Regulation of NAD(P)H:Quinone Oxidoreductase 1 Gene Expression by CYP1A1 Activity

Aryl Hydrocarbon Receptor in Oxidative Stress as a Double Agent and Its Biological and Therapeutic Significance

The aryl hydrocarbon receptor (AhR) has long been implicated in the induction of a battery of genes involved in the metabolism of xenobiotics and endogenous compounds. AhR is a ligand-activated transcription factor necessary for the launch of transcriptional responses important in health and disease. In past decades, evidence has accumulated that AhR is associated with the cellular response to oxidative stress, and this property of AhR must be taken into account during investigations into a mechanism of action of xenobiotics that is able to activate AhR or that is susceptible to metabolic activation by enzymes encoded by the genes that are under the control of AhR. In this review, we examine various mechanisms by which AhR takes part in the oxidative-stress response, including antioxidant and prooxidant enzymes and cytochrome P450. We also show that AhR, as a participant in the redox balance and as a modulator of redox signals, is being increasingly studied as a target for a new class of therapeutic compounds and as an explanation for the pathogenesis of some disorders.

AHR and NRF2 in Skin Homeostasis and Atopic Dermatitis

Skin is constantly exposed to environmental insults, including toxic chemicals and oxidative stress. These insults often provoke perturbation of epidermal homeostasis and lead to characteristic skin diseases. AHR (aryl hydrocarbon receptor) and NRF2 (nuclear factor erythroid 2-related factor 2) are transcription factors that induce a battery of cytoprotective genes encoding detoxication and antioxidant enzymes in response to environmental insults. In addition to their basic functions as key regulators of xenobiotic and oxidant detoxification, recent investigations revealed that AHR and NRF2 also play critical roles in the maintenance of skin homeostasis. In fact, specific disruption of AHR function in the skin has been found to be associated with the pathogenesis of various skin diseases, most prevalently atopic dermatitis (AD). In this review, current knowledge on the roles that AHR and NRF2 play in epidermal homeostasis was summarized. Functional annotations of genetic variants, both regulatory and nonsynonymous SNPs, identified in the AHR and NRF2 loci in the human genome were also summarized. Finally, the possibility that AHR and NRF2 serve as therapeutic targets of AD was assessed.

A prospective pilot study of the T-lymphocyte response to fine particulate matter exposure

Exposure to air pollution is associated with increased morbidity and mortality. Once the fine atmospheric particulate matter (FP) is inhaled, some of its compounds can pass through the lungs and reach the bloodstream where they can come into contact with immune cells. Exposure to FP particularly affects sensitive populations such as the elderly. Aging affects the immune system, making the elderly more vulnerable. The project aims to determine the effects of FP exposure on human T cells while looking for biomarkers associated with exposure. Blood samples from 95 healthy subjects in three different age groups (20-30, 45-55 and 70-85 years) were collected to determine a potential age effect. T lymphocytes were isolated to be exposed ex vivo for 72 hours to 45 μg/mL of FP collected in Dunkirk and chemically characterized. Overexpression of the CYP1A1, CYP1B1 and CYP2S1 genes was therefore measured after exposure of the T cells to FP. These genes code for enzymes known to be involved in the metabolic activation of organic compounds such as polycyclic aromatic hydrocarbons detected in the FP sample. T-cell profiling allowed us to suggest a mixed T-helper 1/2 profile caused by exposure to FP. With regard to the influence of age, we have observed differences in the expression of certain genes, as well as an increase in interleukin-4 and -13 concentrations in the elderly. These results showed that exposure of T lymphocytes to FP causes effects on both transcriptomic and cytokine secretion levels.

Health risk of metal exposure via inhalation of cigarette sidestream smoke particulate matter

Cigarette sidestream smoke particulate matter (CSSP) is a major source of airborne metals in the indoor environment. However, the health impacts of inhalation of CSSP-bound metals are rarely studied. In this study, we quantify the amount of 37 metals discharged through CSSP from a leading Taiwan brand of cigarette, Long Life. We also estimate cancer and non-cancer risks due to inhalation of these metals and investigate possible modes of toxic action. Long Life CSSP exhibits a distinctive carcinogenic metal profile compared with Western brands. When released to a 60-m³ poorly ventilated room, Long Life CSSP metals increase the risk for cancer by a 9.26 or 20.90 in a million chance and the hazard quotient for non-cancer toxicity by 0.496 or 0.286 per cigarette depending on risk estimation system. Cd accounts for more than 90% and 80% of cancer and non-cancer risk, respectively. Long Life CSSP also contains considerable amounts of Al, Ba, and Fe. Metals are not responsible for CSSP-induced cytotoxicity, oxidative stress, and transactivation activity of AhR, Nrf2, and ERα. However, they diminish resveratrol-activated Nrf2 activity and downstream antioxidant gene expression in low-AhR-expressing lung cells. Our results suggest that chronic exposure to Long Life CSSP elevates Cd-associated cancer and non-cancer risks. Furthermore, exposure to Long Life CSSP metals may impair Nrf2-mediated antioxidant protection.

