Hepatocyte differentiation in vitro: initiation of tyrosine aminotransferase expression in cultured fetal rat hepatocytes

Hepatic metabolomic profiling changes along with postnatal liver maturation in breeder roosters

To understand the hepatic metabolic changes during postnatal liver maturation process in breeder roosters, we investigated the hepatic metabolites composition of 1-day-old, 42-day-old, and 35-week-old breeder roosters using gas chromatography-mass spectrometer (GC-MS). Comprehensive multivariate data analyses were applied to identify the distinguishing metabolites of liver. 84 different kinds of distinguishing metabolites were identified between the livers of 1-day-old and 42-day-old breeder roosters, and 58 different kinds of distinguishing metabolites were identified between the livers from 42-day-old and 35-week-old breeder roosters. Further pathway annotations revealed that the hepatic metabolism was extensively remodeled during the postnatal liver maturation process. The antioxidant capacity of the liver and metabolism of carbohydrates, proteins, amino acids, fats, cholesterols, nucleic acids, and vitamins were all significantly changed at different growing periods after birth. Specifically, we found that the hepatic amino acid metabolic function was continuously enhanced from 1-day-old to 35-week-old roosters. However, the glucose and lipid metabolic functions were weakened from 1-day-old to 42-day-old roosters and then elevated from 42-day-old to 35-week-old roosters. In conclusion, the present study revealed that the metabolomic changes are related to the adaption of liver functions in breeder roosters.

Summary: This study revealed that hepatic metabolomic changes are related to functional adaption in breeder roosters, which contribute to a better understanding of the hepatic metabolites composition differences during different periods.

Hepatocytic differentiation of iPS cells on decellularized liver tissue

Decellularized tissues (DETs) have been attracting great attention as scaffolds for tissue-engineering approaches. Recently, some studies have reported that decellularized liver tissues (DLT) can provide an excellent environment for the hepatocytic differentiation of hepatic stem/progenitor cells that were already committed to the hepatocyte lineage. However, the effects of DLT on the hepatocytic differentiation of induced pluripotent stem cells (iPSs) have not yet been established. Here we studied the hepatocytic differentiation of iPSs on DLT and decellularized heart tissues (DHT) in order to determine the tissue-specific effects of DETs on iPSs differentiation. Our results showed that DLTs led to higher gene expression levels of forkhead box A2 (a marker of endoderm) and CCAAT/enhancer binding protein-α (master transcription factor to hepatocyte differentiation), alpha-fetoprotein (a marker of fetal hepatocyte,), and albumin (a marker of fetal and mature hepatocyte) of iPSs than on DHTs. Furthermore, gene expression levels of tyrosine aminotransferase (a marker of mature hepatocyte) were higher on DLT than that on DHT, and immunocytochemical analysis and ELISA assay showed that albumin secretion level of iPSs on DLT was higher than that on DHT. Our study demonstrated that the use of DLTs led to mature hepatocytic differentiation levels of iPSs compared to DHTs, which provides a better niche for iPSs cell engineering and enables the preparation of useful mature cells for regenerative therapy.

Generation of functional hepatocyte-like cells from human deciduous periodontal ligament stem cells

Human deciduous periodontal ligament stem cells have been introduced for as an easily accessible source of stem cells from dental origin. Although recent studies have revealed the ability of these stem cells in multipotential attribute, their efficiency of hepatic lineage differentiation has not been addressed so far. The aim of this study is to investigate hepatic lineage fate competence of periodontal ligament stem cells through direct media induction. Differentiation of periodontal ligament stem cells into hepatocyte-like cells was conducted by the exposure of two phase media induction. First phase was performed in the presence of hepatocyte growth factors to induce a definitive endoderm formation. In the subsequent phase, the cells were treated with oncostatin M and dexamethosone followed by insulin and transferrin to generate hepatocyte-like cells. Hepatic-related characters of the generated hepatocyte-like cells were determined at both mRNA and protein level followed by functional assays. Foremost changes observed in the generation of hepatocyte-like cells were the morphological features in which these cells were transformed from fibroblastic shape to polygonal shape. Temporal expression of hepatic markers ranging from early endodermal up to late markers were detected in the hepatocyte-like cells. Crucial hepatic markers such as glycogen storage, albumin, and urea secretion were also shown. These findings exhibited the ability of periodontal ligament stem cells of dental origin to be directed into hepatic lineage fate. These cells can be regarded as an alternative autologous source in the usage of stem cell-based treatment for liver diseases.

Efficient large-scale generation of functional hepatocytes from mouse embryonic stem cells grown in a rotating bioreactor with exogenous growth factors and hormones

Introduction

Embryonic stem (ES) cells are considered a potentially advantageous source of hepatocytes for both transplantation and the development of bioartificial livers. However, the efficient large-scale generation of functional hepatocytes from ES cells remains a major challenge, especially for those methods compatible with clinical applications.

Methods

In this study, we investigated whether a large number of functional hepatocytes can be differentiated from mouse ES (mES) cells using a simulated microgravity bioreactor. mES cells were cultured in a rotating bioreactor in the presence of exogenous growth factors and hormones to form embryoid bodies (EBs), which then differentiated into hepatocytes.

Results

During the rotating culture, most of the EB-derived cells gradually showed the histologic characteristics of normal hepatocytes. More specifically, the expression of hepatic genes and proteins was detected at a higher level in the differentiated cells from the bioreactor culture than in cells from a static culture. On further growing, the EBs on tissue-culture plates, most of the EB-derived cells were found to display the morphologic features of hepatocytes, as well as albumin synthesis. In addition, the EB-derived cells grown in the rotating bioreactor exhibited higher levels of liver-specific functions, such as glycogen storage, cytochrome P450 activity, low-density lipoprotein, and indocyanine green uptake, than did differentiated cells grown in static culture. When the EB-derived cells from day-14 EBs and the cells’ culture supernatant were injected into nude mice, the transplanted cells were engrafted into the recipient livers.

Conclusions

Large quantities of high-quality hepatocytes can be generated from mES cells in a rotating bioreactor via EB formation. This system may be useful in the large-scale generation of hepatocytes for both cell transplantation and the development of bioartificial livers.

Epithelial-mesenchymal transition delayed by E-cad to promote tissue formation in hepatic differentiation of mouse embryonic stem cells in vitro

Hepatic differentiation of embryonic stem cells (ESCs) usually results in a single cell lineage, and the formation of liver tissues remains difficult. Here, we examine the role of epithelial-mesenchymal transition (EMT) that is regulated by epithelial cadherin (E-cad) expression in hepatic tissue formation from ESCs. E-cad was transfected into mouse ESCs to enable a stable expression of E-cad. Hepatic differentiation of ESCs was then induced by hepatic growth factors. Wnt/β-catenin signaling and EMT speed were examined to determine the differentiation process. Hepatic and angiogenesis markers, as well as differentiated cell-adhesive force were also examined to identify the hepatic tissue differentiation. In our results, E-cad expression gradually decreased in normal ESC (N-ESC) differentiation, but remained stable in the E-cad transfected ESC (EC-ESC) group. In EC-ESC differentiation, expressions of cytoplastic β-catenin and EMT were much lower and significantly prolonged. Angiogenesis markers vascular endothelial growth factor receptor-1 (VEGFR-1) and CD31/PECAM-1 were expressed only on day 5-13 in N-ESC differentiation, whereas VEGFR-1 and CD31/PECAM-1 were expressed prolonged on day 5-17 in the EC-ESC group and were coincident with the expression of hepatic markers. Finally, EC-ESC differentiation maintained multilayer-growth patterns, and abundant vascular network structures appeared and migrated in albumin-positive cell areas. The cellular adhesion forces between embryonic body cells in EC-ESC differentiation during day 13-17 were similar to those of mouse liver tissue. In conclusion, accelerated EMT due to the decreased E-cad expression may partially contribute to the failure of hepatic tissue formation in N-ESC differentiation. E-cad can act in synergy with hepatic growth factors and facilitate the early-stage formation of hepatic tissues through down-regulating Wnt/β-catenin signaling and delaying EMT. This work provides a new insight into hepatic tissue differentiation that is mediated by E-cad from ESC.

