Expression and function of integrin receptors for collagen and laminin in cultured human hepatic stellate cells

Radiotracers for Imaging of Fibrosis: Advances during the Last Two Decades and Future Directions

Fibrosis accompanies various pathologies, and there is thus an unmet medical need for non-invasive, sensitive, and quantitative methods for the assessment of fibrotic processes. Currently, needle biopsy with subsequent histological analysis is routinely used for the diagnosis along with morphological imaging techniques, such as computed tomography (CT), magnetic resonance imaging (MRI), and ultrasound (US). However, none of these imaging techniques are sufficiently sensitive and accurate to detect minor changes in fibrosis. More importantly, they do not provide information on fibrotic activity on the molecular level, which is critical for fundamental understanding of the underlying biology and disease course. Molecular imaging technology using positron emission tomography (PET) offers the possibility of imaging not only physiological real-time activity, but also high-sensitivity and accurate quantification. This diagnostic tool is well established in oncology and has exhibited exponential development during the last two decades. However, PET diagnostics has only recently been widely applied in the area of fibrosis. This review presents the progress of development of radiopharmaceuticals for non-invasive detection of fibrotic processes, including the fibrotic scar itself, the deposition of new fibrotic components (fibrogenesis), or the degradation of existing fibrosis (fibrolysis).

Molecular Mechanisms and Potential New Therapeutic Drugs for Liver Fibrosis

Liver fibrosis is the pathological process of excessive extracellular matrix deposition after liver injury and is a precursor to cirrhosis, hepatocellular carcinoma (HCC). It is essentially a wound healing response to liver tissue damage. Numerous studies have shown that hepatic stellate cells play a critical role in this process, with various cells, cytokines, and signaling pathways engaged. Currently, the treatment targeting etiology is considered the most effective measure to prevent and treat liver fibrosis, but reversal fibrosis by elimination of the causative agent often occurs too slowly or too rarely to avoid life-threatening complications, especially in advanced fibrosis. Liver transplantation is the only treatment option in the end-stage, leaving us with an urgent need for new therapies. An in-depth understanding of the mechanisms of liver fibrosis could identify new targets for the treatment. Most of the drugs targeting critical cells and cytokines in the pathogenesis of liver fibrosis are still in pre-clinical trials and there are hardly any definitive anti-fibrotic chemical or biological drugs available for clinical use. In this review, we will summarize the pathogenesis of liver fibrosis, focusing on the role of key cells, associated mechanisms, and signaling pathways, and summarize various therapeutic measures or drugs that have been trialed in clinical practice or are in the research stage.

High throughput interrogation of human liver stellate cells reveals microenvironmental regulation of phenotype

Liver fibrosis is a common feature of progressive liver disease and is manifested as a dynamic series of alterations in both the biochemical and biophysical properties of the liver. Hepatic stellate cells (HSCs) reside within the perisinusoidal space of the liver sinusoid and are one of the main drivers of liver fibrosis, yet it remains unclear how changes to the sinusoidal microenvironment impact HSC phenotype in the context of liver fibrosis. Cellular microarrays were used to examine and deconstruct the impacts of bio-chemo-mechanical changes on activated HSCs in vitro. Extracellular matrix (ECM) composition and stiffness were found to act individually and in combination to regulate HSC fibrogenic phenotype and proliferation. Hyaluronic acid and collagen III promoted elevated collagen I expression while collagen IV mediated a decrease. Previously activated HSCs exhibited reduced lysyl oxidase (Lox) expression as array substrate stiffness increased, with less dependence on ECM composition. Collagens III and IV increased HSC proliferation, whereas hyaluronic acid had the opposite effect. Meta-analysis performed on these data revealed distinct phenotypic clusters (e.g. low fibrogenesis/high proliferation) as a direct function of their microenvironmental composition. Notably, soft microenvironments mimicking healthy tissue (1 kPa), promoted higher levels of intracellular collagen I and Lox expression in activated HSCs, compared to stiff microenvironments mimicking fibrotic tissue (25 kPa). Collectively, these data suggest potential HSC functional adaptations in response to specific bio-chemo-mechanical changes relevant towards the development of therapeutic interventions. These findings also underscore the importance of the microenvironment when interrogating HSC behavior in healthy, disease, and treatment settings. STATEMENT OF SIGNIFICANCE: In this work we utilized high-throughput cellular microarray technology to systematically interrogate the complex interactions between HSCs and their microenvironment in the context of liver fibrosis. We observed that HSC phenotype is regulated by ECM composition and stiffness, and that these phenotypes can be classified into distinct clusters based on their microenvironmental context. Moreover, the range of these phenotypic responses to microenvironmental stimuli is substantial and a direct consequence of the combinatorial pairing of ECM protein and stiffness signals. We also observed a novel role for microenvironmental context in affecting HSC responses to potential fibrosis therapeutics.

Induced hepatic stellate cell integrin, α8β1, enhances cellular contractility and TGFβ activity in liver fibrosis

No effective therapy exists for fatal fibrosis. New therapeutic targets are needed for hepatic fibrosis because the incidence keeps increasing. The activation and differentiation of fibroblasts into myofibroblasts that causes excessive matrix deposition is central to fibrosis. Here, we investigated whether (and which) integrin receptors for matrix proteins activate hepatic stellate cells (HSCs). First, integrin α‐subunits were investigated systematically for their expression over the course of HSC activation and their distribution on fibroblasts and other systemic primary cells. The most upregulated in plate culture‐activated HSCs and specifically expressed across fibroblast linages was the α8 subunit. An anti‐α8 neutralizing mAb was evaluated in three different murine fibrosis models: for cytotoxic (CCl₄ treatment), non‐alcoholic steatohepatitis‐associated and cholestatic fibrosis. In all models, pathology and fibrosis markers (hydroxyproline and α‐smooth muscle actin) were improved following the mAb injection. We also CCl₄‐treated mice with inducible Itga8−/−; these mice were protected from increased hydroxyproline levels. Furthermore, ITGA8 was upregulated in specimens from 90 patients with liver fibrosis, indicating the relevance of our findings to liver fibrosis in people. Mechanistically, inhibition or ligand engagement of HSC α8 suppressed and enhanced myofibroblast differentiation, respectively, and HSC/fibroblast α8 activated latent TGFβ. Finally, integrin α8β1 potentially fulfils the growing need for anti‐fibrotic drugs and is an integrin not to be ignored. © 2021 The Authors. The Journal of Pathology published by John Wiley & Sons, Ltd. on behalf of The Pathological Society of Great Britain and Ireland.

Space of Disse: a stem cell niche in the liver

Recent evidence indicates that the plasticity of preexisting hepatocytes and bile duct cells is responsible for the appearance of intermediate progenitor cells capable of restoring liver mass after injury without the need of a stem cell compartment. However, mesenchymal stem cells (MSCs) exist in all organs and are associated with blood vessels which represent their perivascular stem cell niche. MSCs are multipotent and can differentiate into several cell types and are known to support regenerative processes by the release of immunomodulatory and trophic factors. In the liver, the space of Disse constitutes a stem cell niche that harbors stellate cells as liver resident MSCs. This perivascular niche is created by extracellular matrix proteins, sinusoidal endothelial cells, liver parenchymal cells and sympathetic nerve endings and establishes a microenvironment that is suitable to maintain stellate cells and to control their fate. The stem cell niche integrity is important for the behavior of stellate cells in the normal, regenerative, aged and diseased liver. The niche character of the space of Disse may further explain why the liver can become an organ of extra-medullar hematopoiesis and why this organ is frequently prone to tumor metastasis.

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

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

[18F]-Alfatide PET imaging of integrin αvβ3 for the non-invasive quantification of liver fibrosis

BACKGROUND & AIMS

The vitronectin receptor integrin αvβ3 drives fibrogenic activation of hepatic stellate cells (HSCs). Molecular imaging targeting the integrin αvβ3 could provide a non-invasive method for evaluating the expression and the function of the integrin αvβ3 on activated HSCs (aHSCs) in the injured liver. In this study, we sought to compare differences in the uptake of [18F]-Alfatide between normal and injured liver to evaluate its utility for assessment of hepatic fibrogenesis.

