The Src family kinase Hck regulates mast cell activation by suppressing an inhibitory Src family kinase Lyn

IgE/antigen-dependent mast cell activation plays a central role in immediate hypersensitivity and other allergic reactions. The Src family tyrosine kinase (SFK) Lyn is activated by the cross-linking of high-affinity IgE receptors (FcRI). Activated Lyn phosphorylates the FcRI subunits, ß and , leading to subsequent activation of various signaling pathways. Lyn also plays a negative regulatory function by activating negative regulatory molecules. Another SFK, Fyn, also contributes to mast cell degranulation by inducing Gab2-dependent microtubule formation. Here we show that a third SFK, Hck, plays a critical role in mast cell activation. Degranulation and cytokine production are reduced in FcRI-stimulated hck^–/– mast cells. The reduced degranulation can be accounted for by defects in Gab2 phosphorylation and microtubule formation. Importantly, Lyn activity is elevated in hck^–/– cells, leading to increased phosphorylation of several negative regulators. However, positive regulatory events, such as activation of Syk, Btk, JNK, p38, Akt, and NF-B, are substantially reduced in hck^–/– mast cells. Analysis of lyn^–/–hck^–/–, lyn^–/–FcRIß^–/–, and hck^–/–FcRIß^–/– cells shows that Hck exerts these functions via both Lyn-dependent and Lyn-independent mechanisms. Thus, this study has revealed a hierarchical regulation among SFK members to fine-tune mast cell activation.