Selenoprotein expression is essential in endothelial cell development and cardiac muscle function

Epitranscriptomic Reprogramming Is Required to Prevent Stress and Damage from Acetaminophen

Epitranscriptomic marks, in the form of enzyme catalyzed RNA modifications, play important gene regulatory roles in response to environmental and physiological conditions. However, little is known with respect to how acute toxic doses of pharmaceuticals influence the epitranscriptome. Here we define how acetaminophen (APAP) induces epitranscriptomic reprogramming and how the writer Alkylation Repair Homolog 8 (Alkbh8) plays a key gene regulatory role in the response. Alkbh8 modifies tRNA selenocysteine (tRNA^Sec) to translationally regulate the production of glutathione peroxidases (Gpx’s) and other selenoproteins, with Gpx enzymes known to play protective roles during APAP toxicity. We demonstrate that APAP increases toxicity and markers of damage, and decreases selenoprotein levels in Alkbh8 deficient mouse livers, when compared to wildtype. APAP also promotes large scale reprogramming of many RNA marks comprising the liver tRNA epitranscriptome including: 5-methoxycarbonylmethyluridine (mcm⁵U), isopentenyladenosine (i⁶A), pseudouridine (Ψ), and 1-methyladenosine (m¹A) modifications linked to tRNA^Sec and many other tRNA’s. Alkbh8 deficiency also leads to wide-spread epitranscriptomic dysregulation in response to APAP, demonstrating that a single writer defect can promote downstream changes to a large spectrum of RNA modifications. Our study highlights the importance of RNA modifications and translational responses to APAP, identifies writers as key modulators of stress responses in vivo and supports the idea that the epitranscriptome may play important roles in responses to pharmaceuticals.

Historical Roles of Selenium and Selenoproteins in Health and Development: The Good, the Bad and the Ugly

Selenium is a fascinating element that has a long history, most of which documents it as a deleterious element to health. In more recent years, selenium has been found to be an essential element in the diet of humans, all other mammals, and many other life forms. It has many health benefits that include, for example, roles in preventing heart disease and certain forms of cancer, slowing AIDS progression in HIV patients, supporting male reproduction, inhibiting viral expression, and boosting the immune system, and it also plays essential roles in mammalian development. Elucidating the molecular biology of selenium over the past 40 years generated an entirely new field of science which encompassed the many novel features of selenium. These features were (1) how this element makes its way into protein as the 21st amino acid in the genetic code, selenocysteine (Sec); (2) the vast amount of machinery dedicated to synthesizing Sec uniquely on its tRNA; (3) the incorporation of Sec into protein; and (4) the roles of the resulting Sec-containing proteins (selenoproteins) in health and development. One of the research areas receiving the most attention regarding selenium in health has been its role in cancer prevention, but further research has also exposed the role of this element as a facilitator of various maladies, including cancer.

Selenium, a Micronutrient That Modulates Cardiovascular Health via Redox Enzymology

Selenium (Se) is a trace nutrient that promotes human health through its incorporation into selenoproteins in the form of the redox-active amino acid selenocysteine (Sec). There are 25 selenoproteins in humans, and many of them play essential roles in the protection against oxidative stress. Selenoproteins, such as glutathione peroxidase and thioredoxin reductase, play an important role in the reduction of hydrogen and lipid hydroperoxides, and regulate the redox status of Cys in proteins. Emerging evidence suggests a role for endoplasmic reticulum selenoproteins, such as selenoproteins K, S, and T, in mediating redox homeostasis, protein modifications, and endoplasmic reticulum stress. Selenoprotein P, which functions as a carrier of Se to tissues, also participates in regulating cellular reactive oxygen species. Cellular reactive oxygen species are essential for regulating cell growth and proliferation, protein folding, and normal mitochondrial function, but their excess causes cell damage and mitochondrial dysfunction, and promotes inflammatory responses. Experimental evidence indicates a role for individual selenoproteins in cardiovascular diseases, primarily by modulating the damaging effects of reactive oxygen species. This review examines the roles that selenoproteins play in regulating vascular and cardiac function in health and disease, highlighting their antioxidant and redox actions in these processes.