Curcumin and its derivatives inhibit 2,3,7,8,-tetrachloro-dibenzo-p-dioxin-induced expression of drug metabolizing enzymes through aryl hydrocarbon receptor-mediated pathway

Certain dioxins, including 2,3,7,8,-tetrachloro-dibenzo-p-dioxin (TCDD), are exogenous ligands for an aryl hydrocarbon receptor (AhR) and induces various drug-metabolizing enzymes. In this study, we examined the effect of curcumin on expression of drug-metabolizing enzymes through the AhR and NF-E2 related factor 2 (Nrf2) pathways. Curcumin dose-dependently inhibited TCDD-induced expression of phase I enzyme cytochrome P450 1A1 (CYP1A1) and phase II enzymes NAD(P)H:quinone oxidoreductase-1 (NQO1) and heme oxygenase 1 (HO-1) but not tert-butyl hydroquinone-induced NQO1 and HO-1, suggesting that curcumin inhibited only AhR pathway, but not Nrf2 one directly. Furthermore, we used 14 curcumin derivatives and obtained the correlation between hydrophobicity of the compounds and suppressive effect against AhR transformation. Results from the quantitative structure active correlative analysis indicated that methoxy groups and β-diketone structure possessing keto-enol tautomerism in curcumin were necessary to inhibit AhR transformation, and the addition of methyl and methoxy group(s) to the curcumin increased the inhibition effect.

Antioxidant Functions of the Aryl Hydrocarbon Receptor

The aryl hydrocarbon receptor (AhR) is a transcription factor belonging to the basic helix-loop-helix/PER-ARNT-SIM family. It is activated by a variety of ligands, such as environmental contaminants like polycyclic aromatic hydrocarbons or dioxins, but also by naturally occurring compounds and endogenous ligands. Binding of the ligand leads to dimerization of the AhR with aryl hydrocarbon receptor nuclear translocator (ARNT) and transcriptional activation of several xenobiotic phase I and phase II metabolizing enzymes. It is generally accepted that the toxic responses of polycyclic aromatic hydrocarbons, dioxins, and structurally related compounds are mediated by activation of the AhR. A multitude of studies indicate that the AhR operates beyond xenobiotic metabolism and exerts pleiotropic functions. Increasing evidence points to a protective role of the AhR against carcinogenesis and oxidative stress. Herein, I will highlight data demonstrating a causal role of the AhR in the antioxidant response and present novel findings on potential AhR-mediated antioxidative mechanisms.

Effects of β-Naphthoflavone on Ugt1a6 and Ugt1a7 Expression in Rat Brain

Uridine 5'-diphosphate-glucuronosyltransferase (UGT) catalyzes a major phase II reaction in a drug-metabolizing enzyme system. Although the UGT1A subfamily is expressed mainly in the liver, it is also expressed in the brain. The purpose of the present study was to elucidate the effect of β-naphthoflavone (BNF), one of the major inducers of drug-metabolizing enzymes, on Ugt1a6 and Ugt1a7 mRNA expression and their glucuronidation in the rat brain. Eight-week-old male Sprague-Dawley rats were treated intraperitoneally with BNF (80 mg/kg), once daily for 7 d. Ugt1a6 and Ugt1a7 mRNA expression increased in the cerebellum and hippocampus (Ugt1a6: 2.1- and 2.3-fold, respectively; Ugt1a7: 1.7- and 2.8-fold, respectively); acetaminophen glucuronidation also increased in the same regions by 4.1- and 2.7-fold, respectively. BNF induced Ugt1a6 and Ugt1a7 mRNA expression and their glucuronidation, and the degree of induction differed among 9 regions. BNF also upregulated CYP1A1, CYP1A2, and CYP1B1 mRNAs in the rat brain. Since the aryl hydrocarbon receptor signaling pathway was activated by BNF, it is indicated that Ugt1a6 and Ugt1a7 were induced via AhR in the rat brain. This study clarified that Ugt1a6 and Ugt1a7 mRNA expression and their enzyme activities were altered by BNF, suggesting that these changes may lead to alteration in the pharmacokinetics of UGT substrate in rat brain.

Influence of dietary fat type on benzo(a)pyrene [B(a)P] biotransformation in a B(a)P-induced mouse model of colon cancer

In the US alone, around 60,000 lives/year are lost due to colon cancer. Diet and environment have been implicated in the development of sporadic colon tumors. The objective of this study was to determine how dietary fat potentiates the development of colon tumors through altered B(a)P biotransformation, using the Adenomatous polyposis coli with Multiple intestinal neoplasia mouse model. Benzo(a)pyrene was administered to mice through tricaprylin, and unsaturated (USF; peanut oil) and saturated (SF; coconut oil) fats at doses of 50 and 100 μg/kg via oral gavage over a 60-day period. Blood, colon, and liver were collected at the end of exposure period. The expression of B(a)P biotransformation enzymes [cytochrome P450 (CYP)1A1, CYP1B1 and glutathione-S-transferase] in liver and colon were assayed at the level of protein, mRNA and activities. Plasma and tissue samples were analyzed by reverse phase high-performance liquid chromatography for B(a)P metabolites. Additionally, DNA isolated from colon and liver tissues was analyzed for B(a)P-induced DNA adducts by the (32)P-postlabeling method using a thin-layer chromatography system. Benzo(a)pyrene exposure through dietary fat altered its metabolic fate in a dose-dependent manner, with 100 μg/kg dose group registering an elevated expression of B(a)P biotransformation enzymes, and greater concentration of B(a)P metabolites, compared to the 50 μg/kg dose group (P<.05). This effect was more pronounced for SF group compared to USF group (P<.05). These findings establish that SF causes sustained induction of B(a)P biotransformation enzymes and extensive metabolism of this toxicant. As a consequence, B(a)P metabolites were generated to a greater extent in colon and liver, whose concentrations also registered a dose-dependent increase. These metabolites were found to bind with DNA and form B(a)P-DNA adducts, which may have contributed to colon tumors in a subchronic exposure regimen.

