Early genes induced in hepatic stellate cells during wound healing

Krüppel-like factor 6 (KLF6) requires its amino terminal domain to promote villous trophoblast cell fusion

INTRODUCTION

Villous cytotrophoblast (vCTB) cells fuse to generate and maintain the syncytiotrophoblast layer required for placental development and function. Krüppel-like factor 6 (KLF6) is a ubiquitous transcription factor with an N-terminal acidic transactivation domain and a C-terminal zinc finger DNA-binding domain. KLF6 is highly expressed in placenta, and it is required for proper placental development. We have demonstrated that KLF6 is necessary for cell fusion in human primary vCTBs, and in the BeWo cell line.

MATERIALS AND METHODS

Full length KLF6 or a mutant lacking its N-terminal domain were expressed in BeWo cells or in primary vCTB cells isolated from human term placentas. Cell fusion, gene and protein expression, and cell proliferation were analyzed. Moreover, Raman spectroscopy and atomic force microscopy (AFM) were used to identify biochemical, topography, and elasticity cellular modifications.

RESULTS

The increase in KLF6, but not the expression of its deleted mutant, is sufficient to trigger cell fusion and to raise the expression of β-hCG, syncytin-1, the chaperone protein 78 regulated by glucose (GRP78), the ATP Binding Cassette Subfamily G Member 2 (ABCG2), and Galectin-1 (Gal-1), all molecules involved in vCTB differentiation. Raman and AFM analysis revealed that KLF6 reduces NADH level and increases cell Young's modulus. KLF6-induced differentiation correlates with p21 upregulation and decreased cell proliferation. Remarkable, p21 silencing reduces cell fusion triggered by KLF6 and the KLF6 mutant impairs syncytialization and decreases syncytin-1 and β-hCG expression.

DISCUSSION

KLF6 induces syncytialization through a mechanism that involves its regulatory transcriptional domain in a p21-dependent manner.

Genetic Polymorphisms and Clinical Features in Diabetic Patients With Fatty Liver: Results From a Single-Center Experience in Southern Italy

Genetic background may be involved in the promotion and progression of non-alcoholic fatty liver disease (NAFLD). Previous studies have suggested that the single nucleotide polymorphisms (SNPs) may be associated with the specific clinical features in the patients with hepatic steatosis; however, data on the patients with diabetes from Southern Italy are lacking. We enrolled 454 patients and 260 of them had type 2 diabetes. We studied the PNPLA3 rs738409, LPIN1 rs13412852, KLF6 rs3750861, SOD2 rs4880, TM6SF2 rs58542926, and ZNF624 rs12603226 SNPs and their distribution in the study population. Lipid profile, liver stiffness, and kidney function were also studied to understand the potential role of the SNPs in the development of clinical phenotypes. No differences were observed in the distribution of polymorphisms between the diabetic and non-diabetic subjects. Carriers of risk allele G for PNPLA3 rs738409 SNP showed a lower mean value of serum triglycerides and a higher liver stiffness. Risk allele for KLF6 rs3750861 and SOD2 rs4880 polymorphism had a lower estimated glomerular filtration rate (eGFR) value, whereas no differences in the glucose and glycated hemoglobin level were observed in the subgroups by the different genotypes. Genetic polymorphisms are useful to identify the patients at higher risk of development of liver fibrosis and lower eGFR values in the patients with diabetes and NAFLD. Their use in clinical practice may help the clinicians to identify the patients who require a more strict follow-up program.

Hepatic stellate cells - from past till present: morphology, human markers, human cell lines, behavior in normal and liver pathology

Hepatic stellate cell (HSC), initially analyzed by von Kupffer, in 1876, revealed to be an extraordinary mesenchymal cell, essential for both hepatocellular function and lesions, being the hallmark of hepatic fibrogenesis and carcinogenesis. Apart from their implications in hepatic injury, HSCs play a vital role in liver development and regeneration, xenobiotic response, intermediate metabolism, and regulation of immune response. In this review, we discuss the current state of knowledge regarding HSCs morphology, human HSCs markers and human HSC cell lines. We also summarize the latest findings concerning their roles in normal and liver pathology, focusing on their impact in fibrogenesis, chronic viral hepatitis and liver tumors.

Two Sides of the Same Coin: The Roles of KLF6 in Physiology and Pathophysiology

The Krüppel-like factors (KLFs) family of proteins control several key biological processes that include proliferation, differentiation, metabolism, apoptosis and inflammation. Dysregulation of KLF functions have been shown to disrupt cellular homeostasis and contribute to disease development. KLF6 is a relevant example; a range of functional and expression assays suggested that the dysregulation of KLF6 contributes to the onset of cancer, inflammation-associated diseases as well as cardiovascular diseases. KLF6 expression is either suppressed or elevated depending on the disease, and this is largely due to alternative splicing events producing KLF6 isoforms with specialised functions. Hence, the aim of this review is to discuss the known aspects of KLF6 biology that covers the gene and protein architecture, gene regulation, post-translational modifications and functions of KLF6 in health and diseases. We put special emphasis on the equivocal roles of its full-length and spliced variants. We also deliberate on the therapeutic strategies of KLF6 and its associated signalling pathways. Finally, we provide compelling basic and clinical questions to enhance the knowledge and research on elucidating the roles of KLF6 in physiological and pathophysiological processes.

