Accelerated DNA adduct formation in the lung of the Nrf2 knockout mouse exposed to diesel exhaust

Regulation of Nrf2, NF-kappaB, and AP-1 signaling pathways by chemopreventive agents

The inhibition of carcinogenesis by chemopreventive agents has been demonstrated in many tumorigenesis animal models. The chemopreventive mechanisms of those phytochemicals have been investigated extensively, though mostly in in vitro cell culture systems. The cellular signaling cascades mediated by transcription factors, including nuclear factor E2-related factor 2 (Nrf2), nuclear factor-kappaB (NF-kappaB), and activator protein-1 (AP-1), have been shown to play pivotal roles in tumor initiation, promotion, and progression processes. Thus, as demonstrated by previous substantive mechanistic studies, they appear to be ideal targets for cancer chemoprevention. In this review, we discuss the current progress and future challenges on our understanding of the molecular mechanisms in cancer chemoprevention by phytochemicals, focusing on the regulation of Nrf2, NF-kappaB, and AP-1 signaling pathways.

Air pollution and cancer: biomarker studies in human populations

Large cohort studies in the U.S. and in Europe suggest that air pollution may increase lung cancer risk. Biomarkers can be useful to understand the mechanisms and to characterize high-risk groups. Here we describe biomarkers of exposure, in particular DNA adducts as well as markers of early damage, including mutagenicity, other endpoints of genotoxicity and molecular biomarkers of cancer. Several studies found an association between external measures of exposure to air pollution and increased levels of DNA adducts, with an apparent levelling-off of the dose-response relationship. Also, numerous experimental studies in vitro and in vivo have provided unambiguous evidence for genotoxicity of air pollution. In addition, due to the organic extracts of particulate matter [especially various polycyclic aromatic hydrocarbon (PAH) compounds], particulate air pollution induces oxidative damage to DNA. The experimental work, combined with the data on frequent oxidative DNA damage in lymphocytes in people exposed to urban air pollution, suggests 8-oxo-dG as one of the important promutagenic lesions. Lung cancer develops through a series of progressive pathological changes occurring in the respiratory epithelium. Molecular alterations such as loss of heterozygosity, gene mutations and aberrant gene promoter methylation have emerged as potentially promising molecular biomarkers of lung carcinogenesis. Data from such studies relevant for emissions rich in PAHs are also summarized, although the exposure circumstances are not directly relevant to outdoor air pollution, in order to shed light on potential mechanisms of air pollution-related carcinogenesis.

Oxidative stress-induced DNA damage by particulate air pollution

Exposure to ambient air particulate matter (PM) is associated with pulmonary and cardiovascular diseases and cancer. The mechanisms of PM-induced health effects are believed to involve inflammation and oxidative stress. The oxidative stress mediated by PM may arise from direct generation of reactive oxygen species from the surface of particles, soluble compounds such as transition metals or organic compounds, altered function of mitochondria or NADPH-oxidase, and activation of inflammatory cells capable of generating ROS and reactive nitrogen species. Resulting oxidative DNA damage may be implicated in cancer risk and may serve as marker for oxidative stress relevant for other ailments caused by particulate air pollution. There is overwhelming evidence from animal experimental models, cell culture experiments, and cell free systems that exposure to diesel exhaust and diesel exhaust particles causes oxidative DNA damage. Similarly, various preparations of ambient air PM induce oxidative DNA damage in in vitro systems, whereas in vivo studies are scarce. Studies with various model/surrogate particle preparations, such as carbon black, suggest that the surface area is the most important determinant of effect for ultrafine particles (diameter less than 100 nm), whereas chemical composition may be more important for larger particles. The knowledge concerning mechanisms of action of PM has prompted the use of markers of oxidative stress and DNA damage for human biomonitoring in relation to ambient air. By means of personal monitoring and biomarkers a few studies have attempted to characterize individual exposure, explore mechanisms and identify significant sources to size fractions of ambient air PM with respect to relevant biological effects. In these studies guanine oxidation in DNA has been correlated with exposure to PM(2.5) and ultrafine particles outdoor and indoor. Oxidative stress-induced DNA damage appears to an important mechanism of action of urban particulate air pollution. Related biomarkers and personal monitoring may be useful tools for risk characterization.

Nrf2 as a target for cancer chemoprevention

Chemical insults, whether of endogenous or exogenous origins, play major roles in the etiopathogenesis of many cancers. As such, strategies to blunt their formation and limit their damage to biomolecules are a central aspect of chemoprevention. Cellular defenses against such insults are regulated in part by the transcription factor Nrf2. Nrf2, in turn, regulates gene expression through interactions with the ARE (antioxidant-response-element) found in the promoter regions of many cytoprotective genes. Under basal conditions, Nrf2 is tethered in the cytoplasm to an actin binding protein Keap1. Pharmacological and food-derived agents such as dithiolethiones and isothiocyanates trigger the release of Nrf2 from Keap1, allowing it to translocate into the nucleus and stimulate gene transcription. Studies using nrf2-deficient mice have revealed that Nrf2 regulates basal and inducible expression of multiple categories of genes, including xenobiotic-metabolizing enzymes, antioxidant enzymes, molecular chaperones/stress response proteins, as well as proteasome subunits, that collectively reflect the complex and important role Nrf2 plays in the cellular defense against carcinogens. Nrf2 knockout mice are greatly predisposed to chemical-induced DNA damage and exhibit higher susceptibility towards cancer development in several models of chemical carcinogenesis. Nrf2 also mediates protection against oxidative stress and influences inflammatory processes, both of which contribute to carcinogenesis. Observations that nrf2-deficient mice are refractory to the protective actions of some chemopreventive agents highlight the importance of the Keap1-Nrf2-ARE signaling pathway as a molecular target for prevention.

Dietary cancer-chemopreventive compounds: from signaling and gene expression to pharmacological effects

The process of cancer development (carcinogenesis leading to advanced metastasized cancers) in humans generally takes many years through initiation, promotion and progression. Because advanced metastasized cancers are almost impossible to treat, cancer chemoprevention for the control and containment of early cancer development is highly desirable. Recent studies have provided strong evidence that many daily-consumed dietary compounds possess cancer-protective properties that might interrupt the carcinogenesis process. These properties include the induction of cellular defense detoxifying and antioxidant enzymes, which can protect against cellular damage caused by environmental carcinogens or endogenously generated reactive oxygen species. These compounds can also affect cell-death signaling pathways, which could prevent the proliferation of tumor cells. In this review, we will summarize current knowledge on dietary cancer-chemopreventive compounds and their induction of detoxifying enzymes and anti-proliferative effects, and discuss the challenges in translating these signaling and gene-expression events to pharmacological effects.

