Hepatic stellate cells' involvement in progenitor-mediated liver regeneration

Hepatic stellate cells maintain liver homeostasis through paracrine neurotrophin-3 signaling that induces hepatocyte proliferation

Organ size is maintained by the controlled proliferation of distinct cell populations. In the mouse liver, hepatocytes in the midlobular zone that are positive for cyclin D1 (CCND1) repopulate the parenchyma at a constant rate to preserve liver mass. Here, we investigated how hepatocyte proliferation is supported by hepatic stellate cells (HSCs), pericytes that are in close proximity to hepatocytes. We used T cells to ablate nearly all HSCs in the murine liver, enabling the unbiased characterization of HSC functions. In the normal liver, complete loss of HSCs persisted for up to 10 weeks and caused a gradual reduction in liver mass and in the number of CCND1+ hepatocytes. We identified neurotrophin-3 (Ntf-3) as an HSC-produced factor that induced the proliferation of midlobular hepatocytes through the activation of tropomyosin receptor kinase B (TrkB). Treating HSC-depleted mice with Ntf-3 restored CCND1+ hepatocytes in the midlobular region and increased liver mass. These findings establish that HSCs form the mitogenic niche for midlobular hepatocytes and identify Ntf-3 as a hepatocyte growth factor.

Preclinical-to-clinical innovations in stem cell therapies for liver regeneration

Acute and chronic liver diseases are the major cause of high morbidity and mortality globally. Liver transplantation is a widely used therapeutic option for liver failure. However, the shortage of availability of liver donors has encouraged research on the alternative approach to liver regeneration. Cell-based regenerative medicine is the best alternative therapy to cater to this need. To date, advanced preclinical approaches have been undertaken on stem cell differentiation and their use in liver tissue engineering for generating efficacious and promising regenerative therapies. Advancements in the bioengineering of stem cells, and organoid generation are the way forward to efficient therapies against liver injury. This review summarizes the recent approaches for stem cell therapy-based liver regeneration and their proof of concepts for clinical application, bioengineering liver organoids to alleviate the liver failure caused due to chronic liver diseases.

Targeting the Hepatic Microenvironment to Improve Ischemia/Reperfusion Injury: New Insights into the Immune and Metabolic Compartments

Hepatic ischemia/reperfusion injury (IRI) is mainly characterized by high activation of immune inflammatory responses and metabolic responses. Understanding the molecular and metabolic mechanisms underlying development of hepatic IRI is critical for developing effective therapies for hepatic IRI. Recent advances in research have improved our understanding of the pathogenesis of IRI. During IRI, hepatocyte injury and inflammatory responses are mediated by crosstalk between the immune cells and metabolic components. This crosstalk can be targeted to treat or reverse hepatic IRI. Thus, a deep understanding of hepatic microenvironment, especially the immune and metabolic responses, can reveal new therapeutic opportunities for hepatic IRI. In this review, we describe important cells in the liver microenvironment (especially non-parenchymal cells) that regulate immune inflammatory responses. The role of metabolic components in the diagnosis and prevention of hepatic IRI are discussed. Furthermore, recent updated therapeutic strategies based on the hepatic microenvironment, including immune cells and metabolic components, are highlighted.

Liver Regeneration in Chronic Liver Injuries: Basic and Clinical Applications Focusing on Macrophages and Natural Killer Cells

BACKGROUND & AIMS

Liver regeneration is a necessary but complex process involving multiple cell types besides hepatocytes. Mechanisms underlying liver regeneration after partial hepatectomy and acute liver injury have been well-described. However, in patients with chronic and severe liver injury, the remnant liver cannot completely restore the liver mass and function, thereby involving liver progenitor-like cells (LPLCs) and various immune cells.

RESULTS

Macrophages are beneficial to LPLCs proliferation and the differentiation of LPLCs to hepatocytes. Also, cells expressing natural killer (NK) cell markers have been studied in promoting both liver injury and liver regeneration. NK cells can promote LPLC-induced liver regeneration, but the excessive activation of hepatic NK cells may lead to high serum levels of interferon-γ, thus inhibiting liver regeneration.

CONCLUSIONS

This review summarizes the recent research on 2 important innate immune cells, macrophages and NK cells, in LPLC-induced liver regeneration and the mechanisms of liver regeneration during chronic liver injury, as well as the latest macrophage- and NK cell-based therapies for chronic liver injury. These novel findings can further help identify new treatments for chronic liver injury, saving patients from the pain of liver transplantations.

The Effects of Growth Factors and Cytokines on Hepatic Regeneration: A Systematic Review

The incidence of liver disease increases throughout the years due to many lifestyle factors; thus, the only definite treatment available for chronic liver disease is a liver transplant. However, the liver has a natural ability to repair itself and regenerate its hepatic tissue from stem cells. It is hypothesized that by inducing the liver with specific growth factors and cytokines such as interleukin 6 (IL-6) compared to general growth factors like growth differentiation factor 15 (GDF-15), it can regenerate, decreasing the need for liver transplant procedures. MEDLINE, the Journal of Hepatology, and Google Scholar were used to find articles. Various studies, including epidemiological studies dated from the year 2000 and greater, were used for the introduction. The results used only randomized control trials, experimental studies, and primary articles published since 2000. This compared the results of manipulating variables to determine the effects of hepatic regeneration by either specific hepatocyte growth factors or general growth factors like GDF-15. A total of 10 collected studies showed increased levels of gene expression and function, improved gross morphology, and histological appearance of the liver when induced by cytokines and specific growth factors versus general growth factors. Overall, the hypothesis was proven. The effects of GDF-15 were not significant compared to the effects of hepatocyte-specific growth factors and cytokines like IL-6 because they have two different effects on the liver after liver injury. Future studies should investigate this topic on the human hepatic regenerative ability, plus compare the effects of general growth factors like GDF-15 and specific hepatocyte growth factors and cytokines such as IL-6 in human liver tissue.

Role of Immune Cells in Biliary Repair

The biliary system is comprised of cholangiocytes and plays an important role in maintaining liver function. Under normal conditions, cholangiocytes remain in the stationary phase and maintain a very low turnover rate. However, the robust biliary repair is initiated in disease conditions, and different repair mechanisms can be activated depending on the pathological changes. During biliary disease, immune cells including monocytes, lymphocytes, neutrophils, and mast cells are recruited to the liver. The cellular interactions between cholangiocytes and these recruited immune cells as well as hepatic resident immune cells, including Kupffer cells, determine disease outcomes. However, the role of immune cells in the initiation, regulation, and suspension of biliary repair remains elusive. The cellular processes of cholangiocyte proliferation, progenitor cell differentiation, and hepatocyte-cholangiocyte transdifferentiation during biliary diseases are reviewed to manifest the underlying mechanism of biliary repair. Furthermore, the potential role of immune cells in crucial biliary repair mechanisms is highlighted. The mechanisms of biliary repair in immune-mediated cholangiopathies, inherited cholangiopathies, obstructive cholangiopathies, and cholangiocarcinoma are also summarized. Additionally, novel techniques that could clarify the underlying mechanisms of biliary repair are displayed. Collectively, this review aims to deepen the understanding of the mechanisms of biliary repair and contributes potential novel therapeutic methods for treating biliary diseases.

