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Sensing of Cellular Signaling by TSC Complex and mTORC1. The TSC complex represents a hub for incoming cellular signals ranging from hypoxia, DNA damage, and energy levels to growth factors, Wnt, and TNF signals. Signaling mediators and/or kinases modulate the GAP activity of the TSC complex to inhibit or activate Rheb. An inactive TSC complex results in active mTORC1 thereby promoting protein and lipid synthesis, energy metabolism, and inhibition of lysosomal biogenesis and autophagy.
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Over the last few years extensive studies have linked the activity of mTORC1 to lysosomal function. These observations propose an intriguing integration of cellular catabolism, sustained by lysosomes, with anabolic processes, largely controlled by mTORC1. Interestingly, lysosomal function directly affects mTORC1 activity and is regulated by ZKSCAN3...
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... has been shown to integrate major intracellular and extracellular signaling pathways including growth factors signaling, amino acids sensing, energy status, and cellular stress (Figure 3). Such mTORC1-mediated Figure 1: Lysosomal Homeostasis. ...
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... TSC complex acts as a central hub and conduit (Figure 3), integrating and conveying several upstream signals that impact mTORC1 activity. Insulin-like growth factor 1 (IGF1) is a well-characterized signal, which stimulates the PI3K and Ras pathways upstream of TSC1. ...
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... thorough studies of the functional role of TNFα, the mechanism by which the cytokine affects mTORC1 activity remains unclear. A recent study of the mechanism in MCF-7 cells indicated that TNFα signals independently of PI3K/Akt to induce IκB kinase (IKK) β-mediated inactivation of TSC1/TSC2 to promote mTORC1 activity (Figure 3; [67]). Indeed, suppression of TSC1 by TNFα may help clarify how inflammation may support tumor angiogenesis. ...
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... Introduction MTORC1 (mechanistic target of rapamycin kinase complex 1) is an evolutionarily conserved serine/threonine kinase that controls organismal growth, and has been found to be deregulated in several devastating human diseases including cancer and neurodegeneration [1,2]. MTORC1 integrates major intracellular and extracellular signaling pathways including growth factor signaling, amino acid sensing, energy status, and cellular stress [1,3]. ...
... MTORC1 integrates major intracellular and extracellular signaling pathways including growth factor signaling, amino acid sensing, energy status, and cellular stress [1,3]. Such MTORC1-mediated signal integration allows for well-controlled regulation of anabolic and catabolic processes such as protein and lipid synthesis, autophagy, lysosomal biogenesis, and energy metabolism that together regulate cellular growth and homeostasis [2,4]. The MTORC1 complex is localized to endolysosomal membranes through an amino acid-dependent mechanism that involves the heterodimeric RRAGA/B-RRAGC/D GTPases. ...
... When positioned on endolysosomal membranes, MTORC1 kinase activity is stimulated by RHEB, a membrane-tethered RAS homolog. Therefore, 2 key sequential events are required for MTORC1 activation: translocation of the complex to endolysosomal membranes, and stimulation of its kinase activity by the GTP-bound RHEB, which is differentially regulated by growth factor signaling [1,2]. MTORC1 controls lysosomal biogenesis and degradation through phosphorylation and inhibition of different members of the basic helix-loop-helix leucine zipper family of transcription factors, including TFEB (transcription factor EB) [1,[5][6][7][8]. ...
The mechanistic target of rapamycin kinase complex 1 (MTORC1) is a central cellular kinase that integrates major signaling pathways, allowing for regulation of anabolic and catabolic processes including macroautophagy/autophagy and lysosomal biogenesis. Essential to these processes is the regulatory activity of TFEB (transcription factor EB). In a regulatory feedback loop modulating transcriptional levels of RRAG/Rag GTPases, TFEB controls MTORC1 tethering to membranes and induction of anabolic processes upon nutrient replenishment. We now show that TFEB promotes expression of endocytic genes and increases rates of cellular endocytosis during homeostatic baseline and starvation conditions. TFEB-mediated endocytosis drives assembly of the MTORC1-containing nutrient sensing complex through the formation of endosomes that carry the associated proteins RRAGD, the amino acid transporter SLC38A9, and activate AKT/protein kinase B (AKT p-T308). TFEB-induced signaling endosomes en route to lysosomes are induced by amino acid starvation and are required to dissociate TSC2, re-tether and activate MTORC1 on endolysosomal membranes. This study characterizes TFEB-mediated endocytosis as a critical process leading to activation of MTORC1 and autophagic function, thus identifying the importance of the dynamic endolysosomal system in cellular clearance.
