Functions of GI-GPx: lessons from selenium-dependent expression and intracellular localization

Selenium Status and Its Antioxidant Role in Metabolic Diseases

Selenium (Se), in the form of selenoproteins, is an essential micronutrient that plays an important role in human health and disease. To date, there are at least 25 selenoproteins in humans involved in a wide variety of biological functions, including mammalian development, metabolic progress, inflammation response, chemoprotective properties, and most notably, oxidoreductase functions. In recent years, numerous studies have reported that low Se levels are associated with increased risk, poor outcome, and mortality of metabolic disorders, mainly related to the limited antioxidant defense resulting from Se deficiency. Moreover, the correlation between Se deficiency and Keshan disease has received considerable attention. Therefore, Se supplementation as a therapeutic strategy for preventing the occurrence, delaying the progression, and alleviating the outcomes of some diseases has been widely studied. However, supranutritional levels of serum Se may have adverse effects, including Se poisoning. This review evaluates the correlation between Se status and human health, with particular emphasis on the antioxidant benefits of Se in metabolic disorders, shedding light on clinical treatment.

The Key Role of Peroxisomes in Follicular Growth, Oocyte Maturation, Ovulation, and Steroid Biosynthesis

The absence of peroxisomes can cause disease in the human reproductive system, including the ovaries. The available peroxisomal gene-knockout female mouse models, which exhibit pathological changes in the ovary and reduced fertility, are listed in this review. Our review article provides the first systematic presentation of peroxisomal regulation and its possible functions in the ovary. Our immunofluorescence results reveal that peroxisomes are present in all cell types in the ovary; however, peroxisomes exhibit different numerical abundances and strong heterogeneity in their protein composition among distinct ovarian cell types. The peroxisomal compartment is strongly altered during follicular development and during oocyte maturation, which suggests that peroxisomes play protective roles in oocytes against oxidative stress and lipotoxicity during ovulation and in the survival of oocytes before conception. In addition, the peroxisomal compartment is involved in steroid synthesis, and peroxisomal dysfunction leads to disorder in the sexual hormone production process. However, an understanding of the cellular and molecular mechanisms underlying these physiological and pathological processes is lacking. To date, no effective treatment for peroxisome-related disease has been developed, and only supportive methods are available. Thus, further investigation is needed to resolve peroxisome deficiency in the ovary and eventually promote female fertility.

The Effect of Acute Oral Galactose Administration on the Redox System of the Rat Small Intestine

Galactose is a ubiquitous monosaccharide with important yet incompletely understood nutritive and physiological roles. Chronic parenteral d-galactose administration is used for modeling aging-related pathophysiological processes in rodents due to its ability to induce oxidative stress (OS). Conversely, chronic oral d-galactose administration prevents and alleviates cognitive decline in a rat model of sporadic Alzheimer's disease, indicating that galactose may exert beneficial health effects by acting in the gut. The present aim was to explore the acute time-response of intestinal redox homeostasis following oral administration of d-galactose. Male Wistar rats were euthanized at baseline (n = 6), 30 (n = 6), 60 (n = 6), and 120 (n = 6) minutes following orogastric administration of d-galactose (200 mg/kg). The overall reductive capacity, lipid peroxidation, the concentration of low-molecular-weight thiols (LMWT) and protein sulfhydryls (SH), the activity of Mn and Cu/Zn superoxide dismutases (SOD), reduced and oxidized fractions of nicotinamide adenine dinucleotide phosphates (NADPH/NADP), and the hydrogen peroxide dissociation rate were analyzed in duodenum and ileum. Acute oral administration of d-galactose increased the activity of SODs and decreased intestinal lipid peroxidation and nucleophilic substrates (LMWT, SH, NADPH), indicating activation of peroxidative damage defense pathways. The redox system of the small intestine can acutely tolerate even high luminal concentrations of galactose (0.55 M), and oral galactose treatment is associated with a reduction rather than the increment of the intestinal OS. The ability of oral d-galactose to modulate intestinal OS should be further explored in the context of intestinal barrier maintenance, and beneficial cognitive effects associated with long-term administration of low doses of d-galactose.

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.

Genetic polymorphism in selenoprotein P modifies the response to selenium-rich foods on blood levels of selenium and selenoprotein P in a randomized dietary intervention study in Danes

Background

Selenium is an essential trace element and is suggested to play a role in the etiology of a number of chronic diseases. Genetic variation in genes encoding selenoproteins, such as selenoprotein P and the glutathione peroxidases, may affect selenium status and, thus, individual susceptibility to some chronic diseases. In the present study, we aimed to (1) investigate the effect of mussel and fish intake on glutathione peroxidase enzyme activity and (2) examine whether single nucleotide polymorphisms in the GPX1, GPX4, and SELENOP genes modify the effect of mussel and fish intake for 26 weeks on whole blood selenium, plasma selenoprotein P concentrations, and erythrocyte GPX enzyme activity in a randomized intervention trial in Denmark.

Results

CC homozygotes of the SELENOP/rs3877899 polymorphism who consumed 1000 g fish and mussels per week for 26 consecutive weeks had higher levels of both selenoprotein P (difference between means - 4.68 ng/mL (95% CI - 8.49, - 0.871)) and whole blood selenium (difference between means - 5.76 (95% CI - 12.5, 1.01)) compared to fish and mussel consuming T-allele carriers although the effect in whole blood selenium concentration was not statistically significant.

Conclusions

Our study indicates that genetically determined variation in SELENOP leads to different responses in expression of selenoproteins following consumption of selenium-rich foods. This study also emphasizes the importance of taking individual aspects such as genotypes into consideration when assessing risk in public health recommendations.

