Existing data suggest that TNF-α stimulates muscle catabolism by activating the ubiqutin/proteasome pathway [
24,
25,
26]. As reviewed elsewhere [
27], the ubiquitin/proteasome pathway degrades the bulk of all intracellular proteins and is responsible for regulated proteolysis in signal transduction, cell-cycle progression, transcriptional regulation, and antigen presentation. Pathway activity depends upon coordinated interactions among several enzyme families. These interventions result in tagging of substrate proteins with polymeric ubiquitin chains that mark the protein for degradation. The marked protein is then degraded by the 26S-proteasome complex, by an ATP-dependent process. In catabolic states, the activity of the ubiquitin/ proteasome pathway is increased by upregulation of selected pathway components [
28]. The level of circulating TNF-α is also elevated in these conditions and is a potential stimulus for pathway upregulation. Acute, intravenous injection of TNF-α causes time-dependent increases in both free and conjugated ubiquitin [
24], and ubiquitin mRNA [
25] in the limb muscles of intact rats. The limited data currently available suggest that TNF-α acts directly on muscle fibers to upregulate the pathway. Llovera
et al [
26] have shown that ubiquitin mRNA levels are elevated in excised muscle exposed to TNF-α
in vitro. Preliminary studies suggest the ubiquitin/proteasome pathway is sensitive to TNF-α signaling events, including elevated ROS levels and NF-κB activation [
29], but this link has yet to be established formally.