Bioactive S-alk(en)yl cysteine sulfoxide metabolites in the genus Allium: the chemistry of potential therapeutic agents

S-Alk(en)yl cysteine sulfoxides are odourless, non-protein sulfur amino acids typically found in members of the family Alliaceae and are the precursors to the lachrymatory and flavour compounds found in the agronomically important genus Allium. Traditionally, Allium species, particularly the onion (Allium cepa) and garlic (A. sativum), have been used for centuries in European, Asian and American folk medicines for the treatment of numerous human pathologies, however it is only recently that any significant progress has been made in determining their mechanisms of action. Indeed, our understanding of the role of Allium species in human health undoubtedly comes from the combination of several academic disciplines including botany, biochemistry and nutrition. During tissue damage, S-alk(en)yl cysteine sulfoxides are converted to their respective thiosulfinates or propanethial-S-oxide by the action of the enzyme alliinase (EC 4.4.1.4). Depending on the Allium species, and under differing conditions, thiosulfinates can decompose to form additional sulfur constituents including diallyl, methyl allyl, and diethyl mono-, di-, tri-, tetra-, penta-, and hexasulfides, the vinyldithiins and (E)- and (Z)-ajoene. Recent reports have shown onion and garlic extracts, along with several principal sulfur constituents, can induce phase II detoxification enzymes like glutathione-S-transferases (EC 2.5.1.18) and quinone reductase (QR) NAD(P)H: (quinine acceptor) oxidoreductase (EC 1.6.99.2) in mammalian tissues, as well as also influencing cell cycle arrest and apoptosis in numerous in vitro cancer cell models. Moreover, studies are also beginning to highlight a role of Allium-derived sulfur compounds in cardiovascular protection. In this review, we discuss the chemical diversity of S-alk(en)yl cysteine sulfoxide metabolites in the context of their biochemical and pharmacological mechanisms.

Nrf2-Keap1 defines a physiologically important stress response mechanism

The transcription factor Nrf2 regulates the basal and inducible expression of numerous detoxifying and antioxidant genes. The cytoplasmic protein Keap1 interacts with Nrf2 and represses its function. Analysis of keap1-knockout mice provides solid evidence that Keap1 acts as a negative regulator of Nrf2 and as a sensor of xenobiotic and oxidative stresses. The simultaneous ablation of the keap1 and nrf2 genes reversed all apparent phenotypes of the Keap1-deficient mice, suggesting that Nrf2 is a primary target of Keap1. The Nrf2-Keap1 system is now recognized as one of the major cellular defence mechanisms against oxidative and xenobiotic stresses. Furthermore, extensive studies have suggested that the Nrf2-Keap1 system contributes to protection against various pathologies, including carcinogenesis, liver toxicity, respiratory distress and inflammation.

Molecular mechanisms activating the Nrf2-Keap1 pathway of antioxidant gene regulation

Several years have passed since NF-E2-related factor 2 (Nrf2) was demonstrated to regulate the induction of genes encoding antioxidant proteins and phase 2 detoxifying enzymes. Following a number of studies, it was realized that Nrf2 is a key factor for cytoprotection in various aspects, such as anticarcinogenicity, neuroprotection, antiinflammatory response, and so forth. These widespread functions of Nrf2 spring from the coordinated actions of various categories of target genes. The activation mechanism of Nrf2 has been studied extensively. Under normal conditions, Nrf2 localizes in the cytoplasm where it interacts with the actin binding protein, Kelch-like ECH associating protein 1 (Keap1), and is rapidly degraded by the ubiquitin-proteasome pathway. Signals from reactive oxygen species or electrophilic insults target the Nrf2-Keap1 complex, dissociating Nrf2 from Keap1. Stabilized Nrf2 then translocates to the nuclei and transactivates its target genes. Interestingly, Keap1 is now assumed to be a substrate-specific adaptor of Cul3-based E3 ubiquitin ligase. Direct participation of Keap1 in the ubiquitination and degradation of Nrf2 is plausible. The Nrf2-Keap1 system is present not only in mammals, but in fish, suggesting that its roles in cellular defense are conserved throughout evolution among vertebrates. This review article recounts recent knowledge of the Nrf2-Keap1 system, focusing especially on the molecular mechanism of Nrf2 regulation.

Evolutionary conserved N-terminal domain of Nrf2 is essential for the Keap1-mediated degradation of the protein by proteasome

Under homeostatic conditions, Nrf2 activity is constitutively repressed. This process is dependent on Keap1, to which Nrf2 binds through the Neh2 domain. Since the N-terminal subdomain of Neh2 (Neh2-NT) contains evolutionarily conserved motifs, we examined the roles they play in the degradation of Nrf2. In Neh2-NT, we defined a novel motif that is distinct from the previously characterized DIDLID motif and designated it DLG motif. Deletion of Neh2-NT or mutation of the DLG motif largely abolished the Keap1-mediated degradation of Nrf2. These mutations were found to enfeeble the binding affinity of Nrf2 to Keap1. The Neh2-NT subdomain directed DLG-dependent, Keap1-independent, degradation of a reporter protein in the nucleus. By contrast, mutation of DLG did not affect the half-life of native Nrf2 protein in the nucleus under oxidative stress conditions. These results thus demonstrate that DLG motif plays essential roles in the Keap1-mediated proteasomal degradation of Nrf2 in the cytoplasm.

Keap1 is a redox-regulated substrate adaptor protein for a Cul3-dependent ubiquitin ligase complex

The bZIP transcription factor Nrf2 controls a genetic program that protects cells from oxidative damage and maintains cellular redox homeostasis. Keap1, a BTB-Kelch protein, is the major upstream regulator of Nrf2 and controls both the subcellular localization and steady-state levels of Nrf2. In this report, we demonstrate that Keap1 functions as a substrate adaptor protein for a Cul3-dependent E3 ubiquitin ligase complex. Keap1 assembles into a functional E3 ubiquitin ligase complex with Cul3 and Rbx1 that targets multiple lysine residues located in the N-terminal Neh2 domain of Nrf2 for ubiquitin conjugation both in vivo and in vitro. Keap1-dependent ubiquitination of Nrf2 is inhibited following exposure of cells to quinone-induced oxidative stress and sulforaphane, a cancer-preventive isothiocyanate. A mutant Keap1 protein containing a single cysteine-to-serine substitution at residue 151 within the BTB domain of Keap1 is markedly resistant to inhibition by either quinone-induced oxidative stress or sulforaphane. Inhibition of Keap1-dependent ubiquitination of Nrf2 correlates with decreased association of Keap1 with Cul3. Neither quinone-induced oxidative stress nor sulforaphane disrupts association between Keap1 and Nrf2. Our results suggest that the ability of Keap1 to assemble into a functional E3 ubiquitin ligase complex is the critical determinant that controls steady-state levels of Nrf2 in response to cancer-preventive compounds and oxidative stress.

