TLR4 phosphorylation at tyrosine 672 activates the ERK/c-FOS signaling module for LPS-induced cytokine responses in macrophages.

Toll-like receptors (TLRs) engage numerous adaptor proteins and signaling molecules, enabling a complex series of post-translational modifications (PTMs) to mount inflammatory responses. TLRs themselves are post-translationally modified following ligand-induced activation, with this being required to relay the full spectrum of pro-inflammatory signaling responses. Here we reveal indispensable roles for TLR4 Y672 and Y749 phosphorylation in mounting optimal lipopolysaccharide (LPS)-inducible inflammatory responses in primary mouse macrophages. LPS promotes phosphorylation at both tyrosine residues, with Y749 phosphorylation being required for maintenance of total TLR4 protein levels and Y672 phosphorylation exerting its pro-inflammatory effects more selectively by initiating ERK1/2 and c-FOS phosphorylation. Our data also support a role for the TLR4-interacting membrane protein SCIMP and SYK kinase axis in mediating TLR4 Y672 phosphorylation to permit downstream inflammatory responses in murine macrophages. The corresponding residue in human TLR4 (Y674) is also required for optimal LPS signaling responses. Our study thus reveals how a single PTM on one of the most widely studied innate immune receptors orchestrates downstream inflammatory responses. This article is protected by copyright. All rights reserved.