Effects of sodium butyrate on the differentiation of pancreatic and hepatic progenitor cells from mouse embryonic stem cells

Recently significant progress has been made in differentiating embryonic stem (ES) cells toward pancreatic cells. However, little is known about the generation and identification of pancreatic progenitor cells from ES cells. Here we explored the influence of sodium butyrate on pancreatic progenitor differentiation, and investigated the different effects of sodium butyrate on pancreatic and hepatic progenitor formation. Our results indicated that different concentration and exposure time of sodium butyrate led to different differentiating trends of ES cells. A relatively lower concentration of sodium butyrate with shorter exposure time induced more pancreatic progenitor cell formation. When stimulated by a higher concentration and longer exposure time of sodium butyrate, ES cells differentiated toward hepatic progenitor cells rather than pancreatic progenitor cells. These progenitor cells could further mature into pancreatic and hepatic cells with the supplement of exogenous inducing factors. The resulting pancreatic cells expressed specific markers such as insulin and C-peptide, and were capable of insulin secretion in response to glucose stimulation. The differentiated hepatocytes were characterized by the expression of a number of liver-associated genes and proteins, and had the capability of glycogen storage. Thus, the current study demonstrated that sodium butyrate played different roles in inducing ES cells toward pancreatic or hepatic progenitor cells. These progenitor cells could be further induced into mature pancreatic cells and hepatocytes. This finding may facilitate the understanding of pancreatic and hepatic cell differentiation from ES cells, and provide a potential source of transplantable cells for cell-replacement therapies.

Hepatic stem cells and liver development

The liver consists of many cell types with specialized functions. Hepatocytes are one of the main players in the organ and therefore are the most vulnerable cells to damage. Since they are not everlasting cells, they need to be replenished throughout life. Although the capacity of hepatocytes to contribute to their own maintenance has long been recognized, recent studies have indicated the presence of both intrahepatic and extrahepatic stem/progenitor cell populations that serve to maintain the normal organ and to regenerate damaged parenchyma in response to a variety of insults.The intrahepatic compartment most likely derives primarily from the biliary tree, particularly the most proximal branches, i.e. the canals of Hering and smallest ductules. The extrahepatic compartment is at least in part derived from diverse populations of cells from the bone marrow. Embryonic stem cells (ES's) are considered as a part of the extrahepatic compartment. Due to their pluripotent capabilities, ES cell-derived cells form a potential future source of hepatocytes, to replace or restore hepatic tissues that have been damaged by disease or injury. Progressing knowledge about stem cells in the liver would allow a better understanding of the mechanisms of hepatic homeostasis and regeneration. Although a human stem cell-derived cell type equivalent to primary hepatocytes does not yet exist, the promising results obtained with extrahepatic stem cells would open the way to cell-based therapy for liver diseases.

What fires prometheus? The link between inflammation and regeneration following chronic liver injury

Liver progenitor cells (LPCs) play a major role in the regeneration process after chronic liver damage, giving rise to hepatocytes and cholangiocytes. Thus, they provide a cell-based therapeutic alternative to organ transplant, the current treatment of choice for end-stage liver disease. In recent years, much attention has focused on unravelling the cytokines and growth factors that underlie this response. Liver regeneration following acute damage is achieved by proliferation of mature hepatocytes; yet similar cytokines, most related to the inflammatory process, are implicated in both acute and chronic liver regeneration. Thus, many recent studies represent attempts to identify LPC-specific factors. This review summarises our current understanding of LPC biology with a particular focus on the liver inflammatory response being associated with the induction of LPCs in the liver. We will describe: (i) the pathways of liver regeneration following acute and chronic damage; (ii) the similarities and differences between the two pathways; (iii) the liver inflammatory environment; (iv) the unique features of liver immunology as well as (v) the interactions between liver immune cells and LPCs. Combining data from studies on the LPC-driven regeneration process with the knowledge in the field of liver immunology will improve our understanding of the LPC response and allow us to regulate these cells in vivo and in vitro for future therapeutic strategies to treat chronic liver disease.

Elevated hepatocyte-specific functions in fetal rat hepatocytes co-cultured with adult rat hepatocytes

Fetal hepatocytes (FHEPs) are a potential source of highly proliferative transplantable cells but express low levels of liver-specific functions. We hypothesized that the microenvironment of adult hepatocytes (AHEPs) may upregulate these functions. Primary FHEPs were seeded on top of collagen-sandwiched AHEPs directly or separated by a porous transwell membrane insert. In direct co-cultures, albumin (ALB) secretion, urea synthesis, and cytochrome P450 (CytP450) activity were all approximately 2 times as high as the sum of the corresponding monocultures. Using a transwell porous insert led to similar results, suggesting a major role for soluble factors. When AHEPs and FHEPs were separated after co-culture, they both initially showed significantly higher ALB secretion than control monocultures, whereas urea synthesis was significantly lower for the FHEPs only. Functions of previously co-cultured FHEPs normalized over the course of a week, but AHEP function remained high even after separation. In conclusion, co-culturing AHEPs with FHEPs increases expression of liver-specific functions in both cell types. The effect on FHEPs, but not AHEPs, was reversible. Unraveling the underlying mechanisms and optimizing this phenomenon will be useful in making fetal liver cells a potential cell source for hepatic tissue-engineering applications.

Keratinocyte serum-free medium maintains long-term liver gene expression and function in cultured rat hepatocytes by preventing the loss of liver-enriched transcription factors

Freshly isolated hepatocytes rapidly lose their differentiated properties when placed in culture. Therefore, production of a simple culture system for maintaining the phenotype of hepatocytes in culture would greatly facilitate their study. Our aim was to identify conditions that could maintain the differentiated properties of hepatocytes for up to 28 days of culture. Adult rat hepatocytes were isolated and attached in Williams' medium E containing 10% serum. The medium was changed to either fresh Williams' medium E or keratinocyte serum-free medium supplemented with dexamethasone, epidermal growth factor and pituitary gland extract. The hepatic phenotype was then analysed using RT-PCR, immunohistochemistry, Western blotting and assays of liver function. Cells cultured in keratinocyte serum-free medium supplemented with dexamethasone, epidermal growth factor and pituitary gland extract maintained their phenotype for 3-4 weeks, based on expression of liver proteins, ureagenesis and response to xenobiotics. In contrast, hepatocytes cultured in Williams' medium E rapidly lost the expression of liver proteins after 3 days. Cells cultured in keratinocyte serum-free medium supplemented with dexamethasone, epidermal growth factor and pituitary gland extract maintained their expression of liver-enriched transcription factors (C/EBPalpha and beta, HNF4alpha and RXRalpha) while expression was either lost or reduced in cells cultured in Williams' medium E. These results suggest that keratinocyte serum-free medium supplemented with dexamethasone, epidermal growth factor and pituitary gland extract can maintain the hepatic phenotype for a prolonged period and that this is probably related to the continued expression of the liver-enriched transcription factors.

