A quantitative analysis of the control of glutamine catabolism in rat liver cells. Use of selective inhibitors

Pivotal role of glutamine synthetase in ammonia detoxification

Glutamine synthetase (GS) catalyzes condensation of ammonia with glutamate to glutamine. Glutamine serves, with alanine, as a major nontoxic interorgan ammonia carrier. Elimination of hepatic GS expression in mice causes only mild hyperammonemia and hypoglutaminemia but a pronounced decrease in the whole-body muscle-to-fat ratio with increased myostatin expression in muscle. Using GS-knockout/liver and control mice and stepwise increments of enterally infused ammonia, we show that ∼35% of this ammonia is detoxified by hepatic GS and ∼35% by urea-cycle enzymes, while ∼30% is not cleared by the liver, independent of portal ammonia concentrations ≤2 mmol/L. Using both genetic (GS-knockout/liver and GS-knockout/muscle) and pharmacological (methionine sulfoximine and dexamethasone) approaches to modulate GS activity, we further show that detoxification of stepwise increments of intravenously (jugular vein) infused ammonia is almost totally dependent on GS activity. Maximal ammonia-detoxifying capacity through either the enteral or the intravenous route is ∼160 μmol/hour in control mice. Using stable isotopes, we show that disposal of glutamine-bound ammonia to urea (through mitochondrial glutaminase and carbamoylphosphate synthetase) depends on the rate of glutamine synthesis and increases from ∼7% in methionine sulfoximine-treated mice to ∼500% in dexamethasone-treated mice (control mice, 100%), without difference in total urea synthesis.

CONCLUSIONS

Hepatic GS contributes to both enteral and systemic ammonia detoxification. Glutamine synthesis in the periphery (including that in pericentral hepatocytes) and glutamine catabolism in (periportal) hepatocytes represents the high-affinity ammonia-detoxifying system of the body. The dependence of glutamine-bound ammonia disposal to urea on the rate of glutamine synthesis suggests that enhancing peripheral glutamine synthesis is a promising strategy to treat hyperammonemia. Because total urea synthesis does not depend on glutamine synthesis, we hypothesize that glutamate dehydrogenase complements mitochondrial ammonia production. (Hepatology 2017;65:281-293).

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.

Glutamine homeostasis and mitochondrial dynamics

Glutamine is a multifaceted amino acid that plays key roles in many metabolic pathways and also fulfils essential signaling functions. Although classified as non-essential, recent evidence suggests that glutamine is a conditionally essential amino acid in several physiological situations. Glutamine homeostasis must therefore be exquisitely regulated and mitochondria represent a major site of glutamine metabolism in numerous cell types. Glutaminolysis is mostly a mitochondrial process with repercussions in organelle structure and dynamics suggesting a tight and mutual control between mitochondrial form and cell bioenergetics. In this review we describe an updated account focused on the critical involvement of glutamine in oxidative stress, mitochondrial dysfunction and tumour cell proliferation, with special emphasis in the initial steps of mitochondrial glutamine pathways: transport into the organelle and hydrolytic deamidation through glutaminase enzymes. Some controversial issues about glutamine catabolism within mitochondria are also reviewed.

Construction of a biological tissue model based on a single-cell model: a computer simulation of metabolic heterogeneity in the liver lobule

An enormous body of information has been obtained by molecular and cellular biology in the last half century. However, even these powerful approaches are not adequate when it comes to higher-level biological structures, such as tissues, organs, and individual organisms, because of the complexities involved. Thus, accumulation of data at the higher levels supports and broadens the context for that obtained on the molecular and cellular levels. Under such auspices, an attempt to elucidate mesoscopic and macroscopic subjects based on plentiful nanoscopic and microscopic data is of great potential value. On the other hand, fully realistic simulation is impracticable because of the extensive cost entailed and enormous amount of data required. Abstraction and modeling that balance the dual requirements of prediction accuracy and manageable calculation cost are of great importance for systems biology. We have constructed an ammonia metabolism model of the hepatic lobule, a histological component of the liver, based on a single-hepatocyte model that consists of the biochemical kinetics of enzymes and transporters. To bring the calculation cost within reason, the porto-central axis, which is an elemental structure of the lobule, is defined as the systems biological unit of the liver, and is accordingly modeled. A model including both histological structure and position-specific gene expression of major enzymes largely represents the physiological dynamics of the hepatic lobule in nature. In addition, heterogeneous gene expression is suggested to have evolved to optimize the energy efficiency of ammonia detoxification at the macroscopic level, implying that approaches like this may elucidate how properties at the molecular and cellular levels, such as regulated gene expression, modify higher-level phenomena of multicellular tissue, organs, and organisms.

