The role of hepatocytes and oval cells in liver regeneration and repopulation

Direct in vivo cell lineage analysis in the retrorsine and 2AAF models of liver injury after genetic labeling in adult and newborn rats

Backgrounds and Aims

When hepatocyte proliferation is impaired, liver regeneration proceeds from the division of non parenchymal hepatocyte progenitors. Oval cells and Small Hepatocyte-like Progenitor Cells (SHPCs) represent the two most studied examples of such epithelial cells with putative stem cell capacity. In the present study we wished to compare the origin of SHPCs proliferating after retrorsine administration to the one of oval cells observed after 2-Acetyl-Amino fluorene (2-AAF) treatment.

Methodology/Principal Findings

We used retroviral-mediated nlslacZ genetic labeling of dividing cells to study the fate of cells in the liver. Labeling was performed either in adult rats before treatment or in newborn animals. Labeled cells were identified and characterised by immunohistochemistry. In adult-labeled animals, labeling was restricted to mature hepatocytes. Retrorsine treatment did not modify the overall number of labeled cells in the liver whereas after 2-AAF administration unlabeled oval cells were recorded and the total number of labeled cells decreased significantly. When labeling was performed in newborn rats, results after retrorsine administration were identical to those obtained in adult-labeled rats. In contrast, in the 2-AAF regimen numerous labeled oval cells were present and were able to generate new labeled hepatocytes. Furthermore, we also observed labeled biliary tracts in 2-AAF treated rats.

Conclusions

Our results srongly suggest that SHPCs are derived from hepatocytes and we confirm that SHPCs and oval cells do not share the same origin. We also show that hepatic progenitors are labeled in newborn rats suggesting future directions for in vivo lineage studies.

The Drosophila hindgut lacks constitutively active adult stem cells but proliferates in response to tissue damage

The adult Drosophila hindgut was recently reported to contain active, tissue-replenishing stem cells, like those of the midgut, but located within an anterior ring so as to comprise a single giant crypt. In contrast to this view, we observed no active stem cells and little cell turnover in adult hindgut tissue based on clonal marking and BrdU incorporation studies. Again contradicting the previous proposal, we showed that the adult hindgut is not generated by anterior stem cells during larval/pupal development. However, severe tissue damage within the hindgut elicits cell proliferation within a ring of putative quiescent stem cells at the anterior of the pylorus. Thus, the hindgut does not provide a model of tissue maintenance by constitutively active stem cells, but has great potential to illuminate mechanisms of stress-induced tissue repair.

Translational research to identify clinical applications of hepatocyte growth factor

Hepatocyte growth factor (HGF), originally purified from the plasma of patients with fulminant hepatic failure, has been shown to carry out various physiological functions. HGF not only stimulates liver regeneration, but also acts as an antiapoptotic factor in in vivo experimental models. Therefore, HGF is a promising therapeutic agent for the treatment of fatal liver diseases, including fulminant hepatic failure. After performing a number of preclinical tests, our group began an investigator-initiated registered phase I/II clinical trial of patients with fulminant hepatic failure to examine the safety and clinical efficacy of recombinant human HGF. In this article, we will discuss the basic research results as well as the translational research that underpins current attempts to use HGF in various clinical settings.

Thymus cell antigen-1-expressing cells in the oval cell compartment

Thymus cell antigen-1 (Thy-1)-expressing cells proliferate in the liver during oval cell (OC)-mediated liver regeneration. We characterized these cells in normal liver, in carbon tetrachloride-injured liver, and in several models of OC activation. The gene expression analyses were performed using reverse-transcriptase polymerase chain reaction (RT-PCR), quantitative RT-PCR (Q-RT-PCR) of cells isolated by fluorescence-activated cell sorting (FACS), and by immunofluorescent microscopy of tissue sections and isolated cells. In normal liver, Thy-1(+) cells are a heterogeneous population: those located in the periportal region do not coexpress desmin or alpha smooth muscle actin (alpha-SMA). The majority of Thy-1(+) cells located at the lobular interface and in the parenchyma coexpress desmin but not alpha-SMA, i.e., they are not resident myofibroblasts. Although Thy-1(+) cells proliferate moderately after carbon tetrachloride injury, in all models of OC-mediated liver regeneration they proliferate quickly and expand significantly and disappear from the liver when the OC response subsides. Activated Thy-1(+) cells do not express OC genes but they express genes known to be expressed in mesenchymal stem cells (CD105, CD73, CD29), genes considered specific for activated stellate cells (desmin, collagen I-a2, Mmp2, Mmp14) and myofibroblasts (alpha-SMA, fibulin-2), as well as growth factors and cytokines (Hgf, Tweak, IL-1b, IL-6, IL-15) that can affect OC growth. Activated in vitro stellate cells do not express Thy-1. Subcloning of Thy-1(+) cells from OC-activated livers yield Thy-1(+) fibroblastic cells and a population of E-cadherin(+) mesenchymal cells that gradually discontinue expression of Thy-1 and begin to express cytokeratins. However, upon transplantation these cells do not differentiate into hepatocytes or cholangiocytes. Activated Thy-1(+) cells produce predominantly latent transforming growth factor beta.

CONCLUSION

Thy-1(+) cells in the OC niche are activated mesenchymal-epithelial cells that are distinct from resident stellate cells, myofibroblasts, and oval cells.

Stem/progenitor cells in liver injury repair and regeneration

Morbidity and mortality from cirrhosis is increasing rapidly in the world. Currently, orthotopic liver transplantation is the only definitive therapeutic option. However, its clinical use is limited, because of poor long-term graft survival, donor organ shortage and high costs associated with the procedure. Stem cell replacement strategies are therefore being investigated as an attractive alternative approach to liver repair and regeneration. In this review we discuss recent preclinical and clinical investigations that explore the therapeutic potential of stem cells in repair of liver injuries. Several types of stem cells. including embryonic stem cells, haematopoietic stem cells and mesenchymal stem cells, can be induced to differentiate into hepatocyte-like cells by defined culture conditions in vitro. Stem cell transplantation has been shown to significantly improve liver function and increase animal survival in experimentally-induced liver-injury models. Moreover, several pilot clinical studies have reported encouraging therapeutic effects in patients treated with stem cells. Although there remain many unresolved issues, the available data support the notion that stem cell technology may lead to the development of effective clinical modalities for human liver diseases.

