Formycin B-resistant mutants of Chinese hamster ovary cells: novel genetic and biochemical phenotype affecting adenosine kinase

Stable mutants which are approximately three- and eightfold resistant to the pyrazolopyrimidine nucleosides formycin A and formycin B (FomR) have been selected in a single step from mutagenized Chinese hamster ovary cells. In cell extracts, the two FomR mutants which were examined were both found to contain no measurable activity of the enzyme adenosine kinase (AK). However, cross-resistance studies with other adenosine analogs such as toyocamycin and tubercidin show that these mutants are distinct from toyocamycin or tubercidin resistant (Toyr) mutants which also contain no measurable AK activity in cell extracts. Studies on the uptake and incorporation of [3H]adenosine and [3H]tubercidin by various mutants and parental cell lines show that unlike the Toyr mutants, which are severely deficient in the phosphorylation of these compounds, the FomR mutants possess nearly normal capacity to phosphorylate these compounds and incorporate them into cellular macromolecules. These results suggest that the FomR mutants contain normal levels of AK activity in vivo. In cell hybrids formed between FomR X FomS cells and FomR X Toyr cells, the formycin-resistant phenotype of both of the FomR mutants behaved codominantly. However, the extracts from these hybrid cells contained either congruent to 50% (FomR X FomS) or no measurable (FomR X Toyr) AK activity, indicating that the lesion in these mutants neither suppresses the wild-type AK activity nor complements the AK deficiency of the Toyr mutants. The presence of AK activity in the FomR mutants in vivo, but not in their cell extracts, along with the codominant behavior of the mutants in hybrids, indicates that the lesions in the FomR mutant are of a novel nature. It is suggested that the genetic lesion in these mutants affects AK activity indirectly and that it may involve an essential cellular function which exists in a complex form with AK. Some implications of these results regarding the mechanism of action of formycin B are discussed.

Quantitative mutagenesis at the adenosine kinase locus in Chinese hamster ovary cells. Development and characteristics of the selection system

Stable mutants exhibiting high degree of resistance (100-1000-fold) to various nucleoside analogs viz, toyocamycin, tubercidin, 6-methyl mercaptopurine riboside (6-MeMPR) and pyrazofurin, are obtained at similar frequency (congruent to 1 X 10(-4] in CHO cells. The mutants resistant to any of the above analogs exhibit similar degree of cross-resistance to the other three nucleoside analogs, and all of the mutants examined contained no measurable activity of the purine salvage pathway enzyme adenosine kinase (AK) which converts these analogs to their phosphorylated derivatives. These results indicate that very similar mutants are selected using any of these analogs. The recovery of AK- mutants in CHO cells is not affected by cell density (up to at least 5 X 10(5) cells per 100-mm diameter dish) and after treatment with mutagen(s) maximum mutagenic effect is observed after 7-8 days, which then remains unchanged for the next several days. Treatment of CHO cells with a number of mutagenic agents e.g. ethyl methanesulfonate, ICR170, ultraviolet light, and benzo[a]pyrene, led to a nearly linear concentration-dependent increase in the frequency of the AK- mutants in cultures. The mutagenic response of the AK locus to these agents compared favorably with that of the HGPRT locus (6-thioguanine resistance) within the same experiments. These results show that the selection system for AK- mutants provides an additional valuable genetic marker for quantitative mutagenesis studies in CHO cells.

Increased expression of one of two adenosine deaminase alleles in a human choriocarcinoma cell line following selection with adenine nucleosides

JEG-3 is a human choriocarcinoma cell line characterized by low levels of adenosine deaminase expression. For the purpose of studying adenosine deaminase gene regulation in the JEG-3 cells, we attempted to select variant cells having increased adenosine deaminase expression. This was accomplished by selecting cells for resistance to the cytotoxic adenosine analogs 9-beta-D-arabinofuranosyl adenine (ara-A) or 9-beta-D-xylofuranosyl adenine (xyl-A), both of which could presumably be detoxified by the action of adenosine deaminase. Single step high dose selection was ineffective in obtaining cells with increased adenosine deaminase. However, multistep selection using either ara-A or xyl-A resulted in cell populations with increased adenosine deaminase activity. Removal of selective pressure resulted in decreased adenosine deaminase levels. Subclones of xyl-A-resistant cells belonged to one of three phenotypic classes characterized by either elevated adenosine deaminase levels, decreased adenosine kinase levels, or both of these features. One subclone (A3-1A7) with unaltered adenosine kinase expression showed a 20-fold increase in adenosine deaminase expression. Further selection of this subclone for increasing xyl-A resistance resulted in an additional 2-fold increase in adenosine deaminase expression, followed by loss of adenosine kinase expression. These adenosine kinase-deficient cells showed no subsequent increase in adenosine deaminase expression in response to further xyl-A selection pressure. These results confirmed that xyl-A toxicity was mediated through its phosphorylated form and indicated that resistance may result from increased adenosine deaminase levels and/or adenosine kinase deficiency. The increased adenosine deaminase expression of the A3-1A7 subclone was exclusively in the ADA 2 allelic form. However, cell fusion experiments between A3-1A7 cells and mouse C1-1D cells established the existence of functional copies of both ADA 1 and ADA 2 allelic genes in the A3-1A7 cells. The increased expression of only one of the two functional ADA alleles, the requirement for a stepwise selection protocol to obtain cells with increased adenosine deaminase, and the instability of the adenosine deaminase phenotype in the absence of selective pressure suggest that the alteration of adenosine deaminase phenotype in the drug-resistant cells was the result of adenosine deaminase gene amplification.

