A ribosomal S-6 kinase-mediated signal to C/EBP-beta is critical for the development of liver fibrosis

Histone methyltransferase Suv39h1 regulates hepatic stellate cell activation and is targetable in liver fibrosis

OBJECTIVE

Liver fibrosis is a prelude to a host of end-stage liver diseases. Hepatic stellate cells (HSCs), switching from a quiescent state to myofibroblasts, are the major source for excessive production of extracellular matrix proteins. In the present study, we investigated the role of Suv39h1, a lysine methyltransferase, in HSC-myofibroblast transition and the implication in liver fibrosis.

DESIGN

HSC-specific or myofibroblast-specific Suv39h1 deletion was achieved by crossbreeding the Suv39h1 f/f mice to the Lrat-Cre mice or the Postn-CreERT2 mice. Liver fibrosis was induced by CCl4 injection or bile duct ligation.

RESULTS

We report that Suv39h1 expression was universally upregulated during HSC-myofibroblast transition in different cell and animal models of liver fibrosis and in human cirrhotic liver tissues. Consistently, Suv39h1 knockdown blocked HSC-myofibroblast transition in vitro. HSC-specific or myofibroblast-specific deletion of Suv39h1 ameliorated liver fibrosis in mice. More importantly, Suv39h1 inhibition by a small-molecule compound chaetocin dampened HSC-myofibroblast transition in cell culture and mitigated liver fibrosis in mice. Mechanistically, Suv39h1 bound to the promoter of heme oxygenase 1 (HMOX1) and repressed HMOX1 transcription. HMOX1 depletion blunted the effects of Suv39h1 inhibition on HSC-myofibroblast transition in vitro and liver fibrosis in vivo. Transcriptomic analysis revealed that HMOX1 might contribute to HSC-myofibroblast transition by modulating retinol homeostasis. Finally, myofibroblast-specific HMOX1 overexpression attenuated liver fibrosis in both a preventive scheme and a therapeutic scheme.

CONCLUSIONS

Our data demonstrate a previously unrecognised role for Suv39h1 in liver fibrosis and offer proof-of-concept of its targetability in the intervention of cirrhosis.

Genetic Ablation of C/EBPα-p300 Pathway Blocks Development of Obese Pregnancy Associated Liver Disorders in Offspring

BACKGROUND & AIMS

The obesity-associated nonalcoholic fatty liver disease represents a common cause of pediatric liver diseases, including the pediatric liver cancer hepatoblastoma. The mechanisms behind the development of fatty liver in children are not yet known. We examined the role of the C/EBPα-p300 pathway in the development of maternal obesity-associated fatty liver phenotype in offspring.

METHODS

Because the ability of C/EBPα to promote fatty liver phenotype is enhanced by CDK4-mediated phosphorylation of C/EBPα at Ser193 and subsequent formation of C/EBPα-p300 complexes, we used wild-type (WT) and C/EBPα-S193D and C/EBPα-S193A mutant mice to study the effects of maternal high-fat diet (HFD) on the liver health of offspring. The females of these mouse lines were fed an HFD before mating, and the pups were further subjected to either an HFD or a normal diet for 12 weeks.

RESULTS

WT female mice on the HFD before and during pregnancy and their subsequent offspring on the HFD had severe fatty liver, fibrosis, and an increased rate of liver proliferation. However, the HFD in C/EBPα-S193A mice did not cause development of these disorders. In HFD-HFD treated WT mice, C/EBPα is phosphorylated at Ser193 and forms complexes with p300, which activate expression of genes involved in development of fatty liver, fibrosis, and proliferation. However, S193A-C/EBPα mice do not have complexes of C/EBPα-S193A with p300, leading to a lack of activation of genes of fatty liver, fibrosis, and proliferation. The mutant C/EBPα-S193D mice have accelerated cdk4-dependent pathway and have developed steatosis at early stages.

CONCLUSIONS

These studies identified the epigenetic cause of obese pregnancy-associated liver diseases and suggest a potential therapy based on inhibition of cdk4-ph-S193-C/EBPα-p300 pathway.

Effect of distinct ECM microenvironments on the genome-wide chromatin accessibility and gene expression responses of hepatic stellate cells

