SEQUENTIAL ACTIVATION OF PHOSHATIDYLINOSITOL 3-KINASE AND PHOSPHOLIPASE C-GAMMA-2 BY THE M-CSF RECEPTOR IS NECESSARY FOR DIFFERENTIATION SIGNALING

Binding of macrophage colony stimulating factor (M-CSF) to its recepto r (Fms) induces dimerization and activation of the tyrosine kinase dom ain of the receptor, resulting in autophosphorylation of cytoplasmic t yrosine residues used as docking sites for SH2-containing signaling pr oteins that relay growth and development signals, To determine whether a distinct signaling pathway is responsible for the Fms differentiati on signal versus the growth signal, we sought new molecules involved i n Fms signaling by performing a two-hybrid screen in yeast using the a utophosphorylated cytoplasmic domain of the wildtype Fms receptor as b ait. Clones containing SH2 domains of phospholipase C-gamma 2 (PLC-gam ma 2) were frequently isolated and shown to interact with phosphorylat ed Tyr721 of the Fms receptor, which is also the binding site of the p 85 subunit of phosphatidylinositol 3-kinase (PI3-kinase), At variance with previous reports, M-CSF induced rapid and transient tyrosine phos phorylation of PLC-gamma 2 in myeloid FDC-P1 cells and this activation required the activity of the PI3-kinase pathway, The Fms Y721F mutati on strongly decreased this activation, Moreover, the Fms Y807F mutatio n decreased both binding and phosphorylation of PLC-gamma 2 but not th at of p85, Since the Fms Y807F mutation abrogates the differentiation signal when expressed in FDC-P1 cells and since this phenotype could b e reproduced by a specific inhibitor of PLC-gamma, we propose that a b alance between the activities of PLC-gamma 2 and PI3-kinase in respons e to M-CSF is required for cell differentiation.