Although cysteinyl
leukotrienes (cysLTs) are known to be principal inflammatory
lipid mediators released from
IgE-stimulated mast cells, the signaling mechanisms involved in the synthesis of cysLTs remain largely unknown. In the present study, therefore, we investigated the signaling pathway by which
IgE induces cysLTs synthesis after binding to its high affinity receptor (FcepsilonRI) in RBL-2H3 mast cells. We found that
IgE-induced cysLT synthesis is completely abolished in RBL-2H3(Rac-N17) cells, a stable cell line expressing Rac(N17), a dominant negative Rac1 mutant; conversely, synthesis was enhanced in cells expressing Rac(V12), a constitutively active Rac1 mutant, suggesting that Rac1 is a key mediator of
IgE signaling to cysLT synthesis. Further analysis aimed at identifying mediators downstream of Rac1 revealed that pretreating cells with a
protein kinase C-delta (PKC-delta) inhibitor or
infection with an adenoviral vector harboring a dominant negative PKC-delta mutant significantly attenuates
IgE-induced ERKs phosphorylation, cytosolic
phospholipase A(2) phosphorylation/translocation, and cysLT synthesis. In addition, the expression of Rac(N17) blocked PKC-delta translocation and impaired the phosphorylation of ERKs and cytosolic
phospholipase A(2) otherwise elicited by
IgE stimulation. Taken together these results suggest that PKC-delta also plays a critical mediatory role in the
IgE signaling pathway leading to cysLT synthesis, acting downstream of Rac1. Finally, the physiological significance of PKC-delta in the
IgE signaling pathway was demonstrated in an Ag (OVA)-challenged in vivo mouse model, in which induced levels of cysLTs and airway responsiveness in lung airways were significantly diminished by prior i.p. injection of a PKC-delta inhibitor.