title: Functional inactivation of the IGF-I receptor delays differentiation of skeletal muscle cells.
abstract: Skeletal myoblasts are inherently programmed to leave the cell cycle and begin the differentiation process following removal of exogenous growth factors. Serum withdrawal results in a marked induction of IGF production which is essential for skeletal muscle differentiation in vitro. However, the potential role of the tyrosine kinase IGF-I receptor (thought to be the principal mediator of both IGF-I and II signaling in skeletal muscle) in the decision of myoblasts to begin differentiation following serum withdrawal is unknown. To explore the role of the IGF-I receptor in this decision by skeletal myoblasts, we functionally inactivated endogenous IGF-I receptors in mouse C2C12 cells using a dominant negative, kinase-inactive IGF-I receptor in which the ATP-binding site lysine (K) at residue 1003 has been mutated to alanine (A). Cell lines with the greatest degree of mutant IGF-I receptor expression (A/K cells) demonstrated functional inactivation of endogenous IGF-I receptors as determined by their impaired ability to phosphorylate the principal substrate of the IGF-I receptor, IRS-1, in response to treatment with IGF-I. In addition, the proliferative response of myoblasts to IGF-I was completely abolished in A/K cells. Following withdrawal of exogenous growth factors, A/K cells demonstrated a marked delay in the induction of the gene expression of myogenin, a skeletal muscle-specific transcription factor essential for differentiation, and a subsequent delay in the induction of muscle creatine kinase activity. Delayed differentiation in A/K cells was associated with prolonged phosphorylation of the cell cycle regulatory retinoblastoma (Rb) protein; it is the un- (or hypo-) phosphorylated form of Rb which is known to promote differentiation in skeletal myoblasts. Thus, the IGF-I receptor regulates the timing of myoblast differentiation induced by serum withdrawal. The delayed differentiation of skeletal myoblasts with functionally inactive IGF-I receptors may result, at least in part, from delayed induction of myogenin gene expression and prolonged phosphorylation of the Rb protein.
authors.affiliation: Department of Pediatrics, University of California, San Francisco, California 94143, USA.
authors.forename: Z Q
authors.initials: ZQ
authors.lastname: Cheng
authors.forename: S
authors.initials: S
authors.lastname: Adi
authors.forename: N Y
authors.initials: NY
authors.lastname: Wu
authors.forename: D
authors.initials: D
authors.lastname: Hsiao
authors.forename: E J
authors.initials: EJ
authors.lastname: Woo
authors.forename: E H
authors.initials: EH
authors.lastname: Filvaroff
authors.forename: T A
authors.initials: TA
authors.lastname: Gustafson
authors.forename: S M
authors.initials: SM
authors.lastname: Rosenthal
mesh_terms: D000818:Animals; D018929:Cell Culture Techniques; D002454:Cell Differentiation / Q000502:physiology; D002455:Cell Division / Q000502:physiology; D016895:Culture Media, Serum-Free; D051379:Mice; D018482:Muscle, Skeletal / Q000166:cytology*; D010766:Phosphorylation; D017526:Receptor, IGF Type 1 / Q000502:physiology*; D016160:Retinoblastoma Protein / Q000378:metabolism; D014162:Transfection
chemical_list: D016895:Culture Media, Serum-Free; D016160:Retinoblastoma Protein; D017526:Receptor, IGF Type 1