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

DDR2 receptor promotes MMP-2-mediated proliferation and invasion by hepatic stellate cells

Type I collagen provokes activation of hepatic stellate cells during liver injury through mechanisms that have been unclear. Here, we tested the role of the discoidin domain tyrosine kinase receptor 2 (DDR2), which signals in response to type I collagen, in this pathway. DDR2 mRNA and protein are induced in stellate cells activated by primary culture or in vivo during liver injury. The receptor becomes tyrosine phosphorylated in response to either endogenous or exogenous type I collagen, whereas its expression is downregulated during cellular quiescence induced by growth on Matrigel. We developed stellate cell lines stably overexpressing either wild-type DDR2, a constitutively active chimeric DDR2 receptor (Fc-DDR2), a truncated receptor expressing the extracellular domain, or a kinase-dead DDR2 Cells overexpressing DDR2 showed enhanced proliferation and invasion through Matrigel, activities that were directly related to increased expression of active matrix metalloproteinase 2 (MMP-2). These data show that DDR2 is induced during stellate cell activation and implicate the phosphorylated receptor as a mediator of MMP-2 release and growth stimulation in response to type I collagen. Moreover, type I collagen-dependent upregulation of DDR2 expression establishes a positive feedback loop in activated stellate cells, leading to further proliferation and enhanced invasive activity.

DDR2 receptor promotes MMP-2-mediated proliferation and invasion by hepatic stellate cells

Type I collagen provokes activation of hepatic stellate cells during liver injury through mechanisms that have been unclear. Here, we tested the role of the discoidin domain tyrosine kinase receptor 2 (DDR2), which signals in response to type I collagen, in this pathway. DDR2 mRNA and protein are induced in stellate cells activated by primary culture or in vivo during liver injury. The receptor becomes tyrosine phosphorylated in response to either endogenous or exogenous type I collagen, whereas its expression is downregulated during cellular quiescence induced by growth on Matrigel. We developed stellate cell lines stably overexpressing either wild-type DDR2, a constitutively active chimeric DDR2 receptor (Fc-DDR2), a truncated receptor expressing the extracellular domain, or a kinase-dead DDR2 Cells overexpressing DDR2 showed enhanced proliferation and invasion through Matrigel, activities that were directly related to increased expression of active matrix metalloproteinase 2 (MMP-2). These data show that DDR2 is induced during stellate cell activation and implicate the phosphorylated receptor as a mediator of MMP-2 release and growth stimulation in response to type I collagen. Moreover, type I collagen-dependent upregulation of DDR2 expression establishes a positive feedback loop in activated stellate cells, leading to further proliferation and enhanced invasive activity.

Integrin up-regulation in chronic liver disease: relationship with inflammation and fibrosis in chronic hepatitis C

In 94 patients with chronic hepatitis C, the pattern of integrin expression was correlated with firstly, the histological activity index, necro-inflammatory grade, and stage of fibrosis; secondly, the expression of inflammatory markers including ICAM-1; and thirdly, the extent and intensity of laminin deposition in the perisinusoidal matrix. Immunohistochemical results were evaluated according to a semi-quantitative scoring system or by image analysis. Increased beta1 expression was observed in 88.2% of cases. The expression of alpha1 and alpha5 was increased in 55% and 58.5% of cases, respectively. alpha6 chain was detected in 78.7% of cases. There were no statistically significant differences in integrin expression level according to Knodell's score, inflammatory grade, or stage of fibrosis. ICAM-1 expression was higher in patients with high scores for beta1 expression, but the differences were not statistically significant. There were significantly more patients with high scores for beta1 expression among those with continuous perisinusoidal deposition of laminin. Moreover, a close statistical correlation was observed between alpha6 induction and perisinusoidal laminin deposition (p<0.001). The results suggest that integrin up-regulation in chronic hepatitis C is more closely related to the fibrotic process than to the inflammatory lesions. This reinforces the idea that integrin induction in chronic liver disease is part of a coordinated process involved in the progression of liver fibrosis.

Laminin deposition to type IV collagen enhances haptotaxis, chemokinesis, and adhesion of hepatoma cells through beta1-integrins

BACKGROUND/AIMS

In hepatocellular carcinoma, laminin deposition to type IV collagen along the sinusoids is observed with the development of arterial network, coinciding with intrahepatic metastasis. We investigated the influence of laminin deposition to type IV collagen on hepatoma cell adhesion, motility and secretion of matrix metalloproteinases (MMPs), which are indispensable behaviors for tumor metastasis.

