Open In App

mTOR Signaling Pathway

Improve
Improve
Like Article
Like
Save
Share
Report

Cell signaling (cell communication in British English) is the capacity of a cell to receive, process, and transmit messages with its surroundings and with itself. Cell signaling is a basic characteristic of all prokaryotic and eukaryotic cellular life. Cell signaling can take place across short or long distances, and is thus classed as autocrine, juxtacrine, paracrine, and endocrine. Signaling molecules can be produced via a variety of biosynthetic pathways and released by passive or active transporters, as well as cell injury. 

Receptors are important in cell signaling because they can sense chemical signals as well as physical inputs. Receptors are proteins that are found on the cell surface or within the cell’s interior, such as the cytoplasm, organelles, and nucleus. Additional enzymatic activity such as proteolytic cleavage, phosphorylation, methylation, and ubiquitinylation may occur as a result of these signaling pathways. Each cell is designed to respond to certain extracellular signal molecules, which serve as the foundation for development, tissue repair, immunology, and homeostasis.

mTOR 

The atypical serine/threonine kinase known as mTOR, which is present in two different complexes, is the target of rapamycin’s molecular action. The first is called mTOR complex 1 (mTORC1) and is suppressed by the drug rapamycin. It is made up of mTOR, Raptor, GL, and DEPTOR. It is a major growth regulator that detects and combines a variety of dietary and environmental stimuli, including growth factors, energy levels, cellular stress, and amino acids. These signals are coupled to the stimulation of cellular growth by phosphorylating substrates that enhance anabolic activities like mRNA translation and lipid synthesis or suppress catabolic processes like autophagy. The tuberous sclerosis heterodimer TSC1/2, the small GTPase Rheb’s GTP-bound state’s GAP, adversely controls the activity of mTORC1 kinase, which is stimulated by the small GTPase Rheb. To control the nucleotide-loading status of Rheb, most upstream inputs are routed through Akt and TSC1/2. On the other hand, amino acids communicate with mTORC1 independently of the PI3K/Akt axis to encourage its movement to the lysosomal surface, where it can be activated in response to Rheb. The coordinated actions of several complexes, particularly the v-ATPase, Ragulator, Rag GTPases, and GATOR1/2, mediate this process. mTOR, Rictor, GL, Sin1, PRR5/Protor-1, and DEPTOR make up the second complex, known as mTOR complex 2 (mTORC2). mTORC2 regulates cytoskeletal dynamics by activating PKC, encourages cellular survival by activating Akt, and promotes ion transport and growth by phosphorylating SGK1. Numerous disease states, such as diabetes, cardiovascular disease, and cancer, are characterized by aberrant mTOR signaling.

m-TOR Signaling

The mammalian target of rapamycin (mTOR), also known as the mechanistic target of rapamycin and also referred to as FK506-binding protein 12-rapamycin-associated protein 1 (FRAP1), is a kinase encoded by the MTOR gene in humans. mTOR is a protein kinase that belongs to the phosphatidylinositol 3-kinase-related kinase family.

mTOR interacts with other proteins and is a key component of two unique protein complexes, mTOR complexes 1 and 2, which govern diverse biological activities. mTOR, in particular, works as a serine/threonine protein kinase that governs cell growth, proliferation, motility, survival, protein synthesis, autophagy, and transcription as a fundamental component of both complexes. mTOR operates as an atyrosine-protein kinase that stimulates the activation of insulin receptors and insulin-like growth factor 1 receptors as a fundamental component of mTORC2. mTORC2 has also been linked to actin cytoskeleton regulation and maintenance.

Types of mTOR

The Mammalian/mechanistic target of rapamycin is referred to as mTOR. In the middle of the 20th century, a bacteria discovered on Easter Island yielded the immunosuppressive medicine rapamycin. The moniker mTOR was coined since one of the first impacts that rapamycin was shown to have was on TOR genes.

In reality, mTOR works as a component of a protein complex that is made up of several other proteins that are joined together and have various inhibitory and activating properties. mTORC1 and mTORC2 are two distinct mTOR complexes. They all stabilize mTOR and aid in its ability to connect to its target receptor, despite having somewhat different proteins in the complex.

