The PICM-19 cell line as an in vitro model of liver bile ductules: effects of cAMP inducers, biopeptides and pH

Characterization of Two Subpopulations of the PICM-19 Porcine Liver Stem Cell Line for use in Cell-Based Extracorporeal Liver Assistance Devices

Two cell lines, PICM-19H and PICM-19B, were derived from the bipotent PICM-19 pig liver stem cell line and assessed for their potential application in artificial liver devices (ALD). The study included assessments of growth rate and cell density in culture, morphological features, serum protein production, γ-glutamyltranspeptidase (GGT) activity and hepatocyte detoxification functions, i.e., inducible P450 activity, ammonia clearance, and urea production. The PICM-19H cell line was derived by temperature selection at 33–34°C. After each passage, PICM-19H cells grew to a nearly confluent monolayer of cells of hepatocyte morphology, i.e., cuboidal cells with centrally located nuclei joined by biliary canaliculi. No differentiation and self-organization into multi-cellular bile ductules, as observed in the parental PICM-19 cell line, occurred within the PICM-19H cell monolayers. The PICM-19H cells contained numerous mitochondria, Golgi apparatus, smooth and rough endoplasmic reticulum, vesicular bodies and occasional lipid vacuoles. The cells had a doubling time of 48–72 h and reached a final density of 1.5 x 105 cells/cm2 at ∼10 d post-passage from a 1:6 split ratio. PICM-19H cells displayed inducible P450 activity, cleared ammonia, and produced urea in a glutamine-free medium. The PICM-19B cells were colony-cloned after spontaneous generation from the PICM-19 parental cell line. PICM-19B cells grew as a tightly knit dome-forming monolayer with no visible biliary canaliculi. Their doubling time was 48–72 h with a final cell density of 2.6 x 105 cells/cm2. Ultrastructural analysis of the PICM-19B monolayers showed the roughly cuboidal cells displayed basal-apical polarization and were joined by tight junction-like complexes. Other ultrastructure features were similar to those of PICM-19H cells except that they possessed numerous cell bodies resembling mucus vacuoles. The PICM-19B cells had relatively high levels of GGT activity, but did retain some inducible P450 activity, and some ammonia clearance and urea synthesis ability. PICM-19B cells produced markedly less serum proteins than PICM-19H cells. These data indicated that both cell lines, either together or alone, may be useful as the cellular substrate for an ALD.

Parasympathetic innervation regulates tubulogenesis in the developing salivary gland

A fundamental question in development is how cells assemble to form a tubular network during organ formation. In glandular organs, tubulogenesis is a multistep process requiring coordinated proliferation, polarization and reorganization of epithelial cells to form a lumen, and lumen expansion. Although it is clear that epithelial cells possess an intrinsic ability to organize into polarized structures, the mechanisms coordinating morphogenetic processes during tubulogenesis are poorly understood. Here, we demonstrate that parasympathetic nerves regulate tubulogenesis in the developing salivary gland. We show that vasoactive intestinal peptide (VIP) secreted by the innervating ganglia promotes ductal growth, leads to the formation of a contiguous lumen, and facilitates lumen expansion through a cyclic AMP/protein kinase A (cAMP/PKA)-dependent pathway. Furthermore, we provide evidence that lumen expansion is independent of apoptosis and involves the cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-regulated Cl(-) channel. Thus, parasympathetic innervation coordinates multiple steps in tubulogenesis during organogenesis.

The importance of being a lumen

Advances in tissue engineering and microtechnology have enabled researchers to more easily generate in vitro tissue models that mimic the tissue geometry and spatial organization found in vivo (e.g., vessel or mammary duct models with tubular structures). However, the widespread adoption of these models for biological studies has been slow, in part due to the lack of direct comparisons between existing 2-dimensional and 3-dimensional cell culture models and new organotypic models that better replicate tissue structure. Using previously developed vessel and mammary duct models with 3-dimensional lumen structures, we have begun to explore this question. In a direct comparison between these next generation organotypic models and more traditional methods, we observed differences in the levels of several secreted growth factors and cytokines. In addition, endothelial vessel geometry profoundly affects the phenotypic behavior of carcinoma cells, suggesting that more traditional in vitro assays may not capture in vivo events. Here, we seek to review and add to the increasing evidence supporting the hypothesis that using cell culture models with more relevant tissue structure influences cell fate and behavior, potentially increasing the relevance of biological findings.

