Transcription factor Nrf2 coordinately regulates a group of oxidative stress-inducible genes in macrophages

Cooperative action of glutamate transporters and cystine/glutamate antiporter system Xc- protects from oxidative glutamate toxicity

Oxidative glutamate toxicity in the neuronal cell line HT22 is a model for cell death by oxidative stress. In this paradigm, an excess of extracellular glutamate blocks the glutamate/cystine-antiporter system Xc-, depleting the cell of cysteine, a building block of the antioxidant glutathione. Loss of glutathione leads to the accumulation of reactive oxygen species and eventually cell death. We selected cells resistant to oxidative stress, which exhibit reduced glutamate-induced glutathione depletion mediated by an increase in the antiporter subunit xCT and system Xc- activity. Cystine uptake was less sensitive to inhibition by glutamate and we hypothesized that glutamate import via excitatory amino acid transporters and immediate re-export via system Xc- underlies this phenomenon. Inhibition of glutamate transporters by l-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) and DL-threo-beta-benzyloxyaspartic acid (TBOA) exacerbated glutamate-induced cell death. PDC decreased intracellular glutamate accumulation and exacerbated glutathione depletion in the presence of glutamate. Transient overexpression of xCT and the glutamate transporter EAAT3 cooperatively protected against glutamate. We conclude that EAATs support system Xc- to prevent glutathione depletion caused by high extracellular glutamate. This knowledge could be of use for the development of novel therapeutics aimed at diseases associated with depletion of glutathione like Parkinson's disease.

Differential effects of the oxidized metabolites of oltipraz on the activation of CCAAT/enhancer binding protein-beta 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.

Oxidative activation of antioxidant defence

Living cells maintain a delicate balance between oxidizing and reducing species, and many disorders such as rheumatoid arthritis and Alzheimer's disease have been associated with a disturbed intracellular 'redox equilibrium'. The past few years have witnessed accelerated research into how natural redox responses and antioxidant defence systems are activated and how they restore a healthy redox balance. To function properly, many of these processes rely on a powerful sulfur redox chemistry, which is best exemplified by the complex, newly emerging cysteine-based redox regulation of the glutathione and thioredoxin pathways. Other redox systems based on oxidatively activated amino acid side chains in proteins are also becoming increasingly important, but are still barely understood or explored.

Peroxiredoxin genes are not induced in myeloid leukemia cells exposed to ionizing radiation

Peroxiredoxins (Prx) comprise an extended family of small antioxidant proteins which conserve a thioredoxin-dependent catalytic function that can contribute to cell protection from reactive oxygen species (ROS). ROS generation is one of the deleterious intracellular effects of ionizing radiation, but the role of Prx during radiation treatment has not been extensively explored. Present experiments measure effects of ionizing radiation on expression of human Prx types I (PAGA), II (NKEF-B) and IV (AOE372) in human myeloid leukemia cells (K562). Prx gene transcription was analyzed by amplifying with RT-PCR cDNAs complementary to each Prx-specific coding sequence and by identifying the derived products with Southern blotting procedure. Transcripts of GAPDH were used as the endogenous standard for semi-quantitative comparisons. No consistent increase in Prx gene expression was detected at time intervals up to 72 h after gamma radiation doses that caused cell cycle arrest and nuclear damage (maximum 20 Gy). Immunoblots also were consistent with a prolonged expression or stability of the Prx I/II proteins. Similarly, a cytotoxic concentration of the oxidant hemin, which stimulates rapid hemoglobinization of K562 cells, caused no induction of Prx gene expression. Our results indicate a high Prx stability in human radio-resistant leukemia cells.

Induction of detoxifying enzymes in rodent white adipose tissue by aryl hydrocarbon receptor agonists and antioxidants

The liver is the main organ of drug metabolism, but the expression and induction by xenobiotics of drug-metabolizing enzymes is also often observed in extrahepatic tissues. Recently, we reported that lipophilic cytochrome P450 inducers, beta-naphthoflavone (BNF), phenobarbital, and dexamethasone, induced CYP1, CYP2B, and CYP3A enzymes, respectively, in rat epididymal white adipose tissue (WAT) at both mRNA and protein levels. To further confirm the xenobiotic-induced expression of drug-metabolizing enzymes in adipose tissue, we studied the induction of CYP1A1 and other detoxifying enzymes by aryl hydrocarbon receptor (AhR) agonists and antioxidants. BNF increased CYP1A1 mRNA levels in several visceral WATs (epididymal, perirenal, and mesenteric) to a greater degree than in subcutaneous WAT in rats. Using C57BL/6 and DBA/2 mice with different responsiveness to aryl hydrocarbons and detecting cytoplasmic levels of AhR proteins, we have demonstrated that AhR mediates this CYP1A1 induction by BNF in WAT. Moreover, the NF-E2-related factor 2 (Nrf2)/antioxidant responsive element pathway is also functional in WAT, since BNF, which is known to activate both AhR and Nrf2, and antioxidants including tert-butylhydroquinone, 1-chloro-2,4-dinitrobenzene, and menadione induced the expression of Nrf2-target genes (NAD-(P)H:quinone oxidoreductase, glutathione S-transferase A subunits, and heme oxygenase-1) in rats and mice. These results suggest that both AhR and Nrf2 pathways are active in WAT and that lipophilic compounds accumulated in WAT can activate these transcription factors to increase detoxification capability in the tissue.

HCV proteins increase expression of heme oxygenase-1 (HO-1) and decrease expression of Bach1 in human hepatoma cells

BACKGROUND/AIMS

Hepatitis C infection induces hepatic oxidative stress. Heme oxygenase (HO), the rate-controlling enzyme of heme catabolism, plays a key role as a protector against oxidative, and other stresses. Other recent work has implicated Bach1, a heme binding protein that represses gene expression, in the regulation of HO-1 gene expression.

METHODS

We investigated the effects of HCV polyprotein expression on expression of HO-1 and Bach1 genes in human hepatoma cells (Huh-7 cells).

RESULTS

HO-1 was up-regulated in the cell line expressing HCV proteins from core up to the aminoterminal domain of NS3. Addition of increasing concentrations of N-acetylcysteine (NAC) led to down-regulation of HO-1 in cells expressing HCV proteins. In contrast, Bach1 was significantly down-regulated in these cells. Sodium arsenite, a strong inducer of oxidative stress and HO-1, reduced Bach1 expression in wild type Huh-7 cells, and NAC partially abrogated this decrease.

CONCLUSIONS

Huh-7 cells expressing HCV proteins show significant up-regulation of the HO-1 gene, and reciprocal down-regulation of the Bach1 gene. Exogenous oxidative stressors and anti-oxidants can modulate expression of these genes. These and other results suggest a key role of down-regulation of Bach1 and up-regulation of HO-1 in diminishing cytotoxic effects of HCV proteins in human hepatocytes.

Ni(II) activates the Nrf2 signaling pathway in human monocytic cells

Nickel is a component of biomedical alloys that is released during corrosion and causes inflammation in tissues by as yet unknown mechanisms. Recent data show that Ni(II) at concentrations of 10-50 microM amplifies lipopolysaccharide-triggered, NFkappaB-mediated cytokine secretion from monocytes. In the current study, we tested the hypothesis that Ni(II) amplifies cytokine secretion by activating the Nrf2 antioxidant pathway rather than by enhancing activity of the NFkappaB signaling pathway. Human THP1 monocytes were exposed to Ni(II) concentrations of 10-30 microM for 6-72 h, then immunoblots of whole-cell lysates or cytosolic and nuclear proteins were used to detect changes in Nrf2 or NFkappaB signaling. Our results show that Ni(II) increased (by 1-2 fold) whole-cell Nrf2 levels and nuclear translocation of Nrf2, and amplified lipopolysaccharide (LPS)-induction of Nrf2 (by 3-5 fold), but had no detectable effect on the initial activation or nuclear translocation of NFkappaB. Because Nrf2 target gene products are known regulators of NFkappaB nuclear activity, our results suggest that Ni(II) may affect cytokine secretion indirectly via modulation of the Nrf2 pathway.

