Clonal identification and characterization of self-renewing pluripotent stem cells in the developing liver

Hepatocytes differentiate into intestinal epithelial cells through a hybrid epithelial/mesenchymal cell state in culture

Hepatocytes play important roles in the liver, but in culture, they immediately lose function and dedifferentiate into progenitor-like cells. Although this unique feature is well-known, the dynamics and mechanisms of hepatocyte dedifferentiation and the differentiation potential of dedifferentiated hepatocytes (dediHeps) require further investigation. Here, we employ a culture system specifically established for hepatic progenitor cells to study hepatocyte dedifferentiation. We found that hepatocytes dedifferentiate with a hybrid epithelial/mesenchymal phenotype, which is required for the induction and maintenance of dediHeps, and exhibit Vimentin-dependent propagation, upon inhibition of the Hippo signaling pathway. The dediHeps re-differentiate into mature hepatocytes by forming aggregates, enabling reconstitution of hepatic tissues in vivo. Moreover, dediHeps have an unexpected differentiation potential into intestinal epithelial cells that can form organoids in three-dimensional culture and reconstitute colonic epithelia after transplantation. This remarkable plasticity will be useful in the study and treatment of intestinal metaplasia and related diseases in the liver.

Stem Cell-Based Strategies: The Future Direction of Bioartificial Liver Development

Acute liver failure (ALF) results from severe liver damage or end-stage liver disease. It is extremely fatal and causes serious health and economic burdens worldwide. Once ALF occurs, liver transplantation (LT) is the only definitive and recommended treatment; however, LT is limited by the scarcity of liver grafts. Consequently, the clinical use of bioartificial liver (BAL) has been proposed as a treatment strategy for ALF. Human primary hepatocytes are an ideal cell source for these methods. However, their high demand and superior viability prevent their widespread use. Hence, finding alternatives that meet the seed cell quality and quantity requirements is imperative. Stem cells with self-renewing, immunogenic, and differentiative capacities are potential cell sources. MSCs and its secretomes encompass a spectrum of beneficial properties, such as anti-inflammatory, immunomodulatory, anti-ROS (reactive oxygen species), anti-apoptotic, pro-metabolomic, anti-fibrogenesis, and pro-regenerative attributes. This review focused on the recent status and future directions of stem cell-based strategies in BAL for ALF. Additionally, we discussed the opportunities and challenges associated with promoting such strategies for clinical applications.

"Small Hepatocytes" in the Liver

Mature hepatocytes (MHs) in an adult rodent liver are categorized into the following three subpopulations based on their proliferative capability: type I cells (MH-I), which are committed progenitor cells that possess a high growth capability and basal hepatocytic functions; type II cells (MH-II), which possess a limited proliferative capability; and type III cells (MH-III), which lose the ability to divide (replicative senescence) and reach the final differentiated state. These subpopulations may explain the liver's development and growth after birth. Generally, small-sized hepatocytes emerge in mammal livers. The cells are characterized by being morphologically identical to hepatocytes except for their size, which is substantially smaller than that of ordinary MHs. We initially discovered small hepatocytes (SHs) in the primary culture of rat hepatocytes. We believe that SHs are derived from MH-I and play a role as hepatocytic progenitors to supply MHs. The population of MH-I (SHs) is distributed in the whole lobules, a part of which possesses a self-renewal capability, and decreases with age. Conversely, injured livers of experimental models and clinical cases showed the emergence of SHs. Studies demonstrate the involvement of SHs in liver regeneration. SHs that appeared in the injured livers are not a pure population but a mixture of two distinct origins, MH-derived and hepatic-stem-cell-derived cells. The predominant cell-derived SHs depend on the proliferative capability of the remaining MHs after the injury. This review will focus on the SHs that appeared in the liver and discuss the significance of SHs in liver regeneration.

Clonal selection parallels between normal and cancer tissues

Clonal selection and drift drive both normal tissue and cancer development. However, the biological mechanisms and environmental conditions underpinning these processes remain to be elucidated. Clonal selection models are centered in Darwinian evolutionary theory, where some clones with the fittest features are selected and populate the tissue or tumor. We suggest that different subclasses of stem cells, each of which is responsible for a distinct feature of the selection process, share common features between normal and cancer conditions. While active stem cells populate the tissue, dormant cells account for tissue replenishment/regeneration in both normal and cancerous tissues. We also discuss potential mechanisms that drive clonal drift, their interactions with clonal selection, and their similarities during normal and cancer tissue development.

Therapeutic Cell Repopulation of the Liver: From Fetal Rat Cells to Synthetic Human Tissues

Progenitor cells isolated from the fetal liver can provide a unique cell source to generate new healthy tissue mass. Almost 20 years ago, it was demonstrated that rat fetal liver cells repopulate the normal host liver environment via a mechanism akin to cell competition. Activin A, which is produced by hepatocytes, was identified as an important player during cell competition. Because of reduced activin receptor expression, highly proliferative fetal liver stem/progenitor cells are resistant to activin A and therefore exhibit a growth advantage compared to hepatocytes. As a result, transplanted fetal liver cells are capable of repopulating normal livers. Important for cell-based therapies, hepatic stem/progenitor cells containing repopulation potential can be separated from fetal hematopoietic cells using the cell surface marker δ-like 1 (Dlk-1). In livers with advanced fibrosis, fetal epithelial stem/progenitor cells differentiate into functional hepatic cells and out-compete injured endogenous hepatocytes, which cause anti-fibrotic effects. Although fetal liver cells efficiently repopulate the liver, they will likely not be used for human cell transplantation. Thus, utilizing the underlying mechanism of repopulation and developed methods to produce similar growth-advantaged cells in vitro, e.g., human induced pluripotent stem cells (iPSCs), this approach has great potential for developing novel cell-based therapies in patients with liver disease. The present review gives a brief overview of the classic cell transplantation models and various cell sources studied as donor cell candidates. The advantages of fetal liver-derived stem/progenitor cells are discussed, as well as the mechanism of liver repopulation. Moreover, this article reviews the potential of in vitro developed synthetic human fetal livers from iPSCs and their therapeutic benefits.

A novel agonist for the HGF receptor MET promotes differentiation of human pluripotent stem cells into hepatocyte-like cells

Hepatocyte growth factor (HGF) is the natural ligand of the MET receptor tyrosine kinase. This ligand-receptor couple is essential for the maturation process of hepatocytes. Previously, the rational design of a synthetic protein based on the assembly of two K1 domains from HGF led to the production of a potent and stable MET receptor agonist. In this study, we compared the effects of K1K1 with HGF during the differentiation of hepatocyte progenitors derived from human induced pluripotent stem cells (hiPSCs). In vitro, K1K1, in the range of 20 to 200 nM, successfully substituted for HGF and efficiently activated ERK downstream signaling. Analysis of the levels of hepatocyte markers showed typical liver mRNA and protein expression (HNF4α, albumin, alpha-fetoprotein, CYP3A4) and phenotypes. Although full maturation was not achieved, the results suggest that K1K1 is an attractive candidate MET agonist suitable for replacing complex and expensive HGF treatments to induce hepatic differentiation of hiPSCs.

Functional characterization of CD49f+ hepatic stem/progenitor cells in adult mice liver

Hepatic Stem/progenitor cells (HSPCs) have gained a large amount of interest for treating acute liver disease. However, the isolation and identification of HSPCs are unclear due to the lack of cell-specific surface markers. To isolate adult HSPCs, we used cell surface-marking antibodies, including CD49f and Sca-1. Two subsets of putative HSPCs, Lin^-CD45^-Sca-1^-CD49f⁺ (CD49f⁺) and Lin^-CD45^-Sca-1⁺CD49f^- (Sca-1⁺) cells, were isolated from adult mice liver by flow cytometry. Robust proliferative activity and clonogenic activity were found in both CD49f⁺ and Sca-1⁺ cells through colony-forming tests and cell cycle analyses. Immunofluorescence staining revealed that CD49f⁺ cells expressed ALB and CK-19 while Sca-1⁺ cells expressed only ALB, indicating that CD49f⁺ cells were bipotential and capable of differentiating into hepatocyte and cholangiocyte. Consequently, PAS stain showed that differentiated CD49f⁺ and Sca-1⁺ cells synthesised glycogen, indicating they could differentiate into functional hepatocytes. mRNA expression profile indicated that both CD49f⁺ and Sca-1⁺ cells showed differential expression of genes that are associated with liver progenitor function such as Sox9 and EpCam. Moreover, two subsets of putative HSPCs were activated by DDC and we found that their abundance and proliferation increased with age. In summary, we hypothesized that CD49f⁺ cells were a type of potential HSPCs and may be utilised for clinical stem cell therapy.

