Maximum activities of some key enzymes of glycolysis, glutaminolysis, Krebs cycle and fatty acid utilization in bovine pulmonary endothelial cells

Cancer-Induced Metabolic Rewiring of Tumor Endothelial Cells

Simple Summary

Angiogenesis, the formation of new blood vessels from preexisting ones, is a complex and demanding biological process that plays an important role in physiological, as well as pathological conditions, including cancer. During tumor growth, the induction of angiogenesis allows tumor cells to grow, invade, and metastasize. Recent evidence supports endothelial cell metabolism as a critical regulator of angiogenesis. However, whether and how tumor endothelial cells rewire their metabolism in cancer remains elusive. In this review, we discussed the metabolic signatures of tumor endothelial cells and their symbiotic, competitive, and mechanical metabolic interactions with tumor cells. We also discussed the recent works that may provide a rationale for attractive metabolic targets and strategies for developing specific therapies against tumor angiogenesis.

Abstract

Cancer is a leading cause of death worldwide. If left untreated, tumors tend to grow and spread uncontrolled until the patient dies. To support this growth, cancer cells need large amounts of nutrients and growth factors that are supplied and distributed to the tumor tissue by the vascular system. The aberrant tumor vasculature shows deep morphological, molecular, and metabolic differences compared to the blood vessels belonging to the non-malignant tissues (also referred as normal). A better understanding of the metabolic mechanisms driving the differences between normal and tumor vasculature will allow the designing of new drugs with a higher specificity of action and fewer side effects to target tumors and improve a patient’s life expectancy. In this review, we aim to summarize the main features of tumor endothelial cells (TECs) and shed light on the critical metabolic pathways that characterize these cells. A better understanding of such mechanisms will help to design innovative therapeutic strategies in healthy and diseased angiogenesis.

The Relationship Between Reactive Oxygen Species and Endothelial Cell Metabolism

Cardiovascular disease (CVD) has been the leading cause of death for many decades, highlighting the importance of new research and treatments in the field. The role of hypoxia and subsequent free radical production [reactive oxygen species (ROS)] have become an area of particular interest in CVD. Interestingly, our laboratory and other laboratories have recently reported positive roles of subcellular ROS in modulating endothelial cell (EC) metabolism, proliferation, and angiogenesis. This bidirectional relationship between ROS and EC metabolism, as well as functional changes, continues to be an area of active research. Interestingly, ECs have been shown to rely on anaerobic processes for ATP generation, despite their direct access to oxygen. This paradox has proven to be beneficial as the major reliance on glycolysis produces ATP faster, preserves oxygen, and results in reduced ROS levels in contrast to oxidative phosphorylation. This review will address the relationship between ROS and carbohydrate, lipid, and nitrogen metabolism in ECs, and their effects on EC phenotype such as sprouting angiogenesis.

The Emerging Role of l-Glutamine in Cardiovascular Health and Disease

Emerging evidence indicates that l-glutamine (Gln) plays a fundamental role in cardiovascular physiology and pathology. By serving as a substrate for the synthesis of DNA, ATP, proteins, and lipids, Gln drives critical processes in vascular cells, including proliferation, migration, apoptosis, senescence, and extracellular matrix deposition. Furthermore, Gln exerts potent antioxidant and anti-inflammatory effects in the circulation by inducing the expression of heme oxygenase-1, heat shock proteins, and glutathione. Gln also promotes cardiovascular health by serving as an l-arginine precursor to optimize nitric oxide synthesis. Importantly, Gln mitigates numerous risk factors for cardiovascular disease, such as hypertension, hyperlipidemia, glucose intolerance, obesity, and diabetes. Many studies demonstrate that Gln supplementation protects against cardiometabolic disease, ischemia-reperfusion injury, sickle cell disease, cardiac injury by inimical stimuli, and may be beneficial in patients with heart failure. However, excessive shunting of Gln to the Krebs cycle can precipitate aberrant angiogenic responses and the development of pulmonary arterial hypertension. In these instances, therapeutic targeting of the enzymes involved in glutaminolysis such as glutaminase-1, Gln synthetase, glutamate dehydrogenase, and amino acid transaminase has shown promise in preclinical models. Future translation studies employing Gln delivery approaches and/or glutaminolysis inhibitors will determine the success of targeting Gln in cardiovascular disease.

Mitochondrial ATP production provides long-range control of endothelial inositol trisphosphate-evoked calcium signaling

Endothelial cells are reported to be glycolytic and to minimally rely on mitochondria for ATP generation. Rather than providing energy, mitochondria in endothelial cells may act as signaling organelles that control cytosolic Ca2+ signaling or modify reactive oxygen species (ROS). To control Ca2+ signaling, these organelles are often observed close to influx and release sites and may be tethered near Ca2+ transporters. In this study, we used high-resolution, wide-field fluorescence imaging to investigate the regulation of Ca2+ signaling by mitochondria in large numbers of endothelial cells (∼50 per field) in intact arteries from rats. We observed that mitochondria were mostly spherical or short-rod structures and were distributed widely throughout the cytoplasm. The density of these organelles did not increase near contact sites with smooth muscle cells. However, local inositol trisphosphate (IP3)-mediated Ca2+ signaling predominated near these contact sites and required polarized mitochondria. Of note, mitochondrial control of Ca2+ signals occurred even when mitochondria were far from Ca2+ release sites. Indeed, the endothelial mitochondria were mobile and moved throughout the cytoplasm. Mitochondrial control of Ca2+ signaling was mediated by ATP production, which, when reduced by mitochondrial depolarization or ATP synthase inhibition, eliminated local IP3-mediated Ca2+ release events. ROS buffering did not significantly alter local Ca2+ release events. These results highlight the importance of mitochondrial ATP production in providing long-range control of endothelial signaling via IP3-evoked local Ca2+ release in intact endothelium.

Measuring Glycolytic and Mitochondrial Fluxes in Endothelial Cells Using Radioactive Tracers

Endothelial cells (ECs) form the inner lining of the vascular network. Although they can remain quiescent for years, ECs exhibit high plasticity in both physiological and pathological conditions, when they need to rapidly form new blood vessels in a process called angiogenesis. EC metabolism recently emerged as an important driver of this angiogenic switch. The use of radioactive tracer substrates to assess metabolic flux rates in ECs has been essential for the discovery that fatty acid, glucose, and glutamine metabolism critically contribute to vessel sprouting. In the future, these assays will be useful as a tool for the characterization of pathological conditions in which deregulation of EC metabolism underlies and/or precedes the disease, but also for the identification of anti-angiogenic metabolic targets. This chapter describes in detail the radioactive tracer substrate assays that have been used for the determination of EC metabolic flux in vitro.

