Formation of mono- and diglucuronides and other glycosides of benzo(a)pyrene-3,6-quinol by V79 cell-expressed human phenol UDP-glucuronosyltransferases of the UGT1 gene complex

Acute toxicities of fluorene, fluorene-1-carboxylic acid, and fluorene-9-carboxylic acid on zebrafish embryos (Danio rerio): Molecular mechanisms of developmental toxicities of fluorene-1-carboxylic acid

In this study, fluorene (FL), FL-1-carboxylic acid (FC-1), and FL-9-carboxylic acid (FC-9) were investigated to understand their acute toxicity by measuring inhibitory effects on hatching rates and developmental processes of zebrafish embryos (Danio rerio). For exposure concentrations up to 3000 μg/L, FC-1 alone showed acute toxicity at 1458 μg/L for LC₅₀ value. FC-1 caused yolk sac and spinal deformities, and pericardial edema. Molecular studies were undertaken to understand FC-1 toxicity examining 61 genes after exposure to 5 μM (equivalent to LC₂₀ value of FC-1) in embryos. In the FC-1-treated embryos, the expression of the cyp7a1 gene, involved in bile acid biosynthesis, was dramatically decreased, while the expression of the Il-1β gene involved in inflammation was remarkably increased. In addition to these findings, in FC-1-treated embryos, the expression of nppa gene related to the differentiation of the myocardium was 3-fold increased. On the other hand, cyp1a, cyp3a, ugt1a1, abcc4, mdr1, and sult1st1 responsible for detoxification of xenobiotics were upregulated in FC-9-treated embryos. Taken together, carboxylation on carbon 1 of FL increased acute toxicity in zebrafish embryos, and its toxicity might be related to morphological changes with modification of normal biological functions and lowered defense ability.

Aryl hydrocarbon receptor (AHR) functions: Balancing opposing processes including inflammatory reactions

Aryl hydrocarbon receptor (AHR) research has shifted from exploring dioxin toxicity to elucidation of physiologic AHR functions. Control of AHR functions is challenged by the fact that AHR is often involved in balancing opposing processes. Two AHR functions are discussed. (i) Microbial defense: intestinal microbiota commensals secrete AHR ligands that are important for maintaining epithelial integrity and generation of anti-inflammatory IL-22 by multiple immune cells. On the other hand, in case of microbial defense, AHR-regulated neutrophils and Th17 cells are involved in generation of bactericidal reactive oxygen species and pro-inflammatory stimuli. However, during the process of infection resolution, 'disease tolerance' is achieved. (ii) Energy, NAD⁺ and lipid metabolism: In obese individuals AHR is involved in either generation or inhibition of fatty liver and associated hepatitis. Inhibition of hepatitis is mainly achieved by regulating NAD⁺-controlled SIRT1, 3 and 6 activity. Interestingly, these enzymes are synergistically modulated by CD38, an NAD-consuming NAD-glycohydrolase. It is proposed that inflammatory responses may be beneficially modulated by AHR agonistic and CD38 inhibiting phytochemicals. Caveats in presence of carcinogenicity have to be taken into account. AHR research is an exciting field but therapeutic options remain challenging.

Aryl hydrocarbon receptor (AHR): From selected human target genes and crosstalk with transcription factors to multiple AHR functions

Accumulating evidence including studies of AHR-deficient mice and TCDD toxicity suggests multiple physiologic AHR functions. Challenges to identify responsible mechanisms are due to marked species differences and dependence upon cell type and cellular context. Transient AHR modulation is often necessary for physiologic functions whereas TCDD-mediated sustained receptor activation has been demonstrated to be responsible for toxic outcomes. To stimulate studies on responsible action mechanisms the commentary is focused on human AHR target genes and crosstalk with transcription factors. Discussed AHR functions include chemical and microbial defense, organ development, modulation of immunity and inflammation, reproduction, and NAD⁺-dependent energy metabolism. Obviously, much more work is needed to elucidate action mechanisms. In particular, studies of pathways leading to NAD⁺-dependent energy metabolism may shed light on the puzzling species differences of TCDD-mediated lethality and provide options for treatment of obesity and age-related degenerative diseases.

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.

