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

Interactions between aryl hydrocarbon receptor (AhR) and hypoxia signaling pathways

Most if not all of the toxic responses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) are mediated through the AhR, which requires ARNT to regulate gene expression. ARNT is also required by HIF-1alpha to enhance the expression of various genes in response to hypoxia. Since both the AhR and hypoxia transcriptional pathways require ARNT, some of the effects of TCDD and similar types of ligands could be explained by interaction between the AhR and hypoxia pathways involving ARNT. The studies on which we report here were conducted to test the hypothesis that there is cross talk between AhR- and HIF-1-mediated transcription pathways. TCDD significantly reduced the hypoxia-mediated reporter gene activity in B-1 cells. Reciprocally, the hypoxia response inducers desferrioxamine or CoCl(2) inhibited AhR-mediated CYP1A1 enzyme activity in B-1 and Hepa 1 cells, and the AhR-mediated luciferase reporter gene activity in H1L1.1c2 cells. The inhibition of AhR-mediated transcription by hypoxia inducers, however, was not observed in H4IIE-luc cells. The interaction between the AhR- and HIF-1-mediated transcription can be attributed to changes in DNA binding activities. TCDD-induced protein binding to dioxin responsive element (DRE) was diminished by desferrioxamine, and TCDD reduced the binding activity to HIF-1 binding site in desferrioxamine-treated Hepa 1 cells. This mutual repression may provide an underlying mechanism for many TCDD-induced toxic responses. The results reported here indicate that there is cross talk between ARNT-requiring pathways. Since ARNT is possibly required by a number of pathways, this type of interaction may explain some of the pleiotropic effects caused by TCDD.

Polymorphic electrophile response elements in the mouse glutathione S-transferase GSTa1 gene that confer increased induction

Induced transcription of a battery of stress response genes in mammals, including several phase I and phase II drug-metabolizing enzymes, is regulated by the electrophile responsive element (EpRE). Because previous directed mutagenesis of nucleotide motifs within the large, composite EpRE were shown to affect transcription factor binding and associated induced expression of dependent genes, we hypothesized that naturally-occurring variation or polymorphism in the EpRE sequence, if found, could affect the induced expression of important protective genes like glutathione S-transferases, and that this could be an important determinant of cancer risk in humans and other mammals. To determine whether this occurred in nature, 32 strains and species of inbred mice were screened to examine the EpRE sequence present in the mGSTa1 promoter. Two species, Mus caroli and Mus spretus, showed TGAC-->TGGC mutations in the tandem TGAC motif. Inducibility (15-fold) of the variant Mus spretus EpRE sequence in a reporter gene construct in HepG2 cells was significantly increased versus the wild-type EpRE sequence (8-fold). A comparison of mGSTa1-induced expression in the livers of Mus spretus, Mus caroli, and BALB/cJ mice showed the highest level of mGSTa1 mRNA in livers from the Mus spretus and Mus carolimice. This naturally-occurring polymorphism within the EpRE domain is the first mutation with an associated phenotype to be reported within a promoter regulatory element of a drug metabolizing gene.

Impact of cellular metabolism on the biological effects of benzo[a]pyrene and related hydrocarbons

Polycyclic aromatic hydrocarbons are ubiquitous contaminants in the environment. Benzo[a]pyrene (BaP), a prototypical member of this class of chemicals, has been extensively studied for its toxic effects in laboratory animals and human populations. BaP toxicity is often mediated by oxidative metabolism to reactive intermediates that interact with macromolecules leading to alterations in target cell structure and function. More recent evidence suggests that disruption of cellular signaling pathways involved in the regulation of growth and differentiation contribute significantly to the toxicity of BaP and its metabolites. This review summarizes recent advances in our understanding of biological mechanisms of BaP toxicity at the molecular level, and the role of metabolic intermediates in carcinogenesis, atherogenesis, and teratogenesis.

The reactive oxygen species--and Michael acceptor-inducible human aldo-keto reductase AKR1C1 reduces the alpha,beta-unsaturated aldehyde 4-hydroxy-2-nonenal to 1,4-dihydroxy-2-nonene

The human aldo-keto reductase AKR1C1 (20alpha(3alpha)-hydroxysteroid dehydrogenase) is induced by electrophilic Michael acceptors and reactive oxygen species (ROS) via a presumptive antioxidant response element (Burczynski, M. E., Lin, H. K., and Penning, T. M. (1999) Cancer Res. 59, 607-614). Physiologically, AKR1C1 regulates progesterone action by converting the hormone into its inactive metabolite 20alpha-hydroxyprogesterone, and toxicologically this enzyme activates polycyclic aromatic hydrocarbon trans-dihydrodiols to redox-cycling o-quinones. However, the significance of its potent induction by Michael acceptors and oxidative stress is unknown. 4-Hydroxy-2-nonenal (HNE) and other alpha,beta-unsaturated aldehydes produced during lipid peroxidation were reduced by AKR1C1 with high catalytic efficiency. Kinetic studies revealed that AKR1C1 reduced HNE (K(m) = 34 microm, k(cat) = 8.8 min(-1)) with a k(cat)/K(m) similar to that for 20alpha-hydroxysteroids. Six other homogeneous recombinant AKRs were examined for their ability to reduce HNE. Of these, AKR1C1 possessed one of the highest specific activities and was the only isoform induced by oxidative stress and by agents that deplete glutathione (ethacrynic acid). Several hydroxysteroid dehydrogenases of the AKR1C subfamily catalyzed the reduction of HNE with higher activity than aldehyde reductase (AKR1A1). NMR spectroscopy identified the product of the NADPH-dependent reduction of HNE as 1,4-dihydroxy-2-nonene. The K(m) of recombinant AKR1C1 for nicotinamide cofactors (K(m) NADPH approximately 6 microm, K(m)(app) NADH >6 mm) suggested that it is primed for reductive metabolism of HNE. Isoform-specific reverse transcription-polymerase chain reaction showed that exposure of HepG2 cells to HNE resulted in elevated levels of AKR1C1 mRNA. Thus, HNE induces its own metabolism via AKR1C1, and this enzyme may play a hitherto unrecognized role in a response mounted to counter oxidative stress. AKRs represent alternative GSH-independent/NADPH-dependent routes for the reductive elimination of HNE. Of these, AKR1C1 provides an inducible cytosolic barrier to HNE following ROS exposure.

TCF11/Nrf1 overexpression increases the intracellular glutathione level and can transactivate the gamma-glutamylcysteine synthetase (GCS) heavy subunit promoter

Gamma-glutamylcysteinylglycine or glutathione (GSH) performs important protective functions in the cell through maintenance of the intracellular redox balance and elimination of xenobiotics and free radicals. The production of GSH involves a number of enzymes and enzyme subunits offering multiple opportunities for regulation. Two members of the CNC subfamily of bZIP transcription factors (TCF11/Nrf1 and Nrf2) have been implicated in the regulation of detoxification enzymes and the oxidative stress response. Here we investigate the potential role of one of these factors, TCF11/Nrf1, in the regulation of GSH levels in the cell and particularly its influence on the expression of one of the enzymatic components necessary for the synthesis of GSH, the heavy subunit of gamma-glutamylcysteine synthetase (GCS(h)). Using overexpression of the transcription factor in COS-1 cells we show that TCF11/Nrf1 stimulates GSH accumulation. Using co-transfection with reporter constructs where reporter expression is driven through the GCS(h) promoter we show that this increase may be mediated in part by induced expression of the GCS(h) gene by TCF11/Nrf1. We further show that a distal portion of the promoter including two antioxidant-response elements (AREs) predominantly mediates the TCF11/Nrf1 transactivation and an electromobility shift assay showed that just one of these AREs specifically binds TCF11/Nrf1 as heterodimers with small Maf proteins. We suggest that TCF11/Nrf1 can operate through a subset of AREs to modulate the expression of GCS(h) together with other components of the pathway and in this way play a role in regulating cellular glutathione levels.

