Mechanism of rat UDP-glucuronosyltransferase 1A6 induction by oltipraz: evidence for a contribution of the Aryl hydrocarbon receptor pathway

Roles of AhR/CYP1s signaling pathway mediated ROS production in uremic cardiomyopathy

PURPOSE

Uremic cardiomyopathy (UCM) is the leading cause of chronic kidney disease (CKD) related mortality. Uremic toxins including indoxyl sulfate (IS) play important role during the progression of UCM. This study was to explore the underlying mechanism of IS related myocardial injury.

METHODS

UCM rat model was established through five-sixths nephrectomy to evaluate its effects on blood pressure, cardiac impairment, and histological changes using echocardiography and histological analysis. Additionally, IS was administered to neonatal rat cardiomyocytes (NRCMs) and the human cardiomyocyte cell line AC16. DHE staining and peroxide-sensitive dye 2',7'-dichlorofluorescein diacetate (H2DCFDA) was conducted to assess the reactive oxygen species (ROS) production. Cardiomyocyte hypertrophy was estimated using wheat germ agglutinin (WGA) staining and immunofluorescence. Aryl hydrocarbon receptor (AhR) translocation was observed by immunofluorescence. The activation of AhR was evaluated by immunoblotting of cytochrome P450 1 s (CYP1s) and quantitative real-time PCR (RT-PCR) analysis of AHRR and PTGS2. Additionally, the pro-oxidative and pro-hypertrophic effects were evaluated using the AhR inhibitor CH-223191, the CYP1s inhibitor Alizarin and the ROS scavenger N-Acetylcysteine (NAC).

RESULTS

UCM rat model was successfully established, and cardiac hypertrophy, accompanied by increased blood pressure, and myocardial fibrosis. Further research confirmed the activation of the AhR pathway in UCM rats including AhR translocation and downstream protein CYP1s expression, accompanied with increasing ROS production detected by DHE staining. In vitro experiment demonstrated a translocation of AhR triggered by IS, leading to significant increase of downstream gene expression. Subsequently study indicated a close relationship between the production of ROS and the activation of AhR/CYP1s, which was effectively blocked by applying AhR inhibitor, CYP1s inhibitor and siRNA against AhR. Moreover, the inhibition of AhR/CYP1s/ROS pathway collectively blocked the pro-hypertrophic effect of IS-mediated cardiomyopathy.

CONCLUSION

This study provides evidence that the AhR/CYP1s pathway is activated in UCM rats, and this activation is correlated with the uremic toxin IS. In vitro studies indicate that IS can stimulate the AhR translocation in cardiomyocyte, triggering to the production of intracellular ROS via CYP1s. This process leads to prolonged oxidative stress stimulation and thus contributes to the progression of uremic toxin-mediated cardiomyopathy.

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.

Role of Hepatic Aryl Hydrocarbon Receptor in Non-Alcoholic Fatty Liver Disease

Numerous nuclear receptors including farnesoid X receptor, liver X receptor, peroxisome proliferator-activated receptors, pregnane X receptor, hepatic nuclear factors have been extensively studied within the context of non-alcoholic fatty liver disease (NAFLD). Following the first description of the Aryl hydrocarbon Receptor (AhR) in the 1970s and decades of research which unveiled its role in toxicity and pathophysiological processes, the functional significance of AhR in NAFLD has not been completely decoded. Recently, multiple research groups have utilized a plethora of in vitro and in vivo models that mimic NAFLD pathology to investigate the functional significance of AhR in fatty liver disease. This review provides a comprehensive account of studies describing both the beneficial and possible detrimental role of AhR in NAFLD. A plausible reconciliation for the paradox indicating AhR as a 'double-edged sword' in NAFLD is discussed. Finally, understanding AhR ligands and their signaling in NAFLD will facilitate us to probe AhR as a potential drug target to design innovative therapeutics against NAFLD in the near future.

Plant Occurring Flavonoids as Modulators of the Aryl Hydrocarbon Receptor

The aryl hydrocarbon receptor (AhR) is a transcription factor deeply implicated in health and diseases. Historically identified as a sensor of xenobiotics and mainly toxic substances, AhR has recently become an emerging pharmacological target in cancer, immunology, inflammatory conditions, and aging. Multiple AhR ligands are recognized, with plant occurring flavonoids being the largest group of natural ligands of AhR in the human diet. The biological implications of the modulatory effects of flavonoids on AhR could be highlighted from a toxicological and environmental concern and for the possible pharmacological applicability. Overall, the possible AhR-mediated harmful and/or beneficial effects of flavonoids need to be further investigated, since in many cases they are contradictory. Similar to other AhR modulators, flavonoids commonly exhibit tissue, organ, and species-specific activities on AhR. Such cellular-context dependency could be probably beneficial in their pharmacotherapeutic use. Flavones, flavonols, flavanones, and isoflavones are the main subclasses of flavonoids reported as AhR modulators. Some of the structural features of these groups of flavonoids that could be influencing their AhR effects are herein summarized. However, limited generalizations, as well as few outright structure-activity relationships can be suggested on the AhR agonism and/or antagonism caused by flavonoids.

Comparative effectiveness of 4 natural and chemical activators of Nrf2 on inflammation, oxidative stress, macrophage polarization, and bactericidal activity in an in vitro macrophage infection model

Inflammation plays a crucial role in the defense response of the innate immune system against pathogen infection. In this study, we selected 4 compounds for their potential or proven anti-inflammatory and/or anti-microbial properties to test on our in vitro model of bacteria-infected THP-1-derived macrophages. We first compared the capacity of sulforaphane (SFN), wogonin (WG), oltipraz (OTZ), and dimethyl fumarate (DMF) to induce the nuclear factor erythroid 2-related factor 2 (Nrf2), a key regulator of the antioxidant, anti-inflammatory response pathways. Next, we performed a comparative evaluation of the antioxidant and anti-inflammatory efficacies of the 4 selected compounds. THP-1-derived macrophages and LPS-stimulated macrophages were treated with each compound and expression levels of genes coding for inflammatory cytokines IL-1β, IL-6, and TNF-α were quantified by RT-qPCR. Moreover, expression levels of genes coding for M1 (IL-23, CCR7, IL-1β, IL-6, and TNF-α) and M2 (PPARγ, MRC1, CCL22, and IL-10) markers were determined in classically-activated M1 macrophages treated with each compound. Finally, the effects of each compound on the intracellular bacterial survival of gram-negative E. coli and gram-positive S. aureus in THP-1-derived macrophages and PBMC-derived macrophages were examined. Our data confirmed the anti-inflammatory and antioxidant effects of SFN, WG, and DMF on LPS-stimulated THP-1-derived macrophages. In addition, SFN or WG treatment of classically-activated THP-1-derived macrophages reduced expression levels of M1 marker genes, while SFN or DMF treatment upregulated the M2 marker gene MRC1. This decrease in expression of M1 marker genes may be correlated with the decrease in intracellular S. aureus load in SFN- or DMF-treated macrophages. Interestingly, an increase in intracellular survival of E. coli in SFN-treated THP-1-derived macrophages that was not observed in PBMC-derived macrophages. Conversely, OTZ exhibited pro-oxidant and proinflammatory properties, and affected intracellular survival of E. coli in THP-1-derived macrophages. Altogether, we provide new potential therapeutic alternatives in treating inflammation and bacterial infection.

