Abstract
Fc gamma RIIa 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 Fc gamma RIIa 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 Fc gamma RIIa has been well studied, it is not clear how SHP-1 is activated by Fc gamma RIIa to mediate its regulatory effect. Here we report that Fc gamma RIIa 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 Fc gamma RIIa, we demonstrate that SHP-1 associates with the phosphorylated amino-terminal ITAM tyrosine of Fc gamma RIIa, whereas the tyrosine kinase Syk associates with the carboxyl-terminal ITAM tyrosine. Association of SHP-1 with Fc gamma RIIa ITAM appears to suppress total cellular tyrosine phosphorylation. Furthermore, Fc gamma RIIa 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 NF kappa B-dependent gene transcription in THP-1 cells activated by clustering Fc gamma RIIa.
Highlights
YXXL motifs that together make the immunoreceptor tyrosine-based activation motif (ITAM)
Stable transfectants expressing immunoreceptor tyrosine-based activation motif (ITAM) tyrosine mutants of Fc␥RIIa, we demonstrate that SHP-1 associates with the phosphorylated amino-terminal ITAM tyrosine of Fc␥RIIa, whereas the tyrosine kinase Syk associates with the carboxyl-terminal ITAM tyrosine
SHP-1 Is Activated by Fc␥RIIa Clustering—To assess whether SHP-1 is activated by Fc␥RIIa, THP-1 cells were stimulated by clustering Fc␥RIIa with Fab fragments of the receptor-specific monoclonal antibody IV.3, followed by secondary cross-linking with goat F(abЈ)2 fragments of anti-mouse Ig antibody
Summary
YXXL motifs that together make the ITAM. The functional significance of this extended spacer is not fully understood. Clustering of Fc␥R by immune complexes initiates a cascade of signaling events, the first of which is the activation of the Src family of tyrosine kinases that phosphorylate the ITAMs of Fc␥R [5, 6]. The phosphorylated ITAMs serve as docking sites for SH2 domain-containing cytosolic enzymes and enzymeadapter complexes including the tyrosine kinase Syk and the p85 adapter subunit of PtdIns 3-kinase [7]. Association of Syk with the phosphorylated ITAM activates the enzyme resulting in autophosphorylation of Syk and tyrosine phosphorylation of multiple cytosolic proteins [8, 9]. Fc␥RIIa 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. Phagocytosis is a complex process that is accompanied by the generation of reactive oxygen radicals and the production of inflammatory cytokines, which results in tissue damage
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call