Liver cell-derived microparticles activate hedgehog signaling and alter gene expression in hepatic endothelial cells

Endothelial and cancer cells interact with mesenchymal stem cells via both microparticles and secreted factors

Tightly associated with blood vessels in their perivascular niche, human mesenchymal stem cells (MSCs) closely interact with endothelial cells (ECs). MSCs also home to tumours and interact with cancer cells (CCs). Microparticles (MPs) are cell-derived vesicles released into the extracellular environment along with secreted factors. MPs are capable of intercellular signalling and, as biomolecular shuttles, transfer proteins and RNA from one cell to another. Here, we characterize interactions among ECs, CCs and MSCs via MPs and secreted factors in vitro. MPs and non-MP secreted factors (Sup) were isolated from serum-free medium conditioned by human microvascular ECs (HMEC-1) or by the CC line HT1080. Fluorescently labelled MPs were prepared from cells treated with membrane dyes, and cytosolic GFP-containing MPs were isolated from cells transduced with CMV-GFP lentivirus. MSCs were treated with MPs, Sup, or vehicle controls, and analysed for MP uptake, proliferation, migration, activation of intracellular signalling pathways and cytokine release. Fluorescently labelled MPs fused with MSCs, transferring the fluorescent dyes to the MSC surface. GFP was transferred to and retained in MSCs incubated with GFP-MPs, but not free GFP. Thus, only MP-associated cellular proteins were taken up and retained by MSCs, suggesting that MP biomolecules, but not secreted factors, are shuttled to MSCs. MP and Sup treatment significantly increased MSC proliferation, migration, and MMP-1, MMP-3, CCL-2/MCP-1 and IL-6 secretion compared with vehicle controls. MSCs treated with Sup and MPs also exhibited activated NF-κB signalling. Taken together, these results suggest that MPs act to regulate MSC functions through several mechanisms.

Human bile contains microRNA-laden extracellular vesicles that can be used for cholangiocarcinoma diagnosis

Cholangiocarcinoma (CCA) presents significant diagnostic challenges, resulting in late patient diagnosis and poor survival rates. Primary sclerosing cholangitis (PSC) patients pose a particularly difficult clinical dilemma because they harbor chronic biliary strictures that are difficult to distinguish from CCA. MicroRNAs (miRs) have recently emerged as a valuable class of diagnostic markers; however, thus far, neither extracellular vesicles (EVs) nor miRs within EVs have been investigated in human bile. We aimed to comprehensively characterize human biliary EVs, including their miR content. We have established the presence of extracellular vesicles in human bile. In addition, we have demonstrated that human biliary EVs contain abundant miR species, which are stable and therefore amenable to the development of disease marker panels. Furthermore, we have characterized the protein content, size, numbers, and size distribution of human biliary EVs. Utilizing multivariate organization of combinatorial alterations (MOCA), we defined a novel biliary vesicle miR-based panel for CCA diagnosis that demonstrated a sensitivity of 67% and specificity of 96%. Importantly, our control group contained 13 PSC patients, 16 with biliary obstruction of varying etiologies (including benign biliary stricture, papillary stenosis, choledocholithiasis, extrinsic compression from pancreatic cysts, and cholangitis), and 3 with bile leak syndromes. Clinically, these types of patients present with a biliary obstructive clinical picture that could be confused with CCA.

CONCLUSION

These findings establish the importance of using extracellular vesicles, rather than whole bile, for developing miR-based disease markers in bile. Finally, we report on the development of a novel bile-based CCA diagnostic panel that is stable, reproducible, and has potential clinical utility.

Cellular and molecular mechanisms in the pathogenesis of liver fibrosis: An update

There have been considerable recent advances towards a better understanding of the complex cellular and molecular network underlying liver fibrogenesis. Recent data indicate that the termination of fibrogenic processes and the restoration of deficient fibrolytic pathways may allow the reversal of advanced fibrosis and even cirrhosis. Therefore, efforts have been made to better clarify the cellular and molecular mechanisms that are involved in liver fibrosis. Activation of hepatic stellate cells (HSCs) remains a central event in fibrosis, complemented by other sources of matrix-producing cells, including portal fibroblasts, fibrocytes and bone marrow-derived myofibroblasts. These cells converge in a complex interaction with neighboring cells to provoke scarring in response to persistent injury. Defining the interaction of different cell types, revealing the effects of cytokines on these cells and characterizing the regulatory mechanisms that control gene expression in activated HSCs will enable the discovery of new therapeutic targets. Moreover, the characterization of different pathways associated with different etiologies aid in the development of disease-specific therapies. This article outlines recent advances regarding the cellular and molecular mechanisms involved in liver fibrosis that may be translated into future therapies. The pathogenesis of liver fibrosis associated with alcoholic liver disease, non-alcoholic fatty liver disease and viral hepatitis are also discussed to emphasize the various mechanisms involved in liver fibrosis.

Hedgehog signaling pathway as key player in liver fibrosis: new insights and perspectives

INTRODUCTION

Activation of hepatic stellate cells (HSCs) is a pivotal cellular event in liver fibrosis. Therefore, improving our understanding of the molecular pathways that are involved in these processes is essential to generate new therapies for liver fibrosis. Greater knowledge of the role of the hedgehog signaling pathway in liver fibrosis could improve understanding of the liver fibrosis pathogenesis.

AREAS COVERED

The aim of this review is to describe the present knowledge about the hedgehog signaling pathway, which significantly participates in liver fibrosis and HSC activation, and look ahead on new perspectives of hedgehog signaling pathway research. Moreover, we will discuss the different interactions with hedgehog signaling pathway-regulated liver fibrosis.

EXPERT OPINION

The hedgehog pathway modulates several important aspects of function, including cell proliferation, activation and differentiation. Targeting the hedgehog pathway can be a promising direction in liver fibrosis treatment. We discuss new perspectives of hedgehog signaling pathway activation in liver fibrosis and HSC fate, including DNA methylation, methyl CpG binding protein 2, microRNA, irradiation and metabolism that influence hedgehog signaling pathway transduction. These findings identify the hedgehog pathway as a potentially important for biomarker development and therapeutic targets in liver fibrosis. Future studies are needed in order to find safer and more effective hedgehog-based drugs.

Molecular mechanism and treatment of viral hepatitis-related liver fibrosis

Hepatic fibrosis is a wound-healing response to various chronic stimuli, including viral hepatitis B or C infection. Activated myofibroblasts, predominantly derived from the hepatic stellate cells (HSCs), regulate the balance between matrix metalloproteinases and their tissue inhibitors to maintain extracellular matrix homeostasis. Transforming growth factor-β and platelet-derived growth factor are classic profibrogenic signals that activate HSC proliferation. In addition, proinflammatory cytokines and chemokines coordinate macrophages, T cells, NK/NKT cells, and liver sinusoidal endothelial cells in complex fibrogenic and regression processes. In addition, fibrogenesis involves angiogenesis, metabolic reprogramming, autophagy, microRNA, and epigenetic regulations. Hepatic inflammation is the driving force behind liver fibrosis; however, host single nucleotide polymorphisms and viral factors, including the genotype, viral load, viral mutation, and viral proteins, have been associated with fibrosis progression. Eliminating the underlying etiology is the most crucial antifibrotic therapy. Growing evidence has indicated that persistent viral suppression with antiviral therapy can result in fibrosis regression, reduced liver disease progression, decreased hepatocellular carcinoma, and improved chances of survival. Preclinical studies and clinical trials are currently examining several investigational agents that target key fibrogenic pathways; the results are promising and shed light on this debilitating illness.