METHODS

Hepatoma cell lines (KYN-1, -2 and -3) were used. The expression of integrin subunit mRNAs in hepatoma cells was confirmed by RT-PCR. The influence of laminin addition to type IV collagen on the adhesion, chemokinesis, and migration of KYN-1, -2 and -3 was evaluated by the haptotactic migration, phagokinetic track motility, and cell adhesion assays. The effects of integrin subunits on these activities were evaluated using the function-blocking antibodies for integrins. Phosphorylation of MEK1/2 and secretion of MMPs were investigated by Western blotting and gelatin zymography.

RESULTS

Integrin alpha1, alpha2, alpha3, alpha6 and beta1 subunit mRNAs were detected. The combination of type IV collagen and laminin enhanced the migration, chemokinesis, and adhesion of hepatoma cells compared to that of type IV collagen when used alone. The enhanced activity was significantly suppressed by function-blocking antibodies for integrin alpha1, alpha2, alpha3, alpha6 and beta1 subunits. Hepatoma cells cultured on the combination of type IV collagen and laminin showed phosphorylation of MEK1/2 and increased secretion of MMPs.

CONCLUSIONS

The addition of laminin to type IV collagen enhances hepatoma cell adhesion and motility through beta1-integrins.

Comparative evaluation of gene delivery devices in primary cultures of rat hepatic stellate cells and rat myofibroblasts

BACKGROUND

The hepatic stellate cell is the primary cell type responsible for the excessive formation and deposition of connective tissue elements during the development of hepatic fibrosis in chronically injured liver. Culturing quiescent hepatic stellate cells on plastic causes spontaneous activation leading to a myofibroblastic phenotype similar to that seen in vivo. This provides a simple model system for studying activation and transdifferentiation of these cells. The introduction of exogenous DNA into these cells is discussed controversially mainly due to the lack of systematic analysis. Therefore, we examined comparatively five nonviral, lipid-mediated gene transfer methods and adenoviral based infection, as potential tools for efficient delivery of DNA to rat hepatic stellate cells and their transdifferentiated counterpart, i.e. myofibroblasts. Transfection conditions were determined using enhanced green fluorescent protein as a reporter expressed under the transcriptional control of the human cytomegalovirus immediate early gene 1 promoter/enhancer.

RESULTS

With the use of chemically enhanced transfection methods, the highest relative efficiency was obtained with FuGENE6 gene mediated DNA transfer. Quantitative evaluation of representative transfection experiments by flow cytometry revealed that approximately 6% of the rat hepatic stellate cells were transfected. None of the transfection methods tested was able to mediate gene delivery to rat myofibroblasts. To analyze if rat hepatic stellate cells and myofibroblasts are susceptible to adenoviral infection, we have inserted the transgenic expression cassette into a recombinant adenoviral type 5 genome as replacement for the E1 region. Viral particles of this replication-deficient Ad5-based reporter are able to infect 100% of rat hepatic stellate cells and myofibroblasts, respectively.

CONCLUSIONS

Our results indicate that FuGENE6-based methods may be optimized sufficiently to offer a feasible approach for gene transfer into rat hepatic stellate cells. The data further demonstrate that adenoviral mediated transfer is a promising approach for gene delivery to these hepatic cells.

Adhesion between cells and extracellular matrix with special reference to hepatic stellate cell adhesion to three-dimensional collagen fibers

Hepatic stellate cells are located in the perisinusoidal space (space of Disse), and extend their dendritic, thin membranous processes and fine fibrillar processes into this space. The stellate cells coexist with a three-dimensional extracellular matrix (ECM) in the perisinusoidal space. In turn the three-dimensional structure of the ECM regulates the proliferation, morphology, and functions of the stellate cell. In this review, the morphology of sites of adhesion between hepatic stellate cells and extracellular matrix is described. Hepatic stellate cells cultured in polystyrene dishes spread well, whereas the cells cultured on or in type I collagen gel become slender and elongate their long cellular processes which adhere directly to the collagen fibers. Cells in type I collagen gel form a large number of adhesive structures, each adhesive area forming a face but not a point. Adhesion molecules, integrins, for the ECM are localized on the cell surface. Elongation of the cellular processes occurs via integrin-binding to type I collagen fibers. The signal transduction mechanism, including protein and phosphatidylinositol phosphorylation, is critical to induce and sustain the cellular processes. Information on the three-dimensional structures of ECM is transmitted via three-dimensional adhesive structures containing the integrins.

