Influence of Different Substrates in Detoxification Activity of Adult Rat Hepatocytes in Long-Term Culture: Implications for Transplantation

Isolation and Culture of Porcine Hepatocytes for Artificial Liver Support

The primary requirement of cells in a liver support system is the preservation of the in vivo metabolic functions that prevent or decrease the progress of hepatic encephalopathy (HE) by providing interim support to liver failure patients. While rodent hepatocytes offer a model for liver assist device (LAD) research, their limited number per animal prohibits direct scale up to human devices. Healthy human liver cells are seldom available in adequate numbers to support clinical LAD use; consequently, a large animal source of liver cells is needed. The study presented here explored the potential of porcine hepatocytes to proliferate and maintain metabolic function in vitro. Porcine hepatocytes were isolated from ~12 kg swine by a modification of Seglen's method. Hepatocytes cultured up to 10 days were shown to metabolize ammonia and maintain both Phase I and II detoxification functions. In addition, the cultures showed proliferative activity both as an increase in total protein content and by thymidine incorporation. Immunocytochemical staining identified cell proliferation through Day 4 to be primarily hepatocytes while Days 6 and 10 showed nonparenchymal cells to be increasing. The detoxification functions measured showed peak activity on Day 4 and gradually declined through Day 10. The ability of porcine hepatocytes to proliferate and maintain a diversity of hepatic functions in culture strongly suggests their potential for use as the biological component of artificial LADs.

Use of Mammalian Liver Cells for Artificial Liver Support

Advances in orthotopic liver transplantation have improved the survival rate of both acute and chronic liver failure patients to nearly 70%. However, the success of this treatment modality has created an international organ shortage. Many patients die while awaiting transplantation in part due to the minimal capacity to store viable transplantable livers beyond 24 h. Additionally, for many areas of the world, routine use of whole liver transplantation to treat liver disease is impractical due to the demands on both financial and technical resources. Potentially, these issues may be alleviated, at least in part, by the use of liver cell transplantation or cellular-based liver assist devices. The well-documented regenerative capacity of the liver may obviate the need for whole organ transplantation in some instances of acute failure, if the patient may be provided temporary metabolic support. Although other patients ultimately may require transplantation, a longer period of time to find a suitable organ for transplantation may be gained by that supportive therapy. The field of liver cell transplantation may offer solutions to patients with inherited metabolic deficiencies or chronic liver disease. The potential to treat an hepatic disorder by using only a fraction of the whole liver would increase the number of whole organs available for orthotopic liver transplantation. Research in the fields of hepatocyte based intra- and extra-corporeal liver support is providing evidence that these therapeutic modalities may ultimately become routine in the treatment of severe liver disease. A historic overview of that technology along with its current status is discussed.

Transplant of Primary Human Hepatocytes Cocultured With Bone Marrow Stromal Cells to SCID Alb-uPA Mice

Hepatocytes are vulnerable to loss of function and viability in culture. Modified culture methods have been applied to maintain their functional status. Heterotypic interactions between hepatocytes and nonparenchymal neighbors in liver milieu are thought to modulate cell differentiation. Cocultivation of hepatocyte with various cell types has been applied to mimic the hepatic environment. Bone marrow stromal cells (BMSC) are plastic cell lines capable of transforming to other cell types. In this study hepatocyte coculture with BMSCs achieved long-term function of human hepatocytes in culture for 4 weeks. In vitro functional status of human hepatocytes in BMSC coculture was compared with fibroblast coculture and collagen culture by measuring albumin, human-α-1-antitrypsin (hAAT), urea secretion, CYP450 activity, and staining for intracellular albumin and glycogen. After 2 weeks in culture hepatocytes were retrieved and transplanted to severe combined immunodeficiency/albumin linked-urokinase type plasminogen activator (SCID Alb-uPA) mice and engraft-ment capacity was analyzed by human hepatic-specific function measured by hAAT levels in mouse serum, and Alu staining of mouse liver for human hepatocytes. Hepatocytes from BMSC coculture had significantly higher albumin, hAAT secretion, urea production, and cytochrome P450 (CYP450) activity than other culture groups. Staining confirmed the higher functional status in BMSC coculture. Transplantation of hepatocytes detached from BMSC cocultures showed significantly higher engraftment function than hepatocytes from other culture groups measured by hAAT levels in mouse serum. In conclusion, BMSC coculture has excellent potential for hepatocyte function preservation in vitro and in vivo after transplant. It is possible to use BMSC hepatocyte coculture as a supply of cell therapy in liver disease.

