Transcriptional regulation of the rat NAD(P)H:quinone reductase gene

High-throughput evaluation of aryl hydrocarbon receptor-binding sites selected via chromatin immunoprecipitation-based screening in Hepa-1c1c7 cells stimulated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Upon binding to ligands such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an aryl hydrocarbon receptor (AhR) is activated to form a heterodimer with an aryl hydrocarbon receptor nuclear translocator (Arnt) and binds to DNA. It has been shown that the binding of AhR to DNA depends on the dioxin response element (DRE) and controls xenobiotic-response genes. AhR-binding DNA fragments from mouse hepatoma Hepa-1c1c7 cells stimulated with TCDD were once enriched in a chromatin immunoprecipitation (ChIP) DNA library and screened through a high-throughput southwestern chemistry-based enzyme-linked immunosorbent assay (SW-ELISA). After screening 1700 fragments, the ChIP-SW-ELISA screening strategy allowed us to isolate 77 fragments tightly interacting with AhR in the presence of TCDD. Only 39 of the 77 fragments appeared to contain a typical DRE, indicating that in some cases the DRE was dispensable for AhR-binding, while 75 fragments were located within promoter-distal regions. Genomic mapping of the 77 fragments enabled us to estimate 121 potential AhR targets including known targets such as Cyp1A1 and Cyp1B1, but only a limited number exhibited an altered expression dependent on TCDD. This study revealed the fact that TCDD-activated AhR frequently binds to promoter-distal regions even without a DRE and is not always involved in transcriptional regulation, suggesting that within the genome DNA-binding of AhR could take place often in many regions without cis-regulatory elements and might not be a key determinant to establish its regulatory function.

Timing is everything: consequences of transient and sustained AhR activity

The aryl hydrocarbon receptor (AhR) was implicated as a mediator of xenobiotic toxicity over three decades ago. Although a complete picture continues to elude us, investigations by many laboratories during the ensuing period have revealed much about AhR biology in normal physiological processes, as well as the toxicities induced by the dioxins and related polychlorinated aromatic hydrocarbons. The findings are captured in numerous excellent reviews. This commentary attempts to inject a new perspective on some new as well as frequently overlooked observations in the context of established receptor properties. Specifically, we examine the impact of transient versus sustained receptor activation on AhR biology, and explore the potential role for cytochrome P450 expression in regulating AhR activity amongst various tissues. The growing recognition that AhR action functions through multiple mechanisms serves to further highlight the importance of limiting prolonged receptor activation.

Nrf2-dependent sulfiredoxin-1 expression protects against cigarette smoke-induced oxidative stress in lungs

Oxidative stress results in protein oxidation and is involved in the pathogenesis of lung diseases such as chronic obstructive pulmonary disorder (COPD). Sulfiredoxin-1 (Srx1) catalyzes the reduction of cysteine sulfinic acid to sulfenic acid in oxidized proteins and protects them from inactivation. This study examined the mechanism of transcriptional regulation of Srx1 and its possible protective role during oxidative stress associated with COPD. Nrf2, a transcription factor known to influence susceptibility to pulmonary diseases, upregulates Srx1 expression during oxidative stress caused by cigarette smoke exposure in the lungs of mice. Disruption of Nrf2 signaling by genetic knockout in mice or RNAi in cells downregulated the expression of Srx1. In silico analysis of the 5'-promoter-flanking region of Srx1 identified multiple antioxidant-response elements (AREs) that are highly conserved. Reporter and chromatin-immunoprecipitation assays demonstrated that ARE1 at -228 is critical for the Nrf2-mediated response. Attenuation of Srx1 expression with RNAi potentiated the toxicity of hydrogen peroxide (H2O2), whereas overexpression of Srx1 protected against H2O2-mediated cell death in vitro. Immunoblot analysis revealed dramatic decreases in Srx1 expression in lungs from patients with COPD relative to nonemphysematous lungs together with a decline in Nrf2 protein. Thus, Srx1, a key Nrf2-regulated gene, contributes to protection against oxidative injury in the lung.

Intracellular iron transport and storage: from molecular mechanisms to health implications

Maintenance of proper "labile iron" levels is a critical component in preserving homeostasis. Iron is a vital element that is a constituent of a number of important macromolecules, including those involved in energy production, respiration, DNA synthesis, and metabolism; however, excess "labile iron" is potentially detrimental to the cell or organism or both because of its propensity to participate in oxidation-reduction reactions that generate harmful free radicals. Because of this dual nature, elaborate systems tightly control the concentration of available iron. Perturbation of normal physiologic iron concentrations may be both a cause and a consequence of cellular damage and disease states. This review highlights the molecular mechanisms responsible for regulation of iron absorption, transport, and storage through the roles of key regulatory proteins, including ferroportin, hepcidin, ferritin, and frataxin. In addition, we present an overview of the relation between iron regulation and oxidative stress and we discuss the role of functional iron overload in the pathogenesis of hemochromatosis, neurodegeneration, and inflammation.

Elevated intracellular calcium increases ferritin H expression through an NFAT-independent post-transcriptional mechanism involving mRNA stabilization

An increase in intracellular Ca2+ is one of the initiating events in T-cell activation. A calcium-mediated signalling cascade in T-cells involves activation of calcineurin and the dephosphorylation and translocation of NFAT (nuclear factor of activated T-cells), resulting in the transcriptional activation of target genes such as IL-2 (interleukin-2). In the present study, we found that increased intracellular calcium leads to induction of the antioxidant protein ferritin H. We previously reported that the ferritin H gene is transcriptionally activated under oxidative stress conditions through an ARE (antioxidant-responsive element). The facts that the ferritin H ARE contains a composite AP-1 (activator protein 1) site and that NFAT collaborates with AP-1 transcription factors led us to test whether calcium-activated NFAT is involved in the ferritin H induction through the ARE. Treatment of Jurkat T-cells with the calcium ionophore, ionomycin, increased ferritin H mRNA and protein expression. Although NFAT translocated to the nucleus and bound a consensus NFAT sequence located in the IL-2 promoter after ionomycin treatment, it did not activate ferritin H transcription despite the presence of a putative NFAT-binding sequence in the ferritin H ARE. In addition, the calcineurin inhibitor cyclosporin A treatment blocked ionomycin-mediated NFAT nuclear translocation but failed to abrogate the increase in ferritin H mRNA. Analysis of mRNA stability after actinomycin D treatment revealed that ionomycin prolongs ferritin H mRNA half-life. Taken together, these results suggest that ionomycin-mediated induction of ferritin H may occur in an NFAT-independent manner but through post-transcriptional stabilization of the ferritin H mRNA.

Sulindac and its metabolites induce carcinogen metabolizing enzymes in human colon cancer cells

Sulindac is a nonsteroidal antiinflammatory drug that has been demonstrated to be a potent chemopreventive agent against colorectal cancer in both human and animal models. In vivo, sulindac may be reversibly reduced to the active antiinflammatory compound, sulindac sulfide, or irreversibly oxidized to sulindac sulfone. Sulindac has also been shown to inhibit polycyclic aromatic hydrocarbon (PAH)-induced cancer, but the molecular mechanisms of its antitumor effect remain unclear. In this study, we investigated the effects of sulindac and its metabolites on the expression of enzymes that metabolize and detoxify PAHs in 2 human colon cancer cell lines, LS180 and Caco-2. Sulindac and sulindac sulfide induced a sustained, concentration-dependent increase in CYP enzyme activity as well as an increase in the mRNA levels of CYP1A1, CYP1A2 and CYP1B1. Sulindac and sulindac sulfide induced the transcription of the CYP1A1 gene, as measured by the level of heterogeneous nuclear CYP1A1 RNA and verified by the use of actinomycin D as a transcription inhibitor. Chromatin immunoprecipitation assays demonstrated that sulindac and sulindac sulfide also increased the nuclear level of activated aryl hydrocarbon receptor, the transcription factor which mediates CYP expression. Additionally, sulindac and both metabolites increased the activity and mRNA expression of the carcinogen detoxification enzyme NAD(P)H:quinone oxidoreductase, as well as the expression of UDP-glucuronosyltransferase mRNA. These results show an overall upregulation of carcinogen metabolizing enzymes in colon cancer cells treated with sulindac, sulindac sulfide and sulindac sulfone that may contribute to the established chemoprotective effects of these compounds.

