Downstream of Kinase, p62 dok , Is a Mediator of FcγRIIB Inhibition of FcεRI Signaling

Abstract

The low-affinity receptor for IgG, FcγRIIB, is expressed widely in the immune system and functions to attenuate Ag-induced immune responses. In mast cells, coaggregation of FcγRIIB with the high-affinity IgE receptor, FcεRI, leads to inhibition of Ag-induced degranulation and cytokine production. FcγRIIB inhibitory activity requires a conserved motif within the FcγRIIB cytoplasmic domain termed the immunoreceptor tyrosine-based inhibition motif. When coaggregated with an activating receptor (e.g., FcεRI, B cell Ag receptor), FcγRIIB is rapidly phosphorylated on tyrosine and recruits the SH2 domain-containing inositol 5-phosphatase (SHIP). However, the mechanisms by which SHIP mediates FcγRIIB inhibitory function in mast cells remain poorly defined. In this report we demonstrate that FcγRIIB coaggregation with FcεRI stimulates enhanced SHIP tyrosine phosphorylation and association with Shc and p62dok. Concurrently, enhanced p62dok tyrosine phosphorylation and association with RasGAP are observed, suggesting that SHIP may mediate FcγRIIB inhibitory function in mast cells via recruitment of p62dok and RasGAP. Supporting this hypothesis, recruitment of p62dok to FcεRI is sufficient to inhibit FcεRI-induced calcium mobilization and extracellular signal-regulated kinase 1/2 activation. Interestingly, both the amino-terminal pleckstrin homology and phosphotyrosine binding domains and the carboxyl-terminal proline/tyrosine-rich region of p62dok can mediate inhibition, suggesting activation of parallel downstream signaling pathways that converge at extracellular signal-regulated kinase 1/2 activation. Finally, studies using gene-ablated mice indicate that p62dok is dispensable for FcγRIIB inhibitory signaling in mast cells. Taken together, these data suggest a role for p62dok as a mediator of FcγRIIB inhibition of FcεRI signal transduction in mast cells.