Extracellular signal-regulated kinase activation differentially regulates platelet-derived growth factor's actions in hepatic stellate cells, and is induced by in vivo liver injury in the rat

Liver fibrosis

Liver fibrosis is the excessive accumulation of extracellular matrix proteins including collagen that occurs in most types of chronic liver diseases. Advanced liver fibrosis results in cirrhosis, liver failure, and portal hypertension and often requires liver transplantation. Our knowledge of the cellular and molecular mechanisms of liver fibrosis has greatly advanced. Activated hepatic stellate cells, portal fibroblasts, and myofibroblasts of bone marrow origin have been identified as major collagen-producing cells in the injured liver. These cells are activated by fibrogenic cytokines such as TGF-beta1, angiotensin II, and leptin. Reversibility of advanced liver fibrosis in patients has been recently documented, which has stimulated researchers to develop antifibrotic drugs. Emerging antifibrotic therapies are aimed at inhibiting the accumulation of fibrogenic cells and/or preventing the deposition of extracellular matrix proteins. Although many therapeutic interventions are effective in experimental models of liver fibrosis, their efficacy and safety in humans is unknown. This review summarizes recent progress in the study of the pathogenesis and diagnosis of liver fibrosis and discusses current antifibrotic strategies.

Liver fibrosis

Liver fibrosis is the excessive accumulation of extracellular matrix proteins including collagen that occurs in most types of chronic liver diseases. Advanced liver fibrosis results in cirrhosis, liver failure, and portal hypertension and often requires liver transplantation. Our knowledge of the cellular and molecular mechanisms of liver fibrosis has greatly advanced. Activated hepatic stellate cells, portal fibroblasts, and myofibroblasts of bone marrow origin have been identified as major collagen-producing cells in the injured liver. These cells are activated by fibrogenic cytokines such as TGF-beta1, angiotensin II, and leptin. Reversibility of advanced liver fibrosis in patients has been recently documented, which has stimulated researchers to develop antifibrotic drugs. Emerging antifibrotic therapies are aimed at inhibiting the accumulation of fibrogenic cells and/or preventing the deposition of extracellular matrix proteins. Although many therapeutic interventions are effective in experimental models of liver fibrosis, their efficacy and safety in humans is unknown. This review summarizes recent progress in the study of the pathogenesis and diagnosis of liver fibrosis and discusses current antifibrotic strategies.

Liver fibrosis

Liver fibrosis is the excessive accumulation of extracellular matrix proteins including collagen that occurs in most types of chronic liver diseases. Advanced liver fibrosis results in cirrhosis, liver failure, and portal hypertension and often requires liver transplantation. Our knowledge of the cellular and molecular mechanisms of liver fibrosis has greatly advanced. Activated hepatic stellate cells, portal fibroblasts, and myofibroblasts of bone marrow origin have been identified as major collagen-producing cells in the injured liver. These cells are activated by fibrogenic cytokines such as TGF-beta1, angiotensin II, and leptin. Reversibility of advanced liver fibrosis in patients has been recently documented, which has stimulated researchers to develop antifibrotic drugs. Emerging antifibrotic therapies are aimed at inhibiting the accumulation of fibrogenic cells and/or preventing the deposition of extracellular matrix proteins. Although many therapeutic interventions are effective in experimental models of liver fibrosis, their efficacy and safety in humans is unknown. This review summarizes recent progress in the study of the pathogenesis and diagnosis of liver fibrosis and discusses current antifibrotic strategies.

Current research of hepatic cirrhosis in China

Hepatic cirrhosis is a common disease that poses a serious threat to public health, and is characterized by chronic, progressive and diffuse hepatic lesions preceded by hepatic fibrosis regardless of the exact etiologies. In recent years, considerable achievements have been made in China in research of the etiopathogenesis, diagnosis and especially the treatment of hepatic fibrosis, resulting in much improved prognosis of hepatic fibrosis and cirrhosis. In this paper, the authors review the current status of research in hepatic fibrosis, cirrhosis and their major complications.

Liver fibrosis

Liver fibrosis is the excessive accumulation of extracellular matrix proteins including collagen that occurs in most types of chronic liver diseases. Advanced liver fibrosis results in cirrhosis, liver failure, and portal hypertension and often requires liver transplantation. Our knowledge of the cellular and molecular mechanisms of liver fibrosis has greatly advanced. Activated hepatic stellate cells, portal fibroblasts, and myofibroblasts of bone marrow origin have been identified as major collagen-producing cells in the injured liver. These cells are activated by fibrogenic cytokines such as TGF-beta1, angiotensin II, and leptin. Reversibility of advanced liver fibrosis in patients has been recently documented, which has stimulated researchers to develop antifibrotic drugs. Emerging antifibrotic therapies are aimed at inhibiting the accumulation of fibrogenic cells and/or preventing the deposition of extracellular matrix proteins. Although many therapeutic interventions are effective in experimental models of liver fibrosis, their efficacy and safety in humans is unknown. This review summarizes recent progress in the study of the pathogenesis and diagnosis of liver fibrosis and discusses current antifibrotic strategies.

