Comparison of cytochrome P450 isoenzyme profiles in rat liver and hepatocyte cultures. The effects of model inducers on apoproteins and biotransformation activities

Temperature-dependent in vitro hepatocytotoxicity of insecticides in bats

Heterothermic insectivorous bats are supposed to experience differential adverse effects of insecticidal pollutants depending on their seasonal and/or daily variation of metabolic and detoxification rates. Here, we investigated effects of imidacloprid, cypermethrin and fipronil on Nyctalus noctula bat-derived hepatocytes through cytotoxicity, cell inhibition and death at different concentrations (0.01, 0.1, 1, 10, 100, 1000 μg/ml), exposure times (10, 24, 48 hrs), incubation temperatures simulating hibernation (8 °C), daily torpor (20 °C), normothermy (37 °C) and active flight (40 °C), and cytochrome P450 addition. Toxic effects were significantly influenced by temperature (p < 0.05), with strong cytotoxicity after 10 hour exposure to fipronil or cypermethrin at 37 and 40 °C, cell replication inhibition (all insecticides at 8 °C) and cellular stimulation, with slight culture proliferation after 48 hours (all insecticides at 40 °C). Replacing protected chiropterans with cell cultures is a way to assess and extrapolate risks of insecticides for bats.

ECITTS: An Integrated Approach to the Application of In Vitro Test Systems to the Hazard Assessment of Chemicals,

As a result of a workshop held at Täljöviken, Åkersberga, Sweden, on 27–29 May 1991, a multicentre collaborative research project was established, with the purpose of developing the concept of integrated in vitro toxicity testing. The first priority was the selection of tests within eight appropriate areas: basal cytotoxicity, irritancy, developmental toxicity, hepatotoxicity, nephrotoxicity, immunotoxicity, neurotoxicity and biokinetics. An ideal battery of tests for each area was identified. Since it was realised that it would not be feasible to include the full ideal list of tests in the project, a minimum test list was also agreed. For each area, ten calibration chemicals were selected. From these 80 compounds, 30 were selected for inclusion, together with 20 of the OECD test programme chemicals, in a first test set of chemicals. The toxicity of these 50 test set chemicals will be investigated in the minimum integrated test scheme.

The aim of the project is to employ non-animal methods to assess the toxicological properties of chemicals, and to improve this assessment through the use of knowledge about mechanisms of toxic action. The information produced will contribute to the establishment of a more-scientific and more-efficient toxicological procedure for hazard assessment. Questions concerning which parameters need to be investigated and combined to make hazard assessments, and which parameters relevant to in vivo toxicity can be determined in non-whole animal test systems, will also be addressed.

Microsomal Metabolism of Prochiral Polychlorinated Biphenyls Results in the Enantioselective Formation of Chiral Metabolites

Polychlorinated biphenyl (PCB) congeners with multiple ortho chlorine substituents and their metabolites exist as stable rotational isomers, or atropisomers, that are nonsuperimposable mirror images of each other. Additionally, the oxidation of certain axially prochiral PCBs, such as 2,2',4,6'-tetrachlorobiphenyl (PCB 51) and 2,2',4,5,6'-pentachlorobiphenyl (PCB 102), in the meta position of the symmetrically substituted phenyl ring is expected to form axially chiral hydroxylated metabolites (OH-PCBs); however, the formation of chiral OH-PCBs from prochiral PCBs has not been demonstrated experimentally. Here, we investigate if the oxidation of PCB 51 and PCB 102 by different microsomal preparations results in the formation of chiral OH-PCBs. Gas chromatographic analysis revealed that PCB 51 and PCB 102 were metabolized to 2,2',4,6'-tetrachlorobiphenyl-3'-ol (OH-PCB 51) and 2,2',4,5,6'-pentachlorobiphenyl-3'-ol (OH-PCB 102), respectively, by liver microsomes from male rats pretreated with different inducers; untreated male monkeys, guinea pigs, rabbits, and hamsters; and female dogs. The formation of both metabolites was inducer- and species-dependent. Both OH-PCB 51 and OH-PCB 102 were chiral and formed enantioselectively by all microsomal preparations investigated. These findings demonstrate that axially chiral PCB metabolites are formed from axially prochiral PCB congeners, a fact that should be considered when studying the environmental fate, transport, and toxicity of OH-PCBs.

Maintenance of in vivo induced cytochrome P-450s in hepatocyte monolayers at non freezing temperatures

Cytochrome P450s (CYPs) induced in rats by 3-methylcholanthrene (3-MC), phenobarbital (PB) and dexamethasone (Dex) were investigated. The inducers had no effect on hepatocyte yield, viability, attachment or spreading on collagen. 3-MC induced ethoxyresorufin deethylase (EROD). Under normothermic conditions the activity fell in culture. However, it was maintained when cells were preserved at 10°C under a gelatin gel. Upon reactivation the activity mirrored that of freshly isolated cells at 37°C. Induced levels were stable for at least 6h , the time to form a confluent monolayer. The investigation was extended to other CYPs by looking at patterns of testosterone metabolism. Phenobarbital had the greatest influence in terms of the quantity and number of metabolites. Culture at 37°C decreased the peaks dramatically within 24 h. All 7 peaks were maintained in the preservation system.

Phenobarbital pretreatment in vivo and in vitro and the effect of hepatotoxicity of d-galactosamine in rat hepatocytes in culture

Galactosamine (GalN) is a known hepatotoxic compound, acting by depletion of uracil nucleotides. The relation between an active cytochrome P-450 system (CYP) and the hepatotoxicity of GalN was studied in rat hepatocytes that were pretreated with phenobarbital (PB) in vivo or in vitro. A 24-hr in vitro pretreatment of cultured hepatocytes with PB resulted in a significant decrease in GalN toxicity as measured by lactate dehydrogenase (LDH) leakage. Furthermore, GalN treatment resulted in an increase in the activity of the PB-induced forms of CYP (namely CYP 2B1/2) as measured by 7-pentoxyresorufin O-depentylase (PROD) activity. This increase was not found after GalN treatment of microsomes. GalN had no effect on the concentration of the apoenzymes. GalN administration to hepatocytes of in vivo PB-pretreated rats resulted in a similar effect of GalN on the activity of the CYP enzymes but PB in vivo had no effect on GalN toxicity. These results suggest that GalN treatment may result in a significant increase in the specific activity of CYP 2B1/2 enzymes (PROD), without an obvious increase in the amount of PB-induced apoenzymes. This phenomenon was measurable only in intact cells. No direct relation is assumed between the activity of the CYP apoenzymes and the decrease in GalN toxicity after PB treatment. The toxicity of Galn was inhibited by PB treatment in vitro.

Melatonin metabolism in the central nervous system

The metabolism of melatonin in the central nervous system is of interest for several reasons. Melatonin enters the brain either via the pineal recess or by uptake from the blood. It has been assumed to be also formed in some brain areas. Neuroprotection by melatonin has been demonstrated in numerous model systems, and various attempts have been undertaken to counteract neurodegeneration by melatonin treatment. Several concurrent pathways lead to different products. Cytochrome P₄₅₀ subforms have been demonstrated in the brain. They either demethylate melatonin to N-acetylserotonin, or produce 6-hydroxymelatonin, which is mostly sulfated already in the CNS. Melatonin is deacetylated, at least in pineal gland and retina, to 5-methoxytryptamine. N¹-acetyl-N²-formyl-5-methoxykynuramine is formed by pyrrole-ring cleavage, by myeloperoxidase, indoleamine 2,3-dioxygenase and various non-enzymatic oxidants. Its product, N¹-acetyl-5-methoxykynuramine, is of interest as a scavenger of reactive oxygen and nitrogen species, mitochondrial modulator, downregulator of cyclooxygenase-2, inhibitor of cyclooxygenase, neuronal and inducible NO synthases. Contrary to other nitrosated aromates, the nitrosated kynuramine metabolite, 3-acetamidomethyl-6-methoxycinnolinone, does not re-donate NO. Various other products are formed from melatonin and its metabolites by interaction with reactive oxygen and nitrogen species. The relative contribution of the various pathways to melatonin catabolism seems to be influenced by microglia activation, oxidative stress and brain levels of melatonin, which may be strongly changed in experiments on neuroprotection. Many of the melatonin metabolites, which may appear in elevated concentrations after melatonin administration, possess biological or pharmacological properties, including N-acetylserotonin, 5-methoxytryptamine and some of its derivatives, and especially the 5-methoxylated kynuramines.

