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

Cryopreservation of Rat and Monkey Liver Slices

A method for the long-term storage of liver slices by cryopreservation was developed. The viability of liver slices was determined by analysing the leakage of alanine aminotransferase, urea production, and the metabolism of testosterone. Rat liver slices were found to be optimally cryopreserved by exposure for 30 minutes to 12% dimethyl sulphoxide (v/v) at 2°C before freezing. Subsequent direct immersion in liquid nitrogen was more effective than a cooling rate of ±1°C/mmute, which reduced viability. Storage at a temperature of -80°C lowered viability compared to storage at -196°C.

These conditions for optimal cryopreservation were used to cryopreserve rat, rhesus monkey and cynomolgus monkey liver slices. The viability of these liver slices was maintained at: 74%, 86% and 85%, respectively, when alanine aminotransferase content was measured; 80%, 109% and 82%, respectively, when urea production was measured; and 109%, 60% and 85%, respectively, when the metabolism of testosterone was measured. Viability was maintained for at least one month. The results show that, by using the method presented here, liver slices from these species can be stored while maintaining viabilities similar to initial values. This method will facilitate the optimal use of liver slices and reduce the number of experimental animals used.

Impact of Percoll purification on isolation of primary human hepatocytes

Research and therapeutic applications create a high demand for primary human hepatocytes. The limiting factor for their utilization is the availability of metabolically active hepatocytes in large quantities. Centrifugation through Percoll, which is commonly performed during hepatocyte isolation, has so far not been systematically evaluated in the scientific literature. 27 hepatocyte isolations were performed using a two-step perfusion technique on tissue obtained from partial liver resections. Cells were seeded with or without having undergone the centrifugation step through 25% Percoll. Cell yield, function, purity, viability and rate of bacterial contamination were assessed over a period of 6 days. Viable yield without Percoll purification was 42.4 × 10⁶ (SEM ± 4.6 × 10⁶) cells/g tissue. An average of 59% of cells were recovered after Percoll treatment. There were neither significant differences in the functional performance of cells, nor regarding presence of non-parenchymal liver cells. In five cases with initial viability of <80%, viability was significantly increased by Percoll purification (71.6 to 87.7%, p = 0.03). Considering our data and the massive cell loss due to Percoll purification, we suggest that this step can be omitted if the initial viability is high, whereas low viabilities can be improved by Percoll centrifugation.

Long-Term Stability of Cryopreserved Human Hepatocytes: Evaluation of Phase I and II Drug-Metabolizing Enzyme Activities and CYP3A4/5 Induction for More than a Decade

We evaluated the long-term stability of hepatocytes stored in the vapor phase of liquid nitrogen for their viability, cytochrome P450 (CYP) 1A2 activity, CYP3A4/5 activity, uridine diphosphate-glucuronosyl transferase (UGT) activity, sulfotransferase (SULT) activity, and CYP3A4/5 induction during 14 years of preservation. No substantial degradation of viability, CYP1A2 activity, UGT activity, or CYP3A4/5 induction was observed. CYP3A4/5 activity showed a slight decrease after 7 years of storage, and SULT activity gradually decreased during storage, although substantial activities remained even after 14 years. These results indicate that cryopreserved human hepatocytes can be stored stably for more than a decade with little or no change in viability, activity of drug-metabolizing enzymes, or CYP3A4/5 induction, and can be widely applicable to qualitative research in drug metabolism.

Equine hepatocytes: isolation, cryopreservation, and applications to in vitro drug metabolism studies

Despite reports of the successful isolation of primary equine hepatocytes, there are no published data regarding the successful cryopreservation of these isolated cells. In this study, a detailed description of the procedures for isolation, cryopreservation, and recovery of equine hepatocytes are presented. Furthermore, the intrinsic clearance (Cl_int) and production of metabolites for three drugs were compared between freshly isolated and recovered cryopreserved hepatocytes. Primary equine hepatocytes were isolated using a two‐step collagenase perfusion method, with an average cell yield of 2.47 ± 2.62 × 10⁶ cells/g of perfused liver tissue and viability of 84.1 ± 2.62%. These cells were cryopreserved with William's medium E containing 10% fetal bovine serum with 10% DMSO. The viability of recovered cells, after a 30% Percoll gradient, was 77 ± 11% and estimated recovery rate was approximately 27%. These purified cells were used to determine the in vitro Cl_int of three drugs used in equine medicine; omeprazole, flunixin, and phenylbutazone, via the substrate depletion method. Cryopreserved suspensions gave a comparable estimation of Cl_int compared to fresh cells for these three drugs as well as producing the same metabolites. This work paves the way for establishing a bank of cryopreserved equine hepatocytes that can be used for estimating pharmacokinetic parameters such as the hepatic metabolic in vivo clearance of a drug as well as producing horse‐specific drug metabolites.

Cryopreserved rat, dog and monkey hepatocytes: measurement of drug metabolizing enzymes in suspensions and cultures

Metabolism in fresh and cryopreserved (CP) rat, dog and monkey hepatocyte suspensions and cultures was measured using midazolam (CYP3A), tolbutamide (CYP2C), dextromethorphan (CYP2D) and p-nitrophenol (glucuronosyl S-transferases (UGT), sulphotransferases (ST)).

