Nonalcoholic Fatty Liver Disease: A Review and Update

The spectrum of nonalcoholic fatty liver disease (NAFLD) ranges from asymptomatic steatosis to nonalcoholic steatohepatitis (NASH) and cirrhosis. Hepatic steatosis occurs when free fatty acids, released in the setting of insulin resistance and the metabolic syndrome, are taken up by the liver. Additional biochemical insults, including oxidative stress, upregulation of inflammatory mediators, and dysregulated apoptosis, can result in inflammation (producing NASH) and fibrosis. Noninvasive methods (e.g., abdominal ultrasonography) are safe ways to support a diagnosis of hepatic steatosis, but advanced liver histopathologic findings including NASH and fibrosis cannot be identified without pursuing liver biopsy. Recent advances in serologic and imaging methods aim to determine severity of inflammation and fibrosis noninvasively. Currently, therapeutic options for NAFLD are limited to medications that reduce risk factors, but the future holds promise for therapies that might slow the progression of this increasingly prevalent disorder.     

Characterization of pathogenic and prognostic factors of nonalcoholic steatohepatitis associated with obesity

BACKGROUND/AIMS: Nonalcoholic steatohepatitis is an emerging clinical problem among the obese population. However, risk factors of progression to advanced forms of liver disease in this particular group of patients remain to be defined. METHODS: The demographics and clinical and histologic features of 46 obese patients were evaluated. The intrahepatic immunological phenotype was assessed in all liver biopsy samples by immunohistochemistry. RESULTS: Histologic findings of nonalcoholic steatohepatitis were observed in 69.5% of the obese population studied and significant fibrosis was evident in 41% of patients with nonalcoholic steatohepatitis. Age (p=0.003), degree of steatosis (p=0.000002), and grade of inflammation (p=0000) at liver biopsy were independent variables positively associated with fibrosis. Intrahepatic expression levels of several immunologic markers of inflammation as well as nitric oxide derivatives were significantly higher in the severe forms of nonalcoholic steatohepatitis than in the mildest forms. CONCLUSIONS: Obese persons with higher age, with greater degrees of hepatic steatosis, and specially those with increased grades of intrahepatic inflammation have the greatest risk for progression to fibrotic liver disease. An oxidative stress-triggered intrahepatic inflammatory response appears to be important in the pathogenesis of nonalcoholic steatohepatitis in obesity.     

Serum ubiquitin levels in patients with nonalcoholic steatohepatitis

BACKGROUND/AIMS: Nonalcoholic steatohepatitis is increasingly recognized as the most common liver disease in patients with elevated liver enzymes. In the pathophysiology of nonalcoholic steatohepatitis, the first step is the lipid accumulation in the liver causing steatosis, the second step involves the endotoxins, cytokines and environmental toxins causing oxidative stress and lipid peroxidation, in time leading to steatohepatitis. Ubiquitin is a molecular chaperone that plays a major role in the degradation of intracellular proteins. Ubiquitin proteasome system is also considered as a cellular defense mechanism that removes damaged proteins generated by oxidative stress. In order to search for the role of ubiquitin in the pathogenesis of nonalcoholic steatohepatitis, serum levels of ubiquitin were studied in patients with nonalcoholic steatohepatitis for the first time in the literature, to our knowledge. METHODOLOGY: Eighteen patients with biopsy proven nonalcoholic steatohepatitis diagnosis (13 males and 5 females with a mean age of 41) and 16 healthy volunteers as a control group (11 males and 5 females, with a mean age of 38) were included in the study. Serum ubiquitin levels were studied by ELISA method. RESULTS: The mean serum ubiquitin level (14.13 +/- 1.46 micrograms/mL) in patients with nonalcoholic steatohepatitis was significantly elevated compared to that of the control group (7.66 +/- 0.40 micrograms/mL) (p < 0.001). No correlation was found among serum ubiquitin levels and hepatic steatosis, inflammation and fibrosis. CONCLUSIONS: Increased serum ubiquitin levels may show that the ubiquitin proteasome pathway actively participates in defending against oxidative stress in nonalcoholic steatohepatitis. Serum ubiquitin concentration may be a marker predicting the intracellular cytoprotective response against oxidative stress rather than the degree of liver damage in pathogenesis of nonalcoholic steatohepatitis. Ubiquitin proteasome system based therapies may have a place in the treatment of patients with nonalcoholic steatohepatitis in the future.     