Krüppel-like factor 6 mediates pulmonary angiogenesis in rat experimental hepatopulmonary syndrome and is aggravated by bone morphogenetic protein 9

Hepatopulmonary syndrome (HPS) is a serious pulmonary vascular disease derived from chronic liver disease, and its key pathogenesis is angiogenesis. Krüppel-like factor 6 (KLF6) mediates physiological repair and remodeling during vascular injury. However, the role of KLF6 in pulmonary microvascular endothelial cells (PMVECs) during angiogenesis of HPS and its underlying mechanism in HPS have not been investigated. Common bile duct ligation (CBDL) in rats can replicate pulmonary vascular abnormalities of human HPS. Here, we found that advanced pulmonary angiogenesis and pulmonary injury score coincided with the increase of KLF6 level in PMVECs of CBDL rat; KLF6 in PMVECs was also induced while cultured with CBDL rat serum in vitro. Inhibition of KLF6 dramatically suppressed PMVEC-mediated proliferation, migration and tube formation in vivo; this may be related to the downregulation of activin receptor-like kinase-1 (ALK1) and endoglin (ENG), which are transacted by KLF6. Bone morphogenetic protein 9 (BMP9) enhanced the expression of KLF6 in PMVECs and was involved in the angiogenesis of HPS. These results suggest that KLF6 triggers PMVEC-mediated angiogenesis of HPS and is aggravated by BMP9, and the inhibition of the BMP9/KLF6 axis may be an effective strategy for HPS treatment.

Summary: Krüppel-like factor 6, which is triggered by pulmonary injury and promoted by bone morphogenetic protein 9, mediates pulmonary angiogenesis in rat experimental hepatopulmonary syndrome and then aggravates lung dysfunction.

Risk Factors for the Development of Nonalcoholic Fatty Liver Disease/Nonalcoholic Steatohepatitis, Including Genetics

Nonalcoholic fatty liver disease is emerging as the most common cause of chronic liver disease worldwide. This trend is, in part, secondary, to the growing incidence of obesity, type 2 diabetes, and metabolic syndrome. Other risk factors include age, gender, race/ethnicity, genetic predisposition, and polycystic ovarian disease. With the introduction of genome-wide association studies, genetic mutations contributing to inherited susceptibility to steatosis have been identified, which hold keys to future improvement in diagnosis and management. This article expands on the aforementioned risk factors and summarizes the current available data on genetic and environmental factors associated with this common entity.

A hepatic stellate cell gene expression signature associated with outcomes in hepatitis C cirrhosis and hepatocellular carcinoma after curative resection

OBJECTIVE

We used an informatics approach to identify and validate genes whose expression is unique to hepatic stellate cells and assessed the prognostic capability of their expression in cirrhosis.

DESIGN

We defined a hepatic stellate cell gene signature by comparing stellate, immune and hepatic transcriptome profiles. We then created a prognostic index using a combination of hepatic stellate cell signature expression and clinical variables. This signature was derived in a retrospective-prospective cohort of hepatitis C-related early-stage cirrhosis (prognostic index derivation set) and validated in an independent retrospective cohort of patients with postresection hepatocellular carcinoma (HCC). We then examined the association between hepatic stellate cell signature expression and decompensation, HCC development, progression of Child-Pugh class and survival.

RESULTS

The 122-gene hepatic stellate cell signature consists of genes encoding extracellular matrix proteins and developmental factors and correlates with the extent of fibrosis in human, mouse and rat datasets. Importantly, association of clinical prognostic variables with overall survival was improved by adding the signature; we used these results to define a prognostic index in the derivation set. In the validation set, the same prognostic index was associated with overall survival. The prognostic index was associated with decompensation, HCC and progression of Child-Pugh class in the derivation set, and HCC recurrence in the validation set.

CONCLUSIONS

This work highlights the unique transcriptional niche of stellate cells, and identifies potential stellate cell targets for tracking, targeting and isolation. Hepatic stellate cell signature expression may identify patients with HCV cirrhosis or postresection HCC with poor prognosis.

Adeno-Associated Virus Vector Mediated Delivery of the HBV Genome Induces Chronic Hepatitis B Virus Infection and Liver Fibrosis in Mice

Liver cirrhosis and hepatocellular carcinomas are major health problems of chronic hepatitis B virus (HBV) infection. To date, rare model has reproduced liver fibrosis associated with long-term HBV infection which in turn has hindered both the understanding of HBV biology and the development of new treatment options. Here, using adeno-associated virus serotype 8 (AAV8) mediated delivery of a 1.2-kb HBV genome, we successfully generated a chronic HBV infectious mouse model that presents the associated liver fibrosis observed following human infection. After AAV8/HBV1.2 vector administration, mice demonstrated effective HBV replication and transcription which resulted in HBV antigen expression and viremia over 6 months. Although no obvious acute inflammatory response was noted, these mice still developed chronic liver disease and hepatic fibrogenesis as demonstrated by increased ground glass-like hepatocytes, an increasing trend of collagen deposition and upregulated fibrosis markers, including type I collagen, type III collagen, tissue inhibitor of metalloproteinase (TIMP), and transforming growth factor-β1(TGF-β1). Taken together, AAV-mediated HBV gene delivery to the mouse liver, induced HBV persistent infection accompanied by liver fibrosis which can serve as a model for investigating the precise mechanisms underlying liver fibrosis following chronic HBV infection as well as for the potential development of novel therapeutics.

Genetic predisposition in NAFLD and NASH: impact on severity of liver disease and response to treatment

Liver fat deposition related to systemic insulin resistance defines non-alcoholic fatty liver disease (NAFLD) which, when associated with oxidative hepatocellular damage, inflammation, and activation of fibrogenesis, i.e. non-alcoholic steatohepatitis (NASH), can progress towards cirrhosis and hepatocellular carcinoma. Due to the epidemic of obesity, NAFLD is now the most frequent liver disease and the leading cause of altered liver enzymes in Western countries. Epidemiological, familial, and twin studies provide evidence for an element of heritability of NAFLD. Genetic modifiers of disease severity and progression have been identified through genome-wide association studies. These include the Patatin-like phosholipase domain-containing 3 (PNPLA3) gene variant I148M as a major determinant of inter-individual and ethnicity-related differences in hepatic fat content independent of insulin resistance and serum lipid concentration. Association studies confirm that the I148M polymorphism is also a strong modifier of NASH and progressive hepatic injury. Furthermore, a few large multicentre case-control studies have demonstrated a role for genetic variants implicated in insulin signalling, oxidative stress, and fibrogenesis in the progression of NAFLD towards fibrosing NASH, and confirm that hepatocellular fat accumulation and insulin resistance are key operative mechanisms closely involved in the progression of liver damage. It is now important to explore the molecular mechanisms underlying these associations between gene variants and progressive liver disease, and to evaluate their impact on the response to available therapies. It is hoped that this knowledge will offer further insights into pathogenesis, suggest novel therapeutic targets, and could help guide physicians towards individualised therapy that improves clinical outcome.