The role of p70S6K in hepatic stellate cell collagen gene expression and cell proliferation

During fibrosis the hepatic stellate cell (HSC) undergoes a complex activation process characterized by increased proliferation and extracellular matrix deposition. The 70-kDa ribosomal S6 kinase (p70S6K) is activated by mitogens, growth factors, and hormones in a phosphatidylinositol 3-kinase-dependent manner. p70S6K regulates protein synthesis, proliferation, and cell cycle control. Because these processes are involved in HSC activation, we investigated the role of p70S6K in HSC proliferation, cell cycle control, and type I collagen expression. Platelet-derived growth factor (PDGF) stimulated p70S6K phosphorylation, which was blocked by LY294002, an inhibitor of phosphatidylinositol 3-kinase. Rapamycin blocked phosphorylation of p70S6K but had no affect on PDGF-induced Akt phosphorylation, positioning p70S6K downstream of Akt. Transforming growth factor-beta, which inhibits HSC proliferation, did not affect PDGF-induced p70S6K phosphorylation. Rapamycin treatment did not affect alpha1(I) collagen mRNA but reduced type I collagen protein secretion. Expression of smooth muscle alpha-actin was not affected by rapamycin treatment, indicating that HSC activation was not altered. Rapamycin inhibited serum-induced DNA synthesis approximately 2-fold. Moreover, rapamycin decreased expression of cyclins D1, D3, and E but not cyclin D2, Rb-Ser780, and Rb-Ser795. Together, p70S6K plays a crucial role in HSC proliferation, collagen expression, and cell cycle control, thus representing a potential therapeutic target for liver fibrosis.

Regulation of PDGF and its receptors in fibrotic diseases

Platelet-derived growth factor (PDGF) isoforms play a major role in stimulating the replication, survival, and migration of myofibroblasts during the pathogenesis of fibrotic diseases. During fibrogenesis, PDGF is secreted by a variety of cell types as a response to injury, and many pro-inflammatory cytokines mediate their mitogenic effects via the autocrine release of PDGF. PDGF action is determined by the relative expression of PDGF alpha-receptors (PDGFRalpha) and beta-receptors (PDGFRbeta) on the surface of myofibroblasts. These receptors are induced during fibrogenesis, thereby amplifying biological responses to PDGF isoforms. PDGF action is also modulated by extracellular binding proteins and matrix molecules. This review summarizes the literature on the role of PDGF and its receptors in the development of fibrosis in a variety of organ systems, including lung, liver, kidney, and skin.

Autocrine/paracrine regulation of the growth of the biliary tree by the neuroendocrine hormone serotonin

BACKGROUND & AIMS

The biliary tree is the target of cholangiopathies that are chronic cholestatic liver diseases characterized by loss of proliferative response and enhanced apoptosis of cholangiocytes, the epithelial cells lining the biliary tree. The endogenous factors that regulate cholangiocyte proliferation are poorly understood. Therefore, we studied the role of the neuroendocrine hormone serotonin as a modulator of cholangiocyte proliferation.

METHODS

The presence of the serotonin 1A and 1B receptors on cholangiocytes was evaluated. We then tested whether the activation of such receptors by the administration of the selective agonists modifies cholangiocyte proliferation and functional activity both in vivo and in vitro. In addition, the intracellular signal mediating the serotonin receptor action in cholangiocytes was characterized. We studied the expression and secretion of serotonin by cholangiocytes and the effects of the neutralization of the secreted hormone on the growth of the biliary tree.

RESULTS

Cholangiocytes express the serotonin 1A and 1B receptors. Their activation markedly inhibits the growth and choleretic activity of the biliary tree in the bile duct-ligated rat, a model of chronic cholestasis. Such changes are mediated by enhanced d -myo-inositol 1,4,5-triphosphate/Ca 2+ /protein kinase C signaling and the consequent inhibition of the adenosine 3',5'-cyclic monophosphate/protein kinase A/Src/extracellular signal-regulated kinase 1/2 cascade. Cholangiocytes secrete serotonin, the blockage of which enhances cholangiocyte proliferation in the course of cholestasis.

CONCLUSIONS

We observed the existence of an autocrine loop based on serotonin that limits the growth of the biliary tree in the course of chronic cholestasis. Our novel findings might open new approaches for the management of cholangiopathies.

Leptin facilitates proliferation of hepatic stellate cells through up-regulation of platelet-derived growth factor receptor

In the present study, we investigated the effect of leptin on proliferation of hepatic stellate cells (HSCs) in vitro. Proliferation of 3-day cultured rat HSCs was assessed by incorporation of 5-bromo-2'-deoxyuridine (BrdU) into the nuclei. The percentages of BrdU-positive cells were increased in the presence of PDGF-BB (5 ng/ml) for 8h as expected. Co-incubation with leptin (10-100 nM) potentiates this PDGF-dependent increase in BrdU positive cells in a dose-dependent manner. Messenger RNA for PDGF receptor alpha and beta subunits was increased almost 2- to 3-fold by incubation with leptin for 6h. Further, pre-incubation with leptin for 6h enhanced PDGF-induced increases in phospho-p44/42 MAP kinase and phospho-Akt levels in a dose-dependent manner. In the same condition, however, leptin per se did not increase phospho-STAT 3 and phospho-p44/42 MAP kinase levels. Instead, leptin increased phospho-Akt levels in HSCs within 30 min, suggesting that the phosphatidylinositol 3 kinase (PI3K)/Akt pathway is involved in the mechanism by which leptin accelerates the proliferation of HSCs. In conclusion, the present study clearly indicated that leptin potentiates PDGF-dependent proliferative responses of HSCs in vitro.

Antisense strategy against PDGF B-chain proves effective in preventing experimental liver fibrogenesis

Hepatic stellate cells (HSCs) and transdifferentiated myofibroblasts are the principal producers of excessive extracellular matrix in liver fibrosis and cirrhosis. Activation of HSC is regulated by several cytokines and growth factors, including platelet-derived growth factor B-chain (PDGF-B), a potent mitogen for HSC, and overexpressed during hepatic fibrogenesis. Previous studies showed that MAPK and phosphatidylinositol 3' kinase are key signaling pathways involved in PDGF-induced stimulation of HSC. Based on the involvement of PDGF-B in fibrogenesis, reducing ligand stimulation of proliferative cytokine- or growth factor receptors interfering with receptor signaling therefore presents an interesting strategy for hepatic fibrosis prevention or interruption. We therefore generated an adenoviral vector serotype 5 (Ad5) expressing an antisense mRNA of the PDGF B-chain (Ad5-CMV-asPDGF) for application in an experimentally induced liver fibrogenesis model. The transgene clearly showed the ability to down-regulate endogenous PDGF B-chain and PDGFRbeta mRNA in culture-activated HSC and rat livers. The asPDGF mRNA also attenuates experimental liver fibrogenesis indicated by reduced levels of alpha-SMA and collagen type I expression.

