Isolation of hydroxyurea-resistant CHO cells with altered levels of ribonucleotide reductase

Differences in the properties of mammalian ribonucleotide reductase toward its substrates

These studies, using three different reagents, show that the substrate properties of ribonucleotide reductase are specific but can be variable depending upon the nature of the interaction of the reagent with the holoenzyme or the individual subunit. Etheno-CDP, which acts as a competitive inhibitor with respect to CDP, interacts with the active site of the holoenzyme. This interaction was the result of rather tight structural requirements as epsilon-ADP did not result in a similar level of inhibition of either CDP or ADP reductase activities. The YL 1/2 antibody which binds very tightly to the NHI subunit has a much greater effect on CDP reductase activity than ADP reductase activity. The nonapeptide that corresponds to the C-terminus amino acid sequence of the NHI subunit and which binds to the EB subunit and aborts the formation of the NHI-EB active complex has a greater effect on ADP reductase activity than on CDP reductase activity. The use of reagents such as these can be helpful in dissecting the subtle but important differences in the substrate properties of mammalian ribonucleotide reductase.

Long-term intravenous hydroxyurea infusions in patients with advanced cancer. A phase I trial

BACKGROUND

Hydroxyurea is an S-phase specific drug. Constant exposure of tumor cells with a low S-phase fraction to the agent may result in improved cell kill. Because of its short half-life, a continuous intravenous infusion may result in better tumor exposure than intake by mouth. The goal of this trial was to find the longest tolerable duration of a continued intravenous infusion of hydroxyurea (HU) given at escalating doses.

METHODS

Eligible patients had histologically confirmed cancer without effective alternate therapy, normal blood counts, liver and kidney function. After giving informed consent, the infusion began via a permanent indwelling catheter utilizing a portable pump. Dose levels (in g/m2/d) were 0.5 for level I, 1.0 for level II, 1.66 for level III, and 2.5 for level IV.

RESULTS

Fourteen patients were entered. Five were men. Median age was 56 years of age (range: 32-67), median performance status 1 (range: 0-2). Diagnoses were as follows: colorectal cancer, seven; unknown primary site, three; breast cancer, two; melanoma, one; and adenoid-cystic carcinoma, one. Nine patients were pretreated with chemotherapy. Three patients were entered per dose level, except on level I, were five were entered. The mean duration of infusion was 12 weeks on level I, 5 weeks on II, 3 on III, 1 on IV. Toxicity included leukopenia below 2.0 K/mm3 in one patient each on levels III and IV, thrombocytopenia below 100 K/mm3 in one patient each on levels II and IV, and stomatitis in three patients (one on level II and two on IV). This toxicity was dose limiting. One patient on level III, with an unknown primary, had an objective response. HU levels were measured by a modification of the Fabricius-Rajewsky method. Mean plasma levels in micrograms per milliliter (SEM) were as follows: level I, 3.6 (0.23); level II, 5.1 (0.57); level III, 10.1 (1.55); and level IV, 16.7 (one point). Fetal hemoglobin rose two-fold and five-fold in two patients on level I after 9 and 16 weeks on therapy, respectively.

CONCLUSIONS

HU as a continuous intravenous infusion is well tolerated; the maximum duration of therapy is related inversely with the dose given. No major antitumor activity was seen. The greatest interest in the drug rests in its future use as a modulator and radiation potentiator. The increase in hemoglobin F was of interest and may be important in the treatment of sickle cell disease.

The gene for a novel protein, a member of the protein disulphide isomerase/form I phosphoinositide-specific phospholipase C family, is amplified in hydroxyurea-resistant cells

Cell lines selected in multiple steps for increasing resistance to hydroxyurea have been shown to have corresponding increases in ribonucleotide reductase activity. We have isolated a number of cDNA clones from a cDNA library constructed from a highly hydroxyurea-resistant hamster cell line, 600H, in which the activity of ribonucleotide reductase is elevated more than 80-fold. These clones correspond to genomic DNA sequences amplified in the 600H cell line compared with the V79 parental line. One of these cDNA clones, termed P5, codes for a 50 kDa protein detected by in vitro translation of poly(A)+ RNA isolated by hybridization/selection. The cDNA sequence contains a single open reading frame of 1317 nucleotides which encodes a polypeptide of 439 amino acids. The amino acid sequence deduced from the cDNA insert contains two copies of the 11-amino-acid sequence Val-Glu-Phe-Tyr-Ala-Pro-Trp-Cys-Gly-His-Cys. Duplicate copies of this sequence also occur in the active site of rat and human protein disulphide isomerase (also known as the beta-subunit of human prolyl 4-hydroxylase, tri-iodothyronine-binding protein) and in Form I phosphoinositide-specific phospholipase C, indicating that P5 falls into this newly defined superfamily of proteins. Genomic sequences similar to the cDNA clone are amplified 10-20-fold in hamster cells selected for resistance to increasing concentrations of hydroxyurea, a phenomenon observed earlier with cDNA clones for the M2 subunit of ribonucleotide reductase and ornithine decarboxylase. RNA blots probed with P5 cDNA show two poly(A)+ RNA species which are elevated in hydroxyurea-resistant cells.

