Intestinal UGTs as potential modifiers of pharmacokinetics and biological responses to drugs and xenobiotics

Uridine 5'-diphosphate-glucuronosyltransferases (UGTs) are the biological catalysts of glucuronidation, a major pathway of conjugative metabolism of drugs and xenobiotics. In addition to the liver and kidney, UGTs are highly expressed in the gastrointestinal tract, where they have the potential to influence the pharmacokinetics and biological effects of ingested drugs and xenobiotics. This paper reviews the current evidence for the contributions of intestinal UGTs to presystemic 'first-pass' metabolism and drug bioavailability, the extent of enterohepatic cycling and the clearance of drugs from plasma, as well as their influence on biological responses to drugs, including drug toxicity. The prediction of the effects of intestinal glucuronidation on these processes depends on knowledge of the types and amounts of UGTs expressed in the small intestine and their specific glucuronidating activities. Whereas the types of UGTs expressed in human gastrointestinal tract are well characterized, further research is needed to understand the absolute amounts of UGTs in the small intestine and the causes of observed high-interindividual variability in the intestinal expression of UGTs.

Gender-Related Differences in Mycophenolate Mofetil-Induced Gastrointestinal Toxicity in Rats

Mycophenolate mofetil (MMF), the prodrug of mycophenolic acid (MPA), is included in current combination immunosuppressive regimens following organ transplant. Treatment with MMF often results in dose-limiting gastrointestinal (GI) side effects. The underlying mechanisms responsible for these side effects are not fully understood, but exposure of the intestinal epithelia to MPA during enterohepatic recycling may be involved. The present study demonstrated that female rats are more susceptible to MMF-induced GI toxicity than male rats. Female Sprague-Dawley rats treated chronically with an oral dose of 50 mg of MPA equivalents/kg/day experienced greater GI toxicity than male rats, as measured by diarrhea grade and weight loss. Intestinal microsomes harvested from the upper jejunum of female rats had approximately 3-fold lower MPA glucuronidation rates compared with male rats. In the remaining areas of the small and large intestine, there was also a trend toward decreased glucuronidation in the female rats. The area under the plasma concentration-time curve (AUC) for MPA following an oral dose of 50 mg of MPA equivalents/kg was roughly similar between genders, whereas the AUC for mycophenolic acid phenolic glucuronide (MPAG) was significantly lower in female rats. Female rats also excreted half of the biliary MPAG as male rats. The greater susceptibility of female rats to MMF-induced gastrointestinal toxicity, despite diminished intestinal MPA exposure via reduced biliary excretion of MPAG, may result from reduced protection of enterocytes by in situ glucuronidation. Likewise, susceptibility to MMF-induced GI toxicity in humans may also result from variable intestinal glucuronidation due to UDP glucuronosyltransferase polymorphisms or differential expression.

Expression of the human UGT1 locus in transgenic mice by 4-chloro-6-(2,3-xylidino)-2-pyrimidinylthioacetic acid (WY-14643) and implications on drug metabolism through peroxisome proliferator-activated receptor alpha activation

The UDP-glucuronosyltransferase (UGT) 1A genes in humans have been shown to be differentially regulated in a tissue-specific fashion. Transgenic mice carrying the human UGT1 locus (Tg-UGT1) were recently created, demonstrating that expression of the nine UGT1A genes closely resembles the patterns of expression observed in human tissues. In the present study, UGT1A1, UGT1A3, UGT1A4, and UGT1A6 have been identified as targets of the peroxisome proliferator-activated receptor (PPAR) alpha in human hepatocytes and Tg-UGT1 mice. Oral administration of the PPARalpha agonist 4-chloro-6-(2,3-xylidino)-2-pyrimidinylthioacetic acid (pirinixic acid, WY-14643) to Tg-UGT1 mice led to induction of these proteins in either the liver, gastrointestinal tract, or kidney. The levels of induced UGT1A3 gene transcripts in liver and UGT1A4 protein in small intestine correlated with induced lamotrigine glucuronidation activity in these tissues. With UGT1A3 previously identified as the major human enzyme involved in human C24-glucuronidation of lithocholic acid (LCA), the dramatic induction of liver UGT1A3 RNA in Tg-UGT1 mice was consistent with the formation of LCA-24G in plasma. Furthermore, PPAR-responsive elements (PPREs) were identified flanking the UGT1A1, UGT1A3, and UGT1A6 genes by a combination of site-directed mutagenesis, specific binding to PPARalpha and retinoic acid X receptor alpha, and functional response of the concatenated PPREs in HepG2 cells overexpressing PPARalpha. In conclusion, these results suggest that oral fibrate treatment in humans will induce the UGT1A family of proteins in the gastrointestinal tract and liver, influencing bile acid glucuronidation and first-pass metabolism of other drugs that are taken concurrently with hypolipidemic therapy.

