Two domains of Nrf2 cooperatively bind CBP, a CREB binding protein, and synergistically activate transcription

Structure of the Keap1:Nrf2 interface provides mechanistic insight into Nrf2 signaling

Keap1 is a BTB-Kelch substrate adaptor protein that regulates steady-state levels of Nrf2, a bZIP transcription factor, in response to oxidative stress. We have determined the structure of the Kelch domain of Keap1 bound to a 16-mer peptide from Nrf2 containing a highly conserved DxETGE motif. The Nrf2 peptide contains two short antiparallel beta-strands connected by two overlapping type I beta-turns stabilized by the aspartate and threonine residues. The beta-turn region fits into a binding pocket on the top face of the Kelch domain and the glutamate residues form multiple hydrogen bonds with highly conserved residues in Keap1. Mutagenesis experiments confirmed the role of individual amino acids for binding of Nrf2 to Keap1 and for Keap1-mediated repression of Nrf2-dependent gene expression. Our results provide a detailed picture of how a BTB-Kelch substrate adaptor protein binds to its cognate substrate and will enable the rational design of novel chemopreventive agents.

DIFFERENTIAL EFFECTS OF THE OXIDIZED METABOLITES OF OLTIPRAZ ON THE ACTIVATION OF CCAAT/ENHANCER BINDING PROTEIN-β AND NF-E2-RELATED FACTOR-2 FOR GSTA2 GENE INDUCTION

Comprehensive mechanistic studies suggest that oltipraz exerts cancer chemopreventive effects through the induction of glutathione S-transferase (GST). Previously, we have shown that the activation of CCAAT/enhancer binding protein-beta (C/EBPbeta), promoted by oltipraz, contributes to the transcriptional induction of the GSTA2 gene. Studies also indicated that exposure of animals to oltipraz triggers nuclear accumulation of NF-E2-related factor-2 (Nrf2) with an increase in Nrf2's antioxidant response element (ARE) binding activity. Given the previous reports that C/EBPbeta activation contributes to oltipraz's induction of the GSTA2 gene and that Nrf2 activation by oltipraz was variable depending on the concentrations, this study investigated whether the major oxidized metabolites of oltipraz induce GSTA2 through the activation of C/EBPbeta and/or Nrf2. Immunoblot analysis revealed that M1 [4-methyl-5-(pyrazin-2-yl)-3H-1,2-dithiol-3-one] and M2 (7-methyl-6,8-bis(methylthio)H-pyrrolo[1,2-a]pyrazine), but not M3 (7-methyl-8-(methylsulfinyl)-6-(methylthio)H-pyrrolo[1,2-a]pyrazine) and M4 (7-methyl-6,8-bis(methylsulfinyl)H-pyrrolo[1,2-a]pyrazine), induced GSTA2 in H4IIE cells. M1 and M2 also increased the luciferase activity from pGL-1651, which contained the luciferase structural gene downstream of the -1.65-kilobase GSTA2 promoter region. Nuclear C/EBPbeta levels were enhanced by the metabolites but not by M3 or M4. Among the oxidized metabolites examined, only M2, which elicited cell death at a relatively high concentration, activated Nrf2, as indicated by nuclear accumulation of Nrf2 and its ARE binding activity. The present study provides evidence that M1 and M2, but not M3 and M4, induce GSTA2 and that M1 induces GSTA2 only via C/EBPbeta activation, whereas M2 does so by activating Nrf2 as well as C/EBPbeta. These results substantiate the differential effects of oltipraz's metabolites on C/EBPbeta- and/or Nrf2-mediated GSTA2 induction.

Role of Nrf2 in the regulation of the Mrp2 (ABCC2) gene

The Nrf2 (nuclear factor-erythroid 2 p45-related factor 2) transcription factor regulates gene expression of the GCLC (glutamate-cysteine ligase catalytic subunit), which is a key enzyme in glutathione synthesis, and GSTs (glutathione S-transferases) via the ARE (antioxidant-response element). The Mrp2 (multidrug-resistance protein 2) pump mediates the excretion of GSH and GSSG excretion as well as endo- and xeno-biotics that are conjugated with GSH, glucuronate or sulphate. Considering that Mrp2 acts synergistically with these enzymes, we hypothesized that the regulation of Mrp2 gene expression is also dependent on Nrf2. Using BHA (butylated hydroxyanisole), which is a classical activator of the ARE-Nrf2 pathway, we observed an increase in the transcriptional activity of Mrp2, GCLC and Gsta1/Gsta2 genes in the mouse liver. A similar pattern of co-induction of Mrp2 and GCLC genes was also observed in mouse (Hepa 1-6) and human (HepG2) hepatoma cells treated with BHA, beta-NF (beta-naphthoflavone), 2,4,5-T (trichlorophenoxyacetic acid) or 2AAF (2-acetylaminofluorene), suggesting that these genes share common mechanism(s) of transcriptional activation in response to exposure to xenobiotics. To define the mechanism of Mrp2 gene induction, the 5'-flanking region of the mouse Mrp2 gene (2.0 kb) was isolated, and two ARE-like sequences were found: ARE-2 (-1391 to -1381) and ARE-1 (-95 to -85). Deletion analyses demonstrated that the proximal region (-185 to +99) contains the elements for the basal expression and xenobiotic-mediated induction of the Mrp2 gene. Gel-shift and supershift assays indicated that Nrf2-protein complexes bind ARE sequences of the Mrp2 promoter, preferentially to the ARE-1 sequence. Overexpression of Nrf2 increased ARE-1-mediated CAT (chloramphenicol acetyltransferase) gene activity, while overexpression of mutant Nrf2 protein repressed the activity. Thus Nrf2 appears to regulate Mrp2 gene expression via an ARE element located at the proximal region of its promoter in response to exposure to xenobiotics.

