TRIF-dependent signaling and its role in liver diseases

Abstract

TIR domain-containing adaptor inducing IFN-β (TRIF) is a crucial adaptor molecule downstream of toll-like receptors 3 (TLR3) and 4 (TLR4). TRIF directly binds to TLR3 through its TIR domain, while it associates with TLR4 indirectly through the bridge adaptor molecule TRIF-related adaptor molecule (TRAM). TRIF plays a pivotal role in regulating interferon beta 1 (IFN-β) response, nuclear factor kappa B (NF-κB) signaling, apoptosis, and necroptosis signaling mediated by TLR3 and TLR4. It accomplishes these by recruiting and activating various kinases or transcription factors via its distinct domains. In this review, we comprehensively summarize the TRIF-dependent signaling pathways mediated by TLR3 and TLR4, elucidating key target molecules and downstream pathways. Furthermore, we provide an overview of TRIF's impact on several liver disorders, including drug-induced liver injury, ischemia-reperfusion liver injury, autoimmune hepatitis, viral hepatitis, alcohol-associated liver disease (ALD), metabolic dysfunction-associated steatotic liver disease (MASLD) and metabolic dysfunction-associated steatohepatitis (MASH). We also explore its effects on liver steatosis, inflammation, fibrosis, and carcinogenesis. A comprehensive understanding of the TRIF-dependent signaling pathways, as well as the intricate relationship between TRIF and liver diseases, can facilitate the identification of potential drug targets and the development of novel and effective therapeutics against hepatic disorders.

Keywords: IFN-β response; NF-κB; TLR3; TLR4; TRIF; apoptosis; liver diseases; necroptosis.

FIGURE 1

TRIF-dependent TLR3 and TLR4 signaling. TLR4 forms a complex with MD-2, a secreted glycoprotein that recognizes and binds LPS through the participation of LBP and CD14. Subsequently, TLR4 is internalized to the membrane of the endosome and relies on bridge adaptor TRAM to initiate TRIF-dependent signaling. TLR3 is localized on the membrane of the endosome and activated by dsRNA, which interacts with TRIF directly. TRIF-dependent TLR3 signaling includes ①-⑧. TRIF-dependent TLR4 signaling includes ①-⑩. ① IFN-β response mediated by the N-terminal of TRIF downstream of TLR3 and TLR4. Upon activation of the TRAF3, the N-terminal of TRIF interacts with TBK1 and IKKε. TBK1 phosphorylates TRIF and then facilitates IRF3 binding to TRIF. Consequently, TBK1 phosphorylates IRF3 and IRF7, facilitating the formation of phosphorylated dimers for both proteins. These dimers subsequently translocate into the nucleus to initiate activation of the IFN-β response. ② NF-κB signaling mediated by the N-terminal of TRIF downstream of TLR3 and TLR4. TRIF recruits TRAF6, which then binds to TAK1 and TAB2 to activate IKKs. Subsequently, activated IKKs phosphorylate IκBα, thereby abolishing its inhibitory function on NF-κB complex and facilitating its activation. ③ NF-κB signaling mediated by the C-terminal of TRIF downstream of TLR3 and TLR4. The C-terminal of TRIF interacts with RIPK1 via their RHIM domains, followed by polyubiquitination events of RIPK1 which then facilitates the combination of TAK1 and TAB2, subsequently promoting IKKs activation, IκBα phosphorylatin, and NF-κB activation. ④ NF-κB signaling mediated by the C-terminal of TRIF downstream of TLR4. The C-terminal of TRIF interacts with RIPK1 through their RHIM domains, facilitating direct binding of RIPK1 to ERK1/2. Subsequently, phosphorylation of ERK1/2 occurs, inducing AP-1 activation and ultimately resulting in the expression of inflammatory factors. ⑤ RIPK3 competitively inhibits TRIF-dependent TLR3/TLR4-RIPK1-NF-κB signaling via its RHIM domain. RIPK3 interacts with RIPK1 through their RHIM domains. This interaction leads to the phosphorylation of RIPK1, thereby inhibiting RIPK1-dependent NF-κB activation processes. ⑥ Apoptosis signaling mediated by the RHIM domain of TRIF downstream of TLR3 and TLR4. The RHIM domain of TRIF allows it to bind to RIPK1. Once bound, RIPK1 recruits FADD through its DD domain, resulting in the activation of caspase8 and subsequently inducing apoptosis. ⑦-⑧ Necroptosis signaling mediated by the RHIM domain of TRIF downstream of TLR3 and TLR4. ⑦ In the presence of z-VAD, TRIF interacts with RIPK3 via their RHIM domains, then facilitates the accumulation of ROS, leading to the initiation of necroptosis. ⑧ When RIPK1 is absent, TRIF promotes the interaction between RIPK3 and MLKL, resulting in the phosphorylation of MLKL, thereby triggering necroptosis. During these processes, the FADD-caspase8 complex inhibits the activation of RIPK3 to restrain necroptosis. ⑨ NLRP3 inflammasome activation and mtDNA synthesis mediated by MyD88 and TRIF downstream of TLR4. MyD88 and TRIF trigger the transcription of CMPK2 with the involvement of IRF1, promoting the synthesis of mtDNA and ox-mtDNA, ultimately resulting in the activation of NLRP3 inflammasome. ⑩ TRIF activates the expression of SAMHD1 which exhibits a negative feedback role in mtDNA synthesis and IL-1β production.

FIGURE 2

Different domains of TRIF and their corresponding functions. The TRIF molecule consists of a TIR domain flanked by an N-terminal extension and a C-terminal extension. The C-terminal RHIM domain is involved in regulating NF-κB activation, apoptosis, and necroptosis; The TIR domain is responsible for binding to the intracellular TIR domain of TLR; The N-terminal TRAF6 binding motif plays a role in NF-κB activation, and the N-terminal TBK1 motif contributes to IFN response regulation.