Toll-like receptor-mediated tyrosine phosphorylation of paxillin via MyD88-dependent and -independent pathways

Toll-like receptor (TLR)-mediated recognition of pathogens represents one of the most important mechanisms of innate immunity. A proximal signaling event of TLR is the direct binding of an adaptor protein MyD88 to TLR and recruitment of the IL-1R-associated kinase (IRAK). In the present study, we examined the effect of several TLR ligands on protein tyrosine phosphorylation in rat macrophages. Macrophage-activating lipopeptide-2 kDa (MALP2) and lipoarabinomannan were used as activators of TLR2, while lipopolysaccharides (LPS) and lipoteichoic acid were used as TLR4 ligands. All these ligands induced tyrosine phosphorylation of proline-rich tyrosine kinase 2 (Pyk2) and its substrate paxillin, an integrin-associated focal adhesion adaptor protein, in the macrophages. PP2, an inhibitor of Src family tyrosine kinases, prevented the TLR-induced phosphorylation of paxillin and Pyk2 without affecting TLR-induced IRAK activation. MALP2 failed to induce paxillin phosphorylation in the macrophages from MyD88-knockout mice. In contrast, the effect of LPS weakened, but was still observed even in the MyD88-deficient cells. Thus, TLR regulate the function of paxillin in an Src family-dependent mechanism through both MyD88-dependent and MyD88-independent pathways.