Rat adult hepatocytes in primary pure and mixed monolayer culture. Comparison of the maintenance of mixed function oxidase and conjugation pathways of drug metabolism

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

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

Effects of chromium picolinate on oxidative damage in primary piglet hepatocytes

Chromium picolinate is a popular nutritional supplement whose safety has been questioned because of the potential risk of oxidative DNA damage. To investigate this possibility, a dose-dependent study was performed in piglet hepatocyte cultures in which low (8 microM), medium (200 microM), and high (400 microM) doses of chromium picolinate were tested and compared to untreated controls. After 48 h incubation, there were no significant differences in the levels of intracellular reactive oxygen species, medium lactate dehydrogenase activity, and comet indicators between the three experimental groups and controls (p > 0.05). In the 8 microM-treated group, the intracellular malondialdehyde content was significantly decreased relative to controls (p < 0.05). All of the studied parameters showed a dose-dependent increase that was statistically significant between the low and high doses (p < 0.05). These results suggest that: (1) chromium picolinate may affect the oxidative status of piglet hepatocytes; (2) the appropriate dose (approximately physiological concentration) of chromium picolinate can inhibit lipid peroxidation, and (3) high doses of chromium picolinate have no significant effects on oxidative damage in piglet hepatocytes, but the existing evidence also imply that exposure to a higher dose appears to be unwarranted.

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

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

[Studies on the activation of complement by encapsulated and non-encapsulated staphylococci after their extraction with guanidinium chloride (author's transl)]

All demonstrable enzymes and toxins of encapsulated staphylococci (KS) were removed by extraction with guanidinium chloride. The capsules, however, remained apparently intact on the extracted (KS-Gu) staphylococci (fig. 1), as well as clumping factor and protein A. KS and KS-Gu failed to activate complement in the absence of specific antibodies. They showed neither immunadherence (table 1) nor agglutination by an antiserum against C3 (table 2). KS and KS-Gu had no significant chemotactic effects in vitro upon bovine granulocytes (fig. 2).

Evaluation of the effect of culture configuration on morphology, survival time, antioxidant status and metabolic capacities of cultured rat hepatocytes

We evaluated the antioxidant status, namely cellular lipid peroxidation, by measuring thiobarbituric acid reactive substances (TBARS), cellular reduced glutathione (GSH) content, glutathione reductase (GSSG-R), glutathione transferase (GST), glutathione peroxidase (GSH-Px) and catalase activities in rat liver, hepatocytes immediately after isolation and in two-dimensional (2D) culture (on non-coated or collagen-coated dishes, as collagen-collagen or collagen-Matrigel sandwich cultures) or three-dimensional (3D) culture on Matrigel-coated dishes. Microsomal cytochrome P450 (CYP)- and UDP-glucuronosyl transferase (UGT)- dependent activities were also assessed in rat livers and hepatocyte cultures. The overall antioxidant status of rat hepatocytes immediately after isolation was not significantly different from that of rat livers. During culture, GSH was increased in 2D but not in 3D cultures in accordance with morphological observations; that is that matrix-cell interactions involving GSH, important in 2D, are minimal in 3D cultures. While UGT- and GST-dependent activities were equivalent in cultured hepatocytes and in rat livers, both catalase and GSH-Px activities decreased with time in all culture configurations. Constitutive CYP-dependent activities were drastically decreased in hepatocytes after isolation and attachment and did not recover in any culture configuration tested. Our results highlight that, although 2D sandwich cultures and 3D cultures on Matrigel allow longevity of rat hepatocyte cultures and optimal induction of CYPs, an imbalance in phase I/phase II detoxication processes in cultured rat hepatocytes occurs, whatever the culture configuration.

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.