Aryl hydrocarbon receptor and intestinal immunity

Aryl hydrocarbon receptor (AhR) is a member of the basic helix-loop-helix-(bHLH) superfamily of transcription factors, which are associated with cellular responses to environmental stimuli, such as xenobiotics and oxygen levels. Unlike other members of bHLH, AhR is the only bHLH transcription factor that is known to be ligand activated. Early AhR studies focused on understanding the role of AhR in mediating the toxicity and carcinogenesis properties of the prototypic ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In recent years, however, it has become apparent that, in addition to its toxicological involvement, AhR is highly receptive to a wide array of endogenous and exogenous ligands, and that its activation leads to a myriad of key host physiological functions. In this study, we review the current understanding of the functions of AhR in the mucosal immune system with a focus on its role in intestinal barrier function and intestinal immune cells, as well as in intestinal homeostasis.

Short-chain chlorinated paraffins (SCCPs) induced thyroid disruption by enhancement of hepatic thyroid hormone influx and degradation in male Sprague Dawley rats

Short-chain chlorinated paraffins (SCCPs) are known to disturb thyroid hormone (TH) homeostasis in rodents. However, the mechanism remains to be fully characterized. In this study, male Sprague Dawley rats received SCCPs (0, 1, 10, or 100mg/kg/day) via gavage once a day for consecutive 28days. Plasma and hepatic TH concentrations, thyrocyte structure, as well as thyroid and hepatic mRNA and protein levels of genes associated with TH homeostasis were examined. Moreover, we performed molecular docking to predict interactions between constitutive androstane receptor (CAR), a key regulator in xenobiotic-induced TH metabolism, with different SCCP molecules. Exposure to SCCPs significantly decreased the circulating free thyroxine (T₄) and triiodothyronine (T₃) levels, but increased thyroid-stimulating hormone (TSH) levels by a feedback mechanism. Decreased hepatic T₄ and increased hepatic T₃ levels were also seen after 100mg/kg/day SCCPs exposure. SCCPs didn't show any significant effects on the expression of thyroid TH synthesis genes or thyrocyte structure. However, stimulation effects were observed for mRNA and protein levels of hepatic uridine diphosphoglucuronosyl transferase (UGT) 1A1 and organic anion transporter 2, suggesting an accelerated TH metabolism in rat liver. The increased cytochrome P450 2B1 but not 1A1 mRNA and protein levels indicated that the CAR signaling was activated by SCCPs exposure. According to docking analysis, SCCPs form hydrophobic interactions with CAR and the binding affinity shows dependency on chlorine content. Overall, our data showed that CAR implicated enhancement of hepatic TH influx and degradation could be the main cause for SCCPs induced TH deficiency in male rats.

The gene expression profile of a drug metabolism system and signal transduction pathways in the liver of mice treated with tert-butylhydroquinone or 3-(3'-tert-butyl-4'-hydroxyphenyl)propylthiosulfonate of sodium

Tert-butylhydroquinone (tBHQ) is a highly effective phenolic antioxidant used in edible oils and fats in foods as well as in medicines and cosmetics. TBHQ has been shown to have both chemoprotective and carcinogenic effects. Furthermore, it has potential anti-inflammatory, antiatherogenic, and neuroprotective activities. TBHQ induces phase II detoxification enzymes via the Keap1/Nrf2/ARE mechanism, which contributes to its chemopreventive functions. Nonetheless, there is growing evidence that biological effects of tBHQ may be mediated by Nrf2-independent mechanisms related to various signaling cascades. Here, we studied changes in gene expression of phase I, II, and III drug metabolizing enzymes/transporters as well as protein levels and activities of cytochromes P450 (CYPs) elicited by tBHQ and its structural homolog TS-13 in the mouse liver. Next, we carried out gene expression analysis to identify signal transduction pathways modulated by the antioxidants. Mice received 100 mg/kg tBHQ or TS-13 per day or only vehicle. The liver was collected at 12 hours and after 7 days of the treatment. Protein and total RNA were extracted. Gene expression was analyzed using Mouse Drug Metabolism and Signal Transduction PathwayFinder RT2Profiler™PCR Arrays. A western blot analysis was used to measure protein levels and a fluorometric assay was employed to study activities of CYPs. Genes that were affected more than 1.5-fold by tBHQ or TS-13 treatment compared with vehicle were identified. Analysis of the gene expression data revealed changes in various genes that are important for drug metabolism, cellular defense mechanisms, inflammation, apoptosis, and cell cycle regulation. Novel target genes were identified, including xenobiotic metabolism genes encoding CYPs, phase II/III drug metabolizing enzymes/transporters. For Cyp1a2 and Cyp2b, we observed an increase in protein levels and activities during tBHQ or TS-13 treatment. Changes were found in the gene expression regulated by NFκB, androgen, retinoic acid, PI3K/AKT, Wnt, Hedgehog and other pathways.

Ah Receptor Pathway Intricacies; Signaling Through Diverse Protein Partners and DNA-Motifs

The Ah receptor is a transcription factor that modulates gene expression via interactions with multiple protein partners; these are reviewed, including the novel NC-XRE pathway involving KLF6.

Expression and functional regulation of the nuclear receptors AHR, PXR, and CAR, and the transcription factor Nrf2 in rat parotid gland

Nuclear receptors and transcription factors regulate the functions of many genes involved in cellular physiology and pathology (e.g. tumorigenesis and autoimmune diseases). The present study was performed to define the expression and the regulation of aryl hydrocarbon receptor (AhR), pregnane X receptor (PXR), constitutive androstane receptor (CAR), and nuclear factor E2-related factor 2 (Nrf2) in the rat parotid gland. Constitutive expression, as well as expression after stimulation with specific inducers for AhR [2,3,7,8-tetrachloro-dibenzylo-p-dioxin (TCDD)], Nrf2(oltipraz), PXR (dexamethasone), and CAR (phenobarbital), was evaluated using the quantitative PCR. Cellular localization of the nuclear receptors and the transcription factor was visualized by immunohistochemical staining. The study revealed constitutive expression of AhR as well as Nrf2, and their induction by TCDD andoltipraz, respectively. Immunohistochemical analysis revealed constitutive, predominantly cytoplasmic, expression of the AhR receptor, especially in interlobular striated duct cells, with nuclear shift upon exposure to TCDD. Inducible expression of Nfr2 was found mainly in the cytoplasm of intralobular striated duct cells. Constitutive expression of PXR and CAR was not found. Bearing in mind the involvement of AhR and Nrf2 in the regulation of many genes, it seems that these factors may play also a role in salivary gland physiology and pathology.

