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

Studies on optimization of growth parameters for L-asparaginase production by Streptomyces ginsengisoli

A species of Streptomyces, Streptomyces ginsengisoli, a river isolate, was evaluated for production of an enzyme, L-asparaginase, with multiple functions mainly anticancer activity. The actinomycete was subjected to submerged fermentation by “shake flask” method. The quantity of L-asparaginase produced was estimated as 3.23 μmol/mL/min. The effect of various culture conditions on L-asparaginase production was studied by adopting a method of variation in one factor at a time. Of the various conditions tested, glucose (followed by starch) and peptone served as good carbon and nitrogen sources, respectively, for maximal production of enzyme at pH 8. The temperature of 30°C and an incubation period of 5 days with 0.05 g% asparagine concentration were found to be optimum for L-asparaginase production.

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.

Pharmacological and clinical evaluation of L-asparaginase in the treatment of leukemia

L-Asparaginase is an effective antineoplastic agent, used in the acute lymphoblastic leukemia chemotherapy. It has been an integral part of combination chemotherapy protocols of pediatric acute lymphoblastic leukemia for almost 3 decades. The potential of L-asparaginase as a drug of leukemia has been a matter of discussion due to the high rate of allergic reactions exhibited by the patients receiving the medication of this enzyme drug. Frequent need of intramuscular injection has been another disadvantage associated with the native preparation. However, of late these clinical complications seem to have been addressed by modified versions of L-asparaginase. PEG-L-asparaginase proves to be most effective in this regard. It becomes important to discuss the efficacy of L-asparaginase as an antileukemic drug vis-a-vis these disadvantages. In this review, an attempt has been made to critically evaluate the pharmacological and clinical potential of various preparations of L-asparaginase as a drug. Advantages of PEG-L-asparaginase over native preparations and historical developments of therapy with l-asparaginase have also been outlined in the review below.

Efficient expression, purification, and characterization of C-terminally tagged, recombinant human asparagine synthetase

Several lines of evidence suggest that up-regulation of asparagine synthetase (AS) in human T-cells results in metabolic changes that underpin the appearance of asparaginase-resistant forms of acute lymphoblastic leukemia (ALL). Inhibitors of human AS therefore have potential as agents for treating leukemia and tools for investigating the cellular basis of AS expression and drug-resistance. A critical problem in developing and characterizing potent inhibitors has been a lack of routine access to sufficient quantities of purified, reproducibly active human AS. We now report an efficient protocol for preparing multi-milligram quantities of C-terminally tagged, wild type human AS in a baculovirus-based expression system. The recombinant enzyme is correctly processed and exhibits high catalytic activity. Not only do these studies offer the possibility for investigating the kinetic behavior of biochemically interesting mammalian AS mutants, but such ready access to large amounts of enzyme also represents a major step in the development and characterization of inhibitors that might have clinical utility in treating asparaginase-resistant ALL.

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.

Methylation and expression of neurofilament genes in tissues and in cell lines of the mouse

The light (NF-L), mid-sized (NF-M) and heavy (NF-H) neurofilament (NF) genes were probed with methylation-sensitive restriction enzymes and patterns of methylation and expression of the NF genes were compared in tissues and cell lines of the mouse. The 5' regions of all three NF genes are identified as CpG islands that remain unmethylated in expressing and non-expressing tissues, although partial methylation occurs at -795 in NF-H and at -525 in NF-M. Methylation of the NF CpG islands is associated with the inactivation of NF genes in L cells and with the selective inactivation of NF-L and NF-M in Neuro 2a cells. We also show that methylation diminishes the ability of the NF promoters to drive transcription of a CAT reporter gene. Hence, the presence of CpG islands may be important in determining patterns of NF transcription in vitro. Moreover, the preservation of CpG islands may be an evolutionary link that bears upon the nature of the NF genes and the mechanisms that have evolved to limit NF expression.

High levels of de novo methylation and altered chromatin structure at CpG islands in cell lines

CpG islands are normally methylation free in cells of the animal, even when the associated gene is transcriptionally silent. In mouse NIH 3T3 and L cells, however, over half of the islands are heavily methylated. Near identity of the methylated subset in the two cell lines suggested that methylation is confined to genes that are nonessential in culture. In agreement with this, islands at several tissue-specific genes, but not at housekeeping genes, have become methylated in many human and mouse cell lines. At the chromatin level, methylated islands are Mspl resistant compared with their nonmethylated counterparts. We suggest that mutation-like gene inactivation due to CpG island methylation is widespread in many cell lines and could explain the loss of cell type-specific functions in culture.

Methylation of mouse adenine phosphoribosyltransferase gene is altered upon cellular differentiation and loss of phenotypic expression

Morphologically differentiated cell lines were previously isolated from a mouse teratocarcinoma stem cell line exhibiting an unstable heterozygous deficiency for adenine phosphoribosyltransferase (APRT) expression. In this study, the methylation sensitive and insensitive isoschizomer restriction endonucleases HpaII and MspI, respectively, were used to demonstrate that the aprt gene in the heterozygous deficient stem cell line was hypomethylated. Loss of APRT activity in this stem cell line was not associated with DNA methylation change. However, differentiation of this stem cell line was associated with hypermethylation of three consecutive HpaII/MspI sites that were located in the second intron and the third exon of the aprt gene. A total of 15 independent APRT homozygous deficient cell lines were isolated from three differentiated heterozygous deficient cell lines, and in all 15 cell lines this differentiation-related methylation pattern was altered. Two classes of alterations were noted: (1) hypomethylation of a site located in the second intron or (2) the apparent spreading of methylation to downstream methylation sites. The CpG-rich promoter region remained hypomethylated in the APRT homozygous deficient differentiated cell lines and a methylation change affecting a specific CpG site upstream of the promoter region was noted in only two of the 15 homozygous deficient cell lines. It is proposed that methylation of the mouse aprt gene may be involved in controlling phenotypic expression in the differentiated cell lines, but not in the stem cell line they were derived from.

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.