Effects of microsomal enzyme inducers upon UDP-glucuronic acid concentration and UDP-glucuronosyltransferase activity in the rat intestine and liver

Induction of mouse UDP-glucuronosyltransferase mRNA expression in liver and intestine by activators of aryl-hydrocarbon receptor, constitutive androstane receptor, pregnane X receptor, peroxisome proliferator-activated receptor alpha, and nuclear factor erythroid 2-related factor 2

UDP-glucuronosyltransferases (UGTs) catalyze the addition of UDP-glucuronic acid to endo- and xenobiotics, enhancing their water solubility and elimination. Many exogenous compounds, such as microsomal enzyme inducers (MEIs), alter gene expression through xenobiotic-responsive transcription factors, namely, the aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), pregnane X receptor (PXR), peroxisome proliferator-activated receptor alpha (PPARalpha), and nuclear factor erythroid 2-related factor 2 (Nrf2). These transcription factors regulate xenobiotic-inducible expression of hepatic and intestinal biotransformation enzymes and transporters. The purpose of this study was to determine hepatic and intestinal inducibility of mouse Ugt mRNA by MEIs. Male C57BL/6 mice were treated for four consecutive days with activators of AhR [2,3,7,8-tetrachlorodibenzodioxin (TCDD), polychlorinated biphenyl 126, and beta-naphthoflavone], CAR [1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP), phenobarbital, and diallyl sulfide], PXR [pregnenolone-16alpha-carbonitrile (PCN), spironolactone, and dexamethasone], PPARalpha (clofibrate, ciprofibrate, and diethylhexylphthalate), and Nrf2 (oltipraz, ethoxyquin, and butylated hydroxyanisole), respectively. Ugt1a1 mRNA expression in liver was induced by activators of all five transcription factor pathways, Ugt1a5 by Nrf2 activators, Ugt1a6 by all the pathways except CAR, and Ugt1a9 by all the pathways except Nrf2. Ugt2b35 mRNA in liver was induced by AhR activators and Ugt2b36 by CAR and PPARalpha activators. Throughout the small and large intestine, the AhR ligand TCDD increased Ugt1a6 and Ugt1a7 mRNA. In small intestine, the PXR activator PCN increased Ugt1a1, Ugt1a6, Ugt1a7, Ugt2b34, and Ugt2b35 mRNA in the duodenum. In conclusion, chemical activation of AhR, CAR, PXR, PPARalpha, and Nrf2 in mouse results in induction of distinct Ugt gene sets in liver and intestine, predominantly the Ugt1a isoforms.

Effects of Food Natural Products on the Biotransformation of PCBs

Many food products, particularly fruits and vegetables, contain natural products that affect biotransformation enzymes. These may be expected to affect the rate of biotransformation of PCBs that are metabolized by the affected enzymes. The first step in PCB metabolism is cytochrome P450-dependent monooxygenation. Natural products present in cruciferous vegetables have been shown to selectively up-regulate CYP1A1 and CYP1A2 isozymes on chronic ingestion, and may lead to increased metabolism of those PCB congeners that are substrates for the induced P450s. On the other hand, several natural products selectively inhibit monooxygenation, especially in the intestine, and may lead to increased bioavailability and reduced metabolism of dietary PCBs. Food natural products are known to affect phase II pathways important in the detoxication of hydroxylated PCBs, namely UDP-glucuronosyltransferase and PAPS-sulfotransferase. Continual dietary exposure to chrysin and quercetin, found in fruits and vegetables, induces UGT1A1 and may reduce exposure to hydroxylated PCBs through increased glucuronidation. These and other natural products are also inhibitors of glucuronidation and sulfonation, potentially leading to transient decreases in the elimination of hydroxylated PCBs. In summary, the expected effects of food natural products on PCB biotransformation are complex and may be biphasic, with initial inhibition followed by enhanced biotransformation through monooxygenation and conjugation pathways.

Glucuronidation of polychlorinated biphenylols and UDP-glucuronic acid concentrations in channel catfish liver and intestine

Polychlorinated biphenylols (OH-PCBs) are potentially toxic polychlorinated biphenyl metabolites that can be eliminated by glucuronidation, catalyzed by UDP-glucuronosyltransferases (UGTs). OH-PCBs with a 3,5-dichloro-4-hydroxy substitution pattern have been detected in blood from humans and wildlife, suggesting slow elimination. In this study we assessed the glucuronidation of 4-OH-PCBs with zero, one, or two chlorine atoms flanking the 4-hydroxyl group and zero to four chlorine atoms in the aphenolic ring in microsomes from channel catfish liver and proximal intestine. Product formation was quantitated with [(14)C]UDP-glucuronic acid (UDPGA). Physiological concentrations of UDPGA were measured in preparations of liver and intestine. When the OH-PCB concentrations were varied in the presence of saturating UDPGA concentrations, glucuronidation V(max) values were higher in hepatic than in intestinal microsomes (0.40-3.4 and 0.12-0.78 nmol/min/mg of protein, respectively), whereas the K(m) values were generally lower for intestine (0.042-0.47 mM) than for liver (0.11-1.64 mM). In both tissues V(max) values with 3,5-dichloro-4-OH-PCBs were lower than with the corresponding 3-chloro-4-OH-PCBs. Varying the UDPGA concentrations in the presence of saturating concentrations of OH-PCB showed that the K(m) for UDPGA was lower in intestine (27 microM) than in liver (690 microM). The measured concentration of UDPGA in catfish liver (246-377 nmol/g) was lower than the K(m) for UDPGA, suggesting that in vivo rates of glucuronidation may be suboptimal, whereas in intestine the measured UDPGA concentration (71-258 nmol/g) was higher than the K(m) for UDPGA. Although liver has a greater glucuronidation capacity than proximal intestine, the properties of intestinal UGTs in channel catfish enable them to efficiently glucuronidate low concentrations of OH-PCBs.

