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

Hepatic regeneration from hematopoietic stem cells

In recent years, numerous investigators have reported novel cellular fates of multipotent stem or progenitor cells. In this review, we discuss the unexpected observations that hematopoietic stem cells can contribute to the hepatocyte lineage in humans and in rodent models of liver disease and regeneration. A key unresolved issue regarding hepatic regeneration from hematopoietic stem cells is whether the mechanism occurs through transdetermination, cell fusion, or other processes. A better understanding of the various stem or progenitor cells of the hepatic lineage may facilitate cellular transplantation approaches for the correction of hepatic function in patients with end-stage liver disease.

Embryonic liver cells and permanent lines as models for hepatocyte and bile duct cell differentiation

Analysis of liver cells during development is facilitated by the possibility of complementing in vivo analysis with experiments on cultured cells. In this review, we discuss results from several laboratories concerning bipotential hepatic stem cells from mouse (HBC-3, H-CFU-C, MMH and BMEL), rat (rhe14321) and primate (IPFLS) embryos. Several groups have used fluorescence-activated cell sorting to identify clonogenic bipotential cells; others have derived bipotential cell lines by plating liver cell suspensions and cloning. The bipotential cells, which probably originate from hepatoblasts, can differentiate as hepatocytes or bile duct cells, and undergo morphogenesis in culture. Disparities in differentiation can be explained by distinct medium compositions, extracellular matrix coated culture surfaces, and gene expression detection methods. Potential applications of these cell lines are discussed.

Pancreatic-hepatic switches in vivo

The existence of a common endodermal progenitor cell to both pancreas and liver is suggested by anatomy of the development of pancreatic and liver buds in embryogenesis. Here we review the large body of evidence that pancreas to liver and liver to pancreas cell differentiation can also occur in adult life. The published data are consistent with the hypothesis that endodermal progenitor cells, capable of giving rise to multiple hepatic and pancreatic cell types continue to persist. These cells may represent a stem cell reservoir with potential in cell therapy applications in the future.

Oval cell-mediated liver regeneration: Role of cytokines and growth factors

In experimental models, which induce liver damage and simultaneously block hepatocyte proliferation, the recruitment of a hepatic progenitor cell population comprised of oval cells is invariably observed. There is a substantial body of evidence to suggest that oval cells are involved in liver regeneration, as they differentiate into hepatocytes and biliary cells. Recently, bone marrow cells were shown to be a source of a stem cells with the capacity to repopulate the liver. Presently, the relationship between bone marrow cells and oval cells is unclear. Investigations will be greatly assisted by the availability of in vitro models based on a knowledge of cytokines that affect oval cells. While the cytokines, which regulate the different hematopoietic lineages, are well characterized, there is relatively little information regarding those that influence oval cells. This review outlines recent developments in the field of oval cell research and focuses on cytokines and growth factors that have been implicated in regulating oval cell proliferation and differentiation.

Identification and propagation of liver stem cells

Although the liver has been known for its enormous regenerative capacity, little is known about the mechanisms responsible for such regeneration.To provide evidence for the existence of liver stem cell, using FACS and single cell-based assays, cells with multi-lineage differentiation potential and self-renewal capability have been prospectively identified. These cells could be clonally propagated in culture where they continuously produced hepatocytes and cholangiocytes as descendants while maintaining primitive stem cells. When the cells clonally expanded in vitro were transplanted into mouse, they morphologically and functionally differentiated into hepatocytes and cholangiocytes. Furthermore, these cells differentiated into pancreatic acinar cells or intestinal epithelial cells upon transplantation into pancreas or duodenal wall. Manipulation of self-renewing liver stem cells may provide new insight into therapies for diseases of the digestive system.

Inducible differentiation and morphogenesis of bipotential liver cell lines from wild-type mouse embryos

This work shows that hepatic cell lines reproducibly can be derived from E14 embryos of many mouse inbred strains. These bipotential mouse embryonic liver (BMEL) cell lines present a mixed morphology, containing both epithelial and palmate-like cells, and an uncoupled phenotype, expressing hepatocyte transcription factors (HNF1alpha, HNF4alpha, GATA4) but not functions (apolipoproteins, albumin). BMEL cells are bipotential: under inducing conditions they express hepatocyte and bile duct functions. In addition, they can undergo morphogenesis in Matrigel culture to form bile duct units. When returned to basal culture conditions, the differentiated cells revert, within a few days, to an undifferentiated state. The ensemble of markers expressed by BMEL cells implies that they originate from hepatoblasts, the endodermal precursors of the liver. In conclusion, the establishment of a simple and reproducible method to isolate from any mouse embryo bipotential hepatic cell lines that exhibit the properties of transit stem cells provides a novel paradigm for investigation of hepatic cell lineage relationships.

