The Differential Role of Protein Kinase C Isozyme in the Rapid Induction of Neurofilament Phosphorylation by Nerve Growth Factor and Phorbol Esters in PC12 Cells

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Abstract
We examined the short‐term regulation of the phosphorylation of the mid‐sized neurofilament subunit (NF‐M) by kinases which were activated in rat pheochromocytoma (PC12) cells by nerve growth factor (NGF) and/or 12‐O‐tet‐radecanoylphorbol 13‐acetate (TPA). We found that NGF and TPA, alone or in combination, increased (a) the incorporation of [32P]Pi into NF‐M and (b) the rate of conversion of NF‐M from a poorly phosphorylated to a more highly phosphorylated form. This was not due to increased synthesis of NF‐M, because NGF alone did not increase NF‐M synthesis and TPA alone or TPA and NGF together inhibited the synthesis of NF‐M. Further, an increase in calcium/phospholipid‐dependent kinase (PKC) activity resulting from the treatment of PC12 cells with NGF and TPA was observed concomitant with the increased phosphorylation of NF‐M. This PKC activity was determined to be derived from the PKCα and PKCβ isozyme. Finally, when PC12 cells were rendered PKC‐deficient by treatment with 1 μM TPA for 24 h, NGF maintained the ability to induce an increase in NF‐M phosphorylation, though not to the level attained in cells which were not PKC‐deficient. These data suggest (hat NGF with or without TPA stimulates NF‐M phosphorylation as a result of a complex series of events which include PKC‐in‐dependent and PKC‐dependent pathways.