Nuclear receptors in the cross-talk of drug metabolism and inflammation

Inflammation and infection have long been known to affect the activity and expression of enzymes involved in hepatic and extrahepatic drug clearance. Significant advances have been made to elucidate the molecular mechanisms underlying the complex cross-talk between inflammation and drug-metabolism alterations. The emergent role of ligand-activated transcriptional regulators, belonging to the nuclear receptor (NR) superfamily, is now well established. The NRs, pregnane X receptor, constitutive androstane receptor, retinoic X receptor, glucocorticoid receptor, and hepatocyte nuclear factor 4, and the basic helix-loop-helix/Per-ARNT-Sim family member, aryl hydrocarbon receptor, are the main regulators of the detoxification function. According to the panel of mediators secreted during inflammation, a cascade of numerous signaling pathways is activated, including nuclear factor kappa B, mitogen-activated protein kinase, and the Janus kinase/signal transducer and activator of transcription pathways. Complex cross-talk is established between these signaling pathways regulating either constitutive or induced gene expression. In most cases, a mutual antagonism between xenosensor and inflammation signaling occurs. This review focuses on the molecular and cellular mechanisms implicated in this cross-talk.

Microsomal epoxide hydrolase (EPHX1) polymorphisms are associated with aberrant promoter methylation of ERCC3 and hematotoxicity in benzene-exposed workers

Benzene is an important industrial chemical and widespread environmental pollutant known to induce leukemia and other blood disorders. To be carcinogenic, benzene must be metabolized to produce toxic metabolites. To investigate whether single nucleotide polymorphisms (SNPs) in the metabolic enzyme genes are associated with benzene-induced alterations in DNA methylation and hematotoxicity, we genotyped four commonly studied SNPs in three metabolic enzymes genes CYP1A1, EPHX1 and NQO1; and analyzed promoter DNA methylation status in 11 genes which have been reported to be associated with benzene-induced hematotoxicity (BLM, CYP1A1, EPHX1, ERCC3, NQO1, NUDT1, p15, p16, RAD51, TP53 and WRAP53) in 77 benzene-exposed workers and 25 unexposed controls in China. ERCC3, a DNA repair gene, showed a small but statistically significant increase of promoter DNA methylation in the exposed group compared with the unexposed group (mean ± SD: 4.73 ± 3.46% vs. 3.63 ± 1.96%, P = 0.048). We also observed that an increased number of C allele for rs1051740 in EPHX1 was associated with decreased ERCC3 methylation levels in benzene-exposed workers (P(trend)  = 0.001), but not in unexposed controls (P(trend)  = 0.379). Interestingly, another EPHX1 SNP (rs2234922) was associated with lower white blood cell (WBC) counts (P(trend)  = 0.044) in benzene-exposed workers. These associations remained the same when ERCC3 promoter methylation and WBCs were dichotomized according to the 90th percentile (≥6%) of methylation levels in controls and a leucopenia cutoff (<4 × 10(9) /L), respectively. Our findings suggest that benzene exposure may be associated with hypermethylation in ERCC3, and that genetic variants in EPHX1 may play an important role in epigenetic changes and hematotoxicity among benzene-exposed workers.

AhR activation underlies the CYP1A autoinduction by A-998679 in rats

Xenobiotic-mediated induction of cytochrome P450 (CYP) drug metabolizing enzymes (DMEs) is frequently encountered in drug discovery and can influence disposition, pharmacokinetic, and toxicity profiles. The CYP1A subfamily of DMEs plays a central role in the biotransformation of several drugs and environmental chemicals. Autoinduction of drugs through CYP3A enzymes is a common mechanism for their enhanced clearance. However, autoinduction via CYP1A is encountered less frequently. In this report, an experimental compound, A-998679 [3-(5-pyridin-3-yl-1,2,4-oxadiazol-3-yl) benzonitrile], was shown to enhance its own clearance via induction of Cyp1a1 and Cyp1a2. Rats were dosed for 5 days with 30, 100, and 200 mg/kg/day A-998679. During the dosing period, the compound's plasma AUC decreased at 30 mg/kg (95%) and 100 mg/kg (80%). Gene expression analysis and immunohistochemistry of the livers showed a large increase in the mRNA and protein levels of Cyp1a, which was involved in the biotransformation of A-998679. Induction of CYP1A was confirmed in primary rat, human, and dog hepatocytes. The compound also weakly inhibited CYP1A2 in human liver microsomes. A-998679 activated the aryl hydrocarbon receptor (AhR) in a luciferase gene reporter assay in HepG2 cells, upregulated expression of genes associated with AhR activation in rat liver and enhanced nuclear migration of AhR in HepG2 cells. Collectively these results demonstrate that A-998679 is an AhR activator that induces Cyp1a1 and Cyp1a2 expression, resulting in an autoinduction phenomenon. The unique properties of A-998679, along with its novel structure distinct from classical polycyclic aromatic hydrocarbons (PAHs), may warrant its further evaluation as a tool compound for use in studies involving AhR biology and CYP1A-related mechanisms of drug metabolism and toxicity.

