Organization of the rat UDP-glucuronosyltransferase, UDPGTr-2, gene and characterization of its promoter

Quantitative Analysis of UDP-Glucuronosyltransferase Ugt1a and Ugt2b mRNA Expression in the Rat Liver and Small Intestine: Sex and Strain Differences

UDP-glucuronosyltransferases (UGTs) catalyze the glucuronidation of numerous endogenous and exogenous compounds to facilitate their excretion from the body. Because rats are commonly used in nonclinical studies, information regarding UGT species differences between rats and humans would be helpful for understanding human pharmacokinetics. In this study, we determined the absolute mRNA expressions of Ugt isoforms in the liver and small intestine of male and female Sprague-Dawley, Fischer 344, and Wistar rats. The sum of the mRNA levels of Ugt isoforms expressed in the liver was significantly (P < 0.005) higher than that in the small intestine regardless of the strain and sex. Ugt2b mRNA levels represented approximately 80% of total Ugt mRNA levels in the liver, whereas Ugt1a mRNA levels accounted for almost 90% in the small intestine. Ugt2b2 mRNA was specifically expressed in Wistar rat liver, resulting in 2-fold higher expression of total hepatic Ugt mRNA in Wistar rats than that in the other strains. Wistar rats showed prominently higher Ugt2b3 and Ugt2b8 mRNA levels in the small intestine than the other strains. The difference between sexes was remarkable with regard to hepatic Ugt1a10 in any of the strains, although slight differences between sexes were also observed in multiple Ugt isoforms. Taken together, this study revealed sex and strain differences in mRNA levels of rat Ugts. The data shown here would be useful for the selection of rat strains in nonclinical studies.

Chemopreventive effect of sinapic acid on 1,2-dimethylhydrazine-induced experimental rat colon carcinogenesis

Sinapic acid (SA) is a naturally occurring phenolic acid found in various herbal plants which is attributed with numerous pharmacological properties. This study was aimed to investigate the chemopreventive effect of SA on 1,2-dimethylhydrazine (DMH)-induced rat colon carcinogenesis. Rats were treated with DMH injections (20 mg kg(-1) bodyweight (b.w.) subcutaneously once a week for the first 4 consecutive weeks and SA (20, 40 and 80 mg kg(-1) b.w.) post orally for 16 weeks. At the end of the 16-week experimental period, all the rats were killed, and the tissues were evaluated biochemically. Our results reveal that DMH alone treatment decreased the levels/activities of lipid peroxidation by-products such as thiobarbituric acid reactive substances, conjugated dienes and antioxidants such as superoxide dismutase, catalase, glutathione reductase, glutathione peroxidase and reduced glutathione in the intestine and colonic tissues which were reversed on supplementation with SA. Moreover, the activities of drug-metabolizing enzymes of phase I (cytochrome P450 and P4502E1) were enhanced and those of phase II (glutathione-S-transferase, DT-diaphorase and uridine diphosphate glucuronosyl transferase) were diminished in the liver and colonic mucosa of DMH alone-treated rats and were reversed on supplementation with SA. All the above changes were supported by the histopathological observations of the rat liver and colon. These findings suggest that SA at the dose of 40 mg kg(-1) b.w. was the most effective dose against DMH-induced colon carcinogenesis, and thus, SA could be used as a potential chemopreventive agent.

Substrate selectivity of human intestinal UDP-glucuronosyltransferases (UGTs): in silico and in vitro insights

The current drug development process aims to produce safe, effective drugs within a reasonable time and at a reasonable cost. Phase II metabolism (glucuronidation) can affect drug action and pharmacokinetics to a considerable extent and so its studies and prediction at initial stages of drug development are very imperative. Extensive glucuronidation is an obstacle to oral bioavailability because the first-pass glucuronidation [or premature clearance by UDP-glucuronosyltransferases (UGTs)] of orally administered agents frequently results in poor oral bioavailability and lack of efficacy. Modeling of new chemical entities/drugs for UGTs and their kinetic data can be useful in understanding the binding patterns to be used in the design of better molecules. This review concentrates on first-pass glucuronidation by intestinal UGTs, including their topology, expression profile, and pharmacogenomics. In addition, recent advances are discussed with respect to substrate selectivity at the binding pocket, structural requirements, and mechanism of enzyme actions.

Repeated exposure to codeine alters morphine glucuronidation by affecting UGT gene expression in the rat

We have previously found that phenantrenic opioids, such as heroin or naltrexone, modulate morphine glucuronidation in the rat. Here we further investigated the effects of phenantrenic opioids on morphine glucuronidation comparing the effects of codeine and heroin. In particular, we measured the synthesis of morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) from morphine: in the liver microsomal preparations obtained from rats repeatedly treated with two different doses of codeine (ex vivo study); in primary cultures of rat hepatocytes previously incubated for 72h with codeine, or heroin (in vitro study); in the latter conditions, the levels of expression of genes coding for uridine-5'-diphosphate-glucuronosyltransferases (UGTs) A1, A6, A7 and 2B1 were also determined; finally, the levels of glucuronic acid in rat hepatocytes previously incubated for 72h with codeine or heroin were assessed. The ex vivo study shows that codeine exposure in vivo stimulated liver microsomal M3G formation and de novo synthesis of M6G. Differently, in primary hepatocyte cultures both codeine and heroin inhibited M3G formation, whereas heroin only stimulated de novo synthesis of M6G; moreover, codeine significantly reduced UGT2B1 expression at 6h and caused a trend toward inhibition of UGT1A1 expression at 72h; heroin enhanced UGT2B1 expression and inhibited that of UGT1A1 at 72h; finally, both codeine and heroin depleted UDPGA content of hepatocytes. In conclusion, codeine affects liver glucuronidation of morphine enlightening the possible contribution of changes in the spectrum of UGT gene expression and co-factor synthesis in this phenomenon.

Sterol glycosyltransferases--the enzymes that modify sterols

Sterols are important components of cell membranes, hormones, signalling molecules and defense-related biotic and abiotic chemicals. Sterol glycosyltransferases (SGTs) are enzymes involved in sterol modifications and play an important role in metabolic plasticity during adaptive responses. The enzymes are classified as a subset of family 1 glycosyltransferases due to the presence of a signature motif in their primary sequence. These enzymes follow a compulsory order sequential mechanism forming a ternary complex. The diverse applications of sterol glycosides, like cytotoxic and apoptotic activity, anticancer activity, medicinal values, anti-stress roles and anti-insect and antibacterial properties, draws attention towards their synthesis mechanisms. Many secondary metabolites are derived from sterol pathways, which are important in defense mechanisms against pathogens. SGTs in plants are involved in changed sensitivity to stress hormones and their agrochemical analogs and changed tolerance to biotic and abiotic stresses. SGTs that glycosylate steroidal hormones, such as brassinosteroids, function as growth and development regulators in plants. In terms of metabolic roles, it can be said that SGTs occupy important position in plant metabolism and may offer future tools for crop improvement.

