Integrative omics analysis in Pandanus odorifer (Forssk.) Kuntze reveals the role of Asparagine synthetase in salinity tolerance

Pandanus odorifer (Forssk) Kuntze grows naturally along the coastal regions and withstands salt-sprays as well as strong winds. A combination of omics approaches and enzyme activity studies was employed to comprehend the mechanistic basis of high salinity tolerance in P. odorifer. The young seedlings of P. odorifer were exposed to 1 M salt stress for up to three weeks and analyzed using RNAsequencing (RNAseq) and LC-MS. Integrative omics analysis revealed high expression of the Asparagine synthetase (AS) (EC 6.3.5.4) (8.95 fold) and remarkable levels of Asparagine (Asn) (28.5 fold). This indicated that salt stress promoted Asn accumulation in P. odorifer. To understand this further, the Asn biosynthesis pathway was traced out in P. odorifer. It was noticed that seven genes involved in Asn bisynthetic pathway namely glutamine synthetase (GS) (EC 6.3.1.2) glutamate synthase (GOGAT) (EC 1.4.1.14), aspartate kinase (EC 2.7.2.4), pyruvate kinase (EC 2.7.1.40), aspartate aminotransferase (AspAT) (EC 2.6.1.1), phosphoenolpyruvate carboxylase (PEPC) (EC 4.1.1.31) and AS were up-regulated under salt stress. AS transcripts were most abundant thereby showed its highest activity and thus were generating maximal Asn under salt stress. Also, an up-regulated Na+/H+ antiporter (NHX1) facilitated compartmentalization of Na+ into vacuoles, suggesting P. odorifer as salt accumulator species.

Effect of post-silking drought on nitrogen partitioning and gene expression patterns of glutamine synthetase and asparagine synthetase in two maize (Zea mays L.) varieties

Glutamine synthetase (GS) and asparagine synthetase (AS) are proposed to have important function in plant nitrogen (N) remobilization, but their roles under drought stress are not well defined. In this study, the expression dynamics of GS and AS genes were analyzed in two maize varieties (ZD958 and NH101) in relation to post-silking drought stress induced nitrogen partitioning. ZD958 was a 'stay-green' variety with 5% nitrogen harvest index (NHI) lower than NH101. From silking to maturity, the amount of nitrogen remobilized from ear-leaves in ZD958 was evidently lower than NH101, and post-silking drought stress increased the nitrogen remobilization for both varieties. In ear-leaves, the expression of ZmGln1-3 was enhanced under drought stress. Three AS genes (ZmAS1, ZmAS2 and ZmAS3) were differentially regulated by post-silking drought treatment, of which the expression of ZmAS3 was stimulated at late stage of leaf senescence. In NH101, the expression level of ZmAS3 was markedly higher than that in ZD958. In developing grains, there were no significant differences in expression patterns of GS and AS genes between well water and drought treated plants. Drought stress altered maize N partitioning at the whole-plant level, and the up-regulation of GS and AS genes may contribute to the higher leaf nitrogen remobilization when exposed to drought treatments.

Overexpression and Purification of Asparagine Synthetase fromEscherichia coli

Overexpression of the asnA gene from Escherichia coli K-12 coding for asparagine synthetase (EC 6.3.1.1) was achieved with a plasmid, pUNAd37, a derivative of pUC18, in E. coli. The plasmid was constructed by optimizing a DNA sequence between the promoter and the ribosome binding region. The enzyme, comprising ca. 15% of the total soluble protein in the E. coli cell, was readily purified to apparent homogeneity by DEAE-Cellulofine and Blue-Cellulofine column chromatographies. The amino-terminal sequence, amino acid composition, and molecular weight of the purified protein agreed with the predicted values based on the DNA sequence of the gene. Furthermore the native molecular weight measured by gel filtration confirmed that asparagine synthetase exists as a dimer of identical subunits.

[Preparation of anti-ASNS monoclonal antibody and detection of ASNS expression in tumor]

AIM

To prepare and characterize monoclonal antibodies against ASNS for the research of ASNS function.

METHODS

To induce the expression of the fusion protein MS2-ASNS, Hybridomas were generated by the fusion with Sp2/0 myelomas and spleen cells, which were obtained from mice immunized with MS2-ASNS recombinant proteins. The specificity and titer of mAb were identified by ELISA methods, and then used to detect the affinity of ASNS in cancer cells by Western blot. The expression of ASNS was detected in some cancer lines and tissues by IHC.

RESULTS

Two hybridoma cell lines F4-15 and F4-16, which stably secret anti-ASNS mAbs were produced. Both cell lines produce IgG2a monoclonal antibody. ELISA demonstrated that anti-ASNS mAbs had high specificity and titer. The titers of anti-ASNS mAbs produced by hybridoma cell lines were up to 1:5×10(5);. Western blot demonstrated that ASNS was expressed in some cancer lines, including human lymphoma cell line K562 and cervical cancer cell line HeLa. The expression of ASNS was also detected by IHC in several tumor cell lines, such as stomach cancer cell line SGC-7901 and liver cancer cell lines SMMC-7721, BEL-7402, HepG2 as well as lung and esophageal carcinoma tissue.

CONCLUSION

The monoclonal antibodies against ASNS have been successfully prepared, which provides a tool for the following research of nasal type NK/T cell lymphoma.

Correlation between asparaginase sensitivity and asparagine synthetase protein content, but not mRNA, in acute lymphoblastic leukemia cell lines

BACKGROUND

Asparaginase (ASNase) is an essential component of most treatment protocols for childhood acute lymphoblastic leukemia (ALL). Although increased asparagine synthetase (ASNS) expression may contribute to ASNase resistance, there is conflicting data from patient samples with regard to correlation between ASNS mRNA content and ASNase sensitivity.

PROCEDURE

Both T-cell and B-cell derived ALL cell lines were treated with ASNase and then monitored for cell proliferation, cell death, and ASNS mRNA and protein expression.

RESULTS

Despite elevated ASNS mRNA following ASNase treatment, different ALL cell lines varied widely in translation to ASNS protein. Although ASNS mRNA levels did not consistently reflect ASNase sensitivity, there was an inverse correlation between ASNS protein and ASNase-induced cell death. Expression of ASNS in an ASNase-sensitive cell line resulted in enhanced ASNase resistance, and conversely, siRNA-mediated inhibition of ASNS expression promoted increased drug sensitivity.

CONCLUSIONS

These observations provide an explanation for the ASNase sensitivity of ALL cells and demonstrate the importance of measuring ASNS protein rather than mRNA in predicting ASNase responsiveness.

