Hepatic glutamine metabolism

An optimized method for estimating glutaminase activity in biological samples

Spectrophotometric methodologies have been used to assess glutaminase activity, for which coloured complexes have been developed that measure spectrophotometry across the visible spectrum using different reagents. The present paper describes a precise, simple and reliable procedure for quantifying glutaminase activity, which is a key enzyme in glutamine hydrolysis and also involved in glutamine metabolism regulation. The procedure presented here measures glutaminase activity by incubating glutaminase enzyme at 37 °C for 20 min with a glutamine substrate dissolved in a buffer (pH 8.6). The enzymatic reaction contains suitable activity of glutamate oxidase, which acts to convert glutamate to hydrogen peroxide and 2-oxoglutarate. To terminate the enzymatic activity, a working solution containing pyridine-2,6-dicarboxylic (PDA) acid and ammonium vanadate (AV) was added following incubation. Oxo-peroxo-pyridine-2,6-dicarboxylato-vanadate (OPDV), a stable orange-coloured chelate complex measuring 435 nm spectrophotometrically, was produced by the interaction between the generated hydrogen peroxide and the supplied reagent. Using the response surface methodology (RSM) as an indicator of the assay's accuracy, we employed the Box-Behnken design (BBD) to improve the method's design (the OPDV-Glutaminase assay). Improvement factors were the volume of working reagent solution (PDA/AV), volume of glutamate oxidase solution (GO), and incubation time. In matched samples, this novel method was verified against a Bland-Altman plot assessment of glutaminase activity using the indophenol methodology. A correlation value of 0.99 between the two methods' comparisons showed that the novel protocol was equally applicable to the reference method.

Impact of L-ornithine L-aspartate on non-alcoholic steatohepatitis-associated hyperammonemia and muscle alterations

BACKGROUND

Metabolic dysfunction-associated fatty liver disease (MAFLD) is the most common chronic liver disease in the world. Progression toward non-alcoholic steatohepatitis (NASH) is associated with alterations of skeletal muscle. One plausible mechanism for altered muscle compartment in liver disease is changes in ammonia metabolism. In the present study, we explored the hypothesis that NASH-associated hyperammonemia drives muscle changes as well as liver disease progression.

MATERIALS AND METHODS

In Alms1-mutant mice (foz/foz) fed a 60% fat diet (HFD) for 12 weeks; we investigated hepatic and muscular ammonia detoxification efficiency. We then tested the effect of an 8 week-long supplementation with L-ornithine L-aspartate (LOLA), a known ammonia-lowering treatment, given after either 4 or 12 weeks of HFD for a preventive or a curative intervention, respectively. We monitored body composition, liver and muscle state by micro computed tomography (micro-CT) as well as muscle strength by four-limb grip test.

RESULTS

According to previous studies, 12 weeks of HFD induced NASH in all foz/foz mice. Increase of hepatic ammonia production and alterations of urea cycle efficiency were observed, leading to hyperammonemia. Concomitantly mice developed marked myosteatosis. First signs of myopenia occurred after 20 weeks of diet. Early LOLA treatment given during NASH development, but not its administration in a curative regimen, efficiently prevented myosteatosis and muscle quality, but barely impacted liver disease or, surprisingly, ammonia detoxification.

CONCLUSION

Our study confirms the perturbation of hepatic ammonia detoxification pathways in NASH. Results from the interventional experiments suggest a direct beneficial impact of LOLA on skeletal muscle during NASH development, though it does not improve ammonia metabolism or liver disease.

Survival strategies of Wuchang bream (Megalobrama amblycephala) juveniles for chronic ammonia exposure: Antioxidant defense and the synthesis of urea and glutamine

This study aimed to explore how Wuchang bream (Megalobrama amblycephala) survive and defend against the toxicity of ambient total ammonia nitrogen (0, 5, 10, 20 and 30 mg/L TA-N) during 30-day exposure. As a result, hepatic malondialdehyde and protein carbonylation as well as histopathological alterations increased with increasing TA-N level, which suggested that chronic ammonia exposure caused oxidative stress and damage in the liver of fish. Meanwhile, the activities of hepatic total superoxide dismutase (T-SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione reductase (GR) and glucose 6-phosphate dehydrogenase (G6PD) as well as the mRNA expression of Cu/Zn sod, cat, gpx and g6pd were elevated significantly along with significant reduction of glutathione (GSH) and nicotinamide adenine dinucleotide phosphate (NADPH) (P < 0.05). These results indicated that hepatic antioxidant responses were activated to alleviate oxidative damages induced by ammonia, in which lower-concentration ammonia only initiate SOD-CAT-GR-G6PDH defense and higher ammonia activated the SOD-CAT-GPx-GSH-GR-G6PDH antioxidant response. In addition, significant increases of serum urea and hepatic ammonia, urea, glutamine, arginase as well as glutamine synthetase were detected with the increase of TA-N (P < 0.05), while serum ammonia levels kept stable (P > 0.05). The present findings further revealed that ammonia could be detoxified directly into glutamine and urea in Wuchang bream to cope with ammonia exposure. In conclusion, under chronic ammonia exposure, enhanced hepatic antioxidant responses as well as increased urea and glutamine synthesis worked in combination to allow Megalobrama amblycephala to defend against environmental ammonia toxicity.

