Stable mutants of mammalian cells that overproduce the first three enzymes of pyrimidine nucleotide biosynthesis

Organization of a multifunctional protein in pyrimidine biosynthesis. A domain hypersensitive to proteolysis

When the multifunctional protein that catalyses the first three steps of pyrimidine biosynthesis in hamster cells is treated with staphylococcal V8 proteinase, a single cleavage takes place. The activities of carbamoyl-phosphate synthetase (EC 6.3.5.5), aspartate carbamoyltransferase (EC 2.1.3.2) and dihydro-orotase (EC 3.5.2.3) and the allosteric inhibition by UTP are unaffected. One fragment, of Mr 182000, has the first and third enzyme activities, whereas the other fragment, of Mr 42000, has aspartate carbamoyltransferase activity and an aggregation site. A similar small fragment is observed in protein digested with low concentrations of trypsin. A similar large fragment is seen after digestion with trypsin and as the predominating form of this protein in certain mutants defective in pyrimidine biosynthesis. These results indicate that a region located adjacent to the aspartate carbamoyltransferase domain is hypersensitive to proteinase action in vitro and may also be sensitive to proteolysis in vivo.

Analysis of CAD Gene Amplification Using a Combined Approach of Molecular Genetics and Cytogenetics

CAD is a multifunctional protein which catalyzes the first three steps of de novo uridine biosynthesis. Rodent cells resistant to PALA, a specific inhibitor of the ATCase activity of CAD, overproduce the CAD protein and CAD mRNA as a direct result of the amplification of the CAD gene. In order to study the mechanism of CAD gene amplification, a functional Syrian hamster CAD gene was inserted into a cosmid vector using molecular cloning techniques. The cloned genes were assayed for biological function by fusing CAD-deficient Chinese hamster ovary (CHO) cell mutants with protoplasts of E. coli containing the CAD cosmids. Two clones with functional CAD genes were isolated and shown to contain inserts 40 and 45 kb long. The cloned genes could also be introduced into wild type CHO cells by selecting for cells which became resistant to high PALA concentrations in a single step. Transformations of mutant and wild type CHO cells contained multiple active copies of the donated Syrian hamster CAD genes in addition to their endogenous CHO CAD genes. The cloned genes in all transformants analyzed are integrated into host cell chromosomes at single locations defined by in situ hybridization. Independently isolated transformants contain the donated genes in different chromosomes. Co-transformation of CHO cells with two different genes by protoplast fusion is also shown to be possible.

Structure of amplified DNA in different Syrian hamster cell lines resistant to N-(phosphonacetyl)-L-aspartate

Syrian hamster cell lines selected in multiple steps for resistance to high levels of N-(phosphonacetyl)-L-aspartate (PALA) contain many copies of the gene coding for the pyrimidine pathway enzyme CAD. Approximately 500 kilobases of additional DNA was coamplified with each copy of the CAD gene in several cell lines. To investigate its structure and organization, we cloned ca. 162 kilobases of coamplified DNA from cell line 165-28 and ca. 68 kilobases from cell line B5-4, using a screening method based solely on the greater abundance of amplified sequences in the resistant cells. Individual cloned fragments were then used to probe Southern transfers of genomic DNA from 12 different PALA-resistant mutants and the wild-type parents. A contiguous region of DNA ca. 44 kilobases long which included the CAD gene was amplified in all 12 mutants. However, the fragments cloned from 165-28 which were external to this region were not amplified in any other mutant, and the external fragments cloned from B5-4 were not amplified in two of the mutants. These results suggest that movement or major rearrangement of DNA may have accompanied some of the amplification events. We also found that different fragments were amplified to different degrees within a single mutant cell line. We conclude that the amplified DNA was not comprised of identical, tandemly arranged units. Its structure was much more complex and was different in different mutants. Several restriction fragments containing amplified sequences were found only in the DNA of the mutant cell line from which they were isolated and were not detected in DNA from wild-type cells or from any other mutant cells. These fragments contained novel joints created by rearrangement of the DNA during amplification. The cloned novel fragments hybridized only to normal fragments in every cell line examined, except for the line from which each novel fragment was isolated or the parental population for that line. This result argues that "hot spots" for forming novel joints are rare or nonexistent.

Properties of single-step mutants of Syrian hamster cell lines resistant to N-(phosphonacetyl)-L-aspartate

Eleven independent lines of Syrian hamster cells were selected by using very low levels of N-(phosphonacetyl)-L-aspartate (PALA), an inhibitor of aspartate transcarbamylase. The protocol employed insured that each resistant cell arose during one of the last divisions before selection was applied. Cells of each mutant line contained an amplification of the structural gene for CAD, a trifunctional protein which includes aspartate transcarbamylase and two other enzymes of UMP biosynthesis. Strikingly, despite the minimal selection employed, the degree of amplification of the CAD gene was 6 to 10 times the normal diploid number in all 11 cases. In situ hybridization indicated that the amplified CAD genes were almost always present at a single chromosomal site in each line. Therefore, one of the two alleles was amplified 11- to 19-fold. The rates at which cells became resistant to PALA, determined by fluctuation analysis, were 100 times less dependent on drug concentration than were the frequencies of resistant cells in steady-state populations. The relatively shallow dependence of this rate upon PALA concentration is consistent with our independent observation that most events gave rise to a similar degree of amplification. In six of six cell lines examined, the levels of CAD mRNA and aspartate transcarbamylase activity were elevated two- to fourfold. These lines were resistant to PALA concentrations 20- to 80-fold higher than the ones used for selection. The organization of amplified DNA was examined by hybridizing Southern blots with cloned DNA fragments containing amplified sequences, previously isolated from two cell lines resistant to high levels of PALA. A contiguous region of DNA approximately 44 kilobases long which included the CAD gene was amplified in five of five single-step mutants examined. Outside this region, these mutants shared amplified sequences with only one of the two highly resistant lines.

