The GI-GPx gene is a target for Nrf2

A diet rich in olive oil phenolics reduces oxidative stress in the heart of SAMP8 mice by induction of Nrf2-dependent gene expression

A Mediterranean diet rich in olive oil has been associated with health benefits in humans. It is unclear if and to what extent olive oil phenolics may mediate these health benefits. In this study, we fed senescence-accelerated mouse-prone 8 (SAMP8, n=11 per group) semisynthetic diets with 10% olive oil containing either high (HP) or low amounts of olive oil phenolics (LP) for 4.5 months. Mice consuming the HP diet had significantly lower concentrations of the oxidative damage markers thiobarbituric acid-reactive substances and protein carbonyls in the heart, whereas proteasomal activity was similar in both groups. Nrf2-dependent gene expression may be impaired during the aging process. Therefore, we measured Nrf2 and its target genes glutathione-S-transferase (GST), γ-glutamyl cysteine synthetase (γ-GCS), nicotinamide adenine dinucleotide phosphate [NAD(P)H]:quinone oxidoreductase (NQO1), and paraoxonase-2 (PON2) in the hearts of these mice. Nrf2 as well as GST, γ-GCS, NQO1, and PON2 mRNA levels were significantly higher in heart tissue of the HP as compared to the LP group. The HP-fed mice had significantly higher PON1 activity in serum compared to those receiving the LP diet. Furthermore, HP feeding increased relative SIRT1 mRNA levels. Additional mechanistic cell culture experiments were performed, and they suggest that the olive oil phenolic hydroxytyrosol present in the HP oil may be responsible for the induction of Nrf2-dependent gene expression and the increase in PON activity. In conclusion, a diet rich in olive oil phenolics may prevent oxidative stress in the heart of SAMP8 mice by modulating Nrf2-dependent gene expression.

Glutathione peroxidase-2 and selenium decreased inflammation and tumors in a mouse model of inflammation-associated carcinogenesis whereas sulforaphane effects differed with selenium supply

Chronic inflammation and selenium deficiency are considered as risk factors for colon cancer. The protective effect of selenium might be mediated by specific selenoproteins, such as glutathione peroxidases (GPx). GPx-1 and -2 double knockout, but not single knockout mice, spontaneously develop ileocolitis and intestinal cancer. Since GPx2 is induced by the chemopreventive sulforaphane (SFN) via the nuclear factor E2-related factor 2 (Nrf2)/Keap1 system, the susceptibility of GPx2-KO and wild-type (WT) mice to azoxymethane and dextran sulfate sodium (AOM/DSS)-induced colon carcinogenesis was tested under different selenium states and SFN applications. WT and GPx2-KO mice were grown on a selenium-poor, -adequate or -supranutritional diet. SFN application started either 1 week before (SFN4) or along with (SFN3) a single AOM application followed by DSS treatment for 1 week. Mice were assessed 3 weeks after AOM for colitis and Nrf2 target gene expression and after 12 weeks for tumorigenesis. NAD(P)H:quinone oxidoreductases, thioredoxin reductases and glutathione-S-transferases were upregulated in the ileum and/or colon by SFN, as was GPx2 in WT mice. Inflammation scores were more severe in GPx2-KO mice and highest in selenium-poor groups. Inflammation was enhanced by SFN4 in both genotypes under selenium restriction but decreased in selenium adequacy. Total tumor numbers were higher in GPx2-KO mice but diminished by increasing selenium in both genotypes. SFN3 reduced inflammation and tumor multiplicity in both Se-adequate genotypes. Tumor size was smaller in Se-poor GPx2-KO mice. It is concluded that GPx2, although supporting tumor growth, inhibits inflammation-mediated tumorigenesis, but the protective effect of selenium does not strictly depend on GPx2 expression. Similarly, SFN requires selenium but not GPx2 for being protective.

Long-term selenium deficiency increases the pathogenicity of a Citrobacter rodentium infection in mice

Citrobacter rodentium is a mouse pathogen that causes infectious colitis and shares characteristics with human enteropathogenic (EPEC) and enterohemorrhagic (EHEC) Escherichia coli, including the ability to cause attaching and effacing lesions in the colon and serves as a useful model to study the pathogenicity of these bacteria. In this study, mice were fed a selenium-deficient diet for 5 or 20 weeks and then infected with C. rodentium. Colonization of the colon by C. rodentium was similar in mice fed adequate or selenium-deficient diets, but total bacterial colonization of the spleen was elevated in mice fed selenium-deficient diet for 20 weeks. Infection-induced changes to the colon included inflammatory cell infiltration, gross changes in crypt architecture, and ulceration and denuding of the epithelial layer that were greatest in mice fed a selenium-deficient diet for 20 weeks. Expression of pro-inflammatory genes was significantly higher 12-days post-infection in mice fed the selenium-deficient diet for 20 weeks compared to mice fed a selenium-adequate diet or selenium-deficient diet for 5 weeks. Diarrhea was prevalent in mice fed the selenium-deficient diet for 20 weeks but not 5 weeks, and this was associated with decreased expression of solute carrier family 26a3 and carbonic anhydrase IV, genes involved in ion transport. These results indicated that selenium played an important role in resistance to the pathological effects of a C. rodentium infection, and therefore, selenium status may be important in the expression of human disease caused by common food-borne bacteria.

Di (2-ethylhexyl) phthalate inhibits growth of mouse ovarian antral follicles through an oxidative stress pathway

Di (2-ethylhexyl) phthalate (DEHP) is a plasticizer that has been shown to inhibit growth of mouse antral follicles, however, little is known about the mechanisms by which DEHP does so. Oxidative stress has been linked to follicle growth inhibition as well as phthalate-induced toxicity in non-ovarian tissues. Thus, we hypothesized that DEHP causes oxidative stress and that this leads to inhibition of the growth of antral follicles. To test this hypothesis, antral follicles isolated from CD-1 mice (age 31-35days) were cultured with vehicle control (dimethylsulfoxide [DMSO]) or DEHP (1-100μg/ml)±N-acetyl cysteine (NAC, an antioxidant at 0.25-1mM). During culture, follicles were measured daily. At the end of culture, follicles were collected and processed for in vitro reactive oxygen species (ROS) assays to measure the presence of free radicals or for measurement of the expression and activity of various key antioxidant enzymes: Cu/Zn superoxide dismutase (SOD1), glutathione peroxidase (GPX) and catalase (CAT). The results indicate that DEHP inhibits the growth of follicles compared to DMSO control and that NAC (0.25-1mM) blocks the ability of DEHP to inhibit follicle growth. Furthermore, DEHP (10μg/ml) significantly increases ROS levels and reduces the expression and activity of SOD1 compared to DMSO controls, whereas NAC (0.5mM) rescues the effects of DEHP on ROS levels and SOD1. However, the expression and activity of GPX and CAT were not affected by DEHP treatment. Collectively, these data suggest that DEHP inhibits follicle growth by inducing production of ROS and by decreasing the expression and activity of SOD1.

Basic principles and emerging concepts in the redox control of transcription factors

Convincing concepts of redox control of gene transcription have been worked out for prokaryotes and lower eukaryotes, whereas the knowledge on complex mammalian systems still resembles a patchwork of poorly connected findings. The article, therefore, reviews principles of redox regulation with special emphasis on chemical feasibility, kinetic requirements, specificity, and physiological context, taking well investigated mammalian transcription factor systems, nuclear transcription factor of bone marrow-derived lymphocytes (NF-κB), and kelch-like ECH-associated protein-1 (Keap1)/Nrf2, as paradigms. Major conclusions are that (i) direct signaling by free radicals is restricted to O(2)•- and •NO and can be excluded for fast reacting radicals such as •OH, •OR, or Cl•; (ii) oxidant signals are H(2)O(2), enzymatically generated lipid hydroperoxides, and peroxynitrite; (iii) free radical damage is sensed via generation of Michael acceptors; (iv) protein thiol oxidation/alkylation is the prominent mechanism to modulate function; (v) redox sensors must be thiol peroxidases by themselves or proteins with similarly reactive cysteine or selenocysteine (Sec) residues to kinetically compete with glutathione peroxidase (GPx)- and peroxiredoxin (Prx)-type peroxidases or glutathione-S-transferases, respectively, a postulate that still has to be verified for putative mammalian sensors. S-transferases and Prxs are considered for system complementation. The impact of NF-κB and Nrf2 on hormesis, management of inflammatory diseases, and cancer prevention is critically discussed.

