Glutamine-stimulated modification and degradation of glutamine synthetase in hepatoma tissue culture cells

Lysine-372-dependent SUMOylation inhibits the enzymatic activity of glutamine synthases

Glutamine synthetase (GS) is a crucial enzyme involved in de novo synthesis of glutamine and participates in several biological processes, including nitrogen metabolism, nucleotide synthesis, and amino acid synthesis. Post-translational modification makes GS more adaptable to the needs of cells, and acetylation modification of GS at double sites has attracted considerable attention. Despite very intensive research, how SUMOylation affects GS activity at a molecular level remains unclear. Here, we report that previously undiscovered GS SUMOylation which is deficient mutant K372R of GS exhibits more bluntness under glutamine starvation. Mechanistically, glutamine deprivation triggers the GS SUMOylation, and this SUMOylation impaired the protein stability of GS, within a concomitant decrease in enzymatic activity. In addition, we identified SAE1, Ubc9, and PIAS1 as the assembly enzymes of GS SUMOylation respectively. Furthermore, Senp1/2 functions as a SUMO-specific protease to reverse the SUMOylation of GS. This study provides the first evidence that SUMOylation serves as a regulatory mechanism for determining the GS enzymatic activity, contributing to understanding the GS regulation roles in various cellular and pathophysiological processes.

Glutamine synthetase expression rescues human dendritic cell survival in a glutamine-deprived environment

Introduction

Glutamine deficiency is a well-known feature of the tumor environment. Here we analyzed the impact of glutamine deprivation on human myeloid cell survival and function.

Methods

Different types of myeloid cells were cultured in the absence or presence of glutamine and/or with L-methionine-S-sulfoximine (MSO), an irreversible glutamine synthetase (GS) inhibitor. GS expression was analyzed on mRNA and protein level. GS activity and the conversion of glutamate to glutamine by myeloid cells was followed by 13C tracing analyses.

Results

The absence of extracellular glutamine only slightly affected postmitotic human monocyte to dendritic cell (DC) differentiation, function and survival. Similar results were obtained for monocyte-derived macrophages. In contrast, proliferation of the monocytic leukemia cell line THP-1 was significantly suppressed. While macrophages exhibited high constitutive GS expression, glutamine deprivation induced GS in DC and THP-1. Accordingly, proliferation of THP-1 was rescued by addition of the GS substrate glutamate and 13C tracing analyses revealed conversion of glutamate to glutamine. Supplementation with the GS inhibitor MSO reduced the survival of DC and macrophages and counteracted the proliferation rescue of THP-1 by glutamate.

Discussion

Our results show that GS supports myeloid cell survival in a glutamine poor environment. Notably, in addition to suppressing proliferation and survival of tumor cells, the blockade of GS also targets immune cells such as DCs and macrophages.

USP15 antagonizes CRL4CRBN-mediated ubiquitylation of glutamine synthetase and neosubstrates

Targeted protein degradation by the ubiquitin-proteasome system represents a new strategy to destroy pathogenic proteins in human diseases, including cancer and neurodegenerative diseases. The immunomodulatory drugs (IMiDs) thalidomide, lenalidomide, and pomalidomide have revolutionized the treatment of patients with multiple myeloma (MM) and other hematologic malignancies, but almost all patients eventually develop resistance to IMiDs. CRBN, a substrate receptor of CUL4-RBX1-DDB1-CRBN (CRL4^CRBN) E3 ubiquitin ligase, is a direct target for thalidomide teratogenicity and antitumor activity of IMiDs (now known as Cereblon E3 ligase modulators: CELMoDs). Despite recent advances in developing potent CELMoDs and CRBN-based proteolysis-targeting chimeras (PROTACs), many questions apart from clinical efficacy remain unanswered. CRBN is required for the action of IMiDs, but its protein expression levels do not correlate with intrinsic resistance to IMiDs in MM cells, suggesting other factors involved in regulating resistance to IMiDs. Our recent work revealed that the CRL4^CRBN-p97 pathway is required for degradation of natural substrate glutamine synthetase (GS) and neosubstrates. Here, I show that USP15 is a key regulator of the CRL4^CRBN-p97 pathway to control stability of GS and neosubstrates IKZF1, IKZF3, CK1-α, RNF166, GSPT1, and BRD4, all of which are crucial drug targets in different types of cancer. USP15 antagonizes ubiquitylation of CRL4^CRBN target proteins, thereby preventing their degradation. Notably, USP15 is highly expressed in IMiD-resistant cells, and depletion of USP15 sensitizes these cells to lenalidomide. Inhibition of USP15 represents a valuable therapeutic opportunity to potentiate CELMoD and CRBN-based PROTAC therapies for the treatment of cancer.

