Maintenance of microsomal cytochromes b5 and P-450 in primary cultures of parenchymal liver cells on collagen membranes

Toxicity and Cell Density Monitoring in Monolayer and Three-dimensional Cultures with the XTT Assay

The application of viability criteria (MTT and XTT tests) to monolayer cultures and immobilised cells in three-dimensional systems was investigated in order to assess cell viability and cell proliferation. The suitability and accuracy of these tests were compared with the conventional criteria (cellular protein and DNA content) used in monolayer cultures for the same purpose. The colorimetric assay based on the metabolic reduction of the tetrazolium salt XTT to a water-soluble formazan proved to be very useful, rapid and sensitive. This automated spectrophotometric enzymatic method, due to its lack of toxicity, also permits repeated nondestructive assays on a single cellular culture for the long-term monitoring of cytotoxicity, cell survival and cell proliferation, and can be performed in 96-well plates with minimal handling. This method could offer a solution for cellular density evaluation in complex cell cultures that do not permit visual examination; it is also the best choice for protein-based, three-dimensional systems such as collagen gels.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

NAD levels in the rat primary cultured hepatocytes affected by peroxisome-proliferators

The effect of peroxisome-proliferators (PPs) on the NAD level in primary cultured rat hepatocytes was investigated and compared with that in the liver of rat administered with PPs. Various PPs, including fibrates, phthalate esters and steroid, increased NAD level in the cultured hepatocytes as in whole animal with a little exception. The NAD level decreased after once reaching the peak by the addition of most PPs. The gradual decrease of NAD observed in primary cultured hepatocytes, was partially inhibited by the addition of poly(ADP-ribose) polymerase and/or NAD glycohydrolase inhibitors. In the presence of these inhibitors, the increase of NAD by PPs still remained. The mechanism of increasing NAD level by PPs will be due to the stimulation of tryptophan (Trp)-NAD biosynthesis, with the possibility of the transcriptional regulation of genes related to Trp-NAD pathway by PPs. This in vitro system, therefore, can be used to clarify detailed mechanism of the stimulation of hepatic NAD biosynthesis in rats administered PPs.

Predictive value of in vitro model systems in toxicology

The application of in vitro model systems to evaluate the toxicity of xenobiotics has significantly enhanced our understanding of drug- and chemical-induced target toxicity. From a scientific perspective, there are several reasons for the popularity of in vitro model systems. From the public perspective, in vitro model systems enjoy increasing popularity because their application may allow a reduction in the number of live animals employed in toxicity testing. In this review, we present an overview of the use of in vitro model systems to investigate target organ toxicity of drugs and chemicals, and provide selective examples of these model systems to better understand cutaneous and ocular toxicity and the role of drug metabolism in the hepatotoxicity of selected agents. We conclude by examining the value and use of in vitro model systems in industrial development of new pharmaceutical agents.

The current status of primary hepatocyte culture

Recently, there have been significant advances toward the development of culture conditions that promote proliferation of primary rodent hepatocytes. There are two major methods for the multiplication of hepatocytes in vitro: one is the use of nicotinamide, the other is the use of a nutrient-rich medium. In the medium containing a high concentration of nicotinamide and a growth factor, primary hepatocytes can proliferate well. In this culture condition small mononucleate cells, which are named small hepatocytes, appear and form colonies. Small hepatocytes have a high potential to proliferate while maintaining hepatic characteristics, and can differentiate into mature ones. On the other hand, combining the nutrient-rich medium with 2% DMSO, the proliferated hepatocytes can recover the hepatic differentiated functions and maintain them for a long time. In this review I describe the culture conditions for the proliferation and differentiation of primary hepatocytes and discuss the small hepatocytes, especially their roles in liver growth.

Use of metabolically competent human hepatoma cells for the detection of mutagens and antimutagens

The human hepatoma line (Hep G2) has retained the activities of various phase I and phase II enzymes which play a crucial role in the activation/detoxification of genotoxic procarcinogens and reflect the metabolism of such compounds in vivo better than experimental models with metabolically incompetent cells and exogenous activation mixtures. In the last years, methodologies have been developed which enable the detection of genotoxic effects in Hep G2 cells. Appropriate endpoints are the induction of 6-TGr mutants, of micronuclei and of comets (single cell gel electrophoresis assay). It has been demonstrated that various classes of environmental carcinogens such as nitrosamines, aflatoxins, aromatic and heterocyclic amines and polycyclic aromatic hydrocarbons can be detected in genotoxicity assays with Hep G2 cells. Furthermore, it has been shown that these assays can distinguish between structurally related carcinogens and non-carcinogens, and positive results have been obtained with rodent carcinogens (such as safrole and hexamethylphosphoramide) which give false negative results in conventional in vitro assays with rat liver homogenates. Hep G2 cells have also been used in antimutagenicity studies and can identify mechanisms not detected in conventional in vitro systems such as induction of detoxifying enzymes, inactivation of endogenously formed DNA-reactive metabolites and intracellular inhibition of activating enzymes.

Formation of bile canaliculi in long-term primary cultures of adult rat hepatocytes on permeable membrane: an ultrastructural study

Adult rat hepatocytes were cultured for 15 days on type I collagen-coated permeable membranes in a hormonally defined Waxman's modified medium supplemented with very low concentrations of insulin, glucagon and dexamethasone. Phase contrast examination showed that 15-day-old cultures still formed a regular monolayer of polygonal cells. In similarly aged cultures, intracellular glycogen was abundant and evenly distributed, while steatosis remained very limited. Scanning and transmission electron microscopy showed that well developed bile canaliculi could be observed on the lateral side of the hepatocyte membrane after 4 days of incubation and persisted for 2 weeks. These canalicular structures probably originated from coalescence of membrane invaginations observed in 1-day-old cultures. Transmission electron microscopy showed that the ultrastructure of the cells was very close to that of normal rat hepatocytes in the intact liver. These results suggest that rat hepatocytes cultured under these experimental conditions are able to develop and maintain tissue-specific cytochemical and morphological properties for at least 15 days.

