Asparagine-requiring tumor cell lines and their non-requiring variants: cytogenetics, biochemistry and population dynamics

An investigation into the mechanism ofL-asparaginase resistance in L5178Y murine leukemia cells

Resistance of leukemia cells toL-asparaginase is presumed to be due to increased expression of asparagine synthetase activity by resistant cells, so they are no longer dependent on an exogenous source ofL-asparagine for growth. The mechanism by which cells acquire the ability for increased enzyme expression, however, has not been clearly defined. Evidence presented here indicates that genomic alterations in the form of translocations, gene amplification, or increased P-glycoprotein expression, do not account for the phenotypic transformation fromL-asparaginase sensitivity toL-asparaginase resistance. Instead, both sensitive and resistant L5178Y cells contain immunoreactive material detected by Western blotting with an antiserum prepared against bovine pancreatic asparagine synthetase. This suggests that the mechanism of resistance might involve modification of asparagine synthetase inL-asparaginase-resistant cells by an as-yet-unidentified mechanism or by inhibition of enzyme activity in theL-asparaginase-sensitive cells.

A review of the genetic effects of ethyl methanesulfonate

Ethyl methanesulfonate (EMS) is a monofunctional ethylating agent that has been found to be mutagenic in a wide variety of genetic test systems from viruses to mammals. It has also been shown to be carcinogenic in mammals. Alkylation of cellular, nucleophilic sites by EMS occurs via a mixed SN1/SN2 reaction mechanism. While ethylation of DNA occurs principally at nitrogen positions in the bases, because of the partial SN1 character of the reaction, EMS is also able to produce significant levels of alkylation at oxygens such as the O6 of guanine and in the DNA phosphate groups. Genetic data obtained using microorganisms suggest that EMS may produce both GC to AT and AT to GC transition mutations. There is also some evidence that EMS can cause base-pair insertions or deletions as well as more extensive intragenic deletions. In higher organisms, there is clear-cut evidence that EMS is able to break chromosomes, although the mechanisms involved are not well understood. An often cited hypothesis is that DNA bases ethylated by EMS (mostly the N-7 position of guanine) gradually hydrolyze from the deoxyribose on the DNA backbone leaving behind an apurinic (or possibly an apyrimidinic) site that is unstable and can lead to single-strand breakage of the DNA. Data also exist that suggest that ethylation of some chromosomal proteins in mouse spermatids by EMS may be an important factor in causing chromosome breakage.

Properties of asparagine synthetase in asparagine-independent variants of Jensen rat sarcoma cells induced by 5-azacytidine

Jensen rat sarcoma cells in culture require L-asparagine for growth and lack detectable levels of asparagine synthetase. Cultures exposed for 24 h to graded concentrations of 5-azacytidine give rise to asparagine-independent variants in high frequency. These prototrophs are stable phenotypically whether maintained in the presence or absence of L-asparagine. Asparagine synthetase activity in several variant clones was uniform in thermolability and several kinetic parameters, as well as in immunological properties. Parental Jensen rat sarcoma cells contained no detectable immunologically cross-reacting material. Our data suggest that transitions between asparagine dependence and independence in these cells are mediated by stable shifts in gene expression rather than by structural gene mutations.

Thymidine resistance in Chinese hamster V79 cells in vitro

Thymidine resistance in V79 Chinese hamster cells has been investigated. Phenotypically stable variant resistant lines occurred at a high frequency, and the mutation rate (2.67 x 10(-3) per cell per generation) to 400 micrograms/ml thymidine resistance as measured by the standard Luria--Delbrück fluctuation analysis was extremely high. Populations of cells maintained for extended periods in F-10 medium spontaneously increased in resistance, possibly as a result of selective pressures due to the thymidine present in F-10 medium since this change was not observed in Dulbecco's medium. The degree of resistance for a given variant was correlated with the amount of thymidine employed in its selection. Metabolic cooperation, resulting in the suppression of the resistant phenotype, was demonstrated in artificial mixtures of sensitive and resistant clonal lines. Clones isolated in high levels of thymidine possessed lowered uptake of [3H]thymidine and the depression in uptake was related to the level of resistance of the particular clone. Although thymidine kinase specific activity levels were slightly depressed in variant cell lines, growth rate and uridine uptake were unaffected. We conclude that thymidine resistance is due to a genetically controlled depression of external thymidine uptake.

