Editorial Open Access
Copyright ©2012 Baishideng Publishing Group Co., Limited. All rights reserved.
World J Gastroenterol. Feb 28, 2012; 18(8): 727-735
Published online Feb 28, 2012. doi: 10.3748/wjg.v18.i8.727
Role of cytokines and chemokines in non-alcoholic fatty liver disease
Vincent Braunersreuther, Giorgio Luciano Viviani, François Mach, Fabrizio Montecucco
Vincent Braunersreuther, François Mach, Fabrizio Montecucco, Division of Cardiology, Foundation for Medical Researches, Faculty of Medicine, Geneva University Hospital, 1211 Geneva, Switzerland
Giorgio Luciano Viviani, Department of Internal Medicine, Adult Diabetes Centre, University of Genoa, 16143 Genoa, Italy
Author contributions: Braunersreuther V, Viviani GL, Mach F and Montecucco F designed the research; Braunersreuther V, Viviani GL, Mach F and Montecucco F drafted the article; Braunersreuther V and Montecucco F wrote the paper; Braunersreuther V, Viviani GL, Mach F, Montecucco F revised and accepted this version of the manuscript.
Supported by The Swiss National Science Foundation, No. 32003B-134963/1 and “Sir Jules Thorn Trust Reg” Foundation, Gustave and Simone Prévot Foundation to Montecucco F; EU FP7 AtheroRemo, No. 201668 and Swiss National Science Foundation, No. 310030B-133127 and Novartis Foundation and Swiss Heart Foundation to Mach F
Correspondence to: Dr. Fabrizio Montecucco, MD, Division of Cardiology, Foundation for Medical Researches, Faculty of Medicine, Geneva University Hospital, Foundation for Medical Researches, 64 Avenue Roseraie, 1211 Geneva, Switzerland. fabrizio.montecucco@unige.ch
Telephone: +41-22-3827238 Fax: +41-22-3827245
Received: June 7, 2011
Revised: July 27, 2011
Accepted: August 4, 2011
Published online: February 28, 2012

Abstract

Non-alcoholic fatty liver disease (NAFLD) includes a variety of histological conditions (ranging from liver steatosis and steatohepatitis, to fibrosis and hepatocarcinoma) that are characterized by an increased fat content within the liver. The accumulation/deposition of fat within the liver is essential for diagnosis of NAFLD and might be associated with alterations in the hepatic and systemic inflammatory state. Although it is still unclear if each histological entity represents a different disease or rather steps of the same disease, inflammatory processes in NAFLD might influence its pathophysiology and prognosis. In particular, non-alcoholic steatohepatitis (the most inflamed condition in NAFLDs, which more frequently evolves towards chronic and serious liver diseases) is characterized by a marked activation of inflammatory cells and the upregulation of several soluble inflammatory mediators. Among several mediators, cytokines and chemokines might play a pivotal active role in NAFLD and are considered as potential therapeutic targets. In this review, we will update evidence from both basic research and clinical studies on the potential role of cytokines and chemokines in the pathophysiology of NAFLD.

