Adipokines and proinflammatory cytokines, the key mediators in the pathogenesis of nonalcoholic fatty liver disease

Table 1 Studies and their findings on adiponectin

Study	Finding	Ref.
Human	Adiponectin levels reduced in obese individuals	[25,34,35]
	Adiponectin levels higher in women	[36]
	Adiponectin levels reduced in T2DMHypoadiponectinemia associated with visceral fat accumulation	[37]
	High concentrations of adiponectin correlated with a decreased risk of developing T2DM	[41-43]
	Adiponectin mRNA decreased in obese T2DM	[40]
	SNP +45T>G genotypes and lower adiponectin level associated with higher FBG, insulin levels and HOMA-IR in obese women	[48]
	SNPs: 3971 A/G (rs822396), +276 G/T (rs1501299), -4522 C/T (rs822393) and Y111H T/C (rs17366743) significantly associated with hypoadiponectinemia	[49]
	High adiponectin/leptin ratio associated with lower plasma triglyceride, HOMA-IR and higher HDL	[44]
	Lower adiponectin levels an independent risk factor for NAFLD	[51]
	In human liver biopsies, hepatic adiponectin receptor mRNAs increased in biopsy-proven NASH	[52]
	Similar levels of adiponectin receptor mRNA in normal, steatotic liver and NASH	[53,54]
	Reduced AdipoR2 protein in NASH compared to steatotic liver	[55]
	Adiponectin levels lower in NASH and correlated with the progression of the disease	[56,70-72]
	HMW adiponectin isoforms increased after biliopancreatic diversion in obese subjects	[77]
Animal	Adiponectin lowers gluconeogenesis in the liver, increases fatty acid oxidation in muscle and reduces IR	[45]
	Disruption of both adiponectin receptors (adipo R1 and R2) increased tissue triglyceride content, inflammation, oxidative stress and IR	[46]
	Adiponectin enhanced the progression of hepatic steatosis, fibrosis, and hepatic tumor formation in NASH	[57]
	Adiponectin prevents lipid accumulation by increasing β-oxydation and by decreasing synthesis of FFA in hepatocytes in NASH	[47,58-60]
	Association of NAFLD and reduced expression of hepatic adiponectin receptors not consistently reported	[61,62]
	Peripheral injection of adiponectin resulted in reduction in body weight and improvement of peripheral IR	[47]
	Adiponectin reduced TNF-α and induced IL-10 release from Kupffer cells	[63]
	Pretreatment with adiponectin ameliorated D-galactosamine/LPS induced elevation of serum AST and ALT levels, and the apoptotic and necrotic changes in hepatocytes	[64]
In vitro	Adiponectin inhibited TNF-α induced expression of endothelial adhesion molecules and decreased LPS induced TNF-α production	[66]
	Adiponectin mediated anti-inflammatory activity by lowering NFκB action	[67]
	Adiponectin increased IL-8 and monocyte chemotactic protein-1 production, and activated the proinflammatory transcription factor NFκB	[68]
	Adiponectin acted antifibrotic through antagonizing leptin-induced STAT3 phosphorylation in activated hepatic stellate cell who promote fibrosis	[73]
	Lower HMW adiponectin closely associated with obesity-related metabolic complications and T2DM	[75]

T2DM: Type 2 diabetes mellitus; HOMA-IR: Homeostasis model assessment-estimated insulin resistance; FBG: Fasting blood glucose; HDL: High-density lipoprotein; NAFLD: Non-alcoholic fatty liver disease; NASH: Nonalcoholic steatohepatitis; HMW: High-molecular weight; IR: Insulin resistance; FFA: Free fatty acids; TNF-α: Tumor necrosis factor alpha; IL-10: Interleukin-10; LPS: Lipopolysaccharide; NF-κB: Nuclear factor kappa-light-chain-enhancer of activated B cells; IL-8: Interleukin-8; STAT-3: Signal transducer and activator of transcription 3.

