The epithelial-to-mesenchymal transition in liver fibrosis: here today, gone tomorrow?

Lack of basic rationale in epithelial-mesenchymal transition and its related concepts

Epithelial-mesenchymal transition (EMT) is defined as a cellular process during which epithelial cells acquire mesenchymal phenotypes and behavior following the downregulation of epithelial features. EMT and its reversed process, the mesenchymal-epithelial transition (MET), and the special form of EMT, the endothelial-mesenchymal transition (EndMT), have been considered as mainstream concepts and general rules driving developmental and pathological processes, particularly cancer. However, discrepancies and disputes over EMT and EMT research have also grown over time. EMT is defined as transition between two cellular states, but it is unanimously agreed by EMT researchers that (1) neither the epithelial and mesenchymal states nor their regulatory networks have been clearly defined, (2) no EMT markers or factors can represent universally epithelial and mesenchymal states, and thus (3) EMT cannot be assessed on the basis of one or a few EMT markers. In contrast to definition and proposed roles of EMT, loss of epithelial feature does not cause mesenchymal phenotype, and EMT does not contribute to embryonic mesenchyme and neural crest formation, the key developmental events from which the EMT concept was derived. EMT and MET, represented by change in cell shapes or adhesiveness, or symbolized by EMT factors, are biased interpretation of the overall change in cellular property and regulatory networks during development and cancer progression. Moreover, EMT and MET are consequences rather than driving factors of developmental and pathological processes. The true meaning of EMT in some developmental and pathological processes, such as fibrosis, needs re-evaluation. EMT is believed to endow malignant features, such as migration, stemness, etc., to cancer cells. However, the core property of cancer (tumorigenic) cells is neural stemness, and the core EMT factors are components of the regulatory networks of neural stemness. Thus, EMT in cancer progression is misattribution of the roles of neural stemness to the unknown mesenchymal state. Similarly, neural crest EMT is misattribution of intrinsic property of neural crest cells to the unknown mesenchymal state. Lack of basic rationale in EMT and related concepts urges re-evaluation of their significance as general rules for understanding developmental and pathological processes, and re-evaluation of their significance in scientific research.

Hepatocellular carcinoma in patients with renal dysfunction: Pathophysiology, prognosis, and treatment challenges

The population of patients with hepatocellular carcinoma (HCC) overlaps to a high degree with those for chronic kidney disease (CKD) and end-stage renal disease (ESRD). The degrees of renal dysfunction vary, from the various stages of CKD to dialysis-dependent ESRD, which often affects the prognosis and treatment choice of patients with HCC. In addition, renal dysfunction makes treatment more difficult and may negatively affect treatment outcomes. This study summarized the possible causes of the high comorbidity of HCC and renal dysfunction. The possible mechanisms of CKD causing HCC involve uremia itself, long-term dialysis status, immunosuppressive agents for postrenal transplant status, and miscellaneous factors such as hormone alterations and dysbiosis. The possible mechanisms of HCC affecting renal function include direct tumor invasion and hepatorenal syndrome. Finally, we categorized the risk factors that could lead to both HCC and CKD into four categories: Environmental toxins, viral hepatitis, metabolic syndrome, and vasoactive factors. Both CKD and ESRD have been reported to negatively affect HCC prognosis, but more research is warranted to confirm this. Furthermore, ESRD status itself ought not to prevent patients receiving aggressive treatments. This study then adopted the well-known Barcelona Clinic Liver Cancer guidelines as a framework to discuss the indicators for each stage of HCC treatment, treatment-related adverse renal effects, and concerns that are specific to patients with pre-existing renal dysfunction when undergoing aggressive treatments against CKD and ESRD. Such aggressive treatments include liver resection, simultaneous liver kidney transplantation, radiofrequency ablation, and transarterial chemoembolization. Finally, focusing on patients unable to receive active treatment, this study compiled information on the latest systemic pharmacological therapies, including targeted and immunotherapeutic drugs. Based on available clinical studies and Food and Drug Administration labels, this study details the drug indications, side effects, and dose adjustments for patients with renal dysfunction. It also provides a comprehensive review of information on HCC patients with renal dysfunction from disease onset to treatment.

SOD3 deficiency induces liver fibrosis by promoting hepatic stellate cell activation and epithelial-mesenchymal transition

Hepatic stellate cell (HSC) activation plays an important role in the pathogenesis of liver fibrosis, and epithelial-mesenchymal transition (EMT) is suggested to potentially promote HSC activation. Superoxide dismutase 3 (SOD3) is an extracellular antioxidant defense against oxidative damage. Here, we found downregulation of SOD3 in a mouse model of liver fibrosis induced by carbon tetrachloride (CCl4 ). SOD3 deficiency induced spontaneous liver injury and fibrosis with increased collagen deposition, and further aggravated CCl4 -induced liver injury in mice. Depletion of SOD3 enhanced HSC activation marked by increased α-smooth muscle actin and subsequent collagen synthesis primarily collagen type I in vivo, and promoted transforming growth factor-β1 (TGF-β1)-induced HSC activation in vitro. SOD3 deficiency accelerated EMT process in the liver and TGF-β1-induced EMT of AML12 hepatocytes, as evidenced by loss of E-cadherin and gain of N-cadherin and vimentin. Notably, SOD3 expression and its pro-fibrogenic effect were positively associated with sirtuin 1 (SIRT1) expression. SOD3 deficiency inhibited adenosine monophosphate-activated protein kinase (AMPK) signaling to downregulate SIRT1 expression and thus involving in liver fibrosis. Enforced expression of SIRT1 inhibited SOD3 deficiency-induced HSC activation and EMT, whereas depletion of SIRT1 counteracted the inhibitory effect of SOD3 in vitro. These findings demonstrate that SOD3 deficiency contributes to liver fibrogenesis by promoting HSC activation and EMT process, and suggest a possibility that SOD3 may function through modulating SIRT1 via the AMPK pathway in liver fibrosis.

Suberoylanilide Hydroxamic Acid (SAHA) Reduces Fibrosis Markers and Deactivates Human Stellate Cells via the Epithelial-Mesenchymal Transition (EMT)

Hepatic fibrosis is known as the accumulation of connective tissue secondary to chronic damage to the liver. Epithelial-mesenchymal transition (EMT) corresponding increase in liver fibrogenesis was shown with immunohistochemistry and PCR-based studies. Suberoylanilide hydroxamic acid (SAHA), a synthetic compound approved as a histone deacetylase inhibitor (HDAC) by the FDA to treat cutaneous T-cell lymphoma is under investigation for the treatment of lung and renal fibrosis. Experimental modeling for hepatic fibrosis can be constructed with an LX2 cell line isolated from human hepatic stellate cells (HSCs). In this study, we aimed to investigate the modulation of SAHA in the pathogenesis of liver fibrosis by detecting the levels of proteins; (E-cadherin (E-cad), N-cadherin (N-cad), Vimentin (Vim), and genes; E-cad, N-cad, Vim, transforming growth factor-beta (TGF-β), alpha-smooth muscle actin (α-SMA), type 1 collagen (COL1A1), type 3 collagen (COL3A1)) that play a significant role in EMT with the LX2 cell line. We also evaluated the action of SAHA with cell proliferation, clonogenic, and migration assay. Cell proliferation was performed by flow cytometry. All the protein levels were determined by Western blot analysis, and gene expression levels were measured by Real-Time PCR. Our study observed that SAHA treatment decreased cell viability, colony formation and migration in LX2 cells. We found that SAHA increased E-cad expression level, while it decreased N-cad, Vim, COL1A1, COL3A1, α-SMA TGF-β genes expression levels. SAHA decreased the level of E-cad, N-cad, and Vim protein levels. We thought that SAHA possesses potent antifibrotic and anti-EMT properties in LX2.

