Regulation of MyD88-dependent signaling events by S nitrosylation retards toll-like receptor signal transduction and initiation of acute-phase immune responses

Nitric oxide (NO) has been thought to regulate the immune system through S nitrosylation of the transcriptional factor NF-kappa B. However, regulatory effects of NO on innate immune responses are unclear. Here, we report that NO has a capability to control Toll-like receptor-mediated signaling through S nitrosylation. We found that the adaptor protein MyD88 was primarily S nitrosylated, depending on the presence of enclothelial NO synthase (eNOS). S nitrosylation at a particular cysteine residue within the TIR domain of MyD88 resulted in slight reduction of the NF-kappa B-activating property. This modification could be restored by the antioxidant glutathione. Through S nitrosylation, NO could negatively regulate the multiple steps of MyD88 functioning, including translocation to the cell membrane after LPS stimulation, interaction with TIRAP, binding to TRAF6, and induction of I kappa B alpha phosphorylation. Interestingly, glutathione could reversely neutralize such NO-derived effects. We also found that an acute febrile response to LPS was precipitated in eNOS-deficient mice, indicating that eNOS-derived NO exerts an initial suppressive effect on inflammatory processes. Thus, NO has a potential to retard induction of MyD88-dependent signaling events through the reversible and oxidative modification by NO, by which precipitous signaling reactions are relieved. Such an effect may reflect appropriate regulation of the acute-phase inflammatory responses in living organisms.