Hollow fibers for hepatocyte encapsulation and transplantation: studies of survival and function in rats

In this study, the feasibility of transplanting hepatocytes using hollow fibers (HF) was investigated. Experiments were carried out in vitro and in vivo to determine the viability and function of hepatocytes encapsulated in four different types of commercially available HF: regenerated cellulose HF (RCHF), polysulfone HF of two different sizes (PSHF-1 and PSHF-2), and polyvinylidine HF (PVDF). Hepatocytes remained viable in all types of HF for at least 1 wk in vitro as measured by light microscopy and their ability to synthesize protein and secrete albumin. However, the levels of protein synthesis and albumin secretion in these cells varied significantly between different HF (RCHF > PSHF-2 > PVDF approximately PSHF-1) and appeared to be inversely related to their internal diameters (215, 500, 1000, and 1100 microns for RCHF, PSHF-2, PVDF, and PSHF-1, respectively). While PSHF-2, PVDF, and PSHF-1 did not support long term viability in vivo, hepatocytes in RCHF survived after implantation in the mesentery. After 24 h in vivo, the hepatocytes appeared morphologically intact and exhibited a similar rate of protein synthesis when compared with cells cultured in parallel. The hepatocytes in RCHF also maintained the ability to synthesize protein after 7 days in vivo. These results suggest that HF of appropriate size may be useful for hepatocyte transplantation applications in which prevascularization is not possible.

Redifferentiation of proliferated rat hepatocytes cultured in L15 medium supplemented with EGF and DMSO

Primary adult rat hepatocytes were cultured in serum-free L15 medium supplemented with 20 mM NaHCO3 and 10 ng/ml epidermal growth factor in a 5% CO2:95% air incubator. The number of cells increased and reached about 180% of the initial value by Day 4, and after 2% dimethyl sulfoxide (DMSO) was added to the culture medium at Day 4, the cells continued to proliferate until Day 6. The number of cells reached about 210% at Day 6 and they were well maintained until Day 18. The cell number gradually decreased with time in culture, but many cells remained for more than 2 mo. On the other hand, without 2% DMSO, the cells proliferated until Day 5, but thereafter they rapidly decreased. After DMSO addition, albumin and transferrin were secreted into the medium and the production of both proteins continued for more than 2 mo. Immunocytochemically both proteins were strongly stained in the cells treated with 2% DMSO. Although the expression of G6Pase in the cells disappeared at Day 6 without DMSO, the cells treated with 2% DMSO recovered G6Pase activity at Day 16. In addition, induction of peroxisomes by 2 mM sodium clofibric acid was clearly shown in the hepatocytes at Day 14 and Day 25 using enzyme-cytochemistry. Ultrastructurally, DMSO-treated hepatocytes had many mitochondria and large peroxisomes with a crystalline nucleoid, and both gap junctions and desmosomes were well developed between the cells even at Day 40. Thus, the number of cells doubled, some differentiated functions of the primary hepatocytes were well restored by the use of 2% DMSO, and these functions were maintained for more than 2 mo.

Effects of interferon, polyriboinosinic acid--polyribocytidilic acid and steroids on the cytochrome P450 system of cultured primary mouse hepatocytes

An earlier study from this laboratory showed that the hepatic murine cytochrome P450 (P450) system was depressed by interferon in vivo but induced in cultured primary hepatocytes. The current investigation attempted to resolve this contradiction. The P450 content of the cells used in the earlier study fell precipitously during the first 24 hr of culture and remained at the same low level throughout another 48 hr of incubation. This failure to maintain the P450 level suggested that the cells may not have been sufficiently viable to support the mechanisms involved in the depressant activity of interferon. Accordingly, a chemically defined medium containing hydrocortisone was devised which supported an acceptable level and function of the P450 system throughout a 72 hr incubation period. Functionality of the P450 system was evaluated by measuring aminopyrine N-demethylase and benzo[a]pyrene hydroxylase activities. When this steroid supplemented medium was used, interferon depressed both activities by about 25%; however, neither activity was affected significantly by poly IC. On the other hand, benzo[a]pyrene hydroxylase activity was depressed by both poly IC and interferon in hepatocytes induced with dexamethasone or with dexamethasone plus 3-methylcholanthrene. These studies emphasize the necessity of maintaining an acceptable level of homeostasis in cultured hepatocytes if one is to derive meaningful interpretations of certain biological events.

Role of the adenylate cyclase, phosphoinositidase C and receptor tyrosyl kinase systems in the control of hepatocyte proliferation by hepatocyte growth factor

Hepatocyte growth factor (HGF) is the most potent known mitogen for hepatocytes in primary culture. However, the mechanisms through which HGF induces hepatocyte proliferation have not been defined. Here we have investigated the role of the adenylate cyclase, phosphoinositidase C and tyrosine kinase signalling systems in the control of hepatocyte proliferation by HGF using freshly isolated or cultured adult rat hepatocytes. We show that human recombinant HGF caused a dose-dependent increase in hepatocyte DNA synthesis with a maximal effect at 10 ng/mL and an EC50 of 5.9 ng/mL. HGF had no effect on hepatocyte adenylate cyclase activity or intracellular cAMP levels. Elevation of hepatocyte cAMP levels resulted in inhibition of HGF-stimulated DNA synthesis. HGF stimulated inositol phospholipid hydrolysis with a maximal effect at 25 ng/mL and potentiated the effect of vasopressin (10(-8) and 10(-9)M). HGF (100 ng/mL) caused an increase in the phosphorylation on tyrosine of an unknown hepatocyte protein with a molecular mass of 36 kDa. Thus, we have shown that HGF, like epidermal growth factor (EGF), can activate the phosphoinositidase C and tyrosine kinase systems in rat hepatocytes. As with EGF, these intracellular signalling systems may underlie HGF-induced hepatocyte proliferation.

