Dependency of the in vitro stabilization of differentiated functions in liver parenchymal cells on the type of cell line used for co-culture

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.

Is hepatitis B-virucidal validation of biocides possible with the use of surrogates?

The hepatitis B virus (HBV) is considered to be a major public health problem worldwide, and a significant number of reports on nosocomial outbreaks of HBV infections have been reported. Prevention of indirect HBV transmission by contaminated objects is only possible through the use of infection-control principles, including the use of chemical biocides, which are proven to render the virus non-infectious. The virucidal activity of biocides against HBV cannot be predicted; therefore, validation of the virucidal action of disinfectants against HBV is essential. However, feasible HBV infectivity assays have not yet been established. Thus, surrogate models have been proposed for testing the efficacy of biocides against HBV. Most of these assays do not correlate with HBV infectivity. Currently, the most promising and feasible assay is the use of the taxonomically related duck hepatitis B virus (DHBV), which belongs to the same Hepadnaviridae virus family. This paper reviews the application of DHBV, which can be propagated in vitro in primary duck embryonic hepatocytes, for the testing of biocides and describes why this model can be used as reliable method to evaluate disinfectants for efficacy against HBV. The susceptibility levels of important biocides, which are often used as ingredients for commercially available disinfectants, are also described.

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.

In vitro cultivation and cryopreservation of duck embryonic hepatocytes

Hepatitis B-virucidal testing of biocides in quantitative suspension tests using duck hepatitis B virus (DHBV) requires primary duck embryonic hepatocytes for viral propagation. To improve the test system and availability of these cells, commercial culture plates with different growth surfaces were tested for cell cultivation and different approaches for cryopreservation of hepatocyte suspension were examined. After 12 days of culture, the largest amounts of hepatocytes were grown in CellBIND and TTP plates and CellBIND surface showed the lowest tendency of monolayer detachment nearly comparable with collagen 1-coated CELLCOAT plates. For cryopreservation of hepatocyte suspension, the use of growth medium supplemented with fetal calf serum (FCS) and dimethyl sulfoxide (ME(2)SO), FCS supplemented with ME(2)SO or cryosafe-1 as cryoprotective agents provided the highest rates of surviving cells after thawing. The freezing-thawing process did not significantly reduce the susceptibility of hepatocytes to infection with DHBV. In conclusion, plates without collagen 1 such as CellBIND are recommended for cultivation of primary duck embryonic hepatocytes in infectivity experiments of DHBV for virucidal testing of biocides. The use of cryopreserved hepatocytes is possible when freshly isolated cells from the liver of duck embryos are not available.

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

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

Exploring interactions between rat hepatocytes and nonparenchymal cells using gene expression profiling

Cocultivation of primary hepatocytes with a plethora of nonparenchymal cells (from within and outside the liver) has been shown to support hepatic functions in vitro. Despite significant investigation into this phenomenon, the molecular mechanism underlying epithelial-nonparenchymal interactions in hepatocyte cocultures remains poorly understood. In this study, we present a functional genomic approach utilizing gene expression profiling to isolate molecular mediators potentially involved in induction of liver-specific functions by nonparenchymal cells. Specifically, primary rat hepatocytes were cocultivated with closely related murine fibroblast cell types (3T3-J2, NIH-3T3, mouse embryonic fibroblasts) to allow their classification as "high," "medium," or "low" inducers of hepatic functions. These functional responses were correlated with fibroblast gene expression profiles obtained using Affymetrix GeneChips. Microarray data analysis provided us with 17 functionally characterized candidate genes in the cell communication category (cell surface, extracellular matrix, secreted factors) that may be involved in induction of hepatic functions. Further analysis using various databases (i.e., PubMed, GenBank) facilitated prioritization of the candidates for functional characterization. We experimentally validated the potential role of two candidates in our coculture model. The cell surface protein, neural cadherin (N-cadherin), was localized to hepatocyte-fibroblast junctions, while adsorbed decorin up-regulated hepatic functions in pure cultures as well as cocultures with low-inducing fibroblasts. In the future, identifying mediators of hepatocyte differentiation may have implications for both fundamental hepatology and cell-based therapies (e.g., bioartificial liver devices). In conclusion, the functional genomic approach presented in this study may be utilized to investigate mechanisms of cell-cell interaction in a variety of tissues and disease states.