Hepatic stellate cells in physiology and pathology

Hepatic stellate cells (HSCs) comprise a minor cell population in the liver but serve numerous critical functions in the normal liver and in response to injury. HSCs are primarily known for their activation upon liver injury and for producing the collagen-rich extracellular matrix in liver fibrosis. In the absence of liver injury, HSCs reside in a quiescent state, in which their main function appears to be the storage of retinoids or vitamin A-containing metabolites. Less appreciated functions of HSCs include amplifying the hepatic inflammatory response and expressing growth factors that are critical for liver development and both the initiation and termination of liver regeneration. Recent single-cell RNA sequencing studies have corroborated earlier studies indictaing that HSC activation involves a diverse array of phenotypic alterations and identified unique HSC populations. This review serves to highlight these many functions of HSCs, and to briefly describe the recent genetic tools that will help to thoroughly investigate the role of HSCs in hepatic physiology and pathology.

Liver regeneration biology: Implications for liver tumour therapies

The liver has remarkable regenerative potential, with the capacity to regenerate after 75% hepatectomy in humans and up to 90% hepatectomy in some rodent models, enabling it to meet the challenge of diverse injury types, including physical trauma, infection, inflammatory processes, direct toxicity, and immunological insults. Current understanding of liver regeneration is based largely on animal research, historically in large animals, and more recently in rodents and zebrafish, which provide powerful genetic manipulation experimental tools. Whilst immensely valuable, these models have limitations in extrapolation to the human situation. In vitro models have evolved from 2-dimensional culture to complex 3 dimensional organoids, but also have shortcomings in replicating the complex hepatic micro-anatomical and physiological milieu. The process of liver regeneration is only partially understood and characterized by layers of complexity. Liver regeneration is triggered and controlled by a multitude of mitogens acting in autocrine, paracrine, and endocrine ways, with much redundancy and cross-talk between biochemical pathways. The regenerative response is variable, involving both hypertrophy and true proliferative hyperplasia, which is itself variable, including both cellular phenotypic fidelity and cellular trans-differentiation, according to the type of injury. Complex interactions occur between parenchymal and non-parenchymal cells, and regeneration is affected by the status of the liver parenchyma, with differences between healthy and diseased liver. Finally, the process of termination of liver regeneration is even less well understood than its triggers. The complexity of liver regeneration biology combined with limited understanding has restricted specific clinical interventions to enhance liver regeneration. Moreover, manipulating the fundamental biochemical pathways involved would require cautious assessment, for fear of unintended consequences. Nevertheless, current knowledge provides guiding principles for strategies to optimise liver regeneration potential.

Dynamic cell contacts between periportal mesenchyme and ductal epithelium act as a rheostat for liver cell proliferation

In the liver, ductal cells rarely proliferate during homeostasis but do so transiently after tissue injury. These cells can be expanded as organoids that recapitulate several of the cell-autonomous mechanisms of regeneration but lack the stromal interactions of the native tissue. Here, using organoid co-cultures that recapitulate the ductal-to-mesenchymal cell architecture of the portal tract, we demonstrate that a subpopulation of mouse periportal mesenchymal cells exerts dual control on proliferation of the epithelium. Ductal cell proliferation is either induced and sustained or, conversely, completely abolished, depending on the number of direct mesenchymal cell contacts, through a mechanism mediated, at least in part, by Notch signaling. Our findings expand the concept of the cellular niche in epithelial tissues, whereby not only soluble factors but also cell-cell contacts are the key regulatory cues involved in the control of cellular behaviors, suggesting a critical role for cell-cell contacts during regeneration.

Hepatic stellate cells - from past till present: morphology, human markers, human cell lines, behavior in normal and liver pathology

Hepatic stellate cell (HSC), initially analyzed by von Kupffer, in 1876, revealed to be an extraordinary mesenchymal cell, essential for both hepatocellular function and lesions, being the hallmark of hepatic fibrogenesis and carcinogenesis. Apart from their implications in hepatic injury, HSCs play a vital role in liver development and regeneration, xenobiotic response, intermediate metabolism, and regulation of immune response. In this review, we discuss the current state of knowledge regarding HSCs morphology, human HSCs markers and human HSC cell lines. We also summarize the latest findings concerning their roles in normal and liver pathology, focusing on their impact in fibrogenesis, chronic viral hepatitis and liver tumors.

Hepatic Stellate Cell Regulation of Liver Regeneration and Repair

The hepatic mesenchyme has been studied extensively in the context of liver fibrosis; however, much less is known regarding the role of mesenchymal cells during liver regeneration. As our knowledge of the cellular and molecular mechanisms driving hepatic regeneration deepens, the key role of the mesenchymal compartment during the regenerative response has been increasingly appreciated. Single-cell genomics approaches have recently uncovered both spatial and functional zonation of the hepatic mesenchyme in homeostasis and following liver injury. Here we discuss how the use of preclinical models, from in vivo mouse models to organoid-based systems, are helping to shape our understanding of the role of the mesenchyme during liver regeneration, and how these approaches should facilitate the precise identification of highly targeted, pro-regenerative therapies for patients with liver disease.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Myofibroblast induces hepatocyte-to-ductal metaplasia via laminin-ɑvβ6 integrin in liver fibrosis

Hepatocytes undergo the metaplasia into ductal biliary epithelial cells (BECs) in response to chronic injury, and subsequently contribute to liver regeneration. The mechanism underlying hepatocyte-to-ductal metaplasia has not been explored until now. In mouse models of liver fibrosis, a florid BEC response was observed in fibrotic liver, and the depletion of myofibroblasts attenuated BEC expansion remarkably. Then, in hepatocyte fate-tracing mouse model, we demonstrated the conversion of mature hepatocytes into ductal BECs in fibrotic liver, and the depletion of myofibroblasts diminished the hepatocyte-to-ductal metaplasia. Finally, the mechanism underlying the metaplasia was investigated. Myofibroblasts secreted laminin-rich extracellular matrix, and then laminin induced hepatocyte-to-ductal metaplasia through ɑvβ6 integrin. Therefore, our results demonstrated myofibroblasts induce the conversion of mature hepatocytes into ductal BECs through laminin-ɑvβ6 integrin, which reveals that the strategy improve regeneration in fibrotic liver through the modification of specific microenvironment.

