Insulin and insulin-like growth factor I induce c-fos expression in postmitotic neurons by a protein kinase C-dependent pathway

Recent studies indicate that insulin and insulin-like growth factors regulate development of the nervous system. The signal transduction pathways that mediate the neurotrophic responses to these peptides are largely unknown. The aims of this study were to examine the regulation of c-fos mRNA expression by insulin and insulin-like growth factor I (IGF-I) in postmitotic neurons derived from fetal chick forebrain and to investigate the role of protein kinase C (PKC) in c-fos induction by these growth factors. Cultured neurons were treated with growth factors or other known activators of c-fos gene expression, and then total cellular RNA was isolated and analyzed by Northern analysis using a full-length mouse c-fos cDNA probe. Fetal calf serum, insulin, IGF-I, 12-O-tetradecanoylphorbol (TPA), and cycloheximide induced a 2.2-kilobase c-fos transcript in neurons. Increased levels of c-fos mRNA were apparent as early as 15 min after the addition of insulin. Levels peaked after 60 min and then remained high up to 180 min. The onset of c-fos induction by IGF-I was similar to insulin; however, the response was more transient. Analyses of dose-response curves indicate that insulin and IGF-I stimulate c-fos expression by interacting with their own receptor type. The decay rate of c-fos mRNA was unaltered by insulin or IGF-I. Pretreatment of neurons with actinomycin D (2 microM) for 5 min prior to the addition of insulin completely blocked the induction of c-fos mRNA. TPA increased c-fos mRNA levels with similar kinetics to that of insulin and IGF-I; however, the attenuation phase more closely paralleled that of insulin. Two inhibitors of PKC, sphingosine and staurosporine, completely blocked the induction of c-fos mRNA by insulin, IGF-I, and TPA. These data indicate that insulin and IGF-I stimulate transcription of the c-fos gene in neurons by a PKC-mediated pathway. Increased AP-1 activity may mediate some of the neurotrophic responses to insulin and IGF-I.