The ribosomal protein S6K is activated by mTORC1, which increases protein synthesis. In addition, mTORC1 suppresses 4EBP1 activity, which typically prevents protein synthesis. Additionally, mTORC1 promotes mitochondrial biogenesis, lipid synthesis, and downregulates autophagy.

mTORC1 and mTORC2

  1. mTOR is a PI3K-related kinase (PIKK) family serine/threonine protein kinase that forms the catalytic component of two different protein complexes known as mTOR Complex 1 (mTORC1) and 2 (mTORC2) (mTORC2).
  2. The three key components of mTORC1 are mTOR, Raptor (a regulatory protein linked with mTOR), and mLST8 (mammalian lethal with Sec13 protein 8, also known as GL).
  3. Raptor enhances substrate recruitment to mTORC1 by binding to the TOR signaling (TOS) motif present on numerous conventional mTORC1 substrates and is needed for mTORC1 subcellular localization.
  4. mLST8, on the other hand, connects with the catalytic domain of mTORC1 and may maintain the kinase activation loop, while genetic studies show it is required for mTORC1 to operate properly.
  5. The rapamycin-FKBP12 combination binds to the FRB domain of mTOR to narrow the catalytic cleft and partially occlude substrates from the active site, according to a crystal structure of the mTOR kinase domain linked to mLST8.

mTOR Signaling

mTOR Pathway

 

Upstream mTORC1

The Akt/PKB Signaling Pathway

  • Insulin-like growth factors can activate mTORC1 via the RTK-Akt/PKB signaling pathway.
  • Finally, Akt phosphorylates TSC2 on serine 939, serine 981, and threonine 1462. 
  • These phosphorylated regions will attract the cytosolic anchoring protein 14-3-3 to TSC2, breaking the TSC1/TSC2 dimer.
  • When TSC2 is not connected with TSC1, it loses its GAP activity and is unable to hydrolyze Rheb-GTP.
  • This leads to sustained stimulation of mTORC1, allowing for protein synthesis via insulin signaling.
  • Akt will also phosphorylate PRAS40, causing it to detach from the Raptor protein on mTORC1.
  • Because PRAS40 blocks Raptor from attracting mTORC1 substrates 4E-BP1 and S6K1, removing it will allow the two substrates to be recruited to mTORC1 and therefore activated in this way.
  • Furthermore, because insulin is a substance released by pancreatic beta cells in response to blood glucose rise, its signaling guarantees that there is enough energy for protein synthesis to occur.
  • S6K1 has the ability to phosphorylate the insulin receptor and reduce its sensitivity to insulin in a negative feedback loop on mTORC1 signaling.
  • This is extremely important in diabetes mellitus, which is caused by insulin resistance.

MAPK/ERK Signaling Pathway

  • Mitogens such as insulin-like growth factor 1 (IGF1) can activate the MAPK/ERK pathway, which inhibits the TSC1/TSC2 complex and so activates mTORC1.
  • The G protein Ras is linked to the plasma membrane by a farnesyl group in this pathway and is in its inactive GDP form.
  • The adaptor protein GRB2 connects with its SH2 domains when a growth factor binds to the nearby receptor tyrosine kinase, which maintains mTORC1 active.
  • RSK has also been demonstrated to phosphorylate the raptor, which aids in its resistance to PRAS40 inhibition.

JNK Signaling Pathway

  • JNK signaling is a component of the mitogen-activated protein kinase (MAPK) signaling system, which is important in stress signaling pathways including gene expression, neural development, and cell survival.
  • Recent research has revealed a direct molecular connection in which JNK phosphorylates Raptor at Ser-696, Thr-706, and Ser-863.
  • As a result, JNK regulates mTORC1 activity.
  • Thus, JNK activation affects protein synthesis via mTORC1 downstream effectors such as S6 kinase and eIFs.

Downstream TORC1

  •  mTORC1 regulates transcription and translation by interacting with S6K1 and 4E-BP1, the eukaryotic initiation factor 4E (eIF4E) binding protein 1, mostly through phosphorylation and dephosphorylation of its downstream targets.
  • In eukaryotic cells, S6K1 and 4E-BP1 regulate translation.
  • Their signals will converge at the translation initiation complex on the 5′ end of mRNA, resulting in translation activation.

