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

Microphysiological systems meet hiPSC technology - New tools for disease modeling of liver infections in basic research and drug development

Complex cell culture models such as microphysiological models (MPS) mimicking human liver functionality in vitro are in the spotlight as alternative to conventional cell culture and animal models. Promising techniques like microfluidic cell culture or micropatterning by 3D bioprinting are gaining increasing importance for the development of MPS to address the needs for more predictivity and cost efficiency. In this context, human induced pluripotent stem cells (hiPSCs) offer new perspectives for the development of advanced liver-on-chip systems by recreating an in vivo like microenvironment that supports the reliable differentiation of hiPSCs to hepatocyte-like cells (HLC). In this review we will summarize current protocols of HLC generation and highlight recently established MPS suitable to resemble physiological hepatocyte function in vitro. In addition, we are discussing potential applications of liver MPS for disease modeling related to systemic or direct liver infections and the use of MPS in testing of new drug candidates.

Strain differences between CD-1 and C57BL/6 mice in expression of metabolic enzymes and DNA methylation modifications of the primary hepatocytes

Primary mouse hepatocyte cultures are widely used in toxicological and pharmacological studies. However, the strain differences in alterations of metabolic enzymes and the regulation of gene expression in response to different stimuli remains unclear. To address this issue, we examined the expression of metabolic enzymes and the regulatory role of DNA methylation in the primary hepatocytes of two mouse strains, CD-1 and C57BL/6. Primary culture of mouse hepatocytes was established using collagen sandwich configuration. Analysis of gene expression of 24 phase I, 18 phase II, and 6 phase III metabolic enzymes on 4 consecutive days after cell seeding revealed that the basal levels of most enzymes in primary cultured hepatocytes differed greatly between the two mouse strains. However, the dynamic changes in most genes were identical between the two strains. In addition, treatment with 3-methylcholanthrene, phenobarbital, and rifampin led to the induction of cytochrome P-450 (cyp) 1a1 and cyp1a2, cyp2b10, cyp3a11. However, induction varied in degree between the two types of primary hepatocytes. The dynamic changes in global DNA methylation and the expression of DNA methylation regulatory factors of the two mouse strains were similar. Of the genes down-regulated over the culture period, hypermethylation of cyp2e1 gene appeared in both mouse strains and led to a suppression of gene expression. Taken together, these results demonstrate that the expression of metabolic enzymes and the response to agonists in primary hepatocytes differ between CD-1 and C57BL/6 mouse strains. Epigenetic regulation might be involved in the suppression of cyp 450s' expression.

A Drug Screen using Human iPSC-Derived Hepatocyte-like Cells Reveals Cardiac Glycosides as a Potential Treatment for Hypercholesterolemia

Efforts to identify pharmaceuticals to treat heritable metabolic liver diseases have been hampered by the lack of models. However, cells with hepatocyte characteristics can be produced from induced pluripotent stem cells (iPSCs). Here, we have used hepatocyte-like cells generated from homozygous familial hypercholesterolemia (hoFH) iPSCs to identify drugs that can potentially be repurposed to lower serum LDL-C. We found that cardiac glycosides reduce the production of apolipoprotein B (apoB) from human hepatocytes in culture and the serum of avatar mice harboring humanized livers. The drugs act by increasing the turnover of apoB protein. Analyses of patient medical records revealed that the treatment of patients with cardiac glycosides reduced serum LDL-C levels. These studies highlight the effectiveness of using iPSCs to screen for potential treatments for inborn errors of hepatic metabolism and suggest that cardiac glycosides could provide an approach for reducing hepatocyte production of apoB and treating hypercholesterolemia.

General Cytotoxicity Assessment by Means of the MTT Assay

Cytotoxicity assays were among the first in vitro bioassay methods used to predict toxicity of substances to various tissues. In vitro cytotoxicity testing provides a crucial means for safety assessment and screening, and for ranking compounds. The choice of using a particular cytotoxicity assay technology may be influenced by specific research goals. As such, four main classes of assays are used to monitor the response of cultured cells after treatment with potential toxicants. These methods measure viability, cell membrane integrity, cell proliferation, and metabolic activity. In this chapter, we focus on the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide tetrazolium reduction colorimetric assay to evaluate detrimental intracellular effects on metabolic activity. This assay is well-characterized, simple to use and remains popular in several laboratories worldwide.

