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

Effect of copper nanoparticles on brain cytochrome P450 enzymes in rats

The aim of the present study was to evaluate the long‑term effect of copper nanoparticles (CuNPs) on cytochrome P450 (CYP450) enzymes in the rat brain. Rats were repeatedly gavaged with different forms of copper sources for 28 days, and the levels of oxidative stress and CYP450 mRNA and protein expression in the rat brain were subsequently analyzed. The results demonstrated that a high dose of CuNPs (200 mg/kg) induced severe oxidative stress in the rat brain along with a decrease in the levels of total superoxide dismutase and glutathione, and an increase in hydroxyl radicals and malondialdehyde. A medium dose of CuNPs reduced CYP450 2C11 and CYP450 3A1 protein expression in the rat brain, whereas high doses of CuNPs resulted in decreased expression of most CYP450 enzyme proteins, and inhibition of pregnane X receptor and constitutive androstane receptor expression. The results suggested that CuNPs may inhibit CYP450 enzyme expression by increasing the levels of oxidative stress and decreasing the expression of nuclear receptors in the rat brain, which affects the metabolism of drugs and endogenous hormones in the brain.

Primary hepatocyte cultures for pharmaco-toxicological studies: at the busy crossroad of various anti-dedifferentiation strategies

Continuously increasing understanding of the molecular triggers responsible for the onset of diseases, paralleled by an equally dynamic evolution of chemical synthesis and screening methods, offers an abundance of pharmacological agents with a potential to become new successful drugs. However, before patients can benefit of newly developed pharmaceuticals, stringent safety filters need to be applied to weed out unfavourable drug candidates. Cost effectiveness and the need to identify compound liabilities, without exposing humans to unnecessary risks, has stimulated the shift of the safety studies to the earliest stages of drug discovery and development. In this regard, in vivo relevant organotypic in vitro models have high potential to revolutionize the preclinical safety testing. They can enable automation of the process, to match the requirements of high-throughput screening approaches, while satisfying ethical considerations. Cultures of primary hepatocytes became already an inherent part of the preclinical pharmaco-toxicological testing battery, yet their routine use, particularly for long-term assays, is limited by the progressive deterioration of liver-specific features. The availability of suitable hepatic and other organ-specific in vitro models is, however, of paramount importance in the light of changing European legal regulations in the field of chemical compounds of different origin, which gradually restrict the use of animal studies for safety assessment, as currently witnessed in cosmetic industry. Fortunately, research groups worldwide spare no effort to establish hepatic in vitro systems. In the present review, both classical and innovative methodologies to stabilize the in vivo-like hepatocyte phenotype in culture of primary hepatocytes are presented and discussed.

Comparison of serotonin glucuronidation activity of UDP-glucuronosyltransferase 1a6a (Ugt1a6a) and Ugt1a6b: evidence for the preferential expression of Ugt1a6a in the mouse brain

Mouse UDP-glucuronosyltransferase (Ugt) 1a6a and Ugt1a6b share 98% sequence homology, but there have been no reports to date that compare their expression levels or enzymatic activities in serotonin glucuronidation. Thus, we designed specific primers for Ugt1a6a and Ugt1a6b to compare their expression in mouse brain regions and livers. Ugt1a6a was dominantly expressed in mouse brains, especially the hippocampus, while both Ugt1a6a and Ugt1a6b were highly expressed in mouse livers, indicating that there are significant differences in the expression patterns of Ugt1a6a and Ugt1a6b among mouse tissues. Glucuronidation of endogenous neurotransmitter serotonin was catalyzed by Ugt1a6b with k(cat)/K(m) (4.5 M(-1)·s(-1)) slightly higher than that of Ugt1a6a (2.4 M(-1)·s(-1)). However, the difference in expression levels between Ugt1a6a and Ugt1a6b in the hippocampus led us to speculate that Ugt1a6a is likely the predominant catalyst of serotonin glucuronidation in the mouse brain. In conclusion, we successfully elucidated the differences between Ugt1a6a and Ugt1a6b expression in the mouse brain. Our new findings indicate that Ugt1a6a and Ugt1a6b play different roles in mice, driven by differences in expression and kinetic properties for serotonin glucuronidation.

