Extracellular glutamate and other metabolites in and around RG2 rat glioma: an intracerebral microdialysis study

Sulfasalazine inhibits the growth of primary brain tumors independent of nuclear factor-kappaB

Nuclear factor-kappaB (NF-kappaB) is a pleiotropic transcription factor that generally enhances cellular resistance to apoptotic cell death. It has been shown to be constitutively active in some cancers and is being pursued as potential anticancer target. Sulfasalazine which is used clinically to treat Crohn's disease has emerged as a potential inhibitor of NF-kappaB and has shown promising results in two pre-clinical studies to target primary brain tumors, gliomas. Once digested, sulfasalazine is cleaved into sulfapyridine and 5-aminosalicylic acid (5-ASA; mesalamine) by colonic bacteria, and the latter, too, is reported to suppress NF-kappaB activity. We now show that glioma cells obtained from patient biopsies or glioma cell lines do not show significant constitutive NF-kappaB activation, unless exposed to inflammatory cytokines. This does not change when gliomas are implanted into the cerebrum of severe combined immun-deficient mice. Nevertheless, sulfasalazine but not its cleaved form 5-ASA caused a dose-dependent inhibition of glioma growth. This effect was entirely attributable to the inhibition of cystine uptake via the system x(c)(-) cystine-glutamate transporter. It could be mimicked by S-4-carboxy-phenylglycine (S-4-CPG) a more specific system x(c)(-) inhibitor, and lentiviral expression of a constitutively active form of IkappaB kinase b was unable to overcome the growth retarding effects of sulfasalazine or S-4-CPG. Both drugs inhibited cystine uptake causing a chronic depletion of intracellular GSH and consequently compromised cellular redox defense which stymied tumor growth. This data suggests that system x(c)(-) is a promising therapeutic target in gliomas and possibly other cancers and that it can be pharmacologically inhibited by Sulfasalazine, an FDA-approved drug.

Decreased expression of the active subunit of the cystine/glutamate antiporter xCT is associated with loss of heterozygosity of 1p in oligodendroglial tumours WHO grade II

AIMS

Oligodendroglial tumours with loss of heterozygosity on 1p (LOH1p) respond better to treatment than oligodendrogliomas without LOH. Previous reports have assigned a crucial role of glutamate metabolism to glioma growth and invasion. The aim was to study the protein expression of different glutamate transporters in relation to LOH1p in low-grade oligodendroglial tumours.

METHODS AND RESULTS

Seventeen oligodendrogliomas World Health Organization (WHO) grade II, 16 oligoastrocytomas WHO grade II and seven astrocytomas WHO grade II were examined. Eleven oligodendrogliomas and five oligoastrocytomas exhibited LOH1p. Immunoreactivity scores (IRS) for glutamate transporters excitatory amino acid transporter (EAAT)-1, -2 and -3 as well as the active cystine/glutamate antiporter subunit xCT were semiquantitatively rated by percentage of positive cells and intensity of immunoreactivity. Reactivity for xCT was lower in tumours with LOH1p than in those without (P = 0.03, Mann-Whitney U-test). No association was found between LOH status and IRS for EAAT-1, -2 or -3. High xCT immunoreactivity was associated with high expression of EAAT-1, -2 or -3.

CONCLUSIONS

Expression of xCT is significantly reduced in low-grade oligodendroglial tumours harbouring LOH1p. Further studies should investigate a potential beneficial effect by inhibiting xCT in low-grade gliomas.

Enhanced expression of the high affinity glutamate transporter GLT-1 in C6 glioma cells delays tumour progression in rat

High grade gliomas are known to release excitotoxic concentrations of glutamate, a process thought to contribute to their malignant phenotype through enhanced autocrine stimulation of their proliferation and destruction of the surrounding nervous tissue. A model of C6 glioma cells in which expression of the high affinity glutamate transporter GLT-1 can be manipulated both in vivo and in vitro was used in order to investigate the consequences of increasing glutamate clearance on tumour progression. These cells were grafted in the striatum of Wistar rats and doxycycline was administered after validation of tumour development by magnetic resonance imaging. Both GLT-1 expression examined by immunohistochemistry and glutamate transport activity measured on synaptosomes appeared robustly increased in samples from doxycycline-treated animals. Moreover, these rats showed extended survival times as compared to vehicle-treated animals, an effect that was consistent with volumetric data revealing delayed tumour growth. As constitutive deficiency in glutamate clearance at the vicinity of brain tumours is well established, these data illustrate the potential benefit that could be obtained by enhancing glutamate transport by glioma cells in order to reduce their invasive behaviour.

