Toll-like receptor 9 promotes steatohepatitis by induction of interleukin-1beta in mice

Sterile inflammation in the liver and pancreas

The ability of tissue injury to result in inflammation is a well-recognized phenomenon and is central to a number of common liver and pancreatic diseases including alcoholic steatohepatitis and pancreatitis, as well as drug-induced liver injury, non-alcoholic steatohepatitis, and pancreatitis from other causes. The requirements of extracellular damage-associated molecules and a cytosolic machinery labeled the inflammasome have been established in in vitro culture systems and in vivo disease models. This has provided a generic insight into the pathways involved, and the challenge now is to understand the specifics of these mechanisms in relation to the particular insults and organs involved. One reason for the excitement in this field is that a number of therapeutic candidates such a toll-like receptor antagonists and interleukin-1R antagonists are either approved or in clinical trials for other indications.

Toll-like receptors in alcoholic liver disease, non-alcoholic steatohepatitis and carcinogenesis

Activation of innate immune systems including Toll-like receptor (TLR) signaling is a key in chronic liver disease. Recent studies suggest that gut microflora-derived bacterial products (i.e. lipopolysaccharide [LPS], bacterial DNA) and endogenous substances (i.e. high-mobility group protein B1 [HMGB1], free fatty acids) released from damaged cells activate hepatic TLRs that contribute to the development of alcoholic (ASH) and non-alcoholic steatohepatitis (NASH) and liver fibrosis. The crucial role of TLR4, a receptor for LPS, has been implicated in the development of ASH, NASH, liver fibrosis, and hepatocellular carcinoma. However, the role of other TLRs, such as TLR2 and TLR9 in chronic liver disease remains less clear. In this review, we will discuss the role of TLR2, 4, and 9 in Kupffer cells and hepatic stellate cells in the development of ASH, NASH, and hepatocarcinogenesis.

From cirrhosis to hepatocellular carcinoma: new molecular insights on inflammation and cellular senescence

Sequential progression from chronic liver disease to fibrosis and to cirrhosis culminates in neoplasia in hepatocellular carcinoma (HCC). The preneoplastic setting of the cirrhotic background provides a conducive environment for cellular transformation. The role of classical inflammation in cirrhosis is widely known, but the exact mechanism linking inflammation and cancer remains elusive. Recent studies have elucidated roles for NF-κB, STAT3 and JNK as possible missing links. In addition, the "inflammasome" (a multiprotein complex and sensor of cellular damage) is a recently identified player in this field. The hallmarks of cirrhosis include necroinflammation, deposition of extracellular matrix and shortening of telomeres, leading to senescence and regeneration. Additionally, the accumulation of genetic/epigenetic changes propels atypical cells toward a malignant phenotype. This review provides recent information on the classical inflammatory pathway, together with a spotlight on inflammasomes and the immunomodulatory role of cellular senescence during the progression from cirrhosis to HCC. Moreover, lacunae in the current knowledge were identified and key questions raised on whether the observed adaptive responses are beneficial or detrimental to tissue homeostasis in a complex organ like liver.

Effects of sodium butyrate and its synthetic amide derivative on liver inflammation and glucose tolerance in an animal model of steatosis induced by high fat diet

Background & Aims

Nonalcoholic fatty liver disease (NAFLD) is the most common form of chronic liver disease. Insulin resistance (IR) appears to be critical in its pathogenesis. We evaluated the effects of sodium butyrate (butyrate) and its synthetic derivative N-(1-carbamoyl-2-phenyl-ethyl) butyramide (FBA) in a rat model of insulin resistance and steatosis induced by high-fat diet (HFD).

Methods

After weaning, young male Sprague-Dawley rats were divided into 4 groups receiving different diets for 6 weeks: 1. control group (standard diet); 2. HFD; 3. HFD plus butyrate (20 mg/kg/die) and 4. HFD plus FBA (42.5 mg/Kg/die, the equimolecular dose of butyrate). Liver tissues of the rats were analyzed by Western blot and real-time PCR. Insulin resistance, liver inflammation and Toll-like pattern modifications were determined.

Results

Evaluation of these two preparations of butyrate showed a reduction of liver steatosis and inflammation in HFD fed animals. The compounds showed a similar potency in the normalisation of several variables, such as transaminases, homeostasis model assessment for insulin resistance index, and glucose tolerance. Both treatments significantly reduced hepatic TNF-α expression and restored GLUTs and PPARs, either in liver or adipose tissue. Finally, FBA showed a higher potency in reducing pro-inflammatory parameters in the liver, via suppression of Toll-like receptors and NF-κB activation.

Conclusions

Our results demonstrated a protective effect of butyrate in limiting molecular events underlying the onset of IR and NAFLD, suggesting a potential clinical relevance for this substance. In particular, its derivative, FBA, could represent an alternative therapeutic option to sodium butyrate, sharing a comparable efficacy, but a better palatability and compliance.

Gut-liver axis and fibrosis in nonalcoholic fatty liver disease: an input for novel therapies

Non-alcoholic fatty liver disease is a multifactorial condition, ranging from simple steatosis to non-alcoholic steatohepatitis with or without fibrosis. In non-alcoholic fatty liver disease, alteration of gut microbiota and increased intestinal permeability increase exposure of the liver to gut-derived bacterial products: lipopolysaccharides and unmethylated CpG DNA. These products stimulate innate immune receptors, namely Toll-like receptors, which activate signalling pathways involved in liver inflammation and fibrogenesis. Currently, there are several studies on the involvement of lipopolysaccharide-activated Toll-like receptor 4 signalling in non-alcoholic fatty liver disease pathogenesis. There has been widespread interest in the study of the involvement of resident hepatic stellate cells and Kupffer cells activation in liver fibrogenesis upon TLR4 stimulation. Although the best evidence to support a role for gut microbiota in non-alcoholic fatty liver disease-induced fibrosis comes largely from animal models, data from human studies are accumulating and could lead to new therapeutic approaches. Therapeutic modulation of gut microflora may be an alternative strategy to develop an anti-fibrotic therapy. In this review, we discuss the relevant role of gut-liver axis in non-alcoholic liver disease-associated liver fibrosis and discuss the evidence on novel anti-fibrotic therapeutic approaches.

Regulation of wound healing and organ fibrosis by toll-like receptors

Chronic injury often triggers maladaptive wound healing responses leading to the development of tissue fibrosis and subsequent organ malfunction. Inflammation is a key component of the wound healing process and promotes the development of organ fibrosis. Here, we review the contribution of Toll-like receptors (TLRs) to wound healing with a particular focus on their role in liver, lung, kidney, skin and myocardial fibrosis. We discuss the role of TLRs on distinct cell populations that participate in the repair process following tissue injury, and the contribution of exogenous and endogenous TLR ligands to the wound healing response. Systemic review of the literature shows that TLRs promote tissue repair and fibrosis in many settings, albeit with profound differences between organs. In particular, TLRs exert a pronounced effect on fibrosis in organs with higher exposure to bacterial TLR ligands, such as the liver. Targeting TLR signaling at the ligand or receptor level may represent a novel strategy for the prevention of maladaptive wound healing and fibrosis in chronically injured organs. This article is part of a Special Issue entitled: Fibrosis: Translation of basic research to human disease.

Intestinal microbiota in patients with nonalcoholic fatty liver disease

Despite evidence that the intestinal microbiota (IM) is involved in the pathogenesis of obesity, the IM composition of patients with nonalcoholic fatty liver disease (NAFLD) has not been well characterized. This prospective, cross-sectional study was aimed at identifying differences in IM between adults with biopsy-proven NAFLD (simple steatosis [SS] or nonalcoholic steatohepatitis [NASH]) and living liver donors as healthy controls (HC). Fifty subjects were included: 11 SS, 22 NASH, and 17 HC. One stool sample was collected from each participant. Quantitative real-time polymerase chain reaction was used to measure total bacterial counts, Bacteroides/Prevotella (herein referred to as Bacteroidetes), Clostridium leptum, C. coccoides, bifidobacteria, Escherichia coli and Archaea in stool. Clinical and laboratory data, food records, and activity logs were collected. Patients with NASH had a lower percentage of Bacteroidetes (Bacteroidetes to total bacteria counts) compared to both SS and HC (P = 0.006) and higher fecal C. coccoides compared to those with SS (P = 0.04). There were no differences in the remaining microorganisms. As body mass index (BMI) and dietary fat intake differed between the groups (P < 0.05), we performed linear regression adjusting for these variables. The difference in C. coccoides was no longer significant after adjusting for BMI and fat intake. However, there continued to be a significant association between the presence of NASH and lower percentage Bacteroidetes even after adjusting for these variables (P = 0.002; 95% confidence interval = -0.06 to -0.02).

