Hematopoietic cells as hepatocyte stem cells: a critical review of the evidence

Fetal liver cell transplantation as a potential alternative to whole liver transplantation?

Because organ shortage is the fundamental limitation of whole liver transplantation, novel therapeutic options, especially the possibility of restoring liver function through cell transplantation, are urgently needed to treat end-stage liver diseases. Groundbreaking in vivo studies have shown that transplanted hepatocytes are capable of repopulating the rodent liver. The two best studied models are the urokinase plasminogen activator (uPA) transgenic mouse and the fumarylacetoacetate hydrolase (FAH)-deficient mouse, in which genetic modifications of the recipient liver provide a tissue environment in which there is extensive liver injury and selection pressure favoring the proliferation and survival of transplanted hepatocytes. Because transplanted hepatocytes do not significantly repopulate the (near-)normal liver, attention has been focused on finding alternative cell types, such as stem or progenitor cells, that have a higher proliferative potential than hepatocytes. Several sources of stem cells or stem-like cells have been identified and their potential to repopulate the recipient liver has been evaluated in certain liver injury models. However, rat fetal liver stem/progenitor cells (FLSPCs) are the only cells identified to date that can effectively repopulate the (near-)normal liver, are morphologically and functionally fully integrated into the recipient liver, and remain viable long-term. Even though primary human fetal liver cells are not likely to be routinely used for clinical liver cell repopulation in the future, using or engineering candidate cells exhibiting the characteristics of FLSPCs suggests a new direction in developing cell transplantation strategies for therapeutic liver replacement. This review will give a brief overview concerning the existing animal models and cell sources that have been used to restore normal liver structure and function, and will focus specifically on the potential of FLSPCs to repopulate the liver.

Surgical Therapy of End-Stage Heart Failure: Understanding Cell-Mediated Mechanisms Interacting with Myocardial Damage

Worldwide, cardiovascular disease results in an estimated 14.3 million deaths per year, giving rise to an increased demand for alternative and advanced treatment. Current approaches include medical management, cardiac transplantation, device therapy, and, most recently, stem cell therapy. Research into cell-based therapies has shown this option to be a promising alternative to the conventional methods. In contrast to early trials, modern approaches now attempt to isolate specific stem cells, as well as increase their numbers by means of amplifying in a culture environment. The method of delivery has also been improved to minimize the risk of micro-infarcts and embolization, which were often observed after the use of coronary catheterization. The latest approach entails direct, surgical, transepicardial injection of the stem cell mixture, as well as the use of tissue-engineered meshes consisting of embedded progenitor cells.

Efficacy of mesenchymal stem cells in suppression of hepatocarcinorigenesis in rats: possible role of Wnt signaling

Background

The present study was conducted to evaluate the tumor suppressive effects of bone marrow derived mesenchymal stem cells (MSCs) in an experimental hepatocellular carcinoma (HCC) model in rats and to investigate the possible role of Wnt signaling in hepato-carcinogenesis.

Methods

Ninety rats were included in the study and were divided equally into: Control group, rats which received MSCs only, rats which received MSCs vehicle only, HCC group induced by diethylnitroseamine (DENA) and CCl₄, rats which received MSCs after HCC induction, rats which received MSCs before HCC induction. Histopathological examination and gene expression of Wnt signaling target genes by real time, reverse transcription-polymerase chain reaction (RT-PCR) in rat liver tissue, in addition to serum levels of ALT, AST and alpha fetoprotein were performed in all groups.

Results

Histopathological examination of liver tissue from animals which received DENA-CCl₄ only, revealed the presence of anaplastic carcinoma cells and macro-regenerative nodules type II with foci of large and small cell dysplasia. Administration of MSCs into rats after induction of experimental HCC improved the histopathological picture which showed minimal liver cell damage, reversible changes, areas of cell drop out filled with stem cells. Gene expression in rat liver tissue demonstrated that MSCs downregulated β-catenin, proliferating cell nuclear antigen (PCNA), cyclin D and survivin genes expression in liver tissues after HCC induction. Amelioration of the liver status after administration of MSCs has been inferred by the significant decrease of ALT, AST and Alpha fetoprotein serum levels. Administration of MSCs before HCC induction did not show any tumor suppressive or protective effect.

Conclusions

Administration of MSCs in chemically induced HCC has tumor suppressive effects as evidenced by down regulation of Wnt signaling target genes concerned with antiapoptosis, mitogenesis, cell proliferation and cell cycle regulation, with subsequent amelioration of liver histopathological picture and liver function.

Anti-fibrogenic strategies and the regression of fibrosis

Liver fibrosis is an outcome of many chronic diseases, and often results in cirrhosis, liver failure, and portal hypertension. Liver transplantation is the only treatment available for patients with advanced stage of fibrosis. Therefore, alternative methods are required to develop new strategies for anti-fibrotic therapy. Available treatments are designed to substitute for liver transplantation or bridge the patients, they include inhibitors of fibrogenic cytokines such as TGF-β1 and EGF, inhibitors of rennin angiotensin system, and blockers of TLR4 signalling. Development of liver fibrosis is orchestrated by many cell types. However, activated myofibroblasts remain the primary target for anti-fibrotic therapy. Hepatic stellate cells and portal fibroblasts are considered to play a major role in development of liver fibrosis. Here we discuss the origin of activated myofibroblasts and different aspects of their activation, differentiation and potential inactivation during regression of liver fibrosis.

[Robust differentiation of fetal hepatocytes from human embryonic stem cells and iPS]

Hepatocyte transplantation is considered as an alternative to organ transplantation in particular for the treatment of liver metabolic diseases. However, due to the difficulties to obtain a large number of hepatocytes, new sources of cells are needed. These cells could be either of hepatic origin (hepatic stem cells) or extrahepatic such as mesenchymal stem cells or pluripotent stem cells (human embryonic stem cells [hESC] or iPS). We developed a new method to differentiate hESCs into fetal hepatocytes. These conditions recapitulate the main liver developmental stages, using fully defined medium devoid of animal products or unknown factors. The differentiated cells express many fetal hepatocytes markers (cytochrome P450 3A7, albumin, alpha-1-antitrypsin, etc.). The cells display specific hepatic functions (ammonia metabolism, excretion of indocyanin green) and are capable to engraft and express hepatic proteins two months after transplantation into newborn uPAxrag2gc-/- mouse liver. We have also showed that this approach is transposable to human iPS, and further studies on animal models will allow us to compare the in vivo potential of these two sources of pluripotent cells. Finally, only studies on large animals such as nonhuman primates will validate an eventual clinical application.

