Activation of Src Family Members Is Not Required for the Platelet-Derived Growth Factor β Receptor To Initiate Mitogenesis

The basal activity of Src family kinases is readily detectable throughout the cell cycle and increases by two- to fivefold upon acute stimulation of cells with growth factors such as platelet-derived growth factor. Previous reports have demonstrated a requirement for Src activity for the G(1)/S and G(2)/M transitions. With a chimeric α-β PDGF receptor (PDGFR) expressed in fibroblasts, we have investigated the importance of the PDGF-mediated increase in Src activity at the G(0)/G(1) transition for subsequent cell cycle events. A mutant PDGFR chimera that was not able to detectably associate with or activate Src was compromised in its ability to mediate tyrosine phosphorylation of receptor-associated signaling molecules and initiated a submaximal activation of Erk. In contrast to these early cell cycle events, later responses such as entry of cells into S phase and cell proliferation proceeded normally when Src activity did not increase following acute stimulation with PDGF. We conclude that the initial burst of Src activity is required for efficient tyrosine phosphorylation of receptor-associated proteins such as PLCγ, RasGAP, Shc, and SHP-2 and for maximal activation of Erk. Surprisingly, these events are not required for PDGF-dependent cell proliferation. Finally, later cell cycle events do not require that Src be activated at the G(0)/G(1) transition and leave open the possibility that events such as the G(1)/S transition require the basal Src activity and/or activation of Src at later times in G(1).