Induction of UDP-glycosyltransferase family 1 genes in rat liver: different patterns of mRNA expression with two inducers, 3-methylcholanthrene and beta-naphthoflavone

The Use of Long-term Hepatocyte Cultures for Detecting Induction of Drug Metabolising Enzymes: The Current Status

In this report, metabolically competent in vitro systems have been reviewed, in the context of drug metabolising enzyme induction. Based on the experience of the scientists involved, a thorough survey of the literature on metabolically competent long-term culture models was performed. Following this, a prevalidation proposal for the use of the collagen gel sandwich hepatocyte culture system for drug metabolising enzyme induction was designed, focusing on the induction of the cytochrome P450 enzymes as the principal enzymes of interest. The ultimate goal of this prevalidation proposal is to provide industry and academia with a metabolically competent in vitro alternative for long-term studies. In an initial phase, the prevalidation study will be limited to the investigation of induction. However, proposals for other long-term applications of these systems should be forwarded to the European Centre for the Validation of Alternative Methods for consideration. The prevalidation proposal deals with several issues, including: a) species; b) practical prevalidation methodology; c) enzyme inducers; and d) advantages of working with independent expert laboratories. Since it is preferable to include other alternative tests for drug metabolising enzyme induction, when such tests arise, it is recommended that they meet the same level of development as for the collagen gel sandwich long-term hepatocyte system. Those tests which do so should begin the prevalidation and validation process.

Disruption of Thyroid Hormone Homeostasis at Weaning of Holtzman Rats by Lactational but Not In Utero Exposure to 2,3,7,8-Tetrachlorodibenzo-p-Dioxin

The purpose of this study is to clarify whether lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is entirely responsible for the perturbation in thyroid hormone homeostasis during the neonatal period. Pregnant Holtzman rats were given a single oral dose of 1.0 mug TCDD/kg body weight on gestational day 15. Half of the litters were cross-fostered with the half of the dams treated with vehicle on postnatal day (PND) 1 to make four groups of rats, control (C/C), prenatal TCDD exposure only (T/C), postnatal TCDD exposure only (C/T), and both prenatal and postnatal TCDD exposure (T/T). On PND 21, the C/T and T/T groups, but not the T/C and C/C groups, showed a significant decrease in serum total thyroxin (TT4) and free thyroxin (FT4) concentrations in both sexes and a significant increase in serum thyroid-stimulating hormone (TSH) levels, particularly male pups. These two groups of male and female pups had significantly higher concentrations of TCDD in the liver, with marked induction of cytochrome P450 (CYP) 1A1 mRNA and intense immunostaining of CYP1A1 in the liver. UDP glycosyltransferase 1 family, polypeptide A6 (UGT1A6) and UGT1A7 mRNAs were induced in their livers, with marked immunostaining of UGT1A6. The transfer of TCDD from dams to the pups was confirmed by the detection of TCDD in mother's milk remaining in the stomachs of lactationally exposed pups on PND 1. The present results demonstrate that lactational, but not in utero, exposure to TCDD was responsible for the disruption of thyroid hormone homeostasis.

Sulfation and glucuronidation of phenols: implications in coenyzme Q metabolism

Phase II conjugation of phenolic compounds constitutes an important mechanism through which exogenous or endogenous toxins are detoxified and excreted. Species differences in the rates of glucuronidation or sulfation can lead to significant variation in the metabolism of this class of compounds. Conjugation of the hydroxyl groups of phenols can occur with glucuronate or sulfate. Quinone metabolism, deactivation, and detoxification are also affected by the same conjugatory systems as phenols; however, reduction of quinones to hydroquinols seems to be a prerequisite. This work reviews current knowledge on phenol conjugation and its implications on hydroquinone metabolism with special consideration for coenzyme Q metabolism.

Differential regulation of alternate UDP-glucuronosyltransferase 1A6 gene promoters by hepatic nuclear factor-1

UDP-glucuronosyltransferase 1A6 (UGT1A6) is a major UGT contributing to the glucuronidation of small phenolic compounds. The gene for rat 1A6 is expressed using two promoters, a distal promoter P1 and a proximal promoter P2. Transcripts from P2 are high in liver, gastrointestinal tract, and kidney, whereas P1 transcripts predominate in other tissues. Here we report evidence for primary control of the P2 promoter by hepatic nuclear factor 1 (HNF1). Transient transfection of a P2 reporter plasmid, p(-1354/+65) 1A6P2-luc, resulted in enhanced luciferase activity in HepG2 but not Hepa1 cells compared to cells transfected with pGL3-Basic control vector. A truncated reporter under the control of -224 to +65 exhibited comparable activity. Footprint analysis of the -224/+65 fragment revealed specific binding by rat liver nuclear protein to a region between bases -60 and -37. The binding activity was also observed with HepG2 cell but not Hepa1 cell extract. Electrophoretic mobility shift assays were consistent with the presence of HNF1 in the binding complexes. The functionality of an HNF1-binding site at -51/-37 is also supported by (1) marked decreases in the activity of P2 reporter plasmids containing a three-base substitution in the proposed HNF1 binding site and (2) the enhancement of P2 reporter activity following cotransfection of an HNF1alpha expression plasmid. The UGT1A6 P1 promoter lacks an HNF1 binding site in the analogous position and showed little response to HNF1 overexpression. Although these data do not strictly rule out an interaction between the P1 promoter and HNF1 bound to -51/-37 of P2, the results suggest a mechanism for the more abundant expression of P2-derived UGT1A6 transcripts in liver and other HNF1-enriched tissues.

