Tyrosine phosphorylation of the vav proto-oncogene product links FcepsilonRI to the Rac1-JNK pathway

Stimulation of high affinity IgE Fc receptors (FcepsilonRI) in basophils and mast cells activates the tyrosine kinases Lyn and Syk and causes the tyrosine phosphorylation of phospholipase C-gamma, resulting in the Ca2+- and protein kinase C-dependent secretion of inflammatory mediators. Concomitantly, FcepsilonRI stimulation initiates a number of signaling events resulting in the activation of mitogen-activated protein kinase (MAPK) and c-Jun NH2-terminal kinase (JNK), which, in turn, regulate nuclear responses, including cytokine gene expression. To dissect the signaling pathway(s) linking FcepsilonRI to MAPK and JNK, we reconstructed their respective biochemical routes by expression of a chimeric interleukin-2 receptor alpha subunit (Tac)-FcepsilonRI gamma chain (Tacgamma) in COS-7 cells. Cross-linking of Tacgamma did not affect MAPK in COS-7 cells, but when coexpressed with the tyrosine kinase Syk, Tacgamma stimulation potently induced Syk and Shc tyrosine phosphorylation and MAPK activation. In contrast, Tacgamma did not signal JNK activation, even when coexpressed with Syk. Ectopic expression of a hematopoietic-specific guanine nucleotide exchange factor (GEF), Vav, reconstituted the Tacgamma-induced, Syk- and Rac1-dependent JNK activation; and tyrosine-phosphorylation of Vav by Syk stimulated its GEF activity for Rac1. Thus, these data strongly suggest that Vav plays a critical role linking FcepsilonRI and Syk to the Rac1-JNK pathway. Furthermore, these findings define a novel signal transduction pathway involving a multimeric cell surface receptor acting on a cytosolic tyrosine kinase, which, in turn, phosphorylates a GEF, thereby regulating its activity toward a small GTP-binding protein and promoting the activation of a kinase cascade.