Mitogens and hepatocyte growth control in vivo and in vitro

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.

Stimulation of in vitro rat hepatocyte proliferation by conditioned medium obtained from an immortalized macrophage cell line

The hepatomitogenic effect of conditioned medium (CDM), obtained from the N-11 mouse macrophage cell line was analysed in rat hepatocyte primary cultures. CDM concentrations from 0.01% to 100% were used and the stimulating action in terms of mitotic index (MI) was evaluated. A clear mitogenic effect was observed only with concentrations higher than 10% with peak effects around 60%. Further increase in CDM concentrations resulted in an MI decrease, and at 100% CDM the effect was totally abolished. Tests addressed to identify the presence of hepatocyte growth factor (HGF) yielded negative results. In order to identify the mitogenic factor(s) involved, we tested CDM obtained after lipopolysaccharide (LPS) stimulation of N-11 cells. Comparison of the results obtained with untreated or LPS stimulated CDMs suggested that macrophage activation does not affect the release of hepatomitogenic activity. To further characterize this macrophage-derived activity, we checked whether CDM could interact with the mitogenic effects of epidermal growth factor (EGF). CDM (10 or 50%) showed no stimulatory effect to hepatocytes cultured in the presence of a maximally stimulatory concentration of EGF. Conversely, both CDM concentrations were able to increase the MI of hepatocyte cultures treated with a suboptimal dose of EGF. These results suggest that macrophages release factor(s) which interact, in hepatocytes, with the EGF signal transduction mechanisms, or with the EGF receptor itself.

The current status of primary hepatocyte culture

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

Growth and maturation of small hepatocytes

Proliferation of adult rat hepatocytes is observed in serum-free Dulbecco's modified Eagle's medium (DMEM) supplemented with 10 mmol/L nicotinamide and 10 ng/mL epidermal growth factor (EGF). The proliferating cells are mainly mononucleate and form small cell colonies surrounded by mature hepatocytes. Although these cells in focal colonies have a less-differentiated appearance, immunocytochemically and ultrastructurally they possess hepatic characteristics. The size of small hepatocytes is one-third to half that of mature hepatocytes. Therefore, we call the cells forming a colony, small hepatocytes. The small hepatocytes can be subcultured for several passages. Furthermore, the cells are rich in the supernatant following 50 g centrifugation for 1 min after collagenase liver perfusion. When the cells are cultured in DMEM supplemented with 10% foetal bovine serum, 10 mmol/L nicotinamide, 1 mmol/L ascorbic acid 2-phosphate, 10 ng/mL EGF and 1% dimethyl sulphoxide, each small hepatocyte can clonally proliferate for more than 3 months. A small hepatocyte divides to form a colony and the number of cells reaches more than 100 within 20 days. With time in culture, cells with a large cytoplasm appear within a colony. They have many mitochondria and large peroxisomes with crystalline nucleoids and are typical, mature hepatocytes. Immunoreactivity to connexin 32 and well-developed bile canaliculus structures are often observed in the cell-cell borders. Thus, we suggest that small hepatocytes may be considered to be 'committed progenitor cells' that can further differentiate into mature hepatocytes.

Identification of a 79-kDa heparin-binding fibroblast growth factor (FGF) receptor in rat hepatocytes and its correlation with the different growth responses to FGF-1 between hepatocyte subpopulations

