Toll-Like Receptor Signaling in the Liver

Toll-like receptors (TLRs) recognize pathogen-associated molecular patterns and are crucially involved in the regulation of innate immune responses. Despite chronic exposure to a high load of bacterial products, the normal liver shows no activation of TLR-signaling pathways. However, under pathologic conditions, Toll-like receptors promote proinflammatory signaling such as the nuclear factor-κB, c-Jun-N-terminal kinase (JNK), p38, and interferon pathways in the liver and regulate antiviral and antibacterial responses, hepatic injury, and wound healing. This review will summarize recent findings on TLR signaling; describe the functional expression of TLRs in resident and nonresident cell populations of the liver; and analyze the role of TLR-mediated signals in hepatic diseases such as hepatitis B and C virus infection, systemic endotoxemia, hepatic ischemia-reperfusion injury, liver regeneration, alcoholic liver injury, nonalcoholic steatohepatitis, primary biliary cirrhosis, and hepatic fibrosis. Toll-like receptors (TLRs) are a group of highly conserved molecules that allow the immune system to sense molecules that are present in most classes of pathogens, but not the host, and to coordinate defense mechanisms against these pathogens. The recognition of pathogen-associated molecular patterns (PAMPs) by Toll-like receptors is a cornerstone of innate immunity and provides a quick and highly efficient response to pathogens in both vertebrate and invertebrate species.1Aderem A. Ulevitch R.J. 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Takeda K. Akira S. Medzhitov R. Toll-like receptors control activation of adaptive immune responses.Nat Immunol. 2001; 2: 947-950Crossref PubMed Scopus (912) Google Scholar Because of its anatomic links to the gut, the liver is constantly exposed to gut-derived bacterial products and functions as a major filter organ and a first line of defense. Eighty percent of intravenously injected endotoxin is detected in the liver within 20–30 minutes.6Mathison J.C. Ulevitch R.J. The clearance, tissue distribution, and cellular localization of intravenously injected lipopolysaccharide in rabbits.J Immunol. 1979; 123: 2133-2143PubMed Google Scholar, 7Ruiter D.J. van der Meulen J. Brouwer A. Hummel M.J. Mauw B.J. van der Ploeg J.C. Wisse E. Uptake by liver cells of endotoxin following its intravenous injection.Lab Invest. 1981; 45: 38-45PubMed Google Scholar Kupffer cells, the resident macrophages of the liver, are able to take up efficiently endotoxin and phagocytose bacteria carried through portal vein blood and are considered to play a major role in the clearance of systemic bacterial infection.8Benacerraf B. Sebestyen M.M. Schlossman S. A quantitative study of the kinetics of blood clearance of P32-labelled Escherichia coli and Staphylococci by the reticuloendothelial system.J Exp Med. 1959; 110: 27-48Crossref PubMed Google Scholar, 9Fox E.S. Thomas P. Broitman S.A. Clearance of gut-derived endotoxins by the liver. Release and modification of 3H, 14C-lipopolysaccharide by isolated rat Kupffer cells.Gastroenterology. 1989; 96: 456-461Abstract Full Text PDF PubMed Google Scholar, 10Gregory S.H. Wing E.J. 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Immunology of the healthy liver old questions and new insights.Gastroenterology. 2001; 120: 250-260Abstract Full Text Full Text PDF PubMed Google Scholar which contribute to a high degree of tolerance as seen by graft survival across major histocompatibility antigen disparities, induction of systemic tolerance to food antigens, and persistence of some viral infections for decades.13Crispe I.N. Hepatic T cells and liver tolerance.Nat Rev Immunol. 2003; 3: 51-62Crossref PubMed Scopus (373) Google Scholar In addition, the healthy liver contains low messenger RNA (mRNA) levels of TLRs such as TLR1, TLR2, TLR4, TLR6, TLR7, TLR8, TLR9, and TLR10 and signaling molecules such as MD-2 and MyD88 in comparison with other organs.14De Creus A. Abe M. Lau A.H. Hackstein H. Raimondi G. Thomson A.W. Low TLR4 expression by liver dendritic cells correlates with reduced capacity to activate allogeneic T cells in response to endotoxin.J Immunol. 2005; 174: 2037-2045Crossref PubMed Google Scholar, 15Lichtman S.N. Wang J. Lemasters J.J. LPS receptor CD14 participates in release of TNF-α in RAW 264.7 and peritoneal cells but not in Kupffer cells.Am J Physiol. 