Key Words: Inflammation, Non-alcoholic fatty liver disease, Cytokine, Chemokine



INTRODUCTION

In the last decade, there has been a remarkable scientific effort to improve our understanding of the pathogenesis, diagnosis, and treatment of non-alcoholic fatty liver disease (NAFLD). Clinical studies revealed dramatically high prevalence of NAFLD worldwide[1,2]. Worrying data on the prevalence of NAFLD in children and adolescents was also revealed[3]. Importantly, in American adolescents followed in the National Health and Nutrition Examination Survey between 1999 and 2004, serum elevation of hepatic enzymes [i.e., alanine aminotransferase (ALT)] was observed in 6% to 11% of subjects (depending of ethnicity)[4]. Furthermore, serum ALT increase was positively associated with waist circumference and insulin resistance, suggesting that NAFLD might be considered as the hepatic manifestation of other epidemic diseases, such as metabolic syndrome and obesity[1,2]. In fact, in obese children and adolescents, NAFLD affects about 20% to 74%, indicating that this disease might start early during life, providing more time for its deleterious evolution[5-7]. However, we believe that NAFLD is limited to patients suffering from obesity, metabolic syndrome, or other fat-related diseases. Although NAFLD has been described as an increased hepatic accumulation of fat (steatosis), a recent scientific consensus defined it as a complex spectrum of diseases, ranging from asymptomatic steatosis with possible aminotransferase alterations to non-alcoholic steato-hepatitis (NASH), cirrhosis, and also hepatocellular carcinoma[8-10]. Whether these conditions are different stages of a common progressive disease or should be considered as different entities, is still an open question. Thus, additional pathophysiological studies on improved animal models are needed to clarify this issue. Indeed, NAFLD is often underestimated, under diagnosed, and not treated in the current medical practice; therefore, its pathophysiological history is at risk of remaining a mystery for several years.

At present, the most suitable area for improving our knowledge of the pathophysiology of NAFLD is represented by the chronic inflammation that underlies all NAFLD entities/stages[11]. Soluble cytokines and chemokines, regulating inflammatory cell function and survival, could be considered as very promising candidates. On the other hand, hormonal axes, adipocytokines, and growth factors have also received attention from NAFLD scientists. In the following paragraphs, we focus on cytokines and chemokines, updating evidence of their role in NAFLD pathophysiology, both in human (Table 1) and animal studies.

