Insulin-like growth factor-II is an autocrine survival factor for differentiating myoblasts

Recent studies indicate that insulin-like growth factor-II (IGF-II) acts as an autocrine differentiation factor for skeletal myoblasts in culture. IGF-II mRNA and protein are induced as early events in muscle differentiation, and the rate and extent of IGF-II secretion correlate with both biochemical and morphological differentiation. Here we show that IGF-II also functions as an essential survival factor during the transition from proliferating to differentiating myoblasts. Stably transfected C2 muscle cell lines were established in which a mouse IGF-II cDNA was expressed in the antisense orientation relative to the constitutively active Moloney sarcoma virus promoter. IGF-II antisense cells proliferated normally in growth medium containing 20% serum but underwent rapid death when placed in low serum differentiation medium. Death was accompanied by characteristic markers of apoptosis with more than 90% of cells showing DNA fragmentation within 12-16 h. Myoblast death was prevented by IGF-I, des [1-3] IGF-I, IGF-II, and insulin with a dose potency consistent with activation of the IGF-I receptor; death also could be blocked by the protein synthesis inhibitor, cycloheximide. Exogenous IGFs additionally stimulated passage through a single cell cycle and subsequently induced terminal differentiation. Cell survival and cell cycle progression also were enhanced by fibroblast growth factor-2 and platelet-derived growth factor-bb, but these peptides did not promote differentiation. Our results define a novel system for studying apoptotic cell death and its prevention by growth factors, underscore the importance of IGF action in minimizing inappropriate cell death, and indicate that shared signal transduction pathways may mediate myoblast survival in vitro.