Tyrphostin [correction of Tryphostin] AG879, a tyrosine kinase inhibitor: prevention of transcriptional activation of the electrophile and the aromatic hydrocarbon response elements

To investigate a possible role of phosphorylation in the signal transduction pathways responsible for transcriptional regulation of drug-metabolizing enzymes, we tested seven specific tyrosine kinase inhibitors (tyrphostins) for their effects on NAD(P)H:quinone oxidoreductase-1 (NQO1) mRNA levels in mouse hepatoma Hepa-1c1c7 (Hepa-1) cells and chose to study AG879 further. The potent electrophile tert-butylhydroquinone (tBHQ) is known to activate NQO1 gene transcription via the electrophile response element (EPRE). Among the tyrphostins tested, tyrphostin AG879 was unique in preventing the accumulation of tBHQ-induced NQO1 mRNA; this effect was dependent on the AG879 dose and was also sensitive to the time when AG879 was added relative to the beginning of tBHQ treatment. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (dioxin; TCDD) is known to activate Cyp1a1 gene transcription by way of aromatic hydrocarbon response elements (AHREs). We found that AG879 also prevents, to a lesser extent, the AHRE-mediated induction of CYP1A1 and NQO1 mRNA by dioxin. Zinc or cadmium is known to activate metallothionein (Mt1) gene transcription via the metal response element (MRE). AG879 induced MT1 mRNA, and AG879 did not block zinc- or cadmium-induced MT1 mRNA, indicating that the effects of AG879 on NQO1 or CYP1A1 mRNA levels cannot be generalized to all transcripts. Using transient transfection of EPRE-, AHRE-, or MRE-driven luciferase reporter gene constructs in Hepa-1 cells, we showed that the inhibitory effects of AG879 occurred at the level of EPRE- and AHRE-mediated transcription, but that AG879 did not affect the MRE-driven transcriptional response. These data suggest that AG879 might inhibit an unknown tyrosine kinase(s) whose activity is essential for EPRE- and AHRE-mediated trans-activation of certain mammalian genes. These results also indicate that some sharing of common signal transduction pathways might exist in the regulation of genes involved in drug metabolism that also respond to oxidative stress.

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.

Inhibition of ERK and p38 MAP kinases inhibits binding of Nrf2 and induction of GCS genes

Genes encoding the catalytic (GCS(h)) and regulatory (GCS(l)) subunits of human gamma-glutamylcysteine synthetase (gammaGCS), which catalyzes the rate limiting step in glutathione synthesis, are up-regulated in response to xenobiotics through Electrophile Response Elements (EpREs). Exposure of HepG2 cells to the GCS-inducing agent, Pyrrolidine dithiocarbamate (PDTC), results in ERK and p38 MAP kinase activation. Inhibition of ERK or p38 kinases by PD98059 or SB202190, respectively, results in approximately 50% reduction in GCS gene induction, while simultaneous inhibition completely eliminates induction. Induction of GCS expression is associated with an increase in Nrf2 and JunD binding to GCS EpREs. Pretreatment with the MAPK inhibitors significantly reduces binding of both transcription factors. These studies indicate that ERK and p38 contribute to the transcriptional up-regulation of the GCS subunit genes following PDTC treatment. Furthermore, supershift analyses suggest that binding of Nrf2 and JunD to the EpRE is a downstream consequence of ERK and p38 phosphorylation events.

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.

Identification and characterization of a novel factor that regulates quinone reductase gene transcriptional activity

The regulation of the quinone reductase (QR) gene as well as other genes involved in detoxification is known to be mediated by an electrophile/antioxidant response element (EpRE/ARE). We have previously observed that QR is up-regulated by the antiestrogen trans-hydroxytamoxifen in breast cancer cells. QR gene regulation by the antiestrogen-occupied estrogen receptor (ER) is mediated by the EpRE-containing region of the human QR gene, and the ER is one of the complex of proteins that binds to the EpRE. In an effort to further understand the mechanism for ER regulation of QR gene we identified other protein factors that regulate QR gene transcriptional activity in breast cancer cells. One of these protein factors, hPMC2 (human homolog of Xenopus gene which prevents mitotic catastrophe), directly binds to the EpRE and interacts with the ER in yeast genetic screening and in vitro assays. Interestingly hPMC2 interacts more strongly to ER beta when compared with ER alpha. In transient transfection assays using reporter constructs containing the EpRE, hPMC2 alone can slightly activate reporter in ER-negative MDA-MB-231 breast cancer cells. The activation of QR gene activity by hPMC2 is enhanced in the presence of ER beta.

Antioxidant regulation of genes encoding enzymes that detoxify xenobiotics and carcinogens

Antioxidants are substances that delay or prevent the oxidation of cellular oxidizable substrates. The various antioxidants exert their effect by scavenging superoxide or by activating a battery of detoxifying/defensive proteins. In this chapter, we have focused on the mechanisms by which antioxidants induce gene expression. Many xenobiotics (e.g., beta-naphthoflavone) activate genes similar to those activated by antioxidants. The promoters of these genes contain a common cis-element, termed the antioxidant response element (ARE), which contains two TRE (TPA response element) or TRE-like elements followed by GC box. Mutational studies have identified GTGAC***GC as the core of the ARE sequence. Many transcription factors, including Nrf, Jun, Fos, Fra, Maf, YABP, ARE-BP1, Ah (aromatic hydrocarbon) receptor, and estrogen receptor bind to the ARE from the various genes. Among these factors, Nrf-Jun heterodimers positively regulate ARE-mediated expression and induction of genes in response to antioxidants and xenobiotics. This Nrf-Jun heterodimerization and binding to the ARE requires unknown cytosolic factors. The mechanism of signal transduction from antioxidants and xenobiotics includes several steps: (1) Antioxidants and xenobiotics undergo metabolism to generate superoxide and related reactive species, leading to the generation of a signal to activate expression of detoxifying/defensive genes. (2) The generation of superoxide and related reactive species is followed by activation of yet to be identified cytosolic factors by unknown mechanism(s). (3). Activated cytosolic factors catalyze modification of Nrf and/or Jun proteins, which bind to the ARE in promoters of the various detoxifying/defensive genes. (4) The transcription of genes encoding detoxifying/defensive proteins is increased. The unknown cytosolic factors are significant molecules because they represent the oxidative sensors within the cells. Identification of the cytosolic factors will be of considerable importance in the field of antioxidants and gene regulation research. Future studies will also be required to completely understand the molecular mechanism of signal transduction from antioxidants and xenobiotics to Nrf-Jun. In addition to the Nrf-Jun pathway, mammalian cells also contain other pathways that activate gene expression in response to oxidative stress. These include NF-KB-, HIF-1-, Mac-1-, and SRF-mediated pathways. It is expected that collectively these pathways increase transcription of more than four dozen genes to protect cells against oxidative stress.

Regulation of genes encoding NAD(P)H:quinone oxidoreductases

NAD(P)H:quinone oxidoreductase (NQO1) and NRH:quinone oxidoreductase (NQO2) are flavoproteins that catalyze two-electron reduction and detoxification of quinones and its derivatives. This leads to the protection of cells against redox cycling, oxidative stress, and neoplasia. NQO1 is expressed ubiquitously in all the tissues. However, the level of expression varied among the human tissues. NQO1 gene is expressed at higher levels in several tumor tissue types, including liver and colon, as compared to normal tissues of similar origin. NQO1 gene expression is coordinately induced with other detoxifying enzyme genes in response to xenobiotics, antioxidants, oxidants, heavy metals, and radiations. Deletion mutagenesis in the NQO1 gene promoter identified several cis-elements including antioxidant response element (ARE), a basal element, and AP-2 element. ARE elements have also been found in the promoter regions of other detoxifying enzyme genes including glutathione S-transferases. ARE is essentially required for expression and coordinated induction of NQO1 and other detoxifying enzyme genes. Nuclear transcription factors Nrf2 and c-Jun bind to the ARE and activate the gene expression. The binding of Nrf2 + c-Jun to the ARE required unknown cytosolic factor(s). In addition to Nrf2 and c-Jun, other nuclear transcription factors including Nrf1, Jun-B, and Jun-D also bind to the ARE and regulate expression and induction of NQO1 gene. A hypothetical model is presented based on the available information on ARE-mediated regulation of detoxifying enzyme genes. Briefly, the Nrf2 is retained in the cytosplasm by a repressor protein Keap1 in untreated normal cells. The treatment of cells with xenobiotics and antioxidants leads to the activation of unknown cytosolic factor(s) that catalyze modification of Nrf2 and/or Keap1. The modification follows dissociation of Nrf2 and Keap1. The free Nrf2 translocates in the nucleus. Nrf2 in the nucleus heterodimerizes with c-Jun and binds to the ARE resulting in the induction of NQO1 and other ARE-regulated genes expression. The identity of cytosolic factor(s) remains unknown.