Heme oxygenase-1/carbon monoxide: from basic science to therapeutic applications

The heme oxygenases, which consist of constitutive and inducible isozymes (HO-1, HO-2), catalyze the rate-limiting step in the metabolic conversion of heme to the bile pigments (i.e., biliverdin and bilirubin) and thus constitute a major intracellular source of iron and carbon monoxide (CO). In recent years, endogenously produced CO has been shown to possess intriguing signaling properties affecting numerous critical cellular functions including but not limited to inflammation, cellular proliferation, and apoptotic cell death. The era of gaseous molecules in biomedical research and human diseases initiated with the discovery that the endothelial cell-derived relaxing factor was identical to the gaseous molecule nitric oxide (NO). The discovery that endogenously produced gaseous molecules such as NO and now CO can impart potent physiological and biological effector functions truly represented a paradigm shift and unraveled new avenues of intense investigations. This review covers the molecular and biochemical characterization of HOs, with a discussion on the mechanisms of signal transduction and gene regulation that mediate the induction of HO-1 by environmental stress. Furthermore, the current understanding of the functional significance of HO shall be discussed from the perspective of each of the metabolic by-products, with a special emphasis on CO. Finally, this presentation aspires to lay a foundation for potential future clinical applications of these systems.

Keap1 recruits Neh2 through binding to ETGE and DLG motifs: characterization of the two-site molecular recognition model

The expression of the phase 2 detoxification enzymes and antioxidant proteins is induced at the transcriptional level by Nrf2 and negatively regulated at the posttranslational level by Keap1 through protein-protein interactions with and subsequent proteolysis of Nrf2. We found that the Neh2 domain of Nrf2 is an intrinsically disordered but biologically active regulatory domain containing a 33-residue central alpha-helix followed by a mini antiparallel beta-sheet. Isothermal calorimetry analysis indicated that one Neh2 molecule interacts with two molecules of Keap1 via two binding sites, the stronger binding ETGE motif and the weaker binding DLG motif. Nuclear magnetic resonance titration study showed that these two motifs of the Neh2 domain bind to an overlapping site on the bottom surface of the beta-propeller structure of Keap1. In contrast, the central alpha-helix of the Neh2 domain does not have any observable affinity to Keap1, suggesting that this region may serve as a bridge connecting the two motifs for the association with the two beta-propeller structures of a dimer of Keap1. Based on these observations, we propose that Keap1 recruits Nrf2 by the ETGE motif and that the DLG motif of the Neh2 domain locks its lysine-rich central alpha-helix in a correct position to benefit ubiquitin signaling.

Activation of the Nrf2-ARE signaling pathway: a promising strategy in cancer prevention

A major protective mechanism against oxidizing substances capable of damaging DNA integrity and initiating carcinogenesis is the induction of phase II detoxification and antioxidant enzymes by chemopreventive agents. A key finding in the field of chemoprevention has been the discovery that the induction of these enzymes is mediated by the cytoplasmic oxidative stress system (Nrf2-Keap1). Under basal (reducing) conditions, Keap1 anchors the Nrf2 transcription factor within the cytoplasm, targeting it for ubiquitination and proteasome degradation, thus repressing its ability to induce phase II genes. When cells are exposed to chemopreventive agents and oxidative stress, however, a signal involving phosphorylation and/or redox modification is transmitted to the Nrf2-Keap1 complex, leading to its dissociation and the nuclear translocation of Nrf2, which, after hetero-dimerically partnering with other transcription factors, binds to the AREs/EpREs present within phase II gene promoters, increasing their transcription. These data should assist in developing new phase II detoxification enzyme inducers as cancer chemopreventive agents within the clinical environment.

The carboxy-terminal Neh3 domain of Nrf2 is required for transcriptional activation

Nrf2 is a transcription factor critical for the maintenance of cellular redox homeostasis. We have previously found that Nrf2 is a labile protein, and its activation in cells under stress involves mechanisms leading to its stabilization. As a modular protein, Nrf2 possesses distinct transactivation and DNA binding domains essential for its transcriptional activity. In this study, we found that the C-terminal "Neh3" domain of Nrf2 is also important for its activity. Deletion of the last 16 amino acids of the protein completely abolishes its ability to activate both reporter and endogenous gene expression. Using site-directed mutagenesis, we have identified a stretch of amino acids within this region that are essential for its activity and that are found to be conserved across species and among other members of the CNC-bZIP family. Importantly, deletion of the final 16 amino acids of Nrf2 does not influence its dimerizing capability, DNA binding activity, or subcellular localization, although it does increase the half-life of the protein. In addition, this region was found to be important for interaction with CHD6 (a chromo-ATPase/helicase DNA binding protein) in a yeast two-hybrid screen. RNA interference-mediated knockdown of CHD6 reduced both the basal and tert-butylhydroquinone-inducible expression of NQO1, a prototypical Nrf2 target gene. These data suggest that the Neh3 domain may act as a transactivation domain and that it is possibly involved in interaction with components of the transcriptional apparatus to affect its transcriptional activity.

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.

