Portal fibroblasts: Underappreciated mediators of biliary fibrosis

Liver fibrogenic cells

Liver fibrogenic cells are a heterogenous population of cells that include α-smooth muscle actin positive myofibroblasts (MFs). MFs promote the progression of chronic liver diseases (CLDs) towards cirrhosis. MFs are highly proliferative and contractile and promote fibrogenesis by means of their multiple phenotypic responses to injury. These include: excess deposition and altered remodelling of extracellular matrix; the synthesis and release of growth factor which sustain and perpetuate fibrogenesis; chronic inflammatory response and neo-angiogenesis. MFs mainly originate from hepatic stellate cells or portal fibroblasts through activation and transdifferentiation. MFs may also potentially differentiate from bone marrow-derived stem cells. It has been suggested that MFs can be derived from hepatocytes or cholangiocytes through a process of epithelial to mesenchymal transition in the liver, however this is controversial. Hepatic MFs may also modulate the immune responses to hepatocellular carcinomas and metastatic cancers through cross talk with hepatic progenitor and tumour cells.

Fibrosis in autoimmune and cholestatic liver disease

Autoimmune and cholestatic liver disease account for a significant part of end-stage liver disease and are leading indications for liver transplantation. Especially cholestatic liver diseases (primary biliary cirrhosis and primary sclerosing cholangitis) appear to be different from other chronic liver diseases with regards to pathogenesis. Portal fibroblasts located in the connective tissue surrounding bile ducts appear to be different from hepatic stellate cells with regards to expression of marker proteins and response the profibrogenic and mitogenic stimuli. In addition there is increasing evidence for a cross talk between activated cholangiocytes and portal myofibroblasts. Several animal models have improved our understanding of the mechanisms underlying these chronic liver diseases. In the present review, we discuss the current concepts and ideas with regards to myofibroblastic cell populations, mechanisms of fibrosis, summarize characteristic histological findings and currently employed animal models of autoimmune and cholestatic liver disease.

Evolution of hepatic fibrosis research

Molecular analysis of hepatic fibrogenesis has progressed with respect to both fibrosis progression and regression by using cell biological, molecular biological and (epi)genetic approaches. Recent researches have revealed sources of collagen-producing cells other than hepatic stellate cells in the liver, and the involvement of the innate immune system and oxidative stress in the fibrotic process has attracted new attention. Together with these advancements in basic knowledge on the cellular and molecular biology of hepatic fibrosis, clinical researches have linked the clarification of the relationship between progression of the fibrosis stage and therapeutic efficacy for chronic viral hepatitis and non-alcoholic steatohepatitis and validation of the regression of advanced fibrosis, even cirrhosis, of appropriate therapies using modern medicines. Furthermore, non-invasive assessment of liver fibrosis using an ultrasound-based modality has become a focus in the clinical diagnosis of liver fibrosis instead of liver biopsy. Taken together, liver fibrosis research has been evolving both basically and clinically in the past three decades.

Septum transversum-derived mesothelium gives rise to hepatic stellate cells and perivascular mesenchymal cells in developing mouse liver

The septum transversum mesenchyme (STM) signals to induce hepatogenesis from the foregut endoderm. Hepatic stellate cells (HSCs) are sinusoidal pericytes assumed to originate from the STM and participate in mesenchymal-epithelial interaction in embryonic and adult livers. However, the developmental origin of HSCs remains elusive due to the lack of markers for STM and HSCs. We previously identified submesothelial cells (SubMCs) beneath mesothelial cells (MCs) as a potential precursor for HSCs in developing livers. In the present study, we reveal that both STM in embryonic day (E) 9.5 and MC/SubMCs in E12.5 share the expression of activated leukocyte cell adhesion molecule (Alcam), desmin, and Wilms tumor 1 homolog (Wt1). A cell lineage analysis using MesP1(Cre) /Rosa26lacZ(flox) mice identifies the mesodermal origin of the STM, HSCs, and perivascular mesenchymal cells (PMCs). A conditional cell lineage analysis using the Wt1(CreERT2) mice demonstrates that Wt1(+) STM gives rise to MCs, SubMCs, HSCs, and PMCs during liver development. Furthermore, we find that Wt1(+) MC/SubMCs migrate inward from the liver surface to generate HSCs and PMCs including portal fibroblasts, smooth muscle cells, and fibroblasts around the central veins. On the other hand, the Wt1(+) STM and MC/SubMCs do not contribute to sinusoidal endothelial cells, Kupffer cells, and hepatoblasts.