Analysis of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced proteome changes in 5L rat hepatoma cells reveals novel targets of dioxin action including the mitochondrial apoptosis regulator VDAC2

As part of a comprehensive survey of the impact of the environmental pollutant and hepatocarcinogen 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the proteome of hepatic cells, we have performed a high resolution two-dimensional gel electrophoresis study on the rat hepatoma cell line 5L. 78 protein species corresponding to 73 different proteins were identified as up- or down-regulated following exposure of the cells to 1 nm TCDD for 8 h. There was an overlap of only nine proteins with those detected as altered by TCDD in our recent study using the non-gel-based isotope-coded protein label method (Sarioglu, H., Brandner, S., Jacobsen, C., Meindl, T., Schmidt, A., Kellermann, J., Lottspeich, F., and Andrae, U. (2006) Quantitative analysis of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced proteome alterations in 5L rat hepatoma cells using isotope-coded protein labels. Proteomics 6, 2407-2421) indicating a strong complementarity of the two approaches. For the majority of the altered proteins, an effect of TCDD on their abundance or posttranslational modifications had not been known before. Several observations suggest that a sizable fraction of the proteins with altered abundance was induced as an adaptive response to TCDD-induced oxidative stress that was demonstrated using the fluorescent probe dihydrorhodamine 123. A prominent group of these proteins comprised various enzymes for which there is evidence that their expression is regulated via the Keap1/Nrf2/antioxidant response element pathway. Other proteins included several involved in the maintenance of mitochondrial energy production and the regulation of the mitochondrial apoptotic pathway. A particularly intriguing finding was the up-regulation of the mitochondrial outer membrane pore protein, voltage-dependent anion channel-selective protein 2 (VDAC2), which was dependent on the presence of a functional aryl hydrocarbon receptor. The regulatability of VDAC2 protein abundance has not been described previously. In view of the recently discovered central role of VDAC2 as an inhibitor of the activation of the proapoptotic protein BAK and the mitochondrial apoptotic pathway, the present data point to a hitherto unrecognized mechanism by which TCDD may affect cellular homeostasis and survival.

Oxidative and electrophilic stress induces multidrug resistance-associated protein transporters via the nuclear factor-E2-related factor-2 transcriptional pathway

Multidrug resistance-associated proteins (Mrps) are adenosine triphosphate-dependent transporters that efflux chemicals out of cells. In the liver, Mrp2 transports bilirubin-glucuronide, glutathione (GSH), and drug conjugates into bile, whereas Mrp3 and Mrp4 efflux these entities into blood. The purpose of this study was to determine whether oxidative conditions (that is, the disruption of hepatic GSH synthesis) or the administration of nuclear factor-E2-related factor-2 (Nrf2) activators (oltipraz and butylated hydroxyanisole) can induce hepatic Mrp transporters and whether that induction is through the Nrf2 transcriptional pathway. Livers from hepatocyte-specific glutamate-cysteine ligase catalytic subunit-null mice had increased nuclear Nrf2 levels, marked gene and protein induction of the Nrf2 target gene NAD(P)H:quinone oxidoreductase 1, as well as Mrp2, Mrp3, and Mrp4 expression. The treatment of wild-type and Nrf2-null mice with oltipraz and butylated hydroxyanisole demonstrated that the induction of Mrp2, Mrp3, and Mrp4 is Nrf2-dependent. In Hepa1c1c7 cells treated with the Nrf2 activator tert-butyl hydroquinone, chromatin immunoprecipitation with Nrf2 antibodies revealed the binding of Nrf2 to antioxidant response elements in the promoter regions of mouse Mrp2 [-185 base pairs (bp)], Mrp3 (-9919 bp), and Mrp4 (-3767 bp).

CONCLUSION

The activation of the Nrf2 regulatory pathway stimulates the coordinated induction of hepatic Mrps.

Evidence for an aryl hydrocarbon receptor-mediated cytochrome p450 autoregulatory pathway

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor responsible for mediating the cellular response to the toxic compound 2,3,7,8,-tetrachlorodibenzo-p-dioxin. An essential role for the AhR in cellular biology has been established previously, but no high-affinity endogenous ligand has yet been identified. We have confirmed the presence of a putative endogenous ligand(s) in CV-1 cells through transient transfection with various cytochrome P450 isoforms. Expression of cytochromes P450 1A1, 1A2, or 1B1 reduced AhR-mediated luciferase reporter activity, whereas cytochrome P450 2E1 exhibited no significant effect. Studies with 2,4,3',5'-tetramethoxystilbene, a potent and specific inhibitor of cytochrome P450 1B1, was able to partially block cytochrome P450 1B1-mediated reduction in reporter gene activity. These results provide evidence of the existence of a possible feedback mechanism in which AhR-regulated cytochromes P450 from the CYP1A and CYP1B families are able to metabolically alter putative endogenous ligand(s). Several experiments were performed to provide initial characterization of these putative endogenous ligands, including electrophoretic mobility shift assay analyses, which demonstrated that these ligands directly activate the AhR. Soluble extracts from various C57BL/6J and Ahr-null mouse tissues were also analyzed for the presence of AhR activators. Studies revealed that Ahr-null mouse lung tissue had a 4-fold increase in AhR-mediated reporter activity in cells. Quantitative polymerase chain reaction analysis revealed that lung tissue exhibits relatively high constitutive CYP1A1 mRNA levels. These results suggest that there is an autoregulatory feedback loop between the AhR and cytochrome P450 1A1 in mouse lung.

Keap1 eye on the target: chemoprevention of liver cancer

Hepatocellular carcinoma (HCC) is one of the most common cancers worldwide, causing nearly 600,000 deaths each year. Increased risk of HCC due to chronic infection with hepatitis B virus (HBV) and exposure to dietary aflatoxins is responsible for many of these deaths. Prevention strategies targeting HBV infection and aflatoxin exposure could dramatically impact the rates of HCC. Universal HBV vaccination programs have begun in some high-risk areas. Strategies to reduce aflatoxin contamination in food stores have also been implemented. However, complete elimination of aflatoxin contamination might not be possible. For this reason, chemoprevention strategies which alter aflatoxin disposition are a practical strategy to reduce the incidence of HCC in populations with high dietary aflatoxin exposure. The mechanisms of aflatoxin-induced hepatocarcinogenesis are well known. This knowledge provides the basis for evaluation of both exposures to aflatoxin, as well as modulation of aflatoxin disposition by chemopreventive agents. Products of aflatoxin DNA damage and toxicity as well as other metabolites can be used as biomarkers to evaluate modulation of aflatoxin disposition. Modulation of aflatoxin disposition can be achieved through induction of conjugating and cytoprotective enzymes. Many of these enzymes are regulated through Kelch ECH-associating protein 1 (Keap1)-NF-E2-related factor 2(Nrf2)-antioxidant response element (ARE) signaling, making this pathway an important molecular target for chemoprevention. Rodent studies have identified several classes of chemopreventive agents which induce cytoprotective genes. These inducers include phenolic antioxidants, dithiolethiones, isothiocyanates, and triterpenoids. Furthermore, clinical interventions have shown that inducers of Keap1-Nrf2- ARE signaling increase cytoprotective enzyme expression, resulting in modulation of aflatoxin disposition. Much work remains to be done in order to take promising chemopreventive agents from preclinical evaluation to application in at-risk populations. However, appropriately designed clinical trials will aid in this process, which can have profound impact on the incidence of HCC.