We reported previously that the potency of heparin-binding fibroblast growth factor-1 (FGF-1) as a mitogen for rat hepatocytes in primary culture is as high as that of epidermal growth factor (EGF) and hepatocyte growth factor. To gain insight into the pathophysiological significance of FGF-1 in hepatocyte growth, we analyzed the cooperative mitogenicity of FGF-1 and EGF. Results from a nuclear labeling assay using [3H]thymidine suggest that most hepatocytes in primary culture consist of two cell populations that differ in response to FGF-1; one is an FGF-1-responsive cell population, and the other is an EGF-responsive (but not FGF-1-responsive) cell population. On the other hand, autoradiographic analysis of 125I-FGF-1 binding demonstrated that high affinity FGF receptors were homogeneously distributed on the surface of all hepatocytes. Cross-linking 125I-FGF-1 to the nonstimulated hepatocyte surface indicated that the high affinity FGF receptors comprise two FGF receptors that differ in molecular mass (128 and 79 kDa). Furthermore, the 79-kDa receptor was preferentially down-regulated when the hepatocytes were stimulated with EGF or hepatocyte growth factor. These data suggest that the abundant expression of the 79-kDa FGF receptor on some populations of hepatocytes is involved in their lack of response to FGF-1. The 128- and 79-kDa FGF receptors were assigned as FGFR2 using an antibody specific to the ectodomain of FGFR2, whereas the 79-kDa receptor was not reactive to the antibody against the carboxyl terminus of FGFR2. This 79-kDa FGF receptor was not tyrosine-phosphorylated in response to FGF-1 stimulation, while the 128-kDa FGF receptor was recognized by anti-phosphotyrosine antibody under the same conditions. Also, the heterodimer of 79- and 128-kDa FGF receptors was less tyrosine-phosphorylated than the homodimer of 128-kDa FGF receptors. These data suggest that the 79-kDa FGF receptor inhibits the function of the 128-kDa FGF receptor through their heterodimerization. Thus, we surmise that the difference in response to FGF-1 between the cell populations of normal rat hepatocytes was caused by the different levels of the 79-kDa FGF receptor in each cell population.

Reduced expression of kan-1 (encoding putative bile acid-CoA-amino acid N-acyltransferase) mRNA in livers of rats after partial hepatectomy and during sepsis

We isolated a cDNA clone, kan-1, from a rat liver cDNA library using a reverse transcriptase PCR cloning method. The kan-1 cDNA encoded a polypeptide of 420 amino acids, and was 70 and 69% identical in nucleotide and amino acid sequences respectively with human liver bile acid-CoA-amino acid N-acyltransferase (BAT). Thus Kan-1 is probably a rat homologue of human BAT (rBAT). Kan-1/rBAT mRNA was mainly expressed in the livers of adult rats and rats immediately after, but not before, birth. It was expressed in the hepatocytes, the sinusoidal endothelial cells and the Kupffer cells of the liver. An anti-Kan-1/rBAT polyclonal antibody detected a protein of molecular mass 46 kDa in the liver. After partial hepatectomy, the levels of Kan-1/rBAT mRNA decreased at 6 and 12 h in the regenerating liver. In a sepsis model, hepatic expression of Kan-1/rBAT mRNA decreased at 6 and 12 h after caecal ligation and puncture. The kinetics of Kan-1/rBAT mRNA expression suggests that it may play a role in acute-phase reactions.

Growth of mouse hepatocytes is stimulated by gastrin

Hepatocyte growth is regulated by various growth factors, including epidermal growth factor (EGF) and insulin. Recently, several additional peptide hormones have been shown to stimulate growth of hepatocyte only in the presence of EGF or insulin and are thus termed secondary mitogens. Gastrin regulates growth of normal and neoplastic gastrointestinal tissues, but the effect on growth of hepatocyte is unknown. We examined the effect of gastrin on growth of a normal mouse hepatocyte (NMH) line established in our laboratory. Effect of gastrin-17 (G-17) (10(-8) to 10(-6) M) on growth of NHM cells was examined in either the presence or absence of EGF in the culture medium. Growth of NMH cells was evaluated by incorporation of either bromodeoxyuridine (BrdU) or 3H-thymidine and by counting cells. Presence of a cell-surface receptor for G-17 was determined by Scatchard analysis using 125I-G-17. In the presence of EGF, gastrin stimulated growth of NMH cells; in the absence of EGF, gastrin did not affect growth. The stimulatory effect of gastrin on NMH cells was blocked by JMV 320, a CCK-B type receptor antagonist. NMH cells possess a single, high affinity binding site for gastrin (Kd = 1.2 nM); EGF increased the gastrin binding capacity compared to non-treated cells (3.5 +/- 0.4 vs. 2.2 +/- 0.6 fmol/10(6) cells). G-17 stimulated growth of NMH cells through a single high affinity receptor for G-17 which pharmcologically appears to be the CCK-B type only in the presence of EGF and thus can be considered a secondary mitogen.