4E-BP1

  • When mTORC1 is activated, it phosphorylates the translation repressor protein 4E-BP1, freeing it from the eukaryotic translation initiation factor 4E. (eIF4E).
  • eIF4E is now free to associate with the eukaryotic translation initiation factors 4G (eIF4G) and 4A (eIF4A) (eIF4A).
  • After binding to the 5′ cap of mRNA, this complex recruits the helicase eukaryotic translation initiation factor A (eIF4A) and its cofactor eukaryotic translation initiation factor 4B. (eIF4B).
  • The helicase is essential to remove hairpin loops that form in the 5′ untranslated regions of mRNA, preventing premature protein translation.

S6K

  • mTOR mediates S6K signaling in a rapamycin-dependent way, with S6K being displaced from the eIF3 complex when mTOR binds to eIF3. 
  • Active S6K1 can then boost protein synthesis by recruiting S6 Ribosomal protein (a ribosomal component) and eIF4B to the pre-initiation complex.
  • S6K1 can interact with mTORC1 in a positive feedback loop by phosphorylating mTOR’s negative regulatory domain at two locations, thr-2446 and ser-2448.
  • Phosphorylation at these locations appears to boost mTOR activity.
  • S6K can also phosphorylate programmed cell death 4 (PDCD4), putting it at risk of degradation by the ubiquitin ligase Beta-TrCP.

mTORC2 Downstream

  • mTORC2 regulates cell survival and proliferation primarily by phosphorylating (PKA/PKG/PKC) protein kinase family.
    mTORC2 controls the actin cytoskeleton via PKC but can also phosphorylate other members of the PKC family that govern cell migration and cytoskeletal remodeling.
  • mTORC2 is involved in the phosphorylation and consequently, activation of Akt, which is a critical signaling component downstream from PI3K once activated, as well as the phosphorylation of SGK1, PKC, and HDACs.

mTORC2’s Upstream

  • mTORC2’s mSin1 subunit, like other PI3K-regulated proteins, has a PH domain that binds phosphoinositides.
  • This domain inhibits mTORC2 catalytic activity in the absence of insulin, which is required for insulin-dependent regulation of mTORC2 activity.
  • The mSin1 subunit can also be phosphorylated by Akt.
  • mTORC1 phosphorylates and thereby activates Grb10, an upstream negative regulator of insulin/IGF-1 receptor signalling.
  • Furthermore, Ric-8B protein and certain lipid metabolites have been discovered as major regulators of mTORC2 activity.

Mechanism of mTOR Signaling Pathway

Growth factors are mostly used in this pathway as the first signaling molecule needed for activation. The pathway is activated when these growth factors bind to tyrosine kinase receptors in the cell membrane. Phosphorylation, in which a phosphate group is placed onto the target enzyme to make it active and extend the route to further targets, activates many of the enzymes involved in this pathway.

Following growth factor binding to membrane receptors:

  1. Phosphorylation activates the PI3 kinase enzyme.
  2. PIP2 is converted to PIP3 by the enzyme PI3 kinase.
  3. PIP3 stimulates the Akt enzyme, which in turn stimulates the mTOR protein.

Mechanism of mTOR downstream

  • Cellular transformation is linked downstream to the mTOR effectors S6 kinase 1 (S6K1), eukaryotic initiation factor 4E (eIF4E), and eukaryotic initiation factor 4E-binding protein 1 (4EBP1).
  • S6K1 phosphorylates several significant targets and is a crucial regulator of cell development.
  • S6K1 and eIF4E are both involved in cellular transformation, and studies have connected their overexpression to a bad prognosis for cancer.

Mechanism of mTOR upstream

  • Upstream, PI3K/AKT signaling is disrupted by a variety of mechanisms, including growth factor receptor overexpression or activation, such as HER-2 (human epidermal growth factor receptor 2) and IGFR (insulin-like growth factor receptor), PI3K mutations, and AKT mutations/amplifications.
  • PTEN, a tumor suppressor and tensin homolog lost on chromosome 10, is a negative regulator of PI3K signaling.
  • PTEN expression is reduced in many malignancies and may be downregulated by a variety of processes including mutations, loss of heterozygosity, methylation, and protein instability.

