Fig. 3. Myostatin and insulin-like growth factor 1 (IGF-1) signaling pathways in skeletal muscle. Myostatin and other transforming growth factor β family members signal via activin receptor II (ActRII), Smad2, and Smad3, which blocks muscle differentiation and leads to muscle atrophy. Inhibition of regulatory-associated protein of mammalian target of rapamycin (mTOR) and mTOR complex 1 (mTORC1) has an additive effect on myostatin signaling. The IGF-1/Akt pathway induces skeletal muscle hypertrophy. ALK, activin receptor-like kinase; GDF11, growth differentiation factor 11; IGFR, IGF receptor; FoxO1, forkhead box protein O1; PI3K, phosphatidylinositol 3-kinase; GSK-3β, glycogen synthase kinase 3β; MuRF, muscle ring finger; p70S6K, p70 S6 kinase.
