Immediate early gene expression in PC12 cells exposed to lead: Requirementfor protein kinase C

We previously demonstrated induction of c-fos mRNA in PC12 cells exposed to lead that was dependent on new transcription. In the current work, we exam ined two signal transduction mechanisms that are activated by lead and have been shown to mediate induction of c-fos mRNA. One mechanism involves prot ein kinase C, and the other requires calmodulin-dependent protein kinase II . Significant increases in the levels of c-fos, c-jun, and egr-1 but not NG FIB mRNA were observed in PC12 cells exposed to lead or phorbol 12-myristat e 13-acetate. In contrast, PC12 cells depolarized with 56 mM K+ displayed a n increase in c-fos, egr-1, and NGFIB but not c-jun mRNA. Similar to other activators of protein kinase C, lead increased AP-1 and Egr-1 DNA binding a ctivity. Additionally, lead increased luciferase activity in cerebellar gra nule cells transfected with an AP-1 luciferase reporter construct. Lead did not increase c-fos mRNA in PC12 cells that were depleted of protein kinase C by a 24-h treatment with phorbol 12,13-dibutyrate or incubated with the protein kinase C inhibitor H-7. In contrast, an inhibitor of calmodulin-dep endent protein kinase, KN-62, and an inhibitor of calmodulin, W-7, did not block the induction of c-fos mRNA by lead. An increase in serum-response el ement DNA-binding activity was observed in nuclear extracts from PC12 cells exposed to lead. It is interesting that lead activated protein kinase C is oforms delta and epsilon, but not isoforms alpha and beta. In conclusion, l ead appears to induce the expression of immediate early genes by a mechanis m that requires protein kinase C.