Hepatic capillariasis in rats: a new model for testing antifibrotic drugs

Rats infected with the helminth Capillaria hepatica regularly develop septal hepatic fibrosis that may progress to cirrhosis in a relatively short time. Because of such characteristics, this experimental model was selected for testing drugs exhibiting antifibrosis potential, such as pentoxifylline, gadolinium chloride and vitamin A. Hepatic fibrosis was qualitatively and quantitatively evaluated in liver samples obtained by partial hepatectomy and at autopsy. The material was submitted to histological, biochemical and morphometric methods. A statistically significant reduction of fibrosis was obtained with pentoxifylline when administered intraperitoneally rather than intravenously. Gadolinium chloride showed moderate activity when administered prophylactically (before fibrosis had started), but showed a poor effect when fibrosis was well advanced. No modification of fibrosis was seen after vitamin A administration. Hydroxyproline content was correlated with morphometric measurements. The model appears to be adequate, since few animals die of the infection, fibrosis develops regularly in all animals, and the effects of different antifibrotic drugs and administration protocols can be easily detected.

Liver fibrosis

Knowledge on the development and progression of liver fibrosis has grown exponentially in the past decade. At present, liver fibrogenesis is referred to as a dynamic process involving complex cellular and molecular mechanisms, resulting from the chronic activation of the tissue repair mechanisms that follows reiterated liver tissue injury. The identification and characterization of the cell types and of the different mediators involved in this process has allowed a "re-visitation" of several issues related to liver cirrhosis and its immediate consequences. Among these, evaluation of the relationships occurring between fibrogenesis and portal hypertension, cholestasis and the development of hepatocellular carcinoma, represent some of the hottest areas of research in this field of hepatology. The elucidation of many of the cellular and molecular mechanisms responsible for the progression of liver fibrosis has provided a sound basis for the development of pharmacological strategies able to modulate this important pathophysiological process.

Nitrovasodilators inhibit platelet-derived growth factor-induced proliferation and migration of activated human hepatic stellate cells

BACKGROUND & AIMS

Nitrovasodilators have been proposed for the treatment of portal hypertension alone or in combination with beta-blockers. In addition to their vasodilatory properties, nitric oxide (NO) donors may exert direct antifibrogenic properties. We evaluated the effect of nitroglycerin (NTG) and S-nitroso-N-acetyl penicillamine (SNAP) on the mitogenic and chemotactic properties of platelet-derived growth factor (PDGF)-BB and the modulation of the relative intracellular signaling pathways in fully activated human hepatic stellate cells (HSCs), a cell type that plays an active role in liver fibrogenesis and portal hypertension.

METHODS & RESULTS

Both NTG and SNAP induced a dose-dependent decrease in PDGF-induced DNA synthesis and cell migration, which was associated with a decrease in PDGF-induced intracellular Ca(2+) increase and extracellular signal-regulated kinase (ERK) activity. These effects were not related to activation of the classic soluble guanylate cyclase (sGC)/guanosine 3',5'-cyclic monophosphate pathway; accordingly, Western blot analysis of HSC lysates revealed the absence of the alpha(1)beta(1) ubiquitous subunits of sGC, whereas they were detectable in quiescent HSCs, freshly isolated from normal human liver. Conversely, both NTG and SNAP induced a more than 10-20-fold increase in prostaglandin E(2) in cell supernatants within 1 minute, associated with an increase in intracellular adenosine 3',5'-cyclic monophosphate levels. Accordingly, the inhibitory effects of NO donors on PDGF action and signaling were eliminated after preincubation with ibuprofen.

CONCLUSIONS

These results suggest that NO donors may exert a direct antifibrogenic action by inhibiting proliferation, motility, and contractility of HSCs in addition to a reduction of fibrillar extracellular matrix accumulation.

The Ras antagonist, farnesylthiosalicylic acid (FTS), inhibits experimentally-induced liver cirrhosis in rats

BACKGROUND/AIMS

Protooncogenes may play an important role, not only in carcinogenesis, but also in the regulation of normal cellular proliferation and differentiation. Several studies have indicated increased expression of the Ras protooncogenes in the liver in animal models and in patients with liver cirrhosis. The aim of the present study was to examine whether a synthetic Ras antagonist, S-farnesylthiosalicylic acid (FTS), which specifically dislodges Ras from the membrane of Ras-transformed fibroblasts (EJ cells), can prevent experimentally-induced liver cirrhosis in rats.

METHODS

Cirrhosis was induced in male Wistar rats by intraperitoneal administration of thioacetamide (200 mg/kg twice weekly for 12 weeks). The Ras antagonist, farnesylthiosalicylic acid (FTS, 5 mg/kg), was administered during the study period 3 times a week. Ras expression in the liver was determined by Western blot analysis with pan anti-Ras antibodies and by immunohistochemistry.