Table 1 Summary of human studies concerning the role of cytokines and chemokines in non-alcoholic fatty liver disease.
Cytokine/chemokineReferencesFindingsApproach/sample sizeTreatment
TNF-αHotamisligil et al[22]Increase of TNF-α in fat tissue of obese subjects18 control and 19 obese premenopausal women-
Correlation between TNF-α and IR
Dandona et al[23]Higher TNF-α levels in obese subjects that contribute to IR30 control and 38 obese women-
Decrease of TNF-α levels and IR with weight loss
Lesmana et al[24]Correlation between TNF-α serum levels and liver fibrosis30 patients with NASH-
Crespo et al[25]Overexpression of TNF-α in liver and adipose tissue in patient with NASH52 obese patients-
Increase of p55 TNF-α receptor expression in fibrotic liver
Enhancement of TNF-α expression with advanced liver fibrosis
Hui et al[26]Increase of TNF-α and TNFR2 in patients with NASH109 patients with NAFLD-
Valenti et al[27]Higher prevalence of 238 TNF-α polymorphism in patients with NAFLD99 subjects with NAFLD-
Zhou et al[28]238 TNF-α polymorphism association with NAFLD susceptibility117 subjects with NAFLD-
Lee et al[29]Reduction of aminotransferase in patients treated with Pentoxifylline20 patients with NASHPentoxifylline 400 mg three time per day
Müller et al[30]No significant increase of TNF-α or its receptor levels in patients with IGT80 subjects with IGT, 152 subjects with type II diabetes and 77 control subjects-
Bruun et al[31]No correlation between IR and TNF-α levels19 obese and 10 lean men-
Ofei et al[32]No effect on insulin sensitivity21 obese NIDDM patientsSingle injection of CDP571 (anti-TNF-α antibody)
Bernstein et al[33]No effect on insulin sensitivity56 subjects with metabolic syndromeEtanercept (TNF-α antagonist) 50 mg 1 time per week, for 4 wk
TGF-β1Annoni et al[40]Enhancement of TGF-β1 expression in man with active liver disease16 patients with active liver disease-
Castilla et al[44]Association of TGF-β1 levels and fibrosis in chronic liver disease46 patients with elevated serum ALT-
Milani et al[45]High TGF-β1 mRNA expression in fibrotic liver2 subjects control, 1 subject with cirrhosis, and 9 subjects with hepatitis B viral liver disease-
Hasegawa et al[47]TGF-β1 levels are useful to differentiate between NAFLD and NASH Benefits of α-tocopherol to treat NASH12 patients with non-alcoholic steatohepatitis and 10 patients with non-alcoholic fatty liverα-tocopherol 300 mg/d during 1 yr
Dixon et al[48]Association of polymorphism inducing angiotensinogen and TGF-β1 and advanced hepatic fibrosis105 obese patients-
IL-6Kopp et al[58]Correlation between IL-6 and IR37 obese patients-
Kugelmas et al[60]Elevated IL-6 concentration in serum of patients with NASH Decrease of IL-6 with the treatment16 patients with NASHVitamin E 800 IU/d
Haukeland et al[61]Higher levels of IL-6 in patients with NAFLD47 patients (22 simple steatosis, 25 NASH) and 30 controls-
Wieckowska et al[63]Higher hepatic IL-6 expression in patients with NASH Association with IL-6 levels and the disease severity Correlation between hepatic IL-6 expression and IR50 patients with suspected NALFD
IL-10Esposito et al[69]Association between low levels of IL-10 and metabolic syndrome50 obese and 50 normal-weight women-
Calcaterra et al[70]No association between metabolic syndrome and low levels of IL-1070 severely obese and 30 non-obese children and adolescents-
CCL2/MCP-1Haukeland et al[61]Elevated levels of CCL2 in patients with NAFLD and NASH47 patients (22 simple steatosis, 25 NASH) and 30 controls-
Westerbacka et al[92]Increase of CCL2 in steatotic liver of patients with NAFLD24 subjects (8 controls, 16 with NAFLD)-
Greco et al[93]Correlation between CCL2 gene expression and liver fat content in patients with NAFLD10 subjects with low and high extremes of fat liver-
CCL5/RANTESWu et al[99]Higher CCL5 expression in adipose tissue of obese patients than in lean controls21 morbidly obese patients, 10 obese patients with metabolic syndrome, and 3 lean controls-
Kirovski et al[100]Upregulation of hepatic and circulating CCL5 levels in patients with NAFLD45 patients with NAFLD and 61 controls with normal liver-
CXCL8/IL-8Bahcecioglu et al[103]Higher serum levels of IL-8 in patients with NASH and cirrhosis compared to control group28 patients with NASH, 14 patients with cirrhosis, and 15 controls-
Torer et al[104]Higher IL-8 serum levels in the patients with NASH than in patients with hepatosteatosis57 patients with NASH and 35 patients with NALFD-
Jarrar et al[105]Higher levels of IL-8 in NAFLD patients compared to obese and non-obese subjects26 patients with NASH, 19 patients with simple steatosis,-
Independent association of IL-8 levels with NASH38 obese controls, and 12 non-obese controls
Abiru et al[106]No differences of IL-8 between NASH, simple steatosis, and control groups23 patients with NASH, 21 patients with simple steatosis, and 18 healthy controls-
CXCL9/MigWasmuth et al[109]Association CXCL9 serum levels and CXCL9 liver expression with liver fibrosis due to NASH441 individuals with HCV
CYTOKINES

Cytokines are soluble molecules that are involved in intercellular communication and are produced by a wide variety of cells in the body, including most types of liver cells[12]. They comprise several subfamilies, including interferons, interleukins, tumor necrosis factors (TNF), transforming growth factors (TGF), colony-stimulating factors, and chemokines. Cytokines mediate several fundamental biological processes, including body growth, adiposity, lactation, hematopoiesis, as well as inflammation and immunity. However, they are also implicated in various pathologies, such as atherosclerosis, rheumatoid arthritis, systemic lupus erythematosus, psoriasis, as well as NAFLD[13-16].

Under physiological conditions, constitutive cytokine generation is absent or minimal in the liver. Nevertheless, pathological stimuli like lipid accumulation induces hepatic cells to produce these inflammatory molecules. Cytokines might play an active role in the development and the potential progression of NAFLD through stimulation of hepatic inflammation, cell necrosis and apoptosis, and induction of fibrosis. Nevertheless, they are also essential for liver regeneration following injury[16]. Evidence on the pathophysiological role of cytokine in NAFLD is reported and discussed below.

