Methylation of the asparagine synthetase promoter in human leukemic cell lines is associated with a specific methyl binding protein

Targeting Asparagine Metabolism in Solid Tumors

Reprogramming of energy metabolism to support cellular growth is a "hallmark" of cancer, allowing cancer cells to balance the catabolic demands with the anabolic needs of producing the nucleotides, amino acids, and lipids necessary for tumor growth. Metabolic alterations, or "addiction", are promising therapeutic targets and the focus of many drug discovery programs. Asparagine metabolism has gained much attention in recent years as a novel target for cancer therapy. Asparagine is widely used in the production of other nutrients and plays an important role in cancer development. Nutritional inhibition therapy targeting asparagine has been used as an anticancer strategy and has shown success in the treatment of leukemia. However, in solid tumors, asparagine restriction alone does not provide ideal therapeutic efficacy. Tumor cells initiate reprogramming processes in response to asparagine deprivation. This review provides a comprehensive overview of asparagine metabolism in cancers. We highlight the physiological role of asparagine and current advances in improving survival and overcoming therapeutic resistance.

The structural insights of L-asparaginase from Pseudomonas aeruginosa CSPS4 at elevated temperatures highlight its thermophilic nature

In the present investigation, a novel thermophilic L-asparaginase (Asn_PA) from Pseudomonas aeruginosa CSPS4 was investigated to explore its structural insights at elevated temperatures. Sequence analysis of Asn_PA depicted three conserved motifs (VVILATGGTIAG, DGIVITHGTDTLEETAYFL, and, LRKQGVQIIRSSHVNAGGF), of them, two motifs exhibit catalytically-important residues i.e., T45 and T125. A homology modelling-based structure model for Asn_PA was generated with 4PGA as the top-matched template. The predicted structure was validated and energy was minimized. Molecular docking was carried out cantered at the active site for asparagine and glutamine as its substrate ligands. The enzyme-substrate interaction analysis showed binding affinities of  - 4.8 and  - 4.1 kcal/mol for asparagine and glutamine respectively. Molecular dynamics (MD) simulation studies showed a better stability of Asn_PA at temperatures of 60 °C, over 40, 50 and, 80 °C, making this enzyme a novel L-asparaginase from other mesophilic P. aeruginosa strain. The trajectory analysis showed that RMSD, Rg, and, SASA values correlate well with each other in the different tested temperatures during the MD analysis. Thus, the present findings encourage extensive characterization of the Asn_PA using laboratory experiments to understand the structural behavior of the active site loop in an open or closed state with and without the substrate molecules.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-024-04072-w.

Bisabosqual A: A novel asparagine synthetase inhibitor suppressing the proliferation and migration of human non-small cell lung cancer A549 cells

Asparagine synthetase (ASNS) is a crucial enzyme for the de novo biosynthesis of endogenous asparagine (Asn), and ASNS shows the positive relationship with the growth of several solid tumors. Most of ASNS inhibitors are analogs of transition-state in ASNS reaction, but their low cell permeability hinders their anticancer activity. Therefore, novel ASNS inhibitors with a new pharmacophore urgently need to be developed. In this study, we established and applied a system for in vitro screening of ASNS inhibitors, and found a promising unique bisabolane-type meroterpenoid molecule, bisabosqual A (Bis A), able to covalently modify K556 site of ASNS protein. Bis A targeted ASNS to suppress cell proliferation of human non-small cell lung cancer A549 cells and exhibited a synergistic effect with L-asparaginase (L-ASNase). Mechanistically, Bis A promoted oxidative stress and apoptosis, while inhibiting autophagy, cell migration and epithelial-mesenchymal transition (EMT), impeding cancer cell development. Moreover, Bis A induced negative feedback pathways containing the GCN2-eIF2α-ATF4, PI3K-AKT-mTORC1 and RAF-MEK-ERK axes, but combination treatment of Bis A and rapamycin/torin-1 overcame the potential drug resistance triggered by mTOR pathways. Our study demonstrates that ASNS inhibition is promising for cancer chemotherapy, and Bis A is a potential lead ASNS inhibitor for anticancer development.

