Contribution of the Ah receptor to the phenolic antioxidant-mediated expression of human and rat UDP-glucuronosyltransferase UGT1A6 in Caco-2 and rat hepatoma 5L cells

The Role of Endocrine Disruption Chemical-Regulated Aryl Hydrocarbon Receptor Activity in the Pathogenesis of Pancreatic Diseases and Cancer

The aryl hydrocarbon receptor (AHR) serves as a ligand-activated transcription factor crucial for regulating fundamental cellular and molecular processes, such as xenobiotic metabolism, immune responses, and cancer development. Notably, a spectrum of endocrine-disrupting chemicals (EDCs) act as agonists or antagonists of AHR, leading to the dysregulation of pivotal cellular and molecular processes and endocrine system disruption. Accumulating evidence suggests a correlation between EDC exposure and the onset of diverse pancreatic diseases, including diabetes, pancreatitis, and pancreatic cancer. Despite this association, the mechanistic role of AHR as a linchpin molecule in EDC exposure-related pathogenesis of pancreatic diseases and cancer remains unexplored. This review comprehensively examines the involvement of AHR in EDC exposure-mediated regulation of pancreatic pathogenesis, emphasizing AHR as a potential therapeutic target for the pathogenesis of pancreatic diseases and cancer.

Drug-drug interactions that alter the exposure of glucuronidated drugs: Scope, UDP-glucuronosyltransferase (UGT) enzyme selectivity, mechanisms (inhibition and induction), and clinical significance

Drug-drug interactions (DDIs) arising from the perturbation of drug metabolising enzyme activities represent both a clinical problem and a potential economic loss for the pharmaceutical industry. DDIs involving glucuronidated drugs have historically attracted little attention and there is a perception that interactions are of minor clinical relevance. This review critically examines the scope and aetiology of DDIs that result in altered exposure of glucuronidated drugs. Interaction mechanisms, namely inhibition and induction of UDP-glucuronosyltransferase (UGT) enzymes and the potential interplay with drug transporters, are reviewed in detail, as is the clinical significance of known DDIs. Altered victim drug exposure arising from modulation of UGT enzyme activities is relatively common and, notably, the incidence and importance of UGT induction as a DDI mechanism is greater than generally believed. Numerous DDIs are clinically relevant, resulting in either loss of efficacy or an increased risk of adverse effects, necessitating dose individualisation. Several generalisations relating to the likelihood of DDIs can be drawn from the known substrate and inhibitor selectivities of UGT enzymes, highlighting the importance of comprehensive reaction phenotyping studies at an early stage of drug development. Further, rigorous assessment of the DDI liability of new chemical entities that undergo glucuronidation to a significant extent has been recommended recently by regulatory guidance. Although evidence-based approaches exist for the in vitro characterisation of UGT enzyme inhibition and induction, the availability of drugs considered appropriate for use as 'probe' substrates in clinical DDI studies is limited and this should be research priority.

Pathogenic mechanisms and regulatory factors involved in alcoholic liver disease

Alcoholism is a widespread and damaging behaviour of people throughout the world. Long-term alcohol consumption has resulted in alcoholic liver disease (ALD) being the leading cause of chronic liver disease. Many metabolic enzymes, including alcohol dehydrogenases such as ADH, CYP2E1, and CATacetaldehyde dehydrogenases ALDHsand nonoxidative metabolizing enzymes such as SULT, UGT, and FAEES, are involved in the metabolism of ethanol, the main component in alcoholic beverages. Ethanol consumption changes the functional or expression profiles of various regulatory factors, such as kinases, transcription factors, and microRNAs. Therefore, the underlying mechanisms of ALD are complex, involving inflammation, mitochondrial damage, endoplasmic reticulum stress, nitrification, and oxidative stress. Moreover, recent evidence has demonstrated that the gut-liver axis plays a critical role in ALD pathogenesis. For example, ethanol damages the intestinal barrier, resulting in the release of endotoxins and alterations in intestinal flora content and bile acid metabolism. However, ALD therapies show low effectiveness. Therefore, this review summarizes ethanol metabolism pathways and highly influential pathogenic mechanisms and regulatory factors involved in ALD pathology with the aim of new therapeutic insights.

Functions of the aryl hydrocarbon receptor (AHR) beyond the canonical AHR/ARNT signaling pathway

The aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor regulating adaptive and maladaptive responses toward exogenous and endogenous signals. Research from various biomedical disciplines has provided compelling evidence that the AHR is critically involved in the pathogenesis of a variety of diseases and disorders, including autoimmunity, inflammatory diseases, endocrine disruption, premature aging and cancer. Accordingly, AHR is considered an attractive target for the development of novel preventive and therapeutic measures. However, the ligand-based targeting of AHR is considerably complicated by the fact that the receptor does not always follow the beaten track, i.e. the canonical AHR/ARNT signaling pathway. Instead, AHR might team up with other transcription factors and signaling molecules to shape gene expression patterns and associated physiological or pathophysiological functions in a ligand-, cell- and micromilieu-dependent manner. Herein, we provide an overview about some of the most important non-canonical functions of AHR, including crosstalk with major signaling pathways involved in controlling cell fate and function, immune responses, adaptation to low oxygen levels and oxidative stress, ubiquitination and proteasomal degradation. Further research on these diverse and exciting yet often ambivalent facets of AHR biology is urgently needed in order to exploit the full potential of AHR modulation for disease prevention and treatment.

Dill Shows Potential for Herb-Drug Interactions via Up-Regulation of CYP1A2, CYP2C19, SULT1A1, NAT2 and ABCB1 in Caco-2 Cells

