Induction of group-1 and group-2 UDP-glucuronosyltransferase in microsomes from the livers of C57 Bl/6 mice

Effects of cytochrome P450 inducers on tamoxifen genotoxicity in female mice in vivo

We recently reported that administration of the antiestrogen tamoxifen (TAM) gives rise to two groups of DNA adducts in female mouse liver in vivo, as measured by 32P-postlabeling, and provided evidence that 4-hydroxytamoxifen and alpha-hydroxytamoxifen are proximate carcinogenic metabolites leading to group I and group II adducts, respectively (Randerath et al., Carcinogenesis 15: 2087-2094, 1994). Because cytochrome P450 (CYP) enzymes play an important role in TAM metabolism, in this investigation we tested the hypothesis that induction of liver CYP enzymes may affect TAM metabolism profoundly, resulting in increased or decreased TAM-DNA adduct formation in vivo. To this end, we treated female ICR mice with TAM either alone or in combination with one of several classic CYP inducers, i.e. phenobarbital (PB), beta-naphthoflavone (BNF), and pregnenolone-16 alpha-carbonitrile (PCN), and determined the levels of 32P-postlabeled TAM-DNA adducts and the activities of several CYP-dependent enzymes. Each of the inducers greatly diminished levels of group II, but did not affect group I adducts. TAM elicited induction of benzphetamine N-demethylase activity in liver, while activities of other enzymes were not affected. TAM, when given in combination with BNF, elicited a synergistic induction of ethoxyresorufin O-deethylase (EROD) (CYP1A1) and methoxyresorufin O-demethylase (MROD) (CYP1A2) activities. Likewise, PCN given along with TAM caused synergistic induction of EROD and ethylmorphine N-demethylase activities. There was no synergism between PB and TAM, however. Overall, the results further support the existence of two pathways of TAM metabolism to DNA-reactive electrophiles and strongly suggest that the classic CYP inducers tested enhance detoxication of TAM to non-genotoxic metabolites.

Effects of indole-3-carbinol on biotransformation enzymes in the rat: in vivo changes in liver and small intestinal mucosa in comparison with primary hepatocyte cultures

Groups of male Wistar rats were fed semi-synthetic diets containing 0, 200 or 500 mg indole-3-carbinol (13C)/kg for 2, 7, 14 or 28 days. After 2 days, P-450 activities were already induced, but the isoenzyme pattern induced was different in the liver and the small intestine. Hepatic P4501A1, P4501A2 and P4502B1 apoprotein levels were dose-relatedly enhanced, whereas in the small intestine induced levels of P4502B1 and P4501A1 were detected but P4501A2 was not induced. Pentoxy- and ethoxyresorufin dealkylation (PROD and EROD) were dose-relatedly enhanced in the liver (5- and 7-fold, respectively, in the higher dose group) as well as in the small intestine (8- and 13-fold, respectively, at 500 mg 13C/kg diet). Testosterone 16 alpha- and 16 beta-hydroxylation in the small intestine were enhanced (6-9-fold) from day 2 onwards, but in the liver these activities were only slightly enhanced from day 7 onwards. Thus, the major forms induced in the liver appear to be P4501A1, P4501A2, P4502B1 and, to a lesser extent, P4503A, whereas in the small intestine all of the effects that were found are associated with only one cytochrome P-450, P4502B1. After 2 days I3C (500 mg/kg) induced glutathione S-transferase in the liver (1.3-fold) and small intestine (1.5-fold). Hepatic glucuronyl transferase (GT1) was induced (about 1.6-fold) after 7, 14 and 28 days. DT-diaphorase was induced in the liver (2.7-fold) and small intestine (1.5-fold) after 14 days of exposure to 500 mg I3C/kg diet. Treatment of rat hepatocytes with indole-3-acetonitrile and 3,3'-diindolylmethane, but not I3C and indole-3-carboxaldehyde, enhanced EROD activity and halved testosterone 16 alpha- and 2 alpha-hydroxylation. All four indoles slightly induced glutathione S-transferase in cultured hepatocytes. Thus, the in vitro studies suggest that the in vivo effects of I3C have to be attributed to indole-condensation products, such as 3,3'-diindolylmethane, but not to I3C itself.

