Maintenance and induction in co-cultured rat hepatocytes of components of the cytochrome P450-mediated mono-oxygenase

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.

Cell and tissue engineering for liver disease

Despite the tremendous hurdles presented by the complexity of the liver's structure and function, advances in liver physiology, stem cell biology and reprogramming, and the engineering of tissues and devices are accelerating the development of cell-based therapies for treating liver disease and liver failure. This State of the Art Review discusses both the near- and long-term prospects for such cell-based therapies and the unique challenges for clinical translation.

Effects of phenobarbital and valproate on the expression of cytochromes P-450 in co-cultured rat hepatocytes

We have investigated, in hepatocytes co-cultured with epithelial cells, the effects of phenobarbital and sodium valproate treatment on members of the cytochrome P-450 superfamily. The expression of the P450IIB and P450IV families was examined, using Western blotting, at 4, 7 and 14 days of co-culture either in the presence or absence of drug treatment. The amount of P450IIB was increased after exposure of the co-cultured cells to either phenobarbital or sodium valproate. In contrast, P450IV amounts were increased only by sodium valproate treatment. The maximal induction of P450IIB (17-fold) was observed at 7 and 14 days of co-culture, whereas P450IV was induced to the same extent in cells co-cultured for 4, 7 and 14 days. We also examined the expression of these two cytochrome P-450 subfamilies in hepatocytes cultured for 4 days in the absence of epithelial cells. The amounts of both P450IIB and P450IV were low or undetectable in these cells, and the induction of these proteins by either phenobarbital or valproate was less than that observed in co-cultured hepatocytes. Our results demonstrate that the co-culture system is a suitable in vitro system for examining the effects of various foreign compounds on the expression of phase I drug metabolizing enzymes.

Effect of Thyroid Hormones onFlavin-containing Monooxygenase Activity in Co-cultured Adult Rat Hepatocytes

The regulation of flavin-containing monooxygenase (FMO) by thyroid hormones was examined under well defined in vitro conditions using adult male rat hepatocytes co-cultured with rat liver epithelial cells of primitive biliary origin. Serum free medium was used to avoid interferences from foetal bovine serum. The effect of thyroxine (T4) and its major metabolite l-triiodothyronine (T3) on FMO activity was estimated spectrophotometrically by measuring the rate of methimazole oxygenation. The highest non-cytotoxic doses of T3 and T4 that could be used in co-cultures were determined by measuring both lactate dehydrogenase leakage into the medium and microsomal protein content of the hepatocytes as a function of culture time. In addition, hormonal responsiveness of the in vitro system used was confirmed by malic enzyme activity measurements. Administration of 10 mum T3 or T4 was found to cause a significant decrease in FMO activity and content, suggesting a suppressive role of both hormones on the regulation of FMO activity in male rats.

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.

Liver-specific functional studies in a microfluidic array of primary mammalian hepatocytes

Nearly half a billion dollars in resources are lost each time a drug candidate is withdrawn from the market by the Food and Drug Administration (FDA) for reasons of liver toxicity. The number of late-phase drug developmental failures due to liver toxicity could potentially be reduced through the use of hepatocyte-based systems capable of modeling the response of in vivo liver tissue to toxic insults. With this article, we report progress toward the goal of realizing an array of primary hepatocytes for use in high-throughput liver toxicity studies. Described herein is the development of a 64 (8 x 8) element array of microfluidic wells capable of supporting micropatterned primary rat hepatocytes in coculture with 3T3-J2 fibroblasts. Each of the wells within the array was continuously perfused with medium and oxygen in a nonaddressable format. The key features of the system design and fabrication are described, including the use of two microfluidic perfusion networks to provide the coculture with an independent and continuous supply of cell culture medium and oxygen. Also described are the fabrication techniques used to selectively pattern hepatocytes and 3T3-J2 fibroblasts within the wells of the array. The functional studies used to demonstrate the synthetic and metabolic capacity of the array are outlined in this article. These studies demonstrate that the hepatocytes contained within the array are capable of continuous, steady-state albumin synthesis (78.4 microg/day, sigma = 3.98 microg/day, N = 8) and urea production (109.8 microg/day, sigma = 11.9 microg/day, N = 8). In the final section of the article, these results are discussed as they relate to the final goal of this research effort, the development of an array of primary hepatocytes for use in physiologically relevant toxicology studies.

