Expression of the thrombin receptor in human liver: up-regulation during acute and chronic injury

Protease-activated receptors in health and disease

Proteases are signaling molecules that specifically control cellular functions by cleaving protease-activated receptors (PARs). The four known PARs are members of the large family of G protein-coupled receptors. These transmembrane receptors control most physiological and pathological processes and are the target of a large proportion of therapeutic drugs. Signaling proteases include enzymes from the circulation; from immune, inflammatory epithelial, and cancer cells; as well as from commensal and pathogenic bacteria. Advances in our understanding of the structure and function of PARs provide insights into how diverse proteases activate these receptors to regulate physiological and pathological processes in most tissues and organ systems. The realization that proteases and PARs are key mediators of disease, coupled with advances in understanding the atomic level structure of PARs and their mechanisms of signaling in subcellular microdomains, has spurred the development of antagonists, some of which have advanced to the clinic. Herein we review the discovery, structure, and function of this receptor system, highlight the contribution of PARs to homeostatic control, and discuss the potential of PAR antagonists for the treatment of major diseases.

Healing gone wrong: convergence of hemostatic pathways and liver fibrosis?

Fibrosis results from a disordered wound healing response within the liver with activated hepatic stellate cells laying down dense, collagen-rich extracellular matrix that eventually restricts liver hepatic synthetic function and causes increased sinusoidal resistance. The end result of progressive fibrosis, cirrhosis, is associated with significant morbidity and mortality as well as tremendous economic burden. Fibrosis can be conceptualized as an aberrant wound healing response analogous to a chronic ankle sprain that is driven by chronic liver injury commonly over decades. Two unique aspects of hepatic fibrosis - the chronic nature of insult required and the liver's unique ability to regenerate - give an opportunity for pharmacologic intervention to stop or slow the pace of fibrosis in patients early in the course of their liver disease. Two potential biologic mechanisms link together hemostasis and fibrosis: focal parenchymal extinction and direct stellate cell activation by thrombin and Factor Xa. Available translational research further supports the role of thrombosis in fibrosis. In this review, we will summarize what is known about the convergence of hemostatic changes and hepatic fibrosis in chronic liver disease and present current preclinical and clinical data exploring the relationship between the two. We will also present clinical trial data that underscores the potential use of anticoagulant therapy as an antifibrotic factor in liver disease.

Congestive Hepatopathy

Liver disease resulting from heart failure (HF) has generally been referred as “cardiac hepatopathy”. One of its main forms is congestive hepatopathy (CH), which results from passive venous congestion in the setting of chronic right-sided HF. The current spectrum of CH differs from earlier reports with HF, due to ischemic cardiomyopathy and congenital heart disease having surpassed rheumatic valvular disease. The chronic passive congestion leads to sinusoidal hypertension, centrilobular fibrosis, and ultimately, cirrhosis (“cardiac cirrhosis”) and hepatocellular carcinoma after several decades of ongoing injury. Contrary to primary liver diseases, in CH, inflammation seems to play no role in the progression of liver fibrosis, bridging fibrosis occurs between central veins to produce a “reversed lobulation” pattern and the performance of non-invasive diagnostic tests of liver fibrosis is poor. Although the clinical picture and prognosis is usually dominated by the underlying heart condition, the improved long-term survival of cardiac patients due to advances in medical and surgical treatments are responsible for the increased number of liver complications in this setting. Eventually, liver disease could become as clinically relevant as cardiac disease and further complicate its management.

Uncovering the Roles of miR-214 in Hepatitis E Virus Replication

Viral pathogenesis is a complex event and its regulation involve dynamic interactions with various host factors, of which microRNAs are the key players. In the current study, we have identified the functional importance of an interplay between hepatitis E virus (HEV) and miR-214. Computational analysis indicated that miR-214 binding site is significantly conserved among HEV and related RNA viruses. Intact miR-214 binding site is imperative for HEV replication. miR-214 is an essential host factor for HEV replication. Herein, we demonstrate that miR-214 interacts directly with HEV RNA to enhance HEV replication and HEV genome translation. Augmented translation results in increased levels of HEV ORF2, which is a factor responsible for upregulation of miR-214. HEV usurps host cellular machinery for improving viral fitness and elevates miR-214 expression for amplifying the expression of proviral host factor intracellular active thrombin. This is because miR-214 represses the expression of the negative regulator of thrombin, i.e., protein C. Another viral factor, HEV ORF3, also contributes to the enhancement of intracellular active thrombin. Furthermore, miR-214 directly targets antiviral host factor 2'-5'-oligoadenylate synthetase. Conclusively, we identified a novel mechanism of positive regulation of HEV replication. miR-214 interacts directly with HEV genome and fine-tunes host factors expression. This results in outweighing the proviral factors on the proviral-antiviral axis probably for generating virus supportive environment.

Interactions of viruses and the humoral innate immune response

The innate immune response is crucial for defense against virus infections where the complement system, coagulation cascade and natural antibodies play key roles. These immune components are interconnected in an intricate network and are tightly regulated to maintain homeostasis and avoid uncontrolled immune responses. Many viruses in turn have evolved to modulate these interactions through various strategies to evade innate immune activation. This review summarizes the current understanding on viral strategies to inhibit the activation of complement and coagulation cascades, evade natural antibody-mediated clearance and utilize complement regulatory mechanisms to their advantage.

Antiapoptotic Effect by PAR-1 Antagonist Protects Mouse Liver Against Ischemia-Reperfusion Injury

BACKGROUND

Coagulation disturbances in several liver diseases lead to thrombin generation, which triggers intracellular injury via activation of protease-activated receptor-1 (PAR-1). Little is known about the thrombin/PAR-1 pathway in hepatic ischemia-reperfusion injury (IRI). The present study aimed to clarify whether a newly selective PAR-1 antagonist, vorapaxar, can attenuate liver damage caused by hepatic IRI, with a focus on apoptosis and the survival-signaling pathway.

METHODS

A 60-min hepatic partial-warm IRI model was used to evaluate PAR-1 expression in vivo. Subsequently, IRI mice were treated with or without vorapaxar (with vehicle). In addition, hepatic sinusoidal endothelial cells (SECs) pretreated with or without vorapaxar (with vehicle) were incubated during hypoxia-reoxygenation in vitro.

RESULTS

In naïve livers, PAR-1 was confirmed by immunohistochemistry and immunofluorescence analysis to be located on hepatic SECs, and IRI strongly enhanced PAR-1 expression. In IRI mice models, vorapaxar treatment significantly decreased serum transaminase levels, improved liver histological damage, reduced the number of apoptotic cells as evaluated by terminal deoxynucleotidyl transferase dUTP nick end labeling staining (median: 135 versus 25, P = 0.004), and induced extracellular signal-regulated kinase 1/2 (ERK 1/2) cell survival signaling (phospho-ERK/total ERK 1/2: 0.96 versus 5.34, P = 0.004). Pretreatment of SECs with vorapaxar significantly attenuated apoptosis and induced phosphorylation of ERK 1/2 in vitro (phospho-ERK/total ERK 1/2: 0.66 versus 3.04, P = 0.009). These changes were abolished by the addition of PD98059, the ERK 1/2 pathway inhibitor, before treatment with vorapaxar.

CONCLUSIONS

The results of the present study revealed that hepatic IRI induces significant enhancement of PAR-1 expression on SECs, which may be associated with suppression of survival signaling pathways such as ERK 1/2, resulting in severe apoptosis-induced hepatic damage. Thus, the selective PAR-1 antagonist attenuates hepatic IRI through an antiapoptotic effect by the activation of survival-signaling pathways.