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.

Bidirectional substrate fluxes through the system N (SNAT5) glutamine transporter may determine net glutamine flux in rat liver

System N (SNAT3 and SNAT5) amino acid transporters are key mediators of glutamine transport across the plasma membrane of mammalian cell types, including hepatocytes and astrocytes. We demonstrate that SNAT5 shows simultaneous bidirectional glutamine fluxes when overexpressed in Xenopus oocytes. Influx and efflux are both apparently Na+ dependent but, since they are not directly coupled, the carrier is capable of mediating net amino acid movement across the cell membrane. The apparent Km values for glutamine influx and efflux are similar (approximately 1 mm) and the transporter behaviour is consistent with a kinetic model in which re-orientation of the carrier from outside- to inside-facing conformations (either empty or substrate loaded) is the limiting step in the transport cycle. In perfused rat liver, the observed relationship between influent (portal) glutamine concentration and net hepatic glutamine flux may be described by a simple kinetic model, assuming the balance between influx and efflux through System N determines net flux, where under physiological conditions efflux is generally saturated owing to high intracellular glutamine concentration. SNAT5 shows a more periportal mRNA distribution than SNAT3 in rat liver, indicating that SNAT5 may have particular importance for modulation of net hepatic glutamine flux.

Molecular and functional analysis of glutamine uptake in human hepatoma and liver-derived cells

Human hepatoma cells take up glutamine at rates severalfold faster than the system N-mediated transport rates observed in normal human hepatocytes. Amino acid inhibition, kinetic, Northern blotting, RT-PCR, and restriction enzyme analyses collectively identified the transporter responsible in six human hepatoma cell lines as amino acid transporter B(0) (ATB(0)), the human ortholog of rodent ASCT2. The majority of glutamine uptake in liver fibroblasts and an immortalized human liver epithelial cell line (THLE-5B) was also mediated by ATB(0). The 2.9-kb ATB(0) mRNA was equally expressed in all cell lines, whereas expression of the system A transporters ATA2 and ATA3 was variable. In contrast, the system N isoforms (SN1 and SN2) were expressed only in well-differentiated hepatomas. ATB(0) mRNA was also expressed in cirrhotic liver and adult and pediatric liver cancer biopsies but was not detectable in isolated human hepatocytes or fetal liver. Although the growth of all hepatomas was glutamine dependent, competitive inhibition of ATB(0)-mediated glutamine uptake blocked proliferation only in poorly differentiated cells lacking SN1 or SN2 expression and exhibiting low glutamine synthetase mRNA levels.

Hepatic glutaminase--a special role in urea synthesis?

OBJECTIVE

To investigate the relationship between hepatic glutaminase and the urea cycle with particular reference to the possibility of the existence of a metabolic channel between glutaminase and carbamylphosphate synthetase I (CPS-I).

METHODS

Rat livers were perfused in the non-recirculating mode with 15-N labeled ammonia and glutamine. The incorporation of 15-N into nitrogenous products was determined by gas chromatography-mass spectrometry.

RESULTS

We devised and validated a theoretical framework that described the incorporation of the 15-N into the various urea mass isotopomers as a function of the isotopic abundance of 15-N in the two precursor molecules, aspartate and citrulline. We then compared the incorporation of 15-N from amino-labeled and amide-labeled glutamine. Glucagon activated incorporation of these labels into products, consistent with an activation of glutaminase. However, the results indicated no metabolic channel between glutaminase and CPS-I.

CONCLUSION

We suggest that glutaminase may play a role in promoting urea production by virtue of N-acetylglutamate synthesis rather than by a channeling mechanism. Glutaminase may provide glutamate, a substrate for the synthesis of N-acetylglutamate which is an obligatory activator of CPS-1.