Effect of ethanol on the expression of hepatic glutathione S-transferase: an in vivo/in vitro study

Ethanol, a human toxicant and a solvent in pharmacological research, is known to interfere with biotransformation of xenobiotics. We compared the in vivo and in vitro long-term effects of ethanol exposure on the expression of glutathione S-transferases (GST, EC 2. 5.1.18) in rat liver. Long-term in vivo ethanol treatment to achieve blood ethanol levels ranging between 10-50 mM was by liquid diet feeding. For in vitro experiments, rat hepatocytes co-cultured with rat liver epithelial cells were exposed to 17 and 68 mM ethanol for up to 10 days. Two weeks of liquid diet ethanol treatment increased total GST activity. Both Mu and Alpha classes and in particular the A1 and A2 subunits and the amount of their corresponding mRNAs were increased. Total GST activity was also increased in co-cultures after exposure to 68 mM ethanol for 10 days. However, the Mu class subunits M1 and M2 and the corresponding mRNAs were increased, rather than the Alpha class subunits. Thus, long-term exposure to ethanol induces hepatic GST both in vivo and in vitro, but different isoenzymes are affected. Consequently, extrapolation of in vitro data on GST expression and regulation to the in vivo situation must be judicious. During xenobiotic metabolism in cell culture, a shift in relative expression and induction of different GST forms may occur, resulting in either an under- or overestimation of effects.

Enhanced cell aggregation and liver functions using polymers modified with a cell-specific ligand in primary hepatocyte cultures

Hepatocytes cultured as multicellular aggregates called spheroids exhibit enhanced liver functions and maintain them over a long period compared with monolayer culture. We previously reported the induction of hepatocyte spheroids using the synthetic polymer Eudragit (a copolymer of methacrylic acid and methylmethacrylate) as an artificial matrix in a cell suspension system (Yamada et al., J. Biochem., 123, 1017-1023, 1998). In this method, hepatocyte aggregation was promoted by the effects of electrostatic and hydrophobic interactions between cells and the polymer. To enhance the cell aggregation ability and cell-specificity of the polymer, in the present study, we prepared hepatocyte-targeting polymers containing lactone, a ligand of the asialoglycoprotein receptor. Addition of the lactone-modified polymers to the medium promoted cell aggregation and spheroid formation more effectively than unmodified Eudragit. The spheroids induced by the polymers exhibited enhanced liver functions, i.e., albumin secretion, ammonia removal, and urea synthesis, from early in the culture. We also investigated the induction of hetero-spheroids composed of various liver constitutive cells by this method. The hetero-spheroids induced by the polymers showed improved liver functions.

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.

Sulfotransferase gene expression in primary cultures of rat hepatocytes

Hepatocyte cultures have been used in pharmacotoxicological studies, and sulfotransferases (ST) are important drug-metabolizing enzymes in liver. The expression of sulfotransferases in hepatocyte cultures has not been examined systematically. In the present study, the mRNA levels of different sulfotransferases in male and female rat hepatocytes were examined by northern-blot analyses. Various culture conditions such as different matrices (collagen, matrigel, collagen sandwich, or co-culture with epithelial cells), medium (Way-mouth's MB 752/1 and Modified Chee's Medium) and glucocorticoid supplementation (dexamethasone, 0.1 microM) were compared. Phenol ST (ST1A1) mRNA levels decreased to about 50% of initial mRNA levels within 10 hr of culture. At 96 hr, ST1A1 mRNA levels were approximately 20% of initial values when cultured on collagen, matrigel or co-culture. The two media did not differ in ability to maintain ST1A1 mRNA levels in the absence of dexamethasone (DEX); however, DEX addition to either medium resulted in ST1A1 mRNA levels greater than 100% of the initial mRNA levels at 96 hr, with the greatest increase observed using the matrigel substratum and Chee's medium. In the absence of DEX, the mRNA levels of N-hydroxy-2-acetylaminoflurene sulfortransferase (ST1C1), estrogen sulfotransferase (ST1E2) and hydroxysteroid sulfotransferase (ST-20/21, ST-40/41, ST-60) fell to approximately 20% of their initial levels within 24 hr, and to less than 5% at 96 hr. The loss of expression of these sulfotransferases was observed with all culture conditions. Addition of DEX to the media resulted in ST-40/41 and ST-60 mRNA expression at 20 and 35% of their initial values, respectively, in cultures maintained on matrigel and Chee's medium at 96 hr. These data suggest that sulfotransferases lose their constitutive expression in hepatocyte culture, but retain their inducibility.