Abbreviations:
CAD: central adrenergic tyrosine hydroxylase-expressing-a-differentiated; ChIP-seq: chromosome immunoprecipitation sequencing; DAPI: 4',6-diamidino-2-phenylindole; DMSO: dimethyl sulfoxide; EDTA: ethylenediaminetetraacetic acid; EEA1: early endosomal antigen 1; EGF: epidermal growth factor; FBS: fetal bovine serum; GFP: green fluorescent protein; GTPase: guanosine triphosphatase; HEK293T: human embryonic kidney 293 cells expressing a temperature-sensitive mutant of the SV40 large T antigen; LAMP: lysosomal-associated membrane protein; LYNUS: lysosomal nutrient-sensing complex; MAP1LC3/LC3: microtubule associated protein 1 light chain 3 alpha/beta; MTOR: mechanistic target of rapamycin kinase; MTORC: mechanistic target of rapamycin kinase complex; OE: overexpression; PH: pleckstrin homology; PtdIns(3,4,5)P3: phosphatidylinositol 3,4,5-trisphosphate; RRAGD: Ras related GTPase binding D; RHEB: Ras homolog enriched in brain; SLC38A9: solute carrier family 38 member 9; SQSTM1: sequestosome 1; TFEB: transcription factor EB; TSC2: tuberous sclerosis 2; TMR: tetramethylrhodamine; ULK1: unc-51 like kinase 1; WT: wild type.
... In light of this evidence, it becomes increasingly clear that such a fundamentally important signalling hub as mTORC1 requires to be tightly controlled in order to maintain healthy balance of anabolic and catabolic processes and ultimately normal physiology at the cellular, tissue, and organismal level. At the very centre of this regulation of mTORC1 signalling is the lysosome, and thereby, this organelle is now considered as a sensor and regulator of the major cellular functions in the cell [44][45][46][47]. ...
The mechanistic target of rapamycin complex 1 (mTORC1) coordinates cellular growth and metabolism with environmental inputs to ensure that cells grow only under favourable conditions. When active, mTORC1 stimulates biosynthetic pathways including protein, lipid and nucleotide synthesis and inhibits cellular catabolism through repression of the autophagic pathway, thereby promoting cell growth and proliferation. The recruitment of mTORC1 to the lysosomal surface has been shown to be essential for its activation. This finding has significantly enhanced our knowledge of mTORC1 regulation and has focused the attention of the field on the lysosome as a signalling hub which coordinates several homeostatic pathways. The intriguing localisation of mTORC1 to the cellular organelle that plays a crucial role in catabolism enables mTORC1 to feedback to autophagy and lysosomal biogenesis, thus leading mTORC1 to enact precise spatial and temporal control of cell growth. This review will cover the signalling interactions which take place on the surface of lysosomes and the cross-talk which exists between mTORC1 activity and lysosomal function.
... So far, no specific allosteric inhibitors are known and both complexes are susceptible to mTOR inhibitors such as Rapamycin or Torin (Hara et al., 2002;Sarbassov et al., 2006). Functionally, mTORC1 has been shown to predominantly tether to lysosomal membranes where it senses the levels of amino acids and growth factors to regulate cellular metabolic processes including protein and lipid synthesis, lysosomal biogenesis, and autophagy (Cafferkey et al., 1993;Kunz et al., 1993;Wullschleger et al., 2006;Nnah et al., 2015). Whether mTORC1 can be activated or localized elsewhere, possibly by differing sets of environmental cues, remains to be determined. ...