Aqueous extract from Spanish black radish (Raphanus sativus L. Var. niger) induces detoxification enzymes in the HepG2 human hepatoma cell line

Spanish black radish (Raphanus sativus L. var. niger) is a member of the Cruciferae family that also contains broccoli and Brussels sprouts, well-known to contain health-promoting constituents. Spanish black radishes (SBR) contain high concentrations of a glucosinolate unique to the radish family, glucoraphasatin, which represents >65% of the total glucosinolates present in SBR. The metabolites of glucosinolates, such as isothiocyanates, are implicated in health promotion, although it is unclear whether glucosinolates themselves elicit a similar response. The crude aqueous extract from 0.3 to 3 mg of dry SBR material increased the activity of the phase II detoxification enzyme quinone reductase in the human hepatoma HepG2 cell line with a maximal effect at a concentration of 1 mg/mL. Treatment of HepG2 cells with the crude aqueous extract of 1 mg of SBR per mL also significantly induced the expression of mRNA corresponding to the phase I detoxification enzymes: cytochrome P450 (CYP) 1A1, CYP1A2, and CYP1B1 as well as the phase II detoxification enzymes: quinone reductase, heme oxygenase 1, and thioredoxin reductase 1. Previous studies have shown that the myrosinase metabolites of different glucosinolates vary in their ability to induce detoxification enzymes. Here, we show that while glucoraphasatin addition was ineffective, the isothiocyanate metabolite of glucoraphasatin, 4-methylthio-3-butenyl isothiocyanate (MIBITC), significantly induced phase II detoxification enzymes at a concentration of 10 microM. These data demonstrate that the crude aqueous extract of SBR and the isothiocyanate metabolite of glucoraphasatin, MIBITC, are potent inducers of detoxification enzymes in the HepG2 cell line.

NF-E2 Related Factor 2 Activation and Heme Oxygenase-1 Induction by tert-Butylhydroquinone Protect against Deltamethrin-Mediated Oxidative Stress in PC12 Cells

Recent findings suggest that oxidative stress caused by pyrethroid pesticides could be closely involved in the neurotoxicity. tert-Butylhydroquinone ( tBHQ) is a known inducer of Nrf2-mediated transcription, and treatment of cells with tBHQ can confer protection against H 2O 2 and 6-hydroxydopamine (6-OHDA). In this study, we investigated the neuroprotective effect of tBHQ against deltamethrin (DM)-induced oxidative stress using rat PC12 adrenal pheochromocytoma cells. The pretreatment of PC12 cells with tBHQ significantly reduced DM-induced generation of reactive oxygen species (ROS) and increased intracellular ionized calcium ([Ca (2+)] i). We also observed that DM or tBHQ induced the expression of NF-E2-related factor 2 (Nrf2) and heme oxygenase-1 (HO-1), a Nrf2-regulated gene. In addition, the Nrf2 antioxidant responsive element (ARE) pathways activated by tBHQ caused a partial inhibition of the DM-induced Nrf2 and HO-1 expression. Altogether, our data clearly indicate that an activation of Nrf2/ARE pathways in PC12 cells by tBHQ treatment protects cells from DM-induced oxidative stress and regulates DM- mediated adaptive responses in PC12 cells via translocation of Nrf2.

2,3,7,8-Tetrachlorodibenzo-p-dioxin modulates functional differentiation of mouse bone marrow-derived dendritic cells Downregulation of RelB by 2,3,7,8-tetrachlorodibenzo-p-dioxin

We have previously shown that benzo(a)pyrene inhibits the growth and functional differentiation of mouse bone marrow (BM)-derived dendritic cells (DCs) [Hwang, J.A., Lee, J.A., Cheong, S.W., Youn, H.J., Park, J.H., 2007. Benzo(a)pyrene inhibits growth and functional differentiation of mouse bone marrow-derived dendritic cells. Downregulation of RelB and eIF3 p170 by benzo(a)pyrene. Toxicol. Lett. 169, 82-90]. Since the toxic effects of benzo(a)pyrene are aryl hydrocarbon receptor (AhR)-dependent, we examined the effects of the very potent AhR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the growth and functional differentiation of mouse BM-derived DCs. Ten nanomolars of TCDD had significant effects on functional differentiation of mouse DCs derived from BM cultured in the presence of GM-CSF and IL-4. The yields of DCs, flow-cytometrically analyzed for co-expression of CD11c/MHCII or CD11c/CD86, were reduced for TCDD-treated cultures, but TCDD itself had no effect on the growth of BM. DCs from TCDD-treated cultures expressed higher levels of MHCII and CD86, whereas expression of CD11c was reduced, compared with vehicle-treated cultures. Production of IL-10, but not IL-12, by the DCs from TCDD-treated cultures was decreased. Allogeneic T-cell stimulating ability of TCDD-treated DCs was increased compared to control DCs. The effects of TCDD were dependent on aryl hydrocarbon receptor (AhR), because alpha-naphthoflavone, an AhR antagonist, suppressed the effects of TCDD on IL-10 production and T-cell stimulating ability. RT-PCR revealed the downregulation of RelB, a transcription factor necessary for DCs differentiation and function. Taken together, although benzo(a)pyrene and TCDD exert their effects via binding to AhR, their effects on the growth and functional differentiation of bone marrow-derived DCs are different.

Lot6p from Saccharomyces cerevisiae is a FMN-dependent reductase with a potential role in quinone detoxification

NAD(P)H:quinone acceptor oxidoreductases are flavoenzymes expressed in the cytoplasm of many tissues and afford protection against the cytotoxic effects of electrophilic quinones by catalyzing a strict two-electron reduction. Such enzymes have been reported from several mammalian sources, e.g. human, mouse and rat, and from plant species. Here, we report identification of Lot6p (YLR011wp), the first soluble quinone reductase from the unicellular model organism Saccharomyces cerevisiae. Localization studies using an antibody raised against Lot6p as well as microscopic inspection of Lot6p-GFP demonstrated accumulation of the enzyme in the cytosol of yeast cells. Despite sharing only 23% similarity to type 1 human quinone reductase, Lot6p possesses biochemical properties that are similar to its human counterpart. The enzyme catalyzes a two-electron reduction of a series of natural and artificial quinone substrates at the expense of either NADH or NADPH. The kinetic mechanism follows a ping-pong bi-bi reaction scheme, with K(M) values of 1.6-11 microm for various quinones. Dicoumarol and Cibacron Marine, two well-known inhibitors of the quinone reductase family, bind to Lot6p and inhibit its activity. In vivo experiments demonstrate that the enzymatic activity of Lot6p is consistent with the phenotype of both Deltalot6 and Lot6p overexpressing strains, suggesting that Lot6p may play a role in managing oxidative stress in yeast.

Coordinate regulation of Phase I and II xenobiotic metabolisms by the Ah receptor and Nrf2

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor with important roles in metabolic adaptation, normal physiology and dioxin toxicology. Metabolic adaptation is based on coordinate regulation of a set of xenobiotic-metabolizing enzymes (XMEs), termed AhR battery. Coordination is achieved by AhR/Arnt-binding to XREs (xenobiotic response elements), identified in the 5' upstream region of AhR target genes. The AhR battery encodes Phase I and II enzymes. Interestingly, these Phase II genes are linked to the Nrf2 gene battery that encodes enzymes that are essential in protection against oxidative/electrophile stress. Nrf2 binds to AREs (antioxidant response elements) in the regulatory region of a large and distinct set of target genes. Functionally characterized response elements such as XREs and AREs in the regulatory region of target genes may provide a genetic basis to understand AhR- and Nrf2-induced genes. Linkage between AhR and Nrf2 batteries is probably achieved by multiple mechanisms, including Nrf2 as a target gene of the AhR, indirect activation of Nrf2 via CYP1A1-generated reactive oxygen species, and direct cross-interaction of AhR/XRE and Nrf2/ARE signaling. Linkage appears to be species- and cell-dependent. However, mechanisms linking XRE- and ARE-controlled Phase II genes need further investigation. Tightened coupling between Phases I and II by AhR- and Nrf2-induced XMEs may greatly attenuate health risks posed by CYP1A1-generated toxic intermediates and reactive oxygen species. Better recognition of coordinate Phase I and II metabolisms may improve risk assessment of reactive toxic intermediates in the extrapolation to low level endo- and xenobiotic exposure.

