Evolution of Inflammation in Nonalcoholic Fatty Liver Disease:The Multiple Parallel Hits Hypothesis
Herbert Tilg and Alexander R.Moschen
Whereas in most cases a fatty liver remains free of inflammation,10%-20%of patients
who have fatty liver develop inflammation andfibrosis(nonalcoholic steatohepatitis
[NASH]).Inflammation may precede steatosis in certain instances.Therefore,NASH
could reflect a disease where inflammation is followed by steatosis.In contrast,NASH sub-
sequent to simple steatosis may be the consequence of a failure of antilipotoxic protection.
reactive oxygen species是什么意思In both situations,many parallel hits derived from the gut and/or the adipose tissue may
promote liver inflammation.Endoplasmic reticulum stress and related signaling networks,
(adipo)cytokines,and innate immunity are emerging as central pathways that regulate key
features of NASH.(H EPATOLOGY2010;52:1836-1846)
N onalcoholic fatty liver disease(NAFLD) includes a disease spectrum ranging from
simple steatosis to nonalcoholic steatohepati-tis(NASH),liverfibrosis,cirrhosis,and hepatocellular carcinoma.1The majority of patients with NAFLD are obese or even morbidly obese and have accompanying insulin resistance.2-4The proportion of patients with NAFLD who have NASH is still not entirely clear but might range from10%-20%.This is relevant because inflammation and/orfibrosis determine the long-term prognosis of this disease,whereas steatosis per se might not adversely affect outcome.5-8Most studies indicate that1%-3%of the Western population might have NASH.The natural history of NAFLD is still poorly understood,and in particular,it is not known why cer-tain patients progress toward inflammation,fibrosis, and cirrhosis and why others do not.One of the burn-ing questions in NAFLD remains which factors could be the driving forces toward a more progressive, inflammatory disease phenotype.Day and colleagues presented more than a decade ago the so-called‘‘two-hit’’model,suggesting that after afirst ,hepatic steatosis)another ,gut-derived endotoxin)is needed to develop NASH.9
Because simple hepatic steatosis is a benign process in the majority of patients,NASH might be a separate disease with a different pathogenesis.Here,we propose a new model suggesting that many hits may act in par-allel,finally resulting in liver inflammation and that especially gut-derived and adipos
e tissue–derived fac-tors may play a central role.Inflammation may pre-cede steatosis in NASH,as inflammatory events may lead to subsequent steatosis.Furthermore,we want to highlight the potential importance of endoplasmic reticulum(ER)stress in various aspects of this disease. Development of Hepatic Steatosis
A fatty liver is the result of the accumulation of var-ious lipids.10Several mechanisms may lead to a fatty liver:(1)increased free fatty acids supply due to increased lipolysis from both visceral/subcutaneous adi-pose tissue and/or increased intake of dietary fat;(2) decreased free fatty oxidation oxidation;(3)increased de novo hepatic lipogenesis(DNL)and(4)decreased hepatic very low density lipoprotein–triglyceride secre-tion.11Free fatty acid delivery to the liver accounts for almost two-thirds of its lipid accumulation.12
Abbreviations:AhR,aryl hydrocarbon receptor;ATF-6,activating transcription factor6;ChREBP,carbohydrate response element-binding protein; DGAT,diacylglycerol acyltransferase;DNL,de novo lipogenesis;ER, endoplasmic reticulum;IKK b,inhibitor of nuclear factor-j B kinase-b;Gpr,G protein–coupled receptor;IL,interleukin;IRE1,inositol-requiring enzyme1; JNK1,c-jun N-terminal protein kinase1;LPS,lipopolysaccharide;mRNA, messenger RNA;PERK,pancreatic ER kinase;PI3K,phosphatidyl inositol3-kinase;patatin-like phospholipase3PNPLA3;PPAR c,peroxisome pr
oliferator-activated receptor-gamma;ROS,reactive oxygen species;SCFA,short chain fatty acid;SOCS3,suppressor of cytokine signaling3;SREBP,sterol regulatory element-binding protein;TLR,toll-like receptor;TNF,tumor necrosis factor; UDCA,ursodeoxycholic acid;UPR,unfolded protein response;XBP1,X-box binding protein1.