Co-amplification of rRNA genes with CAD genes in N-(phosphonacetyl)-L-aspartate-resistant Syrian hamster cells

The amplified CAD genes in N-(phosphonacetyl)-L-aspartate (PALA)-resistant Syrian hamster cells are located in an expanded chromosomal region emanating from the site of the wild-type gene at the tip of the short arm of chromosome B-9. The terminus of B-9 in PALA-sensitive cells contains a cluster of rRNA genes (i.e., a nucleolus organizer, rDNA). We have used a molecular clone containing sequences complementary to Syrian hamster 28S rRNA to investigate whether rDNA is coamplified with CAD genes in the PALA-resistant mutants. In situ hybridization of this probe to metaphase chromosomes demonstrates that rDNA and CAD genes do coamplify in two independently isolated PALA-resistant mutants. The tight linkage of CAD and rDNA genes was demonstrated by their coordinate translocation from B-9 to the end of the long arm of chromosome C-11 in one mutant. Blot hybridization studies substantiate the in situ hybridization results. Both types of analysis indicate that only one or two rDNA genes, on the average, are coamplified per CAD gene. The data are consistent with the model that unequal exchanges between rDNA genes mediate the amplification of CAD genes in the Syrian hamster mutants that were analyzed.

An ultraviolet spectrophotometric assay for aspartate transcarbamylase

A procedure for determining the activity of aspartate transcarbamylase, based upon the greater ultraviolet absorbancy of the products of the reaction catalyzed compared to the reactants, was devised. Extinction coefficients were determined at 205, 210, and 215 nm for the compounds carbamoyl aspartate, acetyl aspartate, and aspartate. These values formed the quantitative basis for a spectrophotometric assay in which an enzymatic reaction is monitored at one of these wavelengths. Use of this procedure was illustrated in four kinetic experiments with the allosteric aspartate transcarbamylase from Escherichia coli, and the nonallosteric catalytic subunit of this enzyme: aspartate saturation curve, arsenate saturation curve (reverse reaction), allosteric activation by a transition-state analog employing acetyl phosphate as substrate, and carbamoyl phosphate progress curve (substrate depletion in the presence of excess cosubstrate). Owing to changes in absorbance on the order of 1000 liter mol-1 cm-1 concomitant with the reaction, the sensitivity of the method is comparable to that of many procedures already in the literature.

Metabolic resistance: the protection of enzymes against drugs which are tight-binding inhibitors by the accumulation of substrate

Blockade of a metabolic pathway by interaction of a drug with a particular 'target enzyme' results in depletion of essential end-products of the pathway and accumulation of intermediates prior to the blockade. Metabolic resistance to a particular drug can arise if the substrate of the inhibited enzyme accumulates to levels sufficiently high to compete effectively with the inhibitor, leading to restoration of full activity of the metabolic pathway after a transitory delay. Such resistance has recently been demonstrated in vitro for the interaction of the tight-binding inhibitor N-phosphonacetyl-L-aspartate (PAcAsp) with the aspartate transcarbamoylase activity of the trifunctional protein which initiates pyrimidine biosynthesis in mammals [Christopherson, R. I. and Jones, M. E. (1980) J. Biol. Chem. 255, 11381-11395]. Carbamoyl phosphate, the product of the carbamoyl phosphate synthetase activity of this trifunctional protein, accumulates to a sufficiently high concentration that the inhibitory effect of PAcAsp is effectively abolished. We have developed a theoretical model for metabolic resistance which quantitatively accounts for these experimental data. This model has been used to simulate the interaction between the following potential or proven anti-cancer drugs and their target enzyme, under conditions similar to those which would occur in vivo: PAcAsp with aspartate transcarbamoylase; various OMP analogues [the 5'-monophosphates of 6-azauridine, pyrazofurin and 1-(beta-D-ribofuranosyl)-barbituric acid] with OMP decarboxylase; 5-fluorodeoxyUMP with thymidylate synthase; methotrexate with dihydrofolate reductase; and deoxycoformycin with adenosine deaminase.

Chromosomal alterations associated with overproduction of asparagine synthetase in albizziin-resistant Chinese hamster ovary cells

The amino acid analog albizziin was used to isolate Chinese hamster ovary cell lines which overproduce asparagine synthetase. Mutants selected in a single step after ethyl methane sulfonate mutagenesis were approximately 10-fold more resistant to the drug than the parental lines and expressed 8- to 17-fold elevations in enzyme activity. The karyotypes of these lines show alterations such as breaks and translocations affecting the long arm of chromosome 1. Cell lines isolated in several steps by growth in progressively increasing concentrations of albizziin were more resistant to the drug and exhibited up to 300-fold enhancement of asparagine synthetase activity. The multistep albizziin-resistant cell lines usually had expanded chromosomal regions which stained somewhat homogeneously, often on the long arm of chromosome 1. These results suggest that resistance to albizziin in the multistep lines may be due to gene amplification.