Nonclassic functions of human topoisomerase I: genome-wide and pharmacologic analyses

The biological functions of nuclear topoisomerase I (Top1) have been difficult to study because knocking out TOP1 is lethal in metazoans. To reveal the functions of human Top1, we have generated stable Top1 small interfering RNA (siRNA) cell lines from colon and breast carcinomas (HCT116-siTop1 and MCF-7-siTop1, respectively). In those clones, Top1 is reduced approximately 5-fold and Top2alpha compensates for Top1 deficiency. A prominent feature of the siTop1 cells is genomic instability, with chromosomal aberrations and histone gamma-H2AX foci associated with replication defects. siTop1 cells also show rDNA and nucleolar alterations and increased nuclear volume. Genome-wide transcription profiling revealed 55 genes with consistent changes in siTop1 cells. Among them, asparagine synthetase (ASNS) expression was reduced in siTop1 cells and in cells with transient Top1 down-regulation. Conversely, Top1 complementation increased ASNS, indicating a causal link between Top1 and ASNS expression. Correspondingly, pharmacologic profiling showed L-asparaginase hypersensitivity in the siTop1 cells. Resistance to camptothecin, indenoisoquinoline, aphidicolin, hydroxyurea, and staurosporine and hypersensitivity to etoposide and actinomycin D show that Top1, in addition to being the target of camptothecins, also regulates DNA replication, rDNA stability, and apoptosis. Overall, our studies show the pleiotropic nature of human Top1 activities. In addition to its classic DNA nicking-closing functions, Top1 plays critical nonclassic roles in genomic stability, gene-specific transcription, and response to various anticancer agents. The reported cell lines and approaches described in this article provide new tools to perform detailed functional analyses related to Top1 function.

Enhanced expression of asparagine synthetase under glucose-deprived conditions protects pancreatic cancer cells from apoptosis induced by glucose deprivation and cisplatin

Although hypovasculature is an outstanding characteristic of pancreatic cancers, the tumor cells survive and proliferate under severe hypoxic, glucose-deprived conditions caused by low blood supply. It is well known that the hypoxia-inducible factor-1 pathway is essential for the survival of pancreatic cancer cells under hypoxic conditions. To discover how pancreatic cancer cells adapt to glucose deprivation as well as hypoxia, we sought glucose deprivation-inducible genes by means of a DNA microarray system. We identified 63 genes whose expression was enhanced under glucose-deprived conditions at >2-fold higher levels than under normal glucose conditions. Among these genes, asparagine synthetase (ASNS) was studied in detail. Although it is known to be associated with drug resistance in leukemia and oncogenesis triggered by mutated p53, its function is yet to be determined. In this study, we found that glucose deprivation induced the overexpression of ASNS through an AMP-activated protein kinase-independent and activating transcription factor-4-dependent manner and that ASNS protects pancreatic cancer cells from apoptosis induced by glucose deprivation itself. ASNS overexpression also induced resistance to apoptosis triggered by cisplatin [cis-diammine-dichloroplatinum (CDDP)] and carboplatin, but not by 5-fluorouracil, paclitaxel, etoposide, or gemcitabine. We show that glucose deprivation induces the activation of c-jun NH(2)-terminal kinase (JNK)/stress-activated protein kinase (SAPK) in a mock transfectant but not in an ASNS transfectant. Consequently, an inhibitor of JNK/SAPK decreased the sensitivity of pancreatic cancer cells to apoptosis by glucose deprivation and CDDP. These results strongly suggest that ASNS is induced by glucose deprivation and may play a pivotal role in the survival of pancreatic cancer cells under glucose-deprived conditions.

TRB3 inhibits the transcriptional activation of stress-regulated genes by a negative feedback on the ATF4 pathway

The integrated stress response (ISR) is defined as a highly conserved response to several stresses that converge to the induction of the activating transcription factor 4 (ATF4). Because an uncontrolled response may have deleterious effects, cells have elaborated several negative feedback loops that attenuate the ISR. In the present study, we describe how induction of the human homolog of Drosophila tribbles (TRB3) attenuates the ISR by a negative feedback mechanism. To investigate the role of TRB3 in the control of the ISR, we used the regulation of gene expression by amino acid limitation as a model. The enhanced production of ATF4 upon amino acid starvation results in the induction of a large number of target genes like CHOP (CAAT/enhancer-binding protein-homologous protein), asparagine synthetase (ASNS), or TRB3. We demonstrate that TRB3 overexpression inhibits the transcriptional induction of CHOP and ASNS whereas TRB3 silencing induces the expression of these genes both under normal and stressed conditions. In addition, transcriptional profiling experiments show that TRB3 affects the expression of many ISR-regulated genes. Our results also suggest that TRB3 and ATF4 belong to the same protein complex bound to the sequence involved in the ATF4-dependent regulation of gene expression by amino acid limitation. Collectively, our data identify TRB3 as a negative feedback regulator of the ATF4-dependent transcription and participates to the fine regulation of the ISR.

CHOP (C/EBP homologous protein) and ASNS (asparagine synthetase) induction in cybrid cells harboring MELAS and NARP mitochondrial DNA mutations

Mitochondrial dysfunction caused by mutations in mitochondrial DNA (mtDNA) is related to a variety of diseases including MELAS and NARP syndromes. However, little is known about the intracellular responses induced by mtDNA mutations. In order to identify genes whose expression is altered as a result of the presence of mtDNA mutations, DNA microarray analysis was performed using human 143B osteosarcoma cells harboring 3243A>G [tRNA-Leu (UUR)] and 8993T>G [ATPase6 Leu156Arg] mtDNA mutations associated with MELAS and NARP syndromes (2SD and NARP3-1 cybrid cells), respectively. We found that mRNA and protein levels of ATF4, CHOP and ASNS were upregulated in 2SD and NARP3-1 cells as compared with parental cells. Reporter assays demonstrated that transcription of CHOP and ASNS genes was upregulated through the AARE (amino acid regulatory element) and NSRE-1 (nutrient-sensing response element-1) enhancer elements to which ATF4 binds, respectively. Furthermore, knockdown of ATF4 by RNA interference reduced CHOP and ASNS transcription in 2SD and NARP3-1 cells. These results suggest that the presence of mtDNA mutations elicits upregulation of CHOP and ASNS genes through the elevation of ATF4 expression and its binding to the AARE and NSRE-1, respectively.

Selective apoptosis of natural killer-cell tumours by l-asparaginase

We examined the effectiveness of various anti-tumour agents to natural killer (NK)-cell tumour cell lines and samples, which are generally resistant to chemotherapy, using flow cytometric terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labelling (TUNEL) assay. Although NK-YS and NK-92 were highly resistant to various anti-tumour agents, l-asparaginase induced apoptosis in these two NK-cell lines. NK-cell leukaemia/lymphoma and acute lymphoblastic leukaemia (ALL) samples were selectively sensitive to l-asparaginase and to doxorubicin (DXR) respectively. Samples of chronic NK lymphocytosis, an NK-cell disorder with an indolent clinical course, were resistant to both drugs. Our study clearly separated two major categories of NK-cell disorders and ALL according to the sensitivity to DXR and l-asparaginase. We examined asparagine synthetase levels by real-time quantitative polymerase chain reaction (RQ-PCR) and immunostaining in these samples. At least in nasal-type NK-cell lymphoma, there was a good correlation among asparagine synthetase expression, in vitro sensitivity and clinical response to l-asparaginase. In aggressive NK-cell leukaemia, although asparagine synthetase expression was high at both mRNA and protein levels, l-asparaginase induced considerable apoptosis. Furthermore, samples of each disease entity occupied a distinct area in two-dimensional plotting with asparagine synthetase mRNA level (RQ-PCR) and in vitrol-asparaginase sensitivity (TUNEL assay). We confirmed rather specific anti-tumour activity of l-asparaginase against NK-cell tumours in vitro, which provides an experimental background to the clinical use of l-asparaginase for NK-cell tumours.

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.

