Mammilian Target of Rapamycins mTOR
The growth of a eukaryotic cell is regulated by both substances present in the extracellular environment (growth factors, hormones etc…) and by the presence, inside the cell itself, of molecules of “nourishment”, such as amino acids and glucose. In both cases, through a rapid succession of events (mainly activation/inactivation of signal protein), determines a modification, more or less temporary, of gene expression in response to the availability or lack of signals. These signals can achieve: 1) the nucleus of the cell, determining changes in the transcription of specific genes, and 2) the cytoplasm, altering the use of specific mRNA. The major route of signal transduction apparatus of protein synthesis includes the kinase mTOR and S6K1. mTOR is a mediator of the rapid phosphorylation-dephosphorylation of S6K1 in response to various signals; S6K1 (ribosomal protein S6 kinase) phosphorylates the ribosomal protein S6 and its activation is correlated with stimulation of cell growth. Furthermore mTOR and S6K1 are also involved in the response of the cell to the availability of amino acids. One of the downstream targets of mTOR pathway-S6K1 is the group of TOP mRNAs whose translation is regulated so dependently by cell growth.
mTOR ( mammalian target of rapamycin) is an enzyme found in 2006 at the Obesity Research Center at the University of Cincinnati (Ohio-USA) by Italian researcher Daniela Cota. The target of rapamycin for mammals (mTOR) is a protein which in humans is encoded by the gene FRAP1. mTOR is a serine and threonine protein kinase that regulates the growth, proliferation, motility and survival of cells, protein synthesis and transcription. mTOR has an important role in the regulation of energy balance and body weight. It is activated by amino acids, glucose, insulin and other hormones that regulate metabolism. mTOR acts as an hypothalamic sensor for leucine in particular, but also for other amino acids. When lecithin is assumed mTOR get into the action and decreases the sensation of appetite.
The mTOR pathway is regulated and integrates the stimuli coming from a large variety of cellular signals, including mitogens, growth factors (such as IGF-1 and IGF-2), hormones such as insulin, nutrients (amino acids, glucose), the cellular energy levels and stress conditions. MTOR is a main path through the PI3K/AKT (v-Akt Murine Thymoma Viral Oncogene homolog-1) of signal transduction, which is critically involved in the mediation of cell survival and proliferation. Signalling through the PI3K/AKT pathway is initiated by stimuli from mitogens and growth factors that bind to cell membrane receptors. These receptors are IGFR (Insulin-like Growth Factor Receptor), PDGFR (Platelet-Derived Growth Factor Receptor), EGFR (Epidermal Growth Factor Receptor) and its family. The signal activated from the receptors is transferred directly to the PI3K/AKT pathway, or, alternatively, can be activated through the activation of growth factor receptors through Ras oncogenes. Ras is another central switch for the signal transduction and it has shown to be a cornerstone activator of MAPK (mitogen- protein kinase activated) of signal transduction. PI3K/AKT pathway can also be activated by insulin through IRS-1/2 (insulin receptor Substrate-1/2). mTOR also modulates protein synthesis through the regulation of RNA polymerase I and III, which are responsible for the transcription of ribosomal RNA and transfer. In the presence of appropriate growth signals such as IGF1, mTOR, together with PI3K and MAPK pathways, modulates the transcription of Pol I- ribosomal RNA. There is also evidence that mTOR may exert its effects on the polymerase through regulation of the phosphorylation status of Rb influencing the stability and expression of cyclin D1 and p27. mTOR as a central modulator of proliferative signal transduction is the ideal therapeutic target against cancer. Through extensive explanations of many signal transduction pathways, it is clear that the mTOR kinase participates in critical events that integrate external signals with internal signals, coordinating cell growth and proliferation. mTOR receives signals that indicate whether the transcription and translational machinery can be up-regulated and efficiently transmits these signals to the appropriate channels. More components of paths through the mTOR signal are deregulated in several types of cancer. The development of inhibitors of mTOR is a rational therapeutic strategy for malignant tumors that are characterized by deregulation paths of the signal through mTOR. The mTOR pathway appears deregulated in several human diseases, especially in certain types of cancer. Rapamycin is an antibiotic that can inhibit mTOR by associating with its intracellular receptor FKBP12 or cyclophilin A. The FKBP12-rapamycin complex binds directly to the binding domain of FKBP12-rapamycin mTOR. It seems that this is the same binding site where mTOR can bind the phosphatidic acid, a second messenger derived from hydrolysis of phosphatidylcholine by phospholipase D. mTOR is the basis of extensive laboratory studies because its pharmacological inhibition was found to be a powerful way to suppress the growth of many types of cancers, such as leukemia, myelodysplasia, glioblastoma, breast, liver and pancreas cancer. In addition to rapamycin, in fact, have already been processed some experimental drugs specific for mTOR that they are studying on tumor cells in culture.
mTOR is the catalytic subunit of two molecular complexes called mTORC1 and mTORC2. mTORC1 mTOR Complex 1 (mTORC1) is composed of mTOR, the G regulatory protein of mTOR called Rheb, a protein similar to the beta subunit of the protein LST8/G of mammals (mLST8/GβL), PRAS40 and DEPTOR, recently identified. This complex is characterized by classical features of mTOR since it works as a sensor for nutrients, energy, redox level and it controls protein synthesis. The activity of this complex is stimulated by insulin, growth factors, serum, phosphatidic acid, amino acids (in particular leucine) and oxidative stress. mTORC1 is inhibited by a low level of nutrients, by a deficiency of growth factors, by reductive stress, by caffeine, by rapamycin, by acid farnesyltyosalicylic and curcumin.