Induction of CYP1 family members under low-glucose conditions requires AhR expression and occurs through the nuclear translocation of AhR

Cross-talk between the aryl hydrocarbon receptor (AhR) pathway and the typical stress response is thought to be an important signal transduction in response to nutrient-stress conditions, such as glucose deprivation in liver cells. In the present study, we demonstrate that reduction of glucose concentration in the medium of HepG2 cells, a human hepatocellular carcinoma cell line, induces the CYP1 family and Nrf2. RNAi for AhR abolishes the induction of expression of CYP1 and Nrf2. These inductions are accompanied by the translocation of AhR into the nucleus in response to low-glucose conditions. Endogenous compounds are recruited as AhR ligands to induce various gene expressions, and our present results suggest that an endogenous AhR ligand is produced under low-glucose conditions and that the role of AhR as a transcription factor is related to the low-glucose response. The recommended glucose concentration (4.5 g/L) in the medium for culture of HepG2 was used as the high-glucose concentration in this study. We adopted 1.0 g/L as the low-glucose condition for elucidation of mechanisms of the stress response. These results will be useful to understand the relationship between drug-metabolizing enzymes and mechanisms of the anti-stress response of tumor cells, and will also be useful for investigating preventive remedies against tumor angiogenesis.

Hepatic coenzyme Q redox balance of fishes as a potential bioindicator of environmental contamination by polycyclic aromatic hydrocarbons

In this communication, we introduce a novel biomarker of aquatic contamination based on the xenobiotic-induced response of the hepatic coenzyme Q (CoQ) redox balance of fishes to polycyclic aromatic hydrocarbons (PAHs). The method is demonstrated by comparing changes in the liver CoQ redox balance with that measured using the CYP1A-based, 7-ethoxyresofurin-O-deethylase activity assay, on administration of benzo[a]pyrene (BaP) and β-naphthoflavone (BNF) to Barramundi (Lates calcarifer). Both assays showed comparable dose-dependent effects in fish treated with BaP or BNF. Perturbation in the constitutive hepatic CoQ redox balance of fishes may thus provide a simple biomarker of aquatic PAH contamination.

Methoxylation of resveratrol: effects on induction of NAD(P)H quinone-oxidoreductase 1 (NQO1) activity and growth inhibitory properties

A series of methoxystilbenes (E and Z isomers) related to resveratrol were investigated for their effects on NQO1 induction in murine hepatoma cells and growth inhibitory effects on human cancer cell lines. Both activities were enhanced in compounds with methoxy groups on rings A and B of resveratrol but methoxylation of the di-meta (3,5) hydroxyl groups on ring A of resveratrol was found to be more critical for improving activity. Strikingly different structure-activity trends were observed, namely the association of E isomers with potent NQO1 induction activity and Z isomers with growth inhibitory properties. The introduction of ortho-methoxy groups on ring A greatly benefited NQO1 induction activity while meta/para methoxy groups on ring A were preferred for potent growth inhibitory effects. These results serve to highlight the contrasting effects on different activities brought about by methoxylation, which is widely employed as a structural modification approach to improve potency and bioavailability of resveratrol. It serves as a timely reminder that in the course of structural modification, a balance between optimizing desired outcomes against unwanted effects is necessary and the most potent analog need not always be the most desirable.

Differential effects of indirubin and 2,3,7,8-tetrachlorodibenzo-p-dioxin on the aryl hydrocarbon receptor (AhR) signalling in liver progenitor cells

In the present study, we investigated the effects of potential endogenous ligand indirubin on the aryl hydrocarbon receptor (AhR) signalling, with a focus on the AhR-dependent gene expression and cell cycle progression in rat liver progenitor cells, and compared them with the effects of a model toxic AhR ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The low (picomolar and nanomolar) doses of indirubin, corresponding to expected endogenous levels, induced a transient translocation of AhR to the nucleus, while high (micromolar) doses induced a long-term AhR nuclear translocation, followed by its degradation, similar to the effects of TCDD. Whereas high doses of indirubin recruited AhR/ARNT1 dimer to rat Cyp1a1 promoter, the low doses did not induce its DNA binding, as revealed by the chromatin immunoprecipitation assay. This corresponded with the fact that the micromolar doses of indirubin significantly increased Cyp1a1/1b1 mRNA in a way similar to TCDD, while the low doses of indirubin were only poor inducers of Cyp1a1/1b1 expression. Comparable patterns of expression were observed also for other AhR gene targets, such as Nqo1 and Nrf2. Also, only micromolar doses of indirubin were able to mimic the effects of TCDD on cell cycle and proliferation of liver progenitor cells or hepatoma cells. Nevertheless, indirubin at low concentrations may have unique effects on gene expression in non-tumorigenic cells. Although both TCDD and the high doses of indirubin repressed plakoglobin (Jup) expression, the picomolar doses of indirubin, unlike the equimolar doses of TCDD, increased mRNA levels of this important desmosomal and adherens junctions constituent. These present data suggest that the outcome of AhR activation induced by indirubin at concentrations expected in vivo may differ from the AhR signalling triggered by exogenous toxic ligands, such as TCDD.