METHODS

PET with [18F]-Alfatide, non-enhanced CT, histopathology, immunofluorescence staining, immunoblotting and gene analysis were performed to evaluate and quantify hepatic integrin αvβ3 levels and liver fibrosis progression in mouse models of fibrosis (carbon tetrachloride [CCl4] and bile duct ligation [BDL]). The liver AUC divided by the blood AUC over 30 min was used as an integrin αvβ3-PET index to quantify fibrosis progression. Ex vivo analysis of frozen liver tissue from patients with fibrosis and cirrhosis verified the animal findings.

RESULTS

Fibrotic mouse livers showed enhanced [18F]-Alfatide uptake and retention compared to control livers. The radiotracer was demonstrated to bind specifically with integrin αvβ3, which is mainly expressed on aHSCs. Autoradiography and histopathology confirmed the PET imaging results. Further, the mRNA and protein level of integrin αvβ3 and its signaling complex were higher in CCl4 and BDL models than controls. The results obtained from analyses on human fibrotic liver sections supported the animal findings.

CONCLUSIONS

Imaging hepatic integrin αvβ3 with PET and [18F]-Alfatide offers a potential non-invasive method for monitoring the progression of liver fibrosis.

LAY SUMMARY

Integrin αvβ3 expression on activated hepatic stellate cells (aHSCs) is associated with HSC proliferation during hepatic fibrogenesis. Herein, we show that a radioactive tracer, [18F]-Alfatide, binds to integrin αvβ3 with high affinity and specificity. [18F]-Alfatide could thus be used as a non-invasive imaging biomarker to track hepatic fibrosis progression.

Liver fibrosis: Direct antifibrotic agents and targeted therapies

Liver fibrosis and in particular cirrhosis are the major causes of morbidity and mortality of patients with chronic liver disease. Their prevention or reversal have become major endpoints in clinical trials with novel liver specific drugs. Remarkable progress has been made with therapies that efficiently address the cause of the underlying liver disease, as in chronic hepatitis B and C. Highly effective antiviral therapy can prevent progression or even induce reversal in the majority of patients, but such treatment remains elusive for the majority of liver patients with advanced alcoholic or nonalcoholic steatohepatitis, genetic or autoimmune liver diseases. Moreover, drugs that would speed up fibrosis reversal are needed for patients with cirrhosis, since even with effective causal therapy reversal is slow or the disease may further progress. Therefore, highly efficient and specific antifibrotic agents are needed that can address advanced fibrosis, i.e., the detrimental downstream result of all chronic liver diseases. This review discusses targeted antifibrotic therapies that address molecules and mechanisms that are central to fibrogenesis or fibrolysis, including strategies that allow targeting of activated hepatic stellate cells and myofibroblasts and other fibrogenic effector cells. Focus is on collagen synthesis, integrins and cells and mechanisms specific including specific downregulation of TGFbeta signaling, major extracellular matrix (ECM) components, ECM-crosslinking, and ECM-receptors such as integrins and discoidin domain receptors, ECM-crosslinking and methods for targeted delivery of small interfering RNA, antisense oligonucleotides and small molecules to increase potency and reduce side effects. With an increased understanding of the biology of the ECM and liver fibrosis and an improved preclinical validation, the translation of these approaches to the clinic is currently ongoing. Application to patients with liver fibrosis and a personalized treatment is tightly linked to the development of noninvasive biomarkers of fibrosis, fibrogenesis and fibrolysis.

The stellate cell system (vitamin A-storing cell system)

Past, present, and future research into hepatic stellate cells (HSCs, also called vitamin A-storing cells, lipocytes, interstitial cells, fat-storing cells, or Ito cells) are summarized and discussed in this review. Kupffer discovered black-stained cells in the liver using the gold chloride method and named them stellate cells (Sternzellen in German) in 1876. Wake rediscovered the cells in 1971 using the same gold chloride method and various modern histological techniques including electron microscopy. Between their discovery and rediscovery, HSCs disappeared from the research history. Their identification, the establishment of cell isolation and culture methods, and the development of cellular and molecular biological techniques promoted HSC research after their rediscovery. In mammals, HSCs exist in the space between liver parenchymal cells (PCs) or hepatocytes and liver sinusoidal endothelial cells (LSECs) of the hepatic lobule, and store 50-80% of all vitamin A in the body as retinyl ester in lipid droplets in the cytoplasm. SCs also exist in extrahepatic organs such as pancreas, lung, and kidney. Hepatic (HSCs) and extrahepatic stellate cells (EHSCs) form the stellate cell (SC) system or SC family; the main storage site of vitamin A in the body is HSCs in the liver. In pathological conditions such as liver fibrosis, HSCs lose vitamin A, and synthesize a large amount of extracellular matrix (ECM) components including collagen, proteoglycan, glycosaminoglycan, and adhesive glycoproteins. The morphology of these cells also changes from the star-shaped HSCs to that of fibroblasts or myofibroblasts.

Integrins and heparan sulfate proteoglycans on hepatic stellate cells (HSC) are novel receptors for HSC-derived exosomes

Exosomes mediate intercellular microRNA delivery between hepatic stellate cells (HSC), the principal fibrosis-producing cells in the liver. The purpose of this study was to identify receptors on HSC for HSC-derived exosomes, which bind to HSC rather than to hepatocytes. Our findings indicate that exosome binding to HSC is blocked by treating HSC with RGD, EDTA, integrin αv or β1 siRNAs, integrin αvβ3 or α5β1 neutralizing antibodies, heparin, or sodium chlorate. Furthermore, exosome cargo delivery and exosome-regulated functions in HSC, including expression of fibrosis- or activation-associated genes and/or miR-214 target gene regulation, are dependent on cellular integrin αvβ3, integrin α5β1, or heparan sulfate proteolgycans (HSPG). Thus, integrins and HSPG mediate the binding of HSC-derived exosomes to HSC as well as the delivery and intracellular action of the exosomal payload.

Cytoglobin expression in the hepatic stellate cell line HSC-T6 is regulated by extracellular matrix proteins dependent on FAK-signalling

BACKGROUND

Fibrosis is a physiological response to cellular injury in the liver and is mediated by the activation of hepatic stellate cells resulting in the replacement of hepatocytes with extracellular matrix comprised principally of collagen 1 to form a hepatic scar. Although the novel hexaco-ordinated globin cytoglobin was identified in activated hepatic stellate cells more than 10 years ago, its role in stellate cell biology and liver fibrosis remains enigmatic.

RESULTS

In the current study, we investigated the role of different extracellular matrix proteins in stellate cell proliferation, activation (alpha smooth muscle actin expression and retinoic acid uptake) and cytoglobin expression. Our results demonstrate that cytoglobin expression is correlated with a more quiescent phenotype of stellate cells in culture and that cytoglobin is regulated by the extracellular matrix through integrin signalling dependent on activation of focal adhesion kinase.

CONCLUSIONS

Although further studies are required, we provide evidence that cytoglobin is a negative regulator of stellate cell activation and therefore may represent a novel target for anti-fibrotic treatments in the future.

A disintegrin and metalloproteinase-12 (ADAM12): function, roles in disease progression, and clinical implications

BACKGROUND

A disintegrin and metalloproteinase-12 (ADAM12) is a member of the greater ADAM family of enzymes: these are multifunctional, generally membrane-bound, zinc proteases for which there are forty genes known (21 of these appearing in humans). ADAM12 has been implicated in the pathogenesis of various cancers, liver fibrogenesis, hypertension, and asthma, and its elevation or decrease in human serum has been linked to these and other physiological/pathological conditions.

SCOPE

In this review, we begin with a brief overview of the ADAM family of enzymes and protein structure. We then discuss the role of ADAM12 in the progression and/or diagnosis of various disease conditions, and we will conclude with an exploration of currently known natural and synthetic inhibitors.

MAJOR CONCLUSION

ADAM12 has potential to emerge as a successful drug target, although targeting the metalloproteinase domain with any specificity will be difficult to achieve due to structural similarity between the members of the ADAM and MMP family of enzymes. Overall, more research is required to establish ADAM12 being as a highly desirable biomarker and drug target of different diseases, and their selective inhibitors as potential therapeutic agents.

GENERAL SIGNIFICANCE

Given the appearance of elevated levels of ADAM12 in various diseases, particularly breast cancer, our understanding of this enzyme both as a biomarker and a potential drug target could help make significant inroads into both early diagnosis and treatment of disease.