Hepatic progenitor cell represents a transitioning cell population between liver epithelium and stroma

Following an acute injury, the liver may maintain its structure and function through mitotic division of mature hepatocytes (i.e. hepatic regeneration). However, the regeneration ability of hepatocytes can be impaired in chronic liver diseases including chronic viral infection and alcohol abuse. Hepatic progenitor cells/oval cells (HPCs/OCs), capable of differentiation into both hepatocytes and cholangiocytes, occur and proliferate during chronic injury. Unfortunately, a use of HPCs for clinical therapy is blocked by the difficulty of exact identity of HPCs in liver. Focusing on the links between phenotype of HPCs and real stem cells originating from fetal liver or bone marrow (BM), the recent studies of HPCs neglect functional analysis and the close relationship between activation of HPCs and extracellular matrix (ECM) remodeling. It is currently widely accepted that mesenchymal-epithelial transition (EMT) and epithelial-mesenchymal transition (MET) play important roles not only in liver development but also in healing of chronic injured adult liver. Co-expression of epithelial/mesenchymal and HPCs markers has been demonstrated in cells undergoing EMT/MET. These cells led to hepatic regeneration after transplanted into rats with chronic liver injury. Notably, there is an increased expression of mesenchymal markers in HPCs after exposure to transforming growth factor-beta1 (TGF-β1). Based on these evidences, we hypothesize that HPCs represent a transitioning cell population undergoing EMT/MET, both parenchymal and mesenchymal cells of liver may be the direct sources of HPCs.

Evolving concepts in cell therapy of liver disease and current clinical perspectives

The clinical use of cells for the treatment of liver disease is not a mere hypothesis. Indeed, it has been known for more than 30 years that, following intraportal infusion, exogenous hepatocytes isolated from a donor liver engraft into the recipient hepatic parenchyma and express metabolic activity. These experimental results encouraged pilot clinical trials using hepatocytes transplantation to treat a variety of liver diseases. More recently, the discovery of liver stem/progenitor cells further fueled the interest in the field. However, it appears that successful liver cell therapy will require better understanding of the mechanisms governing liver regeneration and of their implication in cell transplantation. This review summarizes some recent advances in the field in a bench-to-bedside perspective.

Acute hepatic failure-derived bone marrow mesenchymal stem cells express hepatic progenitor cell genes

Hepatic progenitor cell (HPC) transplantation is a promising alternative to liver transplantation for patients with end-stage liver disease. However, the precise origin of HPCs is unclear. This study aimed to determine whether bone marrow mesenchymal stem cells (BMSCs) isolated from rats in acute hepatic failure (AHF) possess hepatic potential and/or characteristics. BMSCs were isolated from normal rats as well as rats in which AHF was induced by D-galactosamine. HPCs and primary hepatocytes were isolated from normal rats for comparison. The Affymetrix GeneChip Rat Genome 230 2.0 Array was used to perform transcriptome profiling of the AHF-derived BMSCs and HPCs. The results showed that AHF-derived BMSCs had a gene expression profile significantly different from that of control BMSCs. More than 87.7% of the genes/probe sets that were upregulated more than 2-fold in AHF-derived BMSCs were expressed by HPCs, including 12 genes involved in liver development, early hepatocyte differentiation and hepatocyte metabolism. Confirmatory quantitative RT-PCR analysis yielded similar results. In addition, 940 probe sets/genes were expressed in both AHF-derived BMSCs and HPCs but were absent in control cells. Compared to the control cells, AHF-derived BMSCs shared more commonly expressed genes with HPCs. AHF-derived BMSCs have a hepatic transcriptional profile and express many of the same genes expressed by HPCs, strongly suggesting that BMSCs may be a resource for hepatocyte regeneration, and further confirming their potential as a strong source of hepatocyte regeneration during severe hepatic damage.

Model systems and experimental conditions that lead to effective repopulation of the liver by transplanted cells

In recent years, there has been substantial progress in transplanting cells into the liver with the ultimate goal of restoring liver mass and function in both inherited and acquired liver diseases. The basis for considering that this might be feasible is that the liver is a highly regenerative organ. After massive liver injury or surgical removal of two-thirds or more of the liver tissue, the organ can restore its mass with completely normal morphologic structure and function. It has also been found under highly selective conditions that transplanted hepatocytes can fully repopulate the liver and cure a metabolic disorder or deficiency state. Fetal liver cells can also substantially repopulate the normal liver, and it is hoped in the future that effective repopulation will be achievable with cultured cells or cell lines, pluripotent stem cells from other somatic tissues, embryonic stem cells, or induced pluripotent stem cells, which can now be generated in vitro by a variety of methods. The purpose of this review is to present the major systems that have been used for liver repopulation, the variables involved in obtaining successful repopulation and what has been achieved in these various systems to date with different cell types.

CD133+ stem cell mobilization after partial hepatectomy depends on resection extent and underlying disease

BACKGROUND

Bone marrow stem cells (BMSC) can participate to liver regeneration. However, conflicting results have been reported on this topic in patients undergoing liver resection.

AIMS

To assess the impact of liver resection extent and presence of underlying liver disease in modulating BMSC mobilization.

METHODS

We enrolled 29 patients undergoing liver resection of different extents, 5 surgical controls and 10 blood donors. Circulating CD133+ BMSC were measured by flow cytometry at different time-points after surgery. The hepatic commitment of mobilized BMSC was investigated by polymerase chain reaction. Liver specimens were collected during surgery for histopathological analysis. Hepatocyte growth factor and granulocyte-colony stimulating factor serum levels were measured by enzyme-linked immunosorbent assay.

RESULTS

BMSC mobilization was found in patients undergoing major liver resection, especially in the presence of underlying disease. Ductular reactions were noted in patients with chronic hepatopathy and the hepatic progenitor-like cells expressed CD133, NCAM, cytokeratin-19, and alpha-fetoprotein. Hepatocyte growth factor and granulocyte-colony stimulating factor levels increased following liver resection and the contemporaneous presence of liver disease was associated with their highest raise.

CONCLUSIONS

Liver repair is mainly an endogenous process. BMSC become important in case of extensive resection, especially in the presence of underlying hepatopathy and hepatic progenitor-like cells activation. Hepatocyte growth factor and granulocyte-colony stimulating factor seem to be involved in the dynamics underlying hepatic regeneration and BMSC recruitment.

Cell-based therapeutics for liver disorders

INTRODUCTION

Due to a lack of adequate liver donors and post-surgical complications, researchers propose that cell therapy should be an alternative treatment for patients with end-stage liver diseases.

