Insulin-like growth factor-mediated muscle differentiation - Collaborationbetween phosphatidylinositol 3-kinase-Akt-signaling pathways and myogenin

The differentiation and maturation of skeletal muscle require interactions between signaling pathways activated by hormones and growth factors and an intrinsic regulatory network controlled by myogenic transcription factors. Insulin-like growth factors (IGFs) play key roles in muscle development in the embryo and in regeneration in the adult. To study mechanisms of IGF act ion in muscle, we developed a myogenic cell line that overexpresses IGF-bin ding protein-5. C2BP5 cells remain quiescent in low serum differentiation m edium until the addition of IGF-I. Here we use this cell line to identify s ignaling pathways controlling IGF-mediated differentiation. Induction of my ogenin by IGF-I and myotube formation were prevented by the phosphatidylino sitol (PI) 3-kinase inhibitor, LY294002, even when included 2 days after gr owth factor addition, whereas expression of active PI 3-kinase could promot e differentiation in the absence of IGF-I. Differentiation also was induced by myogenin but was blocked by LY294002. The differentiation-promoting eff ects of IGF-I were mimicked by a modified membrane-targeted inducible Akt-1 (iAkt), and iAkt was able to stimulate differentiation of C2 myoblasts and primary mouse myoblasts incubated with otherwise inhibitory concentrations of LY294002. These results show that an IGF-regulated PI 3-kinase-Akt path way controls muscle differentiation by mechanisms acting both upstream and downstream of myogenin.