Selective resistance of L1210 cell lines to inhibitors directed at the subunits of ribonucleotide reductase

L1210 cell lines were generated which were resistant to specific ribonucleotide reductase inhibitors. Hydroxyurea-resistant L1210 cells (HU-7) were cross-resistant to IMPY but sensitive to deoxyadenosine and deoxyguanosine. Deoxyadenosine-resistant L1210 cells (Y-8) were cross-resistant to 2-fluorodeoxyadenosine and showed only a small increase in resistance to hydroxyurea or IMPY. L1210 cells which were generated in the presence of deoxyadenosine/EHNA/IMPY/Desferal were markedly resistant to deoxyadenosine, deoxyguanosine and 2-fluorodeoxyadenosine with moderate increases in resistance to IMPY. The HU-7, Y-8 and ED2 cell lines were sensitive to the inhibitory effects of MAIQ and HAG-IQ. The HU-7 L1210 cell line had elevated levels of ribonucleotide reductase activity and this activity showed normal inhibition by hydroxyurea, IMPY, dATP, dGTP and dTTP. The Y-8 L1210 cell line did not have elevated levels of ribonucleotide reductase activity, but had altered allosteric properties relative to dATP. The ED2 L1210 cell line had elevated levels of ribonucleotide reductase activity and had altered allosteric properties relative to dATP. These data show that resistance to ribonucleotide reductase inhibitors is specifically generated in response to the particular drug. The biochemical basis can be related to either increased levels of ribonucleotide reductase activity or loss of feedback control by dATP or both.

Ribonucleotide reductase--new twists in an old tale

Although they are proliferatively quiescent, the cells in the intact adult rat liver express the gene coding for the M1 subunit of ribonucleotide reductase. But since they do not need deoxyribonucleotides, they promptly inactivate the 88 to 90 kDa M1 products and degrade them into 40 kDa fragments. Partial hepatectomy signals the remaining cells to start proliferating. Two hours before the onset of DNA replication, around 16 to 18 hr after partial hepatectomy, the cells start accumulating a large pool of functional ribonucleotide reductase M2 subunits. Near the end of the G1 build-up the cells step up M1 gene expression, stop inactivating, and reduce the degradation of the M1 products. The accumulating functional 88 to 90 kDa M1 subunits, each with more than one catalytic site, couple with functional M2 subunits to produce active ribonucleotide reductase holoenzyme which accumulates in the outer nuclear membrane from which they supply deoxyribonucleotide precursors to intranuclear replication enzymes. At the end of the S phase, the cell reduces M1 gene expression and resumes degrading 88 to 90 kDa M1 subunits. At least some of the 40 kDa M1 fragments are still active and can form partially active "holoenzymes" when mixed with a standard preparation of functional M2 subunits. The M1 control mechanism appears not to operate in hepatoma cells and Ehrlich ascites tumor cells, both of which maintain a pool of undegraded 88 to 90 kDa M1 components.

Relationships between reversion of hydroxyurea resistance in hamster cells and the co-amplification of ribonucleotide reductase M2 component, ornithine decarboxylase and P5-8 genes

Hydroxyurea is a specific inhibitor of ribonucleotide reductase, which is a rate-limiting enzyme activity in DNA synthesis. Cells selected for resistance to hydroxyurea contain alterations in ribonucleotide reductase activity. An unstable hydroxyurea resistant population of hamster cells has been used to isolate a stable drug resistant cell line, and two stable revertant lines with different sensitivities to hydroxyurea cytotoxicity and different ribonucleotide reductase activity levels. We show for the first time that a decrease in hydroxyurea resistance is accompanied by a parallel decline in gene copies for the M2 component of ribonucleotide reductase, ornithine decarboxylase and a gene of unknown function called p5-8, indicating that the co-amplification of the three genes is associated with drug resistance, and supporting the concept that M2, ornithine decarboxylase and p5-8 are closely linked, and form part of a single amplicon in hamster cells.