Transcriptomic hepatotoxicity signature of chlorpromazine after short- and long-term exposure in primary human sandwich cultures

Drug-induced liver injury is the most frequent reason for market withdrawal of approved drugs, and is difficult to predict in animal models. Here, we analyzed transcriptomic data derived from short- and long-term cultured primary human hepatocytes (PHH) exposed to the well known human hepatotoxin chlorpromazine (CPZ). Samples were collected from five PHH cultures after short-term (1 and 3 days) and long-term (14 days) repeat daily treatment with 0.1 or 0.2 µM CPZ, corresponding to C(max). Two PHH cultures were additionally treated with 1 µM CPZ, and the three others with 0.02 µM CPZ. Differences in the total number of gene changes were seen between donors and throughout treatment. Specific transcriptomic hepatotoxicity signatures were created for CPZ and consisted of inflammation/hepatitis, cholestasis, and liver proliferation in all five donors, as well as fibrosis and steatosis, which were observed in four of five donors. Necrosis was present in three of five donors, and an indicative signature of cirrhosis was observed after long-term 14-day repeat treatment, also in three of five donors. The inter-donor variability in the inflammatory response to CPZ treatment was associated with variability in the strength of the response of the transcriptomic hepatotoxicity signatures, suggesting that features of inflammation could be related to the idiosyncratic hepatotoxic effects of CPZ in humans.

Primary hepatocyte cultures for pharmaco-toxicological studies: at the busy crossroad of various anti-dedifferentiation strategies

Continuously increasing understanding of the molecular triggers responsible for the onset of diseases, paralleled by an equally dynamic evolution of chemical synthesis and screening methods, offers an abundance of pharmacological agents with a potential to become new successful drugs. However, before patients can benefit of newly developed pharmaceuticals, stringent safety filters need to be applied to weed out unfavourable drug candidates. Cost effectiveness and the need to identify compound liabilities, without exposing humans to unnecessary risks, has stimulated the shift of the safety studies to the earliest stages of drug discovery and development. In this regard, in vivo relevant organotypic in vitro models have high potential to revolutionize the preclinical safety testing. They can enable automation of the process, to match the requirements of high-throughput screening approaches, while satisfying ethical considerations. Cultures of primary hepatocytes became already an inherent part of the preclinical pharmaco-toxicological testing battery, yet their routine use, particularly for long-term assays, is limited by the progressive deterioration of liver-specific features. The availability of suitable hepatic and other organ-specific in vitro models is, however, of paramount importance in the light of changing European legal regulations in the field of chemical compounds of different origin, which gradually restrict the use of animal studies for safety assessment, as currently witnessed in cosmetic industry. Fortunately, research groups worldwide spare no effort to establish hepatic in vitro systems. In the present review, both classical and innovative methodologies to stabilize the in vivo-like hepatocyte phenotype in culture of primary hepatocytes are presented and discussed.

Effect of rat serum lipoproteins on mRNA levels and amiodarone metabolism by cultured primary rat hepatocytes

Hyperlipidemia can significantly increase amiodarone (AM) in vivo liver uptake and decrease its velocity of microsomal metabolism. Here, hepatocytes isolated from normolipidemic (NL) and hyperlipidemic rats were incubated with AM in the presence or absence of diluted NL or hyperlipidemic serum. The serum was added either as preincubation before drug, or concurrently with drug; incubations without rat serum were used as controls. The hepatocyte levels of mRNA for several proteins and enzymes were also measured. Disappearance of AM was seen up to 72 h. There was little difference between hepatocytes from NL or hyperlipidemic animals in intrinsic clearance (CL(int) ) of AM. The effect of hyperlipidemic rat serum, either before or with AM, was profound, causing a significant reduction in the CL(int) . Reductions were seen in mRNA for cytochrome P450 1A1, 3A2, and 2D1, some transporters, and low-density lipoprotein receptors after exposure of hepatocytes to lipoprotein-rich sera. In conclusion, exposure of isolated hepatocytes to hyperlipidemic serum caused decreases in AM CL(int) and lower mRNA levels for some proteins involved in the uptake and metabolism of AM. When coincubated with serum, an additional effect of increased binding to lipoproteins seemed to further contribute to a reduced CL of AM.