Coordinate transcriptional and translational regulation of ferritin in response to oxidative stress

The global increase in transcription of cytoprotective genes induced in response to oxidative challenge has been termed the antioxidant response. Ferritin serves as the major iron-binding protein in nonhematopoietic tissues, limiting the catalytic availability of iron for participation in oxygen radical generation. Here we demonstrate that ferritin is a participant in the antioxidant response through a genetically defined electrophile response element (EpRE). The EpRE of ferritin H identified in this report exhibits sequence similarity to EpRE motifs found in antioxidant response genes such as those encoding NAD(P)H:quinone reductase, glutathione S-transferase, and heme oxygenase. However, the EpRE of ferritin H is unusual in structure, comprising two bidirectional motifs arranged in opposing directions on complementary DNA strands. In addition to EpRE-mediated transcriptional activation, we demonstrate that ferritin is subject to time-dependent translational control through regulation of iron-regulatory proteins (IRP). Although IRP-1 is initially activated to its RNA binding (ferritin-repressing) state by oxidants, it rapidly returns to its basal state. This permits the translation of newly synthesized ferritin transcripts and ultimately leads to increased levels of ferritin protein synthesis following oxidant exposure. Taken together, these results clarify the complex transcriptional and translational regulatory mechanisms that contribute to ferritin regulation in response to prooxidant stress and establish a role for ferritin in the antioxidant response.

Transcriptional regulation of the antioxidant response element. Activation by Nrf2 and repression by MafK

The antioxidant response element (ARE) mediates the transcriptional activation of many genes encoding phase II drug-metabolizing enzymes in response to oxidative stress. Recent studies using knockout mice suggest that NF-E2-related factor 2 (Nrf2), along with small Maf proteins, binds and activates the ARE. In this study, using in vitro binding assays, Nrf2/MafK heterodimers were found to interact with high affinity to the ARE. However, distinct differences were observed when this interaction was compared with that formed with nuclear proteins from H4II EC3 or HepG2 cells. Overexpression of Nrf2 activated ARE-mediated transcription in HepG2 cells, and this activation was further increased by tert-butylhydroquinone. In HeLa cells, overexpression of Nrf2 resulted in activation of the ARE, but this activation was no longer induced by tert-butylhydroquinone. Using ARE constructs with point mutations in the core sequence, we found that only mutations at the T or G nucleotides within the core (TGAC) render the ARE unresponsive to Nrf2. Overexpression of MafK led to dose-dependent repression of ARE activity. Activation of the ARE by Nrf2 was similarly antagonized by MafK. These data suggest that Nrf2 plays an important role mediating basal activity of the ARE but that small Maf proteins are repressors and not activators of ARE-mediated transcription.

Signal transduction events elicited by natural products: role of MAPK and caspase pathways in homeostatic response and induction of apoptosis

Many natural products elicit diverse pharmacological effects. Using two classes of potential chemopreventive compounds, the phenolic compounds and the isothiocyanates, we review the potential utility of two signaling events, the mitogen-activated protein kinases (MAPKs) and the ICE/Ced-3 proteases (caspases) stimulated by these agents in mammalian cell lines. Studies with phenolic antioxidants (BHA, tBHQ), and natural products (flavonoids; EGCG, ECG, and isothiocyanates; PEITC, sulforaphane), provided important insights into the signaling pathways induced by these compounds. At low concentrations, these chemicals may activate the MAPK (ERK2, JNK1, p38) leading to gene expression of survival genes (c-Fos, c-Jun) and defensive genes (Phase II detoxifying enzymes; GST, QR) resulting in survival and protective mechanisms (homeostasis response). Increasing the concentrations of these compounds will additionally activate the caspase pathway, leading to apoptosis (potential cytotoxicity). Further increment to suprapharmacological concentrations will lead to nonspecific necrotic cell death. The wider and narrow concentration ranges between the activation of MAPK/gene induction and caspases/cell death exhibited by phenolic compounds and isothiocyanates, respectively, in mammalian cells, may reflect their respective therapeutic windows in vivo. Consequently, the studies of signaling pathways elicited by natural products will advance our understanding of their efficacy and safety, of which many may become important therapeutic drugs of the future.

p38 mitogen-activated protein kinase negatively regulates the induction of phase II drug-metabolizing enzymes that detoxify carcinogens

Phase II drug-metabolizing enzymes, such as glutathione S-transferase and quinone reductase, play an important role in the detoxification of chemical carcinogens. The induction of these detoxifying enzymes by a variety of agents occurs at the transcriptional level and is regulated by a cis-acting element, called the antioxidant response element (ARE) or electrophile-response element. In this study, we identified a signaling kinase pathway that negatively regulates ARE-mediated gene expression. Treatment of human hepatoma HepG2 and murine hepatoma Hepa1c1c7 cells with tert-butylhydroquinone (tBHQ) stimulated the activity of p38, a member of mitogen-activated protein kinase family. Inhibition of p38 activation by its inhibitor, SB203580, enhanced the induction of quinone reductase activity and the activation of ARE reporter gene by tBHQ. In contrast, SB202474, a negative analog of SB203580, had little effect. Consistent with this result, interfering with the p38 kinase pathway by overexpression of a dominant-negative mutant of p38 or MKK3, an immediate upstream regulator of p38, potentiated the activation of the ARE reporter gene by tBHQ, whereas the wild types of p38 and MKK3 diminished such activation. In addition, inhibition of p38 activity augmented the induction of ARE reporter gene activity by tert-butylhydroxyanisole, sulforaphane, and beta-naphthoflavone. Thus, p38 kinase pathway functions as a negative regulator in the ARE-mediated induction of phase II detoxifying enzymes.

Benzo[a]pyrene carcinogenicity is lost in mice lacking the aryl hydrocarbon receptor

The contribution of the aryl hydrocarbon receptor (AhR) in induction of a battery of xenobiotic-metabolizing enzymes has been studied extensively. However, no direct proof has been obtained that it plays a role in modulating carcinogenesis. To address the question of whether AhR is required for tumor induction, we have investigated the response of AhR-deficient mice to benzo[a]pyrene (B[a]P), a widely distributed environmental carcinogen. B[a]P treatment induced expression of the cytochrome P450 gene Cyp1a1 in the skin and liver of AhR-positive mice bearing +/+ and +/- genotypes and did not induce expression of the cytochrome P450 gene Cyp1a1 in AhR-null mice in either skin or liver. In contrast, Cyp1a2 gene expression was positive in liver irrespective of the presence or absence of the AhR gene, or B[a]P treatment, although its inducibility was lost in the AhR(-/-) mouse. All AhR-positive male mice of both +/+ and +/- genotypes that received subcutaneous injection of B[a]P (2 mg) on the first and the eighth days had developed subcutaneous tumors at the site of injection at the end of the 18-week experiment. In contrast, no tumors were apparent in any of the AhR-deficient mice. Likewise, topical application of B[a]P (200 microg) at weekly intervals to the skin of female mice for 25 weeks produced skin tumors only in the AhR-positive mice. Thus the carcinogenic action of B[a]P may be determined primarily by AhR, a transcriptional regulator of the gene for CYP1A1. The results of the present study provide direct evidence that AhR is involved in carcinogenesis.

Cloning and heterologous expression of NAD(P)H:quinone reductase of Arabidopsis thaliana, a functional homologue of animal DT-diaphorase

In higher plants, NAD(P)H:quinone reductase (NQR) is the only flavoreductase known to reduce quinone substrates directly to hydroquinones by a two-electron reaction mechanism. This enzymatic activity is believed to protect aerobic organisms from the oxidative action of semiquinones. For this reason plant NQR has recently been suggested to be related to animal DT-diaphorase. A cDNA clone for NQR of Arabidopsis thaliana was identified, expressed in Escherichia coli, purified and characterized. Its amino acid sequence was found related to a number of putative proteins, mostly from prokaryotes, with still undetermined function. Conversely, in spite of the functional homology, sequence similarity between plant NQR and animal DT-diaphorase was limited and essentially confined to the flavin binding site.