4-Hydroxynonenal induces rat gamma-glutamyl transpeptidase through mitogen-activated protein kinase-mediated electrophile response element/nuclear factor erythroid 2-related factor 2 signaling

Gamma-glutamyl transpeptidase (GGT) plays critical roles in glutathione homeostasis and metabolism. Rat GGT is a single-copy gene from which seven types of GGT mRNA with a common protein encoding sequence, but different 5'-untranslated regions, may be transcribed. We previously showed that type V-2 was the predominant form of GGT mRNA in rat L2 epithelial cells, and that it could be induced by 4-hydroxynonenal (HNE) through the electrophile response element (EpRE) located in GGT promoter 5 (GP5). Here, we report transcription factors binding to GP5 EpRE and the involved signaling pathways. Immunodepletion gel shift assays demonstrated that GP5 EpRE bound JunB, c-Jun, FosB, and Fra2 from unstimulated cells, and that after exposure to HNE, EpRE binding complexes contained nuclear factor erythroid 2-related factor (Nrf) 1, Nrf2, JunB, c-Jun, FosB, c-Fos, Fra1, and Fra2. HNE-induced binding of Nrf2 and c-Jun in GP5 EpRE was confirmed by chromatin immunoprecipitation assays. Using reporter assays and specific inhibitors, we found that HNE induction of rat GGT mRNA V-2 was dependent on activation of extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK), but not protein kinase C or phosphatidylinositol 3-kinase. Pretreatment with ERK and p38MAPK inhibitors also blocked HNE-increased EpRE binding. HNE-increased nuclear content of Nrf1, Nrf2, and c-Jun in L2 cells was partially blocked by inhibition of either ERK1/2 or p38MAPK and completely blocked by simultaneous inhibition of both MAPKs. In conclusion, HNE induces GGT mRNA V-2 through altered EpRE transcription factor binding mediated by both ERK and p38MAPK.

30 some years of heme oxygenase: from a "molecular wrecking ball" to a "mesmerizing" trigger of cellular events

In the beginning, the microsomal HO system was presumed to be made of one isozymes, now known as HO-1, which was cytP450-dependent; and, was thought to be of physiological significance solely in the context of catalysis of hemoglobin heme to bile pigments and CO. A succession of discoveries including characterization of the system as an independent mono-oxygenase, identification of a second form, called HO-2, free radical quenching activity of bile pigments, analogous function of CO in cell signaling to NO, and characterization of the system as HSP32 cognates has led to such an impressive expansion in the number of reports dealing with the HO system that surpass anyone's expectation. This review is a compilation of certain older findings and recent events that together ensure placement of the HO system in the mainstream research for decades to come.

Regulation of glutathione in inflammation and chronic lung diseases

Oxidant/antioxidant imbalance, a major cause of cell damage, is the hallmark for lung inflammation. Glutathione (GSH), a ubiquitous tripeptide thiol, is a vital intra- and extra-cellular protective antioxidant against oxidative stress, which plays a key role in the control of signaling and pro-inflammatory processes in the lungs. The rate-limiting enzyme in GSH synthesis is glutamylcysteine ligase (GCL). GSH is essential for development as GCL knock-out mouse died from apoptotic cell death. The promoter (5'-flanking) region of human GCL is regulated by activator protein-1 (AP-1) and antioxidant response element (ARE), and are modulated by oxidants, phenolic antioxidants, growth factors, inflammatory and anti-inflammatory agents in various cells. Recent evidences have indicated that Nrf2 protein, which binds to the erythroid transcription factor (NF-E2) binding sites, and its interaction with other oncoproteins such as c-Jun, Jun D, Fra1 and Maf play a key role in the regulation of GCL. Alterations in alveolar and lung GSH metabolism are widely recognized as a central feature of many chronic inflammatory lung diseases. Knowledge of the mechanisms of GSH regulation could lead to the pharmacological manipulation of the production and/or gene transfer of this important antioxidant in lung inflammation and injury. This article describes the role of AP-1 and ARE in the regulation of cellular GSH biosynthesis and assesses the potential protective and therapeutic role of glutathione in oxidant-induced lung injury and inflammation.

Glucocorticoid receptor (GR)-associated SMRT binding to C/EBPbeta TAD and Nrf2 Neh4/5: role of SMRT recruited to GR in GSTA2 gene repression

The expression of the glutathione S-transferase gene (GST), whose induction accounts for cancer chemoprevention, is regulated by activation of CCAAT/enhancer binding protein beta (C/EBPbeta) and NF-E2-related factor 2 (Nrf2). The present study investigated the repressing effects of activating glucocorticoid receptor (GR) on C/EBPbeta- and Nrf2-mediated GSTA2 gene induction and the mechanism. Dexamethasone that activates GR inhibited constitutive and oltipraz- or tert-butylhydroquinone (t-BHQ)-inducible GSTA2 expression in H4IIE cells. Also, dexamethasone repressed GSTA2 promoter-luciferase gene activity. Dexamethasone-GR activation did not inhibit nuclear translocation of C/EBPbeta or Nrf2 nor their DNA binding activities induced by oltipraz or t-BHQ. Deletion of the glucocorticoid response element (GRE) in the GSTA2 promoter abolished dexamethasone inhibition of the gene induction. Immunoprecipitation-immunoblotting, chromatin immunoprecipitation, and GST pull-down assays revealed that silencing mediator for retinoid and thyroid hormone receptors (SMRT), a corepressor recruited to steroid-GR complex for histone deacetylation, bound to TAD domain of C/EBPbeta and Neh4/5 domain of Nrf2. The GSTA2 promoter-luciferase activities were decreased by SMRT but not by truncated SMRTs. The small interference RNA (siRNA) against SMRT abolished SMRT repression of the gene induction by C/EBPbeta or Nrf2. The plasmid transfection and siRNA experiments directly evidenced the functional role of SMRT in GSTA2 repression. In conclusion, dexamethasone antagonizes C/EBPbeta- and Nrf2-mediated GSTA2 gene induction via ligand-GR binding to the GRE, and steroid-mediated GSTA2 repression involves inactivation of C/EBPbeta and Nrf2 by SMRT recruited to steroid-GR complex.

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.