Organotypic liver culture models: meeting current challenges in toxicity testing

Prediction of chemical-induced hepatotoxicity in humans from in vitro data continues to be a significant challenge for the pharmaceutical and chemical industries. Generally, conventional in vitro hepatic model systems (i.e. 2-D static monocultures of primary or immortalized hepatocytes) are limited by their inability to maintain histotypic and phenotypic characteristics over time in culture, including stable expression of clearance and bioactivation pathways, as well as complex adaptive responses to chemical exposure. These systems are less than ideal for longer-term toxicity evaluations and elucidation of key cellular and molecular events involved in primary and secondary adaptation to chemical exposure, or for identification of important mediators of inflammation, proliferation and apoptosis. Progress in implementing a more effective strategy for in vitro-in vivo extrapolation and human risk assessment depends on significant advances in tissue culture technology and increasing their level of biological complexity. This article describes the current and ongoing need for more relevant, organotypic in vitro surrogate systems of human liver and recent efforts to recreate the multicellular architecture and hemodynamic properties of the liver using novel culture platforms. As these systems become more widely used for chemical and drug toxicity testing, there will be a corresponding need to establish standardized testing conditions, endpoint analyses and acceptance criteria. In the future, a balanced approach between sample throughput and biological relevance should provide better in vitro tools that are complementary with animal testing and assist in conducting more predictive human risk assessment.

Effects of time culture and prototypical cytochrome P450 3A (CYP3A) inducers on CYP2B22, CYP2C, CYP3A and nuclear receptor (NR) mRNAs in long-term cryopreserved pig hepatocytes (CPHs)

In the present study, transcriptional and post-translational effects of culturing time and prototypical cytochrome P450 3A (CYP3A) inducers on principal nuclear receptors (NRs), CYP2B22, 2C and 3A were investigated in long-term stored (~10 years) cryopreserved pig hepatocytes (CPHs). In the time-course study, a crush and rise effect was observed for pregnane X receptor (NR1I2) and constitutive androstane receptor (NR1I3) mRNAs, while a time-dependent increase of retinoid X receptor alpha (NR2B1) was noticed. Cytochrome P450 gene expression profiles were down-regulated as a function of time. In the induction study, an increase of NR1I2, NR1I3 and NR2B1 mRNAs was observed in dexamethasone-exposed CPHs. About CYPs, an overall up-regulation was seen in CPHs exposed to phenobarbital, while dexamethasone and rifampicin up-regulated only CYP3A. In both studies, transcriptional CYP results were confirmed at the post-translational level (immunoblotting and enzyme activities), except for CYP2B immunoblotting in the induction study. The present data demonstrate that long-term stored CPHs may be used to investigate mechanisms involved in CYPs regulation, expression and function; provide further info about NR regulation of CYPs, and confirm species-differences in these mechanisms of regulation; finally, they suggest the usefulness and relevance of gene expression profiling to early detect any modulation of CYP expression and bioactivity.

Construction of a system that simultaneously evaluates CYP1A1 and CYP1A2 induction in a stable human-derived cell line using a dual reporter plasmid

Human CYP1A1 and CYP1A2 genes are in a head-to-head orientation on chromosome 15 and are separated by a 23-kb intergenic space. To our knowledge, this is the first report on a stable cell line that contains the 23-kb full-length regulatory region and is able to simultaneously assess the transcriptional activation of CYP1A1 and CYP1A2 genes. The stable cell line that constitutively expresses the reporter activities was constructed by inserting the dual reporter plasmid containing the 23-kb region between the CYP1A1 and CYP1A2 genes into the chromosome. Transcriptional activation of the CYP1A1 and CYP1A2 genes was measured simultaneously using luciferase (Luc) and secreted alkaline phosphatase (SEAP) activities, respectively. To demonstrate the utility of the stable cell line, CYP1A1/1A2 induction by the majority of compounds previously identified as CYP1A1/1A2 inducers was measured. The results clearly show that all compounds caused induction of reporter activities. In addition to assessing transcriptional activation of the CYP1A1 and CYP1A2 genes by measuring reporter activities, we determined the intrinsic CYP1A1 and CYP1A2 mRNA levels by treating them with the same compounds. The results suggest that this stable cell line may be used to rapidly and accurately predict CYP1A1/1A2 induction.

