Expression of cytochrome P-450 enzymes in cultured human hepatocytes

Implications for herbal polypharmacy: coumarin-induced hepatotoxicity increased through common herbal phytochemicals astragaloside IV and atractylenolide I

Hepatotoxicity is a well-known adverse effect of many substances, with toxicity often resulting from interactions of drugs with other drug-like substances. With the increased availability of complementary and alternative medicines, including herbal medicines, the likelihood of adverse interactions between drugs and drug-like substances in herbs increases. However, the impact of potential herb-herb interactions is little understood. To assess the potential of two cytochrome P450 enzyme modulating phytochemicals common to many herbal medicines, atractylenolide I (ATR-I) and astragaloside IV (AST-IV), to interact with coumarin, another phytochemical common in many foods, a hepatocyte function model with a liver carcinoma cell line, HepG2, was exposed to these agents. To determine the effects of cytochrome P450 modulation by these phytochemicals certain cells were induced with rifampicin to induce cytochrome P450. Increasing concentrations of ATR-I combined with a fixed, nontoxic concentration of coumarin (200 µM), demonstrated significant additive interactions. 300 µM ATR-I produced a 31% reduction in cell viability (p < 0.01) with coumarin in rifampicin uninduced cells. In rifampicin-induced cells, ATR-I (100-300 µM) produced a significant reduction in cell viability (p < 0.01) with coumarin (200 µM). AST-IV with fixed coumarin (200 µM) showed 27% toxicity at 300 µM AST-IV in rifampicin uninduced cells (p < 0.05) and 30% toxicity in rifampicin induced cells (p < 0.05). However, when fixed coumarin and AST-IV were combined with increasing concentrations of ATR-I no further significant increase in toxicity was observed (p > 0.05). These results demonstrate the potential toxic interactive capabilities of common traditional Chinese herbal medicine phytochemicals and underline the potential importance of coumarin-mediated toxicity.

Human CYP enzyme-activated genotoxicity of 2,2',4,4'-tetrabromobiphenyl ether in mammalian cells

Polybrominated biphenyl ethers (PBDEs) are a group of persistent organic pollutants with endocrine-disrupting, neurotoxic, tumorigenic and DNA-damaging activities. They are hydroxylated by human liver microsomal CYP enzymes, however, their mutagenicity remains unknown. In this study, 2,2',4,4'-tetrabromobiphenyl ether (BDE-47, relatively abundant in human tissues) was investigated for micronuclei induction and DNA damage in mammalian cells. The results indicated that BDE-47 up to 80 μM under a 6 h/18 h (exposure/recovery, covering 2 cell cycles) regime did not induce micronuclei in V79-Mz and V79-derived cell lines expressing human CYP1A1 or 1A2, while it was moderately positive in human CYP2B6-, 2E1-and 3A4-expressing cell lines (V79-hCYP2B6, V79-hCYP2E1-hSULT1A1 and V79-hCYP3A4-hOR, respectively). Following 24 h exposure, BDE-47 induced micronuclei in V79-hCYP2E1-hSULT1A1 and V79-hCYP3A4 cells at increased potencies. In the human hepatoma (HepG2) cells BDE-47 (48 h exposure) was inactive up to 40 μM, however, pretreatment of the cells with ethanol (0.2%, v:v, inducer of CYP2E1) or rifampicin (10 μM, inducer of CYP3A4) led to significant micronuclei formation by BDE-47; pretreatment with bisphenol AF (100 nM) also potentiated BDE-47-induced micronuclei formation (which was blocked by a CYP2E1 inhibitor trans-1,2-dichloroethylene or a CYP3A inhibitor (ketoconazole). Immunofluorescent staining of centromere protein B with the micronuclei formed by BDE-47 in HepG2 cells pretreated with ethanol or rifampicin demonstrated selective formation of centromere-containing micronuclei. The increased phosphorylation of both histones H2AX and H3 in HepG2 by BDE-47 also indicated an aneugenic potential. Therefore, this study suggests that BDE-47 is an aneugen activated by several human CYP enzymes.

Pyrrolizidine alkaloids cause cell cycle and DNA damage repair defects as analyzed by transcriptomics in cytochrome P450 3A4-overexpressing HepG2 clone 9 cells

Pyrrolizidine alkaloids (PAs) are a large group of highly toxic chemical compounds, which are found as cross-contaminants in numerous food products (e.g., honey), dietary supplements, herbal teas, and pharmaceutical herbal medicines. PA contaminations are responsible for serious hepatotoxicity and hepatocarcinogenesis. Health authorities have to set legal limit values to guarantee the safe consumption of plant-based nutritional and medical products without harmful health. Toxicological and chemical analytical methods are conventionally applied to determine legally permitted limit values for PAs. In the present investigation, we applied a highly sensitive transcriptomic approach to investigate the effect of low concentrations of five PAs (lasiocarpine, riddelliine, lycopsamine, echimidine, and monocrotaline) on human cytochrome P450 3A4-overexpressing HepG2 clone 9 hepatocytes. The transcriptomic profiling of deregulated gene expression indicated that the PAs disrupted important signaling pathways related to cell cycle regulation and DNA damage repair in the transfected hepatocytes, which may explain the carcinogenic PA effects. As PAs affected the expression of genes that involved in cell cycle regulation, we applied flow cytometric cell cycle analyses to verify the transcriptomic data. Interestingly, PA treatment led to an arrest in the S phase of the cell cycle, and this effect was more pronounced with more toxic PAs (i.e., lasiocarpine and riddelliine) than with the less toxic monocrotaline. Using immunofluorescence, high fractions of cells were detected with chromosome congression defects upon PA treatment, indicating mitotic failure. In conclusion, the tested PAs revealed threshold concentrations, above which crucial signaling pathways were deregulated resulting in cell damage and carcinogenesis. Cell cycle arrest and DNA damage repair point to the mutagenicity of PAs. The disturbance of chromosome congression is a novel mechanism of Pas, which may also contribute to PA-mediated carcinogenesis. Transcriptomic, cell cycle, and immunofluorescence analyses should supplement the standard techniques in toxicology to unravel the biological effects of PA exposure in liver cells as the primary target during metabolization of PAs.

Generation of proliferating human adult hepatocytes using optimized 3D culture conditions

Generating the proliferation of differentiated normal adult human hepatocytes is a major challenge and an expected central step in understanding the microenvironmental conditions that regulate the phenotype of human hepatocytes in vitro. In this work, we described optimized 3D culture conditions of primary human hepatocytes (PHH) to trigger two waves of proliferation and we identified matrix stiffness and cell-cell interactions as the main actors necessary for this proliferation. We demonstrated that DNA replication and overexpression of cell cycle markers are modulate by the matrix stiffness while PHH cultured in 3D without prior cellular interactions did not proliferate. Besides, we showed that PHH carry out an additional cell cycle after transient inhibition of MAPK MER1/2-ERK1/2 signaling pathway. Collagen cultured hepatocytes are organized as characteristic hollow spheroids able to maintain survival, cell polarity and hepatic differentiation for long-term culture periods of at least 28 days. Remarkably, we demonstrated by transcriptomic analysis and functional experiments that proliferating cells are mature hepatocytes with high detoxication capacities. In conclusion, the advanced 3D model described here, named Hepoid, is particularly relevant for obtaining normal human proliferating hepatocytes. By allowing concomitant proliferation and differentiation, it constitutes a promising tool for many pharmacological and biotechnological applications.

Derivation of CYP3A4 and CYP2B6 degradation rate constants in primary human hepatocytes: A siRNA-silencing-based approach

The first-order degradation rate constant (k_deg) of cytochrome P450 (CYP) enzymes is a known source of uncertainty in the prediction of time-dependent drug-drug interactions (DDIs) in physiologically-based pharmacokinetic (PBPK) modelling. This study aimed to measure CYP k_deg using siRNA to suppress CYP expression in primary human hepatocytes followed by incubation over a time-course and tracking of protein expression and activity to observe degradation. The magnitude of gene knockdown was determined by qPCR and activity was measured by probe substrate metabolite formation and CYP2B6-Glo™ assay. Protein disappearance was determined by Western blotting. During a time-course of 96 and 60 h of incubation, over 60% and 76% mRNA knockdown was observed for CYP3A4 and CYP2B6, respectively. The k_deg of CYP3A4 and CYP2B6 protein was 0.0138 h^-1 (±0.0023) and 0.0375 h^-1 (±0.025), respectively. The k_deg derived from probe substrate metabolism activity was 0.0171 h^-1 (±0.0025) for CYP3A4 and 0.0258 h^-1 (±0.0093) for CYP2B6. The CYP3A4 k_deg values derived from protein disappearance and metabolic activity were in relatively good agreement with each other and similar to published values. This novel approach can now be used for other less well-characterised CYPs.