Enzyme Engineering Strategies for the Bioenhancement of L-Asparaginase Used as a Biopharmaceutical

Over the past few years, there has been a surge in the industrial production of recombinant enzymes from microorganisms due to their catalytic characteristics being highly efficient, selective, and biocompatible. L-asparaginase (L-ASNase) is an enzyme belonging to the class of amidohydrolases that catalyzes the hydrolysis of L-asparagine into L-aspartic acid and ammonia. It has been widely investigated as a biologic agent for its antineoplastic properties in treating acute lymphoblastic leukemia. The demand for L-ASNase is mainly met by the production of recombinant type II L-ASNase from Escherichia coli and Erwinia chrysanthemi. However, the presence of immunogenic proteins in L-ASNase sourced from prokaryotes has been known to result in adverse reactions in patients undergoing treatment. As a result, efforts are being made to explore strategies that can help mitigate the immunogenicity of the drug. This review gives an overview of recent biotechnological breakthroughs in enzyme engineering techniques and technologies used to improve anti-leukemic L-ASNase, taking into account the pharmacological importance of L-ASNase.

In vitro simulation of the acute lymphoblastic leukemia niche: a critical view on the optimal approximation for drug testing

Acute lymphoblastic leukemia with the worst prognosis is related to minimal residual disease. Minimal residual disease not only depends on the individual peculiarities of leukemic clones but also reflects the protective role of the acute lymphoblastic leukemia microenvironment. In this review, we discuss in detail cell-to-cell interactions in the 2 leukemic niches, more explored bone marrow and less studied extramedullary adipose tissue. A special emphasis is given to multiple ways of interactions of acute lymphoblastic leukemia cells with the bone marrow or extramedullary adipose tissue microenvironment, indicating observed differences in B- and T-cell-derived acute lymphoblastic leukemia behavior. This analysis argued for the usage of coculture systems for drug testing. Starting with a review of available sources and characteristics of acute lymphoblastic leukemia cells, mesenchymal stromal cells, endothelial cells, and adipocytes, we have then made an update of the available 2-dimensional and 3-dimensional systems, which bring together cellular elements, components of the extracellular matrix, or its imitation. We discussed the most complex available 3-dimensional systems like "leukemia-on-a-chip," which include either a prefabricated microfluidics platform or, alternatively, the microarchitecture, designed by using the 3-dimensional bioprinting technologies. From our analysis, it follows that for preclinical antileukemic drug testing, in most cases, intermediately complex in vitro cell systems are optimal, such as a "2.5-dimensional" coculture of acute lymphoblastic leukemia cells with niche cells (mesenchymal stromal cells, endothelial cells) plus matrix components or scaffold-free mesenchymal stromal cell organoids, populated by acute lymphoblastic leukemia cells. Due to emerging evidence for the correlation of obesity and poor prognosis, a coculture of adipocytes with acute lymphoblastic leukemia cells as a drug testing system is gaining shape.

Possible mechanism of metabolic and drug resistance with L-asparaginase therapy in childhood leukaemia

L-asparaginase, which hydrolyzes asparagine into aspartic acid and ammonia, is frequently used to treat acute lymphoblastic leukaemia in children. When combined with other chemotherapy drugs, the event-free survival rate is 90%. Due to immunogenicity and drug resistance, however, not all patients benefit from it, restricting the use of L-asparaginase therapy in other haematological cancers. To solve the problem of immunogenicity, several L-ASNase variants have emerged, such as Erwinia-ASNase and PEG-ASNase. However, even when Erwinia-ASNase is used as a substitute for E. coli-ASNase or PEG-ASNase, allergic reactions occur in 3%-33% of patients. All of these factors contributed to the development of novel L-ASNases. Additionally, L-ASNase resistance mechanisms, such as the methylation status of ASNS promoters and activation of autophagy, have further emphasized the importance of personalized treatment for paediatric haematological neoplasms. In this review, we discussed the metabolic effects of L-ASNase, mechanisms of drug resistance, applications in non-ALL leukaemia, and the development of novel L-ASNase.

ASNS can predict the poor prognosis of clear cell renal cell carcinoma

Purpose

Clear cell renal cell carcinoma (ccRCC) is one of the most common malignancies of the urinary system. This study was conducted to discover a new target that can predict the prognosis and promote the treatment of ccRCC.

