Attenuated liver progenitor (oval) cell and fibrogenic responses to the choline deficient, ethionine supplemented diet in the BALB/c inbred strain of mice

BACKGROUND/AIMS

Liver regeneration following chronic injury is associated with inflammation, the proliferation of liver progenitor (oval) cells and fibrosis. Previous studies identified interferon-gamma as a key mediator of oval cell proliferation. Interferon-gamma is known to regulate Th1 cell activities during immune challenge. Therefore, we hypothesised that progenitor cell-mediated regeneration is associated with a Th1 immune response.

METHODS

C57Bl/6 (normal Th1 response) and BALB/c mice (deficient in Th1 signalling) were placed on a carcinogenic diet to induce liver injury, progenitor cell proliferation and fibrosis.

RESULTS

Serum transaminases and mortality were elevated in BALB/c mice fed the diet. Proliferation of liver progenitor cells was significantly attenuated in BALB/c animals. The pattern of cytokine expression and inflammation differed between strains. Liver fibrosis and hepatic stellate cell activation were significantly inhibited in BALB/c mice compared to C57Bl/6. In addition, interferon-gamma knockout mice also showed reduced fibrosis compared to wild type. These findings are in contrast to published results, in which interferon-gamma is shown to be anti-fibrogenic.

CONCLUSIONS

Our data demonstrate that the hepatic progenitor cell response to a CDE diet is inhibited in mice lacking Th1 immune signalling and further show that this inhibition is associated with reduced liver fibrosis.

Jekyll and Hyde: evolving perspectives on the function and potential of the adult liver progenitor (oval) cell

The liver progenitor cell (LPC) has enormous potential for use in cell therapy to treat liver disease. Since liver regenerates readily from pre-existing hepatocytes, a role for LPCs and, indeed, their existence have been questioned. Research during the last decade has established that LPCs are an important alternative source of cells for liver regeneration. Their utility for cell therapy lies in their ability to generate both hepatocytes and cholangiocytes. However, they are observed in liver diseases that often lead to cancer and there is experimental evidence that implicates LPCs as the source of tumours. This article provides a brief history of the studies that established the functional importance of LPCs in liver disease. It focuses on mouse models that have led to the identification of factors that regulate LPC growth and differentiation and discusses LPCs derived from different sources. Recent promising results from both in vitro and vivo studies suggest that LPCs could be useful for cell therapy. In the context of liver disease, LPCs may indeed be the cell of the future and understandably "our favourite cell".

Antiproliferative effects of interferon alpha on hepatic progenitor cells in vitro and in vivo

Hepatic progenitor cells (called oval cells in rodents) proliferate during chronic liver injury. They have been suggested as targets of malignant transformation in chronic liver diseases, including chronic hepatitis C. Interferon alpha therapy reduces the risk of hepatocellular carcinoma (HCC) in chronic hepatitis C regardless of viral clearance. The aim of this study was to determine whether interferon alpha could reduce the risk of HCC by modifying preneoplastic events in the hepatic progenitor cell population. Pre- and post-treatment liver biopsies were evaluated for changes in t he hepaticprogenitor cell population in 16 patients with non-responding chronic hepatitis C Interferon alpha-based treatment significantly reduced the numbers of c-kit-positive hepatic progenitor cells by 50%. To determine the mechanism of cell number reduction, the effects of interferon alpha on murinehepatic progenitor cells were studied in vitro. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) proliferation assay and proliferating cell nuclear antigen staining showed that interferon alpha had a dose-dependent, anti-proliferative effect Interferon alpha stimulated hepatocytic and biliary differentiation of the oval cell lines reflected by increased expression of albumin and cytokeratin19 accompanied by decreased expression of alphafetoprotein and Thy-1. To validatethese results in vivo, mice were placed on the choline-deficient, ethionine-supplemented diet to induce liver injury and oval cell proliferation and treated with pegylated interferon alpha 2b for 2 weeks. This resulted in a significant four-fold reduction in the number of oval cells (P < .05). In conclusion, interferon alpha-based treatment reduced the number of hepatic progenitor cells in chronic liver injury by modulating apoptosis, proliferation, and differentiation. Supplementay material for this article can