Cellular metabolism is regulated by the mTOR kinase, a key component of the molecular nutrient sensor pathway that plays a central role in cellular survival and aging. The mTOR pathway promotes protein and lipid synthesis and inhibits autophagy, a process known for its contribution to longevity in several model organisms. The nutrient-sensing pathway is regulated at the lysosomal membrane by a number of proteins for which deficiency triggers widespread aging phenotypes in tested animal models. In response to environmental cues, this recently discovered lysosomal nutrient-sensing complex regulates autophagy transcriptionally through conserved factors, such as the transcription factors TFEB and FOXO, associated with lifespan extension. This key metabolic pathway strongly depends on nucleocytoplasmic compartmentalization, a cellular phenomenon gradually lost during aging. In this review, we discuss the current progress in understanding the contribution of mTOR-regulating factors to autophagy and longevity. Furthermore, we review research on the regulation of metabolism conducted in multiple aging models, including Caenorhabditis elegans, Drosophila and mouse, and human iPSCs. We suggest that conserved molecular pathways have the strongest potential for the development of new avenues for treatment of age-related diseases.
... Under normal cellular conditions, mTORC1 regulates clearance of misfolded protein aggregates and mTORC1 activity has been directly associated with sporadic AD (22). mTORC1 is regulated by multiple external trophic ligands through the tuberous sclerosis complex and by amino acids through the Rag small GTPase (39,40). Heterodimeric RagA/B and RagC/D complexes are responsible for regulating tethering of mTORC1 to lysosomal membranes in response to increasing intracellular amino acid levels; lysosomal localization of the mTORC1 kinase directly correlates with its activation through Rheb (40). ...
... mTORC1 is regulated by multiple external trophic ligands through the tuberous sclerosis complex and by amino acids through the Rag small GTPase (39,40). Heterodimeric RagA/B and RagC/D complexes are responsible for regulating tethering of mTORC1 to lysosomal membranes in response to increasing intracellular amino acid levels; lysosomal localization of the mTORC1 kinase directly correlates with its activation through Rheb (40). ...
Themolecularmechanisms leading to andresponsible for age-related, sporadic Alzheimer’sdisease (AD)
remain largelyunknown. It iswelldocumented that aging patientswith elevated levels of the amino acidmetabolite
homocysteine (Hcy) are at high risk of developing AD.We investigated the impact of Hcy on molecular clearance
pathways in mammalian cells, including in vitro cultured induced pluripotent stemcell-derived forebrain neurons
and in vivo neurons in mouse brains. Exposure to Hcy resulted in up-regulation of the mechanistic target
of rapamycincomplex 1 (mTORC1)activity, one of themajor kinases incells that is tightly linked to anabolic and
catabolic pathways. Hcy is sensed by a constitutive protein complex composed of leucyl-tRNA-synthetase and
folliculin, which regulates mTOR tethering to lysosomal membranes. In hyper-homocysteinemic human cells
and cystathionine b-synthase-deficient mouse brains, we find an acute and chronic inhibition of the molecular
clearance of protein products resulting in a buildup of abnormal proteins, including b-amyloid and phospho-Tau.
Formation of these protein aggregates leads to AD-like neurodegeneration. This pathology can be prevented by
inhibition of mTORC1 or by induction of autophagy. We conclude that an increase of intracellular Hcy levels predisposes
neurons to develop abnormal protein aggregates, which are hallmarks of AD and its associated onset and
pathophysiology with age.—Khayati, K., Antikainen, H., Bonder, E. M.,Weber, G. F., Kruger, W. D., Jakubowski,
H., Dobrowolski, R. The amino acid metabolite homocysteine activates mTORC1 to inhibit autophagy and form
abnormal proteins in human neurons and mice. FASEB J. 31, 000–000 (2017). www.fasebj.org
... Autophagy is strictly dependent on lysosomal function that is driven by the nutritional status of the cell. Specifically, amino acids are sensed by lysosomes through a protein complex (vacuolar ATPase, Ragulator complex, and the Rag heterodimers A/B and C/D) that tethers the mechanistic target of Rapamycin complex 1 (mTORC1) to their membranes (Laplante and Sabatini, 2012; Nnah et al., 2015 ). The small GTPase Rheb (Ras homolog enriched in brain) activates mTORC1 on lysosomal membranes if TSC1/2 (tuberous sclerosis 1/2 complex) is inactivated by growth factor signaling (Inoki et al., 2003; Tee et al., 2003). ...
