The myofibroblastic conversion of peribiliary fibrogenic cells distinct from hepatic stellate cells is stimulated by platelet-derived growth factor during liver fibrogenesis

Platelet, Antiplatelet Therapy and Metabolic Dysfunction-Associated Steatotic Liver Disease: A Narrative Review

Metabolic dysfunction-associated steatotic liver disease (MASLD) is not only related to traditional cardiovascular risk factors like type 2 diabetes mellitus and obesity, but it is also an independent risk factor for the development of cardiovascular disease. MASLD has been shown to be independently related to endothelial dysfunction and atherosclerosis. MASLD is characterized by a chronic proinflammatory response that, in turn, may induce a prothrombotic state. Several mechanisms such as endothelial and platelet dysfunction, changes in the coagulative factors, lower fibrinolytic activity can contribute to induce the prothrombotic state. Platelets are players and addresses of metabolic dysregulation; obesity and insulin resistance are related to platelet hyperactivation. Furthermore, platelets can exert a direct effect on liver cells, particularly through the release of mediators from granules. Growing data in literature support the use of antiplatelet agent as a treatment for MASLD. The use of antiplatelets drugs seems to exert beneficial effects on hepatocellular carcinoma prevention in patients with MASLD, since platelets contribute to fibrosis progression and cancer development. This review aims to summarize the main data on the role of platelets in the pathogenesis of MASLD and its main complications such as cardiovascular events and the development of liver fibrosis. Furthermore, we will examine the role of antiplatelet therapy not only in the prevention and treatment of cardiovascular events but also as a possible anti-fibrotic and anti-tumor agent.

Endometriotic mesenchymal stem cells promote the fibrosis process of endometriosis through paracrine TGF-β1 mediated RASAL1 inhibition

BACKGROUND

Endometrial-derived stem cells are key players in endometriosis (EMs) pathogenesis, while the mechanism involved is still unclear. Herein, the role and regulatory mechanism of endometriotic mesenchymal stem cells (ecto-MSCs) in regulating fibrosis during EMs progression were investigated.

METHODS

The mRNA and protein expressions were assessed using qRT-PCR, western blot, and immunofluorescence. Flow cytometry was adopted to analyze the markers of MSCs. Transwell assay was adopted to examine endometriotic stromal cells (ESCs) migration and invasion. The interactions between DNMT3A and RASAL1 were analyzed by ChIP assay. In addition, MSP was employed to detect RASAL1 promoter methylation level.

RESULTS

Ecto-MSCs promoted ESCs migration, invasion, and fibrosis process by TGF-β1 paracrine. It was subsequently revealed that TGF-β1 upregulated DNMT3A in ESCs in a SMAD3-dependent manner. As expected, DNMT3A knockdown abolished ecto-MSCs' facilitation on ESCs migration, invasion, and fibrosis process. DNMT3A, as a methyltransferase, reduced RASAL1 expression in TGF-β1-treated ESCs by increasing RASAL1 promoter methylation level. RASAL1, as an antifibrotic protein, was lowly expressed in TGF-β1-treated ESCs, and its overexpression ameliorated TGF-β1-induced increase in ESCs migration, invasion, and fibrosis process.

CONCLUSION

TGF-β1 secreted by ecto-MSCs facilitated fibrogenesis in EMs through SMAD3/DNMT3A-mediated RASAL1 inhibition.

Cellular heterogeneity and plasticity during NAFLD progression

Nonalcoholic fatty liver disease (NAFLD) is a progressive liver disease that can progress to nonalcoholic steatohepatitis (NASH), NASH-related cirrhosis, and hepatocellular carcinoma (HCC). NAFLD ranges from simple steatosis (or nonalcoholic fatty liver [NAFL]) to NASH as a progressive form of NAFL, which is characterized by steatosis, lobular inflammation, and hepatocellular ballooning with or without fibrosis. Because of the complex pathophysiological mechanism and the heterogeneity of NAFLD, including its wide spectrum of clinical and histological characteristics, no specific therapeutic drugs have been approved for NAFLD. The heterogeneity of NAFLD is closely associated with cellular plasticity, which describes the ability of cells to acquire new identities or change their phenotypes in response to environmental stimuli. The liver consists of parenchymal cells including hepatocytes and cholangiocytes and nonparenchymal cells including Kupffer cells, hepatic stellate cells, and endothelial cells, all of which have specialized functions. This heterogeneous cell population has cellular plasticity to adapt to environmental changes. During NAFLD progression, these cells can exert diverse and complex responses at multiple levels following exposure to a variety of stimuli, including fatty acids, inflammation, and oxidative stress. Therefore, this review provides insights into NAFLD heterogeneity by addressing the cellular plasticity and metabolic adaptation of hepatocytes, cholangiocytes, hepatic stellate cells, and Kupffer cells during NAFLD progression.

Cholangiokines: undervalued modulators in the hepatic microenvironment

The biliary epithelial cells, also known as cholangiocytes, line the intra- and extrahepatic bile ducts, forming a barrier between intra- and extra-ductal environments. Cholangiocytes are mostly known to modulate bile composition and transportation. In hepatobiliary diseases, bile duct injury leads to drastic alterations in cholangiocyte phenotypes and their release of soluble mediators, which can vary depending on the original insult and cellular states (quiescence, senescence, or proliferation). The cholangiocyte-secreted cytokines (also termed cholangiokines) drive ductular cell proliferation, portal inflammation and fibrosis, and carcinogenesis. Hence, despite the previous consensus that cholangiocytes are bystanders in liver diseases, their diverse secretome plays critical roles in modulating the intrahepatic microenvironment. This review summarizes recent insights into the cholangiokines under both physiological and pathological conditions, especially as they occur during liver injury-regeneration, inflammation, fibrosis and malignant transformation processes.

The liver in sepsis: molecular mechanism of liver failure and their potential for clinical translation

Liver failure is a life-threatening complication of infections restricting the host's response to infection. The pivotal role of the liver in metabolic, synthetic, and immunological pathways enforces limits the host's ability to control the immune response appropriately, making it vulnerable to ineffective pathogen resistance and tissue damage. Deregulated networks of liver diseases are gradually uncovered by high-throughput, single-cell resolved OMICS technologies visualizing an astonishing diversity of cell types and regulatory interaction driving tolerogenic signaling in health and inflammation in disease. Therefore, this review elucidates the effects of the dysregulated host response on the liver, consequences for the immune response, and possible avenues for personalized therapeutics.

Platelets in Non-alcoholic Fatty Liver Disease

Non alcoholic steatohepatitis (NASH) is the inflammatory reaction of the liver to excessive accumulation of lipids in the hepatocytes. NASH can progress to cirrhosis and hepatocellular carcinoma (HCC). Fatty liver is the hepatic manifestation of metabolic syndrome. A subclinical inflammatory state is present in patients with metabolic alterations like insulin resistance, type-2 diabetes, obesity, hyperlipidemia, and hypertension. Platelets participate in immune cells recruitment and cytokines-induced liver damage. It is hypothesized that lipid toxicity cause accumulation of platelets in the liver, platelet adhesion and activation, which primes the immunoinflammatory reaction and activation of stellate cells. Recent data suggest that antiplatelet drugs may interrupt this cascade and prevent/improve NASH. They may also improve some metabolic alterations. The pathophysiology of inflammatory liver disease and the implication of platelets are discussed in details.