Hepatic stellate cells (HSCs) are one of the primary drivers of liver fibrosis in non-alcoholic fatty liver disease. Although HSC activation in liver disease is associated with changes in extracellular matrix (ECM) deposition and remodeling, it remains unclear how ECM regulates the phenotypic state transitions of HSCs. Using high-throughput cellular microarrays, coupled with genome-wide ATAC and RNA sequencing within engineered ECM microenvironments, we investigated the effect of ECM and substrate stiffness on chromatin accessibility and resulting gene expression in activated primary human HSCs. Cell microarrays demonstrated the cooperative effects of stiffness and ECM composition on H3K4 and H3K9 methylation/acetylation. ATAC sequencing revealed higher chromatin accessibility in HSCs on 1kPa compared to 25kPa substrates for all ECM conditions. Gene set enrichment analysis using RNA sequencing data of HSCs in defined ECM microenvironments demonstrated higher enrichment of NAFLD and fibrosis-related genes in pre-activated HSCs on 1kPa relative to 25kPa. Overall, these findings are indicative of a microenvironmental adaptation response in HSCs, and the acquisition of a persistent activation state. Combined ATAC/RNA sequencing analyses enabled identification of candidate regulatory factors, including HSD11B1 and CEBPb. siRNA-mediated knockdown of HSD11b1 and CEBPb demonstrated microenvironmental controlled reduction in fibrogenic markers in HSCs. STATEMENT OF SIGNIFICANCE: Hepatic stellate cells (HSCs) are one of the primary drivers of liver fibrosis in non-alcoholic fatty liver disease. Although HSC activation in liver disease is associated with changes in extracellular matrix (ECM) deposition and remodeling, it remains unclear how ECM regulates the phenotypic state transitions of HSCs. Using high-throughput cellular microarrays, coupled with genome-wide ATAC and RNA sequencing within engineered ECM microenvironments, we investigated the effect of ECM and substrate stiffness on chromatin accessibility and resulting gene expression in activated primary human HSCs. Overall, these findings were indicative of a microenvironmental adaptation response in HSCs, and the acquisition of a persistent activation state. Combined ATAC/RNA sequencing analyses enabled identification of candidate regulatory factors, including HSD11B1 and CEBPb. siRNA-mediated knockdown of HSD11b1 and CEBPb demonstrated microenvironmental controlled reduction in fibrogenic markers in HSCs.

Pathogenesis of Alcohol-Associated Liver Disease

Excessive alcohol consumption is a global healthcare problem with enormous social, economic, and clinical consequences. While chronic, heavy alcohol consumption causes structural damage and/or disrupts normal organ function in virtually every tissue of the body, the liver sustains the greatest damage. This is primarily because the liver is the first to see alcohol absorbed from the gastrointestinal tract via the portal circulation and second, because the liver is the principal site of ethanol metabolism. Alcohol-induced damage remains one of the most prevalent disorders of the liver and a leading cause of death or transplantation from liver disease. Despite extensive research on the pathophysiology of this disease, there are still no targeted therapies available. Given the multifactorial mechanisms for alcohol-associated liver disease pathogenesis, it is conceivable that a multitherapeutic regimen is needed to treat different stages in the spectrum of this disease.

Immune cell C/EBPβ deficiency is associated with hepatic mononuclear defects and spontaneous hepatitis but not steatohepatitis induced liver fibrosis

BACKGROUND

CCAAT/enhancer-binding protein β (C/EBPβ) is a transcription factor known to be involved in macrophage differentiation and function, steatohepatitis and liver fibrosis.

METHODS

Immune restricted C/EBPβ deficient and control mice were investigated in steady-state and in the CDA-HFD steatohepatitis model. Mice were assessed for weight change, liver biochemical profile, histology and hepatic phagocytes composition.

RESULTS

Flow cytometry analysis of hepatic nonparenchymal cells revealed reduced numbers of hepatic monocytes and Kupffer cells and an increase in hepatic MHC class II positive myeloid cells in immune cells restricted C/EBPβ deficient mice. Immune-restricted C/EBPβ deficiency resulted in decreased weight gain and appearance of mild spontaneous liver inflammation. Nevertheless, In the CDA-HFD steatohepatitis model, immune restricted C/EBPβ deficient and proficient mice exhibit similar grade of hepatic steatosis, liver enzymes levels and fibrosis stage.

CONCLUSIONS

Immune-restricted C/EBPβ deficiency leads to significant alteration in hepatic mononuclear phagocytes composition associated with spontaneous mild hepatitis. Steatohepatitis associated fibrosis is not dependent on C/EBPβ expression by immune cells.

CCAAT/Enhancer-Binding Proteins in Fibrosis: Complex Roles Beyond Conventional Understanding

CCAAT/enhancer-binding proteins (C/EBPs) are a family of at least six identified transcription factors that contain a highly conserved basic leucine zipper domain and interact selectively with duplex DNA to regulate target gene expression. C/EBPs play important roles in various physiological processes, and their abnormal function can lead to various diseases. Recently, accumulating evidence has demonstrated that aberrant C/EBP expression or activity is closely associated with the onset and progression of fibrosis in several organs and tissues. During fibrosis, various C/EBPs can exert distinct functions in the same organ, while the same C/EBP can exert distinct functions in different organs. Modulating C/EBP expression or activity could regulate various molecular processes to alleviate fibrosis in multiple organs; therefore, novel C/EBPs-based therapeutic methods for treating fibrosis have attracted considerable attention. In this review, we will explore the features of C/EBPs and their critical functions in fibrosis in order to highlight new avenues for the development of novel therapies targeting C/EBPs.

Role of Granulocyte Colony-stimulating Factor Therapy in Cirrhosis, 'Inside Any Deep Asking Is the Answering'

Liver cirrhosis progresses through multiple clinical stages which culminate in either death or liver transplantation. Availability of organs, timely listing and prompt receipt of donor-livers pose difficulties in improving transplant-listed and transplant outcomes. In this regard, regenerative therapies, particularly with granulocyte colony-stimulating factor (GCSF), has become a lucrative option for improving transplant-free survival. However, the literature is confusing with regards to patient selection and real outcomes. In this exhaustive review, we describe the basics of liver fibrosis and cirrhosis through novel insights from a therapeutic point of view, discuss preclinical studies on GCSF in advanced liver disease to improve on clinical utility, shed light on the pertinent literature of GCSF in advanced cirrhosis, and provide astute inputs on growth factor therapy in decompensated cirrhosis.