DATA SOURCES

We performed a literature review on cell-based therapy for liver disorders.

AREAS OF AGREEMENT

Due to growing numbers of patients on waiting lists for liver transplantation, a substitute treatment strategy is needed for our patients. Cell therapy can save patients who are in life-threatening situations, enabling them to have more time and increase their chances of survival. Pluripotent stem cells can be a good resource for cell-based therapy after the establishment of efficient differentiation protocols in addition to the settlement of ethical and immunological issues. Cell-based therapy will be applicable after the approval of its efficiency via animal model studies.

AREAS OF CONTROVERSY

Transplanted cells cannot integrate into the recipient liver and lose their functionality after a limited time. The rate of homing and transdifferentiation of transplanted cells into hepatocytes is scant.

GROWING POINTS

Application of autologous bone marrow mononuclear cells (MNCs), hematopoietic and mesenchymal stem cells (HSCs and MSCs) has improved the general conditions of certain patients. Although this improvement is temporary, new studies have focused on increasing their performance. TIMELY AREAS FOR DEVELOPING RESEARCH: The safety, feasibility and efficacy of applying MNCs, HSCs and MSCs in liver disorders have been proven in clinical trials. Patient-specific cell therapy after the production of induced pluripotent stem cells and new discoveries in somatic cell conversion during transdifferentiation are promising insights.

Bone marrow mononuclear cell transplantation improves survival and induces hepatocyte proliferation in rats after CCl(4) acute liver damage

AIM

The aim of this research was to evaluate the effects of bone marrow mononuclear cell (BMC) transplantation in rats with toxic acute liver damage induced by carbon tetrachloride (CCl(4)).

METHODS

Cells from male Wistar rats were obtained using Ficoll density gradient and 0.2 ml (1 × 10(6) cells) were injected into the portal vein of female rats (n = 15) 24 h after damage. Sham group (n = 15) was performed injecting only vehicle in CCl(4)-treated animals. Survival, liver histology, number of mitosis and apoptosis, and identification of stained donor cells were observed 72 h after damage. ALT levels were measured at 0 h, 24 h, 48 h, and 72 h after injury.

RESULTS

Donor cells could be detected in recipient rats' livers by fluorescence staining and Sry PCR. The treated group revealed a significant improvement in survival rate after 72 h (p = 0.003). There was also a significant increase in the number of mitotic events in treated livers (p = 0.029). This result was confirmed using an in vitro cell proliferation assay in isolated hepatocytes treated with conditioned medium from BMC. ALT was reduced in the treated group after 72 h (p = 0.034).

CONCLUSIONS

Results indicate that BMC transplantation has potential as a new therapeutic option for acute liver disease and suggest that these cells may contribute to hepatic recovery through release of mitotic cytokines.

Stem cell-based therapies for liver diseases: state of the art and new perspectives

Millions of patients worldwide suffer from end-stage liver pathologies, whose only curative therapy is liver transplantation (OLT). Given the donor organ shortage, alternatives to OLT have been evaluated, including cell therapies. Hepatocyte transplantation has been attempted to cure metabolic liver disorders and end-stage liver diseases. The evaluation of its efficacy is complicated by the shortage of human hepatocytes and their difficult expansion and cryopreservation. Recent advances in cell biology have led to the concept of "regenerative medicine", based on the therapeutic potential of stem cells (SCs). Different types of SCs are theoretically eligible for liver cell replacement. These include embryonic and fetal SCs, induced pluripotent cells, annex SCs, endogenous liver SCs, and extrahepatic adult SCs. Aim of this paper is to critically analyze the possible sources of SCs suitable for liver repopulation and the results of the clinical trials that have been published until now.

Nf2/Merlin controls progenitor homeostasis and tumorigenesis in the liver

The molecular signals that control the maintenance and activation of liver stem/progenitor cells are poorly understood, and the role of liver progenitor cells in hepatic tumorigenesis is unclear. We report here that liver-specific deletion of the neurofibromatosis type 2 (Nf2) tumor suppressor gene in the developing or adult mouse specifically yields a dramatic, progressive expansion of progenitor cells throughout the liver without affecting differentiated hepatocytes. All surviving mice eventually developed both cholangiocellular and hepatocellular carcinoma, suggesting that Nf2(-/-) progenitors can be a cell of origin for these tumors. Despite the suggested link between Nf2 and the Hpo/Wts/Yki signaling pathway in Drosophila, and recent studies linking the corresponding Mst/Lats/Yap pathway to mammalian liver tumorigenesis, our molecular studies suggest that Merlin is not a major regulator of YAP in liver progenitors, and that the overproliferation of Nf2(-/-) liver progenitors is instead driven by aberrant epidermal growth factor receptor (EGFR) activity. Indeed, pharmacologic inhibition of EGFR blocks the proliferation of Nf2(-/-) liver progenitors in vitro and in vivo, consistent with recent studies indicating that the Nf2-encoded protein Merlin can control the abundance and signaling of membrane receptors such as EGFR. Together, our findings uncover a critical role for Nf2/Merlin in controlling homeostasis of the liver stem cell niche.

Prospects for stem cell transplantation in the treatment of hepatic disease

Stem cell therapy has the potential to provide a valuable adjunct to the management of hepatic disease. Preclinical studies have demonstrated a range of endogenous repair processes that can be exploited through stem cell therapy. Initial translational studies have been encouraging and have suggested improved liver function in advanced chronic liver disease and enhanced liver regeneration after portal vein embolization. This article reviews the potential for stem cell therapies to enhance hepatic regeneration in acute and chronic hepatic disease and is based on a MEDLINE and PubMed search for English language articles investigating mechanisms of hepatic regeneration and delivery of cell therapies. Two main mechanisms of potential stem cell therapy delivery have emerged: (1) a direct contribution to the functional hepatocyte population with embryonic, induced pluripotent, or adult stem cells and (2) the promotion of endogenous regenerative processes with bone marrow-derived stem cells. Bioartificial hepatic support systems may be proven to be an effective method of using ex vivo differentiated hepatocytes and be indicated as a bridging therapy to definitive surgery in acute liver failure. The administration of bone marrow-derived stem cells may enhance liver regeneration in chronic liver disease after portal vein embolization and could facilitate regeneration after partial hepatic resection. Ultimately, the most appropriate hepatic disease targets for stem cell therapies will become apparent as mechanisms of stem involvement in hepatic regeneration are further elucidated.