Methods

The raw data were downloaded from the TCGA database, and the predictive value of ASNS for various clinicopathological features was verified in the following analysis. Then, we analyzed the potential involvement of ASNS in tumor immunity and obtained the possible pathways involving ASNS through GO/KEGG enrichment analysis and GSEA. We also further verified our findings in pathological specimens of ccRCC patients.

Results

ASNS expression was significantly increased in ccRCC, which was associated with advanced clinicopathological characteristics. It was an independent prognostic factor for overall survival in 535 patients with ccRCC. Immune cell infiltration analysis revealed that ASNS expression was related to T lymphocyte infiltration of tumors and poor prognosis. Moreover, we performed relevant functional enrichment analyses of ASNS.

Conclusions

ASNS might play a significant role in the development and immune cell infiltration of ccRCC and serve as a valuable clinical prognostic biomarker.

Novel Insights on the Use of L-Asparaginase as an Efficient and Safe Anti-Cancer Therapy

Simple Summary

L-asparaginase (L-ASNase) therapy is key for achieving the very high cure rate of pediatric acute lymphoblastic leukemia (ALL), yet its use is mostly confined to this indication. One main reason preventing the expansion of today’s FDA-approved L-ASNases to solid cancers is their high toxicity and side effects, which become especially challenging in adult patients. The design of optimized L-ASNase molecules provides opportunities to overcome these unwanted toxicities. An additional challenge to broader application of L-ASNases is how cells can counter the pharmacological effect of this drug and the identification of L-ASNases resistance mechanisms. In this review, we discuss recent insights into L-ASNase adverse effects, resistance mechanisms, and how novel L-ASNase variants and drug combinations can expand its clinical applicability, with a focus on both hematological and solid tumors.

Abstract

L-Asparaginase (L-ASNase) is an enzyme that hydrolyses the amino acid asparagine into aspartic acid and ammonia. Systemic administration of bacterial L-ASNase is successfully used to lower the bioavailability of this non-essential amino acid and to eradicate rapidly proliferating cancer cells with a high demand for exogenous asparagine. Currently, it is a cornerstone drug in the treatment of the most common pediatric cancer, acute lymphoblastic leukemia (ALL). Since these lymphoblasts lack the expression of asparagine synthetase (ASNS), these cells depend on the uptake of extracellular asparagine for survival. Interestingly, recent reports have illustrated that L-ASNase may also have clinical potential for the treatment of other aggressive subtypes of hematological or solid cancers. However, immunogenic and other severe adverse side effects limit optimal clinical use and often lead to treatment discontinuation. The design of optimized and novel L-ASNase formulations provides opportunities to overcome these limitations. In addition, identification of multiple L-ASNase resistance mechanisms, including ASNS promoter reactivation and desensitization, has fueled research into promising novel drug combinations to overcome chemoresistance. In this review, we discuss recent insights into L-ASNase adverse effects, resistance both in hematological and solid tumors, and how novel L-ASNase variants and drug combinations can expand its clinical applicability.

SLC5A3-Dependent Myo-inositol Auxotrophy in Acute Myeloid Leukemia

An enhanced requirement for nutrients is a hallmark property of cancer cells. Here, we optimized an in vivo genetic screening strategy in acute myeloid leukemia (AML), which led to the identification of the myo-inositol transporter SLC5A3 as a dependency in this disease. We demonstrate that SLC5A3 is essential to support a myo-inositol auxotrophy in AML. The commonality among SLC5A3-dependent AML lines is the transcriptional silencing of ISYNA1, which encodes the rate-limiting enzyme for myo-inositol biosynthesis, inositol-3-phosphate synthase 1. We use gain- and loss-of-function experiments to reveal a synthetic lethal genetic interaction between ISYNA1 and SLC5A3 in AML, which function redundantly to sustain intracellular myo-inositol. Transcriptional silencing and DNA hypermethylation of ISYNA1 occur in a recurrent manner in human AML patient samples, in association with IDH1/IDH2 and CEBPA mutations. Our findings reveal myo-inositol as a nutrient dependency in AML caused by the aberrant silencing of a biosynthetic enzyme. SIGNIFICANCE: We show how epigenetic silencing can provoke a nutrient dependency in AML by exploiting a synthetic lethality relationship between biosynthesis and transport of myo-inositol. Blocking the function of this solute carrier may have therapeutic potential in an epigenetically defined subset of AML.This article is highlighted in the In This Issue feature, p. 275.

