H. Lienard et al., Signal regulatory proteins negatively regulate immunoreceptor-dependent cell activation, J BIOL CHEM, 274(45), 1999, pp. 32493-32499
Signal regulatory proteins of the alpha subtype (SIRP alpha) are ubiquitous
molecules of the immunoglobulin superfamily that negatively regulate prote
in tyrosine kinase receptor-dependent cell proliferation. Their intracytopl
asmic domain contains four motifs that resemble immunoreceptor tyrosine-bas
ed inhibition motifs (ITIMs) and that, when tyrosyl-phosphorylated, recruit
cytoplasmic SH2 domain-bearing protein tyrosine phosphatases (SHPs). ITIMs
are borne by molecules that negatively regulate cell activation induced by
receptors bearing immunoreceptor tyrosine-based activation motifs (ITAMs),
Because SIRP alpha are coexpressed with ITAM-bearing receptors in hematopo
ietic cells, we investigated whether SIRP alpha could negatively regulate I
TAM-dependent cell activation. We found SIRPa transcripts in human mast cel
ls, and we show that a chimeric molecule having the transmembrane and intra
cytoplasmic domains of SIRP alpha could inhibit IgE-induced mediator secret
ion and cytokine synthesis by mast cells. Inhibition required that the SIRP
alpha chimera was coaggregated with ITAM-bearing high affinity IgE recepto
rs (Fc epsilon RI). It was correlated with the tyrosyl phosphorylation of t
he SIRP alpha chimera and the recruitment of SHP-1 and SHP-2. The phosphory
lation of Fc epsilon RI ITAMs was decreased; the mobilization of intracellu
lar Ca2+ and the influx of extracellular Ca2+ were reduced, and the activat
ion of the mitogen-activated protein kinases Erk1 and Erk2 was abolished. S
IRP alpha can therefore negatively regulate not only receptor tyrosine kina
se-dependent cell proliferation but also ITAM-dependent cell activation.