The expression of UDP-glucuronosyltransferases of the UGT1 family in human liver and kidney and in response to drugs

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.

Phenytoin alters transcript levels of hormone-sensitive lipase in muscle from horses with hyperkalemic periodic paralysis

In equine hyperkalemic periodic paralysis (HyperPP), there is evidence suggesting that the primary defect in the sodium channel is associated with a secondary alteration in triacylglycerol-associated fatty acid metabolism (TAFAM) in skeletal muscle. Furthermore, TAFAM may be involved in the therapeutic action of phenytoin. The effects of phenytoin treatment on the transcript levels of three key proteins in TAFAM, hormone sensitive lipase (HSL), carnitine palmitoyltransferase (CPT), and fatty acid binding protein (FABP), were examined. These transcripts were quantitated by competitive reverse transcription polymerase chain reaction in undifferentiated and differentiated primary cultures of equine skeletal muscle from control, heterozygous HyperPP, and homozygous-affected HyperPP horses. There was a 10-fold lower level of HSL transcript in both undifferentiated and differentiated cultures from homozygous-affected horses than from horses of the other genotypes. Phenytoin selectively increased the HSL transcript in homozygous-affected differentiated cultures to levels similar to those of the other genotypes. The levels of CPT and FABP transcripts were unaffected by genotype, differentiation, and phenytoin treatment. These results suggest that the primary defect in HyperPP may secondarily decrease HSL transcript levels and that the therapeutic action of phenytoin may include regulation of mRNA transcripts in skeletal muscle.

Drug glucuronidation by human renal UDP-glucuronosyltransferases

The UDP-glucuronosyltransferases catalyse the conjugation of glucuronic acid to a wide variety of endobiotics and xenobiotics, representing one of the major conjugation reactions in the conversion of both exogenous (e.g. drugs and pesticides) and endogenous compounds (e.g. bilirubin and steroid hormones). The liver is the major site of glucuronidation, however a number of extrahepatic tissues exhibit particular UDP-glucuronosyltransferase activities. The present study was undertaken to assess the human renal UDP-glucuronosyltransferase system. Enzymatic analysis of human kidney showed that a limited number of UDP-glucuronosyltransferase isoforms were expressed in this tissue. However the level of renal UGT activity towards the anaesthetic propofol was higher compared with human liver. The glucuronidation of propofol is catalysed by UGT1A8/9 suggesting higher levels of this isoform in the kidney. Immunoblot analysis revealed two major UDP-glucuronosyltransferase immunopositive bands to be present in human kidney as compared to four major bands in human liver. The human kidney was capable of conjugating various structurally diverse drugs and xenobiotics.

Molecular basis for deficient acetaminophen glucuronidation in cats. An interspecies comparison of enzyme kinetics in liver microsomes

Cats are highly susceptible to acetaminophen toxicity because of deficient glucuronidation of this drug in vivo. The enzyme kinetic basis for this defect is unknown. Therefore, the kinetic properties of acetaminophen UDP-glucuronosyltransferase (acetaminophen-UGT) were investigated, using hepatic microsomes from cats (N = 4) compared with those of species that are less sensitive to acetaminophen intoxication including dogs (N = 4), humans (N = 4), and six other mammalian species (one liver from each). Gunn rats were also studied, since they express defective UGT family 1 isoenzymes and are also prone to acetaminophen toxicity. Acetaminophen kinetics were biphasic in all instances with distinct high and low affinity components. Km values for the high affinity activity in cat microsomes (0.31 +/- 0.1 mM; mean +/- SEM) were intermediate between those of dogs (0.11 +/- 0.02 mM) and humans (0.60 +/- 0.06 mM) and other species (0.22 to 6.7 mM; range). On the other hand, high affinity Vmax values were over 10-fold less in cat microsomes (0.025 +/- 0.006 nmol/min/mg) than in dogs (0.92 +/- 0.09 nmol/min/mg) and humans (0.27 +/- 0.09 nmol/min/mg); and over 5-fold less compared with microsomes from other species (range 0.13 to 7.63 nmol/min/mg). Gunn rat microsomes showed a similar 10-fold difference in high affinity Vmax values between the homozygous mutant (0.67 nmol/min/mg) and homozygous normal (6.75 nmol/min/mg) animals. These results demonstrate that, relative to a number of other species, cats have remarkably low hepatic levels of a high affinity acetaminophen-UGT. This difference is sufficient enough to explain poor glucuronidation of acetaminophen in vivo and susceptibility to acetaminophen intoxication.

