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

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

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

Acetylation-deacetylation of the transcription factor Nrf2 (nuclear factor erythroid 2-related factor 2) regulates its transcriptional activity and nucleocytoplasmic localization

Activation of Nrf2 by covalent modifications that release it from its inhibitor protein Keap1 has been extensively documented. In contrast, covalent modifications that may regulate its action after its release from Keap1 have received little attention. Here we show that CREB-binding protein induced acetylation of Nrf2, increased binding of Nrf2 to its cognate response element in a target gene promoter, and increased Nrf2-dependent transcription from target gene promoters. Heterologous sirtuin 1 (SIRT1) decreased acetylation of Nrf2 as well as Nrf2-dependent gene transcription, and its effects were overridden by dominant negative SIRT1 (SIRT1-H355A). The SIRT1-selective inhibitors EX-527 and nicotinamide stimulated Nrf2-dependent gene transcription, whereas resveratrol, a putative activator of SIRT1, was inhibitory, mimicking the effect of SIRT1. Mutating lysine to alanine or to arginine at Lys(588) and Lys(591) of Nrf2 resulted in decreased Nrf2-dependent gene transcription and abrogated the transcription-activating effect of CREB-binding protein. Furthermore, SIRT1 had no effect on transcription induced by these mutants, indicating that these sites are acetylation sites. Microscope imaging of GFP-Nrf2 in HepG2 cells as well as immunoblotting for Nrf2 showed that acetylation conditions resulted in increased nuclear localization of Nrf2, whereas deacetylation conditions enhanced its cytoplasmic rather than its nuclear localization. We posit that Nrf2 in the nucleus undergoes acetylation, resulting in binding, with basic-region leucine zipper protein(s), to the antioxidant response element and consequently in gene transcription, whereas deacetylation disengages it from the antioxidant response element, thereby resulting in transcriptional termination and subsequently in its nuclear export.

Review of molecular mechanisms involved in the activation of the Nrf2-ARE signaling pathway by chemopreventive agents

Human exposures to environmental toxicants have been associated with etiology of many diseases including inflammation, cancer, and cardiovascular and neurodegenerative disorders. To counteract the detrimental effect of environmental insults, mammalian cells have evolved a hierarchy of sophisticated sensing and signaling mechanisms to turn on or off endogenous antioxidant responses accordingly. One of the major cellular antioxidant responses is the induction of antioxidative and carcinogen-detoxification enzymes through the cytoplasmic oxidative stress system (Nrf2-Keap1) activated by a variety of natural and synthetic chemopreventive agents. Under normal conditions, Keap1 anchors the Nrf2 transcription factor within the cytoplasm targeting it for ubiquitination and proteasomal degradation to maintain low levels of Nrf2 that mediate the constitutive expression of Nrf2 downstream genes. When cells are exposed to chemopreventive agents and oxidative stress, a signal involving phosphorylation and/or redox modification of critical cysteine residues in Keap1 inhibits the enzymatic activity of the Keap1-Cul3-Rbx1 E3 ubiquitin ligase complex, resulting in decreased Nrf2 ubiquitination and degradation. As a consequence, free Nrf2 translocates into the nucleus and in combination with other transcription factors (e.g., sMaf, ATF4, JunD, PMF-1) transactivates the antioxidant response elements (AREs)/electrophile response elements (EpREs) of many cytoprotective genes, as well as Nrf2 itself. Upon recovery of cellular redox homeostasis, Keap1 travels into the nucleus to dissociate Nrf2 from the ARE. Subsequently, the Nrf2-Keap1 complex is exported out of the nucleus by the nuclear export sequence (NES) in Keap1. Once in the cytoplasm, the Nrf2-Keap1 complex associates with the Cul3-Rbx1 core ubiquitin machinery, resulting in degradation of Nrf2 and termination of the Nrf2/ARE signaling pathway. The discovery of multiple nuclear localization signals (NLSs) and nuclear export signals (NESs) in Nrf2 also suggests that the nucleocytoplasm translocation of transcription factors is the consequence of a dynamic equilibrium of multivalent NLSs and NESs. On the other hand, Keap1 may provide an additional regulation of the quantity of Nrf2 both in basal and inducible conditions. This chapter summarizes the current body of knowledge regarding the molecular mechanisms through which ARE inducers (chemopreventive agents) regulate the coordinated transcriptional induction of genes encoding phase II and antioxidant enzymes as well as other defensive proteins, via the nuclear factor-erythroid 2 (NF-E2-p45)-related factor 2(Nrf2)/(ARE) signaling pathway.

Discovery of the negative regulator of Nrf2, Keap1: a historical overview

An antioxidant response element (ARE) or an electrophile responsive element (EpRE) regulate the transcriptional induction of a battery of drug-detoxifying enzymes that are protective against electrophiles. Based on the high similarity of the ARE consensus sequence to an erythroid gene regulatory element NF-E2 binding site, we have found that the transcription factor Nrf2 is indispensable for the ARE-mediated induction of drug-metabolizing enzymes. Recent genome-wide analysis demonstrated that Nrf2 regulates hundreds of genes that are involved in the cytoprotective response against oxidative stress. In-depth analysis of Nrf2 regulatory mechanisms has led us to the discovery of a novel protein, which we have named Keap1. Keap1 suppresses Nrf2 activity by specifically binding to its evolutionarily conserved N-terminal Neh2 regulatory domain. In this review article, we summarize the findings and observations that have lead to the discovery of the Nrf2-Keap1 system. Furthermore, we briefly discuss the function of the Nrf2-Keap1 system under the regulation of the endogenous electrophilic compound 15-deoxy-Δ¹²(,)¹⁴-prostaglandin J₂. We propose that Nrf2-Keap1 plays a significant physiological role in the response to endogenous, environmental, and pharmacological electrophiles.

Regulation of NF-E2-related factor 2 signaling for cancer chemoprevention: antioxidant coupled with antiinflammatory

Cancer chemoprevention is a process of using either natural or synthetic compounds to reduce the risk of developing cancer. Observations that NF-E2-related factor 2 (Nrf2)-deficient mice lack response to some chemopreventive agents point to the important role of Nrf2 in chemoprevention. Nrf2 is a member of basic-leucine zipper transcription factor family and has been shown to regulate gene expression by binding to a response element, antioxidant responsive element. It is generally believed that activation of Nrf2 signaling is an adaptive response to the environmental and endogenous stresses. Under homeostatic conditions, Nrf2 is suppressed by association with Kelch-like ECH-associated protein 1 (Keap1), but is stimulated upon exposure to oxidative or electrophilic stress. Once activated, Nrf2 translocates into nuclei and upregulates a group of genes that act in concert to combat oxidative stress. Nrf2 is also shown to have protective function against inflammation, a pathological process that could contribute to carcinogenesis. In this review, we will discuss the current progress in the study of Nrf2 signaling, in particular, the mechanisms of Nrf2 activation by chemopreventive agents. We will also discuss some of the potential caveats of Nrf2 in cancer treatment and future opportunity and challenges on regulation of Nrf2-mediated antioxidant and antiinflammatory signaling in the context of cancer prevention.

