Amino acids and mTORC1: from lysosomes to disease

“In skeletal muscle, glucose import is suppressed and skeletal muscle waste ensues as a consequence (reviewed in [75]).”

“In addition, it is intriguing that only a very few mutations in mTOR have been found in human sporadic cancers [100,101].”

“Furthermore, aberrant mTORC1 potently suppresses autophagy, which may play a role in tumor suppression [62,63,79].”

“The mTOR kinase nucleates two distinct core complexes that have different kinase specificity and distinct protein partners (reviewed in [2]).”

“The DNA damage response, a signal cascade that is initiated by the detection of DNA double-strand breaks and other genetic insults, culminates in activation of the tumor suppressor transcription factor p53, which transactivates AMPK and TSC2, contributing to mTORC1 inhibition [13,14].”

“In parallel, AMPK phosphorylates Raptor [12], leading to direct inhibition of mTORC1, possibly through structural destabilization of the complex.”

“Manipulating mTORC1 activity in this way may also prove important for anti-aging purposes, as suggested by the fact that rapamycin delays aging in mammals [106,107].”

“Despite minimal side effects and outstanding pharmacokinetics, rapamycin has efficacy against only a few pathologies associated with high levels of mTORC1 activity, such as renal cell carcinoma [102] and tuberous sclerosis [103–105].”

“Moreover, it was observed that amino acid withdrawal from cells and organisms triggered autophagy, a process of cellular self-eating where pre-existing proteins and organelles are broken down into simpler metabolites via lysosomal degradation [21].”

“Dietary protein intake is an important regulator of the IGF-1/mTOR network (Efeyan et al, 2012).”

“…effects stem at least in part from the ability of arginine to activate mTORC1, a master growth controller that integrates diverse environmental inputs to coordinate many anabolic and catabolic processes in cells (Ban et al., 2004; Dibble and Manning, 2013; Efeyan et al., 2012; Hara et al., 1998).”

“Although it was clear for over a decade that amino acids were vital for mTORC1 activation, precisely how this signal functioned remained a mystery (32, 33).”

“The mammalian target of rapamycin (mTOR) is central for regulation of the autophagic activity and senses the energy status by integrating signals from, for example, growth factors, amino acids, and glucose (Efeyan et al., 2012).”

“The activation of the pro-aging and procancer mTOR pathways is modulated by energy intake but also by a cocktail of different essential amino acids, with the branched-chain amino acids leucine and isoleucine playing a fundamental role (82).”

“Finally, mutations affecting mammalian amino acid sensing components are linked to immunodeficiency, epilepsy, and cancer (Shimobayashi & Hall, 2015), and mTOR is often deregulated in metabolic disorders such as obesity, diabetes, and cancer (Efeyan et al , 2012; Liko & Hall, 2015).”

“…as a cellular nutrient depot, a function that is evolutionarily conserved from fungi to higher eukaryotes (Chantranupong et al. 2015, Efeyan et al. 2012, Li & Kane 2009). mTORC1 is found on the surfaces of lysosomes and vacuoles in both yeast and metazoan cells, and the mechanisms…”

“However, cells and organisms possess distinct systems to sense fluctuations in nutrients and energy to properly adapt their metabolic state to nutrient availability (feast) or depletion (famine) [1].”

“Gtr1 and Gtr2 are Ras-family GTPases represented in metazoans by the orthologous Rag GTPases, Rag A-D (Sancak et al., 2008; Efeyan et al., 2012).”

“When RagA/B is loaded with GTP, the Rag heterodimer recruits mTORC1 to the lysosomal surface where it binds Rheb and becomes activated (Bar-Peled et al., 2013; Bar-Peled et al., 2012; Efeyan et al., 2012).”

“This process occurs via amino acid-regulated associations between the v-ATPase, Rag-GTPases, and Ragulator, ultimately allowing the v-ATPase to promote lysosomal function while functioning as a signaling molecule for nutrient sensing (Efeyan et al., 2012; Sancak et al., 2010; Sancak et al., 2008).”

“The mTORC1 is a central sensor of cellular amino acids that samples amino acids in both cytosol and lysosome [184, 185].”

“Amino acids must be present in the cellular environment for mTORC1 to be active and are sensed through a signaling pathway that culminates with the nucleotide loading of the Rag GTPases (reviewed in Bar-Peled and Sabatini, 2014; Efeyan et al., 2012; Kim et al., 2013; Yuan et al., 2013).”

“The kinase activity of mTOR is tightly regulated by nutrient availability; when cells are starved of nutrients (specifically glucose and amino acids), they downregulate mTORC1 signaling, while retreatment of cells with these nutrients restores mTORC1-mediated signaling (26).”

“However, long overlooked as a static organelle, lysosomes are now known to be a dynamic hub of cellular activity, including cellular metabolism, nutrient sensing, subcellular localization-dependent pH control, plasma membrane repair, secretion, and spatiotemporal intracellular signaling [52,21,53,22,20].”