Liver stem cells

The concept of a liver stem cell or progenitor cell has not been widely accepted until the last decade. Studies investigating liver regeneration under conditions which totally or substantially preclude hepatocyte proliferation report the proliferation of a subpopulation of small, oval-shaped cells, which are first observed in the portal triad, adjacent to the terminal ducts. These cells, termed liver progenitor oval cells (LPCs) are shown to participate in liver regeneration in a variety of rodent models of chronic liver damage. They express markers common to hepatocytes and cholangiocytes suggesting they are a common precursor of both liver cell lineages. Supporting evidence for liver stem cells has also come from cell tracing studies which show transdifferentiation of bone marrow cells into hepatocytes in both human and animal models. Another important issue is the link between LPCs and hepatocellular carcinoma (HCC). The widening liver donor-recipient gap; a consequence of poor donation rates coupled with increasing incidence of liver disease highlights the importance of establishing the utility of cell transplant as an alternative to treat liver disease. In this regard, liver stem cells and progenitor cells may have a significant role to play. To successfully utilize liver stem cells or LPCs for cell therapy, we have to first develop methods for maintaining and differentiating them in culture. This technology must be based on a thorough understanding of conditions which regulate their behaviour in vitro. In particular, we need to know which growth factors and cytokines affect them and their mechanism of action. Since they are a potential source of HCC, it is also necessary to understand the mechanisms which underlie their transformation to cancer.

Lymphotoxin-beta production following bile duct ligation: possible role for Kupffer cells

BACKGROUND AND AIMS

Lymphotoxin-beta (LT-beta) may play a role in the pathogenesis of chronic liver injury. The aim of this study was to determine in an animal model of bile duct ligation liver injury whether LT-beta expression is induced and whether Kupffer cells are an intrahepatic source of LT-beta.

METHODS

Sprague-Dawley rats were divided into two groups: one group received a single dose of GdCl (a Kupffer cell-blocking agent, 10 mg/kg i.v.), whereas the other group received saline. One day later, the groups underwent bile duct ligation or a sham operation. Liver tissue was obtained on days 1, 3, 5, and 8 for assessment of Kupffer cell numbers, early fibrogenic events and LT-beta gene expression. Kupffer cells were isolated using pronase/collagenase perfusion and centrifugal elutriation.

RESULTS

Hepatic LT-beta mRNA expression increased early following bile duct ligation. Pretreatment of bile duct-ligated animals with GdCl significantly reduced the number of Kupffer cells, delayed the rise in LT-beta expression, but had no effect on fibrogenesis. Recovery of the Kupffer cell population in these animals was accompanied by increased hepatic LT-beta expression. The LT-beta ligand and receptor were expressed by isolated normal Kupffer cells.

CONCLUSIONS

Hepatic LT-beta expression is induced early following bile duct ligation. Kupffer cells may be an intrahepatic source of LT-beta.

Inhibition of adult liver progenitor (oval) cell growth and viability by an agonist of the peroxisome proliferator activated receptor (PPAR) family member gamma, but not alpha or delta

Multifaceted evidence links the development of liver tumours to the activation and proliferation of adult liver progenitor (oval) cells during the early stages of chronic liver injury. The aim of this study was to examine the role of the peroxisome proliferator activated receptors (PPARs): PPARalpha, delta and gamma, in mediating the behaviour of liver progenitor cells during pre-neoplastic disease and to investigate their potential as therapeutic targets for the treatment of chronic liver injury. We observed increased liver expression of PPARalpha and gamma in concert with expanding oval cell numbers during the first 21 days following commencement of the choline deficient, ethionine supplemented (CDE) dietary model of carcinogenic liver injury in mice. Both primary and immortalized liver progenitor cells were found to express PPARalpha, delta and gamma, but not gamma2, the alternate splice form of PPARgamma. WY14643 (PPARalpha agonist), GW501516 (PPARdelta agonist) and ciglitazone (PPARgamma agonist) were tested for their ability to modulate the behaviour of p53-immortalized liver (PIL) progenitor cell lines in vitro. Both PPARdelta and gamma agonists induced dose-dependent growth inhibition and apoptosis of PIL cells. In contrast, the PPARalpha agonist had no effect on PIL cell growth. None of the drugs affected the maturation of PIL cells along either the hepatocytic or biliary lineages, as judged by their patterns of hepatic gene expression prior to and following treatment. Administration of the PPARgamma agonist ciglitazone to mice fed with the CDE diet for 14 days resulted in a significantly diminished oval cell response and decreased fibrosis compared with those receiving placebo. In contrast, GW501516 did not affect oval cell numbers or liver fibrosis, but inhibited CDE-induced hepatic steatosis. In summary, PPARgamma agonists reduce oval cell proliferation and fibrosis during chronic liver injury and may be useful in the prevention of hepatocellular carcinoma.