Hepatic microcirculation and mechanisms of portal hypertension

The liver microcirculatory milieu, mainly composed of liver sinusoidal endothelial cells (LSECs), hepatic stellate cells (HSCs) and hepatic macrophages, has an essential role in liver homeostasis, including in preserving hepatocyte function, regulating the vascular tone and controlling inflammation. Liver microcirculatory dysfunction is one of the key mechanisms that promotes the progression of chronic liver disease (also termed cirrhosis) and the development of its major clinical complication, portal hypertension. In the present Review, we describe the current knowledge of liver microcirculatory dysfunction in cirrhotic portal hypertension and appraise the preclinical models used to study the liver circulation. We also provide a comprehensive summary of the promising therapeutic options to target the liver microvasculature in cirrhosis.

Thrombin and factor Xa link the coagulation system with liver fibrosis

Background

Thrombin activates hepatic stellate cells via protease-activated receptor-1. The role of Factor Xa (FXa) in hepatic fibrosis has not been elucidated. We aimed to evaluate the impact of FXa and thrombin in vitro on stellate cells and their respective inhibition in vivo using a rodent model of hepatic fibrosis.

Methods

HSC-LX2 cells were incubated with FXa and/or thrombin in cell culture, stained for αSMA and relative gene expression and gel contraction calculated. C57BL/6 J mice were administered thioacetamide (TAA) for 8 weeks with Rivaroxaban (n = 15) or Dabigatran (n = 15). Control animals received TAA alone (n = 15). Fibrosis was scored and quantified using digital image analysis and hepatic tissue hydroxyproline estimated.

Results

Stellate cells treated with FXa and thrombin demonstrated upregulation of procollagen, TGF-beta, αSMA and significant cell contraction (43.48%+/− 4.12) compared to culturing with FXa or thrombin alone (26.90%+/− 8.90, p = 0.02; 13.1%+/− 9.84, p < 0.001). Mean fibrosis score, percentage area of fibrosis and hepatic hydroxyproline content (2.46 vs 4.08, p = 0.008; 2.02% vs 3.76%, p = 0.012; 276.0 vs 651.3, p = 0.0001) were significantly reduced in mice treated with the FXa inhibitor compared to control mice. FXa inhibition was significantly more effective than thrombin inhibition in reducing percentage area of fibrosis and hepatic hydroxyproline content (2.02% vs 3.70%,p = 0.031; 276.0 vs 413.1,p = 0.001).

Conclusions

FXa promotes stellate cell contractility and activation. Early inhibition of coagulation using a FXa inhibitor significantly reduces TAA induced murine liver fibrosis and may be a viable treatment for liver fibrosis in patients.

Targeting coagulation factor receptors - protease-activated receptors in idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a lethal lung disease with a 5-year mortality rate of > 50% and unknown etiology. Treatment options remain limited and, currently, only two drugs are available, i.e. nintedanib and pirfenidone. However, both of these antifibrotic agents only slow down the progression of the disease, and do not remarkably prolong the survival of IPF patients. Hence, the discovery of new therapeutic targets for IPF is crucial. Studies exploring the mechanisms that are involved in IPF have identified several possible targets for therapeutic interventions. Among these, blood coagulation factor receptors, i.e. protease-activated receptors (PARs), are key candidates, as these receptors mediate the cellular effects of coagulation factors and play central roles in influencing inflammatory and fibrotic responses. In this review, we will focus on the controversial role of the coagulation cascade in the pathogenesis of IPF. In the light of novel data, we will attempt to reconciliate the apparently conflicting data and discuss the possibility of pharmacologic targeting of PARs for the treatment of fibroproliferative diseases.

Protein C Deficiency in Chronic Hepatitis C

There is accumulating evidence that the coagulation system is involved in the process of fibrogenesis in chronic liver disease. Recent studies postulated a possible connection between plasmatic hypercoagulability and progression of fibrosis. The aim of the study was to investigate disorders of the coagulation system in patients with chronic hepatitis C having different extent of hepatic fibrosis well defined by liver histology. A total of 62 patients with chronic hepatitis C were recruited and categorized into 2 groups according to their histological fibrosis stage : mild/moderate fibrosis group (F0-F3 group, n = 30) and extensive fibrosis/cirrhosis group (F4-F6 group, n = 32). The control group consisted of 31 healthy individuals. The following hemostatic assays were evaluated: antithrombin III (AT), protein C (PC) activity, activated partial thromboplastin time, prothrombin time, plasma fibrinogen as well as conventional liver function test. The PC level exhibited a significant reduction in both patient groups when compared to the normal control group (89.25% ± 10.05% and 48.33% ± 15.86% vs 111.86 ± 10.90; P < .001 and P < .001). The PC was found to be the strongest associated factor to histological fibrosis stage ( r = –.834; P < .0001). Univariate and multivariate analysis showed that AT ( P = .003) and PC ( P = .0001) were the most important factors associated with advanced fibrosis. The PC ( P = .001) was found to be the only predictor of mild fibrosis. In conclusion, PC deficiency occurs in an early stage of liver fibrosis. The severity of deficiency is proportional to extent of fibrosis. The PC may have a key role in linking hypercoagulability with hepatic fibrogenesis in chronic liver disease.

Thrombin activation and liver inflammation in advanced hepatitis C virus infection

Hepatitis C virus (HCV) infection is associated with increased thrombotic risk. Several mechanisms are involved including direct endothelial damage by the HCV virus, with activation of tissue factor, altered fibrinolysis and increased platelet aggregation and activation. In advanced stages, chronic HCV infection may evolve to liver cirrhosis, a condition in which alterations in the portal microcirculation may also ultimately lead to thrombin activation, platelet aggregation, and clot formation. Therefore in advanced HCV liver disease there is an increased prevalence of thrombotic phenomena in portal vein radicles. Increased thrombin formation may activate hepatic stellate cells and promote liver fibrosis. In addition, ischemic changes derived from vascular occlusion by microthrombi favor the so called parenchymal extinction, a process that promotes collapse of hepatocytes and the formation of gross fibrous tracts. These reasons may explain why advanced HCV infection may evolve more rapidly to end-stage liver disease than other forms of cirrhosis.

Enoxaparin reduces hepatic vascular resistance and portal pressure in cirrhotic rats

BACKGROUND & AIMS

Increased hepatic vascular resistance due to fibrosis and elevated hepatic vascular tone is the primary factor in the development of portal hypertension. Heparin may decrease fibrosis by inhibiting intrahepatic microthrombosis and thrombin-mediated hepatic stellate cell activation. In addition, heparin enhances eNOS activity, which may reduce hepatic vascular tone. Our study aimed at evaluating the effects of acute, short-, long-term and preventive enoxaparin administration on hepatic and systemic hemodynamics, liver fibrosis and nitric oxide availability in cirrhotic rats.

METHODS

Enoxaparin (1.8 mg/kg subcutaneously), or its vehicle, was administered to CCl4-cirrhotic rats 24h and 1h before the study (acute), daily for 1 week (short-term) or daily for 3 weeks (long-term) and to thioacetamide-cirrhotic rats daily for 3 weeks with/without thioacetamide (preventive/long-term, respectively). Mean arterial pressure, portal pressure, portal blood flow, hepatic vascular resistance and molecular/cellular mechanisms were evaluated.

RESULTS

No significant changes in hemodynamic parameters were observed in acute administration. However, one-week, three-week and preventive treatments significantly decreased portal pressure mainly due to a decrease in hepatic vascular resistance without significant changes in mean arterial pressure. These findings were associated with significant reductions in liver fibrosis, hepatic stellate cell activation, and desmin expression. Moreover, a reduction in fibrin deposition was observed in enoxaparin-treated rats, suggesting reduced intrahepatic microthrombosis.

