Myostatin and NF-κB regulate skeletal myogenesis through distinct signaling pathways

Myostatin (Mstn) is a potent negative regulator of skeletal development shown to inhibit myoblast proliferation by impinging on cell cycle and suppressing the synthesis of MyoD. Moreover, Mstn causes muscle wasting and its expression is linked with several conditions of muscle loss, mainly dystrophy and cachexia. NF-κB is a transcription factor that is constitutively active in proliferating myoblasts and also plays a role in cell growth control and skeletal muscle differentiation. NF-κB inhibits myogenesis by promoting myoblast growth and inducing loss of MyoD, and NF-κB activity is required in states of muscle wasting. However, the extracellular factors that regulate NF-κB activity to modulate myogenesis are currently not known. Given the similarities in Mstn and NF-κB activities in muscle cells, we investigated the possibility that Mstn-induced regulation of myogenesis may signal via NF-κB. Using a variety of assays to monitor for NF-κB activity, we found that Mstn signaling does not activate NF-κB in differentiating C2C12 myoblasts, nor is the constitutive activity of NF-κB required for Mstn-mediated inhibition of myogenesis. Likewise, in pre-differentiated myotubes, Mstn signaling induces only a modest activation of NF-κB DNA binding activity. We also investigated whether NF-κB inhibition of myogenesis may occur through the regulation of Mstn. However, activation of NF-κB by TNFα or IL-1β failed to induce Mstn expression. These results thus highlight the distinctive differences by which Mstn and NF-κB signal to regulate myogenesis, a finding which broadens our understanding of how these pathways function in both development and disease.