Chemoprevention through the Keap1-Nrf2 signaling pathway by phase 2 enzyme inducers

One successful strategy for cancer chemoprevention is modulation of drug metabolizing enzymes, leading to a facilitated elimination of endogenous and environmental carcinogens. Inducers of phase 2 enzymes such as dithiolethiones inhibit tumorigenesis of environmental carcinogens in various animal models and modulate the metabolism of the carcinogen aflatoxin B1 in human clinical trials. Increasing lines of evidence show that the Keap1-Nrf2 complex is a key molecular target of chemopreventive phase 2 enzyme inducers. The transcription factor Nrf2 is a member of the basic leucine-zipper NF-E2 family and interacts with the antioxidant response element (ARE) in the promoter region of phase 2 detoxifying enzymes. A cytoplasmic actin-binding protein, Keap1, is an inhibitor of Nrf2 that sequesters it in the cytoplasm. Inducers dissociate this complex, allowing Nrf2 to translocate to the nucleus. Disruption of the nrf2 gene in mice leads to the loss of chemopreventive efficacy by inducers. This review focuses on (1) the role of Nrf2 in the regulation of phase 2 and antioxidative genes, (2) the molecular actions of dithiolethiones on the Keap1-Nrf2 pathway, and (3) the contribution of Nrf2-regulated gene families to the cytoprotective actions of dithiolethiones and other inducers. Rapidly accumulating data on this pathway is providing insight into the coordinated mammalian defense systems against electrophiles and oxidative stresses and the means by which it may be targeted by small molecules.

Induction of detoxifying enzymes by garlic organosulfur compounds through transcription factor Nrf2: effect of chemical structure and stress signals

Garlic organosulfur compounds (OSCs) are recognized as a group of potential chemopreventive compounds. It is known that garlic OSCs can modulate drug metabolism systems, especially various phase II detoxifying enzymes, though the mechanism underlying their inductive effect on these enzymes remains largely unknown. In the present study, we investigated the transcriptional levels of NAD(P)H:quinone oxidoreductase 1 (NQO1) and heme oxygenase 1 (HO1) genes, the reporter activity mediated by antioxidant response element (ARE), and the protein level of transcription factor nuclear factor E2-related factor 2 (Nrf2), after administration of three major garlic OSCs--diallyl sulfide (DAS), diallyl disulfide (DADS), and diallyl trisulfide (DATS)--in human hepatoma HepG2 cells. Our results showed that ARE activation and Nrf2 protein accumulation were well correlated with phase II gene expression induction. The structure-activity relationship study indicated that the third sulfur in the structure of OSCs contributed substantially to their bioactivities, and that allyl-containing OSCs were more potent than propyl-containing OSCs. To better understand the signaling events involved in the upregulation of detoxifying enzymes by DATS, ARE activity and Nrf2 protein levels were examined after transient transfection of HepG2 cells with mutant Nrf2, cotreatment with antioxidants, and pretreatment with protein kinase inhibitors. DATS-induced ARE activity was inhibited by dominant-negative Nrf2 Kelch-like ECH-associating protein 1 and constructs. Cotreatment with thiol antioxidants decreased the ARE activity and Nrf2 protein level induced by DATS. Three major mitogen-activated protein kinases (MAPKs)--extracellular signal-regulated protein kinase, c-Jun N-terminal kinase, and p38--were activated by DATS treatment. However, the inhibition of these MAPKs did not affect DATS-induced ARE activity. Pretreatment with various upstream protein kinase inhibitors showed that the protein kinase C pathway was not directly involved in DATS-induced ARE activity, but instead the calcium-dependent signaling pathway appeared to play a role in the DATS-induced cytoprotective effect.

Identification of polymorphisms in the promoter region of the human NRF2 gene

Transcription factor Nrf2 regulates the basal and inducible expression of detoxifying and antioxidant genes. Recent studies using nrf2-null mice suggest that Nrf2 dysfunction might be involved in the pathogenesis of human diseases. To gain insight into the relationship between impairment in the NRF2 gene and human diseases, we attempted to identify polymorphisms in the human NRF2 gene. We determined the structure of the NRF2 gene and found three single nucleotide polymorphisms and one triplet repeat polymorphism in its regulatory region. These results provide a molecular basis for the genetic analysis of the NRF2 gene. The frequency of each polymorphism was examined in two groups of patients with systemic lupus erythematosus and chronic obstructive pulmonary disease. This study did not reveal a close connection between the risk of these diseases and the polymorphisms. However, available lines of evidence suggest the importance of examining the link between NRF2 polymorphisms and other oxidative stress-related diseases.

An important role of Nrf2-ARE pathway in the cellular defense mechanism

The antioxidant responsive element (ARE) is a cis-acting regulatory element of genes encoding phase II detoxification enzymes and antioxidant proteins, such as NAD(P)H: quinone oxidoreductase 1, glutathione S-transferases, and glutamate-cysteine ligase. Interestingly, it has been reported that Nrf2 (NF-E2-related factor 2) regulates a wide array of ARE-driven genes in various cell types. Nrf2 is a basic leucine zipper transcription factor, which was originally identified as a binding protein of locus control region of beta-globin gene. The DNA binding sequence of Nrf2 and ARE sequence are very similar, and many studies demonstrated that Nrf2 binds to the ARE sites leading to up-regulation of downstream genes. The function of Nrf2 and its downstream target genes suggests that the Nrf2-ARE pathway is important in the cellular antioxidant defense system. In support of this, many studies showed a critical role of Nrf2 in cellular protection and anti-carcinogenicity, implying that the Nrf2-ARE pathway may serve as a therapeutic target for neurodegenerative diseases and cancers, in which oxidative stress is closely implicated.

Nrf2 Is Essential for the Chemopreventive Efficacy of Oltipraz against Urinary Bladder Carcinogenesis

The induction of phase 2 detoxifying enzymes, such as UDP-glucuronosyltransferases (UGTs), in response to an array of naturally occurring and synthetic agents, such as oltipraz (4-methyl-5-[2-pyrazinyl]-1,2-dithiole-3-thione), provides an effective means of protection against a variety of carcinogens. Transcription factor Nrf2 is an essential regulator of the inducible expression of detoxifying enzyme genes by chemopreventive agents. In this study, we investigated in Nrf2-deficient mice the susceptibility to the urinary bladder-specific carcinogen N-nitrosobutyl(4-hydroxybutyl)amine (BBN) and the chemopreventive efficacy of oltipraz. The incidence of urinary bladder carcinoma by BBN was significantly higher in Nrf2-/- mice than in wild-type mice; invasive carcinoma was found in 24.0 and 38.5% of wild-type and Nrf2-/- mice, respectively. Oltipraz induced the phase 2 enzymes responsible for BBN detoxification in the liver and urinary bladder in an Nrf2-dependent manner. As expected, therefore, oltipraz decreased the incidence of urinary bladder carcinoma by BBN in wild-type mice but had little effect in Nrf2-/- mice. In wild-type mouse liver, oltipraz significantly induced BBN glucuronidation and decreased the urinary concentration of N-nitrosobutyl(3-carboxypropyl)amine, a proximate carcinogen of BBN. Importantly, BBN was found to suppress the expression of UGT1A specifically in the urinary bladder. This suppression was counteracted by oltipraz in wild-type mice but not in Nrf2-/- mice. These results show that Nrf2 and its downstream target genes are responsible for BBN detoxification. Furthermore, oltipraz prevents carcinogenesis by BBN by enhancing detoxification of this carcinogen in the liver and urinary bladder.