Progress in the emerging role of selenoproteins in cardiovascular disease: focus on endoplasmic reticulum-resident selenoproteins

Cardiovascular diseases represent one of the most important health problems of developed countries. One of the main actors involved in the onset and development of cardiovascular diseases is the increased production of reactive oxygen species that, through lipid peroxidation, protein oxidation and DNA damage, induce oxidative stress and cell death. Basic and clinical research are ongoing to better understand the endogenous antioxidant mechanisms that counteract oxidative stress, which may allow to identify a possible therapeutic targeting/application in the field of stress-dependent cardiovascular pathologies. In this context, increasing attention is paid to the glutathione/glutathione-peroxidase and to the thioredoxin/thioredoxin-reductase systems, among the most potent endogenous antioxidative systems. These key enzymes, belonging to the selenoprotein family, have a well-established function in the regulation of the oxidative cell balance. The aim of the present review was to highlight the role of selenoproteins in cardiovascular diseases, introducing the emerging cardioprotective role of endoplasmic reticulum-resident members and in particular one of them, namely selenoprotein T or SELENOT. Accumulating evidence indicates that the dysfunction of different selenoproteins is involved in the susceptibility to oxidative stress and its associated cardiovascular alterations, such as congestive heart failure, coronary diseases, impaired cardiac structure and function. Some of them are under investigation as useful pathological biomarkers. In addition, SELENOT exhibited intriguing cardioprotective effects by reducing the cardiac ischemic damage, in terms of infarct size and performance. In conclusion, selenoproteins could represent valuable targets to treat and diagnose cardiovascular diseases secondary to oxidative stress, opening a new avenue in the field of related therapeutic strategies.

Selenoproteins in Tumorigenesis and Cancer Progression

Selenium is a micronutrient essential to human health and has long been associated with cancer prevention. Functionally, these effects are thought to be mediated by a class of selenium-containing proteins known as selenoproteins. Indeed, many selenoproteins have antioxidant activity which can attenuate cancer development by minimizing oxidative insult and resultant DNA damage. However, oxidative stress is increasingly being recognized for its "double-edged sword" effect in tumorigenesis, whereby it can mediate both negative and positive effects on tumor growth depending on the cellular context. In addition to their roles in redox homeostasis, recent work has also implicated selenoproteins in key oncogenic and tumor-suppressive pathways. Together, these data suggest that the overall contribution of selenoproteins to tumorigenesis is complicated and may be affected by a variety of factors. In this review, we discuss what is currently known about selenoproteins in tumorigenesis with a focus on their contextual roles in cancer development, growth, and progression.

Selenoprotein H is an essential regulator of redox homeostasis that cooperates with p53 in development and tumorigenesis

Selenium, an essential micronutrient known for its cancer prevention properties, is incorporated into a class of selenocysteine-containing proteins (selenoproteins). Selenoprotein H (SepH) is a recently identified nucleolar oxidoreductase whose function is not well understood. Here we report that seph is an essential gene regulating organ development in zebrafish. Metabolite profiling by targeted LC-MS/MS demonstrated that SepH deficiency impairs redox balance by reducing the levels of ascorbate and methionine, while increasing methionine sulfoxide. Transcriptome analysis revealed that SepH deficiency induces an inflammatory response and activates the p53 pathway. Consequently, loss of seph renders larvae susceptible to oxidative stress and DNA damage. Finally, we demonstrate that seph interacts with p53 deficiency in adulthood to accelerate gastrointestinal tumor development. Overall, our findings establish that seph regulates redox homeostasis and suppresses DNA damage. We hypothesize that SepH deficiency may contribute to the increased cancer risk observed in cohorts with low selenium levels.

