CXC chemokine receptor-1 is expressed by hepatocytes and regulates liver recovery after hepatic ischemia/reperfusion injury

CXC chemokines mediate hepatic inflammation and injury following ischemia/reperfusion (I/R). More recently, signaling through CXC chemokine receptor-2 (CXCR2) was shown to delay liver recovery and repair after I/R injury. The chemokine receptor CXCR1 shares ligands with CXCR2, yet nothing is known about its potential role in liver pathology. In the present study, we examined the role of CXCR1 in the injury and recovery responses to I/R using a murine model. CXCR1 expression was undetectable in livers of sham-operated mice. However, after ischemia CXCR1 expression increased 24 hours after reperfusion and was maximal after 96 hours of reperfusion. CXCR1 expression was localized largely to hepatocytes. In order to assess the function of CXCR1, CXCR2(-/-) mice were treated with the CXCR1/CXCR2 antagonist, repertaxin. Prophylactic treatment with repertaxin had no effect on acute inflammation or liver injury. However, when repertaxin was administered 24 hours postreperfusion there was a significant increase in hepatocellular injury and a delay in recovery compared to control-treated mice. CXCR1(-/-) mice also demonstrated delayed recovery and regeneration after I/R when compared to wild-type mice. In vitro, hepatocytes from CXCR2(-/-) mice that were stimulated to express CXCR1 showed increased proliferation in response to ligand. Hepatocyte proliferation was decreased in CXCR1(-/-) mice in vivo. CONCLUSION: This is the first report to show that CXCR1 expression is induced in hepatocytes after injury. Furthermore, the data suggest that CXCR1 has divergent effects from CXCR2 and appears to facilitate repair and regenerative responses after I/R injury.     

基质细胞衍生因子-1/CXCR4轴与肝脏疾病

基质细胞衍生因子-1(SDF-1)/CXCR4轴及其介导的细胞信号转导通路在肝脏疾病中的作用是国内外研究的热点.研究发现SDF-1/ CXCR4信号转导途径与肝脏再生、炎症、肝硬化以及肿瘤等疾病有关,但其具体机制尚未完全清楚.在细胞微环境中,SDF-1/CXCR4相互作用促进肝癌细胞生长,增强肿瘤的迁移、浸润以及转移能力.本文就SDF-1/CXCR4通路在肝再生、炎症、肿瘤疾病中的病理特征和致病机制的研究进展作一综述.     

Expression of IP-10 chemokine is regulated by pro-inflammatory cytokines in cultured hepatocytes

Chemokines are classified in four distinct groups as CXC, CC, CX3C and C, depending on the presence or absence of a motif called ELR (Arg-Leu-Glu) before the first cysteine residue in their structure. CXC chemokines are also subdivided into ELR+ and ELR-. Increasing evidence has indicated the existence of a chemokine network in the liver which is involved in both physiological responses and, under certain circumstances, pathological and repair processes following hepatic injury. The CXC chemokines play a major role in both these processes, and much attention has been focused on their therapeutic applications to liver disease. The aim of this study was to examine the response of cultured hepatocytes to exogenous inflammatory cytokines (TNF-alpha and IFN-gamma) regarding expression of IP-10 and growth regulatory oncogen (Gro) chemokines. In this study we employed western and northern analysis to measure chemokines at the level of protein and mRNA by hepatocytes in response to pro-inflammatory cytokines. We found that, the pro-inflammatory cytokines, TNF-alpha and IFN-gamma, selectively stimulated expression of IP-10 but were without effect on Gro. This confirms a potential direct involvement of these cytokines in chemokine production by hepatocytes. Thus, IFN-gamma and TNF-alpha may play a role in hepatic injury and inflammation and produce some of their biological effects by localized induction of chemokines by hepatocytes. Given the similarity to an acute phase response, we were able to show that IFN-gamma and TNF-alpha mimicked the effects of cell isolation and culture on induction of IP-10 expression. Further, evidence for linkages between IFN- gamma and TNF- alpha and liver injuries is seen in hepatitis C and hepatitis B in which increased levels of TNF- alpha and its soluble receptor were reported.     

