Liver Steatosis is a Driving Factor of Inflammation

Abstract

See rebuttal on page 1271. See counterpoint on page 1273. See rebuttal on page 1271. See counterpoint on page 1273. Nonalcoholic fatty liver disease (NAFLD) is the most common liver disease associated with comorbidities, such as insulin resistance, cardiovascular, and metabolic diseases. A subset of patients with NAFLD progresses to nonalcoholic steatohepatitis (NASH), characterized by hepatocyte injury, lobular inflammation, and perisinusoidal fibrosis, which may progress to cirrhosis. A significant gap in understanding the causative factors and molecular mechanisms involved in liver injury and inflammation limits the progress in treating NASH. Accumulation of excess free fatty acids in hepatocytes as lipid droplets (called hepatic steatosis) is the stepping stone in the spectrum of NAFLD. Hepatic steatosis is mainly triggered by continuous low-fiber high-fat diets, impaired fatty acid metabolism, sustained adipose tissue lipolysis, and de novo lipogenesis.1Hardy T. Oakley F. Anstee Q.M. Day C.P. et al.Nonalcoholic fatty liver disease: pathogenesis and disease spectrum.Annu Rev Pathol. 2016; 11: 451-496Crossref PubMed Scopus (306) Google Scholar Since the early 1970s, researchers have elegantly demonstrated that the accumulation of oxidizable lipids in hepatocytes increases lipid peroxidation and oxidative stress via peroxisomal β-oxidation. Conversely, the peroxidation of membrane lipids and their products directly contribute to hepatocyte injury by damaging various cellular organelles, such as the endoplasmic reticulum and mitochondria. This leads to activating multiple signaling pathways involved in apoptosis, necrosis, and pyroptosis (inflammasome), resulting in hepatocyte injury and inflammation.2Day C.P. James O.F. Steatohepatitis: a tale of two "hits"?.Gastroenterology. 1998; 114: 842-845Abstract Full Text Full Text PDF PubMed Scopus (3363) Google Scholar Thus, hepatic lipotoxicity acts as the primary insult for initiating injury and inflammation in NASH pathogenesis. The casual role of lipotoxicity in promoting inflammation in the context of NASH is unclear because of the coexistence of steatosis in combination with insulin resistance, adipokines, alteration in the immune system, and dysbiosis.3Schuster S. Cabrera D. Arrese M. Feldstein A.E. et al.Triggering and resolution of inflammation in NASH.Nat Rev Gastroenterol Hepatol. 2018; 15: 349-364Crossref PubMed Scopus (305) Google Scholar It is well acknowledged that intestinal microbial dysbiosis and dysregulated adipokine levels during NASH are closely associated with the severity of hepatic lipotoxicity.4Boursier J. Mueller O. Barret M. Machado M. Fizanne L. Araujo-Perez F. Guy C.D. Seed P.C. Rawls J.F. David L.A. Hunault G. Oberti F. Cales P. Diehl A.M. et al.The severity of nonalcoholic fatty liver disease is associated with gut dysbiosis and shift in the metabolic function of the gut microbiota.Hepatology. 2016; 63: 764-775Crossref PubMed Scopus (654) Google Scholar However, in the setting up of NASH, they are considered secondary offenders. Supporting this concept, Zhang et al5Zhang X. Coker O.O. Chu E.S. Fu K. Lau H.C.H. Wang Y.X. Chan A.W.H. Wei H. Yang X. Sung J.J.Y. Yu J. et al.Dietary cholesterol drives fatty liver-associated liver cancer by modulating gut microbiota and metabolites.Gut. 2021; 70: 761-774Crossref PubMed Scopus (105) Google Scholar demonstrated that lipotoxicity induced by a cholesterol diet sequentially promotes inflammation and hepatocyte injury, associated with gut microbiota dysbiosis. In contrast, decreasing cholesterol levels restored the gut microbiota and completely prevented