Hepatoblastoma: glutamine depletion hinders cell viability in the embryonal subtype but high GLUL expression is associated with better overall survival

Purpose

Glutamine plays an important role in cell viability and growth of various tumors. For the fetal subtype of hepatoblastoma, growth inhibition through glutamine depletion was shown. We studied glutamine depletion in embryonal cell lines of hepatoblastoma carrying different mutations. Since asparagine synthetase was identified as a prognostic factor and potential therapeutic target in adult hepatocellular carcinoma, we investigated the expression of its gene ASNS and of the gene GLUL, encoding for glutamine synthetase, in hepatoblastoma specimens and cell lines and investigated the correlation with overall survival.

Methods

We correlated GLUL and ASNS expression with overall survival using publicly available microarray and clinical data. We examined GLUL and ASNS expression by RT-qPCR and by Western blot analysis in the embryonal cell lines Huh-6 and HepT1, and in five hepatoblastoma specimens. In the same cell lines, we investigated the effects of glutamine depletion. Hepatoblastoma biopsies were examined for histology and CTNNB1 mutations.

Results

High GLUL expression was associated with a higher median survival time. Independent of mutations and histology, hepatoblastoma samples showed strong GLUL expression and glutamine synthesis. Glutamine depletion resulted in the inhibition of proliferation and of cell viability in both embryonal hepatoblastoma cell lines. ASNS expression did not correlate with overall survival.

Conclusion

Growth inhibition resulting from glutamine depletion, as described for the hepatoblastoma fetal subtype, is also detected in established embryonal hepatoblastoma cell lines carrying different mutations. At variance with adult hepatocellular carcinoma, in hepatoblastoma asparagine synthetase has no prognostic significance.

Macrophage Stimulated by Low Ambient Temperature Hasten Tumor Growth via Glutamine Production

Ambient temperature can regulate the immune response and affect tumor growth. Although thermoneutral caging reduces tumor growth via immune activation, little attention has been paid to the tumorigenic effect of low temperature. In the present study, tumor growth was higher at low ambient temperature (4 °C for 8 h/d) than at the standard housing temperature (22 °C) in allograft models. Low temperature-stimulated tumor growth in mice was reduced by monocyte depletion using clodronate liposomes. Proliferation was considerably greater in cancer cells treated with 33 °C-cultured RAW264.7 cell-conditioned media (33CM) than in cells treated with 37 °C-cultured RAW264.7 cell-conditioned media (37CM). Additionally, glutamine levels were markedly higher in 33CM-treated cells than in 37CM-treated cells. We further confirmed that the addition of glutamine into 37CM enhanced its effects on cancer cell proliferation and glutamine uptake inhibition ameliorated the accelerated proliferation induced by 33CM. Consistently, the inhibition of glutamine uptake in the allograft model exposed to low temperature, effectively reduced tumor volume and weight. Collectively, these data suggest that the secretion and utilization of glutamine by macrophages and cancer cells, respectively, are key regulators of low temperature-enhanced cancer progression in the tumor microenvironment.

Multiplexed Proteome Dynamics Profiling Reveals Mechanisms Controlling Protein Homeostasis

Protein degradation plays important roles in biological processes and is tightly regulated. Further, targeted proteolysis is an emerging research tool and therapeutic strategy. However, proteome-wide technologies to investigate the causes and consequences of protein degradation in biological systems are lacking. We developed “multiplexed proteome dynamics profiling” (mPDP), a mass-spectrometry-based approach combining dynamic-SILAC labeling with isobaric mass tagging for multiplexed analysis of protein degradation and synthesis. In three proof-of-concept studies, we uncover different responses induced by the bromodomain inhibitor JQ1 versus a JQ1 proteolysis targeting chimera; we elucidate distinct modes of action of estrogen receptor modulators; and we comprehensively classify HSP90 clients based on their requirement for HSP90 constitutively or during synthesis, demonstrating that constitutive HSP90 clients have lower thermal stability than non-clients, have higher affinity for the chaperone, vary between cell types, and change upon external stimuli. These findings highlight the potential of mPDP to identify dynamically controlled degradation mechanisms in cellular systems.