Proteomic analysis of extracellular matrix from the hepatic stellate cell line LX-2 identifies CYR61 and Wnt-5a as novel constituents of fibrotic liver

Activation of hepatic stellate cells (HSCs) and subsequent uncontrolled accumulation of altered extracellular matrix (ECM) underpin liver fibrosis, a wound healing response to chronic injury, which can lead to organ failure and death. We sought to catalogue the components of fibrotic liver ECM to obtain insights into disease etiology and aid identification of new biomarkers. Cell-derived ECM was isolated from the HSC line LX-2, an in vitro model of liver fibrosis, and compared to ECM from human foreskin fibroblasts (HFFs) as a control. Mass spectrometry analyses of cell-derived ECMs identified, with ≥99% confidence, 61 structural ECM or secreted proteins (48 and 31 proteins for LX-2 and HFF, respectively). Gene ontology enrichment analysis confirmed the enrichment of ECM proteins, and hierarchical clustering coupled with protein–protein interaction network analysis revealed a subset of proteins enriched to fibrotic ECM, highlighting the existence of cell type-specific ECM niches. Thirty-six proteins were enriched to LX-2 ECM as compared to HFF ECM, of which Wnt-5a and CYR61 were validated by immunohistochemistry in human and murine fibrotic liver tissue. Future studies will determine if these and other components may play a role in the etiology of hepatic fibrosis, serve as novel disease biomarkers, or open up new avenues for drug discovery.

Acute alcohol-induced liver injury

Alcohol consumption is customary in most cultures and alcohol abuse is common worldwide. For example, more than 50% of Americans consume alcohol, with an estimated 23.1% of Americans participating in heavy and/or binge drinking at least once a month. A safe and effective therapy for alcoholic liver disease (ALD) in humans is still elusive, despite significant advances in our understanding of how the disease is initiated and progresses. It is now clear that acute alcohol binges not only can be acutely toxic to the liver, but also can contribute to the chronicity of ALD. Potential mechanisms by which acute alcohol causes damage include steatosis, dysregulated immunity and inflammation, and altered gut permeability. Recent interest in modeling acute alcohol exposure has yielded new insights into potential mechanisms of acute injury, which also may well be relevant for chronic ALD. Recent work by this group on the role of PAI-1 and fibrin metabolism in mediating acute alcohol-induced liver damage serve as an example of possible new targets that may be useful for alcohol abuse, be it acute or chronic.

Antiproliferative and proapoptotic effects of tocopherol and tocol on activated hepatic stellate cells

Activated hepatic stellate cells (HSCs) play crucial roles in liver fibrosis. In the course of liver injury, HSCs, which reside in perisinusoidal spaces and lose lipid droplets, morphologically change into a myofibroblastic phenotype and acquire an increased proliferation activity in what is known as the activated state. We have investigated therapeutic strategies for liver fibrosis by promoting spontaneous reversion or inducing apoptosis in activated HSCs. Vitamin E consists of four tocopherols and four tocotrienols, all of which are well-known antioxidants. In this study, the antiproliferative and proapoptotic effects of a tocol, which lacks methyl groups attached to the chromanol ring, and four tocopherols were investigated using activated HSCs. δ-Tocopherol and tocol exhibited relatively high proliferation inhibitory and proapoptotic abilities. However, they did not show proliferation inhibition ability on primary hepatocytes or HepG2 cells. Significant cell detachment was also observed in δ-tocopherol- and tocol-treated HSCs. Decreased protein expressions of α-smooth muscle actin and β1 integrin were observed in a dose-dependent manner. These results indicate that δ-tocopherol and tocol induce anoikis in activated HSCs.

Human platelet antigen genotype is associated with progression of fibrosis in chronic hepatitis C

Although progression of fibrosis in the chronic hepatitis C depends on environmental, viral, and host factors, genetic polymorphisms have been associated recently with this progression, including the expression of integrins, adhesion proteins. Some integrins expressed on the platelet membrane show polymorphic antigenic determinants called human platelet antigens (HPA), where the major ones are HPA-1, -3, -5. The association between HCV infection and HPA-5b has been demonstrated. Similarly, the HPA profile could determine if HPA is related to progression of fibrosis. The goal of this study was to evaluate the association between the frequencies of HPA-1, -3, and -5 and degree of fibrosis in HCV-infected patients. Genomic DNA from 143 HCV-infected patients was used as the source for HPA genotyping by PCR-SSP or PCR-RFLP. Progression of fibrosis was evaluated using the METAVIR scoring system, and the patients were grouped according to degree of fibrosis into G1 (n = 81, with F1, portal fibrosis without septa or F2, few septa) and G2 (n = 62, with F3, numerous septa, or F4, cirrhosis). Statistical analysis was performed using the proportional odds model. The genotypic frequency of HPA-1a/1b was significantly higher in the patients in G2. To evaluate the influence of the time of infection to the development of fibrosis and its effect on the genetic factor HPA-1, 96 patients from 143 studied were evaluated considering the time of HCV infection, and these results suggest that the HPA-1a/1b genotype promotes the development of fibrosis in HCV infection with time.

Hepatic stellate cell (vitamin A-storing cell) and its relative--past, present and future

HSCs (hepatic stellate cells) (also called vitamin A-storing cells, lipocytes, interstitial cells, fat-storing cells or Ito cells) exist in the space between parenchymal cells and liver sinusoidal endothelial cells of the hepatic lobule and store 50-80% of vitamin A in the whole body as retinyl palmitate in lipid droplets in the cytoplasm. In physiological conditions, these cells play pivotal roles in the regulation of vitamin A homoeostasis. In pathological conditions, such as hepatic fibrosis or liver cirrhosis, HSCs lose vitamin A and synthesize a large amount of extracellular matrix components including collagen, proteoglycan, glycosaminoglycan and adhesive glycoproteins. Morphology of these cells also changes from the star-shaped SCs (stellate cells) to that of fibroblasts or myofibroblasts. The hepatic SCs are now considered to be targets of therapy of hepatic fibrosis or liver cirrhosis. HSCs are activated by adhering to the parenchymal cells and lose stored vitamin A during hepatic regeneration. Vitamin A-storing cells exist in extrahepatic organs such as the pancreas, lungs, kidneys and intestines. Vitamin A-storing cells in the liver and extrahepatic organs form a cellular system. The research of the vitamin A-storing cells has developed and expanded vigorously. The past, present and future of the research of the vitamin A-storing cells (SCs) will be summarized and discussed in this review.

Biochemical characterization of the binding of cyclic RGDyK to hepatic stellate cells

Activated hepatic stellate cells (HSCs) play a crucial role in the development of liver fibrosis. Noninvasive monitoring of the activation of HSCs has been challenging due to the lack of specific receptors or motifs on the cells. The present study provides the evidence that integrin alpha v beta 3 expressed on HSCs is a biomarker reflecting the activation of HSCs. Solid-phase synthesis of cRGDyK (Arg-Gly-Asp-(D)Tyr-Lys) peptide and FAM-conjugated peptide were employed for binding to integrin alpha v beta 3. The increased expression of integrin alpha v and beta 3 at mRNA and protein levels was detected during HSC activation. The affinity of cRGDyK to integrin alpha v beta 3 was examined by both radioligand binding assay and FAM-conjugated peptide binding measurements. Quantitative RT-PCR and Western blotting showed a less dramatic, but significant increase in alpha v and beta 3 integrin mRNA and protein expression following activation of rat HSCs. Radioiodinated cRGDyK binds to both purified and membrane-bound integrin alpha v beta 3 with high affinity in a dissociable manner. FAM-conjugated cRGDyK was coupled to activated HSCs in a time- and dose-dependent, receptor-mediated manner. Activated HSCs express sufficient number of integrin alpha v beta 3 receptor. cRGDyK peptide binds to both purified and membrane-bound integrin alpha v beta 3 with high affinity in a reversible fashion. Thus, the cRGDyK peptide represented a new agent potentially useful for the diagnosis of liver fibrosis.