Biokinetic modeling and in vitro-in vivo extrapolations

The introduction of in vitro methodologies in the toxicological risk assessment process requires a number of prerequisites regarding both the toxicodynamics and the biokinetics of the compounds under study. In vitro systems will need to be relevant for measuring those structural and physiological changes that are good indicators for adverse effects. Furthermore, the dose metric found to have an effect in the in vitro system should be relevant. One element in defining the appropriate dose metric is related to the kinetic behavior of the compound in the in vitro system: binding to proteins, binding to plastic, evaporation, and the interaction between the culture medium and the cells. Ways to measure and model "in vitro biokinetics" are described. Second, the appropriate dose metric in vitro, e.g., the effective concentration, will need to be extrapolated to relevant in vivo exposure scenarios. The application of physiologically based biokinetic modelling is essential in such extrapolations. The parameters needed to build these models often can be estimated based on nonanimal data, namely chemical properties (QSARs) and in vitro experiments.

Permissive and suppressive effects of dexamethasone on enzyme induction in hepatocyte co-cultures

1. Steroids are known to act as permissive factors in hepatocytes. This study shows that dexamethasone (DEX) is a permissive factor for induction of CYP2B1/2, CYP3A1, CYP2A1 and probably also CYP2C11 in cultures with primary rat hepatocytes. 2. The induction factor of phenobarbital (PB)-induced formation of 16beta-hydroxytestosterone (OHT), a testosterone biotransformation product predominantly formed by CYP2B1, is increased 18-fold by the addition of 32 nM DEX to the culture medium. Interestingly, higher concentrations of DEX up to 1000 nM led to a concentration-dependent maximally 5-fold decrease (p = 0.002) of phenobarbital-induced 16beta-OHT formation compared with the effect observed with 32 nM DEX. Thus, DEX shows permissive and suppressive effects on enzyme induction depending on the concentration of the glucocorticoid. 3. Qualitatively similar but smaller permissive and suppressive effects of DEX were observed for PB-induced CYP3A1 activity as evidenced by formation of 2beta-, 6beta- and 15beta-OHT. 4. DEX is a permissive factor for induction of CYP2A1 activity by 3-methylcholanthrene (3MC), as evidenced by the formation of 7alpha-OHT. Without addition of DEX, 3MC did not induce formation of 7alpha-OHT, whereas an almost 3-fold induction occurred in the presence of DEX. In contrast to CYP2B and CYP3A, concentrations up to 1000 nM DEX were not suppressive for the induction of CYP2A1. 5. We described recently a technique that allows preparation of cultures from cryopreserved hepatocytes. An almost identical influence of dexamethasone on enzyme induction was observed here in cultures from cryopreserved compared with freshly isolated hepatocytes. 6. Cultures with primary hepatocyte cultures represent a well-established technique for the study of drug-drug interactions. However, a large interlaboratory variation is known. Our study provides evidence that differences in glucocorticoid concentration in the culture medium contribute to this variation.

The contribution of in vitro toxicity data in hazard and risk assessment: current limitations and future perspectives

This paper discusses the role of in vitro toxicology in hazard and risk assessment. A short historical overview of the development of the field of in vitro toxicology is given, showing the possibilities, as well as the limitation for using in vitro methods in screening and in studying mechanisms of toxicity. Although the development of the use of in vitro methods over the last two or three decades is vast, the application in risk assessment is still limited. The need for a more integrated approach, i.e. using in vitro data not in isolation, but in combination with data on structure-activity and in silico biokinetic models is discussed. It is foreseen that the role of in vitro methods in future risk assessments will greatly enhance, also in the light of recent developments in technologies such as genomics and systems biology.

A sandwich-cultured rat hepatocyte system with increased metabolic competence evaluated by gene expression profiling

A rapid decline of cytochrome P450 (CYP450) enzyme activities remains a drawback of rat hepatocyte-based in vitro cultures. Consequently, judgment of the toxic potential of compounds that need bioactivation by CYP450s may not be adequate using this model. In the present study, an improved hepatocyte-based in vitro system was developed with special focus on metabolic competence. Therefore, a mixture of CYP450 inducers, phenobarbital, dexamethasone and beta-naphthoflavone, was added to culture medium of sandwich-cultured rat hepatocytes. The resulting modified model was evaluated by comparing its genome-wide expression profiles with liver and a standard model without the inducer mixture. Metabolic capacity for CYP450 enzymes showed that the modified model resembled more closely the in vivo situation. Gene expression results revealed large differences between in vivo and both in vitro models. The slight differences between the two sandwich models were predominantly represented by gene expression changes in CYP450s. Importantly, in the modified model, expression ratios of the phase I and the majority of phase II genes more closely resembled liver in vivo. The CYP450 enzyme activities corresponded with gene expression data. In conclusion, for toxicological applications using sandwich-cultured hepatocytes, the modified model may be preferred.

Induction of phase I and II drug metabolism in rat small intestine and colon in vitro

The aim of this study was to evaluate drug metabolism in rat small intestinal and colon precision-cut slices during 24 h of incubation and the applicability of these slices for enzyme induction studies. Various parameters were evaluated: intracellular levels of ATP (general viability marker), alkaline phosphatase activity (specific epithelial marker), villin expression (specific epithelial marker), and metabolic rates of 7-ethoxycoumarin (CYP1A), testosterone (CYP3A and CYP2B), and 7-hydroxycoumarin (glucuronide and sulfate conjugation) conversions. ATP and villin remained constant up to, respectively, 5 and 8 h in small intestine and up to 24 h in colon. The metabolic rate remained constant in small intestinal slices up to 8 h and decreased afterward to 24 to 92%, depending on the substrate studied. The inducibility of metabolism in small intestinal and colon slices was tested with several inducers at various concentrations and incubation times. The following inducers were used: 3-methylcholanthrene, beta-naphthoflavone, indirubin, and tert-butylhydroquinone (aryl hydrocarbon receptor ligands), dexamethasone (glucocorticoid receptor/pregnane X receptor ligand) and phenobarbital (constitutive androstane receptor ligand). After incubation with inducers, metabolic rates were evaluated with 7-ethoxycoumarin and testosterone (phase I) and 7-hydroxycoumarin (phase II) as substrate. All inducers elevated the metabolic rates consistent with the available published in vivo induction data. Induction of enzyme activity was already detectable after 5 h (small intestine) and after 8 h (colon) for 3-methylcholanthrene and beta-naphthoflavone and was clearly detectable for all tested inducers after 24 h (up to 20-fold compared with noninduced controls). In conclusion, small intestinal and colon precision-cut slices are useful for metabolism and enzyme induction studies.

Generation of human hepatocytes by stem cell technology: definition of the hepatocyte

Since 1999, numerous articles have reported the generation of hepatocytes from different types of extrahepatic stem or precursor cells. This opens exciting new possibilities for pharmacology and toxicology, as well as for cell therapy. Hepatocyte marker expression, including albumin, cytokeratin 18, c-met, alpha-fetoprotein and cytochrome P450 3A4 and -2B6, has been observed after transplantation of different types of human stem cells into the liver of laboratory animals or in vitro after incubation with cytokines. These intriguing observations have prompted scientists to classify stem cell-derived cell populations as hepatocytes. However, this conclusion may be premature. It has been shown that factors of the liver microenvironment can induce expression of a limited number of hepatocyte marker genes in nonhepatic cell types. To conclude on the grounds of a limited number of markers that these cells are true hepatocytes is not indicated. In this case one should carefully evaluate crucial hepatocyte-defining enzymatic properties. The present article: i) reviews studies describing the fate of extrahepatic human stem and precursor cells in livers of laboratory animals, including the possibility of cell fusion; and ii) critically discusses the phenotype of stem cells after application of various differentiation protocols aimed at generating human hepatocytes. In addition, the necessary criteria needed for defining a true hepatocyte are suggested. Establishing the necessary properties for stem cell-derived hepatocytes is timely and reasonable, and thus avoids further misleading semantic confusion. Finally, it is essential to understand that the definition of a bona fide hepatocyte should not be limited to qualitative assays, such as reverse transcriptase polymerase chain reaction and immunohistochemistry, but has to include a quantitative analysis of enzymatic activities, which allows direct comparison with primary hepatocytes. Although the stem cell-derived-hepatocyte does not yet exist there is a good chance that this aim may be achieved in the future.

