Positive and negative regulation of Fcϵ receptor I‐mediated signaling events by Lyn kinase C‐terminal tyrosine phosphorylation

Abstract

Although it is known that the Src family tyrosine kinase Lyn initiates Fc epsilon receptor I (Fc epsilon RI) signaling by phosphorylation of the receptor subunits, regulation of Lyn kinase activity and its consequences for receptor signaling are incompletely understood. Using a phospho-Lyn-specific antiserum, we show an increased phosphorylation of the Lyn C-terminal regulatory tyrosine and decreased Lyn kinase activity during Fc epsilon RI-mediated mast cell activation. Mutant Lyn, defective in the C-terminal tyrosine, constitutively phosphorylated several substrates in resting cells, but did not cause Fc epsilon RI internalization or spontaneous degranulation. Fc epsilon RI-induced signaling in the presence of constitutively active Lyn exhibited enhanced phosphorylation of the receptor subunits, Syk, LAT, Gab2, phospholipase C (PLC)gamma 1 and PLC gamma 2, and production of phosphatidylinositol 3,4,5-trisphosphate. Although enzymatic activities of PLC gamma 1 and PLC gamma 2 were also up-regulated, amounts of inositol 1,4,5-trisphosphate, mobilization of intracellular calcium and degranulation were suppressed. Additionally, constitutively active Lyn was strikingly less efficient than wild-type Lyn in restoring the receptor-mediated calcium responses in bone marrow mast cells derived from Lyn(-/-) mice. These findings pinpoint the tight regulation of Lyn kinase activity as a critical event in mast cell degranulation.