Regulation of asparagine synthetase gene transcription by the basic region leucine zipper transcription factors ATF5 and CHOP

Asparagine synthetase catalyses the glutamine- and ATP-dependent conversion of aspartic acid to asparagine. In human hepatoma cells cultured in mediumcontaining amino acids, the mRNA of asparagine synthetase is not detectable by RNase protection mapping. However, maintaining the cells in amino acid-free Krebs-Ringer bicarbonate buffer strongly upregulated asparagine synthetase biosynthesis. The effect of amino acid deprivation on asparagine synthetase gene transcription is mediated by a genetic element termed the nutrient-sensing response unit. Previous studies revealed that the basic region leucine zipper (bZIP) transcription factor CREB2/ATF4 is involved in the nutrient deprivation-induced upregulation of asparagine synthetase gene transcription. Here we show that overexpression of the bZIP protein ATF5, a transcriptional activator, stimulates asparagine synthetase promoter/reporter gene transcription via the nutrient-sensing response unit. In contrast, ATF5 does not transactivate cAMP response element (CRE)-containing reporter genes. Overexpression of the C/EBP homologous transcription factor CHOP impaired transcriptional activation of the asparagine synthetase promoter following amino acid deprivation or over-expression of ATF5 or CREB2/ATF4. These data indicate that CHOP functions as a shut-off-device for nutrient deprivation-induced gene transcription.

Asparagine synthetase gene TaASN1 from wheat is up-regulated by salt stress, osmotic stress and ABA

Differences in gene expression between salinity stressed and normally grown wheat seedlings were compared by the differential display (DD) technique. One DD-derived cDNA clone was characterized as a partial sequence of the wheat asparagine ynthetase (AS) gene by sequence analysis and homology search of GenBank databases. Two AS genes of wheat, TaASN1 and TaASN2, were further isolated by the RT-PCR approach. Comparison of the deduced polypeptide of TaASN1 and TaASN2 with AS proteins from other organisms revealed several homologous regions, in particular, the conserved glutamine binding sites and Class-II Glutamine amidotransferases domain. The functionality of TaASN1 was demonstrated by complementing an Escherichia coli asparagine auxotroph. TaASN1 transcripts were detected in roots, shoots, anthers and young spikes by RT-PCR analysis. Abundance of TaASN1 mRNA in young spikes and anthers was higher than that in shoots and roots under normal growth conditions. TaASN1 was dramatically induced by salinity, osmotic stress and exogenous abscisic acid (ABA) in wheat seedlings. TaASN2 transcripts were very low in all detected tissues and conditions and were only slightly induced by ABA in roots.

Amino acid deprivation induces the transcription rate of the human asparagine synthetase gene through a timed program of expression and promoter binding of nutrient-responsive basic region/leucine zipper transcription factors as well as localized histone acetylation

Expression of human asparagine synthetase (ASNS), which catalyzes asparagine and glutamate biosynthesis, is transcriptionally induced following amino acid deprivation. Previous overexpression and electrophoresis mobility shift analysis showed the involvement of the transcription factors ATF4, C/EBPbeta, and ATF3-FL through the nutrient-sensing response element-1 (NSRE-1) within the ASNS promoter. Amino acid deprivation caused an elevated mRNA level for ATF4, C/EBPbeta, and ATF3-FL, and the present study established that the nuclear protein content for ATF4 and ATF3-FL were increased during amino acid limitation, whereas C/EBPbeta-LIP declined slightly. The total amount of C/EBPbeta-LAP protein was unchanged, but changes in the distribution among multiple C/EBPbeta-LAP forms were observed. Overexpression studies established that ATF4, ATF3-FL, and C/EBPbeta-LAP could coordinately modulate the transcription from the human ASNS promoter. Chromatin immunoprecipitation demonstrated that amino acid deprivation increased ATF3-FL, ATF4, and C/EBPbeta binding to the ASNS promoter and enhanced promoter association of RNA polymerase II, TATA-binding protein, and TFIIB of the general transcription machinery. A time course revealed a markedly different temporal order of interaction between these transcription factors and the ASNS promoter. During the initial 2 h, there was a 20-fold increase in ATF4 binding and a rapid increase in histone H3 and H4 acetylation, which closely paralleled the increased transcription rate of the ASNS gene, whereas the increase in ATF3-FL and C/EBPbeta binding was considerably slower and more closely correlated with the decline in transcription rate between 2 and 6 h. The data suggest that ATF3-FL and C/EBPbeta act as transcriptional suppressors for the ASNS gene to counterbalance the transcription rate activated by ATF4 following amino acid deprivation.

Establishment of real-time polymerase chain reaction method for quantitative analysis of asparagine synthetase expression

We established a real-time quantitative PCR (RQ-PCR) with which to measure abundance of the asparagine synthetase (AS) mRNA. The level of AS mRNA paralleled AS enzyme activity, as well as the AS protein level detected by Western blotting and by in situ immunostaining. Cytotoxicity tests in vitro showed that the AS mRNA level also synchronized with cellular resistance to L-asparaginase in cell lines. Cellular levels of AS enzyme activity correlated with resistance to L-asparaginase. These results indicate that the AS mRNA level is an index of resistance to L-asparaginase. RQ-PCR is superior to enzyme assays, Western blotting, and immunostaining in the following ways: less labor and time, accurate and reproducible quantitativity, and broad dynamic range. In addition, RQ-PCR could evaluate differences in L-asparaginase sensitivity although immunostaining could not. And in clinical samples, we analyzed eight pediatric leukemia cases by this RQ-PCR to evaluate whether this method was applicable to clinical laboratories and the expression level of AS mRNA in each case were predictable for the effectiveness of L-asparaginase treatment. Consequently, this method was useful enough in defining candidates for selective therapy that targets an AS deficiency.

Tumor-derived p53 mutants induce oncogenesis by transactivating growth-promoting genes

We have studied the mechanism of mutant p53-mediated oncogenesis using several tumor-derived mutants. Using a colony formation assay, we found that the majority of the mutants increased the number of colonies formed compared to the vector. Expression of tumor-derived p53 mutants increases the rate of cell growth, suggesting that the p53 mutants have 'gain of function' properties. We have studied the gene expression profile of cells expressing tumor-derived p53-D281G to identify genes transactivated by mutant p53. We report the transactivation of two genes, asparagine synthetase and human telomerase reverse transcriptase. Quantitative real-time PCR confirms this upregulation. Transient transfection promoter assays verify that tumor-derived p53 mutants transactivate these promoters significantly. An electrophoretic mobility shift assay shows that tumor-derived p53-mutants cannot bind to the wild-type p53 consensus sequence. The results presented here provide some evidence of a possible mechanism for mutant p53-mediated transactivation.

