Amino acid sufficiency and mTOR regulate p70 S6 kinase and eIF-4E BP1 through a common effector mechanism

Long non-coding RNA UASR1 promotes proliferation and migration of breast cancer cells through the AKT/mTOR pathway

Long non-coding RNAs (lncRNAs) are non-coding RNAs longer than 200 nucleotides that function as regulatory factors in many human diseases, including cancer. However, majority of lncRNAs remain to be characterized. In this study, we characterized a novel lncRNA transcript, named UNC5B antisense RNA1 (UASR1). UASR1 is 647bp in length consisting of two exons. This lncRNA is an antisense of intron 1 of unc-5 netrin receptor B (UNC5B) gene. In breast cancer tissues, UASR1 was upregulated. Ectopic expression of UASR1 promoted proliferation and clonogenic growth of breast cancer cells MCF7 and MDA-MB-231. The migration of these cells also increased as demonstrated by wound healing and transwell assays. In contrast, silencing of UASR1 suppressed cell proliferation and migration. Further studies showed that UASR1 activated AKT and AKT-mediated mTOR signaling pathway to stimulate cell proliferation and growth. In these cells, active pAKT, pTSC2, p4EBP1 and pp70S6K were increased. Taken together, our data suggest that UASR1 plays an oncogenic role in breast cancer cells through activation of the AKT/mTOR signaling pathway, being a novel RNA oncogene.

Sestrin modulator NV-5138 produces rapid antidepressant effects via direct mTORC1 activation

Preclinical studies demonstrate that rapid acting antidepressants, including ketamine require stimulation of mTORC1 signaling. This pathway is regulated by neuronal activity, endocrine and metabolic signals, notably the amino acid leucine, which activates mTORC1 signaling via binding to the upstream regulator sestrin. Here, we examined the antidepressant actions of NV-5138, a novel highly selective small molecule modulator of sestrin that penetrates the blood brain barrier. The results demonstrate that a single dose of NV-5138 produced rapid and long-lasting antidepressant effects, and rapidly reversed anhedonia caused by chronic stress exposure. The antidepressant actions of NV-5138 required BDNF release as the behavioral responses are blocked by infusion of a BDNF neutralizing antibody into the medial prefrontal cortex (mPFC) or in mice with a knock-in of a BDNF polymorphism that blocks activity dependent BDNF release. NV-5138 administration also rapidly increased synapse number and function in the mPFC, and reversed the synaptic deficits caused by chronic stress. Together, the results demonstrate that NV-5138 produced rapid synaptic and antidepressant behavioral responses via activation of the mTORC1 pathway and BDNF signaling, indicating that pharmacological modulation of sestrin is a novel approach for development of rapid acting antidepressants.

mTORC1 Signaling Is Palmitoylation-Dependent in Hippocampal Neurons and Non-neuronal Cells and Involves Dynamic Palmitoylation of LAMTOR1 and mTOR

The mechanistic target of rapamycin (mTOR) Complex 1 (mTORC1) controls growth and proliferation of non-neuronal cells, while during neuronal development mTORC1 responds to glutamate and neurotrophins to promote neuronal migration and dendritic arborization. Recent studies reveal that mTORC1 signaling complexes are assembled on lysosomal membranes, but how mTORC1 membrane targeting is regulated is not fully clear. Our examination of palmitoyl-proteomic databases and additional bioinformatic analyses revealed that several mTORC1 proteins are predicted to undergo covalent modification with the lipid palmitate. This process, palmitoylation, can dynamically target proteins to specific membranes but its roles in mTORC1 signaling are not well described. Strikingly, we found that acute pharmacological inhibition of palmitoylation prevents amino acid-dependent mTORC1 activation in HEK293T cells and brain-derived neurotrophic factor (BDNF)-dependent mTORC1 activation in hippocampal neurons. We sought to define the molecular basis for this finding and found that the mTORC1 proteins LAMTOR1 and mTOR itself are directly palmitoylated, while several other mTORC1 proteins are not palmitoylated, despite strong bioinformatic prediction. Interestingly, palmitoylation of LAMTOR1, whose anchoring on lysosomal membranes is important for mTORC1 signaling, was rapidly increased prior to mTORC1 activation. In contrast, mTOR palmitoylation was decreased by stimuli that activate mTORC1. These findings reveal that specific key components of the mTOR pathway are dynamically palmitoylated, suggesting that palmitoylation is not merely permissive for mTOR activation but is instead actively involved in mTORC1-dependent signaling.

Topotecan induces apoptosis via ASCT2 mediated oxidative stress in gastric cancer

BACKGROUND

Topotecan (TPT) is a Topo I inhibitor and shows obvious anti-cancer effects on gastric cancer. Cancer cells reprogram their metabolic pathways to increase nutrients uptake, which has already been a hallmark of cancer. But the effect of TPT on metabolism in gastric cancer remains unknown.

PURPOSE

To investigate the effect of TPT on metabolism in gastric cancer.

METHODS

ATP production was measured by ATP Assay kit. Glucose and glutamine uptake were measured by Glucose (HK) Assay Kit and Glutamine/Glutamate Determination Kit respectively. To detect glutathione (GSH) concentration and reactive oxygen species (ROS) generation, GSH and GSSG Assay Kit and ROS Assay Kit were adopted. Apoptosis rates, mitochondrial membrane potential (MMP) were determined by flow cytometry and protein levels were analyzed by immumohistochemical staining and western blotting.

RESULTS

TPT increased ATP production. TPT promoted glucose uptake possibly via up-regulation of hexokinase 2 (HK2) or glucose transporter 1 (GLUT1) expression, while decreased glutamine uptake by down-regulation of ASCT2 expression. ASCT2 inhibitor GPNA and ASCT2 knockdown significantly suppressed the growth of gastric cancer cells. Inhibition of ASCT2 reduced glutamine uptake which led to decreased production of GSH and increased ROS level. ASCT2 knockdown induced apoptosis via the mitochondrial pathway and weakened anti-cancer effect of TPT.

CONCLUSION

TPT inhibits glutamine uptake via down-regulation of ASCT2 which causes oxidative stress and induces apoptosis through the mitochondrial pathway. Moreover, TPT inhibits proliferation partially via ASCT2. These observations reveal a previously undescribed mechanism of ASCT2 regulated gastric cancer proliferation and demonstrate ASCT2 is a potential anti-cancer target of TPT.

Inhibition of the amino-acid transporter LAT1 demonstrates anti-neoplastic activity in medulloblastoma

Most cases of medulloblastoma (MB) occur in young children. While the overall survival rate can be relatively high, current treatments combining surgery, chemo- and radiotherapy are very destructive for patient development and quality of life. Moreover, aggressive forms and recurrences of MB cannot be controlled by classical therapies. Therefore, new therapeutic approaches yielding good efficacy and low toxicity for healthy tissues are required to improve patient outcome. Cancer cells sustain their proliferation by optimizing their nutrient uptake capacities. The L-type amino acid transporter 1 (LAT1) is an essential amino acid carrier overexpressed in aggressive human cancers that was described as a potential therapeutic target. In this study, we investigated the therapeutic potential of JPH203, a LAT1-specific pharmacological inhibitor, on two independent MB cell lines belonging to subgroups 3 (HD-MB03) and Shh (DAOY). We show that while displaying low toxicity towards normal cerebral cells, JPH203 disrupts AA homeostasis, mTORC1 activity, proliferation and survival in MB cells. Moreover, we demonstrate that a long-term treatment with JPH203 does not lead to resistance in MB cells. Therefore, this study suggests that targeting LAT1 with JPH203 is a promising therapeutic approach for MB treatment.