Impact of Increased Oxygen Delivery via Bovine Red Blood Cell Supplementation of Culturing Media on Select Metabolic and Synthetic Functions of C3A Hepatocytes Maintained within a Hollow Fiber Bioreactor

Hepatocytes are highly dependent upon appropriate oxygen provision for activity and viability. However, oxygen delivery to hepatocytes cultured within a hollow fiber bioreactor is believed to be problematic because of large diffusion distances, a high hepatocyte oxygen consumption rate and low aqueous media oxygen solubility. Supplementation of bioreactor media with bovine red blood cells (bRBCs) is one means of improving oxygen delivery to hepatocytes as hemoglobin contained within bRBCs binds oxygen. The impact of supplementing hepatocyte culturing media with bRBCs (approximately 5 x 10(8) bRBCs/ml) on hepatocyte activity (albumin and lactate production and glucose consumption) was studied. Decreased hepatocyte lactate production to glucose consumption ratios were found for the case when bRBCs were added to circulating culturing media, which indicated the presence of a more aerobic environment in comparison to the control (no bRBC supplementation). Additionally, albumin synthesis was found to be improved when the circulating media was supplemented with bRBCs. Our results thus support the use of bRBCs to improve oxygen delivery to hepatocytes maintained within a hollow fiber bioreactor.

Sexually dimorphic patterns of episomal rAAV genome persistence in the adult mouse liver and correlation with hepatocellular proliferation

Recombinant adeno-associated virus vectors (rAAVs) show exceptional promise for liver-targeted gene therapy, with phenotype correction in small and large animal disease models being reported with increasing frequency. Success in humans, however, remains a considerable challenge that demands greater understanding of host-vector interactions, notably those governing the efficiency of initial gene transfer and subsequent long-term persistence of gene expression. In this study, we examined long-term enhanced green fluorescent protein (eGFP) expression and vector genome persistence in the mouse liver after rAAV2/8-mediated gene transfer in early adulthood. Two intriguing findings emerged of considerable scientific and clinical interest. First, adult female and male mice showed distinctly different patterns of persistence of eGFP expression across the hepatic lobule after exhibiting similar patterns initially. Female mice retained a predominantly perivenous pattern of expression, whereas male mice underwent inversion of this pattern with preferential loss of perivenous expression and relative retention of periportal expression. Second, these changing patterns of expression correlated with sexually dimorphic patterns of genome persistence that appear linked both spatially and temporally to underlying hepatocellular proliferation. Observation of the equivalent phenomenon in man could have significant implications for the long-term therapeutic efficacy of rAAV-mediated gene transfer, particularly in the context of correction of liver functions showing metabolic zonation.

GABA's control of stem and cancer cell proliferation in adult neural and peripheral niches

Aside from traditional neurotransmission and regulation of secretion, gamma-amino butyric acid (GABA) through GABA(A) receptors negatively regulates proliferation of pluripotent and neural stem cells in embryonic and adult tissue. There has also been evidence that GABAergic signaling and its control over proliferation is not only limited to the nervous system, but is widespread through peripheral organs containing adult stem cells. GABA has emerged as a tumor signaling molecule in the periphery that controls the proliferation of tumor cells and perhaps tumor stem cells. Here, we will discuss GABA's presence as a near-universal signal that may be altered in tumor cells resulting in modified mitotic activity.

Relation between liver progenitor cell expansion and extracellular matrix deposition in a CDE-induced murine model of chronic liver injury

In chronic liver injury, liver progenitor cells (LPCs) proliferate in the periportal area, migrate inside the lobule, and undergo further differentiation. This process is associated with extracellular matrix (ECM) remodeling. We analyzed LPC expansion and matrix accumulation in a choline-deficient, ethionine-supplemented (CDE) model of LPC proliferation. After day 3, CDE induced collagen deposits in the periportal area. Expansion of LPCs as assessed by increased number of cytokeratin 19 (CK19)-positive cells was first observed at day 7, while ECM accumulated 10 times more than in controls. Thereafter, LPCs and ECM increased in parallel. Furthermore, ECM not only accumulates prior to the increase in number of LPCs, but is also found in front of LPCs along the porto-venous gradient of lobular invasion. Double immunostaining revealed that LPCs are embedded in ECM at all times. Moreover, LPCs infiltrating the liver parenchyma are chaperoned by alpha-smooth muscle actin (alpha-SMA)-positive cells. Gene expression analyses confirmed these observations. The expression of CK19, alpha-fetoprotein, E-cadherin, and CD49f messenger RNA (mRNA), largely overexpressed by LPCs, significantly increased between day 7 and day 10. By contrast, at day 3 there was a rapid burst in the expression of components of the ECM, collagen I and laminin, as well as in alpha-SMA and connective tissue growth factor expression.

CONCLUSION

Our data demonstrate that, in a CDE model, ECM deposition and activation of matrix-producing cells occurred as an initial phase, prior to LPC expansion, and in front of LPCs along the porto-venous gradient of lobular invasion. Those observations may reveal a fundamental role for the established hepatic microenvironment or niche during the process of activation and differentiation of liver progenitor cells.

Short-term correction of arginase deficiency in a neonatal murine model with a helper-dependent adenoviral vector

Neonatal gene therapy has the potential to ameliorate abnormalities before disease onset. Our gene knockout of arginase I (AI) deficiency is characterized by increasing hyperammonemia, neurological deterioration, and early death. We constructed a helper-dependent adenoviral vector (HDV) carrying AI and examined for correction of this defect. Neonates were administered 5 x 10(9) viral particles/g and analyzed for survival, arginase activity, and ammonia and amino acids levels. The life expectancy of arg(-/-) mice increased to 27 days while controls died at 14 days with hyperammonemia and in extremis. Death correlated with a decrease in viral DNA/RNA per cell as liver mass increased. Arginase assays demonstrated that vector-injected hepatocytes had ~20% activity of heterozygotes at 2 weeks of age. Hepatic arginine and ornithine in treated mice were similar to those of saline-injected heterozygotes at 2 weeks, whereas ammonia was normal. By 26 days, arginase activity in the treated arg(-/-) livers declined to <10%, and arginine and ornithine increased. Ammonia levels began increasing by day 25, suggesting the cause of death to be similar to that of uninjected arg(-/-) mice, albeit at a later time. These studies demonstrate that the AI deficient newborn mouse can be temporarily corrected and rescued using a HDV.

Intrahepatic transplantation of hepatic oval cells for fulminant hepatic failure in rats

AIM: To evaluate the effect of intrahepatic trans-plantation of hepatic oval cells (HOC) on fulminant hepatic failure (FHF) in rats.