Hepatobiliary Differentiation: Principles from Embryonic Liver Development

Hepatocytes and biliary epithelial cells (BECs), the two endodermal cell types of the liver, originate from progenitor cells called hepatoblasts. Based principally on in vitro data, hepatoblasts are thought to be bipotent stem cells with the potential to produce both hepatocytes and BECs. However, robust in vivo evidence for this model has only recently emerged. We examine the molecular mechanisms that stimulate hepatoblast differentiation into hepatocytes or BECs. In the absence of extrinsic cues, the default fate of hepatoblasts is hepatocyte differentiation. Inductive cues from the hepatic portal vein, however, initiate transcription factor expression in hepatoblasts, driving biliary specification. Defining the mechanisms of hepatobiliary differentiation provides important insights into congenital disorders, such as Alagille syndrome, and may help to better characterize the poorly understood hepatic lineage relationships observed during regeneration from liver injury.

Signaling molecules orchestrating liver regenerative medicine

The liver is in charge of more than 500 functions in the human body, which any damage and failure to the liver can significantly compromise human life. Numerous studies are being carried out in regenerative medicine, as a potential driving force, toward alleviating the need for liver donors and fabrication of a 3D-engineered transplantable hepatic tissue. Liver tissue engineering brings three main factors of cells, extracellular matrix (ECM), and signaling molecules together, while each of these three factors tries to mimic the physiological state of the tissue to direct tissue regeneration. Signaling molecules play a crucial role in directing tissue fabrication in liver tissue engineering. When mimicking the natural in vivo process of regeneration, it is tightly associated with three main phases of differentiation, proliferation (progression), and tissue maturation through vascularization while directing each of these phases is highly regulated by the specific signaling molecules. The understanding of how these signaling molecules guide the dynamic behavior of regeneration would be a tool for further tailoring of bioengineered systems to help the liver regeneration with many cellular, molecular, and tissue-level functions. Hence, the signaling molecules come to aid all these phases for further improvements toward the clinical use of liver tissue engineering as the goal.

Hepatic stellate cells contribute to liver regeneration through galectins in hepatic stem cell niche

BACKGROUND

As a critical cellular component in the hepatic stem cell niche, hepatic stellate cells (HSCs) play critical roles in regulating the expansion of hepatic stem cells, liver regeneration, and fibrogenesis. However, the signaling of HSCs, particularly that involved in promoting hepatic stem cell expansion, remains unclear. While the overexpression of galectins has been identified in regenerating liver tissues, their involvement in cell-cell interactions between HSCs and hepatic stem cells remains to be elucidated.

METHODS

To generate a liver regeneration rat model and establish a hepatic oval cell microenvironment as a stem cell niche, 2-acetylaminofluorene treatment plus partial hepatectomy was performed. Immunofluorescence staining was conducted to detect the emergence of hepatic stem cells and their niche. Liver parenchymal cells, non-parenchymal cells, and HSCs were isolated for gene and protein expression analysis by qPCR or western blotting. To evaluate the effect of galectins on the colony-forming efficiency of hepatic stem cells, c-Kit^-CD29⁺CD49f^+/lowCD45^-Ter-119^- cells were cultured with recombinant galectin protein, galectin antibody, galectin-producing HSCs, and galectin-knockdown HSCs.

RESULTS

Following liver injury, the cytokeratin 19⁺ ductal cells were robustly induced together with the emergence of OV6⁺CD44⁺CD133⁺EpCAM⁺ hepatic stem cells. The activated desmin⁺ HSCs were recruited around the periportal area and markedly enriched in the galectin-positive domain compared to the other non-parenchymal cells. Notably, the HSC fraction isolated from regenerating liver was accompanied by dramatically elevated gene and protein expression of galectins. Hepatic stem cells co-cultured with HSCs significantly enhanced colony-forming efficiency. Conversely, single or double knockdown of galectin-1 and galectin-3 led into a significant function loss, impaired the co-cultured hepatic stem cells to attenuated colony size, inhibited colony frequency, and reduced total cell numbers in colonies. On the other hand, the promotive function of galectins was further confirmed by recombinant galectin protein supplementation and galectins blocking antibodies.

CONCLUSIONS

Our findings, for the first time, demonstrated that galectins from activated HSCs contribute to hepatic stem cell expansion during liver regeneration, suggesting that galectins serve as important stem cell niche components.

Comparative analysis of cell lineage differentiation during hepatogenesis in humans and mice at the single-cell transcriptome level

During embryogenesis, the liver is the site of hepatogenesis and hematopoiesis and contains many cell lineages derived from the endoderm and mesoderm. However, the characteristics and developmental programs of many of these cell lineages remain unclear, especially in humans. Here, we performed single-cell RNA sequencing of whole human and mouse fetal livers throughout development. We identified four cell lineage families of endoderm-derived, erythroid, non-erythroid hematopoietic, and mesoderm-derived non-hematopoietic cells, and defined the developmental pathways of the major cell lineage families. In both humans and mice, we identified novel markers of hepatic lineages and an ID3⁺ subpopulation of hepatoblasts as well as verified that hepatoblast differentiation follows the “default-directed” model. Additionally, we found that human but not mouse fetal hepatocytes display heterogeneity associated with expression of metabolism-related genes. We described the developmental process of erythroid progenitor cells during human and mouse hematopoiesis. Moreover, despite the general conservation of cell differentiation programs between species, we observed different cell lineage compositions during hematopoiesis in the human and mouse fetal livers. Taken together, these results reveal the dynamic cell landscape of fetal liver development and illustrate the similarities and differences in liver development between species, providing an extensive resource for inducing various liver cell lineages in vitro.

Liver Progenitors and Adult Cell Plasticity in Hepatic Injury and Repair: Knowns and Unknowns

The liver is a complex organ performing numerous vital physiological functions. For that reason, it possesses immense regenerative potential. The capacity for repair is largely attributable to the ability of its differentiated epithelial cells, hepatocytes and biliary epithelial cells, to proliferate after injury. However, in cases of extreme acute injury or prolonged chronic insult, the liver may fail to regenerate or do so suboptimally. This often results in life-threatening end-stage liver disease for which liver transplantation is the only effective treatment. In many forms of liver injury, bipotent liver progenitor cells are theorized to be activated as an additional tier of liver repair. However, the existence, origin, fate, activation, and contribution to regeneration of liver progenitor cells is hotly debated, especially since hepatocytes and biliary epithelial cells themselves may serve as facultative stem cells for one another during severe liver injury. Here, we discuss the evidence both supporting and refuting the existence of liver progenitor cells in a variety of experimental models. We also debate the validity of developing therapies harnessing the capabilities of these cells as potential treatments for patients with severe and chronic liver diseases.

Liver Tissue Engineering as an Emerging Alternative for Liver Disease Treatment

Chronic liver diseases affect thousands of lives throughout the world every year. The shortage of liver donors for transplantation has been the main driving force to employ alternative methods such as liver tissue engineering (LTE) in fabricating a three-dimensional transplantable liver tissue or enhancing cell delivery techniques alleviating the need for liver donors. LTE consists of three components, cells, ECM (extracellular matrix), and signaling molecules, which we discuss the first and second. The three most common cell sources used in LTE are human and animal primary hepatocytes, and stem cells for different applications. Two major categories of ECM are used to mimic the microenvironment of these cells, named scaffolds and microbeads. Scaffolds have been made by numerous methods with a wide range of synthetic and natural biomaterials. Cell encapsulation has also been utilized by many polymeric biomaterials. To investigate their functions, many properties have been discussed in the literature, such as biochemical, geometrical, and mechanical properties, in both of these categories. Overall, LTE shows excellent potential in assisting hepatic disorders. However, some challenges exist that prevent the practical use of it clinically, making LTE an ongoing research subject in the scientific society.