The Contribution of Intestinal UDP-Glucuronosyltransferases in Modulating 7-Ethyl-10-hydroxy-camptothecin (SN-38)-Induced Gastrointestinal Toxicity in Rats

Life-threatening diarrhea afflicts a considerable percentage of patients treated with irinotecan, an anticancer agent with effects elicited through its active metabolite 7-ethyl-10-hydroxycamptothecin (SN-38). The primary detoxification pathway for SN-38 is glucuronidation. The purpose of this study was to evaluate the role that intestinal UDP-glucuronosyltransferases (UGTs) have from hepatic UGTs in modulating this diarrhea. To investigate this, Gunn rats devoid of UGT1A activity were injected with recombinant adenoviral vectors expressing UGT1A1, 1A6, and 1A7, resulting in reconstituted hepatic UGT expression comparable to a heterozygote. Hepatic microsome studies indicated that 4 to 7 days after adenoviral injection, transfected Gunn rats (j/jAV) had SN-38 glucuronide (SN-38G) formation rates three times higher than control heterozygote rats (j+AV). The adenovirus did not impart any glucuronidating capacity to the intestine in j/jAV rats, whereas j+AV rats possessed intestinal UGT function. After the administration of 20 mg/kg/day irinotecan i.p. to j/jAV rats 4 days after adenovirus injection, diarrhea ensued before the fourth irinotecan dose. j+AV rats were spared the diarrhea, and the toxicity was mild compared with the j/jAV rats, as measured by diarrhea scores, weight loss, and histological assessments of the cecum and colon. The pharmacokinetics of irinotecan, SN-38, and SN-38G indicate that the systemic exposure of SN-38 and SN-38G was higher and lower, respectively, in j/jAV rats. Despite this, the biliary excretion of irinotecan and metabolites was similar. Because intestinal UGTs are the main discriminating factor between j/jAV and j+AV rats, their presence seems to be critical for the gastrointestinal protection observed in j+AV rats.

Characterization of Rat Intestinal Microsomal UDP-Glucuronosyltransferase Activity toward Mycophenolic Acid

Mycophenolic acid (MPA) is the active immunosuppressive metabolite of the anti-organ rejection drug mycophenolate mofetil (MMF) and is implicated in the gastrointestinal toxicity associated with MMF therapy. Intestinal UDP-glucuronosyltransferases (UGT) have been proposed to provide intrinsic resistance against MMF-induced gastrointestinal toxicity by converting MPA to the inactive MPA 7-O-glucuronide. Using an optimized intestinal microsome preparation method that stabilized the intestinal MPA UGT activity, the MPA UGT activity of male Sprague-Dawley rat intestinal microsomes was characterized. A longitudinal gradient similar to that described for other phenolic compounds was observed, with the activity decreasing from the duodenum to the distal small intestine and colon. The catalytic efficiency of MPA glucuronidation decreased from the proximal to distal intestine as a result of decreasing Vmax and increasing Km. The finding that homozygous Gunn rats lack detectable intestinal MPA UGT activity indicates exclusive roles of UGT1A1, UGT1A6, and/or UGT1A7. Quantitative immunoblotting revealed a parallel between the MPA UGT activity and the content of UGT1A7-like immunoreactivity (18.7 and 7.3 microg/mg for duodenum and colon, respectively). In contrast, the lesser MPA-metabolizing UGT, UGT1A1 and UGT1A6, were lower in abundance (1.6-2.1 and 1.7-2.9 microg/mg, respectively), and their patterns of longitudinal distribution were distinct from the MPA UGT activity. These data suggest a dominant role of a UGT1A7-like enzyme, presumably UGT1A7 itself, in the catalysis of rat intestinal MPA glucuronidation. Studies are ongoing to investigate the relationship between intestinal UGT1A enzymes and susceptibility to MMF-induced gastrointestinal toxicity.

Adenovirus-Mediated Gene Therapy to Restore Expression and Functionality of Multiple UDP-Glucuronosyltransferase 1A Enzymes in Gunn Rat Liver

The Gunn rat has been a valuable model for investigating the effect of UDP-glucuronosyltransferase 1A (UGT1A) deficiencies on drug metabolism and toxicity, but it is limited in some aspects. For example, the native Gunn rat model cannot distinguish between hepatic and extrahepatic UGT1A deficiencies in toxicological mechanisms. To extend the model's utility, we investigated the use of replication-defective recombinant UGT1A adenoviruses for the purpose of selectively restoring hepatic UGT1A function. Mycophenolic acid, the active metabolite of the anti-transplant rejection drug mycophenolate mofetil and suspected gastrointestinal toxicant, was used as a model UGT1A-dependent substrate. Treatment with UGT1A adenoviruses normalized the plasma mycophenolic acid and 7-O-mycophenolate glucuronide (MPAG) (concentration-time curves after mycophenolic acid administration (80 mg/kg intraperitoneally). Functional reconstitution was also apparent in the correction of the mycophenolic acid t(1/2alpha) and the area under the curve (AUC)(MPA,0-8 h)/AUC(MPAG,0-8 h) ratio. Twenty-four hours after administration of mycophenolic acid, severe signs of toxicity were noted in the naive Gunn group, including reduced food consumption. The effect on food consumption was reduced but not completely prevented in the UGT adenovirus-treated Gunn rats. In vitro analyses indicated adenovirus dose-dependent reconstitution of mycophenolic acid UGT activities and UGT1A contents in liver but not intestinal microsomes. In the highest adenovirus dose group, the liver microsomal UGT1A markers exceeded those of the heterozygote controls. The ability to selectively manipulate multiple hepatic UGT1A enzymes in Gunn rats should provide a novel way to assess the importance of intestinal or other extrahepatic UGT1A enzymes in toxicities induced by mycophenolic acid and other cytotoxic drugs and dietary agents.

