Hierarchical analysis of the nerve growth factor‐dependent and nerve growth factor‐independent differentiation signaling pathways in PC12 cells with protein kinase inhibitors

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Abstract
The effects of a series of protein kinase inhibitors on nerve growth factor (NGF)‐dependent and NGF‐independent neurite outgrowth in PC12 cells have established an ordered relationship among those protein kinases sensitive to down regulation by bryostatin, stimulation by staurosporine, inhibition by sphingosine, or inhibition by 6‐thioguanine (6‐TG). Quantitation of the biphasic staurosporin effects on NGF‐induced neurite outgrowth (Hashimoto and Hagino: J Neurochem 53:1675‐1685, 1989) gave an IC50 of 2‐4 nM for inhibition and an EC50 of 15‐20 nM for induction of neurite extension. Both sphingosine and 6‐TG inhibited neurite outgrowth induced by staurosporine and basic fibroblast derived growth factor (bFGF), as well as by NGF; therefore, sphingosine‐ and 6‐TG‐sensitive protein kinase steps occur after the convergence of the NGF, bFGF, and staurosporine signal pathways. Down regulation of protein kinase C by bryostatin chronic treatment, which inhibits NGF‐ and bFGF‐induced neuritogenesis (Singh et al.: Biochemistry 33:542‐551, 1994), did not inhibit the staurosporine‐induced neurite outgrowth. Thus, the bryostatin‐sensitive protein kinase C must occur subsequent to the convergence of the bFGF and NGF pathways, but before (or parallel to) staurosporine initiation of neurite outgrowth. In contrast, low concentrations of phorbol myristoyl acetate (PMA) or bryostatin, which activate protein kinase C activity, enhanced the staurosporine‐ or NGF‐induced neurite extension. These data indicate that stimulation of one or more protein kinase C isozymes can synergistically interact with the signaling pathway to increase the rate of neuritogenesis. Inhibition by 5‐7.5 nM staurosporine acted rapidly to arrest and decrease development of neurites up to 24 hr after NGF treatment, as did K252a and NGF ployclonal antibody addition. Our cellular data support the concept that staurosporine acts to inhibit the NGF receptor Trk (Nye et al.: Mol Biol Cell 3:677‐686, 1992), but that downstream steps can be activated by the higher concentration of staurosporine to bypass Trk and lead to neurite generation. Effects of staurosporine, 6‐TG, and sphingosine on c‐fos gene induction with or without NGF were not correlated with the generation of neurites. The sequence of protein kinases sensitive to these effectors appears to be in the order (but not consecutive) bryostatin, staurosporine, sphingosine, and 6‐TG.