PKC activity and PKC-alpha mRNA content are reduced in serum-derived human neuroblastoma cells without concomitant induction of differentiation.

Protein kinase C (PKC) is a serine/threonine kinase which is thought to play an important role in cellular proliferation and differentiation. PKC activity is stimulated physiologically by diacylglycerol and experimentally by phorbol esters. Long-term exposure of human neuroblastoma cells to phorbol esters results in down-regulation of PKC activity and induction of neuronal differentiation. In this study, we explored the hypothesis that reduced PKC expression is necessary for differentiation of the human neuroblastoma cell line SK-N-SH. PKC activity and PKC-alpha mRNA levels were assayed in cultured SK-N-SH cells over a period of several days in the presence or absence of serum. These determinants of PKC expression were compared with several known markers of neuroblastoma differentiation, including neurite outgrowth and steady-state levels of c-myc and GAP43 mRNA. We observed steady losses of PKC activity and PKC-alpha mRNA content after transfer of cells to serum-free or chemically defined media. However, morphological and biochemical differentiation of SK-N-SH cells occurred only in chemically defined medium, perhaps due to the presence of insulin. We conclude that while loss of PKC may be associated with neuroblastoma differentiation, diminished PKC alone is not sufficient to induce or support the differentiation process.