Differential regulation of rodent hepatocyte and oval cell proliferation by interferon gamma

Hepatocytes and intrahepatic progenitor cells (oval cells) have similar responses to most growth factors but rarely proliferate together. Oval cells constitute a reserve compartment that is activated when hepatocyte proliferation is inhibited. Interferon gamma (IFN-gamma) increases in liver injury that involves oval cell responses, but it is not upregulated during liver regeneration after partial hepatectomy. Based on these observations, we used well-characterized lines of hepatocytes (AML-12 cells) and oval cells (LE-6 cells) to investigate the potential mechanisms that regulate differential growth responses in hepatocytes and oval cells. We show that IFN-gamma blocks hepatocyte proliferation in vivo, and that in combination with either tumor necrosis factor (TNF) or lipopolysaccharide (LPS), it causes cell cycle arrest in hepatocytes but stimulates oval cell proliferation in cultured cells. The hepatocyte cell cycle arrest is reversible, is p53-independent, and is not associated with apoptosis. Treatment of AML-12 hepatocytes with IFN-gamma/LPS or IFN-gamma/TNF, but not with individual cytokines, induced NO synthase and generated NO, while similarly treated oval cells produced little if any NO. Generation of NO by an NO donor reproduced the inhibitory effect of the cytokine combinations on AML-12 cell replication, while NO inhibitors abolish the replication deficiency. In conclusion, we propose that IFN-gamma, in conjunction with TNF or LPS, can both inhibit hepatocyte proliferation through the generation of NO and stimulate oval cell replication. The response of hepatocytes and oval cells to cytokine combinations may contribute to the differential proliferation of these cells in hepatic growth processes.

Differential lymphotoxin-beta and interferon gamma signaling during mouse liver regeneration induced by chronic and acute injury

The liver regenerates after acute injury via hepatocyte cell division; during chronic injury, when hepatocyte replication is impaired or blocked, liver progenitor oval cells mediate liver regeneration. If both regeneration options are blocked in animal models, then liver failure and death ensues. The mechanisms underlying oval cell induction, proliferation, and subsequent liver regeneration remain poorly characterized. In particular, cell-signaling pathways that distinguish the alternative pathways are unknown. This study shows that in a mouse model, hepatic expression of lymphotoxin-beta (LTbeta) and interferon gamma (IFNgamma) transcripts is increased in response to the choline-deficient, ethionine-supplemented (CDE) diet, which induces oval cell-mediated liver regeneration. Oval cells express LTbeta and IFNgamma transcripts, contributing to the increased expression in the liver of mice fed the CDE diet. An attenuated oval cell response to such a diet was observed in LTbeta receptor-, LTbeta-, and IFNgamma-gene targeted mice. Loss of LTbeta and LTbeta receptor signaling reduced the number of oval cells expressing A6 and muscle pyruvate kinase. The lack of IFNgamma signaling reduced muscle pyruvate kinase(+), but not A6(+), oval cells. In contrast, partial hepatectomy suppressed LTbeta and IFNgamma transcripts. We also show that IFNgamma induces STAT-3 phosphorylation in an oval cell line. In conclusion, LTbeta, LTbeta receptor, and IFNgamma are involved in oval cell-mediated, but not hepatocyte-mediated, liver regeneration, and the absence of these pathways impairs the oval cell-dependent regenerative response.

