Staurosporine-induced neurite outgrowth in PC12 cells is independent of protein kinase C inhibition

The protein kinase C (PKC) inhibitor staurosporine, a member of the K252a family of fungal alkaloids that are known as protein kinase inhibitors, induces neurite outgrowth in pheochromocytoma PC12 cells. The progressive staurosporine-induced neurotropic effect (EC₅₀ = 50 nM) has the following characteristics: it is evident after 4 hr of incubation, requires the continuous presence of staurosporine, occurs at 37° but not at 4°, and is not blocked by K252a derivatives. Scanning electron micrographs showed long neurites, ruffling, and dense networks in nerve growth factor (NGF)-treated cells and short neurites, flattening, and smooth cell surface in staurosporine-treated cells. [³H]Staurosporine binding, which was time, temperature, and dose dependent, saturated at 5-10 nM. Other kinase inhibitors were poor competitors. The [³H]staurosporine bound over 20 hr at 37° was poorly dissociated by acetic acid wash or unlabeled staurosporine. These results suggest an uptake process occuring at 37° that is required for the neurotropic effect of staurosporine. NGF did not interfere with staurosporine binding, and staurosporine did not affect NGF receptor binding. At neurotropic concentrations of staurosporine, PKC in PC12 cells was completely inhibited. When PKC activity was down-regulated by prolonged exposure to phorbol myristate acetate, PC12 cells responded to staurosporine with neurite outgrowth similar to that of untreated cells. Although the target and mechanism of the neurotropic effects of staurosporine remain to be determined, the observed effects on PKC-deficient cells indicate that PKC may not be required for the neurotropic effect of this compound in PC12 cells. These results suggest that caution should be taken in the interpretation of staurosporine action in vivo, and they provide a pharmacological tool for the development of potential neurotropic drugs.