Metabolic Inflammation—A Role for Hepatic Inflammatory Pathways as Drivers of Comorbidities in Nonalcoholic Fatty Liver Disease?

Nonalcoholic fatty liver disease (NAFLD) is a global and growing health concern. Emerging evidence points toward metabolic inflammation as a key process in the fatty liver that contributes to multiorgan morbidity. Key extrahepatic comorbidities that are influenced by NAFLD are type 2 diabetes, cardiovascular disease, and impaired neurocognitive function. Importantly, the presence of nonalcoholic steatohepatitis and advanced hepatic fibrosis increase the risk for systemic comorbidity in NAFLD. Although the precise nature of the crosstalk between the liver and other organs has not yet been fully elucidated, there is emerging evidence that metabolic inflammation—in part, emanating from the fatty liver—is the engine that drives cellular dysfunction, cell death, and deleterious remodeling within various body tissues. This review describes several inflammatory pathways and mediators that have been implicated as links between NAFLD and type 2 diabetes, cardiovascular disease, and neurocognitive decline. Nonalcoholic fatty liver disease (NAFLD) is a global and growing health concern. Emerging evidence points toward metabolic inflammation as a key process in the fatty liver that contributes to multiorgan morbidity. Key extrahepatic comorbidities that are influenced by NAFLD are type 2 diabetes, cardiovascular disease, and impaired neurocognitive function. Importantly, the presence of nonalcoholic steatohepatitis and advanced hepatic fibrosis increase the risk for systemic comorbidity in NAFLD. Although the precise nature of the crosstalk between the liver and other organs has not yet been fully elucidated, there is emerging evidence that metabolic inflammation—in part, emanating from the fatty liver—is the engine that drives cellular dysfunction, cell death, and deleterious remodeling within various body tissues. This review describes several inflammatory pathways and mediators that have been implicated as links between NAFLD and type 2 diabetes, cardiovascular disease, and neurocognitive decline. Jörn M. SchattenbergView Large Image Figure ViewerDownload Hi-res image Download (PPT) Nonalcoholic fatty liver disease (NAFLD) and metabolic diseases are closely connected clinical entities. Emerging evidence suggests that these comorbidities do not arise independently of each other but, rather, share pathogenetic features, some of which may emanate primarily from the liver. The closeness of the association between NAFLD and metabolic diseases in clinical cohorts has made it difficult to identify the specific nature and direction of the crosstalk between the hepatic and other compartments. Nevertheless, there are some clues. Importantly, NAFLD is associated with chronic, low-grade inflammation in the liver that causes systemic effects, detectable by systemic alterations in immune cell subsets and humoral factors.1Hotamisligil G.S. Inflammation, metaflammation and immunometabolic disorders.Nature. 2017; 542: 177-185Crossref PubMed Scopus (395) Google Scholar In the liver and extrahepatic organs, these signals can promote cellular dysfunction, cell death, and deleterious tissue remodeling—all in an attempt to maintain structural and functional organ integrity.2Schuppan D. Surabattula R. Wang X.Y. Determinants of fibrosis progression and regression in NASH.J Hepatol. 2018; 68: 238-250Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar The aforementioned mediators are also fundamental regulators of metabolism. As such, they link organ injury directly to metabolic changes, supporting the terms immunometabolic disorder or metabolic inflammation.1Hotamisligil G.S. Inflammation, metaflammation and immunometabolic disorders.Nature. 