Interleukin‐1 and inflammasomes in alcoholic liver... : Hepatology (original) (raw)

Potential conflict of interest: Nothing to report.

Interleukin‐1 (IL‐1), an immune mediator with pleiotropic functions, affects virtually all cells and organs in the body1 and may be a major driver in the pathogenesis of many autoinflammatory, autoimmune, and infectious diseases, as well as in sterile inflammation.5 The IL‐1 family (IL‐1F) cytokines and IL‐1 receptor (IL‐1R) members include IL‐1α, IL‐1β, IL‐1R antagonist (IL‐1Ra), IL‐18, IL‐33, IL‐36, IL‐37, and IL‐38.6 These mediators are mainly proinflammatory but can also be anti‐inflammatory and are important in almost all inflammatory conditions, exerting their specific effects through IL‐1Rs 1‐10.6 They are also involved in the differentiation and function of many innate and lymphoid cells, direct many complex inflammatory processes, and are critical regulators of the acute phase response (Table 1).

Table 1 - IL‐1F Cytokine Members

IL‐1α and IL‐1β bind to the same receptor (IL‐1R type I) and have similar biological functions, mainly proinflammatory activities. IL‐1α is biologically active as a precursor molecule; however, IL‐1β is produced following inflammasome activation. Inflammasomes consist of a protein‐nucleotide‐binding domain and leucine‐rich repeat NLR family pyrin domain containing 3 protein (NLRP3) or cryopyrin, an apoptosis associated speck‐like protein containing CARD, and the pro‐caspase protease caspase‐1.7 Inflammasome activation is thought to be a two‐step process. The first step is up‐regulation of pro‐IL‐1β messenger RNA/protein expression levels and inflammasome components. The second step is typically triggered by ligands of the NLR sensor in the inflammasome leading to inflammasome assembly and functional activation, resulting in cleavage of pro‐caspase‐1 into active caspase‐1 that cleaves pro‐IL‐1β into the mature, secreted IL‐1β8 (Fig. 1).

Figure 1:

Biology of IL‐1. IL‐1α is constitutively expressed by numerous cell types and released after cell death. The plasma membrane‐associated, Ca2+‐dependent protease calpain cleaves pro‐IL‐1α to mature IL‐1α (both forms are biologically active). Pro‐IL‐1β is strongly induced in TLR‐activated monocytes and macrophages.1 Distinct intracellular danger signals activate the NLRP3 inflammasome, resulting in caspase‐1 cleavage of inactive pro‐IL‐1β to active IL‐1β.2 Once released, IL‐1α and IL‐1β bind to IL‐1R type I, which associates with IL‐1R accessory protein. IL‐1 signaling involves the recruitment of adaptor proteins (e.g., MyD88, IL‐1R‐associated kinases 1 and 2) and tumor necrosis factor receptor‐associated factor 6 to activate nuclear factor kappa B or activator protein‐1 through different mitogen‐activated protein kinase family members. IL‐1 signaling is counteracted by naturally occurring antagonists such as the decoy receptor IL‐1R2, which exists as a membrane‐bound and soluble form and binds IL‐1β and IL‐1R accessory protein, and as IL‐1Ra, which binds IL‐1R1 and prevents its association with IL‐1R accessory protein. Canakinumab is a neutralizing human monoclonal antibody targeting IL‐1β and approved for cryopyrin‐associated periodic syndromes. Abbreviations: AP‐1, activator protein 1; ASC, ASC, apoptosis‐associated speck‐like protein containing a CARD; IKK, inhibitor of NF‐κB kinase; IL‐1RAcP, IL‐1R accessory protein; IRAK, IL‐1R‐associated kinase; MAPKF, mitogen‐activated protein kinase family; MyD88, myeloid differentiation protein 88; NF‐κB, nuclear factor kappa B; TAK1, transforming growth factor‐beta‐activated kinase 1; TRAF6, TNF receptor‐associated factor 6.

The IL‐1α precursor, a prototypic alarmin, is released upon cell death by necrosis and is constitutively present in many organs including the liver. This precursor induces a cascade of proinflammatory cytokines and chemokines, resulting in sterile inflammation,10 a feature of many chronic liver disorders especially alcoholic liver disease (ALD) and nonalcoholic fatty liver disease (NAFLD).11 Acute and chronic liver diseases are prototypic cytokine‐driven diseases as several proinflammatory cytokines (IL‐1α, IL‐1β, tumor necrosis factor‐alpha [TNFα], and IL‐6) are critically involved in inflammation, steatosis, fibrosis, and cancer development.12 IL‐1F members are involved in disease processes such as insulin resistance, adipose tissue inflammation, and atherosclerosis, all common features of NAFLD.13 Therefore, IL‐1F members potentially direct and regulate many features of both ALD and NAFLD. IL‐1F members have pleiotropic effects on hepatocytes, stellate cells, and gut integrity, contributing to their role in alcoholic steatohepatitis and NAFLD (Table 2).16 Autoinflammatory diseases and diseases characterized by sterile inflammation are dominantly IL‐1‐mediated, and anti‐IL‐1 strategies have a major impact on some of them.25 Here, we summarize the role of IL‐1F members and certain inflammasomes in ALD and NAFLD and discuss the potential of interfering with these cytokine pathways in these diseases.

Table 2 - IL‐1α and IL‐1β Have Pleiotropic Effects

IL‐1F Cytokine Members and ALD/Acute Alcoholic Hepatitis

Clinical Aspects of Human ALD/Acute Alcoholic Hepatitis

Chronic alcohol consumption causes a spectrum of liver abnormalities, including ALD, which results in >2 million deaths annually.26 The most aggressive form of ALD, acute alcoholic hepatitis/steatohepatitis (AAH), is a severe and often rapidly progressive disease with high mortality rates in patients hospitalized with acutely decompensated liver disease due to AAH.26 These patients typically present with fever (usually modest, <38°C), hepatomegaly, jaundice, anorexia,27 and extensive leukocytosis correlating with the severity of hepatic injury. Key clinical symptoms of AAH (e.g., fever, wasting, anorexia, or leukocytosis) might be mediated by IL‐1α/β and other IL‐1F cytokine members.