CYP3A, CYP2C9, CYP2D6, UGT and ST enzyme functions in fresh and CP rat, dog and monkey hepatocyte suspensions were retained - CP rat hepatocytes lost some CYP2C activity but this was restored by adding NADPH or by placing the cells in culture, suggesting that the enzyme was still functional. Phase 2 activities were equivalent in fresh and CP hepatocyte suspensions. In some cases, incubation conditions increased the rate of metabolism, possibly reflecting de novo cofactor synthesis. However, this effect was substrate and species dependent and was not always the same in fresh and CP cells. CYP3A, CYP2C, CYP2D, UGT and ST activities at 24 hours of culture of rat and monkey hepatocytes were not compromised by cryopreservation. CYP3A, CYP2D but not CYP2C were lower in 24-hour cultures of CP dog hepatocytes than in fresh cells. Despite being lower than fresh cells, UGT activity in dog CP hepatocytes did not decrease from 0 to 24 hours of culture. Speciesspecific metabolism of p-nitrophenol could be demonstrated in both CP cell suspensions and cultures.

In conclusion, these data suggest that the enzyme characteristics of fresh and CP hepatocytes from each species and under specific incubation conditions should be considered when carrying out metabolism studies of new compounds.

Ethanol-induced impairment in the biosynthesis of N-linked glycosylation

Deficiency in N-linked protein glycosylation is a long-known characteristic of alcoholic liver disease and congenital disorders of glycosylation. Previous investigations of ethanol-induced glycosylation deficiency demonstrated perturbations in the early steps of substrate synthesis and in the final steps of capping N-linked glycans in the Golgi. The significance of the biosynthesis of N-glycan precursors in the endoplasmic reticulum, however, has not yet been addressed in alcoholic liver disease. Ethanol-metabolizing hepatoma cells were treated with increasing concentrations of ethanol. Transcript analysis of genes involved in the biosynthesis of N-glycans, activity assays of related enzymes, dolichol-phosphate quantification, and analysis of dolichol-linked oligosaccharides were performed. Upon treatment of cells with ethanol, we found a decrease in the final N-glycan precursor Dol-PP-GlcNAc(2) Man(9) Glc(3) and in C95- and C100-dolichol-phosphate levels. Transcript analysis of genes involved in N-glycosylation showed a 17% decrease in expression levels of DPM1, a subunit of the dolichol-phosphate-mannose synthase, and an 8% increase in RPN2, a subunit of the oligosaccharyl transferase. Ethanol treatment decreases the biosynthesis of dolichol-phosphate. Consequently, the formation of N-glycan precursors is affected, resulting in an aberrant precursor assembly. Messenger RNA levels of genes involved in N-glycan biosynthesis are slightly affected by ethanol treatment, indicating that the assembly of N-glycan precursors is not regulated at the transcriptional level. This study confirms that ethanol impairs N-linked glycosylation by affecting dolichol biosynthesis leading to impaired dolichol-linked oligosaccharide assembly. Together our data help to explain the underglycosylation phenotype observed in alcoholic liver disease and congenital disorders of glycosylation.

Liver slices in in vitro pharmacotoxicology with special reference to the use of human liver tissue

In the early years of research in in vitro pharmacotoxicology liver slices have been used. After a decline in the application of slices in favour of the use of isolated hepatocytes and the isolated perfused liver preparation, the development of the Krumdieck slicer in the 1980s led to a ;comeback' of the technique. This review will focus on the use of human liver, with special reference to the comparison of slices with isolated hepatocytes in in vitro pharmacotoxicology. In addition, an overview on the predictive value of these in vitro systems for drug disposition and toxicity in vivo will be given. Preservation techniques for liver slices and hepatocytes will also be discussed. These techniques ensure an efficient utilization of the scarce human material. For long-term storage of liver slices and hepatocytes, cryopreservation seems most promising. However, cryopreservation is still in its infancy, and reports mainly deal with drug metabolism studies after cryopreservation. Drug toxicity, metabolism and transport data determined in slices and isolated hepatocytes, from both human and animal liver showed good correlation with the corresponding parameters measured in vivo. Therefore, the results obtained in such studies may give rise to more in-depth research on the mechanisms of pharmactoxicology in the human liver.

The effects of intramuscular and intraperitoneal injections of benzo[a]pyrene on selected biomarkers in Clarias gariepinus

This study investigated the dose-dependent and time-course effects of intramuscular (i.m.) and intraperitoneal (i.p.) injection of benzo[a]pyrene (BaP) on the biomarkers EROD activity, GST activity, concentrations of BaP metabolites in bile, and visceral fat deposits (Lipid Somatic Index, LSI) in African catfish (Clarias gariepinus). Intraperitoneal injection resulted in 4.5 times higher accumulation of total selected biliary FACs than i.m. injection. Hepatic GST activities were inhibited by BaP via both injection methods. Dose-response relationships between BaP injection and both biliary FAC concentrations and hepatic GST activities were linear in the i.p. injected group but nonlinear in the i.m. injected fish. Hepatic EROD activity and LSI were not significantly affected by BaP exposure by either injection route. We conclude that i.p. is a more effective route of exposure than i.m. for future ecotoxicological studies of PAH exposure in C. gariepinus.

Optimization of the cryopreservation and thawing protocol for human hepatocytes for use in cell transplantation

Cryopreservation of human hepatocytes is important for their use in hepatocyte transplantation. On thawing, cryopreserved hepatocytes often have reduced viability and metabolic function in comparison with fresh cells. The aim of this study was to modify the different steps in the standard cryopreservation procedure in an attempt to improve the overall outcome. Human hepatocytes with a viability of 69% +/- SD 16% were isolated from donor livers with a collagenase perfusion technique. Different cell densities, concentrations, rates, and methods of addition of dimethyl sulfoxide were tested for the freezing solution. Modified controlled-rate freezer programs were tested to obtain a linear decrease in the temperature. Once they were frozen, the storage time and thawing method for hepatocytes were investigated. The effects on thawed cell viability and attachment, lactate dehydrogenase release, cytochrome P450 1A1/2 activity, and albumin synthesis were determined. The results were used to produce an improved cryopreservation protocol suitable for good manufacturing practice conditions. With a cell density of 10(7) cells/mL in University of Wisconsin solution containing 300 mM glucose, 10% (vol/vol) dimethyl sulfoxide was added dropwise over 5 minutes, and was immediately frozen. Thawing was done rapidly at 37 degrees C, and dilution was performed with Eagle's minimum essential medium containing 300 mM glucose and 4% human serum albumin. Hepatocytes could be stored at -140 degrees C without significant further loss of function for up to 3 years. With this protocol, hepatocytes had a viability of 52% +/- 9%, an attachment efficiency of 48% +/- 8%, and lactate dehydrogenase leakage of 17% +/- 4%. This protocol is currently in use to cryopreserve hepatocytes for use in cell transplantation at our center.