Polarized location of SLC and ABC drug transporters in monolayer-cultured human hepatocytes

Human hepatocytes cultured in a monolayer configuration represent a well-established in vitro model in liver toxicology, notably used in drug transporter studies. Polarized status of drug transporters, i.e., their coordinated location at sinusoidal or canalicular membranes, remains however incompletely documented in these cultured hepatocytes. The present study was therefore designed to analyze transporter expression and location in such cells. Most of drug transporters were first shown to be present at notable mRNA levels in monolayer-cultured human hepatocytes. Cultured human hepatocytes, which morphologically exhibited bile canaliculi-like structures, were next demonstrated, through immunofluorescence staining, to express the influx transporters organic anion transporting polypeptide (OATP) 1B1, OATP2B1 and organic cation transporter (OCT) 1 and the efflux transporter multidrug resistance-associated protein (MRP) 3 at their sinusoidal pole. In addition, the efflux transporters P-glycoprotein and MRP2 were detected at the canalicular pole of monolayer-cultured human hepatocytes. Moreover, canalicular secretion of reference substrates for the efflux transporters bile salt export pump, MRP2 and P-glycoprotein as well as sinusoidal drug transporter activities were observed. This polarized and functional expression of drug transporters in monolayer-cultured human hepatocytes highlights the interest of using this human in vitro cell model in xenobiotic transport studies.

Mechanistic biomarkers in acetaminophen-induced hepatotoxicity and acute liver failure: from preclinical models to patients

INTRODUCTION

Drug hepatotoxicity is a major clinical issue. Acetaminophen (APAP) overdose is especially common. Serum biomarkers used to follow patient progress reflect either liver injury or function, but focus on biomarkers that can provide insight into the basic mechanisms of hepatotoxicity is increasing and enabling us to translate mechanisms of toxicity from animal models into humans.

AREAS COVERED

We review recent advances in mechanistic serum biomarker research in drug hepatotoxicity. Specifically, biomarkers for reactive drug intermediates, mitochondrial dysfunction, nuclear DNA damage, mode of cell death and inflammation are discussed, as well as microRNAs. Emphasis is placed on APAP-induced liver injury.

EXPERT OPINION

Several serum biomarkers of reactive drug intermediates, mitochondrial damage, nuclear DNA damage, apoptosis and necrosis and inflammation have been described. These studies have provided evidence that mitochondrial damage is critical in APAP hepatotoxicity in humans, while apoptosis has only a minor role, and inflammation is important for recovery and regeneration after APAP overdose. Additionally, mechanistic serum biomarkers have been shown to predict outcome as well as, or better than, some clinical scores. In the future, such biomarkers will help determine the need for liver transplantation and, with improved understanding of the human pathophysiology, identify novel therapeutic targets.

Liver slices in in vitro pharmacotoxicology with special reference to the use of human liver tissue

In the early years of research in in vitro pharmacotoxicology liver slices have been used. After a decline in the application of slices in favour of the use of isolated hepatocytes and the isolated perfused liver preparation, the development of the Krumdieck slicer in the 1980s led to a ;comeback' of the technique. This review will focus on the use of human liver, with special reference to the comparison of slices with isolated hepatocytes in in vitro pharmacotoxicology. In addition, an overview on the predictive value of these in vitro systems for drug disposition and toxicity in vivo will be given. Preservation techniques for liver slices and hepatocytes will also be discussed. These techniques ensure an efficient utilization of the scarce human material. For long-term storage of liver slices and hepatocytes, cryopreservation seems most promising. However, cryopreservation is still in its infancy, and reports mainly deal with drug metabolism studies after cryopreservation. Drug toxicity, metabolism and transport data determined in slices and isolated hepatocytes, from both human and animal liver showed good correlation with the corresponding parameters measured in vivo. Therefore, the results obtained in such studies may give rise to more in-depth research on the mechanisms of pharmactoxicology in the human liver.

Expression of Cytochromes P-450 2E1, 3A4 and 1A1/1A2 in Growing and Confluent Human HepG2 Hepatoma Cells-Effect of Ethanol

In cultured human hepatoma HepG2 cells, cytochrome (CYP) 1A-associated 7-ethoxyresorufin-O-deethylase (EROD), CYP 3A-associated benzyloxyresorufin O-debenzylase (BROD) and CYP 2E1-associated p-nitrophenol-hydroxylase (PNPH) decreased during time in culture. The enzyme activities in cells at confluence were 35-60% of the activities in cells 24 hours after seeding. Similarly, CYP 3A and CYP 2E1 proteins were present at higher concentrations in growing (G) than in confluent (C) HepG2 cells. CYP 1A1/1A2 protein was not detected, neither in G nor in C HepG2 cells but was strongly induced by 3-methylcholanthrene (3-MC) treatment. Ethanol (EtOH) was shown to increase CYP 2E1 and CYP 3A proteins and CYP 1A1/1A2-, CYP 2E1- and CYP 3A-associated mixed-function oxidase activities (MFOs) in HepG2 cells, as has been previously reported for primary cultures of human hepatocytes. These effects were observed only at the beginning of culture, in growing HepG2 cells, demonstrating the influence of the growth stage of HepG2 cells on their response to EtOH treatment. This is, to our knowledge, the first report on increases in CYP proteins and associated MFOs by EtOH in HepG2 cells. It suggests that growing HepG2 cells provide a useful in vitro model system in which to study the regulation of human CYPs by EtOH.

Interindividual variability in gene expression profiles in human hepatocytes and comparison with HepaRG cells

Interindividual variations in functions other than drug metabolism activity, remain poorly elucidated in human liver. In the present study, the whole transcriptome of several human hepatocyte populations and the differentiated human HepaRG cell line have been analyzed and compared, using oligonucleotide pangenomic microarrays. We show that, although the variation in the percentages of expressed genes did not exceed 14% among the primary human hepatocyte populations, huge interindividual differences in the transcript levels of many genes were observed. These genes were related to various functions; in addition to drug metabolism, they mainly concerned carbohydrate, amino acid, and lipid metabolism. HepaRG cells expressed from 81 to 92% of the genes active in human hepatocytes and, in addition, a specific gene subset mainly related to their transformed status, some chromosomal abnormalities, and the presence of primitive biliary epithelial cells. Of interest, a relationship was evidenced between abnormal basal expression levels of some target genes and their corresponding previously reported fold changes in one of four human hepatocyte populations treated with the hepatotoxic drug troglitazone and not with other nonhepatotoxic peroxisome proliferator-activated receptor agonists (PLoS One 6:e18816, 2011). Taken together, our results support the view that HepaRG cells express most of the genes active in primary human hepatocytes and show that expression of most human hepatic genes can quantitatively greatly vary among individuals, thereby contributing to explain the huge interindividual variability in susceptibility to drugs and other environmental factors.

General review on in vitro hepatocyte models and their applications

In vitro hepatocyte models represent very useful systems in both fundamental research and various application areas. Primary hepatocytes appear as the closest model for the liver in vivo. However, they are phenotypically unstable, have a limited life span and in addition, exhibit large interdonor variability when of human origin. Hepatoma cell lines appear as an alternative but only the HepaRG cell line exhibits various functions, including major cytochrome P450 activities, at levels close to those found in primary hepatocytes. In vitro hepatocyte models have brought a substantial contribution to the understanding of the biochemistry, physiology, and cell biology of the normal and diseased liver and in various application domains such as xenobiotic metabolism and toxicity, virology, parasitology, and more generally cell therapies. In the future, new well-differentiated hepatocyte cell lines derived from tumors or from either embryonic or adult stem cells might be expected and although hepatocytes will continue to be used in various fields, these in vitro liver models should allow marked advances, especially in cell-based therapies and predictive and mechanistic hepatotoxicity of new drugs and other chemicals. All models will benefit from new developments in throughput screening based on cell chips coupled with high-content imaging and in toxicogenomics technologies.

Stable expression, activity, and inducibility of cytochromes P450 in differentiated HepaRG cells

HepaRG cells possess the unique property to differentiate in vitro and to express various functions of mature hepatocytes, including the major cytochromes P450 (P450s). In the present study, we carefully analyzed mRNA expression and activity of the major P450s and their responsiveness to three prototypical inducers, phenobarbital, rifampicin, and omeprazole, in differentiated HepaRG cell cultures over a 4-week period after low and high seeding. Only minor differences were observed in P450 activities when measured by two cocktails of probe substrates, probably related to the choice and/or concentration of substrates. Similar results were obtained from the two cell seeding conditions. Expression and activities of several P450s were dimethyl sulfoxide-dependent. However, basal P450 expression and activities as well as their responsiveness to the prototypical inducers were well maintained over the 4-week period, and a good correlation was observed between transcript levels and corresponding activities. Thus, CYP1A2, CYP2B6, and CYP3A4 were found to accurately respond to their respective prototypical inducers, i.e., omeprazole, phenobarbital, and rifampicin. Likewise, basal expression of several phase II enzymes, transporters, and nuclear receptors, and response to inducers were also well preserved. More genes were found to be induced in HepaRG cells than in primary human hepatocytes, and no marked variation was noticed between the different passages. Taken together, these data support the conclusion that HepaRG cells represent a promising surrogate to primary human hepatocytes for xenobiotic metabolism and toxicity studies.