TNF-α

TNF-α is an inflammatory mediator secreted by several inflammatory cell types, including monocyte/macrophages, neutrophils, and T-cells, but also by many other tissues, such as the endothelium, adipose tissue, or neuronal tissue. In the liver, TNF-α is secreted directly by hepatocytes and Kupffer cells or indirectly by abdominal fat[17]. Several studies have shown that TNF-α is a key factor in the development of NAFLD and NASH in both humans and animals. Hotamisligil et al[18] showed for the first time a relationship between TNF-α expression and insulin resistance in NASH. The authors stated that adipose tissue represents an important source of obesity-induced inflammation, notably by the expression of TNF-α, which can induce inflammation and insulin resistance. Indeed, in several rodent models of obesity, TNF-α expression in adipose tissue was upregulated as compared to controls[18]. Accordingly with these study, obese mice lacking TNF-α showed an improved insulin sensibility[19]. Recently, mice treated with the anti-TNF-α drug thalidomide showed some improvements in the hepatic alterations mediated by a high-fat diet[20]. Moreover, the use of anti-TNF-α antibodies in an experimental model of NASH decreased inflammation, necrosis, and fibrosis in rats[21].

Although TNF-α inhibition in animal models of NAFLD presents encouraging therapeutic perspectives, in humans, the role of this cytokine remains controversial. In patients, TNF-α levels were shown to be higher in obese than in lean individuals, and were correlated with insulin resistance[22,23]. Moreover, a positive correlation was observed between the degree of liver fibrosis and circulating TNF-α levels in patients with NASH[24]. Another study showed increased TNF-α expression in the liver and adipose tissue in NASH patients with significant fibrosis in comparison with those with a slight or nonexistent fibrosis[25]. More recently, Hui et al[26] strengthened these results, showing increased TNF-α levels in subjects with steatohepatitis as compared to controls. The potential involvement of TNF-α in NAFLD pathophysiology was recently suggested by genetic studies on its polymorphisms[27,28]. Moreover, treatment with pentoxifylline (a molecule inhibiting TNF-α) decreased the serum levels of aminotransferases and displayed hepatic beneficial effects in patients with NASH[29].

Nevertheless, the involvement of TNF-α in insulin resistance and NAFLD is questionable. Some studies did not show any correlation between insulin resistance and TNF-α levels[30,31], whereas two clinical studies, using an antagonist and an anti-TNF-α antibody, did not show any improvements in insulin sensitivity[32,33]. Moreover, in a recent study, Lucero et al[34] did not observe any difference in circulating levels of TNF-α between patients with NAFLD as compared to controls without NAFLD.

TGF-β

TGF-β is a cytokine/growth factor with immunosuppressive, anti-inflammatory, and pro-fibrotic properties[35]. In the liver, TGF-β1 is the most abundant isoform, and it is secreted by immune cells, stellate cells, and epithelial cells[36]. TGF-β1 plays a pivotal role in hepatic fibrosis by mediating the activation of stellate cells and their production of extracellular matrix proteins[37-39]. Indeed, Kupffer and stellate cells produce TGF-β1, which induces the transformation of resting stellate cells to myofibroblasts[40]. In experimental models of hepatic fibrosis induced by CCl4 or schistosomiasis, expression of TGF-β1 is upregulated[41-43]. Moreover, in patients with liver fibrosis, the expression of TGF-β1 mRNA is increased[40,44,45]. Stärkel et al[46] showed that the upregulation of TGF-β1 is an early molecular step in the progressive fibrotic steatohepatitis. A study by Hasegawa and co-workers showed that TGF-β1 levels were increased in patients with NASH as compared to hepatic steatosis. Thus, the measurement of serum levels of TGF-β1 might be useful to distinguish NASH patients in the spectrum of NAFLD[47]. Moreover, polymorphisms that induce high angiotensinogen and TGF-β1 are associated with advanced hepatic fibrosis in obese patients with NAFLD[48].