Constitutive expression of the human peroxiredoxin V gene contributes to protection of the genome from oxidative DNA lesions and to suppression of transcription of noncoding DNA

Peroxiredoxins belong to a family of antioxidant proteins that neutralize reactive oxygen species. One member of this family, peroxiredoxin I (PRDX1), suppresses DNA oxidation. Peroxiredoxin V (PRDX5) has been cloned as a transcriptional corepressor, as a peroxisomal/mitochondrial antioxidant protein, and as an inhibitor of p53-dependent apoptosis. Promoters of mammalian PRDX5 genes contain clusters of antioxidant response elements, which can bind the transcription factor NRF2. However, we found that expression of the human PRDX5 gene in situ was not stimulated by the oxidative agent menadione. Silencing of the NRF2 gene in the absence of oxidative stress by specific siRNA did not decrease PRDX5 protein concentration. We also constructed clones of human lung epithelial cells A549 with siRNA-mediated knockdown of the PRDX5 gene. This led to a significant increase in 8-oxoguanine formation in cell DNA. In the PRDX5 knockdown clone, an increase in transcripts containing sequences of alpha-satellite and satellite III DNAs was also detected, suggesting that this protein may be required for silencing of heterochromatin. Together, these results suggest that constitutively expressed PRDX5 gene plays an important role in protecting the genome against oxidation and may also be involved in the control of transcription of noncoding DNA.

Activation of Nrf2/ARE pathway protects endothelial cells from oxidant injury and inhibits inflammatory gene expression

The antioxidant response element (ARE) is a transcriptional control element that mediates expression of a set of antioxidant proteins. NF-E2-related factor 2 (Nrf2) is a transcription factor that activates ARE-containing genes. In endothelial cells, the ARE-mediated genes are upregulated by atheroprotective laminar flow through a Nrf2-dependent mechanism. We tested the hypothesis that activation of ARE-regulated genes via adenovirus-mediated expression of Nrf2 may suppress redox-sensitive inflammatory gene expression. Expression of Nrf2 in human aortic endothelial cells (HAECs) resulted in a marked increase in ARE-driven transcriptional activity and protected HAECs from H2O2-mediated cytotoxicity. Nrf2 suppressed TNF-α-induced monocyte chemoattractant protein (MCP)-1 and VCAM-1 mRNA and protein expression in a dose-dependent manner and inhibited TNF-α-induced monocytic U937 cell adhesion to HAECs. Nrf2 also inhibited IL-1β-induced MCP-1 gene expression in human mesangial cells. Expression of Nrf2 inhibited TNF-α-induced activation of p38 MAP kinase. Furthermore, expression of a constitutively active form of MKK6 (an upstream kinase for p38 MAP kinase) partially reversed Nrf2-mediated inhibition of VCAM-1 expression, suggesting that p38 MAP kinase, at least in part, mediates Nrf2's anti-inflammatory action. In contrast, Nrf2 did not inhibit TNF-α-induced NF-κB activation. These data identify the Nrf2/ARE pathway as an endogenous atheroprotective system for antioxidant protection and suppression of redox-sensitive inflammatory genes, suggesting that targeting the Nrf2/ARE pathway may represent a novel therapeutic approach for the treatment of inflammatory diseases such as atherosclerosis.

Nrf transcription factors in keratinocytes are essential for skin tumor prevention but not for wound healing

The Nrf2 transcription factor is a key player in the cellular stress response through its regulation of cytoprotective genes. In this study we determined the role of Nrf2-mediated gene expression in keratinocytes for skin development, wound repair, and skin carcinogenesis. To overcome compensation by the related Nrf1 and Nrf3 proteins, we expressed a dominant-negative Nrf2 mutant (dnNrf2) in the epidermis of transgenic mice. The functionality of the transgene product was verified in vivo using mice doubly transgenic for dnNrf2 and an Nrf2-responsive reporter gene. Surprisingly, no abnormalities of the epidermis were observed in dnNrf2-transgenic mice, and even full-thickness skin wounds healed normally. However, the onset, incidence, and multiplicity of chemically induced skin papillomas were strikingly enhanced, whereas the progression to squamous cell carcinomas was unaltered. We provide evidence that the enhanced tumorigenesis results from reduced basal expression of cytoprotective Nrf target genes, leading to accumulation of oxidative damage and reduced carcinogen detoxification. Our results reveal a crucial role of Nrf-mediated gene expression in keratinocytes in the prevention of skin tumors and suggest that activation of Nrf2 in keratinocytes is a promising strategy to prevent carcinogenesis of this highly exposed organ.

Nrf2-deficient mice are highly susceptible to cigarette smoke-induced emphysema

Inflammation, protease/anti-protease imbalance and oxidative stress play important roles in the pathogenesis of emphysema. Nrf2 counteracts oxidative tissue damage and inflammation through transcriptional activation via the anti-oxidant responsive element (ARE). To clarify the protective role of Nrf2 in the development of emphysema, the susceptibility of Nrf2-knockout mice to cigarette smoke (CS)-induced emphysema was examined. In Nrf2-knockout mice, emphysema was first observed at 8 weeks and exacerbated by 16 weeks following CS-exposure, whereas no pathological abnormalities were observed in wild-type mice. Neutrophilic lung inflammation and permeability lung damage were significantly enhanced in Nrf2-knockout mice 8 weeks after CS-exposure. Importantly, neutrophil elastase activity in bronchoalveolar lavage fluids was markedly higher in Nrf2-knockout mice preceding the pronounced neutrophil accumulation. The expression of secretory leukoprotease inhibitor, a potent inhibitor of neutrophil elastase, was inducible in wild-type, but not in Nrf2-knockout mice. This protease/anti-protease imbalance, together with the lack of inducible expression of ARE-regulated anti-oxidant/anti-inflammatory genes, may explain the predisposition of Nrf2-knockout mice to neutrophilic inflammation. Indeed, specific activators of Nrf2 induced the expression of the SLPI gene in macrophages. These results indicate that Nrf2 protects against the development of emphysema by regulating not only the oxidant/anti-oxidant balance, but also inflammation and the protease/anti-protease balance.

Analysis of the interactions of Nrf-2, PMF-1, and CSN-7 with the 5'-flanking sequence of the mouse 4E-BP1 gene

Nuclear factor erythroid 2-related factor 2 (Nrf-2) binds to a specific polyamine responsive element (PRE) in the promoter region of the spermidine-spermine acetyltransferase (SSAT) gene, a key component of the polyamine catabolic pathway. Regulation of SSAT gene transcription requires the additional interaction of Nrf-2 with polyamine modulated factor 1 (PMF-1). Likewise, transcription of the eukaryotic initiation factor 4E binding protein 1 (4E-BP1) gene is regulated in a polyamine-dependent manner, but the actual mechanism has not previously been determined. Analysis of the 5'-flanking sequence of the murine 4E-BP1 gene indicated the presence of several potential PRE sites, which might be involved in regulating its transcription. Our goal in this research was to determine potential interactions between Nrf-2, PMF-1, the human homologue of the Arabidopsis signalosome complex (CSN-7), and these potential PRE sites. Four PCR fragments containing regions with considerable homology (78%) to the human PRE were generated from the 5'-flanking sequence of the mouse 4E-BP1 gene and the fragments were used in electrophoretic gel mobility shift and supershift assays. Purified Nrf-2 interacted with all four of these fragments, and similar gel shifts were observed with both cytoplasmic and nuclear fractions of NIH-3T3 cells. However, polyamine depletion with difluoromethylornithine (DFMO) eliminated the gel shift. Supershift assays indicated that the shift was due to the binding of Nrf-2, and the binding was competitive with a known Nrf-2 binding sequence. Purified PMF-1 did not bind any of the PCR fragments alone, but when added with Nrf-2, decreased the magnitude of the gel shift for one of the fragments (PRE located at -2060 relative to the transcription start site). CSN-7 did not interact with the sequences, nor did it inhibit protein/DNA interaction. These data indicate a possible mechanism by which polyamines enhance the binding of a Nrf-2/PMF-1 complex to the 5'-flanking region of the 4E-BP1 gene. Since polyamines increase expression of the 4E-BP1 gene, it seems likely that formation of this complex is involved in its transcriptional regulation.