Optimization of an isolated perfused rainbow trout liver model: Clearance studies with 7-ethoxycoumarin

To date, research with isolated perfused fish livers has been limited by the relatively short time period during which stable performance can be achieved. In the present study, modifications to existing methods were employed with the goal of extending the usable life of an isolated perfused trout liver preparation. Liver performance was evaluated by measuring O(2) consumption (VO(2)), vascular resistance, K(+) leakage, glucose flux, lactate flux, and clearance of a model metabolic substrate, 7-ethoxycoumarin (CL(H,7-EC)). Livers perfused with solutions containing 15, 38, or 150microM bovine serum albumin (BSA) exhibited relatively stable physiological performance for up to 10h. CL(H,7-EC) decreased rapidly between 1 and 2h in all livers tested, possibly due in part to accumulation of 7-EC within the tissue. CL(H,7-EC) declined slowly thereafter, decreasing by 30-40% between 2 and 10h. A linear equation was subsequently developed to correct measured levels of clearance for this decrease in metabolic activity over time. To illustrate the value of this preparation, experiments were conducted to examine the effects of protein binding on 7-EC clearance. Clearance rates corrected for declining activity (CL(H,7-EC,CORR)) changed in nearly direct proportion to changes in the free concentration of 7-EC efferent to the liver, as predicted by theoretical models of liver function. Additional studies were performed to characterize the concentration-dependence of 7-EC clearance. The rate of substrate disappearance from the perfusate increased in proportion to the total concentration of 7-EC afferent to the liver resulting in constant levels of CL(H,7-EC,CORR). CL(H,7-EC,CORR) values for four livers averaged 12.1+/-2.5mL/h/g-liver (mean+/-SD, n=57 individual determinations) and were in good agreement with an estimate of hepatic clearance obtained by extrapolating published in vitro data from isolated trout hepatocytes. The extended viability of isolated trout livers achieved in this study creates new opportunities for research on hepatic function in fish.

Studying cytochrome P450 kinetics in drug metabolism

BACKGROUND

Determination of cytochrome P450 enzyme-mediated kinetics in vitro can be useful for predicting drug dosing and clearance in humans. Expressed P450s, human liver microsomes, human hepatocytes (both fresh and cryopreserved), and human liver slices are used to estimate K(m) and V(max) values for determination of intrinsic clearance of the drug for scale-up to predict in vivo clearance.

OBJECTIVE

To describe the advantages and disadvantages of the various in vitro systems used to estimate kinetic parameters for disposition of drugs and the various kinetic profiles that can be observed.

METHODS

A review of the literature was conducted to evaluate the utility of the various in vitro preparations, the methods for determining kinetic parameters and the types of kinetic profiles that may be observed.

RESULTS/CONCLUSIONS

The choice of in vitro system for determining kinetic parameters will depend on the objective of the studies, as each system has advantages and disadvantages. Kinetic parameter determinations must be carefully assessed to assure that the correct kinetic model is applied and the most accurate kinetic parameters are determined.

Transcriptomic and phylogenetic analysis of Kpna genes: a family of nuclear import factors modulated in xenobiotic-mediated liver growth

OBJECTIVES

We have identified a member of the karyopherin (importin) alpha family of nuclear import factors as being modulated in rat liver following exposure to the hypolipidaemic and liver growth agent Wy-14,643. To examine the hypothetical role of this protein family as a checkpoint in receptor-mediated signalling, we characterized the rat karyopherin alpha (Kpna) gene family and present cDNA sequences and gene structures for all six rat Kpna genes. Further, we have assembled a comprehensive panel of Kpna coding regions from a range of metazoa, which we have subjected to phylogenetic analysis: This represents by far the most complete phylogenetic study of metazoan karyopherins, including several evolutionary intermediates not previously examined. The phylogeny reveals three Kpna subfamilies with distinct, conserved gene structures, shedding light on the evolutionary origins of this multigene family in metazoa.

METHODS AND RESULTS

Using quantitative PCR, we have analysed Kpna transcript levels in 44 rat tissues; Kpna transcripts show a wide variation in their distribution both in absolute and relative terms, suggestive of specialized roles for each member. We also demonstrate that Kpna genes are regulated in rat liver and isolated hepatocytes in a xenobiotic-specific manner for a number of chemically distinct liver growth agents.

