Inhibition of glutamine synthetase triggers apoptosis in asparaginase-resistant cells

L-asparaginase anti-tumor activity in pancreatic cancer is dependent on its glutaminase activity and resistance is mediated by glutamine synthetase

l-Asparaginase is a cornerstone of acute lymphoblastic leukemia (ALL) therapy since lymphoblasts lack asparagine synthetase (ASNS) and rely on extracellular asparagine availability for survival. Resistance mechanisms are associated with increased ASNS expression in ALL. However, the association between ASNS and l-Asparaginase efficacy in solid tumors remains unclear, thus limiting clinical development. Interestingly, l-Asparaginase also has a glutaminase co-activity that is crucial in pancreatic cancer where KRAS mutations activate glutamine metabolism. By developing l-Asparaginase-resistant pancreatic cancer cells and using OMICS approaches, we identified glutamine synthetase (GS) as a marker of resistance to l-Asparaginase. GS is the only enzyme able to synthesize glutamine, and its expression also correlates with l-Asparaginase efficacy in 27 human cell lines from 11 cancer indications. Finally, we further demonstrated that GS inhibition prevents cancer cell adaptation to l-Asparaginase-induced glutamine starvation. These findings could pave the way to the development of promising drug combinations to overcome l-Asparaginase resistance.

Investigating the role of GLUL as a survival factor in cellular adaptation to glutamine depletion via targeted stable isotope resolved metabolomics

Cellular glutamine synthesis is thought to be an important resistance factor in protecting cells from nutrient deprivation and may also contribute to drug resistance. The application of ‟targeted stable isotope resolved metabolomics” allowed to directly measure the activity of glutamine synthetase in the cell. With the help of this method, the fate of glutamine derived nitrogen within the biochemical network of the cells was traced. The application of stable isotope labelled substrates and analyses of isotope enrichment in metabolic intermediates allows the determination of metabolic activity and flux in biological systems. In our study we used stable isotope labelled substrates of glutamine synthetase to demonstrate its role in the starvation response of cancer cells. We applied ¹³C labelled glutamate and ¹⁵N labelled ammonium and determined the enrichment of both isotopes in glutamine and nucleotide species. Our results show that the metabolic compensatory pathways to overcome glutamine depletion depend on the ability to synthesise glutamine via glutamine synthetase. We demonstrate that the application of dual-isotope tracing can be used to address specific reactions within the biochemical network directly. Our study highlights the potential of concurrent isotope tracing methods in medical research.

Amino Acid Depletion Therapies: Starving Cancer Cells to Death

Targeting tumor cell metabolism is an attractive form of therapy, as it may enhance treatment response in therapy resistant cancers as well as mitigate treatment-related toxicities by reducing the need for genotoxic agents. To meet their increased demand for biomass accumulation and energy production and to maintain redox homeostasis, tumor cells undergo profound changes in their metabolism. In addition to the diversion of glucose metabolism, this is achieved by upregulation of amino acid metabolism. Interfering with amino acid availability can be selectively lethal to tumor cells and has proven to be a cancer specific Achilles' heel. Here we review the biology behind such cancer specific amino acid dependencies and discuss how these vulnerabilities can be exploited to improve cancer therapies.

Pseudomonas aeruginosa recombinant L-asparaginase: Large scale production, purification, and cytotoxicity on THP-1, MDA-MB-231, A549, Caco2 and HCT-116 cell lines

In previous studies Pseudomonas aeruginosal-ASNase complete coding sequence gene, 984 bp (GenBank accession number KU161101.2) was isolated by PCR, cloned into pET28a(+) vector, expressed in E. coli DE3(BL21) pLysS, purified to apparent homogeneity and biochemically characterized. In the present work we highlight large scale production, affinity purification of the recombinant enzyme, effect of osmolytes on the stability of the l-ASNase and cytotoxicity on different cancer cell lines. Successful overexpression was achieved in E. coli as a 6-His-Tag fusion protein after 18 h of induction with lactose at a concentration of 2 g/L in fermentation medium and at 37 °C. The recombinant enzyme was purified to homogeneity using Ni²⁺ chelated Fast Flow Sepharose resin with 19758.8 specific activity and 10.28 purification fold. With respect to the effect of osmolytes on the stability of the purified enzyme, the majority of the tested osmolytes namely 5% maltose, 5% mannitol, 30% glycerol and 5% BSA were found to increase the stability of the recombinant l-ASNase as compared to the free enzyme. Triple negative breast cancer cell line, MDA-MB-231 treated with recombinant l-ASNase showed significant morphological changes and the IC₅₀ of the purified enzyme was found to be 3.1 IU. Human leukemia cell line, THP-1 treated with l-ASNase showed apoptotic bodies and morphological changes with IC₅₀ of the purified enzyme 1.75 IU. Moreover, the purified recombinant l-ASNase was found to induced cytotoxic effects on colorectal adenocarcinoma cell line, Caco-2 with IC₅₀ of 68.28 IU. Results of apoptosis assay on THP-1 cells revealed that the purified l-ASNase induced early and late apoptosis at 14.16% and 7.56 respectively as compared to the control untreated cells.

