Reduced reliance on the trace element selenium during evolution of mammals

Evolution from fish to mammals was accompanied by decreased use of selenocysteine, raising questions about the need for selenium dietary supplements when pathology is not imminent

In silico identification of genes involved in selenium metabolism: evidence for a third selenium utilization trait

<p>Abstract</p> <p>Background</p> <p>Selenium (Se) is a trace element that occurs in proteins in the form of selenocysteine (Sec) and in tRNAs in the form of selenouridine (SeU). Selenophosphate synthetase (SelD) is required for both utilization traits. However, previous research also revealed SelDs in two organisms lacking Sec and SeU, suggesting a possible additional use of Se that is dependent on SelD.</p> <p>Results</p> <p>In this study, we conducted comparative genomics and phylogenetic analyses to characterize genes involved in Se utilization. Candidate genes identified included SelA/SelB and YbbB that define Sec and SeU pathways, respectively, and NADH oxidoreductase that is predicted to generate a SelD substrate. In addition, among 227 organisms containing SelD, 10 prokaryotes were identified that lacked SelA/SelB and YbbB. Investigation of <it>selD </it>neighboring genes in these organisms revealed a SirA-like protein and two hypothetical proteins HP1 and HP2 that were strongly linked to a novel Se utilization. With these new signature proteins, 32 bacteria and archaea were found that utilized these proteins, likely as part of the new Se utilization trait. Metabolic labeling of one organism containing an orphan SelD, <it>Enterococcus faecalis</it>, with ⁷⁵Se revealed a protein containing labile Se species that could be released by treatment with reducing agents, suggesting non-Sec utilization of Se in this organism.</p> <p>Conclusion</p> <p>These studies suggest the occurrence of a third Se utilization trait in bacteria and archaea.</p

Identification of Trace Element-Containing Proteins in Genomic Databases

Development of bioinformatics tools provided researchers with the ability to identify full sets of trace element–containing proteins in organisms for which complete genomic sequences are available. Recently, independent bioinformatics methods were used to identify all, or almost all, genes encoding selenocysteine-containing proteins in human, mouse, and Drosophila genomes, characterizing entire selenoproteomes in these organisms. It also should be possible to search for entire sets of other trace element–associated proteins, such as metal-containing proteins, although methods for their identification are still in development

Abrogated Thioredoxin System Causes Increased Sensitivity to TNF-α-Induced Apoptosis via Enrichment of p-ERK 1/2 in the Nucleus

Thioredoxin (Trx) and thioredoxin reductase 1 (TR1) are among the major redox regulators in mammalian cells and have a wide variety of roles, including removal of intracellular reactive oxygen species (ROS) and prevention of cell death. Tumor necrosis factor-α (TNF-α) induces cancer cell death. Although ROS have been proposed to participate in this process, the role of the thioredoxin system in TNF-α stimulated cell death remains unclear. We investigated the possibility that the thioredoxin system protects against TNF-α-induced cancer cell death by examining whether TR1/Trx1 status controls TNF-α-induced apoptosis in EMT6 murine breast cancer cells. TR1-deficient cells were more sensitive to TNF-α than control cells. Increased sensitivity to TNF-α was most pronounced in Trx1-deficient cells. TNF-α-induced nuclear localization of phosphorylated ERK 1/2 (p-ERK 1/2) correlated with increased apoptosis in TR1- and Trx1-deficient cells, suggesting a pro-apoptotic role for nuclear p-ERK 1/2 in TNF-α-induced apoptosis. In addition, phosphoinositide 3-kinase (PI3K) inhibition dramatically reduced TNF-α-stimulated apoptosis and nuclear localization of p-ERK 1/2. In contrast, inhibition of ROS, MEK, JNK, or p38 did not significantly alter p-ERK 1/2 localization or apoptosis in TR1- and Trx1-deficient cells compared to control cells. Further, NF-κB p65 localization was not changed in TR1- and Trx1-deficient cells in response to TNF-α relative to control cells. Our data suggest that the thioredoxin system plays a critical role in protecting against TNF-α-induced apoptosis by regulating the levels of nuclear p-ERK 1/2 in a PI3K-dependent manner

UGA codon position-dependent incorporation of selenocysteine into mammalian selenoproteins

It is thought that the SelenoCysteine Insertion Sequence (SECIS) element and UGA codon are sufficient for selenocysteine (Sec) insertion. However, we found that UGA supported Sec insertion only at its natural position or in its close proximity in mammalian thioredoxin reductase 1 (TR1). In contrast, Sec could be inserted at any tested position in mammalian TR3. Replacement of the 3′-UTR of TR3 with the corresponding segment of a Euplotes crassus TR restricted Sec insertion into the C-terminal region, whereas the 3′-UTR of TR3 conferred unrestricted Sec insertion into E. crassus TR, in which Sec insertion is normally limited to the C-terminal region. Exchanges of 3′-UTRs between mammalian TR1 and E. crassus TR had no effect, as both proteins restricted Sec insertion. We further found that these effects could be explained by the use of selenoprotein-specific SECIS elements. Examination of Sec insertion into other selenoproteins was consistent with this model. The data indicate that mammals evolved the ability to limit Sec insertion into natural positions within selenoproteins, but do so in a selenoprotein-specific manner, and that this process is controlled by the SECIS element in the 3′-UTR

Thioredoxin Reductase 1 Deficiency Reverses Tumor Phenotype and Tumorigenicity of Lung Carcinoma Cells