CONCLUSIONS

In light of the crucial role of nuclear import in mediating the genomic changes elicited through nuclear receptor activation, we postulate that changes in the levels of specific karyopherins alpha during xenobiotic-mediated liver growth represent an important component of the cellular response to the external stimuli that trigger these events.

Portal collagenase administration reverses carbon tetrachloride-induced rabbit liver cirrhosis

AIM: To investigate whether portal collagenase administration can reverse liver cirrhosis.

METHODS: Four normal controls (group A) received olive oil subcutaneously (sc) for 12 weeks followed by normal saline portal perfusion for 12 weeks. Another four rabbits (group B) received carbon tetrachloride (CCl₄) sc for 12 weeks and then 6 mg of collagenase portally for 12 weeks, while three control rabbits (group C) received CCl₄ for 12 weeks followed by saline for 12 weeks.

RESULTS: After 12 weeks of CCl₄ and another 12 weeks of portal vein perfusion, liver hydroxyproline content in collagenase-treated rabbits was significantly decreased as compared with that in saline-treated controls (177.5 ± 35.6 μg/g vs 446.3 ± 150.1 μg/g; F = 13.78, P < 0.01). Further, liver histology showed complete regression of cirrhosis in the collagenase-treated animals. No toxicity of liver, kidney, lung, brain or heart was observed histologically. Anaphylaxis occurred in 2 animals and one was fatal.

CONCLUSION: Portal administration of collagenase can promote the reversion of established liver cirrhosis in the rabbit CCl₄ model.

GENE EXPRESSION IN HUMAN HEPATOCYTES IN SUSPENSION AFTER ISOLATION IS SIMILAR TO THE LIVER OF ORIGIN, IS NOT AFFECTED BY HEPATOCYTE COLD STORAGE AND CRYOPRESERVATION, BUT IS STRONGLY CHANGED AFTER HEPATOCYTE PLATING

Isolated primary human hepatocytes are a well accepted system for evaluating pharmacological and toxicological effects in humans. However, questions remain regarding how culturing affects the liver-specific functions of the hepatocytes. In addition, cryopreservation could also potentially affect the differentiation state of the hepatocytes. The first aim of the present study was to compare gene expression in freshly isolated primary hepatocytes to that of the liver of origin and to evaluate the expression changes occurring after cryopreservation/thawing, both when maintained in suspension and after plating. The second aim of the present study was to evaluate gene expression in hepatocytes after cold storage of suspensions up to 24 h compared with freshly isolated hepatocytes in suspension. Our results show that the gene expression in freshly isolated human hepatocytes in suspension after isolation is similar to that of the liver of origin. Furthermore, gene expression in primary human hepatocytes in suspension is not affected by hepatocyte cold storage and cryopreservation. However, the gene expression is profoundly affected in monolayer cultures after plating. Specifically, gene expression changes were observed in cultured relative to suspensions of human hepatocytes that are involved in cellular processes such as phase I/II metabolism, basolateral and canalicular transport systems, fatty acid and lipid metabolism, apoptosis, and proteasomal protein recycling. An oxidative stress response may be partially involved in these changes in gene expression. Taken together, these results may aid in the interpretation of data collected from human hepatocyte experiments and suggest additional utility for cold storage and cryopreservation of hepatocytes.

Reversibility of experimental rabbit liver cirrhosis by portal collagenase administration

The regression of cirrhosis is associated with increased intrahepatic collagenolytic enzyme activity. We investigated whether collagenase supplementation via portal vein infusion can retard cirrhosis development and/or reverse cirrhosis. In all, 35 rabbits were initially assigned to study. However, because of high surgical mortality and infection, only 15 animals completed study. Four normal controls (group I) received olive oil subcutaneously (SC) for 12 weeks followed by normal saline portal perfusion for 12 weeks. Four (group II) received CCl(4) SC for 6 weeks followed by portal vein collagenase, 6 mg twice weekly, plus SC CCl(4) for 6 additional weeks and then killed. Four rabbits (group III) received CCl(4) SC for 12 weeks and then 6 mg of collagenase portally for 12 weeks, while three control rabbits (group IV) received CCl(4) for 12 weeks followed by saline for 12 weeks. After 12 weeks of CCl(4), liver hydroxyproline content of collagenase-treated group II (361.1+/-106.6 microg/g) was significantly reduced compared with group III+IV that had not yet received collagenase (589.0+/-162.9 microg/g; P<0.05). In the main comparison, hydroxyproline content of collagenase-treated group III (177.5+/-35.6 microg/g) was significantly decreased compared with saline-treated controls (446.3+/-150.1 microg/g; P<0.01). Further, liver histology showed complete regression of cirrhosis in the collagenase-treated animals. No toxicity of liver, kidney, lung, brain or heart was observed histologically. Anaphylaxis occurred in 2/35 original animals (one fatal). In conclusion, this study provides 'proof of principle' that collagenase portal administration can retard cirrhosis development and speed regression of established cirrhosis in the rabbit CCl(4) model. Potential application to humans is premature, but feasible, if these findings are confirmed in additional animal studies.