IL‐1F Members in Healthy Liver

Most tissues do not constitutively produce cytokines. Transcription of IL‐1β is not detectable in the normal liver, despite its considerable exposure to bacterial endotoxins and other bacterial particles. Interestingly, the IL‐1α precursor is constitutively present in hepatocytes, and this may have major implications for various forms of acute liver diseases/injuries. In acute liver failure induced by various forms of injuries, cell death by necrosis results in IL‐1α precursor release,28 activating cytokine cascades. It may be for this reason, and/or to rapidly antagonize IL‐1α functions, that IL‐1Ra is also highly expressed in healthy livers.30 However, IL‐1Ra is up‐regulated massively during an acute phase response.31 Therefore, a delicate balance between certain IL‐1F members may contribute to healthy liver tissue homeostasis.

Clinical Evidence for a Role of IL‐1F Members in AAH

IL‐1 produces a variety of diverse metabolic events including fever, neutrophilia, activation of monocytes and macrophages, anorexia, altered mineral metabolism, muscle catabolism, and fibroblast proliferation.4 Many of these features are clinical characteristics of AAH, the first human disease where increased IL‐1 activity was detected in serum/plasma.32 In this study, patients with severe AAH had almost 10 times higher serum IL‐1 activity (assessed using a thymocyte costimulator assay after removing serum inhibitors) than healthy controls. ALD and AAH patients also have increased serum levels of other proinflammatory cytokines such as TNFα and IL‐8, a chemokine whose expression is tightly controlled by IL‐1 and TNFα.33 In AAH, IL‐8 serum levels and hepatic CXC chemokine expression correlated with prognosis of these patients.35 Expression of IL‐18, another IL‐1F member, was increased in unstimulated peripheral blood mononuclear cells and serum of patients with ALD.36 In AAH, serum levels of IL‐18 and its natural antagonist, IL‐18 binding protein, are increased and correlate with mortality.37 Patients with various forms of acute hepatic failure and hepatitis, including AAH, demonstrate elevated circulating levels of both IL‐1β and IL‐1Ra.38 Therefore, the balance between proinflammatory and anti‐inflammatory IL‐1F members may be impaired as in other acute and chronic liver diseases39 (Table 3).

Table 3 - Key Properties of IL‐1F Cytokines in ALD and NAFLD

IL‐1F Cytokines and Experimental ALD

Ethanol‐induced liver injury, including liver cirrhosis and AAH, involves various liver cell types and mediators released by many different cell types within and outside the liver, mostly under the control of the innate immune system.

In the liver, Kupffer cells are important in ethanol‐induced liver injury and are among the first to be affected by bacterial or sterile insults. Both acute and chronic ethanol exposure sensitize Kupffer cells to activation by lipopolysaccharide (LPS) through toll‐like receptor 4 (TLR4).41 This sensitization results in the production of various proinflammatory mediators, such as IL‐1β and TNFα, leading to hepatocyte dysfunction, necrosis, apoptosis,12 and the generation of extracellular matrix proteins which can lead to fibrosis/cirrhosis. In a mouse model of ethanol‐induced liver injury using three distinct modalities of IL‐1β deficiency (namely, caspase‐1, apoptosis associated speck‐like protein containing CARD, and IL‐1R type I knockout [KO] mice) and treating mice after ethanol‐induced liver injury with IL‐1Ra, we demonstrated the importance of IL‐1β signaling in this disorder.18 The IL‐1β pathway mediated steatosis, inflammation, and fibrosis; and importantly, caspase‐1 KO mice were protected from fibrosis. Moreover, treatment with IL‐1Ra attenuated steatosis and liver injury even when administered 2 weeks after initiating ethanol feeding. Importantly, IL‐1β synthesized by Kupffer cells after ethanol exposure recruits and activates hepatic invariant natural killer T cells in mice, resulting in liver inflammation and injury.42 In this model, NLRP3 deficiency was associated with decreased steatosis, inflammation, IL‐1β expression, and number of activated natural killer T cells. In a recent study, NLRP3‐deficient mice showed protection from alcohol‐induced liver inflammation and liver injury and steatosis were also attenuated.43

Other cell types in the liver, such as neutrophils, hepatocytes, and hepatic stellate cells, may also be involved in activation of IL‐1F proteins. In human AAH, neutrophils are considerably increased in the liver and periphery and exhibit an altered activation status.44 Chronic plus binge ethanol administration synergistically up‐regulates IL‐1β and TNFα, and associated neutrophil infiltration is mediated by cytokine‐induced induction of E‐selectin.45 Chronic ethanol feeding in rats stimulates IL‐1β release from cultured hepatocytes, suggesting that hepatocytes might also be involved.46 IL‐1β induction results in hepatocyte damage, leukocyte infiltration, and secretion of profibrogenic cytokines such as transforming growth factor‐beta and platelet‐derived growth factor, thereby promoting hepatic stellate cell activation. These studies support a major role for IL‐1F cytokines in experimental alcohol‐induced liver injury and suggest that interfering with the IL‐1β pathway could be a highly attractive therapeutic target in this disease.

An important remaining question is, what are the major cytokine activating factors in this disease? In humans, endotoxin may be a major cytokine‐inducing factor and not alcohol in itself as AAH is usually observed in advanced liver disease where endotoxemia is a common feature. Rat studies suggest that ethanol causes gut leakiness/increased permeability, and subsequent endotoxemia may precede liver inflammation, supporting the concept that ethanol‐induced gut damage is an early event in this disorder.47 In animal models of ethanol‐induced liver injury both direct ethanol effects and endotoxemia (indirect effect) may activate inflammatory pathways. Alcohol‐associated liver injury is inhibited when animals are treated with poorly absorbed oral antibiotics or lactobacillus to decrease endotoxemia.48 This is consistent with the observation that chronic ethanol feeding causes more severe liver injury in wild‐type than in cluster of differentiation 14 (CD14) KO mice.41 These results indicate that gut‐derived endotoxin, acting through its cellular receptor, CD14, is important in the development of early alcohol‐induced liver injury.