Hepatic glutamine metabolism

Expression of high activities of both glutamine synthetase and glutaminase allows the liver to play a major role in the regulation of glutamine homeostasis. The liver shows net glutamine output in metabolic acidosis, in prolonged starvation and animals bearing tumors, net glutamine uptake in the postabsorptive state, on consuming high protein diets, and in uncontrolled diabetes or sepsis. Liver glutamine synthetase is expressed only in a small population of perivenous cells that allows it to salvage any ammonia not incorporated into urea in periportal cells. Hepatic glutaminase is a unique isozyme found only in periportal liver parenchymal cells where it provides glutamate and ammonia for the urea cycle. Control of hepatic glutamine metabolism occurs almost exclusively through changes in the activity of glutaminase, with no change in glutamine synthetase flux.

Recent molecular advances in mammalian glutamine transport

Much has been learned about plasma membrane glutamine transporter activities in health and disease over the past 30 years, including their potential regulatory role in metabolism. Since the 1960s, discrimination among individual glutamine transporters was based on functional characteristics such as substrate specificity, ion dependence, and kinetic and regulatory properties. Within the past two years, several genes encoding for proteins with these defined activities (termed "systems") have been isolated from human and rodent cDNA libraries and found to be distributed among four distinct gene families. The Na(+)-dependent glutamine transporter genes isolated thus far are System N (SN1), System A (ATA1, ATA2), System ASC/B(0) (ASCT2 or ATB(0)), System B(0,+) (ATB(0,+)) and System y(+)L (y(+)LAT1, y(+)LAT2). Na(+)-independent glutamine transporter genes encoding for System L (LAT1, LAT2) and System b(0,+) (b(0,+)AT) have also been recently isolated, and similar to y(+)L, have been shown to function as disulfide-linked heterodimers with the 4F2 heavy chain (CD98) or rBAT (related to b(0,+) amino acid transporter). In this review, the molecular features, catalytic mechanisms and tissue distributions of each are addressed. Although most of these transporters mediate the transmembrane movement of several other amino acids, their potential roles in regulating interorgan glutamine flux are discussed. Most importantly, these newly isolated transporter genes provide the long awaited tools necessary to study their molecular regulation during the catabolic states in which glutamine is considered to be "conditionally essential."

Glutamine and glutathione counteract the inhibitory effects of mediators of sepsis in neonatal hepatocytes

BACKGROUND/PURPOSE

Surgical neonates are at risk of sepsis-associated liver dysfunction. Hydrogen peroxide (H(2)O(2)) and nitric oxide (NO) are important mediators of sepsis, which impair neonatal hepatic metabolism. Glutamine has been shown to have beneficial effects on hepatocyte metabolism during neonatal sepsis. However, the molecular basis of these effects are unknown. The aim of this study was to test the hypotheses that (1) glutamine and its dipeptides counteract the inhibitory effect of septic mediators on neonatal hepatocyte oxygen consumption and (2) the effects of glutamine are specific and not shared by other amino acids. In addition, we wished to determine the metabolic pathways and mediators involved in the action of glutamine.

METHODS

Hepatocytes were isolated from suckling rats, and O(2) consumption measured polarographically. Study A: the ability of 10 mmol/L glutamine to reverse the inhibitory effects of 1.5 mmol/L H(2)O(2) and 300 micromol/L S-Nitroso-N-acetylpenicillamine (SNAP; a nitric oxide donor) on O(2) consumption was examined. Study B: the ability of other amino acids and dipeptides of glutamine to reverse the effects of H(2)O(2) was examined. Study C: various concentrations of glutamine were tested for their ability to reverse the H(2)O(2) inhibition of O(2) consumption. Study D: the mechanism of action of glutamine was examined by incubating hepatocytes with either an inhibitor of entry into the Krebs cycle or an inhibitor of glutathione synthesis. Study E: the ability of glutathione to reverse the inhibitory effects of H(2)O(2) was examined.

