Adaptive Regulation in Skeletal Muscle Glutamine Metabolism in Endotoxin-Treated Rats

Effects of dietary leucine supplementation and immune system stimulation on plasma AA concentrations and tissue protein synthesis in starter pigs

 Immune system stimulation (ISS) adversely affects protein and AA metabolism and reduces productivity in pigs. Leucine (Leu) has a regulatory role in skeletal muscle protein turnover, which may be affected by ISS. The objective of this study was to evaluate the effects of ISS and dietary Leu supplementation on the protein fractional synthesis rate (FSR) of various tissues in pigs. Yorkshire barrows were surgically fitted with jugular vein catheters and assigned to one of three dietary treatments: (i) CON, 1.36% standardized ileal digestible (SID) Leu; (ii) LEU-M, 2.04% SID Leu; and (iii) LEU-H, 2.72% SID Leu. The diets were formulated to contain all essential AA 10% above estimated requirements for maximum whole-body protein deposition for this BW range. At the start of the 36-h challenge period (initial BW = 14.5 ± 0.8 kg), ISS was induced in pigs with lipopolysaccharide (ISS+; n = 7, 8, and 7 for CON, LEU-M, and LEU-H pigs, respectively); a subset of CON pigs was injected with sterile saline (ISS-; n = 6). During challenge period, pigs were fed every 4 h and feed intake of ISS- pigs was kept equal to ISS+ pigs. At the end of the challenge period, FSR of liver, plasma, gastrocnemius, and LD proteins were determined with a flooding dose of l-[ring-2H5]phenylalanine (40 mol%). All essential AA, most nonessential AA, and plasma urea-N peaked at 12 h and declined to baseline levels at 36 h after ISS was induced in ISS+ pigs (P < 0.05), whereas plasma AA and urea-N concentrations were constant in ISS- pigs. At 36 h, dietary Leu supplementation resulted in a linear decline in plasma isoleucine, valine, glutamine, and urea nitrogen concentrations (P < 0.05), whereas plasma Leu concentration was unaffected. Liver protein FSR was increased in ISS+ pigs (P < 0.05), whereas plasma and skeletal muscle protein FSR was not affected by ISS. Dietary Leu supplementation tended to diminish liver protein FSR (linear reduction; P = 0.052) and increase gastrocnemius protein FSR (linear increase; P = 0.085) in ISS+ pigs. Leucine supplementation above estimated requirements may support repartitioning of AA from visceral to peripheral protein deposition during ISS.

Glutamine: Metabolism and Immune Function, Supplementation and Clinical Translation

Glutamine is the most abundant and versatile amino acid in the body. In health and disease, the rate of glutamine consumption by immune cells is similar or greater than glucose. For instance, in vitro and in vivo studies have determined that glutamine is an essential nutrient for lymphocyte proliferation and cytokine production, macrophage phagocytic plus secretory activities, and neutrophil bacterial killing. Glutamine release to the circulation and availability is mainly controlled by key metabolic organs, such as the gut, liver, and skeletal muscles. During catabolic/hypercatabolic situations glutamine can become essential for metabolic function, but its availability may be compromised due to the impairment of homeostasis in the inter-tissue metabolism of amino acids. For this reason, glutamine is currently part of clinical nutrition supplementation protocols and/or recommended for immune suppressed individuals. However, in a wide range of catabolic/hypercatabolic situations (e.g., ill/critically ill, post-trauma, sepsis, exhausted athletes), it is currently difficult to determine whether glutamine supplementation (oral/enteral or parenteral) should be recommended based on the amino acid plasma/bloodstream concentration (also known as glutaminemia). Although the beneficial immune-based effects of glutamine supplementation are already established, many questions and evidence for positive in vivo outcomes still remain to be presented. Therefore, this paper provides an integrated review of how glutamine metabolism in key organs is important to cells of the immune system. We also discuss glutamine metabolism and action, and important issues related to the effects of glutamine supplementation in catabolic situations.

Glutamine: Metabolism and Immune Function, Supplementation and Clinical Translation

Glutamine is the most abundant and versatile amino acid in the body. In health and disease, the rate of glutamine consumption by immune cells is similar or greater than glucose. For instance, in vitro and in vivo studies have determined that glutamine is an essential nutrient for lymphocyte proliferation and cytokine production, macrophage phagocytic plus secretory activities, and neutrophil bacterial killing. Glutamine release to the circulation and availability is mainly controlled by key metabolic organs, such as the gut, liver, and skeletal muscles. During catabolic/hypercatabolic situations glutamine can become essential for metabolic function, but its availability may be compromised due to the impairment of homeostasis in the inter-tissue metabolism of amino acids. For this reason, glutamine is currently part of clinical nutrition supplementation protocols and/or recommended for immune suppressed individuals. However, in a wide range of catabolic/hypercatabolic situations (e.g., ill/critically ill, post-trauma, sepsis, exhausted athletes), it is currently difficult to determine whether glutamine supplementation (oral/enteral or parenteral) should be recommended based on the amino acid plasma/bloodstream concentration (also known as glutaminemia). Although the beneficial immune-based effects of glutamine supplementation are already established, many questions and evidence for positive in vivo outcomes still remain to be presented. Therefore, this paper provides an integrated review of how glutamine metabolism in key organs is important to cells of the immune system. We also discuss glutamine metabolism and action, and important issues related to the effects of glutamine supplementation in catabolic situations.

Effect of supplemental dietary leucine and immune system stimulation on whole-body nitrogen utilization in starter pigs