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

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

Molecular mechanism of phase I and phase II drug-metabolizing enzymes: implications for detoxification

Enzymes that catalyze the biotransformation of drugs and xenobiotics are generally referred to as drug-metabolizing enzymes (DMEs). DMEs can be classified into two main groups: oxidative or conjugative. The NADPH-cytochrome P450 reductase (P450R)/cytochrome P450 (P450) electron transfer systems are oxidative enzymes that mediate phase I reactions, whereas the UDP-glucuronosyltransferases (UGTs) are conjugative enzymes that mediate phase II enzymes. Both enzyme systems are localized to the endoplasmic reticulum (ER) where a number of drugs are sequentially metabolized. DMEs, including P450s and UGTs, generally have a highly plastic active site that can accommodate a wide variety of substrates. The P450 and UGT genes constitute a supergene family, in which UGT proteins are encoded by distinct genes and a complex gene. Both the P450 and UGT genes have evolved to diversify their functions. This chapter reviews advances in understanding the structure and function of the P450R/P450 and UGT enzyme systems. In particular, the coordinate biotransformation of xenobiotics by phase I and II enzymes in the ER membrane is examined.

Possible involvement of oxidative stress in piperonyl butoxide induced hepatocarcinogenesis in rats

To clarify the possible mechanism of non-genotoxic hepatocarcinogenesis induced by piperonyl butoxide (PBO), male F344 rats were administered an i.p. injection of N-diethylnitrosamine (DEN) to initiate hepatocarcinogenesis. Two weeks later, the rats were administered a PBO-containing (0, 1, or 2%) diet for 6 weeks and subjected to a two-third partial hepatectomy 1 week later. After sacrificing them on week 8, their livers were histopathologically examined and analyzed for gene expression using a microarray and real-time RT-PCR. Reactive oxygen species (ROS) products were also measured using liver microsomes. Hepatocytes exhibited centrilobular hypertrophy and increased glutathione S-transferase placental form (GST-P) positive foci formation. ROS products increased significantly in liver microsomes. In the microarray analysis, the expressions of genes related to metabolism and oxidative stress - NAD(P)H dehydrogenase, quinone 1 (Nqo1), UDP-glucuronosyltransferase (UDPGTR-2), glutathione peroxidase 2 (Gpx2), glutathione reductase (GRx) - multidrug resistance associated protein 3 (Abcc3), and solute carrier family 7 (cationic amino acid transporter, y+ system) member 5 (Slc7a5) were up-regulated in the PBO group in comparison to the 0% PBO group; this was confirmed by real-time RT-PCR. Additionally, a significant up-regulation of stress response related genes such as CYP1A1 was observed in PBO-treated groups in real-time RT-PCR. HPLC analysis revealed that the level of 8-OHdG in the 2% PBO group was significantly higher than that in the 0% PBO group. This suggests that PBO has the potential to generate ROS via metabolic pathways and induce oxidative stress, including oxidative DNA damage, resulting in the induction of hepatocellular tumors in rats.

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.

Elevated serum bisphenol A levels under hyperandrogenic conditions may be caused by decreased UDP-glucuronosyltransferase activity

This study was performed to investigate the effect of androgen on the metabolism of bisphenol A (BPA), an endocrine disruptor, in order to clarify the mechanism of the higher levels of serum BPA in men and hyperandrogenemic women compared with normal women. Castrated female rats (OVX) were subcutaneously injected with testosterone propionate (TP) (0.01, 0.1, and 1 mg) every day for 2 weeks. Serum BPA concentrations in OVX rats showed a TP dose-dependent increase and were significantly higher at 0.1 and 1.0 mg of TP. The enzyme reaction of BPA glucuronidation in the rat liver microsomes showed that the ratio of glucuronide in the OVX rats was significantly reduced in a TP dose-dependent manner. Analysis of the mRNA expression of UDP-glucuronosyltransferase 2B1 (UGT2B1) by real-time quantitative RT-PCR revealed that the relative expression level of UGT2B1 mRNA showed a TP dose-dependent decrease. The results of enzyme analyses demonstrated that the ratio of BPA glucuronidation and the expression level of UGT2B1 mRNA were significantly lower under the hyperandrogenemic conditions. The clearance of BPA may be slowed in a TP dose-dependent manner, resulting in an increase of serum BPA concentration under hyperandrogenemic conditions.

Preclinical Development of New Drugs that Enhance Thyroid Hormone Metabolism and Clearance: Inadequacy of Using Rats as an Animal Model for Predicting Human Risks in an IND and NDA

New drugs that enhance metabolism or clearance of thyroid hormones in rats often trigger a sequence of toxicity events during chronic administration: reduction of thyroxine, elevation of thyroid-stimulating hormone (TSH) levels, and thyroid gland hyperfunction/growth. Hepatocellular hypertrophy and thyroid follicular hyperplasia are often observed with increased liver and thyroid organ weights. This unique toxicity profile seems to be species-specific because the thyroxine in rodents is metabolized rapidly, without thyroid hormone-binding globulin that serves as a reserve, as in humans. Thus, elevations of TSH were not reported in humans for drugs such as delavirdine, fluvastatin, nicardipine, phenobarbital, simvastatin, and spironolactone, all of which produce thyroid hyperplasia or tumors in rats. Further, the human thyroid is less sensitive to prolonged TSH stimulation than that of the rat (eg, endemic goiter patients with high TSH due to iodine deficiency do not develop thyroid cancer). In view of the species difference in sensitivity of the thyroid between rodents and humans, using the rat as an animal model to explore target organs of toxicity for a new drug that significantly enhances thyroid hormone metabolism/clearance and increases TSH levels would not be adequate. In this case, a compromised and dysfunctional hypothalamo-pituitary-thyroid system would confound the toxicity profile explored in preclinical toxicity testing and render the model an inadequate risk predictor for the new drug in humans. Under such conditions, IND and NDA sponsors of drugs exhibiting this activity profile should be encouraged to use alternative animal species for toxicity exploration to provide a more meaningful human risk prediction.