Activity of rat UGT1A1 towards benzo[a]pyrene phenols and dihydrodiols

Four UDP-glucuronosyltransferases from the rat UGT1A family were tested for activity towards benzo[a]pyrene phenols and dihydrodiols. UGT1A1 and UGT1A7 were found to be broadly active towards BaP metabolites. Antisera recognizing rat UGT1A1 and UGT1A7 were used to assess UGT levels in relation to UGT activity towards benzo[a]pyrene-7,8-dihydrodiol (BPD). The rank BPD UGT activities were liver=intestine≫kidney, whereas UGT1A1 was highest in liver and UGT1A7 was highest in intestine. Phenobarbital, an inducer of hepatic UGT1A1, only slightly increased BPD UGT activity, whereas UGT1A7 inducers more potently increased the activity. Inhibition studies using the differential UGT1A1 inhibitor, bilirubin, suggest that UGT1A1 is not a major contributor to the constitutive BPD glucuronidating activity of control rat liver microsomes. These data suggest that multiple UGT1A enzymes contribute to glucuronidation of BPD and other BaP metabolites, and that their relative contributions depend on tissue- and environmental-specific factors.

EFFECT OF CHRONIC RENAL INSUFFICIENCY ON HEPATIC AND RENAL UDP-GLUCURONYLTRANSFERASES IN RATS

Significant evidence exists regarding altered CYP450 enzymes in chronic renal insufficiency (CRI), although none exists for the phase II enzymes. The objective of this study was to investigate the effect of CRI on hepatic and renal UDP-glucuronyltransferase (UGT) enzymes. Three groups of rats were included: CRI induced by the 5/6th nephrectomy model, control, and control pair-fed (CPF) rats. UGT activities were determined in liver and kidney microsomes by the 3- and 17-glucuronidation of beta-estradiol (E2-3G and E2-17G), glucuronidation of 4-methylumbelliferone (4-MUG), and 3-glucuronidation of morphine (M3G). UGT isoforms responsible for these catalytic activities were screened using recombinant rat UGT1A1, UGT1A2, UGT1A3, UGT1A7, UGT2B2, UGT2B3, and UGT2B8. UGT protein levels were examined by Western blot analysis using polyclonal antibodies. There was no significant difference between CRI and CPF rats in hepatic and/or renal E2-3G (UGT1A1), E2-17G (UGT2B3), 4-MUG (UGT1A6), and M3G (UGT2B1) formation. Formation of E2-17G and 4-MUG in the liver and E2-3G and 4-MUG in the kidney was significantly reduced (p < 0.05) in CPF and CRI rats compared with control rats. The down-regulated glucuronidation activities were accompanied by corresponding reductions in protein content of specific UGT isoforms. These results suggest that CRI does not seem to influence the protein levels or catalytic activity of most of the major hepatic or renal UGT enzymes. The observed down-regulation of hepatic and renal UGTs in CRI and CPF rats could be caused by restricted food intake in these groups of rats.

Tissue-specific, Inducible, and Hormonal Control of the Human UDP-Glucuronosyltransferase-1 (UGT1) Locus

The human UDP-glucuronosyltransferase 1 (UGT1) locus spans nearly 200 kb on chromosome 2 and encodes nine UGT1A proteins that play a prominent role in drug and xenobiotic metabolism. Transgenic UGT1 (Tg-UGT1) mice have been created, and it has been demonstrated that tissue-specific and xenobiotic receptor control of the UGT1A genes is influenced through circulating humoral factors. In Tg-UGT1 mice, the UGT1A proteins are differentially expressed in the liver and gastrointestinal tract. Gene expression profiles confirmed that all of the UGT1A genes can be targeted for regulation by the pregnane X receptor activator pregnenolone-16alpha-carbonitrile (PCN) or the Ah receptor ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In addition, the selective induction of glucuronidation activity toward lamotrigine, ethinyl estradiol, chenodeoxycholic acid, and lithocholic acid by either PCN or TCDD in small intestine from Tg-UGT1 mice corresponded to expression of the locus in this tissue. Induction of UGT1A1 by PCN and TCDD is believed to be highly dependent upon glucocorticoids, because submicromolar concentrations of dexamethasone actively promote PCN and TCDD induction of UGT1A1 in Tg-UGT1 primary hepatocytes. The role of hormonal control of the UGT1 locus was further verified in pregnant and nursing Tg-UGT1 mice. In maternal 14-day post-conception Tg-UGT1mice, liver UGT1A1, UGT1A4, and UGT1A6 were induced, with the levels returning to near normal by birth. However, maternal liver UGT1A4 and UGT1A6 were dramatically elevated and maintained after birth, indicating that these proteins may play a critical role in maternal metabolism during lactation. With expression of the UGT1 locus confirmed in a variety of mouse tissues, these results suggested that the Tg-UGT1 mice will be a useful model to examine the regulatory and functional properties of human glucuronidation.