Chronic ethanol consumption also induces the production of reactive oxygen species (ROS)49 that induce lipid peroxidation, damaging both plasma and intracellular membranes and leading to the production of reactive aldehydes with potent proinflammatory and profibrotic properties. Oxidative stress and ROS are mainly generated through the induction of cytochrome P450‐2E1 (CYP2E1). Whereas CYP2E1 KO mice exhibit ethanol‐induced liver disease,50 transgenic overexpression of human CYP2E1 in a mouse model increases liver disease severity. The transcription factor nuclear factor‐erythroid 2‐related factor 2 protects cells against xenobiotic and oxidative stress. Nuclear factor‐erythroid 2‐related factor 2 KO mice show increased mortality after ethanol feeding, thus underscoring the role of oxidative stress in ethanol‐induced injury.51 Importantly, ROS are potent inducers of IL‐1F cytokines,52 suggesting that, in addition to endotoxin and alcohol itself, ROS are relevant inducers of the IL‐1 pathway. Furthermore, recent studies in mice and humans show that alcohol results in release of sterile danger signals, uric acid, and extracellular adenosine triphosphate, all known activators of the NLRP3 inflammasome.43 Preclinical models convincingly demonstrate that ethanol‐induced liver injury depends on TNFα, another prototypic proinflammatory cytokine49; however, clinical trials failed to demonstrate any benefit for TNFα neutralization in the treatment of AAH.54 Although both TNFα and IL‐1α/β induce each other's expression,55 neutralization strategies targeting either IL‐1 or TNFα differed with respect to clinical outcome in many diseases. While the herein described cytokine dysregulations in ALD/AAH may reflect secondary phenomena, blockade of these highly proinflammatory IL‐1F cytokines remains an attractive treatment concept, although it has not been studied clinically. A detailed description of the role of other cytokines, ROS, endoplasmic reticulum stress, autophagy, and mitochondrial dysfunction in the pathogenesis is beyond the scope of this review, although these pathways are important in this disease and commonly activated in parallel, in both preclinical models and humans49 (Fig. 2).

Figure 2:

IL‐1 in ALD and NAFLD. In the steady state the liver produces negligible amounts of IL‐1β. In contrast, IL‐1α is transcribed constitutively, stored intracellularly, and released as cells perish, acting as a prototypic alarmin. Acute and chronic alcohol consumption increases gut leakiness for TLR ligands and sensitizes Kupffer cells to activation through TLR4 and CD14 by LPS, which enters the liver through the portal circulation. In this context, Kupffer cells produce high amounts of proinflammatory cytokines including IL‐1β and TNF‐α. Ethanol is metabolized to acetaldehyde and acetate by alcohol dehydrogenase and aldehyde dehydrogenase. Activation of the microsomal ethanol‐oxidizing enzyme CYP2E1 increases ROS levels that damage and activate hepatocytes, which then constitute another source of hepatic IL‐1β (and TNF‐α) and IL‐1α. The HFD also increases gut permeability, and the obesity‐related inflamed adipose tissue produces large amounts of proinflammatory cytokines including IL‐1β, which enter the liver through the hepatic artery. High amounts of circulating saturated free fatty acids, like palmitic acid, synergize with TLR ligands to produce cytokines. IL‐1β and IL‐1α promote numerous processes in fatty liver diseases such as liver steatosis, hepatocyte damage, activation of hepatic stellate cells that promote liver fibrosis, and recruitment of inflammatory cells including neutrophils and invariant natural killer T cells. Abbreviations: ADH, alcohol dehydrogenase; ALDH, aldehyde dehydrogenase; HSC, hepatic stellate cell; iNKT, invariant natural killer T cell; LSEC, liver sinusoidal endothelial cell; MEOS, microsomal ethanol oxidizing system; NAD+, oxidized nicotinamide adenine dinucleotide; NADPH, reduced nicotinamide adenine dinucleotide.

Impaired intestinal permeability is important in ALD/AAH56; however, the mechanisms by which gut permeability occurs are not fully understood, and certain cytokines such as IL‐1F members may be involved. IL‐1β increases intestinal epithelial tight junction permeability by up‐regulation of enterocyte myosin light‐chain kinase.23 Intraperitoneal injection of IL‐1β resulted in a concentration‐dependent increase in mouse gut permeability involving nuclear factor kappa B p65‐induced activation of the mouse enterocyte myosin light‐chain kinase gene.57 Activation of nuclear factor kappa B and myosin light‐chain kinase involves mitogen‐activated protein kinase kinase 1 activation as inhibition of mitogen‐activated protein kinase kinase 1 prevented the IL‐1β‐induced increase in tight junction permeability.24 Whether the mechanisms mediating IL‐1β‐induced gut permeability are also effective in alcohol‐mediated toxicity is currently unknown.

IL‐1 and NAFLD/Nonalcoholic Steatohepatitis

IL‐1F Members and Human Obesity/NAFLD

Increasing evidence supports the idea that adipocytokines and classical cytokines released from the adipose tissue of obese subjects are key players in NAFLD.58 In obese patients, IL‐1Ra (which binds to IL‐1Rs and prevents IL‐1 signal transduction) is markedly up‐regulated in serum, correlates with body mass index and insulin resistance, and is overexpressed in the white adipose tissue.63 Gene expression profiles of subcutaneous white adipose tissue in obese subjects during a low‐calorie diet demonstrate increased IL‐10 and IL‐1Ra messenger RNA expression after substantial weight loss.67 We showed that severely obese patients had significantly higher IL‐1β and IL‐37 expression in subcutaneous/visceral adipose tissue compared to liver expression and that after extensive weight loss, IL‐1β messenger RNA expression decreased significantly, whereas expression of the anti‐inflammatory cytokine IL‐37 increased.61 IL‐37 potently suppresses macrophage production of proinflammatory cytokines, and IL‐37 transgenic mice are protected from LPS‐induced septic shock.68 This suggests that the adipose tissue might be a major source of inflammatory mediators in obesity associated systemic diseases including liver disease.69 Further experiments are needed to understand the in vivo relevance of elevated IL‐1β and its inhibitor, IL‐1Ra, in AAH or nonalcoholic steatohepatitis (NASH) to determine if other confounders are involved, such as resistance to native IL‐1Ra.

IL‐1F Members in Animal Models of NAFLD/NASH

Studies in animal NAFLD models highlight an important role for IL‐1F members. The role of IL‐1α/β in steatosis and steatohepatitis was recently addressed using a diet‐induced model of steatosis/steatohepatitis.70 The authors demonstrated a remarkable increase in hepatic IL‐1α/β expression. Despite less inflammation, IL‐1α‐/‐ mice had increased hepatic and plasma cholesterol levels, suggesting that hepatic fat storage and inflammation might develop in a diverse manner. Importantly, mice deficient in IL‐1α and IL‐1β were protected from inflammation after diet‐induced steatosis. Therefore, these IL‐1F members are crucially involved in the development of liver inflammation. Furthermore, hepatic and not bone marrow‐derived IL‐1α/β deficiency protected against diet‐induced inflammation and fibrosis, supporting the importance of liver‐derived IL‐1F cytokines in this model.70 Experiments with hepatocyte‐specific, Kupffer cell/macrophage‐specific, or adipocyte‐specific IL‐1α/β‐/‐ mice are needed to clarify the impact of these particular cell types.