Regulation of gamma-glutamylcysteine synthetase subunit gene expression by the transcription factor Nrf2

Exposure of HepG2 cells to beta-naphthoflavone (beta-NF) or pyrrolidine dithiocarbamate (PDTC) resulted in the up-regulation of the gamma-glutamylcysteine synthetase catalytic (GCS(h)) and regulatory (GCS(l)) subunit genes. Increased expression was associated with an increase in the binding of Nrf2 to electrophile response elements (EpRE) in the promoters of these genes. Nrf2 overexpression increased the activity of GCS(h) and GCS(l) promoter/reporter transgenes. Overexpression of an MafK dominant negative mutant decreased Nrf2 binding to GCS EpRE sequences, inhibited the inducible expression of GCS(h) and GCS(l) promoter/reporter transgenes, and reduced endogenous GCS gene induction. beta-NF and PDTC exposure also increased steady-state levels of MafG mRNA. In addition to Nrf2, small Maf and JunD proteins were detected in GCS(h)EpRE-protein complexes and, to a lesser extent, in GCS(l)EpRE-protein complexes. The Nrf2-associated expression of GCS promoter/reporter transgenes was inhibited by overexpression of MafG. Inhibition of protein synthesis by cycloheximide partially decreased inducibility by PDTC or beta-NF and resulted in significant increases in GCS mRNA at late time points, when GCS mRNA levels are normally declining. We hypothesize that, in response to beta-NF and PDTC, the GCS subunit genes are transcriptionally up-regulated by Nrf2-basic leucine zipper complexes, containing either JunD or small Maf protein, depending on the particular GCS EpRE target sequence and the inducer. Following maximal induction, down-regulation of the two genes is mediated via a protein synthesis-dependent mechanism.

Antioxidant response element-mediated 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induction of human NAD(P)H:quinone oxidoreductase 1 gene expression

Antioxidant response element (ARE) is required for high basal expression of the human NAD(P)H:quinone oxidoreductase 1 (NQO1) gene in tumor cells and its induction in response to beta-naphthoflavone and phenolic antioxidants. In this study, we have demonstrated that ARE also is required for induction of human NQO1 gene expression in response to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The various results suggest an alternate pathway for TCDD induction of human NQO1 gene expression. This pathway is independent of xenobiotic response element (XRE) and aromatic hydrocarbon (Ah) receptor. It is presumed that TCDD-induced expression of CYP1A1 leads to increased oxidative stress, resulting in transcriptional activation and/or modification of ARE-binding factors and increased expression of the human NQO1 gene.

Lack of an absolute requirement for the native aryl hydrocarbon receptor (AhR) and AhR nuclear translocator transactivation domains in protein kinase C-mediated modulation of the AhR pathway

Protein kinase C (PKC)-mediated modulation of the aryl hydrocarbon receptor (AhR) pathway was examined in CHOK1-derived L10.I cells stably transfected with the pGUDLUC6.1 reporter; pGUDLUC6.1 is solely controlled by four dioxin-responsive enhancer elements. Co treatment of L10.I cells with 10 nM 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) and 81 nM phorbol 12-myristate 13-acetate (PMA), an activator of sn-1,2-diacylglyerol binding PKCs, enhanced transactivation of the reporter construct several-fold relative to cells treated with a saturating 10 nM TCDD dose alone; this effect was dubbed the "PMA effect." A domain swapping and deletional analysis of the native AhR and AhR nuclear translocator (ARNT) protein transactivation domains (TADs) was performed to determine if these domains are absolutely required for the AhR x ARNT dimer-mediated PMA effect in the L10.I model system; controls demonstrate the suitability of the L10.I model for these analyses and that endogenous AhR and ARNT levels are extremely low in this model. Transient coexpression of the AhR and ARNT-474-FLAG, an ARNT protein lacking the native ARNT TAD, in L10.I cells reveals the native ARNT TAD is not absolutely required for the AhR x ARNT-474-FLAG dimer to mediate the PMA effect. Transient coexpression of AhRDeltaCVP, a chimeric AhR protein in which the native AhR TAD has been replaced with the VP16 (herpes simplex virus protein 16) TAD (which control experiments demonstrate is unaffected by PMA), and ARNT in L10.I cells indicates that the native AhR TAD is not absolutely required for this AhRDeltaCVP x ARNT dimer to mediate the PMA effect. These observations strongly suggest that PKC-mediated modulation of the AhR pathway is not absolutely dependent on coactivators recruited to the AhR. ARNT dimer by the native TADs of the AhR and its heterodimerization partner ARNT.

Role of a mitogen-activated protein kinase pathway in the induction of phase II detoxifying enzymes by chemicals

Mitogen-activated protein kinase (MAPK) cascades are activated by diverse extracellular signals and participate in the regulation of an array of cellular programs. In this study, we investigated the roles of MAPKs in the induction of phase II detoxifying enzymes by chemicals. Treatment of human hepatoma (HepG2) and murine hepatoma (Hepa1c1c7) cells with tert-butylhydroquinone (tBHQ) or sulforaphane (SUL), two potent phase II enzyme inducers, stimulated the activity of extracellular signal-regulated protein kinase 2 (ERK2) but not c-Jun N-terminal kinase 1. tBHQ and SUL also activated MAPK kinase. Inhibition of MAPK kinase with its inhibitor, PD98059, abolished ERK2 activation and impaired the induction of quinone reductase, a phase II detoxifying enzyme, and antioxidant response element (ARE)-linked reporter gene by tBHQ and SUL. Overexpression of a dominant-negative mutant of ERK2 also attenuated tBHQ and SUL induction of ARE reporter gene activity. Interestingly, although expression of Ras and its mutant forms showed distinct effects on basal ARE reporter gene activity, they did not affect the activation of reporter gene by the inducers. Furthermore, a dominant-negative mutant of Ras had little effect on ERK2 activation by tBHQ and SUL, implicating a Ras-independent mechanism. Indeed, both tBHQ and SUL were able to stimulate Raf-1 kinase activity in vivo as well as in vitro. Thus, our results indicate that the induction of ARE-dependent phase II detoxifying enzymes is mediated by a MAPK pathway, which may involve direct activation of Raf-1 by the inducers.

Lipoxin A4: a new class of ligand for the Ah receptor

The Ah receptor is a ligand-activated transcription factor that mediates many of the biological actions of a large class of environmental compounds. Support for a role of the Ah receptor in normal physiology also has been reported, but an endogenous regulating ligand has not been identified. We have examined candidate endogenous lipophilic substances and report here the ability of the arachidonic acid metabolite, lipoxin A4, to bind to and activate the Ah receptor in Hepa-1 cells. Lipoxin A4 produced a concentration-dependent response in a DRE-driven CAT reporter construct, with a greater than 10-fold increase in CAT activity at 0. 3 microM. Lipoxin A4 transformed the Ah receptor to an active DRE-binding form in a concentration-dependent manner as indicated by gel mobility shift analysis. Results of Ah receptor competitive binding experiments indicated that at a concentration of 100 nM, lipoxin A4 produced a half-maximum displacement (EC50) of [3H]TCDD binding. Results of Northern blot analyses indicated a transient increase in mRNA levels of the Ah receptor-responsive gene CYP1A1, which peaked at 4 h, consistent with the kinetics observed for lipoxin A4-induced CYP1A1 enzyme activity. Further, lipoxin A4 was found to be a competitive inhibitor for the CYP1A1 enzyme, with a calculated Ki = 1.1 microM. These results establish lipoxin A4 as a new class of Ah receptor ligand, one that differs dramatically from classical Ah receptor ligands.

Redox regulation of c-Ha-ras and osteopontin signaling in vascular smooth muscle cells: implications in chemical atherogenesis

Reduction/oxidation (redox) reactions play a central role in the regulation of vascular cell functions. Recent studies in this laboratory have identified c-Ha-ras and osteopontin genes as critical molecular targets during oxidant-induced atherogenesis. This review focuses on the deregulation of gene transcription by redox-activated trans-acting factors after benzo(a)pyrene challenge and the modulation of extracellular matrix signaling in vascular smooth muscle cells by allylamine-induced oxidative injury. The induction of atherogenic vascular smooth muscle cell phenotypes by chemical injury exhibits remarkable parallels with those seen in other forms of atherogenesis.