Sodium butyrate-treated embryonic stem cells yield hepatocyte-like cells expressing a glycolytic phenotype

Embryonic stem cells serve as a promising technology to obtain specific cell types for a number of biomedical applications. Because traditional techniques, such as embryoid body formation result in a wide array of differentiated cells such as hepatic, neuronal, and cardiac lineages, strategies have been utilized which favor cell-specific differentiation to generate more uniformity. In the present study, we have investigated the use of sodium butyrate in a monolayer culture configuration to mediate hepatocyte differentiation of murine embryonic stem cells. Several functional assays used to characterize hepatocyte function (viz. urea secretion, intracellular albumin content, cytokeratin 18, and glycogen staining) were used to analyze the differentiating cell population, suggesting the presence of an enriched population of hepatocyte-like cells. Since mature hepatocytes mediate energy metabolism predominantly through oxidative means as opposed to hepatocyte precursors, which are primarily glycolytic, we have performed a kinetic analysis of glycolytic and functional capacity to characterize the differentiated cells. In conjunction with mitochondrial mass and activity measurements, we show that Na-butyrate-mediated differentiated cells mediate energy metabolism predominantly through glycolysis. This metabolic and mitochondrial characterization can assist in evaluating stem cell differentiation and may prove useful in identifying key regulatory molecules in mediating further differentiation.

In vitro differentiation of mouse ES cells into hepatocytes with coagulation factors VIII and IX expression profiles

Coagulation factors II, V, VII, VIII, IX and X are produced by hepatocytes. So factors VIII and IX deficiencies, which result in hemophilia A and B, have the potential to respond to cellular replacement therapy. Embryonic stem (ES) cells provide a unique source for therapeutic applications. Here, E14 mouse ES cells have been induced into hepatocytes in vitro. Morphology revealed that ES-derived hepatic-like cells were round or polyhedral shaped with distinct boundary of individual cells, and some arranged in trabeculae. These cells expressed endodermal- or liver-specific mRNA--transthyretin (TTR), alpha1-anti-trypsin (AAT), alpha-fetoprotein (AFP), albumin (ALB), glucose-6-phoshpatase (G6P) and tyrosine aminotransferase (TAT). Approximately (85.1 +/- 0.5)% of the ES-derived cells was stained positive green with ICG uptake. These cells were also stained magenta as a result of PAS reaction. In this paper, expression of coagulation factors VIII and IX mRNA in the ES-derived cells is documented. Therefore, ES cells might be developed as substitute donor cells for the therapy of coagulation factor deficiencies.

In vitro differentiation of human umbilical cord blood-derived mesenchymal stem cells into hepatocyte-like cells

In addition to long-term self-renewal capability, human mesenchymal stem cells (MSCs) possess versatile differentiation potential ranging from mesenchyme-related multipotency to neuroectodermal and endodermal competency. Of particular concern is hepatogenic potential that can be used for liver-directed stem cell therapy and transplantation. In this study, we have investigated whether human umbilical cord blood (UCB)-derived MSCs are also able to differentiate into hepatocyte-like cells. MSCs isolated from UCB were cultured under the pro-hepatogenic condition similar to that for bone marrow (BM)-derived MSCs. Expression of a variety of hepatic lineage markers was analyzed by flow cytometry, RT-PCR, Western blot, and immunofluorescence. The functionality of differentiated cells was assessed by their ability to incorporate DiI-acetylated low-density lipoprotein (DiI-Ac-LDL). As the cells were morphologically transformed into hepatocyte-like cells, they expressed Thy-1, c-Kit, and Flt-3 at the cell surface, as well as albumin, alpha-fetoprotein, and cytokeratin-18 and 19 in the interior. Moreover, about a half of the cells were found to acquire the capability to transport DiI-Ac-LDL. Based on these observations, and taking into account immense advantages of UCB over other stem cell sources, we conclude that UCB-derived MSCs retain hepatogenic potential suitable for cell therapy and transplantation against intractable liver diseases.

Transdifferentiation of mature rat hepatocytes into bile duct-like cells in vitro

We investigated the mechanism of phenotypic plasticity of hepatocytes in a three-dimensional organoid culture system, in which hepatocytic spheroids were embedded within a collagen gel matrix. Hepatocytes expressed several bile duct markers including cytokeratin (CK) 19 soon after culture and underwent branching morphogenesis within the matrix in the presence of insulin and epidermal growth factor. Cultured hepatocytes did not express Delta-like, a specific marker for oval cells and hepatoblasts. Furthermore, hepatocytes isolated from c-kit mutant rats (Ws/Ws), which are defective in proliferation of oval cells, showed essentially the same phenotypic changes as those isolated from control rats. The bile duct-like differentiation of hepatocytes was associated with increased expression of Jagged1, Jagged2, Notch1, and several Notch target genes. CK19 expression and branching morphogenesis were inhibited by dexamethasone, a mitogen-activated protein kinase kinase 1 (MEK1) inhibitor (PD98059), and a phosphatidyl inositol 3-kinase inhibitor (LY294002). After being cultured for more than 3 weeks within the gels, hepatocytes transformed into ductular structures surrounded by basement membranes. Our results suggest that hepatocytes might have the potential to transdifferentiate into bile duct-like cells without acquiring a stem-like phenotype and that this is mediated through specific protein tyrosine phosphorylation pathways.

Hepatocyte-like cells from directed differentiation of mouse bone marrow cells in vitro

AIM

To design the effective directed differentiation medium to differentiate bone marrow cells into hepatocyte-like cells.

METHODS

Bone marrow cells were cultured in the directed differentiation media including fibroblast growth factor-4 (FGF-4) and oncostatin M (OSM). Hepatocyte-like cells from directed differentiation of bone marrow cells were identified through cell morphology, RNA expressions by reverse transcriptase-polymerase chain reaction (RT-PCR), protein expressions by Western blot, and hepatocellular synthesis and metabolism functions by albumin ELISA, Periodic acid-Shiff staining and urea assay.

RESULTS

Some epithelial-like cells or polygonal cells appeared and increased in the course of the cell directed differentiation. Hepatocyte nucleur factor-3beta (HNF-3beta, albumin (ALB), cytokeratin 18 (CK18), transthyretin (TTR), glucose-6-phosphate (G-6-Pase), and tyrosine aminotransferase (TAT) mRNA were expressed in the course of the directed differentiation. The directed differentiated cells on d 21 expressed HNF-3? ALB, and CK18 proteins. The directed differentiated cells produced albumin and synthesized urea in a time-dependent manner. They could also synthesize glycogen.