Mechanism of mTOR2

mTORC2 also controls cellular proliferation and metabolism, in part via modulating IGF-IR, InsR, Akt/PKB, and the serum- and glucocorticoid-induced protein kinase SGK. Furthermore, mTORC2 activity has been linked to autophagy control (macroautophagy and chaperone-mediated autophagy). The specific location of mTORC2 within cells is yet unknown. Some discoveries based on its activity indicate cellular endomembranes, such as mitochondria, as a likely location of mTORC2; however, this might be owing to its interaction with Akt. It is unclear whether these membranes exhibit mTORC2 activity in the cellular setting or contribute to the phosphorylation of mTORC2 substrates.

Regulation of mTOR

Growth factors, as well as amounts of amino acids, glucose, and oxygen, are just a few of the upstream signals that control mTORC1 activity. Most signals regulate mTORC1 largely by one of two mechanisms: direct mTORC1 component alteration or control of Rheb, a small GTPase that, when coupled to GTP, directly interacts with and activates mTORC1. A fascinating process that involves Rheb, but also the Rag GTPases, and is covered in its own section below, is how amino acid signaling to mTORC1 works. Akt phosphorylates TSC2 together with other kinases involved in growth factor signaling, including MAPK and p90 RSK1. The GTPase activating protein (GAP) for Rheb is TSC2, also known as tuberin, and along with TSC1 (also known as hamartin), they make up the heterodimeric tuberous sclerosis complex (TSC).

mTOR Inhibitors

The mechanistic target of rapamycin (mTOR), a serine/threonine-specific protein kinase that is a member of the phosphatidylinositol-3 kinase (PI3K) related kinase family, is inhibited by a class of medications known as mTOR inhibitors (PIKKs). Two protein complexes, mTORC1, and mTORC2, which mTOR forms and signals through, control cellular metabolism, growth, and proliferation. The so-called rapalogs (rapamycin and its analogs), which have demonstrated tumor responses in human studies against a variety of tumor types, are the most well-established mTOR inhibitors.

mTOR inhibitors of the first generation

Rapamycin’s reintroduction as an anticancer drug started with the discovery of temsirolimus in the 1990s (CCI-779). The toxicological profile of this brand-new soluble rapamycin derivative in animals was good. Since then, new rapamycin derivatives with improved pharmacokinetics and fewer immunosuppressive side effects have been developed for the treatment of cancer. These rapalogs include temsirolimus (CCI-779), everolimus (RAD001), and ridaforolimus, which are being examined in cancer clinical studies (AP-23573). Comparable therapeutic advantages to rapamycin are provided by rapamycin analogs. The National Cancer Institute published a list of more than 200 clinical research in 2012 that investigated the anticancer potential of rapalogs as a monotherapy or component of combination therapy for various cancer types. First-generation mTOR inhibitors known as rapalogs are effective in a range of preclinical conditions. Rapalogs are insufficient for generating a broad and powerful anticancer impact, at least when used as monotherapy, due to the previously mentioned restricted mTOR inhibition.

Clinical Significance

  1. Any flaws in mTOR function can result in the development of cancer since mTOR actively participates in the activation of genes linked to cell proliferation. For instance, some genes that mTORC1 and mTORC2 activate can stop cells from dying naturally and enhance food intake, leading to unchecked cell proliferation and tumor development.
  2. Another major cause of heart hypertrophy, which is a major risk factor for cardiac morbidity and cardiac-related mortality, is considered to be hyperactivation of the mTOR pathway.
  3. The mTOR pathway has been demonstrated to play a crucial part in the complicated process of aging, which is influenced by several elements at the cellular level as well as in human living. Regulation and upkeep of mTOR are essential for health due to their function in immune response and cellular senescence. It has also been demonstrated that mTOR signaling is involved in studies that try to lengthen the tissue’s lifetime.

