Journal of Hepatology
Volume 36, Issue 5 , Pages 695-697, May 2002

Liver regeneration: with a little help from marrow

  • Nicolas Ferry

      Affiliations

    • Laboratoire de Thérapie Génique, INSERM ERM 01-05, CHU-Hotel-Dieu, Nantes, France
    • Corresponding Author InformationCorresponding author. Tel.: +33-2-4008-7488
    • ,
  • Michelle Hadchouel

      Affiliations

    • INSERM U347, CHU Kremlin-Bicêtre, Le Kremlin-Bicêtre, France

See Article, pages 653–659

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1. Introduction 

Liver and bone marrow are two organs that have a close relationship. This is obvious during the fetal life, when the stroma of the liver supports proliferation and maturation of both hematopoietic and liver stem cells. After birth, hematopoiesis migrates to the bone marrow and is no longer present in the normal liver, except during some pathologic situations during which extramedullary hepatic hematopoiesis may be reactivated. Therefore, liver and bone marrow are thought to follow separate fates after birth. However, new concepts have emerged during the past 10 years and revolutionized our view of the reciprocal interactions between liver and bone marrow.

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2. Liver as a source of bone marrow 

Liver transplantation experiments revealed that liver grafts contain stem cells capable of repopulating the bone marrow. When supralethally irradiated rats received syngeneic liver grafts, multilineage hematopoietic reconstitution was observed [1]. Subsequently, hematopoietic stem cells were demonstrated to be present in adult mouse liver. These cells expressing particular markers were able to reconstitute bone marrow of lethally irradiated hosts for more than 12 months [2]. The presence of hematopoietic stem cells in the liver is a likely explanation for the long-observed mixed microchimerism of blood cells as well as induction of tolerance after liver transplantation.

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3. Bone marrow as a source of liver cells 

Alternatively, recent studies demonstrated that bone marrow contains stem cells capable of migrating throughout the body and differentiating into various cell types, including liver cells. Almost all liver cell types may originate from bone marrow, although in different settings. Kupffer cells are phagocytic cells present in the liver and their hematopoietic origin has been known for decades. Cholangiocytes, which are epithelial cells lining the bile ducts, may also be derived from bone marrow. Indeed, an intriguing study showed in mice that a single male stem cell transplanted into an irradiated female recipient displayed tremendous differentiating capacity. Male cells of donor origin were present in various epithelial tissues including lung, intestine, skin and cholangiocytes [3]. Cholangiocyte chimerism was also observed in humans after liver transplantation [4]. These studies demonstrated that cholangiocytes may be derived from bone marrow in normal adults in the absence of specific liver injury. More surprisingly, a number of recent reports in mice, rats and humans strengthened the hypothesis that hepatocytes themselves can be derived from bone marrow. The first demonstration was obtained in irradiated female rats from a strain deficient for dipeptidyl peptidase IV (DPPIV) [5]. These animals received bone marrow cells from DPPIV-positive congenic male rats. The authors then stimulated liver regeneration by performing a two-thirds hepatectomy along with administration of a carcinogenic compound, 2-acetyl-aminofluorene, that prevented hepatocyte division. In this setting, proliferation of particular epithelial cells occurs. These cells, termed oval cells, are considered hepatic stem cells able to differentiate along the hepatocyte or biliary lineage. The results obtained by Petersen et al. clearly demonstrated that oval cells arose from hematopoietic precursors and gave rise to male DPPIV-positive hepatocytes [5]. Direct trans-differentiation of hematopoietic cells in hepatocytes was reported by Lagasse et al. after bone marrow transplantation in tyrosinemic fah−/− mice. In this model there is a permanent remodeling of the liver as a consequence of accumulation of the hepatotoxic metabolite, fumaryl-acetoacetate. After bone marrow transplantation of syngeneic purified hematopoietic stem cells from β-galactosidase transgenic mice, clones of β-galactosidase-positive hepatocytes were observed with complete rescue of diseased mice [6]. This suggested that hematopoietic progenitors could give rise to hepatocytes in the absence of carcinogens in a chronic liver disease model. These experiments prompted investigators to look for the presence of the same lineage relationship in humans. To this end, several groups studied retrospectively liver biopsies obtained after cross-gender transplantation of bone marrow and liver. Donor and recipient cells were distinguished using in situ hybridization of Y-chromosome probes. A significant proportion of extrahepatic derived hepatocytes was consistently found in female livers and reached 40% in one published series [7], [8]. This reflects the possibility that particular stem cells present in the bone marrow could trans-differentiate into hepatocytes that normally arise from a non-mesenchymal embryonic origin. Finally, a recent study pointed to the ability of bone marrow cells to give rise to liver endothelial cells [9]. Once again, this result was obtained after cross-gender liver transplantation in humans or bone marrow transplantation in mice with subsequent detection of Y chromosomes in liver endothelial cells. Surprisingly, in contrast to other studies, the authors did not mention the presence of Y-positive cholangiocytes in this setting. Altogether, there is now sufficient evidence that, depending on the presence of liver injury and irradiation, bone marrow cells may give rise to different liver cell types including Kupffer cells, cholangiocytes, endothelial cells and hepatocytes.