A feeder-cell independent subpopulation of the PICM-19 pig liver stem cell line capable of long-term growth and extensive expansion

A method for the feeder-independent culture of PICM-19 pig liver stem cell line was recently devised, but the cell line's growth was finite and the cells essentially ceased dividing after approximately 20 passages over a 1 year culture period. Here we report the isolation, continuous culture, and initial characterization of a spontaneously arising feeder-independent PICM-19 subpopulation, PICM-19FF, that maintained replication rate and hepatocyte functions over an extended culture period. PICM-19FF cells grew to 90-98 % confluency after each passage at 2 week intervals, and the cells maintained a high cell density after 2 years and 48 passages in culture (average of 2.6 × 10(6) cells/T25 flask or 1 × 10(5) cells/cm(2)). Morphologically, the PICM-FF cells closely resembled the finite feeder-independent PICM-19 cultures previously reported, and, as before, no spontaneous formation of 3D multicellular ductules occurred in the cells' monolayer. Their bipotent stem cell nature was therefore not evident. Over extensive passage, cytochrome P450 (EROD) activity was maintained, although urea production was reduced on a per mg protein basis at later passages. Two other attributes of fetal hepatocytes, γ-glutamyl transpeptidase activity and serum-protein secretion, were also shown to be maintained by the PICM-19FF cells. The PICM-19FF cells therefore appear to have indefinite growth potential as a feeder-independent cell line and this should enhance the experimental usefulness of the cell line, in general, and may also improve its application to toxicological/pharmacological assays and artificial liver devices.

Growth and Development Symposium: Development, characterization, and use of a porcine epiblast-derived liver stem cell line: ARS-PICM-19

Totipotent embryonic stem cell lines have not been established from ungulates; however, we have developed a somatic stem cell line from the in vitro culture of pig epiblast cells. The cell line, ARS-PICM-19, was isolated via colony cloning and was found to spontaneously differentiate into hepatic parenchymal epithelial cell types, namely hepatocytes and bile duct cells. Hepatocytes form as monolayers and bile duct cells as 3-dimensional bile ductules. Transmission electron microscopy revealed that the ductules were composed of radially arranged, monociliated cells with their cilia projecting into the lumen of the ductule whereas hepatocytes were arranged in monolayers with lateral canalicular structures containing numerous microvilli and connected by tight junctions and desmosomes. Extensive Golgi and rough endoplasmic reticulum networks were also present, indicative of active protein synthesis. Analysis of conditioned medium by 2-dimensional electrophoresis and mass spectrometry indicated a spectrum of serum-protein secretion by the hepatocytes. The PICM-19 cell line maintains a range of inducible cytochrome P450 activities and, most notably, is the only nontransformed cell line that synthesizes urea in response to ammonia challenge. The PICM-19 cell line has been used for several biomedical- and agricultural-related purposes, such as the in vitro replication of hepatitis E virus, a zoonotic virus of pigs, and a spaceflight experiment to evaluate somatic stem cell differentiation and liver cell function in microgravity. The cell line was also evaluated as a platform for toxicity testing and has been used in a commercial artificial liver rescue device bioreactor. A PICM-19 subclone, PICM-19H, which only differentiates into hepatocytes, was isolated and methods are currently under development to grow PICM-19 cells without feeder cells. Feeder-cell-independent growth will facilitate the study of mesenchymal-parenchymal interactions that influence the divergent differentiation of the PICM-19 cells, enhance our ability to genetically modify the cells, and provide a better model system to investigate porcine hepatic metabolism.

Cytochrome P450 expression profile of the PICM-19H pig liver cell line: potential application to rapid liver toxicity assays

Liver in vitro models are needed to replace animal models for rapid assessment of drug biotransformation and toxicity. The PICM-19 pig liver stem cell line may fulfill this need since these cells have activities associated with xenobiotic phase I and II metabolism lacking in other liver cell lines. The objective of this study was to characterize phase I and II metabolic functions of a PICM-19 derivative cell line, PICM-19H, compared to the tumor-derived human HepG2 C3A cell line and primary cultures of adult porcine hepatocytes. Following exposure of PICM-19H cells to either 3-methylcholanthrene, rifampicin or phenobarbital, the induced activities of cytochrome P450 (CYP450) isozymes CYP-1A, -2, and-3A were assessed. Relative to adult porcine hepatocytes, PICM- 19H cells exhibited 30% and 43%, respectively, of CYP1A and 3A activities, while HepG2 C3A cells exhibited 7% and 0% of those activities. Fluorescent metabolites were extensively conjugated, i.e., 52% and 96% of CYP450-1A and-3A metabolites were released from medium samples following treatment with β-glucuronidase/arylsulfatase. Rifampicin induction of CYP450 isozyme activities was confirmed by conversion of testosterone to 6β-OH-, 2α-OH- and 2β-OH-testosterone, as determined by mass spectrometry. Susceptibility of PICM-19H cells to acetaminophen toxicity was determined; CD50 was calculated to be 14.9±0.9 mM. Toxicity and bioactivation of aflatoxin B1 was determined in 3-methylcholanthrenetreated cultures and untreated controls; CD50 were 1.59 μM and 31 μM, respectively. These results demonstrate the potential use of PICM-19H cells in drug biotransformation and toxicity testing and further support their use in extracorporeal artificial liver device technology.

