Intracellular Glutamine Research Articles

Rational design of ASCT2 inhibitors using an integrated experimental-computational approach

ASCT2 (SLC1A5) is a sodium-dependent neutral amino acid transporter that controls amino acid homeostasis in peripheral tissues. In cancer, ASCT2 is up-regulated where it modulates intracellular glutamine levels, fueling cell proliferation. Nutrient deprivation via ASCT2 inhibition provides a potential strategy for cancer therapy. Here, we rationally designed stereospecific inhibitors exploiting specific subpockets in the substrate binding site using computational modeling and cryo-electron microscopy (cryo-EM). The final structures combined with molecular dynamics simulations reveal multiple pharmacologically relevant conformations in the ASCT2 binding site as well as a previously unknown mechanism of stereospecific inhibition. Furthermore, this integrated analysis guided the design of a series of unique ASCT2 inhibitors. Our results provide a framework for future development of cancer therapeutics targeting nutrient transport via ASCT2, as well as demonstrate the utility of combining computational modeling and cryo-EM for solute carrier ligand discovery.

KRAS-Mutated Myeloma Cells Exploit Autophagy and Macropinocytosis to Tolerate Glutamine-Deprived Conditions

Introduction: Sustained cancer cell growth requires up-regulation of nutrient acquisition mechanisms. Novel approaches can identify “tumor fuel”, the way it is obtained and then altered for cellular usage. Autophagy is a conserved catabolic process that is thought to replenish nutrient pool and thereby starvation survival by providing bioenergetic and biosynthetic substrates. Some KRAS-mutated cancers show heightened dependency to glutamine as a source of energy. Since a large proportion of myeloma tumors are KRAS-mutated, we investigated in the present study the contribution of macropinocytosis and autophagy as the cellular mechanisms involved in protein uptake and degradation, respectively, to growth and survival of myeloma cells under glutamine-deprived condition.Methods: Macropinosomes were visualized using confocal microscopy in KRAS-mutated and WT human myeloma cells utilising TMR-dextran and 5-[N-ethyl-N-isopropyl] amiloride (EIPA) as a marker and specific inhibitor of macropinocytosis, respectively. Internalization and degradation of labelled albumin and its co-localization with TMR-dextran were evaluated in parallel. Bafilomycin was employed to evaluate the dependency of protein degradation on autophagy. Cell sensitivity to glutamine deprivation was examined with an ATP-based viability test. To evaluate the possible role of macropinocytosis and autophagy at glutamine deprivation conditions, the effect of albumin supplementation on cell growth and viability was studied in the presence or absence of EIPA or chloroquine. To test whether the degradation of macropinocytosed albumin results in the production of intracellular glutamine, the intracellular concentrations of glutamine was directly measured.Results: KRAS-mutated KMS28-PE, KMS18 and MM1S cells showed higher levels of EIPA-sensitive macropinocytosis compared to WT-KRAS KMS34, KMS12-BM and TK1 cells. Macropinocytosis led to the internalization of albumin which subsequently underwent bafilomycin-sensitive proteolytic degradation in KRAS-mutated cells. The viability of WT-KRAS cells was reduced in sub-physiological concentrations of glutamine when compared with KRAS-mutated cells that not only remained viable but could continue to slowly proliferate over a period of 7 days (p˂0.05). The compromised growth rate of KRAS-mutated cells at very low concentrations of glutamine was rescued by albumin and this effect was abrogated by EIPA or chloroquine (p˂0.05). The KRAS-mutated cells demonstrated an EIPA-sensitive increase in intracellular concentration of glutamine following albumin supplementation (p˂0.05) that was abrogated following the treatment with chloroquine. In WT cells, in contrast, the level of intracellular glutamine remained almost unchanged following albumin supplementation.Conclusions: Autophagy and macropinocytosis maintain cellular growth and survival in the context of nutrient deprivation by provision of glutamine, and potentially other amino acids, in RAS-mutated myeloma cancer. These data suggest that novel anti-cancer strategies interrupting these cellular mechanisms may represent a promising therapeutic approach to myeloma. DisclosuresSpencer:Janssen: Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding.