Induction of Rat UDP-Glucuronosyltransferases in Liver and Duodenum by Microsomal Enzyme Inducers That Activate Various Transcriptional Pathways

Microsomal enzyme inducers (MEIs) up-regulate phase I biotransformation enzymes, most notably cytochromes P450. Transcriptional up-regulation by MEIs occurs through at least three nuclear receptor mechanisms: constitutive androstane receptor (CAR; CYP2B inducers), pregnane X receptor (PXR; CYP3A inducers), and peroxisome proliferator-activated receptor alpha (PPARalpha; CYP4A inducers). Other mechanisms include transcription factors aryl hydrocarbon receptor (AhR; CYP1A inducers), and nuclear factor erythroid 2 (NF-E2)-related factor 2 (Nrf2; NADPH-quinone oxidoreductase inducers). UDP-glucuronosyltransferases (UGTs) are phase II biotransformation enzymes that are predominantly expressed in liver and intestine. MEIs increase UGT activity; however, transcriptional regulation of individual UGT isoforms is not completely understood. The purpose of this study was to examine inducibility of individual UGT isoforms and potential mechanisms of transcriptional regulation in rat liver and duodenum. UGT mRNA levels were assessed in liver and duodenum of rats treated with MEIs that activate various transcriptional pathways. All four CAR activators induced UGT2B1 in liver, but not duodenum. UGT1A1, 1A5, 1A6, and 2B12 were induced by at least two CAR activators in liver only. Two PXR ligands induced UGT1A2, but only in duodenum. Two PPARalpha ligands induced UGT1A1 and 1A3 in liver only. AhR ligands induced UGT1A6 and 1A7 in liver, but not duodenum. Nrf2 activators increased UGT2B3 and 2B12 in both liver and duodenum, and UGT1A6, 1A7, and 2B1 in liver only. In summary, only UGT1A2 and 1A8 were not inducible in liver by MEIs. MEIs differentially regulate hepatic expression of individual UGT isoforms, although no one transcriptional pathway dominated. In duodenum, MEIs had minimal effects on UGT expression.

Induction of genes for metabolism and transport by trans-stilbene oxide in livers of Sprague-Dawley and Wistar-Kyoto rats

trans-Stilbene oxide (TSO) is a synthetic proestrogen that induces phase I and II drug-metabolizing enzymes in rat liver. The purpose of this study was to determine whether TSO also induces transporter expression in rat liver and whether gene induction in rat liver after TSO occurs in a constitutive androstane receptor (CAR)-dependent manner. Total RNA was isolated from male rat livers after treatment with TSO for up to 4 days (200 mg/kg, i.p., twice daily), and the mRNA levels for each gene were quantified. CYP2B1/2, CYP3A1, epoxide hydrolase, heme oxygenase-1, UGT1A6, UGT2B1, multiple drug resistance protein (Mdr) 1a and 1b, as well as multidrug resistance-associated protein (Mrp) 2, 3, and 4 mRNA were increased in livers after TSO treatment. To determine whether TSO activates gene expression in a CAR-dependent manner, male and female Wistar-Kyoto (WKY) rats were treated with TSO for 3 days. TSO induced CYP2B1/2, UGT2B1, and Mdr1b in males more than in females, suggesting that TSO could increase their expression via CAR. Conversely, TSO induced CYP3A1, epoxide hydrolase, UGT1A6, and Mrp3 similarly in both genders, indicating that induction of these genes occurs independently of CAR. TSO treatment also increased the activity of a CAR binding element luciferase reporter construct in HepG2 cells transfected with rat CAR and in mouse liver. Additionally, TSO increased antioxidant response element/electrophile response element luciferase reporter construct activity in HepG2 cells. In conclusion, in WKY rat liver, TSO increases CYP2B1/2, UGT2B1, and Mdr1b mRNA expression in a gender-dependent manner and CYP3A1, epoxide hydrolase, UGT1A6, and Mrp3 in a gender-independent manner.

