Control of a tumor suppressor PDCD4: Degradation mechanisms of the protein in hepatocellular carcinoma cells

In this study, we demonstrate that EGF inhibits the TGF-β1-induced apoptosis of Huh7 cells. TGF-β1 up-regulates the expression of PDCD4 causing apoptosis, by stimulating the synthesis of PDCD4 mRNA via the Smad signaling pathway. TGF-β1 also inhibits the activation of S6 kinase 1 which phosphorylates the serine 67 residue of PDCD4 and leads to the phosphorylation of serine 71 and serine 76 in the β-TRCP binding sequence. This phosphorylation sequence causes the protein to be degraded in the ubiquitin-proteasome system. EGF activates S6 kinase 1 via the PI3K-Akt-mTOR signaling pathway and stimulates the degradation of PDCD4. EGF also suppresses PDCD4 mRNA levels. As the mTOR inhibitor rapamycin up-regulated PDCD4 mRNA levels, the PI3K-Akt-mTOR signaling pathway may control the transcription of the PDCD4 gene as well as the degradation of the protein. TPA also inhibited the TGF-β1-induced apoptosis of Huh7 cells, stimulating the degradation of the PDCD4-protein. Analyses using PDCD4 mutants with changes of serines 67, 71 and 76 to alanine revealed that the phosphorylation of serine 67 is not essential for the TPA-induced suppression of the protein. The mitogens could not suppress the PDCD4-mutant proteins with changes of serine 71 and/or serine 76 to alanine, however, indicating that phosphorylations at these residues are necessary for the proteasome-mediated degradation of PDCD4. The phosphor-mimic S71/D and S76/D mutants were able to be degraded in the ubiquitin-proteasome system unlike the mutants with changes of serine to alanine. The expression of S71/D mutant was suppressed with EGF but that of S76/D mutant was not indicating that at least partly the phosphorylation of both sites was mediated by different enzymes.