| Abstract: |
The low-affinity receptor for IgG, FcgammaRHB, is expressed widely in the immune system and functions to attenuate Ag-induced immune responses. In mast cells, coaggregation of FcgammaRHB with the high-affinity IgE receptor, FeepsilonRI, leads to inhibition of Ag-induced degranulation and cytokine production. FcgammaRHB inhibitory activity requires a conserved motif within the FcgammaRHB cytoplasmic domain termed the immunoreceptor tyrosine-based inhibition motif. When coaggregated with an activating receptor (e.g., FcepsilonRI, B cell Ag receptor), FcgammaRHB is rapidly phosphorylated on tyrosine and recruits the Slit domain-containing inositol 5-phosphatase (SHIP). However, the mechanisms by which SHIP mediates FcgammaRHB inhibitory function in mast cells remain poorly defined. In this report we demonstrate that FcgammaRHB coaggregation with FcepsilonRI stimulates enhanced SHIP tyrosine phosphorylation and association with She and p62(dok). Concurrently, enhanced p62(dok) tyrosine phosphorylation and association with RasGAP are observed, suggesting that SHIP may mediate FcgammaRHB inhibitory function in mast cells via recruitment of p62(dok) and RasGAP. Supporting this hypothesis, recruitment of p62(dok) to FcepsilonRI is sufficient to inhibit FeepsilonRI-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 p62(dok) 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 p62(dok) is dispensable for FcgammaRHB inhibitory signaling in mast cells. Taken together, these data suggest a role for p62(dok) as a mediator of FcgammaRHB inhibition of FcepsilonRI signal transduction in mast cells. |