Cancer chemoprevention mechanisms mediated through the Keap1-Nrf2 pathway

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

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

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

CD36 participates in a signaling pathway that regulates ROS formation in murine VSMCs

CD36 is a membrane glycoprotein expressed on platelets, monocytes, macrophages, and several other cell types that was recently demonstrated to be involved in platelet activation in response to oxidized phospholipids, including oxidized LDL. Although the role of CD36 in other vascular cells has not been well defined, previous studies have demonstrated that cd36-knockout (cd36-/-) mice have prolonged thrombosis times after vascular injury, which can be protective in the state of hyperlipidemia. Here, we found significantly less ROS in the vessel walls of cd36-/- mice compared with WT after chemically induced arterial injury, suggesting that CD36 may contribute to ROS generation in the VSMCs themselves. Gene expression analysis revealed that the antioxidant enzymes peroxiredoxin-2 (Prdx2) and heme oxygenase-1 were upregulated in cd36-/- VSMCs. Molecular dissection of the pathway in isolated mouse VSMCs revealed CD36 ligand-dependent induction of Fyn phosphorylation, with subsequent phosphorylation and degradation of the redox-sensitive transcription factor Nrf2. Chromatin immunoprecipitation experiments further showed that Nrf2 directly occupied the Prdx2 promoter. The importance of this pathway was evidenced by increased ROS generation in prdx2-/- mice and decreased thrombosis times in both prdx2-/- and nrf2-/- mice after vascular injury. These data suggest that CD36-mediated downregulation of antioxidant systems in VSMCs may contribute to its prothrombotic, proinflammatory, and atherogenic effects.

A small-molecule inducer of the antioxidant response element

Eukaryotic cells counteract oxidative and other environmental stress through the activation of Nrf2, the transcription factor that controls the expression of a host of protective enzymes by binding to the antioxidant response element (ARE). The electrophilic molecules that are able to activate Nrf2 and its downstream target genes have demonstrated therapeutic potential in carcinogen-induced tumor models. Using a high-throughput cellular screen, we discovered a class of ARE activator, which we named AI-1, that activates Nrf2 by covalently modifying Keap1, the negative regulator of Nrf2. Biochemical studies indicated that modification of Cys151 of Keap1 by AI-1 disrupted the ability of Keap1 to serve as an adaptor for Cul3-Keap1 ubiquitin ligase complex, thereby causing stabilization and transcriptional activation of Nrf2. AI-1 and its biotinylated derivative are useful pharmacological probes for investigating the molecular details of the cellular antioxidant response.

Nrf2-ARE signaling pathway and natural products for cancer chemoprevention

BACKGROUND

One of the potential strategies for preventing cancers is using food-based natural products to induce cytoprotective enzymes including phase II and antioxidative enzymes that act in concert to detoxify and eliminate harmful reactive intermediates formed from carcinogens. The antioxidant response element (ARE), which is activated upon binding of the nuclear factor E2-related protein 2 (Nrf2) transcription factor protein, has been identified in the regulatory regions of numerous genes encoding cytoprotective enzymes. Herein, we summarized the current body of knowledge regarding Nrf2 regulation as well as highlighted the Nrf2/ARE activators from natural products, which will potentially be used as chemopreventive agents for cancer patients.

METHODS

Via reviewing Pubmed, we summarized the current progress in the molecular mechanisms of Nrf2 regulation and the major classes of dietary components that act as promising chemopreventive agents through evoking Nrf2-ARE core signaling pathway.

RESULTS

Under basal condition, Nrf2 is at low level, sequestered in the cytoplasm by being tethered to an actin binding Kelch-like ECH associating protein 1 (Keap1). Pharmacological and putative chemopreventive agents trigger the release of Nrf2 from Keap1, allowing it to translocate into the nucleus and drive the gene expression of detoxifying enzymes to perform cancer chemopreventive effect.

CONCLUSION

Augmenting both expression and activity of phase II detoxification and antioxidant enzymes via Nrf2-ARE core signaling pathway would be a rational approach for cancer chemoprevention and the number of novel Nrf2/ARE activators from dietary sources is growing.

Nrf2-Keap1 antioxidant system and hepatic diseases

NF-E2-related factor 2 (Nrf2) is an important transcription factor. When oxidative stress occurs, Nrf2 dissociates from Keap1 (Kelch-like ECH-associating protein 1), translocates to the nucleus, and regulates the expression of genes encoding phase II detoxifying enzymes and antioxidant proteins, thereby increasing the resistance to oxidative stress and electrophilic agents. Reactive oxygen species and oxidative stress play an important role in the development of hepatic diseases. In this article, we will summarize the relationship between the Nrf2-Keap1 system and hepatic diseases.

Nuclear matrix protein (NRP/B) modulates the nuclear factor (Erythroid-derived 2)-related 2 (NRF2)-dependent oxidative stress response

Reactive molecules have diverse effects on cells and contribute to several pathological conditions. Cells have evolved complex protective systems to neutralize these molecules and restore redox homeostasis. Previously, we showed that association of nuclear factor (NF)-erythroid-derived 2 (E2)-related factor 2 (NRF2) with the nuclear matrix protein NRP/B was essential for the transcriptional activity of NRF2 target genes in tumor cells. The present study demonstrates the molecular mechanism by which NRP/B, via NRF2, modulates the transcriptional activity of antioxidant response element (ARE)-driven genes. NRP/B is localized in the nucleus of primary brain tissue and human neuroblastoma (SH-SY5Y) cells. Treatment with hydrogen peroxide (H(2)O(2)) enhances the nuclear colocalization of NRF2 and NRP/B and induces heme oxygenase 1 (HO1). Treatment of NRP/B or NRF2 knockdowns with H(2)O(2) induced apoptosis. Co-expression of NRF2 with members of the Kelch protein family, NRP/B, MAYVEN, or MAYVEN-related protein 2 (MRP2), revealed that the NRF2-NRP/B complex is important for the transcriptional activity of ARE-driven genes HO1 and NAD(P)H:quinine oxidoreductase 1 (NQO1). NRP/B interaction with Nrf2 was mapped to NRF2 ECH homology 4 (Neh4)/Neh5 regions of NRF2. NRP/B mutations that resulted in low binding affinity to NRF2 were unable to activate NRF2-modulated transcriptional activity of the ARE-driven genes, HO1 and NQO1. Thus, the interaction of NRP/B with the Neh4/Neh5 domains of NRF2 is indispensable for activation of NRF2-mediated ARE-driven antioxidant and detoxifying genes that confer cellular defense against oxidative stress-induced damage.

Increased cell migration and plasticity in Nrf2-deficient cancer cell lines

Nuclear Factor (erythroid-derived 2)-like 2 (Nrf2) expression is deregulated in many cancers. Genetic and biochemical approaches coupled with functional assays in cultured cells were used to explore the consequence of Nrf2 repression. Nrf2 suppression by Keap1-directed ubiquitylation or expression of independent shRNA/siRNA sequences enhanced cellular ROS, Smad-dependent tumor cell motility, and growth in soft agar. Loss of Nrf2 was accompanied by concomitant Smad linker region/C-terminus phosphorylation, induction of the E-Cadherin transcriptional repressor Slug, and suppression of the cell-cell adhesion protein E-Cadherin. Ectopic expression of wildtype Nrf2, but not dominant negative Nrf2, suppressed the activity of a synthetic TGF-β1 responsive CAGA-directed luciferase reporter. shRNA knock-down of Nrf2 enhanced the activity of the synthetic CAGA-reporter, as well as the expression of the endogenous Smad target gene plasminogen activator inhibitor-1. Finally, we found that Nrf2/Smad3/Smad4 formed an immunoprecipitable nuclear complex. Thus, loss of Nrf2 increased R-Smad phosphorylation and R-Smad signaling, supporting the hypothesis that loss of Nrf2 in an oncogenic context-dependent manner can enhance cellular plasticity and motility, in part by using TGF-β/Smad signaling.