The increase in circulating pro-inflammatory cytokines following an immune challenge (e.g., with bacterial lipopolysaccharide [LPS]) causes a disruption in normal AA metabolism and increases visceral protein synthesis at the expense of muscle protein synthesis. The objective of this study was to determine the effect of supplemental dietary Leu on the dynamics of whole body nitrogen (N) retention in starter pigs before and after immune system stimulation (ISS) induced by LPS. A total of 28 starter pigs (14.46 ± 0.73 kg BW) were assigned to isoenergetic and isonitrogenous diets formulated to supply essential AA 10% above estimated requirements for maximum whole-body protein deposition (PD) and to contain increasing amounts of Leu: CON (1.36% SID Leu); LEU-M (2.04% SID Leu); and LEU-H (2.72% SID Leu). Pigs were housed in metabolic crates, scale-fed every 4 h based on BW, and adjusted to dietary treatments for 5 d. The 108-h N-balance experiment was divided into two periods: prechallenge (before LPS challenge; six 12-h collections) and challenge (after LPS challenge; three 12-h collections) periods. In both periods, blood was collected to determine plasma AA and urea N concentrations. At the start of the challenge period, one-half of the pigs fed CON and all pigs fed LEU-M and LEU-H were challenged with LPS (ISS+; 30 µg/kg injected intramuscularly); the remaining pigs fed CON were injected with saline (ISS-). Whole-body N retention was determined during subsequent 12-h collections. Plasma free Leu concentration increased linearly with increasing Leu content in the diet before LPS was administered (CON, 124 µmol/L; LEU-M, 185 µmol/L; LEU-H, 227 µmol/L; < 0.01). During the prechallenge period, N retention was lower in pigs fed LEU-M ( < 0.01) and there was no difference between pigs fed CON and LEU-H (7.91, 7.18, and 7.71 g/12 h for CON, LEU-M, and LEU-H, respectively). During the challenge period, N retention in pigs fed CON was higher in ISS- than ISS+ (5.37 vs. 3.83 g/12 h; < 0.01) but was not affected by diet in ISS+ pigs (3.83, 3.21, and 3.45 g/12 h for CON, LEU-M, and LEU-H, respectively; > 0.10). In healthy pigs, feeding a high excess of dietary Leu induced an anabolic response to compensate for reduced N retention that occurred in pigs fed an intermediate excess of dietary Leu. There was no effect of supplemental Leu on N retention in pigs after an LPS challenge.

Exploring the Inflammatory Metabolomic Profile to Predict Response to TNF-α Inhibitors in Rheumatoid Arthritis

In clinical practice, approximately one-third of patients with rheumatoid arthritis (RA) respond insufficiently to TNF-α inhibitors (TNFis). The aim of the study was to explore the use of a metabolomics to identify predictors for the outcome of TNFi therapy, and study the metabolomic fingerprint in active RA irrespective of patients’ response. In the metabolomic profiling, lipids, oxylipins, and amines were measured in serum samples of RA patients from the observational BiOCURA cohort, before start of biological treatment. Multivariable logistic regression models were established to identify predictors for good- and non-response in patients receiving TNFi (n = 124). The added value of metabolites over prediction using clinical parameters only was determined by comparing the area under receiver operating characteristic curve (AUC-ROC), sensitivity, specificity, positive- and negative predictive value and by the net reclassification index (NRI). The models were further validated by 10-fold cross validation and tested on the complete TNFi treatment cohort including moderate responders. Additionally, metabolites were identified that cross-sectionally associated with the RA disease activity score based on a 28-joint count (DAS28), erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP). Out of 139 metabolites, the best-performing predictors were sn1-LPC(18:3-ω3/ω6), sn1-LPC(15:0), ethanolamine, and lysine. The model that combined the selected metabolites with clinical parameters showed a significant larger AUC-ROC than that of the model containing only clinical parameters (p = 0.01). The combined model was able to discriminate good- and non-responders with good accuracy and to reclassify non-responders with an improvement of 30% (total NRI = 0.23) and showed a prediction error of 0.27. For the complete TNFi cohort, the NRI was 0.22. In addition, 88 metabolites were associated with DAS28, ESR or CRP (p<0.05). Our study established an accurate prediction model for response to TNFi therapy, containing metabolites and clinical parameters. Associations between metabolites and disease activity may help elucidate additional pathologic mechanisms behind RA.

Altered glutamine metabolism and therapeutic opportunities for lung cancer

Disordered cancer metabolism was described almost a century ago as an abnormal adaptation of cancer cells to glucose utilization especially in hypoxic conditions; the so-called Warburg effect. Greater research interest in this area in the past two decades has led to the recognition of the critical coupling of specific malignant phenotypes such as increased proliferation and resistance to programmed cell death (apoptosis) with altered metabolic handling of key molecules that are essential for normal cellular metabolism. The altered glucose metabolism frequently encountered in cancer cells has already been exploited for cancer diagnosis and treatment. The role of other glycolytic pathway intermediates and alternative pathways for energy generation and macromolecular synthesis in cancer cells has only become recognized more recently. Especially, the important role of altered glutamine metabolism in the malignant behavior of cancer cells and the potential exploitation of this cellular adaptation for therapeutic targeting has now emerged as an important area of cancer research. Expectedly, attempts to exploit this understanding for diagnostic and therapeutic ends are running apace with the elucidation of the complex metabolic alterations that accompany neoplastic transformation. Because lung cancer is a leading cause of cancer death with limited curative therapy options, careful elucidation of the mechanism and consequences of disordered cancer metabolism in lung cancer is warranted. This review provides a concise, systematic overview of the current understanding of the role of altered glutamine metabolism in cancer, and how these findings intersect with current and future approaches to lung cancer management.

Decreased glutamate, glutamine and citrulline concentrations in plasma and muscle in endotoxemia cannot be reversed by glutamate or glutamine supplementation: a primary intestinal defect?

Endotoxemia affects intestinal physiology. A decrease of circulating citrulline concentration is considered as a reflection of the intestinal function. Citrulline can be produced in enterocytes notably from glutamate and glutamine. The aim of this work was to determine if glutamate, glutamine and citrulline concentrations in blood, intestine and muscle are decreased by endotoxemia, and if supplementation with glutamate or glutamine can restore normal concentrations. We induced endotoxemia in rats by an intraperitoneal injection of 0.3 mg kg(-1) lipopolysaccharide (LPS). This led to a rapid anorexia, negative nitrogen balance and a transient increase of the circulating level of IL-6 and TNF-α. When compared with the values measured in pair fed (PF) animals, almost all circulating amino acids (AA) including citrulline decreased, suggesting a decrease of intestinal function. However, at D2 after LPS injection, most circulating AA concentrations were closed to the values recorded in the PF group. At that time, among AA, only glutamate, glutamine and citrulline were decreased in gastrocnemius muscle without change in intestinal mucosa. A supplementation with 4% monosodium glutamate (MSG) or an isomolar amount of glutamine failed to restore glutamate, glutamine and citrulline concentrations in plasma and muscle. However, MSG supplementation led to an accumulation of glutamate in the intestinal mucosa. In conclusion, endotoxemia rapidly but transiently decreased the circulating concentrations of almost all AA and more durably of glutamate, glutamine and citrulline in muscle. Supplementation with glutamate or glutamine failed to restore glutamate, glutamine and citrulline concentrations in plasma and muscles. The implication of a loss of the intestinal capacity for AA absorption and/or metabolism in endotoxemia (as judged from decreased citrulline plasma concentration) for explaining such results are discussed.