Genetic variation in bilirubin UPD-glucuronosyltransferase gene promoter and Gilbert's syndrome

BACKGROUND

The genetic basis of Gilbert's syndrome is ill-defined. This common mild hyperbilirubinaemia sometimes presents as an intermittent jaundice. A reduced hepatic bilirubin UPD- glucuronosyltransferase (UGT) is associated with this syndrome. We have examined variation in the gene encoding the UGT1*1 enzyme and serum bilirubin levels in a Scottish population.

METHODS

Blood was collected from 12 patients with confirmed or suspected Gilbert's syndrome, from 6 members of a family with 4 Gilbert members, and from 77 non-smoking, alcohol-free, drug-free volunteers recruited from the staff of a teaching hospital in Dundee. Polymerase chain reaction amplification was used to examine sequence variation of the promoter upstream of the UGT1*1 exon I. Genotypes were assigned as follows: 6/6 (homozygous for a common allele bearing the sequence [TA](6)TAA), 7/7 (homozygous for a rarer allele with the sequence [TA](7)TAA), and 6/7 (heterozygous with one of each allele).

FINDINGS

Individuals in the population with the 7/7 genotype had significantly higher bilirubin concentrations than those who had the 6/7 or 6/6 genotype. 14 volunteers underwent a 24 h fasting test to see if they had Gilbert's syndrome, and all four positives had the 7/7 genotype. One confirmed Gilbert's patient, two recurrent jaundice patients (with suspected Gilbert's syndrome), and nine clinically diagnosed cases had the 7/7 genotype. Segregation of the 7/7 genotype with the Gilbert phenotype was also demonstrated in the family with four affected members. The frequency of the 7/7 genotype in this eastern Scottish population was 10-13%.

INTERPRETATION

In a healthy population there was an association between variation in bilirubin concentration and a mutation within the gene encoding the enzyme bilirubin UGT. This and other findings suggest the existence of a mild and a more severe form of Gilbert's syndrome, depending on whether the gene defect lies in the promoter sequence upstream of UGT1*I exon I, as here (mild), or in the coding sequence (severe) of the gene.

Glucuronidation of diflunisal in liver and kidney microsomes of rat and man

1. The glucuronidation of diflunisal to its phenolic (DPG) and acyl glucuronide (DAG) was measured in vitro using microsomes prepared from rat (n = 4) and human (n = 6) liver and kidney tissue. UGT activities towards bilirubin, 4-nitrophenol and (-)-morphine were also determined. 2. beta-Glucuronidase activity towards phenolphthalein glucuronide was much lower in microsomes prepared from human liver (45.2 +/- 3.1 Fishman Units/mg protein), human kidney (22.0 +/- 3.3 FU/mg), and rat kidney (25.1 +/- 2.5 FU/mg) as compared with rat liver (118.7 +/- 8.8 FU/mg). 3. The formation rate of DAG significantly increased when saccharo-1,4-lactone, a beta-glucuronidase inhibitor, was added to the rat liver microsomal incubation medium. beta-Glucuronidase inhibition, however, had little effect on the formation rate of DAG in human liver microsomes, and no effect in rat and human kidney microsomes. The formation of DPG was not affected by the microsomal beta-glucuronidase activity. 4. Unlike rat kidney microsomes, which only formed DAG, human kidney microsomes formed both diflunisal glucuronides. Formation of both diflunisal glucuronides in human kidney microsomes (Vmax = 0.97 +/- 0.21 and 0.27 +/- 0.07 nmol/min/mg for formation of DAG and DPG respectively) represented 60-70% of the activity found in liver microsomes (Vmax = 1.58 +/- 0.32 and 0.40 +/- 0.08 nmol/min/mg for formation of DAG and DPG respectively). 5. These results demonstrate that the in vitro glucuronidation rate of diflunisal may be affected by the microsomal beta-glucuronidase activity particularly when using rat liver microsomes. Our results also demonstrate that the human kidney has an important UGT-activity towards diflunisal.

