P70S6 kinase
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p70S6 Kinase is a serine/threonine kinase. As the name suggests, its target substrate is the S6 ribosomal protein. Phosphorylation of S6 induces protein synthesis at the ribosome
p70S6 kinase is in a signaling pathway that includes mTOR (the mammalian target of rapamycin). mTOR can be activated in distinct ways, thereby activating p70S6 kinase. For example, branced chain amino acids such as leucine are sufficient to activate mTOR, resulting in an increase in p70S6 kinase phosphorylation (and thereby activating it). mTOR is also in a pathway downstream of the kinase AKT. Akt is typically activated upon stimulation of a cell with a growth factor (such as IGF-1). Akt then activates mTOR (by inhibiting the Tsc complex), leading to p70S6 kinase activation.
Physical exercise activates protein synthesis via phosphorylation (activation) of p70S6 kinase (p70S6k) in a pathway that is dependent on mTOR. This has been demonstrated by using an inhibitor of mTOR, rapamycin, to block an increase in muscle mass, despite increases in load (e.g., exercise). Exercise has been shown to increase levels of IGF-1 in muscle, thus inducing the IGF-1/PI 3 Kinase/AKT/P70S6 kinase signaling pathway, and thereby increasing the protein synthesis required to build muscle.
