In vitro inhibitory effects of non-steroidal antiinflammatory drugs on UDP-glucuronosyltransferase 1A1-catalysed estradiol 3beta-glucuronidation in human liver microsomes

The inhibitory potencies of non-steroidal antiinflammatory drugs (NSAID) on UDP-glucuronosyltransferase (UGT) 1A1-catalysed estradiol 3beta-glucuronidation (E3G) were investigated in human liver microsomes (HLM). Inhibitory effects of the following seven NSAID were investigated: acetaminophen, diclofenac, diflunisal, indomethacin, ketoprofen, naproxen and niflumic acid. Niflumic acid had the most potent inhibitory effect on E3G with an IC50 value of 22.2 microM in HLM. The IC50 values of diclofenac, diflunisal, indomethacin for E3G were 60.9, 37.8 and 51.5 microM, respectively, while acetaminophen, ketoprofen and naproxen showed less potent inhibition. Diclofenac inhibited E3G non-competitively with a Ki value of 112 microM in HLM. The IC50 value of diclofenac for 4-methylumbelliferone glucuronidation in recombinant human UGT1A1 was 57.5 microM, similar to that obtained for E3G using HLM. In conclusion, niflumic acid had the most potent inhibitory effects on UGT1A1-catalysed E3G in HLM among seven NSAID investigated.

The Role of Ah Receptor in Induction of Human UDP‐Glucuronosyltransferase 1A1

UDP-glucuronosyltransferases (UGTs) catalyze a major metabolic pathway initiating the transfer of glucuronic acid from uridine 5'-diphosphoglucuronic acid to endogenous and exogenous substances. Endogenous substances include bile acids, steroids, phenolic neurotransmitters, and bilirubin. Xenobiotic substances include dietary substances, therapeutics, and environmental compounds. The versatility in the selection of substrates for glucuronidation results from the multiplicity of the UGTs in addition to the ability of these genes to be regulated. UDP-glucuronosyltransferase 1A1 (UGT1A1), responsible for the glucuronidation of bilirubin, is controlled in a tissue-specific manner and can be regulated following environmental exposure. This chapter describes materials and methods for the examination of molecular interactions that control UGT1A1 expression and induction in response to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Using an in vitro cell culture system, we mapped a regulatory sequence that contains a xenobiotic response element core sequence in the enhancer region of the UGT1A1 gene. Similar to regulation of CYP1A1, the transcriptional activation of UGT1A1 by TCDD is mediated through the aryl hydrocarbon receptor.

Effects of?-estradiol and propofol on the 4-methylumbelliferone glucuronidation in recombinant human UGT isozymes 1A1, 1A8 and 1A9

The effects of beta-estradiol and propofol on human UGT1A1, 1A8 and 1A9 activities were investigated using 4-methylumbelliferone (4-MU) as a substrate for glucuronidation. The formation of 4-MU glucuronide (4-MUG) from 4-MU, in recombinant human UGT 1A1, 1A8 and 1A9 was determined using HPLC with fluorescence detection. The glucuronidation activity of 4-MU was the highest in UGT1A9 with an apparent K(m) value of 8.3 microM, while that in UGT1A1 and 1A8 was linear to at least 100 microM. beta-estradiol had potent inhibitory effects on UGT1A9 as well as on UGT1A1 with IC(50) values of 2.1 and 7.2 microM, respectively. Propofol inhibited UGT1A9 activity with an IC(50) of 55 microM, while the IC(50) value was much higher for UGT1A8. In contrast, beta-estradiol and propofol activated 4-MU glucuronidation in UGT1A1 and 1A8, respectively. This study therefore indicates that the use of beta-estradiol as a specific inhibitor for UGT1A1 should be used with care in the identification of UGT isozymes responsible for glucuronidation in human liver microsomes.