Bioartificial liver support devices: historical perspectives

Fulminant hepatic failure (FHF) is an important cause of death worldwide. Despite significant improvements in critical care therapy there has been little impact on survival with mortality rates approaching 80%. In many patients the cause of the liver failure is reversible and if short-term hepatic support is provided, the liver may regenerate. Survivors recover full liver function and a normal life expectancy. For many years the only curative treatment for this condition has been liver transplantation, subjecting many patients to replacement of a potentially self-regenerating organ, with the lifetime danger of immunosuppression and its attendant complications, such as malignancy. Because of the shortage of livers available for transplantation, many patients die before a transplant can be performed, or are too ill for operation by the time a liver becomes available. Many patients with hepatic failure do not qualify for liver transplantation because of concomitant infection, metastatic cancer, active alcoholism or concurrent medical problems. The survival of patients excluded from liver transplantation or those with potentially reversible acute hepatitis might be improved with temporary artificial liver support. With a view to this, bioartificial liver support devices have been developed which replace the synthetic, metabolic and detoxification functions of the liver. Some such devices have been evaluated in clinical trials. During the last decade, improvements in bioengineering techniques have been used to refine the membranes and hepatocyte attachment systems used in these devices, in the hope of improving function. The present article reviews the history of liver support systems, the attendant problems encountered, and summarizes the main systems that are currently under evaluation.

Culture of porcine hepatocytes: the dogma of exogenous matrix revisited

The use of exogenous matrices has been described as an essential component in securing the viability and functionality of hepatocytes in vitro whether cultured for extracorporeal devices or cell transplantation. Here we report on the in vitro culture of porcine hepatocytes in polystyrene tissue-culture flasks without exogenous matrices showing adequate attachment and viability. Cell proliferation was evidenced by uptake of 5-bromo-2'-deoxyuridine, with peaks at Days 2 (19.7 +/- 8.5%), 15 (20.8 +/- 3.3%), and 35 (21.4 +/- 0.3%). Detoxification capacity was assessed by determination of monoethylglycinexylidide, a product of lidocaine metabolism (highest value 156.5 +/- 10.1 ng/ml at Day 4), and by diazepam clearance (maximum clearance 66.2% at Day 6). Diazepam metabolite levels were highest at Day 4 both for temazepam and oxazepam (6.5 +/- 0.1 and 0.10 +/- 0.01, respectively). These results suggest that the need for an exogenous matrix to achieve sustained proliferative activity and differentiated hepatocyte function should not necessarily be considered a sine qua non condition.

Growth and metabolic activity of immortalized porcine hepatocytes in extracorporeal hollow-fiber liver assist devices

The development of a cell based extracorporeal liver assist device offers a promising clinical approach to bridge individuals suffering from acute liver failure to transplant. However, a major drawback of the existing technology is the lack of a continuous supply of well differentiated hepatocytes. Although some investigators have used primary porcine cells, this approach demands costly, labor-intensive isolation procedures and yields cells with inconsistent detoxification capacity. The limitations of primary cells led us to develop the HepLiu immortalized porcine hepatocyte cell line for use in liver assist devices (LADs). HepLiu cells are nontumorigenic and exhibit multiple hepatic detoxification functions including diazepam and acetaminophen metabolism. To investigate the suitability of HepLiu cells for artificial liver support, morphology, as well as xenobiotic metabolism, was studied in perfused polysulfone hollow-fiber LADs. HepLiu cells were cultured in the intercapillary space of a prototype LAD, and the metabolism of diazepam, acetaminophen, and 7-ethoxycoumarin was evaluated over 25 days in culture. Our results indicated that HepLiu cells proliferated rapidly following inoculation of the LAD until Day 10 when proliferation appeared to cease. Ultrastructural analysis demonstrated that HepLiu cells retained many of the features of primary hepatocytes including desmosomes that sealed bile canalicular-like structures and junctional complexes (intermediate, gap junctions) that appeared concentrated in the paracanalicular areas. Unlike primary porcine hepatocytes, HepLiu cells retained drug metabolic function throughout the 25 day culture period. Diazepam metabolism by HepLiu cells was consistently higher than that of primary cells. Acetaminophen metabolism persisted throughout the 25 day period albeit at a much lower level than the primary cells exhibited on Days 1 or 2. In conclusion, we have shown that HepLiu cells proliferate to occupy the intercapillary space of perfused hollow-fiber LADs following inoculation, and retain their metabolic capacity for Phase I and Phase II detoxification reactions in perfusion culture. Our findings suggest that HepLiu cells may provide an alternative to primary porcine hepatocytes as the cellular component of bioartificial liver support systems.

