Glutamine Synthetase mRNA Levels Research Articles

Starvation alters the activity and mRNA level of glutaminase and glutamine synthetase in the rat intestine

The metabolism of glutamine, the main respiratory fuel of enterocytes, is governed by the activity of glutaminase and glutamine synthetase. Because starvation induces intestinal atrophy, it might alter the rate of intestinal glutamine utilization. This study examined the effect of starvation on the activity, level of mRNA, and distribution of mRNA of glutaminase and glutamine synthetase in the rat intestine. Rats were randomized into groups and were either: (1) fed for 2 days with rat food ad libitum or (2) starved for 2 days. Standardized segments of jejunum and ileum were removed for the estimation of enzyme activity, level of mRNA, and in situ hybridization analysis. The jejunum of the fed rats had a greater activity of both enzymes per centimeter of intestine ( P < 0.01), a greater glutaminase specific activity (1.97 ± 0.45 vs. 1.09 ± 0.34 μmol/hr/mg protein, P < 0.01), and a lower level of glutaminase and glutamine synthetase mRNA. The ileum of the fed rats had a greater activity of glutamine synthetase per centimeter of intestine (162.9 ± 50.6 vs. 91.0 ± 23.1 nmol/hr/cm bowel, P < 0.01), a lower level of glutaminase mRNA, and a greater level of glutamine synthetase mRNA. In situ hybridization analysis showed that starvation does not alter the distribution of glutaminase and glutamine synthetase mRNA in the intestinal mucosa. This study confirms that starvation decreases the total intestinal activity per centimeter of both glutaminase and glutamine synthetase. More importantly, the results indicate that the intestine adapts to starvation by accumulating glutaminase mRNA. This process prepares the intestine for a restoration of intake.

Expression of Glutamine Synthetase in Macrophages

We studied the expression of glutamine synthetase in liver macrophages (Kupffer cells, KCs) in situ and in culture. Glutamine synthetase was detectable at the mRNA and protein level in freshly isolated and short-term-cultured rat liver macrophages. Enzyme activity and protein content were about 9% of that in liver parenchymal cells. In contrast, glutamine synthetase mRNA levels in liver macrophages apparently exceeded those in parenchymal liver cells (PCs). By use of confocal laser scanning microscopy and specific macrophage markers, immunoreactive glutamine synthetase was localized to macrophages in normal rat liver and normal human liver in situ. All liver macrophages stained positive for glutamine synthetase. In addition, macrophages in rat pancreas contained immunoreactive glutamine synthetase, whereas glutamine synthetase was not detectable at the mRNA and protein level in blood monocytes and RAW 264.7 mouse macrophages. No significant amounts of glutamine synthetase were found in isolated rat liver sinusoidal endothelial cells (SECs). The data suggest a constitutive expression of glutamine synthetase not only, as previously believed, in perivenous liver parenchymal cells but also in resident liver macrophages.

Carbon and amino acids reciprocally modulate the expression of glutamine synthetase in Arabidopsis.

In bacteria and yeast, glutamine synthetase (GS) expression is tightly regulated by the metabolic status of the cell, both at the transcriptional and posttranscriptional levels. We discuss the relative contributions of light and metabolic cues on the regulation of members of the GS gene family (chloroplastic GS2 and cytosolic GS1) in Arabidopsis. These studies reveal that the dramatic induction of mRNA for chloroplastic GS2 by light is mediated in part by phytochrome and in part by light-induced changes in sucrose (Suc) levels. In contrast, the modest induction of mRNA for cytosolic GS1 by light is primarily mediated by changes in the levels of carbon metabolites. Suc induction of mRNA for GS2 and GS1 occurs in a time- and dose-dependent manner. Suc-induced changes in GS mRNA levels were also observed at the level of GS enzyme activity. In contrast, amino acids were shown to antagonize the Suc induction of GS, both at the level of mRNA accumulation and that of enzyme activity. For GS2, the gene whose expression was the most dramatically regulated by metabolites, we used a GS2 promoter-beta-glucuronidase fusion to demonstrate that transcriptional control is involved in this metabolic regulation. Our results suggest that the metabolic regulation of GS expression in plants is controlled by the relative abundance of carbon skeletons versus amino acids. This would allow nitrogen assimilation into glutamine to proceed (or not) according to the metabolic status and biosynthetic needs of the plant. This type of GS gene regulation is reminiscent of the nitrogen regulatory system in bacteria, and suggests an evolutionary link between metabolic sensing and signaling in bacteria and plants.