Table 2 Studies and their findings on leptin

Study	Finding	Ref.
Human	Leptin levels higher in obese individuals and increased with overfeeding	[82,83]
	Higher leptin levels in women independent of fat mass	[104-107]
	Body mass index and IR strongly correlated with leptin levels	[84]
	Central obesity correlated with higher leptin levels in comparison with non-central obesity	[86]
	Administration of leptin to individuals with lipoatrophic diabetes resulted in reduction of triacylglycerol concentrations, liver volume, glycated hemoglobin and discontinuation, or a large reduction in antidiabetes therapy	[89]
	Leptin inhibited insulin secretion and transcription of the preproinsulin gene	[87]
	IR associated with elevated plasma leptin levels independently of body fat	[110]
	Leptin/adiponectin ratio predicted T2DM in both sex	[111]
	Leptin C2549A AA genotype found at a higher rate in T2DM	[114]
	Leptin levels significantly higher in NASH, and correlated with the severity of hepatic steatosis, but not with the grade of necroinflammation or fibrosis	[116-118]
	Leptin not found as a predictor of histological severity of NASH	[119]
	No significant difference in leptin concentrations between NASH patients and controls, or in connection to the severity of liver fibrosis	[120,121]
	IR and low leptin levels predictors of steatosis in the liver	[122]
Animal	Mice lacking the ob gene became severely obese	[91]
	Leptin infusion attenuated hepatic steatosis and hyperinsulinemia	[92]
	Mice without leptin signaling had an increased lipid accumulation in liver	[93]
	Leptin prevented lipid accumulation in nonadipose tissue through SREBP-1 modulation	[94]
	After long-term exposure to high-fat diet (> 20 wk), mice resistant to leptin even when directly infused into the brain	[95-98]
	Hyperleptinemia itself contributed to leptin resistance by down regulating cellular response to leptin	[99]
	Mice with poly (ADP-ribose) polymerase-1 deficiency susceptible to diet-induced obesity, hyperleptinemia, and IR	[115]
	Leptin-deficient, insulin-resistant mice developed leptin resistance on a high fat diet independently of hyperleptinemia, c-Jun N-terminal kinase inflammatory pathway relevant in the induction of diet-induced glucose intolerance	[100]
	Leptin increased expression of procollagen-I, transforming growth factor beta1, smooth muscle actin and TNF-α and thus increased liver fibrosis and inflammation	[101]
	Leptin-resistant mice exhibited significantly reduced fibrogenic response	[102,103]
In vitro	Fibrogenic effect of leptin accomplished through hepatic stellate cells, leptin a potent mitogen and apoptosis inhibitor	[23]

IR: Insulin resistance; T2DM: Type 2 diabetes mellitus; NASH: Nonalcoholic steatohepatitis; SREBP-1: Sterol regulatory element-binding protein 1; ADP-ribose: Adenosine diphosphate ribose.

Table 3 Studies and their findings on ghrelin

Study	Finding	Ref.
Human	AG and the AG/DAG ratios positively associated with HOMA-IR in obese children	[128]
	IR obese subjects had elevated AG/DAG ratio compared with non IR obese subjects because of decreased DAG and total ghrelin levels	[129]
	Obese patients with MS had lower total ghrelin and DAG, comparable AG and higher AG/DAG, AG/DAG ratio correlated with IR	[130]
	Ghrelin significantly correlated with HOMA-IR, but was reduced in NAFLD	[131]
	Ghrelin levels were higher in higher stages of fibrosis in morbidly obese patients with NAFLD	[132]
	Higher total ghrelin concentrations in patients with NASH in comparison with steatosis and normal liver	[54]
In vitro	Adipocytes after incubation with AG and DAG significantly increased PPARγ and SREBP-1 mRNA levels and accumulated lipids	[133]
	Ghrelin inhibited AMP-activated protein kinase activity, through which also influenced PPAR-γ in liver and in adipose tissue	[134]
	Administration of ghrelin attenuated NAFLD-induced liver injury, oxidative stress, inflammation, and apoptosis partly through the action of serine/threonine kinase/AMPK and phosphoinositide 3-kinase/protein kinase B pathways in rats	[135]

AG: Acyl-ghrelin; DAG: Des-acyl ghrelin; IR: Insulin resistance; MS: Metabolic syndrome; HOMA-IR: Homeostasis model assessment-estimated insulin resistance; NAFLD: Non-alcoholic fatty liver disease; PPAR-γ: Peroxisome proliferator-activated receptor gamma; SREBP-1: Sterol regulatory element-binding protein 1; AMP: Adenosine monophosphate.