The effects of flubendazole and mebendazole on cytochromes P4501A in pheasant hepatocytes

Many benzimidazoles are known inducers of cytochromes P4501A (CYP1A) in laboratory animals and cell lines. As flubendazole and mebendazole are benzimidazole anthelmintics often used in a pheasant, in the present study an effect of these drugs in primary cultures of pheasant (Phasianus colchicus) hepatocytes was investigated. After 48 h incubation of the hepatocytes with the benzimidazoles (0.2-5 microM), CYP1A activities -- ethoxyresorufin O-deethylation (EROD) and methoxyresorufin O-demethylation (MROD) activities were measured and the CYP1A protein levels were determined by Western blotting. None of the tested benzimidazoles influenced the CYP1A protein content. No pharmacologically significant enhancement of CYP1A after exposure of the hepatocytes to flubendazole and mebendazole was found. Inhibition of the EROD/MROD activities caused by both tested substances was observed only at the highest concentration (5 microM). From a point of view of CYP1A induction or inhibition, the treatment of pheasants by both anthelmintics tested seems to be safe. Our study demonstrates the inter-species differences in CYP1A inducibility and the importance of induction/inhibition studies on target animals.

Modulation of peroxisome proliferator-activated receptors (PPARs) by PPARα- and PPARγ-specific ligands and by 17β-estradiol in isolated zebrafish hepatocytes

Peroxisome proliferation is a phenomenon occurring when responsive animals are exposed to certain compounds so-called peroxisome proliferators and is regulated through a nuclear receptor named peroxisome proliferator-activated receptor (PPAR). PPAR family members exhibit a strong binding affinity for both saturated and unsaturated fatty acids. Activators of PPAR(alpha) include a variety of endogenously present fatty acids, leukotrienes and hydroxyeicosatetraenoic acids (HETEs) and clinically used drugs, such as fibrates. PPAR(beta) activators include fatty acids, prostaglandin A2 (PGA2) and prostacyclin (PGI2). PPAR(gamma) is the most selective receptor and, among others, 15-deoxy-Delta(12,14) prostaglandin J2 (PGJ2) has been described to be a PPAR(gamma)-specific ligand. The aim of the present study was to determine if known PPAR(alpha) and PPAR(gamma) ligands were able to alter the expression of these subtypes in an in vitro model of zebrafish primary hepatocyte culture. With this purpose, a PPAR(alpha) specific ligand (8S-HETE), a PPARgamma specific ligand (PGJ) and a peroxisome proliferator of the fibrate class (clofibrate) were selected. In addition, the female hormone 17beta-estradiol was also used as it is known to interact with PPARs. After cell exposure for 24 h, cells were immunohistochemically stained for both PPARs and immunolabeling was quantified as percentage of positive nuclei and cells. Levels of expression of PPARs were also measured by image analysis as grey level per cell. Expression was induced for both PPAR(alpha) and PPAR(gamma) by clofibrate (at 0.5 mM for PPAR(alpha) and at 1 and 2 mM for PPAR(gamma)), by HETE (1 microM), and by PGJ2 (0.3 and 1 microM for PPAR(alpha) and 0.3 microM for PPAR(gamma)). Expression of PPARgamma was also induced at 10 microM by 17beta-estradiol. The percentage of PPAR(alpha) positive nuclei increased significantly at 1 microM HETE and the percentage of PPAR(gamma) positive cells decreased at 10 microM 17beta-estradiol. As a conclusion, clofibrate, HETE and PGJ2 are able to induce expression of both PPAR(alpha) and PPAR(gamma) in zebrafish primary hepatocyte cultures. Further studies are needed to identify how the expression of different PPAR subtypes is regulated and to elucidate the implication of PPAR subtypes in zebrafish cell functions.

Hepatocytes cultured in alginate microspheres: an optimized technique to study enzyme induction

An important application of hepatocyte cultures is identification of drugs acting as inducers of biotransformation enzymes that alter metabolic clearance of other therapeutic agents. In the present study we optimized an in vitro system with hepatocytes cultured in alginate microspheres that allow studies of enzyme induction with excellent sensitivity. Induction factors obtained with standard inducers, such as 3-methylcholanthrene or phenobarbital, were higher compared to those with conventional hepatocyte co-cultures on collagen coated dishes. This is illustrated by activities of 7-ethoxyresorufin-O-deethylase (EROD) after incubation with 5 microM 3-methylcholanthrene (3-MC), a standard inducer for cytochrome P4501A1 and 1A2. Mean activities for solvent controls and 3-MC exposed cells were 2.99 and 449 pmol/min/mg protein (induction factor: 150) for hepatocytes cultured in microspheres compared to 2.72 and 80.6 pmol/min/mg (induction factor: 29.6) for hepatocytes on collagen coated dishes. To compare these in vitro data to the in vivo situation male Sprague Dawley rats, the same strain that was used also for the in vitro studies, were exposed to 3-MC in vivo using a protocol that guarantees maximal induction. Activities were 29.2 and 1656 pmol/min/mg in liver homogenate of solvent and 3-MC treated animals (induction factor: 56.7). Thus, the absolute activities of 3-MC exposed hepatocytes in microspheres are lower compared to the in vivo situation. However, the induction factor in vitro was even higher compared to the in vivo situation (150-fold versus 56.7-fold). A similar scenario was observed using phenobarbital (0.75 mM) for induction of CYP2B and 3A isoenzymes: induction factors for testosterone hydroxylation in position 16beta were 127.5- and 50.4-fold for hepatocytes in microspheres and conventionally cultured hepatocytes, respectively. The new in vitro system with hepatocytes embedded in solid alginate microspheres offers several technical advantages: (i) the solid alginate microspheres can be liquefied within 60s, allowing a fast and complete harvest of hepatocytes; (ii) alginate capsules are stable allowing transport and mechanical stress; (iii) high numbers of hepatocytes can be encapsulated in short periods; (iv) defined cell numbers between 600 hepatocytes, the approximate number of cells in one capsule, and 18 x 10(6) hepatocytes, the number of hepatocytes in 6 ml alginate, can be transferred to a culture dish or flask. Thus, encapsulated hepatocytes allow a flexible organization of experiments with respect to cell number. In conclusion, we optimized a technique for encapsulation of hepatocytes in alginate microspheres that allows identification of enzyme induction with an improved sensitivity compared to existing systems.

Clofibrate, a peroxisome-proliferator, enhances reverse cholesterol transport through cytochrome P450 activation and oxysterol generation

Fibrates are widely used hypolipidemic agents that activate the peroxisome proliferator-activated receptor a (PPARalpha) and regulate the expression of many genes involved in lipid metabolism. We studied the mechanism of the effect of clofibrate on cholesterol homeostasis. Rats were fed with chow containing clofibrate, cytochrome P-450 inhibitor ketoconazole, or clofibrate plus ketoconazole. Control rats were fed only with normal chow. The levels of six oxysterols in liver microsome were determined. The levels of mRNAs for liver X receptor alpha (LXRalpha), ATP-binding cassette A1 (ABCA1), PPARalpha and cholesterol 7alpha-hydroxylase (CYP7A) in the liver were analyzed by northern blotting. Clofibrate administration decreased plasma levels of total cholesterol and triglyceride and increased high-density lipoprotein-cholesterol (HDL-C). Clofibrate increased the levels of liver microsomal oxysterols including 25- and 27-hydroxycholesterol, which are potent activators of LXRalpha. Clofibrate also enhanced the expression of mRNAs for PPARalpha, LXRalpha, and ABCA1. Simultaneous administration of ketoconazole suppressed the effects of clofibrate on plasma lipids, hepatic oxysterol levels, and the expression of the genes. Clofibrate increases cytochrome P450 content and the resulting oxysterol generation may partly mediate the clofibrate-induced up-regulattion of LXRa and ABCA1, which are related to reverse cholesterol transport.

Liver tissue engineering within alginate scaffolds: effects of cell-seeding density on hepatocyte viability, morphology, and function

Tissue engineering with three-dimensional biomaterials represents a promising approach for developing hepatic tissue to replace the function of a failing liver. Herein, we address cell seeding and distribution within porous alginate scaffolds, which represent a new type of porous biomaterial for tissue engineering. The hydrophilic nature of the alginate scaffold as well as its pore structure and interconnectivity enabled the efficient seeding of hepatocytes into the scaffolds, that is, 70-90% of the initial cells depending on the seeding method. Utilization of centrifugal force during seeding enhanced cell distribution in the porous scaffolds, consequently enabling the seeding of concentrated cell suspensions (>1 x 10(7) cells/mL). Cell density in scaffolds affected hepatocyte viability as judged by MTT assay. At a cell density of 0.28 x 10(6) cells/cm3 scaffold, the number of viable hepatocytes decreased to 33% of its initial value within 7 days, whereas at the denser cultures, 5.7 x 10(6) cells/cm3 scaffold and higher, the cells maintained higher viability while forming a network of connecting spheroids. In the high-density cellular constructs, hepatocellular functions such as albumin and urea secretion, and detoxification (cytochrome P-450 and phase II conjugating enzyme activities), remained high during the 7-day culture. Collectively, the results of the present study highlight the importance of cell density on the hepatocellular functions of three-dimensional hepatocyte constructs as well as the advantages of alginate matrices as scaffoldings.