1998; 275: G39-G46PubMed Google Scholar, 16Zarember K.A. Godowski P.J. Tissue expression of human Toll-like receptors and differential regulation of Toll-like receptor mRNAs in leukocytes in response to microbes, their products, and cytokines.J Immunol. 2002; 168: 554-561PubMed Google Scholar However, under pathologic conditions, TLRs activate inflammatory-signaling pathways in the liver and are actively involved in the pathophysiology in a large number of hepatic diseases. In this article, we will present an overview of recent advances in TLR signaling and review the role of TLRs in the pathophysiology of infectious, toxic, metabolic, and autoimmune liver disease. The human TLR family consists of currently 10 members, which are structurally characterized by the presence of a leucine-rich repeat (LRR) domain in their extracellular domain and a Toll/interleukin (IL)-1 receptor (TIR) domain in their intracellular domain. The existence of a large number of TLRs enables the innate immune system to discriminate between PAMPs that are characteristic of different microbial classes and launch specific defense mechanisms. A comparison of the amino acid sequences of the human TLRs reveals that members of the TLR family can be structurally divided into 5 subfamilies: the TLR3, TLR4, TLR5, TLR2, and TLR9 subfamilies. Whereas the TLR3, TLR4, and TLR5 subfamilies consist of only 1 member, the TLR2 subfamily is composed of TLR1, TLR2, TLR6, and TLR10 and the TLR9 subfamily of TLR7, TLR8, and TLR9. TLRs are able to detect a variety of PAMPs including lipopolysaccharide (LPS; TLR4), lipoproteins (TLR2/TLR1 and TLR2/TLR6 heterodimers), double-stranded RNA (TLR 3) and single-stranded RNA (TLR 7 and TLR8), flagellin (TLR5), and unmethylated CpG-containing DNA (TLR9) (Figure 1). TLR10 is an orphan receptor with currently unknown ligands. TLR1 and TLR2 form heterodimers with TLR6 and TLR10 as well as with each other, which may even broaden the ligand repertoire of these receptors. TLRs that mainly serve to detect bacterial lipopolysaccharides and lipoproteins are located on the cell surface. TLRs such as TLR3, TLR7, TLR8, and TLR9 that mainly recognize viral RNA and bacterial DNA are located in late endosome-lysosomes in which these materials are processed and host DNA is not present, thus avoiding aberrant self-recognition. Although each TLR detects specific PAMPs, many of the signaling molecules that mediate intracellular response are shared by the TLRs and form a complex signaling network that activates several pathways that initiate the transcription of a specific set of genes to induce proinflammatory, antiviral, and antibacterial responses. In addition, TLR ligands also repress the transcription of a large number of genes.17Gao J.J. Diesl V. Wittmann T. Morrison D.C. Ryan J.L. Vogel S.N. Follettie M.T. Regulation of gene expression in mouse macrophages stimulated with bacterial CpG-DNA and lipopolysaccharide.J Leukoc Biol. 2002; 72: 1234-1245PubMed Google Scholar, 18Wells C.A. Ravasi T. Faulkner G.J. Carninci P. Okazaki Y. Hayashizaki Y. Sweet M. Wainwright B.J. Hume D.A. Genetic control of the innate immune response.BMC Immunol. 2003; 4: 5Crossref PubMed Scopus (77) Google Scholar The TLR-signaling pathway shows remarkable similarity to the IL-1 receptor signaling pathway with which it shares many components including highly conserved cytoplasmic TIR domains and several intracellular adapter molecules. Two cell surface molecules, CD14 and MD-2, are involved in addition to TLR4 to transmit signals in response to LPS. Mice that are deficient in CD14 are resistant to the lethal effects of LPS but still able to respond to high concentrations of LPS.19Haziot A. Ferrero E. Kontgen F. Hijiya N. Yamamoto S. Silver J. Stewart C.L. Goyert S.M. Resistance to endotoxin shock and reduced dissemination of gram-negative bacteria in CD14-deficient mice.Immunity. 1996; 4: 407-414Abstract Full Text Full Text PDF PubMed Scopus (530) Google Scholar Recent data suggests that CD14 is not required for all TLR4-mediated signals and that MyD88-dependent signaling may occur in the absence of CD14.20Jiang Z. Georgel P. Du X. Shamel L. Sovath S. Mudd S. Huber M. Kalis C. Keck S. Galanos C. Freudenberg M. Beutler B. CD14 is required for MyD88-independent LPS signaling.Nat Immunol. 2005; 6: 565-570Crossref PubMed Scopus (321) Google Scholar In contrast, mice deficient in MD-2 show an almost completely impaired response to LPS.21Nagai Y. Akashi S. Nagafuku M. Ogata M. Iwakura Y. Akira S. Kitamura T. Kosugi A. Kimoto M. Miyake K. 