Transcriptional regulation of human carboxylesterase 1A1 by nuclear factor-erythroid 2 related factor 2 (Nrf2)

Human carboxylesterase (CES) 1A, which is predominantly expressed in liver and lung, plays an important role in the hydrolysis of endogenous compounds and xenobiotics. CES1A is reported to be induced in human hepatocytes by butylated hydroxyanisole, ticlopidine and diclofenac, and the induction is assumed to be caused by oxidative stress. However, the molecular mechanism remains to be determined. In this study, we sought to investigate whether CES1A is regulated by nuclear factor-erythroid 2 related factor 2 (Nrf2), which is a transcriptional factor activated by oxidative stress, and clarify the molecular mechanism. Real-time reverse transcription-PCR assays revealed that CES1A1 mRNA was significantly induced by tert-butylhydroquinone (tBHQ) and sulforaphane (SFN), which are representative activators of Nrf2 in HepG2, Caco-2 and HeLa cells. The induction was completely suppressed with small interfering RNA for Nrf2. In HepG2 cells, the CES1A protein level and imidapril hydrolase activity, which is specifically catalyzed by CES1A, were also significantly induced by tBHQ and SFN. Luciferase assays revealed that the antioxidant response element (ARE) at -2025 in the CES1A1 gene was responsible for the transactivation by Nrf2. In addition, electrophoretic mobility shift assays and chromatin immunoprecipitation assays revealed that Nrf2 binds to the ARE in the CES1A1 gene. These findings clearly demonstrated that human CES1A1 is induced by Nrf2. This is the first study to demonstrate the molecular mechanism of the inducible regulation of human CES1A1.

Introducing the "TCDD-inducible AhR-Nrf2 gene battery"

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induces genes via the transcription factor aryl hydrocarbon receptor (AhR), including Cyp1a1, NAD(P)H:quinone oxidoreductase 1 (Nqo1), UDP-glucuronosyltransferase 1a6 (Ugt1a6), and glutathione S-transferase a1 (Gsta1). These genes are referred to as the "AhR gene battery." However, Nqo1 is also considered a prototypical target gene of the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2). In mice, TCDD induction of Nrf2 and Nrf2 target, Nqo1, is dependent on AhR, and thus TCDD induction of drug-processing genes may be routed through an AhR-Nrf2 sequence. There has been speculation that Nrf2 may be involved in the TCDD induction of drug-processing genes; however, the data are not definitive. Therefore, to address whether TCDD induction of Nqo1, Ugts, and Gsts is dependent on Nrf2, we conducted the definitive experiment by administering TCDD (50 mug/kg, ip) to Nrf2-null and wild-type (WT) mice and collecting livers 24 h later to quantify the mRNA of drug-processing genes. TCDD induction of Cyp1a1 and Ugt1a1 was similar in WT and Nrf2-null mice, whereas TCDD induction of Ugt1a5 and 1a9 was blunted in Nrf2-null mice. TCDD induced Nqo1, Ugt1a6, 2b34, 2b35, 2b36, UDP-glucuronic acid-synthesizing gene UDP-glucose dehydrogenase, and Gsta1, m1, m2, m3, m6, p2, t2, and microsomal Gst1 in WT mice but not in Nrf2-null mice. Therefore, the present study demonstrates the novel finding that Nrf2 is required for TCDD induction of classical AhR battery genes Nqo1, Ugt1a6, and Gsta1, as well as most Ugt and Gst isoforms in livers of mice.

Functionalized 3-benzylidene-indolin-2-ones: inducers of NAD(P)H-quinone oxidoreductase 1 (NQO1) with antiproliferative activity

Functionalized benzylidene-indolin-2-ones are widely associated with antiproliferative activity. The scaffold is not normally associated with chemoprevention in spite of the presence of a nitrogen-linked Michael acceptor moiety that may predispose members to induction of NQO1, a widely used biomarker of chemopreventive potential. To investigate this possibility, we have synthesized and evaluated a series of functionalized 3-benzylidene-indolin-2-ones for induction of NQO1 in murine Hepa1c1c7 cells as well as antiproliferative activity against two human cancer cell lines (MCF-7, HCT116). The benzylideneindolinones were found to be good inducers of NQO1 activity, with 85% of test compounds able to increase basal NQO1 activity by more than twofold at concentrations of 10 microM. By contrast, fewer compounds (11%) tested at the same concentration were able to reduce cell viability by more than 50%. Structure activity relationships showed that the nitrogen linked Michael acceptor moiety was an essential requirement for both activities. This common feature notwithstanding, substitution of the 3-benzylidene-indolin-2-one core structure affected NQO1 induction and antiproliferative activities in dissimilar ways, underscoring different structural requirements for these two activities. Nonetheless, promising compounds (10, 42, 45-48) were identified that combine selective induction of NQO1 with potent antiproliferative activity. A potential advantage of such agents would be the ability to provide added protection to normal cells by the up-regulation of NQO1 and other phase II enzymes while simultaneously targeting neoplastic cells.