CONCLUSION

our results demonstrate that HSCs and PMCs are derived from MC/SubMCs, which are traced back to mesodermal STM during liver development.

Cholestatic liver fibrosis and toxin-induced fibrosis are exacerbated in matrix metalloproteinase-2 deficient mice

Matrix metalloproteinase (MMP) plays an important role in homeostatic regulation of the extracellular environment and degradation of matrix. During liver fibrosis, several MMPs, including MMP-2, are up-regulated in activated hepatic stellate cells, which are responsible for exacerbation of liver cirrhosis. However, it remains unclear how loss of MMP-2 influences molecular dynamics associated with fibrogenesis in the liver. To explore the role of MMP-2 in hepatic fibrogenesis, we employed two fibrosis models in mice; toxin (carbon tetrachloride, CCl4)-induced and cholestasis-induced fibrosis. In the chronic CCl4 administration model, MMP-2 deficient mice exhibited extensive liver fibrosis as compared with wild-type mice. Several molecules related to activation of hepatic stellate cells were up-regulated in MMP-2 deficient liver, suggesting that myofibroblastic change of hepatic stellate cells was promoted in MMP-2 deficient liver. In the cholestasis model, fibrosis in MMP-2 deficient liver was also accelerated as compared with wild type liver. Production of tissue inhibitor of metalloproteinase 1 increased in MMP-2 deficient liver in both models, while transforming growth factor β, platelet-derived growth factor receptor and MMP-14 were up-regulated only in the CCl4 model. Our study demonstrated, using 2 experimental murine models, that loss of MMP-2 exacerbates liver fibrosis, and suggested that MMP-2 suppresses tissue inhibitor of metalloproteinase 1 up-regulation during liver fibrosis.

Ductal plates in hepatic ductular reactions. Hypothesis and implications. III. Implications for liver pathology

This article discusses on the basis of the ductal plate hypothesis the implication of the concept for several liver abnormalities. The occurrence of ductal plates (DP) during liver growth in childhood would explain the paraportal and parenchymal localizations of von Meyenburg complexes in postnatally developed parts of the liver, and their higher incidence in adulthood versus childhood. It partly clarifies the lack of postnatal intrahepatic bile duct development in Alagille syndrome and the reduced number of portal tracts in this disease. Ductular reactions (DRs) in DP configuration are the predominant type of progenitor cell reaction in fulminant necro-inflammatory liver disease, when lack of sufficient parenchymal regeneration results in liver failure. The concept of dissecting DRs explains the micronodular pattern of advanced biliary and alcoholic cirrhosis. The concept explains the DP patterns of bile ducts in several cases of biliary atresia, with implications for diagnosis and prognosis. The hypothesis also has an impact on concepts about stem/progenitor cells and their niche.

Combination of retinoic acid and ursodeoxycholic acid attenuates liver injury in bile duct-ligated rats and human hepatic cells

Cholestasis leads to liver cell death, fibrosis, cirrhosis, and eventually liver failure. Despite limited benefits, ursodeoxycholic acid (UDCA) is the only Food and Drug Administration-approved treatment for cholestatic disorders. Retinoic acid (RA) is a ligand for nuclear receptors that modulate bile salt homeostasis. RA also possesses immunomodulatory effects and is used to treat acute promyelocytic leukemia and inflammatory disorders such as psoriasis, acne, and rheumatoid arthritis. To test whether the supplementation of RA with UDCA is superior to UDCA alone for treating cholestasis, male Sprague-Dawley rats underwent common bile duct ligation (BDL) for 14 days and were treated with phosphate-buffered saline (PBS), UDCA, all-trans retinoic acid (atRA), or UDCA and atRA by gavage. Treatment with UDCA and atRA substantially improved animal growth rates, significantly reduced liver fibrosis and bile duct proliferation, and nearly eliminated liver necrosis after BDL. Reductions in the bile salt pool size and liver hydroxyproline content were also seen with treatment with atRA or atRA and UDCA versus PBS and UDCA. Furthermore, atRA and UDCA significantly reduced liver messenger RNA and/or protein expression of transforming growth factor β1 (Tgf-β1), collagen 1a1 (Col1A1), matrix metalloproteinase 2 (Mmp2), cytokeratin 19, α-smooth muscle actin (α-SMA), cytochrome P450 7A1 (Cyp7a1), tumor necrosis factor α, and interleukin-β1. The molecular mechanisms of this treatment were also assessed in human hepatocytes, hepatic stellate cells, and LX-2 cells. atRA alone or in combination with UDCA greatly repressed CYP7A1 expression in human hepatocytes and significantly inhibited COL1A1, MMP2, and α-SMA expression and/or activity in primary human hepatic stellate cells and LX-2 cells. Furthermore, atRA reduced TGF-β1-induced Smad2 phosphorylation in LX-2 cells.