TNF/LT? double knockout mice display abnormal inflammatory and regenerative responses to acute and chronic liver injury

Following acute liver injury, hepatocytes divide to facilitate regeneration. However, during chronic injury, hepatocyte proliferation is typically blocked and repair is mediated through liver progenitor (oval) cells. Signalling of the p55 tumour necrosis factor (TNF) receptor is central to these processes. Two ligands for p55 are known: TNF and lymphotoxin-alpha (LTalpha). However, one study suggests that another exists that mediates liver injury following viral challenge. We have therefore investigated whether ligands other than TNF and LTalpha are required for liver regeneration following either acute or chronic injury. Wild-type and double TNF/LTalpha knockout (TNF-/-LTalpha-/-) mice were subjected to either partial hepatectomy (PHx) or a choline-deficient ethionine-supplemented (CDE) diet. Proliferating hepatocytes, oval cells and inflammatory cells were identified and quantified in liver sections by immunohistochemistry. Liver inflammatory cells were characterised by cell surface antigen expression. Liver damage and mortality were monitored. Both hepatocyte and oval cell proliferation was reduced in TNF-/-LTalpha-/- mice. Lymphocyte clusters were evident in all TNF-/-LTalpha-/- livers and were heterogeneous, comprising B and T lymphocytes. PHx evoked liver inflammation in TNF-/-LTalpha-/- but not wild-type mice, whereas no difference was apparent between genotypes in CDE experiments. Thus, TNF/LTalpha signalling mediates liver regeneration involving both hepatocytes and progenitor cells. The hyper-inflammatory response following PHx in TNF-/-LTalpha-/- animals, which is absent following CDE-induced injury, demonstrates that the two forms of liver injury evoke discrete inflammatory responses and provides a model in which such differences can be examined further.

Differential effects of gadolinium chloride on Kupffer cells in vivo and in vitro

Gadolinium chloride (GdCl) is commonly used to study the role of Kupffer cells in liver disease in vivo. The in vitro effects of GdCl on cultured Kupffer cells are poorly characterised. The aim of this study was to characterise rat Kupffer cell TNFalpha production, phagocytic function, and ED1 and ED2 antigen expression following the administration of GdCl. For in vivo experiments, rats received 10mg/kg GdCl IV or sterile saline. Lipopolysaccharide 3mg/kg IP (LPS) was administered 4h prior to sacrifice on Days 1-3, 5 or 8 following GdCl injection. Hepatic ED1 and ED2 positive macrophage numbers and TNFalpha mRNA levels were determined. For in vitro experiments, Kupffer cells were cultured in the presence of 0-270 microM GdCl for 24h following which viability, TNFalpha protein production in response to LPS (10 ng/ml), phagocytosis, and ED1 and ED2 staining were evaluated. In vivo, the proportion of ED1 positive cells which were ED2 positive was reduced from 87 to 3% and hepatic TNFalpha mRNA levels following LPS declined by 60% over Days 1-5 after injection of GdCl (P<0.01). In vitro, phagocytosis declined with increasing concentrations of GdCl. GdCl (0-27 microM) did not effect cultured Kupffer cell viability, TNFalpha production, ED1 or ED2 staining. We conclude that GdCl significantly reduces ED2 expression by Kupffer cells in vivo. In vitro, GdCl has a dose dependent effect on phagocytosis but only effects viability and TNFalpha production at high concentrations. ED2 expression of cultured Kupffer cells is not affected by GdCl.

Histone H4 histidine kinase displays the expression pattern of a liver oncodevelopmental marker

Protein phosphorylation is a vital process in the regulation of mammalian cell division and the protein kinases that catalyze the phosphorylation of proteins on serine, threonine and tyrosine residues have been well characterized. In contrast, little is known about the kinases involved in protein histidine phosphorylation, which have been described in various mammalian cells that are highly proliferative. Histone H4 histidine kinase (HHK) activity is highly active in regenerating rat liver. Using a novel and specific assay, we demonstrate that it is active in human fetal liver, essentially absent in adult liver and highly expressed in liver tumours. 'Normal' liver surrounding the HCC contains low to undetectable levels of HHK. In a rodent model of chronic liver injury that leads to HCC, its activity is induced. Two lines of evidence suggest that liver progenitor (oval) cells, which populate the liver at early stages following induction of liver damage are responsible for the increased activity. Purified oval cells, as well as cell lines established from primary cultures of oval cells express high levels of HHK. We propose that the pattern of expression of histone H4 histidine kinase activity justifies its classification as an oncodevelopmental marker and suggest it may be useful as a diagnostic marker for hepatocellular carcinoma as well for identifying preneoplastic lesions.