2017; 542: 177-185Crossref PubMed Scopus (395) Google Scholar The redundancy of many inflammatory and metabolic signaling pathways, as well as the fact that the entire spectrum of NAFLD comorbidities is not easily modeled in animals, makes it difficult to implicate the liver directly in the extrahepatic complications of NAFLD.3Farrell G. Schattenberg J.M. Leclercq I. et al.Mouse models of nonalcoholic steatohepatitis: toward optimization of their relevance to human nonalcoholic steatohepatitis.Hepatology. 2019; 69: 2241-2257Crossref PubMed Scopus (27) Google Scholar However, experimental evidence for this concept of metabolic inflammation does exist. This arises in the context of individual genetic/epigenetic factors, a range of environmental factors—including dietary/nutritional factors, intestinal gut microbiota, and activity/behavior—and especially crosstalk with the adipose tissue, resulting in variable effect strength to NAFLD and metabolic inflammation. The link between NAFLD and extrahepatic disease is summarized in Figure 1 and will be detailed in this review, with the main focus being on insulin resistance, endothelial dysfunction, and declining neurocognitive function. Epidemiologic studies have identified liver fat content as a primary risk factor for the development of insulin resistance (IR), prediabetes, and type 2 diabetes (T2D).4Kotronen A. Juurinen L. Hakkarainen A. et al.Liver fat is increased in type 2 diabetic patients and underestimated by serum alanine aminotransferase compared with equally obese nondiabetic subjects.Diabetes Care. 2008; 31: 165-169Crossref PubMed Scopus (127) Google Scholar, 5Lonardo A. Ballestri S. Marchesini G. et al.Nonalcoholic fatty liver disease: a precursor of the metabolic syndrome.Dig Liver Dis. 2015; 47: 181-190Abstract Full Text Full Text PDF PubMed Scopus (299) Google Scholar, 6Ballestri S. Zona S. Targher G. et al.Nonalcoholic fatty liver disease is associated with an almost twofold increased risk of incident type 2 diabetes and metabolic syndrome. Evidence from a systematic review and meta-analysis.J Gastroenterol Hepatol. 2016; 31: 936-944Crossref PubMed Scopus (203) Google Scholar The presence of hepatic steatosis on ultrasonography or even an elevated liver function test result doubled the risk of developing T2D over a median follow-up of 5 years.6Ballestri S. Zona S. Targher G. et al.Nonalcoholic fatty liver disease is associated with an almost twofold increased risk of incident type 2 diabetes and metabolic syndrome. Evidence from a systematic review and meta-analysis.J Gastroenterol Hepatol. 2016; 31: 936-944Crossref PubMed Scopus (203) Google Scholar In a retrospective cohort study covering 11 years of follow-up, patients with NAFLD were more likely to develop T2D or metabolic syndrome, although not independently from waist circumference, hypertension, and IR.7Adams L.A. Waters O.R. Knuiman M.W. et al.NAFLD as a risk factor for the development of diabetes and the metabolic syndrome: an eleven-year follow-up study.Am J Gastroenterol. 2009; 104: 861-867Crossref PubMed Scopus (264) Google Scholar This was further supported by studies in patients with prediabetes. Here, liver fat, more so than visceral fat, promoted progression to impaired glucose tolerance.8Kantartzis K. Machann J. Schick F. et al.The impact of liver fat vs visceral fat in determining categories of prediabetes.Diabetologia. 2010; 53: 882-889Crossref PubMed Scopus (90) Google Scholar Importantly, these associations are influenced by ethnic differences. In the Dallas Heart Study, T2D and fatty liver were not as closely associated in black compared with white study participants.9Browning J.D. Szczepaniak L.S. Dobbins R. et al.Prevalence of hepatic steatosis in an urban population in the United States: impact of ethnicity.Hepatology. 