CONCLUSION

There is an inverse and diet-/BMI-independent association between the presence of NASH and percentage Bacteroidetes in the stool, suggesting that the IM may play a role in the development of NAFLD.

Liver Injury and the Activation of the Hepatic Myofibroblasts

Liver fibrosis is a wound healing process, the end result of chronic liver injury elicited by different noxious stimuli. Activated hepatic stellate cells or myofibroblasts and portal myofibroblasts are considered as the main producers of the extracellular matrix in the liver. Upon liver injury the quiescent stellate cells transdifferentiate into myofibroblasts a process highlighted by the loss of vitamin A stores, upregulation of interstitial type collagens, smooth muscle α actin, matrix metalloproteinases, proteoglycans, and the induction of cell survival pathways. Activation of hepatic stellate cells is a result of a complex interplay between the parenchymal cells, immune cells, extracellular matrix mechanics and extrahepatic milieu such as the gut microbiome. In this review we will focus on the pathomechanism of stellate cell activation following chronic liver injury; with the aim of identifying possible treatment targets for anti-fibrogenic agents.

Innate immune receptors: key regulators of metabolic disease progression

The study of the intersection of immunology and metabolism is a growing field fueled by the increased prevalence of obesity-associated pathologies. Importantly, the capacity of the innate immune system to sense metabolic stress induced by nutritional surplus has been linked with the progression of obesity, insulin resistance, type 2 diabetes mellitus, nonalcoholic fatty liver disease, and atherosclerosis. Moreover, it is clear that the innate immune system regulates the composition of the intestinal microbiota, which impacts multiple host metabolic processes. Here we review recent studies in this emerging field with an emphasis on how innate immune receptors determine metabolic disease progression.

The innate immune response during liver inflammation and metabolic disease

The role of the inflammatory response is to combat tissue injury and infection. Innate immune cells recognize cell damage or pathogen invasion with intracellular or surface-expressed pattern recognition receptors (PRRs). Activated PRRs subsequently initiate signaling cascades that trigger the release of factors promoting the inflammatory response. Because the liver is a site where foreign antigens from the gastrointestinal tract encounter the immune system, it is particularly enriched with innate immune cells. These cells can modify and disrupt critical processes implicated in metabolic disease. As such, metabolic stress initiates a feedforward cycle of inflammatory responses, resulting in a state of unresolved chronic inflammation in the liver. Accordingly, the crosstalk between these innate immune cells and the resident parenchymal cells plays an important role in the development of acute and chronic liver disease.

Therapeutic strategies in inflammasome mediated diseases of the liver

Tissue stress and cell death result in inflammation even in the absence of pathogens. Such sterile inflammation is dependent on a cytosolic complex of proteins inside immune cells termed the inflammasome. This complex converts two groups of extracellular signals into an inflammatory response via activation of caspase-1 and secretion of IL-1β and IL-18. Group 1 signals are typically TOLL like receptor agonists and result in transcriptional upregulation of inflammasome components and pro-cytokines. Group 2 signals are diverse, ranging from uric acid to ATP, and lead to assembly and activation of the inflammasome complex. Inflammasome components are required for a wide range of acute and chronic pathologies, including experimental alcoholic and non-alcoholic steatohepatitis, and drug-induced liver injury. Collectively, group 1 and 2 signals, inflammasome components, and cytokine receptors provide a rich source of therapeutic targets. Many of the advances in the field have come from standard reductionist experiments. Progress in the understanding of complex human systems will, however, be dependent on novel strategies such as systems analysis, which analyze large data sets to provide new insights.

Therapeutic potential of interleukin 1 inhibitors in the treatment of alcoholic liver disease

Alcoholic liver disease (ALD) is characterized by steatosis and upregulation of proinflammatory cytokines, including IL-1β. IL-1β, type I IL-1 receptor (IL-1R1), and IL-1 receptor antagonist (IL-1Ra) are all important regulators of the IL-1 signaling complex, which plays a role in inflammation. Furthermore, IL-1β maturation is dependent on caspase-1 (Casp-1). Using IL-1Ratreated mice as well as 3 mouse models deficient in regulators of IL-1β activation (Casp-1 and ASC) or signaling (IL-1R1), we found that IL-1β signaling is required for the development of alcohol-induced liver steatosis, inflammation, and injury. Increased IL-1β was due to upregulation of Casp-1 activity and inflammasome activation. The pathogenic role of IL-1 signaling in ALD was attributable to the activation of the inflammasome in BM-derived Kupffer cells. Importantly, in vivo intervention with a recombinant IL-1Ra blocked IL-1 signaling and markedly attenuated alcohol-induced liver inflammation, steatosis, and damage. Furthermore, physiological doses of IL-1β induced steatosis, increased the inflammatory and prosteatotic chemokine MCP-1 in hepatocytes, and augmented TLR4-dependent upregulation of inflammatory signaling in macrophages. In conclusion, we demonstrated that Casp-1-dependent upregulation of IL-1β and signaling mediated by IL-1R1 are crucial in ALD pathogenesis. Our findings suggest a potential role of IL-1R1 inhibition in the treatment of ALD.

Novel therapeutic targets for nonalcoholic fatty liver disease

INTRODUCTION

Nonalcoholic fatty liver disease (NAFLD) is a serious public health problem. It is now estimated to affect 30% of adults and about 10% of children in the U.S. Hispanics are disproportionably affected with not only higher rates of NAFLD but also more severe disease. Treatment options are currently limited.

AREAS COVERED

In this review, we will focus on a series of novel findings related to the pathobiology of liver damage in nonalcoholic steatohepatitis (NASH) that are attractive targets for development of novel therapeutic strategies for human NASH. In particular, we will discuss four different areas due to their novelty and growing importance including microparticles, the inflammasomes, gut-liver axis and dietary lipids.

EXPERT OPINION

There is an urgent need to develop novel safe and effective therapies for the growing NAFLD epidemic. The data discussed in this article provide strong rational to think out of the box when considering novel therapeutic targets for patients with NAFLD.

The great escape: microbiotal LPS takes a toll on the liver

The interaction between the intestinal microbiota and host is much more complex than previously appreciated, and we are now learning that it can have an impact on extraintestinal human diseases. In this issue of the journal (beginning on page 1090), Lin and colleagues present important data linking the microbiota, lipopolysaccharide (LPS), and toll-like receptor (TLR)4 with hepatitis in a mouse model. These provocative results and those from other recent studies highlight the microbiota as a potential target for therapeutic intervention in several liver diseases.

Tumor-initiating stem-like cells and drug resistance: carcinogenesis through Toll-like receptors, environmental factors, and virus

Neoplasms contain distinct subpopulations of cells known as tumor-initiating stem-like cells (TICs) that have been identified as key drivers of tumor growth and malignant progression with drug resistance. Stem cells normally proliferate through self-renewing divisions in which the two daughter cells differ markedly in their proliferative potential, with one displaying the differentiation phenotypes and another retaining self-renewing activity. Therefore, understanding the molecular mechanisms of hepatocarcinogenesis will be required for the eventual development of improved therapeutic modalities for treating hepatocellular carcinoma (HCC). Hepatitis C virus (HCV) and hepatitis B virus is a major cause of HCC. Compelling epidemiologic evidence identifies obesity and alcohol as co-morbidity factors that can increase the risk of HCV patients for HCC, especially in alcoholics or obese patients. The mechanisms underlying liver oncogenesis, and how environmental factors contribute to this process, are not yet understood. The HCV-Toll-like receptor 4 (TLR4)-Nanog signaling network is established since alcohol/obesity-associated endotoxemia then activates TLR4 signaling, resulting in the induction of the stem cell marker Nanog expression and liver tumors. Liver TICs are highly sensitized to leptin and exposure of TICs to leptin increases the expression and activity of an intrinsic pluripotency-associated transcriptional network comprised of signal transducer and activator of transcription 3, SOX2, OCT4, and Nanog. Stimulation of the pluripotency network may have significant implications for hepatocellular oncogenesis via genesis and maintenance of TICs. It is important to understand how HCV induces liver cancer through genesis of TICs so that better prevention and treatment can be found. This article reviews the oncogenic pathways to generate TICs.

Nonalcoholic fatty liver disease: for better or worse, blame the gut microbiota?

