PC12 cell activation by epidermal growth factor receptor: role of autophosphorylation sites

PC12 cells have been used as a model system for neuronal differentiation due to their ability to alter their phenotype to a sympathetic neuron-like cell in response to nerve growth factor or fibroblast growth factor. Under some conditions, epidermal growth factor (EGF) can also induce PC12 cells to differentiate. To study signaling from the EGF receptor without the confounding effects of endogenous EGF receptors we generated a chimeric receptor comprised of the ectodomain of platelet-derived growth factor (PDGF) receptor in-frame with the transmembrane and cytoplasmic domains of EGF receptor, termed PER. Expression of PER in PC12 cells confers the ability of PDGF to induce differentiation whereas PDGF has no effect on untransfected PC12 cells. This response is kinase activity-dependent since a kinase-deficient mutant (K721M) fails to induce differentiation in response to PDGF. Mutation of five tyrosine residues that are autophosphorylated in response to EGF either individually or in combination had minimal effects on the ability of these receptors to induce morphological PC12 cell differentiation. The PER mutant with all five autophosphorylation sites mutated to phenylalanine (5YF) was equivalently capable of interacting with several important signaling molecules, including Shc, Grb2, Gab1, phospholipase Cgamma, and Cbl. Furthermore, both the phosphatidylinositol 3-kinase (PI3K)/Akt and Ras/Erk pathways were activated in a sustained manner when PER or 5YF-expressing cells were stimulated with PDGF. Our results show that the five autophosphorylation sites in the extra-kinase C-terminal domain of EGFR are not required for the ability of EGFR to induce morphological differentiation of PC12 cells.