Purification and characterization of mouse fetal liver epithelial cells with high in vivo repopulation capacity

Transcriptional dynamics of endodermal organ formation

Although endodermal organs including the liver, pancreas, and intestine are of significant therapeutic interest, the mechanism by which the endoderm is divided into organ domains during embryogenesis is not well understood. To better understand this process, global gene expression profiling was performed on early endodermal organ domains. This global analysis was followed up by dynamic immunofluorescence analysis of key transcription factors, uncovering novel expression patterns as well as cell surface proteins that allow prospective isolation of specific endodermal organ domains. Additionally, a repressive interaction between Cdx2 and Sox2 was found to occur at the prospective stomach-intestine border, with the hepatic and pancreatic domains forming at this boundary, and Hlxb9 was revealed to have graded expression along the dorsal-ventral axis. These results contribute to understanding the mechanism of endodermal organogenesis and should assist efforts to replicate this process using pluripotent stem cells.

Stem cells in liver regeneration, fibrosis and cancer: the good, the bad and the ugly

The worldwide shortage of donor livers to transplant end stage liver disease patients has prompted the search for alternative cell therapies for intractable liver diseases, such as acute liver failure, cirrhosis and hepatocellular carcinoma (HCC). Under normal circumstances the liver undergoes a low rate of hepatocyte 'wear and tear' renewal, but can mount a brisk regenerative response to the acute loss of two-thirds or more of the parenchymal mass. A body of evidence favours placement of a stem cell niche in the periportal regions, although the identity of such stem cells in rodents and man is far from clear. In animal models of liver disease, adopting strategies to provide a selective advantage for transplanted hepatocytes has proved highly effective in repopulating recipient livers, but the poor success of today's hepatocyte transplants can be attributed to the lack of a clinically applicable procedure to force a similar repopulation of the human liver. The activation of bipotential hepatic progenitor cells (HPCs) is clearly vital for survival in many cases of acute liver failure, and the signals that promote such reactions are being elucidated. Bone marrow cells (BMCs) make, at best, a trivial contribution to hepatocyte replacement after damage, but other BMCs contribute to the hepatic collagen-producing cell population, resulting in fibrotic disease; paradoxically, BMC transplantation may help alleviate established fibrotic disease. HCC may have its origins in either hepatocytes or HPCs, and HCCs, like other solid tumours appear to be sustained by a minority population of cancer stem cells.

A novel method of mouse ex utero transplantation of hepatic progenitor cells into the fetal liver

Avoiding the limitations of the adult liver niche, transplantation of hepatic stem/progenitor cells into fetal liver is desirable to analyze immature cells in a hepatic developmental environment. Here, we established a new monitor tool for cell fate of hepatic progenitor cells transplanted into the mouse fetal liver by using ex utero surgery. When embryonic day (ED) 14.5 hepatoblasts were injected into the ED14.5 fetal liver, the transplanted cells expressed albumin abundantly or alpha-fetoprotein weakly, and contained glycogen in the neonatal liver, indicating that transplanted hepatoblasts can proliferate and differentiate in concord with surrounding recipient parenchymal cells. The transplanted cells became mature in the liver of 6-week-old mice. Furthermore, this method was applicable to transplantation of hepatoblast-like cells derived from mouse embryonic stem cells. These data indicate that this unique technique will provide a new in vivo experimental system for studying cell fate of hepatic stem/progenitor cells and liver organogenesis.

Stem cell sources for regenerative medicine

Tissue-resident stem cells or primitive progenitors play an integral role in homeostasis of most organ systems. Recent developments in methodologies to isolate and culture embryonic and somatic stem cells have many new applications poised for clinical and preclinical trials, which will enable the potential of regenerative medicine to be realized. Here, we overview the current progress in therapeutic applications of various stem cells and discuss technical and social hurdles that must be overcome for their potential to be realized.