Dioxin-induced fetal growth retardation: the role of a preceding attenuation in the circulating level of glucocorticoid

Exposure of pregnant rats to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) at a low dose causes developmental disorders such as growth retardation and sexual immaturity in their pups. Our previous studies have demonstrated that TCDD attenuates the expression of pituitary luteinizing hormone in fetuses, resulting in the impairment of sexual behavior after they reach maturity. In this study, we focused on growth disturbance and investigated whether TCDD affects the expression of growth hormone (GH), another pituitary hormone which is essential for normal development in perinatal pups. The result showed that maternal exposure to TCDD (1 µg/kg) at gestational day (GD) 15 reduced the fetal expression of GH from the onset at GD18. In accordance with this, TCDD attenuated the pup weight during the perinatal period. We then examined the effect of TCDD on the serum concentration of corticosterone, which plays a key role in the proliferation of GH-producing cells, and found that TCDD reduces the circulating level of corticosterone in the mothers at GD18 and the male fetuses at GD19. The reduction in fetuses seems to be due to increased inactivation rather than reduced synthesis, because TCDD induces the fetal expression of hepatic enzymes participating in the metabolism of glucocorticoids without changing the expression of steroidogenic proteins in the pituitary-adrenal axis. Supplying corticosterone to TCDD-exposed mothers restored or tended to restore a TCDD-induced reduction in pup weight as well as the levels of pituitary GH mRNA and serum GH. These results suggest that TCDD lowers GH expression and growth retardation owing, at least partially, to a reduction in the circulating level of glucocorticoid in pregnant mothers and their fetuses.

6-shogaol, a major compound in ginger, induces aryl hydrocarbon receptor-mediated transcriptional activity and gene expression

Xenobiotics are usually detoxified by drug-metabolizing enzymes and excreted from the body. The expression of many of drug-metabolizing enzymes is regulated by the aryl hydrocarbon receptor (AHR). Some substances in vegetables have the potential to be AHR ligands. To search for vegetable components that exhibit AHR-mediated transcriptional activity, we assessed the activity of vegetable extracts and identified the active compounds using the previously established stable AHR-responsive HepG2 cell line. Among the hot water extracts of vegetables, the highest activity was found in ginger. The ethyl acetate fraction of the ginger hot water extract remarkably induced AHR-mediated transcriptional activity, and the major active compound was found to be 6-shogaol. Subsequently, the mRNA levels of AHR-targeting drug-metabolizing enzymes (CYP1A1, UGT1A1, and ABCG 2) and the protein level of CYP1A1 in HepG2 cells were shown to be increased by 6-shogaol. This is the first report that 6-shogaol can regulate the expression of detoxification enzymes by AHR activation.

Serum from methimazole-treated patients induces activation of aryl hydrocarbon receptor, a transcription factor that binds to dioxin-response elements

BACKGROUND

The aryl hydrocarbon receptor (AhR) is a transcription factor that is activated by xenobiotic substances such as dioxin. After activation, it binds to dioxin response elements of DNA, thereby inducing transcription of a variety of xenobiotic metabolizing enzymes. To investigate whether AhR-activating substances accumulate in patients with endocrine disorders, we tested serum samples for AhR-stimulating activity.

METHODS

Serum AhR-stimulating activity was evaluated by exposing the HepG2 cells transiently transfected with an AhR-responsive reporter plasmid to serum samples. On the basis of preliminary findings that implicated methimazole (MMI), wild-type and AhR-null mice were treated with MMI, and their plasma AhR-stimulating activities and thyroxine levels were quantified.

RESULTS

In 28 randomly chosen patients, 7 out of 10 Graves' disease patients exhibited increased serum AhR-stimulating activity. The increased activity did not correlate with thyroid hormone status. However, we hypothesized that it might be caused by MMI. Subsequent analyses revealed that in 25 of 26 MMI-treated Graves' patients, serum samples collected after the MMI treatment had significantly higher AhR-stimulating activity compared to samples obtained when the same patients were not on MMI. By contrast, serum AhR-stimulating activity was unchanged in samples from the seven patients on propylthiouracil (PTU) compared to serum taken before the PTU treatment. In vitro experiments demonstrated that an MMI metabolite 3-methyl-2-thiohydantoin, but not MMI, activated AhR. MMI increased plasma AhR-stimulating activities and reduced plasma thyroxine concentrations, in both wild-type and AhR-deficient mice.

CONCLUSIONS

Graves' patients taking MMI have increased serum AhR-stimulating activity, which is unrelated to thyroid hormone status, but correlates with MMI treatment. The AhR activation is likely caused by 3-methyl-2-thiohydantoin. Further studies are required to determine the potency of 3-methyl-2-thiohydantoin as an AhR activator and the significance of the differences between MMI and PTU observed in this study.

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.

Transactivation of genes encoding for phase II enzymes and phase III transporters by phytochemical antioxidants

The induction of phase II enzymes and phase III transporters contributes to the metabolism, detoxification of xenobiotics, antioxidant capacity, redox homeostasis and cell viability. Transactivation of the genes that encode for phase II enzymes and phase III transporters is coordinatively regulated by activating transcription factors in response to external stimuli. Comprehensive studies indicate that antioxidant phytochemicals promote the induction of phase II enzymes and/or phase III transporters through various signaling pathways, including phosphoinositide 3-kinase, protein kinase C, and mitogen-activated protein kinases. This paper focuses on the molecular mechanisms and signaling pathways responsible for the transactivation of genes encoding for these proteins, as orchestrated by a series of transcription factors and related signaling components.

Aryl hydrocarbon receptor and NF-E2-related factor 2 are key regulators of human MRP4 expression

Multidrug resistance protein 4 (MRP4; ABCC4) is an ATP binding cassette transporter that facilitates the excretion of bile salt conjugates and other conjugated steroids in hepatocytes and renal proximal tubule epithelium. MRP4/Mrp4 undergoes adaptive upregulation in response to oxidative and cholestatic liver injury in human and animal models of cholestasis. However, the molecular mechanism of this regulation remains to be determined. The aryl hydrocarbon receptor (AhR) and NF-E2-related factor 2 (Nrf2) play important roles in protecting cells from oxidative stress. Here we examine the role of these two nuclear factors in the regulation of the expression of human MRP4. HepG2 cells and human hepatocytes were treated with the AhR and Nrf2 activators, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 3-methylcholanthrene (3-MC), or oltipraz and other nuclear receptor agonists. TCDD, 3-MC, and oltipraz significantly increased MRP4 expression at mRNA and protein levels. Computer program analysis revealed three Xenobiotic response element (XRE) and one Maf response element sites within the first 500 bp of the MRP4 proximal promoter. Luciferase reporter assay detected strong promoter activity (53-fold higher than vector control) in this region. TCDD and 3-MC also induced promoter activity in the reporter assays. Mutation of any of these XRE sites significantly decreased MRP4 promoter activity in reporter assays, although XRE2 demonstrated the strongest effects on both basal and TCDD-inducible activity. EMSA and chromatin immunoprecipitation assays further confirmed that both AhR and Nrf2 bind to the proximal promoter of MRP4. Our findings indicate that AhR and Nrf2 play important roles in regulating MRP4 expression and suggest that agents that activate their activity may be of therapeutic benefit for cholestasis.

Antioxidant and mitochondrial protective effects of oxidized metabolites of oltipraz

IMPORTANCE OF THE FIELD

Comprehensive studies indicate that oltipraz exerts cancer chemopreventive effects. Oltipraz has other therapeutic potentials, which include anti-fibrotic effect, inhibition of insulin resistance, mitochondrial protection and cytoprotective effect against oxidative stress. Although antioxidant mechanisms may account for its cancer chemopreventive effect, details on the molecular mechanism still remain to be clarified.