Effects of TGF-beta on hyaluronan anabolism in fibroblasts derived from the synovial membrane of the rabbit temporomandibular joint

Hyaluronan (HA) synthesis in the synovial membrane is affected by various chemical mediators. It is hypothesized that transforming growth factor-beta 1 (TGF-beta 1) would be a mediator to modulate HA synthesis in cultured synovial membrane fibroblasts of the temporomandibular joint (TMJ). Fibroblasts were extracted from the TMJ synovial membrane of four-week-old Japanese white rabbits. The amount of HA and expression levels of HA synthase (HAS) mRNAs induced by TGF-beta 1 treatment were analyzed by means of high-performance liquid chromatography and real-time polymerase chain-reaction, respectively. Both medium and large amounts of HA were enhanced by the stimulation of TGF-beta 1. HAS2 mRNA expression was enhanced 13-fold after six-hour stimulation with TGF-beta 1 (10 ng/mL), whereas HAS3 mRNA expression was not changed significantly. These results suggest that TGF-beta 1 enhances the expression of HAS2 mRNA in the TMJ synovial membrane fibroblasts and may contribute to the production of high-molecular-weight HA in the joint fluid.

Synergistic Effect of N-Acetylglucosamine and Retinoids on Hyaluronan Production in Human Keratinocytes

Hyaluronan (HA) is well known to reside in the extracellular matrix as a water-sorbed macromolecule. The aims of this study were twofold: to investigate the regulation of HA synthesis in keratinocytes, and to develop a method to modulate this regulatory process. We found that N-acetylglucosamine (NAG) increased the production of HA by cultured keratinocytes dose dependently, but had no effect on the production by skin fibroblasts. The effect of NAG in keratinocytes was found to be specific for HA production, as there was no change in sulfated glycosaminoglycan formation. The copresence of NAG with either of two retinoids, retinoic acid (RA) or retinol, exerted a synergistic effect on HA production. To investigate whether human HA synthase (HAS) genes were regulated by NAG or retinoids, total RNA extracted from cells treated with these agents was subjected to Northern blot analysis. We observed that RA and retinol markedly induced the expression of HA synthase-3 (HAS3) mRNA. Moreover, beta-carotene, a provitamin A, influenced HA production and HAS3 gene expression in a manner similar to the retinoids. Conversely, NAG had no effect on the expression of HAS3 transcripts. Pretreatment of cells with RA stimulated the activity of membrane-associated HAS, whereas pretreatment with NAG did not. These results suggest that HA production is regulated by at least two pathways: one involving the regulation of HAS gene expression, and the other independent of such a regulatory effect. Taken together, our findings suggest that NAG is a new modulator of HA synthesis.

Induction of rat hepatic and intestinal UDP-glucuronosyltransferases by naturally occurring dietary anticarcinogens

Gastrointestinal tumours are among the most common malignancies in Western society, the majority of which are associated with dietary and lifestyle factors. Many dietary or lifestyle factors have been identified which may have toxic or carcinogenic properties. However, several dietary compounds also able to reduce gastrointestinal cancer rates in both humans and animals have been characterized. Though the exact mechanism leading to the anticarcinogenic action of these compounds is not fully known, it has been demonstrated that this chemopreventive capacity may be due to elevation of the glutathione S-transferase detoxification enzymes. Here we have investigated the effect of several anticarcinogens on the gastrointestinal UDP-glucuronosyltransferase (UGT) enzymes. Diets of male Wistar rats were supplemented with ellagic acid, ferulic acid, Brussels sprouts, quercetin, alpha-angelicalactone, tannic acid, coumarin, fumaric acid, curcumin and flavone, separately, and combinations of alpha-angelicalactone and flavone. Hepatic and intestinal (proximal, mid and distal small intestine and colon) UGT enzyme activities were quantified using 4-nitrophenol and 4-methylumbelliferone as substrates. All anticarcinogens tested increased UGT enzyme activity with both substrates, at one at least of the five different sites investigated. alpha-Angelicalactone, coumarin and curcumin showed enhanced UGT enzyme activities at all five sites. Both small and large intestinal UGT enzyme activities were increased by quercetin, alpha-angelicalactone, coumarin, curcumin and flavone. Except for tannic acid, all agents induced hepatic UGT enzyme activity. Furthermore, dietary administration of alpha-angelicalactone and flavone, given individually or in combination, enhanced the UGT detoxification system in the liver and, to a lesser extent, in intestine. In conclusion, induction of gastrointestinal UGT enzyme activities after consumption of dietary anticarcinogens may contribute to a better detoxification of potentially carcinogenic compounds and subsequently to the prevention of gastrointestinal cancer.