Monosodium Glutamate Supplementation Improves Bone Status in Mice Under Moderate Protein Restriction

Adequate protein intake during development is critical to ensure optimal bone gain and to attain a higher peak bone mass later. Using a mild protein restriction model in Balb/C mice consuming 6% of their total energy intake as soy protein (LP-SOY)-for which we observed a significantly lower femoral cortical thickness, bone volume, trabecular number, and thickness reduction-we evaluated the effects of monosodium glutamate (MSG) supplementation at different concentrations (0.5, 1, 5, 10, and 20 g/kg of diet) on bone characteristics in LP-SOY-fed mice. After 6 and 12 weeks, LP-SOY-fed mice had lower BMD and reduced body weight related to lower lean mass, which was associated with a reduced IGF-1 level. The negative effect of the LP-SOY diet on BMD correlated with impaired bone formation. MSG supplementation, at 5, 10, and 20 g/kg of diet, and PTH injection, used as a positive control, were able to improve BMD and to increase osteoblast activity markers (P1NP and osteocalcin), as well as glutamine plasma concentration. An analysis of bone microarchitecture found that cortical bone was less sensitive to protein restriction than trabecular bone, and that MSG ingestion was able to preserve bone quality through an increase of collagen synthesis, although it did not allow normal bone growth. Our study reinforces the view that glutamate can act as a functional amino acid for bone physiology and support clinical investigation of glutamate supplementation in adults characterized by poor bone status, notably as a result of insufficient protein intake. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Brachyantheraoside A8, a new natural nor-oleanane triterpenoid as a kidney-type glutaminase inhibitor fromStauntonia brachyanthera

With the aim of finding a better kidney-type glutaminase (KGA) inhibitor with potential anti-cancer properties, 18 nor-oleanane triterpenoids fromStauntonia brachyanthera, including 2 new ones, were screened against KGA.

PPARα Downregulates Hepatic Glutaminase Expression in Mice Fed Diets with Different Protein:Carbohydrate Ratios

BACKGROUND

Glutamine is catabolized in the liver by glutaminase 2 (GLS2). Evidence suggests that peroxisome proliferator-activated receptor α (PPARα) represses the expression of several amino acid-catabolizing enzymes, but for Gls2 this is unknown.

OBJECTIVE

The aim of the study was to assess whether PPARα regulates Gls2 expression.

METHODS

For 8 d, 7-9-wk-old male C57BL/6 wild-type (WT) and Ppara-null mice weighing 23.4 ± 0.5 g were fed diets with different dietary protein:carbohydrate (DP:DCH) ratios (6%:77%, 20%:63%, or 50%:33%). Liver samples were obtained after 16 h of feed deprivation or 3 h of refeeding, and microarrays were performed. Hepatic glutaminase expression was measured by quantitative polymerase chain reaction and Western blotting. Cotransfection analyses in hepatocellular carcinoma cell line (HepG2) cells with PPARα and hepatocyte nuclear factor 4α (HNF4α) expression vectors were performed.

RESULTS

The microarray results showed that Gls2 was the only upregulated gene in WT mice, but not in the Ppara-null mice. In the feed-deprived WT mice, the Gls2 mRNA and protein abundances in the 50%:33% group were 2.5- and 1.1-fold greater (P < 0.05), respectively, than those in the 20%:63% group, which were 2.3- and 0.4-fold greater than those in the 6%:77% group (P < 0.01). Gls2 mRNA expression in the 6%:77% group of feed-deprived Ppara-null mice was 33-fold greater than that in the same group of WT mice (P < 0.0001). GLS2 protein abundance in HepG2 cells was 78% greater than that in the controls (P < 0.0001) after HNF4α overexpression, and it was 99% greater after transfection with a short hairpin targeting PPARα.

CONCLUSIONS

In Ppara-null mice, Gls2 mRNA expression was greater than in WT mice, regardless of the DP:DCH ratio. In HepG2 cells overexpressing HNF4α, Gls2 expression increased, an effect repressed by overexpression of PPARα. This suggests that Gls2 depends on the PPARα/HNF4α counterregulatory transcriptional control.

The Metabolic Responses to L-Glutamine of Livers from Rats with Diabetes Types 1 and 2

There are several claims about the beneficial effects of supplementing L-glutamine to both type 1 and type 2 diabetes. The purpose of the present study was to provide detailed knowledge about the fate of this amino acid in the liver, the first organ that receives the compound when ingested orally. The study was done using the isolated perfused rat liver, an experimental system that preserves the microcirculation of the organ and that allows to measured several parameters during steady-state and pre steady-state conditions. L-Glutamine was infused in the portal vein (5 mM) and several parameters were monitored. Livers from type 1 diabetic rats showed an accelerated response to L-glutamine infusion. In consequence of this accelerated response livers from type 1 diabetic rats presented higher rates of ammonia, urea, glucose and lactate output during the first 25–30 minutes following L-glutamine infusion. As steady-state conditions approached, however, the difference between type 1 diabetes and control livers tended to disappear. Measurement of the glycogen content over a period of 100 minutes revealed that, excepting the initial phase of the L-glutamine infusion, the increased glucose output in livers from type 1 diabetic rats was mainly due to accelerated glycogenolysis. Livers from type 2 diabetic rats behaved similarly to control livers with no accelerated glucose output but with increased L-alanine production. L-Alanine is important for the pancreatic β-cells and from this point of view the oral intake of L-glutamine can be regarded as beneficial. Furthermore, the lack of increased glucose output in livers from type 2 diabetic rats is consistent with observations that even daily L-glutamine doses of 30 g do not increase the glycemic levels in well controlled type 2 diabetes patients.

Prevalence of glutamine deficiency in ICU patients: a cross-sectional analytical study

BACKGROUND

Not only is glutamine deficiency an independent predictor of mortality in intensive care unit (ICU) patients, but glutamine supplementation is also recommended for its proven outcome benefits. However, recent data suggest that early glutamine supplementation in certain patient groups increase mortality. The aim of this study was to investigate plasma glutamine levels of adult ICU patients in the South African setting and to determine relationships between glutamine levels, gender, diagnostic categories and selected inflammatory markers. The data from this study will be used as baseline measurement to support a large scale study that will be undertaken in the South African ICU population.

METHODS

This cross-sectional, analytical study included 60 mixed adult ICU patients within 24 h post ICU admission. Plasma glutamine levels were determined on admission. The relationship between glutamine levels, Interleukin-6 (IL-6) and C-reactive protein (CRP); as well as gender- and diagnosis-related differences in glutamine levels were also investigated. A non-parametric ROC curve was computed to determine the CRP concentration cut-off point above which glutamine becomes deficient.