METHODS: HOC obtained from rats were labeled with green fluocescent protein (GFP) or 5, 6-carboxyfluorescein diacetate succinmidyl ester (CFDA-SE). Cell fluorescence was observed under fluorescent microscope at 6, 24, 48 and 72 h after labeling. CFDA-SE labeled HOC (5 × 10⁶ cells each rat) were injected into livers of rats with FHF induced by D-galactosamine. Serum albumin (ALB), alanine aminotransferase (ALT), aspartate aminotransferase (AST) and total bilirubin (TBil) levels were measured at different time points. Liver function of rats was examined on days 3, 7, 14 and 21 after HOC transplantation.

RESULTS: The positive rate of GFP and CFDA-SE labeled HOC was 10% and 90%, respectively, with no significant change in cell viabilities. The survival rate was higher in HOC transplantation group than in control group, especially 48 (9/15 vs 6/15) and 72 h (9/15 vs 4/15) after HOC transplantation. The serum ALT, AST and TBil levels were decreased while the serum Alb level was increased after HOC transplantation. Fluorescence became faded and diffused in liver tissues, suggesting that proliferation and differentiation occur in transplanted HOC.

CONCLUSION: CFDA-SE is superior to GFP in labeling HOC, although fluorescence intensity is decreased progressively with cell division. HOC transplantation can improve the liver function and increase the survival rate of recipients.

The consequences of tetraploidy and aneuploidy

Polyploidy, an increased number of chromosome sets, is a surprisingly common phenomenon in nature, particularly in plants and fungi. In humans, polyploidy often occurs in specific tissues as part of terminal differentiation. Changes in ploidy can also result from pathophysiological events that are caused by viral-induced cell fusion or erroneous cell division. Tetraploidization can initiate chromosomal instability (CIN), probably owing to supernumerary centrosomes and the doubled chromosome mass. CIN, in turn, might persist or soon give way to a stably propagating but aneuploid karyotype. Both CIN and stable aneuploidy are commonly observed in cancers. Recently, it has been proposed that an increased number of chromosome sets can promote cell transformation and give rise to an aneuploid tumor. Here, we review how tetraploidy can occur and describe the cellular responses to increased ploidy. Furthermore, we discuss how the specific physiological changes that are triggered by polyploidization might be used as novel targets for cancer therapy.

Hepatic non-parenchymal cells and extracellular matrix participate in oval cell-mediated liver regeneration

AIM: To elucidate the interaction between non-parenchymal cells, extracellular matrix and oval cells during the restituting process of liver injury induced by partial hepatectomy (PH).

METHODS: We examined the localization of oval cells, non-parenchymal cells, and the extracellular matrix components using immunohistochemical and double immunofluorescent analysis during the proliferation and differentiation of oval cells in N-2-acetylaminofluorene (2-AAF)/PH rat model.

RESULTS: By day 2 after PH, small oval cells began to proliferate around the portal area. Most of stellate cells and laminin were present along the hepatic sinusoids in the periportal area. Kupffer cells and fibronectin markedly increased in the whole hepatic lobule. From day 4 to 9, oval cells spread further into hepatic parenchyma, closely associated with stellate cells, fibronectin and laminin. Kupffer cells admixed with oval cells by day 6 and then decreased in the periportal zone. From day 12 to 15, most of hepatic stellate cells (HSCs), laminin and fibronectin located around the small hepatocyte nodus, and minority of them appeared in the nodus. Kupffer cells were mainly limited in the pericentral sinusoids. After day 18, the normal liver lobule structures began to recover.

CONCLUSION: Local hepatic microenvironment may participate in the oval cell-mediated liver regeneration through the cell-cell and cell-matrix interactions.

Generation and regeneration of cells of the liver and pancreas

Liver and pancreas progenitors develop from endoderm cells in the embryonic foregut. Shortly after their specification, liver and pancreas progenitors rapidly acquire markedly different cellular functions and regenerative capacities. These changes are elicited by inductive signals and genetic regulatory factors that are highly conserved among vertebrates. Interest in the development and regeneration of the organs has been fueled by the intense need for hepatocytes and pancreatic beta cells in the therapeutic treatment of liver failure and type I diabetes. Studies in diverse model organisms have revealed evolutionarily conserved inductive signals and transcription factor networks that elicit the differentiation of liver and pancreatic cells and provide guidance for how to promote hepatocyte and beta cell differentiation from diverse stem and progenitor cell types.

Plasmodium sporozoite passage across the sinusoidal cell layer

Malaria sporozoites must cross at least two cell barriers to reach their initial site of replication in the mammalian host. After transmission into the skin by an infected mosquito, they migrate towards small dermal capillaries, traverse the vascular endothelial layer, and rapidly home to the liver. To infect hepatocytes, the parasites must cross the sinusoidal cell layer, composed of specialized highly fenestrated sinusoidal endothelia and Kupffer cells, the resident macrophages of the liver (Fig. 1). The exact route Plasmodium sporozoites take to hepatocytes has been subject of controversial discussions for many years. Recent cell biological, microscopic, and genetic approaches have considerably enhanced our understanding of the initial events leading to the establishment of a malaria infection in the liver.

Flow cytometric isolation and clonal identification of self-renewing bipotent hepatic progenitor cells in adult mouse liver

The adult liver progenitor cells appear in response to several types of pathological liver injury, especially when hepatocyte replication is blocked. These cells are histologically identified as cells that express cholangiocyte markers and proliferate in the portal area of the hepatic lobule. Although these cells play an important role in liver regeneration, the precise characterization that determines these cells as self-renewing bipotent primitive hepatic cells remains to be shown. Here we attempted to isolate cells that express a cholangiocyte marker from the adult mouse liver and perform single cell-based analysis to examine precisely bilineage differentiation potential and self-renewing capability of these cells. Based on the results of microarray analysis and immunohistochemistry, we used an antibody against CD133 and isolate CD133(+) cells via flow cytometry. We then cultured and propagated isolated cells in a single cell culture condition and examined their potential for proliferation and differentiation in vitro and in vivo. Isolated cells that could form large colonies (LCs) in culture gave rise to both hepatocytes and cholangiocytes as descendants, while maintaining undifferentiated cells by self-renewing cell divisions. The clonogenic progeny of an LC-forming cell is capable of reconstituting hepatic tissues in vivo by differentiating into fully functional hepatocytes. Moreover, the deletion of p53 in isolated LC-forming cells resulted in the formation of tumors with some characteristics of hepatocellular carcinoma and cholangiocarcinoma upon subcutaneous injection into immunodeficient mutant mice. These data provide evidence for the stem cell-like capacity of isolated and clonally cultured CD133(+) LC-forming cells.

CONCLUSION

Our method for prospectively isolating hepatic progenitor cells from the adult mouse liver will facilitate study of their roles in liver regeneration and carcinogenesis.