Aldo-keto reductase-7A protects liver cells and tissues from acetaminophen-induced oxidative stress and hepatotoxicity

Aldo-keto reductase-7A (AKR7A) is an enzyme important for bioactivation and biodetoxification. Previous studies suggested that Akr7a might be transcriptionally regulated by oxidative stress-responsive transcription factor nuclear factor erythroid 2 p45-related factor 2 (Nrf2), a protein highly responsive to acetaminophen (APAP) or its intermediate metabolite, N-acetyl-p-benzoquinoneimine (NAPQI). This study was, therefore, carried out to investigate whether Akr7a is involved in the protection against APAP-induced oxidative stress and hepatotoxicity. We found that in response to APAP or NAPQI exposure, Akr7a3 mRNA and protein were significantly up-regulated in vitro in human HepG2 and LO2 cells. Similarly, strong induction was observed for Akr7a5 in mouse AML12 hepatocytes exposed to APAP. In vivo in wild-type rats, significant up-regulation of hepatic AKR7A1 protein was observed after administration of APAP. On the other hand, depletion of Nrf2 reduced the expression of Akr7a3, suggesting that Nrf2, indeed, contributes significantly to the induction of Akr7a. Moreover, loss of cell viability in Nrf2-depleted cells was significantly rescued by coexpression of AKR7A3. Furthermore, increased AKR7A3 in HepG2 cells was associated with the up-regulation of oxidative stress-related enzymes to enhance cellular antioxidant defense, which appeared to contribute significantly to protection against APAP-induced toxicity. In a line of transgenic rats overexpressing AKR7A1, increased AKR7A1 stimulated the expression of Nrf2 and other Nrf2-regulated genes, but did not better protect rats from APAP insults. In contrast, depletion of Akr7a5 in vitro in cultured AML12 cells or depletion of Akr7a1 in vivo in rat liver greatly increased APAP-induced hepatotoxicity.

CONCLUSION

AKR7A proteins are significantly up-regulated in response to APAP/NAPQI exposure to contribute significantly to protection against APAP-induced hepatotoxicity. AKR7A mediates this protection, in part, through enhancing hepatocellular antioxidant defense.

Glutathione peroxidase-1 in health and disease: from molecular mechanisms to therapeutic opportunities

Reactive oxygen species, such as superoxide and hydrogen peroxide, are generated in all cells by mitochondrial and enzymatic sources. Left unchecked, these reactive species can cause oxidative damage to DNA, proteins, and membrane lipids. Glutathione peroxidase-1 (GPx-1) is an intracellular antioxidant enzyme that enzymatically reduces hydrogen peroxide to water to limit its harmful effects. Certain reactive oxygen species, such as hydrogen peroxide, are also essential for growth factor-mediated signal transduction, mitochondrial function, and maintenance of normal thiol redox-balance. Thus, by limiting hydrogen peroxide accumulation, GPx-1 also modulates these processes. This review explores the molecular mechanisms involved in regulating the expression and function of GPx-1, with an emphasis on the role of GPx-1 in modulating cellular oxidant stress and redox-mediated responses. As a selenocysteine-containing enzyme, GPx-1 expression is subject to unique forms of regulation involving the trace mineral selenium and selenocysteine incorporation during translation. In addition, GPx-1 has been implicated in the development and prevention of many common and complex diseases, including cancer and cardiovascular disease. This review discusses the role of GPx-1 in these diseases and speculates on potential future therapies to harness the beneficial effects of this ubiquitous antioxidant enzyme.

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.

Hormetics: dietary triggers of an adaptive stress response

A series of dietary ingredients and metabolites are able to induce an adaptive stress response either by generation of reactive oxygen species (ROS) and/or via activation of the Nrf2/Keap1 stress response network. Most of the molecules belong to activated Michael acceptors, electrophiles capable to S-alkylate redox sensitive cysteine thiols. This review summarizes recent advances in the (re)search of these compounds and classifies them into distinct groups. More than 60 molecules are described that induce the Nrf2 network, most of them found in our daily diet. Although known as typical antioxidants, a closer look reveals that these molecules induce an initial mitochondrial or cytosolic ROS formation and thereby trigger an adaptive stress response and hormesis, respectively. This, however, leads to higher levels of intracellular glutathione and increased expression levels of antioxidant enzymes such as glutathione peroxidase, thioredoxin reductase, and superoxide dismutase. According to this principle, the author suggests the term hormetics to describe these indirect antioxidants.

Epigallocatechin-3-gallate prevents lupus nephritis development in mice via enhancing the Nrf2 antioxidant pathway and inhibiting NLRP3 inflammasome activation

Patients with lupus nephritis show an impaired oxidative status and increased levels of interleukin (IL)-1β and IL-18, which are closely linked to inflammation and correlated with disease activity. Although epigallocatechin-3-gallate (EGCG), the major bioactive polyphenol present in green tea with antioxidant and free radical scavenging activities, has been reported to have anti-inflammatory effects by inhibiting nuclear factor-kappa B (NF-κB)-mediated inflammatory responses in vivo, its effectiveness for the treatment of lupus nephritis is still unknown. In the present study, 12-week-old New Zealand black/white (NZB/W) F1 lupus-prone mice were treated daily with EGCG by gavage until sacrificed at 34 weeks old for clinical, pathological, and mechanistic evaluation. We found that the administration (1) prevented proteinuria, renal function impairment, and severe renal lesions; (2) increased renal nuclear factor E2-related factor 2 (Nrf2) and glutathione peroxidase activity; (3) reduced renal oxidative stress, NF-κB activation, and NLRP3 mRNA/protein expression and protein levels of mature caspase-1, IL-1β, and IL-18; and (4) enhanced splenic regulatory T (Treg) cell activity. Our data clearly demonstrate that EGCG has prophylactic effects on lupus nephritis in these mice that are highly associated with its effects of enhancing the Nrf2 antioxidant signaling pathway, decreasing renal NLRP3 inflammasome activation, and increasing systemic Treg cell activity.

AHR2 knockdown prevents PAH-mediated cardiac toxicity and XRE- and ARE-associated gene induction in zebrafish (Danio rerio)

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants often present in aquatic systems as complex mixtures. Embryonic fish are sensitive to the developmental toxicity of some PAHs, but the exact mechanisms involved in this toxicity are still unknown. This study explored the role of the aryl hydrocarbon receptor (AHR) in the oxidative stress response of zebrafish to the embryotoxicity of select PAHs. Embryos were exposed to two PAHs, benzo[k]fluoranthene (BkF; a strong AHR agonist) and fluoranthene (FL; a cytochrome P4501A (CYP1A) inhibitor), alone and in combination. CYP1A, CYP1B1, CYP1C1, and redox-responsive genes glutathione s-transferase pi 2 (GSTp2), glutathione peroxidase 1 (GPx1), the glutamate-cysteine ligase catalytic subunit (GCLc), MnSOD and CuZnSOD mRNA expression was examined. CYP1 activity was measured via an in vivo ethoxyresorufin-O-deethlyase (EROD) activity assay, and the area of the pericardium was measured as an index of cardiotoxicity. BkF or FL alone caused no deformities whereas BkF+FL resulted in extreme pericardial effusion. BkF induced CYP activity above controls and co-exposure with FL inhibited this activity. BkF induced expression of all three CYPs, GSTp2, and GCLc. BkF+FL caused greater than additive induction of the three CYPs, GSTp2, GPx1, and GCLc but had no effect on MnSOD or CuZnSOD. AHR2 knockdown protected against the cardiac deformities caused by BkF+FL and significantly inhibited the induction of the CYPs, GSTp2, GPx1, and GCLc after BkF+FL compared to non-injected controls. These results further show the protective role of AHR2 knockdown against cardiotoxic PAHs and the role of AHR2 as a mediator of redox-responsive gene induction.

