Recent evidence supports that tumor necrosis factor (TNF)-, long considered as a catabolic factor, may also have a physiological function in skeletal muscle. The catabolic view, mainly based on correlative studies in human and in vivo animal models, was challenged by experiments with myoblasts, in which TNF- induced differentiation. The biological effects of TNF- in differentiated muscle, however, remain poorly understood. In the present study, we tested whether TNF- has growth-promoting effects in myotubes, and we characterized the mechanisms leading to these effects. Treatment of C₂C₁₂ myotubes with TNF- for 24 h increased protein synthesis and enhanced cellular dehydrogenase activity by 22% and 26%, respectively, without changing cell numbers. These effects were confirmed in myotubes differentiated from primary rat myoblasts. TNF-? activated two signaling cascades: (1) Erk1/2 and its target eIF4E and (2) Akt and its downstream effectors GSK3, p70S6K and 4E-BP1. The TNF--induced phosphorylation of Akt and Erk1/2 was inhibited by an antibody against the TNF-R1. PD98059 pre-treatment abolished TNF--induced phosphorylation of Erk1/2 and eIF4E, while protein synthesis was only partially inhibited. LY294002 completely abolished TNF--induced stimulation of protein synthesis as well as phosphorylation of Akt and its downstream targets GSK3, p70S6K and 4E-BP1. Rapamycin inhibited TNF--induced phosphorylation of the mTORC1 target p70S6K without altering TNF--induced protein synthesis and 4E-BP1 phosphorylation. In conclusion, our results provide evidence that TNF- enhances PS in myotubes, and that this is based on enhanced protein translation mediated by the TNF-R1 and PI3-K/Akt and MEK/Erk signaling cascades.