Nrf2 negatively regulates osteoblast differentiation via interfering with Runx2-dependent transcriptional activation

Nrf2 (nuclear factor E2 p45-related factor 2) is believed to be a transcription factor essential for the regulation of many detoxifying and antioxidative genes in different tissues. In the present study, we investigated the role of Nrf2 in the regulation of osteoblastic differentiation. nrf2 mRNA expression was significantly up-regulated in femur isolated from ovariectomized mice, whereas in situ hybridization analysis revealed that up-regulation of nrf2 mRNA was mainly found in osteoblasts attached on cancellous bone in femur of ovariectomized mice. Expression of Nrf2 protein was also seen in osteoblasts in neonatal mouse tibia and calvaria. In osteoblastic MC3T3-E1 cells stably transfected with nrf2 expression vector, significant inhibition was seen in the maturation-dependent increase in alkaline phosphatase activity as well as the mineralized matrix formation. Stable overexpression of nrf2 significantly impaired Runx2 (runt-related transcription factor 2)-dependent stimulation of osteocalcin promoter activity and recruitment of Runx2 on osteocalcin promoter without affecting the expression of runx2 mRNA. Coimmunoprecipitation and mammalian two-hybrid assay revealed a physical interaction between Runx2 and Nrf2, whereas cellular distribution of endogenous Runx2 was not apparently changed by nrf2 overexpression in MC3T3-E1 cells. Alternatively, Nrf2 bound to antioxidant-responsive element-like-2 sequence of osteocalcin promoter. The inhibition by nrf2 on runx2-dependent osteocalcin promoter activity was partially prevented by the introduction of reporter of deletion mutant for ARE-like-2 sequence of osteocalcin promoter. These data suggest that Nrf2 may negatively regulate cellular differentiation through inhibition of the Runx2-dependent transcriptional activity in osteoblasts.

A Validated System for Simulating Common Carotid Arterial Flow In Vitro: Alteration of Endothelial Cell Response

Pulsations in blood flow alter gene and protein expressions in endothelial cells (EC). A computer-controlled system was developed to mimic the common carotid artery flow waveform and shear stress levels or to provide steady flow of the same mean shear stress in a parallel plate flow chamber. The pseudo-steady state shear stress was determined from real-time pressure gradient measurements and compared to the Navier-Stokes equation solution. Following 24 h of steady flow (SF: 13 dyne/cm2), pulsatile arterial flow (AF: average = 13 dyne/cm2, range = 7-25 dyne/cm2) or static conditions, heme oxygenase-1 (HO-1) and prostaglandin H synthase-2 (PGHS-2) mRNA and protein expressions from human umbilical vein endothelial cells were measured. Relative to steady flow, pulsatile arterial flow significantly attenuated mRNA upregulation of HO-1 (SF: 7.26 +/- 2.70-fold over static, AF: 4.84 +/- 0.37-fold over static; p < 0.01) and PGHS-2 (SF: 6.11+/-1.79-fold over static, AF: 3.54+/-0.79-fold over static; p < 0.001). Pulsatile arterial flow (4.57+/-0.81-fold over static, p < 0.01) also significantly reduced the steady-flow-induced HO-1 protein upregulation (7.99 +/- 1.29-fold over static). These findings reveal that EC can discriminate between different flow patterns of the same average magnitude and respond at the molecular level.

Regulation of the cadmium stress response through SCF-like ubiquitin ligases: comparison between Saccharomyces cerevisiae, Schizosaccharomyces pombe and mammalian cells

Saccharomyces cerevisiae has developed several mechanisms to cope with exposure to cadmium. In particular, the sulfur compound glutathione plays a pivotal role in cadmium detoxification, and exposure to cadmium leads to a wide reorganization of S. cerevisiae transcriptome and proteome, resulting in a significant increase in glutathione synthesis. Met4, the transcriptional activator of the sulfur metabolism enzymes, is a critical actor in this reorganization. Recent work has uncovered a part of the mechanism of cadmium-induced Met4 regulation, and showed that it occurs trough the SCF ubiquitin ligase complex SCF(Met30). We discuss this regulation in S. cerevisiae and compare it with the regulation of two other transcriptional activators involved in cadmium detoxification: the Schizosaccharomyces pombe Zip1, regulated by SCF(Pof1), and the mammalian Nrf2, regulated by the SCF-like ubiquitin ligase Cul3:Rbx1:Keap1.

A small-molecule-inducible Nrf2-mediated antioxidant response provides effective prophylaxis against cerebral ischemia in vivo

The transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) coordinates expression of genes required for free radical scavenging, detoxification of xenobiotics, and maintenance of redox potential. Previously, activation of this pleiotropic response was neuroprotective in cell culture models that simulate components of stroke damage. However, the role of Nrf2 in limiting stroke damage in vivo remained unclear. We report that Nrf2 activation protects the brain from cerebral ischemia in vivo. Acute (1-3 d) intracerebroventricular or intraperitoneal pretreatment with tert-butylhydroquinone (tBHQ), an Nrf2 activity inducer, reduced cortical damage and sensorimotor deficit at 24 h and even 1 month after ischemia-reperfusion in rats. Cortical glutathione levels robustly increased with tBHQ administration to rats and Nrf2-expressing mice, but not Nrf2(-/-) mice. Basal and inducible activities of antioxidant/detoxification enzymes in Nrf2(-/-) mice were reduced when compared with Nrf2(+/+) controls. Interestingly, larger infarcts were observed in Nrf2(-/-) mice at 7 d after stroke, but not at 24 h, suggesting that Nrf2 may play a role in shaping the penumbra well after the onset of ischemia. Neuronal death caused by a "penumbral" model of stroke, using intracortical endothelin-1 microinjection, was attenuated by tBHQ administration to Nrf2(+/+), but not to Nrf2(-/-) mice, confirming the Nrf2-specific action of tBHQ in vivo. We conclude that Nrf2 plays a role in modulating ischemic injury in vivo. Accordingly, Nrf2 activation by small molecule inducers may be a practical preventative treatment for stroke-prone patients.

Flunarizine induces Nrf2-mediated transcriptional activation of heme oxygenase-1 in protection of auditory cells from cisplatin

We investigated the cytoprotective mechanisms of flunarizine in cisplatin-induced death of auditory cells. Concomitant with an increase in viability, treatment with flunarizine resulted in a marked dissociation of Nrf2/Keap1 and subsequent intranuclear translocation of Nrf2, which was mediated by PI3K-Akt signaling. Overexpression of Nrf2 protected cells from cisplatin along with transcriptional activation of ARE to generate heme oxygenase-1 (HO-1). Pretreatment with flunarizine predominantly increased the transcriptional activity of HO-1 among Nrf2-driven transcripts, including HO-1, NQO1, GCLC, GCLM, GST micro-1, and GSTA4. Furthermore, both pharmacological inhibition and siRNA transfection of HO-1 completely abolished the flunarizine-mediated protection of HEI-OC1 cells and the primary rat (P2) organ of Corti explants from cisplatin. These results suggest that Nrf2-driven transcriptional activation of ARE through PI3K-Akt signaling augments the generation of HO-1, which may be a critically important determinant in cellular response toward cisplatin and the cytoprotective effect of flunarizine against cisplatin.

Deficiency of glutathione transferase zeta causes oxidative stress and activation of antioxidant response pathways

Glutathione S-transferase (GST) zeta (GSTZ1-1) plays a significant role in the catabolism of phenylalanine and tyrosine, and a deficiency of GSTZ1-1 results in the accumulation of maleylacetoacetate and its derivatives maleylacetone (MA) and succinylacetone. Induction of GST subunits was detected in the liver of Gstz1(-/-) mice by Western blotting with specific antisera and high-performance liquid chromatography analysis of glutathione affinity column-purified proteins. The greatest induction was observed in members of the mu class. Induction of NAD(P)H:quinone oxidoreductase 1 and the catalytic and modifier subunits of glutamate-cysteine ligase was also observed. Many of the enzymes that are induced in Gstz1(-/-) mice are regulated by antioxidant response elements that respond to oxidative stress via the Keap1/Nrf2 pathway. It is significant that diminished glutathione concentrations were also observed in the liver of Gstz1(-/-) mice, which supports the conclusion that under normal dietary conditions, the accumulation of electrophilic intermediates such as maleylacetoacetate and MA results in a high level of oxidative stress. Elevated GST activities in the livers of Gstz1(-/-) mice suggest that GSTZ1-1 deficiency may alter the metabolism of some drugs and xenobiotics. Gstz1(-/-) mice given acetaminophen demonstrated increased hepatotoxicity compared with wild-type mice. This toxicity may be attributed to the increased GST activity or the decreased hepatic concentrations of glutathione, or both. Patients with acquired deficiency of GSTZ1-1 caused by therapeutic exposure to dichloroacetic acid for the clinical treatment of lactic acidosis may be at increased risk of drug- and chemical-induced toxicity.