Detection of histidine kinases via a filter-based assay and reverse-phase thin-layer chromatographic phosphoamino acid analysis

The methods that detect histidine phosphorylation have largely been either laborious or difficult to apply quantitatively. The major difficulty in assessing for its presence is its alkali-stable, acid-labile nature. While an assay that detects alkali-stable phosphorylation has been developed, it does not distinguish phosphohistidine from other alkali-stable phosphoamino acids. Using this established method, we extend the assay to facilitate the specific detection of phosphohistidine. We use the acid-lability of phosphohistidine as a defining feature in our approach for its detection. In addition, reverse-phase thin-layer chromatography was utilized to conclusively demonstrate the viability of the conditions that we implement in the assay for the selective detection of phosphohistidine. In summary, this report describes a rapid filter-based kinase assay that quantitatively measures histidine kinase activity, even in the presence of tyrosine kinase activity.

In vivo differentiation of mouse embryonic stem cells into hepatocytes

Embryonic stem (ES) cells have been regarded as a powerful resource for cell replacement therapy. In recent reports mouse ES cells have been successfully applied in the treatment of spinal cord injury, hereditary myelin disorder of the central nervous system, and diabetes mellitus. Another type of disease that could benefit from the availability of stem cell therapy is liver disease. However, for this potential to be realized, it is necessary to demonstrate the differentiation of ES cells into hepatocytes. To demonstrate the in vivo differentiation potential of mouse ES cells, we injected ES cells into the spleen of immunosuppressed nude mice. Histological analysis of teratomas derived from injected ES cells revealed that some areas contained typical hepatocytes arranged in a sinusoidal structure. The hepatic nature of these cells was further confirmed by showing that transcripts of liver-specific genes were present in the differentiated teratoma using reverse transcriptase-polymerase chain reaction and immunohistochemistry using several liver-specific antibodies including HEP-PAR, phenylalanine hydroxylase, and mouse N-system aminotransferase to identify the respective proteins in the differentiated hepatocytes. This is the first demonstration that mouse ES cells can differentiate in vivo into a mixed population of hepatocytes of varying maturity. This finding extends the potential use of ES cells in the cell replacement therapy by including its possible application for treating liver diseases.

A modified choline-deficient, ethionine-supplemented diet protocol effectively induces oval cells in mouse liver

Several reliable and reproducible methods are available to induce oval cells in rat liver. Effective methods often involve inhibiting proliferation in hepatocytes using an alkylating agent, then subjecting the rat to partial hepatectomy (PH). The surgery is difficult to perform reproducibly in mice. Approaches that do not include partial hepatectomy, such as administration of D-galactosamine, are ineffective in mice. We found that a choline-deficient, ethionine-supplemented (CDE) diet, which is very effective in rats, leads to high morbidity and mortality when administered to mice. This article outlines an alternative protocol by which a CDE diet can be administered to mice. This diet is shown to be highly effective for oval cell induction, without causing high mortality. It takes less time and is at least as effective as other commonly used protocols for inducing oval cells in mice.

A modified choline-deficient, ethionine-supplemented diet protocol effectively induces oval cells in mouse liver

Several reliable and reproducible methods are available to induce oval cells in rat liver. Effective methods often involve inhibiting proliferation in hepatocytes using an alkylating agent, then subjecting the rat to partial hepatectomy (PH). The surgery is difficult to perform reproducibly in mice. Approaches that do not include partial hepatectomy, such as administration of D-galactosamine, are ineffective in mice. We found that a choline-deficient, ethionine-supplemented (CDE) diet, which is very effective in rats, leads to high morbidity and mortality when administered to mice. This article outlines an alternative protocol by which a CDE diet can be administered to mice. This diet is shown to be highly effective for oval cell induction, without causing high mortality. It takes less time and is at least as effective as other commonly used protocols for inducing oval cells in mice.

