The protein-tyrosine phosphatase SHP-1 associates with the phosphorylated immunoreceptor tyrosine-based activation motif of FcγRIIa to modulate signaling events in myeloid cells

Abstract

FcγRIIa is a low affinity IgG receptor uniquely expressed in human cells that promotes phagocytosis of immune complexes and induces inflammatory cytokine gene transcription. Recent studies have revealed that phagocytosis initiated by FcγRIIa is tightly controlled by the inositol phosphatase SHIP-1, and the protein-tyrosine phosphatase SHP-1. Whereas the molecular nature of SHIP-1 involvement with FcγRIIa has been well studied, it is not clear how SHP-1 is activated by FcγRIIa to mediate its regulatory effect. Here we report that FcγRIIa clustering induces SHP-1 phosphatase activity in THP-1 cells. Using synthetic phosphopeptides, and stable transfectants expressing immunoreceptor tyrosine-based activation motif (ITAM) tyrosine mutants of FcγRIIa, we demonstrate that SHP-1 associates with the phosphorylated amino-terminal ITAM tyrosine of FcγRIIa, whereas the tyrosine kinase Syk associates with the carboxyl-terminal ITAM tyrosine. Association of SHP-1 with FcγRIIa ITAM appears to suppress total cellular tyrosine phosphorylation. Furthermore, FcγRIIa clustering results in the association of SHP-1 with key signaling molecules such as Syk, p85 subunit of PtdIns 3-kinase, and p62dok, suggesting that these molecules may be substrates of SHP-1 in this system. Finally, overexpression of wild-type SHP-1 but not catalytically deficient SHP-1 led to a down-regulation of NFκB-dependent gene transcription in THP-1 cells activated by clustering FcγRIIa.