PITUITARY ADENYLYL CYCLASE-ACTIVATING PEPTIDE STIMULATES EXTRACELLULAR SIGNAL-REGULATED KINASE-1 OR KINASE-2 (ERK1 2) ACTIVITY IN A RAS-INDEPENDENT, MITOGEN-ACTIVATED PROTEIN-KINASE ERK KINASE-1-DEPENDENT OR KINASE-2-DEPENDENT MANNER IN PC12 CELLS/

Sustained activation of extracellular signal-regulated kinase 1/2 (ERK 1/2) 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 ad opt a neuronal phenotype, although the mechanism of this activity is u nclear. PACAP through its type I receptor stimulates a biphasic activa tion of ERK1/2; a > 10-fold increase within 5 min, followed by a > 5-f old increase that is sustained for greater than or equal to 60 min. An equivalent stimulation is seen in PC12 cells expressing a dominant ne gative 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 t he PACAP type I receptor on the ERK1/2 cascade pathway is received dow nstream of Ras, either at Raf or MEK. Down-regulation of protein kinas e 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 i n a Ras-independent manner at either Raf or MEK1/2.