Glutamine: A novel approach to chemotherapy-induced toxicity

Treatment of cancer is associated with short- and long-term side-effects. Cancer produces a state of glutamine deficiency, which is further aggravated by toxic effects of chemotherapeutic agents leading to increased tolerance of tumor to chemotherapy as well as reduced tolerance of normal tissues to the side-effects of chemotherapy. This article reviews the possible role of glutamine supplementation in reducing the serious adverse events in patients treated with anticancer drugs. The literature related to the possible role of glutamine in humans with cancer and the supportive evidence from animal studies was reviewed. Searches were made and the literature was retrieved using PUBMED, MEDLINE, COCHRANE LIBRARY, CENAHL and EMBASE, with a greater emphasis on the recent advances and clinical trials. Glutamine supplementation was found to protect against radiation-induced mucositis, anthracycline-induced cardiotoxicity and paclitaxel-related myalgias/arthralgias. Glutamine may prevent neurotoxicity of paclitaxel, cisplatin, oxaplatin bortezomib and lenolidamide, and is beneficial in the reduction of the dose-limiting gastrointestinal toxic effects of irinotecan and 5-FU-induced mucositis and stomatitis. Dietary glutamine reduces the severity of the immunosuppressive effect induced by methotrexate and improves the immune status of rats recovering from chemotherapy. In patients with acute myeloid leukemia requiring parenteral nutrition, glycyl-glutamine supplementation could hasten neutrophil recovery after intensive myelosuppressive chemotherapy. Current data supports the usefulness of glutamine supplementation in reducing complications of chemotherapy; however, paucity of clinical trials weakens the clear interpretation of these findings.

Molecular cloning and characterization of glutamine synthetase, a tegumental protein from Schistosoma japonicum

Glutamine synthetase catalyzes the synthesis of glutamine, providing nitrogen for the production of purines, pyrimidines, amino acids, and other compounds required in many pivotal cellular events. Herein, a full-length cDNA encoding Schistosoma japonicum glutamine synthetase (SjGS) was isolated from 21-day schistosomes. The entire open reading frame of SjGS contains a 1,095-bp coding region corresponding to 364 amino acids with a calculated molecular weight of 40.7 kDa. NCBIP blast shows that the putative amino acid of SjGS contains a classic β-grasp domain and a catalytic domain of glutamine synthetase. The relative mRNA expression of SjGS was evaluated in 7-, 13-, 21-, 28-, 35-, and 42-day worms of S. japonicum in the final host and higher expression at day 21, and 42 worms were observed. This protein was also detected in worm extracts using Western blot. Immunofluorescence studies indicated that the SjGS protein was mainly distributed on tegument and parenchyma in 28-day adult worms. The recombinant glutamine synthetase with a molecular weight of 45 kDa was expressed in Escherichia coli and purified in its active form. The enzyme activity of the recombinant protein was 3.30 ± 0.67 U.μg-1. The enzyme activity was highly stable over a wide range of pH (6-9) and temperature (25-40 °C) under physiological conditions. The transcription of SjGS was upregulated in praziquantel-treated worms at 2-, 4-, and 24-h posttreatment compared with the untreated control. As a first step towards the clarification of the role of glutamine synthetase in schistosome species, we have cloned and characterized cDNAs encoding SjGS in S. japonicum, and the data presented suggest that SjGS is an important molecule in the development of the schistosome.

Glutamine decreases inflammation in infant rat endotoxemia

Glutamine may have benefits during neonatal sepsis, but its effects on systemic inflammation are unknown. Our aim was to determine whether glutamine affects inflammation in neonatal endotoxemia. Eleven-day rat pups were given intraperitoneal injections of saline (control; C), endotoxin (300 microg/g Escherichia coli lipopolysaccharide) (E), saline with glutamine (2 mmol/g; G), or endotoxin with glutamine (EG). Animals were killed after 2 or 6 hours. Plasma glutamine (mmol/L) was measured enzymatically, and both tumor necrosis factor alpha (pg/mL) and interleukin 10 (IL-10) were measured by enzyme-linked immunosorbent assay. Results, expressed as mean +/- SEM, were analyzed by analysis of variance. Endotoxemia caused a rapid significant decrease in plasma glutamine at 2 hours (C, 0.73 +/- 0.06; E, 0.32 +/- 0.07; mean difference, 0.41 [95% confidence interval {CI, 0.17-0.64}]; P < .001), which was prevented by intraperitoneal glutamine (EG, 0.59 +/- 0.04; mean difference vs E, 0.27 mmol/L [95% CI, 0.03-0.50]; P < .05), indicating glutamine absorption, whereas CG animals had a plasma glutamine of 0.82 +/- 0.07. Tumor necrosis factor alpha was greatly increased by 2-hour endotoxemia (C, 27 +/- 7; E, 2247 +/- 43; mean difference, 2220 pg/mL [95% CI, 2012-2429]; P < .001), and this increase was partly prevented by glutamine (EG, 1991 +/- 91; P < .05 vs E; mean difference, 256; 95% CI, 47-465; P < .05). The effect of glutamine was more pronounced at 6 hours (C, 32 +/- 27; E, 799 +/- 193; EG, 219 +/- 75, C vs E mean difference, 767; 95% CI, 346-1188; P < .001; E vs EG mean difference, 580; 95% CI, 159-1001; P < .01). The IL-10 levels were also greatly increased by 2-hour endotoxemia (C = 55 +/- 21, E = 2429 +/- 58, EG = 1989 +/- 177; C vs E mean difference, 2374; 95% CI, 2740-2008; P < .001; E vs EG mean difference, 440; 95% CI, 74-807; P < .05). Glutamine administration partially prevents the sepsis-induced fall in plasma glutamine levels and reduces the concentration of both proinflammatory and antiinflammatory cytokines.

The equivocal metabolic response to endotoxaemia in type 2 diabetic and obese ZDF rats

AIMS/HYPOTHESIS

The metabolic and endocrine disturbances associated with obesity and type 2 diabetes may impair the normal metabolic response to injury. Our objective was to investigate amino acid metabolism in endotoxaemic type 2 diabetic obese rats.

MATERIALS AND METHODS

A metabolic study was performed over 4 days using male Zucker diabetic fatty (ZDF) rats (fa/fa) and lean littermates (fa/+) divided into three groups: ad libitum-fed groups which underwent no treatment, lipopolysaccharide (LPS)-treated groups receiving E. coli LPS by i.p. injection, and pair-fed groups to the respective LPS groups. We evaluated the effect of endotoxaemia on body weight, food intake and tissue weights. Nitrogen loss and muscular proteolysis were measured daily by determination of urinary 3-methylhistidine (3-MH) excretion. Plasma, intestine and muscle amino acid levels were measured.

RESULTS

The data showed that ad libitum-fed ZDF rats had lower plasma arginine and glutamine levels than ad libitum-fed control rats. Compared with control rats, the LPS-treated ZDF rats presented lower thymic involution, a lower 3-MH:creatinine ratio and higher cumulative nitrogen balance.

