LPS-TLR4 Signaling to IRF-3/7 and NF-κB Involves the Toll Adapters TRAM and TRIF

Abstract

Toll–IL-1–resistance (TIR) domain–containing adaptor-inducing IFN-β (TRIF)–related adaptor molecule (TRAM) is the fourth TIR domain–containing adaptor protein to be described that participates in Toll receptor signaling. Like TRIF, TRAM activates interferon regulatory factor (IRF)-3, IRF-7, and NF-κB-dependent signaling pathways. Toll-like receptor (TLR)3 and 4 activate these pathways to induce IFN-α/β, regulated on activation, normal T cell expressed and secreted (RANTES), and γ interferon–inducible protein 10 (IP-10) expression independently of the adaptor protein myeloid differentiation factor 88 (MyD88). Dominant negative and siRNA studies performed here demonstrate that TRIF functions downstream of both the TLR3 (dsRNA) and TLR4 (LPS) signaling pathways, whereas the function of TRAM is restricted to the TLR4 pathway. TRAM interacts with TRIF, MyD88 adaptor–like protein (Mal)/TIRAP, and TLR4 but not with TLR3. These studies suggest that TRIF and TRAM both function in LPS-TLR4 signaling to regulate the MyD88-independent pathway during the innate immune response to LPS.

NF-κB and the immune response

“…TRAM is required for IRF3 activation and for the delayed phase of NF-kB activation following TLR4 engagement. TLR4-induced IRAK activation by MyD88, however, is unaffected by the absence of TRAM and TRAM does not function in TLR3 TRIFdependent signaling pathways (Fitzgerald et al, 2003;Yamamoto et al, 2003). Therefore, it appears that TRAM is only needed for TRIF signaling downstream of TLR4.…”

“…TRIF signaling to NF-kB and IRF3 also appear to be separately regulated (Figure 5b). Signaling to IRF-3 occurs through two divergent members of the IKK family, IKKi (IKKe) and TBK1 (T2K) (Fitzgerald et al, 2003;Sharma et al, 2003); however, neither kinase is required for NF-kB activation by LPS or TNFa McWhirter et al, 2004). Furthermore, reconstitution of trif À/À cells with mutant TRIF lacking the TRAF-binding domain selectively restores induction of IRF3 but not NF-kB.…”

TLR signalling and activation of IRFs: revisiting old friends from the NF-κB pathway

“…However, TRIF can also interact with TBK1 to promote the activation of IRF3 and IRF7, leading to the induction of type I IFN and IFN-inducible genes. These two TRIF-mediated pathways are triggered by TLR3 (through its direct interaction with TRIF), and TLR4 by its indirect association with TRIF (through the bridging adaptor TRAM) [42,43]. The concomitant activation of NF-kB and IRF3 by TRIF is of biological significance because the activation of the promoters of some genes, such as IFN-b, requires the functional cooperation of these two transcription factors [44].…”

The type I interferon system in systemic lupus erythematosus

“…Thus, TLR-3 exclusively uses Toll/interleukin-1 (IL-1) receptor domain-containing adaptor-inducing IFN␤ (TRIF) and not MyD88 as the adaptor. Furthermore, TLR-4 acts on IFN␤ genes via a TRIF and TRIF-related adaptor molecule pathway (12). The adaptor molecules recruit protein kinases, notably IKK and tumor necrosis factor (TNF) receptor-associated factor family member-associated NF-B activator (TANK)-binding kinase 1 (TBK1).…”