1H MR spectroscopic imaging with short and long echo time to discriminate glycine in glial tumours

OBJECT

To investigate glycine (Gly) concentrations in low- and high-grade gliomas based on (1)H MR spectroscopic imaging (MRSI) with short and long echo time (TE). Myoinositol (MI) and Gly appear at the same resonance frequency of 3.56 ppm, but due to strong coupling the MI signal dephases more rapidly. Therefore, their contribution to the 3.56 ppm signal should be distinguishable comparing MRSI data acquired at short and long TE.

MATERIALS AND METHODS

(1)H MRSI (TE = 30 and 144 ms) was performed at 3 T in 29 patients with histopathological confirmed World Health Organization (WHO) grade II-IV gliomas and in FIVE healthy subjects. All spectra from the gliomas revealed increase of the 3.56 ppm resonance in the short TE spectra. Signal intensities of Gly and MI were differentiated either by analysing the short to long TE ratio of the resonance or by performing a weighted difference. Gly concentrations were compared between high-grade (WHO III-IV) and low-grade gliomas.

RESULTS

High-grade gliomas showed significantly higher Gly concentrations compared to low-grade gliomas.

CONCLUSION

Appropriate data processing of short and long TE (1)H MRSI provides a tool to distinguish and to quantify Gly and MI concentrations in gliomas. As Gly seems to be a marker of malignancy, more dedicated spectroscopic methods to differentiate these metabolites are justified.

Glucose metabolites, glutamate and glycerol in malignant glioma tumours during radiotherapy

OBJECTIVE

The metabolism of malignant glioma was studied in 13 patients. The main objective was to perform a study of the metabolic pattern of glucose, lactate, pyruvate, glutamate and glycerol in tumour tissue during base-line conditions and to detect any changes in the metabolism during radiotherapy.

METHOD

During a stereotactic biopsy, two microdialysis catheters were implanted: one in tumour and one in peri-tumoural tissue. Fasting samples were analysed daily, before and during 5 days of radiotherapy given with 2 Gy fractions.

RESULTS

Base-line levels of glucose and pyruvate were significantly lower in tumour compared to peri-tumoural tissue (P = 0.04 and 0.023, respectively). The lactate/pyruvate ratio was significantly higher in tumour tissue (P = 0.022). In general, the levels of lactate, glutamate and glycerol were higher in tumour tissue, although not statistically significant. Further, we could not detect any significant changes during the 5 days of radiotherapy in any of the metabolites analysed.

CONCLUSION

Radiotherapy up to 10 Gy given in five fractions does not influence the glucose metabolism nor does it induce any acute cytotoxic effect detected with glutamate or glycerol in malignant glioma, as assessed by microdialysis. The study confirms the glycolytic properties of glucose metabolism in malignant glioma.

Glutamate-induced glioma cell proliferation is prevented by functional expression of the glutamate transporter GLT-1

A tetracycline-dependent inducible system was used to achieve controlled expression of the glutamate transporter 1 (GLT-1) in C6 glioma cells. Non-induced cells show modest glutamate uptake and, in the presence of L-cystine, these cells tend to release substantial amounts of glutamate. Overnight exposure to doxycycline increased D-[3H]-aspartate uptake, reaching similar capacity as observed in cultured astrocytes. Efficient clearance of exogenously applied glutamate was evidenced in these cells, even in the presence of l-cystine. The addition of glutamate (100 microM) to the medium of non-induced cells significantly increased their proliferation rate, an effect that was blocked when the expression of GLT-1 was induced. This suggests that impaired glutamate uptake capacity in glioma cells indirectly contributes to their proliferation.

A role for glutamate in growth and invasion of primary brain tumors

The vast majority of primary brain tumors derive from glial cells and are collectively called gliomas. While, they share some genetic mutations with other cancers, they do present with a unique biology and have developed adaptations to meet specific biological needs. Notably, glioma growth is physically restricted by the skull, and, unless normal brain cells are destroyed, tumors cannot expand. To overcome this challenge, glioma cells release glutamate which causes excitotoxic death to surrounding neurons, thereby vacating room for tumor expansion. The released glutamate also explains peritumoral seizures which are a common symptom early in the disease. Glutamate release occurs via system X(c), a cystine-glutamate exchanger that releases glutamate in exchange for cystine being imported for the synthesis of the cellular antioxidant GSH. It protects tumor cells from endogenously produced reactive oxygen and nitrogen species but also endows tumors with an enhanced resistance to radiation- and chemotherapy. Pre-clinical data demonstrates that pharmacological inhibition of system X(c) causes GSH depletion which slows tumor growth and curtails tumor invasion in vivo. An Food and Drug Administration approved drug candidate is currently being introduced into clinical trials for the treatment of malignant glioma.