Assessing the metabolic competence of sandwich-cultured mouse primary hepatocytes

Primary human and rat hepatocyte cultures are well established in vitro systems used in toxicological studies. However, whereas transgenic mouse models provide an opportunity for studying mechanisms of toxicity, mouse primary hepatocyte cultures are less well described. The potential usefulness of a mouse hepatocyte-based in vitro model was assessed in this study by investigating time-dependent competence for xenobiotic metabolism and gene expression profiles. Primary mouse hepatocytes, isolated using two-step collagenase perfusion, were cultured in a collagen sandwich configuration. Gene expression profiles and the activities of various cytochrome P450 (P450) enzymes were determined after 0, 42, and 90 h in culture. Principal component analysis of gene expression profiles shows that replicates per time point are similar. Gene expression levels of most phase I biotransformation enzymes decrease to approximately 69 and 57% of the original levels at 42 and 90 h, respectively, whereas enzyme activities for most of the studied P450s decrease to 59 and 34%. The decrease for phase II gene expression is only to 96 and 92% of the original levels at 42 and 90 h, respectively. Pathway analysis reveals initial effects at the level of proteins, external signaling pathways, and energy production. Later effects are observed for transcription, translation, membranes, and cell cycle-related gene sets. These results indicate that the sandwich-cultured primary mouse hepatocyte system is robust and seems to maintain its metabolic competence better than that of the rat hepatocyte system.

Reproducible chemical-induced changes in gene expression profiles in human hepatoma HepaRG cells under various experimental conditions

The use of in vitro human liver cell models is an attractive approach in toxicogenomic studies designed to analyze gene expression changes induced by a toxic chemical. However, in such studies, reliability, reproducibility and interlaboratory concordance of microarrays, as well as the choice of the most suitable cell model, remain a matter of debate. This work was aimed at evaluating the robustness of microarray technologies and the suitability of the highly differentiated human HepaRG cell line in the investigation of gene expression changes induced by a toxic compound in human liver. The influence of various experimental conditions including cell cultures grown at different test sites, different generations of microarrays, RNA analysis platforms and softwares, was tested on gene expression profiles induced by a 20h treatment with an 8mM concentration of phenobarbital as the toxic compound. As many as 1099 genes (p-value<0.01 and 1.5-fold-change), representing 74% and 30% of the signature genes detected with Agilent 22 and 44K pangenomic microarrays, respectively, were shown to be modulated in common in six independently performed experiments. The most modulated genes included both those known to be regulated by phenobarbital, such as cytochromes P450 and membrane transporters, and those involved in oxidative stress, inflammation and apoptosis, typifying a toxic insult. These data provide strong support for the use of a toxicogenomic approach for the in vitro prediction of chemical toxicity, and for the choice of human HepaRG cells as a promising model system for human hepatotoxicity testing.

Dose- and time-dependent effects of phenobarbital on gene expression profiling in human hepatoma HepaRG cells

Phenobarbital (PB) induces or represses a wide spectrum of genes in rodent liver. Much less is known about its effects in human liver. We used pangenomic cDNA microarrays to analyze concentration- and time-dependent gene expression profile changes induced by PB in the well-differentiated human HepaRG cell line. Changes in gene expression profiles clustered at specific concentration ranges and treatment times. The number of correctly annotated genes significantly modulated by at least three different PB concentration ranges (spanning 0.5 to 3.2 mM) at 20 h exposure amounted to 77 and 128 genes (p< or =0.01) at 2- and 1.8-fold filter changes, respectively. At low concentrations (0.5 and 1 mM), PB-responsive genes included the well-recognized CAR- and PXR-dependent responsive cytochromes P450 (CYP2B6, CYP3A4), sulfotransferase 2A1 and plasma transporters (ABCB1, ABCC2), as well as a number of genes critically involved in various metabolic pathways, including lipid (CYP4A11, CYP4F3), vitamin D (CYP24A1) and bile (CYP7A1 and CYP8B1) metabolism. At concentrations of 3.2 mM or higher after 20 h, and especially 48 h, increased cytotoxic effects were associated with disregulation of numerous genes related to oxidative stress, DNA repair and apoptosis. Primary human hepatocyte cultures were also exposed to 1 and 3.2 mM PB for 20 h and the changes were comparable to those found in HepaRG cells treated under the same conditions. Taken altogether, our data provide further evidence that HepaRG cells closely resemble primary human hepatocytes and provide new information on the effects of PB in human liver. These data also emphasize the importance of investigating dose- and time-dependent effects of chemicals when using toxicogenomic approaches.

Evolving concepts in liver tissue modeling and implications for in vitro toxicology

The development of human cell models stably expressing functional properties of the in vivo cells they are derived from for predicting toxicity of chemicals is a major challenge. For mimicking the liver, a major target of toxic chemicals, primary hepatocytes represent the most pertinent model. Their use is limited by interdonor functional variability and early phenotypic changes although their lifespan can be extended not only by culturing in a 2D dimension under sophisticated conditions but also by the use of synthetic and natural scaffolds as 3D supporting templates that allow cells to have a more stable microenvironment. Hepatocytes derived from stem cells could be the most appropriate alternative but up to now only liver progenitors/hepatoblasts are obtained in vitro. A few hepatocyte cell lines have retained a variable set of liver-specific functions. Among them are the human hepatoma HepaRG cells that express drug metabolism capacity at levels close to those found in primary hepatocytes making them a suitable model for both acute and chronic toxicity studies. New screening strategies are now proposed based on miniaturized and automated systems; they include the use of microfluidic chips and cell chips coupled with high content imaging analysis. Toxicogenomics technologies (particularly toxicotranscriptomics) have emerged as promising in vitro approaches for better identification and discrimination of cellular responses to chemicals. They should allow to discriminate compounds on the basis of the identification of a set of markers and/specific signaling pathways.

Role of cytochrome P450 in drug interactions

Drug-drug interactions have become an important issue in health care. It is now realized that many drug-drug interactions can be explained by alterations in the metabolic enzymes that are present in the liver and other extra-hepatic tissues. Many of the major pharmacokinetic interactions between drugs are due to hepatic cytochrome P450 (P450 or CYP) enzymes being affected by previous administration of other drugs. After coadministration, some drugs act as potent enzyme inducers, whereas others are inhibitors. However, reports of enzyme inhibition are very much more common. Understanding these mechanisms of enzyme inhibition or induction is extremely important in order to give appropriate multiple-drug therapies. In future, it may help to identify individuals at greatest risk of drug interactions and adverse events.

Long-term functional stability of human HepaRG hepatocytes and use for chronic toxicity and genotoxicity studies

The human hepatoma HepaRG cells are able to differentiate in vitro into hepatocyte-like cells and to express various liver-specific functions, including the major cytochromes P450. This study was aimed to determine whether differentiated HepaRG cells retained their specific functional capacities for a long time period at confluence. We show that expression of transcripts encoding CYP1A2, 2B6, 3A4, and 2E1, several phase II and antioxidant enzymes, membrane transporters, including organic cation transporter 1 and bile salt export pump, the nuclear receptors constitutive androstane receptor and pregnane X receptor, and aldolase B remained relatively stable for at least the 4-week confluence period tested. Similarly, activities of CYP3A4 and CYP1A2 and their responsiveness to prototypical inducers were well preserved. Aflatoxin B(1), a potent hepatotoxicant and carcinogen, induced a dose-dependent and cumulative cytotoxicity. Furthermore, at a concentration as low as 0.1 microM, this mycotoxin caused a decrease in both CYP3A4 activity and intracellular ATP associated with morphological alterations, after 14 days following every 2-day exposure. Moreover, using the comet assay, a dose-dependent DNA damage was observed after a 3-h treatment of differentiated HepaRG cells with 1 to 5 microM aflatoxin B(1) in the absence of any cell damage, and this DNA damaging effect was strongly reduced in the presence of ketoconazole, a CYP3A4 inhibitor. These results bring the first demonstration of long-term stable expression of liver-specific markers in HepaRG hepatocyte cultures maintained at confluence and show that these cells represent a suitable in vitro liver cell model for analysis of acute and chronic toxicity as well as genotoxicity of chemicals in human liver.