Integrin alpha V polymorphisms and haplotypes in a Korean population are associated with susceptibility to chronic hepatitis and hepatocellular carcinoma

BACKGROUND/AIMS

Integrins are cell surface receptors for extracellular matrix (ECM) proteins that initiate signalling pathways that modulate proliferation, survival, invasion or metastasis. Consequently, integrins are potential targets for the treatment of cancer. In this study, we investigated whether single nucleotide polymorphisms (SNPs) in integrin alpha(V) (ITGAV) in a Korean population were associated with chronic hepatitis B virus (HBV) infection and HBV-infected hepatocellular carcinoma (HCC).

PATIENTS AND METHODS

Thirteen ITGAV SNPs in 111 cases of chronic HBV infection, 86 cases of HBV-infected HCC and 107 cases of acute self-limited HBV infection were genotyped using Illumina's Sentrix array matrix (SAM) chip.

RESULTS

The ITGAV intron SNPs rs9333289 and rs11685758, the 3'-untranslated region SNP rs1839123 and haplotype 3 (T-T-A) were associated with enhanced susceptibility to HBV-infected HCC (OR=1.75-2.42; P=0.02-0.05), while the intron SNP rs2290083 was associated with both chronic infection and HBV-infected HCC (OR=1.73-2.01; P=0.01-0.04). In addition, both rs2290083 and ht1 (C-C-G) were associated with the age at which chronic infection occurred, as determined by Cox relative hazard analysis (RH=1.39-1.62, P=0.04-0.01)

CONCLUSION

ITGAV SNPs and haplotypes may be genetic factors that increase the susceptibility of Koreans to chronic HBV infection and HBV-infected HCC.

Cell survival, activation and apoptosis of hepatic stellate cells: modulation by extracellular matrix proteins

AIM

Cytokines and growth factors released by various hepatic cells exert both paracrine and autocrine effects on hepatic stellate cell (HSC) activation during liver injury. The aim of the present study was to examine whether the surrounding extracellular matrix (ECM) influences the activation, transdifferentiation and survival of HSCs.

METHODS

An in vitro model system of isolated HSCs maintained in culture on different matrix protein substrata was employed.

RESULTS

The rate of loss of HSC-specific retinol uptake activity and gain of myofibroblast-like activity such as (35)[S] proteoglycan synthesis varied in cells maintained on different matrix proteins and was in the order collagen I > collagen IV >/= laminin. (3)[H]-thymidine incorporation by HSCs maintained on different matrix proteins varied and was in the order collagen I > collagen IV > laminin. MTT assay revealed that the growth inhibition in response to curcumin was significantly low in cells maintained on collagen I. Apoptotic marker activities such as DNA fragmentation, 4',6'-diamidino-2-phenylindole dihydrochloride (DAPI) staining, annexin staining and caspase-3 activities showed that cells maintained on collagen I showed minimal apoptosis than those maintained on collagen IV, laminin and polylysine, showing the influence of ECM on HSC apoptosis. Experiments using blocking antibodies showed that the collagen I effect was mediated through alpha(2)beta(1) integrin.

CONCLUSIONS

These results indicate that ECM influences activation, transdifferentiation and survival of HSCs, and suggest that apart from diffusible factors, the surrounding ECM also influences HSC behavior critical in both the progression of the fibrosis and the restitution of the liver during recovery after hepatic injury.

RACK1, a new ADAM12 interacting protein. Contribution to liver fibrogenesis

ADAM12 belongs to a disintegrin-like and metalloproteinase-containing protein family that possesses multidomain structures composed of a pro-domain, a metalloprotease, disintegrin-like, cysteine-rich, epidermal growth factor-like, and transmembrane domains, and a cytoplasmic tail. Overexpression of several ADAMs has been reported in human cancer, and we recently described the involvement of ADAM12 in liver injury (Le Pabic, H., Bonnier, D., Wewer, U. M., Coutand, A., Musso, O., Baffet, G., Clement, B., and Theret, N. (2003) Hepatology 37, 1056-1066). In this study, we used a yeast two-hybrid screening of a cDNA library from human hepatocellular carcinoma to analyze binding partners of ADAM12. We identify RACK1, a receptor for activated protein kinase C (PKC), as a new ADAM12 interacting protein. RACK1 is up-regulated in patients with hepatocellular carcinoma and is highly expressed by activated hepatic stellate cells. We demonstrate the involvement of RACK1 in mediating the PKC-dependent translocation of ADAM12 to membranes of activated hepatic stellate cells. In particular, treatment of cells with phorbol esters enhances ADAM12 immunostaining in the membrane fractions and the co-immunoprecipitation of ternary complexes containing RACK1, ADAM12, and PKC. By using RNA interference, we demonstrate that inhibition of RACK1 expression diminishes the phorbol 12-myristate 13-acetate-dependent translocation of ADAM12 to membranes of hepatic stellate cells. Finally, hepatic stellate cells cultured on coated type I collagen induces relocalization of ADAM12 in the membrane, suggesting that this major matrix component in liver cancer and fibrogenesis might stimulate ADAM12 translocation to the cell membrane where its shedding activity takes place.

Osteopontin expression in the liver with severe perisinusoidal fibrosis: autopsy case of Down syndrome with transient myeloproliferative disorder

Down syndrome with transient myeloproliferative disorder (TMD) is often associated with perinatal liver fibrosis. The authors recently encountered an autopsy case of this disease with a characteristic severe perisinusoidal liver fibrosis. Osteopontin (OPN) is a molecule that plays an important role in diverse fibro-inflammatory diseases. The purpose of the present report was to examine the involvement of OPN in development of the Down syndrome-associated liver fibrosis. Histology indicated severe perisinusoidal fibrosis and ductular arrangements of hepatocytes in the liver. Appearance of atypical megakaryocytes in the liver, a feature of TMD associated with Down syndrome, was not evident. On immunohistochemistry expression of OPN was observed in hepatocytes often having ductular arrangements and infiltrating macrophages. In contrast, a small number of transforming growth factor-beta1 (TGF-beta1)-positive mononuclear cells were present in the liver. Numerous activated hepatic stellate cells (HSC) expressing alpha-smooth muscle actin (alpha-SMA) were seen in the perisinusoidal area. A recent report indicated that OPN could directly activate the HSC. Thus, it is suggested that OPN produced by hepatocytes and macrophages induces activation of the HSC, and leads to the development of perisinusoidal liver fibrosis.

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.

Differences in regulation of type I collagen synthesis in primary and passaged hepatic stellate cell cultures: the role of alpha5beta1-integrin

Hepatic stellate cells (HSC) differ in their phenotype depending on the initiation and progression of their activation. Our hypothesis was that different mechanisms govern type I collagen synthesis depending on stage of HSC activation. We investigated the role of alpha(5)beta(1)-integrin as a regulator of type I collagen gene COL1A1 expression in primary and passaged HSC cultures using transgenic mouse containing type I collagen gene COL1A1 promoter linked to the chloramphenicol acetyltransferase (CAT) reporter gene. The alpha(5)beta(1) protein levels increased during the activation and were highest in day 6 primary cultures but decreased in passaged HSC. CAT activity, reflecting COL1A1 expression, was upregulated by alpha(5)beta(1)-integrin. Inhibition of alpha(5)beta(1)-integrin by echistatin and blocking antibody resulted in reduced transgene activity only in early primary cultures (compared with the control, 53.3 +/- 12% echistatin and 58.8 +/- 7% blocking antibody, respectively, P < 0.05). Treatment of passaged HSC with either echistatin or blocking antibody had no effect. Fibronectin, an alpha(5)beta(1)-integrin ligand, increased transgene activity in primary (210 +/- 33%, P < 0.05) but not in passaged HSC cultures (119 +/- 8%). This alpha(5)beta(1)-integrin effect appears to be at least in part mediated by CCAAT enhancer binding protein-beta (C/EBPbeta), because fibronectin increased and alpha(5)-gene silencing by small interfering RNA decreased C/EBPbeta levels. In addition, C/EBPbeta knockout mice showed reduced type I collagen synthesis compared with wild-type littermates. Therefore alpha(5)beta(1)-integrin is an important regulator of type I collagen production in early primary HSC cultures but appears to have no direct role once the HSC are fully activated.