The emerging roles of microvesicles in liver diseases

Microvesicles (MVs) are extracellular vesicles released by virtually all cells, under both physiological and pathological conditions. They contain lipids, proteins, RNAs and microRNAs and act as vectors of information that regulate the function of target cells. This Review provides an overview of the studies assessing circulating MV levels in patients with liver diseases, together with an insight into the mechanisms that could account for these changes. We also present a detailed analysis of the implication of MVs in key processes of liver diseases. MVs have a dual role in fibrosis as certain types of MVs promote fibrolysis by increasing expression of matrix metalloproteinases, whereas others promote fibrosis by stimulating processes such as angiogenesis. MVs probably enhance portal hypertension by contributing to intrahepatic vasoconstriction, splanchnic vasodilation and angiogenesis. As MVs can modulate vascular permeability, vascular tone and angiogenesis, they might contribute to several complications of cirrhosis including hepatic encephalopathy, hepatopulmonary syndrome and hepatorenal syndrome. Several results also suggest that MVs have a role in hepatocellular carcinoma. Although MVs represent promising biomarkers in patients with liver disease, methods of isolation and subsequent analysis must be standardized.

Biomarkers of liver cell death

Hepatocyte cell death during liver injury was classically viewed to occur by either programmed (apoptosis), or accidental, uncontrolled cell death (necrosis). Growing evidence from our increasing understanding of the biochemical and molecular mechanisms involved in cell demise has provided an expanding view of various modes of cell death that can be triggered during both acute and chronic liver damage such as necroptosis, pyroptosis, and autophagic cell death. The complexity of non-invasively assessing the predominant mode of cell death during a specific liver insult in either experimental in vivo models or in humans is highlighted by the fact that in many instances there is significant crosstalk and overlap between the different cell death pathways. Nevertheless, the realization that during cell demise triggered by a specific mode of cell death certain intracellular molecules such as proteins, newly generated protein fragments, or MicroRNAs are released from hepatocytes into the extracellular space and may appear in circulation have spurred a significant interest in the development of non-invasive markers to monitor liver cell death. This review focuses on some of the most promising markers, and their potential role in assessing the presence and severity of liver damage in humans.

Microparticles: a new perspective in central nervous system disorders

Microparticles (MPs) are a heterogeneous population of small cell-derived vesicles, ranging in size from 0.1 to 1 μm. They contain a variety of bioactive molecules, including proteins, biolipids, and nucleic acids, which can be transferred between cells without direct cell-to-cell contact. Consequently, MPs represent a novel form of intercellular communication, which could play a role in both physiological and pathological processes. Growing evidence indicates that circulating MPs contribute to the development of cancer, inflammation, and autoimmune and cardiovascular diseases. Most cell types of the central nervous system (CNS) have also been shown to release MPs, which could be important for neurodevelopment, CNS maintenance, and pathologies. In disease, levels of certain MPs appear elevated; therefore, they may serve as biomarkers allowing for the development of new diagnostic tools for detecting the early stages of CNS pathologies. Quantification and characterization of MPs could also provide useful information for making decisions on treatment options and for monitoring success of therapies, particularly for such difficult-to-treat diseases as cerebral malaria, multiple sclerosis, and Alzheimer's disease. Overall, studies on MPs in the CNS represent a novel area of research, which promises to expand the knowledge on the mechanisms governing some of the physiological and pathophysiological processes of the CNS.

MicroRNAs as potential circulating biomarkers of drug-induced liver injury: key current and future issues for translation to humans

Drug-induced liver injury (DILI) is a common form of adverse drug reaction seen within the clinic. Sensitive, specific and non-invasive biomarkers of liver toxicity are required to help diagnose hepatotoxicity and also to identify safety liabilities during drug development. Limitations exist in the current gold standard DILI biomarkers: alanine aminotransferase is not liver-specific and therefore gives rise to false-positive signals. Interest has grown in the potential of microRNAs (miRNAs) as biomarkers of DILI. Some miRNAs display remarkable organ specificity, can be measured sensitively and are stable in a wide range of biofluids. However, little is currently known about the mechanisms through which miRNAs are released from cells. Furthermore, a clinically suitable method to measure miRNAs has not yet been developed. This review aims to highlight the current research surrounding these markers and areas in which further work is required to establish these markers within clinical and pre-clinical settings.

Chicken biliary exosomes enhance CD4(+)T proliferation and inhibit ALV-J replication in liver

Exosomes, which are small membrane vesicles of endocytic origin, carry lipids, RNA/miRNAs, and proteins and have immune modulatory functions. In this study, we isolated exosomes from the bile of specific pathogen-free chickens, 42-43 days of age, by using an ultracentrifugation and filtration method. The density of the exosomes, isolated by sucrose gradient fractionation, was between 1.13 and 1.19 g/mL. Electron microscopic observation of the liver showed that exosomes were present in the space of Disse and bile canaliculus. Chicken biliary exosomes displayed typical saucer-shaped, rounded morphology. Using liquid chromatography mass spectrum methodology, 196 proteins, including exosomal markers and several unique proteins, were identified and compared with mouse biliary exosomes. Noteworthy, CCCH type zinc finger antiviral protein was found on chicken biliary exosomes never described before. Furthermore, our data show that chicken biliary exosomes promote the proliferation of CD4(+) and CD8(+) T cells and monocytes from liver. In addition, chicken biliary exosomes significantly inhibit avian leukosis virus subgroup J, which is an oncogenic retrovirus, from replicating in the DF-1 cell line. These data indicate that chicken biliary exosomes possess the capacity to influence the immune responses of lymphocytes and inhibit avian leukosis virus subgroup J (ALV-J).

Cellular and molecular mechanisms in liver fibrogenesis

Liver fibrogenesis is a dynamic and highly integrated molecular, tissue and cellular process, potentially reversible, that drives the progression of chronic liver diseases (CLD) towards liver cirrhosis and hepatic failure. Hepatic myofibroblasts (MFs), the pro-fibrogenic effector cells, originate mainly from activation of hepatic stellate cells and portal fibroblasts being characterized by a proliferative and survival attitude. MFs also contract in response to vasoactive agents, sustain angiogenesis and recruit and modulate activity of cells of innate or adaptive immunity. Chronic activation of wound healing and oxidative stress as well as derangement of epithelial-mesenchymal interactions are "major" pro-fibrogenic mechanisms, whatever the etiology. However, literature has outlined a complex network of pro-fibrogenic factors and mediators proposed to modulate CLD progression, with some of them being at present highly debated in the field, including the role of epithelial to mesenchymal transition and Hedgehog signaling pathways. Hypoxia and angiogenesis as well as inflammasomes are recently emerged as ubiquitous pro-inflammatory and pro-fibrogenic determinants whereas adipokines are mostly involved in CLD related to metabolic disturbances (metabolic syndrome and/or obesity and type 2 diabetes). Finally, autophagy as well as natural killer and natural killer-T cells have been recently proposed to significantly affect fibrogenic CLD progression.

Liver renewal: detecting misrepair and optimizing regeneration

Cirrhosis and liver cancer, the main causes of liver-related morbidity and mortality, result from defective repair of liver injury. This article summarizes rapidly evolving knowledge about liver myofibroblasts and progenitors, the 2 key cell types that interact to orchestrate effective repair, because deregulation of these cells is likely to be central to the pathogenesis of both cirrhosis and liver cancer. We focus on cirrhosis pathogenesis because cirrhosis is the main risk factor for primary liver cancer. Emerging evidence suggests that the defective repair process has certain characteristics that might be exploited for biomarker development. Recent findings in preclinical models also indicate that the newly identified cellular and molecular targets are amenable to therapeutic manipulation. Thus, recent advances in our understanding about key cell types and fundamental mechanisms that regulate liver regeneration have opened new avenues to improve the outcomes of liver injury.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT01899859.