Rifampicin is only a weak inducer of CYP1A2-mediated presystemic and systemic metabolism: studies with tizanidine and caffeine

OBJECTIVE

Rifampicin greatly reduces the plasma concentrations of many drugs. Our aim was to characterise the inducibility of cytochrome P450 (CYP) 1A2 by rifampicin, using tizanidine and caffeine as probe drugs for presystemic and systemic CYP1A2-mediated metabolism.

METHODS

In a randomised, 2-phase crossover study, ten healthy volunteers were given a 5-day pretreatment with 600 mg rifampicin or placebo once daily. On day 6, a single 4-mg dose of tizanidine was administered orally. Plasma and urine concentrations of parent tizanidine and several of its metabolites (M-3, M-4, M-5, M-9, M-10) and pharmacodynamic variables were measured up to 24 h. A caffeine test was performed in both phases.

RESULTS

Rifampicin moderately reduced the peak plasma concentration (by 51%; P = 0.002) and area under the plasma concentration-time curve [AUC(0-infinity)] (by 54%; P = 0.009) of parent tizanidine, and had no effect on its half-life. The tizanidine/M-3 and tizanidine/M-4 AUC(0-infinity) ratios were slightly (by 30%; P = 0.014; and by 38%; P = 0.007) decreased by rifampicin. Also, the excretion of metabolites M-3, M-4 and M-5 into urine was reduced (P < 0.005), but that of M-10 was increased (P = 0.008) by rifampicin. Rifampicin reduced the tizanidine/M-10 ratio (by 55%; P = 0.047) but had no significant effect on the other tizanidine/metabolite ratios in urine. The caffeine/paraxanthine ratio was reduced by 23% (P = 0.081) by rifampicin. The effect of rifampicin on the caffeine/paraxanthine ratio correlated significantly with the effect of rifampicin on, for example, the AUC(0-infinity) of tizanidine and the tizanidine/M-3 AUC(0-infinity) ratio. The pharmacodynamic effects of tizanidine were reduced by rifampicin.

CONCLUSIONS

Rifampicin moderately decreases the plasma concentrations of tizanidine. The strong inducing effects of rifampicin on other CYP enzymes, e.g. CYP3A4, may have contributed to the findings, and the inducibility of CYP1A2-mediated presystemic (tizanidine) and systemic (tizanidine, caffeine) metabolism by rifampicin is weak at the most. Compared to CYP3A4 substrate drugs, substrates of CYP1A2 are much less susceptible to drug interactions caused by enzyme inducers of the rifampicin type.

Expression of cytochromes P450, conjugating enzymes and nuclear receptors in human hepatoma HepaRG cells

Most human hepatocyte cell lines lack a substantial set of liver-specific functions, especially major cytochrome P450 (P450)-related enzyme activities, making them unrepresentative of in vivo hepatocytes. We have used the HepaRG cells, derived from a human hepatocellular carcinoma, which exhibit a high differentiation pattern after 2 weeks at confluency to determine whether they could mimic human hepatocytes for drug metabolism and toxicity studies. We show that when passaged at low density, these cells reversed to an undifferentiated morphology, actively divided, and, after having reached confluency, formed typical hepatocyte-like colonies surrounded by biliary epithelial-like cells. By contrast, when seeded at high density, hepatocyte-like clusters retained their typical differentiated morphology. Transcripts of various nuclear receptors (aryl hydrocarbon receptor, pregnane X receptor, constitutive androstane receptor, peroxisome proliferator-activated receptor alpha), P450s (CYP1A2, 2C9, 2D6, 2E1, 3A4), phase 2 enzymes (UGT1A1, GSTA1, GSTA4, GSTM1), and other liver-specific functions were estimated by reverse transcriptase-quantitative polymerase chain reaction and were found to be expressed, for most of them, at comparable levels in both confluent differentiated and high-density differentiated HepaRG cells and in cultured primary human hepatocytes. For several transcripts, the levels were strongly increased in the presence of 2% dimethyl sulfoxide. Measurement of basal activities of several P450s and their response to prototypical inducers as well as analysis of metabolic profiles and cytotoxicity of several compounds confirmed the functional resemblance of HepaRG cells to primary cultured human hepatocytes. In conclusion, HepaRG cells constitute the first human hepatoma cell line expressing high levels of the major P450s involved in xenobiotic metabolism and represent a reliable surrogate to human hepatocytes for drug metabolism and toxicity studies.

Expression and induction potential of cytochromes P450 in human cryopreserved hepatocytes

Fresh human hepatocytes are still considered as the "gold standard" to screen in vitro for cytochrome P450 (P450) induction. However, sparse availability of good quality human liver tissue for research purposes and the demand for standardized cell populations, together with the need for proper storage of the cells not immediately required, have resulted in the development of cryopreservation techniques that provide adequate viability and plateability of hepatocytes after thawing. This study aimed at validating cryopreserved human hepatocytes as a model to investigate P450 induction. Cryopreserved cells from four different donors were plated and cultured for 48 h, followed by incubation in the presence of typical P450 inducers. During the experiments, quality of the cultured cells was monitored both physiologically and morphologically. Concomitantly, the activity of CYP1A2, 2B6, 2C9, 2E1, and 3A4 was measured together with their mRNA and protein expression. Determination of CYP1A2, 2B6, 2C9, 2E1, and 3A4 activity in control versus prototypical inducer-treated hepatocytes revealed a maximal significant mean 11.6-, 2.8-, 1.9-, 1.5-, and 9.0-fold induction over their basal expression, respectively. Protein expression analysis of these P450s confirmed these results. Moreover, a mean 44.9-, 3.5-, 3.2-, and 13.8-fold induction of CYP1A2, 2B6, 2C9, and 3A4 mRNA was observed. Our data demonstrate that cryopreserved human hepatocytes are a valuable tool to study the induction of CYP1A2, 2B6, 2C9, 2E1, and 3A4.

Influence of culture time on the expression of drug-metabolizing enzymes in primary human hepatocytes and hepatoma cell line HepG2

Primary cultures of human hepatocytes and hepatoma cell line HepG2 are frequently used to evaluate the hepatic disposition of drugs and other xenobiotics. To check the variability of the expression of drug-metabolizing enzymes in these in vitro models, expression of genes coding for several cytochrome P450 isoforms and phase II enzymes was quantified during culture time by real-time RT-PCR. Gene expression was determined daily for primary hepatocytes maintained in a sandwich culture over 1 week and for HepG2, during the first 10 passages. In primary hepatocytes characteristic expression trends were observed which could be abstracted into three major classes of time curves. Genes of the first and the second class had an expression maximum around day 6 and day 4 in culture, respectively. The third class of genes had two expression peaks: at day 1 and 5 in culture. Surprisingly, also the cell line HepG2 showed significant expression changes during passages. For example, gene expression of cytochrome 1A1 varied 8-fold, that of cytochrome 2B6 30-fold, and that of NADP-quinone reductase 1 more than 200-fold within the first 10 passages. In conclusion, neither primary hepatocytes nor HepG2 cell line display a model for constant expression of drug-metabolizing enzymes.

Pharmacokinetic interactions with rifampicin : clinical relevance

The antituberculosis drug rifampicin (rifampin) induces a number of drug-metabolising enzymes, having the greatest effects on the expression of cytochrome P450 (CYP) 3A4 in the liver and in the small intestine. In addition, rifampicin induces some drug transporter proteins, such as intestinal and hepatic P-glycoprotein. Full induction of drug-metabolising enzymes is reached in about 1 week after starting rifampicin treatment and the induction dissipates in roughly 2 weeks after discontinuing rifampicin. Rifampicin has its greatest effects on the pharmacokinetics of orally administered drugs that are metabolised by CYP3A4 and/or are transported by P-glycoprotein. Thus, for example, oral midazolam, triazolam, simvastatin, verapamil and most dihydropyridine calcium channel antagonists are ineffective during rifampicin treatment. The plasma concentrations of several anti-infectives, such as the antimycotics itraconazole and ketoconazole and the HIV protease inhibitors indinavir, nelfinavir and saquinavir, are also greatly reduced by rifampicin. The use of rifampicin with these HIV protease inhibitors is contraindicated to avoid treatment failures. Rifampicin can cause acute transplant rejection in patients treated with immunosuppressive drugs, such as cyclosporin. In addition, rifampicin reduces the plasma concentrations of methadone, leading to symptoms of opioid withdrawal in most patients. Rifampicin also induces CYP2C-mediated metabolism and thus reduces the plasma concentrations of, for example, the CYP2C9 substrate (S)-warfarin and the sulfonylurea antidiabetic drugs. In addition, rifampicin can reduce the plasma concentrations of drugs that are not metabolised (e.g. digoxin) by inducing drug transporters such as P-glycoprotein. Thus, the effects of rifampicin on drug metabolism and transport are broad and of established clinical significance. Potential drug interactions should be considered whenever beginning or discontinuing rifampicin treatment. It is particularly important to remember that the concentrations of many of the other drugs used by the patient will increase when rifampicin is discontinued as the induction starts to wear off.