RESULTS

The median plasma glutamine level (497 μmol/L) was in the normal range; however, 38.3 % (n = 23) of patients had deficient (<420 μmol/L) and 6.7 % (n = 4) had supra-normal glutamine levels (>930 μmol/L). No significant difference could be detected between glutamine levels and gender or diagnosis categories as a group. When only the medical and surgical categories were compared, the median plasma glutamine level of the medical patients were significantly lower than that of the surgical patients (p = 0.042). Glutamine showed inverse associations with CRP levels (r = -0.44, p < 0.05) and IL-6 concentrations (r = -0.23, p = 0.08). A CRP cut-off value of 95.5 mg/L was determined above which glutamine levels became deficient.

CONCLUSIONS

About a third of patients (38 %) were glutamine deficient on admission to ICU, whereas some presented with supra-normal levels. While glutamine levels correlated inversely with inflammatory markers, and a CRP value of above 95.5 mg/L indicated potential glutamine deficiency, the clinical application of this finding needs further investigation.

Alanyl-glutamine supplementation regulates mTOR and ubiquitin proteasome proteolysis signaling pathways in piglets

OBJECTIVE

The aim of the present study was to investigate the effects of the alanyl-glutamine dipeptide (Ala-Gln) or the combination supplementation of free alanine and glutamine (Ala+Gln) on the mammalian target of rapamycin (mTOR) and ubiquitin-proteasome proteolysis (UPP) signaling pathways in piglets.

METHODS

We randomly allocated 180 piglets to three treatments with three replicates of 20 piglets each, fed with diets containing 0.62% Ala, 0.5% Ala-Gln, 0.21% Ala+0.34% Gln, respectively. The duration of the experiment was 28 d.

RESULTS

The results showed that Ala-Gln increased average daily gain of piglets, and decreased the ratio of feed to gain (P < 0.05). Ala-Gln supplementation increased the concentrations of Gln and glutamate and decreased the activity of glutamine synthetase in liver and skeletal muscle (P < 0.05). Ala-Gln increased the expression of glutaminase and glutamate dehydrogenate (P < 0.05). The increased phosphorylation of eIF-4 E binding protein 1 (4E-BP1) and ribosomal protein S6 kinase 1 (S6K1) in Ala-Gln treatment were associated with phosphorylation of the mTOR in liver and skeletal muscle. Ala+Gln did not affect the phosphorylation abundances of mTOR, 4E-BP1, or S6K1 (P > 0.05). Ala-Gln supplementation inhibited the mRNA expressions of MAFbx and MuRF1 in skeletal muscle of piglets (P < 0.05).

CONCLUSION

Taken together, Ala-Gln supplementation improved the growth performance of piglets, enhanced the metabolism of Gln, upregulated protein synthetic signaling in liver and skeletal muscle and decreased protein degradative signaling in muscle of piglets. Moreover, these effects of Ala-Gln were more effective than those of Ala+Gln.

Glutamine metabolism in advanced age

Glutamine, reviewed extensively in the last century, is a key substrate for the splanchnic bed in the whole body and is a nutrient of particular interest in gastrointestinal research. A marked decrease in the plasma glutamine concentration has recently been observed in neonates and adults during acute illness and stress. Although some studies in newborns have shown parenteral and enteral supplementation with glutamine to be of benefit (by decreasing proteolysis and activating the immune system), clinical trials have not demonstrated prolonged advantages such as reductions in mortality or risk of infections in adults. In addition, glutamine is not able to combat the muscle wasting associated with disease or age-related sarcopenia. Oral glutamine supplementation initiated before advanced age in rats increases gut mass and improves the villus height of mucosa, thereby preventing the gut atrophy encountered in advanced age. Enterocytes from very old rats continuously metabolize glutamine into citrulline, which allowed, for the first time, the use of citrulline as a noninvasive marker of intestinal atrophy induced by advanced age.

Mass spectrometry-based metabolite profiling in the mouse liver following exposure to ultraviolet B radiation

Although many studies have been performed on the effects of ultraviolet (UV) radiation on the skin, only a limited number of reports have investigated these effects on non-skin tissue. This study aimed to describe the metabolite changes in the liver of hairless mice following chronic exposure to UVB radiation. We did not observe significant macroscopic changes or alterations in hepatic cholesterol and triglyceride levels in the liver of UVB-irradiated mice, compared with those for normal mice. In this study, we detected hepatic metabolite changes by UVB exposure and identified several amino acids, fatty acids, nucleosides, carbohydrates, phospholipids, lysophospholipids, and taurine-conjugated cholic acids as candidate biomarkers in response to UVB radiation in the mouse liver by using various mass spectrometry (MS)-based metabolite profiling including ultra-performance liquid chromatography-quadrupole time-of-flight (TOF)-MS, gas chromatography-TOF-MS and nanomate LTQ-MS. Glutamine exhibited the most dramatic change with a 5-fold increase in quantity. The results from altering several types of metabolites suggest that chronic UVB irradiation may impact significantly on major hepatic metabolism processes, despite the fact that the liver is not directly exposed to UVB radiation. MS-based metabolomic approach for determining regulatory hepatic metabolites following UV irradiation will provide a better understanding of the relationship between internal organs and UV light.

Molecular and biochemical modifications of liver glutamine synthetase elicited by daytime restricted feeding

BACKGROUND & AIMS

The circadian clock system in the liver plays important roles in regulating metabolism and energy homeostasis. Restricted feeding schedules (RFS) become an entraining stimulus that promotes adaptations that form part of an alternative circadian clock known as the food entrained oscillator (FEO). The aim of this study was to evaluate the daily variations of glutamine synthetase (GS) in liver under a daytime RFS.