Glutamine synthetase expression in muscle is regulated by transcriptional and posttranscriptional mechanisms.

Skeletal muscle exports glutamine (Gln) and increases the expression of the enzyme glutamine synthetase (GS) in response to physiological stress. Acute stress or direct glucocorticoid administration raises muscle GS mRNA levels dramatically without a parallel increase in GS protein levels. In the lung, this discrepancy is caused by feedback destabilization of the GS protein by its product Gln. It was hypothesized that muscle GS protein levels increase during stress only when the intracellular Gln pool has been depleted. Adult male rats were injected with the glucocorticoid hormone dexamethasone (DEX) to mimic the acute stress response and with the GS inhibitor methionine sulfoximine (MSO) to deplete muscle Gln stores. DEX increased GS mRNA levels by 2.8-fold but increased GS protein levels by an average of only 40%. MSO diminished muscle GLN levels by 68% and caused GS protein levels to rise in accordance with GS mRNA. Chronic stress was mimicked using 6 days of MSO treatment, which produced anorexia, 23% loss of body weight, and 64% decrease in muscle Gln levels, as well as pronounced increases in both muscle GS mRNA (26-fold) and protein levels (35-fold) without elevation of plasma glucocorticoid levels. Calorie-restricted pair-fed animals exhibited lesser increases in muscle GS mRNA (8-fold) and protein levels (5-fold) without a decline in muscle Gln content. Thus regulation of GS expression in both acute and chronic stress involved both transcriptional and posttranscriptional mechanisms, perhaps affected by muscle Gln content.

Glutamine synthetase gene expression in the lungs of endotoxin-treated and adrenalectomized rats.

During sepsis, the lung responds by exporting increased amounts of the amino acid glutamine. This response is accompanied by increased enzymatic activity of glutamine synthetase (GS), which catalyzes the synthesis of glutamine from glutamate and ammonia. It is also known that GS expression in the rat lung can be induced by glucocorticoid hormones. To determine whether the septic response and the response to glucocorticoids are related, we have characterized the induction of GS expression during lipopolysaccharide (LPS)-induced endotoxemia in normal, neutropenic, and adrenalectomized rats. Normal rats exhibited a time- and dose-dependent induction of GS mRNA levels after a single intraperitoneal dose of LPS. Responses to LPS were maximal at doses of 0.1 mg/kg body wt and above. A single 10 mg/kg body wt dose of LPS led to a rapid, transient sevenfold increase in GS mRNA (P < or = 0.1) and a twofold increase in GS protein level 8 h postinjection. Induction of lung GS mRNA 4 h after LPS injection was approximately fivefold in neutropenic (P < or = 0.1) and fourfold in nonneutropenic control rats (P < or = 0.1), suggesting that infiltrating neutrophils or neutrophil-derived factors are not required for GS induction. In response to high-dose, short-term endotoxemia, adrenalectomized rat lung GS mRNA increased twofold (P < or = 0.02) compared with sixfold in sham-operated control rats (P < or = 0.02). However, in response to low-dose, long-term endotoxemia, adrenalectomized rat lung GS mRNA increased threefold (P < or = 0.02) compared with fourfold in sham-operated control rats (P < or = 0.02). Adrenalectomy did not affect the elevation of lung GS mRNA levels in response to dexamethasone. In addition, GS mRNA was induced four- and sixfold in rat microvascular pulmonary endothelial cells exposed to plasma from control and septic rats, respectively. The addition of a glucocorticoid antagonist, RU-38486, completely blocked GS gene induction in the presence of control plasma but only attenuated the response to plasma from septic animals by 30%. These results suggest that GS gene induction during sepsis is only partially mediated by adrenal-derived glucocorticoid hormones.