Selenium and inflammatory bowel disease

Dietary intake of the micronutrient selenium is essential for normal immune functions. Selenium is cotranslationally incorporated as the 21st amino acid, selenocysteine, into selenoproteins that function to modulate pathways involved in inflammation. Epidemiological studies have suggested an inverse association between selenium levels and inflammatory bowel disease (IBD), which includes Crohn's disease and ulcerative colitis that can potentially progress to colon cancer. However, the underlying mechanisms are not well understood. Here we summarize the current literature on the pathophysiology of IBD, which is multifactorial in origin with unknown etiology. We have focused on a few selenoproteins that mediate gastrointestinal inflammation and activate the host immune response, wherein macrophages play a pivotal role. Changes in cellular oxidative state coupled with altered expression of selenoproteins in macrophages drive the switch from a proinflammatory phenotype to an anti-inflammatory phenotype to efficiently resolve inflammation in the gut and restore epithelial barrier integrity. Such a phenotypic plasticity is accompanied by changes in cytokines, chemokines, and bioactive metabolites, including eicosanoids that not only mitigate inflammation but also partake in restoring gut homeostasis through diverse pathways involving differential regulation of transcription factors such as nuclear factor-κB and peroxisome proliferator-activated receptor-γ. The role of the intestinal microbiome in modulating inflammation and aiding in selenium-dependent resolution of gut injury is highlighted to provide novel insights into the beneficial effects of selenium in IBD.

The effect of deposition Se on the mRNA expression levels of GPxs in goats from a Se-enriched county of China

Previous studies revealed that Se was an important regulatory factor for glutathione peroxidase (GPx) genes. However, the relationship between Se concentrations and mRNA expression levels of GPxs were unclear in goats, especially the goats living in natural Se-enriched area. Thus, the aim of this study was to determine the Se concentrations and the mRNA expression levels of GPx-1, GPx-2, GPx-3, and GPx-4 in goats from Ziyang County (ZY-H and ZY-L goats) and Baoji City (BJ-P goats), which were Se-rich region and Se-poor region in China, respectively. Atomic fluorescence spectrometry was used as an essential method to determine the Se concentrations in heart, liver, spleen, lung, kidney, longissimus, biceps femoris, and serum, and the gene expressions were quantified in mRNA samples extracted from the above tissues by real-time quantitative reverse transcription-polymerase chain reaction. We found that the Se concentrations in ZY-H and ZY-L goats were higher than that in BJ-P goats significantly (P < 0.05), and the pertinence relations of Se levels between serum and heart, liver, spleen, and kidney were significant (P < 0.05). The mRNA levels of GPx-1 in ZY-H and ZY-L goats were higher than that in BJ-P goats very significantly (P < 0.01) except for longissimus (P < 0.05). Our results indicated a significant trend for GPx-2 in the direction of increasing mRNA levels with increasing Se concentrations in goats but had no statistical significance (P > 0.05) in our experimental conditions. As to GPx-3, its mRNA expression in spleen, lung, and kidney (P < 0.05) were upregulated and were consensual to high Se contents in ZY-H goats, but no significant effects were observed in heart, liver, longissimus, and biceps femoris among our three groups (P > 0.05). The mRNA levels of GPx-4 in heart, liver, lung, and kidney of ZY-H and ZY-L goats were higher than that of BJ-P goats (P < 0.05), and the difference was very significant in lung especially (P < 0.01), but no change in spleen, longissimus, and biceps femoris (P > 0.05). In summary, these data suggested that the goats living in Ziyang County were rich in Se, and the deposition Se played important roles in the mRNA expression of GPx-1, GPx-3, and GPx-4 in certain tissues of goats differentially.

Effects of different selenium levels on gene expression of a subset of selenoproteins and antioxidative capacity in mice

This study aimed to evaluate how excess selenium induces oxidative stress by determining antioxidant enzyme activity and changes in expression of selected selenoproteins in mice. BALB/c mice (n = 20 per group) were fed a diet containing 0.045 (Se-marginal), 0.1 (Se-adequate), 0.4 (Se-supernutrition), or 0.8 (Se-excess) mg Se/kg. Gene expression was quantified in RNA samples extracted from the liver, kidney, and testis by real-time quantitative reverse transcription-polymerase chain reaction. We found that glutathione peroxidase (GPx) and catalase activities decreased in livers of mice fed the marginal or excess dose of Se as compared to those in the Se-adequate group. Additionally, superoxide dismutase and glutathione reductase activities were significantly reduced only in mice fed the excess Se diet, compared to animals on the adequate Se diet. Se-supernutrition had no effect on hepatic mRNA levels of GPx isoforms 1 and 4 (GPx1 and GPx4), down-regulated GPx isoform 3 (GPx3), and upregulated selenoprotein W (SelW) mRNA expression. The excess Se diet led to decreased hepatic mRNA levels of GPx1, GPx3 and GPx4 but no change in testicular mRNA levels of GPx1, GPx3 or SelW. Dietary Se had no effect on testicular mRNA levels of GPx4. Thus, our results suggest that Se exposure can reduce hepatic antioxidant capacity and cause liver dysfunction. Dietary Se was found to differentially regulate mRNA levels of the GPx family or SelW, depending on exposure. Therefore, these genes may play a role in the toxicity associated with Se.

Selenium status alters the immune response and expulsion of adult Heligmosomoides bakeri worms in mice

Heligmosomoides bakeri is a nematode with parasitic development exclusively in the small intestine of infected mice that induces a potent STAT6-dependent Th2 immune response. We previously demonstrated that host protective expulsion of adult H. bakeri worms from a challenge infection was delayed in selenium (Se)-deficient mice. In order to explore mechanisms associated with the delayed expulsion, 3-week-old female BALB/c mice were placed on a torula yeast-based diet with or without 0.2 ppm Se, and after 5 weeks, they were inoculated with H. bakeri infective third-stage larvae (L3s). Two weeks after inoculation, the mice were treated with an anthelmintic and then rested, reinoculated with L3s, and evaluated at various times after reinoculation. Analysis of gene expression in parasite-induced cysts and surrounding tissue isolated from the intestine of infected mice showed that the local-tissue Th2 response was decreased in Se-deficient mice compared to that in Se-adequate mice. In addition, adult worms recovered from Se-deficient mice had higher ATP levels than worms from Se-adequate mice, indicating greater metabolic activity in the face of a suboptimal Se-dependent local immune response. Notably, the process of worm expulsion was restored within 2 to 4 days after feeding a Se-adequate diet to Se-deficient mice. Expulsion was associated with an increased local expression of Th2-associated genes in the small intestine, intestinal glutathione peroxidase activity, secreted Relm-β protein, anti-H. bakeri IgG1 production, and reduced worm fecundity and ATP-dependent metabolic activity.