Amino acid transporters in the regulation of insulin secretion and signalling

Amino acids are increasingly recognised as modulators of nutrient disposal, including their role in regulating blood glucose through interactions with insulin signalling. More recently, cellular membrane transporters of amino acids have been shown to form a pivotal part of this regulation as they are primarily responsible for controlling cellular and circulating amino acid concentrations. The availability of amino acids regulated by transporters can amplify insulin secretion and modulate insulin signalling in various tissues. In addition, insulin itself can regulate the expression of numerous amino acid transporters. This review focuses on amino acid transporters linked to the regulation of insulin secretion and signalling with a focus on those of the small intestine, pancreatic β-islet cells and insulin-responsive tissues, liver and skeletal muscle. We summarise the role of the amino acid transporter B⁰AT1 (SLC6A19) and peptide transporter PEPT1 (SLC15A1) in the modulation of global insulin signalling via the liver-secreted hormone fibroblast growth factor 21 (FGF21). The role of vesicular vGLUT (SLC17) and mitochondrial SLC25 transporters in providing glutamate for the potentiation of insulin secretion is covered. We also survey the roles SNAT (SLC38) family and LAT1 (SLC7A5) amino acid transporters play in the regulation of and by insulin in numerous affective tissues. We hypothesise the small intestine amino acid transporter B⁰AT1 represents a crucial nexus between insulin, FGF21 and incretin hormone signalling pathways. The aim is to give an integrated overview of the important role amino acid transporters have been found to play in insulin-regulated nutrient signalling.

Translational Control through Differential Ribosome Pausing during Amino Acid Limitation in Mammalian Cells

Limitation for amino acids is thought to regulate translation in mammalian cells primarily by signaling through the kinases mTORC1 and GCN2. We find that a selective loss of arginine tRNA charging during limitation for arginine regulates translation through ribosome pausing at two of six arginine codons. Surprisingly, limitation for leucine, an essential and abundant amino acid in protein, results in little or no ribosome pausing. Chemical and genetic perturbation of mTORC1 and GCN2 signaling revealed that their robust response to leucine limitation prevents ribosome pausing, while an insufficient response to arginine limitation leads to loss of tRNA charging and ribosome pausing. Ribosome pausing decreases protein production and triggers premature ribosome termination without reducing mRNA levels. Together, our results suggest that amino acids that are not optimally sensed by the mTORC1 and GCN2 pathways still regulate translation through an evolutionarily conserved mechanism based on codon-specific ribosome pausing.

Advances in understanding the mechanisms of evasive and innate resistance to mTOR inhibition in cancer cells

The development of drug-resistance by neoplastic cells is recognized as a major cause of targeted therapy failure and disease progression. The mechanistic (previously mammalian) target of rapamycin (mTOR) is a highly conserved Ser/Thr kinase that acts as the catalytic subunit of two structurally and functionally distinct large multiprotein complexes, referred to as mTOR complex 1 (mTORC1) and mTORC2. Both mTORC1 and mTORC2 play key roles in a variety of healthy cell types/tissues by regulating physiological anabolic and catabolic processes in response to external cues. However, a body of evidence identified aberrant activation of mTOR signaling as a common event in many human tumors. Therefore, mTOR is an attractive target for therapeutic targeting in cancer and this fact has driven the development of numerous mTOR inhibitors, several of which have progressed to clinical trials. Nevertheless, mTOR inhibitors have met with a very limited success as anticancer therapeutics. Among other reasons, this failure was initially ascribed to the activation of several compensatory signaling pathways that dampen the efficacy of mTOR inhibitors. The discovery of these regulatory feedback mechanisms greatly contributed to a better understanding of cancer cell resistance to mTOR targeting agents. However, over the last few years, other mechanisms of resistance have emerged, including epigenetic alterations, compensatory metabolism rewiring and the occurrence of mTOR mutations. In this article, we provide the reader with an updated overview of the mechanisms that could explain resistance of cancer cells to the various classes of mTOR inhibitors.

How Viral and Intracellular Bacterial Pathogens Reprogram the Metabolism of Host Cells to Allow Their Intracellular Replication

Viruses and intracellular bacterial pathogens (IBPs) have in common the need of suitable host cells for efficient replication and proliferation during infection. In human infections, the cell types which both groups of pathogens are using as hosts are indeed quite similar and include phagocytic immune cells, especially monocytes/macrophages (MOs/MPs) and dendritic cells (DCs), as well as nonprofessional phagocytes, like epithelial cells, fibroblasts and endothelial cells. These terminally differentiated cells are normally in a metabolically quiescent state when they are encountered by these pathogens during infection. This metabolic state of the host cells does not meet the extensive need for nutrients required for efficient intracellular replication of viruses and especially IBPs which, in contrast to the viral pathogens, have to perform their own specific intracellular metabolism to survive and efficiently replicate in their host cell niches. For this goal, viruses and IBPs have to reprogram the host cell metabolism in a pathogen-specific manner to increase the supply of nutrients, energy, and metabolites which have to be provided to the pathogen to allow its replication. In viral infections, this appears to be often achieved by the interaction of specific viral factors with central metabolic regulators, including oncogenes and tumor suppressors, or by the introduction of virus-specific oncogenes. Less is so far known on the mechanisms leading to metabolic reprogramming of the host cell by IBPs. However, the still scant data suggest that similar mechanisms may also determine the reprogramming of the host cell metabolism in IBP infections. In this review, we summarize and compare the present knowledge on this important, yet still poorly understood aspect of pathogenesis of human viral and especially IBP infections.

Molecular logic of mTORC1 signalling as a metabolic rheostat

The protein kinase complex mechanistic target of rapamycin complex 1 (mTORC1) serves as a key conduit between growth signals and the metabolic processes underlying cell growth. The activation state of mTORC1 is controlled by intracellular nutrients and energy, as well as exogenous hormones and growth factors, thereby integrating local and systemic growth signals. Here we discuss the molecular logic of the mTORC1 signalling network and its importance in coupling growth signals to the control of cellular metabolism. After activation, mTORC1 promotes the conversion of available nutrients and energy into the major macromolecular species contributing to cellular mass, including proteins, nucleic acids and lipids, while suppressing the autophagic recycling of these macromolecules back into their nutrient constituents. Given that uncoupling of mTORC1 from its normal regulatory inputs contributes to many diseases-including cancer, genetic tumour syndromes, metabolic diseases, autoimmune diseases and neurological disorders-understanding the molecular logic of the mTORC1 network and how to modulate it may present therapeutic opportunities for treatment of a broad range of diseases and potentially even for the extension of lifespan.