CONCLUSION

Enoxaparin reduces portal pressure in cirrhotic rats by improving the structural component of increased liver resistance. These findings describe the potentially beneficial effects of enoxaparin beyond the treatment/prevention of portal vein thrombosis in cirrhosis, which deserve further investigation.

Proteinase-activated Receptor 1 Contributed to Up-regulation of Enkephalin in Keratinocytes of Patients with Obstructive Jaundice

Background:

Skin synthesis of endogenous opioids such as enkephalin is considered to be increased in cholestatic rodents, which may induce antinociception in cholestatic liver disease. No studies have reported yet the expression of skin enkephalin in patients with cholestasis.

Methods:

Electrical pain threshold, postoperative morphine consumption, and skin enkephalin expression were measured in patients with jaundice (n = 18) and control patients (n = 16). Male Sprague–Dawley rats (n = 52) and human keratinocyte cell line HaCaT were used in vivo and in vitro studies, respectively. Nociceptive thresholds and plasma and skin levels of methionine-enkephalin were compared in protease-activated receptors-1–antagonized and control bile duct–ligated rats. In in vitro study, the effect on thrombin-induced enkephalin expression was examined and the role of extracellular regulated protein kinases 1/2 and p38 was investigated.

Results:

The authors found that: (1) the electrical pain threshold (mean ± SD) was 1.1 ± 0.1 mA in control patients, whereas it was significantly increased in patients with jaundice (1.7 ± 0.3 mA); 48-h postoperative morphine consumption was approximately 50% higher in the control group than that in the group with jaundice; (2) Skin keratinocytes enkephalin expression was increased in the patients with jaundice; (3) Protease-activated receptors-1 antagonist 1 μg·kg−1·day−1 treatment to the bile duct–ligated rats significantly reduced plasma levels of methionine-enkephalin, nociceptive thresholds, and keratinocytes enkephalin expression; and (4) protease-activated receptors-1 activation induced enkephalin expression through phosphorylation of extracellular regulated protein kinases 1/2 and p38 in keratinocytes.

Conclusion:

Protease-activated receptors-1 activation in peripheral keratinocytes may play an important role in the local synthesis of enkephalin during cholestasis.

Proteinase activated receptor 1 mediated fibrosis in a mouse model of liver injury: a role for bone marrow derived macrophages

Liver fibrosis results from the co-ordinated actions of myofibroblasts and macrophages, a proportion of which are of bone marrow origin. The functional effect of such bone marrow-derived cells on liver fibrosis is unclear. We examine whether changing bone marrow genotype can down-regulate the liver's fibrotic response to injury and investigate mechanisms involved. Proteinase activated receptor 1 (PAR1) is up-regulated in fibrotic liver disease in humans, and deficiency of PAR1 is associated with reduced liver fibrosis in rodent models. In this study, recipient mice received bone marrow transplantation from PAR1-deficient or wild-type donors prior to carbon tetrachloride-induced liver fibrosis. Bone marrow transplantation alone from PAR1-deficient mice was able to confer significant reductions in hepatic collagen content and activated myofibroblast expansion on wild-type recipients. This effect was associated with a decrease in hepatic scar-associated macrophages and a reduction in macrophage recruitment from the bone marrow. In vitro, PAR1 signalling on bone marrow-derived macrophages directly induced their chemotaxis but did not stimulate proliferation. These data suggest that the bone marrow can modulate the fibrotic response of the liver to recurrent injury. PAR1 signalling can contribute to this response by mechanisms that include the regulation of macrophage recruitment.

Mannan binding lectin-associated serine protease 1 is induced by hepatitis C virus infection and activates human hepatic stellate cells

Mannan binding lectin (MBL)-associated serine protease type 1 (MASP-1) has a central role in the lectin pathway of complement activation and is required for the formation of C3 convertase. The activity of MASP-1 in the peripheral blood has been identified previously as a highly significant predictor of the severity of liver fibrosis in hepatitis C virus (HCV) infection, but not in liver disease of other aetiologies. In this study we tested the hypotheses that expression of MASP-1 may promote disease progression in HCV disease by direct activation of hepatic stellate cells (HSCs) and may additionally be up-regulated by HCV. In order to do so, we utilized a model for the maintenance of primary human HSC in the quiescent state by culture on basement membrane substrate prior to stimulation. In comparison to controls, recombinant MASP-1 stimulated quiescent human HSCs to differentiate to the activated state as assessed by both morphology and up-regulation of HSC activation markers α-smooth muscle actin and tissue inhibitor of metalloproteinase 1. Further, the expression of MASP-1 was up-regulated significantly by HCV infection in hepatocyte cell lines. These observations suggest a new role for MASP-1 and provide a possible mechanistic link between high levels of MASP-1 and the severity of disease in HCV infection. Taken together with previous clinical observations, our new findings suggest that the balance of MASP-1 activity may be proinflammatory and act to accelerate fibrosis progression in HCV liver disease.

Antifibrotic effect of heparin on liver fibrosis model in rats

AIM

To evaluate the effect of chronic thrombin inhibition by heparin on experimentally induced chronic liver injury (liver fibrosis) in rats.

METHODS

Chronic liver injury (liver fibrosis) was induced in Wistar rats by oral administration of carbon tetrachloride (CCl4) for 7 wk, an animal model with persistent severe hepatic fibrosis. Intravenous administration of the thrombin antagonist (heparin) started 1 wk after the start of CCl4 intoxication for 6 wk. After completion of treatment (7 wk), markers of hepatic dysfunction were measured and changes evaluated histopathologically.

RESULTS

Higher serum glutamate oxaloacetate transaminase (SGOT), serum glutamate pyruvate transaminase (SGPT), alkaline phosphatase (ALP), total, direct and indirect bilirubin levels, as well as lower fibrinogen levels, were found in CCl4 intoxicated rats. Heparin, silymarin and combination of drug (heparin and silymarin) treatment for 6 wk prevented a rise in SGOT, SGPT, ALP, total, direct and indirect bilirubin levels and improved fibrinogen levels. Deterioration in hepatic function determined by the fibrosis area was retarded, as evident from hepatic histopathology. Total protein levels were not changed in all groups.

CONCLUSION

Heparin, a thrombin antagonist, preserved hepatic function and reduced severity of hepatic dysfunction/fibrogenesis. Combination of heparin and silymarin produced additional benefits on liver fibrosis.

Coagulation and coagulation signalling in fibrosis

Following tissue injury, a complex and coordinated wound healing response comprising coagulation, inflammation, fibroproliferation and tissue remodelling has evolved to nullify the impact of the original insult and reinstate the normal physiological function of the affected organ. Tissue fibrosis is thought to result from a dysregulated wound healing response as a result of continual local injury or impaired control mechanisms. Although the initial insult is highly variable for different organs, in most cases, uncontrolled or sustained activation of mesenchymal cells into highly synthetic myofibroblasts leads to the excessive deposition of extracellular matrix proteins and eventually loss of tissue function. Coagulation was originally thought to be an acute and transient response to tissue injury, responsible primarily for promoting haemostasis by initiating the formation of fibrin plugs to enmesh activated platelets within the walls of damaged blood vessels. However, the last 20years has seen a major re-evaluation of the role of the coagulation cascade following tissue injury and there is now mounting evidence that coagulation plays a critical role in orchestrating subsequent inflammatory and fibroproliferative responses during normal wound healing, as well as in a range of pathological contexts across all major organ systems. This review summarises our current understanding of the role of coagulation and coagulation initiated signalling in the response to tissue injury, as well as the contribution of uncontrolled coagulation to fibrosis of the lung, liver, kidney and heart. This article is part of a Special Issue entitled: Fibrosis: Translation of basic research to human disease.

Antifibrotic effect of heparin on liver fibrosis model in rats

AIM: To evaluate the effect of chronic thrombin inhibition by heparin on experimentally induced chronic liver injury (liver fibrosis) in rats.

