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

Hemin-induced activation of the thioredoxin gene by Nrf2. A differential regulation of the antioxidant responsive element by a switch of its binding factors

Thioredoxin plays an important role in various cellular processes through redox regulation. Here, we have demonstrated that thioredoxin expression is transcriptionally induced in K562 cells by hemin (ferriprotoporphyrin IX) through activation of a regulatory region positioned from -452 to -420 bp of the thioredoxin gene. Insertion of a mutation in the antioxidant responsive element (ARE)/AP-1 consensus binding sequence in this region abolished the response to hemin. With electrophoretic mobility shift and DNA affinity assays, we have shown that the NF-E2p45/small Maf complex constitutively binds to the ARE. The binding of the Nrf2/small Maf complex to ARE was induced by hemin, whereas the binding of Jun/Fos proteins to ARE was induced by phorbol 12-myristate 13-acetate, but not hemin. Hemin induced nuclear translocation of Nrf2 but did not affect nuclear expression of Jun/Fos proteins. Overexpression of Nrf2 augmented the response to hemin in a dose-dependent manner. In contrast, overexpression of the dominant negative mutant of Nrf2 suppressed hemin-induced activation through the ARE. We show here hemin-induced activation of the thioredoxin gene by Nrf2 through the ARE and propose a novel mechanism of the regulation of the ARE through a switch of its binding factors.

Cobalt induces heme oxygenase-1 expression by a hypoxia-inducible factor-independent mechanism in Chinese hamster ovary cells: regulation by Nrf2 and MafG transcription factors

We have shown previously that activation of the heme oxygenase-1 (ho-1) gene by hypoxia in aortic smooth muscle cells is mediated by hypoxia-inducible factor-1 (HIF-1). In mutant (Ka13) Chinese hamster ovary cells lacking HIF activity, accumulation of ho-1 mRNA in response to hypoxia and the hypoxia-mimetic CoCl(2) was similar to that observed in wild type (K1) cells. These results support the existence of HIF-dependent and HIF-independent mechanisms for ho-1 gene activation by hypoxia and CoCl(2). In Ka13 cells, CoCl(2) stimulated expression of a luciferase reporter gene under the control of a 15-kilobase pair mouse ho-1 promoter (pHO15luc). Mutation analyses identified the cobalt-responsive sequences as the stress-response elements (StREs). In electrophoretic mobility shift assays, two specific StRE-protein complexes were observed using extracts from Ka13 cells. In response to cobalt, the level of the slower migrating complex X increased, whereas that of complex Y decreased, in a time-dependent manner. Members of the AP-1 superfamily of basic-leucine zipper factors bind to the StRE. Antibody supershift electrophoretic mobility shift assays did not detect Jun, Fos, or ATF/CREB proteins but identified Nrf2 and the small Maf protein, MafG, as components of complex X. Furthermore, dominant-negative mutants of Nrf2 and small Maf, but not of other bZIP factors, attenuated cobalt-mediated gene activation. Additional experiments demonstrated that induction by cobalt does not result from increased expression of MafG or regulated nuclear translocation of Nrf2 but is dependent on cellular oxidative stress. Unlike cobalt, hypoxia did not stimulate pHO15luc expression and did not increase StRE binding activity, indicating distinct mechanisms for ho-1 gene activation by cobalt and hypoxia in Chinese hamster ovary cells.

xCt cystine transporter expression in HEK293 cells: pharmacology and localization

xCT, the core subunit of the system x(c)(-) high affinity cystine transporter, belongs to a superfamily of glycoprotein-associated amino acid transporters. Although xCT was shown to promote cystine transport in Xenopus oocytes, little work has been done with mammalian cells (Sato, H., Tamba, M., Ishii, T., and Bannai, S. J. Biol. Chem. 274, 11455-11458, 1999). Therefore, we have constructed mammalian expression vectors for murine xCT and its accessory subunit 4F2hc and transfected them into HEK293 cells. We report that this transporter binds cystine with high affinity (81 microM) and displays a pharmacological profile expected for system x(c)(-). Surprisingly, xCT transport activity in HEK293 cells is not dependent on the co-expression of the exogenous 4F2hc. Expression of GFP-tagged xCT indicated a highly clustered plasma membrane and intracellular distribution suggesting the presence of subcellular domains associated with combating oxidative stress. Our results indicate that HEK293 cells transfected with the xCT subunit would be a useful vehicle for future structure-function and pharmacology experiments involving system x(c)(-).