<b>Background and Objective:</b> Dill<i> </i>(<i>Anethum graveolens</i> L.) has the potential to develop as a new alternative medicine due to its pharmacological activities. However, studies into its safety regarding herb-drug interactions have been neglected. This study investigated the risk of dill-induced herb-drug interactions (HDI) by examining its effect on the expression of phase I and II drug-metabolizing enzyme and transporter genes in Caco-2 cells. <b>Materials and Methods:</b> Caco-2 cells (5×10<sup>5</sup> cells/well) were treated with 10 μM ketoconazole, 20 μM rifampicin or dill extract (60-240 μg mL<sup>1</sup>) for 72 hrs. Cell viability was assessed using the resazurin assay and reactive oxygen species (ROS) content was determined with 2 ,7 -dichlorofluorescein diacetate. Aspartate (AST) and alanine aminotransferase (ALT) levels were measured using L-aspartate and L-alanine with α-ketoglutarate as substrate. Expression of phase I (<i>CYP1A2</i>, <i>CYP2C19</i>, <i>CYP2D6</i>, <i>CYP2E1 </i>and <i>CYP3A4</i>) and II (<i>UGT1A6</i>,<i> SULT1A1</i>,<i> NAT1</i>,<i> NAT2 </i>and<i> GSTA1/2</i>) metabolizing genes and transporters (<i>ABCB1</i>,<i> ABCC2</i>,<i> ABCG2 </i>and <i>SLCO1B1</i>) were determined by RT/qPCR. <b>Results:</b> All tested concentrations of dill did not affect cell viability or AST and ALT levels. The highest concentration of dill extract (240 μg mL<sup>1</sup>) significantly lowered the ROS level. Expression of <i>CYP1A2</i>, <i>CYP2C19</i>, <i>SULT1A1</i>, <i>NAT2 </i>and <i>ABCB1 </i>mRNA was significantly up-regulated by dill extract. <b>Conclusion:</b> Dill extract did not directly damage Caco-2 cells but prolonged use of dill may increase the risk of HDI via the up-regulation of the drug-metabolizing genes <i>CYP1A2</i>, <i>CYP2C19</i>, <i>SULT1A1</i>, <i>NAT2 </i>and the transporter <i>ABCB1</i>.

Molecular Mechanisms of the SLC13A5 Gene Transcription

Citrate is a crucial energy sensor that plays a central role in cellular metabolic homeostasis. The solute carrier family 13 member 5 (SLC13A5), a sodium-coupled citrate transporter highly expressed in the mammalian liver with relatively low levels in the testis and brain, imports citrate from extracellular spaces into the cells. The perturbation of SLC13A5 expression and/or activity is associated with non-alcoholic fatty liver disease, obesity, insulin resistance, cell proliferation, and early infantile epileptic encephalopathy. SLC13A5 has been proposed as a promising therapeutic target for the treatment of these metabolic disorders. In the liver, the inductive expression of SLC13A5 has been linked to several xenobiotic receptors such as the pregnane X receptor and the aryl hydrocarbon receptor as well as certain hormonal and nutritional stimuli. Nevertheless, in comparison to the heightened interest in understanding the biological function and clinical relevance of SLC13A5, studies focusing on the regulatory mechanisms of SLC13A5 expression are relatively limited. In this review, we discuss the current advances in our understanding of the molecular mechanisms by which the expression of SLC13A5 is regulated. We expect this review will provide greater insights into the regulation of the SLC13A5 gene transcription and the signaling pathways involved therein.

Xenobiotic Receptors and Their Mates in Atopic Dermatitis

Atopic dermatitis (AD) is the most common inflammatory skin disease worldwide. It is a chronic, relapsing and pruritic skin disorder which results from epidermal barrier abnormalities and immune dysregulation, both modulated by environmental factors. AD is strongly associated with asthma and allergic rhinitis in the so-called 'atopic march.' Xenobiotic receptors and their mates are ligand-activated transcription factors expressed in the skin where they control cellular detoxification pathways. Moreover, they regulate the expression of genes in pathways involved in AD in epithelial cells and immune cells. Activation or overexpression of xenobiotic receptors in the skin can be deleterious or beneficial, depending on context, ligand and activation duration. Moreover, their impact on skin might be amplified by crosstalk among xenobiotic receptors and their mates. Because they are activated by a broad range of endogenous molecules, drugs and pollutants owing to their promiscuous ligand affinity, they have recently crystalized the attention of researchers, including in dermatology and especially in the AD field. This review examines the putative roles of these receptors in AD by critically evaluating the conditions under which the proteins and their ligands have been studied. This information should provide new insights into AD pathogenesis and ways to develop new therapeutic interventions.

The UDP-Glycosyltransferase (UGT) Superfamily: New Members, New Functions, and Novel Paradigms

UDP-glycosyltransferases (UGTs) catalyze the covalent addition of sugars to a broad range of lipophilic molecules. This biotransformation plays a critical role in elimination of a broad range of exogenous chemicals and by-products of endogenous metabolism, and also controls the levels and distribution of many endogenous signaling molecules. In mammals, the superfamily comprises four families: UGT1, UGT2, UGT3, and UGT8. UGT1 and UGT2 enzymes have important roles in pharmacology and toxicology including contributing to interindividual differences in drug disposition as well as to cancer risk. These UGTs are highly expressed in organs of detoxification (e.g., liver, kidney, intestine) and can be induced by pathways that sense demand for detoxification and for modulation of endobiotic signaling molecules. The functions of the UGT3 and UGT8 family enzymes have only been characterized relatively recently; these enzymes show different UDP-sugar preferences to that of UGT1 and UGT2 enzymes, and to date, their contributions to drug metabolism appear to be relatively minor. This review summarizes and provides critical analysis of the current state of research into all four families of UGT enzymes. Key areas discussed include the roles of UGTs in drug metabolism, cancer risk, and regulation of signaling, as well as the transcriptional and posttranscriptional control of UGT expression and function. The latter part of this review provides an in-depth analysis of the known and predicted functions of UGT3 and UGT8 enzymes, focused on their likely roles in modulation of levels of endogenous signaling pathways.

Nrf2 Weaves an Elaborate Network of Neuroprotection Against Stroke

Nuclear factor erythroid 2-related factor 2 (Nrf2) is a neuroprotective transcription factor that has recently attracted increased attention. Stroke, a common and serious neurological disease, is currently a leading cause of death in the USA so far. It is therefore of vital importance to explore how Nrf2 behaves in stroke. In this review, we first introduce the structural features of Nrf2 and Kelch-like ECH-associated protein 1 (Keap1) and briefly depict the activation, inactivation, and regulation processes of the Nrf2 pathway. Next, we discuss the physiopathological mechanisms, upstream modulators, and downstream targets of the Nrf2 pathway. Following this background, we expand our discussion to the roles of Nrf2 in ischemic and hemorrhagic stroke and provide several potential future directions. The information presented here may be useful in the design of future experimental research and increase the likelihood of using Nrf2 as a therapeutic target for stroke in the future.