Self-Renewal Capability of Hepatocytic Parental Progenitor Cells Derived From Adult Rat Liver Is Maintained Long Term When Cultured on Laminin 111 in Serum-Free Medium

In this study, we investigated how the ability of hepatocytic parental progenitor cells (HPPCs) to self‐renew can be maintained and how laminin (LN) isoforms play an important role in their self‐renewal and maturation. Hepatocytes isolated from adult rat livers were cultured on hyaluronic acid to form colonies consisting of CD44⁺ small hepatocytes, which could be passaged on dishes coated with Matrigel. When second‐passage cells were plated on Matrigel, LN111, or LN511, HPPCs appeared on Matrigel and LN111 but not on LN511. We identified two types of cells among the second‐passage cells: Small, round cells and large, flat ones were observed on Matrigel, whereas the former and latter ones were specifically attached on LN111 and LN511, respectively. We hypothesized that small and round cells are the origin of HPPC colonies, and the binding to LN111 could be key to maintaining their self‐renewal capability. Among the integrins involved in LN binding, integrins α3 and β1 were expressed in colonies on LN111 more than in those on LN511, whereas β4 was more strongly expressed in colonies on LN511. Integrin α3^highα6β1^high cells could form HPPC colonies on LN111 but not on LN511, whereas integrin α6β1^low cells could not on either LN111 or LN511. In addition, neutralizing anti‐integrin β1 and anti‐LN111 antibodies inhibited the passaged cells’ ability to attach and form colonies on LN111 by HPPCs. Matrigel overlay induced second‐passage cells growing on LN111 to increase their expression of hepatic functional genes and to form 3‐dimensional colonies with bile canalicular networks, whereas such a shift was poorly induced when they were grown onLN511. Conclusion: These results suggest that the self‐renewal capability of HPPCs depends on LN111 through integrin β1 signaling.

Myofibroblasts control the proliferation of fetal hepatoblasts and their differentiated cholangiocytes during the hepatoblast-to-cholangiocyte transition

Mesenchymal cells in the liver provide the microenvironment for hepatoblasts expansion and differentiation. We have previously demonstrated that myofibroblasts (MFs) promoted hepatoblasts differentiation into cholangiocytes, whereas its role in controlling the proliferation of hepatoblasts and their differentiated cholangiocytes remains elusive. Here, we investigated the role of MFs in regulating the proliferation of hepatoblasts and their differentiated cholangiocytes using an indirect coculture system. When cocultured with hepatoblasts, MFs promoted hepatoblasts differentiation into cholangiocytes and inhibited the proliferation and stemness of hepatoblasts. However, when hepatoblasts already differentiated into cholangiocytes, MFs promoted the differentiated cholangiocytes proliferation. In addition, hepatoblast proliferation genes such as hepatocyte growth factor (HGF), insulin-like growth factor-1 and 2 (IGF-1 and 2), midkine 1 (Mdk1), and pleiotrophin (Ptn) expression in MFs were down-regulated compared with their levels in fibroblasts. Our findings uncover the role of MFs in controlling the proliferation of hepatoblasts and their differentiated cholangiocytes, potentially providing a novel therapeutic strategy for cholangiocyte regeneration.

Fetal liver stem cell transplantation for liver diseases

Stem cell transplantation exhibited a promising lifesaving therapy for various end-stage liver diseases and could serve as a salvaging bridge until curative methods can be performed. In past decades, mature hepatocytes, liver progenitor cells, mesenchymal stem cells and induced pluripotent stem cells have been practiced in above settings. However, long-term survival rates and continuous proliferation ability of these cells in vivo are unsatisfactory, whereas, fetal liver stem cells (FLSCs), given their unique superiority, may be the best candidate for stem cell transplantation technique. Recent studies have revealed that FLSCs could be used as an attractive genetic therapy or regenerative treatments for inherited metabolic or other hepatic disorders. In this study, we reviewed current status and advancements of FLSCs-based treatment.

Hepatocyte-like Cells Derived from Human Pluripotent Stem Cells Can Be Enriched by a Combination of Mitochondrial Content and Activated Leukocyte Cell Adhesion Molecule

Introduction

Non-genetic purification methods for pluripotent stem cell-derived hepatocyte-like cells are useful for liver regenerative therapy and pharmaceutical applications.

Methods

Fluorescent activated cell sorting (FACS) was used to separate cells by combining two parameters: cellular mitochondrial content evaluated by the mitochondrial membrane potential-dependent fluorescent probe (TMRM) and immunocytochemical detection of activated leukocyte cell adhesion molecule (ALCAM). This method was applied to murine fetal, human embryonic stem cell (ESC)-derived, and human induced pluripotent stem cell (iPSC)-derived cell-mixtures. Separately sorted cell fractions were evaluated by quantitative PCR, immunohistochemistry, and cytochemistry for HNF4a, AFP, and albumin mRNA and/or protein expression.

Results

Hepatocyte-like cells were segregated into the high TMRM signal and ALCAM-positive population. The purity of hepatocyte-like cells derived from human iPSCs was 97 ± 0.38% (n = 5).

Conclusions

This hepatocyte-like cell purification method may be applicable to the quality control of cells for liver regenerative cell therapy and pharmaceutical development.

Long-Term Culture of Mouse Fetal Hepatic Stem/Progenitor Cells

Mouse fetal liver includes abundant hepatic stem/progenitor cells (HSPCs). Easy expansion with passage of HSPCs is necessary to obtain steady data. However, it is often difficult to enrich only HSPCs, and HSPCs can die when usual trypsin is used for replating. Here, we introduce serum-free long-term culture with passage of HSPCs using fetal mouse liver without a cell sorter.

Distinct roles of Dlk1 isoforms in bi-potential differentiation of hepatic stem cells

BACKGROUND

Fully understanding the developmental process of hepatic stem cells (HSCs) and the mechanisms of their committed differentiation is essential for optimizing the generation of functional hepatocytes for cell therapy in liver disease. Delta-like 1 homolog (Dlk1), primarily the membrane-bound form (Dlk1^M), is generally used as a surface marker for fetal hepatic stem cell isolation, while its soluble form (Dlk1^S) and the functional roles of different Dlk1 isoforms in HSC differentiation remain to be investigated.

METHODS

Hepatic spheroid-derived cells (HSDCs) were isolated from E12.5 mouse livers to obtain Dlk1⁺ and Dlk1^-subpopulations. Colony formation, BrdU staining, and CCK8 assays were used to evaluate the cell proliferation capacity, and hepatic/cholangiocytic differentiation and osteogenesis/adipogenesis were used to assess the multipotency of the two subpopulations. Transformation of Dlk1⁺ cells into Dlk1^- cells was detected by FACS, and the expression of Dlk1 isoforms were measured by western blot. The distinct roles and regulatory mechanisms of Dlk1 isoforms in HSC differentiation were investigated by overexpressing Dlk1^M.

RESULTS

HSDCs were capable of differentiating into liver and mesenchymal lineages, comprising Dlk1⁺ and Dlk1^- subpopulations. Dlk1⁺ cells expressed both Dlk1^M and Dlk1^S and lost expression of Dlk1^M during passaging, thus transforming into Dlk1^- cells, which still contained Dlk1^S. Dlk1^- cells maintained a self-renewal ability similar to that of Dlk1⁺ cells, but their capacity to differentiate into cholangiocytes was obviously enhanced. Forced expression of Dlk1^M in Dlk1^- cells restored their ability to differentiate into hepatocytes, with an attenuated ability to differentiate into cholangiocytes, suggesting a functional role of Dlk1 in regulating HSC differentiation in addition to acting as a biomarker. Further experiments illustrated that the regulation of committed HSC differentiation by Dlk1 was mediated by the AKT and MAPK signaling pathways. In addition, bFGF was found to serve as an important inducement for the loss of Dlk1^M from Dlk1⁺ cells, and autophagy might be involved.