Influence of broccoli extract and various essential oils on performance and expression of xenobiotic- and antioxidant enzymes in broiler chickens

The aim of our present study was to examine the regulation of xenobiotic- and antioxidant enzymes by phytogenic feed additives in the intestine and the liver of broilers. A total of 240 male Ross-308 broiler chickens (1 d old) were fed a commercial starter diet for 2 weeks. On day 15, the birds were assigned to six treatment groups of forty birds each. The control (Con) group was fed a diet without any additive for 3 weeks. The diet of group sulforaphane (SFN) contained broccoli extract providing 0.075 g/kg SFN, whereas the diets of the other four groups contained 0.15 g/kg essential oils from turmeric (Cuo), oregano (Oo), thyme and rosemary (Ro). Weight gain and feed conversion were slightly impaired by Cuo and Oo. In the jejunum SFN, Cuo and Ro increased the expression of xenobiotic enzymes (epoxide hydrolases 1 and 2 and aflatoxin B1 aldehyde reductase) and of the antioxidant enzyme haeme oxygenase regulated by an 'antioxidant response element' (ARE) compared to group Con. In contrast to our expectations in the liver, the expression of these enzymes was decreased by all the additives. Nevertheless, all the additives increased the Trolox equivalent antioxidant capacity of the jejunum and the liver and reduced Fe-induced lipid peroxidation in the liver. We conclude that the up-regulation of ARE genes in the small intestine reduces oxidative stress in the organism and represents a novel mechanism by which phytogenic feed additives improve the health of farm animals.

Selenium, selenoproteins and the thyroid gland: interactions in health and disease

The trace element selenium is an essential micronutrient that is required for the biosynthesis of selenocysteine-containing selenoproteins. Most of the known selenoproteins are expressed in the thyroid gland, including some with still unknown functions. Among the well-characterized selenoproteins are the iodothyronine deiodinases, glutathione peroxidases and thioredoxin reductases, enzymes involved in thyroid hormone metabolism, regulation of redox state and protection from oxidative damage. Selenium content in selenium-sensitive tissues such as the liver, kidney or muscle and expression of nonessential selenoproteins, such as the glutathione peroxidases GPx1 and GPx3, is controlled by nutritional supply. The thyroid gland is, however, largely independent from dietary selenium intake and thyroid selenoproteins are preferentially expressed. As a consequence, no explicit effects on thyroid hormone profiles are observed in healthy individuals undergoing selenium supplementation. However, low selenium status correlates with risk of goiter and multiple nodules in European women. Some clinical studies have demonstrated that selenium-deficient patients with autoimmune thyroid disease benefit from selenium supplementation, although the data are conflicting and many parameters must still be defined. The baseline selenium status of an individual could constitute the most important parameter modifying the outcome of selenium supplementation, which might primarily disrupt self-amplifying cycles of the endocrine-immune system interface rectifying the interaction of lymphocytes with thyroid autoantigens. Selenium deficiency is likely to constitute a risk factor for a feedforward derangement of the immune system-thyroid interaction, while selenium supplementation appears to dampen the self-amplifying nature of this derailed interaction.

Glucoraphanin does not reduce plasma homocysteine in rats with sufficient Se supply via the induction of liver ARE-regulated glutathione biosynthesis enzymes

Data from human and animal trials have revealed contradictory results regarding the influence of selenium (Se) status on homocysteine (HCys) metabolism. It was hypothesised that sufficient Se reduces the flux of HCys through the transsulphuration pathway by decreasing the expression of glutathione (GSH) synthesising enzymes. Glucoraphanin (GRA) is a potent inducer of genes regulated via an antioxidant response element (ARE), including those of GSH biosynthesis. We tested the hypothesis that GRA supplementation to rat diets lowers plasma HCys levels by increasing GSH synthesis. Therefore 96 weaned albino rats were assigned to 8 groups of 12 and fed diets containing four different Se levels (15, 50, 150 and 450 μg kg(diet)(-1)), either without GRA (groups: C15, C50, C150 and C450) or in combination with 700 μmol GRA kg(diet)(-1) (groups G15, G50, G150 and G450). Rats fed the low Se diets C15 and G15 showed an impressive decrease of plasma HCys. Se supplementation increased plasma HCys and lowered GSH significantly by reducing the expression of GSH biosynthesis enzymes. As new molecular targets explaining these results, we found a significant down-regulation of the hepatic GSH exporter MRP4 and an up-regulation of the HCys exporter Slco1a4. In contrast to our hypothesis, GRA feeding did not reduce plasma HCys levels in Se supplemented rats (G50, G150 and 450) through inducing GSH biosynthesis enzymes and MRP4, but reduced their mRNA in some cases to a higher extent than Se alone. We conclude: 1. That the long-term supplementation of moderate GRA doses reduces ARE-driven gene expression in the liver by increasing the intestinal barrier against oxidative stress. 2. That the up-regulation of ARE-regulated genes in the liver largely depends on GRA cleavage to free sulforaphane and glucose by plant-derived myrosinase or bacterial β-glucosidases. As a consequence, higher dietary GRA concentrations should be used in future experiments to test if GRA or sulforaphane can be established as HCys lowering compounds.