Use of mammalian liver cells for artificial liver support

Advances in orthotopic liver transplantation have improved the survival rate of both acute and chronic liver failure patients to nearly 70%. However, the success of this treatment modality has created an international organ shortage. Many patients die while awaiting transplantation in part due to the minimal capacity to store viable transplantable livers beyond 24 h. Additionally, for many areas of the world, routine use of whole liver transplantation to treat liver disease is impractical due to the demands on both financial and technical resources. Potentially, these issues may be alleviated, at least in part, by the use of liver cell transplantation or cellular-based liver assist devices. The well-documented regenerative capacity of the liver may obviate the need for whole organ transplantation in some instances of acute failure, if the patient may be provided temporary metabolic support. Although other patients ultimately may require transplantation, a longer period of time to find a suitable organ for transplantation may be gained by that supportive therapy. The field of liver cell transplantation may offer solutions to patients with inherited metabolic deficiencies or chronic liver disease. The potential to treat an hepatic disorder by using only a fraction of the whole liver would increase the number of whole organs available for orthotopic liver transplantation. Research in the fields of hepatocyte based intra- and extra-corporeal liver support is providing evidence that these therapeutic modalities may ultimately become routine in the treatment of severe liver disease. A historic overview of that technology along with its current status is discussed.

Isolation and culture of porcine hepatocytes for artificial liver support

The primary requirement of cells in a liver support system is the preservation of the in vivo metabolic functions that prevent or decrease the progress of hepatic encephalopathy (HE) by providing interim support to liver failure patients. While rodent hepatocytes offer a model for liver assist device (LAD) research, their limited number per animal prohibits direct scale up to human devices. Healthy human liver cells are seldom available in adequate numbers to support clinical LAD use; consequently, a large animal source of liver cells is needed. The study presented here explored the potential of porcine hepatocytes to proliferate and maintain metabolic function in vitro. Porcine hepatocytes were isolated from approximately 12 kg swine by a modification of Seglen's method. Hepatocytes cultured up to 10 days were shown to metabolize ammonia and maintain both Phase I and II detoxification functions. In addition, the cultures showed proliferative activity both as an increase in total protein content and by thymidine incorporation. Immunocytochemical staining identified cell proliferation through Day 4 to be primarily hepatocytes while Days 6 and 10 showed nonparenchymal cells to be increasing. The detoxification functions measured showed peak activity on Day 4 and gradually declined through Day 10. The ability of porcine hepatocytes to proliferate and maintain a diversity of hepatic functions in culture strongly suggests their potential for use as the biological component of artificial LADs.

Secretion, surface localization, turnover, and steady state expression of protein disulfide isomerase in rat hepatocytes

Protein disulfide isomerase in isolated rat hepatocytes was present at a concentration of 7 micrograms/mg cell protein, representing a approximately 2-fold enrichment compared to isolated hepatic non-parenchymal cells. Though localized mainly in microsomal fractions of hepatocytes, direct immunofluorescence and cell surface radioiodination followed by immunoprecipitation revealed the presence of M(r) 57,000 disulfide isomerase at the cell surface. Electrostatic interaction of the protein with the cell surface was suggested by susceptibility to carbonate washing. Metabolic radiolabeling and immunoprecipitation studies also indicated that some of the newly synthesized M(r) 57,000 disulfide isomerase was secreted. Treatment of cells with colchicine markedly reduced the recovery of disulfide isomerase from the media, indicating microtubular-directed secretion of the protein. Partial staphlococcal V8 proteolytic digestion of the secreted protein revealed a peptide pattern similar to that of the cellular protein. Immunoprecipitation with antibody specific to the -KDEL peptide retention sequence confirmed the presence of this sequence in the secreted protein. Studies of the turnover of disulfide isomerase revealed a half-life of approximately 96 h. Treatment of cells with tunicamycin or heat shock resulted in an increased recovery of newly synthesized disulfide isomerase from cell lysates but diminished recovery from the media. The secretion and cell surface distribution of disulfide isomerase in hepatocytes may be important for the pathogenesis of immune mediated liver injury.

Primary cultures of rat hepatocytes in hollow fiber chambers

Hepatocyte culture may represent an alternative to the use of animals to study drug detoxification by the liver. An ideal in vitro system should closely mimic the in vivo environment by providing continuous media perfusion and oxygenation, and should facilitate sampling of cells and culture media. To meet these criteria, a hollow fiber bioreactor seeded with isolated rat hepatocytes was developed and tested by measuring the formation of three products of the oxidative metabolism of diazepam and the glucuronidation of phenolsulfonphthalein (PSP). To compare the performance of conventional monolayer culture to that of the bioreactor system, diazepam metabolism was studied for 45 days in both systems. The oxygen dependency of diazepam metabolism was evaluated by perfusing the bioreactor in an oxygen-rich atmosphere (30%). Total diazepam metabolism was twofold higher in the O2-rich perfused hollow fiber cultures than in the cultures perfused under normal conditions, reflecting an increase in temazepam and oxazepam production. Diazepam detoxification activity was significantly enhanced by oxygen (P < or = 0.001) over the life of the perfused cultures. PSP metabolism was similar in all three culture systems. By Day 10, diazepam metabolism in the oxygenated bioreactor system was 44% of the in vivo activity of rat hepatocytes. This activity dropped to 30% by Day 25 of culture. These results justify the use of perfused culture systems for in vitro detoxification studies as an alternative to animal use and emphasize the capacity of a culture device perfused under O2-enriched conditions to maintain long-term P450 activity of rat hepatocytes.