Hepatoblast-like cells populate the adult p53 knockout mouse liver: evidence for a hyperproliferative maturation-arrested stem cell compartment

Although p53 regulates the cell cycle and apoptosis, gross embryonic development is normal in the p53 knockout (-/-) mouse. In this study, we comprehensively assessed liver development in p53 -/- mice (from embryonic day 15 to adult) for evidence of a cell cycle-induced perturbation in differentiation. Liver cell proliferation in the embryo and newborn is similar in p53 -/- and +/+ mice; in contrast, -/- adult hepatocytes divide at twice the rate of wild types. Developmental expression patterns of liver-specific markers that are up-regulated (e.g., phosphoenolpyruvate carboxykinase and aldolase B) and down-regulated (e.g., alpha-fetoprotein) are similar. Therefore, embryonic and perinatal liver development is normal in the absence of p53. However, the p53 -/- adult liver displays small blast-like cells, the majority being hepatic and some lymphoid. These cells appear in periportal regions and can infiltrate the parenchyma. The hepatic blast-like cells express both mature and immature liver markers, suggesting that differentiation of the liver stem cell compartment is blocked.

Oval cell numbers in human chronic liver diseases are directly related to disease severity

The risk of developing hepatocellular carcinoma is significantly increased in patients with genetic hemochromatosis, alcoholic liver disease, or chronic hepatitis C infection. The precise mechanisms underlying the development of hepatocellular carcinoma in these conditions are not well understood. Stem cells within the liver, termed oval cells, are involved in the pathogenesis of hepatocellular carcinoma in animal models and may be important in the development of hepatocellular carcinoma in human chronic liver diseases. The aims of this study were to determine whether oval cells could be detected in the liver of patients with genetic hemochromatosis, alcoholic liver disease, or chronic hepatitis C, and whether there is a relationship between the severity of the liver disease and the number of oval cells. Oval cells were detected using histology and immunohistochemistry in liver biopsies from patients with genetic hemochromatosis, alcoholic liver disease, or chronic hepatitis C. Oval cells were not observed in normal liver controls. Oval cell numbers increased significantly with the progression of disease severity from mild to severe in each of the diseases studied. We conclude that oval cells are frequently found in subjects with genetic hemochromatosis, alcoholic liver disease, or chronic hepatitis C. There is an association between severity of liver disease and increase in the number of oval cells consistent with the hypothesis that oval cell proliferation is associated with increased risk for development of hepatocellular carcinoma in chronic liver disease.

The association between murine cytomegalovirus induced hepatitis and the accumulation of oval cells

The accumulation of oval cells is an early event in the development of hepatocellular carcinoma induced by certain experimental regimes involving hepatocarcinogens. Oval cells have also been observed during chronic hepatitis induced by alcohol and iron overload. In this study, livers of murine cytomegalovirus (MCMV) infected mice were examined to determine whether hepatitis induced by this virus could initiate oval cell proliferation. BALB/c and C57BL/6 mice were infected with MCMV and studied 4, 8, 10 and 12 months later, alongside control (uninfected) mice. The livers were examined histochemically, immunocytochemically and by in situ hybridization to identify oval cells, inflammatory cells and proliferating cells. Oval cells were seen in the periportal regions of livers from MCMV infected BALB/c mice. These increased in number from 4 to 12 months after infection in parallel with increases in the numbers of inflammatory cells, even though cells expressing MCMV antigens were no longer evident in these samples. Proliferating oval cells and hepatocytes were identified by PCNA staining, indicating an increased level of liver regeneration in the infected livers. C57BL/6 mice are less susceptible to persistent MCMV hepatitis and had fewer oval cells than BALB/c mice. Thus the study demonstrates an association between MCMV induced hepatitis, inflammation, and presence of oval cells.