Association of allele-specific methylation of the ASNS gene with asparaginase sensitivity and prognosis in T-ALL

Allele-specific methylation of the ASNS gene is associated with asparaginase sensitivity and therapeutic outcome in T-ALL.

Pediatric T-ALL patients with poor prognostic SPI1 fusion exclusively exhibited ASNS hypomethylation status.

Asparaginase therapy is a key component of chemotherapy for patients with T-cell acute lymphoblastic leukemia (T-ALL). Asparaginase depletes serum asparagine by deamination into aspartic acid. Normal hematopoietic cells can survive due to asparagine synthetase (ASNS) activity, whereas leukemia cells are supposed to undergo apoptosis due to silencing of the ASNS gene. Because the ASNS gene has a typical CpG island in its promoter, its methylation status in T-ALL cells may be associated with asparaginase sensitivity. Thus, we investigated the significance of ASNS methylation status in asparaginase sensitivity of T-ALL cell lines and prognosis of childhood T-ALL. Sequencing of bisulfite polymerase chain reaction products using next-generation sequencing technology in 22 T-ALL cell lines revealed a stepwise allele-specific methylation of the ASNS gene, in association with an aberrant methylation of a 7q21 imprinted gene cluster. T-ALL cell lines with ASNS hypermethylation status showed significantly higher in vitro l-asparaginase sensitivity in association with insufficient asparaginase-induced upregulation of ASNS gene expression and lower basal ASNS protein expression. A comprehensive analysis of diagnostic samples from pediatric patients with T-ALL in Japanese cohorts (N = 77) revealed that methylation of the ASNS gene was associated with an aberrant methylation of the 7q21 imprinted gene cluster. In pediatric T-ALL patients in Japanese cohorts (n = 75), ASNS hypomethylation status was significantly associated with poor therapeutic outcome, and all cases with poor prognostic SPI1 fusion exclusively exhibited ASNS hypomethylation status. These observations show that ASNS hypomethylation status is associated with asparaginase resistance and is a poor prognostic biomarker in childhood T-ALL.

Mutant p53-reactivating compound APR-246 synergizes with asparaginase in inducing growth suppression in acute lymphoblastic leukemia cells

Asparaginase depletes extracellular asparagine in the blood and is an important treatment for acute lymphoblastic leukemia (ALL) due to asparagine auxotrophy of ALL blasts. Unfortunately, resistance occurs and has been linked to expression of the enzyme asparagine synthetase (ASNS), which generates asparagine from intracellular sources. Although TP53 is the most frequently mutated gene in cancer overall, TP53 mutations are rare in ALL. However, TP53 mutation is associated with poor therapy response and occurs at higher frequency in relapsed ALL. The mutant p53-reactivating compound APR-246 (Eprenetapopt/PRIMA-1Met) is currently being tested in phase II and III clinical trials in several hematological malignancies with mutant TP53. Here we present CEllular Thermal Shift Assay (CETSA) data indicating that ASNS is a direct or indirect target of APR-246 via the active product methylene quinuclidinone (MQ). Furthermore, combination treatment with asparaginase and APR-246 resulted in synergistic growth suppression in ALL cell lines. Our results thus suggest a potential novel treatment strategy for ALL.

Asparagine: A Metabolite to Be Targeted in Cancers

Amino acids play central roles in cancer progression beyond their function as building blocks for protein synthesis. Thus, targeting amino acid acquisition and utilization has been proved to be therapeutically beneficial in various pre-clinical models. In this regard, depletion of circulating asparagine, a nonessential amino acid, by L-asparaginase has been used in treating pediatric acute lymphoblastic leukemia (ALL) for decades. Of interest, unlike most solid tumor cells, ALL cells lack the ability to synthesize their own asparagine de novo effectively. However, only until recently, growing evidence suggests that solid tumor cells strive to acquire adequate amounts of asparagine to support tumor progression. This process is subjected to the regulation at various levels, including oncogenic signal, tumor-niche interaction, intratumor heterogeneity and dietary accessibility. We will review the literature on L-asparaginase-based therapy as well as recent understanding of asparagine metabolism in solid tumor progression, with the hope of shedding light into a broader cancer therapeutic strategy by perturbing its acquisition and utilization.