Culture of porcine hepatocytes or bile duct epithelial cells by inductive serum-free media

A serum-free, feeder cell-dependent, selective culture system for the long-term culture of porcine hepatocytes or cholangiocytes was developed. Liver cells were isolated from 1-wk-old pigs or young adult pigs (25 and 63 kg live weight) and were placed in primary culture on feeder cell layers of mitotically blocked mouse fibroblasts. In serum-free medium containing 1% DMSO and 1 μM dexamethasone, confluent monolayers of hepatocytes formed and could be maintained for several wk. Light and electron microscopic analysis showed hepatocytes with in vivo-like morphology, and many hepatocytes were sandwiched between the feeder cells. When isolated liver cells were cultured in medium without dexamethasone but with 0.5% DMSO, monolayers of cholangioctyes formed that subsequently self-organized into networks of multicellular ductal structures, and whose cells had monocilia projecting into the lumen of the duct. Gamma-glutamyl transpeptidase (GGT) was expressed by the cholangiocytes at their apical membranes, i.e., at the inner surface of the ducts. Cellular GGT activity increased concomitantly with the development of ductal structures. Cytochrome P-450 was determined in microsomes following addition of metyrapone to the cultures. In vivo-like levels of P-450s were found in hepatocyte monolayers while levels of P-450 were markedly reduced in cholangiocyte monolayers. Serum protein secretion in conditioned media was analyzed by Western blot and indicated that albumin, transferrin, and haptoglobin levels were maintained in hepatocytes while albumin and haptoglobin declined over time in cholangiocytes. Quantitative RT-PCR analysis showed that serum protein mRNA levels were significantly elevated in the hepatocytes monolayers in comparison to the bile ductule-containing monolayers. Further, mRNAs specific to cholangiocyte differentiation and function were significantly elevated in bile ductule monolayers in comparison to hepatocyte monolayers. The results demonstrate an in vitro model for the study of either porcine hepatocytes or cholangiocytes with in vivo-like morphology and function.

Feeder-independent continuous culture of the PICM-19 pig liver stem cell line

The PICM-19 pig liver stem cell line is a bipotent cell line, i.e., capable of forming either bile ductules or hepatocyte monolayers in vitro, that was derived from the primary culture of pig embryonic stem cells. The cell line has been strictly feeder-dependent in that cell replication, morphology, and function were lost if the cells were cultured without STO feeder cells. A method for the feeder-independent continuous culture of PICM-19 cells (FI-PICM-19) is presented. PICM-19 cells were maintained and grown without feeder cells on collagen I-coated tissue culture plastic for 26 passages (P26) with initial split ratios of 1:3 that diminished to split ratios of less than 1:2 after passage 16. Once plated, the FI-PICM-19 cells were overlaid with a 1:12 to 1:50 dilution of Matrigel or related extracellular matrix product. Growth of the cells was stimulated by daily refeedings with STO feeder-cell conditioned medium. The FI-PICM-19 cells grew to an approximate confluence of 50% prior to each passage at 2-wk intervals. Growth curve analysis showed their average cell number doubling time to be ~96 h. Morphologically, the feeder-independent cells closely resembled PICM-19 cells grown on feeder cells, and biliary canalicui were present at cell-to-cell junctions. However, no spontaneous multicellular ductules formed in the monolayers of FI-PICM-19 cells. Ultrastructural subcellular features of the FI-PICM-19 cells were similar to those of PICM-19 cells cultured on feeder cells. The FI-PICM-19 cells produced a spectrum of serum proteins and expressed many liver/hepatocyte-specific genes. Importantly, cytochrome P450 (EROD) activity, ammonia clearance, and urea production were maintained by the feeder-independent cells. This simple method for the propagation of the PICM-19 cell line without feeder cells should simplify the generation and selection of functional mutants within the population and enhances the cell line's potential for use in toxicological/pharmacological screening assays and for use in an artificial liver device.