Met provides essential signals for liver regeneration

Genetic analysis in mice has demonstrated a crucial role of the Met tyrosine kinase receptor and its ligand, hepatocyte growth factor/scatter factor (HGF/SF), in development of the liver, muscle, and placenta. Here, we use conditional mutagenesis in mice to analyze the function of Met during liver regeneration, using the Mx-cre transgene to introduce the mutation in the adult. After partial hepatectomy in mice carrying the Mx-cre-induced Met mutation, regeneration of the liver is impaired. Comparison of signal transduction pathways in control and mutant livers indicates that Met and other signaling receptors cooperate to fully activate particular signaling molecules, for instance, the protein kinase Akt. However, activation of the Erk1/2 kinase during liver regeneration depends exclusively on Met. Signaling crosstalk is thus an important aspect of the regulation of liver regeneration. Analysis of cell cycle progression of hepatocytes in conditional Met mutant mice indicates a defective exit from quiescence and diminished entry into S phase. Impaired liver regeneration is accompanied by compensatory physiological responses that include prolonged up-regulation of HGF/SF and IL-6 in peripheral blood. Our data demonstrate that the HGF/SF/Met signaling system is essential not only during liver development but also for the regeneration of the organ in the adult.     

Extracellular ATP activates c-jun N-terminal kinase signaling and cell cycle progression in hepatocytes

Partial hepatectomy leads to an orchestrated regenerative response, activating a cascade of cell signaling events necessary for cell cycle progression and proliferation of hepatocytes. However, the identity of the humoral factors that trigger the activation of these pathways in the concerted regenerative response in hepatocytes remains elusive. In recent years, extracellular ATP has emerged as a rapidly acting signaling molecule that influences a variety of liver functions, but its role in hepatocyte growth and regeneration is unknown. In this study, we sought to determine if purinergic signaling can lead to the activation of c-jun N-terminal kinase (JNK), a known central player in hepatocyte proliferation and liver regeneration. Hepatocyte treatment with ATPgammaS, a nonhydrolyzable ATP analog, recapitulated early signaling events associated with liver regeneration-that is, rapid and transient activation of JNK signaling, induction of immediate early genes c-fos and c-jun, and activator protein-1 (AP-1) DNA-binding activity. The rank order of agonist preference, UTP>ATP>ATPgammaS, suggests that the effects of extracellular ATP is mediated through the activation of P2Y2 receptors in hepatocytes. ATPgammaS treatment alone and in combination with epidermal growth factor (EGF) substantially increased cyclin D1 and proliferating cell nuclear antigen (PCNA) protein expression and hepatocyte proliferation in vitro. Extracellular ATP as low as 10 nM was sufficient to potentiate EGF-induced cyclin D1 expression. Infusion of ATP by way of the portal vein directly activated hepatic JNK signaling, while infusion of a P2 purinergic receptor antagonist prior to partial hepatectomy inhibited JNK activation. In conclusion, extracellular ATP is a hepatic mitogen that can activate JNK signaling and hepatocyte proliferation in vitro and initiate JNK signaling in regenerating liver in vivo. These findings have implications for enhancing our understanding of novel factors involved in the initiation of regeneration, liver growth, and development.     

Insulin induces calcium signals in the nucleus of rat hepatocytes

Insulin is an hepatic mitogen that promotes liver regeneration. Actions of insulin are mediated by the insulin receptor, which is a receptor tyrosine kinase. It is currently thought that signaling via the insulin receptor occurs at the plasma membrane, where it binds to insulin. Here we report that insulin induces calcium oscillations in isolated rat hepatocytes, and that these calcium signals depend upon activation of phospholipase C and the inositol 1,4,5-trisphosphate receptor, but not upon extracellular calcium. Furthermore, insulin-induced calcium signals occur in the nucleus, and are temporally associated with selective depletion of nuclear phosphatidylinositol bisphosphate and translocation of the insulin receptor to the nucleus. These findings suggest that the insulin receptor translocates to the nucleus to initiate nuclear, inositol 1,4,5-trisphosphate-mediated calcium signals in rat hepatocytes. This novel signaling mechanism may be responsible for insulin's effects on liver growth and regeneration.     