Intracellular glutamine level determines vascular smooth muscle cell-derived thrombogenicity

Background and aimsThe everolimus-eluting stent (EES), one of the effective stents for in-stent restenosis (ISR), has a lower incidence of stent thrombosis; however, the underlying mechanism remains unknown. This study aimed to identify the effects of everolimus on vascular metabolism and thrombogenicity and examine their mechanistic link. MethodsEESs and bare-metal stents were implanted in rabbit iliac arteries with smooth muscle cell (SMC)-rich neointima induced by endothelial denudation. Four weeks after stent implantation, the stented arteries were examined for histological analysis and metabolomics. Additionally, everolimus effects in coronary artery SMCs metabolism, tissue factor (TF) expression, and procoagulant activity were assessed in vitro. ResultsEES-implanted arteries showed decreased neointima formation, less SMCs infiltration, and reduced TF expression. Concomitantly, they were metabolically characterized by increased levels of metabolites in amino acids, such as glutamine. Similarly, everolimus increased intracellular glutamine levels, decreased TF expression, and reduced procoagulant activity in SMCs in vitro. On the contrary, exogenous glutamine administration also increased intracellular glutamine level, decreased TF expression, and reduced procoagulant activity despite enhanced mammalian target of rapamycin (mTOR) activity. ConclusionsIntracellular glutamine level is likely to determine vascular SMC-related thrombogenicity regardless of mTOR pathway activity. Therefore, increased intracellular glutamine level might contribute partially to the beneficial effect of EES use on stent thrombosis.

The effects of glutaminolysis inhibition on ferroptotic cell death signaling in cardiomyocytes

Mitochondria play a central role in the regulation of cellular redox status. Reduced glutathione (GSH), a highly potent essential antioxidant which directly scavenges various oxidants such as superoxide anion, hydroxyl radical, nitric oxide, and carbon radicals in the cell. Moreover, GSH, through the glutathione peroxidase (GPX) family, is responsible for detoxifying H2O2, peroxynitrites, and lipid peroxides. Conversely, GSH depletion has been shown to trigger programed cell death pathways such as apoptosis and ferroptosis, however the molecular mechanisms underlying these death pathways remain unknown. The amino acids cysteine and glutamine are detrimental for sustaining cellular GSH levels, although during glucose deprivation intracellular glutamine can be transported to the mitochondria and catabolized through the TCA cycle to produce ATP through the process known as glutaminolysis. Glutamine is converted to glutamate through glutaminolysis catalyzed by glutaminase C (GAC), a mitochondrial localized K-type glutaminase, thereby, leading to the formation of 2-oxoglutarate, a TCA cycle intermediate. Excessive metabolism of glutamate by the TCA cycle can lead to GSH depletion and thus, stimulate ferroptosis. In this study, we evaluated if inhibition of glutaminolysis through genetic silencing of mitochondrial GAC can affect ferroptotic cell death and mitochondrial GSH levels/redox status in H9c2 cardiomyocytes. GAC expression was silenced in the cells with a GAC siRNA (GAC-KD cells) in the absence or presence of ferroptosis inducing agent, RSL3 (0.5 µM). Ferrostatin-1 (Fer-1, 2µM), a well-known anti-ferroptotic agent, was used to demonstrate the specificity of ferroptosis. In control group, the cells were treated with a non-coding siRNA (NC) under the same conditions. Cell death (LDH release to culture medium), mtROS production, GSH levels, and mitochondrial membrane potential were analyzed. We found that the cells treated with GAC siRNA showed a 51% reduction in GAC levels compared to NC cells. Analysis of LDH release in NC and GAC-KD cells incubated in the presence of RSL3 and/or Fer-1 for 1 h, 3 h, and 6 h demonstrated that LDH levels were significantly reduced in control, RSL3, and both treatment with GAC-KD compared to NC cells after 1 h incubation however, no differences were found in 3h and 6h of incubation. The GSH level was found a 30% higher in GAC-KD cells compared to NC cells. RSL3 induced a further increase (50%) of the GSH level in NC cells. In conclusion, our data demonstrate inhibition of glutaminolysis provides an early protection against RSL3-induced ferroptosis in H9c2 cardiomyocytes due to a surge in GSH levels suggesting a crucial role of glutaminolysis in early stages of ferroptosis.

NMR-Based Metabolomic Analysis for the Effects of α-Ketoglutarate Supplementation on C2C12 Myoblasts in Different Energy States.