Hepatocyte cryopreservation: Is it time to change the strategy?

Liver cell transplantation presents clinical benefit in patients with inborn errors of metabolism as an alternative, or at least as a bridge, to orthotopic liver transplantation. The success of such a therapeutic approach remains limited by the quality of the transplanted cells. Cryopreservation remains the best option for long-term storage of hepatocytes, providing a permanent and sufficient cell supply. However, isolated adult hepatocytes are poorly resistant to such a process, with a significant alteration both at the morphological and functional levels. Hence, the aim of the current review is to discuss the state of the art regarding widely-used hepatocyte cryopreservation protocols, as well as the assays performed to analyse the post-thawing cell quality both in vitro and in vivo. The majority of studies agree upon the poor quality and efficiency of cryopreserved/thawed hepatocytes as compared to freshly isolated hepatocytes. Intracellular ice formation or exposure to hyperosmotic solutions remains the main phenomenon of cryopreservation process, and its effects on cell quality and cell death induction will be discussed. The increased knowledge and understanding of the cryopreservation process will lead to research strategies to improve the viability and the quality of the cell suspensions after thawing. Such strategies, such as vitrification, will be discussed with respect to their potential to significantly improve the quality of cell suspensions dedicated to liver cell-based therapies.

Evolving concepts in liver tissue modeling and implications for in vitro toxicology

The development of human cell models stably expressing functional properties of the in vivo cells they are derived from for predicting toxicity of chemicals is a major challenge. For mimicking the liver, a major target of toxic chemicals, primary hepatocytes represent the most pertinent model. Their use is limited by interdonor functional variability and early phenotypic changes although their lifespan can be extended not only by culturing in a 2D dimension under sophisticated conditions but also by the use of synthetic and natural scaffolds as 3D supporting templates that allow cells to have a more stable microenvironment. Hepatocytes derived from stem cells could be the most appropriate alternative but up to now only liver progenitors/hepatoblasts are obtained in vitro. A few hepatocyte cell lines have retained a variable set of liver-specific functions. Among them are the human hepatoma HepaRG cells that express drug metabolism capacity at levels close to those found in primary hepatocytes making them a suitable model for both acute and chronic toxicity studies. New screening strategies are now proposed based on miniaturized and automated systems; they include the use of microfluidic chips and cell chips coupled with high content imaging analysis. Toxicogenomics technologies (particularly toxicotranscriptomics) have emerged as promising in vitro approaches for better identification and discrimination of cellular responses to chemicals. They should allow to discriminate compounds on the basis of the identification of a set of markers and/specific signaling pathways.

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.

Long-term maintenance of the drug transport activity in cryopreservation of microencapsulated rat hepatocytes

Transplantation of isolated hepatocytes has been proposed to compensate for essential functions lacking in liver failure or for genetic defects that alter a specific liver metabolic pathway. Hepatocyte utilization for these purposes would be facilitated with a reliable, reproducible, and effective method of long-term hepatocyte storage. We have recently developed a simple new system for cryopreservation of hepatocytes that encapsulates alginate microspheres and maintains liver-specific function. The aim of this study was to elucidate the transport and drug-metabolizing enzyme activities of cryopreserved microencapsulated hepatocytes stored for a long time. Morphological examinations showed there is no apparent injury of the hepatocytes during cryopreservation processes. A drug-metabolizing enzyme (testosterone 6beta-hydroxylase, a specific probe for CYP3A2) and drug transport activities [salicylate, allopurinol, and prostaglandin E2 (PGE2), typical substrates of rOat2] in cryopreserved microencapsulated hepatocytes were maintained up to 120 days. Our results thus demonstrate for the first time that cryopreservation of primary rat hepatocytes by the encapsulation technique allows long-term retention of drug metabolism and drug transport activities.

Characterization of rat and human Kupffer cells after cryopreservation

Kupffer cells (KC) are the resident macrophages of the liver and represent about 80% of the total fixed macrophage population. They are involved in disease states such as endotoxin shock, alcoholic liver diseases and other toxic-induced liver injury. They release physiologically active substances such as eicosanoids and inflammatory cytokines (IL-1, IL-6, TNFalpha), and produce free radical species. Thus, KC are attractive targets for anti-inflammatory therapies and potential candidates responsible for differences in inflammation in liver disease seen between different individuals. However, to perform parallel in vitro experiments with KC from different donors a suitable method for conservation of KC would be necessary. Therefore, the present study evaluated, whether rat and human KC can be frozen, stored and recovered without losing their functional integrity. Rat and human KC were isolated and either cultured under standard conditions (fresh KC) or cryopreserved in special freezing medium (cryopreserved KC). At least 24 h later, cryopreserved KC were thawed, brought into suspension and seeded in the same density as fresh cells for subsequent experiments. Viability of cultured KC was analyzed by trypan blue exclusion. LPS (or PBS as control) stimulation was performed at different time points and cytokine release was analyzed with IL-6 and TNFalpha ELISAs, respectively. Phagocytic capacity was investigated by using a specific phagocytosis assay and FACS analysis. The recovery rate after thawing was around 57% for rat and around 65% for human cryopreserved KC. The results indicate, that KC can successfully be cryopreserved with an adequate recovery rate of viable cells. The properties of fresh and frozen KC can also be compared after thawing. Freshly isolated and cryopreserved cultured KC showed near-normal morphology and did not differ in the cultivation profiles over a period of 72 h. One to three days after seeding, frozen rat or human KC also retained inducible functions such as the production of TNFalpha or IL-6 after LPS challenge. Finally, regardless if they were cryopreserved or not, no differences in the phagocytic activities of the cells were obtained. Taken together, it is concluded that cryopreservation of KC does not change the physiological characteristics of the cells in vitro. Therefore, the method used here for cryopreservation of especially human KC allows the accumulation of KC from several donors for parallel in vitro experiments.