RESULTS

Study A: glutamine reversed the inhibition of hepatocyte O(2) consumption exerted by either H(2)O(2) or NO. Study B: glutamine dipeptides reversed the inhibition of hepatocyte O(2) consumption by H(2)O(2), but other amino acids did not. Study C: the counteracting effect of glutamine was proportional to the dose administered. Study D: blocking entry of glutamine into the Krebs cycle did not abolish the effects of glutamine, but blocking glutathione synthesis completely abolished the effect of glutamine. Study E: exogenous glutathione reversed the inhibitory effect of H(2)O(2) on hepatocyte O(2) consumption.

CONCLUSIONS

This study found that glutamine and its dipeptides are unique in reversing the effects of septic mediators on neonatal rat liver oxidative metabolism. The effectiveness of glutamine appears to be mediated via glutathione synthesis. Addition of glutamine, glutamine dipeptides, or glutathione to total parenteral nutrition (TPN) may be beneficial in preventing liver damage in neonatal sepsis.

Tumor-influenced amino acid transport activities in zonal-enriched hepatocyte populations

Cancer influences hepatic amino acid metabolism in the host. To further investigate this relationship, the effects of an implanted fibrosarcoma on specific amino acid transport activities were measured in periportal (PP)- and perivenous (PV)-enriched rat hepatocyte populations. Na(+)-dependent glutamate transport rates were eightfold higher in PV than in PP preparations but were relatively unaffected during tumor growth. System N-mediated glutamine uptake was 75% higher in PV than in PP preparations and was stimulated up to twofold in both regions by tumor burdens of 9 +/- 4% of carcass weight compared with hepatocytes from pair-fed control animals. Excessive tumor burdens (26 +/- 7%) resulted in hypophagia, loss of PV-enriched system N activities, and reduced transporter stimulation. Conversely, saturable arginine uptake was enhanced fourfold in PP preparations and was induced twofold only after excessive tumor burden. These data suggest that hepatic amino acid transporters are differentially influenced by cancer in a spatial and temporal manner, and they represent the first report of reciprocal zonal enrichment of system N and saturable arginine uptake in the mammalian liver.

Hepatic glutamine transporter activation in burn injury: role of amino acids and phosphatidylinositol-3-kinase

Burn injury elicits a marked, sustained hypermetabolic state in patients characterized by accelerated hepatic amino acid metabolism and negative nitrogen balance. The transport of glutamine, a key substrate in gluconeogenesis and ureagenesis, was examined in hepatocytes isolated from the livers of rats after a 20% total burn surface area full-thickness scald injury. A latent and profound two- to threefold increase in glutamine transporter system N activity was first observed after 48 h in hepatocytes from injured rats compared with controls, persisted for 9 days, and waned toward control values after 18 days, corresponding with convalescence. Further studies showed that the profound increase was fully attributable to rapid posttranslational transporter activation by amino acid-induced cell swelling and that this form of regulation may be elicited in part by glucagon. The phosphatidylinositol-3-kinase (PI3K) inhibitors wortmannin and LY-294002 each significantly attenuated transporter stimulation by amino acids. The data suggest that PI3K-dependent system N activation by amino acids may play an important role in fueling accelerated hepatic nitrogen metabolism after burn injury.

Glutamine and glutamate metabolism across the liver sinusoid

The liver shows net glutamine uptake after a protein-containing meal, during uncontrolled diabetes, sepsis and short-term starvation, but changes to net release during long-term starvation and metabolic acidosis. Some studies report a small net release of glutamate by the liver. The differential expression of glutamine synthetase (perivenous) and glutaminase (periportal) within the liver indicates that glutamine is used for urea synthesis in periportal cells, whereas glutamine synthesis serves to detoxify any residual ammonia in perivenous cells. Experiments in vivo suggest that changes in net hepatic glutamine balance are due predominantly to regulation of glutaminase activity, with the flux through glutamine synthetase being relatively constant.