Pathophysiology and Treatment Options for Hepatic Fibrosis: Can It Be Completely Cured?

Hepatic fibrosis is a dynamic process that occurs as a wound healing response against liver injury. During fibrosis, crosstalk between parenchymal and non-parenchymal cells, activation of different immune cells and signaling pathways, as well as a release of several inflammatory mediators take place, resulting in inflammation. Excessive inflammation drives hepatic stellate cell (HSC) activation, which then encounters various morphological and functional changes before transforming into proliferative and extracellular matrix (ECM)-producing myofibroblasts. Finally, enormous ECM accumulation interferes with hepatic function and leads to liver failure. To overcome this condition, several therapeutic approaches have been developed to inhibit inflammatory responses, HSC proliferation and activation. Preclinical studies also suggest several targets for the development of anti-fibrotic therapies; however, very few advanced to clinical trials. The pathophysiology of hepatic fibrosis is extremely complex and requires comprehensive understanding to identify effective therapeutic targets; therefore, in this review, we focus on the various cellular and molecular mechanisms associated with the pathophysiology of hepatic fibrosis and discuss potential strategies to control or reverse the fibrosis.

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.

Till Death Do Us Part-The Multifaceted Role of Platelets in Liver Diseases

Platelets are tightly connected with the liver, as both their production and their clearance are mediated by the liver. Platelets, in return, participate in a variety of liver diseases, ranging from non-alcoholic fatty liver diseases, (viral) hepatitis, liver fibrosis and hepatocellular carcinoma to liver regeneration. Due to their versatile functions, which include (1) regulation of hemostasis, (2) fine-tuning of immune responses and (3) release of growth factors and cellular mediators, platelets quickly adapt to environmental changes and modulate disease development, leading to different layers of complexity. Depending on the (patho)physiological context, platelets exert both beneficial and detrimental functions. Understanding the precise mechanisms through which platelet function is regulated at different stages of liver diseases and how platelets interact with various resident and non-resident liver cells helps to draw a clear picture of platelet-related therapeutic interventions. Therefore, this review summarizes the current knowledge on platelets in acute and chronic liver diseases and aims to shed light on how the smallest cells in the circulatory system account for changes in the (patho)physiology of the second largest organ in the human body.

A novel model of injured liver ductal organoids to investigate cholangiocyte apoptosis with relevance to biliary atresia

PURPOSE

The fibrogenic process in cholangiopathic diseases such as biliary atresia (BA) involves bile duct injury and apoptosis of cholangiocytes, which leads to the progression of liver fibrosis into liver cirrhosis and can result in end-staged liver disease. Recent advances in the development of organoids or mini-organ structures have allowed us to create an ex vivo injury model of the bile duct that mimics bile duct injury in BA. The aim of this experimental study was to develop a novel model of injured intrahepatic cholangiocytes as this can be relevant to BA. Our new model is important for studying the pathophysiological response of bile ducts to injury and the role of cholangiocytes in initiating the fibrogenic cascade. In addition, it has the potential to be used as a tool for developing new treatment strategies for BA.

METHODS

Liver ductal organoids were generated from the liver of healthy neonatal mouse pups. Intrahepatic bile duct fragments were isolated and cultured in Matrigel dome. Injury was induced in the organoids by administration of acetaminophen in culture medium. The organoids were then evaluated for fibrogenic cytokines expression, cell apoptosis marker and cell proliferation marker.

RESULTS

Organoids generated from intrahepatic bile duct fragments organized themselves into single-layer epithelial spheroids with lumen on the inside mimicking in vivo bile ducts. After 24-h exposure to acetaminophen, cholangiocytes in the organoids responded to the injury by increasing expression of fibrogenic cytokines, transforming growth factor beta-1 (TGF-β1) and platelet-derived growth factor-BB (PDGF-BB). This fibrogenic response of injured organoids was associated with increased cholangiocyte apoptosis and decreased cholangiocyte proliferation.

CONCLUSION

To our knowledge this is the first description of cholangiocyte injury in the organoids derived from intrahepatic bile ducts. Our injury model demonstrated that cholangiocyte apoptosis and its fibrogenic response may play a role in initiation of the fibrogenic process in cholangiopathic diseases such as BA. These findings are important for the development of novel therapy to reduce cholangiocyte apoptosis and to halt the early fibrogenic cascade in liver fibrogenesis. This novel injury model can prove very valuable for future research in biliary atresia.

Review: Pathogenesis of cholestatic liver diseases

Cholestatic liver diseases (CLD) begin to develop after an impairment of bile flow start to affect the biliary tree. Cholangiocytes actively participate in the liver response to injury and repair and the intensity of this reaction is a determinant factor for the development of CLD. Progressive cholangiopathies may ultimately lead to end-stage liver disease requiring at the end orthotopic liver transplantation. This narrative review will discuss cholangiocyte biology and pathogenesis mechanisms involved in four intrahepatic CLD: Primary biliary cholangitis, primary sclerosing cholangitis, cystic fibrosis involving the liver, and polycystic liver disease.

The liver fibrosis niche: Novel insights into the interplay between fibrosis-composing mesenchymal cells, immune cells, endothelial cells, and extracellular matrix

Liver fibrosis is a hepatic wound-healing response caused by chronic liver diseases that include viral hepatitis, alcoholic liver disease, non-alcoholic steatohepatitis, and cholestatic liver disease. Liver fibrosis eventually progresses to cirrhosis that is histologically characterized by an abnormal liver architecture that includes distortion of liver parenchyma, formation of regenerative nodules, and a massive accumulation of extracellular matrix (ECM). Despite intensive investigations into the underlying mechanisms of liver fibrosis, developments of anti-fibrotic therapies for liver fibrosis are still unsatisfactory. Recent novel experimental approaches, such as single-cell RNA sequencing and proteomics, have revealed the heterogeneity of ECM-producing cells (mesenchymal cells) and ECM-regulating cells (immune cells and endothelial cells). These approaches have accelerated the identification of fibrosis-specific subpopulations among these cell types. The ECM also consists of heterogenous components. Their production, degradation, deposition, and remodeling are dynamically regulated in liver fibrosis, further affecting the functions of cells responsible for fibrosis. These cellular and ECM elements cooperatively form a unique microenvironment: a fibrotic niche. Understanding the complex interplay between these elements could lead to a better understanding of underlying fibrosis mechanisms and to the development of effective therapies.

Cholangiocyte pathobiology

Cholangiocytes, the epithelial cells lining the intrahepatic and extrahepatic bile ducts, are highly specialized cells residing in a complex anatomic niche where they participate in bile production and homeostasis. Cholangiocytes are damaged in a variety of human diseases termed cholangiopathies, often causing advanced liver failure. The regulation of cholangiocyte transport properties is increasingly understood, as is their anatomical and functional heterogeneity along the biliary tract. Furthermore, cholangiocytes are pivotal in liver regeneration, especially when hepatocyte regeneration is compromised. The role of cholangiocytes in innate and adaptive immune responses, a critical subject relevant to immune-mediated cholangiopathies, is also emerging. Finally, reactive ductular cells are present in many cholestatic and other liver diseases. In chronic disease states, this repair response contributes to liver inflammation, fibrosis and carcinogenesis and is a subject of intense investigation. This Review highlights advances in cholangiocyte research, especially their role in development and liver regeneration, their functional and biochemical heterogeneity, their activation and involvement in inflammation and fibrosis and their engagement with the immune system. We aim to focus further attention on cholangiocyte pathobiology and the search for new disease-modifying therapies targeting the cholangiopathies.

