mTOR (mammalian rapamycin target) is a serine/threonine protein kinase with a molecular weight of 289 kDa belonging to the phosphatidylositin 3-kinase-associated kinase (PIKK) family. The protein consists of a catalytic kinase domain, an FRB (FKBP12-rapamycin-bound) domain, a predicted self-suppressing domain near the C-terminal (inhibitory subdomain), up to 20 repeat HEAT motifs at the amino terminal, and FAT (FRAP-ATM-TRRAP) and FATC (FAT C-terminal) domains. The C-terminus of toR is highly homologous to the catalytic domain of phosphatidylinositol 3-kinase (PI3K). The TOR protein has evolved conservatively from yeast to humans, and the mTOR protein in humans, mice, and rats is 95% homologous at amino acid levels. The human mTOR gene encodes a protein of 2549 amino acids, and the sequence homology with yeast TOR1 and TOR2 is 42% and 45%, respectively. mTOR plays a central role in signaling pathways involved in controlling cell growth and proliferation (Reference 1).
The mTOR pathway is regulated by a variety of cellular signals, including mitosis growth factor, hormones such as insulin, nutrients (amino acids, glucose), cellular energy levels, and stress conditions. The PI3K/Akt (v-Akt mouse thymomavirus oncogene homology 1) signal transduction pathway is the main pathway for signaling through mTOR and plays an important role in mediating cell survival and proliferation. Signaling through the PI3K/Akt pathway is initiated by mitosis stimulation of growth factors that bind to receptors on the cell membrane. These receptors include IGFR (insulin-like growth factor receptor), PDGFR (platelet-derived growth factor receptor), EGFR (epidermal growth factor receptor), and the HER family. Signals from activated receptors are transmitted directly to the PI3K/Akt pathway, or, alternatively, can be activated by growth factor receptors activated by the oncogenic protein RAS. RAS is another central switch for signal transduction and has been shown to be a key activator of the MAPK (mitogen-activated protein kinase) signal transduction pathway. Insulin can also activate the PI3K/Akt pathway through IRS1/2 (insulin receptor substrate -1/2). Insulin binding activates IR (insulin receptor) tyrosine kinase, phosphorylation of IRS1 or IRS2. PI3K binds to phosphorylated IR via the SH2 (Src-Homology-2) domain in the regulatory subunit P85. This interaction activates the p110 catalytic subunit. Pi3K catalytic membrane-bound PIP2 (phosphatidyl inositol (4,5) diphosphate) is converted to PIP3 (phosphatidyl inositol (3, 4, 5)-triphosphate). PIP3 then binds to Akt's pleckstrin homologous domain, leading to activation of Akt by dimerization and exposure of its catalytic site.
AKT can also be phosphorylated and activated by PDK-1 (phospholipid-dependent kinase-1). AKT directly phosphorylated mTOR. AKT may also act indirectly on mTOR through the action of TSC1/TSC2 (Tuberous Sclerosis Complex). The physical binding of the proteins TSC1 (Hamartin) and TSC2 (Tuberin) produces a functional complex that inhibits mTOR. Recent evidence suggests that the inhibition of TSC1/TSC2 is achieved through the small GTPase Rheb (RasHomolog Enriched In Brain) of the TSC2 inactivated Ras family. TSC2 has GAP (GTPase-Activating Protein) activity on Rheb, and it is speculated that the TSC1/TSC2 complex inhibits mTOR signal transduction by stimulating the GTP hydrolysis of Rheb. RHEB-GTP activates mTOR. PMA (Forbo ester) can also inhibit TSC1/2 complexes through PKC (protein kinase-C) and RSK1 (ribosome-S6 kinase-1), as well as activation of S6K1 by PKC without relying on Akt and resulting in mTOR phosphorylation. AMPK (amp(adenosine 5'-monophosphate)-activated protein kinase) can also modulate mTOR. AMPK is very sensitive to elevated intracellular AMP (5'-adenosine monophosphate)/ATP (adenosine triphosphate) ratio and is therefore a key energy-sensitive kinase. This increase in this proportion promotes phosphorylation and activation of the upstream kinase LKB1, a human tumor suppressor that mutates in Peutz-Jeghers syndrome. Activated AMPK in turn phosphorylates TSC2 (located on a different residue than Akt phosphorylated residue), significantly promoting its activation. This, in turn, inhibits the effect of mTOR activity (References 2, 3 and 5).
Phosphatidic acid (PA) also activates mTOR. There are three different enzymes that produce PA: PLD (phospholipase-D), LPAAT (hemolytic phosphatidyltransferase), and DGK (disaclycerylkinase). PLD is considered to be a major contributor to pa to mTOR signaling. Still, other PA-producing enzymes can also promote mTOR activation; LPAAT has been reportedly elevated in some tumors, and its overexpression can lead to cell transformation. Serum stimulation leads to PLD activation, which is associated with enhanced mTOR signaling. Serum is a mixture of mitogens that function through G protein-coupled receptors (GPCRs) or receptor tyrosine kinases (RTKs). PLD activity increases with stimulation of both receptor types. Lipids such as DAG (diacylglycerol) and PA arise in the membrane domain, where different lipid metabolic pathways maintain close links between them, resulting in appropriate spatiotemporal responses. PLD and DGK can run in parallel, but they can also act as DAG and PAG-generating enzymes in a single pathway. In mammalian cells, the PA produced by the inner membrane (such as the Golgi apparatus) is mainly produced by the PLD action of phosphatidylcholine (PC). This PA can be used both as a messenger to promote vesicle division and as a substrate for phosphatase to convert PA into DAG. Because PC is the most abundant lipid in mammalian membranes, this pathway is a powerful supplier of DAG and can then be used as a substrate for DGK (References 4&5). Therefore, several mechanisms have been proposed to explain how mTOR is regulated by growth factors and cellular energy levels. However, little is known about how mTOR is regulated by stressful conditions. Two stress-inducing proteins, RTP801/Redd1 and RTP801L/Redd2, effectively inhibit signal transduction by mTOR. RTP801 and RTP801L act downstream of AKT and upstream of TSC2 to inhibit mTOR function. Another mTOR inhibitor is rapamycin. When complexed with its cell receptor FKBP12 (FK506-binding protein-12), rapamycin binds directly to TOR to inhibit downstream signaling (References 1, 6, and 7).