Table 4 Studies and their findings on resistin, visfatin and retinol binding protein 4

Adipocytokines	Finding	Ref.
Resistin	Resistin levels increased in morbidly obese humans	[142]
	Resistin levels in T2DM patients 20% higher when compared to non-diabetic patients	[144]
	No correlation between resistin and components of MS on T2DM patients	[145]
	Resistin did not correlate with BMI but significantly correlated with IR	[146]
	G/G -180C>G homozygotes for resistin had significantly higher resistin mRNA levels in abdominal subcutaneous fat	[148]
	Serum resistin levels not associated with the presence of NASH	[149]
	Serum resistin levels higher in NAFLD that in controls and positively correlated with liver inflammation and fibrosis severity	[118,150]
	Resistin serum levels in NAFLD patients were associated with histological severity of the disease but not with IR	[151]
	Expression of resistin in human peripheral-blood mononuclear cells upregulated by TNF-α and IL-6	[152]
Visfatin	Secretion of visfatin enhanced by glucose administration	[153]
	Plasma visfatin elevated in patients with T2DM	[154]
	Visfatin plasma concentrations markedly elevated in obese subjectsBariatric surgery reduced body mass index, visfatin, leptin and increased adiponectin after 6 mo	[155]
	Plasma visfatin levels elevated in subjects with MS	[156]
	Significantly higher visfatin mRNA in visceral fat of obese subjects compared with lean controls, and positively correlated with body mass index	[158]
	Visfatin level lower in NASH compared to NAFLD patients and healthy controls	[159]
	Visfatin level positively correlated with portal inflammation	[160]
Retinol binding protein 4	Serum RBP4 concentration elevated in IR, obese humans, T2DM and in subjects with a strong family history of T2DM	[161,162]
	Strong association of increased circulating RBP4 levels with IR and MS	[163-166]
	No connection of RBP4 with obesity, IR, or components of the MS	[167-171]
	RBP4 levels associated with inflammatory response in obese individuals	[168,172]
	Circulating RBP4 levels higher in subjects with NAFLDRBP4 liver expression higher in moderate/severe NASH compared to mild forms	[173]
	RBP4 level a risk factors for fibrosis ≥ 2 in NASHRBP4 and HOMA-IR independently associated with steatosis in patients with chronic hepatitis C	[174]
	In NAFLD patients, serum RBP4 significantly lower compared with controls, did not correlate with IRRBP4 liver tissue expression enhanced in NAFLD patients and correlated with NAFLD histology	[175]
	Serum RBP4 levels did not correlate with BMI, HOMA-IR, fasting blood glucose, or insulin levels in patients with simple steatosis and NASHPatients with cirrhosis and fibrosis had higher RBP4 compared to controls	[176,177]

T2DM: Type 2 diabetes mellitus; MS: Metabolic syndrome; NASH: Nonalcoholic steatohepatitis; NAFLD: Non-alcoholic fatty liver disease; IR: Insulin resistance; TNF-α: Tumor necrosis factor alpha; IL-6: Interleukin 6; RBP4: Retinol binding protein 4; HOMA-IR: Homeostasis model assessment-estimated insulin resistance.

