Redundant and opposing functions of two tyrosine kinases, Btk and Lyn, in mast cell activation

Protein-tyrosine kinases play crucial roles in mast cell activation through the high-affinity IgE receptor (Fc epsilon RI), In this study, we have mad e the following observations on growth properties and Fc epsilon RI-mediate d signal transduction of primary cultured mast cells from Btk-, Lyn-, and B tk/Lyn-deficient mice. First, Lyn deficiency partially reversed the surviva l effect of Btk deficiency. Second, Fc epsilon RI-induced degranulation and leukotriene release were almost abrogated in Btk/Lyn doubly deficient mast cells while singly deficient cells exhibited normal responses. Tyrosine ph osphorylation of cellular proteins including: phospholipases C-gamma 1 and C-gamma 2 was reduced in Btk/Lyn-deficient mast cells, Accordingly, Fc epsi lon RI-induced elevation of intracellular Ca2+ concentrations and activatio n of protein kinase Cs were blunted in the doubly deficient cells. Third, i n contrast, Btk and Lyn demonstrated opposing roles in cytokine secretion a nd mitogen-activated protein kinase activation. Lyn-deficient cells exhibit ed enhanced secretion of TNF-alpha and IL-2 apparently through the prolonge d activation of extracellular signal-related kinases and c-Jun N-terminal k inase, Potentially accounting for this phenomenon and robust degranulation in Lyn-deficient cells, the activities of protein kinase C alpha and protei n kinase C beta II, low at basal levels, were enhanced in these cells. Four th, cytokine secretion was severely reduced and c-Jun N-terminal kinase act ivation was completely abrogated in Btk/Lyn-deficient mast cells. The data together demonstrate that Btk and Lyn are involved in mast cell signaling p athways in distinctly different ways, emphasizing that multiple signal outc omes must be evaluated to fully understand the functional interactions of i ndividual signaling components.