title: Skeletal myocyte hypertrophy requires mTOR kinase activity and S6K1.
abstract: The protein kinase mammalian target of rapamycin (mTOR) is a central regulator of cell proliferation and growth, with the ribosomal subunit S6 kinase 1 (S6K1) as one of the key downstream signaling effectors. A critical role of mTOR signaling in skeletal muscle differentiation has been identified recently, and an unusual regulatory mechanism independent of mTOR kinase activity and S6K1 is revealed. An mTOR pathway has also been reported to regulate skeletal muscle hypertrophy, but the regulatory mechanism is not completely understood. Here, we report the investigation of mTOR's function in insulin growth factor I (IGF-I)-induced C2C12 myotube hypertrophy. Added at a later stage when rapamycin no longer had any effect on normal myocyte differentiation, rapamycin completely blocked myocyte hypertrophy as measured by myotube diameter. Importantly, a concerted increase of average myonuclei per myotube was observed in IGF-I-stimulated myotubes, which was also inhibited by rapamycin added at a time when it no longer affected normal differentiation. The mTOR protein level, its catalytic activity, its phosphorylation on Ser2448, and the activity of S6K1 were all found increased in IGF-I-stimulated myotubes compared to unstimulated myotubes. Using C2C12 cells stably expressing rapamycin-resistant forms of mTOR and S6K1, we provide genetic evidence for the requirement of mTOR and its downstream effector S6K1 in the regulation of myotube hypertrophy. Our results suggest distinct mTOR signaling mechanisms in different stages of skeletal muscle development: While mTOR regulates the initial myoblast differentiation in a kinase-independent and S6K1-independent manner, the hypertrophic function of mTOR requires its kinase activity and employs S6K1 as a downstream effector.
authors.affiliation: Department of Cell and Structural Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA. ihpark@uiuc.edu
authors.forename: In-Hyun
authors.initials: IH
authors.lastname: Park
authors.forename: Ebru
authors.initials: E
authors.lastname: Erbay
authors.forename: Paul
authors.initials: P
authors.lastname: Nuzzi
authors.forename: Jie
authors.initials: J
authors.lastname: Chen
mesh_terms: D000818:Animals; D049108:Cell Enlargement / Q000187:drug effects; D002478:Cells, Cultured; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D051379:Mice; D018482:Muscle, Skeletal / Q000166:cytology* / Q000502:physiology; D011494:Protein Kinases / Q000378:metabolism*; D038744:Ribosomal Protein S6 Kinases, 90-kDa / Q000378:metabolism*; D015398:Signal Transduction; D058570:TOR Serine-Threonine Kinases
chemical_list: D007334:Insulin-Like Growth Factor I; D011494:Protein Kinases; C546843:mTOR protein, mouse; D038744:Ribosomal Protein S6 Kinases, 90-kDa; C512964:Rps6ka1 protein, mouse; D058570:TOR Serine-Threonine Kinases