Bone marrow injection stimulates hepatic ductular reactions in the absence of injury via macrophage-mediated TWEAK signaling

Phenotypic and functional characterization of macrophages with therapeutic potential generated from human cirrhotic monocytes in a cohort study

BACKGROUND AIMS

Macrophages have complex roles in the liver. The aim of this study was to compare profiles of human monocyte-derived macrophages between controls and cirrhotic patients, to determine whether chronic inflammation affects precursor number or the phenotype, with the eventual aim to develop a cell therapy for cirrhosis.

METHODS

Infusion of human macrophages in a murine liver fibrosis model demonstrated a decrease in markers of liver injury (alanine transaminase, bilirubin, aspartate transaminase) and fibrosis (transforming growth factor-β, α-smooth muscle actin, phosphatidylserine receptor) and an increase in markers of liver regeneration (matrix metalloproteinases [MMP]-9, MMP-12 and TNF-related weak inducer of apoptosis). CD14+ monocytes were then isolated from controls. Monocytes were matured into macrophages for 7 days using a Good Manufacturing Practice-compatible technique.

RESULTS

There was no significant difference between the mean number of CD14+ monocytes isolated from cirrhotic patients (n = 9) and controls (n = 10); 2.8 ± SEM 0.54 × 10(8) and 2.5 ± 0.56 × 10(8), respectively. The mean yield of mature macrophages cultured was also not significantly different between cirrhotic patients and controls (0.9 × 10(8) ± 0.38 × 10(8), with more than 90% viability and 0.65 × 10(8) ± 0.16 × 10(8), respectively. Maturation to macrophages resulted in up-regulation of a number of genes (MMP-9, CCL2, interleukin [IL]-10 and TNF-related weak inducer of apoptosis). A cytokine and chemokine polymerase chain reaction array, comparing the control and cirrhotic macrophages, revealed no statistically significant differences.

CONCLUSIONS

Macrophages can be differentiated from cirrhotic patients' apheresis-derived CD14 monocytes and develop the same pro-resolution phenotype as control macrophages, indicating their suitability for clinical therapy.

Deletion of Wntless in myeloid cells exacerbates liver fibrosis and the ductular reaction in chronic liver injury

Background

Macrophages play critical roles in liver regeneration, fibrosis development and resolution. They are among the first responders to liver injury and are implicated in orchestrating the fibrogenic response via multiple mechanisms. Macrophages are also intimately associated with the activated hepatic progenitor cell (HPC) niche or ductular reaction that develops in parallel with fibrosis. Among the many macrophage-derived mediators implicated in liver disease progression, a key role for macrophage-derived Wnt proteins in driving pro-regenerative HPC activation towards a hepatocellular fate has been suggested. Wnt proteins, in general, however, have been associated with both pro- and anti-fibrogenic activities in the liver and other organs. We investigated the role of macrophage-derived Wnt proteins in fibrogenesis and HPC activation in murine models of chronic liver disease by conditionally deleting Wntless expression, which encodes a chaperone essential for Wnt protein secretion, in LysM-Cre-expressing myeloid cells (LysM-Wls mice).

Results

Fibrosis and HPC activation were exacerbated in LysM-Wls mice compared to littermate controls, in the absence of an apparent increase in myofibroblast activation or interstitial collagen mRNA expression, in both the TAA and CDE models of chronic liver disease. Increased Epcam mRNA levels paralleled the increased HPC activation and more mature ductular reactions, in LysM-Wls mice. Increased Epcam expression in LysM-Wls HPC was also observed, consistent with a more cholangiocytic phenotype. No differences in the mRNA expression levels of key pro-inflammatory and pro-fibrotic cytokines or the macrophage-derived HPC mitogen, Tweak, were observed. LysM-Wls mice exhibited increased expression of Timp1, encoding the key Mmp inhibitor Timp1 that blocks interstitial collagen degradation, and, in the TAA model, reduced expression of the anti-fibrotic matrix metalloproteinases, Mmp12 and Mmp13, suggesting a role for macrophage-derived Wnt proteins in restraining fibrogenesis during ongoing liver injury.

Conclusion

In summary, these data suggest that macrophage-derived Wnt proteins possess anti-fibrogenic potential in chronic liver disease, which may be able to be manipulated for therapeutic benefit.

Electronic supplementary material

The online version of this article (doi:10.1186/s13069-015-0036-7) contains supplementary material, which is available to authorized users.