Effect of oxygen on induction of the cystine transporter by bacterial lipopolysaccharide in mouse peritoneal macrophages

Amino acid transport in mouse peritoneal macrophages is mediated by several membrane carriers with different substrate specificity and sensitivity to environmental stimuli. We reported previously that transport activities of cystine and arginine in the macrophages were induced markedly by low concentrations of bacterial lipopolysaccharide (LPS). It is known that a variety of macrophage functions are affected by ambient oxygen tension. In this study, we have investigated the effects of oxygen on the induction of amino acid transport activity by LPS and found that the induction of cystine, but not arginine, transport activity was dependent on the ambient oxygen tension. When the macrophages were cultured with 2% O(2) in the presence of 1 ng/ml LPS, induction of cystine transport activity was reduced by approximately 70% compared with cells cultured under normoxic conditions. In macrophages, transport of cystine is mediated by a Na(+)-independent anionic amino acid transporter named system x(c)(-). System x(c)(-) is composed of two protein components, xCT and 4F2hc, and the expression of xCT was closely correlated with system x(c)(-) activity. A putative NF-kappaB binding site was found in the 5'-flanking region of the xCT gene, but the enhanced expression of xCT by LPS and oxygen was not mediated by NF-kappaB binding. An increase in intracellular GSH in macrophages paralleled induction of xCT, but not gamma-glutamylcysteine synthetase. These results suggest the importance of system x(c)(-) in antioxidant defense in macrophages exposed to LPS and oxidative stress.

Expression profiling of acetaminophen liver toxicity in mice using microarray technology

Drug-induced hepatotoxicity causes significant morbidity and mortality and is a major concern in drug development. This is due, in large part, to insufficient knowledge of the mechanism(s) of drug-induced liver injury. In order to address this problem, we have evaluated the modulation of gene expression within the livers of mice treated with a hepatotoxic dose of acetaminophen (APAP) using high-density oligonucleotide microarrays capable of determining the expression profile of >11,000 genes and expressed sequence tags (ESTs). Significant alterations in gene expression, both positive and negative, were noted within the livers of APAP-treated mice. APAP-induced toxicity affected numerous aspects of liver physiology causing, for instance, >twofold increased expression of genes that encode for growth arrest and cell cycle regulatory proteins, stress-induced proteins, the transcription factor LRG-21, suppressor of cytokine signaling (SOCS)-2-protein, and plasminogen activator inhibitor-1 (PAI-1). A number of these and other genes and ESTs were detectable within the liver only after APAP treatment suggesting their potential importance in propagating or preventing further toxicity. These data provide new directions for mechanistic studies that may lead to a better understanding of the molecular basis of drug-induced liver injury and, ultimately, to a more rational design of safer drugs.

Preferential expression of antioxidant response element mediated gene expression in astrocytes

Transcriptional control of target genes by antioxidant/electrophile response elements has been well described in peripheral tissues. Genes that are regulated by this mechanism include the antioxidant enzymes NAD(P)H:quinone oxidoreductase, gamma-glutamyl cystine synthetase and glutathione-S-transferase. Antioxidant/electrophile response elements within a gene's promoter confer induction by low-molecular-weight electrophilic compounds such as tert-butylhydroquinone and dimethyl fumarate. We have now examined the ability of antioxidant/electrophile response elements to elicit gene expression in neurons and astrocytes in both brain slices and primary cultures using transient transfection of promoter reporter constructs. Our results using a heat-stable human placental alkaline phosphatase reporter indicate that antioxidant/electrophile response element mediated gene expression is largely restricted to astrocyte cell populations. Placental alkaline phosphatase expression was significantly elevated in astrocytes treated with the antioxidant/electrophile response element inducer dimethyl fumarate. Mutant constructs lacking a functional antioxidant/electrophile response element abolished all placental alkaline phosphatase expression in astrocytes. We suggest that astrocytic metabolic processes that normally aid and/or protect neurons may be controlled via this inducible system.