Posttranslational regulation of tissue inhibitor of metalloproteinase-1 by calcium-dependent vesicular exocytosis

Liver myofibroblasts derived from hepatic stellate cells (HSC) are critical mediators of liver fibrosis. Release of tissue inhibitor of metalloproteinase-1 (TIMP-1) advances liver fibrosis by blocking fibrinolysis. The mechanisms responsible for the posttranslational regulation of TIMP-1 by myofibroblastic HSC are unknown. Here, we demonstrate that TIMP-1 release by HSC is regulated in a posttranslational fashion via calcium-sensitive vesicular exocytosis. To our knowledge, this is the first article to directly examine vesicular trafficking in myofibroblastic HSC, potentially providing a new target to treat and or prevent liver fibrosis.

Role of kruppel-like transcription factors in adipogenesis

The zinc-finger transcription factors of the kruppel-like factor family (KLF) are critical in many physiological and pathological processes including cell proliferation, differentiation, inflammation, and apoptosis. Recently, there is increasing evidence that suggests these KLFs have an important role in fat biology. This review summarizes the role of KLFs in lipid metabolism, especially in adipogenesis, and reveals the relationship networks among members of KLF family in differentiation.

Vascular injury triggers Krüppel-like factor 6 mobilization and cooperation with specificity protein 1 to promote endothelial activation through upregulation of the activin receptor-like kinase 1 gene

RATIONALE

Activin receptor-like kinase-1 (ALK1) is an endothelial transforming growth factor β receptor involved in angiogenesis. ALK1 expression is high in the embryo vasculature, becoming less detectable in the quiescent endothelium of adult stages. However, ALK1 expression becomes rapidly increased after angiogenic stimuli such as vascular injury.

OBJECTIVE

To characterize the molecular mechanisms underlying the regulation of ALK1 on vascular injury.

METHODS AND RESULTS

Alk1 becomes strongly upregulated in endothelial (EC) and vascular smooth muscle cells of mouse femoral arteries after wire-induced endothelial denudation. In vitro denudation of monolayers of human umbilical vein ECs also leads to an increase in ALK1. Interestingly, a key factor in tissue remodeling, Krüppel-like factor 6 (KLF6) translocates to the cell nucleus during wound healing, concomitantly with an increase in the ALK1 gene transcriptional rate. KLF6 knock down in human umbilical vein ECs promotes ALK1 mRNA downregulation. Moreover, Klf6(+/-) mice have lower levels of Alk1 in their vasculature compared with their wild-type siblings. Chromatin immunoprecipitation assays show that KLF6 interacts with ALK1 promoter in ECs, and this interaction is enhanced during wound healing. We demonstrate that KLF6 is transactivating ALK1 gene, and this transactivation occurs by a synergistic cooperative mechanism with specificity protein 1. Finally, Alk1 levels in vascular smooth muscle cells are not directly upregulated in response to damage, but in response to soluble factors, such as interleukin 6, released from ECs after injury.

CONCLUSIONS

ALK1 is upregulated in ECs during vascular injury by a synergistic cooperative mechanism between KLF6 and specificity protein 1, and in vascular smooth muscle cells by an EC-vascular smooth muscle cell paracrine communication during vascular remodeling.

Glucokinase links Krüppel-like factor 6 to the regulation of hepatic insulin sensitivity in nonalcoholic fatty liver disease

The polymorphism, KLF6-IVS1-27A, in the Krüppel-like factor 6 (KLF6) transcription factor gene enhances its splicing into antagonistic isoforms and is associated with delayed histological progression of nonalcoholic fatty liver disease (NAFLD). To explore a potential role for KLF6 in the development of insulin resistance, central to NAFLD pathogenesis, we genotyped KLF6-IVS1-27 in healthy subjects and assayed fasting plasma glucose (FPG) and insulin sensitivities. Furthermore, we quantified messenger RNA (mRNA) expression of KLF6 and glucokinase (GCK), as an important mediator of insulin sensitivity, in human livers and in liver tissues derived from a murine Klf6 knockdown model (DeltaKlf6). Klf6 overexpression studies in a mouse hepatocyte line were utilized to mechanistically link KLF6 with Gck promoter activity. KLF6-IVS1-27Gwt (i.e., less KLF6 splicing) was associated with stepwise increases in FPG and insulin and reduced hepatic insulin sensitivity. KLF6 binds to the liver-specific Gck promoter and activates a GCK promoter-reporter, identifying GCK as a KLF6 direct transcriptional target. Accordingly, in DeltaKlf6 hepatocytes Gck expression was reduced and stable transfection of Klf6 led to up-regulation of Gck. GCK and KLF6 mRNAs correlate directly in human NAFLD tissues and immunohistochemistry studies confirm falling levels of both KLF6 and GCK in fat-laden hepatocytes. In contrast to full-length KLF6, splice variant KLF6-SV1 increases in NAFLD hepatocytes and inversely correlates with glucokinase regulatory protein, which negatively regulates GCK activity.

CONCLUSION

KLF6 regulation of GCK contributes to the development of hepatic insulin resistance. The KLF6-IVS1-27A polymorphism, which generates more KLF6-SV1, combats this, lowering hepatic insulin resistance and blood glucose.