From the Christian Doppler Research Laboratory for Gut Inflammation, Medical University Innsbruck,Innsbruck,Austria
Received July11,2010;accepted September13,2010.
This work was supported by the Christian Doppler Research Society. Address reprint requests to:Herbert Tilg,M.D.,Christian Doppler Research Laboratory for Gut Inflammation,Medical University Innsbruck,Anichstrasse 35,6020Innsbruck,Austria.E-mail:herbert.tilg@i-med.ac.at;fax:þ43512 5046723374.
Copyright V C2010by the American Association for the Study of Liver Diseases. View this article online at wileyonlinelibrary.
DOI10.1002/hep.24001
Potential conflict of interest:Nothing to report.
1836
Elevated peripheral fatty acids and DNL therefore pre-dominantly contribute to the accumulation of hepatic fat in NAFLD.Besides the well-established lipogene-sis-controlling factors such as sterol regulatory ele-ment-binding protein(SREBP)or carbohydrate response element-binding protein(ChREBP),X-box binding protein1(XBP1),known as a key regulator of the unfolded protein response(UPR)secondary to ER stress,is a only recently characterized regulator of hepatic lipogenesis.13
T riglycerides are the main lipids stored in the liver of patients with NAFLD.Although large epidemiological studies suggest triglyceride-mediated pathways might negatively affect disease,14recent evidence indicates that trigylcerides might exert protective functions.Diacyl-glycerol acyltransferase1and2(DGAT1/2)catalyze the final step in triglyceride synthesis.In a model of diet-induced obesity,mice with overexpression of DGAT1in adipocytes and macrophages are protected from macro-phage activation and their accumulation in white adi-pose tissue,from systemic inflammation and insulin re-sistance.15Inhibition of triglyceride synthesis via DGAT2antisense oligonuc
leotides improves liver steato-sis but worsens liver damage,also suggesting that accu-mulation of liver triglycerides could be a protective mechanism.16Hepatic ,triglyceride accu-mulation)is dissociated from insulin resistance in patients with familial hypobetalipoproteinemia,provid-ing further evidence that increased intrahepatic triglycer-ide content might be more a marker rather than a cause of insulin resistance.17In summary,triglyceride synthesis seems to be an adaptive,beneficial response in situations where hepatocytes are exposed to potentially toxic tri-glyceride metabolites.Thus,evidence is increasing that accumulation of fat in the liver in many instances cannot be regarded as a pathology or disease,but rather as a physiologic response to increased caloric consumption.18 Free fatty acids and cholesterol,especially when accumulated in mitochondria,are considered the ‘‘aggressive’’lipids leading to tumor necrosis factor alpha(TNF a)-mediated liver damage and reactive oxy-gen species(ROS)formation.19,20These lipids could also be present in a nonsteatotic liver and act as early ‘‘inflammatory’’hits leading to the whole spectrum of NAFLD pathologies.The concept of lipotoxicity and involved lipid species has been introduced and dis-cussed in several excellent review articles.21,22
Inflammation Preceding Steatosis.Simple hepatic steatosis,which is benign and nonprogressive in the ma-jority of patients,and NASH may reflect different dis-ease entities.Inflammation results in a stress r
esponse of hepatocytes,may lead to lipid accumulation,and there-fore could precede steatosis in NASH.Such a cascade is supported by various studies.Patients with NASH may present without any or much steatosis,suggesting that inflammation could take placefirst.1Anti-TNF anti-body treatment and metformin,an antidiabetic drug that inhibits hepatic TNF a expression,improve steatosis in ob/ob mice.23,24Other proinflammatory mediators might also contribute to the development of steatosis because in some studies hepatic steatosis was not de-pendent on TNF a.25,26In patients with severe alcoholic hepatitis,treatment with infliximab,an anti-TNF anti-body,primarily improves hepatic steatosis.27Loss of Kupffer cells also leads to hepatic steatosis probably via decreased interleukin-10(IL-10)release from Kupffer cells.28Other cell types might also promote hepatic ste-atosis because obesity leads to the hepatic recruitment of a myeloid cell population that further promotes hepatic lipid storage.29In all these situations,hepatic steatosis may be considered as‘‘bystander phenomenon’’subse-quent to inflammatory attacks.Very diverse processes including toxic lipids,nutrients,and other gut-derived and adipose-derived signals(as discussed later)may rep-resent such inflammatory insults.