Nrf2-dependent gene expressions: a molecular toxicological aspect

Although NF-E2 related factor 2 (Nrf2) was found to be a transcriptional regulator that controls an expression of the beta-globin gene, the notion is now widely accepted that this transfactor serves as a master regulator for the gene expression of a battery of proteins acting on anti-oxidative stress and detoxification of electrophiles. The function of Nrf2 that bears transcriptional activation depends solely on its nuclear localization, which is regulated by interaction with the cytosolic anchor protein Keap1 and its own turnover rate. In the present mini-review, we focus on the regulation of Nrf2 function and discuss the physiological and toxicological aspects of this transcriptional factor.

Crystal Structure of the Kelch Domain of Human Keap1

Keap1 is a substrate adaptor protein for an ubiquitin ligase complex that targets the Nrf2 transcription factor for degradation. Keap1 binds Nrf2 through its C-terminal Kelch domain, which contains six copies of the evolutionarily conserved kelch repeat sequence motif. The structure of the Kelch domain from human Keap1 has been determined by x-ray crystallography to a resolution of 1.85 A. The Kelch domain forms a 6-bladed beta-propeller structure, with residues at the C terminus forming the first strand in the first blade. Key structural roles have been identified for the highly conserved glycine, tyrosine, and tryptophan residues that define the kelch repeat sequence motif. In addition, we show that substitution of a single amino acid located within a loop that extends out from the bottom of the beta-propeller structure abolishes binding of Nrf2. The structure of the Kelch domain of Keap1 represents a high quality model for the superfamily of eukaryotic kelch repeat proteins and provides insight into how disease-causing mutations perturb the structural integrity of the Kelch domain.

Nrf2 transcriptionally activates the mafG gene through an antioxidant response element

Nrf2 accumulates in nuclei upon exposure to oxidative stress, heterodimerizes with a small Maf protein, and activates the transcription of stress target genes through antioxidant response elements (AREs). We found that diethyl maleate (DEM), a well known activator of Nrf2, induces one of the small Maf genes, mafG. To elucidate roles MafG might play in the oxidative stress response, we examined transcriptional regulation of the mouse mafG gene. MafG utilizes three independent first exons that are each spliced to second and third coding exons. Among the small maf genes, mafG showed the strongest response to DEM, and of the three first exons, the highest -fold induction was seen with the proximal first exon (Ic). Importantly, one ARE (Ic-ARE) is conserved in the promoter flanking exon Ic of the human and mouse mafG genes. The Nrf2/MafG heterodimer bound the Ic-ARE and activated transcription, whereas DEM failed to activate mafG in nrf2-null mutant cells. Chromatin immunoprecipitation further revealed that both Nrf2 and small Maf proteins associate with the Ic-ARE in vivo. These results demonstrate that mafG is itself an ARE-dependent gene that is regulated by an Nrf2/small Maf heterodimer and suggest the presence of an autoregulatory feedback pathway for mafG transcriptional regulation.

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.

Nrf2 as a novel molecular target for chemoprevention

One of the rational and effective strategies for chemoprevention is the blockade of DNA damage caused by carcinogenic insult. This can be achieved either by reducing the formation of reactive carcinogenic species or stimulating their detoxification. A wide spectrum of xenobiotic metabolizing enzymes catalyze both phase I (oxidation and reduction) and phase II biotransformation (conjugation) reactions involved in carcinogen activation and/or deactivation. Several antioxidant-response element (ARE)-regulated gene products such as glutathione S-transferase, NAD(P)H:quinone oxidoreductase 1, UDP-glucuronosyltransferase, gamma-glutamate cysteine ligase, and hemeoxygenase-1 are known to mediate detoxification and/or to exert antioxidant functions thereby protecting cells from genotoxic damage. The transcription of ARE-driven genes is regulated, at least in part, by nuclear transcription factor erythroid 2p45 (NF-E2)-related factor 2 (Nrf2), which is sequestered in cytoplasm by Kelch-like ECH-associated protein 1 (Keap1). Exposure of cells to ARE inducers results in the dissociation of Nrf2 from Keap1 and facilitates translocation of Nrf2 to the nucleus, where it heterodimerizes with small Maf protein, and binds to ARE, eventually resulting in the transcriptional regulation of target genes. The Nrf2-Keap1-ARE signaling pathway can be modulated by several upstream kinases including phosphatidylinositol 3-kinase, protein kinase C, and mitogen-activated protein kinases. Selected Nrf2-Keap1-ARE activators, such as oltipraz, anethole dithiolethione, sulforaphane, 6-methylsulphinylhexyl isothiocyanate, curcumin, caffeic acid phenethyl ester, 4'-bromoflavone, etc. are potential chemopreventive agents. This mini-review will focus on a chemopreventive strategy directed towards protection of DNA and other important cellular molecules by inducing de novo synthesis of phase II detoxifying or antioxidant genes via the Nrf2-ARE core signaling pathway.

Contribution of NAD(P)H:quinone oxidoreductase 1 to protection against carcinogenesis, and regulation of its gene by the Nrf2 basic-region leucine zipper and the arylhydrocarbon receptor basic helix-loop-helix transcription factors

NAD(P)H:quinone oxidoreductase 1 (NQO1) is a key enzyme involved in defence against reactive forms of oxygen and inhibition of neoplasia. Under conditions of oxidative stress, expression of NQO1 is induced, and the resulting increase in oxidoreductase protein provides the cell with multiple layers of protection against environmental insults. Firstly, the catalytic activity of NQO1 is directed towards the complete reduction and detoxication of highly reactive quinones. Secondly, the oxidoreductase maintains the endogenous lipid-soluble antioxidants, alpha-tocopherol-hydroquinone and ubiquinol in their reduced and active forms. Thirdly, NQO1 is required for the stabilisation of p53 protein in response to DNA-damaging stimuli, and it thereby influences cell fate decisions. In view of the anticarcinogenic actions of NQO1, an understanding of the mechanisms that govern its expression is desirable. The redox sensitivity of NQO1 transcription occurs through a cis-acting antioxidant response element (ARE) located within the regulatory region of the mouse, rat and human genes. This element recruits the positively acting basic leucine zipper (bZip) transcription factor NF-E2 p45-related factor 2 (Nrf2). Under normal constitutive conditions, Nrf2 associates with the cytoskeletal-binding protein Keap1, which regulates the subcellular distribution of the bZip factor and also targets it for proteasome-dependent degradation. Oxidative stress inhibits the Nrf2-Keap1 interaction, thus promoting nuclear accumulation of the transcription factor and transactivation of NQO1 and other ARE-driven genes. Mouse, rat and human NQO1 can also be induced by planar aromatic hydrocarbons through a cis-acting xenobiotic response element (XRE) located in their gene promoters. The XRE recruits the arylhydrocarbon receptor (AhR) and AhR nuclear translocator. Cross-talk may occur between Nrf2 and AhR, but the details of this process remain to be elucidated.