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MD-2 and TLR4 N-linked glycosylations are important for a functional lipopolysaccharide receptor.J Biol Chem. 2002; 277: 1845-1854Crossref PubMed Scopus (132) Google Scholar Several other factors, among them the LPS-binding protein, enhance the response to LPS based on their ability to bind LPS and deliver it to CD14.26Schumann R.R. Leong S.R. Flaggs G.W. Gray P.W. Wright S.D. Mathison J.C. Tobias P.S. Ulevitch R.J. Structure and function of lipopolysaccharide binding protein.Science. 1990; 249: 1429-1431Crossref PubMed Google Scholar Recent evidence suggests that CD36 acts as a co-receptor for a subset of TLR2 ligands. CD36-deficient mice show a severe defect in tumor necrosis factor (TNF)-α production after stimulation with the TLR2/6 ligands macrophage-activating lipopeptide 2 from Mycoplasma pneumoniae and lipoteichoic acid and are hypersusceptible to Staphylococcus aureus infection.27Hoebe K. Georgel P. Rutschmann S. Du X. Mudd S. Crozat K. Sovath S. Shamel L. Hartung T. 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TRAM-deficient mice demonstrate defects in cytokine production in response to LPS but show normal responses to the ligands for TLR2, TLR3, TLR7, and TLR9 and also IL-1.35Yamamoto M. Sato S. Hemmi H. Uematsu S. Hoshino K. Kaisho T. Takeuchi O. Takeda K. Akira S. TRAM is specifically involved in the Toll-like receptor 4-mediated MyD88-independent signaling pathway.Nat Immunol. 2003; 4: 1144-1150Crossref PubMed Scopus (550) Google Scholar MyD88 induces the transcription of proinflammatory and antibacterial mediators by activating the NF-κB, AP-1, IRF5, and p38 pathways (Figure 1). MyD88 activates NF-κB in response to all TLRs, except for TLR3. After stimulation with LPS, MyD88 mediates the early phase strong NF-κB activation, whereas late phase NF-κB activation depends on TRIF.34Yamamoto M. Sato S. Hemmi H. Hoshino K. Kaisho T. Sanjo H. Takeuchi O. Sugiyama M. Okabe M. Takeda K. Akira S. 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Essential function for the kinase TAK1 in innate and adaptive immune responses.Nat Immunol. 2005; 6: 1087-1095Crossref PubMed Scopus (395) Google Scholar MyD88 binds to and activates the transcription factor IRF5 in concert with TRAF6 to induce the transcription of proinflammatory genes such as IL-6, IL-12, and TNF-α but not interferon (IFN) α.39Takaoka A. Yanai H. Kondo S. Duncan G. Negishi H. Mizutani T. Kano S. Honda K. Ohba Y. Mak T.W. Taniguchi T. Integral role of IRF-5 in the gene induction programme activated by Toll-like receptors.Nature. 2005; 434: 243-249Crossref PubMed Scopus (422) Google Scholar Accordingly, IRF5-deficient mice are resistant to lethal shock induced by injection of unmethylated CpG-DNA or LPS.39Takaoka A. Yanai H. Kondo S. Duncan G. Negishi H. Mizutani T. Kano S. Honda K. Ohba Y. Mak T.W. Taniguchi T. 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Li S. Penninger J.M. Wesche H. Ohashi P.S. Mak T.W. Yeh W.C. Severe impairment of interleukin-1 and Toll-like receptor signalling in mice lacking IRAK-4.Nature. 2002; 416: 750-756Crossref PubMed Scopus (442) Google Scholar, 45Honda K. Yanai H. Negishi H. Asagiri M. Sato M. Mizutani T. Shimada N. Ohba Y. Takaoka A. Yoshida N. Taniguchi T. IRF-7 is the master regulator of type-I interferon-dependent immune responses.Nature. 2005; 434: 772-777Crossref PubMed Scopus (942) Google Scholar TRIF mediates antiviral responses after TLR3 and TLR4 stimulation through the transcription factor IRF3, which activates a large set of antiviral genes including IFN-α and IFN-β (Figure 1). The noncanonical IκB kinase (IKK) homologs IKK-ϵ and TANK-binding kinase-1 (TBK1) provide the link between TRIF and IRF3 and mediate IRF3 phosphorylation and the transcription of IRF3-dependent genes.46Fitzgerald K.A. McWhirter S.M. Faia K.L. Rowe D.C. Latz E. Golenbock D.T. Coyle A.J. Liao S.M. Maniatis T. 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Negative regulation of toll-like receptor-mediated immune responses.Nat Rev Immunol. 2005; 5: 446-458Crossref PubMed Scopus (757) Google Scholar These mechanisms act at the receptor level (ST2 and SIGGIR), at the level of adapter molecules (mainly MyD88), at the level of receptor proximal kinases (mainly IRAK), and/or at the level of receptor distal kinases by suppressor of cytokine signaling 1 (SOCS1) (Figure 2). LPS stimulation results in a reduced surface expression of the LPS receptor complex composed of TLR4 and MD-2.55Nomura F. Akashi S. Sakao Y. Sato S. Kawai T. Matsumoto M. Nakanishi K. Kimoto M. Miyake K. Takeda K. Akira S. Cutting edge endotoxin to