mTORC1 Function Research Articles

The roles of mTORC1 in parathyroid gland function in chronic kidney disease-induced secondary hyperparathyroidism: Evidence from male genetic mousemodels and clinical data.

Secondary hyperparathyroidism (SHP) associated with chronic kidney disease (CKD) contributes to morbidity and mortality, yet the related parathyroid signaling pathways are not fully understood. Previous studies have indicated that the parathyroid mTORC1 pathway is activated in both experimental CKD and hypocalcemia-induced SHP. Furthermore, mice with parathyroid-specific mTOR deficiency (PT-mTOR-/-) exhibit disrupted parathyroid glands, but maintain normal serum PTH levels. Conversely, PT-Tsc1-/- mice, with mTORC1 hyperactivation, have enlarged glands and high serum PTH and calcium levels. We now uncover links between mTORC1 function, parathyroid gland morphology, and the response to CKD. Despite impaired gland structure, PT-mTOR-/- mice increased serum PTH to levels similar to controls in response to CKD, but not to acute kidney injury (AKI), highlighting the adaptability of their parathyroid glands to chronic but not acute stimulation. PT-Tsc1-/- mice, with enlarged glands also exhibited a CKD-induced rise in serum PTH comparable to controls, but with a reduced magnitude, suggesting compromised secretion capacity. Parathyroid glands from PT-Tsc1-/- mice displayed sustained high PTH secretion in culture, with no further increase when exposed to calcium-depleted media, unlike control glands. Complementing these findings, human data from 106 healthcare organizations demonstrated that drug-induced mTORC1 inhibition is associated with reduced serum PTH and a lower incidence of SHP in kidney transplant recipients. Collectively, our findings underscore the complex interplay between mTORC1 signaling and gland structure in the pathogenesis of SHP.

Nuclear mTORC1 Live-Cell Sensor nTORSEL Reports Differential Nuclear mTORC1 Activity in Cell Lines.

The mammalian or mechanistic target of rapamycin complex 1 (mTORC1) is activated on the surface of lysosomes and phosphorylates substrates at various subcellular locations, including the lysosome, cytosol, and nucleus. However, the signaling and biological functions of nuclear mTORC1 (nmTORC1) are not well understood, primarily due to limited tools for monitoring mTORC1 activity in the nucleus. In this study, we developed a genetically encoded nmTORC1 sensor, termed nTORSEL, based on the phosphorylation of the eukaryotic initiation factor 4E (eIF4E) binding protein 1 (4EBP1) by mTORC1 within the nucleus. nTORSEL, like its predecessor TORSEL, exhibits a fluorescent punctate pattern in the nucleus through multivalent protein-protein interactions between oligomerized 4EBP1 and eIF4E when nmTORC1 activity is low. We validated nTORSEL using biochemical analyses and imaging techniques across representative cell lines with varying levels of nmTORC1 activity. Notably, nTORSEL specifically detects physiological, pharmacological, and genetic inhibition of nmTORC1 in mouse embryonic fibroblast (MEF) cells but not in HEK293T cells. Therefore, nTORSEL is an effective tool for investigating nuclear mTORC1 signaling in cell lines.

Nutrient control of growth and metabolism through mTORC1 regulation of mRNA splicing

Cellular growth and organismal development are remarkably complex processes that require the nutrient-responsive kinase mechanistic target of rapamycin complex 1 (mTORC1). Anticipating that important mTORC1 functions remained to be identified, we employed genetic and bioinformatic screening in C.elegans to uncover mechanisms of mTORC1 action. Here, we show that during larval growth, nutrients induce an extensive reprogramming of gene expression and alternative mRNA splicing by acting through mTORC1. mTORC1 regulates mRNA splicing and the production of protein-coding mRNA isoforms largely independently of its target p70 S6 kinase (S6K) by increasing the activity of the serine/arginine-rich (SR) protein RSP-6 (SRSF3/7) and other splicing factors. mTORC1-mediated mRNA splicing regulation is critical for growth; mediates nutrient control of mechanisms that include energy, nucleotide, amino acid, and other metabolic pathways; and may be conserved in humans. Although mTORC1 inhibition delays aging, mTORC1-induced mRNA splicing promotes longevity, suggesting that when mTORC1 is inhibited, enhancement of this splicing might provide additional anti-aging benefits.