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.

Selective killing of cancer cells by a small molecule targeting the stress response to ROS

Malignant transformation, driven by gain-of-function mutations in oncogenes and loss-of-function mutations in tumour suppressor genes, results in cell deregulation that is frequently associated with enhanced cellular stress (for example, oxidative, replicative, metabolic and proteotoxic stress, and DNA damage). Adaptation to this stress phenotype is required for cancer cells to survive, and consequently cancer cells may become dependent upon non-oncogenes that do not ordinarily perform such a vital function in normal cells. Thus, targeting these non-oncogene dependencies in the context of a transformed genotype may result in a synthetic lethal interaction and the selective death of cancer cells. Here we used a cell-based small-molecule screening and quantitative proteomics approach that resulted in the unbiased identification of a small molecule that selectively kills cancer cells but not normal cells. Piperlongumine increases the level of reactive oxygen species (ROS) and apoptotic cell death in both cancer cells and normal cells engineered to have a cancer genotype, irrespective of p53 status, but it has little effect on either rapidly or slowly dividing primary normal cells. Significant antitumour effects are observed in piperlongumine-treated mouse xenograft tumour models, with no apparent toxicity in normal mice. Moreover, piperlongumine potently inhibits the growth of spontaneously formed malignant breast tumours and their associated metastases in mice. Our results demonstrate the ability of a small molecule to induce apoptosis selectively in cells that have a cancer genotype, by targeting a non-oncogene co-dependency acquired through the expression of the cancer genotype in response to transformation-induced oxidative stress.

Antroquinonol reduces oxidative stress by enhancing the Nrf2 signaling pathway and inhibits inflammation and sclerosis in focal segmental glomerulosclerosis mice

Oxidative stress, inflammation, and fibrosis are involved in the development and progression of focal segmental glomerulosclerosis (FSGS), a common form of idiopathic nephrotic syndrome that represents a therapeutic challenge because it has a poor response to steroids. Antroquinonol (Antroq), a purified compound, is a major active component of a mushroom, namely Antrodia camphorata, that grows in the camphor tree in Taiwan, and it has inhibitory effects on nitric oxide production and inflammatory reactions. We hypothesized that Antroq might ameliorate FSGS renal lesions by modulating the pathogenic pathways of oxidative stress, inflammation, and glomerular sclerosis in the kidney. We demonstrate that Antroq significantly (1) attenuates proteinuria, renal dysfunction, and glomerulopathy, including epithelial hyperplasia lesions and podocyte injury; (2) reduces oxidative stress, leukocyte infiltration, and expression of fibrosis-related proteins in the kidney; (3) increases renal nuclear factor E2-related factor 2 (Nrf2) and glutathione peroxidase activity; and (4) inhibits renal nuclear factor-κB (NF-κB) activation and decreases levels of transforming growth factor (TGF)-β1 in serum and kidney tissue in a mouse FSGS model. Our data suggest that Antroq might be a potential therapeutic agent for FSGS, acting by boosting Nrf2 activation and suppressing NF-κB-dependent inflammatory and TGF-β1-mediated fibrosis pathways in the kidney.

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.

Nrf2-dependent gene expression is affected by the proatherogenic apoE4 genotype-studies in targeted gene replacement mice

An apoE4 genotype is an important risk factor for cardiovascular and other chronic diseases. The higher cardiovascular disease risk of apoE4 carriers as compared to the apoE3 genotype has been mainly attributed to the differences in blood lipids between the two genotype subgroups. Recently, a potential protective role of the transcription factor Nrf2 in cardiovascular disease prevention has been suggested. In this study, we show that Nrf2-dependent gene expression is affected by the apoE genotype. ApoE4 vs. apoE3 mice exhibited lower hepatic Nrf2 nuclear protein levels. Furthermore, mRNA and protein levels of Nrf2 target genes including glutathione-S-transferase, heme oxygenase-1 and NAD(P)H dehydrogenase, quinone 1 were significantly lower in apoE4 as compared to apoE3 mice. Lower hepatic mRNA levels of phase II enzymes, as observed in apoE4 vs. apoE3 mice, were accompanied by higher mRNA levels of phase I enzymes including Cyp26a1 and Cyp3a16. Furthermore, miRNA-144, miRNA-125b, and miRNA-29a involved in Nrf2 signaling, inflammation, and regulation of phase I enzyme gene expression were affected by the apoE genotype. We provide first evidence that Nrf2 is differentially regulated in response to the apoE genotype.

3,3'-Diindolylmethane but not indole-3-carbinol activates Nrf2 and induces Nrf2 target gene expression in cultured murine fibroblasts

There is increasing interest in the gene-regulatory activity of Brassica vegetable derived phytochemicals such as 3,3'-diindolylmethane (DIM) and indole-3-carbinol (I3C). DIM is formed under acidic conditions by dimerization of I3C. This study compared the Nrf2 activating potential of DIM and I3C in murine fibroblasts (NIH3T3). In contrast to its precursor I3C, DIM induces the transactivation of Nrf2. Furthermore, Nrf2 targets such as HO-1, γGCS and NQO1 were increased on the mRNA and protein levels following DIM treatment. DIM was less potent than sulforaphane (used as positive control) in inducing Nrf2-dependent gene expression. The present data suggest that the dimerization of I3C to DIM increases its Nrf2 inducing activity.

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.

Breakdown products of neoglucobrassicin inhibit activation of Nrf2 target genes mediated by myrosinase-derived glucoraphanin hydrolysis products

Glucosinolates (GLSs) present in Brassica vegetables serve as precursors for biologically active metabolites, which are released by myrosinase and induce phase 2 enzymes via the activation of Nrf2. Thus, GLSs are generally considered beneficial. The pattern of GLSs in plants is various, and contents of individual GLSs change with growth phase and culture conditions. Whereas some GLSs, for example, glucoraphanin (GRA), the precursor of sulforaphane (SFN), are intensively studied, functions of others such as the indole GLS neoglucobrassicin (nGBS) are rather unknown as are functions of combinations thereof. We therefore investigated myrosinase-treated GRA, nGBS and synthetic SFN for their ability to induce NAD(P)H:quinone oxidoreductase 1 (NQO1) as typical phase 2 enzyme, and glutathione peroxidase 2 (GPx2) as novel Nrf2 target in HepG2 cells. Breakdown products of nGBS potently inhibit both GRA-mediated stimulation of NQO1 enzyme and Gpx2 promoter activity. Inhibition of promoter activity depends on the presence of an intact xenobiotic responsive element (XRE) and is also observed with benzo[a]pyrene, a typical ligand of the aryl hydrocarbon receptor (AhR), suggesting that suppressive effects of nGBS are mediated via AhR/XRE pathway. Thus, the AhR/XRE pathway can negatively interfere with the Nrf2/ARE pathway which has consequences for dietary recommendations and, therefore, needs further investigation.

