MyD88-Syk axis is a critical determinant for inflammatory response in macrophages

Abstract

32 Inflammation, a vital immune response to infection and injury, is mediated by macrophage 33 activation. Spleen tyrosine kinase (Syk) and myeloid differentiation primary response 88 34 (MyD88) regulate the inflammatory response in macrophages, however, their roles and the 35 underlying mechanisms are still largely unknown. In this study, the role of the MyD88-Syk 36 axis and the mechanism by which Syk and MyD88 cooperate during macrophage-mediated 37 inflammatory responses were investigated. Inhibition of Syk or MyD88 decreased both 38 generation of reactive oxygen species (ROS)/reactive nitrogen species (RNS) and consequent 39 ROS/RNS-induced phagocytic activity in lipopolysaccharide (LPS)-stimulated RAW264.7 40 cells. Syk inhibition downregulated expression of ROS/RNS-generating enzymes by inhibiting 41 the nuclear factor-kappa B (NF-B) signaling pathway and phagocytic activity by suppressing 42 suppressor of cytokine signaling 1 (SOCS1) via its nitration in the LPS-stimulated RAW264.7 43 cells. Inhibition of ROS/RNS generation suppressed SOCS1 nitration, leading to a decrease in 44 the phagocytic activity of macrophages. Syk was activated by the interaction with MyD88, and 45 the tyrosine 58 residue (Y58) in the hemi-immunoreceptor tyrosine-based activation motif 46 (ITAM) of MyD88 was critical for their interaction and the consequent activation of Syk in 47 macrophages. Src activated MyD88 by phosphorylation at Y58 via the Src kinase domain. 48 Moreover, LPS-induced formation of filamentous actin (F-actin) and Ras-related C3 botulinum 49 toxin substrate 1 (Rac1) activation induced Src activation. Conclusively, these results suggest 50 that the MyD88-Syk axis plays a pivotal role in macrophage-mediated inflammatory responses 51 by inducing ROS/RNS generation and phagocytic activity via activation of Src and its upstream 52 stimulators, F-actin and Rac1. 53 54