Gadolinium chloride suppresses hepatic oval cell proliferation in rats with biliary obstruction

Liver injury due to bile duct ligation (BDL) is histologically characterized by cholestasis, bile ductular proliferation, hepatocellular damage, portal fibrosis, and ultimately biliary cirrhosis. Stem cells within the liver may act as progenitor cells for small epithelial cells termed oval cells that can differentiate into bile duct cells or hepatocytes, whereas myofibroblasts are the principal source of collagen production in fibrosis. The aims of this study were to determine 1) whether BDL induces oval cell proliferation and 2) whether blockade of Kupffer cells affects oval cell proliferation, bile duct proliferation, and myofibroblast transformation in experimental biliary obstruction. Male Sprague-Dawley rats were divided into two groups to receive either a single dose of gadolinium chloride (a selective Kupffer cell blocking agent) or vehicle. One day later, the gadolinium- and vehicle-treated groups were further subdivided to receive either BDL or sham operation. The rats were sacrificed on day 7 postoperatively. Serum was collected for measurement of aspartate aminotransferase, gamma-glutamyl transpeptidase, and bilirubin levels. Liver tissue was taken for evaluation of fibrosis, bile ductular cells, oval cells, and myofibroblasts. BDL resulted in elevated aspartate aminotransferase, gamma-glutamyl transpeptidase, and bilirubin in serum, and gadolinium pretreatment did not modify these effects. BDL induced marked oval cell proliferation, which was completely prevented by gadolinium pretreatment. Gadolinium did not affect the induction of bile duct expansion or myofibroblasts after BDL. We conclude that experimental biliary obstruction induces oval cell proliferation, which can be prevented by gadolinium pretreatment. This suggests that bile ductular proliferation and myofibroblast transformation are not mediated by Kupffer cells and that ductular proliferation can proceed in the absence of oval cells. Alternatively, gadolinium may directly affect oval cell proliferation after BDL.

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.

The oval-shaped cell as a candidate for a liver stem cell in embryonic, neonatal and precancerous liver: identification based on morphology and immunohistochemical staining for albumin and pyruvate kinase isoenzyme expression

Oval cells observed in some experimental models of hepatocarcinogenesis can function as stem cells capable of differentiating into hepatocytes and bile ductular cells. Using markers which characterise embryonic hepatocytes, we showed that oval cells display different patterns of gene expression, suggesting some are more mature than others. In this study we looked for oval cells in developing liver, predicting that they are abundant in embryonic liver and decline in number during development. Albumin (ALB) serves as a liver-specific marker, and the isoenzymes of pyruvate kinase, M2-PK and L-PK, are used to identify immature and mature hepatocytes, respectively. Small oval-shaped cells expressing ALB, M2-PK and L-PK are found near the vascular spaces and portal areas in 20-day gestation (E20), E21, newborn, 3-day and 1-week-old rat liver. Similar cells expressing ALB and M2-PK, but not L-PK are seen only periportally in adult liver. These are abundant in early embryonic liver and decrease in number during development until only a few, located periportally, persist in the adult. Oval cells, located periportally a few days after commencing a choline-deficient, ethionine-supplemented diet, co-express ALB and M2-PK. Their similarity with respect to markers, morphology and location suggests that oval-shaped cells may be the progenitors of oval cells.

Chronic iron overload in rats induces oval cells in the liver

Liver damage induced by a variety of agents including hepatocarcinogens, alcohol, and virus induces proliferation of oval cells. In this study, iron overloading of the liver is used as a means of inducing liver damage over an extended period to ascertain whether it promotes the appearance of oval cells. Rats were fed a 2% carbonyl-iron-supplemented diet for 3 or 6 months. Extensive iron deposits appeared periportally in hepatocytes and some Kupffer cells. Iron deposition was less pronounced pericentrally. Small oval-like cells, morphologically and immunocytochemically similar to CDE-derived oval cells, were identified and quantified. They first emerged periportally and subsequently in small tracts or foci nearer central regions and stained positively for alpha-fetoprotein, pi-class glutathione S-transferase, and the embryonic form of pyruvate kinase. They contained very few iron deposits and were classified as iron free. The major difference between CDE- and iron-overload-derived oval cells was that the latter were negative for transferrin. This study shows that cellular changes occurring in iron-overloaded rat liver are similar to those observed in rats placed on a hepatocarcinogenic diet and in rats chronically exposed to alcohol.

