Identification of the mechanisms regulating the differential activation ofthe MAPK cascade by epidermal growth factor and nerve growth factor in PC12 cells

In PC12 cells, epidermal growth factor (EGF) transiently stimulates the mit ogen-activated protein (MAP) kinases, ERK1 and ERK2, and provokes cellular proliferation. In contrast, nerve growth factor (NGF) stimulation leads to the sustained activation of the MAPKs and subsequently to neuronal differen tiation. It has been shown that both the magnitude and longevity of MAPK ac tivation governs the nature of the cellular response. The activations of MA PKs are dependent upon two distinct small G-proteins, Has and Rap1, that li nk the growth factor receptors to the MAPK cascade by activating c-Raf and B-Raf, respectively. We found that Res was transiently stimulated upon both EGF and NGF treatment of PC12 cells. However, EGF transiently activated Ra p1, whereas NGF stimulated prolonged Rap1 activation. The activation of the ERKs was due almost exclusively (>90%) to the action of B-Raf, The transie nt activation of the MAPKs by EGF was a consequence of the formation of a s hort lived complex assembling on the EGF receptor itself, composed of Crk, C3G, Rap1, and B-Raf, In contrast, NGF stimulation of the cells resulted in the phosphorylation of FRS2. FRS2 scaffolded the assembly of a stable comp lex of Crk, C3G, Rap1, and B-Raf resulting in the prolonged activation of t he MAPKs. Together, these data provide a signaling link between growth fact or receptors and MAPK activation and a mechanistic explanation of the diffe rential MAPK kinetics exhibited by these growth factors.