α-Ketoglutarate (AKG) is attracting much attention from researchers owing to its beneficial effects on anti-aging and cancer suppression, and, more recently, in nutritional supplements. Given that glucose is the main source of energy to maintain normal physiological functions of skeletal muscle, the effects of AKG supplementation for improving muscle performance are closely related to the glucose level in skeletal muscle. The differences of AKG-induced effects in skeletal muscle between two states of normal energy and energy deficiency are unclear. Furthermore, AKG-induced metabolic changes in skeletal muscles in different energy states also remain elusive. Here, we assessed the effects of AKG supplementation on mouse C2C12 myoblast cells cultured both in normal medium (Nor cells) and in low-glucose medium (Low cells), which were used to mimic two states of normal energy and energy deficiency, respectively. We further performed NMR-based metabolomic analysis to address AKG-induced metabolic changes in Nor and Low cells. AKG supplementation significantly promoted the proliferation and differentiation of cells in the two energy states through glutamine metabolism, oxidative stress, and energy metabolism. Under normal culture conditions, AKG up-regulated the intracellular glutamine level, changed the cellular energy status, and maintained the antioxidant capacity of cells. Under low-glucose culture condition, AKG served as a metabolic substrate to reduce the glutamine-dependence of cells, remarkably enhanced the antioxidant capacity of cells and significantly elevated the intracellular ATP level, thereby ensuring the normal growth and metabolism of cells in the state of energy deficiency. Our results provide a mechanistic understanding of the effects of AKG supplements on myoblasts in both normal energy and energy deficiency states. This work may be beneficial to the exploitation of AKG applications in clinical treatments and nutritional supplementations.

Identification and Characterization of IMD-0354 as a Glutamine Carrier Protein Inhibitor in Melanoma.

A key hallmark of cancer, altered metabolism, is central to cancer pathogenesis and therapy resistance. Robust glutamine metabolism is among cellular processes regulating tumor progression and responsiveness to therapy in a number of cancers, including melanoma and breast cancer. Among mechanisms underlying the increase in glutamine metabolism in tumors is enhanced glutamine uptake mediated by the glutamine transporters, with SLC1A5 (also known as ASCT2) shown to play a predominant role. Correspondingly, increased SLC1A5 expression coincides with poorer survival in patients with breast cancer and melanoma. Therefore, we performed an image-based screen to identify small molecules that are able to prevent the localization of SLC1A5 to the plasma membrane without impacting cell shape. From 7,000 small molecules, nine were selected as hits, of which one (IMD-0354) qualified for further detailed functional assessment. IMD-0354 was confirmed as a potent inhibitor of glutamine uptake that attained sustained low intracellular glutamine levels. Concomitant with its inhibition of glutamine uptake, IMD-0354 attenuated mTOR signaling, suppressed two- and three-dimensional growth of melanoma cells, and induced cell-cycle arrest, autophagy, and apoptosis. Pronounced effect of IMD-0354 was observed in different tumor-derived cell lines, compared with nontransformed cells. RNA-sequencing analysis identified the unfolded protein response, cell cycle, and response (DNA damage response pathways) to be affected by IMD-0354. Combination of IMD-0354 with GLS1 or LDHA inhibitors enhanced melanoma cell death. In vivo, IMD-0354 suppressed melanoma growth in a xenograft model. As a modulator of glutamine metabolism, IMD-0354 may serve as an important therapeutic and experimental tool that deserves further examination.

Glutamate is an essential mediator in glutamine-amplified insulin secretion.