Nrf2-dependent activation of the antioxidant responsive element by tert-butylhydroquinone is independent of oxidative stress in IMR-32 human neuroblastoma cells

The antioxidant responsive element (ARE) is a cis-acting regulatory element located in the 5'-flanking region of several genes encoding phase II detoxification enzymes, including NAD(P)H:quinone oxidoreductase (NQO1). We report here that activation of the NQO1 ARE by tert-butylhydroquinone (tBHQ) is dependent on Nrf2 and not oxidative stress in IMR-32 human neuroblastoma cells. Overexpression of wild-type Nrf2 activated ARE in a dose-dependent manner, and ARE activation by tBHQ or diethyl maleate (DEM) was inhibited by dominant/negative Nrf2 not by dominant/negative c-Jun. According to our observation, the palindromic sequence (5' to the core) and the GC box in the ARE core sequence are essential for maximal inducibility by tBHQ or DEM. Overexpression of Nrf2 selectively activated wild-type ARE up to 24 h. In addition, a dramatic nuclear translocation of Nrf2 by tBHQ supports a role for Nrf2 in ARE activation. Although oxidative stress is hypothesized to be a major driving force for ARE activation, pretreatment of antioxidant or antioxidant enzyme did not block tBHQ-mediated ARE activation. In contrast, ARE activation by DEM was inhibited by antioxidants or catalase. These results suggest that ARE activation signals from tBHQ and DEM converge at Nrf2 transcription factor through independent mechanisms.

Effects of hyperoxia and iron on iron regulatory protein-1 activity and the ferritin synthesis in mouse peritoneal macrophages

Ferritin is an intracellular iron storage protein and its translation is inhibited by binding of iron regulatory proteins (IRPs) to the iron-responsive element (IRE) located in the 5' untranslated region of its mRNA. In this paper, we have investigated the effect of hyperoxia and iron on the binding activity of IRP-1 and the ferritin synthesis in mouse peritoneal macrophages. The binding activity of IRP-1 was increased and the ferritin synthesis was suppressed when the macrophages were cultured under hyperoxia, and the reverse occurred under hypoxia. Iron diminished the IRP-1-binding activity and the enhanced synthesis of ferritin. However, this effect was arrested under hyperoxia. Consistently, hypoxia-induced loss of binding activity of IRP-1 and the enhanced synthesis of ferritin were blocked in the presence of an iron chelator deferoxamine. These alterations of the binding activity of IRP-1 in response to oxygen and iron were not reproduced in the cell-free extract. The data suggest that in the macrophages oxygen and iron inversely act on the binding activity of IRP-1 and the ferritin synthesis, and that intracellular mechanism(s) to sense iron and/or oxygen is required for these actions.

Small maf (MafG and MafK) proteins negatively regulate antioxidant response element-mediated expression and antioxidant induction of the NAD(P)H:Quinone oxidoreductase1 gene

The antioxidant response element (ARE) is known to regulate expression and induction of NQO1, GST Ya, and other detoxifying enzyme genes in response to antioxidants and xenobiotics. The nuclear transcription factor Nrf2 and Nrf1 bind to the ARE and positively regulate expression and induction of the NQO1 and GST Ya genes. In this study, we demonstrate that overexpression of small Maf (MafG and MafK) proteins negatively regulate ARE-mediated expression and tert-butyl hydroquinone induction of the NQO1 and GST Ya genes in transfected Hep-G2 cells. In similar experiments, overexpression of small Maf proteins also repressed Nrf2-mediated up-regulation of ARE-mediated NQO1 and GST Ya genes expression in Hep-G2 cells co-transfected with Nrf2 and small Maf proteins. Band and supershift assays with the NQO1 gene ARE and nuclear proteins demonstrate that small MafG and MafK bind to the ARE as Maf-Maf homodimers and Maf-Nrf2 heterodimers. Therefore, Maf-Maf homodimers and possibly Maf-Nrf2 heterodimers play a role in negative regulation of ARE-mediated transcription and antioxidant induction of NQO1 and other detoxifying enzyme genes. In contrast to Maf-Nrf2, the Maf-Nrf1 heterodimers failed to bind with the NQO1 gene ARE and did not demonstrate the repressive effect in transfection assays.

Activation of mitogen-activated protein kinase pathways induces antioxidant response element-mediated gene expression via a Nrf2-dependent mechanism

Antioxidant response element (ARE) regulates the induction of a number of cellular antioxidant and detoxifying enzymes. However, the signaling pathways that lead to ARE activation remain unknown. Here, we report that the expression of mitogen-activated protein (MAP) kinase/extracellular signal-regulated kinase kinase kinase 1 (MEKK1), transforming growth factor-beta-activated kinase (TAK1), and apoptosis signal-regulating kinase (ASK1) in HepG2 cells activated the ARE reporter gene, whereas the expression of their dominant-negative mutants impaired ARE activation by the chemicals sodium arsenite and mercury chloride. Coexpression of downstream kinases, MAP kinase kinase 4, MAP kinase kinase 6, and c-Jun NH(2)-terminal kinase-1, but not MAP kinase kinase 3 and p38, augmented ARE activation by MEKK1, TAK1, and ASK1. The coexpression of a basic leucine zipper transcription factor Nrf2 but not c-Jun also greatly enhanced the activation of reporter gene by MEKK1, TAK1, and ASK1; however, a dominant-negative mutant of Nrf2 (NF-E2-related factor 2) blocked this event. Furthermore, when overexpressed, MEKK1, TAK1, and ASK1 induced the expression of heme oxygenase-1, a gene regulated by ARE, and the cotransfection with the dominant-negative mutant of Nrf2 abolished the induction. Taken together, these results suggest that MAP kinase pathways that are activated by MEKK1, TAK1, and ASK1 may link chemical signals to Nrf2, leading to the activation of ARE-dependent genes.