Comparison of the mutagenic responses of 12 anticancer drugs at the hypoxanthine-guanine phosphoribosyl transferase and adenosine kinase loci in Chinese hamster ovary cells

5he mutagenic responses of 12 anticancer drugs--lomustine, dacarbazine, mitomycin C, chlorambucil, cis-diamminedichloroplatinum (II), busulfan, ellipticine, daunomycin, adriamycin, VM-26, VP16-213, and bleomycin--in Chinese hamster ovary (CHO) cells at two independent genetic loci which affect purine salvage pathway enzymes, hypoxanthine-guanine phosphoribosyl transferase (HGPRT) and adenosine kinase (AK) have been compared within the same experiments. The cellular mutants which lack either HGPRT (HGPRT- mutants) or AK (AK- mutants) activity can be directly selected in CHO cells using 6-thioguanine and 6-methyl mercaptopurineriboside (6-MeMPR) respectively, and optimal conditions for their selection have been described. Results of our studies show that all of the above drugs caused a concentration-dependent increase in the frequencies of both 6-thioguanine- and 6-MeMPR-resistant mutants. However, for a number of these drugs, namely adriamycin, daunomycin, bleomycin, and dacarbazine, which are only weakly mutagenic at the HGPRT locus, a relatively strong and clear mutagenic response was observed at the AK locus. In contrast, VM-26 showed a more pronounced effect at the HGPRT locus. For the remaining compounds, the mutagenic responses of the two loci (as indicated by the slopes of the dose-response curves) were comparable. The observed increase in the frequency of AK- mutants after treatment with various drugs, many of which predominantly induce frameshift and chromosome-breakage type of genetic lesions, suggests that like the HGPRT locus the AK locus is also capable of detecting all the different types of genetic lesions. The favorable response of the AK locus as compared to that of HGPRT locus in these studies indicates that the selection system of AK- mutants should provide a very useful genetic marker for mutagenesis studies in CHO cells.

Chromosome segregation is frequently associated with the expression of recessive mutations in mouse cells

The genes coding for adenosine kinase (ADK; ATP:adenosine 5'-phosphotransferase, EC 2.7.1.20) and esterase-10 (ES-10; carboxylesterase, carboxylic-ester hydrolase, EC 3.1.1.1) are both located on chromosome 14 in the mouse. The near-diploid mouse cell line CAK is heterozygous for two electrophoretic variants of ES-10. Recessive Adk- mutants of CAK have been isolated and analyzed for Es-10 phenotype and karyotypic abnormalities. Two classes of mutants were found with approximatley equal frequencies: those that remained heterozygous in the expression of Es-10 and those that expressed only one Es-10 allele. Of the mutants that lacked one form of ES-10, approximately half were missing most or all of one copy of chromosome 14; the other contained two copies of 14, frequently in the form of an isochromosome. There were no abnormalities of this chromosome found among the mutants that were Es-10 heterozygotes. These results suggest that the expression of an autosomal recessive mutation in near-diploid mouse cells is frequently associated with events that result in the segregation of a physically linked marker and part or all of a chromosome.

Elucidation of aberrant purine metabolism: application to hypoxanthine-guanine phosphoribosylstransferase- and adenosine kinase-deficient mutants, and IMP dehydrogenase- and adenosine deaminase-inhibited human lymphoblasts

We propose that the ratio of [14C]formate-labelled purine nucleosides and bases (both intra and extracellular) to nucleic acid purines provides, in exponentially growing cultures, a sensitive index for comparative studies of purine metabolism. This ratio was 4-fold greater for an HGPRT- mutant than for the parental HGPRT+ human lymphoblast line. The major components of the labelled nucleoside and base fraction were hypoxanthine and inosine. By blocking adenosine deaminase activity with coformycin we found that approx. 90% of inosine was formed directly from IMP rather than the route IMP leads to AMP leads to adenosine leads to inosine. The ratio of labelled base + nucleosides to nucleic acids was essentially unchagned for an AK- lymphoblast line and 2-fold greater than control for an HGPRT(-)-KAK- line, demonstrating that a deficiency of adenosine kinase alone has little effect on the accumulation of purine nucleosides and bases. Although adenosine was a minor component of the nucleoside and base fraction, the adenosine fraction increased from 3 to 13% with the addition of coformycin to the HGPRT(-)-AK- line. In the parental and HGPRT- lines, adenosine was shown to be primarily phosphorylated rather than deaminated at concentrations less than 5 microM. Inhibition of IMP dehydrogenase activity by mycophenolic acid caused a 12- and 3-fold increase in the rate of production of labelled base and nucleoside in the parent and HGPRT- cells respectively. These results suggest that a mutationally induced partial deficiency in the activities converting IMP to guanine nucleotides may result in an increased catabolism of IMP.