Purine deoxyribonucleosides counteract effects of hydroxyurea on deoxyribonucleoside triphosphate pools and DNA synthesis

Inhibition of cell growth and DNA synthesis by hydroxyurea is thought to occur via an effect on the enzyme ribonucleotide reductase leading to a block of deoxyribonucleotide synthesis. Earlier attempts to bypass such a block by delivering deoxyribonucleosides to the medium of cultured cells have given equivocal results. Complications arise in such experiments from the specificity of the phosphorylating enzymes since 3 of the 4 deoxyribonucleosides are substrates for the same enzyme, with widely differing Km values, and from allosteric effects exerted by deoxyribonucleotides. We simplify this situation by using a mutant hamster V79 line that lacks the enzyme dCMP deaminase. The cells contain a 20-fold enlarged dCTP pool and require thymidine for optimal growth. Concentrations of hydroxyurea (50 or 100 microM) that in short-term experiments inhibited DNA synthesis depleted the dATP pool without seriously affecting pyrimidine deoxyribonucleotide pools. The dATP pool could be restored by addition of deoxyadenosine but this depleted the dGTP pool. This depletion could be counteracted by the simultaneous addition of deoxyguanosine but then critically depended on the relative concentrations of the two purine deoxyribonucleosides, with optimal results at 1 microM deoxyadenosine + 100 microM deoxyguanosine. Under those conditions the inhibition of DNA synthesis by hydroxyurea was partially reversed.

Reversion of hydroxyurea resistance, decline in ribonucleotide reductase activity, and loss of M2 gene amplification

A key rate-limiting reaction in the synthesis of DNA is catalyzed by ribonucleotide reductase, the enzyme which reduces ribonucleotides to provide the deoxyribonucleotide precursors of DNA. The antitumor agent, hydroxyurea, is a specific inhibitor of this enzyme and has been used in the selection of drug resistant mammalian cell lines altered in ribonucleotide reductase activity. An unstable hydroxyurea resistant population of mammalian cells with elevated ribonucleotide reductase activity has been used to isolate three stable subclones with varying sensitivities to hydroxyurea cytotoxicity and levels of ribonucleotide reductase activities. These subclones have been analyzed at the molecular level with cDNA probes encoding the two nonidentical subunits of ribonucleotide reductase (M1 and M2). Although no significant differences in M1 mRNA levels or gene copy numbers were detected between the three cell lines, a strong correlation between cellular resistance, enzyme activity, M2 mRNA and M2 gene copies was observed. This is the first demonstration that reversion of hydroxyurea resistance is directly linked to a decrease in M2 mRNA levels and M2 gene copy number, and strongly supports the concept that M2 gene amplification is an important mechanism for achieving resistance to this antitumor agent through elevations in ribonucleotide reductase.

Gene for M1 subunit of ribonucleotide reductase is amplified in hydroxyurea-resistant hamster cells

The hydroxyurea-resistant Chinese hamster cell line 600H has been shown to have greatly elevated quantities of ribonucleotide reductase. This increase in enzyme activity is due to an increased level of both the M1 and M2 subunit activities. The M1 subunit has been purified from the 600H cell line and shown to consist of a series of six protein spots with apparent molecular weights of 88,000 daltons, but with varying isoelectric points in the range of pH 6.5-7.0. Western blot analyses with antisera against the M1 and M2 proteins indicated that both subunit proteins are present in elevated quantities in the 600H cell line when compared to the wild-type V79 cell line. Southern blot analyses with genomic DNA from the series of stepwise-selected hydroxyurea-resistant cell lines leading to 600H showed that, in latter steps of selection, genomic sequences homologous to a mouse M1 cDNA have undergone a fivefold amplification. This was accompanied by a four- to eightfold increase in the single M1 homologous mRNA.

Altered expression of ribonucleotide reductase and role of M2 gene amplification in hydroxyurea-resistant hamster, mouse, rat, and human cell lines

Five hamster, mouse, and rat cell lines resistant to the cytotoxic effects of hydroxyurea have been characterized. All cell lines contained increased ribonucleotide reductase activity, elevated levels of the M2 component of ribonucleotide reductase as judged by electron paramagnetic resonance spectroscopy, and increased copies of M2 mRNA as determined by Northern blot analysis. Two species of M2 mRNA were detected in rodent cell lines, a high-molecular-weight species of approximately 3.4 kb in hamster and rat cells and about 2.1 kb in mouse cells. The low molecular-weight M2 mRNA was about 1.6 kb in all rodent lines. Northern blot analysis showed that the mRNA for the other component of ribonucleotide reductase, M1, was not markedly elevated in the drug-resistant cells and existed as a single 3.1-kb species. Four of the five resistant lines contained an M2 gene amplification as determined by Southern blot analysis, providing direct evidence to support earlier suggestions that hydroxyurea resistance is often accompanied by amplification of a ribonucleotide reductase gene. An increase in gene dosage was detected even in cells exhibiting only modest drug-resistance properties. No evidence for amplification of the M1 gene of ribonucleotide reductase was found. In keeping with these observations with drug-resistant rodent lines, a human (HeLa) cell line resistant to hydroxyurea was also found to contain increased levels of two M2 mRNA species (about 3.4 and 1.6 kb) and exhibited M2 gene amplification. One hamster cell line resembled the other resistant rodent lines in cellular characteristics but did not show amplification of either the M1 or M2 gene, providing an example of a drug-resistant mechanism in which an elevation of M2 mRNA has occurred without a concomitant increase in M2 gene copy number.