The bicarbonate ion is essential for efficient DNA synthesis by primary cultured rat hepatocytes

Bicarbonate in the culture medium is essential for DNA synthesis of primary cultured rat hepatocytes stimulated by epidermal growth factor (EGF). When primary cultured hepatocytes in supplemented Leibovitz L15 medium were placed in a 100% air incubator, no increase in DNA synthesis was observed even after stimulation by EGF. However, when these cells were cultured with NaHCO3 and EGF and placed in a 5% CO2:95% air incubator, a stimulus of DNA synthesis more than 10-fold greater than in cultures in air only was seen, and many mitotic figures could be identified. Furthermore, NaHCO3 added to supplemented DMEM/F12 medium enhanced the DNA synthesis of primary cultured rat hepatocytes in this medium. The ideal pH of the medium for DNA synthesis of cultured hepatocytes was in the range of 7.6 to 8.0. A dose response of NaHCO3 in several media showed that DNA synthesis of the cells increased as the concentration of NaHCO3 increased and that 25 to 30 mM NaHCO3 in the medium was optimal for the replication of DNA by primary cultured rat hepatocytes.

Amino acid-rich medium (Leibovitz L-15) enhances and prolongs proliferation of primary cultured rat hepatocytes in the absence of serum

Multiple rounds of cell division were induced in primary cultured rat hepatocytes in serum-free, modified L-15 medium supplemented with 20 mM NaHCO3 and 10 ng/ml EGF in a 5% CO2/95% air incubator. A 150% increase in cell number and DNA content was observed between day 1 and day 5. The time course of DNA synthesis of hepatocytes cultured in L-15 medium differed from that in DMEM/F12 medium in that there were four peaks of 3H-thymidine incorporation in the L-15 medium, at 60 h, 82 h, 96 h, and 120 h, but only one peak at 48 h in modified DMEM/F12 medium. Labeling studies of the hepatocytes indicated that more than 60% of the cells were stained with antibromodeoxyuridine (BrdU) antibody in the periods of 48-72 h and 72-96 h after plating at densities between 1.5 x 10(5) and 6.0 x 10(5) cells per 35-mm dish. Even at a density of 9.0 x 10(5) cells/dish, about 40% of the cell nuclei were stained with BrdU in the periods of 48-72 h and 72-96 h. In addition, about 20% of the hepatocytes in culture initiated a second round of the cell cycle between 48 and 96 h in culture. Proliferating cells, which were mononucleate with a little cytoplasm, appeared in small clusters or colonies in the culture from day 4. These proliferating cells produced albumin. The addition of essential amino acids to the DMEM/F12 medium enhanced the DNA synthesis of hepatocytes, thus indicating that the higher level of amino acids in L-15 medium may be an important factor in its enhanced ability to support the proliferation of primary cultured rat hepatocytes.

Co-cultures of hepatocytes with epithelial-like cell lines: expression of drug-biotransformation activities by hepatocytes

To improve long-term expression of drug biotransformation activities in hepatocytes, we have examined the suitability of several epithelial-like cell lines (MDCK, MS and L-132) for supporting functional co-cultures with rat hepatocytes. Cells were selected on the basis of their compatibility with hepatocytes, formation of stable monolayers in the absence of serum and lack of drug biotransformation activities. The expression of individual elements of the biotransformation system was evaluated in these co-cultures. Co-cultured hepatocytes remained viable and showed a characteristic polygonal shape for more than a week. Depending on the cell line used, levels of aryl hydrocarbon hydroxylase and 7-ethoxycoumarin O-deethylase activities of co-cultured hepatocytes oscillated between 24-47% of their initial value after 4 days in culture. The highest levels of monooxygenase activity were found in hepatocytes co-cultured with MS cells (41-47%). In contrast, these activities decreased to 6% when hepatocytes were maintained in pure culture for the same period. The activities of the conjugating enzymes UDP-glucuronyltransferase and glutathione S-transferase were maintained at nearly the initial levels during the complete period of study, both in pure and mixed-cultures, regardless of the cell line used. MS cells adapted themselves much better to serum-free culture conditions, and the co-culture with rat hepatocyte was technically easier. After one week, total cytochrome P450 and reduced glutathione in rat hepatocytes/MS co-cultures were 31% and 127% respectively of the day O values, whereas they were undetectable in pure culture. A clear induction of monooxygenase activities by methylcholanthrene, phenobarbital and ethanol could be observed by the 5th day in MS cells/hepatocyte co-cultures. The fact that the results of our work show the suitability of MS cells, an epithelial-derived cell line, for improving the expression of biotransformation enzymes of cultured hepatocytes opens new possibilities of simplifying co-cultures for their use in drug-metabolism studies.

Effect of enzyme inducers on metabolism of 1-nitropyrene in human hepatoma cell line HepG2

We measured the response of HepG2 cells to the classic cytochrome (cyt.) P-450 inducers 3-methylcholanthrene (3-MC) and phenobarbital (PB), by evaluating oxidative and/or reductive metabolism of the nitroarenes, 1-NP and 1,6-dinitropyrene (1,6-DNP), in control and induced cells. In HepG2 cells, 3-MC induces ring-hydroxylation of 1-NP, whereas PB stimulates its nitroreduction. PB induces NADPH-cyt. c reductase, but does not affect other cytosolic and microsomal enzymes which contribute to 1-NP nitroreduction in these cells. However, PB-inducible nitroreductase activity seems to be associated primarily with cyt. P-450 isoenzymatic form(s), as indicated by the requirement for NADPH and the response to specific inhibitors such as alpha-naphthoflavone and CO.