La plasticidad del hepatocito y su relevancia en la fisiología y la patología hepática

El hígado es uno de los principales órganos encargados de mantener la homeostasis en vertebrados,  además de poseer una gran capacidad regenerativa. El hígado está constituido por diversos tipos celulares que de forma coordinada contribuyen para que el órgano funcione eficientemente. Los hepatocitos representan el tipo celular principal de este órgano y llevan a cabo la mayoría de sus actividades; además, constituyen una población heterogénea de células epiteliales con funciones especializadas en el metabolismo. El fenotipo de los hepatocitos está controlado por diferentes vías de señalización, como la vía del TGFβ/Smads, la ruta Hippo/YAP-TAZ y la vía Wnt/β-catenina, entre otras. Los hepatocitos son células que se encuentran normalmente en un estado quiescente, aunque cuentan con una plasticidad intrínseca que se manifiesta en respuesta a diversos daños en el hígado; así, estas células reactivan su capacidad proliferativa o cambian su fenotipo a través de procesos celulares como la transdiferenciación o la transformación, para contribuir a mantener la homeostasis del órgano en condiciones saludables o desarrollar diversas  patologías.

Origin and role of hepatic myofibroblasts in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the most common primary liver cancer and is the second leading cause of cancer-related death worldwide. Fibrosis and cirrhosis are important risk factors for the development of HCC. Hepatic myofibroblasts are the cells responsible for extracellular matrix deposition, which is the hallmark of liver fibrosis. It is believed that myofibroblasts are predominantly derived from hepatic stellate cells (HSCs), also known as Ito cells. Nevertheless, depending on the nature of insult to the liver, it is thought that myofibroblasts may also originate from a variety of other cell types such as the portal fibroblasts (PFs), fibrocytes, hepatocytes, hepatic progenitor cells (HPCs), and mesothelial cells. Liver myofibroblasts are believed to transform into cancer-associated fibroblasts (CAFs) while HCC is developing. There is substantial evidence suggesting that activated HSCs (aHSCs)/cancer-associated fibroblasts (CAFs) may play an important role in HCC initiation and progression. In this paper, we aim to review current literature on cellular origins of myofibroblasts with a focus on hepatitis B virus (HBV)- and hepatitis C virus (HCV)-induced hepatic fibrosis. We also address the role of aHSCs/CAFs in HCC progression through the regulation of immune cells as well as mechanisms of evolvement of drug resistance.

Origin and role of hepatic myofibroblasts in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the most common primary liver cancer and is the second leading cause of cancer-related death worldwide. Fibrosis and cirrhosis are important risk factors for the development of HCC. Hepatic myofibroblasts are the cells responsible for extracellular matrix deposition, which is the hallmark of liver fibrosis. It is believed that myofibroblasts are predominantly derived from hepatic stellate cells (HSCs), also known as Ito cells. Nevertheless, depending on the nature of insult to the liver, it is thought that myofibroblasts may also originate from a variety of other cell types such as the portal fibroblasts (PFs), fibrocytes, hepatocytes, hepatic progenitor cells (HPCs), and mesothelial cells. Liver myofibroblasts are believed to transform into cancer-associated fibroblasts (CAFs) while HCC is developing. There is substantial evidence suggesting that activated HSCs (aHSCs)/cancer-associated fibroblasts (CAFs) may play an important role in HCC initiation and progression. In this paper, we aim to review current literature on cellular origins of myofibroblasts with a focus on hepatitis B virus (HBV)- and hepatitis C virus (HCV)-induced hepatic fibrosis. We also address the role of aHSCs/CAFs in HCC progression through the regulation of immune cells as well as mechanisms of evolvement of drug resistance.

Origin and functional heterogeneity of fibroblasts

The inherent plasticity and resiliency of fibroblasts make this cell type a conventional tool for basic research. But where do they come from, are all fibroblasts the same, and how do they function in disease? The first fibroblast lineages in mammalian development emerge from the ooze of primary mesenchyme during gastrulation. They are cells that efficiently create and negotiate the extracellular matrix of the mesoderm in order to migrate and meet their developmental fate. Mature fibroblasts in epithelial tissues live in the interstitial spaces between basement membranes that spatially delimit complex organ structures. While the function of resident fibroblasts in healthy tissues is largely conjecture, the accumulation of fibroblasts in pathologic lesions offers insight into biologic mechanisms that control their function; fibroblasts are poised to coordinate fibrogenesis in tissue injury, neoplasia, and aging. Here, we examine the developmental origin and plasticity of fibroblasts, their molecular and functional definitions, the epigenetic control underlying their identity and activation, and the evolution of their immune regulatory functions. These topics are reviewed through the lens of fate mapping using genetically engineered mouse models and from the perspective of single-cell RNA sequencing. Recent observations suggest dynamic and heterogeneous functions for fibroblasts that underscore their complex molecular signatures and utility in injured tissues.

Increased Expression of Adherens Junction Components in Mouse Liver following Bile Duct Ligation

Adherens junctions, consisting of cadherins and catenins, are a group of cell-to-cell junctions that mediate mechanistic linkage between neighboring cells. By doing so, adherens junctions ensure direct intercellular contact and play an indispensable role in maintaining tissue architecture. Considering these critical functions, it is not surprising that adherens junctions are frequently involved in disease. In the present study, the effects of bile duct ligation—a surgical procedure to experimentally induce cholestatic and fibrotic liver pathology—on hepatic adherens junctions were investigated in mice. In essence, it was found that liver mRNA and protein levels of E-cadherin, β-catenin and γ-catenin drastically increase following bile duct ligation. These results could suggest a cytoprotective role for hepatic adherens junctions following bile duct ligation.

miR-219-3p regulates the occurrence of hepatic fibrosis by targeting Smad2

Abnormal expression of microRNA (miR)-219-3p has been widely identified in different tumors. However, whether miR-219-3p is involved in the progression of hepatic fibrosis (HF) has never been explored. The present study showed that compared with healthy controls, the levels of miR-291-3p in peripheral blood were decreased in patients with HF. Furthermore, much lower levels of miR-291-3p were identified in fibrotic liver tissues compared with that of normal liver tissues. Receiver operating characteristic curve analysis showed that the levels of miR-291-3p in peripheral blood may screen patients with HF from healthy controls. Reverse transcription quantitative polymerase chain reaction analysis showed that overexpression of miR-291-3p significantly suppressed the mRNA levels of Snai1, vascular endothelial-specific cadherin (VE-cadherin), Vimentin, transforming growth factor (TGF)-β1, and glial fibrillary acidic protein (GFAP). The protein levels of Snai1, VE-cadherin, Vimentin, TGF-β1, and GFAP were also decreased in hepatic stellate cells transfected with miR-291-3p mimics. Further study indicated that mothers against decapentaplegic homolog 2 (Smad2) was a target gene of miR-291-3p. More importantly, silencing of Smad2 could abolish miR-291-3p inhibition-induced TGF-β1 signaling activation. In summary, reduced peripheral blood miR-291-3p may be involved in the progression of HF via targeting Smad2.

Stem Cell Differentiation is Regulated by Extracellular Matrix Mechanics

Stem cells mechanosense the stiffness of their microenvironment, which impacts differentiation. Although tissue hydration anti-correlates with stiffness, extracellular matrix (ECM) stiffness is clearly transduced into gene expression via adhesion and cytoskeleton proteins that tune fates. Cytoskeletal reorganization of ECM can create heterogeneity and influence fates, with fibrosis being one extreme.

