A SECIS binding protein (SBP) is distinct from selenocysteyl-tRNA protecting factor (SePF)

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.

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.

Selenoprotein synthesis: UGA does not end the story

It is well established that the beneficial effects of the trace element selenium are mediated by its major biological product, the amino acid selenocysteine, present in the active site of selenoproteins. These fulfill different functions, as varied as oxidation-reduction of metabolites in bacteria, reduction of reactive oxygen species, control of the redox status of the cell or thyroid hormone maturation. This review will focus on the singularities of the selenocysteine biosynthesis pathway and its unique incorporation mechanism into eukaryal selenoproteins. Selenocysteine biosynthesis from serine is achieved on tRNA(Sec) and requires four proteins. As this amino acid is encoded by an in-frame UGA codon, otherwise signaling termination of translation, ribosomes must be told not to stop at this position in the mRNA. Several molecular partners acting in cis or in trans have been identified, but their knowledge has not enabled yet to firmly establish the molecular events underlying this mechanism. Data suggest that other, so far uncharacterized factors might exist. In this survey, we attempted to compile all the data available in the literature and to describe the latest developments in the field.

Proximal HNF1 element is essential for the induction of human UDP-glucuronosyltransferase 1A1 by glucocorticoid receptor

Previous study showed noinduction of the reporter gene (-3174/+14) of UGT1A1 in HepG2 by bilirubin, but induction by dexamethasone (DEX). This induction was enhanced seven-fold by the co-expression of human glucocorticoid receptor (GR) and was inhibited by a GR antagonist, RU486, indicating stimulation by DEX-GR. Meanwhile, we could not detect stimulation by beta-estradiol, phenobarbital or rifampicin (RIF) in the presence of GR. We investigated the position playing a role in this induction by GR in the promoter region of UGT1A1 using deletion mutants, and clarified the essential sequence (-75/-63) for the binding site of hepatocyte nuclear factor 1 (HNF1). However, GR did not bind directly to this sequence, because UGT-PE2 did not compete for binding to a glucocorticoid responsive element (GRE) probe in an electrophoretic mobility shift assay (EMSA) method. Labeled [(32)P]DNA probe of HNF1 binds with nuclear extracts as shown by the EMSA. This shift of the complex of probe-protein was not inhibited by unlabeled GRE but was inhibited by unlabeled HNF1 element. This shift was not influenced by the addition of anti-GR, but was super-shifted by the addition of anti-HNF1. GR did not stimulate the induction of HNF1, because we detected no-elevation of the mRNA level of HNF1 by reverse transcription-polymerase chain reaction (RT-PCR). Therefore, the induction of UGT1A1 by DEX-GR did not depend on the elevation of HNF1 but on the interaction of GR with HNF1 or the activation of HNF1 through the transcription of other proteins. Also given the lack of evidence of binding of DEX-GR to HNF1 in the EMSA, the data suggest that the mechanism of DEX-GRE effect on HNF1 is indirect by whatever mechanisms.

Selenium, the thyroid, and the endocrine system

Recent identification of new selenocysteine-containing proteins has revealed relationships between the two trace elements selenium (Se) and iodine and the hormone network. Several selenoproteins participate in the protection of thyrocytes from damage by H(2)O(2) produced for thyroid hormone biosynthesis. Iodothyronine deiodinases are selenoproteins contributing to systemic or local thyroid hormone homeostasis. The Se content in endocrine tissues (thyroid, adrenals, pituitary, testes, ovary) is higher than in many other organs. Nutritional Se depletion results in retention, whereas Se repletion is followed by a rapid accumulation of Se in endocrine tissues, reproductive organs, and the brain. Selenoproteins such as thioredoxin reductases constitute the link between the Se metabolism and the regulation of transcription by redox sensitive ligand-modulated nuclear hormone receptors. Hormones and growth factors regulate the expression of selenoproteins and, conversely, Se supply modulates hormone actions. Selenoproteins are involved in bone metabolism as well as functions of the endocrine pancreas and adrenal glands. Furthermore, spermatogenesis depends on adequate Se supply, whereas Se excess may impair ovarian function. Comparative analysis of the genomes of several life forms reveals that higher mammals contain a limited number of identical genes encoding newly detected selenocysteine-containing proteins.

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.

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.

Stability of gastrointestinal glutathione peroxidase mRNA in selenium deficiency depends on its 3'UTR

Selenoproteins decrease upon selenium-deprivation according to their hierarchical ranking. Whereas classical glutathione peroxidase (cGPx) responds to decreased selenium supply with a complete loss of protein and a marked reduction of mRNA levels, gastrointestinal glutathione peroxidase (GI-GPx) remains detectable and its mRNA is stable. The impact of the 3'UTR on cGPx and GI-GPx mRNA stability was studied in stably transfected HepG2 cells with combinations of mutually exchanged coding regions and 3'UTRs of human cGPx and GI-GPx. Stability of chimeric mRNAs was measured by competitive RT-PCR. We found that GI-GPx 3'UTR is sufficient to stabilize its own mRNA but not that of cGPx.