Interleukin-6

The role of interleukin-6 (IL-6) in liver pathology is very complex, and its participation in the development of NAFLD remains unclear. IL-6 activates several cells, such as immune cells, hepatocytes, hematopoietic stem cells, and osteoclasts[49]. Furthermore, IL-6 has a wide range of biological functions, including induction of inflammation and oncogenesis, regulation of immune response, and support of hematopoiesis[49]. IL-6 was initially considered as a hepatoprotector in liver steatosis, capable of reducing oxidative stress and preventing mitochondrial dysfunction[50,51]. Furthermore, this potential hepatoprotective effect of IL-6 was confirmed in other models of liver disease, such as ischemic preconditioning models and in liver regeneration after partial hepatectomy in mice[52-55].

Nevertheless, IL-6 is a key element in the acute phase response, mediating the synthesis of several acute phase proteins (such as C-reactive protein and serum amyloid A)[56]. Thus, we cannot exclude the possibility that IL-6 might also play an indirect deleterious role in NAFLD pathogenesis. In diet-induced obese mice, treatment with IL-6 antibodies improved sensitivity to insulin[57]. Furthermore, IL-6 is considered as a predictor marker of insulin resistance and cardiovascular diseases. In patients undergoing bariatric surgery, decreased IL-6 concentrations were associated with weight loss and insulin resistance improvement[58]. Serum IL-6 levels are higher in animal models and patients with NAFLD[59-61]. Recently, Mas and co-workers showed that diet-induced NASH was reduced in IL-6 knockout mice as compared to controls[62]. In humans with NASH, a positive correlation between IL-6 expression in hepatocytes and the severity of NAFLD was observed [63].

Thus, although IL-6 could improve hepatic regeneration and repair, it could also sensitize the liver to injury, stimulate hepatocyte apoptosis, induce insulin resistance, and participate in NASH development. Recent studies from Yamaguchi illustrated this paradoxical role of IL-6 in NAFLD. Indeed, IL-6 pathway neutralization with tocilizumab, a specific antibody against the IL-6 receptor, enhanced hepatic steatosis, but improved liver damage in mice with methionine choline deficient (MCD) diet-induced NASH[64]. Furthermore, Yamaguchi et al[65], in a second study, showed that not only upregulation of IL-6, but also severe suppression of hepatic IL-6/signal transducer and activator of transcription 3 signaling may lead to the progression of NASH.

IL-10

IL-10 is considered as an anti-inflammatory cytokine that regulates the inflammation in several organs and tissues in physiological or pathological situations[66]. It inhibits T cell-, monocyte-, and macrophage-mediated functions. In the liver, IL-10 has been detected in several cells, including hepatocytes, stellate cells, and Knuppfer cells, but only few studies have been performed to investigate the role of endogenous IL-10 in the progression of NAFLD. A study using IL-10-deficient mice fed on high fat diet, suggested that endogenous IL-10 was protective for hepatic steatosis, but not for concomitant insulin resistance[67]. In another study, Cintra and co-workers observed that the inhibition of IL-10 (either using an anti-IL-10 antibody or an IL-10 antisense oligonucleotide) led to increased expression of pro-inflammatory markers (TNF-α, IL-6, IL-1β, and F4/80) and impaired insulin signal transduction and steatosis[68]. In humans, Esposito and co-workers showed an inverse correlation between IL-10 levels and metabolic syndrome in obese woman, suggesting a potential IL-10-mediated benefit in metabolic syndrome patients also affected by NAFLD[69]. However, Calcaterra et al[70] did not confirm this association in obese children and adolescents.