Induction of CYP3A4 by 1alpha,25-dyhydroxyvitamin D3 in HepG2 cells

CYP3A4, the predominant cytochrome P450 (CYP) expressed in human liver, contributes to the metabolism of approximately half the drugs in use today. In general, human-derived cell lines fail to express CYPs. It was previously shown that CYP3A4 mRNA and CYP3A immunoreactive protein are induced by 1alpha,25-dyhydroxyvitamin D(3) (1alpha,25-(OH)(2)D(3)) in the human colon carcinoma cell line Caco-2. The aim of the present study was to examine whether 1alpha,25-(OH)(2)D(3) regulates CYP3A4 gene expression in HepG2 cells, a human hepatocarcinoma cell line. Treatment with 1alpha,25-(OH)(2)D(3) resulted in an induction of CYP3A4 mRNA and CYP3A4 immunoreactive protein, 1.5-fold and 4.0-fold respectively, when compared to control cultures, in a time-dependent fashion. These observations are in agreement with previous reports suggesting a role of 1alpha,25-(OH)(2)D(3) on CYP3A4 transcription regulation, and demonstrate that this hormone, as in Caco-2 cells, increase CYP3A4 levels in HepG2 cells. In conclusion, HepG2 cell cultures treated with 1alpha,25-(OH)(2)D(3), provides a useful model to study the function of CYP3A4 and its role in drug liver metabolism.

Induction of cytochrome P450 2B6 and 3A4 expression by phenobarbital and cyclophosphamide in cultured human liver slices

PURPOSE

To examine the potential of cultured human liver slices to predict cytochrome P450 (CYP) inducibility, regarding global and zonal CYP expression, together with drug-induced histologic changes.

METHODS

We first assessed whether CYP2B6, 3A4, and 2C9 expression was maintained in cultured liver slices. Cultured hepatocytes were used as the reference culture system. Then we tested the effects of phenobarbital and cyclophosphamide on CYP expression in both models.

RESULTS

Morphologic features are preserved in slices. Basal CYP expression declines with time in culture in both models. Slices display the same region specificity of CYP2B6, 2C9, and 3A4 expression as intact liver. CYP2B6 and 3A4 mRNA, apoprotein, and enzyme-related activities were induced by phenobarbital and cyclophosphamide, whereas CYP2C9 apoprotein was not. Their immunoreactivities were also increased, while their zonal distribution was preserved on slice tissue sections. Microsomal enzyme induction was confirmed by histology.

CONCLUSIONS

Cultured human liver slices are an attractive alternative to hepatocyte culture for the prediction of human CYP isoenzyme induction by xenobiotics.

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.

Pregnane X receptor-dependent and -independent effects of 2-acetylaminofluorene on cytochrome P450 3A23 expression and liver cell proliferation

The arylamide 2-acetylaminofluorene (AAF) is a powerful carcinogen displaying a marked promoting activity, also known to regulate expression of liver detoxifying proteins. In this study we identified CYP3A23, a major inducible cytochrome P-450 (CYP) isoform, as an AAF target in hepatocytes. Indeed, exposure to AAF of primary rat hepatocytes resulted in a marked up-regulation of CYP3A23 expression at both mRNA and protein levels. Using CYP3A23 reporter gene constructs, we further demonstrated that AAF activated the CYP3A23 Direct Repeat 3 (DR3) promoter element interacting with the nuclear pregnane X receptor (PXR). Moreover, the PXR antagonist ecteinascidin-743 fully suppressed AAF-related CYP3A23 induction. Low doses of AAF inhibiting DNA synthesis in hepatocytes however failed to trigger PXR-related CYP3A23 induction and PXR-negative epithelial liver cells remained sensitive to the mito-inhibitory effects of AAF. Such data indicate that AAF up-regulates CYP3A23 through PXR activation but does not require PXR for exerting its carcinogenic promoting properties based on inhibition of cell growth.

Oxidative damage to the hepatocellular proteins after chronic ethanol intake in the rat

BACKGROUND

Protein carbonyl content, a measure of oxidative damage to hepatocellular proteins, and the activities of some thiol-containing proteins were assayed in the liver and plasma, as thiol-containing protein, appear to be targets for free radicals. These may be important in the mechanism of ethanol-induced liver injury.

METHODS

Tap water containing ethanol at the concentration of 25% (v/v) and phenobarbital (500 mg/l) was the only source of drinking water for the experimental rats for 24 months. Another group of rats were administered 25% (v/v) ethanol alone in drinking water for 24 months. Control rats were administered either phenobarbital alone in drinking water or tap water for 24 months. At the end of 24 months, the rats were sacrificed. The protein carbonyl content, activities of glutamine synthase and biotinidase-sulfhydryl group containing enzymes were assayed in the liver along with alkaline protease, an enzyme that degrades oxidized proteins. The total thiol, albumin and the activity of biotinidase were measured in the plasma.

RESULTS

The protein carbonyl content of the liver was increased in the ethanol/phenobarbital-treated rats as well as in the ethanol-treated rats as compared with the controls. The activities of glutamine synthase and biotinidase were decreased significantly in the livers of ethanol/phenobarbital-treated rats as well as the ethanol-treated rats as compared with the controls. The activity of alkaline protease was increased significantly in both the ethanol-treated groups. In the plasma of ethanol/phenobarbital-treated rats as well as the ethanol-treated rats total thiol, albumin and the activity of biotinidase were decreased significantly as compared with the controls. The ethanol/phenobarbital-treated rats as well as the ethanol-treated rats developed fatty liver.

CONCLUSIONS

Damage to proteins occurs upon chronic ethanol intake in the rat, and it may play a role in the pathogenesis of alcohol-induced fatty liver.

Regulation of human CYP2C9 by the constitutive androstane receptor: discovery of a new distal binding site

The CYP2C subfamily metabolizes many clinically important drugs. These genes respond to prototypical inducers such as phenobarbital and rifampicin, yet little has been reported on the mechanisms of induction. This report examines the regulation of CYP2C9 with respect to two specific receptors thought to be involved in phenobarbital (PB) induction, the constitutive androstane receptor (CAR) and pregnane X receptor (PXR). Transfection of either mouse CAR (mCAR) or human CAR (hCAR) into HepG2 cells results in increased CYP2C9 mRNA content. Inducers further increased this response in CAR transfected cells. mCAR but not hCAR conferred drug inducibility to the proximal -2145 bp of the CYP2C9 promoter in luciferase assays. Further examination of a -2925-bp promoter construct revealed that hCAR cotransfection increased activity 20-fold. Gel shift assays confirmed the presence of a distal PB-responsive enhancer module-like enhancer module, CAR-responsive enhancer (CAR-RE), between -2900 and -2841 bp consisting of two DR-5 nuclear receptor binding motifs capable of binding hCAR, mCAR, and, to a lesser extent, human PXR. The majority of binding and hCAR activation is derived from the NR1 portion of the CAR-RE. PB treatment did not further increase the hCAR activation in any of the constructs. In summary, a new CAR/PXR binding site was identified in the CYP2C9 promoter, and this site seems to constitutively regulate transcription via a CAR-dependent mechanism; however, it could not be shown to account for PB inducibility of the gene.

Three-dimensional co-culture of primary human liver cells in bioreactors for in vitro drug studies: effects of the initial cell quality on the long-term maintenance of hepatocyte-specific functions

In vitro culture models that employ human liver cells could be potent tools for predictive studies on drug toxicity and metabolism in the pharmaceutical industry. A bioreactor culture model was developed that permits the three-dimensional co-culture of liver cells under continuous medium perfusion with decentralised mass exchange and integral oxygenation. We tested the ability of the system to support the long-term maintenance and differentiation of primary human liver cells. The effects of the initial cell quality were investigated by comparing cultures from resected, non-preserved liver with cultures from liver graft tissue damaged by long-term preservation. In cultures originating from non-preserved liver, protein and urea synthesis, glucose metabolism, and cytochrome (CYP450) activities were stable over the 2-week culture period, with maximal activities at the end of the first week in culture. Enzyme induction led to increased 7-ethoxyresorufin O-deethylase activities of up to 20 times the basal value. In cultures from preservation-damaged liver, recovery of metabolic activities was detected during bioreactor culture. After two weeks, most biochemical parameters approached those of cultures from non-preserved human liver. Light microscopy demonstrated the three-dimensional reorganisation of hepatocytes and non-parenchymal cells in co-culture. Long-term maintenance, and even the regeneration of specific functional activities of human liver cells, can be achieved in the bioreactor. This could facilitate the introduction into the pharmaceutical industry of in vitro drug testing with primary human liver cells.

