Extended liver-specific functions of porcine hepatocyte spheroids entrapped in collagen gel

Structural polarity and functional bile canaliculi in rat hepatocyte spheroids

Primary hepatocytes self-assemble into spheroids that possess tight junctions and microvilli-lined channels. We hypothesized that polarity develops gradually and that the channels structurally and functionally resemble bile canaliculi. Immunofluorescence labeling of apical and basolateral proteins demonstrated reorganization of the membrane proteins into a polarized distribution during spheroid culture. By means of fluorescent dextran diffusion and confocal microscopy, an extensive network of channels was revealed in the interior of the spheroids. These channels connected over several planes and opened to pores on the surface. To examine the content of apical proteins in the channel membranes, the bile canalicular enzyme dipeptidyl peptidase IV (DPPIV) was localized using a fluorogenic substrate, Ala-Pro-cresyl violet. The results show that DPPIV activity is heterogeneously distributed in spheroids and localized in part to channels. Bile acid excretion was then investigated to demonstrate functional polarity. A fluorescent bile acid analogue, fluorescein isothiocyanate-labeled glycocholate, was taken up into the spheroids and excreted into bile canalicular channels. Due to the structural polarity of spheroids and their ability to excrete bile into channels, they are a unique three-dimensional model of in vitro liver tissue self-assembly. (Videoanimations of some results are available at http://hugroup.cems.umn.edu/research_movies).

Probing enhanced cytochrome P450 2B1/2 activity in rat hepatocyte spheroids through confocal laser scanning microscopy

Cytochrome P450 (CYP450) enzymes are essential for xenobiotic metabolism. Although CYP450s are found in many tissues, CYP2B1/2 are primarily expressed in the rat liver. The constitutive expression in vivo of CYP2B1/2 is low but it is induced in the presence of various drugs such as phenobarbital (PB). In this study, CYP2B1/2 activity in cultured hepatocytes was assessed in situ with the introduction of a fluorogenic substrate, pentoxyresorufin. The product of 7-pentoxyresorufin-O-dealkylation (PROD), which is catalyzed specifically by CYP2B1/2, was detected using confocal laser scanning microscopy (CLSM). Primary hepatocytes cultured as monolayers on collagen-coated surfaces exhibited background PROD activity and minimal PB inducibility after 4 days in culture. In contrast, rat hepatocytes organized in compacted aggregates, or spheroids, exhibited higher levels of PROD activity and retained their ability for PB induction. The results from the CLSM analysis were verified by RT-PCR and Western immunoblotting analysis. Furthermore, CLSM in conjunction with image processing techniques and three-dimensional reconstruction revealed the localization of enhanced PROD activity in the center of spheroids. The results support the use of CLSM as a powerful tool for investigating CYP2B1/2 activity in cultured rat hepatocytes.

Expression of liver-specific functions by rat hepatocytes seeded in treated poly(lactic-co-glycolic) acid biodegradable foams

Techniques of liver replacement would benefit patients awaiting donor livers and may be a substitute for transplantation in patients whose livers can regenerate. Poly(lactic-co-glycolic acid) (PLGA) copolymers are biodegradable and have been shown to be useful as scaffolds for seeding and culturing various types of cells. In this study, foam disks were prepared from PLGA (lactic-to-glycolic mole ratio of 85:15) by lyophilization of benzene (5% w/v) solutions. These disks were then used as scaffolds for rat hepatocyte culture. Foams were coated with either a type I collagen gel (0.1% w/v), coated with gelatin (5% w/v), or treated with oxygen plasma (25 W, 90 s) to modify their surface chemistry and wettability. The disks were then seeded with rat hepatocytes (10(6)/mL) and cultured for a period of 2 weeks. All surface treatments resulted in increased hydrophilicity, the greatest being obtained by collagen treatment (contact angle < 10 degrees ), and a minimal decrease in void fraction (5%). DNA content after a 2-week culture period increased proportionally with the wettability of the treated foam surface. Urea synthesis in untreated foams averaged 15.3 +/- 2.3 microg/h/microg DNA, which was significantly higher than that for controls, whereas gelatin and collagen treated foams exhibited urea synthetic rates below the control levels at all times. The DNA content decreased significantly by about 50% between days 1 and 12. PLGA foams, treated and untreated, represent a promising scaffold for scaling up hepatocyte cultures.