Optimization of conditions for clinical human hepatocyte infusion

Cytotoxicity and apoptosis are common problems in the isolation and storage of human hepatocytes. In vitro environments of hepatocytes during cell infusion may be critical to reducing cellular damage and enhancing cell viability. We examined the effects of donor liver histology (40-50% steatosis vs. normal), incubation time, temperature, and three solutions for infusion on banked primary human hepatocytes, by studying: trypan blue exclusion, AST release, LDH release, MTT assay, detection of DNA ladder, and a hepatocyte proliferation assay. In addition, the microstructure functions of the endoplasmic reticulum and mitochondria of the intact hepatocytes were determined by measuring correlates of UGT 1A1 and cytochrome P-450 3A (CYP3A4) activity. In general, hepatocyte viability decreased significantly within 60 min after thawing. Cells suspended in 5% dextrose lactated Ringers solution (D5LR) maintained greater cell viability. Hepatocytes from normal liver donors showed less AST and LDH enzyme leak in comparison with cells from fatty liver donors. Mild hypothermic temperature (32 degrees C) inhibited cellular damage that otherwise significantly increased at 60 min. Hepatocytes did not proliferate until 12 h from thaw, regardless of supernatant or conditions of suspension. CYP3A4 activity and a marker for UGT 1A1 activity in hepatocytes from normal donor livers were higher than those from steatotic donor livers. These findings suggest that hepatocytes suspended for infusion after isolation from normal liver donors have normal biological functions and less cellular damage/necrosis in contrast with those isolated from fatty liver donors. These damages are inhibited significantly by maintaining hepatocytes at a mild hypothermic temperature (32 degrees C). D5LR alone maintained the best cell viability for up to 60 min. Media of D5LR + adenosine and HMM were able to partially inhibit hepatocyte apoptosis in hepatocytes from steatotic livers.

AN INVESTIGATION OF HUMAN AND RAT LIVER MICROSOMAL MYCOPHENOLIC ACID GLUCURONIDATION: EVIDENCE FOR A PRINCIPAL ROLE OF UGT1A ENZYMES AND SPECIES DIFFERENCES IN UGT1A SPECIFICITY

Mycophenolic acid (MPA; 1,3-dihydro-4-hydroxy-6-methoxy-7-methyl-3-oxo-5-isobenzylfuranyl)-4-methyl-4-hexenoate), the active metabolite of the immunosuppressant prodrug, mycophenolate mofetil, undergoes glucuronidation to its 7-O-glucuronide as a primary route of metabolism. Because differences in glucuronidation may influence the efficacy and/or toxicity of MPA, we investigated the MPA UDP-glucuronosyltransferase (UGT) activities of human liver microsomes (HLMs) and rat liver microsomes with the goal of identifying UGTs responsible for MPA catalysis. HLMs (n = 23) exhibited higher average MPA glucuronidation rates (14.7 versus 6.0 nmol/mg/min, respectively, p < 0.001) and higher apparent affinity for MPA (K(m) = 0.082 mM versus 0.20 mM, p < 0.001) compared with rat liver microsomes. MPA UGT activities were reduced >80% in liver microsomes from Gunn rats. To identify the active enzymes, human and rat UGT1A enzymes were screened for MPA-glucuronidating activity. UGT1A9 was the only human liver-expressed UGT1A enzyme with significant activity and exhibited both high affinity (K(m) = 0.077 mM) and high activity (V(max) = 28 nmol x min(-1) x mg(-1)). Spearman correlation analyses revealed a stronger relationship between HLM MPA UGT activities and 1A9-like content (r(2) = 0.79) relative to 1A1 (r(2) = 0.20), 1A4-like (r(2) = 0.22), and 1A6 (r(2) = 0.41) protein. A different profile was observed for rat with three active liver-expressed UGT1A enzymes: 1A1 (medium affinity/capacity), 1A6 (low affinity/medium capacity), and 1A7 (high affinity/capacity). Our data suggest that UGT1A enzymes are the major contributors to hepatic MPA metabolism in both species, but 1A9 is dominant in human, whereas 1A1 and 1A7 are likely the principal mediators in control rat liver. This information should be useful for interpretation of MPA pharmacokinetic and toxicity data in clinical and animal studies.