The effects of fenbendazole, flubendazole and mebendazole on activities of hepatic cytochromes P450 in pig

Fenbendazole (FBZ), flubendazole (FLBZ) and mebendazole (MBZ) are benzimidazole anthelmintics widely used in veterinary medicine. The effects of these drugs on cytochromes P450 (CYP) were investigated in primary cultures of swine (Sus scrofa f. domestica) hepatocytes. After 48-h incubation of hepatocytes with benzimidazoles (0.1-2.5 microm), ethoxyresorufin O-deethylation (EROD), benzoxyresorufin O-dearylation (BROD), testosterone hydroxylase (6beta-TOH) and testosterone oxidase (17-TO) activities were measured and the CYP1A and 3A protein levels were determined by Western blotting. FBZ produced a significant, concentration-dependent increase of CYP1A activity (EROD) and protein level. No enhancement of CYP1A was observed after exposure to FLBZ and MBZ. All benzimidazoles tested did not cause any induction of CYP3A (BROD, 6beta-TOH, 17-TO activities and protein content). On the other hand, MBZ produced a significant, concentration-dependent decrease of CYP3A (BROD, 6beta-TOH and 17-TO) activities. Pharmacological and toxicological consequences of CYP1A induction and CYP3A inhibition should be taken into account in treatment of pigs with FBZ and MBZ.

Metabolism of CDRI-85/92, a New Potent Anti-ulcer Agent, Involving Cis-Trans Conversion

CDRI 85/92, an anti-ulcer drug, is a new proton pump inhibitor, currently in an advanced stage of drug development. To know more about the drug it was our objective to delineate/identify the metabolic pathway as well as the enzymes responsible for the formation of metabolites. Metabolism of CDRI-85/92 (cis-5-styryl-2-oxazolidinone-4-carboxylic acid) was investigated in rat liver cellular fractions (S9, microsomes and cytosol) using reverse-phase HPLC and mass spectrometry techniques. Two major metabolites were produced by rat liver S9 fractions and reducing factor generating system from either untreated rats or phenobarbitone (PB)-pretreated rats. Incubation of CDRI-85/92 with postmitochondrial fraction (S9) for 24 h resulted in a cis to trans conversion (metabolite M2). Further cis-trans metabolizing capacity was measured separately in the cytosolic and microsomal fractions. Incubation with the cytosolic fraction resulted in an increased rate of cis-trans conversion, while the microsomal fraction showed no cis to trans conversion, thereby restricting the cis to trans conversion to Phase II enzymes, which are mainly located in the cytosol. Studies with PB-pretreated rat liver S9 fractions resulted in an increased rate of cis to trans conversion. Another metabolite was also present (M1) which was identified as an oxygenated metabolite by mass spectrometry. The major urinary metabolite from CDRI-85/92-treated Sprague-Dawley rats (20 mg/kg p.o.) was identified as M2. Studies using sulfobromophthalein and N-ethylmaleimide, as specific inhibitors of GST, showed a complete absence of metabolism, thus indicating the involvement of GST in the metabolism of CDRI-85/92. This study will be helpful in providing clues about factors influencing the bioavailability of CDRI-85/92 as well as drug-drug interactions.

The effects of albendazole and its metabolites on hepatic cytochromes P450 activities in mouflon and rat

Albendazole (ABZ) is a benzimidazole anthelmintic widely used in veterinary medicine. The effects of ABZ on cytochromes P450 were investigated in primary cultures of mouflon (Ovis musimon) and rat (Rattus norvegicus) hepatocytes. Besides ABZ, its two main metabolites (albendazole-sulphoxide, ABZSO and albendazole-sulphone, ABZSOO) were tested to clarify which compound is responsible for the induction potency of this benzimidazole drug. After 48 h incubation of hepatocytes with benzimidazoles (0.2-25 microM), ethoxyresorufin O-deethylation (EROD) and benzoxyresorufin O-dearylation (BROD) were measured and the P4501A and 3A protein levels were determined by Western blotting. All benzimidazoles provoked a significant increase of EROD and BROD activities in rat hepatocytes. ABZSO and ABZSOO seemed to be responsible for the induction effect of ABZ on P450s in rat. In mouflon, no pharmacologically significant induction of EROD and BROD activities by benzimidazoles tested was observed. From this point of view, anthelmintic therapy of mouflons with ABZ seems to be safe.

The effects of benzimidazole anthelmintics on P4501A in rat hepatocytes and HepG2 cells

Benzimidazole anthelmintics including albendazole, fenbendazole, and mebendazole are widely used in veterinary medicine. The effects of these benzimidazoles on cytochrome P4501A were investigated in primary cultures of rat hepatocytes and in the HepG2 cell line. After incubation of rat hepatocytes and HepG2 for 24-, 48-, and 72-h cells with drugs at various concentrations (0.1-50 microM), the enzyme activities associated with P4501A1/2 (7-ethoxyresorufin O-deethylation and 7-methoxyresorufin O-demethylation) were measured. The P4501A1/2 protein levels in both model systems were determined by Western blotting. Although all benzimidazoles provoked a significant increase of P4501A1/2 protein levels and P4501A activities, large differences in the induction response were found which was dependent on drug structure, concentration, and model system used. Based on the results, relationships between induction potency and structure of drug were demonstrated, as well as differences between the in vitro systems used. Therefore, pharmacological and toxicological consequences of cytochrome P4501A induction by benzimidazole drugs should be taken into account in veterinary therapy.

The effects of mebendazole on P4501A activity in rat hepatocytes and HepG2 cells. Comparison with tiabendazole and omeprazole

Mebendazole is a benzimidazole anthelmintic widely used in veterinary and human therapy. Among benzimidazole derivatives, several drugs with inducing effect on cytochromes P450 can be found. However, the induction capacity of mebendazole on P450s has not been explored yet. In this study, the effects of mebendazole on P4501A activity was tested in primary cultures of rat hepatocytes and in human hepatoma HepG2 cell line. Two known P4501A inducers with benzimidazole structure, tiabendazole and omeprazole, were also included in the experiments with the aim of studying structure-induction relationships. After 24-, 48- and 72-h incubation of rat hepatocytes and HepG2 cells with drugs in various concentrations (0.1-100 microM), enzyme activity associated with P4501A1/2 (EROD, MROD) was measured. In addition, the P4501A1/2 protein levels in both in-vitro systems were determined by Western-blotting. Mebendazole provoked a significant increase in P4501A1/2 protein expression and P4501A activity in both in-vitro systems. Omeprazole caused a significant dose-dependent increase of P4501A activity only in HepG2 cells. Although tiabendazole treatment led to significant increase of P4501A protein level, no effect on P4501A activity was observed in either system. The results demonstrate that mebendazole possesses the ability to significantly induce P4501A. Thus, pharmacological and toxicological consequences of P4501A induction should be taken into account in human therapy. The structure-induction relationships and differences between in-vitro systems used are discussed.

Time course of cytochromes P450 decline during rat hepatocyte isolation and culture: effect of L-NAME

The present work describes an isozyme-related effect of collagenase perfusion on hepatocyte microsomal cytochrome (CYP)-dependent monooxygenase activities: CYP 1A1/2-, 2B1/2-, 3A1/2- and 2E1-dependent activities in microsomes from rat hepatocytes after isolation were about 60% of that of liver microsomes, and CYP 4A1-dependent activity was equivalent to liver microsomes. In contrast, the microsomal protein content of the various CYP isoforms was not affected by hepatocyte isolation. This is in accordance with the hypothesis of CYP inactivation during the process of hepatocyte isolation by collagenase digestion. L-NAME (1 mM) was found unable to protect from the decline of CYP-dependent monooxygenase activities following hepatocyte isolation. It is possible that the decrease in glutathione peroxidase activity observed in the presence of L-NAME, namely depression of defense against peroxynitrite, could counteract the beneficial effect of L-NAME on nitric oxide synthesis inhibition. The present work also shows that L-NAME could not avoid the progressive, isoform-specific, most probably turnover-related, decline of CYP proteins and related monooxygenase activities in cultured hepatocytes. Dysregulations in the mechanisms of CYP expression in rat hepatocyte cultures, presently unknown but nitric oxide independent, thus appear to occur in cultured rat hepatocytes.