A rodent model of NASH with cirrhosis, oval cell proliferation and hepatocellular carcinoma

BACKGROUND/AIMS

Hepatocellular carcinoma (HCC) is a well recognized complication of advanced NASH (non-alcoholic steatohepatitis). We sought to produce a rat model of NASH, cirrhosis and HCC.

METHODS

Adult Sprague-Dawley rats, weighing 250-300g, were fed a choline-deficient, high trans-fat diet and exposed to DEN in drinking water. After 16 weeks, the animals underwent liver ultrasound (US), sacrifice and assessment by microscopy, immunohistochemistry and transmission electron microscopy (TEM).

RESULTS

US revealed steatosis and focal lesions in 6 of 7. All had steatohepatitis defined as inflammation, advanced fibrosis and ballooning with Mallory-Denk bodies (MDB) with frank cirrhosis in 6. Areas of more severe injury were associated with anti-CK19 positive ductular reaction. HCC, present in all, were macro-trabecullar or solid with polyhedral cells with foci of steatosis and ballooned cells. CK19 was positive in single or solid nests of oval cells and in neoplastic hepatocytes. TEM showed ballooning with small droplet fat, dilated endoplasmic reticulum and MDB in non-neoplastic hepatocytes and small droplet steatosis in some cancer cells.

CONCLUSIONS

This model replicated many features of NASH including steatohepatitis with ballooning, fibrosis, cirrhosis and hepatocellular carcinoma. Oval cell proliferation was evident and the presence anti-CK 19 positivity in the cancer suggests oval cell origin of the malignancy.

Activation of canonical Wnt signaling pathway promotes proliferation and self-renewal of rat hepatic oval cell line WB-F344 in vitro

AIM: To investigate the effect of activation of canonical Wnt signaling pathway on the proliferation and differentiation of hepatic oval cells in vitro.

METHODS: WB-F344 cells were treated with recombinant Wnt3a (20, 40, 80, 160, 200 ng/mL) in serum-free medium for 24 h. Cell proliferation was measured by Brdu incorporation analysis; untreated WB-F344 cells were taken as controls. After treatment with Wnt3a (160 ng/mL) for 24 h, subcellular localization and protein expression of β-catenin in WB-F344 cells treated and untreated with Wnt3a were examined by immunofluorescence staining and Western blot analysis. CyclinD1 mRNA expression was determined by semi-quantitative reverse-transcript polymerase chain reaction (RT-PCR). The mRNA levels of some phenotypic markers (AFP, CK-19, ALB) and two hepatic nuclear factors (HNF-4, HNF-6) were measured by RT-PCR. Expressions of CK-19 and AFP protein were detected by Western blot analysis.

RESULTS: Wnt3a promoted proliferation of WB-F344 cells. Stimulation of WB-F344 cells with recombinant Wnt3a resulted in accumulation of the transcriptional activator β-catenin, together with its translocation into the nuclei, and up-regulated typical Wnt target gene CyclinD1. After 3 d of Wnt3a treatment in the absence of serum, WB-F344 cells retained their bipotential to express several specific phenotypic markers of hepatocytes and cholangiocytes, such as AFP and CK-19, following activation of the canonical Wnt signaling pathway.

CONCLUSION: The canonical Wnt signaling pathway promotes proliferation and self-renewal of rat hepatic oval cells.

Rad50 is dispensable for the maintenance and viability of postmitotic tissues

The majority of spontaneous chromosome breakage occurs during the process of DNA replication. Homologous recombination is the primary mechanism of repair of such damage, which probably accounts for the fact that it is essential for genome integrity and viability in mammalian cells. The Mre11 complex plays diverse roles in the maintenance of genomic integrity, influencing homologous recombination, checkpoint activation, and telomere maintenance. The complex is essential for cellular viability, but given its myriad influences on genomic integrity, the mechanistic basis for the nonviability of Mre11 complex-deficient cells has not been defined. In this study we generated mice carrying a conditional allele of Rad50 and examined the effects of Rad50 deficiency in proliferative and nonproliferative settings. Depletion of Rad50 in cultured cells caused extensive DNA damage and death within 3 to 5 days of Rad50 deletion. This was not associated with gross telomere dysfunction, suggesting that the telomeric functions of the Mre11 complex are not required for viability. Rad50 was also dispensable for the viability of quiescent liver and postmitotic Purkinje cells of the cerebellum. These findings support the idea that the essential functions of the Mre11 complex are associated with DNA replication and further suggest that homologous recombination is not essential in nondividing cells.

Determinants of pancreatic beta-cell regeneration

Recent studies have revealed a surprising plasticity of pancreatic beta-cell mass. beta-cell mass is now recognized to increase and decrease in response to physiological demand, for example during pregnancy and in insulin-resistant states. Moreover, we and others have shown that mice recover spontaneously from diabetes induced by killing of 70-80% of beta-cells, by beta-cell regeneration. The major cellular source for new beta-cells following specific ablation, as well as during normal homeostatic maintenance of adult beta-cells, is proliferation of differentiated beta-cells. More recently, it was shown that one form of severe pancreatic injury, ligation of the main pancreatic duct, activates a population of embryonic-type endocrine progenitor cells, which can differentiate into new beta-cells. The molecular triggers for enhanced beta-cell proliferation during recovery from diabetes and for activation of embryonic-type endocrine progenitors remain unknown and represent key challenges for future research. Taken together, recent data suggest that regenerative therapy for diabetes may be a realistic goal.

Wnt signaling in liver cancer

Hepatocellular carcinoma (HCC) is a major cause of cancer death worldwide. As in many other types of cancer, aberrant activation of the canonical Wnt/beta-catenin signaling pathway is an important contributor to tumorigenesis. In HCC this frequently occurs through mutations in the N-terminal region of beta-catenin that stabilize the protein and permit an elevated level of constitutive transcriptional activation by beta-catenin/TCF complexes. In this article we review the abundant evidence that Wnt/beta-catenin signaling contributes to liver carcinogenesis. We also discuss what is known about the roles of Wnt signaling in liver development, regeneration, and stem cell behavior, in an effort to understand the mechanisms by which activation of the canonical Wnt pathway promotes tumor formation in this organ. The Wnt/beta-catenin pathway presents itself as an attractive target for developing novel rational therapies for HCC, a disease for which few successful treatment strategies are currently available.