A Novel Function of FOXO4 and mTORC2 for Regulation of Kidney Gluconeogenesis

A Novel Function of FOXO4 and mTORC2 for Regulation of Kidney Gluconeogenesis

The convergence of mTOR signaling and ethanol teratogenesis

Ethanol is one of the most common teratogens and causes of human developmental disabilities. Fetal alcohol spectrum disorders (FASD), which describes the wide range of deficits due to prenatal ethanol exposure, are estimated to affect between 1.1 % and 5.0 % of births in the United States. Ethanol dysregulates numerous cellular mechanisms such as programmed cell death (apoptosis), protein synthesis, autophagy, and various aspects of cell signaling, all of which contribute to FASD. The mechanistic target of rapamycin (mTOR) regulates these cellular mechanisms via sensing of nutrients like amino acids and glucose, DNA damage, and growth factor signaling. Despite an extensive literature on ethanol teratogenesis and mTOR signaling, there has been less attention paid to their interaction. Here, we discuss the impact of ethanol teratogenesis on mTORC1’s ability to coordinate growth factor and amino acid sensing with protein synthesis, autophagy, and apoptosis. Notably, the effect of ethanol exposure on mTOR signaling depends on the timing and dose of ethanol as well as the system studied. Overall, the overlap between the functions of mTORC1 and the phenotypes observed in FASD suggest a mechanistic interaction. However, more work is required to fully understand the impact of ethanol teratogenesis on mTOR signaling.

MTORC1-CTLH E3 ligase regulates the degradation of HMG-CoA synthase 1 through the Pro/N-degron pathway

Mammalian target of rapamycin (mTOR) senses changes in nutrient status and stimulates the autophagic process to recycle amino acids. However, the impact of nutrient stress on protein degradation beyond autophagic turnover is incompletely understood. We report that several metabolic enzymes are proteasomal targets regulated by mTOR activity based on comparative proteome degradation analysis. In particular, 3-hydroxy-3-methylglutaryl (HMG)-coenzyme A (CoA) synthase 1 (HMGCS1), the initial enzyme in the mevalonate pathway, exhibits the most significant half-life adaptation. Degradation of HMGCS1 is regulated by the C-terminal to LisH (CTLH) E3 ligase through the Pro/N-degron motif. HMGCS1 is ubiquitylated on two C-terminal lysines during mTORC1 inhibition, and efficient degradation of HMGCS1 in cells requires a muskelin adaptor. Importantly, modulating HMGCS1 abundance has a dose-dependent impact on cell proliferation, which is restored by adding a mevalonate intermediate. Overall, our unbiased degradomics study provides new insights into mTORC1 function in cellular metabolism: mTORC1 regulates the stability of limiting metabolic enzymes through the ubiquitin system.

Defective Lamtor5 Leads to Autoimmunity by Deregulating v-ATPase and Lysosomal Acidification.

Despite accumulating evidence linking defective lysosome function with autoimmune diseases, how the catabolic machinery is regulated to maintain immune homeostasis remains unknown. Late endosomal/lysosomal adaptor, MAPK and mTOR activator 5 (Lamtor5) is a subunit of the Ragulator mediating mechanistic target of rapamycin complex 1 (mTORC1) activation in response to amino acids, but its action mode and physiological role are still unclear. Here it is demonstrated that Lamtor5 level is markedly decreased in peripheral blood mononuclear cells (PBMCs) of patients with systemic lupus erythematosus (SLE). In parallel, the mice with myeloid Lamtor5 ablation developed SLE-like manifestation. Impaired lysosomal function and aberrant activation of mTORC1 are evidenced in Lamtor5 deficient macrophages and PBMCs of SLE patients, accompanied by blunted autolysosomal pathway and undesirable inflammatory responses. Mechanistically, it is shown that Lamtor5 is physically associated with ATP6V1A, an essential subunit of vacuolar H+-ATPase(v-ATPase), and promoted the V0/V1 holoenzyme assembly to facilitate lysosome acidification. The binding of Lamtor5 to v-ATPase affected the lysosomal tethering of Rag GTPase and weakened its interaction with mTORC1 for activation. Overall, Lamtor5 is identified as a critical factor for immune homeostasis by intergrading v-ATPase activity, lysosome function, and mTOR pathway. The findings provide a potential therapeutic target for SLE and/or other autoimmune diseases.

