title: The phosphatidylinositol 3-kinase/Akt pathway regulates transforming growth factor-{beta} signaling by destabilizing ski and inducing Smad7.
abstract: Ski is an oncoprotein that negatively regulates transforming growth factor (TGF)-beta signaling. It acts as a transcriptional co-repressor by binding to TGF-beta signaling molecules, Smads. Efficient TGF-beta signaling is facilitated by rapid proteasome-mediated degradation of Ski by TGF-beta. Here we report that Ski is phosphorylated by Akt/PKB kinase. Akt phosphorylates Ski on a highly conserved Akt motif at threonine 458 both in vitro and in vivo. The phosphorylation of Ski at threonine 458 is induced by Akt pathway activators including insulin, insulin-like growth factor-1, and hepatocyte growth factor. The phosphorylation of Ski causes its destabilization and reduces Ski-mediated inhibition of expression of another negative regulator of TGF-beta, Smad7. Induction of Smad7 levels leads to inactivation of TGF-beta receptors and TGF-beta signaling cascade, as indicated by reduced induction of TGF-beta target p15. Therefore, Akt modulates TGF-beta signaling by temporarily adjusting the levels of two TGF-beta pathway negative regulators, Ski and Smad7. These novel findings demonstrate that Akt pathway activation directly impacts TGF-beta pathway.
authors.affiliation: Molecular Cancer Biology Program, Biomedicum Helsinki and Haartman Institute, University of Helsinki, 00014 Helsinki, Finland.
authors.forename: Arja M
authors.initials: AM
authors.lastname: Band
authors.forename: Mia
authors.initials: M
authors.lastname: Björklund
authors.forename: Marikki
authors.initials: M
authors.lastname: Laiho
mesh_terms: D000818:Animals; D019556:COS Cells; D045744:Cell Line, Tumor; D002522:Chlorocebus aethiops; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D017228:Hepatocyte Growth Factor / Q000378:metabolism / Q000494:pharmacology; D006801:Humans; D007004:Hypoglycemic Agents / Q000378:metabolism / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000378:metabolism / Q000494:pharmacology; D019869:Phosphatidylinositol 3-Kinases / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects / Q000502:physiology; D055550:Protein Stability / Q000187:drug effects; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt / Q000235:genetics / Q000378:metabolism*; D018125:Receptors, Transforming Growth Factor beta / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D051906:Smad7 Protein / Q000235:genetics / Q000378:metabolism*; D016212:Transforming Growth Factor beta / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D004268:DNA-Binding Proteins; D017228:Hepatocyte Growth Factor; D007004:Hypoglycemic Agents; D007328:Insulin; D007334:Insulin-Like Growth Factor I; D019869:Phosphatidylinositol 3-Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D018125:Receptors, Transforming Growth Factor beta; D051906:Smad7 Protein; D016212:Transforming Growth Factor beta; C066855:HGF protein, human; C497806:SMAD7 protein, human; C060614:SKI protein, human