Murine embryonic stem cell-derived hepatic progenitor cells engraft in recipient livers with limited capacity of liver tissue formation

Directed endodermal differentiation of murine embryonic stem (ES) cells gives rise to a subset of cells with a hepatic phenotype. Such ES cell-derived hepatic progenitor cells (ES-HPC) can acquire features of hepatocytes in vitro, but fail to form substantial hepatocyte clusters in vivo. In this study, we investigated whether this is due to inefficient engraftment or an immature phenotype of ES-HPC. ES cells engrafted into recipient livers of NOD/SCID mice with a similar efficacy as adult hepatocytes after 28 days. Because transplanted unpurified ES-HPC formed teratomas in the spleen and liver, we applied an albumin promoter/enhancer-driven reporter system to purify ES-HPC by cell sorting. RT-PCR analyses for hepatocyte-specific genes showed that the cells exhibited a hepatic phenotype, lacking the expression of the pluripotency marker Oct4, comparable to cells of day 11.5 embryos. Sorted ES-HPC derived from beta-galactosidase transgenic ES cells were injected into fumaryl-acetoacetate-deficient (FAH(-/-)) SCID mice and analyzed after 8 to 12 weeks. Staining with X-gal solution revealed the presence of engrafted cells throughout the liver. However, immunostaining for the FAH protein indicated hepatocyte formation at a very low frequency, without evidence for large hepatocyte cluster formation. In conclusion, the limited repopulation capacity of ES-HPC is not caused by a failure of primary engraftment, but may be due to an immature hepatic phenotype of the transplanted ES-HPC.

Reprogramming of pancreatic beta cells into induced pluripotent stem cells

Induced pluripotent stem (iPS) cells have been derived from fibroblast, stomach, and liver cultures at extremely low frequencies by ectopic expression of the transcription factors Oct4, Sox2, c-myc, and Klf4, a process coined direct or in vitro reprogramming [1-8]. iPS cells are molecularly and functionally highly similar to embryonic stem cells (ESCs), including their ability to contribute to all tissues as well as the germline in mice. The heterogeneity of the starting cell populations and the low efficiency of reprogramming suggested that a rare cell type, such as an adult stem cell, might be the cell of origin for iPS cells and that differentiated cells are refractory to reprogramming. Here, we used inducible lentiviruses [9] to express Oct4, Sox2, c-myc, and Klf4 in pancreatic beta cells to assess whether a defined terminally differentiated cell type remains amenable to reprogramming. Genetically marked beta cells gave rise to iPS cells that expressed pluripotency markers, formed teratomas, and contributed to cell types of all germ layers in chimeric animals. Our results provide genetic proof that terminally differentiated cells can be reprogrammed into pluripotent cells, suggesting that in vitro reprogramming is not restricted to certain cell types or differentiation stages.

Promoter-defined isolation and identification of hepatic progenitor cells from the human fetal liver

Hepatoblasts, which are considered one type of hepatic progenitor cell, reside in the fetal liver. To selectively identify these cells, we transfected primary cultured human fetal liver cells (FLCs) with a pGL3 vector bearing the gene for the enhanced green fluorescence protein (EGFP) under the control of the alpha-fetoprotein (AFP) promoter expressed in hepatoblasts. The FLCs were then sorted by fluorescence-activated cell sorting (FACS) on the basis of AFP promoter-driven EGFP expression. The EGFP-positive cells expressed AFP, albumin, and cytokeratin 19, and could be expanded in vitro. Thus, the AFP promoter-EGFP reporter system is highly useful for identification and isolation of hepatic progenitor cells.

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.

Purification of fetal liver stem/progenitor cells containing all the repopulation potential for normal adult rat liver

BACKGROUND & AIMS

Previously, we showed high-level, long-term liver replacement after transplantation of unfractionated embryonic day (ED) 14 fetal liver stem/progenitor cells (FLSPC). However, for clinical applications, it will be essential to transplant highly enriched cells, while maintaining high repopulation potential.

METHODS

Dlk-1, a member of the delta-like family of cell surface transmembrane proteins, is highly expressed in human and rodent fetal liver. Dlk-1(+) cells, isolated from ED14 fetal liver using immunomagnetic beads, were examined for their hepatic gene expression profile and characteristic properties in vitro and their proliferative and differentiation potential in vivo after transplantation into normal adult rat liver.

