INSULIN-LIKE GROWTH FACTOR-II IS AN AUTOCRINE SURVIVAL FACTOR FOR DIFFERENTIATING MYOBLASTS

Recent studies indicate that insulin-like growth factor-II (IGF-II) ac ts as an autocrine differentiation factor for skeletal myoblasts in cu lture. IGF-II mRNA and protein are induced as early events in muscle d ifferentiation, 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 tra nsfected C2 muscle cell lines were established in which a mouse IGF-II cDNA was expressed in the antisense orientation relative to the const itutively active Moloney sarcoma virus promoter. IGF-II antisense cell s proliferated normally in growth medium containing 20% serum but unde rwent rapid death when placed in low serum differentiation medium. Dea th was accompanied by characteristic markers of apoptosis with more th an 90% of cells showing DNA fragmentation within 12-16 h. Myoblast dea th 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, cycloheximid e. Exogenous IGFs additionally stimulated passage through a single cel l cycle and subsequently induced terminal differentiation. Cell surviv al 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 st udying apoptotic cell death and its prevention by growth factors, unde rscore the importance of IGF action in minimizing inappropriate cell d eath, and indicate that shared signal transduction pathways may mediat e myoblast survival in vitro.