title: Loss of IGF-IEa or IGF-IEb impairs myogenic differentiation.
abstract: Actions of protein products resulting from alternative splicing of the Igf1 gene have received increasing attention in recent years. However, the significance and functional relevance of these observations remain poorly defined. To address functions of IGF-I splice variants, we examined the impact of loss of IGF-IEa and IGF-IEb on the proliferation and differentiation of cultured mouse myoblasts. RNA interference-mediated reductions in total IGF-I, IGF-IEa alone, or IGF-IEb alone had no effect on cell viability in growth medium. However, cells deficient in total IGF-I or IGF-IEa alone proliferated significantly slower than control cells or cells deficient in IGF-IEb in serum-free media. Simultaneous loss of both or specific loss of either splice variant significantly inhibited myosin heavy chain (MyHC) immunoreactivity by 70-80% (P < 0.01) under differentiation conditions (48 h in 2% horse serum) as determined by Western immunoblotting. This loss in protein was associated with reduced MyHC isoform mRNAs, because reductions in total IGF-I or IGF-IEa mRNA significantly reduced MyHC mRNAs by approximately 50-75% (P < 0.05). Loss of IGF-IEb also reduced MyHC isoform mRNA significantly, with the exception of Myh7, but to a lesser degree (∼20-40%, P < 0.05). Provision of mature IGF-I, but not synthetic E peptides, restored Myh3 expression to control levels in cells deficient in IGF-IEa or IGF-IEb. Collectively, these data suggest that IGF-I splice variants may regulate myoblast differentiation through the actions of mature IGF-I and not the E peptides.
authors.affiliation: United States Army Research Institute of Environmental Medicine, Military Performance Division, Natick, Massachusetts 01760, USA. ronald.matheny@us.army.mil
authors.forename: Ronald W
authors.initials: RW
authors.lastname: Matheny
authors.forename: Bradley C
authors.initials: BC
authors.lastname: Nindl
mesh_terms: D017398:Alternative Splicing; D000595:Amino Acid Sequence; D000818:Animals; D015153:Blotting, Western; D002454:Cell Differentiation; D002460:Cell Line; D049109:Cell Proliferation / Q000187:drug effects; D002470:Cell Survival / Q000187:drug effects; D016895:Culture Media, Serum-Free / Q000494:pharmacology; D015870:Gene Expression; D007334:Insulin-Like Growth Factor I / Q000235:genetics* / Q000378:metabolism / Q000494:pharmacology; D051379:Mice; D008969:Molecular Sequence Data; D032446:Myoblasts / Q000166:cytology / Q000378:metabolism*; D018008:Myogenin / Q000235:genetics / Q000378:metabolism; D018995:Myosin Heavy Chains / Q000235:genetics / Q000378:metabolism; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism / Q000494:pharmacology; D051057:Proto-Oncogene Proteins c-akt / Q000235:genetics / Q000378:metabolism; D034622:RNA Interference; D020133:Reverse Transcriptase Polymerase Chain Reaction
chemical_list: D016895:Culture Media, Serum-Free; D018008:Myogenin; D020033:Protein Isoforms; D007334:Insulin-Like Growth Factor I; C494922:Akt2 protein, mouse; D051057:Proto-Oncogene Proteins c-akt; D018995:Myosin Heavy Chains