Effects of Phenobarbital on Expression of UDP-Glucuronosyltransferase 1a6 and 1a7 in Rat Brain

UDP-glucuronosyltransferase (UGT), a phase II drug-metabolizing enzyme, is expressed in the brain and can catalyze glucuronidation of endogenous and exogenous substrates in the brain. Thus, changes in UGT1A expression could affect homeostasis and drug efficacy. Phenobarbital (PB), a typical inducer of drug-metabolizing enzymes, has been reported to induce oxidative stress and epigenetic changes, which could alter UGT expression in the brain. Here, we aimed to clarify the effects of PB on Ugt1a6 and Ugt1a7 gene expression in rat brains. Sprague-Dawley rats were treated intraperitoneally with PB (80 mg/kg), once daily for 7 days. Ugt1a6 and Ugt1a7 mRNA expression levels were increased in the striatum and thalamus (Ugt1a6, 3.0- and 2.9-fold, respectively; Ugt1a7, 2.6- and 2.6-fold, respectively). Acetaminophen glucuronidation was also increased in the medulla oblongata and thalamus by 1.8- and 1.2-fold, respectively. The induction rates within different brain regions were correlated with Ugt1a6 and Ugt1a7 mRNA expression, and the degree of induction also correlated with that of NF-E2-related factor-2 mRNA. Measurement of oxidative stress markers suggested that PB induced oxidative stress in brain regions in which Ugt1a6 and Ugt1a7 mRNAs were increased. Moreover, histone modifications were altered by PB treatment, resulting in increased histone H3 lysine 4 trimethylation in the striatum and thalamus and decreased histone H3 lysine 9 trimethylation in the thalamus. These results suggested that oxidative stress and histone modifications may promote transcriptional activation of Ugt1a6 and Ugt1a7 genes. In summary, Ugt1a6 and Ugt1a7 mRNA levels were increased by PB treatment, which may alter pharmacokinetics in the brain.

Roles of human UDP-glucuronosyltransferases in clearance and homeostasis of endogenous substrates, and functional implications

Human UDP-glucuronosyltransferases (UGTs) are major phase II enzymes in the drug metabolism system. Despite major advances in characterization of UGT gene family members, their role in clearance and homeostasis of endogenous substrates is insufficiently understood. Endobiotic substrates including bilirubin, serotonin, eicosanoids, steroid hormones, bile acids, thyroxine and fat-soluble vitamins A and D are discussed. Species- and tissue/cell-dependent regulation of UGT expression by ligand-activated transcription factors is often involved in endobiotic homeostasis. However, roles of particular UGTs are often difficult to delineate since they function together with other enzymes and transporters. Better knowledge of endobiotic UGT substrates and consequences of their conjugation may help to understand evolutionary conserved UGT functions.

Recovery of redox homeostasis altered by CuNPs in H4IIE liver cells does not reduce the cytotoxic effects of these NPs: an investigation using aryl hydrocarbon receptor (AhR) dependent antioxidant activity

The generation of reactive oxygen species (ROS) and consequent oxidative stress is regarded as a relevant mechanism for nanoparticle toxicity. In cells, the activation of the aryl hydrocarbon receptor (AhR) triggers a cascade of defence responses against oxidative stress. By increasing AhR dependent cellular anti-oxidant activity, we tested the extent to which the cytotoxic effect of copper nanoparticles (CuNPs) is governed by oxidative stress. H4IIE rat hepatoma cells were challenged with high ROS levels after exposure to CuNPs, while the AhR-induced cellular anti-oxidant defence was simultaneously activated by the AhR ligand beta-Naphthoflavone (ßNF). Activation of phase II detoxification enzymes (as glutathione-S-transferases, GSTs) and anti-oxidants (glutathione, GSH) led to a complete abrogation of CuNP-induced ROS production. However, a concurrent reduction in cytotoxicity was not detected, thereby indicating that CuNPs exert non-oxidative stress mediated cytotoxic effects. Transmission electron microscopy analysis pointed to a direct physical perturbation of cellular structures by CuNPs, thus contributing to their cytotoxicity. Our observations highlight that distinct mechanisms underlie the toxicity of ions and NPs and indicate that while ROS elicitation is CuNP-specific, the cytotoxic action of these particles may not be directly related to their pro-oxidative activity. These findings have important implications with respect to the oxidative stress paradigm used to explain NP toxicity.

Transcriptional regulation of human UDP-glucuronosyltransferase genes

Glucuronidation is an important metabolic pathway for many small endogenous and exogenous lipophilic compounds, including bilirubin, steroid hormones, bile acids, carcinogens and therapeutic drugs. Glucuronidation is primarily catalyzed by the UDP-glucuronosyltransferase (UGT) 1A and two subfamilies, including nine functional UGT1A enzymes (1A1, 1A3-1A10) and 10 functional UGT2 enzymes (2A1, 2A2, 2A3, 2B4, 2B7, 2B10, 2B11, 2B15, 2B17 and 2B28). Most UGTs are expressed in the liver and this expression relates to the major role of hepatic glucuronidation in systemic clearance of toxic lipophilic compounds. Hepatic glucuronidation activity protects the body from chemical insults and governs the therapeutic efficacy of drugs that are inactivated by UGTs. UGT mRNAs have also been detected in over 20 extrahepatic tissues with a unique complement of UGT mRNAs seen in almost every tissue. This extrahepatic glucuronidation activity helps to maintain homeostasis and hence regulates biological activity of endogenous molecules that are primarily inactivated by UGTs. Deciphering the molecular mechanisms underlying tissue-specific UGT expression has been the subject of a large number of studies over the last two decades. These studies have shown that the constitutive and inducible expression of UGTs is primarily regulated by tissue-specific and ligand-activated transcription factors (TFs) via their binding to cis-regulatory elements (CREs) in UGT promoters and enhancers. This review first briefly summarizes published UGT gene transcriptional studies and the experimental models and tools utilized in these studies, and then describes in detail the TFs and their respective CREs that have been identified in the promoters and/or enhancers of individual UGT genes.

Aryl hydrocarbon receptor control of adaptive immunity

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that belongs to the family of basic helix-loop-helix transcription factors. Although the AhR was initially recognized as the receptor mediating the pathologic effects of dioxins and other pollutants, the activation of AhR by endogenous and environmental factors has important physiologic effects, including the regulation of the immune response. Thus, the AhR provides a molecular pathway through which environmental factors modulate the immune response in health and disease. In this review, we discuss the role of AhR in the regulation of the immune response, the source and chemical nature of AhR ligands, factors controlling production and degradation of AhR ligands, and the potential to target the AhR for therapeutic immunomodulation.