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Multiplexed proteome dynamics profiling, mPDP, measures changes in proteostasis

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JQ1-PROTAC degrades a key mRNA export factor and blocks protein synthesis

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Raloxifene induces TMEM97 degradation dysregulating cholesterol homeostasis

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Characterization of proteins dependent on HSP90 constitutively or during synthesis

p97/VCP promotes degradation of CRBN substrate glutamine synthetase and neosubstrates

Glutamine synthetase (GS) plays an essential role in metabolism by catalyzing the synthesis of glutamine from glutamate and ammonia. Our recent study showed that CRBN, a direct protein target for the teratogenic and antitumor activities of immunomodulatory drugs such as thalidomide, lenalidomide, and pomalidomide, recognizes an acetyl degron of GS, resulting in ubiquitylation and degradation of GS in response to glutamine. Here, we report that valosin-containing protein (VCP)/p97 promotes the degradation of ubiquitylated GS, resulting in its accumulation in cells with compromised p97 function. Notably, p97 is also required for the degradation of all four known CRBN neo-substrates [Ikaros family zinc finger proteins 1 (IKZF1) and 3 (IKZF3), casein kinase 1α (CK1α), and the translation termination factor GSPT1] whose ubiquitylation is induced by immunomodulatory drugs. Together, these data point to an unexpectedly intimate relationship between the E3 ubiquitin ligase CRL4^CRBN and p97 pathways.

A Brighter Side to Thalidomide: Its Potential Use in Immunological Disorders

Thalidomide and its derivatives are immunomodulatory drugs (IMiDs) known for their sedative, teratogenic, anti-angiogenic, and anti-inflammatory properties. Commonly used in the treatment of cancers such as multiple myeloma and myelodysplastic syndrome (MDS), IMiDs have also been used in the treatment of an inflammatory skin pathology associated with Hansen's disease/leprosy. They have also shown promise in the treatment of autoimmune disorders including systemic lupus erythmatosus (SLE) and inflammatory bowel disease (IBD). Recent structural and experimental observations have revolutionized our understanding of these properties by revealing the fundamental molecular events underpinning IMiD activity. We review these findings, their relevance to IMiD therapy in immunological disorders, and discuss how further research might unlock the vast clinical potential of these compounds.

An Acetyldegron Triggers CRBN to Take Down the "Q"

In this issue of Molecular Cell, Nguyen et al. (2016) show that p300/CBP-mediated acetylation of glutamine synthetase (GS) triggers recognition by the CRL4(CRBN) E3 ubiquitin ligase, resulting in its ubiquitylation and degradation in response to high glutamine concentrations.

Glutamine Triggers Acetylation-Dependent Degradation of Glutamine Synthetase via the Thalidomide Receptor Cereblon

Cereblon (CRBN), a substrate receptor for the cullin-RING ubiquitin ligase 4 (CRL4) complex, is a direct protein target for thalidomide teratogenicity and antitumor activity of immunomodulatory drugs (IMiDs). Here we report that glutamine synthetase (GS) is an endogenous substrate of CRL4(CRBN). Upon exposing cells to high glutamine concentration, GS is acetylated at lysines 11 and 14, yielding a degron that is necessary and sufficient for binding and ubiquitylation by CRL4(CRBN) and degradation by the proteasome. Binding of acetylated degron peptides to CRBN depends on an intact thalidomide-binding pocket but is not competitive with IMiDs. These findings reveal a feedback loop involving CRL4(CRBN) that adjusts GS protein levels in response to glutamine and uncover a new function for lysine acetylation.

Glutamine, insulin and glucocorticoids regulate glutamine synthetase expression in C2C12 myotubes, Hep G2 hepatoma cells and 3T3 L1 adipocytes

The cell-specific regulation of glutamine synthetase expression was studied in three cell lines. In C2C12 myotubes, glucocorticoids increased the abundance of both glutamine synthetase protein and mRNA. Culture in the absence of glutamine also resulted in very high glutamine synthetase protein abundance but mRNA levels were unchanged. Glucocorticoids also increased the abundance of glutamine synthetase mRNA in Hep G2 hepatoma cells but this was not reflected in changes in protein abundance. Culture of Hep G2 cells without glutamine resulted in very high levels of protein, again with no change in mRNA abundance. Insulin was without effect in both C2C12 and Hep G2 cells. In 3T3 L1 adipocytes glucocorticoids increased the abundance of both glutamine synthetase mRNA and protein, insulin added alone had no effect but in the presence of glucocorticoids resulted in lower mRNA levels than seen with glucocorticoids alone, although protein levels remained high under such conditions. In contrast to the other cell lines glutamine synthetase protein levels were relatively unchanged by culture in the absence of glutamine. The results support the hypothesis that in myocytes, and hepatomas, but not in adipocytes, glutamine acts to moderate glutamine synthetase induction by glucocorticoids.