Requirement of 3D extracellular network for maintenance of mature hepatocyte morphology and suppression of alpha-fetoprotein synthesis in vitro

It has been previously shown that adult mouse hepatocytes when co-cultured with rat liver-derived cell lines IAR-2 and IAR-20 can form organotypic hepatocyte islands consisting of cuboidal cells with expressed cell polarity and domain-specific localization of plasma-membrane proteins. The synthesis of alpha-fetoprotein, a fetal-specific antigen, was almost completely suppressed in these islands. It was noticed that organotypic islands were surrounded by fibrils and often covered with a roof of supporting cells. Here we demonstrate that the formation of adult mouse hepatocyte islands in co-cultures depends on the creation of 3D ECM scaffold formed by co-operative activity of hepatocytes and supporting partners. Noteworthy, the collagen type I network was observed exclusively around organotypic islands with complete suppression of alpha-fetoprotein synthesis. In contrast to non-transformed cells, IAR-2 cells, transformed either spontaneously or by transfection with mutant human N-ras(Asn12) cDNA, were completely unable to support in co-culture the differentiated morphology of hepatocytes and to suppress AFP synthesis. Transformed IAR-2 cells were able to produce most of the main ECM components except collagen type I, but failed to form extracellular fibrils both in pure culture and in co-culture with hepatocytes. Together, the data show that 3D ECM scaffold (possibly based on collagen type I) plays an important role in maintaining the differentiated morphology of hepatocytes and suppression of alpha-fetoprotein synthesis in vitro.

Influence of culture system and medium enrichment on sulfotransferase and sulfatase expression in male rat hepatocyte cultures

The expression of sulfotransferase and steroid sulfatase was studied in rat liver using the most promising culture models of hepatocytes, including monolayer culture with a pyruvate (30 mM) enriched medium, co-culture with rat epithelial cells from primitive biliary origin and collagengel sandwich culture. In the latter, addition of dexamethasone (1 microM) to the medium was examined. Phenol sulfotransferase enzymes (SULT1) were studied by measuring activities towards 4-methylphenol and estradiol, hydroxysteroid sulfotransferase (SULT2A) activity was determined towards dehydroepiandrosterone (DHEA). Microsomal steroid sulfatase activity was measured towards estrone sulfate. Western blot analysis was carried out using polyclonal antibodies raised against rat phenol sulfotransferase SULT1A1 (ASTIV), estrogen sulfotransferase SULT1E1 (EST) and hydroxysteroid sulfotransferase (HST). SULT2A activity towards DHEA was maintained at a high level during the whole culture time. In the co-culture it even reached the level of freshly isolated cells. Addition of pyruvate had no positive effect on the activity measured in monolayer cultures. High SULT1A1 activity towards 4-methylphenol was found in the co-culture system. In the monolayer culture, the activity initially decreased with 35% but was then kept at a constant level, while in the sandwich culture low activities were measured. For dexamethasone, an inducing effect on the various SULT activities could not be detected. Independently of the culture model used, the SULT1E1 activity towards estradiol decreased to 20% and 5% of the initial activity after four and seven days of culture, respectively. Microsomal steroid sulfatase activity was best maintained in collagengel sandwich cultures. During the first four days in culture it retained 73% of the initial activity, afterwards it decreased to 40% of the activity found in freshly isolated hepatocytes, irrespective of the culture conditions. High expectations exist for collagengel sandwich cultures, however, in our study the results were rather disappointing. Monolayer is a suitable culture model for short-term purposes. For long-term in vitro biotransformation studies, co-culture is preferred but is rather complex.

Elevated expression of hormone-regulated rat hepatocyte functions in a new serum-free hepatocyte-stromal cell coculture model