Differential rates of glucuronidation for 7-ethyl-10-hydroxy-camptothecin (SN-38) lactone and carboxylate in human and rat microsomes and recombinant UDP-glucuronosyltransferase isoforms

7-ethyl-10-hydroxy-camptothecin (SN-38), the active metabolite of the anti-cancer agent irinotecan, contains a lactone ring that equilibrates with a carboxylate form. Since SN-38 lactone is the active and toxic form, it is prudent to examine whether the more soluble carboxylate is a surrogate for SN-38 lactone conjugation. Therefore, relative rates of glucuronidation and isoform specificity of SN-38 lactone and carboxylate were characterized. The stability of SN-38 lactone and carboxylate in incubation mixtures of microsomes and UDP-glucuronosyltransferase (UGT) isoforms was used to determine optimal incubation times. Microsomal incubations were conducted using rat and human intestinal and hepatic microsomes and human and rat recombinant UGT1A isoforms. Where estimates of lactone and carboxylate glucuronidation rates could not be established due to short incubation times and detection limits, kinetic modeling was used to recover these rate constants. The stability experiments revealed that the lactone was stabilized by rat microsomes, however, the opposite was observed in human microsomes and recombinant isoforms. For all tissues and most UGT isoforms examined, the lactone consistently had catalytic rates up to 6-fold greater than the carboxylate. The rank order of glucuronidation for both SN-38 lactone and carboxylate was 1A7 > 1A1 > 1A9 > 1A8 and 1A7 > 1A8 > 1A1 for human and rat isoforms, respectively. This study provides further support that SN-38 lactone and carboxylate may be considered pharmacokinetically distinct agents. The in vivo impact of this conjugation difference is unknown, since variations in protein binding and transport proteins may affect intracellular concentrations of the lactone or carboxylate.

ANALYSIS OF SUBSTRATE SPECIFICITIES AND TISSUE EXPRESSION OF RAT UDP-GLUCURONOSYLTRANSFERASES UGT1A7 AND UGT1A8

The UGT1 complex codes for a subfamily of homologous "1A7-like" UDP-glucuronosyltransferases (UGTs), including UGT1A7 and UGT1A8. Little information is available regarding either the substrate specificities or regulation of the UGT1A7-like forms from rats. We compared the activities and tissue expression of UGT1A7 and UGT1A8, which exhibit 77% identity in their amino terminal sequence. UGT1A7 shows broad specificity, catalyzing the glucuronidation of 31 of 40 randomly selected substrates (100 muM) at rates >0.1 nmol/mg/min. UGT1A7 substrates included both planar and nonplanar compounds, mono- and polycyclic aromatics, and compounds with bulky side chain ring substitutions. UGT1A8 exhibited a narrower substrate specificity that completely overlapped with UGT1A7. UGT1A8 was most active toward the 1-OH, 4-OH, 5-OH, 6-OH, 7-OH, 10-OH, 11-OH, and 12-OH derivatives of benzo[a]pyrene. Other effective UGT1A8 substrates (>0.1 nmol/mg/min) included 9-OH-benzo[a]pyrene, 1-naphthol, 4-methylumbelliferone, 7-hydroxycoumarin, chrysin, quercetin, 4-nitrophenol, and estriol. In general, substrates preferred by UGT1A8 were polyaromatic planar structures with nonbulky substituents and a superimposable 1-naphtho ring structure. Studies of the tissue expression of the UGT1A7 and 1A8 mRNAs using RNase protection analysis suggested that each is expressed in liver and kidney of control rats. A major difference is the higher expression of UGT1A7 mRNA in intestine. These studies suggest complementary functions of the UGT1A7 and UGT1A8 forms in xenobiotic metabolism. Further studies are necessary to determine whether their relative contributions change as a function of development, hormonal status, or exposure to inducing agents.

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.

Differential regulation of alternate UDP-glucuronosyltransferase 1A6 gene promoters by hepatic nuclear factor-1

UDP-glucuronosyltransferase 1A6 (UGT1A6) is a major UGT contributing to the glucuronidation of small phenolic compounds. The gene for rat 1A6 is expressed using two promoters, a distal promoter P1 and a proximal promoter P2. Transcripts from P2 are high in liver, gastrointestinal tract, and kidney, whereas P1 transcripts predominate in other tissues. Here we report evidence for primary control of the P2 promoter by hepatic nuclear factor 1 (HNF1). Transient transfection of a P2 reporter plasmid, p(-1354/+65) 1A6P2-luc, resulted in enhanced luciferase activity in HepG2 but not Hepa1 cells compared to cells transfected with pGL3-Basic control vector. A truncated reporter under the control of -224 to +65 exhibited comparable activity. Footprint analysis of the -224/+65 fragment revealed specific binding by rat liver nuclear protein to a region between bases -60 and -37. The binding activity was also observed with HepG2 cell but not Hepa1 cell extract. Electrophoretic mobility shift assays were consistent with the presence of HNF1 in the binding complexes. The functionality of an HNF1-binding site at -51/-37 is also supported by (1) marked decreases in the activity of P2 reporter plasmids containing a three-base substitution in the proposed HNF1 binding site and (2) the enhancement of P2 reporter activity following cotransfection of an HNF1alpha expression plasmid. The UGT1A6 P1 promoter lacks an HNF1 binding site in the analogous position and showed little response to HNF1 overexpression. Although these data do not strictly rule out an interaction between the P1 promoter and HNF1 bound to -51/-37 of P2, the results suggest a mechanism for the more abundant expression of P2-derived UGT1A6 transcripts in liver and other HNF1-enriched tissues.