Abstract 5555: Lung squamous cancer cell mTORC2 signaling suppresses antitumor immunity by up-regulation of PSGL-1, a ligand of VISTA/PD-1H checkpoint receptor

Abstract Lung squamous cell carcinoma (LUSC) represents 30% of non-small cell lung cancer (NSCLC) cases and has a high mortality rate with limited targeted therapies. Unlike lung adenocarcinoma (LUAD), which has seen advancements in targeted therapies, LUSC has not benefited from similar breakthroughs despite having similar genetic abnormalities. While immune checkpoint inhibitor therapies have shown promise in some LUSC patients, a significant portion does not respond effectively, leaving limited treatment options. In this study, we investigated vulnerabilities in NSCLC and found that a substantial number of LUSC patients have genetic alterations in the PI3K-mTORC2-AKT signaling pathway. By using CRISPR/CAS-9, we showed that disrupting mTORC2 function in LUSC cells reduced glycolysis and lactate secretion, which in turn reduced tumor growth in vivo. Remarkably, the loss of mTORC2 also enhanced the activity of CD4+ and CD8+ T cells within the tumors. To further understand the metabolic and immune changes associated with mTORC2 loss of function, we performed RNA sequencing comparing wild-type (WT) and mTORC2-deficient cells. Our results confirmed a link between mTORC2 and HIF-2α, while regulating PSGL-1, a ligand of the VISTA/PD-1H checkpoint receptor. Furthermore, introducing a stable, non-degradable form of HIF-2α in mTORC2-deficient tumor cells restored PSGL-1 expression in vitro and in vivo. These results suggest that the interaction between PSGL-1 and VISTA plays a role in suppressing the anti-tumor immune response in LUSC tumors. Indeed, anti-VISTA treatment inhibited the growth of LUSC WT tumors in vivo, suggesting that mTORC2 plays a role in regulating PSGL-1 expression in the tumor microenvironment. Together, these data demonstrate that targeting mTORC2 or inhibiting VISTA checkpoint receptor may serve as a potential therapeutic strategy for improving immunotherapy in LUSC. Citation Format: Verra M. Ngwa, Yoonha Hwang, Deanna N. Edwards, Wenqiang Song, Jin Chen. Lung squamous cancer cell mTORC2 signaling suppresses antitumor immunity by up-regulation of PSGL-1, a ligand of VISTA/PD-1H checkpoint receptor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5555.

Zebrafish raptor mutation inhibits the activity of mTORC1, inducing craniofacial defects due to autophagy-induced neural crest cell death.

The mechanistic target of rapamycin (mTOR) coordinates metabolism and cell growth with environmental inputs. mTOR forms two functional complexes: mTORC1 and mTORC2. Proper development requires both complexes but mTORC1 has unique roles in numerous cellular processes, including cell growth, survival and autophagy. Here, we investigate the function of mTORC1 in craniofacial development. We created a zebrafish raptor mutant via CRISPR/Cas9, to specifically disrupt mTORC1. The entire craniofacial skeleton and eyes were reduced in size in mutants; however, overall body length and developmental timing were not affected. The craniofacial phenotype associates with decreased chondrocyte size and increased neural crest cell death. We found that autophagy is elevated in raptor mutants. Chemical inhibition of autophagy reduced cell death and improved craniofacial phenotypes in raptor mutants. Genetic inhibition of autophagy, via mutation of the autophagy gene atg7, improved facial phenotypes in atg7;raptor double mutants, relative to raptor single mutants. We conclude that finely regulated levels of autophagy, via mTORC1, are crucial for craniofacial development.