The maintenance of cytochrome P-450 in rat hepatocyte culture: some applications of liver cell cultures to the study of drug metabolism, toxicity and the induction of the P-450 system

Treatments affecting the loss of cytochrome P-450 in rat hepatocyte culture are reviewed and the way in which these have produced an understanding of the mechanisms involved are discussed extensively. A simple way to prevent the loss of P-450 in hepatocytes is to culture them with 0.5 mM metyrapone which appears to restore the cytochromes' synthesis and degradation to steady state values. Knowledge of this mechanism has led to the formulation of special culture medium and the application of both culture systems to the study of drug metabolism and toxicity are described. Finally the effect of these culture systems on the expression of the multiple forms of cytochrome P-450 are presented to illustrate the potential of cultured hepatocytes in induction studies.

Multiple sequential periods of DNA synthesis and quiescence in primary hepatocyte cultures maintained on the DMSO-EGF on/off protocol

Repeated periods of DNA synthesis activity (each period consisting of two to three cycles) separated by intervals of quiescence in primary rat hepatocytes can be stimulated by sequential addition and removal of 2% dimethyl sulfoxide (DMSO) in the presence of epidermal growth factor (EGF). Hepatocytes can be kept in nonproliferating cultures for 7 days in media supplemented with 2% DMSO and EGF. If DMSO is removed while EGF is maintained, rat and human hepatocytes enter a 3 to 4 day period of DNA synthesis that declines rapidly by days 4 and 5. If DMSO is reintroduced into cultures at that point, kept on for 3 more days and removed again, hepatocytes reenter into proliferation with another self-limited response of 3 to 4 days. Similar phenomena can seen with hepatocytes maintained in the presence of 3 mM phenobarbital. These protocols demonstrate that loss of responsiveness to mitogens in primary hepatocyte cultures is not an irreversible process. They also raise the possibility that signals for termination of DNA synthesis in hepatocytes emanate from hepatocytes themselves. These studies also suggest for the first time the possibility of designing in vitro systems that will allow clonal expansion of differential hepatocytes.

Heparin-binding growth factor 1 induces the formation of organoid neovascular structures in vivo

One of the promises of modern molecular biology has been the opportunity to use genetically modified human cells in a patient to permanently restore inborn errors of metabolism. Although it has been possible to introduce genes into mammalian cells and to control their expression, it has proven difficult to introduce mammalian cells as carriers of the modified genetic information into hosts. The successful implantation of selective cells cannot be achieved without adequate vascular support, an essential step toward integration and reconstitution of a new biological function. Although a partial solution to this problem has been found by inducing specific site-directed neovessel formation using heparin-binding growth factor 1 (HBGF-1) adsorbed to a collagen matrix, these implants function for only a short period (weeks). We now report the formation of organoid neovascular structures using polytetrafluoroethylene fibers coated with collagen and HBGF-1 implanted in the peritoneal cavity of the rat. The organoid structures contained readily visible vascular lumina and nonvascular structures that resemble nerve tissue. It was also possible to demonstrate that the vascular system on the implant is continuous with the vascular tree of the host. This feature was used to demonstrate that the organoid structures are capable of sustaining the biological function of implanted normal rat hepatocytes over long periods of time (months) in the homozygous Gunn rat, thereby facilitating future applications involving the delivery of new genetic information.

In vitro drug metabolism and pharmacokinetics of diazepam in cynomolgus monkey hepatocytes during culture for six days

Diazepam (DZ), N-desmethyl diazepam (NOR) and temazepam (TEM) were used as substrates in drug metabolism studies to characterize the changes in cytochrome P-450 mono-oxygenase pathways in hepatocytes isolated from cynomolgus monkeys, during culture for 6 days. Hepatocytes were incubated with DZ (20 microM), NOR (6 microM) or TEM (20 microM) for 3 hr at 3, 24, 48, 96 and 144 hr post-isolation in culture, and the profiles of disappearance of DZ, as substrate, and appearance of its metabolites determined. Major metabolites were NOR, TEM and oxazepam (OX). The kinetic profiles for the disappearance of DZ and the accumulation of metabolite were analysed using a four-compartment model and constants for the rates of formation of the metabolites were derived. There were significant changes during the period in culture for the rate constants of DZ demethylation, but no alteration in the 3-hydroxylation activities. Rates of DZ metabolism were unchanged during the initial 2 days in culture and well maintained for the subsequent 4 days, despite a fall in total cytochrome P-450 to 23% of initial values after 6 days. Cynomolgus monkey hepatocytes produce similar metabolite profiles for DZ to those found in man, both in vitro and in vivo, indicating that cynomolgus monkey hepatocytes may represent a relatively stable and valuable model of human drug metabolism.

Gene expression in implanted rat hepatocytes following retroviral-mediated gene transfer

An hepatocyte transplantation-gene transfer protocol has been developed whereby liver cells containing an expressing NeoR gene can be successfully implanted in vivo. Adult primary cultures of rat hepatocytes, after infection with the retroviral vector N2, were grown on a floating solid support (coated with purified collagen IV) in a serum-free hormonally defined medium designed for hepatocytes that also contained G418. Under these conditions, normal adult hepatocytes expressing the NeoR gene could be grown to high density. The solid supports holding the gene-engineered hepatocytes were then implanted into adult rats into subcutaneous and intraperitoneal sites. After one to two weeks, the supports were removed and shown to still contain the gene-engineered hepatocytes expressing the NeoR gene. These results suggest that cells from solid organs, such as the liver, are potential targets for gene transfer and expression studies in vivo.

The expression and metabolic activity of cytochrome P-450 isozymes in control and phenobarbital-induced primary cultures of rat hepatocytes

The expression and activity of the phenobarbital (PB)-inducible P-450 isozymes, P-450b and P-450e, and the major 3-methylcholanthrene (MC)-inducible form, P-450c, were studied in primary cultures of adult rat hepatocytes in T1, Leibovitz L-15 (L-15), and a modification of Waymouth 752/1 (Way) media. P-450 isozymes in initially isolated hepatocytes and control and PB-treated cultures were quantitated by Western blot analysis, and activity was determined with 7,12-dimethylbenz[a]anthracene (DMBA) as substrate. Data from the Western blot analysis correlated well with the metabolic activity toward DMBA. P-450b was consistently induced by PB in hepatocytes in T1 and to a lesser extent in Way. P-450e protein was constitutive in initially isolated cells, expressed in control cultures at a reduced level, and increased or maintained by PB in all three media. DMBA metabolite formation associated with P-450b and P-450e activity was induced by PB in hepatocytes in T1 and Way and was inhibited by antibodies to P-450b. P-450c was only infrequently expressed in freshly prepared hepatocytes, but was detected in all control and PB-treated cultures although at a much higher level in T1. Thus, the amounts of P-450 isozymes, their inducibility by PB, and their activity toward DMBA were found to be dependent on the medium. We have demonstrated enzyme induction and increased activity of the major PB-inducible isozymes in hepatocytes in T1; these are also associated with a change in the control of P-450c expression leading to enhanced constitutive expression and inducibility by phenobarbital.