Analysis of mRNA levels for developmentally regulated prespore specific glutamine synthetase in Dictyostelium discoideum.

The enzyme glutamine synthetase (GS) of Dictyostelium discoideum is developmentally regulated, preferentially localized in prespore cells and is likely to play an important role in controlling the levels of ammonia, a known morphogen, in this organism. To further investigate the regulation of GS, a portion of the GS gene was isolated and used as a probe to examine the changes in GS mRNA throughout development and the level of GS mRNA in the two precursor cell types. The amino acid sequence of the cloned DNA fragment isolated is highly homologous to other eukaryotic GS genes. DNA blot analysis demonstrated that the GS gene exists as a single copy in D. discoideum. Analysis of RNA indicated that there is a single 1.7 kb GS transcript that increased during development to peak at the initial stages of culmination. Furthermore, GS mRNA was highly localized in prespore cells, which is consistent with a proposed source-sink model for ammonia assimilation in this organism.

Regulation of glutamine synthetase in human breast carcinoma cells and experimental tumors

Background. Acute deprivation of extracellular glutamine causes up-regulation of glutamine synthetase (GS) expression by a mechanism involving an increase in GS protein stability. This study examines GS expression in a highly glutamine-dependent and tumorigenic human breast cancer cell line, TSE cells, in response to acute and chronic glutamine deprivation in culture and during tumor formation. Methods. TSE cells were subjected to acute glutamine deprivation, adapted to growth in low glutamine concentrations, and subcutaneously injected into nude mice. GS protein and mRNA levels were assayed by Western and Northern blotting, and intracellular glutamine levels were evaluated by using a colorimetric assay. Results. GS protein levels increased, but GS mRNA levels were unchanged in response to acute glutamine deprivation. Chronic glutamine deprivation in vitro and tumor growth in vivo caused an increase in both GS protein and mRNA levels. Large tumors exhibited lower intracellular glutamine, higher GS protein, and relatively unchanged GS mRNA levels relative to small tumors. Conclusions. TSE tumors exhibit up-regulation of GS protein and mRNA levels and declines in intracellular glutamine content, suggesting that growth in vivo causes a chronic and progressive glutamine deprivation. Up-regulation of GS expression may contribute to adaptation to a nutrient-poor intratumor environment.

Effect of the glutamine synthetase inhibitor, methionine sulfoximine, on the growth and differentiation of Dictyostelium discoideum

To examine further the role of the enzyme glutamine synthetase in Dictyostelium discoideum we report here the effects of a specific glutamine synthetase inhibitor, methionine sulfoximine, on the growth and differentiation of this organism. Vegetative AX3 cells grown in the presence of methionine sulfoximine did not complete culmination in the normal time but were blocked at the finger stage. In these cells glutamine synthetase activity was almost completely abolished. However, methionine sulfoximine did not affect the level of glutamine synthetase mRNA, suggesting that there is no link between glutamine synthetase activity and mRNA transcription. Eventually glutamine synthetase activity reappeared and at the time culmination occurred. These results suggest that glutamine synthetase plays an important role in the assimilation of ammonia during the later stages of development in D. discoideum and that this assimilation is necessary for the completion of culmination.