A genome-wide association study identifies new genes associated with developmental dysplasia of the hip

Developmental dysplasia of the hip (DDH) is one of the most common congenital malformations and covers a spectrum of hip disorders from mild dysplasia to irreducible dislocation. The pathological mechanisms of DDH are poorly understood, which hampers the development of diagnostic tools and treatments. To gain insight into its disease mechanism, we explored the potential biological processes that underlie DDH by integrating pathway analysis tools and performing a genome-wide association study (GWAS). A total of 406 DDH-associated genes (P < 0.001) were identified by our GWAS using a Chinese Han cohort consisting of 386 DDH cases and 500 healthy controls (Set A). We verified the significant loci (P < 10^-5 ) in another Chinese Han cohort consisting of 574 DDH patients and 569 healthy controls (Set B). An intronic Single Nucleotide Polymorphism (SNP) (rs61930502) showed significant association in Set A and Set B (P = 2.65 × 10^-7 and 2.0 × 10^-4 , respectively). The minor allele, rs61930502-A, which tended to prevent DDH showed a dominant effect. Heat shock 70 kDa protein 8 (HSPA8) showed the most direct interactions with other proteins which were coded by DDH-associated genes in the protein-protein interaction analysis. Interestingly, Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis suggested a relation between DDH and the genes involved in type II diabetes mellitus pathway (P = 0.0067). Our genetic and protein interaction evidence could open avenues for future studies of DDH.

Bioabsorbable Drug-Eluting Stent Versus Bare Metal Stent in Iliac Artery Evaluated by Optical Coherence Tomography: An In Vivo Study in Porcine

OBJECTIVE

This study aimed to compare, using optical coherence tomography (OCT), the outcomes of bioabsorbable drug-eluting stent with those of bare metal stent (BMS) following implantation in porcine iliac artery.

METHODS

After the placement of BMS and bioabsorbable drug-eluting stents, we used OCT and digital subtraction angiography to investigate stent appositions, arterial neointima, evagination, and restenosis at 1 and 3 months.

RESULTS

At 1 and 3 months after stent implantation, OCT study was performed to investigate 32 stents and 21 788 struts. Thirty-three malapposed struts were found in the bioabsorbable drug-eluting stent groups and 2 were found in BMS groups. The average neointimal thickness, area, and in-stent stenosis were significantly lower in bioabsorbable drug-eluting stents than in BMS, while the frequency of malapposed struts was higher in the bioresorbable drug-eluting stent groups. Average neointimal thickness was lower in bioabsorbable drug-eluting stents than in BMS at 1 (0.19 ± 0.09 vs 0.67 ± 0.75 mm; P < .001) and 3 months (0.21 ± 0.08 vs 1.52 ± 0.28 mm; P < .001).

CONCLUSION

Our study suggested that bioabsorbable drug-eluting stent is more effective in decreasing arterial restenosis than BMS in animal models.

PDGF-C and PDGF-D signaling in vascular diseases and animal models

Members of the platelet-derived growth factor (PDGF) family are well known to be involved in different pathological conditions. The cellular and molecular mechanisms induced by the PDGF signaling have been well studied. Nevertheless, there is much more to discover about their functions and some important questions to be answered. This review summarizes the known roles of two of the PDGFs, PDGF-C and PDGF-D, in vascular diseases. There are clear implications for these growth factors in several vascular diseases, such as atherosclerosis and stroke. The PDGF receptors are broadly expressed in the cardiovascular system in cells such as fibroblasts, smooth muscle cells and pericytes. Altered expression of the receptors and the ligands have been found in various cardiovascular diseases and current studies have shown important implications of PDGF-C and PDGF-D signaling in fibrosis, neovascularization, atherosclerosis and restenosis.

Cystic Fibrosis-related Liver Disease: Research Challenges and Future Perspectives

OBJECTIVES

Hepatobiliary complications are a leading cause of morbidity and mortality in cystic fibrosis (CF) patients. Knowledge of the underlying pathological aspects and optimal clinical management is, however, sorely lacking.

METHODS

We provide a summary of the lectures given by international speakers at the European Society for Paediatric Gastroenterology, Hepatology, and Nutrition (ESPGHAN) monothematic conference on cystic fibrosis-related liver disease (CFLD) held in Paris in January 2016, to discuss the status of our current knowledge of liver disease in CF patients, to define the critical areas that need to be addressed, and to resolve actions to elucidate relevant mechanisms of disease to optimise future therapeutic options.

CONCLUSIONS

The need for a universal consensus on the definition of CFLD to clarify disease stage and to identify relevant biomarkers to assess disease severity was highlighted. A deeper understanding of the pathophysiology and prognostic factors for the long-term evolution of CFLD is fundamental to move forward and has a strong bearing on identifying potential treatments. Novel experimental models and new treatment options under investigation are discussed and offer hope for the near future of CFLD.

RASAL1 is a potent regulator of hepatic stellate cell activity and liver fibrosis

Liver fibrosis, leading to cirrhosis and liver failure, can occur after chronic liver injury. The transition of hepatic stellate cells (HSCs) from quiescent cells into proliferative and fibrogenic cells is a central event in liver fibrosis. Here, we show that RAS protein activator like-1 (RASAL1), a RAS-GTPase-activating protein, which switches off RAS activity, is significantly decreased during HSC activation, and that HSC activation can be antagonized by forced expression of the RASAL1 protein. We demonstrate that RASAL1 suppresses HSC proliferation by regulating the Ras-MAPK pathway, and that RASAL1 suppresses HSC fibrogenic activity by regulating the PKA-LKB1-AMPK-SRF pathway by interacting with angiotensin II receptor, type 1. We also show that RASAL1-deficient mice are more susceptible to liver fibrosis. These data demonstrate that deregulated RASAL1 expression levels and the affected downstream intracellular signaling are central mediators of perpetuated HSC activation and fibrogenesis in the liver.

Emerging concepts in biliary repair and fibrosis

Chronic diseases of the biliary tree (cholangiopathies) represent one of the major unmet needs in clinical hepatology and a significant knowledge gap in liver pathophysiology. The common theme in cholangiopathies is that the target of the disease is the biliary tree. After damage to the biliary epithelium, inflammatory changes stimulate a reparative response with proliferation of cholangiocytes and restoration of the biliary architecture, owing to the reactivation of a variety of morphogenetic signals. Chronic damage and inflammation will ultimately result in pathological repair with generation of biliary fibrosis and clinical progression of the disease. The hallmark of pathological biliary repair is the appearance of reactive ductular cells, a population of cholangiocyte-like epithelial cells of unclear and likely mixed origin that are able to orchestrate a complex process that involves a number of different cell types, under joint control of inflammatory and morphogenetic signals. Several questions remain open concerning the histogenesis of reactive ductular cells, their role in liver repair, their mechanism of activation, and the signals exchanged with the other cellular elements cooperating in the reparative process. This review contributes to the current debate by highlighting a number of new concepts derived from the study of the pathophysiology of chronic cholangiopathies, such as congenital hepatic fibrosis, biliary atresia, and Alagille syndrome.