Importance of hepatic induction of constitutive androstane receptor and other transcription factors that regulate xenobiotic metabolism and transport

Aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), pregnane X receptor (PXR), peroxisome proliferator-activated receptor-alpha (PPARalpha), and nuclear factor-E2-related factor 2 (Nrf2) are transcription factors that mediate xenobiotic induction of biotransformation enzymes and transporters. The purpose of this study was to determine the tissue distribution and xenobiotic induction of these transcription factors and their associated target genes in mice. Many of these transcription factors were most highly expressed in extrahepatic tissues. CAR expression in female liver was twice that in male liver. This corresponded with greater induction of the CAR target genes Cyp2b10 and multidrug resistance-associated protein (Mrp) 4 by the CAR activator 1,4-bis-[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) in female liver than in male liver. Mice were treated with xenobiotic activators of AhR, CAR, PXR, PPARalpha, or Nrf2 and their associated marker genes were highly induced in liver by these xenobiotic activators. Transcription factor target gene induction occurred with minimal induction of their associated transcription factors. CAR expression was induced by the AhR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), leading to increased basal expression of Cyp2b10 mRNA and enhanced induction of Cyp2b10 by TCPOBOP. Mrp2, 3, and 4 induction was augmented by cotreatment with TCDD and TCPOBOP compared with treatment with either compound alone. These studies illustrate CAR induction by TCDD in mice, indicating that AhR may transcriptionally regulate CAR and thus enhance induction of key metabolism and transporter genes by the CAR activator TCPOBOP. Collectively, these studies illustrate the fact that some xenobiotic inducers may elicit their response through mechanisms involving transcription factor regulation.

Suppression of endogenous antioxidant enzymes by 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced oxidative stress in chicken liver during development

Domestic chickens (Gallus gallus) are an excellent model in which to evaluate developmental toxicity and oxidative stress because of their high sensitivity to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The goal of this study was to measure the effects of environmentally relevant doses of TCDD on endogenous hepatic antioxidant enzyme activity in hatchling chickens. The vehicle (sunflower oil) or 2, 20, or 200 pg/g TCDD was injected into chicken eggs before incubation. On hatching, livers were harvested and quickly frozen. The changes in activity of antioxidant enzymes, including glutathione peroxidase (GPx), glutathione reductase (GRx), copper zinc superoxide dismutase (SOD), and catalase (CAT) were determined as indicators of oxidative stress. TCDD exposure was associated with a significant suppression of the activities of the protective endogenous enzymes GPx, GRx, and SOD in the liver, even at the lowest dose. CAT activity was also suppressed, but not significantly. The measured decreases were 37% to 63% for GPx, 50% to 58% for GRx, 30% to 40% for SOD, and 16% to 24% for CAT. Noncomplex dose-response relationships were evident in GPx and GRx, whereas SOD and CAT curves were U-shaped. These results demonstrate that a decreased ability to scavenge reactive oxygen species may result from developmental TCDD exposure at very low doses, contributing to oxidative stress and thus to the embryotoxicity of TCDD.

The aryl hydrocarbon receptor, more than a xenobiotic-interacting protein

The aryl hydrocarbon (dioxin) receptor (AhR) has been studied for several decades largely because of its critical role in xenobiotic-induced toxicity and carcinogenesis. Albeit this is a major issue in basic and clinical research, an increasing number of investigators are turning their efforts to try to understand the physiology of the AhR under normal cellular conditions. This is an exciting area that covers cell proliferation and differentiation, endogenous mechanisms of activation, gene regulation, tumor development and cell motility and migration, among others. In this review, we will attempt to summarize the studies supporting the implication of the AhR in those endogenous cellular processes.

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.

Genetic polymorphisms of CYP1A1, CYP2E1, GSTM1, and GSTT1 associated with head and neck cancer

BACKGROUND

Alcohol intake and tobacco smoke, in addition to other environmental and genetic factors, have been associated with head and neck cancer. We evaluated the role of metabolic enzyme polymorphisms on the risk of head and neck cancer in a hospital-based case-control study.

METHODS

CYP1A1MspI, CYP2E1PstI, GSTM1, and GSTT1polymorphisms were evaluated in 103 histologically confirmed head and neck cancer cases and 102 controls by means of polymerase chain reaction-restriction fragment length polymorphism methods.

RESULTS

GSTM1null increased the risk of head and neck cancer (odds ratio [OR], 2.2; 95% confidence interval [95% CI], 1.24-3.79), oral cancer (OR, 2.8; 95% CI, 1.28-5.98), and pharyngeal cancer (OR, 2.2; 95% CI, 1.08-4.63). CYP2E1PstI polymorphism indicated a risk for oral cancer (OR, 3.6; 95% CI, 1.29-11.56). The joint effect of GSTM1 null and CYP1A1 polymorphism increased the risk of head and neck cancer (OR, 2.4; 95% CI, 1.13-5.10).

