Little involvement of recycled-amino acids from proteasomal proteolysis in de novo protein synthesis

Myoblast integrity is essential for skeletal muscle regeneration. Many intracellular proteins are degraded by the proteasome and converted to amino acids by aminopeptidases through the protein degradation pathway. Although we previously reported its importance for myoblast integrity, the involved mechanism remains unclear. In this study, we focused on the reusability of proteolytic products to elucidate the regulatory mechanism of protein synthesis mediated by the proteasome and aminopeptidases. Proteasome inhibition decreased protein synthesis, but recycled-amino acids derived from proteasomal proteolysis were not reused for de novo protein synthesis in C2C12 myoblasts. On the other hand, proteasome and aminopeptidase inhibition decreased intracellular ATP levels in C2C12 myoblasts. Therefore, it was indicated that amino acids produced by these proteolytic systems may be reutilized for ATP production through its metabolism, not for de novo protein synthesis. These findings suggested the proteasome and aminopeptidases are thought to be involved in protein synthesis through intracellular energy production by recycled-amino acid metabolism, thereby maintaining myoblast integrity.