CONCLUSIONS

Overall, our study uncovered the differential expression and regulatory roles of Dlk1 isoforms in the commitment of HSC differentiation and suggested that Dlk1 functions as a key regulator that instructs cell differentiation rather than only as a marker of HSCs. Thus, our findings expand the current understanding of the differential regulation of bi-potential HSC differentiation and provide a fine-tuning target for cell therapy in liver disease.

Single-cell murine genetic fate mapping reveals bipotential hepatoblasts and novel multi-organ endoderm progenitors

The definitive endoderm (DE) is the embryonic germ layer that forms the gut tube and associated organs, including thymus, lungs, liver and pancreas. To understand how individual DE cells furnish gut organs, genetic fate mapping was performed using the Rosa26^lacZ Cre-reporter paired with a tamoxifen-inducible DE-specific Cre-expressing transgene. We established a low tamoxifen dose that infrequently induced heritable lacZ expression in a single cell of individual E8.5 mouse embryos and identified clonal cell descendants at E16.5. As expected, only a fraction of the E16.5 embryos contained lacZ-positive clonal descendants and a subset of these contained descendants in multiple organs, revealing novel ontogeny. Furthermore, immunohistochemical analysis was used to identify lacZ-positive hepatocytes and biliary epithelial cells, which are the cholangiocyte precursors, in each clonally populated liver. Together, these data not only uncover novel and suspected lineage relationships between DE-derived organs, but also illustrate the bipotential nature of individual hepatoblasts by demonstrating that single hepatoblasts contribute to both the hepatocyte and the cholangiocyte lineage in vivo.

The susceptibility of human hepatoma-derived oval-like cells to hepatitis B virus infection

Human hepatocytes are a primary site of infection and replication of the hepatitis B virus (HBV). It is tempting to conclude that tissue specificity is controlled via virus-hepatocyte specific interactions at various steps during the viral lifecycle. However, the molecular mechanisms underlying hepatotropism of HBV are not fully clear. To address this issue, this study aims to identify hepatic factors that contribute to the regulation of the lifecycles of hepatitis viruses- especially HBV- and to clarify their regulatory mechanisms. We established oval-like cell lines (Hdo cells) by introducing a set of reprogramming factors (OCT3/4, SOX2, KLF4, LIN28, and NANOG) into human hepatoma HuH7 cells that are susceptible to HBV. Hdo cells exhibit a bi-directional differentiation potential. We found that Hdo cells maintained support for the replication of HBV but not of HCV. The level of particle-associated HBV DNA secreted into the culture medium was higher in the Hdo cells. Still, the HBs antigen level was lower than in parental HuH7 cells, suggesting that the regulation of HBV gene expression was affected by the reprogramming of HuH7 cells. A microarray analysis determined that the expression of host factors was largely comparable among of HuH7 and Hdo cells. In contrast, Hdo cells lost their susceptibility to HCV infection and to replication of the viral subgenome replicon RNA. Our results suggest that epigenetic reprogramming of human hepatoma cells potentially changes their permissivity to HBV. Furthermore, Hdo cells can be used as powerful tools to identify cellular determinants that change their expression during reprogramming or hepatic differentiation.

Known and novel roles of the MET oncogene in cancer: a coherent approach to targeted therapy

The MET oncogene encodes an unconventional receptor tyrosine kinase with pleiotropic functions: it initiates and sustains neoplastic transformation when genetically altered ('oncogene addiction') and fosters cancer cell survival and tumour dissemination when transcriptionally activated in the context of an adaptive response to adverse microenvironmental conditions ('oncogene expedience'). Moreover, MET is an intrinsic modulator of the self-renewal and clonogenic ability of cancer stem cells ('oncogene inherence'). Here, we provide the latest findings on MET function in cancer by focusing on newly identified genetic abnormalities in tumour cells and recently described non-mutational MET activities in stromal cells and cancer stem cells. We discuss how MET drives cancer clonal evolution and progression towards metastasis, both ab initio and under therapeutic pressure. We then elaborate on the use of MET inhibitors in the clinic with a critical appraisal of failures and successes. Ultimately, we advocate a rationale to improve the outcome of anti-MET therapies on the basis of thorough consideration of the entire spectrum of MET-mediated biological responses, which implicates adequate patient stratification, meaningful biomarkers and appropriate clinical end points.

Hepatic stem cells with self-renewal and liver repopulation potential are harbored in CDCP1-positive subpopulations of human fetal liver cells

Background

Mature human hepatocytes are critical in preclinical research and therapy for liver disease, but are difficult to manipulate and expand in vitro. Hepatic stem cells (HpSCs) may be an alternative source of functional hepatocytes for cell therapy and disease modeling. Since these cells play an import role in regenerative medicine, the precise characterization that determines specific markers used to isolate these cells as well as whether they contribute to liver regeneration still remain to be shown.

Method

In this study, human HpSCs were isolated from human primary fetal liver cells (FLCs) by flow cytometry using CDCP1, CD90, and CD66 antibodies. The isolated CDCP1⁺CD90⁺CD66^– HpSCs were cultured on dishes coated with type IV collagen in DMEM nutrient mixture F-12 Ham supplemented with FBS, human γ-insulin, nicotinamide, dexamethasone, and l-glutamine for at least 2 weeks, and were characterized by transcriptomic profiling, quantitative real-time PCR, immunocytochemistry, and in-vivo transplantation.

Results

The purified CDCP1⁺CD90⁺CD66^– subpopulation exhibited clonal expansion and self-renewal capability, and bipotential capacity was further identified in single cell-derived colonies containing distinct hepatocytes and cholangiocytes. Moreover, in-vivo liver repopulation assays demonstrated that human CDCP1⁺CD90⁺CD66^– HpSCs repopulated over 90% of the mouse liver and differentiated into functional hepatocytes with drug metabolism activity.

Conclusions

We identified a human hepatic stem/progenitor population in the CDCP1⁺CD90⁺CD66^– subpopulation in human FLCs, indicating CDCP1 marker could potentially be utilized to identify and isolate HpSCs for further cytotherapy of liver disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-017-0747-3) contains supplementary material, which is available to authorized users.

STAT3 deficiency prevents hepatocarcinogenesis and promotes biliary proliferation in thioacetamide-induced liver injury

AIM

To elucidate the role of STAT3 in hepatocarcinogenesis and biliary ductular proliferation following chronic liver injury.

METHODS

We investigated thioacetamide (TAA)-induced liver injury, compensatory hepatocyte proliferation, and hepatocellular carcinoma (HCC) development in hepatic STAT3-deficient mice. In addition, we evaluated TAA-induced biliary ductular proliferation and analyzed the activation of sex determining region Y-box9 (SOX9) and Yes-associated protein (YAP), which regulate the transdifferentiation of hepatocytes to cholangiocytes.

RESULTS

Both compensatory hepatocyte proliferation and HCC formation were significantly decreased in hepatic STAT3-deficient mice as compared with control mice. STAT3 deficiency resulted in augmentation of hepatic necrosis and fibrosis. On the other hand, biliary ductular proliferation increased in hepatic STAT3-deficient livers as compared with control livers. SOX9 and YAP were upregulated in hepatic STAT3-deficient hepatocytes.

CONCLUSION

STAT3 may regulate hepatocyte proliferation as well as transdifferentiation into cholangiocytes and serve as a therapeutic target for HCC inhibition and biliary regeneration.

