Author(s): KIM, Y., KIM, H., LYM, Y., SHANKARAN, M., YOUNG, J., EVANS, W., HELLERSTEIN, M., WOLFE, R., KIM, I., Institution: GACHON UNIVERSITY, Country: KOREA, SOUTH, Abstract-ID: 2073

INTRODUCTION:
Sarcopenia is an age-related decline of skeletal muscle mass and function that contributes to frailty, loss of independence, metabolic dysfunction, and increased mortality. A key feature is anabolic resistance, defined as blunted stimulation of muscle protein synthesis (MPS) and reduced hypertrophic adaptation to anabolic stimuli such as exercise and nutrient intake. Although the extent varies across conditions, anabolic resistance is frequently observed in aged muscle. However, the molecular basis of these impaired anabolic adaptations remains poorly defined.
METHODS:
Male young (10-week) and aged (24-month) C57BL/6J mice completed 12 weeks of resistance training (RT), with age-matched sedentary controls. To quantify anabolic resistance to RT, cumulative muscle proteome-wide synthesis rates of mixed, myofibrillar, and mitochondrial proteins were measured during the final 2 weeks using deuterium oxide labeling and tandem mass spectrometry. Whole-body functional muscle mass was assessed using the D3-creatine dilution method. To identify candidate genes associated with anabolic resistance, RNA sequencing was performed followed by protein-protein interaction network analysis. To test the role of the candidate gene caveolin-1 (Cav-1), implicated in glucose and fatty acid uptake and metabolism, C2C12 myotubes were transfected with shCav-1 plasmids and stimulated to contract using electrical pulse stimulation (EPS) to induce hypertrophy, while carbon energy flux were quantified using 13C-metabolic flux analysis.
RESULTS:
RT significantly increased harvested muscle mass (+12%), strength (+98%), MPS of mixed (+33%), myofibrillar (+46%), and mitochondrial proteins (+15%) in young mice, but these increases were not observed in aged mice. RNA-sequencing showed that broad RT-induced gene expression changes in young muscle, whereas responses in aged muscle were minimal. Protein-protein interaction analysis identified Cav-1 among candidate hub genes linked to anabolic resistance. Proteome kinetic analysis revealed that synthesis rates of 146 of 159 quantified proteins were lower in aged than young muscle, with the largest reductions in proteins involved in muscle contraction, glycolysis, and oxidative phosphorylation. Cav-1 knockdown abolished EPS-induced hypertrophy and stimulation of MPS, accompanied by reduced glucose uptake (-33.3%) and lactate production (-69.1%), with lower glycolytic flux and ATP production (approximately -51.8% and -41.3%), as determined by 13C-metabolic flux analysis.
CONCLUSION:
Aged mice exhibit anabolic resistance to RT, characterized by failure to muscle hypertrophy and ATP-dependent MPS responses. These data indicate that the absence of hypertrophy and an ATP-dependent MPS response to RT is partly due to reduced glucose uptake, glycolytic carbon flux, and associated ATP production. Cav-1 may therefore serve as a mechanistic link between energy metabolic flux and anabolic remodeling in skeletal muscle.