Nuclear receptors and endobiotics glucuronidation: the good, the bad, and the UGT

The recent progresses in molecular biology and pharmacology approaches allowed the characterization of a series of nuclear receptors (NRs) as efficient regulators of uridine diphosphate glucuronosyltransferase (UGT) genes activity. These regulatory processes ensure an optimized UGT expression in response to specific endo- and/or exogenous stimuli. Many of these NRs are activated by endobiotics that also are substrates for UGTs. Thus, by activating their receptors, these endogenous substances control their own conjugation, leading to the concept that glucuronidation is an important part of feed-forward/feedback mechanisms by which bioactive molecules control their own concentrations. On the other hand, numerous studies have established the pharmacological relevance of NR-UGT regulatory pathways in the response to therapeutic ligands. The present review article aims at providing a comprehensive view of the physiological and pharmacological importance of the NR regulation of the expression and activity of endobiotics-conjugating UGT enzymes. Selected examples will illustrate how the organism profits from the feed-forward/feedback mechanisms involving NR-UGT pathways, but also how such regulatory processes are involved in the initiation and/or progression of several pathological situations. Finally, we will discuss how the present pharmacopeia involves NR-dependent regulation of endobiotics glucuronidation, and whether the unexploited NR-UGT axes could serve as pharmacological targets for novel therapeutics to restore endobiotics homeostasis.

Quantitative phosphoproteomic analysis of early alterations in protein phosphorylation by 2,3,7,8-tetrachlorodibenzo-p-dioxin

A comprehensive quantitative analysis of changes in protein phosphorylation preceding or accompanying transcriptional activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in 5L rat hepatoma cells was performed using the SILAC approach. Following exposure of the cells to DMSO or 1 nM TCDD for 0.5 to 2 h, 5648 phosphorylated peptides corresponding to 2156 phosphoproteins were identified. Eight peptides exhibited a statistically significantly altered phosphorylation because of TCDD exposure and 22 showed a regulation factor of ≥ 1.5 in one of the experiments per time point. The vast majority of the TCCD-induced phosphorylation changes had not been reported before. The transcription factor ARNT, the obligate partner for gene activation by the TCDD-bound Ah receptor, exhibited an up-regulation of its Ser77 phosphorylation, a modification known to control the differential binding of ARNT homodimers and heterodimers to different enhancers suggesting that this phosphorylation represents a novel mechanism contributing to the alteration of gene expression by TCDD. Other proteins with altered phosphorylation included, among others, various transcriptional coregulators previously unknown to participate in TCDD-induced gene activation, regulators of small GTPases of the Ras superfamily, UBX domain-containing proteins and the oncogenic protein LYRIC. The results open up new directions for research on the molecular mechanisms of dioxin action and toxicity.

Identification of novel NRF2-regulated genes by ChIP-Seq: influence on retinoid X receptor alpha

Cellular oxidative and electrophilic stress triggers a protective response in mammals regulated by NRF2 (nuclear factor (erythroid-derived) 2-like; NFE2L2) binding to deoxyribonucleic acid-regulatory sequences near stress-responsive genes. Studies using Nrf2-deficient mice suggest that hundreds of genes may be regulated by NRF2. To identify human NRF2-regulated genes, we conducted chromatin immunoprecipitation (ChIP)-sequencing experiments in lymphoid cells treated with the dietary isothiocyanate, sulforaphane (SFN) and carried out follow-up biological experiments on candidates. We found 242 high confidence, NRF2-bound genomic regions and 96% of these regions contained NRF2-regulatory sequence motifs. The majority of binding sites were near potential novel members of the NRF2 pathway. Validation of selected candidate genes using parallel ChIP techniques and in NRF2-silenced cell lines indicated that the expression of about two-thirds of the candidates are likely to be directly NRF2-dependent including retinoid X receptor alpha (RXRA). NRF2 regulation of RXRA has implications for response to retinoid treatments and adipogenesis. In mouse, 3T3-L1 cells' SFN treatment affected Rxra expression early in adipogenesis, and knockdown of Nrf2-delayed Rxra expression, both leading to impaired adipogenesis.

Human UDP-glucuronosyltransferases: feedback loops between substrates and ligands of their transcription factors

Expression profiles of human adult and fetal hepatic and intestinal UDP-glucuronosyltransferases (UGTs), information about their endo- and xenobiotic substrates, and their transcriptional regulation suggests regulatory circuits between some UGT substrates and ligands of their transcription factors. For examples: (i) bilirubin is solely conjugated by UGT1A1 and activates its transcription factors Ah receptor, PXR and CAR. (ii) Hepatotoxic lithocholic acid (LCA) is oxidized to hyodeoxycholic acid, the latter conjugated by UGT2B4 and UGT2B7. LCA is also an agonist of FXR and PPARα, which are controlling these UGTs. (iii) Similar feedback loops possibly exist between some eicosanoids, PPARα and UGTs. (iv) Regulatory circuits may also have evolved between dietary polyphenols, which are efficient substrates of UGTs and activators of the Ah receptor. Although many newly developed drugs are conjugated by promiscuous UGTs, the discussed regulatory circuits may provide hints to evolutionary important UGT substrates.

Nuclear receptors in the multidrug resistance through the regulation of drug-metabolizing enzymes and drug transporters

Chemotherapy is one of the three most common treatment modalities for cancer. However, its efficacy is limited by multidrug resistant cancer cells. Drug metabolizing enzymes (DMEs) and efflux transporters promote the metabolism, elimination, and detoxification of chemotherapeutic agents. Consequently, elevated levels of DMEs and efflux transporters reduce the therapeutic effectiveness of chemotherapeutics and, often, lead to treatment failure. Nuclear receptors, especially pregnane X receptor (PXR, NR1I2) and constitutive androstane activated receptor (CAR, NR1I3), are increasingly recognized for their role in xenobiotic metabolism and clearance as well as their role in the development of multidrug resistance (MDR) during chemotherapy. Promiscuous xenobiotic receptors, including PXR and CAR, govern the inducible expressions of a broad spectrum of target genes that encode phase I DMEs, phase II DMEs, and efflux transporters. Recent studies conducted by a number of groups, including ours, have revealed that PXR and CAR play pivotal roles in the development of MDR in various human carcinomas, including prostate, colon, ovarian, and esophageal squamous cell carcinomas. Accordingly, PXR/CAR expression levels and/or activation statuses may predict prognosis and identify the risk of drug resistance in patients subjected to chemotherapy. Further, PXR/CAR antagonists, when used in combination with existing chemotherapeutics that activate PXR/CAR, are feasible and promising options that could be utilized to overcome or, at least, attenuate MDR in cancer cells.