Therapeutic potential of human minor salivary gland epithelial progenitor cells in liver regeneration

Liver disease is a serious problem affecting millions of people with continually increasing prevalence. Stem cell therapy has become a promising treatment for liver dysfunction. We previously reported on human minor salivary gland mesenchymal stem cells (hMSGMSCs), which are highly self-renewable with multi-potent differentiation capability. In this study, keratinocyte-like cells with self-regeneration and hepatic differentiation potential were isolated and characterized, and named human minor salivary gland epithelial progenitor cells (hMSG-EpiPCs). hMSG-EpiPCs were easily obtained via minor intraoral incision; they expressed epithelial progenitor/stem cell and other tissue stem cell markers such as CD29, CD49f, cytokeratins, ABCG2, PLET-1, salivary epithelial cell markers CD44 and CD166, and the Wnt target related gene LGR5 and LGR6. The cells were induced into functional hepatocytes in vitro which expressed liver-associated markers ALB, CYP3A4, AAT, and CK18. Upon transplantation in vivo, they ameliorated severe acute liver damage in SCID mice caused by carbon tetrachloride (CCl₄) injection. In a two-thirds partial hepatectomy mouse model, the transplanted cells survived at least 4 weeks and exhibited hepatic potential. These findings demonstrate that hMSG-EpiPCs have potential as a cellular therapy basis for hepatic diseases, physiological and toxicology studies and regenerative medicine.

A unique microenvironment in the developing liver supports the expansion of megakaryocyte progenitors

The fetal liver is the site of a major expansion of the hematopoietic stem cell (HSC) pool and is also a privileged organ to study megakaryocyte progenitor differentiation. We identified in the mouse fetal liver at day 13.5 a discrete stromal cell population harboring a CD45^-TER119^-CD31^-CD51⁺VCAM-1⁺PDGFRα^- (V⁺P^-) phenotype that lacked colony-forming unit fibroblast activity and harbored an hepatocyte progenitor signature. This previously undescribed V⁺P^- population efficiently supported megakaryocyte production from mouse bone marrow HSC and human peripheral blood HSC-myeloid progenitors cultured in the presence of limited cytokine concentrations. Megakaryocytes obtained in V⁺P^- cocultures were polyploid, positive for CD41/CD42c, and efficiently produced proplatelets. Megakaryocyte production appeared to be mediated by an expansion of the progenitor compartment through HSC-stromal cell contact. In conclusion, the fetal liver contains a unique cellular microenvironment that could represent a platform for the discovery of regulators of megakaryopoiesis.

Development of hepatoma-derived, bidirectional oval-like cells as a model to study host interactions with hepatitis C virus during differentiation

Directed differentiation of human stem cells including induced pluripotent stem cells into hepatic cells potentially leads to acquired susceptibility to hepatitis C virus (HCV) infection. However, cellular determinants that change their expression during cell reprogramming or hepatic differentiation and are pivotal for supporting the HCV life cycle remain unclear. In this study, by introducing a set of reprogramming factors, we established HuH-7-derived oval-like cell lines, Hdo-17 and -23, which possess features of bipotential liver precursors. Upon induction of hepatocyte differentiation, expression of mature hepatocyte markers and hepatoblast markers in cells increased and decreased, respectively. In contrast, in response to cholangiocytic differentiation induction, gene expression of epithelium markers increased and cells formed round cysts with a central luminal space. Hdo cells lost their susceptibility to HCV infection and viral RNA replication. Hepatic differentiation of Hdo cells potentially led to recovery of permissiveness to HCV RNA replication. Gene expression profiling showed that most host-cell factors known to be involved in the HCV life cycle, except CD81, are expressed in Hdo cells comparable to HuH-7 cells. HCV pseudoparticle infectivity was significantly but partially recovered by ectopic expression of CD81, suggesting possible involvement of additional unidentified factors in HCV entry. In addition, we identified miR200a-3p, which is highly expressed in Hdo cells and stem cells but poorly expressed in differentiated cells and mature hepatocytes, as a novel negative regulator of HCV replication. In conclusion, our results showed that epigenetic reprogramming of human hepatoma cells potentially changes their permissivity to HCV.

Transplantation of Nonvascularized Kidney Tissue Fragments into the Rat Liver with the Aim of Preserving Renal Function

For research in regenerative medicine, not only the study of cellular pluripotency but also knowledge of the reorganization of tissue structure is crucial. However, the latter will probably be more difficult to acquire. When small fragments of kidney (approx. 1 × 1 mm) were implanted in the liver of syngeneic LEW rats, the tissue survived at least 2 weeks with retention of normal structure including glomeruli and tubules. In contrast, no kidney structure survived when transplanted to subcutaneous sites, omentum, or spleen. Molecules involved in renal tubular function, such as megalin and glut2 transporter protein, were detectable in the implanted tissue by immunohistochemistry, suggesting that the cells were biologically active. Survival of cortex, medulla, and calyx tissues was then compared. All three components were still detectable 8 weeks after transplantation but cortex and medulla were replaced by granuloma at 6 months. Only calyx tissue survived for up to 12 months after transplantation. There was no marked difference in tissue survival, either when the recipient liver was partially resected or when infantile donor kidney was implanted instead of adult kidney. The present method opens new avenues in the development of regenerative medicine (i.e., tissue transplantation) as an intermediate modus between organ transplantation and cell transplantation.

Cryopreserved Fetal Liver Cell Transplants Support the Chronic Failing Liver in Rats with CCl4-Induced Cirrhosis

Hepatocyte transplantation is a promising method for supporting hepatic function in a broad spectrum of liver diseases. The aim of this work was to test the efficacy of human fetal liver cells to support the chronic failing liver in an experimental model of carbon tetrachloride (CCl4)-induced cirrhosis in rats. Liver cirrhosis was induced by intraperitoneal administration of CCl4 at a dose of 0.2 ml (50% v/v solution)/100 g body weight, twice a week for 3 months in rats. Ten days after stopping CCl4 administration (experimental day 0), rats received intrasplenic injection of cryopreserved fetal liver cells (FLC, 1 × 107 cells in 0.3 ml medium). As a cirrhotic control group, CCl4-induced cirrhotic rats were used with intrasplenic injection of an equal volume of medium alone. Animals were sacrificed on experimental day 15. Human fetal liver cell transplantation almost completely prevented the death of cirrhotic animals during the 2 weeks after treatment, while high ongoing mortality was seen in the cirrhotic control group. Cell transplantation into the spleen normalized total bilirubin and TBARSs levels and increased albumin levels in blood serum, as well as restoring mitochondrial function and liver detoxification function (assessed by cytochrome P450 contents and activity) compared with the activities seen in the cirrhosis control group. In parallel with this restoration of biochemical and functional liver indices, morphological patterns of liver recovery or regeneration after liver cell transplantation were demonstrated in day 15 samples by light microscopy. These were absent in the group that had received only medium alone.

The Role of Integrin α6 (CD49f) in Stem Cells: More than a Conserved Biomarker

Stem cells have the capacity for self-renewal and differentiation into specialized cells that form and repopulated all tissues and organs, from conception to adult life. Depending on their capacity for differentiation, stem cells are classified as totipotent (ie, zygote), pluripotent (ie, embryonic stem cells), multipotent (ie, neuronal stem cells, hematopoietic stem cells, epithelial stem cells, etc.), and unipotent (ie, spermatogonial stem cells). Adult or tissue-specific stem cells reside in specific niches located in, or nearby, their organ or tissue of origin. There, they have microenvironmental support to remain quiescent, to proliferate as undifferentiated cells (self-renewal), and to differentiate into progenitors or terminally differentiated cells that migrate from the niche to perform specialized functions. The presence of proteins at the cell surface is often used to identify, classify, and isolate stem cells. Among the diverse groups of cell surface proteins used for these purposes, integrin α6, also known as CD49f, may be the only biomarker commonly found in more than 30 different populations of stem cells, including some cancer stem cells. This broad expression among stem cell populations indicates that integrin α6 may play an important and conserved role in stem cell biology, which is reaffirmed by recent demonstrations of its role maintaining self-renewal of pluripotent stem cells and breast and glioblastoma cancer stem cells. Therefore, this review intends to highlight and synthesize new findings on the importance of integrin α6 in stem cell biology.

iTRAQ-Based Membrane Proteomics Reveals Plasma Membrane Proteins Change During HepaRG Cell Differentiation

HepaRG cell, a stabilized bipotent liver progenitor cell line, exhibits hepatocyte functions only after differentiation. However, the mechanism of transition from nondifferentiated to differentiated states, accompanied by proliferation migration and differentiation, remains poorly understood, particularly those proteins residing in the plasma membrane. In this study, the membrane protein expression change of HepaRG cell during differentiation were systematically analyzed using an iTRAQ labeled quantitative membrane proteomics approach. A total of 70 membrane proteins were identified to be differentially expressed among 849 quantified membrane proteins. Function and disease clustering analysis proved that 11 of these proteins are involved in proliferation, migration, and differentiation. Two key factors (MMP-14 and OCLN) were validated by qRT-PCR and Western blot. Blockade of MMP-14 further demonstrated its important function during tumor cell migration. The data sets have been uploaded to ProteomeXchange with the identifier PXD004752.

