Calcium influx activates extracellular-regulated kinase/mitogen-activated protein kinase pathway through a calmodulin-sensitive mechanism in PC12 cells

Evidence suggests that membrane depolarization is able to promote neuronal survival through a sustained, although moderate, increase in the intracellu lar calcium. We have used the PC12 cell line to study the possible intracel lular pathways that can be activated by calcium influx. Previously, we obse rved that membrane depolarization-induced calcium influx was able to activa te the extracellular-regulated kinase (ERK)/mitogen-activated protein kinas e pathway and most of this activation was calmodulin-dependent. We demonstr ated that a part of the ERK activation is due to the phosphorylation of the epidermal growth factor receptor. Here, we show that both the epidermal gr owth factor receptor phosphorylation and the Shc-Grb2-Ras activation are no t calmodulin-modulated. Moreover, dominant negative mutant Ha-ras (Asn-17) prevents the activation on ERKs by membrane depolarization, suggesting that Pas and calmodulin are both necessaries to activate ERKs by membrane depol arization. We failed to observe any significant induction and/or modulation of the A-Raf, B-Raf or c-Raf-l kinase activities, thus suggesting the exis tence of a MEK kinase different from the classical Raf kinases that directl y or indirectly can be modulated by Ca2+/calmodulin.