Regulation of hepatic stellate cell activation and growth by transcription factor myocyte enhancer factor 2

BACKGROUND & AIMS

Hepatic stellate cells (HSCs) undergo activation during the development of liver fibrosis. Transcriptional regulation plays a key role in this process. We studied the role of transcription factor myocyte enhancer factor 2 (MEF2) during HSC activation.

METHODS

Culture of HSCs isolated from rat liver on plastic dishes and HSC-T6 on a basement membrane-like matrix were used as models of HSC activation and deactivation, respectively. The expression and activity of MEF2 were correlated with HSC activation. The roles of MEF2 during HSC activation were assessed in vitro and in vivo by animal models of fibrosis.

RESULTS

Early induction of MEF2 messenger RNA and protein accompanied culture-induced HSC activation. This was associated with enhanced MEF2 DNA binding and transactivation activity. p38 mitogen-activated protein kinase but not extracellular signal-regulated kinase pathway was required for increased MEF2 activity during HSC activation. Increased MEF2 protein also correlated with fibrosis in vivo. Reversal of HSC activation was paralleled by a marked decrease in MEF2 protein and activity. Functionally, enhancing MEF2 significantly increased the expression of alpha-smooth muscle actin (alpha-SMA), activated collagen I promoter activity, and stimulated HSC proliferation. MEF2 interference RNA significantly inhibited expression of alpha-SMA, collagen alpha1(I), and proliferating cell nuclear antigen.

CONCLUSIONS

The studies provide the first evidence for the presence of MEF2 in the liver and show that MEF2 regulates multiple aspects of HSC activation. These studies show a novel role of MEF2 as a key nuclear mediator that may participate in the pathologic process of liver fibrogenesis in vivo.

Differential requirement of members of the MAPK family for CCL2 expression by hepatic stellate cells

Hepatic stellate cells (HSC) coordinate the liver wound-healing response through secretion of several cytokines and chemokines, including CCL2 (formerly known as monocyte chemoattractant protein-1). In this study, we evaluated the role of different proteins of the MAPK family (ERK, p38(MAPK), and JNK) in the regulation of CCL2 expression by HSC, as an index of their proinflammatory activity. Several mediators activated all three MAPK, including TNF, IL-1, and PDGF. To assess the relative role of the different MAPKs, specific pharmacological inhibitors were used; namely, SB203580 (p38(MAPK)), SP600125 (JNK), and PD98059 (MEK/ERK). The efficacy and specificity of the different inhibitors in our cellular system were verified analyzing the enzymatic activity of the different MAPKs using in vitro kinase assays and/or testing the inhibition of phosphorylation of downstream substrates. SB203580 and SP600125 dose-dependently inhibited CCL2 secretion and gene expression induced by IL-1 or TNF. In contrast, inhibition of ERK did not affect the upregulation of CCL2 induced by the two cytokines. Finally, activin A was also found to stimulate CCL2 expression and to activate ERK, JNK, p38, and their downstream targets. Unlike in cells exposed to proinflammatory cytokines, all three MAPKs were required to induce CCL2 secretion in response to activin. We conclude that members of the MAPK family differentially regulate cytokine-induced chemokine expression in human HSC.

Inhibitory effect of soluble PDGF-beta receptor in culture-activated hepatic stellate cells

Following liver injury, hepatic stellate cells undergo phenotypic transformation with acquisition of myofibroblast-like features, characterized by increased cell proliferation, motility, contractility, and extracellular matrix production. Activation of hepatic stellate cells is regulated by several cytokines and growth factors, including platelet-derived growth factor B-chain, a potent mitogen for HSC, overexpressed during hepatic fibrogenesis. This pleiotropic mediator exerts cellular effects by binding to specific receptors, inducing receptor dimerization and tyrosine-autophosphorylation. Activated receptor phosphotyrosines recruit signal transduction molecules, initiating various signaling pathways. We produced a soluble PDGFbeta-receptor (sPDGFRbeta) consisting of an extracellular domain connected to the IgG-Fc part of human immunoglobulin heavy chain. This soluble, chimeric receptor inhibits PDGF signaling and PDGF-induced proliferation in culture-activated hepatic stellate cells. Furthermore, sPDGFR decreased collagen type I (alphaI) mRNA expression and inhibits autocrine-looping in PDGF-BB mRNA production. In summary, sPDGFRbeta clearly shows effective inhibitory properties in early HSC activation, suggesting potential therapeutic impact for anti-PDGF intervention in liver fibrogenesis.

Dominant-negative soluble PDGF-beta receptor inhibits hepatic stellate cell activation and attenuates liver fibrosis

Hepatic fibrogenesis is a consequence of hepatic stellate cells that become activated and transdifferentiate into a myofibroblastic phenotype with the ability to proliferate and synthesize large quantities of extracellular matrix components. In this process, platelet-derived growth factor (PDGF) is the most potent stimulus for hepatic stellate cell proliferation and migration, and is overexpressed during active hepatic fibrogenesis. This cytokine binds to the PDGF receptor type beta, activates Ras and sequentially propagates the stimulatory signal sequentially via phosphorylation of Raf-1, MEK and the extracellular-signal regulated kinases ERK1/ERK2. Hepatic injury is associated with both increased autocrine PDGF signaling and upregulation of PDGF receptor. In this study, we report that a dominant-negative soluble PDGF-beta receptor consisting of a chimeric IgG containing the extracellular portion of the PDGF receptor type beta blocks HSC activation and attenuates fibrogenesis induced by ligation of the common bile duct in rats. In culture-activated hepatic stellate cells, the soluble receptor blocks phosphorylation of endogenous PDGF receptor, phosphorylation of the ERK1/EKR2 signal and reduces proliferative activities of HSC. In vivo, both the delivery of the purified soluble PDGF antagonist and the administration of adenoviruses expressing the artificial transgene were able to reduce significantly the expression of collagen and alpha-smooth muscle actin. Our results demonstrate that PDGF plays a critical role in the progression and initiation of experimental liver fibrogenesis, and suggest that early anti-PDGF intervention should have a therapeutical impact on the treatment of liver fibrogenesis.