Induction of Glutamine Synthetase Expression after Major Burn Injury is Tissue Specific and Temporally Variable

Major burn injury results in a translocation of amino acids from peripheral tissues to the abdominal viscera. Glutamine is a major participant in this event. Thermal injury causes a depletion of plasma and muscle glutamine pools as well as activation of proteolysis and release of glutamine from skeletal muscle. De novo synthesis of glutamine is regulated by the expression of the enzyme glutamine synthetase (GS). We studied the tissue-specific regulation of GS expression after thermal injury. Burn injury of rats was produced by scalding of 25 or 40% of skin surface. In normal rats, four organs, including lung, muscle, kidney, and liver were assayed for relative GS messenger RNA content by Northern blotting 8 and 24 hours after 40% area burn. The effect of adrenalectomy on GS mRNA induction in muscle was assessed 24 hours after 25% area burn injury. GS mRNA levels were increased 2.3-fold in lung at 8 hours and 7.3-fold in muscle at 24 hours after burn injury. No appreciable increase in GS mRNA level was observed in kidney or liver. Muscle GS mRNA levels were lower than sham-operated controls in both burned and unburned adrenalectomized rats. However, adrenalectomy did not attenuate relative GS mRNA induction in muscle at 24 hours after burn injury. Burn injury causes an induction in GS mRNA levels in a tissue-specific fashion. Adrenalectomy greatly reduced GS mRNA levels, but did not completely block the induction of GS express in muscle after burn injury. This finding suggests that glucocorticoid hormones together with a unknown factor of nonadrenal origin influence this metabolic response to burn injury.

Glutamine synthetase induction by glucocorticoids is preserved in skeletal muscle of aged rats.

Glutamine synthetase (GS) is a glucocorticoid-inducible enzyme that has a key role for glutamine synthesis in muscle. We hypothesized that the glucocorticoid induction of GS could be altered in aged rats, because alterations in the responsiveness of some genes to glucocorticoids were reported in aging. We compared the glucocorticoid-induced GS in fast-twitch and slow-twitch skeletal muscles (tibialis anterior and soleus, respectively) and heart from adult (age 6-8 mo) and aged (age 22 mo) female rats. All animals received dexamethasone (Dex) in their drinking water (0.77 +/- 0.10 and 0.80 +/- 0.08 mg/day per adult and aged rat, respectively) for 5 days. Dex caused an increase in both GS activity and GS mRNA in fast-twitch and slow-twitch skeletal muscles from adult and aged rats. In contrast, Dex increased GS activity in heart of adult rats, without any concomitant change in GS mRNA levels. Furthermore, Dex did not affect GS activity in aged heart. Thus the responsiveness of GS to an excess of glucocorticoids is preserved in skeletal muscle but not in heart from aged animals.

Tissue-specific regulation of glutamine synthetase gene expression in acute pancreatitis is confirmed by using interleukin-1 receptor knockout mice

Acute pancreatitis causes a pronounced depletion of plasma and muscle glutamine pools. In several other catabolic disease states expression of the enzyme glutamine synthetase (GS) is induced in lung and muscle to support glutamine secretion by these organs. The hormonal mediators of GS induction have not been conclusively identified. We used mice deficient for the expression of the type 1 interleukin-1 receptor (IL-1R1 knockout mice) to investigate the expression of GS during acute edematous pancreatitis. Acute edematous pancreatitis was induced in adult male wild-type and IL-1R1 knockout mice by means of the intraperitoneal administration of cerulein, and their conditions were monitored. Five organs, including lung, liver, gastrocnemius muscle, spleen, and pancreas, were assayed for relative GS messenger RNA (mRNA) content by Northern blotting. The ultimate severity of pancreatitis was reduced by IL-1R1 deficiency. GS mRNA levels increased during progression of pancreatitis in lung, spleen, and muscle tissue from each group. No consistent increase in GS mRNA level was observed in liver. IL-1R1 deficiency did not affect GS mRNA expression in lung tissue but consistently retarded GS induction in the spleens of knockout animals. IL-1R deficiency altered the kinetics of GS induction in muscle. Cerulein-induced experimental pancreatitis causes an induction in GS mRNA levels in a tissue-specific fashion. IL-1R1 deficiency reduced the ultimate severity of the condition and altered the induction of GS mRNA in the spleen and muscle.

