Whey protein-derived exosomes increase protein synthesis and hypertrophy in C_2­C₁₂ myotubes

We sought to examine potential amino acid independent mechanisms whereby hydrolyzed whey protein (WP) affects muscle protein synthesis (MPS) and anabolism in vitro. Specifically, we tested (1) whether 3-h and 6-h treatments of WP, essential amino acids, or L-leucine (Leu) affected MPS, and whether 6-h treatments with low-, medium-, or high doses of WP versus Leu affected MPS; (2) whether knockdown of the primary Leu transporter affected WP- and Leu-mediated changes in MPS, mammalian target of rapamycin (mTOR) signaling responses, or both, following 6-h treatments; (3) whether exosomes isolated from WP (WP-EXO) affected MPS, mTOR signaling responses, or both, compared with untreated (control) myotubes, following 6-h, 12-h, and 24-h treatments, and whether they affected myotube diameter following 24-h and 48-h treatments. For all treatments, 7-d post-differentiated C₂C₁₂ myotubes were examined. In experiment 1, 6-h WP treatments increased MPS compared with control (+46%), Leu (+24%), and essential amino acids (+25%). Moreover, the 6-h low-, medium-, and high WP treatments increased MPS by approximately 40 to 50% more than corresponding Leu treatments. In experiment 2 (LAT short hairpin RNA-transfected myotubes), 6-h WP treatments increased MPS compared with control (+18%) and Leu (+19%). In experiment 3, WP-EXO treatments increased MPS over controls at 12 h (+18%) and 24 h (+45%), and myotube diameters increased with 24-h (+24%) and 48-h (+40%) WP-EXO treatments compared with controls. The WP-EXO treatments did not appear to operate through mTOR signaling; instead, they increased mRNA and protein levels o eukaryotic initiation factor 4A. Bovine-specific microRNA following 24-h WP-EXO treatments were enriched in myotubes (chiefly miR-149-3p, miR-2881), but were not related to hypertrophic gene targets. To summarize, hydrolyzed WP-EXO increased skeletal MPS and anabolism in vitro, and this may be related to an unknown mechanism that increases translation initiation factors rather than enhancing mTOR signaling or the involvement of bovine-specific microRNA.