CONCLUSIONS

GSTM1 null alone or associated with CYP1A1 increased the risk of head and neck cancer; the CYP2E1PstI mutated allele increased the risk for only oral cancer.

The role of aryl hydrocarbon receptor in the pathogenesis of cardiovascular diseases

Numerous experimental and epidemiological studies have demonstrated that polycyclic aromatic hydrocarbons (PAHs), which are major constituents of cigarette tobacco tar, are strongly involved in the pathogenesis of the cardiovascular diseases (CVDs). Knowing that PAH-induced toxicities are mediated by the activation of a cytosolic receptor, aryl hydrocarbon receptor (AhR), which regulates the expression of a group of xenobiotic metabolizing enzymes (XMEs) such as CYP1A1, CYP1A2, CYP1B1, NQO1, and GSTA1, suggests a direct link between AhR-regulated XMEs and CVDs. Therefore, identifying the localization and expression of the AhR and its regulated XMEs in the cardiovascular system (CVS) is of major importance in understanding their physiological and pathological roles. Generally, it was believed that the levels of AhR-regulated XMEs are lower in the CVS than in the liver; however, it has been shown that similar or even higher levels of expression are demonstrated in the CVS in a tissue- and species-specific manner. Moreover, most, if not all, AhR-regulated XMEs are differentially expressed in most of the CVS, particularly in the endothelium cells, aorta, coronary arteries, and ventricles. Although the exact mechanisms of PAH-mediated cardiotoxicity are not fully understood, several mechanisms are proposed. Generally, induction of CYP1A1, CYP1A2, and CYP1B1 is considered cardiotoxic through generating reactive oxygen species (ROS), DNA adducts, and endogenous arachidonic acid metabolites. However the cardioprotective properties of NQO1 and GSTA1 are mainly attributed to the antioxidant effect by decreasing ROS and increasing the levels of endogenous antioxidants. This review provides a clear understanding of the role of AhR and its regulated XMEs in the pathogenesis of CVDs, in which imbalance in the expression of cardioprotective and cardiotoxic XMEs is the main determinant of PAH-mediated cardiotoxicity.

CYP1A in TCDD toxicity and in physiology-with particular reference to CYP dependent arachidonic acid metabolism and other endogenous substrates

Toxicologic and physiologic roles of CYP1A enzyme induction, the major biochemical effect of aryl hydrocarbon receptor activation by TCDD and other receptor ligands, are unknown. Evidence is presented that CYP1A exerts biologic effects via metabolism of endogenous substrates (i.e., arachidonic acid, other eicosanoids, estrogens, bilirubin, and melatonin), production of reactive oxygen, and effects on K(+) and Ca(2+) channels. These interrelated pathways may connect CYP1A induction to TCDD toxicities, including cardiotoxicity, vascular dysfunction, and wasting. They may also underlie homeostatic roles for CYP1A, especially when transiently induced by common chemical exposures and environmental conditions (i.e., tryptophan photoproducts, dietary indoles, and changes in oxygen tension).

Transcriptional regulation of the NAD(P)H:quinone oxidoreductase 1 and glutathione S-transferase ya genes by mercury, lead, and copper

Recently, we demonstrated the ability of heavy metals, particularly Hg2+, Pb2+, and Cu2+, to differentially modulate in Hepa 1c1c7 cells the expression of the phase II xenobiotic metabolizing enzymes NAD(P)H:quinone oxidoreductase 1 (Nqo1) and glutathione S-transferase subunit Ya (Gst ya) genes, yet the mechanisms involved remain unknown. To investigate the molecular mechanisms involved in the regulation of Nqo1 and Gst ya genes by heavy metals, Hepa 1c1c7 cells were treated with Hg2+, Pb2+, or Cu2+ in the presence and absence of 2,3,7,8-tetrachlorodibenzo-p-dioxin, a potent inducer of Nqo1, Gst ya, and Cyp1a1 genes. Analysis of the time-dependent effect of heavy metals revealed that Hg2+ and Pb2+ increased whereas Cu2+ inhibited the constitutive and inducible expression of Nqo1 and Gst ya mRNAs in a time-dependent manner. The RNA synthesis inhibitor actinomycin D significantly inhibited the Nqo1 and Gst ya mRNA induction in response to metals, indicating a requirement of de novo RNA synthesis. The protein synthesis inhibitor cycloheximide significantly inhibited metal-mediated induction of Nqo1 and Gst ya mRNAs, which coincided with a decrease in the nuclear factor erythroid 2-related factor 2 (Nrf2) protein expression, implying the requirement of Nrf2 protein synthesis for the induction of these genes. Furthermore, inhibition of Nrf2 protein degradation by carbobenzoxy-L-leucyl-L-leucyl-leucinal (MG-132), a 26S proteasome inhibitor, significantly reversed the cycloheximide-mediated inhibition of Nqo1 and Gst ya mRNAs, which coincided with an increase in the expression of Nrf2, confirming that a transcriptional mechanism is involved. Nqo1 and Gst ya mRNA and protein decay experiments revealed lack of post-transcriptional and post-translational mechanisms. This is the first demonstration that heavy metals regulate the expression of Nqo1 and Gst ya genes through a transcriptional mechanism.