the NASH development. Similarly, dysbiosis occurs in steatosis-prone leptin-deficient (Lepob/ob) mice, independent of dietary regimens, indicating that alterations in the host (lipid) metabolism is the primary event in the spectrum of liver diseases.6Nagpal R. Newman T.M. Wang S. Jain S. Lovato J.F. Yadav H. et al.Obesity-linked gut microbiome dysbiosis associated with derangements in gut permeability and intestinal cellular homeostasis independent of diet.J Diabetes Res. 2018; 20183462092Crossref PubMed Scopus (75) Google Scholar In addition, inhibiting the accumulation of saturated fatty acids in the hepatocytes through pharmacologic or genetic approaches improves insulin sensitivity and attenuates inflammation,7Friedman S.L. Neuschwander-Tetri B.A. Rinella M. Sanyal A.J. et al.Mechanisms of NAFLD development and therapeutic strategies.Nat Med. 2018; 24: 908-922Crossref PubMed Scopus (1188) Google Scholar suggesting that hepatic lipotoxicity is sufficient to trigger the injury and inflammatory response in the liver. How does lipotoxicity induce inflammation and injury in NASH? Hepatocytes enriched with mitochondria metabolizes fatty acids into acetyl-CoA via fatty acid β-oxidation. However, increased fatty acid delivery endorses lipotoxicity by generating excess reactive oxygen species, which causes mitochondrial damage. Under physiological conditions, mitophagy removes damaged mitochondria; however, lipotoxicity impairs mitophagy resulting in the accumulation of damaged mitochondria leading to hepatocellular injury (Figure 1).8Mansouri A. Gattolliat C.H. Asselah T. Mitochondrial dysfunction and signaling in chronic liver diseases.Gastroenterology. 2018; 155: 629-647Abstract Full Text Full Text PDF PubMed Scopus (261) Google Scholar The injured hepatocytes release danger signals, such as damage-associated molecular patterns (DAMPs), including mitochondrial DNA and high-mobility group box 1. Intracellular DAMPs are recognized by pattern recognition receptors, whereas extracellular DAMPs act through the receptors of advanced glycation end products and toll-like receptor 4 and 9 signaling. Activation of these receptors in hepatocytes and immune cells co-operatively triggers a sterile inflammation through diverse pathways, such as nuclear factor-κB, mitogen-activated protein kinase (p38 MAPK), (p42/44 MAPK), and c-Jun N-terminal kinase signaling cascades (Figure 1).9Yang R. Tonnesseen T.I. DAMPs and sterile inflammation in drug hepatotoxicity.Hepatol Int. 2019; 13: 42-50Crossref PubMed Scopus (32) Google Scholar,10Han H. Desert R. Das S. Song Z. Athavale D. Ge X. Nieto N. et al.Danger signals in liver injury and restoration of homeostasis.J Hepatol. 2020; 73: 933-951Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar For instance, studies show that ablation of toll-like receptor 4 and 9 signaling in the hepatocytes or immune cells similarly attenuate high-fat-induced hepatic steatosis and inflammation, suggesting toll-like receptor signaling in parenchymal and nonparenchymal cells coordinates obesity-associated fatty liver disease.11Garcia-Martinez I. Santoro N. Chen Y. Hoque R. Ouyang X. Caprio S. Shlomchik M.J. Coffman R.L. Candia A. Mehal W.Z. et al.Hepatocyte mitochondrial DNA drives nonalcoholic steatohepatitis by activation of TLR9.J Clin Invest. 2016; 126: 859-864Crossref PubMed Scopus (270) Google Scholar, 12Jia L. Chang X. Qian S. Liu C. Lord C.C. Ahmed N. Lee C.E. Lee S. Gautron L. Mitchell M.C. Horton J.D. Scherer P.E. Elmquist J.K. et al.Hepatocyte toll-like receptor 4 deficiency protects against alcohol-induced fatty liver disease.Mol Metab. 