METHODS: Chronic liver injury (liver fibrosis) was induced in Wistar rats by oral administration of carbon tetrachloride (CCl₄) for 7 wk, an animal model with persistent severe hepatic fibrosis. Intravenous administration of the thrombin antagonist (heparin) started 1 wk after the start of CCl₄ intoxication for 6 wk. After completion of treatment (7 wk), markers of hepatic dysfunction were measured and changes evaluated histopathologically.

RESULTS: Higher serum glutamate oxaloacetate transaminase (SGOT), serum glutamate pyruvate transaminase (SGPT), alkaline phosphatase (ALP), total, direct and indirect bilirubin levels, as well as lower fibrinogen levels, were found in CCl₄ intoxicated rats. Heparin, silymarin and combination of drug (heparin and silymarin) treatment for 6 wk prevented a rise in SGOT, SGPT, ALP, total, direct and indirect bilirubin levels and improved fibrinogen levels. Deterioration in hepatic function determined by the fibrosis area was retarded, as evident from hepatic histopathology. Total protein levels were not changed in all groups.

CONCLUSION: Heparin, a thrombin antagonist, preserved hepatic function and reduced severity of hepatic dysfunction/fibrogenesis. Combination of heparin and silymarin produced additional benefits on liver fibrosis.

What we should know about portal vein thrombosis in cirrhotic patients: A changing perspective

Portal vein thrombosis (PVT) is one of the most common complications occurring during the natural course of liver cirrhosis. Even though PVT is often asymptomatic, the worsening of liver function, an unexpected episode of gastrointestinal bleeding or ascitic decompensation may be landmarks of PVT development. Beyond these clinical manifestations, it is debated whether PVT really has an impact on liver cirrhosis natural history or rather represents only one of its consequences. Probably PVT development should not only be considered as a matter of impaired blood flow or pro-coagulation tendency. On one hand, PVT seems a consequence of the worsening in portal vein outflow due to the increased hepatic resistance in cirrhotic livers. On the other hand, vascular microthrombosis secondary to necroinflammation may cause liver ischemia and infarction, with loss of hepatic tissue (parenchymal extinction) which is replaced by fibrotic tissue. Therefore, PVT might also be considered as the overt manifestation of the liver fibrosing process evolution and anticoagulant therapy may thus have microscopic indirect effects also on the progression of liver disease. At present, a connection between PVT development and the progression of liver fibrosis/cirrhosis has not yet been demonstrated. Nevertheless, it is not clear if PVT development may worsen cirrhotic patients’ outcome by itself. Some authors tried to assess liver transplant benefit in PVT cirrhotic patients but data are contrasting. In this review, we will try to answer these questions, providing a critical analysis of data reported in literature.

Regulation of protease-activated receptor-1 expression in human buccal fibroblasts stimulated with arecoline

BACKGROUND

The purpose of this study was to compare the major thrombin receptor protease-activated receptor-1 (PAR-1) expression in normal human buccal mucosa and oral submucous fibrosis (OSF) specimens and further explore the potential mechanisms that may lead to induce PAR-1 expression.

METHODS

Thirty OSF and 10 normal buccal mucosa specimens were examined by immunohistochemistry. Buccal mucosal fibroblasts (BMFs) were challenged with arecoline by using Western blot analysis. N-acetyl-L-cysteine (NAC), LY294002, herbimycin A, NS-398, and PD98059 were added to find the possible regulatory mechanisms.

RESULTS

PAR-1 expression was significantly higher in OSF specimens (p < .05). Arecoline was found to elevate PAR-1 expression in a dose-dependent and time-dependent manner (p < .05). The addition of NAC, LY294002, herbimycin A, NS398, and PD98059 markedly inhibited the arecoline-induced PAR-1 expression (p < .05).

CONCLUSION

PAR-1 expression is significantly upregulated in areca quid chewing-associated OSF. Arecoline-induced PAR-1 expression was downregulated by NAC, LY294002, herbimycin A, NS398, and PD98059.

The role of hypercoagulability in liver fibrogenesis

The development of hepatic fibrosis on a background of chronic liver injury represents a complex disease trait modulated through the interaction of host genetic factors and environmental influences. Early observations that hepatic inflammation and cirrhosis are associated with the presence of microthrombi within the hepatic vasculature and fibrin/fibrinogen deposition were followed by epidemiological studies showing that carriage of the Factor V Leiden (FvL) mutation, protein C deficiency and increased expression of factor VIII are associated with accelerated progression to cirrhosis in a chronic hepatitis C infection. Additional data suggest that these factors may influence fibrogenesis in many forms of chronic liver disease and extra-hepatic fibrotic processes. Drawing evidence both from liver research and studies of fibrogenesis in other organ systems, two hypotheses may explain how activity of the coagulation cascade influences the rate of hepatic fibrogenesis: tissue ischaemia and parenchymal extinction and direct thrombin mediated stellate cell activation via PAR-1 cleavage. Drawing on preclinical and clinical studies we discuss the evidence for a role for coagulation cascade activity in hepatic fibrogenesis and explore the proposed pathogenic mechanisms that lead to stellate cell activation. The corollary of an association between hypercoagulation and increased fibrosis is that interference with the coagulation cascade may reduce hepatic fibrosis. We conclude this article by examining the implications for future therapeutic intervention.

Hypercoagulability in cirrhosis: causes and consequences

Decreased levels of most coagulation factors and thrombocytopenia are the main haemostatic abnormalities of cirrhosis. As a consequence, this condition was, until recently, considered as the prototype acquired coagulopathy responsible for bleeding. However, recent evidence suggests that it should, rather, be regarded as a condition associated with normal or even increased thrombin generation. The bleeding events that occur in these patients should, therefore, be explained by the superimposed conditions that frequently occur in this setting. Due to elevated levels of factor VIII (procoagulant driver) in combination with decreased protein C (anticoagulant driver), which are typically found in patients with cirrhosis, a procoagulant imbalance, defined as a partial resistance to the in vitro anticoagulant action of thrombomodulin, can be demonstrated. Whether this in vitro hypercoagulability is truly representative of what occurs in vivo remains to be established. However, the hypothesis that it may have clinical consequences is attractive and deserves attention. The possible consequences that we discuss herein include whether (i) cirrhosis is a condition associated with increased risk of venous thromboembolism or portal vein thrombosis; (ii) the hypercoagulability associated with cirrhosis has any other role outside coagulation (i.e. progression of liver fibrosis); and (iii) anticoagulation should be used in cirrhosis. Although apparently provocative, considering anticoagulation as a therapeutic option in patients with cirrhosis is now supported by a rationale of increasing strength. There may be subgroups of patients who benefit from anticoagulation to treat or prevent thrombosis and to slow hepatic fibrosis. Clinical studies are warranted to explore these therapeutic options.

Fibrosis severity and mannan-binding lectin (MBL)/MBL-associated serine protease 1 (MASP-1) complex in HCV-infected patients

BACKGROUND AND STUDY AIMS

Mannan-binding lectin (MBL) is a collectin synthesised in the liver and secreted into the bloodstream. It binds micro-organisms via interactions with glycans on the target surface. Bound MBL subsequently activates MBL-associated serine protease proenzymes (MASPs). Several studies have investigated the possible role for MBL in hepatitis C virus (HCV) infection by examining MBL levels and polymorphisms in relation to disease progression and in response to treatment. The aim of this study was to investigate the relation of the activity of MBL and MBL/MASP-1 complex in sera of patients with mild and severe chronic HCV infection and outcome of HCV infection.