Nrf2-ARE stress response mechanism: a control point in oxidative stress-mediated dysfunctions and chronic inflammatory diseases

Nrf2, a redox sensitive transcription factor, plays a pivotal role in redox homeostasis during oxidative stress. Nrf2 is sequestered in cytosol by an inhibitory protein Keap1 which causes its proteasomal degradation. In response to electrophilic and oxidative stress, Nrf2 is activated, translocates to nucleus, binds to antioxidant response element (ARE), thus upregulates a battery of antioxidant and detoxifying genes. This function of Nrf2 can be significant in the treatment of diseases, such as cancer, neurodegenerative, cardiovascular and pulmonary complications, where oxidative stress causes Nrf2 derangement. Nrf2 upregulating potential of phytochemicals has been explored, in facilitating cure for various ailments while, in cancer cells, Nrf2 upregulation causes chemoresistance. Therefore, Nrf2 emerges as a key regulator in oxidative stress-mediated diseases and Nrf2 silencing can open avenues in cancer treatment. This review summarizes Nrf2-ARE stress response mechanism and its role as a control point in oxidative stress-induced cellular dysfunctions including chronic inflammatory diseases.

Ascorbic acid partly antagonizes resveratrol mediated heme oxygenase-1 but not paraoxonase-1 induction in cultured hepatocytes - role of the redox-regulated transcription factor Nrf2

BACKGROUND

Both resveratrol and vitamin C (ascorbic acid) are frequently used in complementary and alternative medicine. However, little is known about the underlying mechanisms for potential health benefits of resveratrol and its interactions with ascorbic acid.

METHODS

The antioxidant enzymes heme oxygenase-1 and paraoxonase-1 were analysed for their mRNA and protein levels in HUH7 liver cells treated with 10 and 25 μmol/l resveratrol in the absence and presence of 100 and 1000 μmol/l ascorbic acid. Additionally the transactivation of the transcription factor Nrf2 and paraoxonase-1 were determined by reporter gene assays.

RESULTS

Here, we demonstrate that resveratrol induces the antioxidant enzymes heme oxygenase-1 and paraoxonase-1 in cultured hepatocytes. Heme oxygenase-1 induction by resveratrol was accompanied by an increase in Nrf2 transactivation. Resveratrol mediated Nrf2 transactivation as well as heme oxygenase-1 induction were partly antagonized by 1000 μmol/l ascorbic acid.

CONCLUSIONS

Unlike heme oxygenase-1 (which is highly regulated by Nrf2) paraoxonase-1 (which exhibits fewer ARE/Nrf2 binding sites in its promoter) induction by resveratrol was not counteracted by ascorbic acid. Addition of resveratrol to the cell culture medium produced relatively low levels of hydrogen peroxide which may be a positive hormetic redox-signal for Nrf2 dependent gene expression thereby driving heme oxygenase-1 induction. However, high concentrations of ascorbic acid manifold increased hydrogen peroxide production in the cell culture medium which may be a stress signal thereby disrupting the Nrf2 signalling pathway.

Loss of GPx2 increases apoptosis, mitosis, and GPx1 expression in the intestine of mice

Localization of glutathione peroxidase 2 (GPx2), the gastrointestinal form of GPx's, in the intestinal crypt epithelium points to a specific but so-far unknown function of this particular GPx. Therefore, the consequences of a GPx2 knockout were tested in mice fed a selenium-restricted, Se-adequate, or Se-supplemented diet. An unexpected increase in total GPx activity was found throughout the intestine in selenium-fed GPx2 knockout (KO) animals. Immunohistochemistry revealed a strong increase in GPx1 in the colon and ileum, especially in crypt bases where typically GPx2 is localized. GPx1 mRNA was not enhanced in GPx2 KO, indicating that up-regulation most probably occurs at the translational level. Loss of GPx2 was accompanied by an increase in apoptotic cells at colonic crypt bases, an area essential for the self-renewal of the intestinal epithelium, particularly under selenium restriction. Additionally, mitotic cells increased in the middle parts of the crypts, indicating an extension of the proliferative area. These findings corroborate a role for GPx2 in regulating mucosal homeostasis. In GPx2 KO mice, an increase in GPx1 can only partially compensate for GPx2, even under selenium supplementation, indicating that GPx2 is the major antiapoptotic GPx in the colon. These data explain why spontaneous ileocolitis becomes manifested only if both Gpx2 and Gpx1 are deleted.

Cancer chemoprevention mechanisms mediated through the Keap1-Nrf2 pathway

The cap'n'collar (CNC) bZIP transcription factor Nrf2 controls expression of genes for antioxidant enzymes, metal-binding proteins, drug-metabolising enzymes, drug transporters, and molecular chaperones. Many chemicals that protect against carcinogenesis induce Nrf2-target genes. These compounds are all thiol-reactive and stimulate an adaptive response to redox stress in cells. Such agents induce the expression of genes that posses an antioxidant response element (ARE) in their regulatory regions. Under normal homeostatic conditions, Nrf2 activity is restricted through a Keap1-dependent ubiquitylation by Cul3-Rbx1, which targets the CNC-bZIP transcription factor for proteasomal degradation. However, as the substrate adaptor function of Keap1 is redox-sensitive, Nrf2 protein evades ubiquitylation by Cul3-Rbx1 when cells are treated with chemopreventive agents. As a consequence, Nrf2 accumulates in the nucleus where it heterodimerizes with small Maf proteins and transactivates genes regulated through an ARE. In this review, we describe synthetic compounds and phytochemicals from edible plants that induce Nrf2-target genes. We also discuss evidence for the existence of different classes of ARE (a 16-bp 5'-TMAnnRTGABnnnGCR-3' versus an 11-bp 5'-RTGABnnnGCR-3', with or without the embedded activator protein 1-binding site 5'-TGASTCA-3'), species differences in the ARE-gene battery, and the identity of critical Cys residues in Keap1 required for de-repression of Nrf2 by chemopreventive agents.

The cap'n'collar transcription factor Nrf2 mediates both intrinsic resistance to environmental stressors and an adaptive response elicited by chemopreventive agents that determines susceptibility to electrophilic xenobiotics

Transcription factor Nrf2 regulates genes encoding drug-metabolising enzymes and drug transporters, as well as enzymes involved in the glutathione, thioredoxin and peroxiredoxin antioxidant pathways. Using mouse embryonic fibroblast (MEF) cells from Nrf2(+/+) and Nrf2(-/-) mice, in conjunction with the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) cytotoxicity assay, we have shown that loss of Nrf2 diminishes the intrinsic resistance of mutant fibroblasts towards isothiocyanates (i.e. sulforaphane), epoxides (i.e. (2S,3S)-(-)-3-phenylglycidol, ethyl 3-phenylglycidate and styrene-7,8-epoxide), peroxides, hydroquinones and quinones (i.e. tert-butylhydroperoxide, tert-butylhydroquinone and 2,3-dimethoxynaphthoquinone), NaAsO(2), and various mutagens, including β-propiolactone, cisplatin, mechlorethamine and methyl methanesulfonate to ∼50% of that observed in equivalent wild-type cells. Exposure of Nrf2(+/+) fibroblasts, but not Nrf2(-/-) fibroblasts, to a non-toxic dose (3μmol/l) of the chemopreventive agent sulforaphane (Sul) stimulated an adaptive response that, 18h after first being subjected to the isothiocyanate, caused an induction of between 2- and 10-fold in the levels of mRNA for glutamate-cysteine ligase catalytic (Gclc) and modifier (Gclm) subunits, glutathione S-transferases and NAD(P)H:quinone oxidoreductase-1 (Nqo1); this was accompanied by an increase in total glutathione of between 1.5- and 1.9-fold. Pre-treatment of Nrf2(+/+) MEF cells with 3μM Sul for 18h prior to challenge with xenobiotics, conferred between 2.0- and 4.0-fold protection against isothiocyanates, reactive carbonyls, peroxides, quinones, NaAsO(2), and the anticancer nitrogen mustard chlorambucil, but pre-treatment with 3μM Sul produced no such increased tolerance in Nrf2(-/-) MEF cells. The inducible resistance towards acrolein, cumene hydroperoxide and chlorambucil, produced by pre-treating wild-type fibroblasts with 3μM Sul, was dependent on glutathione because simultaneous pre-treatment with 5μmol/l buthionine sulfoximine abolished the increased tolerance of these xenobiotics. However, inducible resistance towards menadione that occurred upon pre-treatment with 3μM Sul was independent of glutathione and may be due to upregulation of Nqo1. Thus Nrf2 controls cellular resistance against electrophiles.