Molecular and Cellular Functions Distinguish Superior Therapeutic Efficiency of Bone Marrow CD45 Cells Over Mesenchymal Stem Cells in Liver Cirrhosis

Liver fibrosis is strongly associated with chronic inflammation. As an alternative to conventional treatments for fibrosis, mesenchymal stem cells (MSCs) therapy is found to be attractive due to its immunomodulatory functions. However, low survival rate and profibrogenic properties of MSCs remain the major concerns, leading to skepticism in many investigators. Here, we have asked the question whether bone marrow (BM)-derived CD45 cells is the better candidate than MSCs to treat fibrosis, if so, what are the molecular mechanisms that make such distinction. Using CCl4 -induced liver fibrosis mouse model of a Metavir fibrosis score 3, we showed that BM-CD45 cells have better antifibrotic effect than adipose-derived (AD)-MSCs. In fact, our study revealed that antifibrotic potential of CD45 cells are compromised by the presence of MSCs. This difference was apparently due to significantly high level expressions of matrix metalloproteinases-9 and 13, and the suppression of hepatic stellate cells' (HpSCs) activation in the CD45 cells transplantation group. Mechanism dissection studied in vitro supported the above opposing results and revealed that CD45 cell-secreted FasL induced apoptotic death of activated HpSCs. Further analyses suggest that MSC-secreted transforming growth factor β and insulin-like growth factor-1 promoted myofibroblastic differentiation of HpSCs and their proliferation. Additionally, the transplantation of CD45 cells led to functional improvement of the liver through repair and regeneration. Thus, BM-derived CD45 cells appear as a superior candidate for the treatment of liver fibrosis due to structural and functional improvement of CCl4 -induced fibrotic liver, which were much lower in case of AD-MSC therapy.

Kupffer cell-monocyte communication is essential for initiating murine liver progenitor cell-mediated liver regeneration

Liver progenitor cells (LPCs) are necessary for repair in chronic liver disease because the remaining hepatocytes cannot replicate. However, LPC numbers also correlate with disease severity and hepatocellular carcinoma risk. Thus, the progenitor cell response in diseased liver may be regulated to optimize liver regeneration and minimize the likelihood of tumorigenesis. How this is achieved is currently unknown. Human and mouse diseased liver contain two subpopulations of macrophages with different ontogenetic origins: prenatal yolk sac-derived Kupffer cells and peripheral blood monocyte-derived macrophages. We examined the individual role(s) of Kupffer cells and monocyte-derived macrophages in the induction of LPC proliferation using clodronate liposome deletion of Kupffer cells and adoptive transfer of monocytes, respectively, in the choline-deficient, ethionine-supplemented diet model of liver injury and regeneration. Clodronate liposome treatment reduced initial liver monocyte numbers together with the induction of injury and LPC proliferation. Adoptive transfer of monocytes increased the induction of liver injury, LPC proliferation, and tumor necrosis factor-α production.

CONCLUSION

Kupffer cells control the initial accumulation of monocyte-derived macrophages. These infiltrating monocytes are in turn responsible for the induction of liver injury, the increase in tumor necrosis factor-α, and the subsequent proliferation of LPCs.

M1 Muscarinic Receptor Deficiency Attenuates Azoxymethane-Induced Chronic Liver Injury in Mice

Cholinergic nervous system regulates liver injury. However, the role of M1 muscarinic receptors (M1R) in modulating chronic liver injury is uncertain. To address this gap in knowledge we treated M1R-deficient and WT mice with azoxymethane (AOM) for six weeks and assessed liver injury responses 14 weeks after the last dose of AOM. Compared to AOM-treated WT mice, M1R-deficient mice had attenuated liver nodularity, fibrosis and ductular proliferation, α-SMA staining, and expression of α1 collagen, Tgfβ-R, Pdgf-R, Mmp-2, Timp-1 and Timp-2. In hepatocytes, these findings were associated with reductions of cleaved caspase-3 staining and Tnf-α expression. In response to AOM treatment, M1R-deficient mice mounted a vigorous anti-oxidant response by upregulating Gclc and Nqo1 expression, and attenuating peroxynitrite generation. M1R-deficient mouse livers had increased expression of Trail-R2, a promotor of stellate cell apoptosis; dual staining for TUNNEL and α-SMA revealed increased stellate cells apoptosis in livers from M1R-deficient mice compared to those from WT. Finally, pharmacological inhibition of M1R reduced H₂O₂-induced hepatocyte apoptosis in vitro. These results indicate that following liver injury, anti-oxidant response in M1R-deficient mice attenuates hepatocyte apoptosis and reduces stellate cell activation, thereby diminishing fibrosis. Therefore, targeting M1R expression and activation in chronic liver injury may provide therapeutic benefit.

Hepatic progenitor cells of biliary origin with liver repopulation capacity

Hepatocytes and cholangiocytes self-renew following liver injury. Following severe injury hepatocytes are increasingly senescent, but whether hepatic progenitor cells (HPCs) then contribute to liver regeneration is unclear. Here, we describe a mouse model where the E3 ubiquitin ligase Mdm2 is inducibly deleted in more than 98% of hepatocytes, causing apoptosis, necrosis and senescence with nearly all hepatocytes expressing p21. This results in florid HPC activation, which is necessary for survival, followed by complete, functional liver reconstitution. HPCs isolated from genetically normal mice, using cell surface markers, were highly expandable and phenotypically stable in vitro. These HPCs were transplanted into adult mouse livers where hepatocyte Mdm2 was repeatedly deleted, creating a non-competitive repopulation assay. Transplanted HPCs contributed significantly to restoration of liver parenchyma, regenerating hepatocytes and biliary epithelia, highlighting their in vivo lineage potency. HPCs are therefore a potential future alternative to hepatocyte or liver transplantation for liver disease.