Certain Dietary Factors:A Direct Roadmap to Lipotoxic-ity?The consumption of trans-fatty acids has increased dramatically in the last decades and mice fed trans-fatty acids develop larger livers with NA
SH-like lesions and in-sulin resistance.30Although virtually absent from our diet in the past,fructose has now become a major constituent of modern diet.When obese subjects consumed glucose-or fructose-sweetened beverages for10weeks,fasting plasma glucose and insulin levels increased and insulin sensitivity decreased in subjects consuming fructose but not in those consuming glucose.31Daily fructose con-sumption is associated with increased hepatic inflamma-tion andfibrosis in humans.32The aryl hydrocarbon receptor(AhR)is a ligand-activated transcription factor sensing xenotoxicants such as dioxin.This pathway may play a major role in inflammatory processes.33Many AhR agonists are present in the diet such as indolo-(3,2-b)-carbazole and3,30-diindolylmethane(metabolized from indole3-carbinol),orflavonoids.T ransgenic mice with constitutively activated AhR develop spontaneous hepatic steatosis and increased hepatic oxidative stress.34 It remains to be identified how certain nutrients might directly lead to liver inflammation.
Gut-Derived Signals Beyond Endotoxin May Promote Liver Inflammation Conventionalization of germ-free mice with a nor-mal microbiota leads to weight gain,obesity,and
HEPATOLOGY,Vol.52,No.5,2010TILG AND MOSCHEN1837
insulin resistance,which suggests that the microbiota and/or microbiota-regulated host factors might in
flu-ence energy absorption,adiposity,systemic inflamma-tion,and development of insulin resistance.35,36 Endotoxin and Its Role in Obesity.Endotoxin(li-popolysaccharide[LPS]),a key constituent of many bacteria present in our microbiota,plays a central role in innate immune responses and has been considered the so-called‘‘second hit’’in previous NASH models.9 Manipulation at the gut surface,including dietary ingredients,may affect LPS metabolism and result in increased circulating plasma levels.It has been demon-strated that intake of a high-fat or a high-carbohydrate diet in humans over only3days leads to an increase in circulating LPS ,‘‘second hit’’).37 Endotoxemia,however,might not only lead to sys-temic inflammation but might also worsen obesity itself.38When endotoxemia was induced for4weeks in lean mice,liver and adipose tissue weight gain were increased similarly as after a high-fat diet.This weight gain was paralleled by hepatic insulin resistance,and could be prevented by antibiotic therapy.Patients with NAFLD demonstrate increased gut permeability,which importantly has been associated with the severity of liver steatosis but not with the degree of inflammation (NASH).39This study therefore suggests that gut-derived factors/signals such as endotoxin might also affect accumulation of hepatic fat.