2004; 40: 1387-1395Crossref PubMed Scopus (2501) Google Scholar Thus, targeting liver fat and its secreted products was suggested to be more promising than visceral fat in the prevention and treatment of impaired glucose metabolism.8Kantartzis K. Machann J. Schick F. et al.The impact of liver fat vs visceral fat in determining categories of prediabetes.Diabetologia. 2010; 53: 882-889Crossref PubMed Scopus (90) Google Scholar Hepatic fat accumulation is associated with lipotoxicity from high levels of free fatty acids (FFAs), cholesterol, and other lipid metabolites, leading to oxidative stress, endoplasmic reticulum (ER) stress, and mitochondrial dysfunction, which suppress hepatic insulin sensitivity and promote de novo lipogenesis. The connection between these pathophysiological processes and IR are supported by mechanistic studies that link hepatic fat and inflammation causally to IR. These data are summarized in the section below and illustrated in Figure 2. The stress-activated protein kinase c-Jun N-terminal kinase (JNK) and its downstream signaling cascade are abnormally activated in patients with NAFLD and, in particular, NASH.10Cazanave S.C. Mott J.L. Elmi N.A. et al.JNK1-dependent PUMA expression contributes to hepatocyte lipoapoptosis.J Biol Chem. 2009; 284: 26591-26602Crossref PubMed Scopus (128) Google Scholar Murine models of NASH have proven the mechanistic involvement of JNK in NASH pathogenesis.11Schattenberg J.M. Singh R. Wang Y. et al.JNK1 but not JNK2 promotes the development of steatohepatitis in mice.Hepatology. 2006; 43: 163-172Crossref PubMed Scopus (284) Google Scholar Sustained JNK activation—with different effects of the JNK1 and JNK2 isoforms—can be induced by saturated FFAs and other lipids such as diacylglycerol (DAG) and ceramide, but also nonlipids including fructose, cytokines, and gut microbial metabolites. These compounds impair insulin signaling and alter phosphoinositide-3-kinase (PI3K) activation.10Cazanave S.C. Mott J.L. Elmi N.A. et al.JNK1-dependent PUMA expression contributes to hepatocyte lipoapoptosis.J Biol Chem. 2009; 284: 26591-26602Crossref PubMed Scopus (128) Google Scholar The hyperglycemia and dyslipidemia that result from JNK-mediated hepatic IR can propagate peripheral IR and the decline of pancreatic β-cell function.12Poitout V. Robertson R.P. Glucolipotoxicity: fuel excess and β-cell dysfunction.Endocr Rev. 2008; 29: 351-366Crossref PubMed Scopus (0) Google Scholar Table 1 summarizes the mechanistic studies that have shown the involvement of hepatic JNK in the regulation of whole-body insulin sensitivity. Aberrant JNK activation, however, is not restricted to the liver in metabolically burdened individuals. In hematopoietic cells, in the adipose tissue and even in the brain, JNK activation has been implicated in the development of IR.13Sabio G. Das M. Mora A. et al.A stress signaling pathway in adipose tissue regulates hepatic insulin resistance.Science. 2008; 322: 1539-1543Crossref PubMed Scopus (0) Google Scholar,14Belgardt B.F. Mauer J. Wunderlich F.T. et al.Hypothalamic and pituitary c-Jun N-terminal kinase 1 signaling coordinately regulates glucose metabolism.Proc Natl Acad Sci U S A. 2010; 107: 6028-6033Crossref PubMed Scopus (112) Google Scholar In these situations, JNK appears to influence IR primarily by promoting inflammation, which can inhibit insulin signaling. This hypothesis is supported by studies of JNK inhibition using small molecules, dominant-negative JNK constructs, and RNA interference-mediated blockade of JNK activity, all of which