trans-Stilbene oxide induces expression of genes involved in metabolism and transport in mouse liver via CAR and Nrf2 transcription factors

trans-Stilbene oxide (TSO) induces drug metabolizing enzymes in rat and mouse liver. TSO is considered a phenobarbital-like compound because it induces Cyp2B mRNA expression in liver. Phenobarbital increases Cyp2B expression in liver via activation of the constitutive androstane receptor (CAR). The purpose of this study was to determine whether TSO induces gene expression in mouse liver via CAR activation. TSO increased CAR nuclear localization in mouse liver, activated the human Cyp2B6 promoter in liver in vivo, and activated a reporter plasmid that contains five nuclear receptor 1 (NR1) binding sites in HepG2 cells. TSO administration increased expression of Cyp2b10, NAD(P)H:quinone oxidoreductase (Nqo1), epoxide hydrolase, heme oxygenase-1, UDP-glucuronosyl-transferase (Ugt) 1a6 and 2b5, and multidrug resistance-associated proteins (Mrp) 2 and 3 mRNA in livers from male mice. Cyp2b10 and epoxide hydrolase induction by TSO was decreased in livers from CAR-null mice, compared with wild-type mice, suggesting CAR involvement. In contrast, TSO administration induced Nqo1 and Mrp3 mRNA expression equally in livers from wild-type and CAR-null mice, suggesting that TSO induces expression of some genes through a mechanism independent of CAR. TSO increased nuclear staining of the transcription factor Nrf2 in liver, and activated an antioxidant/electrophile response element luciferase reporter construct that was transfected into HepG2 cells. In summary, in mice, TSO increases Cyp2b10 and epoxide hydrolase expression in mice via CAR, and potentially induces Nqo1 and Mrp3 expression via Nrf2. Moreover, our data demonstrate that a single compound can activate both CAR and Nrf2 transcription factors in liver.

Kinetics of genistein and its conjugated metabolites in pregnant Sprague-Dawley rats following single and repeated genistein administration

Diets high in soy-based products are well known for their estrogenic activity. Genistein, the predominant phytoestrogen present in soy, is known to interact with estrogen receptors (ER) alpha and beta and elicits reproductive effects in developing rodents. In the rat, genistein is metabolized predominantly to glucuronide and sulfate conjugates, neither of which is capable of activating ER. Therefore, it is critical to understand the delivery of free and conjugated genistein across the placenta to the fetus following maternal genistein exposure such that the potential fetal exposure to free genistein can be assessed. Genistein (4 or 40 mg/kg) was administered to pregnant Sprague-Dawley rats by oral gavage daily from gestation day (GD) 5 through 19 or on GD 19 alone. Maternal and GD 19 fetal tissues were collected 0.5, 1, 2, 4, 6, 8, 12, and 24 h following administration of the final dose on GD 19. Concentrations of genistein, genistein glucuronide, and genistein sulfate were quantitated by LC-MS/MS. In maternal plasma, genistein glucuronide was the predominant metabolite. In the fetal plasma, genistein glucuronide and genistein sulfate were the primary metabolites. Genistein levels in maternal and fetal plasma were much lower than its conjugates. The concentration of genistein in placental tissue was higher than either conjugate. Fetal concentrations of unconjugated genistein following administration of 40 mg/kg were above the EC50 for ERbeta activation. Repeated administration of 40 mg/kg genistein resulted in minor changes in genistein kinetics in the pregnant rat compared to single administration of the same dose. These data suggest that conjugated forms of genistein are not transported across the placenta. High placental concentrations of genistein indicate the placenta is a potential target organ for genistein action during gestation.

Renal damage, metabolism and covalent binding following administration of the nephrotoxicant N-(3,5-dichlorophenyl)succinimide (NDPS) to male Fischer 344 rats

In vivo metabolism, nephrotoxicity and covalent binding to proteins were evaluated in male Fischer 344 rats that received [2,3-14C]-N-(3,5-dichlorophenyl)succinimide (14C-NDPS). Some animals were pretreated with the enzyme inducer phenobarbital (PB, 80 mg/kg per day, for 3 days, i.p. in saline) prior to receiving a non-nephrotoxic dose of 14C-NDPS (0.2 mmol/kg, i.p. in corn oil). Other rats were pretreated with the cytochrome P450 inhibitor 1-aminobenzotriazole (ABT, 100 mg/kg, 1 h prior to NDPS, i.p. in saline) before administration of a non-toxic or a toxic dose (0.2 or 0.6 mmol/kg, respectively, i.p. in corn oil) of 14C-NDPS. Non-pretreated animals received either dose of 14C-NDPS, but did not receive PB or ABT. All rats were sacrificed 6 h after administration of 14C-NDPS. Nephrotoxicity was monitored by measuring urine volume, urine protein concentrations, blood urea nitrogen levels, and kidney weights. The NDPS metabolic profile in tissue, blood, and urine was analyzed by HPLC. Covalent binding of 14C-NDPS-derived radioactivity to tissue proteins was also measured. Compared with non-pretreated rats, PB-pretreatment potentiated the toxicity of the non-toxic dose of 14C-NDPS. In contrast, ABT-pretreatment protected the rats against NDPS nephrotoxicity. The amount of N-(3,5-dichlorophenyl)-2-hydroxysuccinamic acid (2-NDHSA), an oxidative, nephrotoxic metabolite of NDPS, was elevated in kidney homogenates and urine by PB-pretreatment (0.2 mmol/mg NDPS). ABT pretreatment inhibited NDPS metabolism at both doses. Covalent binding of 14C-NDPS (0.2 mmol/kg)-derived radioactivity to renal and plasma proteins was higher in the PB-pretreated rats than in the non-pretreated animals. In contrast, ABT-pretreatment partially inhibited covalent binding at both doses of 14C-NDPS. Our results suggest that there is a relationship between oxidative metabolism of NDPS, covalent binding of an NDPS metabolite to renal proteins, and NDPS-induced nephrotoxicity in rats.

