title: Constitutive expression of insulin receptor substrate (IRS)-1 inhibits myogenic differentiation through nuclear exclusion of Foxo1 in L6 myoblasts.
abstract: Insulin-like growth factors (IGFs) are well known to play essential roles in enhancement of myogenic differentiation. In this report we showed that initial IGF-I signal activation but long-term IGF-1 signal termination are required for myogenic differentiation. L6 myoblast stably transfected with myc-epitope tagged insulin receptor substrate-1, myc-IRS-1 (L6-mIRS1) was unable to differentiate into myotubes, indicating that IRS-1 constitutive expression inhibited myogenesis. To elucidate the molecular mechanisms underlying myogenic inhibition, IGF-I signaling was examined. IGF-I treatment of control L6 cells for 18 h resulted in a marked suppression of IGF-I stimulated IRS-1 association with the p85 PI 3-kinase and suppression of activation of Akt that correlated with a down regulation of IRS-1 protein. L6-mIRS1 cells, in contrast, had sustained high levels of IRS-1 protein following 18 h of IGF-I treatment with persistent p85 PI 3-kinase association with IRS-1, Akt phosphorylation and phosphorylation of the downstream Akt substrate, Foxo1. Consistent with Foxo1 phosphorylation, Foxo1 protein was excluded from the nuclei in L6-mIRS1 cells, whereas Foxo1 was localized in the nuclei in control L6 cells during induction of differentiation. In addition, L6 cells stably expressing a dominant-interfering form of Foxo1, Δ256Foxo1 (L6-Δ256Foxo1) were unable to differentiate into myotubes. Together, these data demonstrate that IGF-I regulation of Foxo1 nuclear localization is essential for the myogenic program in L6 cells but that persistent activation of IGF-1 signaling pathways results in a negative feedback to prevent myogenesis.
authors: Department of Animal Sciences, Graduate School of Agriculture and Life Sciences, The University of Tokyo, Bunkyo-ku, Tokyo, Japan.
authors: Fumihiko
authors: F
authors: Hakuno
authors: Yoko
authors: Y
authors: Yamauchi
authors: Gen
authors: G
authors: Kaneko
authors: Yosuke
authors: Y
authors: Yoneyama
authors: Jun
authors: J
authors: Nakae
authors: Kazuhiro
authors: K
authors: Chida
authors: Tatsuhiko
authors: T
authors: Kadowaki
authors: Keitaro
authors: K
authors: Yamanouchi
authors: Masugi
authors: M
authors: Nishihara
authors: Shin-Ichiro
authors: S
authors: Takahashi
mesh_terms: D000818:Animals; D002454:Cell Differentiation / Q000187:drug effects; D002467:Cell Nucleus / Q000187:drug effects / Q000378:metabolism*; D049109:Cell Proliferation / Q000187:drug effects; D004789:Enzyme Activation / Q000187:drug effects; D051858:Forkhead Transcription Factors / Q000378:metabolism*; D005799:Genes, Dominant / Q000235:genetics; D055504:Insulin Receptor Substrate Proteins / Q000378:metabolism*; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology; D024510:Muscle Development / Q000187:drug effects; D009154:Mutation / Q000235:genetics; D032446:Myoblasts / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism*; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D021381:Protein Transport / Q000187:drug effects; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D051381:Rats; D032496:Satellite Cells, Skeletal Muscle / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D000071161:Forkhead Box Protein O1
chemical_list: D051858:Forkhead Transcription Factors; D055504:Insulin Receptor Substrate Proteins; D007334:Insulin-Like Growth Factor I; D009419:Nerve Tissue Proteins; D051057:Proto-Oncogene Proteins c-akt; C074639:Foxo1 protein, rat; D000071161:Forkhead Box Protein O1