The deleterious interplay between tumor epithelia and stroma in cholangiocarcinoma

Prognosis of cholangiocarcinoma, a devastating liver epithelial malignancy characterized by early invasiveness, remains very dismal, though its incidence has been steadily increasing. Evidence is mounting that in cholangiocarcinoma, tumor epithelial cells establish an intricate web of mutual interactions with multiple stromal components, largely determining the pervasive behavior of the tumor. The main cellular components of the tumor microenvironment (i.e. myofibroblasts, macrophages, lymphatic endothelial cells), which has been recently termed as 'tumor reactive stroma', are recruited and activated by neoplastic cells, and in turn, deleteriously mold tumor behavior by releasing a huge variety of paracrine signals, including cyto/chemokines, growth factors, morphogens and proteinases. An abnormally remodeled and stiff extracellular matrix favors and supports these cell interactions. Although the mechanisms responsible for the generation of tumor reactive stroma are largely uncertain, hypoxia presumably plays a central role. In this review, we will dissect the intimate relationship among the different cell elements cooperating within this complex 'ecosystem', with the ultimate goal to pave the way for a deeper understanding of the mechanisms underlying cholangiocarcinoma aggressiveness, and possibly, to foster the development of innovative, combinatorial therapies aimed at halting tumor progression. This article is part of a Special Issue entitled: Cholangiocytes in Health and Diseaseedited by Jesus Banales, Marco Marzioni, Nicholas LaRusso and Peter Jansen.

Portal fibroblasts marked by the surface antigen Thy1 contribute to fibrosis in mouse models of cholestatic liver injury

Liver fibrosis, a condition that is characterized by excessive production and accumulation of extracellular matrix, including collagen, is the most common outcome of chronic liver injuries of different etiologies. Vitamin A‐storing hepatic stellate cells (HSCs) are considered to be the main source of this collagen production, with activation in response to liver injury. In contrast, the contribution of other cell types to this fibrogenic response remains largely elusive due to the lack of specific surface markers to identify and isolate these cells for detailed analysis. Here, we identify a mesenchymal population of thymus cell antigen 1 (Thy1)⁺ CD45⁻ cells (Thy1 MCs) in the mouse liver; these cells reside near the portal vein in vivo and indicate profibrogenic characteristics in vitro, shown by their expression of collagen and α‐smooth muscle actin. Flow cytometric analysis of mouse liver nonparenchymal cells revealed that vitamin A storage and Thy1 expression were mutually exclusive, indicating that Thy1 MCs are distinct from HSCs. Importantly, Thy1 MCs reacted and contributed to the development of liver fibrosis specifically in mouse models of cholestatic liver injury. With the occurrence of cholestatic liver injury, collagen‐producing Thy1 MCs expanded in cell number and inhibited collagen degradation through up‐regulation of matrix metalloproteinase inhibitor Timp1 expression, thereby promoting the accumulation of extracellular matrix in the periportal area. Conclusion: This study establishes Thy1 as a useful cell surface marker to prospectively identify and isolate periportal fibroblasts and further highlights a significant contribution of these cells to the pathogenesis of liver fibrosis caused by cholestatic liver injuries. We suggest that Thy1 MCs may be an interesting therapeutic target for treating liver fibrosis in addition to the well‐characterized HSCs. (Hepatology Communications 2017;1:198‐214)

Regulators of Cholangiocyte Proliferation

Cholangiocytes, a small population of cells within the normal liver, have been the focus of a significant amount of research over the past two decades because of their involvement in cholangiopathies such as primary sclerosing cholangitis and primary biliary cholangitis. This article summarizes landmark studies in the field of cholangiocyte physiology and aims to provide an updated review of biliary pathogenesis. The historical approach of rodent extrahepatic bile duct ligation and the relatively recent utilization of transgenic mice have led to significant discoveries in cholangiocyte pathophysiology. Cholangiocyte physiology is a complex system based on heterogeneity within the biliary tree and a number of signaling pathways that serve to regulate bile composition. Studies have expanded the list of neuropeptides, neurotransmitters, and hormones that have been shown to be key regulators of proliferation and biliary damage. The peptide histamine and hormones, such as melatonin and angiotensin, angiotensin, as well as numerous sex hormones, have been implicated in cholangiocyte proliferation during cholestasis. Numerous pathways promote cholangiocyte proliferation during cholestasis, and there is growing evidence to suggest that cholangiocyte proliferation may promote hepatic fibrosis. These pathways may represent significant therapeutic potential for a subset of cholestatic liver diseases that currently lack effective therapies.

Knockout of microRNA-21 reduces biliary hyperplasia and liver fibrosis in cholestatic bile duct ligated mice

Cholestasis is a condition that leads to chronic hepatobiliary inflammation, fibrosis, and eventually cirrhosis. Many microRNAs (miRs) are known to have a role in fibrosis progression; however, the role of miR-21 during cholestasis remains unknown. Therefore, the aim of this study was to elucidate the role of miR-21 during cholestasis-induced biliary hyperplasia and hepatic fibrosis. Wild-type (WT) and miR-21^-/- mice underwent Sham or bile duct ligation (BDL) for 1 week, before evaluating liver histology, biliary proliferation, hepatic stellate cell (HSC) activation, fibrotic response, and small mothers against decapentaplegic 7 (Smad-7) expression. In vitro, immortalized murine biliary cell lines (IMCLs) and human hepatic stellate cell line (hHSC) were treated with either miR-21 inhibitor or control before analyzing proliferation, apoptosis, and fibrotic responses. In vivo, the levels of miR-21 were increased in total liver and cholangiocytes after BDL, and loss of miR-21 decreased the amount of BDL-induced biliary proliferation and intrahepatic biliary mass. In addition, loss of miR-21 decreased BDL-induced HSC activation, collagen deposition, and expression of the fibrotic markers transforming growth factor-β1 and α-smooth muscle actin. In vitro, IMCL and hHSCs treated with miR-21 inhibitor displayed decreased proliferation and expression of fibrotic markers and enhanced apoptosis when compared with control treated cells. Furthermore, mice lacking miR-21 show increased Smad-7 expression, which may be driving the decrease in biliary hyperplasia and hepatic fibrosis. During cholestatic injury, miR-21 is increased and leads to increased biliary proliferation and hepatic fibrosis. Local modulation of miR-21 may be a therapeutic option for patients with cholestasis.