DNA adducts of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) in fetal tissues of patas monkeys after transplacental exposure

Transplacental genotoxicity of the heterocyclic amine food-derived mutagen/carcinogen 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) has been investigated by (32)P-postlabeling assay for IQ-DNA adducts in maternal liver, placenta, and several fetal tissues of patas monkeys, after exposure to 15, 35, or 50 mg/kg IQ near the end of gestation or to the highest dose in the first or second trimester. Dose-dependent adduct formation occurred in all tissues, with the highest levels occurring in maternal liver. Adduct amounts were similar among fetal tissues and placenta, except for lower levels in fetal brain and slightly more adducts in fetal liver. Adducts in placenta, fetal liver, lung, kidney, skin, and adrenal gland, but not in maternal liver or fetal brain, increased significantly as gestation progressed. Pretreatment with phenobarbital, which induces CYP enzymes that detoxify IQ, decreased adducts in maternal liver and possibly placenta, but not in fetal tissues. The CYP inducer beta-naphthoflavone caused a significant increase in IQ-DNA adducts in fetal lungs. Regression analysis suggested that IQ activation in maternal and fetal liver and possibly placenta contributed to adduct formation in fetal tissues; adducts in placenta and/or fetal liver were strong predictors for those in most fetal tissues. The results indicate that exposure of pregnant primates to IQ results in DNA adduct formation in most fetal tissues, especially late in gestation; that upregulation of maternal detoxification does not provide fetal protection; and that adducts in placenta indicate adduct levels in fetal tissues.

Distribution and inducibility by 3-methylcholanthrene of family 1 UDP-glucuronosyltransferases in the rat gastrointestinal tract

UDP-Glucuronosyltransferases (UGT) catalyze the glucuronidation of a broad spectrum of endobiotic and xenobiotic substrates. The resulting glucuronides are more hydrophilic, facilitating renal and biliary excretion. Apart from hepatic glucuronidation, high rates of gastrointestinal glucuronidation have been observed. The aim of this study was to characterize the expression of family 1 UGTs (UGT1A) in liver, kidney, and all parts of the rat gastrointestinal tract by reverse transcription polymerase reaction (RT-PCR), Northern blot, and xenobiotic induction experiments. RT-PCR experiments were performed with primers specific for all known rat UGT1A mRNAs. UGT1A1, UGT1A6, and UGT1A7 were expressed in liver, kidney, and the gastrointestinal tract. UGT1A5 transcripts were detected in liver, but not in kidney or gastrointestinal tissue. In contrast, UGT1A2 and UGT1A3 were not expressed in liver or kidney, but were detected in intestine. Low levels of UGT1A3 were detectable in duodenum and jejunum. UGT1A2 was abundantly expressed in the small intestine; expression levels in the stomach and the large intestine were low. Quantitative evaluation of RNA levels by Northern blot revealed expression in gradients, with highest UGT1A mRNA levels in duodenum and decreasing levels in the small and large intestine. Only UGT1A6 was expressed at high levels in the rectum. Rats treated with 3-methylcholanthrene (3-MC) displayed a 10-fold induction of hepatic UGT1A6 and UGT1A7 mRNAs. In gastric tissues and in intestine, induction was 4-fold and 2-fold, respectively. In contrast to the constitutive expression of UGT1A7 in kidney, UGT1A6 was inducible in the liver. Effects of 3-MC on UGT1A1 expression revealed downregulation in the liver and highly variable effects in duodenum and stomach. This study demonstrates tissue-specific expression and tissue-specific induction patterns in rat liver, kidney, and gastrointestinal tract, which may represent the physiological basis of tissue-specific glucuronidation in rats.