Intestinal Epithelium:Linking Nutrients to Meta-bolic Diseases.Our microbiota might influence sys-temic immune responses.Such an effect might take place via their capacity to digest dietaryfiber resulting in t
he production of short-chain fatty acids(SCFA). SCFAs have anti-inflammatory functions in various models of colitis and human ulcerative colitis probably via interaction with its receptor,the G protein–coupled receptor43(Gpr43).40Gpr43À/Àmice show systemic inflammation in various tissues,41similar to germ-free wild-type mice devoid of bacterial ferment-ing capacity and hence with almost absent SCFAs in the gut.Various other ,fasting-induced adipose factor;Gpr41)have been characterized that might interfere with metabolism/adiposity,highlighting how the intestinal microbiota and its products might directly regulate host gene expression and affect sys-temic inflammation.42-45These pathways involve the intestinal epithelium as‘‘sensor’’of the microbiota, implicating a major role for the intestinal epithelium in determining systemic metabolic functions(for details,see Fig.1).Interference with our microbiota via probiotics or prebiotics might therefore be benefi-cial and improve systemic inflammation/metabolic function.So far,only a few animal studies have been performed that suggest that this might indeed be the case.23,46,47
Toll-Like Receptors and Role of Innate Immunity in Obesity-Related Inflammation.Toll-like receptors (TLRs),also expressed on the gut epithelium,can respond to nutritional lipids such as free fatty acids and might thereby have a role in the pathogenesis of obesity-associated inflammation/insulin resistance.48 The recognition of fatty acids by TLR4can induce the production of proinflammatory cyto
kines in macro-phages and epithelial cells.49TLR-4–deficient mice are protected from high-fat diet-induced inflammation and insulin resistance.50It is,however,not universally accepted whether saturated free fatty acids are ligands for certain TLRs because it has been demonstrated that saturated fatty acids might not directly stimulate TLR-dependent signaling.51Therefore,observed effects in the above discussed in vivo study49could also be accounted by gut-derived endotoxin or by endotoxin contamination of the lipids employed.
Other TLRs may also be involved in obesity-related inflammation.TLR9promotes steatohepatitis because TLR9-deficient mice are protected from liver inflam-mation.52The importance of the gut as‘‘metabolic organ’’has been convincingly demonstrated by a recent report indicating that mice deficient in TLR5develop all features of metabolic syndrome including hyperpha-gia,obesity,insulin resistance,pancreatic inflamma-tion,and hepatic steatosis.53TLR5deficiency affected the composition of the gut microbiota and,remark-ably,transfer of the microbiota from TLR5À/Àmice to healthy mice resulted in transfer of disease.There are two major implications of this work:(1)the innate immune system plays a critical role in the development of the metabolic syndrome and(2)transfer of the gut microbiota to wild-type germ-free mice results in sev-eral features of de novo ,metabolic syn-drome),again supporting a major role for our micro-biota in metabolic inflammation.
Adipose Tissue-Derived Signals:The Adipose Tissue Attacks the Liver
Adipose tissue has appeared in the last decade as a highly active endocrine and immune organ with the capacity of producing various mediators including adi-pocytokines and cytokines both in health and disease. The balance/imbalance of an adipose tissue‘‘mediator cocktail’’may profoundly affect not only the situation in the adipose tissue but especially in important target organs such as the liver(Fig.1).