Regulation of the human bilirubin UDP-glucuronosyltransferase gene

The human UGT1 gene is a single copy gene consisting of four common exons and more than 13 variable exons which span more than 200 kb of the human genome. A single variable exon is spliced to the four common exons to form the mRNA for synthesis of a single UDP-glucuronosyltransferase (UGT) isoenzyme. Treatment of humans or hepatoma cell lines with drugs such as phenobarbital causes the induction of hepatic bilirubin UGT by increased transcription from the UGT1 gene. The upstream region of UGT1*1 (bilirubin UGT) was sequenced and found to contain consensus sequences for several transcriptional regulatory elements including a 'BARBIE box'. An unusual 'TATA' promoter sequence A(TA)6TAA was also observed. The 5' region flanking the UGT1*1 exon when cloned into reporter constructs and transfected into four cells lines was capable of promoting reporter gene expression, but not when transfected into monkey kidney cell fibroblasts (COS-7 cells) indicating a cell specific expression. Sequential deletion of the 5' flanking region in the plasmid constructs did not cause any significant reduction in reporter expression. Treatment of cells transfected with these plasmid constructs with drugs did not cause a significant increase in reporter expression except with retinoic acid plus WY 14643. Introduction of an additional two base pairs (TA) into the 'TATA' box of the 5' gene sequence (as observed in Gilbert's patients) did not significantly change reporter expression levels. The regulation of the biliruibin UGT gene by drugs is not yet understood and it will be important to identify additional genetic elements possibly further than -2kb upstream of the UGT1*1 coding region, which regulate the expression of this gene.

Glucuronidation of propofol in microsomal fractions from various tissues and species including humans: effect of different drugs

This in vitro study was conducted to evaluate propofol glucuronidation and the effect of concomitantly administered drugs in various species. Propofol glucuronidation was studied in microsomal fractions from rat, rabbit, and human livers. Extrahepatic metabolism was investigated using lung and kidney microsomes. The propofol-uridine diphosphate-glucuronosyltransferase (UGT) activity measured in liver microsomes was higher in rabbit than in rat. Among the three tested species, human livers exhibited the highest activity, with only small variability in the three samples studied. Animal kidney, but not lung (animal or human), microsomes were able to glucuronidate propofol, meaning that extrahepatic metabolism of propofol exists, at least in the kidney, in the tested species (rat and rabbit). Since metabolic interactions are potential sources of prolonged drug effect or overdose, we screened the effect of 21 compounds (known substrates of various UGT or potentially coadministered drugs) on the glucuronidation of propofol by human liver microsomes. Inhibitions obtained with chemicals or drugs glucuronidated by either UGT1 or UGT2 families (1-naphtol, 4-hydroxybiphenyl, carvacrol, n-propylgallate, ketoprofen, chloramphenicol, acetylsalicylic acid) indicated that at least two UGT isoforms are involved in propofol glucuronidation. Inhibition was observed with several drugs potentially coadministered during pre-, per, or postoperative periods (e.g., acetylsalicyclic acid, ketoprofen, oxazepam, fentanyl). Although not directly transposable to the in vivo situation, these results indicate that such interactions are theoretically possible.(ABSTRACT TRUNCATED AT 250 WORDS)

Specificity of human UDP-glucuronosyltransferases and xenobiotic glucuronidation

Several human liver UDP-Glucuronosyltransferases (UGTs) have been cloned and the cDNAs expressed in heterologous cell lines. This technological advance has allowed the assessment of the functional substrate specificity of these UGTs. The problems which may be encountered with the latency and assay of UGTs are briefly described. The data accumulated to date indicate that the Km, and possibly the Vmax/Km, for individual substrates are the best parameters to assess the specificity of the enzymes towards xenobiotic molecules. The substrate specificity of seven UGTs has been summarised from the currently available information. Of these, UGT1*02 and UGT2B8 appear to be key isoforms in the glucuronidation of a wide range of xenobiotic substrates. Additional UGTs have yet to be identified and characterised and their future inclusion may provide further insights. Finally, the functional role of each UGT in vivo has to be determined.

Discrimination between Crigler-Najjar type I and II by expression of mutant bilirubin uridine diphosphate-glucuronosyltransferase

Crigler-Najjar (CN) disease is classified into two subtypes, type I and II. The molecular basis for the difference between these types is not well understood. Several mutations in the bilirubin UDP-glucuronosyl-transferase (B-UGT) gene of six CN type I and two CN type II patients were identified. Recombinant cDNAs containing these mutations were expressed in COS cells. B-UGT activity was measured using HPLC and the amount of expressed protein was quantitated using a sandwich ELISA. This enabled us to determine the specific activities of the expressed enzymes. All type I patients examined had mutations in the B-UGT1 gene that lead to completely inactive enzymes. The mutations in the B-UGT1 gene of patients with CN type II only partially inactivated the enzyme. At saturating concentrations of bilirubin (75 microM) CN type II patient A had 4.4 +/- 2% residual activity and CN type II patient B had 38 +/- 2% residual activity. Kinetic constants for the glucuronidation of bilirubin were determined. The affinities for bilirubin of B-UGT1 expressed in COS cells and B-UGT from human liver microsomes were similar with Km of 5.1 +/- 0.9 microM and 7.9 +/- 5.3 microM, respectively. B-UGT1 from patient B had a tenfold decreased affinity for bilirubin, Km = 56 +/- 23 microM. At physiological concentrations of bilirubin both type II patients will have a strongly reduced conjugation capacity, whereas type I patients have no B-UGT activity. We conclude that CN type I is caused by a complete absence of functional B-UGT and that in CN type II B-UGT activity is reduced.