Nonalcoholic fatty liver disease (NAFLD) is a major clinical consequence for people with obesity and metabolic syndrome and is also associated with enteral and parenteral nutrition. Early studies suggested that altered gut microbiota might contribute to obesity by affecting energy harvest from the diet and energy storage in the host. Recent evidence in humans as well as in animal models has linked gut microbiota to the development of NAFLD through the gut-liver axis. With bacterial overgrowth and increased intestinal permeability observed in patients with NAFLD and in animal models, gut-derived bacterial products such as endotoxin (lipopolysaccharide) and bacterial DNA are being delivered to the liver through the portal vein and then activate Toll-like receptors (TLRs), mainly TLR4 and TLR9, and their downstream cytokines and chemokines, leading to the development and progression of NAFLD. Given the limited data in humans, the role of gut microbiota in the pathogenesis of NAFLD is still open to discussion. Prebiotics and probiotics have been attempted to modify the microbiota as preventive or therapeutic strategies on this pathological condition. Their beneficial effects on NALFD have been demonstrated in animal models and limited human studies. However, prospective, appropriately powered, randomized, controlled clinical trials are needed to determine whether prebiotics and probiotics and other integrated strategies to modify intestinal microbiota are efficacious therapeutic modalities to treat NALFD.

A novel small-molecule enantiomeric analogue of traditional (-)-morphinans has specific TLR9 antagonist properties and reduces sterile inflammation-induced organ damage

TLR9 is a key determinant of the innate immune responses in both infectious and sterile injury. Specific antagonism of TLR9 is of great clinical interest to reduce tissue damage in a wide range of pathologies, and has been approached by modification of nucleic acids, the recognized ligand for TLR9. Such oligonucleotide-derived pharmacotherapeutics have limitations in specificity for nucleic acid receptors, significant potential for immunologic recognition with generation of innate and adaptive immune responses, and limited bioavailability. We have identified enantiomeric analogues of traditional (-)-morphinans as having TLR9 antagonist properties on reporter cell lines. One of these analogues (COV08-0064) is demonstrated to be a novel small-molecule antagonist of TLR9 with greater specificity for TLR9 than oligo-based antagonists. COV08-0064 has wide bioavailability, including the s.c. and oral routes. It specifically inhibits the action of TLR9 antagonists on reporter cells lines and the production of cytokines by TLR9 agonists from primary cells. It also has efficacy in limiting TLR9-mediated sterile inflammation in in vivo models of acute liver injury and acute pancreatitis. The identification of a morphinan-based novel small-molecule structure with TLR9 antagonism is a significant step in expanding therapeutic strategies in the field of sterile inflammatory injury.

The role of gut microbiota on insulin resistance

The development of obesity and insulin resistance has been extensively studied in the last decades, but the mechanisms underlying these alterations are still not completely understood. The gut microbiota has been identified as a potential contributor to metabolic diseases. It has been shown that obese individuals present different proportions of bacterial phyla compared with lean individuals, with an increase in Firmicutes and Actinobacteria and a decrease in Bacteroidetes. This alteration seems to interfere with intestinal permeability, increasing the absorption of lipopolysaccharide (LPS), which reaches circulation and initiates activation of Toll-like receptor (TLR) 4 and 2 and LPS receptor CD14, leading to increased activation of inflammatory pathways. With these activations, an impairment of the insulin signaling is observed, with decreased phosphorylation of the insulin receptor, insulin receptor substrate (IRS) and Akt, as well as increased inhibitory serine phosphorylation of IRS-1. Altered proportions of bacterial phyla have also been demonstrated to interfere with host’s biochemical pathways, increasing energy extraction and depot in adipose tissue. Therefore, understanding the mechanisms by which the alteration in the gut microbiota produces different signaling activations and phenotype changes may offer an interesting opportunity for the treatment of obesity and type 2 diabetes.

Tumor progression locus 2/Cot is required for activation of extracellular regulated kinase in liver injury and toll-like receptor-induced TIMP-1 gene transcription in hepatic stellate cells in mice

Toll-like receptors (TLRs) function as key regulators of liver fibrosis and are able to modulate the fibrogenic actions of nonparenchymal liver cells. The fibrogenic signaling events downstream of TLRs on Kupffer cells (KCs) and hepatic stellate cells (HSCs) are poorly defined. Here, we describe the MAP3K tumor progression locus 2 (Tpl2) as being important for the activation of extracellular regulated kinase (ERK) signaling in KCs and HSCs responding to stimulation of TLR4 and TLR9. KCs lacking Tpl2 display defects with TLR induction of cytokines interleukin (IL)-1β, IL-10, and IL-23. tpl2(-/-) HSCs were unable to increase expression of fibrogenic genes IL-1β and tissue inhibitor of metalloproteinase 1 (TIMP-1), with the latter being the result of defective stimulation of TIMP-1 promoter activity by TLRs. To determine the in vivo relevance of Tpl2 signaling in liver fibrosis, we compared the fibrogenic responses of wild-type (WT) and tpl2(-/-) mice in three distinct models of chronic liver injury. In the carbon tetrachloride and methionine-choline-deficient diet models, we observed a significant reduction in fibrosis in mice lacking Tpl2, compared to WT controls. However, in the bile duct ligation model, there was no effect of tpl2 deletion, which may reflect a lesser role for HSCs in wounding response to biliary injury.

CONCLUSION

We conclude that Tpl2 is an important signal transducer for TLR activation of gene expression in KCs and HSCs by the ERK pathway and that suppression of its catalytic activity may be a route toward suppressing fibrosis caused by hepatocellular injuries. (HEPATOLOGY 2013).

Caspase-1 as a central regulator of high fat diet-induced non-alcoholic steatohepatitis

Nonalcoholic steatohepatitis (NASH) is associated with caspase activation. However, a role for pro-inflammatory caspases or inflammasomes has not been explored in diet-induced liver injury. Our aims were to examine the role of caspase-1 in high fat-induced NASH. C57BL/6 wild-type and caspase 1-knockout (Casp1(-/-)) mice were placed on a 12-week high fat diet. Wild-type mice on the high fat diet increased hepatic expression of pro-caspase-1 and IL-1β. Both wild-type and Casp1(-/-) mice on the high fat diet gained more weight than mice on a control diet. Hepatic steatosis and TG levels were increased in wild-type mice on high fat diet, but were attenuated in the absence of caspase-1. Plasma cholesterol and free fatty acids were elevated in wild-type, but not Casp1(-/-) mice, on high fat diet. ALT levels were elevated in both wild-type and Casp1(-/-) mice on high fat diet compared to control. Hepatic mRNA expression for genes associated with lipogenesis was lower in Casp1(-/-) mice on high fat diet compared to wild-type mice on high fat diet, while genes associated with fatty acid oxidation were not affected by diet or genotype. Hepatic Tnfα and Mcp-1 mRNA expression was increased in wild-type mice on high fat diet, but not in Casp1(-/-) mice on high fat diet. αSMA positive cells, Sirius red staining, and Col1α1 mRNA were increased in wild-type mice on high fat diet compared to control. Deficiency of caspase-1 prevented those increases. In summary, the absence of caspase-1 ameliorates the injurious effects of high fat diet-induced obesity on the liver. Specifically, mice deficient in caspase-1 are protected from high fat-induced hepatic steatosis, inflammation and early fibrogenesis. These data point to the inflammasome as an important therapeutic target for NASH.

Toll-like receptor 2 and palmitic acid cooperatively contribute to the development of nonalcoholic steatohepatitis through inflammasome activation in mice

Innate immune signaling associated with Toll-like receptors (TLRs) is a key pathway involved in the progression of nonalcoholic steatohepatitis (NASH). Here we show that both TLR2 and palmitic acid are required for activation of the inflammasome, interleukin (IL)-1α, and IL-1β, resulting in the progression of NASH. Wild-type (WT) and TLR2(-/-) mice were fed a choline-deficient amino acid-defined (CDAA) diet for 22 weeks to induce NASH. Bone marrow-transplanted TLR2 chimeric mice were generated after the recipient mice were lethally irradiated. Kupffer cells and hepatic stellate cells (HSCs) were isolated from WT mice and stimulated with TLR2 ligand and/or palmitic acid. WT mice on the CDAA diet developed profound steatohepatitis and liver fibrosis. In contrast, TLR2(-/-) mice had suppressed progression of NASH. Although both Kupffer cells and HSCs respond to TLR2 ligand, TLR2 bone marrow chimeric mice demonstrated that Kupffer cells were relatively more important than HSCs in TLR2-mediated progression of NASH. In vitro, palmitic acid alone did not increase TLR2 signaling-target genes, including cytokines and inflammasome components in Kupffer cells and HSCs. The TLR2 ligand increased Nod-like receptor protein 3, an inflammasome component, in Kupffer cells but not in HSCs. In the presence of TLR2 ligand, palmitic acid did induce caspase-1 activation and release of IL-1α and IL-1β in Kupffer cells; however, these effects were not observed in HSCs. In vivo, WT mice on the CDAA diet showed increased caspase-1 activation in the liver and elevated serum levels of IL-1α and IL-1β levels, which were suppressed in TLR2(-/-) mice.

