Regulation of protein synthesis associated with skeletal muscle hypertrophy by insulin-, amino acid- and exercise-induced signalling

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Abstract
Although insulin, amino acids and exercise individually activate multiple signal transduction pathways in skeletal muscle, one pathway, the phosphatidylinositol 3-kinase (PI3K)–mammalian target of rapamycin (mTOR) signalling pathway, is a target of all three. Activation of the PI3K–mTOR signal transduction pathway results in both acute (i.e. occurring in minutes to hours) and long-term (i.e. occurring in hours to days) up-regulation of protein synthesis through modulation of multiple steps involved in mediating the initiation of mRNA translation and ribosome biogenesis respectively. In addition, changes in gene expression through altered patterns of mRNA translation promote cell growth, which in turn promotes muscle hypertrophy. The focus of the present discussion is to review current knowledge concerning the mechanism(s) through which insulin, amino acids and resistance exercise act to activate the PI3K–mTOR signal transduction pathway and thereby enhance the rate of protein synthesis in muscle.