Enhancing productivity of Chinese hamster ovary (CHO) cells: synergistic strategies combining low-temperature culture and mTORC1 signaling engineering

Introduction: The growing demand for recombinant proteins in medicine has prompted biopharmaceutical companies to seek ways to maximize the manufacturing process. Despite its known negative impact on cell growth, temperature shift (TS) has emerged as a cost-effective strategy to enhance protein quantity and quality in Chinese Hamster Ovary cells (CHO). As cells adapt their growth and protein synthesis rate to the environment through influencing mTOR complex 1 (mTORC1), here we evaluated the potential of mTORC1 signaling engineering to improve the production of granulocyte-macrophage colony-stimulating factor (GM-CSF) protein in stable CHO cells at low temperature.Methods: First, the expression of genes that negatively control mTORC1 functions in response to environmental fluctuations, including TSC1, AMPK, MAPKAPK5, and MARK4 genes, was assessed via real-time qPCR in CHO-K1 after a temperature shift from 37°C to 30°C. Then, plasmids harboring the shRNAs targeting these genes were constructed into the PB513B-1 plasmid with expression driven by either the constitutive CMV promoter or the cold-inducible HSP90 promoter. Finally, the impact of transient gene downregulation was evaluated on GM-CSF and mTOR proteins productivity in GM-CSF-producing CHO-K1 cells using ELISA and Western-blot assays, respectively. The growth rate of the transfected cells at the two temperatures was evaluated using flow cytometry.Results: Hypothermic conditions promote the upregulation of mTORC1 inhibitor genes, especially TSC1 and MAPKAPK5, while downregulating S6K, a key effector of the mTORC1 signaling pathway, in CHO-K1 cells. Transcription and protein levels of mTOR increased upon transfection, “pB513-b CMV-P/4shRNAs/GFP” plasmid, “pB513-bHSP90-P/4sh-RNAs/GFP” and pB513B-1 plasmid as mock group in GM-CSF-producing CHO-K1 cells (approximately 60%), along with a high transcript level of S6K. Cell growth-related characteristics were improved, albeit with distinct effects at different temperatures. Notably, these changes were more efficient at 30°C when utilizing the HSP90 promoter, resulting in a three-fold increase in GM-CSF production after 3 days.Conclusion: This study highlights the importance of temperature regulation and mTORC1 modulation in CHO cellular processes, particularly in recombinant protein production. Understanding these mechanisms paves the way for developing innovative strategies to enhance cell growth, protein synthesis, and overall bioprocess performance, particularly in manufacturing human therapeutic proteins.

PPARγ activation by fisetin mitigates vascular smooth muscle cell senescence via the mTORC2-FoxO3a-autophagy signaling pathway

Cellular senescence is caused by diverse stimuli and contributes to cardiovascular diseases. Several studies have indicated that PPARγ acts as a key mediator of lipid metabolism and shown that it has a protective effect on vascular biology. Nevertheless, the mechanism responsible for the anti-aging effects of PPARγ has not been fully elucidated in vascular smooth muscle cell (VSMC). Furthermore, although mTOR complex 2 (mTORC2) is known to be involved in cellular senescence and autophagy, relatively few studies have investigated its effects as compared with mTOR complex 1 (mTORC1). Therefore, we focused on mTORC2 function and investigated the relationship between PPARγ and mTORC2, and the anti-aging mechanism in VSMC. We found PPARγ activation dose-dependently mitigated the hydrogen peroxide (H2O2)-induced senescence. Treatment of fisetin induced the translocation of PPARγ from cytosol to nuclear and inhibited VSMC senescence. Moreover, activated PPARγ increased PTEN transcription, leading to inhibition of the mTORC2 signaling pathway. We determined mTORC2 activation contributed to senescence by suppressing the FoxO3a-autophagy signaling pathway, and dual knockdown of mTORC1 and mTORC2 decreased cellular senescence and increased autophagy activation more than respective single knockdown. Finally, fisetin acted as a PPARγ activator and inhibited VSMC senescence through the mTORC2-FoxO3a-autophagy signaling pathway. These results demonstrate PPARγ is associated with cellular senescence and that fisetin has an anti-aging effect via PPARγ activation and mTORC2 inhibition in VSMC. These results demonstrate that the mTORC2 signaling pathway regulates autophagy and cellular senescence, which suggests mTORC2 should be considered a significant target for preventing cellular senescence and age-related diseases.