METHODS

Hepatic GS properties were analysed at 3-h intervals over a 24-h period in adult male Wistar rats maintained in a 12:12 h light–dark cycle. RFS group: food access for 2-h in light phase, during 3 weeks. AL group: feeding ad libitum. Fa group: acute fast (21 h). Fa–Re group: acute fast followed by refed 2 h.mRNA expression was measured by RT-qPCR, protein presence by Western-blot and immunohistochemistry, enzyme activity by a spectrophotometric assay, and glutamine by high pressure liquid chromatography.

RESULTS AND CONCLUSIONS

Restricted feeding schedule induced circadian rhythmicity inmRNA levels of GS and the loss of the rhythmic pattern in mitochondrial GS activity. GS activity in liver homogenates displayed a robust rhythmic pattern in AL that was not modified by RFS. The presence of GS and its zonal distribution did not show rhythmic pattern in both groups. However, acute Fa and Fa–Re diminished GS protein and activity in liver homogenates. Hepatic glutamine concentrations showed a 24-h rhythmic pattern in both groups, in an antiphasic pattern. In conclusion, daytime RFS influences the liver GS system at different levels, that could be part of rheostatic adaptations associated to the FEO, and highlight the plasticity of this system.

Glutamine flux imaging using genetically encoded sensors

Genetically encoded sensors allow real-time monitoring of biological molecules at a subcellular resolution. A tremendous variety of such sensors for biological molecules became available in the past 15 years, some of which became indispensable tools that are used routinely in many laboratories. One of the exciting applications of genetically encoded sensors is the use of these sensors in investigating cellular transport processes. Properties of transporters such as kinetics and substrate specificities can be investigated at a cellular level, providing possibilities for cell-type specific analyses of transport activities. In this article, we will demonstrate how transporter dynamics can be observed using genetically encoded glutamine sensor as an example. Experimental design, technical details of the experimental settings, and considerations for post-experimental analyses will be discussed.

Visualization of glutamine transporter activities in living cells using genetically encoded glutamine sensors

Glutamine plays a central role in the metabolism of critical biological molecules such as amino acids, proteins, neurotransmitters, and glutathione. Since glutamine metabolism is regulated through multiple enzymes and transporters, the cellular glutamine concentration is expected to be temporally dynamic. Moreover, differentiation in glutamine metabolism between cell types in the same tissue (e.g. neuronal and glial cells) is often crucial for the proper function of the tissue as a whole, yet assessing cell-type specific activities of transporters and enzymes in such heterogenic tissue by physical fractionation is extremely challenging. Therefore, a method of reporting glutamine dynamics at the cellular level is highly desirable. Genetically encoded sensors can be targeted to a specific cell type, hence addressing this knowledge gap. Here we report the development of Föster Resonance Energy Transfer (FRET) glutamine sensors based on improved cyan and yellow fluorescent proteins, monomeric Teal Fluorescent Protein (mTFP)1 and venus. These sensors were found to be specific to glutamine, and stable to pH-changes within a physiological range. Using cos7 cells expressing the human glutamine transporter ASCT2 as a model, we demonstrate that the properties of the glutamine transporter can easily be analyzed with these sensors. The range of glutamine concentration change in a given cell can also be estimated using sensors with different affinities. Moreover, the mTFP1-venus FRET pair can be duplexed with another FRET pair, mAmetrine and tdTomato, opening up the possibility for real-time imaging of another molecule. These novel glutamine sensors will be useful tools to analyze specificities of glutamine metabolism at the single-cell level.

Differential expression of the glutamate transporter GLT-1 in pancreas

The glutamate uptake transporter GLT-1 is best understood for its critical role in preventing brain seizures. Increasing evidence argues that GLT-1 also modulates, and is modulated by, metabolic processes that influence glucose homeostasis. To investigate further the potential role of GLT-1 in these regards, the authors examined GLT-1 expression in pancreas and found that mature multimeric GLT-1 protein is stably expressed in the pancreas of wild-type, but not GLT-1 knockout, mice. There are three primary functional carboxyl-terminus GLT-1 splice variants, called GLT-1a, b, and c. Brain and liver express all three variants; however, the pancreas expresses GLT-1a and GLT-1b but not GLT-1c. Quantitative real time-PCR further revealed that while GLT-1a is the predominant GLT-1 splice variant in brain and liver, GLT-1b is the most abundant splice variant expressed in pancreas. Confocal microscopy and immunohistochemistry showed that GLT-1a and GLT-1b are expressed in both islet β- and α-cells. GLT-1b was also expressed in exocrine ductal domains. Finally, glutamine synthetase was coexpressed with GLT-1 in islets, which suggests that, as with liver and brain, one possible role of GLT-1 in the pancreas is to support glutamine synthesis.

Proteome analysis of fatty liver in feed-deprived dairy cows reveals interaction of fuel sensing, calcium, fatty acid, and glycogen metabolism

The liver of dairy cows is involved in signaling the current hepatic metabolic state to the brain via metabolites and nerval afferents to control and adjust feed intake. Feed deprivation may result in mobilization of body reserves favoring hepatic steatosis. While the overall metabolic changes are well characterized, specific regulatory mechanisms are not readily understood. To identify molecular events associated with metabolic adaptation and the control of energy homeostasis, liver specimens from six ad libitum-fed and six feed-deprived cows were analyzed for selected metabolites, for the activation of AMP kinase, and for regulatory/regulated proteins using two-dimensional gel electrophoresis and MALDI-TOF-MS. Feed deprivation increased total liver fat and the calcium content, as well as augmented AMPK phosphorylation, while it decreased the contents of protein, glucose, glycogen, and cholesterol when expressed as a percentage of dry matter. Among 34 differentially expressed proteins identified, we found downregulation of proteins associated with fatty acid oxidation, glycolysis, electron transfer, protein degradation, and antigen processing, as well as cytoskeletal rearrangement. Proteins upregulated after feed deprivation included enzymes of the urea cycle, fatty acid or cholesterol transport proteins, an inhibitor of glycolysis, and previously unknown changes in calcium signaling network. Direct correlation was found between expression of glycolytic enzymes and glucose/glycogen content, whereas inverse correlation exists between expression of β-oxidative enzymes and total liver fat content. In conclusion, the regulatory response of identified proteins may help to explain development and consequences of hepatic lipidosis but also offers novel candidates potentially involved in signaling for maintaining energy homeostasis.

