Regulation of tumor suppressor PDCD4 by novel protein kinase C isoforms

Transforming growth factor-β1 (TGF-β1) induces apoptosis in normal hepatocytes and hepatoma cells. PDCD4 is involved in TGF-β1-induced apoptosis via the Smad pathway. The tumor promoter 12-O-tetradecanoylphorbor-13-acetate (TPA), a protein kinase C stimulator, inhibits TGF-β1-induced apoptosis. However, the mechanisms of TPA action on PDCD4 expression remain to be elucidated. Therefore. the regulatory mechanism of PDCD4 expression by PKC was investigated. The treatment of the human hepatoma cell line, Huh7 with TPA suppressed PDCD4 protein expression and TGF-β1 failed to increase the PDCD4 protein expression. PKC inhibitors Ro-31-8425 or bisindolylmaleimide-1-hydrocholoride (pan-PKC inhibitors) and rottlerin (PKCδ inhibitor), but not Go6976 (PKCα inhibitor), enhanced the induction of PDCD4 protein by TGF-β1. Furthermore, siRNA-mediated knockdown of PKCδ and ε, but not PKCα, augmented the TGF-β1-stimulated PDCD4 protein expression. However, TPA or pan-PKC inhibitor did not alter the PDCD4 mRNA expression either under basal- and TGF-β1-treated conditions. The down-regulation of PDCD4 by TPA was restored by treatment with the proteasome inhibitor MG132. These data suggest that two isoforms of PKCs are involved in the regulation of the PDCD4 protein expression related to the proteasomal degradation pathway.