Energy and Glucose Sensing

By lowering ATP levels, the decreased glycolytic flux brought on by glucose restriction suppresses mTORC1. mTORC1 is inhibited by 2-Deoxy-Glucose (2DG), a hexokinase inhibitor, or the mitochondrial uncoupler FCCP in wild-type cells but not in cells lacking TSC2. Low ATP levels are sent to TSC2 by the 5′AMP-activated protein kinase (AMPK). Reduced ATP production and an increase in the AMP/ATP ratio cause the heterotrimeric kinase AMPK to become active. When there is a shortage of glucose, AMPK phosphorylates TSC2 directly, activating it by an unknown method and suppressing mTORC1 in the process. For cells to survive, this procedure is crucial.

Oxygen Detection

Low oxygen levels, or hypoxia, inhibit mTORC1 signaling in a variety of mechanisms. Hypoxia also inhibits mTORC1 via the actions of Redd1/RTP801, which was first discovered in Drosophila as Scylla and Charybdis. These genes generate hypoxia-inducible RNAs that enigmatically suppress the activity of mTORC1. A similar pathway connecting AMPK and PI3K, Redd1’s activation of TSC2, and the ensuing inhibition of mTORC1 have been discovered through genetic research. Injuries other than hypoxia, such as energy deprivation, DNA damage, glucocorticoids, and oxidizing agents, activate Redd1, showing that Redd1 performs a variety of roles in sending stress signals to mTORC1. In addition to AMPK and Redd1, there are alleged other pathways by which hypoxia decreases mTORC1, but these need to be verified and further studied.

Detecting Amino Acids

A lack of amino acids puts cells under stress, and they react by initiating and inhibiting a variety of activities. Low concentrations of amino acids suppress TORC1 signaling in a wide range of organisms, including yeast and humans. Deletion of dTOR in flies mimics amino acid withdrawal by imitating the breakdown and mobilization of nutritional stores, which prevents larval development and causes the accumulation of lipid vesicles in the larval fat body. In C. elegans, the deletion of the intestinal amino-acid transporter pep-2 disrupts insulin and ceTOR signaling, which shortens body length and reduces the number of offspring via sluggish post-embryonic growth. When ceTOR is inhibited, worms are better able to withstand environmental hazards including heat and oxidative damage.

Functions of mTOR in Healthy Brain Development

Because mTOR is required for the development, division, and migration of every cell, it is thought to be essential for organism growth. In fact, mTOR was found to be crucial for good development and survival in studies employing knockout (KO) mice. The first genetic evidence that mTOR is essential for brain development came from a mouse model with ethyl-nitroso-urea-induced mutations. This mutant, called flat-top, was a loss-of-function mTOR mutant brought on by misplacing; it showed a deficit in the development of the telencephalon and died in the midst of pregnancy. The milder phenotype compared to complete KO may be due to the partial loss of mTOR function. In actuality, the mutant mouse’s p70S6K activity was still 10% lower than that of the wild-type mouse.

Functions of mTOR in the formation of Dendrites

The impact of mTOR on neurite formation is thoroughly morphologically investigated by culture studies. Particularly, when constitutively active or dominantly negative forms of PI3K, Akt, and Ras were transfected, it was demonstrated that PI3K-Akt accelerated the development of the soma and dendrites. Ras and PI3K-Akt coupling leads to more complex dendrites in hippocampal neurons. Similar to this, siRNA-mediated inhibition of the phosphatase and tensin homolog (PTEN), the phosphatase for Akt, induced hippocampal dendritic arborization. The dendritic formation induced by these therapies was reduced by chronic rapamycin therapy, siRNA-mediated inhibition of mTOR and p70S6K, and overexpression of phosphorylation-defective mutant 4EBP. These results imply that dendritic maturation and development depend on mTOR, namely mTORC1.

Functions of mTOR in axon extension

Axon orientation is controlled throughout development by a balance of chemotactic inputs from attracting and repellent chemicals. Semaphorin-3 and netrin-1 have been shown to cause growth cone collapse and repulsive turning in Xenopus retinal neurons. The process was stopped by rapamycin as well as the protein synthesis inhibitors cycloheximide and anisomycin. The phosphorylation of 4EBP by growth cones in response to semaphorin-3 and netrin-1 has been observed. Slit2 has been shown to have rapamycin-sensitive effects on 4EBP phosphorylation and growth cone collapse, albeit at a later stage. The aberrant retinogeniculate projection caused by TSC2 haploinsufficiency in mice (TSC-/+) served as a demonstration of the disruption of axon guidance. Since it is known that ephrin-eph communication is crucial for this tract’s axon guidance, the effect of ephrinA on the mTOR pathway was examined.