AREAS COVERED IN THIS REVIEW

Two major metabolic pathways of oltipraz include oxidative desulfuration of the thione to yield 4-methyl-5-(pyrazin-2-yl)-3H-1,2-dithiol-3-one and molecular rearrangement to 7-methyl-6,8-bis(methylthio)H-pyrrolo[1,2-a]pyrazine. In addition to the diverse pharmacological effects of oltipraz, the oxidized metabolites may have distinct biological effects on cell survival. The AMP-activated protein kinase pathway has been recognized as a key cascade for mitochondrial protection and cell survival events, which can be activated by the oxidized metabolites of oltipraz.

WHAT THE READER WILL GAIN

In this review, the metabolic activation of oltipraz and the role of the cell signaling pathways in regulating the expression of Phase II genes and antioxidant activity are discussed with particular reference to their effects on mitochondrial protection and cell survival.

TAKE HOME MESSAGE

In terms of therapeutic potential, the findings reviewed here demonstrate a therapeutic potential for oxidized metabolite of oltipraz and offer comparison of antioxidant capacity between metabolites and parent compound.

Short-term exposure to triclosan decreases thyroxine in vivo via upregulation of hepatic catabolism in Young Long-Evans rats

Triclosan (5-chloro-2-(2,4-dichlorophenoxy)-phenol) is a chlorinated phenolic antibacterial compound found in consumer products. In vitro human pregnane X receptor activation, hepatic phase I enzyme induction, and decreased in vivo total thyroxine (T4) suggest adverse effects on thyroid hormone homeostasis. Current research tested the hypothesis that triclosan decreases circulating T4 via upregulation of hepatic catabolism and transport. Weanling female Long-Evans rats received triclosan (0-1000 mg/kg/day) by gavage for 4 days. Whole blood and liver were collected 24 h later. Total serum T4, triiodothyronine (T3), and thyroid-stimulating hormone (TSH) were measured by radioimmunoassay. Hepatic microsomal assays measured ethoxyresorufin-O-deethylase, pentoxyresorufin-O-deethylase (PROD), and uridine diphosphate glucuronyltransferase enzyme activities. The messenger RNA (mRNA) expression of cytochrome P450s 1a1, 2b1/2, and 3a1/23; UGTs 1a1, 1a6, and 2b5; sulfotransferases 1c1 and 1b1; and hepatic transporters Oatp1a1, Oatp1a4, Mrp2, and Mdr1b was measured by quantitative reverse transcriptase PCR. Total T4 decreased dose responsively, down to 43% of control at 1000 mg/kg/day. Total T3 was decreased to 89 and 75% of control at 300 and 1000 mg/kg/day. TSH did not change. Triclosan dose dependently increased PROD activity up to 900% of control at 1000 mg/kg/day. T4 glucuronidation increased nearly twofold at 1000 mg/kg/day. Cyp2b1/2 and Cyp3a1/23 mRNA expression levels were induced twofold and fourfold at 300 mg/kg/day. Ugt1a1 and Sult1c1 mRNA expression levels increased 2.2-fold and 2.6-fold at 300 mg/kg/day. Transporter mRNA expression levels were unchanged. These data denote important key events in the mode of action for triclosan-induced hypothyroxinemia in rats and suggest that this effect may be partially due to upregulation of hepatic catabolism but not due to mRNA expression changes in the tested hepatic transporters.

In Vivo Acute Exposure to Polychlorinated Biphenyls: Effects on Free and Total Thyroxine in Rats

Hypothyroxinemia in rats has been well documented as a result of exposure to polychlorinated biphenyls (PCBs). Hypothetical mechanisms include induction of hepatic catabolic enzymes and cellular hormone transporters, and/or interference with plasma transport proteins. We hypothesized that if thyroxine displacement from transport proteins by PCBs occurs in vivo, it would result in increased free thyroxine (FT4). This study investigates the effects of a single oral dose of 2,2’,4,4’,5,5'-hexachlorobiphenyl (PCB 153 at 60 mg/kg) or 3,3’,4,4’,5,5'-hexachlorobiphenyl (PCB 169 at 1 mg/kg) on rats at 28 or 76 days of age. Total thyroxine (TT4) and FT4 were measured at 0.5, 1, 2, 4, 8, 24, or 48 hours post -dosing. Microsomal ethoxy- and pentoxy-resorufin-O-deethylase (EROD and PROD) activity and uridine diphosphoglucuronosyl transferase (UGT) activity were determined. No significant increase in TT4 or FT4 concentrations was seen at any time point. PCB 153 significantly decreased TT4 and FT4 in young and adult rats, with young rats showing a time-by-treatment interaction from 2 to 48 hours post -dosing in serum FT4. With PCB 169 exposure, young rats showed a decrease in FT4 only, whereas adult rats showed decreases in TT4 only. Hepatic EROD and PROD activities were both dramatically increased following PCB 169 and 153, respectively. Uridine diphosphoglucuronosyl transferase activity was increased only after PCB 169 exposure. These data demonstrate that neither PCB 153 nor PCB169 increased FT4, which supports the conclusion that these PCBs do not displace thyroxine from serum TTR, or if it does occur, there is no subsequent increase in serum FT4 in vivo.

Therapeutic potential of dithiolethiones for hepatic diseases

Comprehensive studies support the notion that oltipraz [4-methyl-5-(2-pyrazynyl)-1,2-dithiole-3-thione] and its congeners exert cancer chemopreventive effects by the prevention, inhibition or reversal of carcinogenic processes. Recently, it was found that dithiolethione compounds had the activities to prevent or treat fibrosis, insulin resistance, and mitochondrial protective effects in the liver by a mechanism involving AMP-activated protein kinase (AMPK) and/or 70-kDa ribosomal protein S6 kinase 1 (S6K1). Moreover, chemical regulation of the AMPK-S6K1 pathway was found to affect Liver X receptor (LXR) activity and lipogenesis, leading to the identification of AMPK and S6K1 as targets for treating hepatic steatosis. These biological activities of dithiolethiones may offer a novel approach to pharmaceutical intervention. This review focuses on the interaction between oltipraz and the AMPK-mTOR-S6K1 pathway, which regulates genes that confer hepatocyte protection from intoxication, disrupted energy metabolism, and inflammation. In terms of therapeutic potential, the findings reviewed here demonstrate a new therapeutic potential for dithiolethiones, which function in a unique manner, and offer the possibility of new treatments for hepatic diseases.

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.

Variability and function of family 1 uridine-5'-diphosphate glucuronosyltransferases (UGT1A)

The substrate spectrum of human UDP-glucuronosyltransferase 1A (UGT1A) proteins includes the glucuronidation of non-steroidal anti-inflammatory drugs, anticonvulsants, chemotherapeutics, steroid hormones, bile acids, and bilirubin. The unique genetic organization of the human UGT1A gene locus, and an increasing number of functionally relevant genetic variants define tissue specificity as well as a broad range of interindividual variabilities of glucuronidation. Genetic UGT1A variability has been conserved throughout the protein's evolution and shows ethnic diversity. It is the biochemical and genetic basis for clinical phenotypes such as Gilbert's syndrome and Crigler-Najjar's disease as well as for the potential for severe, unwanted drug side effects such as in irinotecan treatment. UGT1A variants influence the metabolic effects of xenobiotic exposure and therefore have been linked to cancer risk. Detailed knowledge of the organization, function, and pharmacogenetics of the human UGT1A gene locus is likely to significantly contribute to the improvement of drug safety and efficacy as well as to the provision of steps toward the goal of individualized drug therapy and disease risk prediction.

