Loss of 4E-BPs Prevents the Hindlimb Immobilization-Induced Decrease in Protein Synthesis in Skeletal Muscle.

The present study was designed to test the hypothesis that upregulating protein synthesis attenuates the loss of muscle mass in a model of disuse atrophy. The studies compared the effect of unilateral hindlimb immobilization in wild-type (WT) mice and double-knockout (DKO) mice lacking the translational regulators 4E-BP1 and 4E-BP2. Immobilization-induced downregulation of protein synthesis occurred in both groups of mice, but protein synthesis was higher in gastrocnemius muscle from the immobilized hindlimb of fasted DKO compared to WT mice. Surprisingly, although protein synthesis was partially elevated in DKO compared to WT mice, atrophy occurred to the same extent in both groups of animals. This may be partially due to impaired leucine-induced stimulation of protein synthesis in DKO compared to WT mice due to down-regulated eukaryotic initiation factor eIF4E expression in muscle of DKO compared to WT mice. Expression of the E3 ubiquitin ligases MAFbx and MuRF-1 mRNAs and total protein ubiquitylation were upregulated in the immobilized compared with the non-immobilized hindlimb of both WT and DKO mice, with little difference in the magnitude of the upregulation between genotypes. Analysis of newly synthesized proteins revealed a downregulation of several glycolytic enzymes in the gastrocnemius of DKO mice compared to WT mice, as well as in the immobilized compared to the non-immobilized hindlimb. Overall, the results suggest that the elevated rate of protein synthesis during hindlimb immobilization in fasted DKO mice is insufficient to prevent disuse-induced muscle atrophy, probably due to induction of compensatory mechanisms including downregulation of eIF4E expression.