CHEMOKINES

Chemokines (chemotactic cytokines) are small heparin-binding proteins known to induce mainly leukocyte trafficking, growth, and activation in inflammatory sites[71,72]. Many cell types, including endothelial cells, smooth muscle cells, leukocytes, hepatocytes, and stellate cells, can secrete them. Approximately 50 currently identified chemokines are classified in four subfamilies (C, CC, CXC, CX3C) according to their structural arrangement of N-terminal conserved cysteine residues. Chemokines need to bind to their coupled seven transmembrane protein G coupled receptors on target cells to induce cellular changes. Chemokines and their receptors have been implicated in multiple inflammatory diseases, such as atherosclerosis, multiple sclerosis, psoriasis, and insulin resistance[73]. Expression of several chemokines and chemokine receptors has been shown to be upregulated in the livers of obese patients with severe steatosis and NASH[74]. Inflammatory processes are crucial in the potential progression of NAFLD; therefore, chemokines might also play a pivotal role in NAFLD pathophysiology[75].

CCL2/monocyte chemotactic protein-1

CCL2 is a potent chemoattractant that is principally secreted by macrophages and, to a lesser extent, by activated endothelial cells, smooth muscle cells, and hepatic stellate cells[76-79]. It activates target cells (mainly macrophages) through binding with its receptor, CCR2[76-79]. It is widely secreted in adipose tissue and plasma of obese mice[80]. Monocyte/macrophage infiltration in adipose tissue has been observed in animal models and humans[81]. Monocyte/macrophage accumulation in the steatotic liver was reduced in mice fed on high-fat diet and who were deficient for CCL2 or CCR2 genes[82]. However, the role of CCL2 is actually more complex, extending far beyond the monocyte/macrophage chemoattractant effect. For instance, low density lipoprotein receptor and CCL2 double knockout mice showed alterations in glucose and lipid metabolism induced by high-fat diet[83]. When fed with a normal chow diet, these mice are characterized by lower alterations in the lipid and glucose profile. However, obesity is also reduced under normal diet, suggesting that different food intake might regulate CCL2-mediated inflammation in mice prone to develop obesity and atherosclerosis. Importantly, CCL2 deficiency in mice fed on a high-fat diet decreases insulin resistance and hepatic steatosis[84]. On the other hand, mice overexpressing CCL2 in adipose tissue presented increased insulin resistance and hepatic triglyceride levels[84].

Interestingly, CCL2 was also upregulated in the livers of animals with high-fat diet-induced NASH[85]. This pathophysiological aspect of CCL2 directly contributed to the lipid accumulation in hepatocytes via the activation of peroxisome proliferator-activated receptor α gene expression[86]. More recently, Obstfeld and co-workers showed that hepatic myeloid cells might play a crucial role in the promotion of obesity-induced hepatic steatosis. Indeed, they observed that obesity upregulates CCL2 expression in hepatocytes, leading to the recruitment of CCR2-positive myeloid cells and thus, promoting hepatosteatosis[87]. Pharmacological treatments inhibiting the CCL2/CCR2 pathway in several mouse models of metabolic diseases significantly improved obesity, insulin resistance, hepatic steatosis, and inflammation in the adipose tissue[88-90]. These benefits were not confirmed by other studies. For example, CCL2 deletion in an experimental model of MCD diet-induced steatosis did not improve liver fat accumulation and associated inflammation[91].

In humans, only few studies have been performed to investigate the role of CCL2 in NAFLD pathology. Haukeland et al[61] showed that patients with NAFLD had low-grade systemic inflammation and presented with higher serum levels of CCL2 compared to controls. Moreover, CCL2 has been confirmed to also be elevated in the steatotic livers of NAFLD patients[92]. More recently, another study showed that CCL2 expression was positively correlated with liver fat content in patients with NAFLD[93]. These studies suggest an important participation of CCL2 in the potential progression of simple steatosis to NASH. Therefore, although the role of CCL2 in metabolic diseases requires further investigation, these studies suggest a potential direct role of CCL2 in NAFLD and, in particular, in NASH.