Effect of culture conditions on the expression and function of Bsep, Mrp2, and Mdr1a/b in sandwich-cultured rat hepatocytes

Rat hepatocytes cultured in a sandwich configuration form functional canalicular networks. The influence of extracellular matrix configuration, medium composition, and confluency on the expression and function of Bsep, Mrp2, and Mdr1a/b in sandwich-cultured (SC) rat hepatocytes was examined. Primary rat hepatocytes were: (1) maintained in various extracellular matrix sandwich configurations, (2) cultured in Dulbecco's modified Eagle's medium (DMEM), Modified Chee's medium (MCM) or Williams' E medium (WME), and/or (3) plated at decreasing cell density. Bsep, Mrp2, and Mrdr1a/b expression in day 4 SC rat hepatocytes was assessed by Western blot; function was measured by accumulation of taurocholate, 5(and 6)-carboxy-2',7'-dichlorofluorescein, and rhodamine 123, respectively, in canalicular networks. In general, the extracellular matrix conditions examined resulted in similar protein expression and function. Function of Bsep, Mrp2, and Mdr1a/b was higher in SC rat hepatocytes maintained in DMEM or WME. Mrp2 and Mdr1a/b expression, representative of total cellular content, did not always correlate directly with function, which should be reflective of canalicular membrane expression. Mrp2 expression decreased significantly as cell density decreased in SC hepatocytes. Low plating density in Biocoat plates resulted in poor canalicular network formation and reduced function of Mrp2 and Mdr1a/b. Expression and/or function of Mrp2 and Mdr1a/b in rat hepatocytes cultured in a sandwich configuration may be influenced by plating density and media type.

Microstructured scaffolds for liver tissue cultures of high cell density: Morphological and biochemical characterization of tissue aggregates

Very high cell densities and optimal vascularization characterize among others organs and tissues in vivo. In order to study organ-specific functions in vitro or to make use of them in medical devices/treatments in the future, this natural architecture should be rebuilt. An important aspect in this context is the appropriate ratio of medium to cell volume being so far not optimally reestablished in most of the currently available in vitro systems. To improve such culture conditions, we constructed a microstructure to culture hepatocytes and (without any addition of extracellular matrix material) characterized liver tissue in the form of evenly sized aggregates. The liver-specific differentiation status of such aggregates was monitored by their ability to perform CYP450 dependent xenobiotic metabolism along with the measurement of albumin secretion. Freshly isolated adult rat hepatocytes show an initial loss of total CYP450 content and of associated activities (mixed function oxidases). However, in the aggregate system, this level did not decrease further but remained stable or even increased throughout the culture period of 10-13 days. The CYP450 dependent metabolism of the hepatocytes is able to respond to classic inducing agents. The described culture efficiently supports liver-specific functions of adult rat hepatocytes and seems to be suited not only for use in an extracorporeal liver device but also for the formation of evenly sized small aggregates to be of use in transplantation of differentiated liver tissue. Moreover, after design variations, the microstructure can be applied for functional analysis of metabolically active hepatocytes as well as for toxicological and pharmacological validation.

Cell-, tissue-, sex- and developmental stage-specific expression of mouse flavin-containing monooxygenases (Fmos)

The cell-, tissue-, sex- and developmental stage-specific expression profiles of five members of the flavin-containing monooxygenase (FMO) family, FMO1, 2, 3, 4 and 5, were investigated in 129/SV mice, using isoform-specific antisense RNA probes. In situ hybridization localized FMO1 and 5 mRNAs to the perivenous, and FMO 2, 3 and 4 mRNAs to the periportal, regions of the liver. In kidney, each FMO mRNA is localized to the distal and proximal tubules and collecting ducts; FMO1 mRNA is present also in the glomerulus. In lung, FMO1 and 3 mRNAs are expressed in the terminal bronchiole, and FMO1 mRNA also in the alveoli. FMO1 mRNA is present in neurons of the cerebrum and in the choroid plexus. RNase protection assays showed that the most abundant isoform in newborn liver, lung, kidney and brain, and in adult lung and kidney is FMO1, but in adult liver FMO5 is present in greatest amounts. In liver, lung and kidney, expression of Fmo1, 3 and 5 peaks at 3 or 5 weeks of age, but in the brain, Fmo1 expression is greatest in newborns. In the kidney, FMO5 mRNA abundance is fourfold greater in males than in females, at all stages of development. Our results demonstrate that Fmo1, 2, 3, 4 and 5 exhibit distinct cell-, tissue-, sex- and developmental stage-specific patterns of expression.