The role of glucuronidation in N-(3,5-dichlorophenyl)succinimide (NDPS) nephrotoxicity: nephrotoxic potential of NDPS and NDPS metabolites in Gunn, Wistar, and Fischer 344 rats

The agricultural fungicide N-(3,5-dichlorophenyl)succinimide (NDPS) is an acute nephrotoxicant in rats. Although the mechanism of NDPS nephrotoxicity is not clear, our previous studies have strongly suggested that glucuronide conjugation of NDPS metabolite(s) is an important biotransformation reaction leading to the ultimate nephrotoxicant metabolite(s) mediating NDPS nephrotoxicity. In this study, the nephrotoxic potential of NDPS and its nephrotoxicant metabolites, N-(3,5-dichlorophenyl)-2-hydroxysuccinimide (NDHS) and N-(3,5-dichlorophenyl)-2-hydroxysuccinamic acid (NDHSA), was examined in Gunn rats, which contain a genetic deficiency in bilirubin uridine diphosphate-glucuronosyltransferase (UDPGT), to explore further the role of glucuronidation in NDPS nephrotoxicity. The nephrotoxic potential of NDPS, NDHS, and NDHSA was also examined in Wistar rats, the parent strain for Gunn rats and which generally have normal UDPGT activity. Comparisons were then made with the nephrotoxicity induced by these compounds in Fischer 344 (F344) rats. Age-matched male F344, homozygous (j/j) Gunn, and Wistar rats were used. Rats (four to eight rats/group) of each strain were administered NDPS (0.4 mmol/kg ip), NDHS (0.1 or 0.2 mmol/kg ip), NDHSA (0.1 mmol/kg ip), or vehicle, and renal effects were monitored functionally and morphologically for 48 h. NDPS and its nephrotoxicant metabolites, NDHS and NDHSA, were much weaker nephrotoxicants in Gunn rats than in F344 rats, while Wistar rats were susceptible to the nephrotoxicity induced by NDPS, NDHS, or NDHSA. These results suggest that the lack of NDPS nephrotoxicity observed in Gunn rats is due to the deficiency in UDPGT in this strain rather than the parent Wistar strain being inherently nonresponsive to NDPS nephrotoxicity. Therefore, it appears that glucuronide metabolite(s) of NDHS and/or NDHSA contribute(s) to NDPS nephrotoxicity, although the exact nature of the nephrotoxicant glucuronide metabolite(s) of NDPS remains to be determined.

Effect of glucuronidation substrates/inhibitors on N-(3,5-dichlorophenyl)succinimide nephrotoxicity in Fischer 344 rats

The agricultural fungicide N-(3,5-dichlorophenyl)succinimide (NDPS) is an acute nephrotoxicant in rats. Our previous studies have strongly suggested that glucuronide conjugation of NDPS metabolites might be a bioactivation step mediating NDPS nephrotoxicity. In this study, effects of substrates and/or inhibitors of primarily glucuronidation on NDPS nephrotoxicity were examined to explore further the role of glucuronidation in NDPS nephrotoxicity. Male Fischer 344 rats (4-6/group) were administered one of the following intraperitoneal (i.p.) pretreatments (dose, pretreatment time) prior to NDPS (0.4 mmol/kg) or NDPS vehicle (sesame oil, 2.5 ml/kg): (1) no pretreatment; (2) borneol (900 mg/kg, 30 min); (3) eugenol (500 mg/kg per day, 3 days); (4) clofibric acid (400 mg/kg, 15 min before (1/2 dose) and 3 h after (1/2 dose)), or (5) valproic acid, sodium salt (1.0 mmol/kg, 15 min). Following NDPS or NDPS vehicle administration, renal function was monitored at 24 and 48 h. Pretreatment with borneol or eugenol, substrates for ether glucuronidation and sulfation (mainly glucuronidation), afforded complete protection against NDPS nephrotoxicity. Substrates for acyl glucuronidation, clofibric acid or valproic acid, mildly reduced or had little effect on NDPS nephrotoxicity, respectively. These results suggest that ether glucuronide conjugates of NDPS metabolites, rather than acyl glucuronide conjugates, may be the primary ultimate nephrotoxicant species mediating NDPS nephrotoxicity.

Enhancement of intestinal UDP-glucuronosyltranferase activity in partially hepatectomized rats

To evaluate whether a temporary hepatic insufficiency may affect intestinal glucuronidation, we determined UDP-glucuronosyltransferase activity towards bilirubin and p-nitrophenol in rat jejunum and liver after partial hepatectomy. Enzyme assays were performed in native, and in UDP-N-acetylglucosamine- or palmitoyl lysophosphatidylcholine-activated microsomes at different times post-hepatectomy. Content of enzyme was analyzed by Western blot. Microsomal cholesterol/phospholipid ratio, phospholipid and total fatty acid classes were also determined to evaluate the possible influence on enzyme activity. The results show that while hepatic microsomes exhibited no change in UDP-glucuronosyltransferase activity (for both substrates) with respect to shams at any time of study, intestinal activities increased significantly 48 h after surgery, returning to sham values 96-h post-hepatectomy. Western blotting confirmed the increase (about 50% for both substrates 48-h post-hepatectomy) in intestinal UDP-glucuronosyltransferase activity. No variations were observed in hepatic and intestinal microsomal lipid composition in agreement with the absence of modification in the percent of activation by palmitoyl lysophosphatidylcholine. In conclusion, jejunum but not liver, was able to produce a compensatory increase in conjugation capacity during a transitory loss of hepatic mass. The phenomenon is associated to a modification in the amount of UDP-glucuronosyltransferase, rather than to changes in the characteristics of the enzyme environment.

