TNF-α increases protein content in C2C12 and primary myotubes by enhancing protein translation via the TNF-R1, PI3K, and MEK.

Recent evidence supports that TNF-α, long considered 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-α induced differentiation. The biological effects of TNF-α in differentiated muscle, however, remain poorly understood. In the present study, we tested whether TNF-α has growth-promoting effects in myotubes, and w& characterized the mechanisms leading to these effects. Treatment of C2C12 myotubes with TNF-α for 24 h increased protein synthesis (PS) 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-α activated two signaling cascades: 1) ERK1/2 and its target eIF4E and 2) Akt and its downstream effectors GSK-3, p70S6K and 4E-BP1. TNF-α-induced phosphorylation of Akt, and ERK1/2 was inhibited by an antibody against TNF-α receptor 1 (TNF-R1). PD-98059 pretreatment abolished TNF-α-induced phosphorylation of ERK1/2 and eIF4E, whereas PS was only partially inhibited. LY-294002 completely abolished TNF-α-induced stimulation of PS as well as phosphorylation of Akt and its downstream targets GSK-3, p70S6K, and 4E-BP1. Rapamycin inhibited TNF-α-induced phosphorylation of the mTOR C1 target p70S6K without altering TNF-α-induced PS and 4E-BP1 phosphorylation. In conclusion, our results provide evidence that TNF-α enhances PS in myotubes and that this is based on enhanced protein translation mediated by the TNF-R1 and PI3K-Akt and MEK-ERK signaling cascades. [ABSTRACT FROM AUTHOR]