Toll-like receptor 4 signaling in liver injury and hepatic fibrogenesis

Abstract

Toll-like receptors (TLRs) are a family of transmembrane pattern recognition receptors (PRR) that play a key role in innate and adaptive immunity by recognizing structural components unique to bacteria, fungi and viruses. TLR4 is the most studied of the TLRs, and its primary exogenous ligand is lipopolysaccharide, a component of Gram-negative bacterial walls. In the absence of exogenous microbes, endogenous ligands including damage-associated molecular pattern molecules from damaged matrix and injured cells can also activate TLR4 signaling. In humans, single nucleotide polymorphisms of the TLR4 gene have an effect on its signal transduction and on associated risks of specific diseases, including cirrhosis. In liver, TLR4 is expressed by all parenchymal and non-parenchymal cell types, and contributes to tissue damage caused by a variety of etiologies. Intact TLR4 signaling was identified in hepatic stellate cells (HSCs), the major fibrogenic cell type in injured liver, and mediates key responses including an inflammatory phenotype, fibrogenesis and anti-apoptotic properties. Further clarification of the function and endogenous ligands of TLR4 signaling in HSCs and other liver cells could uncover novel mechanisms of fibrogenesis and facilitate the development of therapeutic strategies.

Figure 1

Schematic overview of Toll-like receptor (TLR)4 signaling pathway. LPS interacts with circulating LPS-binding protein (LBP) and binds to TLR4 on the cell membrane with two co-receptors (CD14 and myeloid differentiation protein (MD)2), activating myeloid differentiation factor (MyD)88-dependent and (MyD)88-independent TLR4 signaling via different adaptor proteins. The MyD88-dependent pathway signals through activation of iκB kinase (IKK) and mitogen activated protein kinase (MAPK) pathways, which in turn leads to activation of transcription factors nuclear factor (NF)-κB and activator protein (AP)-1, respectively, and controls the expression of pro-inflammatory cytokines and other immune related genes. In addition, phosphatidylinositol 3-kinase (PI3K) and AKT are also important factors downstream of MyD88 that mediate NF-κB activation. The MyD88-independent pathway is mediated by the TIR domain-containing adaptor inducing interferon-β (TRIF), which activates interferon regulatory (IRF)3 and induces the expression of interferon (IFN)-β and IFN-responsive genes

Figure 2

Activation of the Toll-like receptor (TLR)4 signaling in liver injuries. Exogenous and endogenous ligand(s) of TLR4 are increased during liver injuries caused by various etiologies such as nonalcoholic steatohepatitis (NASH). Activation of TLR4 by both pathogen-associated molecular patterns (PAMPs; exogenous ligands, for example, lipopolysaccharide (LPS)) and damage-associated molecular patterns (DAMPs; endogenous ligands, for example, high mobility group box (HMGB)1) induces pre-inflammatory cytokines and chemokines, thereby enhancing liver inflammation, injury and fibrogenesis. TLR4 activation also enhances interferon (IFN)-β and IFN-stimulated gene expression, which have inhibitory effects on hepatitis viruses.

Figure 3

Hypothesis: fibrogenic signaling induced by Toll-like receptor (TLR)4 in hepatic stellate cells (HSCs) and Kupffer cells (KCs), and the effect of TLR4 single nucleotide polymorphisms (SNPs). HSCs have intact TLR4 signaling. Both pathogen-associated molecular patterns (PAMPs; exogenous ligands, for example, LPS) and damage-associated molecular patterns (DAMPs; endogenous ligands, for example, HMGB1) may activate TLR4 in HSCs, signaling through NF-κB, leading to downregulation of the transmembrane inhibitory TGF-β1 pseudoreceptor bone morphogenic protein and active membrane bound inhibitor (BAMBI) on HSCs, a key fibrogenic cell in liver. As a result, net TGF-β1 activity increases, enhancing extracellular matrix production. Concurrently, TLR4 signaling in HSCs also increases the activity and migration of KCs. Activation of TLR4 signaling on KCs enhances the interaction within KCs and HSCs by activating the production of profibrogenic factors. Polymorphisms of the TLR4 gene, which decrease the recognition of PAMPs and DAMPs by TLR4 on HSCs and KCs, decrease the activation of TLR4 signaling and downstream production of proinflammatory cytokines, chemokines and fibrogenic proteins such as BAMBI. The TLR4 single nucleotide polymorphisms (SNPs) confer an overall protective effect against inflammation and fibrogenesis.