Alkaline intracellular pH (pHi) increases PI3K activity to promote mTORC1 and mTORC2 signaling and function during growth factor limitation

The conserved protein kinase mTOR (mechanistic target of rapamycin) responds to diverse environmental cues to control cell metabolism and promote cell growth, proliferation, and survival as part of two multiprotein complexes, mTOR complex 1 (mTORC1) and mTORC2. Our prior work demonstrated that an alkaline intracellular pH (pHi) increases mTORC2 activity and cell survival in complete media in part by activating AMP-activated protein kinase, a kinase best known to sense energetic stress. It is important to note that an alkaline pHi represents an underappreciated hallmark of cancer cells that promotes their oncogenic behaviors. In addition, mechanisms that control mTORC1 and mTORC2 signaling and function remain incompletely defined, particularly in response to stress conditions. Here, we demonstrate that an alkaline pHi increases phosphatidylinositide 3-kinase (PI3K) activity to promote mTORC1 and mTORC2 signaling in the absence of serum growth factors. Alkaline pHi increases mTORC1 activity through PI3K-Akt signaling, which mediates inhibitory phosphorylation of the upstream proteins tuberous sclerosis complex 2 and proline-rich Akt substrate of 40 kDa and dissociates tuberous sclerosis complex from lysosomal membranes, thus enabling Rheb-mediated activation of mTORC1. Thus, alkaline pHi mimics growth factor-PI3K signaling. Functionally, we also demonstrate that an alkaline pHi increases cap-dependent protein synthesis through inhibitory phosphorylation of eIF4E binding protein 1 and suppresses apoptosis in a PI3K- and mTOR-dependent manner. We speculate that an alkaline pHi promotes a low basal level of cell metabolism (e.g., protein synthesis) that enables cancer cells within growing tumors to proliferate and survive despite limiting growth factors and nutrients, in part through elevated PI3K-mTORC1 and/or PI3K-mTORC2 signaling.

Research progress in diseases associated with genetic variants of GATOR1 complex

The GATOR1 complex is located at the upstream of the mTOR signal pathway and can regulate the function of mTORC1. Genetic variants of the GATOR1 complex are closely associated with epilepsy, developmental delay, cerebral cortical malformation and tumor. This article has reviewed the research progress in diseases associated with genetic variants of the GATOR1 complex, with the aim to provide a reference for the diagnosis and treatment of such patients.

Raptor levels are critical for β-cell adaptation to a high-fat diet in male mice

ObjectiveThe essential role of raptor/mTORC1 signaling in β-cell survival and insulin processing has been recently demonstrated using raptor knock-out models. Our aim was to evaluate the role of mTORC1 function in adaptation of β-cells to insulin resistant state. MethodHere, we use mice with heterozygous deletion of raptor in β-cells (βraHet) to assess whether reduced mTORC1 function is critical for β-cell function in normal conditions or during β-cell adaptation to high-fat diet (HFD). ResultsDeletion of a raptor allele in β-cells showed no differences at the metabolic level, islets morphology, or β-cell function in mice fed regular chow. Surprisingly, deletion of only one allele of raptor increases apoptosis without altering proliferation rate and is sufficient to impair insulin secretion when fed a HFD. This is accompanied by reduced levels of critical β-cell genes like Ins1, MafA, Ucn3, Glut2, Glp1r, and specially PDX1 suggesting an improper β-cell adaptation to HFD. ConclusionThis study identifies that raptor levels play a key role in maintaining PDX1 levels and β-cell function during the adaptation of β-cell to HFD. Finally, we identified that Raptor levels regulate PDX1 levels and β-cell function during β-cell adaptation to HFD by reduction of the mTORC1-mediated negative feedback and activation of the AKT/FOXA2/PDX1 axis. We suggest that Raptor levels are critical to maintaining PDX1 levels and β-cell function in conditions of insulin resistance in male mice.