Amino acid deprivation and endoplasmic reticulum stress induce expression of multiple activating transcription factor-3 mRNA species that, when overexpressed in HepG2 cells, modulate transcription by the human asparagine synthetase promoter

Transcription from the ASNS (asparagine synthetase) gene is increased in response to either amino acid (amino acid response) or glucose (endoplasmic reticulum stress response) deprivation. These two independent pathways converge on the same set of genomic cis-elements within the ASNS promoter, referred to as nutrient-sensing response element-1 and -2. Chromatin immunoprecipitation analysis provides the first in vivo evidence for activating transcription factor (ATF)-3 binding to the proximal ASNS promoter containing the nutrient-sensing response element-1 sequence. Overexpression of the full-length ATF3 protein caused a concentration-dependent biphasic response in ASNS promoter-driven transcription. Both amino acid limitation and activation of the endoplasmic reticulum stress response by glucose deprivation caused an increase in ATF3 mRNA content. However, reverse transcriptase-PCR analysis revealed that the increase in the ATF3 mRNA species detected by Northern analysis actually encoded both full-length ATF3 and two predicted truncated ATF3 isoforms (ATF3deltaZip2c and ATF3deltaZip3). Based on sequence analysis, one of the predicted truncated proteins (ATF3deltaZip3) is likely incapable of binding DNA; and yet, exogenous expression of the cDNA enhanced starvation-induced or ATF4-activated ASNS transcription, possibly by sequestering corepressor proteins. Collectively, the results provide evidence for a potential role of multiple predicted ATF3 isoforms in the transcriptional regulation of the ASNS gene in response to nutrient deprivation.

Molecular cloning and characterisation of two genes encoding asparagine synthetase in barley (Hordeum vulgare L.)

Two different cDNA clones encoding asperagine synthetase (AS: EC 6.3.5.4.) were cloned from barley (Hordeum vulgare L. cv. Alexis). The corresponding genes were designated HvAS1 (GenBank no AF307145) and HvAS2 (GenBank no AY193714). Chromosomal mapping using wheat-barley addition lines revealed that the HvAS1 gene is located on the long arm of barley chromosome 5, while the HvAS2 gene maps to the short arm of chromosome 3. Both genes are expressed in barley leaves according to RT-PCR analysis but only the HvAS1 gene expression can be detected in roots. Northern blots show no expression of HvAS1 in plants grown under a normal 16 h light/8 h dark cycle but after 10 h of continuous darkness, transcript appears and mRNA accumulates over a 48-h period of dark treatment. In roots, low-level expression of HvAS1 could be detected and the expression level appears to be unaffected by light. A polyclonal antibody was raised against the HvAS1 protein and used in Western blot analysis. The AS protein accumulated during a 48-h period of dark treatment, following the increase in HvAS1 transcript.

Sensitivity to L-asparaginase is not associated with expression levels of asparagine synthetase in t(12;21)+ pediatric ALL

The (12;21) translocation resulting in TEL/AML1 gene fusion is present in about 25% of childhood precursor B-lineage acute lymphoblastic leukemia (ALL) and is associated with a good prognosis and a high cellular sensitivity to L-asparaginase (L-Asp). ALL cells are thought to be sensitive to L-Asp due to lower asparagine synthetase (AS) levels. Resistance to L-Asp may be caused by an elevated cellular level of AS or by the ability of resistant cells to rapidly induce the expression of the AS gene on L-Asp exposure. AS may be a target regulated by t(12;21). We studied the relationship between t(12;21) and the mRNA level of AS to investigate a possible mechanism underlying L-Asp sensitivity. Real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis surprisingly revealed that 30 patients positive for t(12;21) expressed 5-fold more AS mRNA compared with 17 patients negative for t(12;21) (P =.008) and 11 samples from healthy controls (P =.016). The mRNA levels of AS between t(12;21)(-) ALL and healthy controls did not differ. No difference was found between ALL patients positive or negative for t(12;21) in the capacity to up-regulate AS after in vitro L-Asp exposure, excluding a defective capacity for t(12;21) cells in up-regulating AS on L-Asp exposure. Moreover, no correlation was observed between AS mRNA expression and sensitivity to L-Asp. We conclude that the sensitivity of t(12;21)(+) childhood ALL to L-Asp is not associated with the expression level of the AS gene. Furthermore, we contradict the general thought that leukemic cells specifically lack AS compared with normal bone marrow and blood cells.

ATF4 is a mediator of the nutrient-sensing response pathway that activates the human asparagine synthetase gene

Transcription from the asparagine synthetase (A.S.) gene is increased in response to either amino acid (amino acid response) or glucose (endoplasmic reticulum stress response) deprivation. These two independent pathways converge on the same set of genomic cis-elements within the A.S. promoter referred to as nutrient-sensing response elements (NSRE) 1 and 2, both of which are necessary for gene activation. The NSRE-1 sequence was used to screen ATF/CREB family members by electrophoresis mobility shift assays and supershift by specific antibodies. The results indicated that ATF4 binds to the NSRE-1 sequence and that the amount of the ATF4 complex was increased when extracts from amino acid-deprived or glucose-deprived cells were tested. Using electrophoresis mobility shift assay experiments and a probe that contained both NSRE-1 and NSRE-2, mutation of the NSRE-1 sequence completely prevented formation of the ATF4-containing complexes, whereas mutation of the NSRE-2 sequence did not. Overexpression of ATF4 increased A.S. promoter-driven transcription, whereas an inhibitory dominant negative ATF4 mutant blocked both basal and starvation-enhanced transcription. Collectively, the results provide both in vitro and in vivo evidence for a role of ATF4 in the transcriptional activation of the A.S. gene in response to nutrient deprivation.

Role of Sp1 and Sp3 in the nutrient-regulated expression of the human asparagine synthetase gene

The human asparagine synthetase (AS) gene responds to depletion of mammalian cells for either amino acids or carbohydrates. Five specific cis-elements have been implicated: three GC boxes (GC-I, GC-II and GC-III) and two nutrient-sensing response elements (NSRE-1, -2). This study shows that all three GC boxes are required to maintain basal transcription and to obtain maximal induction of the AS gene by amino acid limitation. However, there is not complete redundancy among the three GC boxes, and there is a hierarchy of importance with regard to transcription (GC-III > GC-II > GC-I). In vitro, two GC boxes formed protein-DNA complexes (GC-II and GC-III) with Sp1 and Sp3. Although transcription of the AS gene is elevated by nutrient limitation, the absolute amount of these protein-DNA complexes and the total pools of Sp1 and Sp3 did not increase. A small, but detectable portion of Sp1 was modified by phosphorylation following amino acid deprivation. In vivo, expression of Sp1 and Sp3 in Drosophila SL2 cells increased AS promoter activity. Sp1 expression increased basal transcription but did not cause a further increase when SL2 cells were amino acid-deprived. Sp3 expression enhanced both the basal and the starvation-induced transcription.

CCAAT/enhancer-binding protein-beta is a mediator of the nutrient-sensing response pathway that activates the human asparagine synthetase gene

Transcription from the human asparagine synthetase (AS) gene is increased in response to either amino acid (amino acid response) or glucose (unfolded protein response) deprivation. These two independent pathways converge on the same set of genomic cis-elements within the AS promoter, which are referred to as nutrient-sensing response element (NSRE)-1 and -2, both of which are absolutely necessary for gene activation. The NSRE-1 sequence was used to identify the corresponding transcription factor by yeast one-hybrid screening. Based on those results, electrophoretic mobility shift assays for individual CCAAT/enhancer-binding protein-beta (C/EBP) family members were performed to test for supershifting of complexes by specific antibodies. The results indicated that of all the family members, C/EBPbeta bound to the NSRE-1 sequence to the greatest extent and that the absolute amount of this complex was increased when extracts from amino acid- or glucose-deprived cells were tested. Using electrophoretic mobility shift assays, mutation of the NSRE-1 sequence completely prevented formation of the C/EBPbeta-containing complexes. In contrast, mutation of the NSRE-2 sequence did not block C/EBPbeta binding. Overexpression in HepG2 hepatoma cells of the activating isoform of C/EBPbeta increased AS promoter-driven transcription, whereas the inhibitory dominant-negative isoform of C/EBPbeta blocked enhanced transcription following amino acid or glucose deprivation. Collectively, the results provide both in vitro and in vivo evidence for a role of C/EBPbeta in the transcriptional activation of the AS gene in response to nutrient deprivation.

