TWEAK cytokine has been implicated in several biological responses including inflammation, angiogenesis, and osteoclastogenesis. We have investigated the role of TWEAK in regulating skeletal muscle mass. Addition of soluble TWEAK protein to cultured myotubes reduced the mean myotube diameter and enhanced the degradation of specific muscle proteins such as CK and MyHCf. The effect of TWEAK on degradation of MyHCf was stronger than its structural homologue, TNF-. TWEAK increased the ubiquitination of MyHCf and the transcript levels of atrogin-1 and MuRF1 ubiquitin ligases. TWEAK inhibited phosphorylation of Akt kinase and its downstream targets GSK-3, FOXO1, mTOR, and p70S6K. Furthermore, TWEAK increased the activation of NF-B transcription factor in myotubes. Adenoviral-mediated overexpression of IBN (a degradation-resistant mutant of NF-B inhibitory protein IB) in myotubes blocked the TWEAK-induced degradation of MyHCf. Chronic administration of TWEAK in mice resulted in reduced body and skeletal muscle weight with an associated increase in the activity of ubiquitin-proteasome system and NF-B. Finally, muscle-specific transgenic overexpression of TWEAK decreased the body and skeletal muscle weight in mice. Collectively, our data suggest that TWEAK induces skeletal muscle atrophy through inhibition of the PI3K/Akt signaling pathway and activation of the ubiquitin-proteasome and NF-B systems.--Dogra, C., Changotra, H., Wedhas, N., Qin, X., Wergedal, J. E., Kumar, A. TNF-related weak inducer of apoptosis (TWEAK) is a potent skeletal muscle-wasting cytokine.