Unresolved issues in the study of mammalian ribonucleotide reductase

Although research on mammalian ribonucleotide reductase and its role in cell replication has been intensified in recent years, there remain several areas in which there is not uniform agreement with respect to several of its important properties. The major issues include: 1) whether there is one enzyme which catalyzes the reduction of all four substrates or several enzymes for the four substrates; 2) whether the two subunits are coordinately regulated as the cells pass from G1 to S during the cell cycle; if not which subunit represents the limiting component and what are the respective half-lives of the individual subunits; 3) whether the allosteric regulation which has been demonstrated in the test tube is the actual mechanism in the intact cells; and 4) is mammalian ribonucleotide reductase part of an enzyme complex which channels ribonucleoside diphosphates into DNA. The data which have appeared in the literature are discussed in the context of these unresolved questions.

Dominant mutation in mouse cells associated with resistance to Hoechst 33258 dye, but sensitivity to ultraviolet light and DNA base-damaging compounds

A spontaneous derivative of murine L tk- cells has been isolated which has gained a resistance to the cytostatic/lethal effects of high concentrations of Hoechst 33258. The resistant clone HoeR-415 was at least 20-fold more resistant to the dye (D10 dose). HoeR-415 cells have a normal response to X-rays and mitomycin-C and colchicine but were found to show a small sensitivity to UV light, 4NQO, and EMS (1.4, 1.6, and 1.6-fold lower D10 doses, respectively). HoeR-415 cells do not show an increased mutability by EMS. The HoeR phenotype was found to be codominant in hybrids. In order to explain these various characteristics, we suggest that the HoeR-415 mutation may result in an altered topoisomerase activity. Consistent with this we find HoeR-415 cells have an increased sensitivity to novobiocin.

Characterization of a mouse cell line selected for hydroxyurea resistance by a stepwise procedure: drug-dependent overproduction of ribonucleotide reductase activity

Hydroxyurea was used as a selective agent in culture, to isolate by a stepwise procedure, a unique mouse L cell line called LHF which exhibited a stable resistance to high concentrations of drug (5 mM). LHF cells contained an elevation in ribonucleotide reductase activity which depended upon whether cells were previously cultured in the presence or absence of hydroxyurea. M1 immunoprecipitation and M2 titration experiments indicated that both ribonucleotide reductase subunits were elevated in drug-resistant cells. Interestingly, a very large drug-dependent change in the M2 activity (about a 100-fold) was observed. Studies on enzyme activity with cycloheximide and actinomycin D indicated that the hydroxyurea-dependent increase in activity required de novo protein synthesis and transcriptional activity. These results are different from other ribonucleotide reductase overproducing cell lines previously described, and indicate that hydroxyurea modulates enzyme activity by an interesting mechanism.

Comparison of glycine metabolism in mouse lymphoma cells either sensitive or resistant to L-asparaginase

Previous work suggested a relationship between glycine metabolism and the effect of L-asparaginase upon tumor cells. Therefore, L5178Y (sensitive) or L5178Y/L-ASE (resistant) ascites lymphoma cells were incubated with 14C-labeled glyoxylate, glycine, serine, or asparagine, and the metabolism to other amino acids was measured by high performance liquid chromatography. Metabolic differences between the two cells lines were found. Under control conditions, the interconversion rate of glycine and serine via serine hydroxymethyltransferase (SHMT) was higher in sensitive than in resistant cells. The transformation rate of glyoxylate to serine was also higher in sensitive cells. These results may indicate a difference in the activity of SHMT. An alternate explanation would be that transport or diffusion of serine and glycine into sensitive cells is greater than into resistant cells. Several crucial metabolic differences were observed between the two cell types when L-asparaginase was added. A key difference is the decrease of glycine synthesis from glyoxylate observed in the sensitive cells compared to resistant cells which show no change. This suggests that asparagine is used for transamination of glyoxylate. Also, only sensitive cells appear to compensate for L-asparaginase-induced loss of glycine formation from glyoxylate by increasing glycine synthesis from serine. Alterations in sensitive tumor glycine metabolism may be an important function of L-asparaginase anticancer activity.