RESULTS

Rats treated with thioacetamide and the Ras antagonist, farnesylthiosalicylic acid (FTS), for 12 weeks had lower histopathologic scores of fibrosis and inflammation (p-values of 0.003 and 0.008, respectively) than those treated with thioacetamide only. There were no differences between the histopathologic scores in vehicle (control) and in Ras-antagonist (FTS) only treatments. Analysis of hepatic hydroxyproline levels from the two thioacetamide-treated groups and controls confirmed the histopathologic scores (7.7+/-0.9 mg/g protein in the TAA-treated vs. 3.8+/-0.5 mg/g protein in the TAA+FTS treated group, p = 0.007). Ras levels, determined by Western blot analysis, were markedly increased in the livers treated with TAA (17-fold over control) and significantly decreased (by about 70%) in the livers of rats treated with TAA and FTS. Studies in isolated human hepatic stellate cells demonstrated that FTS inhibited both DNA synthesis and migration of those cells (p<0.05).

CONCLUSION

These results indicate that inhibition of Ras expression in the liver during fibrogenesis, prevents the development of experimentally-induced hepatic cirrhosis.

Integrin alpha6beta1 plays a significant role in the attachment of hepatoma cells to laminin

BACKGROUND/AIMS

Tumor invasion and metastasis consist of a series of complex events. During this process, the ability of tumor cells to adhere to laminin, a major component of basement membranes, is required at various steps. The expression of laminin-binding integrins and the extent of tumor metastasis and progression appear to be related. In hepatocellular carcinoma, increased expression of laminin-binding integrins is observed. However, little is known concerning the possible functional interactions between laminin-binding integrins and laminin. Therefore, we investigated the participation of laminin-binding integrins in the attachment of hepatoma cells to laminin.

METHODS

Human hepatoma cell lines (KIM-1, KYN-1, 2) were used. We investigated the expression of integrin alpha1, alpha2, alpha3, alpha6, beta1, and beta4 subunits on hepatoma cells by immunocytochemical and flow cytometric analysis. Participation of these integrin subunits in the attachment of hepatoma cells to laminin was evaluated by an inhibition of cell adhesion assay.

RESULTS

Integrin alpha1, alpha2, alpha3, alpha6 and beta1 subunits were expressed at the marginal areas of hepatoma cells, while the integrin beta4 subunit was scarcely detected. Laminin promoted the attachment of hepatoma cells in a dose-dependent manner. Although anti-integrin alpha1, alpha2, beta3 and beta4 subunit antibodies did not inhibit cell attachment to laminin, anti-integrin alpha6 and beta1 subunit antibodies inhibited the attachment by 50% or more.

CONCLUSIONS

These findings indicate that integrin alpha6beta1 is very important in the attachment of hepatoma cells to laminin, suggesting the participation of this integrin in metastasis and invasion of hepatoma cells.

Intracellular signaling pathways in stellate cell activation

Pathological fibrogenesis in the liver is mediated by activated stellate cells. These cells have a myofibroblastic phenotype with the ability to proliferate and synthesize large quantities of extracellular matrix components. A number of factors have been proposed to initiate and perpetuate the fibrogenic process in stellate cells, including inflammatory cytokines, alterations in the extracellular matrix, growth factors, and oxidative stress. Some recent research has focused on the intracellular signaling pathways that are stimulated by these factors in stellate cells, including mitogen-activated protein kinases, phosphatidylinositol 3-kinase, focal adhesion kinase, and protein kinase C. This paper will summarize the experimental evidence that implicates these pathways in stellate cell activation, focusing on the effects of exposure to platelet-derived growth factor, tumor necrosis factor-alpha, and fibronectin. Implications for alcohol-induced hepatic fibrosis and future directions for research will also be discussed.

MMP2 activation by collagen I and concanavalin A in cultured human hepatic stellate cells

Fibrosis occurs in most chronic liver injuries and results from changes in the balance between synthesis and degradation of extracellular matrix components. In fibrotic livers, there is a markedly increased activity of matrix metalloproteinase 2 (MMP2), a major enzyme involved in extracellular matrix remodeling. We have previously shown that hepatic stellate cells secrete latent MMP2 and that MMP2 activation occurs in coculture of hepatic stellate cells and hepatocytes concomitantly with matrix deposition. In the present work we investigated the effects of various extracellular matrix components and concanavalin A, an inducer of immune-mediated liver injuries, on MMP2 activation in cultured human hepatic stellate cells. Collagen I induced a dose-dependent MMP2 activation, which was not blocked by both actinomycin and cycloheximide. Collagen VI, laminin, and a reconstituted basement membrane (matrigel) were ineffective in inducing activation. Specific antibodies against the subunits of alpha2beta1 integrins, the major collagen I receptor, induced partial inhibition of MMP2 activation. Treatment of cells with concanavalin A resulted in a marked activation of MMP2 that correlated with the proteolytic processing of MT1-MMP, the MMP2 activator, from a Mr=60 kd toward a Mr=43 kd polypeptide. Actinomycin and cycloheximide inhibited the MMP2 activation induced by concanavalin A. Recombinant tissue inhibitor of metalloproteinase-2 and the MMP inhibitor BB-3103, but not PMSF, blocked MMP2 activation induced by collagen I or concanavalin A, and MT1-MMP processing to its Mr-43 kd form. These results suggest that the accumulation of collagen I may specifically contribute to the remodeling of extracellular matrix in fibrotic livers by inducing MMP2 activation.