Mammalian Krüppel-like factors in health and diseases

The Krüppel-like factor (KLF) family of transcription factors regulates diverse biological processes that include proliferation, differentiation, growth, development, survival, and responses to external stress. Seventeen mammalian KLFs have been identified, and numerous studies have been published that describe their basic biology and contribution to human diseases. KLF proteins have received much attention because of their involvement in the development and homeostasis of numerous organ systems. KLFs are critical regulators of physiological systems that include the cardiovascular, digestive, respiratory, hematological, and immune systems and are involved in disorders such as obesity, cardiovascular disease, cancer, and inflammatory conditions. Furthermore, KLFs play an important role in reprogramming somatic cells into induced pluripotent stem (iPS) cells and maintaining the pluripotent state of embryonic stem cells. As research on KLF proteins progresses, additional KLF functions and associations with disease are likely to be discovered. Here, we review the current knowledge of KLF proteins and describe common attributes of their biochemical and physiological functions and their pathophysiological roles.

Pediatric nonalcoholic fatty liver disease: A clinical and laboratory challenge

The true prevalence of pediatric nonalcoholic fatty liver disease (NAFLD) is unknown. Challenges in determining the population prevalence of NAFLD include the type of test (and the reference intervals used to define normal and abnormal), the type of population (general population, hospital series), the demographic characteristics of the population sampled, and the nature of the study design. The natural history of pediatric NAFLD remains uncertain. The issue of when to perform a liver biopsy in children with suspected NAFLD remains controversial. Children with NAFLD but normal alanine aminotransferase are rarely investigated. However, evidence of alterations in glucose metabolism parameters should prompt a better understanding of the natural history of pediatric NAFLD not only in terms of the progression of liver disease but also regarding its potential relationship with other health outcomes such as type 2 diabetes mellitus and cardiovascular disease. This evidence could make liver biopsy mandatory in the majority of cases at risk of progressive and severe hepatic and extrahepatic disease. This conclusion, however, raises the question of the feasibility of liver biopsy assessment in an extremely large at risk population, and of the cost/effectiveness of this policy. There is a considerable, continuous interest in reliable, noninvasive alternatives that will allow the prognosis of pediatric NAFLD to be followed in large community or population-based studies.

Hepatic stellate cell and myofibroblast-like cell gene expression in the explanted cirrhotic livers of patients undergoing liver transplantation

BACKGROUND

Hepatic stellate cells (HSC) are involved in hepatic fibrogenesis. Cell signaling associated with an insult to the liver affects an HSC transdifferentiation to fibrogenic myofibroblast-like cells.

AIMS

To investigate the transcriptional expression distinguishing HSC and myofibroblast-like cells between livers with and without cirrhosis.

METHODS

Tissue from ten cirrhotic livers (undergoing transplant) and four non-cirrhotic livers from the National Disease Research Interchange underwent cell separation to extract HSC and myofibroblast-like cell populations. Separated cell types as well as LI-90 cells were subjected to microarray analysis. Selected microarray results were verified by quantitative real-time PCR.

RESULTS

Differential expression of some genes, such as IL-1beta, IL-1alpha, and IL-6, was associated with both transdifferentiation and disease. Other genes, such as fatty acid 2-hydroxylase only show differential expression in association with disease. Functional analysis supported these findings, indicating some signal transduction pathways (IL-6) are involved in disease and activation, whereas retinoid X receptor signaling in HSC from cirrhotic and non-cirrhotic livers varies in scope and quality.

CONCLUSIONS

These findings indicate distinct phenotypes for HSC from cirrhotic and non-cirrhotic livers. Furthermore, coordinated differential expression between genes involved in the same signal transduction pathways provides some insight into the mechanisms that may control the balance between fibrogenesis and fibrolysis.

TGF-beta regulates the expression of transcription factor KLF6 and its splice variants and promotes co-operative transactivation of common target genes through a Smad3-Sp1-KLF6 interaction

KLF6 (Krüppel-like factor 6) is a transcription factor and tumour suppressor with a growing range of biological activities and transcriptional targets. Among these, KLF6 suppresses growth through transactivation of TGF-beta1 (transforming growth factor-beta1). KLF6 can be alternatively spliced, generating lower-molecular-mass isoforms that antagonize the full-length WT (wild-type) protein and promote growth. A key target gene of full-length KLF6 is endoglin, which is induced in vascular injury. Endoglin, a homodimeric cell membrane glycoprotein and TGF-beta auxiliary receptor, has a pro-angiogenic role in endothelial cells and is also involved in malignant progression. The aim of the present work was to explore the effect of TGF-beta on KLF6 expression and splicing, and to define the contribution of TGF-beta on promoters regulated by co-operation between KLF6 and Sp1 (specificity protein 1). Using co-transfection, co-immunoprecipitation and fluorescence resonance energy transfer, our data demonstrate that KLF6 co-operates with Sp1 in transcriptionally regulating KLF6-responsive genes and that this co-operation is further enhanced by TGF-beta1 through at least two mechanisms. First, in specific cell types, TGF-beta1 may decrease KLF6 alternative splicing, resulting in a net increase in full-length, growth-suppressive KLF6 activity. Secondly, KLF6-Sp1 co-operation is further enhanced by the TGF-beta-Smad (similar to mothers against decapentaplegic) pathway via the likely formation of a tripartite KLF6-Sp1-Smad3 complex in which KLF6 interacts indirectly with Smad3 through Sp1, which may serve as a bridging molecule to co-ordinate this interaction. These findings unveil a finely tuned network of interactions between KLF6, Sp1 and TGF-beta to regulate target genes.

The Kruppel-like factor 6 genotype is associated with fibrosis in nonalcoholic fatty liver disease

BACKGROUND & AIMS

Although nonalcoholic fatty liver disease (NAFLD) is increasingly common, only a minority of affected individuals develop fibrotic liver disease. Based on its role in liver growth and repair, we explored whether Kruppel-like factor 6 (KLF6) plays a role in NAFLD progression.