The Nrf2 system as a potential target for the development of indirect antioxidants

Oxidative stress causes damage to multiple cellular components such as DNA, proteins, and lipids, and is implicated in various human diseases including cancer, neurodegeneration, inflammatory diseases, and aging. In response to oxidative attack, cells have developed an antioxidant defense system to maintain cellular redox homeostasis and to protect cells from damage. The thiol-containing small molecules (e.g. glutathione), reactive oxygen species-inactivating enzymes (e.g. glutathione peroxidase), and phase 2 detoxifying enzymes (e.g. NAD(P)H: quinine oxidoreductase 1 and glutathione-S-transferases) are members of this antioxidant system. NF-E2-related factor 2 (Nrf2) is a CNC-bZIP transcription factor which regulates the basal and inducible expression of a wide array of antioxidant genes. Following dissociation from the cytosolic protein Keap1, a scaffolding protein which binds Nrf2 and Cul3 ubiquitin ligase for proteasome degradation, Nrf2 rapidly accumulates in the nucleus and transactivates the antioxidant response element in the promoter region of many antioxidant genes. The critical role of Nrf2 has been demonstrated by various animal studies showing that mice with a targeted disruption of the nrf2 gene are prone to develop lesions in response to environmental toxicants/carcinogens, drugs, and inflammatory insults. In this review, we discuss the role of the Nrf2 system, with particular focus on Nrf2-controlled target genes and the potential pleiotropic effects of Nrf2 activation of indirect antioxidants.

Hepatitis B virus induces expression of antioxidant response element-regulated genes by activation of Nrf2

The expression of a variety of cytoprotective genes is regulated by short cis-acting elements in their promoters, called antioxidant response elements (AREs). A central regulator of ARE-mediated gene expression is the NF-E2-related factor 2 (Nrf2). Human hepatitis B virus (HBV) induces a strong activation of Nrf2/ARE-regulated genes in vitro and in vivo. This is triggered by the HBV-regulatory proteins (HBx and LHBs) via c-Raf and MEK. The Nrf2/ARE-mediated induction of cytoprotective genes by HBV results in a better protection of HBV-positive cells against oxidative damage as compared with control cells. Furthermore, there is a significantly increased expression of the Nrf2/ARE-regulated proteasomal subunit PSMB5 in HBV-positive cells that is associated with a decreased level of the immunoproteasome subunit PSMB5i. In accordance with this finding, HBV-positive cells display a higher constitutive proteasome activity and a decreased activity of the immunoproteasome as compared with control cells even after interferon α/γ treatment. The HBV-dependent induction of Nrf2/ARE-regulated genes might ensure survival of the infected cell, shape the immune response to HBV, and thereby promote establishment of the infection.

Ionizing radiation activates the Nrf2 antioxidant response

The transcription factor NF-E2-related factor 2 (Nrf2) binds the antioxidant DNA response element (ARE) to activate important cellular cytoprotective defense systems. Recently several types of cancers have been shown to overexpress Nrf2, but its role in the cellular response to radiation therapy has yet to be fully determined. In this study, we report that single doses of ionizing radiation from 2 to 8 Gy activate ARE-dependent transcription in breast cancer cells in a dose-dependent manner, but only after a delay of five days. Clinically relevant daily dose fractions of radiation also increased ARE-dependent transcription, but again only after five days. Downstream activation of Nrf2-ARE-dependent gene and protein markers, such as heme oxygenase-1, occurred, whereas Nrf2-deficient fibroblasts were incapable of these responses. Compared with wild-type fibroblasts, Nrf2-deficient fibroblasts had relatively high basal levels of reactive oxygen species that increased greatly five days after radiation exposure. Further, in vitro clonogenic survival assays and in vivo sublethal whole body irradiation tests showed that Nrf2 deletion increased radiation sensitivity, whereas Nrf2-inducing drugs did not increase radioresistance. Our results indicate that the Nrf2-ARE pathway is important to maintain resistance to irradiation, but that it operates as a second-tier antioxidant adaptive response system activated by radiation only under specific circumstances, including those that may be highly relevant to tumor response during standard clinical dose-fractionated radiation therapy.

Enhanced hepatic Nrf2 activation after ursodeoxycholic acid treatment in patients with primary biliary cirrhosis

The cytoprotective mechanisms of ursodeoxycholic acid (UDCA) in primary biliary cirrhosis (PBC) have not been fully clarified. UDCA has some antioxidant properties. Nuclear factor-E2-related factor-2 (Nrf2) plays a critical role in protecting a variety of tissues against oxidative stress. Therefore, to investigate the potential antioxidant effects of UDCA in PBC, we determined the intracellular status of both oxidant stress and antioxidant defenses in paired pre- and posttreatment liver biopsies from 13 PBC patients by immunodetection of 8-hydroxydeoxyguanosine (8-OHdG), Nrf2-, and Nrf2-mediated antioxidant proteins. After UDCA treatment, the number of 8-OHdG-positive hepatocytes or bile duct cells decreased with improvement of hepatic injury. The hepatic levels of both total and phosphorylated Nrf2 protein were increased, along with upregulation of nuclear phosphorylated Nrf2 expression in bile duct cells. In addition, the levels of both thioredoxin (TRX) and thioredoxin reductase 1 (TrxR1) protein were increased in the liver after UDCA. The upregulation of hepatic TRX or TrxR1 protein expression positively correlated with that of total Nrf2 protein expression. In conclusion, UDCA treatment can enhance hepatic Nrf2 activation and upregulate hepatic TRX and TrxR1 protein expression. Hepatic Nrf2 activation may play a role in the therapeutic response to UDCA in PBC.

PAC1 gene knockout reveals an essential role of chaperone-mediated 20S proteasome biogenesis and latent 20S proteasomes in cellular homeostasis

The 26S proteasome, a central enzyme for ubiquitin-dependent proteolysis, is a highly complex structure comprising 33 distinct subunits. Recent studies have revealed multiple dedicated chaperones involved in proteasome assembly both in yeast and in mammals. However, none of these chaperones is essential for yeast viability. PAC1 is a mammalian proteasome assembly chaperone that plays a role in the initial assembly of the 20S proteasome, the catalytic core of the 26S proteasome, but does not cause a complete loss of the 20S proteasome when knocked down. Thus, both chaperone-dependent and -independent assembly pathways exist in cells, but the contribution of the chaperone-dependent pathway remains unclear. To elucidate its biological significance in mammals, we generated PAC1 conditional knockout mice. PAC1-null mice exhibited early embryonic lethality, demonstrating that PAC1 is essential for mammalian development, especially for explosive cell proliferation. In quiescent adult hepatocytes, PAC1 is responsible for producing the majority of the 20S proteasome. PAC1-deficient hepatocytes contained normal amounts of the 26S proteasome, but they completely lost the free latent 20S proteasome. They also accumulated ubiquitinated proteins and exhibited premature senescence. Our results demonstrate the importance of the PAC1-dependent assembly pathway and of the latent 20S proteasomes for maintaining cellular integrity.