Amino Acid Degrading Enzymes and their Application in Cancer Therapy

BACKGROUND

Amino acids are essential components in various biochemical pathways. The deprivation of certain amino acids is an antimetabolite strategy for the treatment of amino acid-dependent cancers which exploits the compromised metabolism of malignant cells. Several studies have focused on the development and preclinical and clinical evaluation of amino acid degrading enzymes, namely L-asparaginase, L-methionine γ-lyase, L-arginine deiminase, L-lysine α-oxidase. Further research into cancer cell metabolism may therefore define possible targets for controlling tumor growth.

OBJECTIVE

The purpose of this review was to summarize recent progress in the relationship between amino acids metabolism and cancer therapy, with a particular focus on Lasparagine, L-methionine, L-arginine and L-lysine degrading enzymes and their formulations, which have been successfully used in the treatment of several types of cancer.

METHODS

We carried out a structured search among literature regarding to amino acid degrading enzymes. The main aspects of search were in vitro and in vivo studies, clinical trials concerning application of these enzymes in oncology.

RESULTS

Most published research are on the subject of L-asparaginase properties and it's use for cancer treatment. L-arginine deiminase has shown promising results in a phase II trial in advanced melanoma and hepatocellular carcinoma. Other enzymes, in particular Lmethionine γ-lyase and L-lysine α-oxidase, were effective in vitro and in vivo.

CONCLUSION

The findings of this review revealed that therapy based on amino acid depletion may have the potential application for cancer treatment but further clinical investigations are required to provide the efficacy and safety of these agents.

Assessment of l-Asparaginase Pharmacodynamics in Mouse Models of Cancer

l-asparaginase (ASNase) is a metabolism-targeted anti-neoplastic agent used to treat acute lymphoblastic leukemia (ALL). ASNase’s anticancer activity results from the enzymatic depletion of asparagine (Asn) and glutamine (Gln), which are converted to aspartic acid (Asp) and glutamic acid (Glu), respectively, in the blood. Unfortunately, accurate assessment of the in vivo pharmacodynamics (PD) of ASNase is challenging because of the following reasons: (i) ASNase is resilient to deactivation; (ii) ASNase catalytic efficiency is very high; and (iii) the PD markers Asn and Gln are depleted ex vivo in blood samples containing ASNase. To address those issues and facilitate longitudinal studies in individual mice for ASNase PD studies, we present here a new LC-MS/MS bioanalytical method that incorporates rapid quenching of ASNase for measurement of Asn, Asp, Gln, and Glu in just 10 µL of whole blood, with limits of detection (s:n ≥ 10:1) estimated to be 2.3, 3.5, 0.8, and 0.5 µM, respectively. We tested the suitability of the method in a 5-day, longitudinal PD study in mice and found the method to be simple to perform with sufficient accuracy and precision for whole blood measurements. Overall, the method increases the density of data that can be acquired from a single animal and will facilitate optimization of novel ASNase treatment regimens and/or the development of new ASNase variants with desired kinetic properties.

Drug-induced amino acid deprivation as strategy for cancer therapy

Cancer is caused by uncontrollable growth of neoplastic cells, leading to invasion of adjacent and distant tissues resulting in death. Cancer cells have specific nutrient(s) auxotrophy and have a much higher nutrient demand compared to normal tissues. Therefore, different metabolic inhibitors or nutrient-depleting enzymes have been tested for their anti-cancer activities. We review recent available laboratory and clinical data on using various specific amino acid metabolic pathways inhibitors in treating cancers. Our focus is on glutamine, asparagine, and arginine starvation. These three amino acids are chosen due to their better scientific evidence compared to other related approaches in cancer treatment. Amino acid-specific depleting enzymes have been adopted in different standard chemotherapy protocols. Glutamine starvation by glutaminase inhibitior, transporter inhibitor, or glutamine depletion has shown to have significant anti-cancer effect in pre-clinical studies. Currently, glutaminase inhibitor is under clinical trial for testing anti-cancer efficacy. Clinical data suggests that asparagine depletion is effective in treating hematologic malignancies even as a single agent. On the other hand, arginine depletion has lower toxicity profile and can effectively reduce the level of pro-cancer biochemicals in patients as shown by ours and others’ data. This supports the clinical use of arginine depletion as anti-cancer therapy but its exact efficacy in various cancers requires further investigation. However, clinical application of these enzymes is usually hindered by common problems including allergy to these foreign proteins, off-target cytotoxicity, short half-life and rapidly emerging chemoresistance. There have been efforts to overcome these problems by modifying the drugs in different ways to circumvent these hindrance such as (1) isolate human native enzymes to reduce allergy, (2) isolate enzyme isoforms with higher specificities and efficiencies, (3) pegylate the enzymes to reduce allergy and prolong the half-lives, and (4) design drug combinations protocols to enhance the efficacy of chemotherapy by drug synergy and minimizing resistance. These improvements can potentially lead to the development of more effective anti-cancer treatment with less adverse effects and higher therapeutic efficacy.

[Expression of asparagine synthetase in relapsed or refractory extranodal NK/T cell lymphoma]

OBJECTIVE

To detect the expression level of asparagine synthetase (ASNS) in patients with relapsed or refractory extranodal NK/T cell lymphoma and explore its clinical significance.