Myofibroblasts and lung fibrosis induced by carbon nanotube exposure

Carbon nanotubes (CNTs) are newly developed materials with unique properties and a range of industrial and commercial applications. A rapid expansion in the production of CNT materials may increase the risk of human exposure to CNTs. Studies in rodents have shown that certain forms of CNTs are potent fibrogenic inducers in the lungs to cause interstitial, bronchial, and pleural fibrosis characterized by the excessive deposition of collagen fibers and the scarring of involved tissues. The cellular and molecular basis underlying the fibrotic response to CNT exposure remains poorly understood. Myofibroblasts are a major type of effector cells in organ fibrosis that secrete copious amounts of extracellular matrix proteins and signaling molecules to drive fibrosis. Myofibroblasts also mediate the mechano-regulation of fibrotic matrix remodeling via contraction of their stress fibers. Recent studies reveal that exposure to CNTs induces the differentiation of myofibroblasts from fibroblasts in vitro and stimulates pulmonary accumulation and activation of myofibroblasts in vivo. Moreover, mechanistic analyses provide insights into the molecular underpinnings of myofibroblast differentiation and function induced by CNTs in the lungs.

In view of the apparent fibrogenic activity of CNTs and the emerging role of myofibroblasts in the development of organ fibrosis, we discuss recent findings on CNT-induced lung fibrosis with emphasis on the role of myofibroblasts in the pathologic development of lung fibrosis. Particular attention is given to the formation and activation of myofibroblasts upon CNT exposure and the possible mechanisms by which CNTs regulate the function and dynamics of myofibroblasts in the lungs. It is evident that a fundamental understanding of the myofibroblast and its function and regulation in lung fibrosis will have a major influence on the future research on the pulmonary response to nano exposure, particle and fiber-induced pneumoconiosis, and other human lung fibrosing diseases.

New approaches for fibrosis regression in alcoholic cirrhosis

Liver fibrosis is a dynamic process of fibrinogenesis and fibrinolysis. It is sequelae of recurrent injury and inflammation to the liver. Only recently has there been significant progress in understanding the pathophysiology behind liver fibrosis. This has allowed for the development of identifiable targets for potential therapies. In this article we will discuss the underlying general cellular mechanisms that play a key role in the pathway of fibrinogenesis and fibrinolysis and then focus on the mechanisms that are key in alcohol-induced liver fibrosis. Challenges in formulating potential fibrosis therapies as well as current potential targets for liver fibrosis will be reviewed as well.

Culture Model of Rat Portal Myofibroblasts

Myofibroblasts are matrix-producing cells with contractile properties, usually characterized by de novo expression of alpha-smooth muscle actin, that arise in fibrotic diseases. Hepatic stellate cells (HSCs), known as perisinusoidal cells containing auto-fluorescent vitamin A, are the major although not exclusive source of myofibroblasts in the injured liver. Portal myofibroblasts (PMFs) have been defined as liver myofibroblasts derived from cells that are distinct from HSCs and located in the portal tract. Here, we describe the protocol we have established to obtain rat PMFs in culture. In this method, the biliary tree is (i) separated from the liver parenchyma by in situ enzymatic perfusion of the liver, (ii) minced and further digested in vitro, until bile duct segments are isolated by sequential filtration. Bile duct isolates free of HSC contaminants, form small cell clusters, which initially comprise a large majority of epithelial cells. In culture conditions (fetal bovine serum) that provide a growth advantage to mesenchymal cells over epithelial cells, the epithelial cells die and detach from the substrate, while spindle-shaped cells outgrow from the periphery of the cell clusters, as shown by video-microscopy. These cells are highly proliferative and after 4–5 days, the culture is composed exclusively of fully differentiated myofibroblasts, which express alpha-smooth muscle actin and collagen 1, and secrete abundant collagen. We found no evidence for epithelial-mesenchymal transition, i.e., no co-expression of alpha-smooth muscle actin and cytokeratin at any stage, while cytokeratin becomes undetectable in the confluent cells. PMFs obtained by this method express the genes that were previously reported to be overexpressed in non-HSC or portal fibroblast-derived liver myofibroblasts as compared to HSC-derived myofibroblasts, including the most discriminant, collagen 15, fibulin 2, and Thy-1. After one passage, PMFs retain the same phenotypic features as in primary culture. In conclusion, this straightforward and reproducible method of PMF culture, can be used to identify new markers of PMFs at different stages of differentiation, to compare their phenotype with those of HSC-MFs and ultimately determine their progenitors and specific functions in liver wound-healing.

The Role of NADPH Oxidases (NOXs) in Liver Fibrosis and the Activation of Myofibroblasts

Chronic liver injury, resulted from different etiologies (e.g., virus infection, alcohol abuse, nonalcoholic steatohepatitis (NASH) and cholestasis) can lead to liver fibrosis characterized by the excess accumulation of extracellular matrix (ECM) proteins (e.g., type I collagen). Hepatic myofibroblasts that are activated upon liver injury are the key producers of ECM proteins, contributing to both the initiation and progression of liver fibrosis. Hepatic stellate cells (HSCs) and to a lesser extent, portal fibroblast, are believed to be the precursor cells that give rise to hepatic myofibroblasts in response to liver injury. Although, much progress has been made toward dissecting the lineage origin of myofibroblasts, how these cells are activated and become functional producers of ECM proteins remains incompletely understood. Activation of myofibroblasts is a complex process that involves the interactions between parenchymal and non-parenchymal cells, which drives the phenotypic change of HSCs from a quiescent stage to a myofibroblastic and active phenotype. Accumulating evidence has suggested a critical role of NADPH oxidase (NOX), a multi-component complex that catalyzes reactions from molecular oxygen to reactive oxygen species (ROS), in the activation process of hepatic myofibroblasts. NOX isoforms, including NOX1, NOX2 and NOX4, and NOX-derived ROS, have all been implicated to regulate HSC activation and hepatocyte apoptosis, both of which are essential steps for initiating liver fibrosis. This review highlights the importance of NOX isoforms in hepatic myofibroblast activation and the progression of liver fibrosis, and also discusses the therapeutic potential of targeting NOXs for liver fibrosis and associated hepatic diseases.

Epithelial-mesenchymal transition in liver fibrosis

Liver fibrosis is the result of a sustained wound healing response to sustained chronic liver injury, which includes viral, alcoholic and autoimmune hepatitis. Hepatic regeneration is the dominant outcome of liver damage. The outcomes of successful repair are the replacement of dead epithelial cells with healthy epithelial cells, and reconstruction of the normal hepatic structure and function. Prevention of the development of epithelial-mesenchymal transition (EMT) may control and even reverse liver fibrosis. EMT is a critical process for an epithelial cell to undergo a conversion to a mesenchymal phenotype, and is believed to be an inflammation-induced response, which may have a central role in liver fibrosis. The origin of fibrogenic cells in liver fibrosis remains controversial. Numerous studies have investigated the origin of all fibrogenic cells within the liver and the mechanism of the signaling pathways that lead to the activation of EMT programs during numerous chronic liver diseases. The present study aimed to summarize the evidence to explain the possible role of EMT in liver fibrosis.