Transcription factors in the cellular signaling network as prime targets of chemopreventive phytochemicals

Accumulating evidence from epidemiologic and laboratory studies support an inverse relationship between a regular consumption of fruits and vegetables and the risk of specific cancers. Numerous phytochemicals derived from edible plants have been reported to possess ability to interfere with a specific stage of carcinogenic process. Multiple mechanisms have been proposed to account for the anti-carcinogenic actions of dietary constituents, but more attention has recently focussed on intracellular signaling cascades as common molecular targets of a wide variety of chemopreventive phytochemicals.

Regulation of heme oxygenase-1 gene transcription: recent advances and highlights from the International Conference (Uppsala, 2003) on Heme Oxygenase

Recent investigations into the regulation of heme oxygenase-1 gene (hmox-1) transcription have exposed mechanisms of increasing diversity and complexity worthy of a gene whose expression is modulated by a seemingly endless array of physiological, pathophysiological, and nonphysiological agents and conditions. For instance, contrary to initial and prevalent assumptions that inducer-dependent gene stimulation is mediated principally by the positive action of transcription activators, it now appears that such induction may occur secondarily to deactivation of the repressor protein, Bach1. As a further complication, heme and cadmium, two potent inducers of the hmox-1 gene, inhibit Bach1 function by different mechanisms-by inhibition of DNA binding or promotion of nuclear export, respectively. Bach1 also plays a role in signal-dependent hmox-1 gene repression, an increasingly appreciated phenomenon that is manifested in a species- and cell-specific manner. Although extreme concentrations of the heme oxygenase-1 protein resulting from the opposing phenomena of gene activation and repression have physiological consequences, even minor modulation in the level of this enzyme, as elicited by variations in the length of a dinucleotide repeat region within the human hmox-1 promoter, may be of clinical relevance. Finally, mechanistic diversity is also apparent in the type and combination of protein kinase-dependent, signal transduction pathways used during hmox-1 gene activation.

Functional and placental expression analysis of the human NRF3 transcription factor

Members of the Maf protooncogene and cap'n' collar families of basic-leucine zipper transcription factors play important roles in development, differentiation, oncogenesis, and stress signaling. In this study, we performed an in vivo protein-protein interaction screen to search for novel partners of the small Maf proteins. Using full-length human MAFG protein as bait, we identified the human basic-leucine zipper protein NRF3 [NF-E2 (nuclear factor erythroid 2)-related factor 3] as an interaction partner. Transfection studies confirmed that NRF3 is able to dimerize with MAFG. The resulting NRF3/MAFG heterodimer recognizes nuclear factor-erythroid 2/Maf recognition element-type DNA-binding motifs. Functional analysis revealed the presence of a strong transcriptional activation domain in the center region of the NRF3 protein. We found that NRF3 transcripts are present in placental chorionic villi from at least week 12 of gestation on through term. In particular, NRF3 is highly expressed in primary placental cytotrophoblasts, but not in placental fibroblasts. The human choriocarcinoma cell lines BeWo and JAR, derived from trophoblastic tumors of the placenta, also strongly express NRF3 transcripts. We generated a NRF3-specific antiserum and identified NRF3 protein in placental choriocarcinoma cells. Furthermore, we showed that NRF3 transcript and protein levels are induced by TNF-alpha in JAR cells. Our functional studies suggest that human NRF3 is a potent transcriptional activator. Finally, our expression and induction analyses hint at a possible role of Nrf3 in placental gene expression and development.

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.

Nuclear factor E2-related factor 2-dependent antioxidant response element activation by tert-butylhydroquinone and sulforaphane occurring preferentially in astrocytes conditions neurons against oxidative insult

Binding of the transcription factor nuclear factor E2-related factor 2 (Nrf2) to the antioxidant response element (ARE) in neural cells results in the induction of a battery of genes that can coordinate a protective response against a variety of oxidative stressors. In this study, tert-butylhydroquinone (tBHQ) and sulforaphane were used as activators of this pathway. Consistent with previous studies, treatment of primary cortical cultures from ARE reporter mice revealed selective promoter activity in astrocytes. This activation protected neurons from hydrogen peroxide and nonexcitotoxic glutamate toxicity. tBHQ treatment of cultures from Nrf2 knock-out animals resulted in neither ARE activation nor neuroprotection. By reintroducing Nrf2 via infection with a replication-deficient adenovirus (ad), both the genetic response and neuroprotection were rescued. Conversely, infection with adenovirus encoding dominant-negative (DN) Nrf2 (ad-DN-Nrf2) or pretreatment with the selective phosphatidylinositol-3 kinase inhibitor LY294002 inhibited the tBHQ-mediated promoter response and corresponding neuroprotection. Interestingly, the adenoviral infection showed a high selectivity for astrocytes over neurons. In an attempt to reveal some of the cell type-specific changes resulting from ARE activation, cultures were infected with adenovirus encoding green fluorescent protein (GFP) (ad-GFP) or ad-DN-Nrf2 (containing GFP) before tBHQ treatment. A glia-enriched population of GFP-infected cells was then isolated from a population of uninfected neurons using cell-sorting technology. Microarray analysis was used to evaluate potential glial versus neuron-specific contributions to the neuroprotective effects of ARE activation and Nrf2 dependence. Strikingly, the change in neuronal gene expression after tBHQ treatment was dependent on Nrf2 activity in the astrocytes. This suggests that Nrf2-dependent genetic changes alter neuron-glia interactions resulting in neuroprotection.