Activation of the unfolded protein response pathway induces human asparagine synthetase gene expression

The gene for the amino acid biosynthetic activity asparagine synthetase (AS) is induced by both amino acid and glucose deprivation of cells. The data reported here document that the human AS gene is induced following activation of the Unfolded Response Pathway (UPR), also known as the Endoplasmic Reticulum Stress Response (ERSR) in mammals. Increased AS transcription occurs in response to glucose deprivation, tunicamycin, or azetidine-2-carboxylate, all known to activate the UPR/ERSR pathway. Previously identified ERSR target genes contain multiple copies of a single highly conserved cis-element. In contrast, the human AS gene does not contain the ERSR element, as it has been described for other responsive genes. Instead, AS induction requires an Sp1-like sequence, a sequence previously shown to be associated with amino acid control of transcription, and possibly, a third region containing no consensus sequences for known transcription factors. Oligonucleotides covering each of these regions form DNA-protein complexes in vitro, and for some the amount of these complexes is greater when nuclear extracts from glucose-starved cells are tested. These results document that a wider range of metabolic activities are activated by the UPR/ERSR pathway than previously recognized and that genomic elements other than those already described can serve to enhance transcription of specific target genes.

Light-induced transcriptional repression of the pea AS1 gene: identification of cis-elements and transfactors

Here, we examine the cis-elements and trans-factors affecting the expression of asparagine synthetase (AS) genes whose transcription is negatively regulated by light. The promoters for the AS1 and AS2 genes of pea were isolated, sequenced, and functionally dissected for their ability to confer regulated expression to the GUS reporter gene in transgenic tobacco. Histochemical analysis of transgenic plants demonstrated that the AS1 and AS2 promoters show identical patterns of cell-specific expression. The more highly active AS1 promoter was further demonstrated to confer negative light-regulation to the GUS gene in transgenic tobacco. Deletion analysis and gain-of-function experiments showed that 124 bp of the AS1 promoter was sufficient to confer light-activated repression to a heterologous promoter. Potential conserved transcription regulatory elements, Box B, Box C, and Box C' within this region were shown to bind to nuclear proteins by gel shift analysis. A light-specific DNA:protein interaction was detected with Box B. The nuclear factors that bind to Box C and C' elements of AS1 are competed by a putative repressor element 'RE1' defined previously in the oat phytochrome gene whose transcription is also repressed by light. The Box B and C/C'-Box/RE1-binding factors were found in nuclear extracts of tobacco, pea, and Arabidopsis and may therefore be universal factors involved in light-activated transcriptional repression.

Phytochrome-regulated repression of gene expression requires calcium and cGMP

The plant photoreceptor phytochrome A utilizes three signal transduction pathways, dependent upon calcium and/or cGMP, to activate genes in the light. In this report, we have studied the phytochrome A regulation of a gene that is down-regulated by light, asparagine synthetase (AS1). We show that AS1 is expressed in the dark and repressed in the light. Repression of AS1 in the light is likely controlled by the same calcium/cGMP-dependent pathway that is used to activate other light responses. The use of the same signal transduction pathway for both activating and repressing different responses provides an interesting mechanism for phytochrome action. Using complementary loss- and gain-of-function experiments we have identified a 17 bp cis-element within the AS1 promoter that is both necessary and sufficient for this regulation. This sequence is likely to be the target for a highly conserved phytochrome-generated repressor whose activity is regulated by both calcium and cGMP.

Molecular cloning and expression of two cDNAs encoding asparagine synthetase in soybean

Two cDNA clones (SAS1 and SAS2) encoding different isoforms of asparagine synthetase (AS; EC 6.3.5.4) were isolated. Their DNA sequences were determined and compared. The amino-terminal residues of the predicted SAS1 and SAS2 proteins were identical to those of the glutamine binding domain of AS from pea, asparagus, Arabidopsis and human, suggesting that SAS1 and SAS2 cDNAs encode the glutamine-dependent form of AS. The open reading frames of SAS1 and SAS2 encode a protein of 579 and 581 amino acids with predicted molecular weights of 65182 and 65608 Da respectively. Similarity of the deduced amino acid sequences of SAS1 and SAS2 with other known AS sequences were 92% and 93% for pea AS1; 91% and 96% for pea AS2; 88% and 91% for asparagus; 88% and 90.5% for Arabidopsis; 70.5% and 72.5% for E. coli asnB and 61% and 63% for man. A plasmid, pSAS2E, was constructed to express the soybean AS protein in Escherichia coli. Complementation experiments revealed that the soybean AS protein was functional in E. coli. Southern blot analysis indicated that the soybean AS is part of a small gene family. AS transcript was expressed in all tissues examined, but higher levels were seen in stem and root of light-grown tissue and leaves of dark-treated tissue.

Sugar regulation of harvest-related genes in asparagus

The signals controlling the abundance of transcripts up-regulated (pTIP27, pTIP31, and pTIP32) or down-regulated (pTIP20 and pTIP21) after harvest in asparagus (Asparagus officinalis L.) spears were examined. pTIP27 and pTIP31 are known to encode asparagine synthetase (AS) and a beta-galactosidase (beta-gal) homolog, respectively. The nucleotide sequences of pTIP20, pTIP21, and pTIP32 were determined, and they encode histone 3, histone 2B, and an unknown product, respectively. Changes in respiration, soluble sugars, and abundance of the five mRNAs were similar in the tips stored as 30-mm lengths or as part of 180-mm spears. We previously hypothesized that sugars may regulate the level of AS transcripts in asparagus tissue. Asparagus cell cultures were used to test the role of sugar status may regulate the level of AS transcripts in asparagus tissue. Asparagus cell cultures were used to test the role of sugar status in regulating gene expression. Transcript abundance for AS, beta-gal, and pTIP32 was low in cells in sugar-containing medium but increased within 12 h after transferring cells to a sugar-free medium. Histone 3 and histone 2B transcripts were, in general, abundant in cells on sugar-containing medium but declined in abundance when transferred to sugar-free medium. When cells were returned to sugar-containing medium the abundance of transcripts for histone 3 and histone 2B increased, whereas that for AS, beta-gal, and pTIP32 decreased. Soluble sugar levels are known to decline rapidly in the tips of harvested spears. Metabolic regulation by sugar status may have a major influence on gene expression in asparagus spears and other tissue after harvest.

Lactobacillus bulgaricus asparagine synthetase and asparaginyl-tRNA synthetase: coregulation by transcription antitermination?

Genes encoding the ammonia-dependent asparagine synthetase (asnA) and asparaginyl-tRNA synthetase (asnS) have been cloned from Lactobacillus bulgaricus ATCC 11842. The nucleotide sequence suggests that asnA and asnS are organized as one operon and regulated by the tRNA-directed transcription antitermination mechanism (T. M. Henkin, Mol. Microbiol. 13:381-387, 1994).