Use of hepatocyte cultures for liver support bioreactors

Hybrid artificial liver systems are being developed as extracorporeal temporary liver support therapy. Here, an overview is given with emphasis on hepatocyte culture models for bioreactors, in vitro studies, animal studies and the clinical application of hybrid liver support systems. In vitro studies show long term external metabolic functions of primary isolated hepatocytes in bioreactors. These systems are capable of supporting essential liver functions. Animal experiments show the possibility of upscaling the bioreactors for clinical treatment. Since there is no reliable animal model for investigations on the treatment of acute liver failure, the promising results of these studies have limited relevance. The small number of clinical studies are not sufficient to give statements about a clinical improvement of therapy of acute liver failure. Although important progress has been made in the development of the systems, multiple different hepatocyte culture models and bioreactor constructions are discussed in the literature, indicating competition in this field of medical research.

Studies of tamoxifen as a promoter of hepatocarcinogenesis in female Fischer F344 rats

Tamoxifen, an antiestrogen used in the treatment of breast cancer, was assessed for carcinogenic potential in the two-stage model of experimental hepatocarcinogenesis. Groups of female Fisher F344 rats were initiated with a non-necrogenic, subcarcinogenic dose of diethylnitrosamine (DEN; 10 mg/kg, po) and fed tamoxifen at a concentration of 250 mg per kg of AIN-76A diet for 6 or 15 months. The livers of these animals exhibited an increase in size and number of altered hepatic foci compared with those animals which were initiated with DEN but not exposed to tamoxifen. This finding indicates that tamoxifen may have a carcinogenic potential in the rat liver. After 6 months of treatment, neoplastic nodules were observed in 3/8 rats in the DEN-initiated, tamoxifen-treated group. In the initiated group provided with tamoxifen for 15 months, neoplastic nodules were observed in 7/8 rats and hepatocellular carcinomas in 3/8 rats. The serum level of tamoxifen in these rats was 200-300 ng/ml. The ratio of tamoxifen, 4-hydroxy tamoxifen, and N-desmethyl tamoxifen was 1:0.1:0.5-1 in the serum. When adjusted for age-related weight increases, the serum and liver levels of tamoxifen and its N-desmethyl metabolite did not change over the 15 months. In the rat liver, the level of tamoxifen and its N-desmethyl metabolite was 10-29 micrograms/g liver after 6 or 15 months of chronic dietary administration. The ratio of tamoxifen:4-hydroxy tamoxifen:N-desmethyl tamoxifen was 1:0.1.3-3.3 in the liver. Therefore, the liver had 20- to 30-fold more tamoxifen and 4-hydroxy tamoxifen and at least 100-fold more N-desmethyl tamoxifen than the serum (assuming 1 gram of tissue is equivalent to 1 ml of serum). These results indicate that tamoxifen is a promoting agent for the rat liver at serum levels found in patients given the usual therapeutic course of tamoxifen. The high concentrations of tamoxifen attained in the rat liver indicate that actions other than its known estrogenicity for liver could contribute to its promoting action. In addition, these results indicate that the pharmacodynamic differences in tamoxifen metabolism in rats and humans and at low versus high doses should be determined. Thus, the therapeutic indications for tamoxifen should be balanced by the potential risk it may present as a promoting agent in mammalian liver.

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.

Rat hepatocyte cultures and co-cultures in biotransformation studies of xenobiotics

Long-term cultures of hepatocytes could represent a suitable in vitro model for biotransformation studies of xenobiotics. At present however, no ideal culture system can be proposed since, in all existing models, phenotypic changes occur, affecting selectively some components of phase I and/or phase II xenobiotic metabolism. From the authors' own results and recent studies of several other investigators, carried out on rat hepatocytes, it becomes clear that four groups of factors may affect biotransformation capacity: soluble medium factors, extracellular matrix components, cell-cell interactions and factors affecting replication. For the maintenance of liver-specific functions, it seems of utmost importance that the tridimensional shape of the hepatocytes is kept. Usually, phase II enzymatic activity is better kept than that of phase I. The cytochrome P450 dependent monoxygenases, in particular, are easily lost. Interesting is the observation that co-cultures of rat hepatocytes with rat liver epithelial cells exhibit higher and much better preserved phase I and phase II biotransformation than monocultures. Clearly, further research is needed to improve this promising in vitro model.