SOX9 predicts progression toward cirrhosis in patients while its loss protects against liver fibrosis

Fibrosis and organ failure is a common endpoint for many chronic liver diseases. Much is known about the upstream inflammatory mechanisms provoking fibrosis and downstream potential for tissue remodeling. However, less is known about the transcriptional regulation in vivo governing fibrotic matrix deposition by liver myofibroblasts. This gap in understanding has hampered molecular predictions of disease severity and clinical progression and restricted targets for antifibrotic drug development. In this study, we show the prevalence of SOX9 in biopsies from patients with chronic liver disease correlated with fibrosis severity and accurately predicted disease progression toward cirrhosis. Inactivation of Sox9 in mice protected against both parenchymal and biliary fibrosis, and improved liver function and ameliorated chronic inflammation. SOX9 was downstream of mechanosignaling factor, YAP1. These data demonstrate a role for SOX9 in liver fibrosis and open the way for the transcription factor and its dependent pathways as new diagnostic, prognostic, and therapeutic targets in patients with liver fibrosis.

Involvement of epithelial-mesenchymal transition in liver fibrosis

Fibrosis of the liver is an inherent wound healing response to chronic liver injury. Regeneration of liver epithelium and restoration of normal liver structure were generally involved in this process. Although the liver has a striking capacity to adapt to damage through tissue repair, excessive accumulation of extracellular matrix during this process often leads to scar tissue formation and subsequent fibrosis. Epithelial to mesenchymal transition (EMT) enables a polarized epithelial cell to undergo multiple changes biochemically and to bear a mesenchymal cell phenotype. EMT plays a critical role in tissue and organ development and embryogenesis. In the liver, it is proposed that epithelial cells can acquire fibroblastic phonotype via EMT and contribute to fibrogenesis. This made EMT a potential target for antifibrotic strategies. Following an original passion, many investigators devote themselves to exploring this mechanism in liver fibrosis. However, as research continues, this hypothesis became highly controversial. The exact contribution of EMT to fibrogenesis was challenged due to the contradictory results from related studies. In this review, we summarized the recent advances regarding EMT in hepatic fibrosis and discussed the potentially involved liver cell types and pathways in order to reach rational and helpful conclusions.

Fate tracing of hepatocytes in mouse liver

Hepatocytes perform most of the functions of the liver and are considered terminally differentiated cells. Recently, it has been suggested that hepatocytes might have the potential to transdifferentiate or dedifferentiate under physiological or pathological conditions in vivo. Epithelial-mesenchymal transition of hepatocytes in liver fibrosis has also been proposed. However, these findings have not been fully confirmed. In this study, hepatocytes were genetically labelled for cell fate tracing using lacZ via the tamoxifen-induced CreERT/loxP system. After induction with tamoxifen, alb + cells were permanently marked by lacZ expression, and all progeny lacZ + cells were derived from a single source with no interference. We did not observe transdifferentiation or dedifferentiation of hepatocytes into cholangiocytes or hepatic progenitor cells under conditions of liver homeostasis or following a 2/3 partial hepatectomy. Meanwhile, lacZ/OPN-positive cells were observed in livers of 3,5-diethoxycarbonyl-1,4-dihydrocollidine-fed mice, and lacZ/alpha-smooth muscle actin-positive cells were detected in carbon tetrachloride-induced chronic liver injury models. These results suggested that some existing differentiated alb + cells might have the potential of transdifferentiation/dedifferentiation or epithelial-to-mesenchymal transition in vivo in some liver injury models, but the proportion of these alb + cells in liver was very low, and their significance and actual function during the pathological process remains to be elucidated.

Can a fibrotic liver afford epithelial-mesenchymal transition?

The question whether epithelial-mesenchymal transition (EMT) occurs during liver fibrogenesis is a controversial issue. In vitro studies confirm that hepatocytes or cholangiocytes undergo EMT upon transforming growth factor β (TGF-β) stimulation, whereas in vivo experiments based on genetic fate mapping of specific cell populations suggest that EMT does not occur in fibrotic animal models. In this review we present current data supporting or opposing EMT in chronic liver disease and discuss conditions for the occurrence of EMT in patients. Based on the available data and our clinical observations we hypothesize that EMT-like alterations in liver cirrhosis are a side effect of high levels of TGF-β and other pro-fibrotic mediators rather than a biological process converting functional parenchyma, i.e., hepatocytes, into myofibroblasts at a time when essential liver functions are deteriorating.

Fibronectin fibrils regulate TGF-β1-induced Epithelial-Mesenchymal Transition

Epithelial-Mesenchymal Transition (EMT) is a dynamic process through which epithelial cells transdifferentiate from an epithelial phenotype into a mesenchymal phenotype. Previous studies have demonstrated that both mechanical signaling and soluble growth factor signaling facilitate this process. One possible point of integration for mechanical and growth factor signaling is the extracellular matrix. Here we investigate the role of the extracellular matrix (ECM) protein fibronectin (FN) in this process. We demonstrate that inhibition of FN fibrillogenesis blocks activation of the Transforming Growth Factor-Beta (TGF-β) signaling pathway via Smad2 signaling, decreases cell migration and ultimately leads to inhibition of EMT. Results show that soluble FN, FN fibrils, or increased contractile forces are insufficient to independently induce EMT. We further demonstrate that inhibition of latent TGF-β1 binding to FN fibrils via either a monoclonal blocking antibody against the growth factor binding domain of FN or through use of a FN deletion mutant that lacks the growth factor binding domains of FN blocks EMT progression, indicating a novel role for FN in EMT in which the assembly of FN fibrils serves to localize TGF-β1 signaling to drive EMT.

Adverse outcome pathway development from protein alkylation to liver fibrosis

In modern toxicology, substantial efforts are undertaken to develop alternative solutions for in vivo toxicity testing. The adverse outcome pathway (AOP) concept could facilitate knowledge-based safety assessment of chemicals that does not rely exclusively on in vivo toxicity testing. The construction of an AOP is based on understanding toxicological processes at different levels of biological organisation. Here, we present the developed AOP for liver fibrosis and demonstrate a linkage between hepatic injury caused by chemical protein alkylation and the formation of liver fibrosis, supported by coherent and consistent scientific data. This long-term process, in which inflammation, tissue destruction, and repair occur simultaneously, results from the complex interplay between various hepatic cell types, receptors, and signalling pathways. Due to the complexity of the process, an adequate liver fibrosis cell model for in vitro evaluation of a chemical's fibrogenic potential is not yet available. Liver fibrosis poses an important human health issue that is also relevant for regulatory purposes. An AOP described with enough mechanistic detail might support chemical risk assessment by indicating early markers for downstream events and thus facilitating the development of an in vitro testing strategy. With this work, we demonstrate how the AOP framework can support the assembly and coherent display of distributed mechanistic information from the literature to support the use of alternative approaches for prediction of toxicity. This AOP was developed according to the guidance document on developing and assessing AOPs and its supplement, the users' handbook, issued by the Organisation for Economic Co-operation and Development.