Mechanisms governing the expression of the enzymes of glutamine metabolism--glutaminase and glutamine synthetase

Whether on the scale of a single cell, organ or organism, glutamine homeostasis is to a large extent determined by the activities of glutaminase (GA, EC 3.5.1.2) and glutamine synthetase (GS, EC 6.3.1.2), the two enzymes that are the focus of this report. GA and GS each provide examples of regulation of gene expression at many different levels. In the case of GA, two different genes (hepatic- and kidney-type GA) encode isoforms of this enzyme. The expression of hepatic GA mRNA is increased during starvation, diabetes and high protein diet through a mechanism involving increased gene transcription. In contrast, the expression of kidney GA mRNA is increased post-transcriptionally by a mechanism that increases mRNA stability during acidosis. We found recently that several isoforms of rat and human kidney-type GA are formed by tissue-specific alternative RNA splicing. Although the implications of this post-transcriptional processing mechanism for GA activity are not yet clear, it allows for the expression of different GA isoforms in different tissues and may limit the expression of GA activity in muscle tissues by diverting primary RNA transcripts to a spliceform that produces a nonfunctional translation product. The expression of GS enzyme is also regulated by both transcriptional and post-transcriptional mechanisms. For example, the GS gene is transcriptionally activated by glucocorticoid hormones in a tissue-specific fashion. This hormonal response allows GS mRNA levels to increase in selected organs during catabolic states. However, the ultimate level of GS enzyme expression is further governed by a post-transcriptional mechanism regulating GS protein stability. In a unique form of product feedback, GS protein turnover is increased by glutamine. This mechanism appears to provide a means to index the production of glutamine to its intracellular concentration and, therefore, to its systemic demand. Herein, we also provide experimental evidence that GS protein turnover is dependent upon the activity of the 26S proteosome.

Degradation of DNA topoisomerase I by a novel trypsin-like serine protease in proliferating human T lymphocytes

DNA topoisomerase I (Topo I) contributes to various important biological functions, and its activity is therefore likely regulated in response to different physiological conditions. Increases in both the synthesis and degradation of Topo I were previously shown to accompany phytohemagglutinin stimulation of proliferation in human peripheral T lymphocytes. The mechanism of this degradation of Topo I has now been investigated with both in vivo and in vitro assays. The activity of a nuclear protease that specifically degrades Topo I was induced in proliferating T lymphocytes. The full-length Topo I protein (100 kDa) was sequentially degraded to 97- and 82-kDa fragments both in vivo and in vitro. The initial site of proteolytic cleavage was mapped to the NH(2)-terminal region of the enzyme. The degradation of Topo I in vitro was inhibited by aprotinin or soybean trypsin inhibitor, suggesting that the enzyme responsible is a trypsin-like serine protease. Furthermore, Topo I degradation by this protease was Mg(2+)-dependent. The Topo I-specific protease activity induced during T lymphocytes proliferation was not detected in Jurkat (human T cell leukemia) cells and various other tested human cancer cell lines, possibly explaining why the abundance of Topo I is increased in tumor cells.

Glutamine synthetase expression in rat lung is regulated by protein stability

During physiological stress, the lung increases production of the amino acid glutamine (Gln) using the enzyme Gln synthetase (GS) to maintain Gln homeostasis. Glucocorticoid hormones are considered the principal mediators of GS expression during stress. However, whereas animal studies have shown that glucocorticoids increase lung GS mRNA levels 500-700%, GS activity levels rise only 20-45%. This discrepancy suggests that a posttranscriptional control mechanism(s) ultimately determines GS expression. We hypothesized that the level of GS protein in the lung is governed by the intracellular Gln concentration through a mechanism of protein destabilization, a feedback regulatory mechanism that has been observed in vitro. To test this hypothesis, Sprague-Dawley rats were treated with a Gln-free diet and the GS inhibitor methionine sulfoximine (MSO) to deplete tissue Gln levels and prevent this feedback regulation. Exposure to Gln-free chow and MSO (100 mg/kg body wt) for 6 days decreased plasma Gln levels 50% (P < 0.01) and decreased lung tissue Gln levels by 70% (P < 0.01). Although lung GS mRNA levels were not influenced by Gln depletion, there was a sevenfold (P < 0.01) increase in GS protein. A parenteral Gln infusion (200 mM, 1.5 ml/h) for the last 2 days of MSO treatment replenished lung Gln levels to 65% of control level and blunted the increase in GS protein levels by 33% (P < 0.05) compared with rats receiving an isomolar glycine solution. The acute effects of glucocorticoid and MSO administration on lung GS expression were also measured. Whereas dexamethasone (0.5 mg/kg) and MSO injections individually augmented lung GS protein levels twofold and fourfold (P < 0.05), respectively, the combination of dexamethasone and MSO produced a synergistic, 12-fold induction (P < 0.01) in lung GS protein over 8 h. The data suggest that, whereas glucocorticoids are potent mediators of GS transcriptional activity, protein stability greatly influences the ultimate expression of GS in the lung.