Transcription factor Nrf2 plays a pivotal role in protection against elastase-induced pulmonary inflammation and emphysema

Emphysema is one of the major pathological abnormalities associated with chronic obstructive pulmonary disease. The protease/antiprotease imbalance and inflammation resulting from oxidative stress have been attributed to the pathogenesis of emphysema. Nrf2 is believed to protect against oxidative tissue damage through the transcriptional activation of a battery of antioxidant enzymes. In this study, we investigated the protective role of Nrf2 in the development of emphysema using elastase-induced emphysema as our model system. We found that elastase-provoked emphysema was markedly exacerbated in Nrf2-knockout (KO) mice compared with wild-type mice. The severity of emphysema in Nrf2-KO mice correlated intimately with the degree of lung inflammation in the initial stage of elastase treatment. The highly inducible expression of antioxidant and antiprotease genes observed in wild-type alveolar macrophages was significantly attenuated in the lungs of Nrf2-KO mice. Interestingly, transplantation of wild-type bone marrow cells into Nrf2-KO mice retarded the development of initial lung inflammation and subsequent emphysema, and this improvement correlated well with the appearance of macrophages expressing Nrf2-regulated antiprotease and antioxidant genes. Thus, Nrf2 appears to exert its protective effects through the transcriptional activation of antiprotease and antioxidant genes in alveolar macrophages.

Global Gene Expression Analyses of Mouse Fibroblast L929 Cells Exposed to IC50 MMA by DNA Microarray and Confirmation of Four Detoxification Genes' Expression by Real-time PCR

Methyl methacrylate (MMA) is the main component of methyl methacrylic resin, which is widely used in dentistry. Previous studies have investigated whether MMA has any adverse effects on growth and gene expression in mouse fibroblast L929 cells. The present study was designed to further understand the effects of MMA by focusing on cDNA microarray data after L929 cells were exposed to MMA. MMA was found to inhibit cell growth and induce detoxification response genes in L929 cells. One of the most highly up-regulated genes was glutathione S-transferase, alpha 1 (Ya) (Gsta1), which has recently been shown to participate in Nrf2 regulation and is considered to be related to detoxification response. Molecular biological data obtained in the present study may therefore provide useful insights into the effects of MMA on living tissue.

Heme oxygenase-1 gene enhancer manifests silencing activity in a chromatin environment prior to oxidative stress

The expression of heme oxygenase-1 (HO-1) is regulated by E1 and E2 enhancers, both of which contain multiple Maf recognition elements (MAREs). In living cells, MAREs are bound by Bach1/MafK heterodimers, hence maintaining a quiescent state of the HO-1 gene (hmox-1). However, in transient transfection assays, they act as transcriptional enhancers. Therefore MAREs may manifest their function only in a chromatin environment. By using NIH3T3 cell pools stably transfected with EGFP reporter genes driven by the wild-type or mutated E2 enhancer, we demonstrate that the E2 MAREs function as transcriptional silencers depending on the binding of Bach1/MafK heterodimer in vivo only in a chromatin environment. After cadmium treatment, they switched into transcriptional enhancers. Surprisingly, single MARE site did not exhibit such function. Furthermore, by using DNase I hypersensitivity assay, we demonstrate that simple chromatin condensations were not involved in the Bach1-mediated repression. We conclude that, in a chromatin environment, the E2 MAREs function as transcriptional silencers depending on binding of Bach1/MafK heterodimer.

Two-site substrate recognition model for the Keap1-Nrf2 system: a hinge and latch mechanism

Cells are equipped with a number of transcriptional factors that safeguard against various environmental insults. Proteasomal protein degradation plays an important role in the Keap1-Nrf2 cytoprotection system, with molecular machinery similar to that for other environmental defense systems such as inflammatory and hypoxic responses. While Nrf2 protein stabilization is known to be redox-sensitive, the transcription factors NF-kappaB and HIF-1alpha for inflammatory and hypoxic responses, respectively, are also influenced by the cellular redox conditions. In this review we present the recently proposed two-site substrate recognition model of the Keap1-Nrf2 system, which regulates the cellular responses against oxidative and xenobiotic stresses. The implications of two destructive motifs in Nrf2, the ETGE and DLG motifs, which appear to function as a hinge and latch attenuating Keap1 activity in different redox states, are discussed.

The heme-Bach1 pathway in the regulation of oxidative stress response and erythroid differentiation

Heme--as a prosthetic group of proteins required for oxygen transport and storage, respiration, and biosynthetic pathways--is essential for practically all forms of life. Additionally, the degradation products of heme (i.e., carbon monoxide, biliverdin, and bilirubin) produced by the enzymatic actions of heme oxygenase (HO) and biliverdin reductase, possess various biological activities in vivo. In mammalian cells, heme also functions as an intracellular regulator of gene expression by virtue of its ability to bind to Bach1, a transcription factor that functions in association with small Maf proteins. Normally, such complexes function as repressors by binding to specific target sequences, the Maf recognition element (MARE), within enhancers of genes encoding proteins such as HO-1 and beta-globin. By binding to Bach1, heme induces selective removal of the repressor from the gene enhancers permitting subsequent occupancy of the MAREs by activators that, interestingly, also contain small Maf proteins. Thus small Maf proteins play dual functions in gene expression: complexes with Bach1 repress MARE-dependent gene expression, whereas heterodimers with NF-E2 p45 or related factors (Nrf1, Nrf2, and Nrf3) activate MARE-driven genes. By modulating the equilibrium of the small Maf heterodimer network, heme regulates expression of the cytoprotective enzyme HO-1 during the stress response and of beta-globin during erythroid differentiation. Implications of such heme-regulated gene expression in human diseases including atherosclerosis are discussed.

Induction of endogenous Nrf2/small maf heterodimers by arsenic-mediated stress in placental choriocarcinoma cells

Exposure to inorganic arsenic has been associated with various forms of cancer, nervous system pathogenesis, and vascular diseases, as well as reproductive and developmental toxicity. Here, the effect of inorganic arsenic on placental JAR choriocarcinoma cells was assessed. The nuclear protein levels of the CNC transcription factor Nrf2 were strongly induced in the presence of arsenic. Dosage response experiments showed that 0.5 microM of arsenic is sufficient to augment Nrf2 levels. The expression of the Nrf2 dimerization partners MafG and MafK appeared not to be modulated by arsenic, whereas MafF protein levels were slightly increased. Arsenic also induced the binding of endogenous Nrf2/small Maf DNA-binding complexes to a stress response element (StRE) recognition site. In addition, arsenic caused oxidative stress in the choriocarcinoma cell model as evidenced by an increase in intracellular H2O2 levels. Expression of the enzyme heme oxygenase-1 (HO-1), a known Nrf2 target gene, was upregulated by exposure of JAR cells to arsenic. These results suggest that Nrf2/small Maf heterodimers may play an important role in the response to arsenic-mediated stress in placental cells.

Apomorphine protects against 6-hydroxydopamine-induced neuronal cell death through activation of the Nrf2-ARE pathway

NF-E2-related factor-2 (Nrf2), a basic leucine zipper transcription factor, is involved in the expression of numerous detoxifying and antioxidant genes via the antioxidant response element (ARE). Apomorphine (Apo), a dopamine D(1)/D(2) receptor agonist, is used for clinical therapy of Parkinson's disease. On the other hand, Apo is a potent radical scavenger and has protective effects on oxidative stress-induced cell death. Previously, we have reported that pretreatment of human neuroblastoma SH-SY5Y cells with Apo enhances protection against 6-hydroxydopamine (6-OHDA)-induced cell death. In this study, we investigated whether the Nrf2-ARE system is involved in the protection by Apo. Pretreatment of SH-SY5Y cells with Apo suppressed 6-OHDA-induced cell death in a dose-dependent manner. However, neither SCH23390, a dopamine D(1) receptor antagonist, nor sulpiride, a dopamine D(2) receptor antagonist, prevented the protective effect of Apo. Apo stimulated the translocation of Nrf2 into the nucleus and the transactivation of the ARE. The expression of heme oxygenase-1 (HO-1) was dose dependently induced by Apo. Moreover, we found that the activation of the ARE and the induction of HO-1 mRNA caused by Apo were suppressed in the presence of the antioxidant N-acetylcysteine and also that Apo produced intracellular reactive oxygen species (ROS), indicating that the low level of ROS produced by Apo may play a critical role in this phenomenon. Taken together, our findings suggest that not only the function as a radical scavenger but also the function as an Nrf2-ARE pathway activator may be involved in the neuroprotective effects of Apo.