PATIENTS AND METHODS

Serum level of MBL and functional assays for MBL/MASP-1 complex activity were assayed in sera of 80 patients with chronic HCV infection. Patients were divided into two groups according to the results of the liver biopsy, group I (40 HCV patients had mild hepatic fibrosis, Ishak fibrosis stages 0-1) and group II (40 HCV patients had severe hepatic fibrosis, Ishak fibrosis stages 5-6), in addition to 20 control subjects as group III. The analysis of the MBL/MASP-1 complex activity at 0, 3 and 6 months was performed in all patients.

RESULTS

Serum levels of MBL and MBL/MASP-1 complex activity were higher in sera of patients with chronic HCV liver disease compared to those in control subjects. There was a correlation between the activity of the MBL/MASP-1 complex and the severity of fibrosis (P=0.003). MBL/MASP-1 complex activity was associated more significantly with severe fibrosis in comparison to MBL concentration.

CONCLUSION

MBL and MBL/MASP-1 complex activities play a key role in first-line host defence mechanism against certain infectious agents including HCV infection. However, it is also likely that the role of MBL and MBL/MASP-1 complex activity extends beyond this restricted infection-related view in that it appears to be a key regulator of inflammation.

Mechanisms of hepatic fibrogenesis

Multiple etiologies of liver disease lead to liver fibrosis through integrated signaling networks that regulate the deposition of extracellular matrix. This cascade of responses drives the activation of hepatic stellate cells (HSCs) into a myofibroblast-like phenotype that is contractile, proliferative and fibrogenic. Collagen and other extracellular matrix (ECM) components are deposited as the liver generates a wound-healing response to encapsulate injury. Sustained fibrogenesis leads to cirrhosis, characterized by a distortion of the liver parenchyma and vascular architecture. Uncovering the intricate mechanisms that underlie liver fibrogenesis forms the basis for efforts to develop targeted therapies to reverse the fibrotic response and improve the outcomes of patients with chronic liver disease.

Mechanisms of hepatic fibrogenesis

Multiple etiologies of liver disease lead to liver fibrosis through integrated signaling networks that regulate the deposition of extracellular matrix. This cascade of responses drives the activation of hepatic stellate cells (HSCs) into a myofibroblast-like phenotype that is contractile, proliferative and fibrogenic. Collagen and other extracellular matrix (ECM) components are deposited as the liver generates a wound-healing response to encapsulate injury. Sustained fibrogenesis leads to cirrhosis, characterized by a distortion of the liver parenchyma and vascular architecture. Uncovering the intricate mechanisms that underlie liver fibrogenesis forms the basis for efforts to develop targeted therapies to reverse the fibrotic response and improve the outcomes of patients with chronic liver disease.

Mechanisms of hepatic fibrogenesis

Multiple etiologies of liver disease lead to liver fibrosis through integrated signaling networks that regulate the deposition of extracellular matrix. This cascade of responses drives the activation of hepatic stellate cells (HSCs) into a myofibroblast-like phenotype that is contractile, proliferative and fibrogenic. Collagen and other extracellular matrix (ECM) components are deposited as the liver generates a wound-healing response to encapsulate injury. Sustained fibrogenesis leads to cirrhosis, characterized by a distortion of the liver parenchyma and vascular architecture. Uncovering the intricate mechanisms that underlie liver fibrogenesis forms the basis for efforts to develop targeted therapies to reverse the fibrotic response and improve the outcomes of patients with chronic liver disease.

Silencing tissue inhibitors of metalloproteinases (TIMPs) with short interfering RNA reveals a role for TIMP-1 in hepatic stellate cell proliferation

Myofibroblastic, activated hepatic stellate cells (HSC) play a pivotal role in the development of liver fibrosis through the secretion of fibrillar collagens and the tissue inhibitors of metalloproteinase (TIMP)-1 and -2. TIMPs are believed to promote hepatic fibrosis by inhibiting both matrix degradation and apoptosis of HSC. In other cell types, there is evidence that TIMP-1 has effects on proliferation, however the role of TIMPs in the regulation of HSC proliferation remains unexplored. Therefore, we have used short interfering RNA (siRNA) to investigate the effects of autocrine TIMP-1 and -2 on HSC proliferation. TIMP-1 and -2 siRNA were highly effective, producing peak target protein knockdown compared to negative control siRNA of 92% and 63%, respectively. Specific silencing of TIMP-1, using siRNA, significantly reduced HSC proliferation. TIMP-1 was localised in part to the HSC nucleus and TIMP-1 siRNA resulted in loss of both cytoplasmic and nuclear TIMP-1. Attenuated proliferation was associated with reduced Akt phosphorylation and was partially rescued by addition of recombinant TIMP-1. We have revealed a novel autocrine mitogenic effect of TIMP-1 on HSC, which may involve Akt-dependent and specific nuclear mechanisms of action. We suggest that TIMP-1 might promote liver fibrosis by means other than its previously described anti-apoptotic effect on HSC. Moreover, these findings, together with our previous reports and the emerging data from in vivo studies of TIMP inhibition, provide strong evidence that TIMP-1 is mechanistically central to liver fibrosis and an important potential therapeutic target.

Paediatric cholestatic liver disease: Diagnosis, assessment of disease progression and mechanisms of fibrogenesis

Cholestatic liver disease causes significant morbidity and mortality in children. The diagnosis and management of these diseases can be complicated by an inability to detect early stages of fibrosis and a lack of adequate interventional therapy. There is no single gold standard test that accurately reflects the presence of liver disease, or that can be used to monitor fibrosis progression, particularly in conditions such as cystic fibrosis. This has lead to controversy over how suspected liver disease in children is detected and diagnosed. This review discusses the challenges in using commonly available methods to diagnose hepatic fibrosis and monitor disease progression in children with cholestatic liver disease. In addition, the review examines the mechanisms hypothesised to be involved in the development of hepatic fibrogenesis in paediatric cholestatic liver injury which may ultimately aid in identifying new modalities to assist in both disease detection and therapeutic intervention.

Viral cirrhosis: an overview of haemostatic alterations and clinical consequences

Viral hepatitis is a major health problem worldwide, the principal cause of cirrhosis and hepatocarcinoma. Once cirrhosis occurs, the consequences of liver dysfunction and portal hypertension become evident and, sometimes, life threatening for patients. Among the various complications of liver cirrhosis, the alteration of haemostatic balance is often a hard challenge for the clinician, since it is capable to predispose both to bleeding or thrombosis. In this review, we analyze the principal aspects of procoagulant, anticoagulant and fibrinolytic capacity of cirrhotic patients, which appears to be variably altered in all these aspects, not only in the direction of a tendency to bleeding. Laboratory investigations, at present, may provide only a partial representation of this condition, because of the impossibility to obtain a test capable to furnish a global overview of the haemostatic system and to reproduce in vivo conditions. Furthermore, we describe the pathophysiological mechanisms underlying bleeding manifestations and thrombosis development in cirrhotic patients, which should be considered not only as obvious consequences of the advanced liver disease but, rather, as the result of a complex interaction between inherited and acquired factors.

The coagulation factor Xa/protease activated receptor-2 axis in the progression of liver fibrosis: a multifaceted paradigm

Hepatic fibrosis is a common response to virtually all forms of chronic liver injury independent of the etiologic agent. Despite the relatively large population of patients suffering from hepatic fibrosis and cirrhosis, no efficient and well-tolerated drugs are available for the treatment of this disorder. The lack of efficient treatment options is at least partly because the underlying cellular mechanisms leading to hepatic fibrosis are only partly understood. It is thus of pivotal importance to better understand the cellular processes contributing to the progression of hepatic fibrosis. Interestingly in this perspective, a common feature of fibrotic disease of various organs is the activation of the coagulation cascade and hepatic fibrosis is also accompanied by a local hypercoagulable state. Activated blood coagulation factors directly target liver cells by activating protease-activated receptors (PAR) thereby inducing a plethora of cellular responses like (among others) proliferation, migration and extracellular matrix production. Coagulation factor driven PAR activation thus establishes a potential link between activation of the coagulation cascade and the progression of fibrosis. The current review focuses on blood coagulation factor Xa and summarizes the variety of cellular functions induced by factor Xa-driven PAR-2 activation and the subsequent consequences for tissue repair and hepatic fibrosis.