Xenobiotic-responsive element for the transcriptional activation of the rat Cu/Zn superoxide dismutase gene

Cu/Zn superoxide dismutase (SOD1) catalyzes the dismutation of superoxide radicals produced from biological oxidation and environmental stresses. A number of xenobiotics are toxic because they generate free radicals, such as superoxide and hydroxyl radicals, through a redox cycle. The xenobiotic responsive element (XRE) was located between the nt -268 and -262 region of the 5'-flanking sequence of the SOD1 gene. Functional analyses of this element by deletion, mutations, and heterologous promoter systems confirmed that the expression of the SOD1 gene was induced by a xenobiotic through the XRE. Gel mobility shift assays showed the xenobiotic inducible binding of the receptor-ligand complex to XRE. The cytoplasmic fraction from nontreated HepG2 cells also contains the factor as a cryptic form and prominently reveals its DNA-binding activity by incubation with betaNF in vitro. These results suggest that the XRE participates in the induction of the rat SOD1 gene by xenobiotics.

Activation of antioxidant/electrophile-responsive elements in IMR-32 human neuroblastoma cells

The present investigation demonstrates distinct patterns of activation for antioxidant/electrophile-responsive elements (ARE/EpREs) in cells of neuronal versus hepatic origin suggesting the possibility of cell-/tissue-specific signaling pathways and/or transcription factors required for ARE/EpRE activation. The ARE/EpRE is a cis-acting regulatory element found in 5'-flanking regions of numerous genes including NAD(P)H:quinone oxidoreductase (QR) and glutathione S-transferases. Insomuch as ARE/EpRE activation has been studied primarily in hepatoma cell lines there is little information on how these responsive elements and corresponding genes are regulated in brain. ARE/EpRE-reporter constructs were transiently transfected into IMR-32 human neuroblastoma cells. Activation of ARE/EpRE sequences by tert-butylhydroquinone (tBHQ), a redox-cycling compound, in IMR-32 cells (20- to 30-fold) is dramatically different from the minimal response seen in HepG2 human hepatoma cells (2- to 3-fold). beta-napthoflavone, an ARE/EpRE inducer in HepG2 cells, as well as the oxidants hydrogen peroxide and tert-butyl hydroperoxide did not induce the ARE/EpRE in IMR-32 cells. In addition, we show that the core sequence containing a complete 5' palindrome is necessary for maximal activation of the ARE/EpRE in IMR-32 cells. Mutations within this palindromic sequence decrease basal level expression and block induction by tBHQ but not phorbol 12-myristate 13-acetate. Furthermore, activation of the hQR-ARE/EpRE by tBHQ correlates with induction of endogenous QR activity in IMR-32 neuroblastoma cells (15-fold). Thus, elucidating the mechanism of ARE/EpRE activation in this human neuroblastoma cell line may identify unknown transcription factors or signal transduction cascades regulating ARE/EpRE-driven gene expression.

A novel positive regulatory element that enhances hamster CYP2A8 gene expression mediated by xenobiotic responsive element

CYP2A8 is a major form of cytochrome P-450 inducible by 3-methylcholanthrene in Syrian hamster liver. To identify DNA elements necessary for the transcriptional activation of the CYP2A8 gene, we analyzed the regulatory region of the CYP2A8 gene and conducted transient transfection experiments of CYP2A8-luciferase fusion plasmids in primary cultures of hamster hepatocytes. We analyzed up to -5 kb of the 5'-flanking region and found the region sufficient for the 3-methylcholanthrene-inducible gene expression. This region contained a consensus sequence for xenobiotic responsive element (XRE) between -2366 and -2349, which was shown to be essential for induction of the gene expression. Furthermore, we found a novel positive regulatory element for XRE-mediated gene expression (PREX) located upstream of the XRE. This element is not identified in any genes inducible by 3-methylcholanthrene so far reported. Without PREX, the XRE-mediated promoter activity was enhanced nearly 10-fold, whereas with PREX, the activity was enhanced 20-fold over the basal level. Gel mobility shift assays revealed specific binding of nuclear proteins to PREX. Mutations and deletions of PREX caused a loss of the binding and promoter-enhancing activities, respectively. Moreover, transient expression experiments showed that the enhancing activity of PREX was not observed in Drosophila Schneider's line 2 cells, which were shown to lack the PREX binding proteins.

Chemoprotective properties of phenylpropenoids, bis(benzylidene)cycloalkanones, and related Michael reaction acceptors: correlation of potencies as phase 2 enzyme inducers and radical scavengers

Induction of phase 2 enzymes (e.g., glutathione transferases, NAD(P)H:quinone reductase, glucuronosyltransferases, epoxide hydrolase) is a major strategy for reducing the susceptibility of animal cells to neoplasia and other forms of electrophile toxicity. In a search for new chemoprotective enzyme inducers, a structure-activity analysis was carried out on two types of naturally occurring and synthetic substituted phenylpropenoids: (a) Ar-CH=CH-CO-R, where R is OH, OCH3, CH3, or Ar, including cinnamic, coumaric, ferulic, and sinapic acid derivatives, their ketone analogues, and chalcones; and (b) bis(benzylidene)cycloalkanones, Ar-CH=C(CH2)n(CO)C=CH-Ar, where n = 5, 6, or 7. The potencies of these compounds in inducing NAD(P)H:quinone reductase activity in murine hepatoma cells paralleled their Michael reaction acceptor activity (Talalay, P.; De Long, M. J.; Prochaska, H. J. Proc. Natl. Acad. Sci. U.S.A. 85, 1988, 8261-8265). Unexpectedly, the bis(benzylidene)cycloalkanones also powerfully quenched the lucigenin-derived chemiluminescence evoked by superoxide radicals. Introduction of o-hydroxyl groups on the aromatic rings of these phenylpropenoids dramatically enhanced their potencies not only as inducers for quinone reductase but also as quenchers of superoxide. These potentiating o-hydroxyl groups are hydrogen-bonded, as shown by moderate downfield shift of their proton NMR resonances and their sensitivities to the solvent environment. The finding that the potencies of a series of bis(benzylidene)cycloalkanones in inducing quinone reductase appear to be correlated with their ability to quench superoxide radicals suggests that the regulation of phase 2 enzymes may involve both Michael reaction reactivity and radical quenching mechanisms.

Induction of a cytosolic 54 kDa protein in rat liver that is highly homologous to selenium-binding protein

We have previously shown that a 54 kDa protein in rat liver is highly homologous to selenium-binding protein (SeBP) or acetaminophen-binding protein (APBP) in mice and is highly inducible by treatment with 3,3',4,4',5-pentachlorobiphenyl or 3-methylcholanthrene. In this study, we examine the effect of six typical inducers, 3-methylcholanthrene (MC), isosafrole (ISO), phenobarbital (PB), dexamethasone (DEX), clofibrate (CLO), pyrazole (PYR) and butylated hydroxytoluene (BHT), on the expression level of this 54 kDa protein. Male Wistar rats were given each inducer following a predetermined schedule. Among these inducers, the 54 kDa protein was inducible by MC and BHT. The response to MC and BHT was compared with that of NAD(P)H: quinone oxidoreductase and ethoxyresorufin O-deethylase activities. The induction mechanisms and physiological role of the 54 kDa protein are discussed in the light of our results.

Differential chemoprotection against acetaminophen-induced hepatotoxicity by latentiated L-cysteines

Novel thiazolidine prodrugs were prepared by the condensation of L-cysteine with aldose disaccharides. Using a disaccharide in prodrug construction allows for a terminal cyclic sugar moiety to be present on the prodrug, which may allow the delivery of the agent to specific receptors, such as the asialoglycoprotein receptor (ASGPR) of hepatocytes, that require specific structural motifs for recognition. Three L-cysteine prodrugs were synthesized with a pendant cyclic galactose moiety; two related glucose-bearing prodrugs were synthesized for comparison. The prodrugs were designed to release L-cysteine, which is then available to support glutathione (GSH) biosynthesis and provide cytoprotection against a variety of toxic insults. Protection studies in Swiss-Webster mice used acetaminophen (575 mg/kg), a well-documented hepatotoxin which depletes GSH at overdose. Three prodrugs performed exceptionally well against acetaminophen-induced hepatotoxicity, as measured by increased survival and improved histological profiles of liver tissue after 48 h. In further experimentation, two of the disaccharide-based prodrugs, prepared from alpha- and beta-lactose, were compared with the monosaccharide-based compound prepared from ribose. Co-administration of the selected prodrugs with a 400 mg/kg dose of acetaminophen to Swiss-Webster mice prevented the short-term depletion in hepatic GSH and also reduced hepatotoxicity as determined by histological damage and serum levels of alanine aminotransferase. A single dose of the prodrugs alone had no effect on hepatic drug metabolizing enzymes [glutathione S-transferase (GST), NAD(P)H:quinone oxidoreductase (QOR), UDP-glucuronosyltransferase (UGT), and cytochrome P450], but, concordant with the reduction of hepatotoxicity, the latentiated forms prevented the significant elevation in QOR activity and mRNA and GST mRNA elicited by acetaminophen itself. GST activity, UGT activity and mRNA, and cytochrome P450 concentration were all unaffected by acetaminophen or the prodrugs. These studies identified novel L-cysteine prodrugs with potentially useful hepatoprotective activity. However, no structure-activity relationships were obvious. In addition, the occurrence of targeted delivery to hepatocytes remains ambiguous.