Expression and induction of CYP2C P450 enzymes in primary cultures of human hepatocytes

Although CYP2C8, CYP2C9, and CYP2C19 play an important role in drug biotransformation, factors influencing the expression and activity of these CYP2C P450s in human liver remain largely undefined. We used primary cultures of human hepatocytes from 15 subjects to assess the inducibility of CYP2C enzyme expression by prototypical inducer agents, including rifampicin, dexamethasone, and phenobarbital. After culture for 72 h in serum-free medium on collagen, Western blotting revealed that CYP2C9 was the only CYP2C enzyme expressed at appreciable levels in untreated hepatocytes. Subsequent treatment with 25 microMrifampicin for 48 h elicited marked increases in CYP2C8 (700 +/- 761%), CYP2C19 (854%), and CYP2C9 (209 +/- 176%) protein content versus a 550 +/- 170% enhancement of CYP3A4 enzyme levels. Parallel increases in CYP2C mRNAs, measured by Northern blotting and/or RNase protection, were found in rifampicin-treated hepatocytes, with CYP2C8, CYP2C9, and CYP2C19 transcripts exhibiting increases of 688 +/- 635, 207 +/- 49, and 230 +/- 60%, respectively, versus an 8.8-fold enhancement of CYP3A4 mRNA levels. Dexamethasone (10 microM) treatment enhanced CYP2C8 mRNA (360 +/- 100%) and protein (274%) content, although this steroid had less effect on CYP2C9 and CYP2C19 transcripts (23 +/- 21% and 21 +/- 36%, respectively) and enzyme levels (55 and 143%, respectively). Phenobarbital (100 microM) was a powerful inducer of CYP2C9 (850%) and CYP2C19 (735%) mRNA content, and also increased CYP2C8 (610%) and CYP3A4 (205%) transcripts. Our results show that CYP2C enzyme expression in human hepatocytes is highly inducible by rifampicin, dexamethasone, and phenobarbital. Because these xenobiotics are ligands and/or activators of the pregnane X receptor and/or constitutive androstane receptor, such orphan nuclear receptors and their response elements may partake in regulating CYP2C gene expression in humans.

Cytochrome P450 expression in human hepatocytes and hepatoma cell lines: molecular mechanisms that determine lower expression in cultured cells

1. Cultured hepatic cells have reduced cytochrome P450 (CYP) activities in comparison with human liver, but the mechanism(s) that underlies this circumstance is not clear. We investigated the causes of this low CYP activity by analysing the activity, protein, mRNA and heterologous nuclear RNA contents of the most important CYPs involved in drug metabolism (1A1, 1A2, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1, 3A4, 3A5) in cultured human hepatocytes, and in HepG2 and Mz-Hep-1 hepatoma cell lines. 2. After 24 h of culture, hepatocytes retained most of their CYP activities and protein contents, but the mRNA decreased 20-fold. However, the mRNA content of most CYPs in 24-h hepatocytes was still 400-fold higher than in hepatoma cells. When we examined the transcriptional activity of the CYP genes, this decreased during culture time in hepatocytes and it was poor in hepatoma cell lines. 3. We investigated the abundance of key hepatic transcription factors that govern CYP transcription (C/EBP-beta: LAP and LIP, HNF-3alpha, HNF-4alpha, RXR-alpha) and observed that the expression of some factors was altered in the hepatoma cells. 4. In conclusion, the loss of biotransformation activity in cultured hepatic cells is caused by a decrease in CYP transcription, which correlates with an alteration in the expression of key transcription factors.

Transcriptional activation of cytochrome P450 CYP2C45 by drugs is mediated by the chicken xenobiotic receptor (CXR) interacting with a phenobarbital response enhancer unit

Cytochromes P450 (CYP)-2C enzymes fulfill an important role in xenobiotic metabolism and therefore have extensively been studied in rodents and humans. However, no CYP2C genes have been described in avian species to date. In this paper, we report the cloning, functional analysis, and regulation of chicken CYP2C45. The sequence shares up to 58% amino acid identity with CYP2Cs in other species. The overexpression of CYP2C45 in chicken hepatoma cells leghorn male hepatoma (LMH) led to increased scoparone metabolism. CYP2C45 regulation was studied in LMH cells at the mRNA level and in reporter gene assays using a construct containing 2.6 kb of its 5'-flanking region. Exposure of LMH cells to phenobarbital or metyrapone led to a 95- or 210-fold increase in CYP2C45 mRNA and a 140- or 290-fold increase in reporter gene expression, respectively. A phenobarbital response enhancer unit (PBRU) of 239 bp containing a DR-4 nuclear receptor binding site was identified within the 2.6-kb fragment. Site-specific mutation of the DR-4 revealed the requirement of this motif for CYP2C45 induction by drugs. The chicken xenobiotic receptor CXR interacted with the PBRU in electromobility shift and transactivation assays. Furthermore, the related nuclear receptors, mouse PXR and mouse CAR, transactivated this enhancer element, suggesting evolutionary conservation of nuclear receptor-DNA interactions in CYP2C induction.

Pharmacokinetic aspects of treating infections in the intensive care unit: focus on drug interactions

Pharmacokinetic interactions involving anti-infective drugs may be important in the intensive care unit (ICU). Although some interactions involve absorption or distribution, the most clinically relevant interactions during anti-infective treatment involve the elimination phase. Cytochrome P450 (CYP) 1A2, 2C9, 2C19, 2D6 and 3A4 are the major isoforms responsible for oxidative metabolism of drugs. Macrolides (especially troleandomycin and erythromycin versus CYP3A4), fluoroquinolones (especially enoxacin, ciprofloxacin and norfloxacin versus CYP1A2) and azole antifungals (especially fluconazole versus CYP2C9 and CYP2C19, and ketoconazole and itraconazole versus CYP3A4) are all inhibitors of CYP-mediated metabolism and may therefore be responsible for toxicity of other coadministered drugs by decreasing their clearance. On the other hand, rifampicin is a nonspecific inducer of CYP-mediated metabolism (especially of CYP2C9, CYP2C19 and CYP3A4) and may therefore cause therapeutic failure of other coadministered drugs by increasing their clearance. Drugs frequently used in the ICU that are at risk of clinically relevant pharrmacokinetic interactions with anti-infective agents include some benzodiazepines (especially midazolam and triazolam), immunosuppressive agents (cyclosporin, tacrolimus), antiasthmatic agents (theophylline), opioid analgesics (alfentanil), anticonvulsants (phenytoin, carbamazepine), calcium antagonists (verapamil, nifedipine, felodipine) and anticoagulants (warfarin). Some lipophilic anti-infective agents inhibit (clarithromycin, itraconazole) or induce (rifampicin) the transmembrane transporter P-glycoprotein, which promotes excretion from renal tubular and intestinal cells. This results in a decrease or increase, respectively, in the clearance of P-glycoprotein substrates at the renal level and an increase or decrease, respectively, of their oral bioavailability at the intestinal level. Hydrophilic anti-infective agents are often eliminated unchanged by renal glomerular filtration and tubular secretion, and are therefore involved in competition for excretion. Beta-lactams are known to compete with other drugs for renal tubular secretion mediated by the organic anion transport system, but this is frequently not of major concern, given their wide therapeutic index. However, there is a risk of nephrotoxicity and neurotoxicity with some cephalosporins and carbapenems. Therapeutic failure with these hydrophilic compounds may be due to haemodynamically active coadministered drugs, such as dopamine, dobutamine and furosemide, which increase their renal clearance by means of enhanced cardiac output and/or renal blood flow. Therefore, coadministration of some drugs should be avoided, or at least careful therapeutic drug monitoring should be performed when available. Monitoring may be especially helpful when there is some coexisting pathophysiological condition affecting drug disposition, for example malabsorption or marked instability of the systemic circulation or of renal or hepatic function.