Effects of cooked brussels sprouts on cytochrome P-450 profile and phase II enzymes in liver and small intestinal mucosa of the rat

Male Wistar rats were given semi-synthetic diets supplemented with 0, 2.5, 5 and 20% cooked Brussels sprouts for 2, 7, 14 or 28 days. The effects on several cytochrome P-450 enzymes and phase II enzymes (glutathione S-transferase (GST), glucuronyl transferases 1 and 2 (GT1 and GT2) and DT-diaphorase (DTD)) in the liver and small intestinal mucosa were investigated. From 2 days of exposure onwards Brussels sprouts induced P4501A2 and--to a lesser extent--P4501A1 apoprotein levels in the liver, whereas in the small intestine markedly enhanced P4502B apoprotein levels could be detected. No enhanced P4503A apoprotein levels were observed. The 5 and 20% sprouts diets increased the intestinal pentoxyresorufin depentylation (PROD, 4.5-9-fold), and the hydroxylation of testosterone at the 16 alpha- and 16 beta-site (2.6-4.2-fold) after 2 days of exposure. In addition, the 20% sprouts died also enhanced the intestinal ethoxyresorufin deethylation (EROD) activity (c. 5-fold), the hepatic EROD and PROD activities (c. 2-fold) and the formation of 6 beta-hydroxytestosterone (c. 1.6-fold); the formation of 2 alpha-hydroxytestosterone in the liver was decreased (to c. 70% of the control value). GST activity was induced both in the liver (5 and 20% diet) and intestine (20% diet only) throughout the experiment. The 20% sprouts diet enhanced the hepatic DTD and GT1 activities, whereas the GT2 activity was decreased. The induction of DTD in the small intestine after 2 days (2.5-3.2-fold with 5 and 20% sprouts diets, respectively) diminished during the experiment. These results indicate that dietary exposure to cooked Brussels sprouts for only 2 days can change the metabolic activities of several phase II enzymes and cytochrome P-450 enzymes, of which P4502B is the predominant form induced in the small intestine.

Hepatic UDP-glucuronyltransferase(s) activity toward thyroid hormones in rats: induction and effects on serum thyroid hormone levels following treatment with various enzyme inducers

Induction of hepatic UDP-glucuronyltransferase(s) (hUDP-GT(s] activity toward thyroid hormones and the relationship between the activity and the serum thyroid hormones or the thyroid stimulating hormone (TSH) level were examined in male Sprague-Dawley rats after four consecutive ip doses of various hepatic enzyme inducers at 75-150 mg/kg/day. hUDP-GT activity toward thyroxine (T4; hUDP-GT-T4) was induced by treatment with beta-naphthoflavone, 3-methylcholanthrene (3-MC), polychlorinated biphenyls, or pregnenolone-16 alpha-carbonitrile. However, no significant induction was observed for isosafrole administration and in the cases of phenobarbital and 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane slight decreases were found. The induction profile of hUDP-GT-T4 for these inducers was approximately the same as that of hUDP-GT activity toward triiodothyronine (T3; hUDP-GT-T3), indicating that these two thyroid hormones (T4 and T3) are glucuronidated by the same hUDP-GT(s). Moreover, the induction profile of both hUDP-GT-T4 and hUDP-GT-T3 was similar to that of hUDP-GT toward 1-naphthol, but not chloramphenicol, suggesting that T4 and T3 belong to the so-called group-1 substrates which are preferentially glucuronidated by hUDP-GT(s) inducible by treatment with 3-MC. Decreases in serum T4 levels clearly correlated with an increase in hUDP-GT-T4 activity, indicating that serum T4 levels are directly affected by hUDP-GT-T4 activity. However, no direct correlation between decrease in thyroid hormone levels and compensatory increase in TSH levels was found.

Heterogeneity of hepatic microsomal UDP-glucuronosyltransferases activities: use and comparison of differential inductions in some mammalian species

1. The co-injection in rats of the inducers 3-methylcholanthrene or phenobarbital and of a protein synthesis inhibitor (cycloheximide) shows that two clusters of hepatic UDP-Glucuronosyltransferases (GT1 and GT2) are under separate genomic expression and differentially regulated. 2. The administration of cycloheximide alone even suggests a distinct turn-over for these two groups of isoenzymes. 3. Indirect evidence for a UDPGT isoform specialized for some structurally-related exogenous substrates, the monoterpenoid alcohols, is brought. Their conjugation exhibits a small deficiency and a marked response to phenobarbital treatment in the Gunn rat and an exclusive inducibility by phenobarbital in the guinea-pig.

