Regulation of targets of mTOR (mammalian target of rapamycin) signalling by intracellular amino acid availability

Abstract

In mammalian cells, amino acids affect the phosphorylation state and function of several proteins involved in mRNA translation that are regulated via the rapamycin-sensitive mTOR (mammalian target of rapamycin) pathway. These include ribosomal protein S6 kinase, S6K1, and eukaryotic initiation factor 4E-binding protein, 4E-BP1. Amino acids, especially branched-chain amino acids, such as leucine, promote phosphorylation of 4E-BP1 and S6K1, and permit insulin to further increase their phosphorylation. However, it is not clear whether these effects are exerted by extracellular or intracellular amino acids. Inhibition of protein synthesis is expected to increase the intracellular level of amino acids, whereas inhibiting proteolysis has the opposite effect. We show in the present study that inhibition of protein synthesis by any of several protein synthesis inhibitors tested allows insulin to regulate 4E-BP1 or S6K1 in amino-acid-deprived cells, as does the addition of amino acids to the medium. In particular, insulin activates S6K1 and promotes initiation factor complex assembly in amino-acid-deprived cells treated with protein synthesis inhibitors, but cannot do so in the absence of these compounds. Their effects occur at concentrations commensurate with their inhibition of protein synthesis and are not due to activation of stress-activated kinase cascades. Inhibition of protein breakdown (autophagy) impairs the ability of insulin to regulate 4E-BP1 or S6K1 under such conditions. These and other data presented in the current study are consistent with the idea that it is intracellular amino acid levels that regulate mTOR signalling.

Autophagy and signaling: their role in cell survival and cell death

“…Although a separate signaling pathway starting at the plasma membrane cannot be excluded, the available evidence indicates that mTOR-dependent signaling is stimulated by intracellular amino acids, rather than extracellular amino acids (leucine in particular). 41 In another study, carried out with C2C12 myotubes, 42 similar observations were made. Rapamycin was found to stimulate autophagic proteolysis, but could not abolish the Figure 1 Diagram of autophagy signaling.…”

“…As indicated above, in a variety of cell types, including hepatocytes, mTOR has an absolute requirement for amino acids, and is not activated by insulin alone. Sufficient amounts of amino acids from endogenous origin must apparently have been present in these studies (also see Beugnet et al 41 ). According to the authors, 40 amino acids use an mTOR-independent pathway which is initiated by binding one or more amino acids, presumably leucine, to an aminoacid receptor protein in the plasma membrane, which then somehow leads to an inhibition of autophagy that is independent of mTOR.…”

New strategies in sport nutrition to increase exercise performance

“…Furthermore, only a fraction of maximal mTORC1 activity (~30%) is required to fully saturate muscle protein synthesis [98,99]. The amino acid sensing by mTORC1 also seems to be driven not by extracellular, but instead by intracellular amino acid concentrations [100]. The mechanisms by which this occurs are incredibly complex and not yet fully defined.…”

Recycle or die: The role of autophagy in cardioprotection

“…The mTOR (mammalian target of rapamycin) has been identified as a negative regulator of autophagy in mammalian cells [38,39]. Many studies have demonstrated that rapamycin is a potent stimulator of autophagy by inhibiting mTOR [39,40], and rapamycin treatment has been shown to provide protection against I/R injury in Langendorff perfused rat hearts [41]. However, mTOR regulates multiple metabolic pathways in addition ot autophagy.…”