CONCLUSIONS/INTERPRETATION

Against our working hypothesis, ZDF rats did not show an impaired metabolic response, and even appeared to be less sensitive to the stress.

NOS3 is involved in the increased protein and arginine metabolic response in muscle during early endotoxemia in mice

Sepsis is a severe catabolic condition. The loss of skeletal muscle protein mass is characterized by enhanced release of the amino acids glutamine and arginine, which (in)directly affects interorgan arginine and the related nitric oxide (NO) synthesis. To establish whether changes in muscle amino acid and protein kinetics are regulated by NO synthesized by nitric oxide synthase-2 or -3 (NOS2 or NOS3), we studied C57BL6/J wild-type (WT), NOS2-deficient (NOS2-/-), and NOS3-deficient (NOS3-/-) mice under control (unstimulated) and lipopolysaccharide (LPS)-treated conditions. Muscle amino acid metabolism was studied across the hindquarter by infusing the stable isotopes L-[ring-2H5]phenylalanine, L-[ring-2H2]tyrosine, L-[guanidino-15N2]arginine, and L-[ureido-13C,2H2]citrulline. Muscle blood flow was measured using radioactive p-aminohippuric acid dilution. Under baseline conditions, muscle blood flow was halved in NOS2-/- mice (P < 0.1), with simultaneous reductions in muscle glutamine, glycine, alanine, arginine release and glutamic acid, citrulline, valine, and leucine uptake (P < 0.1). After LPS treatment, (net) muscle protein synthesis increased in WT and NOS2-/- mice [LPS vs. control: 13 +/- 3 vs. 8 +/- 1 (SE) nmol.10 g(-1).min(-1) (WT), 18 +/- 5 vs. 7 +/- 2 nmol.10 g(-1).min(-1) (NOS2-/-); P < 0.05 for LPS vs. control]. This response was absent in NOS3-/- mice (LPS vs. control: 11 +/- 4 vs. 10 +/- 2 nmol.10 g(-1).min(-1)). In agreement, the increase in muscle arginine turnover after LPS was also absent in NOS3-/- mice. In conclusion, disruption of the NOS2 gene compromises muscle glutamine release and muscle blood flow in control mice, but had only minor effects after LPS. NOS3 activity is crucial for the increase in muscle arginine and protein turnover during early endotoxemia.

The effect of inflammatory stimuli on NMDA-related activation of glutamine synthase in human cultured astroglial cells

Removal of glutamate from the synaptic cleft by astroglial glutamine synthase (GS) is a crucial step in the regulation of glutamate turnover and metabolism, thus participating in endogenous neuroprotective processes occurring within brain tissues. Here we investigated on the effect of inflammatory cytokines on GS activity in astroglial cells undergoing NMDA receptors stimulation. Incubation of human cultured astroglial cells with NMDA (100 microM) enhanced GS expression, an effect driven by the generation of nitric oxide (NO) since l-NAME (500 microM), an inhibitor of NO synthase, reversed this effect. NMDA-related increase of GS activity and glutamine concentration was antagonised by previous incubation of astroglial cells with a mixture of LPS plus gammaIFN, an effect counteracted by dexamethasone, the latter effect being accompanied by inhibition of inducible NO synthase. These results show that LPS plus gammaIFN inhibit elevation of GS activity subsequent to NMDA receptor stimulation in astroglial cells via enhancement of inducible NO synthase, and this may represent the site of interaction between pro-inflammatory and excitotoxic stimuli in the brain.

Characterization of the alteration of nutritional state in brain injury induced by fluid percussion in rats

OBJECTIVE

Patients suffering from traumatic brain injury (TBI) undergo rapid weight loss with negative nitrogen balance and enhanced whole-body protein breakdown, with protein wasting causing morbidity and increased mortality. Many experimental models of TBI have been used to evaluate strategies to improve the outcome of these patients, but nutritional status has not been considered in experiments published to date, although this may have great importance and influence the results obtained with TBI models. This study characterized the hypercatabolism level and nutritional status of TBI rats.

DESIGN

Twenty-four male Wistar rats were randomized into three groups. Rats from the TBI group were anesthetized and fluid percussion was applied. The pair-fed (PF) group was healthy but was pair-fed to the TBI group. The ad libitum (AL) group was healthy and fed ad libitum. The study was performed over 10 days post-TBI.

MEASUREMENTS AND RESULTS

TBI in rats was characterized by remarkable long-lasting anorexia, renal failure (creatinine clearance: AL 1.8+/-0.2 and PF 1.5+/-0.1 vs. TBI 0.9+/-0.1 l/24 hour), anorexia (appetite depressed throughout the study), increased myofibrillar proteolysis (3-methylhistidine/creatinine ratio (day 2: AL 36+/-1 and PF 38+/-2 vs. TBI 54+/-5 micromol/mmol), and intestinal atrophy (ileum: AL 29.3+/-2.5 and PF 28.7+/-1.1 vs. TBI 22.5+/-1.4 mg/cm). In addition, anorexia led to muscular atrophy and decreased nitrogen balance. The metabolic alterations described above can increase morbidity and mortality.

CONCLUSIONS

TBI by fluid percussion in rats is a model reproducing the metabolic and nutritional alterations observed in clinical practice and is suitable for further studies exploring the efficacy of optimized nutritional support.

II. Glutamine and glutamate

Glutamine and glutamate with proline, histidine, arginine and ornithine, comprise 25% of the dietary amino acid intake and constitute the "glutamate family" of amino acids, which are disposed of through conversion to glutamate. Although glutamine has been classified as a nonessential amino acid, in major trauma, major surgery, sepsis, bone marrow transplantation, intense chemotherapy and radiotherapy, when its consumption exceeds its synthesis, it becomes a conditionally essential amino acid. In mammals the physiological levels of glutamine is 650 micromol/l and it is one of the most important substrate for ammoniagenesis in the gut and in the kidney due to its important role in the regulation of acid-base homeostasis. In cells, glutamine is a key link between carbon metabolism of carbohydrates and proteins and plays an important role in the growth of fibroblasts, lymphocytes and enterocytes. It improves nitrogen balance and preserves the concentration of glutamine in skeletal muscle. Deamidation of glutamine via glutaminase produces glutamate a precursor of gamma-amino butyric acid, a neurotransmission inhibitor. L-Glutamic acid is a ubiquitous amino acid present in many foods either in free form or in peptides and proteins. Animal protein may contain from 11 to 22% and plants protein as much as 40% glutamate by weight. The sodium salt of glutamic acid is added to several foods to enhance flavor. L-Glutamate is the most abundant free amino acid in brain and it is the major excitatory neurotransmitter of the vertebrate central nervous system. Most free L-glutamic acid in brain is derived from local synthesis from L-glutamine and Kreb's cycle intermediates. It clearly plays an important role in neuronal differentiation, migration and survival in the developing brain via facilitated Ca++ transport. Glutamate also plays a critical role in synaptic maintenance and plasticity. It contributes to learning and memory through use-dependent changes in synaptic efficacy and plays a role in the formation and function of the cytoskeleton. Glutamine via glutamate is converted to alpha-ketoglutarate, an integral component of the citric acid cycle. It is a component of the antioxidant glutathione and of the polyglutamated folic acid. The cyclization of glutamate produces proline, an amino acid important for synthesis of collagen and connective tissue. Our aim here is to review on some amino acids with high functional priority such as glutamine and to define their effective activity in human health and pathologies.