CONCLUSION

Our findings indicate that the addition of RA to UDCA reduces the bile salt pool size and liver fibrosis and might be an effective supplemental therapy with UDCA for cholestatic diseases.

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.

Oral methylthioadenosine administration attenuates fibrosis and chronic liver disease progression in Mdr2-/- mice

BACKGROUND

Inflammation and fibrogenesis are directly related to chronic liver disease progression, including hepatocellular carcinoma (HCC) development. Currently there are few therapeutic options available to inhibit liver fibrosis. We have evaluated the hepatoprotective and anti-fibrotic potential of orally-administered 5'-methylthioadenosine (MTA) in Mdr2(-/-) mice, a clinically relevant model of sclerosing cholangitis and spontaneous biliary fibrosis, followed at later stages by HCC development.

METHODOLOGY

MTA was administered daily by gavage to wild type and Mdr2(-/-) mice for three weeks. MTA anti-inflammatory and anti-fibrotic effects and potential mechanisms of action were examined in the liver of Mdr2(-/-) mice with ongoing fibrogenesis and in cultured liver fibrogenic cells (myofibroblasts).

PRINCIPAL FINDINGS

MTA treatment reduced hepatomegaly and liver injury. α-Smooth muscle actin immunoreactivity and collagen deposition were also significantly decreased. Inflammatory infiltrate, the expression of the cytokines IL6 and Mcp-1, pro-fibrogenic factors like TGFβ2 and tenascin-C, as well as pro-fibrogenic intracellular signalling pathways were reduced by MTA in vivo. MTA inhibited the activation and proliferation of isolated myofibroblasts and down-regulated cyclin D1 gene expression at the transcriptional level. The expression of JunD, a key transcription factor in liver fibrogenesis, was also reduced by MTA in activated myofibroblasts.

CONCLUSIONS/SIGNIFICANCE

Oral MTA administration was well tolerated and proved its efficacy in reducing liver inflammation and fibrosis. MTA may have multiple molecular and cellular targets. These include the inhibition of inflammatory and pro-fibrogenic cytokines, as well as the attenuation of myofibroblast activation and proliferation. Downregulation of JunD and cyclin D1 expression in myofibroblasts may be important regarding the mechanism of action of MTA. This compound could be a good candidate to be tested for the treatment of (biliary) liver fibrosis.

Expression and functional analysis of endoglin in isolated liver cells and its involvement in fibrogenic Smad signalling

Endoglin is an accessory component of the TGF-β-binding receptor complex that differentially modulates TGF-β and BMP responses. The existence of two splice variants L- and S-endoglin which differ in their cytoplasmic domain has already been shown in human and mice. Endoglin is located on the cell surfaces of cultured hepatic stellate cells and transdifferentiated myofibroblasts suggesting that this receptor might be associated with the profibrogenic attributes of these liver cell subpopulations. We now show that endoglin expression is increased in transdifferentiating hepatic stellate cells and in two models of liver fibrosis (i.e. bile duct ligation and carbon tetrachloride model) and further detectable in cultured portal fibroblasts representing another important fibrogenic cell type but not in hepatocytes. In respect to TGF-β1-signalling, we demonstrate that endoglin interacts with and is phosphorylated by TβRII. In hepatic stellate cells, TGF-β1 upregulates endoglin expression most likely via the ALK5 pathway and requires the SP1 transcription factor. We further identified a novel rat splice variant that is structurally and functionally different from that identified in human and mouse. Transient overexpression of endoglin resulted in a strong increase of TGF-β1-driven Smad1/5 phosphorylation and α-smooth muscle actin expression in a hepatic stellate cell line. In supernatants of respective cultures, we could detect the ectodomain of endoglin suggesting that shedding is a further key process involved in the regulation of this surface receptor.