The effects of space flight and microgravity on the growth and differentiation of PICM-19 pig liver stem cells

The PICM-19 pig liver stem cell line was cultured in space for nearly 16 d on the STS-126 mission to assess the effects of spaceflight on the liver's parenchymal cells-PICM-19 cells to differentiate into either monolayers of fetal hepatocytes or 3-dimensional bile ductules (cholangiocytes). Semi-quantitative data included light microscopic assessments of final cell density, cell morphology, and response to glucagon stimulation and electron microscopic assessment of the cells' ultrastructural features and cell-to-cell connections and physical relationships. Quantitative assessments included assays of hepatocyte detoxification functions, i.e., inducible P450 activities and urea production and quantitation of the mRNA levels of several liver-related genes. Three post-passage age groups were included: 4-d-, 10-d-, and 14-d-old cultures. In comparing flight vs. ground-control cultures 17 h after the space shuttle's return to earth, no differences were found between the cultures with the exception being that some genes were differentially expressed. By light microscopy both young and older cultures, flight and ground, had grown and differentiated normally in the Opticell culture vessels. The PICM-19 cells had grown to approximately 75% confluency, had few signs of apoptosis or necrosis, and had either differentiated into monolayer patches of hepatocytes with biliary canaliculi visible between the cells or into 3-dimensional bile ductules with well-defined lumens. Ultrastructural features between flight and ground were similar with the PICM-19 cells displaying numerous mitochondria, Golgi apparatus, smooth and rough endoplasmic reticulum, vesicular bodies, and occasional lipid vacuoles. Cell-to-cell arrangements were typical in both flight and ground-control samples; biliary canaliculi were well-formed between the PICM-19 cells, and the cells were sandwiched between the STO feeder cells. PICM-19 cells displayed inducible P450 activities. They produced urea in a glutamine-free medium and produced more urea in response to ammonia. The experiment's aim to gather preliminary data on the PICM-19 cell line's suitability as an in vitro model for assessments of liver function in microgravity was demonstrated, and differences between flight and ground-control cultures were minor.

A gel-based reference map of the porcine hepatocyte proteome

The overall goal of our research is to characterize and identify gene expression profiles of porcine hepatic cells. In this study, we have prepared two-dimensional electrophoresis maps of cytosol and membrane fractions from freshly prepared hepatocytes which were pooled from three crossbred pigs (35-69kg). Following isoelectric focusing with three pH range immobilized pH gradient strips (pH 3-6, 5-8 and 7-10) and staining the second dimension gels with colloidal Coomassie blue, 728 protein spots were picked and digested with trypsin. Extracted tryptic peptides were initially subjected to matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF-MS) analysis for identification of proteins by peptide mass fingerprinting (PMF). Proteins which were not identified by PMF were analyzed by liquid chromatography-tandem MS. Utilizing publicly available databases [NCBInr, Swiss Prot and expressed sequence tags (EST)], 648 proteins were identified. Of those, 282 were unique proteins and greater than 90% of proteins spots contained single proteins. These data represent the first comprehensive proteomic analysis of porcine hepatocytes and will provide a database for future investigations of endocrine regulation of gene expression and metabolic processes in vitro.

Bioartificial liver systems: why, what, whither?

Acute liver disease is a life-threatening condition for which liver transplantation is the only recognized effective therapy. While etiology varies considerably, the clinical course of acute liver failure is common among the etiologies: encephalopathy progressing toward coma and multiple organ failure. Detoxification processes, such as molecular adsorbent recirculating system (MARS®) and Prometheus, have had limited success in altering blood chemistries positively in clinical evaluations, but have not been shown to be clinically effective with regard to patient survival or other clinical outcomes in any Phase III prospective, randomized trial. Bioartificial liver systems, which use liver cells (hepatocytes) to provide metabolic support as well as detoxification, have shown promising results in early clinical evaluations, but again have not demonstrated clinical significance in any Phase III prospective, randomized trial. Cell transplantation therapy has had limited success but is not practicable for wide use owing to a lack of cells (whole-organ transplantation has priority). New approaches in regenerative medicine for treatment of liver disease need to be directed toward providing a functional cell source, expandable in large quantities, for use in various applications. To this end, a novel bioreactor design is described that closely mimics the native liver cell environment and is easily scaled from microscopic (<1 ml cells) to clinical (∼600 ml cells) size, while maintaining the same local cell environment throughout the bioreactor. The bioreactor is used for study of primary liver cell isolates, liver-derived cell lines and stem/progenitor cells.