Regulation of the spatiotemporal pattern of expression of the glutamine synthetase gene

Glutamine synthetase, the enzyme that catalyzes the ATP-dependent conversion of glutamate and ammonia into glutamine, is expressed in a tissue-specific and developmentally controlled manner. The first part of this review focuses on its spatiotemporal pattern of expression, the factors that regulate its levels under (patho)physiological conditions, and its role in glutamine, glutamate, and ammonia metabolism in mammals. Glutamine synthetase protein stability is more than 10-fold reduced by its product glutamine and by covalent modifications. During late fetal development, translational efficiency increases more than 10-fold. Glutamine synthetase mRNA stability is negatively affected by cAMP, whereas glucocorticoids, growth hormone, insulin (all positive), and cAMP (negative) regulate its rate of transcription. The signal transduction pathways by which these factors may regulate the expression of glutamine synthetase are briefly discussed. The second part of the review focuses on the evolution, structure, and transcriptional regulation of the glutamine synthetase gene in rat and chicken. Two enhancers (at -6.5 and -2.5 kb) were identified in the upstream region and two enhancers (between +156 and +857 bp) in the first intron of the rat glutamine synthetase gene. In addition, sequence analysis suggests a regulatory role for regions in the 3' untranslated region of the gene. The immediate-upstream region of the chicken glutamine synthetase gene is responsible for its cell-specific expression, whereas the glucocorticoid-induced developmental appearance in the neural retina is governed by its far-upstream region.

Sepsis increases lung glutamine synthetase expression in the tumor-bearing host

Acute stresses such as trauma or endotoxemia augment GLN demand and are associated with increased release of this amino acid from skeletal muscle and lung as well as increased expression of glutamine synthetase (GS, the principal enzyme of GLN synthesis) in these tissues. Muscle GLN release is also increased during chronic catabolic states which are associated with depletion of lean body mass, such as starvation or malignancy. We hypothesized that the expression of GS in response to an acute stress would be altered in tumor-bearing rats (TBR) experiencing severe cachexia and therefore a previously heightened GLN demand. Male Fischer 344 rats were implanted with methylcholanthrene-induced fibrosarcoma tumors or underwent sham operations and pair-feeding (sham) with TBR partners. When tumor burden reached approximately 15% of carcass weight, animals received injections of either Escherichia coli lipopolysaccharide (LPS, 1 mg/kg body wt) or saline vehicle. Rats were sacrificed 8 h after injection and lung and muscle tissue were analyzed for GS mRNA and protein via Northern and Western blot techniques, respectively. LPS injection caused an equivalent 4- to 6-fold increase in lung and muscle GS mRNA in both TBR and sham rats (P < 0.01). LPS did not produce a significant increase in GS protein level in muscle tissue of either group or in lung tissue of sham rats. In contrast, endotoxin did lead to a 3.5-fold increase in GS protein levels in lung tissue of TBRs (P < 0.05). This increase in lung GS protein may signify the importance of the lung in maintaining GLN homeostasis during chronic catabolic states where muscle mass is diminished.

