Rac and phosphatidylinositol 3-kinase regulate the protein kinase B in FcεRI signaling in RBL 2H3 mast cells

Fc epsilon RI signaling in rat basophilic leukemia cells depends on phosphatidylinositol 3-kinase (PU-kinase) and the small GTPase pac, Here, we studied the functional relationship among PI3-kinase, its effector protein kinase B (PKB), and Rac using inhibitors of PI3-kinase and toxins inhibiting Rac. Wortmannin, an inhibitor of PI3-kinase, blocked Fc epsilon RI-mediated tyrosine phosphorylation of phospholipase C gamma, inositol phosphate formation, calcium mobilization, and secretion of hexosaminidase. Similarly, Clostridium difficile toxin B, which inactivates all Rho GTPases including Rho, Rac and Cdc42, and Clostridium sordellii lethal toxin, which inhibits Rac (possibly Cdc42) but not Rho, blocked these responses. Stimulation of the FceRI receptor induced a rapid increase in the GTP-bound form of Rac. Whereas toxin B inhibited the Rac activation, PI3-kinase inhibitors (wortmannin and LY294002) had no effect on activation of Rac. In line with this, wortmannin had no effect on tyrosine phosphorylation of the guanine nucleotide exchange factor Vav. Wortmannin, toxin B, and lethal toxin inhibited phosphorylation of PKB on Ser(473). Similarly, translocation of the pleckstrin homology domain of PKB tagged with the green fluorescent protein to the membrane, which was induced by activation of the FceRI receptor, was blocked by inhibitors of PU-kinase and Rac inactivation. Our results indicate that in rat basophilic leukemia cells Rac and PU-kinase regulate PKB and suggest that Rac is functionally located upstream and/or parallel of PI3-kinase/PKB in Fc epsilon -RI signaling.