CYCLIC-AMP ACTIVATES THE MITOGEN-ACTIVATED PROTEIN-KINASE CASCADE IN PC12 CELLS

Mitogen-activated protein (MAP) kinases are activated in response to a large variety of extracellular signals, including growth factors, hor mones, and neurotransmitters, which activate distinct intracellular si gnaling pathways. Their activation by the cAMP-dependent pathway, howe ver, has not been reported. In rat pheochromocytoma PC12 cells, we dem onstrate here a stimulation of the MAP kinase isozyme extracellular si gnal-regulated kinase 1 (ERK1) following elevation of intracellular cA MP after exposure of the cells to isobutyl-methylxanthine, cholera tox in, forskolin, or cAMP-analogues. cAMP acted synergistically with phor bol ester, an activator of protein kinase C, in the stimulation of ERK 1. In accordance with this observation, the peptide neurotransmitter p ituitary adenylate cyclase-activating polypeptide 38 (PACAP38), which stimulates cAMP production as well as phosphatidylinositol breakdown i n PC12 cells, was an efficient activator of ERK1. In combination with various growth factors, cAMP acted in a more than additive manner on E RK1 activity. Elevation of intracellular cAMP increased in vivo P-32-l abeling of ERK1, suggesting that cAMP stimulated ERK1 by activating MA P kinase kinase, an immediate upstream activator of ERK1 in the MAP ki nase cascade. Supporting this view, forskolin and a cAMP analogue were found to increase the activity of MAP kinase kinase in PC12 cells, al one as well as in combination with phorbol ester. PACAP38 also stimula ted in vivo P-32-labeling of ERK1 and MAP kinase kinase activity. Fina lly, cAMP or PACAP38 increased by 3-fold nerve growth factor-stimulate d neurite formation in PC12 cells, which may be correlated with the po tentiating effect of these agents on nerve growth factor-stimulated ER K1 activity.