Antioxidant and mitochondrial protective effects of oxidized metabolites of oltipraz

IMPORTANCE OF THE FIELD

Comprehensive studies indicate that oltipraz exerts cancer chemopreventive effects. Oltipraz has other therapeutic potentials, which include anti-fibrotic effect, inhibition of insulin resistance, mitochondrial protection and cytoprotective effect against oxidative stress. Although antioxidant mechanisms may account for its cancer chemopreventive effect, details on the molecular mechanism still remain to be clarified.

AREAS COVERED IN THIS REVIEW

Two major metabolic pathways of oltipraz include oxidative desulfuration of the thione to yield 4-methyl-5-(pyrazin-2-yl)-3H-1,2-dithiol-3-one and molecular rearrangement to 7-methyl-6,8-bis(methylthio)H-pyrrolo[1,2-a]pyrazine. In addition to the diverse pharmacological effects of oltipraz, the oxidized metabolites may have distinct biological effects on cell survival. The AMP-activated protein kinase pathway has been recognized as a key cascade for mitochondrial protection and cell survival events, which can be activated by the oxidized metabolites of oltipraz.

WHAT THE READER WILL GAIN

In this review, the metabolic activation of oltipraz and the role of the cell signaling pathways in regulating the expression of Phase II genes and antioxidant activity are discussed with particular reference to their effects on mitochondrial protection and cell survival.

TAKE HOME MESSAGE

In terms of therapeutic potential, the findings reviewed here demonstrate a therapeutic potential for oxidized metabolite of oltipraz and offer comparison of antioxidant capacity between metabolites and parent compound.

Oncogenic NRF2 mutations in squamous cell carcinomas of oesophagus and skin

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

Stress-activated cap'n'collar transcription factors in aging and human disease

Cap'n'collar (Cnc) transcription factors are conserved in metazoans and have important developmental and homeostatic functions. The vertebrate Nrf1, Nrf2, and Nrf3; the Caenorhabditis elegans SKN-1; and the Drosophila CncC comprise a subgroup of Cnc factors that mediate adaptive responses to cellular stress. The most studied stress-activated Cnc factor is Nrf2, which orchestrates the transcriptional response of cells to oxidative stressors and electrophilic xenobiotics. In rodent models, signaling by Nrf2 defends against oxidative stress and aging-associated disorders, such as neurodegeneration, respiratory diseases, and cancer. In humans, polymorphisms that decrease Nrf2 abundance have been associated with various pathologies of the skin, respiratory system, and digestive tract. In addition to preventing disease in rodents and humans, Cnc factors have life-span-extending and anti-aging functions in invertebrates. However, despite the pro-longevity and antioxidant roles of stress-activated Cnc factors, their activity paradoxically declines in aging model organisms and in humans suffering from progressive respiratory disease or neurodegeneration. We review the roles and regulation of stress-activated Cnc factors across species, present all reported instances in which their activity is paradoxically decreased in aging and disease, and discuss the possibility that the pharmacological restoration of Nrf2 signaling may be useful in the prevention and treatment of age-related diseases.

Effects of atmospheric pollutants on the Nrf2 survival pathway

BACKGROUND, AIM, AND SCOPE

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

MAIN FEATURES

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

RESULTS AND DISCUSSION

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

CONCLUSIONS

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

PERSPECTIVES

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

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

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

NF-E2 domination over Nrf2 promotes ROS accumulation and megakaryocytic maturation

In megakaryocytes, the maturation process and oxidative stress response appear to be closely related. It has been suggested that increased oxygen tension and reactive oxygen species (ROS) promote megakaryopoiesis and that the expression of stress-responsive genes responsible for ROS elimination declines during megakaryocytic maturation. NF-E2 p45 is an essential regulator of megakaryopoiesis, whereas Nrf2 is a key activator of stress-responsive genes. Because p45 and Nrf2 have similar DNA-binding specificities, we hypothesized that p45 competes with Nrf2 to repress stress-responsive genes and achieves favorable intracellular conditions to allow ROS to be efficiently used as signaling molecules. We conducted comprehensive gene expression profiling with wild-type and p45-null megakaryocytes and examined the functional relationship between p45 and Nrf2. We found that 2 characteristic gene clusters are defined within p45 target genes: platelet genes and cytoprotective genes. The former are unique targets activated by p45, whereas the latter are common targets of p45 and Nrf2. Further analysis suggested that, as a less efficacious activator, p45 maintains moderate expression of cytoprotective genes through competing with Nrf2 and promotes ROS accumulation. Increased ROS enhanced platelet gene expression. These results suggest that p45 dominates over Nrf2 to enhance megakaryocytic maturation by promoting ROS accumulation.

Signaling pathways involved in phase II gene induction by alpha, beta-unsaturated aldehydes

Phase II enzymes are induced primarily through the common electrophile response element (EpRE) signaling. Studies performed in different cell types and with different inducer appear to indicate variation in the upstream signaling pathways involved in the induction of these phase II genes. Nonetheless, whether variation in signaling among phase II genes in the same cell with the same inducer is unclear. This study is designed to answer this question using human bronchial epithelial cells (HBE1 cells) as a model and screening with a variety of protein kinase inhibitors with varying degrees of specificity. Two electrophiles, 4-hydroxynonenal (HNE) and acrolein, induced the expression of phase II genes (GCLC, GCLM, NQO1, NQO2, HO-1, and GSTM-1). Nrf2 silencing significantly decreased the induction of all of these genes, confirming the involvement of Nrf2-EpRE signaling. ERK and p38MAPK inhibitors had no effect, while a JNK inhibitor abrogated the GCLC and GCLM induction by HNE, but not that by acrolein. Among the PKC inhibitors used, one eliminated gene induction by HNE and acrolein, while two others showed no effects. One PI3K inhibitor decreased the induction of GCLM, NQO1, NQO2 and HO-1, but not GCLC and GST-M1; on the other hand, the inhibitory effects of another PI3K inhibitor on gene induction seems to be gene- and inducer- specific. In conclusion, our data suggest that although phase II genes are coordinately induced through Nrf2-EpRE signaling by electrophiles, the upstream signaling pathways involved are gene- and inducer- specific. It is also suggested that commercial kinase inhibitors may produce non-specific effects on phase II gene expression via mechanisms unrelated to their purported specificity.

Targeting the Nrf2 pathway against cardiovascular disease

Nuclear factor E2-related factor 2 (Nrf2) is a transcription factor that controls the basal and inducible expression of a battery of antioxidant genes and other cytoprotective Phase II detoxifying enzymes. Nrf2 is ubiquitously expressed in the cardiovascular system. While several Nrf2 downstream genes have been implicated in protection against the pathogenesis of cardiovascular diseases, the precise role of Nrf2 in the cardiovascular system remains to be elucidated. Nevertheless, mounting evidence has revealed that Nrf2 is a critical regulator of cardiovascular homeostasis via the suppression of oxidative stress, a major causative factor for the development and progression of cardiovascular diseases. Therefore, Nrf2 promises to be an attractive therapeutic target for the treatment of cardiovascular disease. Herein, we review the current literature that suggests that Nrf2 is a valuable therapeutic target for cardiovascular disease, as well as experiments that illustrate the mechanisms of Nrf2 cardioprotection.