Interleukin-18 up-regulates amino acid transporters and facilitates amino acid-induced mTORC1 activation in natural killer cells

Upon inflammation, natural killer (NK) cells undergo metabolic changes to support their high energy demand for effector function and proliferation. The metabolic changes are usually accompanied by an increase in the expression of nutrient transporters, leading to increased nutrient uptake. Among various cytokines inducing NK cell proliferation, the mechanisms underlying the effect of interleukin (IL)-18 in promoting NK cell proliferation are not completely understood. Here, we demonstrate that IL-18 is a potent cytokine that can enhance the expression of the nutrient transporter CD98/LAT1 for amino acids independently of the mTORC1 pathway and thereby induce a dramatic metabolic change associated with increased proliferation of NK cells. Notably, treatment of IL-18-stimulated NK cells with leucine activates the metabolic sensor mTORC1, indicating that the high expression of amino acid transporters induces amino acid-driven mTORC1 activation. Inhibition of the amino acid transporter CD98/LAT1 abrogated the leucine-driven mTORC1 activation and reduced NK cell effector function. Taken together, our study identified a novel role of IL-18 in up-regulating nutrient transporters on NK cells and thereby inducing metabolic changes, including the mTORC1 activation by amino acids.

Phosphatidylinositol-5-Phosphate 4-Kinases Regulate Cellular Lipid Metabolism By Facilitating Autophagy

While the majority of phosphatidylinositol-4, 5-bisphosphate (PI-4, 5-P₂) in mammalian cells is generated by the conversion of phosphatidylinositol-4-phosphate (PI-4-P) to PI-4, 5-P₂, a small fraction can be made by phosphorylating phosphatidylinositol-5-phosphate (PI-5-P). The physiological relevance of this second pathway is not clear. Here, we show that deletion of the genes encoding the two most active enzymes in this pathway, Pip4k2a and Pip4k2b, in the liver of mice causes a large enrichment in lipid droplets and in autophagic vesicles during fasting. These changes are due to a defect in the clearance of autophagosomes that halts autophagy and reduces the supply of nutrients salvaged through this pathway. Similar defects in autophagy are seen in nutrient-starved Pip4k2a^-/-Pip4k2b^-/- mouse embryonic fibroblasts and in C. elegans lacking the PI5P4K ortholog. These results suggest that this alternative pathway for PI-4, 5-P₂ synthesis evolved, in part, to enhance the ability of multicellular organisms to survive starvation.

Leucine Signals to mTORC1 via Its Metabolite Acetyl-Coenzyme A

The mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) is a master regulator of cell growth and metabolism. Leucine (Leu) activates mTORC1 and many have tried to identify the mechanisms whereby cells sense Leu in this context. Here we describe that the Leu metabolite acetyl-coenzyme A (AcCoA) positively regulates mTORC1 activity by EP300-mediated acetylation of the mTORC1 regulator, Raptor, at K1097. Leu metabolism and consequent mTORC1 activity are regulated by intermediary enzymes. As AcCoA is a Leu metabolite, this process directly correlates with Leu abundance, and does not require Leu sensing via intermediary proteins, as has been described previously. Importantly, we describe that this pathway regulates mTORC1 in a cell-type-specific manner. Finally, we observed decreased acetylated Raptor, and inhibited mTORC1 and EP300 activity in fasted mice tissues. These results provide a direct mechanism for mTORC1 regulation by Leu metabolism.

Infection of HeLa cells with Chlamydia trachomatis inhibits protein synthesis and causes multiple changes to host cell pathways

The obligate intracellular bacterium Chlamydia trachomatis replicates in a cytosolic vacuole in human epithelial cells. Infection of human cells with C. trachomatis causes substantial changes to many host cell-signalling pathways, but the molecular basis of such influence is not well understood. Studies of gene transcription of the infected cell have shown altered transcription of many host cell genes, indicating a transcriptional response of the host cell to the infection. We here describe that infection of HeLa cells with C. trachomatis as well as infection of murine cells with Chlamydia muridarum substantially inhibits protein synthesis of the infected host cell. This inhibition was accompanied by changes to the ribosomal profile of the infected cell indicative of a block of translation initiation, most likely as part of a stress response. The Chlamydia protease-like activity factor (CPAF) also reduced protein synthesis in uninfected cells, although CPAF-deficient C. trachomatis showed no defect in this respect. Analysis of polysomal mRNA as a proxy of actively transcribed mRNA identified a number of biological processes differentially affected by chlamydial infection. Mapping of differentially regulated genes onto a protein interaction network identified nodes of up- and down-regulated networks during chlamydial infection. Proteomic analysis of protein synthesis further suggested translational regulation of host cell functions by chlamydial infection. These results demonstrate reprogramming of the host cell during chlamydial infection through the alteration of protein synthesis.

Cryo-EM insight into the structure of MTOR complex 1 and its interactions with Rheb and substrates

The mechanistic target of rapamycin (MTOR) is a giant protein kinase that, together with the accessory proteins Raptor and mLst8, forms a complex of over 1 MDa known as MTOR complex 1 (MTORC1). MTORC1, through its protein kinase activity, controls the accretion of cell mass through the regulation of gene transcription, mRNA translation, and protein turnover. MTORC1 is activated in an interdependent manner by insulin/growth factors and nutrients, especially amino acids, and is inhibited by stressors such as hypoxia and by the drug rapamycin. The action of insulin/growth factors converges on the small GTPase Rheb, which binds directly to the MTOR polypeptide in MTORC1 and, in its GTP-bound state, initiates kinase activation. Biochemical studies established that MTORC1 exists as a dimer of the MTOR/Raptor/mLst8 trimer, and progressive refinements in cryo-electron microscopy (cryo-EM) have enabled an increasingly clear picture of the architecture of MTORC1, culminating in a deep understanding of how MTORC1 interacts with and phosphorylates its best-known substrates-the eIF-4E binding protein/4E-BP, the p70 S6 kinase/S6K1B, and PRAS40/AKT1S1-and how this is inhibited by rapamycin. Most recently, Rheb-GTP has been shown to bind to MTORC1 in a cooperative manner at an allosteric site remote from the kinase domain that twists the latter into a catalytically competent configuration. Herein, we review the recent cryo-EM and associated biochemical studies of MTORC1 and seek to integrate these new results with the known physiology of MTORC1 regulation and signaling.

Neuroanatomical Framework of the Metabolic Control of Reproduction

A minimum amount of energy is required for basic physiological processes, such as protein biosynthesis, thermoregulation, locomotion, cardiovascular function, and digestion. However, for reproductive function and survival of the species, extra energy stores are necessary. Production of sex hormones and gametes, pubertal development, pregnancy, lactation, and parental care all require energy reserves. Thus the physiological systems that control energy homeostasis and reproductive function coevolved in mammals to support both individual health and species subsistence. In this review, we aim to gather scientific knowledge produced by laboratories around the world on the role of the brain in integrating metabolism and reproduction. We describe essential neuronal networks, highlighting key nodes and potential downstream targets. Novel animal models and genetic tools have produced substantial advances, but critical gaps remain. In times of soaring worldwide obesity and metabolic dysfunction, understanding the mechanisms by which metabolic stress alters reproductive physiology has become crucial for human health.