Glutamine and arginine affect Caco-2 cell proliferation by promotion of nucleotide synthesis

OBJECTIVE

We tested our hypothesis that 1) the major effect of Gln is as a nitrogen donor, not an energy source, for nucleotides (NT) and 2) the supplementation of culture medium with arginine (Arg) decreases the flux of glutamine (Gln) for conversion to Arg, thus accelerating NT synthesis.

METHODS

Various concentrations of nucleosides (NS+NT) Gln, and glutamate (Glu) in culture were tested for their effect on Caco-2 cell proliferation. (Arg was tested in media with and without Gln to evaluate the Gln pathway. The incorporation of (15)N from L-[5-(15)N]-Gln into NTs of DNA was measured under different NS + NT and Arg concentrations.)

RESULTS

The proliferation of Caco-2 cells was increased by NS + NT and Gln supplementation, but not by Glu. The effective concentration of NS + NT was 100-fold smaller than that of Gln. An Arg effect was observed only in the presence of Gln. The NT synthesis from Gln, as indicated by (15)N incorporation from L-[5-(15)N]-Gln, was increased by Arg supplementation and decreased by NS + NT supplementation.

CONCLUSION

These results support our hypothesis that the effects of Gln and Arg on Caco-2 cell proliferation are by the promotion of NT synthesis and that the major role of Gln is not energy supply.

Glutamine metabolism in sepsis and infection

Severe infection causes marked derangements in the flow of glutamine among organs, and these changes are accompanied by significant alterations in regional cell membrane transport and intracellular glutamine metabolism. Skeletal muscle, the major repository of glutamine, exhibits a twofold increase in glutamine release during infection, which is associated with a significant increase in endogenous glutamine biosynthesis. Despite an increase in glutamine synthetase activity in skeletal muscle, the intracellular glutamine pool becomes depleted, indicating that release rates exceed rates of synthesis. Simultaneously, the circulating pool of glutamine does not increase, indicating accelerated uptake by other organs. The liver appears to be the major organ of glutamine uptake in severe infection; studies in endotoxemic rodents have shown net hepatic glutamine uptake to increase by as much as 8- to 10-fold. This increase is due partially to increases in liver blood flow, but also to a three- to fourfold increase in hepatocyte System N activity in the liver. Cytokines and glucocorticoids mediate the increased uptake of glutamine by the liver in septic states as well as other compounds. Sepsis does not appear to induce an increase in System N gene expression, indicating that the increase in hepatic glutamine transport observed during severe infection is probably regulated at the protein level. The bowel displays a decrease in glutamine utilization during sepsis, a response that may be related to the decrease in circulating insulin-like growth factor-1 (IGF-1) levels that is characteristic of sepsis. Recent studies suggest that IGF-1 has a direct effect on stimulating glutamine transport across the gut lumen and thus may represent a therapeutic avenue for improving gut nutrition during severe infection. The cells of the immune system (lymphocytes, macrophages) are also major glutamine consumers during inflammatory states in which cell proliferation is increased. Under these conditions, glutamine availability can become rate limiting for key cell functions, such as phagocytosis and antibody production.

Can a glutamate-enriched diet counteract glutamine depletion in endotoxemic rats?

The study evaluated whether a glutamate-enriched diet would restore glutamine tissue pools and maintain tissue trophicity in endotoxemic rats. For this purpose, young male Sprague-Dawley rats received an intraperitoneal injection of lipopolysaccharide (LPS) from Escherichia coli at 3 mg/kg body weight. After 24 hours of food deprivation, the rats were enterally refed for 48 hours using Osmolite enriched with glutamate at 4 g/kg/d (LPS-Glu group, n = 7) or glycine isonitrogenous to glutamate (LPS-Gly group, n = 7). A control group (healthy group, n = 7) had free access to a standard rodent diet. Tissue weights and protein contents were significantly lower in both LPS-treated groups than in the healthy group. No plasma or tissue accumulation of glutamate was observed except in the liver. Glutamine concentrations were increased in the jejunum, liver, and plasma in the LPS-Glu group versus the other two groups (P < 0.05). Conversely, they were depleted in muscles of the endotoxemic groups versus the healthy group (P < 0.05). Villus height was significantly greater in the LPS-Glu group than in the LPS-Gly group in the jejunum (P < 0.05), but not in the ileum. In conclusion, a glutamate-enriched diet administered enterally to endotoxemic rats can counteract glutamine depletion in the splanchnic area but not in muscles. In addition, glutamate displayed a trophic effect restricted to the jejunum.

Identification of glucocorticoid-responsive elements that control transcription of rat glutamine synthetase

Basal expression of glutamine synthetase (GS) is very low in rat lung and muscle and remarkably enhanced by glucocorticoid hormones during trauma and catabolic states. Although this response is believed to be transcriptionally regulated, the genetic elements responsible for tissue-specific glucocorticoid induction of GS expression have not been identified. A rat lung epithelial cell line (L2) and a glucocorticoid receptor-deficient human prostate cancer cell line (PC3), together with GS reporter gene constructs, were utilized in gene transfer experiments to identify two regions within the rat genomic clone gGS3 that imparted dexamethasone (Dex) responsiveness to both the homologous GS promoter and the heterologous herpes simplex virus thymidine kinase promoter in glucocorticoid receptor-dependent fashions. One region lies nearly 6 kb upstream of the GS transcription initiation site, and the other lies within the first intron of the GS gene. Dex responsiveness was localized to a 325-bp fragment of the intron region containing a canonical glucocorticoid response element and to a 225-bp fragment of the far-upstream region containing three separate glucocorticoid response element half-sites. The GS promoter exhibited relatively high basal activity that was repressed by inclusion of the far-upstream or the intron glucocorticoid-responsive region. Dex treatment negated this repression. A model is suggested in which the glucocorticoid-receptor unit causes derepression of lung and muscle GS transcription during trauma and catabolic states.

