Insulin-Like Growth Factor Signaling during Myogenesis.

Abstract

Myogenesis is the process of muscle development and growth. It involves a series of highly ordered cellular events – myoblast proliferation, cell cycle withdrawal, terminal differentiation, and cell fusion. Myogenesis requires the coordinated actions of various transcription factors and growth factors. Insulin-like growth factors (IGFs) stimulate both myoblast proliferation and differentiation. It remains elusive how these two mutually exclusive cellular responses can be elicited by the same growth factor. In order to address this question, I investigated the regulation of IGF signaling actions by both local IGF binding proteins (IGFBPs) and the cellular microenvironment. First, I examined how IGF actions are regulated by the local IGFBPs during myogenesis. IGF-II stimulates myoblast differentiation and its own gene expression by a positive feedback mechanism via the PI3K-Akt pathway. IGFBP-5 is the major IGFBP expressed during myogenesis. Its expression is rapidly elevated and precedes the appreciable increase of IGF-II transcription. IGFBP-5 knockdown impairs myogenesis and suppresses IGF-II gene expression. Adding exogenous IGF or constitutively active Akt rescues the defects caused by IGFBP-5 knockdown. Furthermore, IGFBP-5 regulates IGF-II actions by binding to IGF. Together, these data suggest that under physiological conditions, IGFBP-5 promotes myoblast differentiation and potentiates IGF-II actions by binding to and switching on the IGF-II auto-regulation loop. Second, I addressed how cellular microenvironment specifies IGF signaling outcomes. While IGF promotes myoblast differentiation under normoxia, it stimulates proliferation under hypoxia. IGF downstream signaling pathways – Akt-mTOR, p38, and Erk MAPK, are differentially regulated under hypoxia. Hypoxia inhibits the myogenic Akt-mTOR and p38 pathways by suppressing Akt signaling and inducing negative regulators. In contrast, Erk activity is enhanced under hypoxia leading to increased proliferation and decreased differentiation. p38 inhibition directly contributes to IGF-stimulated proliferation. Moreover, the altered IGF signaling outcomes are linked to the profoundly altered cellular metabolism in hypoxia. IGF increases cell proliferation under hypoxia by further enhancing HIF-1-mediated glycolysis. In conclusion, IGF signaling during myogenesis is intricately regulated by both local IGFBPs and cellular microenvironment. These mechanisms determine whether cells respond to IGF signaling by proliferating or differentiating.