RESULTS

Rat ED14 FLSPC were purified to 95% homogeneity and exhibited cell culture and gene expression characteristics expected for hepatic stem/progenitor cells. Rat ED14 FLSPC are alpha-fetoprotein(+)/cytokeratin-19(+) or alpha-fetoprotein(+)/cytokeratin-19(-) and contain all of the normal liver repopulation capacity found in fetal liver. Hematopoietic stem cells, a major component in crude fetal liver cell preparations that engraft in other organs, such as bone marrow, spleen, and lung, are totally removed by Dlk-1 selection, and Dlk-1 purified FLSPC repopulate only the liver.

CONCLUSIONS

This is the first study reporting purification of hepatic stem/progenitor cells from fetal liver that are fully capable of repopulating the normal adult liver. This represents a major advance toward developing protocols that will be essential for clinical application of liver cell transplantation therapy.

New primary culture systems to study the differentiation and proliferation of mouse fetal hepatoblasts

Hepatoblasts have the potential to differentiate into both hepatocytes and biliary epithelial cells through a differentiation program that has not been fully elucidated. With the aim to better define the mechanism of differentiation of hepatoblasts, we isolated hepatoblasts and established new culture systems. We isolated hepatoblasts from E12.5 fetal mouse liver by using E-cadherin. The E-cadherin+ cells expressed alpha-fetoprotein (AFP) and albumin (Alb) but not cytokeratin 19 (CK19). Transplantation of the E-cadherin+ cells into mice that had been subjected to liver injury or biliary epithelial injury led to differentiation of the cells into hepatocytes or biliary epithelial cells, respectively. In a low-cell-density culture system in the absence of additional growth factors, E-cadherin+ cells formed colonies of various sizes, largely comprising Alb-positive cells. Supplementation of the culture medium with hepatocyte growth factor and epidermal growth factor promoted proliferation of the cells. Thus the low-cell-density culture system should be useful to identify inductive factors that regulate the proliferation and differentiation of hepatoblasts. In a high-cell-density system in the presence of oncostatin M+dexamethasone, E14.5, but not E12.5, E-cadherin+ cells differentiated into mature hepatocytes, suggesting that unidentified factors are involved in hepatic maturation. Culture of E-cadherin+ cells derived from E12.5 or E14.5 liver under high-cell-density conditions should allow elucidation of the mechanism of hepatic differentiation in greater detail. These new culture systems should be of use to identify growth factors that induce hepatoblasts to proliferate or differentiate into hepatocytes and biliary epithelial cells.

Activation of stem cells in hepatic diseases

The liver has enormous regenerative capacity. Following acute liver injury, hepatocyte division regenerates the parenchyma but, if this capacity is overwhelmed during massive or chronic liver injury, the intrinsic hepatic progenitor cells (HPCs) termed oval cells are activated. These HPCs are bipotential and can regenerate both biliary epithelia and hepatocytes. Multiple signalling pathways contribute to the complex mechanism controlling the behaviour of the HPCs. These signals are delivered primarily by the surrounding microenvironment. During liver disease, stem cells extrinsic to the liver are activated and bone-marrow-derived cells play a role in the generation of fibrosis during liver injury and its resolution. Here, we review our current understanding of the role of stem cells during liver disease and their mechanisms of activation.

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.

Stem cells, cell transplantation and liver repopulation

Liver transplantation is currently the only therapeutic option for patients with end-stage chronic liver disease and for severe acute liver failure. Because of limited donor availability, attention has been focused on the possibility to restore liver mass and function through cell transplantation. Stem cells are a promising source for liver repopulation after cell transplantation, but whether or not the adult mammalian liver contains hepatic stem cells is highly controversial. Part of the problem is that proliferation of mature adult hepatocytes is sufficient to regenerate the liver after two-thirds partial hepatectomy or acute toxic liver injury and participation of stem cells is not required. However, under conditions in which hepatocyte proliferation is blocked, undifferentiated epithelial cells in the periportal areas, called "oval cells", proliferate, differentiate into hepatocytes and restore liver mass. These cells are referred to as facultative liver stem cells, but they do not repopulate the normal liver after their transplantation. In contrast, epithelial cells isolated from the early fetal liver can effectively repopulate the normal liver, but they are already traversing the hepatic lineage and may not be true stem cells. Mesenchymal stem cells and embryonic stem cells can be induced to differentiate along the hepatic lineage in culture, but at present these cells are inefficient in repopulating the liver. This review will characterize these various cell types and compare the properties of these cells and the conditions under which they do or do not repopulate the liver following their transplantation.