Quantitative genetic analysis of tumor progression

Metastasis and resistance to chemotherapy are common features of progressed cancers. With respect to the latter phenotype, it is thought that during tumor growth drug-resistant cells arise spontaneously at rates characteristic of the genetic alterations involved. On application of chemotherapy, such variant tumor cells are more likely to survive, and they may eventually dominate, resulting in a non-responsive malignancy. Aspects of this model have been confirmed in a number of experimental systems and in patients. In contrast to our understanding of drug resistance, steps involved in the progression to metastatic spread of tumor cells are much less well-understood. In this review we describe methodologies of quantitative genetic analysis with reference to development of drug resistance. We then describe attempts by ourselves and others to use a similar approach to investigate metastatic properties. Based on these studies, we have proposed the quantitative 'dynamic heterogeneity' model of tumor metastasis, which is presented here. Using an 'experimental' metastasis assay and Luria-Delbruck fluctuation analysis, we determined that in murine KHT fibrosarcoma and B16 melanoma lines, 'metastatic' variants with a distinct phenotype are generated at high rates. These variants are relatively unstable resulting in a dynamic equilibrium between generation and loss of metastatic variants. The metastatic ability of such a tumor population is thus dependent on the frequency of a subpopulation of metastatic variants which are turning over rapidly. This dynamic heterogeneity model is able to quantitatively provide a unifying explanation for a wide range of observations concerning tumor heterogeneity and clonal instability. Genetic mechanisms involving rapid rates have been characterized in drug-resistant variants. We speculate that similar processes may be involved in different aspects of tumor progression such as those resulting in metastasis.

Chromosome-mediated gene transfer of hydroxyurea resistance and amplification of ribonucleotide reductase activity

Metaphase chromosomes purified from a hydroxyurea-resistant Chinese hamster cell line were able to transform recipient wild-type cells to hydroxyurea resistance at a frequency of 10(-6). Approximately 60% of the resulting transformant clones gradually lost hydroxyurea resistance when cultivated for prolonged periods in the absence of drug. One transformant was subjected to serial selection in higher concentrations of hydroxyurea. The five cell lines generated exhibited increasing relative plating efficiency in the presence of the drug and a corresponding elevation in their cellular content of ribonucleotide reductase. The most resistant cell line had a 163-fold increase in relative plating efficiency and a 120-fold increase in enzyme activity when compared with the wild-type cell line. The highly hydroxyurea-resistant cell lines had strong electron paramagnetic resonance signals characteristic of an elevated level of the free radical present in the M2 subunit of ribonucleotide reductase. Two-dimensional electrophoresis of cell-free extracts from one of the resistant cell lines indicated that a 53,000-dalton protein was present in greatly elevated quantities when compared with the wild-type cell line. These data suggest that the hydroxyurea-resistant cell lines may contain an amplification of the gene for the M2 subunit of ribonucleotide reductase.

Chromosome-mediated gene transfer of hydroxyurea resistance and amplification of ribonucleotide reductase activity

Metaphase chromosomes purified from a hydroxyurea-resistant Chinese hamster cell line were able to transform recipient wild-type cells to hydroxyurea resistance at a frequency of 10(-6). Approximately 60% of the resulting transformant clones gradually lost hydroxyurea resistance when cultivated for prolonged periods in the absence of drug. One transformant was subjected to serial selection in higher concentrations of hydroxyurea. The five cell lines generated exhibited increasing relative plating efficiency in the presence of the drug and a corresponding elevation in their cellular content of ribonucleotide reductase. The most resistant cell line had a 163-fold increase in relative plating efficiency and a 120-fold increase in enzyme activity when compared with the wild-type cell line. The highly hydroxyurea-resistant cell lines had strong electron paramagnetic resonance signals characteristic of an elevated level of the free radical present in the M2 subunit of ribonucleotide reductase. Two-dimensional electrophoresis of cell-free extracts from one of the resistant cell lines indicated that a 53,000-dalton protein was present in greatly elevated quantities when compared with the wild-type cell line. These data suggest that the hydroxyurea-resistant cell lines may contain an amplification of the gene for the M2 subunit of ribonucleotide reductase.

Tunicamycin-resistant mutations in mouse FM3A cells

Tunicamycin is an antibiotic that inhibits the oligosaccharide synthesis of glycoproteins. It greatly suppressed the growth of cultured mouse mammary carcinoma FM3A cells, when added to growth medium at concentrations of more than 0.1 microgram/ml. We have developed a single-step selection system for quantitatively detecting mutations resistant to the antibiotic in FM3A cells. Mutant colonies resistant to 1-1.2 micrograms tunicamycin per ml (the optimal concentration of the selecting agent) appeared at a frequency of 10(-4) to 10(-5) in an unmutagenized population, but they increased over 50-fold in the population mutagenized with 0.5 microgram N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) per ml for 2 h and selected under optimal conditions for the time of mutation expression and cell density in selective medium. Fluctuation analysis, by the method of Luria and Delbrück, revealed that tunicamycin-resistant mutations occurred at random during proliferation in normal medium at a rate of 1.2 x 10(-6) per cell per generation. So far 45 spontaneous and MNNG-induced mutant lines have been isolated and serially passaged in the absence of tunicamycin. These mutant lines all inherited their resistance for more than 60 generations. The mutants examined in detail were 12- to 26-fold more resistant than wild-type cells in terms of the D10 value, the concentration of tunicamycin reducing the plating efficiency to 10% of the control. In the hybrids between wild-type and mutant cells the tunicamycin resistance behaved in a co-dominant manner. Tunicamycin inhibited the incorporation of [3H]mannose into the acid-insoluble cell fraction; in this respect, mutant cells were over 30-fold more resistant than wild-type cells. Possible mechanisms of tunicamycin resistance are discussed.