LRP6 Knockdown Ameliorates Insulin Resistance via Modulation of Autophagy by Regulating GSK3β Signaling in Human LO2 Hepatocytes

Recent studies suggest that autophagy is highly involved in insulin resistance (IR). Inhibition of the PI3K/AKT/mTOR signaling pathway induces autophagy activation. Additionally, depletion of LRP6 has been shown to increase insulin sensitivity but its mechanism is still not clear. We hypothesized that LRP6 contributes to IR by regulating mTOR mediated autophagy through GSK3β in hepatocytes. LO2 hepatocytes were treated with palmitate (PA) and insulin to induced IR. Levels of LRP6 mRNA and protein expression were measured by real time-PCR and western blot analysis. LRP6 knock down was achieved by adenovirus mediated Si-LRP6 expression and its roles in IR, glucose, GSK3β, mTOR signaling, and autophagy were explored. Finally, GSK3β was overexpressed and its involvement in autophagy and IR was examined. We found that PA treatment led to a reduced glucose uptake and IR in hepatocytes, which was accompanied by an upregulation of LRP6 expression. Knocking down of LRP6 enhanced glucose uptake and insulin sensitivity in PA treated cells, probably through increasing GSK3b activity. Overexpression of GSK3b mimicked LRP6 reduction by enhancing autophagy and ameliorating IR. Our study revealed a significant molecular mechanism connecting LRP6 to insulin sensitivity through GSK3β-mTOR mediated autophagy.

DEPTOR modulates activation responses in CD4+ T cells and enhances immunoregulation following transplantation

DEPTOR is an evolutionarily conserved cell-intrinsic binding partner of mTOR that functions as a negative regulator of signaling responses. In this study, we show that DEPTOR is expressed within CD4⁺ T cells, and we observed that its relative level of expression modulates differentiation as well as glucose utilization within CD4⁺ T effectors in vitro. Using knock-in mice, we also find that induced expression of DEPTOR within CD4⁺ T regulatory cells stabilizes Foxp3 expression, shifts metabolism toward oxidative phosphorylation, and increases survival and suppressive function. In vivo, fully MHC mismatched cardiac allograft survival is significantly prolonged in knock-in recipients and sustained recipient expression of DEPTOR in combination with costimulatory blockade induces long-term graft survival. Furthermore, we show that the induced expression of DEPTOR in CD4⁺ T effectors fails to inhibit acute allograft rejection. Rather, prolonged survival is dominantly mediated via induced expression and function of DEPTOR within recipient CD4⁺ T regulatory cells. These collective findings identify DEPTOR as a novel protein that functions in CD4⁺ T cells to augment immunoregulation in vitro and in vivo.

Selective inhibition of carbonic anhydrase-IX by sulphonamide derivatives induces pH and reactive oxygen species-mediated apoptosis in cervical cancer HeLa cells

Selective inhibition with sulphonamides of carbonic anhydrase (CA) IX reduces cell proliferation and induces apoptosis in human cancer cells. The effect on CA IX expression of seven previously synthesised sulphonamide inhibitors, with high affinity for CA IX, as well as their effect on the proliferation/apoptosis of cancer/normal cell lines was investigated. Two normal and three human cancer cell lines were used. Treatment resulted in dose- and time-dependent inhibition of the growth of various cancer cell lines. One compound showed remarkably high toxicity towards CA IX-positive HeLa cells. The mechanisms of apoptosis induction were determined with Annexin-V and AO/EB staining, cleaved caspases (caspase-3, caspase-8, caspase-9) and cleaved PARP activation, reactive oxygen species production (ROS), mitochondrial membrane potential (MMP), intracellular pH (pHi), extracellular pH (pHe), lactate level and cell cycle analysis. The autophagy induction mechanisms were also investigated. The modulation of apoptotic and autophagic genes (Bax, Bcl-2, caspase-3, caspase-8, caspase-9, caspase-12, Beclin and LC3) was measured using real time PCR. The positive staining using γ-H2AX and AO/EB dye, showed increased cleaved caspase-3, caspase-8, caspase-9, increased ROS production, MMP and enhanced mRNA expression of apoptotic genes, suggesting that anticancer effects are also exerted through its apoptosis-inducing properties. Our results show that such sulphonamides might have the potential as new leads for detailed investigations against CA IX-positive cervical cancers.

Saccharomyces cerevisiae adapted to grow in the presence of low-dose rapamycin exhibit altered amino acid metabolism

BACKGROUND

Rapamycin is a potent inhibitor of the highly conserved TOR kinase, the nutrient-sensitive controller of growth and aging. It has been utilised as a chemotherapeutic agent due to its anti-proliferative properties and as an immunosuppressive drug, and is also known to extend lifespan in a range of eukaryotes from yeast to mammals. However, the mechanisms through which eukaryotic cells adapt to sustained exposure to rapamycin have not yet been thoroughly investigated.

METHODS

Here, S. cerevisiae response to long-term rapamycin exposure was investigated by identifying the physiological, transcriptomic and metabolic differences observed for yeast populations inoculated into low-dose rapamycin-containing environment. The effect of oxygen availability and acidity of extracellular environment on this response was further deliberated by controlling or monitoring the dissolved oxygen level and pH of the culture.

RESULTS

Yeast populations grown in the presence of rapamycin reached higher cell densities complemented by an increase in their chronological lifespan, and these physiological adaptations were associated with a rewiring of the amino acid metabolism, particularly that of arginine. The ability to synthesise amino acids emerges as the key factor leading to the major mechanistic differences between mammalian and microbial TOR signalling pathways in relation to nutrient recognition.

CONCLUSION

Oxygen levels and extracellular acidity of the culture were observed to conjointly affect yeast populations, virtually acting as coupled physiological effectors; cells were best adapted when maximal oxygenation of the culture was maintained in slightly acidic pH, any deviation necessitated more extensive readjustment to additional stress factors.

Silencing the FOLR2 Gene Inhibits Cell Proliferation and Increases Apoptosis in the NCI-H1650 Non-Small Cell Lung Cancer Cell Line via Inhibition of AKT/Mammalian Target of Rapamycin (mTOR)/Ribosomal Protein S6 Kinase 1 (S6K1) Signaling

Background

The FOLR2 gene encodes folate receptor-beta (FR-beta), which is expressed by tumor-associated macrophages. The effects of FOLR2 gene expression in non-small cell lung cancer (NSCLC) remains unknown. This study aimed to investigate the effects of FOLR2 gene expression and gene silencing in human NSCLC cell lines and normal human bronchial epithelial (HBE) cells in vitro.

Material/Methods

Quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot were performed to detect the expression of the FOLR2 gene, cell cycle and apoptosis-associated genes in normal HBE cells and the NSCLC cell lines, A549, NCI-H1299, NCI-H1650, and NCI-H460. Using small interfering RNA (siRNA), or silencing RNA, FOLR2 gene silencing was performed for NCI-H1650 cells. Cell counting kit-8 (CCK-8) was used to measure cell viability. Cell cycle and apoptosis were determined using flow cytometry. Western blot evaluated the expression of Akt, mTOR, and S6K1 signaling.