Enrichment of a bipotent hepatic progenitor cell from naïve adult liver tissue

BACKGROUND/AIM

Recent interest in the liver stem cell field has led to the identification and characterization of several hepatic progenitor cell populations from fetal and adult tissues. We isolated a hepatic progenitor cell from naïve adult liver and the current studies focus on differentiation and growth.

RESULTS

A Sca-1(+) hepatic progenitor cell was identified within the liver parenchyma. This cell expresses numerous liver related genes and transcription found in the developing and/or adult liver. It is located in the peri-portal region and expresses markers associated with undifferentiated hepatic cell populations, mature hepatocytes and biliary cells which distinguish it from the Sca-1(-) fraction.

CONCLUSION

This hepatic progenitor cell from uninjured liver has features of both hepatocytic and biliary populations and demonstrates proliferative potential. Further studies will focus on sca-HPC subsets and conditions that regulate differentiation towards hepatic or biliary lineages.

Application of liver stem cells for cell therapy

The worldwide shortage of donor livers to transplant end stage liver disease patients has prompted the search for alternative cell therapies for intractable liver disease. Embryonic stem cells can be readily differentiated into hepatocytes, and their transplantation into animals has improved liver function in the absence of teratoma formation: their use in bioartificial liver support is an obvious application. In animal models of liver disease, adopting strategies to provide a selective advantage for transplanted foetal or adult hepatocytes have proved highly effective in repopulating recipient livers, but the poor success of today's hepatocyte transplants can be attributed to the lack of a clinically applicable procedure to force a similar repopulation of the human liver. The activation of bipotential hepatic progenitor cells is clearly vital for survival in many cases of acute liver failure, but surprisingly little progress has been made with these cells in terms of transplantation. Finally there is the controversial subject of autologous bone marrow, and while the contribution of these indigenous cells to liver turnover seems at best, trivial, results from a small number of phase 1 studies of transplantation of bone marrow to cirrhotic patients have been moderately encouraging.

New cell surface markers for murine fetal hepatic stem cells identified through high density complementary DNA microarrays

Isolation of hepatic stem cells from the adult liver (AL) has not yet been achieved due to the lack of specific cell surface markers. To identify new surface markers for hepatic stem cells, we analyzed differences in the gene expression profile of embryonic day (ED) 13.5 fetal liver stem/progenitor cells (FLSPC) versus AL by complementary DNA (cDNA) microarray technology. Using FLSPC purified to >90% by immunomagnetic selection for E-cadherin and high density (27k) mouse cDNA microarrays, we identified 474 genes that are more strongly expressed in FLSPC (FLSPC-up genes) and 818 genes that are more strongly expressed in AL (AL-up genes). The most highly overrepresented gene ontology (GO) categories for FLSPC-up genes are nucleus, cellular proliferation, and cell cycle control. AL-up genes are overrepresented for genes in metabolic pathways for specific hepatic functions. We identified 24 FLSPC-up gene surface markers and 69 AL-up gene surface markers. Western blot studies confirmed the expression of the FLSPC-up gene neighbor of Punc E11 (Nope) in fetal liver, but expression was not detectable in AL. Immunohistochemistry, confocal microscopy, and fluorescence-activated cell sorting (FACS) analysis of fetal liver demonstrated that Nope is specifically expressed on the surface of FLSPC within the fetal liver.

CONCLUSION

This is the first microarray study to analyze the specific gene expression profile of purified murine FLSPC. Our analysis identified 24 new/potential cell surface markers for murine fetal hepatic stem cells, of which Nope may be particularly useful in future studies to identify, characterize and isolate hepatic stem cells from the AL.