CONCLUSION

Our differentiation media, including FGF-4 and OSM, are effective to differentiate bone marrow cells into hepatocyte-like cells, which could be used for hepatocyte resources for bioartificial liver or hepatocyte transplantation.

TNF-alpha regulates mouse fetal hepatic maturation induced by oncostatin M and extracellular matrices

Fetal hepatic maturation consists of multisteps and is regulated by several cytokines and cell-cell or cell-matrices interactions. In the mid-to-late fetal stage, hepatocytes have few metabolic functions associated with adult liver homeostasis. Cultured fetal hepatocytes acquire the expression of several mature liver-specific genes through stimulation with hepatic maturation factor oncostatin M (OSM) and matrigel. Tumor necrosis factor-alpha (TNFalpha) regulates fetal hepatic maturation stimulated by OSM and matrigel. TNFalpha suppressed expression of mature liver-specific genes such as tyrosine aminotransferase and apolipoproteins. In addition, the expression of hematopoietic cytokines and cyclin A2, repressed by OSM and matrigel, is induced by TNFalpha in the fetal hepatic cultures coincident with cell division. TNFalpha inhibited the induction of hepatocyte nuclear factor 4alpha induced by OSM and matrigel, suggesting that down-regulation of hepatocyte nuclear factor 4alpha expression is involved in the mechanism of suppression of hepatic maturation by TNFalpha. Interestingly, TNFalpha is expressed in the prenatal and postnatal liver but not in adult liver, whereas TNFR1, a TNFalpha receptor, is expressed in both fetal and adult livers. In conclusion, TNFalpha is a suppressive factor of hepatic maturation. The balance between hepatic maturation factor (OSM and extracellular matrices) and TNFalpha is important for liver development.

Role of the hepatocyte nuclear factor 4alpha in control of the pregnane X receptor during fetal liver development

The fetal liver, the major site of hematopoiesis during embryonic development, acquires additional functions near birth. Among the important liver functions is the response to xenobiotic exposure due to expression of several cytochromes P450 (CYP) and drug efflux transporters. Expression of these genes is regulated by nuclear receptors such as the pregnane X receptor (PXR). In this study, regulation of xenobiotic responses during fetal liver development was analyzed using a fetal hepatocyte primary culture system derived from embryonic day 15 (E15) livers. Hepatocyte nuclear factor (HNF) 4alpha regulates the expression of many genes preferentially in the liver. Expression of several xenobiotic response genes as well as HNF4alpha was increased in fetal hepatocytes stimulated by the hepatic maturation factors oncostatin M (OSM) and Matrigel. To determine the contribution of HNF4alpha to xenobiotic responses in the fetal liver, fetal hepatocytes containing floxed HNF4alpha alleles were cultured and the HNF4alpha gene was inactivated by infection with an adenovirus containing the Cre gene. Expression of CYP3A11 and PXR was suppressed by inactivation of HNF4alpha. An HNF4alpha binding site was characterized in the PXR promoter and found to be required for activation of the PXR promoter in fetal hepatocytes. In conclusion, HNF4alpha is the key transcription factor regulating responses to xenobiotics through activation of the PXR gene during fetal liver development.

Embryoid-body cells derived from a mouse embryonic stem cell line show differentiation into functional hepatocytes

Embryonic stem (ES) cells have a potential to differentiate into various progenitor cells. Here we investigated the differentiation capacity of mouse ES cells into hepatocytes both in vitro and in vivo. During the culture of embryoid bodies (EBs) derived from ES cells, albumin (ALB) messenger RNA (mRNA) was expressed within 12 days after removal of leukemia inhibitory factor, and alpha-fetoprotein (AFP) mRNA was observed within 9 days without additional exogenous growth factors. In ES cells and early EBs, by contrast, neither ALB mRNA nor AFP mRNA was observed. ALB protein was first detected at day 15 and the level increased with the culture period. The differentiation of EBs facilitated the synthesis of urea with the culture period, whereas early EBs and ES cells produced no urea. These results suggest that cultured EBs contain hepatocytes capable of producing ALB and urea. ES cells and the isolated cells from EBs were transplanted through portal vein to the liver after 30% partial hepatectomy of female mice pretreated with 2-acetylaminofluorene. Four weeks after transplantation with isolated cells from day-9 EBs, ES-derived cells containing Y-chromosome in the liver were positive for ALB (0.2% of total liver cells), whereas teratoma was found in mice transplanted with ES cells or EBs up to day 6. The incidence of teratoma was decreased with the culture duration and no teratoma was observed in the liver transplanted with isolated cells from day-9 EBs. In conclusion, our in vitro and in vivo experiments revealed that cultured EBs contain functional hepatocytes or hepatocyte-like cells.

Maturation of fetal hepatocytes in vitro by extracellular matrices and oncostatin M: induction of tryptophan oxygenase

Previously, we described that embryonic day 14.5 (E14.5) mouse fetal hepatocytes differentiate to express tyrosine amino transferase (TAT) and glucose-6-phosphatase, which are expressed in the perinatal liver, in response to oncostatin M (OSM) or in high-cell-density culture. However, under such conditions, fetal hepatic cells failed to express genes for adult liver-specific enzymes, such as tryptophan oxygenase (TO). Although phenobarbital (PB) and dimethylsulfoxide (DMSO) have been known to maintain the functions of adult hepatocytes in vitro, they failed to induce TO expression in fetal hepatic cells. Thus far, no system has been developed that reproduces terminal differentiation of fetal hepatocytes in vitro. Here, we describe that extracellular matrices derived from Engelbreth-Holm-Swarm sarcoma (EHS) in combination with OSM or high-cell-density culture induced expression of TO as well as cytochrome P450 genes that are involved in detoxification. However, EHS alone was insufficient to induce expression of TO, although it induced TAT expression in fetal hepatocytes. In addition, high-density culture further augmented differentiation. In conclusion, the combination of signals by cytokines, cell-cell contact, and cell-matrix interaction is required for induction of adult liver functions in fetal hepatocytes in vitro. This primary culture system will be useful for studying the mechanism of liver development.

In vitro differentiation of WB-F344 rat liver epithelial cells into the biliary lineage

Differentiation of hepatic precursor cells in the biliary lineage has rarely been investigated, owing to the lack of convenient in vitro models. In this study, we used sodium butyrate and culture on Matrigel to promote differentiation of WB-F344 rat liver epithelial cells along the biliary phenotype. This differentiation was assessed by following the expression of phenotypic markers at the protein or mRNA level. Sodium butyrate induced cytokeratin 19 expression and gamma-glutamyltranspeptidase activity, together with a large increase in gamma-glutamyltranspeptidase mRNA IV, a transcript expressed at high levels in biliary cells. We also observed an increase in aquaporin-1 and beta4 integrin mRNAs, encoding two proteins expressed in adult biliary cells. Culture on Matrigel increased cytokeratin 19, gamma-glutamyltranspeptidase, and BDS7 expression in WB-F344 cells which still expressed aquaporin-1 and beta4 integrin. These results show that WB-F344 cells are able to differentiate in vitro along the biliary pathway, making them a candidate model for analyzing the molecular events associated with the hepatoblast-biliary cell transition.