Regulation of mouse organic anion-transporting polypeptides (Oatps) in liver by prototypical microsomal enzyme inducers that activate distinct transcription factor pathways

Drug-metabolizing enzymes and transporters are key factors that affect disposition of xenobiotics. Phase I enzyme induction by classes of microsomal enzyme inducers occurs via activation of transcription factors such as aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), pregnane X receptor (PXR), peroxisome proliferator-activated receptor alpha (PPARalpha), and nuclear factor erythroid 2-related factor 2 (Nrf2). However, regulation of organic anion-transporting polypeptide (Oatp) uptake transporters by these factors is poorly understood. Hepatic Oatp uptake of some chemicals must occur prior to biotransformation; thus, we hypothesize that expression of Oatps and biotransformation enzymes is coordinately regulated in liver. In the present study, the effects of known chemical activators of AhR, CAR, PXR, PPARalpha, and Nrf2 on the hepatic mRNA expression of mouse Oatps and drug-metabolizing enzymes were quantified by the branched DNA assay. All chemicals increased the expression of their well characterized target drug-metabolizing enzymes: AhR ligands increased Cyp1A1, CAR activators increased Cyp2B10, PXR ligands increased Cyp3A11, PPARalpha ligands increased Cyp4A14, and Nrf2 activators induced NAD(P)H:quinone oxidoreductase 1. AhR ligands (2,3,7,8-tetrachlorodibenzo-p-dioxin, polychlorinated biphenyl 126, and beta-naphthoflavone) increased Oatp2b1 and 3a1 mRNA expression in liver. CAR activators [phenobarbital, 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene, and diallyl sulfide] decreased Oatp1a1 mRNA expression. Two PXR ligands [pregnenolone-16alpha-carbonitrile (PCN) and spironolactone] increased Oatp1a4 mRNA expression in liver, whereas PXR ligands (PCN, spironolactone, and dexamethasone) and PPARalpha ligands (clofibrate, ciprofibrate, and diethylhexylphthalate) decreased Oatp1a1, 1b2, 2a1, and 2b1 mRNA expression in liver. Nrf2 activators (oltipraz, ethoxyquin, and butylated hydroxyanisole) down-regulated Oatp1a1 but up-regulated Oatp2b1 mRNA expression. Therefore, only a few transcription factor activators increased Oatp expression, and, surprisingly, many decreased Oatp expression.

Aging-associated changes in gene expression in the ACI rat prostate: Implications for carcinogenesis

BACKGROUND

Prostate cancer is the most frequently diagnosed neoplasm and the second leading cause of cancer-related mortality in men. Although the incidence of prostate cancer increases with age, the link between aging and prostate cancer is poorly understood.

METHODS

Affymetrix oligonucleotide microarrays were used to analyze the mRNA expression levels in the dorsolateral prostates from 6- and 18-month-old ACI rats. Real-time RT-PCR and immunohistochemistry was performed to validate microarray data in a select set of genes.

RESULTS

Microarray analysis revealed changes in gene expression associated with inflammation, oxidative stress, tissue remodeling, and energy metabolism. Most of these changes have been related to increased proliferative status of the prostate, anti-apoptosis, activated stroma, and alteration of the energy metabolism.

CONCLUSIONS

Age-associated alterations in the gene expression profile may put the aging prostate in risk for the initiation, promotion, and progression of neoplastic transformation in both our animal model and humans.

2,3,7,8-Tetrachlorodibenzo-p-dioxin Induces Insulin-Like Growth Factor Binding Protein-1 Gene Expression and Counteracts the Negative Effect of Insulin

Recent epidemiological studies have revealed a possible correlation between exposure to high levels of dioxins or dioxin-like compounds and diabetes. Yet the interaction between insulin and dioxin actions remains elusive. We studied the regulation of insulin-like growth factor binding protein-1 (IGFBP-1), a protein involved in glucose homeostasis and whose expression is down-regulated by insulin. We showed that 2,3,7,8-tetrachorodibenzo-p-dioxin (TCDD) specifically induced IGFBP-1 mRNA in human hepatocytes and HepG2 human hepatoma cells (2.5- and 8-fold, respectively). Cellular and secreted IGFBP-1 protein levels were also up-regulated. Transfection and reporter assays showed that the IGFBP-1 promoter was activated by TCDD and that this activation was dependent on the integrity of a proximal xenobiotic-responsive element (XRE). This XRE, located near the insulin-glucocorticoid regulatory region, binds the aryl-hydrocarbon receptor. In agreement with previous studies, the IGFBP-1 promoter was down-regulated by insulin (50%); we show here that although TCDD activated the IGFBP-1 promoter 5- to 6-fold, the combination of TCDD and insulin led to an expression level of IGFBP-1 that was higher than basal level (2- to 3-fold activation). Similar regulations were observed for the endogenous IGFBP-1 mRNA. These data suggest that the xenobiotic-hormonal regulatory region of the IGFBP-1 promoter mediates an up-regulation of IGFBP-1 expression by TCDD even in the presence of insulin. Because IGFBP-1 modulates blood glucose levels, the up-regulation of IGFBP-1 by dioxins might account for the disruptive effects of these pollutants on glucose metabolism.