METHODS

Ten patients with relapsed or refractory extranodal NK/T cell lymphoma admitted in our department from January, 2013 to January, 2016 were analyzed. The diagnoses were confirmed by pathological and immunohistochemical examination following failed chemotherapies in all cases. Branched DNA-liquidchip technique (bDNA-LCT) was used for detecting ASNS mRNA expression in paraffin-embedded tissue sections in the 10 cases of relapsed or refractory extranodal NK/T cell lymphoma and in 5 cases of chronic rhinitis. The correlations were analyzed between ASNS expression and the clinicopathological features and outcomes of the patients with failed chemotherapy regimens containing asparaginasum.

RESULTS

Six out of the 10 patients with relapsed or refractory extranodal NK/T cell lymphoma died due to diseaseprogression. The expression level of ASNS was significantly higher in the lymphoma tissues than in tissue specimens of chronic rhinitis (P<0.05). The expression level of ASNS was associated with the International Prognostic Index (P=0.023) in patients with relapsed or refractory extranodal NK/T cell lymphoma, and Kaplan-Meier curve showed that a high ASNS expression was correlated with a reduced overall survival and progression-free survival of the patients.

CONCLUSION

Asparaginasum-based chemotherapy regimens are recommended for treatment of relapsed or refractory extranodal NK/T cell lymphoma with low ASNS expressions.

Cancer metabolism and oxidative stress: Insights into carcinogenesis and chemotherapy via the non-dihydrofolate reductase effects of methotrexate

Methotrexate has been in use as an anti-cancer agent for over 60 years. Though inhibition of dihydrofolate reductase is its best known mechanisms of action, its non-dihydrofolate reductase dependent mechanisms disrupt metabolic pathways resulting in a depletion of NAD(P)H and increasing oxidative stress. These mechanisms highlight a novel dependence of cancer cells on their metabolic abnormalities to buffer oxidative stress and chemotherapeutic agents interfere with these cellular abilities. Mitochondria appear to play a significant role in maintaining cancer cell viability and alterations in metabolism seen in cancer cells aid this mitochondrial ability. Further research is needed to understand the effects of other chemotherapeutic agents on these pathways.

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Methotrexate inhibits multiple enzymes beyond dihydrofolate reductase.

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Mitochondria serve a critical role in buffering cellular oxidative stress.

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Metabolic derangements seen in cancer fuel this mitochondrial rescue function.

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Methotrexate toxicity occurs secondary to a handicapping of this buffering function.

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Many chemotherapeutic agents appear to have mechanisms affecting these pathways.

Differential mechanisms of asparaginase resistance in B-type acute lymphoblastic leukemia and malignant natural killer cell lines

Bacterial L-asparaginase (ASNase), hydrolyzing L-asparagine (Asn), is an important drug for treating patients with acute lymphoblastic leukaemia (ALL) and natural killer (NK) cell lymphoma. Although different native or pegylated ASNase-based chemotherapy are efficient, disease relapse is frequently observed, especially in adult patients. The neo-synthesis of Asn by asparagine synthetase (AsnS) following ASNase treatment, which involves the amino acid response and mitogen-activated protein kinase kinase/extracellular signal-regulated kinase pathways, is believed to be the basis of ASNase-resistance mechanisms. However, AsnS expression has not emerged as an accurate predictive factor for ASNase susceptibility. The aim of this study was to identify possible ASNase sensitivity/resistance-related genes or pathways using a new asparaginase, namely a pegylated r-crisantaspase, with a focus on classic Asn-compensatory responses and cell death under conditions of Asn/L-glutamine limitation. We show that, for B-ALL cell lines, changes in the expression of apoptosis-regulatory genes (especially NFκB-related genes) are associated with ASNase susceptibility. The response of malignant NK cell lines to ASNase may depend on Asn-compensatory mechanisms and other cellular processes such as cleavage of BCL2A1, a prosurvival member of the Bcl-2 protein family. These results suggest that according to cellular context, factors other than AsnS can influence ASNase susceptibility.

Glutamine depletion by crisantaspase hinders the growth of human hepatocellular carcinoma xenografts

Background:

A subset of human hepatocellular carcinomas (HCC) exhibit mutations of β-catenin gene CTNNB1 and overexpress Glutamine synthetase (GS). The CTNNB1-mutated HCC cell line HepG2 is sensitive to glutamine starvation induced in vitro with the antileukemic drug Crisantaspase and the GS inhibitor methionine-L-sulfoximine (MSO).

Methods:

Immunodeficient mice with subcutaneous xenografts of the CTNNB1-mutated HCC cell lines HepG2 and HC-AFW1 were treated with Crisantaspase and/or MSO, and tumour growth was monitored. At the end of treatment, tumour weight and histology were assessed. Serum and tissue amino acids were determined by HPLC. Gene and protein expression were estimated with RT-PCR and western blot and GS activity with a colorimetric method. mTOR activity was evaluated from the phosphorylation of p70S6K1.

Results:

Crisantaspase and MSO depleted serum glutamine, lowered glutamine in liver and tumour tissue, and inhibited liver GS activity. HepG2 tumour growth was significantly reduced by either Crisantaspase or MSO, and completely suppressed by the combined treatment. The combined treatment was also effective against xenografts of the HC-AFW1 cell line, which is Crisantaspase resistant in vitro.

