P0, an acidic protein component of the ribosomal protein in eukaryotic 60 S ribosomal subunit, plays an important role in polypeptide chain elongation during translation. To investigate the role of protein kinase C in thyroid cell protein synthesis, we examined the effect of 12-O-tetradecanoyl-phorbol-13-acetate (TPA) on the expression of P0 mRNA and protein. RNA slot blot hybridization revealed that TPA induced the accumulation of P0 mRNA in FRTL-5 cells in a time- and dose-dependent manner. A maximal increase of 2-fold was observed 18 h after addition of TPA. Cycloheximide markedly inhibited the TPA-induced accumulation of P0 mRNA. Nuclear runoff transcription assays using nuclei prepared from TPA-treated FRTL-5 cells revealed that TPA increased the transcription of P0 mRNA but not of beta-actin. Immunoblotting experiments using anti-P protein antibody showed that TPA also increased the protein amount of P0. These results suggest that TPA activates protein synthesis in thyroid cells by inducing the expression of ribosomal proteins.