Indomethacin increases liver damage in a murine model of liver injury from alpha-1-antitrypsin deficiency

Homozygous (PIZZ) alpha-1-antitrypsin (alpha(1)-AT) deficiency is associated with the development of liver damage in children as well as chronic liver injury and hepatocellular carcinoma in adults. The alpha(1)-AT mutant Z gene encodes a mutant protein that accumulates in the endoplasmic reticulum of hepatocytes rather than being secreted appropriately into serum. Liver injury is caused by the accumulation of alpha(1)-AT mutant Z protein in hepatocytes, which triggers downstream intracellular injury pathways. However, development of clinical liver disease among PIZZ homozygotes is highly variable, suggesting other genetic or environmental factors contribute to liver injury. In this study, we tested whether nonsteroidal anti-inflammatory drugs (NSAIDs) could be a comorbid factor in the development of liver injury in alpha(1)-AT deficiency using the PiZ mouse. This mouse model is transgenic for the mutant Z allele of the human alpha(1)-AT gene, in which alpha(1)-ATZ expression is regulated by the human promoter regulatory sequences. Our results showed that administration of indomethacin to PiZ mice resulted in increased hepatic injury, indicated by increased hepatocellular proliferation and increased activation of caspase 9. This indomethacin-induced injury was associated with activation of IL-6-STAT3 signaling, increased expression of alpha(1)-AT mRNA, and greater accumulation of mutant polymerized alpha(1)-ATZ protein in livers of indomethacin-treated PiZ mice compared to vehicle-treated PiZ animals. In conclusion, environmental factors, such as exogenous medication administration, can significantly potentiate the liver injury associated with alpha(1)-ATZ hepatic accumulation; NSAIDs may be especially injurious to patients with alpha(1)-AT deficiency, possibly by increasing the expression and accumulation of the hepatotoxic mutant protein.

Cryopreservation of isolated human hepatocytes for transplantation: State of the art

Hepatocytes isolated from unused donor livers are being used for transplantation in patients with acute liver failure and liver-based metabolic defects. As large numbers of hepatocytes can be prepared from a single liver and hepatocytes need to be available for emergency and repeated treatment of patients it is essential to be able to cryopreserve and store cells with good thawed cell function. This review considers the current status of cryopreservation of human hepatocytes discussing the different stages involved in the process. These include pre-treatment of cells, freezing solution, cryoprotectants and freezing and thawing protocols. There are detrimental effects of cryopreservation on hepatocyte structure and metabolic function, including cell attachment, which is important to the engraftment of transplanted cells in the liver. Cryopreserved human hepatocytes have been successfully used in clinical transplantation, with evidence of replacement of missing function. Further optimisation of hepatocyte cryopreservation protocols is important for their use in hepatocyte transplantation.

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

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

Cryopreservation of viable hepatocyte monolayers in cryoprotectant media with high serum content: metabolism of testosterone and kaempherol post-cryopreservation

Little work in the literature focuses on the cryopreservation of primary hepatocytes as monolayer cultures, yet this technique offers many distinct advantages over other cryopreservation systems, including high recovery, high post-thaw nutrient penetration, and low numbers of trapped dead cells. This article investigates the cryopreservation of primary rat hepatocytes at -78 degrees C attached as monolayers to collagen coated culture dishes, and describes efforts to increase post-thaw viability and function through manipulation of the freeze/thaw protocol. Different concentrations of foetal calf serum (FCS) with 10% (v/v) dimethyl sulphoxide (ME2SO) were tested as cryopreservation media, and high cryoprotectant serum levels were found to be important in maintaining membrane integrity and function in the cryopreserved rat hepatocyte monolayer cultures. Cultures cryopreserved with 90% (v/v) FCS plus 10% (v/v) ME2SO maintain 79.7+/-6.5% of the monolayer area as viable cells with normal morphology (by image analysis), 112.7+/-14.2% protein concentration, 55.4+/-4.2% carboxyfluorescein diacetate de-acetylation, 27.2+/-7.5% kaempherol glucuronidation (a measure of UDP-glucuronosyl transferase activity), and 39.3+/-7.3% testosterone hydroxylation (a measure of cytochrome P-450 activity) compared with non-cryopreserved controls. This method of cryopreservation may provide a simple, convenient means of long-term storage of hepatocytes for in vitro metabolism studies.

Hypothermic preservation of hepatocytes

This paper presents a review of recent research on the hypothermic storage of hepatocytes. The first focus is on the diversity of methodologies currently employed in this area. The cell damage caused by hypothermic preservation and its possible mechanism are then investigated on both morphological and molecular biology. Later, the gene expressions on a mRNA level or enzyme level after hypothermic preservation are further discussed. Finally, the improvement of hypothermic storage by preconditioning, such as by increasing temperature, is explored.