Common pathway signature in lung and liver fibrosis

Fibrosis, a progressive accumulation of extracellular matrix components, encompasses a wide spectrum of distinct organs, and accounts for an increasing burden of morbidity and mortality worldwide. Despite the tremendous clinical impact, the mechanisms governing the fibrotic process are not yet understood, and to date, no clinically reliable therapies for fibrosis have been discovered. Here we applied Regeneration Intelligence, a new bioinformatics software suite for qualitative analysis of intracellular signaling pathway activation using transcriptomic data, to assess a network of molecular signaling in lung and liver fibrosis. In both tissues, our analysis detected major conserved signaling pathways strongly associated with fibrosis, suggesting that some of the pathways identified by our algorithm but not yet wet-lab validated as fibrogenesis related, may be attractive targets for future research. While the majority of significantly disrupted pathways were specific to histologically distinct organs, several pathways have been concurrently activated or downregulated among the hepatic and pulmonary fibrosis samples, providing new evidence of evolutionary conserved pathways that may be relevant as possible therapeutic targets. While future confirmatory studies are warranted to validate these observations, our platform proposes a promising new approach for detecting fibrosis-promoting pathways and tailoring the right therapy to prevent fibrogenesis.

Pleiotrophin regulates the ductular reaction by controlling the migration of cells in liver progenitor niches

OBJECTIVE

The ductular reaction (DR) involves mobilisation of reactive-appearing duct-like cells (RDC) along canals of Hering, and myofibroblastic (MF) differentiation of hepatic stellate cells (HSC) in the space of Disse. Perivascular cells in stem cell niches produce pleiotrophin (PTN) to inactivate the PTN receptor, protein tyrosine phosphatase receptor zeta-1 (PTPRZ1), thereby augmenting phosphoprotein-dependent signalling. We hypothesised that the DR is regulated by PTN/PTPRZ1 signalling.

DESIGN

PTN-GFP, PTN-knockout (KO), PTPRZ1-KO, and wild type (WT) mice were examined before and after bile duct ligation (BDL) for PTN, PTPRZ1 and the DR. RDC and HSC from WT, PTN-KO, and PTPRZ1-KO mice were also treated with PTN to determine effects on downstream signaling phosphoproteins, gene expression, growth, and migration. Liver biopsies from patients with DRs were also interrogated.

RESULTS

Although quiescent HSC and RDC lines expressed PTN and PTPRZ1 mRNAs, neither PTN nor PTPRZ1 protein was demonstrated in healthy liver. BDL induced PTN in MF-HSC and increased PTPRZ1 in MF-HSC and RDC. In WT mice, BDL triggered a DR characterised by periportal accumulation of collagen, RDC and MF-HSC. All aspects of this DR were increased in PTN-KO mice and suppressed in PTPRZ1-KO mice. In vitro studies revealed PTN-dependent accumulation of phosphoproteins that control cell-cell adhesion and migration, with resultant inhibition of cell migration. PTPRZ1-positive cells were prominent in the DRs of patients with ductal plate defects and adult cholestatic diseases.

CONCLUSIONS

PTN, and its receptor, PTPRZ1, regulate the DR to liver injury by controlling the migration of resident cells in adult liver progenitor niches.

Revisiting Epithelial-to-Mesenchymal Transition in Liver Fibrosis: Clues for a Better Understanding of the "Reactive" Biliary Epithelial Phenotype

Whether liver epithelial cells contribute to the development of hepatic scarring by undergoing epithelial-to-mesenchymal transition (EMT) is a controversial issue. Herein, we revisit the concept of EMT in cholangiopathies, a group of severe hepatic disorders primarily targeting the bile duct epithelial cell (cholangiocyte), leading to progressive portal fibrosis, the main determinant of liver disease progression. Unfortunately, therapies able to halt this process are currently lacking. In cholangiopathies, fibrogenesis is part of ductular reaction, a reparative complex involving epithelial, mesenchymal, and inflammatory cells. Ductular reactive cells (DRC) are cholangiocytes derived from the activation of the hepatic progenitor cell compartment. These cells are arranged into irregular strings and express a “reactive” phenotype, which enables them to extensively crosstalk with the other components of ductular reaction. We will first discuss EMT in liver morphogenesis and then highlight how some of these developmental programs are partly reactivated in DRC. Evidence for “bona fide” EMT changes in cholangiocytes is lacking, but expression of some mesenchymal markers represents a fundamental repair mechanism in response to chronic biliary damage with potential harmful fibrogenetic effects. Understanding microenvironmental cues and signaling perturbations promoting these changes in DRC may help to identify potential targets for new antifibrotic therapies in cholangiopathies.

Immunohistochemical study for the origin of ductular reaction in chronic liver disease

The appearance of proliferating bile ductular structures, which is called the "atypical ductular reaction" is frequently observed in various chronic liver diseases associated. However, the origin of these increased bile ductules has been a matter of controversy. In this study, we investigated the origin of ductular cells as an aspect of relation between epithelial to mesenchymal transition (EMT) and epithelial members of liver parenchyme, such as hepatocyte and cholangiocyte by immunohistochemical staining of human liver. Thirteen specimens of surgically resected liver with biliary cirrhosis were selected. Three sets of double immunohistochemical stains were done; Hep-Par 1 - cytokeratin 19 (CK19), Hep-Par 1 - α-sm ooth mus cle actin (α-SMA) and CK19 - α-SMA. As a result, we investigated the dual expression of the markers of hepatocyte and cholangiocyte in the same cell; in ductular cell and surrounding hepatocyte. However, there seems to be no dual expression of markers for EMT with epithelial markers. This study suggests a possibility of phenotypic change of mature hepatocyte into cholangiocyte. Future studies will be necessary to determine the role that proliferating cholangiocytes play in the pathogenesis of biliary fibrosis and how cholangiocytes interact with other cell types of the liver such as hepatic stellate cells or Kupffer cells.

Mechanisms of fibrogenesis in liver cirrhosis: The molecular aspects of epithelial-mesenchymal transition

Liver injuries are repaired by fibrosis and regeneration. The cause of fibrosis and diminished regeneration, especially in liver cirrhosis, is still unknown. Epithelial-mesenchymal transition (EMT) has been found to be associated with liver fibrosis. The possibility that EMT could contribute to hepatic fibrogenesis reinforced the concept that activated hepatic stellate cells are not the only key players in the hepatic fibrogenic process and that other cell types, either hepatic or bone marrow-derived cells could contribute to this process. Following an initial enthusiasm for the discovery of this novel pathway in fibrogenesis, more recent research has started to cast serious doubts upon the real relevance of this phenomenon in human fibrogenetic disorders. The debate on the authenticity of EMT or on its contribution to the fibrogenic process has become very animated. The overall result is a general confusion on the meaning and on the definition of several key aspects. The aim of this article is to describe how EMT participates to hepatic fibrosis and discuss the evidence of supporting this possibility in order to reach reasonable and useful conclusions.

Proteolytic and non-proteolytic activation of keratinocyte-derived latent TGF-β1 induces fibroblast differentiation in a wound-healing model using rat skin

Transforming growth factor-β1 (TGF-β1) reportedly causes the differentiation of fibroblasts to myofibroblasts during wound healing. We investigated the mechanism underlying the activation of latent TGF-β1 released by keratinocytes in efforts to identify promising pharmacological approaches for the prevention of hypertrophic scar formation. A three-dimensional collagen gel matrix culture was prepared using rat keratinocytes and dermal fibroblasts. Stratified keratinocytes promoted the TGF receptor-dependent increase in α-smooth muscle actin (α-SMA) immunostaining and mRNA levels in fibroblasts. Latent TGF-β1 was found to be localized suprabasally and secreted. α-SMA expression was inhibited by an anti-αv-integrin antibody and a matrix metalloproteinase (MMP) inhibitor, GM6001. In a two-dimensional fibroblast culture, α-SMA expression depended on the production of endogenous TGF-β1 and required αv-integrin or MMP for the response to recombinant latent TGF-β1. In keratinocyte-conditioned medium, MMP-dependent latent TGF-β1 secretion was detected. Applying this medium to the fibroblast culture enhanced α-SMA production. This effect was decreased by GM6001, the anti-αv-integrin antibody, or the preabsorption of latent TGF-β1. These results indicate that keratinocytes secrete latent TGF-β1, which is liberated to fibroblasts over distance and is activated to produce α-SMA with the aid of a positive-feedback loop. MMP inhibition was effective for targeting both keratinocytes and fibroblasts in this model.