Regulation of the antioxidant response element by protein kinase C-mediated phosphorylation of NF-E2-related factor 2

A coordinated cellular response to oxidative stress occurs in part through transcriptional regulation via a cis-acting sequence known as the antioxidant response element (ARE). NF-E2-related factor 2 (Nrf2), a member of the Cap'n'Collar family of basic region-leucine zipper (bZIP) transcription factors, has been implicated as an essential component of an ARE-binding transcriptional complex, but the signaling pathway leading to its activation has remained unclear. Using a reporter gene assay, we found that ARE-directed transcription was activated by phorbol 12-myristate 13-acetate (PMA), but completely suppressed by staurosporine and Ro-32-0432, selective inhibitors of protein kinase C (PKC). Immunocytochemistry and subcellular fractionation revealed that PMA, like tert-butylhydroquinone (tBHQ), promoted the nuclear localization of Nrf2, a process that was blocked by staurosporine or Ro-32-0432. We showed that Nrf2, a previously unidentified kinase target, was phosphorylated in HepG2 cells. PMA transiently activated Nrf2 phosphorylation, whereas the addition of tBHQ or beta-naphthoflavone (betaNF) led to a persistent stimulation, which was abolished by staurosporine, but not by U0126 and SB203580, respective inhibitors of MEK and p38 kinases. Purified Nrf2 was phosphorylated in vitro by the catalytic subunit of PKC, or by PKC immunoprecipitated from cell lysates. Significantly, PKC precipitated from tBHQ- or betaNF-treated cells showed enhanced activity against Nrf2. These findings indicate an important role of the PKC pathway in the ARE-mediated gene expression, and suggest that PKC-directed phosphorylation of Nrf2 may be a critical event for the nuclear translocation of this transcription factor in response to oxidative stress.

Mechanism of heme oxygenase-1 gene activation by cadmium in MCF-7 mammary epithelial cells. Role of p38 kinase and Nrf2 transcription factor

The mouse heme oxygenase-1 (HO-1) gene, ho-1, contains two inducible enhancers, E1 and E2. Of several cell lines tested, induction of an E1/luciferase fusion construct, pE1-luc, by CdCl(2) is most pronounced in MCF-7 cells. In these cells, E1, but not E2, is necessary and sufficient for ho-1 gene activation. Exposure of MCF-7 cells to 10 micrometer CdCl(2) stimulates phosphorylation of ERK, JNK, and p38 mitogen-activated protein kinases, implicating one or more of these signaling pathways in ho-1 gene induction. SB203580, an inhibitor of p38, diminishes cadmium-stimulated pE1-luc expression and HO-1 mRNA levels by up to 70-80%. PD098059, an ERK pathway inhibitor, does not affect HO-1 mRNA induction at the highest concentration (40 micrometer) tested. Similarly, co-expression of a dominant-negative mutant of p38alpha, but not of ERK1, ERK2, JNK1, or JNK2, reduces basal and cadmium-induced pE1-luc activity. E1 contains binding sites for the activator protein-1 (Fos/Jun), Cap'n'Collar/basic leucine zipper (CNC-bZIP), and CCAAT/enhancer-binding protein (C/EBP) families of transcription factors. A dominant-negative mutant of Nrf2 (a CNC-bZIP member), but not of c-Jun or C/EBPbeta, inhibits pE1-luc activation by cadmium. Induction of the endogenous ho-1 gene is also inhibited by the Nrf2 mutant. Mutations of E1 that inhibit cadmium inducibility also suppress the trans-activation and DNA binding activities of Nrf2, and SB203580, but not PD098059, attenuates Nrf2-mediated trans-activation of pE1-luc. Taken together, these results indicate that cadmium induces ho-1 gene expression via sequential activation of the p38 kinase pathway and Nrf2.