Protein kinase C-dependent activation of P44/42 mitogen-activated protein kinase and heat shock protein 70 in signal transduction during hepatocyte ischemic preconditioning

AIM: To investigate the significance of protein kinase C (PKC), P44/42 mitogen-activated protein kinase (MAPKs) and heat shock protein (HSP)70 signal transduction during hepatocyte ischemic preconditioning.

METHODS: In this study we used an in vitro ischemic preconditioning (IP) model for hepatocytes and an in vivo model for rat liver to investigate the significance of protein kinase C (PKC), P44/42 mitogen-activated protein kinase (P44/42 MAPKs) and heat shock protein 70 (HSP70) signal transduction in IP. Through a normal liver cell hypoxic preconditioning (HP) model in which cultured normal liver cells were subjected to 3 cycles of 5 min of incubation under hypoxic conditions followed by 5 min of reoxygenation and subsequently exposed to hypoxia and reoxygenation for 6 h and 9 h respectively. PKC inhibitor, activator and MEK inhibitor were utilized to analyze the phosphorylation of PKC, the expression of P44/42 MAPKs and HSP70. Viability and cellular ultrastructure were also observed. By using rat liver as an in vivo model of liver preconditioning (3 cycles of 10-min occlusion and 10-min reperfusion), in vivo phosphorylation of PKC and P44/42MAPKs, HSP70 expression were further analyzed. AST/ALT concentration, cellular structure and ultrastruture were also observed. All the data were statistically analyzed.

RESULTS: Similar results were obtained in both in vivo and in vitro IP models. Compared with the control without IP (or HP), the phosphorylation of PKC and P44/42 MAPKs and the expression of HSP70 were obviously increased in IP (or HP) treated model in which cytoprotection could be found. The effects of preconditioning were mimicked by stimulating PKC with 4β phorobol-12-myristate13-acetate (PMA). Conversely, inhibiting PKC with chelerythrine abolished the protection given by preconditioning. PD98059, inhibitor of MEK (the upstream kinase of P44/42MAPKs), also reverted the cytoprotection exerted by preconditioning.

CONCLUSION: The results demonstrate that preconditioning induces a rapid activation of P44/42MAPKs and PKC activation plays a pivotal role in the activation of P44/42 MAPKs pathway that participates in the preservation of liver cells. HSP expression is regulated by signals in PKC dependent P44/ 42 MAPKs pathway.

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.

Chinese herbs Kangxian ruangan keli inhibits expression of MEK-1 and c-fos in hepatic stellate cell indused by PDGF

AIM: To investigate the effect of Kangxian ruangan keli (KXR) on the expression of MEK-1 and c-fos in hepatic stellate cell (HSC) indused by PDGF.

METHODS: In a serum-free culture system, HSC was treated with a KXR preparation for 24 hours, followed by stimulation with PDGF-BB for 24 hours. Then the cells were incubated again in the medium containing KXR for 3 hours stimulated with PDGF-BB for 5 minutes, and collected. The proliferation of HSC was examined using an MTT assay. MEK-1 was detected with Western blotting and visualized by the enhenced chemiluminescent (ECL) method. The expression of c-fos mRNA was analyzed with in situ hybridization.

RESULTS: The A values for the HSC growing in the media without and with addition of PDGF were 0.170±0.060 and 0.820±0.050, respectively. The PDGF-induced increase was hindered remarkably by KXR preparation in a dose-dependent manner. Reaction values for the systems with 5 g/L and 1.25 g/L of KXR were 0.280±0.030 and 0.430±0.040 respectively, lower significantly than that in the culture free of KXR (0.820±0.050, P < 0.01). In addition, values for MEK-1 in HSC treated with 5 mg/mL and 1.25 mg/mL of KXR were 0.143±0.013, and 0.170±0.007, respectively, being lower than that in the cells treated only with PDGF-BB (0.186±0.010, P < 0.01). The expression level of c-fos mRNA was 0.152±0.010 and 0.163±0.005, respectively, also lower than that of the PDGF group (0.183±0.014, P < 0.01).

CONCLUSION: Within the dose range used in the present study, KXR preparation shows an inhibitory effect on HSC proliferation induced by PDGF. The mechanism of this process may involve interference with Ras-MEK-MAPK singal transduction mediated by PDGF.