The UDP-glucuronosyltransferases of the blood-brain barrier: their role in drug metabolism and detoxication

UDP-glucuronosyltransferases (UGTs) form a multigenic family of membrane-bound enzymes expressed in various tissues, including brain. They catalyze the formation of β-D-glucuronides from structurally unrelated substances (drugs, other xenobiotics, as well as endogenous compounds) by the linkage of glucuronic acid from the high energy donor, UDP-α-D-glucuronic acid. In brain, UGTs actively participate to the overall protection of the tissue against the intrusion of potentially harmful lipophilic substances that are metabolized as hydrophilic glucuronides. These metabolites are generally inactive, except for important pharmacologically glucuronides such as morphine-6-glucuronide. UGTs are mainly expressed in endothelial cells and astrocytes of the blood brain barrier (BBB). They are also associated to brain interfaces devoid of BBB, such as circumventricular organ, pineal gland, pituitary gland and neuro-olfactory tissues. Beside their key-role as a detoxication barrier, UGTs play a role in the steady-state of endogenous compounds, like steroids or dopamine (DA) that participate to the function of the brain. UGT isoforms of family 1A, 2A, 2B and 3A are expressed in brain tissues to various levels and are known to present distinct but overlapping substrate specificity. The importance of these enzyme species with regard to the formation of toxic, pharmacologically or physiologically relevant glucuronides in the brain will be discussed.

Geometric attributes of retaining glycosyltransferase enzymes favor an orthogonal mechanism

Retaining glycosyltransferase enzymes retain the stereochemistry of the donor glycosidic linkage after transfer to an acceptor molecule. The mechanism these enzymes utilize to achieve retention of the anomeric stereochemistry has been a matter of much debate. Re-analysis of previously released structural data from retaining and inverting glycosyltransferases allows competing mechanistic proposals to be evaluated. The binding of metal-nucleotide-sugars between inverting and retaining enzymes is conformationally unique and requires the donor substrate to occupy two different orientations in the two types of glycosyltransferases. The available structures of retaining glycosyltransferases lack appropriately positioned enzymatic dipolar residues to initiate or stabilize the intermediates of a dissociative mechanism. Further, available structures show that the acceptor nucleophile and anomeric carbon of the donor sugar are in close proximity. Structural features support orthogonal (front-side) attack from a position lying ≤90° from the C1-O phosphate bond for retaining enzymes. These structural conclusions are consistent with the geometric conclusions of recent kinetic and computational studies.

UGT1A6 polymorphisms modulated lung cancer risk in a Chinese population

Uridine diphosphoglucuronosyltransferases (UGTs) 1A6 is the only UGT1A isoform expressed in lung tissue. It is responsible for the detoxification of carcinogens such as benezo[a]pyrene from cigarette smoke. The purpose of this study was to evaluate the association of UGT1A6 polymorphisms and haplotypes with lung cancer risk and to evaluate the functional significance of UGT1A6 polymorphisms. Genomic DNA was isolated from leukocytes. Eight UGT1A6 polymorphisms were sequenced in a test set of 72 Chinese lung cancer patients and 62 healthy controls. Potential risk modifying alleles were validated in a separate set of 95 Chinese lung cancer patients and 100 healthy controls. UGT1A6 19T>G, 541A>G and 552A>C showed significant association with increased lung cancer risk, while UGT1A6 105C>T and IVS1+130G>T were significantly associated with reduced lung cancer risk. Multivariate logistic regression analysis demonstrated a significant association of lung cancer with UGT1A6 541A>G (OR: 3.582, 95% CI: 1.27–10.04, p = 0.015), 552A>C (OR: 5.364, 95% CI: 1.92–14.96, p = 0.001) and IVS1+130G>T (OR: 0.191, 95% CI: 0.09–0.36, p<0.001). Functional test demonstrated that UGT1A6 105C>T increased mRNA stability, providing a plausible explanation of its association with reduced lung cancer risk. Thus UGT1A6 polymorphisms may be used to identify people with increased risk of developing lung cancer.

Ah receptor- and Nrf2-gene battery members: modulators of quinone-mediated oxidative and endoplasmic reticulum stress

Quinones are ubiquitously present in mammals and their environment. They are involved in physiologic functions such as electron transport but are also toxic compounds. In particular, quinone-quinol redox cycles may lead to oxidative stress, and arylating quinones have been demonstrated to activate endoplasmic reticulum (ER) stress. To detoxify quinones coordinately regulated Ah receptor and Nrf2 gene batteries evolved. Two pathways are emphasized: (i) glutathione S-transferases, and (ii) NAD(P)H:quinone oxidoreductases NQO1 and NQO2 acting together with UDP-glucuronosyltransferases and sulfotransferases. Coupling between these enzymes may prevent oxidative and ER stress in a tissue-dependent manner, as discussed using benzo[a]pyrene detoxification in enterocytes, catecholestrogen metabolism in breast tissue and endometrium, and aminochromes in neurones and astrocytes. Possible consequences of chronic ER stress such as apoptosis and inflammation as well as therapeutic possibilities of modulating Ah receptor and Nrf2 are discussed. In conclusion, tight coupling of Ah receptor- and Nrf2-regulated enzymes may prevent quinone-mediated oxidative and ER stress.