Results

Expression of the FOLR2 gene was increased in NSCLC cells compared with normal HBE cells. Silencing of the expression of the FOLR2 gene in NCI-H1650 cells reduced cell viability, increased cell apoptosis, and arrested cells in the G1 phase of the cell cycle, decreased the expression of cyclin D1, upregulated expression of cell cycle inhibitors, p21 and p27, upregulated the expression of Bax/Bcl-2, and inhibited phosphorylation of AKT, mTOR, and S6K1.

Conclusions

Silencing of the FOLR2 gene inhibited phosphorylation of AKT, mTOR, and S6K1, inhibited cell proliferation and increased apoptosis in the NCI-H1650 human NSCLC cell line.

The tuberous sclerosis complex subunit TBC1D7 is stabilized by Akt phosphorylation-mediated 14-3-3 binding

The tuberous sclerosis complex (TSC) is a negative regulator of mTOR complex 1, a signaling node promoting cellular growth in response to various nutrients and growth factors. However, several regulators in TSC signaling still await discovery and characterization. Using pulldown and MS approaches, here we identified the TSC complex member, TBC1 domain family member 7 (TBC1D7), as a binding partner for PH domain and leucine-rich repeat protein phosphatase 1 (PHLPP1), a negative regulator of Akt kinase signaling. Most TBC domain-containing proteins function as Rab GTPase-activating proteins (RabGAPs), but the crystal structure of TBC1D7 revealed that it lacks residues critical for RabGAP activity. Sequence analysis identified a putative site for both Akt-mediated phosphorylation and 14-3-3 binding at Ser-124, and we found that Akt phosphorylates TBC1D7 at Ser-124. However, this phosphorylation had no effect on the binding of TBC1D7 to TSC1, but stabilized TBC1D7. Moreover, 14-3-3 protein both bound and stabilized TBC1D7 in a growth factor-dependent manner, and a phospho-deficient substitution, S124A, prevented this interaction. The crystal structure of 14-3-3ζ in complex with a phospho-Ser-124 TBC1D7 peptide confirmed the direct interaction between 14-3-3 and TBC1D7. The sequence immediately upstream of Ser-124 aligned with a canonical β-TrCP degron, and we found that the E3 ubiquitin ligase β-TrCP2 ubiquitinates TBC1D7 and decreases its stability. Our findings reveal that Akt activity determines the phosphorylation status of TBC1D7 at the phospho-switch Ser-124, which governs binding to either 14-3-3 or β-TrCP2, resulting in increased or decreased stability of TBC1D7, respectively.

Glucose promotes cell growth by suppressing branched-chain amino acid degradation

Glucose and branched-chain amino acids (BCAAs) are essential nutrients and key determinants of cell growth and stress responses. High BCAA level inhibits glucose metabolism but reciprocal regulation of BCAA metabolism by glucose has not been demonstrated. Here we show that glucose suppresses BCAA catabolism in cardiomyocytes to promote hypertrophic response. High glucose inhibits CREB stimulated KLF15 transcription resulting in downregulation of enzymes in the BCAA catabolism pathway. Accumulation of BCAA through the glucose-KLF15-BCAA degradation axis is required for the activation of mTOR signaling during the hypertrophic growth of cardiomyocytes. Restoration of KLF15 prevents cardiac hypertrophy in response to pressure overload in wildtype mice but not in mutant mice deficient of BCAA degradation gene. Thus, regulation of KLF15 transcription by glucose is critical for the glucose-BCAA circuit which controls a cascade of obligatory metabolic responses previously unrecognized for cell growth.

Signal integration in the (m)TORC1 growth pathway

Background

The protein kinase Target Of Rapamycin (TOR) is a nexus for the regulation of eukaryotic cell growth. TOR assembles into one of two distinct signalling complexes, TOR complex 1 (TORC1) and TORC2 (mTORC1/2 in mammals), with a set of largely non-overlapping protein partners. (m)TORC1 activation occurs in response to a series of stimuli relevant to cell growth, including nutrient availability, growth factor signals and stress, and regulates much of the cell's biosynthetic activity, from proteins to lipids, and recycling through autophagy. mTORC1 regulation is of great therapeutic significance, since in humans many of these signalling complexes, alongside subunits of mTORC1 itself, are implicated in a wide variety of pathophysiologies, including multiple types of cancer, neurological disorders, neurodegenerative diseases and metabolic disorders including diabetes.

Methodology

Recent years have seen numerous structures determined of (m)TOR, which have provided mechanistic insight into (m)TORC1 activation in particular, however the integration of cellular signals occurs upstream of the kinase and remains incompletely understood. Here we have collected and analysed in detail as many as possible of the molecular and structural studies which have shed light on (m)TORC1 repression, activation and signal integration.

Conclusions

A molecular understanding of this signal integration pathway is required to understand how (m)TORC1 activation is reconciled with the many diverse and contradictory stimuli affecting cell growth. We discuss the current level of molecular understanding of the upstream components of the (m)TORC1 signalling pathway, recent progress on this key biochemical frontier, and the future studies necessary to establish a mechanistic understanding of this master-switch for eukaryotic cell growth.

Increased autophagy signaling but not proteasome activity in human skeletal muscle after prolonged low-intensity exercise with negative energy balance

Little is known about the molecular regulation of skeletal muscle protein turnover during exercise in field conditions where energy is intake inadequate. Here, 17 male and 7 female soldiers performed an 8 days long field-based military operation. Vastus lateralis muscle biopsies, in which autophagy, the ubiquitin-proteasome system, and the mTORC1 signaling pathway were studied, were collected before and after the operation. The 187 h long operation resulted in a 15% and 29% negative energy balance as well as a 4.1% and 4.6% loss of body mass in women and men, respectively. After the operation protein levels of ULK1 as well as the phosphorylation of ULK1^Ser317 and ULK1^Ser555 had increased by 11%, 39%, and 13%, respectively, and this was supported by a 17% increased phosphorylation of AMPK^Thr172 (P < 0.05). The LC3b-I/II ratio was threefold higher after compared to before the operation (P < 0.05), whereas protein levels of p62/SQSTM1 were unchanged. The β1, β2, and β5 activity of the proteasome and protein levels of MAFbx did not change, whereas levels of MuRF-1 were slightly reduced (6%, P < 0.05). Protein levels and phosphorylation status of key components in the mTORC1 signaling pathway remained at basal levels after the operation. Muscle levels of glycogen decreased from 269 ± 12 to 181 ± 9 mmol·kg dry·muscle^-1 after the exercise period (P < 0.05). In conclusion, the 8 days of field-based exercise resulted in induction of autophagy without any increase in proteasome activity or protein ubiquitination. Simultaneously, the regulation of protein synthesis through the mTORC1 signaling pathway was maintained.