Recent advances in liver stem cell therapy

PURPOSE OF REVIEW

Patients with liver cirrhosis often require liver transplantation, which remains the only effective treatment of the end-stage cirrhosis. Here we briefly summarize the current concepts in treatment of liver diseases based on the transplantation of intrahepatic liver cells, capable of repopulating the injured liver. These cells include hepatocytes, oval cells (bipotential intrahepatic progenitor cells), bone marrow hematopoietic and mesenchymal stem cells, and induced pluripotent stem (iPS) cells.

RECENT FINDINGS

Although liver transplantation remains the only conventional treatment, liver cell transplantation is an experimental procedure which has been successfully used in clinical trials in patients with acute liver failure, chronic liver disease with end-stage cirrhosis. Extraordinary progress has been made in the field of hepatic progenitors and iPS. Liver precursor cells (oval cells) are recognized as bipotential precursor cells in the damaged liver. They can rapidly proliferate, change their cellular composition, and differentiate into hepatocytes and cholangiocytes to compensate for the cellular loss and maintain liver homeostasis in animal models of liver injury. Similarly, iPS are somatic cells obtained from patients and differentiated into hepatocytes in vitro. Future studies of iPS are designed to develop of specific conditions to expand and in vitro differentiate somatic cells into functionally mature liver cells.

SUMMARY

The current review defines and discusses different populations of hepatic cells which can be potentially used for liver cell transplantation to advance the therapy of hepatic cirrhosis.

Autologous hematopoietic stem cell transplantation in 48 patients with end-stage chronic liver diseases

The only presently viable treatment for end-stage liver disease is whole organ transplantation. However, there are insufficient livers available. The aim of the present study is to provide autologous bone marrow-derived stem cells as a potential therapeutic for patients with end-stage cirrhosis. This is a retrospective chart review of autologous stem cell treatment in 48 patients, 36 with chronic end-stage hepatitis C-induced liver disease and 12 with end-stage autoimmune liver disease. For all patients, granulocyte colony-stimulating factor was administered to mobilize their hematopoietic stem cells. Following leukapheresis, CD34(+) stem cells were isolated, amplified, and partially differentiated in culture, then reinjected into each subject via their hepatic artery or portal vein. Treatment was generally well tolerated with the expected moderate but transient bone pain from G-CSF in less than half of the patients. Three patients had serious treatment-related complications, and only 20.8% of these end-stage liver disease patients died during 12 months of follow up. For all patients there was a statistically significant decrease in ascites. There was clinical and biochemical improvement in a large percentage of patients who received the transplantation. In the viral group, there were marked changes in albumin (p = 0.0003), bilirubin (p = 0.04), INR (p = 0.0003), and ALT levels (p = 0.02). In the autoimmune group, values also improved significantly for albumin (p = 0.001), bilirubin (p = 0.002), INR (p = .0005), and ALT levels (p = 0.003). These results suggest that autologous CD34(+) stem cell transplantation may be safely administered and appears to offer some therapeutic benefit to patients with both viral and autoimmune-induced end-stage liver disease.

Epithelial-mesenchymal transition: from molecular mechanisms, redox regulation to implications in human health and disease

Epithelial to mesenchymal transition (EMT) is a fundamental process, paradigmatic of the concept of cell plasticity, that leads epithelial cells to lose their polarization and specialized junctional structures, to undergo cytoskeleton reorganization, and to acquire morphological and functional features of mesenchymal-like cells. Although EMT has been originally described in embryonic development, where cell migration and tissue remodeling have a primary role in regulating morphogenesis in multicellular organisms, recent literature has provided evidence suggesting that the EMT process is a more general biological process that is also involved in several pathophysiological conditions, including cancer progression and organ fibrosis. This review offers first a comprehensive introduction to describe major relevant features of EMT, followed by sections dedicated on those signaling mechanisms that are known to regulate or affect the process, including the recently proposed role for oxidative stress and reactive oxygen species (ROS). Current literature data involving EMT in both physiological conditions (i.e., embryogenesis) and major human diseases are then critically analyzed, with a special final focus on the emerging role of hypoxia as a relevant independent condition able to trigger EMT.

Portal venous endothelium in developing human liver contains haematopoietic and epithelial progenitor cells

Future treatments for chronic liver disease are likely to involve manipulation of liver progenitor cells (LPCs). In the human, data characterising the regenerative response is limited and the origin of adult LPCs is unknown. However, these remain critical factors in the design of cell-based liver therapies. The developing human liver provides an ideal model to study cell lineage derivation from progenitors and to understand how foetal haematopoiesis and liver development might explain the nature of the adult LPC population. In 1st trimester human liver, portal venous endothelium (PVE) expressed adult LPC markers and markers of haematopoietic progenitor cells (HPCs) shared with haemogenic endothelium found in the embryonic dorsal aorta. Sorted PVE cells were able to generate hepatoblast-like cells co-expressing CK18 and CK19 in addition to Dlk/pref-1, E-cadherin, albumin and fibrinogen in vitro. Furthermore, PVE cells could initiate haematopoiesis. These data suggest that PVE shares phenotypical and functional similarities both with adult LPCs and embryonic haemogenic endothelium. This indicates that a temporal relationship might exist between progenitor cells in foetal liver development and adult liver regeneration, which may involve progeny of PVE.

Transplantation of human amnion epithelial cells reduces hepatic fibrosis in immunocompetent CCl₄-treated mice

Chronic liver injury and inflammation lead to hepatic fibrosis, cirrhosis, and liver failure. Embryonic and mesenchymal stem cells have been shown to reduce experimental liver fibrosis but have potential limitations, including the formation of dysplastic precursors, tumors, and profibrogenic cells. Other stem-like cells may reduce hepatic inflammation and fibrosis without tumor and profibrogenic cell formation. To test this hypothesis we transplanted human amnion epithelial cells (hAEC), isolated from term delivered placenta, into immunocompetent C57/BL6 mice at week 2 of a 4-week regimen of carbon tetrachloride (CCl₄) exposure to induce liver fibrosis. Two weeks following hAEC infusion, intact cells expressing the human-specific markers inner mitochondrial membrane protein and human leukocyte antigen-G were found in mouse liver without evidence of host rejection of the transplanted cells. Human albumin, known to be produced by hAEC, was detected in sera of hAEC-treated mice. Human DNA was detected in mouse liver and also spleen, lungs, and heart of some animals. Following hAEC transplantation, CCl₄-treated animals showed decreased serum ALT levels and reduced hepatocyte apoptosis, compared to controls. hAEC-treated mouse liver had lower TNF-α and IL-6 protein levels and higher IL-10 compared to animals given CCl₄ alone. Compared to CCl₄ controls, hAEC-treated mice showed fewer activated collagen-producing hepatic stellate cells and less fibrosis area and collagen content. Reduced hepatic TGF-β levels in conjunction with a twofold increase in the active form of the collagen-degrading enzyme matrix metalloproteinase-2 in hAEC-treated mice compared to CCl₄ controls may account for the reduction in fibrosis. hAEC transplantation into immunocompetent mice leads to cell engraftment, reduced hepatocyte apoptosis, and decreased hepatic inflammation and fibrosis.

