several decades. This is attributed to low molecular weight selenium compounds, as well as to its presence within at least 25 proteins, named selenoproteins, in the form of the amino acid selenocysteine (Sec). Incorporation of Sec into selenoproteins employs a unique mechanism that involves decoding of the UGA codon. This process requires multiple features such as the selenocysteine insertion sequence (SECIS) element and several protein factors including a specific elongation factor EFSec and the SECIS binding protein 2, SBP2. The function of most selenoproteins is currently unknown; however, thioredoxin reductases (TrxR), glutathione peroxidases (GPx) and thyroid hormone deiodinases (DIO) are well characterised selenoproteins involved in redox regulation of intracellular signalling, redox homeostasis and thyroid hormone metabolism. Recent evidence points to a role for selenium compounds as well as selenoproteins in the prevention of some forms of cancer. A number of clinical trials are either underway or being planned to examine the effects of selenium on cancer incidence. In this review we describe some of the recent progress in our understanding of the mechanism of selenoprotein synthesis, the role of selenoproteins in human health and disease and the therapeutic potential of some of these proteins.

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.

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.

A regulatory role for Sec tRNA[Ser]Sec in selenoprotein synthesis

Selenium is biologically active through the functions of selenoproteins that contain the amino acid selenocysteine. This amino acid is translated in response to in-frame UGA codons in mRNAs that include a SECIS element in its 3' untranslated region, and this process requires a unique tRNA, referred to as tRNA([Ser]Sec). The translation of UGA as selenocysteine, rather than its use as a termination signal, is a candidate restriction point for the regulation of selenoprotein synthesis by selenium. A specialized reporter construct was used that permits the evaluation of SECIS-directed UGA translation to examine mechanisms of the regulation of selenoprotein translation. Using SECIS elements from five different selenoprotein mRNAs, UGA translation was quantified in response to selenium supplementation and alterations in tRNA([Ser]Sec) levels and isoform distributions. Although each of the evaluated SECIS elements exhibited differences in their baseline activities, each was stimulated to a similar extent by increased selenium or tRNA([Ser]Sec) levels and was inhibited by diminished levels of the methylated isoform of tRNA([Ser]Sec) achieved using a dominant-negative acting mutant tRNA([Ser]Sec). tRNA([Ser]Sec) was found to be limiting for UGA translation under conditions of high selenoprotein mRNA in both a transient reporter assay and in cells with elevated GPx-1 mRNA. This and data indicating increased amounts of the methylated isoform of tRNA([Ser]Sec) during selenoprotein translation indicate that it is this isoform that is translationally active and that selenium-induced tRNA methylation is a mechanism of regulation of the synthesis of selenoproteins.

Microflora trigger colitis in mice deficient in selenium-dependent glutathione peroxidase and induce Gpx2 gene expression

Selenium-dependent glutathione peroxidase isoenzymes-1 and -2 are the major glutathione-dependent H2O2-reducing activities in the epithelium of the mid- to lower gastrointestinal tract. The two isoenzymes protect mice against ileocolitis. We have found that luminal microflora are required for colitis to develop in mice deficient in GPX-1 and GPX-2 activity (GPX-DKO). Within 7 days of association with microflora, previously asymptomatic germ-free GPX-DKO mice developed severe acute colitis while their littermates with at least one wild-type Gpx1 or Gpx2 gene remained virtually symptom-free. Microflora also affected Gpx2 gene expression. Gpx2, but not Gpx1, mRNA levels were elevated 4-5 fold in the ileum and colon in conventionally reared or microflora-associated adult mice compared with germ-free mice. Since the gastrointestinal tract microflora undergo major changes 2-3 weeks after birth, from relatively benign to a potentially stressful composition, we examined postnatal Gpx2 gene expression. The jejunal and ileal GPX-2 activity levels were low in two to three week-old mice and increased 5-7 fold during the next two weeks. GPX-2 activity levels were correlated with the mRNA levels. Colon Gpx2 mRNA levels held steady at about 50% of adult levels from 12-21 days of age but were several times higher than ileal levels. Our results suggest that ileal Gpx2 mRNA and GPX-2 activity levels are induced by luminal microflora. This response is consistent with a role for GPX as an anti-inflammatory activity.

Trends in selenium biochemistry

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