Tumor necrosis factor alpha (TNF-α) inactivates the PI3-kinase/PKB pathway and induces atrophy and apoptosis in L6 myotubes

Muscle atrophy poses a serious concern to patients inflicted with inflammatory diseases. An increasing body of evidence implies that TNF-α plays a critical role in muscle atrophy in a number of these clinical settings. The mechanisms mediating its effects are not completely understood and conflicting data regarding its anabolic and catabolic actions exists. To examine the functional significance and detailed morphological characteristics of TNF-α-induced muscle proteolysis, differentiated L6 myotubes were subjected to increasing concentrations of recombinant TNF-α for 24 and 48 h. Data analysis of cell death showed that TNF-α induced a combination of apoptosis and necrosis in high concentrations. TNF-R1, rather than TNF-R2, was significantly upregulated. In addition, the transcription factors, NF-κB and FKHR were rapidly activated thus leading to increased expression of ubiquitin ligases, MuRF-1 and MAFbx. Muscle fiber diameter decreased with increasing TNF-α concentrations and was associated with attenuation of the PI3-K/Akt pathway as well as significant reductions in differentiation markers. Furthermore, treatment of L6 myotubes with exogenous TNF-α strongly potentiates its proteolytic effects through certain MAPKs that are activated. These observations suggest that TNF-α induces muscle proteolysis in a dose-dependent manner via various signal transduction pathways.