Dietary selenium has potent cancer prevention activity. Both low molecular weight selenocompounds and selenoproteins are implicated in this effect. Thioredoxin reductase 1 (TR1) is one of the major antioxidant and redox regulators in mammals that supports p53 function and other tumor suppressor activities. However, this selenium-containing oxidoreductase is also overexpressed in many malignant cells and has been proposed as a target for cancer therapy. To further assess the role of TR1 in the malignancy process, we used RNA interference technology to decrease its expression in mouse lung carcinoma (LLC1) cells. Stable transfection of LLC1 cells with a small interfering RNA construct that specifically targets TR1 removal manifested a reversal in the morphology and anchorage-independent growth properties of these cancer cells that made them similar to those of normal cells. The expression of at least two cancer-related protein mRNAs, Hgf and Opn1, were reduced dramatically in the TR1 knockdown cells. Mice injected with the TR1 knockdown showed a dramatic reduction in tumor progression and metastasis compared with those mice injected with the corresponding control vector. In addition, tumors that arose from injected TR1 knockdown cells lost the targeting construct, suggesting that TR1 is essential for tumor growth in mice. These observations provide direct evidence that the reduction of TR1 levels in malignant cells is antitumorigenic and suggest that the enzyme is a prime target for cancer therapy

Supporting Information for: A highly efficient form of the selenocysteine insertion sequence element in protozoan parasites and its use in mammalian cells

Selenoproteins are an elite group of proteins containing a rare amino acid, selenocysteine (Sec), encoded by the codon, UGA. In eukaryotes, incorporation of Sec requires a Sec insertion sequence (SECIS) element, a stem–loop structure located in the 3\u27-untranslated regions of selenoprotein mRNAs. Here we report identification of a noncanonical form of SECIS element in Toxoplasma gondii and Neospora canine, single-celled apicomplexan parasites of humans and domestic animals. This SECIS has a GGGA sequence in the SBP2-binding site in place of AUGA previously considered invariant. Using a combination of computational and molecular techniques, we show that Toxoplasma and Neospora possess both canonical and noncanonical SECIS elements. The GGGA-type SECIS element supported Sec insertion in mammalian HEK 293 and NIH 3T3 cells and did so more efficiently than the natural mammalian SECIS elements tested. In addition, mammalian type I and type II SECIS elements mutated into the GGGA forms were functional but manifested decreased Sec insertion efficiency. We carried out computational searches for both AUGA and GGGA forms of SECIS elements in Toxoplasma and detected five selenoprotein genes, including one coding for a previously undescribed selenoprotein, designated SelQ, and two containing the GGGA form of the SECIS element. In contrast, the GGGA-type SECIS elements were not detected in mammals and nematodes. As a practical outcome of the study, we developed pSelExpress1, a vector for convenient expression of selenoproteins in mammalian cells. It contains an SBP2 gene and the most efficient tested SECIS element: an AUGA mutant of the GGGA-type Toxoplasma SelT structure

Evolutionary dynamics of eukaryotic selenoproteomes: large selenoproteomes may associate with aquatic life and small with terrestrial life

In silico and metabolic labeling studies of the selenoproteomes of several eukaryotes revealed distinct selenoprotein patterns as well as an ancient origin of selenoproteins and massive, independent losses in land plants, fungi, nematodes, insects and some protists, suggesting that the environment plays an important role in selenoproteome evolution

Targeting Thioredoxin Reductase 1 Reduction in Cancer Cells Inhibits Self-Sufficient Growth and DNA Replication

Thioredoxin reductase 1 (TR1) is a major redox regulator in mammalian cells. As an important antioxidant selenoprotein, TR1 is thought to participate in cancer prevention, but is also known to be over-expressed in many cancer cells. Numerous cancer drugs inhibit TR1, and this protein has been proposed as a target for cancer therapy. We previously reported that reduction of TR1 levels in cancer cells reversed many malignant characteristics suggesting that deficiency in TR1 function is antitumorigenic. The molecular basis for TR1's role in cancer development, however, is not understood. Herein, we found that, among selenoproteins, TR1 is uniquely overexpressed in cancer cells and its knockdown in a mouse cancer cell line driven by oncogenic k-ras resulted in morphological changes characteristic of parental (normal) cells, without significant effect on cell growth under normal growth conditions. When grown in serum-deficient medium, TR1 deficient cancer cells lose self-sufficiency of growth, manifest a defective progression in their S phase and a decreased expression of DNA polymerase α, an enzyme important in DNA replication. These observations provide evidence that TR1 is critical for self-sufficiency in growth signals of malignant cells, that TR1 acts largely as a pro-cancer protein and it is indeed a primary target in cancer therapy

Comparative Analysis of Selenocysteine Machinery and Selenoproteome Gene Expression in Mouse Brain Identifies Neurons as Key Functional Sites of Selenium in Mammals

Although dietary selenium (Se) deficiency results in phenotypes associated with selenoprotein depletion in various organs, the brain is protected from Se loss. To address the basis for the critical role of Se in brain function, we carried out comparative gene expression analyses for the complete selenoproteome and associated biosynthetic factors. Using the Allen Brain Atlas, we evaluated 159 regions of adult mouse brain and provided experimental analyses of selected selenoproteins. All 24 selenoprotein mRNAs were expressed in the mouse brain. Most strikingly, neurons in olfactory bulb, hippocampus, cerebral cortex, and cerebellar cortex were exceptionally rich in selenoprotein gene expression, in particular in GPx4, SelK, SelM, SelW, and Sep15. Over half of the selenoprotein genes were also expressed in the choroid plexus. A unique expression pattern was observed for one of the highly expressed selenoprotein genes, SelP, which we suggest to provide neurons with Se. Cluster analysis of the expression data linked certain selenoproteins and selenocysteine machinery genes and suggested functional linkages among selenoproteins, such as that between SelM and Sep15. Overall, this study suggests that the main functions of selenium in mammals are confined to certain neurons in the brain