Engineering liver therapies for the future

Treatment of liver disease has been greatly improved by the advent and evolution of liver transplantation. However, as demand for donor organs continues to increase beyond their availability, the need for alternative liver therapies is clear. Several approaches including extracorporeal devices, cell transplantation, and tissue-engineered constructs have been proposed as potential adjuncts or even replacements for transplantation. Simultaneously, experience from the liver biology community have provided valuable insight into tissue morphogenesis and in vitro stabilization of the hepatocyte phenotype. The next generation of cellular therapies must therefore consider incorporating cell sources and cellular microenvironments that provide both a large population of cells and strategies to maintain liver-specific functions over extended time frames. As cell-based therapies evolve, their success will require contribution from many diverse disciplines including regenerative medicine, developmental biology, and transplant medicine.

Immunohistochemical characterization of canine intestinal epithelial and mesenchymal tumours with a monoclonal antibody to hepatocyte paraffin 1 (Hep Par 1)

Monoclonal antibody hepatocyte paraffin 1 (Hep Par 1) is reported to be highly specific and sensitive for hepatocellular tumours of humans and dogs. However, in a previous study we observed immunoreactivity for Hep Par 1 in an intestinal adenocarcinoma metastatic to the liver. In this paper, we examined normal intestine and 57 canine intestinal tumours including adenocarcinomas of the small and large intestine, colonorectal polyps, mucinous carcinomas, stromal tumours, leiomyosarcomas, fibrosarcoma, plasmacytomas, and osteosarcoma for immunoreactivity for Hep Par 1. Normal intestinal epithelial cells were strongly labelled, particularly at the base of crypts. However, epithelial cells in small intestinal neoplasms had variable reactivity for Hep Par 1. In contrast, normal colonic epithelium was seldom labelled, but hyperplastic or neoplastic colonic epithelium was usually reactive to Hep Par 1. Five of seven small intestinal adenocarcinomas, two of three large intestinal adenocarcinomas, two of three mucinous adenocarcinomas, and all (29) rectal polyps were positive for Hep Par 1. The staining was diffuse, granular and cytoplasmic. The only non-epithelial tumours positive for Hep Par 1 were two of four leiomyosarcomas. Hep Par 1 can be used to differentiate hepatocellular from biliary neoplasms but its value in differentiating metastatic tumours to the liver is questionable due to the reactivity of Hep Par 1 in many intestinal tumours.

Hepatic stem cells

The liver in an adult healthy body maintains a balance between cell gain and cell loss. Though normally proliferatively quiescent, hepatocyte loss such as that caused by partial hepatectomy, uncomplicated by virus infection or inflammation, invokes a rapid regenerative response to restore liver mass. This restoration of moderate cell loss and 'wear and tear' renewal is largely achieved by hepatocyte self-replication. Furthermore, hepatocyte transplants in animals have shown that a certain proportion of hepatocytes can undergo significant clonal expansion, suggesting that hepatocytes themselves are the functional stem cells of the liver. More severe liver injury can activate a potential stem cell compartment located within the intrahepatic biliary tree, giving rise to cords of bipotential so-called oval cells within the lobules that can differentiate into hepatocytes and biliary epithelial cells. A third population of stem cells with hepatic potential resides in the bone marrow; these haematopoietic stem cells can contribute to the albeit low renewal rate of hepatocytes, make a more significant contribution to regeneration, and even completely restore normal function in a murine model of hereditary tyrosinaemia. How these three stem cell populations integrate together to achieve a homeostatic balance is not known. This review focuses on two major aspects of liver stem cell biology: firstly, the identity of the liver stem cells, and secondly, their potential value in the treatment of major liver disease.

Adult stem cell plasticity

Observations made in the last few years support the existence of pathways, in adult humans and rodents, that allow adult stem cells to be surprisingly flexible in their differentiation repertoires. Termed plasticity, this property allows adult stem cells, assumed, until now, to be committed to generating a fixed range of progeny, to switch, when they have been relocated, to make other specialized sets of cells appropriate to their new niche. Reprogramming of some adult stem cells can occur in vivo; the stem cells normally resident in bone marrow appear particularly flexible and are able to contribute usefully to multiple recipient organs. This process produces cells with specialized structural and metabolic adaptations commensurate with their new locations. In a few examples, the degree of support is sufficient to assist or even rescue recipient mice from genetic defects. Some studies provide evidence for the expansion of the reprogrammed cells locally, but in most it remains possible that cells arrive and redifferentiate, but are no longer stem cells. Nevertheless, the fact that appropriately differentiated cells are delivered deep within organs simply by injection of bone marrow cells should make us think differently about the way that organs regenerate and repair. Migratory pathways for stem cells in adult organisms may exist that could be exploited to effect repairs using an individual's own stem cells, perhaps after gene therapy. Logical extensions of this concept are that a transplanted organ would become affected by the genetic susceptibilities of the recipient, alleles that re-express themselves via marrow-derived stem cells, and that plasticity after bone marrow transplantation would also transfer different phenotypes, affecting important parameters such as susceptibility to long-term complications of diabetes, or the ability to metabolize drugs in the liver. This article reviews some of the evidence for stem cell plasticity in rodents and man.