Involvement of ribonucleotide reductase activity in the senescence of normal human diploid fibroblasts

The levels of intracellular ribonucleotide reductase activity, a highly regulated rate-limiting step in DNA synthesis, were investigated during serial subculture of normal human diploid fibroblasts in vitro. This key enzyme activity was found to decline significantly during cellular senescence. This observation along with previous findings of a mutator gene associated with mammalian ribonucleotide reductase suggests a possible mutation mechanism for aging which involves changes in reductase activity during cellular senescence. Furthermore, in keeping with the decrease in enzyme activity, we show that cell resistance to the antitumor agent hydroxyurea, whose site of action is ribonucleotide reductase, decreases progressively with increasing passage numbers. This indicates that an important factor to be considered in drug therapy aimed at the reductase is the increased sensitivity of normal cells to drug with cell age, due to a decline in enzyme activity. Much remains to be determined about age-dependent factors involved in drug therapy; cultured normal human diploid fibroblasts provide a useful system in which to investigate these important parameters.

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.

Hydroxyurea inhibition of growth and DNA synthesis in Toxoplasma gondii: characterization of a resistant mutant

Hydroxyurea inhibited the growth and DNA synthesis of Toxoplasma gondii growing in human fibroblast cells. A concentration of 18 micrograms/ml totally suppressed plaque formation. The synthesis of T. gondii RNA was not acutely inhibited. The parasite was equally sensitive to hydroxyurea when grown in wild type or hydroxyurea resistant host cells. With the aid of chemical mutagenesis, we isolated a stable hydroxyurea resistant mutant of T. gondii. This mutant showed no increased ability to incorporate [3H]uracil into its pyrimidine deoxynucleotide pool. Hydroxyurea depressed the [3H]uracil labeling of the pyrimidine deoxynucleotide pool in the wild type parasite but not in the mutant, suggesting that the mutant ribonucleotide reductase was resistant to the inhibitory effect of the drug.

Bromodeoxyuridine resistance in CHO cells occurs in three discrete steps

Four independent mutants were isolated from mutagenized cultures of CHO cells by sib selection on the basis of resistance to a low concentration (2.6 x 10(-5) M) of BrdU. All four lines were stable, but all had about 100% of the wild-type (WT) specific activity of thymidine kinase (TK). None of the four yielded derivatives resistant to a high level of BrdU (2 x 10(-4) M) in one step even after mutagenesis, but variants resistant to 4-6 x 10(-5) M BrdU could be isolated at frequencies of about 2 x 10(-5)/cell. At frequencies of 10(-4)-10(-5), the second-step mutants gave colonies resistant to 2 x 10(-4) M BrdU. The second and third steps of resistance were correlated with partial and complete reduction, respectively, in the specific activity of TK, suggesting that the variants may be genotypically heterozygous and homozygous-negative at the tk locus. The first step of BrdU resistance was dominant and appeared to result from a mutation in the gene from ribonucleotide reductase, since in vitro assays on partially purified preparations showed that the reductase activity in mutant cells was less sensitive to BrdUTP than in WT cells.

Bromodeoxyuridine mutagenesis, ribonucleotide reductase activity, and deoxyribonucleotide pools in hydroxyurea-resistant mutants

Ribonucleotide reductase activity and deoxyribonucleoside triphosphate (dNTP) pools were examined in several Syrian hamster melanoma cell mutants which are resistant to hydroxyurea (HU), an inhibitor of ribonucleotide reductase, and which also show increased resistance to bromdeoxyuridine (BrdU) mutagenesis. For most of the mutants, resistance to HU and BrdU mutagenesis is associated with increased levels of ribonucleotide reductase activity. No evidence was found for qualitative alterations in the ribonucleotide reductase in the mutant cells. The dNTP pools in the mutants are somewhat resistant to the perturbations can be produced in wild-type cells by the addition of BrdU, although significant perturbations can be produced in the mutants by higher concentrations of BrdU. The decrease in BrdU-induced nucleotide pool perturbations may account for the resistance of the mutants to BrdU mutagenesis.