Fibrotic liver has prompt recovery after ischemia-reperfusion injury

Hepatic ischemia-reperfusion (I/R) is a major complication of liver resection, trauma, and liver transplantation; however, liver repair after I/R in diseased liver has not been studied. The present study sought to determine the manner in which the fibrotic liver repairs itself after I/R. Liver fibrosis was established in mice by CCl4 administration for 6 wk, and then liver I/R was performed to investigate liver injury and subsequent liver repair in fibrotic and control livers. After I/R, fibrotic liver had more injury compared with nonfibrotic, control liver; however, fibrotic liver showed rapid resolution of liver necrosis and reconstruction of liver parenchyma. Marked accumulation of hepatic stellate cells and macrophages were observed specifically in the fibrotic septa in early reparative phase. Fibrotic liver had higher numbers of hepatic stellate cells, macrophages, and hepatic progenitor cells during liver recovery after I/R than did control liver, but hepatocyte proliferation was unchanged. Fibrotic liver also had significantly greater number of phagocytic macrophages than control liver. Clodronate liposome injection into fibrotic mice after I/R caused decreased macrophage accumulation and delay of liver recovery. Conversely, CSF1-Fc injection into normal mice after I/R resulted in increased macrophage accumulation and concomitant decrease in necrotic tissue during liver recovery. In conclusion, fibrotic liver clears necrotic areas and restores normal parenchyma faster than normal liver after I/R. This beneficial response appears to be directly related to the increased numbers of nonparenchymal cells, particularly phagocytic macrophages, in the fibrotic liver.NEW & NOTEWORTHY This study is the first to reveal how diseased liver recovers after ischemia-reperfusion (I/R) injury. Although it was not completely unexpected that fibrotic liver had increased hepatic injury after I/R, a novel finding was that fibrotic liver had accelerated recovery and repair compared with normal liver. Enhanced repair after I/R in fibrotic liver was associated with increased expansion of phagocytic macrophages, hepatic stellate cells, and progenitor cells.

Reduced production of laminin by hepatic stellate cells contributes to impairment in oval cell response to liver injury in aged mice

Aged liver is usually impaired in response to hepatic injury. Tissue-specific stem cells participate in the repair of tissue injury. However, how oval cells (OCs) respond to injury and how the process is regulated by tissue microenvironment in aged mice have not been fully understood. In this study, taking advantage of well-established murine OC activation model, we demonstrated that OCs were less activated upon injury in aged mice and the impairment was mainly attributed to dysfunction in their niche. Through analyzing global gene expression, we found that the genes differentially expressed in damaged young and aged mouse liver tissues were predominantly those required for the formation and remodeling of extracellular matrix. As one of the most important extracellular matrix components in the OC niche, laminin was shown to promote the proliferation of OCs. Not surprisingly, laminin was downregulated with aging. Consistent with the downregulation of genes encoding DNA-dependent protein kinase (DNA-PK) proteins in aged hepatic stellate cells (HSCs), inhibition of DNA-PK also led to reduced expression of laminin in HSCs. Moreover, impairment in OC activation caused by less supporting from DNA-damaged HSCs could be rescued by laminin. This study reveals a new cellular mechanism underlying impaired OCs functionality during aging.

TGF-β1 signaling activates hepatic stellate cells through Notch pathway

Hepatic stellate cells (HSCs), as the most important stromal cells in the liver microenvironment, play crucial roles in hepatic fibrosis, hepatocellular carcinoma, liver regeneration and fetal liver development after transdifferentiating into myofibroblasts (MFs). Transforming growth factor β1 (TGF-β1), as an important polyergic cytokine, is involved in HSCs activation process. However, the specific mechanisms of HSCs transdifferentiation process are not clearly demonstrated. Here we added exogenous recombinant TGF-β1 protein and transforming growth factor β receptor 1 (TGF-βR1) inhibitor SB431542 into mouse HSCs to detect the detailed impact of TGF-β1 signaling on HSCs activation. TGF-β1 signaling significantly increased phosphorylated (P)-Smad2/3 level and promoted Smad2/3 translocation from the cytoplasm to the nucleus, which also caused transdifferentiation of HSCs into MFs. Importantly, TGF-β1 signaling also resulted in high expression of Notch pathway markers Notch1, Jagged1, Hes1 in HSCs. In contrast, expression of those above markers in mouse HSCs were obviously decreased after hampering TGF-β1 signaling via TGF-βR1 inhibitor SB431542. To further examine the effect of Notch pathway on HSCs activation process, TGF-β1-stimulated HSCs and control HSCs were treated with or without LY450139, a specific inhibitor of Notch pathway. LY450139 evidently decreased the expression of Notch1 and MFs marker α-smooth muscle actin (α-SMA) expression in HSCs. These above results may provide a novel insight that TGF-β1 signaling controls HSCs activation process through regulating the expression of Notch pathway markers.

A comparative study of subsequent liver cirrhosis risk in non-Helicobacter pylori-infected peptic ulcer patients with and without vagotomy: An Asian population cohort study

OBJECTIVES

Hepatic parasympathetic nerves branch off the vagus nerve. The vagal and hepatic nervous systems are important in liver physiological processes and some diseases such as diabetes, obesity, and liver cirrhosis. We were interested in vagal nerve integrity and subsequent diseases in peptic ulcer patients. Herein, we used National Health Insurance database in Taiwan and retrospectively assessed the risk of developing liver cirrhosis in peptic ulcer patients with and without complications by surgical treatments.

METHODS

A cohort of 357 423 peptic ulcer patients without Helicobacter pylori, hepatitis B/C virus infection, and alcoholism from 2001 to 2008 was established. A randomly selected cohort of 357 423 people without peptic ulcer that matched by age, gender, comorbidities, and index year was used for comparison. The risks of developing liver cirrhosis were assessed both in cohorts and in peptic ulcer patients with and without vagotomy at the end of 2011.

RESULTS

Peptic ulcer patients were with higher incidence of liver cirrhosis than those without peptic ulcer (2.63 vs 0.96 per 1000 person-years) and with a 2.79-fold adjusted hazard ratio (HR) (95% confidence interval = 2.66-2.93) based on the multivariable Cox proportional hazards regression analysis. Comparing with different peptic ulcer management strategies, the HR value for subsequent liver cirrhosis risk was the lowest in vagotomy group (HR = 0.46, 95% confidence interval = 0.33-0.64).

CONCLUSIONS

Peptic ulcer patients have an increased risk of developing liver cirrhosis. Moreover, there were association of vagotomy and decreased risk of subsequent liver cirrhosis in complicated peptic ulcer patients. However, further studies are warranted.

Ductular Reaction in Liver Diseases: Pathological Mechanisms and Translational Significances

Ductular reaction (DR) is characterized by the proliferation of reactive bile ducts induced by liver injuries. DR is pathologically recognized as bile duct hyperplasia and is commonly observed in biliary disorders. It can also be identified in various liver disorders including nonalcoholic fatty liver disease. DR is associated with liver fibrosis and damage, and the extent of DR parallels to patient mortality. DR raises scientific interests because it is associated with transdifferentiation of liver cells and may play an important role in hepatic regeneration. The origin of active cells during DR can be cholangiocytes, hepatocytes, or hepatic progenitor cells, and associated signaling pathways could differ depending on the specific liver injury or animal models used in the study. Although further studies are needed to elucidate detailed mechanisms and the functional roles in liver diseases, DR can be a therapeutic target to inhibit liver fibrosis and to promote liver regeneration. This review summarizes previous studies of DR identified in patients and animal models as well as currently understood mechanisms of DR.