Conclusions:

The combination of Crisantaspase and MSO reduces glutamine supply to CTNNB1-mutated HCC xenografts and hinders their growth.

Role of ion transport in control of apoptotic cell death

Cell shrinkage is a hallmark and contributes to signaling of apoptosis. Apoptotic cell shrinkage requires ion transport across the cell membrane involving K(+) channels, Cl(-) or anion channels, Na(+)/H(+) exchange, Na(+),K(+),Cl(-) cotransport, and Na(+)/K(+)ATPase. Activation of K(+) channels fosters K(+) exit with decrease of cytosolic K(+) concentration, activation of anion channels triggers exit of Cl(-), organic osmolytes, and HCO3(-). Cellular loss of K(+) and organic osmolytes as well as cytosolic acidification favor apoptosis. Ca(2+) entry through Ca(2+)-permeable cation channels may result in apoptosis by affecting mitochondrial integrity, stimulating proteinases, inducing cell shrinkage due to activation of Ca(2+)-sensitive K(+) channels, and triggering cell-membrane scrambling. Signaling involved in the modification of cell-volume regulatory ion transport during apoptosis include mitogen-activated kinases p38, JNK, ERK1/2, MEKK1, MKK4, the small G proteins Cdc42, and/or Rac and the transcription factor p53. Osmosensing involves integrin receptors, focal adhesion kinases, and tyrosine kinase receptors. Hyperosmotic shock leads to vesicular acidification followed by activation of acid sphingomyelinase, ceramide formation, release of reactive oxygen species, activation of the tyrosine kinase Yes with subsequent stimulation of CD95 trafficking to the cell membrane. Apoptosis is counteracted by mechanisms involved in regulatory volume increase (RVI), by organic osmolytes, by focal adhesion kinase, and by heat-shock proteins. Clearly, our knowledge on the interplay between cell-volume regulatory mechanisms and suicidal cell death is still far from complete and substantial additional experimental effort is needed to elucidate the role of cell-volume regulatory mechanisms in suicidal cell death.

The eukaryotic initiation factor 2 kinase GCN2 protects against hepatotoxicity during asparaginase treatment

Asparaginase is an important drug in the treatment regimen for acute lymphoblastic leukemia. Asparaginase depletes circulating asparagine and glutamine, activating an amino acid stress response (AAR) involving phosphorylation of eukaryotic initiation factor 2 (eIF2) by general control nonderepressible kinase 2 (GCN2). We hypothesized that GCN2 functions to mitigate hepatic stress during asparaginase therapy by activating the AAR. To test this idea, C57BL/6J wild-type mice (Gcn2(+/+)) and those deleted for Gcn2 (Gcn2(-/-)) were injected with asparaginase or saline excipient one time daily for 1 or 6 days. In liver, increased phosphorylation of eIF2 and mRNA expression of AAR target genes activating transcription factor 4, asparagine synthetase, eIF4E-binding protein 1, and CAAT enhancer-binding protein homologous protein were significantly blunted or blocked in the liver of Gcn2(-/-) mice. Loss of AAR during asparaginase coincided with increases in mammalian target of rapamycin signaling, hepatic triglyceride accumulation, and DNA damage in association with genetic markers of oxidative stress (glutathione peroxidase) and inflammation (tumor necrosis factor alpha-α). Although asparaginase depleted circulating asparagine in both Gcn2(+/+) and Gcn2(-/-) mice, all other amino acids, including plasma glutamine, were elevated in the plasma of Gcn2(-/-) mice. This study shows that loss of GCN2 promotes oxidative stress and inflammatory-mediated DNA damage during asparaginase therapy, suggesting that patients with reduced or dysfunctional AAR may be at risk of developing hepatic complications during asparaginase treatment.

Inhibiting glutamine uptake represents an attractive new strategy for treating acute myeloid leukemia

Cancer cells require nutrients and energy to adapt to increased biosynthetic activity, and protein synthesis inhibition downstream of mammalian target of rapamycin complex 1 (mTORC1) has shown promise as a possible therapy for acute myeloid leukemia (AML). Glutamine contributes to leucine import into cells, which controls the amino acid/Rag/mTORC1 signaling pathway. We show in our current study that glutamine removal inhibits mTORC1 and induces apoptosis in AML cells. The knockdown of the SLC1A5 high-affinity transporter for glutamine induces apoptosis and inhibits tumor formation in a mouse AML xenotransplantation model. l-asparaginase (l-ase) is an anticancer agent also harboring glutaminase activity. We show that l-ases from both Escherichia coli and Erwinia chrysanthemi profoundly inhibit mTORC1 and protein synthesis and that this inhibition correlates with their glutaminase activity levels and produces a strong apoptotic response in primary AML cells. We further show that l-ases upregulate glutamine synthase (GS) expression in leukemic cells and that a GS knockdown enhances l-ase-induced apoptosis in some AML cells. Finally, we observe a strong autophagic process upon l-ase treatment. These results suggest that l-ase anticancer activity and glutamine uptake inhibition are promising new therapeutic strategies for AML.

