title: TNF-alpha increases protein content in C2C12 and primary myotubes by enhancing protein translation via the TNF-R1, PI3K, and MEK.
abstract: Recent evidence supports that TNF-alpha, 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-alpha induced differentiation. The biological effects of TNF-alpha in differentiated muscle, however, remain poorly understood. In the present study, we tested whether TNF-alpha has growth-promoting effects in myotubes, and we characterized the mechanisms leading to these effects. Treatment of C(2)C(12) myotubes with TNF-alpha 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-alpha activated two signaling cascades: 1) ERK1/2 and its target eIF4E and 2) Akt and its downstream effectors GSK-3, p70(S6K), and 4E-BP1. TNF-alpha-induced phosphorylation of Akt, and ERK1/2 was inhibited by an antibody against TNF-alpha receptor 1 (TNF-R1). PD-98059 pretreatment abolished TNF-alpha-induced phosphorylation of ERK1/2 and eIF4E, whereas PS was only partially inhibited. LY-294002 completely abolished TNF-alpha-induced stimulation of PS as well as phosphorylation of Akt and its downstream targets GSK-3, p70(S6K), and 4E-BP1. Rapamycin inhibited TNF-alpha-induced phosphorylation of the mTOR C1 target p70(S6K) without altering TNF-alpha-induced PS and 4E-BP1 phosphorylation. In conclusion, our results provide evidence that TNF-alpha 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.
authors.affiliation: Institute of Physiology, Department of Biomedicine, University and University Hospital of Basel, Basel, Switzerland.
authors.forename: Isabelle
authors.initials: I
authors.lastname: Plaisance
authors.forename: Christian
authors.initials: C
authors.lastname: Morandi
authors.forename: Claire
authors.initials: C
authors.lastname: Murigande
authors.forename: Marijke
authors.initials: M
authors.lastname: Brink
mesh_terms: D000818:Animals; D015153:Blotting, Western; D002460:Cell Line; D002470:Cell Survival / Q000187:drug effects; D051379:Mice; D008931:Mitochondria, Muscle / Q000187:drug effects / Q000201:enzymology; D020928:Mitogen-Activated Protein Kinases / Q000235:genetics / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000378:metabolism*; D009124:Muscle Proteins / Q000096:biosynthesis*; D051058:Oncogene Protein v-akt / Q000378:metabolism; D010088:Oxidoreductases / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases / Q000235:genetics / Q000502:physiology*; D010766:Phosphorylation; D014176:Protein Biosynthesis / Q000187:drug effects; D047888:Receptors, Tumor Necrosis Factor, Type I / Q000235:genetics / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction / Q000187:drug effects; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology*
chemical_list: D009124:Muscle Proteins; D047888:Receptors, Tumor Necrosis Factor, Type I; D014409:Tumor Necrosis Factor-alpha; D010088:Oxidoreductases; D019869:Phosphatidylinositol 3-Kinases; D051058:Oncogene Protein v-akt; D020928:Mitogen-Activated Protein Kinases