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

The utility of oltipraz as a cancer chemopreventive agent is thought to depend on the induction of enzymes involved in phase 2 xenobiotic detoxification. Although studies of some enzymes induced by oltipraz implicate a novel transcriptional activating pathway involving Nrf2 and antioxidant-response elements (AREs), the mechanism of phenol UGT induction has remained unclear. Previous work showed that UGT1A6 is transcribed from two promoters, P1 and P2, that are both induced by oltipraz in rat liver. The effect also occurs in rat hepatocytes treated with oltipraz (concentrations >3 microM). To investigate the mechanism, luciferase reporter plasmids under the control of P1 [p(-1078/+27)1A6P1-luc] or P2 [p(-1354/+65)1A6P2-luc] were transfected into rat hepatocytes and tested for inducibility. P1, but not P2, showed responsiveness to oltipraz (2- to 5-fold increase) and 3-methylcholanthrene (10- to 30-fold increase). Because P1 contained no visible AREs, the role of a xenobiotic response element (XRE) centered between bases -134 and -129 was evaluated. Mutation of the XRE core reduced the effects of both oltipraz and 3-methylcholanthrene on the P1 reporter. The 1A6 XRE conferred oltipraz responsiveness on the simian virus 40 promoter of pGL3-Promoter. Comparative effects of oltipraz and 3-methylcholanthrene on transfected cytochrome P4501A1 reporters support the general but relatively weak XRE-stimulating activity of oltipraz. The involvement of the aryl hydrocarbon receptor (AHR) and aryl hydrocarbon nuclear translocator (ARNT) in mediating the effects of oltipraz on the XRE is supported by electrophoretic mobility supershift data and AHR/ARNT overexpression studies. These data raise questions about the contribution of AHR and other secondary induction pathways in the mechanism of oltipraz.

Glucuronidation of acetaminophen catalyzed by multiple rat phenol UDP-glucuronosyltransferases

Gunn rats glucuronidate acetaminophen (APAP) at reduced rates and show increased susceptibility to APAP-induced hepatotoxicity. This defect is presumed to involve UDP-glucuronosyltransferase (UGT) 1A6, which is nonfunctional in Gunn rats, but it is currently unclear whether other 1A family members are also involved. In humans, two 1A isoforms are known to be active (1A6 and 1A9) but 1A6 form has a 25-fold lower apparent K(m) (2 mM). Rat liver microsomal APAP UGT activity is induced by in vivo treatment with beta-naphthoflavone or oltipraz, an effect correlating with induction of 1A6 and 1A7. To address a possible role of 1A7 in APAP glucuronidation relative to other 1A forms, cDNAs encoding UGTs 1A1, 1A5, 1A6, 1A7, and 1A8 were expressed in human embryonic kidney cells and the contents of expressed enzyme in prepared membrane fractions determined by quantitative immunoblotting. At 2.5 mM APAP, 1A7 showed the highest specific activity (2.8 nmol/min/nmol 1A7 protein), followed by 1A6 (1.1 nmol/min/nmol), and 1A8 (0.27 nmol/min/nmol). 1A1 and 1A5 were essentially inactive. Kinetic comparisons indicated 1A7 had a similar apparent K(m) as 1A6 (4.7 versus 3.9 mM, respectively) but a 2.4-fold higher catalytic activity. These data suggest that in rats, 1A7 plays a major role in APAP glucuronidation and contributes to protection against APAP-induced hepatotoxicity. The involvement of other UGTs besides 1A6 is further underscored by the presence of significant residual APAP-glucuronidating activity by Gunn rat hepatocytes, indicating the activity of an unknown UGT2 family member.

An alternative promoter contributes to tissue- and inducer-specific expression of the rat UDP-glucuronosyltransferase 1A6 gene

UDP-glucuronosyltransferase 1A6 (UGT1A6), a key enzyme catalyzing the glucuronidation of small planar phenols and amines, is expressed in a tissue- and inducer-dependent manner. Expression is high in kidney, gastrointestinal tract, and induced liver, with low expression in spleen, lung, and ovary. Exposure to certain chemicals, such as 3-methylcholanthrene, benzo[a]pyrene, beta-naphthoflavone, and oltipraz elevates UGT1A6 mRNA in liver and to a lesser extent gastrointestinal tract and kidney, but not in other tissues. The mechanisms underlying this complex pattern of expression have been elusive. We have identified a new type of UGT1A6 mRNA (class 2) that differs in its 5' untranslated sequence. The class 2 transcript is the more abundant type expressed in liver, gastrointestinal tract, and kidney. Transcription of the class 2 mRNA is initiated 107 bases 5' of the UGT1A6 coding exon. The promoter region flanking the transcription start site contains an HNF1-like binding site identical to that in the human UGT1A6 gene. Both class 1 and class 2 mRNAs were elevated in liver by 3-methylcholanthrene, benzo[a]pyrene, beta-naphthoflavone, and oltipraz, with preferential elevation of class 1 occurring after 3-methylcholanthrene and benzo[a]pyrene treatment. These data suggest that transcription from a second promoter contributes to tissue- and inducer-specific expression of rat UGT1A6.