Perifusion of co-cultured hepatocytes: optimization of studies on drug metabolism and cytotoxicity in vitro

The combination of co-cultivation of hepatocytes and epithelial cell lines with a newly developed perifusion system was used for in vitro studies on drug metabolism and cytotoxicity. This approach improved the viability and enhanced the induction of the biotransforming capacity of the hepatocytes. As demonstrated for the induction of 7-ethoxyresorufin O-deethylase activity by 3-methylcholanthrene or benzanthracene, co-cultured hepatocytes in the perifusion system responded more sensitively to these inducers than without perifusion, most likely owing to stable (steady-state) concentrations of the inducers under the former conditions and rapidly declining concentrations under the latter conditions. The perifusion approach rendered it possible to determine the kinetics of drug metabolism during single or sequential incubations. After induction with 3-methylcholanthrene and phenobarbital, phase I metabolism of lonazolac to the monohydroxylated product in perifused co-cultures closely (87%) approached the values reported for the in vivo production, whereas in stationary co-cultures only 52% could be reached. Likewise, cytotoxic effects could be detected more precisely in the perifused co-cultures. If cells were pretreated with 0.2 mmol/L galactosamine for 3 h, perifusion with increasing concentrations of menadione differentially killed epithelial RL-ET-14 cells and hepatocytes at low and high concentrations, respectively, while in stationary co-cultures no differential effect was observed and only the higher concentrations were cytotoxic for both cells. Prevention by incubation with S-adenosylmethionine of menadione cytotoxicity up to a menadione concentration of 250 micromol/L was seen only in the perifused co-cultures, whereas in stationary cultures only a slight shift of the cytotoxic concentration exerting 50% cell damage to higher values was noted. These results demonstrate the versatile application of perifused co-cultures for studies on drug metabolism including induction of cytochrome P450-dependent enzymes and steady-state kinetics of biotransformation, as well as cytotoxic and protective effects of different drugs.

Detection of elevated reactive oxygen species level in cultured rat hepatocytes treated with aflatoxin B1

Accumulating evidence demonstrates that oxidative damage is one of the underlying mechanisms to the cytotoxicity and carcinogenicity of AFB1. The main objective of this study is to show that AFB1 increases reactive oxygen species (ROS) formation in hepatocytes. The ROS level was detected using a fluorescence probe, 2',7'-dichlorofluorescin diacetate (DCFH-DA), which could be converted to highly fluorescent dichlorofluorescein (DCF) with the presence of intracellular ROS. It was found that AFB1 exposure significantly enhanced DCF fluorescence formation in cultured rat hepatocytes. A dose-response of AFB1 was also observed within the range of 10 nM to 1000 nM. Catalase (CAT) was able to completely prevent the increase of DCF fluorescence in AFB1-treated cells in a dose-dependent manner (from 500 to 2000 U/ml). Moreover, the significant inhibitory effects of desferrioxamine (DFO) and dimethyl sulfoxide (DMSO) on DCF fluorescence formation were also observed in both control and AFB1-treated hepatocytes. Therefore, results from the present study provide in vitro evidence indicating the generation of ROS in cultured rat hepatocytes caused by AFB1 exposure. It is postulated that the metabolic process of AFB1 by cytochrome P450 might be the possible source of the elevated ROS level in AFB1-treated hepatocytes. The enhanced level of ROS may be responsible for the oxidative damage caused by AFB1, which may ultimately contribute to the cytotoxic and carcinogenic effects of AFB1.