Hepatocyte polarity

Hepatocytes, like other epithelia, are situated at the interface between the organism's exterior and the underlying internal milieu and organize the vectorial exchange of macromolecules between these two spaces. To mediate this function, epithelial cells, including hepatocytes, are polarized with distinct luminal domains that are separated by tight junctions from lateral domains engaged in cell-cell adhesion and from basal domains that interact with the underlying extracellular matrix. Despite these universal principles, hepatocytes distinguish themselves from other nonstriated epithelia by their multipolar organization. Each hepatocyte participates in multiple, narrow lumina, the bile canaliculi, and has multiple basal surfaces that face the endothelial lining. Hepatocytes also differ in the mechanism of luminal protein trafficking from other epithelia studied. They lack polarized protein secretion to the luminal domain and target single-spanning and glycosylphosphatidylinositol-anchored bile canalicular membrane proteins via transcytosis from the basolateral domain. We compare this unique hepatic polarity phenotype with that of the more common columnar epithelial organization and review our current knowledge of the signaling mechanisms and the organization of polarized protein trafficking that govern the establishment and maintenance of hepatic polarity. The serine/threonine kinase LKB1, which is activated by the bile acid taurocholate and, in turn, activates adenosine monophosphate kinase-related kinases including AMPK1/2 and Par1 paralogues has emerged as a key determinant of hepatic polarity. We propose that the absence of a hepatocyte basal lamina and differences in cell-cell adhesion signaling that determine the positioning of tight junctions are two crucial determinants for the distinct hepatic and columnar polarity phenotypes.

Experimental liver fibrosis research: update on animal models, legal issues and translational aspects

Liver fibrosis is defined as excessive extracellular matrix deposition and is based on complex interactions between matrix-producing hepatic stellate cells and an abundance of liver-resident and infiltrating cells. Investigation of these processes requires in vitro and in vivo experimental work in animals. However, the use of animals in translational research will be increasingly challenged, at least in countries of the European Union, because of the adoption of new animal welfare rules in 2013. These rules will create an urgent need for optimized standard operating procedures regarding animal experimentation and improved international communication in the liver fibrosis community. This review gives an update on current animal models, techniques and underlying pathomechanisms with the aim of fostering a critical discussion of the limitations and potential of up-to-date animal experimentation. We discuss potential complications in experimental liver fibrosis and provide examples of how the findings of studies in which these models are used can be translated to human disease and therapy. In this review, we want to motivate the international community to design more standardized animal models which might help to address the legally requested replacement, refinement and reduction of animals in fibrosis research.

Recent advances in the morphological and functional heterogeneity of the biliary epithelium

This review focuses on the recent advances related to the heterogeneity of different-sized bile ducts with regard to the morphological and phenotypical characteristics, and the differential secretory, apoptotic and proliferative responses of small and large cholangiocytes to gastrointestinal hormones/peptides, neuropeptides and toxins. We describe several in vivo and in vitro models used for evaluating biliary heterogeneity. Subsequently, we discuss the heterogeneous proliferative and apoptotic responses of small and large cholangiocytes to liver injury and the mechanisms regulating the differentiation of small into large (more differentiated) cholangiocytes. Following a discussion on the heterogeneity of stem/progenitor cells in the biliary epithelium, we outline the heterogeneity of bile ducts in human cholangiopathies. After a summary section, we discuss the future perspectives that will further advance the field of the functional heterogeneity of the biliary epithelium.

Hepatic stellate cells in liver development, regeneration, and cancer

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

Role of stem/progenitor cells in reparative disorders

Adult stem cells are activated to proliferate and differentiate during normal tissue homeostasis as well as in disease states and injury. This activation is a vital component in the restoration of function to damaged tissue via either complete or partial regeneration. When regeneration does not fully occur, reparative processes involving an overproduction of stromal components ensure the continuity of tissue at the expense of its normal structure and function, resulting in a “reparative disorder”. Adult stem cells from multiple organs have been identified as being involved in this process and their role in tissue repair is being investigated. Evidence for the participation of mesenchymal stromal cells (MSCs) in the tissue repair process across multiple tissues is overwhelming and their role in reparative disorders is clearly demonstrated, as is the involvement of a number of specific signaling pathways. Transforming growth factor beta, bone morphogenic protein and Wnt pathways interact to form a complex signaling network that is critical in regulating the fate choices of both stromal and tissue-specific resident stem cells (TSCs), determining whether functional regeneration or the formation of scar tissue follows an injury. A growing understanding of both TSCs, MSCs and the complex cascade of signals regulating both cell populations have, therefore, emerged as potential therapeutic targets to treat reparative disorders. This review focuses on recent advances on the role of these cells in skeletal muscle, heart and lung tissues.

Host responses in tissue repair and fibrosis

Myofibroblasts accumulate in the spaces between organ structures and produce extracellular matrix (ECM) proteins, including collagen I. They are the primary "effector" cells in tissue remodeling and fibrosis. Previously, leukocyte progenitors termed fibrocytes and myofibroblasts generated from epithelial cells through epithelial-to-mesenchymal transition (EMT) were considered the primary sources of ECM-producing myofibroblasts in injured tissues. However, genetic fate mapping experiments suggest that mesenchyme-derived cells, known as resident fibroblasts, and pericytes are the primary precursors of scar-forming myofibroblasts, whereas epithelial cells, endothelial cells, and myeloid leukocytes contribute to fibrogenesis predominantly by producing key fibrogenic cytokines and by promoting cell-to-cell communication. Numerous cytokines derived from T cells, macrophages, and other myeloid cell populations are important drivers of myofibroblast differentiation. Monocyte-derived cell populations are key regulators of the fibrotic process: They act as a brake on the processes driving fibrogenesis, and they dismantle and degrade established fibrosis. We discuss the origins, modes of activation, and fate of myofibroblasts in various important fibrotic diseases and describe how manipulation of macrophage activation could help ameliorate fibrosis.

NADPH oxidase NOX4 mediates stellate cell activation and hepatocyte cell death during liver fibrosis development

A role for the NADPH oxidases NOX1 and NOX2 in liver fibrosis has been proposed, but the implication of NOX4 is poorly understood yet. The aim of this work was to study the functional role of NOX4 in different cell populations implicated in liver fibrosis: hepatic stellate cells (HSC), myofibroblats (MFBs) and hepatocytes. Two different mice models that develop spontaneous fibrosis (Mdr2(-/-)/p19(ARF-/-), Stat3(Δhc)/Mdr2(-/-)) and a model of experimental induced fibrosis (CCl(4)) were used. In addition, gene expression in biopsies from chronic hepatitis C virus (HCV) patients or non-fibrotic liver samples was analyzed. Results have indicated that NOX4 expression was increased in the livers of all animal models, concomitantly with fibrosis development and TGF-β pathway activation. In vitro TGF-β-treated HSC increased NOX4 expression correlating with transdifferentiation to MFBs. Knockdown experiments revealed that NOX4 downstream TGF-β is necessary for HSC activation as well as for the maintenance of the MFB phenotype. NOX4 was not necessary for TGF-β-induced epithelial-mesenchymal transition (EMT), but was required for TGF-β-induced apoptosis in hepatocytes. Finally, NOX4 expression was elevated in patients with hepatitis C virus (HCV)-derived fibrosis, increasing along the fibrosis degree. In summary, fibrosis progression both in vitro and in vivo (animal models and patients) is accompanied by increased NOX4 expression, which mediates acquisition and maintenance of the MFB phenotype, as well as TGF-β-induced death of hepatocytes.