1838TILG AND MOSCHEN HEPATOLOGY,November2010
Adiponectin:Prototypic Adipocytokine in Health and Disease.Adiponectin is an anti-inflammatory adi-pocytokine that signals through two receptors.54-56Obesity is associated with hypoadiponectinemia,and adiponectin levels increase after weight loss.55Adipo-nectin induces extracellular Ca 2þinflux by adiponectin receptor 1,which is necessary for activation of adeno-sine monophosphate–activated protein kinase (AMPK)and Sirtuin 1(Sirt1).57Hepatocyte-specific deletion of Sirt1leads not only to hepatic steatosis but also to ER stress and liver inflammation.58Genetically obese lep-tin-deficient ob/ob mice exhibit a reversal of
the
Fig.1.The multiple parallel hits model.Lipotoxicity:(1)A liver loaded with lipids consisting primarily of trigylcerides might reflect a benign process because trigylcerides might exert mostly protective effects.Furthermore,hyperleptinemia leads to oxidation of hepatic lipids,thereby also protecting this organ from lipotoxicity.When the capacity of peripheral and central organs of detoxifying ‘‘aggressive lipids’’fails,lipotoxic attack of the liver might begin.Inflammation may precede steatosis in NASH.Gut-derived signals:Many signals beyond endotoxin might affect hepatic steatosis and inflammation.Several pathways have been identified how the gut microbiota might influence host energy metabolism:(2)Absence of the microbiota in germ-free mice correlates with increased activity of phosphorylated AMPK in the liver and the muscle (not shown).(3)Some of the breakdown products of polysaccharides are metabolized to SCFAs.SCFAs such as propionate and acetate are ligands for the G protein–coupled receptors Gpr41and Gpr43.Shortage of SCFAs might allow the evolution of systemic inflammatory events.Such mechanisms elegantly combine diet,microbiota,and the epithelial cell as ‘‘nutrient sensor.’’(4)The microbiota decreases epithelial expression of fasting-induced adipo-cyte factor (Fiaf ),which functions as a circulating lipoprotein lipase (LPL)inhibitor and therefore is an important regulator of peripheral fat stor-age.(5)Several TLRs,such as TLR5or TLR9,are not only able t
o affect microbiota but also to regulate metabolism,systemic inflammation,and insulin resistance,thus highlighting the role of the innate immune system in metabolic inflammation as observed in NASH.(6)Various nutrients such as trans fatty acids (TFAs),fructose or aryl hydrocarbon receptor (AhR)ligands such as 2,3,7,8-tetrachlorodibenzodioxin (TCDD)may directly lead to steatosis/liver inflammation.Adipose tissue–derived signals:Signals derived from the adipose tissue beyond toxic lipids might play a central role in NAFLD/NASH.(7)Here,adipocytokines such as adiponectin and leptin,certain proinflammatory cytokines such as TNF a or IL-6,and others (the death receptor Fas,PPAR c )are of key relevance.The cytokine/adipocytokine milieu might be critical because ob/ob -adiponectin tg mice,although becoming severely obese,are not insulin-resistant.This suggests that in the hierarchy of processes soluble mediators play the central role.Adipose-derived mediators might indeed affect target organs such as the liver,because JNK1adipose-deficient mice are protected from diet-induced obesity,and experiments have demonstrated that this effect is mediated mainly by IL-6(a cytokine),which is of key impor-tance in human obesity.
HEPATOLOGY,Vol.52,No.5,2010TILG AND MOSCHEN 1839
diabetic phenotype with normalization of glucose and insulin levels upon transgenic overexpression of the full-length isoform of adiponectin,despite retaining the obese phenotype.59This report convincingly
dem-onstrates that,despite massive expansion of subcutane-ous adipose tissue,high-level expression of adipose tis-sue adiponectin reduces liver fat content and improves insulin resistance.Therefore,also in humans,a suffi-cient production of adiponectin might play a central role in establishing a balance where local and systemic/ liver inflammation is prevented.60In the hierarchy of processes in the adipose tissue,soluble mediators such as adiponectin might be the‘‘big players.’’
Leptin.Because adipocytes expand with triglycer-ides,leptin secretion increases proportionally.61Hyper-leptinemia reduces fat content in peripheral organs. Because leptin stimulates fatty acid oxidation,adipo-cytes would be oxidizing,rather than storing fat if the endogenous leptin they synthesize acts on them.62,63 Such an autocrine/paracrine relationship between lep-tin and its secreting cell,the adipocyte,is prevented by a progressive decline of adipocyte leptin receptor expression.It is assumed that leptin’s capacity to oxi-dize lipids is fully operative in the liver,thereby mini-mizing ectopic lipid accumulation,at least temporarily. Whether such a mechanism is operative in NAFLD is not known.