A model for Sec incorporation with the regions upstream of the UGA Sec codon to play a key role

For eukaryotic selenoprotein mRNAs, it has been proposed that the SECIS element in the 3'-UTR is required for recognition of UGA as a Sec codon. Some proteins which bind to SECIS (SBP) have been reported. However, it is not clear how the SECIS element in the 3'-UTR can mediate Sec insertion far at the in-frame UGA Sec codons. The idea that there must be a signal near the UGA Sec codon is still being considered. Therefore, we searched for a protein which binds to an RNA sequence surrounding the UGA Sec codon on human GPx mRNA. We found a protein, prepared from bovine brain microsomes, which strongly bound to the RNA fragment upstream of the UGA Sec codon but not to the RNA sequence downstream of the UGA codon. This protein also bound to the SECIS sequence in the 3'-UTR of human GPx, and this binding to SECIS was competed with the RNA fragment upstream of the UGA Sec codon. We also obtained the similar results with the RNA fragments of type I iodothyronine 5'-deiodinase (5'DI) mRNAs. Comparison of such RNA fragments with SECIS fragments revealed similarities in the region upstream of the in-frame UGA Sec codon of several Se-protein mRNAs. The study thus favors a novel model of Sec incorporation at the UGA Sec codon that involves the regions upstream of the UGA codon of mRNAs of mammalian selenoproteins. This model explains that the stem-loop structure covering the UGA codon is recognized by SBP and how the UGA Sec codon escapes from attack by eRF.

Characterization of mSelB, a novel mammalian elongation factor for selenoprotein translation

Decoding of UGA selenocysteine codons in eubacteria is mediated by the specialized elongation factor SelB, which conveys the charged tRNA(Sec) to the A site of the ribosome, through binding to the SECIS mRNA hairpin. In an attempt to isolate the eukaryotic homolog of SelB, a database search in this work identified a mouse expressed sequence tag containing the complete cDNA encoding a novel protein of 583 amino acids, which we called mSelB. Several lines of evidence enabled us to establish that mSelB is the bona fide mammalian elongation factor for selenoprotein translation: it binds GTP, recognizes the Sec-tRNA(Sec) in vitro and in vivo, and is required for efficient selenoprotein translation in vivo. In contrast to the eubacterial SelB, the recombinant mSelB alone is unable to bind specifically the eukaryotic SECIS RNA hairpin. However, complementation with HeLa cell extracts led to the formation of a SECIS-dependent complex containing mSelB and at least another factor. Therefore, the role carried out by a single elongation factor in eubacterial selenoprotein translation is devoted to two or more specialized proteins in eukaryotes.

Structural analysis of new local features in SECIS RNA hairpins

Decoding of the UGA selenocysteine codon for selenoprotein translation requires the SECIS element, a stem-loop motif in the 3'-UTR of the mRNA carrying short or large apical loops. In previous structural studies, we derived a secondary structure model for SECIS RNAs with short apical loops. Work from others proposed that intra-apical loop base pairing can occur in those SECIS that possess large apical loops, yielding form 2 SECIS versus the form 1 with short loops. In this work, SECIS elements arising from eight different selenoprotein mRNAs were assayed by enzymatic and/or chemical probing showing that seven can adopt form 2. Further, database searches led to the discovery in drosophila and zebrafish of SECIS elements in the selenophosphate synthetase 2, type 1 deiodinase and SelW mRNAs. Alignment of SECIS sequences not only highlighted the predominance of form 2 but also made it possible to classify the SECIS elements according to the type of selenoprotein mRNA they belong to. Interestingly, the alignment revealed that an unpaired adenine, previously thought to be invariant, is replaced by a guanine in four SECIS elements. Tested in vivo, neither the A to G nor the A to U changes at this position greatly affected the activity while the most detrimental effect was provided by a C. The putative contribution of the various SECIS motifs to function and ligand binding is discussed.

Recognition and binding of the human selenocysteine insertion sequence by nucleolin

Prokaryotic and eukaryotic cells cotranslationally incorporate the unusual amino acid selenocysteine at a UGA codon, which conventionally serves as a termination signal. Translation of selenoprotein gene transcripts in eukaryotes depends upon a "selenocysteine insertion sequence" in the 3'-untranslated region. We have previously shown that DNA-binding protein B specifically binds this sequence element. We now report the identification of nucleolin as a partner in the selenoprotein translation complex. In RNA electromobility shift assays, nucleolin binds the selenocysteine insertion sequence from the human cellular glutathione peroxidase gene, competes with binding activity from COS cells, and shows diminished affinity for probes with mutations in functionally important, conserved sequence elements. Antibody to nucleolin interferes with the gel shift activity of COS cell extract. Antibody to DNA-binding protein B co-extracts nucleolin from HeLa cell cytosol, and the two proteins co-sediment in glycerol gradient fractions of ribosomal high salt extracts. Thus, nucleolin appears to join DNA-binding protein B and possibly other partners to form a large complex that links the selenocysteine insertion sequence in the 3'-untranslated region to other elements in the coding region and ribosome to translate the UGA "stop" codon as selenocysteine.

The many routes of bacterial transfer RNAs after aminoacylation

Subsequent to their aminoacylation, tRNAs are subject to specific maturation and/or correction processes. Aminoacylated tRNAs ready for use in translation are then specifically channelled to the ribosomal A or P sites. Structural and biochemical studies have opened the way towards furthering our understanding of these routes to the ribosome, which involve a strict distinction between initiator and elongator tRNAs.