Quantitative reverse transcriptase/PCR assay for the measurement of induction in cultured hepatocytes

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.

The suitability of rat hepatoma cell line H4IIE for evaluating the potentials of compounds to induce CYP3A23 expression

To investigate the suitability of H4IIE cells for detecting cytochrome P450 (CYP) induction in vitro, we compared CYP induction by typical CYP inducers in H4IIE cells and rat primary hepatocytes by examining gene expression and enzyme activity, and by immunocytochemistry. The cells were preincubated with 0.1 μM of dexamethasone (DEX) for 24 h, followed by 48 h of exposure to 10 μM of beta-naphthoflavone (bNF), 100 μM of phenobarbital (PB) and 10 μM of DEX. Cyp1a1, Cyp2b1/2 and Cyp3a23/3a1 (Cyp3a23) expressions in H4IIE cells were up-regulated 280-, 1.5- and 65-fold relative to those in vehicle-treated cells, respectively. The fold inductions of those expressions in rat primary hepatocytes were 80-, 33- and 152-fold, respectively. Comprehensive gene expression analysis using DNA microarrays showed that Cyp3a23, Gsta2, Ugt2b12, Udpgt and Sult2a1 expressions were up-regulated in H4IIE cells exposed to 10 μM of DEX. CYP3A activity was not increased, but some H4IIE cells exposed to DEX were stained strongly with anti-CYP3A antibody. We cloned these cells and obtained cloned H4IIE (cH4IIE) cells with expression level of Cyp3a23 higher than those of vehicle-treated cells. It was confirmed that preincubation with 0.1 μM of DEX increased pregnane X receptor (Pxr) expression level and enhanced the Cyp3a23 induction effects of test compounds significantly. Retrospective examination of in vitro CYP induction assay using cH4IIE cells resulted in 80% correlation with the data from in vivo rat toxicity studies. These results suggested that cH4IIE cells are suitable for evaluating the potentials of a compound to induce CYP3A23 expression.

Does CYP2E1 play a major role in the aggravation of isoniazid toxicity by rifampicin in human hepatocytes?

Isoniazid and rifampicin are first-line anti-tubercular drugs. In a recent paper, Shen et al. provided interesting findings that rifampicin exacerbated isoniazid toxicity in human hepatocytes but not in rat hepatocytes. The main conclusion was that the difference in cytochrome P450 2E1 (CYP2E1) induction by rifampicin between rat and human hepatocytes accounted for the difference in exacerbation of isoniazid hepatotoxicity by rifampicin. 4-Nitrophenol hydroxylase (4-NP) activity was the only probe of CYP2E1 activity used in the paper. The authors presented data showing that rifampicin enhanced 4-NP activity and CYP2E1 mRNA expression in human hepatocytes, but not in rat hepatocytes. However, CYP3A also makes a significant contribution to 4-NP activity in humans and rats, which has been confirmed by both CYP3A-specific inducer and inhibitors. Rifampicin is a strong inducer of human CYP3A; thus, the increase in 4-NP activity in human hepatocytes could be due to the induction of CYP3A. Rifampicin did not increase 4-NP activities in rat hepatocytes, which could reflect a lack of the induction of rat CYP3A by rifampicin. Additionally, more experiments are needed to support the conclusion that rifampicin increased CYP2E1 mRNA expression in human hepatocytes because of the small sample size and the limitations of semi-quantitative RT-PCR. The study by Shen et al. suggests that another drug-metabolizing enzyme rather than CYP2E1 could be involved in the aggravation of isoniazid toxicity by rifampicin in human hepatocytes.