Nrf2 plays an important role in coordinated regulation of Phase II drug metabolism enzymes and Phase III drug transporters

The nuclear transcription factor E2-related factor 2 (Nrf2) has been shown to play pivotal roles in preventing xenobiotic-related toxicity and carcinogen-induced carcinogenesis. These protective roles of Nrf2 have been attributed in part to its involvement in the induction of Phase II drug conjugation/detoxification enzymes as well as antioxidant enzymes through the Nrf2-antioxidant response element (ARE) signaling pathways. This review summarizes the current research status of the identification of Nrf2-regulated drug metabolism enzymes (DMEs), especially Phase II DMEs, and Phase III drug transporters. In addition, the molecular mechanisms underlying the coordinated regulation of Phase II DMEs and Phase III transporters will also be discussed based on findings published in the literature.

Regulation of the rat UGT1A6 by glucocorticoids involves a cryptic glucocorticoid response element

Glucocorticoids precociously induce fetal rat UGT1A6 and potentiate polycyclic aromatic hydrocarbon (PAH)-dependent induction of this enzyme in vivo and in isolated rat hepatocytes. To establish whether induction was due to glucocorticoid receptor (GR), luciferase reporter vectors were tested in transfection assays with HepG2 cells. Using a reporter construct containing approximately 2.26 kilobases of the 5'-flanking region of the UGT1A6-noncoding leader exon (A1*), dexamethasone increased basal activity 3- to 7-fold in cells cotransfected with an expression plasmid for GR. PAH increased gene expression 23-fold, but the presence of dexamethasone only induced PAH-dependent expression by 1.5-fold, suggesting interaction between GR and the aryl hydrocarbon (Ah) receptor. Furthermore, the GR antagonist RU 38486 [17beta-hydroxy-11beta-(4-dimethylamino-phenyl)-17alpha-(prop-1-ynyl)-estra-4,9-dien-3-one] was a partial agonist that increased, rather than inhibited, basal activity 3-fold. 5'-deletion analysis defined the 5'-boundary for a functional glucocorticoid-responsive unit between base pairs -141 and -118 relative to the transcription start site. This region contains the Ah receptor response element (AhRE), and both PAH and glucocorticoid-dependent gene activation were lost when this area was deleted. Mutation of a single base pair located in the AhRE region simultaneously reduced induction by PAH and increased glucocorticoid induction. Thus, the sequences of both the AhRE and glucocorticoid response elements seem to overlap, suggesting that Ah receptor binding may decrease glucocorticoid-dependent induction due to interactions of these two cis-acting elements. Mutation of a putative GRE located between base pair -81 and -95 reduced, but did not completely eliminate, glucocorticoid-dependent induction of the reporter, suggesting that a nonclassic mechanism of induction is involved in this response.

UGT1A6 polymorphism and salicylic acid glucuronidation following aspirin

OBJECTIVES

In vivo, aspirin (acetylsalicylic acid) is rapidly deacetylated to form salicylic acid, which then undergoes primary or secondary glucuronidation catalyzed by UDP-glucuronosyltransferases (UGTs). The variant UGT1A6*2 (T181A, R184S) is associated with altered enzyme function. Our objective was to compare salicylic acid glucuronidation in individuals with different UGT1A6 genotypes.

METHODS

Following orally dosing with 650 mg aspirin, saliva and urine samples were collected over a period of 24 h from healthy individuals with homozygous wild-type UGT1A6 *1/*1 (n=19) and homozygous variant UGT1A6 *2/*2 (T181A, R184S) (n=9) genotypes.

RESULTS

No statistically significant differences were observed in salivary pharmacokinetic parameters. Urinary excretion of the sum of aspirin and its metabolites (salicyluric acid, salicyluric acid phenolic glucuronide, salicyl phenolic glucuronide, salicyl acyl glucuronide, salicylic acid) during the early period of 2-4 h of collection was significantly lower in UGT1A6 *1/*1 than in UGT1A6 *2/*2 individuals. Further, UGT1A6 *1/*1 individuals excreted a lower percentage of aspirin and its metabolites in the first 12 h and a greater percentage after 12 h than UGT1A6 *2/*2 individuals.

CONCLUSIONS

The variant UGT1A6*2 or polymorphisms in other UGTs that are in linkage disequilibrium with UGT1A6*2 may confer more rapid glucuronidation of salicylic acid than the wild-type UGT1A6 *1/*1.