Novel mechanism of inhibition of rat kidney-type glutaminase by bis-2-(5-phenylacetamido-1,2,4-thiadiazol-2-yl)ethyl sulfide (BPTES)

The release of GA (mitochondrial glutaminase) from neurons following acute ischaemia or during chronic neurodegenerative diseases may contribute to the propagation of glutamate excitotoxicity. Thus an inhibitor that selectively inactivates the released GA may limit the accumulation of excess glutamate and minimize the loss of neurological function that accompanies brain injury. The present study examines the mechanism of inactivation of rat KGA (kidney GA isoform) by the small-molecule inhibitor BPTES [bis-2-(5-phenylacetamido-1,2,4-thiadiazol-2-yl)ethyl sulfide]. BPTES is a potent inhibitor of KGA, but not of the liver GA isoform, glutamate dehydrogenase or gamma-glutamyl transpeptidase. Kinetic studies indicate that, with respect to glutamine, BPTES has a K(i) of approx. 3 microM. Moreover, these studies suggest that BPTES inhibits the allosteric activation caused by phosphate binding and promotes the formation of an inactive complex. Gel-filtration chromatography and sedimentation-velocity analysis were used to examine the effect of BPTES on the phosphate-dependent oligomerization of KGA. This established that BPTES prevents the formation of large phosphate-induced oligomers and instead promotes the formation of a single oligomeric species with distinct physical properties. Sedimentation-equilibrium studies determined that the oligomer produced by BPTES is a stable tetramer. Taken together, the present work indicates that BPTES is a unique and potent inhibitor of rat KGA and elucidates a novel mechanism of inactivation.

Effect of maternal dexamethasone and alpha-ketoglutarate administration on skeletal development during the last three weeks of prenatal life in pigs

BACKGROUND

The effect of dexamethasone (Dex) on postnatal bone formation processes is known to decrease the synthesis of collagen and bone matrix, but the effect of alpha-ketoglutarate (AKG) is to induce positive effects on growth and skeletal development during postnatal life. However, the effects of Dex and AKG treatment on the prenatal processes of skeletal development have not been investigated so far.

OBJECTIVE

The aim of this study was to determine the effect of Dex and AKG administered separately or simultaneously to sows during the last three weeks of pregnancy on the skeletal development in fetuses.

METHODS

Immediately after birth blood samples were collected from non-suckling piglets for alkaline phosphatase and osteocalcin determinations, and the humeri were isolated. Bone mineral density (BMD) and bone mineral content (BMC) of humeri and the geometric and mechanical properties were evaluated.

RESULTS

Dex and AKG administered separately to pregnant sows during the last 24 days of prenatal life decreased BMD, BMC, and geometric and mechanical parameters of humeri in the newborns. Simultaneous administration of Dex and AKG significantly increased the analyzed properties of humeri.

CONCLUSION

The bone mineral density and mechanical and geometric properties of humeri indicate an inverse effect of maternal separate or simultaneous administration of AKG and Dex to sows on bone development during the last 24 days of prenatal life.

Hepatocellular expression of glutamine synthetase: an indicator of morphogen actions as master regulators of zonation in adult liver

Glutamine synthetase (GS) has long been known to be expressed exclusively in pericentral hepatocytes most proximal to the central veins of liver lobuli. This enzyme as well as its peculiar distribution complementary to the periportal compartment for ureogenesis plays an important role in nitrogen metabolism, particularly in homeostasis of blood levels of ammonium ions and glutamine. Despite this fact and intensive studies in vivo and in vitro, many aspects of the regulation of its activity on the protein and on the genetic level remained enigmatic. Recent experimental advances using transgenic mice and new analytic tools have revealed the fundamental role of morphogens such as wingless-type MMTV integration site family member signals (Wnt), beta-catenin, and adenomatous polyposis coli in the regulation of this particular enzyme. In addition, novel information concerning the structure of transcription factor binding sites within regulatory regions of the GS gene and their interactions with signalling pathways could be collected. In this review we focus on all aspects of the regulation of GS in the liver and demonstrate how the new findings have changed our view of the determinants of liver zonation. What appeared as a simple response of hepatocytes to blood-derived factors and local cellular interactions must now be perceived as a fundamental mechanism of adult tissue patterning by morphogens that were considered mainly as regulators of developmental processes. Though GS may be the most obvious indicator of morphogen action among many other targets, elucidation of the complex regulation of the expression of the GS gene could pave the road for a better understanding of the mechanisms involved in patterning of liver parenchyma. Based on current knowledge we propose a new concept of how morphogens, hormones and other factors may act in concert, in order to restrict gene expression to small subpopulations of one differentiated cell type, the hepatocyte, in different anatomical locations. Although many details of this regulatory network are still missing, and an era of exciting new discoveries is still about to come, it can already be envisioned that similar mechanisms may well be active in other organs contributing to the fine-tuning of organ-specific functions.

Anaplerotic diet therapy in inherited metabolic disease: therapeutic potential

Beginning with phenylketonuria, dietary therapy for inborn errors has focused primarily on the restriction of the precursor to an affected catabolic pathway in an attempt to limit the production of potential toxins. Anaplerotic therapy is based on the concept that there may exist an energy deficit in these diseases that might be improved by providing alternative substrate for both the citric acid cycle (CAC) and the electron transport chain for enhanced ATP production. This article focuses on this basic problem, as it may relate to most catabolic disorders, and provides our current experience involving inherited diseases of mitochondrial fat oxidation, glycogen storage, and pyruvate metabolism using the anaplerotic compound triheptanoin. The observations have led to a realization that 'inter-organ' signalling and 'nutrient sensors' such as adenylate monophosphate mediated-protein kinase (AMPK) and mTOR (mammalian target of rapamycin) appear to play a significant role in the intermediary metabolism of these diseases. Activated AMPK turns on catabolic pathways to augment ATP production while turning off synthetic pathways that consume ATP. Information is provided regarding the inter-organ requirements for more normal metabolic function during crisis and how anaplerotic therapy using triheptanoin, as a direct source of substrate to the CAC for energy production, appears to be a more successful approach to an improved quality of life for these patients.