Association of aberrant ASNS imprinting with asparaginase sensitivity and chromosomal abnormality in childhood BCP-ALL

Karyotype is an important prognostic factor in childhood B-cell precursor acute lymphoblastic leukemia (BCP-ALL), but the underlying pharmacogenomics remain unknown. Asparaginase is an integral component in current chemotherapy for childhood BCP-ALL. Asparaginase therapy depletes serum asparagine. Normal hematopoietic cells can produce asparagine by asparagine synthetase (ASNS) activity, but ALL cells are unable to synthesize adequate amounts of asparagine. The ASNS gene has a typical CpG island in its promoter. Thus, methylation of the ASNS CpG island could be one of the epigenetic mechanisms for ASNS gene silencing in BCP-ALL. To gain deep insights into the pharmacogenomics of asparaginase therapy, we investigated the association of ASNS methylation status with asparaginase sensitivity. The ASNS CpG island is largely unmethylated in normal hematopoietic cells, but it is allele-specifically methylated in BCP-ALL cells. The ASNS gene is located at 7q21, an evolutionally conserved imprinted gene cluster. ASNS methylation in childhood BCP-ALL is associated with an aberrant methylation of the imprinted gene cluster at 7q21. Aberrant methylation of mouse Asns and a syntenic imprinted gene cluster is also confirmed in leukemic spleen samples from ETV6-RUNX1 knockin mice. In 3 childhood BCP-ALL cohorts, ASNS is highly methylated in BCP-ALL patients with favorable karyotypes but is mostly unmethylated in BCP-ALL patients with poor prognostic karyotypes. Higher ASNS methylation is associated with higher L-asparaginase sensitivity in BCP-ALL through lower ASNS gene and protein expression levels. These observations demonstrate that silencing of the ASNS gene as a result of aberrant imprinting is a pharmacogenetic mechanism for the leukemia-specific activity of asparaginase therapy in BCP-ALL.

Targeting extracellular nutrient dependencies of cancer cells

BACKGROUND

Cancer cells rewire their metabolism to meet the energetic and biosynthetic demands of their high proliferation rates and environment. Metabolic reprogramming of cancer cells may result in strong dependencies on nutrients that could be exploited for therapy. While these dependencies may be in part due to the nutrient environment of tumors, mutations or expression changes in metabolic genes also reprogram metabolic pathways and create addictions to extracellular nutrients.

SCOPE OF REVIEW

This review summarizes the major nutrient dependencies of cancer cells focusing on their discovery and potential mechanisms by which metabolites become limiting for tumor growth. We further detail available therapeutic interventions based on these metabolic features and highlight opportunities for restricting nutrient availability as an anti-cancer strategy.

MAJOR CONCLUSIONS

Strategies to limit nutrients required for tumor growth using dietary interventions or nutrient degrading enzymes have previously been suggested for cancer therapy. The best clinical example of exploiting cancer nutrient dependencies is the treatment of leukemia with l-asparaginase, a first-line chemotherapeutic that depletes serum asparagine. Despite the success of nutrient starvation in blood cancers, it remains unclear whether this approach could be extended to other solid tumors. Systematic studies to identify nutrient dependencies unique to individual tumor types have the potential to discover targets for therapy.

Promoter demethylation of the asparagine synthetase gene is required for ATF4-dependent adaptation to asparagine depletion

Tumor cells adapt to nutrient-limited environments by inducing gene expression that ensures adequate nutrients to sustain metabolic demands. For example, during amino acid limitations, ATF4 in the amino acid response induces expression of asparagine synthetase (ASNS), which provides for asparagine biosynthesis. Acute lymphoblastic leukemia (ALL) cells are sensitive to asparagine depletion, and administration of the asparagine depletion enzyme l-asparaginase is an important therapy option. ASNS expression can counterbalance l-asparaginase treatment by mitigating nutrient stress. Therefore, understanding the mechanisms regulating ASNS expression is important to define the adaptive processes underlying tumor progression and treatment. Here we show that DNA hypermethylation at the ASNS promoter prevents its transcriptional expression following asparagine depletion. Insufficient expression of ASNS leads to asparagine deficiency, which facilitates ATF4-independent induction of CCAAT-enhancer-binding protein homologous protein (CHOP), which triggers apoptosis. We conclude that chromatin accessibility is critical for ATF4 activity at the ASNS promoter, which can switch ALL cells from an ATF4-dependent adaptive response to ATF4-independent apoptosis during asparagine depletion. This work may also help explain why ALL cells are most sensitive to l-asparaginase treatment compared with other cancers.

