Analysis of the Stimulative Effect of Arginine on Translation Initiation of Protein Synthesis in Skeletal Muscle

Background: Arginine (Arg) is thought to potentially stimulate protein synthesis. Although the detailed mechanism by which Arg regulates protein synthesis is not fully known, it is believed to occur primarily through the mechanistic target of rapamycin complex 1 (mTORC1)-dependent activation of translation initiation. The aim of this study was to evaluate the ability of Arg to stimulate translation initiation to upregulate protein synthesis and identify the possible signaling pathways involved in the stimulatory effect of Arg on mRNA translation in skeletal muscle. Methods: Overnight-fasted mice were intraperitoneally injected with Arg, sacrificed 1 h later, and then the gastrocnemius muscles were excised. In addition, to determine the mechanism by which Arg stimulates translation initiation in skeletal muscle, we used mouse-derived C2C12 myotubes. Cells were preincubated with several inhibitors of intracellular signaling or the G protein–coupled receptor, Class C, group 6, subtype A (GPRC6A) antagonist, and then added to the culture with Arg. Phosphorylation of 4E-binding protein 1 (4E-BP1) and ribosomal protein S6 kinase (S6K1) as markers of mTORC1-dependent protein synthesis activity was measured. Results: Intraperitoneal injection of Arg increased 4E-BP1 and S6K1 phosphorylation. In C2C12 myotubes, Arg addition significantly increased the phosphorylation of 4E-BP1 and S6K1, and this upregulation was attenuated by pretreatment with the mTORC1 inhibitor rapamycin. In addition, pretreatment with the PI3K inhibitor LY294002, the AKT inhibitor MK-2206, and the GPRC6A antagonist calindol completely inhibited Arg-upregulated 4E-BP1 and S6K1 phosphorylation. Conclusions: The findings of this study suggest that Arg stimulates the initiation of mRNA translation via the GPRC6A/PI3K/AKT/mTORC1 signaling pathway, thereby stimulating protein synthesis in skeletal muscle.