Overproduction of the free radical of ribonucleotide reductase in hydroxyurea-resistant mouse fibroblast 3T6 cells

Hydroxyurea inhibits the activity of ribonucleotide reductase (ribonucleoside-diphosphate reductase; 2'-deoxy-ribonucleoside-diphosphate:oxidized-thioredoxin 2'-oxidoreductase, EC 1.17.4.1) in bacteria and mammalian cells. The reductase from Escherichia coli consists of two nonidentical subunits (B1 and B2) and hydroxyurea acts by specifically destroying a tyrosine free radical of B2 required for enzyme activity. The mammalian enzyme also consists of two nonidentical subunits (M1 and M2), only one of which (M1) has been obtained in pure form. By continuous culture at stepwise increasing drug concentrations, we have now obtained a 3T6 mouse fibroblast cell line with a 100-fold increased resistance to hydroxyurea. Extracts from resistant cells showed a 3- to 15-fold increase in reductase activity. The amount of M1 protein was not increased. The amount of M2 protein could not be measured directly, but the M2 activity in extracts from resistant cells (but not normal cells) showed an EPR spectrum very similar to that of the tyrosine radical of the bacterial B2 subunit. We propose that resistance to hydroxyurea is caused either by overproduction of the complete M2 subunit or by increased generation of the tyrosine radical within the M2 protein. It seems that either alternative mirrors a possible normal regulatory mechanism for the activity of the reductase.

N-carbamoyloxyurea-resistant Chinese hamster ovary cells with elevated levels of ribonucleotide reductase activity

We describe the isolation and characterization of a Chinese hamster ovary cell line selected for resistance to N-carbamoyloxyurea. Using the mammalian cell permeabilization assay developed in our laboratory, a detailed analysis of the target enzyme, ribonucleotide reductase (EC 1.17.4.1), was carried out. Both drug-resistant and parental wild-type cells required the same optimum conditions for enzyme activity. The Ki values for N-carbamoyloxyurea inhibition of CDP reduction were 2.0 mM for NCR-30A cells and 2.3 mM for wild-type cells, while the Ki value for ADP reduction was 2.3 mM for both cell lines. Although the Ki values remained essentially unchanged, the Vmax values for NCR-30A cells were 1.01 nmoles dCDP formed/5 X 10(6) cells/hour and 1.83 nmoles dADP/5 X 10(6) cells/hour, while those for the wild-type cells were 0.49 nmoles dCDP produced/5 X 10(6) cells/hour and 1.00 nmoles dADP/5 X 10(6) cells/hour. This approximate twofold increase in reductase activity as least partially accounts for a 2.6-fold increase in D10 value for cellular resistance to N-carbamoyloxyurea exhibited by NCR-30A cells. The NCR-30A cell line was also cross-resistant to the antitumor agents, hydroxyurea and guanazole. No differences in Ki values for inhibition of CDP and ADP reduction by these two drugs were detected and cellular resistance could be entirely accounted for by the elevation in activity of the reductase in the NCR-30A cell line. The properties of N-carbamoyloxyurea-resistance cells indicate they should be useful for further investigations into the regulation of mammalian enzyme activity.

Alteration of ribonucleotide reductase in aphidicolin-resistant mutants of mouse FM3A cells with associated resistance to arabinosyladenine and arabinosylcytosine

Aphidicolin-resistant mutants of mouse FM3A cells were isolated and characterized. Most of the mutants were of a type showing associated resistance to arabinosyladenine, arabinosylcytosine, deoxyadenosine, and excess thymidine. This phenotype could also be observed in a variant line selected by resistance to a low level of arabinosylcytosine. In cell-cell hybrids, aphidicolin resistance as well as this cross-resistance behaved a codominant traits. The mutants had an increased dATP pool and decreased ability to incorporate labeled deoxycytidine into macromolecules. Genetic and biochemical evidence suggested that the mutation conferring the pleiotropic phenotype resulted from a change in ribonucleotide reductase activity such that the enzyme was desensitized to the allosteric negative effector dATP. This alteration of the enzyme could account for the marked change in deoxynucleotide pools and for the aphidicolin resistance of the mutants.

