The ubiquitin-proteasome pathway as a therapeutic target for muscle wasting

Atrophy of skeletal muscle is common to a number of conditions, including cancer, sepsis, AIDS, renal failure, diabetes, severe trauma, and burns. In all cases, protein synthesis in skeletal muscle is depressed, whereas protein degradation is increased through an increase in activity and expression of the ubiquitin-proteasome proteolytic pathway. This pathway is not responsive to simple nutritional intervention. Certain agents, including glucocorticoids, cytokines, proteolysis-inducing factor (PIF), and oxidative stress, are thought to be responsible for the induction of the ubiquitin-proteasome pathway in skeletal muscle in catabolic conditions. Insulin suppresses activation of this pathway, and loss of insulin action in diabetes leads to muscle wasting. Cytokines, PIF, and reactive oxygen species (ROS) are thought to induce proteasome expression through activation of the transcription factor nuclear factor kappa B (NF-κB). Targets for therapeutic intervention include antagonists of the inducers of proteasome expression, intracellular signaling pathways leading to activation of NF-κB, and the enzymes inducing ubiquitin conjugation to the substrate protein (myosin), as well as the proteasome itself. Anticytokine and anti-PIF antibodies are effective in attenuating muscle protein degradation in certain experimental animal models, and glucocorticoid receptor antagonists are effective in the treatment of sepsis. Agents that inhibit NF-κB activation, such as resveratrol, thalidomide, ibuprofen, eicosapentaenoic acid, and β-hydroxy-β-methylbutyrate, are effective in the preservation of skeletal muscle mass in cachexia. These results suggest that the ubiquitin-proteasome pathway is an appropriate therapeutic target to prevent muscle wasting.