Coordinate regulation of cytochrome P450 1a1 expression in mouse liver by the aryl hydrocarbon receptor and the beta-catenin pathway

The expression of cytochrome P450 (CYP) 1a1 and other drug-metabolizing enzymes is controlled by the aryl hydrocarbon receptor (AhR), which is activated by dioxin-type inducers leading to transcriptional induction of target genes. Here, we show that a second level of transcriptional control exists in hepatocytes, which is tightly linked to the Wnt/β-catenin/T-cell factor (TCF) signaling pathway. In transgenic mice, hepatic expression of CYP1A (and other CYP isoforms) is stimulated by the expression of mutationally activated β-catenin(S33Y) in the absence of AhR-activating compounds but repressed after knockout of β-catenin. These effects were further analyzed in vitro, and the stimulatory role of β-catenin was ascribed to a TCF-binding site within the CYP1A1 promoter. Moreover, β-catenin signaling acted cooperatively with AhR agonists via AhR-binding sites on the DNA during the induction of Cyp1a1 in vivo and in vitro. Activation of β-catenin enhanced the transactivation potential of ligand-activated AhR at its DNA-binding sites without altering the total amount of DNA-bound AhR. Coimmunoprecipitation demonstrated a physical interaction between AhR and β-catenin. Furthermore, the present results suggest that transcriptional induction of the AhR by β-catenin does not play a major role in β-catenin-dependent regulation of Cyp1a1 expression and that inhibition of β-catenin signaling by ligand-activated AhR, as recently observed in the intestine does not occur in mouse liver. In conclusion, signaling through β-catenin activates basal CYP1A1 expression and augments CYP1A1 induction by AhR ligands through enhancement of the transactivation potential of the AhR.

Breakdown products of neoglucobrassicin inhibit activation of Nrf2 target genes mediated by myrosinase-derived glucoraphanin hydrolysis products

Glucosinolates (GLSs) present in Brassica vegetables serve as precursors for biologically active metabolites, which are released by myrosinase and induce phase 2 enzymes via the activation of Nrf2. Thus, GLSs are generally considered beneficial. The pattern of GLSs in plants is various, and contents of individual GLSs change with growth phase and culture conditions. Whereas some GLSs, for example, glucoraphanin (GRA), the precursor of sulforaphane (SFN), are intensively studied, functions of others such as the indole GLS neoglucobrassicin (nGBS) are rather unknown as are functions of combinations thereof. We therefore investigated myrosinase-treated GRA, nGBS and synthetic SFN for their ability to induce NAD(P)H:quinone oxidoreductase 1 (NQO1) as typical phase 2 enzyme, and glutathione peroxidase 2 (GPx2) as novel Nrf2 target in HepG2 cells. Breakdown products of nGBS potently inhibit both GRA-mediated stimulation of NQO1 enzyme and Gpx2 promoter activity. Inhibition of promoter activity depends on the presence of an intact xenobiotic responsive element (XRE) and is also observed with benzo[a]pyrene, a typical ligand of the aryl hydrocarbon receptor (AhR), suggesting that suppressive effects of nGBS are mediated via AhR/XRE pathway. Thus, the AhR/XRE pathway can negatively interfere with the Nrf2/ARE pathway which has consequences for dietary recommendations and, therefore, needs further investigation.

Nrf2-ARE stress response mechanism: a control point in oxidative stress-mediated dysfunctions and chronic inflammatory diseases

Nrf2, a redox sensitive transcription factor, plays a pivotal role in redox homeostasis during oxidative stress. Nrf2 is sequestered in cytosol by an inhibitory protein Keap1 which causes its proteasomal degradation. In response to electrophilic and oxidative stress, Nrf2 is activated, translocates to nucleus, binds to antioxidant response element (ARE), thus upregulates a battery of antioxidant and detoxifying genes. This function of Nrf2 can be significant in the treatment of diseases, such as cancer, neurodegenerative, cardiovascular and pulmonary complications, where oxidative stress causes Nrf2 derangement. Nrf2 upregulating potential of phytochemicals has been explored, in facilitating cure for various ailments while, in cancer cells, Nrf2 upregulation causes chemoresistance. Therefore, Nrf2 emerges as a key regulator in oxidative stress-mediated diseases and Nrf2 silencing can open avenues in cancer treatment. This review summarizes Nrf2-ARE stress response mechanism and its role as a control point in oxidative stress-induced cellular dysfunctions including chronic inflammatory diseases.

Effects of tert-butylhydroquinone on intestinal inflammatory response and apoptosis following traumatic brain injury in mice

Traumatic brain injury (TBI) can induce intestinal inflammatory response and mucosal injury. Antioxidant transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) has been shown in our previous studies to prevent oxidative stress and inflammatory response in gut after TBI. The objective of this study was to test whether tert-butylhydroquinone (tBHQ), an Nrf2 inducer, can protect against TBI-induced intestinal inflammatory response and mucosal injury in mice. Adult male ICR mice were randomly divided into three groups: (1) sham + vehicle group, (2) TBI + vehicle group, and (3) TBI + tBHQ group (n = 12 per group). Closed head injury was adopted using Hall's weight-dropping method. Intestinal mucosa apoptosis and inflammatory-related factors, such as nuclear factor kappa B (NF-κB), tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), interleukin-6 (IL-6) and intercellular adhesion molecule-1 (ICAM-1), were investigated at 24 h after TBI. As a result, we found that oral treatment with 1% tBHQ prior to TBI for one week markedly decreased NF-κB activation, inflammatory cytokines production, and ICAM-1 expression in the gut. Administration of tBHQ also significantly attenuated TBI-induced intestinal mucosal apoptosis. The results of the present study suggest that tBHQ administration could suppress the intestinal inflammation and reduce the mucosal damage following TBI.

Transcriptional factor aryl hydrocarbon receptor (Ahr) controls cardiovascular and respiratory functions by regulating the expression of the Vav3 proto-oncogene

Aryl hydrocarbon receptor (Ahr) is a transcriptional factor involved in detoxification responses to pollutants and in intrinsic biological processes of multicellular organisms. We recently described that Vav3, an activator of Rho/Rac GTPases, is an Ahr transcriptional target in embryonic fibroblasts. These results prompted us to compare the Ahr(-/-) and Vav3(-/-) mouse phenotypes to investigate the implications of this functional interaction in vivo. Here, we show that Ahr is important for Vav3 expression in kidney, lung, heart, liver, and brainstem regions. This process is not affected by the administration of potent Ahr ligands such as benzo[a]pyrene. We also report that Ahr- and Vav3-deficient mice display hypertension, tachypnea, and sympathoexcitation. The Ahr gene deficiency also induces the GABAergic transmission defects present in the Vav3(-/-) ventrolateral medulla, a main cardiorespiratory brainstem center. However, Ahr(-/-) mice, unlike Vav3-deficient animals, display additional defects in fertility, perinatal growth, liver size and function, closure, spleen size, and peripheral lymphocytes. These results demonstrate that Vav3 is a bona fide Ahr target that is in charge of a limited subset of the developmental and physiological functions controlled by this transcriptional factor. Our data also reveal the presence of sympathoexcitation and new cardiorespiratory defects in Ahr(-/-) mice.