FISETIN ENHANCES PPAR GAMMA EXPRESSION TO PREVENT VASCULAR SMOOTH MUSCLE CELL SENESCENCE VIA MTORC2-AKT-FOXO3A SIGNALING PATHWAY

Objective: Cellular senescence is caused by diverse stimuli and is contributed to cardiovascular diseases such as atherosclerosis, coronary heart disease, and hypertension. Several studies have indicated that PPAR gamma is mainly a key mediator in lipid metabolism and adipocyte differentiation. In addition, PPAR gamma has a protective effect in vascular biology and regulates the inflammation and reactive oxygen species (ROS). Nevertheless, the anti-aging mechanism of PPAR gamma has not been elucidated in vascular smooth muscle cell (VSMC). Although mTORC2 is involved in the cellular senescence and autophagy, there are significantly fewer studies than mTORC1. Therefore, we focused on mTORC2 function and then aimed to investigate the correlation between PPAR gamma and mTORC2, and to identify the anti-aging mechanism in VSMC. Design and method: To generate the oxidative stress-induced VSMC senescence, we treated with hydrogen peroxide (H2O2). The anti-aging effect and underlying mechanism of fisetin were evaluated by Western blot analysis, senescence-associated beta-galactosidase (SA-beta-gal) staining, flow cytometry, immunofluorescence analysis, and immunoprecipitation analysis. Results: We found hydrogen peroxide (H2O2) induced not only PPAR gamma translocation from nuclear to cytosol but also mTORC2 activation with VSMC senescence. We determined mTORC2 activation contributed to the senescence by suppressing FoxO3a-autophagy signaling pathway. Furthermore, the double knockdown of mTORC1 and mTORC2 decreased the cellular senescence and increased the autophagy activation more than single knockdown. However, fisetin acted as a PPAR gamma activator, leading to the inhibition of VSMC senescence through mTORC2-Akt (Ser473)-FoxO3a signaling pathway which is related to autophagy. Conclusions: These results demonstrate PPAR gamma is associated with cellular senescence and fisetin has anti-aging effect via PPAR gamma activation and mTORC2 inhibition. Moreover, mTORC2 signaling pathway could regulate autophagy, which suggests mTORC2 is considered a significant target for preventing cellular senescence and aging-related disease.

De novo missense variants in RRAGC lead to a fatal mTORopathy of early childhood

PurposeMechanistic target of rapamycin (mTOR) complex 1 (mTORC1) regulates cell growth in response to nutritional status. Central to the mTORC1 function is the Rag-GTPase heterodimer. One component of the Rag heterodimer is RagC (Ras-related GTP-binding protein C), which is encoded by the RRAGC gene. MethodsGenetic testing via trio exome sequencing was applied to identify the underlying disease cause in 3 infants with dilated cardiomyopathy, hepatopathy, and brain abnormalities, including pachygyria, polymicrogyria, and septo-optic dysplasia. Studies in patient-derived skin fibroblasts and in a HEK293 cell model were performed to investigate the cellular consequences. ResultsWe identified 3 de novo missense variants in RRAGC (NM_022157.4: c.269C>A, p.(Thr90Asn), c.353C>T, p.(Pro118Leu), and c.343T>C, p.(Trp115Arg)), which were previously reported as occurring somatically in follicular lymphoma. Studies of patient-derived fibroblasts carrying the p.(Thr90Asn) variant revealed increased cell size, as well as dysregulation of mTOR-related p70S6K (ribosomal protein S6 kinase 1) and transcription factor EB signaling. Moreover, subcellular localization of mTOR was decoupled from metabolic state. We confirmed the key findings for all RRAGC variants described in this study in a HEK293 cell model. ConclusionThe above results are in line with a constitutive overactivation of the mTORC1 pathway. Our study establishes de novo missense variants in RRAGC as cause of an early-onset mTORopathy with unfavorable prognosis.

Glutamine Produces Ammonium to Tune Lysosomal pH and Regulate Lysosomal Function.