Hepatic maturation in differentiating embryonic stem cells in vitro

We investigated the potential of mouse embryonic stem (ES) cells to differentiate into hepatocytes in vitro. Differentiating ES cells expressed endodermal-specific genes, such as alpha-fetoprotein, transthyretin, alpha 1-anti-trypsin and albumin, when cultured without additional growth factors and late differential markers of hepatic development, such as tyrosine aminotransferase (TAT) and glucose-6-phosphatase (G6P), when cultured in the presence of growth factors critical for late embryonic liver development. Further, induction of TAT and G6P expression was induced regardless of expression of the functional SEK1 gene, which is thought to provide a survival signal for hepatocytes during an early stage of liver morphogenesis. The data indicate that the in vitro ES differentiation system has a potential to generate mature hepatocytes. The system has also been found useful in analyzing the role of growth factors and intracellular signaling molecules in hepatic development.

Oncostatin M and hepatocyte growth factor induce hepatic maturation via distinct signaling pathways

Liver development is regulated by soluble factors as well as cell-cell contacts. We previously reported that oncostatin M (OSM) induced hepatic maturation in a primary culture of embryonic day 14 liver cells. While OSM expression in the liver starts in mid gestation and decreases in postnatal stages, hepatocyte growth factor (HGF) is mainly expressed in the liver in the first few days after birth. In this study, we compared the effect of OSM and HGF on the differentiation of fetal hepatic cells in vitro. Like OSM, HGF in the presence of dexamethasone induced expression of glucose-6-phosphatase, tyrosine amino transferase and carbamoyl-phosphate synthase, and accumulation of glycogen in fetal hepatic cells, although to a lesser extent than OSM. Interestingly, while both OSM and HGF up-regulated production of albumin, secretion of albumin occurred only in response to OSM. In addition, although hepatic maturation induced by OSM depends on STAT3, HGF failed to activate STAT3 and HGF-induced differentiation was independent of STAT3. These results indicate that OSM and HGF induce hepatic maturation through different signaling pathways.

Cell density-dependent regulation of hepatic development by a gp130-independent pathway

We previously demonstrated that oncostatin M (OSM) promotes hepatic development in concert with glucocorticoid. The livers from mice deficient for gp130, a signaling subunit of the OSM receptor, displayed reduced expression of hepatic differentiation marker and defective glycogenic function. However, these phenotypes were not completely abolished in gp130(-/-) mice, suggesting that there is an alternative pathway regulating hepatic development in vivo. To test this possibility, we cultured gp130(-/-) fetal hepatic cells and investigated a signal that induces hepatic differentiation. When hepatocytes were forced to interact with each other by inoculating cells at high densities, hepatic differentiation was induced even in the absence of gp130. Moreover, cells stimulated with OSM and/or cultured at a high density possess many other metabolic functions. These observations suggest that fetal hepatic cells acquire multiple characteristics of differentiated hepatocytes in response to the signals generated by cell-cell contacts as well as by OSM.

The relationship between differentiation and proliferation in late gestation fetal rat hepatocytes

Hepatocyte proliferation and differentiation occur simultaneously during late mammalian gestation. We hypothesized that regulation of hepatocyte growth and differentiation would be coordinated in late gestation fetal hepatocyte cultures such that proliferation would be most active in a population of less well-differentiated cells. Cultured fetal hepatocytes (embryonic d 19 and 21; E19 and E21) were studied using double staining immunofluorescent microscopy. Differentiation was assessed as staining for alpha-fetoprotein (AFP), three markers of enzymic differentiation (glucokinase [GK], phosphoenolpyruvate carboxykinase [PEPCK], and carbamoyl phosphate synthase [CPS]), and a hepatocyte cell-cell adhesion molecule (C-CAM). Proliferation was assessed using immunocytochemical detection of proliferating cell nuclear antigen (PCNA) or 5-bromo-2'-deoxy-uridine (BrdU) incorporation into DNA. Fetal hepatocyte cultures consisted of a heterogeneous population of cells, slightly more than half of which were proliferative under defined, growth factor-free conditions. These cultures were heterogeneous for AFP expression. There was no correlation between the expression of AFP and PCNA or AFP and S-phase entry (BrdU staining) during the first 48 h in culture. Similar results were obtained in staining for the enzymic differentiation markers and C-CAM. In addition, the differentiation status of cultured fetal hepatocytes was unrelated to a presumed indicator of mature growth regulation, mitogenic responsiveness to transforming growth factor alpha (TGFalpha), and hepatocyte growth factor (HGF). Finally, absence of any correlation between proliferation and differentiated phenotype was supported by in vivo studies using staining for PCNA, AFP, CPS, and PEPCK in liver sections. These results indicate that the developmental program governing differentiation of late gestation fetal rat hepatocytes is independent from mechanisms controlling proliferation.

Hepatic differentiation induced by oncostatin M attenuates fetal liver hematopoiesis

Embryonic liver is a transient site for definitive hematopoiesis. Along with maturation of the bone marrow and spleen, hematopoietic cells relocate from the liver to their final destinations while the liver starts organizing its own structure and develops numerous metabolic functions toward adult. Recently, it was demonstrated that the signal exerted by oncostatin M (OSM) through gp130 plays a pivotal role in the maturation process of the liver both in vitro and in vivo. However, the molecular basis underlying the termination of embryonic hematopoiesis remains unknown. In this study, we report that primary culture of fetal hepatic cells from embryonic day 14.5 murine embryos supported expansion of blood cells from Lin-Sca-1(+)c-Kit+ cells, giving rise to myeloid, lymphoid, and erythroid lineages. Of interest, promotion of hepatic development by OSM and glucocorticoid strongly suppressed in vitro hematopoiesis. Consistent with these results, hepatic culture from the embryonic day 18.5 liver no longer supported hematopoiesis. These data together with the previous observations suggest that the signals exerted by OSM and glucocorticoid induce hepatic differentiation, which in turn terminate embryonic hematopoiesis and promote relocation of hematopoietic cells.

Fetal liver development requires a paracrine action of oncostatin M through the gp130 signal transducer

Fetal liver, the major site of hematopoiesis during embryonic development, acquires additional various metabolic functions near birth. Although liver development has been characterized biologically as consisting of several distinct steps, the molecular events accompanying this process are just beginning to be characterized. In this study, we have established a novel culture system of fetal murine hepatocytes and investigated factors required for development of hepatocytes. We found that oncostatin M (OSM), an interleukin-6 family cytokine, in combination with glucocorticoid, induced maturation of hepatocytes as evidenced by morphological changes that closely resemble more differentiated hepatocytes, expression of hepatic differentiation markers and intracellular glycogen accumulation. Consistent with these in vitro observations, livers from mice deficient for gp130, an OSM receptor subunit, display defects in maturation of hepatocytes. Interestingly, OSM is expressed in CD45(+) hematopoietic cells in the developing liver, whereas the OSM receptor is expressed predominantly in hepatocytes. These results suggest a paracrine mechanism of hepatogenesis; blood cells, transiently expanding in the fetal liver, produce OSM to promote development of hepatocytes in vivo.

