Cultured PC12 cells provide a model well suited for investigating events associated with peptide factor-stimulated neuronal differentiation. PC12 cells possess high affinity receptors for/and respond to the neurotrophin, nerve growth factor (NGF). The neural differentiative molecular signals elicited by NGF are primarily mediated through protein kinase cascades initiated by ligand-induced activation of the high affinity, NGF receptor tyrosine kinase p140trkA. In addition to p140trkA, PC12 cells are decorated with receptors for numerous other peptides including the neuroactive peptide, pituitary adenylate cyclase activating protein (PACAP). In contrast to the unimembrane-spanning, trkA-NGF tyrosine kinase receptor, PACAP binds to and activates receptors that belong to the seven membrane-spanning, G-binding protein coupled family of receptors. Addition of PACAP to PC12 cells results in activation of both adenylate cyclase and phospholipase C resulting in subsequent stimulation of cyclic AMP-dependent protein kinase A (PKA) and protein kinase C (PKC)phosphorylating activities. Treatment of PC12 cells with PACAP induces sprouting of neuritic processes that are qualitatively dissimilar to the more robust, complexely arborized neurites elicited by NGF. The morphologic, neurodifferentiative effects of PACAP are achieved without usurping molecules involved in the initiation of signaling cascades activated by the well-characterized neurotrophic factor, NGF. The focus of current research is on ligand-mediated activation of the Ras/Raf/MEK/MAPK signal transduction pathway demonstrated to play a critical role in transduction of neurodifferentiative signals elicited by NGF. As is the case for NGF, intracellular signaling pathways activated by PACAP converge on this protein kinase signal transduction cassette. Although the effects of PACAP in PC12 cells have been linked to activation of PKA, pretreatment with the PKA-selective antagonist, H-89, failed to inhibit PACAP-elicited stimulation of MAPK and the induction of neurite outgrowth. However, pharmacologic inhibition of protein kinase C blocks activation of MAPK and prevents the outgrowth of neuritic processes, a hallmark feature of morphologic neuronal differentiation. Furthermore, in contrast to the situation for NGF, the effects of PACAP38 on PC12 cell morphology occur independently from the activities of both Ras and Src. More recently, experimental results demonstrate that PACAP triggers a heterologous upregulation of the trkA-NGF tyrosine kinase receptor. This novel observation provides important new information regarding the manner in which expression of the NGF-trkA receptor is regulated.
