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

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.

Changes of plasma glutamine concentration and hepatocyte membrane system N transporters expression in early endotoxemia

BACKGROUND

Glutamine plays important roles in health and critical illness. During endotoxemia, glutamine metabolism, including its plasma level and transport, changes markedly. Previous studies have demonstrated that system N transporters in hepatocytes play a major role in hepatic glutamine transport. However, little is known about the changes of mRNA and protein expression of system N transporters in hepatocyte plasma membrane. Furthermore, the alteration of plasma glutamine concentration during endotoxemia is still controversial. In this study, we investigated the changes in early endotoxemic rats by intraperitoneal injection of lipopolysaccharide (LPS).

MATERIALS AND METHODS

Three, 6, 12 mg/kg body weight doses of LPS were injected intraperitoneally to establish the endotoxemic rat model; equal volume of 0.9% saline was used as the control. Before and 2, 4, 6, 12, 24h after injections, plasma glutamine concentration, mRNA, and protein expression of SNAT3 and SNAT5 transporters in hepatocyte plasma membrane were detected by high performance liquid chromatography, real-time PCR, and Western blot, respectively.

RESULTS

LPS injection resulted in a marked increase of the plasma glutamine concentration from 4 to 12h (3mg/kg) and 2 to 6h (6 mg/kg, 12 mg/kg) after the injection compared with its physiologic level, and a significant decrease in 6, 12 mg/kg groups at 24h. Both the mRNA and protein expression of SNAT3 and SNAT5 were enhanced by LPS in a time- and dose-dependent manner.

CONCLUSIONS

The plasma glutamine concentration in endotoxemic rat increased transiently during early endotoxemia but subsequently decreased over time. The effect of LPS on system N expression occurs not only at the protein level, but also at the mRNA level. It is reasonable to supplement glutamine for patients with sepsis or endotoxemia begin at 6 to 12h after the development of disease.

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.

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 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.

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."

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 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.

Glutamine, exercise and immune function. Links and possible mechanisms

Glutamine is the most abundant free amino acid in human muscle and plasma and is utilised at high rates by rapidly dividing cells, including leucocytes, to provide energy and optimal conditions for nucleotide biosynthesis. As such, it is considered to be essential for proper immune function. During various catabolic states including surgical trauma, infection, starvation and prolonged exercise, glutamine homeostasis is placed under stress. Falls in the plasma glutamine level (normal range 500 to 750 mumol/L after an overnight fast) have been reported following endurance events and prolonged exercise. These levels remain unchanged or temporarily elevated after short term, high intensity exercise. Plasma glutamine has also been reported to fall in patients with untreated diabetes mellitus, in diet-induced metabolic acidosis and in the recovery period following high intensity intermittent exercise. Common factors among all these stress states are rises in the plasma concentrations of cortisol and glucagon and an increased tissue requirement for glutamine for gluconeogenesis. It is suggested that increased gluconeogenesis and associated increases in hepatic, gut and renal glutamine uptake account for the depletion of plasma glutamine in catabolic stress states, including prolonged exercise. The short term effects of exercise on the plasma glutamine level may be cumulative, since heavy training has been shown to result in low plasma glutamine levels (< 500 mumol/L) requiring long periods of recovery. Furthermore, athletes experiencing discomfort from the overtraining syndrome exhibit lower resting levels of plasma glutamine than active healthy controls. Therefore, physical activity directly affects the availability of glutamine to the leucocytes and thus may influence immune function. The utility of plasma glutamine level as a marker of overtraining has recently been highlighted, but a consensus has not yet been reached concerning the best method of determining the level. Since injury, infection, nutritional status and acute exercise can all influence plasma glutamine level, these factors must be controlled and/or taken into consideration if plasma glutamine is to prove a useful marker of impending overtraining.

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.

Stimulation of rat hepatic amino acid transport by burn injury

Burn injury accelerates hepatic amino acid metabolism, but the role of transmembrane substrate delivery in this response has not been investigated. We therefore studied the effects of cutaneous scald injury on the Na+-dependent transport of glutamine and alanine in isolated rat liver plasma membrane vesicles. Scald injury resulted in liver damage and a 1.4- to 2.3-fold and 1.5- to 2.8-fold stimulation of hepatic transport rates for glutamine and alanine, respectively, proportional to the total burned surface area (TBSA) after 24 hours. Enhanced uptake of glutamine and alanine was attributable to increases in the maximum velocity (Vmax) of system N and system A activities, respectively. Hepatic amino acid transport activity remained elevated in vesicles from burned animals after 72 hours, but the degree of stimulation (1.3- to 1.7-fold for glutamine and 1.3- to 1.6-fold for alanine) was less than that observed 24 hours after thermal injury. Liver function tests returned to control values after 72 hours as well, indicating rectification of hepatic damage. In contrast to the induction of hepatic system A and system N activity in catabolic states such as cancer and endotoxemia, further studies showed that tumor necrosis factor (TNF) failed to play a significant role in burn-stimulated amino acid transport rates. When combined with plasma liver enzyme profiles, early transient hepatic amino acid transporter stimulation may support amino acid-dependent pathways involved in the repair of burn-dependent hepatic damage.

Hepatic glutaminase gene expression in the tumor-bearing rat

Previous studies have documented an increase in hepatic plasma membrane glutamine transport in the tumor-bearing rat, but the effects of tumor burden on hepatic glutaminase expression have not been carefully studied. The purpose of this study was to examine the effects of tumor burden and food intake on hepatic glutaminase expression. Rats were implanted with syngeneic methylcholanthrene-induced fibrosarcoma tumor tissue; control rats were sham operated and pair-fed every 24 hr. Northern blotting was used to assay the effect of tumor burden and fasting on hepatic glutaminase mRNA levels, using beta-actin mRNA as a control. Hepatic glutaminase mRNA levels in livers of pair-fed controls were found to be 4-fold greater than levels in livers of tumor-bearing animals. Examination of food intake patterns in these animals indicated that pair-fed controls ate their allotted chow quickly while tumor-bearing rats ate small amounts throughout each 24 hr period. This observation suggested that the differences in glutaminase mRNA levels may be due to a period of fasting by pair-fed animals which was not experienced by the tumor-bearing group. Hepatic glutaminase mRNA levels rapidly increased in normal rats during acute fasting to levels 5.5-fold greater than fed animals. Glucose feeding and insulin injection rapidly reversed the effect of fasting on hepatic glutaminase mRNA levels in normal rats. Tumor-bearing rats also exhibited upregulation of hepatic glutaminase mRNA levels in response to fasting.

CONCLUSIONS

(1) Tumor burden itself does not alter hepatic glutaminase expression, at least at the pre-translational level. Instead, differences in hepatic glutaminase mRNA content are due to differences in food intake patterns. (2) Hepatic glutaminase mRNA levels are rapidly upregulated in response to fasting, an effect which appears to be linked to a decrease in plasma insulin concentrations. Because tumor-bearing rats eat regularly over a 24 hr period (albeit in small increments), thereby maintaining the plasma insulin concentration, hepatic glutaminase mRNA may not rise as it does in pair-fed controls whose daily chow intake is complete within hours of food allocation. (3) This study indicates that differences in the timing of food intake between tumor-bearing rats and pair-fed controls can alter the expression of genes that are influenced by nutrient availability. These differences should be taken into account when designing studies which involve pair-feeding to control nutrient intake.