The PDGF system and its antagonists in liver fibrosis

Platelet derived growth factor (PDGF) signaling plays an important role in activated hepatic stellate cells and portal fibroblast proliferation, chemotaxis, migration and cell survival. PDGF receptors and ligands are upregulated in experimental liver fibrotic models as well as in human liver fibrotic diseases. Blocking of PDGF signaling ameliorates experimental liver fibrogenesis. The plurality of molecular and cellular activities of PDGF and its involvement in initiation, progression and resolution of hepatic fibrogenesis offers an infinite number of therapeutic possibilities. These include the application of therapeutic antibodies (e.g. AbyD3263, MOR8457) which specifically sequester individual PDGF isoforms or the inhibition of PDGF isoforms by synthetic aptamers. In particular, the isolation of innovative slow off-rate modified aptamers (e.g., SOMAmer SL1 and SL5) that carry functional groups absent in natural nucleic acids by the Systematic Evolution of Ligands by EXponential (SELEX) enrichment technique offers the possibility to design high affinity aptamers that target PDGF isoforms for clinical purposes. Dominant-negative soluble PDGF receptors are also effective in attenuation of hepatic stellate cell proliferation and hepatic fibrogenesis. Moreover, some multikinase inhibitors targeting PDGF signaling have been intensively tested during the last decade and are on the way into advanced preclinical studies and clinical trials. This narrative review aims to gauge the recent progression of research into PDGF systems and liver fibrosis.

Role of Hypoxia-Inducible Factors in the Development of Liver Fibrosis

Liver fibrosis remains a significant clinical problem in the United States and throughout the world. Although important advances in the understanding of this disease have been made, no effective pharmacologic agents have been developed that directly prevent or reverse the fibrotic process. Many of the successes in liver fibrosis treatment have been targeted toward treating the cause of fibrosis, such as the development of new antivirals that eradicate hepatitis virus. For many patients, however, this is not feasible, so a liver transplant remains the only viable option. Thus, there is a critical need to identify new therapeutic targets that will slow or reverse the progression of fibrosis in such patients. Research over the last 16 years has identified hypoxia-inducible factors (HIFs) as key transcription factors that drive many aspects of liver fibrosis, making them potential targets of therapy. In this review, we discuss the latest work on HIFs and liver fibrosis, including the cell-specific functions of these transcription factors in the development of liver fibrosis.

Human liver myofibroblasts during development and diseases with a focus on portal (myo)fibroblasts

Myofibroblasts are stromal cells mainly involved in tissue repair. These cells present contractile properties and play a major role in extracellular matrix deposition and remodeling. In liver, myofibroblasts are found in two critical situations. First, during fetal liver development, especially in portal tracts, myofibroblasts surround vessels and bile ducts during their maturation. After complete development of the liver, myofibroblasts disappear and are replaced in portal tracts by portal fibroblasts. Second, during liver injury, myofibroblasts re-appear principally deriving from the activation of local stromal cells such as portal fibroblasts and hepatic stellate cells or can sometimes emerge by an epithelial-mesenchymal transition process. After acute injury, myofibroblasts play also a major role during liver regeneration. While myofibroblastic precursor cells are well known, the spectrum of activation and the fate of myofibroblasts during disease evolution are not fully understood. Some data are in accordance with a possible deactivation, at least partial, or a disappearance by apoptosis. Despite these shadows, liver is definitively a pertinent model showing that myofibroblasts are pivotal cells for extracellular matrix control during morphogenesis, repair and fibrous scarring.

Bone marrow-derived fibrocytes contribute to liver fibrosis

Chronic liver injury often leads to hepatic fibrosis, a condition associated with increased levels of circulating TGF-β1 and lipopolysaccharide, activation of myofibroblasts, and extensive deposition of extracellular matrix, mostly collagen Type I. Hepatic stellate cells are considered to be the major(1) but not the only source of myofibroblasts in the injured liver.(2) Hepatic myofibroblasts may also originate from portal fibroblasts, mesenchymal cells, and fibrocytes.(3) Since the discovery of fibrocytes in 1994 by Dr. Bucala and colleagues, this bone marrow (BM)-derived collagen Type I-producing CD45(+) cells remain the most fascinating cells of the hematopoietic system. Due to the ability to differentiate into collagen Type I producing cells/myofibroblasts, fibrocytes were implicated in the pathogenesis of liver, skin, lung, and kidney fibrosis. However, studies of different organs often contain controversial results on the number of fibrocytes recruited to the site of injury and their biological function. Furthermore, fibrocytes were implicated in the pathogenesis of sepsis and were shown to possess antimicrobial activity. Finally, in response to specific stimuli, fibrocytes can give rise to fully differentiated macrophages, suggesting that in concurrence with the high plasticity of hematopoietic cells, fibrocytes exhibit progenitor properties. Here, we summarize our current understanding of the role of CD45(+)Collagen Type I(+) BM-derived cells in response to fibrogenic liver injury and septicemia and discuss the most recent evidence supporting the critical role of fibrocytes in the mediation of pro-fibrogenic and/or pro-inflammatory responses.

Antifibrotic therapies in the liver

Significant progress has been made in understanding the principles underlying the development of liver fibrosis. This includes appreciating its dynamic nature, the importance of active fibrolysis in fibrosis regression, and the plasticity of cell populations endowing them with fibrogenic or fibrolytic properties. This is complemented by an increasing array of therapeutic targets with known roles in the progression or regression of fibrosis. With a key role for fibrosis in determining clinical outcomes and encouraging data from recently Food and Drug Administration-approved antifibrotics for pulmonary fibrosis, the development and validation of antifibrotic therapies has taken center stage in translational hepatology. In addition to summarizing the recent progress in antifibrotic therapies, the authors discuss some of the challenges ahead, such as achieving a better understanding of the interindividual heterogeneity of the fibrotic response, how to match interventions with the ideal patient population, and the development of better noninvasive methods to assess the dynamics of fibrogenesis and fibrolysis. Together, these advances will permit a better targeting and dose titration of individualized therapies. Finally, the authors discuss combination therapy with different antifibrotics as possibly the most potent approach for treating fibrosis in the liver.

Novel therapeutic targets in primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is a chronic immune-mediated liver disease characterized by progressive cholestasis, biliary fibrosis and eventually cirrhosis. It results in characteristic symptoms with marked effects on life quality. The advent of large patient cohorts has challenged the view of PBC as a benign condition treated effectively by the single licensed therapy-ursodeoxycholic acid (UDCA). UDCA nonresponse or under-response has a major bearing on outcome, substantially increasing the likelihood that liver transplantation will be required or that patients will die of the disease. In patients with high-risk, treatment-unresponsive or highly symptomatic disease the need for new treatment approaches is clear. Evolution in our understanding of disease mechanisms is rapidly leading to the advent of new and re-purposed therapeutic agents targeting key processes. Notable opportunities are offered by targeting what could be considered as the 'upstream' immune response, 'midstream' biliary injury and 'downstream' fibrotic processes. Combination therapy targeting several pathways or the development of novel agents addressing multiple components of the disease pathway might be required. Ultimately, PBC therapeutics will require a stratified approach to be adopted in practice. This Review provides a current perspective on potential approaches to PBC treatment, and highlights the challenges faced in evaluating and implementing those treatments.