Functional characterization of human UDP-glucuronosyltransferase 1A9 variant, D256N, found in Japanese cancer patients

SN-38 (7-ethyl-10-hydroxycamptothecin), an active metabolite of the antitumor prodrug irinotecan, is conjugated and detoxified to SN-38 10-O-beta-d-glucuronide by hepatic UDP-glucuronosyltransferase (UGT) 1A1. Recent studies have revealed that other UGT1A isoforms, UGT1A7 and UGT1A9, also participate in SN-38 glucuronidation. Although several genetic polymorphisms are reported for UGT1A1 and UGT1A7 that affect the SN-38 glucuronidation activities, no such polymorphisms have been identified for UGT1A9. In the present study, UGT1A9 exon 1 and its flanking regions were sequenced from 61 Japanese cancer patients who were all treated with irinotecan. A novel nonsynonymous single nucleotide polymorphism was identified in UGT1A9 exon 1, heterozygous 766G>A resulting in the amino acid substitution of D256N. The wild-type and D256N UGT1A9s were transiently expressed at similar protein levels in COS-1 cells, and their membrane fractions were characterized in vitro for the glucuronidation activities toward SN-38. The apparent Km values were 19.3 and 44.4 microM, and the Vmax values were 2.94 and 0.24 pmol/min/mg of membrane protein for the wild-type and D256N variant, respectively. The SN-38 glucuronidation efficiency (normalized Vmax/Km) of D256N was less than 5% that of wild-type UGT1A9. These results clearly indicate that the D256N variant is essentially nonfunctional with regard to SN-38 glucuronidation. These findings highlight the importance of further studies into the potential influence of UGT1A9 D256N variant to irinotecan metabolism in vivo.

Irinotecan treatment in cancer patients with UGT1A1 polymorphisms

At the present time, pharmacogenetic investigation of irinotecan (CPT-11, Camptosar) therapy is mainly focused on the clinical relevance of genetic variation in the UDP-glucuronosyltransferase (UGT1A1) gene. The glucuronidation of the potent topoisomerase I inhibitor SN-38 is a major inactivation pathway of irinotecan metabolism. UGT1A1 genotypes associated with Gilbert's syndrome (a mild intermittent hyperbilirubinemia) are characterized by reduced glucuronidation of SN-38. Such UGT1A1 genetic variants have different distribution across individuals of different ethnicity. The (TA)n TAA polymorphism in the promoter is more frequent in Caucasians as compared to Asians, in whom missense polymorphisms in the exons are more common. Two recent pharmacogenetic trials (one performed in the United States and the other in Japan) investigated the clinical significance of UGT1A1 gene mutations for both the pharmacokinetics of irinotecan metabolites and the toxicity profile. The results of these association studies showed that preliminary genotyping of the (TA)n polymorphism might predict the occurrence of toxicity in genetically predisposed patients.

The effect of valproic acid on drug and steroid glucuronidation by expressed human UDP-glucuronosyltransferases

Valproic acid glucuronidation kinetics were carried our with three human UGT isoforms: UGT1A6, UGT1A9, and UGT2B7 as well as human liver and kidney microsomes. The glucuronidation of valproic acid was typified by high K(m) values with microsomes and expressed UGTs (2.3-5.2mM). The ability of valproic acid to interact with the glucuronidation of drugs, steroids and xenobiotics in vitro was investigated using the three UGT isoforms known to glucuronidate valproic acid. In addition to this the effect of valproic acid was investigated using two other UGT isoforms: UGT1A1 and UGT2B15 which do not glucuronidate valproic acid. Valproic acid inhibited UGT1A9 catalyzed propofol glucuronidation in an uncompetitive manner and UGT2B7 catalyzed AZT glucuronidation competitively (K(i)=1.6+/-0.06mM). Valproate significantly inhibited UGT2B15 catalyzed steroid and xenobiotic glucuronidation although valproate was not a substrate for this UGT isoform. No significant inhibition of UGT1A1 or UGT1A6 by valproic acid was observed. These data indicate that valproic acid inhibition of glucuronidation reactions is not always due to simple competitive inhibition of substrates.