The glypican 3 hepatocellular carcinoma marker regulates human hepatic stellate cells via Hedgehog signaling

BACKGROUND

Hepatocellular carcinoma (HCC) frequently represents two diseases as it often arises in the setting of cirrhosis caused by the proliferation and activation of hepatic stellate cells (HSCs). Previously, we identified that Hedgehog (Hh) signaling regulates HSC viability and fibrinogenesis, as well as HCC tumorigenesis. Although it is increasingly recognized that HSCs and HCCs communicate via paracrine signaling, Hh's role in this process is just emerging. We hypothesized that a secreted HCC tumor marker and Hh mediator, glypican 3 (GPC3), may regulate HSC.

METHODS

Using three human HCC lines (Hep3B, PLC/PRF/5 and SK-Hep-1) and one Hh-responsive human HSC line (LX-2), we developed two in vitro models of HCC-to-HSC paracrine signaling using a Transwell coculture system and HCC-conditioned media. We then evaluated the effects of these models, as well as GPC3, on HSC viability and gene expression.

RESULTS

Using our coculture and conditioned media models, we demonstrate that the three HCC lines decrease HSC viability. Furthermore, we demonstrate that recombinant GPC3 dose-dependently decreases the LX-2 viability while inhibiting the expression of Hh target genes that regulate HSC viability. Finally, GPC3's inhibitory effects on cell viability and Hh target gene expression are partially abrogated by heparin, a competitor for GPC3 binding.

CONCLUSIONS

For the first time, we show that GPC3, an HCC biomarker and Hh mediator, regulates human HSC viability by regulating Hh signaling. This expands on existing data suggesting a role for tumor-stroma interactions in the liver and suggests that GPC3 plays a role in this process.

Physiology of cholangiocytes

Cholangiocytes are epithelial cells that line the intra- and extrahepatic ducts of the biliary tree. The main physiologic function of cholangiocytes is modification of hepatocyte-derived bile, an intricate process regulated by hormones, peptides, nucleotides, neurotransmitters, and other molecules through intracellular signaling pathways and cascades. The mechanisms and regulation of bile modification are reviewed herein.

Lipid-induced toxicity stimulates hepatocytes to release angiogenic microparticles that require Vanin-1 for uptake by endothelial cells

Angiogenesis is a key pathological feature of experimental and human steatohepatitis, a common chronic liver disease that is associated with obesity. We demonstrated that hepatocytes generated a type of membrane-bound vesicle, microparticles, in response to conditions that mimicked the lipid accumulation that occurs in the liver in some forms of steatohepatitis and that these microparticles promoted angiogenesis. When applied to an endothelial cell line, medium conditioned by murine hepatocytes or a human hepatocyte cell line exposed to saturated free fatty acids induced migration and tube formation, two processes required for angiogenesis. Medium from hepatocytes in which caspase 3 was inhibited or medium in which the microparticles were removed by ultracentrifugation lacked proangiogenic activity. Isolated hepatocyte-derived microparticles induced migration and tube formation of an endothelial cell line in vitro and angiogenesis in mice, processes that depended on internalization of microparticles. Microparticle internalization required the interaction of the ectoenzyme Vanin-1 (VNN1), an abundant surface protein on the microparticles, with lipid raft domains of endothelial cells. Large quantities of hepatocyte-derived microparticles were detected in the blood of mice with diet-induced steatohepatitis, and microparticle quantity correlated with disease severity. Genetic ablation of caspase 3 or RNA interference directed against VNN1 protected mice from steatohepatitis-induced pathological angiogenesis in the liver and resulted in a loss of the proangiogenic effects of microparticles. Our data identify hepatocyte-derived microparticles as critical signals that contribute to angiogenesis and liver damage in steatohepatitis and suggest a therapeutic target for this condition.

Exosomes in the pathogenesis, diagnostics and therapeutics of liver diseases

Exosomes are small (30-100 nm in diameter) extracellular membrane-enclosed vesicles released by different cell types into the extracellular space or into biological fluids by exocytosis as a result of fusion of intracellular multivesicular bodies with the plasma membrane. The primary function of exosomes is intercellular communication with both beneficial (physiological) and harmful (pathological) potential outcomes. Liver cells are exosome-releasing cells as well as targets for endogenous exosomes and exosomes derived from cells of other organs. Despite limited studies on liver exosomes, initial observations suggest that these vesicles are important in liver physiology and pathophysiology. In this review, we briefly summarize the recent findings on liver exosomes, their functions and significance for novel diagnostic and therapeutic approaches.

Vascular biology of the biliary epithelium

Cholangiocytes are involved in a variety of processes essential for liver pathophysiology. To meet their demanding metabolic and functional needs, bile ducts are nourished by their own arterial supply, the peribiliary plexus. This capillary network originates from the hepatic artery and is strictly arranged around the intrahepatic bile ducts. Biliary and vascular structures are linked by a close anatomic and functional association necessary for liver development, normal organ physiology, and liver repair. This strong association is finely regulated by a range of angiogenic signals, enabling the cross talk between cholangiocytes and the different vascular cell types. This review will briefly illustrate the "vascular" properties of cholangiocytes, their underlying molecular mechanisms and the relevant pathophysiological settings.

Cellular mechanisms of tissue fibrosis. 5. Novel insights into liver fibrosis

Liver fibrosis is the common scarring reaction associated with chronic liver injury that results from prolonged parenchymal cell injury and/or inflammation. The fibrogenic response is characterized by progressive accumulation of extracellular matrix components enriched in fibrillar collagens and a failure of matrix turnover. This process is driven by a heterogeneous population of hepatic myofibroblasts, which mainly derive from hepatic stellate cells and portal fibroblasts. Regression of fibrosis can be achieved by the successful control of chronic liver injury, owing to termination of the fibrogenic reaction following clearance of hepatic myofibroblasts and restoration of fibrolytic pathways. Understanding of the complex network underlying liver fibrogenesis has allowed the identification of a large number of antifibrotic targets, but no antifibrotic drug has as yet been approved. This review will highlight recent advances regarding the mechanisms that regulate liver fibrogenesis and fibrosis regression, with special focus on novel signaling pathways and the role of inflammatory cells. Translation of these findings to therapies will require continued efforts to develop multitarget therapeutic approaches that will improve the grim prognosis of liver cirrhosis.

Transcriptome of extracellular vesicles released by hepatocytes

The discovery that the cells communicate through emission of vesicles has opened new opportunities for better understanding of physiological and pathological mechanisms. This discovery also provides a novel source for non-invasive disease biomarker research. Our group has previously reported that hepatocytes release extracellular vesicles with protein content reflecting the cell-type of origin. Here, we show that the extracellular vesicles released by hepatocytes also carry RNA. We report the messenger RNA composition of extracellular vesicles released in two non-tumoral hepatic models: primary culture of rat hepatocytes and a progenitor cell line obtained from a mouse foetal liver. We describe different subpopulations of extracellular vesicles with different densities and protein and RNA content. We also show that the RNA cargo of extracellular vesicles released by primary hepatocytes can be transferred to rat liver stellate-like cells and promote their activation. Finally, we provide in vitro and in vivo evidence that liver-damaging drugs galactosamine, acetaminophen, and diclofenac modify the RNA content of these vesicles. To summarize, we show that the extracellular vesicles secreted by hepatocytes contain various RNAs. These vesicles, likely to be involved in the activation of stellate cells, might become a new source for non-invasive identification of the liver toxicity markers.

Circulating microparticles: square the circle

Background

The present review summarizes current knowledge about microparticles (MPs) and provides a systematic overview of last 20 years of research on circulating MPs, with particular focus on their clinical relevance.

