Muscle Protein Signaling in C2C12 Cells Is Stimulated to Similar Degrees by Diverse Commercial Food Protein Sources and Experimental Soy Protein Hydrolysates

Dietary protein stimulates muscle protein synthesis and is essential for muscle health. We developed a screening assay using C2C12 mouse muscle cells to assess the relative abilities of diverse commercial protein sources and experimental soy protein hydrolysates (ESH), after simulated gut digestion (SGD), to activate the mechanistic target of rapamycin complex I (mTORC1) muscle protein synthesis signaling pathway (p70S6K((Thr389)) phosphorylation). Activation of mTORC1 was expressed as a percentage of a maximal insulin response. The bioactivities of proteins grouped by source including fish (81.3 +/- 10.6%), soy (66.2 +/- 4.7%), dairy (61.8 +/- 4.3%), beef (53.7 +/- 8.6%), egg (52.3 +/- 10.6%), soy whey (43.4 +/- 8.6%), and pea (31.4 +/- 10.6%) were not significantly different from each other. Bioactivity for ESH ranged from 28.0 +/- 7.5 to 98.2 +/- 6.6%. The results indicate that both the protein source and processing conditions are key determinants for mTORC1 activation. Regression analyses demonstrated that neither leucine nor total branched-chain amino acid content of proteins is the sole predictor of mTORC1 activity and that additional factors are necessary.