The Chemopreventive Effect of Taxifolin Is Exerted through ARE-Dependent Gene Regulation

Phase II detoxification enzymes are responsible for the detoxification and elimination of activated carcinogens, and thus act as important biomarkers for chemoprevention. In this study, we tested the chemopreventive activity of taxifolin, a flavanon compound purified from a mongolian medicinal plant, by measuring quinone reductase (QR) activity in HCT 116 cells. Taxifolin induced significant QR activity, but displayed relatively low cytotoxicity in cells (chemoprevention index=5.75). To identify the target genes regulated by taxifolin, DNA microarray was performed with a 3K human cancer chip containing 3096 human genes associated with carcinogenesis. Significant analysis of microarray (SAM) revealed 428 differentially expressed (DE) genes as statistically significant, with a false discovery rate (FDR) of 57.2% (delta=0.3366). Sixty-five genes, including a few detoxification enzymes (NQO1, GSTM1) and an antioxidant enzyme (TXNRD1), were up-regulated and 363 genes were down-regulated in the presence of 60 microM taxifolin. In view of the finding that selected genes of interest contained antioxidant response element (ARE), we hypothesize that taxifolin modulates chemopreventive genes through activation of the ARE. Transient transfection experiments using the ARE QR-CAT construct demonstrate that taxifolin significantly activates ARE, but not xenobiotic response element (XRE). In conclusion, taxifolin acts as a potential chemopreventive agent by regulating genes via an ARE-dependent mechanism.

Mechanisms of a tumor marker, glutathione transferase P, expression during hepatocarcinogenesis of the rat

The molecular mechanism of any tumor marker expression may shed a light on the mechanism of the particular tumorigenesis. This idea in mind, we have been pursuing the mechanism of specific induction of the placental type glutathione transferase (GST-P) gene during hepatocarcinogenesis of the rat. Making use of advanced technologies of molecular biology including proteomic analysis, gene cloning and production of specific transgenic rats etc., we were able to identify the enhancer and the activator proteins responsible for this tumor marker expression. Negative regulatory regions and modulatory proteins were also found. The overview of this long range study and the future outlook of the problem will be discussed.

Glutathione peroxidase 2, the major cigarette smoke-inducible isoform of GPX in lungs, is regulated by Nrf2

Disruption of NF-E2-related factor (Nrf2), a redox-sensitive basic leucine zipper transcription factor, causes early-onset and more severe emphysema due to chronic cigarette smoke. Nrf2 determines the susceptibility of lungs to cigarette smoke-induced emphysema in mice through the transcriptional induction of numerous antioxidant genes. The lungs of Nrf2-/- mice have higher oxidative stress as evident from the increased levels of lipid peroxidation (4-hydroxy-2-nonenal) and oxidative DNA damage (7,8-dihydro-8-Oxo-2'deoxyguanosine) in response to cigarette smoke. Glutathione peroxidases (GPX) are the primary antioxidant enzymes that scavenge hydrogen peroxide and organic hydroperoxides. Among the five GPX isoforms, expression of GPX2 was significantly induced at both mRNA and protein levels in the lungs of Nrf2+/+ mice, in response to cigarette smoke. Activation of Nrf2 by specific knock down of the cytosolic inhibitor of Nrf2, Keap1, by small inhibitory RNA (siRNA) upregulated the expression of GPx2, whereas Nrf2 siRNA down-regulated the expression of GPX2 in lung epithelial cells. An ARE sequence located in the 5' promoter-flanking region of exon 1 that is highly conserved between mouse, rat, and human was identified. Mutation of this ARE core sequence completely abolished the activity of promoter-reporter gene construct. The binding of Nrf2 to the GPX2 antioxidant response element was confirmed by chromatin immunoprecipation, electrophoretic mobility shift assays, and site-directed mutagenesis. This study shows that GPX2 is the major oxidative stress-inducible cellular GPX isoform in the lungs, and that its basal as well as inducible expression is dependent on Nrf2.

Novel n-3 fatty acid oxidation products activate Nrf2 by destabilizing the association between Keap1 and Cullin3

Consumption of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) can mitigate the progression of diseases in which oxidative stress represents a common underlying biochemical process. Nrf2-regulated gene expression regulates detoxification of reactive oxygen species. EPA and DHA were subjected to an in vitro free radical oxidation process that models in vivo conditions. Oxidized n-3 fatty acids reacted directly with the negative regulator of Nrf2, Keap1, initiating Keap1 dissociation with Cullin3, thereby inducing Nrf2-directed gene expression. Liquid chromatography-tandem mass spectrometry analyses of oxidized EPA demonstrated the presence of novel cyclopentenone-containing molecules termed J3-isoprostanes in vitro and in vivo and were shown to induce Nrf2-directed gene expression. These experiments provide a biochemical basis for the hypothesis that formation of J-ring compounds generated from oxidation of EPA and DHA in vivo can reach concentrations high enough to induce Nrf2-based cellular defense systems.

Hepatic metabolism, phase I and II biotransformation enzymes in Atlantic salmon (Salmo Salar, L) during a 12 week feeding period with graded levels of the synthetic antioxidant, ethoxyquin

The synthetic antioxidant ethoxyquin (EQ) is a widely used additive in animal feeds, including farmed fish feed. The use of EQ as food additive is prohibited and it is also undesirable in farmed meat and fish products. The possible negative aspects of EQ in fish feeds, such as modulation of hepatic detoxifying enzymes and possible effects through "carry-over" to edible parts of fish are not known. In addition, the subsequent consequences for human consumers have not been previously studied. In the present work, the alteration in gene and protein expression patterns, and catalytic activities of phase I and II hepatic biotransformation enzymes due to prolonged exposure to graded levels of dietary EQ in the range of 11-1800 mg EQ/kg feed were studied. The kinetics of parent EQ and its major metabolite, ethoxyquin dimer (EQDM) was also studied. In general two weeks seem to be the critical point in the entire toxicological response of salmon to dietary consumed EQ. Biotransformation of EQ to EQDM is shown to be a rapid process. However, the decrease in biotransformation rate results in the accumulation of EQ metabolites, high concentration of which was postulated to alter translation and post-translational modification of CYP3A, GST and UDPGT at feeding day 14 and 42, with subsequent decreases in the biotransformation of consumed EQ. Decrease in the biotransformation of consumed EQ produced the retention of un-metabolized EQ rather than metabolites in salmon liver. This may be considered as undesirable effect, since it could lead to the transport and accumulation in other organs and edible tissues. It may also cause a new wave of biotransformation with formation of metabolites inhibiting detoxifying enzymes. In general, these processes may prolong the excretion of dietary EQ from the fish body and produce EQ-derived residues in the ready-to-consume salmon or fish products. These EQ residues may have higher toxicological effects for human consumers than the parent compound and therefore need to be studied in more detail.

Investigating the role of the aryl hydrocarbon receptor in benzene-initiated toxicity in vitro

Chronic occupational exposure to benzene has been correlated with aplastic aneamia and acute myelogenous leukemia, however mechanisms behind benzene toxicity remain unknown. Interestingly, benzene-initiated hematotoxicity is absent in mice lacking the aryl hydrocarbon receptor (AhR) suggesting an imperative role for this receptor in benzene toxicities. This study investigated two potential roles for the AhR in benzene toxicity using hepa 1c1c7 wild type and AhR deficient cells. Considering that many toxic effects of AhR ligands are dependent on AhR activation, our first objective was to determine if benzene, hydroquinone (HQ) or benzoquinone (BQ) could activate the AhR. Secondly, because the AhR regulates a number of enzymes involved in oxidative stress pathways, we sought to determine if the AhR had a role in HQ and BQ induced production of reactive oxygen species (ROS). Dual luciferase assays measuring dioxin response element (DRE) activation showed no significant change in DRE activity after exposure to benzene, HQ or BQ for 24h. Immunofluorescence staining showed cytosolic localization of the AhR after 2h incubations with benzene, HQ or BQ. Western blot analysis of cells exposed to benzene, HQ or BQ for 1, 12 and 24h did not demonstrate induction of CYP1A1 protein expression. Dichlorodihydrofluorescein staining of cells exposed to benzene, HQ or BQ revealed that the presence of the AhR did not affect BQ and HQ induced ROS production. These results indicate that the involvement of the AhR in benzene toxicity does not seem to be through classical activation of this receptor or through interference of oxidative stress pathways.