Heterogeneity of hepatic microsomal UDP-glucuronosyltransferase activities. Conjugations of phenolic and monoterpenoid aglycones in control and induced rats and guinea pigs

In this report we present evidence that the heterogeneity of hepatic microsomal UDP-glucuronosyltransferase(s) (UDPGT) activities depends on the chemical structures of the aglycones as well as their biophysical constants. Three animal models were used: Wistar rats, which have active UDPGTs; Gunn rats, in which some of the UDPGT activities are reduced, but which can be induced by phenobarbital; and guinea pigs. In Wistar rats, we found that some coumarins were poor substrates of UDPGT (GT1) and that twenty monoterpenoid alcohol activities showed typical phenobarbital-inducible behavior. In Gunn rats, we showed that substitution of the phenolic aglycone by bulky (alkyl- or methoxy-) groups in the 2-position of the phenolic ring decreased UDPGT (GT1) activity, whereas substitution in the 4-position resulted in an increase in this activity. We also showed that, in this particular strain, activities toward terpenes were less affected than activities toward flat (aromatic) aglycones. Induction by phenobarbital in Gunn rats increased the activity and limited the deficiency for monoterpenoid alcohols. In guinea pigs, we confirmed that phenobarbital selectively increased the activities of UDPGT towards twenty monoterpenoid alcohols without affecting other typical phenobarbital-induced activities such as those for conjugation of morphine. Finally, we showed that orientation of the aglycone molecule in the active site was apparently related to its dipole moment and that the distance between "acceptor-oxygen" (hydroxyl) and the carbons out of the general plane of the molecule was an important factor. These studies clearly suggest that rat and guinea pig contain a UDPGT(monoterpenoid alcohols) with restricted specificities and also that UDPGT(GT1) comprises at least two or three different isoenzymes, each with a slightly different restricted specificity towards flat aromatic aglycones.

Separation of different UDP glucuronosyltransferase activities according to charge heterogeneity by chromatofocusing using mouse liver microsomes. Three major types of aglycones

Hepatic UDP glucuronosyltransferase (EC 2.4.1.17) (GT) enzymes in control, phenobarbital- and 3-methylcholanthrene-induced microsomes from C57BL/6N mice have been fractionated according to charge heterogeneity on a chromatofocusing system using a pH 9.5 to 6 gradient. Transferase activities for eleven different substrates were determined on column fractions. Activities toward 3-hydroxybenzo[a]pyrene, phenolphthalein and estrone (type 1 substrates) were enhanced by both effector compounds and always eluted primarily at pH 8.5. In control and phenobarbital-induced microsomes, activities toward testosterone, 4-hydroxybiphenyl, morphine, naphthol and 9-hydroxybenzo[a]pyrene (type 2 substrates) eluted primarily at about pH 6.7. Activities toward p-nitrophenol, 4-methylumbelliferone and 2-hydroxybiphenyl (type 3 substrates) in control and phenobarbital-induced microsomes exhibited two peaks which eluted at pH 8.5 and 6.7. 3-Methylcholanthrene treatment increased almost exclusively activities which eluted at pH 8.5 for each of the three types of substrates. The pH value of elution corresponds to the approximate isoelectric point of the eluted protein. Immunoabsorption studies with an antibody preparation raised against a purified low pI form confirmed that a 51,000-dalton transferase form, GTM1, eluted primarily at pH 6.7 and that a 54,000-dalton form, GTM2, eluted at pH 8.5. A mathematical treatment of the ratios of activity after 3-methylcholanthrene treatment to that after phenobarbital treatment versus pH produced six patterns of activity. A minimum of two enzymes at the low pH region and one enzyme at the high pH region, all with broad-substrate specificity, could account for these patterns.