Glutamine transport and human hepatocellular transformation

Among other functions, the liver serves to regulate both glucose and nitrogen economy in the body, and in humans, the amino acid glutamine is a major gluconeogenic substrate and the primary extrahepatic ammonia shuttle. Accordingly, the liver acinus possesses a unique heterogeneous metabolic architecture suited to carry out these functions with glutamine-consuming urea cycle and gluconeogenic enzymes in the periportal hepatocytes and a high capacity for glutamine synthesis in the perivenous hepatocytes, resulting in net glutamine balance across the hepatic bed under most conditions. Cytoplasmic levels of glutamine are significantly governed by the activity of the System N transporter in the plasma membrane of parenchymal cells; in this capacity, this glutamine carrier has been shown to represent a rate-limiting step in metabolism via glutaminase. The unique properties of System N allow it to rapidly adapt in support of the dynamic demands of whole body ammonia and glucose homeostasis. In contrast to System N in normal hepatocytes, human hepatoma cells take up glutamine at rates several-fold faster through a broad-specificity higher affinity transporter with characteristics of System ASC or B0. It is currently hypothesized that the expression of this high activity carrier by hepatoma cells combined with accelerated metabolism and tumor-induced derangements in hepatocellular architecture result in net glutamine consumption, and may underlie the diminished plasma glutamine levels observed in patients with hepatocellular carcinoma (HCC). The transport of glutamine through System ASC has been shown to regulate growth in some human hepatoma cells, which suggests this transporter may warrant consideration as a therapeutic target for HCC.

Rat liver endothelial cell glutamine transporter and glutaminase expression contrast with parenchymal cells

Despite the central role of the liver in glutamine homeostasis in health and disease, little is known about the mechanism by which this amino acid is transported into sinusoidal endothelial cells, the second most abundant hepatic cell type. To address this issue, the transport of L-glutamine was functionally characterized in hepatic endothelial cells isolated from male rats. On the basis of functional analyses, including kinetics, cation substitution, and amino acid inhibition, it was determined that a Na+-dependent carrier distinct from system N in parenchymal cells, with properties of system ASC or B0, mediated the majority of glutamine transport in hepatic endothelial cells. These results were supported by Northern blot analyses that showed expression of the ATB0 transporter gene in endothelial but not parenchymal cells. Concurrently, it was determined that, whereas both cell types express glutamine synthetase, hepatic endothelial cells express the kidney-type glutaminase isozyme in contrast to the liver-type isozyme in parenchymal cells. This represents the first report of ATB0 and kidney-type glutaminase isozyme expression in the liver, observations that have implications for roles of specific cell types in hepatic glutamine homeostasis in health and disease.

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.

Hepatic glutamine transport and metabolism

Although the liver was long known to play a major role in the uptake, synthesis, and disposition of glutamine, metabolite balance studies across the whole liver yielded apparently contradictory findings suggesting that little or no net turnover of glutamine occurred in this organ. Efforts to understand the unique regulatory properties of hepatic glutaminase culminated in the conceptual reformulation of the pathway for glutamine synthesis and turnover, especially as regards the role of sub-acinar distribution of glutamine synthetase and glutaminase. This chapter describes these processes as well as the role of glutamine in hepatocellular hydration, a process that is the consequence of cumulative, osmotically active uptake of glutamine into cells. This topic is also examined in terms of the effects of cell swelling on the selective stimulation or inhibition of other far-ranging cellular processes. The pathophysiology of the intercellular glutamine cycle in cirrhosis is also considered.

The oxidative metabolism of glutamine. A modulator of reactive oxygen intermediate-mediated cytotoxicity of tumor necrosis factor in L929 fibrosarcoma cells

Treatment of the mouse fibrosarcoma cell line L929 with tumor necrosis factor (TNF) induces necrotic cell death. A crucial step in the cytotoxic action mechanism of TNF involves perturbation of mitochondrial functions leading to the formation of reactive oxygen intermediates (ROI). L929 cells have energy requirements adapted to a high proliferation rate. Glutamine (Gln) is utilized as a major energy source and drives mitochondrial ATP formation, while glucose is mainly converted to lactate through glycolysis. We investigated the role of the bioenergetic pathways involved in substrate utilization on the cytotoxic action of TNF and established a link between Gln oxidation and TNF-induced mitochondrial distress. Omission of Gln from the medium desensitizes the cells to TNF cytotoxicity, while the lack of glucose in the medium does not alter the TNF response. Sudden depletion of Gln from the culture medium results in a sharp decline in mitochondrial respiration in the cells, which might explain the decreased TNF responsiveness. However, when L929 cells are adapted to long term growth under conditions without Gln, these so-called L929/Gln- cells have restored respiration, but they still display a decreased sensitivity to TNF cytotoxicity. Thus the TNF responsiveness of L929 cells depends on bioenergetic reactions that are specifically involved in the oxidation of Gln. This is further confirmed by the desensitizing effect of specific inhibitors of these Gln-linked enzyme reactions on TNF cytotoxicity in the parental cells, but not in the L929/Gln- cells. Analysis of the induction of mitochondrial ROI formation by TNF in parental and L929/Gln- cells suggests that the effect of Gln on the sensitivity to TNF cytotoxicity involves a mechanism that renders the mitochondria more susceptible to TNF-induced mediators, resulting in enhanced ROI production and accelerated cytotoxicity.

