Fatty liver: Role of inflammation and fatty acid nutrition

Nonalcoholic fatty liver disease (NAFLD) refers to a wide spectrum of liver damage, ranging from simple steatosis to nonalcoholic steatohepatitis (NASH), advanced fibrosis, and cirrhosis. NAFLD is strongly associated with insulin resistance and is defined by accumulation of liver fat >5% per liver weight in the presence of <10 g of daily alcohol consumption. The exact prevalence of NAFLD is uncertain because of the absence of simple noninvasive diagnostic tests to facilitate an estimate of prevalence but in subgroups of people such as those with type 2 diabetes, the prevalence may be as high as 70%. NASH is an important subgroup within the spectrum of NAFLD that progresses over time with worsening fibrosis and cirrhosis, and NASH is associated with increased risk for cardiovascular disease. It is, therefore, important to understand the pathogenesis of NASH specifically, to develop strategies for interventions to treat this condition. The purpose of this review is to discuss the roles of inflammation, fatty acids and fatty acids in nutrition, in the pathogenesis and potential treatment of NAFLD.     

The relationship between oxidative stress and nonalcoholic fatty liver disease: Its effects on the development of nonalcoholic steatohepatitis

Abstract

Nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) are the most common underlying causes of chronic liver injury. They are associated with a wide spectrum of hepatic disorders including basic steatosis, steatohepatitis, and cirrhosis. The molecular and cellular mechanisms underlying hepatic injury in NAFLD and NASH are still unknown. This review describes the roles of oxidative stress and inflammatory responses in the pathogenesis of NAFLD and its progression to NASH.     

Pyruvate dehydrogenase kinase 4 mediates lipogenesis and contributes to the pathogenesis of nonalcoholic steatohepatitis

Nonalcoholic steatohepatitis (NASH) is a progressive disease and poses a high risk of severe liver damage. However, the pathogenesis of NASH is still unclear. Accumulation of lipid droplets and insulin resistance is the hallmark of NASH. Pyruvate dehydrogenase kinase isoenzyme 4 (PDK4) plays key role in glucose metabolism via regulating the activity of pyruvate dehydrogenase complex (PDC). Here, we demonstrated a novel of PDK4 in NASH by regulating hepatic steatosis and insulin signaling pathway in methionine and choline deficient (MCD) diet induced NASH model. Hepatic PDK4 levels were highly induced in human patients with NASH and MCD diet fed mice, as well as in hepatocytes treated with oleic acid. The glucose and lipid metabolism were impaired in Pdk4^?/? mice. Pdk4 deficiency ameliorated the hepatic steatosis significantly in NASH mice. Pdk4^?/?-MCD mice had reduced liver weights and triglyceride (TG) levels. And Pdk4 deficiency dramatically reduced the expression of genes related to fatty acid uptake, synthesis and gluconeogenesis. In addition, elevated phosphorylated AMPK (p-AMPK), p-SAPK/JNK and diminished p-ERK, p-P38, p-Akt and p-mTOR/p-4EBP1 proteins were observed. In conclusion, our data indicated that PDK4 potentially contributes to the hepatic steatosis in NASH via regulating several signaling pathway and PDK4 may be a new therapeutic strategy against NAFLD.     

The expanding role of fish models in understanding non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is a condition in which excessive fat accumulates in the liver of an individual who has not consumed excessive alcohol. Non-alcoholic steatohepatitis (NASH), a severe form of NAFLD, can progress to hepatic cirrhosis and/or hepatocellular carcinoma (HCC). NAFLD is considered to be a hepatic manifestation of metabolic syndrome, and its incidence has risen worldwide in lockstep with the increased global prevalence of obesity. Over the last decade, rodent studies have yielded an impressive list of molecules associated with NAFLD and NASH pathogenesis. However, the identification of currently unknown metabolic factors using mammalian model organisms is inefficient and expensive compared with studies using fish models such as zebrafish (Danio rerio) and medaka (Oryzias latipes). Substantial advances in unraveling the molecular pathogenesis of NAFLD have recently been achieved through unbiased forward genetic screens using small fish models. Furthermore, these easily manipulated organisms have been used to great advantage to evaluate the therapeutic effectiveness of various chemical compounds for the treatment of NAFLD. In this Review, we summarize aspects of NAFLD (specifically focusing on NASH) pathogenesis that have been previously revealed by rodent models, and discuss how small fish are increasingly being used to uncover factors that contribute to normal hepatic lipid metabolism. We describe the various types of fish models in use for this purpose, including those generated by mutation, transgenesis, or dietary or chemical treatment, and contrast them with rodent models. The use of small fish in identifying novel potential therapeutic agents for the treatment of NAFLD and NASH is also addressed.     