Human hepatic stellate cells express CCR5 and RANTES to induce proliferation and migration

Activated hepatic stellate cells (HSCs) are the main producers of extracellular matrix in the fibrotic liver and are involved in the regulation of hepatic inflammation. The aim of this study was to characterize the role of regulated on activation, normal T-cell expressed, and presumably secreted (RANTES) in activated HSCs. RANTES mRNA and protein secretion were strongly induced after stimulating HSCs with TNF-alpha, IL-1beta, or CD40L. RANTES production was NF-kappaB dependent, because inhibitor-kappaB (IkappaB) superrepressor and dominant-negative IkappaB kinase-2 almost completely blocked RANTES expression. NF-kappaB activation was sufficient to drive RANTES expression as demonstrated by the strong induction of RANTES in HSCs expressing NF-kappaB-inducing kinase. The JNK/activator protein-1 pathway also contributed to RANTES expression as demonstrated by the blocking effects of the JNK inhibitor SP600125. HSCs responded to stimulation with recombinant human (rh)RANTES with an increase in intracellular calcium concentration and a rapid increase in free radical formation. Furthermore, rhRANTES induced ERK phosphorylation, ERK-dependent [3H]thymidine incorporation, and HSC proliferation. Additionally, rhRANTES induced focal adhesion kinase phosphorylation and a substantial increase in HSC migration. HSCs functionally expressed chemokine receptor-5 (CCR5), as shown by flow-cytometric analysis and RT-PCR, and the inhibitory effects of a blocking CCR5 antibody on rhRANTES-induced ERK activation, proliferation, and migration. Diphenylene iodonium and N-acetylcysteine inhibited rhRANTES-induced ERK activation and HSC proliferation, indicating that NADPH oxidase-dependent production of reactive oxygen species was required. In conclusion, RANTES and CCR5 represent potential mediators of 1) HSC migration and proliferation and 2) a cross-talk between HSCs and leukocytes during fibrogenesis.

Regulation of ERK/JNK/p70S6K in two rat models of liver injury and fibrosis

BACKGROUND/AIMS

The regulation of three major intracellular signalling protein kinases was investigated in two models of liver injury leading to hepatic fibrosis, dimethylnitrosamine administration (DMN) and bile duct ligation (BDL).

METHODS

Extracellular signal-regulated kinases (ERK)1/2, c-Jun terminal kinase (JNK) and p70S6-kinase (p70(S6K)) were studied in vivo in the whole liver, in liver sections and in isolated hepatocytes, cholangiocytes and hepatic stellate cells (HSC).

RESULTS

In the whole liver, activation of these kinases occurred with a different kinetic pattern in both models of liver injury. By immunohistochemistry and Western blot in isolated cells, phosphorylated kinases were detected in proliferating cells (i.e. hepatocytes and cholangiocytes after DMN and BDL, respectively), in addition to stellate-like elements. ERK1/2, JNK and p70(S6K) activation was associated with hepatocytes proliferation after DMN, while JNK activation was not associated with cholangiocytes proliferation after BDL. In HSC isolated from injured livers, protein kinases were differentially activated after BDL and DMN. Kinases activation in HSC in vivo preceded cell proliferation and alpha-smooth muscle actin appearance, a marker of HSC transformation in myofibroblast-like cells, and collagen deposition.

CONCLUSIONS

Our findings indicate that these kinases are coordinately regulated during liver regeneration and suggest that their modulation could be considered as a future therapeutic approach in the management of liver damage.

The chemokine CCL21 modulates lymphocyte recruitment and fibrosis in chronic hepatitis C

BACKGROUND AND AIMS

The chemokines CCL19 and CCL21 bind CCR7, which is involved in the organization of secondary lymphoid tissue and is expressed during chronic tissue inflammation. We investigated the expression of CCL21 and CCR7 in chronic hepatitis C. The effects of CCL21 on hepatic stellate cells (HSCs) were also studied.

METHODS

Expression of CCL21 was assessed by in situ hybridization and immunohistochemistry. CCR7 on T cells was analyzed by flow cytometry. Cultured human HSCs were studied in their activated phenotype.

RESULTS

In patients with chronic hepatitis C, expression of CCL21 and CCR7 was up-regulated. CCL21 was detected in the portal tracts and around inflammatory lymphoid follicles, in proximity to T lymphocytes and dendritic cells, which contributed to expression of this chemokine. Expression of CCR7 was also increased in patients with primary biliary cirrhosis. Intrahepatic CD8(+) T lymphocytes isolated from patients with chronic hepatitis C had a significantly higher percentage of positivity for CCR7 than those from healthy controls, and the expression of CCR7 was associated with that of CXCR3. Cultured HSCs expressed functional CCR7, the activation of which stimulated cell migration and accelerated wound healing in an in vitro model. Exposure of HSCs to CCL21 triggered several signaling pathways, including extracellular signal-regulated kinase, Akt, and nuclear factor kappaB, resulting in induction of proinflammatory genes.

CONCLUSIONS

Expression of CCL21 during chronic hepatitis C is implicated in the recruitment of T lymphocytes and the organization of inflammatory lymphoid tissue and may promote fibrogenesis in the inflamed areas via activation of CCR7 on HSCs.

Kangxian ruangan keli inhibits hepatic stellate cell proliferation mediated by PDGF

AIM: To investigate the effect of Kangxian ruangan keli (KXR) on hepatic stellate cell (HSC) proliferation mediated by platelet-derived growth factor (PDGF) and the underlying mechanism.

METHODS: In a serum-free culture system, HSCs were treated with a KXR preparation for 24 h, followed by stimulation with PDGF-BB for 24 h. Then the cells were incubated again in the medium containing KXR for 3 h stimulated with PDGF-BB for 5 minutes, and collected. The proliferation of HSC was examined using an MTT assay and flow cytometry. Tyrosine phosphorylation was detected with Western blotting and visualized by the enhenced chemiluminescent (ECL) method.

RESULTS: The OD values for the HSCs growing in the media without and with addition of PDGF were 0.17 ± 0.06 and 0.82 ± 0.05, respectively. The PDGF-induced increase was hindered remarkably by KXR preparation in a dose-dependent manner. The reaction values for the systems with 5 mg/mL, 2.5 mg/mL and 1.25 mg/mL of KXR were 0.28 ± 0.03, 0.37 ± 0.02 and 0.43 ± 0.04, respectively. Moreover, the percentages of S-phase cells in these KXR-containing culture systems were 10.95 ± 1.35, 32.76 ± 1.07 and 43.19 ± 1.09, respectively, all of which were significantly lower than that in the culture free of KXR (68.24 ± 2.72). In addition, the values for tyrosine-phosphorylated protein in HSCs treated with 5 mg/mL and 1.25 mg/mL of KXR were 0.1349 ± 0.0072 and 0.1658 ± 0.0025, respectively, which were smaller than that in the cells treated only with PDGF-BB (0.1813 ± 0.0117).