The serum amyloid A3 promoter-driven luciferase reporter mice is a valuable tool to image early renal fibrosis development and shows the therapeutic effect of glucosyl-hesperidin treatment

Tubulointerstitial fibrosis is a progressive process affecting the kidneys, causing renal failure that can be life-threatening. Thus, renal fibrosis has become a serious concern in the ageing population; however, fibrotic development cannot be diagnosed early and assessed noninvasively in both patients and experimental animal models. Here, we found that serum amyloid A3 (Saa3) expression is a potent indicator of early renal fibrosis; we also established in vivo Saa3/C/EBPβ-promoter bioluminescence imaging as a sensitive and specific tool for early detection and visualization of tubulointerstitial fibrosis. Saa3 promoter activity is specifically upregulated in parallel with tumor necrosis factor α (TNF-α) and fibrotic marker collagen I in injured kidneys. C/EBPβ, upregulated in injured kidneys and expressed in tubular epithelial cells, is essential for the increased Saa3 promoter activity in response to TNF-α, suggesting that C/EBPβ plays a crucial role in renal fibrosis development. Our model successfully enabled visualization of the suppressive effects of a citrus flavonoid derivative, glucosyl-hesperidin, on inflammation and fibrosis in kidney disease, indicating that this model could be widely used in exploring therapeutic agents for fibrotic diseases.

Hypoxia-induced ADAM 17 expression is mediated by RSK1-dependent C/EBPβ activation in human lung fibroblasts

Hypoxia was identified as a mediator of lung fibrosis in patients with chronic obstructive asthma (COA). Overexpression of a disintegrin and metalloproteinase 17 (ADAM 17) and connective tissue growth factor (CTGF) leads to development of tissue fibrosis. However, the signaling pathway in hypoxia-induced ADAM 17 expression remains poorly defined. In this study, we investigated the roles that ribosomal S-6 kinase 1 (RSK1)/CCAAT/enhancer-binding protein β (C/EBPβ)-dependent ADAM 17 expression plays in hypoxia-induced CTGF expression in human lung fibroblasts. We observed that hypoxia caused increases in ADAM 17 expression and ADAM 17-luciferase activity in WI-38 cells. Hypoxia-induced CTGF-luciferase activity and CTGF expression were reduced in cells transfected with small interfering (si)RNA of ADAM 17 in WI-38 cells. Moreover, hypoxia-induced ADAM 17 expression was reduced by RSK1 siRNA and C/EBPβ siRNA. Hypoxia caused time-dependent increases in RSK1 phosphorylation at Thr359/Ser363. Exposure of cells to hypoxia resulted in increased C/EBPβ phosphorylation at Thr266 and C/EBPβ-luciferase activity in time-dependent manners, and these effects were suppressed by RSK1 siRNA. Hypoxia induced recruitment of C/EBPβ to the ADAM 17 promoter. Furthermore, CTGF-luciferase activity induced by hypoxia was attenuated by RSK1 siRNA and C/EBPβ siRNA. These results suggest that hypoxia instigates the RSK1-dependent C/EBPβ signaling pathway, which in turn initiates binding of C/EBPβ to the ADAM 17 promoter and ultimately induces ADAM 17 expression in human lung fibroblasts. Moreover, RSK1/C/EBPβ-dependent ADAM 17 expression is involved in hypoxia-induced CTGF expression. Our results suggest possible therapeutic approaches for treating hypoxia-mediated lung fibrosis in COA.

Ribosomal S6 kinase (RSK) modulators: a patent review

INTRODUCTION

The p90 ribosomal S6 kinases (RSK) are a family of Ser/Thr protein kinases that are downstream effectors of MEK1/2-ERK1/2. Increased RSK activation is implicated in the etiology of multiple pathologies, including numerous types of cancers, cardiovascular disease, liver and lung fibrosis, and infections.

AREAS COVERED

The review summarizes the patent and scientific literature on small molecule modulators of RSK and their potential use as therapeutics. The patents were identified using World Intellectual Property Organization and United States Patent and Trademark Office databases. The compounds described are predominantly RSK inhibitors, but a RSK activator is also described. The majority of the inhibitors are not RSK-specific.

EXPERT OPINION

Based on the overwhelming evidence that RSK is involved in a number of diseases that have high mortalities it seems surprising that there are no RSK modulators that have pharmacokinetic properties suitable for in vivo use. MEK1/2 inhibitors are in the clinic, but the efficacy of these compounds appears to be limited by their side effects. We hypothesize that targeting the downstream effectors of MEK1/2, like RSK, are an untapped source of drug targets and that they will generate less side effects than MEK1/2 inhibitors because they regulate fewer effectors.

Kinase analysis in alcoholic hepatitis identifies p90RSK as a potential mediator of liver fibrogenesis

OBJECTIVE

Alcoholic hepatitis (AH) is often associated with advanced fibrosis, which negatively impacts survival. We aimed at identifying kinases deregulated in livers from patients with AH and advanced fibrosis in order to discover novel molecular targets.

DESIGN

Extensive phosphoprotein analysis by reverse phase protein microarrays was performed in AH (n=12) and normal human livers (n=7). Ribosomal S6 kinase (p90RSK) hepatic expression was assessed by qPCR, Western blot and immunohistochemistry. Kaempferol was used as a selective pharmacological inhibitor of the p90RSK pathway to assess the regulation of experimentally-induced liver fibrosis and injury, using in vivo and in vitro approaches.