Multi-SNP Analysis of GWAS Data Identifies Pathways Associated with Nonalcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD) is a common liver disease; the histological spectrum of which ranges from steatosis to steatohepatitis. Nonalcoholic steatohepatitis (NASH) often leads to cirrhosis and development of hepatocellular carcinoma. To better understand pathogenesis of NAFLD, we performed the pathway of distinction analysis (PoDA) on a genome-wide association study dataset of 250 non-Hispanic white female adult patients with NAFLD, who were enrolled in the NASH Clinical Research Network (CRN) Database Study, to investigate whether biologic process variation measured through genomic variation of genes within these pathways was related to the development of steatohepatitis or cirrhosis. Pathways such as Recycling of eIF2:GDP, biosynthesis of steroids, Terpenoid biosynthesis and Cholesterol biosynthesis were found to be significantly associated with NASH. SNP variants in Terpenoid synthesis, Cholesterol biosynthesis and biosynthesis of steroids were associated with lobular inflammation and cytologic ballooning while those in Terpenoid synthesis were also associated with fibrosis and cirrhosis. These were also related to the NAFLD activity score (NAS) which is derived from the histological severity of steatosis, inflammation and ballooning degeneration. Eukaryotic protein translation and recycling of eIF2:GDP related SNP variants were associated with ballooning, steatohepatitis and cirrhosis. Il2 signaling events mediated by PI3K, Mitotic metaphase/anaphase transition, and Prostanoid ligand receptors were also significantly associated with cirrhosis. Taken together, the results provide evidence for additional ways, beyond the effects of single SNPs, by which genetic factors might contribute to the susceptibility to develop a particular phenotype of NAFLD and then progress to cirrhosis. Further studies are warranted to explain potential important genetic roles of these biological processes in NAFLD.     

Genome-Wide Analysis of Copy Number Variation Identifies Candidate Gene Loci Associated with the Progression of Non-Alcoholic Fatty Liver Disease

Between 10 and 25% of individuals with non-alcoholic fatty liver disease (NAFLD) develop hepatic fibrosis leading to cirrhosis and hepatocellular carcinoma (HCC). To investigate the molecular basis of disease progression, we performed a genome-wide analysis of copy number variation (CNV) in a total of 49 patients with NAFLD [10 simple steatosis and 39 non-alcoholic steatohepatitis (NASH)] and 49 matched controls using high-density comparative genomic hybridization (CGH) microarrays. A total of 11 CNVs were found to be unique to individuals with simple steatosis, whilst 22 were common between simple steatosis and NASH, and 224 were unique to NASH. We postulated that these CNVs could be involved in the pathogenesis of NAFLD progression. After stringent filtering, we identified four rare and/or novel CNVs that may influence the pathogenesis of NASH. Two of these CNVs, located at 13q12.11 and 12q13.2 respectively, harbour the exportin 4 (XPO4) and phosphodiesterase 1B (PDE1B) genes which are already known to be involved in the etiology of liver cirrhosis and HCC. Cross-comparison of the genes located at these four CNV loci with genes already known to be associated with NAFLD yielded a set of genes associated with shared biological processes including cell death, the key process involved in ‘second hit’ hepatic injury. To our knowledge, this pilot study is the first to provide CNV information of potential relevance to the NAFLD spectrum. These data could prove invaluable in predicting patients at risk of developing NAFLD and more importantly, those who will subsequently progress to NASH.     