Gene deletion (gene knockout)

A gene knockout is a genetic procedure in which one of an organism’s genes is rendered inactive (abbreviated as KO) (“knocked out” of the organism). However, the term “KO” can also apply to the creature that contains a gene knockout or the knockout gene itself. Gene function is studied using knockout creatures, or simply knockouts, often by examining the consequences of gene deletion. Researchers deduce conclusions from the distinction between the knockout organism and healthy people.

A gene knock-in is essentially the reverse of the KO approach. A double knockout occurs when two genes are simultaneously deleted from an organism (DKO). In a similar manner, three or four knocked-out genes are referred to as triple knockouts (TKO) or quadruple knockouts (QKO), respectively.

 Inhibition by Rapamycin

The mechanism underlying the cell specificity of rapamycin-induced mTORC2 inhibition is unknown, which is particularly significant given that many of the negative metabolic side effects of rapamycin reported in mouse studies and human clinical trials have recently been attributed to mTORC2 inhibition. The expression levels of several FK506-binding proteins (FKBPs), especially FKBP12 and FKBP51, are identified as important factors for rapamycin-mediated inhibition of mTORC2. In support of this, reducing FKBP12 totally changes a cell line that is sensitive to mTORC2 inhibition to an insensitive cell line, while increasing FKBP12 expression can augment mTORC2 inhibition.

Further FKBP12 decrease in cell lines with already low FKBP12 levels totally prevents rapamycin-induced mTORC1 inhibition, demonstrating that relative FKBP12 levels are important for both mTORC1 and mTORC2 inhibition, although at different levels. FKBP51 deficiency, on the other hand, makes cells more susceptible to mTORC2 inhibition. Our findings show that the expression of FKBP12 and FKBP51 is the rate-limiting factor that defines a cell line’s or tissue’s rapamycin responsiveness. These discoveries have implications for treating particular diseases such as neurodegeneration and cancer, as well as general anti-aging efforts. 06-binding proteins (FKBPs), namely FKBP12 and FKBP51, as important determinants of rapamycin-mediated mTORC2 inhibition

FAQs on mTOR Signaling

Question 1: How does mTOR inhibition work?

Answer:

The regulatory-associated protein of mTOR (raptor) binding to mTOR is prevented by the suppression of mTOR, despite the fact that this binding is required for the phosphorylation of S6K1 and 4EBP1 downstream. S6K1 consequently dephosphorylates, reducing protein production as well as cell size and mortality.

Question 2: How exactly does mTOR control metabolism?

Answer:

The mechanistic target of rapamycin (mTOR), which integrates signals from nutrition, growth factors, and other environmental cues, regulates cellular metabolism. mTOR is a component of the two evolutionarily conserved protein complexes, mTORC1, and mTORC2, found in yeast and humans.

Question 3: What takes place when mTOR is turned on?

Answer:

The transcription of genes involved in glycolysis, the pentose-phosphate pathway (PPP), and de novo lipogenesis are encouraged after mTORC1 is activated. Hypoxia-inducible factor 1 (HIF1) is a transcription factor that upregulates glycolysis.

Question 4: What is the purpose of mTOR signaling?

Answer:

The mTOR signaling pathway, which is frequently activated in tumors, is crucial for tumor metabolism because it not only controls protein synthesis and gene transcription to control immune cell differentiation and cell proliferation.

Question 5: How is the mTOR pathway stimulated?

Answer:

IGF-I/insulin, mechanical stimulation, and amino acids (blue lines) activate mTORC1 whereas glucocorticoids and myostatin suppress it (red lines). Skeletal muscle protein synthesis is increased when mTORC1 is activated.

Question 6: How is mTOR activated?

Answer:

Amino acids, insulin, and growth hormones are known to trigger mTOR signaling, which is inhibited by dietary or energy deprivation. mTOR is important for cell physiology.



Last Updated : 12 Jan, 2024
Like Article
Save Article
Previous
Next
Share your thoughts in the comments
Similar Reads