Liver fibrogenesis and the role of hepatic stellate cells: new insights and prospects for therapy

Hepatic fibrosis is a wound-healing response to chronic liver injury, which if persistent leads to cirrhosis and liver failure. Exciting progress has been made in understanding the mechanisms of hepatic fibrosis. Major advances include: (i) characterization of the components of extracellular matrix (ECM) in normal and fibrotic liver; (ii) identification of hepatic stellate cells as the primary source of ECM in liver fibrosis; (iii) elucidation of key cytokines, their cellular sources, modes of regulation, and signalling pathways involved in liver fibrogenesis; (iv) characterization of key matrix proteases and their inhibitors; (v) identification of apoptotic mediators in stellate cells and exploration of their roles during the resolution of liver injury. These advances have helped delineate a more comprehensive picture of liver fibrosis in which the central event is the activation of stellate cells, a transformation from quiescent vitamin A-rich cells to proliferative, fibrogenic and contractile myofibroblasts. The progress in understanding fibrogenic mechanisms brings the development of effective therapies closer to reality. In the future, targeting of stellate cells and fibrogenic mediators will be a mainstay of antifibrotic therapy. Points of therapeutic intervention may include: (i) removing the injurious stimuli; (ii) suppressing hepatic inflammation; (iii) down-regulating stellate cell activation; and (iv) promoting matrix degradation. The future prospects for effective antifibrotic treatment are more promising than ever for the millions of patients with chronic liver disease worldwide.

Role of Rho small GTP binding protein in the regulation of actin cytoskeleton in hepatic stellate cells

BACKGROUND/AIMS

In the fibrotic response to liver injury, hepatic stellate cells are activated, leading to the myofibroblastic cell shape, with actin cytoskeletal reorganization and increased extracellular matrix production. The reorganization of actin cytoskeleton suggests that the small GTP binding protein Rho might modulate the process of this myofibroblastic change. The aim of this study was to investigate the role of Rho in the phenotypic changes of hepatic stellate cells.

METHODS

The phenotypic changes were investigated by the overexpression of Rho regulator, Rho GDI or dominant negative mutant of Rho in mouse hepatic stellate cell line, GRX cells. In activated rat hepatic stellate cells, the effects of microinjection of Botulinus toxin C3, which is the specific inhibitor for Rho, were analyzed. Furthermore, the effect of C3 on the type I collagen accumulation in hepatic stellate cells was investigated.

RESULTS

Overexpression of Rho GDI or the dominant negative mutant of Rho caused the shrinkage cell shape and suppressed stress fiber formation. Microinjection of toxin C3 caused a markedly distorted cell shape and the disappearance of stress fibers in rat stellate cells. In addition, C3 strongly suppressed collagen accumulation in activated stellate cells.

CONCLUSIONS

These results suggest that Rho regulates the actin cytoskeletal reorganization, and may be implicated in the collagen accumulation in activated stellate cells. These findings provide evidence for the role of Rho in the myofibroblastic phenotype in hepatic stellate cells.

Induction of apoptosis in rat hepatic stellate cells by disruption of integrin-mediated cell adhesion

Cell-matrix adhesion is recognized as a physiologic determinant of cell growth and survival. Integrin occupancy seems to be a primary role. We sought to investigate the signal transduction pathways for integrin effects on cell survival in hepatic stellate cells. Integrin function was antagonized by the soluble integrin recognition sequence pentapeptide Gly-Arg-Gly-Asp-Ser (GRGDS) in primary cultures of rat hepatic stellate cells. Integrin antagonism with GRGDS peptide induced apoptosis. To investigate signal transduction mechanisms for the effect of integrins on cell survival in hepatic stellate cells, the expression of p53, Bcl-2, and Bax was analyzed. Incubation with soluble GRGDS peptide resulted in increased expression of p53 and decreased the Bcl-2/Bax ratio. In conclusion, these findings indicate that the abrogation of cell adhesion with soluble GRGDS peptide plays a critical role in the induction of apoptosis of rat hepatic stellate cells.