Aims/IntroductionGlutamine is the most abundant amino acid in the circulation. In this study, we investigated cell signaling in the amplification of insulin secretion by glutamine.Materials and MethodsClonal pancreatic β‐cells MIN6‐K8, wild‐type B6 mouse islets, glutamate dehydrogenase (GDH) knockout clonal β‐cells (Glud1KOβCL), and glutamate‐oxaloacetate transaminase 1 (GOT1) knockout clonal β‐cells (Got1KOβCL) were studied. Insulin secretion from these cells and islets was examined under various conditions, and intracellular glutamine metabolism was assessed by metabolic flux analysis. Intracellular Ca2+ concentration ([Ca2+]i) was also measured.ResultsGlutamine dose‐dependently amplified insulin secretion in the presence of high glucose in both MIN6‐K8 cells and Glud1KOβCL. Inhibition of glutaminases, the enzymes that convert glutamine to glutamate, dramatically reduced the glutamine‐amplifying effect on insulin secretion. A substantial amount of glutamate was produced from glutamine through direct conversion by glutaminases. Glutamine also increased [Ca2+]i at high glucose, which was abolished by inhibition of glutaminases. Glutamic acid dimethylester (dm‐Glu), a membrane permeable glutamate precursor that is converted to glutamate in cells, increased [Ca2+]i as well as induced insulin secretion at high glucose. These effects of glutamine and dm‐Glu were dependent on calcium influx. Glutamine also induced insulin secretion in clonal β‐cells MIN6‐m14, which otherwise exhibit no insulin secretory response to glucose.ConclusionsGlutamate converted from glutamine is an essential mediator that enhances calcium signaling in the glutamine‐amplifying effect on insulin secretion. Our data also suggest that glutamine exerts a permissive effect on glucose‐induced insulin secretion.

Effect of glutamine metabolism on chemoresistance and its mechanism in tumors.

The metabolic reprogramming of tumor cells is characterized by increased uptake of various nutrients including glutamine. Glutamine metabolism provides the required substances for glycolysis and oxidative phosphorylation and affects the homeostasis of carbohydrate，fat and protein metabolism to induce the chemoresistance of tumor cells. Combination of chemotherapeutic agents with inhibitors specific to different components of glutamine metabolic pathway has obtained favorable clinical results on various tumors. Glutamine metabolic pathway plays a role in drug resistance of tumor cells in various ways. Firstly，the dynamic change of glutamine transporters can directly affect intracellular glutamine content thereby causing drug resistance; secondly，tumor stromal cells including adipocyte，fibroblast and metabolite from tumor microenvironment would give rise to immune-mediated drug resistance; thirdly，the expression and activity of key enzymes in glutamine metabolism also has a critical role in drug resistance of tumors. This article reviews the effects of glutamine metabolic pathway in the development of tumor chemoresistance，in terms of transporters，tumor microenvironment and metabolic enzymes，to provide insight for improving the therapeutic efficacy for drug-resistant tumors.

Monoethanolamine-induced glucose deprivation promotes apoptosis through metabolic rewiring in prostate cancer.

Rationale: Cancer cells rely on glucose metabolism for fulfilling their high energy demands. We previously reported that monoethanolamine (Etn), an orally deliverable lipid formulation, reduced intracellular glucose and glutamine levels in prostate cancer (PCa). Glucose deprivation upon Etn treatment exacerbated metabolic stress in PCa, thereby enhancing cell death. Moreover, Etn was potent in inhibiting tumor growth in a PCa xenograft model. However, the precise mechanisms underlying Etn-induced metabolic stress in PCa remain elusive. The purpose of the present study was to elucidate the mechanisms contributing to Etn-mediated metabolic rewiring in PCa.Methods: Glucose transporters (GLUTs) facilitate glucose transport across the plasma membrane. Thus, we assessed the expression of GLUTs and the internalization of GLUT1 in PCa. We also evaluated the effects of Etn on membrane dynamics, mitochondrial structure and function, lipid droplet density, autophagy, and apoptosis in PCa cells.Results: Compared to other GLUTs, GLUT1 was highly upregulated in PCa. We observed enhanced GLUT1 internalization, altered membrane dynamics, and perturbed mitochondrial structure and function upon Etn treatment. Etn-induced bioenergetic stress enhanced lipolysis, decreased lipid droplet density, promoted accumulation of autophagosomes, and increased apoptosis.Conclusion: We provide the first evidence that Etn alters GLUT1 trafficking leading to metabolic stress in PCa. By upregulating phosphatidylethanolamine (PE), Etn modulates membrane fluidity and affects mitochondrial structure and function. Etn also induces autophagy in PCa cells, thereby promoting apoptosis. These data strongly suggest that Etn rewires cellular bioenergetics and could serve as a promising anticancer agent for PCa.

Targeting Glutamine Synthesis Inhibits Stem Cell Adipogenesis in Vitro.