Induction of mouse UDP-glucuronosyltransferase mRNA expression in liver and intestine by activators of aryl-hydrocarbon receptor, constitutive androstane receptor, pregnane X receptor, peroxisome proliferator-activated receptor alpha, and nuclear factor erythroid 2-related factor 2

UDP-glucuronosyltransferases (UGTs) catalyze the addition of UDP-glucuronic acid to endo- and xenobiotics, enhancing their water solubility and elimination. Many exogenous compounds, such as microsomal enzyme inducers (MEIs), alter gene expression through xenobiotic-responsive transcription factors, namely, the 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). These transcription factors regulate xenobiotic-inducible expression of hepatic and intestinal biotransformation enzymes and transporters. The purpose of this study was to determine hepatic and intestinal inducibility of mouse Ugt mRNA by MEIs. Male C57BL/6 mice were treated for four consecutive days with activators of AhR [2,3,7,8-tetrachlorodibenzodioxin (TCDD), polychlorinated biphenyl 126, and beta-naphthoflavone], CAR [1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP), phenobarbital, and diallyl sulfide], PXR [pregnenolone-16alpha-carbonitrile (PCN), spironolactone, and dexamethasone], PPARalpha (clofibrate, ciprofibrate, and diethylhexylphthalate), and Nrf2 (oltipraz, ethoxyquin, and butylated hydroxyanisole), respectively. Ugt1a1 mRNA expression in liver was induced by activators of all five transcription factor pathways, Ugt1a5 by Nrf2 activators, Ugt1a6 by all the pathways except CAR, and Ugt1a9 by all the pathways except Nrf2. Ugt2b35 mRNA in liver was induced by AhR activators and Ugt2b36 by CAR and PPARalpha activators. Throughout the small and large intestine, the AhR ligand TCDD increased Ugt1a6 and Ugt1a7 mRNA. In small intestine, the PXR activator PCN increased Ugt1a1, Ugt1a6, Ugt1a7, Ugt2b34, and Ugt2b35 mRNA in the duodenum. In conclusion, chemical activation of AhR, CAR, PXR, PPARalpha, and Nrf2 in mouse results in induction of distinct Ugt gene sets in liver and intestine, predominantly the Ugt1a isoforms.

Dithiolethiones for cancer chemoprevention: where do we stand?

Dithiolethiones are a well-known class of cancer chemopreventive agents; the key mechanism of action of dithiolethiones involves activation of Nrf2 signaling and induction of phase II enzymes. In the past, attention has been focused mainly on 4-methyl-5-pyrazinyl-3H-1,2-dithiole-3-thione (oltipraz), which showed ability as a wide-spectrum inhibitor of chemical carcinogenesis in preclinical models. However, clinical trials of oltipraz have shown questionable efficacy, and at the high doses employed in such studies, significant side effects were observed. Dithiolethiones that are markedly more effective and potent than oltipraz in both induction of phase II enzymes and inhibition of chemical carcinogenesis in preclinical studies have been identified, and these compounds have shown pronounced organ specificity in vivo. Further investigation of these compounds may lead to development of effective and safe agents for cancer prevention in humans.

Effects of perinatal PBDE exposure on hepatic phase I, phase II, phase III, and deiodinase 1 gene expression involved in thyroid hormone metabolism in male rat pups

Previous studies demonstrated that perinatal exposure to polybrominated diphenyl ethers (PBDEs), a major class of brominated flame retardants, may affect thyroid hormone (TH) concentrations by inducing hepatic uridinediphosphate-glucoronosyltransferases (UGTs). This study further examines effects of the commercial penta mixture, DE-71, on genes related to TH metabolism at different developmental time points in male rats. DE-71 is predominately composed of PBDE congeners 47, 99, 100, 153, 154 with low levels of brominated dioxin and dibenzofuran contaminants. Pregnant Long-Evans rats were orally administered 1.7 (low), 10.2 (mid), or 30.6 (high) mg/kg/day of DE-71 in corn oil from gestational day (GD) 6 to postnatal day (PND) 21. Serum and liver were collected from male pups at PND 4, 21, and 60. Total serum thyroxine (T(4)) decreased to 57% (mid) and 51% (high) on PND 4, and 46% (mid) dose and 25% (high) on PND 21. Cyp1a1, Cyp2b1/2, and Cyp3a1 enzyme and mRNA expression, regulated by aryl hydrocarbon receptor, constitutive androstane receptor, and pregnane xenobiotic receptor, respectively, increased in a dose-dependent manner. UGT-T(4) enzymatic activity significantly increased, whereas age and dose-dependent effects were observed for Ugt1a6, 1a7, and 2b mRNA. Sult1b1 mRNA expression increased, whereas that of transthyretin (Ttr) decreased as did both the deiodinase I (D1) enzyme activity and mRNA expression. Hepatic efflux transporters Mdr1 (multidrug resistance), Mrp2 (multidrug resistance-associated protein), and Mrp3 and influx transporter Oatp1a4 mRNA expression increased. In this study the most sensitive responses to PBDEs following DE-71 exposure were CYP2B and D1 activities and Cyb2b1/2, d1, Mdr1, Mrp2, and Mrp3 gene expression. All responses were reversible by PND 60. In conclusion, deiodination, active transport, and sulfation, in addition to glucuronidation, may be involved in disruption of TH homeostasis due to perinatal exposure to DE-71 in male rat offspring.

Mode of Action: Neurotoxicity Induced by Thyroid Hormone Disruption During Development—Hearing Loss Resulting From Exposure to PHAHs

An increasing incorporation of mode of action (MOA) information into risk assessments has led to examination of animal MOAs to determine relevance to humans. We examined a specific MOA for developmental neurotoxicity using the MOA/Human Relevance Framework (Meek et al., 2003). The postulated MOA of ototoxicity in rats involves early postnatal exposure to polychlorinated biphenyls (PCBs) via lactation, an upregulation of hepatic uridine diphosphoglucuronyltransferases (UGTs), and subsequent hypothyroxinemia during a critical period of cochlear development, with the ultimate neurotoxic consequence of hearing loss. This review concludes with high confidence in the animal MOA and medium confidence for the interspecies concordance for the key events in the MOA. Possible interspecies differences in toxicodynamic factors moderate confidence in some key events. In addition, there is a question of whether ambient human exposures are large enough to cause human fetal hypothyroxinemia to the degree needed to cause hearing loss. Data gaps identified by this analysis include a need to characterize the induciblity of human fetal UGTs and the comparative sensitivity of UGT induction by xenobiotics in rats and humans. Research on these areas of uncertainty will increase confidence that this MOA for PCBs is not likely to not occur in humans, assuming normal conditions of limited ambient exposure.

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.

