The Tyrosine Kinase Syk Differentially Regulates Toll-like Receptor Signaling Downstream of the Adaptor Molecules TRAF6 and TRAF3

Abstract

Toll-like receptors (TLRs) are a major family of pattern recognition receptors, and they play a crucial role in innate immune responses. Activation of TLR4 signaling at the plasma membrane by its ligand lipopolysaccharide (LPS) stimulates a proinflammatory pathway dependent on the E3 ubiquitin ligase TRAF6 (tumor necrosis factor receptor-associated factor 6) and the kinase TAK1 (transforming growth factor β-activated kinase 1), whereas TLR4 signaling at endosomes stimulates the production of type I interferons (IFNs) through a pathway that depends on TRAF3 and the kinase TBK1 (TANK-binding kinase-1). We found that the nonreceptor tyrosine kinase Syk partially mediated the endocytosis of TLR4, but it also played a dual role in TLR4-mediated signaling. LPS-dependent stimulation of TLR4 in Syk-deficient macrophages led to enhanced activation of TAK1 and increased production of proinflammatory cytokines compared to that in wild-type macrophages. In contrast, Syk-deficient macrophages exhibited decreased TLR4-dependent activation of TBK1 signaling and production of type I IFNs. We found that Syk was present in both TRAF6- and TRAF3-containing signaling complexes; however, the LPS-dependent, lysine 63-linked ubiquitination of TRAF6 and TRAF3 was oppositely regulated by Syk. We identified the domains of Syk that interacted with TRAF3, TRAF6, TAK1, and TBK1, factors activated by multiple TLRs, which suggests that Syk may act as a common regulator of various TLR responses. Together, our results demonstrate the opposing regulatory roles of Syk in TLR-mediated TRAF6 and TRAF3 signaling pathways, which suggests that Syk may fine-tune the innate immune response to lessen inflammation.

Syk Mediates IL−17-Induced CCL20 Expression by Targeting Act1-Dependent K63-Linked Ubiquitination of TRAF6

“…Combining with previous data that TAK1 is associated with TRAF6 after IL-17R activation (Takaesu et al, 2000;Qian et al, 2007;Walsh et al, 2008), we speculate that Syk is recruited to the complex of Act1/TRAF6 rather than TRAF6/ TAK1 in IL-17R signaling. This suggestion is different from our previous findings on TLRs signaling in murine macrophages where activated Syk is recruited to TRAF6/TAK1 (Lin et al, 2013). Therefore, we suggest that Syk might be a common regulator of TRAF6-dependent signaling and cellular function, whereas its action mode is cell type and stimuli dependent.…”

“…The specific primers for b-actin were forward primer 5 0 -CGGGGACCTG ACTGACTACC-3 0 and reverse primer 5 0 -AGGAAGGCTGGAAGA GTGC-3 0 . The experiment and data analysis were conducted as we previously reported (Lin et al, 2013).…”

“…When ligands (e.g., major histocompatibility complex (MHC), antigens, IgE) attach to immunoreceptors (such as BCR, FcgRI), the Src family kinases Lyn and/or Fyn can phosphorylate the immunoreceptor tyrosine-based activation motif of immunoreceptors or their associated adaptors, in turn recruiting Syk through its N-terminal SH2 domains binding to the phosphorylated immunoreceptor tyrosine-based activation motif for Syk activation (Turner et al, 2000;Tohyama and Yamamura, 2009;Mocsai et al, 2010;de Castro, 2011). Besides activation by receptors whose activation relies on immunoreceptor tyrosine-based activation motif, Syk also can be activated by integrins (Hamerman et al, 2009;Mocsai et al, 2010), toll-like receptors (Lee et al, 2009;Zanoni et al, 2011;Lu et al, 2012;Lin et al, 2013), and tumor necrosis factor (TNF) receptor. Syk activation has an essential role in TNF-induced activation of mitogen-activated protein kinases, NF-kB, and apoptosis in Jurkat T cells (Takada and Aggarwal, 2004), matrix metalloproteinase generation in synoviocytes (Chae et al, 2006), and nitric oxide production in airway epithelial cells (Takada and Aggarwal, 2004;Ulanova et al, 2006).…”

Promising Role of Toll-Like Receptor 8 Agonist in Concert with Prostratin for Activation of Silent HIV

“…8A and B). SYK is a key downstream molecule in various immunoreceptor signaling events, in pre-T cells, mature B cells (90,120,121), and malignancies (122,123): e.g., J-lat cells. MEK, belonging to the mitogen-activated protein kinase (MAPK) pathway, bridged the signaling events induced by prostratin to latency reversion in J-lat cells and in coculture.…”

“…8B and C). This phenomenon might be explained by a SYKdependent inhibition of JNK and p38 phosphorylation (90). Interestingly, inhibition of phospholipase C (PLC) led to enhanced HIV transcription, especially in prostratintreated J-lat monoculture (Fig.…”