Increased expression of T cell chemokines and their receptors in chronic hepatitis C: relationship with the histological activity of liver disease

OBJECTIVES: Although chemokines seem to be important in certain inflammatory disorders, little is known about the role of these proteins in chronic hepatitis C. METHODS: Expression of selected CXC and CC chemokines and their receptors was assessed by immunohistochemistry and flow cytometry in chronic hepatitis C. Tissue samples from normal liver and that of sustained responders were also evaluated. A comparative analysis between the histological grading and the intrahepatic expression level of chemokines was performed. RESULTS: The majority of liver-derived T lymphocytes expressed CXCR3 and CCR5 chemokine receptors, representing high enrichment over levels of CXCR3 + and CCR5 + T cells in blood from chronic hepatitis C. An intense intrahepatic expression of their respective ligands, the CXC chemokine Mig, and RANTES, was detected in the same patients studied, being restricted to the sinusoidal endothelium and to hepatocytes, respectively. A statistically significant association between the intrahepatic chemokine expression level and the inflammatory activity of chronic hepatitis C was found. Of note was the marked expression of both CXCR3 and its ligand Mig on endothelial cells from portal neovessels in chronic hepatitis C. CONCLUSIONS: Intrahepatic chemokine signaling could play a key role regulating significant pathological events during chronic hepatitis C, opening new avenues for therapeutic interventions based on chemokine activities.     

Intact signaling by transforming growth factor beta is not required for termination of liver regeneration in mice

Transforming growth factor beta (TGF-beta) is a potent inhibitor of hepatocyte proliferation in vitro and is suggested to be a key negative regulator of liver growth. To directly address the role of TGF-beta signaling in liver regeneration in vivo, the TGF-beta type II receptor gene (Tgfbr2) was selectively deleted in hepatocytes by crossing "floxed" Tgfbr2 conditional knockout mice with transgenic mice expressing Cre under control of the albumin promoter. Hepatocytes isolated from liver-specific Tgfbr2 knockout (R2LivKO) mice were refractory to the growth inhibitory effects of TGF-beta1. The peak of DNA synthesis after 70% partial hepatectomy occurred earlier (36 vs. 48 hours) and was 1.7-fold higher in R2LivKO mice compared with controls. Accelerated S-phase entry by proliferating R2LivKO hepatocytes coincided with the hyperphosphorylation of Rb protein and the early upregulation of cyclin D1 and cyclin E. However, by 120 hours after partial hepatectomy, hepatocyte proliferation was back to baseline in both control and R2LivKO liver. Regenerating R2LivKO liver showed evidence of increased signaling by activin A and persistent activity of the Smad pathway. Blockage of activin A signaling by the specific inhibitor follistatin resulted in increased hepatocyte proliferation at 120 hours, particularly in R2LivKO livers. In conclusion, TGF-beta regulates G(1) to S phase transition of hepatocytes, but intact signaling by TGF-beta is not required for termination of liver regeneration. Increased signaling by activin A may compensate to regulate liver regeneration when signaling through the TGF-beta pathway is abolished, and may be a principal factor in the termination of liver regeneration.     

Amphiregulin: an early trigger of liver regeneration in mice

BACKGROUND AND AIMS: Liver regeneration is a unique response directed to restore liver mass after resection or injury. The survival and proliferative signals triggered during this process are conveyed by a complex network of cytokines and growth factors acting in an orderly manner. Activation of the epidermal growth factor receptor is thought to play an important role in liver regeneration. Amphiregulin is a member of the epidermal growth factor family whose expression is not detectable in healthy liver. We have investigated the expression of amphiregulin in liver injury and its role during liver regeneration after partial hepatectomy. METHODS: Amphiregulin gene expression was examined in healthy and cirrhotic human and rat liver, in rodent liver regeneration after partial hepatectomy, and in primary hepatocytes. The proliferative effects and intracellular signaling of amphiregulin were studied in isolated hepatocytes. The in vivo role of amphiregulin in liver regeneration after partial hepatectomy was analyzed in amphiregulin-null mice. RESULTS: Amphiregulin gene expression is detected in chronically injured human and rat liver and is rapidly induced after partial hepatectomy in rodents. Amphiregulin expression is induced in isolated hepatocytes by interleukin 1beta and prostaglandin E(2), but not by hepatocyte growth factor, interleukin 6, or tumor necrosis factor alpha. We show that amphiregulin behaves as a primary mitogen for isolated hepatocytes, acting through the epidermal growth factor receptor. Finally, amphiregulin-null mice display impaired proliferative responses after partial liver resection. CONCLUSIONS: Our findings indicate that amphiregulin is an early-response growth factor that may contribute to the initial phases of liver regeneration.     