Steatosis and steatohepatitis in diabetic patient

Nonalcoholic fatty liver disease (NAFLD) is an increasingly recognized condition of excess fat deposition within the liver. NAFLD includes a spectrum of liver pathology ranging from bland hepatic steatosis to steatohepatitis and cirrhosis. Nonalcoholic steatohepatitis (NASH) is an inflammatory and fibrosing condition of the liver thought to be an intermediate stage of NAFLD that may progress to endstage liver disease, liver-related death and hepatocellular carcinoma. Nonalcoholic steatohepatitis (NASH) is a common liver disease that is characterized histologically by hepatic steatosis, lobular inflammation, and hepatocellular ballooning, it can progress to cirrhosis in up to 15% of patients. There is currently no therapy that is of proven benefit for nonalcoholic steatohepatitis. The disease is closely associated with insulin resistance and features of the metabolic syndrome such as obesity (increased waist circumference), hypertriglyceridemia, and type 2 diabetes. The pathologic criteria are now well established and the diagnosis can only be made once the absence or limited use of alcohol is confirmed. In addition to insulin resistance, oxidative stress has been implicated as a key factor contributing to hepatic injury in patients with nonalcoholic steatohepatitis. Thus, both insulin resistance and oxidative stress are attractive targets for therapy in patients with this disease. Several pilot studies have provided evidence that insulin sensitizers such as thiazolidinediones and antioxidants such as vitamin E improve clinical and histologic features of nonalcoholic steatohepatitis. The medical evidence of a benefit, however, is limited, because these studies had small samples and were performed at single centers. Moreover, a recent multicenter trial showed a reduction in hepatic steatosis but no improvement in markers of cell injury after a year of rosiglitazone therapy. The value of these remains uncertain. Until now the best trial was done by Sanyal, who studied 240 patients divided into 3 groups (pioglitazone versus vitamin E versus placebo)--multicenter, randomized, double-blind clinical trial in non-diabetics.     

Lipidomics in non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD), the most common chronic liver disorder in Western countries, comprises steatosis to nonalcoholic steatohepatitis (NASH), with the latter having the potential to progress to cirrhosis. The transition from isolated steatosis to NASH is still poorly understood, but lipidomics approach revealed that the hepatic lipidome is extensively altered in the setting of steatosis and steatohepatitis and these alterations correlate with disease progression. Recent data suggest that both quantity and quality of the accumulated lipids are involved in pathogenesis of NAFLD. Changes in glycerophospholipid, sphingolipid, and fatty acid composition have been described in both liver biopsies and plasma of patients with NAFLD, implicating that specific lipid species are involved in oxidative stress, inflammation, and cell death. In this article, we summarize the findings of main human lipidomics studies in NAFLD and delineate the currently available information on the pathogenetic role of each lipid class in lipotoxicity and disease progression.     

Mitochondrial DNA from hepatocytes as a ligand for TLR9: Drivers of nonalcoholic steatohepatitis?

Nonalcoholic fatty liver disease(NAFLD) is the most common liver disease worldwide, affecting approximately one third of the Western world. It consists of a wide spectrum of liver disorders, ranging from fatty liver to nonalcoholic steatohepatitis(NASH), which consists of steatosis, ballooning injury and inflammation. Despite an alarming growth in the statistics surrounding NAFLD, there are as yet no effective therapies for its treatment. Innate immune signaling has been thought to play a significant role in initiating and augmenting hepatic inflammation, contributing to the transition from nonalcoholic fatty liver to NASH. An immune response is triggered by countless signals called damage-associated molecular patterns(DAMPs) elicited by lipid-laden and damaged hepatocytes, which are recognized by pattern recognition receptors(PRRs) on hepatic immune cells to initiate inflammatory signaling. In this editorial, in addition to summarizing innate immune signaling in NAFLD and discussing potential therapies that target innate immune pathways, we have described a recent study that demonstrated that mitochondrial DNA serves as a DAMP activating a hepatic PRR, TLR9, in mice and in the plasma of NASH patients. In addition to identifying a new ligand for TLR9 during NASH progression, the study shows that blocking TLR9 reverses NASH, paving the way for the development of future NASH therapy.     

Molecular mechanisms of lipotoxicity in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is characterized by insulin resistance, which results in elevated serum concentration of free fatty acids (FFAs). Circulating FFAs provide the substrate for triacylglycerol formation in the liver, and may also be directly cytotoxic. Hepatocyte apoptosis is a key histologic feature of NAFLD, and correlates with progressive inflammation and fibrosis. The molecular pathways leading to hepatocyte apoptosis are not fully defined; however, recent studies suggest that FFA-induced apoptosis contributes to the pathogenesis of nonalcoholic steatohepatitis. FFAs directly engage the core apoptotic machinery by activating the proapoptotic protein Bax, in a c-jun N-terminal kinase-dependent manner. FFAs also activate the lysosomal pathway of cell death and regulate death receptor gene expression. The role of ER stress and oxidative stress in the pathogenesis of nonalcoholic steatohepatitis has also been described. Understanding the molecular mediators of liver injury should promote development of mechanism-based therapeutic interventions.     