In a diet‐induced model of obesity, TLR9 affected steatohepatitis through IL‐1β, and IL‐1β increased lipid accumulation in hepatocytes and regulated inflammation, hepatic insulin resistance, and fibrosis.71 IL‐1α‐/‐ mice have lower fasting glucose and insulin levels and improved insulin sensitivity compared to wild‐type controls.72 In another study, mice with a selective deficiency of IL‐1α in Kupffer cells demonstrated reduced liver inflammation and expression of IL‐1β, TNFα, and IL‐6 when fed an atherogenic diet.73 IL‐1β KO mice fed a high‐fat diet (HFD) exhibited only minor adipose tissue inflammation compared to wild‐type mice and had larger fat deposits and increased levels of several adipogenesis genes (e.g., peroxisome proliferator‐activated receptor‐gamma [PPARγ] or fatty acid binding protein 4) after HFD, which coincided with smaller livers, less hepatic steatosis, and intact insulin sensitivity.74 IL‐1Ra administration to obese mice significantly reduces hepatic steatosis and hepatic lipogenic gene expression, underscoring a role for IL‐1F cytokines in this process.20 These data collectively suggest that IL‐1β might promote adipose tissue inflammation and favor fat accumulation both in the liver and in adipose tissue macrophages, supporting our clinical data that “adipose tissue‐liver crosstalk” plays a role in these diseases.61

IL‐1R1 KO mice preserve insulin sensitivity after an HFD and exhibit down‐regulation of various antioxidant proteins and endoplasmic reticulum stress markers, suggesting that disruption of IL‐1R1 is metabolically beneficial.76 Caspase‐1 is key for IL‐1β processing, and as expected, caspase‐1 KO mice are also protected from HFD‐induced hepatic steatosis, inflammation, and fibrogenesis.77 Caspase‐1 also regulates fibrosis development in diet‐induced steatohepatitis.78 In this study, selective depletion of Kupffer cells potently suppressed methionine and choline deficiency‐induced caspase‐1 activation and consequent fibrosis development. Wild‐type mice on a choline‐deficient, amino acid‐defined diet showed increased hepatic caspase‐1 activity and elevated serum levels of IL‐1α and IL‐1β,79 which were not observed in TLR2 KO mice. In addition, both TLR2 activation and palmitic acid (a saturated fatty acid) were required for inflammasome activation, IL‐1α/β release, and progression of NASH. Together with our previous findings that palmitic acid activates inflammasomes resulting in sensitization to LPS‐induced IL‐1β production in hepatocytes, it appears that both Kupffer cells and hepatocytes produce IL‐1β and are involved in the pathogenesis of metabolic inflammation in NASH.80 After induction of intestinal inflammation through application of dextran sulfate sodium, hepatic inflammation initiated by an HFD worsened, supporting a role for gut‐derived signals as key players in NASH81 (Fig. 2).

Certain inflammasomes that process IL‐1F cytokines are suggested to have a role in metabolic inflammation. Caspase‐1 and IL‐1β activities increase in adipose tissue of diet‐induced as well as genetically modified obese mice.82 In this study, mice defective in caspase‐1 had increased insulin sensitivity, and treatment of obese mice with a caspase‐1 inhibitor improved insulin sensitivity in vivo. IL‐18, another proinflammatory IL‐1F member,83 exists as an inactive precursor (pro‐IL‐18) and requires caspase‐1 for activation. IL‐18 is also up‐regulated in adipose tissue of obese mice and humans.84 Ablation of NLRP3 in mice prevents obesity‐induced inflammasome activation in adipose and liver tissue, reduces IL‐18 adipose tissue expression, and improves metabolic functions.86 A recent article showed that both bone marrow‐derived and liver parenchymal cells contribute to NLRP3 inflammasome activation in methionine and choline‐deficient diet‐induced steatohepatitis.87 Both NLRP3 inflammasome activation and levels of NLRP3‐stimulating sterile danger signals, uric acid and adenosine triphosphate, were increased together with increased levels of serum LPS in steatohepatitis in mice after a prolonged high‐fat, high‐cholesterol, high‐sugar diet, indicating a role for NLRP3 and its ligands in NASH.83 Whereas loss of NLRP3 function improves diet‐induced steatohepatitis, NLRP3 inflammasome gain of function aggravates liver disease, resulting in severe liver fibrosis and highlighting this pathway in the pathogenesis of NAFLD.88 The role of NLRP3 remains controversial because another study found that NLRP3 ablation worsened fatty liver disease by promoting gut dysbiosis and chronic inflammation.89 In summary, there is increasing evidence that IL‐1F members and certain inflammasomes contribute significantly to NAFLD in various experimental studies. It remains to be defined whether mainly hepatic or extrahepatic IL‐1 sources contribute to NAFLD development as extrahepatic sources could critically contribute to hepatic and overall systemic inflammation.

IL‐1 Suppressing Strategies as a Therapeutic Principle in NAFLD/NASH

A large study (Canakinumab Anti‐Inflammatory Thrombosis Outcome Study [CANTOS]) is under way to examine whether IL‐1 neutralization by a monoclonal antibody against IL‐1β can reduce rates of recurrent myocardial infarction, stroke, cardiovascular death, and type 2 diabetes among stable patients with coronary disease.90 This study will definitively prove whether IL‐1β is of clinical relevance in metabolic inflammation and atherosclerosis. Specific IL‐1 neutralizing studies have not been initiated in NASH, but positive findings in the aforementioned trial could pave the way for such protocols.

