title: Interactions between p-Akt and Smad3 in injured muscles initiate myogenesis or fibrogenesis.
abstract: In catabolic conditions such as aging and diabetes, IGF signaling is impaired and fibrosis develops in skeletal muscles. To examine whether impaired IGF signaling initiates muscle fibrosis, we generated IGF-IR(+/-) heterozygous mice by crossing loxP-floxed IGF-IR (exon 3) mice with MyoD-cre mice. IGF-IR(+/-) mice were studied because we were unable to obtain homozygous IGF-IR-KO mice. In IGF-IR(+/-) mice, both growth and expression of myogenic genes (MyoD and myogenin; markers of satellite cell proliferation and differentiation, respectively) were depressed. Likewise, in injured muscles of IGF-IR(+/-) mice, there was impaired regeneration, depressed expression of MyoD and myogenin, and increased expression of TGF-β1, α-SMA, collagen I, and fibrosis. To uncover mechanisms stimulating fibrosis, we isolated satellite cells from muscles of IGF-IR(+/-) mice and found reduced proliferation and differentiation plus increased TGF-β1 production. In C2C12 myoblasts (a model of satellite cells), IGF-I treatment inhibited TGF-β1-stimulated Smad3 phosphorylation, its nuclear translocation, and expression of fibronectin. Using immunoprecipitation assay, we found an interaction between p-Akt or Akt with Smad3 in wild-type mouse muscles and in C2C12 myoblasts; importantly, IGF-I increased p-Akt and Smad3 interaction, whereas TGF-β1 decreased it. Therefore, in muscles of IGF-IR(+/-) mice, the reduction in IGF-IR reduces p-Akt, allowing for dissociation and nuclear translocation of Smad3 to enhance the TGF-β1 signaling pathway, leading to fibrosis. Thus, strategies to improve IGF signaling could prevent fibrosis in catabolic conditions with impaired IGF signaling.
authors.affiliation: Nephrology Division, Department of Medicine, Baylor College of Medicine, Houston, Texas, USA.
authors.forename: Yanjun
authors.initials: Y
authors.lastname: Dong
authors.forename: Ronak
authors.initials: R
authors.lastname: Lakhia
authors.forename: Sandhya S
authors.initials: SS
authors.lastname: Thomas
authors.forename: Yanlan
authors.initials: Y
authors.lastname: Dong
authors.forename: Xiaonan H
authors.initials: XH
authors.lastname: Wang
authors.forename: Kleiton Augusto Santos
authors.initials: KA
authors.lastname: Silva
authors.forename: Liping
authors.initials: L
authors.lastname: Zhang
mesh_terms: D000818:Animals; D002454:Cell Differentiation / Q000187:drug effects; D049109:Cell Proliferation; D002469:Cell Separation; D005355:Fibrosis / Q000473:pathology; D007150:Immunohistochemistry; D047468:Immunoprecipitation; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology; D051379:Mice; D018345:Mice, Knockout; D024510:Muscle Development / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D018482:Muscle, Skeletal / Q000254:growth & development / Q000293:injuries*; D017570:MyoD Protein / Q000096:biosynthesis / Q000235:genetics; D051058:Oncogene Protein v-akt / Q000502:physiology*; D060888:Real-Time Polymerase Chain Reaction; D017526:Receptor, IGF Type 1 / Q000096:biosynthesis / Q000235:genetics; D012038:Regeneration; D032496:Satellite Cells, Skeletal Muscle / Q000502:physiology; D051900:Smad3 Protein / Q000502:physiology*; D053773:Transforming Growth Factor beta1 / Q000037:antagonists & inhibitors / Q000494:pharmacology
chemical_list: D017570:MyoD Protein; C057363:MyoD1 myogenic differentiation protein; D051900:Smad3 Protein; C497791:Smad3 protein, mouse; D053773:Transforming Growth Factor beta1; D007334:Insulin-Like Growth Factor I; D017526:Receptor, IGF Type 1; D051058:Oncogene Protein v-akt
keywords: Smad3; fibrosis; insulin-like growth factor I; myogenesis; satellite cells; transforming growth factor-β1