IL-6and TNF a:Key(Adipo)cytokines.Expression of IL-6and TNF a,two important proinflammatory cytokines,is profoundly increased in human fat cells from obese subjects and patients with insulin resist-ance.64IL-6serum levels are elevated in obese patients and weight loss results in decreased IL-
6serum lev-els.65,66Enhanced TNF a expression in adipose tissue of obese subjects decreases following weight loss.67 Insulin resistance is an important feature of NAFLD and is caused by a variety of factors,including soluble mediators derived from immune cells and/or adipose tissue.68Insulin resistance may augment inflammation in NASH because patients with type2diabetes melli-tus are often worse in terms of histopathological changes such as ballooning,apoptosis,and lobular and/or portal inflammation.1Serine phosphorylation of insulin receptor substrate by inflammatory signal transducers such as c-jun N-terminal protein kinase1 (JNK1)or inhibitor of nuclear factor-j B kinase-b (IKK b)is considered one of the key aspects that dis-rupt insulin signaling.Sabio ported that JNK1 signaling specifically in adipose tissue consequent to a high-fat diet causes hyperinsulinemia,hepatic steatosis, and hepatic insulin resistance.69Importantly,this distal effect of adipose tissue on the liver was mediated via increased JNK1-dependent IL-6secretion from adipo-cytes,proving that adipose tissue–derived IL-6regu-lates distal metabolic effects in the liver.It has to be stated that in this and other models,a high-fat diet is a prerequisite to induce‘‘pathology,’’telling us that indeed‘‘an inflammatory diet’’might exist that drives certain processes including liver inflammation at the end.
We recently demonstrated that such a mechanism as suggested by Sabio et al.might also be operative
in human obesity.70In this study,IL-6expression has been more than100-fold higher in adipose tissue(sub-cutaneous and visceral)compared to its liver expres-sion,suggesting that in severe obesity,the adipose tis-sue is indeed the major source of IL-6.Weight loss resulted in a dramatic decrease,especially of IL-6and TNF a expression with subsequent reduced expression of hepatic suppressor of cytokine signaling3(SOCS3) expression and improved insulin sensitivity,and hence evidence of hepatic consequences of these alterations in adipose tissue.The liver might be a key target organ for adipose tissue–derived IL-6and TNF a,because continuous IL-6/TNF a exposure affects hepatic insulin ,via up-regulation of SOCS3.71Impor-tantly,enhanced expression of proinflammatory cyto-kines in adipose tissue was observed,although liver inflammation was still absent,suggesting that adipose tissue inflammation could precede liver inflamma-tion.70Peroxisome proliferator-activated receptor-gamma(PPAR c),a member of the nuclear receptor family,plays a major role in adipogenesis,atherosclero-sis,inflammation,and glucose metabolism.Adipose tissue–specific deletion of PPAR c results in diminished weight gain despite hyperphagia,diminished serum concentrations of leptin/adiponectin,and insulin resist-ance.72,73Mice with a deficiency of the death receptor Fas specifically in adipocytes are not only protected from adipose tissue inflammation(induced by a high-fat diet)but also from hepatic steatosis and hepatic in-sulin resistance.74
Many human studies suggest that the amount of vis-ceral fat directly correlates with degree of hepatic stea-tosis and inflammation.Hepatic inflammation andfi-brosis correlate with the amount of visceral fat.75 T runk fat has been shown to be a major factor leading to increased serum alanine aminotransferase levels, which might reflect more advanced disease such as NASH.76This large clinical study further supports the important association between adipose tissue and liver disease.Besides certain adipocytokines/immune media-tors,the cellular infiltrate in the adipose tissue is also of major importance because ablation of adipose
1840TILG AND MOSCHEN HEPATOLOGY,November2010
版权声明：本站内容均来自互联网，仅供演示用，请勿用于商业和其他非法用途。如果侵犯了您的权益请与我们联系QQ:729038198，我们将在24小时内删除。
发表评论