Functions and transcriptional regulation of adult human hepatic UDP-glucuronosyl-transferases (UGTs): mechanisms responsible for interindividual variation of UGT levels

Ten out of 19 UDP-glucuronosyltransferases (UGTs) are substantially expressed in adult human liver (>1% of total UGTs); 5 UGT1 isoforms (UGT1A1, 1A3, 1A4, 1A6 and 1A9) and 5 UGT2 family members (UGT2B4, 2B7, 2B10, 2B15 and 2B17) (Izukawa et al. [11]). Surprisingly, UGT2B4 and UGT2B10 mRNA were found to be abundant in human liver suggesting an underestimated role of the liver in detoxification of their major substrates, bile acids and eicosanoids. Among factors responsible for high interindividual variation of hepatic UGT levels (genetic diversity including polymorphisms and splice variants, regulation by liver-enriched transcription factors such as HNF1 and HNF4, and ligand-activated transcription factors) nuclear receptors (PXR, CAR, PPARalpha, etc.), and the Ah receptor are discussed. Unraveling the mechanisms responsible for interindividual variation of UGT expression will be beneficial for drug therapy but still remains a major challenge.

Possible involvement of oxidative stress in fenofibrate-induced hepatocarcinogenesis in rats

To clarify whether oxidative stress is involved in the development of hepatocellular preneoplastic foci induced by fenofibrate (FF), a peroxisome proliferator-activated receptor alpha agonist, male F344/N rats were fed a diet containing 6,000, 3,000, or 0 ppm of FF for 13 weeks after N-diethylnitrosamine initiation. Two-third partial hepatectomy was performed 1 week after the FF treatment. Histopathologically, the number of hepatocellular altered foci significantly increased in the FF-treated groups with a concomitant increase in the number of hepatocytes positive for anti-Ki-67 antibody, but the number and area of glutathione S-transferase placental form (GST-P)-positive foci decreased in these groups, as compared to those in the controls. Microarray analysis or quantitative real-time reverse transcription-polymerase chine reaction demonstrated the significant up-regulations of Aco and Cyp4a1 (genes related to lipid metabolism); Gpx2, Yc2, Cat, Cyp2b15, and Ugt1a6 (metabolic oxidative stress-related genes); Apex1, Mgmt, Xrcc5, Nbn, and Gadd45a (DNA repair-related genes); and Ccnd1 (cell cycle-related genes) in the FF-treated groups, and the significant down-regulations of Cyp1a2, Gsta2, Gstm2, and Gstm3 (phase I or II metabolism-related genes); Mlh1 and Top1 (DNA repair-related genes); and Cdkn1a, Cdkn1b, Chek2, and Gadd45b (cell cycle/apoptosis-related genes) in these rats. FF-treatment increased the activity of enzymes such as carnitine acetyltransferase, carnitine palmitoyltransferase, fatty acyl-CoA oxidizing system, and catalase in the liver, but not superoxide dismutase in the liver. In addition, 8-OHdG level in liver DNA, lipofuscin deposition in hepatocytes, and in vitro reactive oxygen species production in microsomes significantly increased due to FF treatment. These results suggest that oxidative stress is involved in the development of FF-induced hepatocellular preneoplastic foci in rats.

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.

Joint Effects between UDP-Glucuronosyltransferase 1A7 Genotype and Dietary Carcinogen Exposure on Risk of Colon Cancer

The UDP-glucuronosyltransferase 1A7 (UGT1A7) gene is polymorphic and encodes an enzyme involved in the detoxification of heterocyclic amines (HCA) and polycyclic aromatic hydrocarbons (PAH). Consumption of pan-fried and well-done meat are surrogates for HCA and PAH exposure and are possibly associated with colon cancer. We have evaluated whether UGT1A7 allelic variations are associated with colon cancer and whether UGT1A7 genotype modified associations among meat intake, exposure to HCAs and PAHs, and colon cancer in a population-based case-control study of African Americans (197 cases and 202 controls) and whites (203 cases and 210 controls). As part of a 150-item food frequency questionnaire, meat intake was assessed by cooking method and doneness and used to estimate individual HCA and PAH exposure. UGT1A7 alleles (UGT1A7*1, UGT1A7*2, UGT1A7*3, and UGT1A7*4) were measured and genotypes were categorized into predicted activity groups (high: *1/*1, *1/*2, *2/*2; intermediate: *1/*3, *1/*4, *2/*3; low: *3/*3, *3/*4, *4/*4). There was no association with UGT1A7 low versus high/intermediate genotype [odds ratio (OR), 1.1; 95% confidence interval (95% CI), 0.7-1.8], regardless of race. Greater than additive joint effects were observed for UGT1A7 low genotype and HCA-related factors. For example, equal to or greater than the median daily intake of the HCA, 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline (DiMeIQx) and having UGT1A7 low genotype was positively associated with colon cancer (OR, 2.4; 95% CI, 1.2-4.8), compared with less than the median daily intake and UGT1A7 high/intermediate genotypes. These data suggest that the associations among cooked meat-derived compound exposure, and colon cancer are modified by the UGT1A7 genotype.