Cryopreservation of adult human hepatocytes obtained from resected liver biopsies

Isolated human hepatocytes have been shown to represent a valuable in vitro model to investigate the metabolism and cytotoxicity of xenobiotics. In addition, human hepatocyte transplantation and artificial liver support systems using isolated human hepatocytes are currently investigated as treatment for acute and chronic hepatic failure. In this regard, human hepatocyte banking by cryopreservation would be of great interest. In the present study, freshly isolated hepatocytes from resected liver biopsies of 28 separate donors (viability: 88 +/- 2%; plating efficiency: 79 +/- 5%) were cryopreserved using two different protocols, stepwise freezing (SF) or progressive freezing (PF), in combination (PF(+), SF(+)) or not (PF(-), SF(-)) with a 30 min preincubation in culture medium at 37 degrees C. Total recovery was higher after PF (38 +/- 3%) than after SF (12 +/- 2%). Preincubation prior to SF had no effect on plating efficiency of thawed hepatocytes (SF(-): 38 +/- 6% versus SF(+): 46 +/- 7%) while preincubation prior to PF increased plating efficiency of thawed hepatocytes (PF(-): 42 +/- 6% versus PF(+): 64 +/- 4%, p < 0.05). In attached cultured human cryopreserved/thawed hepatocytes (CH) from the PF(+) group, albumin production and glutathione content were not significantly different from those of the freshly isolated hepatocyte (FIH) cultures. Cells in CH monolayers appeared smaller than cells in FIH monolayers. In addition, the pattern of cytochrome P450- and UDP-glucuronosyl transferase-dependent isoenzyme activities and GST activity were different, suggesting a variability in the resistance to cryopreservation of the various liver hepatocyte populations. Taken all together, the results of the present study suggest that recovery of human hepatocytes after isolation prior to progressive freezing should allow human hepatocyte banking for use in pharmacotoxicology and cell therapy research purposes.

Cryopreservation of rat hepatocyte monolayers: cell viability and cytochrome P450 content in post-thaw cultures

Cryopreservation of primary hepatocyte monolayers may provide a means of long-term storage of the cells for in vitro studies of xenobiotic metabolism and toxicity. Rat hepatocytes can be stored at -70 degrees C as simple monolayers attached to collagen-coated dishes, and post-thaw cultures can be continued for up to 72 h. Throughout this post-thaw period viability of the cells was demonstrated by retention of intracellular fluorescence after exposure to carboxyfluorescein diacetate (CFDA) and examination by confocal laser scanning microscopy (CLSM). CLSM images revealed an uneven distribution of CFDA-derived fluorescence within hepatocytes post-thaw, particularly in Williams' E medium, indicating generation and retention of carboxyfluorescein within the intracellular organelles. The membranes of the intracellular organelles appear to be less sensitive to freeze/thaw damage than the cell membrane. Viability was not compromised with storage for up to 28 days at -70 degrees C. Cytochrome P450 content was retained in post-thaw culture to a similar extent as in non-frozen cultures. Cryopreserved rat hepatocyte monolayers may provide a useful in vitro model for studying xenobiotic metabolism and toxicity.

Which hepatocyte will it be? Hepatocyte choice for bioartificial liver support systems

Liver failure, notwithstanding advances in medical management, remains a cause of considerable morbidity and mortality in the developed world. Although bioartificial liver (BAL) support systems offer the potential of significant therapeutic benefit for such patients, many issues relating to their use are still to be resolved. In this review, these issues are examined in terms of the functions required, the cells of choice in such a system, and the most appropriate environment to optimize the function of such cells. The major functions identified to date for a BAL are ammonia detoxification and biotransformation of toxic compounds, although this somewhat belies the complexity of the functions required. Two practical choices for cell type within such a system are xenogenic hepatocytes and immortalized human hepatocyte lines. Both these choices have drawbacks, such as the transmission of zoonoses and malignant infiltration, respectively. Finally, improvements in culture conditions, such as supplemented media, biodegradable scaffolds, and coculture, offer the possibility of prolonging the differentiated function of hepatocytes in a BAL.

Metabolic activity of fresh and cryopreserved cynomolgus monkey (Macaca fascicularis) hepatocytes

1. The effect of cryopreservation on the metabolic capacity of monkey hepatocytes over 4 h in suspension and 24 h in culture was determined. Hepatocytes were diluted in a buffer containing 10% DMSO and frozen in a computer-controlled chamber. 2. Initial ethoxyresorufin and ethoxycoumarin O-deethylase (ECOD) activities were the same in fresh and cryopreserved (CP) hepatocytes. ECOD activity in suspensions declined over 4 h but was the same in fresh and CP hepatocytes. 3. The formation of testosterone hydroxy (OHT) metabolites (namely 6beta-OHT, 2beta-OHT, 16beta-OHT, 16alpha-OHT, 15beta-OHT, 2alpha-OHT and 6beta-OHT) was unaffected by cryopreservation. The loss of OHT activities over 4 h in CP and fresh whole cell suspensions was attributed to a loss of cofactor. CP hepatocyte cultures had equivalent OHT activities to freshly isolated hepatocytes. 4. Initial UDP-glucuronyltransferase (UGT) activities, using the substrates 4-methylumbelliferone, ethoxycoumarin and hydroxycoumarin, were equivalent in fresh and CP whole hepatocytes. At later times, UGT activity was lower in CP than fresh hepatocytes but this was due to a loss of UDPGA. Initial sulphotransferase (SULT) activities, using the substrates 2-naphthol, ethoxycoumarin and hydroxycoumarin, were equivalent in fresh and CP hepatocytes. SULT activities were less stable than UGT activities but were the same in fresh and CP hepatocytes throughout the 4-h incubation. 5. Initial glutathione S-transferase activities (using 1-chloro-2,4-dinitrobenzene) were the same in fresh and CP hepatocytes and both did not decrease over 4 h. 6. CP monkey hepatocytes are a useful model for metabolic and cytotoxicity studies. These cells can be can be used either in suspension or in culture.