Differentiation of human hepatoma cells during confluence as revealed by gene expression profiling

Certain human hepatocarcinoma cells undergo differentiation when grown at confluence. In order to understand the basis for this differentiation, we investigated the phenotypic changes occurring during confluent growth of the human hepatoma B16A2 cell line. The global gene expression profile of B16A2 cells grown during confluence for 5 weeks was investigated using microarrays containing complementary sequences corresponding to approximately 10,000 genes, and compared with profiles of adult human liver and HepG2 cells. The major part of gene products detected were shared by all three systems and the hepatoma cell lines expressed surprisingly high levels of liver-enriched transcription factors. During confluence of B16A2 cells, the majority of transcriptional changes monitored were directed towards the phenotype of adult human liver in vivo, although the changes accounted for less than 10% of those necessary to acquire a native hepatic phenotype. Several markers of liver differentiation and regeneration were changed in similar manner as observed in developing liver and during liver regeneration. In conclusion, the data indicate that differentiation in vitro of the B16A2 cell line during confluence partially resembles that of hepatic differentiation and regeneration in vivo, implying a partial normalization of a low differentiated phenotype.

Long-term maintenance of cytochrome P450 activities by rat hepatocyte/3T3 cell co-cultures in heparinized human plasma

Little information on the effect of plasma on hepatocyte cytochrome P450 (CYP) activities is currently available. We characterized the effect of plasma on CYPs of hepatocyte-mesenchymal cell co-cultures, which exhibit stable liver specific functions and may be potentially useful for bioartificial liver design. Rat hepatocyte-mouse 3T3-J2 cell co-cultures were maintained for 6 days in medium, and then switched to heparinized human plasma containing 3-methylcholanthrene (3MC; 2 microM), phenobarbital (PB; 1 mM), or no inducer for up to 7 days. CYP activities were measured in situ based on the o-dealkylation of ethoxy- (EROD), methoxy- (MROD), pentoxy- (PROD), or benzyloxy- (BROD) resorufin. Plasma alone increased PROD/BROD but not EROD/MROD. The endogenous inducer was in the high molecular weight fraction (>5 kD) of plasma and inhibited by >5 nM okadaic acid and >10 microM dibutyryl cyclic AMP, two inhibitors of PB-inducible CYPs. Furthermore, plasma increased CYP1A1 and CYP2B1/2 mRNA levels. In plasma, 3MC induced EROD/MROD to about 60% of the level induced in culture medium while PB induced PROD/BROD that were three- to 10-fold above levels induced in medium. CYP activities decreased between days 2 and 7 of plasma exposure, but were enhanced by plasma supplementation with amino acids, insulin, glucagon, and hydrocortisone.

Quantification and cellular localization of expression in human skin of genes encoding flavin-containing monooxygenases and cytochromes P450

The expression, in adult human skin, of genes encoding flavin-containing monooxygenases (FMOs) 1, 3, 4, and 5 and cytochromes P450 (CYPs) 2A6, 2B6, and 3A4 was determined by RNase protection. Each FMO and CYP exhibits inter-individual variation in expression in this organ. Of the individuals analysed, all contained CYP2B6 mRNA in their skin, 90% contained FMO5 mRNA and about half contained mRNAs encoding FMOs 1, 3, and 4, and CYPs 2A6 and 3A4. The amount of each of the FMO and CYP mRNAs in skin is much lower than in the organ in which it is most highly expressed, namely the kidney (for FMO1) and the liver (for the others). In contrast to the latter organs, in the skin FMO mRNAs are present in amounts similar to, or greater than, CYP mRNAs. Only the mRNA encoding CYP2B6 decreased in abundance in skin with increasing age of the individual. All of the mRNAs were substantially less abundant in cultures of keratinocytes than in samples of skin from which the cells were derived. In contrast, an immortalized human keratinocyte cell line, HaCaT, expressed FMO3, FMO5, and CYP2B6 mRNAs in amounts that fall within the range detected in the whole skin samples analysed. FMO1, CYP2A6, and CYP3A4 mRNAs were not detected in HaCaT cells, whereas FMO4 expression was markedly increased in this cell line compared to whole skin. In situ hybridization showed that the expression of each of the FMOs and CYPs analysed was localized to the epidermis, sebaceous glands and hair follicles.