Glutaminase-1 stimulates the proliferation, migration, and survival of human endothelial cells

Glutaminase-1 (GLS1) is a mitochondrial enzyme found in endothelial cells (ECs) that metabolizes glutamine to glutamate and ammonia. Although glutaminolysis modulates the function of human umbilical vein ECs, it is not known whether these findings extend to human ECs beyond the fetal circulation. Furthermore, the molecular mechanism by which GLS1 regulates EC function is not defined. In this study, we show that the absence of glutamine in the culture media or the inhibition of GLS1 activity or expression blocked the proliferation and migration of ECs derived from the human umbilical vein, the human aorta, and the human microvasculature. GLS1 inhibition arrested ECs in the G0/G1 phase of the cell cycle and this was associated with a significant decline in cyclin A expression. Restoration of cyclin A expression via adenoviral-mediated gene transfer improved the proliferative, but not the migratory, response of GLS1-inhibited ECs. Glutamine deprivation or GLS1 inhibition also stimulated the production of reactive oxygen species and this was associated with a marked decline in heme oxygenase-1 (HO-1) expression. GLS1 inhibition also sensitized ECs to the cytotoxic effect of hydrogen peroxide and this was prevented by the overexpression of HO-1. In conclusion, the metabolism of glutamine by GLS1 promotes human EC proliferation, migration, and survival irrespective of the vascular source. While cyclin A contributes to the proliferative action of GLS1, HO-1 mediates its pro-survival effect. These results identify GLS1 as a promising therapeutic target in treating diseases associated with aberrant EC proliferation, migration, and viability.

Endothelial Cell Metabolism

Endothelial cells (ECs) are more than inert blood vessel lining material. Instead, they are active players in the formation of new blood vessels (angiogenesis) both in health and (life-threatening) diseases. Recently, a new concept arose by which EC metabolism drives angiogenesis in parallel to well-established angiogenic growth factors (e.g., vascular endothelial growth factor). 6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3-driven glycolysis generates energy to sustain competitive behavior of the ECs at the tip of a growing vessel sprout, whereas carnitine palmitoyltransferase 1a-controlled fatty acid oxidation regulates nucleotide synthesis and proliferation of ECs in the stalk of the sprout. To maintain vascular homeostasis, ECs rely on an intricate metabolic wiring characterized by intracellular compartmentalization, use metabolites for epigenetic regulation of EC subtype differentiation, crosstalk through metabolite release with other cell types, and exhibit EC subtype-specific metabolic traits. Importantly, maladaptation of EC metabolism contributes to vascular disorders, through EC dysfunction or excess angiogenesis, and presents new opportunities for anti-angiogenic strategies. Here we provide a comprehensive overview of established as well as newly uncovered aspects of EC metabolism.

Role of glutamine and interlinked asparagine metabolism in vessel formation

Endothelial cell (EC) metabolism is emerging as a regulator of angiogenesis, but the precise role of glutamine metabolism in ECs is unknown. Here, we show that depriving ECs of glutamine or inhibiting glutaminase 1 (GLS1) caused vessel sprouting defects due to impaired proliferation and migration, and reduced pathological ocular angiogenesis. Inhibition of glutamine metabolism in ECs did not cause energy distress, but impaired tricarboxylic acid (TCA) cycle anaplerosis, macromolecule production, and redox homeostasis. Only the combination of TCA cycle replenishment plus asparagine supplementation restored the metabolic aberrations and proliferation defect caused by glutamine deprivation. Mechanistically, glutamine provided nitrogen for asparagine synthesis to sustain cellular homeostasis. While ECs can take up asparagine, silencing asparagine synthetase (ASNS, which converts glutamine-derived nitrogen and aspartate to asparagine) impaired EC sprouting even in the presence of glutamine and asparagine. Asparagine further proved crucial in glutamine-deprived ECs to restore protein synthesis, suppress ER stress, and reactivate mTOR signaling. These findings reveal a novel link between endothelial glutamine and asparagine metabolism in vessel sprouting.

Glutamine fuels proliferation but not migration of endothelial cells

Endothelial metabolism is a key regulator of angiogenesis. Glutamine metabolism in endothelial cells (ECs) has been poorly studied. We used genetic modifications and ¹³C tracing approaches to define glutamine metabolism in these cells. Glutamine supplies the majority of carbons in the tricyclic acid (TCA) cycle of ECs and contributes to lipid biosynthesis via reductive carboxylation. EC-specific deletion in mice of glutaminase, the initial enzyme in glutamine catabolism, markedly blunts angiogenesis. In cell culture, glutamine deprivation or inhibition of glutaminase prevents EC proliferation, but does not prevent cell migration, which relies instead on aerobic glycolysis. Without glutamine catabolism, there is near complete loss of TCA intermediates, with no compensation from glucose-derived anaplerosis. Mechanistically, addition of exogenous alpha-ketoglutarate replenishes TCA intermediates and rescues cellular growth, but simultaneously unveils a requirement for Rac1-dependent macropinocytosis to provide non-essential amino acids, including asparagine. Together, these data outline the dependence of ECs on glutamine for cataplerotic processes; the need for glutamine as a nitrogen source for generation of biomass; and the distinct roles of glucose and glutamine in EC biology.

Endothelial cell metabolism in health and disease: impact of hypoxia

In contrast to the general belief, endothelial cell (EC) metabolism has recently been identified as a driver rather than a bystander effect of angiogenesis in health and disease. Indeed, different EC subtypes present with distinct metabolic properties, which determine their function in angiogenesis upon growth factor stimulation. One of the main stimulators of angiogenesis is hypoxia, frequently observed in disease settings such as cancer and atherosclerosis. It has long been established that hypoxic signalling and metabolism changes are highly interlinked. In this review, we will provide an overview of the literature and recent findings on hypoxia-driven EC function and metabolism in health and disease. We summarize evidence on metabolic crosstalk between different hypoxic cell types with ECs and suggest new metabolic targets.

Ammonia promotes endothelial cell survival via the heme oxygenase-1-mediated release of carbon monoxide

Although endothelial cells produce substantial quantities of ammonia during cell metabolism, the physiologic role of this gas in these cells is not known. In this study, we investigated if ammonia regulates the expression of heme oxygenase-1 (HO-1), and if this enzyme influences the biological actions of ammonia on endothelial cells. Exogenously administered ammonia, given as ammonium chloride or ammonium hydroxide, or endogenously generated ammonia stimulated HO-1 protein expression in cultured human and murine endothelial cells. Dietary supplementation of ammonia also induced HO-1 protein expression in murine arteries. The increase in HO-1 protein by ammonia in endothelial cells was first detected 4h after ammonia exposure and was associated with the induction of HO-1 mRNA, enhanced production of reactive oxygen species (ROS), and increased expression and activity of NF-E2-related factor-2 (Nrf2). Ammonia also activated the HO-1 promoter and this was blocked by mutating the antioxidant responsive element or by overexpressing dominant-negative Nrf2. The induction of HO-1 expression by ammonia was dependent on ROS formation and prevented by N-acetylcysteine or rotenone. Finally, prior treatment of endothelial cells with ammonia inhibited tumor necrosis factor-α-stimulated cell death. However, silencing HO-1 expression abrogated the protective action of ammonia and this was reversed by the administration of carbon monoxide but not bilirubin or iron. In conclusion, this study demonstrates that ammonia stimulates the expression of HO-1 in endothelial cells via the ROS-Nrf2 pathway, and that the induction of HO-1 contributes to the cytoprotective action of ammonia by generating carbon monoxide. Moreover, it identifies ammonia as a potentially important signaling gas in the vasculature that promotes endothelial cell survival.