Glutamine is the most abundant amino acid in the body and has a metabolic role as a precursor for protein, amino sugar and nucleotide synthesis. After glucose, glutamine is the main source of energy in cells and has recently been shown to be an important carbon source for de novo lipogenesis. Glutamine is synthesized by the enzyme glutamine synthetase, a mitochondrial enzyme that is active during adipocyte differentiation suggesting a regulatory role in this process. The aim of our study was therefore to investigate whether glutamine status impacts on the differentiation of adipocytes and lipid droplet accumulation. Mouse mesenchymal stem cells (MSCs) were submitted to glutamine deprivation (i.e. glutamine-free adipogenic medium in conjunction with irreversible glutamine synthetase inhibitor, methionine sulfoximine - MSO) during differentiation and their response was compared with MSCs differentiated in glutamine-supplemented medium (5, 10 and 20 mM). Differentiated MSCs were assessed for lipid content using Oil Red O (ORO) staining and gene expression was analysed by qPCR. Intracellular glutamine levels were determined using a colorimetric assay, while extracellular glutamine was measured using liquid chromatography-mass spectrometry (LC-MS). Glutamine deprivation largely abolished adipogenic differentiation and lipid droplet formation. This was accompanied with a reduction in intracellular glutamine concentration, and downregulation of gene expression for classical adipogenic markers including PPARγ. Furthermore, glutamine restriction suppressed isocitrate dehydrogenase 1 (IDH1) gene expression, an enzyme which produces citrate for lipid synthesis. In contrast, glutamine supplementation promoted adipogenic differentiation in a dose-dependent manner. These results suggest that the glutamine pathway may have a previously over-looked role in adipogenesis. The underlying mechanism involved the glutamine-IDH1 pathway and could represent a potential therapeutic strategy to treat excessive lipid accumulation and thus obesity.

T cell-specific deletion of Pgam1 reveals a critical role for glycolysis in T cell responses

Although the important roles of glycolysis in T cells have been demonstrated, the regulatory mechanism of glycolysis in activated T cells has not been fully elucidated. Furthermore, the influences of glycolytic failure on the T cell-dependent immune response in vivo remain unclear. We therefore assessed the role of glycolysis in the T cell-dependent immune response using T cell-specific Pgam1-deficient mice. Both CD8 and CD4 T cell-dependent immune responses were attenuated by Pgam1 deficiency. The helper T cell-dependent inflammation was ameliorated in Pgam1-deficient mice. Glycolysis augments the activation of mTOR complex 1 (mTORC1) and the T-cell receptor (TCR) signals. Glutamine acts as a metabolic hub in activated T cells, since the TCR-dependent increase in intracellular glutamine is required to augment glycolysis, increase mTORC1 activity and augment TCR signals. These findings suggest that mTORC1, glycolysis and glutamine affect each other and cooperate to induce T cell proliferation and differentiation.

The E3 ligase HUWE1 inhibition as a therapeutic strategy to target MYC in multiple myeloma

Proteasome inhibitors have provided a significant advance in the treatment of multiple myeloma (MM). Consequently, there is increasing interest in developing strategies to target E3 ligases, de-ubiquitinases, and/or ubiquitin receptors within the ubiquitin proteasome pathway, with an aim to achieve more specificity and reduced side-effects. Previous studies have shown a role for the E3 ligase HUWE1 in modulating c-MYC, an oncogene frequently dysregulated in MM. Here we investigated HUWE1 in MM. We identified elevated expression of HUWE1 in MM compared with normal cells. Small molecule-mediated inhibition of HUWE1 resulted in growth arrest of MM cell lines without significantly effecting the growth of normal bone marrow cells, suggesting a favorable therapeutic index. Studies using a HUWE1 knockdown model showed similar growth inhibition. HUWE1 expression positively correlated with MYC expression in MM bone marrow cells and correspondingly, genetic knockdown and biochemical inhibition of HUWE1 reduced MYC expression in MM cell lines. Proteomic identification of HUWE1 substrates revealed a strong association of HUWE1 with metabolic processes in MM cells. Intracellular glutamine levels are decreased in the absence of HUWE1 and may contribute to MYC degradation. Finally, HUWE1 depletion in combination with lenalidomide resulted in synergistic anti-MM activity in both in vitro and in vivo models. Taken together, our data demonstrate an important role of HUWE1 in MM cell growth and provides preclinical rationale for therapeutic strategies targeting HUWE1 in MM.