Efficacy of theanine is connected with theanine metabolism by any enzyme, not only drug metabolizing enzymes

Theanine increases the antitumor effect of doxorubicin (DOX) with decreasing adverse reaction. We clarified the mechanism by which theanine decreases the adverse reaction of DOX on any metabolizing enzymes of theanine. There was no change in the activity of any CYPs and the cytochrome P450 content by theanine treatment. Namely, it was considered that the decrease of DOX adverse reactions by theanine was not connected with CYP activity. In other words, it is shown that theanine has no effect on the metabolism of other medicines and is safe as a food (tea) or supplement. Glutathione S-transferase activity did not change in the theanine-alone group whereas increased in the theanine and DOX-combined group. These results suggested that theanine combination increased the conjugate with DOX and GSH, promoted the efflux of GS-DOX conjugates from the liver, and decreased DOX concentration in the liver. In medium containing theanine with glutaminase in vitro, glutamate gradually generated, showing that glutaminase reacted with theanine. Furthermore, the generation of glutamate increased by reaction of theanine and gamma-glutamyltranspeptidase (gamma-GTP), showed that gamma-GTP converted theanine to glutamate. It is expected that theanine metabolism occurred by hydrolysis and rearrangement reaction by gamma-GTP in the liver. Namely, it is suggested that the metabolism of theanine mediated by glutaminase and gamma-GTP and the increase of glutamate mediated GSH is important for theanine-induced action. In conclusion, it appeared that theanine does not change the biodistribution of combined drugs but it modulates biodistribution or damage to the relative site of GSH, and shows preventive effects in tissue.

Fragmentation pathway for glutamine identification: loss of 73 Da from dimethylformamidine glutamine isobutyl ester

A fragmentation mechanism for the neutral loss of 73 Da from dimethylformamidine glutamine isobutyl ester is investigated. Understanding this mechanism will allow to improve the identification and quantification of 15N-labeled and unlabeled glutamine and the distinguishing of glutamine and glutamic acid by electrospray ionization (ESI)-tandem mass spectrometry. Before mass spectrometry analysis, glutamine and glutamic acid are derivatized with dimethylformamide dimethyl acetal and isobutanol to form dimethylformamidine isobutyl ester. Derivatization conditions are modified based on an existing method to ensure complete derivatization of glutamic acid and to prevent the hydrolysis of glutamine. The fragmentation mechanism of dimethylformamidine glutamine isobutyl ester is studied and possible fragmentation pathways are proposed. Based on the fragmentation mechanism, a quantification method is developed to quantify both 15N-labeled and unlabeled glutamine and glutamic acid at a series of different neutral losses by performing multiple-reaction monitoring (MRM) scans in a triple-quadrupole mass spectrometer. Labeled glutamine includes 15N-amide labeled, 15N-amine labeled glutamine and glutamine 15N-labeled at both amide and amine positions. Deuterium labeled glutamine and glutamic acid are used as internal standards. Isotope effects are characterized for 15N labeled and deuterium labeled glutamine. It is found that the same method can be used to distinguish aspartic acid from asparagine. This study will improve the application of MS/MS for amino acid quantification and stable isotope labeling metabolism studies.

Differential Regulation of Amino Acid Transporter SNAT3 by Insulin in Hepatocytes

The liver is a metabolism and transfer center of amino acids as well as the prime target organ of insulin. In this report, we characterized the regulation of system N/A transporter 3 (SNAT3) in the liver of dietary-restricted mice and in hepatocytes treated with serum starvation and insulin. The expression of SNAT3 was up-regulated in dietary-restricted mice. The expression of SNAT3 protein was detected on the plasma membrane of hepatocyte-like H2.35 cells with a half-life of 6-8 h. When H2.35 cells were depleted of serum, the expression of SNAT3 was increased. An increased concentration of insulin, however, suppressed SNAT3 expression. Interestingly, the down-regulation of SNAT3 expression by insulin was blocked by the specific phosphoinositide 3-kinase inhibitor LY294002 and mammalian target of rapamycin inhibitor, but not by MAPK inhibitor PD98059, suggesting that insulin exerts its effect on SNAT3 through phosphoinositide 3-kinase-mammalian target of rapamycin signaling. Surface biotinylation assay showed an increased level of SNAT3 on the cell surface after 0.5 h of insulin treatment, although no effect was observed after 24 h of treatment. Consistently, the transport of the substrate l-histidine was increased with short, but not long, treatment by insulin in both H2.35- and SNAT3-transfected COS-7 cells. The L-histidine uptake was inhibited significantly by L-histidine followed by 2-endoamino-bicycloheptane-2-carboxylic acid and L-cysteine and to a lesser extent by L-alanine and aminoisobutyric acid, but was not inhibited by alpha-(methylamino)isobutyric acid, implying that uptake of L-histidine in H2.35 cells is primarily mediated by system N transporters. In conclusion, differential regulation of SNAT3 by insulin and serum starvation reinforces the functional significance of this transporter in liver physiology.