Pathogenesis and prognosis of hepatocellular carcinoma at the cellular and molecular levels

Different approaches predict the outcome for patients with hepatocellular carcinoma (HCC). The expression of biliary-hepatic progenitor cell markers generally correlates with poor prognosis. This article focuses on the pathogenesis of HCC, how differentiation or dedifferentiation leads to a phenotype switch, and heterogeneity in the same tumor. A tumor cell decides its fate based on a complex interplay of signaling pathways. Interaction with the microenvironment decides whether it will invade, proliferate, or enter survival mode. Several signaling pathways contribute to stemness features, reflecting a small chemoresistant subpopulation of the tumor that expresses biliary-hepatic progenitor cell markers.

Cell therapy for liver disease: From liver transplantation to cell factory

Work over several decades has laid solid foundations for the advancement of liver cell therapy. To date liver cell therapy in people has taken the form of hepatocyte transplantation for metabolic disorders with a hepatic basis, and for acute or chronic liver failure. Although clinical trials using various types of autologous cells have been implemented to promote liver regeneration or reduce liver fibrosis, clear evidence of therapeutic benefits have so far been lacking. Cell types that have shown efficacy in preclinical models include hepatocytes, liver sinusoidal endothelial cells, mesenchymal stem cells, endothelial progenitor cells, and macrophages. However, positive results in animal models have not always translated through to successful clinical therapies and more realistic preclinical models need to be developed. Studies defining the optimal repopulation by transplanted cells, including routes of cell transplantation, superior engraftment and proliferation of transplanted cells, as well as optimal immunosuppression regimens are required. Tissue engineering approaches to transplant cells in extrahepatic locations have also been proposed. The derivation of hepatocytes from pluripotent or reprogrammed cells raises hope that donor organ and cell shortages could be overcome in the future. Critical hurdles to be overcome include the production of hepatocytes from pluripotent cells with equal functional capacity to primary hepatocytes and long-term phenotypic stability in vivo.

Role of liver progenitors in liver regeneration

During massive liver injury and hepatocyte loss, the intrinsic regenerative capacity of the liver by replication of resident hepatocytes is overwhelmed. Treatment of this condition depends on the cause of liver injury, though in many cases liver transplantation (LT) remains the only curative option. LT for end stage chronic and acute liver diseases is hampered by shortage of donor organs and requires immunosuppression. Hepatocyte transplantation is limited by yet unresolved technical difficulties. Since currently no treatment is available to facilitate liver regeneration directly, therapies involving the use of resident liver stem or progenitor cells (LPCs) or non-liver stem cells are coming to fore. LPCs are quiescent in the healthy liver, but may be activated under conditions where the regenerative capacity of mature hepatocytes is severely impaired. Non-liver stem cells include embryonic stem cells (ES cells) and mesenchymal stem cells (MSCs). In the first section, we aim to provide an overview of the role of putative cytokines, growth factors, mitogens and hormones in regulating LPC response and briefly discuss the prognostic value of the LPC response in clinical practice. In the latter section, we will highlight the role of other (non-liver) stem cells in transplantation and discuss advantages and disadvantages of ES cells, induced pluripotent stem cells (iPS), as well as MSCs.

EpCAM and the biology of hepatic stem/progenitor cells

Epithelial cell adhesion molecule (EpCAM) is a transmembrane glycoprotein, which is frequently and highly expressed on carcinomas, tumor-initiating cells, selected tissue progenitors, and embryonic and adult stem cells. During liver development, EpCAM demonstrates a dynamic expression, since it can be detected in fetal liver, including cells of the parenchyma, whereas mature hepatocytes are devoid of EpCAM. Liver regeneration is associated with a population of EpCAM-positive cells within ductular reactions, which gradually lose the expression of EpCAM along with maturation into hepatocytes. EpCAM can be switched on and off through a wide panel of strategies to fine-tune EpCAM-dependent functional and differentiative traits. EpCAM-associated functions relate to cell-cell adhesion, proliferation, maintenance of a pluripotent state, regulation of differentiation, migration, and invasion. These functions can be conferred by the full-length protein and/or EpCAM-derived fragments, which are generated upon regulated intramembrane proteolysis. Control by EpCAM therefore not only depends on the presence of full-length EpCAM at cellular membranes but also on varying rates of the formation of EpCAM-derived fragments that have their own regulatory properties and on changes in the association of EpCAM with interaction partners. Thus spatiotemporal localization of EpCAM in immature liver progenitors, transit-amplifying cells, and mature liver cells will decisively impact the regulation of EpCAM functions and might be one of the triggers that contributes to the adaptive processes in stem/progenitor cell lineages. This review will summarize EpCAM-related molecular events and how they relate to hepatobiliary differentiation and regeneration.