[Adipose tissue-derived stem cells: hepatic plasticity]

Currently, the only effective treatment for end-stage liver disease is liver transplantation. The number of patients on the waiting list increases considerably each year, giving rise to a wide imbalance between supply and demand for healthy livers. Knowledge of stem cells and their possible use have awakened great interest in the field of hepatology, these cells being one of the most promising short-term alternatives. Hepatic stem cell therapy consists of the implantation of healthy cells capable of performing the functions that damaged cells are unable to carry out. Recent observations indicate that several stem cells can differentiate into distinct cell lineages. Hepatic differentiation of adult stem cells from several origins has yielded highly promising results. Adipose tissue in adults contains a reservoir of stem cells that can be induced and differentiated into different types of cells, showing a high degree of plasticity. Because of its abundance and easy access, adipose tissue is a promising source of adult stem cells for hepatic stem cell therapy. The present article reviews the progress made in the differentiation of adult stem cells from adipose tissue into cells with hepatic phenotype. We also discuss the potential application of this technique as a therapy for temporary metabolic support in patients with end-stage liver failure awaiting whole organ transplantation, as a method to support liver function and facilitate regeneration of the native liver in cases of fulminant hepatic failure, and as a treatment in patients with genetic metabolic defects in vital liver functions.

Therapy with bone marrow cells reduces liver alterations in mice chronically infected by Schistosoma mansoni

AIM: To investigate the potential of bone marrow mononuclear cells (BM-MCs) in the regeneration of hepatic lesions induced by Schistosoma mansoni (S.mansoni) chronic infection.

METHODS: Female mice chronically infected with S.mansoni were treated with BM-MCs obtained from male green fluorescent protein (GFP) transgenic mice by intravenous or intralobular injections. Control mice received injections of saline in similar conditions. Enzyme-linked immunosorbent assay (ELISA) assay for transforming growth factor-beta (TGF-β), polymerase chain reaction (PCR) for GFP DNA, immunofluorescence and morphometric studies were performed.

RESULTS: Transplanted GFP⁺ cells migrated to granuloma areas and reduced the percentage of liver fibrosis. The presence of donor-derived cells was confirmed by Fluorescence in situ hybridization (FISH) analysis for detection of cells bearing Y chromosome and by PCR analysis for detection of GFP DNA. The levels of TGF-β, a cytokine associated with fibrosis deposition, in liver fragments of mice submitted to therapy were reduced. The number of oval cells in liver sections of S.mansoni-infected mice increased 3-4 fold after transplantation. A partial recovery in albumin expression, which is decreased upon infection with S.mansoni, was found in livers of infected mice after cellular therapy.

CONCLUSION: In conclusion, transplanted BMCs migrate to and reduce the damage of chronic fibrotic liver lesions caused by S.mansoni.

Autologous bone marrow cell infusion therapy for liver cirrhosis

The plasticity of bone marrow cells (BMC) has been confirmed by autopsy results of female recipients of BMC from male donors. To establish new clinical therapies for patients with liver cirrhosis using autologous BMC, we developed a new in vivo murine model using green fluorescent protein (GFP) and repeated carbon tetrachloride (CCl(4)) injection. We found that BMC infused through the tail vein, efficiently repopulated cirrhotic liver tissue and, under the influence of persistent liver damage induced by carbon tetrachloride, differentiated into albumin-producing hepatocytes. Moreover, such BMC infusions into mice with cirrhosis improved liver function and reduced mortality. The latter observation correlated with the strong expression of matrix metalloproteinases (MMP), particularly MMP-9, and reduced hepatic fibrosis. The results from the 'GFP/CCl(4) model' showed that cell therapy using autologous BMC has the potential to become an effective treatment for patients with liver failure due to advanced liver cirrhosis. This review summarizes previous findings plus these recent experimental results, as well as recent clinical trials of BMC transfusion into patients with end-stage chronic liver disease.

C-kit inhibition by imatinib mesylate attenuates progenitor cell expansion and inhibits liver tumor formation in mice

BACKGROUND & AIMS

Numerous studies have linked the proliferation of liver progenitor cells (LPCs) during chronic liver disease to the risk for development of hepatocellular carcinoma. Thus, selective inhibition of LPC growth during preneoplastic injury may prevent or delay the onset of liver cancer. Rats carrying a germ-line mutation in c-kit have an impaired LPC response to liver injury. Therefore, we hypothesized that the c-kit inhibitor imatinib mesylate (IM) would suppress LPC growth and, therefore, may exert antitumorigenic effects in the liver.

METHODS

Expression of IM target proteins was examined in chronically injured rodent and human livers. The effect of IM was examined in vitro using LPC lines and in vivo in mice fed a choline-deficient, ethionine-supplemented (CDE) diet. Livers were examined following short-term (up to 1 month) or long-term (up to 14 months) feeding of CDE diet and drug treatments.

RESULTS

C-kit was significantly up-regulated in chronic injury and expressed by LPCs. IM was antiproliferative to LPC lines, and knockdown of c-kit reduced this response. IM treatment inhibited the LPCs response and early fibrogenesis induced by a short-term CDE diet. On the longer term, IM treatment reduced the extent of fibrosis and significantly inhibited tumor formation.

CONCLUSIONS

Tyrosine kinase inhibitors, such as IM, may be suited for the prevention of hepatocellular carcinoma in the setting of chronic liver injury via antiproliferative effects on c-kit-expressing LPCs.

Hepatic progenitor cell surface markers and biological function

Hepatic progenitor cells have the bipotential capable of differentiation into mature hepatocytes and biliary epithelial cells when hepatocyte proliferation is inhibited and liver regeneration compromised. This review focuses on the surface markers and biological function of hepatic progenitor cells and the existed questions in this field are also discussed.

Stem cell-based therapy in gastroenterology and hepatology

Protagonists of a new scientific era, stem cells are promising tools on which regenerative medicine relies for the treatment of human pathologies. Stem cells can be obtained from various sources, including embryos, fetal tissues, umbilical cord blood, and also terminally differentiated organs. Once forced to expand and differentiate into functional progenies, stem cells may become suitable for cell replacement and tissue engineering. The manipulation and/or stimulation of adult stem cells seems to be particularly promising, as it could improve the endogenous regenerative potential without risks of rejection and overcome the ethical and political issues related to embryonic stem cell research. Stem cells are already leaving the bench and reaching the bedside, despite an incomplete knowledge of the genetic control program driving their fate and plasticity. In gastroenterology and hepatology, the first attempts to translate stem cell basic research into novel therapeutic strategies have been made for the treatment of several disorders, such as inflammatory bowel diseases, diabetes mellitus, celiachy and acute or chronic hepatopaties. Nonetheless, critical aspects need to be further addressed, including the long-term safety, tolerability and efficacy of cell-based treatments, as well as their carcinogenic potential. Aim of this review is to summarize the state-of-the-arts on gastrointestinal and hepatic stem cells and on stem cell-based therapies in gastroenterology and hepatology, highlighting both the benefits and the potential risks of these new tools for the treatment and prevention of human diseases.