Rapid spontaneous dihydrofolate reductase gene amplification shown by fluorescence-activated cell sorting

We have determined whether the gene encoding dihydrofolate reductase (5,6,7,8-tetrahydrofolate: NADP+ oxidoreductase, EC 1.5.1.3) varies spontaneously in gene copy number in cells in vitro. Cells were stained under nonselective conditions with fluoresceinated methotrexate, which binds quantitatively to dihydrofolate reductase. Cells with the highest fluorescence were collected by a fluorescence-activated cell sorter and subsequently grown in the absence of methotrexate. At no time during the experiment were the cells placed under metabolic stress. After 10 successive rounds of growth and sorting, the derived population showed a 50-fold increase in fluorescence intensity, was highly resistant to methotrexate, and was amplified 40-fold in content of dihydrofolate reductase gene. We also found that cells already having amplified genes can undergo increases or decreases in their fluorescence and in gene copy number even more rapidly (at rates as high as 3 X 10(-2) amplification events per cell division) than do parental cells (ca. 10(-3) events per division). We therefore conclude that gene amplification can occur spontaneously in cells and that the rate of its occurrence varies with gene copy number.

Stepwise isolation and properties of unstable Chinese hamster cell variants that overproduce adenylate deaminase

Addition of coformycin (0.5 microgram/ml) to a culture medium containing adenine causes in Chinese hamster fibroblasts a lethal depletion of IMP. Resistant variants have been recovered, some of which exhibit increased adenylate deaminase activity. (Debatisse et al., J. Cell. Physiol., 106:1-11, 1981). The selective medium was made more specific for the isolation of this class of variants by supplementation with azaserine. The hyperactive variants remained sensitive to coformycin concentrations above that used for their selection and were unstable. Their frequency was not increased by ethyl methane sulfonate mutagenesis. The resistant phenotype and the increased activity of adenylate deaminase behaved as semidominant traits in hybrids. No change was detected in the Km for AMP, the cofactor requirement, or the chromatographic properties of adenylate deaminase in the variants. Through stepwise selection in media supplemented with increasing coformycin concentrations, unstable clones with adenylate deaminase activity up to 150-fold the wild-type level were isolated; from an unstable clone, a stable subclone with reduced resistance and enzyme activity was recovered. Evidence that increased adenylate deaminase activity is the manifestation of overaccumulation of the enzyme protein was supplied by the correlation of enzyme activity with the intensity of a protein band comigrating with purified adenylate deaminase during sodium dodecyl sulfate-polyacrylamide gel electrophoresis of cell extracts. Several unidentified additional bands showed comparable quantitative changes. The striking similarity between the adenylate deaminase-overproducing lines and unstable dihydrofolate reductase-overproducing lines generated by gene amplification strongly suggests that the coformycin-resistant variants also resulted from amplification of an adenylate deaminase gene.

Chromosomal alterations associated with overproduction of asparagine synthetase in albizziin-resistant Chinese hamster ovary cells

The amino acid analog albizziin was used to isolate Chinese hamster ovary cell lines which overproduce asparagine synthetase. Mutants selected in a single step after ethyl methane sulfonate mutagenesis were approximately 10-fold more resistant to the drug than the parental lines and expressed 8- to 17-fold elevations in enzyme activity. The karyotypes of these lines show alterations such as breaks and translocations affecting the long arm of chromosome 1. Cell lines isolated in several steps by growth in progressively increasing concentrations of albizziin were more resistant to the drug and exhibited up to 300-fold enhancement of asparagine synthetase activity. The multistep albizziin-resistant cell lines usually had expanded chromosomal regions which stained somewhat homogeneously, often on the long arm of chromosome 1. These results suggest that resistance to albizziin in the multistep lines may be due to gene amplification.

Thymidylate synthetase overproduction in 5-fluorodeoxyuridine-resistant mouse fibroblasts

We describe the isolation and characterization of a series of 5-fluorodeoxyuridine (FdUrd)-resistant mouse 3T6 cell lines that overproduce thymidylate synthetase (TS) by up to 50-fold compared with the parental cells. The resistant cells were selected by growing 3T6 cells or a methotrexate-resistant 3T6 cell line (M50L3, isolated previously in our laboratory) in gradually increasing concentrations of FdUrd. Uridine and cytidine were included in the culture medium to reduce toxicity from metabolic products of FdUrd. Cells that were resistant to the drug by virtue of loss of thymidine kinase activity were eliminated by selection in medium containing hypoxanthine, methotrexate, and thymidine. M50L3 cells were found to adapt to FdUrd more readily than 3T6 cells. A number of clones were isolated that were able to grow in the presence of 3 microM (M50L3 derived) or 0.3 microM (3T6 derived) FdUrd. Several were found to overproduce TS by 10 to 50-fold compared with normal 3T6 cells. All were found to have thymidine kinase activity, although the enzyme level was significantly reduced in some clones. The overproduced TS was inactivated by 5-fluorodeoxyuridylic acid at the same concentration as the enzyme from 3T6 cells. TS was purified from the LU3-7 clone (50-fold overproducer) by affinity chromatography on methotrexate-polyacrylamide. The monomer molecular weight was about 38,000, which was the same as the molecular weight of the monomer in 3T6 cells. The overproduction trait was gradually lost (half-life, 3 weeks) when LU3-7 cells were grown in the absence of FdUrd. The overproducing cells will provide an abundant supply of TS and (very likely) its mRNA and may serve as a convenient model system for detailed studies of the regulation of TS gene expression during the cell cycle.