Xenobiotic induction of cytochrome P450 2B1 (CYP2B1) is mediated by the orphan nuclear receptor constitutive androstane receptor (CAR) and requires steroid co-activator 1 (SRC-1) and the transcription factor Sp1

The constitutive androstane receptor (CAR) activates the expression of a reporter gene attached to the phenobarbital-response element (PBRE) of the cytochrome P450 2B1 (CYP2B1) gene in response to the barbiturate phenobarbital and the plant product picrotoxin. The xenobiotic-mediated increase in transactivation occurs in transfected primary hepatocytes and in liver transfected by biolistic-particle-mediated DNA transfer, but not in the transformed cell lines HepG2, CV-1 and HeLa, which support only constitutive activation of gene expression by CAR. Steroid co-activator 1 (SRC-1) enhances both constitutive and xenobiotic-induced CAR-mediated transactivation via the CYP2B1 PBRE in transfected primary hepatocytes. The nuclear receptor 1 (NR1) site of the PBRE is sufficient for CAR-mediated transactivation, but additional sequences within the PBRE, and hence the proteins that bind to them, are required for the interaction of CAR with SRC-1. The NR2 site of the PBRE binds proteins other than CAR, including an unidentified nuclear receptor heterodimerized with retinoid X receptor alpha. By binding to the proximal promoter of CYP2B1, the transcription factor Sp1 increases both basal transcription and xenobiotic-induced expression via the PBRE. Thus induction of CYP2B1 expression by xenobiotics is mediated by the nuclear receptor CAR and, for optimal expression, requires SRC-1 and Sp1.

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.

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).

Insulin-mediated modulation of cytochrome P450 gene induction profiles in primary rat hepatocyte cultures

In this investigation, we examined the effects of insulin on gene induction responsiveness in primary rat hepatocytes. Cells were cultured for 72 hours either in the absence or presence of 1 microM insulin and then exposed to increasing concentrations of phenobarbital (PB; 0.01-3.5 mM). Culturing in the absence of insulin produced 1.5-2-fold increases in the induction magnitude of CYP2B1 and CYP2B2 mRNA expression resulting from PB exposures, without altering the bell-shaped dose-response curve characteristic of this agent. However, for the CYP3A1 gene, insulin removal led to a pronounced shift in both the PB-induction magnitude and dose-response relationships of the induction response, with higher levels of CYP3A1 expression resulting from exposures to lower concentrations of inducer. Insulin removal also reduced the time required to attain maximal induction of CYP2B1/2 and CYP3A1 gene expression. The insulin effects were not specific for PB induction, as insulin deprivation similarly enhanced both dexamethasone- and beta-naphthoflavone-inducible CYP3A1 and CYP1A1 expression profiles, respectively. In contrast, the level of albumin mRNA expression was reduced considerably in cells deprived of insulin. We conclude that insulin is an important regulator of inducible and liver-specific gene expression in primary rat hepatocytes.

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.

Effect of cell-cell interactions in preservation of cellular phenotype: cocultivation of hepatocytes and nonparenchymal cells

Heterotypic cell interaction between parenchymal cells and nonparenchymal neighbors has been reported to modulate cell growth, migration, and/or differentiation. In both the developing and adult liver, cell-cell interactions are imperative for coordinated organ function. In vitro, cocultivation of hepatocytes and nonparenchymal cells has been used to preserve and modulate the hepatocyte phenotype. We summarize previous studies in this area as well as recent advances in microfabrication that have allowed for more precise control over cell-cell interactions through 'cellular patterning' or 'micropatterning'. Although the precise mechanisms by which nonparenchymal cells modulate the hepatocyte phenotype remain unelucidated, some new insights on the modes of cell signaling, the extent of cell-cell interaction, and the ratio of cell populations are noted. Proposed clinical applications of hepatocyte cocultures, typically extracorporeal bioartificial liver support systems, are reviewed in the context of these new findings. Continued advances in microfabrication and cell culture will allow further study of the role of cell communication in physiological and pathophysiological processes as well as in the development of functional tissue constructs for medical applications.

The flavin-containing monooxygenase 2 gene (FMO2) of humans, but not of other primates, encodes a truncated, nonfunctional protein

Flavin-containing monooxygenases (FMOs) are NADPH-dependent flavoenzymes that catalyze the oxidation of heteroatom centers in numerous drugs and xenobiotics. FMO2, or "pulmonary" FMO, one of five forms of the enzyme identified in mammals, is expressed predominantly in lung and differs from other FMOs in that it can catalyze the N-oxidation of certain primary alkylamines. We describe here the isolation and characterization of cDNAs for human FMO2. Analysis of the sequence of the cDNAs and of a section of the corresponding gene revealed that the major FMO2 allele of humans encodes a polypeptide that, compared with the orthologous protein of other mammals, lacks 64 amino acid residues from its C terminus. Heterologous expression of the cDNA revealed that the truncated polypeptide was catalytically inactive. The nonsense mutation that gave rise to the truncated polypeptide, a C --> T transition in codon 472, is not present in the FMO2 gene of closely related primates, including gorilla and chimpanzee, and must therefore have arisen in the human lineage after the divergence of the Homo and Pan clades. Possible mechanisms for the fixation of the mutation in the human population and the potential significance of the loss of functional FMO2 in humans are discussed.