Oxidative stress induced lung cancer and COPD: opportunities for epigenetic therapy

Reactive oxygen species (ROS) form as a natural by-product of the normal metabolism of oxygen and play important roles within the cell. Under normal circumstances the cell is able to maintain an adequate homeostasis between the formation of ROS and its removal through particular enzymatic pathways or via antioxidants. If however, this balance is disturbed a situation called oxidative stress occurs. Critically, oxidative stress plays important roles in the pathogenesis of many diseases, including cancer. Epigenetics is a process where gene expression is regulated by heritable mechanisms that do not cause any direct changes to the DNA sequence itself, and disruption of epigenetic mechanisms has important implications in disease. Evidence is emerging that histone deacetylases (HDACs) play decisive roles in regulating important cellular oxidative stress pathways including those involved with sensing oxidative stress and those involved with regulating the cellular response to oxidative stress. In particular aberrant regulation of these pathways by HDACs may play critical roles in cancer progression. In this review we discuss the current evidence linking epigenetics and oxidative stress and cancer, using chronic obstructive pulmonary disease and non-small cell lung cancer to illustrate the importance of epigenetics on these pathways within these disease settings.

Transcriptional regulation of rat gamma-glutamate cysteine ligase catalytic subunit gene is mediated through a distal antioxidant response element

Despite it being a quintessential Phase II detoxification gene, the transcriptional regulation of the rat gamma-glutamate cysteine ligase catalytic subunit (GCLC) is controversial. Computer-based sequence analysis identified three putative antioxidant response elements (AREs) at positions -889 to -865 (ARE1), -3170 to -3146 (ARE2) and -3901 to -3877 (ARE3) in the 5'-flanking region of the transcriptional start site. Transfections of individual ARE-luciferase reporter gene constructs into H4IIE cells, a rat hepatoma cell line, identified ARE3 as the functional promoter. Chromatin immunoprecipitation assays using primary rat hepatocytes showed that the transcription factor Nrf2, which is known to regulate ARE-mediated genes, is associated with ARE3. Co-transfection of H4IIE cells with luciferase reporter plasmids containing Gclc ARE3 and an Nrf2 expression plasmid resulted in a 3-fold activation of ARE3-mediated transcription relative to controls. "Loss-of-function" analysis for Nrf2 by small interfering RNA (siRNA) revealed that ARE3-mediated expression was significantly impaired while site-directed mutagenesis of the ARE3-luciferase reporter abolished Nrf2-mediated induction. Treatment with two known Nrf2 inducers, R-(alpha)-lipoic acid and anetholedithiolethione, showed that the inducible expression of the GCLC gene was also regulated by the ARE3 element. Taken together, these results show that Nrf2 regulates the constitutive expression of rat Gclc through a distal ARE present in its 5'-flanking region. This is the first report showing that rat Gclc is under the transcriptional control of the Nrf2-ARE pathway on a constitutive basis.

The Nrf2-antioxidant response element signaling pathway and its activation by oxidative stress

A major mechanism in the cellular defense against oxidative or electrophilic stress is activation of the Nrf2-antioxidant response element signaling pathway, which controls the expression of genes whose protein products are involved in the detoxication and elimination of reactive oxidants and electrophilic agents through conjugative reactions and by enhancing cellular antioxidant capacity. At the molecular level, however, the regulatory mechanisms involved in mediating Nrf2 activation are not fully understood. It is well established that Nrf2 activity is controlled, in part, by the cytosolic protein Keap1, but the nature of this pathway and the mechanisms by which Keap1 acts to repress Nrf2 activity remain to be fully characterized and are the topics of discussion in this minireview. In addition, a possible role of the Nrf2-antioxidant response element transcriptional pathway in neuroprotection will also be discussed.

NRF2 and KEAP1 mutations: permanent activation of an adaptive response in cancer

Transcription factor nuclear factor-erythroid 2-related factor 2 (NRF2) controls cellular adaptation to oxidants and electrophiles by inducing antioxidant and detoxification genes in response to redox stress. NRF2 is negatively regulated by Kelch-like ECH-associated protein 1 (KEAP1). Tumours from approximately 15% of patients with lung cancer harbour somatic mutations in KEAP1 that prevent effective NRF2 repression. Recently, two NRF2 mutation 'hot-spots' were identified in approximately 10% of patients with lung cancer, enabling the transcription factor to evade KEAP1-mediated repression. Somatic mutations in KEAP1 and NRF2 provide an insight into the molecular mechanisms by which NRF2 is regulated. Moreover, constitutive NRF2 activation might cause drug resistance in tumours, and an understanding of how the transcription factor is regulated indicates ways in which this could be overcome.

Acetylation of Nrf2 by p300/CBP augments promoter-specific DNA binding of Nrf2 during the antioxidant response

To maintain intracellular redox homeostasis, genes encoding many antioxidants and detoxification enzymes are transcriptionally upregulated upon deleterious oxidative stress through the cis antioxidant responsive elements (AREs) in their promoter regions. Nrf2 is the critical transcription factor responsible for ARE-dependent transcription. We and others have previously demonstrated that Nrf2 is targeted for ubiquitin-mediated degradation by Keap1 in a redox-sensitive manner through modifications of distinct cysteine residues of Keap1. Here, we report that p300/CBP directly acetylates Nrf2 in response to arsenite-induced stress. We have identified multiple acetylated lysine residues within the Nrf2 Neh1 DNA-binding domain. Combined lysine-to-arginine mutations on the acetylation sites, with no effects on Nrf2 protein stability, compromised the DNA-binding activity of Nrf2 in a promoter-specific manner. These findings demonstrated that acetylation of Nrf2 by p300/CBP augments promoter-specific DNA binding of Nrf2 and established acetylation as a novel regulatory mechanism that functions in concert with Keap1-mediated ubiquitination in modulating the Nrf2-dependent antioxidant response.

The Nrf1 CNC/bZIP protein is a nuclear envelope-bound transcription factor that is activated by t-butyl hydroquinone but not by endoplasmic reticulum stressors

In rat liver RL-34 cells, endogenous Nrf1 (nuclear factor-erythroid 2 p45 subunit-related factor 1) is localized in the ER (endoplasmic reticulum) where it exists as a glycosylated protein. Electron microscopy has demonstrated that ectopic Nrf1 in COS-1 cells is located in the ER and the NE (nuclear envelope). Subcellular fractionation, together with a membrane proteinase protection assay, revealed that Nrf1 is an integral membrane protein with both luminal and cytoplasmic domains. The N-terminal 65 residues of Nrf1 direct its integration into the ER and NE membranes and tether it to a Triton X-100-resistant membrane microdomain that is associated with lipid rafts. The activity of Nrf1 was increased by the electrophile tBHQ (t-butyl hydroquinone) probably through an N-terminal domain-dependent process. We found that the NST (Asn/Ser/Thr-rich) domain, along with AD1 (acidic domain 1), contributes positively to the transactivation activity of full-length Nrf1. Furthermore, the NST domain contains seven putative -Asn-Xaa-Ser/Thr- glycosylation sites and, when glycosylation was prevented by replacing all of the seven asparagine residues with either glutamine (Nrf1(1-7xN/Q)) or aspartic acid (Nrf1(1-7xN/D)), the former multiple point mutant possessed less activity than the wild-type factor, whereas the latter mutant exhibited substantially greater activity. Lastly, the ER stressors tunicamycin, thapsigargin and Brefeldin A were found to inhibit basal Nrf1 activity by approximately 25%, and almost completely prevented induction of Nrf1-mediated transactivation by tBHQ. Collectively, these results suggest that the activity of Nrf1 critically depends on its topology within the ER, and that this is modulated by redox stressors, as well as by its glycosylation status.