CONCLUSION: Within the dose range used in the present study, KXR preparation shows an inhibitory effect on HSC proliferation induced by PDGF. The mechanism of this process may involve interference with tyrosine phosphorylation mediated by PDGF.

Prolonged infusion of angiotensin II into normal rats induces stellate cell activation and proinflammatory events in liver

Recent evidence indicates that angiotensin II (ANG II) plays an important role in liver fibrogenesis. However, the underlying mechanisms are largely unknown. In advanced chronic liver diseases, circulating levels of ANG II are frequently elevated. We investigated the hepatic effects of prolonged systemic infusion of ANG II in normal rats. Saline or ANG II at subpressor and pressor doses (15 and 50 ng.kg-1.min-1, respectively) were infused to normal rats for 4 wk through a subcutaneous osmotic pump. Infusion of ANG II resulted in liver injury, as assessed by elevated serum liver enzymes. Livers from ANG II-perfused rats showed activation of JNK and ERK as well as increased NF-kappaB and activating protein-1 DNA-binding activity. Moreover, ANG II perfusion induced oxidative stress, increased concentration of proinflammatory cytokines, and upregulated the inflammatory proteins inducible nitric oxide synthase and cyclooxygenase-2. Histological examination of the livers from ANG II-infused rats showed mild portal inflammation as well as thickening and thrombosis of small hepatic vessels. ANG II-treated livers showed accumulation of CD43-positive inflammatory cells and activated hepatic stellate cells (HSCs) at the pericentral areas. A slight increase in collagen synthesis was observed, as assessed by Sirius red staining and hepatic hydroxyproline. All of these effects were observed when ANG II was perfused at subpressor and pressor doses. ANG II also accelerated the activation of primary cultured rat HSCs. In conclusion, increased systemic ANG II can induce liver injury by promoting proinflammatory events and vascular damage. ANG II-induced hepatic effects are not dependent on increase in arterial pressure.

Effects of PD98059 on proliferation of rat cultured hepatic stellate cells stimulated by acetaldehyde

AIM

To study the effects of PD98059, the specific blocking agent of MEK1, on the proliferation of hepatic stellate cells and expression of Proliferating Cell Nuclear Antigen in rat hepatic stellate cells (HSC).

METHODS

HSC stimulated by acetaldehyde were cultured. The cell growth was evaluated by MTT colorimetric assay. Proliferating cell nuclear antigen (PCNA) was examined by immunocytochemical staining.

RESULTS

PD98059 of 20 μmol/L had an inhibitory effect on proliferation of HSC (P<0.05, 0.109±0.020 vs 0.146±0.030), which was more obvious when cells exposed to PD98059 at 50 and 100 μmol /L (P<0.05, 0.081±0.010, 0.056±0.020 vs 0.146±0.030), and the expression of PCNA also showed a descending tendency with the increase of PD98059 concentration (P<0.05, 0.62±0.09, 0.47±0.04, 0.34±0.04 vs 0.740.05)

CONCLUSION

PD98059 inhibits proliferation of HSC and expression of PCNA, which is correlated with the decreased activity of PCNA.

PPARgamma ligand attenuates PDGF-induced mesangial cell proliferation: role of MAP kinase

BACKGROUND

Mesangial proliferation is a key feature in the pathogenesis of a number of renal diseases and can be experimentally induced by the mitogen platelet-derived growth factor (PDGF). Mitogen-activated protein kinase (MAPK) signaling plays a key role in mesangial cell proliferation. In the present study we examined whether peroxisome proliferator-activated receptor gamma (PPARgamma) activators/ligands, thiazolidinediones such as ciglitazone, troglitazone, and rosiglitazone, can inhibit cell proliferation by modulating individual steps in the MAPK pathway.

METHODS

Mouse mesangial cells were made quiescent and proliferation was measured following the application of PDGF. Using ciglitazone as the model compound, the mechanism of the antiproliferative effect of PPARgamma activators on MAPK and specific cell cycle regulatory proteins were examined by Western blot analysis and transfection studies.

RESULTS

Ciglitazone inhibited PDGF-induced mesangial cell proliferation in a dose-dependent manner (1 to 20 micromol/L). The inhibitory effect was blocked by a peroxisome proliferator-activated receptor element (PPRE) decoy oligonucleotide, indicating that the observed effect of ciglitazone was via PPARgamma activation. Ciglitazone (1 to 20 micromol/L) did not affect extracellular signal-regulated protein kinase (ERK) activation but inhibited the activation of serum response element (SRE) by 85 +/- 6% (P < 0.01). This effect was associated with a reduction in c-fos expression (80 +/- 9%, P < 0.01). Ciglitazone (1, 10, and 20 micromol/L) also inhibited cyclin D1 expression by 37 +/- 8%, 79 +/- 15%, and 87 +/- 12%, respectively (P < 0.001 to 0.001), and p21 expression by 45 +/- 6% (P < 0.01), 61 +/- 10% (P < 0.001), and 72 +/- 8% (P < 0.001), respectively. Ciglitazone inhibited PDGF-mediated up-regulation of p27. In addition, the antiproliferative effect of ciglitazone was potentiated by PD98059, a mitogen-activated protein (MAP) kinase kinase (MEK) inhibitor that acts at a step upstream from ERK.

CONCLUSION

These data indicate that PPARgamma activation may inhibit mesangial growth directly by affecting MAPK and cell cycle regulatory proteins. Furthermore, a MAP kinase inhibitor can potentiate the antiproliferative effect.