Understanding the Causes and Implications of Endothelial Metabolic Variation in Cardiovascular Disease through Genome-Scale Metabolic Modeling

High-throughput biochemical profiling has led to a requirement for advanced data interpretation techniques capable of integrating the analysis of gene, protein, and metabolic profiles to shed light on genotype-phenotype relationships. Herein, we consider the current state of knowledge of endothelial cell (EC) metabolism and its connections to cardiovascular disease (CVD) and explore the use of genome-scale metabolic models (GEMs) for integrating metabolic and genomic data. GEMs combine gene expression and metabolic data acting as frameworks for their analysis and, ultimately, afford mechanistic understanding of how genetic variation impacts metabolism. We demonstrate how GEMs can be used to investigate CVD-related genetic variation, drug resistance mechanisms, and novel metabolic pathways in ECs. The application of GEMs in personalized medicine is also highlighted. Particularly, we focus on the potential of GEMs to identify metabolic biomarkers of endothelial dysfunction and to discover methods of stratifying treatments for CVDs based on individual genetic markers. Recent advances in systems biology methodology, and how these methodologies can be applied to understand EC metabolism in both health and disease, are thus highlighted.

Metabolic signatures of Besnoitia besnoiti-infected endothelial host cells and blockage of key metabolic pathways indicate high glycolytic and glutaminolytic needs of the parasite

Besnoitia besnoiti is an obligate intracellular and emerging coccidian parasite of cattle with a significant economic impact on cattle industry. During acute infection, fast-proliferating tachyzoites are continuously formed mainly in endothelial host cells of infected animals. Given that offspring formation is a highly energy and cell building block demanding process, the parasite needs to exploit host cellular metabolism to meet its metabolic demands. Here, we analyzed the metabolic signatures of B. besnoiti-infected endothelial host cells and aimed to influence parasite proliferation by inhibitors of specific metabolic pathways. The following inhibitors were tested: fluoro 2-deoxy-D-glucose and 2-deoxy-D-glucose (FDG, DG; inhibitors of glycolysis), 6-diazo-5-oxo-L-norleucin (DON; inhibitor of glutaminolysis), dichloroacetate (DCA; inhibitor of pyruvate dehydrogenase kinase which favorites channeling of glucose carbons into the TCA cycle) and adenosine-monophosphate (AMP; inhibitor of ribose 5-P synthesis). Overall, B. besnoiti infections of bovine endothelial cells induced a significant and infection rate-dependent increase of glucose, lactate, glutamine, glutamate, pyruvate, alanine, and serine conversion rates which together indicate a parasite-triggered up-regulation of glycolysis and glutaminolysis. Thus, addition of DON, FDG, and DG into the cultivation medium of B. besnoiti infected endothelial cells led to a dose-dependent inhibition of parasite replication (4 μM DON, 99.5 % inhibition; 2 mM FDG, 99.1 % inhibition; 2 mM DG, 93 % inhibition; and 8 mM DCA, 71.9 % inhibition). In contrast, AMP had no significant effects on total tachyzoite production up to a concentration of 20 mM. Together, these data may open new strategies for the development of therapeutics for B. besnoiti infections.

Mitochondrial Regulation of the Muscle Microenvironment in Critical Limb Ischemia

Critical limb ischemia (CLI) is the most severe clinical presentation of peripheral arterial disease and manifests as chronic limb pain at rest and/or tissue necrosis. Current clinical interventions are largely ineffective and therapeutic angiogenesis based trials have shown little efficacy, highlighting the dire need for new ideas and novel therapeutic approaches. Despite a decade of research related to skeletal muscle as a determinant of morbidity and mortality outcomes in CLI, very little progress has been made toward an effective therapy aimed directly at the muscle myopathies of this disease. Within the muscle cell, mitochondria are well positioned to modulate the ischemic cellular response, as they are the principal sites of cellular energy production and the major regulators of cellular redox charge and cell death. In this mini review, we update the crucial importance of skeletal muscle to CLI pathology and examine the evolving influence of muscle and endothelial cell mitochondria in the complex ischemic microenvironment. Finally, we discuss the novelty of muscle mitochondria as a therapeutic target for ischemic pathology in the context of the complex co-morbidities often associated with CLI.

Principles of targeting endothelial cell metabolism to treat angiogenesis and endothelial cell dysfunction in disease

The endothelium is the orchestral conductor of blood vessel function. Pathological blood vessel formation (a process termed pathological angiogenesis) or the inability of endothelial cells (ECs) to perform their physiological function (a condition known as EC dysfunction) are defining features of various diseases. Therapeutic intervention to inhibit aberrant angiogenesis or ameliorate EC dysfunction could be beneficial in diseases such as cancer and cardiovascular disease, respectively, but current strategies have limited efficacy. Based on recent findings that pathological angiogenesis and EC dysfunction are accompanied by EC-specific metabolic alterations, targeting EC metabolism is emerging as a novel therapeutic strategy. Here, we review recent progress in our understanding of how EC metabolism is altered in disease and discuss potential metabolic targets and strategies to reverse EC dysfunction and inhibit pathological angiogenesis.

Proteomics-based metabolic modeling reveals that fatty acid oxidation (FAO) controls endothelial cell (EC) permeability

Endothelial cells (ECs) play a key role to maintain the functionality of blood vessels. Altered EC permeability causes severe impairment in vessel stability and is a hallmark of pathologies such as cancer and thrombosis. Integrating label-free quantitative proteomics data into genome-wide metabolic modeling, we built up a model that predicts the metabolic fluxes in ECs when cultured on a tridimensional matrix and organize into a vascular-like network. We discovered how fatty acid oxidation increases when ECs are assembled into a fully formed network that can be disrupted by inhibiting CPT1A, the fatty acid oxidation rate-limiting enzyme. Acute CPT1A inhibition reduces cellular ATP levels and oxygen consumption, which are restored by replenishing the tricarboxylic acid cycle. Remarkably, global phosphoproteomic changes measured upon acute CPT1A inhibition pinpointed altered calcium signaling. Indeed, CPT1A inhibition increases intracellular calcium oscillations. Finally, inhibiting CPT1A induces hyperpermeability in vitro and leakage of blood vessel in vivo, which were restored blocking calcium influx or replenishing the tricarboxylic acid cycle. Fatty acid oxidation emerges as central regulator of endothelial functions and blood vessel stability and druggable pathway to control pathological vascular permeability.