Gender difference in serum bisphenol A levels may be caused by liver UDP-glucuronosyltransferase activity in rats

Gender difference in human bisphenol A (BPA) concentrations was revealed by determining serum BPA. We studied the serum concentrations and the metabolism of BPA in rats by an HPLC system. Rat serum BPA concentrations were significantly higher in males (24.9+/-7.38 ng/ml, P=0.026, n=10) than in females (8.27+/-3.11 ng/ml, n=10), as in humans. The resultant enzyme reaction products of BPA glucuronidation in the rat liver microsomes fraction were analyzed by an HPLC system. The ratio of BPA glucuronidation in the microsome reaction was significantly higher (P=0.015) in female than in male rats. The mRNA expression of UDP-glucuronosyltransferase 2B1 (UGT2B1), an isoform of UGT related to BPA glucuronidation, in the rat liver was analyzed by a real-time quantitative RT-PCR. The relative expression level of UGT2B1 mRNA was significantly higher (P<0.001) in female than in male rat livers. The gender difference in serum BPA concentrations may be explained by the difference in clearance based on the UGT activities.

UDP-glucuronosyltransferases: gene structures of UGT1 and UGT2 families

In human, rat, and mice, a UGT1 complex locus provides for developmental-, inducer-, and cell-specific synthesis of a family of chemical-detoxifying isozymes, UDP-glucuronosyltransferases, which prevent toxicities, mutagenesis, and/or carcinogenesis. Between 10 and 14 first exons with individual promoter elements are tandemly arrayed upstream of 4 shared exons so as to synthesize independently as many overlapping primary transcripts. RNA splice sites allow a lead exon to join the common exons to generate mRNAs with unique 5' ends, but common 3' ends. Intra- and interspecies comparisons of amino acid sequences encoded by first exons show an evolutionary continuum; also, recognizable bilirubin- and phenol-specific catalytic units are differentially regulated by model compounds, phenobarbital, and/or aromatic hydrocarbons. Whereas UGT1 loci allow minimal changes to achieve new isozymes, a single deleterious mutation in a common exon negatively impacts the arrangement by inactivating the entire family of isozymes compared to an event at independent loci as seen in the UGT2 family. In humans, lethal hyperbilirubinemic Crigler-Najjar type 1 and milder diseases/syndromes are due to deleterious to mildly deleterious mutations in the bilirubin-specific UGT1A1 or a common exon. In addition, the number of TA repeats (N(5-8)) in the UGT1A1 proximal TATA box affects transcriptional rate and, thus, activity. Evidence also shows that polymorphisms in nonbilirubin-specific first exons also impact chemical detoxifications and other diseases.

Glucuronidation of carboxylic acid containing compounds by UDP-glucuronosyltransferase isoforms

Glucuronide conjugation of xenobiotics containing a carboxylic acid moiety represents an important metabolic pathway for these compounds in humans. Several human UDP-glucuronosyltransferases (UGTs) have been shown to catalyze the formation of acyl-glucuronides, including UGT2B7, UGT1A3, and UGT1A9. In this study, recombinant expressed UGT isoforms were investigated with many structurally related carboxylic acid analogues, and the UGT rank order for catalyzing the glucuronidation of carboxylic acids was UGT2B7?UGT1A3 approximately UGT1A9. Despite being a poor substrate with UGT1A3, coumarin-3-carboxylic acid was not a substrate for any other UGT isoform tested in this study, suggesting that it could be a specific substrate for UGT1A3. Interestingly, UGT1A7 and UGT1A10 also react with several carboxylic acid aglycones. Kinetic analysis showed that UGT2B7 exhibits much higher glucuronidation efficiency (Vmax/Km) with ibuprofen, ketoprofen, and others, compared to UGT1A3. These data indicate that UGT2B7 could be the major isoform involved in the glucuronidation of carboxylic acid compounds in humans.

Glucuronidation and the UDP-glucuronosyltransferases in health and disease

This article is an updated report of a symposium held at the June 2000 annual meeting of the American Society for Pharmacology and Experimental Therapeutics in Boston. The symposium was sponsored by the ASPET Divisions for Drug Metabolism and Molecular Pharmacology. The report covers research from the authors' laboratories on the structure and regulation of UDP-glucuronosyltransferase (UGT) genes, glucuronidation of xenobiotics and endobiotics, the toxicological relevance of UGTs, the role of UGT polymorphisms in cancer susceptibility, and gene therapy for UGT deficiencies.

Six novel UDP-glucuronosyltransferase (UGT1A3) polymorphisms with varying activity

Human UDP-glucuronosyltransferase (UGT) is a part of a major excretion pathway for endobiotics and xenobiotics. The UGT family of genes is highly polymorphic, and our aim is to describe novel polymorphisms at the UGT1A3 locus and determine how they alter substrate metabolism and drug reactions. One hundred healthy Japanese adults volunteered for the present study. We sequenced PCR-amplified fragments of the gene directly, and calculated the frequency of the genetic variations detected. To measure variant enzyme activity, we constructed five expression models and used estrone as the substrate in the assays. We identified six novel single nucleotide polymorphisms (SNPs). Of these, four caused amino acid substitutions (17A-->G: Q6R, 31T-->C: W11R, 133C-->T: R45W, and 140T-->C: V47A) and the remaining two were silent (81G-->A: E27E and 447A-->G: A159A). We found five types of alleles having differing SNP combinations: wild type (frequency=0.61), W11R-E27E-A159A (0.10), Q6A-W11R-E27E-A159A (0.055), W11R-E27E-V47A-A159A (0.125), and R45W (0.11). Expression studies found that the variants changed the enzyme efficiencies ( Km/ Vmax) to 121% of the wild type for W11R, 86% for Q6R-W11R, 369% for W11R-V47A, and 70% for R45W. Several UGT 1A3 polymorphisms exist in the Japanese population, having different levels of activity. These polymorphisms are capable of affecting the steady state levels of estrogens, and may increase sensitivity to adverse drug effects.

Tissue mRNA expression of the rat UDP-glucuronosyltransferase gene family

UDP-Glucuronosyltransferases (UGTs) are phase II biotransformation enzymes that glucuronidate numerous endobiotic and xenobiotic substrates. Glucuronidation increases the water solubility of the substrate and facilitates renal and biliary excretion of the resulting glucuronide conjugate. UGTs have been divided into two gene families, UGT1 and UGT2. Tissue distribution of UGTs has not been thoroughly examined, and such data could provide insight into the importance of individual UGT isoforms in specific tissues and to the pharmacokinetics and target organ toxicity of UGT substrates. Therefore, the aim of this study was to determine mRNA levels of rat UGT1 and UGT2 family members in liver, kidney, lung, stomach, duodenum, jejunum, ileum, large intestine, cerebellum, and cerebral cortex, as well as nasal epithelium for UGT2A1. Tissue levels of UGT mRNA were detected using branched DNA signal amplification analysis. Three UGT isoforms, UGT1A1, UGT1A6, and UGT2B12, were detected in many tissues, whereas distribution of other UGT isoforms was more tissue-specific. For example, UGT2A1 was detected predominantly in nasal epithelium. Additionally, UGT1A5, UGT2B1, UGT2B2, UGT2B3, and UGT2B6 were detected primarily in liver. Furthermore, detection of UGT1A2, UGT1A3, UGT1A7, and UGT2B8 was somewhat specific to gastrointestinal (GI) tract. However, not all of these UGTs were detected in all portions of the GI tract. UGT1A8 was unique in that it was barely detectable in any of the tissues examined. In conclusion, some UGT isoforms were expressed in multiple tissues, whereas other UGT isoforms were predominantly expressed in a certain tissue such as nasal epithelium, liver, or GI tract.