Glucuronidation of carcinogenic arylamine metabolites by rat liver microsomes

Since 2-acetylaminofluorene (2-AAF), 4-acetylaminobiphenyl (4-AABP) and 2-aminonaphthalene (2-AN) display varying degrees of carcinogenicity in the rat, which is capable of N-acetylating arylamines, an attempt was made to correlate the difference in carcinogenicity of these compounds with the ease of O-glucuronidation of their hydroxamic acids by rat hepatic microsomes, a reaction believed to be a detoxification mechanism. UDP-glucuronosyltransferase activity of rat hepatic microsomes was activated by Triton X-100. Glucuronidation by Triton X-100 activated microsomes of the N-hydroxy derivative of 2-AN was approximately 1.5 and 1.8 times faster than the corresponding derivatives of 2-aminofluorene (2-AF) and 4-aminobiphenyl (4-ABP) respectively. However, glucuronidation of the N-hydroxy-N-acetyl derivative of 2-AN was 40 and 17 times faster than the corresponding derivatives of 2-AF and 4-ABP respectively. Aroclor 1254 and 3-methylcholanthrene, but not phenobarbital, acetanilide and butylated hydroxytoluene, induced the enzyme for the glucuronidation of 2-AN derivatives. The present study (1) demonstrates an inverse relationship between the carcinogenicity of 2-AN, 4-AABP and 2-AAF and the ease of glucuronidation of their hydroxamic acid derivatives, and (2) suggests that, in addition to N- and C-hydroxylation, glucuronidation may play an important role in determining the carcinogenicity of arylamines and arylacetamides in the rat.

Purification and immunochemical characterization of a low-pI form of UDP glucuronosyltransferase from mouse liver

A liver UDP glucuronosyltransferase (GT) enzyme from either phenobarbital- or 3-methylcholanthrene-treated C57BL/6N mice was isolated by phenyl-Sepharose, DEAE-ion exchange, and UDP hexanolamine chromatographic steps. This enzyme had a broad substrate specificity and was mainly responsible for the microsomal capacity to glucuronidate testosterone, 1-naphthol, and morphine. This UDP glucuronosyltransferase ( GTM1 ) appeared to be at least 95% homogeneous and had a subunit molecular weight of 51,000 using sodium dodecyl sulfate-polyacrylamide gel and two-dimensional gel electrophoreses. Antibodies prepared against the purified protein developed a single immunoprecipitin line by double-diffusion analysis with purified antigen and with solubilized microsomes from both control and drug-induced C57BL/6N and DBA/2N mice. A precipitin line was also observed with microsomal proteins which isoelectrofocused at approximately pH 6.7, but not with those which isoelectrofocused at approximately pH 8.5. GTM1 was, therefore, designated at low-pI form. Immunopurified antibody preferentially inhibited and immunoprecipitated GT activities toward testosterone, 1-naphthol, and morphine. To a lesser extent, activities toward phenolphthalein, 3-hydroxybenzo[a]pyrene, and estrone were inhibited while activities toward 4-nitrophenol and 4-methylumbelliferone were not affected. All activities, however, were immunoadsorbed in the presence of protein A-Sepharose. This observation can be explained by the following results. Immunoprecipitates from labeled microsomes contained primarily a 51,000-Da protein. When the immune complexes were adsorbed with protein A-Sepharose, a 54,000-Da protein as well as the expected 51,000-Da GTM1 was detected. This 54,000-Da protein was associated with the glucuronidation of 3-hydroxybenzo[a]pyrene and 4-nitrophenol, and was designated GTM2 .

Assessment of the Mulder and Van Doorn kinetic procedure and rapid centrifugal analysis of UDP-glucuronosyltransferase activities

The optimal experimental conditions of the enzyme assay described by Mulder and Van Doorn (1975, Biochem J. 151, 131-140) for the measurement of UDP-glucuronosyltransferase activities were tested towards structurally different aglycones. This assessment of this assay revealed that addition of Triton X-100 as enzyme activator was necessary because of its apparent inhibitory effects on interfering reactions. Under these conditions, accordance of the data with results published in the literature was obtained. We present for the first time an UDP-glucuronosyltransferase assay adapted on a fast analyser centrifuge which allows a rapid and sensitive measurement of enzyme activity that is very useful for kinetic constant determination, without consuming a large volume of reagents.

Studies of UDP-glucuronosyltransferase activity toward eugenol, using a gas chromatographic method of measurement

A method for the assay of uridine diphosphate (UDP)-glucuronosyltransferase activities toward some phenolic compounds and monoterpenoid alcohols is described. The method is based on the disappearance of the free substrate after incubation with microsomes and UDP-glucuronate. This disappearance is recorded using a gas chromatographic process. This method has been used, for example, to characterize the glucuronidation process of eugenol (4-allyl-2-methoxyphenol). The method could be extended to other substrates. Analytical conditions are given for some of them, especially monoterpenoid alcohols since the studies of their conjugations are a growing field of interest in evaluation of heterogeneity of UDP-glucuronosyltransferase. The method could also be used with other biological materials including cell suspension and crude liver biopsies.