A crucial role of Nrf2 in in vivo defense against oxidative damage by an environmental pollutant, pentachlorophenol

Our goal was to elucidate roles of Nrf2 in in vivo defense against pentachlorophenol (PCP), an environmental pollutant and hepatocarcinogen in mice. We examined oxidative stress and cell proliferation, along with other hepatotoxicological parameters, in the livers of nrf2-deficient (wild:+/+, heterozygous:+/-, homozygous:-/-) animals fed PCP in their diet at doses of 0, 150, 300, 600, or 1200 ppm for 4 weeks. For measurement of methoxyresorufin-O-demethylase (CYP 1A2), NAD(P):quinone oxidoreductase 1 (NQO1), and UDP-glucuronosyltransferase (UDP-GT), an additional study was performed with all but the 150-ppm dose. Significant elevation of 8-hydroxydeoxyguanosine (8-OH-dG) levels in the liver DNA was observed only in -/- mice treated with PCP at 1200 ppm. Levels of thiobarbituric-acid-reactive substances (TBARS) were also raised significantly compared to those of the relevant +/+ mice. Bromodeoxyuridine labeling indices (BrdU-LIs) of hepatocytes in -/- mice were significantly higher at all doses than those in the relevant +/+ mice. Relative liver weights were unchanged in mice lacking Nrf2, whereas liver weight in +/+ and +/- mice was increased. Significant elevations of serum ALP activity, but not ALT and AST activity, occurred at 600 ppm and above in -/- mice compared to the relevant +/+ mice. Histopathologically, centrilobular hepatocyte necrosis was severe in the -/- mice that received 600 ppm. Although CYP 1A2 activity was elevated in all treated mice, increases in NQO1 levels and UDP-GT activities did not occur only in -/- mice. These data suggest that Nrf2 plays a key role in prevention of PCP-induced oxidative stress and cell proliferation.

Novel cytoprotective mechanism of anti-parkinsonian drug deprenyl: PI3K and Nrf2-derived induction of antioxidative proteins

Neuroprotection has received considerable attention as a strategy for the treatment of Parkinson's disease (PD). Deprenyl (Selegiline) is a promising candidate for neuroprotection; however, its cytoprotective mechanism has not been fully clarified. Here, we report a novel cytoprotective mechanism of deprenyl involving PI3K and Nrf2-mediated induction of oxidative stress-related proteins. Deprenyl increased the expression of HO-1, PrxI, TrxI, TrxRxI, gammaGCS, and p62/A170 in SH-SY5Y cells. Deprenyl also induced the nuclear accumulation of Nrf2 and increased the binding activity of Nrf2 to the enhancer region of human genomic HO-1. The Nrf2-mediated induction of antioxidative molecules was controlled by PI3K. Indeed, furthermore, neurotrophin receptor TrkB was identified as an upstream signal for PI3K-Nrf2 activation by deprenyl. These results suggest that the cytoprotective effect of deprenyl is, in part, dependent on Nrf2-mediated induction of antioxidative proteins, suggesting that activation of the PI3K-Nrf2 system may be a useful therapeutic strategy for PD.

p62/SQSTM1 forms protein aggregates degraded by autophagy and has a protective effect on huntingtin-induced cell death

Autophagic degradation of ubiquitinated protein aggregates is important for cell survival, but it is not known how the autophagic machinery recognizes such aggregates. In this study, we report that polymerization of the polyubiquitin-binding protein p62/SQSTM1 yields protein bodies that either reside free in the cytosol and nucleus or occur within autophagosomes and lysosomal structures. Inhibition of autophagy led to an increase in the size and number of p62 bodies and p62 protein levels. The autophagic marker light chain 3 (LC3) colocalized with p62 bodies and co-immunoprecipitated with p62, suggesting that these two proteins participate in the same complexes. The depletion of p62 inhibited recruitment of LC3 to autophagosomes under starvation conditions. Strikingly, p62 and LC3 formed a shell surrounding aggregates of mutant huntingtin. Reduction of p62 protein levels or interference with p62 function significantly increased cell death that was induced by the expression of mutant huntingtin. We suggest that p62 may, via LC3, be involved in linking polyubiquitinated protein aggregates to the autophagy machinery.

Tissue Prx I in the protection against Fe-NTA and the reduction of nitroxyl radicals

Peroxiredoxin I (Prx I) is a key cytoplasmic peroxidase that reduces intracellular hydroperoxides in concert with thioredoxin. To study the role of tissue Prx I in protection from oxidative stress, we generated Prx I-/- mice by gene trapping. We then evaluated the acute-phase tissue damage caused by ferric-nitrilotriacetate (Fe-NTA). Increases in serum aspartate aminotransferase and alanine aminotransferase levels were significantly greater in Prx I-/- than wild-type mice, 4 and 12 h after the injection of Fe-NTA. Using real-time EPR imaging, we examined the reduction of the stable paramagnetic nitroxyl radical 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl in vivo, and found that the half-life of this spin probe in the liver and kidney was significantly prolonged in the Prx I-/- mice. These results demonstrate that Prx I-/- mice have less reducing activity and are more susceptible to the damage mediated by reactive oxygen species in vivo than wild-type mice.

2-Cys peroxiredoxin function in intracellular signal transduction: therapeutic implications

H(2)O(2) is a reactive oxygen species that has drawn much interest because of its role as a second messenger in receptor-mediated signaling. Mammalian 2-Cys peroxiredoxins have been shown to eliminate efficiently the H(2)O(2) generated in response to receptor stimulation. 2-Cys peroxiredoxins are members of a novel peroxidase family that catalyze the H(2)O(2) reduction reaction in the presence of thioredoxin, thioredoxin reductase and NADPH. Several lines of evidence suggest that 2-Cys peroxiredoxins have dual roles as regulators of the H(2)O(2) signal and as defenders of oxidative stress. In particular, 2-Cys peroxiredoxin appears to provide selective, specific and localized control of receptor-mediated signal transduction. Thus, the therapeutic potential of 2-Cys peroxiredoxins is clear for diseases, such as cancer and cardiovascular diseases, that involve reactive oxygen species.

Effect of targeted deletion of the heme oxygenase-2 gene on hemoglobin toxicity in the striatum

The heme oxygenase (HO) enzymes catalyze the rate-limiting step in the breakdown of heme to iron, carbon monoxide, and biliverdin. A prior cell culture study demonstrated that deletion of HO-2, the isoform constitutively expressed in neurons, attenuated hemoglobin (Hb) neurotoxicity. The present study tested the hypothesis that HO-2 gene deletion is cytoprotective in a model of Hb toxicity in vivo. Stereotactic injection of 6 microL stroma-free Hb (SFHb) into the striatum significantly increased protein oxidation in wild-type mice at 24 to 72 h, as detected by an assay for carbonyl groups. At 72 h, carbonylation was increased 2.5-fold compared with that in the contralateral striatum. In HO-2 knockout mice, protein oxidation was not increased at 24 h, and was increased by only 1.7-fold at 72 h. Similarly, striatal lipid peroxidation, as detected by the malondialdehyde assay, was significantly greater in the SFHb-injected striata of wild-type mice than in knockout mice. Striatal cell viability, determined by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, was 45.0%+/-6.3% of that in contralateral striata in wild-type mice at 72 h; it was increased to 85%+/-8% in knockouts. Heme oxygenase-2 gene deletion did not alter weight loss or mortality after SFHb injection. Baseline striatal HO-1 expression was similar in knockout and wild-type mice; induction after SFHb injection occurred more rapidly in the latter. These results suggest that HO-2 gene deletion protects striatal cells from the oxidative toxicity of Hb in vivo. Pharmacologic or genetic strategies that target HO-2 may be beneficial after central nervous system hemorrhage, and warrant further investigation.