Role of Fn14 in acute alcoholic steatohepatitis in mice

TNF-like weak inducer of apoptosis (TWEAK) is a growth factor for bipotent liver progenitors that express its receptor, fibroblast growth factor-inducible 14 (Fn14), a TNF receptor superfamily member. Accumulation of Fn14(+) progenitors occurs in severe acute alcoholic steatohepatitis (ASH) and correlates with acute mortality. In patients with severe ASH, inhibition of TNF-α increases acute mortality. The aim of this study was to determine whether deletion of Fn14 improves the outcome of liver injury in alcohol-consuming mice. Wild-type (WT) and Fn14 knockout (KO) mice were fed control high-fat Lieber deCarli diet or high-fat Lieber deCarli diet with 2% alcohol (ETOH) and injected intraperitoneally with CCl₄ for 2 wk to induce liver injury. Mice were euthanized 3 or 10 days after CCl₄ treatment. Survival was assessed. Liver tissues were analyzed for cell death, inflammation, proliferation, progenitor accumulation, and fibrosis by quantitative RT-PCR, immunoblot, hydroxyproline content, and quantitative immunohistochemistry. During liver injury, Fn14 expression, apoptosis, inflammation, hepatocyte replication, progenitor and myofibroblast accumulation, and fibrosis increased in WT mice fed either diet. Mice fed either diet expressed similar TWEAK/Fn14 levels, but ETOH-fed mice had higher TNF-α expression. The ETOH-fed group developed more apoptosis, inflammation, fibrosis, and regenerative responses. Fn14 deletion did not reduce hepatic TNF-α expression but improved all injury parameters in mice fed the control diet. In ETOH-fed mice, Fn14 deletion inhibited TNF-α induction and increased acute mortality, despite improvement in liver injury. Fn14 mediates wound-healing responses that are necessary to survive acute liver injury during alcohol exposure.

Galectin-3 regulates hepatic progenitor cell expansion during liver injury

OBJECTIVE

Following chronic liver injury or when hepatocyte proliferation is impaired, ductular reactions containing hepatic progenitor cells (HPCs) appear in the periportal regions and can regenerate the liver parenchyma. HPCs exist in a niche composed of myofibroblasts, macrophages and laminin matrix. Galectin-3 (Gal-3) is a β-galactoside-binding lectin that binds to laminin and is expressed in injured liver in mice and humans.

DESIGN

We examined the role of Gal-3 in HPC activation. HPC activation was studied following dietary induced hepatocellular (choline-deficient ethionine-supplemented diet) and biliary (3,5-diethoxycarbonyl-1,4-dihydrocollidine supplemented diet) injury in wild type and Gal-3(-/-) mice.

RESULTS

HPC proliferation was significantly reduced in Gal-3(-/-) mice. Gal-3(-/-) mice failed to form a HPC niche, with reduced laminin formation. HPCs isolated from wild type mice secrete Gal-3 which enhanced adhesion and proliferation of HPCs on laminin in an undifferentiated form. These effects were attenuated in Gal3(-/-) HPCs and in wild type HPCs treated with the Gal-3 inhibitor lactose. Gal-3(-/-) HPCs in vitro showed increased hepatocyte function and prematurely upregulated both biliary and hepatocyte differentiation markers and regulated cell cycle genes leading to arrest in G0/G1.

CONCLUSIONS

We conclude that Gal-3 is required for the undifferentiated expansion of HPCs in their niche in injured liver.

Advances in hepatic stem/progenitor cell biology

The liver is famous for its strong regenerative capacity, employing different modes of regeneration according to type and extent of injury. Mature liver cells are able to proliferate in order to replace the damaged tissue allowing the recovery of the parenchymal function. In more severe scenarios hepatocytes are believed to arise also from a facultative liver progenitor cell compartment. In human, severe acute liver failure and liver cirrhosis are also both important clinical targets in which regeneration is impaired, where the role of this stem cell compartment seems more convincing. In animal models, the current state of ambiguity regarding the identity and role of liver progenitor cells in liver physiology dampens the enthusiasm for the potential use of these cells in regenerative medicine. The aim of this review is to give the basics of liver progenitor cell biology and discuss recent results vis-à-vis their identity and contribution to liver regeneration.

Hypoxia inducible factors in liver disease and hepatocellular carcinoma: current understanding and future directions

Hypoxia inducible transcription factors (HIFs) activate diverse pathways that regulate cellular metabolism, angiogenesis, proliferation, and migration, enabling a cell to respond to a low oxygen or hypoxic environment. HIFs are regulated by oxygen-dependent and independent signals including: mitochondrial dysfunction, reactive oxygen species, endoplasmic reticular stress, and viral infection. HIFs have been reported to play a role in the pathogenesis of liver disease of diverse aetiologies. This review explores the impact of HIFs on hepatocellular biology and inflammatory responses, highlighting the therapeutic potential of targeting HIFs for an array of liver pathologies.