Basal steroidogenic activity of adrenocortical cells is increased 10-fold by coculture with chromaffin cells

Historically, catecholamine-producing chromaffin cells and steroid-producing adrenocortical cells have been regarded as two independent endocrine systems that are united under a common capsule to form the adrenal gland. There is increasing evidence for bidirectional interactions, with regulatory influences of adrenocortical secretory products on adrenomedullary functions and vice versa. However, the direct involvement of chromaffin cells on the regulation and maintenance of cortical function has not yet been demonstrated. Therefore, we analyzed glucocorticoid secretion and P450 messenger RNA (mRNA) expression in bovine adrenocortical cells in cocultures with chromaffin cells compared with those in pure cortical cell cultures. Cortisol release from cortical cells in coculture with chromaffin cells was 10 times as high (mean +/- SEM, 1035 +/- 119%) as that from the same number of isolated cortical cells (100 +/- 11%). By a [3H]thymidine incorporation assay, it was demonstrated that this effect was not due to a higher proliferation rate. Northern analysis revealed an increasing expression of P450(17alpha) mRNA in the coculture from days 1-5, whereas in isolated cortical cells, P450(17alpha) mRNA decreased, leading to a 6-fold difference on day 5. Inhibitors of protein (cycloheximide) or RNA (actinomycin D) synthesis completely annulled the observed increase in cortisol release, indicating that de novo protein synthesis is required for this activation of adrenocortical steroidogenesis. Addition of the cyclooxygenase inhibitor indomethacin reduced the stimulatory effect, suggesting that this stimulation is in part mediated by PGs. Locally produced ACTH, catecholamines, and interleukin-1 accounted for 43% of the effect. Secretory products of chromaffin cells that act in concert are believed to be responsible for the stimulation of steroidogenesis in the coculture. The coculture system is an in vitro model that corresponds to the in vivo situation in the intact adrenal gland, where both endocrine cell systems are in close contact. Our data demonstrate the requirement of intraadrenal cellular communication for the full strength of the adrenocortical hormonal response.

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.

Characterization of phenobarbital-inducible mouse Cyp2b10 gene transcription in primary hepatocytes

The mouse phenobarbital (PB)-inducible Cyp2b10 gene promoter has been isolated and sequenced, and control of its expression has been characterized. The 1405-base pair (bp) Cyp2bl0 promoter sequence is 83% identical to the corresponding region from the rat CYP2B2 gene. In addition to the lack of CA repeats, differences include insertion of 42 base pairs (-123/-82 bp) into the middle of a consensus sequence to the so-called "Barbie box." In this report, we have developed a primary mouse hepatocyte culture system in which endogenous 2B10 mRNA as well as Cyp2b10-driven CAT activity were induced by PB and 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP), but not by the 3-chloro derivative of TCPOBOP. Deletion analysis of the Cyp2b10 promoter identified a basal transcription element at -64/-34 bp and a negative element at -971/-775 bp. Sequences contained within the -1404/-971 bp region are responsible for the induced CAT activity. DNase I protection and gel shift assays detected five major protein binding sites within the -1404/-971 bp fragment, one of which shared high sequence identity with a portion of a regulatory element in CYP2B2 gene (Trottier, E., Belzil, A., Stoltz, C., and Anderson, A. (1995) Gene 158, 263-268). Our results indicate that sequences important for PB-induced transcription of Cyp2b10 gene are located in the distal promoter.

Differential developmental and tissue-specific regulation of expression of the genes encoding three members of the flavin-containing monooxygenase family of man, FMO1, FMO3 and FM04

We have previously described the isolation and sequencing of cDNA clones encoding flavin-containing monooxygenases (FMOs) 1 and 4 of man [Dolphin, C., Shephard, E. A., Povey, S., Palmer, C. N. A., Ziegler, D. M., Ayesh, R., Smith, R. L. & Phillips, I. R. (1991) J. Biol. Chem. 266, 12379-12385; Dolphin, C., Shephard E. A., Povey, S., Smith, R. L. & Phillips, I. R. (1992) Biochem. J. 287, 261-267]. We present here the isolation of a cDNA for FM03 of man. The sequence of this CDNA and the amino acid sequence deduced from it differ substantially from those previously reported for this member of the FMO family of man. In addition, we have investigated, by quantitative RNase protection assays, the expression in several foetal and adult human tissues of genes encoding FMO1, FMO3 and FMO4, Our results demonstrate that, in the adult, FMO1 is expressed in kidney but not in liver, whereas in the foetus it is expressed in both organs. The lack of expression of FMO1 in adult human liver is in marked contrast to the situation in other mammals, such as pig and rabbit, in which FMO1 constitutes a major form of the enzyme in the liver of the adult animal. The mRNA encoding FMO3 is abundant in adult liver and is also present, in low abundance, in some foetal tissues. Thus, FMO1 and FMO3 are both subject to developmental and tissue-specific regulation, with a developmental switch in the expression of the genes taking place in the liver. FMO4 mRNA is present in low abundance in several foetal and adult tissues and thus the corresponding gene appears to be expressed constitutively.