In vivo pharmacokinetics, activation of MAPK signaling and induction of phase II/III drug metabolizing enzymes/transporters by cancer chemopreventive compound BHA in the mice

Phenolic antioxidant butylated hydroxyanisole (BHA) is a commonly used food preservative with broad biological activities, including protection against chemical-induced carcinogenesis, acute toxicity of chemicals, modulation of macromolecule synthesis and immune response, induction of phase II detoxifying enzymes, as well as its undesirable potential tumor-promoting activities. Understanding the molecular basis underlying these diverse biological actions of BHA is thus of great importance. Here we studied the pharmacokinetics, activation of signaling kinases and induction of phase II/III drug metabolizing enzymes/transporter gene expression by BHA in the mice. The peak plasma concentration of BHA achieved in our current study after oral administration of 200 mg/kg BHA was around 10 microM. This in vivo concentration might offer some insights for the many in vitro cell culture studies on signal transduction and induction of phase II genes using similar concentrations. The oral bioavailability (F) of BHA was about 43% in the mice. In the mouse liver, BHA induced the expression of phase II genes including NQO-1, HO-1, gamma-GCS, GST-pi and UGT 1A6, as well as some of the phase III transporter genes, such as MRP1 and Slcolb2. In addition, BHA activated distinct mitogen-activated protein kinases (MAPKs), c-Jun N-terminal kinase (JNK), extracellular signal-regulated protein kinase (ERK), as well as p38, suggesting that the MAPK pathways may play an important role in early signaling events leading to the regulation of gene expression including phase II drug metabolizing and some phase III drug transporter genes. This is the first study to demonstrate the in vivo pharmacokinetics of BHA, the in vivo activation of MAPK signaling proteins, as well as the in vivo induction of Phase II/III drug metabolizing enzymes/transporters in the mouse livers.

Genome-wide screening of dioxin-responsive genes in fetal brain: bioinformatic and experimental approaches

Many of the effects of dioxins, which are potent environmental pollutants and teratogens, are mediated through the aryl hydrocarbon receptor, also known as the dioxin receptor. The purpose of the present study was to characterize dioxin-responsive genes in a comprehensive manner using two complementary approaches: bioinformatic analysis and microarray analysis. First, we characterized the overall distribution of the cis-regulatory element for the dioxin-responsive element sequence (DRE) 'gcgtg' within putative promoter regions. We assembled the upstream sequences 10 kb from the transcription start site and evaluated their location and frequency in the human and mouse genomes. Second, we characterized the expression profile of mouse embryonic day 12 fetal brain exposed to 2,3,7,8-tetrarchlorodibenzo-p-dioxin. The distributions of 26,680 DREs among 2,843 human genes and 98,711 DREs among 18,541 mouse genes were examined. In both species, the DREs tended to be located close to the transcription start site. Forty genes exhibited significant induction or repression following dioxin exposure in fetal mice. The set of genes exhibited a strong functional coherence, with statistically significant enrichment in organogenesis and the DNA-dependent regulation of transcription, according to Gene Ontology annotations. In both humans and mice, DREs were preferentially distributed close to transcription start sites. Evolutionary conservation of this unique DRE distribution pattern suggests that DREs may be involved in transcriptional regulation. In mice, prenatal dioxin exposure altered the expression of 10 transcription factors, many of which have been documented to play a role in organogenesis. These genes may represent potential mediators of dioxin's effects in fetal tissues.

Bucillamine induces glutathione biosynthesis via activation of the transcription factor Nrf2

The properties of bucillamine, a synthetic antioxidant, have been attributed mainly to the donation of thiol groups to glutathione (GSH). We recently demonstrated that glutamate-cysteine ligase catalytic subunit (GCLC), the rate-limiting enzyme of GSH biosynthesis, and the multidrug-resistance-associated protein 2 (Mrp2/MRP2) are coordinately induced in response to xenobiotic through the activation of the antioxidant-response element (ARE) by nuclear factor-erythroid 2 p45-related factor (Nrf2). We tested the hypothesis that bucillamine and its oxidized metabolite SA 981 also activate the Nrf2 pathway, thereby increasing glutathione biosynthesis in human HepG2 and murine Hepa 1-6 hepatoma cell lines, through the induction of the GCLC enzyme as well as the Mrp2/MRP2 transporter, which mediates the excretion of glutathione and its conjugates from hepatocytes. Both bucillamine and SA 981 produced a significant dose-dependent increase in the mRNA levels of Mrp2/MRP2 and GCLC after 24 h. The levels of the transcription factor Nrf2 in the nuclei were maximal at 3 h, remained elevated at 6 h, and decreased to control values at 24 h in both cell lines. Moreover, both bucillamine and SA 981 significantly increased the expressions of Mrp2/MRP2 and GCLC proteins in both cell lines. Finally, in both cell lines, bucillamine and SA 981 increased the GSH content two- to three-fold. These results demonstrate that bucillamine and SA 981 activate the ARE-ARE pathway increasing the expression of ARE-driven genes such as those of GCLC and Mrp2/MRP2. The role of bucillamine as a chemopreventive agent against cancer remains to be elucidated.

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.

Using U0126 to dissect the role of the extracellular signal-regulated kinase 1/2 (ERK1/2) cascade in the regulation of gene expression by endothelin-1 in cardiac myocytes

The hypertrophic agonist endothelin-1 rapidly but transiently activates the extracellular signal-regulated kinase 1/2 (ERK1/2) cascade (and other signalling pathways) in cardiac myocytes, but the events linking this to hypertrophy are not understood. Using Affymetrix rat U34A microarrays, we identified the short-term (2-4 h) changes in gene expression induced in neonatal myocytes by endothelin-1 alone or in combination with the ERK1/2 cascade inhibitor, U0126. Expression of 15 genes was significantly changed by U0126 alone, and expression of an additional 78 genes was significantly changed by endothelin-1. Of the genes upregulated by U0126, four are classically induced through the aryl hydrocarbon receptor (AhR) by dioxins suggesting that U0126 activates the xenobiotic response element in cardiac myocytes potentially independently of effects on ERK1/2 signalling. The 78 genes showing altered expression with endothelin-1 formed five clusters: (i) three clusters showing upregulation by endothelin-1 according to time course (4 h > 2 h; 2 h > 4 h; 2 h approximately 4 h) with at least partial inhibition by U0126; (ii) a cluster of 11 genes upregulated by endothelin-1 but unaffected by U0126 suggesting regulation through signalling pathways other than ERK1/2; (iii) a cluster of six genes downregulated by endothelin-1 with attenuation by U0126. Thus, U0126 apparently activates the AhR in cardiac myocytes (which must be taken into account in protracted studies), but careful analysis allows identification of genes potentially regulated acutely via the ERK1/2 cascade. Our data suggest that the majority of changes in gene expression induced by endothelin-1 are mediated by the ERK1/2 cascade.