SDF-1α-Mediated Tissue Repair by Stem Cells: A Promising Tool in Cardiovascular Medicine?

Circulating smooth muscle progenitor cells have been identified as a source for neointimal smooth muscle cells after various types of injuries to the vessel wall contributing to neointimal hyperplasia, implying a fundamental role of these progenitor cells in the vascular response to injury. Recent studies have provided insight into the molecular mechanisms of mobilization and local recruitment of smooth muscle progenitor cells. The CXC chemokine SDF-1alpha and its receptor CXCR4 have been identified as the central signaling axis regulating the homing of smooth muscle progenitor cells into the injured vessel wall. This review discusses the unique biologic functions of this chemokine and its contribution to stem-cell-based vascular repair and disease.     

Effect of liver regeneration on malignant hepatic tumors

Liver regeneration after major surgery may activate occult micrometastases and facilitate tumor growth,leading to liver tumor recurrence.Molecular changes during liver regeneration can provide a microenvironment that stimulates intrahepatic tumor propagation through alterations in cellular signaling pathways,where activation and proliferation of mature hepatocytes,hepatic progenitor cells,non-parenchymal liver cells might favor both liver regeneration and tumor growth.This review highlights recent advances of tumor growth and development in the regenerating liver,possible mechanisms and clinical implications.     

FXR促进肝脏再生作用的研究进展

肝脏具有再生的能力是众所周知的,他在部分切除或损伤后通过代偿增生的方式能够实现自我再生.法尼酯X受体(farnesoid X receptor,FXR)是配体激活转录因子的核激素受体超家族中的一员,胆汁酸是FXR的内源性配体.作为一种代谢调节因子,FXR在调节胆汁酸、脂质和葡萄糖的代谢中起到了至关重要的作用.近来,研究发现胆汁酸与其受体FXR结合激活细胞间的信号转导对肝再生起重要作用,胆汁酸/FXR信号通路是正常肝再生所必需的.此外,FXR能促进肝脏损伤后的修复,且FXR的活化能够减轻年龄相关性的肝脏再生缺陷.这些新的发现提示FXR介导的胆汁酸信号在正常肝再生中是一个不可或缺的组成部分,同时也突出了FXR配体对促进部分肝移植或肝癌切除术后的肝脏再生的潜在应用.这篇综述概述了FXR的基本资料和对肝脏再生作用的研究进展.     

CXC chemokine IP-10: a key actor in liver disease?

Interferon-γ-inducible protein 10 (IP-10), or C-X-C motif chemokine (CXCL10), is a small cytokine belonging to the non-ELR CXC chemokine family. By binding to its specific receptor CXCR3, IP-10 recruits activated CXCR3+ T cells to the liver parenchyma and plays a pivotal role in liver disease initiation and progression. IP-10 is mainly secreted by hepatocytes and liver sinusoidal endothelium. Different IP-10 forms exert different functions: long-length IP-10 directs CXCR3+ T cell migration and is associated with inflammation, while short IP-10 is a CXCR3 antagonist, thereby playing protective role in liver injury. IP-10 levels are positively associated with the severity of liver inflammation, fibrosis stage and acute graft rejection. High IP-10 levels are closely related to anti-HCV therapy failure. Thus, IP-10 may be both a potential prognostic tool and a therapeutic target for the treatment of patients with HCV or HIV/HCV co-infection. The purpose of this review is to highlight the growing advances in basic knowledge and clinical interest of IP-10 in liver disease.     