Myofibroblasts Derived from Hepatic Progenitor Cells Create the Tumor Microenvironment

Hepatic progenitor cells (HPCs) appear in response to several types of chronic injury in the human and rodent liver that often develop into liver fibrosis, cirrhosis, and primary liver cancers. However, the contribution of HPCs to the pathogenesis and progression of such liver diseases remains controversial. HPCs are generally defined as cells that can differentiate into hepatocytes and cholangiocytes. In this study, however, we found that HPCs isolated from the chronically injured liver can also give rise to myofibroblasts as a third type of descendant. While myofibroblast differentiation from HPCs is not significant in culture, during tumor development, HPCs can contribute to the formation of the tumor microenvironment by producing abundant myofibroblasts that might form a niche for tumor growth and survival. Thus, HPCs can be redefined as cells with a potential for differentiation into myofibroblasts that is specifically activated during tumor formation.

Suppression of lethal-7b and miR-125a/b Maturation by Lin28b Enables Maintenance of Stem Cell Properties in Hepatoblasts

In liver development, hepatoblasts that act as hepatic stem/progenitor cells proliferate and differentiate into both hepatocytes and cholangiocytes to form liver tissues. Although numerous factors contribute to this event, little is known about the roles of microRNAs in hepatoblast proliferation and differentiation. In this study, we focused on the lineage-28 (Lin28) family proteins, which are required for microRNA regulation in pluripotent stem cells and cancer cells, and investigated their roles as regulatory factors for the properties of hepatoblasts.

CONCLUSION

Lin28b was specifically expressed in hepatoblasts, and its suppression induced growth arrest and cholangiocyte differentiation of hepatoblasts; mechanistically, Lin28b positively regulates the expression of Lin28b itself and cell cycle-related proteins in hepatoblasts by suppressing the maturation of target microRNAs, lethal-7b and miR-125a/b, enabling maintenance of the stem cell properties of hepatoblasts, such as their capabilities for proliferation and bi-lineage differentiation, during liver development. (Hepatology 2016;64:245-260).

Investigation of bipotent differentiation of hepatoblasts using inducible diphtheria toxin receptor-transgenic mice

AIM

Hepatic progenitor cells, called hepatoblasts, are highly proliferative and exhibit bipotential differentiation into hepatocytes and cholangiocytes in the fetal liver. Thus, they are the ideal source for transplantation therapy. Although several studies have been performed in vitro, the molecular mechanisms regulating hepatoblast differentiation in vivo following transplantation remain poorly understood. The aim of this study was to investigate an in vivo model to analyze hepatoblast bipotency and proliferative ability.

METHODS

Hepatic transplantation model using Cre-inducible diphtheria toxin receptor-transgenic mice (iDTR), and albafpCre mice expressing Cre under the control of albumin and α-fetoprotein (AFP) regulatory elements were established. Fresh hepatoblasts were transplanted into diphtheria toxin (DT)-injected iDTRalbafpCre mice and we analyzed their differentiation and proliferation abilities by immunostaining and gene expression profiles.

RESULTS

Fresh hepatoblasts transplanted into DT-injected iDTRalbafpCre mice engrafted and differentiated into both hepatocytes and cholangiocytes. Additionally, the number of engrafted hepatoblast-derived hepatocytes increased following partial hepatectomy and serial DT injections. Expression levels of hepatic functional genes in transplanted hepatoblast-derived hepatocytes were similar to that of normal hepatocytes.

CONCLUSION

In our iDTRalbafpCre transplantation model, fresh hepatoblasts could differentiate into hepatocytes and cholangiocytes. In addition, these donor cells were induced to proliferate by the following liver injury stimulation. This result suggests that this model is valuable for investigating hepatoblast differentiation pathways in vivo.

Dysregulation of Bmi1 promotes malignant transformation of hepatic progenitor cells

Adult hepatic progenitor cells (HPCs) are involved in a wide range of human liver diseases, including hepatocellular carcinoma (HCC). Bmi1 has been reported to have vital roles in stem cell self-renewal and carcinogenesis. We have previously demonstrated that Bmi1 is upregulated in HCC with bile duct tumor thrombi, a subtype of HCC characterized by profuse expression of hepatic stem cell markers. However, the function of Bmi1 in HPCs has not yet been well elucidated. The current study was designed to investigate the effects of Bmi1 on the biological properties of rat HPCs. To accomplish this, Bmi1 was silenced or enhanced in two HPC cell lines (WB-F344 and OC3) by, respectively, using either small interfering RNA against Bmi1 or a forced Bmi1 expression retroviral vector. The biological functions of Bmi1 in HPCs were investigated through cell proliferation assays, colony-formation assays, cell cycle analysis and invasion assays, as well as through xenograft-formation assays. In this study, genetic depletion of Bmi1 repressed cell proliferation, colony formation and invasion in both assessed HPC cell lines relative to controls. Conversely, forced expression of Bmi1 in two HPCs cell lines promoted cell proliferation, colony formation and invasion in vitro. Aldehyde dehydrogenase (ALDH) assay revealed a significant increase in the number of ALDH-positive cells following the forced expression of Bmi1 in HPCs. Most importantly, transplantation of forced Bmi1 expression HPCs into nude mice resulted in the formation of tumors with histological features of poorly differentiated HCC. Taken together, our findings indicate that forced expression of Bmi1 promotes the malignant transformation of HPCs, suggesting Bmi1 might be a potential molecular target for the treatment of HCC.

Concise review: cell therapies for hereditary metabolic liver diseases-concepts, clinical results, and future developments

The concept of cell-based therapies for inherited metabolic liver diseases has been introduced for now more than 40 years in animal experiments, but controlled clinical data in humans are still not available. In the era of dynamic developments in stem cell science, the "right" cell for transplantation is considered as an important key for successful treatment. Do we aim to transplant mature hepatocytes or do we consider the liver as a stem/progenitor-driven organ and replenish the diseased liver with genetically normal stem/progenitor cells? Although conflicting results from cell tracing and transplantation experiments have recently emerged about the existence and role of stem/progenitor cells in the liver, their overall contribution to parenchymal cell homeostasis and tissue repair is limited. Accordingly, engraftment and repopulation efficacies of extrahepatic and liver-derived stem/progenitor cell types are considered to be lower compared to mature hepatocytes. On the basis of these results, we will discuss the current clinical cell transplantation programs for inherited metabolic liver diseases and future developments in liver cell therapy.

Acyclic retinoid induces differentiation and apoptosis of murine hepatic stem cells

Introduction

The therapeutic potential of acyclic retinoid (ACR), a synthetic retinoid, has been confirmed in experimental and clinical studies. Therapeutic targets include precancerous and cancer stem cells. As ACR is also involved in developmental processes, its effect on normal hepatic stem cells (HpSCs) should be investigated for understanding the underlying mechanisms. Here, we examined effects of the acyclic retinoid peretinoin on fresh isolated murine HpSCs.

Methods

We isolated c-kit⁻CD29⁺CD49f^+/lowCD45⁻Ter119⁻ cells from murine fetal livers using flow cytometry. To evaluate the effect of ACR, we traced clonal expansion and analyzed cell differentiation as well as apoptosis during the induction process by immunofluorescent staining and marker gene expression.

Results

ACR dose-dependently inhibited HpSCs expansion. Stem cell clonal expansion was markedly inhibited during the culture period. Moreover, ACR showed a significant promotion of HpSC differentiation and induction of cellular apoptosis. The expression of stem cell marker genes, Afp, Cd44, and Dlk, was downregulated, while that of mature hepatocyte genes, Alb and Tat, and apoptosis-related genes, Annexin V and Caspase-3, were upregulated. Flow cytometry showed that the proportion of Annexin V-positive cells increased after ACR incubation compared with the control. Data obtained by immunofluorescent staining for albumin and Caspase-3 corroborated the data on gene expression. Finally, we found that ACR directly regulates the expression of retinoic acid receptors and retinoid X receptors.

