Involvement of phosphatidylinositol 3-kinase and Akt in the induction of muscle protein degradation by proteolysis-inducing factor

The role of Akt/PKB in proteolysis-inducing factor (PIF) induced protein degradation has been investigated in murine myotubes. PIF induced transient phosphorylation of Akt at Ser 473}, within 30min, which was attenuated by the phosphoinositide 3-kinase (PI3-K) inhibitor LY294002, and the tyrosine kinase inhibitor genestein. Protein degradation was attenuated in myotubes expressing a dominant negative mutant of Akt (DNAkt), compared with the wild-type variant, while it was enhanced in myotubes containing a constitutively active Akt construct (Myr Akt). A similar effect was observed on the induction of the ubiquitin-proteasome pathway. Phosphorylation of Akt has been linked to upregulation of the ubiquitin-proteasome pathway through activation of NF-κB in a PI3-K dependent process. Protein degradation was attenuated by rapamycin, a specific inhibitor of mammalian target of rapamycin (mTOR), when added before, or up to 30 min after addition of PIF. PIF induced transient phosphorylation of mTOR and the 70kDa ribosomal protein S6 kinase. These results suggest that transient activation of Akt results in an increased protein degradation through activation of NF-κB and that this also allows for a specific synthesis of proteasome subunits.