RESULTS

Proteomic analysis identified p90RSK as one of the most deregulated kinases in AH. Hepatic p90RSK gene and protein expression was also upregulated in livers with chronic liver disease. Immunohistochemistry studies showed increased p90RSK staining in areas of active fibrogenesis in cirrhotic livers. Therapeutic administration of kaempferol to carbon tetrachloride-treated mice resulted in decreased hepatic collagen deposition, and expression of profibrogenic and proinflammatory genes, compared to vehicle administration. In addition, kaempferol reduced the extent of hepatocellular injury and degree of apoptosis. In primary hepatic stellate cells, kaempferol and small interfering RNA decreased activation of p90RSK, which in turn regulated key profibrogenic actions. In primary hepatocytes, kaempferol attenuated proapoptotic signalling.

CONCLUSIONS

p90RSK is upregulated in patients with chronic liver disease and mediates liver fibrogenesis in vivo and in vitro. These results suggest that the p90RSK pathway could be a new therapeutic approach for liver diseases characterised by advanced fibrosis.

C/EBPβ-Thr217 Phosphorylation Stimulates Macrophage Inflammasome Activation and Liver Injury

Amplification of liver injury is mediated by macrophages but the signaling by which the macrophage inflammasome enhances liver injury is not completely understood. The CCAAT/Enhancer Binding Protein-β (C/EBPβ) is a critical signaling molecule for macrophages because expression of a dominant inhibitor of C/EBPβ DNA-binding sites or a targeted deletion of C/EBPβ results in impaired macrophage differentiation. We reported that expression of the phosphorylation-mutant C/EBPβ-Glu217, which mimics phosphorylated C/EBPβ-Thr217, was sufficient to confer macrophage survival to Anthrax lethal toxin. Here, using primary hepatocytes, primary liver macrophages, dominant positive and negative transgenic mice of the C/EBPβ-Thr217 phosphoacceptor, macrophage ablation, and an inhibitory peptide of C/EBPβ-Thr217 phosphorylation, we determined that this phosphorylation is essential for the activation of the inflammasome in liver macrophages and for the hepatocyte apoptosis induced by hepatotoxins that results in liver injury. Similar findings were observed in the livers of patients with acute injury induced by Toxic Oil Syndrome.

Liquiritigenin Protects Rats from Carbon Tetrachloride Induced Hepatic Injury through PGC-1α Pathway

The lack of effective treatment for liver cirrhosis and hepatocellular carcinomas imposes serious challenges to the healthcare system. Here, we investigated the efficacy and mechanism of liquiritigenin involved in preventing or retarding the progression of liver diseases in a rat model with chronic carbon tetrachloride (CCl4) exposure. Sprague Dawley rats were given CCl4 and lliquiritigenin alone or simultaneously for 8 weeks before liver was harvested to check histological changes by Hematoxylin and Eosin (H&E) staining, apoptosis by TUNEL assay, ROS by dihydroethidium staining, antioxidant enzyme activities and malondialdehyde using specific kits, and gene expression by quantitative real-time PCR and western blot. Chronic CCl4 exposure caused profound changes in liver histology with extensive hepatocyte death (necrosis and apoptosis), fat accumulation, and infiltration of inflammatory cells, accompanied by depressed activities of antioxidant enzymes, increased oxidative stress, elevated expression of inflammation and fibrotic genes, and downregulation of PGC-1α, ND1, and Bcl-x in rat liver. All these changes were abolished or alleviated by lliquiritigenin. The results demonstrated that liquiritigenin is effective in protecting liver from injury or treating chronic liver diseases. The modulation of PGC-1α and its downstream genes might play a critical role in relieving CCl4-induced hepatic pathogenesis by liquiritigenin.

Strategies to prevent and reverse liver fibrosis in humans and laboratory animals

Liver fibrosis results from chronic damage to the liver in conjunction with various pathways and is mediated by a complex microenvironment. Based on clinical observations, it is now evident that fibrosis is a dynamic, bidirectional process with an inherent capacity for recovery and remodeling. The major mechanisms involved in liver fibrosis include the repetitive injury of hepatocytes, the activation of the inflammatory response after injury stimulation, and the activation and proliferation of hepatic stellate cells (HSCs), which represents the major extracellular matrix (ECM)-producing cells, stimulated by hepatocyte injury and inflammation. The microenvironment in the liver is synergistically regulated abnormal ECM deposition, scar formation, angiogenesis, and fibrogenesis. Moreover, recent studies have clarified novel mechanism in fibrosis such as epigenetic regulation of HSCs, the leptin and PPARγ pathways, the coagulation system, and even autophagy. Uncovering the mechanisms of liver fibrogenesis provides a basis to develop potential therapies to reverse and treat the fibrotic response, thereby improving the outcomes of patients with chronic liver disease. Although both scientific and clinical challenges remain, emerging studies attempt to reveal the ideal anti-fibrotic drug that could be easily delivered to the liver with high specificity and low toxicity. This review highlights the mechanisms, including novel pathways underlying fibrogenesis that may be translated into preventive and treatment strategies, reviews both current and novel agents that target specific pathways or multiple targets, and discusses novel drug delivery systems such as nanotechnology that can be applied in the treatment of liver fibrosis. In addition, we also discuss some current treatment strategies that are being applied in animal models and in clinical trials.