CONCLUSION

TLR2 and palmitic acid cooperatively activate the inflammasome in Kupffer cells and/or macrophages in the development of NASH.

Role of gut microbiota in liver diseases

The liver constantly encounters food-derived antigens and bacterial components such as lipopolysaccharide translocated from the gut into the portal vein. Bacterial components stimulate Toll-like receptors (TLR), which are expressed on Kupffer cells, biliary epithelial cells, hepatocytes, hepatic stellate cells, endothelial cells and dendritic cells and recognize specific pathogen-associated molecular patterns. The signaling of TLR to its main ligand triggers inflammation. Usually, in order to protect against hyperactivation of the immune system and to prevent organ failure by persistent inflammation, TLR tolerance to repeated stimuli is induced. In chronic liver diseases, a breakdown in TLR tolerance occurs. Furthermore, Kupffer cells, hepatic stellate cells and natural killer T cells are key components of innate immunity. Decreased numbers and impaired ability of these cells lead to failures in immune tolerance, resulting in persistent inflammation. Recently, the activation of inflammasome was revealed to control the secretion of pro-inflammatory cytokines such as interleukin-1β in response to bacterial pathogens. Innate immunity seems to be an important contributor to the pathogenesis of fatty liver disease and autoimmune liver disease. Recently, probiotics were reported to affect various liver diseases via shifts in gut microbiota and the stability of intestinal permeability. However, many unresolved questions remain. Further analysis will be needed to gain a more comprehensive understanding of the association of innate immunity with the pathogenesis of various liver diseases.

Toll-like receptors in liver disease

Activation of inflammatory signaling pathways is of central importance in the pathogenesis of alcoholic liver disease (ALD) and nonalcoholic steatohepatitis (NASH). Recent studies demonstrated that Toll-like receptors, the sensors of microbial and endogenous danger signals, are expressed and activated in innate immune cells as well as in parenchymal cells in the liver and thereby contribute to ALD and NASH. In this review, we emphasize the importance of gut-derived endotoxin and its recognition by TLR4 in the liver. The significance of TLR-induced intracellular signaling pathways and cytokine production as well as the contribution of individual cell types to the inflammation is evaluated. The contribution of TLR signaling to the induction of liver fibrosis and to the progression of liver pathology mediated by viral pathogens is reviewed in the context of ALD and NASH.

Circulating microRNAs in exosomes indicate hepatocyte injury and inflammation in alcoholic, drug-induced, and inflammatory liver diseases

MicroRNAs are fine tuners of diverse biological responses and are expressed in various cell types of the liver. Here we hypothesized that circulating microRNAs (miRNAs) may serve as biomarkers of liver damage and inflammation. We studied miRNA-122, which is abundant in hepatocytes, and miR-155, -146a, and -125b, which regulate inflammation in immune cells in mouse models of alcoholic liver disease (ALD), drug (acetaminophen, APAP)-induced liver injury (DILI), and Toll-like receptor (TLR) 9+4 ligand-induced inflammatory cell-mediated liver damage. We found that serum/plasma miR-122 correlated with alanine aminotransferase (ALT) increases in the liver damage caused by alcohol, APAP, and TLR9 (CpG)+4 (LPS) ligands. MiR-155, a regulator of inflammation, was increased in serum/plasma in alcoholic and inflammatory liver injury. Alcohol failed to increase serum miR-122 in TLR4-deficient and p47phox-deficient mice that were protected from ALD. We found the most robust increase in plasma miR-122 in DILI and it correlated with the highest ALT levels. Consistent with the massive inflammatory cell infiltration in the liver, plasma miR-155 and miR-146a were significantly elevated after CpG+LPS administration. We show for the first time that, depending on the type of liver injury, circulating miRNAs are associated either with the exosome-rich or protein-rich compartments. In ALD and in inflammatory liver injury, serum/plasma miR-122 and miR-155 were predominantly associated with the exosome-rich fraction, whereas in DILI/APAP injury these miRNAs were present in the protein-rich fraction.

CONCLUSION

Our results suggest that circulating miRNAs may serve as biomarkers to differentiate between hepatocyte injury and inflammation and the exosome versus protein association of miRNAs may provide further specificity to mechanisms of liver pathology.

Sterile inflammation in the liver

Inflammation In the absence of pathogens occurs in all tissues in response to a wide range of stimuli that cause tissue stress and injury. Such sterile inflammation (SI) is a key process in drug-induced liver injury, nonalcoholic steatohepatitis, and alcoholic steatohepatitis and is a major determinant of fibrosis and carcinogenesis. In SI, endogenous damage-associated molecular patterns (DAMPS), which are usually hidden from the extracellular environment, are released on tissue injury and activate receptors on immune cells. More than 20 such DAMPS have been identified and activate cellular pattern recognition receptors, which were originally identified as sensors of pathogen-associated molecular patterns. Activation of pattern recognition receptors by DAMPS results in a wide range of immune responses, including production of proinflammatory cytokines and localization of immune cells to the site of injury. DAMPS result in the assembly of a cytosolic protein complex termed the inflammasome, which activates the serine protease caspase-1, resulting in activation and secretion of interleukin-1β and other cytokines. SI-driven liver diseases are responsible for the majority of liver pathology in industrially developed countries and lack specific therapy. Identification of DAMPS, their receptors, signaling pathways, and cytokines now provides a wide range of therapeutic targets for which many antagonists are already available.

Kuppfer cells trigger nonalcoholic steatohepatitis development in diet-induced mouse model through tumor necrosis factor-α production

BACKGROUND

The mechanisms triggering nonalcoholic steatohepatitis (NASH) remain poorly defined.

RESULTS

Kupffer cells are the first responding cells to hepatocyte injuries, leading to TNFα production, chemokine induction, and monocyte recruitment. The silencing of TNFα in myeloid cells reduces NASH progression.

CONCLUSION

Increase of TNFα-producing Kupffer cells is crucial for triggering NASH via monocyte recruitment.

SIGNIFICANCE

Myeloid cells-targeted silencing of TNFα might be a tenable therapeutic approach. Nonalcoholic steatohepatitis (NASH), characterized by lipid deposits within hepatocytes (steatosis), is associated with hepatic injury and inflammation and leads to the development of fibrosis, cirrhosis, and hepatocarcinoma. However, the pathogenic mechanism of NASH is not well understood. To determine the role of distinct innate myeloid subsets in the development of NASH, we examined the contribution of liver resident macrophages (i.e. Kupffer cells) and blood-derived monocytes in triggering liver inflammation and hepatic damage. Employing a murine model of NASH, we discovered a previously unappreciated role for TNFα and Kupffer cells in the initiation and progression of NASH. Sequential depletion of Kupffer cells reduced the incidence of liver injury, steatosis, and proinflammatory monocyte infiltration. Furthermore, our data show a differential contribution of Kupffer cells and blood monocytes during the development of NASH; Kupffer cells increased their production of TNFα, followed by infiltration of CD11b(int)Ly6C(hi) monocytes, 2 and 10 days, respectively, after starting the methionine/choline-deficient (MCD) diet. Importantly, targeted knockdown of TNFα expression in myeloid cells decreased the incidence of NASH development by decreasing steatosis, liver damage, monocyte infiltration, and the production of inflammatory chemokines. Our findings suggest that the increase of TNFα-producing Kupffer cells in the liver is crucial for the early phase of NASH development by promoting blood monocyte infiltration through the production of IP-10 and MCP-1.

Kuppfer cells trigger nonalcoholic steatohepatitis development in diet-induced mouse model through tumor necrosis factor-α production

BACKGROUND

The mechanisms triggering nonalcoholic steatohepatitis (NASH) remain poorly defined.

RESULTS

Kupffer cells are the first responding cells to hepatocyte injuries, leading to TNFα production, chemokine induction, and monocyte recruitment. The silencing of TNFα in myeloid cells reduces NASH progression.