Effects of sodium sulfate on acute N-(3,5-dichlorophenyl)succinimide (NDPS) nephrotoxicity in the Fischer 344 rat

The agricultural fungicide N-(3,5-dichlorophenyl)succinimide (NDPS) induces acute polyuric renal failure in rats. Results of previous studies have suggested that NDPS may induce nephrotoxicity via conjugates of NDPS metabolites. Thus, the purpose of this study was to examine if administered sodium sulfate could alter NDPS nephrotoxicity. Male Fischer 344 rats (four rats per group) were administered a single intraperitoneal (i.p.) injection of sodium sulfate (0.035, 0.07, 0.35 or 3.5 mmol/kg) or sodium chloride (7.0 mmol/kg) 20 min before NDPS (0.2, 0.4 or 0.8 mmol/kg) or NDPS vehicle (sesame oil, 2.5 ml/kg) and renal function monitored at 24 and 48 h. High dose sodium sulfate (3.5 mmol/kg) markedly attenuated NDPS nephrotoxicity, while sodium chloride had no effect on NDPS-induced renal effects. NDPS nephrotoxicity was also attenuated by a pretreatment dose of 0.35 mmol/kg sodium sulfate, while 0.07 mmol/kg sodium sulfate pretreatment potentiated NDPS 0.2 mmol/kg to produce nephrotoxicity without markedly attenuating NDPS 0.4 mmol/kg to induce renal effects. A dose of 0.035 mmol/kg sodium sulfate did not potentiate NDPS 0.2 mmol/kg to induce nephrotoxicity. These results suggest that sulfate conjugates of NDPS metabolites might contribute to NDPS nephrotoxicity.

Intestinal UDP-glucuronosyltransferase activities in rat and rabbit

1. Tissues other than the liver can contribute significantly to the drug-metabolizing capacity of an animal. In the current study, the glucuronidation of several aglycones in microsomes from the small intestinal mucosa of rat and rabbit has been investigated and compared with glucuronidation in liver microsomes. 2. UDP-glucuronosyltransferase activities in intestinal microsomes were generally higher in rabbit when compared with rat, ranging from 200 to 300% for 1-naphthol, 2-naphthol, 4-methylumbelliferone, 2-hydroxybiphenyl and 4-hydroxybiphenyl. 3. In contrast, hepatic activities were much higher in rat than in rabbit, ranging from 300 to 400% for 1-naphthol, 2-naphthol, 4-methylumbelliferone, 2-hydroxybiphenyl and testosterone; and from 150 to 250% for 4-nitrophenol and diclofenac. 4. In rabbit, activities in the small intestinal mucosa were comparable (70-100%) with hepatic activities for most aglycones. In rat, intestinal mucosa activities were much lower than in liver, with activities toward 1-naphthol, 2-naphthol, 4-nitrophenol, 4-methylumbelliferone, 2-hydroxybiphenyl and 4-hydroxybiphenyl in the small intestine representing 5-15% of hepatic activities. 5. With a higher intestine:liver activity ratio, intestinal UDP-glucuronosyltransferases could be anticipated to contribute more to overall drug glucuronidation in rabbit as compared with rat, thereby contributing more to reducing drug bioavailability.

Differential induction of isozymes of drug-metabolizing enzymes by butylated hydroxytoluene in mice and Chinese hamsters

Induction of isozymes of drug-metabolizing enzymes by butylated hydroxytoluene (BHT) was studied in the male ddY mouse and Chinese hamster. In mice given 0.05 and 0.15% BHT in the diet for 14 days cytochrome P-450 contents and the activities of uridine diphosphate-glucuronyl transferase (UDP-GT) and pentoxyresorufin O-dealkylase were markedly increased, while in those fed 0.15% BHT testosterone 6 alpha-, 16 alpha- and 16 beta-hydroxylases were greatly increased, which indicated induction of cytochrome P-450 isozymes of the CYP2B family. Western blot analysis also showed an increased level of the isozyme immunorelated to rat CYP2B2 by BHT feeding. The activities of aryl hydrocarbon hydroxylase, ethoxycoumarin O-deethylase (ECOD), erythromycin N-demethylase and glutathione S-transferase (GST) remained unchanged. In Chinese hamsters given 0.05 and 0.15% BHT in the diet for 14 days activities of ECOD and GST were induced, but cytochrome P-450 contents and the activities of other enzymes were unaffected. Testosterone 15 alpha-hydroxylase was induced in hamsters fed 0.15% BHT. These findings suggested that BHT administration in the hamster induced CYP2A2-type isozyme, which was confirmed by Western blot analysis. BHT treatment enhanced activation of benzo[a] pyrene (B[a]P) as determined by the mutagenicity test, especially in Chinese hamsters. The results suggest that BHT treatment induces specific isozymes of drug-metabolizing enzymes and might modify the expression of toxicities of other chemicals.