Roles of glucuronidation and UDP-glucuronosyltransferases in xenobiotic bioactivation reactions

Glucuronide conjugates represent one of the major types of naturally occurring phase 2 metabolites of xenobiotics and endobiotics. The process underlying their formation, glucuronidation, is normally considered detoxifying, because glucuronides usually possess less intrinsic biological or chemical activity than their parent aglycones and they are rapid excreted. However, a number of glucuronide conjugates are known that are active and may contribute to pharmacological activities or toxicities associated with their parent compounds. These include two classes of glucuronides with electrophilic chemical reactivity (N-O-glucuronides of hydroxamic acids and acyl glucuronides of carboxylic acids) and several types of glucuronides that impart biological effects through non-covalent interactions (morphine 6-O-glucuronide, retinoid glucuronides, and D-ring glucuronides of estrogens). Glucuronides may thus contribute to clinically significant effects, including environmental arylamine-induced carcinogenesis, drug hypersensitivity and other toxicities associated with carboxylic acid drugs, morphine analgesia, and cholestasis from estrogens. This review summarizes the rat and human UDP-glucuronosyltransferases that may be involved in the formation of bioactive glucuronides, including their substrate- and tissue-specificity and genetic and environmental influences on their activity. This knowledge may be useful for enhancing the therapeutic efficacy and minimizing the risk of adverse effects associated with xenobiotics that undergo bioactivating glucuronidation reactions.

Structural heterogeneity at the UDP-glucuronosyltransferase 1 locus: functional consequences of three novel missense mutations in the human UGT1A7 gene

One of the most important mechanisms involved in host defense against xenobiotic chemicals and endogenous toxins is the glucuronidation catalysed by UDP-glucuronosyltransferase enzymes (UGT). The role of genetic factors in determining variable rates of glucuronidation is not well understood, but phenotypic evidence in support of such variation has been reported. In the present study, six single nucleotide polymorphisms were discovered in the first exon of the UGT1A7 gene, which codes for the putative substrate-binding domain, revealing a high structural heterogeneity at the UGT1 gene locus. The new UGT1A7 proteins differ in their primary structure at amino acid positions 129, 131 and 208, creating four distinct UGT1A7 allelic variants in the human population: UGT1A7*1 (N129 R131 W208), *2 (K129 K131 W208), *3 (K129 K131 R208), and *4 (N129 R131 R208). In functional studies, HEK cells stably transfected to express the four allelic UGT1A7 variants exhibited significant differences in catalytic activity towards 3-, 7-, and 9-hydroxy-benzo(a)pyrene. UGT1A7*3 exhibited a 5.8-fold lower relative Vmax compared to wild-type *1, whereas *2 and *4 had a 2.6- and 2.8-fold lower relative Vmax than *1, respectively, suggesting that these mutations confer slow glucuronidation phenotype. Kinetic characterization suggested that these differences were primarily attributable to altered Vmax. Additionally, it suggested that each amino acid substitutions can independently affect the UGT1A7 catalytic activity, and that their effects are additive. The expression pattern of UGT1A7 studied herein and its catalytic activity profile suggest a possible role of UGT1A7 in the detoxification and elimination of carcinogenic products in lung. A population study demonstrated that a considerable proportion of the population (15.3%) was found homozygous for the low activity allele containing all three missense mutations, UGT1A7*3. These findings suggest that further studies are needed to investigate the impact of the low UGT1A7 conjugator genotype on individual susceptibility to chemical-induced diseases and responses to therapeutic drugs.

Induction of UDP-glucuronosyltransferase UGT1A1 by the flavonoid chrysin in the human hepatoma cell line hep G2

The UDP-glucuronosyltransferases (UGTs) have long been known to be inducible by various chemicals, including drugs, although the extent of induction in general has been modest. In the present study, we determined the ability of the dietary flavonoid chrysin to induce UGT activity, protein and mRNA. When pretreating human hepatoma Hep G2 cells with 25 microM chrysin, the glucuronidation of chrysin itself increased 4.2-fold when measured in the intact cell and 14-fold in the cell homogenate, i.e., autoinduction. Microsomes from chrysin-treated cells probed with specific antibodies in Western analyses showed marked induction of the UGT1A family of proteins. Isoform-specific induction of the important hepatic UGT1A1 protein was observed but not of UGT1A6 or UGT2B7. The strong induction of UGT1A1 was confirmed by Northern analyses of total RNA as well as mRNA, using a specific probe. UGT1A1 message as well as protein was detectable also in untreated Hep G2 cells. In catalytic activity assays with recombinant UGT1A1, 1A4, 1A6 and 1A9, chrysin was found to be a high affinity substrate for UGT1A1 (K(m) 0.35 microM). Catalytic activity was also found for UGT1A9 and 1A6 but not for 1A4. Further studies demonstrated a 20-fold induction of the glucuronidation of bilirubin by the chrysin-treated cells and a 7. 9-fold induction of the glucuronidation of the oral contraceptive drug ethinylestradiol, two of the best known and specific UGT1A1 substrates, demonstrating the potential importance of this induction. In view of these findings, it will be important to extend these studies to other dietary flavonoids.