Transcribe to survive: transcriptional control of antioxidant defense programs for neuroprotection in Parkinson's disease

Parkinson's disease (PD) is a progressive, primarily motor disorder that is characterized by loss of dopaminergic (DA) neurons within the substantia nigra (SN). Cell death in PD has been associated with impaired mitochondrial function and increased oxidative stress. Strategies to reduce the oxidative load in DA cells may be beneficial in slowing the progression of PD. The transcription factor nuclear factor-erythroid 2 (NF-E2) related factor 2 (NRF2) is emerging as a master regulator of antioxidant defense systems, which makes it an attractive target for manipulations that aim to increase cellular resistance to oxidative stress. Peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator-1 alpha (PGC1alpha) is a regulator of mitochondrial biogenesis genes that simultaneously upregulates many genes known to protect against oxidative stress. Pgc-1alpha knockout mice show enhanced susceptibility to SN neuronal loss following MPTP exposure, whilst overexpression of Pgc-1alpha appears to protect against oxidative stress in vitro. This makes PGC-1alpha a highly attractive target for neuroprotective therapies in PD. This review will explore the mechanisms behind the induction of NRF2 and PGC-1alpha in response to oxidative stress and identify common pathways that may provide targets for upregulating antioxidant defense programs.

Molecular mechanisms of Nrf2-mediated antioxidant response

Nrf2 is the key transcription factor regulating the antioxidant response. Nrf2 signaling is repressed by Keap1 at basal condition and induced by oxidative stress. Keap1 is recently identified as a Cullin 3-dependent substrate adaptor protein. A two-sites binding "hinge & latch" model vividly depicts how Keap1 can efficiently present Nrf2 as substrate for ubiquitination. Oxidative perturbation can impede Keap1-mediated Nrf2 ubiquitination but fail to disrupt Nrf2/Keap1 binding. Nrf2 per se is a redox-sensitive transcription factor. A new Nrf2-mediated redox signaling model is proposed based on these new discoveries. Free floating Nrf2 protein functions as a redox-sensitive probe. Keap1 instead functions as a gate keeper to control the availability of Nrf2 probes and thus regulates the overall sensitivity of the redox signaling.

Pharmacogenomic profile of soy isoflavone concentrate in the prostate of Nrf2 deficient and wild-type mice

The involvement of Nrf2-a bZip transcription factor in soy isoflavones induced protection against oxidative stress and cancer has been reported. To gain better insight into the role of Nrf2 in prostate cancer chemoprevention by soy isoflavones, we examined the pharmacogenomics and gene expression profiles elicited by soy isoflavones in the prostates of C57BL/6J/Nrf2(-/-) and C57BL6J/Nrf2(+/+) wildtype. The profiles were analyzed using 45000 Affymetrix mouse genome 430-2.0 array and Genespring-7.2 software. The results obtained from microarray were further validated by real-time reverse transcription-PCR. Clusters of genes that were induced or suppressed more than twofold were identified as Nrf2 regulated soy isoflavone induced or suppressed genes. Classification based on their biological function revealed that genes mainly belonging to the categories of electron transport, phase II metabolizing enzymes, cell growth and differentiation, apoptosis, cell cycle, transcription factors, transport, mRNA processing, and carbohydrate homeostasis were either induced or suppressed by soy isoflavone and regulated by Nrf2. In addition, modulation of novel target genes such as LATS2 and GREB1 were identified to be mediated by Nrf2. Thus our current study provides a potential link between cancer chemopreventive properties of soy derived phytochemicals, the transcription factor Nrf2 and prevention of prostate cancer.

Dual roles of Nrf2 in cancer

In response to oxidative stress, the transcription factor NF-E2-related factor 2 (Nrf2) controls the fate of cells through transcriptional upregulation of antioxidant response element (ARE)-bearing genes, including those encoding endogenous antioxidants, phase II detoxifying enzymes, and transporters. Expression of the Nrf2-dependent proteins is critical for ameliorating or eliminating toxicants/carcinogens to maintain cellular redox homeostasis. As a result, activation of the Nrf2 pathway, by naturally-occurring compounds or synthetic chemicals at sub-toxic doses, confers protection against subsequent toxic/carcinogenic exposure. Thus, the use of dietary compounds or synthetic chemicals to boost the Nrf2-dependent adaptive response to counteract environmental insults has emerged to be a promising strategy for cancer prevention. Interestingly, recent emerging data has revealed the "dark" side of Nrf2. Nrf2 and its downstream genes are overexpressed in many cancer cell lines and human cancer tissues, giving cancer cells an advantage for survival and growth. Furthermore, Nrf2 is upregulated in resistant cancer cells and is thought to be responsible for acquired chemoresistance. Therefore, it may be necessary to inhibit the Nrf2 pathway during chemotherapy. This review is primarily focused on the role of Nrf2 in cancer, with emphasis on the recent findings indicating the cancer promoting function of Nrf2 and its role in acquired chemoresistance.

Network inference algorithms elucidate Nrf2 regulation of mouse lung oxidative stress

A variety of cardiovascular, neurological, and neoplastic conditions have been associated with oxidative stress, i.e., conditions under which levels of reactive oxygen species (ROS) are elevated over significant periods. Nuclear factor erythroid 2-related factor (Nrf2) regulates the transcription of several gene products involved in the protective response to oxidative stress. The transcriptional regulatory and signaling relationships linking gene products involved in the response to oxidative stress are, currently, only partially resolved. Microarray data constitute RNA abundance measures representing gene expression patterns. In some cases, these patterns can identify the molecular interactions of gene products. They can be, in effect, proxies for protein-protein and protein-DNA interactions. Traditional techniques used for clustering coregulated genes on high-throughput gene arrays are rarely capable of distinguishing between direct transcriptional regulatory interactions and indirect ones. In this study, newly developed information-theoretic algorithms that employ the concept of mutual information were used: the Algorithm for the Reconstruction of Accurate Cellular Networks (ARACNE), and Context Likelihood of Relatedness (CLR). These algorithms captured dependencies in the gene expression profiles of the mouse lung, allowing the regulatory effect of Nrf2 in response to oxidative stress to be determined more precisely. In addition, a characterization of promoter sequences of Nrf2 regulatory targets was conducted using a Support Vector Machine classification algorithm to corroborate ARACNE and CLR predictions. Inferred networks were analyzed, compared, and integrated using the Collective Analysis of Biological Interaction Networks (CABIN) plug-in of Cytoscape. Using the two network inference algorithms and one machine learning algorithm, a number of both previously known and novel targets of Nrf2 transcriptional activation were identified. Genes predicted as novel Nrf2 targets include Atf1, Srxn1, Prnp, Sod2, Als2, Nfkbib, and Ppp1r15b. Furthermore, microarray and quantitative RT-PCR experiments following cigarette-smoke-induced oxidative stress in Nrf2(+/+) and Nrf2(-/-) mouse lung affirmed many of the predictions made. Several new potential feed-forward regulatory loops involving Nrf2, Nqo1, Srxn1, Prdx1, Als2, Atf1, Sod1, and Park7 were predicted. This work shows the promise of network inference algorithms operating on high-throughput gene expression data in identifying transcriptional regulatory and other signaling relationships implicated in mammalian disease.

