MSTN and muscular atrophy: Akt represses expression of the muscular atrophy inducing E3-ubiquitin ligases MAFbx and MuRF1 through phosphorylation of FoxO transcription factors [19] and conversely the repressive action of Akt on MAFbx and MuRF1 can be lifted by myostatin through SMAD2 [20], [21].