Fetal and adult liver stem cells for liver regeneration and tissue engineering

For the development of innovative cell-based liver directed therapies, e.g. liver tissue engineering, the use of stem cells might be very attractive to overcome the limitation of donor liver tissue. Liver specific differentiation of embryonic, fetal or adult stem cells is currently under investigation. Different types of fetal liver (stem) cells during development were identified, and their advantageous growth potential and bipotential differentiation capacity were shown. However, ethical and legal issues have to be addressed before using fetal cells. Use of adult stem cells is clinically established, e.g. transplantation of hematopoietic stem cells. Other bone marrow derived liver stem cells might be mesenchymal stem cells (MSC). However, the transdifferentiation potential is still in question due to the observation of cellular fusion in several in vivo experiments. In vitro experiments revealed a crucial role of the environment (e.g. growth factors and extracellular matrix) for specific differentiation of stem cells. Co-cultured liver cells also seemed to be important for hepatic gene expression of MSC. For successful liver cell transplantation, a novel approach of tissue engineering by orthotopic transplantation of gel-immobilized cells could be promising, providing optimal environment for the injected cells. Moreover, an orthotopic tissue engineering approach using bipotential stem cells could lead to a repopulation of the recipients liver with healthy liver and biliary cells, thus providing both hepatic functions and biliary excretion. Future studies have to investigate, which stem cell and environmental conditions would be most suitable for the use of stem cells for liver regeneration or tissue engineering approaches.

Molecular Mechanism of Liver Development and Regeneration

The liver is the central organ for metabolism and has strong regenerative capability. Although the liver has been studied mostly biochemically and histopathologically, genetic studies using gene-targeting technology have identified a number of cytokines, intracellular signaling molecules, and transcription factors involved in liver development and regeneration. In addition, various in vitro systems such as fetal liver explant culture and primary culture of fetal liver cells have been established, and the combination of genetic and in vitro studies has accelerated investigation of liver development. Identification of the cell-surface molecules of liver progenitors has made it possible to identify and isolate liver progenitors, making the liver a unique model for stem cell biology. In this review, we summarize progresses in understanding liver development and regeneration.

BMP-4 is required for hepatic specification of mouse embryonic stem cell-derived definitive endoderm

When differentiated in the presence of activin A in serum-free conditions, mouse embryonic stem cells efficiently generate an endoderm progenitor population defined by the coexpression of either Brachyury, Foxa2 and c-Kit, or c-Kit and Cxcr4. Specification of these progenitors with bone morphogenetic protein-4 in combination with basic fibroblast growth factor and activin A results in the development of hepatic populations highly enriched (45-70%) for cells that express the alpha-fetoprotein and albumin proteins. These cells also express transcripts of Afp, Alb1, Tat, Cps1, Cyp7a1 and Cyp3a11; they secrete albumin, store glycogen, show ultrastructural characteristics of mature hepatocytes, and are able to integrate into and proliferate in injured livers in vivo and mature into hepatocytes expressing dipeptidyl peptidase IV or fumarylacetoacetate hydrolase. Together, these findings establish a developmental pathway in embryonic stem cell differentiation cultures that leads to efficient generation of cells with an immature hepatocytic phenotype.

Cholangiocyte marker-positive and -negative fetal liver cells differ significantly in their ability to regenerate the livers of adult rats exposed to retrorsine

We have used monoclonal antibodies against cell-surface developmental epitopes in combination with micromagnetic beads to isolate phenotypically defined subpopulations of cholangiocyte marker-positive fetal liver epithelial cells (CMP-FLEC). Differentiation potential was evaluated by injecting cell isolates from dipeptidyl peptidase IV (DPPIV) positive (DPPIV+) Fischer donor rats into the spleen of partially hepatectomized, DPPIV negative (DPPIV-)Fischer host rats exposed to retrorsine. At various time points, liver tissue was harvested and cells in DPPIV+ colonies were phenotyped by immunofluorescence and histochemical protocols. Functional differentiation and liver replacement were determined by comparing donor and host hepatocyte protein expression patterns and DPPIV enzyme activity in extracts from livers of host rats receiving CMP-FLEC. Our results showed that bipotentiality was retained during differentiation and maturation of CMP-FLEC, indicating that the acquisition of ductal morphology and phenotype were not indicative of lineage commitment. CMP-FLEC transplanted into the adult rat liver lost ductal and gained hepatocyte markers, and acquired protein expression patterns in 2D gels with a close similarity (>75% spot match) to host hepatocytes but differing significantly from the transplanted CMP-FLEC cell isolate (<25%spot match). The average size of donor hepatocyte colonies increased with time so that by 1 year, up to 70% of the host rat liver was replaced by CMP-FLEC derived DPPIV+ hepatocytes. Depletion of CMP-FLEC from fetal liver isolates resulted in a marked decrease in adult liver colonization, suggesting that a high percentage of the hepatocyte colonies in animals receiving total fetal liver isolates are derived from CMP-FLEC.