Table 5 Studies and their findings on tumor necrosis factor alpha

Study	Finding	Ref.
Human	Healthy subjects with highest serum TNF-α levels had significantly greater risk of developing NAFLD	[180]
	TNF-α infusion in healthy humans impaired insulin signaling via increased phosphorylation of p70 S6 kinase, extracellular signal-regulated kinase-1/2, c-Jun NH(2)-terminal kinase, and serine phosphorylation of IRS-1 as well as impaired phosphorylation of Akt substrate 160 thereby GLUT4 translocation and glucose uptake in skeletal muscle	[181]
	TNF-α gene polymorphism in the -238 A allele associated with susceptibility to NAFLD, correlated with IR and increased BMI in Chinese population	[202,203]
	TNF-α polymorphism at position 1031C and 863A in a Japanese population associated with NASH without significant difference between NAFLD patients and controls	[188]
	TNF-α and soluble TNFR2 plasma levels increased in NASH patients, independently of IR, compared to controls, but not among different stages of NAFLD	[187],[192]
	Serum TNF-α/TNFR1 increased in NASH patients as compared with other stages	[189]
	In obese NASH patients expression of liver and adipose TNF-α mRNA and its p55 receptor increased and correlated with advanced fibrosis	[190]
	In children serum TNF-α and leptin associated with a NAFLD activity score of 5 or more	[191]
	TNF-α mRNA cut-off of 100 ng/mL predicted NASH	[192]
	In morbidly obese NASH patients high TNF-α mRNA expression in liver correlated with plasma levels of LPS-binding protein	[200]
	Treatment with TNF-α inhibitor (pentoxifylline) for 6 mo reduced liver enzymes, serum TNF-α level and improved IR	[204]
	In NAFLD/NASH patients probiotic therapy decreased TNF-α levels	[208]
	Patients with MS with or without NAFLD treated with fish oil for 6 mo resulted in the reduction of oxidative stress and production of proinflammatory cytokines (TNF-α and IL-6)	[214]
Animal	Prolonged infusion of TNF-α in rats decreased ability of insulin to suppress hepatic gluconeogenesis and stimulate peripheral glucose utilization	[178]
	Obese mice with impaired TNF-α signaling protected from obesity-derived IR in peripheral tissues and had lower levels of circulating free fatty acids	[179]
	Mice deficient in both TNF-α receptors fed with MCD diet had attenuated liver steatosis, fibrosis and number of recruited Kupffer cellsTNF-α administration induced tissue inhibitors of metalloproteinases 1 mRNA expression in activated HSC and suppressed their apoptosis	[193]
	On MCD-diet induced NASH mice model NASH developed independently of TNF-α synthesis	[186]
	Fructose overfeeding in mice led to endotoxemia, increased TNF-α and liver steatosis that was reduced after treatment with antibiotics	[197]
	Mice lacking TNFR1 were resistant to fructose-induced steatosis (increased phospho AMPK and AKT levels, decreased SREBP-1 and FAS expression in the liver as well as RBP4 plasma levels)	[198]
	Dietary oleate reduced hepatic steatosis, inflammation, fibrosis and mRNA expression of TNF-α in MCD diet-induced NASH animal model	[216]
	TNF-α levels in liver were lower in dietary induced NASH animal model treated with glutamine	[217]
	α- and γ-tocopherol protected against LPS-triggered NASH in an obese mouse model, by decreasing liver necroinflammatory activity, levels of TNF-α, without affecting body mass or hepatic steatosis	[219]
	Obese mice on a HFD treated with thalidomide (100 mg/kg per day for 10 d) showed improvements in insulin sensitivity, through restoration of the hepatic insulin IRS-1 and AKT phosphorylation, an improvement in hepatic steatosis was also noticed, which correlated with reduced TNF-a levels	[218]
	Statins (rosuvastatin and pioglitazon) in diet-induced NASH rat models decreased serum TNF-α level	[212,213]
	Treatment with anti-TNF antibodies in ob/ob mice fed with HFD improved liver steatosis, insulin sensitivity, and serum ALT levels	[209]
	Treatment of HFD-rat with monoclonal TNF-α antibody, infliximab, reduced proinflammatory markers (TNF-α, IL-6, IL-1β), activity of JNK and IKK-B, SOCS-3 expression, and improved insulin signaling through JAK2/STAT-3 and IRS/AKT/FOXO1 pathway in the liverThis all led to reduced IR, fat liver accumulation and inflammation	[210]
	LPS derived TNF-α production enhanced expression of SREBP-1 mRNA leading to hepatic steatosis	[201]
In vitro	JNK2-/- hepatocytes resistant to TNF-α induced apoptosis	[183]
	Tiazolidinediones reversed TNF-α induced IR	[211]
	Quercetin decreased TNF-α expression in oleic acid induced steatotic HepG2 cells	[215]