Enzymes of de novo pyrimidine biosynthesis in Babesia rodhaini

The pathway of de novo pyrimidine biosynthesis in the rodent parasitic protozoa Babesia rodhaini has been investigated. Specific activities of five of the six enzymes of the pathway were determined: aspartate transcarbamylase (ATCase: E.C. 2.1.3.2); dihydroorotase (DHOase: E.C. 3.5.2.3); dihydroorotate dehydrogenase (DHO-DHase: E.C. 1.3.3.1); orotate phosphoribosyltransferase (OPRTase: E.C. 2.4.2.10); and orotidine-5'-phosphate decarboxylase (ODCase: E.C. 4.1.1.23). Michaelis constants for ATCase, DHO-DHase, OPRTase, and ODCase were determined in whole homogenates. Several substrate analogs were also investigated as inhibitors and inhibitor constants determined. N-(phosphonacetyl)-L-aspartate was shown to be an inhibitor of the ATCase with an apparent Ki of 7 microM. Dihydro-5-azaorotate inhibited the DHO-DHase (Ki, 16 microM) and 5-azaorotate (Ki, 21 microM) was an inhibitor of the OPRTase. The UMP analog, 6-aza-UMP (Ki, 0.3 microM) was a potent inhibitor of ODCase, while lower levels of inhibition were found with the product, UMP (Ki, 120 microM) and the purine nucleotide, XMP (Ki, 95 microM). Additionally, menoctone, a ubiquinone analog, was shown to inhibit DHO-DHase.

Stepwise isolation and properties of unstable Chinese hamster cell variants that overproduce adenylate deaminase

Addition of coformycin (0.5 microgram/ml) to a culture medium containing adenine causes in Chinese hamster fibroblasts a lethal depletion of IMP. Resistant variants have been recovered, some of which exhibit increased adenylate deaminase activity. (Debatisse et al., J. Cell. Physiol., 106:1-11, 1981). The selective medium was made more specific for the isolation of this class of variants by supplementation with azaserine. The hyperactive variants remained sensitive to coformycin concentrations above that used for their selection and were unstable. Their frequency was not increased by ethyl methane sulfonate mutagenesis. The resistant phenotype and the increased activity of adenylate deaminase behaved as semidominant traits in hybrids. No change was detected in the Km for AMP, the cofactor requirement, or the chromatographic properties of adenylate deaminase in the variants. Through stepwise selection in media supplemented with increasing coformycin concentrations, unstable clones with adenylate deaminase activity up to 150-fold the wild-type level were isolated; from an unstable clone, a stable subclone with reduced resistance and enzyme activity was recovered. Evidence that increased adenylate deaminase activity is the manifestation of overaccumulation of the enzyme protein was supplied by the correlation of enzyme activity with the intensity of a protein band comigrating with purified adenylate deaminase during sodium dodecyl sulfate-polyacrylamide gel electrophoresis of cell extracts. Several unidentified additional bands showed comparable quantitative changes. The striking similarity between the adenylate deaminase-overproducing lines and unstable dihydrofolate reductase-overproducing lines generated by gene amplification strongly suggests that the coformycin-resistant variants also resulted from amplification of an adenylate deaminase gene.

Structure of the gene for CAD, the multifunctional protein that initiates UMP synthesis in Syrian hamster cells

Two adjacent fragments of genomic DNA spanning the gene for CAD, which encodes the first three enzymes of UMP biosynthesis, were cloned from a mutant Syrian hamster cell line containing multiple copies of this gene. The mutant was selected for resistance to N-(phosphonacetyl)-L-aspartate, a potent and specific inhibitor of aspartate transcarbamylase, the second enzyme in the pathway. The sizes and positions of about 37 intervening sequences within the 25-kilobase CAD gene were mapped by electron microscopy, and the locations of the 5' and 3' ends of the 7.9-kilobase CAD mRNA were established by electron microscopy and by other hybridization methods. The coding sequences are small (100 to 400 bases), as are most of the intervening sequences (50 to 300 bases). However, there are also several large intervening sequences of up to 5,000 bases each. Two small cytoplasmic polyadenylated RNAs are transcribed from a region just beyond the 5' end of the CAD gene, and their abundance reflects the degree of gene amplification.

General method for cloning amplified DNA by differential screening with genomic probes

Mutant Syrian hamster cell lines resistant to N-(phosphonacetyl)-L-aspartate, a potent and specific inhibitor of aspartate transcarbamylase, have amplified the gene coding for the multifunctional protein (CAD) that includes this activity. The average amount of DNA amplified is approximately 500 kilobases per gene copy, about 20 times the length of the CAD gene itself. A differential screening method which uses genomic DNAs as probes was developed to isolate recombinant phage containing fragments of amplified DNA. One probe was prepared by reassociating fragments of total genomic DNA from 165-28, a mutant cell line with 190 times the wild-type complement of CAD genes, until all of the sequences repeated about 200 times were annealed and then isolating the double-stranded DNA with hydroxyapatite. This DNA was highly enriched in sequences from the entire amplified region, whereas the same sequences were very rare in DNA prepared similarly from wild-type cells. After both DNAs were labeled by nick translation, highly repeated sequences were removed by hybridization to immobilized total genomic DNA from wild-type cells. A library of cloned DNA fragments from mutant 165-28 was screened with both probes, and nine independent fragments containing about 165 kilobases of amplified DNA, including the CAD gene, have been isolated so far. These cloned DNAs can be used to study the structure of the amplified region, to evaluate the nature of the amplification event, and to investigate gene expression from the amplified DNA. For example, one amplified fragment included a gene coding for a 3.8-kilobase, cytoplasmic, polyadenylated RNA which was overproduced greatly in cells resistant to N-(phosphonacetyl)-L-aspartate. The method for cloning amplified DNA is general and can be used to evaluate the possible involvement of gene amplification in phenomena such as drug resistance, transformation, or differentiation. DNA fragments corresponding to any region amplified about 10-fold or more can be cloned, even if no function for the region is known. The method for removing highly repetitive sequences from genomic DNA probes should also be of general use.