Heterogeneity of hepatic microsomal UDP-glucuronosyltransferase(s) activities: a new kinetic approach for the study of induction and specificity

UDP-glucuronosyltransferase (UGT) activities have been described as heterogeneous, i.e. supported by a family of isoenzymes, each of them being capable of conjugating a given chemical family of aglycons, including steroids, coumarines, and phenols. Some of these isoenzymes are specifically induced by xenobiotics. In order to discriminate between the different isoenzymes, we propose a new in situ approach that combines induction (gene regulation) and catalytic activities (specificity). The characterization of one isoenzyme is obtained by (i) increasing its amount by specific inductive stimulation and (ii) studying simultaneously the glucuronidation kinetics of a series of alternative substrates. Provided the substrates are of similar structure, a linear relationship can be established between their glucuronidation rates before versus after induction. We developed a simple mathematical model to analyze the kinetic behaviors observed. With this method, it is possible to know (i) the exact extent of induction of a given isoenzyme by a given inducer (induction factor, n) and (ii) its strict specificity. This in situ methodology is complementary to isolated protein or cDNAs, for the characterization of the real in situ substrate specificity of differentially regulated UGT isoenzymes.

Human UDP-glucuronosyl transferases: chemical defence, jaundice and gene therapy

Human UDP-glucuronosyltransferases (UDPGTs) are a family of enzymes which detoxify many hundreds of compounds by their conjugation to glucuronic acid, rendering them both harmless and more water soluble, hence, excretable. The level of expression of each UDPGT isoform in the body is the result of interplay between temporal, tissue-specific and environmental regulators. This complexity contributes to the difficulty in predicting the metabolic fate of compounds. Genetic defects and polymorphisms affecting individual isoform activities have deleterious and potentially lethal effects, as exemplified by the severe hyperbilirubinaemia observed in Crigler-Najjar Syndrome. Such severe genetic defects in bilirubin glucuronidation are obvious candidates for antenatal screening and gene therapy.

Heterogeneity of hepatic UDP-glucuronosyltransferase activities: investigations of isoenzymes involved in p-nitrophenol glucuronidation

1. UDP-Glucuronosyltransferase (UGT, EC 2.1.4.17) has been measured routinely with para-nitrophenol (pNP) because the UGT assay using this substrate is easy to assess and run. 2. This compound has been used in several studies as a substrate for purification of the enzyme. 3. In the present work, we characterize the para-nitrophenol-conjugating activity. 4. An analysis of kinetics of para-nitrophenol conjugation obtained from various biological sources (various tissues and various species) leads us to the conclusion that at least three isoenzymes are responsible for this activity in the rat. 5. Both UGT-(testosterone) and the 3-methylcholanthrene-inducible form previously described in the literature, may be responsible for the activity, whilst a highly specific form (UGT-phenol) is reported here for the first time. 6. This work is intended to lay down the basis of further investigations, including purification of the highly specific isoform.

Paracetamol glucuronidation by recombinant rat and human phenol UDP-glucuronosyltransferases

Stably expressed human and rat phenol UDP-glucuronosyltransferases (UGTs) of the UGT1 complex (HlugP1, HlugP4 and 3-methylcholanthrene-inducible rat UGT1A1, the latter considered to be an orthologous enzyme to HlugP1) have been used to investigate the role of UGTs in paracetamol glucuronidation. Kinetic analysis of recombinant UGTs was compared to that of total UGT activities in liver microsomes. Paracetamol was found to be an overlapping substrate of several UGTs. It shows higher affinity for HlugP1 and rat UGT1A1 (apparent Km values of 2 and 3 mM, respectively) than for HlugP4 (Km = 50 mM) and other UGTs present in liver microsomes (Km values of > 12 mM). Glucuronidation of paracetamol with HlugP1 contrasts with that of 6-hydroxychrysene and of 4-methylumbelliferone, which are conjugated with higher affinity by HlugP4 than by HlugP1. Due to the wide tissue distribution of rat UGT1A1, paracetamol glucuronidation was also investigated in extrahepatic rat and human tissues. Paracetamol UGT activity was present and inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin in rat kidney, lung and spleen. It was also detected in human kidney. A selective cDNA probe for exon 1 of HlugP1 cross-reacted with mRNA from both human liver and kidney. The results demonstrate that paracetamol is conjugated by HlugP1 and its rat orthologue UGT1A1 with higher affinity than by HlugP4 and other UGTs.