Persistent induction of hepatic and pulmonary phase II enzymes by 3-methylcholanthrene in rats

We reported earlier that exposure of rats to 3-methylcholanthrene (MC) causes sustained induction of hepatic cytochrome P450 (CYP)1A expression for up to 45 days by mechanisms other than persistence of the parent MC (Moorthy, J. 2000. Pharmacology. Exp. Ther. 294, 313-322). The CYP1A genes are members of the Ah gene battery that also encode CYP1B1 and phase II enzymes such as glutathione S-transferase (GST-alpha), UDP glucuronyl transferase (UGT)1A, NAD(P)H (nicotinamide adenine dinucleotide phosphate, reduced):quinone oxidoreductase I (NQO1), aldehyde dehydrogenase (ALDH), etc. Therefore, in this investigation, we tested the hypothesis that MC elicits persistent induction of CYP1B1 and phase II genes, which are in part regulated by the Ah receptor (AHR). Female Sprague-Dawley rats were treated with MC (100 mumol/kg), ip, once daily for 4 days, and expression of CYP1B1 and several phase II (e.g., GST-alpha, NQO1) genes and their corresponding proteins were determined in lung and liver. The major finding was that MC persistently induced (3- to 10-fold) the expression of several phase II enzymes, including GST-alpha, NQO1, UGT1A1, ALDH, and epoxide hydrolase in both tissues for up to 28 days. However, MC did not elicit sustained induction of CYP1B1. Our results thus support the hypothesis that MC elicits coordinated and sustained induction of phase II genes presumably via persistent activation of the AHR, a phenomenon that may have implications for chemical-induced carcinogenesis and chemopreventive strategies in humans.

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.

Induction of human UGT1A1 by a complex of dexamethasone-GR dependent on proximal site and independent of PBREM

UDP-glucuronosyltransferase 1A1 (UGT1A1) plays a key role to conjugate bilirubin and prevent jaundice. There are two major elements for the induction of UGT1A1, such as PBREM (-3483/-3194), far from the promoter site, and HNF1 (-75/-63), near to the promoter site. In a previous report, we showed that the proximal HNF1 site is essential for the induction of UGT1A1 by glucocorticoid receptor (GR). In this report, we investigated the influence of PBREM on the induction of the UGT1A1 reporter gene by GR and PXR with dexamethasone (DEX). We confirmed that GR was transferred from cytosol into the nucleus in 15-30 min by DEX stimulation, but HNF1 was not. We constructed a reporter gene containing PBREM to compare the induction of the reporter gene without PBREM by DEX-GR. The results show that induction of the reporter gene with PBREM by DEX at 100 muM is the same level as the induction of the reporter gene without PBREM, although PBREM contains GRE. Co-transfection of hGR with the reporter gene did not show any influence of the induction of the reporter gene between the vector with and without PBREM. Meanwhile, by co-transfection of hPXR, the induction of the reporter gene with PBREM was significantly more than the induction of the reporter gene without PBREM at 100 microM DEX. This supports that hPXR induced UGT1A1 through PBREM by DEX. These results showed that PBREM has no relation with the induction by DEX-GR but the proximal site of UGT1A1 may function in stimulation by DEX-GR.

Induction of phase I and II drug metabolism in rat small intestine and colon in vitro

The aim of this study was to evaluate drug metabolism in rat small intestinal and colon precision-cut slices during 24 h of incubation and the applicability of these slices for enzyme induction studies. Various parameters were evaluated: intracellular levels of ATP (general viability marker), alkaline phosphatase activity (specific epithelial marker), villin expression (specific epithelial marker), and metabolic rates of 7-ethoxycoumarin (CYP1A), testosterone (CYP3A and CYP2B), and 7-hydroxycoumarin (glucuronide and sulfate conjugation) conversions. ATP and villin remained constant up to, respectively, 5 and 8 h in small intestine and up to 24 h in colon. The metabolic rate remained constant in small intestinal slices up to 8 h and decreased afterward to 24 to 92%, depending on the substrate studied. The inducibility of metabolism in small intestinal and colon slices was tested with several inducers at various concentrations and incubation times. The following inducers were used: 3-methylcholanthrene, beta-naphthoflavone, indirubin, and tert-butylhydroquinone (aryl hydrocarbon receptor ligands), dexamethasone (glucocorticoid receptor/pregnane X receptor ligand) and phenobarbital (constitutive androstane receptor ligand). After incubation with inducers, metabolic rates were evaluated with 7-ethoxycoumarin and testosterone (phase I) and 7-hydroxycoumarin (phase II) as substrate. All inducers elevated the metabolic rates consistent with the available published in vivo induction data. Induction of enzyme activity was already detectable after 5 h (small intestine) and after 8 h (colon) for 3-methylcholanthrene and beta-naphthoflavone and was clearly detectable for all tested inducers after 24 h (up to 20-fold compared with noninduced controls). In conclusion, small intestinal and colon precision-cut slices are useful for metabolism and enzyme induction studies.

Nrf2-Keap1 signaling pathway regulates human UGT1A1 expression in vitro and in transgenic UGT1 mice

The formation of beta-D-glucopyranosides (glucuronides) by the UDP-glucuronosyltransferases (UGTs) is a significant metabolic pathway that facilitates the elimination of small hydrophobic molecules such as drugs, dietary constituents, steroids, and bile acids. We elucidate here that an anti-oxidative response leads to induction of UGT1A1 through the Nrf2-Keap1 pathway. When human HepG2 cells were treated with the prooxidants tert-butylhydroquinone and beta-naphthoflavone, cellular UGT1A1 glucuronidation activities were increased. The induction of UGT1A1 proceeded following the overexpression of Nrf2 and was blocked following overexpression of Keap1, demonstrating that Keap1 suppresses Nrf2 activation of the UGT1A1 gene. Loss of function analysis for Nrf2 conducted by small interfering RNA revealed that induction of UGT1A1 was not seen in Nrf2 knock-out cells. To examine the contribution of oxidants toward the regulation of human UGT1A1 in vivo, transgenic mice bearing the human UGT1 locus (Tg-UGT1) were treated with tert-butylhydroquinone. Human UGT1A1 was markedly increased in small and large intestines as well as in liver. Gene mapping experiments including transfections of UGT1A1 reporter gene constructs into HepG2 cells coupled with functional analysis of Nrf2 expression and binding to anti-oxidant-response elements (ARE) resulted in identification of an ARE in the phenobarbital-response enhancer module region of the UGT1A1 gene. The ARE flanks the recently identified Ah receptor xenobiotic-responsive element. The results suggest that Nrf2-Keap1-dependent UGT1A1 induction by prooxidants might represent a key adaptive response to cellular oxidative stress that defends against a variety of environmental insults, including electrophile attacks and chemical carcinogenesis.

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.

