TRIF mediates TLR3 signaling and TLR4-induced MyD88-independent pathway, such as delayed NF-κB activation 11–13. The interaction between TRIF and TLR4 is mediated by TRAM 14–16. As a newly discovered member of the TLR-adaptor family, the function of SARM is relatively unknown, yet it is the most conserved TIR domain-containing protein, having homologues in Drosophila17, zebrafish

18, Caenorhabditis Selleckchem PD0325901 elegans19 and horseshoe crab 20. These homologues share a common domain architecture constituted of N-terminal Armadillo motifs (ARM), two sterile α motif (SAM) domains and a C-terminal TIR domain 21. The unique combination of three protein–protein interaction domains in SARM suggests that amongst the family of TLR adaptors, SARM probably functions differently from the other adaptor molecules 21, 22. In fact, SARM seems to exhibit multiple CHIR99021 roles, and its functions differ in different species and under different circumstances. SARM negatively regulates NF-κB and IRF3-mediated TLR3 and TLR4 signaling, both in the human 23 and in the horseshoe crab 20. These earlier studies showed that such inhibition is restricted to the TRIF pathway. It was reported that the overexpression of SARM blocks the induction of TRIF-dependent, but not MyD88-dependent genes,

and that this interaction is enhanced by LPS 23, suggesting that SARM is specifically responsible for downregulating TRIF-mediated TLR signaling during Gram-negative bacterial infection. Some recent findings add further complexity to the function of SARM, indicating upregulation 24 or downregulation 25 of its expression upon immune activation. Yet another

study showed a viral infection-mediated immune activation of SARM in the mouse brain 26. Besides immune function, SARM has also been implicated in the neuronal system 27, 28. Overall, the conundrum of the function of SARM remains unsolved. Besides NF-κB and IRF3, AP-1 is another transcription factor activated by TLR signaling. Although SARM specifically inhibits TRIF-dependent activation of NF-κB and IRF3, GNE-0877 it is unknown whether SARM also inhibits AP-1, and whether it is also restricted to the TRIF pathway. Since the TLR-mediated pathway for AP-1 activation is distinctive from those which activate NF-κB and IRF3 29, it is possible that SARM uses different mechanisms to regulate AP-1 signaling. In neuronal stress, SARM recruits activated JNK3 into the mitochondria 27, suggesting its potential involvement in MAPK signaling to promote neuronal apoptosis. In C. elegans, the SARM homolog, TIR-1, functions through a p38 MAPK signal transduction cascade 30, 31. However, the role of human SARM in MAPK pathway is unmapped. Here, we demonstrate that human SARM is capable of blocking the LPS-induced MyD88- and TRIF-mediated AP-1 activation. The effect of SARM against the LPS-mediated AP-1 activation was verified by suppression of endogenous SARM with siRNA, which resulted in increased basal AP-1 level.