Influence of preculture on the prefreeze and postthaw characteristics of hepatocytes

Recent studies performed in our laboratory have shown that a brief period of preculture prior to cryopreservation improves the postthaw viability of hepatocytes. The purpose of this investigation is to characterize specific metabolic and biochemical characteristics of the hepatocytes (both frozen and nonfrozen) to help elucidate the role of preculture on the postthaw viability. Fresh and thawed hepatocytes were cultured in a bioartificial liver (BAL) to determine albumin secretion as a function of time in culture. In addition, cell extracts were analyzed using nuclear magnetic resonance (NMR) spectroscopy to quantify changes in cell membrane composition and energetics as a function of time in culture prefreeze and postthaw. The results of these studies showed an increase in albumin concentration in the culture medium with time in culture for the period tested for both fresh and frozen and thawed hepatocytes. NMR spectroscopy of lipid extracts indicates that in vitro culture of hepatocytes results in an increase in cholesterol relative to membrane phospholipid. Moreover, the NMR results also indicate phospholipid interconversion, via specific lipases in cultured hepatocytes, and these changes are consistent with water permeability measurements performed previously. Significant changes in phosphoenergetics were also observed, with the net energy charge for the cells increasing significantly with time in culture. In addition, NMR spectra show increased levels of 6-phosphogluconate, another indicator of the cellular response to the stresses of isolation and ex vivo culture. These results suggest that energetic considerations may be a significant factor in the ability of hepatocytes to survive the stresses of freezing and thawing. Significant shifts in membrane phospholipids may also influence membrane permeability and postthaw survival.

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.

Intensive promotion of spheroid formation by soluble factors in a hepatocyte-conditioned medium

We developed a hybrid artificial liver and a drug metabolism simulator using polyurethane foam (PUF) in which primary hepatocytes spontaneously form functional spheroids. Gel filtration liquid chromatography analysis of a hepatocyte-conditioned medium during spheroid formation showed that some substances secreted by primary rat hepatocytes accumulated advantageously inside the pores of PUF compared with outside. Similar substances were detected in a hepatocyte-conditioned medium from a positively-charged surface by concentrating the substances using an ultrafiltration membrane of a molecular weight-cutoff of 50 kD. These substances were shown to act as soluble factors on freshly isolated primary rat hepatocytes to promote spontaneous and rapid spheroid formation, depending on their concentration by preventing them from initially attaching and spreading on a positively-charged surface. In particular, using 50-fold concentrated substances, about 80% of total hepatocytes formed the floating spheroids within 72 h of culture. The resulting spheroids had a diameter distribution mainly ranging from 40 to 70 microm and expressed high-level liver-specific functions compared with a conventional monolayer.

Postthaw viability of precultured hepatocytes

Hepatocytes are being studied for a wide variety of applications, including drug metabolism studies, gene therapy, and use in liver-assist devices for temporary liver support. The ability to cryopreserve isolated hepatocytes would permit the pooling of cells to reach the required therapeutic coordination of the cell supply with patient care regimes and the completion of safety and quality-control testing. The objective of this investigation was to develop a method of cryopreserving isolated hepatocytes that will retain high levels of function and facilitate the use of the cells in different applications. Freshly isolated hepatocytes were cultured in a spinner flask for different periods of time, up to 48 h. The cells were cryopreserved by use of a range of solution concentrations and cooling rates. For fresh, nonfrozen hepatocytes precultured for 24 h prior to being plated on collagen, the albumin secretion rate was 0.88 +/- 0.62 mg/ml/h. When the cells were precultured for 24 h, frozen in a solution containing 10% Me2SO with a cooling rate of 1 degrees C/min, thawed, plated on collagen, and cultured, the albumin secretion rate was 0.21 +/- 0.24 microg/ml/h. In contrast, freshly isolated hepatocytes cryopreserved without preculture and cultured on collagen had an albumin secretion rate of 0.07 +/- 0.08 mg/ml/h. The influences of different solution compositions and cooling rates on postthaw function of precultured hepatocytes were also determined. These results indicate that the use of a preliminary culture step prior to cryopreservation can enhance the postthaw function of hepatocytes.

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.

Optimization of rat hepatocyte culture in citrated human plasma

BACKGROUND

Maintenance of liver-specific functions in hepatocyte cultures during plasma exposure is critical for the clinical application of bioartificial liver assist systems. Sodium citrate is a common anticoagulant but has been shown to be cytotoxic to hepatocytes. We have tested the effect of various supplements on the viability and function of adult primary rat hepatocytes exposed to citrated plasma.