Molecular basis of heme oxygenase-1 induction: implications for chemoprevention and chemoprotection

Heme oxygenase (HO)-1, involved in the heme degradation process, is an important antioxidant enzyme. The induction of HO-1 gene expression, in response to diverse oxidative stimuli, represents a critical event in adaptive cellular response. Experimental models of various diseases, including acute inflammation, atherosclerosis, degenerative diseases, and carcinogenesis, have demonstrated that the induction of HO-1 can prevent or mitigate the symptoms associated with these ailments. Recent progress in our understanding of cellular signaling networks as critical modulators of gene transcription sheds light on the molecular basis of HO-1 gene expression. A panel of redox-sensitive transcription factors such as activator protein-1, nuclear factor- kappaB, and nuclear factor E2-related factor-2, and some of the upstream kinases have been identified as regulators of HO-1 gene induction. The scope of this review is limited to focus on molecular mechanisms underlying HO-1 expression and the significance of targeted induction of HO-1 as a strategy to achieve chemoprevention and chemoprotection.

Molecular mechanisms involved in enhancing HO-1 expression: de-repression by heme and activation by Nrf2, the "one-two" punch

Heme oxygenase (HO)-1 is a stress response protein, which confers cytoprotection against oxidative injury and provides a vital function in maintaining tissue homeostasis. Molecular mechanisms involved in the inducible transcription of ho-1 occurring in response to numerous and diverse stressful conditions have remained elusive. Since the discovery of E1 and E2, the two upstream enhancers regulating induction of ho-1 transcription in 1989, there have been many studies dealing with molecular mechanisms involved in enhancing HO-1 expression. In this commentary, recent advances in our understanding of the mechanisms involved in the induction of HO-1 expression in mammalian cells are summarized with some supportive results reported by others. Currently available data indicate that activation of ho-1 transcription involves both the heme (native substrate)-dependent selective alleviation of repressor and the oxidative stress-dependent activation of transcriptional activator. The stress-released free-heme (HO-1 substrate) from hemoproteins involved in causing oxidative stress itself appears to act as a molecular switch controlling the repressor- activator antagonism on the enhancer sequences of ho-1. Thus, induction of HO-1 appears to operate in a manner like a simple feedback loop. dox Signal. 7, 1674-1687.

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.

Genetic evidence that small maf proteins are essential for the activation of antioxidant response element-dependent genes

While small Maf proteins have been suggested to be essential for the Nrf2-mediated activation of antioxidant response element (ARE)-dependent genes, the extent of their requirement remains to be fully documented. To address this issue, we generated mafG::mafF double-mutant mice possessing MafK as the single available small Maf. Induction of the NAD(P)H:quinone oxidoreductase 1 (NQO1) gene was significantly impaired in double-mutant mice treated with butylated hydroxyanisole, while other ARE-dependent genes were less affected. Similarly, in a keap1-null background, where many of the ARE-dependent genes are constitutively activated in an Nrf2-dependent manner, only a subset of ARE-dependent genes, including NQO1, were sensitive to a simultaneous deficiency in MafG and MafF. Examination of single and double small maf mutant cells revealed that MafK also contributes to the induction of ARE-dependent genes. To obtain decisive evidence, we established mafG::mafK::mafF triple-mutant fibroblasts that completely lack small Mafs and turned out to be highly susceptible to oxidative stress. We found that induction in response to diethyl maleate was abolished in a wider range of ARE-dependent genes in the triple-mutant cells. These data explicitly demonstrate that small Mafs play critical roles in the inducible expression of a significant portion of ARE-dependent genes.

Glutathione-mediated formation of oxygen free radicals by the major metabolite of oltipraz

The major metabolite of the cancer chemopreventive oltipraz (1), a pyrrolopyrazine thione, 4, has been shown to be a phase two enzyme inducer, an activity thought to be a key to the cancer chemopreventive action of the parent compound. To understand the possible mechanism by which the metabolite acts as an inducer, a study of its potential to generate free radicals was undertaken. Electron paramagnetic resonance (EPR) spin trapping studies using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were performed with 7-methyl-6,8-bis-methyldisulfanyl-pyrrolo[1,2-a]pyrazine, 5, a synthetic precursor to the metabolite in aqueous and organic solvents. In the presence of GSH, which rapidly liberates the metabolite from the precursor, a 1:2:2:1 quartet spectrum with hyperfine coupling constants a(N) = a(H) = 14.9 G, characteristic of the hydroxyl radical adduct of DMPO, was observed in the presence of oxygen. No signal was seen under anaerobic conditions. This signal was quenched by the addition of the superoxide scavenging enzyme Cu,Zn-superoxide dismutase. In aqueous dimethyl sulfoxide (80 vol % DMSO), the metabolite precursor 5, GSH, and DMPO exhibited an EPR spectrum with the hyperfine values of a(N) = 12.7 G, a(H1) = 10.3 G, and a(H2) = 1.3 G, corresponding to the superoxide radical adduct of DMPO. The amount of superoxide radical adduct formed from the reaction of 5 and GSH increases with GSH concentration in phosphate buffer solution. Kinetic studies show that the formation of superoxide radical anion is first-order with respect to GSH. The formation of superoxide radical anion by the metabolite in the presence of GSH is linear at lower concentrations of 5 but becomes nonlinear at high concentrations. Overall, these studies suggest a mechanism in which GSH reduces the metabolite 4 to 4. , presumably a radical anion, that in turn donates an electron to oxygen resulting in superoxide radical anion formation. This GSH stimulated redox cycle of the metabolite 4 suggests a possible mechanism by which the parent compound oltipraz might effect the cancer chemopreventive increase in the transcription of phase two enzymes that is mediated by transcription factor Nrf2.

Lipopolysaccharide-Induced Heme Oxygenase-1 Expression in Human Monocytic Cells Is Mediated via Nrf2 and Protein Kinase C

Monocytes play a key role in mobilization of the immune response during sepsis. In response to LPS, monocytes produce both proinflammatory mediators and regulatory proteins that counteract the inflammation and oxidative stress. In murine macrophages, LPS stimulates expression of heme oxygenase 1 (HO-1), a cytoprotective enzyme that catalyzes the degradation of heme. The HO-1 5'-untranslated region, similarly to other cytoprotective genes, contains antioxidant-response elements (AREs) that can bind the transcription factor NF-E2-related factor 2 (Nrf2). At present, the role of Nrf2 in LPS-induced HO-1 expression in monocytic cells has not been investigated. In this study, LPS induced HO-1 mRNA and protein expression in human monocytes and THP-1 cells. Nrf2 translocated from the cytosol to the nucleus in response to LPS and bound to the ARE site in the human HO-1 promoter. In addition, a dominant negative Nrf2 mutant inhibited LPS-induced HO-1 mRNA expression but not TNF-alpha mRNA expression in THP-1 cells. Ro-31-8220, a pan-protein kinase C (PKC) inhibitor, and Go6976, a classical PKC inhibitor, blunted LPS-induced HO-1 mRNA expression in monocytes and THP-1 cells. Both PKC inhibitors also blocked LPS-induced Nrf2 binding to the ARE. These results indicate that LPS-induced HO-1 expression in human monocytic cells requires Nrf2 and PKC.

Heme Oxygenase and Atherosclerosis

Overproduction of reactive oxygen species under pathophysiological conditions, including dyslipidemia, hypertension, diabetes, and smoking, is integral in the development of cardiovascular diseases (CVD). The reactive oxygen species released from all types of vascular cells regulate various signaling pathways that mediate not only vascular inflammation in atherogenesis but also antioxidative and antiinflammatory responses. One such protective and stress-induced protein is heme oxygenase (HO). HO is the first rate-limiting enzyme in heme breakdown to generate equimolar quantities of carbon monoxide, biliverdin, and free ferrous iron. Accumulating evidence has shown that inducible HO (HO-1) and its products function as adaptive molecules against oxidative insults. The proposed mechanisms by which HO-1 exerts its cytoprotective effects include its abilities to degrade the pro-oxidative heme, to release biliverdin and subsequently convert it bilirubin, both of which have antioxidant properties, and to generate carbon monoxide, which has antiproliferative and antiinflammatory as well as vasodilatory properties. Herein, I highlight the relationship of HO and cardiovascular disease, especially atherosclerosis, gene-targeting approaches in animal models, and the potential for and concern about HO-1 as a novel therapeutic target for cardiovascular diseases.