Selenoproteins and oxidative stress-induced inflammatory tumorigenesis in the gut

Selenium is an essential micronutrient that is incorporated into at least 25 selenoproteins encoded by the human genome, many of which serve antioxidant functions. Because patients with inflammatory bowel disease (IBD) demonstrate nutritional deficiencies and are at increased risk for colon cancer due to heightened inflammation and oxidative stress, selenoprotein dysfunction may contribute to disease progression. Over the years, numerous studies have analyzed the effects of selenoprotein loss and shown that they are important mediators of intestinal inflammation and carcinogenesis. In particular, recent work has focused on the role of selenoprotein P (SEPP1), a major selenium transport protein which also has endogenous antioxidant function. These experiments determined SEPP1 loss altered immune and epithelial cellular function in a murine model of colitis-associated carcinoma. Here, we discuss the current knowledge of SEPP1 and selenoprotein function in the setting of IBD, colitis, and inflammatory tumorigenesis.

Paradoxical Roles of Antioxidant Enzymes: Basic Mechanisms and Health Implications

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are generated from aerobic metabolism, as a result of accidental electron leakage as well as regulated enzymatic processes. Because ROS/RNS can induce oxidative injury and act in redox signaling, enzymes metabolizing them will inherently promote either health or disease, depending on the physiological context. It is thus misleading to consider conventionally called antioxidant enzymes to be largely, if not exclusively, health protective. Because such a notion is nonetheless common, we herein attempt to rationalize why this simplistic view should be avoided. First we give an updated summary of physiological phenotypes triggered in mouse models of overexpression or knockout of major antioxidant enzymes. Subsequently, we focus on a series of striking cases that demonstrate "paradoxical" outcomes, i.e., increased fitness upon deletion of antioxidant enzymes or disease triggered by their overexpression. We elaborate mechanisms by which these phenotypes are mediated via chemical, biological, and metabolic interactions of the antioxidant enzymes with their substrates, downstream events, and cellular context. Furthermore, we propose that novel treatments of antioxidant enzyme-related human diseases may be enabled by deliberate targeting of dual roles of the pertaining enzymes. We also discuss the potential of "antioxidant" nutrients and phytochemicals, via regulating the expression or function of antioxidant enzymes, in preventing, treating, or aggravating chronic diseases. We conclude that "paradoxical" roles of antioxidant enzymes in physiology, health, and disease derive from sophisticated molecular mechanisms of redox biology and metabolic homeostasis. Simply viewing antioxidant enzymes as always being beneficial is not only conceptually misleading but also clinically hazardous if such notions underpin medical treatment protocols based on modulation of redox pathways.

Selenium and selenocysteine: roles in cancer, health, and development

The many biological and biomedical effects of selenium are relatively unknown outside the selenium field. This fascinating element, initially described as a toxin, was subsequently shown to be essential for health and development. By the mid-1990s selenium emerged as one of the most promising cancer chemopreventive agents, but subsequent human clinical trials yielded contradictory results. However, basic research on selenium continued to move at a rapid pace, elucidating its many roles in health, development, and in cancer prevention and promotion. Dietary selenium acts principally through selenoproteins, most of which are oxidoreductases involved in diverse cellular functions.

Regulation of inflammation by selenium and selenoproteins: impact on eicosanoid biosynthesis

Uncontrolled inflammation is a contributing factor to many leading causes of human morbidity and mortality including atherosclerosis, cancer and diabetes. Se is an essential nutrient in the mammalian diet that has some anti-inflammatory properties and, at sufficient amounts in the diet, has been shown to be protective in various inflammatory-based disease models. More recently, Se has been shown to alter the expression of eicosanoids that orchestrate the initiation, magnitude and resolution of inflammation. Many of the health benefits of Se are thought to be due to antioxidant and redox-regulating properties of certain selenoproteins. The present review will discuss the existing evidence that supports the concept that optimal Se intake can mitigate dysfunctional inflammatory responses, in part, through the regulation of eicosanoid metabolism. The ability of selenoproteins to alter the biosynthesis of eicosanoids by reducing oxidative stress and/or by modifying redox-regulated signalling pathways also will be discussed. Based on the current literature, however, it is clear that more research is necessary to uncover the specific beneficial mechanisms behind the anti-inflammatory properties of selenoproteins and other Se metabolites, especially as related to eicosanoid biosynthesis. A better understanding of the mechanisms involved in Se-mediated regulation of host inflammatory responses may lead to the development of dietary intervention strategies that take optimal advantage of its biological potency.

