EGF in combination with depolarization or cAMP produces morphological but not physiological differentiation in PC12...

In response to nerve growth factor (NGF) or basic fibroblast growth factor (bFGF) receptor activated Ras/extracellular signal-regulated kinase (ERK) signaling, PC12 cells undergo a prototypical neuronal differentiation program, characterized by neurite extension and upregulation of voltage-gated ion channels. The epidermal growth factor (EGF) receptor also activates Ras/ERK signaling, but produces proliferation instead of differentiation. In the presence of depolarizing concentrations of KCl, however, EGF elicits neurite outgrowth through the synergistic actions of the Ras/ERK and cAMP signaling pathways. To assess if EGF and KCl/cAMP elicit the same suite of differentiation events as does NGF and bFGF, we used patch clamp recording to determine if EGF in the presence of KCl or a cAMP agonist also induced physiological differentiation as defined by upregulation of ion channels. Chronic NGF treatment of PC12 cell cultures elicited robust morphological differentiation, a threefold increase in mean calcium channel current density, and an eightfold increase in mean sodium channel current density. Sibling cultures chronically treated with EGF in the presence of high KCl or a cAMP agonist also displayed morphological differentiation, but had calcium channel current densities which were no larger than untreated, undifferentiated cells. Additionally, the increase in mean sodium channel current density induced by EGF in the presence of KCl or cAMP was no greater than the increase observed with EGF alone. Thus, although EGF in the presence of KCl or cAMP is sufficient to induce morphological differentiation as defined by neurite outgrowth, synergism of the Ras/ERK and cAMP/PKA signaling pathways is not sufficient to promote the fully physiologically differentiated PC12 phenotype.