Advance in hepatic stem cells and liver injury

The liver diseases remain major causes of death all over the world. Although orthotopic liver transplantation is an effective treatment for end-stage liver diseases. However, shortage of healthy livers for transplantation worldwide have urgently limited the use of liver transplantation for acute and chronic liver diseases. Stem cells play an important role in the concert of liver regeneration. Hepatic stem cells have been shown experimentally to participate in liver proliferation. Furthermore, it has been postulated that hepatic stem cells are able to transdifferentiate into both hepatocytes and bole duct cells. These data indicate a possible role and therapeutic potential of hepatic stem cells in liver diseases. In this paper, we reviewed the application of stem cells in liver diseases.

Improved Xenogenic Hepatocyte Implantation into Nude Mouse Liver Parenchyma with Acute Liver Failure when Followed by Repeated Anti-Fas Antibody (Jo2) Treatment

Hepatocyte transplantation is a promising therapy for acute liver failure in humans. Recently, we succeeded in inducing various acute and chronic liver failures in nude mice. Engraftment of transplanted xenogeneic rat hepatocytes, visualized in the host liver by anti-MHC class I immunohistochemistry, revealed that liver repopulation was limited, and equivalent in nude mice with and without acute liver failure. In the present study, acute liver failure was induced in nude mice by a single injection of sublethal anti-Fas antibody Jo2, followed 24 h later by rat hepatocyte transplantation and than by a weekly repeated injection of Jo2. Rat hepatocyte engraftment into the recipient liver parenchyma 3 weeks following hepatocyte transplantation was about sevenfold increased when nude mice were subsequently subjected to weekly repeated Jo2 injection. Genomic analysis of these mice showed an overall transcriptome profile of upregulation of cellular cycle blocking transcripts, activation of liver injury inducing IFN-γ/STAT1 pathway, and circadian transcript signature of antiproliferative cell status compared to mice submitted to hepatocyte transplantation only. The findings of the present study suggest that the induction of cell proliferation blockade in recipient livers could promote sufficient engraftment of transplanted hepatocytes to allow transient or definitive treatment of liver failure in humans.

Progenitor gene DLK1 might be an independent prognostic factor of liver cancer

BACKGROUND

Delta-like 1 homolog (DLK1) is a marker for progenitor cells of the liver. The gene encoding DLK1 is expressed early during embryonic development but, importantly, it is also expressed in some human liver cancers. However, the prognostic value of the DLK1 gene has not been investigated.

OBJECTIVES

To examine the association between the DLK1 gene and survival time and whether high levels of expression of DLK1 are a prognostic factor for liver cancer.

METHODS

We evaluated 60 cases of primary liver cancer, and investigated the link between the expression of DLK1 and patient survival. Clinical characteristics of the cases used for our study, such as tumor size, differentiation and staging, are statistically evenly distributed. Using RT-PCR, western blotting and immunohistochemistry, we analyzed the expression of DLK1 in the tumor samples and evaluated the results statistically.

RESULTS

DLK1 was expressed in 22 of the 60 cases (36.7%), and analysis of the survival of the patients revealed that DLK1-positive patients had a shorter survival time than DLK1-negative patients. Cox regression analysis also showed that DLK1 is a risk factor. However, DLK1 expression does not seem to correlate with other classic prognostic factors such as alpha-fetoprotein (AFP), tumor-node-metastasis (TNM) and vascular invasion, which implies that it is an independent prognostic factor.

Deletion of the Met tyrosine kinase in liver progenitor oval cells increases sensitivity to apoptosis in vitro

The hepatocyte growth factor (HGF)/Met signaling system is essential for liver development, homeostasis, and function. In this study, we took advantage of a liver-specific, Met-conditional knockout mouse generated in our laboratory to address the molecular mechanisms of HGF/Met signaling in adult liver progenitor cell (oval cell) biology. For this purpose, we isolated oval cells from 3,5-diethoxycarbonyl-1,4-dihydro-collidine-treated Met(flx/flx) mice and established oval cell-derived cell lines that carried either functional (Met(flx/flx)) or a nonfunctional (Met(-/-)) met gene using virus-mediated Cre-loxP recombination. Oval cells lacking Met tyrosine kinase activity displayed neither Met phosphorylation nor activation of downstream targets and were refractory to HGF stimulation. Although Met(-/-) and Met(flx/flx) cells proliferated at similar rates under 10% serum, Met-deficient cells demonstrated decreased cell viability and were more prone to apoptosis when challenged with either serum starvation or the pro-apoptotic cytokine transforming growth factor-beta. Treatment with HGF reduced transforming growth factor-beta-mediated cell death in Met(flx/flx) but not Met(-/-) cells. Importantly, Met(flx/flx) and Met(-/-) cells both constitutively expressed hgf, and conditioned medium from serum-starved oval cells exhibited anti-apoptotic activity in Met(flx/flx) cells. Furthermore, serum-starved Met(flx/flx) cells showed persistent activation of the Met tyrosine kinase, suggesting HGF/Met autocrine regulation. In conclusion, these data reveal a critical, functional role for Met in oval cell survival through an autocrine mechanism.

Adult-derived human liver mesenchymal-like cells as a potential progenitor reservoir of hepatocytes?

It is currently accepted that adult tissues may develop and maintain their own stem cell pools. Because of their higher safety profile, adult stem cells may represent an ideal candidate cell source to be used for liver cell therapies. We therefore evaluated the differentiation potential of mesenchymal-like cells isolated from adult human livers. Mesenchymal-like cells were isolated from enzymatically digested adult human liver and expanded in vitro. Cell characterization was performed using flow cytometry, RT-PCR, and immunofluorescence, whereas the differentiation potential was evaluated both in vitro after incubation with specific media and in vivo after intrasplenic transplantation of uPA(+/+)-SCID and SCID mice. Adult-derived human liver mesenchymal-like cells expressed both hepatic and mesenchymal markers among which albumin, CYP3A4, vimentin, and alpha-smooth muscle actin. In vitro differentiation studies demonstrated that these mesenchymal-like cells are preferentially determined to differentiate into hepatocyte-like cells. Ten weeks following intrasplenic transplantation into uPA(+/+)-SCID mice, recipient livers showed the presence of human hepatocytic cell nodules positive for human albumin, prealbumin, and alpha-fetoprotein. In SCID transplanted liver mice, human hepatocyte-like cells were mostly found near vascular structures 56 days posttransplantation. In conclusion, the ability of isolated adult-derived liver mesenchymal stem-like cells to proliferate and differentiate into hepatocyte-like cells both in vitro and in vivo leads to propose them as an attractive expandable cell source for stem cell therapy in human liver diseases.