Glutamine stimulates mTORC1 independent of the cell content of essential amino acids

Glutamine and leucine are important mTORC1 modulators, although their roles are not precisely defined. In HepG2 and HeLa cells glutamine-free incubation lowers mTORC1 activity, although cell leucine is not decreased. mTORC1 activity, suppressed by amino acid-free incubation, is completely rescued only if essential amino acids (EAA) and glutamine are simultaneously restored, although cell leucine is higher in the absence than in the presence of glutamine. Thus, glutamine stimulates mTORC1 independent of cell leucine, suggesting the existence of two distinct stimulatory signals from either glutamine or EAA.

The non-proteinogenic amino acids L-methionine sulfoximine and DL-phosphinothricin activate mTOR

L-Methionine sulfoximine (MSO) and DL-Phosphinothricin (PPT), two non-proteinogenic amino acids known as inhibitors of Glutamine Synthetase, cause a dose-dependent increase in the phosphorylation of the mTOR substrate S6 kinase 1. The effect is particularly evident in glutamine-depleted cells, where mTOR activity is very low, but is detectable for PPT also in the presence of glutamine. The stimulation of mTOR activity by either MSO or PPT is strongly synergized by essential amino acids. Thus, the non-proteinogenic amino acids MSO and PPT are mTOR activators.

Binding of nickel to testicular glutamate-ammonia ligase inhibits its enzymatic activity

Exposure to nickel has been shown to cause damage to the testis in several animal models. It is not known if the testis expresses protein(s) that can bind nickel. To test this, we used a nickel-binding assay to isolate testicular nickel-binding proteins. We identified glutamate-ammonia ligase (GLUL) as a prominent nickel-binding protein by mass spectrometry. Protein analysis and reverse transcriptase polymerase chain reaction showed that GLUL is expressed in the testis, predominantly in interstitial cells. We determined that GLUL has a higher affinity for nickel than for its regular co-factor manganese. We produced an enzymatically active, recombinant GLUL protein. Upon binding, nickel interferes with the manganese-catalyzed enzymatic activity of recombinant GLUL protein. We also determined that GLUL activity in testes of animals exposed to nickel sulfate is reduced. Our results identify testicular GLUL as the first testicular protein shown to be affected by nickel exposure.

Cell-cycle inhibition by Helicobacter pylori L-asparaginase

Helicobacter pylori (H. pylori) is a major human pathogen causing chronic gastritis, peptic ulcer, gastric cancer, and mucosa-associated lymphoid tissue lymphoma. One of the mechanisms whereby it induces damage depends on its interference with proliferation of host tissues. We here describe the discovery of a novel bacterial factor able to inhibit the cell-cycle of exposed cells, both of gastric and non-gastric origin. An integrated approach was adopted to isolate and characterise the molecule from the bacterial culture filtrate produced in a protein-free medium: size-exclusion chromatography, non-reducing gel electrophoresis, mass spectrometry, mutant analysis, recombinant protein expression and enzymatic assays. L-asparaginase was identified as the factor responsible for cell-cycle inhibition of fibroblasts and gastric cell lines. Its effect on cell-cycle was confirmed by inhibitors, a knockout strain and the action of recombinant L-asparaginase on cell lines. Interference with cell-cycle in vitro depended on cell genotype and was related to the expression levels of the concurrent enzyme asparagine synthetase. Bacterial subcellular distribution of L-asparaginase was also analysed along with its immunogenicity. H. pylori L-asparaginase is a novel antigen that functions as a cell-cycle inhibitor of fibroblasts and gastric cell lines. We give evidence supporting a role in the pathogenesis of H. pylori-related diseases and discuss its potential diagnostic application.

The eIF2 kinase GCN2 is essential for the murine immune system to adapt to amino acid deprivation by asparaginase

Amino acid starvation by asparaginase (ASNase) enhances phosphorylation of eukaryotic initiation factor 2 (eIF2) by general control nonderepressible 2 (GCN2) kinase, leading to reduced global mRNA translation rates. This conserves energy and allows cells time to reprogram stress-related gene expression to alleviate cell injury. This study addressed the importance of GCN2 for the immune system to adapt to amino acid starvation by ASNase. GCN2(+/+) and GCN2(-/-) mice were injected once daily with ASNase or saline for up to 7 d. In both thymus and spleen, activation of amino acid stress response genes to ASNase, such as asparagine synthetase and CAAT enhancer binding protein homologous protein, required GCN2. ASNase reduced food intake and body weight in both genotypes, but spleen and thymus wet weights and total cell numbers in thymus, spleen, bone marrow, and mesenteric lymph nodes were less in GCN2(-/-) mice treated with ASNase (genotype x ASNase, P < 0.05). In the thymus, GCN2(-/-) mice treated with ASNase demonstrated enhanced apoptosis and fewer cells in all subpopulations examined (CD3+, CD4-8-, CD4+8+, CD4+8-, CD4-8+) compared with GCN2(+/+) mice treated with ASNase (genotype x ASNase, P < 0.05). In the spleen, GCN2 deletion magnified ASNase-induced reductions in CD4+ T cells, CD8+ T cells, CD19+ B cells, and CD11b+ leukocytes (genotype x ASNase, P < 0.05). These results indicate that loss of GCN2 enhances immunosuppression by ASNase and that this eIF2 kinase is broadly required for amino acid stress management in the immune system.