affected multiple cell types and resulted in increased insulin sensitivity and improved systemic metabolism.15Nakatani Y. Kaneto H. Kawamori D. et al.Modulation of the JNK pathway in liver affects insulin resistance status.J Biol Chem. 2004; 279: 45803-45809Crossref PubMed Scopus (0) Google Scholar,16Kaneto H. Nakatani Y. Miyatsuka T. et al.Possible novel therapy for diabetes with cell-permeable JNK-inhibitory peptide.Nat Med. 2004; 10: 1128-1132Crossref PubMed Scopus (277) Google Scholar Interestingly, targeting ER stress through chaperones normalized hepatic JNK signaling and restored glucose homeostasis in a mouse model of T2D, including resolution of NAFLD.17Ozcan U. Yilmaz E. Ozcan L. et al.Chemical chaperones reduce ER stress and restore glucose homeostasis in a mouse model of type 2 diabetes.Science. 2006; 313: 1137-1140Crossref PubMed Scopus (1697) Google Scholar These mechanistic findings have supported the concept that targeting JNK would be beneficial to limit IR. However, JNK inhibition in patients has not been achieved yet. As a matter of fact, a study that explored apoptosis signal-regulating kinase-1 (Ask-1) inhibition—which is located upstream of JNK—did not show a benefit on hepatic IR or NAFLD. Thus, therapeutic exploitation of JNK inhibition in patients with NAFLD appears to require a detailed understanding of the cell type– and compartment-specific effects of this signaling pathway in patients with metabolic diseases.Table 1Mechanistic Proof Linking Hepatic Inflammation to Insulin ResistanceInflammatory pathway in the liverEvidence and modelReferencesJNKHFD feeding in rats results in hepatic steatosis without significant visceral or skeletal muscle fat and induces hepatic IR from JNK1 and PKC-ε activation.158Samuel V.T. Liu Z.X. Qu X. et al.Mechanism of hepatic insulin resistance in non-alcoholic fatty liver disease.J Biol Chem. 2004; 279: 32345-32353Crossref PubMed Scopus (901) Google ScholarInhibition of JNK1 using antisense oligonucleotides in HFD-fed mice ameliorates steatohepatitis and normalizes insulin sensitivity (glucose, insulin, and homeostasis model assessment). JNK2 antisense oligonucleotide treatment improves whole-body insulin sensitivity, whereas hepatitis is worsened due to increased activation of mitochondrial death signaling in hepatocytes after JNK2 knockdown.159Singh R. Wang Y. Xiang Y. et al.Differential effects of JNK1 and JNK2 inhibition on murine steatohepatitis and insulin resistance.Hepatology. 2009; 49: 87-96Crossref PubMed Scopus (155) Google ScholarLiver-specific knockdown of JNK1 using short hairpin RNA in mice enhances hepatic insulin sensitivity (Akt phosphorylation) and improves hyperinsulinemia and hyperglycemia in diet-induced obese mice. In parallel, circulating triglyceride levels increase, implicating JNK1 in triglyceride metabolism independently of IR.160Yang R. Wilcox D.M. Haasch D.L. et al.Liver-specific knockdown of JNK1 up-regulates proliferator-activated receptor gamma coactivator 1 beta and increases plasma triglyceride despite reduced glucose and insulin levels in diet-induced obese mice.J Biol Chem. 2007; 282: 22765-22774Crossref PubMed Scopus (0) Google ScholarHepatocyte-specific ablation of Jnk1 results in glucose intolerance, IR, and hepatic steatosis under HFD conditions. In this model, JNK1 is implicated in opposing actions in liver and adipose tissue to both promote and prevent hepatic steatosis.161Sabio G. Cavanagh-Kyros J. Ko H.J. et al.Prevention of steatosis by hepatic JNK1.Cell Metab. 