Hepatic progenitor cells in liver regeneration: current advances and clinical perspectives

When there is a massive loss of hepatocytes and/or an inhibition in the proliferative capacity of the mature hepatocytes, activation of a dormant cell population of resident hepatic progenitor cells (HPCs) occurs. Depending on the type of liver damage HPCs generate new hepatocytes and biliary cells to repopulate the liver placing them as potential candidates for cell therapy in human liver failure. Liver injury specific mechanisms through which HPCs differentiate towards mature epithelial cell types are recently become understood. Such new insights will enable us not only to direct HPCs behaviour for therapeutic purposes, but also to develop clinically feasible methods for in vivo differentiation of other stem cell types towards functional hepatocytes. This review aimed to provide the current improved knowledge of the role of HPCs niche and its signals in directing the behaviour and fate of HPCs and to translate this basic knowledge of HPCs activation/differentiation into its clinical applications.

Polysialic acid/neural cell adhesion molecule modulates the formation of ductular reactions in liver injury

In severe liver injury, ductular reactions (DRs) containing bipotential hepatic progenitor cells (HPCs) branch from the portal tract. Neural cell adhesion molecule (NCAM) marks bile ducts and DRs, but not mature hepatocytes. NCAM mediates interactions between cells and surrounding matrix; however, its role in liver development and regeneration is undefined. Polysialic acid (polySia), a unique posttranslational modifier of NCAM, is produced by the enzymes, ST8SiaII and ST8SiaIV, and weakens NCAM interactions. The role of polySia with NCAM synthesizing enzymes ST8SiaII and ST8SiaIV were examined in HPCs in vivo using the choline-deficient ethionine-supplemented and 3,5-diethoxycarbonyl-1,4-dihydrocollidine diet models of liver injury and regeneration, in vitro using models of proliferation, differentiation, and migration, and by use of mouse models with gene defects in the polysialyltransferases (St8sia 2+/-4+/-, and St8sia2-/-4-/-). We show that, during liver development, polySia is required for the correct formation of bile ducts because gene defects in both the polysialyltransferases (St8sia2+/-4+/- and St8sia2-/-4-/- mice) caused abnormal bile duct development. In normal liver, there is minimal polySia production and few ductular NCAM+ cells. Subsequent to injury, NCAM+ cells expand and polySia is produced by DRs/HPCs through ST8SiaIV. PolySia weakens cell-cell and cell-matrix interactions, facilitating HGF-induced migration. Differentiation of HPCs to hepatocytes in vitro results in both transcriptional down-regulation of polySia and cleavage of polySia-NCAM. Cleavage of polySia by endosialidase (endoN) during liver regeneration reduces migration of DRs into parenchyma.

CONCLUSION

PolySia modification of NCAM+ ductules weakens cell-cell and cell-matrix interactions, allowing DRs/HPCs to migrate for normal development and regeneration. Modulation of polySia levels may provide a therapeutic option in liver regeneration.

Preparing the ground for tissue regeneration: from mechanism to therapy

Chronic diseases confer tissue and organ damage that reduce quality of life and are largely refractory to therapy. Although stem cells hold promise for treating degenerative diseases by 'seeding' injured tissues, the regenerative capacity of stem cells is influenced by regulatory networks orchestrated by local immune responses to tissue damage, with macrophages being a central component of the injury response and coordinator of tissue repair. Recent research has turned to how cellular and signaling components of the local stromal microenvironment (the 'soil' to the stem cells' seed), such as local inflammatory reactions, contribute to successful tissue regeneration. This Review discusses the basic principles of tissue regeneration and the central role locally acting components may play in the process. Application of seed-and-soil concepts to regenerative medicine strengthens prospects for developing cell-based therapies or for promotion of endogenous repair.

Myofibroblastic cells function as progenitors to regenerate murine livers after partial hepatectomy

OBJECTIVE

Smoothened (SMO), a coreceptor of the Hedgehog (Hh) pathway, promotes fibrogenic repair of chronic liver injury. We investigated the roles of SMO+ myofibroblast (MF) in liver regeneration by conditional deletion of SMO in α smooth muscle actin (αSMA)+ cells after partial hepatectomy (PH).

DESIGN

αSMA-Cre-ER(T2)×SMO/flox mice were treated with vehicle (VEH) or tamoxifen (TMX), and sacrificed 24-96 h post-PH. Regenerating livers were analysed for proliferation, progenitors and fibrosis by qRT-PCR and quantitative immunohistochemistry (IHC). Results were normalised to liver segments resected at PH. For lineage-tracing studies, αSMA-Cre-ER(T2)×ROSA-Stop-flox-yellow fluorescent protein (YFP) mice were treated with VEH or TMX; livers were stained for YFP, and hepatocytes isolated 48 and 72 h post-PH were analysed for YFP by flow cytometric analysis (FACS).

RESULTS

Post-PH, VEH-αSMA-SMO mice increased expression of Hh-genes, transiently accumulated MF, fibrosis and liver progenitors, and ultimately exhibited proliferation of hepatocytes and cholangiocytes. In contrast, TMX-αSMA-SMO mice showed loss of whole liver SMO expression, repression of Hh-genes, enhanced accumulation of quiescent HSC but reduced accumulation of MF, fibrosis and progenitors, as well as inhibition of hepatocyte and cholangiocyte proliferation, and reduced recovery of liver weight. In TMX-αSMA-YFP mice, many progenitors, cholangiocytes and up to 25% of hepatocytes were YFP+ by 48-72 h after PH, indicating that liver epithelial cells were derived from αSMA-YFP+ cells.