Up-regulation of soyasaponin biosynthesis by methyl jasmonate in cultured cells of Glycyrrhiza glabra

Exogenously applied methyl jasmonate (MeJA) stimulated soyasaponin biosynthesis in cultured cells of Glycyrrhiza glabra (common licorice). mRNA level and enzyme activity of beta-amyrin synthase (bAS), an oxidosqualene cyclase (OSC) situated at the branching point for oleanane-type triterpene saponin biosynthesis, were up-regulated by MeJA, whereas those of cycloartenol synthase, an OSC involved in sterol biosynthesis, were relatively constant. Two mRNAs of squalene synthase (SQS), an enzyme common to both triterpene and sterol biosyntheses, were also up-regulated by MeJA. In addition, enzyme activity of UDP-glucuronic acid: soyasapogenol B glucuronosyltransferase, an enzyme situated at a later step of soyasaponin biosynthesis, was also up-regulated by MeJA. Accumulations of bAS and two SQS mRNAs were not transient but lasted for 7 d after exposure to MeJA, resulting in the high-level accumulation (more than 2% of dry weight cells) of soyasaponins in cultured licorice cells. In contrast, bAS and SQS mRNAs were coordinately down-regulated by yeast extract, and mRNA accumulation of polyketide reductase, an enzyme involved in 5-deoxyflavonoid biosynthesis in cultured licorice cells, was induced transiently by yeast extract and MeJA, respectively.

Glucuronidation of odorant molecules in the rat olfactory system: activity, expression and age-linked modifications of UDP-glucuronosyltransferase isoforms, UGT1A6 and UGT2A1, and relation to mitral cell activity

The aim of the present study was to examine the glucuronidation of a series of odorant molecules by homogenates prepared either with rat olfactory mucosa, olfactory bulb or brain. Most of the odorant molecules tested were efficiently conjugated by olfactory mucosa, whereas olfactory bulb and brain homogenates displayed lower activities and glucuronidated only a few molecules. Important age-related changes in glucuronidation efficiency were observed in olfactory mucosa and bulb. Therefore, we studied changes in expression of two UDP-glucuronosyltransferase isoforms, UGT1A6 and UGT2A1, in 1-day, 1- and 2-week-, 3-, 12- and 24-month-old rats. UGT1A6 was expressed at the same transcriptional level in the olfactory mucosa, bulb and brain, throughout the life period studied. UGT2A1 mRNA was expressed in both olfactory mucosa and olfactory bulb, in accordance with previous results [Mol. Brain Res. 90 (2001) 83], but UGT2A1 transcriptional level was 400-4000 times higher than that of UGT1A6. Moreover, age-dependent variations in UGT2A1 mRNA expression were observed. As it has been suggested that drug metabolizing enzymes could participate in olfactory function, mitral cell electrical activity was recorded during exposure to different odorant molecules in young, adult and old animals. Age-related changes in the amplitude of response after stimulation with several odorant molecules were observed, and the highest responses were obtained with molecules that were not efficiently glucuronidated by olfactory mucosa. In conclusion, the present work presents new evidence of the involvement of UGT activity in some steps of the olfactory process.

Long-term effects of a novel phosphorothionate (RPR-II) on detoxifying enzymes in brain, lung, and kidney rats

The effects of a phosphorothionate, 2-butenoic acid-3-(diethoxyphosphinothioyl) methyl ester (RPR-II), on the activities of glutathione S-transferase (GST) and UDP-glucuronyltransferase (UDPGT) and the level of glutathione (GSH) were evaluated in rats after administration of RPR-II at 0.014 (low), 0.028 (medium), and 0.042 (high) mgkg(-1)day(-1) for 90 days and also at 28 days (withdrawal) after stopping treatment. Brain GST activity and GSH level decreased significantly at the high dose on the 45th and 90th days of treatment. Dose- and time-dependent decreases in GST activity and GSH was level were observed in lung at medium and high doses and in kidneys at all three doses on both the 45th and 90th days. UDPGT activity increased significantly in kidneys at the medium and high doses at 45 and 90 days. Brain and lung did not display any significant variations in UDPGT activity when compared with the control. Interestingly, the withdrawal study revealed that the effect was reversible within 28 days of cessation of treatment, when enzyme activity reverted to levels close to those of controls. The study revealed that RPR-II affected the GSH- and GST-dependent detoxification system of the treated tissues of rat and its potential to modulate the enzymes is in the order kidneys>lung>>brain. The present subacute study suggests that RPR-II may bring about physiological upsets by altering GSH- and GST-dependent events in different tissues of exposed organisms.