Nrf2 Is a Key Transcription Factor That Regulates Antioxidant Defense in Macrophages and Epithelial Cells: Protecting against the Proinflammatory and Oxidizing Effects of Diesel Exhaust Chemicals

The proinflammatory effects of particulate pollutants, including diesel exhaust particles (DEP), are related to their content of redox cycling chemicals and their ability to generate oxidative stress in the respiratory tract. An antioxidant defense pathway, which involves phase II enzyme expression, protects against the pro-oxidative and proinflammatory effects of DEP. The expression of enzymes, including heme oxygenase-1 (HO-1) and GST, is dependent on the activity of a genetic antioxidant response element in their promoters. In this study we investigated the mechanism by which redox cycling organic chemicals, prepared from DEP, induce phase II enzyme expression as a protective response. We demonstrate that aromatic and polar DEP fractions, which are enriched in polycyclic aromatic hydrocarbons and quinones, respectively, induce the expression of HO-1, GST, and other phase II enzymes in macrophages and epithelial cells. We show that HO-1 expression is mediated through accumulation of the bZIP transcription factor, Nrf2, in the nucleus, and that Nrf2 gene targeting significantly weakens this response. Nrf2 accumulation and subsequent activation of the antioxidant response element is regulated by the proteasomal degradation of Nrf2. This pathway is sensitive to pro-oxidative and electrophilic DEP chemicals and is also activated by ambient ultrafine particles. We propose that Nrf2-mediated phase II enzyme expression protects against the proinflammatory effects of particulate pollutants in the setting of allergic inflammation and asthma.

Nrf2 deficiency causes tooth decolourization due to iron transport disorder in enamel organ

Rodents have brownish-yellow incisors whose colour represents their iron content. Iron is deposited into the mature enamel by ameloblasts that outline enamel surface of the teeth. Nrf2 is a basic region-leucine zipper type transcription factor that regulates expression of a range of cytoprotective genes in response to oxidative and xenobiotic stresses. We found that genetically engineered Nrf2-deficient mice show decolourization of the incisors. While incisors of wild-type mice were brownish yellow, incisors of Nrf2-deficient mice were greyish white in colour. Micro X-ray imaging analysis revealed that the iron content in Nrf2-deficient mouse incisors were significantly decreased compared to that of wild-type mice. We found that iron was aberrantly deposited in the papillary layer cells of enamel organ in Nrf2-deficient mouse, suggesting that the iron transport from blood vessels to ameloblasts was disturbed. We also found that ameloblasts of Nrf2-null mouse show degenerative atrophy at the late maturation stage, which gives rise to the loss of iron deposition to the surface of mature enamel. Our results thus demonstrate that the enamel organ of Nrf2-deficient mouse has a reduced iron transport capacity, which results in both the enamel cell degeneration and disturbance of iron deposition on to the enamel surface.

A strategy for cancer prevention: Stimulation of the Nrf2-ARE signaling pathway

Many genes, with products involved in the protection of cells against carcinogens, oxidants, and other toxic chemicals, are under the transcriptional control of a simple DNA regulatory element [i.e., the antioxidant response element (ARE)]. One or more functional AREs have been confirmed or are believed to exist in the upstream region of many anticarcinogenic/antioxidant genes and have been shown to mediate the coordinate transcriptional up-regulation of these genes by many chemical agents [i.e., the ARE-mediated inducers]. There is strong evidence that increased expression of ARE-regulated genes inhibits cancer development. The signaling system leading to ARE activation has been partly elucidated, and nuclear factor erythroid 2–related factor 2 (Nrf2) has been identified as the key transcriptional factor that serves to transmit the inducer signal to ARE. It is now known that nuclear factor erythroid 2–related factor 2, which is normally sequestered in the cytoplasm by Kelch-like ECH-associated protein 1, dissociates from Kelch-like ECH-associated protein 1 on exposure to ARE-mediated inducers, translocates to the nucleus, complexes with other nuclear factors, and binds to ARE. Rapid and simple assays have been devised to identify chemical agents that can stimulate this signaling pathway. Moreover, many ARE-mediated inducers have been identified, and several of them have shown promising cancer preventive activity.

Redox-regulated Turnover of Nrf2 Is Determined by at Least Two Separate Protein Domains, the Redox-sensitive Neh2 Degron and the Redox-insensitive Neh6 Degron

The Nrf2 transcription factor is more rapidly turned over in cells grown under homeostatic conditions than in those experiencing oxidative stress. The variable turnover of Nrf2 is accomplished through the use of at least two degrons and its redox-sensitive interaction with the Kelch-repeat protein Keap1. In homeostatic COS1 cells, the Neh2 degron confers on Nrf2 a half-life of less than 10 min. Analyses of deletion mutants of a Gal4(HA)mNeh2 fusion protein and full-length mNrf2 indicate that full redox-sensitive Neh2 destabilizing activity depends upon two separate sequences within this N-terminal domain. The DIDLID element (amino acids 17-32) is indispensable for Neh2 activity and appears necessary to recruit a ubiquitin ligase to the fusion protein. A second motif within Neh2, the ETGE tetrapeptide (amino acids 79-82), allows the redox-sensitive recruitment of Nrf2 to Keap1. This interaction, which occurs only in homeostatic cells, enhances the capacity of the Neh2 degron to direct degradation by functioning downstream of ubiquitination mediated by the DIDLID element. By contrast with the situation under homeostatic conditions, the Neh2 degron is neither necessary nor sufficient to account for the characteristic half-life of Nrf2 in oxidatively stressed cells. Instead, the previously uncharacterized, redox-insensitive Neh6 degron (amino acids 329-379) is essential to ensure that the transcription factor is still appropriately turned over in stressed cells, albeit with an increased half-life of 40 min. A model can now be proposed to explain how the turnover of this protein adapts in response to alterations in cellular redox state.

The transcription factor NRF2 protects against pulmonary fibrosis

The molecular mechanisms of pulmonary fibrosis are poorly understood, although reactive oxygen species are thought to have an important role. NRF2 is a transcription factor that protects cells and tissues from oxidative stress by activating protective antioxidant and detoxifying enzymes. We hypothesized that NRF2 protects lungs from injury and fibrosis induced by bleomycin, an anti-neoplastic agent that causes pulmonary fibrosis in susceptible patients. To test this hypothesis, mice with targeted deletion of Nrf2 (Nrf2-/-) and wild-type (Nrf2+/+) mice were treated with bleomycin or vehicle, and pulmonary injury and fibrotic responses were compared. Bleomycin-induced increases in lung weight, epithelial cell death, and inflammation were significantly greater in Nrf2-/- mice than in Nrf2+/+ mice. Indices of lung fibrosis (hydroxyproline content, collagen accumulation, fibrotic score, cell proliferation) were significantly greater in bleomycin-treated Nrf2-/- mice, compared with Nrf2+/+ mice. NRF2 expression and activity were elevated in Nrf2+/+ mice by bleomycin. Bleomycin caused greater up-regulation of several NRF2-inducible antioxidant enzyme genes and protein products in Nrf2+/+ mice compared with Nrf2-/- mice. Further, bleomycin-induced transcripts and protein levels of lung injury and fibrosis markers were significantly attenuated in Nrf2+/+ mice compared with Nrf2-/- mice. Results demonstrated that NRF2 has a critical role in protection against pulmonary fibrosis, presumably through enhancement of cellular antioxidant capacity. This study has important implications for the development of intervention strategies against fibrosis.