The metabolism of clopidogrel is catalyzed by human cytochrome P450 3A and is inhibited by atorvastatin

The prodrug clopidogrel (Plavix) is activated by cytochrome p450 (p450) to a metabolite that inhibits ADP-induced platelet aggregation. Clopidogrel is frequently administered to patients in conjunction with the CYP3A4 substrate atorvastatin (Lipitor). Since clinical studies indicate that atorvastatin inhibits the antiplatelet activity of clopidogrel, we investigated whether CYP3A4 metabolized clopidogrel in vitro. Microsomes prepared from dexamethasone-pretreated rats metabolized clopidogrel at a rate of 3.8 nmol min(-1) nmol of p450(-1), which is 65 and 1270% faster than the rate of metabolism by microsomes from control and beta-napthoflavone-treated rats, respectively. To identify the human p450s responsible for clopidogrel oxidation, genetically engineered microsomes containing a single human p450 isozyme were tested for their ability to oxidize clopidogrel. CYP3A4 and 3A5 metabolized clopidogrel at a significantly higher rate than eight other p450 isozymes, suggesting that CYP3A4 and 3A5 are primarily responsible for in vivo clopidogrel metabolism. Clopidogrel interacts with human CYP3A4 with a spectral dissociation constant (K(s)), K(m), and V(max) of 12 microM, 14 +/- 1 microM and 6.7 +/- 1 nmol min(-1) nmol p450(-1), respectively. Atorvastatin lactone, the physiologically relevant substrate, inhibits clopidogrel with a K(i) of 6 microM. When clopidogrel and atorvastatin are present at equimolar concentrations, clopidogrel metabolism is inhibited by greater than 90%. Since CYP3A4 and 3A5 metabolize clopidogrel faster than other human p450 isozymes and are the most abundant p450s in human liver, they are predicted to be predominantly responsible for the activation of clopidogrel in vivo.

Time-dependent variations of drug-metabolising enzyme activities (DMEs) in primary cultures of rabbit hepatocytes

In the present study, time-dependent variations of drug-metabolising enzyme activities (DMEs) in primary cultures of rabbit hepatocytes, a species of economic importance in Mediterranean countries, were investigated. Cross-bred rabbits were anesthetised and their livers perfused in situ by a two-step collagenase technique; cells suspensions were filtered, seeded in collagen-coated dishes and cultivated at 37 degrees C in a controlled atmosphere for 24 and 72 h. Cytochrome P450 and b(5) contents as well as the catalytic activity of some P450-dependent monooxygenases were measured in subcellular fractions obtained by differential ultracentrifugation; microsomal proteins were also subjected to immunoblotting, using antibodies to rat P4501A, 2B, 2E1 and 3A isoforms. The activity of some microsomal hydrolytic enzymes was also determined. As regards conjugative enzymes, glutathione content and activities of glutathione S-transferase, uridindiphosphoglucuronosyl-transferase, acetyl-transferase and 1,2-epoxibuthane glutathione transferase were assayed. An overall reduction of the catalytic activity was observed 72 h after plating, reaching in certain instances the level of statistical significance. On the whole, our data confirm those previously reported with hepatocytes obtained from other species; however, the evidence that DMEs were still measurable after 72 h supports the usefulness of this in vitro method for drug metabolism studies in the rabbit as well.

Derivatives of dehydrocrotonin, a diterpene lactone isolated from Croton cajucara: cytotoxicity in rat cultured hepatocytes and in V79 cells

Derivatives of dehydrocrotonin (DHC; Compound I) with different anti-ulcerogenic properties but less toxicity were produced by reducing the cyclohexenone moiety of DHC with NaBH4 (Compound II), reducing the cyclohexenone and lactone moieties with LiAlH4 (Compound III) and transforming the lactone moiety into an amide (Compound IV) using dimethylamine. Derivatives of DHC were assayed in cultured hepatocytes and V79 fibroblasts. Three independent endpoints assays for cytotoxicity were used, namely, the DNA content, tetrazolium reduction (MTT) and neutral red uptake (NRU). Compound III was less toxic than the other DHC derivatives in both cell cultures. ICso values ranging from 250 to 600 microM were obtained for Compounds II and IV in the NRU and DNA content tests evaluated in 4-hour hepatocyte cultures. Although Compound II showed relatively low cytotoxicity in rat hepatocytes based on the NRU and DNA content assays, a very high toxicity (IC50=10 microM) was observed in the MTT test. Metabolites of Compound II in conditioned medium from 4-hour old hepatocyte cultures enhanced the MTT-reducing ability of V79 fibroblasts. The cytotoxicity of the derivatives was greater in recently isolated hepatocytes (only a 4-hour incubation for cell attachment prior to treating with the derivatives) than in hepatocytes previously cultured (24-hour incubation) before the treatment. Thus, aging reduced the cytotoxic effects of DHC derivatives in isolated hepatocytes, suggesting that P450-mediated biotransformation of such derivatives may lead to the formation of more toxic metabolites.

Methods of in vitro toxicology

In vitro methods are common and widely used for screening and ranking chemicals, and have also been taken into account sporadically for risk assessment purposes in the case of food additives. However, the range of food-associated compounds amenable to in vitro toxicology is considered much broader, comprising not only natural ingredients, including those from food preparation, but also compounds formed endogenously after exposure, permissible/authorised chemicals including additives, residues, supplements, chemicals from processing and packaging and contaminants. A major promise of in vitro systems is to obtain mechanism-derived information that is considered pivotal for adequate risk assessment. This paper critically reviews the entire process of risk assessment by in vitro toxicology, encompassing ongoing and future developments, with major emphasis on cytotoxicity, cellular responses, toxicokinetics, modelling, metabolism, cancer-related endpoints, developmental toxicity, prediction of allergenicity, and finally, development and application of biomarkers. It describes in depth the use of in vitro methods in strategies for characterising and predicting hazards to the human. Major weaknesses and strengths of these assay systems are addressed, together with some key issues concerning major research priorities to improve hazard identification and characterisation of food-associated chemicals.

Cytochrome P4501A induction and testosterone hydroxylation in cultured hepatocytes of four fish species

Primary hepatocytes of the rainbow trout (Oncorhynchus mykiss), flounder (Platychthis flesus), dab (Limanda limanda) and lemon sole (Microstomus kitt) were exposed to 3,3'4,4'5 pentachlorobiphenyl (PCB 126) or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) for two days. This resulted in a dose-dependent induction of cytochrome P4501A (CYP1A) activity, measured as ethoxyresorufin O-deethylase (EROD), or methoxyresorufin O-deethylase (MROD) activity. In all species, a linear relationship was observed between EROD and MROD activities, suggesting that the same CYP1A enzyme metabolizes the two alkoxy-resorufin substrates. Exposures of hepatocytes of flounder or dab to TCDD, resulted in a 59-fold and 8.2-fold induction of EROD activity, respectively. This did not concur with a change in the in vitro testosterone hydroxylation profiles of both species. These and other in vitro data indicate that TCDD exposure does not influence monooxygenase activities involved in testosterone hydroxylation. Furthermore, CYP1A is of minor importance for testosterone hydroxylation in these fish species.

In vitro metabolism of a new H(+)/K(+) ATPase inhibitor DBM-819 in liver microsomes using HPLC and electrospray mass spectrometry

The metabolism of 1-(2-methyl-4-methoxyphenyl)-4-[(3-hydroxypropyl)amino]-6-methyl-2,3-dihydropyrrolo[3,2c]quinoline (DBM-819), a new H(+)/K(+) ATPase inhibitor, has been studied by HPLC with spectrometric detection and on-line LC-electrospray mass spectrometry. In vitro incubation of DBM-819 with rat liver microsomes in the presence of NADPH resulted in the production of four metabolites (M1-4), whereas DBM-819 was oxidized to two metabolites, M2 and M4, by human liver microsomes. M2, M3 and M4 were identified as O-demethyl-DBM-819, 8-hydroxy-DBM-819 and N-dehydroxypropyl-DBM-819, respectively, based on LC/MS/MS analysis with authentic standards. M1 was tentatively identified as 1-(hydroxy-2-methyl-4-methoxyphenyl)-4-[(3-hydroxypropyl)amino]-6-methyl-2,3-dihydropyrrolo[3,2c]quinoline. Rat liver CYP1A1/2 catalyzed the oxidation of DBM-819 to 8-hydroxy-DBM-819 and N-dehydroxypropyl-DBM-819. Human CYP3A4 was a major isozyme for the formation of O-demethyl-DBM-819 as well as N-dehydroxypropyl-DBM-819.