2018; 14: 121-129Crossref PubMed Scopus (28) Google Scholar, 13Jia L. Vianna C.R. Fukuda M. Berglund E.D. Liu C. Tao C. Sun K. Liu T. Harper M.J. Lee C.E. Lee S. Scherer P.E. Elmquist J.K. et al.Hepatocyte Toll-like receptor 4 regulates obesity-induced inflammation and insulin resistance.Nat Commun. 2014; 5: 3878Crossref PubMed Scopus (190) Google Scholar Thus, lipotoxicity-mediated elevation in reactive oxygen species and mitochondrial damage sets a stage to trigger hepatic inflammation. Lipotoxicity-mediated mitochondrial dysfunction and reactive oxygen species generation also activate nod-like receptor protein (NLRP3) inflammasome signaling by promoting NLRP3 oligomerization and inflammasome assembly. The NLRP3-inflammasome pathway acts as an adaptive mechanism to restore hepatocellular homeostasis during acute stress; however, its sustained activation from persistent injury promotes liver injury, pyroptosis, and fibrosis (Figure 1).14Wree A. Eguchi A. McGeough M.D. Pena C.A. Johnson C.D. Canbay A. Hoffman H.M. Feldstein A.E. et al.NLRP3 inflammasome activation results in hepatocyte pyroptosis, liver inflammation, and fibrosis in mice.Hepatology. 2014; 59: 898-910Crossref PubMed Scopus (531) Google Scholar, 15Jo E.K. Kim J.K. Shin D.M. Sasakawa C. et al.Molecular mechanisms regulating NLRP3 inflammasome activation.Cell Mol Immunol. 2016; 13: 148-159Crossref PubMed Scopus (684) Google Scholar, 16Zhang N.P. Liu X.J. Xie L. Shen X.Z. Wu J. et al.Impaired mitophagy triggers NLRP3 inflammasome activation during the progression from nonalcoholic fatty liver to nonalcoholic steatohepatitis.Lab Invest. 2019; 99: 749-763Crossref PubMed Scopus (65) Google Scholar Studies revealed that NLRP3 signaling in parenchymal cells is crucial in NASH pathogenesis because global but not myeloid-specific activation of NLRP3 increases hepatocyte death and injury.14Wree A. Eguchi A. McGeough M.D. Pena C.A. Johnson C.D. Canbay A. Hoffman H.M. Feldstein A.E. et al.NLRP3 inflammasome activation results in hepatocyte pyroptosis, liver inflammation, and fibrosis in mice.Hepatology. 2014; 59: 898-910Crossref PubMed Scopus (531) Google Scholar,17Han C.Y. Rho H.S. Kim A. Kim T.H. Jang K. Jun D.W. Kim J.W. Kim B. Kim S.G. et al.FXR inhibits endoplasmic reticulum stress-induced NLRP3 inflammasome in hepatocytes and ameliorates liver injury.Cell Rep. 2018; 24: 2985-2999Abstract Full Text Full Text PDF PubMed Scopus (83) Google Scholar Mechanistically, NLRP3 activates caspase-1 in hepatocytes, cleaving prointerleukin (IL)1β and pro-IL18β into their mature forms (IL-1β and IL-18). These proinflammatory cytokines induce hepatocyte pyroptosis via nuclear factor-κB activation.18Petrasek J. Bala S. Csak T. Lippai D. Kodys K. Menashy V. Barrieau M. Min S.Y. Kurt-Jones E.A. Szabo G. et al.IL-1 receptor antagonist ameliorates inflammasome-dependent alcoholic steatohepatitis in mice.J Clin Invest. 2012; 122: 3476-3489Crossref PubMed Scopus (446) Google Scholar Conflicting evidence also suggests that hepatocyte-derived saturated fatty acids, DAMPs, and stress molecules, such as uric acid and cholesterol crystals, activate the NLRP3 inflammasome in nonparenchymal cells leading to sterile inflammation and hepatocellular injury. For example, mitochondrial DNA released from the damaged hepatocytes activates the NLRP3 inflammasome in hepatic Kupffer cells leading to an inflammatory response.19Gaul S. Leszczynska A. Alegre F. Kaufmann B. Johnson C.D. Adams L.A. Wree A. Damm G. Seehofer D. Calvente C.J. Povero D. Kisseleva T. Eguchi A. McGeough M.D. Hoffman H.M. Pelegrin P. Laufs U. Feldstein A.E. et al.Hepatocyte pyroptosis and release of inflammasome particles induce stellate cell activation and liver fibrosis.J Hepatol. 