Molecular mechanism activating Nrf2-Keap1 pathway in regulation of adaptive response to electrophiles

Electrophile responsive element (EpRE)-mediated gene induction is a pivotal mechanism of cellular defense against the toxicity of electrophiles and reactive oxygen species (ROS). Nrf2, which belongs to the cap'-n'-collar family of basic region-leucine zipper transcription factors, has emerged as an essential component of an EpRE-binding transcriptional complex. Detailed analysis of the regulatory mechanism governing Nrf2 activity led to the identification of Keap1, which represses Nrf2 activity by directly binding to the N-terminal Neh2 domain. Keap1 interaction with Neh2 leads to the sequestration of Nrf2 in the cytoplasm and to the enhancement of Nrf2 degradation by proteasomes conferring tight regulation on the response. Electrophiles act to counteract sequestration of Nrf2 by Keap1 and provoke Nrf2 activation. Constitutive activation of Nrf2-regulated transcription in Keap1 knockout mice clearly demonstrated that the disruption of Keap1 repression is sufficient for the activation of Nrf2. These observations indicated that the mechanism that modulates Nrf2-Keap1 interaction is pivotal for the cellular sensing mechanism for electrophiles. Recent analyses argue that the redox mechanism that modifies cysteine residues of Keap1 governs the Keap1-Nrf2 interaction and therefore is critical for sensing of electrophiles.

Small Maf proteins serve as transcriptional cofactors for keratinocyte differentiation in the Keap1-Nrf2 regulatory pathway

The small Maf proteins, MafF, MafG, and MafK, possess a leucine zipper (Zip) domain that is required for homodimer or heterodimer complex formation with other bZip transcription factors. In this study we sought to determine the identity of the specific constituent that collaboratively interacts with Nrf2 to bind to the Maf recognition element in vivo. Studies in vitro suggested that Nrf2 forms heterodimers with small Maf proteins and then bind to Maf recognition elements, but the bona fide partner molecules supporting Nrf2 activity in vivo have not been definitively identified. Nrf2 activity is usually suppressed by a cytoplasmic repressor, Keap1, so disruption of the keap1 gene causes constitutive activation of Nrf2. Nrf2 hyperactivity results in hyperproliferation of keratinocytes in the esophagus and forestomach leading to perinatal lethality. However, simultaneous disruption of nrf2 rescued keap1-null mice from the lethality. We exploited this system to investigate whether small Mafs are required for Nrf2 function. We generated keap1 and small maf compound mutant mice and examined whether keratinocyte abnormalities persisted in these animals. The data show that loss of mafG and mafF in the keap1-null mice reversed the lethal keratinocyte dysfunction and rescued the keap1-null mutant mice from perinatal lethality. This rescue phenotype of mafG::mafF::keap1 triple compound mutant mice phenocopies that of the nrf2::keap1 compound mutant mice, indicating that the small Maf proteins MafG and MafF must functionally cooperate with Nrf2 in vivo.

Transcription factor Nrf2 regulates inflammation by mediating the effect of 15-deoxy-Delta(12,14)-prostaglandin j(2)

Activated macrophages express high levels of Nrf2, a transcription factor that positively regulates the gene expression of antioxidant and detoxication enzymes. In this study, we examined how Nrf2 contributes to the anti-inflammatory process. As a model system of acute inflammation, we administered carrageenan to induce pleurisy and found that in Nrf2-deficient mice, tissue invasion by neutrophils persisted during inflammation and the recruitment of macrophages was delayed. Using an antibody against 15-deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)), it was observed that macrophages from pleural lavage accumulate 15d-PGJ(2). We show that in mouse peritoneal macrophages 15d-PGJ(2) can activate Nrf2 by forming adducts with Keap1, resulting in an Nrf2-dependent induction of heme oxygenase 1 and peroxiredoxin I (PrxI) gene expression. Administration of the cyclooxygenase 2 inhibitor NS-398 to mice with carrageenan-induced pleurisy caused persistence of neutrophil recruitment and, in macrophages, attenuated the 15d-PGJ(2) accumulation and PrxI expression. Administration of 15d-PGJ(2) into the pleural space of NS-398-treated wild-type mice largely counteracted both the decrease in PrxI and persistence of neutrophil recruitment. In contrast, these changes did not occur in the Nrf2-deficient mice. These results demonstrate that Nrf2 regulates the inflammation process downstream of 15d-PGJ(2) by orchestrating the recruitment of inflammatory cells and regulating the gene expression within those cells.

Free radical biology - terminology and critical thinking

What is an antioxidant? Can one, at a cellular level, speak of direct and indirect antioxidants? Can oxidative stress be quantified and characterized? What are the oxidant species that may have regulatory functions in a cell? Since the above concepts have become of frequent use in all Journals, it may be appropriate if some critical thinking outlined in this review could become available to a broad public.

Antioxidants enhance mammalian proteasome expression through the Keap1-Nrf2 signaling pathway

Proteasomes degrade damaged proteins formed during oxidative stress, thereby promoting cell survival. Neurodegenerative and other age-related disorders are associated with reduced proteasome activity. We show herein that expression of most subunits of 20S and 19S proteasomes, which collectively assemble the 26S proteasome, was enhanced up to threefold in livers of mice following treatment with dithiolethiones, which act as indirect antioxidants. Subunit protein levels and proteasome activity were coordinately increased. No induction was seen in mice where the transcription factor Nrf2 was disrupted. Promoter activity of the PSMB5 subunit of the 20S proteasome increased with either Nrf2 overexpression or treatment with antioxidants in mouse embryonic fibroblasts. Tandem antioxidant response elements in the proximal promoter of PSMB5 that controlled these responses were identified. We propose that induction of the 26S proteasome through the Nrf2 pathway represents an important indirect action of these antioxidants that can contribute to their protective effects against chronic diseases.