DNA strand breaking capacity of acrylamide and glycidamide in mammalian cells

We compared the DNA damaging potency of acrylamide (AA) and its metabolite glycidamide (GA) in the comet assay in cell systems differing with respect to species origin and cytochrome P450-depended monooxygenase (CYP2E1) expression (V79, Caco-2, primary rat hepatocytes). Only after 24 h incubation in the highest concentration of AA (6 mM) a slight but significant increase in DNA damage was observed in V79 and Caco-2 cells. In primary rat hepatocytes, however, expressing substantial amounts of CYP2E1, no induction of DNA strand breaks was found. At the end of the incubation time period (24 h), still 67+/-19% of the CYP2E1 protein was detected by Western blotting. Direct treatment with GA resulted in a significant increase in DNA damage in V79 cells and primary rat hepatocytes at concentrations > or =100 microM (24 h). Caco-2 cells were found to be less sensitive, exhibiting an increase in DNA strand breaks at concentrations > or 300 microM GA. These data confirm the higher genotoxic potential of GA compared to AA but also indicate that high expression of CYP2E1 per se is not necessarily associated with increased genotoxicity of AA. We, therefore, investigated whether the intracellular glutathione (GSH) level might be a critical determinant for the genotoxicity of AA in cells with different CYP2E1 status. Depletion of intracellular GSH by dl-buthionine-[S,R]-sulfoxime (BSO) in rat hepatocytes and V79 cells resulted in a significant induction of DNA strand breaks after incubation with 1 mM AA. However, at higher concentrations (> or =1.25 mM) a strong increase in cytotoxicity, resulting in a severe loss of viability, was observed. In summary, the DNA strand breaking effect of AA appeared not to be directly correlated with the CYP2E1 status of the cells. Depletion of GSH is associated with an increase in AA genotoxicity but seems also to lead to a substantial enhancement of cytotoxicity.

IMPACT OF TRANSCRIPTION FACTOR PROFILE AND CHROMATIN CONFORMATION ON HUMAN HEPATOCYTE CYP3A GENE EXPRESSION

Recent data have made it increasingly clear that the gene expression profile of a cell system, and its alteration in response to external stimuli, is highly dependent on both the higher order chromatin structure of the genome and the interaction of gene products in interpreting stimuli. To further explore this phenomenon, we have examined the role of both of these factors in controlling xenobiotic-mediated gene expression changes in primary and transformed human hepatocytes (HuH7). Using quantitative polymerase chain reaction, expression levels of several transcription factors implicated in the liver-specific regulation of the CYP3A gene family were examined in human adult and fetal liver RNA samples. These expression profiles were then compared with those obtained from both primary and transformed human hepatocytes, showing that, in general, cultured cells exhibit a distinct profile compared with either the fetal or adult samples. Transcriptome profiles before and after exposure to the CYP3A transcriptional activators rifampicin, dexamethasone, pregnane-16alpha-carbonitrile, and phenobarbital were subsequently examined. Whereas exposure to these compounds elicited a dose-dependent increase in CYP3A transcription in primary hepatocytes, no alteration in expression levels was observed for the hepatoma cell line HuH7. Alteration in the expression levels of pregnane X receptor and chicken ovalbumin upstream promoter transcription factor I, and the disruption of higher order chromatin within HuH7 cells altered CYP3A expression and/or activation by xenobiotics toward that observed in primary hepatocytes. These data provide potential roles for these two processes in regulating CYP3A expression in vivo.