Neural control of glutamine synthetase activity in rat skeletal muscles

The mechanism of glutamine synthetase induction in rat skeletal muscle after denervation or limb immobilization was investigated. Adult male rats were subjected to midthigh section of the sciatic nerve. At 1, 2, and 5 h and 1, 2, and 7 days after denervation, rats were killed and denervated, and contralateral control soleus and plantaris muscles were excised, weighted, homogenized, and assayed for glutamine synthetase. Glutamine synthetase activity increased approximately twofold 1 h after denervation in both muscles. By 7 days postdenervation enzyme activity had increased to three times the control level in plantaris muscle and to four times the control level in soleus muscle. Increased enzyme activity after nerve section was associated with increased maximum velocity with no change in apparent Michaelis constant. Immunotitration with an antiglutamine synthetase antibody suggested that denervation caused an increase in the number of glutamine synthetase molecules in muscle. However, Northern-blot analysis revealed no increase in the steady-state level of glutamine synthetase mRNA after denervation. A mixing experiment failed to yield evidence for the presence of a soluble factor involved in regulating the activity of glutamine synthetase in denervated muscle. A combination of denervation and dexamethasone injections resulted in additive increases in glutamine synthetase. Thus the mechanism underlying increased glutamine synthetase after denervation appears to be posttranscriptional and is distinct from that of the glucocorticoid-mediated glutamine synthetase induction previously described by us.

Murine glutamine synthetase: cloning, developmental regulation, and glucocorticoid inducibility

We have cloned the murine glutamine synthetase (GS) gene and measured GS enzyme activity and mRNA in five tissues (retina, brain, liver, kidney, and skeletal muscle) during perinatal development. Retinal GS enzyme activity increases 200-fold between Day 1 and Day 21 and is accompanied by an increase in the level of GS mRNA; developmental regulation in other tissues is much less dramatic. Based on Southern blotting analysis, a single GS gene gives rise to the tissue-specific patterns of GS mRNA expression. The increase in murine retinal GS observed during perinatal development is similar in magnitude to that observed in the chicken retina just prior to hatching. In the embryonic chicken retina, glucocorticoid hormones mediate a large increase in the level of GS mRNA. However, although glucocorticoids induce a 12-fold increase in GS mRNA in murine skeletal muscle, expression of the retinal enzyme and mRNA is only modestly glucocorticoid-inducible in the mouse. Therefore, despite the hormonal responsiveness of the murine GS gene, it is not likely that glucocorticoids are important physiological modulators of the developmental rise in murine retinal GS.

Multiple mechanisms by which glutamine synthetase levels are controlled in murine tissue culture cells

We report the isolation of a complimentary DNA (cDNA) clone encoding glutamine synthetase, derived from a population of methionine sulfoxime-resistant mouse GF1 fibroblasts. When GF1 cells are incubated for 48 h in the presence of the glucocorticoid hormone dexamethasone, the specific activity of glutamine synthetase (GS), assayed as glutamyltransferase activity, increases by threefold. Based on dot hybridization analysis, hormonal treatment also produces a similar increase in the level of GS mRNA. When GF1 cells or mouse Neuro 2A neuroblastoma cells are transferred from medium containing 4 mM glutamine to glutamine-free medium, glutamyltransferase activity increases by at least fivefold. However, the presence or absence or glutamine in the medium does not affect the relative level of glutamine synthetase mRNA in either cell line. With both GF1 and Neuro 2A cells, the half-time for the decline in glutamine synthetase enzyme activity on addition of glutamine to the medium is approximately 1.5 h. This rapid decline, coupled with the lack of effect of glutamine on the level of GS messenger RNA in Neuro 2A cells, renders it unlikely that neural cells alter glutamine synthetase levels in response to glutamine by a biosynthetic mechanism, as suggested by previous authors [L. Lacoste, K.D. Chaudhary, and J. Lapointe (1982) J. Neurochem. 39, 78-85].

Genetic determinants of glutamine synthetase in Drosophila melanogaster: a gene for glutamine synthetase I resides in the 21B3-6 region

Recombinational and deletion mapping of electrophoretic variants of the glutamine synthetase I isozyme (GSI) in Drosophila melanogaster locates the gene in the 21B region on the second chromosome. We have conducted a genetic analysis of the region extending cytologically from 21A to 21B4-6. Recessive lethal mutations were generated by ethyl methanesulfonate (EMS) and ethyl nitrosourea (ENU) mutagenesis and by hybrid dysgenesis (HD). These lethals fall into seven functional groups, which were partially ordered by complementation with cytologically defined deficiencies of this region generated by hybrid dysgenesis. Two of the EMS- and two of the ENU-induced lethals fulfill biochemical criteria expected for null alleles of the GSI gene.