CONCLUSION

Increase of TNFα-producing Kupffer cells is crucial for triggering NASH via monocyte recruitment.

SIGNIFICANCE

Myeloid cells-targeted silencing of TNFα might be a tenable therapeutic approach. Nonalcoholic steatohepatitis (NASH), characterized by lipid deposits within hepatocytes (steatosis), is associated with hepatic injury and inflammation and leads to the development of fibrosis, cirrhosis, and hepatocarcinoma. However, the pathogenic mechanism of NASH is not well understood. To determine the role of distinct innate myeloid subsets in the development of NASH, we examined the contribution of liver resident macrophages (i.e. Kupffer cells) and blood-derived monocytes in triggering liver inflammation and hepatic damage. Employing a murine model of NASH, we discovered a previously unappreciated role for TNFα and Kupffer cells in the initiation and progression of NASH. Sequential depletion of Kupffer cells reduced the incidence of liver injury, steatosis, and proinflammatory monocyte infiltration. Furthermore, our data show a differential contribution of Kupffer cells and blood monocytes during the development of NASH; Kupffer cells increased their production of TNFα, followed by infiltration of CD11b(int)Ly6C(hi) monocytes, 2 and 10 days, respectively, after starting the methionine/choline-deficient (MCD) diet. Importantly, targeted knockdown of TNFα expression in myeloid cells decreased the incidence of NASH development by decreasing steatosis, liver damage, monocyte infiltration, and the production of inflammatory chemokines. Our findings suggest that the increase of TNFα-producing Kupffer cells in the liver is crucial for the early phase of NASH development by promoting blood monocyte infiltration through the production of IP-10 and MCP-1.

IL-1 receptor antagonist ameliorates inflammasome-dependent alcoholic steatohepatitis in mice

Alcoholic liver disease (ALD) is characterized by steatosis and upregulation of proinflammatory cytokines, including IL-1β. IL-1β, type I IL-1 receptor (IL-1R1), and IL-1 receptor antagonist (IL-1Ra) are all important regulators of the IL-1 signaling complex, which plays a role in inflammation. Furthermore, IL-1β maturation is dependent on caspase-1 (Casp-1). Using IL-1Ra-treated mice as well as 3 mouse models deficient in regulators of IL-1β activation (Casp-1 and ASC) or signaling (IL-1R1), we found that IL-1β signaling is required for the development of alcohol-induced liver steatosis, inflammation, and injury. Increased IL-1β was due to upregulation of Casp-1 activity and inflammasome activation. The pathogenic role of IL-1 signaling in ALD was attributable to the activation of the inflammasome in BM-derived Kupffer cells. Importantly, in vivo intervention with a recombinant IL-1Ra blocked IL-1 signaling and markedly attenuated alcohol-induced liver inflammation, steatosis, and damage. Furthermore, physiological doses of IL-1β induced steatosis, increased the inflammatory and prosteatotic chemokine MCP-1 in hepatocytes, and augmented TLR4-dependent upregulation of inflammatory signaling in macrophages. In conclusion, we demonstrated that Casp-1-dependent upregulation of IL-1β and signaling mediated by IL-1R1 are crucial in ALD pathogenesis. Our findings suggest a potential role of IL-1R1 inhibition in the treatment of ALD.

Inflammasomes in liver diseases

Inflammation is a common element in the pathogenesis of most chronic liver diseases that lead to fibrosis and cirrhosis. Inflammation is characterized by activation of innate immune cells and production of pro-inflammatory cytokines IL-1α, IL-1β, and TNFα. Inflammasomes are intracellular multiprotein complexes expressed in both parenchymal and non-parenchymal cells of the liver that in response to cellular danger signals activate caspase-1, and release IL-1β and IL-18. The importance of inflammasome activation in various forms of liver diseases in relation to liver damage, steatosis, inflammation and fibrosis is discussed in this review.

Exercise training attenuates hepatic inflammation, fibrosis and macrophage infiltration during diet induced-obesity in mice

Nonalcoholic steatohepatitis, which is considered the hepatic event in metabolic syndrome, was recently associated with the innate immune system. Although regular exercise reduces hepatic injury markers like serum alanine aminotransferase (ALT) levels, the mechanisms regulating the effects of exercise on steatohepatitis are unclear. This study aimed to clarify whether exercise training suppresses hepatic injury, inflammation, and fibrosis by suppressing macrophage infiltration. Male C57BL/6J (4-week old) mice were randomly divided into four groups: normal diet (ND) control (n=7), ND exercise (n=5), high-fat diet and high-fructose water (HFF) control (n=11), and HFF exercise (n=11) groups. Mice were fed the ND or HFF from 4 to 20 weeks of age. The exercise groups were trained on a motorized treadmill for 60 min/day, five times/week. The nonalcoholic fatty liver disease (NAFLD) activity score and plasma ALT activity, indicators of liver injury, were increased in HFF control mice but were attenuated in HFF exercise mice. Hepatic inflammation, indicated by hepatic tumor necrosis factor (TNF)-α levels and hepatic resident macrophage infiltration, was significantly lower in HFF exercise mice than in HFF control mice. Hepatic fibrosis markers (histological hepatic fibrosis detected by Sirius red and α-smooth muscle actin staining and tissue inhibitor of matrix metalloproteinase-1 mRNA) were attenuated in HFF exercise mice compared with HFF control mice. These results suggest that exercise training reduces hepatic inflammation, injury, and fibrosis by suppressing macrophage infiltration.

A myriad of pathways to NASH

Nonalcoholic steatohepatitis (NASH) is defined histopathologically by the presence of macrovesicular steatosis, cellular ballooning, and inflammation. NASH represents a complex multifactorial disease that typically occurs within the context of the metabolic syndrome. NASH lacks homogeneity, and other forms of NASH can present atypically. Less than 50% of patients with NASH respond to pharmacologic treatment, which speaks to this heterogeneity. The authors discuss drugs, disease entities, and nutritional states that can cause or exacerbate underlying NASH indirectly through worsening insulin resistance or directly by interfering with lipid metabolism, promoting oxidative injury, or activating inflammatory pathways.

Liver fibrogenesis in non-alcoholic steatohepatitis

Non-alcoholic fatty liver disease (NAFLD) is emerging as one of the most common chronic liver diseases in developed western countries. Non-alcoholic steatohepatitis (NASH) is the most severe form of NAFLD, and can progress to more severe forms of liver disease, including fibrosis, cirrhosis, and even hepatocellular carcinoma. The activation of hepatic stellate cells plays a critical role in NASH-related fibrogenesis. Multiple factors, such as insulin resistance, oxidative stress, pro-inflammatory cytokines and adipokines, and innate immune responses, are known to contribute to the development of NASH-related fibrogenesis. Furthermore, these factors may share synergistic interactions, which could contribute to the process of liver fibrosis. Given the complex etiology of NASH, combined treatment regimes that target these different factors provide potential treatment strategies for NASH-related liver fibrosis.

Toxic lipids stored by Kupffer cells correlates with their pro-inflammatory phenotype at an early stage of steatohepatitis

BACKGROUND & AIMS

Non-alcoholic steatohepatitis (NASH) is characterized by steatosis associated with liver inflammation. Steatosis causes recruitment of lymphocytes into the liver and this is worsened by lipopolysaccharides (LPS). As macrophages may be involved in the lymphocyte homing, we studied the role of lipids in determining the phenotype of Kupffer cells (KCs) at the stage of steatosis.

METHODS

Steatosis was induced in mice by a high fat diet. The turnover and the recruitment of KCs were analyzed in vivo by flow cytometry. KCs phenotype was assessed by optical and electron microscopy, cell culture and lymphocyte recruitment by in vitro chemotaxis. Lipidomic analysis was carried out by mass-spectrometry and gene expression analysis by TaqMan low density array.

RESULTS

Although the number of KCs was not modified in steatotic livers compared to normal livers, their phenotypes were different. Electron microscopy demonstrated that the KCs from fatty livers were enlarged and loaded with lipid droplets. Lipid synthesis and trafficking were dysregulated in fat-laden KCs and toxic lipids accumulated. Fat-laden KCs recruited more CD4+ T and B lymphocytes in response to LPS stimulation than did control KCs and produced high levels of pro-inflammatory cytokines/chemokines, which could be reversed by inhibition of lipogenesis.

