Leucine and Mammalian Target of Rapamycin-Dependent Activation of Muscle Protein Synthesis in Aging

The preservation or restoration of muscle mass is of prime importance for healthy aging. However, aging has been repeatedly shown to be associated with resistance of muscle to the anabolic effects of feeding. Leucine supplementation has been proposed as a possible strategy because of its regulatory role on protein homeostasis. Indeed, it acts independently of growth factors and leads to enhanced cap-dependent mRNA translation initiation and increased protein synthesis. Leucine acts as a signaling molecule directly at the muscle level via the activation of mammalian/mechanistic target of rapamycin complex 1 (mTORC1). However, in aged muscle, mTORC1 activation seems to be impaired, with decreased sensitivity and responsiveness of muscle protein synthesis to amino acids, whereas the phosphorylation state of several components of this signaling pathway appears to be higher in the basal state. This may stem from specific age related impairment of muscle signaling and from decreased nutrient and growth factor delivery to the muscle. Whether aging per se affects mTORC1 signaling remains to be established, because aging is frequently associated with inadequate protein intake, decreased insulin sensitivity, inactivity, inflammatory processes, etc. Whatever its origin, this anabolic resistance to feeding can be mitigated by quantitative and qualitative manipulation of protein supply, such as leucine supplementation; however, there remains the question of possible adverse effects of long-term, high-dose leucine supplementation in terms of insulin resistance and tumorigenesis.