Liver fibrosis

Knowledge on the development and progression of liver fibrosis has grown exponentially in the past decade. At present, liver fibrogenesis is referred to as a dynamic process involving complex cellular and molecular mechanisms, resulting from the chronic activation of the tissue repair mechanisms that follows reiterated liver tissue injury. The identification and characterization of the cell types and of the different mediators involved in this process has allowed a "re-visitation" of several issues related to liver cirrhosis and its immediate consequences. Among these, evaluation of the relationships occurring between fibrogenesis and portal hypertension, cholestasis and the development of hepatocellular carcinoma, represent some of the hottest areas of research in this field of hepatology. The elucidation of many of the cellular and molecular mechanisms responsible for the progression of liver fibrosis has provided a sound basis for the development of pharmacological strategies able to modulate this important pathophysiological process.

Inhibition of integrin signaling with Arg-Gly-Asp motifs in rat hepatic stellate cells

BACKGROUND/AIMS

Activation of hepatic stellate cells plays a key role in liver fibrogenesis. Disruption of normal hepatic stellate cell-matrix interactions may contribute to this process. However, little is known about the molecular events leading from integrin-extracellular matrix interaction to hepatic stellate cell function. Therefore, we investigated the role of integrin signaling in tyrosine phosphorylation of focal adhesion kinase and cytoskeletal assembly in rat hepatic stellate cells using soluble Arg-Gly-Asp containing peptides.

METHODS

Hepatic stellate cells were isolated from normal rat livers. Integrin alpha5beta1 expression in hepatic stellate cells was analyzed by immunoprecipitation and immunocytochemistry. The cytoskeletal assembly and tyrosine phosphorylation of focal adhesion kinase were determined by immunocytochemistry and immunoblotting. We also analyzed the effect of Arg-Gly-Asp containing peptides on the expression of smooth muscle alpha actin by immunocytochemistry and immunoblotting.

RESULTS

We identified integrin alpha5beta1 in rat hepatic stellate cells. Stress fiber formation and cell shape were different when hepatic stellate cells were plated on various extracellular matrix components. Treatment of hepatic stellate cells with soluble Arg-Gly-Asp peptides diminished the adhesion-induced tyrosine phosphorylation of focal adhesion kinase and inhibited the formation of stress fibers. The peptides also reduced the expression of smooth muscle alpha actin.

CONCLUSIONS

Our results demonstrate that adhesion to extracellular matrix induces tyrosine phosphorylation of focal adhesion kinase and promotes actin stress fiber formation and focal adhesion assembly in rat hepatic stellate cells, and that these events are disturbed by soluble Arg-Gly-Asp peptides.

Rho directs activation-associated changes in rat hepatic stellate cell morphology via regulation of the actin cytoskeleton

Hepatic stellate cell activation, thought to play a key role in fibrosis of the liver, is characterized by changes in cellular morphology. The intracellular signals regulating morphological alterations associated with stellate cell activation are uncertain. The ras-like guanosine triphosphate-binding protein, rho, has recently emerged as an important regulator of the actin cytoskeleton, and consequently cell morphology. The aim of this study was to test the hypothesis that rho signaling pathways direct activation-associated morphological changes in stellate cells by regulating the actin cytoskeleton. The morphology and actin cytoskeleton of primary rat hepatic stellate cells were studied with phase contrast, differential interference contrast, and epifluorescence microscopy. Immunohistochemistry and immunoblot analysis were used to examine rho expression and activity, respectively. Quiescent and activated stellate cells were investigated in the absence and presence of C3 transferase, a bacterial toxin that specifically inhibits rho. Stellate cell activation was characterized by the development of prominent intracellular fibers, and the loss of dendrite-like processes and perinuclear retinoid droplets. Moreover, activation was accompanied by the formation of prominent actin stress fibers and focal adhesions. Both rho expression and activity were demonstrated in stellate cells. C3 transferase blocked and reversed, both activation-associated morphological alterations and activation-associated changes in the actin cytoskeleton, in quiescent and activated stellate cells, respectively. These results indicate that rho directs activation-associated changes in rat hepatic stellate cell morphology via regulation of the actin cytoskeleton.

Knockout of alpha6 beta1-integrin expression reverses the transformed phenotype of hepatocarcinoma cells

BACKGROUND & AIMS

Hepatocellular carcinoma is a common complication in liver cirrhosis. The integrin alpha6 beta1, a receptor for the laminin family of extracellular matrix proteins, has been found to be overexpressed in hepatocarcinoma. In an effort to further characterize the involvement of alpha6 beta1-integrin in hepatocarcinoma progression and to study alpha6 beta1-mediated functions, a human hepatocarcinoma cell line, HepG2, that express high surface levels of alpha6 beta1 and uses only this integrin to mediate adhesion on laminin was identified.