CONCLUSIONS

Lipid accumulation in fat-laden KCs is due to a dysregulation of lipid metabolism and trafficking. Fat-laden KCs are "primed" to recruit lymphocytes and exhibit a pro-inflammatory phenotype, which is reversible with inhibition of lipogenesis.

Stat3-mediated activation of microRNA-23a suppresses gluconeogenesis in hepatocellular carcinoma by down-regulating glucose-6-phosphatase and peroxisome proliferator-activated receptor gamma, coactivator 1 alpha

Considerable effort has been made in elucidating the mechanism and functional significance of high levels of aerobic glycolysis in cancer cells, commonly referred to as the Warburg effect. Here we investigated whether the gluconeogenic pathway is significantly modulated in hepatocarcinogenesis, resulting in altered levels of glucose homeostasis. To test this possibility, we used a mouse model (mice fed a choline-deficient diet) that develops nonalcoholic steatohepatitis (NASH), preneoplastic nodules, and hepatocellular carcinoma (HCC), along with human primary HCCs and HCC cells. This study demonstrated marked reduction in the expressions of G6pc, Pepck, and Fbp1 encoding the key gluconeogenic enzymes glucose-6-phosphatase, phosphoenolpyruvate carboxykinase, fructose-1,6-phosphatase, respectively, and the transcription factor Pgc-1α in HCCs developed in the mouse model that correlated with reduction in serum glucose in tumor-bearing mice. The messenger RNA (mRNA) levels of these genes were also reduced by ≈80% in the majority of primary human HCCs compared with matching peritumoral livers. The expression of microRNA (miR)-23a, a candidate miR targeting PGC-1α and G6PC, was up-regulated in the mouse liver tumors as well as in primary human HCC. We confirmed PGC-1α and G6PC as direct targets of miR-23a and their expressions negatively correlated with miR-23a expression in human HCCs. G6PC expression also correlated with tumor grade in human primary HCCs. Finally, this study showed that the activation of interleukin (IL)-6-Stat3 signaling caused the up-regulation of miR-23a expression in HCC.

CONCLUSION

Based on these data, we conclude that gluconeogenesis is severely compromised in HCC by IL6-Stat3-mediated activation of miR-23a, which directly targets PGC-1α and G6PC, leading to decreased glucose production.

Gut--liver axis: the impact of gut microbiota on non alcoholic fatty liver disease

AIM

To examine the impact of gut microbiota on non alcoholic fatty liver disease (NAFLD) pathogenesis.

DATA SYNTHESIS

Emerging evidence suggests a strong interaction between gut microbiota and liver. Receiving approximately 70% of its blood supply from the intestine, the liver represents the first line of defence against gut-derived antigens. Intestinal bacteria play a key role in the maintenance of gut-liver axis health. Disturbances in the homeostasis between bacteria- and host-derived signals at the epithelial level lead to a break in intestinal barrier function and may foster "bacterial translocation", defined as the migration of bacteria or bacterial products from the intestinal lumen to mesenteric lymph nodes or other extraintestinal organs and sites. While the full repertoire of gut-derived microbial products that reach the liver in health and disease has yet to be explored, the levels of bacterial lipopolysaccharide, a component of the outer membrane of Gram-negative bacteria, are increased in the portal and/or systemic circulation in several types of chronic liver diseases. Derangement of the gut flora, particularly small intestinal bacterial overgrowth, occurs in a large percentage (20-75%) of patients with chronic liver disease. In addition, evidence implicating the gut-liver axis in the pathogenesis of metabolic liver disorders has accumulated over the past ten years.

CONCLUSIONS

Complex metabolic diseases are the product of multiple perturbations under the influence of triggering factors such as gut microbiota and diet, thus, modulation of the gut microbiota may represent a new way to treat or prevent NAFLD.

Hepatic recruitment of macrophages promotes nonalcoholic steatohepatitis through CCR2

Inflammatory cell infiltration in the liver is a hallmark of nonalcoholic steatohepatitis (NASH). The chemokine-chemokine receptor interaction induces inflammatory cell recruitment. CC-chemokine receptor (CCR)2 is expressed on hepatic macrophages and hepatic stellate cells. This study aims to investigate the therapeutic potential of CCR2 to NASH. Twenty-two weeks on a choline-deficient amino acid-defined (CDAA) diet induced steatosis, inflammatory cell infiltration, and liver fibrosis with increased CCR2 and monocyte chemoattractant protein (MCP)-1 expression in the wild-type livers. The infiltrated macrophages expressed CD68, CCR2, and a marker of bone marrow-derived monocytes, Ly6C. CCR2(-/-) mice had less steatosis, inflammatory cell infiltration, and fibrosis, and hepatic macrophages expressing CD68 and Ly6C were decreased. Toll-like receptor (TLR)4(-/-), TLR9(-/-), and MyD88(-/-) mice had reduced hepatic macrophage infiltration with decreased MCP-1 and CCR2 expression because TLR signaling is a potent inducer of MCP-1. To assess the role of Kupffer cells at the onset of NASH, Kupffer cells were depleted by liposomal clodronate. The Kupffer cell depletion ameliorated steatohepatitis with a decrease in the MCP-1 expression and recruitment of Ly6C-expressing macrophages at the onset of NASH. Finally, to test the therapeutic potential of targeting CCR2, a CCR2 inhibitor was administered to mice on a CDAA diet. The pharmaceutical inhibition of CCR2 prevented infiltration of the Ly6C-positive macrophages, resulting in an inhibition of liver inflammation and fibrosis. We concluded that CCR2 and Kupffer cells contribute to the progression of NASH by recruiting bone marrow-derived monocytes.

Toll-like receptors in liver fibrosis: cellular crosstalk and mechanisms

Toll-like receptors (TLRs) are pattern recognition receptors that distinguish conserved microbial products, also known as pathogen-associated molecular patterns (PAMPs), from host molecules. Liver is the first filter organ between the gastrointestinal tracts and the rest of the body through portal circulation. Thus, the liver is a major organ that must deal with PAMPs and microorganisms translocated from the intestine and to respond to the damage associated molecular patterns (DAMPs) released from injured organs. These PAMPs and DAMPs preferentially activate TLR signaling on various cell types in the liver inducing the production of inflammatory and fibrogenic cytokines that initiate and prolong liver inflammation, thereby leading to fibrosis. We summarize recent findings on the role of TLRs, ligands, and intracellular signaling in the pathophysiology of liver fibrosis due to different etiology, as well as to highlight the potential role of TLR signaling in liver fibrosis associated with hepatitis C infection, non-alcoholic and alcoholic steatoheoatitis, primary biliary cirrhosis, and cystic fibrosis.

Bias in macrophage activation pattern influences non-alcoholic steatohepatitis (NASH) in mice

In humans, there is large inter-individual variability in the evolution of NAFLD (non-alcoholic fatty liver disease) to NASH (non-alcoholic steatohepatitis). To investigate this issue, NASH was induced with an MCD (methionine-choline-deficient) diet in C57BL/6 and Balb/c mice that are characterized by different biases in Th1/Th2 and macrophage (M1/M2) responses. Following 4 weeks on the MCD diet, steatosis and lobular inflammation were prevalent in C57BL/6 (Th1, M1 oriented) than in Balb/c (Th2, M2 oriented) mice. Consistently, hepatic TNFα (tumour necrosis factor α) mRNA expression and circulating TNFα levels were higher in MCD-fed C57BL/6 than in MCD-fed Balb/c mice. The Th1/Th2 bias did not account for the increased NASH severity, as in both strains MCD feeding did not significantly modify the liver mRNA expression of the Th1 markers IFNγ (interferon γ) and T-bet or that of the Th2 markers IL-4 (interleukin 4) and GATA-3. Conversely, MCD-fed C57BL/6 mice displayed higher liver mRNAs for the macrophage M1 activation markers iNOS (inducible NO synthase), IL-12p40 and CXCL10 (CXC chemokine ligand 10) than similarly treated Balb/c mice, without effects on the M2 polarization markers IL-10 and MGL-1 (macrophage galactose-type C-type lectin-1). Circulating IL-12 was also higher in MCD-fed C57BL/6 than in MCD-fed Balb/c mice. The analysis of macrophages isolated from the livers of MCD-fed animals confirmed an enhanced expression of M1 markers in C57BL/6 mice. Among all of the MCD-treated mice, liver iNOS, IL-12p40 and CXCL10 mRNA levels positively correlated with the frequency of hepatic necro-inflammatory foci. We concluded that the macrophage M1 bias in C57BL/6 mice may account for the increased severity of NASH in this strain, suggesting macrophage responses as important contributors to NAFLD progression.