Conclusions

These findings indicate that ACR inhibits the clonal expansion of normal HpSCs in vitro and promotes the differentiation of immature cells by regulating receptors of retinoic acid.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-015-0046-9) contains supplementary material, which is available to authorized users.

Stem and progenitor cell systems in liver development and regeneration

The liver comprises two stem/progenitor cell systems: fetal and adult liver stem/progenitor cells. Fetal hepatic progenitor cells, derived from foregut endoderm, differentiate into mature hepatocytes and cholangiocytes during liver development. Adult hepatic progenitor cells contribute to regeneration after severe and chronic liver injuries. However, the characteristics of these somatic hepatic stem/progenitor cells remain unknown. Culture systems that can be used to analyze these cells were recently established and hepatic stem/progenitor cell-specific surface markers including delta-like 1 homolog (DLK), cluster of differentiation (CD) 13, CD133, and LIV2 were identified. Cells purified using antibodies against these markers proliferate for an extended period and differentiate into mature cells both in vitro and in vivo. Methods to force the differentiation of human embryonic stem and induced pluripotent stem (iPS) cells into hepatic progenitor cells have been recently established. We demonstrated that the CD13(+) CD133(+) fraction of human iPS-derived cells contained numerous hepatic progenitor-like cells. These analyses of hepatic stem/progenitor cells derived from somatic tissues and pluripotent stem cells will contribute to the development of new therapies for severe liver diseases.

KDR identifies a conserved human and murine hepatic progenitor and instructs early liver development

Understanding the fetal hepatic niche is essential for optimizing the generation of functional hepatocyte-like cells (hepatic cells) from human embryonic stem cells (hESCs). Here, we show that KDR (VEGFR2/FLK-1), previously assumed to be mostly restricted to mesodermal lineages, marks a hESC-derived hepatic progenitor. hESC-derived endoderm cells do not express KDR but, when cultured in media supporting hepatic differentiation, generate KDR+ hepatic progenitors and KDR- hepatic cells. KDR+ progenitors require active KDR signaling both to instruct their own differentiation into hepatic cells and to non-cell-autonomously support the functional maturation of cocultured KDR- hepatic cells. Analysis of human fetal livers suggests that similar progenitors are present in human livers. Lineage tracing in mice provides in vivo evidence of a KDR+ hepatic progenitor for fetal hepatoblasts, adult hepatocytes, and adult cholangiocytes. Altogether, our findings reveal that KDR is a conserved marker for endoderm-derived hepatic progenitors and a functional receptor instructing early liver development.

Downregulation of miR122 by grainyhead-like 2 restricts the hepatocytic differentiation potential of adult liver progenitor cells

Late fetal and adult livers are reported to contain bipotential liver stem/progenitor cells (LPCs), which share surface markers, including epithelial cell adhesion molecule (EpCAM), with cholangiocytes and differentiate into both hepatocytes and cholangiocytes. However, recent results do not necessarily support the idea that LPCs contribute significantly to cellular turnover and regeneration by supplying new hepatocytes. Here, we examined the colony-forming capability of EpCAM(+) cells isolated from mouse livers between E17 and 11 weeks of age. We found that the number of bipotential colonies was greatly reduced between 1 and 6 weeks, indicating that the number of LPCs decreases during postnatal development. Moreover, bipotential colonies derived from adult LPCs contained substantially fewer albumin(+) cells than those from neonatal LPCs. We further examined the differentiation potential of neonatal and adult LPCs by transplantation and found that neonatal cells differentiated into mature hepatocytes in recipient livers more frequently than adult LPCs. Since we previously reported that the transcription factor grainyhead-like 2 (GRHL2) expressed in EpCAM(+) cells inhibits hepatocytic differentiation, we examined whether targets of GRHL2 might block hepatocytic differentiation. DNA and microRNA microarrays revealed that miR122, the expression of which correlates with hepatocytic differentiation, was greatly reduced in adult as compared with neonatal EpCAM(+) cells. Indeed, GRHL2 negatively regulates the promoter/enhancer activity of the Mir122 gene. Our results indicate that neonatal but not adult EpCAM(+) LPCs have great potential to produce albumin(+) hepatocytes. GRHL2 suppresses transcription of miR122 and thereby restricts the differentiation potential of adult LPCs.

Mesenchymal progenitor cells in mouse foetal liver regulate differentiation and proliferation of hepatoblasts

BACKGROUND & AIMS

Hepatoblasts are somatic progenitor cells of the foetal liver that possess high proliferative capacity and bi-potency for differentiation into both hepatocytes and cholangiocytes. Although mesenchymal cells are known to be important for liver ontogeny, current understanding of their interaction with hepatoblasts remains obscure. Mesenchymal cell populations in the developing liver were purified and their potential to support proliferation and differentiation of hepatoblasts was examined.

METHODS

Foetal liver cells were fractionated with a flow cytometer using antibodies against cell surface markers. Gene expression of mesenchymal-specific transcripts and morphological characteristics were analysed. The ability of the mesenchymal cells to support hepatoblast function was analysed using a transwell and direct coculture system.

RESULTS

CD45(-) Ter119(-) CD71(-) Dlk1(mid) PDGFRα(+) cells from the mid-foetal stage liver expressed the mesenchymal cell-specific transcription factors H2.0-like homeobox 1 and LIM homeobox 2 at high levels. Foetal mesenchymal cells make contact with hepatoblasts in vivo and possess the potential to differentiate into chondrocytes, osteocytes and adipocytes under appropriate cell culture conditions, indicating that these cells are possible candidates for mesenchymal stem/progenitor cells. Foetal mesenchymal cells expressed pleiotrophin, hepatocyte growth factor and midkine 1, which are involved in the growth of hepatoblasts. Using the coculture system with hepatoblasts and foetal mesenchymal cells, these cells were shown to support proliferation and maturation of hepatoblasts through indirect and direct interactions respectively.

CONCLUSIONS

Dlk1(mid) PDGFRα(+) cells in non-haematopoetic fraction derived from the foetal liver exhibit mesenchymal stem/progenitor cell characteristics and have abilities to support proliferation and differentiation of hepatoblasts.

Application of Nanoscaffolds in Mesenchymal Stem Cell-Based Therapy

Regenerative medicine is an alternative solution for organ transplantation. Stem cells and nanoscaffolds are two essential components in regenerative medicine. Mesenchymal stem cells (MSCs) are considered as primary adult stem cells with high proliferation capacity, wide differentiation potential, and immunosuppression properties which make them unique for regenerative medicine and cell therapy. Scaffolds are engineered nanofibers that provide suitable microenvironment for cell signalling which has a great influence on cell proliferation, differentiation, and biology. Recently, application of scaffolds and MSCs is being utilized in obtaining more homogenous population of MSCs with higher cell proliferation rate and greater differentiation potential, which are crucial factors in regenerative medicine. In this review, the definition, biology, source, characterization, and isolation of MSCs and current report of application of nanofibers in regenerative medicine in different lesions are discussed.

The CD133+CD44+ precancerous subpopulation of oval cells is a therapeutic target for hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a malignant tumor associated with a generally poor prognosis and a high rate of recurrence. HCC usually develops in the context of chronic liver diseases, and long-lasting premalignant conditions precede cancer development. A promising therapeutic approach is to eliminate precancerous cells, which are considered as the precursors of cancer stem cells, to prevent further malignant transformation. In this study, we identified a subpopulation of precancerous cells in a rat liver carcinogenesis model, which were enriched in CD133(+)CD44(+)CD45(-)HIS49(-) cells that formed part of the hepatic oval cells fraction. Prospective isolation of the precancerous cells using flow cytometry identified stem cell properties such as the ability to expand clonally and differentiate into bi-lineage cell types. Furthermore, an acyclic retinoid, which was recently shown to improve overall survival after HCC resection, directly inhibited the extensive expansion of the isolated precancerous cells in vitro and decreased the emergence of the precancerous cells and their progeny in vivo. Long-term follow-up after the acyclic retinoid treatment confirmed reduction in precancerous changes, ultimately resulting in suppression of HCC development. These findings, together with data from recent clinical trials showing marked reduction in intrahepatic recurrence, suggest that acyclic retinoid directly prevents de novo HCC by inhibiting the development of precancerous cells. Given recent advances in diagnostic techniques and the establishment of surveillance programs, the targeting of precancerous cells may have a huge impact on preventative cancer therapies.