Pharmacological inhibition of the vitronectin receptor abrogates PDGF-BB-induced hepatic stellate cell migration and activation in vitro

BACKGROUND/AIMS

Activated hepatic stellate cells (HSC) play a central role in the development of liver fibrosis. Platelet-derived growth factor (PDGF)-BB and the integrin alphavbeta3 mediate mesenchymal cell migration and proliferation. However, their contribution and interaction during fibrogenic activation of HSC remains unclear. To this aim we investigated if PDFGF-BB and alphavbeta3 interact, and how far small molecular inhibitors of alphavbeta3 modulate PDGF-BB and serum-induced migration, proliferation and fibrogenic activation of HSC.

METHODS

Rat and human HSC were subjected to migration and proliferation assays in the presence or absence of a peptide or a nonpeptide alphavbeta3 inhibitor. Activation of mitogen-activated protein kinases (ERK1/2, p38), Akt, focal adhesion kinase (FAK), paxillin and beta3 integrin was evaluated by phospho-specific Western blotting. Fibrosis related transcripts were determined by quantitative real-time PCR.

RESULTS

PDGF-BB-stimulated HSC migration which was blocked dose-dependently by the alphavbeta3 antagonists, with complete inhibition at 10(-6)M. alphavbeta3 blockage did not affect cell viability or proliferation, while it decreased phosphorylation of FAK, paxillin, beta3 integrin and p38, but not of ERK1/2 or Akt. alphavbeta3 inhibition led to downregulation of certain profibrogenic transcripts, while it upregulated fibrolytic MMP-13 mRNA.

CONCLUSIONS

Inhibition of integrin alphavbeta3 leads to abrogation of migration of HSC stimulated with PDGF-BB and to an antifibrogenic gene expression pattern.

Osmosensing by integrins in rat liver

Changes in hepatocyte hydration are induced not only by ambient hypo- or hyperosmolarity, but also under isosmotic condition by hormones, substrates, and oxidative stress. The perfused rat liver is a well-established intact organ model with preservation of the three-dimensional hepatocyte anchoring to the extracellular matrix and/or adjacent cells, parenchymal cell polarity, liver cell heterogeneity, acinar construction, and gene expression gradients. Originally, data from the perfused rat liver indicated that changes of cell hydration independent of their origin critically contribute to the control of autophagic proteolysis and canalicular bile acid excretion. Meanwhile, the concept that cell hydration changes trigger signal transduction processes that control metabolism, gene expression, transport, and the susceptibility to stress is well accepted. This chapter summarizes evidence obtained from experiments with the perfused rat liver that integrins are osmosensors in the liver and thereby critically contribute to the Src- and MAP-kinase-dependent inhibition of autophagic proteolysis, stimulation of canalicular taurocholate excretion, and regulatory volume decrease as induced by hypoosmotic swelling. Moreover, integrin-dependent sensing of hepatocyte swelling is essential for signaling and proteolysis inhibition by insulin and glutamine. These findings define a novel role of integrins in insulin and glutamine signaling and set an example for mechanotransduction as an integral part of overall growth factor and nutrient signaling.

Liver fibrosis induced by hepatic overexpression of PDGF-B in transgenic mice

BACKGROUND/AIMS

In hepatic fibrogenesis, stellate cells are activated leading to production and deposition of extracellular matrix. To clarify the role of PDGF-B in liver fibrogenesis, we overexpressed PDGF-B in the liver of transgenic mice.

METHODS

Transgenic mice for the conditional overexpression of PDGF-B in the liver under control of an albumin promoter were generated utilising the Cre/loxP system. Constitutive PDGF-B expression was achieved after breeding with mice expressing Cre-recombinase under actin promoter control. Tamoxifen inducible expression was achieved after breeding with mice expressing Cre under transthyretin receptor promoter control. Levels of fibrosis were assessed and the expression of regulators of matrix remodelling was measured.

RESULTS

PDGF-B expression caused hepatic stellate cell and myofibroblast activation marked by alpha-smooth muscle actin and PDGFR-beta expression. Liver fibrosis was verified macroscopically, histologically and by collagen I mRNA quantification in 4-6 week-old animals. MMP-2, MMP-9 and TIMP-1 were upregulated whereas TGF-beta expression was unchanged.

CONCLUSIONS

We identified PDGF-B as a proliferative and profibrogenic stimulus and potential inducer of stellate cell transdifferentiation in vivo. PDGF-B overexpression causes liver fibrosis without significantly upregulating TGF-beta1, suggesting a TGF-beta-independent mechanism. The established model provides a tool for testing anti-PDGF-B therapeutic strategies in liver fibrosis in vivo.

Collagen binding alpha2beta1 and alpha1beta1 integrins play contrasting roles in regulation of Ets-1 expression in human liver myofibroblasts

Activation of hepatic stellate cells from quiescence to myofibroblast-like cells (MFBs) is a pivotal event in hepatic fibrogenesis. Plastic-cultured stellate cells (an established in vitro model of the activated phenotype) recultured on Matrigel revert to quiescence. In the present study we analyzed the molecular mechanism underlying this process, focusing on the effect of collagen receptors alpha(2)beta(1) and alpha(1)beta(1) integrin signaling on the expression of Ets-1 transcription factor and its target gene MMP1 in cultured human MFBs. Cells grown in 3-dimensional (3D) substrates (Matrigel) or collagen type I gel) markedly upregulated Ets-1 and MMP1 messages, in comparison to cells cultured on plastic. A similar effect but less intense was mimicked by stimulation of alpha(2)beta(1) or blocking of alpha(1)beta(1) integrin in cells grown on plastic. We observed increased expression of MMP1 transcripts with parallel changes in MMP1 promoter activity, and in mRNA and protein levels of upstream transcription factors Ets-1 and c-Jun. Interference with alpha(2)beta(1) and alpha(1)beta(1) integrin function in cells cultured in a 3D collagen substrate resulted in an even greater effect. Morphologically, stimulation of alpha(2)beta(1) integrin resulted in formation of multicellular networks, probably by facilitation of cell migration. Thus, we report the novel observation that in cultured human MFBs reverting to quiescence, the expression of transcription factor Ets-1 and its downstream target MMP1 can be modulated by changes in the microenvironment, which are mediated, at least in part, by the balance between collagen receptor integrin alpha(2)beta(1) and alpha(1)beta(1) activities.

[Hepatic stellate cells: it's role in normal and pathological conditions]

Hepatic fibrosis is a dynamic and sophisticatedly regulated wound healing response to chronic hepatocellular injury. This fibrotic process results from the accumulation of extracellular matrix (ECM) including collagen, proteoglycan, and adhesive glycoproteins which are principally produced by hepatic stellate cells (HSC), a mesenchymal cell type located between parenchymal cell plates and sinusoidal endothelial cells in the space of Disse. In physiological conditions, quiescent HSCs play important roles in the regulation of retinoid homeostasis and ECM remodeling by producing ECM components as well as metalloproteases and its inhibitor. However during hepatic fibrogenesis, HSCs are known to be activated or "transdifferentiated" to myofibroblast-like cells which play a pivotal role in ECM remodeling and hepatic blood flow regulation. Activation of HSC is now well established as the key process involved in the development of hepatic fibrosis. Both basic morphology and functions of HSCs in normal conditions and its role in pathological fibrosis will be discussed in this review.

Activation of nuclear factor kappa B (NF-kappaB) by connective tissue growth factor (CCN2) is involved in sustaining the survival of primary rat hepatic stellate cells

Background/Aims

Connective tissue growth factor (CCN2) is a matricellular protein that plays a role in hepatic stellate cell (HSC)-mediated fibrogenesis. The aim of this study was to investigate the regulation by CCN2 of cell survival pathways in primary HSC.

Methods

Primary HSC were obtained by in situ enzymatic perfusion of rat liver. NF-κB activation was assessed by immunoblotting for IκBα phosphorylation and degradation and by NF-κB p50 or p65 nuclear accumulation. NF-κB DNA-binding activity was determined by gel mobility shift assay while NF-κB response gene expression was evaluated using a luciferase reporter. Cell viability was assessed by Trypan blue staining or ATP luminescent assay while apoptosis was evaluated by caspase-3 activity.

Results

CCN2 induced IκBα phosphorylation and degradation as well as nuclear accumulation of NF-κB. Activated NF-κB comprised three dimers, p65/p65, p65/p50 and p50/p50, that individually bound to DNA-binding sites and subsequently triggered transcriptional activity. This was confirmed by showing that CCN2 promoted activity of a NF-κB luciferase reporter. CCN2 promoted survival of serum-starved HSC and protected the cells from death induced by blocking the NF-κB signaling pathway using Bay-11-7082, a specific inhibitor of IκBα phosphorylation.