Histone 2A stimulates glucose-6-phosphatase activity by permeabilization of liver microsomes

Histone 2A increases glucose-6-phosphatase activity in liver microsomes. The effect has been attributed either to the conformational change of the enzyme, or to the permeabilization of microsomal membrane that allows the free access of substrate to the intraluminal glucose-6-phosphatase catalytic site. The aim of the present study was the critical reinvestigation of the mechanism of action of histone 2A. It has been found that the dose-effect curve of histone 2A is different from that of detergents and resembles that of the pore-forming alamethicin. Inhibitory effects of EGTA on glucose-6-phosphatase activity previously reported in histone 2A-treated microsomes have been also found in alamethicin-permeabilized vesicles. The effect of EGTA cannot therefore simply be an antagonization of the effect of histone 2A. Histone 2A stimulates the activity of another latent microsomal enzyme, UDP-glucuronosyltransferase, which has an intraluminal catalytic site. Finally, histone 2A renders microsomal vesicles permeable to non-permeant compounds. Taken together, the results demonstrate that histone 2A stimulates glucose-6-phosphatase activity by permeabilizing the microsomal membrane.

Inhibition of hyaluronan synthesis in Streptococcus equi FM100 by 4-methylumbelliferone

As observed previously in cultured human skin fibroblasts, a decrease of hyaluronan production was also observed in group C Streptococcus equi FM100 cells treated with 4-methylumbelliferone (MU), although there was no effect on their growth. In this study, the inhibition mechanism of hyaluronan synthesis by MU was examined using Streptococcus equi FM100, as a model. When MU was added to a reaction mixture containing the two sugar nucleotide donors and a membrane-rich fraction as an enzyme source in a cell-free hyaluronan synthesis experiment, there was no change in the production of hyaluronan. On the contrary, when MU was added to the culture medium of FM100 cells, hyaluronan production in the isolated membranes was decreased in a dose-dependent manner. However, when the effect of MU on the expression level of hyaluronan synthase was examined, MU did not decrease either the mRNA level of the has operon containing the hyaluronan synthase gene or the protein level of hyaluronan synthase. Solubilization of the enzyme from membranes of MU-treated cells and addition of the exogenous phospholipid, cardiolipin, rescued hyaluronan synthase activity. In the mass spectrometric analysis of the membrane phospholipids from FM100 cells treated with MU, changes were observed in the distribution of only cardiolipin species but not of the other major phospholipid, PtdGro. These results suggest that MU treatment may cause a decrease in hyaluronan synthase activity by altering the lipid environment of membranes, especially the distribution of different cardiolipin species, surrounding hyaluronan synthase.

Accelerated degradation of mislocalized UDP-glucuronosyltransferase family 1 (UGT1) proteins in Gunn rat hepatocytes

Gunn rat is a hyperbilirubinemic rat strain that is inherently deficient in the activity of UDP-glucuronosyltransferase form 1A1 (UGT1A1). A premature termination codon is predicted to produce truncated UGT1 proteins that lack the COOH-terminal 116 amino acids in Gunn rat. Pulse-chase experiments using primary cell cultures showed that the truncated UGT1A1 protein in Gunn rat hepatocytes was synthesized similarly to wild-type UGT1A1 protein in normal Wistar rat hepatocytes. However, the truncated UGT1A1 protein was degraded rapidly with a half-life of about 50 min, whereas the wild-type UGT1A1 protein had a much longer half-life of about 10 h. The rapid degradation of truncated UGT1A1 protein was inhibited partially but not completely by treating Gunn rat hepatocytes with proteasome inhibitors such as carbobenzoxy-Leu-Leu-leucinal and lactacystin. By contrast, neither the lysosomal cysteine protease inhibitor nor the calpain inhibitor slowed the degradation. Our findings show that the absence of UGT1 protein from Gunn rat hepatocytes is due to rapid degradation of the truncated UGT1 protein by the proteasome and elucidate the molecular basis underlying the deficiency in bilirubin glucuronidation.