CD38-mediated Ca2+ signaling contributes to angiotensin II-induced activation of hepatic stellate cells: attenuation of hepatic fibrosis by CD38 ablation

CD38 is a type II glycoprotein that is responsible for the synthesis and hydrolysis of cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP), Ca(2+)-mobilizing second messengers. The activation of hepatic stellate cells (HSCs) is a critical event in hepatic fibrosis because these cells are the main producers of extracellular matrix proteins in the liver. Recent evidence indicates that the renin-angiotensin system plays a major role in liver fibrosis. In this study, we showed that angiotensin II (Ang II) evoked long lasting Ca(2+) rises and induced NAADP or cADPR productions via CD38 in HSCs. Inositol 1,4,5-trisphosphate as well as NAADP-induced initial Ca(2+) transients were prerequisite for the production of cADPR, which was responsible for later sustained Ca(2+) rises in the Ang II-treated HSCs. Ang II-mediated inositol 1,4,5-trisphosphate- and NAADP-stimulated Ca(2+) signals cross-talked in a dependent manner with each other. We also demonstrated that CD38 plays an important role in Ang II-induced proliferation and overproduction of extracellular matrix proteins in HSCs, which were reduced by an antagonistic cADPR analog, 8-bromo-cADPR, or in CD38(-/-) HSCs. Moreover, we presented evidence to implicate CD38 in the bile duct ligation-induced liver fibrogenesis; infiltration of inflammatory cells and expressions of alpha-smooth muscle actin, transforming growth factor-beta1, collagen alphaI(1), and fibronectin were reduced in CD38(-/-) mice compared with those in CD38(+/+) mice. These results demonstrate that CD38-mediated Ca(2+) signals contribute to liver fibrosis via HSCs activation, suggesting that intervention of CD38 activation may help prevent hepatic fibrosis.

Thrombotic risk factors and liver histologic lesions in non-alcoholic fatty liver disease

BACKGROUND/AIMS

The pathogenetic mechanisms of development of non-alcoholic steatohepatitis (NASH) and fibrosis are not clear, although thrombosis of small intrahepatic veins has been suggested to trigger liver tissue remodelling and thrombotic risk factors have been associated with more advanced fibrosis in chronic viral hepatitis (CVH). We evaluated the prevalence of thrombotic risk factors (RFs) in non-alcoholic fatty liver disease (NAFLD) and their possible association with fatty liver or NASH.

METHODS

We included 60 patients with histologically documented NAFLD and a historical cohort of 90 patients with chronic hepatitis B (n=39) or C (n=51). Thrombophilic factors were evaluated on the day of the liver biopsy.

RESULTS

One or more thrombotic RFs were detected in 37% of NAFLD patients, and >or= 2 RFs were detected in 12% of NAFLD patients, being less frequently present than in CVH patients (37% and 68%, respectively; P <or= 0.001). Among NAFLD cases, one or more thrombotic RFs were significantly more frequently present in NASH than simple fatty liver (56% vs. 8%; odds ratio [OR]: 13.8, 2.8-67.4, P<0.001). In multivariate analysis, NASH was independently associated with moderate to severe steatosis (adjusted OR: 24.3; P=0.001) and the presence of one or more thrombotic RFs (adjusted OR: 38.7; P=0.002). Fibrosis stage was worse in NASH patients with than without thrombotic RFs (2.5+/-1.1 vs. 1.3+/-1.1; P=0.002).

CONCLUSIONS

Thrombotic RFs are frequently present in patients with NAFLD and are associated with NASH and more advanced fibrosis. Such an association may have significant clinical implications, even though it is not clear yet whether it represents a primary or secondary phenomenon.

Parenchymal extinction: coagulation and hepatic fibrogenesis

Observations that hepatic inflammation and cirrhosis are associated with the presence of thrombi within the hepatic microvasculature and fibrin-fibrinogen deposition have led to epidemiologic studies showing that carriage of the factor V Leiden mutation, protein C deficiency, and increased expression of factor VIII are associated with rapid progression to cirrhosis in a chronic hepatitis C virus. Additional data suggest that this process may extend more broadly to progression in many forms of chronic liver disease. This article discusses the evidence for a role for coagulation cascade activity in hepatic fibrogenesis and explores the proposed pathogenic mechanisms including the downstream events of thrombin activation. Interference with either the generation of thrombin or its downstream activity may reduce hepatic fibrosis. Also examined are the implications for future therapeutic intervention.

Pulmonary epithelium is a prominent source of proteinase-activated receptor-1-inducible CCL2 in pulmonary fibrosis

RATIONALE

Studies in patients and experimental animals provide compelling evidence of the involvement of the major thrombin receptor, proteinase-activated receptor-1 (PAR(1)), and the potent chemokine, chemokine (CC motif) ligand-2 (CCL2)/monocyte chemotactic protein-1, in the pathogenesis of idiopathic pulmonary fibrosis (IPF). PAR(1) knockout mice are protected from bleomycin-induced lung inflammation and fibrosis and this protection is associated with marked attenuation in CCL2 induction.

OBJECTIVES

The aim of this study was to determine which cell types represent the major source of PAR(1)-inducible CCL2 in the fibrotic lung.

METHODS

Using immunohistochemistry and dual immunofluorescence, we examined PAR(1) and CCL2 expression in the bleomycin model and human IPF lung. PAR(1) and CCL2 gene expression was also assessed in laser-captured alveolar septae from patients with IPF. The ability of PAR(1) to induce CCL2 production by lung epithelial cells was also examined in vitro.

MEASUREMENTS AND MAIN RESULTS

We report for the first time that PAR(1) and CCL2 are coexpressed and co-up-regulated on the activated epithelium in fibrotic areas in IPF. Similar observations were found in bleomycin-induced lung injury. Furthermore, we show that thrombin is a potent inducer of CCL2 gene expression and protein release by cultured lung epithelial cells via a PAR(1)-dependent mechanism.

CONCLUSIONS

These data support the notion that PAR(1) activation on lung epithelial cells may represent an important mechanism leading to increased local CCL2 release in pulmonary fibrosis. Targeting PAR(1) on the pulmonary epithelium may offer a unique opportunity for therapeutic intervention in pulmonary fibrosis and other inflammatory and fibroproliferative conditions associated with excessive local generation of thrombin and CCL2 release.

Advances in antifibrotic therapy

Sustained progress in defining the molecular pathophysiology of hepatic fibrosis has led to a comprehensive framework for developing antifibrotic therapies. Indeed, the single greatest limitation in bringing new drugs to the clinical setting is a lack of clarity regarding clinical trial and treatment end points, not a lack of promising agents. A range of treatments, including those developed for other indications, as well as those specifically developed for hepatic fibrosis, are nearing or in clinical trials. Most are focused on attacking features of either hepatic injury and/or activated stellate cells and myofibroblasts, which are the primary sources of extracellular matrix (scar) proteins. Thus, features of injury and stellate cell activation provide a useful template for classifying these emerging agents and point to a new class of therapies for patients with fibrosing liver disease.

Effect of a thrombin receptor (protease-activated receptor 1, PAR-1) gene polymorphism in chronic hepatitis C liver fibrosis

BACKGROUND AND AIM

Tissue injury leads to activation of coagulation and generation of thrombin. Inhibition of thrombin receptor protease-activated receptor 1 (PAR-1) has been shown to reduce liver fibrosis in animals. This study aimed to evaluate the effect of PAR-1 gene polymorphism on rate of liver fibrosis (RF) in chronic hepatitis C.