Autocrine glutamate signaling promotes glioma cell invasion

Malignant gliomas have been shown to release glutamate, which kills surrounding brain cells, creating room for tumor expansion. This glutamate release occurs primarily via system xC, a Na+-independent cystine-glutamate exchanger. We show here, in addition, that the released glutamate acts as an essential autocrine/paracrine signal that promotes cell invasion. Specifically, chemotactic invasion and scrape motility assays each show dose-dependent inhibition of cell migration when glutamate release was inhibited using either S-(4)-CPG or sulfasalazine, both potent blockers of system xC. This inhibition could be overcome by the addition of exogenous glutamate (100 micromol/L) in the continued presence of the inhibitors. Migration/invasion was also inhibited when Ca2+-permeable alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors (AMPA-R) were blocked using GYKI or Joro spider toxin, whereas CNQX was ineffective. Ca2+ imaging experiments show that the released glutamate activates Ca2+-permeable AMPA-R and induces intracellular Ca2+ oscillations that are essential for cell migration. Importantly, glioma cells release glutamate in sufficient quantities to activate AMPA-Rs on themselves or neighboring cells, thus acting in an autocrine and/or paracrine fashion. System xC and the appropriate AMPA-R subunits are expressed in all glioma cell lines, patient-derived glioma cells, and acute patient biopsies investigated. Furthermore, animal studies in which human gliomas were xenographed into scid mice show that chronic inhibition of system xC-mediated glutamate release leads to smaller and less invasive tumors compared with saline-treated controls. These data suggest that glioma invasion is effectively disrupted by inhibiting an autocrine glutamate signaling loop with a clinically approved candidate drug, sulfasalazine, already in hand.

Glutamate receptor expression in multiple sclerosis lesions

Blockade of receptors for the excitatory neurotransmitter glutamate ameliorates neurological clinical signs in models of the CNS inflammatory demyelinating disease multiple sclerosis (MS). To investigate whether glutamate excitoxicity may play a role in MS pathogenesis, the cellular localization of glutamate and its receptors, transporters and enzymes was examined. Expression of glutamate receptor (GluR) 1, a Ca(++)-permeable ionotropic AMPA receptor subunit, was up-regulated on oligodendrocytes in active MS lesion borders, but Ca(++)-impermeable AMPA GluR2 subunit levels were not increased. Reactive astrocytes in active plaques expressed AMPA GluR3 and metabotropic mGluR1, 2/3 and 5 receptors and the GLT-1 transporter, and a subpopulation was immunostained with glutamate antibodies. Activated microglia and macrophages were immunopositive for GluR2, GluR4 and NMDA receptor subunit 1. Kainate receptor GluR5-7 immunostaining showed endothelial cells and dystrophic axons. Astrocyte and macrophage populations expressed glutamate metabolizing enzymes and unexpectedly the EAAC1 transporter, which may play a role in glutamate uptake in lesions. Thus, reactive astrocytes in MS white matter lesions are equipped for a protective role in sequestering and metabolizing extracellular glutamate. However, they may be unable to maintain glutamate at levels low enough to protect oligodendrocytes rendered vulnerable to excitotoxic damage because of GluR1 up-regulation.

Expression of the Water Channel Protein Aquaporin-9 in Malignant Brain Tumors

Recently, many studies seen concerning the expression and distribution of aquaporins and K channels in the central nervous system, and their physiological and pathophysiologic roles in water and ion homeostasis. Whereas most data were collected on aquaporin-4 (AQP4) in astrocytes, only little attention was paid to AQP9 which is a water channel transporting glycerol, mannitol, and urea as well. This is the first study describing AQP9 in human brain and human brain tumors. For comparison, we also investigated the immunohistochemical distribution of AQP9 in the rat glioma RG2. Whereas in the normal rat brain AQP9 is only weakly expressed by astrocytes, the anti-AQP9 immunoreactivity was found to be increased at the tumor border, but not within the tumor. In contrast, in human glioblastoma, most glioma cells throughout the tumor revealed a strong anti-AQP9 immunoreactivity across the whole surface of the cell. In the discussion, the increase of the anti-AQP9 immunoreactivity in glioma cells is suggested to reflect an upregulation and to counteract the glioma-associated lactic acidosis by clearance of glycerol and lactate from the extracellular space. In addition, the increased level of AQP9 immunoreactivity could be involved in the energy metabolism of the glioma and/or surrounding neuronal cells.