Induction of glutamine synthetase in rat astrocytes by co-cultivation with embryonic chick neurons

Co-cultivation of confluent rat astrocyte cultures with embryonic chick neurons resulted in induction of glutamine synthetase activity in the astrocytes. This induction of glutamine synthetase in astrocytes by neurons was independent of induction by hydrocortisone and forskolin, but was dependent on the length of co-cultivation and the number of neurons present in the co-culture. Cycloheximide and actinomycin D inhibited the induction of glutamine synthetase in astrocytes by neurons, whereas cytosine arabinoside had no apparent effect. Results suggest that this induction of glutamine synthetase in astrocytes is mediated by cell contact with neurons and may represent a specific neuronal and glial interaction.

Effects of medium glutamine, glutamate, and ammonia on glutamine synthetase activity in cultured mouse astroglial cells

Mouse astroglial cells were grown during the last week of culture in either glutamine-free or glutamine-containing medium. The addition of cortisol to the glutamine-containing medium resulted in a doubling of astroglial glutamine synthetase (GS) activity. Withdrawal of glutamine from the medium resulted in a 50% elevation of GS and addition of cortisol to such a medium resulted in a further increase in GS which was not additive to glutamine withdrawal. Both in glutamine-free and glutamine-containing medium, the addition of glutamate resulted in a depression of both basal and cortisol induced GS activity. The simultaneous addition of ammonia plus glutamate to the culture medium ameliorated the glutamate mediated depressive effects on cortisol induced but not basal GS activity. Glutamine withdrawal from the culture medium resulted in an astroglial protein deficit. The addition of ammonia to the medium considerably reduced this deficit and the addition of glutamate completely eliminated this protein deficit.

The cloning and nucleotide sequence of cDNA for an amplified glutamine synthetase gene from the Chinese hamster

The nucleotide sequence for a glutamine synthetase (GS) mRNA from gene-amplified Chinese hamster (CHO) cells was determined from recombinant cDNA clones obtained from both pBR322 and lambda gt10 libraries and by primer extension. The sequence obtained contains about 1400 bp corresponding to a minor species of mRNA terminated by a poly A sequence. The mRNA contains 146 nucleotides of 5'-noncoding region, 1119 bp of coding sequence, and 108 bp of 3'-noncoding sequence with a 32 bp poly(A) tail. The polyadenylation site used shows little homology with efficient polyadenylation sites, but has considerable complementarity with U4 RNA. The predicted amino acid sequence, starting from an initiation codon with the preferred sequence surrounding it, indicates that Chinese hamster GS has high homology with published bovine brain GS peptides and enabled an ordering of these peptides. There is homology between the mammalian GS enzymes and glutamine synthetases obtained from plants and cyanobacteria but no obvious homology between the CHO cell GS sequence and that of other ATP hydrolysing enzymes.

Glutamine synthetase induction by glucocorticoids in the glucocorticoid-sensitive human leukemic cell line CEM-C7

Treatment of CEM-C7 cells with glucocorticoids produces a 2.5-fold increase in the activity of the enzyme glutamine synthetase (GS). This increase is specific for steroids with glucocorticoid activity adn occurs over a range of steroid concentrations consistent with a receptor-mediated mechanism. Half-maximal and maximal inductions by dexamethasone (dex) occur at 2 X 10(-8) M and 2 X 10(-7) M dex, respectively, concentrations approximately equal to those necessary to produce half and full occupancy of glucocorticoid receptors. GS activity began to increase 1 hour after dex treatment and was complete by 12 hours. This is well before any of the growth inhibitory or cytolytic effects of dex on this cell line occur. This increase was dependent on the presence of glucocorticoid receptors and required both RNA and protein synthesis. Removal of dex following stimulation to maximal levels resulted in a decrease of GS activity to preinduced levels with a half-time of 5 hours. Glutamine deprivation of cells resulted in increased GS activity. However, even in the total absence of glutamine, dex treatment elicited a 2.0-2.5-fold increase in GS activity, ruling out inhibition of glutamine uptake as a mechanism for the dex-induced increase. Experiments with 5'-bromodeoxyuridine (BrdU) demonstrated that GS elevation was sensitive to BrdU substitution of DNA, while dex-induced growth inhibition was not. Therefore GS elevation and growth inhibition in this cell line appear to be independently expressed steroid responses.

