Maturation of the myogenic program is induced by postmitotic expression ofinsulin-like growth factor I

The molecular mechanisms underlying myogenic induction by insulin-like grow th factor I (IGF-I) are distinct from its proliferative effects on myoblast s. To determine the postmitotic role of IGF-I on muscle cell differentiatio n, we derived L6E9 muscle cell lines carrying a stably transfected rat IGF- I gene under the control of a myosin light chain (MLC) promoter-enhancer ca ssette. Expression of MLC-IGF-I exclusively in differentiated L6E9 myotubes , which express the embryonic form of myosin heavy chain (MyHC) and no endo genous IGF-I, resulted in pronounced myotube hypertrophy, accompanied by ac tivation of the neonatal MyHC isoform. The hypertrophic myotubes dramatical ly increased expression of myogenin, muscle creatine kinase, beta-enolase, acid IGF binding protein 5 and activated the myocyte enhancer factor 2C gen e which is normally silent in this cell line. MLC-IGF-I induction in differ entiated L6E9 cells also increased the expression of a transiently transfec ted LacZ reporter driven by the myogenin promoter, demonstrating activation of the differentiation program at the transcriptional level. Nuclear reorg anization, accumulation of skeletal actin protein, and an increased express ion of beta 1D integrin were also observed. Inhibition of the phosphatidyl inositol (PI) 3-kinase intermediate in IGF-I-mediated signal transduction c onfirmed that the PI 3-kinase pathway is required only at early stages for IGF-I-mediated hypertrophy and neonatal MyHC induction in these cells. Expr ession of IGF-I in postmitotic muscle may therefore play an important role in the maturation of the myogenic program.