Symptom management and supportive care of the patient with brain metastases

The focus of care for patients with brain metastases will always be on therapeutic options such as surgery, radiotherapy, and chemotherapy. However, proper symptom management and supportive care of non-therapeutic issues will be equally as important, including treatment of seizures, use of anticonvulsants, corticosteroids, and gastric acid inhibitors, assessment of swallowing dysfunction, treatment of thromboembolic events, appropriate use, and safe application of anticoagulation, and evaluation of psychiatric issues. Appropriate management of these supportive aspects of patient care will improve overall quality of life and allow the patient and family to more easily concentrate on treatment.

Inhibition of cystine uptake disrupts the growth of primary brain tumors

Glial cells play an important role in sequestering neuronally released glutamate via Na+-dependent transporters. Surprisingly, these transporters are not operational in glial-derived tumors (gliomas). Instead, gliomas release glutamate, causing excitotoxic death of neurons in the vicinity of the tumor. We now show that glutamate release from glioma cells is an obligatory by-product of cellular cystine uptake via system xc-, an electroneutral cystine-glutamate exchanger. Cystine is an essential precursor for the biosynthesis of glutathione, a major redox regulatory molecule that protects cells from endogenously produced reactive oxygen species (ROS). Glioma cells, but not neurons or astrocytes, rely primarily on cystine uptake via system xc- for their glutathione synthesis. Inhibition of system xc- causes a rapid depletion of glutathione, and the resulting loss of ROS defense causes caspase-mediated apoptosis. Glioma cells can be rescued if glutathione status is experimentally restored or if glutathione is substituted by alternate cellular antioxidants, confirming that ROS are indeed mediators of cell death. We describe two potent drugs that permit pharmacological inhibition of system xc-. One of these drugs, sulfasalazine, is clinically used to treat inflammatory bowel disease and rheumatoid arthritis. Sulfasalazine was able to reduce glutathione levels in tumor tissue and slow tumor growth in vivo in a commonly used intracranial xenograft animal model for human gliomas when administered by intraperitoneal injection. These data suggest that inhibition of cystine uptake into glioma cells through the pharmacological inhibition of system xc- may be a viable therapeutic strategy with a Food and Drug Administration-approved drug already in hand.

The excitatory amino acid transporter-2 induces apoptosis and decreases glioma growth in vitro and in vivo

Accumulating evidence suggests that glutamate plays a key role in the proliferation and invasion of glioblastoma tumors. Astrocytic tumors have been shown to release glutamate at high levels, which may stimulate tumor cell proliferation and motility via activation of glutamate receptors. Excess glutamate has also been found to facilitate tumor invasion by causing excitotoxic damage to normal brain thereby paving a pathway for tumor migration. Results from tissue microarray analyses showed decreased excitatory amino acid transporter-2 (EAAT-2) expression in high-grade glial tumors compared with low-grade astrocytomas and normal brain. EAAT-2 expression was inversely correlated with tumor grade, implicating its potential role in glial tumor progression, which was reflected by an undetectable level of EAAT-2 protein in glioma cell lines. In this study, we sought to investigate the effect of reconstituted EAAT-2 on glioma cell growth in vitro and in vivo by adenoviral-mediated gene transfer. Infection of glioma cells with Ad-EAAT-2 resulted in a physiologic level of functional EAAT-2, and a subsequent dose-dependent reduction in cell proliferation in all glioma cell lines tested compared with controls. Interestingly, results from analyses of Annexin V staining, detection of poly(ADP-ribose)polymerase cleavage and caspase-3 activation all indicated that Ad-EAAT-2 infection elicited apoptosis in glioma cells. Ex vivo experiments in nude mice showed a total suppression of tumor growth at sites that received Ad-EAAT-2-infected cells. Collectively, our results uncovered a new function of EAAT-2 in controlling glioma proliferation. Further studies will improve our knowledge of the role of glutamate in glioma growth and may provide useful prognostic information and alternative therapeutic targets for the treatment of glioma.

