PC12 cells provide a model well suited for the study of events associated with peptide factor-stimulated neuronal differentiation. PC12 cells possess high affinity receptors for/and respond to the neurotrophin, nerve growth factor (NGF). Upon addition of NGF to PC12 cultures, cellular replication ceases and once-round cells develop a phenotype characteristic of sympathetic neurons replete with electrically excitable neuritic processes possessing neurotransmitter-containing axonal terminals. While the precise sequence of events responsible for mediating this profound, neurodifferentiative biologic signal remains to be determined, it is clear that protein phosphorylation is a critical component. The cascade of events elicited by NGF is initiated by ligand-induced activation of the high affinity, NGF receptor, p140trkA. This proto-oncogene product is a tyrosine kinase. Once activated, the receptor dimerizes, autophosphorylates on tyrosine residues and recruits important signal transducing molecules through direct protein-protein interactions. In addition to the neurotrophin receptor, p140trkA, PC12 cells express receptors for numerous other peptides including the mitogen, epidermal growth factor (EGF) and the neuroactive peptides, vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase activating protein (PACAP). Like p140trkA, the receptor for EGF is a tyrosine kinase. Upon addition of EGF to PC12 cells, virtually all the biochemical events that immediately follow, particularly protein tyrosine phosphorylation of key signalling substrates, are identical to those observed for NGF. However, unlike NGF, EGF is not capable of stimulating PC12 cells to undergo neuronal differentiation. Addition of VIP or PACAP to PC12 cells also stimulates protein phosphorylation. Phosphate is added primarily to serine residues within the polypeptide substrate and this presumably is mediated by activation of adenylate cyclase, resulting in enhanced generation of cyclic AMP leading to activation of cyclic AMP-dependent protein kinase A (PKA). Commensurate with the addition of VIP or PACAP is the sprouting of short, neuritic processes. Qualitatively, the processes induced by VIP or PACAP are distinct from the more fully extended, more complexely arborized neurites elicited by NGF. Neither EGF nor VIP/PACAP, alone, elicits a robust neuronal differentiative response when added to PC12 cells, however, when added together, PC12 cells undergo a morphologic differentiation analogous to that observed for NGF. In addition, activation of MAP kinase and induction of neurofilaments comparable to that observed following NGF treatment has been noted. Currently, regulation of tyrosine hydroxylase expression and the trophic effects of EGF in combination with VIP/PACAP are being evaluated in order to determine the extent to which these non-neurotrophic peptides replicate the effects of NGF.