Three-dimensional coculture of primary hepatocytes and stellate cells in silk scaffold improves hepatic morphology and functionality in vitro

A vigorous in vitro model of liver that could recapitulate hepatic phenotype and functionality in vivo would exclusively improve the efficiency of bioartificial liver, drug discovery, or even transplantation therapy. Owing to the indispensable role of three-dimensional (3D) microenvironment in supporting viability and function of hepatocytes in vitro, much effort recently has been focused on improving reproducibility and standardization of primary hepatocyte cultures with a paradigm shift to 3D culture system, In the present study, an improved 3D coculture system of hepatocytes was established in which rat primary hepatocytes were cocultured with hepatic stellate cells in silk porous scaffolds. Silk scaffolds with incorporated extracellular matrix provided a suitable microenvironment for maintaining the viability, morphology and gene expression of the primary hepatocyte in vitro. The presence of stromal cells promoted primary hepatocyte to generate cellular aggregates with well-organized 3D architecture after 3 days of coculture in vitro. These aggregates exhibited proper morphology similar to liver tissue in vivo. Consistent with their phenotypic appearance, well-maintained functionality of hepatocytes was also observed in the cocultures, where albumin secretion/expression, urea synthesis as well as messenger ribonucleic acid expression of multiple cytochrome Ps (CYPs) enzymes increased significantly compared to either the 3D monocultures or monolayer cultures. Additionally, this 3D multicellular coculture model displayed an improved metabolic activity of CYPs enzymes to the probe drugs treatment. Thus, this culture system would not only contribute to the construction of micro-organoid tissue of liver but also potentially provide a robust tool for drug metabolism evaluation in vitro. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 2171-2180, 2018.

The diversity and plasticity of adult hepatic progenitor cells and their niche

The liver is a unique organ for homoeostasis with regenerative capacities. Hepatocytes possess a remarkable capacity to proliferate upon injury; however, in more severe scenarios liver regeneration is believed to arise from at least one, if not several facultative hepatic progenitor cell compartments. Newly identified pericentral stem/progenitor cells residing around the central vein is responsible for maintaining hepatocyte homoeostasis in the uninjured liver. In addition, hepatic progenitor cells have been reported to contribute to liver fibrosis and cancers. What drives liver homoeostasis, regeneration and diseases is determined by the physiological and pathological conditions, and especially the hepatic progenitor cell niches which influence the fate of hepatic progenitor cells. The hepatic progenitor cell niches are special microenvironments consisting of different cell types, releasing growth factors and cytokines and receiving signals, as well as the extracellular matrix (ECM) scaffold. The hepatic progenitor cell niches maintain and regulate stem cells to ensure organ homoeostasis and regeneration. In recent studies, more evidence has been shown that hepatic cells such as hepatocytes, cholangiocytes or myofibroblasts can be induced to be oval cell-like state through transitions under some circumstance, those transitional cell types as potential liver-resident progenitor cells play important roles in liver pathophysiology. In this review, we describe and update recent advances in the diversity and plasticity of hepatic progenitor cell and their niches and discuss evidence supporting their roles in liver homoeostasis, regeneration, fibrosis and cancers.

Polaprezinc inhibits liver fibrosis and proliferation in hepatocellular carcinoma

Hepatic fibrosis is defined as a pathological process, and activation of hepatic stellate cells (HSCs) is believed to be the key event of liver fibrosis. Additionally, activated HSCs may participate in the formation of the tumor microenvironment. Polaprezinc, a protector of the gastric mucosa, has been recently demonstrated to be an inhibitor of liver fibrosis in a mouse model. Proliferation and colony formation assays were performed to determine the inhibitory effects of polaprezinc on the growth of LX‑2 and hepG2 cells. A migration assay was used to evaluate the change in mobility of LX‑2 cells and quantitative polymerase chain reaction was performed to detect the expression levels of key markers of fibrosis. Finally, a gene chip assay for polaprezinc‑treated hepG2 cells was performed to evaluate the effect of polaprezinc on the hepG2 gene expression profile. The proliferation assay indicated that polaprezinc may inhibit the LX‑2 cell proliferation and the migration assays confirmed the inhibition of mobility. The expression levels of fibrotic markers such as collagen I, fibronectin and α‑smooth muscle actin were downregulated following polaprezinc treatment. The proliferation activity of polaprezinc‑treated hepG2 cells was reduced and the gene chip assay indicated that series of gene expression changes associated with cancer migration, cell skeletal organization and proliferation had occurred. In conclusion, polaprezinc treatment mayinhibit the proliferation of hepatocellular carcinoma cells and reverse liver fibrosis by deactivating HSCs. The present findings suggest that polaprezinc provides a novel treatment for patients with gastritis complicated with cirrhosis.

Early activated hepatic stellate cell-derived paracrine molecules modulate acute liver injury and regeneration

The effects of paracrine action from early activated hepatic stellate cells (HSCs) on resident liver epithelium cells are not clear. Here, we investigated whether a systemic infusion of early activated HSC-derived paracrine factors (HSC-CM) would evoke an enhanced liver protective response in acetaminophen (APAP)-induced acute liver injury (ALI) in mice and explored the possible underlying mechanisms. The survival rate, liver injury, and liver regeneration were analyzed in mice with or without HSC-CM treatment in vivo. A systemic infusion of HSC-CM provided a significant survival benefit in APAP-induced ALI. HSC-CM therapy resulted in a reduction of hepatocellular death and increased numbers of both proliferating hepatocytes and adult hepatic progenitor cells (AHPCs) with up-regulation of liver regeneration relevant genes. The HSC-CM treatment reduced leukocyte infiltration and down-regulated systemic inflammation with decreases in IFN-γ, IL-1ra, IL-1β, TNF-α, and increases in IL-10. The direct anti-death and pro-regeneration effects of HSC-CM on AHPCs were demonstrated using in vitro assays. Treatment with HSC-CM promoted AHPCs proliferation and resulted in increased pAkt expression in vitro, and this effect was abolished by the PI3K/Akt inhibitor LY294002. These data provide evidence that early activated HSC-CM therapy offered trophic support to the acutely injured liver by inhibiting liver cell death and stimulating regeneration, potentially creating a new method for the treatment of ALI.

Immunohistochemical study of hepatic fibropoiesis associated with canine visceral leishmaniasis

Hepatic fibropoiesis has been confirmed in canine visceral leishmaniasis. In fibrotic disease, hepatic stellate cells (HSC) play an important role in fibropoiesis, undergoing activation by TGF-β to acquire characteristics of myofibroblasts. These cells show extensive capacity for proliferation, motility, contractility, collagen synthesis and extracellular matrix component synthesis. The aim of this work was to identify markers of HSC activation in 10 symptomatic and 10 asymptomatic dogs naturally infected with Leishmania (Leishmania) infantum. Eight uninfected dogs were used as controls. Alpha-actin (α-SMA), vimentin and cytokeratin were investigated by immunohistochemistry as HSC markers. The cytokine TGF-β in tissue was also evaluated by immunohistochemistry. All infected dogs showed higher numbers of reticular fibres than controls. Fibropoiesis found in infected dogs was always associated with the presence of parasites and chronic granulomatous hepatitis. Positive correlation was found among fibropoiesis, parasite tissue load and expression of α-SMA. There was no correlation between fibropoiesis, vimentin and cytokeratin markers. The expression of cytokine TGF-β was higher in infected dogs than in controls, but not significantly different between symptomatic and asymptomatic dogs. These results confirm previous work describing the intense hepatic fibropoiesis in dogs naturally infected with Leishmania infantum, but now associated them with overexpression of TGF-β, where α-SMA may be a superior marker for activated HSC cells in CVL.