Ribonucleotide reduction in intact human diploid fibroblasts

There have been very few studies on ribonucleotide reductase activity in human tissue. In this report we describe a rapid and convenient procedure for determining purine and pyrimidine ribonucleotide reduction in normal human diploid fibroblasts and use the method to examine some general properties of the activity in these cells. ADP and CDP reductase was characterized for its response to the positive effectors, ATP and dGTP, the negative effector dATP, and the reducing agent dithiothreitol. Apparent Km values for ADP and CDP were determined to be 0.1 mM and 0.04 mM respectively. THe antitumor agent hydroxyurea inhibited both purine and pyrimidine reductase in a noncompetitive fashion, giving Ki value of 0.40 mM and 0.41 mM for ADP and CDP respectively. These Ki estimates are about four to five times higher than those reported for some permanent cell lines. An examination of the cytotoxic effects of hydroxyurea indicated a close correlation between the concentration of drug which inhibited enzyme activity and decreased colony-forming ability. Clearly the ability to investigate ribonucleotide reduction in low numbers of normal human diploid cells will be useful for genetic and biochemical studies.

Parameters governing the transfer of the genes for thymidine kinase and dihydrofolate reductase into mouse cells using metaphase chromosomes or DNA

The conditions necessary to achieve high frequency transfer of the thymidine kinase and dihydrofolate reductase genes from hamster cells into mouse cells were investigated. Of the parameters examined, the length of adsorption time, input gene dosage, and treatment with dimethylsulfoxide (DMSO) were found to significantly alter the transfer frequency using either metaphase chromosomes or purified DNA as the transfer vehicle. With the mouse cell line as a recipient, the optimal adsorption period for DNA or chromosomes from MtxRIII cells was found to vary from 8 to 16 h in those experiments where the recipient cells were subsequently treated with DMSO. Without DMSO, similar frequencies could be obtained by extending the period of adsorption. Increasing the dosage of DNA or chromosomes resulted in an almost linear increase in the number of transformants. The optimal conditions for transfer did not significantly differ for the two genes studied. On the average, the optimal conditions yielded 1.5 x 10(3) transformants per 10(7) recipient cells with chromosomes; with DNA an average of only 60 transformants were observed. In general, DNA transformants grown in the absence of methotrexate were unstable; whereas, under the same conditions about 20% of the transformants from the chromosome experiments were stable.

Hydroxyurea-resistant mouse L cells with elevated levels of drug-resistant ribonucleotide reductase activity

We describe the isolation and partial characterization of a mouse L-cell line which is resistant to normally highly cytotoxic concentrations of hydroxyurea. A detailed analysis of the target enzyme ribonucleotide reductase in both wild-type and hydroxyurea-resistant enzyme preparations suggests that the drug-resistant cells form a ribonucleotide reductase enzyme which contains a structural alteration, rendering it less sensitive to inhibition by hydroxyurea. K1 values for hydroxyurea inhibition of ribonucleotide reduction in enzyme preparations from hydroxyurea-resistant cells were significantly higher than corresponding values from preparations from wild-type cells. The Km for CDP reduction in enzyme preparations of drug-resistant cells was approximately threefold higher than the corresponding parental wild-type value. In addition, in vivo enzyme assays detected a major difference between the temperature profiles of ribonucleotide reduction in nucleotide-permeable drug-resistant and wild-type cells. When levels of ribonucleotide reductase activity were measured in vivo, it was found that the drug-resistant cells contained approximately 3 times the wild-type level of CDP reductase activity and twice wild-type level of GDP reductase activity. This combination of enhanced enzyme levels plus an altered sensitivity to drug inhibition can easily account for the drug-resistance phenotype. The properties of these hydroxyurea-resistant cells indicate that they will be useful for genetic and biochemical studies.

Resistance to bromodeoxyuridine mutagenesis and toxicity in mammalian cells selected for resistance to hydroxyurea

Mutant cell lines resistant to hydroxyurea (HU), an inhibitor of the enzyme ribonucleotide reductase, were selected from a line of Syrian hamster melanoma cells. Mutant lines were selected for resistance to 0.3 mM HU, and from these lines, second-step mutants were selected for resistance to 1.9 mM HU. The HUr lines were tested ffor their responses to 5-bromodeoxyuridine (brdU), in terms of toxicity, mutagenesis, and incorporation of BrdU into DNA. All of the HUr lines showed increased resistance to the toxic effects of BrdU. in addition, the HUr lines all were resistant to BrdU mutagenesis. Overall, there was good correlation among the levels of resistance to HU toxicity, BrdU toxicity, and BrdU mutagenesis in the HUr lines. These tests were carried out under conditions such that the parental and HUr cells incorporated equal amounts of BrdU into nuclear DNA. Therefore, the resistance of the HUr cells to the effects of BrdU cannot be attributed to decreased incorporation of BrdU into DNA. These results suggest that the HUr cells have an lateration in ribonucleotide reductase activity that simultaneously confers resistance to HU and BrdU. The properties of the HUr cells suggest that the perturbation of deoxcytidine metabolism by BrdU. The properties of the HUr cells suggest that the perturbation of deoxcytidine metabolism by BrdU triphosphate inhibition of ribonucleotide reductase activity plays a key role in the toxic and mutagenic effects of BrdU in mammalian cells.