Effect of protein kinase C during hepatocyte hypoxic precondition

AIM

To investigate the effects of protein kinase C (PKC) on hypoxic preconditioning (HP) for hepatocyte.

METHODS

Through a normal liver cell HP model, PKC inhibitor and activator were utilized to analyze the phosphorylation of PKC. The cellular structure and viability were also observed. All the data were statistically analyzed.

RESULTS

Compared with the phosphorylation of PKC in the control without HP [(710.5±78.8) fkat/g], the phosphorylation of PKC was obviously increased in HP treated model [(1823.7±268.2) fkat/g] and PMA treated model [(2 541.2±326.5) fkat/g] (P<0.01). Cellular changes were less. In addition, opposite changes were found in PKC inhibited groups, and the phosphorylation of PKC was [(1 088.0±89.3) fkat/g] (P<0.01).

CONCLUSION

The activation of PKC is the important chain of HP in the preservation of liver cell, and its mechanism may be involved in protein phosphorylation.

Extracellular signal-regulated kinase in liver fibrogenesis of rat

AIM

To explore the role of ERK signal transduction pathway in the pathogenesis of liver fibrosis via investigating the expression and distribution of ERK1 in rats with liver fibrosis.

METHODS

Liver fibrosis model of rats were made by subcutaneously injecting with CCl4. Thirty-two male SD rats (weight 250-300 g) were randomly scarified at 1, 4 and 8 weeks after injection of CCl4 respectively, and their liver were used to detect ERK1 expression by immunohistochemical staining.

RESULTS

The expression of ERK1 in rats after injection with CCl4 were found chiefly in hepatic stellate cells(HSC)and all significantly higher than those in normal rats(P<0.05). Moreover, it presented with a progressive tendency for the expression of ERK1 in rats respectively at 1st, 4th and 8th week after injection with CCl4 (P<0.05).

CONCLUSION

The activation of ERK signal transduction pathway enhances HSC proliferation, and it may play an important role in liver fibrogenesis in rat.

Inhibition of pancreatic stellate cell activation by the hydroxymethylglutaryl coenzyme A reductase inhibitor lovastatin

Pancreatic stellate cells (PSCs) play a key role in pancreatic fibrosis, a constant feature of chronic pancreatitis. PSC activation occurs in response to profibrogenic mediators such as cytokines and involves proliferation, transition towards a myofibroblastic phenotype and enhanced production of extracellular matrix proteins. Previously, we have shown that PSC activation correlates with the activity of the Ras-Raf-ERK (extracellular signal-regulated kinase) signalling cascade [Gut 51 (2002) 579]. Using a rat culture model of PSCs, we have now evaluated the effects of lovastatin, a hydroxymethylglutaryl coenzyme A reductase inhibitor that interferes with protein isoprenylation, on PSC viability and activation as well as on signalling through Ras proteins. Apoptotic cells were detected applying the TUNEL assay. Proliferation of PSCs was quantitated using the bromodeoxyuridine DNA incorporation assay. Expression of alpha-smooth muscle actin (an indicator of the myofibroblastic phenotype), ERK activation and membrane translocation of the Ras superfamily member RhoA were analysed by immunoblotting. Lovastatin inhibited serum- and platelet-derived growth factor-stimulated PSC proliferation in a dose-dependent manner. At drug concentrations above the level required for growth inhibition, a strong increase of apoptotic cells was observed. Furthermore, lovastatin inhibited induction of alpha-smooth muscle actin expression in the course of primary culture. Immunoblot experiments indicated that lovastatin suppressed both Ras-mediated ERK 1/2 activation and platelet-derived growth factor-induced membrane translocation of RhoA. Together, our data suggest that lovastatin, through the interruption of Ras signalling, interferes with PSC activation. The antifibrotic efficiency of statins should be tested in animal models of chronic pancreatitis.

The role of focal adhesion kinase-phosphatidylinositol 3-kinase-akt signaling in hepatic stellate cell proliferation and type I collagen expression

Following a fibrogenic stimulus, the hepatic stellate cell (HSC) undergoes a complex activation process associated with increased cell proliferation and excess deposition of type I collagen. The focal adhesion kinase (FAK)-phosphatidylinositol 3-kinase (PI3K)-Akt signaling pathway is activated by platelet-derived growth factor (PDGF) in several cell types. We investigated the role of the FAK-PI3K-Akt pathway in HSC activation. Inhibition of FAK activity blocked HSC migration, cell attachment, and PDGF-induced PI3K and Akt activation. Both serum- and PDGF-induced Akt phosphorylation was inhibited by LY294002, an inhibitor of PI3K. A constitutively active form of Akt stimulated HSC proliferation in serum-starved HSCs, whereas LY294002 and dominant-negative forms of Akt and FAK inhibited PDGF-induced proliferation. Transforming growth factor-beta, an inhibitor of HSC proliferation, did not block PDGF-induced Akt phosphorylation, suggesting that transforming growth factor-beta mediates its antiproliferative effect downstream of Akt. Expression of type I collagen protein and alpha1(I) collagen mRNA was increased by Akt activation and inhibited when PI3K activity was blocked. Therefore, FAK is important for HSC migration, cell attachment, and PDGF-induced cell proliferation. PI3K is positioned downstream of FAK. Signals for HSC proliferation are transduced through FAK, PI3K, and Akt. Finally, expression of type I collagen is regulated by the PI3K-Akt signaling pathway.