Increased Circulating Levels of Alpha-Ketoglutarate in Morbidly Obese Women with Non-Alcoholic Fatty Liver Disease

BackgroundNon-alcoholic fatty liver disease (NAFLD) causes a wide spectrum of liver damage, ranging from simple steatosis to cirrhosis. However, simple steatosis (SS) and steatohepatitis (NASH) cannot yet be distinguished by clinical or laboratory features. The aim of this study was to assess the relationship between alpha-ketoglutarate and the degrees of NAFLD in morbidly obese patients.ResultsWe found that serum levels of alpha-ketoglutarate were significantly higher in morbidly obese women than in normal-weight women. We showed that circulating levels of alpha-ketoglutarate were lower in lean controls and morbidly obese patients without NAFLD. We also found that alpha-ketoglutarate serum levels were higher in both SS and NASH than in normal liver of morbidly obese patients. However, there was no difference between SS and NASH. Moreover, we observed that circulating levels of alpha-ketoglutarate were associated with glucose metabolism parameters, lipid profile, hepatic enzymes and steatosis degree. In addition, diagnostic performance of alpha-ketoglutarate has been analyzed in NAFLD patients. The AUROC curves from patients with liver steatosis exhibited an acceptable clinical utility. Finally, we showed that the combination of biomarkers (AST, ALT and alpha-ketoglutarate) had the highest accuracy in diagnosing liver steatosis.ConclusionThese findings suggest that alpha-ketoglutarate can determine the presence of non-alcoholic fatty liver in morbidly obese patients but it is not valid a biomarker for NASH.     

Lipidomics in pathogenesis,progression and treatment of nonalcoholic steatohepatitis (NASH): Recent advances

Nonalcoholic fatty liver disease (NAFLD) is a chronic liver disease affecting up to 30% of the general adult population. NAFLD encompasses a histological spectrum ranging from pure steatosis to non-alcoholic steatohepatitis (NASH). NASH can progress to cirrhosis and is becoming the most common indication for liver transplantation, as a result of increasing disease prevalence and of the absence of approved treatments. Lipidomic readouts of liver blood and urine samples from experimental models and from NASH patients disclosed an abnormal lipid composition and metabolism. Collectively, these changes impair organelle function and promote cell damage, necro-inflammation and fibrosis, a condition termed lipotoxicity. We will discuss the lipid species and metabolic pathways leading to NASH development and progression to cirrhosis, as well as and those species that can contribute to inflammation resolution and fibrosis regression. We will also focus on emerging lipid-based therapeutic opportunities, including specialized proresolving lipid molecules and macrovesicles contributing to cell-to-cell communication and NASH pathophysiology.     

The role of fructose-enriched diets in mechanisms of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) currently affects 20%-30% of adults and 10% of children in industrialized countries, and its prevalence is increasing worldwide. Although NAFLD is a benign form of liver dysfunction, it can proceed to a more serious condition, nonalcoholic steatohepatitis (NASH), which may lead to liver cirrhosis and hepatocellular carcinoma. NAFLD is accompanied by obesity, metabolic syndrome and diabetes mellitus, and evidence suggests that fructose, a major caloric sweetener in the diet, plays a significant role in its pathogenesis. Inflammatory progression to NASH is proposed to occur by a two-hit process. The first "hit" is hepatic fat accumulation owing to increased hepatic de novo lipogenesis, inhibition of fatty acid beta oxidation, impaired triglyceride clearance and decreased very-low-density lipoprotein export. The mechanisms of the second "hit" are still largely unknown, but recent studies suggest several possibilities, including inflammation caused by oxidative stress associated with lipid peroxidation, cytokine activation, nitric oxide and reactive oxygen species, and endogenous toxins of fructose metabolites.     