DETERMINATION OF DRUG GLUCURONIDATION AND UDP-GLUCURONOSYLTRANSFERASE SELECTIVITY USING A 96-WELL RADIOMETRIC ASSAY

A rapid and sensitive radiometric assay for UDP-glucuronosyltransferase (UGT) is described. UGT substrates are incubated in 96-well plates with microsomes in the presence of [14C]UDP-glucuronic acid, and 14C-labeled glucuronidation products are separated from the unreacted nucleotide sugar by solid-phase extraction using 96-well extraction plates. The assay was validated with 15 structurally diverse UGT substrates containing acidic, phenolic, and hydroxyl reacting groups. Glucuronidation velocities for these compounds were determined using human, rat, and dog liver microsomes, and reaction kinetics were studied with 1-naphthol and 4-methylumbelliferone. Results obtained with the new assay confirmed the previously reported rank order of glucuronidation velocity of several typical UGT substrates and the finding that the glucuronidation of most of these compounds is significantly faster in dog than in human liver microsomes. UGT specificity of five compounds was determined using recombinant human UGTs. The major UGT isoforms identified were UGT1A6, UGT1A7, and UGT1A9 for 4-methylumbelliferone; UGT1A6 and UGT1A8 for 1-naphthol; UGT2B7 for naloxone; UGT1A3 and UGT2B7 for ketoprofen; and UGT1A4 for trifluoperazine. Identical results were obtained with a conventional high-performance liquid chromatography method coupled to mass spectrometric detection. The new assay should prove valuable for rapidly benchmarking recombinant UGTs and microsomal preparations from different species and tissues, identifying high-turnover compounds during drug discovery, and reaction phenotyping studies.

UDP-glucuronosyltransferase 1A6: structural, functional, and regulatory aspects

Glucuronidation, catalyzed by two families of UDP-glucuronosyltransferases (UGTs), represents a major phase II reaction of endo- and xenobiotic biotransformation. UGT1A6 is the founding member of the rat and human UGT1 family. It is expressed in liver and extrahepatic tissues, such as intestine, kidney, testis, and brain, and conjugates planar phenols and arylamines. Serotonin has been identified as a selective endogenous substrate of the human enzyme. UGT1A6 is also involved in conjugation of the drug paracetamol (acetaminophen) and of phenolic metabolites of benzo[a]pyrene (together with rat UGT1A7 and human UGT1A9). High interindividual variability of human liver protein levels is due to a number of influences, including genetic, tissue-specific, and environmental factors. Evidence shows that homo- and heterozygotic expression of UGT1A6 alleles markedly affects enzyme activity. HNF1 may be responsible for tissue-specific UGT1A6 expression. Multiple environmental factors controlling UGT1A6 expression have been identified, including the pregnane X receptor, the constitutive androstane receptor, the aryl hydrocarbon receptor, and Nrf2, a bZIP transcription factor mediating stress responses. However, marked differences have been noted in the expression of rat and human UGT1A6. Regulatory factors have been studied in detail in the human Caco-2 colon adenocarcinoma cell model.

Coordinate Regulation of Drug Metabolism by Xenobiotic Nuclear Receptors: UGTs Acting Together with CYPs and Glucuronide Transporters

Xenobiotic nuclear receptors (PXR, CAR, and the Ah receptor) coordinately induce genes involved in all phases of xenobiotic metabolism including oxidative metabolism, conjugation, and transport. The comment--dedicated to honor the memory of Herbert Remmer, mentor of the author K. W. B.--discusses mechanistic, functional, and evolutionary aspects of xenobiotic nuclear receptors which induce UGTs together with CYPs and glucuronide transporters in human and rodent liver and intestine. Recent findings on regulation of CYPs, UGTs, and transporters suggest that while nuclear receptor signaling induces different CYPs, regulation may converge on single UGTs and transporters. Functional consequences of co-regulation are discussed using examples from the metabolism of xeno- and endobiotics (drugs, bilirubin, bile salts, steroid hormones, and carcinogens). Animal-plant interactions may have been a major driving force in the evolutionary divergence of CYPs and UGTs in mammals and insects as well as in their regulation by nuclear receptors. In addition, regulation by nuclear receptors was probably shaped by the need for homeostatic control of endobiotic signals in the evolution of multicellular organisms.

Vertebrate UDP-glucuronosyltransferases: functional and evolutionary aspects

UDP-glucuronosyltransferases (UGTs) represent major phase II drug metabolizing enzymes. They are part of a rapidly growing, sequence similarly based superfamily of UDP-glycosyltransferases, including a number of enzymes, which presumably are functionally unrelated to UGTs. The present commentary discusses evolutionary aspects of the large glycosyltransferase superfamily emphasizing functionally related members which share roles in detoxication and elimination of endo- and xenobiotics. The discussion starts with the two human UGT families and polymorphism frequencies in different populations. These families probably evolved in vertebrates as a result of the struggle against toxic phytoalexins at the hepatogastrointestinal barrier. Co-regulation of some UGTs with other drug metabolizing enzymes may also have evolved in the course of 'animal-plant warfare'. Related UDP-glucosyltransferases evolved in insects. Even in plants and bacteria UDP-glucosyltransferases have been characterized which may be functionally related.