Inhibition and active sites of UDP-glucuronosyltransferases 2B7 and 1A1

Two human UDP-glucuronosyltransferases (UGTs), UGT2B7 and UGT1A1, catalyze the glucuronidation of many endo- and xenobiotics. Although UGT1A1 uniquely catalyzes the glucuronidation of the endobiotic, bilirubin, and UGT2B7 uniquely catalyzes the glucuronidation of morphine to both the 3-0 glucuronide and the 6-0 glucuronide, both catalyze the glucuronidation of the mixed opioid agonist/antagonist buprenorphine with high efficiency. Etonitazenyl, a mu opioid receptor antagonist, was found to inhibit competitively opioid, steroid, and other substrate glucuronidation reactions catalyzed by UGT2B7. Data showing several benzodiazepines and alternative substrates interacting competitively support previous work, which indicates a single binding domain within UGT2B7. Etonitazenyl also competitively inhibited the glucuronidation of buprenorphine catalyzed by UGT1A1. However, neither etonitazenyl nor buprenorphine inhibited bilirubin glucuronidation except at very high concentrations. Therefore, it is unlikely that buprenorphine therapy for opioid or other drug addiction would influence bilirubin glucuronidation and lead to hyperbilirubenmia. Anthraflavic acid and catechol estrogen glucuronidation, catalyzed by UGT1A1, was also not inhibited by etonitazenyl or buprenorphine. Reactions catalyzed by UGT1A6 were not affected by etonitazenyl. These studies indicate that UGT2B7 has one binding site and that UGT1A1 has two or more binding sites for xenobiotics and endobiotics.

The homeodomain Pbx2-Prep1 complex modulates hepatocyte nuclear factor 1alpha-mediated activation of the UDP-glucuronosyltransferase 2B17 gene

UDP glucuronosyltransferases (UGT) are expressed in a wide range of tissues in which their levels of expression and distribution are dependent on cell-type specific regulatory mechanisms. The presence of a hepatocyte nuclear factor (HNF) 1 binding site in the proximal promoters of several UGT2B genes has been shown to contribute to their expression in liver cells and possibly other HNF1-containing cell types. In some of these UGT2B genes, a putative pre-B cell homeobox (Pbx) transcription factor binding site is found directly adjacent to the functional HNF1 site. To determine whether this putative Pbx site contributes to the regulation of UGT2B expression, we chose the UGT2B17 gene and investigated the capacity of its Pbx site to bind specific transcription factors and alter promoter activity. The UGT2B17 Pbx site matches a consensus Pbx site known to bind members of the Pbx, Hox, Meis, and Prep1 families of homeodomain-containing proteins and has previously been shown to bind nuclear proteins in DNaseI footprint assays. In this study, we used gel shift and functional assays to show that a Pbx2-Prep1 heterodimer can bind to the UGT2B17 Pbx site and interfere with the binding of HNF1alpha to its site adjacent to the Pbx site. This interaction of Pbx2-Prep1 and HNF1alpha results in down-regulation of HNF1alpha-mediated activation of the UGT2B17 promoter. Modulation of transcription by restricting the binding of transcriptional effectors to their target site is a novel role for Pbx2-Prep1 complexes.

Increase in rat liver UDP-glucuronosyltransferase mRNA by microsomal enzyme inducers that enhance thyroid hormone glucuronidation

Treatment of rats with the microsomal enzyme inducers pregnenolone-16alpha-carbonitrile (PCN), 3-methylcholanthrene (3-MC), and Aroclor 1254 [PCB (polychlorinated biphenyl)] has been shown to decrease circulating levels of thyroid hormones as well as increase microsomal glucuronidation of thyroxine (T(4)). In addition, PCN increases triiodothyronine (T(3)) uridine diphosphate glucuronosyltransferase (UGT) activity. Members of the UGT1A family are believed to glucuronidate T(4), specifically UGT1A1 and UGT1A6, whereas the UGT2 family is believed to glucuronidate T(3), namely UGT2B2. The purpose of this study was to determine whether the aforementioned microsomal enzyme inducers increase the mRNAs that encode these and other UGT enzymes in rat liver. Male Sprague-Dawley rats were fed a control diet or a diet containing PCN (1000 ppm), 3-MC (250 ppm), or PCB (100 ppm) for 7 days, at which time livers were collected. Increases in mRNA were detected by QuantiGene branched DNA signal amplification. A 3-fold increase in UGT1A1 mRNA was produced by PCN in addition to increases in UGT1A2 (4-fold) and UGT1A5 (2-fold) mRNA. PCN affected neither UGT2B2 nor any other UGT2B mRNA level. 3-MC and PCB increased UGT1A6 mRNA 6- and 4-fold, respectively. 3-MC and PCB each increased UGT1A7 mRNA 4-fold but did not significantly increase any other UGT mRNAs. These findings suggest that PCN enhances T(4) UGT activity by increased expression of UGT1A1 and that 3-MC and PCB enhance T(4) UGT activity by increased expression of UGT1A6. These findings also suggest that increased T(3) UGT activity produced by PCN is due to a mechanism other than increased transcription of UGT2B2, possibly increased UGT2B2 protein or induction of another UGT enzyme.