Inflammation in alcoholic liver disease

Frank Burr Mallory's landmark observation in 1911 on the histopathology of alcoholic liver disease (ALD) was the first identification of a link between inflammation and ALD. In this review, we summarize recent advances regarding the origins and roles of various inflammatory components in ALD. Metabolism of ethanol generates a number of metabolites, including acetate, reactive oxygen species, acetaldehyde, and epigenetic changes, that can induce inflammatory responses. Alcohol and its metabolites can also initiate and aggravate inflammatory conditions by promoting gut leakiness of microbial products, by sensitizing immune cells to stimulation, and by activating innate immune pathways, such as complement. Chronic alcohol consumption also sensitizes nonimmune cells, e.g., hepatocytes, to inflammatory signals and impairs their ability to respond to protective signals. Based on these advances, a number of inflammatory targets have been identified with potential for therapeutic intervention in ALD, presenting new opportunities and challenges for translational research.

The phenotypic fate and functional role for bone marrow-derived stem cells in liver fibrosis

Liver fibrosis is an outcome of chronic liver injury of any etiology. It is manifested by extensive deposition of extracellular matrix (ECM) proteins that produce a fibrous scar in the injured liver. Bone marrow (BM) cells may play an important role in pathogenesis and resolution of liver fibrosis. BM cells contribute to the inflammatory response by TGF-β1 secretion and activation of liver resident myofibroblasts. Moreover, BM itself can serve as a source of collagen expressing cells, e.g. BM-derived fibrocytes and mesenchymal progenitors, which in turn, have a potential to in situ differentiate into fibrogenic myofibroblasts and facilitate fibrosis. Finally, BM cells play an active part in resolution of liver fibrosis after cessation of fibrogenic stimuli. While natural killer (NK) cells are implicated in apoptosis of activated hepatic stellate cells/myofibroblasts, cells of myelo-monocitic lineage secrete matrix metalloproteinases to actively degrade the fibrous scar. The focus of this review is on the current understanding of the role of different subsets of BM cells in the onset, development and resolution of liver fibrosis.

NASH is an Inflammatory Disorder: Pathogenic, Prognostic and Therapeutic Implications

While non-alcoholic fatty liver disease (NAFLD) is highly prevalent (15% to 45%) in modern societies, only 10% to 25% of cases develop hepatic fibrosis leading to cirrhosis, end-stage liver disease or hepatocellular carcinoma. Apart from pre-existing fibrosis, the strongest predictor of fibrotic progression in NAFLD is steatohepatitis or non-alcoholic steatohepatitis (NASH). The critical features other than steatosis are hepatocellular degeneration (ballooning, Mallory hyaline) and mixed inflammatory cell infiltration. While much is understood about the relationship of steatosis to metabolic factors (over-nutrition, insulin resistance, hyperglycemia, metabolic syndrome, hypoadiponectinemia), less is known about inflammatory recruitment, despite its importance for the perpetuation of liver injury and fibrogenesis. In this review, we present evidence that liver inflammation has prognostic significance in NAFLD. We then consider the origins and components of liver inflammation in NASH. Hepatocytes injured by toxic lipid molecules (lipotoxicity) play a central role in the recruitment of innate immunity involving Toll-like receptors (TLRs), Kupffer cells (KCs), lymphocytes and neutrophils and possibly inflammasome. The key pro-inflammatory signaling pathways in NASH are nuclear factor-kappa B (NF-κB) and c-Jun N-terminal kinase (JNK). The downstream effectors include adhesion molecules, chemokines, cytokines and the activation of cell death pathways leading to apoptosis. The upstream activators of NF-κB and JNK are more contentious and may depend on the experimental model used. TLRs are strong contenders. It remains possible that inflammation in NASH originates outside the liver and in the gut microbiota that prime KC/TLR responses, inflamed adipose tissue and circulating inflammatory cells. We briefly review these mechanistic considerations and project their implications for the effective treatment of NASH.

Genetically modified mouse models for the study of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is associated with obesity, insulin resistance, and type 2 diabetes. NAFLD represents a large spectrum of diseases ranging from (1) fatty liver (hepatic steatosis); (2) steatosis with inflammation and necrosis; to (3) cirrhosis. The animal models to study NAFLD/nonalcoholic steatohepatitis (NASH) are extremely useful, as there are still many events to be elucidated in the pathology of NASH. The study of the established animal models has provided many clues in the pathogenesis of steatosis and steatohepatitis, but these remain incompletely understood. The different mouse models can be classified in two large groups. The first one includes genetically modified (transgenic or knockout) mice that spontaneously develop liver disease, and the second one includes mice that acquire the disease after dietary or pharmacological manipulation. Although the molecular mechanism leading to the development of hepatic steatosis in the pathogenesis of NAFLD is complex, genetically modified animal models may be a key for the treatment of NAFLD. Ideal animal models for NASH should closely resemble the pathological characteristics observed in humans. To date, no single animal model has encompassed the full spectrum of human disease progression, but they can imitate particular characteristics of human disease. Therefore, it is important that the researchers choose the appropriate animal model. This review discusses various genetically modified animal models developed and used in research on NAFLD.

Inflammasome-mediated dysbiosis regulates progression of NAFLD and obesity

Non-alcoholic fatty liver disease (NAFLD) is the hepatic manifestation of metabolic syndrome and the leading cause of chronic liver disease in the Western world. Twenty per cent of NAFLD individuals develop chronic hepatic inflammation (non-alcoholic steatohepatitis, NASH) associated with cirrhosis, portal hypertension and hepatocellular carcinoma, yet the causes of progression from NAFLD to NASH remain obscure. Here, we show that the NLRP6 and NLRP3 inflammasomes and the effector protein IL-18 negatively regulate NAFLD/NASH progression, as well as multiple aspects of metabolic syndrome via modulation of the gut microbiota. Different mouse models reveal that inflammasome-deficiency-associated changes in the configuration of the gut microbiota are associated with exacerbated hepatic steatosis and inflammation through influx of TLR4 and TLR9 agonists into the portal circulation, leading to enhanced hepatic tumour-necrosis factor (TNF)-α expression that drives NASH progression. Furthermore, co-housing of inflammasome-deficient mice with wild-type mice results in exacerbation of hepatic steatosis and obesity. Thus, altered interactions between the gut microbiota and the host, produced by defective NLRP3 and NLRP6 inflammasome sensing, may govern the rate of progression of multiple metabolic syndrome-associated abnormalities, highlighting the central role of the microbiota in the pathogenesis of heretofore seemingly unrelated systemic auto-inflammatory and metabolic disorders.

DSS induced colitis increases portal LPS levels and enhances hepatic inflammation and fibrogenesis in experimental NASH

BACKGROUND & AIMS

Intestinal bacterial overgrowth and increased permeability are features of non alcoholic steatohepatitis (NASH). Bacterial endotoxin has been shown to promote NASH progression. Application of dextran sulfate sodium (DSS) is a colitis model in mice characterized by damage of the intestinal barrier. This study was designed to investigate if application of DSS aggravates experimental NASH.

METHODS

Male C57bl/6 mice were allocated into four experimental groups receiving either (I) standard chow (SC), (II) a high fat (HF) diet, (III) SC+DSS (1% in the drinking water), and (IV) HF+DSS for 12 weeks.

RESULTS

DSS treatment caused inflammation and proinflammatory gene expression (IL-1β, IL-17, TNF) in the colon. Expression of colonic antimicrobial peptide Cramp was significantly induced in SC+DSS mice, whereas expression was blocked in the HF+DSS group. Endotoxin levels were elevated in SC+DSS and HF mice but further augmented in the HF+DSS group. In line with this, increased hepatic TLR4 and TLR9 mRNA levels were detected in HF+DSS mice. The histological analysis revealed hepatic steatosis in both HF groups. Hepatic inflammation was more severe in HF+DSS mice, reflected by histology and analysis of proinflammatory gene expression (TNF and MCP-1). HF+DSS mice showed increased hepatic fibrosis by sirius red staining, hepatic collagen I expression, and α-SMA positive cells accompanied by higher p47(phox), TIMP-1, TGF-β, Pai-1, and α-SMA mRNA expression.

CONCLUSIONS

Induction of an intestinal inflammation in experimental NASH promotes LPS translocation, hepatic inflammation, and fibrogenesis probably due to inhibition of intestinal antimicrobial peptides. These findings underscore the pathophysiological role of the gut-liver axis in the progression of NASH.