Single-copy and amplified CAD genes in Syrian hamster chromosomes localized by a highly sensitive method for in situ hybridization

Syrian hamster cells resistant to N-(phosphonacetyl)-L-aspartate (PALA), a specific inhibitor of the aspartate transcarbamylase activity of the multifunctional protein CAD, overproduce this protein as a result of amplification of the CAD gene. We have used a sensitive in situ hybridization technique to localize CAD genomes in spreads of metaphase chromosomes from several independent PALA-resistant lines and from wild-type PALA-sensitive cells. The amplified genes were always found within chromosomes, usually in an expanded region of the short arm of chromosome B9. In wild-type cells, the CAD gene was also on the short arm of chromosome B9. In one mutant line, 90 to 100 CAD genes were found within an expanded B9 chromosome and 10 to 15 more were near the distal end of one arm of several different chromosomes. Another line contained most the genes in a telomeric chromosome or large chromosome fragment. The amplified genes were in chromosomal regions that were stained in a banded pattern by trypsin-Giemsa. A few double minute chromosomes were observed in a very small fraction of the total spreads examined. The it situ hybridizations were performed in the presence of 10% dextral sulfate 500, which increases the signal by as much as 100-fold. Using recombinant DNA plasmids nick-translated with [125I]dCTP to high specific radioactivity, 10 CAD genes in a single chromosomal region were revealed after 1 week of autoradiographic exposure, and the position of the unique gene could be seen after 1 month.

Cloning and restriction mapping of the yeast URA2 gene coding for the carbamyl phosphate synthetase aspartate-transcarbamylase complex

Two yeast DNA pools inserted in a hybrid Escherichia coli-yeast vector pFL1 were used to transform E. coli and yeast aspartate-transcarbamylase-less strains to prototrophy. From the first pool--a BamHI yeast DNA digest--a 6.4 kb BamHI fragment was recovered that gave good complementation of the E. coli auxotrophy but poor complementation of the yeast auxotrophy. From the second pool--a partial Sau3A yeast DNA digest--five independent plasmids complementing either E. coli, yeast, or both were recovered. Each of the five plasmids possessed sequences in common with the 6.4 kb BamHI fragment. One of these plasmids, which complemented the two URA2 activities in yeast and which produced a carbamyl-phosphate synthetase, aspartate-transcarbamylase complex sensitive to UTP feedback inhibition contained the full URA2 gene. A restriction map of the URA2 gene has been constructed and seven different consecutive segments have been recloned in pBR322 to measure their hybridization with URA2 messenger RNA, allowing us to estimate the limits of the gene.

Selection of tunicamycin-resistant Chinese hamster ovary cells with increased N-acetylglucosaminyltransferase activity

Chinese hamster ovary (CHO) cells resistant to the antibiotic tunicamycin (TM) have been isolated by a stepwise selection procedure with progressive increments of TM added to the medium. TM inhibits asparagine-linked glycoprotein biosynthesis by blocking the transfer of N-acetylglucosamine-1-phosphate from UDP-N-acetylglucosamine to the lipid carrier. The TM-resistant cells exhibited a 200-fold increase in their LD50 for TM and were morphologically distinct from the parental cells. The rate of asparagine-linked glycoprotein biosynthesis was the same for wild-type and TM-resistant cells. Membrane preparations from TM-resistant cells cultured for 16 d in the absence of TM had a 15-fold increase in the specific activity of the UDP-N-acetylglucosamine:dolichol phosphate N-acetylglucosamine-1-phosphate transferase as compared to membranes of wild-type cells. The products of the in vitro assay were N-acetylglucosaminylpyrophosphoryl-lipid and N,N'-diacetylchitobiosylpyrophosphoryl-lipid for membranes from both TM-resistant and wild-type cells. The transferase activity present in membrane preparations from wild-type of TM-resistant cells was inhibited by comparable levels of TM. The data presented are consistent with overproduction of enzyme as the mechanism of resistance in these variant CHO cells.

Thymidylate synthetase overproduction in 5-fluorodeoxyuridine-resistant mouse fibroblasts

We describe the isolation and characterization of a series of 5-fluorodeoxyuridine (FdUrd)-resistant mouse 3T6 cell lines that overproduce thymidylate synthetase (TS) by up to 50-fold compared with the parental cells. The resistant cells were selected by growing 3T6 cells or a methotrexate-resistant 3T6 cell line (M50L3, isolated previously in our laboratory) in gradually increasing concentrations of FdUrd. Uridine and cytidine were included in the culture medium to reduce toxicity from metabolic products of FdUrd. Cells that were resistant to the drug by virtue of loss of thymidine kinase activity were eliminated by selection in medium containing hypoxanthine, methotrexate, and thymidine. M50L3 cells were found to adapt to FdUrd more readily than 3T6 cells. A number of clones were isolated that were able to grow in the presence of 3 microM (M50L3 derived) or 0.3 microM (3T6 derived) FdUrd. Several were found to overproduce TS by 10 to 50-fold compared with normal 3T6 cells. All were found to have thymidine kinase activity, although the enzyme level was significantly reduced in some clones. The overproduced TS was inactivated by 5-fluorodeoxyuridylic acid at the same concentration as the enzyme from 3T6 cells. TS was purified from the LU3-7 clone (50-fold overproducer) by affinity chromatography on methotrexate-polyacrylamide. The monomer molecular weight was about 38,000, which was the same as the molecular weight of the monomer in 3T6 cells. The overproduction trait was gradually lost (half-life, 3 weeks) when LU3-7 cells were grown in the absence of FdUrd. The overproducing cells will provide an abundant supply of TS and (very likely) its mRNA and may serve as a convenient model system for detailed studies of the regulation of TS gene expression during the cell cycle.