Metabolic regulation of asparagine synthetase gene expression in maize (Zea mays L.) root tips

Differential hybridization of a cDNA library constructed with poly(A)+ mRNA from 24 h starved maize (Zea mays L.) root tips, resulted in the isolation of a cDNA (called pZSS1) that was highly induced during glucose deprivation. The nucleotide sequence analysis of the full-length cDNA allowed its identification by comparison with sequence data bases. The 586 amino acid sequence encoded by pZSS1 was shown to be about 60% identical to sequences of asparagine synthetases (EC 6.3.5.4) from Asparagus officinalis, Pisum sativum, Arabidopsis thaliana and Brassica oleracea. Southern blot analysis of maize genomic DNA showed that asparagine synthetase may be encoded by at least two genes. The use of a specific probe for the 3' untranslated region of pZSS1 in Northern blot experiments, revealed that the isolated AS gene was essentially expressed in roots of maize seedlings. Time course analysis revealed that maximal expression of the gene corresponding to pZSS1 occurs between 18 and 24 h after the onset of the starvation treatment. The steady-state levels of transcripts in maize root tips were found to change under various incubation conditions. Exogenous supply of metabolizable sugars downregulated the gene expression, while carbohydrate deprivation or feeding with non-metabolizable sugars resulted in the induction of gene expression. In addition to carbohydrate deprivation, the effects of nitrogen metabolite supply and stress conditions indicate that gene expression might be under metabolic control in maize root tips. The intracellular nitrogen to carbon ratio might be an important factor for the regulation of asparagine synthetase gene expression.

Cloning of rat asparagine synthetase and specificity of the amino acid-dependent control of its mRNA content

A full-length cDNA clone for rat asparagine synthetase (AS) was isolated from a cDNA library enriched for amino acid-regulated sequences. The AS cDNA was used to investigate the amino acid-dependent repression of AS mRNA content in rat Fao hepatoma cells. In response to complete amino acid starvation, there was an approximately 10-fold increase in the level of AS mRNA. Three species of mRNA, of approx. sizes 2.0, 2.5 and 4.0 kb, were detected and each was simultaneously regulated to the same degree. The expression of AS mRNA increased by 6 h after removal of amino acids, reached a plateau after 9 h, and was blocked by either actinomycin D or cycloheximide. Partial repression of the AS mRNA content was maintained by the presence of a single amino acid in the culture medium, but the degree of effectiveness for each one varied widely. Glutamine showed the greatest ability to repress the AS mRNA content, even at an extracellular concentration 10 times below its plasma level. Other effective repressors included the amino acids asparagine, histidine and leucine, as well as ammonia. Depletion of selected single amino acids from an otherwise complete culture medium also caused up-regulation. In particular, removal of histidine, threonine or tryptophan from the medium, or the addition of histidinol to inhibit histidinyl-tRNA synthetase, resulted in a significant increase in AS mRNA content. The data indicate that nutrient regulation of AS mRNA occurs by a general control mechanism that is responsive to a spectrum of amino acids.

Metabolic regulation of the gene encoding glutamine-dependent asparagine synthetase in Arabidopsis thaliana

Here, we characterize a cDNA encoding a glutamine-dependent asparagine synthetase (ASN1) from Arabidopsis thaliana and assess the effects of metabolic regulation on ASN1 mRNA levels. Sequence analysis shows that the predicted ASN1 peptide contains a purF-type glutamine-binding domain. Southern blot experiments and cDNA clone analysis suggest that ASN1 is the only gene encoding glutamine-dependent asparagine synthetase in A. thaliana. The ASN1 gene is expressed predominantly in shoot tissues, where light has a negative effect on its mRNA accumulation. This negative effect of light on ASN1 mRNA levels was shown to be mediated, at least in part, via the photoreceptor phytochrome. We also investigated whether light-induced changes in nitrogen to carbon ratios might exert a metabolic regulation of the ASN1 mRNA accumulation. These experiments demonstrated that the accumulation of ASN1 mRNA in dark-grown plants is strongly repressed by the presence of exogenous sucrose. Moreover, this sucrose repression of ASN1 expression can be partially rescued by supplementation with exogenous amino acids such as asparagine, glutamine, and glutamate. These findings suggest that the expression of the ASN1 gene is under the metabolic control of the nitrogen to carbon ratio in cells. This is consistent with the fact that asparagine, synthesized by the ASN1 gene product, is a favored compound for nitrogen storage and nitrogen transport in dark-grown plants. We have put forth a working model suggesting that when nitrogen to carbon ratios are high, the gene product of ASN1 functions to re-direct the flow of nitrogen into asparagine, which acts as a shunt for storage and/or long-distance transport of nitrogen.

Induction of asparagine synthetase by follicle-stimulating hormone in primary cultures of rat Sertoli cells

We studied effects of various agents, including follicle-stimulating hormone (FSH), on changes in the level of asparagine synthetase in cultured rat Sertoli cells. FSH, dibutylyl cAMP (Bt2cAMP), and cAMP-increasing agents, such as forskolin and theophylline, increased enzyme activity within 24 h. Western blot analysis revealed the increase in cellular protein content of asparagine synthetase by these agents to a degree similar to that of the enhanced enzyme activity. Northern blot analysis also showed elevation of the level of mRNA for asparagine synthetase in FSH- and Bt2cAMP-treated cells. Testosterone, insulin, prostaglandin E2, or 12-O-tetradecanoyl phorbol-13-acetate did not stimulate the enzyme. The results suggest that asparagine synthetase is induced via a cAMP-dependent signal transduction pathway in Sertoli cells. Increased production of asparagine in FSH- and Bt2cAMP-treated cells was indicated by extracellular accumulation of more asparagine around the treated cells than around the controls. The basal level of asparagine synthetase assessed in extracts from whole testes increased with age of animals toward 60-70 days, but the degree of hormonal stimulation of the enzyme in isolated Sertoli cells decreased during Days 20 to 30 postpartum. This suggested some different mechanism for regulation of the enzyme. The enhanced activity of asparagine synthetase by FSH in Sertoli cell may be important for maturation and function of Sertoli cells.

Isolation and characterization of a cDNA clone for a harvest-induced asparagine synthetase from Asparagus officinalis L

A full-length cDNA clone (pTIP27) encoding asparagine synthetase (AS; EC 6.3.5.4) was isolated from a cDNA library prepared from the tip section (apex to 30 mm) of Asparagus officinalis L. spears. The cDNA clone encodes an mRNA of 1978 bp, giving a derived protein of 66.5 kD molecular mass. The derived amino acid sequence is 81% homologous to AS from Pisum sativum. Only low levels of transcript for AS could be detected in growing spears, roots, or ferns. However, AS mRNA levels began to increase in the tips of harvested spears after 2 h at 20 degrees C, and in the other sections of the spear after 4 h, suggesting that all sections of the spear were responding to the same postharvest signal. The results are discussed in relation to metabolic changes occurring in harvested spears.