Deficiency of the Nrf1 and Nrf2 transcription factors results in early embryonic lethality and severe oxidative stress

Nrf1 and Nrf2 are members of the CNC family of bZIP transcription factors that exhibit structural similarities, and they are co-expressed in a wide range of tissues during development. Nrf2 has been shown to be dispensable for growth and development in mice. Nrf2-deficient mice, however, are impaired in oxidative stress defense. We previously showed that loss of Nrf1 function in mice results late gestational embryonic lethality. To determine whether Nrf1 and Nrf2 have overlapping functions during early development and in the oxidative stress response, we generated mice that are deficient in both Nrf1 and Nrf2. In contrast to the late embryonic lethality in Nrf1 mutants, compound Nrf1, Nrf2 mutants die early between embryonic days 9 and 10 and exhibit extensive apoptosis that is not observed in the single mutants. Loss of Nrf1 and Nrf2 leads to marked oxidative stress in cells that is indicated by elevated intracellular reactive oxygen species levels and cell death that is reversed by culturing under reduced oxygen tension or the addition of antioxidants. Compound mutant cells also show increased levels of p53 and induction of Noxa, a death effector p53 target gene, suggesting that cell death is potentially mediated by reactive oxygen species activation of p53. Moreover, we show that expression of genes related to antioxidant defense is severely impaired in compound mutant cells compared with single mutant cells. Together, these findings indicate that the functions of Nrf1 and Nrf2 overlap during early development and to a large extent in regulating antioxidant gene expression in cells.

Use of proteomics to demonstrate a hierarchical oxidative stress response to diesel exhaust particle chemicals in a macrophage cell line

Epidemiological studies demonstrate an association between short term exposure to ambient particulate matter (PM) and cardiorespiratory morbidity and mortality. Although the biological mechanisms of these adverse effects are unknown, emerging data suggest a key role for oxidative stress. Ambient PM and diesel exhaust particles (DEP) contain redox cycling organic chemicals that induce pro-oxidative and pro-inflammatory effects in the lung. These responses are suppressed by N-acetylcysteine (NAC), which directly complexes to electrophilic DEP chemicals and exert additional antioxidant effects at the cellular level. A proteomics approach was used to study DEP-induced responses in the macrophage cell line, RAW 264.7. We demonstrate that in the dose range 10-100 microg/ml, organic DEP extracts induce a progressive decline in the cellular GSH/GSSG ratio, in parallel with a linear increase in newly expressed proteins on the two-dimensional gel. Using matrix-assisted laser desorption ionization time-of-flight mass spectrometry and electrospray ionization-liquid chromatography/mass spectrometry/mass spectrometry analysis, 32 newly induced/NAC-suppressed proteins were identified. These include antioxidant enzymes (e.g. heme oxygenase-1 and catalase), pro-inflammatory components (e.g. p38MAPK and Rel A), and products of intermediary metabolism that are regulated by oxidative stress. Heme oxygenase-1 was induced at low extract dose and with minimal decline in the GSH/GSSG ratio, whereas MAP kinase activation required a higher chemical dose and incremental levels of oxidative stress. Moreover, at extract doses >50 microg/ml, there is a steep decline in cellular viability. These data suggest that DEP induce a hierarchical oxidative stress response in which some of these proteins may serve as markers for oxidative stress during PM exposures.

Keap1-null mutation leads to postnatal lethality due to constitutive Nrf2 activation

Transcription factor Nrf2 (encoded by Nfe2l2) regulates a battery of detoxifying and antioxidant genes, and Keap1 represses Nrf2 function. When we ablated Keap1, Keap1-deficient mice died postnatally, probably from malnutrition resulting from hyperkeratosis in the esophagus and forestomach. Nrf2 activity affects the expression levels of several squamous epithelial genes. Biochemical data show that, without Keap1, Nrf2 constitutively accumulates in the nucleus to stimulate transcription of cytoprotective genes. Breeding to Nrf2-deficient mice reversed the phenotypic Keap1 deficiencies. These experiments show that Keap1 acts upstream of Nrf2 in the cellular response to oxidative and xenobiotic stress.

Involvement of reactive oxygen species in the metabolic pathways triggered by diesel exhaust particles in human airway epithelial cells

Diesel exhaust particles (DEP) induce a proinflammatory response in human bronchial epithelial cells (16HBE) characterized by the release of proinflammatory cytokines after activation of transduction pathways involving MAPK and the transcription factor NF-kappaB. Because cellular effects induced by DEP are prevented by antioxidants, they could be mediated by reactive oxygen species (ROS). Using fluorescent probes, we detected ROS production in bronchial and nasal epithelial cells exposed to native DEP, organic extracts of DEP (OE-DEP), or several polyaromatic hydrocarbons. Carbon black particles mimicking the inorganic part of DEP did not increase ROS production. DEP and OE-DEP also induced the expression of genes for phase I [cytochrome P-450 1A1 (CYP1A1)] and phase II [NADPH quinone oxidoreductase-1 (NQO-1)] xenobiotic metabolization enzymes, suggesting that DEP-adsorbed organic compounds become bioavailable, activate transcription, and are metabolized since the CYP1A1 enzymatic activity is increased. Because NQO-1 gene induction is reduced by antioxidants, it could be related to the ROS generated by DEP, most likely through the activation of the stress-sensitive Nrf2 transcription factor. Indeed, DEP induced the translocation of Nrf2 to the nucleus and increased protein nuclear binding to the antioxidant responsive element. In conclusion, we show that DEP-organic compounds generate an oxidative stress, activate the Nrf2 transcription factor, and increase the expression of genes for phase I and II metabolization enzymes.

Induction of phase 2 enzymes by serum oxidized polyamines through activation of Nrf2: effect of the polyamine metabolite acrolein

The naturally occurring polycationic polyamines including putrescine, spermidine, and spermine play an important role in cell growth, differentiation, and gene expression. However, circulating polyamines are potential substrates for several oxidizing enzymes including copper-containing serum amine oxidase. These enzymes are capable of oxidizing serum polyamines to several toxic metabolites including aldehydes and H(2)O(2). In this study, we investigated the effects of polyamines as inducers of phase 2 enzymes and other genes that promote cell survival in a cell culture system in the presence of bovine serum. Spermidine and spermine (50 microM) increased NAD(P)H quinone oxidoreductase (NQO1) activity up to 3-fold in murine keratinocyte PE cells. Transcript levels for glutathione S-transferase (GST) A1, GST M1, NQO1, gamma-glutamylcysteine ligase regulatory subunit, and UDP-glucuronyltransferase 1A6 were significantly increased by spermidine and this effect was mediated through the antioxidant response element (ARE). The ARE from the mouse GST A1 promoter was activated about 9-fold by spermine and 5-fold by spermidine treatment, but could be inhibited by the amine oxidase inhibitor, aminoguanidine, suggesting that acrolein or hydrogen peroxide generated from polyamines by serum amine oxidase may be mediators for phase 2 enzyme induction. Elevations of ARE-luciferase expression and NQO1 enzyme activity by spermidine were not affected by catalase, while both were completely repressed by aldehyde dehydrogenase treatment. Direct addition of acrolein to PE cells induced multiple phase 2 genes and elevated nuclear levels of Nrf2, a transcription factor that binds to the ARE. Expression of mutant Nrf2 repressed the activation of the ARE-luciferase reporter by polyamines and acrolein. These results indicate that spermidine and spermine increase the expression of phase 2 genes in cells grown in culture through activation of the Nrf2-ARE pathway by generating the sulfhydryl reactive aldehyde, acrolein.