Role of innate immunity and the microbiota in liver fibrosis: crosstalk between the liver and gut

Liver fibrosis occurs as a wound-healing scar response following chronic liver inflammation including alcoholic liver disease, non-alcoholic steatohepatitis, viral hepatitis, cholestatic liver disease and autoimmune liver diseases. The liver has a unique vascular system within the gastrointestinal tract, as the majority of the liver's blood supply comes from the intestine through the portal vein. When the intestinal barrier function is disrupted, an increase in intestinal permeability leads to the translocation of intestine-derived bacterial products such as lipopolysaccharide (LPS) and unmethylated CpG containing DNA to the liver via the portal vein. These gut-derived bacterial products stimulate innate immune receptors, namely Toll-like receptors (TLRs), in the liver. TLRs are expressed on Kupffer cells, endothelial cells, dendritic cells, biliary epithelial cells, hepatic stellate cells, and hepatocytes. TLRs activate these cells to contribute to acute and chronic liver diseases. This review summarizes recent studies investigating the role of TLRs, intestinal microbiota and bacterial translocation in liver fibrosis, alcoholic liver disease and non-alcoholic steatohepatitis.

Lack of interleukin-1α or interleukin-1β inhibits transformation of steatosis to steatohepatitis and liver fibrosis in hypercholesterolemic mice

BACKGROUND & AIMS

The identification of the cellular and molecular pathways that mediate the development of non-alcoholic steatohepatitis is of crucial importance. Cytokines produced by liver-resident and infiltrating inflammatory cells, play a pivotal role in liver inflammation. The role of the proinflammatory cytokines IL-1α and IL-1β in steatohepatitis remains elusive.

METHODS

We employed IL-1α and IL-1β-deficient mice and transplanted marrow cells to study the role of liver-resident and bone marrow-derived IL-1 in steatosis and its progression to steatohepatitis.

RESULTS

Atherogenic diet-induced steatohepatitis in wild-type mice was associated with 16 and 4.6 fold-elevations in mRNA levels of hepatic IL-1α and IL-1β, respectively. In mice deficient in either IL-1α or IL-1β the transformation of steatosis to steatohepatitis and liver fibrosis was markedly reduced. This protective effect in IL-1α-deficient mice was noted despite increased liver cholesterol levels. Deficiency of IL-1α markedly reduced plasma serum amyloid A and steady-state levels of mRNA coding for inflammatory genes (P-selectin, CXCL1, IL-6, and TNFα) as well as pro-fibrotic genes (MMP-9 and Collagen) and particularly a 50% decrease in TGFβ levels (p = 0.004). IL-1α mRNA levels were two-folds lower in IL-1β-deficient mice, and IL-1β transcripts were three-folds lower in IL-1α-deficient compared to wild-type mice. Hepatic cell derived IL-1α rather than from recruited bone marrow-derived cells was required for steatohepatitis development.

CONCLUSIONS

These data demonstrate the critical role of IL-1α and IL-1β in the transformation of steatosis to steatohepatitis and liver fibrosis in hypercholesterolemic mice. Therefore, the potential of neutralizing IL-1α and/or IL-1β to inhibit the development of steatohepatitis should be explored.

Nuclear factor high-mobility group box1 mediating the activation of Toll-like receptor 4 signaling in hepatocytes in the early stage of nonalcoholic fatty liver disease in mice

One of the challenges surrounding nonalcoholic fatty liver disease (NAFLD) is to discover the mechanisms that underlie the initiation of it. The aim of the present study was to elucidate the effects of Toll-like receptor 4 (TLR4) signaling in liver parenchymal cells during the early stage of NAFLD. Male TLR4-wildtype, TLR4-knockout, TLR2-knockout, MyD88-knockout, and TRIF-knockout mice were fed a normal diet or high-fat diet (HFD). Liver steatosis, alanine aminotransferase levels, nuclear translocation of nuclear factor kappa B (NF-κB) (p65), macrophage accumulation, and neutrophil infiltration were assessed. Using Kupffer cell depletion or bone marrow transplantation, we examined the potential role of Kupffer cells and myeloid infiltrating cells during the initiation of NAFLD. Immunohistochemistry and western blotting were implemented to determine the release of high-mobility group box1 (HMGB1). The neutral-antibody against HMGB1 was used to block the activity of free HMGB1. Here we report that the activation of TLR4 signaling in hepatocytes, accompanied with the relocation of P65 in nucleus, was proven to play an important role during the initiation of NAFLD. Importantly, HMGB1 releasing from hepatocytes in response to free fatty acid (FFA) infusion was first reported as the key molecule for the TLR4/MyD88 activation and cytokines expression in vitro and in vivo. Treatment with neutralizing antibody to HMGB1 protects against FFA-induced tumor necrosis factor alpha and interleukin-6 production.

CONCLUSION

Our study supports the notion that TLR4/MyD88 signaling in liver parenchymal cells plays a pivotal role during the early progression of HFD-induced NAFLD, in which free HMGB1 served as a positive component mediating TLR4 activation.

New insights into the role of macrophages in adipose tissue inflammation and Fatty liver disease: modulation by endogenous omega-3 Fatty Acid-derived lipid mediators

Obesity is causally linked to a chronic state of “low-grade” inflammation in adipose tissue. Prolonged, unremitting inflammation in this tissue has a direct impact on insulin-sensitive tissues (i.e., liver) and its timely resolution is a critical step toward reducing the prevalence of related co-morbidities such as insulin resistance and non-alcoholic fatty liver disease. This article describes the current state-of-the-art knowledge and novel insights into the role of macrophages in adipose tissue inflammation, with special emphasis on the progressive changes in macrophage polarization observed over the course of obesity. In addition, this article extends the discussion to the contribution of Kupffer cells, the liver resident macrophages, to metabolic liver disease. Special attention is given to the modulation of macrophage responses by omega-3-PUFAs, and more importantly by resolvins, which are potent anti-inflammatory and pro-resolving autacoids generated from docosahexaenoic and eicosapentaenoic acids. In fact, resolvins have been shown to work as endogenous “stop signals” in inflamed adipose tissue and to return this tissue to homeostasis by inducing a phenotypic switch in macrophage polarization toward a pro-resolving phenotype. Collectively, this article offers new views on the role of macrophages in metabolic disease and their modulation by endogenously generated omega-3-PUFA-derived lipid mediators.

Cannabinoid CB2 receptors protect against alcoholic liver disease by regulating Kupffer cell polarization in mice

Activation of Kupffer cells plays a central role in the pathogenesis of alcoholic liver disease. Because cannabinoid CB2 receptors (CB2) display potent anti-inflammatory properties, we investigated their role in the pathogenesis of alcoholic liver disease, focusing on the impact of CB2 on Kupffer cell polarization and the consequences on liver steatosis. Wild-type (WT) mice fed an alcohol diet showed an induction of hepatic classical (M1) and alternative (M2) markers. Cotreatment of alcohol-fed mice with the CB2 agonist, JWH-133, decreased hepatic M1 gene expression without affecting the M2 profile. In keeping with this, genetic ablation of CB2 enhanced hepatic induction of M1 gene signature and blunted the induction of M2 markers. CB2 also modulated alcohol-induced fatty liver, as shown by the reduction of hepatocyte steatosis in JWH-133-treated mice and its enhancement in CB2-/- animals. Studies in isolated Kupffer cells and cultured macrophages further demonstrated that CB2 inhibits M1 polarization and favors the transition to an M2 phenotype. In addition, conditioned-medium experiments showed that preventing M1 polarization in CB2-activated macrophages protects from lipid accumulation in hepatocytes. Heme oxygenase-1 (HO-1) mediated the anti-inflammatory effects of CB2 receptors. Indeed, alcohol-fed mice treated with JWH-133 showed increased hepatic expression of macrophage HO-1, as compared to vehicle-treated counterparts. In keeping with this, JWH-133 induced HO-1 expression in cultured macrophages, and the HO-1 inhibitor, zinc protoporphyrin, blunted the inhibitory effect of JWH-133 on lipopolysaccharide-induced nuclear factor-kappa B activation and M1 polarization. Altogether, these findings demonstrate that CB2 receptors display beneficial effects on alcohol-induced inflammation by regulating M1/M2 balance in Kupffer cells, thereby reducing hepatocyte steatosis via paracrine interactions between Kupffer cells and hepatocytes. These data identify CB2 agonists as potential therapeutic agents for the management of alcoholic liver disease.