Liver progenitor cells-mediated liver regeneration in liver cirrhosis

Cirrhosis is defined as the histological development of regenerative nodules surrounded by fibrous bands in response to chronic liver injury. In cirrhotic liver where hepatocytes proliferation is compromised, liver progenitor cells (LPCs) are activated and then differentiated into hepatocytes and cholangiocytes, leading to the generation of regenerative nodules and functional restoration. Here, we summarize and discuss recent findings on the mechanisms underlying LPCs-mediated regeneration in liver cirrhosis. Firstly, we provide recent research on the mechanism underlying LPCs activation in severe or chronic liver injury. Secondly, we present new and exciting data on exploring the origin of LPCs, which reveal that the hepatocytes give rise to duct-like progenitors that then differentiate back into hepatocytes in chronic liver injury or liver cirrhosis. Finally, we highlight recent findings from the literature exploring the role of LPCs niche in directing the behavior and fate of LPCs. This remarkable insight into the cellular and molecular mechanisms of LPCs-mediated regeneration in liver cirrhosis will provide a basis for translating this knowledge into clinical application.

The adverse outcome pathway for rodent liver tumor promotion by sustained activation of the aryl hydrocarbon receptor

An Adverse Outcome Pathway (AOP) represents the existing knowledge of a biological pathway leading from initial molecular interactions of a toxicant and progressing through a series of key events (KEs), culminating with an apical adverse outcome (AO) that has to be of regulatory relevance. An AOP based on the mode of action (MOA) of rodent liver tumor promotion by dioxin-like compounds (DLCs) has been developed and the weight of evidence (WoE) of key event relationships (KERs) evaluated using evolved Bradford Hill considerations. Dioxins and DLCs are potent aryl hydrocarbon receptor (AHR) ligands that cause a range of species-specific adverse outcomes. The occurrence of KEs is necessary for inducing downstream biological responses and KEs may occur at the molecular, cellular, tissue and organ levels. The common convention is that an AOP begins with the toxicant interaction with a biological response element; for this AOP, this initial event is binding of a DLC ligand to the AHR. Data from mechanistic studies, lifetime bioassays and approximately thirty initiation-promotion studies have established dioxin and DLCs as rat liver tumor promoters. Such studies clearly show that sustained AHR activation, weeks or months in duration, is necessary to induce rodent liver tumor promotion--hence, sustained AHR activation is deemed the molecular initiating event (MIE). After this MIE, subsequent KEs are 1) changes in cellular growth homeostasis likely associated with expression changes in a number of genes and observed as development of hepatic foci and decreases in apoptosis within foci; 2) extensive liver toxicity observed as the constellation of effects called toxic hepatopathy; 3) cellular proliferation and hyperplasia in several hepatic cell types. This progression of KEs culminates in the AO, the development of hepatocellular adenomas and carcinomas and cholangiolar carcinomas. A rich data set provides both qualitative and quantitative knowledge of the progression of this AOP through KEs and the KERs. Thus, the WoE for this AOP is judged to be strong. Species-specific effects of dioxins and DLCs are well known--humans are less responsive than rodents and rodent species differ in sensitivity between strains. Consequently, application of this AOP to evaluate potential human health risks must take these differences into account.

Early activated hepatic stellate cell-derived molecules reverse acute hepatic injury

AIM: To test whether hepatic stellate cells (HSCs) at different activation stages play different roles in acetaminophen (APAP)-induced acute liver injury (ALI).

METHODS: HSCs were isolated from mouse liver and cultured in vitro. Morphological changes of initiation HSCs [HSCs (5d)] and perpetuation HSCs [HSCs (p3)] were observed by immunofluorescence and transmission electron microscopy. The protective effects of HSC-derived molecules, cell lysates and HSC-conditioned medium (HSC-CM) were tested in vivo by survival and histopathological analyses. Liver injury was determined by measuring aminotransferase levels in the serum and by histologic examination of tissue sections under a light microscope. Additionally, to determine the molecular mediators of the observed protective effects of initiation HSCs, we examined HSC-CM using a high-density protein array.

RESULTS: HSCs (5d) and HSCs (p3) had different morphological and phenotypic traits. HSCs (5d) presented a star-shaped appearance with expressing α-SMA at non-uniform levels between cells. However, HSCs (p3) evolved into myofibroblast-like cells without lipid droplets and expressed a uniform and higher level of α-SMA. HSC-CM (5d), but not HSC-CM (p3), provided a significant survival benefit and showed a dramatic reduction of hepatocellular necrosis and panlobular leukocyte infiltrates in mice exposed to APAP. However, this protective effect was abrogated at higher cell masses, indicating a therapeutic window of effectiveness. Furthermore, the protein array screen revealed that HSC-CM (5d) was composed of many chemokines and growth factors that correlated with inflammatory inhibition and therapeutic activity. When compared with HSC-CM (p3), higher levels of monocyte chemoattractant protein-1, macrophage inflammatory protein-1γ, hepatocyte growth factor, interleukin-10, and matrix metalloproteinase-2, but lower levels of stem cell factor and Fas-Ligand were observed in HSC-CM (5d).

CONCLUSION: These data indicated that initiation HSCs and perpetuation HSCs were different in morphology and protein expression, and provided the first experimental evidence of the potential medical value of initiation HSC-derived molecules in the treatment of ALI.

Hepatic stellate cells and liver fibrosis

Hepatic stellate cells are resident perisinusoidal cells distributed throughout the liver, with a remarkable range of functions in normal and injured liver. Derived embryologically from septum transversum mesenchyme, their precursors include submesothelial cells that invade the liver parenchyma from the hepatic capsule. In normal adult liver, their most characteristic feature is the presence of cytoplasmic perinuclear droplets that are laden with retinyl (vitamin A) esters. Normal stellate cells display several patterns of intermediate filaments expression (e.g., desmin, vimentin, and/or glial fibrillary acidic protein) suggesting that there are subpopulations within this parental cell type. In the normal liver, stellate cells participate in retinoid storage, vasoregulation through endothelial cell interactions, extracellular matrix homeostasis, drug detoxification, immunotolerance, and possibly the preservation of hepatocyte mass through secretion of mitogens including hepatocyte growth factor. During liver injury, stellate cells activate into alpha smooth muscle actin-expressing contractile myofibroblasts, which contribute to vascular distortion and increased vascular resistance, thereby promoting portal hypertension. Other features of stellate cell activation include mitogen-mediated proliferation, increased fibrogenesis driven by connective tissue growth factor, and transforming growth factor beta 1, amplified inflammation and immunoregulation, and altered matrix degradation. Evolving areas of interest in stellate cell biology seek to understand mechanisms of their clearance during fibrosis resolution by either apoptosis, senescence, or reversion, and their contribution to hepatic stem cell amplification, regeneration, and hepatocellular cancer.