Ontogeny of Glutamine Synthetase in Rat Small Intestine

Glutamine synthetase (GS) is a key enzyme involved in the endogenous biosynthesis of glutamine, an amino acid known to be essential for small intestinal metabolism and function. This study describes the ontogeny of rat small intestinal GS from fetal life through adulthood with enzyme activities, protein immunoblotting, and steady state levels of GS mRNA by RNA gel blots and dot blots. Enzyme activities progressively increased from 21 d of fetal life to 32 d postnatally, then decreased in adulthood. The amount of GS immunoreactive protein in the small intestine increased from fetal life to 10-day-old infants and persisted into adulthood. GS mRNA, as quantified by dot blots was highest at 19 d postnatally. The ontogenic changes in rat small intestinal GS appear to correspond temporally with rapid growth and weaning. The steady increase in GS enzyme activity up to 32 d of age with a subsequent drop in adulthood is not paralleled by an increase in GS mRNA or protein. These findings suggest an apparent complex regulation of the enzyme activity at a transcriptional or translational levels.

Differentiation of serum‐free mouse embryo cells into astrocytes is accompanied by induction of glutamine synthetase activity

Serum-free mouse embryo (SFME) cells derived in a defined serum-free medium have been cultured for more than 200 generations and display properties of neural progenitor cells. SFME cells express the neuroepithelial stem cell marker nestin in defined serum-free medium. Exposure of SFME cells to transforming growth factor beta (TGF-beta) or serum decreases nestin expression and induces the astrocyte marker glial fibrillary acidic protein, suggesting that SFME cells differentiate into astrocytes upon exposure to TGF-beta or serum. We examined the expression by SFME cells of the functional central nervous system (CNS) astrocyte marker glutamine synthetase (GS). GS activity is induced in SFME cells upon exposure to TFG-beta or serum. The induction of GS activity was dose- and time-dependent and was reversible. Retinoic acid, hydrocortisone, and dibutyryl cyclic AMP also induced GS expression. The induction of GS activity was accompanied by an increase in the level of GS mRNA and protein. This work provides further evidence that SFME cells represent neural progenitor cells which differentiate into functional astrocytes upon exposure to TGF-beta or serum.

Glucocorticoids Regulate Rat Glutamine Synthetase Expression in a Tissue-Specific Manner

During stress states, organismal glutamine production is augmented secondary to an increase in the activity of glutamine synthetase (GS) in the lung and skeletal muscle. Because glucocorticoids are key regulators of the metabolic response to stress, we undertook a survey of glucocorticoid induction of GS expression in rat organs in response to dexamethasone. Male adult rats were injected with glucocorticoid or vehicle and 4 hr later, 10 organs were assayed for GS messenger RNA and protein contents by Northern and Western blotting. We observed a 20-fold range of GS mRNA levels in organs of control animals. Blotting detected two GS RNA species of approximately 2.8- and 1.4-kb sizes in all tissue except testis, where an additional 2-kb RNA species was observed. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mBNA levels were also assayed and used as a normalization factor. An approximately 10-fold range of GAPDH mRNA levels was observed. Four hours after dexamethasone injection, a nearly a 5-fold increase in glutamine synthetase mRNA levels in lung and muscle, as well as an approximately 2-fold increase in heart were observed. Relative to GAPDH mRNA, a significant decrease in GS mRNA levels was observed in the liver. A wide range of glutamine synthetase protein contents were observed in rat organs. Comparison of Northern and Western blotting results revealed a dichotomy in the ratio of relative GS mRNA and protein level in rat organs, suggesting that tissue-specific posttranscriptional processes determine GS protein levels. Four hours after dexamethasone injection, an apparent increase in GS protein was observed in the lung, muscle, and thymus; however, significant induction of GS protein was demonstrated only in lung. The differential response of GS mRNA and protein levels to dexamethasone suggests that GS expression is induced by glucocorticoids in a tissue-specific manner.