Counter-regulatory hormones and mechanisms in amino acid metabolism with special reference to the catabolic response in skeletal muscle

There is evidence that both counter-regulatory hormones, in particular glucocorticoids, and cytokines influence amino acid and protein metabolism in skeletal muscle, and that these two groups of regulators interact in the development of muscle catabolism. Glucocorticoids stimulate muscle proteolysis during sepsis and also in other catabolic conditions. In addition, glucocorticoids regulate muscle glutamine metabolism, resulting in increased glutamine release and reduced glutamine concentrations in skeletal muscle. Glucocorticoids inhibit the glutamine transporter in skeletal muscle and stimulate glutamine synthetase activity. Proinflammatory cytokines, in particular tumor necrosis factor and interleukin-1, inhibit muscle amino acid transport by system A, and these cytokine effects are probably indirect. Most of the catabolic effects of tumor necrosis factor in skeletal muscle, including stimulated protein degradation and inhibited amino acid uptake, are mediated by glucocorticoids.

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.

Plasma and muscle amino acid levels in relation to resting energy expenditure and inflammation in stable chronic obstructive pulmonary disease

In patients with chronic obstructive pulmonary disease (COPD), muscle wasting can occur independently of fat loss, suggesting disturbances in protein metabolism. In order to provide more insight in amino-acid (AA) metabolism in patients with stable COPD, we examined arterial plasma and anterior tibialis muscle AA levels, comparing 12 COPD patients with eight age-matched healthy control subjects. We also studied relationships between AA levels, the acute phase response as measured by lipopolysaccharide-binding protein (LBP), and resting energy expenditure (REE). In contrast to findings in acute diseases associated with muscle wasting, we found increased muscle glutamine (GLN) levels in our patient group (mean +/- SEM = 10,782 +/- 770 versus 7,844 +/- 293 micromol/kg wet weight, p < 0. 01). Furthermore, muscle arginine, ornithine, and citrulline were significantly increased in the patient group, whereas glutamic acid was decreased. In plasma, the sum of all AA (SumAA) was decreased in the patient group (2,595 +/- 65 versus 2,894 +/- 66 micromol/L, p < 0.01), largely because of decreased levels of alanine (254 +/- 10 versus 375 +/- 25 micromol/L, p < 0.0001), GLN (580 +/- 17 versus 641 +/- 17 micromol/L, p < 0.05), and glutamic acid (91 +/- 5 versus 130 +/- 10 micromol/L, p < 0.01). LBP levels were increased in COPD patients as compared with controls (11.7 +/- 4.5 versus 8.6 +/- 1.0 mg/L, p < 0.05), and showed a positive correlation with REE (r = 0. 49, p = 0.03), a negative correlation with the SumAA in plasma (r = -0.76, p < 0.0001), and no correlation with muscle AA levels. In conclusion, various disturbances in plasma and muscle AA levels were found in COPD patients. A relationship between the observed decreased plasma AA levels and inflammation was suggested.

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.

Effects of endotoxin challenge on hepatic amino acid transport during cancer

BACKGROUND

The hepatic uptake of amino acids is increased in both sepsis and cancer, and this response appears to be both global and essential in the catabolic host. Because immunocompromised cancer patients are susceptible to episodes of gram-negative sepsis, we examined the capacity of hepatocytes from normal and tumor-influenced livers to respond to the additional challenge of endotoxemia via increases in the Na+-dependent uptake of glutamine and zwitterionic amino acids by System N and System A, respectively.

MATERIALS AND METHODS

Fischer 344 rats were implanted with methylcholanthrene-induced fibrosarcomas. Control rats were sham-operated and pair-fed. Animal pairs (tumor burden = 8-32% carcass weight) were injected intraperitoneally with either Escherichia coli endotoxin (10 mg/kg) or PBS, and after 4 h, hepatocytes were isolated from the livers of the animals via collagenase perfusion and placed in primary culture. Three hours later, amino acid transport rates were measured using radiolabeled glutamine for System N and alpha-methylaminoisobutyric acid (MeAIB), a nonmetabolizable substrate specific for System A.

RESULTS

Cancer-independent of tumor size-and endotoxin each elicited similar 1.5- to 2-fold inductions of System N activity. When combined, their effects were additive rather than synergistic. In contrast, endotoxin induced an insignificant increase in System A activity, whereas cancer stimulated this carrier 2-fold in either the absence or the presence of endotoxin.

CONCLUSIONS

The primary glutamine and alanine carriers in hepatocytes are differentially influenced during catabolic states, and the tumor-influenced liver is competent to further increase glutamine uptake in response to additional catabolic insults.

Measurement of glutamine synthetase activity in rat muscle by a colorimetric assay

Glutamine synthetase catalyses the formation of L-Gln from L-Glu and NH4+. This enzyme also exerts a glutamyl-transferase activity that produces gamma-glutamyl-hydroxamate from Gln and hydroxylamine. This gamma-glutamyl-transfer reaction can be used to determine glutamine synthetase activity by colorimetric assay. This method has never been applied to rat muscle. The aim of this work was to study and optimize the glutamine synthetase assay conditions in rat muscle. Enzyme activity was linear with time of incubation (30 min at 37 degrees C) and linear with enzyme concentration in the incubation medium. The method was specific. In addition, this assay correlated well with a radiometric assay (y = 0.76x + 340, where x and y are the glutamine synthetase activities measured by radiometry and colorimetry respectively; r = 0.94; P = 0.05). Finally, no glutamine synthetase activity was found in muscles of rats treated with methionine sulfoximine, an inhibitor of glutamine synthetase, and activity dramatically rose in muscles from rats treated with dexamethasone, an activator of glutamine synthetase (in extensor digitorum longus: 2717 +/- 54 nmol/min/g protein in dexamethasone-treated rats versus 1228 +/- 114 nmol/min/g protein in control rats, P < 0.0001). In conclusion, the method presented here is accurate and reliable for measurement of glutamine synthetase activity in muscles.

Characterization of glutamine synthetase transcript, protein, and enzyme activity in the human placenta

This study characterizes the molecular mechanisms necessary for glutamine synthesis in the human placenta. RNA hybridization and protein immunoblotting were used to verify the presence of glutamine synthetase (GS) transcripts and protein, respectively. Additionally, the presence of GS was determined by immunohistochemistry. RNA hybridization demonstrated the presence of 1.8- and 2.8-kB transcripts and protein immunoblotting yielded a single 49-kDa band, characteristics of GS transcripts and protein, respectively. The mean (+/- s.d.) specific activity of placental GS, expressed as mumol gamma-glutamyl hydroxamic acid/mg protein/h was 1.80 +/- 0.59, which is comparable to other organs which are net glutamine producers. Immunohistochemical analysis indicated the presence of GS within the cytotrophoblast and mesenchyme layers of placental villi, but not in the syncytiotrophoblast. Although these results suggest that the human placenta is capable of synthesizing glutamine, the fate of glutamine produced by this organ remains speculative.