CXCL5 plasma levels decrease in patients with chronic liver disease

Background and Aims: CXCL5 (chemokine [C‐X‐C motif] ligand 5, also known as epithelial neutrophil‐activating peptide 78 [ENA78]) belongs to the CXC chemokine family and has been shown to have promitotic effects on hepatocytes. The aim of our study was to assess CXCL5 plasma levels in patients with chronic liver disease. Methods: CXCL5 plasma levels were measured in 111 patients with chronic liver disease and 98 healthy controls. The gene expression of CXCL5 and its main receptor, CXC receptor‐2, were also determined in liver biopsies from 46 patients. Results: CXCL5 levels were correlated with clinical presentation, laboratory parameters, and liver histology. Plasma CXCL5 levels in patients with liver cirrhosis were lower than those in healthy controls, and correlated with hepatic biosynthetic capacity, Child–Pugh and model for end‐stage liver disease scores. Patients with hepatic necroinflammation and fibrosis on liver histology showed lower plasma CXCL5 levels. In patients with typical clinical complications of cirrhosis, CXCL5 levels were found to be decreased. Intrahepatically, CXCL5 expression was increased in patients with advanced fibrosis and cirrhosis. The isolation of different cellular compartments from mouse livers suggested that hepatic stellate cells and sinusoidal endothelial cells are the main sources of hepatic CXCL5. Conclusions: Plasma CXCL5 levels are lower in patients with chronic liver disease, suggesting that CXCL5 might be involved in the pathogenesis of chronic liver disease. CXCL5 could serve as an additional biomarker for hepatic necroinflammation and fibrosis.     

Thrombospondin-1 is a novel negative regulator of liver regeneration after partial hepatectomy through transforming growth factor-beta1 activation in mice

The matricellular protein, thrombospondin-1 (TSP-1), is prominently expressed during tissue repair. TSP-1 binds to matrix components, proteases, cytokines, and growth factors and activates intracellular signals through its multiple domains. TSP-1 converts latent transforming growth factor-beta1 (TGF-β1) complexes into their biologically active form. TGF-β plays significant roles in cell-cycle regulation, modulation of differentiation, and induction of apoptosis. Although TGF-β1 is a major inhibitor of proliferation in cultured hepatocytes, the functional requirement of TGF-β1 during liver regeneration remains to be defined in vivo. We generated a TSP-1-deficient mouse model of a partial hepatectomy (PH) and explored TSP-1 induction, progression of liver regeneration, and TGF-β-mediated signaling during the repair process after hepatectomy. We show here that TSP-1-mediated TGF-β1 activation plays an important role in suppressing hepatocyte proliferation. TSP-1 expression was induced in endothelial cells (ECs) as an immediate early gene in response to PH. TSP-1 deficiency resulted in significantly reduced TGF-β/Smad signaling and accelerated hepatocyte proliferation through down-regulation of p21 protein expression. TSP-1 induced in ECs by reactive oxygen species (ROS) modulated TGF-β/Smad signaling and proliferation in hepatocytes in vitro, suggesting that the immediately and transiently produced ROS in the regenerating liver were the responsible factor for TSP-1 induction. CONCLUSIONS: We have identified TSP-1 as an inhibitory element in regulating liver regeneration by TGF-β1 activation. Our work defines TSP-1 as a novel immediate early gene that could be a potential therapeutic target to accelerate liver regeneration.     

Hepatocyte growth factor inhibits insulin-stimulated glycogen synthesis in primary cultured hepatocytes

BACKGROUND/AIMS: Hepatocyte growth factor (HGF) plays an important role as a mitogen in liver regeneration. However, little is known about the metabolic effects of HGF in the liver. Studies were performed to examine whether HGF influences carbohydrate metabolism, which is drastically changed in the early course of the regeneration. METHODS: Primary cultured rat hepatocytes were treated with glucoregulatory hormones such as insulin, glucagon and adrenaline in the presence or absence of HGF. Cellular glycogen deposition and activities of its metabolic enzymes were compared. RESULTS: HGF inhibited insulin-stimulated glycogen deposition, but had no effect on glycogen degradation stimulated by glucagon and adrenaline. HGF decreased glycogen synthase activity and increased glycogen phosphorylase activity in insulin-stimulated hepatocytes, resulting in the inhibition of glycogen synthesis. Experiments with immunoprecipitation revealed that HGF had no effect on the upstream of insulin signaling including an activation of its receptor and association of insulin receptor substrate with phosphatidylinositol 3-kinase, indicating that HGF presumably affects further downstream of these events. CONCLUSIONS: These results demonstrate that HGF interacts with insulin on glucose metabolism in hepatocytes. HGF may be involved in glucose regulation, and contribute to cell growth and maturation in addition to its mitogenic action during liver regeneration.