Induction of Rat UDP-Glucuronosyltransferases in Liver and Duodenum by Microsomal Enzyme Inducers That Activate Various Transcriptional Pathways

Microsomal enzyme inducers (MEIs) up-regulate phase I biotransformation enzymes, most notably cytochromes P450. Transcriptional up-regulation by MEIs occurs through at least three nuclear receptor mechanisms: constitutive androstane receptor (CAR; CYP2B inducers), pregnane X receptor (PXR; CYP3A inducers), and peroxisome proliferator-activated receptor alpha (PPARalpha; CYP4A inducers). Other mechanisms include transcription factors aryl hydrocarbon receptor (AhR; CYP1A inducers), and nuclear factor erythroid 2 (NF-E2)-related factor 2 (Nrf2; NADPH-quinone oxidoreductase inducers). UDP-glucuronosyltransferases (UGTs) are phase II biotransformation enzymes that are predominantly expressed in liver and intestine. MEIs increase UGT activity; however, transcriptional regulation of individual UGT isoforms is not completely understood. The purpose of this study was to examine inducibility of individual UGT isoforms and potential mechanisms of transcriptional regulation in rat liver and duodenum. UGT mRNA levels were assessed in liver and duodenum of rats treated with MEIs that activate various transcriptional pathways. All four CAR activators induced UGT2B1 in liver, but not duodenum. UGT1A1, 1A5, 1A6, and 2B12 were induced by at least two CAR activators in liver only. Two PXR ligands induced UGT1A2, but only in duodenum. Two PPARalpha ligands induced UGT1A1 and 1A3 in liver only. AhR ligands induced UGT1A6 and 1A7 in liver, but not duodenum. Nrf2 activators increased UGT2B3 and 2B12 in both liver and duodenum, and UGT1A6, 1A7, and 2B1 in liver only. In summary, only UGT1A2 and 1A8 were not inducible in liver by MEIs. MEIs differentially regulate hepatic expression of individual UGT isoforms, although no one transcriptional pathway dominated. In duodenum, MEIs had minimal effects on UGT expression.

Regulation of theCYP1A1Gene by 2,3,7,8-Tetrachlorodibenzo-p-dioxin but Not by β-Naphthoflavone or 3-Methylcholanthrene Is Altered in Hepatitis C Virus Replicon-Expressing Cells

Exposure to hepatitis C virus (HCV) can lead to the development of cirrhosis and hepatocellular carcinoma. To examine the effects of long-term HCV infection on hepatic cytochrome P450 1A1 (CYP1A1) expression and function, we used a human hepatoma cell line expressing the HCV subgenomic replicon (Huh.8) to evaluate CYP1A1 induction by the aryl hydrocarbon receptor (AhR) ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In this study, we demonstrate that the induction of CYP1A1 expression in Huh.8 cells by TCDD but not by beta-naphthoflavone or 3-methylcholanthrene was significantly diminished. TCDD exposure of Huh.8 cells resulted in greater than 55% suppression of CYP1A1 transcription compared with the parent cell line Huh7, whereas protein levels and enzyme activities were further diminished. Suppression of CYP1A1 mRNA expression in TCDD-treated Huh.8 cells was partially reversed after pretreatment with the antioxidants N-acetylcysteine and nordihydroguaiaretic acid, suggesting a role for oxidative stress. Induced CYP1A1 message, protein, and enzyme activity were partially restored in an Huh7 cell line expressing the HCV replicon containing a deletion in the nonstructural protein NS5A. Furthermore, adenoviral expression of NS5A in Huh7 partially suppressed TCDD-induced CYP1A1 protein and enzyme activity, implicating this protein in the mechanism of suppression. These findings demonstrate that TCDD-mediated AhR signaling is impaired in hepatocytes in which HCV is present and that NS5A alone or in the presence of other nonstructural proteins of the subgenomic replicon is in part responsible.

Nrf2 defends the lung from oxidative stress

Nuclear factor, erythroid 2 related factor 2 (Nrf2) belongs to the Cap'n'collar/basic region leucine zipper (CNC-bZIP) transcription factor family, and is activated by diverse oxidants, pro-oxidants, antioxidants, and chemopreventive agents. After phosphorylation and dissociation from the cytoplasmic inhibitor, Kelch-like ECH-associated protein 1 (Keap1), Nrf2 translocates to the nucleus and binds to an antioxidant response element (ARE). Through transcriptional induction of ARE-bearing genes that encode antioxidant-detoxifying proteins, Nrf2 activates cellular rescue pathways against oxidative injury, inflammation/immunity, apoptosis, and carcinogenesis. ARE-driven genes include direct antioxidants (e.g., GPx), thiol metabolism-associated detoxifying enzymes (e.g., GSTs), stress-response genes (e.g., HO-1), and others (e.g., PSMB5). Application of nrf2 germ-line mutant mice elucidated protective roles for Nrf2 in various models of human disorders in the liver, lung, kidney, brain, and circulation. In the lung, deficiency of nrf2 augmented injury caused by bleomycin and environmental oxidants including hyperoxia, diesel exhaust particles, and cigarette smoke. Microarray analyses of lungs from nrf2-deficient and -sufficient mice identified Nrf2-dependent genes that might be critical in pulmonary protection. Observations from these studies highlight the importance of the Nrf2-antioxidant pathway and may provide new therapeutic strategies for acute respiratory distress syndrome, idiopathic pulmonary fibrosis, cancer, and emphysema in which oxidative stress is implicated.

Proximal HNF1 element is essential for the induction of human UDP-glucuronosyltransferase 1A1 by glucocorticoid receptor

Previous study showed noinduction of the reporter gene (-3174/+14) of UGT1A1 in HepG2 by bilirubin, but induction by dexamethasone (DEX). This induction was enhanced seven-fold by the co-expression of human glucocorticoid receptor (GR) and was inhibited by a GR antagonist, RU486, indicating stimulation by DEX-GR. Meanwhile, we could not detect stimulation by beta-estradiol, phenobarbital or rifampicin (RIF) in the presence of GR. We investigated the position playing a role in this induction by GR in the promoter region of UGT1A1 using deletion mutants, and clarified the essential sequence (-75/-63) for the binding site of hepatocyte nuclear factor 1 (HNF1). However, GR did not bind directly to this sequence, because UGT-PE2 did not compete for binding to a glucocorticoid responsive element (GRE) probe in an electrophoretic mobility shift assay (EMSA) method. Labeled [(32)P]DNA probe of HNF1 binds with nuclear extracts as shown by the EMSA. This shift of the complex of probe-protein was not inhibited by unlabeled GRE but was inhibited by unlabeled HNF1 element. This shift was not influenced by the addition of anti-GR, but was super-shifted by the addition of anti-HNF1. GR did not stimulate the induction of HNF1, because we detected no-elevation of the mRNA level of HNF1 by reverse transcription-polymerase chain reaction (RT-PCR). Therefore, the induction of UGT1A1 by DEX-GR did not depend on the elevation of HNF1 but on the interaction of GR with HNF1 or the activation of HNF1 through the transcription of other proteins. Also given the lack of evidence of binding of DEX-GR to HNF1 in the EMSA, the data suggest that the mechanism of DEX-GRE effect on HNF1 is indirect by whatever mechanisms.

