Novel roles of Akt and mTOR in suppressing TGF-beta/ALK5-mediated Smad3 activation.
Insulin-like growth factor-I inhibits transforming growth factor-beta (TGF-beta) signaling by blocking activation of Smad3 (S3), via a phosphatidylinositol 3-kinase (PI3K)/Akt-dependent pathway. Here we provide the first report that the kinase activity of Akt is necessary for its ability to suppress many TGF-beta responses, including S3 activation and induction of apoptosis. Wild-type and myristoylated Akts (Akt(WT) and Akt(Myr)) suppress TGF-beta-induced phospho-activation of S3 but not Smad2 (S2), whereas kinase-dead Akt1 (Akt1K179M) or dominant-negative PI3K enhances TGF-beta-induced phospho-activation of both S2 and S3. Using siRNA, rapamycin (Rap), and adenoviral expression for FKBP12-resistant and constitutively active TGF-beta type I receptor (ALK5), we demonstrate that mammalian target of Rap (mTOR) mediates Akt1 suppression of phospho-activation of S3. These and further data on Akt1-S3 binding do not support a recently proposed model that Akt blocks S3 activation through physical interaction and sequestration of S3 from TGF-beta receptors. We propose a novel model whereby Akt suppresses activation of S3 in an Akt kinase-dependent manner through mTOR, a likely route for loss of tumor suppression by TGF-beta in cancers.
Song
Kyung
K
Case Comprehensive Cancer Center Research Laboratories and Department of Pharmacology, Case Western Reserve University, Cleveland, OH 44106, USA.
Wang
Hui
H
Krebs
Tracy L
TL
Danielpour
David
D
D030201:Activin Receptors, Type I / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D000818:Animals; D017209:Apoptosis; D002478:Cells, Cultured; D006801:Humans; D009154:Mutation; D010766:Phosphorylation; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D034741:RNA, Small Interfering / Q000235:genetics / Q000494:pharmacology; D051381:Rats; D000077293:Receptor, Transforming Growth Factor-beta Type I; D018125:Receptors, Transforming Growth Factor beta / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D020123:Sirolimus / Q000494:pharmacology; D051899:Smad2 Protein / Q000378:metabolism; D051900:Smad3 Protein / Q000378:metabolism*; D058570:TOR Serine-Threonine Kinases; D022061:Tacrolimus Binding Protein 1A / Q000378:metabolism; D016212:Transforming Growth Factor beta / Q000037:antagonists & inhibitors* / Q000378:metabolism
D034741:RNA, Small Interfering; D018125:Receptors, Transforming Growth Factor beta; D051899:Smad2 Protein; D051900:Smad3 Protein; D016212:Transforming Growth Factor beta; D011494:Protein Kinases; C546842:MTOR protein, human; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D058570:TOR Serine-Threonine Kinases; D030201:Activin Receptors, Type I; D000077293:Receptor, Transforming Growth Factor-beta Type I; C000625841:TGFBR1 protein, human; C000625843:Tgfbr1 protein, rat; D022061:Tacrolimus Binding Protein 1A; D020123:Sirolimus