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

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

BRCA1 Modulates Xenobiotic Stress-inducible Gene Expression by Interacting with ARNT in Human Breast Cancer Cells

Previously, we have reported that BRCA1 regulates the expression of various classes of genes, including genes involved in xenobiotic stress responses (Bae, I., Fan, S., Meng, Q., Rih, J. K., Kim, H. J., Kang, H. J., Xu, J., Goldberg, I. D., Jaiswal, A. K., and Rosen, E. M. (2004) Cancer Res. 64, 7893-7909). In the present study, we have investigated the effects of BRCA1 on xenobiotic stress-inducible gene expression. In response to aryl hydrocarbon receptor (AhR) ligands, cytoplasmic AhR becomes activated and then translocates to the nucleus where it forms a complex with the aryl hydrocarbon receptor nuclear translocator (ARNT). Subsequently, the AhR.ARNT complex binds to the enhancer or promoter of genes containing a xenobiotic stress-responsive element and regulates the expression of multiple target genes including cytochrome P450 subfamily polypeptide 1 (CYP1A1). In this study, we have found that endogenous and overexpressed exogenous wild-type BRCA1 affect xenobiotic stress-induced CYP1A1 gene expression. Using a standard chromatin immunoprecipitation assay, we have demonstrated that BRCA1 is recruited to the promoter regions of CYP1A1 and CYP1B1 along with ARNT and/or AhR following xenobiotic exposure. Our findings suggest that BRCA1 may be physiologically important for mounting a normal response to xenobiotic insults and that it may function as a coactivator for ARNT activity. Using immunoprecipitation, Western blotting, and glutathione S-transferase capture assays, a xenobiotic-independent interaction between BRCA1 and ARNT has been identified, although it is not yet known whether this is a direct or indirect interaction. We have also found that the inducibility of CYP1A1 and CYP1B1 transcripts following xenobiotic stress was significantly attenuated in BRCA1 knockdown cells. This reduced inducibility is associated with an altered stability of ARNT and was almost completely reversed in cells transfected with an ARNT expression vector. Finally, we have found that xenobiotic (TCDD) treatments of breast cancer cells containing reduced levels of BRCA1 cause the transcription factor ARNT to become unstable.

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

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

Nrf2: a potential molecular target for cancer chemoprevention by natural compounds

One of the most prominent strategies of cancer chemoprevention might be protecting cells or tissues against various carcinogens and carcinogenic metabolites derived from exogenous or endogenous sources. This protection could be achieved through the induction of phase 2 detoxifying enzymes and antioxidant enzymes such as glutathione S-transferase, NAD(P)H quinone oxidoreductase 1, and heme oxygenase-1, a process that is mediated mainly by the antioxidant response elements (ARE) within the promoter regions of these genes. Nuclear factor-erythroid 2-related factor 2 (Nrf2), a member of the Cap 'n' collar (CNC) family of basic region-leucine zipper transcription factors, plays a key role in ARE-mediated gene expression. Under normal condition, Nrf2 is sequestered in the cytoplasm by an actin-binding protein, Kelch-like ECH associating protein 1 (Keap1), and upon exposure of cells to inducers such as oxidative stress and certain chemopreventive agents, Nrf2 dissociates from Keap1, translocates to the nucleus, binds to AREs, and transactivates phase 2 detoxifying and antioxidant genes. Several upstream signaling pathways including mitogen-activated protein kinases, protein kinase C, phosphatidylinositol 3-kinase, and transmembrane kinase are implicated in the regulation of Nrf2/ARE activity. Furthermore, many natural chemopreventive agents are known to induce Nrf2/ARE-dependent gene expression, also in part by regulating the turnover of the Nrf2 protein itself. This review discusses our current understanding of the Nrf2/ARE pathway as a potential molecular target for cancer chemoprevention, as well as the feasibility of screening natural compounds for activation of this pathway and as potential cancer preventive agents for human use.

Increased expression of nuclear factor E2 p45-related factor 2 (NRF2) in head and neck squamous cell carcinomas

BACKGROUND

Head and neck squamous cell carcinoma (HNSCC) continues to cause significant morbidity and mortality. Overexpression of specific phase II gene products may represent an important biomarker. One regulator of phase II gene expression is the transcription factor nuclear factor E2 p45-related factor 2 (Nrf2). Nrf2 expression was evaluated in HNSCC, to determine whether it might serve as a biomarker for early detection of disease.

METHODS

A tissue microarray was constructed of 141 HNSCC biopsy cores from 47 HNSCCs. In addition, histologically normal squamous mucosa was obtained from 7 patients. Protein expression was evaluated by immunohistochemistry.

RESULTS

Nrf2 expression was increased in 91.5% of tumors. Expression of thioredoxin, a Nrf2-reguated gene product, was elevated in 75% of tumors. Keap1, which regulates the rate of Nrf2 ubiquitination and degradation, was relatively overexpressed in HNSCC compared with normal mucosa.

CONCLUSIONS

Nrf2 expression may be a possible HNSCC candidate biomarker.

A speed congenic rat strain bearing the tongue cancer susceptibility locus Tscc1 from Dark-Agouti rats

We previously reported that Dark-Agouti (DA) rats are highly susceptible to 4-nitroquinoline 1-oxide (4NQO)-induced tongue cancer (TC), whereas Wistar/Furth (WF) rats are barely susceptible. Linkage analysis of reciprocal (DAxWF)F2 rats demonstrated five quantitative trait loci, Tongue squamous cell carcinoma 1-5 (Tscc1-5) determining the size and number of the TCs. The major susceptibility locus Tscc1 is mapped on rat chromosome 19. In the present study, we used a marker-assisted speed congenic procedure to construct WF.DA-Tscc1 (WF-T1D) rats, i.e. WF rats carrying a DA-derived Tscc1 chromosomal segment, and evaluated the effect of a single Tscc1 on 4NQO-induced tongue carcinogenesis. In WF-T1D rats, the incidence, number and size of 4NQO-induced TCs were significantly higher than those in WF rats, indicating that the introgressed segment contains one of the susceptibility loci for 4NQO-induced TCs from DA rats. Detection of a single nucleotide polymorphism in NQO1, one of the Tscc1 candidate genes, enabled us to map NQO1 in the Tscc1 segment between D19Wox8 and D19Wox7 on chromosome 19. Possible relevance of NQO1 polymorphism to TC susceptibility is discussed.

Identification of Sensor Cysteines in Human Keap1 Modified by the Cancer Chemopreventive Agent Sulforaphane

The chemopreventive agent sulforaphane is an isothiocyanate derived from cruciferous vegetables. Sulforaphane exerts cancer chemopreventive effects by inducing antioxidant/electrophile response element (ARE)-regulated phase 2 enzyme and antioxidant genes through activation of the transcription factor nuclear factor-E2-related factor 2 (Nrf2), which is regulated by the thiol-rich sensor protein Kelch-like ECH-associated protein 1 (Keap1). Sulforaphane is an electrophile that can react with protein thiols to form thionoacyl adducts. We hypothesized that, like other electrophilic Nrf2 activators, sulforaphane activates this system through specific modifications of the Keap1 protein. However, thionoacyl adducts are labile to hydrolysis and transacylation reactions, which complicate the identification of the sulforaphane adduct sites on Keap1. In this study, we characterized the stability of sulforaphane thionoacyl adducts and developed a liquid chromatography-tandem mass spectrometry method to map labile sulforaphane adduct sites formed on Keap1 in vitro. Sulforaphane displays a distinctly different pattern of Keap1 modification than previously studied ARE inducers that modify Keap1 by alkylation. Sulforaphane modified Keap1 most readily in the Kelch domain, rather than in the central linker domain, which is targeted by previously characterized ARE inducers. Also, in contrast to previously studied ARE inducers and as reported recently [Zhang, et al. (2005) J. Biol. Chem. 280, 30091-30099], sulforaphane treatment in vivo does not lead to the accumulation of ubiquitinated Keap1. Our observations suggest a novel mechanism for Nrf2 stabilization by sulforaphane-Keap1 thionoacyl adduct formation.

Molecular mechanism of nrf2 activation by oxidative stress

The capacity of cells to maintain homeostasis during oxidative stress resides in activation or induction of protective enzymes. Nuclear-factor-E2-related factor (Nrf)-2 as a member of bZIP transcription factors is expressed in a variety of tissues. Transcriptional activation of antioxidant genes through an antioxidant response element (ARE) is largely dependent upon Nrf2. The genes that contain a functional ARE include those encoding GSTA1, GSTA2, NAD(P)H:quinone reductase, and gamma-glutamylcysteine synthetase heavy and light subunits that play a role in defense against oxidative stress. Previously, we showed that phosphatidylinositol 3-kinase (PI3-kinase) controls nuclear translocation of Nrf2 in response to oxidative stress, which involves rearrangement of actin microfilaments. Now, we report that PI3-kinase is responsible for the rise of cellular Ca(2+), which is requisite for nuclear translocation of Nrf2. Immunocytochemistry and subcellular fractionation analyses revealed that Nrf2 relocated from the cytoplasm to the plasma membrane prior to its nuclear translocation. We further found that CCAAT/enhancer binding protein-beta (C/EBPbeta), peroxisome proliferatoractivated receptor-gamma (PPARgamma), and retinoid X receptor (RXR) heterodimer serve as the activating transcription factors for the phase II gene induction. Hence, PI3-kinase-mediated Nrf2 activation in combination with activating PPARgamma-RXR and C/EBPbeta contributes to antioxidant phase II enzyme induction via coordinate gene transactivation.