A Comparison of Hepatic Enzyme Activities and their Modulation by Dexamethazone in Freshly Isolated and Cultured Hepatocytes and in the Differentiated Hepatoma Cell Line, 2sFou

Rodent hepatocytes are mitotically inhibited and lose hepatospecific functions over time in culture. In contrast, some differentiated hepatoma cell lines express stable hepatospecific functions in culture, but at much lower levels than those initially found in primary hepatocytes. A number of hepatospecific functions were measured in freshly isolated and cultured rat hepatocytes; these were compared to activities found in the differentiated Reuber hepatoma cell line, 2sFou. The effects of dexamethazone on these activities were also investigated, since dexamethazone is reported to enhance the expression of organotypic functions. The P450-related activities (ethoxyresorufin-O-deethylase activity and pentoxyresorufin-O-depenty-lase activity) and glucose-6-phosphatase activity declined in hepatocytes with increasing time in culture. The same activities in 2sFou cells were similar to those in hepatocytes which had been cultured for 72 hours. Tyrosine amino transferase (TAT) activity declined in hepatocyte cultures with time, but dexamethazone (1μM) restored activity up to freshly isolated cell values. TAT activity in hepatoma cells exceeded that in hepatocytes and was highly inducible by dexamethazone. γ-Glutamyl transpeptidase activity increased with culture time in hepatocytes and was also highly expressed in 2sFou cells. In hepatocytes, the activity of a high affinity alcohol dehydrogenase (ADH) declined with time in culture. In 2sFou cells, there was evidence of a low affinity (extra-hepatic or fetal) form of ADH, which was not evident in cultured hepatocytes.

Induction of CYP3A and associated terfenadine N-dealkylation in rat hepatocytes cocultured with 3T3 cells

Long-term culture of hepatocytes has been challenged by the loss of differentiated functions. In particular, there is a rapid decline in cytochrome P450 (CYP). In this study, we cocultured rat hepatocytes with 3T3 fibroblasts for 10 days, and examined hepatocyte viability, morphology, and expression of CYP3A. Terfenadine was incubated with the cultures, and its biotransformation was quantitatively analyzed by HPLC. Terfenadine is metabolized by two major pathways: C-hydroxylation to an alcohol metabolite which is further oxidized to a carboxylic acid, and N-dealkylation to azacyclonol. In rat liver, only the N-dealkylation pathway appears to be mediated by CYP3A since anti-rat CYP3A antibody inhibited azacyclonol but not alcohol metabolite formation in incubations of terfenadine with liver microsomes. Freshly isolated rat hepatocytes were seeded on top of confluent 3T3 cells. Cultures were maintained in Williams' E medium supplemented with 10% fetal bovine serum and either 0.1 mumol/L or 5 mumol/L dexamethasone. In pure hepatocyte cultures, viability, as determined by lactate dehydrogenase (LDH) activity, decreased steadily to less than 30% of initial levels by day 10. In cocultures, LDH activity remained high and was 70% of initial levels on day 10. The half-life of terfenadine disappearance was optimally maintained in cocultures treated with 5 mumol/L dexamethasone, and was associated with the increased formation of azacyclonol. On day 5, nearly 50% of added 5 mumol/L terfenadine was converted to azacyclonol within 6 h, whereas the conversion was only 4% on day 1. Western and RNA-slot blot analyses confirmed that treatment with 5 mumol/L dexamethasone induced CYP3A mRNA expression and CYP3A protein expression. This coculture system could offer a useful approach in the study of drugs and xenobiotics metabolized by CYP3A.

Cytochrome P450 activities in pure and co-cultured rat hepatocytes. Effects of model inducers