Surviving Stress: Modulation of ATF4-Mediated Stress Responses in Normal and Malignant Cells

Activating transcription factor 4 (ATF4) is a stress-induced transcription factor that is frequently upregulated in cancer cells. ATF4 controls the expression of a wide range of adaptive genes that allow cells to endure periods of stress, such as hypoxia or amino acid limitation. However, under persistent stress conditions, ATF4 promotes the induction of apoptosis. Recent advances point to a role for post-translational modifications (PTMs) and epigenetic mechanisms in balancing these pro- and anti-survival effects of ATF4. We review here how PTMs and epigenetic modifiers associated with ATF4 may be exploited by cancer cells to cope with cellular stress conditions that are intrinsically associated with tumor growth. Identification of mechanisms that modulate ATF4-mediated transcription and its effects on cellular metabolism may uncover new targets for cancer treatment.

Identification of a cytogenetic and molecular subgroup of acute myeloid leukemias showing sensitivity to L-Asparaginase

L-Asparaginase (L-Asp) is an enzyme that catalyzes the hydrolysis of L-asparagine to L-aspartic acid, and its depletion induces leukemic cell death. L-Asp is an important component of treatment regimens for Acute Lymphoblastic Leukemia (ALL). Sensitivity to L-Asp is due to the absence of L-Asparagine synthetase (ASNS), the enzyme that catalyzes the biosynthesis of L-asparagine. ASNS gene is located on 7q21.3, and its increased expression in ALLs correlates with L-Asp resistance. Chromosome 7 monosomy (-7) is a recurrent aberration in myeloid disorders, particularly in adverse-risk Acute Myeloid Leukemias (AMLs) and therapy-related myeloid neoplasms (t-MN), that leads to a significant downregulation of the deleted genes, including ASNS. Therefore, we hypothesized that -7 could affect L-Asp sensitivity in AMLs. By treating AML cell lines and primary cells from pediatric patients with L-Asp, we showed that -7 cells were more sensitive than AML cells without -7. Importantly, both ASNS gene and protein expression were significantly lower in -7 AML cell lines, suggesting that haploinsufficiency of ASNS might induce sensitivity to L-Asp in AMLs. To prove the role of ASNS haploinsufficiency in sensitizing AML cells to L-Asp treatment, we performed siRNA-knockdown of ASNS in AML cell lines lacking -7, and observed that ASNS knockdown significantly increased L-Asp cytotoxicity. In conclusion, -7 AMLs showed high sensitivity to L-Asp treatment due to low expression of ASNS. Thus, L-Asp may be considered for treatment of AML pediatric patients carrying -7, in order to improve the outcome of adverse-risk AMLs and t-MN patients.

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.

ATF4 activity: a common feature shared by many kinds of slow-aging mice

ATF4, a DNA-binding factor that modulates responses to amino acid availability and ribosomal function, has been shown to be altered in both liver and fibroblasts from two strains of long-lived mice, i.e. Snell dwarf and PAPP-A knockout mice. New data now show elevated ATF4 levels, and elevation of ATF4-dependent proteins and mRNAs, in liver of mice treated with acarbose or rapamycin, calorically restricted mice, methionine-restricted mice, and mice subjected to litter crowding. Elevation of ATF4, at least in liver, thus seems to be a shared feature of diets, drugs, genes, and developmental alterations that extend maximum lifespan in mice.