The use of jejunal transplants to treat a genetic enzyme deficiency

INTRODUCTION

The Gunn rat is an excellent animal model of Crigler-Najjar syndrome, type 1. The liver and small intestine synthesize no functional bilirubin uridine diphosphoglucuronosyl transferase and, consequently, the animals cannot conjugate bilirubin. In prior studies, the authors have shown that 15- to 20-cm jejunal transplants from normal Wistar rats lowered but did not normalize serum bilirubin levels. Phenobarbital has been used to increase enzyme conjugation of bilirubin.

HYPOTHESIS

Phenobarbital treatment of Gunn recipients of jejunal transplants from Wistar rats normalizes serum bilirubin levels.

METHODS

Forty-three Gunn recipients of jejunal transplants from Wistar rats were divided into four groups: 1) heterotopically placed grafts (Thiry-Vella loops), saline-treated, n = 14; 2) heterotopically placed grafts, phenobarbital-treated (80 mg/kg/day), n = 17; 3) orthotopically placed (in intestinal continuity) grafts, saline-treated, n = 5; and 4) orthotopically placed grafts, phenobarbital-treated, n = 7. Serum was collected before operation and weekly for 8 weeks for measurement of serum total, indirect, and direct bilirubin levels. Animals received cyclosporine, 5 micrograms/kg, daily intramuscularly.

RESULTS

Phenobarbital significantly augmented the bilirubin-lowering effect of heterotopic jejunal transplants (group 2). Mean total serum bilirubin fell from 9.14 +/- 0.01 to a nadir of 1.63 +/- 0.11 mg/dL at 6 weeks, after which time, levels began to rise toward baseline (as noted previously). Serum indirect bilirubin levels behaved in a similar fashion. Phenobarbital treatment "normalized" serum bilirubin levels in recipients of orthotopic Wistar jejunal grafts (group 4). Mean total serum bilirubin plummeted from 8.41 +/- 0.20 to 0.76 +/- 0.15 mg/dL at 1 week, and levels remained within the normal range for the entire 8-week study period. Identical changes were observed for serum indirect bilirubin levels.

CONCLUSIONS

The combination of phenobarbital treatment and orthotopic small bowel transplantation may be an appropriate therapeutic alternative to liver transplantation in the management of Crigler-Najjar syndrome, type 1.

Comparative effects of omeprazole on xenobiotic metabolizing enzymes in the rat and human

Omeprazole induces CYP1A in the human liver and gut, which has led to concern about possible side effects. The purpose of this study was to compare the effects of omeprazole on phase 1 and phase 2 enzymes in the rat and human. Male rats were treated with intraperitoneal (40 or 80 mg/kg) or oral omeprazole (40 mg/kg) for 5 or 14 days, respectively. The activities and amounts of CYP1A, uridine diphosphate-glucuronosyltransferase, and glutathione transferase were determined in liver and gut. Enzyme activities were also determined in duodenal biopsy specimens from six healthy human volunteers before and after treatment with omeprazole (20 mg/day) for 10 days. Treatment with intraperitoneal omeprazole (40 mg/kg; 80 mg/kg) coinduced uridine diphosphate-glucuronosyltransferase (36%; 66%), glutathione transferase (22%; 50%), and CYP1A (26%; 50%) in rat liver. In rat small intestine, comparable levels of induction were observed for uridine diphosphate-glucuronosyltransferase and glutathione transferase; CYP1A was unaffected. Oral omeprazole had similar effects. Immunoblotting showed corresponding changes in the amounts of these enzymes. Omeprazole increased the activities of CYP1A (19% to 167%; p = 0.014) and uridine diphosphate-glucuronosyltransferase (11% to 68%; p = 0.04) in the duodenal biopsy specimens of all six human volunteers; glutathione transferase was unaffected. Thus, omeprazole coinduced multiple xenobiotic metabolizing enzymes in the rat and human. The pattern of induction differed in the rat and human, consistent with known differences in genetic regulatory elements in the two species.

Subchronic effects of 2,3,7,8-TCDD or PCBs on thyroid hormone metabolism: use in risk assessment

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 3,3',4,4',5-pentachlorobiphenyl (PCB 126), or 2,3,3',4,4',5-hexachlorobiphenyl (PCB 156) on thyroid hormone metabolism were studied in 13-week feeding studies in female Sprague-Dawley rats. The diets were supplemented with the compounds tested at concentrations ranging from 0.2 to 20 micrograms/kg diet for TCDD, 7 to 180 micrograms/kg diet for PCB 126, or 1.2 to 12 mg/kg diet for PCB 156, respectively. Significant correlations were found for all three compounds between reductions in plasma total thyroxine (TT4) levels and inductions of the microsomal phase II enzyme UDP-glucuronosyltransferase by using T4 as a substrate (T4UGT). Furthermore, the coinduction of certain phase I and II isozymes, i.c., cytochrome P450 1A1 (CYP1A1) and UGT1A1, by these compounds, clearly suggests the involvement of an Ah receptor-mediated mechanism in the disturbance of thyroid hormone metabolism by these polyhalogenated aromatic compounds. These results provide a mechanistic base for the use of certain effects on thyroid hormone metabolism by polyhalogenated aromatic compounds in risk assessment. By using these effects, potencies of PCB 126 and PCB 156 relative to TCDD ranged from 0.008 to 0.1 for PCB 126, and from 0.00007 to 0.004 for PCB 156, respectively. These values correspond very well with relative potencies of PCB 126 and PCB 156 by using some other well-known Ah receptor-mediated toxic and biochemical parameters.