“For instance, budding yeast lacks a canonical insulin-PI3K pathway, thus negating the need for a coincidence detection mechanism (Efeyan et al., 2012). mTORC1 activation by the PI3K-RHEB axis The small GTPase Ras homologue enriched in brain (RHEB) contains a C-terminal farnesylation motif that…”

“As most mTORC1 functions are high energy consuming, regulation of mTORC1 activity is tightly coupled to the energy status of the cell and regulated by nutrients, energy, stress and growth factors, thereby ensuring that cells stop growing under unfavorable conditions 1,8,21,22 .”

“These discoveries set the stage for several important reports detailing how amino acids contribute to mTORC1 activation in animal cells (Efeyan et al., 2012).”

“, mitochondria in quiescent HSCs) reduces ROS levels, possibly modulates amino acid efflux and mTOR activation toward HSC priming (Figure S7C, model), and contributes to carbon mass for cell proliferation (Efeyan et al., 2012; Hosios et al., 2016).”

“Activation of the mTORC1 pathway by AAs correlates with the translocation of mTORC1 from undefined locations to lysosomes where it encounters RHEB, a potent mTORC1 activator (Efeyan et al., 2012; Sancak et al., 2008).”

“As a consequence, increased lysosomal rotein degradation ensues resulting in amino acid replenishment, eactivation of MTORC1, and cessation of autophagosome biogensis [117,118].”

“…level, starvation is translated into signaling through the large, serine/threonine kinase mTOR (mammalian target of rapamycin). mTOR is found in two different complexes (reviewed in Efeyan et al. 2012) and in the form of mTORC1 (mTOR complex I) acts as a negative regulator of macroautophagy.”

“In turn, impaired proteolysis could affect lysosomal amino acid sensors , such as mTORC1, thereby affecting lysosomal signalling 132 (FIG.”

“Here, in this brief review,wewill focus the discussion on this ambiguous role of autophagy in the development and progression of cancer.”

“the participation of other amino acid transporters in the lysosomal membrane cannot be ruled out (Efeyan et al. 2012).”

“Excessive intake of energy and essential amino acids stimulates this pathway, whereas calorie and protein restriction inhibits its activity (Giovannucci et al., 2003; Fontana et al., 2008, 2010; Efeyan et al., 2012; Mercken et al., 2013).”

“…in LSD may include the following: storage of cholesterol in the endolysosome membranes 45 ; luminal oxidation of lysosomal thiols by free cystine 10 ; and accumulation of cystine sustaining mTORC1 activity and negatively regulating lysosome biogenesis by suppressing TFE/MiTF signaling 46,47 .”

“Therefore, it is possible that the degree of mTORC1 inhibition will be reflected from differential responses of the mRNAs that are reported as being under the translational control of mTORC1-dependent signaling (78, 79).”

“The nutrient-sensing mTOR is localized to the lysosomal surface (Efeyan et al., 2012; Betz and Hall, 2013), where it modulates autophagy (Yu et al., 2010; Zoncu et al., 2011).”

“ATG9 (6) and LC3 (7) are also regulated by phosphorylation.”

“The Rag GTPases bind raptor and mediate amino acid signaling to mTORC1.”

“MTORC1 is a kinase that regulates the cellular response to amino acids, growth factors, and changing energy levels within the cell.(46) We have previously shown that in astrocytes, CRYBA1 can regulate MTOR signaling by modulating IGF2 (insulin-like growth factor 2 [somatomedin A]).”

“The reader is directed elsewhere for further information regarding upstream signaling mechanisms of autophagy through mTORC1 (He and Klionsky, 2009; Efeyan et al., 2012; Shanware et al., 2013; Sergin and Razani, 2014).”

“…differences between WT and Gpr65 (cid:1) / (cid:1) BMDMs identified differential regulation of genes implicated in lysosomal function ( Atp6v1d and Atp6v1e1 ) and vesicular transport ( Snx10 ), suggesting potential dysregulation of these processes (Figure S5A; Efeyan et al., 2012; Qin et al., 2006).”

“Growth factors or insulin activates classI-phosphatidylinosital-3 kinase (PI3K) and AKT, which leads to mTOR activation through the tuberous sclerosis complex protein (TSC)-Rheb signalling cascade.(142) Activation of mTOR then shuts down autophagy, which is a cellular catabolic process, by regulating the ULK1-FIP200At…”

“the role of Rag GTPases see (Efeyan et al. 2012)].”

“Only if sufficient amino acids are present, full mTORC1 activation by growth factors or PRR ligands occurs (142, 143).”

“…to be regulated by glucose in yeast andmammals (Csibi et al., 2013; Efeyan et al., 2013; Fingar and Blenis, 2004; Kalender et al., 2010; Urban et al., 2007), it is Cytosolic pH Regulates Cell Growth best known for its function in sensing amino acid availability (Broach, 2012; Efeyan et al., 2012).”

“This is an open access article under the CC BY license. essential amino acids activate mTOR via Rag GTPases at the lysosomal membrane (27), it was surprising that suppression of mTOR blocked cell cycle progression at a site later in G1 than the checkpoints that monitor the presence of essential amino acids and Gln (25…”

“High circulating levels of IGF-1 are associated with increased risk of PCa and BC [9-11], Moreover, multiple lines of evidence have shown that activation of the PI3K/ AKT/mTOR pathway, through insulin/IGF-1 stimulation and/or high levels of essential amino acids, play a crucial role in maintaining the malignant phenot…”

“We discovered a large number of translationally upregulated mRNAs that compose the interface of mTORC1 signaling with the lysosome (Efeyan et al., 2012; Bar-Peled and Sabatini, 2014) (Fig.”

