A Crucial Role for the Mitogen-Activated Protein Kinase Pathway in Nicotinic Cholinergic Signaling to Secretory Protein Transcription in Pheochromocytoma Cells

Abstract

The mitogen-activated protein kinase (MAPK) pathway plays a pivotal role in intracellular signaling, and this cascade may impinge on cAMP response elements (CREs) of target genes. Both the MAPK pathway and chromogranin A expression may be activated by cytosolic calcium influx, and calcium-dependent signals map onto the chromogranin A promoter proximal CRE. We therefore probed the role of the MAPK pathway in chromogranin A biosynthesis after secretory stimulation of PC12 pheochromocytoma cells by the nicotinic cholinergic pathway, the physiological secretory trigger. Chemical inhibition of either MAPK or MAPK kinase blocked the response of a transfected chromogranin A promoter to nicotine or protein kinase C activation [by phorbol-12-myristate-13-acetate (PMA)], although nicotine-evoked catecholamine secretion was unaffected. Activation of the MAP kinase cascade (Ras, Raf, MAPK, or CREB kinase) by cotransfection of pathway components stimulated the chromogranin A promoter. Cotransfection of MAPK pathway dominant negative mutants (for Raf, MAPK, or CREB kinase) blocked nicotinic or PMA activation of chromogranin A, although a dominant negative Ras mutant was without effect. MAPK pathway enzymatic activity was stimulated by both nicotine and PMA. Point mutations of the chromogranin A CRE suggested that this element was necessary incis for stimulation by nicotine, PMA, or chemical activation of the MAPK pathway. Transfer of the CRE to a heterologous promoter conferred inducibility by not only nicotine or cAMP but also MAPK activation. Expression of the CREB antagonist KCREB blocked the response of the chromogranin A promoter to nicotine, cAMP, or MAPK pathway activation by either chemical stimulation or cotransfection of active cascade components. Chromogranin A mRNA responded to MAPK pathway manipulation in a fashion similar to the transfected chromogranin A promoter, in both direction and magnitude. We conclude that the MAPK pathway is a necessary intermediate in signaling from the nicotinic receptor to secretory protein transcription, although not to catecholamine secretion. In trans, this response seems to involve the following signal cascade: protein kinase C → Raf → MAPK kinase → MAPK → CREB kinase → CREB. In cis, activation by the cascade maps onto the chromogranin A promoter proximal CRE, which is both necessary and sufficient to confer the response.