Complementary roles for toxicologic pathology and mathematics in toxicogenomics, with special reference to data interpretation and oscillatory dynamics

Toxicogenomics is an emerging multidisciplinary science that will profoundly impact the practice of toxicology. New generations of biologists, using evolving toxicogenomics tools, will generate massive data sets in need of interpretation. Mathematical tools are necessary to cluster and otherwise find meaningful structure in such data. The linking of this structure to gene functions and disease processes, and finally the generation of useful data interpretation remains a significant challenge. The training and background of pathologists make them ideally suited to contribute to the field of toxicogenomics, from experimental design to data interpretation. Toxicologic pathology, a discipline based on pattern recognition, requires familiarity with the dynamics of disease processes and interactions between organs, tissues, and cell populations. Optimal involvement of toxicologic pathologists in toxicogenomics requires that they communicate effectively with the many other scientists critical for the effective application of this complex discipline to societal problems. As noted by Petricoin III et al (Nature Genetics 32, 474-479, 2002), cooperation among regulators, sponsors and experts will be essential for realizing the potential of microarrays for public health. Following a brief introduction to the role of mathematics in toxicogenomics, "data interpretation" from the perspective of a pathologist is briefly discussed. Based on oscillatory behavior in the liver, the importance of an understanding of mathematics is addressed, and an approach to learning mathematics "later in life" is provided. An understanding of pathology by mathematicians involved in toxicogenomics is equally critical, as both mathematics and pathology are essential for transforming toxicogenomics data sets into useful knowledge.

Glutamine metabolism: Role in acid-base balance*

The intent of this review is to provide a broad overview of the interorgan metabolism of glutamine and to discuss in more detail its role in acid-base balance. Muscle, adipose tissue, and the lungs are the primary sites of glutamine synthesis and release. During normal acid-base balance, the small intestine and the liver are the major sites of glutamine utilization. The periportal hepatocytes catabolize glutamine and convert ammonium and bicarbonate ions to urea. In contrast, the perivenous hepatocytes are capable of synthesizing glutamine. During metabolic acidosis, the kidney becomes the major site of glutamine extraction and catabolism. This process generates ammonium ions that are excreted in the urine to facilitate the excretion of acids and bicarbonate ions that are transported to the blood to partially compensate the acidosis. The increased renal extraction of glutamine is balanced by an increased release from muscle and liver and by a decreased utilization in the intestine. During chronic acidosis, this adaptation is sustained, in part, by increased renal expression of genes that encode various transport proteins and key enzymes of glutamine metabolism. The increased levels of phosphoenolpyruvate carboxykinase result from increased transcription, while the increase in glutaminase and glutamate dehydrogenase activities result from stabilization of their respective mRNAs. Where feasible, this review draws upon data obtained from studies in humans. Studies conducted in model animals are discussed where available data from humans is either lacking or not firmly established. Because there are quantitative differences in tissue utilization and synthesis of glutamine in different mammals, the review will focus more on common principles than on quantification.

Intestinal glutaminase activity is increased in liver cirrhosis and correlates with minimal hepatic encephalopathy

BACKGROUND/AIMS

We performed the current study to assess the intestinal activity of enterocyte phosphate-activated glutaminase (PAG) in cirrhosis.

METHODS

Forty-nine cirrhotic patients and 36 control subjects underwent endoscopic duodenal biopsies. Minimal hepatic encephalopathy (MHE) was evaluated using three psychometric tests. Oral glutamine challenge (OGC) was performed and MELD, Child-Pugh and the presence of esophageal varices were recorded. PAG was measured by enzymatic methods. Cerebral magnetic resonance spectroscopy was performed in 10 cirrhotics.

RESULTS

PAG was found to be higher in cirrhotics than control subjects 2.4+/-1.51 vs. 0.68+/-0.57IU/mg protein (P<0.001). PAG was also increased in patients with MHE and correlated with MELD, INR, esophageal varices and serum bile acids. A negative correlation was observed between PAG activity and intra-cerebral choline/creatine ratio (r=-0.67; P=0.035) and a positive correlation with glutamine plus glutamate/creatine ratio (r=0.78; P=0.007). In multivariate analysis using backward logistic regression, presence of MHE was the only variable independently related to altered enterocyte PAG.

CONCLUSIONS

Enterocyte PAG is increased in cirrhotic patients and correlates with MHE. These data support a possible role for intestinal glutaminase in the pathogenesis of hepatic encephalopathy (HE) and could be a new target for future therapies.

The role of glutamine transaminase K (GTK) in sulfur and alpha-keto acid metabolism in the brain, and in the possible bioactivation of neurotoxicants

Glutamine transaminase K (GTK), which is a freely reversible glutamine (methionine) aromatic amino acid aminotransferase, is present in most mammalian tissues, including brain. Quantitatively, the most important amine donor in vivo is glutamine. The product of glutamine transamination (i.e., alpha-ketoglutaramate; alphaKGM) is rapidly removed by cyclization and/or conversion to alpha-ketoglutarate. Transamination is therefore "pulled" in the direction of glutamine utilization. Major biological roles of GTK are to maintain low levels of phenylpyruvate and to close the methionine salvage pathway. GTK also catalyzes the transamination of cystathionine, lanthionine, and thialysine to the corresponding alpha-keto acids, which cyclize to ketimines. The cyclic ketimines and several metabolites derived therefrom are found in brain. It is not clear whether these compounds have a biological function or are metabolic dead-ends. However, high-affinity binding of lanthionine ketimine (LK) to brain membranes has been reported. Mammalian tissues possess several enzymes capable of catalyzing transamination of kynurenine in vitro. Two of these kynurenine aminotransferases (KATs), namely KAT I and KAT II, are present in brain and have been extensively studied. KAT I and KAT II are identical to GTK and alpha-aminoadipate aminotransferase, respectively. GTK/KAT I is largely cytosolic in kidney, but mostly mitochondrial in brain. The same gene codes for both forms, but alternative splicing dictates whether a 32-amino acid mitochondrial-targeting sequence is present in the expressed protein. The activity of KAT I is altered by a missense mutation (E61G) in the spontaneously hypertensive rat. The symptoms may be due in part to alteration of kynurenine transamination. However, owing to strong competition from other amino acid substrates, the turnover of kynurenine to kynurenate by GTK/KAT I in nervous tissue must be slow unless kynurenine and GTK are sequestered in a compartment distinct from the major amino acid pools. The possibility is discussed that the spontaneous hypertension in rats carrying the GTK/KAT I mutation may be due in part to disruption of glutamine transamination. GTK is one of several pyridoxal 5'-phosphate (PLP)-containing enzymes that can catalyze non-physiological beta-elimination reactions with cysteine S-conjugates containing a good leaving group attached at the sulfur. These elimination reactions may contribute to the bioactivation of certain electrophiles, resulting in toxicity to kidney, liver, brain, and possibly other organs. On the other hand, the beta-lyase reaction catalyzed by GTK may be useful in the conversion of some cysteine S-conjugate prodrugs to active components in vivo. The roles of GTK in (a) brain nitrogen, sulfur, and aromatic amino acid/kynurenine metabolism, (b) brain alpha-keto acid metabolism, (c) bioactivation of certain electrophiles in brain, (d) prodrug targeting, and (e) maintenance of normal blood pressure deserve further study.