Glutamine is one of the most abundant amino acids in the cell. In mitochondria, glutaminases 1 and 2 (GLS1/2) hydrolyze glutamine to glutamate, which serves as the precursor of multiple metabolites. Here, we show that ammonium generated during GLS1/2-mediated glutaminolysis regulates lysosomal pH and in turn lysosomal degradation. In primary human skin fibroblasts BJ cells and mouse embryonic fibroblasts, deprivation of total amino acids for 1 h increased lysosomal degradation capacity as shown by the increased turnover of lipidated microtubule-associated proteins 1A/1B light chain 3B (LC3-II), several autophagic receptors, and endocytosed DQ-BSA. Removal of glutamine but not any other amino acids from the culture medium enhanced lysosomal degradation similarly as total amino acid starvation. The presence of glutamine in regular culture media increased lysosomal pH by >0.5 pH unit and the removal of glutamine caused lysosomal acidification. GLS1/2 knockdown, GLS1 antagonist, or ammonium scavengers reduced lysosomal pH in the presence of glutamine. The addition of glutamine or NH4Cl prevented the increase in lysosomal degradation and curtailed the extension of mTORC1 function during the early time period of amino acid starvation. Our findings suggest that glutamine tunes lysosomal pH by producing ammonium, which regulates lysosomal degradation to meet the demands of cellular activities. During the early stage of amino acid starvation, the glutamine-dependent mechanism allows more efficient use of internal reserves and endocytosed proteins to extend mTORC1 activation such that the normal anabolism is not easily interrupted by a brief disruption of the amino acid supply.

Perturbation of Intermediary Metabolism Inhibits Glycolysis, Oxidative Phosphorylation, Amino Acid Synthesis and Prevents Lymphoma Development in Mice

Alpha-ketoglutarate (αKG) is an essential intermediate metabolite. The sources of cellular αKG are the oxidation of isocitrate by IDHs (isocitrate dehydrogenases 1-3), glutaminolysis and the action of the metabolic repair enzymes D-2/L-2- hydroxyglutarate dehydrogenase (D2/L2HGDH), which oxidizes 2-HG into αKG. As a TCA cycle intermediate, αKG participates in the generation of reducing equivalents for energy production, is a source of precursors for synthesis of macromolecules, acts as a nitrogen scavenger, and is an obligatory co-substrate for multiple dioxygenases. Earlier, we discovered loss-of-function mutation of D2HGDH in diffuse large B cell lymphomas (DLBCL; Lin et al, Nat Commun 2015) and found that the main metabolic consequence of these mutations was depletion of αKG, rather than accumulation of D-2-HG, which typifies IDH1/2 mutant tumors. Here, we examined how D-2-HG/αKG homeostasis influence DLBCL metabolism and if exogenous modulation of these intermediate metabolites plays a role in lymphoma biology. We first created KO models of D2HGDH in DLBCL cell lines and queried its impact on oxidative phosphorylation and glycolysis using a seahorse assay. Deletion of D2HGDH increased oxygen consumption rate (OCR) and extracellular acidification rate (ECAR), reflecting elevated mitochondrial respiration and glycolysis, and ATP production. This gain in energy production translated in increased proliferation of multiple (n=10) DLBCL cell lines exposed to the cell permeable octyl-D-2-HG metabolite. Considering that D-2-HG competitively inhibits αKG-dependent activities, and that αKG has been recently shown to suppress epithelial cancers (Tran et al, Nat Cancer, 2020), we examined the effect of αKG on energy metabolism in DLBCL. We exposed 12 DLBCL cell lines to various cell-permeable formulations of αKG (dimethyl-αKG [DMαKG] 5mM, or octyl-αKG 1mM) and detected a rapid (1h) and significant suppression of OCR (mean = 89%, 81-93%) and ECAR (mean = 58%, 35-66%) in all models. This collapse in energy generation had severe repercussions on cell growth and viability; exposure to αKG suppressed DLBCL proliferation (mean = 72% vs. vehicle-treated cells, 47%-99%) and induced apoptosis (mean = 60% vs. vehicle-treated cells, 28%-93%). To gain insight into how αKG supplementation drives the collapse of both cell respiration and glycolysis we used mass spectrometry (MS) to quantify central carbon metabolites in 12 DLBCL cell lines exposed to DMαKG for 30' and 24h. Following the 30' pulse, we detected a significant elevation in all measured metabolites (αKG, succinate, fumarate, malate, citrate, pyruvate and lactate). Conversely, at 24h, while the levels of αKG, succinate, fumarate, malate, citrate remained significantly elevated, the ability to generate lactate and/or pyruvate was blunted in all models, possibly secondary to the inhibition of glycolysis detected with the seahorse assays. We also used MS to measure the impact of αKG supplementation on amino acid levels. We detected a rapid and sustained (30'-24h) depletion of the branched-chain amino acids (BCAAs) isoleucine, leucine and valine (Log2 fold change, 1.37, 1.04 and 0.44 respectively, p<0.001). We attributed the depletion of BCAAs to increased activity of the αKG-dependent BCAA transaminases, BCAT1/2. Importantly, this hitherto unrecognized effect of αKG resulted in broad inhibition of MTORC1 function (secondary to leucine depletion) in DLBCL cell lines and primary lymphomas. Next, we tested the relevance of these findings in vivo. Herein, starting at 5 weeks of age, we randomized Eµ-Myc mice (n=45,) to receive drinking water supplemented with 15mg/ml DMαKG or normal water. The primary endpoint of this trial was time to lymphoma development (and death). Mice exposed to αKG supplementation had a significantly longer survival and protracted lymphoma diagnosis (Figure 1). Remarkably, OCR and ECAR were significantly suppressed (37% and 26%, respectively, p<0.01) in splenocytes of αKG-supplemented vs control mice (n=11). Further, at 12 months of αKG exposure, the mice displayed no clinical toxicity and had normal blood counts and biochemistry. We concluded that αKG displays anti-lymphoma activity, which mechanistically associates with a collapse in energy generation, depletion of BCAAs and suppression of MTORC1. The benefit of αKG supplementation in the context of cancer deserves further examination in humans. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Phosphoproteomic Analysis Defines BABAM1 as mTORC2 Downstream Effector Promoting DNA Damage Response in Glioblastoma Cells.