Xanthohumol isolated from Humulus lupulus Inhibits menadione-induced DNA damage through induction of quinone reductase

The female parts of hops (Humulus lupulus L.) show estrogenic effects as well as cancer chemopreventive potential. We analyzed the chemopreventive mechanism of hops by studying its antioxidative activities and its effect on the detoxification of a potentially toxic quinone (menadione). The detoxification enzyme quinone reductase [(NAD(P)H:quinone oxidoreductase, QR] protects against quinone-induced toxicity and has been used as a marker in cancer chemoprevention studies. Although the hop extract was only a weak quencher of free radicals formed from 1,1-diphenyl-2-picrylhydrazyl, it demonstrated strong QR induction in Hepa 1c1c7 cells. In addition, compounds isolated from hops including xanthohumol (XH) and 8-prenylnaringenin were tested for QR induction. Among these, XH was the most effective at inducing QR with a concentration required to double the specific activity of QR (CD value) of 1.7 +/- 0.7 microM. In addition, pretreatment of Hepa1c1c7 cells with XH significantly inhibited menadione-induced DNA single-strand breaks. The QR inhibitor dicumarol reversed the protective effect of XH against menadione-induced DNA damage. Because the expression of QR and other detoxifying enzymes is known to be upregulated by binding of the transcription factor Nrf2 to the antioxidant response element (ARE), the reporter activity mediated by ARE in HepG2-ARE-C8 cells was investigated after incubation with XH for 24 h. Under these conditions, XH increased ARE reporter activity in a dose-dependent manner. One mechanism by which XH might induce QR could be through interaction with Keap1, which sequesters Nrf2 in the cytoplasm, so that it cannot activate the ARE. Using LC-MS-MS, we demonstrated that XH alkylates human Keap1 protein, most likely on a subset of the 27 cysteines of Keap1. This suggests that XH induces QR by covalently modifying the Keap1 protein. Therefore, XH and hops dietary supplements might function as chemopreventive agents, through induction of detoxification enzymes such as QR.

Nrf2 as a target for cancer chemoprevention

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

Transcriptional regulation of thioredoxin reductase 1 expression by cadmium in vascular endothelial cells: role of NF-E2-related factor-2

Thioredoxin reductase (TrxR) is a selenoprotein that catalyzes the reduction of the active site disulfide of thioredoxin (Trx), which regulates the redox status of the cells. In the present study, we found that TrxR1, one of the three TrxR isozymes, was induced by cadmium as well as tumor necrosis factor alpha (TNFalpha) in bovine arterial endothelial cells (BAEC), and investigated the mechanism of cadmium-induced TrxR1 expression. We here showed that cadmium, differently from TNFalpha, enhanced the promoter activity of the 5'-flanking region of human TrxR1 gene (nucleotides -1692 to +49). Deletion and site-directed mutation of antioxidant responsive element (ARE) (nucleotides -62 to -48) in this region abolished the response to cadmium. Overexpression of NF-E2-related factor-2 (Nrf2) augmented the TrxR1 promoter activity. In contrast, overexpression of the dominant negative mutant of Nrf2 suppressed cadmium-induced activation of TrxR1 promoter through the ARE. Chromatin immunoprecipitation (ChIP) assays showed that anti-Nrf2 antibody precipitated ARE from the chromatin of the cadmium-treated cells. These results indicated that cadmium-induced TrxR1 gene expression is mediated by the activation of Nrf2 transcription factor and its binding to ARE in the TrxR1 gene promoter. We further found that in addition to cadmium, the activators of Nrf2, such as diethyl maleate (DEM) and arsenite, induced both TrxR1 and Trx gene expression in BAEC. Nrf2 might play an important role in the regulation of the cellular Trx system consisting of Trx and TrxR.

Gene expression analysis in mice with elevated glial fibrillary acidic protein and Rosenthal fibers reveals a stress response followed by glial activation and neuronal dysfunction

Alexander disease is a fatal neurodegenerative disorder resulting from missense mutations of the intermediate filament protein, GFAP. The pathological hallmark of this disease is the formation of cytoplasmic protein aggregates within astrocytes known as Rosenthal fibers. Transgenic mice engineered to over-express wild-type human GFAP develop an encephalopathy with identical aggregates, suggesting that elevated levels of GFAP in addition to mutant protein contribute to the pathogenesis of this disorder. To study further the effects of elevated GFAP and Rosenthal fibers per se, independent of mutations, we performed gene expression analysis on olfactory bulbs of transgenic mice at two different ages to follow the progression of pathology. The expression profiles reveal a stress response that includes genes involved in glutathione metabolism, peroxide detoxification and iron homeostasis. Many of these genes are regulated by the transcription factor Nfe2l2, which is also increased in expression at 3 weeks. An immune-related response occurs with activation of cytokine and cytokine receptor genes, complement components and acute phase response genes. These transcripts are further elevated with age, with additional induction of macrophage-specific markers such as Mac1 and CD68, suggesting activation of microglia. At 4 months, decreased expression of genes for microtubule-associated proteins, vesicular trafficking proteins and neurotransmitter receptors becomes apparent. Interneuron-specific transcription factors including Dlx family members and Pax6 are downregulated as well as Gad1 and Gad2, suggesting impairment of GABAergic granule cells. Together, these data implicate an initial stress response by astrocytes, which results in the activation of microglia and compromised neuronal function.

Modifying specific cysteines of the electrophile-sensing human Keap1 protein is insufficient to disrupt binding to the Nrf2 domain Neh2

The risks of cancer and other degenerative diseases caused by reactive oxygen species and electrophiles can be reduced by the up-regulation of detoxifying enzymes. A major mechanism whereby these protective enzymes are induced occurs through activation of the antioxidant response element (ARE) by the oxidative-stress sensor protein Kelch-like ECH-associated protein 1 (Keap1) and the transcription factor NF-E2-related factor 2 (Nrf2). Under basal conditions, Keap1 sequesters Nrf2 in the cytoplasm by binding to its Neh2 domain. Chemical inducers such as sulforaphane are known to react with Keap1 cysteine residues, thereby promoting Nrf2 nuclear accumulation and hence ARE activation. A widely accepted model for Nrf2 nuclear accumulation is that modification of Keap1 cysteines leads directly to dissociation of the Keap1-Nrf2 complex. This model is based on studies with mouse proteins and has served as the experimental basis and hypothesis for numerous investigations. Through a combination of chemical, mass spectrometry, and isothermal titration calorimetry methods, we have tested the direct-dissociation model using a series of ARE inducers: sulforaphane, isoliquiritigenin, 15-deoxy-Delta12,14-prostaglandin-J2, menadione, 1-Cl-2,4-dinitrobenzene, and biotinylated iodoacetamide. Surprisingly, these data suggest that the direct disruption model for Keap1-Nrf2 is incorrect. The relative reactivity of human Keap1 cysteines was determined. In addition to the same five cysteines identified for mouse Keap1, two highly reactive and previously unobserved cysteines were identified. Based on these results, a model is proposed that should aid in the understanding of Keap1-Nrf2 signaling mechanisms.

Nrf2 controls constitutive and inducible expression of ARE-driven genes through a dynamic pathway involving nucleocytoplasmic shuttling by Keap1

Nrf2 regulates the expression of genes encoding antioxidant proteins involved in cellular redox homeostasis. Previous studies have suggested that activation of Nrf2 is mediated by mechanisms promoting its dissociation from Keap1, a cytosolic repressor that acts to sequester the transcription factor in the cytoplasm. As a short-lived protein, Nrf2 is also activated by mechanisms leading to its stabilization in cells under stress, and recent evidence indicates that Keap1 has an active role in the control of its stability. In this report, using immunocytochemistry, cell fractionation, and chromatin immunoprecipitation analyses, we found that Nrf2 is primarily a nuclear protein and that it is expressed and recruited to the chromatin constitutively to drive basal gene expression. Furthermore, we found evidence indicating that Keap1 may repress Nrf2 activity by transiently shuttling into the nucleus to promote its ubiquitylation. The data suggested that the steady-state level of Nrf2 is maintained by a dynamic pathway that balances its constitutive expression with a Keap1-regulated degradation process downstream of its role as a transcriptional activator. We suggest that the stabilization of Nrf2 in cells under stress represents the central regulatory response mediated by mechanisms that interfere with its interaction with Keap1, leading to the induction of antioxidant enzymes important to maintain cellular redox homeostasis.