Hepatic progenitors for liver disease: current position

Liver regeneration restores the original functionality of hepatocytes and cholangiocytes in response to injury. It is regulated on several levels, with different cellular populations contributing to this process, eg, hepatocytes, liver precursor cells, intrahepatic stem cells. In response to injury, mature hepatocytes have the capability to proliferate and give rise to new hepatocytes and cholangiocytes. Meanwhile, liver precursor cells (oval cells) have become the most recognized bipotential precursor cells in the damaged liver. They rapidly proliferate, change their cellular composition, and differentiate into hepatocytes and cholangiocytes to compensate for the cellular loss and maintain liver homeostasis. There is a growing body of evidence that oval cells originate from the intrahepatic stem cell(s), which in turn give(s) rise to epithelial, including oval cells, and/or other hepatic cells of nonepithelial origin. Since there is a close relationship between the liver and hematopoiesis, bone marrow derived cells can also contribute to liver regeneration by the fusion of myeloid cells with damaged hepatocytes, or differentiation of mesenchymal stem cells into hepatocyte-like cells. The current review discusses the contribution of different cells to liver regeneration and their characteristics.

Architectural and immunohistochemical characterization of biliary ductules in normal human liver

The canals of Hering or biliary ductules have been described to connect the bile canaliculi with the interlobular bile ducts, and thus forming the distal part of the biliary tree. Studies in the last two decades suggested that the cells constructing these ductules could behave as hepatic progenitor cells. The canals of Hering are confined to the periportal space in the rat, while they have been reported to spread beyond the limiting plate in human liver. The distribution of the distal biliary ductules in normal human hepatic tissue has been investigated in our recent experiments. We could demonstrate the presence of interlobular connective tissue septa in a rudimentary form in healthy livers. The canals of Hering run in these septa in line with the terminal branches of the portal vein and hepatic arteries. This arrangement develops in the postnatal period but regresses after early childhood. The canals of Hering can be identified by the unique epithelial membrane antigen (EMA)-/CD56+/CD133+ immunophenotype. The canals of Hering leave the periportal space and spread into the liver parenchyma along rudimentary interlobular septa outlining the hepatic lobules. Our observations refine the original architectural description of the intraparenchymal portion of the canals of Hering in the human liver. The distinct immunophenotype supports their unique biological function.

Stem cells in hepatocarcinogenesis: evidence from genomic data

Increasing evidence suggests that many, perhaps all solid tumors contain a subset of cells that possess functional properties similar to the normal tissue stem cells, including self-renewal, unlimited proliferative capacity, and pluripotency. The hierarchical cancer model that places a cancer stem cell (CSC) population at the apex of tumor formation is based on this notion. The cancer stem cell hypothesis posits that CSCs are responsible not only for tumor initiation, but also generation of metastasis and local recurrence after therapy. Current definitions of the CSC are based only on functional properties regardless of potential cellular origin. Histopathology investigations of chronic liver diseases and experimental studies support the existence of CSCs in liver cancer. In particular, recent advances in microarray technologies utilizing integrative comparative genomic analysis of human hepatocellular carcinoma specimens, cancer cell lines, and transgenic models establish the molecular similarities between CSC and normal tissue stem cells and highlight the importance of CSC for the prognosis of liver cancer patients. The results have also uncovered the key "stemness" and oncogenic pathways frequently disrupted during hepatocarcinogenesis providing the basis for identifying novel therapeutic targets against CSC.

Bone marrow-derived stem cells in liver repair: 10 years down the line

Hematopoietic stem cells have potential in the field of regenerative medicine because of their capacity to form cells of different lineages. Bone marrow stem cells have been shown to contribute to parenchymal liver cell populations, and although this may not be functionally significant, it has sparked interest in the field of autologous stem cell infusion as a possible treatment for cirrhosis. In this review, we will examine the evidence for the contribution of bone marrow-derived cells to populations of liver cells and for the functional contribution of bone marrow-derived cells to both liver fibrosis and repair. The mechanisms by which cells are trafficked from the bone marrow to the liver are complex; the stromal derived factor-1/CXC receptor 4 axis is central to this process. There are limited data in liver injury, but we will examine findings from the bone marrow transplantation literature and discuss their relevance to liver disease. Stromal derived factor-1 also has a role in endogenous liver stem cell accumulation. Some groups have already started infusing autologous bone marrow cells into patients with cirrhosis. We will review these trials in the context of the basic science that we have discussed, and we will consider targets for investigation in the future.

The identification of stem cells in human liver diseases and hepatocellular carcinoma

Liver stem cells are thought to preside in bile ducts and the canals of Hering. They extend into the liver parenchyma at a time when normal liver cell proliferation is suppressed and liver regeneration is stimulated. In the present study 69 liver biopsies and surgically excised liver tumors were studied for the presence of liver stem cells. It was found that human cirrhotic livers and hepatocellular carcinomas (HCC) frequently exhibited isolated single scattered hepatocyte stem cells within the liver parenchyma rather than in the portal tract, bile duct or the canal of Hering. These cells expressed liver stem cell markers. HCCs also contained isolated tumor cell which expressed the same stem cell markers. The markers used were GST-P, OV-6, CK-19, Oct-3/4 and FAT10. They were identified by immunofluorescent antibody staining. HGF, EGF, CK19, AIR, H19, Nanog, Oct-3/4 and FAT10 were identified by RNA-FISH. H19 is a non-coding RNA, which is expressed in most HCCs.

RESULTS

Immunohistochemistry and RNA-FISH performed on human livers identified isolated stem cells in liver parenchyma as follows: Stem cells identified by immunohistochemical markers (OV-6 and GST-P) and RNA-FISH markers (HGF, EGF, CK19 and H19) were found scattered in the liver parenchyma of cirrhotic livers and within hepatocellular carcinomas (HCCs). Precirrhotic ASH or NASH all stained negative for these stem cells. In HCCs, 13 out of 15 had stem cells located within the tumor (78%). In cirrhotic livers, 12 out of 28 (37%) had liver parenchymal stem cells present. In one case of stage 3 precirrhosis, stem cells were also found. Double staining for the markers showed colocalization of the markers in stem cells. Stem cells were found in 33% of HBV, 47% of HCV, 25% of alcoholic steatohepatitis (ASH) and 17% of non-alcoholic steatohepatitis (NASH). The frequency of stem cells found in the different disease categories correlates with the frequency of HCC occurring in these different diseases.