Tissue specific differences in the regulation of the UDP glucuronosyltransferase 2B17 gene promoter

The human UDP glucuronosyltransferase UGT2B17, glucuronidates androgens and is expressed in the liver and the prostate. Although evidence suggests that variations in UGT2B17 expression between tissues may be a critical determinant of androgen response, the factors that regulate UGT2B17 expression in the liver and prostate are unknown. In this study, we have isolated a 596 bp promoter of the UGT2B17 gene and studied its regulation in the liver cell line, HepG2 and the prostate cell line, LNCaP. The transcription start site of UGT2B17 was mapped and proteins that bound to the proximal promoter were detected by DNase1 footprint analysis. A region (-40 to -52 bp) which resembled a hepatocyte nuclear factor 1 (HNF1) binding site bound proteins in nuclear extracts from HepG2 cells, but did not bind proteins from LNCaP nuclear extracts. In HepG2 cells, HNF1alpha bound to this region and activated the UGT2B17 promoter, as assessed by functional and gel shift assays. HNF1alpha activation of the promoter was prevented by mutation or deletion of the putative HNF1 site. The related transcription factor HNF1beta, which is present in HepG2 cells, did not activate the promoter. The UGT2B17 promoter could also be activated by exogenous HNF1alpha in LNCaP cells. However, because these cells do not contain HNF1alpha, other transcription factors must regulate the UGT2B17 promoter. Cotransfection experiments showed that HNF1beta, elevates promoter activity in LNCaP cells. This activation did not involve the putative HNF1 region (-40 to -52 bp) since mutation of this region did not affect promoter activation by HNF1beta. These results suggest that the UGT2B17 promoter is regulated by different factors in liver-derived HepG2 and prostate-derived LNCaP cells.

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.

Genomic organization of the UGT2b gene cluster on human chromosome 4q13

The UDP-glucuronosyltransferases (UGTs) comprise a large family of proteins capable of detoxifying a wide variety of both endogenous and exogenous substrates. The primary function of this gene superfamily is to catalyze the glycosylation of substrates such as biogenic amines, steroids, bile acids, phenolic compounds and various other pharmacologically relevant compounds, including numerous carcinogens, toxic environmental pollutants and prescription drugs. This conjugation increases the solubility of these compounds, allowing them to be excreted more readily through hepatic or renal mechanisms. This paper describes the genomic characterization and chromosomal localization of three UGT2B genes which together comprise part of a large cluster of related sequences, including pseudogenes found on human chromosome 4q13. A genomic map spanning approximately 500-1000 kb of this region reveals the presence of three previously described UGT2B genes, at least two previously uncharacterized pseudogenes and a significant number of remnant gene fragments and places UGT2B4 between UGT2B7 and UGT2B15. Additionally, access to a large reference DNA bank allowed us to calculate allele frequencies for two UGT2B SNPs: D85R in UGT2B15 and Q458D in UGT2B4 amongst 803 unrelated individuals representing five ethnic populations. The data presented here suggest a recent evolutionary history of gene duplication, mutation and rearrangement. Furthermore, they suggest that a re-evaluation of the current description of the UGT2B gene family with respect to the number of specific genes, degree of allelic diversity and molecular evolution may be necessary.

Activation of the mouse TATA-less and human TATA-containing UDP-glucuronosyltransferase 1A1 promoters by hepatocyte nuclear factor 1

UDP-glucuronosyltransferase (UGT) 1A1 (UGT1A1) catalyzes the glucuronidation of bilirubin in liver. Among all UGT isoforms identified to date, it is the only relevant bilirubin-glucuronidating enzyme in human. Because glucuronoconjugation is the major route of bilirubin elimination, any genetic alteration that affects bilirubin glucuronosyltransferase activity may result in a more or less severe hyperbilirubinemia. In this study, we report the cloning and characterization of the transcriptional regulation of the mouse UGT1A1 gene. Primary-structure analysis of the mouse Thymidine Adevice promoter revealed marked differences with its human homolog. First, the mouse promoter lacks the highly polymorphic thymidine/adenine repeat occurring in the human promoter, which has been associated with some forms of hyperbilirubinemia. Second, an L1 transposon element, which is absent in the human promoter, is found 480 bp upstream of the transcription start site in mouse. Using the electromobility shift and DNase I footprinting experiments, we have identified a hepatocyte nuclear factor 1-binding site in the mouse UGT1A1 promoter that confers responsiveness to both factors HNF1alpha and HNF1beta in HEK293 cells. Furthermore, we show that this element, which is conserved in the human promoter, also confers strong HNF1 responsiveness to the human UGT1A1 gene. Together, these results provide evidence for a major regulatory function of this liver-enriched transcription factor in UGT1A1 activity in both rodents and human.

Expression of UDP-glucuronosyltransferases (UGTs) 2B7 and 1A6 in the human brain and identification of 5-hydroxytryptamine as a substrate

The extrahepatic expression of UDP-glucuronosyltransferases (UGTs) is important in the detoxification of a number of endogenous and exogenous compounds, including 5-hydroxytryptamine and morphine. Studies were designed to investigate the extrahepatic expression of human UGTs using RT-PCR techniques and to determine the UGTs involved in the glucuronidation of 5-hydroxytryptamine. Human UGT2B7 expression was found in the human liver, kidney, pancreas, and brain, while UGT1A6 expression is found in the liver, kidney, and brain. This is the first observation of UGTs present in the human central nervous system. Using glucuronidation assays, a significant amount of 5-hydroxytryptamine glucuronide was found to be catalyzed by UGT1A6. These studies suggest that UGT2B7 may play an important role in the overall contribution of morphine analgesia by serving to generate the potent morphine-6-O-glucuronide in situ. UGT1A6 could play an important role in the glucuronidation of 5-hydroxytryptamine in vivo, therefore terminating the actions of the neurotransmitter.

Cloning and expression of human UDP-glucuronosyltransferase (UGT) 1A8

The mRNA expression of human UDP-glucuronosyltransferase 1A8 (UGT1A8) has been found in jejunum, ileum, and colon but not in liver. A cDNA with a complete UGT1A8 coding region was amplified from total human ileal RNA by reverse transcriptase-polymerase chain reaction and inserted into the mammalian expression vector, pcDNA3. Lysates of HK293 cells expressing UGT1A8 revealed the expression of a protein with a molecular mass of 56 kDa by Western blot analysis. Transiently expressed human UGT1A8 shows glucuronidation activities with coumarins, anthraquinones, flavonoids, phenolic compounds, catechol estrogens, 17-hydroxyandrogens, primary amines such as the carcinogen 4-aminobiphenyl, and certain opioids. This UGT may play an important role in the detoxification of xenobiotics in human intestine.

New perspectives on the function of myelin galactolipids

A defining feature of the vertebrate nervous system is the ensheathment of axons by myelin, a multilamellar membrane containing a small group of proteins and an abundance of the galactolipid galactocerebroside (GalC) and its sulfated derivative sulfatide. Several in vitro studies have suggested that these galactolipids transduce developmental signals, facilitate protein trafficking and stabilize membranes. In addition, mice lacking the ability to synthesize GalC or sulfatide form dysfunctional and unstable myelin. These findings suggest that the galactolipids are essential components of myelin, and that functional and structural properties of myelin result from the combined contributions of galactolipids and proteins.

