OVEREXPRESSION OF SHC PROTEINS POTENTIATES THE PROLIFERATIVE RESPONSETO THE GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR AND RECRUITMENT OF GRB2 SOS AND GRB2/P140 COMPLEXES TO THE BETA-RECEPTOR SUBUNIT/

The high affinity receptor for GM-CSF consists of a unique a subunit a nd a beta subunit that is shared with receptors for IL-3 and IL-5. Act ivation of GM-CSF receptor (GMR) triggers two distinct cytoplasmic sig nalling pathways, JAK2, and Pas, and is sufficient to maintain prolife ration of growth factor-dependent cell lines, She proteins are phospho rylated upon activation of GMR and may be involved in the transmission of GM-CSF signals to Pas. To define the role of She proteins in cells stimulated with GM-CSF, we investigated both the network of interacti ons that involve She after GM-CSF stimulation and the effects of overe xpressing She proteins on the proliferative response to GM-CSF. Two cy toplasmic complexes, Grb2/Sos and Grb2/p140 bind through the Grb2 SH2 domain to phosphorylated She, and are thereby recruited to the beta su bunit, Both complexes are stable, even in the absence of ligand, and d epend on the direct association of p140 and Sos respectively with the SH3 domains of Grb2, p140 is an uncharacterized protein constitutively phosphorylated on tyrosine and, in its Grb2-bonnd form, expressed onl y in hematopoietic cells, the oligomeric complex formed by phosphoryla ted beta subunit-phosphorylated Shc-Grb2-SoS-p140 is also induced by I L-3 and L-5 stimulation of growth-factor dependent cell lines, Overexp ression of wild-type She proteins in growth factor-dependent cells inc reases both MAP kinase activation and proliferation in response to GM- CSF, These effects require the association of She with Grb2, Taken tog ether these results indicate that phosphorylation of She proteins is a crucial step in the transmission of GM-CSP proliferative stimuli, sin ce it creates a high affinity binding site for the Grb2/SoS complex, w hose function is to activate Ras and, for the Grb2/p140 complex, whose function remains unknown.