Induction of multidrug resistance-1 and cytochrome P450 mRNAs in human mononuclear cells by rifampin

Reverse transcription-polymerase chain reaction (RT-PCR) and quantitative, competitive RT-PCR were used to examine the capability of rifampin to induce the expression of mRNA derived from multidrug resistance-1 (MDR1) and drug-metabolizing cytochrome P450 (P450) genes in the mononuclear fraction (lymphocytes) of human blood. A total of 50 healthy volunteers (age, 18-74) participated in two studies in which 600 mg of rifampin was administered orally once daily in the evening for 7 days. Twenty of these individuals also received fexofenadine before and after rifampin dosing. MDR1 and CYP2C8 mRNAs were expressed in 100% (50 of 50) and 95% (35 of 37) of individuals, respectively, at baseline. A significant (P < 0.05; n = 37) increase in the expression of MDR1 mRNA from 176,900 +/- 122,000 to 248,500 +/- 162,300 molecules/microg of RNA was observed following rifampin administration in the human lymphocytes. There was no significant (P > 0.05) difference in MDR1 mRNA expression between males and females at baseline. Interestingly, 58% of the individuals (n = 29) demonstrated a 120% increase [95% confidence interval (CI); 120%; range, 81-153%; responders] in MDR1 mRNA expression. In contrast, the remaining 42% of individuals (n = 21) exhibited a mean decrease of -5.2% (95% CI; -5.2%; range, -15 to +4%; nonresponders). Rifampin steady-state trough serum concentrations were not significantly different (P > 0.05) between responders and nonresponders. Likewise, there was no relationship between the observed induction in MDR1 mRNA expression in lymphocytes and the observed increase in fexofenadine oral clearance in twenty volunteers. The mRNA of CYP2E1, CYP3A5, CYP3A7, CYP4A11, and CYP4B1 genes were variably expressed at baseline and following rifampin treatment. In contrast, CYP2C9 and CYP3A4 mRNAs were undetectable in lymphocytes both before and after rifampin dosing. Interindividual variability in baseline expression and inducibility of MDR1 and P450 mRNA in human lymphocytes appeared to be substantial and may not reflect the expression of these enzymes in other tissues.

Longterm stability of phase I and phase II enzymes of porcine liver cells in flat membrane bioreactors

Recently, researchers have focused on the use of bioartificial liver devices to support patients with fulminant hepatic failure. Our team developed a cell-based flat membrane bioreactor (FMB). In this, porcine liver cells were maintained in 3D-coculture between two gel layers in a sandwich configuration for 3 weeks to study the influence of this bioreactor technique on the preservation of basic, not induced activities of phase I and phase II enzymes. First, the time and substrate dependencies of the following enzymes were measured: ethoxyresorufin-O-deethylase (EROD, CYP 1A1/1A2) and ethoxycoumarin-O-deethylase (ECOD, CYP 2B6) as phase I enzymes, and glutathione-S-transferase (GST), UDP-glucuronosyltransferase (UGT) and sulfotransferase (ST) as phase II enzymes. To find optimal test conditions Michaelis-Menten kinetics were calculated. Next, different potential inducers were tested to find out the most effective compounds. Based on these results, the basic, not induced levels of the different enzymes were determined in the flat membrane bioreactor. Furthermore, the response of these enzyme activities to the chosen inducers was investigated to examine whether the cells keep their ability for drug-drug interactions. Basic, not induced activities of both phase I enzymes and the phase II enzymes GST and UGT were maintained at nearly the initial levels during the complete period of study. In addition, it was possible to induce these enzymes twice or three times in a weekly interval. In contrast, the basic, not induced activity of ST increased during the first 10 days of culture. It stabilized then and was maintained steady. As in short-term investigations, no reaction of the ST-activity towards any inducer could be obtained. These results prove that porcine liver cells preserve their phase I and phase II activities and respond to inducing drugs over 3 weeks in culture. Therefore, the flat membrane bioreactor is not only suitable for investigating drug metabolism, drug-drug interactions, and enzyme induction but also for supporting liver functions.

Human hepatocyte culture systems for the in vitro evaluation of cytochrome P450 expression and regulation

Primary cultures of human hepatocytes have been used extensively by both academic and industrial laboratories for evaluating the hepatic disposition of drugs and other xenobiotics. Their primary utility has been for assessing the induction potential of new chemical entities (NCEs) and they continue to serve as the gold standard. Primary considerations for conducting in vitro drug testing utilizing cultures of human hepatocytes, such as the effects of culture and study conditions, are discussed. The maintenance of normal cellular physiology and intercellular contacts in vitro is of particular importance for optimal phenotypic gene expression and response to drugs and other xenobiotics. Significant advances in our understanding of cytochrome P450 (CYP450) enzyme regulation have been made with the recent identification of the nuclear receptors mediating the induction of CYP2B and CYP3A enzymes. In particular, the activation of pregnane X receptor (PXR) by prototypical inducers of CYP3A has been found to correlate well with the species-specific modulation of CYP3A by various drugs and other xenobiotics. Concomitant with the discovery of PXR has been the identification of compounds that may act synergistically or antagonistically on multiple receptors (e.g., co-repressors and/or co-activators of the receptor) introducing novel mechanisms of drug-drug interactions. Differential expression of the individual isoforms of the major CYP450 enzymes over time in culture suggest that this model system is not reflective of in vivo profiles and, therefore, may be limited in its application for drug metabolism studies. Overall, primary cultures of human hepatocytes can serve as a sensitive and selective model for predicting the regulation of CYP450 modulation by drugs and other xenobiotics. Considerations and recommendations for standardizing testing conditions and choosing relevant endpoint(s) are presented.

Species comparison in P450 induction: effects of dexamethasone, omeprazole, and rifampin on P450 isoforms 1A and 3A in primary cultured hepatocytes from man, Sprague-Dawley rat, minipig, and beagle dog

Induction of P450 isoforms 1A (CYP1A) and 3A (CYP3A) by model inducers dexamethasone, omeprazole and rifampin was evaluated in primary cultured hepatocytes from man and laboratory animals. Inducer-specific species-differences were observed. Results with human hepatocytes from six human donors consistently show that both rifampin and dexamethasone were inducers of CYP3A activity (measured as testosterone 6beta-hydroxylase activity), with rifampin being more potent. Conversely, in rat hepatocytes, dexamethasone was a potent CYP3A inducer while rifampin was not an inducer. Rifampin but not dexamethasone induced CYP3A in minipig and beagle dog hepatocytes. Omeprazole was a potent inducer of CYP1A activity (measured as ethoxyresorufin-O-deethylase activity) in human, beagle dog and minipig hepatocytes, and not an inducer in rat hepatocytes. The species-differences observed suggest that human hepatocytes represent the most appropriate preclinical experimental system for the evaluation of P450 induction in human.

Induction of CYP1A by Benzo[k]fluoranthene in Human Hepatocytes: CYP1A1 or CYP1A2?

While fresh human hepatocyte cultures are widely used to model hepatic cytochrome P450 (CYP) regulation and activity, their CYP1A subfamily composition induced by, e.g., polycyclic aromatic hydrocarbons is ambiguous. CYP1A1, CYP1A2, or both have been reported to be expressed, and their varied roles in chemical carcinogenesis makes resolution of which CYPs are expressed essential. We have used an immunoblot system with Bis-Tris-HCl-buffered polyacrylamide gel, which clearly resolves human CYP1A1 and CYP1A2, and polyclonal goat anti-human CYP1A1/CYP1A2 and rabbit anti-human CYP1A2 antibodies to probe the expressed CYP1A1 and CYP1A2 composition of seven individual human hepatocyte cultures induced with 5 microM benzo[k]fluoranthene (BKF) for 24 h. In six of the cultures only CYP1A1 was detected, and in the seventh both CYPs were detected. In most vehicle-treated hepatocyte cultures, neither CYP1A1 nor CYP1A2 was detected. In three additional hepatocyte cultures treated individually with BKF and 2,3,7,8,-tetrachlorodibenzo-p-dioxin (TCDD), the resultant induced CYP1A1/1A2 profiles were essentially not influenced by the nature of the inducing agents. To develop an activity-based assay to differentiate between CYP1A1 and CYP1A2 expression in human hepatocytes, our previously published R warfarin assay (Drug Metab. Disp. (1995) 23, 1339-1345) was applied to TCDD (10 nM)-treated hepatocyte culture. The low concentration of TCDD did not produce inhibition of the warfarin metabolism-such inhibition could confound the results. Based on the ratios of 6- to 8-hydroxywarfarin formed in two cultures, the ratios of CYP1A1/CYP1A2 expressed in these cultures were determined and they agreed with the ratios determined by immunoblot analysis. Thus each individual human hepatocyte culture must be characterized for induced CYP1A1 and CYP1A2 expression in studies of CYP1A activity. The warfarin assay provides a means of characterizing the cultures.

Drug biotransformation by human hepatocytes. In vitro/in vivo metabolism by cells from the same donor

BACKGROUND/AIMS

Cultured human hepatocytes are considered a close model to human liver. However, the fact that hepatocytes are placed in a microenvironment that differs from that of the cell in the liver raises the question: to what extent does drug metabolism in vitro reflect that of the liver in vivo? This issue was examined by investigating the in vitro and in vivo metabolism of aceclofenac, an analgesic/anti-inflammatory drug.