Cytochrome P450 activity in control and induced long-term cultures of rat hepatocyte spheroids

Long-term events such as enzyme induction or chronic toxicity require long-term liver culture models that maintain activity of xenobiotic metabolising enzymes. The levels of these enzyme activities and their responsiveness to chemical induction was studied in rat hepatocyte spheroids, a potential long-term hepatocyte culture model. In comparison with other long-term liver culture models, the basal metabolic activity of spheroids has not been well studied. Additionally, no existing data on the induction of CYP3A activity in spheroids could be found. The basal xenobiotic metabolising activity of rat hepatocyte spheroids was monitored over 14 days in culture, using testosterone as a probe substrate. When spheroids from days 2-14 in culture were compared to 24-h control spheroids, there was a differential maintenance of basal CYP activity. CYP2A and CYP3A activities were maintained over the culture period, while there were time-related decreases in CYP2C11 and CYP2C/CYP2B1/2 activities. The responsiveness of rat hepatocyte spheroids to chemical induction was studied following treatment with phenobarbitone (PB) or dexamethasone (DEX). PB treatment induced CYP2A, CYP2C, CYP2B1/2 and CYP3A activities. DEX treatment resulted in an induction of CYP3A and CYP2C11 activities. The results demonstrate that rat hepatocyte spheroids retained some of the liver-specific functions essential in a long-term hepatocyte culture model, thus making spheroids comparable to other long-term culture models available.

Immobilisation on polystyrene of diazirine derivatives of mono- and disaccharides: biological activities of modified surfaces

The potential of surface glycoengineering for biomaterials and biosensors originates from the importance of carbohydrate-protein interactions in biological systems. The strategy employed here utilises carbene generated by illumination of diazirine to achieve covalent bonding of carbohydrates. Here, we describe the synthesis of an aryl diazirine containing a disaccharide (lactose). Surface analysis techniques [X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectroscopy (ToF-SIMS)] demonstrate its successful surface immobilisation on polystyrene (PS). Results are compared to those previously obtained with an aryl diazirine containing a monosaccharide (galactose). The biological activity of galactose- or lactose-modified PS samples is studied using rat hepatocytes, Allo A lectin and solid-phase semi-synthesis with alpha-2,6-sialyltransferase. Allo A shows some binding to galactose-modified PS but none to lactose-modified surfaces. Similar results are obtained with rat hepatocytes. In contrast, sialylation of lactose-modified PS is achieved but not with galactose-modified surfaces. The different responses indicate that the biological activity depends not only on the carbohydrate per se but also on the structure and length of the spacer.

Characterization of biotransformation enzyme activities in primary rat proximal tubular cells

The proximal tubule is a frequent target for nephrotoxic compounds due to it's ability to transport and accumulate xenobiotics and their metabolites, as well as by the presence of an organ-selective set of biotransformation enzymes. The aim of the present study was to characterize the activities of different biotransformation enzymes during primary culturing of rat proximal tubular cells (PT cells). Specific marker substrates for determining cytochrome P450 (CYP450) activity of primary cultured PT cells include 7-ethoxyresorufin (CYP1A1), caffeine (CYP1A), testosterone (CY2B/C, CYP3A), tolbutamide (CYP2C) and dextromethorphan (CYP2D1). Activities of the CYP450 isoenzymes decreased considerably during culture with the greatest loss in activity within 24 h of culture. In addition, expression of CYP450 apoprotein, including CYP1A, CYP2C, CYP2D, CYP2E and CYP4A, was detected in microsomes from freshly isolated PT cells by immunoblotting using specific antibodies. CYP2B and CYP3A apoprotein could not be detected. Activity of the phase II biotransformation enzymes GST, GGT, beta-lyase and UGT was determined with 1-chloro-2,4-dinitrobenzene, L-glutamic acid gamma-(7-amido-4-methyl-coumarin), S-(1,1,2,2-tetrafluoroethyl)-L-cysteine and 1-naphthol, respectively, as marker substrates. Activity of the phase II enzymes remained more stable and, in contrast to CYP450 activity, significant activity was still expressed after 1 week of PT cell culture. Thus, despite the obvious advantages of PT cells as an in-vitro model for studies of biotransformation mediated toxicity, the strong time dependency of especially phase I and, to a lesser extent, phase II biotransformation activities confers limitations to their application.

Comparison of the stability of some major cytochrome P450 and conjugation reactions in rat, dog and human hepatocyte monolayers

The stability of four major cytochrome P450 isoenzymes (CYPIA, CYP2B, CYP2E1 and CYP3A) and of two phase II conjugation enzymes (glucuronyl- and sulfotransferases) was investigated in primary cultures of rat, dog and human hepatocytes in the same conditions. 7-ethoxyresorufin deethylation (EROD), 7-methoxycoumarin demethylation (MCOD), chlorzoxazone (CLOX) 6-hydroxylation, 1'- and 4-hydroxylation of midazolam (MDZ), and p-nitrophenol glucuronidation and sulfation, were used respectively. The EROD activity was stable over 72 hours in rat and dog and only 48 hours in human hepatocytes. The MCOD activity was also stable in rat but decreased in dog by 30% within 72 hours The CLOX hydroxylase activity was most stable in human whereas in rat and dog it fell down to 30% within 72 and 24 hours, respectively. The MDZ hydroxylase activity showed the same unstability profile in the three species investigated. Both conjugation reactions were either stable or showed an increase by up to 60-70% in all three species over 72 hours. The enzymes tested showed different stabilities in rat, dog and human hepatocytes over 72 hours, thus demonstrating the limitations of hepatocyte monolayers as models for metabolic investigations and emphasising the need for validation/characterization studies before routine use.

Xenobiotic metabolism by cultured primary porcine hepatocytes

Considering the large yield of viable cells comparable to human liver, primary porcine hepatocytes offer a valuable resource for constructing a bioartificial liver device. In this study, the ability of cultured primary porcine hepatocytes to detoxify xenobiotics has been examined using various known substrates of cytochrome P450 isoenzymes and UDP-glucuronosyltransferases. Present investigation demonstrated the stability of the isoenzymes responsible for the metabolism of diazepam in native state and stabilization of other isoenzymes, as judged by ethoxycoumarin o-dealkylase (ECOD), ethoxyresorufin o-dealkylase (EROD), benzyloxyresorufin o-dealkylase (BROD), and pentoxyresorufin o-dealkylase (PROD) activities following induction in culture environment, for a period of 8 days. Resorufin O-dealkylase activities were found to be the most unstable and deteriorated within first 5 days in culture. These activities were restored following induction with 3-methylcholanthrene (3-MC) or sodium phenobarbital (PB) to 20-fold of 1 activity for EROD, and 60 and 174% of day 1 activity for PROD and BROD on day 8, respectively. Metabolism of methoxyresorufin was most strikingly increased following induction with 3-MC to approximately 60-fold of day 1 activity, on day 8. UDP-glucuronosyltransferase-dependent glucuronidation of phenol red, however, stayed intact during the course of our study without induction. Our study indicated that porcine hepatocytes in vitro maintain many important liver-specific functions including detoxification (steady state and inducibility).

Stability of cytochrome P450 proteins in cultured precision-cut rat liver slices

The objective of this study was to evaluate the stability of individual, xenobiotic-metabolising, cytochrome P450 proteins in precision-cut rat liver slices cultured for up to 72 h using the multiwell plate system. This was achieved using established diagnostic probes (O-dealkylation of methoxy-, ethoxy- and pentoxy-resorufin, testosterone 2alpha-hydroxylase, debrisoquine 4-hydroxylase, aniline p-hydroxylase and lauric acid hydroxylase) and immunologically using Western blotting. All cytochrome P450 activities declined in culture, the most rapid loss occurring at about 8-12 h of culture; in all cases no detectable activity was present in the 72-h cultured slices. Isoform-specific differences in the stability of various cytochrome P450 proteins were observed, with CYP2E1 being the most stable. When cytochrome P450 expression was determined immunologically, a different picture emerged. High levels of apoprotein were retained in the slices even when activity was very low. In the case of CYP2B, apoprotein levels even increased following the culture of hepatic slices. It is concluded, that for tissue slices to become an acceptable in vitro alternative system for long-term incubations, the culturing conditions must be improved to ensure that cytochrome P450 activities are better maintained.