Colitis locus on chromosome 2 impacting the severity of early-onset disease in mice deficient in GPX1 and GPX2

BACKGROUND

Genetic background has a profound effect on inflammatory bowel disease. The Gpx1 and Gpx2 double knockout (GPX1/2-DKO) mice on a mixed C57BL/6 (B6) and 129S1/SvimJ (129) background exhibit spontaneous ileocolitis. The DKO mice on a B6 background have mild ileocolitis. We characterized the 129 DKO mice to identify a genetic locus affecting disease severity.

METHODS

We backcrossed B6;129 DKO mice to 129 and analyzed for ileocolitis penetrance and severity at N5, N7, and N10. By correlating disease severity with single-nucleotide polymorphism (SNP) markers, we identified a colitis locus.

RESULTS

As early as 9 days of age, 129 DKO N5 and N10 mice showed disease signs and morbidity. The N10 DKO mice had the severest colitis with nearly complete penetrance and high morbidity compared with other generations or backgrounds. 129 DKO mice had elevated colonic KC and SAA3 expression, shorter colon length, and cecal E. coli overgrowth compared to B6 DKO mice. Analysis of the B6 loci in 129 N5, N7, and N10 cohorts pointed to a region of chromosome 2: 119 Mbp contributing to mild symptoms.

CONCLUSIONS

GPX1/2-DKO mice on 129 genetic background have the most aggressive colitis compared to B6;129 and B6 colonies. A B6 locus significantly contributing the resistance resides on chromosome 2: 119 Mbp. This region coincides with cytokine-deficiency-induced colitis susceptibility, Cdcs3, identified in the resistant B6 and sensitive C3H/HeJBir (C3Bir) with IL-10 deficiency. A three-way SNP analysis between 129, B6, and C3Bir locus points the major candidate genes to B2m, Dnajc17, Duox2, Pla2g4b, Pla2g4e, Pla2g4f and Slc30a4.

Adaptive dysfunction of selenoproteins from the perspective of the triage theory: why modest selenium deficiency may increase risk of diseases of aging

The triage theory proposes that modest deficiency of any vitamin or mineral (V/M) could increase age-related diseases. V/M-dependent proteins required for short-term survival and/or reproduction (i.e., "essential") are predicted to be protected on V/M deficiency over other "nonessential" V/M-dependent proteins needed only for long-term health. The result is accumulation of insidious damage, increasing disease risk. We successfully tested the theory against published evidence on vitamin K. Here, we review about half of the 25 known mammalian selenoproteins; all of those with mouse knockout or human mutant phenotypes that could be used as criteria for a classification of essential or nonessential. Five selenoproteins (Gpx4, Txnrd1, Txnrd2, Dio3, and Sepp1) were classified as essential and 7 (Gpx1, Gpx 2, Gpx 3, Dio1, Dio2, Msrb1, and SelN) nonessential. On modest selenium (Se) deficiency, nonessential selenoprotein activities and concentrations are preferentially lost, with one exception (Dio1 in the thyroid, which we predict is conditionally essential). Mechanisms include the requirement of a special form of tRNA sensitive to Se deficiency for translation of nonessential selenoprotein mRNAs except Dio1. The same set of age-related diseases and conditions, including cancer, heart disease, and immune dysfunction, are prospectively associated with modest Se deficiency and also with genetic dysfunction of nonessential selenoproteins, suggesting that Se deficiency could be a causal factor, a possibility strengthened by mechanistic evidence. Modest Se deficiency is common in many parts of the world; optimal intake could prevent future disease.

Redox control systems in the nucleus: mechanisms and functions

Proteins with oxidizable thiols are essential to many functions of cell nuclei, including transcription, chromatin stability, nuclear protein import and export, and DNA replication and repair. Control of the nuclear thiol-disulfide redox states involves both the elimination of oxidants to prevent oxidation and the reduction of oxidized thiols to restore function. These processes depend on the common thiol reductants, glutathione (GSH) and thioredoxin-1 (Trx1). Recent evidence shows that these systems are controlled independent of the cytoplasmic counterparts. In addition, the GSH and Trx1 couples are not in redox equilibrium, indicating that these reductants have nonredundant functions in their support of proteins involved in transcriptional regulation, nuclear protein trafficking, and DNA repair. Specific isoforms of glutathione peroxidases, glutathione S-transferases, and peroxiredoxins are enriched in nuclei, further supporting the interpretation that functions of the thiol-dependent systems in nuclei are at least quantitatively distinct, and probably also qualitatively distinct, from similar processes in the cytoplasm. Elucidation of the distinct nuclear functions and regulation of the thiol redox pathways in nuclei can be expected to improve understanding of nuclear processes and also to provide the basis for novel approaches to treat aging and disease processes associated with oxidative stress in the nuclei.

Selenium-enriched milk proteins and selenium yeast affect selenoprotein activity and expression differently in mouse colon

Certain forms of dietary Se may have an advantage in improving Se status and reducing cancer risk. The present study compared the effects of an Se-enriched milk protein product (dairy-Se) with an Se yeast (yeast-Se) on selenoprotein activity and expression in the mouse colon. Mice were fed four diets for 4 weeks: a control milk protein diet (Se at 0.068 parts per million (ppm)), dairy-Se diets with Se at 0.5 and 1 ppm, and a yeast-Se diet with Se at 1 ppm. Cytosolic glutathione peroxidase-1 (GPx-1) activity, mRNA of selenoprotein P (SeP), GPx-1, gastrointestinal glutathione peroxidase-2 (GPx-2) and thioredoxin reductase-1 (TrxR-1) were examined in the mouse colon. Dairy-Se diets did not significantly affect GPx-1 mRNA and GPx-1 activity but produced a dose-dependent increase in SeP and GPx-2 mRNA, with a significantly higher level achieved at 1 ppm Se (P < 0.05). Yeast-Se at 1 ppm significantly increased GPx-1 mRNA and GPx-1 activity (P < 0.01) but not GPx-2 mRNA. Neither Se supplement had any effect on TrxR-1. The present study indicates that selenoprotein levels in the mouse colon are regulated differently depending on the Se supplement. As we have previously shown that dairy-Se at 1 ppm was protective against colorectal cancer (CRC) in an azoxymethane-induced CRC mouse model, this up-regulation of colonic GPx-2 and SeP with Se supplementation may be crucial to its chemopreventive action.