L-Asparaginase: A Promising Chemotherapeutic Agent

This article comprises detailed information about L-asparaginase, encompassing topics such as microbial and plant sources of L-asparaginase, treatment with L-asparaginase, mechanism of action of L-asparaginase, production, purification, properties, expression and characteristics of l-asparaginase along with information about studies on the structure of L-asparaginase. Although L-asparaginase has been reviewed by Savitri and Azmi (2003), our effort has been to include recent and updated information about the enzyme covering new aspects such as structural modification and immobilization of L-asparaginase, recombinant L-asparaginase, resistance to L-asparaginase, methods of assay of L-asparagine and L-asparaginase activity using the biosensor approach, L-asparaginase activity in soil and the factors affecting it. Also, side-effects of L-asparaginase treatment in acute lymphoblastic leukemia (ALL) have been discussed in the current review. L-asparaginase has been and is still one of the most widely studied therapeutic enzymes by researchers and scientists worldwide.

Glutamine synthetase is essential for proliferation of fetal skin fibroblasts

Background. Glutamine synthetase (GS) is ubiquitously expressed in the human and plays a major role for many metabolic pathways. However, little is known about its role during the fetal period. Methods. Cultured skin fibroblasts derived from an aborted fetus deficient in GS activity due to a R324C exchange as well as fetal and mature controls were used to determine the level of GS-expression, apoptosis, and proliferation in presence or absence of exogenous glutamine. Results. Glutamine synthetase can be found at early gestational stages. Loss of GS activity either inherited or induced through l-methionine sulfoximine leads to an upregulation of the GS protein but not of the GS mRNA and results in a significant drop in the proliferation rate but has no effect on apoptosis. Exogenous glutamine does not influence the rate of apoptosis but increases proliferation rates of the fetal but not the mature fibroblasts. Conclusion. GS can be found during early human fetal stages when it displays a significant effect on cell proliferation.

Pharmacokinetic, pharmacodynamic and intracellular effects of PEG-asparaginase in newly diagnosed childhood acute lymphoblastic leukemia: results from a single agent window study

L-asparaginase is an effective drug for treatment of children with acute lymphoblastic leukemia (ALL). The effectiveness is thought to result from depletion of asparagine in serum and cells. We investigated the clinical response in vivo of 1000 IU/m(2) pegylated (PEG)-asparaginase and its pharmacokinetic, pharmacodynamic and intracellular effects in children with newly diagnosed ALL before start of combination chemotherapy. The in vivo window response was significantly related to immunophenotype and genotype: 26/38 common/pre B-ALL cases, especially those with hyperdiploidy and TELAML1 rearrangement, demonstrated a good clinical response compared to 8/17 T-ALL (P=0.01) and BCRABL-positive ALL (P=0.04). A poor in vivo clinical window response was related to in vitro resistance to L-asparaginase (P=0.02) and both were prognostic factors for long-term event-free survival (hazard ratio 6.4, P=0.004; hazard ratio 3.7, P=0.01). After administration of one in vivo dose of PEG-asparaginase no changes in apoptotic parameters or in intracellular levels of twenty amino acids in leukemic cells could be measured, in contradiction to the changes found after in vitro exposure. This may be explained by the rapid removal of apoptotic cells from the circulation in vivo. One additional dose of PEG-asparaginase upfront ALL treatment did not lead to other severe toxicities.

Reduced Ca2+ entry and suicidal death of erythrocytes in PDK1 hypomorphic mice

The phosphoinositide-dependent kinase PDK1 is a key element in the phosphoinositol-3-kinase signalling pathway, which is involved in the regulation of ion channels, transporters, cell volume and cell survival. Eryptosis, the suicidal death of erythrocytes, is characterized by decrease in cell volume, cell membrane blebbing and phospholipids scrambling with phosphatidylserine exposure at the cell surface. Oxidative stress, osmotic shock or Cl- removal trigger eryptosis by activation of Ca2+-permeable cation channels and subsequent increase in cytosolic Ca2+ activity. To explore the impact of PDK1 for erythrocyte survival, eryptosis was analysed in hypomorphic mice (pdk1hm) expressing only some 25% of PDK1 and in their wild-type littermates (pdk1wt). Cell volume was estimated from forward scatter and phosphatidylserine exposure from annexin-V binding in fluorescence activated cell sorter analysis. Forward scatter was smaller in pdk1hm than in pdk1wt erythrocytes. Oxidative stress (100 microM tert-butylhydroperoxide), osmotic shock (+300 mM sucrose) and Cl- removal (replacement of Cl- with gluconate) all decreased forward scatter and increased the percentage of annexin-V-binding erythrocytes from both pdk1hm and pdk1wt mice. After treatment, the forward scatter was similar in both genotypes, but the percentage of annexin-V binding was significantly smaller in pdk1hm than in pdk1wt erythrocytes. According to Fluo-3 fluorescence, cytosolic Ca2+ activity was significantly smaller in pdk1hm than in pdk1wt erythrocytes. Treatment with Ca2+-ionophore ionomycin (1 microM) was followed by an increase in annexin-V binding to similar levels in pdk1hm and pdk1wt erythrocytes. The experiments reveal that PDK1 deficiency is associated with decreased Ca2+ entry into erythrocytes and thus with blunted eryptotic effects of oxidative stress, osmotic shock and Cl- removal.