Evolution of the bioartificial liver: the need for randomized clinical trials

The pursuit of a bioartificial liver is well documented in the literature. Early techniques of artificial liver support that have undergone clinical testing included simple exchange transfusions, extracorporeal xenogeneic or allogeneic liver perfusion, cross-circulation, hemodialysis, charcoal hemoperfusion, and plasmapheresis with plasma exchange. These techniques failed because they were unable to adequately support those hepatic functions essential for survival and because they lacked a back-up therapy, such as liver transplantation, for irreversible forms of liver disease. The concept evolved that hepatic functions essential for survival would be best performed by hepatocytes in an apparatus that allowed sustained or repetitive application. The best results have been achieved with bioartificial liver technologies that employ hepatocytes as implantable systems or extracorporeal devices. Implantable bioartificial liver systems include hepatocytes that have been on coated microcarrier beads, within microencapsulated gel droplets, within biodegradable polymeric substrates, or as spheroid hepatocyte aggregates. Extracorporeal systems include hepatocytes in suspension, on flat plates, and in hollow fiber bioreactors. Several extracorporeal systems have undergone extensive animal testing and are entering the early stages of human clinical trials. Randomized trials are needed to establish the value of bioartificial liver support in the treatment of patients with acute hepatic failure or as a bridge to liver transplantation.

Xenobiotic-modulated expression of hepatic glutathione S-transferase genes in primary rat hepatocyte culture

CYP 2B1/B2 and 1A1 expression in primary rat hepatocytes plated on a substratum of Vitrogen using Chee's Essential Medium has been reported to be responsive to xenobiotic treatment (Jauregui, H.O., Ng, S.F., Gann, K.L. and Waxman, D.J. (1991) Xenobiotica 21, 1091-1106). Class alpha, mu and pi glutathione S-transferase (GST) gene expression in response to xenobiotic treatment using this primary hepatocyte culture system was examined and the results compared with those obtained for P4502B1/B2 and 1A1 expression. Cytosolic GST activity decreased approx. 75% during the first 48 h of culture relative to freshly isolated hepatocytes and subsequently, increased, attaining a level at 96 h that was 134% of the activity at 48 h post-plating. Treatment of the hepatocyte cultures with phenobarbital (2 mM) or 3-methylcholanthene (5 microM) for 24, 48, or 72 h, beginning 24 h after plating, resulted in significant increases in glutathione S-transferase activity relative to control, with maximal increases of 158 and 164% measured at 72 h following phenobarbital or 3-methylcholanthrene treatment, respectively. SDS-PAGE analysis of cytosolic proteins showed a substantial increase in the intensities of protein bands migrating in the region of the GSTs following phenobarbital, beta-naphthoflavone or 3-methylcholanthrene treatment. Immunoblot analysis of cytosolic fractions using affinity-purified class-specific GST IgGs confirmed that alpha, mu and pi-class GST isozymes were elevated approx. 1.5- to 2-fold following phenobarbital, or beta-naphthoflavone treatment; 3-methylcholanthrene was less effective in enhancing GST expression in cultured hepatocytes as compared to phenobarbital or beta-naphthoflavone. Although GST pi was below the limit of detection in freshly-isolated hepatocytes, enhanced expression of this form was observed in untreated hepatocytes cultured for longer than 72 h. Immunoblot analysis of microsomal fractions revealed that cytochrome P-4502B1/2B2 and 1A1 levels were increased significantly in hepatocyte cultures treated with phenobarbital or 3-methylcholanthrene, respectively, relative to the undetectable levels found in untreated controls. Northern blot analysis of poly(A)+ mRNA isolated from cultures that had been treated with phenobarbital or 3-methylcholanthrene showed an approx. 2- and 4-fold increase in the expression of alpha and pi class glutathione S-transferase mRNAs, respectively, as compared to untreated cells. The level of P-4501A1 or 2B1 mRNA was also markedly elevated following 3-methylcholanthrene or phenobarbital treatment, respectively. The results of this study demonor the first time, that expression of alpha, mu and pi-class glutathione S-transferase genes is effectively modulated in primary yet culture system by different classes of xenobiotics.