Extracellular matrix and liver disease

SIGNIFICANCE

The extracellular matrix (ECM) is a dynamic microenvironment that undergoes continuous remodeling, particularly during injury and wound healing. Chronic liver injury of many different etiologies such as viral hepatitis, alcohol abuse, drug-induced liver injury, obesity and insulin resistance, metabolic disorders, and autoimmune disease is characterized by excessive deposition of ECM proteins in response to persistent liver damage.

CRITICAL ISSUES

This review describes the main collagenous and noncollagenous components from the ECM that play a significant role in pathological matrix deposition during liver disease. We define how increased myofibroblasts (MF) from different origins are at the forefront of liver fibrosis and how liver cell-specific regulation of the complex scarring process occurs.

RECENT ADVANCES

Particular attention is paid to the role of cytokines, growth factors, reactive oxygen species, and newly identified matricellular proteins in the regulation of fibrillar type I collagen, a field to which our laboratory has significantly contributed over the years. We compile data from recent literature on the potential mechanisms driving fibrosis resolution such as MF' apoptosis, senescence, and reversal to quiescence.

FUTURE DIRECTIONS

We conclude with a brief description of how epigenetics, an evolving field, can regulate the behavior of MF and of how new "omics" tools may advance our understanding of the mechanisms by which the fibrogenic response to liver injury occurs.

Mesenchymal-specific deletion of C/EBPβ suppresses pulmonary fibrosis

The CCAAT/enhancer-binding protein β (C/EBPβ) regulates a variety of factors and cellular responses associated with pulmonary fibrosis. To distinguish its role in the mesenchyme from that in other compartments, the effects of mesenchymal-specific deletion of C/EBPβ on pulmonary fibrosis was examined. Crossing of mice with the floxed C/EBPβ gene with α2(I) collagen enhancer-CreER(T)-bearing mice successfully generated progeny with a conditional knockout (CKO) of C/EBPβ in collagen I-expressing ("mesenchymal") cells only on treatment with tamoxifen (C/EBPβ CKO). When treated with an endotracheal bleomycin injection, C/EBPβ CKO mice showed significant attenuation of pulmonary fibrosis relative to control C/EBPβ-intact mice. C/EBPβ CKO mice also had reduced myofibroblasts in the lung. However, no significant differences in inflammatory/immune cell influx were noted in the mutant mice relative to the control mice. DNA microarray and real-time PCR analyses identified a series of myofibroblast differentiation regulators as novel target genes of C/EBPβ. Interestingly, C/EBPβ deficiency caused a marked induction of matrix metalloproteinase 12 expression, suggesting its potential role as a repressor, which could account for the noted reduction in fibrosis in the C/EBPβ-deficient mice. Thus, these findings indicate an essential role for C/EBPβ in the mesenchymal compartment in pulmonary fibrosis that is independent of its effects on inflammation or immune cell infiltration.

Expression2Kinases: mRNA profiling linked to multiple upstream regulatory layers

MOTIVATION

Genome-wide mRNA profiling provides a snapshot of the global state of cells under different conditions. However, mRNA levels do not provide direct understanding of upstream regulatory mechanisms. Here, we present a new approach called Expression2Kinases (X2K) to identify upstream regulators likely responsible for observed patterns in genome-wide gene expression. By integrating chromatin immuno-precipitation (ChIP)-seq/chip and position weight matrices (PWMs) data, protein-protein interactions and kinase-substrate phosphorylation reactions, we can better identify regulatory mechanisms upstream of genome-wide differences in gene expression. We validated X2K by applying it to recover drug targets of food and drug administration (FDA)-approved drugs from drug perturbations followed by mRNA expression profiling; to map the regulatory landscape of 44 stem cells and their differentiating progeny; to profile upstream regulatory mechanisms of 327 breast cancer tumors; and to detect pathways from profiled hepatic stellate cells and hippocampal neurons. The X2K approach can advance our understanding of cell signaling and unravel drugs mechanisms of action.

AVAILABILITY

The software and source code are freely available at: http://www.maayanlab.net/X2K.

CONTACT

avi.maayan@mssm.edu

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

C/EBPβ-Thr217 phosphorylation signaling contributes to the development of lung injury and fibrosis in mice

Background

Although C/EBPβ^ko mice are refractory to Bleomycin-induced lung fibrosis the molecular mechanisms remain unknown. Here we show that blocking the ribosomal S-6 kinase (RSK) phosphorylation of the CCAAT/Enhancer Binding Protein (C/EBP)-β on Thr217 (a RSK phosphoacceptor) with either a single point mutation (Ala217), dominant negative transgene or a blocking peptide containing the mutated phosphoacceptor ameliorates the progression of lung injury and fibrosis induced by Bleomycin in mice.