Oxidative-stress potency of organic extracts of diesel exhaust and urban fine particles in rat heart microvessel endothelial cells

Exposure to fine particulate materials is associated with an increase in mortality rate of cardiovascular diseases. Particles deposited in the lung may affect the vascular system both directly (leaching of soluble components from particles) and indirectly (via cytokines and mediators). The present study addressed cytotoxicity and oxidative stress potency of organic extracts of diesel exhaust particles (OE-DEP) and urban fine particles (OE-UFP) in rat heart microvessel endothelial (RHMVE) cells. The LC(50) values of OE-DEP and OE-UFP were calculated to be 17 and 34 microg/ml, respectively, suggesting that OE-DEP was more cytotoxic than OE-UFP. The viability of OE-DEP- and OE-UFP-exposed cells was ameliorated by N-acetyl-L-cysteine (NAC). The cell monolayer was exposed to 0 (control), 1, 3, and 10 microg/ml OE-DEP for 6 h and mRNA levels of antioxidant enzymes such as heme oxygenase-1 (HO-1), thioredoxin peroxidase 2 (TRPO), glutathione S-transferase P subunit (GST-P), and NADPH dehydrogenase (NADPHD) were quantitated by northern analysis. All those mRNA levels increased dose-dependently with OE-DEP and HO-1 mRNA showed the most marked response to OE-DEP. mRNA levels of those antioxidant enzymes and heat shock protein 72 (HSP72) in OE-DEP-exposed cells were higher than those of OE-UFP-exposed cells as compared at the same concentration. The transcription levels of HO-1 and HSP72 in OE-DEP- and OE-UFP-exposed cells were also reduced by NAC. Those results suggest that the organic fraction of particulate materials in the urban air has a potency to cause oxidative stress to endothelial cells and may be implicated in cardiovascular diseases through functional changes of endothelial cells.

Keap1 regulates both cytoplasmic-nuclear shuttling and degradation of Nrf2 in response to electrophiles

BACKGROUND

Transcription factor Nrf2 regulates the expression of a set of detoxifying and anti-oxidant enzyme genes. Several lines of evidence suggest that electrophiles and reactive oxygen species liberate Nrf2 from its cytoplasmic repressor Keap1 and provoke the accumulation of Nrf2 in the nucleus. To elucidate the molecular mechanisms as to how Nrf2 is activated by inducers, we examined the cytoplasmic-nuclear shuttling and turnover of Nrf2.

RESULTS

We found that Nrf2 is rapidly degraded through the proteasome pathway, while electrophiles cause Nrf2 nuclear translocation with concomitant stabilization. Crucial to the inducible accumulation of Nrf2 is the enfeebling of the Nrf2-Keap1 interaction by electrophiles. Exploiting mice which have the LacZ reporter gene knocked into the nrf2 locus, we revealed that the inducible accumulation of Nrf2 protein by electrophiles in macrophages and intestinal epithelia could be recapitulated by the Nrf2 N-terminal region in combination with a nuclear localization signal. We also found constitutive Nrf2 nuclear accumulation in Keap1-deficient mouse macrophages.

CONCLUSIONS

Our results highlight the fact that Nrf2 protein turnover is regulated by Keap1 mediated subcellular compartmentalization.

Gene expression of detoxifying enzymes in AhR and Nrf2 compound null mutant mouse

The arylhydrocarbon receptor (AhR) regulates the expression of cytochrome P450 (CYP)-1 gene family members which catalyze xenobiotic Phase I metabolism, while Nrf2 exerts the concerted regulation of Phase II enzyme genes. We generated AhR and Nrf2 compound null mutant mice to examine the integrated function of AhR- and Nrf2-regulated enzymes in detoxification. Furthermore, we used this mouse model, by administering three different classes of chemical inducers, to examine how xenobiotic metabolism may be influenced in the absence of signals transduced by AhR or Nrf2. The compound mutant mice responded only weakly to AhR ligand or Phase II inducer, while they displayed a clear response to phenobarbital, an inducer of the CYP2B family through another, unrelated transcription factor. Here, we report an initial characterization of the AhR-Nrf2 double mutant mice, which may serve as a simplified bioassay system to evaluate xenobiotic toxicity and metabolic biotransformation of various drugs and environmental chemicals.

Modulation of gene expression by cancer chemopreventive dithiolethiones through the Keap1-Nrf2 pathway. Identification of novel gene clusters for cell survival

Enzyme inducers such as 3H-1,2-dithiole-3-thione (D3T) enhance the detoxication of environmental carcinogens and protect against neoplasia. The putative molecular sensor for inducers is Keap1, a sulfhydryl-rich protein that sequesters the transcription factor Nrf2 in the cytoplasm. Expression of these detoxication enzymes is blunted in nrf2-deficient mice; moreover, these mice are more sensitive to carcinogenesis, and the protective actions of dithiolethiones are lost with nrf2 disruption. Hepatic gene expression profiles were examined by oligonucleotide microarray analysis in vehicle- or D3T-treated wild-type mice as well as in nrf2 single and keap1-nrf2 double knockout mice to identify those genes regulated by the Keap1-Nrf2 pathway. Transcript levels of 292 genes were elevated in wild-type mice 24 h after treatment with D3T; 79% of these genes were induced in wild-type, but not nrf2-deficient mice. These nrf2-dependent, D3T-inducible genes included known detoxication and antioxidative enzymes. Unexpected clusters included genes for chaperones, protein trafficking, ubiquitin/26 S proteasome subunits, and signaling molecules. Gene expression patterns in keap1-nrf2 double knockout mice were similar to those in nrf2-single knockout mice. D3T also led to nrf2-dependent repression of 31 genes at 24 h; principally genes related to cholesterol/lipid biosynthesis. Collectively, D3T increases the expression of genes through the Keap1-Nrf2 signaling pathway that directly detoxify toxins and generate essential cofactors such as glutathione and reducing equivalents. Induction of nrf2-dependent genes involved in the recognition and repair/removal of damaged proteins expands the role of this pathway beyond primary control of electrophilic and oxidative stresses into secondary protective actions that enhance cell survival.