2009; 10: 491-498Abstract Full Text Full Text PDF PubMed Scopus (90) Google ScholarCompound ablation of both Jnk1 and Jnk2 in hepatocytes provides protection from HFD-induced IR. Increased insulin sensitivity is apparent in the liver, adipose tissue, and skeletal muscle, as is reduced pancreatic islet hypertrophy. Hepatic JNK1/2 ablation promotes the oxidation of fatty acids through PPAR-α activation and results in increased FGF-21. FGF-21 partially mediates the beneficial metabolic effects of hepatic JNK1/2 deficiency.162Vernia S. Cavanagh-Kyros J. Garcia-Haro L. et al.The PPARα-FGF21 hormone axis contributes to metabolic regulation by the hepatic JNK signaling pathway.Cell Metab. 2014; 20: 512-525Abstract Full Text Full Text PDF PubMed Scopus (68) Google ScholarOverexpression of dominant-negative type JNK in the liver of obese diabetic mice improves IR related to a decrease in the expression of key gluconeogenic enzymes to decrease hepatic glucose production. Conversely, expression of wild-type JNK in the liver of nonobese mice decreases insulin sensitivity.15Nakatani Y. Kaneto H. Kawamori D. et al.Modulation of the JNK pathway in liver affects insulin resistance status.J Biol Chem. 2004; 279: 45803-45809Crossref PubMed Scopus (0) Google ScholarOverexpression of dual-specificity phosphatase 9 (DUSP-9) in the liver of ob/ob mice reduces hepatic steatosis; suppresses the activation of MAPKs, including JNK; and reduces hyperglycemia related to decreased expression of gluconeogenic and lipogenic genes.163Emanuelli B. Eberle D. Suzuki R. et al.Overexpression of the dual-specificity phosphatase MKP-4/DUSP-9 protects against stress-induced insulin resistance.Proc Natl Acad Sci U S A. 2008; 105: 3545-3550Crossref PubMed Scopus (0) Google ScholarNF-κBExpression of constitutively active IKKβ in hepatocytes impairs insulin signaling in hepatocytes and myocytes from HFD feeding. Liver IKKβ activation is associated with increased expression of the NF-κB target gene IL6, which regulates muscle insulin sensitivity.19Cai D. Yuan M. Frantz D.F. et al.Local and systemic insulin resistance resulting from hepatic activation of IKK-beta and NF-kappaB.Nat Med. 2005; 11: 183-190Crossref PubMed Scopus (1546) Google ScholarDepletion of IKKβ in hepatocytes (IkbkbΔhep) or myeloid cells (IkbkbΔmye) protects mice from hepatic IR from HFD and inflammation.18Arkan M.C. Hevener A.L. Greten F.R. et al.IKK-β links inflammation to obesity-induced insulin resistance.Nat Med. 2005; 11: 191-198Crossref PubMed Scopus (1294) Google ScholarHepatocyte-specific deletion of the NF-κB p65 gene improves hepatic insulin sensitivity, decreases hepatic gluconeogenesis, and inhibits the cAMP/PKA pathway.164Ke B. Zhao Z. Ye X. et al.Inactivation of NF-κB p65 (RelA) in liver improves insulin sensitivity and inhibits cAMP/PKA pathway.Diabetes. 2015; 64: 3355-3362Crossref PubMed Google ScholarLiver-specific inactivation of the NF-κB essential modulator (NEMO) in mice prevents IR but synergizes with HFD feeding in the development of hepatic steatosis, inflammation, and hepatocyte apoptosis, favoring liver tumorigenesis.76Usynin I.F. Khar’kovsky A.V. Balitskaya N.I. et al.Gadolinium chloride-induced Kupffer cell blockade increases uptake of oxidized low-density lipoproteins by rat heart and aorta.Biochemistry (Mosc). 1999; 64: 620-624PubMed Google ScholarHepatic blockage of receptor activated of NF-κB ligand (RANKL) in genetic and nutritional mouse models of T2D results in a marked improvement of hepatic insulin sensitivity and hyperglycemia.165Kiechl S. Wittmann J. Giaccari A. et al.Blockade of receptor activator of nuclear factor-kappaB (RANKL) signaling improves hepatic insulin resistance and prevents development of diabetes mellitus.Nat Med. 2013; 19: 358-363Crossref PubMed Scopus (124) Google ScholarHepatic overexpression of B-cell leukemia (Bcl) 3—a coactivator of NF-κB—in mice exacerbates the dysmetabolic and inflammatory phenotype in NAFLD, triggering metabolic inflammation in the liver and adipose tissue and whole-body IR.166Gehrke N. Worns M.A. Huber Y. et al.Hepatic B cell leukemia-3 promotes hepatic steatosis and inflammation through insulin-sensitive metabolic transcription factors.J Hepatol. 