Transcriptional regulation of the human quinone reductase gene by antiestrogen-liganded estrogen receptor-alpha and estrogen receptor-beta

We have previously reported that antiestrogens stimulate quinone reductase (NAD(P)H:(quinone-acceptor) oxidoreductase (QR or NQO1); EC 1.6.99.2) enzymatic activity, an action that may provide protective effects against the toxicity and mutagenicity caused by quinones. We have now investigated the transcriptional regulation of the QR gene by antiestrogens. In transfection experiments employing the 5'-flanking (863-base pair) region of the human QR gene promoter with its electrophile/antioxidant response element (EpRE/ARE) or deleted or mutated constructs, we observe that antiestrogens induced an increase in QR gene promoter reporter activity in estrogen receptor (ER) negative breast cancer and endometrial cancer cells transfected with ER, and this induction by antiestrogens was repressed by estradiol. The stimulation of QR transcriptional activity required the 31-base pair electrophile-responsive region from the human QR gene promoter and a functional ER. Intriguingly, antiestrogens were stronger activators of the QR EpRE via the ER subtype ERbeta than ERalpha. Oligonucleotide gel mobility and antibody shift assays reveal that the ER binds to the EpRE but is only a minor component of the proteins bound to the EpRE in ER-containing MCF-7 breast cancer cells. While binding of ERbeta to the estrogen response element was weaker when compared with ERalpha, ERbeta and ERalpha showed similar binding to the EpRE. Together these findings provide evidence that QR gene regulation by the antiestrogen-occupied ER is mediated by the EpRE-containing region of the human QR gene and indicate that the ER is one of the complex of proteins that binds to the EpRE. In addition, that ERbeta is a more potent activator at EpRE elements than is ERalpha suggests that the different levels of these two receptors in various estrogen target cells could impact importantly on the antioxidant potency of antiestrogens in different target cells. These findings have broad implications regarding the potential beneficial effects of antiestrogens since EpREs mediate the transcriptional induction of numerous genes, including QR, which encode chemoprotective detoxification enzymes.

Induction of UDP-glycosyltransferase family 1 genes in rat liver: different patterns of mRNA expression with two inducers, 3-methylcholanthrene and beta-naphthoflavone

Uridine diphosphate (UDP)-glucuronosyltransferases (UGTs), presently called UDP-glycosyltransferases, catalyse the detoxification of many toxic and carcinogenic compounds. Glucuronidation is also a major metabolic pathway for numerous drugs. The UGT1A6 gene (formerly known as UGT1*06 and UGT1A1) has been suggested to belong to the aryl hydrocarbon (Ah) gene battery, which consists of several genes encoding for drug-metabolising enzymes regulated by dioxin and other ligands of the Ah receptor. In this study, we analysed the localisation of UGT1A6 expression in rat liver by in situ hybridisation to mRNA. Two different RNA probes were used, one which was specific to UGT1A6 and the other against the C terminal sequence shared by all UGT1 genes. In this study, no UGT1A6 mRNA was detected in the control animals. However, other gene(s) of the UGT1 family were expressed in the perivenous region surrounding the central veins as detected by hybridisation with the probe against the common region of the UGT1 genes. Treatment with the lower dose (5 mg/kg) of 3-methylcholanthrene (3MC) induced expression of UGT1A6 perivenously. Treatment with the higher dose (25 mg/kg) of 3-Methylcholanthrene resulted in a more panacinar expression pattern. In contrast to the perivenous induction observed with 3-methylcholanthrene, treatment with 15 mg/kg of beta-naphthoflavone (BNF) resulted in strong induction in the periportal region. The results reveal an inducer-specific pattern of UGT1A6 expression similar to that demonstrated earlier for other Ah battery genes, namely CYP1A1, CYP1A2, GSTYalpha and ALDH3. The finding further supports the notion that common factors regulate the regional hepatic expression of Ah battery genes.

Modulation of antioxidant responses by arsenic in maize

The effects of arsenic on the expression of the antioxidant genes encoding superoxide dismutase, catalase, and glutathione S-transferase, as well as the activity of SOD and CAT enzymes, were examined at different developmental stages and in different tissues. Both CAT and SOD activities increased in response to low concentrations (0.01-0.1 mM) of arsenic in developing maize embryos. In germinating embryos both CAT and SOD activities increased in response to a wide range of arsenic concentrations (0.01-10 mM). Cat1 transcript increased in response to arsenic in developing and germinating embryos and in young leaves. Conversely, Cat2 increased at low concentrations of arsenic only in germinating embryos. Cat3 transcript levels increased in response to low concentrations of arsenic only in developing embryos. Sod3 transcript increased at low concentrations of arsenic in developing, germinating embryos and in leaves. The cytosolic Sod4 and Sod4A increased in response to arsenic in germinating embryos, while only Sod4 transcript increased in response to arsenic in leaves. Expression of Gst1 was similar to that of Cat1 in all tissues examined. These results indicate that arsenic triggers tissue and developmental stage specific defense responses of antioxidant and detoxification related genes in maize.

A pharmacodynamic analysis of TCDD-induced cytochrome P450 gene expression in multiple tissues: dose- and time-dependent effects

The ability of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) to alter gene expression and the demonstration that the induction of CYP1A2 is responsible for hepatic TCDD sequestration suggest that both pharmacokinetic and pharmacodynamic events must be incorporated for a quantitative description of TCDD disposition. In this paper, a biologically based pharmacodynamic (BBPD) model for TCDD-induced biochemical responses in multiple tissues was developed. The parameters responsible for tissue response were estimated simultaneously with a refined physiologically based pharmacokinetic (PBPK) model developed by Wang et al. (1997a), by using the time-dependent effects of TCDD on induced CYP1A1/CYP1A2 gene expression in multiple target tissues (liver, lungs, kidneys, and skin) of female Sprague-Dawley rats treated with 10 microgram TCDD/kg for 30 min, 1, 3, 8, or 24 h, or 7, 14, or 35 days. This refined BBPD model developed based on the time-course of TCDD-induced CYP1A1/CYP1A2 protein expression, and associated enzymatic activities well described the dose-dependent effects of TCDD on cytochrome P450 protein expression and associated enzyme activities in the multiple tissues of female Sprague-Dawley rats at 3 days following a single exposure to TCDD (0.01-30.0 micromgram TCDD/kg). This is the first BBPD model to quantitatively describe the time- and dose-dependent effects of TCDD on induced CYP1A1/CYP1A2 protein expression and associated enzyme activities in multiple target tissues for TCDD-induced biochemical responses.

An electrophile responsive element (EpRE) regulates beta-naphthoflavone induction of the human gamma-glutamylcysteine synthetase regulatory subunit gene. Constitutive expression is mediated by an adjacent AP-1 site

Exposure of HepG2 cells to beta-naphthoflavone (beta-NF) results in time- and dose-dependent increase in the steady-state mRNA levels for both the catalytic (GCSh) and regulatory (GCS1) subunits of gamma-glutamylcysteine synthetase (GCS) which catalyzes the rate-limiting step in the de novo synthesis of the cellular antioxidant glutathione (GSH) (Mulcahy, R. T., Wartman, M. A., Bailey, H. B., and Gipp, J. J. (1997) J. Biol. Chem. 272, 7445-7454). Cloning and sequencing of the GCS1 promoter region is reported. Regulatory sequences mediating basal and beta-NF induced expression of the GCSl gene were identified using a series of promoter/reporter fusion genes transfected into HepG2 cells. Sequences directing basal and beta-NF induced expression were localized between nucleotides -344 and -242 (numbered relative to the translation start site). Mutational analyses indicate that basal expression of the GCSl gene is directed by a consensus AP-1-binding site located 33 base pairs upstream of a consensus electrophile responsive element (EpRE) sequence; both cis-elements are capable of supporting beta-NF inducibility. Elimination of the inducible response requires simultaneous mutation of both sequences, however, in the presence of an intact EpRE the upstream AP-1 site is irrelevant to induction. Regulation of expression of both human GCS subunit genes in response to beta-NF is therefore mediated by cis-elements satisfying the consensus core EpRE motif.