Flavokawain A inhibits prostate cancer cells by inducing cell cycle arrest and cell apoptosis and regulating the glutamine metabolism pathway

Flavokawain A (FKA), a major chalcone in kava extracts, has exhibited anti-proliferative and apoptotic effects in the prostate cancer. However, the molecular mechanism of FKA remains unclear. In this study, FKA induces cell apoptosis and cell cycle arrest in a G2M phase to prostate cancer cells. FKA interferes with tubulin polymerization and inhibits survivin expression in PC3 cells. Molecular docking simulation experiment finds that FKA can bind to colchicine binding sites that inhibit tubulin polymerization. FKA treatment regulates the glutamine metabolism pathway in PC3 cells by reducing intracellular glutamine, glutamic and proline. FKA treatment also decreases the GSH content by decreasing the activity of GSH synthetase (GSS) and increasing the activity of glutathione thiol transferase (GSTP1), which subsequently induces ROS production and PC3 cell apoptosis.

Improving cytosolic aspartate biosynthesis increases glucoamylase production in Aspergillus niger under oxygen limitation

BackgroundGlucoamylase is one of the most industrially applied enzymes, produced by Aspergillus species, like Aspergillus niger. Compared to the traditional ways of process optimization, the metabolic engineering strategies to improve glucoamylase production are relatively scarce.ResultsIn the previous study combined multi-omics integrative analysis and amino acid supplementation experiment, we predicted four amino acids (alanine, glutamate, glycine and aspartate) as the limited precursors for glucoamylase production in A. niger. To further verify this, five mutants namely OE-ala, OE-glu, OE-gly, OE-asp1 and OE-asp2, derived from the parental strain A. niger CBS 513.88, were constructed respectively for the overexpression of five genes responsible for the biosynthesis of the four kinds of amino acids (An11g02620, An04g00990, An05g00410, An04g06380 and An16g05570). Real-time quantitative PCR revealed that all these genes were successfully overexpressed at the mRNA level while the five mutants exhibited different performance in glucoamylase production in shake flask cultivation. Notably, the results demonstrated that mutant OE-asp2 which was constructed for reinforcing cytosolic aspartate synthetic pathway, exhibited significantly increased glucoamylase activity by 23.5% and 60.3% compared to CBS 513.88 in the cultivation of shake flask and the 5 L fermentor, respectively. Compared to A. niger CBS 513.88, mutant OE-asp2 has a higher intracellular amino acid pool, in particular, alanine, leucine, glycine and glutamine, while the pool of glutamate was decreased.ConclusionOur study combines the target prediction from multi-omics analysis with the experimental validation and proves the possibility of increasing glucoamylase production by enhancing limited amino acid biosynthesis. In short, this systematically conducted study will surely deepen the understanding of resources allocation in cell factory and provide new strategies for the rational design of enzyme production strains.

Identification of the main glutamine and glutamate transporters in Staphylococcus aureus and their impact on c-di-AMP production.

A Staphylococcus aureus strain deleted for the c‐di‐AMP cyclase gene dacA is unable to survive in rich medium unless it acquires compensatory mutations. Previously identified mutations were in opuD, encoding the main glycine‐betaine transporter, and alsT, encoding a predicted amino acid transporter. Here, we show that inactivation of OpuD restores the cell size of a dacA mutant to near wild‐type (WT) size, while inactivation of AlsT does not. AlsT was identified as an efficient glutamine transporter, indicating that preventing glutamine uptake in rich medium rescues the growth of the S. aureus dacA mutant. In addition, GltS was identified as a glutamate transporter. By performing growth curves with WT, alsT and gltS mutant strains in defined medium supplemented with ammonium, glutamine or glutamate, we revealed that ammonium and glutamine, but not glutamate promote the growth of S. aureus. This suggests that besides ammonium also glutamine can serve as a nitrogen source under these conditions. Ammonium and uptake of glutamine via AlsT and hence likely a higher intracellular glutamine concentration inhibited c‐di‐AMP production, while glutamate uptake had no effect. These findings provide, besides the previously reported link between potassium and osmolyte uptake, a connection between nitrogen metabolism and c‐di‐AMP signalling in S. aureus.