Regulation of uridine diphosphate glucuronosyltransferase during the acute-phase response

The acute-phase response is associated with profound effects on oxidative drug metabolism. However, the effects on glucuronidation are poorly characterized. The aim of the present study was to determine the role of mediators of the acute-phase response in the regulation of hepatic uridine diphosphate glucuronosyltransferase (UGT) expression. Family 1 and family 2 UGT isoforms were studied in turpentine-injected rats and in primary hepatocyte cultures exposed to cytokines and/or dexamethasone. In the in vivo model, glucuronidation of p-nitrophenol was unaffected, while testosterone glucuronidation was reduced to 65% of control (P<0.01). In contrast, the mRNA level of UGT1*1 (which metabolizes bilirubin, not phenols) was depressed to 16% of control (P<0.002), while the mRNA level of UGT2B3 (which metabolizes testosterone) was reduced to 53% (P<0.05). In primary hepatocyte culture, dexamethasone treatment resulted in a 3.4-fold induction of UGT1*1 mRNA levels (P<0.001) but only a 1.5-fold induction of UGT2B3 (P=0.1). Interleukin-6 in the presence of dexamethasone resulted in a marked dose-dependent suppression of both UGT1*1 and UGT2B3, although to different degrees. Interleukin-1 had no effect on UGT mRNA levels. Thus, inflammatory mediators, such as cytokines and glucocorticoids, may be important determinants of both oxidative and conjugative drug metabolism by the liver.

Chromosomal localization, structure, and regulation of the UGT2B17 gene, encoding a C19 steroid metabolizing enzyme

UGT2B17 is a UDP-glucuronosyltransferase enzyme expressed in several extrahepatic steroid target tissues, including the human prostate, where it glucuronidates C19 steroids such as dihydrotestosterone (DHT), androsterone (ADT), and androstane-3alpha, 17beta-diol (3alpha-diol). To determine if UGT2B17 is regulated by physiological effectors of the human prostate, DHT and epidermal growth factor (EGF) were demonstrated to specifically down-regulate the steady-state levels of UGT2B17 transcript and protein in LNCaP cells (Guillemette et al., 1997). These results implicate regulation of UGT2B17 at the level of gene transcription, therefore, a P-1-derived artificial chromosome (PAC) clone of 120 kb containing the entire UGT2B17 gene was isolated. The gene is comprised of six exons spanning approximately 30 kb, and fluorescence in situ hybridization of the UGT2B17 PAC clone to normal human lymphocyte chromosomes, mapped the gene to chromosome 4q13. To determine if the 5'-flanking DNA of the UGT2B17 gene is sufficient to confer gene expression, a 2,942-bp fragment was subcloned into a luciferase reporter plasmid and yielded an activity of 25-fold over background when transfected in LNCaP cells. However, transfection of the construct into HK-293, MCF-7, JEG-3, and HepG2 cells yielded only a moderate activity of two- to five-fold over background. Treatment of transfected LNCaP cells with 10 nM R1881, a nonmetabolizable analog of DHT, and 10 ng/ml EGF decreased the luciferase activity by 60%. This suggests that at least part, if not all, of the inhibitory effect of EGF and DHT on UGT2B17 is at the level of transcription. Progressive 5' deletions of the UGT2B17 5'-flanking region in the luciferase constructs alleviated the inhibition by R1881 and EGF, and revealed several potential responsive elements that may confer the observed regulation of the UGT2B17 gene. This study demonstrates regulation of the UGT2B17 gene by physiological effectors of the human prostate and supports the hypothesis that UGT enzymes are involved in steroid metabolism in extrahepatic tissues.

Characterization of rabbit UDP-glucuronosyltransferase UGT1A7: tertiary amine glucuronidation is catalyzed by UGT1A7 and UGT1A4

A rabbit liver UDP-glucuronosyltransferase cDNA that is related to human and rat UGT1A7 has been identified. The predicted amino acid sequence of the UGT1A7l displays 80% similarity to that encoded by human HP4 (UGT1A9), but 81% to that predicted for human UGT1A7 and 77% to the rat UGT1A7 (UGTA2). The exons encoding human UGT1A7 and rat UGTA2 are the seventh of the series of cassette exons that flank the 3' common exon series of the UGT1A locus. Southern blot analysis demonstrates that the exon sequence encoding UGT1A7l is part of a larger cluster of highly related genes. The UGT1A7l RNA is expressed in both neonatal and adult liver, and unlike rat UGT1A2 which is inducible with Ah receptor ligands such as polycyclic aromatic hydrocarbons, rabbit UGT1A7l is not regulated when animals are exposed to these inducers. Following expression of UGT1A7l in COS-1 cells, glucuronidation activity was identified for small phenolic molecules like 4-nitrophenol, bulky phenols as represented by 4-hydroxybiphenol and octylgallate, as well as 4-hydroxyestrone. In addition, UGT1A7l possesses catalytic activity toward tertiary amines like the tricyclic antidepressant imipramine. The pattern of UGT1A7l glucuronidation is similar to that observed for human UGT1A9, except tertiary amines are not subject to glucuronidation by human UGT1A9. Glucuronidation of tertiary amines is catalyzed principally by human UGT1A4 as well as rabbit UGT1A4. Although rabbit UGT1A7l catalyzes the formation of quarternary ammonium glucuronides, the Vmax is considerably less than that observed for rabbit UGT1A4. Overall, the characterization of rabbit UGT1A7l suggests that this protein represents the ortholog of the human UGT1A7, which to date has not been identified.

Expression and functional domains of rabbit liver UDP-glucuronosyltransferase 2B16 and 2B13