Structure of the human allelic glutathione S-transferase-pi gene variant, hGSTP1 C, cloned from a glioblastoma multiforme cell line

We recently reported the cloning of full-length cDNAs corresponding to mRNAs of three GST-pi genes, hGSTP1*A, hGSTP1*B and hGSTP1*C, as well as, the isolation of the full-length hGSTP1*C, of the human glutathione S-transferase-pi (GST-pi) gene that is characterized by a A-->G transition at +1404 in exon 5 and a C-->T transition at +2294 in exon 6. Although the promoter of the isolated gene was identical to that of the previously described GST-pi gene isolated from the MCF 7 and the HPB-ALL cell lines, both of which were hGSTP1*A, a number of structural differences were observed, including, nucleotide transitions, transversions, deletions and insertions, some of which created new restriction enzyme cleavage sites. A guanine insertion in the insulin response element, IRE, in intron 1 created an additional site for 5'-cytosine methylation. Seven repeat retinoic acid response element (RARE) consensus half sites, A(G)GG(T)TC(G)A at +1521 to +1644 were identified in the cloned hGSTP1*C. Five of the RARE half-sites had the minimal spacer nucleotide requirement for functionality and DNA mobility shift analysis with different pairs of the RARE half-sites and supershift studies using antibodies against RAR-beta showed significant binding of nuclear protein complexes from RA-treated cells to these RAREs. GST-pi gene expression was increased significantly in cells transfected with the GST-pi gene and treated with all-trans RA. These results contrast with those in a previous report in which RA was shown to suppress the GST-pi promoter, and indicate a complex mechanism of RA-mediated GST-pi gene regulation in tumor cells.

Expression of functional aromatic hydrocarbon receptor and aromatic hydrocarbon nuclear translocator proteins in murine bone marrow stromal cells

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) acting through the aromatic hydrocarbon receptor (AhR) and its dimerization partner, the AhR nuclear translocator protein (arnt), elicits numerous toxicological effects including immunosuppression and thymic atrophy. Previous work has shown that TCDD alters bone marrow prothymocyte populations. These effects could be mediated at the lymphocyte level directly and/or through effects on bone marrow stromal cells, a population important in the support of lymphopoiesis. The purpose of this study was to characterize AhR and arnt expression in three murine bone marrow stromal cell lines (S17, M2-10B4, and BMS2) and in primary stromal cell cultures. Immunoblot analysis detected AhR protein in M2-10B4 and BMS2 cells. AhR protein was also detected in the primary cultures. Arnt protein could be detected in all cell cultures. Electrophoretic mobility shift assays detected TCDD-dependent dioxin-responsive element (DRE) binding in all three cell lines. DNA binding was sequence-specific and dependent on AhR, as demonstrated by the addition of unlabeled DRE DNA or of anti-AhR antibody. Results obtained with the primary cultures paralleled those seen with the stromal cell lines. The ED50 for induction of TCDD-dependent DRE binding in M2-10B4 cells was 0.21 nM. TCDD treatment did not induce stromal P4501A1 mRNA expression but did increase P4501B1 mRNA levels in all three cell lines and in the primary cultures. These results indicate that murine bone marrow stromal cells express AhR and arnt proteins. Furthermore, these proteins are functional in terms of their DRE-binding ability and potential to regulate mRNA levels in a gene-specific fashion.

Genomic cloning of hGSTP1*C, an allelic human Pi class glutathione S-transferase gene variant and functional characterization of its retinoic acid response elements

The complete hGSTP1*C, consisting of 7 exons and 6 introns contained in 3116 base pairs, was isolated from a cosmid genomic library of a glioblastoma multiforme cell line. Although the promoter of hGSTP1*C was identical to that of the previously reported GST-Pi gene, several of its structural features had not been previously described. These include several nucleotide transitions and transversions. Transitions of A --> G at +1404 and C --> T at +2294 in exons 5 and 6, respectively, changed codons Ile104 to Val104 and Ala113 to Val113. The gene also contained a guanine insertion at +51 in the insulin response element in intron 1 and six tandem repeats and one palindromic retinoic acid response element (RARE) consensus half-sites, A(G)GG(T)TC(G)A in intron 5. Retinoic acid (RA) treatment increased GST-Pi gene expression significantly in MGR3 cells. GST-Pi gene constructs with and without RARE deletion were used to show the RARE requirement for GST-Pi gene induction by RA. The isolation of the hGSTP1*C gene and the evidence that it contains functional RAREs should contribute to a better understanding of the molecular regulation of the GST-Pi gene in human cells.

Redox regulation of genes that protect against carcinogens

Most carcinogens require activation to electrophilic metabolites or species that generate reactive oxygen in order to initiate the tumorigenic process. These reactive intermediates can, in turn, be detoxified by endogenous enzyme systems that and in the protection of cells from either toxic or mutagenic product formation. The levels of many of these enzymes are elevated by numerous compounds found in the diet, or by antioxidants. Recent evidence describes the mechanism for this induction of carcinogen detoxication enzymes to be regulated at the transcriptional level. Nuclear transcription factors bound to sites common among these carcinogen detoxication genes are activated by as yet unknown signal transduction pathways. The activity of these nuclear transcription factors are modulated by pro- and antioxidant reagents, suggesting that a redox-sensitive component governs the induction of enzymes involved in carcinogen metabolism. In this review, evidence for the redox regulation of the genes encoding carcinogen detoxication enzymes is presented. Evidence is also presented suggesting the participation of nuclear factor kappa B (NF-kappa B), mitogen-activated protein (MAP) kinase, and basic leucine zipper (bZIP) proteins and their activation pathways in this induction.

Comprehensive analysis of proteins which interact with the antioxidant responsive element: correlation of ARE-BP-1 with the chemoprotective induction response

Transcriptional activation of the mouse glutathione S-transferase Ya gene by chemoprotective molecules is mediated through the interaction of trans-acting factors with an antioxidant responsive element (ARE) in the promoter region of this gene. In a step toward identifying those factors which bind productively to the GST Ya ARE, all of the discernible, specific ARE-binding proteins (ARE-BP) in nuclear extracts from HepG2 cells were systematically characterized. By gel-mobility-shift analysis, seven specific ARE-BPs, termed ARE-BP-1 through 7 in order of increasing mobility, were observed that did not vary in concentration or migration between induced and uninduced cell extracts. The molecular weights of the individual ARE-BP subunits were determined by a two-dimensional electrophoresis protocol. Ferguson gel analysis of native protein size indicated that several of the ARE-BP-DNA complexes are composed of multiple protein subunits. Wild-type AREs and GST Ya ARE fragments and mutant sequences were evaluated for their ability to mediate induction in a reporter gene system in HepG2 cells. This same panel of sites was tested in an in vitro binding assay for the ability to compete for the ARE-BPs. A binding profile for each ARE-BP was compiled. Correlation between the ARE-BP binding profiles and induction results indicated that: (i) the ARE-BP-1 and ARE-BP-2 complexes formed only with AREs that supported induction, and (ii) the ARE-BP-4 complex formed with all inducible AREs, but it also bound to ARE mutants that failed to support induction. Based on the studies, an early composite regulatory element model for ARE-mediated expression is presented. ARE-BP-1 is proposed to be the mediator of the ARE's unique induction response to chemoprotective agents.