Aminopyrine metabolism in primary monolayer cultures of diabetic rat hepatocytes

1. A new support system has been used which provides long-term maintenance of rat liver parenchymal cells in monolayer cultures. The cells, maintained on collagen gel/polychlorinated vinylidene film, expressed aminopyrine metabolizing activity for up to 5 days. This culture system was utilized to study the metabolism of aminopyrine in the liver cells isolated from normal, alloxan- and streptozotocin-diabetic rats. 2. Aminopyrine was metabolized at a slower rate in both types of cultured diabetic rat hepatocytes than in cultured normal rat hepatocytes, as judged from higher levels of the unchanged drug in the culture medium. 3. The formation of the metabolites 4-monoaminoantipyrine, 4-acetylaminoantipyrine and 4-formylaminoantipyrine decreased in the cultured diabetic rat hepatocytes, while that of 4-aminoantipyrine was at the same levels as controls. In contrast, 3-hydroxymethyl-2-methyl-4-dimethylamino-1-phenyl-3-pyrazolin-5-on e (AM-CH2OH) formation in the cultured diabetic rat hepatocytes increased over control value. These findings agree with in vivo results which have been reported by the authors. 4. The increase in AM-CH2OH was prevented by insulin in a dose-dependent manner. However, insulin did not affect the formation of other metabolites. These findings indicate that the amount of cytochrome P-450 isozyme involved in the oxidation of 3-methyl group may be regulated by insulin. 5. The present results, indicate that this primary culture system is a useful tool for the study of drug metabolism in diabetes.

Presence of hexobarbital in primary cultures of rat hepatocytes maintains cytochrome P-450 levels and drug metabolizing enzyme activities

Addition of hexobarbital (1 mM) to the culture medium of rat hepatocytes protected against the rapid decline in the level of cytochrome P-450 and the activities of various drug metabolizing enzymes. While the hepatocytes cultured for 72 hr without hexobarbital had only 30% of their original level of cytochrome P-450, the cells maintained with hexobarbital had 75% of the initial level of the hemoprotein. After 72 hr in culture, the activities of aminopyrine N-demethylase and biphenyl 4-hydroxylase were 22-24% of the original rate for the nontreated cells and 73-78% for the hexobarbital treated cells. The activities of 7-ethoxycoumarin O-deethylase and aryl hydrocarbon hydroxylase in the cultures of treated cells were even higher than those of the freshly isolated hepatocytes. Additions of other substrates of hepatic mixed function oxidase to the culture medium did not protect against the loss of cytochrome P-450 and enzyme activities.

Long-term maintenance of hepatocytes in primary culture in the presence of DMSO: further characterization and effect of nafenopin, a peroxisome proliferator

The addition of 2% dimethyl sulfoxide to adult rat hepatocytes cultured in a chemically defined medium at Day 1 after cell plating resulted in maintenance of the cytochrome P-450 content and the cyanide-insensitive palmitoyl-CoA beta-oxidation activity at 66 and 70% of the initial Day 1 values. The addition of phenobarbital, 3-methylcholanthrene, or nafenopin from Day 3 to Day 6 increased the contents of cytochrome P-450 to 128, 239, and 251%, respectively, compared to untreated controls at Day 3. In addition, nafenopin also caused a pronounced and time-dependent increase in palmitoyl-CoA beta-oxidation activity but was found to have only a weak stimulating effect on replicative DNA synthesis (2-fold) when compared to that of epidermal growth factor (6.5-fold). In the presence of dimethyl sulfoxide the hepatocyte cultures could be kept alive for more than 1 month. Exposure of such cultures to nafenopin from Day 1 do Day 37 resulted in survival which was even better than that of their untreated counterparts. This effect was accompanied by the appearance of abundant endoplasmic reticulum membranes and an increased number of peroxisomes.

Species differences in the hepatotoxicity of paracetamol are due to differences in the rate of conversion to its cytotoxic metabolite

The cytotoxicity of paracetamol and of its putative toxic metabolite, N-acetyl-p-benzo-quinoneimine (NABQI) have been investigated in hepatocytes from hamster, mouse, rat and human liver. Whereas paracetamol readily caused cell blebbing and a loss of viability in hepatocytes from mouse and hamster, human and rat hepatocytes were much more resistant to these effects. In marked contrast, there were no significant differences in the sensitivity of the cells from any species to the toxic effects of NABQI. Glutathione depletion by NABQI and paracetamol correlated very well with the toxic effects of these compounds. It is concluded that species differences in sensitivity to the hepatotoxicity of paracetamol are due almost entirely to differences in the rate of formation of NABQI, and not to any intrinsic differences in sensitivity or in any difference in the fate of NABQI once formed. Further, man appears to be relatively resistant to the hepatotoxic effects of paracetamol, and the results in hepatocytes were confirmed by both in vitro and in vivo analyses.