The specific performance of the adult hepatic parenchymal cell is maintained and controlled by factors deriving from the stromal bed; the chemical nature of these factors is unknown. This study aimed to develop a serum-free hierarchical hepatocyte-nonparenchymal (stromal) cell coculture system. Hepatic stromal cells proliferated on crosslinked collagen in serum-free medium with epidermal growth factor, basic fibroblast growth factor, and hepatocyte-conditioned medium; cell type composition changed during the 2-wk culture period. During the first wk, the culture consisted of proliferating sinusoidal endothelial cells with well-preserved sieve plates, proliferating hepatic stellate cells, and partially activated Kupffer cells. The number of endothelial cells declined thereafter; stellate cells and Kupffer cells became the prominent cell types after 8 d. Hepatocytes were seeded onto stromal cells precultured for 4-14 d; they adhered to stellate and Kupffer cells, but spared the islands of endothelial cells. Stellate cells spread out on top of the hepatocytes; Kupffer cell extensions established multiple contacts to hepatocytes and stellate cells. Hepatocyte viability was maintained by coculture; the positive influence of stromal cell signals on hepatocyte differentiation became evident after 48 h; a strong improvement of cell responsiveness toward hormones could be observed in cocultured hepatocytes. Hierarchial hepatocyte coculture enhanced the glucagon-dependent increases in phosphoenolpyruvate carboxykinase activity and messenger ribonucleic acid (mRNA) content three- and twofold, respectively; glucagon-activated urea production was elevated twofold. Coculturing also stimulated glycogen deposition; basal synthesis was increased by 30% and the responsiveness toward insulin and glucose was elevated by 100 and 55%, respectively. The insulin-dependent rise in the glucokinase mRNA content was increased twofold in cocultured hepatocytes. It can be concluded that long-term signals from stromal cells maintain hepatocyte differentiation. This coculture model should, therefore, provide the technical basis for the investigation of stroma-derived differentiation factors.

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.

Multicellular spheroids: a three-dimensional in vitro culture system to study tumour biology

The growth of tumour cells as three-dimensional multicellular spheroids in vitro has led to important insights in tumour biology, since properties of the in vivo-tumour such as proliferation or nutrient gradients, can be studied under controlled conditions. While this review starts with an update of recent data on spheroid monocultures, especially concerning tumour microenvironment and therapeutic modalities, the main emphasis is put on the spectrum of heterologous cultures which have evolved in previous years. This type of culture includes tumour cell interaction with endothelial, fibroblast or immunocompetent cells. The relation of the spheroid culture model to other types of three-dimensional culture and our critical evaluation and presentation of the technical aspects of growing and analysing spheroids are included in the text. These topics are chosen to help the experimental pathologist design experiments with tumour spheroids and to stimulate discussion.

Viability, attachment efficiency, and xenobiotic metabolizing enzyme activities are well maintained in EDTA isolated rat liver parenchymal cells after hypothermic preservation for up to 3 days in University of Wisconsin solution

Rat liver parenchymal cells were isolated by EDTA perfusion and were subsequently purified by Percoll centrifugation. The freshly isolated liver cells had a mean viability of 95% as judged by trypan blue exclusion. Isolated liver parenchymal cells were then stored at 0 degrees C for up to 1 wk in University of Wisconsin solution (UW). During this hypothermic preservation, the viability was only slightly reduced to 92% after 1 d and to 85% after 3 d at 0 degrees C. Thereafter, the viability decreased rapidly. After cold storage for up to 3 d, it was possible to use the parenchymal liver cells either in short-term suspension or in cell culture. The attachment efficiency in cell culture was the same for freshly isolated liver cells (84%) and after 2 d cold preservation (81%). The cytochrome P450 content and the enzyme activities of soluble epoxide hydrolase, UDP-glucuronosyl transferase, phenol sulfotransferase, and glutathione S-transferase were not significantly different between freshly isolated cells and cells after 3 d of hypothermic preservation. Furthermore, freshly isolated and intact liver cells stored for 3 d were used in the cell-mediated Salmonella mutagenicity test as a metabolizing system. Both fresh and stored liver parenchymal cells metabolized benzo(a)pyrene,2-aminoanthracene, and cyclophosphamide to their ultimate mutagens.(ABSTRACT TRUNCATED AT 250 WORDS)

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).

Preservation of the rate and profile of xenobiotic metabolism in rat hepatocytes stored in liquid nitrogen