Tissue- and gender-specific mRNA expression of UDP-glucuronosyltransferases (UGTs) in mice

UDP-glucuronosyltransferases (UGTs) catalyze phase II biotransformation reactions in which lipophilic substrates are conjugated with glucuronic acid to increase water solubility and enhance excretion. Currently, little information regarding tissue- or gender-specific expression of mouse UGTs is available. Mice are increasingly popular models in biomedical research, and therefore, thorough characterization of murine drug metabolism is desired. The purpose of the present study was to determine both tissue- and gender-specific UGT gene expression profiles in mice. RNA from 14 tissues was isolated from male and female C57BL/6 mice and UGT expression was determined by the branched DNA signal amplification assay. UGTs highly expressed in mouse liver include Ugt1a1, Ugt1a5, Ugt1a6, Ugt1a9, Ugt2a3, Ugt2b1, Ugt2b5/37/38, Ugt2b34, Ugt2b35, and Ugt2b36. Several isoforms were expressed in the gastrointestinal (GI) tract, including Ugt1a6, Ugt1a7c, Ugt2a3, Ugt2b34, and Ugt2b35. In kidney, Ugt1a2, Ugt1a7c, Ugt2b5/37/38, Ugt2b35, and Ugt3a1/2 were expressed. UGT expression was also observed in other tissues: lung (Ugt1a6), brain (Ugt2b35), testis and ovary (Ugt1a6 and Ugt2b35), and nasal epithelia (Ugt2a1/2). Male-predominant expression was observed for Ugt2b1 in liver, Ugt2b5/37/38 in kidney, and Ugt1a6 in lung. Female-predominant expression was observed for Ugt1a1 and Ugt1a5 in liver, Ugt1a2 in kidney, Ugt2b35 in brain, and Ugt2a1/2 in nasal epithelia. UDP-glucose pyrophosphorylase was highly expressed in liver, kidney, and GI tract, whereas UDP-glucose dehydrogenase was highly expressed in the GI tract. In conclusion, marked differences in tissue- and gender-specific expression patterns of UGTs exist in mice, potentially influencing drug metabolism and pharmacokinetics.

Bound Sugars in Hepatic Glycoproteins from Male Rats During Dietary Citrus Bioflavonoid and/or Ascorbic Acid Supplementation

The purpose of these experiments was to determine whether quantitative changes take place in the attached sugars of hepatic (postmitochondrial) glycoproteins isolated from rats fed a diet supplemented with citrus bioflavonoids (an equal mixture of rutin, naringin, and hesperidin) (B) and/or ascorbic acid (C) for 40 days and 90 days. Statistically significant increases in body weights (P < .05) were observed in the B-only groups and liver weights in the CB group (P < .01-.05) after 40 and 90 days of feeding the experimental diets, while liver weights were decreased in the B-only groups after 40 days of feeding (P < .05). In the acid-soluble glycoprotein fraction, statistically significant decreases were seen in bound hexoses and fucose (P < .05) in the CB group after 40 days, and in bound fucose only after 90 days (P < .05). In the acid-insoluble glycoprotein fraction, statistically significant changes were seen in bound hexoses and fucose (P < .05) in the CB group after 40 days, and in bound fucose only in the CB group after 90 days (P < .05). There is an apparent overall decrease in the sugar-rich acid-soluble glycoprotein fraction that is accentuated even further by combined CB supplementation. This decrease is more probable after 40 days than it is after 90 days of dietary supplementation. An adaptive phenomenon is suggested for maintaining the intracellular environment during periods of dietary citrus bioflavonoid and/or ascorbic acid supplementation.

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

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

Identification and Characterization of Functional Hepatocyte Nuclear Factor 1‐Binding Sites in UDP‐Glucuronosyltransferase Genes

The hepatocyte nuclear factor 1 (HNF1) transcription factor family is composed of two closely related homeodomain proteins with similar but distinct expression profiles. Homodimers and heterodimers of these transcription factors, HNF1alpha and HNF1beta, increase transcription from target genes through direct physical interaction with one or more elements of sufficient similarity to a 13 nucleotide-inverted dyad consensus-binding sequence. Potential HNF1-binding sites have been found in the proximal upstream regulatory regions of most known human UDP-glucuronosyltransferase (UGT) genes. As the liver and gastrointestinal tract are both important sites of glucuronidation and express significant levels of one or both HNF1 proteins, it is thought that these homeoproteins may play a role in transcriptional regulation of UGTs. This chapter explores the current evidence that HNF1 transcription factors are explicitly involved in the transcription of mammalian UGT genes. Most data supporting this hypothesis come from in vitro reporter assays, site-directed mutagenesis, and electrophoretic mobility-shift assays, for which methods are detailed. However, as in vitro functionality of transcription factors does not necessarily imply significance in vivo, some of the limitations of these techniques are also examined. In addition, available in vivo data are discussed, with particular attention given to contributions made by HNF1alpha knockout mouse models and microarray studies of human tissue. Finally, possible scenarios in which HNF1-mediated regulation of UGT expression may be clinically relevant are suggested.