Phorbol esters down-regulate alpha-fetoprotein gene expression without affecting growth in fetal hepatocytes in primary culture

The effects of phorbol esters (phorbol-12,13-dibutyrate, PDB) on alpha-fetoprotein expression and cell growth were assayed by using fetal hepatocytes in primary culture. PDB acts synergistically with epidermal growth factor (EGF) to specifically decrease alpha-fetoprotein (AFP) mRNA levels, without affecting the expression of other genes of the same family, such as albumin and Vitamin D-binding protein (DBP). This effect is PDB-dose dependent, maximal effects being at 10 ng/ml. The implication of protein kinase C (PKC) in this effect seems clear since bisindolylmaleimide (BIS), a specific PKC inhibitor, completely blocks the PDB effect on AFP expression. Nuclear run-on experiments show that the decrease in AFP mRNA levels is mainly due to an inhibition in the transcription rate of the gene. Determination of PKC activities shows that fetal hepatocytes contain mainly Ca(2+)-independent isoenzymes, which patterns of activation was not modified by EGF plus PDB treatment with respect to PDB treatment. We have found that MAPK and JNK activities, c-jun and c-fos mRNA levels and AP-1 binding activity are notably increased when cells are incubated with both EGF and PDB, PDB does not stimulate growth of fetal hepatocytes, measured either as [3H]-thymidine incorporation into DNA or by cell cycle analysis using flow cytometry. All these results suggest that activation of PKC may affect liver gene expression rather than cell growth in fetal hepatocytes.

Spatial distribution of cytoskeleton intermediate filaments during fetal rat hepatocyte differentiation

The construction of the liver parenchyma throughout fetal development depends on the elaboration of intercellular contacts between epithelial cells and between epithelial and mesenchymal cells. During this time, the spatial distribution of cytokeratins in hepatocytes shows a striking evolution as demonstrated by confocal microscopy and image analysis. In the early stages of fetal rat development, the liver is mainly a hematopoietic organ and hepatocytes represent fewer than 40% of all liver cells. At this time, cytokeratin filaments are scarce and are randomly distributed inside the cytoplasm. A coexpression of desmin and cytokeratin is found in some cells. Intercellular contacts between epithelial and mesenchymal cells are more numerous than between epithelial cells. Later in development, hepatocytes are arranged in a "muralium duplex" architecture (two-cell-thick sheets). Contacts between hepatocytes become more numerous and bile canaliculi become well developed. The density of cytokeratin filaments increases and appears to be very high near the bile canaliculi. In adult liver, hepatocytes are arranged in a "muralium simplex" architecture. Cytokeratin filaments show a symmetrical distribution in relation to the nuclear region. The highest density of filaments is found near the cytoplasmic membrane. Variations of the spatial distribution of intermediate filaments throughout hepatocyte differentiation were investigated in a pilot study using computerized image analysis. We found significant differences between the filament networks in fetal and adult hepatocytes.

5' sequences direct developmental expression and hormone responsiveness of tyrosine aminotransferase in primary cultures of fetal rat hepatocytes

Tyrosine aminotransferase (TyrAT) is one of several gluconeogenic enzymes which appear postnatally in humans and rodents in response to increased glucocorticoid and glucagon levels and decreased insulin. Primary cultured fetal rat hepatocytes older than day 15 of gestation (>E15) transcribe the TyrAT gene in response to the synergistic effect of dexamethasone and N6,2'-O-dibutyryl-adenosine 3',5'-monophosphate (Bt2cAMP), whereas less mature hepatocytes (<E15) do not [Shelly, L. L. & Yeoh, G. C. T. (1991) Eur. J. Biochem. 199, 475-481]. Therefore, we consider >E15 hepatocytes, and not <E15 hepatocytes, to be determined. This study reports that 11.1 kb of sequences upstream of the TyrAT transcription start site, which include a cAMP-responsive element (CRE) and a glucocorticoid-responsive element (GRE), are required for correct developmental regulation of gene expression in determined fetal hepatocytes. In contrast, the TyrAT CRE alone does not have this capability. Dexamethasone augments basal and Bt2cAMP-stimulated activity of the TyrAT CRE alone, suggesting that synergism may be due to interaction between the glucocorticoid and cAMP-signaling pathways. However, Bt2cAMP does not further increase dexamethasone-induced activity of the 11.1 kb 5' sequences when the TyrAT CRE is removed, thus excluding interaction of Bt2cAMP with the glucocorticoid pathway. Finally, insulin inhibition of dexamethasone-induced gene transcription is shown to be conferred by TyrAT 5' sequences. This study shows that cellular components, other than those which mediate hormonal regulation of genes, are required for determination of hepatocytes with respect to TyrAT. Since this phenomenon is observed with transient transfections, it is unlikely to involve higher-order chromatin structure.

Calcium phosphate transfection and cell-specific expression of heterologous genes in primary fetal rat hepatocytes

In order to study transcriptional regulation of hepatic genes during development, a method for transfer of fusion genes to primary cultures of fetal hepatocytes was required. The aim of this study was to assess currently available transfection methods and optimize the best method for use with cultured fetal hepatocytes. The Rous sarcoma virus 5' long terminal repeat controlling transcription of the beta-galactosidase reporter gene (pRSV lac Z II) was used to assess electroporation, lipofection, DEAE-dextran and calcium phosphate transfection in cultured primary fetal hepatocytes. The success of transfection was determined by histochemical detection and quantitation of beta-galactosidase activity. Results showed that calcium phosphate transfection was optimal for fetal hepatocytes with respect to beta-galactosidase activity and cell survival. For maximum transfection of cells, 10 micrograms/ml DNA, HEPES buffered saline transfection buffer at pH 7.05 and a 24 hr expression period for the reporter gene were employed. Glycerol shock did not increase transfection efficiency significantly. The method was simplified by adding calcium chloride solution to DNA diluted in transfection buffer and the resulting co-precipitate added directly to the medium covering the cells. Transfection 24 hr after initial culture and a precipitate incubation time of 20 hr were optimal. The suitability of this method was confirmed with a liver-specific promoter controlling beta-galactosidase and chloramphenicol acetyltransferase expression. In conclusion this study shows that a modified calcium phosphate transfection method is most effective for transferring DNA to primary cultured fetal hepatocytes. It is concluded that this method is appropriate for use with fetal hepatocytes and will facilitate studies of gene regulation during liver development.

The effect of iron status on glyceraldehyde 3-phosphate dehydrogenase expression in rat liver

The influence of iron status on glyceraldehyde 3-phosphate dehydrogenase (GAPDH) gene transcription, mRNA levels and distribution was determined in whole liver samples from adult Wistar rats. While iron loading did not alter GAPDH expression, iron deficiency evoked a 2.3-fold increase in the steady-state level of liver GADPH mRNA, but did not affect gene transcription or intracellular localisation of the message. Therefore, the over-expression of GAPDH mRNA in iron deficiency is probably due to increased message stability.