Artificial cell microencapsulated stem cells in regenerative medicine, tissue engineering and cell therapy

Adult stem cells, especially isolated from bone marrow, have been extensively investigated in recent years. Studies focus on their multiple plasticity oftransdifferentiating into various cell lineages and on their potential in cellular therapy in regenerative medicine. In many cases, there is the need for tissue engineering manipulation. Among the different approaches of stem cells tissue engineering, microencapsulation can immobilize stem cells to provide a favorable microenvironment for stem cells survival and functioning. Furthermore, microencapsulated stem cells are immunoisolated after transplantation. We show that one intraperitoneal injection of microencapsulated bone marrow stem cells can prolong the survival of liver failure rat models with 90% of the liver removed surgically. In addition to transdifferentiation, bone marrow stem cells can act as feeder cells. For example, when coencapsulated with hepatocytes, stem cells can increase the viability and function of the hepatocytes in vitro and in vivo.

Infusion of autologous bone marrow mononuclear cells through hepatic artery results in a short-term improvement of liver function in patients with chronic liver disease: a pilot randomized controlled study

AIM

This randomized controlled study evaluated the effect of autologous infusion of bone marrow cells (BMC) in patients with hepatic cirrhosis.

METHODS

Thirty patients on the liver transplant waiting list were randomly assigned to receive BMC therapy or no treatment. They were followed up for 1 year. The study was nonblinded. Autologous mononuclear-enriched BMC were infused into the hepatic artery; liver function scores/tests were chosen as endpoints to assess efficacy. Statistical analysis calculated mean relative changes (RC) from baseline and fitted a random-effects model.

RESULTS

Mean age, baseline model for end-stage liver disease, and Child-Pugh score were similar in both groups. Child-Pugh score improved in the first 90 days in the cell therapy group compared with controls (P = 0.017, BMC group RC = -8%, controls RC = +5%). The model for end-stage liver disease score remained stable in the treated patients (RC -2 to +6%), whereas it increased during follow-up in the control group (RC +6 to +18%). Albumin levels improved in the treatment arm, whereas they remained stable among controls in the first 90 days (P = 0.034; BMC group RC = +16%, control group RC = +2%). Bilirubin levels increased among controls, whereas they decreased in the therapy arm during the first 60 days; INR RC differences between groups reached up to 10%. The changes observed did not persist beyond 90 days.

CONCLUSION

Transplantation of autologous BMC into the hepatic artery improved liver function in patients with advanced cirrhosis in the first 90 days. However, larger studies are necessary to define the role of BMC therapy in cirrhotic patients. Repeated autologous BMC infusions or combination therapy with granulocyte-colony-stimulating factor might improve or sustain the treatment response.

Bone marrow transplantation demonstrates medullar origin of CD34+ fibrocytes and ameliorates hepatic fibrosis in Abcb4-/- mice

Bone marrow (BM)-derived stem cells and CD34(+) fibrocytes are associated with fibrogenesis in several organs. In an Abcb4(-/-) mouse model for sclerosing cholangitis alpha-smooth muscle actin-positive (alpha-SMA(+)) myofibroblasts are thought to play a pivotal role in hepatic fibrogenesis. The aim of this study was 2-fold: (1) to demonstrate that the origin of an important fibrogenetic cell population is the BM; and (2) to investigate whether transplantation of BM (BM-Tx) affects liver function, staging, and grading. Surrogate markers for fibrogenesis and regulation of hepatic stellate cells (HSC) as well as progenitor-cell-derived fibrocytes in liver tissue were analyzed by quantitative real-time polymerase chain reaction (PCR) and immunohistology. After lethal irradiation of recipient mice, BM-Tx was carried out by way of tail vein injection of BM cells from marker protein donors (green fluorescent protein, GFP(+)) or Abcb4(-/-) mice as control (syngeneic Tx). Parameters of liver function were assessed serologically and histologically. Activated HSC of alpha-SMA(+)/CRP2(+) phenotype were expressed in approximately 50% of proliferating bile ducts, whereas fibrotic liver parenchyma showed no expression thereof. Epithelial mesenchymal transfer (EMT) was visualized in the areas of proliferating bile ducts. The hematopoietic origin of CD34(+) fibrocytes was demonstrated immunohistologically in livers of BM chimeric mice. These CD34(+) cells infiltrated hepatic lobules from portal fields and developed a desmin(+) phenotype expressing collagen type I in fibrotic parenchyma as well as in vitro after isolation by magnetic cell separation. Transplantation of GFP(+)/Abcb4(+) BM improved liver function and staging compared with sham transplantation, but no significant differences were noticed among allogeneic and syngeneic Tx.

CONCLUSION

The present study is the first to identify that both BM-derived fibrocytes and HSC are involved in biliary fibrogenesis in Abcb4(-/-) mice. Our data suggest that changes in immunity subsequent to BM-Tx may alter hepatic fibrosis.

Potentials of regenerative medicine for liver disease

Liver transplantation is still the only effective treatment for end-stage liver disease. However, because of the serious worldwide shortage of donated organs, an alternative cellular therapy would be desirable. Animal studies and preclinical trials have indicated that hepatocyte transplantation can serve as an alternative to liver transplantation. Unfortunately, however, the harvesting of hepatocytes is associated with the same problem as organ transplantation, i.e., a lack of a suitable cell source. Therefore, current stem cell technology, which is attempting to establish an unlimited supply of hepatocytes, would facilitate the clinical application of hepatocyte transplantation. This review summarizes current knowledge of embryonic and adult stem cell differentiation into hepatocytes and discusses how liver stem cells could be applied clinically in the future.

Negligible contribution of bone marrow-derived cells to collagen production during hepatic fibrogenesis in mice

BACKGROUND & AIMS

Recent studies have reported that bone marrow (BM)-derived cells migrating into fibrotic liver tissue exhibit a myofibroblast-like phenotype and may participate in the progression of liver fibrosis. However, their contribution to collagen production has not been fully verified yet. We revisited this issue by using 2 mechanistically distinct liver fibrosis models introduced into transgenic collagen reporter mice and their BM recipients.