METHODS

KLF6 expression in 31 fibrosis scored NAFLD liver biopsy specimens was assessed by real-time polymerase chain reaction. Transfected minigene constructs were used to study the effect of a polymorphism, KLF6-IVS1-27G>A, that promotes KLF6 alternative splicing in vitro. We genotyped KLF6-IVS1-27G>A in 3 groups of patients (UK group 1, n = 306; Italian group 2, n = 109; trio group 3, n = 61 children and parents).

RESULTS

KLF6 expression was increased in association with increased steatosis, inflammation, and fibrosis in NAFLD livers. KLF6-IVS1-27G>A promoted alternative splicing of KLF6 and abrogated the up-regulation of both alpha-smooth muscle actin and collagen 1 in LX-2 cells. Group 1 genotyping identified KLF6-IVS1-27G>A in 44 of 306 (14.4%) patients. Notably, KLF6-IVS1-27G>A was associated significantly with milder NAFLD, with only 25% having more advanced fibrosis compared with 45% of wild-type (wt) individuals. This trend was confirmed in group 2. A linear regression analysis including all 415 patients, adjusted for age, sex, body mass index, and blood glucose level, confirmed that presence of the wt KLF6 allele was an independent predictor of fibrotic NAFLD. Furthermore, we have shown preferential transmission of the wt allele to children with fibrotic NAFLD.

CONCLUSIONS

We report a functional polymorphism in the KLF6 gene associated with advanced NAFLD and believe further study of KLF6 may enhance our understanding of this disease process.

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

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

Role of Krüppel-Like Transcription Factors in Endothelial Biology

Krüppel-like factors are members of the zinc finger family of transcription factors that have been implicated as playing key roles in regulating cellular differentiation and tissue development. Studies over the past several years support an important role for this family of factors in vascular biology. This review summarizes the role of Krüppel-like factors in endothelial cell biology.

Gene expression profiles of hepatic cell-type specific marker genes in progression of liver fibrosis

AIM: To determine the gene expression profile data for the whole liver during development of dimethylni-trosamine (DMN)-induced hepatic fibrosis.

METHODS: Marker genes were identified for different types of hepatic cells, including hepatic stellate cells (HSCs), Kupffer cells (including other inflammatory cells), and hepatocytes, using independent temporal DNA microarray data obtained from isolated hepatic cells.

RESULTS: The cell-type analysis of gene expression gave several key results and led to formation of three hypotheses: (1) changes in the expression of HSC-specific marker genes during fibrosis were similar to gene expression data in in vitro cultured HSCs, suggesting a major role of the self-activating characteristics of HSCs in formation of fibrosis; (2) expression of mast cell-specific marker genes reached a peak during liver fibrosis, suggesting a possible role of mast cells in formation of fibrosis; and (3) abnormal expression of hepatocyte-specific marker genes was found across several metabolic pathways during fibrosis, including sulfur-containing amino acid metabolism, fatty acid metabolism, and drug metabolism, suggesting a mechanistic relationship between these abnormalities and symptoms of liver fibrosis.

CONCLUSION: Analysis of marker genes for specific hepatic cell types can identify the key aspects of fibrogenesis. Sequential activation of inflammatory cells and the self-supporting properties of HSCs play an important role in development of fibrosis.

Transcriptional regulation of stellate cell activation

The identification of activated hepatic stellate cells and related cell types as key fibrogenic effectors during liver injury has led to intense evaluation of transcriptional events underlying their behavior. While initial studies focused on characterizing interactions between transcription factors and regulatory regions within gene promoters, epigenetic mechanisms have emerged as major determinants of gene activation and repression, in particular histone acetylation and promoter methylation, as well as other complex conditional interactions that underlie global changes in gene expression. Three examples are provided that illustrate how stellate cell activation may be controlled by widely divergent regulatory pathways, including alternative splicing of a growth inhibitory transcription factor (Kruppel-like factor-6), epigenetic regulation of a factor regulating stellate cell survival (nuclear factor kappaB), and regulation of a transcription factor whose expression maintains stellate cell quiescence (LIM homeobox gene 2 [Lhx2]). These complex cascades illustrate how clarifying the finely tuned interdependent layers of transcriptional, translational, post-translational and epigenetic gene regulation in stellate cells is raising new prospects for therapy of hepatic fibrosis.

Platelet-derived growth factor C induces liver fibrosis, steatosis, and hepatocellular carcinoma

Members of the platelet-derived growth factor (PDGF) ligand family are known to play important roles in wound healing and fibrotic disease. We show that both transient and stable expression of PDGF-C results in the development of liver fibrosis consisting of the deposition of collagen in a pericellular and perivenular pattern that resembles human alcoholic and nonalcoholic fatty liver disease. Fibrosis in PDGF-C transgenic mice, as demonstrated by staining and hydroxyproline content, is preceded by activation and proliferation of hepatic stellate cells, as shown by collagen, alpha-smooth muscle actin and glial fibrillary acidic protein staining and between 8 and 12 months of age is followed by the development of liver adenomas and hepatocellular carcinomas. The hepatic expression of a number of known profibrotic genes, including type beta1 TGF, PDGF receptors alpha and beta, and tissue inhibitors of matrix metalloproteinases-1 and -2, increased by 4 weeks of age. Increased PDGF receptor alpha and beta protein levels were associated with activation of extracellular regulated kinase-1 and -2 and protein kinase B. At 9 months of age, PDGF-C transgenic mice had enlarged livers associated with increased fibrosis, steatosis, cell dysplasia, and hepatocellular carcinomas. These studies indicate that hepatic expression of PDGF-C induces a number of profibrotic pathways, suggesting that this growth factor may act as an initiator of fibrosis. Moreover, PDGF-C transgenic mice represent a unique model for the study of hepatic fibrosis progressing to tumorigenesis.