Analysis of the Long-Lived Responses Induced by Immunostimulants and Their Effects on a Viral Infection in Zebrafish (Danio rerio)

In recent years, the innate immune response has gained importance since evidence indicates that after an adequate priming protocol, it is possible to obtain some prolonged and enhanced immune responses. Nevertheless, several factors, such as the timing and method of administration of the immunostimulants, must be carefully considered. An inappropriate protocol can transform the treatments into a double-edged sword for the teleost immune system, resulting in a stressful and immunosuppressive state. In this work, we analyzed the long-term effects of different stimuli (β-glucans, lipopolysaccharide, and polyinosinic:polycytidylic acid) on the transcriptome modulation induced by Spring Viremia Carp Virus (SVCV) in adult zebrafish (Danio rerio) and on the mortality caused by this infection. At 35 days post-immunostimulation, the transcriptome was found to be highly altered compared to that of the control fish, and these stimuli also conditioned the response to SVCV challenge, especially in the case of β-glucans. No protection against SVCV was found with any of the stimuli, and non-significant higher mortalities were even observed, especially with β-glucans. However, in the short term (pre-stimulation with β-glucan and infection after 7 days), slight protection was observed after infection. The transcriptome response in the zebrafish kidney at 35 days posttreatment with β-glucans revealed a significant response associated with stress and immunosuppression. The identification of genes that were differentially expressed before and after the infection seemed to indicate a high energy cost of the immunostimulation that was prolonged over time and could explain the lack of protection against SVCV. Differential responses to stress and alterations in lipid metabolism, the tryptophan–kynurenine pathway, and interferon-gamma signaling seem to be some of the mechanisms involved in this response, which represents the end of trained immunity and the beginning of a stressful state characterized by immunosuppression.

Abnormal mTOR Activation in Autism

The mechanistic target of rapamycin (mTOR) is an important signaling hub that integrates environmental information regarding energy availability and stimulates anabolic molecular processes and cell growth. Abnormalities in this pathway have been identified in several syndromes in which autism spectrum disorder (ASD) is highly prevalent. Several studies have investigated mTOR signaling in developmental and neuronal processes that, when dysregulated, could contribute to the development of ASD. Although many potential mechanisms still remain to be fully understood, these associations are of great interest because of the clinical availability of mTOR inhibitors. Clinical trials evaluating the efficacy of mTOR inhibitors to improve neurodevelopmental outcomes have been initiated.

mTORC1 Controls Phase Separation and the Biophysical Properties of the Cytoplasm by Tuning Crowding

Macromolecular crowding has a profound impact on reaction rates and the physical properties of the cell interior, but the mechanisms that regulate crowding are poorly understood. We developed genetically encoded multimeric nanoparticles (GEMs) to dissect these mechanisms. GEMs are homomultimeric scaffolds fused to a fluorescent protein that self-assemble into bright, stable particles of defined size and shape. By combining tracking of GEMs with genetic and pharmacological approaches, we discovered that the mTORC1 pathway can modulate the effective diffusion coefficient of particles ≥20 nm in diameter more than 2-fold by tuning ribosome concentration, without any discernable effect on the motion of molecules ≤5 nm. This change in ribosome concentration affected phase separation both in vitro and in vivo. Together, these results establish a role for mTORC1 in controlling both the mesoscale biophysical properties of the cytoplasm and biomolecular condensation.

PATs and SNATs: Amino Acid Sensors in Disguise

Solute Carriers (SLCs) are involved in the transport of substances across lipid bilayers, including nutrients like amino acids. Amino acids increase the activity of the microenvironmental sensor mechanistic Target of Rapamycin Complex 1 (mTORC1) to promote cellular growth and anabolic processes. They can be brought in to cells by a wide range of SLCs including the closely related Proton-assisted Amino acid Transporter (PAT or SLC36) and Sodium-coupled Neutral Amino acid Transporter (SNAT or SLC38) families. More than a decade ago, the first evidence emerged that members of the PAT family can act as amino acid-stimulated receptors, or so-called "transceptors," connecting amino acids to mTORC1 activation. Since then, further studies in human cell models have suggested that other PAT and SNAT family members, which share significant homology within their transmembrane domains, can act as transceptors. A paradigm shift has also led to the PATs and SNATs at the surface of multiple intracellular compartments being linked to the recruitment and activation of different pools of mTORC1. Much focus has been on late endosomes and lysosomes as mTORC1 regulatory hubs, but more recently a Golgi-localized PAT was shown to be required for mTORC1 activation. PATs and SNATs can also traffic between the cell surface and intracellular compartments, with regulation of this movement providing a means of controlling their mTORC1 regulatory activity. These emerging features of PAT and SNAT amino acid sensors, including the transceptor mechanism, have implications for the pharmacological inhibition of mTORC1 and new therapeutic interventions.

The mTORC1 Signaling Network Senses Changes in Cellular Purine Nucleotide Levels

Mechanistic (or mammalian) target of rapamycin complex 1 (mTORC1) integrates signals from growth factors and nutrients to control biosynthetic processes, including protein, lipid, and nucleic acid synthesis. We find that the mTORC1 pathway is responsive to changes in purine nucleotides in a manner analogous to its sensing of amino acids. Depletion of cellular purines, but not pyrimidines, inhibits mTORC1, and restoration of intracellular adenine nucleotides via addition of exogenous purine nucleobases or nucleosides acutely reactivates mTORC1. Adenylate sensing by mTORC1 is dependent on the tuberous sclerosis complex (TSC) protein complex and its regulation of Rheb upstream of mTORC1, but independent of energy stress and AMP-activated protein kinase (AMPK). Even though mTORC1 signaling is not acutely sensitive to changes in intracellular guanylates, long-term depletion of guanylates decreases Rheb protein levels. Our findings suggest that nucleotide sensing, like amino acid sensing, enables mTORC1 to tightly coordinate nutrient availability with the synthesis of macromolecules, such as protein and nucleic acids, produced from those nutrients.

Signaling Pathways Involved in the Regulation of mRNA Translation

Translation is a key step in the regulation of gene expression and one of the most energy-consuming processes in the cell. In response to various stimuli, multiple signaling pathways converge on the translational machinery to regulate its function. To date, the roles of phosphoinositide 3-kinase (PI3K)/AKT and the mitogen-activated protein kinase (MAPK) pathways in the regulation of translation are among the best understood. Both pathways engage the mechanistic target of rapamycin (mTOR) to regulate a variety of components of the translational machinery. While these pathways regulate protein synthesis in homeostasis, their dysregulation results in aberrant translation leading to human diseases, including diabetes, neurological disorders, and cancer. Here we review the roles of the PI3K/AKT and MAPK pathways in the regulation of mRNA translation. We also highlight additional signaling mechanisms that have recently emerged as regulators of the translational apparatus.