Phosphorylation of Nrf2 in the transcription activation domain by casein kinase 2 (CK2) is critical for the nuclear translocation and transcription activation function of Nrf2 in IMR-32 neuroblastoma cells

The antioxidant-activated transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) regulates the induction of cytoprotective genes against chemical toxicity and oxidative injuries. The role of phosphorylation in Nrf2 activation has been suggested but remains elusive. We report that phenolic antioxidant/pro-oxidant tert-butylhydroquinone (tBHQ) induced two forms of the Nrf2 protein in neuroblastoma cells (IMR-32), which migrated as distinctive bands on SDS-PAGE. In vitro treatment with lambda phosphatase eliminated the slower migrating form and increased the amount of the faster migrating form of Nrf2. In vivo (32)Pi-phosphorylation resulted in (32)Pi-labeling of the Nrf2 protein in the presence of tBHQ that can be dephosphorylated by lambda phosphotase, indicating that the slower migrating form is a phosphorylated Nrf2 protein and the faster form an unphosphorylated Nrf2. Unphosphorylated Nrf2 predominated in the cytoplasm, whereas the phosphorylated form preferentially localized in the nucleus. Nuclear Nrf2 can be dephosphorylated by lambda phosphotase in vitro and be converted to the faster migrating form, implicating phosphorylation of Nrf2 in the cytoplasmic-nuclear translocation of the protein. Deletional analyses from both the carboxyl- and amino-ends revealed the transcription activation (TA) domains Neh4 (Nrf2-ECH homology 4) and Neh5 (Nrf2-ECH homology 5) as a major region necessary for the phosphorylation. The TA domains are characterized by the presence of multiple phosphorylation sites of casein kinase 2 (CK2). Moreover, CK2 phosphorylated the TA domains in vitro. Treatment with CK2 inhibitor 2-dimethylamino-4,5,6,7,-tetrabromo-1H-benzimidazole (DMAT) blocked the induction of endogenous target genes of Nrf2 in cells and inhibited the TA activities of both the full length and the TA domains of Nrf2 to a large extent. Finally, phosphorylation of the TA domains correlated with the nuclear translocation of Nrf2 that was inhibited by DMAT in a concentration-dependent manner. The findings demonstrated that phosphorylation of Nrf2 at the TA domains by CK2 is an integral component of Nrf2 activation necessary for the nuclear localization and transcription activation function of Nrf2 in neuroblastoma cells.

Kavalactones protect neural cells against amyloid beta peptide-induced neurotoxicity via extracellular signal-regulated kinase 1/2-dependent nuclear factor erythroid 2-related factor 2 activation

One hallmark of Alzheimer's disease is the accumulation of amyloid beta-peptide (AP), which can initiate a cascade of oxidative events that may result in neuronal death. Because nuclear factor erythroid 2-related factor 2 (Nrf2) is the major regulator for a battery of genes encoding detoxifying and antioxidative enzymes via binding to the antioxidant response element (ARE), it is of great interest to find nontoxic activators of Nrf2 rendering neuronal cells more resistant to AP toxicity. Using ARE-luciferase assay and Western blot, we provide evidence that the kavalactones methysticin, kavain, and yangonin activate Nrf2 time- and dose-dependently in neural PC-12 and astroglial C6 cells and thereby up-regulate cytoprotective genes. Viability and cytotoxicity assays demonstrate that Nrf2 activation is able to protect neural cells from amyloid beta-(1-42) induced neurotoxicity. Down-regulation of Nrf2 by small hairpin RNA as well as extracellular signal-regulated kinase 1/2 inhibition abolishes cytoprotection. We further give evidence that kavalactone-mediated Nrf2 activation is not dependent on oxidative stress production. Our results demonstrate that kavalactones attenuate amyloid beta-peptide toxicity by inducing protective gene expression mediated by Nrf2 activation in vitro. These findings indicate that the use of purified kavalactones might be considered as an adjunct therapeutic strategy to combat neural demise in Alzheimer disease and other oxidative stress-related diseases.

Expression of Nrf2 and its downstream gene targets in hibernating 13-lined ground squirrels, Spermophilus tridecemlineatus

Mammalian hibernation is associated with wide variation in heart rate, blood flow, and oxygen delivery to tissues and is used as a model of natural ischemia/reperfusion. In non-hibernators, ischemia/reperfusion is typically associated with oxidative stress but hibernators seem to deal with potential oxidative damage by enhancing antioxidant defenses in an anticipatory manner. The present study assesses the role of the Nrf2 transcription factor in the regulation of antioxidant defenses during hibernation. Nrf2 mRNA and protein expression were enhanced in selected organs of 13-lined ground squirrels, Spermophilus tridecemlineatus during hibernation. Furthermore, Nrf2 protein in heart was elevated by 1.4-1.5 fold at multiple stages over a torpor-arousal bout including during entry, long term torpor, and early arousal. Levels returned to euthermic values when squirrels were fully aroused in interbout. Protein levels of selected downstream target genes under Nrf2 control were also measured via immunoblotting over the torpor-arousal cycle in heart. Cu/Zn superoxide dismutase and aflatoxin aldehyde reductase levels increased significantly during entry into torpor and then gradually declined falling to control levels or below in fully aroused animals. Heme oxygenase-1 also showed the same trend. This suggests a role for Nrf2 in regulating the antioxidant defenses needed for hibernation success. Heart nrf2 was amplified by PCR and sequenced. The deduced amino acid sequence showed high identity with the sequence from other mammals but with selected unique substitutions (e.g., proline residues at positions 111 and 230) that might be important for conformational stability of the protein at near 0 degrees C body temperatures in the torpid state.

Activating transcription factor 3 is a novel repressor of the nuclear factor erythroid-derived 2-related factor 2 (Nrf2)-regulated stress pathway

The transcription factor nuclear factor erythroid-derived 2-related factor 2 (Nrf2) regulates induction of an extensive cellular stress response network when complexed with the cAMP-responsive element binding protein (CBP) at antioxidant response elements (ARE) located in the promoter region of target genes. Activating transcription factor 3 (ATF3) can repress Nrf2-mediated signaling in a manner that is not well understood. Here, we show that ATF3-mediated suppression is a consequence of direct ATF3-Nrf2 protein-protein interactions that result in displacement of CBP from the ARE. This work establishes ATF3 as a novel repressor of the Nrf2-directed stress response pathway.

NF-kappaB/p65 antagonizes Nrf2-ARE pathway by depriving CBP from Nrf2 and facilitating recruitment of HDAC3 to MafK

Constitutively activated NF-kappaB occurs in many inflammatory and tumor tissues. Does it interfere with anti-inflammatory or anti-tumor signaling pathway? Here, we report that NF-kappaB p65 subunit repressed the Nrf2-antioxidant response element (ARE) pathway at transcriptional level. In the cells where NF-kappaB and Nrf2 were simultaneously activated, p65 unidirectionally antagonized the transcriptional activity of Nrf2. In the p65-overexpressing cells, the ARE-dependent expression of heme oxygenase-1 was strongly suppressed. However, p65 inhibited the ARE-driven gene transcription in a way that was independent of its own transcriptional activity. Two mechanisms were found to coordinate the p65-mediated repression of ARE: (1) p65 selectively deprives CREB binding protein (CBP) from Nrf2 by competitive interaction with the CH1-KIX domain of CBP, which results in inactivation of Nrf2. The inactivation depends on PKA catalytic subunit-mediated phosphorylation of p65 at S276. (2) p65 promotes recruitment of histone deacetylase 3 (HDAC3), the corepressor, to ARE by facilitating the interaction of HDAC3 with either CBP or MafK, leading to local histone hypoacetylation. This investigation revealed the participation of NF-kappaB p65 in the negative regulation of Nrf2-ARE signaling, and might provide a new insight into a possible role of NF-kappaB in suppressing the expression of anti-inflammatory or anti-tumor genes.