METHODS

Polymorphisms studied: C > T transition 1426 bp upstream of translation start site (-1426C/T), 13 bp repeat of preceding -506 5'-CGGCCGCGGGAAG-3' sequence (-506I/D), and A > T transversion in intervening sequence (IVS) 14 bp upstream of exon-2 start site (IVS-14A/T). A total of 287 European and 90 Brazilian patients were studied.

RESULTS

1426C/T polymorphism: There was a trend to higher RF in patients with the TT genotype (P = 0.06) and an association between genotype CC and slow fibrosis (P = 0.03) in Europeans. In males, RF was significantly higher in those with the TT genotype compared to CT (P = 0.003) and CC (P = 0.007). There was a significant association between TT and fast fibrosis (P = 0.04). This was confirmed in an independent cohort of Brazilians where RF was higher in TT than in CC (P = 0.03). Analysis of -506I/D showed no difference in RF and distribution of slow/fast fibrosis among different genotypes in both populations. Analysis of IVS-14A/T showed no difference between genotypes.

CONCLUSION

In conclusion, these findings suggest that PAR-1 receptor polymorphisms influence the progression of liver fibrosis.

Coagulation status modulates murine hepatic fibrogenesis: implications for the development of novel therapies

BACKGROUND

There is strong evidence demonstrating that coagulation system activation contributes to wound healing and promotes organ fibrosis. Several epidemiological studies have now shown that prothrombotic status, including carriage of the factor (F)V Leiden mutation, is associated with rapid progression of hepatic fibrosis.

OBJECTIVES

To assess the effect of a procoagulant state on progression of hepatic fibrosis in a controlled environment and to test whether anticoagulation could attenuate fibrogenesis.

METHODS

We investigated the effects of coagulation status on liver fibrosis development in a mouse model of chronic toxic liver injury. Prothrombotic FV Leiden mutant mice, C57BL/6 control animals and anticoagulated mice were studied after chronic exposure to carbon tetrachloride.

RESULTS

Carriage of the FV Leiden mutation caused a significant increase in hepatic fibrosis. Anticoagulation with warfarin significantly reduced fibrosis progression in wild-type mice but was less effective against the profibrotic FV Leiden mutation. Changes in the fibrosis scores were mirrored by changes in liver hydroxyproline content and hepatic stellate cell activation detected by alpha-smooth muscle actin expression.

CONCLUSIONS

These results demonstrate that coagulation status has a strong influence on hepatic fibrogenesis. It is likely that thrombin signaling through the proteinase-activated receptor 1 (PAR(1)) receptor expressed on hepatic stellate cells is responsible for this relationship. These results represent the first reported use of anticoagulation to slow hepatic fibrogenesis and suggest a potential novel anti-fibrotic therapeutic approach for the future.

Hepatic fibrosis -- overview

The study of hepatic fibrosis, or scarring in response to chronic liver injury, has witnessed tremendous progress in the past two decades. Clarification of the cellular sources of scar, and emergence of hepatic stellate cells not only as a fibrogenic cell type, but also as a critical immunomodulatory and homeostatic regulator are among the most salient advances. Activation of hepatic stellate cells remains a central event in fibrosis, complemented by evidence of additional sources of matrix-producing cells including bone marrow, portal fibroblasts, and epithelial-mesenchymal transition from both hepatocytes and cholangiocytes. A growing range of cytokines and their receptors and inflammatory cell subsets have further expanded our knowledge about this dynamic process. Collectively, these findings have laid the foundation for continued elucidation of underlying mechanisms, and more importantly for the implementation of rationally based approaches to limit fibrosis, accelerate repair and enhance liver regeneration in patients with chronic liver disease.

Mechanisms of hepatic fibrogenesis

Substantial improvements in the treatment of chronic liver disease have accelerated interest in uncovering the mechanisms underlying hepatic fibrosis and its resolution. Activation of resident hepatic stellate cells into proliferative, contractile, and fibrogenic cells in liver injury remains a dominant theme driving the field. However, several new areas of rapid progress in the past 5-10 years also have taken root, including: (1) identification of different fibrogenic populations apart from resident stellate cells, for example, portal fibroblasts, fibrocytes, and bone-marrow-derived cells, as well as cells derived from epithelial mesenchymal transition; (2) emergence of stellate cells as finely regulated determinants of hepatic inflammation and immunity; (3) elucidation of multiple pathways controlling gene expression during stellate cell activation including transcriptional, post-transcriptional, and epigenetic mechanisms; (4) recognition of disease-specific pathways of fibrogenesis; (5) re-emergence of hepatic macrophages as determinants of matrix degradation in fibrosis resolution and the importance of matrix cross-linking and scar maturation in determining reversibility; and (6) hints that hepatic stellate cells may contribute to hepatic stem cell behavior, cancer, and regeneration. Clinical and translational implications of these advances have become clear, and have begun to impact significantly on the management and outlook of patients with chronic liver disease.

Hepatic stellate cells: protean, multifunctional, and enigmatic cells of the liver

The hepatic stellate cell has surprised and engaged physiologists, pathologists, and hepatologists for over 130 years, yet clear evidence of its role in hepatic injury and fibrosis only emerged following the refinement of methods for its isolation and characterization. The paradigm in liver injury of activation of quiescent vitamin A-rich stellate cells into proliferative, contractile, and fibrogenic myofibroblasts has launched an era of astonishing progress in understanding the mechanistic basis of hepatic fibrosis progression and regression. But this simple paradigm has now yielded to a remarkably broad appreciation of the cell's functions not only in liver injury, but also in hepatic development, regeneration, xenobiotic responses, intermediary metabolism, and immunoregulation. Among the most exciting prospects is that stellate cells are essential for hepatic progenitor cell amplification and differentiation. Equally intriguing is the remarkable plasticity of stellate cells, not only in their variable intermediate filament phenotype, but also in their functions. Stellate cells can be viewed as the nexus in a complex sinusoidal milieu that requires tightly regulated autocrine and paracrine cross-talk, rapid responses to evolving extracellular matrix content, and exquisite responsiveness to the metabolic needs imposed by liver growth and repair. Moreover, roles vital to systemic homeostasis include their storage and mobilization of retinoids, their emerging capacity for antigen presentation and induction of tolerance, as well as their emerging relationship to bone marrow-derived cells. As interest in this cell type intensifies, more surprises and mysteries are sure to unfold that will ultimately benefit our understanding of liver physiology and the diagnosis and treatment of liver disease.

Protease-activated receptor 1 knockout reduces experimentally induced liver fibrosis

Thrombin inhibition protects against liver fibrosis. However, it is not known whether the thrombin profibrogenic effect is due to effects on blood coagulation or to signaling via protease-activated receptors (PARs). We took advantage of the lack of blood coagulation defects in PAR-1-knockout mice. Acute carbon tetrachloride (CCl(4)) toxicity was similar in wild-type (WT), PAR-1(-/-), and PAR-1(+/-) mice as judged by aminotransferase levels, area of liver necrosis, and liver peroxidation measured by Fourier-transformed infrared spectroscopy. Fifteen mice/group received CCl(4) or its solvent for 6 wk (300 microl/kg, 3 times a week). Fibrosis area was increased 10-fold by CCl(4) treatment in WT mice. PAR-1 deficiency protected against fibrosis, with 36% and 56% decrease in PAR-1(+/-) and PAR-1(-/-) mice, respectively (P < 0.001). Similar results were obtained for area of activated fibrogenic cells (64% and 79% decrease in PAR-1(+/-) and PAR-1(-/-) mice, respectively, P < 0.001). These findings were corroborated by measurements of type I collagen, matrix metalloproteinase-2, and PDGF-beta receptor mRNA levels. There was also a significant decrease in T lymphocyte infiltration in PAR-1-deficient mice. Altogether, these results suggest that thrombin profibrogenic effects are independent of effects on blood coagulation and are instead due to direct effects on fibrogenic cells and possibly on T lymphocytes.