Various compounds interfere with the IL‐1 pathway. Resveratrol improves hepatic inflammation including decreased IL‐1β production, serum and liver triglyceride levels, and glucose control in diet‐induced obesity in mice,91 suggesting potential therapeutic activity in NAFLD. However, at least 8 weeks of therapy with resveratrol did not improve any features of NAFLD in a small patient cohort.92 PPARs could be attractive targets for NASH therapy. PPAR‐γ agonists (e.g., pioglitazone) have shown some efficacy in the treatment of NASH, and activation of PPAR‐δ improved fatty acid oxidation in the liver paralleled by suppression of hepatic de novo lipogenesis and glucose production.93 PPAR‐δ agonists also improved hepatic inflammation by suppressing the synthesis of proinflammatory cytokines (e.g., IL‐1β94) and protected mice from liver fibrosis.95 The metabolic and anti‐inflammatory effects of PPAR‐δ agonists make them attractive therapeutic targets in NASH. Overall, these studies suggest that IL‐1F members are important mediators in metabolic inflammation. Therefore, it seems reasonable that neutralization of IL‐1 or interference with its synthesis using various drugs could be a promising strategy in the treatment of human NAFLD.

Conclusions

IL‐1F members and inflammasomes are critically involved in ALD and NAFLD pathogenesis. IL‐1F cytokines are potent proinflammatory mediators of the innate immune system and are responsible for early defense after infectious and noninfectious insults such as those initated by ethanol or obesity. Released anti‐inflammatory IL‐1F cytokines such as IL‐1Ra and IL‐37 might not fully neutralize the potent proinflammatory features of IL‐1α/β and others. IL‐1F cytokines are classical pleiotropic mediators that regulate very diverse features of human ALD and NAFLD such as fever, leukocytosis, acute phase protein synthesis, liver inflammation, fibrosis, and metabolic inflammation. These diverse features and the likelihood that this group of cytokines is crucially involved in these diseases make them attractive treatment candidates. IL‐1Ra and anti‐IL‐1 antibodies are attractive candidates in ALD and NAFLD, and a US clinical trial using IL‐1Ra in the treatment of severe alcoholic steatohepatitis is under way. IL‐1Ra seems especially attractive as it is currently believed that this treatment is safe even in advanced disease and not immuosuppressive.96 Large studies using canakinumab have begun in the field of atherosclerosis and metabolic inflammation, which will provide important information on how targeting IL‐1β affects underlying mechanisms and a foundation for IL‐1 neutralizing clinical trials in both human ALD and NAFLD.

REFERENCES

1. Dinarello CA, Rosenwasser LJ, Wolff SM. Demonstration of a circulating suppressor factor of thymocyte proliferation during endotoxin fever in humans. J Immunol 1981;127:2517‐2519.

2. Auron PE, Webb AC, Rosenwasser LJ, Mucci SF, Rich A, Wolff SM, et al. Nucleotide sequence of human monocyte interleukin 1 precursor cDNA. Proc Natl Acad Sci USA 1984;81:7907‐7911.

3. Dinarello CA. Immunological and inflammatory functions of the interleukin‐1 family. Annu Rev Immunol 2009;27:519‐550.

4. Dinarello CA. Anti‐inflammatory agents: present and future. Cell 2010;140:935‐950.

5. Sims JE, Smith DE. The IL‐1 family: regulators of immunity. Nat Rev Immunol 2010;10:89‐102.

6. Garlanda C, Dinarello CA, Mantovani A. The interleukin‐1 family: back to the future. Immunity 2013;39:1003‐1018.

7. Hoffman HM, Mueller JL, Broide DH, Wanderer AA, Kolodner RD. Mutation of a new gene encoding a putative pyrin‐like protein causes familial cold autoinflammatory syndrome and Muckle‐Wells syndrome. Nat Genet 2001;29:301‐305.

8. Martinon F, Burns K, Tschopp J. The inflammasome: a molecular platform triggering activation of inflammatory caspases and processing of proIL‐beta. Mol Cell 2002;10:417‐426.

9. Szabo G, Csak T. Inflammasomes in liver diseases. J Hepatol 2012;57:642‐654.

10. Rider P, Carmi Y, Guttman O, Braiman A, Cohen I, Voronov E, et al. IL‐1alpha and IL‐1beta recruit different myeloid cells and promote different stages of sterile inflammation. J Immunol 2011;187:4835‐4843.

11. Kubes P, Mehal WZ. Sterile inflammation in the liver. Gastroenterology 2012;143:1158‐1172.

12. Tilg H, Diehl AM. Cytokines in alcoholic and nonalcoholic steatohepatitis. N Engl J Med 2000;343:1467‐1476.

13. Tilg H, Moschen AR. Inflammatory mechanisms in the regulation of insulin resistance. Mol Med 2008;14:222‐231.

14. Tack CJ, Stienstra R, Joosten LA, Netea MG. Inflammation links excess fat to insulin resistance: the role of the interleukin‐1 family. Immunol Rev 2012;249:239‐252.

15. Freigang S, Ampenberger F, Weiss A, Kanneganti TD, Iwakura Y, Hersberger M, et al. Fatty acid‐induced mitochondrial uncoupling elicits inflammasome‐independent IL‐1alpha and sterile vascular inflammation in atherosclerosis. Nat Immunol 2013;14:1045‐1053.

16. Szabo G, Petrasek J. Inflammasome activation and function in liver disease. Nat Rev Gastroenterol Hepatol 2015;12:387‐400.

17. Denes A, Lopez‐Castejon G, Brough D. Caspase‐1: is IL‐1 just the tip of the ICEberg? Cell Death Dis 2012;3:e338.

18. Petrasek J, Bala S, Csak T, Lippai D, Kodys K, Menashy V, et al. IL‐1 receptor antagonist ameliorates inflammasome‐dependent alcoholic steatohepatitis in mice. J Clin Invest 2012;122:3476‐3489.

19. Stienstra R, Saudale F, Duval C, Keshtkar S, Groener JE, van Rooijen N, et al. Kupffer cells promote hepatic steatosis via interleukin‐1beta‐dependent suppression of peroxisome proliferator‐activated receptor alpha activity. Hepatology 2010;51:511‐522.

20. Negrin KA, Roth Flach RJ, DiStefano MT, Matevossian A, Friedline RH, Jung D, et al. IL‐1 signaling in obesity‐induced hepatic lipogenesis and steatosis. PLoS One 2014;9:e107265.

21. Gieling RG, Wallace K, Han YP. Interleukin‐1 participates in the progression from liver injury to fibrosis. Am J Physiol Gastrointest Liver Physiol 2009;296:G1324‐G1331.