Ubiquitination of Keap1, a BTB-Kelch substrate adaptor protein for Cul3, targets Keap1 for degradation by a proteasome-independent pathway

Keap1 is a BTB-Kelch protein that functions as a substrate adaptor protein for a Cul3-dependent E3 ubiquitin ligase complex. Keap1 targets its substrate, the Nrf2 transcription factor, for ubiquitination and subsequent degradation by the 26 S proteasome. Inhibition of Keap1-dependent ubiquitination of Nrf2 increases steady-state levels of Nrf2 and enables activation of cytoprotective Nrf2-dependent genes. In this report, we demonstrate that Keap1 and three other BTB-Kelch proteins, including GAN1, ENC1, and Sarcosin, are ubiquitinated by a Cul3-dependent complex. Ubiquitination of Keap1 is markedly increased in cells exposed to quinone-induced oxidative stress, occurs in parallel with inhibition of Keap1-dependent ubiquitination of Nrf2, and results in decreased steady-state levels of Keap1, particularly in cells that are unable to synthesize glutathione. Degradation of Keap1 is independent of the 26 S proteasome, because inhibitors of the 26 S proteasome do not prevent loss of Keap1 following exposure of cells to quinone-induced oxidative stress. Our results suggest that a switch from substrate to substrate adaptor ubiquitination is a critical regulatory step that controls steady-state levels of both BTB-Kelch substrate adaptor proteins and their cognate substrates.

Effects of genetic ablation of bach1 upon smooth muscle cell proliferation and atherosclerosis after cuff injury

Bach1 is a transcriptional repressor of the cytoprotective enzyme heme oxygenase-1 (HO-1). Although HO-1 protects against atherosclerosis, the function of Bach1 in this process is poorly understood. We isolated peritoneal macrophages and aortic smooth muscle cells (SMC) from wild-type and bach1-deficient mice. bach1-Deficient macrophages expressed increased levels of HO-1 and showed elevated phagocytic activity when incubated with 0.75 microm microspheres. In SMC, bach1-ablation resulted in increased expression of HO-1 and decreased proliferation in bromodeoxyuridine incorporation assay as compared with wild-type cells. The up-regulated phagocytic activity and reduced SMC proliferation of bach1-deficient cells were not restored by Zinc (II) protoporphyrin IX, an inhibitor of HO, suggesting that HO-independent mechanisms are also involved in the regulation of phagocytosis of macrophages and proliferation of SMC by Bach1. In wild-type mice, cuff placement around femoral artery caused pronounced intimal proliferation without affecting the media, thus resulting in intimal to medial (I/M) volume ratio of 65.6%. bach1-deficient mice had less degree of intimal growth (I/M ratio of 45.6%). These results indicate that Bach1 plays a critical role in the regulation of HO-1 expression, macrophage function, SMC proliferation and neointimal formation. Bach1 may regulate gene expression in these cells during inflammation and atherogenesis.

Keap1 regulates the oxidation-sensitive shuttling of Nrf2 into and out of the nucleus via a Crm1-dependent nuclear export mechanism

Keap1 is a negative regulator of Nrf2, a transcription factor essential for antioxidant response element (ARE)-mediated gene expression. We find that Keap1 sequesters Nrf2 in the cytoplasm, not by docking it to the actin cytoskeleton but instead through an active Crm1/exportin-dependent nuclear export mechanism. Deletion and mutagenesis studies identified a nuclear export signal (NES) in the intervening region of Keap1 comprised of hydrophobic leucine and isoleucine residues in agreement with a traditional NES consensus sequence. Mutation of the hydrophobic amino acids resulted in nuclear accumulation of both Keap1 and Nrf2, as did treatment with the drug leptomycin B, which inactivates Crm1/exportin. ARE genes were partially activated under these conditions, suggesting that additional oxidation-sensitive elements are required for full activation of the antioxidant response. Based on these data, we propose a new model for regulation of Nrf2 by Keap1. Under normal conditions, Keap1 and Nrf2 are complexed in the cytoplasm where they are targeted for degradation. Oxidative stress inactivates Keap1's NES, allowing entry of both Keap1 and Nrf2 into the nucleus and transcriptional transactivation of ARE genes.

Role of 15-DeoxyΔ12,14Prostaglandin J2and Nrf2 Pathways in Protection against Acute Lung Injury

RATIONALE

Acute lung injury (ALI) is a disease process that is characterized by diffuse inflammation in the lung parenchyma. Recent studies demonstrated that cyclooxygenase-2 (COX-2) induced at the late phase of inflammation aids in the resolution of inflammation by generating 15-deoxy-delta(12,14)-prostaglandin J2 (15d-PGJ2). Transcription factor Nrf2 is activated by electrophiles and exerts antiinflammatory effects by inducing the gene expression of antioxidant and detoxification enzymes.

OBJECTIVES

Because 15d-PGJ2 is an endogenous electrophile, we hypothesized that it protects against ALI by activating Nrf2.

METHODS

To test this hypothesis, we generated a reversible ALI model by intratracheal injection of carrageenin, an inducer of acute inflammation, whose stimulation has been known to induce COX-2.

MAIN RESULTS

We found that ALI induced by carrageenin was markedly exacerbated in Nrf2-knockout mice, compared with wild-type mice. Analysis of bronchoalveolar lavage fluids also revealed that the magnitude and the duration of acute inflammation, indicated by albumin concentration and the number of neutrophils, were significantly enhanced in Nrf2-knockout mice. Treatment of wild-type mice with NS-398, a selective COX-2 inhibitor, significantly exacerbated ALI to the level of Nrf2-knockout mice. In the lungs of NS-398-treated wild-type mice, both the accumulation of 15d-PGJ2 and the induction of Nrf2 target antioxidant genes were significantly attenuated. Exogenous administration of 15d-PGJ2 reversed the exacerbating effects of NS-398 with the induction of antioxidant genes.

CONCLUSIONS

These results demonstrated in vivo that 15d-PGJ2 plays a protective role against ALI by exploiting the Nrf2-mediated transcriptional pathway.

Differential responses of the Nrf2-Keap1 system to laminar and oscillatory shear stresses in endothelial cells

The Nrf2-Keap1 system coordinately regulates cytoprotective gene expression via the antioxidant responsive element (ARE). The expression of several ARE-regulated genes was found to be up-regulated in endothelial cells by laminar shear stress, suggesting that Nrf2 contributes to the anti-atherosclerosis response via the ARE. To gain further insight into the roles that Nrf2 plays in the development of atherosclerosis, we examined how Nrf2 regulates gene expression in response to anti-atherogenic laminar flow (L-flow) or pro-atherogenic oscillatory flow (O-flow). Exposure of human aortic endothelial cells (HAECs) to L-flow, but not to O-flow, induced the expression of cytoprotective genes, such as NAD(P)H quinone oxidoreductase 1 (NQO1) by 5-fold and heme oxygenase-1 by 8-fold. The critical contribution of Nrf2 to the expression induced by L-flow was ascertained in siRNA-mediated knock-down experiments. Two cyclooxygenase-2 (COX-2) specific inhibitors attenuated Nrf2 nuclear accumulation in the acute phase of L-flow exposure. A downstream product of COX-2, 15-deoxy-Delta(12,14)-prostaglandin J2 (15d-PGJ2), activated the Nrf2 regulatory pathway in HAECs through binding to the cysteines of Keap1. These results demonstrate that 15d-PGJ2 is essential for L-flow to activate Nrf2 and induce anti-atherosclerotic gene expression. Whereas both L-flow and O-flow induced the nuclear accumulation of Nrf2 to comparable levels, chromatin immunoprecipitation analysis revealed that Nrf2 binding to the NQO1 ARE was significantly diminished in the case of O-flow compared with that of L-flow. These results suggest that O-flow inhibits Nrf2 activity at the DNA binding step, thereby suppressing athero-protective gene expression and hence predisposing the blood vessels to the formation of atherosclerosis.