The role of insulin in the stimulation of protein synthesis occurs at the level of translational initiation and elongation and is exerted primarily via a cascade leading to the activation of mechanistic target of rapamycin, mTOR, a protein with homology to a family of proteins first identified in yeast that bind to the immunosuppressant drug, rapamycin. Rapamycin gets its name from the fact that the compound was isolated from the bacterium Streptomyces hygroscopicus discovered on Easter Island (Rapa Nui). mTOR is a kinase whose catalytic domain shares significant homology with lipid kinases of the PI3K family.
mTOR is actually a component of two distinct multiprotein complexes termed mTORC1 and mTORC2 (mTOR complex 1 and mTOR complex 2). The activity of mTORC1 is sensitive to inhibition by by rapamycin whereas mTORC2 is not. Within the context of mTOR activity, mTORC1 is the central complex as it is responsible for integrating a diverse series of signal transduction cascades initiated by changes in both intra- and extracellular events. Activation and/or regulation of mTORC1 is involved in the control of cell proliferation, survival, metabolism and stress responses. These events can be triggered by nutrient availability, glucose, oxygen, and numerous different types of cell surface receptor activation, each of which eventually impinge on the activity of mTORC1. The mTORC1 is the major mTOR-containing complex that regulates cellular responses to nutrient deprivation and various forms of cellular stress. The mTORC1 is composed of mTOR, RAPTOR, mTOR associated protein yeast LST8 homolog, DEPTOR, and PRAS40. The mTOR protein is encoded by the MTOR gene. RAPTOR is derived from Regulatory Associated Protein of mTOR complex 1 and the protein is encoded by the RPTOR gene. The mTOR associated protein yeast LST8 homolog is also called mammalian lethal with SEC13 protein 8 and is encoded by the MLST8 gene. DEPTOR is derived from DEP domain containing mTOR interacting protein which is encoded by the DEPTOR gene. The DEP domain is a globular domain consisting of around 80 amino acids whose name is derived from the three proteins in which it was originally identified (Dishevelled, EGL-10, and Pleckstrin). In humans the EGL-10 homolog is encoded by the RGS7 (regulator of G-protein signaling 7) gene. PRAS40 is derived from Protein-Rich AKT Substrate 40 kDa originally isolated from D. melanogaster. PRAS40 in humans is correctly identified as AKT1 substrate 1 which is encoded by the AKT1S1 gene. Both DEPTOR and PRAS40 serve as negative regulators of the mTORC1. mTORC2 is involved in the control of the activity of serum- and glucocorticoid-induced kinase (SGK). Full activation of PKB/AKT requires the involvement of mTORC2.
Insulin-mediated cascade leading to enhanced translation: (not intended to be a complete description of all of the targets of insulin action that affect translation rates). Also shown is the effect of an increase in the AMP to ATP ratio which activates AMP-activated kinase, AMPK. STK11-LKB1-PJS: serine-threonine kinase 11, Peutz-Jeghers syndrome gene. IRS1: insulin receptor substrate-1; PI3K: phosphatidylinositol-3-kinase; PIP2: phosphatidylinositol-4,5-bisphosphate; PTEN: phosphatase and tensin homolog deleted on chromosome 10; PDK1: PIP3-dependent protein kinase; Tsc1 and Tsc2: Tuberous sclerosis tumor suppressors 1 (hamartin) and 2 (tuberin); Rheb: Ras homolog enriched in brain; mTOR: mammalian target of rapamycin. PKB/AKT: protein kinase B; GSK3: glycogen synthase kinase-3; 4EBP1: eIF-4E binding protein; p70S6K: 70kDa ribosomal protein S6 kinase, also called S6K. Arrows denote either direction of flow or positive effects, red T-lines represent inhibitory effects.
Insulin action leads to an increase in the activity of PI3K which in turn phosphorylates membrane phospholipids generating phosphatidylinositol-3,4,5-trisphophate (PIP3) from phosphatidylinositol-4,5-bisphosphate (PIP2). PIP3 then activates the kinase PDK1 which in turn phosphorylates and activates PKB/AKT. Activated PKB/AKT will phosphorylate TSC2 (tuberin) of the TSC1/TSC2 complex on two residues (S939 and T1462) resulting in altered activity of the complex. The TSC1/TSC2 complex functions as a GTPase-activating protein (GAP) which increases GTP hydrolyzing activity of Rheb. The GAP activity resides in the C-terminal portion of tuberin. The faster the GTPase action of Rheb the faster will be the reduction in Rheb activation of mTOR. When TSC1/TSC2 is phosphorylated by PKB/AKT it is less effective at stimulating the GTPase activity of Rheb and therefore Rheb activation of mTOR will remain high as is the case in response to insulin action.
AMPK phosphorylates TSC2 at two sites (T1271 and S1387) that are distinct from the sites that are the PKB/AKT targets for phosphorylation. Evidence indicates that the AMPK-mediated phosphorylation of TSC2 promotes the GTPase activity of Rheb resulting in inhibition of mTOR and thus a decrease in protein synthesis. Recent evidence has shown that PKB/AKT actually phosphorylates tuberin at 4 sites (S939, S1130, S1132, T1462) all of which result in inhibition of the Rheb-GAP activity of the TSC1/TSC2 complex.
The ultimate activation of mTOR leads to phosphorylation and activation of p70S6K which in turn leads to increased phosphorylation of eEF2 kinase. eEF2 kinase normally phosphorylates eEF2 leading to a decrease in its role in translation elongation. When phosphorylated by p70S6K, eEF2 kinase is less active at phosphorylating eEF2, thus eEF2 is much more active in response to insulin action. In addition, insulin action leads to a rapid dephosphorylation of eEF-2 via activation of protein phosphatase 2A (PP2A). Taken together, reduced eEF2K-mediated phosphorylation and increased eEF-2 dephosphorylation lead to increased protein synthesis.
Both mTOR and p70S6K have been shown to phosphorylate the regulator of translation initiation, eIF-4E binding protein, 4EBP1. Phosphorylation of 4EBP1 prevents it from binding to eIF-4E. Binding of 4EBP1 to eIF-4E prevents eIF-4E from interaction with the cap structure of mRNAs which is necessary for translational initiation. Thus, the consequences of 4EBP1:eIF-4E interaction is a reduction in translation initiation. As a consequence of the concerted actions of mTOR and p70S6K, insulin results in increased protein synthesis.
PKB/AKT activation will also lead to phosphorylation and inhibition of glycogen synthase kinase-3 (GSK3). One of the targets of GSK3, relative to translation, is eIF2B. Phosphorylation of eIF2B prevents it from performing its GTPase activating (GAP) function in association with eIF2 (see the Protein Synthesis page for more details) and as a consequence results in reduced translational initiation. However, when GSK3 is inhibited by PKB/AKT phosphorylation the GAP activity of eIF2B remains high and consequently the rate of translational initiation by eIF2 remains high so protein synthesis is favored.
Insulin also has profound effects on the transcription of numerous genes, effects that are primarily mediated by regulated function of sterol-regulated element binding protein, SREBP. These transcriptional effects include (but are not limited to) increases in glucokinase, pyruvate kinase, lipoprotein lipase (LPL), fatty acid synthase (FAS) and acetylCoA carboxylase (ACC) and decreases in glucose 6-phosphatase, fructose 1,6-bisphosphatase and phosphoenolpyruvate carboxykinase (PEPCK).