Embelin inhibits endothelial mitochondrial respiration and impairs neoangiogenesis during tumor growth and wound healing

In the normal quiescent vasculature, only 0.01% of endothelial cells (ECs) are proliferating. However, this proportion increases dramatically following the angiogenic switch during tumor growth or wound healing. Recent evidence suggests that this angiogenic switch is accompanied by a metabolic switch. Here, we show that proliferating ECs increasingly depend on mitochondrial oxidative phosphorylation (OxPhos) for their increased energy demand. Under growth conditions, ECs consume three times more oxygen than quiescent ECs and work close to their respiratory limit. The increased utilization of the proton motif force leads to a reduced mitochondrial membrane potential in proliferating ECs and sensitizes to mitochondrial uncoupling. The benzoquinone embelin is a weak mitochondrial uncoupler that prevents neoangiogenesis during tumor growth and wound healing by exhausting the low respiratory reserve of proliferating ECs without adversely affecting quiescent ECs. We demonstrate that this can be exploited therapeutically by attenuating tumor growth in syngenic and xenograft mouse models. This novel metabolic targeting approach might be clinically valuable in controlling pathological neoangiogenesis while sparing normal vasculature and complementing cytostatic drugs in cancer treatment.

Role of endothelial cell metabolism in vessel sprouting

Endothelial cells (ECs) are quiescent for years but can plastically switch to angiogenesis. Vascular sprouting relies on the coordinated activity of migrating tip cells at the forefront and proliferating stalk cells that elongate the sprout. Past studies have identified genetic signals that control vascular branching. Prominent are VEGF, activating tip cells, and Notch, which stimulates stalk cells. After the branch is formed and perfused, ECs become quiescent phalanx cells. Now, emerging evidence has accumulated indicating that ECs not only adapt their metabolism when switching from quiescence to sprouting but also that metabolism regulates vascular sprouting in parallel to the control by genetic signals.

Role of PFKFB3-driven glycolysis in vessel sprouting

Vessel sprouting by migrating tip and proliferating stalk endothelial cells (ECs) is controlled by genetic signals (such as Notch), but it is unknown whether metabolism also regulates this process. Here, we show that ECs relied on glycolysis rather than on oxidative phosphorylation for ATP production and that loss of the glycolytic activator PFKFB3 in ECs impaired vessel formation. Mechanistically, PFKFB3 not only regulated EC proliferation but also controlled the formation of filopodia/lamellipodia and directional migration, in part by compartmentalizing with F-actin in motile protrusions. Mosaic in vitro and in vivo sprouting assays further revealed that PFKFB3 overexpression overruled the pro-stalk activity of Notch, whereas PFKFB3 deficiency impaired tip cell formation upon Notch blockade, implying that glycolysis regulates vessel branching.

Endothelial cell metabolism and tumour angiogenesis: glucose and glutamine as essential fuels and lactate as the driving force

Angiogenic endothelial cells and tumour cells can survive under hypoxic conditions and even proliferate and migrate in a low-oxygen environment. In both cell types, high rates of glycolysis (i.e. conversion of glucose to lactate) and glutaminolysis provide most of the required biosynthetic intermediates and energy to support sprouting and cell division without coupling to oxidative phosphorylation. This metabolic preference is observed under hypoxic conditions, but also in situations in which oxygen is present. In the case of tumour cells, this is known as the Warburg effect and is largely governed by oncogenes. In endothelial cells lining tumour blood vessels, the option of respiration-independent metabolism allows the neovasculature to resist the hostile environment of fluctuating oxygen tension (ranging from severe hypoxia to quasi-normal levels of oxygen). In addition, accumulation in tumours of lactate, the end-product of glycolysis, largely contributes to the angiogenic phenotype through inhibition of prolyl hydroxylase 2 and the activation of HIF1α and NFκB. Activation of the latter in a hypoxia-independent manner leads to the increased production of interleukin-8/CXCL8 which drives the autocrine stimulation of endothelial cell proliferation and maturation of neovessels. In conclusion, the addiction of proliferating endothelial cells for glucose and glutamine as fuels and the driving force of lactate to promote angiogenesis provide novel potential treatment options without the disadvantages of conventional anti-angiogenic drugs.

Porcine liver sinusoidal endothelial cells contribute significantly to intrahepatic ammonia metabolism

Ammonia metabolism in the liver has been largely credited to hepatocytes (HCs). We have shown that liver nonparenchymal cells that include liver sinusoidal endothelial cells (LSECs) produce ammonia. To address the limited knowledge regarding a role for LSECs in ammonia metabolism, we investigated the ammonia metabolism of isolated LSECs and HCs under three different conditions: (1) bioreactors containing LSECs (LSEC-bioreactors), (2) bioreactors containing HCs (HC-bioreactors), and (3) separate bioreactors containing LSECs and HCs connected in sequence (Seq-bioreactors). Our results showed that LSEC-bioreactors released six-fold more ammonia (22.2 nM/hour/10(6) cells) into the growth media than HC-bioreactors (3.3 nM/hour/10(6) cells) and Seq-bioreactors (3.8 nM/hour/10(6) cells). The glutamate released by LSEC-bioreactors (32.0 nM/hour/10(6) cells) was over four-fold larger than that released by HC-bioreactors and Seq-bioreactors (<7 nM/hour/10(6) cells). LSEC-bioreactors and HC-bioreactors consumed large amounts of glutamine (>25 nM/hour/10(6) cells). Glutaminase is known for catalyzing glutamine into glutamate and ammonia. To determine if this mechanism may be responsible for the large levels of glutamate and ammonia found in LSEC-bioreactors, immunolabeling of glutaminase and messenger RNA expression were tested. Our results demonstrated that glutaminase was present with colocalization of an LSEC-specific functional probe in lysosomes of LSECs. Furthermore, using a nucleotide sequence specific for kidney-type glutaminase, reverse-transcription polymerase chain reaction revealed that this isoform of glutaminase was expressed in porcine LSECs.

CONCLUSION

LSECs released large amounts of ammonia, perhaps due to the presence of glutaminase in lysosomes. The ammonia and glutamate released by LSECs in Seq-bioreactors were used by hepatocytes, suggesting an intrahepatic collaboration between these two cell types.

Pathways involved in alanyl-glutamine-induced changes in neutrophil amino- and α-keto acid homeostasis or immunocompetence

We examined the effects of DON [glutamine-analogue and inhibitor of glutamine-requiring enzymes], alanyl-glutamine (regarding its role in neutrophil immunonutrition) and alanyl-glutamine combined with L-NAME, SNAP, DON, beta-alanine and DFMO on neutrophil amino and alpha-keto acid concentrations or important neutrophil immune functions in order to establish whether an inhibitor of *NO-synthase [L-NAME], an *NO donor [SNAP], an analogue of taurine and a taurine transport antagonist [beta-alanine], an inhibitor of ornithine-decarboxylase [DFMO] as well as DON could influence any of the alanyl-glutamine-induced effects. In summary, irrespective of which pharmacological, metabolism-inhibiting or receptor-mediated mechanisms were involved, our results showed that impairment of granulocytic glutamine uptake, modulation of intracellular glutamine metabolisation and/or de novo synthesis as well as a blockade of important glutamine-dependent metabolic processes may led to significant modifications of physiological and immunological functions of the affected cells.

