Both tumor necrosis factor- (TNF-)/interferon- (IFN-) and angiotensin II (Ang II) induced an increase in total protein degradation in murine myotubes, which was completed attenuated by treatment with -hydroxy--methylbutyrate (HMB; 50µM). There was an increase in formation of reactive oxygen species (ROS) within 30min, as well as an increase in the activity of both caspase-3 and -8, and both effects were attenuated by HMB. Moreover, inhibitors of caspase-3 and -8 completely attenuated both ROS formation and total protein degradation induced by TNF-/IFN- and Ang II. There was an increased autophosphorylation of dsRNA-dependent protein kinase (PKR), which was attenuated by the specific caspase-3 and -8 inhibitors. Neither ROS formation or protein degradation occurred in myotubes expressing a catalytically inactive PKR variant, PKR6, in response to TNF-/IFN-, in comparison with myotubes expressing wild-type PKR, although there was still activation of caspases-3 and -8. HMB also attenuated activation of PKR, suggesting that it was important in protein degradation. Formation of ROS was attenuated by rotenone, an inhibitor of the mitochondrial electron transport chain, L-nitro arginine methyl ester, an inhibitor of nitric oxide synthase and SB203580, a specific inhibitor of p38 mitogen activated protein kinase (p38 MAPK), which also attenuated total protein degradation. Activation of p38MAPK by PKR provides the link to ROS formation. These results suggest that TNF-/IFN- and Ang II induce muscle protein degradation by a common signaling pathway, which is attenuated by HMB, and that this involves the initial activation of caspase-3 and -8, followed by autophosphorylation and activation of PKR, which then leads to increased ROS formation via activation of p38MAPK. Increased ROS formation is known to induce protein degradation through the ubiquitin-proteasome pathway.