Synergetic engineering of central carbon and nitrogen metabolism for the production of N-acetylglucosamine in Bacillus subtilis.

N-acetylglucosamine (GlcNAc) is a nitrogen-containing compound, which is widely used as a nutraceutical and pharmaceutical. In our previous work, we constructed a recombinant Bacillus subtilis strain for the biosynthesis of GlcNAc by engineering the central carbon metabolism. However, nitrogen is also required for the synthesis of GlcNAc. Hence, it is necessary to simultaneously coordinate the carbon and nitrogen metabolism to improve production of GlcNAc. In this work, we attempted to enhance GlcNAc production in B. subtilis by increasing supply of precursors N-acetylglucosamine 6-phosphate (GlcNAc6P) and glutamate. The expression of a key enzyme, GlcNAc6P N-acetyltransferase (GNA1), was enhanced by engineering the promoter and ribosome binding site to enhance the production of GlcNAc6P. Next, we examined the effect of different nitrogen sources on GlcNAc synthesis. We observed that urea can promote nitrogen assimilation for GlcNAc synthesis. The glutamate synthesis was improved by deleting the two endogenous glutamate dehydrogenase genes (rocG and gudB) and by integrating one exogenous glutamate dehydrogenase gene (gdh). This strategy enhanced the intracellular glutamate and glutamine by 69.8% and 46.9%, respectively. The synergetic engineering of central carbon and nitrogen metabolisms increased the GlcNAc titer from 14.0 to 22.2g/L in the shaker flask. Hence, our study demonstrated the importance of carbon and nitrogen metabolism coordination in the production of nitrogen-containing compounds.

Ammonia Induces a Myostatin-Mediated Atrophy in Mammalian Myotubes, but Induces Hypertrophy in Avian Myotubes

Ammonia, a byproduct of protein catabolism that is generally regarded as toxic, is processed by the liver for excretion. In diseases resulting in hepatic insufficiency, circulating ammonia levels increase dramatically, ensuing secondary disorders. Sarcopenia, or loss of muscle mass, is commonly associated with hyperammonemia. In mammalian models of cirrhosis, increased myostatin is consistent, contributes to muscle autophagy, and reduces satellite cell activation and differentiation, whereas, avian species show a positive myogenic response to ammonia. The objective of the study was to elucidate the effect of ammonia in chicken, mouse, and rat derived myotubes. Primary myoblasts were isolated from the pectoralis major (breast) and biceps femoris (thigh) of embryonic day 17 chicken embryos, and from the hindlimbs of 3-day-old rat pups. C2C12 cells were used for mouse myoblasts. Myotubes were exposed to 10mM ammonium acetate (AA) or 10mM sodium acetate (SA) for 24 hours to determine myogenic response to ammonia. Relative expression of myostatin mRNA, determined by quantitative real-time PCR, was significantly higher in mammalian myotubes compared to chicken myotubes (P < 0.001). Western blot analysis of myostatin protein confirmed a significant increase in ammonia treated rat myotubes, while chicken breast myotubes showed a significant decrease in myostatin (P < 0.05). Myotube diameter significantly increased in chicken breast and thigh cultures treated with ammonia, while diameter was significantly reduced in mouse and rat myotubes (P < 0.05). Intracellular glutamine is significantly higher in chicken thigh, but not breast, myotubes treated with AA compared to SA treated myotubes (P < 0.05). To investigate fiber type differences in ammonia metabolism, Western blot analysis of protein from AA and SA treated myotubes was examined for fast and slow myosin heavy chain isoforms. AA treatment resulted in a higher ratio of fast to slow isoforms of myosin heavy chain in both types of chicken myotubes, while fast isoforms were decreased in AA treated mouse and rat myotubes. These data demonstrate that chicken myotubes respond positively to ammonia while rodent myotubes respond negatively. Further, there is evidence that ammonia induces a fast fiber type shift in avian muscle, but a slow phenotype shift in mammalian muscle.