TNF-α: Tumor necrosis factor alpha; NAFLD: Non-alcoholic fatty liver disease; BMI: Body mass index; IRS-1: Insulin receptor substrate 1; GLUT4: Glucose transporter type 4; IR: Insulin resistance; NASH: Nonalcoholic steatohepatitis; TNFR1: Tumor necrosis factor receptor 1; IL-6: Interleukin 6; LPS: Lipopolysaccharide; HFD: High-fat diet; JNK: c-Jun N-terminal kinase; MS: Metabolic syndrome; MCD: Methionine and choline deficient diet; HSC: Hepatic stellate cells; TNFR2: Tumor-necrosis factor receptor 2; AMPK: AMP-activated protein kinase; SOCS-3: Suppressors of cytokine signaling-3; Akt: Protein kinase B; SREBP-1: Sterol regulatory element-binding protein 1; FAS: Fatty acid synthase; RBP4: Retinol binding protein 4; IKK-B: Inhibitor of nuclear factor kappa-B kinase subunit beta; STAT3: Signal transducer and activator of transcription 3.

Table 6 Studies and their findings on interleukin-6

Study	Finding	Ref.
Human	Increased plasma IL-6 in T2DM	[222]
	Elevated basal IL-6 levels in healthy humans present high relative risk of developing T2DM	[224]
	Obese patients after bariatric surgery who lost weight had decreased IR and IL-6	[225][226]
	IL-6 174C polymorphism associated with NASH and IR	[239]
	IL-6 levels higher in NAFLD patients, especially with advance stages, compared to ones with hepatitis B	[240]
	Increased serum IL-6 levels in biopsy proven NAFLD compared to controls	[241]
	No difference in IL-6 levels among T2DM patients with NASH/advanced fibrosis compared to those without NASH or light fibrosis	[242]
	No difference in serum IL-6 and its intrahepatic mRNA expression between NASH and steatosis	[243,244]
	In morbidly obese patients serum IL-6 levels correlated with progression of steatosis but in NASH declinedIL-6 > 4.81 pg/mL predicted liver steatosis	[245]
	Hepatocyte IL-6 expression positively correlated with degree of inflammation, stage of fibrosis and IR	[246]
	Increased circulating IL-6 and its soluble receptor in NASH patients compared with steatosis and healthy volunteers	[189]
	Normal IL-6 values exclude NASH	[247]
	IL-6, total cytokeratin-18 (M65) and adiponectin - a new panel for predicting NASH	[248]
	Decreased IL-6 levels after lifestyle changes and vitamin E administration	[249]
Animal	Chronic administration of IL-6 suppressed hepatic insulin signaling without effect on skeletal muscle	[231]
	Lep(ob) mice neutralized with IL-6 antibody showed increased insulin receptor signaling in the liver but not in peripheral tissues	[232]
	IL-6 decreases overall IR and hepatic inflammation	[233]
	Hepatoprotective and hepatoproliferative role of short-term exposure to IL-6 in ischaemic preconditioning models	[234]
	Treatment of IL-6-deficient mice acutely with IL-6 restored STAT3 binding and hepatocyte proliferation	[235]
	Chronic liver exposure to IL-6 led to cell death via Bax induction, activation of Fas agonist derived caspase-9 and cytochrome c release	[236]
	IL-6 showed inflammatory and antisteatotic effects in liver on mouse NASH model	[237]
	Hepatoprotective role of IL-6 by STAT3 activation in severe NASH model	[238]
In vitro	LPS through TLR receptors stimulated macrophages to produce TNF-α that up-regulated IL-6 production in adipocytes and macrophages	[220]
	IL-6 inhibited insulin-induced glycogenesis in hepatocytes	[227]
	IL-6 promoted IR in hepatocytes and HepG2 via decreased tyrosine phosphorylation of IRS-1, impaired association of the p85 subunit of phosphatidylinositol 3-kinase with IRS-1, inhibition of Akt and glycogen synthesis	[228]
	IL-6 impaired insulin signaling in 3T3-L1 adipocytes through inhibition of gene transcription of IRS-1, GLUT-4 and PPARγ	[229]
	IL-6-dependent IR mediated by induction of SOCS-3 protein in HepG2 cells	[230]

IL-6: Interleukin 6; T2DM: Type 2 diabetes mellitus; IR: Insulin resistance; NASH: Nonalcoholic steatohepatitis; NAFLD: Non-alcoholic fatty liver disease; STAT3: Signal transducer and activator of transcription 3; LPS: Lipopolysaccharide; TLR: Toll-like receptor; IRS-1: Insulin receptor substrate 1; Akt: Protein kinase B; GLUT4: Glucose transporter type 4; PPAR-γ: Peroxisome proliferator-activated receptor gamma.