Effect of dibutyryl cyclic AMP and dexamethasone on glutamine synthetase gene expression in rat astrocytes in culture

Astrocytes are the primary site of glutamate conversion to glutamine in the brain. We examined the effects of treatment with either dibutyryl cyclic AMP and/or the synthetic glucocorticoid dexamethasone on glutamine synthetase enzyme activity and steady-state mRNA levels in cultured neonatal rat astrocytes. Treatment of cultures with dibutyryl cyclic AMP alone (0.25 mM-1.0 mM) increased glutamine synthetase activity and steady state mRNA levels in a dose-dependent manner. Similarly, treatment with dexamethasone alone (10(-7)-10(-5) M) increased glutamine synthetase mRNA levels and enzyme activity. When astrocytes were treated with both effectors, additive increases in glutamine synthetase activity and mRNA were obtained. However, the additive effects were observed only when the effect of dibutyryl cyclic AMP alone was not maximal. These findings suggest that the actions of these effectors are mediated at the level of mRNA accumulation. The induction of glutamine synthetase mRNA by dibutyryl cyclic AMP was dependent on protein synthesis while the dexamethasone effect was not. Glucocorticoids and cyclic AMP are known to exert their effects on gene expression by different molecular mechanisms. Possible crosstalk between these effector pathways may occur in regulation of astrocyte glutamine synthetase expression.

Glutamine synthetase modulation in the brain of rats subjected to deprivation of paradoxical sleep

Changes in the level of glutamine synthetase (GS), an enzyme mainly located in astrocytes, were investigated in rat brain after deprivation of paradoxical sleep (PSD) and during recovery. An immunotitration method was used to evaluate the relative level of GS in brain tissue. At the end of a 24 h PSD, a significant increase in GS protein was observed both in the frontoparietal cortex (CX) and in the locus coeruleus area (LC). Four hours later during recovery, the level of GS protein returned to normal level in the CX but fell below control levels in the LC. In contrast, in the CX, the level of glial fibrillary acidic protein, an astroglial marker, did not change after PSD or during recovery. GS mRNA was quantified in the entire cortex by northern blot hybridization using of an oligonucleotidic GS-cDNA probe. We observed an increase in the GS mRNA level in the cortex of PSD rats of the same magnitude as the increase in GS protein. Both GS mRNA and GS protein tended to return to control values 4 h later during recovery. These results are discussed with particular attention to stress effects and possible physiological mechanisms regarding the regulation of amino acid levels by neurotransmitters during prolonged waking or neuronal excitation.

Molecular basis for differential expression of glutamine synthetase in retina glia and neurons

Glutamine synthetase (GS) is a differentiation marker of retina glial cells. It is expressed in the chicken neural retina at a particularly high level, is inducible by glucocorticoids and is always confined to Müller glia. This study investigated the molecular basis for tissue and cell-type specific expression of the GS gene. A high level of GS expression in the retina was found to coincide with the accumulation of a relatively high level of GS mRNA in this tissue. The gliatoxic agent α-aminoadipic acid, which can selectively destroy glia cells, was used to demonstrate that restriction of GS induction to Müller glia is controlled at a transcriptional level. Cortisol could induce accumulation of GS mRNA and transcription of the GS gene in Müller glia but not in retina neurons. Glia and neurons were also found to differ in their ability to express the glucocorticoid inducible CAT construct, pΔG46TCO, which is controlled by a ‘simple GRE’ promoter. When introduced into cells of retina tissue, this construct was cortisol-inducible in glia whereas in neurons it was only slightly inducible or not at all. Introduction of a glucocorticoid receptor expression vector into the cells facilitated induction of the CAT construct in neurons. Analysis by immunoblotting revealed that expression of the glucocorticoid receptor protein is predominantly restricted to Müller glia. These results suggest that differential levels of glucocorticoid receptor expression in glia and neurons might be the basis for cell-type specific induction of GS.