Fatty Liver Through the Ages: Nonalcoholic Steatohepatitis

ObjectiveThe global epidemic of obesity and type 2 diabetes mellitus is the main driver of the growing global prevalence of nonalcoholic fatty liver disease (NAFLD). We aimed to review the current literature on NAFLD and nonalcoholic steatohepatitis (NASH) as it impacts children and adults.MethodsWe performed a literature search on fatty liver specifically NAFLD and NASH among children and adults.ResultsThe prevalence of NAFLD in children ranges from 8% to 12%, while the prevalence in adults ranges 25% to 48%. The prevalence of NASH among children with NAFLD is 23%, while it ranges from 13% to 65% in adults. There are similar risk factors for NAFLD among children and adults. However, in children, the diagnostic tests in the studies of NAFLD are limited to the elevation of the alanine aminotransferase level or a liver biopsy. In adults, additional diagnostic modalities, including noninvasive tests, have been used. From the spectrum of NAFLD, patients with NASH are predominantly at risk of progressive liver disease to cirrhosis and liver-related mortality. NAFLD is associated with impairment of health-related quality of life and substantial economic burden.ConclusionThe comprehensive burden (clinical, health-related quality of life, and economic) of NAFLD is high and increasing.     

Vismodegib Suppresses TRAIL-mediated Liver Injury in a Mouse Model of Nonalcoholic Steatohepatitis

Hedgehog signaling pathway activation has been implicated in the pathogenesis of NASH. Despite this concept, hedgehog pathway inhibitors have not been explored. Thus, we examined the effect of vismodegib, a hedgehog signaling pathway inhibitor, in a diet-induced model of NASH. C57BL/6 mice were placed on 3-month chow or FFC (high saturated fats, fructose, and cholesterol) diet. One week prior to sacrifice, mice were treated with vismodegib or vehicle. Mice fed the FFC diet developed significant steatosis, which was unchanged by vismodegib therapy. In contrast, vismodegib significantly attenuated FFC-induced liver injury as manifested by reduced serum ALT and hepatic TUNEL-positive cells. In line with the decreased apoptosis, vismodegib prevented FFC-induced strong upregulation of death receptor DR5 and its ligand TRAIL. In addition, FFC-fed mice, but not chow-fed animals, underwent significant liver injury and apoptosis following treatment with a DR5 agonist; however, this injury was prevented by pre-treatment with vismodegib. Consistent with a reduction in liver injury, vismodegib normalized FFC-induced markers of inflammation including mRNA for TNF-α, IL-1β, IL-6, monocyte chemotactic protein-1 and a variety of macrophage markers. Furthermore, vismodegib in FFC-fed mice abrogated indices of hepatic fibrogenesis. In conclusion, inhibition of hedgehog signaling with vismodegib appears to reduce TRAIL-mediated liver injury in a nutrient excess model of NASH, thereby attenuating hepatic inflammation and fibrosis. We speculate that hedgehog signaling inhibition may be salutary in human NASH.     

The role of AMPKα2 in the HFD-induced nonalcoholic steatohepatitis

Nonalcoholic fatty liver disease (NAFLD) is associated with hepatic steatosis, inflammation and liver fibrosis and has become one of the leading causes of hepatocellular carcinoma and liver failure. However, the underlying molecular mechanism of hepatic steatosis and the progression to nonalcoholic steatohepatitis (NASH) are not fully understood. Herein, we discovered that AMPKα2 catalytic subunit showed reduced expression in the liver following high fat diet (HFD) feeding to mice. Importantly, knockout of AMPKα2 in mice aggravated NAFLD, hepatic steatosis, inflammation and fibrosis. On the other hand, hepatocyte-targeted overexpression of AMPKα2 prevented or reversed NAFLD indications. In vivo mechanistic studies revealed that increased phosphorylation of IKKα/β and NF-κB in HFD-fed AMPKα2^−/− mice compared to WT mice, and treatment of these mouse cohorts with an inhibitor of NF-κB signaling for 4 weeks, effectively attenuated the progression of steatohepatitis and metabolic disorder features. In summary, AMPKα2 provides a protective role in the process of hepatic steatosis to NASH progression through suppression of liver NF-κB signaling.     