Enhanced susceptibility to suicidal death of erythrocytes from transgenic mice overexpressing erythropoietin

Eryptosis, a suicidal death of mature erythrocytes, is characterized by decrease of cell volume, cell membrane blebbing, and breakdown of cell membrane asymmetry with phosphatidylserine exposure at the cell surface. Triggers of eryptosis include increased cytosolic Ca(2+) activity, which could result from activation of Ca(2+)-permeable cation channels. Ca(2+) triggers phosphatidylserine exposure and activates Ca(2+)-sensitive K(+) channels, leading to cellular K(+) loss and cell shrinkage. The cation channels and thus eryptosis are stimulated by Cl(-) removal and inhibited by erythropoietin. The present experiments explored eryptosis in transgenic mice overexpressing erythropoietin (tg6). Erythrocytes were drawn from tg6 mice and their wild-type littermates (WT). Phosphatidylserine exposure was estimated from annexin binding and cell volume from forward scatter in fluorescence-activated cell sorting (FACS) analysis. The percentage of annexin binding was significantly larger and forward scatter significantly smaller in tg6 than in WT erythrocytes. Transgenic erythrocytes were significantly more resistant to osmotic lysis than WT erythrocytes. Cl(-) removal and exposure to the Ca(2+) ionophore ionomycin (1 microM) increased annexin binding and decreased forward scatter, effects larger in tg6 than in WT erythrocytes. The K(+) ionophore valinomycin (10 nM) triggered eryptosis in both tg6 and WT erythrocytes and abrogated differences between genotypes. An increase of extracellular K(+) concentration to 125 mM blunted the difference between tg6 and WT erythrocytes. Fluo-3 fluorescence reflecting cytosolic Ca(2+) activity was larger in tg6 than in WT erythrocytes. In conclusion, circulating erythrocytes from tg6 mice are sensitized to triggers of eryptosis but more resistant to osmotic lysis, properties at least partially due to enhanced Ca(2+) entry and increased K(+) channel activity.

The inhibition of glutamine synthetase sensitizes human sarcoma cells to L-asparaginase

PURPOSE

To evaluate the activity of the antitumor enzyme L: -asparaginase (ASNase) on tumor cells of mesenchymal origin and the contribution of glutamine synthetase (GS) to the adaptation to the metabolic stress caused by the anti-tumor enzyme.

METHODS

We studied the effects of ASNase in six human sarcoma cell lines: HT1080 (fibrosarcoma); RD (rhabdomyosarcoma); SW872 (liposarcoma); HOS, SAOS-2, and U2OS (osteosarcoma) in the absence or in the presence of the GS inhibitor methionine L: -sulfoximine (MSO).

RESULTS

HT1080 and SW872 cells were highly sensitive to ASNase-dependent cytotoxicity. In contrast, RD, SAOS-2, HOS, and U2OS cells exhibited only a partial growth suppression upon treatment with the anti-tumor enzyme. In these cell lines ASNase treatment was associated with increased levels of GS. When ASNase was used together with MSO, the proliferation of the poorly sensitive cell lines was completely blocked and a significant decrease in the IC(50) for ASNase was observed. Moreover, when ASNase treatment was carried on in the presence of MSO, HOS and U2OS osteosarcoma cells exhibited a marked cytotoxicity, with increased apoptosis.

CONCLUSIONS

In human sarcoma cells (1) GS markedly contributes to the metabolic adaptation of tumor cells to ASNase and (2) the inhibition of GS activity enhances the antiproliferative and cytotoxic effects of ASNase. The two-step interference with glutamine metabolism, obtained through the combined treatment with ASNase and MSO, may provide a novel therapeutic approach that should be further investigated in human tumors of mesenchymal origin.

Plasma membrane ion channels in suicidal cell death

The machinery leading to apoptosis includes altered activity of ion channels. The channels contribute to apoptotic cell shrinkage and modify intracellular ion composition. Cl(-) channels allow the exit of Cl(-), osmolytes and HCO(3)(-) leading to cell shrinkage and cytosolic acidification. K(+) exit through K(+) channels contributes to cell shrinkage and decreases intracellular K(+) concentration, which in turn favours apoptotic cell death. K(+) channel activity further determines the cell membrane potential, a driving force for Ca(2+) entry through Ca(2+) channels. Ca(2+) may enter through unselective cation channels. An increase of cytosolic Ca(2+) may stimulate several enzymes executing apoptosis. Specific ion channel blockers may either promote or counteract suicidal cell death. The present brief review addresses the role of ion channels in the regulation of suicidal cell death with special emphasis on the role of channels in CD95 induced apoptosis of lymphocytes and suicidal death of erythrocytes or eryptosis.