N-(phosphonacetyl)-L-aspartate (PALA) in advanced malignant melanoma: a phase II trial of the EORTC Malignant Melanoma Cooperative Group

Thirty-nine patients with measurable advanced malignant melanoma were entered in a phase II trial with PALA. Among the 36 evaluable patients there were 18 men and 18 women, with a median age of 53 yr (29-73) and a median performance status (Karnofsky) of 100 (50-100). Indicator lesion consisted essentially of soft tissue lesions (29 patients) and/or lung metastases (9 patients). Only three patients had received prior chemotherapy. PALA was given as a 60-min i.v. infusion at a daily dose of 2.5 g/m2 for two consecutive days. Courses were repeated every two weeks. A median number of 3 courses (2-8) were administered. Partial response (greater than 50%) was obtained in 4 patients for 6-17 weeks. Eight patients had stable disease after 3 courses of PALA and 24 had progressive disease. Toxic effects were generally mild to moderate and mainly included cutaneous toxicity, nausea and vomiting, stomatitis, and diarrhea. Myelosuppression was rare and negligible. It is concluded that PALA given at the dose schedule selected for this trial is fairly well tolerated and has borderline antitumor activity in good-risk patients with advanced malignant melanoma.

Isolation and characterization of Chinese hamster ovary cell mutants resistant to the amino acid analog beta-aspartyl hydroxamate

Chinese hamster ovary cell lines which are resistant to an amino acid analog, beta-aspartyl hydroxamate, have been isolated and characterized. Mutants resistant to 100-150 microM beta-aspartyl hydroxamate arose from ethyl methane sulfonate-treated parental lines at frequencies of 3.4 x 10(-6) to 1.3 x 10(-7). The mutants fell into at least two genetic classes: 18% of the mutants behaved codominantly in hybrids, the others recessively. Complementation studies indicated that all the recessive mutants belonged to the same class. Mutants selected after one step of mutagenesis overproduce the enzyme asparagine synthetase constitutively with four- to sixfold increases in specific activities over the basal levels of the parental lines. beta-Aspartyl hydroxamate-resistant cell lines with up to 20-fold elevations in asparagine synthetase activity have been isolated after two steps of mutageneis. In addition, highly resistant lines have been selected by long-term growth of a dominant mutant in increasing concentrations of the drug. Resistance in the latter appears to be due not only to overproduction of asparagine synthetase but also to an alteration in the affinity of the enzyme for beta-aspartyl hydroxamate.

Isolation of deoxycoformycin-resistant cells with increased levels of adenosine deaminase

Deoxycoformycin (dCF) is a specific inhibitor of adenosine deaminase (ADA). Rat hepatoma cells deficient in adenosine kinase and growing on adenosine as the sole carbon source are sensitive to the lethal action of dCF. Mutants resistant to dCF arise spontaneously with a frequency of 1.7 x 10(-6). This frequency is increased to 2.6 x 10(-5) by prior mutagenesis with ethyl methane sulfonate. Initially, dCF-resistant cell lines have 3-10 times the level of adenosine deaminase when compared to sensitive parental cells. Subsequent selection of mutants resistant to increased concentrations of dCF results in cells with a 15- to 30-fold increase in ADA levels. Quantitative immunoprecipitation tests indicate that the increase in enzyme activity in one line tested is due to an increase in the number of ADA molecules. These dCF' cell lines may serve as a model system to study the human disease state, hereditary hemolytic anemia, which is associated with increased levels of ADA.

General method for cloning amplified DNA by differential screening with genomic probes

Mutant Syrian hamster cell lines resistant to N-(phosphonacetyl)-L-aspartate, a potent and specific inhibitor of aspartate transcarbamylase, have amplified the gene coding for the multifunctional protein (CAD) that includes this activity. The average amount of DNA amplified is approximately 500 kilobases per gene copy, about 20 times the length of the CAD gene itself. A differential screening method which uses genomic DNAs as probes was developed to isolate recombinant phage containing fragments of amplified DNA. One probe was prepared by reassociating fragments of total genomic DNA from 165-28, a mutant cell line with 190 times the wild-type complement of CAD genes, until all of the sequences repeated about 200 times were annealed and then isolating the double-stranded DNA with hydroxyapatite. This DNA was highly enriched in sequences from the entire amplified region, whereas the same sequences were very rare in DNA prepared similarly from wild-type cells. After both DNAs were labeled by nick translation, highly repeated sequences were removed by hybridization to immobilized total genomic DNA from wild-type cells. A library of cloned DNA fragments from mutant 165-28 was screened with both probes, and nine independent fragments containing about 165 kilobases of amplified DNA, including the CAD gene, have been isolated so far. These cloned DNAs can be used to study the structure of the amplified region, to evaluate the nature of the amplification event, and to investigate gene expression from the amplified DNA. For example, one amplified fragment included a gene coding for a 3.8-kilobase, cytoplasmic, polyadenylated RNA which was overproduced greatly in cells resistant to N-(phosphonacetyl)-L-aspartate. The method for cloning amplified DNA is general and can be used to evaluate the possible involvement of gene amplification in phenomena such as drug resistance, transformation, or differentiation. DNA fragments corresponding to any region amplified about 10-fold or more can be cloned, even if no function for the region is known. The method for removing highly repetitive sequences from genomic DNA probes should also be of general use.