22. Han YP, Zhou L, Wang J, Xiong S, Garner WL, French SW, et al. Essential role of matrix metalloproteinases in interleukin‐1‐induced myofibroblastic activation of hepatic stellate cell in collagen. J Biol Chem 2004;279:4820‐4828.

23. Al‐Sadi R, Ye D, Dokladny K, Ma TY. Mechanism of IL‐1beta‐induced increase in intestinal epithelial tight junction permeability. J Immunol 2008;180:5653‐5661.

24. Al‐Sadi R, Ye D, Said HM, Ma TY. IL‐1beta‐induced increase in intestinal epithelial tight junction permeability is mediated by MEKK‐1 activation of canonical NF‐kappaB pathway. Am J Pathol 2010;177:2310‐2322.

25. Dinarello CA, Simon A, van der Meer JW. Treating inflammation by blocking interleukin‐1 in a broad spectrum of diseases. Nat Rev Drug Discov 2012;11:633‐652.

26. Lucey MR, Mathurin P, Morgan TR. Alcoholic hepatitis. N Engl J Med 2009;360:2758‐2769.

27. Mathurin P, Lucey MR. Management of alcoholic hepatitis. J Hepatol 2012;56(Suppl. 1):S39‐S45.

28. Chen CJ, Kono H, Golenbock D, Reed G, Akira S, Rock KL. Identification of a key pathway required for the sterile inflammatory response triggered by dying cells. Nat Med 2007;13:851‐856.

29. Sakurai T, He G, Matsuzawa A, Yu GY, Maeda S, Hardiman G, et al. Hepatocyte necrosis induced by oxidative stress and IL‐1 alpha release mediate carcinogen‐induced compensatory proliferation and liver tumorigenesis. Cancer Cell 2008;14:156‐165.

30. Matsukawa A, Fukumoto T, Maeda T, Ohkawara S, Yoshinaga M. Detection and characterization of IL‐1 receptor antagonist in tissues from healthy rabbits: IL‐1 receptor antagonist is probably involved in health. Cytokine 1997;9:307‐315.

31. Gabay C, Smith MF, Eidlen D, Arend WP. Interleukin 1 receptor antagonist (IL‐1Ra) is an acute‐phase protein. J Clin Invest 1997;99:2930‐2940.

32. McClain CJ, Cohen DA, Dinarello CA, Cannon JG, Shedlofsky SI, Kaplan AM. Serum interleukin‐1 (IL‐1) activity in alcoholic hepatitis. Life Sci 1986;39:1479‐1485.

33. Tilg H, Wilmer A, Vogel W, Herold M, Nolchen B, Judmaier G, et al. Serum levels of cytokines in chronic liver diseases. Gastroenterology 1992;103:264‐274.

34. Bird GL, Sheron N, Goka AK, Alexander GJ, Williams RS. Increased plasma tumor necrosis factor in severe alcoholic hepatitis. Ann Intern Med 1990;112:917‐920.

35. Dominguez M, Miquel R, Colmenero J, Moreno M, Garcia‐Pagan JC, Bosch J, et al. Hepatic expression of CXC chemokines predicts portal hypertension and survival in patients with alcoholic hepatitis. Gastroenterology 2009;136:1639‐1650.

36. Hanck C, Manigold T, Bocker U, Kurimoto M, Kolbel CB, Singer MV, et al. Gene expression of interleukin 18 in unstimulated peripheral blood mononuclear cells of patients with alcoholic cirrhosis. Gut 2001;49:106‐111.

37. Spahr L, Garcia I, Bresson‐Hadni S, Rubbia‐Brandt L, Guler R, Olleros M, et al. Circulating concentrations of interleukin‐18, interleukin‐18 binding protein, and gamma interferon in patients with alcoholic hepatitis. Liver Int 2004;24:582‐587.

38. Sekiyama KD, Yoshiba M, Thomson AW. Circulating proinflammatory cytokines (IL‐1 beta, TNF‐alpha, and IL‐6) and IL‐1 receptor antagonist (IL‐1Ra) in fulminant hepatic failure and acute hepatitis. Clin Exp Immunol 1994;98:71‐77.

39. Gramantieri L, Casali A, Trere D, Gaiani S, Piscaglia F, Chieco P, et al. Imbalance of IL‐1 beta and IL‐1 receptor antagonist mRNA in liver tissue from hepatitis C virus (HCV)‐related chronic hepatitis. Clin Exp Immunol 1999;115:515‐520.

40. Ludwiczek O, Vannier E, Moschen A, Salazar‐Montes A, Borggraefe I, Gabay C, et al. Impaired counter‐regulation of interleukin‐1 by the soluble IL‐1 receptor type II in patients with chronic liver disease. Scand J Gastroenterol 2008;43:1360‐1365.

41. Hritz I, Mandrekar P, Velayudham A, Catalano D, Dolganiuc A, Kodys K, et al. The critical role of toll‐like receptor (TLR) 4 in alcoholic liver disease is independent of the common TLR adapter MyD88. Hepatology 2008;48:1224‐1231.

42. Cui K, Yan G, Xu C, Chen Y, Wang J, Zhou R, et al. Invariant NKT cells promote alcohol‐induced steatohepatitis through interleukin‐1beta in mice. J Hepatol 2015;62:1311‐1318.

43. Petrasek J, Iracheta‐Vellve A, Saha B, Satishchandran A, Kodys K, Fitzgerald KA, et al. Metabolic danger signals, uric acid and ATP, mediate inflammatory cross‐talk between hepatocytes and immune cells in alcoholic liver disease. J Leukoc Biol 2015;98:249‐256.

44. Mookerjee RP, Stadlbauer V, Lidder S, Wright GA, Hodges SJ, Davies NA, et al. Neutrophil dysfunction in alcoholic hepatitis superimposed on cirrhosis is reversible and predicts the outcome. Hepatology 2007;46:831‐840.

45. Bertola A, Park O, Gao B. Chronic plus binge ethanol feeding synergistically induces neutrophil infiltration and liver injury in mice: a critical role for E‐selectin. Hepatology 2013;58:1814‐1823.

46. Valles SL, Blanco AM, Azorin I, Guasch R, Pascual M, Gomez‐Lechon MJ, et al. Chronic ethanol consumption enhances interleukin‐1‐mediated signal transduction in rat liver and in cultured hepatocytes. Alcohol Clin Exp Res 2003;27:1979‐1986.