The crucial role of cholangiocytes in cholangiopathies

Cholangiopathies are diseases involving the intrahepatic biliary tree. They appear to involve, chronic inflammation of the bile ducts, which can lead to the development of bile duct cholestasis, proliferation/ductopenia, biliary fibrosis, and malignant transformation. Sustained stimulatory insults to biliary epithelial cells can induce a ductular reaction, which has a key role in the initiation and progression of cholangiopathies. The epithelial-mesenchymal interaction between reactive cholangiocytes and mesenchymal cells with the inflammatory infiltrates plays a major role in this pathogenesis. Cytokines, chemokines, growth factors and morphogens mediate these interactions in an autocrine or paracrine manner. The main hepatic myofibroblasts (MFs) in cholangiopathies originate from portal fibroblasts. Hepatic stellate cells and fibrocytes also transform into MFs. Whether cholangiocytes or hepatocytes are a source of MFs via the epithelial-mesenchymal transition (EMT) remains a matter of controversy. Although there have been numerous indirect findings supporting the theory of a cholangiocyte EMT in human tissues, recent studies using lineage tracing methods have demonstrated strong evidence against the EMT. Understanding the pathogenic mechanisms involved in cholangiopathies can allow for better-targeted anti-fibrotic therapies in animal models. Before anti-fibrotic therapies can translate into clinical trials, improved monitoring of the fibrotic progression of cholangiopathies and an accurate assessment regarding the effectiveness of the proposed treatments must be achieved.

Cellular and molecular mechanisms of chronic inflammation-associated organ fibrosis

Organ fibrosis is a pathological condition associated with chronic inflammatory diseases. In fibrosis, excessive deposition of extracellular matrix (ECM) severely impairs tissue architecture and function, eventually resulting in organ failure. This process is mediated primarily by the induction of myofibroblasts, which produce large amounts of collagen I, the main component of the ECM. Accordingly, the origin, developmental pathways, and mechanisms of myofibroblast regulation are attracting increasing attention as potential therapeutic targets. The fibrotic cascade, from initial epithelial damage to eventual myofibroblast induction, is mediated by complex biological processes such as macrophage infiltration, a shift from Th1 to Th2 phenotype, and by inflammatory mediators such as transforming growth factor-β. Here, we review the current understanding of the cellular and molecular mechanisms underlying organ fibrosis.

Targeting PDGFR-β in Cholangiocarcinoma

BACKGROUND

Cholangiocarcinomas (CCAs) are highly desmoplastic neoplasms with a tumour microenvironment plentiful in myofibroblasts (MFBs). MFB-derived PDGF-BB survival signalling is a mediator of CCA cell resistance to apoptotic stimuli. This raises the concept that targeting PDGFR-β, a cognate receptor of PDGF-BB, represents a potential strategy for the treatment of human CCA.

AIMS

Herein, we examine a role for inhibiting PDGFR-β in restoring CCA cell sensitivity to apoptotic stimuli in vitro and in vivo.

METHODS

We employed human CCA samples from 41 patients (19 intrahepatic and 22 extrahepatic CCA samples), the human CCA cell lines KMCH-1 and HUCCT-1 as well as shPDGFR-β-KMCH-1 and human myofibroblastic LX-2 cells for these studies. In vivo-experiments were conducted using a syngeneic rat orthotopic CCA model.

RESULTS

Of several MFB-derived growth factors profiled, PDGF-BB and CTGF were most abundantly expressed; however, only PDGF-BB attenuated tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) cytotoxicity. Co-culturing CCA cells with PDGF-BB-secreting MFBs significantly decreased TRAIL-induced CCA cell apoptosis when compared with monoculture conditions; this cytoprotective effect was abrogated in the presence of the tyrosine kinase inhibitors imatinib mesylate or linifanib, which inhibit PDGFR-β. Consistent with these findings, MFB-imparted cytoprotection also was abolished when PDGFR-β was knocked down as demonstrated in shPDGFR-β-KMCH-1 cells. Finally, administration of imatinib mesylate increased CCA cell apoptosis and reduced tumour growth in a rodent in vivo-CCA model that mimics the human disease.

CONCLUSIONS

Targeting PDGFR-β sensitizes CCA cells to apoptotic stimuli and appears to be therapeutic in vivo.

The origin of interstitial myofibroblasts in chronic kidney disease

Chronic kidney diseases (CKD), independent of their primary cause, lead to progressive, irreversible loss of functional renal parenchyma. Renal pathology in CKD is characterized by tubulointerstitial fibrosis with excessive matrix deposition produced by myofibroblasts. Because blocking the formation of these scar-forming cells represents a logical therapeutic target for patients with progressive fibrotic kidney disease, the origin of renal myofibroblasts is a subject of intense investigation. Although the traditional view holds that resident fibroblasts are the myofibroblast precursor, for the last 10 years, injured epithelial cells have been thought to directly contribute to the myofibroblast pool by the process of epithelial-to-mesenchymal transition (EMT). The recent application of genetic fate mapping techniques in mouse fibrosis models has provided new insights into the cell hierarchies in fibrotic kidney disease and results cast doubt on the concept that EMT is a source of myofibroblast recruitment in vivo, but rather point to the resident pericyte/perivascular fibroblast as the myofibroblast progenitor pool. This review will highlight recent findings arguing against EMT as a direct contributor to the kidney myofibroblast population and review the use of genetic fate mapping to elucidate the cellular mechanisms of kidney homeostasis and disease.

Ligand-independent transforming growth factor-β type I receptor signalling mediates type I collagen-induced epithelial-mesenchymal transition

Evidence suggests epithelial-mesenchymal transition (EMT) as one potential source of fibroblasts in idiopathic pulmonary fibrosis. To assess the contribution of alveolar epithelial cell (AEC) EMT to fibroblast accumulation in vivo following lung injury and the influence of extracellular matrix on AEC phenotype in vitro, Nkx2.1-Cre;mT/mG mice were generated in which AECs permanently express green fluorescent protein (GFP). On days 17-21 following intratracheal bleomycin administration, ~4% of GFP-positive epithelial-derived cells expressed vimentin or α-smooth muscle actin (α-SMA). Primary AECs from Nkx2.1-Cre;mT/mG mice cultured on laminin-5 or fibronectin maintained an epithelial phenotype. In contrast, on type I collagen, cells of epithelial origin displayed nuclear localization of Smad3, acquired spindle-shaped morphology, expressed α-SMA and phospho-Smad3, consistent with activation of the transforming growth factor-β (TGFβ) signalling pathway and EMT. α-SMA induction and Smad3 nuclear localization were blocked by the TGFβ type I receptor (TβRI, otherwise known as Alk5) inhibitor SB431542, while AEC derived from Nkx2.1-Cre;Alk5(flox/KO) mice did not undergo EMT on collagen, consistent with a requirement for signalling via Alk5 in collagen-induced EMT. Inability of a pan-specific TGFβ neutralizing antibody to inhibit effects of collagen together with absence of active TGFβ in culture supernatants is consistent with TGFβ ligand-independent activation of Smad signalling. These results support the notion that AECs can acquire a mesenchymal phenotype following injury in vivo and implicate type I collagen as a key regulator of EMT in AECs through signalling via Alk5, likely in a TGFβ ligand-independent manner.