Portal myofibroblasts promote vascular remodeling underlying cirrhosis formation through the release of microparticles

Liver fibrosis expanding from portal tracts and vascular remodeling are determinant factors in the progression of liver diseases to cirrhosis. In the present study, we examined the potential contribution of portal myofibroblasts (PMFs) to the vascular changes leading to cirrhosis. The analyses of liver cells based on the transcriptome of rat PMFs, compared to hepatic stellate cell HSC-derived myofibroblasts in culture, identified collagen, type XV, alpha 1 (COL15A1) as a marker of PMFs. Normal liver contained rare COL15A1-immunoreactive cells adjacent to the bile ducts and canals of Hering in the portal area. A marked increase in COL15A1 expression occurred together with that of the endothelial marker, von Willebrand factor, in human and rat liver tissue, at advanced stages of fibrosis caused by either biliary or hepatocellular injury. In cirrhotic liver, COL15A1-expressing PMFs adopted a perivascular distribution outlining vascular capillaries proximal to reactive ductules, within large fibrotic septa. The effect of PMFs on endothelial cells (ECs) was evaluated by in vitro and in vivo angiogenesis assays. PMF-conditioned medium increased the migration and tubulogenesis of liver ECs as well as human umbilical vein ECs and triggered angiogenesis within Matrigel plugs in mice. In coculture, PMFs developed intercellular junctions with ECs and enhanced the formation of vascular structures. PMFs released vascular endothelial growth factor (VEGF)A-containing microparticles, which activated VEGF receptor 2 in ECs and largely mediated their proangiogenic effect. Cholangiocytes potentiated the angiogenic properties of PMFs by increasing VEGFA expression and microparticle shedding in these cells.

CONCLUSION

PMFs are key cells in hepatic vascular remodeling. They signal to ECs through VEGFA-laden microparticles and act as mural cells for newly formed vessels, driving scar progression from portal tracts into the parenchyma.

PDGFRα in liver pathophysiology: emerging roles in development, regeneration, fibrosis, and cancer

Platelet-derived growth factor receptor α (PDGFRα) is an isoform of the PDGFR family of tyrosine kinase receptors involved in cell proliferation, survival, differentiation, and growth. In this review, we highlight the role of PDGFRα and the current evidence of its expression and activities in liver development, regeneration, and pathology-including fibrosis, cirrhosis, and liver cancer. Studies elucidating PDGFRα signaling in processes ranging from profibrotic signaling, angiogenesis, and oxidative stress to epithelial-to-mesenchymal transition point toward PDGFRα as a potential therapeutic target in various hepatic pathologies, including hepatic fibrosis and liver cancer. Furthermore, PDGFRα localization and modulation during liver development and regeneration may lend insight into its potential roles in various pathologic states. We will also briefly discuss some of the current targeted treatments for PDGFRα, including multi receptor tyrosine kinase inhibitors and PDGFRα-specific inhibitors.

Extrahepatic platelet-derived growth factor-β, delivered by platelets, promotes activation of hepatic stellate cells and biliary fibrosis in mice

BACKGROUND & AIMS

Platelet-derived growth factor-β (PDGFB) is a mitogen for hepatic stellate cells (HSCs). We studied the cellular sources of PDGFB and the effects of a high-affinity monoclonal antibody against PDGFB (MOR8457) in mouse models of biliary fibrosis.

METHODS

Cellular sources of PDGFB were identified using quantitative reverse-transcription polymerase chain reaction, biochemical, and immunohistologic methods. Mice with advanced biliary fibrosis, MDR2(Abcb4)-null mice, and C57Bl/6 (control) mice were placed on 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-supplemented diets and were given weekly intraperitoneal injections of MOR8457. Platelets were depleted from MDR2-null mice by injection of an antibody against CD41, or inhibited with diets containing low-dose aspirin. Liver tissues were collected and analyzed by quantitative reverse-transcription PCR and histologic and biochemical analyses.

RESULTS

Levels of PDGFB protein, but not messenger RNA, were increased in fibrotic livers of MDR2-null mice, compared with control mice. Platelet clusters were detected in the hepatic endothelium, in close proximity to HSCs, and were identified as a source of PDGFB protein in MDR2-null mice. Levels of the PDGFB were increased in serum samples from patients with early stages of liver fibrosis of various etiologies (F1-2, n = 16; P < .05), compared with nonfibrotic liver tissue (F0, n = 12). Depletion of platelets from MDR2-null mice normalized hepatic levels of PDGFB within 48 hours, reducing levels of a marker of HSC activation (α-smooth muscle actin) and expression of genes that promote fibrosis. Diets supplemented with low-dose aspirin reduced circulating serum and hepatic levels of PDGFB and significantly reduced progression of fibrosis in MDR2-null mice over 1 year. MOR8457 produced a dose-dependent decrease in liver fibrosis in MDR2-null mice, reducing collagen deposition by 45% and expression of fibrosis-associated genes by 50%, compared with mice given a control antibody. In vitro, platelets activated freshly isolated HSCs (induction of α-smooth muscle actin and fibrosis-associated genes) via a PDGFB-dependent mechanism. MOR8457 also reduced liver fibrosis in mice placed on DDC-supplemented diets.

CONCLUSIONS

Platelets produce PDGFB to activate HSC and promote fibrosis in MDR2-null mice and mice on DDC-supplemented diets. Antiplatelet therapy or selective inhibition of PDGFB might reduce biliary fibrosis in patients with liver disease.

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.

Portal Fibroblasts in Biliary Fibrosis

Portal fibroblasts are a minor population in the normal liver, found in the periportal mesenchyme surrounding the bile ducts. While many researchers have hypothesized that they are an important myofibroblast precursor population in biliary fibrosis, responsible for matrix deposition in early fibrosis and for recruiting hepatic stellate cells, the role of portal fibroblasts relative to hepatic stellate cells is controversial. Several papers published in the past year have addressed this point and have identified other potential roles for portal fibroblasts in biliary fibrosis. The goal of this review is to critically assess these recent studies, to highlight gaps in our knowledge of portal fibroblasts, and to suggest directions for future research.

Extracellular matrix and liver disease

SIGNIFICANCE

The extracellular matrix (ECM) is a dynamic microenvironment that undergoes continuous remodeling, particularly during injury and wound healing. Chronic liver injury of many different etiologies such as viral hepatitis, alcohol abuse, drug-induced liver injury, obesity and insulin resistance, metabolic disorders, and autoimmune disease is characterized by excessive deposition of ECM proteins in response to persistent liver damage.

CRITICAL ISSUES

This review describes the main collagenous and noncollagenous components from the ECM that play a significant role in pathological matrix deposition during liver disease. We define how increased myofibroblasts (MF) from different origins are at the forefront of liver fibrosis and how liver cell-specific regulation of the complex scarring process occurs.

RECENT ADVANCES

Particular attention is paid to the role of cytokines, growth factors, reactive oxygen species, and newly identified matricellular proteins in the regulation of fibrillar type I collagen, a field to which our laboratory has significantly contributed over the years. We compile data from recent literature on the potential mechanisms driving fibrosis resolution such as MF' apoptosis, senescence, and reversal to quiescence.

FUTURE DIRECTIONS

We conclude with a brief description of how epigenetics, an evolving field, can regulate the behavior of MF and of how new "omics" tools may advance our understanding of the mechanisms by which the fibrogenic response to liver injury occurs.