MATERIALS AND METHODS

Freshly isolated rat hepatocytes were cultured in the collagen gel sandwich configuration in culture medium for 6 days followed by exposure to citrated human plasma with various supplements for 1 week. Controls were left in culture medium throughout. Viability and synthetic functions were evaluated.

RESULTS

Hepatocytes exposed to unsupplemented citrated plasma lost significant viability and function within the first 2 days. Cells cultured in plasma supplemented with a fivefold concentrate of standard hepatocyte culture medium maintained urea (1. 2-2.1 micromol/day/10(6) cells) and albumin (51-62 microg/day/10(6) cells) synthesis rates equal to or higher than those of controls. Among the various components of the concentrated medium supplement, calcium chloride (1.8 mM), magnesium sulfate (0.8 mM), amino acids (fourfold Basal Medium Eagle amino acids including 4 mM glutamine), and glucagon (14 ng/ml) were found to be essential in maintaining urea synthesis. Maintenance of a high albumin synthesis rate also required the addition of hydrocortisone (7.5 microg/ml) and insulin (0.5 U/ml).

CONCLUSIONS

Appropriate metabolic and hormonal supplementation of citrated human plasma prevents its cytotoxic effects and may be used in conjunction with in vivo use of bioartificial liver assist systems.

Evaluation of the effect of culture matrices on induction of CYP3A isoforms in cultured porcine hepatocytes

Several bioartificial liver devices have been developed as temporary therapy for patients suffering from fulminant hepatic failure. Some of these devices contain porcine hepatocytes entrapped in collagen matrices. In order to improve the function of these BAL devices, there exists a need to optimize metabolic function of cultured hepatocytes. The goal of these investigations was to evaluate the effect of altering culture conditions on rifampin-mediated induction of CYP3A isoforms in cultured porcine hepatocytes. Midazolam metabolism was compared in porcine hepatocytes cultured in a monolayer configuration on collagen gels, in a sandwich configuration between collagen gels and a Matrigel overlay, and in spheroidal cultures. The effect of culture conditions was evaluated, by measuring CYP3A-mediated metabolism of midazolam and by immunoblotting to detect CYP3A proteins, in control cultures and in rifampin-treated cultures. Results obtained by normalizing the metabolism rate data to cell numbers (based on DNA content) present at the end of the culture experiment, showed that there was no difference between the different culture conditions tested. Our results suggest that culturing porcine hepatocytes as spheroids or in a sandwich configuration between collagen and Matrigel, offers no advantage in terms of CYP3A-mediated metabolic function on a per cell basis compared to culturing on collagen gels.

Microencapsulation of normal and transfected L929 fibroblasts in a HEMA-MMA copolymer

Mouse L929 fibroblasts transfected to express a secreted form of human alkaline phosphatase (SEAP) were encapsulated in approximately 400-microm poly(hydroxyethyl methacrylate-co-methyl methacrylate) (HEMA-MMA) microcapsules as a baseline for the use of genetically engineered cells in encapsulation therapy. Although incubation of microcapsules with serum-containing medium resulted in maintaining the number of live encapsulated cells with the passage of time, incubation in a serum-free medium resulted in a three-fold proliferation of the encapsulated cells within a 3-week observation period. Similar to the results for incubation with serum-containing medium, co-encapsulation with a bovine dermal type I collagen, i.e., the inclusion of a matrix in the core of the capsules, resulted in maintenance of the initial number of live cells with the passage of time. SEAP measurements indicated that the transfected cells not only continued to express the transgene product after encapsulation, but also adapted to the capsule microenvironment to secrete SEAP at progressively larger amounts with the passage of time. However, SEAP expression only occurred when the transfected cells (encapsulated or non-encapsulated) were cultivated in serum-containing medium.

Characterization of drug metabolizing activities in pig hepatocytes for use in bioartificial liver devices: comparison with other hepatic cellular models

BACKGROUND/AIMS

The pig is considered the best donor of hepatocytes for bioartificial liver devices, but little is known about the metabolic capability of pig hepatocytes. Therefore, we have evaluated drug metabolizing activities in pig hepatocytes and liver microsomes and compared the results with those of man and other animal hepatic cellular models that are potential sources of cells for bioreactors, such as rat, rabbit and dog hepatocytes and hepatoma cell lines.