Results

MPs are a heterogeneous population of cell-derived vesicles, with sizes ranging between 50 and 1000 nm. MPs are capable of transferring peptides, proteins, lipid components, microRNA, mRNA, and DNA from one cell to another without direct cell-to-cell contact. Growing evidence suggests that MPs present in peripheral blood and body fluids contribute to the development and progression of cancer, and are of pathophysiological relevance for autoimmune, inflammatory, infectious, cardiovascular, hematological, and other diseases. MPs have large diagnostic potential as biomarkers; however, due to current technological limitations in purification of MPs and an absence of standardized methods of MP detection, challenges remain in validating the potential of MPs as a non-invasive and early diagnostic platform.

Conclusions

Improvements in the effective deciphering of MP molecular signatures will be critical not only for diagnostics, but also for the evaluation of treatment regimens and predicting disease outcomes.

Novel therapeutic targets for nonalcoholic fatty liver disease

INTRODUCTION

Nonalcoholic fatty liver disease (NAFLD) is a serious public health problem. It is now estimated to affect 30% of adults and about 10% of children in the U.S. Hispanics are disproportionably affected with not only higher rates of NAFLD but also more severe disease. Treatment options are currently limited.

AREAS COVERED

In this review, we will focus on a series of novel findings related to the pathobiology of liver damage in nonalcoholic steatohepatitis (NASH) that are attractive targets for development of novel therapeutic strategies for human NASH. In particular, we will discuss four different areas due to their novelty and growing importance including microparticles, the inflammasomes, gut-liver axis and dietary lipids.

EXPERT OPINION

There is an urgent need to develop novel safe and effective therapies for the growing NAFLD epidemic. The data discussed in this article provide strong rational to think out of the box when considering novel therapeutic targets for patients with NAFLD.

Origins and functions of liver myofibroblasts

Myofibroblasts combine the matrix-producing functions of fibroblasts and the contractile properties of smooth muscle cells. They are the main effectors of fibrosis in all tissues and make a major contribution to other aspects of the wound healing response, including regeneration and angiogenesis. They display the de novo expression of α-smooth muscle actin. Myofibroblasts, which are absent from the normal liver, are derived from two major sources: hepatic stellate cells (HSCs) and portal mesenchymal cells in the injured liver. Reliable markers for distinguishing between the two subpopulations at the myofibroblast stage are currently lacking, but there is evidence to suggest that both myofibroblast cell types, each exposed to a particular microenvironment (e.g. hypoxia for HSC-MFs, ductular reaction for portal mesenchymal cell-derived myofibroblasts (PMFs)), expand and exert specialist functions, in scarring and inflammation for PMFs, and in vasoregulation and hepatocellular healing for HSC-MFs. Angiogenesis is a major mechanism by which myofibroblasts contribute to the progression of fibrosis in liver disease. It has been clearly demonstrated that liver fibrosis can regress, and this process involves a deactivation of myofibroblasts, although probably not to a fully quiescent phenotype. This article is part of a Special Issue entitled: Fibrosis: Translation of basic research to human disease.

Hedgehog signalling regulates liver sinusoidal endothelial cell capillarisation

OBJECTIVE

Vascular remodelling during liver damage involves loss of healthy liver sinusoidal endothelial cell (LSEC) phenotype via capillarisation. Hedgehog (Hh) signalling regulates vascular development and increases during liver injury. This study therefore examined its role in capillarisation.

DESIGN

Primary LSEC were cultured for 5 days to induce capillarisation. Pharmacological, antibody-mediated and genetic approaches were used to manipulate Hh signalling. Effects on mRNA and protein expression of Hh-regulated genes and capillarisation markers were evaluated by quantitative reverse transcription PCR and immunoblot. Changes in LSEC function were assessed by migration and tube forming assay, and gain/loss of fenestrae was examined by electron microscopy. Mice with acute or chronic liver injury were treated with Hh inhibitors; effects on capillarisation were assessed by immunohistochemistry.

RESULTS

Freshly isolated LSEC expressed Hh ligands, Hh receptors and Hh ligand antagonist Hhip. Capillarisation was accompanied by repression of Hhip and increased expression of Hh-regulated genes. Treatment with Hh agonist further induced expression of Hh ligands and Hh-regulated genes, and upregulated capillarisation-associated genes; whereas Hh signalling antagonist or Hh ligand neutralising antibody each repressed expression of Hh target genes and capillarisation markers. LSEC isolated from Smo(loxP/loxP) transgenic mice that had been infected with adenovirus expressing Cre-recombinase to delete Smoothened showed over 75% knockdown of Smoothened. During culture, Smoothened-deficient LSEC had inhibited Hh signalling, less induction of capillarisation-associated genes and retention of fenestrae. In mice with injured livers, inhibiting Hh signalling prevented capillarisation.

CONCLUSIONS

LSEC produce and respond to Hh ligands, and use Hh signalling to regulate complex phenotypic changes that occur during capillarisation.

Macrophage-derived Hedgehog ligands promotes fibrogenic and angiogenic responses in human schistosomiasis mansoni

BACKGROUND

Schistosomiasis mansoni is a major cause of portal fibrosis and portal hypertension. The Hedgehog pathway regulates fibrogenic repair in some types of liver injury.

AIMS

Determine if Hedgehog pathway activation occurs during fibrosis progression in schistosomiasis and to determine if macrophage-related mechanisms are involved.

METHODS

Immunohistochemistry was used to characterize the cells that generate and respond to Hedgehog ligands in 28 liver biopsies from patients with different grades of schistosomiasis fibrosis staged by ultrasound. Cultured macrophages (RAW264.7 and primary rat Kupffer cells) and primary rat liver sinusoidal endothelial cells (LSEC) were treated with schistosome egg antigen (SEA) and evaluated using qRT-PCR. Inhibition of the Hedgehog pathway was used to investigate its role in alternative activation of macrophages (M2) and vascular tube formation.

RESULTS

Patients with schistosomiasis expressed more ligands (Shh and Ihh) and target genes (Patched and Gli2) than healthy individuals. Activated LSEC and myofibroblasts were Hedgehog responsive [Gli2(+)] and accumulated in parallel with fibrosis stage (P < 0.05). Double IHC for Ihh/CD68 showed that Ihh(+) cells were macrophages. In vitro studies demonstrated that SEA-stimulated macrophages to express Ihh and Shh mRNA (P < 0.05). Conditioned media from such macrophages induced luciferase production by Shh-LightII cells (P < 0.001) and Hedgehog inhibitors blocked this effect (P < 0.001). SEA-treated macrophages also up-regulated their own expression of M2 markers, and Hh pathway inhibitors abrogated this response (P < 0.01). Inhibition of the Hedgehog pathway in LSEC blocked SEA-induced migration and tube formation.

CONCLUSION

SEA stimulates liver macrophages to produce Hh ligands, which promote alternative activation of macrophages, fibrogenesis and vascular remodelling in schistosomiasis.

Signals and cells involved in regulating liver regeneration

Liver regeneration is a complex phenomenon aimed at maintaining a constant liver mass in the event of injury resulting in loss of hepatic parenchyma. Partial hepatectomy is followed by a series of events involving multiple signaling pathways controlled by mitogenic growth factors (HGF, EGF) and their receptors (MET and EGFR). In addition multiple cytokines and other signaling molecules contribute to the orchestration of a signal which drives hepatocytes into DNA synthesis. The other cell types of the liver receive and transmit to hepatocytes complex signals so that, in the end of the regenerative process, complete hepatic tissue is assembled and regeneration is terminated at the proper time and at the right liver size. If hepatocytes fail to participate in this process, the biliary compartment is mobilized to generate populations of progenitor cells which transdifferentiate into hepatocytes and restore liver size.