Pregnane X receptor-agonists down-regulate hepatic ATP-binding cassette transporter A1 and scavenger receptor class B type I

Pregnane X receptor (PXR) is the molecular target for a wide variety of endogenous and xenobiotic compounds. It regulates the expression of genes central to the detoxification (cytochrome P-450 enzymes) and excretion (xenobiotic transporters) of potentially harmful compounds. The aim of the present investigation was to determine the role of PXR in regulation of high-density lipoprotein (HDL) cholesterol metabolism by studying its impact on ATP-binding cassette transporter A1 (ABCA1) and scavenger receptor class B type I (SR-BI) expression in hepatocytes. ABCA1 and SR-BI are major factors in the exchange of cholesterol between cells and HDL. Expression analyses were performed using Western blotting and quantitative real time RT-PCR. Luciferase reporter gene assays were used to measure promoter activities. Total cholesterol was measured enzymatically after lipid extraction (Folch's method). The expression of ABCA1 and SR-BI was inhibited by the PXR activators rifampicin and lithocholic acid (LCA) in HepG2 cells and pregnenolone 16alpha-carbonitrile (PCN) in primary rat hepatocytes. Thus, PXR appears to be a regulator of hepatic cholesterol transport by inhibiting genes central to cholesterol uptake (SR-BI) and efflux (ABCA1).

Metabolism and Disposition Kinetics of Nicotine

Nicotine is of importance as the addictive chemical in tobacco, pharmacotherapy for smoking cessation, a potential medication for several diseases, and a useful probe drug for phenotyping cytochrome P450 2A6 (CYP2A6). We review current knowledge about the metabolism and disposition kinetics of nicotine, some other naturally occurring tobacco alkaloids, and nicotine analogs that are under development as potential therapeutic agents. The focus is on studies in humans, but animal data are mentioned when relevant to the interpretation of human data. The pathways of nicotine metabolism are described in detail. Absorption, distribution, metabolism, and excretion of nicotine and related compounds are reviewed. Enzymes involved in nicotine metabolism including cytochrome P450 enzymes, aldehyde oxidase, flavin-containing monooxygenase 3, amine N-methyltransferase, and UDP-glucuronosyltransferases are represented, as well as factors affecting metabolism, such as genetic variations in metabolic enzymes, effects of diet, age, gender, pregnancy, liver and kidney diseases, and racial and ethnic differences. Also effects of smoking and various inhibitors and inducers, including oral contraceptives, on nicotine metabolism are discussed. Due to the significance of the CYP2A6 enzyme in nicotine clearance, special emphasis is given to the effects and population distributions of CYP2A6 alleles and the regulation of CYP2A6 enzyme.

The Quantitative Prediction of In Vivo Enzyme-Induction Caused by Drug Exposure from In Vitro Information on Human Hepatocytes

There have been no reports of the quantitative prediction of induction for drug-metabolizing enzymes in humans. We have tried to predict such enzyme induction in humans from in vitro data obtained using human hepatocytes. The in vitro and in vivo data on enzyme induction by inducers, such as rifampicin, phenobarbital and omeprazole, were collected from the published literature. The degree of enzyme induction in humans was compared with that predicted from in vitro data on human hepatocytes. Using the in vivo data, we calculated the hepatic intrinsic clearance of typical CYP substrates, such as midazolam and caffeine, before and after inducer treatment and estimated the induction ratios of hepatic intrinsic clearance following treatment. In the in vitro studies, the amount of mRNA or enzyme and enzyme activity in human hepatocytes, with or without an inducer, were compared and the induction ratios were estimated. The unbound mean concentration was taken as an index of drug exposure and the induction ratios in the in vivo and in vitro studies were compared. The unbound mean concentrations of inducers used in the in vitro studies were higher than those in the in vivo studies. The maximum induction ratios by inducers in the in vitro studies were higher than those in the in vivo studies. The induction ratio for rifampicin, omeprazole, troglitazone, dexamethasone and phenobarbital increased as the unbound mean concentration increased to reach a constant value. The induction of CYP3A and 1A was analyzed by the Emax model. The maximum induction ratio (Emax) and the concentration at half maximum induction (EC50) for rifampicin, omeprazole, troglitazone, dexamethasone and phenobarbital were 12.3, 0.847 micromol/L, 2.36, 0.225 micromol/L, 6.86, 0.002 micromol/L, 8.30, 9.32 micromol/L, and 7.62, 58.4 micromol/L, respectively. The Emax and EC50 of omeprazole for CYP1A were 12.02 and 0.075 micromol/L, respectively. The predicted induction ratio of all those inducers, except for omeprazole, based on the Emax and EC50 values obtained from the in vitro data were similar to the observed values. On the whole, a good correlation between the observed and predicted induction ratio of omeprazole was observed (r=0.768, p<0.05), although the predicted induction ratio was higher than the observed value. In conclusion, the present study suggests that it is possible to predict quantitatively the CYP3A enzyme induction from hepatocyte data.