The story so far: Molecular regulation of the heme oxygenase-1 gene in renal injury

Heme oxygenases (HOs) catalyze the rate-limiting step in heme degradation, resulting in the formation of iron, carbon monoxide, and biliverdin, the latter of which is subsequently converted to bilirubin by biliverdin reductase. Recent attention has focused on the biological effects of product(s) of this enzymatic reaction, which have important antioxidant, anti-inflammatory, and cytoprotective functions. Two major isoforms of the HO enzyme have been described: an inducible isoform, HO-1, and a constitutively expressed isoform, HO-2. A third isoform, HO-3, closely related to HO-2, has also been described. Several stimuli implicated in the pathogenesis of renal injury, such as heme, nitric oxide, growth factors, angiotensin II, cytokines, and nephrotoxins, induce HO-1. Induction of HO-1 occurs as an adaptive and beneficial response to these stimuli, as demonstrated by studies in renal and non-renal disease states. This review will focus on the molecular regulation of the HO-1 gene in renal injury and will highlight the interspecies differences, predominantly between the rodent and human HO-1 genes.

Regulation of Nrf2 transactivation domain activity. The differential effects of mitogen-activated protein kinase cascades and synergistic stimulatory effect of Raf and CREB-binding protein

Transcription factor NF-E2-related factor 2 (Nrf2) regulates the induction of Phase II detoxifying enzymes as well as anti-oxidative enzymes. In this study, we investigated the transactivation potential of different Nrf2 transactivation domain regions by using the Gal4-Nrf2 chimeras and Gal4-Luc reporter co-transfection assay system in HepG2 cells. The results indicated that chimera Gal4-Nrf2-(1-370), which contains the full transactivation domain showed very potent transactivation activity. The high transactivation activity of Gal4-Nrf2-(113-251) and the diminished transactivation activities of chimera Gal4-Nrf2-(1-126) and Gal4-Nrf2-(230-370) suggested that the Nrf2 N-terminal 113-251 amino acids region is critical in maintaining its transactivation activity. Overexpression of upstream MAPKs such as Raf, MEKK1, TAK1-DeltaN, and ASK1 up-regulated the transactivation activities of Gal4-Nrf2-(1-370) and Gal4-Nrf2-(113-251) in a dose-dependent manner. Further investigation on the effects of the three MAPK pathways on Nrf2 transactivation domain activity demonstrated that both ERK and JNK signaling pathways stimulated the Gal4-Nrf2-(1-370) transactivation activity while the p38 pathway played a negative role. Site-directed mutagenesis studies on potential MAPK phosphorylation sites of Gal4-Nrf2-(113-251) showed no significant effect on its basal transactivation activity or the fold of induction by Raf. Interestingly, the nuclear transcription coactivator CREB-binding protein (CBP), which can bind to Nrf2 transactivation domain and can be activated by ERK cascade, showed synergistic stimulation with Raf on the transactivation activities of both the chimera Gal4-Nrf2-(1-370) and the full-length Nrf2. Taken together, this study clearly demonstrated that different segments of Nrf2 transactivation domain have different transactivation potential and different MAPKs have differential effects on Nrf2 transcriptional activity. It also suggested that the up-regulation of Nrf2 transactivation domain activity by upstream MAPKs such as Raf may not be mediated by direct phosphorylation of the Nrf2 transactivation domain, but rather by regulation of the transcriptional activity of coactivator CBP.

Mouse MafA, homologue of zebrafish somite Maf 1, contributes to the specific transcriptional activity through the insulin promoter

Large Maf transcription factors, which are members of the basic leucine zipper (b-Zip) superfamily, have been reported to be involved in embryonic development and cell differentiation. Previously, we isolated a novel zebrafish large Maf cDNA, somite Maf1 (SMaf1), which possesses transactivational activity within its N-terminus domain. To elucidate SMaf1 function in mammals, we tried to isolate the mouse homologue of zebrafish SMaf1. We isolated the mouse homologue of zebrafish SMaf1, which is the same molecule as the recently reported MafA. MafA mRNA was detected in formed somites, head neural tube, and liver cells in the embryos. In the adult mouse, MafA transcript was amplified in the brain, lung, spleen, and kidney by RT-PCR. MafA mRNA was also detectable in beta-cell line. Next, we analyzed the transcriptional activity of MafA using rat insulin promoters I and II (RIPI and II), since a part of RIP sequence was similar to the Maf recognition element (MARE) and MafA was expressed in pancreatic beta cells. MafA was able to activate transcription from RIPII, but not RIPI, in a dose dependent manner and the activity was dependent on RIPE3b/C1 sequences. In addition, the amount of MafA protein was regulated by glucose concentration. These results indicate that MafA is the homologue of zebrafish SMaf1 and acts as a transcriptional activator of the insulin gene promoter through the RIPE3b element.