Hyperoxia sensing: from molecular mechanisms to significance in disease

Oxygen therapy using mechanical ventilation with hyperoxia is necessary to treat patients with respiratory failure and distress. However, prolonged exposure to hyperoxia leads to the generation of excessive reactive oxygen species (ROS), causing cellular damage and multiple organ dysfunctions. As the lungs are directly exposed, hyperoxia can cause both acute and chronic inflammatory lung injury and compromise innate immunity. ROS may contribute to pulmonary oxygen toxicity by overwhelming redox homeostasis, altering signaling cascades that affect cell fate, ultimately leading to hyperoxia-induced acute lung injury (HALI). HALI is characterized by pronounced inflammatory responses with leukocyte infiltration, injury, and death of pulmonary cells, including epithelia, endothelia, and macrophages. Under hyperoxic conditions, ROS mediate both direct and indirect modulation of signaling molecules such as protein kinases, transcription factors, receptors, and pro- and anti-apoptotic factors. The focus of this review is to elaborate on hyperoxia-activated key sensing molecules and current understanding of their signaling mechanisms in HALI. A better understanding of the signaling pathways leading to HALI may provide valuable insights on its pathogenesis and may help in designing more effective therapeutic approaches.

An autoregulatory loop between Nrf2 and Cul3-Rbx1 controls their cellular abundance

The INrf2 (Keap1)/Cul3-Rbx1 complex constantly degrades Nrf2 under normal conditions. When a cell encounters oxidative or electrophilic stress, Nrf2 dissociates from the INrf2/Cul3-Rbx1 complex and translocates into the nucleus. In the nucleus, Nrf2 activates a myriad of antioxidant and defensive genes that protect cells. Nrf2 is then exported out of the nucleus and degraded. INrf2 serves as a substrate adaptor to link Nrf2 to Cul3 and Rbx1. Cul3 and Rbx1 make up the ubiquitin ligase complex that is responsible for the ubiquitination and degradation of Nrf2. Previously we have shown a feedback autoregulatory loop between Nrf2 and INrf2 indicating that Nrf2 regulates INrf2 by controlling its transcription. Here we are extending this research by demonstrating the presence of another feedback autoregulatory loop between Cul3-Rbx1 and Nrf2. Experiments using Hepa-1 and HepG2 cells indicate that Nrf2 controls its own degradation by regulating expression and induction of Cul3-Rbx1 genes. Treatment with the antioxidant tert-Butylhydroquinone (t-BHQ) leads to induction of Cul3-Rbx1 genes. Mutagenesis and transfection experiments identified an antioxidant response element in the forward and reverse strands of the proximal Cul3 and Rbx1 promoters, respectively, that Nrf2 binds and regulates expression and antioxidant induction of the Cul3-Rbx1 genes. In addition, short interfering RNA inhibition and overexpression of Nrf2 led to a respective decrease and increase in Cul3-Rbx1 gene expression. The increase in Cul3-Rbx1 leads to ubiquitination and degradation of Nrf2. These data suggest that Nrf2 regulates Cul3-Rbx1 by controlling regulation of expression and induction of Cul3-Rbx1. The induction of Cul3-Rbx1 control Nrf2 by increasing degradation.

A noncanonical mechanism of Nrf2 activation by autophagy deficiency: direct interaction between Keap1 and p62

In response to stress, cells can utilize several cellular processes, such as autophagy, which is a bulk-lysosomal degradation pathway, to mitigate damages and increase the chances of cell survival. Deregulation of autophagy causes upregulation of p62 and the formation of p62-containing aggregates, which are associated with neurodegenerative diseases and cancer. The Nrf2-Keap1 pathway functions as a critical regulator of the cell's defense mechanism against oxidative stress by controlling the expression of many cellular protective proteins. Under basal conditions, Nrf2 is ubiquitinated by the Keap1-Cul3-E3 ubiquitin ligase complex and targeted to the 26S proteasome for degradation. Upon induction, the activity of the E3 ubiquitin ligase is inhibited through the modification of cysteine residues in Keap1, resulting in the stabilization and activation of Nrf2. In this current study, we identified the direct interaction between p62 and Keap1 and the residues required for the interaction have been mapped to 349-DPSTGE-354 in p62 and three arginines in the Kelch domain of Keap1. Accumulation of endogenous p62 or ectopic expression of p62 sequesters Keap1 into aggregates, resulting in the inhibition of Keap1-mediated Nrf2 ubiquitination and its subsequent degradation by the proteasome. In contrast, overexpression of mutated p62, which loses its ability to interact with Keap1, had no effect on Nrf2 stability, demonstrating that p62-mediated Nrf2 upregulation is Keap1 dependent. These findings demonstrate that autophagy deficiency activates the Nrf2 pathway in a noncanonical cysteine-independent mechanism.

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.

Oncogenic NRF2 mutations in squamous cell carcinomas of oesophagus and skin

Nuclear factor erythroid-related factor 2 (NRF2) encodes a transcription factor that induces expression of cytoprotective proteins upon oxidative stress and oncogenic NRF2 mutations have been found in lung and head/neck cancers that inactivate KEAP1-mediated degradation of NRF2. The aim of this study was to catalogue NRF2 mutations in other human cancers. For this, we analysed 1145 cancer tissues from carcinomas from oesophagus, skin, uterine cervix, lung, larynx, breast, colon, stomach, liver, prostate, urinary bladder, ovary, uterine cervix, and kidney, and meningiomas, multiple myelomas, and acute leukaemias by single-strand conformation polymorphism (SSCP) assay. We detected NRF2 mutations in oesophagus (8/70; 11.4%), skin (1/17; 6.3%), lung (10/125; 8.0%), and larynx (3/23; 13.0%) cancers. Of note, all of the 22 mutations except one were found in squamous cell carcinomas (SCCs) (95.5%). The mutations were observed within or near DLG and ETGE motifs that are important in NRF2 and KEAP1 interaction. All of the oesophageal SCCs and skin SCCs with the NRF2 mutations showed increased NRF2 expression in the nuclei. However, none of the SCCs from oesophagus and skin harboured KEAP1 mutation. Our study demonstrated here that NRF2 mutation occurs not only in lung and head/neck cancers, but also in oesophageal and skin cancers. Our data suggest that the NRF2 mutation plays a role in the development of SCC and is a feature of SCC.

Role of antioxidants in the skin: anti-aging effects

Intracellular and extracellular oxidative stress initiated by reactive oxygen species (ROS) advance skin aging, which is characterized by wrinkles and atypical pigmentation. Because UV enhances ROS generation in cells, skin aging is usually discussed in relation to UV exposure. The use of antioxidants is an effective approach to prevent symptoms related to photo-induced aging of the skin. In this review, the mechanisms of ROS generation and ROS elimination in the body are summarized. The effects of ROS generated in the skin and the roles of ROS in altering the skin are also discussed. In addition, the effects of representative antioxidants on the skin are summarized with a focus on skin aging.

Effects of atmospheric pollutants on the Nrf2 survival pathway

BACKGROUND, AIM, AND SCOPE

Atmospheric pollution is a worldwide problem. Exposure to atmospheric pollutants causes toxic cellular effects. One of the mechanisms of toxicity by these pollutants is the promotion of oxidative stress. Several signaling pathways control cellular redox homeostasis. In this respect, nuclear factor erythroid 2-related factor 2 (Nrf2) is a crucial transcription factor in the cell's response to oxidative stress.

MAIN FEATURES

In cellular animal models, exposure to atmospheric pollutants activates Nrf2, attenuating its toxic and even its carcinogenic effects. Therefore, we have reviewed the scientific literature in order to indicate that air pollutants, such as particulate matter, polycyclic aromatic hydrocarbons, and gaseous matter, are Nrf2 pathway inductors, triggering self-defense through the establishment of proinflammatory and antioxidant responses.