Overexpression of asparagine synthetase in albizziin-resistant murine diploid embryonic stem cells

Gene amplification is commonly observed in primary tumors and established drug-resistant cell lines, both of which are generally aneuploid. However, this process is undetectable (frequency < 10(-9) in normal diploid mammalian cell lines. To investigate whether gene amplification can occur in pluripotent diploid cells, we have selected drug-resistant mutants of mouse embryonic stem (ES) cells. We had previously found that Chinese hamster ovary (CHO) and human cell lines selected in albizziin (Alb), an amino acid analog of L-glutamine, overexpress asparagine synthetase (AS) due to gene amplification. The same drug selection system was applied to ES cells to isolate single-step and multistep drug-resistant mutants. Albizziin-resistant ES cells exhibited elevated levels of AS; however, drug resistance in ES cells was associated with mRNA overexpression without gene amplification. AS gene amplification was observed in only one drug-resistant cell line and was preceded by AS mRNA overexpression. Gene amplification in the latter coincided with the loss of the pluripotent nature of the ES cells.

Overproduction, preparation of monoclonal antibodies and purification of E. coli asparagine synthetase A

In order to explore the structure--function relationship of the Escherichia coli asparagine synthetase A it was necessary to devise a system for overexpression of the gene and purification of the gene product. The E. coli asparagine synthetase A structural gene was fused to the 3' end of the human carbonic anhydrase II structural gene and overexpressed in E. coli. The gene product, a 66 kDa fusion protein, which exhibited asparagine synthetase activity, was purified in a single step by affinity chromatography and used as the antigen for the production of monoclonal antibodies. The monoclonal antibodies were screened by ELISA. Colonies were chosen which were positive for purified fusion protein and negative for purified human carbonic anhydrase II. The E. coli asparagine synthetase A gene was then overexpressed and the gene product was used without purification for the final screen. The antibodies selected were used for immunoaffinity chromatography to purify the recombinant overexpressed E. coli asparagine synthetase A. Thus, a procedure is now available so that asparagine synthetase A can be purified to homogeneity in a single step.

High-level expression of human asparagine synthetase and production of monoclonal antibodies for enzyme purification

In order to obtain large quantities of extremely pure human asparagine synthetase for detailed kinetic and structural studies, its gene was cloned into a 2mu plasmid (pBS24.1GAS) suitable for replication in a Saccharomyces cerevisiae cir0 strain (AB116). In this construct, the transcription of the asparagine synthetase gene is regulated by the alcohol dehydrogenase II/glyceraldehyde-3-phosphate dehydrogenase promoter, which is subject to glucose repression. The expression of the enzyme was allowed to take place in yeast minimal medium containing D-galactose as the only sugar nutrient. Eleven monoclonal antibodies to recombinant human asparagine synthetase were produced and one of them was selected to make immunoaffinity resins. After single-step immunoaffinity chromatography, more than 1.2 mg of homogeneous enzyme was obtained from the total cell extract from a 100-ml yeast culture. The yield of pure enzyme was over 100-fold higher than that of a previously reported yeast expression system. SDS-PAGE analysis showed the enzyme to be extremely pure and isoelectric focusing gel electrophoresis showed that the enzyme has an isoelectric point of 7.5. Immunoaffinity-purified recombinant human asparagine synthetase demonstrated both glutamine-dependent and ammonia-dependent asparagine synthetase activities, as well as glutaminase activity.

Light represses transcription of asparagine synthetase genes in photosynthetic and nonphotosynthetic organs of plants

Asparagine synthetase (AS) mRNA in Pisum sativum accumulates preferentially in plants grown in the dark. Nuclear run-on experiments demonstrate that expression of both the AS1 and AS2 genes is negatively regulated by light at the level of transcription. A decrease in the transcriptional rate of the AS1 gene can be detected as early as 20 min after exposure to light. Time course experiments reveal that the levels of AS mRNA fluctuate dramatically during a "normal" light/dark cycle. This is due to a direct effect of light and not to changes associated with circadian rhythm. A novel finding is that the light-repressed expression of the AS1 gene is as dramatic in nonphotosynthetic organs such as roots as it is in leaves. Experiments demonstrate that the small amount of light which passes through the soil is sufficient to repress AS1 expression in roots, indicating that light has a direct effect on AS1 gene expression in roots. The negative regulation of AS gene expression by light was shown to be a general phenomenon in plants which also occurs in nonlegumes such as Nicotiana plumbaginifolia and Nicotiana tabacum. Thus, the AS genes can serve as a model with which to dissect the molecular basis for light-regulated transcriptional repression in plants.

Hypomethylation and reactivation of the asparagine synthetase gene induced by L-asparaginase and ethyl methanesulfonate

Successful chemotherapeutic treatment of drug-responsive cancers can be compromised by the acquisition of drug resistance. Standard remission induction therapy for childhood acute lymphoblastic leukemia includes L-asparaginase, since the leukemic cells lack asparagine synthetase (AS) activity and require exogenous asparagine. We have used the Chinese hamster ovary cell line N3, which lacks AS activity, as a model to examine a novel mechanism involved in the development of drug resistance in acute lymphoblastic leukemia. Expression of AS in Chinese hamster ovary cells is associated with hypomethylation in the 5' region of the gene. Activation of AS in concert with hypomethylation occurs spontaneously at a frequency of about 10(-6); we have found that treatment with the hypomethylating drug 5-azacytidine induces a reversion frequency of 10(-2). To investigate the possibility that chemotherapeutic drugs induce similar changes, the asparagine auxotrophic cell line N3 was treated with the chemotherapeutic agents L-asparaginase, vincristine, and 1-beta-D-arabinofuranosylcytosine and with the mutagen ethyl methanesulfonate. Both L-asparaginase and ethyl methanesulfonate increased the frequency of reversion to asparagine prototrophy to about 10(-5), whereas vincristine and 1-beta-D-arabinofuranosylcytosine had no such effect. Asparagine prototrophy correlated with the demethylation of CpG sites in the 5' region of the AS gene and with the appearance of AS mRNA in revertants. In addition to the specific effect seen with the AS gene, L-asparaginase and ethyl methanesulfonate induced global reductions in methylation of up to 25 and 10%, respectively. The ability of chemotherapeutic drugs to inhibit DNA methylation and thereby activate previously silent genes may enable them to promote the aggressiveness of cancers in vivo, including the expression of drug resistance.

Molecular and genetic characterization of human cell lines resistant to L-asparaginase and albizziin

Human cell lines resistant to L-asparaginase or albizziin were isolated by multistep selection of HT1080 fibrosarcoma and MIA PaCa-2 pancreatic carcinoma cells. Mutants were cross-resistant to both drugs, but more resistant to the drug used for selection. The drug-resistant cell lines expressed elevated levels of asparagine synthetase activity and protein, up to 17-fold over that of the parental cells. Enzyme overproduction was due to gene amplification in the albizziin-resistant cells, whereas increased expression without amplification was observed in L-asparaginase-resistant cells.