47. Keshavarzian A, Farhadi A, Forsyth CB, Rangan J, Jakate S, Shaikh M, et al. Evidence that chronic alcohol exposure promotes intestinal oxidative stress, intestinal hyperpermeability and endotoxemia prior to development of alcoholic steatohepatitis in rats. J Hepatol 2009;50:538‐547.

48. Adachi Y, Moore LE, Bradford BU, Gao W, Thurman RG. Antibiotics prevent liver injury in rats following long‐term exposure to ethanol. Gastroenterology 1995;108:218‐224.

49. Tilg H, Moschen AR. IL‐1 cytokine family members and NAFLD: neglected in metabolic liver inflammation. J Hepatol 2011;55:960‐962.

50. Lu Y, Zhuge J, Wang X, Bai J, Cederbaum AI. Cytochrome P450 2E1 contributes to ethanol‐induced fatty liver in mice. Hepatology 2008;47:1483‐1494.

51. Lamle J, Marhenke S, Borlak J, von Wasielewski R, Eriksson CJ, Geffers R, et al. Nuclear factor‐eythroid 2‐related factor 2 prevents alcohol‐induced fulminant liver injury. Gastroenterology 2008;134:1159‐1168.

52. Lo YY, Wong JM, Cruz TF. Reactive oxygen species mediate cytokine activation of c‐Jun NH2‐terminal kinases. J Biol Chem 1996;271:15703‐15707.

53. Iracheta‐Vellve A, Petrasek J, Satishchandran A, Gyongyosi B, Saha B, Kodys K, et al. Inhibition of sterile danger signals, uric acid and ATP, prevents inflammasome activation and protects from alcoholic steatohepatitis in mice. J Hepatol 2015;63:1147‐1155.

54. Boetticher NC, Peine CJ, Kwo P, Abrams GA, Patel T, Aqel B, et al. A randomized, double‐blinded, placebo‐controlled multicenter trial of etanercept in the treatment of alcoholic hepatitis. Gastroenterology 2008;135:1953‐1960.

55. Dinarello CA, Cannon JG, Wolff SM, Bernheim HA, Beutler B, Cerami A, et al. Tumor necrosis factor (cachectin) is an endogenous pyrogen and induces production of interleukin 1. J Exp Med 1986;163:1433‐1450.

56. Sanyal AJ, Gao B, Szabo G. Gaps in knowledge and research priorities for alcoholic hepatitis. Gastroenterology 2015;149:4‐9.

57. Al‐Sadi R, Guo S, Dokladny K, Smith MA, Ye D, Kaza A, et al. Mechanism of interleukin‐1beta induced‐increase in mouse intestinal permeability in vivo. J Interferon Cytokine Res 2012;32:474‐484.

58. Anstee QM, Targher G, Day CP. Progression of NAFLD to diabetes mellitus, cardiovascular disease or cirrhosis. Nat Rev Gastroenterol Hepatol 2013;10:330‐344.

59. Yki‐Jarvinen H. Non‐alcoholic fatty liver disease as a cause and a consequence of metabolic syndrome. Lancet Diabetes Endocrinol 2014;2:901‐910.

60. Moschen AR, Molnar C, Geiger S, Graziadei I, Ebenbichler CF, Weiss H, et al. Anti‐inflammatory effects of excessive weight loss: potent suppression of adipose interleukin 6 and tumour necrosis factor alpha expression. Gut 2010;59:1259‐1264.

61. Moschen AR, Molnar C, Enrich B, Geiger S, Ebenbichler CF, Tilg H. Adipose and liver expression of interleukin (IL)‐1 family members in morbid obesity and effects of weight loss. Mol Med 2011;17:840‐845.

62. Rosen ED, Spiegelman BM. What we talk about when we talk about fat. Cell 2014;156:20‐44.

63. Meier CA, Bobbioni E, Gabay C, Assimacopoulos‐Jeannet F, Golay A, Dayer JM. IL‐1 receptor antagonist serum levels are increased in human obesity: a possible link to the resistance to leptin? J Clin Endocrinol Metab 2002;87:1184‐1188.

64. Juge‐Aubry CE, Somm E, Giusti V, Pernin A, Chicheportiche R, Verdumo C, et al. Adipose tissue is a major source of interleukin‐1 receptor antagonist: upregulation in obesity and inflammation. Diabetes 2003;52:1104‐1110.

65. Somm E, Cettour‐Rose P, Asensio C, Charollais A, Klein M, Theander‐Carrillo C, et al. Interleukin‐1 receptor antagonist is upregulated during diet‐induced obesity and regulates insulin sensitivity in rodents. Diabetologia 2006;49:387‐393.

66. Jung C, Gerdes N, Fritzenwanger M, Figulla HR. Circulating levels of interleukin‐1 family cytokines in overweight adolescents. Mediators Inflamm 2010;2010:958403.

67. Clement K, Viguerie N, Poitou C, Carette C, Pelloux V, Curat CA, et al. Weight loss regulates inflammation‐related genes in white adipose tissue of obese subjects. FASEB J 2004;18:1657‐1669.

68. Nold MF, Nold‐Petry CA, Zepp JA, Palmer BE, Bufler P, Dinarello CA. IL‐37 is a fundamental inhibitor of innate immunity. Nat Immunol 2010;11:1014‐1022.

69. Tilg H, Moschen AR. Evolution of inflammation in nonalcoholic fatty liver disease: the multiple parallel hits hypothesis. Hepatology 2010;52:1836‐1846.

70. Kamari Y, Shaish A, Vax E, Shemesh S, Kandel‐Kfir M, Arbel Y, et al. Lack of interleukin‐1alpha or interleukin‐1beta inhibits transformation of steatosis to steatohepatitis and liver fibrosis in hypercholesterolemic mice. J Hepatol 2011;55:1086‐1094.

71. Miura K, Kodama Y, Inokuchi S, Schnabl B, Aoyama T, Ohnishi H, et al. Toll‐like receptor 9 promotes steatohepatitis by induction of interleukin‐1beta in mice. Gastroenterology 2010;139:323‐334.

72. Lagathu C, Yvan‐Charvet L, Bastard JP, Maachi M, Quignard‐Boulange A, Capeau J, et al. Long‐term treatment with interleukin‐1beta induces insulin resistance in murine and human adipocytes. Diabetologia 2006;49:2162‐2173.