Is there a role for adaptive immunity in nonalcoholic steatohepatitis?

The growing diffusion of nonalcoholic fatty liver disease (NAFLD) is a consequence of the worldwide increase in the prevalence of obesity. Oxidative stress is widely recognized to play a pivotal role in NAFLD evolution to nonalcoholic steatohepatitis (NASH). Here we review recent evidence suggesting that oxidative stress-derived antigens originating within fatty livers stimulate both humoral and cellular adaptive immune responses and the possible mechanisms involved in sustaining hepatic inflammation in NASH.     

Galectin-3 and IL-33/ST2 axis roles and interplay in dietinduced steatohepatitis

Immune reactivity and chronic low-grade inflammation(metaflammation) play an important role in the pathogenesis of obesity-associated metabolic disorders, including type 2 diabetes and nonalcoholic fatty liver disease(NAFLD), a spectrum of diseases that include liver steatosis, nonalcoholic steatohepatitis(NASH), fibrosis, and cirrhosis. Increased adiposity and insulin resistance contribute to the progression from hepatic steatosis to NASH and fibrosis through the development of proinflammatory and profibrotic processes in the liver, including increased hepatic infiltration of innate and adaptive immune cells, altered balance of cytokines and chemokines, increased reactive oxygen species generation and hepatocellular death. Experimental models of dietary-induced NAFLD/NASH in mice on different genetic backgrounds or knockout mice with different immune reactivity are used for elucidating the pathogenesis of NASH and liver fibrosis. Galectin-3(Gal-3), a unique chimera-type β-galactoside-binding protein of the galectin family has a regulatory role in immunometabolism and fibrogenesis. Mice deficient in Gal-3 develop pronounced adiposity, hyperglycemia and hepatic steatosis, as well as attenuated liver inflammation and fibrosis when fed an obesogenic high-fat diet. Interleukin(IL)-33, a member of the IL-1 cytokine family, mediates its effects through the ST receptor, which is present on immune and nonimmune cells and participates in immunometabolic and fibrotic disorders. Recent evidence, including our own data, suggests a protective role for the IL-33/IL-33R(ST2) signaling pathway in obesity, adipose tissue inflammation and atherosclerosis, but a profibrotic role in NASH development. The link between Gal-3 and soluble ST2 in myocardial fibrosis and heart failure progression has been demonstrated and we have recently shown that Gal-3 and the IL-33/ST2 pathway interact and both have a profibrotic role in diet-induced NASH. This review discusses the current evidence on the roles of Gal-3 and the IL-33/ST2 pathway and their interplay in obesity-associated hepatic inflammation and fibrogenesis that may be of interest in the development of therapeutic interventions to prevent and/or reverse obesity-associated hepatic inflammation and fibrosis.     

Growth hormone reverses nonalcoholic steatohepatitis in a patient with adult growth hormone deficiency

BACKGROUND AND AIMS: Nonalcoholic steatohepatitis (NASH) is an emerging progressive hepatic disease and demonstrates steatosis, inflammation, and fibrosis. Insulin resistance is a common feature in the development of NASH. Molecular pathogenesis of NASH consists of 2 steps: triglyceride accumulation in hepatocytes with insulin resistance and an enhanced oxidative stress caused by reactive oxygen species. Interestingly, NASH demonstrates a striking similarity to the pathologic conditions observed in adult growth hormone deficiency (AGHD). AGHD is characterized by decreased lean body mass, increased visceral adiposity, abnormal lipid profile, and insulin resistance. Moreover, liver dysfunctions with hyperlipidemia and nonalcoholic fatty liver disease (NAFLD) are frequently observed in patients with AGHD, and it is accompanied by metabolic syndrome. METHODS: We studied a case diagnosed as NASH with hyperlipidemia in AGHD. The effect of GH-replacement therapy on the patient was analyzed. RESULTS: Six months of GH-replacement therapy in the patient drastically ameliorated NASH and the abnormal lipid profile concomitant with a marked reduction in oxidative stress. CONCLUSIONS: These results suggest that GH plays an essential role in the metabolic and redox regulation in the liver.     