A transient treatment of hippocampal neurons with alpha-tocopherol induces a long-lasting protection against oxidative damage via a genomic action

Neuroprotection exerted by alpha-tocopherol against oxidative stress was investigated in cultured rat hippocampal neurons. In addition to its direct action as a radical scavenger revealed at concentrations above 10 microM, a transient application of 1 microM alpha-tocopherol phosphate (alpha-TP) to neurons induced a complete delayed long-lasting protection against oxidative insult elicited by exposure to Fe2+ ions, but not against excitotoxicity. A minimal 16-h application of alpha-TP was required to observe the protection against subsequent oxidative stress. This delayed protection could last up to a week after the application of alpha-TP, even when medium was changed after the alpha-TP treatment. Cycloheximide, added either 2 h before or together with alpha-TP, prevented the delayed neuroprotection, but not the acute. However, cycloheximide applied after the 16-h alpha-TP pretreatment did not alter the delayed neuroprotection. Neither Trolox, a cell-permeant analogue of alpha-tocopherol, nor other antioxidants, such as epigallocatechin-gallate and N-acetyl-L-cysteine, elicited a similar long-lasting protection. Only tert-butylhydroquinone could mimic the alpha-TP effect. Depletion of glutathione (GSH) by L-buthionine sulfoximine did not affect the delayed alpha-TP protection. Thus, in addition to its acute anti-radical action, alpha-TP induces a long-lasting protection of neurons against oxidative damage, via a genomic action on antioxidant defenses apparently unrelated to GSH biosynthesis.

DNA and mRNA elements with complementary responses to hemin, antioxidant inducers, and iron control ferritin-L expression

Ferritins, an ancient family of protein nanocages, concentrate iron in iron-oxy minerals for iron-protein biosynthesis and protection against oxy radical damage. Of the two genetic mechanisms that regulate rates of ferritin-L synthesis, DNA transcription and mRNA translation, more is known about mRNA regulation where iron targets complexes of an mRNA structure, the iron-responsive element (IRE) sequence, and ferritin IRE repressors (iron regulatory proteins 1 and 2). Neither the integration of mRNA and DNA regulation nor the ferritin-L DNA promoter are well studied. We now report the combined effects of DNA transcription and mRNA translation regulation of ferritin-L synthesis. First, the promoter of human ferritin-L, encoding the animal-specific subunit associated with human diseases, was identified, and contained an overlapping Maf recognition element (MARE) and antioxidant responsive element (ARE) that was positively regulated by tert-butylhydroquinone, sulforaphane, and hemin with responses comparable to thioredoxin reductase (ARE regulator) or quinone reductase (MARE/ARE regulator). Iron, a poor regulator of the ferritin-L promoter, was 800 times less effective than sulforaphane. Combining the ferritin-L MARE/ARE and IRE produced a response to hemin that was 3-fold greater than the sum of responses of the MARE/ARE or IRE alone. Regulation of ferritin-L by a MARE/ARE DNA sequence emphasizes the importance of ferritin-L in oxidative stress that complements the mRNA regulation in iron stress. Combining DNA and mRNA mechanisms of regulation, as for ferritin-L, illustrates the advantages of using two types of genetic targets to achieve sensitive responses to multiple signals.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) alters the mRNA expression of critical genes associated with cholesterol metabolism, bile acid biosynthesis, and bile transport in rat liver: a microarray study

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent hepatotoxin that exerts its toxicity through binding to the aryl hydrocarbon receptor (AhR) and the subsequent induction or repression of gene transcription. In order to further identify novel genes and pathways that may be associated with TCDD-induced hepatotoxicity, we investigated gene changes in rat liver following exposure to single oral doses of TCDD. Male Sprague-Dawley rats were administered single doses of 0.4 microg/kg bw or 40 microg/kg bw TCDD and killed at 6 h, 24 h, or 7 days, for global analyses of gene expression. In general, low-dose TCDD exposure resulted in greater than 2-fold induction of genes coding for a battery of phase I and phase II metabolizing enzymes including CYP1A1, CYP1A2, NADPH quinone oxidoreductase, UGT1A6/7, and metallothionein 1. However, 0.4 microg/kg bw TCDD also altered the expression of Gadd45a and Cyclin D1, suggesting that even low-dose TCDD exposure can alter the expression of genes indicative of cellular stress or DNA damage and associated with cell cycle control. At the high-dose, widespread changes were observed for genes encoding cellular signaling proteins, cellular adhesion, cytoskeletal and membrane transport proteins as well as transcripts coding for lipid, carbohydrate and nitrogen metabolism. In addition, decreased expression of cytochrome P450 7A1, short heterodimer partner (SHP; gene designation nr0b2), farnesyl X receptor (FXR), Ntcp, and Slc21a5 (oatp2) were observed and confirmed by RT-PCR analyses in independent rat liver samples. Altered expression of these genes implies major deregulation of cholesterol metabolism and bile acid synthesis and transport. We suggest that these early and novel changes have the potential to contribute significantly to TCDD induced hepatotoxicity and hypercholesterolemia.

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.

Redox regulation of the transcriptional repressor Bach1

Bach1 is a transcriptional repressor of heme oxygenase-1, one of the most inducible phase 2 proteins. Bach1 binds in conjunction with a small Maf protein to tandem repeats of the antioxidant response element (ARE) and quenches the target gene expression. On the other hand, the transactivator Nrf2 binds and up-regulates the ARE-governed gene expression. By using a sulfhydryl oxidizing agent, diamide, here we provide evidence which indicates that the Bach1 function is regulated by the redox state. Diamide showed restricted Nrf2 nuclear translocation and ARE-driven reporter activity but reversed the ARE transcriptional activity suppressed by ectopically expressed Bach1. Substitution of the conserved cysteine residue in the DNA binding domain of Bach1 to serine (C574S mutant) caused a refractory response to the diamide-mediated reactivation of the Bach1-suppressed reporter activity. Moreover, diamide induced cytoplasmic translocation of the GFP-Bach1 fusion protein but failed to translocate the fusion protein consisting of the C574S mutant. These data suggest that redox regulation of Bach1 is an alternative mechanism to induce multiple ARE-governed genes.

Induction of NAD(P)H Quinone Oxidoreductase by Vegetables Widely Consumed in Catalonia, Spain

Monofunctional inducers (MIs) enhance phase 2 enzymes such as nicotinamide-adenine-dinucleotide-phosphate [NAD(P)H] quinone oxidoreductase (NQO1) without modifying oxidation enzymes. The induction of these protective enzymes appears to be mediated by genetic regulatory elements in their promoter regions known as the antioxidant response element (ARE). The aim of this study was to identify, through an in vitro study, which of the 30 fruits and vegetables commonly consumed in Catalonia, Spain, contain MIs of NQO1. We assayed the capacity of extracts of these fruits and vegetables to induce NQO1 [by more than 1.5-fold: ratio of induction (cells treated/control) >1.5, 8-mg/ml dose] in two murine hepatoma cell lines: Hepa 1c1c7 and BPrC1, a modified cell line that possesses a nonfunctional aryl hydrocarbon receptor nuclear translocator system and is thus nonresponsive to bifunctional inducers. We also used a third cell line, papiloma (PE) murine keratinocytes, a stably transfected cell line with an ARE-luc+ plasmid (AREPE cell line) for verifying induction through the ARE with a simple luminescence screening assay. Broccoli (Hepa 1c1c7, ratio=5.5; BPrC1, ratio=2.3), calcot (Allium cepa L.) (Hepa 1c1c7, ratio=4.7; BPrC1, ratio=.5), green onion (Hepa 1c1c7, ratio=4.6; BPrC1, ratio=2), green cabbage (Hepa 1c1c7, ratio=3.6; BPrC1, ratio=2.7), purple cabbage (Hepa 1c1c7, ratio=3.4; BPrC1, ratio=2), and black cabbage (Hepa 1c1c7, ratio=3; BPrC1, ratio=3) were active NQO1 inducers in both murine hepatoma cell lines. Extracts from broccoli (ratio=3.5), calcot (ratio=4.8), cauliflower (ratio=4.2), cabbage (ratio=2.2), green onion (ratio=3.2), green cabbage (ratio=3.6), black cabbage (ratio=4.5), and purple cabbage (ratio=3.7) were confirmed to contain MIs in the AREPE cell line. These results are very similar to those described for vegetables consumed in the United States, with the exception of calcot, which is common in Catalonia but is not grown or consumed widely in the United States.