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

UDP-glucuronosyltransferases (UGTs) represent major phase II enzymes of drug metabolism which are regulated in a tissue-specific manner by endogenous and environmental factors. Among the latter, aryl hydrocarbon receptor (AhR) agonists such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and phenolic antioxidants such as tert-butylhydroquinone (tBHQ) are known to induce the expression of human UGT1A6 in Caco-2 cells. While binding of the TCDD-activated AhR to one xenobiotic response element (XRE) in the 5'-flanking regulatory region of UGT1A6 was characterised previously, the mechanism responsible for tBHQ induction is unknown. Therefore, it was investigated whether antioxidant response elements (AREs) are involved in tBHQ induction of UGT1A6. Transfectants of 3 kb of its regulatory region and its deletion mutants were treated with tBHQ. These studies suggested a region with approximately 2-fold induction, including an ARE-like motif, 15 bp downstream of the previously characterised XRE. Transfectants of the point-mutated ARE-like motif showed marginally reduced response to tBHQ, but surprisingly, loss of response to TCDD, suggesting interference of flanking proteins with the AhR/Arnt complex. Coordinate responses of UGT activity after treatment with TCDD or tBHQ were also observed in rat hepatoma 5L cells, mutants without the AhR and with recomplemented AhR. The results suggest a contribution of the AhR pathway and of proteins binding to the XRE flanking region to the induction of human UGT1A6 by both AhR agonists and phenolic antioxidants.

Acyl glucuronidation and glucosidation of a new and selective endothelin ET(A) receptor antagonist in human liver microsomes

Compound A [(+)-(5S,6R,7R)-2-isopropylamino-7-[4-methoxy-2-((2R)-3-methoxy-2-methylpropyl)-5-(3,4-methylenedioxyphenyl) cyclopenteno [1,2-b] pyridine 6-carboxylic acid] is a new and selective endothelin ET(A) receptor antagonist. It underwent significant acyl glucuronidation and acyl glucosidation in human liver microsomes supplemented with UDP-glucuronic acid (UDPGA) and UDP-glucose (UDPG). These two conjugations were observed in a panel of human liver microsomal samples (n = 16) that gave rise to varying activities but with no significant correlation with each other in the native and activator-treated microsomal preparations (r(2) <or= 0.4, p > 0.05). The lack of correlation may be explained by the involvement of multiple UDP-glucuronosyltransferases (UGTs; UGT1A1, 1A3, 1A9, 2B7 and 2B15) in the glucuronidation but essentially solely UGT2B7 in the glucosidation. Both reactions conformed to monophasic Michaelis-Menten kinetics in human liver microsomes. The glucuronidation reaction exhibited apparent K(m) values (mean +/- S.E.) for compound A and UDPGA of 8.4 +/- 0.6 and 605 +/- 35 microM, respectively, whereas the values for the glucosidation reaction were 10.2 +/- 1.5 and 670 +/- 120 microM, respectively. In both pooled human liver microsomes and expressed UGT2B7, UDPG and UDPGA competitively inhibited their counterpart conjugations with K(i) values close to their K(m) values, indicating a comparable affinity of the enzyme toward these two nucleotide sugars. We herein report a drug acyl glucoside formed in human liver microsomes at a considerable turnover rate and provide the evidence for a UGT isoform (UGT2B7) capable of transferring both glucuronic acid and glucose from UDPGA and UDPG to an aglycone.

Human UDP-glucuronosyltransferases: metabolism, expression, and disease

In vertebrates, the glucuronidation of small lipophilic agents is catalyzed by the endoplasmic reticulum UDP-glucuronosyltransferases (UGTs). This metabolic pathway leads to the formation of water-soluble metabolites originating from normal dietary processes, cellular catabolism, or exposure to drugs and xenobiotics. This classic detoxification process, which led to the discovery nearly 50 years ago of the cosubstrate UDP-glucuronic acid (19), is now known to be carried out by 15 human UGTs. Characterization of the individual gene products using cDNA expression experiments has led to the identification of over 350 individual compounds that serve as substrates for this superfamily of proteins. This data, coupled with the introduction of sophisticated RNA detection techniques designed to elucidate patterns of gene expression of the UGT superfamily in human liver and extrahepatic tissues of the gastrointestinal tract, has aided in understanding the contribution of glucuronidation toward epithelial first-pass metabolism. In addition, characterization of the UGT1A locus and genetic studies directed at understanding the role of bilirubin glucuronidation and the biochemical basis of the clinical symptoms found in unconjugated hyperbilirubinemia have uncovered the structural gene polymorphisms associated with Crigler-Najjar's and Gilbert's syndrome. The role of the UGTs in metabolism and different disease states in humans is the topic of this review.