Gut endotoxin restriction prevents catabolic changes in glutamine metabolism after surgery in the bile duct-ligated rat

OBJECTIVE

The objective of this study was to investigate the role of gut-derived endotoxemia in postoperative glutamine (GLN) metabolism of bile duct-ligated rats.

SUMMARY BACKGROUND DATA

Postoperative complications in patients with obstructive jaundice are associated with gut-derived endotoxemia. In experimental endotoxemia, catabolic changes in GLN metabolism have been reported. Glutamine balance is considered important in preventing postsurgical complications.

METHODS

Male Wistar rats were treated orally with the endotoxin binder cholestyramine (n = 24, 150 mg/day) or saline (n = 24). On day 7, groups received a SHAM operation or a bile duct ligation (BDL). On day 21, all rats were subjected to a laparotomy followed 24 hours later by blood flow measurements and blood sampling. Glutamine organ handling was determined for the gut, liver, and one hindlimb. Intracellular GLN muscle concentrations were determined.

RESULTS

Compared to the SHAM groups, BDL rats showed lower gut uptake of GLN (28%, p < 0.05); a reversal of liver GLN release to an uptake (p < 0.05); higher GLN release from the hindlimb (p < 0.05); and lower intracellular muscle GLN concentration (32%, p < 0.05). Cholestyramine treatment in BDL rats maintained GLN organ handling and muscle GLN concentrations at SHAM levels.

CONCLUSIONS

Disturbances in postoperative GLN metabolism in BDL rats can be prevented by gut endotoxin restriction. Gut-derived endotoxemia after surgery in obstructive jaundice dictates GLN metabolism.

Alanyl-glutamine-supplemented total parenteral nutrition improves survival and protein metabolism in rat protracted bacterial peritonitis model

BACKGROUND

The effects of glutamine-enriched total parenteral nutrition (TPN) solution on survival, and protein turnover in the whole body and in individual organs were investigated in a rat protracted peritonitis model.

METHODS

Twenty-three rats underwent venous catheter insertion. Osmotic pumps were implanted in the peritoneal cavity to allow continuous delivery of Escherichia coli (4 x 10(8) CFU/d). The conventional TPN group received a conventional amino acid solution. The Ala-Gln TPN group received an alanyl-glutamine-enriched TPN solution. The two TPN solutions were isocaloric and isonitrogenous.

RESULTS

Over the 5 days of TPN treatment, the survival rate of the Ala-Gln group was significantly higher than that of the conventional group. The Ala-Gln group tended to have increased whole-body protein turnover compared with the conventional group. Fractional protein synthetic rates (FSR) in the liver and gastrocnemius muscle of the Ala-Gln group were significantly higher than those of the conventional group. The serum glutamine concentration correlated positively with the FSR of both liver and muscle. The Ala-Gln group showed significantly greater mucosal height and mitoses per crypt, in the small intestine, than did the conventional group.

CONCLUSIONS

Our results suggested that, in comparison with standard glutamine-free TPN, Ala-Gln-supplemented TPN increases protein synthesis in the liver and skeletal muscle, protects the morphology of the intestinal mucosa, and improves survival in protracted bacterial peritonitis. Ala-Gln supplementation may be useful in septic patients.

Ornithine alpha-ketoglutarate counteracts thymus involution and glutamine depletion in endotoxemic rats

This work studied the action of ornithine a-ketoglutarate (OKG) supplementation in an experimental model of endotoxemia in the rat. Male Wistar rats were injected intraperitoneally with lipopolysaccharide (LPS) from Escherichia coli (0127:B8). They were fasted for 24 h, then refed for 48 h with an enteral diet supplemented with either OKG (66 mg N x kg(-1) x d(-1)) or glycine, isonitrogenous to the OKG group. A control (sham) group was also studied. LPS treatment induced a decrease in thymus and muscle weights compared to controls, and a decrease in glutamine and arginine concentrations in the anterior tibialis muscle. Supplementation with OKG restored thymus weight and muscle arginine level and increased muscle glutamine concentration, when compared to controls. We conclude that OKG counteracts the thymic involution that occurs with endotoxemia, and restores the muscular content of glutamine and arginine, both of which are involved in the regulation of immune function.

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

BACKGROUND

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Starvation and endotoxin act independently and synergistically to coordinate hepatic glutamine transport

OBJECTIVE

Because hepatic glutamine transport is markedly enhanced during critical illness, we tested the hypothesis that nutrient starvation and endotoxemia act coordinately to augment transport activity.

DESIGN

Fed or starved (48 hours) rats received Escherichia coli endotoxin (LPS, 10 mg/kg of body weight, intraperitoneally) or saline before hepatocyte isolation for measurement of glutamine transport.

MATERIALS AND METHODS

Hepatocytes were isolated from fed or fasted rats 4 hours after LPS treatment. [3H]glutamine uptake was measured and normalized to cellular protein. Data (mean +/- standard deviation, three separate determinations) were analyzed by Student's t test and analysis of variance.

MAIN RESULTS

Starvation induced a 1.6-fold increase in glutamine transport, while LPS treatment of fed rats increased transport activity 2.6-fold. Treatment of fasted animals with LPS induced a sixfold increase in glutamine transport. Kinetically, this effect in endotoxemic starved rats was mediated by both an increase in System N Vmax and the induction of a high affinity System A amino acid carrier which transports glutamine.

CONCLUSIONS

Starvation and endotoxemia regulate hepatocyte glutamine transport independently and synergistically. This hepatic response provides glutamine and other amino acids to support key metabolic pathways in the liver during critical illness.

Glutamine

Glutamine is the most abundant free amino acid in the circulation. It is a primary fuel for rapidly dividing cells and plays a key role in the transport of nitrogen between organs. Although glutamine is absent from conventional regimens aimed at nutritional support, glutamine deficiency can occur during periods of metabolic stress; this has led to the reclassification of glutamine as a conditionally essential amino acid. Experiments with various animal models have demonstrated that the provision of glutamine can result in better nitrogen homoeostasis, with conservation of skeletal muscle. There is also considerable evidence that glutamine can enhance the barrier function of the gut. This review concludes by discussing the clinical evidence that supports the inclusion of stable forms of glutamine in solutions of nutrients.