Attenuation of early liver fibrosis by pharmacological inhibition of smoothened receptor signaling

Hedgehog (Hh) signaling is involved in the pathogenesis of liver fibrosis. It has been previously shown that Hh-inhibitor cyclopamine (CYA) can reduce liver fibrosis in rats. However, CYA is not stable in vivo, which limits its clinical application. This study compares the antifibrotic potential of two known Hh antagonists, vismodegib (GDC-0449, abbreviated to GDC) and CYA. GDC is a synthetic molecule presently in clinical cancer trials and has been reported to be safe and efficacious. These drugs attenuated early liver fibrosis in common bile duct ligated rats, improved liver function, and prevented hepatic stellate cell (HSC) activation, thereby suppressing epithelial to mesenchymal transition (EMT). While both CYA and GDC increased the number of proliferating cell nuclear antigen positive liver cells in vivo, only CYA increased Caspase-3 expression in HSCs in rat livers, suggesting that while GDC and CYA effectively attenuate early liver fibrosis, their hepatoprotective effects may be mediated through different modes of action. Thus, GDC has the potential to serve as a new therapeutic agent for treating early liver fibrosis.

Mechanisms of disease progression in NASH: new paradigms

The incidence of nonalcoholic fatty liver disease is increasing at an astonishing rate in the US population. Although only a small proportion of these patients develop steatohepatitis (NASH), those who do have a greater likelihood of developing end-stage liver disease and complications. Research on liver fibrosis and NASH progression shows that hedgehog (Hh) is reactivated after liver injury to assist in liver repair and regeneration. When the process of tissue repair and regeneration is prolonged or when Hh ligand and related genes are aberrantly regulated and excessive, tissue repair goes awry and NASH progresses to cirrhosis and hepatocellular carcinoma.

Circulating microparticles as disease-specific biomarkers of severity of inflammation in patients with hepatitis C or nonalcoholic steatohepatitis

BACKGROUND & AIMS

Microparticles released into the bloodstream upon activation or apoptosis of CD4(+) and CD8(+) T cells correlate with inflammation as determined by histologic analysis in patients with chronic hepatitis C (CHC). Patients with nonalcoholic fatty liver (NAFL) or nonalcoholic steatohepatitis (NASH) can be differentiated from those with CHC based on activation of distinct sets of immune cells in the liver.

METHODS

We compared profiles of circulating microparticles from patients with NAFL and NASH (n = 67) to those of CHC (n = 42), with healthy individuals (controls) using flow cytometry; the profiles were correlated with inflammation grade and fibrosis stage based on histologic analyses. We assessed the ability of the profiles to determine the severity of inflammation and fibrosis based on serologic and histologic analyses.

RESULTS

Patients with CHC had increased levels of microparticles from CD4(+) and CD8(+) T cells; the levels correlated with disease severity based on histologic analysis and levels of alanine aminotransferase. Patients with NAFL or NASH had significant increases in numbers of microparticles from invariant natural killer T cells and macrophages/monocytes (CD14(+)), which mediate pathogenesis of NASH. Microparticles from CD14(+) and invariant natural killer T cells correlated with levels of alanine aminotransferase and severity of NASH (based on histology). Levels of microparticles could differentiate between patients with NAFL or NASH and those with CHC, or either group of patients and controls (area under the receiver operating characteristic curves ranging from 0.56 to 0.99).

CONCLUSIONS

Quantification of immune cell microparticles from serum samples can be used to assess the extent and characteristics of hepatic inflammation in patients with chronic liver disease.

Liver extracellular vesicles in health and disease

Extracellular vesicles (EVs) play an important role in cell-to-cell communication. Although there are different kinds of vesicles, each with their own secretion and capture biology, all of them carry a cargo of proteins, lipids, metabolites and nucleic acids. They act as vehicles for exchange of biological materials and signals and are involved in the regulation of various physiological processes. Liver is an essential organ containing different cell populations fulfilling various functions, which need to be strictly controlled and coordinated. There are a few articles showing the role of liver-derived EVs. On the basis of them, we present here a hypothesis of the implication of such vesicles in the physiology of the liver. Different liver cell types, including hepatocytes, cholangiocytes and stellate cells, secrete and capture EVs and interact with them. Liver injury changes the abundance and cargo of EVs; these changes are likely to be important for the outcome of stress response. Although a substantial effort has been put into the characterization of EVs in isolated populations, it is only recently that some more comprehensive information has begun to appear. In this article, we hypothesize about the role of EVs in liver microenvironment and their possible function using published data from both hepatic and non-hepatic systems.

Hedgehog pathway activation parallels histologic severity of injury and fibrosis in human nonalcoholic fatty liver disease

The Hedgehog (HH)-signaling pathway mediates several processes that are deregulated in patients with metabolic syndrome (e.g., fat mass regulation, vascular/endothelial remodeling, liver injury and repair, and carcinogenesis). The severity of nonalcoholic fatty liver disease (NAFLD) and metabolic syndrome generally correlate. Therefore, we hypothesized that the level of HH-pathway activation would increase in parallel with the severity of liver damage in NAFLD. To assess potential correlations between known histologic and clinical predictors of advanced liver disease and HH-pathway activation, immunohistochemistry was performed on liver biopsies from a large, well-characterized cohort of NAFLD patients (n = 90) enrolled in the Nonalcoholic Steatohepatitis Clinical Research Network (NASH CRN) Database 1 study. Increased HH activity (evidenced by accumulation of HH-ligand-producing cells and HH-responsive target cells) strongly correlated with portal inflammation, ballooning, and fibrosis stage (each P < 0.0001), supporting a relationship between HH-pathway activation and liver damage. Pathway activity also correlated significantly with markers of liver repair, including numbers of hepatic progenitors and myofibroblastic cells (both P < 0.03). In addition, various clinical parameters that have been linked to histologically advanced NAFLD, including increased patient age (P < 0.005), body mass index (P < 0.002), waist circumference (P < 0.0007), homeostatic model assessment of insulin resistance (P < 0.0001), and hypertension (P < 0.02), correlated with hepatic HH activity.

CONCLUSION

In NAFLD patients, the level of hepatic HH-pathway activity is highly correlated with the severity of liver damage and with metabolic syndrome parameters that are known to be predictive of advanced liver disease. Hence, deregulation of the HH-signaling network may contribute to the pathogenesis and sequelae of liver damage that develops with metabolic syndrome.

Sinusoidal endothelial dysfunction precedes inflammation and fibrosis in a model of NAFLD

Non-alcoholic fatty liver disease (NAFLD) is the hepatic manifestation of the metabolic syndrome. Most morbidity associated with the metabolic syndrome is related to vascular complications, in which endothelial dysfunction is a major pathogenic factor. However, whether NAFLD is associated with endothelial dysfunction within the hepatic vasculature is unknown. The aims of this study were to explore, in a model of diet-induced overweight that expresses most features of the metabolic syndrome, whether early NAFLD is associated with liver endothelial dysfunction. Wistar Kyoto rats were fed a cafeteria diet (CafD; 65% of fat, mostly saturated) or a control diet (CD) for 1 month. CafD rats developed features of the metabolic syndrome (overweight, arterial hypertension, hypertryglyceridemia, hyperglucemia and insulin resistance) and liver steatosis without inflammation or fibrosis. CafD rats had a significantly higher in vivo hepatic vascular resistance than CD. In liver perfusion livers from CafD rats had an increased portal perfusion pressure and decreased endothelium-dependent vasodilation. This was associated with a decreased Akt-dependent eNOS phosphorylation and NOS activity. In summary, we demonstrate in a rat model of the metabolic syndrome that shows features of NAFLD, that liver endothelial dysfunction occurs before the development of fibrosis or inflammation.