Strategies for using in vitro screens in drug metabolism

In vitro assays are increasingly being used in drug metabolism studies to screen novel chemicals. Their advantages are twofold: first, they allow testing early in the drug discovery phase, providing important information on chemical characteristics; second, human cells or cell constituents can be utilized, increasing the relevance to man. However, the process of isolation, transformation or storage of these cell systems may alter their phenotype (and, in the case of tumour-derived cell lines, genotype as well). A review of the systems currently employed shows that, whereas all systems have their own caveats, it is possible to find an appropriate system for any particular question that is asked.

Tissue distribution of mRNA expression of human cytochrome P450 isoforms assessed by high-sensitivity real-time reverse transcription PCR

Pairs of forward and reverse primers and TaqMan probes specific to each human cytochrome P450 isoform were prepared. Analysis of the mRNA level of each CYP isoform in total RNA from pooled specimens of various human organs was performed by real-time reverse transcription PCR using an ABI PRISM 7700 sequence detector system. The expression of CYP3A4 mRNA was similar to that of CYP3A7 mRNA in the fetal liver, and CYP3A4 mRNA levels in the fetal liver were about 0.1 times lower than in the adult liver. CYP2E1 showed the highest level of mRNA expression in the liver. The mRNA expression of 30 CYP isoforms (CYP1A1, 1A2, 1B1, 2A6, 2A7, 2B6, 2C8, 2C9, 2C18, 2C19, 2D6, 2E1, 2J2, 3A4, 3A5, 3A7, 4A11, 4F2, 4F3, 5A1, 7B1, 8A1, 8B1, 17, 26A1, 27, 27B1, 39A1, 46, and 51) in the liver was successfully detected by this method. CYP2F1, 4B1, 4F8, 11s (11A, 11B1, and 11B2), 19, and 24 mRNA levels were the highest in the lung, lung, prostate, adrenal gland, placenta, and kidney, respectively; however, the mRNA expression of these eight CYP isoforms in the liver was not detected by this method. The mRNA levels of the CYP isoforms determined in various human tissues were in good agreement with previously reported data. The method described here has the advantages of high specificity and excellent quantification over a wide range of mRNA concentrations, making it suitable for the evaluation of a large number of samples in the assessment of the expression profile of drug-metabolizing enzymes.

Effects of prototypical microsomal enzyme inducers on cytochrome P450 expression in cultured human hepatocytes