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.

Vertebrate UDP-glucuronosyltransferases: functional and evolutionary aspects

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

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.

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.

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.

Validation of serotonin (5-hydroxtryptamine) as an in vitro substrate probe for human UDP-glucuronosyltransferase (UGT) 1A6

Investigation of human UDP-glucuronosyltransferase (UGT) isoforms has been limited by a lack of specific substrate probes. In this study serotonin was evaluated for use as a probe substrate for human UGT1A6 using recombinant human UGTs and tissue microsomes. Of the 10 commercially available recombinant UGT isoforms, only UGT1A6 catalyzed serotonin glucuronidation. Serotonin-UGT activity at 40 mM serotonin concentration varied more than 40-fold among human livers (n = 54), ranging from 0.77 to 32.9 nmol/min/mg of protein with a median activity of 7.1 nmol/min/mg of protein. Serotonin-UGT activity kinetics of representative human liver microsomes (n = 7) and pooled human kidney, intestinal and lung microsomes and recombinant human UGT1A6 typically followed one enzyme Michaelis-Menten kinetics. Serotonin glucuronidation activity in these human liver microsomes had widely varying V(max) values ranging from 0.62 to 51.3 nmol/min/mg of protein but very similar apparent K(m) values ranging from 5.2 to 8.8 mM. Pooled human kidney, intestine, and lung microsomes had V(max) values (mean +/- standard error of the estimates) of 8.8 +/- 0.4, 0.22 +/- 0.00, and 0.03 +/- 0.00 nmol/min/mg of protein (respectively) and apparent K(m) values of 6.5 +/- 0.9, 12.4 +/- 2.0, and 4.9 +/- 3.3 mM (respectively). In comparison, recombinant UGT1A6 had a V(max) of 4.5 +/- 0.1 nmol/min/mg of protein and an apparent K(m) of 5.0 +/- 0.4 mM. A highly significant correlation was found between immunoreactive UGT1A6 protein content and serotonin-UGT activity measured at 4 mM serotonin concentration in human liver microsomes (R(s) = 0.769; P < 0.001) (n = 52). In conclusion, these results indicate that serotonin is a highly selective in vitro probe substrate for human UGT1A6.

Induction of UDP-glucuronosyltransferase 1A8 mRNA by 3-methylcholanthene in rat hepatoma cells

UDP-glucuronosyltransferases (UGTs) catalyze the glucuronidation of a broad spectrum of endobiotic and xenobiotic compounds, which leads to the excretion of hydrophilic glucuronides via bile or urine. By a mechanism of exon sharing, isoforms of the UGT1 family are made from the complex gene locus by an alternative combination of one of the unique first exons with the other commonly used exons. This study demonstrates that the expression of the UGT1 gene UGT1A6, 1A7 and 1A8 is regulated at the transcriptional level by 3-methylcholanthene (3-MC) in rat hepatoma H-4-II-E cells. Following 3-MC treatment, there is a gradual increase in the amount of UGT1A6 and UGT1A7 mRNA to the maximum levels after 16hr of treatment. The induction effect of 3-MC led to the expression of UGT1A8 which has not been reported before. This induction is suppressed by the RNA synthesis inhibitor actinomycin D, indicating that the inducer does not act at the level of mRNA stabilization. Northern blot analysis showed a 4-fold increase in UGT1A8 transcription after treatment with 3-MC. The prolonged treatment with the protein synthesis inhibitor did not affect the induction process. The results provide experimental evidence for a transcriptional control of UGT1A8 synthesis. Transcriptional activation of the UGT1A8 by 3-MC does not appear to require de novo protein synthesis. 3-MC dependent activation is probably the result of a direct action of the compound on the aryl hydrocarbon receptor complex (AhR).