Activation of mTOR leads to phosphorylation of several downstream targets. For the protein mTOR to activate its signaling cascade, the ternary complexes mTORC1 (mTOR complex-1) and mTORC2 (mTOR complex-2) must be formed. Rapamycin-sensitive mTORC1 controls several pathways that collectively determine cell mass (size). Rapamycin-insensitive mTORC2 controls the actin cytoskeleton, which determines the shape of the cell. mTORC1 (and possibly mTORC2) is a multimer, although it is plotted as a monomer. mTORC1 is a ternary complex consisting of mTOR, RAPTOR (mTOR regulation-related protein), and G-BetaL (G-protein β subunit-like protein). The mTORC2 complex, on the other hand, consists of mTOR, G-BetaL, and Rictor. The most well-known effectors studied downstream of mTOR are two signaling pathways that act in parallel and control the translation of mRNA. Activated mTOR mediates phosphorylation of eIF4EBP1 (eukaryotic translation initiation factor-4E binding protein-1) and ribosome proteins p70S6K or S6K1 (S6 kinase). 4EBP1 (also known as PHAS1) is a small molecule protein that inhibits the activity of the eIF4F (eukaryotic cell initiation factor-4) complex. In the non-phosphorylated state, 4EBP1/PHAS1 binds to the mRNA cap of the eIF4F complex in combination with the subunit eIF4E (eukaryotic translation initiation factor -4E), thereby inhibiting the activity of eIF4E initiation protein synthesis. mTOR phosphorylates 4EBP1, reducing its affinity with eIF4E and dissociates the two proteins. eIF4E can then bind to other components of eIF4F, including the large scaffold protein eIF4G (eukaryotic translation initiation factor -4-γ), the RNA helicase eIF4A (eukaryotic translation initiation factor -4A) and eIF4B (eukaryotic translation initiation factor -4B) dependent on adenosine triphosphate to form an active complex. This complex facilitates the translation of hat-dependent proteins. The net effect is an increase in translation of a subset of mRNAs with 5'-untranslated regions that typically encode proteins associated with proliferative responses in the cell cycle and transition from the G1 phase to the S phase. These mRNAs include those encoding c-Myc, CCND1 (Cyclin-D1), and ornithine decarboxylase. Cyclin-D1 binds to CDK4 to form the complex required for phosphorylation of Rb (retinoblastoma protein), facilitating cell cycle and DNA replication. Deprivation of growth factors or inhibition of mTOR results in 4EBP1 dephosphorylation and rebinding with eIF4E, followed by a decrease in cap-specific translation. mTOR may also indirectly affect the phosphorylated state of 4EBP1 by regulating the activity of PP2A (protein phosphatase-2A). The second major effector downstream of mTOR is S6K1 serine/threonine kinase. After receiving the PI3K/Akt pathway-mediated proliferation upstream signal, mTOR phosphorylates and activates S6K1. In turn, S6K1 phosphorylates and activates the 40S ribosome S6 protein, promoting the recruitment of the 40S ribosome subunit into the activated translational polymer. In particular, the translation of mRNAs with 5'-top (5'-T-terminal oligopyrimidine) sequences is enhanced. These mRNAs with 5'-TOP primarily encode ribosomal proteins, elongation factors, and IGF-II (insulin-like growth factor-II). Dephosphorylation of S6K1 reduces the synthesis of various components of the protein translation system, resulting in a significant reduction in protein synthesis. mTORC1 also regulates VEGF (vascular endothelial growth factor) by phosphorylatizing HIF1Alpha (hypoxia-inducible factor-1-alpha subunit) (References 8, 9, and 10).
In addition to its effect on translation, mTOR also regulates protein synthesis by modulating RNA polymerases I and III, which are responsible for the transcription of ribosomes and transported RNA. In the presence of appropriate growth signals such as IGF1, mTOR, together with the PI3K and MAPK pathways, regulates the transcription of Pol I-mediated ribosome RNA. There is also evidence that mTOR may modulate the phosphorylation state of Rb by influencing the stability and expression of Cyclin-D1 and p27 that regulate cdK upstream of Rb, thereby having an effect on polymerase. mTORC2 may signal to the actin cytoskeleton through a small Rho-type GTPase and PFC. In addition, mTORC2 controls the formation of activated, GTP-bound rac1 in a growth factor-dependent manner. mTORC2 also α controls phosphorylation and activation of protein Kinase-C-Alpha (PKC-Alpha). As a central regulator of proliferative signal transduction, mTOR is an ideal target for the treatment of tumors. Through extensive elucidation of many signal transduction pathways, mTOR kinases are involved in key events that integrate external and internal signals, coordinating cell growth and proliferation. mTOR receives signals that indicate whether transcription and translation mechanisms should be upregulated, and then effectively transmits those signals to the appropriate pathways. In many cancer types, multiple components of the signaling pathway that transmit signals through mTOR are out of balance. The development of mTOR inhibitors is a rational treatment strategy for the treatment of malignant tumors characterized by dysregulation of the mTOR signaling pathway (Reference 9&11).
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This article was originally produced by The Scientific Assistant (ID: SciRes).