title: mTOR kinase-dependent, but raptor-independent regulation of downstream signaling is important for cell cycle exit and myogenic differentiation.
abstract: Myogenic differentiation in the C2C12 myoblast model system reflects a concerted and controlled activation of transcription and translation following the exit of cells from the cell cycle. Previously we have shown that the mTORC1 signaling inhibitor, RAD001, decreased protein synthesis rates, delayed C2C12 myoblast differentiation, decreased p70S6K activity but did not affect the hypermodification of 4E-BP1. Here we have further investigated the modification of 4E-BP1 during the early phase of differentiation as cells exit the cell cycle, using inhibitors to target mTOR kinase and siRNAs to ablate the expression of raptor and rictor. As predicted, inhibition of mTOR kinase activity prevented p70S6K, 4E-BP1 phosphorylation and was associated with an inhibition of myogenic differentiation. Surprisingly, extensive depletion of raptor did not affect p70S6K or 4E-BP1 phosphorylation, but promoted an increase in mTORC2 activity (as evidenced by increased Akt Ser473 phosphorylation). These data suggest that an mTOR kinase-dependent, but raptor-independent regulation of downstream signaling is important for myogenic differentiation.
authors.affiliation: a Department of Biochemistry, School of Life Sciences ; University of Sussex ; Brighton , UK.
authors.forename: Hilary J
authors.initials: HJ
authors.lastname: Pollard
authors.forename: Mark
authors.initials: M
authors.lastname: Willett
authors.forename: Simon J
authors.initials: SJ
authors.lastname: Morley
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism*; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism; D002453:Cell Cycle; D018797:Cell Cycle Proteins; D002454:Cell Differentiation; D039642:Eukaryotic Initiation Factors; D051379:Mice; D032342:Muscle Cells / Q000166:cytology / Q000378:metabolism*; D024510:Muscle Development / Q000502:physiology; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D000076223:Regulatory-Associated Protein of mTOR; D015398:Signal Transduction; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism*
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; C499527:Eif4ebp1 protein, mouse; D039642:Eukaryotic Initiation Factors; D010750:Phosphoproteins; D000076223:Regulatory-Associated Protein of mTOR; C540162:Rptor protein, mouse; C546843:mTOR protein, mouse; D058570:TOR Serine-Threonine Kinases
keywords: 4E-BP1; C2C12; initiation factor; myoblasts; raptor; signaling; translation