CONCLUSIONS

Hh signalling promotes transition of quiescent hepatic stellate cells to fibrogenic MF, some of which become progenitors that regenerate the liver epithelial compartment after PH. Hence, scarring is a component of successful liver regeneration.

Bone marrow stem cell therapy for liver disease

Liver disease is a rising cause of mortality and morbidity, and treatment options remain limited. Liver transplantation is curative but limited by donor organ availability, operative risk and long-term complications. The contribution of bone marrow (BM)-derived stem cells to tissue regeneration has been recognised and there is considerable interest in the potential benefits of BM stem cells in patients with liver disease. In chronic liver disease, deposition of fibrous scar tissue inhibits hepatocyte proliferation and leads to portal hypertension. Although initial reports had suggested transdifferentiation of stem cells into hepatocytes, the beneficial effects of BM stem cells are more likely derived from the ability to breakdown scar tissue and stimulate hepatocyte proliferation. Studies in animal models have yielded promising results, although the exact mechanisms and cell type responsible have yet to be determined. Small-scale clinical studies have quickly followed and, although primarily designed to examine safety and feasibility of this approach, have reported improvements in liver function in treated patients. Well-designed, controlled studies are required to fully determine the benefits of BM stem cell therapy.

Hematopoietic stem cells and liver regeneration: differentially acting hematopoietic stem cell mobilization agents reverse induced chronic liver injury

Bone marrow (BM) could serve as a source of cells facilitating liver repopulation in case of hepatic damage. Currently available hematopoietic stem cell (HSC) mobilizing agents, were comparatively tested for healing potential in liver fibrosis. Carbon tetrachloride (CCl4)-injured mice previously reconstituted with Green Fluorescent Protein BM were mobilized with Granulocyte-Colony Stimulating Factor (G-CSF), Plerixafor or G-CSF+Plerixafor. Hepatic fibrosis, stellate cell activation and oval stem cell frequency were measured by Gomori and by immunohistochemistry for a-Smooth Muscle Actin and Cytokeratin-19, respectively. Angiogenesis was evaluated by ELISA and immunohistochemistry. Quantitative real-time PCR was used to determine the mRNA levels of liver Peroxisome Proliferator-Activated Receptor gamma (PPAR-γ), Interleukin-6 (IL-6) and Tumor Necrosis-alpha (TNFα). BM-derived cells were tracked by double immunofluorescence. The spontaneous migration of mobilized HSCs towards injured liver and its cytokine secretion profile was determined in transwell culture systems. Either single-agent mobilization or the combination of agents significantly ameliorated hepatic damage by decreasing fibrosis and restoring the abnormal vascular network in the liver of mobilized mice compared to CCl4-only mice. The degree of fibrosis reduction was similar among all mobilized mice despite that G-CSF+Plerixafor yielded significantly higher numbers of circulating HSCs over other agents. The liver homing potential of variously mobilized HSCs differed among the agents. An extended G-CSF treatment provided the highest anti-fibrotic effect over all tested modalities, induced by the proliferation of hepatic stem cells and decreased hepatic inflammation. Plerixafor-mobilized HSCs, despite their reduced liver homing potential, reversed fibrosis mainly by increasing hepatic PPAR-γ and VEGF expression. In all groups, BM-derived mature hepatocytes as well as liver-committed BM stem cells were detected only at low frequencies, further supporting the concept that alternative mechanisms rather than direct HSC effects regulate liver recovery. Overall, our data suggest that G-CSF, Plerixafor and G-CSF+Plerixafor act differentially during the wound healing process, ultimately providing a potent anti-fibrotic effect.

Regenerative medicine. Knocking on the door to successful hepatocyte transplantation

With the ongoing shortage of livers available for transplantation, attention has turned to cell-based approaches to support liver function and enable liver regeneration. However, hepatocyte transplantation is beset with problems and a clinically adoptable strategy is lacking. How can a plentiful supply of hepatocyte-like cells with long-term proliferation be generated?

The portal inflammatory infiltrate and ductular reaction in human nonalcoholic fatty liver disease

Although nonalcoholic fatty liver disease (NAFLD) is conventionally assessed histologically for lobular features of inflammation, development of portal fibrosis appears to be associated with disease progression. We investigated the composition of the portal inflammatory infiltrate and its relationship to the ductular reaction (DR), a second portal phenomenon implicated in fibrogenesis. The portal inflammatory infiltrate may contribute directly to fibrogenesis as well as influence the fate of the DR hepatic progenitor cells (HPCs), regulating the balance between liver repair and fibrosis. The presence of portal inflammation in NAFLD was strongly correlated with disease severity (fibrosis stage) and the DR. The portal infiltrate was characterized by immunostaining NAFLD liver biopsy sections (n = 33) for broad leukocyte subset markers (CD68, CD3, CD8, CD4, CD20, and neutrophil elastase) and selected inflammatory markers (matrix metalloproteinase 9 and interleukin [IL]-17). Cells expressing all markers examined were identified throughout the liver lobules and in portal tracts, although portal tracts were more densely populated (P < 0.01), and dominated by CD68(+) macrophages and CD8(+) lymphocytes, at all stages of disease. An increase in portal macrophages in NAFLD patients with steatosis alone (P < 0.01) was the earliest change detected, even before elevated expression of the proinflammatory cytokines, IL1B and TNF, in patients with early NASH (P < 0.05). Portal and periductal accumulation of all other cell types examined occurred in progressed NASH (all P < 0.05).