Regulation of amino acid and glucose transporters in endothelial and smooth muscle cells

While transport processes for amino acids and glucose have long been known to be expressed in the luminal and abluminal membranes of the endothelium comprising the blood-brain and blood-retinal barriers, it is only within the last decades that endothelial and smooth muscle cells derived from peripheral vascular beds have been recognized to rapidly transport and metabolize these nutrients. This review focuses principally on the mechanisms regulating amino acid and glucose transporters in vascular endothelial cells, although we also summarize recent advances in the understanding of the mechanisms controlling membrane transport activity and expression in vascular smooth muscle cells. We compare the specificity, ionic dependence, and kinetic properties of amino acid and glucose transport systems identified in endothelial cells derived from cerebral, retinal, and peripheral vascular beds and review the regulation of transport by vasoactive agonists, nitric oxide (NO), substrate deprivation, hypoxia, hyperglycemia, diabetes, insulin, steroid hormones, and development. In view of the importance of NO as a modulator of vascular tone under basal conditions and in disease and chronic inflammation, we critically review the evidence that transport of L-arginine and glucose in endothelial and smooth muscle cells is modulated by bacterial endotoxin, proinflammatory cytokines, and atherogenic lipids. The recent colocalization of the cationic amino acid transporter CAT-1 (system y(+)), nitric oxide synthase (eNOS), and caveolin-1 in endothelial plasmalemmal caveolae provides a novel mechanism for the regulation of NO production by L-arginine delivery and circulating hormones such insulin and 17beta-estradiol.

Glutamine metabolism in endothelial cells: ornithine synthesis from glutamine via pyrroline-5-carboxylate synthase

L-Glutamine (the most abundant free amino acid in plasma and the body) is a potent inhibitor of endothelial NO synthesis. However, little is known about glutamine metabolism in endothelial cells (EC). As an initial step toward understanding the role of glutamine in endothelial physiology, the present study was conducted to quantify glutamine catabolism in microvascular, aortic and venous EC. For metabolic studies, EC were incubated for 1 h in Krebs bicarbonate buffer containing 5 mM glucose and 0.5-4 mM L-[U-(14)C]-glutamine. For enzymological studies, cell extracts and mitochondrial fractions were prepared to determine the activities of glutamine-degrading enzymes. Our results reveal extensive hydrolysis of glutamine to glutamate and ammonia in a concentration-dependent manner via phosphate-dependent glutaminase in all EC studied. In addition, both metabolic and enzymological evidence indicate a novel pathway for endothelial synthesis of ornithine from glutamine via pyrroline-5-carboxylate synthase. This new knowledge of glutamine metabolism may pave a new path for understanding the physiological role of glutamine in vascular function.

ATP converts necrosis to apoptosis in oxidant-injured endothelial cells

Cell death due to necrosis results in acute inflammation, while death by apoptosis generally does not. The effect of adenosine triphosphate (ATP) on the pattern of cell death induced by oxidants was examined in bovine endothelial cells. ATP levels were altered by hydrogen peroxide (H2O2), glutamine (Gln), and metabolic inhibition (MI), to determine if necrosis can be shifted to apoptosis during oxidant injury. The form of cell death was determined by fluorescence microscopic techniques and the pattern of DNA degradation on agarose gels. ATP levels were measured using the luciferase-luciferin assay. Apoptosis occurred with 100 microM H2O2 without an alteration in ATP levels. ATP was significantly lowered with 5 mM H2O2, and necrosis occurred. MI, in combination with 100 microM H2O2, decreased ATP and resulted in necrosis. MI alone, however, did not cause cell death. Gln partially restored ATP levels in cells injured with 5 mM H2O2 and resulted in a significant increase in apoptosis. DNA laddering on agarose gels confirmed the apoptotic changes seen by fluorescence microscopy. In summary, a threshold level of ATP 25% of basal levels is required for apoptosis to proceed after oxidant stress, otherwise necrosis occurs. Agents like glutamine that enhance ATP levels in oxidant-stressed cells may be potent means of shifting cell death during inflammation to the noninflammatory form of death--apoptosis.

Glutamine transport by the blood-brain barrier: a possible mechanism for nitrogen removal

Glutamine and glutamate transport activities were measured in isolated luminal and abluminal plasma membrane vesicles derived from bovine brain endothelial cells. Facilitative systems for glutamine and glutamate were almost exclusively located in luminal-enriched membranes. The facilitative glutamine carrier was neither sensitive to 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid inhibition nor did it participate in accelerated amino acid exchange; it therefore appeared to be distinct from the neutral amino acid transport system L1. Two Na-dependent glutamine transporters were found in abluminal-enriched membranes: systems A and N. System N accounted for approximately 80% of Na-dependent glutamine transport at 100 microM. Abluminal-enriched membranes showed Na-dependent glutamate transport activity. The presence of 1) Na-dependent carriers capable of pumping glutamine and glutamate from brain into endothelial cells, 2) glutaminase within endothelial cells to hydrolyze glutamine to glutamate and ammonia, and 3) facilitative carriers for glutamine and glutamate at the luminal membrane may provide a mechanism for removing nitrogen and nitrogen-rich amino acids from brain.

Growth requirements of endothelial cells in culture: variations in serum and amino acid concentrations

Endothelial cell growth in vitro is limited to the availability of nutrients from commercially available media and added serum. Nutrients, such as amino acids, are chiefly derived from the cell culture medium, rather than from added serum, and optimal endothelial cell growth may be dependent on amino acid levels in the culture media. To test this hypothesis, porcine pulmonary artery-derived endothelial cells were exposed to culture medium 199 (M199), amino acid-deficient M199 (dM199), as well as dM199 supplemented with amino acids. Cell protein was similar in cells cultured for 3 d in M199 supplemented with 1, 3, 5 or 10% bovine calf serum, respectively. Addition of amino acid solutions (L-amino acids [Laa], DL-amino acids [DLaa], 2Laa, or Laa+glutamine) to dM199 demonstrated a cell dependence for optimal growth on the type of amino acids as well as on the total available nitrogen in the media. Compared with M199, dM199 supplemented with Laa only partially supported long-term growth of endothelial cells in culture. On the other hand, dM199 supplemented with either 2Laa, DLaa, or Laa+ glutamine was superior over M199 with regard to endothelial cell growth. The addition of Laa+glutamine to dM199 was most growth-supporting, with an increase of over 2.6-fold in total cell protein compared with cells cultured with M199. These results suggest that, in addition to the presence of essential amino acids, total available nitrogen in culture media may be a critical factor for optimal endothelial cell growth.