Cell volume regulatory ion channels in cell proliferation and cell death

Alterations of cell volume are key events during both cell proliferation and apoptotic cell death. Cell proliferation eventually requires an increase of cell volume, and apoptosis is typically paralleled by cell shrinkage. Alterations of cell volume require the participation of ion transport across the cell membrane, including appropriate activity of Cl(-) and K(+) channels. Cl(-) channels modify cytosolic Cl(-) activity and mediate osmolyte flux, and thus influence cell volume. Most Cl(-) channels allow exit of HCO(3)(-), leading to cytosolic acidification, which in turn inhibits cell proliferation and favors apoptosis. K(+) exit through K(+) channels decreases cytosolic K(+) concentration, which may sensitize the cell for apoptotic cell death. K(+) channel activity further maintains the cell membrane potential, a critical determinant of Ca(2+) entry through Ca(2+) channels. Ca(2+) may, in addition, enter through Ca(2+)-permeable cation channels, which, in some cells, are activated by hyperosmotic shock. Increases of cytosolic Ca(2+) activity may trigger both mechanisms required for cell proliferation and mechanisms, leading to apoptosis. Thereby cell proliferation and apoptosis depend on magnitude and temporal organization of Ca(2+) entry, as well as activity of other signaling pathways. Accordingly, the same ion channels may participate in the stimulation of both cell proliferation and apoptosis. Specific ion channel blockers may thus abrogate both cellular mechanisms, depending on cell type and condition.

Role of glutamine depletion in directing tissue-specific nutrient stress responses to L-asparaginase

L-asparaginase is important in the induction regimen for treating acute lymphoblastic leukemia. Cytotoxic complications are clinically significant problems lacking mechanistic insight. To reveal tissue-specific molecular responses to this drug, mice were administered asparaginase from either Escherichia coli (clinically used) or Wolinella succinogenes (novel, glutaminase-free form). Both enzymes abolished serum asparagine, but only the E. coli form reduced circulating glutamine. E. coli asparaginase reduced protein synthesis in liver and spleen but not pancreas via increased phosphorylation of the translation factor eIF2. In contrast, treatment with Wolinella caused no untoward changes in protein synthesis in any tissue examined. Treating mice deleted for the eIF2 kinase, GCN2, with the E. coli enzyme showed eIF2 phosphorylation to be GCN2-dependent, but only initially. Furthermore, although eIF2 phosphorylation was not increased in the pancreas or by Wolinella asparaginase, expression of the amino acid stress response genes, asparagine synthetase and CHOP/GADD153, increased as a result of both enzymes, even in tissues demonstrating no change in eIF2 phosphorylation. Finally, signaling downstream of the mammalian target of rapamycin kinase was repressed in liver and pancreas by E. coli but not Wolinella asparaginase. These data demonstrate that the nutrient stress response to asparaginase is tissue-specific and exacerbated by glutamine depletion. Importantly, increased expression of asparagine synthetase and CHOP does not require eIF2 phosphorylation, signifying alternate or auxiliary means of inducing gene expression under conditions of amino acid depletion in the whole animal.

Inborn error of amino acid synthesis: human glutamine synthetase deficiency

Glutamine synthetase (GS) is ubiquitously expressed in human tissues, being involved in ammonia detoxification and interorgan nitrogen flux. Inherited systemic deficiency of glutamine based on a defect of glutamine synthetase was recently described in two newborns with an early fatal course of disease. Glutamine was largely absent in their serum, urine and cerebrospinal fluid. Each of the patients had a homozygous mutation in the glutamine synthetase gene and enzymatic investigations confirmed that these mutations lead to a severely reduced glutamine synthetase activity. From the observation in the first patients with congenital glutamine synthetase deficiency, brain malformation can be expected as one of the leading signs. In addition, other organ systems are probably involved as observed in one of the index patients who suffered from severe enteropathy and necrolytic erythema of the skin. Deficiency of GS has to be added to the list of inherited metabolic disorders as a rare example of a defect in the biosynthesis of an amino acid.

Ion channels in cell proliferation and apoptotic cell death

Cell proliferation and apoptosis are paralleled by altered regulation of ion channels that play an active part in the signaling of those fundamental cellular mechanisms. Cell proliferation must--at some time point--increase cell volume and apoptosis is typically paralleled by cell shrinkage. Cell volume changes require the participation of ion transport across the cell membrane, including appropriate activity of Cl- and K+ channels. Besides regulating cytosolic Cl- activity, osmolyte flux and, thus, cell volume, most Cl- channels allow HCO3- exit and cytosolic acidification, which inhibits cell proliferation and favors apoptosis. K+ exit through K+ channels may decrease intracellular K+ concentration, which in turn favors apoptotic cell death. K+ channel activity further maintains the cell membrane potential, a critical determinant of Ca2+ entry through Ca2+ channels. Cytosolic Ca2+ may trigger mechanisms required for cell proliferation and stimulate enzymes executing apoptosis. The switch between cell proliferation and apoptosis apparently depends on the magnitude and temporal organization of Ca2+ entry and on the functional state of the cell. Due to complex interaction with other signaling pathways, a given ion channel may play a dual role in both cell proliferation and apoptosis. Thus, specific ion channel blockers may abrogate both fundamental cellular mechanisms, depending on cell type, regulatory environment and condition of the cell. Clearly, considerable further experimental effort is required to fully understand the complex interplay between ion channels, cell proliferation and apoptosis.