Methodology/Principal Findings

Mice expressing the non-phosphorylatable C/EBPβ-Ala217 transgene had a marked reduction in lung injury on day-13 after Bleomycin exposure, compared to C/EBPβ^wt mice, judging by the decrease of CD68⁺ activated monocytes/macrophages, bone marrow-derived CD45⁺ cells and lung cytokines as well as by the normal surfactant protein-C expression by lung pneumocytes. On day-21 after Bleomycin treatment, C/EBPβ^wt mice but not mice expressing the dominant negative C/EBPβ-Ala217 transgene developed severe lung fibrosis as determined by quantitative collagen assays. All mice were of identical genetic background and back-crossed to the parental wild-type inbreed FVB mice for at least ten generations. Treatment of C/EBPβ^wt mice with a cell permeant, C/EBPβ peptide that inhibits phosphorylation of C/EBPβ on Thr217 (40 µg instilled intracheally on day-2 and day-6 after the single Bleomycin dose) also blocked the progression of lung injury and fibrosis induced by Bleomycin. Phosphorylation of human C/EBPβ on Thr266 (human homologue phosphoacceptor) was induced in collagen-activated human lung fibroblasts in culture as well as in activated lung fibroblasts in situ in lungs of patients with severe lung fibrosis but not in control lungs, suggesting that this signaling pathway may be also relevant in human lung injury and fibrosis.

Conclusions/Significance

These data suggest that the RSK-C/EBPβ phosphorylation pathway may contribute to the development of lung injury and fibrosis.

Elevated activation of ERK1 and ERK2 accompany enhanced liver injury following alcohol binge in chronically ethanol-fed rats

BACKGROUND

Binge drinking after chronic ethanol consumption is one of the important factors contributing to the progression of steatosis to steatohepatitis. The molecular mechanisms of this effect remain poorly understood. We have therefore examined in rats the effect of single and repeat ethanol binge superimposed on chronic ethanol intake on liver injury, activation of mitogen-activated protein kinases (MAPKs), and gene expression.

METHODS

Rats were chronically treated with ethanol in liquid diet for 4 weeks followed by single ethanol binge (5 gm/kg body weight) or 3 similar repeated doses of ethanol. Serum alcohol and alanine amino transferase (ALT) levels were determined by enzymatic methods. Steatosis was assessed by histology and hepatic triglycerides. Activation of MAPK, 90S ribosomal kinase (RSK), and caspase 3 were evaluated by Western blot. Levels of mRNA for tumor necrosis factor alpha (TNFα), early growth response-1 (egr-1), and plasminogen activator inhibitor-1 (PAI-1) were measured by real-time qRT-PCR.

RESULTS

Chronic ethanol treatment resulted in mild steatosis and necrosis, whereas chronic ethanol followed by binge group exhibited marked steatosis and significant increase in necrosis. Chronic binge group also showed significant increase (compared with chronic ethanol alone) in the phosphorylation of extracellular regulated kinase 1 (ERK1), ERK2, and RSK. Phosphorylation of c-Jun N-terminal kinase (JNK) and p38 MAPK did not increase by the binge. Ethanol binge, after chronic ethanol intake, caused increase in mRNA for egr-1 and PAI-1, but not TNFα.

CONCLUSIONS

Chronic ethanol exposure increases the susceptibility of rat liver to increased injury by 1 or 3 repeat binge. Among other alterations, the activated levels of ERK1, and more so ERK2, were remarkably amplified by binge suggesting a role of these isotypes in the binge amplification of the injury. In contrast, p38 MAPK and JNK1/2 activities were not amplified. These binge-induced changes were also reflected in the increases in the RNA levels for egr-1 and PAI-1. This study offers chronic followed by repeat binge as a model for the study of progression of liver injury by ethanol and highlights the involvement of ERK1 and ERK2 isotypes in the amplification of liver injury by binge ethanol.

Pathogenesis of liver fibrosis

Liver fibrosis is a major cause of morbidity and mortality worldwide due to chronic viral hepatitis and, more recently, from fatty liver disease associated with obesity. Hepatic stellate cell activation represents a critical event in fibrosis because these cells become the primary source of extracellular matrix in liver upon injury. Use of cell-culture and animal models has expanded our understanding of the mechanisms underlying stellate cell activation and has shed new light on genetic regulation, the contribution of immune signaling, and the potential reversibility of the disease. As pathways of fibrogenesis are increasingly clarified, the key challenge will be translating new advances into the development of antifibrotic therapies for patients with chronic liver disease.

Evolving challenges in hepatic fibrosis

Continued elucidation of the mechanisms of hepatic fibrosis has yielded a comprehensive and nuanced portrait of fibrosis progression and regression. The paradigm of hepatic stellate cell (HSC) activation remains the foundation for defining events in hepatic fibrosis and has been complemented by progress in a number of new areas. Cellular sources of extracellular matrix beyond HSCs have been identified. In addition, the role of chemokine, adipokine, neuroendocrine, angiogenic and NAPDH oxidase signaling in the pathogenesis of hepatic fibrosis has been uncovered, as has the contribution of extracellular matrix stiffness to fibrogenesis. There is also increased awareness of the contribution of innate immunity and greater understanding of the complexity of gene regulation in HSCs and myofibroblasts. Finally, both apoptosis and senescence have been recognized as orchestrated programs that eliminate fibrogenic cells during resolution of liver fibrosis. Ironically, the progress that has been made has highlighted the growing disparity between advances in the experimental setting and their translation into new diagnostic tools and treatments. As a result, focus is shifting towards overcoming key translational challenges in order to accelerate the development of new therapies for patients with chronic liver disease.