UDP-glucuronic acid:soyasapogenol glucuronosyltransferase involved in saponin biosynthesis in germinating soybean seeds

We detected UDP-glucuronic acid:soyasapogenol glucuronosyltransferase (UGASGT) activity in the microsomal fraction from germinating soybean (Glycine max [L.] Merr.) seed. A microsomal fraction was isolated from germinating soybean seed and treated with various detergents to solubilize the enzyme. UGASGT activity was monitored throughout purification using UDP-[U-(14)C]glucuronic acid and soyasapogenol B as substrates. Purification of UGASGT was achieved by HiTrap Q, Superdex 200, and HiTrap Blue chromatography procedures. This resulted in >205-fold enrichment relative to the starting homogenate. UGASGT was found to require divalent cations for activity. Studies on the substrate specificity of UGASGT demonstrated that the specificity for the sugar residue transferred was very high, as activity was scarcely found when UDP-glucuronic acid was replaced by other UDP sugars: UDP-glucose and UDP-galactose. Soyasapogenols, which are the aglycons of soybean saponin, are usable acceptors, but glycyrrhetinic acid, sophoradiol, beta-amyrin, and flavonoids are not. These findings suggest that this UGASGT was a specific enzyme for UDP-glucuronic acid as a donor and soyasapogenols as acceptors, and that it was related to the biosynthesis of the sugar chain in soybean saponin. This study provides a basis for the molecular characterization of a key enzyme in saponin biosynthesis in soybean. The isolation of the gene may enable its use in the elucidation of the biosynthesis and physiological role of saponins in soybean.

Tissue-specific regulation of canine intestinal and hepatic phenol and morphine UDP-glucuronosyltransferases by beta-naphthoflavone in comparison with humans

UDP-glucuronosyltransferases (UGTs) are regulated in a species- and tissue-dependent manner by endogenous and environmental factors. The present study was undertaken to further our knowledge about regulation of UGTs in dogs, a species widely used in preclinical safety evaluation. beta-Naphthoflavone (BNF) was selected as a known aryl hydrocarbon receptor agonist and antioxidant-type inducer. The latter group of inducers is intensively investigated as dietary chemoprotectants against colon cancer. Dog UGTs were investigated in comparison with related human UGTs by examples, (i) expression of dog UGT1A6, the first sequenced dog phenol UGT, and (ii) morphine UGT activities, responsible for intestinal and hepatic first-pass metabolism of morphine. The following results were obtained: (i) dog UGT1A6 was found to be constitutively expressed in liver and marginally increased by BNF treatment. Expression was low in small intestine but ca. 6-fold higher in colon than for example in jejunum. Conjugation of 4-methylumbelliferone, one of the substrates of dog UGT1A6, was also enhanced 7-fold in colonic compared to jejunal microsomes. (ii) Compared to the corresponding human tissues, canine 3-O- and 6-O-morphine UGT activities were found to be >10-fold higher in dog liver and ca. 10-fold lower in small intestinal microsomes. Small intestinal morphine and 4-hydroxybiphenyl UGT activities appeared to be moderately (2- to 3-fold) induced by oral treatment with BNF. (iii) In contrast to dogs, morphine UGT activities were found to be similar in homogenates from human enterocytes and liver. The results suggest marked differences in tissue-specific regulation of canine vs. human hepatic and intestinal phenol or morphine UGTs.

Liver phase I and phase II enzymatic induction and genotoxic responses of beta-naphthoflavone water-exposed sea bass

