Skeletal muscle IL-15/IL-15R and myofibrillar protein synthesis after resistance exercise

In vitro and in vivo studies described the myokine IL-15 and its receptor IL-15R as anabolic/anti-atrophy agents, however, the protein expression of IL-15R has not been measured in human skeletal muscle and data regarding IL-15 expression remain inconclusive. The purpose of the study was to determine serum and skeletal muscle IL-15 and IL-15R responses to resistance exercise session and to analyze their association with myofibrillar protein synthesis (MPS). Fourteen participants performed a bilateral leg resistance exercise composed of four sets of leg press and four sets of knee extension at 75% 1RM to task failure. Muscle biopsies were obtained at rest, 0, 4 and 24hours post-exercise and blood samples at rest, mid-exercise, 0, 0.3, 1, 2, 4 and 24hours post-exercise. Serum IL-15 was increased by similar to 5.3-fold immediately post-exercise, while serum IL-15R decreased similar to 75% over 1hour post-exercise (P<.001). Skeletal muscle IL-15R mRNA and protein expression were increased at 4hours post-exercise by similar to 2-fold (P<.001) and similar to 1.3-fold above rest (P=.020), respectively. At 24hours post-exercise, IL-15 (P=.003) and IL-15R mRNAs increased by similar to 2-fold (P=.002). Myofibrillar fractional synthetic rate between 0-4hours was associated with IL-15R mRNA at rest (r=.662, P=.019), 4hours (r=.612, P=.029), and 24hours post-exercise (r=.627, P=.029). Finally, the muscle IL-15R protein up-regulation was related to Leg press 1RM (r=.688, P=.003) and total weight lifted (r=.628, P=.009). In conclusion, IL-15/IL-15R signaling pathway is activated in skeletal muscle in response to a session of resistance exercise.