Tryptanthrins: a novel class of agonists of the aryl hydrocarbon receptor

2,3,7,8-Tetrachlorodibenzo-p-dioxin and related environmental pollutants exert most of their adverse effects such as immunosuppression, induction of endocrine dysfunction, tumor promotion, and teratogenicity via the aryl hydrocarbon or dioxin receptor. While most potent agonists of the aryl hydrocarbon receptor are of synthetic origin, an increasing number of natural compounds are now recognized as receptor agonists. Our findings demonstrated that some tryptanthrin derivatives biosynthesized in incubations of Candida lipolytica with tryptophan and anthranilic acid or its derivatives were agonists of the aryl hydrocarbon receptor. The biosynthetic products 8-methyltryptanthrin, 8-chlorotryptanthrin, and 8-bromotryptanthrin induced cytochrome P4501A1 mRNA and protein in rat hepatocytes in primary culture, characteristic features of aryl hydrocarbon receptor agonists. Log-probit analysis of the catalytic activity of cytochrome P4501A1, 7-ethoxyresorufin O-deethylase (EROD), revealed EC50 induction values of 1.7, 0.25, and 0.17 microM for 8-methyltryptanthrin, 8-chlorotryptanthrin, and 8-bromotryptanthrin, respectively. Interestingly, the nonsubstituted tryptanthrin molecule, biosynthesized from the common physiological precursors tryptophan and anthranilic acid, was also active as an inducer. The specificity of the inducing effect of tryptanthrins was demonstrated in gel retardation experiments in Hepa-1 mouse hepatoma cells, showing the characteristic interaction of the activated aryl hydrocarbon receptor with an oligonucleotide containing a xenobiotic-responsive element. It is suggested that the receptor may be part of a defense system protecting higher organisms from secondary metabolites formed by the microflora of the host or its environment.

Regulation of human apolipoprotein A-I gene expression by gramoxone

To induce oxidative stress, HepG2 cells were exposed to a compound known as gramoxone. This compound undergoes a one-electron reduction to form a stable free radical which is capable of generating reactive oxygen species. We demonstrated that exposure of HepG2 cells to gramoxone (0.1 microM) resulted in a 2-fold decrease in apoA-I mRNA with no significant change in apoB and apoE mRNA levels. To examine if increased rates of mRNA degradation were responsible for the reduction in apoA-I mRNA levels, mRNA half-lives were measured in the presence of actinomycin D with and without gramoxone treatment. These studies revealed a 4-fold increase in the rate of apoA-I mRNA degradation in cells exposed to gramoxone. In similarly treated cells, nuclear run-off assays indicated that the transcription rate of the apoA-I gene was also increased 2-fold. Consistent with nuclear run-off assays, transient transfection experiments using a series of pGL2-derived luciferase reporter plasmids containing the human apoAI proximal promoter demonstrated that gramoxone treatment increased apoA-I promoter activity 2-fold. We have identified a potential "antioxidant response element" (ARE) in the apoA-I promoter region that may be responsible for the increase in apoA-I transcriptional activity by gramoxone. Gel mobility shift assays with an ARE oligonucleotide revealed increased levels of a specific protein-DNA complex that formed with nuclear extracts from gramoxone-treated cells. UV cross-linking experiments with the ARE and nuclear extracts from either untreated or gramoxone-treated cells detected proteins of approximately 100 and 115 kDa. When a single copy of the ARE was inserted upstream of the SV40 promoter in a luciferase reporter plasmid, a significant 2-fold induction in luciferase activity was observed in HepG2 cells in the presence of gramoxone. In contrast, a plasmid containing a mutant apoAI-ARE did not confer responsiveness to gramoxone. Furthermore, pGL2 (apoAI-250 mutant ARE), in which point mutations eliminated the ARE in the apoAI promoter, showed no increase in luciferase activity in response to gramoxone. Taken together, the data suggest that gramoxone affects apoA-I mRNA levels by both transcriptional and post-transcriptional mechanisms.

2,3,7,8-Tetrafluorodibenzo-p-dioxin: a potent agonist of the murine dioxin receptor

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related polychlorinated aromatic hydrocarbons exert a pattern of toxicity related to their binding to a common receptor, the Ah (aryl hydrocarbon) or dioxin receptor. No information is available, however, on the toxicological properties of 2,3,7,8-tetrafluorodibenzo-p-dioxin (TFDD). In our experiments, TFDD was found to act as a highly potent dioxin receptor agonist leading to a transient induction of cytochrome P450(CYP)1A1 mRNA and protein in receptor-proficient mouse Hepa-1 hepatoma cells treated with 10(-10) M TFDD. However, no significant induction of 7-ethoxyresorufin O-deethylase (EROD) activity was observable in TFDD-treated Hepa-1 cells or mouse hepatocytes in primary culture, suggesting an interference with the catalytic activity of CYP1A1. Parallel experiments with 10(-10) M TCDD showed a sustained induction of CYP1A1 mRNA and protein, and of EROD activity. When a reporter gene construct comprising a xenobiotic-responsive element (XRE) in 5'-position was transfected in Hepa-1c-1c-7 cells, 5×10(-8) M TFDD and 5×10(-9) M TCDD induced transcription to a comparable extent. Both inducers were inactive when a mutant XRE with a guanine replaced by thymine was transfected. In metabolism studies in mouse liver homogenate, TFDD was rapidly degraded in the presence of an NADPH-regenerating system, and metabolism was enhanced in liver homogenate from β-naphthoflavone-pretreated mice indicating that TFDD is metabolized in a CYP-catalyzed pathway. The open ring products dihydroxytetrafluororbiphenyl ether, and 1,2-difluoro-o-benzoquinone, probably derived from 1,2-difluorocatechol, were identified by GC-MS analysis of the incubation mixtures, whereas no phenolic metabolites/and or metabolites with an intact dioxin ring could be found. It is concluded that TFDD, in contrast to its chlorinated analogue, is metabolically unstable, and thus currently does not fulfill the criteria for the recommendation of a TCDD or toxicity equivalency factor (TEF).

Identification of a motif within the 5' regulatory region of pS2 which is responsible for AP-1 binding and TCDD-mediated suppression

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related compounds modulate several endocrine systems by altering hormone synthesis, enhancing ligand metabolism, and down-regulating receptor levels/binding activity. Previous studies have demonstrated that TCDD inhibits the 17beta-estradiol (E2)-induction of pS2, a human breast cancer prognostic marker. This inhibition occurs at the gene expression level and is Ah receptor (AhR)-mediated. Analysis of the 5' regulatory region has identified three motifs which resemble dioxin response element (DRE) core sequences. pS2-regulated luciferase deletion constructs identified the DRE-like motif located at -527 to -514 as being required for TCDD-mediated suppression. A point mutation within this core motif (T-518C) abolished the inhibition by TCDD while UV-induced protein-DNA cross-linking and competitive gel retardation assays demonstrated AhR complex binding to this motif. Further study of this region also revealed an adjacent putative AP-1 site, diverging by one base pair from the consensus sequence. Gel retardation assays using TPA-treated MCF-7 cell nuclear extracts showed an induced complex binding to the AP-1-like site. Competition studies and antibody supershifts confirmed that the retarded complex consists of AP-1-like proteins. pS2-regulated luciferase constructs containing mutations specific to the AP-1-like motif greatly diminished the inducibility in response to E2. These results suggest that an interaction between AhR complexes and AP-1-like proteins may be responsible for TCDD-mediated inhibition of E2-induced pS2 expression.

Functional antioxidant responsive elements

Exposure of human and rodent cells to a wide variety of chemoprotective compounds confers resistance against a broad set of carcinogens. For a subset of the chemoprotective compounds, protection is generated by an increase in the abundance of protective enzymes like glutathione S-transferases (GST). Antioxidant responsive elements (AREs) mediate the transcriptional induction of a battery of genes which comprise much of this chemoprotective response system. Past studies identified a necessary ARE "core" sequence of RTGACnnnGC, but this sequence alone is insufficient to mediate induction. In this study, the additional sequences necessary to define a sufficient, functional ARE are identified through systematic mutational analysis of the murine GST Ya ARE. Introduction of the newly identified necessary nucleotides into the regions flanking a nonresponsive, ARE-like, GST-Mu promoter sequence produced an inducible element. A screen of the GenBank database with the newly identified ARE consensus identified 16 genes which contained the functional ARE consensus sequence in their promoters. Included within this group was an ARE sequence from the murine ferritin-L promoter that mediated induction when tested. In an electrophoretic mobility-shift assay, the ferritin-L ARE was bound by ARE-binding protein 1, a protein previously identified as the likely mediator of the chemoprotective response. A three-level ARE classification system is presented to account for the distinct induction strengths observed in our mutagenesis studies. A model of the ARE as a composite regulatory site, where multiple transcription factors interact, is presented to account for the complex characteristics of ARE-mediated chemoprotective gene expression.