title: A growth stimulus is needed for IGF-1 to induce skeletal muscle hypertrophy in vivo.
abstract: Here, we characterise new strains of normal and dystrophic (mdx) mice that overexpress Class 2 IGF-1 Ea in skeletal myofibres. We show that transgenic mice have increased muscle levels of IGF-1 (approximately 13-26 fold) and show striking muscle hypertrophy (approximately 24-56% increase in mass). Adult normal muscles were resistant to elevated IGF-1; they reached adult steady state and maintained the same mass from 3 to 12 months. By contrast, dystrophic muscles from mdx/IGF-1(C2:Ea) mice continued to increase in mass during adulthood. IGF-1 signalling was evident only in muscles that were growing as a result of normal postnatal development (23-day-old mice) or regenerating in response to endogenous necrosis (adult mdx mice). Increased phosphorylation of Akt at Ser473 was not evident in fasted normal adult transgenic muscles, but was 1.9-fold higher in fasted normal young transgenic muscles compared with age-matched wild-type controls and fourfold higher in fasted adult mdx/IGF-1(C2:Ea) compared with mdx muscles. Muscles of adult mdx/IGF-1(C2:Ea) mice showed higher p70(S6K)(Thr421/Ser424) phosphorylation and both young transgenic and adult mdx/IGF-1(C2:Ea) mice had higher phosphorylation of rpS6(Ser235/236). The level of mRNA encoding myogenin was increased in normal young (but not adult) transgenic muscles, indicating enhanced myogenic differentiation. These data demonstrate that elevated IGF-1 has a hypertrophic effect on skeletal muscle only in growth situations.
authors.affiliation: School of Anatomy and Human Biology, The University of Western Australia, Perth 6009, WA, Australia. tshavlakadze@anhb.uwa.edu.au
authors.forename: Thea
authors.initials: T
authors.lastname: Shavlakadze
authors.forename: Jinfen
authors.initials: J
authors.lastname: Chai
authors.forename: Kirsten
authors.initials: K
authors.lastname: Maley
authors.forename: Greg
authors.initials: G
authors.lastname: Cozens
authors.forename: Griffin
authors.initials: G
authors.lastname: Grounds
authors.forename: Nadine
authors.initials: N
authors.lastname: Winn
authors.forename: Nadia
authors.initials: N
authors.lastname: Rosenthal
authors.forename: Miranda D
authors.initials: MD
authors.lastname: Grounds
mesh_terms: D000375:Aging / Q000473:pathology; D000818:Animals; D001835:Body Weight; D005786:Gene Expression Regulation; D006984:Hypertrophy; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism; D051379:Mice; D018101:Mice, Inbred mdx; D018485:Muscle Fibers, Skeletal / Q000201:enzymology / Q000473:pathology; D018482:Muscle, Skeletal / Q000201:enzymology / Q000254:growth & development* / Q000473:pathology*; D009137:Muscular Dystrophy, Animal / Q000097:blood / Q000201:enzymology / Q000473:pathology; D018008:Myogenin / Q000235:genetics / Q000378:metabolism; D009929:Organ Size; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000378:metabolism; D015398:Signal Transduction; D058570:TOR Serine-Threonine Kinases
chemical_list: D047908:Intracellular Signaling Peptides and Proteins; D018008:Myogenin; D012333:RNA, Messenger; D007334:Insulin-Like Growth Factor I; C546842:MTOR protein, human; C546843:mTOR protein, mouse; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D058570:TOR Serine-Threonine Kinases