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.

Dual phenotypic expression of hepatocytes and bile ductular markers in developing and preneoplastic rat liver

This study supports the existence of a pluripotent liver stem cell population which has the potential to differentiate into hepatocytes and bile ductular cells. We compared the expression of hepatocyte-specific and bile ductular-specific markers in fetal and preneoplastic rat liver. L-pyruvate kinase (L-PK) and alpha glutathione S-transferase (GST) were used as adult hepatocyte-specific markers, while cytokeratin 19 (CK19) was used as a bile ductular-specific marker. pi GST and M2-pyruvate kinase (M2-PK), which are fetal hepatocyte-specific and expressed at high levels in the oval and duct-like cells, were also used. We characterized fetal liver derived from 13-21 days of gestation (E13-E21). pi GST was detected in the E18 hepatoblasts, which form the intrahepatic bile ducts, while CK19 was detected at E19. Some of these cells express alpha GST and L-PK from E19 to E21. Oval, duct-like and bile ductular cells in rats treated with a choline-deficient diet containing 0.07% ethionine (CDE diet) for up to 8 weeks were characterized by double immunocytochemistry. L-PK and alpha GST are absent from bile ductular cells in the normal adult liver and up to 3 weeks of CDE treatment. After 4-5 weeks on CDE treatment, the majority of bile ductular cells express L-PK, while at 6 weeks some co-express L-PK and alpha GST. There are two populations of oval cells, a major population expressing only the fetal hepatocyte markers, while a minor population expresses the fetal hepatocyte, adult hepatocyte and bile ductular markers. There are at least three different duct-like cell populations which co-express different markers and have characteristics of fetal hepatocytes at sequential stages of differentiation. One population co-expresses pi GST and M2-PK and is similar to fetal hepatocytes derived from E13-E14 fetuses. The second expresses the two fetal markers and L-PK, and this reflects characteristics of E15 hepatocytes. The third expresses pi GST, M2-PK, L-PK and alpha GST which is characteristic of E16-E19 hepatocytes. Upon withdrawal of the CDE diet, autoradiography using tritiated thymidine shows that oval and duct-like cells differentiate into hepatocytes. This study demonstrates that oval and duct-like cells express both hepatocytic and bile ductular markers, and have the capacity to differentiate into hepatocytes, characteristics similar to hepatoblasts in the developing rat liver.

Appearance of oval cells in the liver of rats after long-term exposure to ethanol

Epidemiological studies show an increased risk of developing liver cancer among alcoholics. There is some agreement that ethanol itself is not carcinogenic, but it may enhance the tumorigenic process by inducing drug-metabolizing enzymes, suppression of the immune system or by affecting DNA repair enzymes. Precisely how ethanol predisposes or promotes the development of hepatoma is unknown. Hepatocarcinogenesis induced by a choline-deficient, ethionine-supplemented (CDE) diet produces extensive alteration of the liver architecture with the emergence and rapid proliferation of oval cells. This study examines whether chronic alcohol consumption induces the proliferation of oval cells. Oval cells induced in rats maintained on a 5% ethanol liquid diet (ELD) for up to 24 months, or fed a CDE diet for up to 4 weeks, are compared using a panel of liver-specific markers. In CDE-treated rats, oval cells staining positively for alpha-fetoprotein (AFP), pi-class glutathione S-transferase (pi GST), and the embryonic form of pyruvate kinase (M2-PK) are observed after 1 week. Similar cells are seen in ELD-treated rats after 2 months. Their numbers increase with time, and incorporation of [3H]thymidine confirms they are a dividing population. Acute damage induced by partial hepatectomy and CCI4 poisoning did not induce the appearance of oval cells. We conclude that chronic ethanol consumption induces oval cell proliferation. We suggest that, in addition to other proposed mechanisms, an alteration in cellular composition of the liver be considered as an explanation for the increased incidence of liver cancer among alcoholics.