Developing strategies for liver fibrosis treatment

Liver fibrosis represents a major worldwide healthcare burden. Current therapy is limited to removing the causal agent. This approach is successful in some diseases; particularly haemochromatosis and chronic viral hepatitis. However, for many patients treatment is not possible, while other patients present to medical attention at an advanced stage of fibrosis. There is therefore a great need for novel therapies for liver fibrosis. The hepatic stellate cell has been recognised to be responsible for most of the excess extracellular matrix observed in chronic liver fibrosis. The detailed understanding of hepatic stellate cell biology has allowed the rational design of novel antifibrotic therapies. This review describes for the general reader the novel emerging therapies for liver fibrosis.

p38 MAP kinase mediates platelet-derived growth factor-stimulated migration of hepatic myofibroblasts

Although the migration of hepatic myofibroblasts (HMFs) contributes to the development of fibrosis, the signals regulating migration of these cells are poorly understood. In this study, we tested the hypothesis that HMF migration is stimulated by platelet-derived growth factor-BB (PDGF-BB) through p38 mitogen-activated protein (MAP) kinase and extracellular signal-regulated kinase (ERK) signaling pathways. This hypothesis was addressed by directly visualizing the migration of cultured human HMFs into a wound. PDGF-BB stimulated membrane ruffling, migration, and proliferation. PDGF-BB also induced activation of p38 MAP kinase, its downstream effector, heat shock protein (HSP) 27, ERK 1 and ERK 2, and p125 focal adhesion kinase (FAK). Selective antagonism of p38 MAP kinase blocked PDGF-BB-stimulated HSP 27 phosphorylation, membrane ruffling, and migration, but did not alter PDGF-BB-induced proliferation. Selective antagonism of ERK kinase inhibited PDGF-BB-induced ERK phosphorylation and proliferation, but did not affect PDGF-BB-stimulated migration. Concentrations of PDGF-BB that stimulated migration and proliferation did not influence myosin-dependent contractility. Neither selective inhibition of p38 MAP kinase nor ERKs altered PDGF-BB-induced activation of FAK. In conclusion, these results provide novel evidence indicating that (1) HMF migration is stimulated by PDGF-BB through the regulation of membrane ruffling by a p38 MAP kinase signaling pathway, (2) whereas p38 MAP kinase mediates PDGF-BB-stimulated migration, but not proliferation, ERKs mediate PDGF-induced proliferation, but not migration, and (3) increases in myosin-dependent contractility are not required for PDGF-BB-stimulated migration.

Signal transduction by the chemokine receptor CXCR3: activation of Ras/ERK, Src, and phosphatidylinositol 3-kinase/Akt controls cell migration and proliferation in human vascular pericytes

Hepatic stellate cells (HSC) and glomerular mesangial cells (MC) are tissue-specific pericytes involved in tissue repair, a process that is regulated by members of the chemokine family. In this study, we explored the signal transduction pathways activated by the chemokine receptor CXCR3 in vascular pericytes. In HSC, interaction of CXCR3 with its ligands resulted in increased chemotaxis and activation of the Ras/ERK cascade. Activation of CXCR3 also stimulated Src phosphorylation and kinase activity and increased the activity of phosphatidylinositol 3-kinase and its downstream pathway, Akt. The increase in ERK activity was inhibited by genistein and PP1, but not by wortmannin, indicating that Src activation is necessary for the activation of the Ras/ERK pathway by CXCR3. Inhibition of ERK activation resulted in a decreased chemotactic and mitogenic effect of CXCR3 ligands. In MC, which respond to CXCR3 ligands with increased DNA synthesis, CXCR3 activation resulted in a biphasic stimulation of ERK activation, a pattern similar to the one observed in HSC exposed to platelet-derived growth factor, indicating that this type of response is related to the stimulation of cell proliferation. These data characterize CXCR3 signaling in pericytes and clarify the relevance of downstream pathways in the modulation of different biologic responses.

Ligands of peroxisome proliferator-activated receptor gamma modulate profibrogenic and proinflammatory actions in hepatic stellate cells

BACKGROUND & AIMS

Proliferation and migration of hepatic stellate cells (HSCs) and expression of chemokines are involved in the pathogenesis of liver inflammation and fibrogenesis. Peroxisome proliferator-activated receptor (PPAR)-gamma is a receptor transcription factor that controls growth and differentiation in different tissues. We explored the effects of PPAR-gamma agonists on the biological actions of cultured human HSCs.

METHODS

HSCs were isolated from normal human liver tissue and used in their myofibroblast-like phenotype or immediately after isolation. Activation of PPAR-gamma was induced with 15-deoxy-Delta(12, 14)-prostaglandin J(2) or with troglitazone.

RESULTS

PPAR-gamma agonists dose-dependently inhibited HSC proliferation and chemotaxis induced by platelet-derived growth factor. This effect was independent of changes in postreceptor signaling or expression of c-fos and c-myc and was associated with inhibition of cell cycle progression beyond the G(1) phase. Activation of PPAR-gamma also resulted in a complete inhibition of the expression of monocyte chemotactic protein 1 at the gene and protein levels. Comparison of quiescent and culture-activated HSCs revealed a marked decrease in PPAR-gamma expression in activated cells.

CONCLUSIONS

Activation of PPAR-gamma modulates profibrogenic and proinflammatory actions in HSCs. Reduced PPAR-gamma expression may contribute to confer an activated phenotype to HSCs.

Fibrogenesis I. New insights into hepatic stellate cell activation: the simple becomes complex

Hepatic stellate cell activation is a complex process. Paradoxes and controversies include the origin(s) of hepatic stellate cells, the regulation of membrane receptor signaling and transcription, and the fate of the cells once liver injury resolves. Major themes have emerged, including the dominance of autocrine signaling and the identification of counterregulatory stimuli that oppose key features of activated cells. Advances in analytical methods including proteomics and gene array, coupled with powerful bioinformatics, promise to revolutionize how we view cellular responses. Our understanding of stellate cell activation is likely to benefit from these advances, unearthing modes of regulating cellular behavior that are not even conceivable on the basis of current paradigms.