The two best characterized targets of mTORC1 are the protein-kinase 1 p70 S6 (S6K1) and 4E-BP1, or the protein that binds the eukaryotic initiation factor 4E. mTORC1 phosphorylates S6K1 on at least two residues causing mostly the modification of a threonine residue (T389). This event stimulates the subsequent phosphorylation of S6K1 by PDK1. The active S6K1 can now stimulate the initiation of protein synthesis through the activation of the ribosomal protein S6, a component of the ribosome and other components of the transcriptional apparatus.
S6K1 can also participate in a positive feedback loop with mTORC1 by phosphorylating the negative regulatory domain of mTOR on two sites, which appears to stimulate the activity of mTOR. It has been demonstrated that mTORC1 phosphorylates at least four residues of 4E-BP1 in a hierarchical way. Non-phosphorylated 4E-BP1 binds tightly to the starting factor of transcription eIF4E and prevents its binding to mRNA and their recruitment to the ribosomal initiation complex. Under phosphorylation by mTORC1, 4E-BP1 releases eIF4E, allowing it to perform its function. The activity of mTORC1 appears to be regulated by a dynamic interaction between mTOR and Raptor, mediated by GβL. Raptor and mTOR share a strong interaction in the region N-term and a weak interaction to C-term close to the domino kinase mTOR. When you are warned stimulatory signals, such as high levels of nutrients or energy, the interaction between mTOR and Raptor in C-term is weakened and possibly completely lost, allowing the activation of the kinase activity of mTOR. When the stimulatory signals are removed, for example if there are low levels of nutrients, the interaction between mTOR and Raptor to C term is reinforced, strongly deactivating the function of mTOR kinase. In addition to amino acids and glucose, fatty acids also can adjust the mTORC1 complex. In the heart, for example, free fatty acids are potent activators of the cascade that leads to its activation. In this case the activation of mTOR is the consequence of inhibition of protein kinase activated by adenosine monophosphate (AMPKalpha), involved in the control of cellular energy. In cardiac tissue, an increased oxidation of fatty acids leads to a ratio reduction of AMP/ATP, which inhibits the kinase AMPKalpha. This in turn, can modulate the transduction TORC1-dependent with at least two modes. In the first case it phosphorylates TSC2 protein; in the second, directly phosphorylates mTOR. In both cases, there is activation of the function of these proteins.
In the case of physical exercise, in addition to stimulation due to increased uptake of amino acids at the level of the muscle cells, the control of TORC-1 may depend in part from the axis phospholipase D/phosphatidic acid (PLD2/PA). It is reported from a study, in fact, that the mechanical stimulation of muscle preparations ex vivo leads to activate the function of the enzyme PLD, phosphatidic acid accumulation and increased operation of complex TORC-1
mTOR Complex2 (mTORC2) consists of mTOR, by Rictor, from GβL and from protein 1 that interacts with the mammalian kinase activated by stress (MAPK/APK-1). It has been shown that mTOR has an important regulatory function of the cytoskeleton through the stimulation of fibres of F-actin. mTORC2 also seems to hold the asset that was previously attributed to a protein kinase known as “PDK2” or second phosphoinositide-dependent kinase (Phosphoinositide-Dependent Kinase 2). mTORC2 kinase phosphorylates serine/threonine Akt/PKB on serine residue S473. The phosphorylation of this serine stimulates the phosphorylation of Akt by a threonine (T308) of PDK1 and involves full activation of Akt. Curcumin inhibits both preventing phosphorylation on serine.
mTORC2 seems to be regulated by insulin, growth factors, serum and nutrient levels. MTORC2 was originally identified as an entity not sensitive to rapamycin, since a high exposure to rapamycin did not affect the activity of mTORC2 or the phosphorylation of Akt. Anyway, subsequent studies have shown that, at least in some cell lines, chronic exposure to rapamycin, although not giving effects on pre-existing mTORC2, can bind to free mTOR, thus inhibiting the formation of new mTORC2.
In both cases, through a rapid succession of events (mainly activation/inactivation of protein signal), determines a modification, more or less temporary, of gene expression in response to the availability or lack of signals. These signals can achieve: 1) the nucleus of the cell, determining changes in the transcription of specific genes and 2) the cytoplasm, altering the use of specific mRNA. The major route of signal transduction to the apparatus of protein synthesis includes the kinase mTOR and S6K1.
mTOR is a mediator of the rapid phosphorylation-dephosphorylation of S6K1 in response to various signals; S6K1 (ribosomal protein S6 kinase) phosphorylates the ribosomal protein S6 and its activation is correlated with stimulation of cell growth. Furthermore, mTOR and S6K1 are also involved in the response of the cell to the availability of amino acids. One of the downstream targets of mTOR pathway-S6K1 is the group of TOP mRNAs whose translation is regulated dependently by cell growth.