METHODS

To assess the role of alpha6 beta1 in these cells, a cytoplasmic domain deletion mutant of the beta4-integrin subunit by complementary DNA transfection was expressed. The expression of the mutant beta4 subunit in association with endogenous alpha6 showed a dominant-negative effect on alpha6 beta1 expression.

RESULTS

Stable transfectants of HepG2 that expressed the mutant beta4 subunit showed a reduced ability to adhere and migrate on laminin matrices and to invade Matrigel. Furthermore, transfected cells showed significantly lower growth rates and reduced anchorage-independent growth compared with mock-transfected cells.

CONCLUSIONS

These findings on the expression and function of alpha6 beta1 in hepatocarcinoma cells emphasize the potential contribution of this laminin receptor in the neoplastic transformation of hepatocytes.

Morphology of sites of adhesion between hepatic stellate cells (vitamin A-storing cells) and a three-dimensional extracellular matrix

BACKGROUND

Hepatic stellate cells lie in the perisinusoidal space in a three-dimensionally distributed extracellular matrix (ECM). This three-dimensional structure of the ECM regulates the proliferation, morphology, and functions of the stellate cell. To investigate how the three-dimensional structure of ECM regulates behavior of the cells, we cultured stellate cells two- or three-dimensionally and examined the morphology of the cells in both cases as well as the localization of cell-surface adhesion molecules specific for the ECM.

METHODS

Isolated rat stellate cells and human stellate cells were cultured in Dulbecco's modified Eagle's medium. Rat stellate cells were cultured in non-coated polystyrene culture dishes, or on or in type I collagen gels. The morphology of cell-ECM adhesion was examined under transmission and scanning electron microscopes. Localization of integrin alpha2 and integrin beta1 in human stellate cells was examined by immunoelectron microscopy. Immunostaining was performed with a mouse monoclonal anti-human integrin alpha2 or integrin beta1 antibody and goat anti-mouse IgG coupled with 10-nm immunogold.

RESULTS

Hepatic stellate cells cultured in polystyrene dishes spread well. However, the cells cultured on or in the type I collagen gel became slender. The cells extended long cellular processes onto or into the gel. The cellular processes were entangled three-dimensionally with the type I collagen fibers and directly adhered to these fibers. The cells inoculated in type I collagen gels formed a large number of adhesive structures that resembled focal adhesions. These adhesive structures were distributed not only on the lower side but also on the upper side of both the cell bodies and cellular processes. Moreover, each adhesive area formed a face but not a point. Integrin alpha2 and integrin beta1 were detected on the surfaces of cell bodies, cellular processes, and microprojections.

CONCLUSIONS

The cells cultured in type I collagen gel develop a three-dimensional adhesive structure.

Signal transduction in hepatic stellate cells

Hepatic stellate cells (HSC) are presently regarded as one of the key cell types involved in the progression of liver fibrosis and in the related pathophysiological and clinical complications. Following acute or chronic liver tissue damage, HSC undergo a process of activation towards a phenotype characterised by increased proliferation, motility, contractility and synthesis of extracellular matrix (ECM) components. Several factors have been shown to play a key role in the promotion of the full-blown picture of activated HSC. These include extensive changes in the composition and organisation of the ECM, the secretion of several growth factors, cytokines, chemokines, products of oxidative stress and other soluble factors. It is evident that each cellular response to extracellular stimuli must be framed in a scenario where different forces modulate one another and result in a prevalent biological effect. Along these lines, the identification and characterisation of intracellular signalling pathways activated by different stimuli in HSC represent a mandatory step. In this review article we have made an attempt to summarise recent acquisitions to our knowledge of the involvement of different intracellular signalling pathways in key aspects of HSC biology.

The role of alpha1beta1 integrin in wound contraction. A quantitative analysis of liver myofibroblasts in vivo and in primary culture

An unresolved question in wound contraction concerns the identity of integrins mediating the attachment of tissue myofibroblasts to matrix in the injury site. Previous studies with cell lines have focussed on alpha1beta1 and alpha2beta1, the principal collagen-binding integrins, but have yielded conflicting data. We have examined this issue in wound healing in the liver, isolating the myofibroblast population (activated stellate cells) and quantitating expression of the alpha1 and alpha2 integrin subunits during the in vivo injury. Normal stellate cells displayed alpha1 but no detectable alpha2. During injury, alpha1 expression was maintained; alpha2 became detectable at the mRNA level but at all times was <8% of alpha1 mRNA. Contraction of collagen lattices, studied with 24-h cultured cells and initiated by endothelin 1, was blocked 70% by anti-alpha1 and 30% by anti-alpha2 (both significant, p < 0.05). The inhibition by anti-alpha2, which was unexpected, was attributable to culture-induced change in integrin expression; both the mRNA and protein for alpha2 increased strikingly within 24 h of plating stellate cells on a collagen gel. We conclude that alpha1beta1 is the sole integrin utilized by contracting myofibroblasts in vivo. Although alpha2beta1 is capable of mediating contraction, its expression by myofibroblasts occurs largely, if not exclusively, in response to culture.