Coordinate induction by antioxidants of UDP-glucuronosyltransferase UGT1A6 and the apical conjugate export pump MRP2 (multidrug resistance protein 2) in Caco-2 cells

Treatment of Caco-2 cells with the antioxidants quercetin or t-butylhydroquinone led to induced protein levels of UDP-glucuronosyltransferase UGT1A6 (ca. 3-fold over controls) and of the apical conjugate export pump multidrug resistance protein 2 (MRP2; 1.9-fold over controls). In contrast to UGT1A6, MRP2 (symbol ABCC2) was not inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin. Immunocytochemistry demonstrated that MRP2 was only expressed at the brush border domain of Caco-2 cell monolayers. The results indicate that UGT1A6 and MRP2 are coordinately induced by antioxidants, facilitating chemoprotection against phenolic toxins and excretion of conjugates into the intestinal lumen.

Differential protection by rat UDP-glucuronosyltransferase 1A7 against Benzo[a]pyrene-3,6-quinone- versus Benzo[a]pyrene-induced cytotoxic effects in human lymphoblastoid cells

UDP-glucuronosyltransferase 1A7 (UGT1A7) is a polyaromatic hydrocarbon (PAH)-inducible UGT with activity toward various benzo[a]pyrene (B[a]P) metabolites. To investigate the influence of rat UGT1A7 on B[a]P-induced cytotoxicity, human lymphoblastoid L3 cells were transfected with pMF6 (control expression vector), p167Dtk2 (microsomal epoxide hydrolase expression vector), or p167Dtk2-1A7 (epoxide hydrolase/UGT1A7 coexpression vector), and the cell populations were compared for sensitivity to B[a]P-induced effects. B[a]P inhibited cell proliferation and decreased relative cell survival of p167Dtk2 and p167Dtk2-1A7 cells to a similar extent. Metabolism studies using [(3)H]B[a]P revealed increased formation of glucuronide conjugates of B[a]P-4,5-diol, 3-OH-, or 9-OH-B[a]P and an unidentified metabolite by p167Dtk2-1A7 cells, but the presence of unconjugated metabolites suggested that glucuronidation capacity may be limited. No differences between p167Dtk2 and p167Dtk2-1A7 L3 cells were observed in the growth inhibitory effects of 3-OH-B[a]P or B[a]P-7,8-diol, but p167Dtk2-1A7-expressing cells were found to be less sensitive to B[a]P-3,6-quinone-induced effects on cell proliferation and relative cell survival. The effect was also observed in AHH-1 lymphoblastoid cells expressing UGT1A7 without epoxide hydrolase. The UGT1A7-expressing AHH-1 cells were also less sensitive to growth inhibition by B[a]P-1,6-quinone and B[a]P-6,12-quinone. Flow cytometric analysis of vehicle and B[a]P-3, 6-quinone-exposed cell populations showed an association between UGT1A7 expression and resistance to B[a]P-3,6-quinone-induced apoptosis and loss of cell viability. These data suggest that UGT1A7 may be preferentially active toward B[a]P-quinones and that UGT1A7 may represent the PAH-inducible UGT activity previously implicated in protection against toxic redox cycling by B[a]P-3,6-quinone.

Characterization of conjugated metabolites of benzo[a]pyrene in germ-free rat urine by liquid chromatography/electrospray tandem mass spectrometry

The characterization of conjugated metabolites of benzo[a]pyrene (BP) in the urine of male germ-free rats given a single intraperitoneal dose of [(14)C]BP is described. Urinary metabolites, constituting 9% of the administered radioactivity, were extracted on a Sep-Pak C(18) cartridge and separated by lipophilic ion-exchange chromatography into neutral and acidic fractions (fractions I-V). Metabolites in the latter fractions, constituting more than 80% of the urinary radioactivity, were characterized by reversed-phase HPLC and capillary column liquid chromatography/electrospray mass spectrometry (LC/ESMS) and tandem mass spectrometry (MS/MS). Relative quantities of BP metabolites were estimated from the distribution of radioactivity. Some coeluting compounds were semiquantified from the ion current chromatograms obtained in the capillary column LC/ESMS analyses. The major conjugated metabolites in fraction II, containing about 50% of the urinary radioactivity, consisted of three tetrahydrotrihydroxy-BP-S-N-acetylcysteines, the major isomer being 7,8,9,10-tetrahydro-8,9, 10-trihydroxy-BP-7-S-N-acetylcysteine, two dihydrotrihydroxy-BP-S-N-acetylcysteines, and a tetrahydrotetrahydroxy-BP-S-N-acetylcysteine. Fraction II also contained three apparently unconjugated compounds whose structures will be described elsewhere. Metabolites characterized in fractions III and IV, containing about 30% of the urinary radioactivity, included three BP-O,O'-disulfates, two monohydroxy-BP-O-sulfates, three dihydrodihydroxy-BP-O-sulfates, three BP-O,O'-diglucuronides, and a BP-O-sulfate-O'-glucuronide. Trace levels of a tetrahydrotrihydroxy-BP-S-glutathione conjugate were detected in fraction V.