Nrf2 regulates thromboxane synthase gene expression in human lung cells

Thromboxane A(2) synthase (TXAS) converts prostaglandin H(2) to thromboxane A(2), a potent inducer of vaso-constriction and platelet aggregation. TXAS expression level is cell type preferential; high in hematopoietic cells and low in nonhematopoietic cells. We previously showed that p45 NF-E2 activated the TXAS promoter in hematopoietic cells via binding to the nucleotides -86/-77 from the transcriptional start site [Yaekashiwa and Wang (2002) J. Biol. Chem. 277, 22497-22508]. We reported here that, by transient transfection analysis, this region was also critical for TXAS trans-activation in the A549 and WI-38 lung cells. Mutation of the NF-E2 site greatly reduced TXAS promoter activity in these two types of cells. Using stably transfected A549 cells, we showed that an NF-E2 mutation retained only 0.25% of the wild-type promoter activity. Ecotopic expression of NF-E2 related factors showed that Nrf2, but not Nrf1, Nrf3, or Bach1, activated TXAS promoter in a dose-dependent manner. Furthermore, chromatin immunoprecipitation assay using the stably transfected A549 cells demonstrated that Nrf2 bound the TXAS NF-E2 site in vivo. TXAS gene thus utilizes the same cis-acting element but different trans-acting factors to confer cell-preferential expression. We also showed that forced expression of p300 upregulated TXAS gene in a dose-dependent manner. Mutation of NF-E2 site, but not TATA or initiator site, abolished the p300-mediated activation of TXAS gene.

Nrf2, not the estrogen receptor, mediates catechol estrogen-induced activation of the antioxidant responsive element

The antioxidant responsive element (ARE) plays an important role in the gene expression of phase II detoxification enzymes, such as NAD(P)H:quinone oxidoreductase 1 (NQO1), and NF-E2-related factor2 (Nrf2) is the transcription factor for the ARE-driven genes. Interestingly, estrogen receptor (ER) was reported to increase NQO1 gene expression through the ARE. In this study, we investigated the role of ER and Nrf2 in ARE activation using IMR-32 cells and mouse primary astrocytes. Among tested estrogen-related compounds, only catechol estrogens (i.e. 4-hydroxyestradiol) activated the ARE. Since 4-hydroxyestradiol-induced ARE activation was not inhibited by either 17beta-estradiol or tamoxifen, and overexpression of ER-alpha decreased 4-hydroxyestradiol-induced ARE activation, ARE activation by catechol estrogen was independent of ER. Nrf2, however, was very important in the 4-hydroxyestradiol-induced ARE activation. 4-Hydroxyestradiol did not activate the ARE in Nrf2 knockout (-/-) primary astrocytes, but did activate the ARE when Nrf2 was transfected into Nrf2-/- astrocytes. In addition, dominant negative Nrf2 completely blocked 4-hydroxyestradiol-induced ARE activation in Nrf2+/+ astrocytes, and only 4-hydroxyestradiol induced Nrf2 nuclear translocation in IMR-32 cells. A selective phosphatidylinositol 3-kinase (PI3-kinase) inhibitor (LY294002) blocked 4-hydroxyestradiol-induced Nrf2 nuclear translocation and NQO1 activity induction in IMR-32 cells. Taken together, these observations suggest that 4-hydroxyestradiol activates the ARE by a PI3-kinase-Nrf2 dependent mechanism, not involving ER.

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.

Identification of a novel Nrf2-regulated antioxidant response element (ARE) in the mouse NAD(P)H:quinone oxidoreductase 1 gene: reassessment of the ARE consensus sequence

NQO1 [NAD(P)H:quinone oxidoreductase 1] has an integral role in cellular responses to oxidative stress. The expression of NQO1 is up-regulated in the mouse following challenge with electrophilic chemicals, in an Nrf2 (NF-E2 p45-related factor 2)-dependent fashion, but the molecular basis for this observation remains unexplained. Through characterization of the murine nqo1 5'-upstream region, we now show that Nrf2 regulates this gene directly via an ARE (antioxidant response element) that lies within a 24 bp region spanning nt -444 to -421. A comprehensive mutation study of this ARE revealed that it does not conform to the currently accepted ARE consensus sequence [(5'-TMAnnRTGAYnnnGCRwwww-3', with essential nucleotides shown in capitals); two cytosine residues (shown in bold in the following sequence) that have been designated 'n' previously because they were thought to be redundant (5'-gagTcA C aGTgAGt C ggCAaaatt-3') have now been found to be essential for enhancer activity; two guanines (also shown in bold) previously regarded as essential for ARE function (5'-gagTcACaGT g AGtCg g CAaaatt-3') have proven to be dispensable]. Examination of wild-type and nrf2 (-/-) mouse embryonic fibroblasts demonstrated that Nrf2 is essential for both constitutive expression of NQO1 and its induction by sulphoraphane. Electrophoretic mobility-shift and chromatin immunoprecipitation assays revealed that Nrf2 associates, in low amounts, with the nqo1 ARE under constitutive conditions, and following sulphoraphane challenge of cells, Nrf2 is recruited to the ARE in substantially greater quantities, as a heterodimer with the small Maf (musculoaponeurotic fibrosarcoma virus) protein, MafK. Also, MafK was found to bind the nqo1 ARE in an Nrf2-independent fashion, and may contribute to transcriptional repression of the oxidoreductase gene. These findings allow a model for transcriptional control of nqo1 through the ARE to be proposed. Furthermore, our results indicate that distinct AREs have differential sequence requirements, and a universally applicable consensus sequence cannot be derived.