2016; 65: 1188-1197Abstract Full Text Full Text PDF PubMed Scopus (3) Google Scholar Open table in a new tab The hepatic nuclear factor κB (NF-κB) pathway is a central mediator of immune and stress responses, and it is critical for liver tissue homeostasis. Many stimuli activate the NF-κB signaling cascade in hepatocytes, including saturated FFAs, toxic lipid metabolites such as DAG and ceramide, advanced glycated end products, modified low-density lipoprotein (LDL) particles, cytokines, lipopolysaccharide, and other pathogen- and damage-associated molecular patterns (DAMPs).18Arkan M.C. Hevener A.L. Greten F.R. et al.IKK-β links inflammation to obesity-induced insulin resistance.Nat Med. 2005; 11: 191-198Crossref PubMed Scopus (1294) Google Scholar The pivotal role of the hepatic NF-κB signaling cascade in determining insulin sensitivity was shown in mice with liver-specific overexpression of the inhibitor of NF-κB kinase subunit β (IKKβ).19Cai D. Yuan M. Frantz D.F. et al.Local and systemic insulin resistance resulting from hepatic activation of IKK-beta and NF-kappaB.Nat Med. 2005; 11: 183-190Crossref PubMed Scopus (1546) Google Scholar IKKβ is a subunit of the inhibitor of κB kinase (IKK) complex, which phosphorylates IκB molecules for degradation, thereby freeing NF-κB to activate transcription of target genes regulating inflammation and insulin signaling. Also, IKK itself is capable of inhibitory phosphorylation of insulin receptor substrate 1.20Gao Z. Hwang D. Bataille F. et al.Serine phosphorylation of insulin receptor substrate 1 by inhibitor κB kinase complex.J Biol Chem. 2002; 277: 48115-48121Crossref PubMed Scopus (0) Google Scholar Inhibition of liver IKKβ, which has been accomplished with the anti-inflammatory drug salicylate, systemic neutralization of interleukin (IL) 6, and hepatic expression of an IκBα superrepressor has been effective in improving both hepatic and peripheral IR.19Cai D. Yuan M. Frantz D.F. et al.Local and systemic insulin resistance resulting from hepatic activation of IKK-beta and NF-kappaB.Nat Med. 2005; 11: 183-190Crossref PubMed Scopus (1546) Google Scholar These data suggest that a strong inflammatory response in hepatocytes contributes to whole-body glucose homeostasis. This hypothesis was validated and extended in mice with depletion of IKKβ in either hepatocytes (IkbkbΔhep) or myeloid cells (IkbkbΔmye).18Arkan M.C. Hevener A.L. Greten F.R. et al.IKK-β links inflammation to obesity-induced insulin resistance.Nat Med. 2005; 11: 191-198Crossref PubMed Scopus (1294) Google Scholar This second study showed that deletion of IKKβ from hepatocytes yielded modest effects on peripheral insulin signaling, whereas stronger effects were seen when IKKβ was deleted in myeloid cells. In fact, liver-resident rather than circulating myeloid cells were believed to be the primary population responsible for modulating peripheral insulin sensitivity in IkbkbΔhep mice. The more robust contribution of liver-resident than circulating myeloid cells to peripheral IR was attributed to intrahepatic activation of myeloid cells by inflammatory cytokines such as IL1 or tumor necrosis factor–α (TNF-α) generated by insulin-resistant hepatocytes. The concept that activation of immune cell subsets within the liver affects insulin responses in peripheral tissues is at the heart of the metabolic inflammation theory. It is supported by studies with tissue-specific IKKβ transgenic