Suppression of nerve growth factor-directed neurite outgrowth in PC12 cells by sphingosine, an inhibitor of protein kinase C.

The formation of vertebrate neural circuitry is regulated in part by neurotrophic agents, such as nerve growth factor (NGF); however, the biochemical mechanisms involved in neurite outgrowth have yet to be completely resolved. Phorbol ester tumor promoters are known to influence the extension of neurites in a variety of neurodevelopmental systems, and protein kinase C, the major phorbol ester receptor, has been implicated in this process. In the present study, sphingosine, a specific pharmacological inhibitor of protein kinase C, was employed to investigate the role of this enzyme in the elaboration of neurites in PC12 pheochromocytoma cells. Normally, PC12 cells respond to NGF by morphologically differentiating into sympathetic neuron-like cells, exhibiting a marked hypertrophy, and extending slender neurites piloted by well defined growth cones. The elaboration of NGF-induced neurites was found to be reversibly inhibited by sphingosine in a dose-dependent manner (IC50 = 2.5-5 microM), while similar concentrations of several structural analogs were inactive. The suppression of neurite outgrowth by sphingosine was antagonized by the addition of 12-O-tetradecanoylphorbol 13-acetate (TPA), which binds to and directly activates protein kinase C. In the presence of NGF, TPA treatment increased the incidence of neurite outgrowth, and this increase, in turn, was antagonized by sphingosine. The binding of [3H]phorbol 12,13-dibutyrate to specific phorbol ester binding sites in PC12 cells was inhibited by sphingosine at concentrations similar to those which inhibited neurite outgrowth. The effects of sphingosine on TPA-directed protein phosphorylation were examined in situ, revealing inhibition of [32P]phosphate incorporation into cellular proteins. The specific TPA-directed phosphorylation of tyrosine hydroxylase was inhibited by sphingosine, as was the resulting increase in enzyme activity. The effects of sphingosine on the levels of alpha- and beta-tubulin mRNAs were also examined in an effort to delimit the locus of protein kinase C action. Concentrations of sphingosine which suppressed neurite outgrowth did not inhibit the NGF-directed elevation of tubulin transcript levels. Taken together, these results reveal the presence of a sphingosine-sensitive pathway in neurite outgrowth and indicate that protein kinase C plays a role in mediating the neuritogenic effects of NGF. Furthermore, the results suggest that protein kinase C acts at a distal segment of the neurite growth pathway.