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.

The influence of medium composition on the maintenance of cytochrome P-450, glutathione content and urea synthesis: a comparison of rat and sheep primary hepatocyte cultures

Rat and sheep primary hepatocytes have been cultured in four different medium formulations: Williams' E, Chee's, Medium 199 and Modified Earle's. The total cytochrome P450 content, intracellular concentration of reduced glutathione, rate of urea synthesis and total protein content of cultures of cells from both species in each medium have been determined. Modified Earle's and Chee's medium proved to be the most favourable formulations for the culture of rat hepatocytes. After 48 h, cells cultured in Modified Earle's had significantly more cytochrome P450 and a significantly greater rate of urea synthesis than cells in any other medium. After 6 days in culture the difference in cytochrome P450 levels between rat hepatocytes in Chee's medium and those in Modified Earle's medium was abrogated. The difference in the rate of urea synthesis between rat hepatocytes cultured in each of these two media was shown to be more dependent on the medium in which the cells were maintained during the period of urea synthesis measurement than on the medium in which the cells had been previously cultured. Sheep hepatocytes cultured in Chee's medium ruptured and died within 24 h. Apart from this, sheep cells were less sensitive to changes in medium formulation than were rat hepatocytes. The initial plating efficiency was lower in sheep cells. Total cytochrome P450 content was the most discriminatory of the four parameters for evaluating the status of rat hepatocyte cultures. However, urea synthesis may be the most useful parameter for assessment of hepatocyte function in hybrid liver devices such as bioartificial liver support systems where access to the cells during operation of the device is restricted.

Metabolism of pimobendan in long-term human hepatocyte culture: in vivo-in vitro comparison

The aim of this study was to investigate further the potential of a new hepatocyte culture based on the hypothesis that liver cells in an appropriate in vitro environment (immobilizing gel technique) maintain high metabolic activity comparable with that in vivo. Pimobendan (UD-CG 115), a pyridazinone derivative, is a cardiotonic vasodilator that increases myocardial contractility through calcium sensitization and relaxation of vascular smooth muscle, probably due to phosphodiesterase inhibition. In man, pimobendan is O-demethylated to UD-CG 212. This latter is metabolized to O- and N-glucuronides. Pimobendan itself is also glucuronidated to a N-glucuronide. Human hepatocytes immobilized in collagen gel were incubated with pimobendan to investigate their metabolic activity in the long-term and to compare the results to the data from clinical trials. 14C-labelled pimobendan was incubated at two concentrations (10 and 100 microM) at day 3, 11 and 22 of culture, and samples were analysed after 4, 24 and 48-h incubation. Metabolic patterns were evaluated by hplc with radioactivity-, diode array-, and mass spectral-detection. In vitro, pimobendan was O-demethylated and subsequently O-glucuronidated. The rate of metabolism of pimobendan could be maintained in this culture system for > 3 weeks. However, the relative amount of a putative N-glucuronide under in vitro conditions was lower than in vivo.

Effects of the anticonvulsant, valproate, on the expression of components of the cytochrome-P-450-mediated monooxygenase system and glutathione S-transferases

It has been shown previously that the anticonvulsant agent, sodium valproate, induces certain cytochrome P-450 monooxygenase activities and decreases glutathione S-transferase activity. We have used Western blotting, RNase protection assays and Northern blot hybridization to determine the effects of valproate on the abundance of individual components of the cytochrome P-450 monooxygenase and of glutathione S-transferase subunits. Due to the short half-life of the drug in rats we have used an in vitro experimental system comprised of rat hepatocytes co-cultured with rat primitive biliary epithelial cells. Valproate was shown to be a potent inducer of two members of the cytochrome P-450 (CYP)2B subfamily, CYP2B1 and 2B2. The induction of the proteins was mediated at the level of the mRNAs, with the mRNA for CYP2B1 being more highly induced than that for CYP2B2. The drug also induced, but to a much lesser extent, two important components of the cytochrome-P-450-mediated monooxygenase system, NADPH-dependent cytochrome P-450 reductase and cytochrome b5, and their corresponding mRNAs. Thus, the effects of valproate on cytochromes P-450 and other components of the cytochrome-P450-mediated monooxygenase system mimic those of another, structurally diverse, antiepileptic drug, phenobarbital. However, in contrast to phenobarbital, which induces glutathione S-transferase subunits 1, 2, 3, 4 and 7, valproate selectively decreases the abundance of subunits 3 and/or 4. It has been shown previously that CYP2B1 is involved in the production of metabolites of valproate implicated in hepatotoxicity. The induction of this protein by valproate would thus contribute substantially to the hepatotoxic effects associated with the drug.