DNA adduct burden and tobacco carcinogenesis

DNA adducts associated with tobacco smoking could provide a marker of biologically effective dose of tobacco carcinogens and improve individual cancer risk prediction. A significant number of clinical and epidemiologic studies have reported associations of increased DNA adduct levels with the occurrence of the prevalent tobacco related cancers including cancer of the lung, head and neck, and bladder. The inducibility of DNA adducts following in vitro treatments using blood lymphocytes also appears to be a risk factor in the development of lung and head and neck cancer. Corroborative evidence pointing to the importance of DNA adducts in tobacco carcinogenesis include numerous studies showing associations of tobacco smoke exposure with the induction of DNA adducts in humans in vivo. Further effort is necessary, however, to more fully characterize the dose-response relationship between smoking and DNA adducts in exposed target and surrogate tissues. The relationship between gene polymorphisms thought to modify tobacco-related cancer risk and DNA adduct levels is complex. Results of some DNA adduct studies (both in vitro and in vivo) appear inconsistent with the epidemiologic findings. This is evident for polymorphisms involving both carcinogen metabolism (e.g. GSTP1) and DNA repair (e.g. XRCC1). Molecular studies of human tumors suggest associations of p53 mutation with DNA adducts and have revealed correlations of DNA adduct levels with somatic alterations (e.g. 3p21 LOH) that are thought to occur at the very earliest stages of tobacco carcinogenesis. More research is needed to assess the relationship between endogenous sources of DNA adducts and tobacco smoke exposure and the relative oncogenic effects of chemically stable versus unstable DNA adducts. Many potentially fruitful new avenues of cancer research are emerging that integrate DNA adduct analyses with assessments of smoking, genetics, diet and ambient air quality. These investigations aim to understand the multifactorial nature of interindividual variability in response to tobacco carcinogens. As these trends continue a variety of innovative study designs and approaches will become important in human populations.

The Keap1 BTB/POZ dimerization function is required to sequester Nrf2 in cytoplasm

Transactivation of phase II detoxification enzymes and antioxidant proteins is mediated by the Cap'N'Collar transcription factor, Nrf2, which is sequestered in the cytoplasm by the actin-binding protein Keap1. Mutation of a conserved serine (S104A) within the Keap1 BTB/POZ domain disrupts Keap1 dimerization and eliminates the ability of Keap1 to sequester Nrf2 in the cytoplasm and repress Nrf2 transactivation. Disruption of endogenous Keap1 dimerization using BTB/POZ dominant negative proteins also inhibits the ability of Keap1 to retain Nrf2 in the cytoplasm. Exposure to an electrophilic agent that induces Nrf2 release and nuclear translocation disrupts formation of a Keap1 complex in vivo. Collectively, these data support the conclusion that Keap1 dimerization is required for Nrf2 sequestration and transcriptional repression. Furthermore, exposure to inducing agents disrupts the Keap1 dimerization function and results in Nrf2 release.

Identification of the interactive interface and phylogenic conservation of the Nrf2-Keap1 system

BACKGROUND

The transcription factor Nrf2 and its negative regulator Keap1 play important roles in transcriptional induction of a set of detoxifying and anti-oxidant enzymes. To gain an insight into our present enigma as to how cells receive oxidative and electrophilic signals and transduce them to Nrf2, we have developed a zebrafish model system for molecular toxicological studies.

RESULTS

We systematically cloned zebrafish cytoprotective enzyme cDNAs and found their expression to be efficiently induced by electrophilic agents. We consequently identified the presence of Nrf2 and Keap1 in zebrafish. Both loss- and gain-of-function analyses demonstrated that Nrf2 is the primary regulator of a subset of cytoprotective enzyme genes, while Keap1 suppresses Nrf2 activity in zebrafish. An ETGE motif, critical for the Nrf2-Keap1 interaction, was identified in the Neh2 domain of Nrf2 by reverse two-hybrid screening and found to be indispensable for the regulation of Nrf2 activity in zebrafish.

CONCLUSION

Taken together, these results indicate that the Nrf2-Keap1 system is highly conserved among vertebrates and that the interface between Nrf2 and Keap1 forms an important molecular basis of this regulatory system.

Loss of the Nrf2 transcription factor causes a marked reduction in constitutive and inducible expression of the glutathione S-transferase Gsta1, Gsta2, Gstm1, Gstm2, Gstm3 and Gstm4 genes in the livers of male and female mice

Mice that lack the Nrf2 basic-region leucine-zipper transcription factor are more sensitive than wild-type (WT) animals to the cytotoxic and genotoxic effects of foreign chemicals and oxidants. To determine the basis for the decrease in tolerance of the Nrf2 homozygous null mice to xenobiotics, enzyme assay, Western blotting and gene-specific real-time PCR (TaqMan) have been used to examine the extent to which hepatic expression of GSH-dependent enzymes is influenced by the transcription factor. The amounts of protein and mRNA for class Alpha, Mu and Pi glutathione S-transferases were compared between WT and Nrf2 knockout (KO) mice of both sexes under both constitutive and inducible conditions. Among the class Alpha and class Mu transferases, constitutive expression of Gsta1, Gsta2, Gstm1, Gstm2, Gstm3, Gstm4 and Gstm6 subunits was reduced in the livers of Nrf2 mutant mice to between 3% and 60% of that observed in WT mice. Induction of these subunits by butylated hydroxyanisole (BHA) was more marked in WT female mice than in WT male mice. TaqMan analyses showed the increase in transferase mRNA caused by BHA was attenuated in Nrf2(-/-) mice, with the effect being most apparent in the case of Gsta1, Gstm1 and Gstm3. Amongst class Pi transferase subunits, the constitutive hepatic level of mRNA for Gstp1 and Gstp2 was not substantially affected in the KO mice, but their induction by BHA was dependent on Nrf2; this was more obvious in female mutant mice than in male mice. Nrf2 KO mice exhibited reduced constitutive expression of the glutamate cysteine ligase catalytic subunit, and, to a lesser extent, the expression of glutamate cysteine ligase modifier subunit. Little variation was observed in the levels of glutathione synthase in the different mouse lines. Thus the increased sensitivity of Nrf2(-/-) mice to xenobiotics can be partly attributed to a loss in constitutive expression of multiple GSH-dependent enzymes, which causes a reduction in intrinsic detoxification capacity in the KO animal. These data also indicate that attenuated induction of GSH-dependent enzymes in Nrf2(-/-) mice probably accounts for their failure to adapt to chronic exposure to chemical and oxidative stress.

Integration and diversity of the regulatory network composed of Maf and CNC families of transcription factors

Recent progress in the analysis of transcriptional regulation has revealed the presence of an exquisite functional network comprising the Maf and Cap 'n' collar (CNC) families of regulatory proteins, many of which have been isolated. Among Maf factors, large Maf proteins are important in the regulation of embryonic development and cell differentiation, whereas small Maf proteins serve as obligatory heterodimeric partner molecules for members of the CNC family. Both Maf homodimers and CNC-small Maf heterodimers bind to the Maf recognition element (MARE). Since the MARE contains a consensus TRE sequence recognized by AP-1, Jun and Fos family members may act to compete or interfere with the function of CNC-small Maf heterodimers. Overall then, the quantitative balance of transcription factors interacting with the MARE determines its transcriptional activity. Many putative MARE-dependent target genes such as those induced by antioxidants and oxidative stress are under concerted regulation by the CNC family member Nrf2, as clearly proven by mouse germline mutagenesis. Since these genes represent a vital aspect of the cellular defense mechanism against oxidative stress, Nrf2-null mutant mice are highly sensitive to xenobiotic and oxidative insults. Deciphering the molecular basis of the regulatory network composed of Maf and CNC families of transcription factors will undoubtedly lead to a new paradigm for the cooperative function of transcription factors.