Metabolic acidosis stimulates intestinal glutamine absorption

Glutamine is an essential nutrient for cell integrity during acidotic states such as shock, but the effect of extracellular pH on intestinal mucosal cell glutamine uptake is poorly understood. The purpose of this in vitro study was to investigate the intracellular signaling pathways involved in controlling intestinal glutamine transport during acidosis. Lowering the pH in the cell culture medium resulted in an increase in glutamine transport activity in a time- and pH-dependent fashion. Chronic acidosis (pH 6.6 for 48 hours) resulted in a twofold increase in glutamine transport activity (1.63+/-0.25 nmole/mg protein/minute in acidosis vs. 0.78+/-0.11 nmole/mg protein/minute in control) and a threefold increase in glutamine transport gene ATB(0) messenger RNA levels. This acidosis-induced increase in glutamine transport activity was due to a stimulation of transporter maximal transport capacity (V(max) 13.6+/-0.73 nmole/mg protein/minute in acidosis vs. 6.3+/-0.46 nmole/mg protein/minute in control) rather than a change in transporter affinity (K(m)=0.23+/-0.02 mmol/L glutamine in acidosis vs. 0.19+/-0.02 mmol/L glutamine in control). This acidosis-stimulated glutamine transport activity was blocked by actinomycin-D or cycloheximide. Cellular mitogen-activated protein kinase (MAPK) MEK1/2 and p42/44 levels were elevated in acidotic cells, and the acidosis-induced glutamine transport activity was blocked by the MAPK MEK 1 inhibitor PD 98059. Acidosis stimulates glutamine transport in Caco-2 cells via signaling pathways that lead to transcription of the glutamine transporter gene and translation of functional transporters. Mitogen-activated protein kinases are key intracellular regulators involved in this signal transduction cascade. An increased availability of glutamine to cells subjected to redox stress may help in maintaining cellular integrity.

Role of specific amino acids in nutritional support

Attempts have been made over the past decade to evaluate the role of specific amino acids in the nutritional support of surgical patients. Particular attention has been paid to the branched-chain amino acids, glutamine, and a collection of compounds that have been referred to as 'immunonutrients'. The present review provides an overview of the rationale behind the use of these nutrients and reviews information about their role in patients undergoing surgery. In general, although the use of immunonutrients enhances the nutritional status of patients, and although a number of clinical trials have claimed clinical advantages, there are still doubts about their clinical utility and concerns about their high cost.

Bacterial expression, purification, and characterization of rat kidney-type mitochondrial glutaminase

The human gene that encodes the kidney-type glutaminase (KGA) spans 84-kb, contains 19 exons, and encodes two alternatively spliced mRNAs. Various segments of the rat KGA cDNA were PCR amplified and cloned into a bacterial expression vector to determine whether the N- and C- terminal ends of the glutaminase protein were essential for activity. A recombinant glutaminase, lacking the coding sequence contained in exon 1, was found to be fully active. In contrast, proteins that lacked sequences from exons 1 and 2 and exons 1-3 were inactive. An additional construct that corresponded to the sequence encoded by exons 2-14 also retained full activity. Both of the fully active, truncated proteins were purified to apparent homogeneity using an incorporated N-terminal His(6)-tag and Ni(2+)-affinity chromatography. The K(M) values for glutamine of the native and recombinant forms of glutaminase were nearly identical. However, the two truncated forms of the glutaminase exhibit the characteristic phosphate activation profile only when dialyzed into a buffer lacking phosphate. Dialysis versus 10mM Tris-phosphate was sufficient to form an active tetramer. Thus, the deleted N-terminal sequence may contribute to the phosphate-dependent oligomerization and activation of the native glutaminase.

Mouse system-N amino acid transporter, mNAT3, expressed in hepatocytes and regulated by insulin-activated and phosphoinositide 3-kinase-dependent signalling

Amino acid transporters are essential for normal cell function and physiology. In the present study, we report the identification and functional and regulatory characterization of a mouse system-N amino acid transporter, mNAT3. Expression of mNAT3 in Xenopus oocytes revealed that the strongest transport activities were preferred for L-alanine. In addition, mNAT3 is an Na(+)- and pH-dependent low-affinity transporter and it partially tolerates substitution of Na(+) by Li(+). mNAT3 has been found to be expressed predominantly in the liver, where it is localized to the plasma membrane of hepatocytes, with the strongest expression in those cells adjacent to the central vein, decreasing gradually towards the portal tract. Treatment of mouse hepatocyte-like H2.35 cells with insulin led to a significant increase in the expression of mNAT3, and this stimulation was associated closely with an increase in the uptake of L-alanine. Interestingly, this insulin-induced stimulatory effect on mNAT3 expression was attenuated by the phosphoinositide 3-kinase inhibitor LY294002, but not by the mitogen-activated protein kinase inhibitor PD98059, although both kinases were fully activated by insulin. The results suggest that insulin-mediated regulation of mNAT3 is likely to be mediated through a phosphoinositide 3-kinase-dependent signalling pathway. The unique expression pattern and insulin-mediated regulatory properties of mNAT3 suggest that this transporter may play an important role in liver physiology.