Comprehensive analysis of differential gene expression profiles on carbon tetrachloride-induced rat liver injury and regeneration

Microarray analysis of RNA from carbon tetrachloride (CCl4)-administered rat livers was performed at various time points to establish a global gene expression profile during injury and regeneration stages. A single dose of 1 ml/kg of CCl4 was given by ip injection, and the liver samples were obtained after 6, 24, 48 h, and 2 weeks. Histopathologic, biochemical, and immunohistochemical studies enabled the classification of the CCl4 effect into injury (6 and 24 h) and regeneration (48 h and 2 weeks) stages. The expression levels of 5180 clones on a custom rat gene microarray were analyzed and 587 clones yielded changeable gene expression on at least single time point. One hundred seventy-nine clones were classified as injury-specific clones, while 38 clones as regeneration-specific clones. Characteristic gene expression profiles could be associated with CCl4-induced gene expression with the disruption of lipid metabolism, which is known to cause the fatty liver induced by CCl4 treatment. In addition, induction of the transcripts for many ribosomal proteins was detected during the injury stage, particularly at the 24-h time point, despite the previous report of decreased protein synthesis rate upon CCl4 treatment. Several genes with known functions were also identified as CCl4-regulated genes. In conclusion, we established a global gene expression profile utilizing microarray analysis in rat liver upon acute CCl4 administration with a full chronological profile that not only covers injury stage but also later points of regeneration stage.

Lhx2-/- mice develop liver fibrosis

Liver fibrosis is a wound-healing response to chronic injury of any type and is characterized by a progressive increase in deposition of extracellular matrix (ECM) proteins, the major source of which are activated hepatic stellate cells (HSCs). Because the LIM homeobox gene Lhx2 is expressed in HSCs and liver development in Lhx2(-/-) mice is disrupted, we analyzed liver development in Lhx2(-/-) embryos in detail. Lhx2(-/-) embryos contain numerous activated HSCs and display a progressively increased deposition of the ECM proteins associated with liver fibrosis, suggesting that Lhx2 inhibits HSC activation. Transfection of Lhx2 cDNA into a human HSC line down-regulates expression of genes characteristic of activated HSCs. Moreover, the Lhx2(-/-) liver display a disrupted cellular organization and an altered gene expression pattern of the intrahepatic endodermal cells, and the increased deposition of ECM proteins precedes these abnormalities. Collectively these results show that Lhx2 negatively regulates HSC activation, and its inactivation in developing HSCs appears therefore to mimic the signals that are triggered by the wound-healing response to chronic liver injury. This study establishes a spontaneous and reproducible animal model for hepatic fibrosis and reveals that Lhx2 expression in HSCs is important for proper cellular organization and differentiation of the liver.

Transcriptional activation of endoglin and transforming growth factor-beta signaling components by cooperative interaction between Sp1 and KLF6: their potential role in the response to vascular injury

Endoglin is an endothelial membrane glycoprotein involved in cardiovascular morphogenesis and vascular remodeling. It associates with transforming growth factor-beta (TGF-beta) signaling receptors to bind TGF-beta family members, forming a functional receptor complex. Arterial injury leads to up-regulation of endoglin, but the underlying regulatory events are unknown. The transcription factor KLF6, an immediate-early response gene induced in endothelial cells during vascular injury, transactivates TGF-beta, TGF-beta signaling receptors, and TGF-beta-stimulated genes. KLF6 and, subsequently, endoglin were colocalized to vascular endothelium (ie, expressed in the same cell type) following carotid balloon injury in rats. After endothelial denudation, KLF6 was induced and translocated to the nucleus; this was followed 6 hours later by increased endoglin expression. Transient overexpression of KLF6, but not Egr-1, stimulated endogenous endoglin mRNA and transactivated the endoglin promoter. This transactivation was dependent on a GC-rich tract required for basal activity of the endoglin promoter driven by the related GC box binding protein, Sp1. In cells lacking Sp1 and KLF6, transfected KLF6 and Sp1 cooperatively transactivated the endoglin promoter and those of collagen alpha1(I), urokinase-type plasminogen activator, TGF-beta1, and TGF-beta receptor type 1. Direct physical interaction between Sp1 and KLF6 was documented by coimmunoprecipitation, pull-down experiments, and the GAL4 one-hybrid system, mapping the KLF6 interaction to the C-terminal domain of Sp1. These data provide evidence that injury-induced KLF6 and preexisting Sp1 may cooperate in regulating the expression of endoglin and related members of the TGF-beta signaling complex in vascular repair.

In vivo and in vitro interactions between human colon carcinoma cells and hepatic stellate cells

Stromal reaction is important for the growth of cancer both in primary and metastatic sites. To demonstrate this reaction during the hepatic metastasis of human colon carcinoma, we histologically investigated alterations to the distribution and phenotype of hepatic stellate cells (HSCs), the only mesenchymal cells in the liver parenchyma, using a nude mouse model. Intrasplenically injected colon carcinoma LM-H3 cells migrated into the space of Disse and underwent proliferation, in close association with hepatocytes and HSCs, at 2 days. At 14 days, HSCs were accumulated around the tumor mass and expressed alpha-smooth muscle actin, a marker for HSC activation. We next investigated in vitro the growth factors involved in the interactions between LM-H3 cells and HSCs. Conditioned medium of rat HSCs which underwent culture-induced activation contained platelet-derived growth factor (PDGF)-AB, hepatocyte growth factor (HGF) and transforming growth factor (TGF)-beta, and could augment LM-H3-cell proliferation and migration. Neutralizing antibodies against PDGF-AA and PDGF-BB and those against PDGF-BB and HGF inhibited proliferation and migration, respectively, of LM-H3 cells, whereas antibody against TGF-beta had no effect. LM-H3 cells expressed PDGF receptors-alpha and -beta and c-met. Conditioned medium of LM-H3 cells contained PDGF-AB, and could enhance HSC proliferation and migration. This augmenting effect was suppressed by treatment with anti-PDGF-AB antibody. The present study has demonstrated that bidirectional interactions involving PDGF and HGF take place in vitro between colon carcinoma cells and HSCs, raising the possibility that similar interactions might be involved in the stromal reaction during hepatic metastasis.