Deamination of amino acids as a source for ammonia production in human skeletal muscle during prolonged exercise

1. The influence of pre-exercise muscle glycogen content on ammonia production, adenine nucleotide breakdown and amino acid metabolism was investigated during prolonged exercise in six subjects having one leg with a normal and one leg with a low muscle glycogen content. One-leg knee-extensor exercise was performed for 90 min, at a workload of 60-65% of the maximal power output, first with one leg and then with the other. 2. During exercise ammonia was released in gradually increasing amounts and plateaued after 1 h exercise at a rate of approximately 80 mumol min-1. The total ammonia production was 9.1 +/- 0.4 and 9.5 +/- 1.4 mmol (kg dry muscle)-1 in the normal and low glycogen content leg, respectively. 3. Levels of muscle phosphocreatine (PC), total adenine nucleotides and inosine monophosphate (IMP) were similar at rest and after 90 min of exercise. 4. Only minor differences were observed between rest and exercise and between legs for the muscle concentrations of glutamine, alanine and the branched-chain amino acids. Muscle glutamate concentration decreased by 60-70% within the first 10 min of exercise. Glutamate consumption over 90 min quantitatively equalled ammonia production. Most of the glutamate was consumed within the first 10 min of exercise, while ammonia production gradually increased during exercise. Therefore deamination of glutamate cannot be the main source of ammonia production during the later stage of exercise. 5. It is concluded that during prolonged one-leg exercise at moderate intensity: (a) ammonia production is not affected by pre-exercise muscle glycogen content, (b) ammonia production exceeds by far the breakdown of adenine nucleotides to IMP and therefore has to be derived from alternative sources, and (c) deamination of amino acids is a likely source of ammonia production during prolonged exercise.

Interactions between L-arginine and L-glutamine change endothelial NO production. An effect independent of NO synthase substrate availability

The effect of extracellular L-arginine and L-glutamine on nitric oxide (NO) release was studied in cultured bovine aortic endothelial cells and in rabbit aortic rings. Increasing L-arginine (0.01 to 10 mM) did not alter NO release from cultured endothelial cells or modify endothelium-dependent relaxation to acetylcholine in isolated vessels. L-Glutamine (0.6 and 2 mM) inhibited NO release from cultured cells (in response to bradykinin) and from aortic rings (in response to acetylcholine or ADP). L-Arginine (0.1-10 mM) dose-dependently reversed the L-glutamine inhibition of receptor-stimulated NO release in both models. In contrast to its inhibitory response to receptor-mediated stimuli, glutamine alone slightly potentiated NO release in both models when the calcium ionophore, A23187, was added. Furthermore, cultured cells incubated with L-arginine (0.01-10 mM), in the presence or absence of glutamine, released similar amounts of NO in response to A23187. L-Glutamine did not affect intracellular L-arginine levels. Neither D-glutamine nor D-arginine affected NO release or endothelium-dependent vascular relaxation. L-Glutamine had no effect on the activity of endothelial NOS assessed by L-arginine to L-citrulline conversion. These findings show that in the absence of L-glutamine, manipulating intracellular L-arginine levels over a wide range does not affect NO release. L-Glutamine in concentrations circulating in vivo may tonically inhibit receptor-mediated NO release by interfering with signal transduction. One mechanism by which L-arginine may enhance NO release is via reversal of the inhibitory effect of L-glutamine, but apparently independently of enhancing NO synthase substrate.

Reduced PO2 alters the behavior of Fura-2 and Indo-1 in bovine pulmonary artery endothelial cells

Calcium-sensitive fluorophores are used to estimate cytosolic free Ca2+ in many cell types under various conditions. We tested the effect of reduced PO2 on the behavior of Fura-2 and Indo-1 in cultured bovine pulmonary artery endothelial cells. Reduced PO2 (PO2 25-35 mmHg) caused a significant upward shift of in vivo calibration curves for both fluorophores. The in vivo emission spectrum of Fura-2 indicated that the effect was principally due to attenuated emission at the Ca(2+)-unbound 380 nm wavelength, with no shift in position of the emission maxima for either Ca(2+)-bound or unbound forms of the fluorophore. Reduced PO2 did not directly alter the behavior of the dyes, as no shift of in vitro calibration curves was seen. Neither decreased photobleaching nor altered autofluorescence accounted for the shift. We investigated several potential indirect effects, including cellular acidification, reduced viscosity, inhibition of oxidative energy production and reductive stress. In contrast to lowered PO2, acidification in vitro produced a leftward but not an upward shift. Estimation of intracellular pH with SNAFL-calcein under reduced PO2 showed no apparent acidification in these cells, further strengthening the argument that altered intracellular pH was not causing the shift. Others have shown that decreases in viscosity in vitro may shift the calibration curve for Fura-2 upward, similar to our finding with reduced PO2. However, for Indo-1 we found that decreased viscosity in vitro attenuated fluorescence emission at the Ca(2+)-bound 405 nm wavelength, thus producing the opposite effect on fluorescence ratio and indicating that reduced PO2 was not acting through changes in cellular microviscosity.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin stimulates glycolysis and pentose cycle activity in bovine microvascular endothelial cells

Glucose metabolism via the pentose cycle, glycolysis and the Krebs cycle was quantified in bovine microvascular endothelial cells. The major measured end-product of glucose was L-lactate, with relatively small amounts of glucose carbons converted to CO2 and pyruvate. The pentose cycle accounted for less than 4% of the glucose utilized. About 60-70% of the metabolized glucose carbons could not be accounted for by lactate, pyruvate and CO2. Insulin stimulated glycolysis and pentose cycle activity, but had no effect on glucose oxidation via the Krebs cycle. As the pentose cycle is a major source of NADPH which is required for the synthesis of nitric oxide (the endothelium relaxing factor), insulin may play a role in regulating NO generation in endothelial cells by modulating the pentose cycle activity.

Nutrition, endothelial cell metabolism, and atherosclerosis

The vascular endothelium that forms an interface between the blood and the surrounding tissues is continuously exposed to both physiologic and pathophysiologic stimuli. These stimuli are often mediated by nutrients that can contribute to the overall function of the endothelial cell in the regulation of vascular tone, coagulation and fibrinolysis, cellular growth and differentiation, and immune and inflammatory responses. Therefore, nutrient-mediated functional changes of the endothelium and the underlying tissues may be significantly involved in the atherosclerotic disease process. There is evidence that individual nutrients or nutrient derivatives may either provoke or prevent metabolic and physiologic perturbations of the vascular endothelium. Preservation of nutrients that exhibit antiatherogenic properties may, therefore, be a critical issue in the preparation and processing of foods. This review focuses on selected nutrients as they affect endothelial cell metabolism and their possible implications in atherosclerosis.