CONCLUSION

Knowledge of the complex cellular composition of the portal inflammatory infiltrate and HPC/DR niche in NAFLD will shape future functional studies to elucidate the contribution of portal inflammation to HPC differentiation and NAFLD pathogenesis.

Liver regeneration by stem/progenitor cells

The liver is renowned for its strong, robust regenerative capacity, employing different modes of regeneration according to type and extent of injury. The process of compensatory hypertrophy of the liver upon partial hepatectomy has been standing as a classical model for studying organ regeneration in mammals and a subject of exhaustive analyses. Meanwhile, in view of the physiological relevance for many of the human liver pathologies induced upon toxic insults or hepatitis, other injury models have recently drawn increasing attention. In those damaged livers where hepatocyte proliferation is compromised, adult liver stem/progenitor cells (LPCs) are activated and differentiate to hepatocytes and cholangiocytes, leading to functional recovery of the organ. Here, we summarize and discuss recent findings on the mechanisms underlying the regeneration process of the liver. Whereas the primary focus of this article is on those related to LPC-mediated regeneration, we also introduce topics on compensatory hypertrophy, where application of new technologies and molecular genetics approaches in mice has gained a paradigm shift. Identification of various markers for LPC populations has expedited their characterization and enabled us to examine their differentiation potential in vivo using genetic lineage-tracing approaches. Comprehensive studies regarding intercellular signaling pathways and their modes of action have succeeded in elucidating novel frameworks for the LPC-niche interaction functioning in the regenerating liver.

CONCLUSION

Advancing our understanding of the cellular and molecular mechanisms for liver regeneration should provide a basis for developing therapeutic strategies to treat patients with liver disease.

TWEAK and LTβ Signaling during Chronic Liver Disease

Chronic liver diseases (CLD) such as hepatitis B and C virus infection, alcoholic liver disease, and non-alcoholic steatohepatitis are associated with hepatocellular necrosis, continual inflammation, and hepatic fibrosis. The induced microenvironment triggers the activation of liver-resident progenitor cells (LPCs) while hepatocyte replication is inhibited. In the early injury stages, LPCs regenerate the liver by proliferation, migration to sites of injury, and differentiation into functional biliary epithelial cells or hepatocytes. However, when this process becomes dysregulated, wound healing can progress to pathological fibrosis, cirrhosis, and eventually hepatocellular carcinoma. The other key mediators in the pathogenesis of progressive CLD are fibrosis-driving, activated hepatic stellate cells (HSCs) that usually proliferate in very close spatial association with LPCs. Recent studies from our group and others have suggested the potential for cytokine and chemokine cross-talk between LPCs and HSCs, which is mainly driven by the tumor necrosis factor (TNF) family members, TNF-like weak inducer of apoptosis (TWEAK) and lymphotoxin-β, potentially dictating the pathological outcomes of chronic liver injury.

TWEAK/Fn14 signaling is required for liver regeneration after partial hepatectomy in mice

Background & Aims

Pro-inflammatory cytokines are important for liver regeneration after partial hepatectomy (PH). Expression of Fibroblast growth factor-inducible 14 (Fn14), the receptor for TNF-like weak inducer of apoptosis (TWEAK), is induced rapidly after PH and remains elevated throughout the period of peak hepatocyte replication. The role of Fn14 in post-PH liver regeneration is uncertain because Fn14 is expressed by liver progenitors and TWEAK-Fn14 interactions stimulate progenitor growth, but replication of mature hepatocytes is thought to drive liver regeneration after PH.

Methods

To clarify the role of TWEAK-Fn14 after PH, we compared post-PH regenerative responses in wild type (WT) mice, Fn14 knockout (KO) mice, TWEAK KO mice, and WT mice treated with anti-TWEAK antibodies.

Results

In WT mice, rare Fn14(+) cells localized with other progenitor markers in peri-portal areas before PH. PH rapidly increased proliferation of Fn14(+) cells; hepatocytic cells that expressed Fn14 and other progenitor markers, such as Lgr5, progressively accumulated from 12–8 h post-PH and then declined to baseline by 96 h. When TWEAK/Fn14 signaling was disrupted, progenitor accumulation, induction of pro-regenerative cytokines, hepatocyte and cholangiocyte proliferation, and over-all survival were inhibited, while post-PH liver damage and bilirubin levels were increased. TWEAK stimulated proliferation and increased Lgr5 expression in cultured liver progenitors, but had no effect on either parameter in cultured primary hepatocytes.