RSK in tumorigenesis: connections to steroid signaling

The Ser/Thr kinase family, RSK, has been implicated in numerous types of hormone-dependent and -independent cancers. However, there has been little consideration of RSKs as downstream mediators of steroid hormone non-genomic effects or of their ability to facilitate steroid receptor-mediated gene expression. Steroid hormone signaling can directly stimulate the MEK/ERK/RSK pathway to regulate cellular proliferation and survival in transformed cells. To date, multiple mechanisms of RSK and steroid hormone receptor-mediated proliferation/survival have been elucidated. For example, RSK enhances proliferation of breast and prostate cancer cells via its ability to control the levels of the estrogen receptor co-activator, cyclin D1. While in lung and other tumors RSK may control apoptosis via estrogen-mediated regulation of mitochondrial integrity. Thus the RSKs could be important anti-cancer therapeutic targets in many different transformed tissues. The recent discovery of RSK-specific inhibitors will advance our current understanding of RSK in transformation and drive these studies into animal and clinical models. In this review we explore the mechanisms associated with RSK in tumorigenesis and their relationship to steroid hormone signaling.

RSK2 Binding Models Delineate Key Features for Activity

Due to its overexpression and activation in human cancer cells and tissues, an emerging molecular target in cancer therapeutics is p90 ribosomal s6 kinase 2 (RSK2). While a growing number of RSK2 inhibitors have been reported in the literature, only the crystal structure of RSK2 in complex with an AMP analogue provides a structural basis for understanding RSK2 inhibition. To remedy this, we used our fluorescence polarization assay to determine the RSK2 activity for a set of structurally diverse compounds, and followed this by modeling their binding modes in an all-atom, energy refined crystal structure of RSK2. These binding models reveal that Val131 and Leu147 are key interaction sites for potent RSK2 inhibition. This structure-based pharmacophore is an important tool for new lead discovery and refinement.

Decreased Jun-D and myogenin expression in muscle wasting of human cachexia

Muscle wasting is a critical feature of patients afflicted by acquired immune deficiency syndrome (AIDS), cancer, or chronic inflammatory diseases. In a mouse model of muscle wasting, TNF-alpha induces oxidative stress and nitric oxide synthase-2 (NOS2) and decreases myogenin, Jun-D, and creatinine kinase muscle isoform (CKM) expression. Here, we studied 12 patients with muscle wasting due to cancer (N = 10) or AIDS (N = 2) and 4 control subjects. We show that in skeletal muscle of cachectic patients there is 1) increased expression and activity of the TNF-alpha signaling, including TNF-alpha mRNA, activation of TNFR1, and TNF-alpha-associated to TNFR1; 2) increased oxidative stress, as determined by the presence of malondialdehyde-lysine adducts; 3) increased NOS2 mRNA and protein; 4) decreased expression of Jun-D, myogenin, myosin, and CKM mRNA and protein; 5) impaired CKM-E box binding activities, associated with decreased Jun-D/myogenin activities; and 6) oxidative modification and ubiquitination of Jun-D. These studies show that these molecular pathways are modulated in association with muscle wasting in patients with cancer or AIDS, and whether or not they cause muscle wasting remains to be determined.

Transcriptional regulation of hepatic stellate cells

Hepatic stellate cell (HSC) activation is a process of cellular transdifferentiation in which, upon liver injury, the quiescent vitamin A storing perisinusoidal HSC is converted into a wound-healing myofibroblast and acquires potent pro-inflammatory and pro-fibrogenic activities. This remarkable phenotypic transformation is underpinned by changes in the expression of a vast number of genes. In this review we survey current knowledge of the transcription factors that either control HSC activation or which regulate specific fibrogenic functions of the activated HSC such as collagen expression, proliferation and resistance to apoptosis.

Involvement of 90-kuD ribosomal S6 kinase in collagen type I expression in rat hepatic fibrosis

AIM: To investigate the relationship between 90-kuD ribosomal S6 kinase (p90RSK) and collagen type I expression during the development of hepatic fibrosis in vivo and in vitro.

METHODS: Rat hepatic fibrosis was induced by intraperitoneal injection of dimethylnitrosamine. The protein expression and cell location of p90RSK and their relationship with collagen type I were determined by co-immunofluoresence and confocal microscopy. Subsequently, RNAi strategy was employed to silence p90RSK mRNA expression in HSC-T6, an activated hepatic stellate cell (HSC) line. The expression of collagen type I in HSC-T6 cells was assessed by Western blotting and real-time polymerase chain reaction. Furthermore, HSCs were transfected with expression vectors or RNAi constructs of p90RSK to increase or decrease the p90RSK expression, then collagen type I promoter activity in the transfected HSCs was examined by reporter assay. Lastly HSC-T6 cells transfected with p90RSK siRNA was treated with or without platelet-derived growth factor (PDGF)-BB at a final concentration of 20 &mgr;g/L and the cell growth was determined by MTS conversion.

RESULTS: In fibrotic liver tissues, p90RSK was over-expressed in activated HSCs and had a significant positive correlation with collagen type I levels. In HSC-T6 cells transfected with RNAi targeted to p90RSK, the expression of collagen type I was down-regulated (61.8% in mRNA, P < 0.01, 89.1% in protein, P < 0.01). However, collagen type I promoter activity was not increased with over-expression of p90RSK and not decreased with low expression either, compared with controls in the same cell line (P = 0.076). Furthermore, p90RSK siRNA exerted the inhibition of HSC proliferation, and also abolished the effect of PDGF on the HSC proliferation.