Drug interactions

Drugs for allergy are often taken in combination with other drugs, either to treat allergy or other conditions. In common with many pharmaceuticals, most such drugs are subject to metabolism by P450 enzymes and to transmembrane transport. This gives rise to considerable potential for drug-drug interactions, to which must be added consideration of drug-diet interactions. The potential for metabolism-based drug interactions is increasingly being taken into account during drug development, using a variety of in silico and in vitro approaches. Prediction of transporter-based interactions is not as advanced. The clinical importance of a drug interaction will depend upon a number of factors, and it is important to address concerns quantitatively, taking into account the therapeutic index of the compound.

Heterolytic reduction of fatty acid hydroperoxides by cytochrome c/cardiolipin complexes: antioxidant function in mitochondria

Cytochrome c (cyt c), a mitochondrial intermembrane electron shuttle between complexes III and IV, can, upon binding with an anionic phospholipid, cardiolipin (CL), act as a peroxidase that catalyzes cardiolipin oxidation. H(2)O(2) was considered as a source of oxidative equivalents for this reaction, which is essential for programmed cell death. Here we report that peroxidase cyt c/CL complexes can utilize free fatty acid hydroperoxides (FFA-OOH) at exceptionally high rates that are approximately 3 orders of magnitude higher than for H(2)O(2). Similarly, peroxidase activity of murine liver mitochondria was high with FFA-OOH. Using EPR spin trapping and LC-MS techniques, we have demonstrated that cyt c/CL complexes split FFA-OOH predominantly via a heterolytic mechanism, yielding hydroxy-fatty acids, whereas H(2)O(2) (and tert-butyl hydroperoxide, t-BuOOH) undergo homolytic cleavage. Computer simulations have revealed that Arg(38) and His(33) are important for the heterolytic mechanism at potential FFA-OOH binding sites of cyt c (but not for H(2)O(2) or t-BuOOH). Regulation of FFA-OOH metabolism may be an important function of cyt c that is associated with elimination of toxic FFA-OOH and synthesis of physiologically active hydroxy-fatty acids in mitochondria.

Hierarchical regulation of selenoprotein expression and sex-specific effects of selenium

The expression of selenoproteins is controlled on each one of the textbook steps of protein biosynthesis, i.e., during gene transcription, RNA processing, translation and posttranslational events as well as via control of the stability of the involved intermediates and final products. Selenoproteins are unique in their dependence on the trace element Se which they incorporate as the 21st proteinogenic amino acid, selenocysteine. Higher mammals have developed unique pathways to enable a fine-tuned expression of all their different selenoproteins according to developmental stage, actual needs, and current availability of the trace element. Tightly controlled and dynamic expression patterns of selenoproteins are present in different tissues. Interestingly, these patterns display some differences in male and female individuals, and can be grossly modified during disease, e.g. in cancer, inflammation or neurodegeneration. Likewise, important health issues related to the selenium status show unexpected sexual dimorphisms. Some detailed molecular insights have recently been gained on how the hierarchical Se distribution among the different tissues is achieved, how the selenoprotein biosynthesis machinery discriminates among the individual selenoprotein transcripts and how impaired selenoprotein biosynthesis machinery becomes phenotypically evident in humans. This review tries to summarize these fascinating findings and highlights some interesting and surprising sex-specific differences.

Variation in the selenoenzyme genes and risk of advanced distal colorectal adenoma

BACKGROUND

Epidemiologic and animal studies provide evidence for a chemopreventive effect of selenium on colorectal cancer, which may be mediated by the antioxidative and anti-inflammatory properties of selenoenzymes. We therefore investigated whether genetic variants in selenoenzymes abundantly expressed in the colon are associated with advanced colorectal adenoma, a cancer precursor.

METHODS

Cases with a left-sided advanced adenoma (n = 772) and matched controls (n = 777) screen negative for polyps based on sigmoidoscopy examination were randomly selected from participants in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. The underlying genetic variation was determined by resequencing. We genotyped 44 tagging single nucleotide polymorphisms (SNP) in six genes [glutathione peroxidase 1-4 (GPX1, GPX2, GPX3, and GPX4), selenoprotein P (SEPP1), and thioredoxin reductase 1 (TXNRD1)] to efficiently predict common variation across these genes.

RESULTS

Four variants in SEPP1 were significantly associated with advanced adenoma risk. A rare variant in the 5' region of SEPP1 (-4166C>G) was present in nine cases but in none of the controls (exact P = 0.002). Three SNPs located in the 3' region of SEPP1, which is overlapping with the promoter region of an antisense transcript, were significantly associated with adenoma risk: homozygotes at two SEPP1 loci (31,174 bp 3' of STP A>G and 43,881 bp 3' of STP G>A) were associated with increased adenoma risk [odds ratio (OR), 1.48; 95% confidence interval (95% CI), 1.00-2.19 and OR, 1.53; 95% CI, 1.05-2.22, respectively] and the variant SEPP1 44,321 bp 3' of STP C>T was associated with a reduced adenoma risk (CT versus CC OR, 0.85; 95% CI, 0.63-1.15). Furthermore, we observed a significant 80% reduction for advanced colorectal adenoma risk for carriers of the variant allele at TXNRD1 IVS1-181C>G (OR, 0.20; 95% CI, 0.07-0.55; P trend = 0.004). Consistent with the individual SNP results, we observed a significant overall association with adenoma risk for SEPP1 and TXNRD1 (global P = 0.02 and 0.008, respectively) but not for the four GPX genes.