Development of dexamethasone-inducible tyrosine aminotransferase activity in WB-F344 rat liver epithelial stemlike cells cultured in the presence of sodium butyrate

Sodium butyrate acts as a differentiation-promoting agent for a wide variety of cell types, including some tumor cell lines. In this study, we examined the effects of sodium butyrate (SB) on the functional differentiation of cultured WB-F344 rat liver epithelial stemlike cells. Treatment of WB-F344 cells with 3.75 mM SB resulted in an inhibition of cellular proliferation, alterations to normal cellular morphology (increased cell size and decreased nuclear/cytoplasmic ratio), and significant increases in cellular protein synthesis. The SB-mediated changes in cell morphology, proliferative status, and protein catabolism were accompanied by development of dexamethasone-inducible tyrosine aminotransferase (TAT) enzyme activity. Culture of WB-F344 cells in growth medium containing SB and dexamethasone (DEX; 1 x 10(-6) M) resulted in greater than sevenfold increase in the basal TAT activity compared with control cultures. An additional sixfold increase in TAT activity was observed when cells cultured in medium containing SB and DEX were exposed to 1 x 10(-7) M DEX during the last 24 hours of culture. The DEX-inducible TAT activity developed by SB-treated WB-F344 cells responded to the modulating effects of insulin and L-tyrosine in a manner that closely resembled that reported for cultured hepatocytes and hepatoma cell lines. These studies show that treatment of WB-F344 rat liver epithelial stemlike cells with the differentiation-promoting agent SB in vitro leads to expression of the differentiation-specific hepatocyte enzyme TAT.

The efficiency of nuclear processing of the tyrosine aminotransferase mRNA transcript increases after partial hepatectomy

Following a two-thirds partial hepatectomy, an approximately fivefold increase in the levels of nuclear and total mRNA for tyrosine aminotransferase was observed at 1 h and 1.5 h, respectively, and a return to the levels of the quiescent state, i.e. the levels found in non-operated livers from adrenalectomized rats, was established 16 h post-hepatectomy. The increase in mRNA levels was not accounted for by a comparable change in the rate of transcription of the gene which, at 0.5 h post-hepatectomy, reached a maximum value that amounted to only 1.4-fold the value for quiescent liver. Subsequent changes in the transcription rate largely accounted for the changes in mRNA levels observed later on. Although tyrosine aminotransferase mRNA levels were equal in quiescent and 16-h-regenerating liver, the rate of transcription of the gene in quiescent liver was threefold higher than the rate in 16-h-regenerating liver. The maintenance of a higher rate of gene transcription in quiescent liver, as compared to regenerating liver, was shown to depend on ongoing protein synthesis. The possibility that the high rate of gene transcription was due to blockage or pausing during transcript elongation in quiescent liver was excluded. The inference is that the pronounced increase in tyrosine aminotransferase mRNA levels within 1 h of partial hepatectomy is largely due to a rapid increase in the efficiency of nuclear processing of the primary transcript.

Regulation of malic enzyme gene expression by nutrients, hormones, and growth factors in fetal hepatocyte primary cultures

The culture of fetal hepatocytes for 64 h in medium supplemented with 5 mM glucose, T3, insulin, and dexamethasone resulted in the coordinate precocious expression of malic enzyme mRNA, protein, and specific activity. T3 was the main inducer; meanwhile, insulin exerted a small synergistic effect when added with T3. Dexamethasone had a potentiation effect on the T3 response of malic enzyme mRNA expression regardless of the presence of insulin. This effect of dexamethasone on T3 response of malic enzyme mRNA expression was time (64 h) and glucose dependent. Glucagon, and to a greater degree dibutyryl-cAMP, repressed malic enzyme mRNA as well as protein expression by T3 and dexamethasone, in the absence of insulin. Glucose and other carbon sources such as lactate-pyruvate or dihydroxyacetone induced the abundance of malic enzyme mRNA in the absence of hormones. Insulin and T3 produced a high accumulation of malic enzyme mRNA in lactate-pyruvate medium, this effect being decreased by dexamethasone. EGF suppressed the induction produced by T3 and dexamethasone on malic enzyme mRNA, while the expression of beta-actin mRNA remained essentially unmodified.

Differentiation of functional hepatocytes and biliary epithelial cells from immature hepatocytes of the fetal mouse in vitro

Differentiation of functional hepatocytes and biliary epithelial cells from immature hepatocytes was analysed in vitro. When fetal mouse liver fragments containing immature hepatocytes but no bile ducts were cultured organotypically, the immature hepatocytes differentiated into large hepatocytes. Some of these expressed bile duct markers such as cytokeratin and Dolichos biflorus agglutinin-binding sites, though only to a small extent, and typical intrahepatic bile duct cells failed to differentiate. Dexamethasone stimulated immature hepatocytes to differentiate into both mature hepatocyte and biliary epithelial cell lineages. Especially in the liver fragments cultured on Matrigel, dexamethasone stimulated the expression of bile duct markers (such as cytokeratin and binding sites for two types of lectin) in the immature hepatocytes. These results support the idea that immature hepatocytes can differentiate into both mature hepatocytes and biliary epithelial cells during normal development of the mouse liver, and suggest that glucocorticoids stimulate both these differentiation pathways. It also seems that basal laminar components may play a role in bile duct differentiation.

Transcriptional- and post-transcriptional-dependent regulation of glutathione S-transferase expression in rat hepatocytes as a function of culture conditions

Transcriptional activity of the glutathione S-transferase (GST) alpha (subunits 1 and 2), mu (subunits 3 and 4) and pi (subunit 7) gene families has been analyzed using the nuclear 'run-on' technique on adult rat hepatocytes maintained for 4 days in conventional culture and for 4 and 12 days in co-culture with rat liver epithelial cells. Several medium conditions are included in this study, namely with or without fetal calf serum and with nicotinamide or dimethylsulphoxide. Hepatocytes co-cultured for 4 days maintain approximately 30-70% of the alpha gene family transcriptional activity, whatever the medium conditions, when compared to freshly isolated hepatocytes. A marked decrease is observed after 12 days of co-culture or when hepatocytes are maintained in conventional culture. The transcriptional activity of the mu gene family is maintained at 40-160% when hepatocytes are cultured with or without fetal calf serum, and is inducible by nicotinamide (approximately 4-fold) and dimethylsulphoxide (approximately 2-fold) in conventional culture and/or in co-culture. In contrast to freshly isolated hepatocytes, GST pi gene transcriptional activity is observed in conventional and co-cultured hepatocytes, irrespective of the medium conditions. Dimethylsulphoxide treatment however, represses the expression of GST 7 in vitro. These results demonstrate that the expression of GST alpha, mu and pi genes in conventional and co-cultured rat hepatocytes is controlled primarily at the level of transcription. It cannot be excluded, however, that dimethylsulphoxide stabilizes the GST mRNA levels in vitro.