Gastrointestinal stem cells

Turnover of the epithelial cell lineages within the gastrointestinal tract is a constant process, occurring every 2-7 days under normal homeostasis and increasing after damage. This process is regulated by multipotent stem cells, which give rise to all gastrointestinal epithelial cell lineages and can regenerate whole intestinal crypts and gastric glands. The stem cells of the gastrointestinal tract are as yet undefined, although it is generally agreed that they are located within a 'niche' in the intestinal crypts and gastric glands. Studies of allophenic tetraparental chimeric mice and targeted stem cell mutations suggest that a single stem cell undergoes asymmetrical division to produce an identical daughter cell, and thus replicate itself, and a committed progenitor cell which further differentiates into an adult epithelial cell type. The discovery of stem cell plasticity in many tissues, including the ability of transplanted bone marrow to transdifferentiate into intestinal subepithelial myofibroblasts, provides a potential use of bone marrow cells to deliver therapeutic genes to damaged tissues, for example, in treatment of mesenchymal diseases in the gastrointestinal tract, such as fibrosis and Crohn's disease. Studies are beginning to identify the molecular pathways that regulate stem cell proliferation and differentiation into adult gastrointestinal cell lineages, such as the Wnt and Notch/Delta signalling pathways, and the importance of mesenchymal-epithelial interactions in normal gastrointestinal epithelium and in development and disease.

Recent developments on human cell lines for the bioartificial liver

Most bioartificial liver (BAL) devices contain porcine primary hepatocytes as their biological component. However, alternatives are needed due to xenotransplantation associated risks. Human liver cell lines have excellent growth characteristics and are therefore candidates for application in BAL devices. Tumour-derived cell lines HepG2 and C3A express a variety of liver functions, but some specific liver functions, like ammonia detoxification and ureagenesis are insufficient. Immortalised human hepatocytes might offer better prospects. The balance between immortalisation and transformation with dedifferentiation of cells seems controllable by conditional immortalisation and/or the use of telomerase as immortalising agent. Another promising approach will be the use of embryonic or adult human stem cells. Rodent stem cells have been directed to hepatic differentiation in vitro, which might be applicable to human stem cells. However, both functionality and safety of immortalised human liver cell lines and differentiated stem cells should be improved before successful use in BAL devices becomes reality.

How cells change their phenotype

Recent attention has focused on the remarkable ability of adult stem cells to produce differentiated cells from embryologically unrelated tissues. This phenomenon is an example of metaplasia and shows that embryological commitments can be reversed or erased under certain circumstances. In some cases, even fully differentiated cells can change their phenotype (transdifferentiation). This review examines recently discovered cases of metaplasia, and speculates on the potential molecular and cellular mechanisms that underlie the switches, and their significance to developmental biology and medicine.

Stem cells: hype and reality

This update discusses what is known regarding embryonic and adult tissue-derived pluripotent stem cells, including the mechanisms underlying self-renewal without senescence, differentiation in multiple cell types both in vitro and in vivo, and future potential clinical uses of such stem cells. In Section I, Dr. Lansdorp reviews the structure and function of telomerase, the enzyme that restores telomeric ends of chromosomes upon cell division, highly present in embryonic stem cells but not adult stem cells. He discusses the structure and function of telomerase and signaling pathways activated by the enzyme, with special emphasis on normal and leukemic hematopoietic stem cells. In Section II, Dr. Pera reviews the present understanding of mammalian pluripotent embryonic stem cells. He discusses the concept of pluripotentiality in its embryonic context, derivation of stem cells from embryonic or fetal tissue, the basic properties of the stem cells, and methods to produce specific types of differentiated cell from stem cells. He examines the potential applications of stem cells in research and medicine and some of the barriers that must be crossed to achieve these goals. In Section III, Dr. Verfaillie reviews the present understanding of pluripotency of adult stem cells. She discusses the concept of stem cell plasticity, a term used to describe the greater potency described by several investigators of adult tissue-derived stem cells, critically reviews the published studies demonstrating stem cell plasticity, and possible mechanisms underlying such plasticity, and examines the possible role of pluripotent adult stem cells in research and medicine.