Positive and negative regulation of mast cell activation by Lyn via the FcεRI

Aggregation of the high affinity receptor for IgE (Fc epsilon RI) induces activation of mast cells. In this study we show that upon low intensity stimulation of Fc epsilon RI with monomeric IgE, IgE plus anti-IgE, or IgE plus low Ag, Lyn (a Src family kinase) positively regulates degranulation, cytokine production, and survival, whereas Lyn works as a negative regulator of high intensity stimulation with IgE plus high Ag. Low intensity stimulation suppressed Lyn kinase activity and its association with Fc epsilon RI beta subunit, whereas high intensity stimulation enhanced Lyn activity and its association with Fc epsilon RI beta. The latter induced much higher levels of Fc epsilon RI beta phosphorylation and Syk activity than the former. Downstream positive signaling molecules, such as Akt and p38, were positively and negatively regulated by Lyn upon low and high intensity stimulations, respectively. In contrast, the negative regulators, SHIP and Src homology 2 domain-containing protein tyrosine phosphatase-1, interacted with Fc epsilon RI beta, and their phosphorylation was controlled by Lyn. Therefore, we conclude that Lyn-mediated positive vs negative regulation depends on the intensity of the stimuli. Studies of mutant Fc epsilon RI beta showed that Fc epsilon RI beta subunit-ITAM (ITAM motif) regulates degranulation and cytokine production positively and negatively depending on the intensity of Fc epsilon RI stimulation. Furthermore, Lyn-mediated negative regulation was shown to be exerted via the Fc epsilon RI beta-ITAM.