Hepatic progenitor cells in canine and feline medicine: potential for regenerative strategies

New curative therapies for severe liver disease are urgently needed in both the human and veterinary clinic. It is important to find new treatment modalities which aim to compensate for the loss of parenchymal tissue and to repopulate the liver with healthy hepatocytes. A prime focus in regenerative medicine of the liver is the use of adult liver stem cells, or hepatic progenitor cells (HPCs), for functional recovery of liver disease. This review describes recent developments in HPC research in dog and cat and compares these findings to experimental rodent studies and human pathology. Specifically, the role of HPCs in liver regeneration, key components of the HPC niche, and HPC activation in specific types of canine and feline liver disease will be reviewed. Finally, the potential applications of HPCs in regenerative medicine of the liver are discussed and a potential role is suggested for dogs as first target species for HPC-based trials.

The canine hepatic progenitor cell niche: molecular characterisation in health and disease

Hepatic progenitor cells (HPCs) are an adult stem cell compartment in the liver that contributes to liver regeneration when replication of mature hepatocytes is insufficient. In this study, laser microdissection was used to isolate HPC niches from the livers of healthy dogs and dogs with lobular dissecting hepatitis (LDH), in which HPCs are massively activated. Gene expression of HPC, hepatocyte and biliary markers was determined by quantitative reverse transcriptase PCR. Expression and localisation of selected markers were further studied at the protein level by immunohistochemistry and immunofluorescent double staining in samples of normal liver and liver from dogs with LDH, acute and chronic hepatitis, and extrahepatic cholestasis. Activated HPC niches had higher gene expression of the hepatic progenitor markers OPN, FN14, CD29, CD44, CD133, LIF, LIFR and BMI1 compared to HPCs from normal liver. There was lower expression of albumin, but activated HPC niches were positive for the biliary markers SOX9, HNF1β and keratin 19 by immunohistochemistry and immunofluorescence. Laminin, activated stellate cells and macrophages are abundant extracellular matrix and cellular components of the canine HPC niche. This study demonstrates that the molecular and cellular characteristics of canine HPCs are similar to rodent and human HPCs, and that canine HPCs are distinctively activated in different types of liver disease.

The roles of transforming growth factor-β, Wnt, Notch and hypoxia on liver progenitor cells in primary liver tumours (Review)

Primary liver tumours have a high incidence and mortality. The most important forms are hepatocellular carcinoma and intrahepatic cholangiocarcinoma, both can occur together in the mixed phenotype hepatocellular-cholangiocarcinoma. Liver progenitor cells (LPCs) are bipotential stem cells activated in case of severe liver damage and are capable of forming both cholangiocytes and hepatocytes. Possibly, alterations in Wnt, transforming growth factor-β, Notch and hypoxia pathways in these LPCs can cause them to give rise to cancer stem cells, capable of driving tumourigenesis. In this review, we summarize and discuss current knowledge on the role of these pathways in LPC activation and differentiation during hepatocarcinogenesis.

The functional role of microRNAs in alcoholic liver injury

The function of microRNAs (miRNAs) during alcoholic liver disease (ALD) has recently become of great interest in biological research. Studies have shown that ALD associated miRNAs play a crucial role in the regulation of liver-inflammatory agents such as tumour necrosis factor-alpha (TNF-α), one of the key inflammatory agents responsible for liver fibrosis (liver scarring) and the critical contributor of alcoholic liver disease. Lipopolysaccharide (LPS), a component of the cell wall of gram-negative bacteria, is responsible for TNF-α release by Kupffer cells. miRNAs are the critical mediators of LPS signalling in Kupffer cells, hepatocytes and hepatic stellate cells. Certain miRNAs, in particular miR-155 and miR-21, show a positive correlation in up-regulation of LPS signalling when they are exposed to ethanol. ALD is related to enhanced gut permeability that allows the levels of LPS to increase, leads to increased secretion of TNF-α by the Kupffer cells and subsequently promotes alcoholic liver injury through specific miRNAs. Meanwhile, two of the most frequently dysregulated miRNAs in steatohepatitis, miR-122 and miR-34a are the critical mediators in ethanol/LPS activated survival signalling during ALD. In this review, we summarize recent findings regarding the experimental and clinical aspects of functions of specific microRNAs, focusing mainly on inflammation and cell survival after ethanol/LPS treatment, and advances on the role of circulating miRNAs in human alcoholic disorders.

Molecular constituents of the extracellular matrix in rat liver mounting a hepatic progenitor cell response for tissue repair

BACKGROUND

Tissue repair in the adult mammalian liver occurs in two distinct processes, referred to as the first and second tiers of defense. We undertook to characterize the changes in molecular constituents of the extracellular matrix when hepatic progenitor cells (HPCs) respond in a second tier of defense to liver injury.

RESULTS

We used transcriptional profiling on rat livers responding by a first tier (surgical removal of 70% of the liver mass (PHx protocol)) and a second tier (70% hepatectomy combined with exposure to 2-acetylaminofluorene (AAF/PHx protocol)) of defense to liver injury and compared the transcriptional signatures in untreated rat liver (control) with those from livers of day 1, day 5 and day 9 post hepatectomy in both protocols. Numerous transcripts encoding specific subunits of collagens, laminins, integrins, and various other extracellular matrix structural components were differentially up- or down-modulated (P < 0.01). The levels of a number of transcripts were significantly up-modulated, mainly in the second tier of defense (Agrn, Bgn, Fbn1, Col4a1, Col8a1, Col9a3, Lama5, Lamb1, Lamb2, Itga4, Igtb2, Itgb4, Itgb6, Nid2), and their signal intensities showed a strong or very strong correlation with Krt1-19, a well-established marker of a ductular/HPC reaction. Furthermore, a significant up-modulation and very strong correlation between the transcriptional profiles of Krt1-19 and St14 encoding matriptase, a component of a novel protease system, was found in the second tier of defense. Real-time PCR confirmed the modulation of St14 transcript levels and strong correlation to Krt-19 and also showed a significant up-modulation and strong correlation to Spint1 encoding HAI-1, a cognate inhibitor of matriptase. Immunodetection and three-dimensional reconstructions showed that laminin, Collagen1a1, agrin and nidogen1 surrounded bile ducts, proliferating cholangiocytes, and HPCs in ductular reactions regardless of the nature of defense. Similarly, matriptase and HAI-1 were expressed in cholangiocytes regardless of the tier of defense, but in the second tier of defense, a subpopulation of HPCs in ductular reactions co-expressed HAI-1 and the fetal hepatocyte marker Dlk1.

CONCLUSION

Transcriptional profiling and immunodetection, including three-dimensional reconstruction, generated a detailed overview of the extracellular matrix constituents expressed in a second tier of defense to liver injury.