Fat and the liver--a molecular overview

Nonalcoholic fatty liver disease (NAFLD) is a chronic liver disease that occurs in nondrinkers but which cannot be distinguished from alcohol-induced liver disease histologically. There are no diagnostic blood tests for NAFLD but the disease is associated with several insulin-resistant states, including obesity, type 2 diabetes mellitus and dyslipidemia. Most of the liver-related morbidity and mortality that accompany NAFLD occur in patients who develop cirrhosis. The latter is most likely to occur in individuals who have progressed from simple steatosis (fatty liver) to steatohepatitis, a chronic inflammatory liver lesion. The mechanisms that promote the transition from steatosis to nonalcoholic steatohepatitis appear to involve multiple cellular adaptations to the oxidative stress that occurs when fatty acid metabolism is deranged during insulin resistance. A better understanding of these mechanisms is desired to target treatments to prevent and/or reverse nonalcoholic steatohepatitis, thereby aborting the evolution of cirrhosis.     

JNK1 but not JNK2 promotes the development of steatohepatitis in mice

Nonalcoholic fatty liver disease (NAFLD) is characterized by hepatic steatosis and varying degrees of necroinflammation. Although chronic oxidative stress, inflammatory cytokines, and insulin resistance have been implicated in the pathogenesis of NAFLD, the mechanisms that underlie the initiation and progression of this disease remain unknown. c-Jun N-terminal kinase (JNK) is activated by oxidants and cytokines and regulates hepatocellular injury and insulin resistance, suggesting that this kinase may mediate the development of steatohepatitis. The presence and function of JNK activation were therefore examined in the murine methionine- and choline-deficient (MCD) diet model of steatohepatitis. Activation of hepatic JNK, c-Jun, and AP-1 signaling occurred in parallel with the development of steatohepatitis in MCD diet-fed mice. Investigations in jnk1 and jnk2 knockout mice demonstrated that jnk1, but not jnk2, was critical for MCD diet-induced JNK activation. JNK promoted the development of steatohepatitis as MCD diet-fed jnk1 null mice had significantly reduced levels of hepatic triglyceride accumulation, inflammation, lipid peroxidation, liver injury, and apoptosis compared with wild-type and jnk2 -/- mice. Ablation of jnk1 led to an increase in serum adiponectin but had no effect on serum levels of tumor necrosis factor-alpha. In conclusion, JNK1 is responsible for JNK activation that promotes the development of steatohepatitis in the MCD diet model. These findings also provide additional support for the critical mechanistic involvement of JNK1 overactivation in conditions associated with insulin resistance and the metabolic syndrome.     

Roles of adipose restriction and metabolic factors in progression of steatosis to steatohepatitis in obese, diabetic mice

Background and Aims: We previously reported that steatohepatitis develops in obese, hypercholesterolemic, diabetic foz/foz mice fed a high‐fat (HF) diet for 12 months. We now report earlier onset of steatohepatitis in relation to metabolic abnormalities, and clarify the roles of dietary fat and bodily lipid partitioning on steatosis severity, liver injury and inflammatory recruitment in this novel non‐alcoholic steatohepatitis (NASH) model. Methods: Foz/foz (Alms1 mutant) and wild‐type (WT) mice were fed a HF diet or chow, and metabolic characteristics and liver histology were studied at 2, 6, 12 and 24 weeks. Results: After 12 weeks HF‐feeding, foz/foz mice were obese and diabetic with approximately 70% reduction in serum adiponectin. Hepatomegaly developed at this time, corresponding to a plateau in adipose expansion and increased adipose inflammation. Liver histology showed mild inflammation and hepatocyte ballooning as well as steatosis. By 24 weeks, HF‐fed foz/foz mice developed severe steatohepatitis (marked steatosis, alanine aminotransferase elevation, ballooning, inflammation, fibrosis), whereas dietary and genetic controls showed only simple steatosis. While steatosis was associated with hepatic lipogenesis, indicated by increased fatty acid synthase activity, steatohepatitis was associated with significantly higher levels of CD36, indicating active fatty acid uptake, possibly under the influence of peroxisome proliferator‐activated receptor‐γ. Conclusion: In mice genetically predisposed to obesity and diabetes, HF feeding leads to restriction of adipose tissue for accommodation of excess energy, causing lipid partitioning into liver, and transformation of simple steatosis to fibrosing steatohepatitis. The way in which HF feeding ‘saturates’ adipose stores, decreases serum adiponectin and causes hepatic inflammation in steatohepatitis may provide clues to pathogenesis of NASH in metabolic syndrome.     