METHODS

Total cytochrome P450 levels, six phase 1 activities representative of the most relevant cytochrome P450 enzymes (7-ethoxycoumarin O-deethylase, 7-ethoxy-, 7-methoxy- and 7-benzoxyresorufin O-dealkylases, coumarin 7-hydroxylase and p-nitrophenol hydroxylase), two phase 2 activities (glutathione S-transferase and UDP-glucuronyltransferase) and CYP-dependent regioselective testosterone metabolism were evaluated in in vitro models of different species.

RESULTS

The pattern of specific cytochrome P450 activities and the metabolic profile of testosterone in intact hepatocytes were essentially the same as those measured in liver microsomes. Relatively low ethoxy-, methoxy-, and benzoxyresorufin O-dealkylation rates were found in pig liver microsomes and hepatocytes as compared to hepatic in vitro human models. However, in contrast with the other species studied, stereoselective testosterone oxidation profiles were practically identical in human and pig models. Finally, the metabolic capability of hepatoma cell lines was very limited in comparison with that of hepatocytes.

CONCLUSIONS

Pig hepatocytes are able to maintain in culture the phase 1 and phase 2 activities found in liver microsomes. The high metabolic similarities found between pig and human hepatocytes lend support to the use of pig hepatocytes in bioartificial liver devices.

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.

Receding cytochrome P450 activity in disassembling hepatocyte spheroids

Primary rat hepatocytes can self-assemble to form multicellular spheroids when plated onto Primaria petri dishes or suspended in stirred vessels. These spheroids exhibit prolonged viability, enhanced liver-specific functions and differentiated ultrastructure compared to monolayer cultures. Upon transfer to collagen coated surface, or upon the addition of fetal bovine serum (FBS) to the culture, these spheroids began to disassemble and spread on the surface. The dynamics of cytochrome P450 CYP1A1/2 activity in the course of spheroid disassembly was examined in situ by detection of the fluorescent product, resorufin, of ethoxyresorufin O-dealkylation. Optical sectioning of the disassembling spheroids by confocal microscopy demonstrated that hepatocytes that reverted to monolayer exhibited markedly lower CYP1A1/2 activity than those that remained in a multilayered structure. This occurred whether the disassembly was caused by incubation with FBS-containing medium or by cultivation on a collagen-coated surface. When spheroids were cultured on the surface of agar, the disassembly process was retarded even in the presence of FBS. However, even in those intact spheroids, the exposure to FBS markedly decreased CYP1A1/2 activity. The decreased CYP1A1/2 activity was correlated to a diminished smooth endoplasmic reticulum as seen in the transmission electron micrograph. The results clearly demonstrate that the disassembly of hepatocyte spheroids led to decreased CYP1A1/2 activity. Furthermore, FBS contained a factor that caused CYP1A1/2 to decrease even in intact spheroids.

Evaluation of a hybrid artificial liver using a polyurethane foam packed-Bed culture system in dogs

BACKGROUND AND AIMS

We developed a polyurethane foam packed-bed culture system of hepatocyte spheroids as a hybrid artificial liver (PUF-HAL), which was effective for recovery from liver failure in rat experiments. In this report, the design of a scaled-up PUF-HAL for dogs is described and evaluated using a dog acute liver failure model.

METHODS

Warm ischemic liver failure was induced with a portocaval shunt in each dog. The dogs were divided into two groups: (1) a control group (N = 4), in which each dog was attached to a PUF-HAL without hepatocytes for 9 h, and (2) a HAL group (N = 5), in which each dog was attached to a PUF-HAL with hepatocytes. Blood pressure, blood ammonia, blood glucose, serum creatinine, and other parameters related to liver function were compared between the two groups.

RESULTS

In the HAL group, blood ammonia and serum creatinine levels were significantly lower, and blood pressure and blood glucose levels significantly higher, than those in the control group.

CONCLUSIONS

The scaled-up PUF-HAL developed for large animals is useful as a liver support system in the dog acute liver failure model.

A model of low-temperature water transport for hepatocyte spheroids

Spheroids are multicellular aggregates that exhibit a more tissue-like morphology and function when compared to monolayer cultures of the same cells. Hepatocyte spheroids are presently under investigation for use of an artificial liver. The ability to cryopreserve hepatocyte spheroids is essential for their clinical and commercial application. A multicompartment model was formulated to predict water content as a function of temperature during freezing. The theoretical predictions of water transport indicate that there will be spatial differences in water content of the spheroid during freezing and that due to the rapid decrease in water transport with decreasing temperature, the undercooling of the intracellular solution during freezing will increase steadily. These results indicate that conventional freezing of hepatocyte spheroids will be difficult to accomplish due to transport limitations in the spheroids.

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

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