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.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Transforming growth factor β induces expression of connective tissue growth factor in hepatic progenitor cells through Smad independent signaling

Hepatic progenitor cells (HPCs) are activated in the chronic liver injury and are found to participate in the progression of liver fibrosis, while the precise role of HPCs in liver fibrosis remains largely elusive. In this study, by immunostaining of human liver sections, we confirmed that HPCs were activated in the cirrhotic liver and secreted transforming growth factor β (TGF-β) and connective tissue growth factor (CTGF), both of which were important inducers of liver fibrosis. Besides, we used HPC cell lines LE/6 and WB-F344 as in vitro models and found that TGF-β induced secretion of CTGF in HPCs. Moreover, TGF-β signaling was intracrine activated and contributed to autonomous secretion of CTGF in HPCs. Furthermore, we found that TGF-β induced expression of CTGF was not mediated by TGF-β activated Smad signaling but mediated by TGF-β activated Erk, JNK and p38 MAPK signaling. Taken together, our results provide evidence for the role of HPCs in liver fibrosis and suggest that the production of CTGF by TGF-β activated MAPK signaling in HPCs may be a therapeutic target of liver fibrosis.

Hepatic stem cell niches

Stem cell niches are special microenvironments that maintain stem cells and control their behavior to ensure tissue homeostasis and regeneration throughout life. The liver has a high regenerative capacity that involves stem/progenitor cells when the proliferation of hepatocytes is impaired. In recent years progress has been made in the identification of potential hepatic stem cell niches. There is evidence that hepatic progenitor cells can originate from niches in the canals of Hering; in addition, the space of Disse may also serve as a stem cell niche during fetal hematopoiesis and constitute a niche for stellate cells in adults.

Hepatic stellate cells in liver development, regeneration, and cancer

Hepatic stellate cells are liver-specific mesenchymal cells that play vital roles in liver physiology and fibrogenesis. They are located in the space of Disse and maintain close interactions with sinusoidal endothelial cells and hepatic epithelial cells. It is becoming increasingly clear that hepatic stellate cells have a profound impact on the differentiation, proliferation, and morphogenesis of other hepatic cell types during liver development and regeneration. In this Review, we summarize and evaluate the recent advances in our understanding of the formation and characteristics of hepatic stellate cells, as well as their function in liver development, regeneration, and cancer. We also discuss how improved knowledge of these processes offers new perspectives for the treatment of patients with liver diseases.

Hepatocyte growth factor/c-met signaling is required for stem-cell-mediated liver regeneration in mice

Hepatocyte growth factor (HGF)/c-Met supports a pleiotrophic signal transduction pathway that controls stem cell homeostasis. Here, we directly addressed the role of c-Met in stem-cell-mediated liver regeneration by utilizing mice harboring c-met floxed alleles and Alb-Cre or Mx1-Cre transgenes. To activate oval cells, the hepatic stem cell (HSC) progeny, we used a model of liver injury induced by diet containing the porphyrinogenic agent, 3,5-diethocarbonyl-1,4-dihydrocollidine (DDC). Deletion of c-met in oval cells was confirmed in both models by polymerase chain reaction analysis of fluorescence-activated cell-sorted epithelial cell adhesion molecule (EpCam)-positive cells. Loss of c-Met receptor decreased the sphere-forming capacity of oval cells in vitro as well as reduced oval cell pool, impaired migration, and decreased hepatocytic differentiation in vivo, as demonstrated by double immunofluorescence using oval- (A6 and EpCam) and hepatocyte-specific (i.e. hepatocyte nuclear factor 4-alpha) antibodies. Furthermore, lack of c-Met had a profound effect on tissue remodeling and overall composition of HSC niche, which was associated with greatly reduced matrix metalloproteinase (MMP)9 activity and decreased expression of stromal-cell-derived factor 1. Using a combination of double immunofluorescence of cell-type-specific markers with MMP9 and gelatin zymography on the isolated cell populations, we identified macrophages as a major source of MMP9 in DDC-treated livers. The Mx1-Cre-driven c-met deletion caused the greatest phenotypic impact on HSCs response, as compared to the selective inactivation in the epithelial cell lineages achieved in c-Met(fl/fl); Alb-Cre(+/-) mice. However, in both models, genetic loss of c-met triggered a similar cascade of events, leading to the failure of HSC mobilization and death of the mice.

CONCLUSION

These results establish a direct contribution of c-Met in the regulation of HSC response and support a unique role for HGF/c-Met as an essential growth-factor-signaling pathway for regeneration of diseased liver.

Cross-species comparisons of transcriptomic alterations in human and rat primary hepatocytes exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin

A toxicogenomics approach was used to qualitatively and quantitatively compare the gene expression changes in human and rat primary hepatocytes exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Hepatocytes from five individual rats and five individual humans were exposed for 24 h to 11 concentrations of TCDD ranging from 0.00001 to 100nM and a vehicle control. Gene expression changes were analyzed using whole-genome microarrays containing 13,002 orthologs. Significant changes in expression of individual orthologs at any concentration (fold change [FC] ± 1.5 and false discovery rate < 0.05) were higher in the rat (1547) compared with human hepatocytes (475). Only 158 differentially expressed orthologs were common between rats and humans. Enrichment analysis was performed on the differentially expressed orthologs in each species with 49 and 34 enriched human and rat pathways, respectively. Only 12 enriched pathways were shared between the two species. The results demonstrate significant cross-species differences in expression at both the gene and pathway level. Benchmark dose analysis of gene expression changes showed an average 18-fold cross-species difference in potency among differentially expressed orthologs with the rat more sensitive than the human. Similar cross-species differences in potency were observed for signaling pathways. Using the maximum FC in gene expression as a measure of efficacy, the human hepatocytes showed on average a 20% lower efficacy among the individual orthologs showing differential expression. The results provide evidence for divergent cross-species gene expression changes in response to TCDD and are consistent with epidemiological and clinical evidence showing humans to be less sensitive to TCDD-induced hepatotoxicity.

Liver precursor cells increase hepatic fibrosis induced by chronic carbon tetrachloride intoxication in rats

Hepatic fibrosis, the major complication of virtually all types of chronic liver damage, usually begins in portal areas, and its severity has been correlated to liver progenitor cells (LPC) expansion from periportal areas, even if the primary targets of injury are intralobular hepatocytes. The aim of this study was to determine the potential fibrogenic role of LPC, using a new experimental model in which rat liver fibrosis was induced by chronic carbon tetrachloride (CCl(4)) administration for 6 weeks, in combination with chronic acetylaminofluorene treatment (AAF), which promotes activation of LPC compartment. Treatment with CCl(4) alone caused a significant increase in serum transaminase activity as well as liver fibrosis initiating around central veins and leading to formation of incomplete centro-central septa with sparse fibrogenic cells expressing α-smooth muscle actin (αSMA). In AAF/CCl(4)-treated animals, the fibrogenic response was profoundly worsened, with formation of multiple porto-central bridging septa leading to cirrhosis, whereas hepatocellular necrosis and inflammation were similar to those observed in CCl(4)-treated animals. Enhanced fibrosis in AAF/CCl(4) group was accompanied by ductule forming LPC expanding from portal areas, αSMA-positive cells accumulation in the fibrotic areas and increased expression of hepatic collagen type 1, 3 and 4 mRNA. Moreover, CK19-positive LPC expressed the most potent fibrogenic cytokine transforming growth factor-β (TGFβ) without any expression of αSMA, desmin or fibroblast-specific protein-1, demonstrating that LPC did not undergo an epithelial-mesenchymal transition. In this new experimental model, LPC, by expressing TGFβ, contributed to the accumulation of αSMA-positive myofibroblasts in the ductular reaction leading to enhanced fibrosis but also to disease progression and to a fibrotic pattern similar to that observed in humans.