Endothelial replicative senescence delayed by the inhibition of MTORC1 signaling involves MicroRNA-107

Accumulation of senescent endothelial cells can contribute to endothelium dysfunction. Suppression of MTOR signaling has been shown to delay senescence but the mechanism that underpins this effect, particularly one that involves miRNAs, remains to be further defined. This study sought to identify miRNAs involved in MTORC1-mediated inhibition of replicative senescence in endothelial cells. Pre-senescent HUVECs were prolonged treated with low dose rapamycin (1 nM), an MTOR inhibitor. Rapamycin treatment down-regulated the phosphorylated MTOR, RPS6 and 4EBP1 expressions, which confirmed MTORC1 suppression. Prolonged low dose rapamycin treatment has significantly reduced the percentage of senescence-associated beta galactosidase (SA-β gal) positively stained senescent cells and P16INK4A expression in these cells. On the contrary, the percentage of BrdU-labelled proliferating cells has significantly increased. RPTOR, a positive regulator of MTORC1 was knockdown using RPTOR siRNA to inhibit MTORC1 activation. RPTOR knockdown was evidenced by significant suppressions of RPTOR mRNA and protein expression levels. In these cells, the expression of miR-107 was down-regulated whereas miR-145-5p and miR-217 were up-regulated. Target gene prediction revealed PTEN as the target of miR-107 and this was confirmed by biotin pull-down assay. Over-expression of miR-107 has decreased PTEN expression, increased MTORC1 activity, induced cell cycle arrest at G0/G1 phase and up-regulated P16INK4A expression but mitigated tube formation. Collectively, our findings revealed that delayed endothelial replicative senescence caused by the inhibition of MTORC1 activation could be modulated by miR-107 via its influence on PTEN.

Effects of single amino acid deficiency on mRNA translation are markedly different for methionine versus leucine

Although amino acids are known regulators of translation, the unique contributions of specific amino acids are not well understood. We compared effects of culturing HEK293T cells in medium lacking either leucine, methionine, histidine, or arginine on eIF2 and 4EBP1 phosphorylation and measures of mRNA translation. Methionine starvation caused the most drastic decrease in translation as assessed by polysome formation, ribosome profiling, and a measure of protein synthesis (puromycin-labeled polypeptides) but had no significant effect on eIF2 phosphorylation, 4EBP1 hyperphosphorylation or 4EBP1 binding to eIF4E. Leucine starvation suppressed polysome formation and was the only tested condition that caused a significant decrease in 4EBP1 phosphorylation or increase in 4EBP1 binding to eIF4E, but effects of leucine starvation were not replicated by overexpressing nonphosphorylatable 4EBP1. This suggests the binding of 4EBP1 to eIF4E may not by itself explain the suppression of mRNA translation under conditions of leucine starvation. Ribosome profiling suggested that leucine deprivation may primarily inhibit ribosome loading, whereas methionine deprivation may primarily impair start site recognition. These data underscore our lack of a full understanding of how mRNA translation is regulated and point to a unique regulatory role of methionine status on translation initiation that is not dependent upon eIF2 phosphorylation.

Licorice flavonoid oil enhances muscle mass in KK-Ay mice

AIMS

Muscle mass is regulated by the balance between the synthesis and degradation of muscle proteins. Loss of skeletal muscle mass is associated with an increased risk of developing metabolic diseases such as obesity and type 2 diabetes mellitus. The aim of this study was to clarify the effects of licorice flavonoid oil on muscle mass in KK-A^y/Ta mice.

MAIN METHODS

Male genetically type II diabetic KK-A^y/Ta mice received 0, 1, or 1.5 g/kg BW of licorice flavonoid oil by mouth once daily for 4 weeks. After 4 weeks, the femoral and soleus muscles were collected for western blotting for evaluation of the mTOR/p70 S6K, p38/FoxO3a, and Akt/FoxO3a signaling pathways.

KEY FINDINGS

Ingestion of licorice flavonoid oil significantly enhanced femoral muscle mass without affecting body weight in KK-A^y/Ta mice. Licorice flavonoid oil also decreased expression of MuRF1 and atrogin-1, which are both markers of muscle atrophy. The mechanisms by which licorice flavonoid oil enhances muscle mass include activation of mTOR and p70 S6K, and regulation of phosphorylation of FoxO3a.

SIGNIFICANCE

Ingestion of licorice flavonoids may help to prevent muscle atrophy.

Depletion of the mRNA translation initiation inhibitor, programmed cell death protein 4 (PDCD4), impairs L6 myotube formation

The mechanistic (mammalian) target of rapamycin complex 1 (mTORC1) signaling is vital for optimal muscle mass and function. Although the significance of mTORC1 in stimulating muscle growth is unequivocal, evidence in support of its role during muscle regeneration is less clear. Here, we showed that the abundance (protein and mRNA) of the mTORC1/S6K1 substrate, programmed cell death protein 4 (PDCD4), is upregulated at the onset of differentiation of L6 and C2C12 cells. The increase in PDCD4 was not associated with any changes in S6K1 activation, but the abundance of beta transducing repeat‐containing protein (β‐TrCP), the ubiquitin ligase that targets PDCD4 for degradation, increased. Myoblasts lacking PDCD4 showed impaired myotube formation and had markedly low levels of MHC‐1. Analysis of poly (ADP‐ribose) Polymerase (PARP), caspase 7 and caspase 3 indicated reduced apoptosis in PDCD4‐deficient cells. Our data demonstrate a role for PDCD4 in muscle cell formation and suggest that interventions that target this protein may hold promise for managing conditions associated with impaired myotube formation.

Effect of Nutrition on Statural Growth

In children, proper growth and development are often regarded as a surrogate marker for good health. A complex system controls the initiation, rate, and cessation of growth, and thus gives a wonderful example of the interactions between genetics, epigenetics, and environmental factors (especially stress and nutrition). Malnutrition is considered a leading cause of growth attenuation in children. This review summarizes our current knowledge regarding the mechanisms linking nutrition and skeletal growth, including systemic factors, such as insulin, growth hormone, insulin-like growth factor-1, fibroblast growth factor-21, etc., and local mechanisms, including mTOR, miRNAs, and epigenetics. Studying the molecular mechanisms regulating skeletal growth may lead to the establishment of better nutritional and therapeutic regimens for more effective linear growth in children with malnutrition and growth abnormalities. 
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Modulation of the secretory pathway by amino-acid starvation

As a major anabolic pathway, the secretory pathway needs to adapt to the demands of the surrounding environment and responds to different exogenous signals and stimuli. In this context, the transport in the early secretory pathway from the endoplasmic reticulum (ER) to the Golgi apparatus appears particularly regulated. For instance, protein export from the ER is critically stimulated by growth factors. Conversely, nutrient starvation also modulates functions of the early secretory pathway in multiple ways. In this review, we focus on amino-acid starvation and how the function of the early secretory pathway is redirected to fuel autophagy, how the ER exit sites are remodeled into novel cytoprotective stress assemblies, and how secretion is modulated in vivo in starving organisms. With the increasingly exciting knowledge on mechanistic target of rapamycin complex 1 (mTORC1), the major nutrient sensor, it is also a good moment to establish how the modulation of the secretory pathway by amino-acid restriction intersects with this major signaling hub.

mTORC1 and Nutrient Homeostasis: The Central Role of the Lysosome

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.