METHODS

Hepatocytes isolated from programmed liver biopsies were incubated with aceclofenac, and the metabolites formed were investigated by HPLC. During the course of clinical recovery, patients were given the drug, and the metabolites, largely present in the urine, were analyzed. In vitro and in vivo data of the same individual were compared.

RESULTS

The relative abundance of oxidized metabolites in vitro (i.e. 4'OH-aceclofenac + 4'OH-diclofenac vs. total hydroxylated metabolites; Spearman's p = 0.855), as well the hydrolysis of aceclofenac (4'OH-diclofenac vs. 4'OH-aceclofenac + 4'OH-diclofenac; p = 0.691) correlated well with in vivo data. The conjugation of the drug in vitro (24.6 +/- 7.6%) was lower than that in vivo (44.9 +/- 5.3%). The rate of 4'OH-aceclofenac formation in vitro correlated with the amount of metabolites excreted in urine after 16 h (p = 0.95).

CONCLUSIONS

The in vitro/in vivo metabolism of the drug was surprisingly similar in each patient. The variability observed in vitro reflected an existing phenotypic variability among donors.

Gene structure of CYP2C8 and extrahepatic distribution of the human CYP2Cs

Extrahepatic tissue distribution of the mRNAs for the four human CYP2Cs (2C8, 2C9, 2C18, and 2C19) was examined in kidney, testes, adrenal gland, prostate, brain, uterus, mammary gland, ovary, lung, and duodenum. CYP2C mRNAs were detected by RT-PCR using specific primers for each individual CYP2C. CYP2C8 mRNA was detected in the kidney, adrenal gland, brain, uterus, mammary gland, ovary, and duodenum. CYP2C9 mRNA was detected in the kidney, testes, adrenal gland, prostate, ovary, and duodenum. CYP2C18 mRNA was found only in the brain, uterus, mammary gland, kidney, and duodenum and CYP2C19 mRNA was found only in the duodenum. Immunoblot analysis of small intestinal microsomes detected both 2C9 and 2C19 proteins. In addition, genomic clones for CYP2C8 were sequenced, and long-distance PCR was performed to determine the complete gene structure. CYP2C8 spanned a 31 kb region. Comparative analysis of the 2.4 kb upstream region of CYP2C8 with CYP2C9 revealed two previously unidentified transcription factors sites, C/EBP and HPF-1, and the latter might be involved in hepatic expression. Although CYP2C8 has been shown to be phenobarbital inducible, neither a barbiturate-responsive regulatory sequence (a Barbie box) nor a phenobarbital-responsive enhancer module (PBREM) was found within the upstream region analyzed.

Expression and regulation of cytochrome P450 enzymes in primary cultures of human hepatocytes

The aim of this study was to test suitable culture conditions for maintaining normal cellular cytoarchitecture and inducibility of P450 enzymes in primary cultures of human hepatocytes by prototypical inducers. The selectivity and sensitivity of a sandwich culture system were determined by treating cultures with a number of clinically relevant drugs that are known to be inducers of either rodent and/or human P450 enzymes. The results showed that considerable induction of CYP3A4 activity is observed at DMSO concentrations greater than 0.1% (v/v). No differences in P450 induction response were observed between cultures maintained under different matrix conditions. However, the matrix condition considered to be optimal for maintaining cellular integrity, protein yields, and P450 enzyme induction was a sandwich configuration in combination with modified Chee's medium containing insulin (6.25 microg/mL) and dexamethasone (< or =0.1 microM). Under these conditions, induction of CYP3A4 occurred at clinically relevant drug concentrations, and maximal activities were achieved after 3 days of exposure. Overall, the response of human hepatocyte cultures to treatment with both positive and negative modulators was found to reflect that observed in vivo with respect to both enzyme specificity and potency of enzyme induction, although considerable sample-to-sample variability was observed in the magnitude of induction.

Detoxifying activity in pig livers and hepatocytes intended for xenotherapy

BACKGROUND

Both livers and hepatocytes from pigs have been proposed for the treatment of end-stage liver diseases, as an alternative to allogeneic liver transplants. However, little is known of the capability of porcine hepatocytes to fulfill the biotransformation pathways of toxic compounds, including those released from livers in acute failure. We have studied the activity and expression of detoxifying enzymes in porcine livers and in cultured hepatocytes and their induction by phenobarbital.

METHODS

Cytochromes P450 (CYP) 1A, 2B, and 3A and GST-like activities were tested with the following specific substrates: 7-ethoxyresorufin, 7-pentoxyresorufin, nifedipine, testosterone, 1-chloro-2,4-dinitrobenzene, 1,2-dichloro-4-nitrobenzene, and ethacrinic acid. CYP 1A1/2-, 2B1/2-, 2E1- and 3A4-related and GSTalpha proteins were analyzed by Western blotting and CYP 1A1/2, 2B1/2, 2C6, 2E1, and 3A4, aldehyde dehydrogenase, epoxide hydrolase, and GSTalpha-like RNA by Northern blotting.

RESULTS

Enzymatic activities reflecting the expression of CYP 1A-, CYP 2B-, CYP 2E1-, and CYP 3A-like genes, that is, ethoxyresorufin-O-deethylase, pentoxyresorufin-O-deethylase, nifedipine oxidase and testosterone 6beta-hydroxylase, and chlorzoxazone 6-hydroxylase, were identified in pig livers. CYP 1A and CYP 2E1, GSTalpha-like proteins, CYP 1A, 2C, and 2E, epoxide hydrolase, aldehyde dehydrogenase, and GST like RNA were expressed in vivo and in vitro. CYP 2B and CYP 3A RNA and proteins, and their associated activities were induced by phenobarbital.

CONCLUSIONS

Porcine hepatocytes express the most important biotransformation enzymes and their corresponding activities and RNA. Thus, livers and hepatocytes from pigs can detoxify a large spectrum of exogenous and endogenous compounds, which makes them a convenient substitute for allogeneic transplants for patients with liver failure.

Cytochrome P-450 3A4: regulation and role in drug metabolism

Cytochrome P-450 (P-450) 3A4 is the most abundant P-450 expressed in human liver and small intestine. P-450 3A4 contributes to the metabolism of approximately half the drugs in use today, and variations in its catalytic activity are important in issues of bioavailability and drug-drug interactions. The gene is known to be inducible by barbiturates, glucocorticoids, and rifampicin in humans and in isolated hepatocytes, although the mechanism remains unclear. The 5'-untranslated region includes putative basal transcription element, hepatocyte nuclear factor, p53, AP-3, glucocorticoid regulatory element, pregnane X receptor, and estrogen receptor element sequences. Recently, the GRE element has been shown to act in a classic glucocorticoid response. Several issues remain to be resolved regarding the catalytic activity of the P-450 3A4 protein, including rate-limiting steps and the need for cytochrome b5, divalent cations, and acidic phospholipid systems for optimal activity. Another issue involves the basis of the homotropic and heterotropic cooperativity seen with the enzyme. The in vivo significance of these findings remains to be further established. In addition to more basic studies on P-450 3A4, several areas of practical interest to the pharmaceutical industry require development.

Phenobarbital responsiveness conferred by the 5'-flanking region of the rat CYP2B2 gene in transgenic mice

Phenobarbital (PB) is a prototype for a class of agents that produce marked transcriptional activation of a number of genes, including certain cytochrome P-450s. We used transgenic mouse approaches and multiple gene reporters to assess the functional consequences of specific deletions and site-specific mutations within the 2.5kb 5'-flanking region of the rat CYP2B2 gene. Protein-DNA interactions at the PBRU domain also were characterized. Using the transgenic models, we demonstrate that sequences between -2500 and -1700bp of the CYP2B2 gene are critical for PB induction; mice with 1700 or 800bp of 5'-flanking CYP2B2 sequence are not PB responsive. DNA affinity enrichment techniques and immunoblotting and electromobility shift assays were used to determine that nuclear factor 1 (NF-1) interacts strongly with a site centered at -2200bp in the PB responsive unit (PBRU) of CYP2B2. To test the functional contribution of NF-1 in PB activation, we introduced specific mutations within the PBRU NF-1 element and demonstrated that these mutations completely ablate the binding interaction. However, transgenic mice incorporating the mutant NF-1 sequence within an otherwise wild-type -2500/CYP2B2 transgene maintained full PB responsiveness. These results indicate that, despite the avidity of the respective DNA-protein interaction within the PBRU in vitro, NF-1 interaction is not an essential factor directing PB transcriptional activation in vivo.