Functional role of chemokines in liver disease models

Chemokines are a class of small cytokine-like molecules that orchestrate immune cell infiltration into the liver in response to acute and chronic injuries. Apart from their chemotactic effect, however, chemokines seem to mediate many other aspects of liver diseases, including a direct activation of stellate cells, the modulation of hepatocyte proliferation and angiogenesis. The identification of specific biological functions for chemokines in liver diseases has been hampered by the finding that resident and infiltrating cells in the liver are often a source, as well as a target, of chemokines. Furthermore, chemokines might cause differing effects depending on the etiology of liver damage, their local concentrations and their ability to form multimers and heterodimers. Nevertheless, the functions of a number of important chemokines and their associated receptors have been identified in both in vivo and in vitro studies. Indeed, harmful (proinflammatory, profibrogenic) and beneficial (antifibrogenic, antiangiogenic) effects of chemokines have been discovered in experimental liver disease models. In this Review, the current knowledge of chemokines in experimental liver disease models is summarized. Advances that might lead to preclinical applications are discussed, as are the roles of chemokine receptors as promising pharmacologically targetable molecules.

Apoptosis as a mechanism for liver disease progression

Hepatocyte injury is ubiquitous in clinical practice, and the mode of cell death associated with this injury is often apoptosis, especially by death receptors. Information from experimental systems demonstrates that hepatocyte apoptosis is sufficient to cause liver hepatic fibrogenesis. The mechanisms linking hepatocyte apoptosis to hepatic fibrosis remain incompletely understood, but likely relate to engulfment of apoptotic bodies by professional phagocytic cells and stellate cells, and release of mediators by cells undergoing apoptosis. Inhibition of apoptosis with caspase inhibitors has demonstrated beneficial effects in murine models of hepatic fibrosis. Recent studies implicating Toll-like receptor 9 in liver injury and fibrosis are also of particular interest. Engulfment of apoptotic bodies is one mechanism by which the TLR9 ligand (CpG DNA motifs) could be delivered to this intracellular receptor. These concepts suggest therapy focused on interrupting the cellular mechanisms linking apoptosis to fibrosis would be useful in human liver diseases.

The coagulation system contributes to alphaVbeta6 integrin expression and liver fibrosis induced by cholestasis

Chronic injury to intrahepatic bile duct epithelial cells (BDECs) elicits expression of various mediators, including the αVβ6 integrin, promoting liver fibrosis. We tested the hypothesis that tissue factor (TF)-dependent thrombin generation and protease activated receptor-1 (PAR-1) activation contribute to liver fibrosis induced by cholestasis via induction of αVβ6 expression. To test this hypothesis, mice deficient in either TF or PAR-1 were fed a diet containing 0.025% α-naphthylisothiocyanate (ANIT), a BDEC-selective toxicant. In genetically modified mice with a 50% reduction in liver TF activity fed an ANIT diet, coagulation cascade activation and liver fibrosis were reduced. Similarly, liver fibrosis was significantly reduced in PAR-1(-/-) mice fed an ANIT diet. Hepatic integrin β6 mRNA induction, expression of αVβ6 protein by intrahepatic BDECs, and SMAD2 phosphorylation were reduced by TF deficiency and PAR-1 deficiency in mice fed the ANIT diet. Treatment with either an anti-αVβ6 blocking antibody or soluble transforming growth factor-β receptor type II reduced liver fibrosis in mice fed the ANIT diet. PAR-1 activation enhanced transforming growth factor-β1-induced integrin β6 mRNA expression in both transformed human BDECs and primary rat BDECs. Interestingly, TF and PAR-1 mRNA levels were increased in livers from patients with cholestatic liver disease. These results indicate that a TF-PAR-1 pathway contributes to liver fibrosis induced by chronic cholestasis by increasing expression of the αVβ6 integrin, an important regulator of transforming growth factor-β1 activation.