Table 7 Studies and their findings on interleukin-1α, interleukin-1β, interleukin-1Ra and interleukin-18

Cytokines	Finding	Ref.
IL-1α	Acute treatment of 3T3-L1 adipocytes with IL-1α led to transient IR at IRS-1 level, mediated by its serine phosphorylation	[254]
IL-1β
Human	Weight loss in severely obese patients led to decreased IL-1β in subcutaneous adipose tissue and in liver without effect on adipose IL-1α IL-1β was significantly higher in subcutaneous/visceral adipose tissue than in liver	[253]
	IL1-β genetic variants in Japanese population associated with NASH	[259]
Animal	Hepatic IL-1α and IL-1β increased in NASH animal models Mice deficient in either cytokine not prone to NASH and fibrosis development	[255]
	In experimental models TLR2 and palmitic acid activated inflammasome in Kupffer cells and produced IL-1α and IL-1β	[257]
	IL-1β/ApoE-deficient mice had less pronounced atherosclerosis	[262]
	Treatment with an IL-1β antibody improved glycemic control and β cell function in diet-induced obese mice	[263]
	Animal NASH model showed increased macrophage infiltration in adipose tissue as well as in liver accompanied with increased expression of IL-1β	[264]
	Hepatic steatosis partially mediated by Kupffer cells that produced IL-1β which suppressed PPAR-α	[266]
	In diet induced NASH mice probiotics decreased hepatic IL-1β mRNA	[268]
In vitro	IL-1β inhibited insulin-induced phosphorylation of the insulin receptor beta subunit, IRS1, protein kinase B and extracellular regulated kinase 1/2 in murine and human adipocytes that lead to IR and inhibition of lipogenesis IL-1β decreased adiponectin	[260]
	IL-1β promoted hepatic fibrosis by upregulating TIMMP-1 in rat HSC mediated by p38 mitogen-activated protein kinases and JNK	[267]
IL-1Ra	IL-1Ra decreased glucose uptake in muscle and was upregulated in WAT of diet-induce obese mice	[270]
	Atherogenic diet in IL-1Ra deficient mice caused severe liver steatosis, inflammation and portal fibrosis	[272]
IL-18	In obese women IL-18 positively correlated with body weight and visceral fat	[275]
	In T2DM patients and non-diabetic controls IL-18 plasma levels positively correlated with HOMA-IR	[276]
	In male patients with NAFLD, IL-18 alone in the absence of metabolic risks cannot contribute to evolution of NAFLD	[278]
	IL-18 enhanced cytokine production by stimulating TNF-α synthesis in immune cells	[279]
	Il-18 administrated with IL-12 induced mouse fatty liver in an IFN-γ dependent manner	[280]
	Rosiglitazone in NAFLD rat model reduced IL-18 and caspase-1 in liver as well as improved histology	[277]

IL-1α: Interleukin 1 alfa; IR: Insulin resistance; IRS-1: Insulin receptor substrate 1; IL-1β: Interleukin 1 beta; IL-18: Interleukin 18; NASH: Nonalcoholic steatohepatitis; TLR2: Toll-like receptor 2; PPAR-α: Peroxisome proliferator-activated receptor alfa; TIMMP-1: Tissue inhibitor of matrix metalloproteinase-1; HSC: Hepatic stellate cells; JNK: c-Jun N-terminal kinases; WAT: White adipose tissue; T2DM: Type 2 diabetes mellitus; HOMA-IR: Homeostasis model assessment-estimated insulin resistance; NAFLD: Non-alcoholic fatty liver disease; TNF-α: Tumor necrosis factor alpha; IL-12: Interleukin 12; IFNγ: Interferon gamma.