Effect of dexamethasone on cytochrome P-450 mediated metabolism of 2-acetylaminofluorene in cultured rat hepatocytes

The metabolism of 2-acetylaminofluorene (AAF) to its six oxidative metabolites has been used to investigate the effect of dexamethasone on cytochrome P-450 activity in cultured rat hepatocytes. In control hepatocytes the metabolism of AAF to its 1-, 5-, 7-, 9- and N-hydroxylated metabolites rapidly declined in culture over the first 24 hr while 3-hydroxylation remained relatively constant. These activities either remained unchanged or increased slightly during the next 48 hr in culture. The addition of dexamethasone (100 nM) to the culture medium had little effect in arresting the initial decline but by 72 hr the 7-, 5- and 3-hydroxylations increased to values 2.5, 16 and 21 times the respective 24-hr values. The inductive effect of dexamethasone on the 3- and 5-hydroxylations of AAF was maximal at 100 nM whereas the 7-hydroxylation increased linearly as a function of the dexamethasone concentration up to 1 microM. Cortisol and corticosterone and the non-glucocorticoids fluoxymesterone and methyltestosterone induced a pattern of AAF metabolism resembling that in dexamethasone-treated cultures, suggesting that a range of steroids not restricted to glucocorticoids may induce multiple cytochrome P-450 isozymes via related mechanisms. Pregnenolone 16 alpha-carbonitrile induced only the 7-hydroxylation of AAF probably reflecting induction of cytochrome P-450p. While dexamethasone was a strong inducer of the 3- and 5-hydroxylations of AAF in hepatocyte culture, assay of these activities in freshly isolated cells after in vivo treatment with dexamethasone showed a strong induction of 7-hydroxylation but only small effects on 3- and 5-hydroxylations. Indeed the profile of AAF metabolism induced in culture by dexamethasone resembles more closely the profile induced by 3-methylcholanthrene in vivo. These data suggest that factors yet to be identified strongly influence the steroid-induced pattern of cytochrome P-450 gene expression.

Carbon tetrachloride and trichloroethylene toxicities to rat hepatocytes in primary monolayer culture: its relationship to the level of cytochrome P-450

The effects of carbon tetrachloride (CCl4) and trichloroethylene (TCE) on the synthesis and the secretion of triacylglycerols (TGs) in primary cultured rat hepatocytes were investigated in relation to the level of cellular cytochrome P-450 (P-450). Pretreatment of rats with phenobarbital (PB) and plating the hepatocytes in the presence of metyrapone attained a marked preservation of P-450 during the preparation of monolayer. CCl4 was able to cause the accumulation of cellular TG in the hepatocytes when the content of P-450 was retained at the level equivalent to that in the liver in vivo.

Enhancement of aldehyde dehydrogenase activity in human and rat hepatocyte cultures by 3-methylcholanthrene

Aldehyde dehydrogenase was measured in primary cultures of hepatocytes obtained with a two-step collagenase perfusion either from human hepatic tissue or from livers of Fisher rats. Basal enzyme activity declines gradually as a function of time in culture, but remains at all times higher when measured with propionaldehyde and NAD (P/NAD) than with benzaldehyde and NADP (B/NADP). Treatment of the cultures with 2 microM of 3-methylcholanthrene for four days significantly increased the B-NADP activity of human and rat hepatocytes (tenfold and eightfold respectively). In human hepatocytes 3-methylcholanthrene increases also the P/NAD activity, but to a lesser extent (twofold), compared to the B/NADP activity. Due to the significant enhancement of B/NADP activity in cultures of human and rat hepatocytes after application of 3-methylcholanthrene, the initial difference in the basal activity levels between the P/NAD and B/NADP forms diminishes or, in the case of human hepatocytes, is even inverted. These results show for the first time that aldehyde dehydrogenase activity is increased in cultured human hepatocytes. This biochemical property is preserved in human and rat hepatocyte cultures, despite the rather quick loss of the basal aldehyde dehydrogenase activity.

Use of a low-speed, iso-density percoll centrifugation method to increase the viability of isolated rat hepatocyte preparations

A simple yet effective method (iso-density percoll centrifugation) has been developed for consistently preparing isolated rat liver parenchymal cells with over 98% initial viability. The method has been applied to cells isolated by a variety of collagenase digestion techniques. This procedure involves the low-speed centrifugation (50 X g) of the initial cell suspension through a percoll medium having a density of 1.06 g/ml and results in the separation of single and viable parenchymal cells from cell aggregates, debris, and nonparenchymal cells. The enriched parenchymal cells have been shown to be superior to untreated cells by a number of criteria including: preparation homogeneity, cell morphology, maintenance of cytochrome P-450, hormonal responsiveness (measured by the induction of tyrosine aminotransferase after treatment with glucagon or dexamethasone, or both), plasma membrane integrity (determined by both trypan blue exclusion and leakage of glutamic-oxaloacetic transaminase), and the DNA repair capability after treatment with benzo[a]pyrene or 2-acetylaminofluorene.

Isolation and culture of hepatocytes from human liver of immediate autopsy

Human livers were removed at immediate autopsy (IA) from brain death patients within 1 h after cessation of cardiac function. Viable hepatocytes were isolated successfully from these IA livers by perfusion of an intact lobe with collagenase or by digestion of a small tissue wedge with collagenase-dispase. The yields of hepatocytes ranged from 1 to 3 X 10(6) cells/g liver in the five cases studied. Approximately 70 to 90% of the cells excluded trypan blue dye. In the isolated hepatocytes, 632 pmol/mg protein of cytochrome p450 and 536 pmol/mg protein cytochrome b5 were measured. The cells attached to the dishes in 4 h and produced monolayer cultures with a high success rate. The cells maintained in primary cultures for several days and developed ultrastructural features characteristic of human hepatocytes in vivo. The cultured hepatocytes can hydroxylate benzo[a]pyrene, conjugate the metabolites, and have a benzo[a]pyrene hydroxylase activity of 48.7 pmol/mg DNA per h, which is comparable to that of rat hepatocytes. The liver cells repaired DNA damage caused by exposures to aminofluorene and acetylaminofluorene in culture.

