Figure 6. Proposed model of signaling associated with Fst-mediated
skeletal muscle hypertrophy. Fst inhibits the activation of the TGF- signaling
pathway by binding to multiple extracellular ligands including
myostatin and Activin A. Inhibiting the actions of myostatin and Activin A
diminishes phosphorylation of Smad3, which alters the transcription of
Smad target genes, and potentiates Akt/mTOR signaling that stimulates
protein synthesis. Increased phosphorylation of Akt contributes to the inhibition
of Smad3-dependent signals that otherwise repress muscle growth
(arrow with broken line), thereby maximizing protein synthesis networks
regulated by Fst.