METHODS

BM of wild-type mice was replaced by cells obtained from transgenic animals harboring tissue-specific enhancer/promoter sequences of alpha2(I) collagen gene (COL1A2) linked to enhanced green fluorescent protein (EGFP) or firefly luciferase (LUC) gene. Liver fibrosis was introduced into those mice by repeated carbon tetrachloride injections or ligation of the common bile duct. Activation of COL1A2 promoter was assessed by confocal microscopic examination detecting EGFP signals and luciferase assays of liver homogenates.

RESULTS

The tissue-specific COL1A2 enhancer/promoter was activated in hepatic stellate cells following a single carbon tetrachloride injection or during primary culture on plastic. A large number of EGFP-positive collagen-expressing cells were observed in liver tissue of transgenic COL1A2/EGFP mice in both liver fibrosis models. In contrast, there were few EGFP-positive BM-derived collagen-producing cells detected in fibrotic liver tissue of COL1A2/EGFP recipients. Luciferase assays of liver tissues from COL1A2/LUC-recipient mice further indicated that BM-derived cells produced little collagen in response to fibrogenic stimuli.

CONCLUSIONS

By using a specific and sensitive experimental system, which detects exclusively BM-derived collagen-producing cells, we conclude an unexpectedly limited role of BM-derived cells in collagen production during hepatic fibrogenesis.

Transplanted endothelial progenitor cells ameliorate carbon tetrachloride-induced liver cirrhosis in rats

Cirrhosis is the most common end stage of liver diseases, and there are no effective treatment methods. Here we evaluated the effect of endothelial progenitor cell (EPC) transplantation from rat bone marrow (BM) on the development of cirrhosis induced by carbon tetrachloride (CCl(4)). Ex vivo generated, characterized, and cultivated rat BM-derived EPCs were identified by their vasculogenic properties in vitro. EPCs from male rats were transplanted into female rats via the intraportal vein 12 weeks after they had been challenged with CCl(4), and the rats were killed 16 weeks later. The control rats received only a saline infusion. The fibrosis index and donor cell engraftment were assessed after EPC transplantation. After transplantation via the portal vein, PKH26 labeling, polymerase chain reaction, and in situ hybridization analysis revealed that the donor EPCs had adhered to the vasolateral surfaces of blood vessels and established in the liver. EPCs reduced the expression of alpha-smooth muscle actin, collagen III, and transforming growth factor beta (P < 0.05) as well as levels of aspartate aminotransferase, alanine aminotransferase, and total bilirubin in the serum (P < 0.05), but at the same time they increased the levels of albumin and Ki67. CCl(4) treatment increased the international prothrombin ratio (P < 0.05) and reduced albumin levels, whereas EPCs restored these parameters to normal levels. These results suggest that EPC transplantation could play a role in regulating hepatocyte regeneration and ameliorating established liver cirrhosis.

Clinical implications of advances in the basic science of liver repair and regeneration

Recent advances in our understanding of the basic mechanisms that control liver regeneration and repair will produce the next generation of therapies for human liver disease. Insights gained from large-scale genetic analysis are producing a new framework within which to plan interventions. Identification of specific molecules that drive regeneration will increase the options for live-donor liver transplantation, and help treat patients with small-for-size syndrome or large tumors who would otherwise have inadequate residual mass after resection. In a complementary fashion, breakthroughs in the ability to manipulate various cell types to adopt the hepatocyte or cholangiocyte phenotype promise to revolutionize therapy for acute liver failure and metabolic liver disease. Finally, elucidating the complex interactions of liver cells with each other and various matrix components during the response to injury is essential for fabricating a liver replacement device. This focused review will discuss how a variety of important scientific advances are likely to impact the treatment of specific types of liver disease.

Rationale and targets for antifibrotic therapies

We have made striking progress in our understanding of the biochemistry and cell biology that underlies liver fibrosis and cirrhosis, including the development of strategies and agents to prevent and reverse fibrosis and incipient cirrhosis. However, translation of this knowledge into clinical practice has been hampered by the limitation of many in vitro and in vivo models to confirm mechanisms and to test antifibrotic agents, as well as the lack of sensitive methodologies to quantify the degree of liver fibrosis and the dynamics of fibrosis progression or reversal. Furthermore, while cirrhosis and subsequent decompensation are accepted hard clinical end-points, fibrosis and fibrosis progression alone are merely plausible surrogates for future clinical deterioration. This review focuses on basic mechanisms that underlay liver fibrosis progression and reversal and optimized strategies for preclinical antifibrotic drug development and validation. Therapies include several drugs that are of proven safety for other indications, agents that interfere with major fibrogenic or fibrolytic mechanisms, targeted drug delivery to the fibrogenic liver cells, and their potential combinations with hepatocyte or stem cell replenishment.

Bone marrow and non-bone marrow TLR4 regulates hepatic ischemia/reperfusion injury

Hepatic ischemia-reperfusion injury (IRI) is a highly coordinated process often observed during liver transplantation, liver surgery, and hemorrhagic shock. Signaling through toll-like receptor 4 (TLR4), which is widely expressed on all kinds of liver cells, appears critical in the pathogenesis of IRI. Although the role of TLR4 expressed on non-parenchymal cells (NPCs) of the liver, including Kupffer cells and neutrophils, in IRI has been widely studied, TLR4 signaling on liver sinusoidal endothelial cells (LSECs) or hepatocytes in the process of IRI, and their coordination with bone marrow derived TLR4 in the late reperfusion stage, is largely unknown. We produced TLR4 chimeric mice that received hepatic IRI, and examined the degree of liver injury and the underlying mechanisms of injury. Results indicated that mutation of TLR4 on bone-marrow or non-bone marrow derived cells reduced hepatic IRI in the late reperfusion stage via cytokine release and neutrophil infiltration, while non-bone marrow derived TLR4 regulated the expression of ICAM-1 on hepatocytes and LSECs, exacerbating their injury. In conclusion, both TLR4 on bone marrow derived and non-bone marrow derived cells were necessary in the process of hepatic IRI.

Liver cancer stem cells: implications for a new therapeutic target

Hepatocellular carcinoma (HCC) is an aggressive tumour with a poor prognosis. Current therapeutic strategies against this disease target mostly rapidly growing differentiated tumour cells. However, the result is often dismal due to the chemoresistant nature of this tumour type. Recent research efforts on stem cells and cancer biology have shed light on new directions for the eradication of cancer stem cells (CSCs) in HCC. The liver is a distinctive organ with the ability of tissue renewal in response to injury. Based on the hypothesis that cancer development is derived from the hierarchy of the stem cell system, we will briefly discuss the origin of liver stem cells and its relation to HCC development. We will also summarize the current CSC markers in HCC and discuss their relevance to the treatment of this deadly disease.