Stimulation of oval cell and hepatocyte proliferation by exogenous bombesin and neurotensin in partially hepatectomized rats

AIM: To investigate the effect of the neuropeptides bombesin (BBS) and neurotensin (NT) on oval cell proliferation in partially hepatectomized rats not pretreated with a known hepatocyte inhibitor.

METHODS: Seventy male Wistar rats were randomly divided into five groups: I = controls, II = sham operated, III = partial hepatectomy 70% (PHx), IV = PHx + BBS (30 μg/kg per day), V = PHx + NT (300 μg/kg per day). Forty eight hours after liver resection, portal endotoxin levels and hepatic glutathione redox state were determined. α-fetoprotein (AFP) mRNA (in situ hybridisation), cytokeratin-19 and Ki67 antigen expression (immunohistochemistry) and apoptosis (TUNEL) were evaluated on liver tissue samples. Cells with morphological features of oval cells that were cytokeratin-19 (+) and AFP mRNA (+) were scored in morphometric analysis and their proliferation was recorded. In addition, the proliferation and apoptotic rates of hepatocytes were determined.

RESULTS: In the control and sham operated groups, oval cells were significantly less compared to groups III, IV and V (P < 0.001). The neuropeptides BBS and NT significantly increased the proliferation of oval cells compared to group III (P < 0.001). In addition, BBS and NT induced a significant increase of hepatocyte proliferation (P < 0.001), whereas it decreased their apoptotic activity (P < 0.001) compared to group III. BBS and NT significantly decreased portal endotoxemia (P < 0.001) and increased the hepatic GSH: GSSG ratio (P < 0.05 and P < 0.001, respectively) compared to group III.

CONCLUSION: BBS and NT stimulated oval cell proliferation in a model of liver regeneration, without use of concomitant suppression of hepatocyte proliferation as oval cell activation stimuli, and improved the hepatocyte regenerative response. This peptides-induced combined stimulation of oval cell and hepatocyte proliferation might serve as a possible treatment modality for several liver diseases.

Kupffer cells influence parenchymal invasion and phenotypic orientation, but not the proliferation, of liver progenitor cells in a murine model of liver injury

Activation of myofibroblasts (MF) and extracellular matrix (ECM) deposition predispose the expansion and differentiation of liver progenitor cells (LPC) during chronic liver injury. Because Kupffer cells (KC) are active modulators of tissue response and fibrosis, we analyzed their role in a model of LPC proliferation. A choline-deficient diet, supplemented by ethionine (CDE) was administrated to C57Bl/6J mice that were depleted of KC by repeated injections of clodronate (CLO) and compared to PBS-injected mice. On CDE, massive KC activation was observed in the PBS group, but this was blunted in CLO-treated mice. The depletion of KC did not influence LPC proliferation but reduced their invasive behavior. Instead of being found far into the parenchyma, as was found in the PBS group (mean distance from portal vein: 209 μm), LPC of CLO mice remained closer to the portal area (138 μm), forming aggregates and phenotypically resembling cells of biliary lineage. Notably, removal of KC was also associated with a significant decrease in amount of MF and ECM and in the expression of profibrotic factors. Thus, besides ECM and MF, KC are also a significant component of the microenvironmental changes preceding LPC expansion. Depletion of KC may limit the LPC parenchymal invasion through a deficiency in chemoattracting factors, reduced activation of MF, and/or a paucity of the ECM framework necessary for cell motility.

Genome-wide expression profiling of hepatic oval cells after partial hepatectomy in rats

To examine the changes of biological activities in hepatic oval cells (HOCs) elicited by 70% partial hepatectomy (PH) and understand the relationship between this cell and liver regeneration (LR), this study isolated and obtained the high purity HOCs (≥ 95%) from rat regenerating livers, and then monitored gene expression profiling of rat hepatic oval cells following surgical operation. Results showed that there were LR-related 1059 genes. These genes were grossly classified into three groups using a fold change cut-off threshold of three-fold: up-regulation, down-regulation and up/down regulation. Analyses of gene expression patterns combined with gene functional categorization suggested that genes in the categories "nucleic acid metabolism" and "cell cycle" were dominated by up-regulated expression. Genes in the functional groups "cell metabolism" and "oxidation reduction" were significantly enriched in expression pattern characterized by down-regulation. According to above mentioned results, the synchronized induction of DNA replication and cell proliferation-involved genes suggested that the peak of oval cell proliferation might occur between 30 and 36 h post-PH. The amino acid transformation-involved genes were down-regulated at the early phase of LR, which perhaps trigger the storage of those amino acids essential for protein synthesis. Reduced oxidative-reduction activity at early phase might be related to negative influence of surgical operation on its detoxification capacity. Conclusively, the genome-wide transcriptional analysis of oval cells would contribute to our understanding of the nature of LR at cell level.

Epigenetic cell fate regulation of hepatic stellate cells

Research in the past three decades has identified key mediators and signaling mechanisms responsible for myofibroblastic transdifferentiation (MTD) of hepatic stellate cells (HSC), the pivotal event in liver fibrogenesis. Yet, fundamental understanding of the MTD from the viewpoint of cell fate or lineage regulation has been elusive. Recent studies using genetic cell fate mapping techniques demonstrate HSC are derived from mesoderm and at least in part via septum transversum and mesothelium. HSC express markers for different cell types derived from multipotent mesenchymal progenitors. A regulatory commonality between differentiation of adipocytes and that of HSC is shown, and a shift from adipogenic to myogenic or neuronal phenotype characterizes HSC MTD. Central to this shift is a loss of expression of the master adipogenic regulator peroxisome proliferator activated receptor-γ (PPAR-γ). Restored expression of PPAR-γ and/or other adipogenic transcription factors reverses myofibroblastic HSC to differentiated cells. In MTD, Pparγ is epigenetically repressed by induction of methyl-CpG binding protein 2 and its enrichment to the promoter and polycomb repressive complex-facilitated histone H3 lysine 27 di/tri-methylation at the 3' exons. Blocking canonical wingless-related MMTV integration site (Wnt) signaling in myofibroblastic HSC with the co-receptor antagonist Dickkopf-1, abrogates these epigenetic mechanisms, restores PPAR-γ expression and HSC differentiation. Necdin, a melanoma antigen family protein, is identified as an upstream mediator for induction of the canonical Wnt10b and consequent Pparγ repression and HSC MTD. The identified morphogen-induced epigenetic regulation of Pparγ and HSC fate may serve as a novel target for manipulation of liver fibrosis and mesenchymal-epithelial interactions in liver regeneration.