Effect of PCB 126 on Hepatic Metabolism of Thyroxine and Perturbations in the Hypothalamic-Pituitary-Thyroid Axis in the Rat

The objective of this research was to examine the time- and dose- dependent disturbances in the hypothalamic-pituitary-thyroid (HPT) axis of adult male rats administered a potent coplanar (non-ortho) PCB, 3,3',4,4',5-pentachlorobiphenyl (PCB 126). Adult male Sprague-Dawley rats were administered a single oral bolus dose of 0, 7.5, 75, or 275 microg PCB 126/kg bw dissolved in corn oil. The rats were sacrificed periodically over 22 days. The 7.5-microg/kg dose induced hepatic ethoxyresorufin-O-deethylation EROD activity, but no changes were observed in hepatic uridine diphosphate glucuronyl transferases (UDPGTs) activity or serum TSH, T4, or fT4 concentrations. The two highest doses caused a modest decline in weight gain, induced hepatic EROD and UDPGT activities, increased serum TSH concentrations, and decreased serum T4 and fT4 concentrations. The amount of thyroxine glucuronide formed daily (pM/mg protein) increased linearly with the area-under-the-concentration-curve (AUCC) for PCB 126 in liver (microg/kg/day) and then slowed at the 275-microg/kg PCB 126 dose. Perturbations in the HPT axis were nonlinear with respect to PCB 126 dosing. As expected, an inverse relationship between the AUCC for serum T4 (microg/dl/day) and the AUCC for serum TSH (ng/dl/day) was observed; however, the relationship was highly nonlinear. These data support a mode of action for PCB 126 involving induction of hepatic UDPGTs by the aryl hydrocarbon receptor AhR. However, the dose-response characteristics of the HPT axis are nonlinear and complex, requiring sophisticated tools, such as PBPK models, to characterize dose response.

Nuclear receptors and drug disposition gene regulation

In this minireview, the role of various nuclear receptors and transcription factors in the expression of drug disposition genes is summarized. Specifically, the molecular aspects and functional impact of the aryl hydrocarbon receptor (AhR), nuclear factor-E2 p45-related factor 2 (N(r)f2), hepatocyte nuclear factor 1alpha (HNF1alpha), constitutive androstane receptor (LAR), pregnane X receptor (PXR), farnesoid X receptor (FXR), peroxisome proliferator-activated receptor alpha (PPAR(alpha)), hepatocyte nuclear factor 4alpha (HNF4alpha), vitamin D receptor (VDR), liver receptor homolog 1 (LRH1), liver X receptor (LXR(alpha)), small heterodimer partner-1 (SHP-1), and glucocorticoid receptor (GR) on gene expression are detailed. Finally, we discuss some current topics and themes in nuclear receptor-mediated regulation of drug metabolizing enzymes and drug transporters.

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.

INDUCTION OF THE MULTIDRUG RESISTANCE-ASSOCIATED PROTEIN FAMILY OF TRANSPORTERS BY CHEMICAL ACTIVATORS OF RECEPTOR-MEDIATED PATHWAYS IN MOUSE LIVER

The multidrug resistance-associated proteins (Mrp) are ATP-dependent transporters that export a variety of conjugated and unconjugated compounds out of cells. There are nine identified Mrp transporters in humans, with murine orthologs for all except Mrp8. Because nuclear receptors mediate induction of phase I enzymes, Mrp transporter expression might be similarly regulated by these receptors to coordinate metabolism and export of chemicals from liver. To test the hypothesis that Mrp expression may be coordinately regulated with phase I enzyme expression in liver, 15 different compounds were given representing known transcriptionally mediated pathways: aryl hydrocarbon receptor (AhR), pregnane X receptor (PXR), constitutive androstane receptor (CAR), peroxisome proliferator-activated receptor alpha (PPARalpha), and nuclear factor-E2-related factor 2 (Nrf2). Each of these compounds induced expression of their respective target enzyme in liver, demonstrating that the chemical regimens were effective. The AhR ligands [2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polychlorinated biphenyl 126 (PCB126), and beta-naphthoflavone] induced Mrp2, -3, -5, and -6 mRNA expression. The CAR activator 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) induced Mrp2, -3, -4, -6, and -7 mRNA expression. Mrp3 was also induced by two other CAR activators phenobarbital and diallyl sulfide, two PXR ligands, pregnenalone-16alpha-carbonitrile and spironolactone, and the PPARalpha ligands clofibrate, ciprofibrate, and diethylhexylphthalate. The Nrf2 activators (butylated hydroxyanisole, oltipraz, and ethoxyquin) induced Mrp2-6. In conclusion, a variety of mechanisms are suggested for Mrp3 induction, including AhR, CAR, PXR, PPARalpha, and Nrf2, whereas on a whole, a predominant role for AhR and Nrf2 in hepatic induction of the Mrp family was observed. Thus, these specific transcription factors are implicated in regulation of both drug metabolism and efflux transport.

UDP glucuronosyltransferase (UGT) 1A6 pharmacogenetics: I. Identification of polymorphisms in the 5'-regulatory and exon 1 regions, and association with human liver UGT1A6 gene expression and glucuronidation

UDP glucuronosyltransferase (UGT) 1A6 is a major isoform in human liver that glucuronidates numerous drugs, toxins, and endogenous substrates with high interindividual variability. The molecular basis for this variability remains unknown, although it likely involves genetic and environmental factors. Phenotype-genotype studies were conducted using a well characterized human liver bank (n = 54) and serotonin glucuronidation as a UGT1A6-specific phenotype marker. A positive moderate-to-heavy alcohol use history (>14 drinks per week) was the only demographic factor examined that correlated with phenotype and was associated with 2-fold higher serotonin glucuronidation (p < 0.001), UGT1A6 protein content (p = 0.004), and UGT1A6 mRNA content (p = 0.025). UGT1A6 gene resequencing identified three nonsynonymous polymorphisms (S7A, T181A, and R184S) in exon 1 and eight novel polymorphisms in the 5'-regulatory region (to -2052 base pairs). S7A was in complete linkage disequilibrium with three 5'-regulatory region polymorphisms (-1710c-->g, -1310del5, and -652g-->a). Initial univariate analyses did not identify any significant phenotype-genotype associations. However, in livers without substantial alcohol exposure, 50% lower UGT1A6 mRNA levels (p = 0.026) were found in carriers of the linked S7A-enhancer polymorphisms compared with noncarriers but without significant effect on UGT1A6 protein content or glucuronidation activities. Three major haplotypes, including (*)1A (reference), (*)1B (-1535g-->a and -427g-->c), and (*)2 (-1710c-->g, -1310del5, -652g-->a, S7A, T181A, and R184S), were identified, accounting for 90% of alleles. No association of haplotype with any of the phenotype measures could be discerned. In conclusion, although the identified UGT1A6 polymorphisms did not explain the observed glucuronidation variability, there does seem to be a significant role for environmental factors associated with alcohol consumption.

A cell-based system to identify and characterize the molecular mechanism of drug-metabolizing enzyme (DME) modulators

Many naturally occurred or synthetic compounds can modulate the body's drug-metabolizing enzymes to enhance carcinogen detoxification, and some have demonstrated remarkable cancer prevention effects. Understanding the molecular mechanism behind each candidate agent is critically important in designing rational cancer chemoprevention strategies. In this work, we have employed a set of molecular mechanism-based assays and characterized eight classes of known drug-metabolizing enzyme (DME) modulators in a cellular system. Examination of mRNA and protein levels of representative phase I and phase II enzymes validated the results obtained in our cell-based system. Our data confirmed that the antioxidant ethoxyquin (EQ) and the isothiolcyanate sulfurophane (SFP) exclusively activate the antioxidant response element (ARE), and thus represent monofunctional inducers. We were also able to reclassify some compounds, and to use the system to identify structure-activity relationships among structurally related but different compounds. Finally, this cell-based system permitted us to identify a potential novel mechanism for cross-talk between the ARE and the xenobiotic response element (XRE)-mediated pathways.