The Protein Tyrosine Phosphatase SHP-1 Associates with the Phosphorylated ITAM of FcγRIIa to Modulate Signaling Events in Myeloid Cells

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. While 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 ITAM tyrosine mutants of FcγRIIa, we demonstrate that SHP-1 associates with the phosphorylated amino-terminal ITAM tyrosine of FcγRIIa, while the tyrosine kinase Syk associates with the carboxy-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-3kinase and p62dok, suggesting that these molecules may be substrates of SHP-1 in this system. Finally, over-expression of wildtype SHP-1 but not catalytic-deficient SHP-1 led to a down regulation of NFκBdependent gene transcription in THP-1 cells activated by clustering FcγRIIa. 3 by gest on O cber 5, 2017 hp://w w w .jb.org/ D ow nladed from