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.

Extracorporeal support and hepatocyte transplantation in acute liver failure and cirrhosis

The relative shortage of donor organs and lack of immediate availability mean that many patients with acute liver failure die before orthotopic liver transplantation can be performed. An effective temporary liver support system could improve the chance of survival with or without a transplant being ultimately carried out. Recent technological advances resulting in improved maintenance of hepatocyte viability and function in culture and bioreactor designs which facilitate adequate perfusion of the cellular component and removal of products of cellular metabolism have led to the development of a number of bioartificial devices for liver support. Three such devices have undergone preliminary clinical evaluation in the setting of acute liver failure, with a statistically significant reduction in raised intracerebral pressure along with improvements in consciousness level and some biochemical parameters associated with treatment with one of these. Several other devices with different characteristics have shown promise in vitro and/or in animal models but await clinical evaluation. Several new totally artificial systems have also been described, along with the emergence of isolated hepatocyte transplantation, with reports of successful 'bridging' to liver transplantation. Controlled trials on a multicentre basis in well-defined patient groups and with standardized outcome measures will be required to properly evaluate the clinical value of each of these approaches to providing liver support in acute liver failure and cirrhosis. A better understanding of mechanisms underlying multiorgan failure and of factors inhibiting liver regeneration, thereby allowing a more targeted approach, will be essential to the further development of effective liver support strategies in these settings.

Survival and function of isolated hepatocytes after cryopreservation

Cryopreservation in liquid nitrogen is presently the only way for long-term storage of isolated hepatocytes. Freeze-thaw conditions are not well defined yet; the most critical parameters appear to be the choice of the cryoprotectant, composition of the freezing medium, and cooling and thawing rates. Comparable results have been obtained with hepatocytes from various species, including man. Cryopreservation usually results in low cell recovery and early alterations of functional activities. However, both phase I and phase II xenobiotic metabolism is still active after thawing, at least during a short period. Moreover, survival and function of cryopreserved hepatocytes can be improved when these cells have a high energy status, are cryopreserved after immobilization in a gel, separated from dead cells on a Percoll gradient or placed in more favorable culture conditions (e.g. in coculture with liver non parenchymal cells). Additional studies are needed to improve freeze-thaw protocols and to better characterize liver parenchymal cells after storage, including evaluation of their responsiveness to specific inducers.

Induction of cytochrome-P450 in cryopreserved rat and human hepatocytes

Our laboratory has been routinely using suspended and cultured human hepatocytes for predicting drug metabolism and enzyme induction by drug candidates to aid drug discovery. Increasing limitation and irregular availability of human tissue has indicated the need for maximizing the use of this valuable resource. Cryopreservation of surplus hepatocytes after isolation would greatly increase the potential of this model. However, cryopreservation of hepatocytes by various methods has resulted in cells with poor metabolic activity and unacceptably low survival rates in culture. Recently, Zaleski et al. (Biochem. Pharmacol. 46 (1993) 111-116) reported that cryopreserved rat hepatocytes retained metabolic capacity similar to fresh hepatocytes when the cells were preincubated for 30 min at 37 degrees C in Krebs Ringer bicarbonate buffer prior to freezing. To further explore this methodology, both the functional capacity of the cells in culture as well as their ability to retain CYP inducibility were investigated with thawed cryopreserved hepatocytes. Although human hepatocytes were used in this study the initial work focused on rat hepatocytes as a cell model. Our results showed that while the preincubation step did not appear to effect the initial viability of cryopreserved hepatocytes, survival of the cells in culture was greatly enhanced. Plating efficiencies for nonpreincubated cryopreserved hepatocytes were decreased to approximately 15% of fresh cells after 48 h in culture. In contrast, cells that had been preincubated prior to freezing had an excellent plating efficiency (approximately 60%) and responded to classical CYP inducers dexamethasone, beta-naphthoflavone and phenobarbital in a manner indistinguishable from that of fresh hepatocytes. Experiments with human hepatocytes have also demonstrated similar results. This is the first time to our knowledge that cryopreserved hepatocytes from both rat and human have been shown to reproducibly respond to CYP inducers in culture.

Effect of cold and warm ischaemia on drug metabolism in isolated hepatocytes and slices from human and monkey liver

1. The influence of short-term cold storage in University of Wisconsin organ preservation solution (UW) on the ability to metabolize lidocaine, testosterone and 7-ethoxycoumarin in isolated human and cynomolgus monkey (Macaca fascicularis) hepatocytes and liver slices has been investigated. 2. The human liver tissue was obtained from two different sources, i.e. healthy liver tissue from patients undergoing partial hepatectomy because of metastases of colorectal carcinoma (PH livers) and donor tissue remaining as surgical waste after reduced size or split liver transplantation (Tx livers). Tx livers were perfused in situ with ice-cold UW avoiding warm ischaemia. This in contrast with PH livers, where the operation caused warm ischaemia for 5-90 min. 3. Liver slices and hepatocytes from cynomolgus monkey liver showed comparable metabolic rates for the substrates tested, indicating that all hepatocytes in the slice are participating in the biotransformation of the substrates. These monkey liver preparations can be stored up to 18 h with only a slight loss of their metabolic capacity. 4. Liver slices and isolated hepatocytes from the Tx livers as well as isolated cells from the PH livers could also be stored up to 18 h without losing metabolic capacity. However, for liver slices prepared from PH livers cold storage is not recommended, because metabolic function was reduced by approximately 40% after 18 h.