Fibronectin and cytokines increase JNK, ERK, AP-1 activity, and transin gene expression in rat hepatic stellate cells

Cytokines, growth factors, and alterations in the extracellular matrix composition may play a role in maintaining hepatic stellate cells (HSC) in the activated state that is responsible for hepatic fibrogenesis. However, the signal transduction pathways that are stimulated by these factors in HSC remain to be fully elucidated. Recent evidence indicates that the mitogen-activated protein kinase (MAPK) family, including c-Jun NH2-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK), plays an important role in the cellular response to stress. The aims of this study were to investigate whether fibronectin (FN) or the inflammatory cytokines interleukin-1alpha (IL-1alpha) and tumor necrosis factor-alpha (TNF-alpha) activate JNK, ERK, and AP-1 activity in HSC and induce the gene expression of the matrix metalloproteinase transin. Treatment of HSC with FN resulted in an up to 4.5-fold increase in ERK activity and a 2.1-fold increase in JNK activity. IL-1alpha and TNF-alpha produced up to a fourfold increase in JNK activity and a twofold increase in ERK activity. We then compared the effects of FN, IL-1alpha, and TNF-alpha on AP-1 activity and metalloproteinase mRNA induction. All three compounds increased AP-1 binding and promoter activity, and transin mRNA levels were increased 1.8-fold by FN, 2.2-fold by IL-1alpha, and 2.8-fold by TNF-alpha. Therefore, FN and inflammatory cytokines increase MAPK activity, stimulate AP-1 activity, and increase transin gene expression in HSC. Signal transduction pathways involving the MAPK family may play an important role in the regulation of matrix metalloproteinase expression by cytokines and FN in HSC.

Integrin-mediated stimulation of monocyte chemotactic protein-1 expression

We investigated whether activation of integrin receptors could modulate the expression of monocyte chemotactic protein-1 (MCP-1) in human hepatic stellate cells (HSC), mesenchymal cells responsible for extracellular matrix synthesis within the liver. When compared to non-adherent cells, HSC plated on collagen types I or IV, or fibronectin, showed increased MCP-1 gene expression and protein secretion in the conditioned medium. Increased MCP-1 secretion was also observed when cells were plated on dishes coated with a monoclonal antibody directed against the beta1-integrin subunit, demonstrating that ligation of beta1-integrins is sufficient to stimulate MCP-1 expression. Conversely, integrin-independent cell adhesion on poly-L-lysine did not modify MCP-1 secretion. Disruption of the actin cytoskeleton by cytochalasin D blocked the collagen-dependent increase in MCP-1 secretion. Chemotactic assay of HSC-conditioned medium showed that HSC plated on collagen secrete higher amounts of chemotactic factors for lymphomonocytes, and that MCP-1 accounts for the great majority of this effect. These findings indicate a novel mechanism of MCP-1 regulation possibly relevant in those conditions where HSC interact with an altered extracellular matrix.

The integrin alpha 6 beta 4 and the biology of carcinoma

The integrin family of adhesion receptors plays a major role in epithelial organization and function. Moreover, the altered expression and function of specific integrins most likely contributes significantly to carcinoma progression. The integrin alpha 6 beta 4, the focus of this review, is a receptor for several members of the laminin family and is preferentially expressed at the basal surface of most epithelia, where it contributes to basement membrane interactions. Mounting evidence suggests that the alpha 6 beta 4 integrin plays a key role in carcinoma cell biology. Several histopathological studies have established a correlation between alpha 6 beta 4 integrin expression and tumor progression. The importance of alpha 6 beta 4 expression in tumors in underscored by the findings that invading fronts of several carcinomas are enriched in the expression of alpha 6 beta 4 integrin ligands, such as laminin-1 and laminin-5. The participation of the alpha 6 beta 4 integrin in invasion is supported further by in vitro functional studies using carcinoma cells that have been transfected with the beta 4 cDNA. The mechanisms by which alpha 6 beta 4 contributes to tumor progression are probably related to its mechanical and signaling properties and are currently under intense study.