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4. Bone marrow contribution to liver regeneration 

The potential contribution of bone marrow cells to liver regeneration is an important issue that is now addressed by Fujii et al. in this issue [10]. One distinctive feature of the liver is its capacity to regenerate after injury or partial hepatectomy. The mechanisms responsible for the initiation of regeneration are still not fully understood. Two cytokines, interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α) are required for normal liver regeneration and IL-6−/− mice as well as TNF-α receptor knockout mice have a defective liver regeneration [11], [12]. Recent studies suggest that Kupffer cells or cells from bone marrow are the main source of these cytokines [13], [14]. In their study, Fujii et al. studied the contribution of bone marrow cells in liver regeneration. They used lethally irradiated mice that were grafted with bone marrow cells from a transgenic donor that constitutively expresses the marker green fluorescent protein (GFP). Four weeks after bone marrow transplantation, the recipient mice were subjected to partial hepatectomy and the presence of GFP-positive cells in the liver was analyzed by fluorescent microscopy as well as FACS analysis. Four weeks after hepatectomy, GFP-positive cells that co-expressed PECAM-1, a marker of endothelial and Kupffer cells, were present in the liver. Further analysis using flow cytometry as well as phagocytosis of latex microspheres allowed a precise quantification of each particular cell type. Almost 12% of the non-parenchymal cells were GFP-positive after partial hepatectomy. Among them, 70% were endothelial cells and 30% Kupffer cells. Therefore, the use of a marker gene that is easily detected allowed Fujii et al. to quantify trans-differentiation in non-parenchymal cells, an issue that has not been addressed in other studies.

Another interesting finding of this study is that neither cholangiocytes nor hepatocytes expressing GFP were found in regenerating livers. This is divergent from previous studies in which cholangiocytes were present after bone marrow transplantation [3], [4] and confirm that regrowth of the hepatic lobule after hepatectomy results from proliferation of remaining cholangiocytes and hepatocytes. Also, this is in keeping with the hypothesis that generation of hepatocytes from bone marrow requires chronic hepatic injury. Liver regeneration following an acute toxic injury may not trigger a sufficient stimulus to induce significant repopulation of hepatocytes from bone marrow cells (H. Gilgenkrantz, personal communication). Therefore, it appears that the stimuli that promote bone marrow cells to differentiate or trans-differentiate into various liver cell types are not completely defined and deserve further attention. In this respect, Fujii et al. suggest that vascular endothelial growth factor (VEGF) may be one of the key factors involved in migration of endothelial progenitor cells from bone marrow. Although VEGF is involved in vascular regeneration, a complex set of cytokines and growth factors receptors also contributes to revascularization of regenerating liver [15]. Since bone marrow stem cells are now considered as normally circulating cells [16], the real issue should be which of these numerous factors are responsible for making circulating stem cells stop in the liver.

The blossoming of publications showing trans-differentiation of bone marrow cells into other tissues suggest that this phenomenon is real and has been observed using various strategies of cell lineage analysis. Although transfer of genetic material between cells has been described, this mechanism appears unlikely due to the high number of cells observed in many studies including the present one. A new research field is developing that challenges old embryology concepts and presents exciting clinical applications. In extreme situations, the liver may ask for the bone marrow's help by secreting cytokines to improve its own regeneration. This may explain surprising findings such as lower recovery from fulminant hepatitis in patients with aplastic anemia and may lead to new therapeutic strategies using bone marrow stem cells for the treatment of liver diseases.

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Acknowledgements 

We thank Hélène Gilgenkrantz and Mark Haskins for critical reading of the manuscript.

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References 

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PII: S0168-8278(02)00054-5

Journal of Hepatology
Volume 36, Issue 5 , Pages 695-697, May 2002

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