Thrombin inhibits migration of human hepatic myofibroblasts

Several lines of data recently pointed out a role of the serine proteinase thrombin in liver fibrogenesis, but its mechanism of action is unknown. The aim of this study was to evaluate the effect of thrombin on the migration of human liver myofibroblasts. We show here that thrombin inhibits both basal migration and platelet-derived growth factor (PDGF)-BB-induced migration of myofibroblasts. By using a thrombin antagonist, a protease-activated receptor (PAR)-1 mimetic peptide, and a PAR-1 antibody, we show that this effect is dependent on the catalytic activity of thrombin and on PAR-1 activation. Thrombin's effect on basal migration was dependent on cyclooxygenase 2 (COX-2) activation because it was blocked by the COX-2 inhibitors NS-398 and nimesulide, and pharmacological studies showed that it was relayed through prostaglandin E(2) and its EP(2) receptor. On the other hand, thrombin-induced inhibition of PDGF-BB-induced migration was not dependent on COX-2. We show that thrombin inhibits PDGF-induced Akt-1 phosphorylation. This effect was consecutive to inhibition of PDGF-beta receptor activation through active dephosphorylation. Thus thrombin, through two distinct mechanisms, inhibits both basal- and PDGF-BB-induced migration of human hepatic liver myofibroblasts. The fine tuning of myofibroblast migration may be one of the mechanisms used by thrombin to regulate liver fibrogenesis.

α-Thrombin inhibits DNA synthesis in rat hepatocytes but not in hepatoma cells by receptor activation and proteolysis

Prothrombin is a plasma protein, which after tissue injury is converted to alpha-thrombin and is mainly involved in blood clot formation. It has also been shown to have a mitogenic effect on primary endothelial cells, vascular smooth muscle cells, fibroblasts and some tumor cells, but is an inhibitor of rat hepatocyte DNA synthesis on fibronectin matrix in cell culture. We now report that prothrombin is converted to alpha-thrombin by primary cultures of normal adult rat hepatocytes and alpha-thrombin is also a potent inhibitor of hepatocytes DNA synthesis. In contrast, rat hepatoma cells cultured under similar conditions were resistant to alpha-thrombin mediated DNA synthesis inhibition. The inhibitory effect of alpha-thrombin on DNA synthesis was antagonized by hirudin and antithrombin, two specific alpha-thrombin inhibitors or by the presence of collagen-I matrix. A thrombin receptor activating peptide (TRAP6) also inhibited EGF-mediated rat hepatocyte DNA synthesis, suggesting a role of the thrombin receptors in this process. Matrix fibronectin was degraded by alpha-thrombin. However, no appreciable cell detachment was observed. These results suggest a role of alpha-thrombin as a potent growth inhibitor of normal hepatocytes, possibly through control of fibronectin or other matrix protein(s).

Severe fibrosis in hepatitis C virus-infected patients is associated with increased activity of the mannan-binding lectin (MBL)/MBL-associated serine protease 1 (MASP-1) complex

Mannan-binding lectin (MBL) binds microorganisms via interactions with glycans on the target surface. Bound MBL subsequently activates MBL-associated serine protease proenzymes (MASPs). A role for MBL in hepatitis C virus (HCV) infection had been indicated by previous studies examining MBL levels and polymorphisms in relation to disease progression and response to treatment. We undertook this study to investigate a possible relationship between disease progression and functional MBL/MASP-1 complex activity. A functional assay for MBL/MASP-1 complex activity was employed to examine serum samples from patients with chronic HCV infection, non-HCV liver disease and healthy controls. Intrapatient consistency of MBL/MASP-1 complex activity levels was assessed in sequential samples from a subgroup of patients. Median values of MBL/MASP-1 complex activity were higher in sera from patients with liver disease compared with healthy controls. MBL/MASP-1 complex activity levels correlate with severity of fibrosis after adjusting for confounding factors (P = 0.003). MBL/MASP-1 complex activity was associated more significantly with fibrosis than was MBL concentration. The potential role of MBL/MASP-1 complex activity in disease progression is worthy of further study to investigate possible mechanistic links.

Resistance to activated protein C is a risk factor for fibrostenosis in Crohn’s disease

AIM: To evaluate the effect of resistance to activated protein C (aPCR), the most common known inherited thrombophilic disorder, on the risk of intestinal operation of fibrostenosis in patients with Crohn’s disease (CD).

METHODS: In a previous study, we assessed the prevalence of aPCR in CD. In a retrospective case-controlled study, 8 of these CD patients with aPCR were now compared with 24 CD patients without aPCR, matched by gender, age at diagnosis and duration of disease in a 1:3 fashion. The primary end point was the occurrence of an intestinal CD-related operation with evidence of fibrostenosis in the bowel resection specimen.

RESULTS: The Kaplan-Meier analysis revealed that patients with aPCR had a lower probability of remaining free of operation with fibrostenosis than patients without aPCR (P = 0.0372; exact log-rank test) resulting in a significantly shorter median time interval from diagnosis of CD to the first operation with fibrostenosis (32 vs 160 mo). At 10 years, the likelihood of remaining free of operation with fibrostenosis was 25% for patients with aPCR and 57.8% for patients without aPCR.

CONCLUSION: CD patients with aPCR are at higher risk to undergo intestinal operation of fibrostenosis than those without aPCR. This supports our hypothesis of aPCR being a possible risk factor for fibrostenosis in CD.

Hepatic fibrosis, stellate cells, and portal hypertension

Hepatic fibrogenesis is the common result of injury to the liver. It is believed to be a critical factor that leads to hepatic dysfunction and may be important in portal hypertension. The fibrogenic response is a complex process in which accumulation of extracellular matrix proteins, tissue contraction, and alteration in blood flow are prominent. A critical event in fibrogenesis is activation of resident perisinusoidal cells that are termed "hepatic stellate cells". Stellate cell activation is characterized by many important phenotypes, including enhanced extracellular matrix synthesis and prominent contractility. Given the central role of stellate cell activation in hepatic fibrogenesis (and portal hypertension), effective therapy for hepatic fibrogenesis is most likely will be directed at this event.

Global analysis of host tissue gene expression in the invasive front of colorectal liver metastases

Host cell reactions are a crucial determinant for tumor invasion. We analyzed on a genomewide scale gene expression differences between microdissected tissues taken from unaffected liver tissue of a human colorectal tumor (LS174) growing in the livers of nude mice and tissue from the host part of the invasive front. Due to the low degree of interspecies cross-hybridization of 15% as determined on Affymetrix microarrays, our xenograft model allowed for the distinction of genes of murine versus human origin even if the respective tissues could not be isolated separately. Using the gene ontology (GO) classification, we were able to determine patterns of up- and downregulated genes in the liver part of the invasive front. We observed a pronounced overrepresentation, e.g., of the GO terms "extracellular matrix," "cell communication," "response to biotic stimulus," "structural molecule activity" and "cell growth," indicating a very pronounced host cell response to tumor invasion. On the single gene level, hepatic stellate cell (HSC) activation markers were overrepresented in the liver part of the invasion front. Immunohistochemistry and qPCR confirmed an activation of HSC as well as an increased number of HSC in the invasive front as compared to the noninvaded liver tissue. In summary, our data demonstrate the feasibility of an interspecies differential gene expression approach on a genomewide scale.