Functions and transcriptional regulation of PAH-inducible human UDP-glucuronosyltransferases

Functions and regulation of selected human UDP-glucuronosyltransferases (UGT1A1, UGT1A4, UGT1A6, UGT1A9, UGT2B7, UGT2B15) are summarized. Evidence for at least two PAH-inducible UGTs (UGT1A6 and UGT1A9) is presented, which, however, are also constitutively expressed in a tissue- and cell-specific manner. These isoforms have recently been characterized to conjugate planar and bulky phenols, respectively. Using a selective RT-PCR method, UGT1A6 expression was detected in a variety of tissues (liver, kidney, lung, intestine, and pharyngeal mucosa). PAH-inducible UGTs may cooperate in the metabolism of phenolic metabolites of benzo(a)pyrene. Studies with stably expressed isoforms suggest that UGT1A9 is responsible for the formation of benzo(a)pyrene-3.6-diphenol diglucuronide, the major biliary metabolite of benzo(a)pyrene.

AH receptor-controlled transcriptional regulation and function of rat and human UDP-glucuronosyltransferase isoforms

Transcriptional regulation and function of rat and human PAH-inducible UDP-glucuronosyltransferase (UGT) isoforms have been studied. 1. At least two PAH-inducible UGT isoforms are expressed in a variety of tissues, the rat isoforms UGT1A6 and UGT1A7, and the human isoforms UGT1A6 and UGT1A9. 2. For the rat and human UGT1A6 isoforms two modes of tissue- and cell-specific regulation were found, PAH-inducible and constitutive expression. 3. Transient transfection studies, using human UGT1A6/CAT fusion constructs and colon carcinoma Caco-2 cells, revealed that PAH induction of human UGT1A6 is mediated by the Ah receptor. 4. Cell-expressed UGT isoforms were used to study their function in PAH metabolism. Rat UGT1A7 and human UGT1A9 appear to be more efficient than the corresponding UGT1A6 isoforms in catalyzing glucuronide formation of PAH phenols and diphenols. Several isoforms may act together in the formation of benzo(a)pyrene-3.6-diol diglucuronide, the major glucuronide found in rat bile. The results suggest complex modes of transcriptional regulation of PAH-inducible UGTs. They also suggest a major role of these UGT isoforms in detoxication of PAHs.

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

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

alpha-N-acetylgalactosamine-capping of chondroitin sulfate core region oligosaccharides primed on xylosides

We previously reported that cultured mammalian cells incubated with 4-methylumbelliferyl (MU) or p -nitrophenyl (pNP) beta-xyloside synthesize an alpha-GalNAc-terminated pentasaccharide resembling the glycosaminoglycan-core protein linkage region. Here we show that human melanoma M21 cells and human neuroblastoma cells incubated with Xylbeta-MU/pNP also make an alpha-GalNAc-terminated heptasaccharide containing one chondroitin disaccharide repeat. High performance liquid chromatography and matrix-assisted laser desorption ionization mass spectrometry analysis of intact or glycosidase-digested xyloside showed the structure as: GalNAcalphaGlcAbeta1,3GalNAcbeta1,4GlcAbeta1,3Galbe ta1,3Galbeta1, 4Xylbeta-MU/pNP. The alpha-GalNAc-terminated xylosides can account for approximately 10% of the total Xylbeta-MU/pNP products ( approximately 1.5 nmol/h/mg). These results show that GalNAcalphaGlcAbeta-modification is relatively abundant, but not unique to the GAG-linkage tetrasaccharide. alpha-GalNAc addition to the GlcA residue does not appear to be an extension of general phase II detoxification of xenobiotics that involve glucuronidation, since M21 cells incubated with MU synthesize only 0.3 pmol GlcAbeta-MU/h/mg protein, and undetectable amount of GalNAcalphaGlcAbeta-MU (<40 fmol/h/mg). Further, subcellular fractionation shows that the alpha- N- acetylgalactosaminyltransferase activity colocalizes in the Golgi with other glycosyl transferases and not in the ER, where xenobiotic detoxification glucuronosyltransferases are found. Although GalNAcalphaGlcAbeta-terminal modification has not been detected on naturally occurring GAG chains, the substantial amount of alpha-GalNAc transferase activity suggests that the alpha-GalNAc transferase could utilize other GlcA-containing glycoconjugates as acceptors.