CONCLUSION

Our study suggests that genetic variants at or near the SEPP1 and TXNRD1 loci may be associated with advanced colorectal adenoma. As this is the first study to comprehensively investigate this hypothesis, confirmation in independent study populations is needed.

Functional effects of a common single-nucleotide polymorphism (GPX4c718t) in the glutathione peroxidase 4 gene: interaction with sex

BACKGROUND

Selenium is essential for health in humans. Selenium is present as selenocysteine in selenoproteins such as the glutathione peroxidases (GPx). Selenocysteine incorporation requires specific structures in the 3'untranslated region (3'UTR) of selenoprotein mRNAs.

OBJECTIVE

This study investigated the functional significance of the single-nucleotide polymorphism (SNP) GPx4c718t within the 3'UTR of the GPx4 gene.

DESIGN

A selenium supplementation trial was carried out with prospectively genotyped individuals of both homozygote genotypes for this SNP. Blood samples were analyzed at baseline, after a 6-wk supplementation with 100 mug Se as sodium selenite/d, and during a 6-wk washout period. RNA-protein binding studies were carried out in vitro.

RESULTS

Both lymphocyte GPx1 protein concentrations and plasma GPx3 activity increased significantly after selenium supplementation in CC but not TT participants. After selenium withdrawal, there was a significant fall in both lymphocyte GPx4 protein concentrations and GPx4 activity in TT but not in CC participants; this effect was modulated by sex. RNA-protein binding assays showed that both T and C variants of transcripts corresponding to the GPx4 3'UTR formed complexes in vitro and that the C variant bound more strongly than did either the T variant or the GPx1 3'UTR.

CONCLUSIONS

The GPX4c718t SNP both alters protein binding to the 3'UTR in vitro and influences the concentration of lymphocyte GPx4 and other selenoproteins in vivo. The latter is consistent with competition for selenium in selenoprotein synthesis, and, at low selenium intake, the SNP thus may influence susceptibility to disease.

Molecular biology of glutathione peroxidase 4: from genomic structure to developmental expression and neural function

Selenoproteins have been recognized as modulators of brain function and signaling. Phospholipid hydroperoxide glutathione peroxidase (GPx4/PHGPx) is a unique member of the selenium-dependent glutathione peroxidases in mammals with a pivotal role in brain development and function. GPx4 exists as a cytosolic, mitochondrial, and nuclear isoform derived from a single gene. In mice, the GPx4 gene is located on chromosome 10 in close proximity to a functional retrotransposome that is expressed under the control of captured regulatory elements. Elucidation of crystallographic data uncovered structural peculiarities of GPx4 that provide the molecular basis for its unique enzymatic properties and substrate specificity. Monomeric GPx4 is multifunctional: it acts as a reducing enzyme of peroxidized phospholipids and thiols and as a structural protein. Transcriptional regulation of the different GPx4 isoforms requires several isoform-specific cis-regulatory sequences and trans-activating factors. Cytosolic and mitochondrial GPx4 are the major isoforms exclusively expressed by neurons in the developing brain. In stark contrast, following brain trauma, GPx4 is specifically upregulated in non-neuronal cells, i.e., reactive astrocytes. Molecular approaches to genetic modification in mice have revealed an essential and isoform-specific function for GPx4 in development and disease. Here we review recent findings on GPx4 with emphasis on its molecular structure and function and consider potential mechanisms that underlie neural development and neuropathological conditions.

Activation of the glutathione peroxidase 2 (GPx2) promoter by β-catenin

GPx2, formerly named gastrointestinal glutathione peroxidase, is highly expressed in the proliferative area of the intestinal crypt-to-villus axis and in Paneth cells. Additionally, GPx2 is transiently up-regulated during development of gastrointestinal adenocarcinomas. Because both normal proliferation and differentiation of intestinal epithelial cells as well as carcinogenesis are regulated by the Wnt pathway, it was tested whether GPx2 may be a target of the β-catenin/TCF complex which transfers Wnt signals. The GPx2 promoter contains five putative β-catenin/TCF binding sites. Accordingly, the promoter was active in two cell lines with a constitutively active Wnt pathway, HepG2 and SW480, but not in BHK-21 cells in which the pathway is silent. Overexpression of β-catenin/TCF activated the GPx2 promoter in all three cell lines. Overexpression of wild-type adenomatous polyposis coli (APC) in SW480 cells which harbor a mutated APC gene decreased basal GPx2 promoter activity. Truncation of the promoter identified one β-catenin/TCF binding site that was sufficient for activation. Mutation of this site reduced the response to β-catenin/TCF by more than 50%. These findings suggest a function of GPx2 in the maintenance of normal renewal of the intestinal epithelium. Whether up-regulation of GPx2 during carcinogenesis supports tumor growth or can rather be considered as a counteracting effect remains to be investigated.

Drug metabolism-related genes as potential biomarkers: analysis of expression in normal and tumour breast tissue

The complex role of drug metabolism-related enzymes and their possible influence in cancer development, treatment and outcome has not yet been completely elucidated. There is evidence that these enzymes can activate certain environmental procarcinogens to more toxic derivatives and thus a role has been proposed for them in carcinogenesis. The fact that they can also inactivate a number of chemotherapeutic drugs has raised the possibility of these enzymes influencing the sensitivity of tumour cells to anticancer agents. In this report, we analyse the expression of drug metabolism-related genes within a whole genome microarray study of 104 breast cancer and 17 normal breast specimens. Kaplan-Meier survival curves, Chi-squared, and Cox Regression analyses were used to identify associations between expression of gene transcripts and patients' clinicopathological and survival data. Our results show that several of these genes are significantly expressed in both normal and tumour tissue; in many cases, expression is altered in the tumour specimens as compared to normal breast tissue. Moreover, expression of ARNT2 and GST A1 was correlated with prognosis. Kaplan-Meier analysis showed expression of ARNT2 mRNA to correlate significantly with favourable disease outcome for patients, in terms of both their disease-free survival (P = 0.0094) and overall survival (P = 0.0018) times from diagnosis, while detection of GST A1 mRNA correlated with shortened disease-free survival (P = 0.0131) and overall survival (P = 0.0028). Multivariate analysis indicated GST A1 expression to be an independent prognostic factor for overall survival (P = 0.045). Our results suggest a possible use of ARNT2 and GST A1 as prognostic breast cancer biomarkers.