Structure of the gene for CAD, the multifunctional protein that initiates UMP synthesis in Syrian hamster cells

Two adjacent fragments of genomic DNA spanning the gene for CAD, which encodes the first three enzymes of UMP biosynthesis, were cloned from a mutant Syrian hamster cell line containing multiple copies of this gene. The mutant was selected for resistance to N-(phosphonacetyl)-L-aspartate, a potent and specific inhibitor of aspartate transcarbamylase, the second enzyme in the pathway. The sizes and positions of about 37 intervening sequences within the 25-kilobase CAD gene were mapped by electron microscopy, and the locations of the 5' and 3' ends of the 7.9-kilobase CAD mRNA were established by electron microscopy and by other hybridization methods. The coding sequences are small (100 to 400 bases), as are most of the intervening sequences (50 to 300 bases). However, there are also several large intervening sequences of up to 5,000 bases each. Two small cytoplasmic polyadenylated RNAs are transcribed from a region just beyond the 5' end of the CAD gene, and their abundance reflects the degree of gene amplification.

Single-copy and amplified CAD genes in Syrian hamster chromosomes localized by a highly sensitive method for in situ hybridization

Syrian hamster cells resistant to N-(phosphonacetyl)-L-aspartate (PALA), a specific inhibitor of the aspartate transcarbamylase activity of the multifunctional protein CAD, overproduce this protein as a result of amplification of the CAD gene. We have used a sensitive in situ hybridization technique to localize CAD genomes in spreads of metaphase chromosomes from several independent PALA-resistant lines and from wild-type PALA-sensitive cells. The amplified genes were always found within chromosomes, usually in an expanded region of the short arm of chromosome B9. In wild-type cells, the CAD gene was also on the short arm of chromosome B9. In one mutant line, 90 to 100 CAD genes were found within an expanded B9 chromosome and 10 to 15 more were near the distal end of one arm of several different chromosomes. Another line contained most the genes in a telomeric chromosome or large chromosome fragment. The amplified genes were in chromosomal regions that were stained in a banded pattern by trypsin-Giemsa. A few double minute chromosomes were observed in a very small fraction of the total spreads examined. The it situ hybridizations were performed in the presence of 10% dextral sulfate 500, which increases the signal by as much as 100-fold. Using recombinant DNA plasmids nick-translated with [125I]dCTP to high specific radioactivity, 10 CAD genes in a single chromosomal region were revealed after 1 week of autoradiographic exposure, and the position of the unique gene could be seen after 1 month.

N-(phosphonacetyl)-L-aspartate (PALA) in advanced soft tissue sarcoma: a phase II trial of the EORTC soft tissue sarcoma group

Thirty-six patients with measurable or evaluable advanced soft tissue sarcoma were entered in a phase II trial with PALA. Among the 27 evaluable patients, 15 were men, the median age was 55 yr (16-69) and the median performance status (Karnofsky) was 80 (50-100). Most patients had leiomyosarcoma (8), liposarcoma (3), neurofibrosarcoma (3), synovial cell sarcoma (3), or undifferentiated sarcoma (3). Indicator lesions consisted essentially of lung metastases (21) and/or soft tissue lesions (14). All patients had received prior chemotherapy with 1-5 regimens and 6 had achieved objective response with these previous treatments. PALA was given as a 60-min i.v. infusion at a daily dose of 2.5 g/m2 for two consecutive days. Courses were repeated every two weeks. A median number of 3 courses (2-17) were administered. Partial remission (greater than 50%) was obtained in one patient with a liposarcoma who had also responded to prior combination chemotherapy. This single response to PALA lasted 6 weeks from initiation of therapy. Four patients had unchanged disease after 6+ courses of PALA and 22 had progressive disease. Toxic effects were generally mild to moderate and included cutaneous toxicity (17), diarrhea (14), stomatitis (13), ocular manifestations, consisting of conjunctivitis, corneal ulceration and/or photophobia (11), nausea and vomiting (6) and, possibly, seizures (2). There was no evidence of drug-related myelosuppression. It is concluded that PALA given at the dose schedule selected for this trial has no significant antitumor activity in advanced soft tissue sarcoma previously treated with chemotherapy.

Effects of acivicin and PALA, singly and in combination, on de novo pyrimidine biosynthesis

The inhibitory activities of two oncolytic amino acid analogs, acivicin and N-(phosphonacetyl)-L-aspartate, on pyrimidine biosynthesis have been examined in a murine tumor line, the Lewis lung carcinoma. Acivicin, an antimetabolite elaborated by Streptomyces sviceus, inhibits a spectrum of L-glutamine utilizing enzymes including carbamoyl phosphate synthetase II, the inaugurating enzyme of de novo pyrimidine biosynthesis. Profound inhibition of carbamoyl phosphate synthetase II activity by acivicin is demonstrated in vitro as well as in vivo. N-(Phosphonacetyl)-L-aspartate, a rationally-designed transition-state analog of the reaction catalyzed by L-aspartate transcarbamylase, the second enzyme of the pathway, is a potent and specific inhibitor of L-aspartate transcarbamylase. Both agents, at therapeutic doses, exert marked inhibitions of their respective target enzymes and impede flux through the pathway as monitored by inhibition of pyrazofurin-provoked accumulation of orotate and orotidine. Additionally, synergistic effects are observed when acivicin and N-(phosphonacetyl)-L-aspartate are used in combination, both in terms of biochemical and therapeutic endpoints. The salient features of the actions of these drugs on pyrimidine biosynthesis in the Lewis lung carcinoma are summarized in Table 6. Comparison of the effects of acivicin with those of N-(phosphonacetyl)-L-aspartate suggest divergent actions on nucleotide biosynthesis. In spite of its pronounced sensitivity to acivicin, carbamoyl phosphate synthetase II appears not to be a critical target for the antineoplastic activity of this drug.