Detection of a functional promoter/enhancer in an intron-less human gene encoding a glutamine synthetase-like enzyme

A human genomic clone, psi GS, containing an intron-less glutamine synthetase (GS)-encoding pseudogene, was isolated by screening a human library. A sequence of 3004 bp, containing the GS coding region and both the 5' and 3' flanking sequences, was identified that exhibits all the characteristics of a processed pseudogene. The coding region shows 93% identity with the human GS cDNA (hGS) sequence and contains two frame-shifts and two termination codons. The coding sequence is flanked by a 9-bp AT repeat and a putative polyadenylation site, AATAAA, at the 3' end. Primer extension analysis and S1 nuclease mapping showed a transcription start point (tsp) 62 bp upstream from the start codon indicating a shorter untranslated region than hGS. Transfection of HeLa cells with cat constructs containing portions of the 5' flanking sequence showed the presence of a functional promoter/enhancer within 200 bp of the tsp, independent of its orientation.

Cytokine control of nutrition and metabolism in critical illness

During the last two decades, major advances in technology and in our fundamental understanding of the biologic aspects of sepsis and cancer cachexia have dramatically affected the therapeutic strategies available to the surgeon to care for critically ill patients. It is clear, however, that cytokines affect whole body nutrition and metabolism and are responsible for many of the clinically observed nutritional effects of injury, infection, and cancer, including fever, hypermetabolism, anorexia, protein catabolism, cachexia, and altered fat, glucose, and trace mineral metabolism. These metabolic and nutritional effects of cytokines are influenced by the nutritional status of the host, which is generally altered during the course of the critical illness. In the future, the use of specialized diets and the use of selective cytokine blockade are likely to be important components of the overall care of the catabolic patient.

Glutamine and cancer

OBJECTIVE

This overview on glutamine and cancer discusses the importance of glutamine for tumor growth, summarizes the alterations in interorgan glutamine metabolism that develop in the tumor-bearing host, and reviews the potential benefits of glutamine nutrition in the patient with cancer.

SUMMARY BACKGROUND DATA

Glutamine is the most abundant amino acid in the blood and tissues. It is essential for tumor growth and marked changes in organ glutamine metabolism are characteristic of the host with cancer. Because host glutamine depletion has adverse effects, it is important to study the regulation of glutamine metabolism in cancer and to evaluate the impact of glutamine nutrition in the tumor-bearing state.

METHODS

Data from a variety of investigations on glutamine metabolism and nutrition related to the host with cancer were compiled and summarized.

RESULTS

Numerous studies on glutamine metabolism in cancer indicate that many tumors are avid glutamine consumers in vivo and in vitro. As a consequence of progressive tumor growth, host glutamine depletion develops and becomes a hallmark. This glutamine depletion occurs in part because the tumor behaves as a "glutamine trap" but also because of cytokine-mediated alterations in glutamine metabolism in host tissues. Animal and human studies that have investigated the use of glutamine-supplemented nutrition in the host with cancer suggest that pharmacologic doses of dietary glutamine may be beneficial.

CONCLUSIONS

Understanding the control of glutamine metabolism in the tumor-bearing host not only improves the knowledge of metabolic regulation in the patient with cancer but also will lead to improved nutritional support regimens targeted to benefit the host.

Glutamine peptide-supplemented long-term total parenteral nutrition: effects on intracellular and extracellular amino acid patterns, nitrogen economy, and tissue morphology in growing rats

Glutamine (GLN) is a nonessential amino acid that is not included in current regimens for parenteral nutrition because of its chemical instability. This study tested the hypothesis that GLN supplementation during long-term total parenteral nutrition (TPN) (3 weeks) would enhance GLN availability, thereby improving nitrogen economy and growth in a growing rat model: Standard TPN delivering 300 kcal/kg per day (lipid:carbohydrate = 1.1) including 2.1 g of nitrogen per kilogram per day in an all-in-one solution was compared with an isonitrogenous, isocaloric, and isovolemic TPN regimen with 0.29 g of nitrogen per kilogram per day substituted by GLN derived from the dipeptides glycyl-GLN and alanyl-GLN (TPN GLN). Enterally fed controls were included. Analysis was confined to nonbacteremic animals with negative blood culture, in which extracellular and intracellular amino acid concentrations including GLN, nitrogen balance, serum protein concentrations, growth, and histologic sections of liver and small-bowel mucosa (light and scanning electron microscopy) were evaluated. Hepatic intracellular GLN concentrations were significantly lower, in animals receiving GLN-free TPN (11.7 +/- 1.6 nmol/mg fat-free dry and solid tissue mass, n = 9) compared with both GLN-supplemented TPN (16.0 +/- 3.0, n = 7) and enteral feeding (18.2 +/- 1.8, n = 6) (p < .001). Corresponding results were found for intracellular GLN concentrations in skeletal muscle (TPN standard 12.5 +/- 3.1, TPN GLN 14.7 +/- 3.1, enteral control 17.3 +/- 2.3, p < .05), intestinal mucosa, and spleen as well as for plasma concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of progressive tumor growth on glutamine metabolism in skeletal muscle and kidney

OBJECTIVE

The effects of progressive malignant growth on glutamine metabolism in skeletal muscle and in kidney were investigated.

SUMMARY BACKGROUND DATA

Fast-growing tumors consume considerable quantities of glutamine and lead to a decrease in circulating glutamine concentrations.

METHODS

Experiments were performed at various stages of tumor growth in rats implanted subcutaneously with the non-metastasizing methylcholanthrene-induced (MCA) fibrosarcoma and in pair-fed non tumor-bearing controls.

RESULTS

Tumor growth stimulated a twofold increase in hindquarter (muscle) glutamine release, which was not due to an increase in blood flow, but rather to a doubling in the fractional release rate. Consequently, a progressive decrease in skeletal muscle glutamine concentrations was observed over time. Simultaneously, the activity of glutamine synthetase (GS), the principal enzyme of de novo glutamine biosynthesis, increased more than twofold. This increase in muscle GS activity was accompanied by an increase in GS mRNA but the augmentation in GS expression apparently could not match the increased rate of efflux since muscle depletion developed. In rats with large tumors and severe glutamine depletion, GS activity was not elevated. Glutamine feeding increased muscle glutamine concentrations and glutamine synthetase specific activity. Although tumor growth led to the development of mild systemic acidemia, the classic renal adaptations normally observed, i.e., elevated glutaminase activity and accelerated renal glutamine utilization, were not present in acidotic tumor-bearing rats. Instead, renal GS activity was increased in tumor-bearing animals and ammoniagenesis was enhanced, in spite of a reduction in net renal glutamine uptake.

CONCLUSIONS

These data suggest that marked alterations in muscle and renal glutamine handling occur in the host with cancer; the enhanced muscle glutamine release in conjunction with no increase in renal consumption is consistent with increased glutamine uptake in other organs, most likely the tumor itself and the liver.