Glioblastoma (GBM) is a devastating primary brain cancer with a poor prognosis. GBM is associated with an abnormal mechanistic target of rapamycin (mTOR) signaling pathway, consisting of two distinct kinase complexes: mTORC1 and mTORC2. The complexes play critical roles in cell proliferation, survival, migration, metabolism, and DNA damage response. This study investigated the aberrant mTORC2 signaling pathway in GBM cells by performing quantitative phosphoproteomic analysis of U87MG cells under different drug treatment conditions. Interestingly, a functional analysis of phosphoproteome revealed that mTORC2 inhibition might be involved in double-strand break (DSB) repair. We further characterized the relationship between mTORC2 and BRISC and BRCA1-A complex member 1 (BABAM1). We demonstrated that pBABAM1 at Ser29 is regulated by mTORC2 to initiate DNA damage response, contributing to DNA repair and cancer cell survival. Accordingly, the inactivation of mTORC2 significantly ablated pBABAM1 (Ser29), reduced DNA repair activities in the nucleus, and promoted apoptosis of the cancer cells. Furthermore, we also recognized that histone H2AX phosphorylation at Ser139 (γH2AX) could be controlled by mTORC2 to repair the DNA. These results provided a better understanding of the mTORC2 function in oncogenic DNA damage response and might lead to specific mTORC2 treatments for brain cancer patients in the future.

Can We Use mTOR Inhibitors for COVID-19 Therapy?

Infection by the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) provokes acute inflammation due to extensive replication of the virus in the epithelial cells of the upper and lower respiratory system. The mammalian target of rapamycin (mTOR) is a l signalling protein with critical functions in cell growth, metabolism, and proliferation. It is known for its regulatory functions in protein synthesis and angiogenesis cascades. The structure of mTOR consists of two distinct complexes (mTORC1 and mTORC2) with diverse functions at different levels of the signalling pathway. By activating mRNA translation, the mTORC1 plays a key role in regulating protein synthesis and cellular growth. On the other hand, the functions of mTORC2 are mainly associated with cell proliferation and survival. By using an appropriate inhibitor at the right time, mTOR modulation could provide immunosuppressive opportunities as antirejection regimens in organ transplantation as well as in the treatment of autoimmune diseases and solid tumours. The mTOR also has an important role in the inflammatory process. Inhibitors of mTOR might indeed be promising agents in the treatment of viral infections. They have further been successfully used in patients with severe influenza A/H1N1 pneumonia and acute respiratory failure. The officially accepted mTOR inhibitors that have undergone clinical testing are sirolimus, everolimus, temsirolimus, and tacrolimus. Thus, further studies on mTOR inhibitors for SARS-CoV-2 infection or COVID-19 therapy are well merited.