73. Olteanu S, Kandel‐Kfir M, Shaish A, Almog T, Shemesh S, Barshack I, et al. Lack of interleukin‐1alpha in Kupffer cells attenuates liver inflammation and expression of inflammatory cytokines in hypercholesterolaemic mice. Dig Liver Dis 2014;46:433‐439.

74. Nov O, Shapiro H, Ovadia H, Tarnovscki T, Dvir I, Shemesh E, et al. Interleukin‐1beta regulates fat‐liver crosstalk in obesity by auto‐paracrine modulation of adipose tissue inflammation and expandability. PLoS One 2013;8:e53626.

75. Ress C, Moschen AR, Sausgruber N, Tschoner A, Graziadei I, Weiss H, et al. The role of apolipoprotein A5 in non‐alcoholic fatty liver disease. Gut 2011;60:985‐991.

76. de Roos B, Rungapamestry V, Ross K, Rucklidge G, Reid M, Duncan G, et al. Attenuation of inflammation and cellular stress‐related pathways maintains insulin sensitivity in obese type I interleukin‐1 receptor knockout mice on a high‐fat diet. Proteomics 2009;9:3244‐3256.

77. Dixon LJ, Flask CA, Papouchado BG, Feldstein AE, Nagy LE. Caspase‐1 as a central regulator of high fat diet‐induced non‐alcoholic steatohepatitis. PLoS One 2013;8:e56100.

78. Dixon LJ, Berk M, Thapaliya S, Papouchado BG, Feldstein AE. Caspase‐1‐mediated regulation of fibrogenesis in diet‐induced steatohepatitis. Lab Invest 2012;92:713‐723.

79. Miura K, Yang L, van Rooijen N, Brenner DA, Ohnishi H, Seki E. Toll‐like receptor 2 and palmitic acid cooperatively contribute to the development of nonalcoholic steatohepatitis through inflammasome activation in mice. Hepatology 2013;57:577‐589.

80. Csak T, Ganz M, Pespisa J, Kodys K, Dolganiuc A, Szabo G. Fatty acid and endotoxin activate inflammasomes in mouse hepatocytes that release danger signals to stimulate immune cells. Hepatology 2011;54:133‐144.

81. Gabele E, Dostert K, Hofmann C, Wiest R, Scholmerich J, Hellerbrand C, et al. DSS induced colitis increases portal LPS levels and enhances hepatic inflammation and fibrogenesis in experimental NASH. J Hepatol 2011;55:1391‐1399.

82. Stienstra R, Joosten LA, Koenen T, van Tits B, van Diepen JA, van den Berg SA, et al. The inflammasome‐mediated caspase‐1 activation controls adipocyte differentiation and insulin sensitivity. Cell Metab 2010;12:593‐605.

83. Ganz M, Bukong TN, Csak T, Saha B, Park JK, Ambade A, et al. Progression of non‐alcoholic steatosis to steatohepatitis and fibrosis parallels cumulative accumulation of danger signals that promote inflammation and liver tumors in a high fat‐cholesterol‐sugar diet model in mice. J Transl Med 2015;13:193.

84. Membrez M, Ammon‐Zufferey C, Philippe D, Aprikian O, Monnard I, Mace K, et al. Interleukin‐18 protein level is upregulated in adipose tissue of obese mice. Obesity (Silver Spring) 2009;17:393‐395.

85. Madsen EL, Bruun JM, Skogstrand K, Hougaard DM, Christiansen T, Richelsen B. Long‐term weight loss decreases the nontraditional cardiovascular risk factors interleukin‐18 and matrix metalloproteinase‐9 in obese subjects. Metabolism 2009;58:946‐953.

86. Vandanmagsar B, Youm YH, Ravussin A, Galgani JE, Stadler K, Mynatt RL, et al. The NLRP3 inflammasome instigates obesity‐induced inflammation and insulin resistance. Nat Med 2011;17:179‐188.

87. Csak T, Pillai A, Ganz M, Lippai D, Petrasek J, Park JK, et al. Both bone marrow‐derived and non‐bone marrow‐derived cells contribute to AIM2 and NLRP3 inflammasome activation in a MyD88‐dependent manner in dietary steatohepatitis. Liver Int 2014;34:1402‐1413.

88. Wree A, McGeough MD, Pena CA, Schlattjan M, Li H, Inzaugarat ME, et al. NLRP3 inflammasome activation is required for fibrosis development in NAFLD. J Mol Med (Berl) 2014;92:1069‐1082.

89. Henao‐Mejia J, Elinav E, Jin C, Hao L, Mehal WZ, Strowig T, et al. Inflammasome‐mediated dysbiosis regulates progression of NAFLD and obesity. Nature 2012;482:179‐185.

90. Ridker PM, Thuren T, Zalewski A, Libby P. Interleukin‐1beta inhibition and the prevention of recurrent cardiovascular events: rationale and design of the Canakinumab Anti‐inflammatory Thrombosis Outcomes Study (CANTOS). Am Heart J 2011;162:597‐605.

91. Yang SJ, Lim Y. Resveratrol ameliorates hepatic metaflammation and inhibits NLRP3 inflammasome activation. Metabolism 2014;63:693‐701.

92. Chachay VS, Macdonald GA, Martin JH, Whitehead JP, O'Moore‐Sullivan TM, Lee P, et al. Resveratrol does not benefit patients with nonalcoholic fatty liver disease. Clin Gastroenterol Hepatol 2014;12:2092‐2103.

93. Qin X, Xie X, Fan Y, Tian J, Guan Y, Wang X, et al. Peroxisome proliferator‐activated receptor‐delta induces insulin‐induced gene‐1 and suppresses hepatic lipogenesis in obese diabetic mice. Hepatology 2008;48:432‐441.

94. Liu S, Hatano B, Zhao M, Yen CC, Kang K, Reilly SM, et al. Role of peroxisome proliferator‐activated receptor δ/β in hepatic metabolic regulation. J Biol Chem 2011;286:1237‐1247.

95. Iwaisako K, Haimerl M, Paik YH, Taura K, Kodama Y, Sirlin C, et al. Protection from liver fibrosis by a peroxisome proliferator‐activated receptor delta agonist. Proc Natl Acad Sci USA 2012;109:E1369‐E1376.

96. Mertens M, Singh JA. Anakinra for rheumatoid arthritis: a systematic review. J Rheumatol 2009;36:1118‐1125.

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