title: mTOR function in skeletal muscle hypertrophy: increased ribosomal RNA via cell cycle regulators.
abstract: The purpose of this study was to identify the potential downstream functions associated with mammalian target of rapamycin (mTOR) signaling during myotube hypertrophy. Terminally differentiated myotubes were serum stimulated for 3, 6, 12, 24, and 48 h. This treatment resulted in significant myotube hypertrophy (protein/DNA) and increased RNA content (RNA/DNA) with no changes in DNA content or indices of cell proliferation. During myotube hypertrophy, the increase in RNA content was accompanied by an increase in tumor suppressor protein retinoblastoma (Rb) phosphorylation and a corresponding increase in the availability of the ribosomal DNA transcription factor upstream binding factor (UBF). Serum stimulation also induced an increase in cyclin D1 protein expression in the differentiated myotubes with a concomitant increase in cyclin D1-dependent cyclin-dependent kinase (CDK)-4 activity toward Rb. The increases in myotube hypertrophy and RNA content were blocked by rapamycin treatment, which also prevented the increase in cyclin D1 protein expression, CDK-4 activity, Rb phosphorylation, and the increase in UBF availability. Our findings demonstrate that activation of mTOR is necessary for myotube hypertrophy and suggest that the role of mTOR is in part to modulate cyclin D1-dependent CDK-4 activity in the regulation of Rb and ribosomal RNA synthesis. On the basis of these results, we propose that common molecular mechanisms contribute to the regulation of myotube hypertrophy and growth during the G1 phase of the cell cycle.
authors.affiliation: School of Kinesiology, The University of Illinois at Chicago, Chicago, Illinois, USA.
authors.forename: Gustavo A
authors.initials: GA
authors.lastname: Nader
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: McLoughlin
authors.forename: Karyn A
authors.initials: KA
authors.lastname: Esser
mesh_terms: D000818:Animals; D002453:Cell Cycle / Q000502:physiology*; D049108:Cell Enlargement / Q000187:drug effects; D002478:Cells, Cultured; D003470:Culture Media; D019938:Cyclin D1 / Q000378:metabolism; D051358:Cyclin-Dependent Kinase 4 / Q000378:metabolism; D006984:Hypertrophy; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000473:pathology / Q000502:physiology; D018482:Muscle, Skeletal / Q000187:drug effects / Q000473:pathology / Q000502:physiology*; D032446:Myoblasts / Q000187:drug effects / Q000473:pathology / Q000502:physiology*; D010766:Phosphorylation; D035463:Pol1 Transcription Initiation Complex Proteins / Q000378:metabolism; D011494:Protein Kinases / Q000502:physiology*; D012335:RNA, Ribosomal / Q000096:biosynthesis*; D051381:Rats; D016160:Retinoblastoma Protein / Q000378:metabolism; D044967:Serum; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases
chemical_list: D003470:Culture Media; D035463:Pol1 Transcription Initiation Complex Proteins; D012335:RNA, Ribosomal; D016160:Retinoblastoma Protein; C063844:transcription factor UBF; D019938:Cyclin D1; D011494:Protein Kinases; D058570:TOR Serine-Threonine Kinases; D051358:Cyclin-Dependent Kinase 4; D020123:Sirolimus