IGF-1 induces human myotube hypertrophy by increasing cell recruitment.

Insulin-like growth factor-1 (IGF-1) has been shown in rodents (i) in vivo to induce muscle fiber hypertrophy and to prevent muscle mass decline with age and (ii) in vitro to enhance the proliferative life span of myoblasts and to induce myotube hypertrophy. In this study, performed on human primary cultures, we have shown that IGF-1 has very little effect on the proliferative life span of human myoblasts but does delay replicative senescence. IGF-1 also induces hypertrophy of human myotubes in vitro, as characterized by an increase in the mean number of nuclei per myotube, an increase in the fusion index, and an increase in myosin heavy chain (MyHC) content. In addition, muscle hypertrophy can be triggered in the absence of proliferation by recruiting more mononucleated cells. We propose that IGF-1-induced hypertrophy can involve the recruitment of reserve cells in human skeletal muscle.