Southern blot analysis has demonstrated that the 5' portion of the rabbit liver dexamethasone-inducible UDP-glucuronosyltransferase (UGT) 2B13 RNA is related in sequence to a family of UGT genes (Tukey, R. H., Pendurthi, U. R., Nguyen, N. T., Green, M. D., and Tephly, T. R. (1993) J. Biol. Chem. 268, 15260-15266). To identify these additional gene transcripts, rabbit liver cDNA libraries were screened with a 5' conserved 330-base pair UGT2B13 cDNA fragment, resulting in the isolation and characterization of several rabbit liver UGT cDNAs. One such clone, called pGT11, encodes a putative glycoprotein that is 78% similar to rabbit UGT2B13. The new UGT has been designated UGT2B16. The UGT2B16 gene is expressed as a single 4200-base RNA transcript that is regulated only in adult rabbits. The predicted NH2-terminal 25 amino acids of UGT2B16 are identical to that of rabbit liver UGT2B13, with the remainder of the protein being 77% similar to UGT2B13. Expressed UGT2B16 protein in COS-1 cells was active toward 4-hydroxybiphenyl, similar to that of UGT2B13. However, UGT2B16 efficiently conjugated 4-hydroxyestrone and 4-tert-butylphenol, substrates that are not efficiently catalyzed by UGT2B13. To further characterize the structural domains of UGT2B16 and UGT2B13, a series of chimeric cDNAs were constructed that contained portions of both UGT2B16 and UGT2B13. Chimeric 2B163002B13531, which contained the amino-terminal UGT2B16 amino acids 1-300 followed by amino acids 301-531 of UGT2B13, as well as chimeric 2B163582B13531 and 2B164342B13531 proteins, catalyzed the glucuronidation of 4-hydroxyestrone, indicating that the carboxyl terminus of UGT2B13 could substitute for those same regions on UGT2B16. However, the replacement of the carboxyl end of UGT2B13 with 2B16300-531 or 2B16434-531 dramatically impaired the catalytic function of the chimeric proteins. These results indicate that the carboxyl end of UGT2B13 plays an important role in the functional and possible conformational state of the protein.

HNF1 alpha activates the rat UDP glucuronosyltransferase UGT2B1 gene promoter

The rat UDP glucuronosyltransferase UGT2B1 is expressed mainly in the liver where it glucuronidates steroids and environmental toxins and carcinogens. A region between -42 and -55 bp upstream from the UGT2B1 gene transcription start site was previously identified as sharing sequence similarity with the hepatocyte nuclear factor 1 (HNF1) consensus binding site. In this study, the importance of this region in the regulation of the UGT2B1 gene was confirmed by functional and DNA binding assays. A minimal UGT2B1 gene promoter containing the putative HNF1 binding site was fused to the CAT reporter gene and transfected into HepG2 cells. Only low levels of CAT activity were detected. This activity was increased 50-fold when an HNF1 alpha expression vector was co-transfected with the UGT2B1 promoter CAT construct but was not altered when a HNF1 beta expression vector was used. A UGT2B1 promoter construct with the HNF1-like region deleted was not activated by either co-transfected HNF1 expression vector. DNase 1 footprinting and gel-shift analysis demonstrated that nuclear proteins present in both HepG2 cells and rat liver bind to the HNF1-like element. The presence of HNF1 alpha in these nuclear proteins that bind to the HNF1-like element was confirmed by supershift analysis with antisera to HNF1 alpha. Specific binding of nuclear proteins to the HNF1-like element was not seen in extracts from three cell lines derived from nonhepatic tissues. These data strongly suggest that the liver-enriched factor HNF1 alpha binds to, and activates, the UGT2B1 gene promoter

Regulatory mechanisms involved in activator-protein-1 (AP-1)-mediated activation of glutathione-S-transferase gene expression by chemical agents

Induction of murine glutathione-S-transferase (GST) Ya gene expression by a variety of chemical agents is mediated by a regulatory element, EpRE, composed of an Ets and two adjacent activator protein-1 (AP-1)-like sites and activated by the Fos/Jun heterodimeric complex (AP-1). The mechanism of this induction was examined in the present study. We find that the regulation of EpRE-mediated GST Ya gene expression by 3-methylcholanthrene, tert-butylhydroquinone and beta-naphthoflavone is associated with an induction of AP-1 DNA-binding activity and that the AP-1 complex induced in hepatoma cells by these chemicals contains members of the Fos and Jun protein families. We show that tert-butylhydroquinone induces c-fos gene expression and indicate the formation of a transcriptionally active AP-1 complex that contains Fos/Jun heterodimer. In F9 cells, which are considered to lack AP-1 complex, a careful examination reveals that tert-butylhydroquinone induces a low level of an AP-1-related activity responsible for the enhanced expression of EpRE as well as of AP-1 reporter constructs. We find that protein phosphorylations mediate the activation of the GST Ya gene by chemical agents since okadaic acid, an inhibitor of protein phosphatases, can mimic this activation while protein kinase inhibitors abolish it. Evidence is presented that 3-methylcholanthrene, tert-butylhydroquinone and beta-naphthoflavone use a signal transduction pathway to Fos/Jun-dependent GST Ya gene expression via Ras and protein-tyrosine kinase activity. Furthermore, we find that activation by phorbol 12-myristate 13-acetate, which uses both protein kinase C and protein-tyrosine kinase activities, may share a common pathway with these chemicals downstream of Ras.

Identification of differentially expressed genes during hepatocytes development and characterization of their prenatal hormonal induction

Upon birth, the liver acquires new functions as a result of the initiation of expression of key enzymes. One example is the initiation of gluconeogenesis which depends on the induced appearance of phosphoenolpyruvate carboxykinase (P-pyruvate-CK) at birth. To characterize other genes that undergo such regulation, a differential screening was performed on a cDNA library from well-differentiated hepatoma cells. The pattern of tissue-specific and developmental-specific expression was determined for seven genes. Three clones, out of which two encode for the known genes alcohol dehydrogenase class I (ADH) and phenylalanine 4-monooxygenase (PAH) and a new gene (clone 116-3), exhibited a pattern of expression similar to that of the P-pyruvate-CK gene, i.e. their expression was liver and kidney specific and induced in the liver upon birth. Determination of the sequence of clone 116-3 revealed that it belonged to the UDP-glucuronosyltransferases type 2 (UGT2) family and thus was named UGT2B-rH4. To examine whether expression of the various genes could be prematurely induced by hormones in the fetal liver, either high levels of cAMP or low levels of insulin were induced in utero. The results demonstrated that cAMP induced a marked expression only of the genes for P-pyruvate-CK and ADH but not of those for PAH or UGT2B-rH4, while insulin deficiency induced premature expression of all four genes. We suggest that a set of genes whose expression is specifically induced in the liver upon birth can be prematurely induced by the hormones in utero.

Recognition of uridine diphosphate glucuronosyl transferases by LKM-3 antibodies in chronic hepatitis D

Patients with chronic hepatitis D often have liver-kidney microsomal antibodies type 3 (LKM-3). These antibodies react with several microsomal antigens that have a molecular weight of 55 KDa and an isoelectric point of about 8. We studied the molecular nature of the antigen and, by immunoscreening a human liver cDNA expression library with KM-3 sera, found that uridine diphosphate glucuronosyl transferases (UGT) appeared as candidate antigens. We confirmed the identity of UGT as an antigen by reacting the sera with recombinant rabbit liver UGT proteins. Some sera reacted with rabbit UGT-2 proteins, but UGT-1 proteins were more sensitive and specific in detecting LKM-3 autoantibodies in patient sera. Anti-UGT-1 antibodies were detected in all LKM-3 positive sera from patients with hepatitis D and 1 out of 11 patients with autoimmune hepatitis type 2. Sera from patients who had hepatitis B only did not react with UGT proteins. The UGT proteins are part of the phase II enzymes of drug metabolism and are the first such enzymes to be identified as human autoantigens.