The molecular biology of the flavin-containing monooxygenases of man

cDNA clones encoding five distinct members of the FMO family of man (FMOs 1, 2, 3, 4 and 5) were isolated by a combination of library screening and reverse transcription-polymerase chain reaction techniques. The deduced amino acid sequences of the human FMOs have 82-87% identity with their known orthologues in other mammal but only 51-57% similarity to each other. The hydropathy profiles of the proteins are very similar. From the calculated rate of evolution of FMOs (a 1% change in sequence per 6 million years) it would appear that individual members of the FMO gene family arose by duplication of a common ancestral gene some 250-300 million years ago. Each of the FMO genes was mapped by the polymerase chain reaction to the long arm of human chromosome 1. The localization of the FMO1 gene was further refined to 1q23-q25 by in situ hybridization of human metaphase chromosomes. RNase protection assays demonstrated that in man each FMO gene displays a distinct developmental and tissue-specific pattern of expression. In the adult, FMO1 is expressed in kidney but not in liver, whereas in the foetus its mRNA is abundant in both organs. FMO3 expression is essentially restricted to the liver in the adult and the mRNA is either absent, or present in low amounts, in foetal tissues. FMO4 is expressed more constitutively. Human FMO1 and FMO3 cDNAs were functionally expressed in prokaryotic and eukaryotic cells. FMO1 and FMO3, expressed in either system, displayed product stereoselectivity in their catalysis of the N-oxidation of the pro-chiral tertiary amines, N-ethyl-N-methylaniline (EMA) and pargyline. Both enzymes were stereoselective with respect to the production of the (-)-S-enantiomer of EMA N-oxide. But in the case of pargyline, the enzymes displayed opposite stereoselectivity, FMO1 producing solely the (+)-enantiomer and FMO3 predominantly the (-)-enantiomer of the N-oxide.

The gap junctional intercellular communication is no prerequisite for the stabilization of xenobiotic metabolizing enzyme activities in primary rat liver parenchymal cells in vitro

In primary monocultures of adult rat liver parenchymal cells (PC), the activities of the xenobiotic metabolizing enzymes microsomal epoxide hydrolase (mEHb), soluble epoxide hydrolase (sEH), glutathione S-transferases (GST), and phenolsulfotransferase (ST) were reduced after 7 d to values below 33% of the initial activities. Furthermore, the gap junctional intercellular communication (GJIC), measured after microinjection by dye transfer, decreased from 90% on Day 1 to undetectable values after 5 d in monoculture. Co-culture of PC with nonparenchymal rat liver epithelial cells (NEC) increased (98% on Day 1) and stabilized (82% on Day 7) the homotypic GJIC of PC. Additionally, most of the measured xenobiotic metabolizing enzyme activities were well stabilized over 1 wk in co-culture. Because GJIC is one of several mechanisms playing an important role in cell differentiation, the importance of GJIC for the stabilization of xenobiotic metabolizing enzymes in PC was investigated. PC in monoculture were, therefore, treated with 2% dimethyl sulfoxide (DMSO), a differentiation promoting factor, and 1,1,1-trichloro-2,2,-bis (p-chlorophenyl) ethane (DDT) (10 micrograms/ml), a liver tumor promotor and inhibitor of GJIC, was given to co-cultures of PC with NEC. DMSO significantly stabilized (68% on Day 7), while DDT significantly inhibited (8% on Day 7) homotypic GJIC of PC in the respective culture systems. In contrast, the activities of mEHb, sEH, GST, and ST were not affected in the presence of DMSO or DDT. These results lead to the assumption that the differentiation parameters measured in this study (i.e., homotypic GJIC and the activities of xenobiotic metabolizing enzymes) are independently regulated in adult rat liver PC.