Keap1-dependent proteasomal degradation of transcription factor Nrf2 contributes to the negative regulation of antioxidant response element-driven gene expression

Keap1 is a negative regulator of Nrf2, a bZIP transcription factor that mediates adaptation to oxidative stress. Previous studies suggested this negative regulation is a consequence of Keap1 controlling the subcellular distribution of Nrf2. We now report that Keap1 also controls the total cellular level of Nrf2 protein. In the RL34 non-transformed rat liver cell line, Nrf2 was found to accumulate rapidly in response to oxidative stress caused by treatment with sulforaphane, and the accumulation resulted from inhibition of proteasomal-mediated degradation of the bZIP protein. By heterologously expressing in COS1 cells epitope-tagged Nrf2 and an Nrf2DeltaETGE mutant lacking the Keap1-binding site, in both the presence and absence of Keap1 we demonstrate that Nrf2 is subject to ubiquitination and proteasomal degradation independently of both Keap1 and the redox environment of the cell. In oxidatively stressed cells, this is the sole mechanism responsible for Nrf2 degradation. However, under homeostatic conditions Nrf2 is subject to a substantially more rapid mode of proteasomal degradation than it is in oxidatively stressed cells, and this rapid turnover of Nrf2 requires it to interact with Keap1. Within Nrf2, the N-terminal Neh2 domain is identified as the redox-sensitive degron. These data suggest that Keap1 negatively regulates Nrf2 by both enhancing its rate of proteasomal degradation and altering its subcellular distribution.

Identification of the NF-E2-related factor-2-dependent genes conferring protection against oxidative stress in primary cortical astrocytes using oligonucleotide microarray analysis

The antioxidant responsive element (ARE) mediates transcriptional regulation of phase II detoxification enzymes and antioxidant proteins such as NAD(P)H:quinone oxidoreductase (NQO1), glutathione S-transferases, and glutamate-cysteine ligase. In this study, we demonstrate that NF-E2-related factor-2 (Nrf2) plays a major role in transcriptional activation of ARE-driven genes and identify Nrf2-dependent genes by oligonucleotide microarray analysis using primary cortical astrocytes from Nrf2(+/+) and Nrf2(-/-) mice. Nrf2(-/-) astrocytes had decreased basal NQO1 activity and no induction by tert-butylhydroquinone compared with Nrf2(+/+) astrocytes. Similarly, both basal and induced levels of human NQO1-ARE-luciferase expression in Nrf2(-/-) astrocytes were significantly lower than in Nrf2(+/+) astrocytes. Furthermore, human NQO1-ARE-luciferase expression in Nrf2(-/-) astrocytes was restored by overexpression of Nrf2, whereas ARE activation in Nrf2(+/+) astrocytes was completely blocked by dominant-negative Nrf2. In addition, we observed that Nrf2-dependent genes protected primary astrocytes from H(2)O(2)- or platelet-activating factor-induced apoptosis. In support of these observations, we identified Nrf2-dependent genes encoding detoxification enzymes, glutathione-related proteins, antioxidant proteins, NADPH-producing enzymes, and anti-inflammatory genes using oligonucleotide microarrays. Proteins within these functional categories are vital to the maintenance and responsiveness of a cell defense system, suggesting that an orchestrated change in gene expression via Nrf2 and the ARE gives a synergistic protective effect against oxidative stress.

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.

Nrf2 mediates the induction of ferritin H in response to xenobiotics and cancer chemopreventive dithiolethiones

Ferritin is a ubiquitous intracellular iron storage protein that consists of 24 subunits of the H and L type. The ability to sequester iron from participation in oxygen free radical formation is consistent with a cytoprotective role for ferritin. Here we demonstrate that ferritins H and L are induced in cells treated with beta-napthoflavone (beta-NF) and chemopreventive dithiolethiones. Induction of ferritin H by beta-NF and the dithiolethiones oltipraz and 1,2-dithiole-3-thione (D3T) occurs via a transcriptional mechanism that is mediated by the ferritin H electrophile/antioxidant-responsive element (EpRE/ARE). The murine ferritin H gene contains five potential xenobiotic-responsive element (XRE) sequences in its 5'-promoter region. However, deletion analysis demonstrates that these XRE sequences are not functional in inducing ferritin H in response to beta-NF. Electrophoretic mobility shift assays demonstrate that the ferritin H EpRE/ARE binds Nrf2. Transfection of chimeric ferritin H reporter genes with Nrf2 expression vectors and Nrf2 dominant-negative mutants indicate that Nrf2 functions at the EpRE/ARE to mediate transcriptional activation of ferritin H. Induction of ferritin H and L was not seen in Nrf2 knockout cells, demonstrating that this transcription factor is required for the induction of ferritin in response to polycyclic aromatic xenobiotics and chemopreventive agents. Nrf2 may also play a role in basal transcription of both ferritin H and L. These results provide a mechanistic link between regulation of the iron storage protein ferritin and the cancer chemopreventive response.

Phosphorylation of Nrf2 at Ser-40 by protein kinase C regulates antioxidant response element-mediated transcription

Nrf2, a basic leucine zipper transcription factor, is an essential activator of the coordinated transcription of genes encoding antioxidant enzymes and phase II detoxifying enzymes through the regulatory sequence termed antioxidant response element (ARE). Recently we reported evidence for the involvement of protein kinase C (PKC) in phosphorylating Nrf2 and triggering its nuclear translocation in response to oxidative stress. We show here that phosphorylation of purified rat Nrf2 by the catalytic subunit of PKC was blocked by a synthetic peptide mimicking one of the potential PKC sites. Accordingly, Nrf2 bearing a Ser to Ala mutation at amino acid 40 (S40A) could not be phosphorylated by PKC. The S40A mutation did not affect in vitro binding of Nrf2/MafK to the ARE. However, it partially impaired Nrf2 activation of ARE-driven transcription in a reporter gene assay when Keap1 was overexpressed. In vitro transcribed/translated Keap1 could be coimmunoprecipitated with Nrf2. Phosphorylation of wild-type Nrf2 by PKC promoted its dissociation from Keap1, whereas the Nrf2-S40A mutant remained associated. These findings together with our prior studies suggest that the PKC-catalyzed phosphorylation of Nrf2 at Ser-40 is a critical signaling event leading to ARE-mediated cellular antioxidant response.

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.

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.