Examining the endogenous antioxidant response through immunofluorescent analysis of Nrf2 in tissue

As organisms designed to depend upon oxygen to sustain life, humans are necessarily and continually exposed to damaging oxidizing agents. As a vital protective measure, oxygen-dependent organisms have developed a highly evolutionarily conserved mechanism for preventing oxidative stress. NF-E2 (nuclear factor (erythroid-derived 2))-related factor-2 (Nrf2) is the primary regulator of this endogenous antioxidant response. Many diseases that plague human society, ranging from various cancers to neurodegenerative diseases, have oxidative stress as a component of their etiology, and thus, much disease research has focused on Nrf2, both as a potential point of biological failure and as a promising therapeutic target. As a transcription factor, Nrf2 is active only when it is nuclear, and is regulated largely by its subcellular distribution. Thus, Nrf2 protein levels and subcellular localization are both key pieces of information when studying the endogenous antioxidant response. Immunofluorescent analysis (IFA) of Nrf2 in human tissue is a particularly powerful tool in the study of Nrf2 in disease, because it allows examination of both of these regulatory mechanisms that modulate Nrf2 activity.

The Nrf2-Keap1 defence pathway: role in protection against drug-induced toxicity

The metabolic biotransformation of xenobiotics to chemically reactive metabolites can, in some instances, underlie the pathogenesis of certain adverse drug reactions, due to the development of chemical or oxidative stress. In order to guard against such stresses, mammalian cells have evolved multi-faceted, highly-regulated defence systems, one of the most important being that which is regulated by the transcription factor Nrf2. Through regulating the expression of numerous cytoprotective genes, Nrf2 serves as a critical determinant of a cell's capacity to survive, or succumb, to a toxic insult. The aim of this review is to summarise our current understanding of the biochemistry that underlies the Nrf2 defence pathway, and highlight the important role of this transcription factor in the protection against drug-induced toxicity, primarily through the examination of recent investigations that have demonstrated an increased vulnerability to various toxins in animals lacking Nrf2.

Nrf2 Neh5 domain is differentially utilized in the transactivation of cytoprotective genes

The transcription factor Nrf2 (nuclear factor erythroid 2-related factor 2) contains two transcription activation domains, Neh4 (Nrf2 ECH homology 4) and Neh5, which co-ordinately regulate transactivation of cytoprotective genes. In the present study we aimed to clarify the role of the Neh5 domain in Nrf2-mediated gene regulation. Deletion of the complete Neh5 domain reduces expression of endogenous Nrf2 target genes, such as HO-1 (haem oxygenase 1), NQO1 [NAD(P)H:quinone oxidoreductase 1] and GCLM (glutamate cysteine ligase modulatory subunit), in human kidney epithelial cells. Furthermore, the deletion of Neh5 markedly repressed CBP [CREB (cAMP-response-element-binding protein)-binding protein] and BRG1 (Brahma-related gene 1) from associating with Nrf2, diminishing their co-operative enhancement of HO-1 promoter activity. Mutational analysis of the Neh5 domain revealed a motif that shares significant homology with beta-actin and ARP1 (actin-related protein 1). Mutagenesis of this motif selectively decreased HO-1, but not NQO1 and GCLM, expression. Taken together, these results indicate that the Neh5 domain has the ability to regulate Nrf2 target gene transcription, yet the role of the Neh5 domain in transcription varies from gene to gene.

Identification of protein interaction antagonists using the repressed transactivator two-hybrid system

The repressed transactivator (RTA) yeast two-hybrid system was developed to enable genetic identification of interactions with transcriptional activator proteins. We have devised modifications of this system that enable its use in screening for inhibitors of protein interactions from small molecule compound libraries. We show that inhibition of protein interactions can be measured by monitoring growth in selective medium containing 3-aminotriazole (3-AT) and using this assay have identified inhibitors of four independent protein interactions in screens with a 23,000 small molecule compound library. Compounds found to inhibit one of the tested interactions between FKBP12 and the transforming growth factor β receptor (TGFβ-R) were validated in vivo and found to inhibit calcineurin-dependent signaling in T cells. One of these compounds was also found to cause elevated basal expression of a TGFβ-R/SMAD-dependent reporter gene. These results demonstrate the capability of the RTA small molecule screening assay for discovery of potentially novel therapeutic compounds.

Natural dietary anti-cancer chemopreventive compounds: redox-mediated differential signaling mechanisms in cytoprotection of normal cells versus cytotoxicity in tumor cells

Many dietary phytochemicals exhibit health-beneficial effects including prevention of diseases such as cancer, as well as neurological, cardiovascular, inflammatory, and metabolic diseases. Evolutionarily, herbivorous and omnivorous animals have been ingesting plants. This interaction between "animal-plant" ecosystems has resulted in an elaborate system of detoxification and defense mechanisms evolved by animals including humans. Mammalian cells, including human cells, respond to these dietary phytochemicals by "non-classical receptor sensing" mechanisms of electrophilic chemical-stress typified by "thiol-modulated" cellular signaling events primarily leading to the gene expression of pharmacologically beneficial effects, but sometimes unwanted cytotoxicity also. Our laboratory has been studying two groups of dietary phytochemical cancer-chemopreventive compounds (isothiocyanates and polyphenols), which are effective in chemical-induced, as well as genetically-induced, animal carcinogenesis models. These compounds typically generate "cellular stress" and modulate gene expression of phase II detoxifying/antioxidant enzymes. Electrophiles, reactive oxygen species, and reactive nitrogen species are known to act as second messengers in the modulation of many cellular signaling pathways leading to gene expression changes and pharmacological responses. Redox-sensitive transcription factors such as nuclear factor-E2-related factor 2 (Nrf2), AP-1, NF-kappaB, to cite a few examples, sense and transduce changes in the cellular redox status and modulate gene expression responses to oxidative and electrophilic stresses, presumably via sulfhydryl modification of critical cysteine residues found on these proteins and/or other upstream redox-sensitive molecular targets. In the current review, we will explore dietary cancer chemopreventive phytochemicals, discuss the link between oxidative/electrophilic stresses and the redox circuitry, and consider different redox-sensitive transcription factors. We will also discuss the kelch-like erythroid Cap'n'Collar homologue-associated protein 1 (Keap1)-Nrf2 axis in redox signaling of induction of phase II detoxifying/antioxidant defense mechanisms, an important target and preventive strategy for normal cells against carcinogenesis, and the converse inhibition of cell growth/inflammatory signaling pathways that would confer therapeutic intervention in many types of cancers. Finally, we will summarize the Nrf2 paradigm in gene expression, the pharmacotoxicogenomic relevance of redox-sensitive Nrf2, and the redox regulation of cell death mechanisms.

