FcepsilonRI beta-chain ITAM amplifies PI3K-signaling to ensure synergistic degranulation response via FcepsilonRI and adenosine receptors.

Simultaneous stimulation with antigen and adenosine in mast cells induces a synergistic degranulation response at a low antigen dose that is insufficient to cause secretion by itself. This kind of stimulation is thought to be relevant to the immediate asthmatic response upon bronchial challenge with low-dose allergen. In this context, FcepsilonRI- and adenosine receptor-mediated signalings cooperate to increase degranulation in mast cells. In the present study, we prepared mast cells that have mutations (Y219F/Y225F/Y229F) in three tyrosine residues of the FcepsilonRI beta-chain (FcRbeta)-ITAM in order to elucidate the molecular mechanisms of degranulation response synergistically elicited by costimulation with low-dose antigen and adenosine. Introduction of mutations in the FcRbeta-ITAM abolished the synergistic degranulation response. Upon costimulation with low-dose antigen and adenosine, tyrosine phosphorylation of Grb2-associated binder 2, which is located upstream of PI3K-signaling, was significantly increased, but severely diminished in FcRbeta-ITAM mutant cells. These findings indicate that FcRbeta acts as a critical element in mast cell synergistic degranulation response through FcepsilonRI and adenosine receptors, and that PI3K-signaling through FcRbeta-ITAM is a crucial participant in augmentation of FcepsilonRI-mediated degranulation by adenosine.