Conclusions

TWEAK-FN14 signaling is necessary for the healthy adult liver to regenerate normally after acute partial hepatectomy.

A novel llama antibody targeting Fn14 exhibits anti-metastatic activity in vivo

Expression of fibroblast growth factor (FGF)-inducible 14 (Fn14), a member of the tumor necrosis factor receptor superfamily, is typically low in healthy adult organisms, but strong Fn14 expression is induced in tissue injury and tissue remodeling. High Fn14 expression is also observed in solid tumors, which is why this receptor is under consideration as a therapeutic target in oncology. Here, we describe various novel mouse-human cross-reactive llama-derived recombinant Fn14-specific antibodies (5B6, 18D1, 4G5) harboring the human IgG1 Fc domain. In contrast to recombinant variants of the established Fn14-specific antibodies PDL192 and P4A8, all three llama-derived antibodies efficiently bound to the W42A and R56P mutants of human Fn14. 18D1 and 4G5, but not 5B6, efficiently blocked TNF-like weak inducer of apoptosis(TWEA K) binding at low concentrations (0.2–2 μg/ml). Oligomerization and Fcγ receptor (FcγR) binding converted all antibodies into strong Fn14 agonists. Variants of 18D1 with enhanced and reduced antibody-dependent cell-mediated cytotoxicity (ADCC) activity were further analyzed in vivo with respect to their effect on metastasis. In a xenogeneic model using human colon carcinoma cancer cells, both antibody variants were effective in reducing metastasis to the liver. In contrast, only the 18D1 variant with enhanced ADCC activity, but not its ADCC-defective counterpart, suppressed lung metastasis in the RE NCA model. In sum, this suggests that Fn14 targeting might primarily act by triggering of antibody effector functions, but also by blockade of TWEA K-Fn14 interaction in some cases

Accurate prediction of drug-induced liver injury using stem cell-derived populations

Despite major progress in the knowledge and management of human liver injury, there are millions of people suffering from chronic liver disease. Currently, the only cure for end-stage liver disease is orthotopic liver transplantation; however, this approach is severely limited by organ donation. Alternative approaches to restoring liver function have therefore been pursued, including the use of somatic and stem cell populations. Although such approaches are essential in developing scalable treatments, there is also an imperative to develop predictive human systems that more effectively study and/or prevent the onset of liver disease and decompensated organ function. We used a renewable human stem cell resource, from defined genetic backgrounds, and drove them through developmental intermediates to yield highly active, drug-inducible, and predictive human hepatocyte populations. Most importantly, stem cell-derived hepatocytes displayed equivalence to primary adult hepatocytes, following incubation with known hepatotoxins. In summary, we have developed a serum-free, scalable, and shippable cell-based model that faithfully predicts the potential for human liver injury. Such a resource has direct application in human modeling and, in the future, could play an important role in developing renewable cell-based therapies.

Stem cells in translation: impression of the ISSCR Regional Meeting in Florence

The International Society for Stem Cell Research hosted an exciting meeting on stem cell-based translational medicine in Florence, Italy in September 2013. This report gives an overview of recent advances and breakthroughs presented at the meeting.

IFN-γ inhibits liver progenitor cell proliferation in HBV-infected patients and in 3,5-diethoxycarbonyl-1,4-dihydrocollidine diet-fed mice

BACKGROUND & AIMS

Proliferation of liver progenitor cells (LPCs) is associated with inflammation and fibrosis in chronic liver diseases. However, how inflammation and fibrosis affect LPCs remains obscure.

METHODS

We examined the role of interferon (IFN)-γ, an important pro-inflammatory and anti-fibrotic cytokine, in LPC expansion in HBV-infected patients and in mice challenged with 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)- or choline-deficient, ethionine-supplemented (CDE) diet as well as in primary LPCs and LPC cell line.

RESULTS

The CK19 staining scores correlated with inflammation and fibrosis grades in the livers from 110 HBV-infected patients. Nine-month IFN-γ treatment decreased LPC numbers, inflammation, and fibrosis in these HBV-infected patients. Similarly, a two-week IFN-γ treatment also decreased LPC activation in DDC-treated mice. Disruption of IFN-γ or its signaling components (e.g., IFNGR, STAT1, and IRF-1) increased LPC proliferation and liver fibrosis in DDC-fed mice. In contrast, deletion of IFN-γ did not increase, but rather slightly reduced LPC proliferation in CDE-fed mice. In vitro, IFN-γ attenuated proliferation of the LPC cell line BMOL and of primary LPCs from wild type mice, but not STAT1(-/-) or IRF-1(-/-) mice. Furthermore, co-culture assays suggest that IFN-γ can indirectly promote LPC proliferation via the activation of macrophages but attenuate it via the inhibition of hepatic stellate cells.

CONCLUSIONS

IFN-γ inhibits LPC expansion via the direct inhibition of LPC proliferation and indirect attenuation of liver fibrosis in the DDC model, but it may also enhance LPC expansion via the promotion of inflammation in the CDE model; thereby playing dual roles in regulating LPC proliferation in vivo.