Differential Responses to Nerve Growth Factor and Epidermal Growth Factor in Neurite Outgrowth of PC12 Cells Are Determined by Rac1 Activation Systems

Neurite outgrowth of PC12 cells is induced by nerve growth factor (NGF) but not by epidermal growth factor (EGF). This differential response has been explained by the duration of mitogen-activated protein kinase (MAPK) activation; NGF induces sustained MAPK activation but EGF leads short-lived activation. However, precise mechanisms have not yet been understood. Here we demonstrate the difference between NGF and EGF in regulation of Rac1, a small GTPase involved in neurite outgrowth, in PC12 cells. NGF phosphoinositide 3-kinase dependently induces transient activation of Rac1 and accumulation of active Rac1 at protrusion sites on the cell surface, inducing filamentous actin-rich protrusions and subsequent neurite formation in a Rac1-dependent manner. On the other hand, EGF phosphoinositide 3-kinase independently induces more transient Rac1 activation but neither accumulates active Rac1 nor forms Rac1- and filamentous actin-rich protrusions. Difference in the Rac1 localization between NGF and EGF was also observed with the localization of exogenously expressed green fluorescent protein-tagged Rac1. The Rac1-mediated protrusion by NGF is independent of MAPK cascade, but the subsequent neurite extension requires the cascade. Thus, the differential activation of Rac1 and localization of active Rac1 contribute to the difference in the ability of NGF and EGF to induce neurite outgrowth, and we propose that the MAPK cascade-independent prompt activation of Rac1 and recruitment of active Rac1 at the protrusion sites trigger the initiation of neurite formation.