The stability and inducibility of several P450 activities (namely, P450 1A1; 2A1, 2B1/2, 2C11, and 3A1) were studied in rat hepatocytes co-cultured with the MS epithelial cell line derived from monkey kidney. The results revealed that these monooxygenase activities were systematically higher in co-cultures than in conventional hepatocyte cultures. Pure cultures showed a rapid loss of monooxygenase activities, which were undetectable after 5 days. In contrast, all isozymes assayed were measurable in co-cultured hepatocytes on Day 7 (about 15 to 40% of the initial activities of Day 0 of culture). The beneficial effects of the co-culture system seemed to be more selective for certain cytochrome P450 isoforms, with P450 1A1 and 3A1 being the best stabilized isozymes after 1 wk. A clear response to inducers was observed in co-cultures, each isozyme showing a different induction pattern. 3-Methylcholanthrene produced a strong increase in P450 1A1 (7-ethoxyresorufin O-deethylase) activity and a low increase in P450 2A1 (testosterone 7 alpha-hydroxylation), whereas no changes were observed in the other activities. Phenobarbital treatment resulted in increases in P450 2B1/2 (7-pentoxyresorufin O-depentylase and 16 alpha- and 16 beta-hydroxylation of testosterone) activities, while minor effects were observed on P450 3A1 (testosterone 6 beta-hydroxylation) activity. Dexamethasone markedly increased P450 3A1 (testosterone 6 beta- and 15 beta-hydroxylation) activity and, to a lesser extent, P450 2B1/2 (16 beta-hydroxylation).

Phase I and II biotransformations in living CaCo 2 cells cultivated under serum-free conditions. Selective apical excretion of reaction products

CaCo 2 cells, cultivated in a synthetic, serum-free nutritive medium on poly (ethylene terephthalate) membranes, form a confluent monolayer of differentiated cells, with the apical and basolateral poles exposed to the upper and lower compartments, respectively, of bicameral culture inserts (Halleux and Schneider, In Vitro Cell Dev Biol, 27A: 293-302, 1991). This cell culture system allows the passage of intact mannitol by the paracellular route and the transcellular diffusion of testosterone which appears mainly as a biotransformed unconjugated metabolite. When ethoxyresorufin is added to either the apical or basolateral poles of living CaCo 2 cells, resorufin is formed, and more than 80% is excreted at the apical pole. Under our experimental conditions, no detectable amounts of glucurono- or sulfconjugates are found. Methylcholanthrene and phenobarbital increase the biotransformation of ethoxyresorufin 50 and 3 times, respectively, and induce that of benzoxyresorufin, but not of pentoxyresorufin which remains absent under all conditions. These substances do not affect the proportion of resorufin recovered at the apical role. Verapamil inhibits by 25% the release of resorufin but does not affect its distribution. Chlorodinitrobenzene is conjugated with glutathione and at least two-thirds of the product is excreted at the apical pole; methylcholanthrene and phenobarbital do not increase this activity. These results demonstrate that differentiated CaCo 2 cells, under serum-free conditions, perform phase I and II reactions and that the biotransformation products are selectively excreted at the apical pole.

Rat hepatocyte cultures and co-cultures in biotransformation studies of xenobiotics

Long-term cultures of hepatocytes could represent a suitable in vitro model for biotransformation studies of xenobiotics. At present however, no ideal culture system can be proposed since, in all existing models, phenotypic changes occur, affecting selectively some components of phase I and/or phase II xenobiotic metabolism. From the authors' own results and recent studies of several other investigators, carried out on rat hepatocytes, it becomes clear that four groups of factors may affect biotransformation capacity: soluble medium factors, extracellular matrix components, cell-cell interactions and factors affecting replication. For the maintenance of liver-specific functions, it seems of utmost importance that the tridimensional shape of the hepatocytes is kept. Usually, phase II enzymatic activity is better kept than that of phase I. The cytochrome P450 dependent monoxygenases, in particular, are easily lost. Interesting is the observation that co-cultures of rat hepatocytes with rat liver epithelial cells exhibit higher and much better preserved phase I and phase II biotransformation than monocultures. Clearly, further research is needed to improve this promising in vitro model.