CONCLUSION: p90RSK is over-expressed in activated HSCs and involved in regulating the abnormal expression of collagen type I through initiating the proliferation of HSCs.

Severe hepatocellular injury with apoptosis induced by a hepatitis C polymerase inhibitor

GOALS

To describe the mechanisms of severe hepatocellular injury with apoptosis in 2 patients receiving hepatitis C virus (HCV)-796.

BACKGROUND

HCV-796 is a hepatitis C polymerase inhibitor approved by the US Food and Drug Administration for a phase 2 study of the treatment of hepatitis C in combination with PEG-Interferon and ribavirin.

RESULTS

The injury occurred after more than 12 weeks of treatment, with a >20-fold increase in serum alanine aminotransferase and aspartate aminotransferase, and a marked increase in total (and direct) bilirubin in the absence of cholestasis. There was no evidence of autoimmune or viral hepatitis. Involvement of the mitochondrial apoptotic pathway was demonstrated by (1) release of cytochrome C into the cytosol; (2) association of cytochrome C with apoptotic protease activating factor-1 in the cytosol; (3) activation of initiator caspase 9; (4) activation of effector caspase 3; (5) increased serum caspase-3 cleaved cytokeratin-18 peptide; (6) nuclear fragmentation; (7) mitochondrial structural abnormalities; (8) expression of light chain 3 B, an indicator of autophagy; (9) probable autophagy of mitochondria by autophagosomes; and (10) probable phagocytosis of apoptotic hepatocytes by activated macrophages. Immunoglobulin G immune complexes were identified in the hepatocytes and localized to the endoplasmic reticulum and Golgi of these patients after the drug-induced liver disease, reflecting a primary or secondary target. Hepatitis C treatment was discontinued at weeks 15 and 19 in patients 1 and 2, respectively. After more than 6 months off the medication, both patients normalized the serum alanine aminotransferase, aspartate aminotransferase, and total bilirubin with undetectable HCV RNA.

CONCLUSIONS

HCV-796 may cause severe hepatocellular injury and apoptosis, with a marked immune reaction in susceptible patients.

Current and future anti-fibrotic therapies for chronic liver disease

Chronic injury results in a wound healing response that eventually leads to fibrosis. The response is generalized, with features common among multiple organ systems. In the liver, various different types of injury lead to fibrogenesis, implying a common pathogenesis. Although several specific therapies for patients who have different liver diseases have been successfully developed, including antiviral therapies for those who have hepatitis B and hepatitis C virus infection, specific and effective antifibrotic therapy remains elusive. Over the past 2 decades, great advances in the understanding of fibrosis have been made and multiple mechanisms underlying hepatic fibrogenesis uncovered. Elucidation of these mechanisms has been of fundamental importance in highlighting novel potential therapies. Preclinical studies have indicated several putative therapies that might abrogate fibrogenesis. This article emphasizes mechanisms underlying fibrogenesis and reviews available and future therapeutics.

Targeting liver myofibroblasts: a novel approach in anti-fibrogenic therapy

Chronic liver disease results in a liver-scarring response termed fibrosis. Excessive scarring leads to cirrhosis, which is associated with high morbidity and mortality. The only treatment for liver cirrhosis is liver transplantation; therefore, much attention has been directed toward therapies that will slow or reverse fibrosis. Although anti-fibrogenic therapies have been shown to be effective in experimental animal models, licensed therapies have yet to emerge. A potential problem for any anti-fibrogenic therapy in the liver is the existence of the body’s major drug metabolising cell (the hepatocyte) adjacent to the primary fibrosis-causing cell, the myofibroblast. This article reviews the development of a human recombinant single-chain antibody (scAb) that binds to the surface of myofibroblasts. This antibody binds specifically to myofibroblasts in fibrotic mouse livers. When conjugated with a compound that stimulates myofibroblast apoptosis, the antibody directs the specific apoptosis of myofibroblasts with greater specificity and efficacy than the free compound. The antibody also reduces the adverse effect of liver macrophage apoptosis and—in contrast to the free compound—reversed fibrosis in the sustained injury model used. These data suggest that specifically stimulating the apoptosis of liver myofibroblasts using a targeting antibody has potential in the treatment of liver fibrosis.

Direct infection and replication of naturally occurring hepatitis C virus genotypes 1, 2, 3 and 4 in normal human hepatocyte cultures

BACKGROUND

Hepatitis C virus (HCV) infection afflicts about 170 million individuals worldwide. However, the HCV life cycle is only partially understood because it has not been possible to infect normal human hepatocytes in culture. The current Huh-7 systems use cloned, synthetic HCV RNA expressed in hepatocellular carcinoma cells to produce virions, but these cells cannot be infected with naturally occurring HCV obtained from infected patients.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we describe a human hepatocyte culture permissible to the direct infection with naturally occurring HCV genotypes 1, 2, 3 and 4 in the blood of HCV-infected patients. The culture system mimics the biology and kinetics of HCV infection in humans, and produces infectious virions that can infect naïve human hepatocytes.

CONCLUSIONS/SIGNIFICANCE

This culture system should complement the existing systems, and may facilitate the understanding of the HCV life cycle, its effects in the natural host cell, the hepatocyte, as well as the development of novel therapeutics and vaccines.