Microvesicles as novel biomarkers and therapeutic targets in transplantation medicine

Microvesicles (MVs) including exosomes are emerging new biomarkers and potential regulators of inflammation and immunological processes. Such particles contain proteins and genetic information including DNA and microRNAs that may be of importance for cell/cell communication. However, their role during and after organ transplantation and immunomodulatory effects is only in its beginning of understanding. We here, in brief, introduce generation and biological importance of MVs, describe their (patho)physiological roles and their potential use as future biomarkers and therapeutic agents in transplantation medicine. Circulating MVs may have a great potential to detect possible immune rejections and MV modulation may emerge as a therapeutic approach in organ rejection therapy.

Sinusoidal endothelial dysfunction precedes inflammation and fibrosis in a model of NAFLD

Non-alcoholic fatty liver disease (NAFLD) is the hepatic manifestation of the metabolic syndrome. Most morbidity associated with the metabolic syndrome is related to vascular complications, in which endothelial dysfunction is a major pathogenic factor. However, whether NAFLD is associated with endothelial dysfunction within the hepatic vasculature is unknown. The aims of this study were to explore, in a model of diet-induced overweight that expresses most features of the metabolic syndrome, whether early NAFLD is associated with liver endothelial dysfunction. Wistar Kyoto rats were fed a cafeteria diet (CafD; 65% of fat, mostly saturated) or a control diet (CD) for 1 month. CafD rats developed features of the metabolic syndrome (overweight, arterial hypertension, hypertryglyceridemia, hyperglucemia and insulin resistance) and liver steatosis without inflammation or fibrosis. CafD rats had a significantly higher in vivo hepatic vascular resistance than CD. In liver perfusion livers from CafD rats had an increased portal perfusion pressure and decreased endothelium-dependent vasodilation. This was associated with a decreased Akt-dependent eNOS phosphorylation and NOS activity. In summary, we demonstrate in a rat model of the metabolic syndrome that shows features of NAFLD, that liver endothelial dysfunction occurs before the development of fibrosis or inflammation.

Sonic Hedgehog on microparticles and neovascularization

Neovascularization represents a pivotal process consisting in the development of vascular network during embryogenesis and adult life. Postnatally, it arises mainly through angiogenesis, which has physiological and pathological roles in health and disease. Blood vessel formation results as tightly regulated multistep process which needs coordination and precise regulation of the balance of proangiogenic and antiangiogenic factors. Sonic Hedgehog (SHH), a morphogen belonging to Hedgehog (HH) family proteins, is implicated in a remarkably wide variety of process, including vessel development. Recent evidence demonstrate that, in addition to the classic factors, microvesicles (MVs), both microparticles (MPs) and exosomes, small vesicles released distinct cellular compartments, are involved in modulation of neovascularization. MPs generated from T lymphocytes undergoing both activation and apoptosis harbor at their surface SHH and play a crucial role in modulation of neovascularization. They are able to modulate the different steps implicated in angiogenesis process in vitro and to enhance postischemic neovascularization in vivo. As the consequence, we suggest that the MPs carrying SHH contribute to generation of a vascular network and may represent a new therapeutic approach to treat pathologies associated with failed angiogenesis.

Hedgehog signaling antagonist promotes regression of both liver fibrosis and hepatocellular carcinoma in a murine model of primary liver cancer

OBJECTIVE

Chronic fibrosing liver injury is a major risk factor for hepatocarcinogenesis in humans. Mice with targeted deletion of Mdr2 (the murine ortholog of MDR3) develop chronic fibrosing liver injury. Hepatocellular carcinoma (HCC) emerges spontaneously in such mice by 50-60 weeks of age, providing a model of fibrosis-associated hepatocarcinogenesis. We used Mdr2(-/-) mice to investigate the hypothesis that activation of the hedgehog (Hh) signaling pathway promotes development of both liver fibrosis and HCC.

METHODS

Hepatic injury and fibrosis, Hh pathway activation, and liver progenitor populations were compared in Mdr2(-/-) mice and age-matched wild type controls. A dose finding experiment with the Hh signaling antagonist GDC-0449 was performed to optimize Hh pathway inhibition. Mice were then treated with GDC-0449 or vehicle for 9 days, and effects on liver fibrosis and tumor burden were assessed by immunohistochemistry, qRT-PCR, Western blot, and magnetic resonance imaging.

RESULTS

Unlike controls, Mdr2(-/-) mice consistently expressed Hh ligands and progressively accumulated Hh-responsive liver myofibroblasts and progenitors with age. Treatment of aged Mdr2-deficient mice with GDC-0449 significantly inhibited hepatic Hh activity, decreased liver myofibroblasts and progenitors, reduced liver fibrosis, promoted regression of intra-hepatic HCCs, and decreased the number of metastatic HCC without increasing mortality.

CONCLUSIONS

Hh pathway activation promotes liver fibrosis and hepatocarcinogenesis, and inhibiting Hh signaling safely reverses both processes even when fibrosis and HCC are advanced.

Increased production of sonic hedgehog by ballooned hepatocytes

Ballooned hepatocytes distinguish non-alcoholic steatohepatitis (NASH) from steatosis. Such cells contain dilated endoplasmic reticulum and ubiquitin aggregates, characteristics of endoplasmic reticulum stress. Hepatocyte ballooning increases the risk for fibrosis in NASH, suggesting that ballooned hepatocytes release pro-fibrogenic factors. Hedgehog ligands function as pro-fibrogenic factors in liver diseases, but mechanisms for hedgehog ligand production remain poorly understood. We evaluated the hypothesis that endoplasmic reticulum stress induces hepatocyte production of hedgehog ligands that provide paracrine pro-fibrogenic signals to neighbouring cells. In livers from NASH patients, keratin 8/18 and ubiquitin staining demonstrated enlarged, keratin 8/18-negative/ubiquitin-positive hepatocytes (ballooned hepatocytes) that were positive for Sonic hedgehog. In order to model endoplasmic reticulum stress in vitro, primary mouse hepatocytes were treated with tunicamycin. Compared to vehicle, tunicamycin significantly increased Sonic hedgehog and Indian hedgehog expression. Furthermore, conditioned medium from tunicamycin-treated hepatocytes increased Gli-luciferase reporter activity 14-fold more than conditioned medium from vehicle-treated hepatocytes. Cyclopamine (hedgehog signalling inhibitor) abrogated the effect of conditioned medium from tunicamycin-treated hepatocytes, verifying that soluble hepatocyte-derived factors activate hedgehog signalling. Ballooned hepatocytes in NASH patients did not express the hedgehog target gene, Gli2, α-smooth muscle actin or vimentin, but were surrounded by Gli2-positive stromal cells expressing these myofibroblast markers. Trichrome staining demonstrated the accumulation of ballooned hepatocytes in areas of matrix deposition, and numbers of Sonic hedgehog-positive hepatocytes correlated with the degree of ballooning and fibrosis stage. Hepatocytes undergoing endoplasmic reticiulum stress generate hedgehog ligands which act as paracrine pro-fibrogenic factors for hedgehog-responsive stromal cells. These results help to explain why fibrosis stage correlates with hepatocyte ballooning in NASH.

Hedgehog signaling in cholangiocytes

PURPOSE OF REVIEW

Cells lining the biliary tree are targets of injury, but also orchestrate liver repair. The latter involves autocrine/paracrine signaling that enhances the viability and growth of residual ductular cells and promotes accumulation of inflammatory and myofibroblastic cells. The mechanisms mediating this so-called 'ductular reaction' need to be better understood to improve injury outcomes. Studies are revealing that ductular cells produce and respond to hedgehog (Hh) ligands, developmental morphogens that control progenitor cell fate and tissue construction during embryogenesis. Because this has potential implications for liver repair, this review will summarize current knowledge about Hh signaling and cholangiocytes.