mice confirming, that IKKβ contributes to IR when it is activated in liver18Arkan M.C. Hevener A.L. Greten F.R. et al.IKK-β links inflammation to obesity-induced insulin resistance.Nat Med. 2005; 11: 191-198Crossref PubMed Scopus (1294) Google Scholar,19Cai D. Yuan M. Frantz D.F. et al.Local and systemic insulin resistance resulting from hepatic activation of IKK-beta and NF-kappaB.Nat Med. 2005; 11: 183-190Crossref PubMed Scopus (1546) Google Scholar or resident myeloid cells,18Arkan M.C. Hevener A.L. Greten F.R. et al.IKK-β links inflammation to obesity-induced insulin resistance.Nat Med. 2005; 11: 191-198Crossref PubMed Scopus (1294) Google Scholar but not in skeletal muscle21Cai D. Frantz J.D. Tawa Jr., N.E. et al.IKKbeta/NF-kappaB activation causes severe muscle wasting in mice.Cell. 2004; 119: 285-298Abstract Full Text Full Text PDF PubMed Scopus (898) Google Scholar or the adipose tissue22Jiao P. Feng B. Ma J. et al.Constitutive activation of IKKβ in adipose tissue prevents diet-induced obesity in mice.Endocrinology. 2012; 153: 154-165Crossref PubMed Scopus (26) Google Scholar (see Table 1). The concept is further supported by findings in a translational study showing that activation of macrophages in the hepatic compartment correlated with the degree of adipose tissue IR.23Rosso C. Kazankov K. Younes R. et al.Crosstalk between adipose tissue insulin resistance and liver macrophages in non-alcoholic fatty liver disease.J Hepatol. 2019; 71: 1012-1021Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar Overall, these data indicate that targeting hepatic NF-κB for inhibition might be a strategy to restore hepatic and whole-body insulin sensitivity in patients with NAFLD. Although multiple studies have supported the finding that treatment of patients with T2D with compounds that inhibit systemic NF-κB activity—including salicylates, which are nonspecific NF-κB inhibitors—improves glycemic control, lipid levels, and adiponectin,24Hundal R.S. Petersen K.F. Mayerson A.B. et al.Mechanism by which high-dose aspirin improves glucose metabolism in type 2 diabetes.J Clin Invest. 2002; 109: 1321-1326Crossref PubMed Google Scholar no studies have yet validated an effect of NF-κB inhibitors on IR in patients with NAFLD. A prominent role of liver-resident immune cells, particularly Kupffer cells (KCs), in regulating insulin sensitivity and inflammation in NAFLD has been shown. This immune cell subset is very sensitive to activators of the toll-like receptor (TLR) family (including TLR4 and TLR9)25Mridha A.R. Haczeyni F. Yeh M.M. et al.TLR9 is up-regulated in human and murine NASH: pivotal role in inflammatory recruitment and cell survival.Clin Sci (Lond). 2017; 131: 2145-2159Crossref PubMed Scopus (21) Google Scholar and the stimulator of interferon genes (STING) signaling pathway.26Yu Y. Liu Y. An W. et al.STING-mediated inflammation in Kupffer cells contributes to progression of nonalcoholic steatohepatitis.J Clin Invest. 2019; 129: 546-555Crossref PubMed Scopus (24) Google Scholar After activation and polarization to an M1 (inflammatory) phenotype, these M1 KCs produce proinflammatory and profibrotic cytokines, including TNF-α, IL1β, and transforming growth factor–β, which amplify hepatic inflammation and impair insulin sensitivity in hepatocytes.27Jager J. Aparicio-Vergara M. Aouadi M. Liver innate immune cells and insulin resistance: the multiple facets of Kupffer cells.J Intern Med. 2016; 280: 209-220Crossref PubMed Scopus (0) Google Scholar Moreover, KC-derived cyclooxygenase products, such as prostaglandin E2, can directly induce IR in hepatocytes synergistically with IL6.28Henkel J. Neuschafer-Rube F. Pathe-Neuschafer-Rube A. et al.Aggravation by prostaglandin E2 of interleukin-6-dependent insulin resistance in hepatocytes.Hepatology. 