Effect of phenobarbital and other model inducers on cytochrome P450 isoenzymes in primary culture of dog hepatocytes

1. The effects of phenobarbital (PB), beta-naphthoflavone (beta-NF), omeprazole (Omep) and rifampicin (Rif) on drug-metabolizing activities in dog hepatocytes, cultured with William's medium E, were examined. 2. The drug metabolizing activities of the hepatocytes decreased during culture; 7-ethoxycoumarin O-deethylase (ECOD) activity was nearly 70% of initial value at 72 h, but 7-methoxycoumarin O-demethylase (MCOD), 7-propoxycoumarin O-depropylase (PCOD), progesterone 6 beta-hydroxylase (6 beta-OH-P), progesterone 16 alpha-hydroxylase (16 alpha-OH-P), progesterone 21-hydroxylase (21-OH-P), 7-ethoxyresorufin O-deethylase (EROD) activities and total cytochrome P450 content were approx. 50%. 3. When the hepatocytes were cultured with PB, the enzyme activities increased time- and dose-dependently. MCOD, ECOD and PCOD activities increased 5-8 fold with 2 mM PB in 96 h. Similar results were obtained for 6 beta-OH-P, 16 alpha-OH-P and 21-OH-P activities, and total cytochrome P450. The effect of PB was abolished when 2.5 microM cycloheximide or 0.1 microM actinomycin D was included in the culture. 4. Treatment of hepatocytes with 40 microM beta-NF for 72 h resulted in 25-fold elevation of EROD activity. beta-NF enhanced PCOD activity approx. six-fold, while ECOD increased only slightly, and 7-MCOD negligibly. 5. Omep (100 microM) increased EROD activity nearly 10-fold, and 25 microM Rif increased 6 beta-OH-P activity approx. 8-fold, but ECOD only slightly. 6. Western blot analysis of microsomes from cultured dog hepatocytes with anti-rat CYP 2B1 antibodies indicated that PB increased an immunochemically-reactive protein. The protein showed the same mobility as the major dog P450 isozyme (cytochrome P450 PBD-2 or CYP 2B11) purified from liver microsomes of PB-treated male beagle dog. In a similar manner, induction of cytochrome P450 PBD-1 (CYP 3A12) by PB was confirmed.

An in vitro model of rodent nongenotoxic hepatocarcinogenesis

An in vitro model of liver in which rat hepatocytes are maintained as cocultures with nonparenchymal epithelial cells (NPC) derived from liver has been developed and characterized with respect to maintenance of hepatocyte viability and differentiated function. The system was then evaluated as a model for studying peroxisome proliferator-induced rodent liver nongenotoxic carcinogenesis. Within the coculture model, hepatocyte viability and morphology were maintained for 1 month or more within a system that is both easily accessible for microscopic examination and is free of any additives that may lead to artifacts. Even after 1 month or more, hepatocyte cocultures retained expression of the constitutive liver marker albumin. In addition, they maintained the ability to show induction of the peroxisome proliferator-inducible enzymes peroxisomal bifunctional enzyme (PBE) and cytochrome P450IVA1 in response to the peroxisome proliferator nafenopin. After 4 weeks, NPC cocultures showed a six- and a fourfold induction of PBE and cytochrome P450IVA1 expression, respectively, which compared well with the three- and fivefold induction seen in freshly isolated cells. This was paralleled by an increase in the cytoplasmic volume fraction of peroxisomes averaging eightfold. Interestingly, great heterogeneity was exhibited between adjacent hepatocytes in terms of the degree of peroxisome proliferation, a finding reflected by immunocytochemical staining which indicated heterogeneity in the level of expression of the peroxisome proliferator-inducible enzymes. Other cell lines representing different tissue types, morphologies, and species were also examined for their ability to support hepatocyte survival but were found to be ineffective, with the exception of a bovine corneal endothelial cell line. This line supported hepatocyte survival and maintenance of differentiated function but to a lesser extent than that observed with NPC. Ultrastructural examination of NPC cocultures revealed extensive interhepatocyte junctional complexes and interdigitation of adjacent membranes together with the presence of bile canalicular structures. There were no junctional complexes between the hepatocytes and the supporting feeder cells with any contact being limited to a close association of the hepatocytes with the extracellular matrix presumably produced by the NPC. The data demonstrate that hepatocytes maintained in vitro within an NPC coculture system retain differentiated function and the ability to respond to the peroxisome proliferator class of nongenotoxic carcinogens. Cocultures will provide us with a model system for the study of changes in hepatocyte growth regulation during rodent liver nongenotoxic carcinogenesis.