MicroRNAs in the Pathogenesis of Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD), or, more accurately, metabolic associated fatty liver disease, accounts for a large proportion of chronic liver disorders worldwide and is closely associated with other conditions such as cardiovascular disease, obesity, and type 2 diabetes mellitus. NAFLD ranges from simple steatosis to nonalcoholic steatohepatitis (NASH) and can progress to cirrhosis and, eventually, also hepatocellular carcinoma. The morbidity and mortality associated with NAFLD are increasing rapidly year on year. Consequently, there is an urgent need to understand the etiology and pathogenesis of NAFLD and identify effective therapeutic targets. MicroRNAs (miRNAs), important epigenetic factors, have recently been proposed to participate in NAFLD pathogenesis. Here, we review the roles of miRNAs in lipid metabolism, inflammation, apoptosis, fibrosis, hepatic stellate cell activation, insulin resistance, and oxidative stress, key factors that contribute to the occurrence and progression of NAFLD. Additionally, we summarize the role of miRNA-enriched extracellular vesicles in NAFLD. These miRNAs may comprise suitable therapeutic targets for the treatment of this condition.     

Nonalcoholic fatty liver disease: predisposing factors and the role of nutrition

More than 20% of Americans have nonalcoholic fatty liver disease (NAFLD), and this is, by far, the leading cause of abnormal liver enzymes in the United States. Nonalcoholic steatohepatitis (NASH), a more serious form of NAFLD, can proceed to cirrhosis and even hepatocellular carcinoma. These liver diseases represent the hepatic component of the metabolic syndrome, and this spectrum of liver disease represents a major health problem both in the United States and worldwide. Hepatic steatosis is closely linked to nutrition, including obesity, possibly high-fructose corn syrup consumption and consumption of certain types of fats. There are a variety of second insults or "hits" that appear to transform simple steatosis into NASH, with some of these second hits including certain proinflammatory cytokines, oxidative stress and possibly industrial toxins. In certain underdeveloped countries, it appears likely that industrial toxins play a role in NASH, and there is increasing interest in the potential interaction of industrial toxins and nutrients. Moreover, optimal therapy for NAFLD appears to include lifestyle modification with exercise, diet and weight loss. Certain nutrients may also be of benefit. Important areas for future research are the effect(s) of nutritional supplements on NAFLD/NASH and the effects of industrial toxins.     

Inhibition of EGFR attenuates fibrosis and stellate cell activation in diet-induced model of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver disease. NAFLD begins with steatosis and advances to nonalcoholic steatohepatitis (NASH) and cirrhosis. The molecular mechanisms involved in NAFLD progression are not understood. Based on recent studies showing dysregulation of epidermal growth factor receptor (EGFR) in animal models of liver injury, we sought to determine if inhibition of EGFR mitigates liver fibrosis and HSC activation in NAFLD. We utilized the high fat diet (HFD)-induced murine model of liver injury to study the role of EGFR in NAFLD. The lipid accumulation, oxidative stress, hepatic stellate cell (HSC) activation and matrix deposition were examined in the liver tissues. We also evaluated the EGFR signaling pathway, ROS activation and pro-fibrogenic phenotype in oxidized low density lipoproteins (ox-LDL) challenged cultured HSCs. We demonstrate that EGFR was phosphorylated in liver tissues of HFD murine model of NAFLD. Inhibition of EGFR prevented diet-induced lipid accumulation, oxidative stress, and HSC activation and matrix deposition. In cultured HSCs, we show that ox-LDL caused rapid activation of the EGFR signaling pathway and induce the production of reactive oxygen species. EGFR also mediated HSC activation and promoted a pro-fibrogenic phenotype. In conclusion, our data demonstrate that EGFR plays an important role in NAFLD and is an attractive target for NAFLD therapy.     