“406 Cell Reports 10, 398–413, January 20, 2015 a2015 The Authors is integral to this (Efeyan et al., 2012).”

“The mTORC1 complex includes the regulatory proteins RAPTOR (regulatory-associated protein of mTOR), mLST8 (mammalian lethal with SEC13 protein), and DEPTOR (DEP domain-containing mTOR-interacting protein) (Efeyan et al. 2012).”

“Because the amino-acid-induced activation of mTORC1 requires the formation of heterodimers of RagA with RagC or RagD (Sekiguchi et al., 2001), we determined whether RagA ubiquitination is affected by the presence of RagC.”

“Althoughmuchisknownaboutthedependence of mTORC1 on amino acids (4), less is known about the ability of mTOR to respond to the presence of other nutrients essential for cell growth, such as glucose and lipids.”

“regulating mTOR activity, serum IGF-1 concentrations, and longevity [2, 38, 49-54].”

“It is well established that Leu uptake or feeding increases protein synthesis in cells and tissues via mechanisms that activate mTORC1 (48, 49).”

“The exact mechanism by which leucine activates mTORC1 signalling is still not fully resolved but both cytosolic and lysosomal leucine-sensors have been described [1,2].”

“When RagA, RagB, or Gtr1 is bound to GTP, and RagC, RagD, or Gtr2 to GDP, the respective heterodimer is in its active, TORC1-stimulating conformation.8-11 In mammalian cells, Rag GTPases do not directly activate TORC1, but trigger TORC1 relocalization from the cytoplasm to the limiting membrane of the lysosome, where…”

“I3K and mTOR signalling plays a key role in mediating cellular responses to growth factor and nutrient availability 1,2 .”

“In particular, the amino acid arginine, a known activator of mTORC1 (Ban et al., 2004; Dibble and Manning, 2013; Efeyan et al., 2012; Hara et al., 1998), was remarkably decreased (Figure S1F).”

“Cell growth is regulated by mechanistic target of rapamycin complex-1 (mTORC1), a complex of cytosolic proteins that is activated by growth factor receptor signaling, tumor-promoting phorbol esters, and increased levels of amino acids inside endolysosomes (Roux et al., 2004; Efeyan et al., 2012).”

“Energy and amino acids restriction cause an inhibition of mTORC1 activity, which in turn enhances autophagy, improves proteostasis and stem cell function 61 .”

“In addition to being utilized in various metabolic pathways, BCAA regulate gene expression and signal transduction for protein synthesis (Efeyan et al., 2012; Bai et al., 2015).”

“1 3 acids act to stimulate protein synthesis through mTOR is less well understood, however, it is widely accepted that amino acids stimulate protein synthesis through a pathway independent of the insulin signaling cascade (Efeyan et al. 2012).”

“Heterodimeric complex of tuberous sclerosis complex 1 (TSC1 or hamartin) and TSC2 (or tuberin) is one of the most important sensors involved in the negative regulation of mTORC1 activity.(16) Mice lacking Tsc1 or Tsc2 genes in oocytes all showed an identical phenotype with premature activation of all primordial follic…”

“Last but not least, activation and deregulation of mTORC1 affecting autophagy and lysosomal biogenesis have been implicated in malignant transformation and in sustaining cancer growth, but this subject has been extensively discussed in several reviews (McCarty, 2011; Efeyan et al., 2012; Kimmelman and White, 2017; Sax…”

“While transcripts for Tsc1 that is an upstream negative regulator of mTOR signaling [63] were highly reduced in Foxo3 mutant”

“The mammalian (or mechanistic) target of rapamycin (mTOR) is a master growth regulator (Laplante & Sabatini, 2012) that functions as a signal integrator combining regulatory inputs from nutrients, growth factors, energy levels and stress signals (Efeyan et al., 2012).”

“, p70S6K1) because the kinase activity of mTORC1 has multiple inputs (beyond phosphorylation), including cellular localization and varying affinity to its binding partners (89).”

“Amino acids negatively regulate autophagy via mammalian target of rapamycin (mTOR) signaling, a negative regular of autophagy [14].”

“for Clinical Nutrition and Metabol cell growth or activate energy sparing processes (for a more detailed review see [3,4]).”

“The multiprotein complex mTORC1, which regulates cellular growth and metabolism, localizes in the cytosol under low amino acids conditions, and is recruited on the lysosomal membrane when amino acids are abundant [149,150].”

“The mTORC1 pathway regulates many major cellular processes and is implicated in an increasing number of pathological conditions, including cancer, obesity, type 2 diabetes and neurodegeneration (Efeyan et al., 2012).”

“Activation of mTORC1 by nutrients and/or growth factors typically promotes protein synthesis while, at the same time, suppressing catabolic processes [79,80].”