Resistance to methyl mercaptopurine riboside in cultured hamster cells. Preliminary characterization of resistant cells and conditions affecting their selection in quantitative mutation studies

Cells resistant to high concentrations of methyl mercaptopurine riboside (MMPR), an anlogue of adenosine, were found at a frequency of about 6 x 10(-6) per viable cell in untreated cultures of V79 Chinese hamster cells. Resistant cells were selected less efficiently if purines were added to the MMPR-medium, but high cell densities had little effect upon selection. 6 independently-isolated spontaneous MMPR-resistant sublines were characterized by their resistance to the toxicity of different purines, rate of purine excretion, incorporation of radioactive adenosine, electrophoresis of cell extracts, and expression of resistance in hybrids to an MMPR-sensitive line. 5 of these sublines showed recessive expression of MMPR-resistance in hybrids and had characteristics consistent with loss of adenosine kinase activity, while the remaining subline was much less resistant to MMPR and showed semi-dominant expression of resistance without loss of adenosine kinase activity. Cells with high resistance to MMPR were not found in a "tetraploid" (hybrid) V79 line or in freshly-isolated human cell cultures, but occurred at a comparable frequency to V79 in another commonly-used aneuploid hamster line, CHO-K1. The frequency of MMPR-resistant cells in V79 cultures was increased to a similar extent by treatment with gamma-rays or with ethyl methanesulphonate, providing a suitable post-treatment interval was allowed for the expression of resistance. A genetic interpretation of these data is given in which it is proposed that resistance most usually arises through mutation of an autosomally-linked gene of which one copy has been inactivated or lost in V79 and in CHO-K1 cells. In comparison to published data on the selection of "mutants" resistant to 6-thioguanine, it is argued that MMPR could be as useful a selective agent as thioguanine and may select a different range of types of mutagenic event.

Genetic and biochemical studies with the adenosine analogs toyocamycin and tubercidin: mutation at the adenosine kinase locus in Chinese hamster cells

The pyrrolopyrimidine nucleosides toyocamycin and tubercidin show several unique features of growth inhibition in Chinese hamster ovary (CHO) cells. Stable mutants which are more than 600-fold resistant to these drugs are obtained in CHO cells at a strikingly high frequency of approximately 10(-3), in the absence of mutagenesis. The mutants resistant to toyocamycin (Toyr) and tubercidin (Tubr) exhibit similar cross-resistance patterns to the two selective drugs as well as to adenosine and 6-methyl mercaptopurine riboside, indicating that the same lesion is probably involved in all cases. The mutants examined were found to be deficient in the enzyme adenosine kinase (AK), indicating that the phosphorylation of these analogs is an essential first step in their toxic action. The above mutants (AK-) behaved recessively in cell hybrids, and segregation studies indicate that the AK locus is not linked to the X chromosome. The frequencies of similar Toyr mutants in other Chinese hamster lines, e.g., V79, CHW, M3-1, GM7, and CHO-K1, varied from similar to more than three logs less than that observed for CHO cells, indicating that various cell lines probably differ in the number of functional gene copies for this locus.

Adenosine kinase as a new selective marker in somatic cell genetics: isolation of adenosine kinase--deficient mouse cell lines and human--mouse hybrid cell lines containing adenosine kinase

A new selective system for isolating somatic cell hybrids, using adenosine kinase as the selective marker, has been developed. The selective medium for forward selection (to select for cells containing adenosine kinase) contains alanosine, adenosine and uridine. To survive in the presence of alanosine, cells must have adenosine kinase in order to utilize exogenous adenosine as the sole source of AMP. Uridine is added to the selective medium to prevent the toxic effects of adenosine on cultured mammalian cells. The selective medium for reverse selection (to select for cells lacking adenosine kinase) contains 2-fluoroadenosine, an analogue of adenosine, which is converted to a toxic nucleotide by the action of adenosine kinase. Mouse mutant cell lines deficient in adenosine kinase have been derived. Human--mouse hybrid cells containing the kinase have been prepared from one of these mutant lines. Karyotype data of these hygrid lines and their adenosine kinase-minus sublines are consistent with assignment by others of the human gene for adenosine kinase on chromosome 10.

Assignment of the gene for adenosine kinase to chromosome 14 in Mus musculus by somatic cell hybridization

Evidence is presented for the assignment of the gene for adenosine kinase to Mus musculus chromosome 14 by synteny testing and karyotypic analysis of mouse X Chinese hamster somatic cell hybrid clones. ADOK and two enzymes previously mapped to mouse chromosome 14, NP and ES-10, were expressed concordantly in 29 hybrid clones. Chromosome analysis confirmed this assignment. Syntenic evidence is also presented using several clones of a gene transfer system in which the gene for human HPRT had integrated into modified mouse chromosome 14's.