Drug and xenobiotic glucuronidation catalysed by cloned human liver UDP-Glucuronosyltransferases stably expressed in tissue culture cell lines

Two human UDP-Glucuronosyltransferase (UGT) cDNA clones were stably integrated into V79 chinese hamster fibroblast cells and the functional enzymes were expressed in this heterologous environment. More than 100 drugs and xenobiotics were used as substrates for glucuronidation, catalysed by the cloned UGTs to determine the chemical structures accepted as substrates. UGT HP1 exhibited a limited specificity for planar phenolic compounds, whereas UGT HP4 was more promiscuous in acceptance of non-planar phenols, anthraquinones, flavones, aliphatic alcohols, aromatic carboxylic acids, steroids and many drugs of varied structure. These conclusions are illustrated here by using a series of alkyl- and halophenols. This work indicates the considerable potential value in use of these recombinant cell lines to study human drug glucuronidation.

Steroid UDP glucuronosyltransferases

The glucuronidation of steroids is a major process necessary for their elimination in the bile and urine. In general, steroid glucuronides are biologically less reactive than their parent steroids. However, in some cases often associated with disease and steroid therapy, more reactive or toxic glucuronides may be formed. The concentrations of specific steroid glucuronides in the blood may also indicate hormonal imbalances and may funnction as diagnostic markers of genetic defects in steroid synthesis and metabolism. In this review, the forms of UDP glucuronosyltransferase involved in steroid glucuronidation are described in terms of their specificities, functional domains and regulation. The available evidence suggests that steroid glucuronidation is mainly carried out by members of the UGT2B subfamily which are encoded by genes containing 6 exons. Members of this subfamily exhibit a regioselectively in their glucuronidation of steroids that is mediated by domains in the amino-terminal half on the protein encoded by exons 1 and 2. Although much of this review will describe studies in the rat, preliminary evidence indicates that a similar situation may exist in humans.

Monoclonal antibodies against 4-hydroxybiphenyl-UDP-glucuronosyltransferase

Monoclonal antibodies against purified rat liver 4-hydroxybiphenyl UDP-glucuronosyltransferase were developed using the hybridoma technology. In immunoblot analysis the antibodies specifically reacted with purified 4-hydroxybiphenyl-UDPGT but not with other purified UDPGT enzyme fractions. One single band was detected with microsomes of rat liver and small intestine but not with microsomes of kidney and lung. The reactive protein was also found in dog and human liver microsomes. It could be shown that there was no increase of immunoreactive protein after pretreatment with phenobarbital or 5,6-benzoflavone. This supports the hypothesis that more than one 4-hydroxybiphenyl-UDPGT exist in rat liver which are differently inducible.

Large deletion of androsterone UDP-glucuronosyltransferase gene in the inherited deficient strain of Wistar rats

LA Wistar rats have a deficiency of androsterone UDP-glucuronosyltransferase (UDPGT) and are present in Wistar rat colonies around the world. In order to clarify the molecular mechanism of the deficiency, androsterone UDPGT cDNA clone, pGT2 was isolated from rat liver cDNA library and was digested with restriction enzymes to afford three probes for Northern and Southern blot analyses in HA (normal), heterozygous LA and LA Wistar rats. In Northern blot analysis, androsterone UDPGT mRNA was totally absent in LA Wistar rat liver. Southern blot analysis suggested a large deletion of androsterone UDPGT gene in the rats. Genomic DNA amplifications with synthetic primers which have nucleotide sequences corresponding to the 5'-region of androsterone UDPGT cDNA, suggested that androsterone UDPGT gene has exon 1 with a length of some 700 bp and that this exon is deleted in LA Wistar rats. Based on these lines of evidence, it is concluded that the large portion of androsterone UDPGT gene is deleted in LA Wistar rats, which results in the absence of androsterone UDPGT mRNA and consequently the corresponding enzyme protein.

Isolation, sequence, and developmental expression of rat UGT2B2: the gene encoding a constitutive UDP glucuronosyltransferase that metabolizes etiocholanolone and androsterone

The UDP glucuronosyltransferase gene UGT2B2 is constitutively expressed in rat liver, and the enzyme has been shown to conjugate glucuronic acid with various endogenous steroids, especially etiocholanolone and androsterone. We have cloned and sequenced much of the UGT2B2 gene and 5'-flanking (247 bp) and 3'-flanking (734 bp) regions. The gene contains six exons spanning about 15.3 kb. Translation begins at nucleotide 36 of exon 1 and terminates with 280 coding nucleotides into exon 6, encoding a protein of 530 amino acids (calculated Mr of the unmodified chain = 60,913). We have determined that the UGT2B2 full-length cDNA is 1,974 bp. Northern hybridization revealed that the hepatic UGT2B2 transcript is detectable 4 days before birth, becomes markedly elevated in the neonate, and is even further increased at 3 and 12 weeks of age in the liver of both male and female rats.

The UDP glucuronosyltransferase gene superfamily: suggested nomenclature based on evolutionary divergence

A nomenclature system for the UDP glucuronosyltransferase superfamily is proposed, based on divergent evolution of the genes. A total of 26 distinct cDNAs in five mammalian species have been sequenced to date. Comparison of the deduced amino acid sequences leads to the definition of two families and a total of three subfamilies. For naming each gene, we propose that the root symbol UGT for human (Ugt for mouse), representing "UDP glucuronosyltransferase," be followed by an Arabic number denoting the family, a letter designating the subfamily, and an Arabic numeral representing the individual gene within the family or subfamily (hyphen before the Arabic number for mouse), e.g., human UGT2B1 and murine Ugt2b-1. Whereas the gene and cDNA should be italicized, the corresponding transcript, protein, and enzyme activity should not be written with lowercase letters or in italics, e.g., human or murine UGT2B1. Recent experimental evidence suggests that several exons of the UGT1 gene might be shared, indicating that distinct UGT1 transcripts and proteins may arise via alternative splicing; the gene and gene product of alternative splicing will be designated with an asterisk, e.g., UGT1*6 and UGT1*6, respectively. When an orthologous gene between species cannot be identified with certainty, as occurs in the UGT2B subfamily, we recommend sequential naming of the genes chronologically as they become characterized. We suggest that the human nomenclature system be used for species other than the mouse. We anticipate that this UGT gene nomenclature system will require updating on a regular basis.