Conclusion

CCN2 contributes to the survival of primary HSC through the NF-κB pathway.

Immortalization of pancreatic stellate cells as an in vitro model of pancreatic fibrosis: deactivation is induced by matrigel and N-acetylcysteine

Tissue fibrosis is one of the characteristics of chronic pancreatitis and pancreatic adenocarcinoma. Activated pancreatic stellate cells (PSC) play a central role in this process. However, analysis of the molecular mechanisms leading to PSC activation is hampered by the lack of an established human PSC line. To overcome this problem, we immortalized and characterized primary human PSC. The cells were isolated by the outgrowth method and were immortalized by transfection with SV40 large T antigen and human telomerase (hTERT). Primary human PSC served as controls. An immortalized line, RLT-PSC, was analyzed for the expression of stellate cell markers. Moreover, the effects of transforming growth factor beta 1(TGFbeta1) or platelet-derived growth factor stimulation and of cultivation on basement membrane components or N-acetylcysteine (NAC) treatment on gene and protein expression and proliferation were analyzed. Immortal RLT-PSC cells retained the phenotype of activated PSC proven by the expression of alpha-smooth muscle actin (alphaSMA), vimentin, desmin and glial fibrillary acidic protein (GFAP). TGFbeta1 treatment upregulated the expression of alphaSMA, collagen type I (Col I), fibronectin and TGFbeta1. Incubation of RLT-PSC cells and primary human activated PSC on Matrigel plus NAC treatment resulted in a deactivated phenotype as evidenced by a decrease of alphaSMA, connective tissue growth factor and Col I expression and by a decreased proliferation of the cells. Moreover, this treatment restored the ability of the cells to store vitamin A in cytoplasmic vesicles. In conclusion, we have established an immortal pancreatic stellate cell line, without changing the characteristic phenotype. Importantly, we were able to demonstrate that besides soluble factors, the matrix surrounding PSC plays a pivotal role in the maintenance of the activation process of PSC. Cultivation of activated PSC on a reconstituted basement membrane plus treatment with NAC was able to deactivate the cells, thus pointing to the possibility of an antifibrosis therapy in chronic pancreatitis.

rAAV-mediated stable expression of heme oxygenase-1 in stellate cells: a new approach to attenuate liver fibrosis in rats

Liver fibrosis is the consequence of activation of hepatic stellate cells mediated by persistent or recurrent liver injury, where oxidative stress or inflammatory response resulting from immune cells and cytokines are involved. Targeting of hepatic stellate cells could be an important strategy for the therapy of liver fibrosis. In this study, we showed a tropism of recombinant adeno-associated virus (rAAV, serotype 2) with high efficiency in transduction of a homeostatic gene, heme oxygenase-1 (HO-1), to activated stellate cells. The binding of rAAVs to stellate cells increased significantly after serum-stimulated activation compared with quiescent status. Portal injection of rAAVs to normal or carbon tetrachloride (CCl(4))-induced liver fibrosis showed a distinct distribution of rAAV binding. The majority of injected rAAVs bound to the cells in fibrotic areas that were associated with higher expression levels of fibroblast growth factor receptor-1alpha at 2 hours after administration. Isolation of different types of cells from CCl(4)-induced fibrotic livers showed predominant expression of transgene in stellate cells after rAAV/HO-1 administration on day 3 and remained stable for 12 weeks. In addition, HO-1-transduced stellate cells showed reduced transcript levels of type 1 collagen and impaired proliferative ability compared with controls. With this approach, the severity of established micronodular cirrhosis was markedly reduced. In conclusion, these findings suggest a new approach for the treatment of liver fibrosis using adeno-associated virus-mediated gene transfer.

Hyaluronic acid inhibits adhesion of hepatic stellate cells in spite of its stimulation of DNA synthesis

Unbalanced accumulation of fibers in extracellular matrix (ECM) results from attachment and activation of hepatic stellate cells (HSCs) during chronic liver diseases, in which the content of hyaluronic acid (HA), a glycosaminoglycan, in ECM changes. No information is available on the effect of HA on adhesion and activation of HSCs although that of collagen (Col) on HSCs was extensively studied. This study investigated the effects of HA with or without Col on adhesion of HSCs or the rate of DNA synthesis. Attachment of primary cultured HSCs was microscopically monitored in the plate simultaneously coated with HA or other ECM components. HA inhibited adhesion of quiescent HSCs at least up to 7 days after seeding, whereas HSCs were adherent to plastic or type I collagen (Col-I), type III collagen (Col-III), type IV collagen (Col-IV) or fibronectin. Both microscopy and alpha-smooth muscle actin immunocytochemistry revealed that the number of HSCs, which had been re-seeded after 15 days of culture, attached to HA-coated area was remarkably lower compared to that of HSCs on Col-I or plastic. Incorporation of HA into Col-I prevented adhesion of activated HSCs to matrix film. The number of HSCs adherent to HA at early times after seeding was minimal and significantly lower than that of the cells adherent to plastic. In contrast, either Col-I or Col-IV increased the number of adherent cells. Attachment of HSCs to plastic was inhibited by soluble HA in culture medium. CD44, the cell surface receptor to which HA binds, was immunochemically detected in HSCs. Adhesion of HSCs to plastic, HA or Col-I was not changed by anti-CD44 antibody. Either HA or Col increased the basal or platelet-derived growth factor-inducible rate of thymidine incorporation into DNA in HSCs. In conclusion, HA inhibits adhesion of quiescent or activated HSCs in spite of its stimulation of DNA synthesis, whereas Col increases HSC attachment and DNA synthesis, and inhibition of HSC adhesion by HA does not involve CD44.

Liver fibrosis: from the bench to clinical targets

Progressive liver fibrosis is the main cause of organ failure in chronic liver diseases of any aetiology. Fibrosis develops with different spatial patterns and is a consequence of different prevalent mechanisms according to the diverse causes of parenchymal damage. Indeed, fibrosis, observed as a consequence of chronic viral infection is initially concentrated within and around the portal tract, while fibrosis secondary to toxic/metabolic damage is located mainly in the centrolobular areas. In addition, it is increasingly evident that different cell types are involved in the deposition of fibrillar extracellular matrix during active hepatic fibrogenesis: hepatic stellate cells are mainly involved when hepatocellular damage is limited or concentrated within the liver lobule, whereas portal myofibroblasts and fibroblasts provide a predominant contribution when the damage is located in the proximity of the portal tracts. In the later stages of evolution (septal fibrosis) it is likely that all extracellular matrix-producing cells contribute to fibrogenesis. Recruitment and activation of extracellular matrix-producing cells to the site of tissue damage can be due to different major mechanisms: (1) Chronic activation of the tissue repair process. In this case, as a consequence of the reiterated damage, accumulation of fibrillar extracellular matrix reflects the impossibility of an effective remodelling and regeneration. (2) Effect of oxidative stress products, including reactive oxygen intermediates and reactive aldehydes. These products, whose concentration become critical in toxic/metabolic liver injury, are able to induce the synthesis of fibrillar extracellular matrix even in the absence of significant hepatocyte damage and inflammation. (3) Derangement of normal the epithelial/mesenchymal interaction. This typically occurs in all conditions characterised by cholangiocyte damage/proliferation, where a consensual proliferation of extracellular matrix-producing cells and progressive fibrogenesis is commonly observed. A major advancement towards the understanding of the molecular mechanisms of fibrogenesis is derived from a consistent number of in vitro studies investigating the biological role of growth factors/cytokines and other soluble factors and their intracellular signalling pathways. The relevance of these factors has been confirmed by studies performed on animal models and by studies performed on pathological human liver. Along these lines, the elucidation of a consistent number of cellular and molecular mechanisms responsible for the progression of liver fibrosis has provided sound basis for the development of pharmacological strategies able to modulate this important pathophysiological process. Finally, there are several clinically relevant issues that need re-evaluation and/or further investigation, and in particular: (1) the need of an accurate and effective monitoring of the fibrotic progression of chronic liver diseases and of the effectiveness of the currently proposed treatments; (2) the identification of general or individual factors potentially relevant for a faster progression of the disease.