Impact of serum on clearance predictions obtained from suspensions and primary cultures of rat hepatocytes

The objective of the present study was to compare two configurations of the hepatocyte model namely suspensions (SH) and conventional primary cultures (CPC) for their ability to predict the hepatic clearance in vivo in the rat and, to investigate the impact of serum on the prediction accuracy. The metabolic competences of several cytochrome P450 isoenzymes were investigated both in CPC and SH in the presence or absence of serum. Under the same conditions, the in vitro intrinsic clearance of six test compounds metabolised by a variety of phase I and phase II enzymes (antipyrine, RO-X, mibefradil, midazolam, naloxone and oxazepam) were derived from Vmax/Km scaled up to the corresponding in vivo hepatic metabolic clearance. CYP activities were shown to be stable in both CPC and SH for up to 6 h of incubation, except for the CYP 3A1 activity that decreased in CPC even in the presence of serum. Moreover, the clearances predicted from SH in the presence of serum were closer to the in vivo values than those obtained from CPC. SH represent a convenient model to assess the hepatic metabolism of xenobiotics, the presence of serum in the incubation medium significantly improved in several instances the quality of the predictions.

Strategies for using in vitro screens in drug metabolism

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

Activation of signalling pathways during hepatocyte isolation: relevance to toxicology in vitro

The "Holy Grail" of in vitro toxicology is to develop assay systems that mimic the in vivo situation and hence reduce the need for toxicity tests employing experimental animals. However a major problem to be overcome with cell culture models is the rapid loss of differentiated phenotype that markedly limits extrapolation of results to the whole animal (i.e. human) situation. This limitation is most obvious in the application of hepatocyte cultures to predict pathways of metabolism mediated toxicity and results from the rapid loss of cytochrome P450 content. Here we demonstrate that changes in hepatocyte gene expression (e.g. MAP kinase and NF-kappaB activation) occur very early into the well established hepatocyte isolation procedure employing collagenase suggesting that hepatocytes are undergoing a pro-inflammatory ('acute phase') response before they are cultured. Data is presented indicating that the stimulus is, in part, due to oxidative stress but the demonstration of endotoxins in collagenase preparations is likely to exacerbate the situation. Thus appreciation of these early events during hepatocyte isolation represents the surest foundation for the successful application of cultured hepatocytes to toxicology rather than relying on traditional manipulations of hepatocyte culture medium/substratum once differentiated phenotype has already been lost.

Magnesium deficiency induces apoptosis in primary cultures of rat hepatocytes

The effects of extracellular magnesium (Mg) concentration on the rate of apoptosis in rat hepatocytes in primary culture were examined. After overnight attachment, incubations were conducted for up to 72 h in serum-free media containing low (0-0.4 mmol/L), physiological (0.8 mmol/L) or high (2 and 5.6 mmol/L) Mg concentrations. At 72 h, we observed numerous rounded hepatocytes on top of a shrunken cell monolayer at extracellular Mg concentrations < 0.8 mmol/L. These morphological features were associated with Mg-dependent differences in the total protein levels. The various Mg concentrations did not affect DNA synthesis; however, at a concentration < 0.8 mmol/L, the susceptibility of cultured rat hepatocytes to oxidative stress was increased as shown by the reduced glutathione concentration (10.6 +/- 2.8 vs. 37.3 +/- 4.1 nmol/mg protein with 0 and 0.8 mmol/L, respectively; P < 0.05) and increased lipid peroxidation (0.36 +/- 0.03 vs. 0.21 +/- 0.01 nmol malondialdehyde/mg protein with 0 and 0.8 mmol/L, respectively; P < 0.05). Fluorescence microscopy after Hoechst dye staining revealed numerous apoptotic figures in Mg-free monolayers compared with 0.8 and 5.6 mmol/L Mg conditions. These observations were confirmed quantitatively by flow-cytometric analysis after propidium iodide staining. The proportion of subdiploid cells decreased with increasing Mg concentration; for example, it was greater at 72 h in Mg-free cultures (76%) than in cultures containing 0.8 mmol/L or 5.6 mmol/L Mg (28%; P < 0.05). Caspase-3 was highly activated in Mg-free cultures after 48 h of treatment compared with 0.8 and 5.6 mmol/L conditions (P < 0.05). Overall, these results show that extracellular Mg deficiency has a negative effect on the survival of cultured rat hepatocytes by inducing apoptosis; however, supplementation of extracellular Mg did not reduce the spontaneous apoptosis that occurred over time in rat hepatocyte cultures.