Understanding selenoprotein function and regulation through the use of rodent models

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

Reproductive function of Selenoprotein M in Chinese mitten crabs (Eriocheir sinesis)

Selenoproteins are present in all major forms of life, including eukaryotes, bacteria and archaea. In eukaryotic animals, selenoproteins often function as antioxidants, but rare or absent in other phyla, such as plants and fungi (except for the green alga Chlamydomonas). Selenoprotein M (SelM) is a selenocysteine containing protein with redox activity, which is involved in the antioxidant response. However, information remains limited about SelM physiology and function in marine invertebrates, particularly in crustaceans. Hence, we investigated the reproductive functionality of SelM in the Chinese mitten crab (Eriocheir sinensis), which is a commercially important yet disease vulnerable aquaculture species. The full-length SelM cDNA (928bp) strand was cloned by using PCR, based on an initial expressed sequence tag (EST) that was isolated from a hepatopancreatic cDNA library. The SelM cDNA contained a 390bp open reading frame (ORF) that encoded a putative 129 amino acid (aa) protein. SelM mRNA expression in E. sinensis was (a) tissue-specific, with the highest expression observed in the hepatopancreas, testis, ovaries and intestines. Based on this information, we then detected the different stages of tissue expression for SelM in the testis, ovary, and male crab hepatopancreas and hemolymph, and the enzyme activity of SelM in the testis. Overall, SelM was isolated successfully from the Chinese mitten crab, and its involvement in the regulation of reproduction during the period of rapid development in E. sinensis was confirmed.

The Roles of Glutathione Peroxidases during Embryo Development

Embryo development relies on the complex interplay of the basic cellular processes including proliferation, differentiation, and apoptotic cell death. Precise regulation of these events is the basis for the establishment of embryonic structures and the organ development. Beginning with fertilization of the oocyte until delivery the developing embryo encounters changing environmental conditions such as varying levels of oxygen, which can give rise to reactive oxygen species (ROS). These challenges are met by the embryo with metabolic adaptations and by an array of anti-oxidative mechanisms. ROS can be deleterious by modifying biological molecules including lipids, proteins, and nucleic acids and may induce abnormal development or even embryonic lethality. On the other hand ROS are vital players of various signaling cascades that affect the balance between cell growth, differentiation, and death. An imbalance or dysregulation of these biological processes may generate cells with abnormal growth and is therefore potentially teratogenic and tumorigenic. Thus, a precise balance between processes generating ROS and those decomposing ROS is critical for normal embryo development. One tier of the cellular protective system against ROS constitutes the family of selenium-dependent glutathione peroxidases (GPx). These enzymes reduce hydroperoxides to the corresponding alcohols at the expense of reduced glutathione. Of special interest within this protein family is the moonlighting enzyme glutathione peroxidase 4 (Gpx4). This enzyme is a scavenger of lipophilic hydroperoxides on one hand, but on the other hand can be transformed into an enzymatically inactive cellular structural component. GPx4 deficiency - in contrast to all other GPx family members - leads to abnormal embryo development and finally produces a lethal phenotype in mice. This review is aimed at summarizing the current knowledge on GPx isoforms during embryo development and tumor development with an emphasis on GPx4.

Effect of dietary selenium on the progression of heart failure in the ageing spontaneously hypertensive rat

Oxidative stress has been directly implicated in hypertension and myocardial remodelling, two pathologies fundamental to the development of chronic heart failure. Selenium (Se) can act directly and indirectly as an antioxidant and a lowered Se status leads to a higher risk of cardiovascular disease. This study examined the role of Se on the development of hypertension and subsequent progression to chronic heart failure in spontaneously hypertensive rats (SHR). Three dietary groups were studied: (i) Se-free; (ii) normal Se (50 μg Se/kg food); and (iii) high Se (1000 μg Se/kg food). Systolic blood pressure and echocardiography were used to detect cardiac changes in vivo. At study end, cardiac tissues were assayed for glutathione peroxidase activity, thioredoxin reductase activity, and protein carbonyls. The major finding of this study was the high heart failure-related mortality rate in SHRs fed an Se-free diet (70%). Normal and high levels of dietary Se resulted in higher survival rates of 78 and 100%, respectively. Furthermore, high dietary Se was clearly associated with lower levels of cardiac oxidative damage and increased antioxidant expression, as well as a reduction in disease severity and mortality in the SHR.