Hepatocyte function in a hollow fiber bioreactor: a potential bioartificial liver

We have developed a novel hepatocyte loaded hollow fiber bioreactor as a potential bioartificial liver. Freshly harvested rat hepatocytes were entrapped in a three-dimensional gel matrix within hollow fibers in a perfused bioreactor. Gel entrapment allowed cells to be cultured at high density while maintaining tissue-specific function. Hepatocyte function was evaluated in 10 bioreactors, each containing approximately 5 x 10(7) cells. Oxygen consumption averaged 0.32 pmole/cell/hr, albumin appearance averaged 0.60 pg/cell/hr, and lidocaine clearance (a measure of the P-450 activity) averaged 0.74 pg/cell/hr. Function persisted for the 7 days of the study. Electron microscopy at 7 days showed the distinctive ultrastructure of viable, differentiated hepatocytes: bile canaliculi, intercellular junctions, peroxisomes, abundant mitochondria, and glycogen granules. Maintenance of tissue specific function and ultrastructure suggests that this bioreactor configuration has potential as a device to support patients in liver failure, as well as to study hepatocytes in vitro.

Differential expression of the transfected liver-specific alpha 1-inhibitor III gene in normal hepatocytes and hepatoma cells in culture

Normal and malignant hepatocytes were transfected during log phase culture with a nested series of DNA plasmids containing 5'-flanking regions of the rat liver-specific acute phase plasma proteinase alpha 1-inhibitor III (alpha 1 I3) gene. Under these conditions, luciferase reporter gene expression in primary adult rat and mouse hepatocytes was 10-fold higher than luciferase expression in hepatoma lines (human HepG2 and Hep3B; rat FAZA). Optimal expression in primary rat hepatocytes required regions stretching 2214 bp 5'-upstream of the transcription start site. Shorter 5'-flanking sequences were optimal for expression in hepatoma cells (-1025 and -186 bp for rat and human lines, respectively) and primary mouse hepatocytes (-225 bp). In contrast, regions from -186 to -225 bp drove luciferase expression in primary rat hepatocytes, but only 20-75% of optimal levels. Qualitative differences were unaccounted for by non-equivalent uptake of plasmid DNA, suggesting that tissue specific gene expression is regulated differently in normal and malignant cells, and with apparent species specificity.

Primary culture of rat hepatocytes entrapped in cylindrical collagen gels: An in vitro system with application to the bioartificial liver

A static culture model employing cylindrical collagen-hepatocyte gels is reported for large scale testing of conditions relevant to the three compartment hollow fiber bioartificial liver. High density hepatocyte cultivation was achieved by cell entrapment within the collagen-hepatocyte gel. Hepatocyte viability was assessed by vital staining, gel contraction, and insulin utilization. Measures of hepatocyte-specific function included albumin synthesis, ureagenesis, lidocaine biotransformation, and cholate conjugation. Although hepatocyte viability remained stable through the seven day incubation period, hepatocyte functions were not uniformly preserved. Albumin synthesis remained stable, while representative P-450 and conjugation activities decreased with time. This static culture system will facilitate the development of a hollow fiber bioartificial liver which utilizes cylindrical collagen-hepatocyte gels.

Cell-density dependent expression of the c-myc gene in primary cultured rat hepatocytes

During culture of mature rat hepatocytes as monolayers, c-myc mRNA was found to be expressed transiently within 2 h, decreasing rapidly to the basal level at 10 h. Then its level increased again to over 10-fold the basal level at 24 h, and remained at this high level during culture. The increase of c-myc mRNA in the second phase was shown by nuclear run-off experiments to be due to an increase of its transcription. The second, but not the first, increase in c-myc expression was inversely proportional to the cell density in culture. The expression of c-myc mRNA was not affected by various hormones including growth factors. These results indicate that hepatocytes in culture at lower cell density tend to move from the Go phase to the G1 phase, but remain in the Go phase when cultured at high cell density.