MAP4K3 mediates amino acid-dependent regulation of autophagy via phosphorylation of TFEB

Autophagy is the major cellular pathway by which macromolecules are degraded, and amino acid depletion powerfully activates autophagy. MAP4K3, or germinal-center kinase-like kinase, is required for robust cell growth in response to amino acids, but the basis for MAP4K3 regulation of cellular metabolic disposition remains unknown. Here we identify MAP4K3 as an amino acid-dependent regulator of autophagy through its phosphorylation of transcription factor EB (TFEB), a transcriptional activator of autophagy, and through amino acid starvation-dependent lysosomal localization of MAP4K3. We document that MAP4K3 physically interacts with TFEB and MAP4K3 inhibition is sufficient for TFEB nuclear localization, target gene transactivation, and autophagy, even when mTORC1 is activated. Moreover, MAP4K3 serine 3 phosphorylation of TFEB is required for TFEB interaction with mTORC1-Rag GTPase-Ragulator complex and TFEB cytosolic sequestration. Our results uncover a role for MAP4K3 in the control of autophagy and reveal MAP4K3 as a central node in nutrient-sensing regulation.

Amino acids stimulate cell growth and depletion in a cell activates autophagy, yet how this is regulated is unclear. Here, the authors show that MAP4K3 (also known as germinal-center kinase-like kinase) acts as an amino acid-dependent regulator of autophagy, via phosphorylation of the transcription factor EB.

Combining the mammalian target of rapamycin inhibitor, rapamycin, with resveratrol has a synergistic effect in multiple myeloma

Rapamycin is known to inhibit the mammalian target of rapamycin complex (mTORC)1 signaling pathway, but it is unable to effectively inhibit mTORC2, resulting in activation of protein kinase B in multiple myeloma (MM) cell lines. Additionally, certain studies have suggested that resveratrol has an effect on human MM cells, and that rapamycin in combination with resveratrol may be useful in cancer therapy. The present study aimed to investigate the combined treatment effect of resveratrol and rapamycin on the MM MM1.S cell line. The results demonstrated that combined treatment with rapamycin and resveratrol effectively inhibited cell viability in the MM1.S cell line through inhibition of the mTORC1 and mTORC2 signaling pathways, compared with resveratrol or rapamycin monotherapy. In addition, cyclin D1 levels were decreased and the activation of caspase-3 and poly (ADP-ribose) polymerase was increased. These results suggested that downregulation of the mTOR signaling cascades is likely to be a crucial mediator in the impairment of viability and the induction of apoptosis resulting from combined therapy with resveratrol and rapamycin in MM1.S cells.

Nutrient Sensing in Cancer

Cell-intrinsic mechanisms of nutrient sensing are intimately linked to adaptive metabolic responses, and these pathways play critical roles in the complex and dynamic nutrient environment of a growing tumor. Nutrient-responsive transcription factors (e.g., HIF, SREBP, ATF4) and signaling pathways (e.g., mTORC1, AMPK) allow tumor cells to tune their metabolic output and strategies to fluctuations in nutrient availability, thus balancing tumor cell proliferation and survival with a combination of anabolic and adaptive responses. Coupling these nutrient-sensing mechanisms to the control of recycling and scavenging processes, such as autophagy and macropinocytosis, further enhances the adaptability to nutrients within tumors. Here, we discuss the key nutrient-sensing pathways active in cancer cells, how oncogenic events influence these pathways, and their likely contributions to tumor growth and survival. A better understanding of nutrient-sensing strategies and metabolic adaptations within the tumor microenvironment is critical to defining and targeting metabolic vulnerabilities in cancer.

Hexokinase 2 is a molecular bridge linking telomerase and autophagy

Autophagy is systematically regulated by upstream factors and nutrients. Recent studies reported that telomerase and hexokinase 2 [HK2) regulate autophagy through mTOR and that telomerase has the capacity to bind to the HK2 promoter. However, the molecular linkage among telomerase, HK2, and autophagy is not fully understood. Here, we show that HK2 connects telomerase to autophagy. HK2 inhibition in HepG2 cells suppressed TERT-induced autophagy activation and further enhancement by glucose deprivation. The HK2 downstream factor mTOR was responsible for the TERT-induced autophagy activation under glucose deprivation, implying that TERT promotes autophagy through an HK2-mTOR pathway. TERC played a role similar to that of TERT, and simultaneous expression of TERT and TERC synergistically enhanced HK2 expression and autophagy. At the gene level, TERT bound to the HK2 promoter at a specific region harboring the telomerase-responsive sequence ‘TTGGG.’ Mutagenesis of TERC and the TERT-responsive element in the HK2 promoter revealed that TERC is required for the binding of TERT to the HK2 promoter. We demonstrate the existence of a telomerase-HK2-mTOR-autophagy axis and suggest that inhibition of the interaction between telomerase and the HK2 promoter diminishes glucose starvation-induced autophagy.

PTEN negatively regulates the cell lineage progression from NG2+ glial progenitor to oligodendrocyte via mTOR-independent signaling

Oligodendrocytes (OLs), the myelin-forming CNS glia, are highly vulnerable to cellular stresses, and a severe myelin loss underlies numerous CNS disorders. Expedited OL regeneration may prevent further axonal damage and facilitate functional CNS repair. Although adult OL progenitors (OPCs) are the primary players for OL regeneration, targetable OPC-specific intracellular signaling mechanisms for facilitated OL regeneration remain elusive. Here, we report that OPC-targeted PTEN inactivation in the mouse, in contrast to OL-specific manipulations, markedly promotes OL differentiation and regeneration in the mature CNS. Unexpectedly, an additional deletion of mTOR did not reverse the enhanced OL development from PTEN-deficient OPCs. Instead, ablation of GSK3β, another downstream signaling molecule that is negatively regulated by PTEN-Akt, enhanced OL development. Our results suggest that PTEN persistently suppresses OL development in an mTOR-independent manner, and at least in part, via controlling GSK3β activity. OPC-targeted PTEN-GSK3β inactivation may benefit facilitated OL regeneration and myelin repair.

Sphingomyelin in microdomains of the plasma membrane regulates amino acid-stimulated mTOR signal activation

Sphingomyelin (SM) is required for cells to proliferate, but the reason is not fully understood. In order to asses this question, we employed a cell line, ZS, which lacks both SMS1 and SMS2, isolated from mouse embryonic fibroblasts in SMS1 and 2 double knockout mouse, and SMS1 or SMS2 re-expressing cells, ZS/SMS1 or ZS/SMS2, respectively. We investigated regulation of SM in activating the mammalian target of rapamycin (mTOR) signal induced by essential amino acids (EAA), using these cells. EAA-stimulated mTOR signal was more activated in ZS/SMS1 and ZS/SMS2 cells than in controls. Treatment with methyl-b-cyclodextrin dramatically inhibited the activation. Interestingly, we found that the expression of CD98, LAT-1 and ASCT-2, amino acid transporters concerned with mTOR activation, was down-regulated in ZS cells. Transporters localized in microdomains and formed a functional complex. Our results indicate that SM affect proliferation through the transport of amino acids via SM-enriched microdomains.