... Autophagy is strictly dependent on lysosomal function that is driven by the nutritional status of the cell. Specifically, amino acids are sensed by lysosomes through a protein complex (vacuolar ATPase, Ragulator complex, and the Rag heterodimers A/B and C/D) that tethers the mechanistic target of Rapamycin complex 1 (mTORC1) to their membranes (Laplante and Sabatini, 2012;Nnah et al., 2015). The small GTPase Rheb (Ras homolog enriched in brain) activates mTORC1 on lysosomal membranes if TSC1/2 (tuberous sclerosis 1/2 complex) is inactivated by growth factor signaling (Inoki et al., 2003;Tee et al., 2003). ...
Attenuated auto-lysosomal system has been associated with Alzheimer disease (AD), yet all underlying molecular mechanisms leading to this impairment are unknown. We show that the amino acid sensing of mechanistic target of rapamycin complex 1 (mTORC1) is dysregulated in cells deficient in presenilin, a protein associated with AD. In these cells, mTORC1 is constitutively tethered to lysosomal membranes, unresponsive to starvation, and inhibitory to TFEB-mediated clearance due to a reduction in Sestrin2 expression. Normalization of Sestrin2 levels through overexpression or elevation of nuclear calcium rescued mTORC1 tethering and initiated clearance. While CLEAR network attenuation in vivo results in buildup of amyloid, phospho-Tau, and neurodegeneration, presenilin-knockout fibroblasts and iPSC-derived AD human neurons fail to effectively initiate autophagy. These results propose an altered mechanism for nutrient sensing in presenilin deficiency and underline an importance of clearance pathways in the onset of AD.
... Autophagy is strictly dependent on lysosomal function that is driven by the nutritional status of the cell. Specifically, amino acids are sensed by lysosomes through a protein complex (vacuolar ATPase, Ragulator complex, and the Rag heterodimers A/B and C/D) that tethers the mechanistic target of Rapamycin complex 1 (mTORC1) to their membranes (Laplante and Sabatini, 2012;Nnah et al., 2015). The small GTPase Rheb (Ras homolog enriched in brain) activates mTORC1 on lysosomal membranes if TSC1/2 (tuberous sclerosis 1/2 complex) is inactivated by growth factor signaling (Inoki et al., 2003;Tee et al., 2003). ...
After several years of extensive research, the main cause of aging is yet elusive. There are some theories about aging, such as stem cell aging, senescent cells accumulation, and neuro-endocrine theories. None of them is able to explain all changes that happen in cells and body through aging. By finding out the main cause of aging, it will be much easier to control, prevent and even reverse the aging process. Our cells, regardless of their replicative capacity, get old through aging and they have almost the same epigenetic age. Different cell signaling pathways contribute to aging. The most important one is mTORC1 that becomes hyperactive in cells that undergo aging. Other significant changes with age are lysosome accumulation, impaired autophagy, and mitophagy. Immune system undergoes gradual changes through aging including a shift from lymphoid to myeloid lineage production as well as increased IL-6 and TNF-α which lead to age-related weight loss and meta-inflammation. Additionally, our endocrine system also experiences some changes that should be taken into consideration when looking for the main cause of aging in the human body. In this review, we planned to summarize some of the changes that happen in cells and the body through aging.
Targeting autophagy and lysosome may serve as a promising strategy for cancer therapy. Tea polysaccharide (TP) has shown promising antitumor effects. However, its mechanism remains elusive. Here, TP was found to have a significant inhibitory effect on the proliferation of colon cancer line HCT116 cells. RNA-seq analysis showed that TP upregulated autophagy and lysosome signal pathways, which was further confirmed through experiments. Immunofluorescence experiments indicated that TP activated transcription factor EB (TFEB), a key nuclear transcription factor modulating autophagy and lysosome biogenesis. In addition, TP inhibited the activity of mTOR, while it increased the expression of Lamp1. Furthermore, TP ameliorated the lysosomal damage and autophagy flux barrier caused by Baf A1 (lysosome inhibitor). Hence, our data suggested that TP repressed the proliferation of HCT116 cells by targeting lysosome to induce cytotoxic autophagy, which might be achieved through mTOR-TFEB signaling. In summary, TP may be used as a potential drug to overcome colon cancer.