Amphibian cells in culture. II. Isolation of drug-resistant variants and an asparagine-independent variant

With L-15 as the base medium, drug-resistant variants were isolated from two amphibian tissue culture strains: the Xenopus laevis A8 diploid cell line and the ICR 2A cell line of Rana pipiens. Four different classes of variants were obtained: (1) A8 cells resistant to chloramphenicol, an inhibitor of mitochondrial protein synthesis; (2) A8 cells resistant to ouabain, an inhibitor of the Na+/K+-activated ATPase of the plasma membrane;(3) ICR 2A cells resistant to low (20 microgram/ml) and high (300 microgram/ml) levels of bromodeoxyuridine (BUdR), a thymidine analog which interferes with the pyrimidine salvage pathway; and (4) ICR 2A cells resistant to 2,6-diaminopurine (DAP), an adenine analog which interferes with the purine salvage pathway. Unlike the other variants, isolation of BUdR resistant cells is a 2-step process. Resistance to low levels of BUdR is phenotypically expressed by a reduction in thymidine transport activities while resistance to high levels of this compound is evidenced by greatly reduced levels of thymidine kinase activity. DAP-resistant cells, which are characterized by reduced levels of adenine phosphoribosyl transferase (APRT) activity, do not die in AAT (adenine, aminopterin, thymidine) selection medium. This suggests that these cells utilize adenine efficiently. With MEM as the base medium, an asparagine independent clone was isolated from the ICR 2A cell line. When compared with the wild type, this variant exhibited a slightly reduced growth rate in the presence or absence of asparagine.

A role for asparaginyl-tRNA in the regulation of asparagine synthetase in a mammalian cell line

The expression of asparagine synthetase activity [L-aspartate:ammonia ligase (AMP-forming), EC 6.3.1.1] in cultured Chinese hamster ovary (CHO) cells is regulated by asparagine. After transfer of CHO cells from an asparagine-supplemented medium to a medium lacking asparagine, activity increases 1.5- to 2-fold. If asparagine is added back to the medium, activity returns to control levels. To test the possible involvement of Asn-tRNAAsn in regulating the levels of asparagine synthetase, we have examined the levels of asparagine synthetase in a mutant of CHO cells containing a temperature-sensitive asparaginyl-tRNA synthetase [L-asparagine:tRNA ligase (AMP-forming), EC 6.1.1.22]. Under conditions of limited asparaginyl-tRNA synthetase activity in the mutant, there is a 2- to 3-fold increase in the level of asparagine synthetase activity. Under identical conditions, there is no change in asparagine synthetase activity in the wild type. This correlation between asparaginyl-tRNA synthetase activity and asparagine synthetase levels may be a consequence of a direct role of tRNAAsn in the regulation of the in vivo expression of the asparagine synthetase structural gene.

Mutagenesis studies on cultured mammalian cells. The sensitivity of the asparagine-requiring phenotype to several chemical agents

The effect of several chemical agents on the mutation frequency from asparagine dependence to asparagine independence has been studied in Jensen sarcoma cells. It was found that ethylmethanesulfonate brought about a dramatic exponential increase, while nitrosoguanidine was not lighly effective as a mutagen, causing only a modest increase in mutation frequency, and quinacrine HCl was ineffective. The results presented here are compared with those obtained in other systems and with our previous work on the effects of UV on mutation induction in the asparagine system. They suggest that the basis of the asparagine requirement of mammalian cell lines resides in a specific genetic alteration in nuclear DNA which is corrected by the mutagenic action of the agents tested here.

On the relationship between spontaneous mutation rates in vivo and in vitro

Recent estimates of spontaneous mutation rates in man, in which previous sources of bias are corrected, indicate that the average is about 3 x 10(-7) per locus per generation, a much lower figure than is generally accepted. Assuming 100 to 1000 cell divisions between each gametic union, this information predicts that cellular mutation rats should be in the order of 10(-9) per locus per generation. Since none of the mutation rates measured in cultured cells are this low (average for seven characters equals 7 x 10(-7)), the size of mutation rates in cultured cells cannot be used to substantiate the claim of epigenetic inheritance. Furthermore, this information suggests that in multicellular organisms the germinal tissue is sequestered from mutagenic insult or subjected to selection against mutational damage so as to keep the genetic load of a species at a tolerable level. Alternatively, cell culture environments may present an extremely abnormal situation to somatic cells, thus elevating the mutation rate.

Effect of ploidy on spontaneous mutation rate to asparagine non-requirement in cultured cells

Variations in ploidy do not affect the spontaneous mutation rate to asparagine non-requirement in Jensen rat sarcoma cells cultivated in vitro.