Regulation of drug-metabolizing enzymes by xenobiotic receptors: PXR and CAR

Drug-metabolizing enzymes (DMEs) and transporters play pivotal roles in the disposition and detoxification of numerous foreign and endogenous chemicals. To accommodate chemical challenges, the expression of many DMEs and transporters is up-regulated by a group of ligand-activated transcription factors namely nuclear receptors (NRs). The importance of NRs in xenobiotic metabolism and clearance is best exemplified by the most promiscuous xenobiotic receptors: pregnane X receptor (PXR, NR1I2) and constitutive androstane/activated receptor (CAR, NR1I3). Together, these two receptors govern the inductive expression of a largely overlapping array of target genes encoding phase I and II DMEs, and drug transporters. Moreover, PXR and CAR also represent two distinctive mechanisms of NR activation, whereby CAR demonstrates both constitutive and ligand-independent activation. In this review, recent advances in our understanding of PXR and CAR as xenosensors are discussed with emphasis placed on the differences rather than similarities of these two xenobiotic receptors in ligand recognition and target gene regulation.

Expression of UDP-glucuronosyltransferase 1A6 isoform in Caco-2 cells stimulated with lipopolysaccharide

Glucuronidation is an important metabolic process of detoxification in all vertebrates. The reaction is catalyzed by a multigene family of UDP-glucuronosyltransferases (UGTs) able to convert many xenobiotics and endobiotics (hydrophobic substances) to inactive, water-soluble glucuronides. The UGTs play a protective role, facilitating the elimination of potentially toxic metabolites via urine, bile and feces; therefore, impairment of UGTs may have important toxicological consequences. The regulation of UGTs during bacterial infection or inflammation is not well described. In this study, we investigated the in vitro effect of lipopolysaccharide (LPS) on the expression of the UGT1A6 isoform in human colon carcinoma Caco-2 cells. Results demonstrated a significant down-regulation of UGT1A6 expression, both in terms of mRNA and protein levels, and a reduced UGT activity after LPS exposure of cell cultures, suggesting a role for endotoxins on UGT regulation mechanisms.

Activation of xenobiotic receptors: driving into the nucleus

IMPORTANCE OF THE FIELD

Xenobiotic receptors (XRs) play pivotal roles in regulating the expression of genes that determine the clearance and detoxification of xenobiotics, such as drugs and environmental chemicals. Recently, it has become increasingly evident that most XRs shuttle between the cytoplasm and nucleus, and activation of such receptors is directly associated with xenobiotic-induced nuclear import.

AREAS COVERED IN THIS REVIEW

The scope of this review covers research literature that discusses nuclear translocation and activation of XRs, as well as unpublished data generated from this laboratory. Specific emphasis is given to the constitutive androstane receptor (CAR), the pregnane X receptor and the aryl hydrocarbon receptor.

WHAT THE READERS WILL GAIN

A number of molecular chaperons presumably associated with cellular localization of XRs have been identified. Primary hepatocyte cultures have been established as a unique model retaining inactive CAR in the cytoplasm. Moreover, several splicing variants of human CAR exhibit altered cellular localization and chemical activation.

TAKE HOME MESSAGE

Nuclear accumulation is an essential step in the activation of XRs. Although great strides have been made, much remains to be understood concerning the mechanisms underlying intracellular localization and trafficking of XRs, which involve both direct ligand-binding and indirect pathways.

The regulation of UDP-glucuronosyltransferase genes by tissue-specific and ligand-activated transcription factors

Elucidation of the mechanisms regulating UGT genes is of prime importance if the adverse effects of interactions between drugs primarily eliminated by glucuronidation are to be minimized, and if UGT expression is to be manipulated for therapeutic effect. The factors controlling UGT gene expression in the liver include the liver-enriched transcription factors, HNF-1alpha and HNF-4alpha, several members of the nuclear-receptor family (CAR, PXR, FXR, LXR, and PPAR), the arylhydrocarbon receptor, and transcription factors involved in stress responses (Nrf2, Maf). HNF-1alpha, in concert with the intestine-specific transcription factor, Cdx2, and Sp1 regulate UGT gene expression in the gastrointestinal tract, whereas the genes for the major androgen-glucuronidating enzymes, UGT2B15 and UGT2B17, are upregulated by estrogens in breast cell lines and downregulated by androgens in prostate-derived cells. Despite this knowledge, the complex interactions between these transcription factors and their coregulators has not been determined, and the mechanisms regulating UGT gene expression in organs and tissues, other than the liver, gastrointestinal tract, breast, and prostate, remain to be elucidated.

Nrf2 plays an important role in coordinated regulation of Phase II drug metabolism enzymes and Phase III drug transporters

The nuclear transcription factor E2-related factor 2 (Nrf2) has been shown to play pivotal roles in preventing xenobiotic-related toxicity and carcinogen-induced carcinogenesis. These protective roles of Nrf2 have been attributed in part to its involvement in the induction of Phase II drug conjugation/detoxification enzymes as well as antioxidant enzymes through the Nrf2-antioxidant response element (ARE) signaling pathways. This review summarizes the current research status of the identification of Nrf2-regulated drug metabolism enzymes (DMEs), especially Phase II DMEs, and Phase III drug transporters. In addition, the molecular mechanisms underlying the coordinated regulation of Phase II DMEs and Phase III transporters will also be discussed based on findings published in the literature.

Interaction between oxidative stress sensor Nrf2 and xenobiotic-activated aryl hydrocarbon receptor in the regulation of the human phase II detoxifying UDP-glucuronosyltransferase 1A10

The defense against oxidative stress is a critical feature that prevents cellular and DNA damage. UDP-glucuronosyltransferases (UGTs) catalyze the glucuronidation of xenobiotics, mutagens, and reactive metabolites and thus act as indirect antioxidants. Aim of this study was to elucidate the regulation of UGTs expressed in the mucosa of the gastrointestinal tract by xenobiotics and the main mediator of antioxidant defense, Nrf2 (nuclear factor erythroid 2-related factor 2). Xenobiotic (XRE) and antioxidant (ARE) response elements were detected in the promoters of UGT1A8, UGT1A9, and UGT1A10. Reporter gene experiments demonstrated XRE-mediated induction by dioxin in addition to tert-butylhydroquinone (ARE)-mediated induction of UGT1A8 and UGT1A10, which are expressed in extrahepatic tissue in humans in vivo. The responsible XRE and ARE motifs were identified by mutagenesis. Small interfering RNA knockdown, electrophoretic mobility shifts, and supershifts identified a functional interaction of Nrf2 and the aryl hydrocarbon receptor (AhR). Induction of UGT1A8 and UGT1A10 requires Nrf2 and AhR. It proceeds by utilizing XRE- as well as ARE-binding motifs. In summary, we demonstrate the coordinated AhR- and Nrf2-dependent transcriptional regulation of human UGT1As. Cellular protection by glucuronidation is thus inducible by xenobiotics via AhR and by oxidative metabolites via Nrf2 linking glucuronidation to cellular protection and defense against oxidative stress.

