Negative regulation of Fc RI-mediated mast cell activation by a ubiquitin-protein ligase Cbl-b

Aggregation of the high-affinity immunoglobulin E (IgE) receptor (Fc RI) on mast cells induces a number of biochemical events, including protein-tyrosine phosphorylation leading to degranulation and multiple cytokine gene transcription. Here, we have demonstrated that a second member of the Cbl family of ubiquitinprotein ligase Cbl-b translocates into the lipid raft after Fc RI engagement. Overexpression of Cbl-b in the lipid raft inhibits Fc RI-mediated degranulation and cytokine gene transcription through the distinct mechanism. A point mutation of Cys373 in the RING finger domain of Cbl-b abrogates the suppression of Fc RImediated degranulation but not cytokine gene transcription. The antigen-induced tyrosine phosphorylation of Fc RI, Syk, phospholipase C(PLC), activation of c-Jun N-terminal kinase (JNK), extracellular signal regulated kinase (ERK), inhibitor of nuclear factor B kinase (IKK), and Ca influx were all suppressed in the cells overexpressing Cbl-b in the lipid raft. In particular, the expression amount of Gab2 protein and thereby its Fc RImediated tyrosine phosphorylation were dramatically down-regulated by ubiquitinprotein ligase activity of Cbl-b. These results suggest that Cbl-b is a negative regulator of both Lyn-Syk-LAT and Gab2mediated complementary signaling pathways in Fc RI-mediated mast cell activation. (Blood. 2004;103:1779-1786)