Preclinical factors influencing the relative contributions of Phase I and II enzymes to the metabolism of the experimental anti-cancer drug 5,6-dimethylxanthenone-4-acetic acid

It is important to determine the relative contribution of each metabolic pathway (f(p)) and of enzymes to the net metabolism of a drug. The aim of this study was to investigate, using a human liver bank, the f(p) of the anti-cancer drug 5,6-dimethylxanthenone-4-acetic acid (DMXAA) and the effects of various inhibitors and inducers on f(p). The mean apparent K(m) and V(max) values (N=14) were 21+/-5 microM and 0.04+/-0.02 nmol/min/mg, respectively, for 6-methylhydroxylation, and 143+/-79 microM and 0.71+/-0.52 nmol/min/mg, respectively, for acyl glucuronidation in human liver microsomes. 6-Methylhydroxylation and acyl glucuronidation contributed 26 and 74%, respectively, to DMXAA metabolism at 5 microM; values were 7 and 93% at 350 microM DMXAA. There was a significant relationship between the ratio of metabolic activity by Phase II and I reactions (R(II/I)) and uridine diphosphate glucuronosyltransferase (UGT2B7) protein level (r=0.605, P=0.022), whereas a reverse correlation between R(II/I) and cytochrome P450 (CYP1A) protein level was observed (r=-0.540, P=0.046). Various compounds inhibited either DMXAA glucuronidation or 6-methylhydroxylation, or both pathways. Pretreatment of rats with beta-naphthoflavone, but not phenobarbitone and cimetidine, increased the percentage of the contribution by 6-methylhydroxylation to 17% from 4% of control at 5 microM DMXAA. Our results indicate that the f(p) of DMXAA is subject to substrate concentration, inhibition, induction, and the protein levels of enzymes that biotransform DMXAA. However, clinical studies are important to verify the conclusions drawn from in vitro data.

Enterohepatic circulation: physiological, pharmacokinetic and clinical implications

Enterohepatic recycling occurs by biliary excretion and intestinal reabsorption of a solute, sometimes with hepatic conjugation and intestinal deconjugation. Cycling is often associated with multiple peaks and a longer apparent half-life in a plasma concentration-time profile. Factors affecting biliary excretion include drug characteristics (chemical structure, polarity and molecular size), transport across sinusoidal plasma membrane and canniculae membranes, biotransformation and possible reabsorption from intrahepatic bile ductules. Intestinal reabsorption to complete the enterohepatic cycle may depend on hydrolysis of a drug conjugate by gut bacteria. Bioavailability is also affected by the extent of intestinal absorption, gut-wall P-glycoprotein efflux and gut-wall metabolism. Recently, there has been a considerable increase in our understanding of the role of transporters, of gene expression of intestinal and hepatic enzymes, and of hepatic zonation. Drugs, disease and genetics may result in induced or inhibited activity of transporters and metabolising enzymes. Reduced expression of one transporter, for example hepatic canalicular multidrug resistance-associated protein (MRP) 2, is often associated with enhanced expression of others, for example the usually quiescent basolateral efflux MRP3, to limit hepatic toxicity. In addition, physiologically relevant pharmacokinetic models, which describe enterohepatic recirculation in terms of its determinants (such as sporadic gall bladder emptying), have been developed. In general, enterohepatic recirculation may prolong the pharmacological effect of certain drugs and drug metabolites. Of particular importance is the potential amplifying effect of enterohepatic variability in defining differences in the bioavailability, apparent volume of distribution and clearance of a given compound. Genetic abnormalities, disease states, orally administered adsorbents and certain coadministered drugs all affect enterohepatic recycling.

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).

Para-nitrophenol glucuronidation and sulfation in rat and human liver slices

Para-nitrophenol (PNP) is a well-known substrate for both phase I (hydroxylation at cytochrome P450) and phase II reactions (glucuronidation and sulfation). HPLC separation of PNP conjugates has already been described, but not for respective studies with liver slices, which nowadays have proven to be a suitable model for metabolic studies. Therefore we adapted an HPLC method for the simultaneous measurement of PNP glucuronidation (PNP-G) and sulfation (PNP-S) in this in vitro system. Both activities are substantially maintained over an incubation period of 24 h. PNP-G activity, however, seems to be better preserved, as indicated by stable values for PNP-G but reduced PNP-S values after 48 h liver slice preincubation. 24 h exposure of the slices to beta-naphthoflavone or phenobarbital does not change PNP-G or PNP-S activities.

In situ hybridization studies of UDP-glucuronosyltransferase UGT1A6 expression in rat testis and brain

UDP-glucuronosyltransferases (UGTs), in addition to their role in overall pharmacokinetics, play important roles in local protection of cells against toxins and in the control of endogenous receptor ligands. UGT1A6, which conjugates planar phenols, appears to be expressed in many organs, but information on cell-specific expression in these organs is controversial or absent. Therefore, a non-isotopic in situ hybridization method was developed and applied to localize UGT1A6 expression in rat testis and brain. It was found that UGT1A6 is expressed in Sertoli cells and spermatogonia of rat testis and in brain neurons, in particular in hippocampal pyramidal cells and Purkinje cells of the cerebellum.