CCL5/regulated on activation normal T-cell expressed and secreted

CCL5 is involved in several chronic immune-inflammatory diseases, such as atherosclerosis, acute myocardial infarction, myocarditis, rheumatoid arthritis, and multiple sclerosis[94,95]. It is secreted by various cells, such as endothelial cells, smooth muscle cells, macrophages, or hepatic stellate cells. This chemokine is mainly involved in migration of T cells, monocytes, neutrophils, and dendritic cells through binding to its cognate transmembrane receptors, CCR1, 3 and 5. The receptor CCR5 has been identified on isolated hepatic stellate cells, suggesting that these hepatic cells are both the target and source of CCL5[96,97]. The association of CCL5 with NAFLD was shown recently in humans and mice. Indeed, two studies showed that obesity increased hepatic expression of CCL5 in a murine model of NASH and in obese patients[98,99]. Hepatocytes are the major source of serum and hepatic CCL5 in NAFLD[100]. CCL5 release in the liver is mediated by hepatocellular lipid accumulation, suggesting that hepatic steatosis per se has pathophysiological relevance[100]. CCL5 is also involved in the progression of hepatic fibrosis in mice via CCR1 and CCR5 triggering[97]. More recently, Berres and co-workers defined CCL5 as a critical mediator of experimental liver fibrosis. Indeed, antagonism of CCL5 on receptor CCR5 improved experimental liver fibrosis in mice, indicating that CCL5 is a promising therapeutic target to reduce NAFLD[101].

CXCL8/IL-8

CXCL8/IL-8 is a CXC chemokine produced by several cell types, including inflammatory and endothelial cells[102]. The major role of this chemokine is to orchestrate neutrophil recruitment within inflamed tissues. There is little data documenting its potential role in NAFLD. Serum levels of CXCL8 were significantly higher in subjects with NASH as compared to hepatosteatosis or healthy control group[103,104]. More recently, Jarrar and co-workers showed that serum levels of CXCL8 were higher in NAFLD patients as compared to obese and non-obese patients[105]. In addition, CXCL8 serum levels were independently associated with NASH[105]. Conversely, the study from Abiru and co-workers did not confirm this association or any significant differences in serum CXCL8[106].

Other CXC chemokines

Chemokines CXCL9/monokine induced by interferon-γ, and CXCL10/interferon inducible protein-10, which bind the common receptor CXCR3, are generally not detectable in most non-lymphoid tissues under physiological conditions. However, in some inflammatory conditions, interferon gamma might increase their release. CXCR3 is found at high levels on activated T cells, memory T cells, and natural killer cells[107]. CXCL9 and CXCL10 mainly induce the migration of these cell types. In the liver, endothelial cells highly express CXCL9 leading to the transmigration of the CXCR3-expressing lymphocytes[108]. Recently, high levels of CXCL9 were found in the livers of patients with NASH[109]. In this study, Wasmuth and co-workers identified the CXCL9-CXCR3 axis as a potential anti-fibrotic pathway in the liver in both humans and animals.

CONCLUSION

The take-home message of the present update on the inflammatory pathophysiology of NAFLD do not recommend any optimistic insights for the near future. Much research remains to be done to clarify the pathophysiology of NAFLD and to identify selective targets for treatment. The involvement of cytokines and chemokines and their receptors in the pathogenesis of NAFLD is only partially understood. Although the first studies attempting therapeutic strategies targeting the chemokine system have been recently published, we believe that scientific interest in NAFLD should be increased. In particular, effort is required to improve the consideration of NAFLD as a dangerous condition that should not be underestimated or by-passed. Another crucial aspect is represented by the identification of common cytokines and chemokines between NAFLD and metabolic or cardiovascular diseases. The most promising mediators (such as TNF-α, CCL2 and CCL5) also require more selective inhibitory drugs to safely improve NAFLD, limiting the potential risk of deleterious immune-suppression.

Footnotes

Peer reviewers: Maria Concepción Gutiérrez-Ruiz, PhD, Department of Health Sciences, Universidad Autónoma Metropolitana-Iztapalapa, DCBS, Av San Rafael Atlixco 186, Colonia Vicentina, México, DF 09340, Mexico; Astrid van der Velde, PhD, AMC Liver Center, Meibergdreef 69-71, 1105 BK Amsterdam, The Netherlands

S- Editor Tian L L- Editor Stewart GJ E- Editor Li JY

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