The cloning and reintroduction into animal cells of a functional CAD gene, a dominant amplifiable genetic marker

Rodent cells resistant to PALA, a specific inhibitor of the aspartate transcarbamylase activity of the multifunctional CAD protein, overproduce CAD as a result of amplification of the CAD gene. We cloned a functional CAD gene from Syrian hamster cells using a cosmid vector. Two independently isolated cosmids containing CAD genes have inserts 40 and 45 kb long. We introduced the cloned genes into CAD-deficient Chinese hamster ovary (CHO) cell mutants by fusing them with protoplasts of Escherichia coli containing the cosmids. We also introduced the cloned genes into wild-type CHO cells by selecting cells that became resistant to high concentrations of PALA following protoplast fusion. The transformants of the mutant and wild-type CHO cells contain multiple active copies of the donated Syrian hamster CAD genes. The cloned genes in three independent transformants are integrated into host-cell chromosomes at single locations identified by in situ hybridization. In two of these transformants, the genes are located in one X chromosome or in a chromosome resembling the X. In the third case, the genes are located in a small metacentric or rearranged chromosome.

Controlled proteolysis of the multifunctional protein that initiates pyrimidine biosynthesis in mammalian cells: evidence for discrete structural domains

The multifunctional protein that initiates de novo pyrimidine biosynthesis in mammalian cells carries carbamoylphosphate synthetase, aspartate transcarbamylase (aspartate carbamoyltransferase), and dihydro-orotase activities on a single 215,000-dalton polypeptide chain. Kinetic studies of the controlled proteolysis of the molecule by elastase showed that the protein was not attacked at random by the protease but rather was successively cleaved into at least six well-defined proteolytic fragments. The initial cleavage converted the intact molecule into a 190,000-dalton species which appeared to retain all of the catalytic and regulatory functions of the native protein. This species was subsequently cleaved into two fragments, 150,000 and 40,000 daltons. The 40,000-dalton species, which carried the aspartate transcarbamylase activity, was resistant to further proteolysis; the 150,000-dalton polypeptide, which carried carbamoyl-phosphate synthetase and dihydro-orotase activities, underwent further digestion to 140,000 daltons. Continued proteolysis produced two species, 79,000 and 45,000 daltons; like the 40,000-dalton species, these were stable against further elastase digestion. The aspartate transcarbamylase and dihydro-orotase activities and the regulatory functions were preserved throughout the course of digestion; the carbamoylphosphate synthetase activity was more labile. By using sucrose gradient centrifugation and ion exchange chromatography, the 40,000- and 45,000-dalton species have been isolated. The 40,000-dalton fragment was found to have only aspartate transcarbamylase activity; the 45,000-dalton fragment has only dihydro-orotase activity. These experiments showed that this multifunctional protein is organized as discrete structural domains in which regions of the polypeptide chain are autonomously folded into separate functional units.

Genetic and biochemical properties of revertants at the rudimentary locus in Drosophila melanogaster

The rudimentary (r) locus of Drosophila melanogaster encodes the first three enzymes for the de novo synthesis of pyrimidines: carbamyl phosphate synthase (CPSase), aspartyl transcarbamylase (ATCase), and dihydroorotase (DHOase). Revertants of leaky r mutants deficient in CPSase have been recovered. Of those examined, most (28/30) appear to result from reversion of the original mutation or a second site mutation in the mutant gene: They map to the r locus, have levels of CPSase and Km's for glutamine similar to wild type, and have comparatively little change in ATCase and DHOase activity. Two of the revertants appear to involve regulatory mutations that lead to an increase in defective CPSase molecules: They map to the r locus, have Km's for glutamine like that of the CPSase in the progenitor stock, and lead to relatively small increases in CPSase activity that are roughly paralleled by increases in ATCase and DHOase. The recovery of these putative regulatory mutations increasing the activity of the three enzymes supports the conclusion that the three activities are part of a trifunctional polypeptide or that their genes are transcribed together as parts of a multicistronic transcript.

The potentiation of adenine toxicity to Chinese hamster cells by coformycin: suppression in mutants with altered regulation of purine biosynthesis or increased adenylate-deaminase activity

When added to medium containing coformycin (2 microM or above), adenine is lethal to Chinese hamster fibroblasts at the concentration inhibiting de novo purine biosynthesis (Debatisse and Buttin, '77b). Rescue by hypoxanthine suggested that cells die of IMP starvation when the analog can turn off deamination of both adenosine and adenylate. As predicted from this hypothesis, two classes of variants resistant to the mixture of coformycin + adenine have been isolated: Class 1 variants have altered control of de novo IMP biosynthesis; they fall into two subclasses on the basis of their resistance to adenosine. Class 2 variants have a 6-10-fold increased level of AMP-deaminase (E.C.: 3.5.4.6); their growth in the selective medium is temperature-dependent, a property accounted for by the observation that cell growth in the presence of coformycin imposes a gradual thermodependent decay of specific AMP-deaminase activity in both wild-type and variant lines. This control by coformycin of AMP-deaminase activity is unaltered in mutants deficient in the four activities of adenosine-kinase. APRT, HGPRT and deoxycytidine-kinase. Most of the resistant variants are unstable and exhibit either increased or reduced resistance, depending on prolonged growth in selective or normal medium.