Long-term maintenance of drug-metabolizing enzyme activities in rat hepatocytes after cryopreservation

Recent studies have demonstrated that freshly isolated adult hepatocytes from various species can be hypothermically preserved for a short period or cryopreserved for a prolonged period before seeding in primary culture. This study was designed to determine whether rat hepatocytes could be maintained functional for a prolonged period after either hypothermic preservation or cryopreservation. Cold storage was carried out in University of Wisconsin solution (UW) and freezing in Leibovitz medium added with 10% fetal calf serum and 16% dimethyl sulfoxide. Rat hepatocytes were then set up either in pure conventional culture or in coculture with rat liver epithelial cells. Various functions were measured over 4- and 15-day periods, i.e., albumin secretion rate, deethylation of ethoxyresorufin and phenacetin, dealkylation of pentoxyresorufin, glucuronidation and sulfoconjugation of paracetamol, and N-acetylation of procainamide. No major differences were observed between unfrozen, frozen, and UW-preserved cells. While in pure culture all the functions tested were markedly decreased after 3 or 4 days, they remained high over the 15-day period in coculture, being either maintained or increased after 7-12 days compared to initial values. These results clearly demonstrate that when maintained under suitable culture conditions, rat hepatocytes can fully recover after hypothermic preservation or cryopreservation and therefore represent a suitable in vitro model system for pharmacotoxicological studies.

Comparison of five incubation systems for rat liver slices using functional and viability parameters

Precision-cut liver slices are presently used for various research objects, e.g. to study metabolism, transport, and toxicity of xenobiotics. Various incubation systems are presently employed, but a systematic comparison between these incubation systems with respect to preservation of slice function has not been performed yet. Therefore, we started a comparative study to evaluate five of these systems: the shaken flask (an Erlenmeyer in a shaking water bath), the stirred-well (24-well culture plate equipped with grids and magnetic stirrers), rocker platform (6-well culture plate with Netwell insert rocked on a platform), the roller system (dynamic organ culture rolled on an insert in a glass vial), and the 6-well shaker (6-well culture plate in a shaking water bath). The liver slices were incubated in these incubation systems for 0.5, 1.5, and 24.5 h and subsequently subjected to viability and metabolic function tests. The viability of the incubated liver slices was evaluated by: potassium content, MTT assay, energy charge, histomorphology, and LDH leakage. Their metabolic functions were studied by determination of the metabolism of lidocaine, testosterone, and antipyrine. Up to 1.5 h of incubation all five incubation systems gave similar results with respect to viability and metabolic function of the liver slices. However, after 24 h, the shaken flask, the rocker platform, and the 6-well shaker incubation systems appeared to be superior to the stirred well and the roller incubation systems.

Influence of 48 hours of cold storage in University of Wisconsin organ preservation solution on metabolic capacity of rat hepatocytes

BACKGROUND/AIMS

Suspensions of isolated hepatocytes are a valuable tool to study liver functions. For optimal use of the isolated hepatocytes, methods are needed to preserve the hepatocytes while maintaining their viability, metabolic and transport functions. Until now, little has been known about the maintenance of the drug metabolism capacity and energy state, measured by the so-called energy charge (ATP+1/2ADP)/(ATP+ADP+AMP), in hepatocytes after storage in University of Wisconsin cold storage solution (UW). Consequently, we investigated whether UW, originally designed to preserve organs for transplantation, was suitable for preservation of isolated rat hepatocytes with respect to the maintenance of drug metabolism and levels of energy-rich substrates.

METHODS

Viability of the isolated rat hepatocytes was determined by trypan blue exclusion, ATP content and energy charge after 24 and 48 h of storage in UW at 0 degrees C. Phase I and II metabolic functions of the cells were studied by measuring the cytochrome P450 content and the metabolic rate of lidocaine and 7-ethoxycoumarin.

RESULTS

During 48 h of storage of hepatocytes in UW both phase I and phase II metabolism are preserved at control levels. After storage, the viability of the hepatocytes was not changed significantly, and the cells maintained proper cellular ATP content and overall energy charge.

CONCLUSIONS

These results imply that hepatocytes from a single isolation can be stored in UW solution and used for metabolism experiments for 3 consecutive days, allowing a reduction in the use of experimental animals.

Use of fresh and cryopreserved hepatocytes to study the metabolism of pesticides in food-producing animals and rats

1. The possibility of using hepatocytes from food-producing animals in order to determine the metabolic routes of pesticides has been studied using a strobilurin fungicide (BAS 490 F). Hepatocytes suspensions were prepared from goat, pig, hen, and rat and the major metabolites were compared with those obtained in vivo. 2. The hepatocytes gave metabolite patterns matching qualitatively with in vivo results, but no good quantitative correlation was found. 3. A freezing and thawing method was developed using liquid nitrogen and a programmable freezer, which allows acceptable recoveries of functional cells as assessed by glutathione and cytochrome P450 contents, and phase I and II enzymatic activities (including 7-ethoxycoumarin-O-deethylase, ethoxyresorufin-O-deethylase, glutathione-S-transferase, and UDP-glucuronosyl transferase), with 60-70% viability. 4. The cells were damaged through freezing as indicated by the efflux of glutathione (40-60% of the intracellular content), but remained able to metabolize BAS 490 F, partially like fresh cells. A good qualitative but no quantitative matching of the metabolite patterns before and after cryopreservation was found, indicating that the metabolic activities are affected to variable extents during the freezing process. 5. The use of fresh and cryopreserved cells as models for metabolism and species comparison, and as a versatile tool to synthesize metabolites, is discussed.