Sea bass were exposed to 0 (control), 0.1, 0.3, 0.9, and 2.7 microM beta-naphthoflavone (BNF) for 0, 2, 8, and 16 h in order to assess the chronological and concentration relationships between BNF phase I and II biotransformation responses, such as liver cytochrome P450 (P450) content, ethoxyresorufin-O-deethylase (EROD), uridine diphosphate-glucuronosyl transferase (UDP-GT), and the genotoxic effects, measured either by erythrocytic micronuclei (EMN) or erythrocytic nuclear abnormalities (ENA) tests. Liver alanine aminotransferase (ALT) activity and liver somatic index (LSI) were also measured. A significant liver EROD activity was found at 8 h exposure, respectively, to 0.1, 0.3, 0.9, and 2.7 microM BNF. Maximal liver EROD activity increase was observed at 16 h exposure to 0.9 microM BNF, whereas the highest liver P450 was reached at 8 h exposure to 2.7 microM BNF. Liver UDP-GT activity was significantly increased at 2 h exposure to 0.1 and 0.3 microM BNF and at 8 h exposure to 0.1, 0.3, and 0.9 microM BNF, decreasing at 16 h, for every exposure concentration. Significant ENA increase was observed at 2h exposure, respectively, to 0.3, 0.9, and 2.7 microM BNF. Maximal ENA increase was observed at 16 h exposure to 0.9 microM BNF. The MN was significantly increased at 8 and 16 h exposure, respectively, to 2.7 and 0.9 microM BNF. Liver ALT activity significantly increases at 8 h exposure to 0.1 and 0.3 microM BNF, whereas liver somatic index was significantly increased from 2 to 16 h exposure for every BNF concentration. A slight liver EROD activity increase with a concomitant lack of liver UDP-GT activity is able to induce significant erythrocytic genotoxic effects. Liver UDP-GT high levels are important in sea bass BNF detoxification. However, high liver UDP-GT activity is not enough to prevent the BNF metabolite genotoxic effects on sea bass erythrocytes when liver EROD activity is induced at 2 and 8 h exposure to 0.3 and 0.9 microM BNF. The genotoxic effects measured as EMN and ENA suggest that the balance between the rates of liver BNF reactive and conjugated metabolites seems to be critical.

Hydroxylated polychlorinated biphenyls as inhibitors of the sulfation and glucuronidation of 3-hydroxy-benzo[a]pyrene

Polychlorinated biphenyls (PCBs) can be metabolized by cytochromes P450 to hydroxylated biotransformation products. In mammalian studies, some of the hydroxylated products have been shown to be strong inhibitors of steroid sulfotransferases. As a part of ongoing research into the bioavailability of environmental pollutants in catfish intestine, we investigated the effects of a series of hydroxylated PCBs (OH-PCBs) on two conjugating enzymes, phenol-type sulfotransferase and glucuronosyltransferase. We incubated cytosolic and microsomal samples prepared from intestinal mucosa with 3-hydroxy-benzo[a]pyrene and appropriate cosubstrates and measured the effect of OH-PCBs on the formation of BaP-3-glucuronide and BaP-3-sulfate. We used PCBs with 4, 5, and 6 chlorine substitutions and the phenolic group in the ortho, meta, and para positions. OH-PCBs with the phenolic group in the ortho position were weak inhibitors of sulfotransferase; the median inhibitory concentration (IC50) ranged from 330 to 526 microM. When the phenol group was in the meta or para position, the IC50 was much lower (17.8-44.3 microM). The OH-PCBs were more potent inhibitors of glucuronosyltransferase, with IC50s ranging from 1.2 to 36.4 microM. The position of the phenolic group was not related to the inhibitory potency: the two weakest inhibitors of sulfotransferase, with the phenolic group in the ortho position, were 100 times more potent as inhibitors of glucuronosyltransferase. Inhibition of glucuronosyltransferase by low concentrations of OH-PCBs has not been reported before and may have important consequences for the bioavailability, bioaccumulation, and toxicity of other phenolic environmental contaminants.

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).

[Effect of soybean isoflavones on antioxidant status in rats fed diet with oxidized linseed oil]

The maintenance of male Wistar rats on semi-synthetic diets containing oxidized flaxseed oil for 4 weeks caused decrease in serum total antioxidant activity, significant suppression of microsomal UDP-glucuronosyl transferase activity and benzpyrene hydroxylase activity, increase in reduced glutathione level and catalase activity of liver. Supplementation of diets with 0.05% of soy isoflavones has resulted in normalization of investigated characteristics.