Unveiling the molecular mechanisms behind selenium-related diseases through knockout mouse studies

Selenium (Se), in the form of the 21st amino acid selenocysteine, is an integral part of selenoproteins and essential for mammals. While a large number of health claims for Se has been proposed in a diverse set of diseases, little is known about the precise molecular mechanisms and the physiological roles of selenoproteins. With the recent and rigorous application of reverse genetics in the mouse, great strides have been made to address this on a more molecular level. In this review, we focus on results obtained from the application of mouse molecular genetics in mouse physiology and discuss these insights into the physiological actions of selenoproteins in light of evidence from human genetics.

New Strategy of Functional Analysis of PHGPx Knockout Mice Model Using Transgenic Rescue Method and Cre-LoxP System

Phospholipid hydroperoxide glutathione peroxidase (PHGPx) is an intracellular antioxidant enzyme that directly reduces peroxidized phospholipids. PHGPx is transcribed from one gene into three types of mRNA, mitochondrial, non-mitochondrial and nucleolar PHGPx by alternative transcription. In this review, we focus on our recent experiments on the regulation of promoter activity of the types of PHGPx and on the novel strategy of functional analysis of a PHGPx knockout mice model using the transgenic rescue method and Cre-LoxP system. PHGPx is especially high in testis and spermatozoa. A deficiency is implicated in human infertility. We established spermatocyte-specific PHGPx knockout (KO) mice using a Cre-loxP system. Targeted disruption of all exons of the PHGPx gene in mice by homologous recombination caused embryonic lethality at 7.5 days post coitum. The PHGPx-loxP transgene rescued PHGPx KO mice from embryonic lethality. These rescued floxed PHGPx mice were mated with spermatocyte specific Cre expressing mice. All the spermatocyte-specific PHGPx KO male mice were infertile and displayed a significant decrease in the number of spermatozoa and significant reductions in forward motility by mitochondrial dysfunction of spermatozoa. These results demonstrate that depletion of PHGPx in spermatozoa may be one of the causes of male infertility in mice and humans.

The construction of a cDNA library enriched for immune genes and the analysis of 7535 ESTs from Chinese mitten crab Eriocheir sinensis

Chinese mitten crab Eriocheir sinensis is one of the most important aquaculture crustacean species in China. A cDNA library was constructed from hemocytes of E. sinensis challenged with the mixture of Listonella anguillarum and Staphylococcus aureus, and randomly sequenced to collect genomic information and identify genes involved in immune defense response. Single-pass 5' sequencing of 10368 clones yielded 7535 high quality ESTs (Expressed Sequence Tags) and these ESTs were assembled into 2943 unigenes. BLAST analysis revealed that 1706 unigenes (58.0% of the total) or 4593 ESTs (61.0% of the total) were novel genes that had no significant matches to any protein sequences in the public databases. The rest 1237 unigenes (42.0% of the total) were closely matched to the known genes or sequences deposited in public databases, which could be classed into 20 or 23 classifications according to "molecular function" or "biological process" respectively based on the Gene Ontology (GO). And 221 unigenes (7.5% of all 2943 unigenes, 17.9% of matched unigenes) or 969 ESTs (12.9% of all 7535 ESTs, 32.9% of matched ESTs) were identified to be immune genes. The relative higher proportion of immune-related genes in the present cDNA library than that in the normal library of E. sinensis and other crustaceans libraries, and the differences and changes in percentage and quantity of some key immune-related genes especially the immune inducible genes between two E. sinensis cDNA libraries may derive from the bacteria challenge to the Chinese mitten crab. The results provided a well-characterized EST resource for the genomics community, gene discovery especially for the identification of host-defense genes and pathways in crabs as well as other crustaceans.

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.