RECENT FINDINGS

Diverse types of liver injury stimulate cholangiocytes to generate Hh ligands, and cholangiocyte-derived Hh ligands interact with receptors on cholangiocytes and neighboring cells to modulate virtually every aspect of the ductular reaction to injury. Excessive Hh signaling promotes dysfunctional repair and results in chronic hepatic inflammation, fibrogenesis, and carcinogenesis.

SUMMARY

The Hh pathway is part of the complex signaling network that orchestrates liver repair. How other pathways and posttranscriptional mechanisms modulate Hh signaling in ductular cells remains unclear. Further research in this area may identify novel therapeutic targets for the treatment of cholangiopathies and cholangiocarcinoma.

Hedgehog activity, epithelial-mesenchymal transitions, and biliary dysmorphogenesis in biliary atresia

Biliary atresia (BA) is notable for marked ductular reaction and rapid development of fibrosis. Activation of the Hedgehog (Hh) pathway promotes the expansion of populations of immature epithelial cells that coexpress mesenchymal markers and may be profibrogenic. We examined the hypothesis that in BA excessive Hh activation impedes ductular morphogenesis and enhances fibrogenesis by promoting accumulation of immature ductular cells with a mesenchymal phenotype. Livers and remnant extrahepatic ducts from BA patients were evaluated by quantitative reverse-transcription polymerase chain reaction (QRT-PCR) and immunostaining for Hh ligands, target genes, and markers of mesenchymal cells or ductular progenitors. Findings were compared to children with genetic cholestatic disease, age-matched deceased donor controls, and adult controls. Ductular cells isolated from adult rats with and without bile duct ligation were incubated with Hh ligand-enriched medium ± Hh-neutralizing antibody to determine direct effects of Hh ligands on epithelial to mesenchymal transition (EMT) marker expression. Livers from pediatric controls showed greater innate Hh activation than adult controls. In children with BA, both intra- and extrahepatic ductular cells demonstrated striking up-regulation of Hh ligand production and increased expression of Hh target genes. Excessive accumulation of Hh-producing cells and Hh-responsive cells also occurred in other infantile cholestatic diseases. Further analysis of the BA samples demonstrated that immature ductular cells with a mesenchymal phenotype were Hh-responsive. Treating immature ductular cells with Hh ligand-enriched medium induced mesenchymal genes; neutralizing Hh ligands inhibited this.

CONCLUSION

BA is characterized by excessive Hh pathway activity, which stimulates biliary EMT and may contribute to biliary dysmorphogenesis. Other cholestatic diseases show similar activation, suggesting that this is a common response to cholestatic injury in infancy.

Hedgehog signaling in the liver

Reactivation of Hedgehog (Hh), a morphogenic signaling pathway that controls progenitor cell fate and tissue construction during embryogenesis occurs during many types of liver injury in adult. The net effects of activating the Hedgehog pathway include expansion of liver progenitor populations to promote liver regeneration, but also hepatic accumulation of inflammatory cells, liver fibrogenesis, and vascular remodeling. All of these latter responses are known to be involved in the pathogenesis of cirrhosis. In addition, Hh signaling may play a role in primary liver cancers, such as cholangiocarcinoma and hepatocellular carcinoma. Study of Hedgehog signaling in liver cells is in its infancy. Additional research in this area is justified given growing experimental and clinical data supporting a role for the pathway in regulating outcomes of liver injury.

Osteopontin is induced by hedgehog pathway activation and promotes fibrosis progression in nonalcoholic steatohepatitis

Nonalcoholic steatohepatitis (NASH) is a leading cause of cirrhosis. Recently, we showed that NASH-related cirrhosis is associated with Hedgehog (Hh) pathway activation. The gene encoding osteopontin (OPN), a profibrogenic extracellular matrix protein and cytokine, is a direct transcriptional target of the Hh pathway. Thus, we hypothesize that Hh signaling induces OPN to promote liver fibrosis in NASH. Hepatic OPN expression and liver fibrosis were analyzed in wild-type (WT) mice, Patched-deficient (Ptc(+/-) ) (overly active Hh signaling) mice, and OPN-deficient mice before and after feeding methionine and choline-deficient (MCD) diets to induce NASH-related fibrosis. Hepatic OPN was also quantified in human NASH and nondiseased livers. Hh signaling was manipulated in cultured liver cells to assess direct effects on OPN expression, and hepatic stellate cells (HSCs) were cultured in medium with different OPN activities to determine effects on HSC phenotype. When fed MCD diets, Ptc(+/-) mice expressed more OPN and developed worse liver fibrosis (P < 0.05) than WT mice, whereas OPN-deficient mice exhibited reduced fibrosis (P < 0.05). In NASH patients, OPN was significantly up-regulated and correlated with Hh pathway activity and fibrosis stage. During NASH, ductular cells strongly expressed OPN. In cultured HSCs, SAG (an Hh agonist) up-regulated, whereas cyclopamine (an Hh antagonist) repressed OPN expression (P < 0.005). Cholangiocyte-derived OPN and recombinant OPN promoted fibrogenic responses in HSCs (P < 0.05); neutralizing OPN with RNA aptamers attenuated this (P < 0.05).

CONCLUSION

OPN is Hh-regulated and directly promotes profibrogenic responses. OPN induction correlates with Hh pathway activity and fibrosis stage. Therefore, OPN inhibition may be beneficial in NASH.

Human T cell microparticles circulate in blood of hepatitis patients and induce fibrolytic activation of hepatic stellate cells

Microparticles (MPs) are small cell membrane vesicles that are released from cells during apoptosis or activation. Although circulating platelet MPs have been studied in some detail, the existence and functional role of T cell MPs remain elusive. We show that blood from patients with active hepatitis C (alanine aminotransferase [ALT] level >100 IU/mL) contains elevated numbers of T cell MPs compared with patients with mild hepatitis C (ALT <40 IU/mL) and healthy controls. T cell MPs fuse with cell membranes of hepatic stellate cells (HSCs), the major effector cells for excess matrix deposition in liver fibrosis and cirrhosis. MP uptake is partly intercellular adhesion molecule 1-dependent and leads to activation of nuclear factor kappa B and extracellular signal-regulated kinases 1 and 2 and subsequent up-regulation of fibrolytic genes in HSCs, down-regulation of procollagen α1(I) messenger RNA, and blunting of profibrogenic activities of transforming growth factor β1. Ex vivo, the induced fibrolytic activity is evident in MPs derived from activated CD4+ T cells and is highest in MPs derived from activated and apoptotic CD8+ T cells. Mass spectrometry, fluorescence-activated cell sorting analysis, and function blocking antibodies revealed CD147/Emmprin as a candidate transmembrane molecule in HSC fibrolytic activation by CD8+ T cell MPs.

CONCLUSION

Circulating T cell MPs are a novel diagnostic marker for inflammatory liver diseases, and in vivo induction of T cell MPs may be a novel strategy to induce regression of liver fibrosis.