Differential regulation of expression of cytosolic and mitochondrial thioredoxin reductase in rat liver and kidney

Adequate supply of selenium (Se) is critical for synthesis of selenoproteins through selenocysteine insertion mechanism. To explore this process we investigated the expression of the cytosolic and mitochondrial isoenzymes of thioredoxin reductase (TrxR1 and TrxR2) in response to altered Se supply. Rats were fed diets containing different quantities of selenium and the levels of TrxR1 and TrxR2 protein and their corresponding mRNAs were determined in liver and kidney. Expression of the two isoenzymes was differentially affected, with TrxR1 being more sensitive to Se depletion than TrxR2 and greater changes in liver than kidney. In order to determine if the selenocysteine incorporation sequence (SECIS) element was critical in this response liver and kidney cell lines (H4 and NRK-52E) were transfected with reporter constructs in which expression of luciferase required read-through at a UGA codon and which contained either the TrxR1 or TrxR2 3'UTR, or a combination of the TrxR1 5' and 3'UTRs. Cell lines expressing constructs with the TrxR1 3'UTR demonstrated no response to restricted Se supply. In comparison the Se-deficient cells expressing constructs with the TrxR2 3'UTR showed considerably less luciferase activity than the Se-adequate cells. No disparity of response to Se supply was observed in the constructs containing the different TrxR1 5'UTR variants. The data show that there is a prioritisation of TrxR2 over TrxR1 during Se deficiency such that TrxR1 expression is more sensitive to Se supply than TrxR2 but this sensitivity of TrxR1 was not fully accounted for by TrxR1 5' or 3'UTR sequences when assessed using luciferase reporter constructs.

Does cross-reactivity between mycobacterium avium paratuberculosis and human intestinal antigens characterize Crohn's disease?

BACKGROUND & AIMS

Most Crohn's disease (CD) patients show seroreactivity against Mycobacterium avium paratuberculosis (MAP), suggesting a pathogenic role for this organism. Our aim was to seek amino acid similarities between MAP and intestinal proteins that, through molecular mimicry, could serve as targets for cross-reactive immunity in CD.

METHODS

Fifty-three peptides comprising 23 sets of MAP/human intestinal peptidyl mimics chosen for maximal homology were constructed and tested for immunologic cross-reactivity by enzyme-linked immunosorbent assay in 50 patients with CD, 50 with ulcerative colitis, and 38 healthy controls.

RESULTS

Antibody reactivity was present in only 7 of 23 peptide sets. MAP/self-reactivity in at least 1 of the 7 reactive sets was present in 21 (42%) CD patients but was virtually absent in the controls. Significant double-reactivity was found against MAP glycosyl transferase d (gsd)(230-244)/human gastrointestinal glutathione peroxidase (GPg)(111-125) homologues in 15 of 50 (30%) CD patients; MAP alkylohydroperoxidase C (ahpC)(20-34)/human tumor overexpressed protein (TOG)(637-651) double-reactivity was present in 10 (20%) CD patients, but in none of the controls. Inhibition studies confirmed that simultaneous reactivity to mimics was caused by cross-reactivity. Three-dimensional modeling predicts GPg(111-125) will be exposed in a solvent-accessible surface region of the protein compatible with antibody recognition. Antibody affinity was greater for the MAP mimics than for the self-sequences, suggesting that reactivity to the mycobacterial sequences precedes that against self-sequences.

CONCLUSIONS

We describe MAP/self-mimics as targets of cross-reactive antibody responses characterizing patients with CD. Our findings indicate gastrointestinal glutathione peroxidase as a novel autoantigen in CD.

Cyclophosphamide suppresses thioredoxin reductase in bladder tissue and its adaptive response via inductions of thioredoxin reductase and glutathione peroxidase

Mammalian thioredoxin reductase (TrxR) catalyzes the reduction of oxidized thioredoxin in a NADPH-dependent manner, and contains a selenocysteine residue near the C-terminus. Glutathione peroxidase (GPx) is one of the primary antioxidant enzymes that scavenge hydrogen peroxide and organic hydroperoxides. Both TrxR and GPx play an important role in protecting against oxidative stress. Cyclophosphamide (CTX), one of the most widely prescribed antineoplastic drugs, could cause cystitis. We found that 4 h after a bolus dose of CTX (30, 90, 150, 300 and 450 mg/kg) were administrated intraperitoneally, TrxR activity was significantly decreased in a dose-dependent manner, by 32%, 44%, 68%, 87% and 99%, respectively, in comparison with control group. When fixing CTX dose at 150 mg/kg, TrxR activity changed over time, significantly reduced to 68% of the activity in comparison with control tissue at 2 h, and gradually recovered to normal level within 24 h. In addition, we found that GPx activity was induced significantly after 4h. The results of the present study suggest that marked suppression of TrxR activity could be involved in the mechanism of CTX-induced cystitis, bladder may have a protective system against tissue damage by CTX via upregulation of TrxR and GPx, which is an adaptive response to oxidative stress.