The hepatocyte-specific phenotype of murine liver cells correlates with high expression of connexin32 and connexin26 but very low expression of connexin43

This investigation was initiated in order to find out whether expression of the hepatocyte-specific phenotype is accompanied by expression of certain connexin genes coding for gap junctional protein subunits. Several clones of mouse embryonic hepatocytes immortalized in serum-free MX83 medium by infection with recombinant retrovirus-expressed transcripts for connexin32, connexin26, albumin, alpha-fetoprotein, tyrosine aminotransferase, as well as aldolase A and B, at more than half of the levels found in primary mouse hepatocytes. In addition the immortalized hepatocyte clones contained low levels of connexin43 mRNA of which only trace amounts were detected in primary embryonic mouse hepatocytes and in rat liver. Two of the immortalized hepatocyte clones were shifted from serum-free MX83 medium to Dulbecco's modified Eagle medium (DMEM) containing 10% fetal calf serum and, after 2, 14, or 180 days, back to MX83 medium. We found that expression of connexin32 and connexin26 mRNAs as well as transcripts of other liver-specific proteins was reversibly decreased in serum-containing medium, whereas the expression level of connexin43 transcripts was increased in serum-containing DMEM compared to serum-free MX83 medium. The expression levels of connexin26, connexin32, or connexin43 mRNAs were altered by the addition of fetal calf serum or arginine or by the absence of hydrocortisone in MX83 medium, all of which contributed to the shift in phenotype. Furthermore several dedifferentiated cell lines derived from rat or mouse liver and cultivated in serum-containing medium were found to express little connexin32 or connexin26 mRNA but relatively high levels of connexin43 mRNA.

Regulation of albumin expression in fetal rat hepatocytes cultured under proliferative conditions: role of epidermal growth factor and hormones

Sustained production of plasma proteins, notably albumin, is a reliable indicator of the differentiated state of hepatocytes. In this work, we have developed a fetal hepatocyte culture system where studying the regulation of albumin expression in proliferating liver cells. Our results show that under proliferative conditions (i.e., in the presence of EGF) fetal hepatocytes maintain albumin production above control quiescent non-treated cells. Glucagon and noradrenaline have no effect on the proliferation induced by EGF in cultured fetal hepatocytes; however, they act synergistically with the growth factor, increasing intracellular albumin levels. The maximum response is obtained by treatment of cells with EGF and noradrenaline. The stimulatory noradrenergic effect is mimicked by agents that increase cyclic AMP levels (forskolin plus IBMX). However, vasopressin or phorbol esters have no effect on albumin production, neither alone nor in combination with EGF. Dexamethasone, which does not alter the proliferative induction of EGF, increases albumin content. This effect is independent of the proliferative status of the cells and is not enhanced by glucagon, noradrenaline, or cyclic AMP increasing agents. The hormonal changes observed in albumin production partially correlate with changes in mRNA levels. This is the first time that cyclic AMP increasing agents are shown to act synergistically with EGF, increasing the expression of this liver specific gene.

Expression of glutathione S-transferase during rat liver development

The ontogeny of rat liver glutathione S-transferase (EC 2.5.1.18) (GSTs) during foetal and postnatal development was investigated. The GSTs are dimers, the subunits of which belong to three multigene families, Alpha (subunits 1, 2, 8 and 10), Mu (subunits 3, 4, 6, 9 and 11) and Pi (subunit 7) [Mannervik, Alin, Guthenberg, Jennsson, Tahir, Warholm & Jörnvall (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 7202-7206; Kispert, Meyer, Lalor, Coles & Ketterer (1989) Biochem. J. 260, 789-793]. There is considerable structural homology within each gene family, with the result that whereas reverse-phase h.p.l.c. successfully differentiates individual subunits, immunocytochemical and Northern-blotting analyses may only differentiate families. Enzymic activity, h.p.l.c. and Northern blotting indicated that expression of GST increased from very low levels at 12 days of foetal growth to substantial amounts at day 21. At birth, GST concentrations underwent a dramatic decline and remained low until 5-10 days post partum, after which they increased to adult levels. During the period under study, GST subunits underwent differential expression. The Mu family had a lower level of expression than the Alpha family, and, within the Alpha family, subunit 1 was more dominant in the adult than the foetus. Subunit 2 is the major form in the foetus. Most noteworthy were subunits 7 and 10, which were prominent in the foetus, but present at low levels post partum. Immunocytochemical analysis of the 17-day foetal and newborn rat livers showed marked differences in the distribution of GSTs in hepatocytes. In the 17-day foetal liver Pi greater than Alpha greater than Mu whereas in the newborns Alpha greater than Mu much greater than Pi. Erythropoietic cells were not stained for any of the three GST families. Steady-state mRNA concentrations in the foetus correlated with the relative transcription of the Alpha, Mu and Pi class genes. However, in those genes expressed post partum, namely the Alpha and Mu class, low transcriptional activity was associated with high concentrations of mRNA. This suggests that there is a switch from transcriptional control to post-transcriptional control at birth. GST 7-7 appears to be regulated predominantly by transcription throughout the period of liver development under observation.

Effects of dexamethasone and cAMP on tyrosine aminotransferase expression in cultured fetal rat hepatocytes

Fetal hepatocyte cultures were used to investigate tyrosine aminotransferase (TyrAT) expression during development. Previous studies showed that TyrAT is synthesized by hepatocytes isolated from 15-day-gestation fetuses maintained in culture for two or more day, then exposed to dexamethasone. TyrAT expression was essentially undetectable on the first day of culture of hepatocytes derived from 15-day-gestation, or less mature, fetuses. Dexamethasone and cAMP are potent inducers of TyrAT and they synergistically induce TyrAT to extremely high levels when added simultaneously to cultured fetal hepatocytes. The effects of dibutyryl-cAMP (Bt2cAMP) alone and in combination with dexamethasone on TyrAT expression are investigated. Hepatocytes isolated from 15-day-gestation fetuses exposed to both inducers possessed detectable levels of TyrAT activity and mRNA on day 1 of culture, and this increased by day 3. In contrast, hepatocytes exposed to either inducing agent alone were essentially negative on day 1, but positive on day 3. This was shown to be a consequence of transcription. When 13-day-gestation hepatocytes were maintained in culture under identical conditions detectable levels of TyrAT mRNA were evident on day 1, and this increased by day 3. Immunocytochemical studies revealed that the appearance and subsequent increase in TyrAT production elicited by dexamethasone and Bt2cAMP were due to changes in the proportion of hepatocytes expressing the enzyme. Therefore, in the presence of both dexamethasone and Bt2cAMP, TyrAT expression can be detected in some cells at an earlier stage of liver development than reported previously.

Regulation of glutathione S-transferase gene expression by phenobarbital in cultured adult rat hepatocytes

Previous studies, by using Northern blotting analyses, showed that phenobarbital (PB) affects the steady-state mRNA levels of glutathione S-transferase (GST) subunits 1/2, 3/4 and 7 in both conventional cultures of adult rat hepatocytes and co-cultures, with rat liver epithelial cells [Vandenberghe et al., 1989, FEBS Lett. 251, 59-64; Morel et al., 1989, FEBS Lett. 258, 99-102]. To determine whether PB acts at the transcriptional level, nuclear 'run on' experiments using cDNA probes hybridizing to GST subunits 1/2, 3/4 and 7 mRNA were performed on purified nuclei isolated from control and PB treated hepatocytes seeded under conventional and co-culture conditions. Data from this study demonstrate that the increase in steady-state mRNA levels observed in both conventional culture and co-culture after 4 days PB exposure results from an increased transcriptional activity of the GST genes. However, a substantial increase in steady-state mRNA levels in the absence of a commensurate increase in transcriptional activity at 12 days of co-culture, indicates that the barbiturate has also a stabilizing effect in vitro on the GST mRNAs.