RESULTS AND DISCUSSION

Exposure to reactive molecules as atmospheric pollutants causes the activation of Nrf2 and the subsequent regulation of the expression of cytoprotective and detoxifying enzymes, as well as antioxidants. Moreover, induction of Nrf2 prior to exposure reduces the harmful effects of pollutants. The present article discusses the protective role of the Nrf2 pathway against different atmospheric pollutant insults.

CONCLUSIONS

Nrf2 regulates the expression of numerous cytoprotective genes that function to detoxify reactive species produced during atmospheric pollutant metabolic reactions. From the papers highlighted in this review, we conclude that Nrf2 has an important role in the defense against atmospheric pollutant-induced toxicity.

PERSPECTIVES

Further studies are needed to understand the signaling events that turn on the system in response to atmospheric pollutant stress. This could allow for the possibility of targeting the pathway for prevention benefits in the near future.

Nrf2 target genes are induced under marginal selenium-deficiency

A suboptimal selenium supply appears to prevail in Europe. The current study, therefore, was focused on the changes in gene expression under a suboptimal selenium intake. Previous microarray analyses in the colon of mice fed either a selenium-adequate or a moderately deficient diet revealed a change in genes of several pathways. Severe selenium-deficiency has been found previously to influence Nrf2-regulated genes of the adaptive response. Since the previous pathway analyses were done with a program not searching for Nrf2 target genes, respective genes were manually selected and confirmed by qPCR. qPCR revealed an induction of phase II (Nqo1, Gsts, Sult1b1 and Ugt1a6) and antioxidant enzymes (Hmox1, Mt2, Prdx1, Srxn1, Sod1 and Gclc) under the selenium-poor diet, which is considered to compensate for the loss of selenoproteins. The strongest effects were observed in the duodenum where preferentially genes for antioxidant enzymes were up-regulated. These also include the mRNA of the selenoproteins TrxR1 and GPx2 that would enable their immediate translation upon selenium refeeding. The down-regulation of Gsk3β in moderate selenium-deficiency observed in the previous paper provides a possible explanation for the activation of the Nrf2 pathway, because inhibition of GSK3β results in the nuclear accumulation of Nrf2.

Changing paradigms in thiology from antioxidant defense toward redox regulation

The history of free radical biochemistry is briefly reviewed in respect to major trend shifts from the focus on radiation damage toward enzymology of radical production and removal and ultimately the role of radicals, hydroperoxides, and related fast reacting compounds in metabolic regulation. Selected aspects of the chemistry of radicals and hydroperoxides, the enzymology of peroxidases, and the biochemistry of adaptive responses and regulatory phenomena are compiled and discussed under the perspective of how the fragments of knowledge can be merged to biologically meaningful concepts of regulation. It is concluded that (i) not radicals but H(2)O(2), hydroperoxides, and peroxynitrite are the best candidates for oxidant signals, (ii) peroxidases of the GPx and Prx family or functionally equivalent proteins have the chance to specifically sense hydroperoxides and to transduce the oxidant signal, (iii) redox signaling proceeds via reactions known from thiol peroxidase and redoxin chemistry, (iv) proximal targets are proteins that are modified at SH groups, and (v) redoxins are documented signal transducers but also used as terminators. The importance of kinetics for forward signaling and for sensitivity modulation by competition is emphasized and ways to restore resting conditions are discussed. Research needs to validate emerging concepts are outlined.

The keap1-nrf2 cellular defense pathway: mechanisms of regulation and role in protection against drug-induced toxicity

Adverse drug reactions pose a significant public health problem. In some cases, the process of drug metabolism can contribute to the onset of toxicity through the bioactivation of a parent molecule to a chemically reactive intermediate. In order to maintain a favorable balance between bioactivation and detoxification, mammalian cells have evolved an inducible cell defense system known as the antioxidant response pathway. The activity of this cytoprotective pathway is largely regulated by the transcription factor Nrf2, which governs the expression of many phase II detoxification and antioxidant enzymes. In turn, the activity of Nrf2 is regulated by the cysteine-rich cytosolic inhibitor Keap1, which acts as a "sensor" for chemical/oxidative stress. This article summarizes our current understanding of the molecular mechanisms that regulate the function of the Keap1-Nrf2 pathway and highlights the importance of Nrf2 in the protection against drug-induced toxicity.

Molecular targets of dietary phenethyl isothiocyanate and sulforaphane for cancer chemoprevention

Development of cancer is a long-term and multistep process which comprises initiation, progression, and promotion stages of carcinogenesis. Conceivably, it can be targeted and interrupted along these different stages. In this context, many naturally occurring dietary compounds from our daily consumption of fruits and vegetables have been shown to possess cancer preventive effects. Phenethyl isothiocyanate (PEITC) and sulforaphane (SFN) are two of the most widely investigated isothiocyanates from the crucifers. Both have been found to be very potent chemopreventive agents in numerous animal carcinogenesis models as well as cell culture models. They exert their chemopreventive effects through regulation of diverse molecular mechanisms. In this review, we will discuss the molecular targets of PEITC and SFN potentially involved in cancer chemoprevention. These include the regulation of drug-metabolizing enzymes phase I cytochrome P450s and phase II metabolizing enzymes. In addition, the signaling pathways including Nrf2-Keap 1, anti-inflammatory NFkappaB, apoptosis, and cell cycle arrest as well as some receptors will also be discussed. Furthermore, we will also discuss the similarities and their potential differences in the regulation of these molecular targets by PEITC and SFN.

Human-specific induction of glutathione peroxidase-3 by proteasome inhibition in cardiovascular cells

Glutathione peroxidase-3 (GPx-3) is a key antioxidant enzyme in the plasma. GPx-3 was previously identified as the major antioxidative enzyme that was induced upon nontoxic proteasome inhibition in endothelial cells. Here, we investigated the determinants of the proteasome inhibitor-induced expression of GPx-3. Nontoxic proteasome inhibition massively upregulates GPx-3 RNA and protein in human umbilical cord vein cells within 24 h. Surprisingly, induction of GPx-3 was species-specific for human cells. The exponential upregulation of GPx-3 is mediated by transcriptional activation of the human GPx-3 promoter and, in addition, stabilization of GPx-3 mRNA: in reporter gene assays with full-length and deleted variants of the human GPx-3 promoter we identified a putative antioxidative response element (ARE) as essential and also sufficient for transcriptional activation of GPx-3 by proteasome inhibition. However, the ARE-specific antioxidative transcription factor Nrf2 is not involved in the activation of GPx-3. UV-crosslinking using the 3'UTR of GPx-3 revealed an altered protein binding pattern in the presence of proteasome inhibitors, thus indicating regulation of mRNA stability of human GPx-3. As GPx-3 is secreted into the plasma, our data point toward a borderline defense mechanism of endothelial cell-derived GPx-3 to protect the vasculature from oxidative stress.

Nrf2-dependent and -independent responses to nitro-fatty acids in human endothelial cells: identification of heat shock response as the major pathway activated by nitro-oleic acid

Electrophilic fatty acid derivatives, including nitrolinoleic acid and nitro-oleic acid (OA-NO(2)), can mediate anti-inflammatory and pro-survival signaling reactions. The transcription factor Nrf2, activated by electrophilic fatty acids, suppresses redox-sensitive pro-inflammatory gene expression and protects against vascular endothelial oxidative injury. It was therefore postulated that activation of Nrf2 by OA-NO(2) accounts in part for its anti-inflammatory actions, motivating the characterization of Nrf2-dependent and -independent effects of OA-NO(2) on gene expression using genome-wide transcriptional profiling. Control and Nrf2-small interfering RNA-transfected human endothelial cells were treated with vehicle, oleic acid, or OA-NO(2), and differential gene expression profiles were determined. Although OA-NO(2) significantly induced the expression of Nrf2-dependent genes, including heme oxygenase-1 and glutamate-cysteine ligase modifier subunit, the majority of OA-NO(2)-regulated genes were regulated by Nrf2-independent pathways. Moreover, gene set enrichment analysis revealed that the heat shock response is the major pathway activated by OA-NO(2), with robust induction of a number of heat shock genes regulated by the heat shock transcription factor. Inasmuch as the heat shock response mediates anti-inflammatory and cytoprotective actions, this mechanism is proposed to contribute to the protective cell signaling functions of nitro-fatty acids and other electrophilic fatty acid derivatives.