Microdialysis

Microdialysis is a probe-based sampling method, which, if linked to analytical devices, allows for the measurement of drug concentration profiles in selected tissues. During the last two decades, microdialysis has become increasingly popular for preclinical and clinical pharmacokinetic studies. The advantage of in vivo microdialysis over traditional methods relates to its ability to continuously sample the unbound drug fraction in the interstitial space fluid (ISF). This is of particular importance because the ISF may be regarded as the actual target compartment for many drugs, e.g. antimicrobial agents or other drugs mediating their action through surface receptors. In contrast, plasma concentrations are increasingly recognised as inadequately predicting tissue drug concentrations and therapeutic success in many patient populations. Thus, the minimally invasive microdialysis technique has evolved into an important tool for the direct assessment of drug concentrations at the site of drug delivery in virtually all tissues. In particular, concentrations of transdermally applied drugs, neurotransmitters, antibacterials, cytotoxic agents, hormones, large molecules such as cytokines and proteins, and many other compounds were described by means of microdialysis. The combined use of microdialysis with non-invasive imaging methods such as positron emission tomography and single photon emission tomography opened the window to exactly explore and describe the fate and pharmacokinetics of a drug in the body. Linking pharmacokinetic data from the ISF to pharmacodynamic information appears to be a straightforward approach to predicting drug action and therapeutic success, and may be used for decision making for adequate drug administration and dosing regimens. Hence, microdialysis is nowadays used in clinical studies to test new drug candidates that are in the pharmaceutical industry drug development pipeline.

[18F]-2-fluoro-2-deoxyglucose transport kinetics as a function of extracellular glucose concentration in malignant glioma, fibroblast and macrophage cells in vitro

FDG-PET is used to measure the metabolic rate of glucose. Transport and phosphorylation determine the amount of hexose analog that is phosphorylated and trapped. Competition occurs for both events, such that extracellular glucose concentration affects the FDG image. This study investigated the effect of glucose concentration on the rate of FDG accumulation in three cell lines. The results show that extracellular glucose concentration has a greater impact on the rate of FDG accumulation than the relative abundance of GLUT transporter subtypes.

Malignant gliomas: perverting glutamate and ion homeostasis for selective advantage

The malignant transformation of astrocytes, oligodendrocytes or their progenitor cells gives rise to tumors that are collectively called gliomas. Although the vast majority of these tumors are believed to be of astrocytic origin, the actual cell of origin remains unknown. And whereas gliomas present with some genetic alterations in tumor-suppressor genes or oncogenes that are common among cancers, their biology differs significantly from other neoplasms. Most notably, they grow in a confined space defined by the bones of the skull and spinal column, and they spread by active cell migration rather than by passive, humorous spread. Recent research suggests that astrocyte-derived tumors co-opt neurotransmitters, their transporters and ion channels to support their unusual growth and invasive migration.

Baseline levels of glucose metabolites, glutamate and glycerol in malignant glioma assessed by stereotactic microdialysis

The metabolism of high grade astrocytoma was studied in 15 patients using intra-tumoural microdialysis. Two catheters were implanted during a stereotactic biopsy procedure: one in the tumour and one in the peri-tumoural tissue. The patients were fully mobilized the same day as the operation. Microdialysis samples were collected the next day and subsequently analysed for glucose, lactate, pyruvate, glutamate and glycerol. The main objective was to establish base-line levels of the studied substances. In addition, an in vitro experiment was performed in order to estimate recovery for the flow rates and catheters used. Glucose showed a tendency to be lower in tumour than in peri-tumoural tissue, indicating a high energy demand of the tumour. Lactate was significantly higher in tumour tissue. This supports previous reports that high grade astrocytomas use glycolysis rather than respiration to meet their energy demand. The tumours were also classified into necrotic and non-necrotic, according to the radiological finding. The necrotic tumours showed significantly higher levels of glutamate. They also presented a tendency to higher levels of glycerol than the non-necrotic tumours. These findings might be explained by the release of intracellular glutamate and of cell-membrane glycerol by cell destruction. We believe that microdialysis in awake and mobilized patients will prove to be a valuable tool in investigating metabolic events in malignant brain tumours especially during therapy.

Glutamate release promotes growth of malignant gliomas

Glutamate neurotoxicity has been implicated in stroke, head trauma, multiple sclerosis and neurodegenerative diseases. Although recent data show that cultured glioma cells secrete glutamate, the growth potential of brain tumors has not yet been linked to an excitotoxic mechanism. Using bioluminescence detection of glutamate release from freshly prepared brain slices, we show that implanted glioma cells continue to secrete glutamate. Moreover, gliomas with high glutamate release have a distinct growth advantage in host brain that is not present in vitro. Treatment with the NMDA receptor antagonists MK801 or memantine slowed the growth of glutamate-secreting tumors in situ, suggesting that activation of NMDA receptors facilitates tumor expansion. These findings support a new approach for therapy of brain tumors, based upon antagonizing glutamate secretion or its target receptors.