Unilateral Hindlimb Immobilization Decreases Muscle Mass in Part by Proteasome- Dependent Proteolysis but Independent of Protein Synthesis and Translation Initiation Factors

The hypothesis of the present study was that rats subjected to short-term unilateral hindlimb immobilization would incur skeletal muscle wasting and concomitant alterations in protein synthesis, controllers of translation, and indices of protein degradation. Rats were unilaterally casted for 1, 3, or 5 days to avoid complications associated with other disuse models. In the casted limb, gastrocnemius wet weight decreased 12% after 3 days and thereafter remained constant. In contrast, the contralateral control leg displayed a steady growth rate over time. The rate of protein synthesis and translational efficiency were unchanged in the immobilized muscle at day 5. The total amount and phosphorylation state of eIF2α, S6k1, eEF1A, and eEF2, as well as the distribution of eIF4E between eIF4G and 4E-BP1 complexes were unaltered. Muscle wasting occurred independently of an alteration in acute IGF – I or insulin responsiveness, or elevated inflammatory cytokine expression. The mRNA contents of polyubiquitin and the ubiquitin ligases MAFbx/Atrogin-1 and MuRF1 were elevated in immobilized muscle at all time points with peak expression occurring at day 3. Daily injection of the type II glucocorticoid receptor antagonist RU486 did not prevent decreases in gastrocnemius wet weight nor increases in mRNA for MAFbx/Atrogin-1 and MuRF1. However, in vivo administration of the proteasome inhibitor Velcade prevented 53% of wet weight loss associated with 3 days of immobilization. These data suggest the loss of skeletal muscle mass in this model of disuse is the result of integrated atrophy and growth failure, and is distinct from previous studies in that there is no defect in protein synthesis and essential regulators of translational initiation and elongation at day 5. The loss of muscle weight appears glucocorticoid-independent, can be partially rescued with a potent proteasome inhibitor, and is associated with enhanced mRNA expression of multiple factors that contribute to ubiquitin-proteasome dependent degradation and are likely to control the remodeling of immobilized skeletal muscle during atrophy.