Comprehensive proteomic and transcriptomic analysis reveals early induction of a protective anti-oxidative stress response by low-dose proteasome inhibition

Effective inhibition of the proteasome by high doses of proteasome inhibitors induces apoptotic cell death. In contrast, partial proteasome inhibition by low inhibitor doses mediates a protective cellular stress response. The early targets and mediators of these dose-dependent effects of proteasome inhibitors are unknown. Primary human umbilical cord vein endothelial cells were treated with low and high doses of the proteasome inhibitor MG132 for 2 h. In a combined 2-DE and MS approach, we identified more than 20 new targets of proteasome inhibition. These proteins are involved in cell cycle regulation, signaling, cytoskeletal rearrangement, and cellular stress response. Accompanying Affymetrix analysis revealed that these proteins are not regulated on the transcriptional level but are mainly stabilized by proteasome inhibition. The proteasome-dependent accumulation of the anti-oxidative sensor proteins DJ-1, peroxiredoxin-1 and -6 was accompanied by dose-dependent induction of oxidative stress after 2 h of proteasome inhibition and contributed to the differential transcriptional stress response to low- and high-dose proteasome inhibition: Whereas low-dose proteasome inhibition induces a transcriptional profile reminiscent of a physiological stress response that preconditions and protects endothelial cells from oxidative stress, high inhibitor doses induce massive transcriptional dysregulation and pronounced oxidative stress triggering apoptosis.

Nrf2 promotes neuronal cell differentiation

The transcription factor Nrf2 has emerged as a master regulator of the endogenous antioxidant response, which is critical in defending cells against environmental insults and in maintaining intracellular redox balance. However, whether Nrf2 has any role in neuronal cell differentiation is largely unknown. In this report, we have examined the effects of Nrf2 on cell differentiation using a neuroblastoma cell line, SH-SY5Y. Retinoic acid (RA) and 12-O-tetradecanoylphorbol 13-acetate, two well-studied inducers of neuronal differentiation, are able to induce Nrf2 and its target gene NAD(P)H quinone oxidoreductase 1 in a dose- and time-dependent manner. RA-induced Nrf2 up-regulation is accompanied by neurite outgrowth and an induction of two neuronal differentiation markers, neurofilament-M and microtubule-associated protein 2. Overexpression of Nrf2 in SH-SY5Y cells promotes neuronal differentiation, whereas inhibition of endogenous Nrf2 expression inhibited neuronal differentiation. More remarkably, the positive role of Nrf2 in neuronal differentiation was verified ex vivo in primary neuron culture. Primary neurons isolated from Nrf2-null mice showed a retarded progress in differentiation, compared to those from wild-type mice. Collectively, our data demonstrate a novel role for Nrf2 in promoting neuronal cell differentiation, which will open new perspectives for therapeutic uses of Nrf2 activators in patients with neurodegenerative diseases.

Protection of vascular cells from oxidative stress by proteasome inhibition depends on Nrf2

AIMS

Increased levels of reactive oxygen species cause oxidative stress and severely damage lipids, proteins, and DNA. We have previously shown that partial proteasome inhibition induces an antioxidative gene pattern in endothelial cells. Here, we elucidate the mechanisms of proteasome inhibitor-mediated upregulation of antioxidative enzymes and cytoprotection.

METHODS AND RESULTS

Non-toxic proteasome inhibition upregulated mRNA and protein expression of superoxide dismutase 1 (SOD1) and haem oxygenase 1 (HO1) in several human endothelial and vascular smooth muscle cell types. Transcriptional activation of these enzymes was shown by inhibition of RNA polymerase II and nuclear run-on assays. Transfection of endothelial cells with luciferase reporter constructs revealed that upregulation can be largely confined to an antioxidant response element (ARE), which proved to be sufficient for transcriptional activation of SOD1 and HO1. Co-transfection studies and bandshift analyses confirmed binding of the antioxidative transcription factor NF-E2-related factor 2 (Nrf2)-which was stabilized by proteasome inhibition as shown by immunoblots-to the ARE site of HO1. Experiments with aortic endothelial and smooth muscle cells from Nrf2 wild-type and knockout mice revealed an essential role of Nrf2: in wild-type cells, proteasome inhibitor-mediated induction of SOD1 and HO1 was accompanied by protection of vascular cells against oxidative stress as determined by lactate dehydrogenase release assays. In contrast, proteasome inhibitor-mediated induction of antioxidative enzymes and cytoprotection were completely lost in cells from Nrf2 knockout mice.

CONCLUSION

Nrf2-dependent transcriptional activation of antioxidative enzymes is crucial for proteasome inhibitor-mediated protection of vascular cells against oxidative stress.

Redox mechanisms involved in the selective activation of Nrf2-mediated resistance versus p53-dependent apoptosis in adenocarcinoma cells

We have investigated the role of reactive oxygen species and thiol-oxidizing agents in the induction of cell death and have shown that adenocarcinoma gastric (AGS) cells respond differently to the oxidative challenge according to the signaling pathways activated. In particular, apoptosis in AGS cells is induced via the mitochondrial pathway upon treatment with thiol-oxidizing agents, such as diamide. Apoptosis is associated with persistent oxidative damage, as evidenced by the increase in carbonylated proteins and the expression/activation of DNA damage-sensitive proteins histone H2A.X and DNA-dependent protein kinase. Resistance to hydrogen peroxide is instead associated with Keap1 oxidation and rapid translocation of Nrf2 into the nucleus. Sensitivity to diamide and resistance to hydrogen peroxide are correlated with GSH redox changes, with diamide severely increasing GSSG, and hydrogen peroxide transiently inducing protein-GSH mixed disulfides. We show that p53 is activated in response to diamide treatment by the oxidative induction of the Trx1/p38(MAPK) signaling pathway. Similar results were obtained with another carcinoma cell line, CaCo2, indicating that these findings are not limited to AGS cells. Our data suggest that thiol-oxidizing agents could be exploited as inducers of apoptosis in tumor histotypes resistant to ROS-producing chemotherapeutics.

Part of the Series: From dietary antioxidants to regulators in cellular signaling and gene regulation

The association of decreased cancer risk with intake of cruciferous vegetables and selenium is stronger than that reported for fruits and vegetables in general. An active constituent in cruciferae is sulforaphane. Chemopreventive effects of both, sulforaphane and selenium have been attributed to an antioxidant action which certainly is too simplicistic. Sulforaphane induces via activation of the Nrf2/Keap1 system phase 2 enzymes that protect against carcinogens and oxidants. Induced enzymes comprise the selenoproteins thioredoxin reductase-1 (TrxR1) and gastrointestinal glutathione peroxidase (GI-GPx, GPx2), which contain antioxidant response elements (ARE) in their promoter regions. Translational realisation of the enhanced transcripts depends on adequate selenium supply, which explains the synergism of Nrf2 activators and selenium. Regarding tumorigenesis the role of TrxR1 is ambiguous: it is essential for fast tumor cell growth but also diminishes vascularisation of tumors. The anticarcinogenic role of GI-GPx is evident from enhanced gastrointestinal tumor formation in gpx2/gpx1 double KO mice.