Pituitary adenylyl cyclase-activating peptide stimulates extracellular signal-regulated kinase 1 or 2 (ERK1/2) activity in a ras-independent, mitogen-activated protein kinase ERK kinase 1 or 2-dependent manner in PC12 cells

Sustained activation of extracellular signal-regulated kinase 1/2 (ERK1/2) is critical for initiating differentiation of the PC12 cell to a sympathetic-like neurone. The neuropeptide, pituitary adenylyl cyclase-activating peptide (PACAP), has been demonstrated to cause cell to adopt a neuronal phenotype, although the mechanism of this activity is unclear. PACAP through its type I receptor stimulates a biphasic activation of ERK1/2; a > 10-fold increase within 5 min, followed by a > 5-fold increase that is sustained for greater than or equal to 60 min. An equivalent stimulation is seen in PC12 cells expressing a dominant negative Ras mutant. However, the mitogen-activated kinase/ERK kinase 1/2 (MEK1/2) inhibitor PD98059 blocked both PACAP-induced stimulation of ERK1/2 activity and neurite outgrowth. Thus, activation signal from the PACAP type I receptor on the ERK1/2 cascade pathway is received downstream of Ras, either at Raf or MEK. Down-regulation of protein kinase C or its inhibition by calphostin C blocked the ability of PACAP to stimulate ERK1/2. We conclude that activation of PACAP type I receptor activates protein kinase C, which then activates the ERK1/2 cascade in a Ras-independent manner at either Raf or MEK1/2.