Pathogenesis of liver fibrosis: role of oxidative stress

In the liver, the progressive accumulation of connective tissue, a complex and dynamic process termed fibrosis, represents a very frequent event following a repeated or chronic insult of sufficient intensity to trigger a "wound healing"-like reaction. The fibrotic process recognises the involvement of various cells and different factors in bringing about an excessive fibrogenesis with disruption of intercellular contacts and interactions and of extracellular matrix composition. However, Kupffer cells, together with recruited mononuclear cells, and hepatic stellate cells are by far the key-players in liver fibrosis. Their cross-talk is triggered and favoured by a series of chemical mediators, with a prominent role played by the transforming growth factor beta. Both expression and synthesis of this inflammatory and pro-fibrogenic cytokine are mainly modulated through redox-sensitive reactions. Further, involvement of reactive oxygen species and lipid peroxidation products can be clearly demonstrated in other fundamental events of hepatic fibrogenesis, like activation and effects of stellate cells, expression of metalloproteinases and of their specific inhibitors. The important outcome of such findings as regards the pathogenesis of liver fibrosis derives from the observation of a consistent and marked oxidative stress condition in many if not all chronic disease processes affecting hepatic tissue. Hence, reactive oxidant species likely contribute to both onset and progression of fibrosis as induced by alcohol, viruses, iron or copper overload, cholestasis, hepatic blood congestion.

New aspects of hepatic fibrosis

Hepatic stellate cells are the major source of extracellular matrix proteins in hepatic fibrosis, including Type I collagen. In response to liver injury, the hepatic stellate cells change from a quiescent to an activated phenotype. This activation process includes a phenotypic change to a myofibroblast-like cell, increased proliferation rate, loss of retinoid stores, increased production of extracellular matrix proteins, chemokines, and cytokines, and contractility. Ongoing studies are characterizing the genes that are differentially expressed in the quiescent and activated hepatic stellate cells. We have also investigated the regulation of Type I collagen expression, the cleavage of collagen propeptides, and the formation of collagen cross-links. Understanding these pathways may provide new insights into the molecular pathogenesis of hepatic fibrosis.

Stellate cell activation in alcoholic fibrosis--an overview

There has been remarkable progress in our understanding of how chronic alcohol ingestion may lead to hepatic injury and scarring, or fibrosis. Hepatic fibrosis represents the liver's wound healing response and is characterized by accumulation of interstitial matrix, or scar. Fibrosis in the liver results from the activation of stellate cells, or resident mesenchymal cells. Stellate cell activation is a dramatic phenotype transition whose net effect is the replacement of normal liver matrix by scar. Features of stellate cell activation include increased cell accumulation from proliferation and directed migration, increased matrix production, enhanced contractility, accelerated degradation of the normal liver matrix, release of profibrogenic cytokines, and loss of cellular vitamin A. Alcohol may enhance fibrogenesis through stimulation of stellate cells by hypoxia, generation of lipid peroxides from damaged hepatocytes, production of acetaldehyde that may have direct fibrogenic activity, and through activation of Kupffer cells or resident macrophages. Unanswered questions remain to be studied, but the clarification of underlying mechanisms of fibrosis portends continued progress in our ability to treat alcoholic liver fibrosis.

The role of TGFbeta1 in initiating hepatic stellate cell activation in vivo

BACKGROUND/AIMS

The activation of hepatic stellate cells is a key initiating event in hepatic fibrogenesis. Although TGFbeta1 is a potent inducer of collagen alpha1(I) expression in vitro and elevated levels of TGFbeta1 are found in patients and experimental animals with hepatic fibrosis and cirrhosis, the role of increased TGFbeta1 in the initiation of hepatic stellate cell activation in vivo is unknown. We used two experimental approaches to study this relationship: 1) Induction of an acute liver injury with carbon tetrachloride (CCl4) in normal and TGFbeta1-knockout (ko) mice, and 2) overexpression of TGFbeta1 in the liver of wild-type mice using a recombinant replication-deficient adenovirus encoding human TGFbeta1 (Ad-TGFbeta1).

METHODS

TGFbeta1-ko mice (n=6) and normal mice (n=6) were injected once intraperitoneally (i.p.) with CCl4 (1 microl/g BW) or mineral oil. Wild-type mice (n=3) were injected intravenously with Ad-TGFbeta1 (10(10) pfu) or a control virus expressing beta-galactosidase (Ad-LacZ, 10(10) pfu). Animals were sacrificed after 3 days and total liver RNA was prepared. The expression of collagen alpha1(I) mRNA normalized to GAPDH mRNA was measured by RNase protection assay, asmooth muscle actin (alpha-sma) protein expression was analyzed by Western blotting. The expression of TGFbeta1, TGFbeta2, and TGFbeta3 mRNAs were determined semi-quantitatively with RT-PCR.

RESULTS

The collagen alpha1(I) mRNA was increased 10-fold in CCl4-treated wild-type mice compared to the controls. This increase was reduced about 80% in the TGFbeta1-ko mice. The TGFbeta1 mRNA levels in the wild-type mice were proportional to the collagen alpha1(I) mRNA levels. a-sma, a marker of hepatic stellate cell activation, was expressed earlier and at a higher level in wild-type mice than TGFbeta-ko mice after CCl4 treatment. The Ad-TGFbeta1 infected mice had 14-fold higher hepatic TGFbeta protein levels and 15-fold higher collagen alpha1(I) mRNA levels than the Ad-LacZ-infected control mice. Collagen alpha1(I) mRNA levels were proportional to the transgenic TGFbeta1 mRNA levels, while the endogenous TGFbeta1 was only slightly higher than in the controls. TGFbeta2 and TGFbeta3 mRNA levels were elevated in CCl4-treated wild-type and TGFbeta1-ko mice and in Ad-TGFbeta1-infected mice compared to the controls.