Cryopreservation of rat hepatocyte monolayer cultures

1. Most previous attempts to cryopreserve hepatocytes have used suspensions stored at either -70 degrees C or in liquid nitrogen, and the major problem is that these do not, on subsequent thawing, attach well in culture. This limits their use in studies of drug metabolism and xenobiotic-induced toxicity. In this manuscript we demonstrate successful cryopreservation of rat hepatocytes as monolayers attached to a collagen film. 2. Monolayers can be frozen and thawed without significant loss of cells, and although damage to the internal and plasma membranes is evident immediately post-thaw, a remarkable repair process takes place over 24-48 h post-thaw. Immediately post-thaw only 10% of the cells exclude Trypan Blue, but by 48 h 80-90% of the thawed cells are viable, indicating that repair of the plasma membranes has taken place. 3. The cells post-thaw retain aspects of liver-specific function including cytochrome P450 content and albumin synthesis. However, cytosolic proteins are lost through the damaged membranes and, probably because of this, urea synthesis from ammonia is retained at only 25% of pre-freeze values. 4. A cryopreservation method based on adherent hepatocytes on a collagen substrate overcomes the problems encountered with culture of cryopreserved hepatocyte suspensions, and may provide a practical means of establishing a 'bank' of hepatocytes from several donors and species.

Cryopreserved and hypothermically stored rat liver parenchymal cells as metabolizing system in the Salmonella mutagenicity assay

Freshly isolated and preserved rat liver parenchymal cells were used as metabolizing system in the Salmonella mutagenicity assay. The liver cells were isolated with EDTA perfusion without the addition of collagenase and had a viability of 96% as judged by trypan blue exclusion. When freshly isolated liver parenchymal were cryopreserved with a computer controlled freezing protocol and stored at -196 degrees C they had a mean viability of 89% after thawing. Furthermore, freshly isolated cells were stored at 0 degree C in University of Wisconsin organ transplantation solution. After 1 day of hypothermic storage they had a viability of 95%. Four different indirect mutagens, 2-aminoanthracene, benzo[a]pyrene, 7,12-dimetylbenz[a]anthracene and cyclophosphamide, were used with the liver cells as metabolizing system in the preincubation assay with Salmonella typhimurium TA100. After cryopreservation, liver parenchymal cells were able to activate all tested indirect mutagens to ultimate mutagens. However, the induction of revertants was lower with three of the four tested compounds. Only 2-aminoanthracene was activated to the same extent by freshly isolated and cryopreserved liver cells. 7-Hydroxymethyl-12-methylbenz[a]anthracene, which is activated to its ultimate mutagen by sulfotransferase, also induced a reduced mutagenic effect with cryopreserved liver cells in comparison to freshly isolated liver parenchymal cells. This indicates that phase I and phase II enzyme activities are effected by cryopreservation. However, identical mutation frequencies were obtained when freshly isolated liver parenchymal cells or 1 day hypothermically preserved liver parenchymal cells were used in the cell-mediated Salmonella mutagenicity test. The use of hypothermic short-time storage of liver parenchymal cells could help to make the liver cell-mediated genotoxicity test simpler and thereby more attractive.

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)

Cell systems for use in studies on the relationship between foreign compound metabolism and toxicity

Since the metabolism of most foreign compounds is predominantly controlled by hepatic in metabolism, isolated hepatocytes in most cases quite well predict the pattern of the overall metabolism of a given compound. Methods have been developed for cryopreserving isolated hepatocytes from man and other species with satisfactory maintenance of foreign compound metabolizing enzyme activities. The installation of a bank of cryopreserved hepatocytes from different species is possible and may be used for rational species extrapolation. It is necessary for some toxicological investigations to have hepatocytes which retain their differentiated status in culture for a sufficient time period. This might be achieved by co-culturing hepatocytes with diverse cell lines. However, from one cell line to the other differences in the pattern of stabilization of individual hepatocyte functions are found. In addition, questions on metabolic action of individual isoenzymes can also be addressed by the use of genetically engineered cell lines. All the in vitro systems mentioned, especially those which contain differentiated human cells or human isoenzymes are helpful in the rational species extrapolation of toxic effects from animal to man.

Cryopreservation of human adult hepatocytes for use in drug metabolism and toxicity studies

1. Human hepatocytes were cryopreserved for up to 14 days at -80 degrees C and the cryoprotection offered by different media investigated in terms of post-thaw cell viability and function. 2. Optimal cryoprotection was offered by a solution containing dimethylsulphoxide, propylene glycol, acetamide and polyethylene glycol 8000 in Leibowitz L15 medium. 3. The cytochrome P450 content and activities of the microsomal P450 dependent mixed function oxidase system were well maintained at above 70% of fresh cell values throughout the cryopreservation period. However, the activities of the cytosolic enzymes studied, glutathione S-transferase and glutathione reductase, were not well maintained; they declined to < 40% of fresh cell values after storage of cells for 14 days at -80 degrees C. The membrane environment may protect microsomal enzymes from denaturation by freeze-thaw damage. 4. After cryopreservation, viability of human hepatocytes was higher than that of rat hepatocytes preserved under identical conditions. For human cells maximum post-cryopreservation viability was 67% after 24 h at -80 degrees C; this declined to 49% after 14 days storage at -80 degrees C. In addition post-cryopreservation human hepatocytes remained > 70% viable when incubated at 37 degrees C in suspension compared with only 46% of rat hepatocytes. This indicates that human hepatocytes can withstand freeze-thaw damage better than those from rat. 5. The results of this study define optimal conditions for cryopreserving human hepatocytes. Although microsomal enzyme activities are retained post-cryopreservation, the decrease in viability of thawed cells upon incubation at 37 degrees C suggests that caution should be exercized when using cryopreserved cells to study integrated drug metabolizing pathways in man in vitro.

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.