Aryl hydrocarbon receptor-inducible or constitutive expression of human UDP glucuronosyltransferase UGT1A6

Transcriptional regulation of human UGT1A6, a UDP glucuronosyltransferase isoform conjugating a wide variety of planar phenols, has been studied using transfection experiments with plasmids containing its 3-kb 5' upstream region and chloramphenicol acetyltransferase as reporter gene. Previously, two modes of expression of the isoform have been described: in colon carcinoma Caco-2 cells UGT1A6 was found to be TCDD-inducible, whereas in lung carcinoma A549 cells it was constitutively expressed. Therefore functional analysis of UGT1A6 regulation was carried out using these two cell lines. In the upstream region of human UGT1A6 one xenobiotic-responsive element (XRE) was found between-1498 and -1502 bp. In Caco-2 cells the reporter gene activity of the entire plasmid and of deletion mutants containing the XRE were TCDD-inducible, in contrast to experiments with a deletion mutant which did not contain the XRE. TCDD induction was marginal in transfection studies with A549 cells. Gel mobility shift analysis indicated that the aryl hydrocarbon receptor and its partner Arnt bind to the XRE. Furthermore, primer extension studies suggest cell-specific use of multiple TATA boxes. Hence, regulation of human UGT1A6 appears to be cell-specific including both constitutive and aryl hydrocarbon receptor-controlled expression.

Drug-metabolizing enzymes in pharyngeal mucosa and in oropharyngeal cancer tissue

Cytochrome P4501A1 (CYP1A1) and the UDP-glucuronosyltransferase isoform UGT1A6 were studied in pharyngeal mucosa and squamous cancer tissue obtained from 27 male subjects (10 healthy nonsmoking volunteers, 10 smokers, and 7 smokers with pharyngeal cancer). CYP1A activity (7-ethoxyresorufin O-deethylase) was significantly induced in smokers as compared to nonsmokers (2.3 +/- 1.1 and 0.8 +/- 0.4 pmol x min[-1] x mg protein[-1], respectively). Immunoblot analysis demonstrated enhanced CYP1A1 protein in smokers. UGT activity towards 4-methylumbelliferone and 1-naphthol was also detectable in oropharyngeal mucosa. RT-PCR (reverse transcriptase-polymerase chain reaction) analysis indicated that UGT activity was at least in part due to the expression of UGT1A6. In cancer tissue, CYP1A activity was decreased in comparison with surrounding healthy mucosa (1.2 +/- 0.9 in tumor tissue vs. 2.2 +/- 0.7 pmol x min[-1] x mg protein[-1], respectively), whereas means and medians of UGT activity were unchanged. The results suggest that phase I and II drug-metabolizing enzymes are detectable in oropharyngeal mucosa and that CYP1A activity is inducible by constituents of cigarette smoke.

Characterization of 1-hydroxypyrene as a novel marker substrate of 3-methylcholanthrene-inducible phenol UDP-glucuronosyltransferase(s)

Rats were treated with acetone, pyrazole, phenobarbital, 4,4'-methylenebis-(2-chloroaniline) (MOCA), 3-methylcholanthrene, creosote oil, or a mixture of polychlorinated biphenyls (Aroclor 1254) to study the inducibility and enzyme kinetics of UDP-glucuronosyltransferases towards 1-hydroxypyrene, which is a human metabolite and a urinary biomarker of exposure to pyrene. The rate of 1-hydroxypyrene glucuronidation was analyzed in rat liver microsomes by a fluorometric HPLC assay of the formed glucuronide. The apparent K(m) and Vmax values in untreated controls (K(m) = 0.27 mM; Vmax = 31 nmol/min./mg protein) did not differ markedly from those in rats treated with acetone, pyrazole or phenobarbital, whereas the significantly decreased K(m) and increased Vmax values of the rats treated with the carcinogenic chemicals, MOCA (0.11; 51), creosote (0.06; 137), 3-methylcholanthrene (0.07; 141) or the Aroclor-1254 polychlorinated biphenyl (PCB) mixture (0.08; 226), implicated major changes in the hepatic expression of UDP-glucuronosyltransferases. 1-Hydroxypyrene proved to be a high affinity substrate and a sensitive marker of the polycyclic aromatic hydrocarbon (PAH) metabolizing UDP-glucuronosyltransferase(s). Catalytically, the most efficient isoforms were induced in creosote, 3-methylcholanthrene and PCB-treated rats showing Vmax/K(m) ratios which were 22-27 times greater than in untreated controls. Our findings suggest the existence of a 3-methylcholanthrene type inducible and a functionally efficient low-K(m)/ high-Vmax form(s) of UDP-glucuronosyltransferase(s) that detoxify 1-hydroxypyrene and probably other polycyclic aromatic hydrocarbons as well.