β-catenin activation in a novel liver progenitor cell type is sufficient to cause hepatocellular carcinoma and hepatoblastoma

Hepatocellular carcinoma (HCC) was thought historically to arise from hepatocytes, but gene expression studies have suggested that it can also arise from fetal progenitor cells or their adult progenitor progeny. Here, we report the identification of a unique population of fetal liver progenitor cells in mice that can serve as a cell of origin in HCC development. In the transgenic model used, mice carry the Cited1-CreER(TM)-GFP BAC transgene in which a tamoxifen-inducible Cre (CreER(TM)) and GFP are controlled by a 190-kb 5' genomic region of Cited1, a transcriptional coactivator protein for CBP/p300. Wnt signaling is critical for regulating self-renewal of progenitor/stem cells and has been implicated in the etiology of cancers of rapidly self-renewing tissues, so we hypothesized that Wnt pathway activation in CreER(TM)-GFP(+) progenitors would result in HCC. In livers from the mouse model, transgene-expressing cells represented 4% of liver cells at E11.5 when other markers were expressed, characteristic of the hepatic stem/progenitor cells that give rise to adult hepatocytes, cholangiocytes, and SOX9(+) periductal cells. By 26 weeks of age, more than 90% of Cited1-CreER(TM)-GFP;Ctnnb1(ex3(fl)) mice with Wnt pathway activation developed HCC and, in some cases, hepatoblastomas and lung metastases. HCC and hepatoblastomas resembled their human counterparts histologically, showing activation of Wnt, Ras/Raf/MAPK, and PI3K/AKT/mTOR pathways and expressing relevant stem/progenitor cell markers. Our results show that Wnt pathway activation is sufficient for malignant transformation of these unique liver progenitor cells, offering functional support for a fetal/adult progenitor origin of some human HCC. We believe this model may offer a valuable new tool to improve understanding of the cellular etiology and biology of HCC and hepatoblastomas and the development of improved therapeutics for these diseases.

Human fetal hepatic progenitor cells are distinct from, but closely related to, hematopoietic stem/progenitor cells

Much controversy surrounds the identity and origin of human hepatic stem and progenitor cells in part because of a lack of small animal models in which the developmental potential of isolated candidate cell populations can be functionally evaluated. We show here that adoptive transfer of CD34(+) cells from human fetal liver into sublethally irradiated NOD-SCID Il2rg(-/-) (NSG) mice leads to an efficient development of not only human hematopoietic cells but also human hepatocyte-like cells in the liver of the recipient mice. Using this simple in vivo assay in combination with cell fractionation, we show that CD34(+) fetal liver cells can be separated into three distinct subpopulations: CD34(hi) CD133(hi), CD34(lo) CD133(lo), and CD34(hi) CD133(neg). The CD34(hi) CD133(hi) population contains hematopoietic stem/progenitor cells (HSPCs) as they give rise to T cells, B cells, NK cells, dendritic cells, and monocytes/macrophages in NSG mice and colony-forming unit (CFU)-GEMM cells in vitro. The CD34(lo) CD133(lo) population does not give rise to hematopoietic cells, but reproducibly generates hepatocyte-like cells in NSG mice and in vitro. The CD34(hi) CD133(neg) population only gives rise to CFU-GM and burst-forming unit-erythroid in vitro. Furthermore, we show that the CD34(lo) CD133(lo) cells express hematopoietic, hepatic, and mesenchymal markers, including CD34, CD133, CD117, epithelial cell adhesion molecule, CD73, albumin, α-fetal protein, and vimentin and transcriptionally are more closely related to HSPCs than to mature hepatocytes. These results show that CD34(lo) CD133(lo) fetal liver cells possess the hepatic progenitor cell properties and that human hepatic and hematopoietic progenitor cells are distinct, although they may originate from the same precursors in the fetal liver.

Mesenchymal stem cells support hepatocyte function in engineered liver grafts

Recent studies suggest that organ decellularization is a promising approach to facilitate the clinical application of regenerative therapy by providing a platform for organ engineering. This unique strategy uses native matrices to act as a reservoir for the functional cells which may show therapeutic potential when implanted into the body. Appropriate cell sources for artificial livers have been debated for some time. The desired cell type in artificial livers is primary hepatocytes, but in addition, other supportive cells may facilitate this stem cell technology. In this context, the use of mesenchymal stem cells (MSC) is an option meeting the criteria for therapeutic organ engineering. Ideally, supportive cells are required to (1) reduce the hepatic cell mass needed in an engineered liver by enhancing hepatocyte function, (2) modulate hepatic regeneration in a paracrine fashion or by direct contact, and (3) enhance the preservability of parenchymal cells during storage. Here, we describe enhanced hepatic function achieved using a strategy of sequential infusion of cells and illustrate the advantages of co-cultivating bone marrow-derived MSCs with primary hepatocytes in the engineered whole-liver scaffold. These co-recellularized liver scaffolds colonized by MSCs and hepatocytes were transplanted into live animals. After blood flow was established, we show that expression of adhesion molecules and proangiogenic factors was upregulated in the graft.

Re-evaluation of liver stem/progenitor cells

Liver stem/progenitor cells (LPCs) are defined as cells that supply two types of liver epithelial cells, hepatocytes and cholangiocytes, during development, cellular turnover, and regeneration. Hepatoblasts, which are fetal LPCs derived from endoderm stem cells, robustly proliferate and differentiate into hepatocytes and cholangiocytes during fetal life. Between mid-gestation and the neonatal period, some cholangiocytes function as LPCs. Although LPCs in adult livers can be enriched in cells positive for cholangiocyte markers, their tissue localization and functions in cellular turnover remain obscure. On the other hand, it is well known that liver regeneration under conditions suppressing hepatocyte proliferation is supported by LPCs, though their origin has not been clearly identified. Recently many groups took advantage of new techniques including prospective isolation of LPCs by fluorescence-activated cell sorting and genetic lineage tracing to facilitate our understanding of epithelial supply in normal and injured livers. Those works suggest that, in normal livers, the turnover of hepatocytes mostly depends on duplication of hepatocytes. It is also demonstrated that liver epithelial cells as well as LPCs have great plasticity and flexible differentiation capability to respond to various types of injuries by protecting or repairing liver tissues.

Engineering of human hepatic tissue with functional vascular networks

Although absolute organ shortage highlights the needs of alternative organ sources for regenerative medicine, the generation of a three-dimensional (3D) and complex vital organ, such as well-vascularized liver, remains a challenge. To this end, tissue engineering holds great promise; however, this approach is significantly limited by the failure of early vascularization in vivo after implantation. Here, we established a stable 3D in vitro pre-vascularization platform to generate human hepatic tissue after implantation in vivo. Human fetal liver cells (hFLCs) were mixed with human umbilical vein endothelial cells (HUVECs) and mesenchymal stem cells (hMSCs) and were implanted into a collagen/fibronectin matrix composite that was used as a 3-D carrier. After a couple of days, the fluorescent HUVECs developed premature vascular networks in vitro, which were stabilized by hMSCs. The establishment of functional vessels inside the pre-vascularized constructs was proven using dextran infusion studies after implantation under a transparency cranial window. Furthermore, dynamic morphological changes during embryonic liver cell maturation were intravitaly quantified with high-resolution confocal microscope analysis. The engineered human hepatic tissue demonstrated multiple liver-specific features, both structural and functional. Our new techniques discussed here can be implemented in future clinical uses and industrial uses, such as drug testing.