PHOSPHOLIPASE-C-GAMMA-1 AND PHOSPHATIDYLINOSITOL-3 KINASE ARE THE DOWNSTREAM MEDIATORS OF THE PDGF RECEPTORS MITOGENIC SIGNAL

Upon ligand-induced tyrosine phosphorylation, the platelet-derived growth factor (PDGF) receptor (PDGFR) beta subunit associates with PLC-gamma1, RasGAP, PI3K, and a 64 kd protein. To determine the relative role of each of these associated proteins in PDGFR signaling, we constructed a PDGFR mutant (F5) unable to bind any of them and a panel of ''add-back'' mutants that could bind only one of the receptor-associated proteins. F5 PDGFR failed to activate PLC-gamma1, PI3K, or Ras and was unable to trigger DNA synthesis. Permitting association of F5 PDGFR with either PLC-gamma1 or PI3K restored Ras activation and a mitogenic response. Surprisingly, even though binding of the 64 kd protein almost fully restored Ras activation, it did not rescue the receptor's ability to trigger DNA synthesis. Thus Ras activation is insufficient to trigger PDGF-dependent DNA synthesis, and PLC-gamma1 and PI3K are independent downstream mediators of PDGFs mitogenic signal.