ASCT2 (SLC1A5)-dependent glutamine uptake is involved in the progression of head and neck squamous cell carcinoma

BackgroundGlutamine is an abundant and versatile nutrient in cancer cells. Head and neck squamous cell carcinoma (HNSCC) was reported to be dependent on mainly glucose, not glutamine, for producing the energy required for survival and proliferation.MethodsThe roles of ASCT2 (SLC1A5) and associated glutamine metabolism were determined by the MTT, colony formation, glutamine uptake, intracellular glutathione, ROS detection, immunofluorescence, immunohistochemistry, and apoptosis enzyme-linked immunosorbent assays as well as animal studies.ResultsWe found that glutamine is also critical for HNSCC. In this study, ASCT2, an amino acid transporter responsible for glutamine transport, in addition to LAT1 and GLS, is overexpressed in HNSCC and associated with poor survival. Using both in vivo and in vitro models, we found that knocking down ASCT2 by shRNAs or miR-137 or the combination of silencing ASCT2 and pharmacologically inhibiting SNAT2 via a small-molecule antagonist called V-9302 significantly suppressed intracellular glutamine levels and downstream glutamine metabolism, including glutathione production; these effects attenuated growth and proliferation, increased apoptosis and autophagy, and increased oxidative stress and mTORC1 pathway suppression in HNSCC. Additionally, silencing ASCT2 improved the response to cetuximab in HNSCC.ConclusionsIn summary, ASCT2-dependent glutamine uptake and subsequent glutamine metabolism are essential for HNSCC tumorigenesis, and the combination of glutamine uptake inhibitors and cetuximab presents a promising strategy for improving the outcomes of HNSCC patients.

Acidosis-induced metabolic reprogramming in tumor cells enhances the anti-proliferative activity of the PDK inhibitor dichloroacetate.

Altered metabolic pathways in cancer such as exacerbated glycolytic flux and increased glutamine metabolism are promising targets for anti-cancer therapies. While commonly observed in glycolytic tumors, extracellular acidosis has never been considered as a potential modulator of anti-metabolic drug activity such as dichloroacetate (DCA). Using cancer cells from various origins selected for their ability to proliferate under acidic conditions, we found that DCA exerts greater inhibitory effects on the growth of these acid-adapted cells than in parental cells. Moreover, daily DCA administration to mice led to a significant decrease in tumor growth from acid-adapted cells but not from parental cells. 13C-tracer studies revealed that DCA induced a double metabolic shift, diminishing glycolysis and increasing intracellular glutamine in acid-adapted cells. As a consequence, DCA reduced the pentose phosphate pathway activity more extensively and increased apoptosis in acid-adapted cells. Finally, the combination of DCA with a glutaminase inhibitor significantly enhanced the cytotoxic effects of DCA. Overall, the interplay between acidosis and DCA exposure leads to metabolic reprogramming that considerably alters cellular fitness.

Anti-tumor effects of an antagonistic mAb against the ASCT2 amino acid transporter on KRAS-mutated human colorectal cancer cells.

KRAS mutations are detected in numerous human cancers, but there are few effective drugs for KRAS‐mutated cancers. Transporters for amino acids and glucose are highly expressed on cancer cells, possibly to maintain rapid cell growth and metabolism. Alanine‐serine‐cysteine transporter 2 (ASCT2) is a primary transporter for glutamine in cancer cells. In this study, we developed a novel monoclonal antibody (mAb) recognizing the extracellular domain of human ASCT2, and investigated whether ASCT2 can be a therapeutic target for KRAS‐mutated cancers. Rats were immunized with RH7777 rat hepatoma cells expressing human ASCT2 fused to green fluorescent protein (GFP). Splenocytes from the immunized rats were fused with P3X63Ag8.653 mouse myeloma cells, and selected and cloned hybridoma cells secreting Ab3‐8 mAb were established. This mAb reacted with RH7777 transfectants expressing ASCT2‐GFP proteins in a GFP intensity‐dependent manner. Ab3‐8 reacted with various human cancer cells, but not with non‐cancer breast epithelial cells or ASCT2‐knocked out HEK293 and SW1116 cells. In SW1116 and HCT116 human colon cancer cells with KRAS mutations, treatment with Ab3‐8 reduced intracellular glutamine transport, phosphorylation of AKT and ERK, and inhibited in vivo tumor growth of these cells in athymic mice. Inhibition of in vivo tumor growth by Ab3‐8 was not observed in HT29 colon and HeLa uterus cancer cells with wild‐type KRAS. These results suggest that ASCT2 is an excellent therapeutic target for KRAS‐mutated cancers.