DNA methylation patterns associated with asparagine synthetase expression in asparagine-overproducing and -auxotrophic cells

In Chinese hamster ovary cells, the gene for asparagine synthetase, which spans 20 kilobase pairs, was found to contain a cluster of potential sites for CpG methylation in a 1-kilobase-pair region surrounding the first exon. Fourteen of the sites that could be assayed for methylation by MspI-HpaII digestions were found in this region, with an additional nine MspI sites spread throughout the remainder of the gene. The methylation status of the gene was analyzed in a series of cell lines that differed in the amount of asparagine synthetase activity. The level of expression showed a direct correlation with the extent of methylation of a subset of the MspI sites found in the 5' region of the gene. The rest of the gene was completely methylated in most cell lines. Wild-type cells, which expressed a basal level of asparagine synthetase activity, were partially demethylated in the 5' region. In contrast, asparagine-requiring N3 cells, which lacked detectable mRNA for asparagine synthetase, were methylated throughout the entire gene. Spontaneous revertants of strain N3, selected for growth in asparagine-free medium, exhibited extensive hypomethylation of the asparagine synthetase gene. The methylation pattern of the gene in cell lines that overproduced the enzyme was also examined. Albizziin-resistant cell lines, which had amplified copies of the gene, were extensively demethylated in the 5' region. Overexpression of asparagine synthetase in beta-aspartyl hydroxamate-resistant lines without amplified copies of the gene was also correlated with DNA hypomethylation.

Expression of human asparagine synthetase in Escherichia coli

Human asparagine synthetase was expressed in Escherichia coli. Synthesis of the enzyme was demonstrated by immunoblotting and by complementation of asparagine auxotrophy in E. coli. The recombinant enzyme was shown to have both the ammonia- and glutamine-dependent asparagine synthetase activity in vitro. Compared to asparagine synthetase isolated from beef pancreas, the one expressed in E. coli migrated at a slightly slower rate on a denaturing protein gel. In contrast with previous reports, the data obtained here strongly suggest that the active enzyme is a homodimer. The production of soluble and active enzyme was shown to be highly temperature-dependent. Expression at 37 degrees C yielded no soluble enzyme, whereas growth at 30 and 21 degrees C favored the production of soluble asparagine synthetase. The incubation temperature was also important for complementation of asparagine auxotrophy in E. coli, as growth in the absence of asparagine occurred at 30 degrees C and not at 37 degrees C.

Isolation of human cDNAs for asparagine synthetase and expression in Jensen rat sarcoma cells

Asparagine synthetase cDNAs containing the complete coding region were isolated from a human fibroblast cDNA library. DNA sequence analysis of the clones showed that the message contained one open reading frame encoding a protein of 64,400 Mr, 184 nucleotides of 5' untranslated region, and 120 nucleotides of 3' noncoding sequence. Plasmids containing the asparagine synthetase cDNAs were used in DNA-mediated transfer of genes into asparagine-requiring Jensen rat sarcoma cells. The cDNAs containing the entire protein-coding sequence expressed asparagine synthetase activity and were capable of conferring asparagine prototrophy on the Jensen rat sarcoma cells. However, cDNAs which lacked sequence for as few as 20 amino acids at the amino terminal could not rescue the cells from auxotrophy. The transferant cell lines contained multiple copies of the human asparagine synthetase cDNAs and produced human asparagine synthetase mRNA and asparagine synthetase protein. Several transferants with numerous copies of the cDNAs exhibited only basal levels of enzyme activity. Treatment of these transferant cell lines with 5-azacytidine greatly increased the expression of asparagine synthetase mRNA, protein, and activity.

AsnC: an autogenously regulated activator of asparagine synthetase A transcription in Escherichia coli

The regulation of the asparagine synthetase A gene of Escherichia coli was studied in vitro with a coupled transcription-translation system. It was shown that the 17-kilodalton gene, which is transcribed divergently from the adjacent asnA gene, codes for an activator of asnA transcription. The synthesis of the 17-kilodalton protein, which we now call AsnC, is autogenously regulated. The stimulating effect of AsnC on asnA transcription is abolished by asparagine, while the autoregulation of asnC is not affected by asparagine. The N-terminal part of the asnC protein, inferred from the DNA sequence, is homologous to the DNA-binding domain of regulatory proteins like catabolite gene activator, cro, and cI. This homology and direct repeats found in the region of the two asn promoters suggest that the asnC protein regulates transcription by binding to DNA. The asn promoters were defined by mapping of the mRNA start sites of in vitro-generated transcripts.

Characterization of single step albizziin-resistant Chinese hamster ovary cell lines with elevated levels of asparagine synthetase activity

The amino acid analog, albizziin, which acts as a competitive inhibitor of asparagine synthetase with respect to glutamine was used to isolate mutants of Chinese hamster ovary cells with alterations in levels of the target enzyme. These mutant lines have been characterized biochemically and genetically. Mutants selected in a single step are up to 40-fold more resistant to the drug than the parental line, express levels of asparagine synthetase activity 6-17-fold greater than that of wild type cells, and act co-dominantly in hybrids. Several classes of mutations can be distinguished on the basis of cross-resistance to beta-aspartyl hydroxamate, another amino acid analog. Studies on asparagine synthetase indicate that resistance to albizziin may be due to altered regulation of asparagine synthetase, structural mutations of the enzyme, and gene amplification.

Asparaginyl-tRNA aminoacylation levels and asparagine synthetase expression in cultured Chinese hamster ovary cells

Changes in ventricular size and brain parenchyma were documented in 40 preterm neonates with intracranial hemorrhage (ICH), who were serially examined for 3 weeks or more. Sonography disclosed a close relation between the severity of the intracranial hemorrhage and the development of progressive ventricular dilatation. Eighty percent of preterm neonates with minor degrees of intracranial hemorrhage (localized subependymal hemorrhage or subependymal hemorrhage with small intraventricular hemorrhage [subependymal/intraventricular hemorrhage]) did not develop significant ventricular dilatation, whereas all of the neonates with intraventricular and/or intraparenchymal hemorrhage developed moderate or severe ventricular dilatation. Spontaneous resolution of moderate and/or severe ventricular dilatation occurred by the end of the third week in about one-third of neonates with intracranial hemorrhage. Progressive ventricular dilatation was documented in 10 of 15 neonates with major intraventricular and/or intraparenchymal hemorrhage. The therapeutic implications of the findings are discussed.

Elevated levels of asparagine synthetase activity in physiologically and genetically derepressed Chinese hamster ovary cells are due to increased rates of enzyme synthesis

The activity of asparagine synthetase in Chinese hamster ovary (CHO) cells is increased in response to asparagine deprivation or decreased aminoacylation of several tRNAs (Andrulis, I. L., Hatfield, G. W., and Arfin, S. M. (1979) J. Biol. Chem. 254, 10629-10633). CHO cells resistant to beta-aspartylhydroxamate have up to 5-fold higher levels of asparagine synthetase than the parental line (Gantt, J. S., Chiang, C. S., Hatfield, G. W., and Arfin, S. M. (1980) J. Biol. Chem. 255, 4808-4813). We have investigated the basis for these differences in enzyme activity by combined radiochemical and immunological techniques. The asparagine synthetase of beef pancreas was purified to apparent homogeneity. Antibodies raised against the purified protein cross-react with the asparagine synthetase of CHO cells. Immunotitrations show that the amount of enzyme protein in physiologically or genetically derepressed CHO strains is proportional to the level of enzyme activity. Measurement of the relative rates of asparagine synthetase synthesis by pulse-labeling experiments demonstrate that the difference in the number of asparagine synthetase molecules is closely correlated with the rate of enzyme synthesis. In contrast, the half-life of asparagine synthetase in wild type cells and in physiologically or genetically derepressed cells is very similar. It appears that the increased levels of asparagine synthetase can be attributed solely to an increased rate of enzyme synthesis.