The selenocysteine tRNA STAF-binding region is essential for adequate selenocysteine tRNA status, selenoprotein expression and early age survival of mice

STAF [Sec (selenocysteine) tRNA gene transcription activating factor] is a transcription activating factor for a number of RNA Pol III- and RNA Pol II-dependent genes including the Trsp [Sec tRNA gene], which in turn controls the expression of all selenoproteins. Here, the role of STAF in regulating expression of Sec tRNA and selenoproteins was examined. We generated transgenic mice expressing the Trsp transgene lacking the STAF-binding site and made these mice dependent on the transgene for survival by removing the wild-type Trsp. The level of Sec tRNA was unaffected or slightly elevated in heart and testis, but reduced approximately 60% in liver and kidney, approximately 70% in lung and spleen and approximately 80% in brain and muscle compared with the corresponding organs in control mice. Moreover, the ratio of the two isoforms of Sec tRNA that differ by methylation at position 34 (Um34) was altered significantly, and the Um34-containing form was substantially reduced in all tissues examined. Selenoprotein expression in these animals was most affected in tissues in which the Sec tRNA levels were most severely reduced. Importantly, mice had a neurological phenotype strikingly similar to that of mice in which the selenoprotein P gene had been removed and their life span was substantially reduced. The results indicate that STAF influences selenoprotein expression by enhancing Trsp synthesis in an organ-specific manner and by controlling Sec tRNA modification in each tissue examined.

The phenotype and long-term follow-up in 11 patients with juvenile selenoprotein N1-related myopathy

The selenoprotein N1-related myopathies comprise rigid spine muscular dystrophy, the "classical" form of multiminicore disease, a desmin-related myopathy with Mallory body like inclusions and a form of congenital fiber-type disproportion. To define the phenotype and long-term clinical course in juvenile Selenoprotein N1-related myopathies 11 juvenile patients from eight families with SEPN1 mutations were assessed over a mean period of 7.2 years. Clinical findings, histomorphological studies, respiratory investigations and genetic data were analyzed: age of manifestation varied within the first 2 years of life with muscle hypotonia, lag of head control and delayed motor development. Further gross motor development was normal in 9/11 patients. All patients were ambulant for at least 1000 m at a mean age of 13.7 years. Eight patients exhibited a rigid spine diagnosed at a mean age of 10 years. All patients had respiratory impairment with a vital capacity ranging from 18% to 65%. Four patients were intermittently nocturnally ventilated at a mean age of 11 years. Body mass index was below 20 (kgm(-2)) in all patients. Muscle biopsies of eight individuals revealed multiminicores (n=2), congenital fiber-type disproportion (n=1), myopathic changes with single cores (n=2) and unspecific myopathic features (n=3). Mutations were distributed throughout the entire SEPN1 gene. Although the phenotype of juvenile selenoprotein N1-related myopathies is homogenous regarding the main symptoms we describe a variable degree of clinical severity. Major complications were early respiratory failure, impaired increase in weight and orthopedic problems. There seems to be no correlation between skeletal muscle weakness and respiratory failure.

Loss of housekeeping selenoprotein expression in mouse liver modulates lipoprotein metabolism

Selenium is incorporated into proteins as selenocysteine (Sec), which is dependent on its specific tRNA, designated tRNA([Ser]Sec). Targeted removal of the tRNA([Ser]Sec) gene (Trsp) in mouse hepatocytes previously demonstrated the importance of selenoproteins in liver function. Herein, analysis of plasma proteins in this Trsp knockout mouse revealed increases in apolipoprotein E (ApoE) that was accompanied by elevated plasma cholesterol levels. The expression of genes involved in cholesterol biosynthesis, metabolism and transport were also altered in knockout mice. Additionally, in two transgenic Trsp mutant mouse lines (wherein only housekeeping selenoprotein synthesis was restored), the expression of ApoE, as well as genes involved in cholesterol biosynthesis, metabolism and transport were similar to those observed in wild type mice. These data correlate with reports that selenium deficiency results in increased levels of ApoE, indicating for the first time that housekeeping selenoproteins have a role in regulating lipoprotein biosynthesis and metabolism.

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.