Engagement of alphavbeta3 integrin regulates proliferation and apoptosis of hepatic stellate cells

Hepatic stellate cells are the major source of the extracellular matrix that accumulates in fibrotic liver. During progressive liver fibrosis, hepatic stellate cells proliferate, but during resolution of fibrosis there is extensive stellate cell apoptosis that coincides with degradation of the liver scar. We have examined the possibility that the fate of stellate cells is influenced by the extracellular matrix through the intermediary of alpha(v)beta(3) integrin. alpha(v)beta(3) integrin was expressed by activated, myofibroblastic rat and human stellate cells in culture. Antagonism of this integrin using neutralizing antibodies, echistatin, or small inhibitory RNA to silence alpha(v) subunit expression inhibited stellate cell proliferation and their expression of proliferating cell nuclear antigen and activated forms of p44 and p42 MAPK. These alpha(v)beta(3) antagonists also increased apoptosis of cultured stellate cells, and this was associated with an increase in the BAX/BCL-2 protein ratio, induction of nuclear DNA fragmentation, and activation of intracellular caspase-3. Expression of tissue inhibitor of metalloproteinases-1 by activated stellate cells was reduced by the alpha(v)beta(3) antagonists, while matrix metalloproteinase-9 synthesis was enhanced. Stellate cells incubated with active recombinant matrix metalloproteinase-9 showed enhanced apoptosis, while cells treated with a synthetic inhibitor of this protease showed increased survival. Our studies suggest that alpha(v)beta(3) integrin regulates the fate of hepatic stellate cells. Degradation of alpha(v)beta(3) ligands surrounding activated stellate cells during resolution of liver fibrosis might decrease alpha(v)beta(3) integrin ligation, suppressing stellate cell proliferation and inducing a fibrolytic, matrix metalloproteinase-secreting phenotype that may prime stellate cells for apoptosis.

Connective tissue growth factor (CCN2) induces adhesion of rat activated hepatic stellate cells by binding of its C-terminal domain to integrin alpha(v)beta(3) and heparan sulfate proteoglycan

Connective tissue growth factor (CCN2, also known as CTGF) is a matricellular protein that appears to play an important role in hepatic stellate cell (HSC)-mediated fibrogenesis. After signal peptide cleavage, the full-length CCN2 molecule comprises four structural modules (CCN2(1-4)) and is susceptible to proteolysis by HSC yielding isoforms comprising essentially modules 3 and 4 (CCN2(3-4)) or module 4 alone (CCN2(4)). In this study we show that rat activated HSC are capable of adhesion to all three CCN2 isoforms via the binding of module 4 to integrin alpha(v)beta(3), a process that is dependent on interactions between module 4 and cell surface heparan sulfate proteoglycans (HSPGs). These findings are based on several lines of evidence. First, integrin alpha(v)beta(3) was detected in HSC lysates by immunoprecipitation and Western blot, and CCN2(4)-mediated HSC adhesion was blocked by anti-integrin alpha(v)beta(3) antibody. Second, as assessed by immunoprecipitation and solid phase binding assay, CCN2(4) bound directly to integrin alpha(v)beta(3) in cell-free systems. Third, destruction or inhibition of synthesis of cell surface HSPGs with, respectively, heparinase or sodium chlorate abrogated HSC adhesion to CCN2(4). Fourth, prior occupancy of heparin-binding sites on CCN2(4) with soluble heparin completely blocked HSC adhesion. These findings indicate that integrin alpha(v)beta(3) functions as a co-receptor with HSPGs for CCN2(4)-mediated HSC adhesion. Furthermore, by peptide mapping and site-directed mutagenesis we demonstrated that the sequence IRTPKISKPIKFELSG within CCN2(4) is a unique binding domain for integrin alpha(v)beta(3) that is sufficient to mediate integrin alpha(v)beta(3)- and HSPG-dependent HSC adhesion. These findings offer the possibility of developing novel antifibrotic therapies that target the integrin-binding domain.

Liver extracellular matrix in health and disease

Liver fibrosis is the hallmark of every chronic liver disease. It is also the major factor of morbidity and mortality due to the development of cirrhosis and its complications including hepatocellular carcinoma. But even at the beginning of the process of liver fibrosis and due to the strategic position of the extracellular matrix at the interface between blood flow and epithelial compartment, any quantitative or qualitative modification of extracellular matrix will rapidly affect structure and function of the liver. The development of several animal models of liver fibrosis as well as isolation and cultivation of hepatic stellate cells, the major fibrogenic cell type in the liver, led to the gathering of recent knowledge on the mechanism of liver fibrosis. Activation of hepatic stellate cells is a key event in this process and many details on this finely tuned mechanism are now available. In addition to these experimental data, experience from chronic hepatitis C now allows the development of new concepts and perspectives such as liver fibrosis regression and antifibrotic therapies.

Initial signaling of the fibronectin receptor (alpha5beta1 integrin) in hepatic stellate cells is independent of tyrosine phosphorylation

BACKGROUND/AIMS

Activation of hepatic stellate cells (HSC) plays an integral role in hepatic fibrosis. HSC activation increases fibronectin (alpha(5)beta(1)) receptor expression and interactions between alpha(5)beta(1) and the extracellular matrix increase collagen synthesis. It is unclear how signaling by the alpha(5)beta(1) receptor initiates these changes. We aimed to determine the signaling cascade after alpha(5)beta(1) stimulation in activated HSC.

METHODS

HSC were isolated from male Sprague-Dawley rats. Activated HSC were exposed to beads coated with fibronectin (ligand for alpha(5)beta(1)) or D-polylysine (inert control). HSC were stained with FTC-labeled antibodies against classes of signaling molecules. Tyrosine phosphorylation was blocked using genistein or herbimycin A. The fraction of beads with localized immunostaining (indicating accumulation of signaling protein) was determined.

RESULTS

The majority of cytoskeletal proteins, Src substrates, Src kinases and members of the ERK and JNK signaling molecule families require actin cytoskeletal organization and tyrosine-kinase-mediated phosphorylation to accumulate. Several proteins (e.g. tensin, FAK) accumulated in the absence of tyrosine phosphorylation.

CONCLUSIONS

The alpha(5)beta(1) integrin-ligand interaction induces accumulation of cytoskeletal molecules, activating multiple kinase pathways. Initial integrin signaling by alpha(5)beta(1) are associated with cytoskeletal proteins and are independent of tyrosine phosphorylation. We suggest that there may be cytoskeletal changes that may be targeted to diminish HSC activation.

Basement membrane-like matrix inhibits proliferation and collagen synthesis by activated rat hepatic stellate cells: evidence for matrix-dependent deactivation of stellate cells

During liver fibrosis hepatic stellate cells become activated, transforming into proliferative myofibroblastic cells expressing type I collagen and alpha-smooth muscle actin. They become the major producers of the fibrotic neomatrix in injured liver. This study examines if activated stellate cells are a committed phenotype, or whether they can become deactivated by extracellular matrix. Stellate cells isolated from normal rat liver proliferated and expressed mRNA for activation markers, alpha-smooth muscle actin, type I procollagen and tissue inhibitor of metalloproteinases-1 following 5-7 day culture on plastic, but culture on Matrigel suppressed proliferation and mRNA expression. Activated stellate cells were recovered from plastic by trypsinisation and replated onto plastic, type I collagen films or Matrigel. Cells replated on plastic and type I collagen films proliferated and remained morphologically myofibroblastic, expressing alpha-smooth muscle actin and type I procollagen. However, activated cells replated on Matrigel showed <30% of the proliferative rate of these cells, and this was associated with reduced cellular expression of proliferating cell nuclear antigen and phosphorylation of mitogen-activated protein kinase in response to serum. Activated HSC replated on Matrigel for 3-7 days progressively reduced their expression of mRNA for type I procollagen and alpha-smooth muscle actin and both became undetectable after 7 days. We conclude that basement membrane-like matrix induces deactivation of stellate cells. Deactivation represents an important potential mechanism mediating recovery from liver fibrosis in vivo where type I collagen is removed from the liver and stellate cells might re-acquire contact with their normal basement membrane-like pericellular matrix.