非酒精性脂肪肝与脂肪性肝炎动物模型研究进展

非酒精性脂肪肝是无酒精滥用的包括单纯性脂肪肝、脂肪性肝炎、脂肪性肝纤维化和肝硬化的肝病综合征,目前已成为广受关注的肝病医学难题。随着抗脂肪肝药物的深入研究,动物模型制作得到很好发展。近年来,在大鼠、沙鼠、小鼠、兔和小猪等动物种属成功地建立了食物、胃肠外营养与蛋氨酸胆碱缺乏等诱导的单纯性脂肪肝和脂肪性肝炎动物模型,这些模型为研究脂肪肝和脂肪性肝炎的发病机理与治疗提供了机会。每种动物模型各有优缺点,合理应用动物模型能更好地开展脂肪肝病的实验和临床研究。本文综述了非酒精性脂肪肝及脂肪性肝炎动物模型制作方法的若干研究进展。     

The role of fatty acids in the development and progression of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) has emerged as a serious obesity-related disorder. NAFLD encompasses a wide spectrum of hepatic derangements ranging from a surfeit of fat in the liver (steatosis) to lipid surplus accompanied by fibrosis and cellular death (nonalcoholic steatohepatitis or NASH). The most widely accepted model to explain the progression from simple NAFLD to NASH is the "two-hit hypothesis," wherein fat over accumulation per se is not sufficient to induce the progression to statohepatitis, but renders the liver more susceptible to "second hits" that, once imposed upon the steatotic liver, cause further aberrations that culminate in the development of NASH. However, in light of recent data from our laboratory and elsewhere, we propose that an increased ratio of saturated-to-unsaturated fatty acids delivered to or stored within the liver may, in part, mediate the progression from simple steatosis to NASH. The molecular mechanisms that mediate the effect of saturated fatty acids are unclear, although proinflammatory cytokines, reactive oxygen species, and endoplasmic reticulum stress may all play a role. Collectively, these data suggest that saturated fatty acids may represent an intrinsic second hit to the liver that hastens the development of NASH.     

Amelioration of late-onset hepatic steatosis in IDH2-deficient mice

Nonalcoholic fatty liver disease (NAFLD) has a high prevalence in the general population and can evolve into nonalcoholic steatohepatosis (NASH), cirrhosis, and complications such as liver failure and hepatocellular carcinoma. Recently, we reported that mitochondrial NADP⁺-dependent isocitrate dehydrogenase, encoded by the IDH2, plays an important role in the regulation of redox balance and oxidative stress levels, which are tightly associated with intermediary metabolism and energy production. In the present study, we showed that in mice targeted disruption of IDH2 attenuates age-associated hepatic steatosis by the activation of p38/cJun NH2-terminal kinase (JNK) and p53, presumably induced by the elevation of mitochondrial reactive oxygen species (ROS), which in turn resulted in the suppression of hepatic lipogenesis and inflammation via the upregulation of fibroblast growth factor 21 (FGF21) and the inhibition of NFκB signaling pathways. Our finding uncovers a new mechanism involved in hepatocellular steatosis and IDH2 may be a valuable therapeutic target for the management of NAFLD.     

Mouse Models of Diet-Induced Nonalcoholic Steatohepatitis Reproduce the Heterogeneity of the Human Disease

MethodsMice were fed standard chow, MCD diet for 8 weeks, or Western diet (45% energy from fat, predominantly saturated fat, with 0.2% cholesterol, plus drinking water supplemented with fructose and glucose) for 16 weeks. Liver pathology and metabolic profile were compared.ResultsThe metabolic profile associated with human NASH was better mimicked by Western diet. Although hepatic steatosis (i.e., triglyceride accumulation) was also more severe, liver non-esterified fatty acid content was lower than in the MCD diet group. NASH was also less severe and less reproducible in the Western diet model, as evidenced by less liver cell death/apoptosis, inflammation, ductular reaction, and fibrosis. Various mechanisms implicated in human NASH pathogenesis/progression were also less robust in the Western diet model, including oxidative stress, ER stress, autophagy deregulation, and hedgehog pathway activation.ConclusionFeeding mice a Western diet models metabolic perturbations that are common in humans with mild NASH, whereas administration of a MCD diet better models the pathobiological mechanisms that cause human NAFLD to progress to advanced NASH.