Activation of Toll-like receptors induces macrophage migration via the iNOS/Src/FAK pathway

Macrophage motility is crucial in innate immunity. Toll-like receptors (TLRs), members of the interleukin-1 receptor superfamily, are a family of transmembrane pattern recognition receptors that are expressed primarily in antigen-expressing cells such as macrophages. These interleukin-1 receptors play a key role in immune response by recognizing highly conserved microbial molecules known as pathogen-associated molecular patterns. Studies have shown that when the macrophage-like cell line RAW264.7, peritoneal macrophages, and bone marrow-derived macrophages are exposed to lipopolysaccharide (LPS, TLR4 ligand), peptidoglycan (PGN, TLR2 ligand), polyinosinic-polycytidylic acid (polyI:C, TLR3 ligand), or CpG-oligodeoxynucleotides (CpG, TLR9 ligand), there is a marked increase in cell motility and in levels of activated Src (but not Fgr, Hck, and Lyn). Attenuation of Src suppressed LPS-, PGN-, polyI:C-, and CpG-elicited movement as well as the level of FAK Pi-Tyr861, which can be reversed by reintroduction of siRNA-resistant Src. Furthermore, siRNA knockdown of FAK decreased the degree of TLR-mediated motility of macrophages. Strikingly, LPS-, PGN-, polyI:C-, and CpG-enhanced Src expression, FAK Pi-Tyr861, and cell motility were greatly reduced in macrophages devoid of inducible nitric oxide synthase (iNOS, a NF-κB target), which can be induced by the aforementioned TLR ligands. Because NO can upregulate the expression and activity of Src, and because Src can mediate NF-κB activation, we hypothesize that there is a loop of signal amplification that influences the iNOS/Src/FAK axis for macrophage locomotion in response to engagement of TLRs.