Pulmonary fibrosis: cellular and molecular events

Connective tissue remodeling of the interstitium is an important feature of chronic lung diseases encompassing interstitial inflammatory changes and subsequent pulmonary fibrosis. The early inflammatory phase is usually associated with the release of several cytokines and chemokines by activated resident cells and infiltrating cells which, in turn, help further recruit inflammatory mononuclear cells. Cytokines and growth factors secreted by inflammatory cells and by interstitial cells (fibroblasts and myofibroblasts) play an important role in the fibrogenic phase of pulmonary fibrosis by inducing matrix synthesis. In addition, matrix-degrading enzymes and their inhibitors also contribute to extracellular matrix (ECM) remodeling in pulmonary fibrosis. This review addresses the pathophysiology of wound healing and different phases of pulmonary fibrosis.

Approaches for treatment of liver fibrosis in chronic hepatitis C

In the past 20 years, the elucidation of the mechanisms responsible for liver fibrogenesis has provided many potential targets for antifibrotic treatments. Difficulty has arisen, however, from the fact that fibrogenesis is part of a general beneficial wound healing process. To be successful, an antifibrotic treatment of HCV might need to be delivered selectively to the hepatic site of fibrogenesis or targeted precisely at an HCV-specific regulatory mechanism. It is likely that in the future, besides viral eradication, another treatment goal in chronic HCV infection will be to reverse existing fibrosis, but considerable work is necessary before making this a reality.

Stimulation of Pro-MMP-2 Production and Activation by Native Form of Extracellular Type I Collagen in Cultured Hepatic Stellate Cells

Cultured hepatic stellate cells (HSCs) are known to change their morphology and function with respect to the production of extracellular matrices (ECMs) and matrix metalloproteinases (MMPs) in response to ECM components. We examined the regulatory role of the native form of type I collagen fibrils in pro-MMP-2 production and activation in cultured HSCs. Gelatin zymography of the conditioned media revealed that pro- and active form of MMP-2 was increased in the HSCs cultured on type I collagen gel but not on type I collagen-coated surface, gelatin-coated surface, type IV collagen-coated surface, or Matrigel, suggesting the importance of the native form of type I collagen fibrils in pro-MMP-2 production and activation. The induction of active MMP-2 by extracellular type I collagen was suppressed by the blocking antibody against integrin beta1 subunits, indicating the involvement of integrin signaling in pro-MMP-2 activation. RT-PCR analysis indicated that MMP-2, membrane type-1 MMP (MT1-MMP) and tissue inhibitor of metalloproteinase-2 (TIMP-2) mRNA levels were elevated in HSCs cultured on type I collagen gel. The increased MT1-MMP proteins were localized on the cell surface of HSCs cultured on type I collagen gel. In contrast to the expression of MMP-2, HSCs showed a great decline in MMP-13 expression in HSCs cultured on type I collagen gel. These results indicate that the native fibrillar (polymerized) but not monomeric form of type I collagen induced pro-MMP-2 production and activation through MT1-MMP and TIMP-2 in cultured HSCs, suggesting an important role of HSCs in ECM remodeling in the hepatic perisinusoidal spaces.

Expression of transmembrane 4 superfamily (TM4SF) proteins and their role in hepatic stellate cell motility and wound healing migration

BACKGROUND/AIMS

Migration of activated hepatic stellate cells (HSC) is a key event in the progression of liver fibrosis. Little is known about transmembrane proteins involved in HSC motility. Tetraspanins (TM4SF) have been implicated in cell development, differentiation, motility and tumor cell invasion. We evaluated the expression and function of four TM4SF, namely CD9, CD81, CD63 and CD151, and their involvement in HSC migration, adhesion, and proliferation.

METHODS/RESULTS

All TM4SF investigated were highly expressed at the human HSC surface with different patterns of intracellular distribution. Monoclonal antibodies directed against the four TM4SF inhibited HSC migration induced by extracellular matrix proteins in both wound healing and haptotaxis assays. This inhibition was independent of the ECM substrates employed (collagen type I or IV, laminin), and was comparable to that obtained by incubating the cells with an anti-beta1 blocking mAb. Importantly, cell adhesion was unaffected by the incubation with the same antibodies. Co-immunoprecipitation studies revealed different patterns of association between the four TM4SF studied and beta1 integrin. Finally, anti-TM4SF antibodies did not affect HSC growth.

CONCLUSIONS

These findings provide the first characterization of tetraspanins expression and of their role in HSC migration, a key event in liver tissue wound healing and fibrogenesis.

Dystroglycan expression in hepatic stellate cells: role in liver fibrosis

Dystroglycan is a membrane component of the dystrophin-glycoprotein transmembrane complex. Its expression is required for the spatial organization of laminin on the cell surface and for basement membrane assembly. In view of the constitution of a perisinusoidal basement membrane during liver fibrosis, we studied dystroglycan expression in liver fibrosis. Dystroglycan mRNA and protein expression were investigated by immunofluorescence, Western blot, and quantitative RT-PCR (TaqMan) in normal human and rat liver and in isolated rat hepatic stellate cells. On Western blot, a 43-kd band corresponding to beta-dystroglycan was observed in protein extracted from normal and fibrotic human and rat livers. The specific 43-kd protein was also detected in lysates from rat hepatic stellate cells but not from hepatocytes. By immunofluorescence, patchy deposits of beta-dystroglycan were detected on membrane of hepatic stellate cells in culture. On Western blot and quantitative RT-PCR, an up-regulation of beta-dystroglycan was shown during spontaneous activation of hepatic stellate cells in culture. Direct evidence for the role of dystroglycan in laminin-hepatic stellate cell interaction was shown because specific antibody directed against alpha-dystroglycan inhibited partially hepatic stellate cell adhesion on laminin-coated plates. This mechanism was calcium dependent because EDTA inhibited cell/laminin adhesion, an effect reversed by addition of Ca(2+). This study shows that dystroglycan is expressed on the membrane of hepatic stellate cells and is up-regulated during liver fibrosis. Dystroglycan interaction with laminin should be implicated in the concentration of pericellular laminin and in the constitution of a perisinusoidal basement membrane during liver fibrosis.

The motility of hepatic Ito cells can be acquired by their myofibroblastic transformation

We investigated the relationship between the motility of hepatic Ito cells and their myofibroblastic transformation in cultures. Ito cells were freshly isolated from rat liver and cultured in a 10% FBS-supplemented medium. On day 2 after beginning the culture, transmission electron microscopy and oil red O staining showed that Ito cells possessed numerous lipid droplets but not actin filaments in the cytoplasm. On day 8, actin filaments were abundantly found in the cytoplasm, whereas lipid droplets dramatically decreased in number. Western blot analysis also demonstrated that protein levels of alpha-smooth muscle actin in the cell markedly increased with time, but no obvious change was detected in those of desmin and tubulin beta. In Boyden's chamber assay, the migration of Ito cells, which was induced by platelet-derived growth factor-BB (PDGF-BB), was activated in a time-dependent manner. This migration of transformed Ito cells was independent of the degree of their adhesion to various substances of the extracellular matrix. Among these molecules, laminin showed the highest effect upon the migratory activity. The migration of transformed Ito cells on laminin was completely inhibited by cytochalasin D, colchicine, or taxol. Furthermore, their adhesion to the matrix was also decreased by cytochalasin D or colchicine, but not by taxol. These findings indicate that the motility of Ito cells is acquired in conjunction with their myofibroblastic transformation, which is accompanied by morphological changes with a new organization of actin filaments. The results also suggest that microtubules as well as the extracellular matrix are deeply associated with the motility of Ito cells.

[Mechanisms of hepatic fibrogenesis]

Hepatic fibrosis is a scaring process leading to cirrhosis, a major complication of numerous chronic liver diseases. Hepatic stellate cells play a central role in the fibrotic process. After parenchymal or biliary injury, cytokines and growth factors allow the recruitment, proliferation, and activation, of stellate cells toward myofibroblasts, which secrete the extracellular matrix. Fibrosis, resulting from the failure of the balance between synthesis and degradation of extracellular matrix, is an evolutive and potentially reversible process. Histological examination is the main investigation to quantify fibrosis. Serological tests are warranted to allow a non invasive follow up of patients. Development of antifibrotic therapies should soon permit to slow down the evolution toward cirrhosis, limiting the needs for hepatic transplantation.