7-Ethoxycoumarin deethylase activity as a convenient measure of liver drug metabolizing enzymes: regulation in cultured rat hepatocytes

Assays of 7-ethoxycoumarin O-deethylase (ECD) activity in intact cells were used as a sensitive and convenient measure of the drug-metabolizing activity of rat hepatocytes maintained for up to 4 days in primary culture. A combination of nicotinamide or other pyridines with dexamethasone was shown to maintain ECD at or above the activity of untreated livers in vivo and to potentiate induction by xenobiotics. Inductions in vivo and in culture were quantitatively similar but differed qualitatively as judged by the proportion of ECD activity inhibitable by metyrapone. A survey of possible endogenous regulators of liver monooxygenases established that: dexamethasone and other glucocorticoids induced ECD and potentiated induction by xenobiotics, particularly phenobarbitone; other steroids including testosterone, 17 beta-estradiol and pregnenolone 16 alpha-carbonitrile caused small inductions; insulin lowered both ECD activity and the proportion of activity inhibitable by metyrapone; dibutyryl cyclic AMP or glucagon lowered ECD; and high concentrations of aminolevulinate partly repressed induction by xenobiotics. Based on these findings, hepatocyte culture conditions which maintain ECD activity and inducibility at or above in vivo levels are defined.

Detection of DNA single-strand breaks induced by procarcinogens in Chinese hamster ovary cells cocultured with rat hepatocytes

DNA single-strand breaks induced by procarcinogens were detected in Chinese hamster ovary (CHO) cells cocultured with adult rat hepatocytes. Freshly isolated adult rat hepatocytes were added to the CHO cell culture prelabeled with [3H]thymidine. After allowing the hepatocytes to attach on or near the CHO cells, aflatoxin B1 or benzo[a]pyrene was added to the culture and incubated for the desired time. DNA single-strand breaks in CHO cells were measured by the alkaline elution technique. Aflatoxin B1 induced some DNA single-strand breaks in CHO cells cultured alone, but in coculture system with hepatocytes the number of DNA single-strand breaks increased greatly. The magnitude of the increase was related to the dose and the time of exposure to aflatoxin B1. Addition of proteinase-K to the cell lysates increased the elution of DNA compared to that of samples without proteinase-K. Benzo[a]pyrene did not induce any DNA single-strand breaks in CHO cells in the absence of liver cells, but a significant number of single-strand breaks were detected in the coculture system.

Hamster hepatocytes in culture as a model for acetaminophen toxicity studies with inhibitors of drug metabolism

A hamster hepatocyte system was developed for use in studying the toxicity of acetaminophen (APAP). The cells were isolated and placed in culture conditions in petri dishes containing a film of collagen. Hepatocytes, after attachment to collagen, were exposed for various periods of time to different concentrations of APAP. Hepatocytes exposed to APAP exhibited concentration- and time-dependent GSH depletion followed by cytoplasmic enzyme leakage and an increase in malondialdehyde content (TBA-reactive material). These effects were reduced by the drug metabolism inhibitors metyrapone, piperonyl butoxide, and dithiocarb. Removal of APAP and its unbound metabolites from cells prior to 1.5 hr followed by culture in drug-free medium resulted in no observable damage to the cells over a 24-hr period. Removal of drug after longer exposure times followed by culture in fresh medium resulted in eventual cell damage. This finding showed that deleterious changes caused by APAP occurred over a 1.5-hr period after which eventual hepatocyte damage could not be reversed by removal of the drug. Further experiments showed that metyrapone and dithiocarb had some protective effect when added after APAP had been completely removed from damaged cells. This result indicates that these agents have a protective action separate from, and in addition to, their ability to inhibit APAP oxidation via cytochromes P-450.

Status of reduced glutathione in primary cultures of rat hepatocytes and the effect on conjugation of benzo[a]pyrene-7,8-dihydrodiol-9,10-oxide

The intracellular level of reduced glutathione (GSH) and GSH conjugation have been investigated in primary cell cultures of hepatocytes isolated from control rats, phenobarbitone (PB) and 3-methylcholanthrene (MC) treated rats. The data demonstrate that in all cell cultures the GSH concentrations show a triphasic pattern: (i) within 1 h of culture an initial marked decrease to 50% of the levels found in fresh hepatocytes; (ii) recovery of GSH concentrations to above the levels observed in fresh cells. This occurs after 6 h in culture with control cells and after 10-24 h with cells from either PB or MC treated rats and was most prominent in cells from PB-treated rats. (iii) A slow decline to between 30 and 40 nmol GSH/mg protein from 24 to 96 h in culture. Synthesis of GSH was slower in cultured cells from PB treated rats and this was confirmed by the resynthesis rates when diethylmaleate (DEM) was used to deplete GSH. The formation of GSH conjugates with racemic 7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) was measured in control cells in suspension and after 3 and 24 h in culture. Despite the decrease in GSH concentrations observed between 1 and 4 h after culture, the conjugation rates were not decreased.

Drug metabolism activities of isolated rat hepatocytes in monolayer culture

The levels of cytochrome P-450 in hepatocytes cultured as monolayers for 22 hrs in Dulbecco's modified Eagle medium supplemented with serum and insulin was reduced to approximately 40% of initial values of freshly isolated hepatocytes. In correspondence with this the activities of the cytochrome P-450 monooxygenases aryl hydrocarbon (benzo(a)pyrene) hydroxylase (AHH) and ethylmorphine (EM) N-demethylase were reduced to 40 and 22% of their initial activities, respectively. Modifying the culture medium through omission of cysteine and cystine, and adding dexamethazone and delta-amino levulinic acid, increased the content of cytochrome P-450 to 59% and EM N-demethylase to 46% of initial values, but was without effect on AHH activity. However, further modifications by adding high concentrations of asparagine and leucine increased AHH activity to 62% of initial values, but did not further enhance the total content of cytochrome P-450 or the EM N-demethylase activity. The activities of cytochrome P-450 reductase, flavin containing monooxygenase, epoxide hydrolase and glutathione S-transferase decreased less (to about 70-80% of initial values) than cytochrome P-450 associated monooxygenase activities, whereas UDP-glucuronyl transferase decreased to about 50% of initial values. In contrast to what was observed regarding cytochrome P-450 and associated monooxygenase activities, modification of the incubation conditions did not affect the non-cytochrome P-450 enzymatic activities.