Injury mechanism dictates contribution of bone marrow-derived cells to murine hepatic vascular regeneration

Stem and progenitor cells derived from adult marrow have been shown to regenerate vascular cells in response to injury. However, it is unclear whether the type of injury dictates the contribution of such cells to neovascularization and which subpopulations of cells contribute to vascular regeneration. To address these questions, we determined the extent that hematopoietic stem cells (HSC) contributed to blood vessel formation in response to two types of liver injury, partial hepatectomy (PH) and toxin-induced injury. Lac-Z-labeled HSC were engrafted into lethally irradiated, genetically matched recipients. After 14 d, we identified transplanted cells engrafted within the vascular endothelium of toxin-damaged liver, but not in the vasculature of liver regenerated in response to PH. Engraftment of HSC-derived cells occurred in a gradient fashion with the highest activity in the severely injured areas. Although HSC-derived cells contributed to both microvessels and large vessels, the large caliber vessels trended toward higher engraftment levels. Thus, the contribution of marrow-derived cells to hepatic neovascularization is dependent upon the type of injury sustained. Furthermore, following toxin-induced liver injury, engraftment rates trended higher in large vessels compared with capillaries, suggesting that remodeling of existing vessels is a predominant mechanism of repair, relative to the formation of new microvasculature.

Non invasive high resolution in vivo imaging of alpha-naphthylisothiocyanate (ANIT) induced hepatobiliary toxicity in STII medaka

A novel transparent stock of medaka (Oryzias latipes; STII), homozygous recessive for all four pigments (iridophores, xanthophores, leucophores, melanophores), permits transcutaneous, high resolution (<1 microm) imaging of internal organs and tissues in living individuals. We applied this model to in vivo investigation of alpha -naphthylisothiocyanate (ANIT) induced hepatobiliary toxicity. Distinct phenotypic responses to ANIT involving all aspects of intrahepatic biliary passageways (IHBPs), particularly bile preductular epithelial cells (BPDECs), associated with transitional passageways between canaliculi and bile ductules, were observed. Alterations included: attenuation/dilation of bile canaliculi, bile preductular lesions, hydropic vacuolation of hepatocytes and BPDECs, mild BPDEC hypertrophy, and biliary epithelial cell (BEC) hyperplasia. Ex vivo histological, immunohistochemical, and ultrastructural studies were employed to aid in interpretation of, and verify, in vivo findings. 3D reconstructions from in vivo investigations provided quantitative morphometric and volumetric evaluation of ANIT exposed and untreated livers. The findings presented show for the first time in vivo evaluation of toxicity in the STII medaka hepatobiliary system, and, in conjunction with prior in vivo work characterizing normalcy, advance our comparative understanding of this lower vertebrate hepatobiliary system and its response to toxic insult.

Wnt/beta-catenin signaling mediates oval cell response in rodents

Adult hepatic stem cells or oval cells are facultative stem cells in the liver that are activated during regeneration only during inhibition of innate hepatocyte proliferation. On the basis of its involvement in liver cancer, regeneration, and development, we investigated the role of the Wnt/beta-catenin pathway in oval cell response, which was initiated in male Fisher rats with 2-acetylaminofluorine and two-third partial hepatectomy (PHX). Extensive oval cell activation and proliferation were observed at 5 and 10 days post-PHX, as indicated by hematoxylin-eosin and proliferating cell nuclear antigen analysis. A noteworthy increase in total and active beta-catenin was observed at this time, which was localized to the oval cell cytoplasm and nuclei by immunohistochemistry and confirmed by double immunofluorescence. A concomitant increase in Wnt-1 in hepatocytes along with increased expression of Frizzled-2 in oval cells was observed. This paracrine mechanism coincided with a decrease in Wnt inhibitory factor-1 and glycogen synthase kinase-3beta down-regulation leading to beta-catenin stabilization. To strengthen its role, beta-catenin conditional knockout mice were treated with 3,5-diethoxycarbonyl-1,4-dihydrocollidine to induce oval cell activation. A dramatic decrease in the A6-positive oval cell numbers in the absence of beta-catenin demonstrated a critical role of beta-catenin in oval cell biology.

CONCLUSION

The Wnt/beta-catenin pathway plays a key role in the normal activation and proliferation of adult hepatic stem cells.

Stem cells in liver regeneration and therapy

The liver has adapted to the inflow of ingested toxins by the evolutionary development of unique regenerative properties and responds to injury or tissue loss by the rapid division of mature cells. Proliferation of the parenchymal cells, i.e. hepatocytes and epithelial cells of the bile duct, is regulated by numerous cytokine/growth-factor-mediated pathways and is synchronised with extracellular matrix degradation and restoration of the vasculature. Resident hepatic stem/progenitor cells have also been identified in small numbers in normal liver and implicated in liver tissue repair. Their putative role in the physiology, pathophysiology and therapy of the liver, however, is not yet precisely known. Hepatic stem/progenitor cells also known as "oval cells" in rodents have been implicated in liver tissue repair, at a time when the capacity for hepatocyte and bile duct replication is exhausted or experimentally inhibited (facultative stem/progenitor cell pool). Although much more has to be learned about the role of stem/progenitor cells in the physiology and pathophysiology of the liver, experimental analysis of the therapeutic value of these cells has been initiated. Transplantation of hepatic stem/progenitor cells or in vivo pharmacological activation of the pool of hepatic stem cells may provide novel modalities for the therapy of liver diseases. In addition, extrahepatic stem cells (e.g. bone marrow cells) are being investigated for their contribution to liver regeneration. Hepatic progenitor cells derived from embryonic stem cells are included in this review, which also discusses future perspectives of stem cell-based therapies for liver diseases.

Immunobiology and pathogenesis of viral hepatitis

Among the many viruses that are known to infect the human liver, hepatitis B virus (HBV) and hepatitis C virus (HCV) are unique because of their prodigious capacity to cause persistent infection, cirrhosis, and liver cancer. HBV and HCV are noncytopathic viruses and, thus, immunologically mediated events play an important role in the pathogenesis and outcome of these infections. The adaptive immune response mediates virtually all of the liver disease associated with viral hepatitis. However, it is becoming increasingly clear that antigen-nonspecific inflammatory cells exacerbate cytotoxic T lymphocyte (CTL)-induced immunopathology and that platelets enhance the accumulation of CTLs in the liver. Chronic hepatitis is characterized by an inefficient T cell response unable to completely clear HBV or HCV from the liver, which consequently sustains continuous cycles of low-level cell destruction. Over long periods of time, recurrent immune-mediated liver damage contributes to the development of cirrhosis and hepatocellular carcinoma.