Mechanistic studies of the Nrf2-Keap1 signaling pathway

Since eukaryotic cells constantly encounter various environmental insults, they have evolved defense mechanisms to cope with toxicant- and carcinogen-induced oxidative stress or electrophiles. One of the most important cellular defense mechanisms against oxidative stress or electrophiles is mediated by the transcription factor Nrf2. Under the basal condition, Nrf2-dependent transcription is repressed by a negative regulator Keap1. When cells are exposed to oxidative stress, electrophiles, or chemopreventive agents, Nrf2 escapes Keap1-mediated repression and activates antioxidant responsive element (ARE)-dependent gene expression to maintain cellular redox homeostasis. Beyond its antioxidant function, Nrf2 has recently been recognized as a key factor regulating an array of genes that defend cells against the deleterious effects of environmental insults. Since this Nrf2-dependent cellular defense response is able to protect multi-organs or multi-tissues, activation of Nrf2 has been implicated in conferring protection against many human diseases, including cancer, neurodegenerative diseases, cardiovascular diseases, acute and chronic lung injury, autoimmune diseases, and inflammation. Therefore, understanding of Nrf2 regulation is crucial in the development of drugs for therapeutic intervention. This review will discuss recent progress in the field of the Nrf2-Keap1 signaling pathway, with emphasis on the mechanistic studies of Nrf2 regulation by Keap1, oxidative stress, or chemopreventive compounds.

Genetic Variations and Haplotype Structures of Transcriptional Pactor Nrf2 and Its Cytosolic Reservoir Protein Keap1 in Japanese

Transcriptional factor Nrf2 and its cytosolic reservoir protein Keap1 play important roles in induction of the expression of genes for xenobiotic metabolism and disposition, many of which are involved in protection from oxidative stress. In this study, 5 NFE2L2 (encoding Nrf2) and 6 KEAP1 exons and their flanking introns were comprehensively screened for genetic variations in 84 Japanese subjects. As for NFE2L2, 14 genetic variations were found, including 9 novel ones: 7 were located in the 5'-flanking region, 1 in the 5'-untranslated region (5'-UTR), 3 (1 synonymous and 2 nonsynonymous) in the coding exons, 1 in the intron, and 2 in the 3'-UTR. Two novel nonsynonymous variations, 697C>T (Pro233Ser) and 1094G>T (Ser365Ile), were heterozygously found with allele frequencies of 0.012 and 0.006, respectively. Regarding KEAP1, 18 genetic variations were detected, including 13 novel ones: 2 were located in the 5'-flanking region, 4 in the coding exons (4 synonymous), 5 in the introns, 4 in the 3'-UTR, and 3 in the 3'-flanking region. Based on the linkage disequilibrium (LD) profiles, both genes were analyzed as single LD blocks, where 14 (NFE2L2) and 18 (KEAP1) haplotypes were inferred. Six (NFE2L2) and 5 (KEAP1) haplotypes were relatively prevalent (>or=0.03 frequencies) and accounted for >or=88% of the inferred haplotypes. Haplotype-tagging variations of each gene were identified to capture these prevalent haplotypes. These data would be fundamental and useful information for pharmacogenetic studies on Nrf2-regulated genes for xenobiotic metabolism and disposition.

Active defense under oxidative stress. The antioxidant responsive element

This review considers the mechanisms and factors that stimulate transcription of genes regulated by the antioxidant responsive element (ARE). The latter is important for cell defense under conditions of oxidative stress and also for detoxification of electrophilic xenobiotics. There are differences in regulation of intracellular homeostasis involving Nrf2-mediated activation of ARE and other redox-sensitive factors (NF-kappaB and AP-1).

Ethanol sensitivity: a central role for CREB transcription regulation in the cerebellum

Background

Lowered sensitivity to the effects of ethanol increases the risk of developing alcoholism. Inbred mouse strains have been useful for the study of the genetic basis of various drug addiction-related phenotypes. Inbred Long-Sleep (ILS) and Inbred Short-Sleep (ISS) mice differentially express a number of genes thought to be implicated in sensitivity to the effects of ethanol. Concomitantly, there is evidence for a mediating role of cAMP/PKA/CREB signalling in aspects of alcoholism modelled in animals. In this report, the extent to which CREB signalling impacts the differential expression of genes in ILS and ISS mouse cerebella is examined.

Results

A training dataset for Machine Learning (ML) and Exploratory Data Analyses (EDA) was generated from promoter region sequences of a set of genes known to be targets of CREB transcription regulation and a set of genes whose transcription regulations are potentially CREB-independent. For each promoter sequence, a vector of size 132, with elements characterizing nucleotide composition features was generated. Genes whose expressions have been previously determined to be increased in ILS or ISS cerebella were identified, and their CREB regulation status predicted using the ML scheme C4.5. The C4.5 learning scheme was used because, of four ML schemes evaluated, it had the lowest predicted error rate. On an independent evaluation set of 21 genes of known CREB regulation status, C4.5 correctly classified 81% of instances with F-measures of 0.87 and 0.67 respectively for the CREB-regulated and CREB-independent classes. Additionally, six out of eight genes previously determined by two independent microarray platforms to be up-regulated in the ILS or ISS cerebellum were predicted by C4.5 to be transcriptionally regulated by CREB. Furthermore, 64% and 52% of a cross-section of other up-regulated cerebellar genes in ILS and ISS mice, respectively, were deemed to be CREB-regulated.

Conclusion

These observations collectively suggest that ethanol sensitivity, as it relates to the cerebellum, may be associated with CREB transcription activity.

Nrf2 Possesses a Redox-sensitive Nuclear Exporting Signal in the Neh5 Transactivation Domain

NF-E2-related factor 2 (Nrf2) is the key transcription factor regulating the antioxidant response. Previous studies identified a nuclear localization signal (NLS) in the basic region and a nuclear exporting signal (NES) in the leucine zipper domain of Nrf2. In this study, we characterize a new functional NES (175LLSIPELQCLNI186) in the transactivation (TA) domain of Nrf2. A green fluorescence protein (GFP)-tagged Nrf2 segment (amino acids162-295) called GFP-NESTA exhibited a cytosolic distribution that could be disrupted by L184A mutation or leptomycin B treatment. Chimeric expression of this NESTA with a nuclear protein GAL4DBD could expel GAL4DBD into the cytoplasm. A variety of oxidants, including sulforaphane, tert-butylhydroquinone, and H2O2, could effectively induce nuclear translocation of GFP-NESTA. Mutational studies showed that cysteine 183 may mediate the redox response of NESTA. The discovery of multiple NLS/NES motifs in Nrf2 and the redox sensitivity of NESTA imply Nrf2 may be self-sufficient to sense and transduce oxidative signals into the nucleus, consequently initiating antioxidant gene transcription.

BRG1 interacts with Nrf2 to selectively mediate HO-1 induction in response to oxidative stress

NF-E2-related factor 2 (Nrf2) regulates antioxidant-responsive element-mediated induction of cytoprotective genes in response to oxidative stress. The purpose of this study was to determine the role of BRG1, a catalytic subunit of SWI2/SNF2-like chromatin-remodeling complexes, in Nrf2-mediated gene expression. Small interfering RNA knockdown of BRG1 in SW480 cells selectively decreased inducible expression of the heme oxygenase 1 (HO-1) gene after diethylmaleate treatment but did not affect other Nrf2 target genes, such as the gene encoding NADPH:quinone oxidoreductase 1 (NQO1). Chromatin immunoprecipitation analysis revealed that Nrf2 recruits BRG1 to both HO-1 and NQO1 regulatory regions. However, BRG1 knockdown selectively decreased the recruitment of RNA polymerase II to the HO-1 promoter but not to the NQO1 promoter. HO-1, but not other Nrf2-regulated genes, harbors a sequence of TG repeats capable of forming Z-DNA with BRG1 assistance. Similarly, replacement of the TG repeats with an alternative Z-DNA-forming sequence led to BRG1-mediated activation of HO-1. These results thus demonstrate that BRG1, through the facilitation of Z-DNA formation and subsequent recruitment of RNA polymerase II, is critical in Nrf2-mediated inducible expression of HO-1.