Protein Kinase C and Its 80-Kilodalton Substrate Protein in Neuroblastoma Cell Neurite Outgrowth

A potential role of the protein kinase C (PKC) system in differentiation of human neuroblastoma cell line LA-N-5 was investigated. It was found that neurite outgrowth induced by 12-O-tetradecanoylphorbol 13-acetate (TPA, 81 nM) was associated with a down-regulation of PKC as determined independently by immunocytochemistry, immunoblot, and enzyme activity assay. Down-regulation of PKC in cells induced to differentiate by retinoic acid (1 microM) was less pronounced, whereas it was undetected in cells induced to differentiate by nerve growth factor (100 ng/ml). The in vitro phosphorylation of an 80-kilodalton protein present in control LA-N-5 cells or in cells treated with TPA, retinoic acid, or nerve growth factor for 1 day decreased to various extents at days 4 or 7 concomitant with neuritogenesis. Pretreatment of LA-N-5 cells with a high concentration (1 microM) of TPA to deplete cellular PKC rendered the cells unresponsive to the differentiating effect of the agents. It was observed that CHP-100 cells, another human neuroblastoma line shown to be resistant to differentiation induced by the agents, had a reduced PKC level and the amount of in vitro phosphorylation of the 80-kilodalton protein was greatly reduced in control cells and remained relatively unchanged when the cells were treated with the agents for up to 7 days. The present studies suggested that PKC and its 80-kilodalton substrate protein were likely involved in initiation and/or progression of LA-N-5 cell differentiation induced by TPA and that separate PKC-independent pathways might also be involved in the differentiating effect of retinoic acid or nerve growth factor.