Hepatic accumulation of Hedgehog-reactive progenitors increases with severity of fatty liver damage in mice

Progenitors regenerate fatty livers but the mechanisms involved are uncertain. The Hedgehog pathway regulates mesendodermal progenitors and modulates mesenchymal-epithelial interactions during tissue remodeling. To determine if Hedgehog signaling increases in liver progenitors during fatty liver injury, we compared expression of Hedgehog ligands and target genes across a spectrum of injury. Leptin-deficient ob/ob mice with fatty livers and their healthy lean littermates were studied before and after exposure to the hepatotoxin, ethionine. At baseline, ob/ob mice had greater liver damage than controls. Ethionine induced liver injury in both ob/ob and lean mice, with greater injury occurring in ob/ob mice. After ethionine, the ob/ob mice developed liver atrophy and fibrosis. Liver injury increased hepatic accumulation of progenitors, including ductular cells that produced and responded to Hedgehog ligands. A dose-response relationship was demonstrated between liver injury and expansion of Hedgehog-responsive progenitors. In severely damaged, atrophic livers, nuclei in mature-appearing hepatocytes accumulated the Hedgehog-regulated mesenchymal transcription factor, Gli2 and lost expression of the liver epithelial transcription factor, hepatocyte nuclear factor 6 (HNF-6). Hepatic levels of collagen mRNA and pericellular collagen fibrils increased concomitantly. Hence, fatty liver injury increases Hedgehog activity in liver progenitors, and this might promote epithelial-mesenchymal transitions that result in liver fibrosis.

Origin of cancer stem cells: the role of self-renewal and differentiation

BACKGROUND

Self-renewal and differentiation potential is the feature of stem cells. Differentiation is usually considered to be a one-way process of specialization as cells develop the functions of their ultimate fate and lose their immature characteristics, such as self-renewal. Progenitor cells, the products of stem cells losing the activity of self-renewal, could differentiate to mature cells, which have the feature of differentiation and lose the activity of self-renewal. The roles for cancer stem cells have been demonstrated for some cancers. However, the origin of the cancer stem cells remains elusive.

METHODS

This review focuses on current scientific controversies related to the establishment of the cancer stem cells--in particular, how self-renewal and differentiation block might contribute to the evolution of cancer.

RESULTS

Cancer stem cells may be caused by transforming mutations occurring in multi-potential stem cells, tissue-specific stem cells, progenitor cells, mature cells and cancer cells. Progenitor cells obtain the self-renewal activity by activating the self-renewal-associated genes rather than dedifferentiate to tissue special stem cells. The transform multi-potential stem cells gain the differentiation feature of special tissue by differentiating to cancer cells. Mature cells and cancer cells may dedifferentiate or reprogram to cancer stem cells by genetic and / or epigenetic events to gain the self-renewal activity and lose some features of differentiation. The cancer-derived stem cells are not the "cause", but the "consequence" of carcinogenesis. The genetic program controlling self-renewal and differentiation is a key unresolved issue.

CONCLUSION

Cancer stem cells may be caused by disturbance of self-renewal and differentiation occurring in multi-potential stem cells, tissue-specific stem cells, progenitor cells, mature cells and cancer cells.

Bile duct destruction by 4,4'-diaminodiphenylmethane does not block the small hepatocyte-like progenitor cell response in retrorsine-exposed rats

Liver regeneration after surgical partial hepatectomy (PH) in retrorsine-exposed rats is accomplished through the outgrowth and expansion of small hepatocyte-like progenitor cells (SHPCs). The cells of origin for SHPCs and their tissue niche have not been identified. Nevertheless, some investigators have suggested that SHPCs may represent an intermediate or transitional cell type between oval cells and mature hepatocytes, rather than a distinct progenitor cell population. We investigated this possibility through the targeted elimination of oval cell proliferation secondary to bile duct destruction in retrorsine-exposed rats treated with 4,4'-diaminodiphenylmethane (DAPM). Fischer 344 rats were treated with 2 doses (30 mg/kg body weight) retrorsine (at 6 and 8 weeks of age) followed by PH 5 weeks later. Twenty-four hours before PH, select animals were given a single dose of DAPM (50 mg/kg). Treatment of rats with DAPM produced severe bile duct damage but did not block liver regeneration. Oval cells were never seen in the livers of DAPM-treated retrorsine-exposed rats after PH. Rather, liver regeneration in these rats was mediated by the proliferation of SHPCs, and the cellular response was indistinguishable from that observed in retrorsine-exposed rats after PH. SHPC clusters emerge 1 to 3 days post-PH, expand through 21 days post-PH, with normalization of the liver occurring by the end of the experimental interval.

CONCLUSION

These results provide direct evidence that SHPC-mediated liver regeneration does not require oval cell activation or proliferation. In addition, these results provide strong evidence that SHPCs are not the progeny of oval cells but represent a distinct population of liver progenitor cells.

Murine gallbladder epithelial cells can differentiate into hepatocyte-like cells in vitro

We determined whether extrahepatic biliary epithelial cells can differentiate into cells with phenotypic features of hepatocytes. Gallbladders were removed from transgenic mice expressing hepatocyte-specific beta-galactosidase (beta-Gal) and cultured under standard conditions and under experimental conditions designed to induce differentiation into a hepatocyte-like phenotype. Gallbladder epithelial cells (GBEC) cultured under standard conditions exhibited no beta-Gal activity. beta-Gal expression was prominent in 50% of cells cultured under experimental conditions. Similar morphological changes were observed in GBEC from green fluorescent protein transgenic mice cultured under experimental conditions. These cells showed higher levels of mRNA for genes expressed in hepatocytes, but not in GBEC, including aldolase B, albumin, hepatocyte nuclear factor-4alpha, aldehyde dehydrogenase 1, and glutamine synthetase, and they synthesized bile acids. Additional functional evidence of a hepatocyte-like phenotype included LDL uptake and enhanced benzodiazepine metabolism. Connexin-32 expression was evident in murine hepatocytes and in cells cultured under experimental conditions, but not in cells cultured under standard conditions. Notch 1, 2, and 3 and Notch ligand Jagged 1 mRNAs were downregulated in these cells compared with cells cultured under standard conditions. CD34, alpha-fetoprotein, and Sca-1 mRNA were not expressed in cells cultured under standard conditions, suggesting that the hepatocyte-like cells did not arise from hematopoietic stem cells or oval cells. These results point to future avenues for investigation into the potential use of GBEC in the treatment of liver disease.