Cultures with cryopreserved hepatocytes: applicability for studies of enzyme induction

The use of hepatocyte cultures is well established for the study of drug-drug interactions. However, the major hindrance for the use of human hepatocyte cultures is that human hepatocytes are only occasionally available. This problem could be overcome by cryopreservation. Although cryopreserved hepatocytes have been recommended for short term applications in suspension, studies on induction of enzyme activity, requiring a more prolonged maintenance of cryopreserved hepatocytes in culture, represent a new field of research. In the present study, we established a technique that allows preparation of rat hepatocyte co-cultures, using cryopreserved hepatocytes. After incubation with phenobarbital (0.75 mM; 72 h) induction factors for the isoenzyme-dependent regio and stereoselective testosterone hydroxylations were 1.6, 2.2, 1.0, 2.1, 5.6, 2.4, 3.6, 4.5 and 0.9 for 2alpha-, 2beta-, 6alpha-, 6beta-, 7alpha-, 15beta-, 16alpha- and 16beta-hydroxytestosterone and 4-androsten-3,17 dione. Regarding induction factors of less than 2-fold, as questionable these induction factors were similar to those of cultures with freshly isolated hepatocytes and the induction pattern of the individual hydroxylation products was similar to the in vivo situation. In addition 3-methylcholanthrene (5 microM; 72 h) induced exclusively the formation of 7alpha-hydroxytestosterone (6.6-fold) in cultures with cryopreserved hepatocytes. This specificity also correlates to that obtained in rats. Although these induction factors were clearly satisfactory in cryopreserved cultures, the absolute activities of the main testosterone hydroxylation products were reduced when compared to fresh cultures. For instance, 6beta-hydroxytestosterone, the main metabolite in solvent controls was reduced to 79%, 7alpha-hydroxytestosterone, the main metabolite after induction with 3-MC, was reduced to 66% and 16beta-hydroxytestosterone, the main metabolite after induction with PB, was reduced to 52%. Similarly, EROD activity after induction with 3-methylcholanthrene in cryopreserved cultures was reduced to 62%, compared with that in fresh cultures. Although further optimization and validation is required, the data show that cytochrome P450 activities can clearly be induced in co-cultures of cryopreserved hepatocytes, in a fashion which for the investigated inducers, is similar to that in cultures from freshly isolated hepatocytes and similar to the in vivo situation.

Cryopreserved primary hepatocytes as a constantly available in vitro model for the evaluation of human and animal drug metabolism and enzyme induction

The use of primary hepatocytes is now well established for both studies of drug metabolism and enzyme induction. Cryopreservation of primary hepatocytes decreases the need for fresh liver tissue. This is especially important for research with human hepatocytes because availability of human liver tissue is limited. In this review, we summarize our research on optimization and validation of cryopreservation techniques. The critical elements for successful cryopreservation of hepatocytes are (1) the freezing protocol, (2) the concentration of the cryoprotectant [10% dimethyl-sulfoxide (DMSO)], (3) slow addition and removal of DMSO, (4) carbogen equilibration during isolation of hepatocytes and before cryopreservation, and (5) removal of unvital hepatocytes by Percoll centrifugation after thawing. Hepatocytes of human, monkey, dog, rat, and mouse isolated and cryopreserved by our standard procedure have a viability > or = 80%. Metabolic capacity of cryopreserved hepatocytes determined by testosterone hydroxylation, 7-ethoxyresorufin-O-de-ethylase (EROD), 7-ethoxycoumarin-O-deethylase (ECOD), glutathione S-transferase, UDP-glucuronosyl transferase, sulfotransferase, and epoxide hydrolase activities is > or = 60% of freshly isolated cells. Cryopreserved hepatocytes in suspension were successfully applied in short-term metabolism studies and as a metabolizing system in mutagenicity investigations. For instance, the complex pattern of benzo[a]pyrene metabolites including phase II metabolites formed by freshly isolated and cryopreserved hepatocytes was almost identical. For the study of enzyme induction, a longer time period and therefore cryopreserved hepatocyte cultures are required. We present a technique with cryopreserved hepatocytes that allows the induction of testosterone metabolism with similar induction factors as for fresh cultures. However, enzyme activities of induced hepatocytes and solvent controls were smaller in the cryopreserved cells. In conclusion, cryopreserved hepatocytes held in suspension can be recommended for short-term metabolism or toxicity studies. Systems with cryopreserved hepatocyte cultures that could be applied for studies of enzyme induction are already in a state allowing practical application, but may be further optimized.

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.

Role of hypoxia and constitutionally different resistance to hypoxia/stress as the determiners of individual profile of cytochrome P450 isozyme activity

The hepatic cytochromes P450 1A1, 1A2, 2B1, and 2E1 activities have been investigated in the sublines of Wistar rats with principally different (high or low) resistance to hypoxia/stress. Repeated measurements in normoxic conditions showed a significant prevalence of total cytochrome P450 content, CYP 1A1, and CYP 2E1 activities in rats with low-resistance (LR) to hypoxia compared to rats with high-resistance to hypoxia (HR), whereas in HR rats the CYP 1A2 activity was 63% higher (p < 0.001) than in LR rats. In the conditions of acute hypobaric hypoxia these differences were manifested distinctly: in HR rats an enhancement of CYP 1A2 activity by 49% of aerobic value (p < 0.01) was observed, in LR rats the total P450 content, CYP 1A1 and 2E1 activities (p < 0.05-0.001) were increased. The 30-min total liver ischemia formed an individual response of the drug-metabolizing system: in HR rats CYP 1A2 and 2B1 activities were decreased in the early postischemic period and were not restored by the 21st day, whereas in LR rats CYP 1A2 activity was not affected and was induced more than 2-fold of aerobic value in the late post-ischemic period. The CYP 2B1 activity was induced almost 1.5-fold during the whole postischemic period. These data suggest that acute hypoxia and individual resistance to hypoxia/stress, one of the cardinal constitutional features, provide an individual reaction of drug-metabolizing system and enzymes of P450, in particular. The individual constitutional resistance to hypoxia/stress may be a serious criterion for an individual approach in pharmacotherapy of hypoxic states, diseases, as well as for prognosis and prevention of early and distant complications of irrational pharmacotherapy.

Kupffer cell-mediated differential down-regulation of cytochrome P450 metabolism in rat hepatocytes

Nonparenchymal cells, particularly Kupffer cells, might play an important role in the modulation of xenobiotic metabolism in liver and its pharmacological and toxicological consequences. This intercellular communication via the exchange of soluble factors was investigated in primary rat Kupffer cells and hepatocytes. Freshly isolated rat Kupffer cells were seeded onto cell culture inserts and cocultured with 5 day old serum-free rat hepatocyte monolayer cultures at a ratio of 1:1 for 2 days. Hepatocyte cultures, Kupffer cell cultures or cocultures were treated with 0.1 ng/ml-10 microg/ml lipopolysaccharide (LPS). Within this concentration range, no significant toxicity was observed in either cell type. In LPS-exposed cocultures, tumor necrosis factor alpha (TNFalpha) levels rose up to 5 ng/ml within 5 h; nitric oxide (NO) levels increased up to 70 microM within 48 h of treatment, both in a dose-dependent fashion. The release of negative (albumin) and positive (alpha1-acid-glycoprotein) acute phase proteins from the hepatocytes was strongly down- and up-regulated, respectively. The simultaneous treatment of the cocultures with phenobarbital and LPS (10 ng/ml) or 3-methylcholanthrene and LPS (10 ng/ml) resulted in a strong down-regulation (85%) of the phenobarbital-induced cytochrome P450 (CYP) isoform CYP2B1 in the hepatocytes whereas the 3-methylcholanthrene-induced isoform CYP1A1 was only weakly affected (15%). This specific down-regulation of CYP2B1 was mediated exclusively by TNFalpha, released from the Kupffer cells. It was not linked with NO release from or inducible NO synthase activity in the hepatocytes. The TNFalpha release was not affected by the two xenobiotics. Acetaminophen tested in these cocultures showed no direct interaction with the Kupffer cells. The use of liver cell cocultures is therefore a useful approach to investigate the influence of intercellular communication on xenobiotic metabolism in liver.