Glutamine synthetase activity in the developing secondary palate and induction by dexamethasone

Glutamine synthetase (EC 6.3.1.2) (GS) and glutamyltransferase (EC 2.3.2.1) (GT) specific activity were examined in developing A/Jax and C57BL/6J (C57) mouse fetal secondary palates. In addition, the induction of palatal GS was also examined after maternal injection of dexamethasone. Palatal GT activity was uniformly higher in A/J than C57 palates with both strains showing highest activity late on day 13 of gestation and a drop in activity by early day 14. In contrast, A/J palatal GS activity peaked transiently late on day 13, dropped by early day 14 and remained lower throughout the remaining period of palatal development. Palatal GS activity in C57 mouse fetuses, although failing to show a discrete transient peak of activity, remained at a constant elevated level from early day 13 to late day 14 and did not decrease until day 15 of gestation. These elevated levels of palatal GS and GT activity correspond to the gestation period of maximal palatal glycoconjugate biosynthesis. Thus, palatal GS activity may play an important regulatory role in the synthesis of these macromolecules. A/J and C57BL/6J mice exhibit different susceptibilities to glucocorticoid-induced cleft palate. However, maternal administration of a non-teratogenic dose of dexamethasone on either late day 12 or late day 13 resulted in a dramatic stimulation of both A/J and C57 fetal palatal GS but not GT activity when assay 18 h later. A/J palatal tissue responded to dexamethasone with greater induction of palatal GS activity than enzyme activity in C57 palates. Palatal GS, sensitive to glucocorticoid stimulation, may thus be an important link in expressing hormonal control of normal palatal differentiation.

Unlinked control of multiple glucocorticoid-induced processes in HTC cells

HTC cell variants chosen for their lack of tyrosine aminotransferase (EC 2.6.1.5) (TAT) induction by glucocorticoids were tested for interrelated effects on other glucocorticoid responses: TAT induction by dibutyryl cyclic AMP (dBcAMP) +/- dexamethasone, glutamine synthetase (GS) induction, cyclic nucleotide phosphodieterase (PDE) suppression, inhibition of alpha-aminoisobutyric acid (AIB) uptake, inhibition of plasminogen activator (PA), and induction of mouse mammary tumor virus (MTV). Loss of TAT induction by steroid was accompanied by loss of TAT induction by dBcAMP and of PDE suppression by steroid. In addition, subclones of MTV-infected cells were examined for the effect of the virus on glutamine synthetase (GS) and TAT induction. The virus had no effect on their induction in wild-type cells and no effect on GS induction in the variants. One MTV-infected subclone from a TAT variant, however, showed significant return of TAT induction.

Nitrogen source regulates glutamine synthetase mRNA levels in Neurospora crassa

Neurospora crassa glutamine synthetase mRNA was measured by its capacity to direct the synthesis of the specific protein in a cell-free system derived from rabbit reticulocytes. N. crassa cultures grown on glutamate as the sole nitrogen source had higher mRNA activities than did those grown on glutamine. The differences were about 10-fold when polysomal RNA was used for translation and about 5-fold when either total cellular RNA or polyadenylic acid-enriched cellular RNA was used. These data indicate that in exponentially growing N. crassa, the nitrogen source regulates glutamine synthetase by adjusting specific mRNA levels.

Effects of glutamine, methionine sulfone and dexamethasone on rates of synthesis of glutamine synthetase in cultured hepatoma cells

Glutamine synthetase (EC 6.3.1.2) activity of hepatoma tissue culture cells is elevated by corticosteroids and depressed by glutamine (Kulka, R.G., Tomkins, G.M. and Crook, R.B. (1972) J. Cell Biol., 54, 175--179). The transfer of cells from high (1--5 mM) to low (0.2--0.4 mM) concentrations of glutamine causes a marked increase in glutamine synthetase activity. The addition of a glutamine antagonist, methionine sulfone (1 mM) to cells suspended in high (1 mM) concentrations of glutamine also causes an increase of glutamine synthetase activity which is greater than that elicited by the transfer of cells to low concentrations of glutamine. Rates of synthesis of glutamine synthetase have been measured by radioimmunoprecipitation in hepatoma tissue culture cells incubated under various conditions. Incubation of cells with the synthetic corticosteroid hormone, dexamethasone, markedly stimulates the relative rate of glutamine synthetase biosynthesis. Glutamine, or its analogue, methionine sulfone, have no effect on the relative rate of synthesis of the enzyme. However, total protein and RNA synthesis increase markedly with increasing external glutamine concentration in the range 0--1 mM. Methionine sulfone (1 mM) inhibits the degradation of glutamine synthetase in the presence of 1 mM glutamine. The data are consistent with the conclusion that the corticosteroid, dexamethasone, elevates glutamine synthetase activity by stimulating its rate of synthesis, whereas methionine sulfone elevates glutamine synthetase activity by inhibiting the glutamine-stimulated degradation of preformed enzyme.