Dexamethasone increases jejunal glutamine synthetase expression via translational regulation

Glutamine provides energy and precursors for nucleotide biosynthesis for the gut mucosa, and it is essential for intestinal metabolism and function. During stress states, glutamine uptake of circulating and luminal glutamine may be diminished, but the ability of the gut mucosa to synthesize glutamine de novo in response to this decreased delivery remains undefined. Since the glucocorticoids play an important role in regulating interorgan glutamine metabolism during catabolic states, we hypothesized that these hormones induce the expression of gut mucosal glutamine synthetase (GS), the enzyme that catalyzes the intracellular biosynthesis of glutamine. Adult rats were treated with dexamethasone (DEX, 0.5 mg/kg intraperitoneally) or saline (controls). At various times after treatment (4, 12, 24, 48, and 72 hours), jejunal mucosal GS-specific activity was assayed, and total RNA was extracted. GS transcripts were detected by Northern blot analysis, using a radiolabeled rat GS cDNA probe. Transcripts were quantitated by phospho-imaging and normalized to β-actin. An anti-GS polyclonal antibody was used to quantitate GS protein concentrations by Western blot analysis. The relative quantities of GS translated were measured using a cell-free protein-synthesizing system (reticulocyte lysate assay). Data were analyzed using analysis of variance and were considered statistically significant for p < 0.05. DEX increased GS activity by 45% 12 hours after administration. Western blot analysis revealed an increase in the concentration of the GS protein in the jejunum of DEX-treated animals. Northern blot analysis demonstrated no significant change in GS mRNA levels after DEX treatment, indicating the possibility of posttranscriptional regulation. In vitro translational experiments demonstrated that the quantity of GS translated was increased by 25% after the administration of DEX. These data suggest that glucocorticoids may increase jejunal mucosal GS levels by accelerating protein translation. This adaptive response could provide ghitamine for the gut mucosa during stress, when exogenous glutamine supplies may be rate limiting.

A glutamatergic mechanism for aluminum toxicity in astrocytes.

The effect of aluminum on the metabolism of glutamate and glutamine in astrocytes was studied to provide information about a possible biochemical mechanism for aluminum neurotoxicity and its potential contribution to neurodegenerative disease. Exposure of cultured rat brain astrocytes for 3-4 d to 5-7.5 mM aluminum lactate increased glutamine synthetase activity by 100-300% and diminished glutaminase activity by 50-85%. Increased glutamine synthetase enzyme activity was accompanied by an elevated level of glutamine synthetase mRNA. Alterations in glutaminase and glutamine synthetase following aluminum exposure caused increased intracellular glutamine levels, decreased intracellular glutamate levels, and increased conversion of glutamate to glutamine and the release of the latter into the extracellular space. The results of these changes may alter the availability of neurotransmitter glutamate in vivo and may be a mechanism for the aluminum neurotoxicity observed in individuals exposed to the metal during dialysis procedures and other situations.

Dexamethasone regulates glutamine synthetase expression in rat lung

Since the lungs play a central role in maintaining glutamine homeostasis in normal and catabolic disease states, we studied the regulation of glutamine synthetase (GS) expression by dexamethasone in rat lung. Adult rats received saline (controls) or dexamethasone (0.5 mg/kg). Lung total RNA was extracted for Northern hybridization and labeled with an α- 32P rat GS cDNA probe. The mRNA of the constitutively expressed gene β-actin was the control for RNA loading. GS transcripts were measured by laser densitometry and normalized to actin, and GS specific activity was also determined. Following a single injection of dexamethasone (0.5 mg/kg), lung GS activity increased by 40% at 4 hours and by 75% at 8 hours. The dexamethasone-mediated increase in GS activity was associated with a marked increase in GS mRNA levels, which preceded the increase in enzyme activity by approximately 2 hours. Serial daily dexamethasone administration for 3 and 6 days caused an even greater increase in GS mRNA levels and specific activity. No effect was seen on β-actin levels, demonstrating that the expression of GS was not part of a global response to steroids. Therefore, glucocorticoids stimulate GS expression in rat lung. This regulation appears to be one mechanism by which lung glutamine release is augmented during critical illness.