To examine the signals that regulate NeuroD expression, we analyzed the effects of activation of two major signal pathways, the protein kinase A (PKA) pathway and the protein kinase C (PKC) pathway, on the expression of NeuroD in Y79human retinoblastoma cells. Activation of PKC resulted in marked induction of NeuroD mRNA and NeuroD protein. NeuroD mRNA induction was inhibited by calphostin C, an inhibitor of PKC. On the other hand, stimulation of PKA by forskolin had a weak suppressive effect on NeuroD mRNA expression. Induction of NeuroD expression was followed by enhancement of expression of the AK1 gene, one of the target genes of NeuroD, which encodes adenylate kinase isozyme 1, an important enzyme in the cellular adenine nucleotide homeostasis. Our results indicate that NeuroD expression is regulated, at least in part, by the PKC pathway and not by the PKA pathway.