GPX2, a direct target of p63, inhibits oxidative stress-induced apoptosis in a p53-dependent manner

The p53 family consists of p53, p63, and p73, each of which has multiple isoforms due to transcription at two separate promoters and alternative splicing. Although p53 is a bona fide tumor suppressor, p63 appears to have a Janus-faced function as a tumor suppressor and an oncogene. To address the two opposing functions of p63, we analyzed its target genes. Here, we found that GPX2, which encodes a glutathione peroxidase, is up-regulated by p63 but not p53. Accordingly, a unique responsive element was found in the promoter of the GPX2 gene that can be activated and bound by p63 but not p53. We also found that upon overexpression, GPX2 alleviates the apoptotic response of MCF7 cells to oxidative stresses. Interestingly, the protective function of GPX2 is p53 dependent. Likewise, we showed that a deficiency in GPX2 renders MCF7 cells susceptible to oxidative stress-induced apoptosis. Given that the deltaN isoform of p63 is frequently overexpressed in tumor cells, the observations here provide an insight into the mechanism by which some isoforms of p63 serve as a pro-survival factor by up-regulating GPX2 to reduce the p53-dependent oxidative stress-induced apoptotic response.

Glutathione peroxidases and redox-regulated transcription factors

Analysis of the selenoproteome identified five glutathione peroxidases (GPxs) in mammals: cytosolic GPx (cGPx, GPx1), phospholipid hydroperoxide GPx (PHGPX, GPx4), plasma GPx (pGPX, GPx3), gastrointestinal GPx (GI-GPx, GPx2) and, in humans, GPx6, which is restricted to the olfactory system. GPxs reduce hydroperoxides to the corresponding alcohols by means of glutathione (GSH). They have long been considered to only act as antioxidant enzymes. Increasing evidence, however, suggests that nature has not created redundant GPxs just to detoxify hydroperoxides. cGPx clearly acts as an antioxidant, as convincingly demonstrated in GPx1-knockout mice. PHGPx specifically interferes with NF-kappaB activation by interleukin-1, reduces leukotriene and prostanoid biosynthesis, prevents COX-2 expression, and is indispensable for sperm maturation and embryogenesis. GI-GPx, which is not exclusively expressed in the gastrointestinal system, is upregulated in colon and skin cancers and in certain cultured cancer cells. GI-GPx is a target for Nrf2, and thus is part of the adaptive response by itself, while PHGPx might prevent cancer by interfering with inflammatory pathways. In conclusion, cGPx, PHGPx and GI-GPx have distinct roles, particularly in cellular defence mechanisms. Redox sensing and redox regulation of metabolic events have become attractive paradigms to unravel the specific and in part still enigmatic roles of GPxs.

The GI-GPx gene is a target for Nrf2

The gastrointestinal glutathione peroxidase (GI-GPx, GPx2) is a selenoprotein that was suggested to act as barrier against hydroperoxide absorption but has also been implicated in the control of inflammation and malignant growth. In CaCo-2 cells, GI-GPx was induced by t-butyl hydroquinone (tBHQ) and sulforaphane (SFN), i.e., "antioxidants" known to activate the "antioxidant response element" (ARE) via electrophilic thiol modification of Keap1 in the Nrf2/Keap1 system. The functional significance of a putative ARE in the GI-GPx promoter was validated by transcriptional activation of reporter gene constructs upon exposure to electrophiles (tBHQ, SFN, and curcumin) or overexpression of Nrf2 and by reversal of these effects by mutation of the ARE in the promoter and by overexpressed Keap1. Binding of Nrf2 to the ARE sequence in authentic gpx2 was corroborated by chromatin immunoprecipitation. Thus, the presumed natural antioxidants sulforaphane and curcumin may exert their anti-inflammatory and anticarcinogenic effects not only by induction of phase 2 enzymes but also by the up-regulation of the selenoprotein GI-GPx.

Effects of Se-depletion on glutathione peroxidase and selenoprotein W gene expression in the colon

Selenium (Se)-containing proteins have important roles in protecting cells from oxidative damage. This work investigated the effects of Se-depletion on the expression of the genes encoding selenoproteins in colonic mucosa from rats fed diets of different Se content and in human intestinal Caco-2 cells grown in Se-adequate or Se-depleted culture medium. Se-depletion produced statistically significant (P<0.05) falls in glutathione peroxidase (GPX) 1 mRNA (60-83%) and selenoprotein W mRNA (73%) levels, a small but significant fall in GPX4 mRNA (17-25%) but no significant change in GPX2. The data show that SelW expression in the colon is highly sensitive to Se-depletion.

Phospholipid hydroperoxides are detoxified by phospholipase A2 and GSH peroxidase in rat gastric mucosa

The aim of this study was to determine the metabolic fate of phospholipid hydroperoxides (PLOOH) in rat gastric mucosa. Here we report evidence concerning the mechanism for PLOOH detoxification in gastric mucosa homogenate. Analysis by the TLC blot technique showed that the gastric mucosa has the highest potential to eliminate 1-palmitoyl-2-linoleoyl-phosphatidylcholine hydroperoxides (PL-PtdChoOOH) compared with the intestinal mucosa and liver. Major products detected after incubation with gastric mucosa were the partially reduced linoleic acid hydroperoxides (LAOOH) and lysophosphatidylcholine, indicating the involvement of phospholipase A2 (PLA2) in the elimination pathway. Using unilamellar vesicles, we demonstrated that gastric mucosal PLA2 does not distinguish between PLOOH and intact phospholipids. Although gastric mucosal PLA2 activity efficiently eliminated excess amounts of PLOOH, the complete reduction of LAOOH was dependent on the supply of exogenous GSH. In a separate experiment, administration of egg yolk PtdChoOOH to rats for 6 d significantly elevated GSH peroxidase (GPx) activity in the gastric mucosa. We concluded that excess amounts of PLOOH are efficiently eliminated through the hydrolysis by PLA2, and the subsequent reduction of FA hydroperoxide by GPx is the critical step for complete detoxification of oxidized phospholipids in the stomach.

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.