Cultured human hepatocytes are a valuable in vitro system for evaluating new molecular entities as inducers of cytochrome P450 (P450) enzymes. The present study summarizes data obtained from 62 preparations of cultured human hepatocytes that were treated with vehicles (saline or dimethylsulfoxide, 0.1%), beta-naphthoflavone (33 microM), phenobarbital (100 or 250 microM), isoniazid (100 microM) and/or rifampin (20 or 50 microM), and examined for the expression of P450 enzymes based on microsomal activity toward marker substrates, or in the case of CYP2C8, the level of immunoreactive protein. The results show that CYP1A2 activity was markedly induced by beta-naphthoflavone (on average 13-fold, n = 28 preparations), and weakly induced by phenobarbital (1.9-fold, n = 25) and rifampin (2.3-fold, n = 22); CYP2A6 activity tended to be increased with phenobarbital (n = 7) and rifampin (n = 3) treatments, but the effects were not statistically significant; CYP2B6 was induced by phenobarbital (6.5-fold, n = 13) and rifampin (13-fold, n = 14); CYP2C8 was induced by phenobarbital (4.0-fold, n = 4) and rifampin (5.2-fold, n = 4); CYP2C9 was induced by phenobarbital (1.8-fold, n = 14) and rifampin (3.5-fold, n = 10); CYP2C19 was markedly induced by rifampin (37-fold, n = 10), but relatively modestly by phenobarbital (7-fold, n = 9); CYP2D6 was not significantly induced by phenobarbital (n = 5) or rifampin (n = 5); CYP2E1 was induced by phenobarbital (1.7-fold, n = 5), rifampin (2.2-fold, n = 5), and isoniazid (2.3-fold, n = 5); and, CYP3A4 was induced by phenobarbital (3.3-fold, n = 42) and rifampin (10-fold, n = 61), but not by beta-naphthoflavone. Based on these observations, we generalize that beta-naphthoflavone induces CYP1A2 and isoniazid induces CYP2E1, whereas rifampin and, to a lesser extent phenobarbital, tend to significantly and consistently induce enzymes of the CYP2A, CYP2B, CYP2C, CYP2E, and CYP3A subfamilies but not the 2D subfamily.

A review of the common properties of drugs with idiosyncratic hepatotoxicity and the "multiple determinant hypothesis" for the manifestation of idiosyncratic drug toxicity

Idiosyncratic drug toxicity is generally believed to be a phenomenon that cannot be readily evaluated experimentally. Reasons for this difficulty include the following: 1. It is a rare event (<1/5,000) and therefore impossible to be studied in clinical trials; 2. It is a human-specific event not detectable in experimental animals. To aid the understanding of idiosyncratic toxicity and to develop an experimental strategy for this phenomenon, a hypothesis is proposed. The hypothesis states that the low frequency of idiosyncratic drug toxicity is due to the requirements for the occurrence of multiple critical and discrete events, with the probability for the occurrence of idiosyncratic drug toxicity as a product of the probabilities of each event. The key determinants of these critical events are proposed to be: 1. Chemical properties; 2. exposure; 3. environmental factors; and 4. genetic factors. Based on this hypothesis, idiosyncratic drug toxicity can be evaluated experimentally via studying these key determinants. The chemical properties critical to idiosyncratic drug toxicity are identified via a review of the common properties of drugs that cause idiosyncratic liver toxicity. These properties include: 1. Formation of reactive metabolites. 2. Metabolism by P450 isoforms. 3. Preponderance of P450 inducers, and 4. Occurrence of clinically significant pharmacokinetic interactions with co-administered drugs. Based on this review, it is proposed that these common properties may be useful experimental endpoints for the prediction and therefore avoidance of the selection of drug candidates with idiosyncratic drug toxicity for further development.

Expression of N-deacetylase/sulfotransferase and 3-O-sulfotransferase in rat alveolar type II cells

Basal laminae beneath alveolar type I cells are suggested to contain highly sulfated heparan sulfate-containing proteoglycans (PGs), and cultured type II cells accumulate highly sulfated matrices. To characterize the regulation of PG synthesis during the transition from type II cells to type I cells, we examined mRNA expression of N-deacetylase/sulfotransferase (NST) and 3-O-sulfotransferase (3-OST), two enzymes specific for heparan sulfate synthesis. We found that both freshly isolated and cultured type II cells expressed NST and 3-OST as shown by in situ hybridization. Expression of surfactant-associated protein A, B, and C mRNAs, determined by semiquantitative PCR, decreased during culture. Expression of type I cell marker T1alpha mRNA increased except in cells cultured on an Engelbrecht-Holm-Swarm gel. Expression of NST was dependent on cell density and matrix and was intense in conditions where cells spread fully, whereas 3-OST expression was unchanged in the conditions examined. The PG sulfation inhibitor sodium chlorate significantly inhibited cultured type II cell spreading, and this inhibition was reversed by sodium sulfate. These results suggest that highly sulfated PGs modified by NST are necessary for the spreading of cells during transdifferentiation of type II cells to mature type I cells.