Raptor-rictor axis in TGFbeta-induced protein synthesis.
Transforming growth factor-beta (TGFbeta) stimulates pathological renal cell hypertrophy for which increased protein synthesis is critical. The mechanism of TGFbeta-induced protein synthesis is not known, but PI 3 kinase-dependent Akt kinase activity is necessary. We investigated the contribution of downstream effectors of Akt in TGFbeta-stimulated protein synthesis. TGFbeta increased inactivating phosphorylation of Akt substrate tuberin in a PI 3 kinase/Akt dependent manner, resulting in activation of mTOR kinase. mTOR activity increased phosphorylation of S6 kinase and the translation repressor 4EBP-1, which were sensitive to inhibition of both PI 3 kinase and Akt. mTOR inhibitor rapamycin and a dominant negative mutant of mTOR suppressed TGFbeta-induced phosphorylation of S6 kinase and 4EBP-1. PI 3 kinase/Akt and mTOR regulated dissociation of 4EBP-1 from eIF4E to make the latter available for binding to eIF4G. mTOR and 4EBP-1 modulated TGFbeta-induced protein synthesis. mTOR is present in two multi protein complexes, mTORC1 and mTORC2. Raptor and rictor are part of mTORC1 and mTORC2, respectively. shRNA-mediated downregulation of raptor inhibited TGFbeta-stimulated mTOR kinase activity, resulting in inhibition of phosphorylation of S6 kinase and 4EBP-1. Raptor shRNA also prevented protein synthesis in response to TGFbeta. Downregulation of rictor inhibited serine 473 phosphorylation of Akt without any effect on phosphorylation of its substrate, tuberin. Furthermore, rictor shRNA increased phosphorylation of S6 kinase and 4EBP-1 in TGFbeta-independent manner, resulting in increased protein synthesis. Thus mTORC1 function is essential for TGFbeta-induced protein synthesis. Our data also provide novel evidence that rictor negatively regulates TORC1 activity to control basal protein synthesis, thus conferring tight control on cellular hypertrophy.
Das
Falguni
F
Department of Medicine, University of Texas Health Science Center, San Antonio, Texas 78229-3900, USA.
Ghosh-Choudhury
Nandini
N
Mahimainathan
Lenin
L
Venkatesan
Balachandar
B
Feliers
Denis
D
Riley
Daniel J
DJ
Kasinath
Balakuntalam S
BS
Choudhury
Goutam Ghosh
GG
D002352:Carrier Proteins / Q000378:metabolism*; D004789:Enzyme Activation / Q000187:drug effects; D039561:Eukaryotic Initiation Factor-4E / Q000378:metabolism; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism*; D050527:Mesangial Cells / Q000187:drug effects / Q000201:enzymology; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D014176:Protein Biosynthesis / Q000187:drug effects*; D011494:Protein Kinases / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D034741:RNA, Small Interfering / Q000378:metabolism; D019893:Ribosomal Protein S6 Kinases / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D058570:TOR Serine-Threonine Kinases; D016212:Transforming Growth Factor beta / Q000494:pharmacology*; D000077005:Tuberous Sclerosis Complex 2 Protein; D025521:Tumor Suppressor Proteins / Q000378:metabolism
D002352:Carrier Proteins; D039561:Eukaryotic Initiation Factor-4E; D047908:Intracellular Signaling Peptides and Proteins; D019869:Phosphatidylinositol 3-Kinases; D011494:Protein Kinases; D051057:Proto-Oncogene Proteins c-akt; D034741:RNA, Small Interfering; D019893:Ribosomal Protein S6 Kinases; D058570:TOR Serine-Threonine Kinases; D016212:Transforming Growth Factor beta; D000077005:Tuberous Sclerosis Complex 2 Protein; D025521:Tumor Suppressor Proteins