Contribution of human hematopoietic stem cells to liver repair

Immune-deficient mouse models of liver damage allow examination of human stem cell migration to sites of damage and subsequent contribution to repair and survival. In our studies, in the absence of a selective advantage, transplanted human stem cells from adult sources did not robustly become hepatocytes, although some level of fusion or hepatic differentiation was documented. However, injected stem cells did home to the injured liver tissue and release paracrine factors that hastened endogenous repair and enhanced survival. There were significantly higher levels of survival in mice with a toxic liver insult that had been transplanted with human stem cells but not in those transplanted with committed progenitors. Transplantation of autologous adult stem cells without conditioning is a relatively safe therapy. Adult stem cells are known to secrete bioactive factors that suppress the local immune system, inhibit fibrosis (scar formation) and apoptosis, enhance angiogenesis, and stimulate recruitment, retention, mitosis, and differentiation of tissue-residing stem cells. These paracrine effects are distinct from the direct differentiation of stem cells to repair tissue. In patients at high risk while waiting for a liver transplant, autologous stem cell therapy could be considered, as it could delay the decline in liver function.

Human progenitor cells with high aldehyde dehydrogenase activity efficiently engraft into damaged liver in a novel model

Human cord blood stem cells (hCBSCs) have been reported to generate hepatocyte-like cells and thus hold promise for repairing damaged liver. However, the frequency of hCBSC-derived hepatocytes varies tremendously between different studies, and it is still controversial as to whether hCBSC-derived cells can transdifferentiate into hepatocytes or simply fuse to recipient hepatocytes. We used the beta-glucuronidase-deficient nonobese diabetic/severe combined immunodeficient/mucopolysaccharidosis type VII (NOD/SCID/MPSVII) mouse model for better identification of engrafted cells. We transplanted lineage-depleted human umbilical cord blood-derived cells with high aldehyde dehydrogenase activity (ALDH(hi)Lin(-)) into irradiated NOD/SCID/MPSVII mice followed by carbon tetrachloride administration to induced liver damage. ALDH(hi)Lin(-) cells were efficiently engrafted in the recipient mouse livers and improved recovery of the mice from toxic insult. The percentage of human cells in these livers ranged between 3% and 14.2% using quantitative real-time polymerase chain reaction. Furthermore, human-originated cells expressing liver-specific alpha1-antitrypsin messenger RNA, albumin and hepatocyte nuclear factor 1 protein were detected in the recipient livers. Interestingly, human versus murine centromeric fluorescent in situ hybridization analysis on the liver sections demonstrated that most human cells were not fused to mouse cells. However, the majority of the human originated albumin-expressing cells also carried mouse genetic material, hence were the product of cell fusion.

CONCLUSION

hCBSCs or their progeny may home to the injured liver and release trophic factors that hasten tissue repair, whereas fusion of these cells with hepatocytes may occur rarely and contribute to a lesser extent to liver repair.

Passage of bone-marrow-derived liver stem cells in a proliferating culture system

AIM: To explore the feasibility of passage of bone-marrow-derived liver stem cells (BDLSCs) in culture systems that contain cholestatic serum.

METHODS: Whole bone marrow cells of rats were purified with conditioning selection media that contained 50 mL/L cholestatic serum. The selected BDLSCs were grown in a proliferating culture system and a differentiating culture system. The culture systems contained factors that stimulated the proliferation and differentiation of BDLSCs. Each passage of the proliferated stem cells was subjected to flow cytometry to detect stem cell markers. The morphology and phenotypic markers of BDLSCs were characterized using immunohistochemistry, reverse transcription polymerase chain reaction (RT-PCR) and electron microscopy. The metabolic functions of differentiated cells were also determined by glycogen staining and urea assay.

RESULTS: The conditioning selection medium isolated BDLSCs directly from cultured bone marrow cells. The selected BDLSCs could be proliferated for six passages and maintained stable markers in our proliferating system. When the culture system was changed to a differentiating system, hepatocyte-like colony-forming units (H-CFUs) were formed. H-CFUs expressed markers of embryonic hepatocytes (alpha-fetoprotein, albumin and cytokeratin 8/18), biliary cells (cytokeratin 19), hepatocyte functional proteins (transthyretin and cytochrome P450-2b1), and hepatocyte nuclear factors 1α and -3β). They also had glycogen storage and urea synthesis functions, two of the critical features of hepatocytes.

CONCLUSION: BDLSCs can be selected directly from bone marrow cells, and pure BDLSCs can be proliferated for six passages. The differentiated cells have hepatocyte-like phenotypes and functions. BDLSCs represent a new method to provide a readily available alternate source of cells for clinical hepatocyte therapy.

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.

Transdifferentiation of stem cells: a critical view

Recently a large amount of new data on the plasticity of stem cells of various lineages have emerged, providing new perspectives especially for the therapeutic application of adult stem cells. Previously unknown possibilities of cell differentiation beyond the known commitment of a given stem cell have been described using keywords such as "blood to liver," or "bone to brain." Controversies on the likelihood, as well as the biological significance, of these conversions almost immediately arose within this young field of stem cell biology. This chapter will concentrate on these controversies and focus on selected examples demonstrating the technical aspects of stem cell transdifferentiation and the evaluation of the tools used to analyze these events.

Therapeutic potential of adult bone marrow stem cells in liver disease and delivery approaches

Hematopoietic stem cells (HSCs) and mesenchymal stem cell (MSCs) are two main subtypes of bone marrow stem cells. Extensive studies have been carried out to investigate the therapeutic potential of BMSCs in liver disease. A number of animal and human studies demonstrated that either HSCs or MSCs could be applied to therapeutic purposes in certain liver diseases. The diseased liver may recruit migratory stem cells, particularly from the bone marrow, to generate hepatocyte-like cells either by transdifferentiation or cell fusion. Transplantation of BMSCs has therapeutic effects of restoration of liver mass and function, alleviation of fibrosis and correction of inherited liver diseases. There are still controversial results over the potential effects of BMSCs on liver diseases, and some of the discrepancies are thought to be lied in the differences of experimental protocols, differences in individual research laboratory, and the uncertainties of the techniques employed. Several potential approaches for BMSCs delivery in liver diseases have been proposed in animal studies and human trials. BMSCs can be delivered via intraportal vein, systemic infusion, intraperitoneal, intrahepatic, intrasplenic. The optimal stem cells delivery should be easy to perform, less invasive and traumatic, minimum side effects, and with high cells survival rate. In this review, we focus on the up-to-date evidence of therapeutic effects of BMSCs on liver disease, the characteristics of various delivery approaches, and the considerations for future studies.