A simple procedure for cryopreservation of rat hepatocytes that allows recovery of viable cells retaining activities of phase I and phase II drug metabolism equivalent to freshly isolated cells is described. The cooling process was initiated 30 min after incubation of freshly isolated hepatocytes at 37 degrees in Krebs-Ringer bicarbonate buffer containing 15 mM glucose to allow for metabolic equilibration. At the end of this period, hepatocyte suspensions were supplemented with 1.7% albumin, 13.3% dimethyl sulfoxide, and the synthetic buffers, 3-[N-morpholino]propanesulfonic acid (MOPS) and N-[2-hydroxyethyl]piperazine-N'-[2-ethanesulfonic acid] (HEPES). Hepatocytes were cooled in a stepwise manner to -196 degrees by holding the cells for 1 hr at -20 degrees and then for 1 hr at -70 degrees before transfer into liquid nitrogen. After thawing and removal of damaged cells by centrifugation in Percoll, the total recovery of viable hepatocytes subjected to freezing was about 42%. The contents of ATP, ADP, and AMP were not altered significantly in cells stored in liquid nitrogen. The metabolic competence of cryopreserved hepatocytes was further confirmed by their ability to synthesize urea from NH4Cl and ornithine at the same high rate that was observed in freshly isolated cells (693 +/- 68 and 740 +/- 68 nmol.mg dry wt-1 x hr-1, respectively). Similarly, cryopreservation did not affect drug-metabolizing systems as indicated by the metabolism of benzo[a]pyrene and 7-ethoxycoumarin, two model substrates. In both freshly isolated and cryopreserved hepatocytes, 7-ethoxycoumarin was O-deethylated to 7-hydroxycoumarin at essentially the same rates (8.66 +/- 0.75 and 8.25 +/- 0.53 nmol.mg dry wt-1.hr-1, respectively) and 7-hydroxycoumarin accumulated in hepatocyte suspensions almost exclusively in the conjugated form. The storage of hepatocytes in liquid nitrogen also did not affect the complex metabolism of benzo[a]pyrene to total oxygenated metabolites and, more importantly, to metabolites conjugated with glutathione, glucuronic acid, and sulfuric acid. Thus, cryopreserved hepatocytes represent a valid and convenient model to study drug biotransformation in intact cells.

Xenobiotic-metabolizing enzyme activities in hybrid cell lines established by fusion of primary rat liver parenchymal cells with hepatoma cells

1. The activities of xenobiotic-metabolizing enzymes were determined in hybrid cell lines (hepatocytoma, HPCT) which have been established by fusion of liver parenchymal cells from adult rat (PC) with cells from a Reuber hepatoma cell line (FAO). 2. Cytochrome P450 was not measurable spectrophotometrically in FAO and HPCT. P450-dependent conversion of testosterone was below the detection limit in FAO and only marginally present in HPCT. 3. Microsomal and cytosolic epoxide hydrolase, glutathione S-transferase and phenol sulphotranserase were low or even below detection limit in FAO. These enzyme activities were significantly higher in HPCT and correspond to about 1-10% the activities measured in PC. 4. 1-Naphthol UPD-glucuronosyl transferase activity was about 20% in FAO and about 100% in HPCT compared to PC. 5. Metabolic conversion of benzo[a]pyrene was low in FAO, high in PC, and intermediate in HPCT. The presented data, however, do not allow the conclusion whether this intermediate rate is catalyzed by similar P450 isoenzymes as in PC. 6. Due to the easily measurable phase II-metabolizing enzyme activities HPCT may, however, be useful for in vitro enzyme induction or repression studies.

Characterization of cryopreserved rat liver parenchymal cells by metabolism of diagnostic substrates and activities of related enzymes

The metabolism of testosterone and benzo(a)pyrene (BaP) which is mediated by diverse enzymes was determined in cryopreserved rat liver parenchymal cells and compared with that found in freshly isolated cells. In addition, the activities of single xenobiotic-metabolizing enzymes were measured by using specific substrates. The cytochrome P450 (P450)-mediated total metabolic conversion of testosterone was reduced to 55% in cryopreserved cells. The metabolite profile, i.e. the formation of single metabolites compared with total metabolic conversion, was however unchanged when compared with freshly isolated cells. A concomitant reduction in the activities of the involved P450 isoenzymes can therefore be postulated. The amount of detected phase I-metabolites of BaP was unaffected by the cryopreservation method. The formation of phase II-metabolites and total metabolic conversion of BaP in cryopreserved cells was however reduced to about 50-60%. The reduced glutathione S-transferase and more obviously phenol sulfotransferase activities measured in cryopreserved cells, may explain the impaired conjugation of BaP. The ratio between phase I- and phase II-metabolites was thus changed by cryopreservation. Density separation on Percoll yielded cryopreserved cells with a viability and metabolic capacity not measurably different from freshly isolated cells. To this extent, cryopreserved, Percoll-purified liver parenchymal cells are a useful in vitro system for drug metabolism studies. However due to the extensive loss in cell number during this procedure (recovery = 22% of freshly isolated cells) the application of this system is limited.