Biliary tree stem/progenitor cells in glands of extrahepatic and intraheptic bile ducts: an anatomical in situ study yielding evidence of maturational lineages

Stem/progenitors have been identified intrahepatically in the canals of Hering and extrahepatically in glands of the biliary tree. Glands of the biliary tree (peribiliary glands) are tubulo-alveolar glands with mucinous and serous acini, located deep within intrahepatic and extrahepatic bile ducts. We have shown that biliary tree stem/progenitors (BTSCs) are multipotent, giving rise in vitro and in vivo to hepatocytes, cholangiocytes or pancreatic islets. Cells with the phenotype of BTSCs are located at the bottom of the peribiliary glands near the fibromuscular layer. They are phenotypically heterogeneous, expressing transcription factors as well as surface and cytoplasmic markers for stem/progenitors of liver (e.g. SOX9/17), pancreas (e.g. PDX1) and endoderm (e.g. SOX17, EpCAM, NCAM, CXCR4, Lgr5, OCT4) but not for mature markers (e.g. albumin, secretin receptor or insulin). Subpopulations co-expressing liver and pancreatic markers (e.g. PDX1(+)/SOX17(+)) are EpCAM(+/-), and are assumed to be the most primitive of the BTSC subpopulations. Their descendants undergo a maturational lineage process from the interior to the surface of ducts and vary in the mature cells generated: pancreatic cells in hepatopancreatic ducts, liver cells in large intrahepatic bile ducts, and bile duct cells along most of the biliary tree. We hypothesize that there is ongoing organogenesis throughout life, with BTSCs giving rise to hepatic stem cells in the canals of Hering and to committed progenitors within the pancreas. The BTSCs are likely to be central to normal tissue turnover and injury repair and to be key elements in the pathophysiology of liver, pancreas and biliary tree diseases, including oncogenesis.

Myofibroblast-derived PDGF-BB promotes Hedgehog survival signaling in cholangiocarcinoma cells

Cholangiocarcinoma (CCA) cells paradoxically express the death ligand, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and, therefore, are dependent upon potent survival signals to circumvent TRAIL cytotoxicity. CCAs are also highly desmoplastic cancers with a tumor microenvironment rich in myofibroblasts (MFBs). Herein, we examine a role for MFB-derived CCA survival signals. We employed human KMCH-1, KMBC, HuCCT-1, TFK-1, and Mz-ChA-1 CCA cells, as well as human primary hepatic stellate and myofibroblastic LX-2 cells, for these studies. In vivo experiments were conducted using a syngeneic rat orthotopic CCA model. Coculturing CCA cells with myofibroblastic human primary hepatic stellate cells or LX-2 cells significantly decreased TRAIL-induced apoptosis in CCA cells, a cytoprotective effect abrogated by neutralizing platelet-derived growth factor (PDGF)-BB antiserum. Cytoprotection by PDGF-BB was dependent upon Hedgehog (Hh) signaling, because it was abolished by the smoothened (SMO; the transducer of Hh signaling) inhibitor, cyclopamine. PDGF-BB induced cyclic adenosine monophosphate-dependent protein kinase-dependent trafficking of SMO to the plasma membrane, resulting in glioma-associated oncogene (GLI)2 nuclear translocation and activation of a consensus GLI reporter gene-based luciferase assay. A genome-wide messenger RNA expression analysis identified 67 target genes to be commonly up- (50 genes) or down-regulated (17 genes) by both Sonic hedgehog and PDGF-BB in a cyclopamine-dependent manner in CCA cells. Finally, in a rodent CCA in vivo model, cyclopamine administration increased apoptosis in CCA cells, resulting in tumor suppression.

CONCLUSIONS

MFB-derived PDGF-BB protects CCA cells from TRAIL cytotoxicity by a Hh-signaling-dependent process. These results have therapeutical implications for the treatment of human CCA.

Current status of novel antifibrotic therapies in patients with chronic liver disease

Fibrosis accumulation is a dynamic process resulting from a wound-healing response to acute or chronic liver injury of all causes. The cascade starts with hepatocyte necrosis and apoptosis, which instigate inflammatory signaling by chemokines and cytokines, recruitment of immune cell populations, and activation of fibrogenic cells, culminating in the deposition of extracellular matrix. These key elements, along with pathways of transcriptional and epigenetic regulation, represent fertile therapeutic targets. New therapies include drugs specifically designed as antifibrotics, as well as drugs already available with well-established safety profiles, whose mechanism of action may also be antifibrotic. At the same time, the development of noninvasive fibrogenic markers, and techniques (e.g. fibroscan), as well as combined scoring systems incorporating serum and clinical features will allow improved assessment of therapy response. In aggregate, the advances in the elucidation of the biology of fibrosis, combined with improved technologies for assessment will provide a comprehensive framework for design of antifibrotics and their analysis in well-designed clinical trials. These efforts may ultimately yield success in halting the progression of, or reversing, liver fibrosis.

Molecular pathogenesis of hepatic fibrosis and current therapeutic approaches

The pathogenesis of hepatic fibrosis involves significant deposition of fibrilar collagen and other extracellular matrix proteins. It is a rather dynamic process of wound healing in response to a variety of persistent liver injury caused by factors such as ethanol intake, viral infection, drugs, toxins, cholestasis, and metabolic disorders. Liver fibrosis distorts the hepatic architecture, decreases the number of endothelial cell fenestrations and causes portal hypertension. Key events are the activation and transformation of quiescent hepatic stellate cells into myofibroblast-like cells with the subsequent up-regulation of proteins such as α-smooth muscle actin, interstitial collagens, matrix metalloproteinases, tissue inhibitor of metalloproteinases, and proteoglycans. Oxidative stress is a major contributing factor to the onset of liver fibrosis and it is typically associated with a decrease in the antioxidant defense. Currently, there is no effective therapy for advanced liver fibrosis. In its early stages, liver fibrosis is reversible upon cessation of the causative agent. In this review, we discuss some aspects on the etiology of liver fibrosis, the cells involved, the molecular pathogenesis, and the current therapeutic approaches.

Keratin 19 epithelial patterns in cirrhotic stroma parallel hepatocarcinogenesis

Cirrhotic septa harbor vessels and inflammatory, fibrogenic, and ductular epithelial cells, collectively referred to as the ductular reaction (DR). Lack of the DR in the stromal compartment around hepatocellular carcinoma (HCC) has been documented; however, the relationship of epithelial keratin 19 (K19) structures to progression of intralesional carcinogenesis has not been explored. K19 immunoreactivity in the stromal compartment around 176 nodules in cirrhotic explants was examined. Quantitative differences (P < 0.0001) were manifested in three distinct histologically identifiable patterns: "complex" around cirrhotic nodules (CN), "attenuated" around dysplastic nodules (DN), and "absent" around HCC. Markers of necrosis or apoptosis could not explain the perinodular K19 epithelial loss; however, multicolor immunolabeling for K19, vimentin, E-Cadherin, SNAIL, and fibroblast-specific protein 1 (FSP-1) demonstrated discrepancies in immunophenotype and cytomorphologic features. Variability of cellular features was accompanied by an overall decrease in epithelial markers and significantly increased fractions of SNAIL- and FSP-1-positive cells in the DR around DN when compared with CN (P < 0.0001). Immunolabeling of transforming growth factor-β signaling components (TGFβR1, SMAD3, and pSMAD2/3) demonstrated increased percentages of pSMAD2/3 around DN when compared with CN (P < 0.0001). These findings collectively suggest marked alterations in cellular identity as an underlying mechanism for the reproducible extralesional K19 pattern that parallels progressive stages of intranodular hepatocarcinogenesis. Paracrine signaling is proposed as a link that emphasizes the importance of the epithelial-stromal compartment in malignant progression of HCC in cirrhosis.