Total parenteral nutrition with glutamine in bone marrow transplantation and other clinical applications (a randomized, double-blind study)

In a paper by Ziegler et al (Ann Intern Med 116: 821-828, 1992), total parenteral nutrition supplemented with L-glutamine (TPN/GLN) was reported beneficial in patients receiving bone marrow transplantation (BMT) for hematologic malignancies. By using a similar protocol, we studied 29 patients with both hematologic malignancies and solid tumors, and with both allogeneic and autologous BMTs. In a double-blind, randomized approach, patients were given isocaloric, isonitrogenous TPN after BMT until they consumed 50% of their required diet orally. Total body water and extracellular water were measured before and after TPN in 10 patients. Total body water increased in patients receiving standard TPN and decreased significantly in patients receiving TPN/GLN. Length of hospital stay after BMT was significantly (5.8 days) less in patients receiving TPN/GLN. Incidence of positive bacterial cultures, clinical infections, and mortality did not differ significantly between the two groups. When the groups were subdivided into patients with hematologic malignancies and those with solid tumors, there were no significant differences in the above variables associated with TPN/GLN. In 17 of 30 additional hospitalized patients receiving standard TPN, substitution of TPN/GLN did not have discernible clinical or laboratory effects but appeared to be safe. Inclusion of patients with solid tumors and a higher mortality in our patients may have obscured beneficial effects of TPN/GLN observed by others.

Lipid metabolism of myocardial endothelial cells

The vascular endothelium can be regarded as a widely distributed organ, interposed between the intravascular and extravascular spaces, with a pluripotent function in the regulation of capillary diameter, vascular homeostasis, lipoprotein metabolism and the vascular response to injury. In the basal physiological state these processes provide a non-thrombotic, non-inflammatory vascular lining preventing uncontrolled inflammation and coagulation. Endothelial cells respond to potential harmful conditions (mechanical stress, anoxia, ischemia and oxidative stress) and a variety of hormones and vasoactive mediators by inducing coagulation and production of inflammatory mediators through the production of 'bioactive' lipids. Although the number of studies in isolated myocardial endothelial cells is limited, from the presumed metabolic analogy with endothelial cells isolated (and cultured) from other organs, one may conclude that the bioactive lipids include oxygenated arachidonate metabolites (eicosanoids) and the platelet activating factor (1--O-alkyl-2-acetyl-sn-glycerol-3-phosphocholine; PAF). All aspects of lipid metabolism, related to the production of eicosanoids and PAF, are present within myocardial endothelial cells. There is uptake and incorporation of fatty acids by endothelial cells and liberation from endogenous triacylglycerol and (membrane) phospholipid stores by (phospho)lipases. Endothelial cells oxidize fatty acids in a carnitine-dependent, mitochondrial, pathway. Endothelial cells actively interact with high density lipoprotein (HDL) and low density lipoprotein (LDL) leading to uptake of cholesterol(esters) that undergo intracellular hydrolysis, and re-esterification to phospho- and neutral lipids, and leaving the LDL-particle modified in a way that makes them bind to the scavenger receptor on macrophages.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of endotoxin on glutamine transport by pulmonary artery endothelial cells

The effects of endotoxin on glutamine transport by cultured pulmonary artery endothelial cells (PAECs) were studied in order to gain further insight into the regulation of the altered lung glutamine metabolism that characterizes severe infection. Incubation of PAECs with endotoxin (1 micrograms/ml) resulted in a significant increase in System ASC-mediated glutamine transport which did not occur for 8 hr and was maximal after 12 hr of exposure. Kinetic studies indicated that the increase in carrier-mediated activity was not due to a change in Km (101 +/- 6 microM in controls vs 97 +/- 4 microM in endotoxin-treated cells, P = NS), but rather to a 73% increase in Vmax (840 +/- 60 pmole/mg protein/30 sec in controls vs 1450 +/- 80 in endotoxin-treated cells, P less than 0.001). The increase in glutamine uptake by PAECs was completely blocked by actinomycin D and cycloheximide, indicating that the accelerated glutamine transport was most probably due to an increase in transporter synthesis. Endotoxin stimulates glutamine uptake by PAECs, either directly or indirectly, an adaptive response which may be necessary to support cellular metabolism, structure, and function.

The effects of diet on the maximal activities of glutaminase, citrate synthase, hexokinase, 6-phosphofructokinase and lactate dehydrogenase in the skin of haired and hairless mice of various ages

1. The maximal activities of hexokinase (HK), 6-phosphofructokinase (PFK), lactate dehydrogenase (LDH), citrate synthase (CS) and glutaminase (GLU) which provide a quantitative indices of flux through several important pathways have been measured in the skin of haired Balb/c and hairless Balb/c (nu/nu) mice under normal and dietary stress. 2. The skin of old haired mice exhibited higher PFK and LDH activities with lower HK, CS and GLU activities. All activities of enzymes associated with energy metabolism in the skin of old hairless mice were higher than those in the skin of haired mice. 3. HK, LDH, CS and GLU activities were maintained at normal levels in the skin of haired mice when these mice were fed diets deficient in energy or protein components (HPLE, LPNE). These enzymes however were severely suppressed when mice were fed a diet deficient in both energy and protein components (LPLE). Recovery of activities of these enzymes to the control level was observed when mice were refed with the normal diet for a week.

Effects of hypoxia and metabolic inhibitors on production of prostacyclin and endothelium-derived relaxing factor by pig aortic endothelial cells

1. The content of adenosine triphosphate (ATP) and basal and bradykinin-stimulated production of prostacyclin and endothelium-derived relaxing factor (EDRF) was measured in primary cultures of porcine aortic endothelial cells under normoxic (14.4% O2) and hypoxic (2.8% O2) conditions, and following treatment with rotenone and 2-deoxy glucose, which inhibit oxidative and glycolytic metabolism, respectively. 2. ATP content and basal and bradykinin-stimulated production of prostacyclin were similar under normoxic and hypoxic conditions. EDRF production, assessed as endothelial guanosine 3':5'-cyclic monophosphate (cyclic GMP) content, was also similar under both conditions. 3. Treatment with rotenone (0.3 microM) had no effect on ATP content or basal or bradykinin-stimulated production of prostacyclin or of EDRF, measured as endothelial cyclic GMP content. Elevation of cyclic GMP content by atriopeptin II was also unaffected. 4. Treatment with 2-deoxy glucose (20 mM) in glucose-free Krebs solution lowered ATP content, reduced bradykinin-stimulated production of prostacyclin and abolished the bradykinin-stimulated elevation of cyclic GMP content. Resting production of prostacyclin was unaffected but basal content of cyclic GMP was lowered in some experiments. Elevation of cyclic GMP content by atriopeptin II was abolished. 5. Combined treatment with rotenone (0.3 microM) and 2-deoxy glucose (20 mM) lowered ATP content more than with 2-deoxy glucose alone. Basal production of prostacyclin rose slightly and bradykinin-stimulated production was powerfully inhibited. Basal content of cyclic GMP was unaffected, but bradykinin-stimulated production was abolished. Elevation of cyclic GMP by atriopeptin II was also abolished. 6. Cascade bioassay experiments using endothelium-denuded rings of rabbit aorta as a detector system confirmed that bradykinin-stimulated production of EDRF was blocked by 2-deoxy glucose, but not by rotenone. 7. These data indicate that porcine aortic endothelial cells in culture operate under mainly glycolytic metabolism and this probably explains why production of prostacyclin and EDRF is unaffected under hypoxic conditions. They also indicate that glycolytic metabolism is required for agonist-stimulated production of prostacyclin and EDRF by these cells.