Asparagine synthetase: regulation by cell stress and involvement in tumor biology

Asparagine synthetase (ASNS) catalyzes the conversion of aspartate and glutamine to asparagine and glutamate in an ATP-dependent reaction. The enzyme is ubiquitous in its organ distribution in mammals, but basal expression is relatively low in tissues other than the exocrine pancreas. Human ASNS activity is highly regulated in response to cell stress, primarily by increased transcription from a single gene located on chromosome 7. Among the genomic elements that control ASNS transcription is the C/EBP-ATF response element (CARE) within the promoter. Protein limitation or an imbalanced dietary amino acid composition activate the ASNS gene through the amino acid response (AAR), a process that is replicated in cell culture through limitation for any single essential amino acid. Endoplasmic reticulum stress also increases ASNS transcription through the PERK-eIF2-ATF4 arm of the unfolded protein response (UPR). Both the AAR and UPR lead to increased synthesis of ATF4, which binds to the CARE and induces ASNS transcription. Elevated expression of ASNS protein is associated with resistance to asparaginase therapy in childhood acute lymphoblastic leukemia and may be a predictive factor in drug sensitivity for certain solid tumors as well. Activation of the GCN2-eIF2-ATF4 signaling pathway, leading to increased ASNS expression appears to be a component of solid tumor adaptation to nutrient deprivation and/or hypoxia. Identifying the roles of ASNS in fetal development, tissue differentiation, and tumor growth may reveal that ASNS function extends beyond asparagine biosynthesis.

Functional analysis of a novel DNA polymorphism of a tandem repeated sequence in the asparagine synthetase gene in acute lymphoblastic leukemia cells

Asparagine synthetase (ASNS) is an enzyme expressed ubiquitously in mammalian cells. Here, we discovered two 14-bp tandem repeat (2R, wild-type) sequences in the first intron of the gene. The 14-bp sequence is similar to the three GC-boxes (GC-I, -II, and -III) found in the promoter region of the ASNS gene, as well as, the binding site of transcription factor Sp-1. Approximately 75% of acute lymphoblastic leukemia (ALL) samples had the 2R sequence in both allele; however, 20% and 3% ALL samples had three (3R) and four (4R) 14-bp tandem repeats in one allele, respectively; the other allele had 2R. The tandem repeat sequence was not specific to the leukemia cells but represents a novel germline polymorphism. Interestingly, the 14-bp sequence functioned as a transcriptional enhancer element as shown by reporter analysis and formed a protein-DNA complex in vitro. Our data for the first time show that the ASNS gene has tandem repeated sequences as a polymorphism, and it can function as a transcriptional element; increased number of tandem repeat producing increased activity. Clinical significance in ALL requires further studies.

Poor prognosis in acute lymphoblastic leukemia may relate to promoter hypermethylation of cancer-related genes

The hallmark of acute lymphoblastic leukemia (ALL) is a progressive appearance of malignant cell behavior that is triggered by the evolution of altered gene function. ALL has traditionally been viewed as a genetic disease; however, epigenetic defects also play an important role. DNA promoter methylation has gained increasing recognition as an important mechanism for transcriptional silencing of tumor-suppressor genes. Hypermethylation may contribute to the pathogenesis of leukemias providing an alternative route to gene mutation. We have reported that gene methylation in ALL cells is the most important way to inactivate cancer-related genes in this disease. In fact, this epigenetic event can help to inactivate tumor-suppressive apoptotic or growth-arresting responses and has prognostic impact in B- and T-ALL. The presence in individual tumors of multiple genes simultaneously methylated is an independent factor of poor prognosis in both childhood and adult ALL in terms of disease-free survival and overall survival. Moreover, methylation status is able to redefine the prognosis of selected ALL groups with well-established prognostic features.

Epigenetic changes in the repression and induction of asparagine synthetase in human leukemic cell lines

In common with certain other lymphoid neoplasms, cells of the human lymphocytic leukemia lines 1873 and 1929 are asparagine (ASN) auxotrophs. Asparagine synthetase (ASY), which is a housekeeping gene, is repressed and the promoting region of the gene is highly methylated. We now demonstrate in these cells multiple levels in control of the expression of this gene, in a system of cocultivation with macrophages and other cell types. In this system, mediated by cell-to-cell contact, ASY becomes expressed by the leukemic cells and they become prototrophic. Demethylation of ASY occurs; it follows expression and is permanent over multiple cell generations, but the cells return to auxotrophy with rapid repression of ASY on removal from cell contact. With ASY expression, the associated histone H3 at lysine position 9 (H3K9) becomes acetylated and H3K4, methylated. In contrast to other systems, H3K9 methylation does not characterize the repressed state. The changes leading from repression to induction of ASY and demethylation parallel the physiological changes specific to functional maturation of normal lymphoid precursors. The lability of expression of ASY has potential significance in determining the sensitivity of leukemic cells to L-asparaginase.