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HIGH AFFINITY IgE RECEPTOR MEDIATED PROTEIN TYROSINE PHOSPHORYLATION: EVIDENCE FOR A NOVEL SIGNAL TRANSDUCTION PATHWAY

The rat basophilic leukemia cell line (RBL) is an excellent model for the investigation of IgE-mediated signal transduction. Recently, we demonstrated that aggregation of the high affinity IgE receptor (FcєRI) results in rapid tyrosine phosphorylation of a 72 kDa molecule (pp72). Here we investigated the relationship of pp72 phosphorylation to previously known signal transduction signals, in particular G protein activation and phosphatidylinositol (PI) hydrolysis by phospholipase C (PLC). Activation of G proteins by sodium fluoride or the non-hydrolysable GTP analog GTPrS induced both PI hydrolysis and histamine secretion without tyrosine phosphorylation of pp72. Similarly, in RBL cells transfected with the G protein coupled muscarinic acetylcholine receptor, stimulation with the agonist carbachol activated phospholipase C and histamine secretion without pp72 phosphorylation. Therefore, pp72 phosphorylation was not induced by G protein activation, or as a consequence of PI hydrolysis. To investigate whether pp72 phosphorylation precedes the activation of phospholipase C, we studied the effect of the tyrosine kinase inhibitor genistein. Preincubation of the cells with genistein decreased in parallel antigen-induced tyrosine phosphorylation of pp72 (IC50 = 3μg/ml) and secretion of histamine (IC50 =34μg/ml). In contrast, genistein in concentrations of up to 60 μg/ml did not inhibit PI hydrolysis nor did it change the amount of the calcium releasing secondary messenger inositol (1, 4, 5) trisphosphate. These results suggest that pp72 phosphorylation represents a novel, independent signal transduction pathway induced specifically by activation of FcєRl.