2021; 74: 156-167Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar Remarkably, various classes of lipids elicit a differential effect on inflammasome activation during NASH progression.20Liang J.J. Fraser I.D.C. Bryant C.E. Lipid regulation of NLRP3 inflammasome activity through organelle stress.Trends Immunol. 2021; 42: 807-823Abstract Full Text Full Text PDF PubMed Scopus (2) Google Scholar Therefore, understanding the lipid mediators of inflammasome activation may lead to novel therapeutic targets to treat NASH. Recent studies show that hepatocytes communicate with neighboring cells via extracellular vehicles (EVs), composed of cargo in the form of proteins, lipids, and nucleic acids. Notably, the circulating levels of EVs are significantly elevated in human and mouse models of NASH.21Ipsen D.H. Tveden-Nyborg P. Extracellular vesicles as drivers of non-alcoholic fatty liver disease: small particles with big impact.Biomedicines. 2021; 9: 93Crossref PubMed Scopus (7) Google Scholar It is also evident that the composition of the EVs cargo varies considerably between normal patients and patients with NASH, contributing to the worsening of liver injury and inflammation. Several mechanisms regulate EV release, including inositol-requiring enzyme 1α and death receptor 5. Studies show that lipids control the EV release and its cargo composition. For example, death receptor 5 proapoptotic signaling induced by the saturated fatty acids increases the release of EVs bearing tumor necrosis factor–related apoptosis-inducing ligand from the hepatocytes, which then activates the release of proinflammatory cytokines from the macrophages.22Hirsova P. Ibrahim S.H. Krishnan A. Verma V.K. Bronk S.F. Werneburg N.W. Charlton M.R. Shah V.H. Malhi H. Gores G.J. et al.Lipid-induced signaling causes release of inflammatory extracellular vesicles from hepatocytes.Gastroenterology. 2016; 150: 956-967Abstract Full Text Full Text PDF PubMed Scopus (254) Google Scholar Similarly, EVs with integrin β1 as a cargo released from the lipotoxic hepatocytes mediates monocyte adhesion to liver sinusoidal endothelial cells, promoting hepatic inflammation.23Guo Q. Furuta K. Lucien F. Gutierrez Sanchez L.H. Hirsova P. Krishnan A. Kabashima A. Pavelko K.D. Madden B. Alhuwaish H. Gao Y. Revzin A. Ibrahim S.H. et al.Integrin beta1-enriched extracellular vesicles mediate monocyte adhesion and promote liver inflammation in murine NASH.J Hepatol. 2019; 71: 1193-1205Abstract Full Text Full Text PDF PubMed Scopus (60) Google Scholar Moreover, EVs barring the proinflammatory miR-1 released from the lipid-laden hepatocytes suppress KLF4 and activates nuclear factor-κB in the endothelial cells (Figure 1).24Jiang F. Chen Q. Wang W. Ling Y. Yan Y. Xia P. et al.Hepatocyte-derived extracellular vesicles promote endothelial inflammation and atherogenesis via microRNA-1.J Hepatol. 2020; 72: 156-166Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar However, it remains unclear how a mere lipid overload regulates EV release and composition. EVs are the center of interest for future evaluation of novel therapeutic approaches in various diseases. Therefore, deciphering the mechanisms underlying lipid regulation of EV homeostasis will benefit in combating NASH. Despite the remarkable progress in understanding the complex relationship between lipotoxicity and NASH progression, “the chicken or the egg paradox” still prevails on the causal relationship between hepatic lipotoxicity and inflammation. A better understanding of the critical determinants of hepatic injury and inflammation driven by hepatic steatosis could help identify novel therapeutic targets for NASH.

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