Urinary ascorbic acid--HPLC determination and application as a noninvasive biomarker of hepatic response

A high-performance liquid chromatograph (HPLC) procedure has been developed for the determination of rat urinary ascorbic acid, a major metabolite of the hepatic glucuronic acid pathway. The presence of EDTA and HCl effectively inhibited degradation of ascorbic acid during the collection of urine specimens. The reliability of the procedure was demonstrated by its high recovery (90%), specificity (characteristic absorption maximum and discrimination from isoascorbic acid), and reproducibility (2-3% coefficient of variation). The usefulness of this assay as an indicator of hepatic response was demonstrated in preliminary experiments where increases in urinary ascorbic acid excretion were detected in male rats treated with PCB 126 (3,3',4,4',5-pentachlorobiphenyl) or PCB 105 (2,3,3',4,4'-pentachlorobiphenyl). The HPLC measurement also showed that the two PCB congeners differed markedly in their potency in stimulating urinary ascorbic acid excretion. For example, 10 micrograms/kg bw/day of PCB 126 was sufficient to cause a fourfold increase in urinary ascorbic excretion while 5000 micrograms/kg bw/day of PCB 105 was required for a sevenfold increase. In response to the administration of PCB 105 or PCB 126, urinary ascorbic acid appeared to increase to the same extent as increases in hepatic ethoxyresorufin O-deethylase (EROD) and UDP-glucuronosyltransferase (UGT) activities, and to a much higher extent than changes in liver weight and hematological and serum clinical chemical parameters. The sensitivity and specificity, the ease in obtaining timed specimens, and the noninvasive nature make this assay a useful biomarker of hepatic response in dose-finding and various acute and chronic studies.

Dietary lipids induce phase 2 enzymes in rat small intestine

We examined the role of dietary lipids in determining the activities of glutathione S-transferase (GST) and UDPglucuronosyl-transferase (UGT) in rat small intestine. Male Wistar rats were fed a fat-free (FF) diet or isocaloric control diet containing 5% corn oil (CO) or 5% fish oil (FO) for 3 weeks. The activities of these enzymes were about 2-fold higher in rats fed the FO diet vs. the FF diet. Intermediate levels of enzyme activity were found in rats fed the CO diet. Diet-induced differences in enzyme levels were shown by immunoblotting. The highest levels of glutathione S-transferase and UDPglucuronosyltransferase were detected in rats fed the FO diet. The lowest levels of these enzymes were found in rats fed the FF diet. Intermediate levels of enzyme were detected in rats fed the CO diet. Thus, diet-induced differences in enzyme activities were paralleled by changes in enzyme levels. Fatty acid analysis of mucosal lipids showed that the FF and FO diets were associated with decreased levels of linoleic and arachidonic acids as compared with the CO diet.

Effect of microsomal enzyme modulators on N-(3,5-dichlorophenyl)-2-hydroxysuccinimide (NDHS)-induced nephrotoxicity in the Fischer 344 rat

We have previously reported that phenobarbital (PB) pretreatment enhances and piperonyl butoxide (PIBX) pretreatment or cobalt chloride (CoCl2) pretreatment decreases the nephrotoxicity induced by the model nephrotoxicant N-(3,5-dichlorophenyl)succinimide (NDPS) in the Fischer 344 rat. The objective of this study was to determine the effect of a microsomal enzyme inducer (PB) or microsomal enzyme inhibitor (PIBX or CoCl2) on a single intraperitoneal (i.p.) injection of N-(3,5-dichlorophenyl)-2-hydroxysuccinimide (NDHS, 0.05, 0.1 or 0.2 mmol/kg), a nephrotoxicant metabolite of NDPS, or vehicle (sesame oil, 2.5 ml/kg). Renal function was monitored at 24 and 48 h post-NDHS for PB pretreated rats and at 24 h only for PIBX and CoCl2 pretreated rats, due to lethality at 48 h in PIBX pretreated rats. PB pretreatment potentiated the renal toxicity induced by a non-toxic dose of NDHS (0.05 mmol/kg), inducing diuresis and elevated proteinuria, hematuria, glucosuria, blood urea nitrogen (BUN) concentration and kidney weight. PB pretreatment also enhanced some monitored renal effects of a toxic dose (0.1 mmol/kg) of NDHS, including reduced organic ion transport by renal cortical slices. PIBX and CoCl2 pretreatments did not markedly affect the increased kidney weight, proteinuria, glucosuria, BUN concentration or altered organic ion transport induced by NDHS (0.2 mmol/kg) treatment. We conclude that PB potentiates NDHS-induced nephrotoxicity via a mechanism not influenced by CoCl2 or PIBX.