Establishment of two rat hepatoma cell strains produced by a carcinogen initiation, phenobarbital promotion protocol

Two Fischer 344 rat hepatoma cell strains, JM1 and JM2, have been isolated from a primary hepatocellular carcinoma. Primary tumor formation was induced in a two-thirds partially hepatectomized rat by a single low dose (70 mg/kg of diethylnitrosamine followed by chronic phenobarbital administration (0.1 g/100 ml drinking water). The primary tumors were passed three times by subcutaneous implantation of tumor fragments into the inguinal region of syngeneic recipients. The fourth pass was by injection of tumor cells directly into the livers of recipient rats. Several weeks later, the tumor-containing rat livers were subjected to collagenase perfusion. Two cell lines emerged from tissue culture of the cells isolated by perfusion. Each cell line was cloned by serial dilution. Cells JM1 and JM2 were tumorigenic when injected into syngeneic rats. The tumors, which arose from injected cell strains, exhibited several characteristics of hepatocellular carcinoma. Morphology was examined by light and electron microscopy. Histochemical studies of JM1 and JM2 cells grown in vitro and in vivo were done. The levels of tyrosine aminotransferase and three microsomal enzymes of importance to drug and carcinogen metabolism were investigated. To our knowledge, this is the first report of cell strains derived from an initiation promotion protocol in rats.

Induction of lipogenic enzymes in primary cultures of rat hepatocytes. Relationship between lipogenesis and carbohydrate metabolism

Using primary cultures of adult rat hepatocytes, the regulation of the following lipogenic enzymes was studied: glucose-6-phosphate dehydrogenase, malic enzyme, ATP-citrate lyase, acetyl-CoA carboxylase, fatty acid synthetase, and stearoyl-CoA desaturase. The addition to the culture medium of either insulin or triiodothyronine produced a 2-3-fold increase in each of the individual enzyme activities whereas glucagon slightly decreased enzyme activities. The addition to the medium of 8-bromoguanosine 3,'5'-monophosphate had no effect on any of the enzyme activities unless glucose was also added to the culture medium. Glucose addition alone to the culture medium was without any effect; however, glucose enhanced the stimulation of enzyme activity due to insulin. The addition of fructose or glycerol, even in the absence of insulin, increased the activities of each of the enzymes studied 2-3-fold. The increases in enzyme activity brought about by insulin or fructose were apparently the result of de novo enzyme synthesis, as indicated by the observation that the increases were not noted in the presence of cordycepin or cycloheximide. Immunoprecipitation of ATP-citrate lyase from hepatocytes pulse-labeled with [3H]leucine indicated that the induction of this enzyme in response to the addition of fructose or glycerol to the culture medium was the result of an increase in the rate of synthesis of the enzyme. These results indicate that the activity and synthesis of individual enzymes involved in lipogenesis are increased in response to the metabolism of carbohydrate independently in part from hormonal effects.

Effect of hydrocortisone and nicotinamide on gamma glutamyltransferase in primary cultures of rat hepatocytes

Isolated rat hepatocytes cultured on collagen coated plates exhibit a gradual fetal phenotypic change during time in culture. The fetal liver marker gamma glutamyltransferase (GGT) was used to follow this change. Inasmuch as a significant overgrowth of nonparenchymal liver derived cells is seen frequently in primary cultures of hepatocytes, a technique was utilized that corrects for the presence of nonparenchymal cells. In media supplemented with either hydrocortisone (10(-5) M) or nicotinamide (25 mM) the original epithelial morphology of hepatocytes was preserved for a longer period of time than in unsupplemented media. Hepatocytes in unsupplemented media exhibited an increase in GGT specific activity over time. Hydrocortisone (10(-5) M) induced an increase in GGT activity compared to controls. Nicotinamide (25 mM) inhibited the increase in GGT activity compared to the unsupplemented hepatocytes. Our results indicate that GGT is regulated by hydrocortisone and nicotinamide.

Stimulation of de novo synthesis of cytochrome P-450 by phenobarbital in primary nonproliferating cultures of adult rat hepatocytes

Primary monolayer cultures of nonproliferating parenchymal cells prepared from adult rat liver and maintained in serum-free medium responded to additions of phenobarbital with concentration-dependent increases in synthesis and accumulation of a cytochrome P-450 protein immunochemically and catalytically indistinguishable from that found in the livers of adult rats treated with phenobarbital. Maximal stimulation of the rate of synthesis of this cytochrome protein by phenobarbital, as much as 20-fold higher than in control cultures (1.01% of the rate of synthesis of total cellular protein), could be achieved when the drug was first added to cultures no older than 24 hr and then was maintained in the medium for 96 hr. In addition to phenobarbital, chemicals classified as "phenobarbital-like" inducers in vivo (mephenytoin, mirex, 2,2',4,4',5,5'-hexabromobiphenyl) induced synthesis in culture of this same immunoreactive protein. Supplementation of the medium with 0.1 microM H2SeO3 plus phenobarbital produced an average 2-fold enhancement in the rate of synthesis of this inducible cytochrome protein as compared to that in cultures receiving phenobarbital alone. Inasmuch as there was a decline in selenium content and in the activity of the seleno-enzyme glutathione peroxidase in hepatocyte cultures maintained in standard culture medium for more than 24 hr, the added selenium appears to correct a spontaneously acquired cellular deficiency in selenium. Contrary to the concept that liver cells placed in culture promptly dedifferentiate with general loss of specialized functions such as cytochrome P-450, our data demonstrate that expression of the phenobarbital-inducible form of cytochrome P-450 is not extinguished in culture, but rather it is masked transiently and is attenuated as the cells adapt to the imperfect conditions of the culture environment.