Glutamine transport by vesicles isolated from tumour-cell mitochondrial inner membrane

Mitochondrial-inner-membrane vesicles, isolated from Ehrlich ascites carcinoma cells by titration with detergents, accumulated L-glutamine by a very efficient transport system. The vesicles lack any phosphate-activated glutaminase activity, allowing measurement of transport rates without interference by L-glutamine metabolism. The time course of the transport was linear for the first 60 s, reaching a steady state after 120 min. L-Glutamine transport showed co-operativity, with a Hill coefficient of 2.2; the kinetic parameters S0.5 and Vmax had values of 5 mM and 26 nmol/30 s per mg of protein respectively. The pH-dependence curve showed a bell shape, with a pH optimum about 8.0. The uptake of L-glutamine was not affected by the presence of a 50-fold molar excess of D-glutamine, L-cysteine, L-histidine, L-alanine, L-serine and L-leucine, whereas L-glutamate behaved as a poor inhibitor. The structural analogue L-glutamate gamma-hydroxamate (5mM) inhibited the net uptake by 68%; interestingly, other analogues (6-diazo-5-oxo-L-norleucine, acivicin and L-glutamate gamma-hydrazide) were ineffective. The impermeant thiol reagent p-chloromercuriphenylsulphonic acid (0.5mM) completely abolished the mitochondrial L-glutamine uptake; in contrast, other thiol reagents (mersalyl and N-ethylmaleimide) did not significantly affect the transport. These data confirm the existence of a specific transport system with high capacity for L-glutamine in the mitochondrial inner membrane, a step preceding the highly operative glutaminolysis in tumour cells.

Activation of hepatic glutaminase by spermine

Glutaminase activity in intact mitochondria from rat liver is activated by spermine, as indicated both by increased glutamate production from glutamine and by increased respiration with glutamine as sole substrate. Glutaminase activity assayed in membranes from frozen-thawed mitochondria, is activated by spermine about 6-fold at physiological concentrations of its other effectors (NH4+ at 0.7 mM, Pi 5 mM) and at pH 7.4. Spermine decreased the apparent Km for glutamine from 38 to 15 mM at 5 mM Pi, and increased the sensitivity of the enzyme for phosphate activation so that the concentration required for 50% stimulation decreased from 15 to 4 mM. Half-maximal spermine effects occurred at 0.15 mM, which is in the physiological range. Spermine was effective in the presence of physiological concentrations of Mg2+. We suggest that spermine may be a physiological activator of hepatic glutaminase.

Hormonal control of hepatic glutaminase

(1) Glucagon activates hepatic glutaminase in vivo. Mitochondria from glucagon-injected rats retain an enhanced capacity to catabolize glutamine and this is more sensitive to activation by inorganic phosphate. The glucagon-elicited stimulation of glutaminase is not evident in broken mitochondria. A similar activation of glutaminase occurs in a number of situations which are associated with elevated glucagon levels in vivo, i.e., after a high-protein meal, after injection of bacterial endotoxin and in diabetes mellitus. (2) Studies in isolated hepatocytes revealed that glutaminase could be activated, not only by glucagon, but also by a cell-permeable protein kinase A activator (Sp-cAMPS) and by a cell-permeable protein phosphatase 1 and 2A inhibitor (okadaic acid). However, the activation of glutaminase by glucagon was not inhibited by a cell-permeable protein kinase A inhibitor (Rp-8-Br-cAMPS). We suggest that the signalling pathway, for glutaminase activation by glucagon, is complex and possibly contains redundant elements.