Sequence analysis and functional characterization of the 5'-flanking region of the rat multidrug resistance protein 2 (mrp2) gene

Gene expression of the canalicular conjugate transporter mrp2 is inducible by treatment with the DNA-damaging agents 2-acetylaminofluorene (50 and 100 microM), and cisplatin (20 microM) in primary rat hepatocytes as well as in the rat hepatoma cell line H4IIE. Furthermore, phenobarbital (1 and 2 mM) induces mrp2 gene expression, probably explaining the increase in bile-salt-independent bile flow caused by phenobarbital, while the cholestatic drug ethinyl estradiol (10(-6) M) leads to an increase in mrp2 mRNA but decreases Mrp2 protein level probably via a posttranscriptional mechanism. The 5'-flanking region of the rat mrp2 gene was sequenced and cloned into a luciferase reporter vector. Transient transfection assays with reporter vectors containing unidirectionally deleted 5'-flanking regions using H4IIE cells indicate that two different sequences of 17 and 37 bases comprising a Y-Box and a GC-Box are required for mrp2 gene basal expression. Sequences mediating 2-AAF induction are located within a region 250 bases upstream of the translation start site while the inducing effect of phenobarbital seems to be mediated by another domain located further upstream.

Peroxisome proliferators increase the formation of BPDE-DNA adducts in isolated rat hepatocytes

Peroxisome proliferators are known to modulate the activity of xenobiotic-metabolising enzymes, including glutathione S-transferase (GST) and cytochrome P-450 (CYP). In this study the effect of peroxisome proliferators silvex and di(2-ethylhexyl)phthalate (DEHP) on the formation of (+)-anti-benzo(a)pyrene -7,8-dihydrodiol-9,10-epoxide (BPDE)-DNA adducts from a proximate mutagen and carcinogen (-)-transbenzo(a)pyrene-7,8-dihydrodiol (BPDD) has been investigated. Rat CYP1A1 metabolises BPDD to mutagenic BPDE, which may form DNA adducts or, alternatively, be detoxified by hydrolysis or glutathione conjugation. In this experiment the formation of BPDE-DNA adducts was significantly increased in hepatocytes isolated from all silvex treated rats and two out of four DEHP treated rats (14 day treatment). The activity of CYP1A1 was increased whereas GST was reduced by the peroxisome proliferator silvex. These changes were more significant than those induced by DEHP. We have hypothesised that the formation of BPDE-DNA adducts was primarily due to the increased BPDD activation to BPDE versus reduced detoxication of BPDE. Other hepatic changes induced by the peroxisome proliferators, e.g. peroxisome proliferation per se and increased mitotic activity of the liver could have an effect on the outcome of BPDD exposure.

Activity and inducibility of drug-metabolizing enzymes in immortalized hepatocyte-like cells (mhPKT) derived from a L-PK/Tag1 transgenic mouse

This report describes the establishment and characterization of the mhPKT cell line derived from the liver of a transgenic mouse harboring the simian virus (SV40) large T and small t antigens placed under the control of the 5' regulatory sequence of the rat L-type pyruvate kinase (L-PK) gene. mhPKT cells had a prolonged life span, expressed the SV40-encoded nuclear large T antigen when grown in glucose-enriched medium, and induced tumors when injected subcutaneously into athymic (nu-nu) mice. Growth on petri dishes or filters yielded multiple layers of cuboid cells, with numerous spaces between adjacent cells that were closed by junctional complexes. These bile canaliculi-like structures exhibited numerous microvilli in which villin, an actin-binding brush-border protein, colocalized with actin. These bile canaliculi-like structures appeared to be functional as they accumulated fluorescein. mhPKT cells conserved the expression of the liver-specific transcription factors HNF1, HNF3, HNF4, and DBP together with substantial levels of L-PK and albumin but not alpha-fetoprotein mRNA transcripts. mhPKT cells mainly metabolized testosterone into androstenedione and 6beta-hydroxytestosterone, as in vivo. 3-Methylcholanthrene and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) markedly increased ethoxyresorufin-O-deethylase activity and the related cytochrome P450 (CYP) 1A1/2 protein, whereas alpha-naphtoflavone antagonized the TCDD-elicited induction. Phenobarbital slightly increased the CYP2B-mediated activities of pentoxyresorufin-O-depentylase, 2beta- and 16beta-testosterone hydroxylase. mhPKT cells also had substantial sulfotransferase, UDP-glucuronyltransferase, and glutathione S-transferase activities. This model may serve as a tool for long-term in vitro studies of xenobiotic metabolism, potent CYP inducers, and hepatocyte damage due to drugs and other factors.

Peroxisome proliferator nafenopin potentiated cytotoxicity and genotoxicity of cyclophosphamide in the liver and bone marrow cells

Peroxisome proliferators are ubiquitous rodent hepatocarcinogens, known to modulate the activities of xenobiotic-metabolising enzymes such as glutathione S-transferases (GST) and mixed-function oxidase (cytochrome P-450). In addition these compounds induce pleiotropic changes in the liver of rodents even after a short-term treatment. It has been hypothesised that the enzymatic and cellular changes induced by peroxisome proliferators may alter the toxicity of other compounds activated by cytochrome P-450 and detoxified by GST isoenzymes. The effect of nafenopin-induced changes in the liver of rats on the toxicity of an anti-cancer drug cyclophosphamide was studied using cyto- and geno-toxicity parameters in the liver and bone marrow cells. The administration of cyclophosphamide (10 or 20 mg/kg bw) to the rats pre-treated with 80 mg/kg bw of nafenopin for 2 days resulted in significantly increased cytotoxic response in bone marrow cells. However, genotoxicity of cyclophosphamide was increased only in the liver of nafenopin pre-treated rats. Low level of genotoxicity in bone marrow could be accounted for potentiated cytotoxicity of cyclophosphamide. These events coincided with a significant, up to 5-fold, increase in indirect activation-detoxication index for cyclophosphamide, determined as a ratio of ECOD and GST activities, in nafenopin treated rats. This resulted from the induction of ECOD responsible for the formation of reactive metabolites of cyclophosphamide and reduced activity of GST responsible for their detoxication. In addition, mitotic activity of hepatocytes was increased in nafenopin treated rats that might also have an impact on the genotoxicity of cyclophosphamide in this organ.

Regulation of the major detoxication functions by phenobarbital and 3-methylcholanthrene in co-cultures of rat hepatocytes and liver epithelial cells

In the present study, we analysed the expression of monooxygenase activities and mRNAs associated with cytochrome P-450 (CYP), including CYP1A1/2, CYP2B1/2, CYP2C6, CYP2E1, CYP3A1/2, glutathione transferase alpha (GST alpha), aldehyde dehydrogenase and epoxide hydrolase in co-cultures of primary rat hepatocytes and rat liver epithelial cells. We observed that pentoxyresorufin O-deethylation activity was well maintained and ethoxyresorufin O-deethylation activity gradually decreased during co-culture time. In addition, we showed that phenobarbital and 3-methylcholanthrene treatments resulted in a significant increase of these activities. Two general patterns of accumulation of liver-specific mRNAs were observed. CYP1A1/2, CYP2B1/2, CYP3A1/2, GST alpha, aldehyde dehydrogenase and epoxide hydrolase mRNAs were maintained at a stable level, whereas CYP2C6 and CYP2E1 mRNAs showed a continuous decline. In addition, we observed a strong increase of CYP1A1/2 (13.6-fold) and GST alpha (3.9-fold) mRNA expression in 3-methylcholanthrene-treated co-cultures and induction of CYP2B1/2 (19-fold), CYP2C6 (10-fold), CYP3A1/2 (11.2-fold), GST alpha (9-fold), aldehyde dehydrogenase (6-fold) and epoxide hydrolase (5-fold) mRNA expression in phenobarbital-treated co-cultures. Furthermore, we demonstrated that liver-specific gene expression was restricted to hepatocytes, with the notable exception of epoxide hydrolase and CYP2E1 which were expressed in both cell types during the co-culture, as shown by the selective recovery of both hepatocytes and rat liver epithelial cells. Finally, to investigate whether co-cultures could be used to study the molecular mechanisms regulating CYP transcription, we performed transfection of hepatocytes, before the establishment of the co-culture, with large CYP2B1 (3.9 kb) or CYP2B2 (4.5 kb) promoter chloramphenicol acetyltransferase constructs or with a construct containing a 163-bp DNA sequence element reported to confer phenobarbital responsiveness. A 2-3-fold increase over the basal level of chloramphenicol acetyltransferase activity was observed in phenobarbital-treated co-cultures transfected with the phenobarbital-responsive element construct, although phenobarbital had no effect on large CYP2B1 or CYP2B2 promoter fragments. Our results demonstrate that the co-culture system provides a good tool for studying drug metabolism, and shows promise as a new tool for analysing transcriptional regulation under the influence of xenobiotics within primary hepatocytes.