Involvement of hepatocyte nuclear factor 1 in the regulation of the UDP-glucuronosyltransferase 1A7 (UGT1A7) gene in rat hepatocytes

UDP-glucuronosyltransferase 1A7 (UGT1A7) is a major UGT contributing to the glucuronidation of xenobiotic phenols in rats. Its expression in rat liver is tightly regulated, with low constitutive and high inducible expression in response to aryl hydrocarbon receptor ligands and oltipraz. Previously, we reported the absence of 3-methylcholanthrene- or oltipraz-responsive elements in the 1.6-kbp region flanking the UGT1A7 promoter. However, potential binding sites were noted for several liver-enriched transcription factors. Here we show that deletion of the hepatic nuclear factor (HNF)3, HNF4, and CCAAT-enhancer binding protein-like binding sites had no effect on the expression of a UGT1A7 reporter plasmid, p(-965/+56)1A7-Luc, in primary rat hepatocytes. The full activity of the promoter was contained in the region between bases -157 and +76. Two sites of binding by rat liver nuclear proteins were detected in this region by DNase footprinting. PR-1 corresponded to the HNF1-like binding site between bases -52 and -38, whereas PR-2 was located between -30 to -6. Gel retardation studies supported the presence of HNF1alpha in the PR-1 DNA-liver nuclear protein complex. Mutation of PR-1 inhibited binding in the gel shift assay, prevented activation by overexpressed HNF1 in human embryonic kidney cells, and reduced by >80% the maximal luciferase activities expressed from basal and 3-methylcholanthrene-responsive UGT1A7 gene reporter constructs in primary rat hepatocytes. These data provide evidence for an important stimulatory role of HNF1 in promoting UGT1A7 gene expression in rat liver.

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.

Expression and inducibility of the human bilirubin UDP-glucuronosyltransferase UGT1A1 in liver and cultured primary hepatocytes: evidence for both genetic and environmental influences

In Crigler-Najjar type II patients and, recently, in Crigler-Najjar type I patients treated with human hepatocyte cell therapy, phenobarbital has been used for reducing the serum bilirubin load. Its effect is attributed to induction of the enzyme required for hepatic bilirubin elimination, UDP-glucuronosyltransferase, UGT1A1. This study investigated the expression and inducibility of UGT1A1 in human donor livers and their corresponding primary hepatocyte cultures. Immunoblot analysis using a specific antibody directed against the amino terminal of the human UGT1A1 isoform showed that 5 hepatocyte donors exhibited a >50-fold difference in UGT1A1 level. UGT1A1 protein level correlated strongly with both liver microsomal bilirubin UGT activity and liver UGT1A1 mRNA level (r(2) =.82 and.72, respectively). Of the 4 patients with the lowest UGT1A1 levels, 3 were homozygotes for the UGT1A1 promoter variant sequence associated with Gilbert's syndrome, and the fourth was a heterozygote. The 3 donors with the highest levels had a history of phenytoin exposure. Hepatocytes isolated from the phenytoin-exposed donors exhibited marked declines in UGT1A1 mRNA levels during culturing. Induction studies using hepatocytes treated for 48 hours with phenobarbital (2 mmol/L), oltipraz (50 micromol/L), or 3-methylcholanthrene (2.5 micromol/L) revealed UGT1A1-inducing effects of phenobarbital, oltipraz, and, in particular, 3-methylcholanthrene. Our data suggest that both genetic and environmental factors play an important role in the marked interindividual variability in UGT1A1 expression. An understanding of these mechanisms could lead to advances in the pharmacological therapy of life-threatening unconjugated hyperbilirubinemia.

Mono- and Diglucuronide formation from benzo[a]pyrene and chrysene diphenols by AHH-1 cell-expressed UDP-glucuronosyltransferase UGT1A7

Polycyclic aromatic hydrocarbon (PAH)-type compounds induce at least two rat UDP-glucuronosyltransferase isoforms, UGT1A6 and UGT1A7. Among the glucuronidation reactions of PAH metabolites studied, mono- and diglucuronide formation of benzo[a]pyrene and chrysene-3,6-diphenol showed the highest induction factors in rat liver microsomes. Availability of AHH-1 cells stably expressing UGT1A7 allowed us to study whether this PAH-inducible isoform could catalyze benzo[a]pyrene and chrysene-3,6-diphenol glucuronidation. It was found that UGT1A7 indeed catalyzed mono- and diglucuronide formation of both benzo[a]pyrene and chrysene 3,6-diphenols. V79 cell-expressed rat UGT1A6 also catalyzed these reactions, except for chrysene diphenol diglucronide formation (Bock et al., Mol Pharmacol 42: 613-618, 1992). Enzyme kinetic studies of the glucuronidation of 6-hydroxychrysene (used as a stable PAH phenol) indicated that UGT1A7 conjugated this compound with a lower apparent Km value (0.1 microM) than UGT1A6 (10 microM). The results suggest that the two PAH-inducible UGTs may cooperate in conjugating PAH metabolites, but that UGT1A7 is more efficient.