Phorbol ester stimulation of protein kinase C activity and ribosomal DNA transcription. Role in hypertrophic growth of cultured cardiomyocytes.

The mechanism by which phorbol esters induce hypertrophic growth of cardiomyocytes was investigated. Control and 4 alpha-phorbol 12,13-didecanoate-treated myocytes demonstrated a slow rate of growth as measured by the protein/DNA ratio and cell area. In contrast, treatment with phorbol 12-myristate 13-acetate (PMA) stimulated protein accumulation by 34%, while cell area was increased by 68% over control myocytes after 72 h. RNA content in PMA-treated myocytes was 33% higher than in control cells and 4 alpha-phorbol 12,13-didecanoate-treated cells after 72 h. Membrane-associated protein kinase C activity was transiently increased after PMA treatment but returned to normal by 48 h. Cytosolic protein kinase C activity was not significantly altered by PMA. Membrane-associated and cytosolic protein kinase C activities were not altered by 4 alpha-phorbol 12,13-didecanoate. Protein kinase C activity, RNA polymerase I activity, and the transcriptional rate of ribosomal DNA (rDNA) were increased in nuclei isolated from PMA-treated cells. However, consistent with a high rate of processing of pre-ribosomal RNA (pre-rRNA), the pool size of pre-rRNA relative to the 28 S rRNA was unaltered by PMA treatment. These data demonstrated that PMA-induced hypertrophic growth of cardiomyocytes was due to an increase in the capacity for protein synthesis (rRNA), and suggest that this results from protein kinase C mediated increase in the rate of transcription of rDNA.