Hepatic Stem Cells: In Search of

The field of stem cell biology has exploded with the study of a wide range of cellular populations involving endodermal, mesenchymal, and ectodermal organs. One area of extensive study has included the identification of hepatic stem and progenitor cell subpopulations. Liver stem cells provide insights into the potential pathways involving liver regeneration that are independent of mature hepatocytes. Hepatic progenitor cells are either bipotent or multipotent and capable of multiple rounds of replication. They have been identified in fetal as well as adult liver. Various injury models have been used to expand this cellular compartment. The nomenclature, origin, and function of the hepatic progenitor cell populations are areas of ongoing debate. In this review, we will discuss the different definitions and functions of hepatic progenitor cells as well as the current research efforts examining their therapeutic potential.

Liver stem cells and prospects for liver reconstitution by transplanted cells

Although it was proposed almost 60 years ago that the adult mammalian liver contains hepatic stem cells, this issue remains controversial. Part of the problem is that no specific marker gene unique to the adult hepatic stem cell has yet been identified, and regeneration of the liver after acute injury is achieved through proliferation of adult hepatocytes and does not require activation or proliferation of stem cells. Also, there are differences in the expected properties of stem versus progenitor cells, and we attempt to use specific criteria to distinguish between these cell types. We review the evidence for each of these cell types in the adult versus embryonic/fetal liver, where tissue-specific stem cells are known to exist and to be involved in organ development. This review is limited to studies directed toward identification of hepatic epithelial stem cells and does not address the controversial issue of whether stem cells derived from the bone marrow have hepatocytic potential, a topic that has been covered extensively in other recent reviews.

Isolation and characterization of bipotent liver progenitor cells from adult mouse

Liver progenitor cells have drawn a great deal of attention both for their therapeutic potential and for their usefulness in exploring the molecular events surrounding liver development and regeneration. Despite the intensive studies on liver progenitors from rats, equivalent progenitor cells derived from mice are relatively rare. We used retrosine treatment followed by partial hepatectomy to elicit liver progenitors in mice. From these animals showing prominent ductular reactions, mouse-derived liver progenitor cell lines (LEPCs) were isolated by single-cell cloning. Phenotypic and lineage profiling of the LEPC clones were performed using immunochemistry, reverse transcription-polymerase chain reaction, and a dual-color system comprising the reporter EGFP under the control of the cytokeratin 19 promoter and the DsRed reporter under the control of the albumin promoter. LEPCs expressed liver progenitor cell markers. LEPCs also expressed some markers shared by bone marrow-derived hematopoietic stem cells c-Kit and Thy-1 but not CD34 and CD45. When cultured as aggregates in Matrigel, LEPCs differentiated into hepatocyte upon treatment with 50 ng/ml epithelial growth factor or differentiated into biliary lineage cells upon treatment with 20 ng/ml hepatocyte growth factor. In the presence of 2% dimethyl sulfoxide and 2% Matrigel, LEPCs acquired predominantly bile lineage phenotypes, with occasional patches of cells exhibiting hepatocyte phenotypes. Upon transplantation into CCl4-injured-liver, LEPCs engrafted into liver parenchyma and differentiated into hepatocytes. Considering the amenability of the mouse to genetic manipulation, these mouse-derived LEPCs may be useful tools as in vitro models to study molecular events in liver development and regeneration and can shed light in studying the therapy potential of liver stem cells.