2009; 50: 781-790Crossref PubMed Scopus (0) Google Scholar By contrast, selective depletion or functional inhibition of KCs through clodronate liposomes improves fasting hyperglycemia and IR in high-fat diet (HFD)–fed mice and rats.29Huang W. Metlakunta A. Dedousis N. et al.Depletion of liver Kupffer cells prevents the development of diet-induced hepatic steatosis and insulin resistance.Diabetes. 2010; 59: 347-357Crossref PubMed Scopus (272) Google Scholar Activated KCs also promote the expansion of the liver macrophage pool by recruitment of inflammatory C-C chemokine receptor type 2+ (CCR2+) monocytes, which contribute to decreased hepatic insulin sensitivity30Morinaga H. Mayoral R. Heinrichsdorff J. et al.Characterization of distinct subpopulations of hepatic macrophages in HFD/obese mice.Diabetes. 2015; 64: 1120-1130Crossref PubMed Scopus (0) Google Scholar and correlate with systemic IR in patients with NAFLD.31Parker R. Weston C.J. Miao Z. et al.CC chemokine receptor 2 promotes recruitment of myeloid cells associated with insulin resistance in nonalcoholic fatty liver disease.Am J Physiol Gastrointest Liver Physiol. 2018; 314: G483-G493Crossref PubMed Scopus (11) Google Scholar The soluble CD163 (sCD163) has been shown to be a marker of hepatic macrophage activation, and, interestingly, the reduction of its concentration in plasma after gastric bypass surgery in humans has been associated with reductions in intrahepatic fat and IR.32Fjeldborg K. Pedersen S.B. Moller H.J. et al.Intrahepatic fat content correlates with soluble CD163 in relation to weight loss induced by Roux-en-Y gastric bypass.Obesity (Silver Spring). 2015; 23: 154-161Crossref PubMed Scopus (0) Google Scholar A role of the M1 inflammatory hepatic milieu for the development of IR was also supported by data related to the myeloid cell-specific deletion of the zinc finger protein 36 (Zfp36), which resulted in improved glucose tolerance in hyperinsulinemic-euglycemic clamp studies.33Caracciolo V. Young J. Gonzales D. et al.Myeloid-specific deletion of Zfp36 protects against insulin resistance and fatty liver in diet-induced obese mice.Am J Physiol Endocrinol Metab. 2018; 315: E676-E693Crossref PubMed Scopus (1) Google Scholar An attempt to affect M1-polarization in the hepatic compartment using the antifibrotic drug pirfenidone in a high-cholesterol and high-fat diet model showed some rationale for insulin sensitivity.34Chen G. Ni Y. Nagata N. et al.Pirfenidone prevents and reverses hepatic insulin resistance and steatohepatitis by polarizing M2 macrophages.Lab Invest. 2019; 99: 1335-1348Crossref PubMed Scopus (0) Google Scholar Likewise, in animal models, dietary antioxidants affected the balance between M1 and M2 polarization of intrahepatic macrophages, which, in turn, affected insulin sensitivity.35Ni Y. Nagashimada M. Zhan L. et al.Prevention and reversal of lipotoxicity-induced hepatic insulin resistance and steatohepatitis in mice by an antioxidant carotenoid, beta-cryptoxanthin.Endocrinology. 2015; 156: 987-999Crossref PubMed Scopus (0) Google Scholar,36Ni Y. Nagashimada M. Zhuge F. et al.Astaxanthin prevents and reverses diet-induced insulin resistance and steatohepatitis in mice: a comparison with vitamin E.Sci Rep. 2015; 5: 17192Crossref PubMed Scopus (91) Google Scholar Limitations to the strict separation of the M1 and M2 phenotypes arise from heterogeneous activation states that macrophages can exhibit, and thus, caution is needed when interpreting their multivalent function in metabolic liver disease. Nevertheless, the aforementioned findings are interesting, because one of the earliest studies in NASH, the PIVENS