Aryl hydrocarbon receptor-mediated regulation of the human estrogen and bile acid UDP-glucuronosyltransferase 1A3 gene

UDP-glucuronosyltransferases contribute to the detoxification of drugs by forming water soluble beta-D-glucopyranosiduronic acids. The human UGT1A3 protein catalyzes the glucuronidation of estrogens, bile acids and xenobiotics including non-steroidal anti-inflammatory drugs and lipid lowering drugs. Regulation of UGT1A3 by xenobiotic response elements is likely, but the responsible elements are yet uncharacterized. In addition, genetic promoter variants may affect UGT1A3 regulation and potential induction by xenobiotics. The UGT1A3 promoter was analyzed by mutagenesis, reporter gene, and mobility shift analyses. Three hundred and eighty-nine blood donors were genotyped for promoter single nucleotide polymorphisms (SNPs) showing an allelic frequency of 42% of variants at -66 (T to C) and -204 (A to G). A xenobiotic response element regulating aryl hydrocarbon receptor (AhR)-mediated UGT1A3 transcription was identified and characterized. UGT1A3 transcription was reduced in the presence of promoter SNPs. These data demonstrate xenobiotic induced regulation of the UGT1A3 gene by the AhR, which shows genetic variability.

Genetic variability of aryl hydrocarbon receptor (AhR)-mediated regulation of the human UDP glucuronosyltransferase (UGT) 1A4 gene

UDP glucuronosyltransferases (UGTs) play an important role for drug detoxification and toxicity. UGT function is genetically modulated by single nucleotide polymorphisms (SNPs) which lead to the expression of functionally altered protein, or altered expression levels. UGT1A4 activity includes anticonvulsants, antidepressants and environmental mutagens. In this study the induction of the human UGT1A4 gene and a potential influence of genetic variation in its promoter region were analyzed. SNPs at bp -219 and -163 occurred in 9% among 109 blood donors reducing UGT1A4 transcription by 40%. UGT1A4 transcription was dioxin inducible. Reporter gene experiments identified 2 xenobiotic response elements (XRE), which were functionally confirmed by mutagenesis analyses, and binding was demonstrated by electromobility shift assays. Constitutive human UGT1A4 gene expression and induction was aryl hydrocarbon receptor (AhR)-dependent, and reduced in the presence of SNPs at bp -219 and -163. AhR-mediated regulation of the human UGT1A4 gene by two XRE and a modulation by naturally occurring genetic variability by SNPs is demonstrated, which indicates gene-environment interaction with potential relevance for drug metabolism.

Phytochemical regulation of UDP-glucuronosyltransferases: implications for cancer prevention

Uridine 5'-diphospho-glucuronosyltransferases (UGTs) are Phase II biotransformation enzymes that metabolize endogenous and exogenous compounds, some of which have been associated with cancer risk. Many phytochemicals have been shown to induce UGTs in humans, rodents, and cell culture systems. Because UGTs maintain hormone balance and facilitate excretion of potentially carcinogenic compounds, regulation of their expression and activity may affect cancer risk. Phytochemicals regulate transcription factors such as the nuclear factor-erythroid 2-related factor 2 (Nrf2), aryl hydrocarbon, and pregnane X receptors as well as proteins in several signal transduction cascades that converge on Nrf2 to stimulate UGT expression. This induction can be modified by several factors, including phytochemical dose and bioavailability and interindividual variation in enzyme expression. In this review, we summarize the knowledge of dietary modulation of UGTs, particularly by phytochemicals, and discuss the potential mechanisms by which phytochemicals regulate UGT transcription.

Role of mitogen-activated protein kinases in aryl hydrocarbon receptor signaling

Human populations are increasingly exposed to a number of environmental pollutants such as polycyclic aromatic hydrocarbons, polychlorinated biphenyls and dioxins. These compounds are activators of the aryl hydrocarbon receptor (AhR) that controls the expression of many genes including those for detoxification enzymes. The regulatory mechanisms of AhR are multi-factorial and include phosphorylation by various protein kinases. Significant progress in the research of mitogen-activated protein kinases (MAPKs) has been achieved in the last decade. Isolated reports have been published on the role of MAPKs in AhR functions and vice versa, with activation of MAPKs by AhR ligands. This mini-review summarizes current knowledge on the mutual interactions between MAPKs and AhR. The majority of studies has been done on cancer-derived cell lines that have impaired cell cycle regulation and lacks the complete detoxification apparatus. We emphasize the importance of the future studies that should be done on non-transformed cells to distinguish the role of MAPKs in cancer and normal cells. Primary cultures of human or rodent hepatocytes that are equipped with a fully functional biotransformation battery or xenobiotics-metabolizing extra-hepatic tissues should be the models of choice, as the results in our experiments confirm.

Induction of metabolism and transport in human intestine: validation of precision-cut slices as a tool to study induction of drug metabolism in human intestine in vitro

Induction of drug enzyme activity in the intestine can strongly determine plasma levels of drugs. It is therefore important to predict drug-drug interactions in human intestine in vitro. We evaluated the applicability of human intestinal precision-cut slices for induction studies in vitro. Morphological examination and intracellular ATP levels indicated tissue integrity up to 24 h of incubation, whereas in proximal jejunum slices, the metabolic rate toward most substrates remained at 40 to 50% of initial values. In colon slices, the cytochrome P450 conversions were below the detection limit, but conjugation rates remained relatively constant during incubation. The inducibility of drug-metabolizing enzymes and P-glycoprotein was evaluated using prototypical inducers for five induction pathways. beta-Naphthoflavone (aryl hydrocarbon receptor ligand) induced CYP1A1 (132-fold in colon and 362-fold in proximal jejunum) and UDP glucuronosyltransferase (UGT) 1A6 mRNA (9.8-fold in colon and 3.2-fold in proximal jejunum). In proximal jejunum, rifampicin (RIF) [pregnane X receptor (PXR) ligand] induced CYP3A4 (5.2-fold), CYP2B6 (2-fold), UGT1A6 (2.2-fold), and multidrug resistance-1 (MDR1)/ABCB1 mRNA (2.7-fold), whereas 6beta-hydroxytestosterone formation (CYP3A4) increased 2-fold. In colon, RIF induced UGT1A6 32-fold and MDR1 2.2-fold. Dexamethasone (glucocorticoid receptor and PXR ligand) induced CYP3A4 mRNA (3.5-fold) and activity (5-fold) in proximal jejunum. Phenobarbital (constitutive androstane receptor activator) induced CYP3A4 (4.1-fold, only in jejunum), CYP2B6 (4.9-fold in colon and 2.3-fold in proximal jejunum), and MDR1/ABCB1 mRNA and CYP3A4 activity (2-fold only proximal jejunum). Quercetin (nuclear factor-E2-related factor 2 activator) induced UGT1A6 mRNA (6.7-fold in colon and 2.2-fold in proximal jejunum). In conclusion, this study shows that human intestinal precision-cut slices are useful to study induction of drug-metabolizing enzymes and transporters in the human intestine.

