Editors' ChoiceInflammation

TRAF6 phosphorylation inhibits inflammation

Sci. Signal.  03 Mar 2015:
Vol. 8, Issue 366, pp. ec49
DOI: 10.1126/scisignal.aab0085

Toll-like receptors (TLRs) recognize pathogen- and host-associated molecular patterns to stimulate the transcription factor nuclear factor κB (NF-κB), which leads to the production of proinflammatory cytokines. The E3 ubiquitin ligase and adaptor protein TRAF6 is activated downstream of TLRs and undergoes homo-oligomerization and nondegradative autoubiquitylation, which is required for the recruitment of effectors that activate NF-κB signaling. Jiao et al. found that exposure of THP-1 cells, a human monocytic cell line, to the TLR4 ligand lipopolysaccharide (LPS) resulted in an early decrease in the abundance of mRNA encoding the serine and threonine kinase MST4, which was accompanied by the increased abundance of mRNAs encoding the proinflammatory cytokines tumor necrosis factor (TNF) and interleukin-6 (IL-6). A subsequent reduction in cytokine-encoding gene expression occurred concurrently with an increase in MST4 expression. Overexpression of MST4 in transfected THP-1 cells decreased the amounts of TNF and IL-6 produced in response to LPS, whereas knockdown of MST4 in these cells had the opposite effect. This inhibitory effect of MST4 on TLR4 signaling required its kinase activity. In transfected cells, MST4 inhibited the TRAF6-dependent activation of NF-κB. Coimmunoprecipitation experiments with THP-1 cells showed that MST4 interacted with TRAF6 and that the interaction decreased soon after stimulation of the cells with LPS, but increased at later time points. Overexpression of MST4 inhibited the LPS-stimulated autoubiquitylation of TRAF6. In vitro kinase assays showed that MST4 phosphorylated TRAF6 at two threonine residues in its C-terminal TRAF domain, and these residues were necessary for MST4-mediated inhibition of TRAF6 auto-ubiquitylation. Furthermore, MST4-dependent phosphorylation of TRAF6 prevented homo-oligomerization. Finally, mice in which MST4 was knocked down by injection of siRNA (MST4-KD mice) showed increased lung damage when subjected to septic shock, which was associated with the increased abundance of proinflammatory cytokines. Cell-depletion analysis of LPS-treated MST4-KD mice suggested that MST4 inhibited TLR signaling mostly in macrophages. Together, these data suggest that MST4 limits inflammatory responses by blocking the function of TRAF6.

S. Jiao, Z. Zhang, C. Li, M. Huang, Z. Shi, Y. Wang, X. Song, H. Liu, C. Li, M. Chen, W. Wang, Y. Zhao, Z. Jiang, H. Wang, C. C. L. Wong, C. Wang, Z, Zhou, The kinase MST4 limits inflammatory responses through direct phosphorylation of the adaptor TRAF6. Nat. Immunol. 16, 246–257 (2015). [PubMed]