Derivation and characterization of pluripotent cell lines from pig embryos of different origins

Embryonic stem cells (ESCs) hold great promise for therapeutic use and represent a unique tool for investigating the process of self-renewal and differentiation. The properties that make ESCs unique are their capacity of unlimited self-renewal coupled with the property of re-entering the developmental process if returned inside a blastocyst. Such plasticity enable ESCs to form all embryonic tissues including germ cells. However, these remarkable properties, at present, have been demonstrated only for mouse ESCs even if cells with somehow more limited capacities have been derived in many different species including humans. The isolation of pluripotent embryonic cells lines from human embryos marked a crucial change of perspective in evaluating the properties defining an embryonic stem cell lines moving the focus from the generation of a germ-line chimera, obviously not feasible nor desirable in human, to the capacity of these cells to differentiate both in vivo and in vitro in fully mature and functional cell types of all kinds. Therefore, ESCs properties in species different from the mouse are being reassessed and re-evaluated, in view of their potential use as experimental models for the development of clinical applications. Among the species that may play a useful role in this field, the pig has a long-standing history as a prime animal model for pre-clinical biomedical applications and therefore, pig ESCs are attracting renewed interest. In this review, we will summarize the current knowledge on this topic and will contrast the relatively limited data available in this species with the much larger wealth of information available for mouse and human ESCs, in an attempt to assess whether or not pig ESCs can actually become a useful tool in the fast growing field of cell therapy.

Challenges and prospects for the establishment of embryonic stem cell lines of domesticated ungulates

Embryonic stem (ES) cell lines provide an invaluable research tool for genetic engineering, developmental biology and disease models. These cells can be maintained indefinitely in culture and yet maintain competence to produce all the cells within a fetus. While mouse ES cell lines were first established over two decades ago and primate ES cells in the 1990 s, validated ES cell lines have yet to be established in ungulates. Why competent, pluripotent ES cells can be established from certain strains of mice and from primates, and not from cows, sheep, goats or pigs is an on-going topic of interest to animal reproduction scientists. The identification of appropriate stem cell markers, functional cytokine pathways, and key pluripotency-maintaining factors along with the release of more comprehensive bovine and porcine genomes, provide encouragement for establishment of ungulate ES cell lines in the near future.

Analysis of the expression of aquaporin-1 and aquaporin-9 in pig liver tissue: comparison with rat liver tissue

Aquaporins (AQPs) are cellular proteins involved with the movement of water across cell membranes and are fundamentally important to the fluid transport in the bile ducts and ductules of the liver. An immunohistochemical analysis of AQP-1 and AQP-9 was undertaken to describe their expression in fetal and adult pig liver, while immunoreagents specific to some other AQPs were screened for their efficacy on pig liver tissues. Anti-AQP-1 antibody reacted with the bile duct of the portal space and the bile ductules at the periphery of the liver lobules. Histological identification of bile ductules was confirmed by positive reactivity with anti-cytokeratin-7 and antilaminin immunostaining. Anti-AQP-1 signals were also pronounced in the endothelium of the portal space blood vessels and peripheral distributing venules. Antibody to AQP-9 reacted strongly with small ductules peripheral to the liver lobules, but only weakly with the bile ducts of the portal space. Anti-AQP-adipose antibody bound to the smooth muscle cells of the arteries in the portal space and sporadically with certain binucleated cells in the liver lobule. Antibodies to AQP-3, AQP-4, AQP-7, and AQP-8 were nonreactive with any of the tissues of the adult pig liver. For comparative purposes, immunohistochemical analysis of rat liver tissue was done with the anti-AQP-1 and AQP-9 antibodies. Anti-AQP-1 reacted weakly with the rat liver's bile ducts, but robustly with the endothelium of the liver's veins and arteries. It also reacted strongly with the central vein of the rat liver lobules, and, because the staining was continuous with hepatic sinusoids, it appeared that the reactivity was specific to the endothelial cells. Anti-AQP-9 antibodies reacted with rat hepatocytes and was not associated with the canaliculi, as judged by concurrent phalloidin staining of actin. The results indicate that specific AQPs are expressed in the tissues of the pig liver and that AQP-9 expression is distinct from its expression in the rat liver.