Transcriptional regulation of NF-E2 p45-related factor (NRF2) expression by the aryl hydrocarbon receptor-xenobiotic response element signaling pathway: direct cross-talk between phase I and II drug-metabolizing enzymes

The aryl hydrocarbon receptor (AHR) and NF-E2 p45-related factor (NRF2) are two distinct transcription factors involved in the regulation of drug-metabolizing enzymes. Increasing evidence from several studies implies that AHR and NRF2 have direct links, but the molecular mechanism remains unknown. In this work we demonstrate for the first time that Nrf2 gene transcription is directly modulated by AHR activation. DNA sequence analyses of the mouse Nrf2 promoter revealed one xenobiotic response element (XRE)-like element (XREL1) located at -712 and two additional XRE-like elements located at +755 (XREL2) and +850 (XREL3). Functional analysis using luciferase assay showed that XREL1, XREL2, and XREL3 are all inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment, with XREL2 being the most potent. The functionality of these XRE-like elements was further confirmed by mutagenesis and gel shift experiments. Finally, we used chromatin immunoprecipitation assay to show a direct binding of AHR to the Nrf2 promoter. Cells with silenced AHR expression using siRNA also lost NRF2 mRNA induction by 2,3,7,8-tetrachlorodibenzo-p-dioxin. These new data position NRF2-antioxidant response element downstream in the AHR-XRE pathway. Moreover, direct regulation of NRF2 by AHR contributes to couple phase I and II enzymes into an integrated system facilitating more effective xenobiotic and carcinogen detoxification.

The hypoxia-inducible factor and tumor progression along the angiogenic pathway

The hypoxia-inducible factor (HIF) is a transcription factor that plays a key role in the response of cells to oxygen levels. HIF is a heterodimer of alpha- and beta-subunits where the alpha-subunit is translated constitutively but has a very short half-life under normal oxygen concentrations. Negative regulation of the half-life and activity of the alpha-subunit is dependent on its posttranslational hydroxylation by hydroxylases that are dependent on oxygen for activity. Thus under low oxygen (hypoxic) conditions the hydroxylases are inactive and the alpha-subunit is stable and able to interact with the beta-subunit to bind and induce transcription of target genes. Hypoxic conditions are encountered in development and in disease states such as cancer. Tumors that have outstripped their blood supply become hypoxic and express high levels of HIF. HIF in turn targets genes that induce survival, glycolysis, and angiogenesis, a form of neovascularization, which ensures the tumor with a continued supply of oxygen and nutrients for further growth.

Smad3-ATF3 signaling mediates TGF-beta suppression of genes encoding Phase II detoxifying proteins

This study provides evidence that in mammary epithelial cells the pluripotent cytokine TGF-beta1 repressed expression of multiple genes involved in Phase II detoxification. GCLC, the gene that encodes the catalytic subunit of the enzyme glutamate cysteine ligase, the rate-limiting enzyme in the biosynthesis of glutathione, was used as a molecular surrogate for investigating the mechanisms by which TGF-beta suppressed Phase II gene expression. TGF-beta was found to suppress luciferase reporter activity mediated by the human GCLC proximal promoter, as well as reporter activity mediated by the GCLC antioxidant response element, ARE4. TGF-beta downregulated expression of endogenous GCLC mRNA and GCLC protein. TGF-beta suppression of the Phase II genes correlated with a decrease in cellular glutathione and an increase in cellular reactive oxygen species. Ectopic expression of constitutively active Smad3E was sufficient to inhibit both reporters in the absence of TGF-beta, whereas dominant negative Smad3A blocked TGF-beta suppression. Smad3E suppressed Nrf2-mediated activation of the GCLC reporter. We demonstrate that TGF-beta increased ATF3 protein levels, as did transient overexpression of Smad3E. Ectopic expression of ATF3 was sufficient to suppress the GCLC reporter activity, as well as endogenous GCLC expression. These results demonstrate that Smad3-ATF3 signaling mediates TGF-beta repression of ARE-dependent Phase II gene expression and potentially provide critical insight into mechanisms underlying TGF-beta1 function in carcinogenesis, tissue repair, and fibrosis.

Regulation of GST-P gene expression during hepatocarcinogenesis

Placental glutathione S-transferase (GST-P), a member of glutathione S-transferase, is known for its specific expression during rat hepatocarcinogenesis and has been used as a reliable tumor marker for experimental rat hepatocarcinogenesis. To explain the molecular mechanism underlying its specific expression concomitant with the malignant transformation, we have analyzed the regulatory element of the GST-P gene and the transcription factor that binds to this element. From the extensive analyses by the establishment of the transgenic rat lines having various regions of GST-P gene, we could identify the GPE1 as an essential enhancer element for specific GST-P expression. Next, we examined the transcription factor that binds and activates the GPE1, specifically in the early stage of hepatocarcinogenesis and in the hepatoma. Electrophoresis gel mobility shift assay, reporter transfection analysis, and the chromatin immunoprecipitation analysis indicate that the Nrf2/MafK heterodimer binds and activates GPE1 element in preneoplastic lesions and hepatomas but not in the normal liver cells. In this chapter, we describe details of the transgenic rat analyses and the identification of a factor responsible for the specific expression of the GST-P gene and discuss a possible molecular scenario for malignant transformation and tumor marker gene expression.

Nrf2 deficiency improves autoimmune nephritis caused by the fas mutation lpr

BACKGROUND

Nrf2 is a basic leucine zipper transcriptional activator essential for the coordinate transcriptional induction of antioxidant and phase II drug metabolizing enzymes. We previously reported that Nrf2-deficient female mice develop lupus-like autoimmune nephritis (Kidney Int 60:1343-1353, 2001). The result suggested that nrf2 is a possible candidate gene in determining susceptibility to autoimmune diseases. MRL/lpr mice, defective in Fas-mediated apoptosis, develop glomerulonephritis due to the production of autoantibodies.

METHODS

To investigate the mechanism whereby Nrf2 contributes to the susceptibility to autoimmune diseases, we generated nrf2-/-lpr/lpr mice.

RESULTS

Unexpectedly, the lifespan of nrf2-/-lpr/lpr female mice was markedly prolonged and these mice showed an improvement in nephritis compared to nrf2+/+lpr/lpr female mice. Immunologic abnormalities and hypergammaglobulinemia were also alleviated in nrf2-/-lpr/lpr female mice. Furthermore, lymphadenopathy was suppressed as a result of increased apoptosis. To elucidate the molecular mechanism causing a stimulation of apoptosis, we analyzed the response made by nrf2-/-lpr/lpr mice to death signals. We show that nrf2-/-lpr/lpr mice are sensitive to tumor necrosis factor-alpha (TNF-alpha)-mediated apoptosis. Since intracellular glutathione levels are decreased in Nrf2-deficient cells, it is probable that a prolonged depletion in glutathione levels leads to the enhancement in TNF-alpha-mediated apoptosis.

CONCLUSION

These results indicate that a deficiency in Nrf2 enhances TNF-alpha-mediated apoptosis which in-turn ameliorates the abnormal apoptotic response that arises from a mutation in the lpr gene. Therefore, Nrf2 deficiency acts as a suppressor of the autoimmune accelerating gene lpr.