title: Muscle-specific overexpression of the type 1 IGF receptor results in myoblast-independent muscle hypertrophy via PI3K, and not calcineurin, signaling.
abstract: The insulin-like growth factors (IGF-I and IGF-II), working through the type 1 IGF receptor (IGF-1R), are key mediators of skeletal muscle fiber growth and hypertrophy. These processes are largely dependent on stimulation of proliferation and differentiation of muscle precursor cells, termed myoblasts. It has not been rigorously determined whether the IGFs can also mediate skeletal muscle hypertrophy in a myoblast-independent fashion. Similarly, although the phosphatidylinositol 3-kinase (PI3K) and calcineurin signaling pathways have been implicated in skeletal muscle hypertrophy, these pathways are also involved in skeletal myoblast differentiation. To determine whether the IGFs can stimulate skeletal muscle hypertrophy in a myoblast-independent fashion, we developed and validated a retroviral expression vector that mediated overexpression of the human IGF-1R in rat L6 skeletal myotubes (immature muscle fibers), but not in myoblasts. L6 myotubes transduced with this vector accumulated significantly higher amounts of myofibrillar proteins, in a ligand- and receptor-dependent manner, than controls and demonstrated significantly increased rates of protein synthesis. Stimulation of myotube hypertrophy was independent of myoblast contributions, inasmuch as these cultures did not exhibit increased levels of myoblast proliferation or differentiation. Experiments with PI3K and calcineurin inhibitors indicated that myoblast-independent myotube hypertrophy was mediated by PI3K, but not calcineurin, signaling. This study demonstrates that IGF can mediate skeletal muscle hypertrophy in a myoblast-independent fashion and suggests that muscle-specific overexpression of the IGF-1R or stimulation of its signaling pathways could be used to develop strategies to ameliorate muscle wasting without stimulating proliferative pathways leading to carcinogenesis or other pathological sequelae.
authors.affiliation: Department of Gerontology, University of Washington, Seattle, WA 98493, USA. quinnL@u.washington.edu
authors.forename: Lebris S
authors.initials: LS
authors.lastname: Quinn
authors.forename: Barbara G
authors.initials: BG
authors.lastname: Anderson
authors.forename: Stephen R
authors.initials: SR
authors.lastname: Plymate
mesh_terms: D000199:Actins / Q000378:metabolism; D000818:Animals; D019703:Calcineurin / Q000378:metabolism*; D065095:Calcineurin Inhibitors; D002454:Cell Differentiation / Q000187:drug effects / Q000235:genetics; D049108:Cell Enlargement / Q000187:drug effects; D002460:Cell Line; D002867:Chromones / Q000494:pharmacology; D016572:Cyclosporine / Q000494:pharmacology; D003907:Dexamethasone / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D015870:Gene Expression / Q000187:drug effects; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000378:metabolism / Q000494:pharmacology; D009025:Morpholines / Q000494:pharmacology; D018485:Muscle Fibers, Skeletal / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D032448:Myoblasts, Skeletal / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D018995:Myosin Heavy Chains / Q000378:metabolism; D009218:Myosins / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010766:Phosphorylation / Q000187:drug effects; D014176:Protein Biosynthesis / Q000187:drug effects; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D051381:Rats; D017526:Receptor, IGF Type 1 / Q000235:genetics* / Q000378:metabolism / Q000502:physiology; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D014162:Transfection
chemical_list: D000199:Actins; D019703:Calcineurin; D065095:Calcineurin Inhibitors; D002867:Chromones; D016572:Cyclosporine; D003907:Dexamethasone; D004791:Enzyme Inhibitors; D007334:Insulin-Like Growth Factor I; D009025:Morpholines; D018995:Myosin Heavy Chains; D009218:Myosins; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D051057:Proto-Oncogene Proteins c-akt; D017526:Receptor, IGF Type 1; C085911:2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one