Viral factors induce Hedgehog pathway activation in humans with viral hepatitis, cirrhosis, and hepatocellular carcinoma

Hedgehog (Hh) pathway activation promotes many processes that occur during fibrogenic liver repair. Whether the Hh pathway modulates the outcomes of virally mediated liver injury has never been examined. Gene-profiling studies of human hepatocellular carcinomas (HCCs) demonstrate Hh pathway activation in HCCs related to chronic infection with hepatitis B virus (HBV) or hepatitis C virus (HCV). Because most HCCs develop in cirrhotic livers, we hypothesized that Hh pathway activation occurs during fibrogenic repair of liver damage due to chronic viral hepatitis, and that Hh-responsive cells mediate disease progression and hepatocarciongenesis in chronic viral hepatitis. Immunohistochemistry and qRT-PCR analysis were used to analyze Hh pathway activation and identify Hh-responsive cell types in liver biopsies from 45 patients with chronic HBV or HCV. Hh signaling was then manipulated in cultured liver cells to directly assess the impact of Hh activity in relevant cell types. We found increased hepatic expression of Hh ligands in all patients with chronic viral hepatitis, and demonstrated that infection with HCV stimulated cultured hepatocytes to produce Hh ligands. The major cell populations that expanded during cirrhosis and HCC (ie, liver myofibroblasts, activated endothelial cells, and progenitors expressing markers of tumor stem/initiating cells) were Hh responsive, and higher levels of Hh pathway activity associated with cirrhosis and HCC. Inhibiting pathway activity in Hh-responsive target cells reduced fibrogenesis, angiogenesis, and growth. In conclusion, HBV/HCV infection increases hepatocyte production of Hh ligands and expands the types of Hh-responsive cells that promote liver fibrosis and cancer.

The role of Hedgehog signaling in fibrogenic liver repair

Repair of adult liver, like many tissues, involves the coordinated response of a number of different cell types. In adult livers, fibroblastic cells, ductular cells, inflammatory cells, and progenitor cells contribute to this process. Our studies demonstrate that the fates of such cells are dictated, at least in part, by Hedgehog, a fetal morphogenic pathway that was once thought to be active mainly during embryogenesis. Studies of injured adult human and rodent livers demonstrate that injury-related activation of the Hedgehog pathway modulates several important aspects of repair, including the growth of hepatic progenitor populations, hepatic accumulation of myofibroblasts, repair-related inflammatory responses, vascular remodeling, liver fibrosis and hepatocarcinogenesis. These findings identify the Hedgehog pathway as a potentially important target for biomarker development and therapeutic manipulation, and emphasize the need for further research to advance knowledge about how this pathway is regulated by and interacts with other signals that regulate adult liver repair.

Biliary exosomes influence cholangiocyte regulatory mechanisms and proliferation through interaction with primary cilia

Exosomes are small extracellular vesicles that are thought to participate in intercellular communication. Recent work from our laboratory suggests that, in normal and cystic liver, exosome-like vesicles accumulate in the lumen of intrahepatic bile ducts, presumably interacting with cholangiocyte cilia. However, direct evidence for exosome-ciliary interaction is limited and the physiological relevance of such interaction remains unknown. Thus, in this study, we tested the hypothesis that biliary exosomes are involved in intercellular communication by interacting with cholangiocyte cilia and inducing intracellular signaling and functional responses. Exosomes were isolated from rat bile by differential ultracentrifugation and characterized by scanning, transmission, and immunoelectron microscopy. The exosome-ciliary interaction and its effects on ERK1/2 signaling, expression of the microRNA, miR-15A, and cholangiocyte proliferation were studied on ciliated and deciliated cultured normal rat cholangiocytes. Our results show that bile contains vesicles identified as exosomes by their size, characteristic "saucer-shaped" morphology, and specific markers, CD63 and Tsg101. When NRCs were exposed to isolated biliary exosomes, the exosomes attached to cilia, inducing a decrease of the phosphorylated-to-total ERK1/2 ratio, an increase of miR-15A expression, and a decrease of cholangiocyte proliferation. All these effects of biliary exosomes were abolished by the pharmacological removal of cholangiocyte cilia. Our findings suggest that bile contains exosomes functioning as signaling nanovesicles and influencing intracellular regulatory mechanisms and cholangiocyte proliferation through interaction with primary cilia.

Overview of extracellular microvesicles in drug metabolism

IMPORTANCE OF THE FIELD

Liver is the major body reservoir for enzymes involved in the metabolism of endogenous and xenobiotic compounds. Recently, it has been shown that hepatocytes release exosome-like vesicles to the extracellular medium, and the proteomic characterization of these hepatocyte-secreted exosomes has revealed the presence of several of these enzymes on them.

AREAS COVERED IN THIS REVIEW

A systematic bibliographic search focused on two related aspects: i) xenobiotic-metabolizing enzymes that have been detected in microvesicles (MVs); and ii) MVs that are in the blood stream or secreted by cell types with clear interactions with this fluid.

WHAT THE READER WILL GAIN

A discussion of these hepatocyte-secreted vesicles along with other MVs as enzymatic carriers in the context of extrahepatic drug-metabolizing systems.

TAKE HOME MESSAGE

The contribution of many tissues including the liver to the MV plasma population is supported by several reports. On the other hand, many enzymes involved in the metabolism of drugs have been detected in MVs. Together, these observations support a role of hepatic-MVs in spreading the liver metabolizing activities through the body contributing in this manner to extrahepatic drug metabolism systems what could be relevant for body homeostasis and pharmaceutical interests.

Vascular factors, angiogenesis and biliary tract disease

PURPOSE OF REVIEW

Recent studies have brought to light that angiogenesis and the expression of pro-angiogenic factors such as vascular endothelial growth factors (VEGFs) participate in the pathogenesis of biliary tract diseases. This review summarizes recent progress that has been accomplished in the field, which expands our understanding of the relationship between vascular growth and the biliary tract, particularly the molecular mechanisms that underlie the pathogenesis of biliary tract diseases.

RECENT FINDINGS

Angiogenesis and the expression of vascular factors play a key role in the pathogenesis of primary biliary cirrhosis, cholangiocarcinoma, liver cysts, and in the progression of biliary fibrosis in animal models. Inhibition of angiogenesis limits fibrosis in animal models, whereas the bile acid, taurocholate, has protective effects in animal models of bile duct and peribiliary vascular plexus damage.

SUMMARY

A widening body of information indicates that the expression of pro-angiogenic factors such as VEGFs and angiogenesis play an important role in a variety of biliary tract diseases. Further characterization of the link between angiogenesis and vascular growth factor expression will help in elucidating the mechanisms regulating the pathogenesis of biliary tract diseases and in devising new treatment approaches for these devastating diseases.

Signals from dying hepatocytes trigger growth of liver progenitors

OBJECTIVE

The death rate of mature hepatocytes is chronically increased in various liver diseases, triggering responses that prevent liver atrophy, but often cause fibrosis. Mice with targeted disruption of inhibitor kappa B kinase (Ikk) in hepatocytes (HEP mice) provide a model to investigate this process because inhibiting Ikk-nuclear factor-kappaB (NF-kappaB) signalling in hepatocytes increases their apoptosis.

METHODS

Cell proliferation, apoptosis, progenitors, fibrosis and production of Hedgehog (Hh) ligands (progenitor and myofibroblast growth factors) were compared in HEP and control mice before and after feeding methionine choline-deficient ethionine-supplemented (MCDE) diets. Ikkbeta was deleted from primary hepatocytes to determine the effects on Hh ligand production; Hh signalling was inhibited directly in progenitors to determine the effects on viability. Liver sections from patients were examined to assess relationships between hepatocyte production of Hh ligands, accumulation of myofibroblastic cells and liver fibrosis.

RESULTS

Disrupting the Ikk-NF-kappaB pathway in hepatocytes inhibited their proliferation but induced their production of Hh ligands. The latter provided viability signals for progenitors and myofibroblasts, enhancing accumulation of these cell types and causing fibrogenesis. Findings in the mouse models were recapitulated in diseased human livers.

CONCLUSION

Dying mature hepatocytes produce Hh ligands which promote the compensatory outgrowth of progenitors and myofibroblasts. These results help to explain why diseases that chronically increase hepatocyte death promote cirrhosis.