CONCLUSIONS

Absence of TGFbeta1 inhibits hepatic collagen alpha1(I) mRNA and alpha-sma protein expression by the toxic stimulus CCl4, and targeted TGFbeta1 overexpression increases collagen alpha1(I) mRNA and alpha-sma protein levels in the liver in vivo. Other TGFbeta family members do not compensate for the TGFbeta1 deficiency. This indicates that TGFbeta1 accelerates, but is not absolutely required, for the activation of hepatic stellate cells.

Transcriptional activation of transforming growth factor beta1 and its receptors by the Kruppel-like factor Zf9/core promoter-binding protein and Sp1. Potential mechanisms for autocrine fibrogenesis in response to injury

We have explored the regulation of transforming growth factor beta (TGF-beta) activity in tissue repair by examining the interactions of Zf9/core promoter-binding protein, a Kruppel-like zinc finger transcription factor induced early in hepatic stellate cell (HSC) activation, with promoters for TGF-beta1 and TGF-beta receptors, types I and II. Nuclear extracts from culture-activated HSCs bound avidly by electrophoretic mobility shift assay to two tandem GC boxes within the TGF-beta1 promoter but minimally to a single GC box; these results correlated with transactivation by Zf9 of TGF-beta1 promoter-reporters. Zf9 transactivated the full-length TGF-beta1 promoter in either primary HSCs, HSC-T6 cells (an SV40-immortalized rat HSC line), Hep G2 cells, or Drosophila Schneider (S2) cells. Recombinant Zf9-GST also bound to GC box sequences within the promoters for the types I and II TGF-beta receptors. Both type I and type II TGF-beta receptor promoters were also transactivated by Zf9 in mammalian cells but not in S2 cells. In contrast, Sp1 significantly transactivated both receptor promoters in S2 cells. These results suggest that (a) Zf9/core promoter-binding protein may enhance TGF-beta activity through transactivation of both the TGF-beta1 gene and its key signaling receptors, and (b) transactivating potential of Zf9 and Sp1 toward promoters for TGF-beta1 and its receptors are not identical and depend on the cellular context.

Expression of cellular prion protein in activated hepatic stellate cells

Suppression subtractive hybridization was used to clone genes associated with the activation of hepatic stellate cells and 13 genes were found to be dominantly expressed in activated stellate cells. Among them, one was identical to the 421-837th base pairs of cDNA sequence reported for rat prion-related protein (PrP). In cultured stellate cells, PrP mRNA expression increased in a time-dependent manner in parallel with smooth muscle (SM) alpha-actin mRNA expression. In situ hybridization demonstrated that PrP mRNA was localized in and around the fibrous septa of carbon tetrachloride (CCl4)-treated liver. Cellular PrP (PrPc) was produced by culture-activated stellate cells, and immunohistochemically detected in the fibrous septa of CCl4-damaged liver and sinusoidal linings of common bile duct-ligated liver, consistent with the localization of SM alpha-actin. Immunoelectron microscopy revealed that PrPc resided on the plasma membrane of stellate cells. These results indicate that PrP expression is closely related to stellate cell activation associated with fibrogenic stimuli.

Cloning the cDNA for a new human zinc finger protein defines a group of closely related Krüppel-like transcription factors

We have identified a novel zinc finger protein that has been named ubiquitous Krüppel-like factor (UKLF) based on structural considerations and the pattern of gene expression. UKLF was isolated by the polymerase chain reaction approach using degenerate oligonucleotides corresponding to the DNA-binding domain of erythroid Krüppel-like factor (EKLF) and cDNA prepared from human vascular endothelial cells. The carboxyl-terminal portion of UKLF contains three zinc fingers of the Cys2-His2 type and binds in vitro to the CACCC motif of the beta-globin promoter and to the Sp1 recognition sequence. The amino-terminal portion of UKLF consists of a hydrophobic region rich in serines and a negatively charged segment with several glutamic acid residues. The first 47 amino acids of the acidic region are nearly identical to the amino-terminal portion of another Krüppel-like factor, the so-called core promoter-binding protein (CPBP) or Zf9. Like CPBP/Zf9, UKLF can function as a transcription activator in co-transfection assays. However, this activity is lost when the highly conserved amino-terminal segment is deleted. These findings indicate that UKLF and CPBP/Zf9 represent a distinct subgroup of closely related Krüppel-like activators of transcription. Mapping of the UKLF gene to chromosome 2 suggested that UKLF and CPBP/Zf9 translocated to different chromosomes following duplication from an ancestral gene.

Zf9, a Kruppel-like transcription factor up-regulated in vivo during early hepatic fibrosis

Wound repair in the liver induces altered gene expression in stellate cells (resident mesenchymal cells) in a process known as "activation." A zinc finger transcription factor cDNA, zf9, was cloned from rat stellate cells activated in vivo. Zf9 expression and biosynthesis are increased markedly in activated cells in vivo compared with cells from normal rats ("quiescent" cells). The factor is localized to the nucleus and the perinuclear zone in activated but not quiescent cells. Zf9 mRNA also is expressed widely in nonhepatic adult rat tissues and the fetal liver. The zf9 nucleotide sequence predicts a member of the Kruppel-like family with a unique N-terminal domain rich in serine-proline clusters and leucines. The human zf9 gene maps to chromosome 10P near the telomere. Zf9 binds specifically to a DNA oligonucleotide containing a GC box motif. The N-terminal domain of Zf9 (amino acids 1-201) is transactivating in the chimeric GAL4 hybrid system. In Drosophila schneider cells, full length Zf9 transactivates a reporter construct driven by the SV40 promoter/enhancer, which contains several GC boxes. A physiologic role for Zf9 is suggested by its transactivation of a collagen alpha1(I) promoter reporter. Transactivation of collagen alpha1(I) by Zf9 is context-dependent, occurring strongly in stellate cells, modestly in Hep G2 cells, and not at all in D. schneider cells. Our results suggest that Zf9 may be an important signal in hepatic stellate cell activation after liver injury.

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.