Maximal activities of glutaminase, citrate synthase, hexokinase, 6-phosphofructokinase and lactate dehydrogenase in skin of immune-competent Balb/c and immune-deficient Balb/c (nu/nu) mice during wound healing

1. The maximal activities of hexokinase (HK), 6-phosphofructokinase (PFK), lactate dehydrogenase, citrate synthase (CS) and glutaminase (GLU) which provide quantitative and qualitative indices of flux through several important metabolic pathways have been examined in the wounded skin of haired immune competent Balb/c mice and hairless immune deficient Balb/c (nu/nu) mice of various ages during the first ten days of wound healing. 2. The potential for glucose utilization and for aerobic metabolism as suggested by the maximal activities of HK, PFK, CS, were raised in the skin of Balb/c mice of various ages on all post wounding days. Increases in the maximal activity of GLU was observed only in the skin of 6 and 10 weeks old Balb/c mice during wound healing. 3. There was no evidence of a contribution to the maximal activity of GLU by infiltrating cells of the immune system to the wound site in the skin of either haired or hairless mice.

The metabolism of L-arginine and its significance for the biosynthesis of endothelium-derived relaxing factor: cultured endothelial cells recycle L-citrulline to L-arginine

We have investigated the mechanism by which cultured endothelial cells generate L-arginine (L-Arg), the substrate for the biosynthesis of endothelium-derived relaxing factor. When Arg-depleted endothelial cells were incubated in Krebs' solution for 60 min, L-Arg levels were significantly (9.7-fold) elevated. The generation of L-Arg coincided with a substantial decrease (90%) in intracellular L-glutamine (L-Gln), whereas all other amino acids were virtually unaffected. Changes in calcium, pH, or oxygen tension had no effect on L-Arg generation, which was, however, prevented when the cells were incubated in culture medium containing L-Gln. L-Arg generated by endothelial cells labeled with L-[14C]Arg was derived from an unlabeled intracellular source, for the specific activity of the intracellular L-Arg pool decreased substantially (8.8-fold) over 60 min. Arg-depleted endothelial cells did not form urea or metabolize L-ornithine but converted L-citrulline (L-Cit) to L-Arg possibly via formation of L-argininosuccinic acid. Nondepleted cells stimulated with the calcium ionophore A23187 showed only a transient accumulation of L-Cit, indicating that L-Cit is recycled to L-Arg during the biosynthesis of endothelium-derived relaxing factor. The generation of L-Arg by Arg-depleted endothelial cells was partially (45%) blocked by protease inhibitors, and various Arg-containing dipeptides were rapidly cleaved to yield L-Arg. Thus, cultured endothelial cells recycle L-Cit to L-Arg and possibly liberate peptidyl L-Arg. The Arg-Cit cycle appears to be the equivalent in the endothelial cell to the formation of urea by the liver. The biosynthesis of endothelium-derived relaxing factor may, therefore, not only produce a powerful vasodilator but also relieve the endothelial cell of excess nitrogen.

The metabolism of L-arginine and its significance for the biosynthesis of endothelium-derived relaxing factor: L-glutamine inhibits the generation of L-arginine by cultured endothelial cells

The mechanism by which L-glutamine (L-Gln) inhibits the release of endothelium-derived relaxing factor from bovine aortic cultured endothelial cells was investigated. The intracellular concentration of L-arginine (L-Arg) in Arg-depleted endothelial cells was inversely related to the level of L-Gln. Removal of L-Gln from the culture medium (usually containing L-Gln at 2 mM) abolished the inhibitory effect of the culture medium on L-Arg generation. L-Gln (0.2 and 2 mM) but not D-Gln inhibited the generation of L-Arg by both Arg-depleted and nondepleted endothelial cells. L-Gln did not interfere with the uptake of L-Arg or the metabolism of L-Arg-L-Phe to L-Arg but inhibited the formation of L-Arg from L-citrulline (L-Cit), L-Cit-L-Phe, and NG-monomethyl-L-arginine. L-Gln also inhibited the conversion of L-[14C]Cit to L-[14C]Arg by Arg-depleted endothelial cells. However, L-Gln did not inhibit the conversion of L-argininosuccinic acid to L-Arg by endothelial cell homogenates. Thus, L-Gln interferes with the conversion of L-Cit to L-Arg probably by acting on argininosuccinate synthetase rather than argininosuccinate lyase. L-Gln also inhibited the generation of L-Arg by the monocyte-macrophage cell line J774 but had no effect on the conversion of L-Cit to L-Arg by these cells. As the release of endothelium-derived relaxing factor from cultured and non-cultured endothelial cells is limited by the availability of L-Arg, endogenous L-Gln may play a regulatory role in the biosynthesis of endothelium-derived relaxing factor.

Inhibition by L-glutamine of the release of endothelium-derived relaxing factor from cultured endothelial cells

L-Glutamine (0.02-2 mM) but not D-glutamine (0.2 mM and 2 mM) inhibited the release of endothelium-derived relaxing factor (EDRF) from bovine aortic endothelial cells cultured in the presence or absence of L-arginine. Inhibition was maximal at a concentration of 200 microns, and was reversed by L-arginine (50 microns) but not D-arginine (100 microns). L-Glutamic acid (2 mM) or ammonium chloride (1 mM), putative products of the metabolism of L-glutamine in endothelial cells, had no effect on EDRF release. L-Glutamine (0.2 mM and 2 mM) but not D-glutamine (2 mM), L-glutamic acid (2 mM) or ammonium chloride (1 mM) also inhibited the generation of L-arginine in endothelial cells. Thus, L-glutamine inhibits EDRF release by preventing the generation of L-arginine.

Effects of physiological and pathological levels of glucocorticoids on skeletal muscle glutamine metabolism in the rat

The effects of physiological and pathological concentrations of glucocorticoids were investigated using the glucocorticoid antagonist RU486 and the synthetic glucocorticoid dexamethasone, respectively. The effects of these treatments on the concentrations of glutamine and other amino acids in skeletal muscle and plasma and on the rates of release of glutamine and alanine from incubated preparations of skeletal muscle of the rat were investigated. Dexamethasone treatment increased the concentration of glutamine and the rate of release of this amino acid from incubated soleus muscle preparations. This treatment decreased the concentration of glutamine in both gastrocnemius and EDL muscles, but was without effect on the rate of glutamine release from EDL muscles. In contrast, administration of the glucocorticoid antagonist RU486 decreased the rate of glutamine release from muscle. It is concluded that glucocorticoids have marked effects on the metabolism of glutamine by skeletal muscle per se and that these hormones may be important in the control of the rate of glutamine release from muscle in both physiological and pathological conditions.