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

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

Emerging role of Nrf2 in protecting against hepatic and gastrointestinal disease

Transcription factor NF-E2-related factor 2 (Nrf2) belongs to the basic region-leucine zipper family and is activated in response to electrophiles and reactive oxygen species. Nrf2 coordinately regulates the constitutive and inducible transcription of a wide array of genes involved in drug metabolism, detoxification, and antioxidant defenses. During periods of oxidative stress, Nrf2 is released from sequestration in the cytoplasm and translocates to the nucleus. Nrf2 binds antioxidant response elements (AREs) in the regulatory regions of target genes and activates transcription. Genetically modified mice lacking Nrf2 serve as a useful tool for identifying new ARE-regulated genes and assessing the ability of Nrf2 to confer protection against a variety of pathologies in numerous organs including the liver, intestine, lung, skin, and nervous system. With regards to the liver and gastrointestinal tract, Nrf2 knockout mice are more susceptible to acetaminophen-induced hepatocellular injury, benzo[a]pyrene-induced tumor formation and Fas- and TNFalpha -mediated hepatocellular apoptosis. The higher sensitivity of Nrf2 knockout mice to chemical toxicity is due in part to reduced basal and inducible expression of detoxification enzymes. Nrf2 may also be important in protecting against liver fibrosis, gallstone development, and formation of aberrant crypt foci. Research of Nrf2 has opened up new opportunities in understanding how antioxidant defense pathways are regulated, how oxidative stress contributes to disease progression and may serve as a novel target for designing therapies to prevent and treat diseases in which oxidative stress is implicated.

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

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

The 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.

Ah receptor: Dioxin-mediated toxic responses as hints to deregulated physiologic functions

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor and member of the bHLH/PAS (basic Helix-Loop-Helix/Per-Arnt-Sim) family of chemosensors and developmental regulators. It represents a multifunctional molecular switch regulating endo- and xenobiotic metabolism as well as cell proliferation and differentiation. Physiologic functions of the AhR are beginning to be understood, including functions in vascular development, and in detoxification of endo- and xenobiotics. The AhR is also recognized as the culprit for most toxic responses observed after exposure to dioxins and related compounds such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The non-metabolizable AhR agonist TCDD has to be distinguished from the myriad of metabolizable agonists present as dietary contaminants and plant constituents as well as endogenous toxins. The hypothesis is emerging that the diverse tissue-specific, TCDD-mediated toxicities are due to sustained and inappropriate AhR activation leading to deregulated physiologic functions. In support of this hypothesis recent observations in the context of some TCDD-mediated toxic responses are discussed, such as chloracne, cleft palate, thymus involution and in particular carcinogenesis. Major open questions are addressed, such as ligand-independent AhR activation by phosphorylation and the large differences in species-dependent susceptibility to toxic responses. Though important issues remain unresolved, the commentary is intended to stimulate efforts to understand dioxin-mediated toxic responses with emphasis on carcinogenesis in comparison with AhR-mediated physiologic functions.

Quantitative analysis of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced proteome alterations in 5L rat hepatoma cells using isotope-coded protein labels

In an effort to contribute to a better understanding of the hepatic toxicity of the ubiquitous environmental pollutant and hepatocarcinogen 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a comprehensive quantitative proteome analysis was performed on 5L rat hepatoma cells exposed to 1 nM TCDD for 8 h. Changes in the abundances of individual protein species in TCDD-treated cells as compared to untreated cells were analysed using the nongel-based isotope-coded protein label (ICPL) method [Schmidt, A., Kellermann, J., Lottspeich, F., Proteomics 2005, 5, 4-15]. 89 proteins were identified as up- or down-regulated by TCDD. For the majority of the altered proteins, an impact of TCDD on their abundance had not been known before. Due to the physicochemical properties or the translational regulation of a large number of the affected proteins, their alteration would have escaped detection by gel-based methods for proteome analysis and by standard mRNA expression profiling, respectively. The identified proteins with TCDD-altered abundance include several proteins implicated in cell cycle regulation, growth factor signalling and the control of apoptosis. The results thus provide new starting-points for the investigation of specific aspects of the toxicity and carcinogenicity of dioxin in liver.

Activation of coupled Ah receptor and Nrf2 gene batteries by dietary phytochemicals in relation to chemoprevention

The Ah receptor (AhR) is a ligand-activated transcription factor and member of the bHLH/PAS (basic helix-loop-helix/Per-Arnt-Sim) family of chemosensors and developmental regulators. It represents a multifunctional molecular switch involved in regulation of endo- and xenobiotic metabolism, in vascular development and in dioxin-mediated toxicities. Recently, the oxidative stress-protecting Nrf2 has been shown to be a downstream target of the AhR [Miao W, Hu L, Scrivens PJ, Batist G. Transcriptional regulation of NF-E2 p45-regulated factor (NRF2) expression by the aryl hydrocarbon receptor-xenobiotic response element signaling pathway. J Biol Chem 2005;280:20340-8]. This finding offers the possibility that distinct but partially overlapping AhR and Nrf2 gene batteries of Phase II xenobiotic-metabolizing enzymes can be synergistically activated by a number of phytochemicals, acting as selective or mixed activators of target genes. In addition, it is conceivable that AhR-mediated oxidative/electrophile stress may be attenuated by coupled Nrf2 activation. The commentary discusses potentials and limitations of (i) selective Nrf2 and of (ii) synergistic AhR plus Nrf2 activation by phytochemicals in efforts towards chemoprevention of cancer and degenerative diseases, and describes clinical trials providing the expectation that chemopreventive measures may favorably modulate unavoidable endo- and exogenous toxin exposures in high risk populations.