Myofibroblast differentiation: main features, biomedical relevance, and the role of reactive oxygen species

SIGNIFICANCE

Myofibroblasts are prototypical fibrotic cells, which are involved in a number of more or less pathological conditions, from foreign body reactions to scarring, from liver, kidney, or lung fibrosis to neoplastic phenomena. The differentiation of precursor cells (not only of fibroblastic nature) is characterized by a complex interplay between soluble factors (growth factors such as transforming growth factor β1, reactive oxygen species [ROS]) and material properties (matrix stiffness).

RECENT ADVANCES

The last 15 years have seen very significant advances in the identification of appropriate differentiation markers, in the understanding of the differentiation mechanism, and above all, the involvement of ROS as causative and persistence factors.

CRITICAL ISSUES

The specific mechanisms of action of ROS remain largely unknown, although evidence suggests that both intracellular and extracellular phenomena play a role.

FUTURE DIRECTIONS

Approaches based on antioxidant (ROS-scavenging) principles and on the potentiation of nitric oxide signaling hold much promise in view of a pharmacological therapy of fibrotic phenomena. However, how to make the active principles available at the target sites is yet a largely neglected issue.

The portal fibroblast: not just a poor man's stellate cell

Portal fibroblasts, the resident fibroblasts of the portal tract, are found in the mesenchyme surrounding the bile ducts. Their roles in liver homeostasis and response to injury are undefined and controversial. Although portal fibroblasts almost certainly give rise to myofibroblasts during the development of biliary fibrosis, recent lineage tracing studies suggest that their contribution to fibrogenesis is limited compared with that of hepatic stellate cells. Other functions of portal fibroblasts include participation in the peribiliary stem cell niche, regulation of cholangiocyte proliferation, and deposition of specific matrix proteins. Portal fibroblasts synthesize elastin and other components of microfibrils; these may serve structural roles, providing stability to ducts and the vasculature under conditions of increased ductal pressure, or could regulate the bioavailability of the fibrogenic transforming growth factor β in response to injury. Viewing portal fibroblasts in the context of fibroblast populations throughout the body and studying their niche-specific roles in matrix deposition and epithelial regulation could yield new insights into their contributions in the normal and injured liver. Understanding the functions of portal fibroblasts will require us to view them as more than just an alternative to hepatic stellate cells in fibrosis.

The effect of antifibrotic drugs in rat precision-cut fibrotic liver slices

Two important signaling pathways in liver fibrosis are the PDGF- and TGFβ pathway and compounds inhibiting these pathways are currently developed as antifibrotic drugs. Testing antifibrotic drugs requires large numbers of animal experiments with high discomfort. Therefore, a method to study these drugs ex vivo was developed using precision-cut liver slices from fibrotic rat livers (fPCLS), representing an ex vivo model with a multicellular fibrotic environment. We characterized the fibrotic process in fPCLS from rat livers after 3 weeks of bile duct ligation (BDL) during incubation and tested compounds predominantly inhibiting the TGFβ pathway (perindopril, valproic acid, rosmarinic acid, tetrandrine and pirfenidone) and PDGF pathway (imatinib, sorafenib and sunitinib). Gene expression of heat shock protein 47 (Hsp47), α smooth muscle actin (αSma) and pro-collagen 1A1 (Pcol1A1) and protein expression of collagens were determined. During 48 hours of incubation, the fibrosis process continued in control fPCLS as judged by the increased gene expression of the three fibrosis markers, and the protein expression of collagen 1, mature fibrillar collagen and total collagen. Most PDGF-inhibitors and TGFβ-inhibitors significantly inhibited the increase in gene expression of Hsp47, αSma and Pcol1A1. Protein expression of collagen 1 was significantly reduced by all PDGF-inhibitors and TGFβ-inhibitors, while total collagen was decreased by rosmarinic acid and tetrandrine only. However, fibrillar collagen expression was not changed by any of the drugs. In conclusion, rat fPCLS can be used as a functional ex vivo model of established liver fibrosis to test antifibrotic compounds inhibiting the PDGF- and TGFβ signalling pathway.

Monitoring fibrogenic progression in the liver

The clinical course of chronic liver diseases is significantly dependent on the progression rate of fibrosis which is the unstructured replacement of injured parenchyma by extracellular matrix. Despite intensive studies, the clinical opportunities for patients with fibrosing liver diseases have not improved. This will be changed by increasing knowledge of new pathogenetic mechanisms, which complement the "canonical principle" of fibrogenesis. The latter is based on the activation of hepatic stellate cells and their transdifferentiation to myofibroblasts induced by hepatocellular injury and consecutive inflammatory mediators such as TGF-β. Stellate cells express a broad spectrum of matrix components. New mechanisms indicate that the heterogeneous pool of (myo-)fibroblasts can be supplemented by epithelial-mesenchymal transition (EMT) from cholangiocytes and potentially also from hepatocytes to fibroblasts, by influx of bone marrow-derived fibrocytes in the damaged liver tissue and by differentiation of a subgroup of monocytes to fibroblasts after homing in the damaged tissue. These processes are regulated by the cytokines TGF-β and BMP-7, chemokines, colony-stimulating factors, metalloproteinases and numerous trapping proteins. They offer innovative diagnostic and therapeutic options. As an example, modulation of TGF-β/BMP-7 ratio changes the rate of EMT, and so the simultaneous determination of these parameters and of the connective tissue growth factor (CTGF) in serum might provide information on fibrogenic activity. Also, proteomic and glycomic approaches of serum are under investigation to set up specific protein profiles in patients with liver fibrosis. The aim of this article is to present the current pathogenetic concepts of liver fibrosis and to discuss established and novel diagnostic approaches to reflect the process of hepatic fibrogenesis in the medical laboratory.

Depletion of apoptosis signal-regulating kinase 1 prevents bile duct ligation-induced necroinflammation and subsequent peribiliary fibrosis

Apoptosis signal-regulating kinase 1 (ASK1), also known as mitogen-activated protein kinase kinase kinase (MAP3K), is ubiquitously expressed and situated in an important upstream position of many signal transduction pathways. ASK1 plays a pivotal role in stressor-induced cell survival and inflammatory reactions. To ascertain the regulatory functions of ASK1 in bile duct ligation (BDL)-induced liver injury, we examined the net effects of ASK1 depletion on hepatic necroinflammation and/or fibrosis. We subjected C57BL/6 wild-type (WT) or ASK1-deficient (ASK1(-/-)) mice to sham or BDL surgery for 14 days. In day 3 BDL animals, ASK1(-/-) mice had significantly fewer bile infarcts along with more reduced interlobular or portal inflammatory infiltrate of various immune cells, including neutrophils, compared with WT mice in which ASK1 expression was markedly activated. Morphologically apoptotic hepatocytes or cholangiocytes were negligible in both the sham and BDL animals. In contrast, ASK1(-/-) mice had significantly less proliferating activity of not only hepatocytes but also large cholangiocytes than WT mice. Day 14 BDL ASK1(-/-) mice manifested potential antifibrogenic aspects of ASK1 deficiency, characterized by significantly fewer activated peribiliary fibrogenic cells and peribiliary fibrosis. These observations indicate that ASK1-mediated hepatic necroinflammation and proliferation, but not apoptosis, are closely linked to liver fibrosis and fibrogenesis. A specific ASK1 pathway blocker or inhibitor might offer a therapeutic strategy against human cholestatic diseases.