非酒精性脂肪性肝病分子机制研究进展

非酒精性脂肪性肝病(NAFLD)是一种以肝细胞脂肪变性和脂质沉积为特征,但无过量饮酒史的临床综合征。NAFLD疾病谱包括非酒精性单纯性脂肪肝、非酒精性脂肪性肝炎(NASH)、NASH相关肝纤维化和肝硬化。NAFLD的发病机制较为复杂,至今尚未完全阐明。本文就近年NAFLD分子机制研究的新进展,包括胰岛素抵抗(IR)、肝脂肪变性、氧化应激等方面作一综述。     

Mechanisms Linking Nonalcoholic Fatty Liver Disease with Coronary Artery Disease

The most common cause of death in patients with nonalcoholic fatty liver disease (NAFLD) is coronary artery disease (CAD), not chronic liver disease. Fatty liver increases cardiovascular risk by classical (dyslipidemia, hypertension, diabetes) and by less conventional mechanisms. Common pathways involved in the pathogenesis of fatty liver and CAD includes hepatic insulin resistance and sub clinical inflammation. The hepatic insulin resistance state of fatty liver infiltration is characterized by increased FFA, which causes lipotoxicity and impairs endothelium-dependent vasodilatation, increases oxidative stress, and has a cardio toxic effect. Additional metabolic risk factors include leptin, adiponectin, pro inflammatory cytokines [such as IL-6, C-reactive protein and plasminogen activator inhibitor-1 (PAI-1)], which together lead to increased oxidative stress and endothelial dysfunction, finally promoting coronary artery disease (CAD). When classical risk factors are superimposed on fatty liver accumulation, they may further increase the new metabolic risk factors, exacerbating CAD. The clinical implication is that patients with NAFLD are at higher risk (steatohepatitis, diabetes, obesity, atherogenic dyslipidemia) and should undergo periodic cardiovascular risk assessment including the Framingham score, cardiac effort test, and measurement of intimae-media thickening of the carotids arteries. This may improve risk stratification for CAD.     

Inflammation and fibrogenesis in steatohepatitis

Nonalcoholic fatty liver disease consists of a range of disorders characterized by excess accumulation of triglyceride within the liver. Whereas simple steatosis is clinically benign, nonalcoholic steatohepatitis (NASH) often progresses to cirrhosis. Inflammation and fibrogenesis are closely inter-related and are major targets of NASH research. Experimental data have shown that inflammation in NASH is caused by insulin resistance, systemic lipotoxicity due to overnutrition, lipid metabolites, the production of proinflammatory cytokines and adipokines by visceral adipose tissue, gut-derived bacteria, and oxidative stress. In NASH-associated fibrosis, the principal cell type responsible for extracellular matrix production is recognized as the hepatic stellate cell. Although the fibrotic mechanisms underlying NASH are largely similar to those observed in other chronic liver diseases, the altered patterns of circulating adipokines, the generation of oxidative stress, and the hormonal profile associated with the metabolic syndrome might play unique roles in the fibrogenesis associated with the disease. Information on the basic pathogenesis of NASH with a focus on the generation of inflammation and fibrosis will be discussed.     

Lipid-induced oxidative stress causes steatohepatitis in mice fed an atherogenic diet

Recently, nonalcoholic steatohepatitis (NASH) was found to be correlated with cardiovascular disease events independently of the metabolic syndrome. The aim of this study was to investigate whether an atherogenic (Ath) diet induces the pathology of steatohepatitis necessary for the diagnosis of human NASH and how cholesterol and triglyceride alter the hepatic gene expression profiles responsible for oxidative stress. We investigated the liver pathology and plasma and hepatic lipids of mice fed the Ath diet. The hepatic gene expression profile was examined with microarrays and real-time polymerase chain reactions. The Ath diet induced dyslipidemia, lipid peroxidation, and stellate cell activation in the liver and finally caused precirrhotic steatohepatitis after 24 weeks. Cellular ballooning, a necessary histological feature defining human NASH, was observed in contrast to existing animal models. The addition of a high-fat component to the Ath diet caused hepatic insulin resistance and further accelerated the pathology of steatohepatitis. A global gene expression analysis revealed that the Ath diet up-regulated the hepatic expression levels of genes for fatty acid synthesis, oxidative stress, inflammation, and fibrogenesis, which were further accelerated by the addition of a high-fat component. Conversely, the high-fat component down-regulated the hepatic gene expression of antioxidant enzymes and might have increased oxidative stress. CONCLUSION: The Ath diet induces oxidative stress and steatohepatitis with cellular ballooning. The high-fat component induces insulin resistance, down-regulates genes for antioxidant enzymes, and further aggravates the steatohepatitis. This model suggests the critical role of lipids in causing oxidative stress and insulin resistance leading to steatohepatitis.