Lineage tracing demonstrates no evidence of cholangiocyte epithelial-to-mesenchymal transition in murine models of hepatic fibrosis

Whether or not cholangiocytes or their hepatic progenitors undergo an epithelial-to-mesenchymal transition (EMT) to become matrix-producing myofibroblasts during biliary fibrosis is a significant ongoing controversy. To assess whether EMT is active during biliary fibrosis, we used Alfp-Cre × Rosa26-YFP mice, in which the epithelial cells of the liver (hepatocytes, cholangiocytes, and their bipotential progenitors) are heritably labeled at high efficiency with yellow fluorescent protein (YFP). Primary cholangiocytes isolated from our reporter strain were able to undergo EMT in vitro when treated with transforming growth factor-β1 alone or in combination with tumor necrosis factor-α, as indicated by adoption of fibroblastoid morphology, intracellular relocalization of E-cadherin, and expression of α-smooth muscle actin (α-SMA). To determine whether EMT occurs in vivo, we induced liver fibrosis in Alfp-Cre × Rosa26-YFP mice using the bile duct ligation (BDL) (2, 4, and 8 weeks), carbon tetrachloride (CCl(4) ) (3 weeks), and 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC; 2 and 3 weeks) models. In no case did we find evidence of colocalization of YFP with the mesenchymal markers S100A4, vimentin, α-SMA, or procollagen 1α2, although these proteins were abundant in the peribiliary regions.

CONCLUSION

Hepatocytes and cholangiocytes do not undergo EMT in murine models of hepatic fibrosis.

Epithelial-mesenchymal transition (EMT) in kidney fibrosis: fact or fantasy?

Epithelial-mesenchymal transition (EMT) has become widely accepted as a mechanism by which injured renal tubular cells transform into mesenchymal cells that contribute to the development of fibrosis in chronic renal failure. However, an increasing number of studies raise doubts about the existence of this process in vivo. Herein, we review and summarize both sides of this debate, but it is our view that unequivocal evidence supporting EMT as an in vivo process in kidney fibrosis is lacking.

Epithelial-mesenchymal interactions in biliary diseases

In most cholangiopathies, liver diseases of different etiologies in which the biliary epithelium is the primary target in the pathogenic sequence, the central mechanism involves inflammation. Inflammation, characterized by pleomorphic peribiliary infiltrate containing fibroblasts, macrophages, lymphocytes, as well as endothelial cells and pericytes, is associated to the emergence of "reactive cholangiocytes." These biliary cells do not possess bile secretory functions, are in contiguity with terminal cholangioles, and are of a less-differentiated phenotype. They have acquired several mesenchymal properties, including motility and ability to secrete a vast number of proinflammatory chemo/cytokines and growth factors along with de novo expression of a rich receptor machinery. These functional properties enable reactive cholangiocytes to establish intimate contacts and to mutually exchange a variety of paracrine signals with the different mesenchymal cell types populating the portal infiltrate. The extensive crosstalk between the epithelial and mesenchymal compartments is the driver of liver repair mechanisms in cholangiopathies, ultimately evolving toward portal fibrosis. Herein, the authors first review the properties of the different cell types involved in their interaction, and then analyze the underlying molecular mechanisms as they relate to liver repair in cholangiopathies.

New concepts in liver regeneration

The unique ability of the liver to regenerate itself has fascinated biologists for years and has made it the prototype for mammalian organ regeneration. Harnessing this process has great potential benefit in the treatment of liver failure and has been the focus of intense research over the past 50 years. Not only will detailed understanding of cell proliferation in response to injury be applicable to other dysfunction of organs, it may also shed light on how cancer develops in a cirrhotic liver, in which there is intense pressure on cells to regenerate. Advances in molecular techniques over the past few decades have led to the identification of many regulatory intermediates, and pushed us onto the verge of an explosive era in regenerative medicine. To date, more than 10 clinical trials have been reported in which augmented regeneration using progenitor cell therapy has been attempted in human patients. This review traces the path that has been taken over the last few decades in the study of liver regeneration, highlights new concepts in the field, and discusses the challenges that still stand between us and clinical therapy.

Insulin resistance and necroinflammation drives ductular reaction and epithelial-mesenchymal transition in chronic hepatitis C

OBJECTIVE

To study the mechanism(s) linking insulin resistance (IR) to hepatic fibrosis and the role of the epithelial component in tissue repair and fibrosis in chronic hepatitis C (CHC).

DESIGN

Prospective observational study.

SETTING

Tertiary care academic centre.

PATIENTS

78 consecutive patients with CHC.

MAIN OUTCOME MEASURES

IR, calculated by the oral glucose insulin sensitivity during oral glucose tolerance test; necroinflammatory activity and fibrosis, defined according to Ishak's score; steatosis, graded as 0 (<5% of hepatocytes), 1 (5-33%), 2 (33-66%) and 3 (>66%). To evaluate the role of the epithelial component in tissue repair and fibrosis, the expansion of the ductular reaction (DR) was calculated by keratin-7 (CK7) morphometry. Nuclear expression of Snail, downregulation of E-cadherin and expression of fibroblast specific protein-1 (FSP1) and vimentin by CK7-positive cells were used as markers of epithelial-mesenchymal transition in DR elements.

RESULTS

IR, the degree of necroinflammation and expansion of the DR (stratified as reactive ductular cells (RDCs), hepatic progenitor cells and intermediate hepatobiliary cells according to morphological criteria) were all associated with the stage of fibrosis. Nuclear Snail expression, E-cadherin downregulation and vimentin upregulation were observed in RDCs. By dual immunofluorescence for CK7 and FSP1, the number of RDCs undergoing epithelial-mesenchymal transition progressively increased together with the necroinflammatory score. By multivariate analysis, total inflammation and insulin resistance were the only factors significantly predicting the presence of advanced fibrosis (Ishak score ≥3) and the expansion of RDCs.

CONCLUSION

This study indicates that IR is associated with the degree of necroinflammatory injury in CHC and contributes to hepatic fibrosis by stimulating the expansion of RDCs that express epithelial-mesenchymal transition markers.

Evolving challenges in hepatic fibrosis

Continued elucidation of the mechanisms of hepatic fibrosis has yielded a comprehensive and nuanced portrait of fibrosis progression and regression. The paradigm of hepatic stellate cell (HSC) activation remains the foundation for defining events in hepatic fibrosis and has been complemented by progress in a number of new areas. Cellular sources of extracellular matrix beyond HSCs have been identified. In addition, the role of chemokine, adipokine, neuroendocrine, angiogenic and NAPDH oxidase signaling in the pathogenesis of hepatic fibrosis has been uncovered, as has the contribution of extracellular matrix stiffness to fibrogenesis. There is also increased awareness of the contribution of innate immunity and greater understanding of the complexity of gene regulation in HSCs and myofibroblasts. Finally, both apoptosis and senescence have been recognized as orchestrated programs that eliminate fibrogenic cells during resolution of liver fibrosis. Ironically, the progress that has been made has highlighted the growing disparity between advances in the experimental setting and their translation into new diagnostic tools and treatments. As a result, focus is shifting towards overcoming key translational challenges in order to accelerate the development of new therapies for patients with chronic liver disease.

Epithelial-mesenchymal transitions and hepatocarcinogenesis

Epithelial-mesenchymal transitions (EMTs) are believed to play a role in invasion and metastasis of many types of tumors. In this issue of the JCI, Chen et al. show that a gene that has been associated with aggressive biology in hepatocellular carcinomas initiates a molecular cascade that results in EMT.