Alterations in hepatic detoxifying enzymes induced by new organophosphorus insecticides following subchronic exposure in rats

The present study examined the structure-toxicity relationship of two novel phosphorothionates, a methyl ester (RPR-II) and an ethyl ester (RPR-V), with regard to their interaction with certain biochemical indices in rat. Male rats were treated orally with 10% (low), 20% (medium) and 30% (high) doses of the two compounds (14, 28 and 42 microg kg(-1) day (-1) of RPR-II and 33, 66 and 99 microg kg(-1) day(-1) of RPR-V) daily for 90 days. The activity of hepatic glutathione S-transferase (GST) and UDP-glucuronyl transferase (UDPGT) and the level of glutathione (GSH) were estimated at 0, 45 and 90 days of treatment as well as 28 days after cessation of treatment. RPR-II caused statistically significant depletion of GSH after 45 and 90 days of treatment at the high dose, whereas RPR-V depleted GSH only after 90 days at the high dose. RPR-II inhibited GST after 45 and 90 days at medium and high doses, whereas RPR-V caused inhibition of GST after 45 and 90 days only at the high dose. Significantly, UDPGT activity was increased only by the high dose of RPR-II after 90 days. However, a dose and time-dependent increase in UDPGT activity was observed at all three doses of RPR-V after 45 and 90 days. There was no modulation in any of the three indices at low doses of the two organophosphorus insecticides. The withdrawal study revealed that induced changes in hepatic parameters were reversible 28 days after cessation of treatment. The results indicated that the two insecticides had different potential to modulate hepatic GST, UDPGT and GSH due to subchronic exposure and that these metabolic alterations are quite reversible after withdrawal of treatment.

Molecular basis of perinatal changes in UDP-glucuronosyltransferase activity in maternal rat liver

The molecular basis of perinatal changes occurring in major UDP-glucuronosyltransferase (UGT) family 1 isoforms and in UGT2B1, a relevant isoform belonging to family 2, was analyzed in rat liver. Nonpregnant, pregnant (19-20 days of pregnancy), and two groups of postpartum animals corresponding to early and middle stages of lactation (2-4 and 10-12 days after delivery, respectively) were studied. UGT activity determined in UDP-N-acetylglucosamine-activated microsomes revealed that bilirubin, p-nitrophenol, and ethynylestradiol (17beta-OH and 3-OH) but not androsterone and estrone glucuronidation rates, were decreased in pregnant rats. Decreased enzyme activities returned to control values after delivery. p-Nitrophenol, androsterone, and estrone conjugation rate increased in postpartum rats. Western blot analysis performed with anti-peptide-specific (anti-1A1, 1A5, 1A6, and 2B1) antibodies revealed decreased levels of all family 1 isoforms and UGT2B1 during pregnancy. In postpartum animals, protein level recovered (1A5 and 2B1) or even increased (1A1 and 1A6) with respect to control rats. Northern blot analysis suggested that expression of UGT proteins is down-regulated at a post-translational level during pregnancy and that increased levels of 1A1 and 1A6 observed in postpartum rats were associated to increased mRNA. To establish whether prolactin is involved in up-regulation of UGT1A1 and 1A6 postpartum, ovariectomized rats were treated with 300 microg of ovine prolactin per day for 7 days. The data indicated that prolactin was able to increase expression of UGT1A6 (protein and mRNA) but not 1A1. Thus, prolactin is the likely mediator of the increased expression of UGT1A6 observed in maternal liver postpartum.

The phenobarbital response enhancer module in the human bilirubin UDP-glucuronosyltransferase UGT1A1 gene and regulation by the nuclear receptor CAR

The UDP-glucuronosyltransferase, UGT1A1, is the critical enzyme responsible for detoxification of the potentially neurotoxic bilirubin by conjugating it with glucuronic acid. For decades, phenobarbital (PB) treatment for hyperbilirubinemia has been known to increase expression of the UGT1A1 gene in liver. We have now delineated the PB response activity to a 290-bp distal enhancer sequence (-3483/-3194) of the UGT1A1 gene. The enhancer contains 3 putative nuclear receptor motifs, and it was activated by the nuclear orphan receptor, human constitutive active receptor (hCAR), in cotransfected HepG2 cells. Bacterially expressed hCAR, acting as a heterodimer with in vitro-translated retinoid X receptor (RXRalpha), only bound to 1 of the 3 NR motifs, named gtNR1 in a gel-shift assay. Consistently, mutations of the gtNR1 site significantly decreased the activation by hCAR of the 290-bp DNA in transfection assays. Moreover, the 290-bp DNA was effectively activated in mouse primary hepatocytes in response to PB, offering an excellent clinical test for the examination of the responsiveness of the UGT1A1 to PB in the human population, particularly individuals with hyperbilirubinemia.