Nutritional and hormonal control of protein breakdown.

Multiple lines of evidence suggest that the ubiquitin-proteasome-dependent proteolytic pathway is the major degradative process responsible for the loss of muscle proteins seen in various pathological states and following food deprivation. The first step in this pathway is the covalent attachment of polyubiquitin chains to protein substrates. This signal targets the substrates for subsequent hydrolysis into peptides by the 26S proteasome. Several metabolic abnormalities (reduced food intake, impaired mobility, and perturbations in the production or responsiveness of catabolic and anabolic hormones, cytokines and/or proteolysis inducing factors) act in concert to contribute to muscle wasting in disease states. We cite recent evidence that insulin, glucocorticoids, thyroid hormones, and nutrients regulate the rates of ubiquitinylation of protein substrates and of proteasome-dependent proteolysis in skeletal muscle.