A specific microassay for evaluating hepatic LDH activity in co-cultures of hepatocytes with other cells

This study describes the development of a simple, rapid and reproducible microassay for determining the intracellular LDH activity of rat hepatocytes present in a co-culture system with other cells. The procedure involves treatment of cellular homogenates with an anti-LDH antiserum that specifically inhibits the LDH activity of rat hepatocytes. The assay is performed in 96-well plates and LDH activity can be measured directly in the same wells using a colorimetric method. The difference in LDH activity values measured before and after antiserum incubation reflects the LDH content of the hepatocytes in the sample. The advantages of this method are the small number of cells required, a reduction in sample handling and the possibility of differentiating LDH activity in hepatic and non-hepatic cells. The possible applications of this technique as a parameter for biochemical data and as a test for cytotoxicity studies in co-cultures are also discussed.

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).

Advantages and disadvantages of using human cells for pharmacological and toxicological studies

1. Marked species differences in the distribution and affinity of drug receptors, and in the patterns of biotransformation and susceptibility to the toxicity of xenobiotics, provide the impetus for using human tissues for pharmacological and toxicological studies. 2. Studies with intact cells facilitate the correlation of xenobiotic metabolism with cellular indices of toxicity, which can provide the mechanistic basis for understanding species differences in toxicity. 3. Human cells in suspension or primary culture reflect the variability in susceptibility to toxicity in a population. 4. The current limitation to these studies is scarcity of human material, the need for improved (cryo)preservation techniques for human hepatocytes/precision-cut slices and difficulties in predicting in vivo exposure-risk relationships from in vitro dose-response relationships.

Interaction of proteins with a cytochrome P450 2B2 gene promoter: identification of two DNA sequences that bind proteins that are enriched or activated in response to phenobarbital

Cytochromes P450 (CYPs) are of central importance in the metabolism of foreign hydrophobic compounds. Members of the CYP2B subfamily are inducible at the transcriptional level by the barbiturate, phenobarbital. Owing to the lack of a suitable phenobarbital-responsive cell line, very little is known regarding the mechanisms by which phenobarbital induces the expression of these genes. We report the use of gel retardation and DNase I footprinting to investigate the presence of regulatory protein binding sites within a CYP2B2 gene promoter. Two DNA sequences, located between -183 to -199 and -31 to -72, have been identified that bind rat liver nuclear proteins that are enriched or activated in vivo by phenobarbital. Gel retardation competition experiments demonstrated that the two sequences bound different proteins. In vitro transcription competition experiments demonstrated that the sequences and the proteins with which they interact are involved in regulating CYP2B2 gene transcription. These two DNA sequences and their cognate binding proteins may play a role in the induction of CYP2B2 gene expression in response to phenobarbital.

The pyruvate dehydrogenase complex: cloning of the rat somatic E1 alpha subunit and its coordinate expression with the mRNAs for the E1 beta, E2, and E3 catalytic subunits in developing rat brain

We report the isolation of cDNA clones encoding the somatic form of the E1 alpha subunit of the pyruvate dehydrogenase complex of rat. The deduced amino acid sequence has 99.5, 98, and 97% identity, respectively, with the orthologous proteins of mouse, human, and pig and 98.5% identity with a rat E1 alpha sequence reported previously. The cDNAs isolated in this and earlier studies predict different E1 alpha subunit mRNA sizes and amino acid sequences. These differences have been investigated by PCR, northern blot hybridization, and RNase protection. We have used our E1 alpha cDNA, in conjunction with cDNA probes to the E1 beta, E2, and E3 catalytic subunits of rat pyruvate dehydrogenase complex and also to rat citrate synthase, to perform RNase protection assays of developing rat whole brain RNA. The results show a 2.5-fold increase in the concentration of each of the subunit mRNAs and a 1.2-fold increase in citrate synthase mRNA from late foetal stage to 5 days post partum. Thereafter, the mRNA levels remained constant. These data indicate that the respective six- and threefold increases in the amounts of pyruvate dehydrogenase complex and citrate synthase found to occur in rat brain between birth and adulthood are mediated principally by translational and/or posttranslational mechanisms.

Characterization of cDNAs encoding the rat testis-specific E1 alpha subunit of the pyruvate dehydrogenase complex: comparison of expression of the corresponding mRNA with that of the somatic E1 alpha subunit

cDNA clones encoding the testis-specific form of the rat pyruvate dehydrogenase complex E1 alpha subunit have been isolated. Comparison of the predicted amino acid sequence with those of the somatic and testis-specific E1 alpha forms of man and mouse and the somatic E1 alpha form of rat indicates the change of a serine residue, believed to be phosphorylated in vivo by pyruvate dehydrogenase E1 alpha-specific kinase, to an alanine at position 233. The implications of this change are discussed. Northern blot analysis and RNase protection assays indicate that the expression of mRNA encoding testis-specific E1 alpha subunit is restricted to testis whereas mRNA for the somatic form is found in all tissues analyzed, albeit in very small amounts in testis.

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.