Genetic characterization of hydroxyurea-resistance in Chinese hamster ovary cells

Antisense oligodeoxyribonucleotide inhibition of TGF-beta 1 gene expression and alterations in the growth and malignant properties of mouse fibrosarcoma cells

Transforming growth factor (TGF-beta) is a family of multifunctional signalling molecules that play a fundamental role in both normal and malignant cell behavior. Procedures that alter mouse TGF-beta 1 gene expression provide an important approach for analyzing the complex regulatory processes associated with this member of the growth factor family. Therefore, we have designed oligodeoxyribonucleotides (oligos) in an antisense orientation, which are complementary to regions of the TGF-beta 1 message, in an attempt to obtain an oligo sequence that specifically reduces TGF-beta 1 synthesis. We observed that oligos containing a mixture of phosphorothioate and phosphodiester linkages were less toxic and more specific when compared to those only containing phosphorothioate. A non-toxic sequence was identified that markedly reduced the levels of TGF-beta 1 in oligo-treated malignant mouse fibrosarcoma cells. The invasive and metastatic properties of these fibrosarcoma cells were also significantly decreased following treatment with the antisense oligo. The results indicate an important role for altered TGF-beta 1 expression in the regulation of malignant cell proliferation, invasion and metastasis. These results also indicate that this oligo sequence is a useful tool for studies directed towards understanding the complex relationships between TGF-beta 1 and cellular regulation.

Lack of correlation between deoxyribonucleotide pool sizes, spontaneous mutation rates and malignant potential in Chinese hamster ovary cells

To examine the relationship between altered spontaneous mutation rates and malignant characteristics of cells, two hydroxyurea-resistant Chinese hamster ovary cell lines, with alterations in ribonucleotide reductase, were examined for their rates of spontaneous mutation to 6-thioguanine and ouabain resistance, tumor growth rates and their ability to form experimental lung metastases. The most resistant cell line, HR-R2T, showed no changes in the rate of spontaneous mutation to 6-thioguanine or ouabain resistance compared to the parental wild-type cell line; however, the mutant line formed lung metastases in experimental metastasis assays with BALB/c nu/nu mice, and exhibited metastatic abilities significantly different from the wild-type population. Furthermore, the HR-R2T population did not show imbalances in any of the deoxyribonucleoside triphosphate pool sizes, which are frequently observed in cells altered in ribonucleotide reductase activity. The second hydroxyurea-resistant line, HNR-AT, had gross alterations in dCTP and dGTP pools and although the rate of spontaneous mutation to 6-thioguanione resistance was unaltered, it showed a moderate decrease in the rate of spontaneous mutation to ouabain resistance when compared to the parental wild-type population. Interestingly, the HNR-AT cell line did not form any lung metastases in the experimental metastasis assay. Both mutant cell lines, HR-R2T, and HNR-AT, had increased tumor growth rates in C57 BALB/c "beige" nude (nu/nu) mice as compared to the parental wild-type population. In total, the results obtained with the two mutant cell lines question the association of altered mutation rates with increased metastatic potential. Although several explanations are possible for the altered malignant properties exhibited by HR-R2T and HNR-AT cells, it is interesting to note that the results are consistent with earlier suggestions that changes in ribonucleotide reductase may accompany modifications in the malignant characteristics of cells.

Molecular and cellular characterization of drug resistant hamster cell lines with alterations in ribonucleotide reductase

Ribonucleotide reductase consists of 2 protein components frequently called M1 and M2. Hydroxyurea specifically inhibits DNA synthesis by interacting with the M2 protein and destroying a unique tyrosyl-free radical. We have carried out a molecular and cellular characterization of 2 Chinese hamster ovary cell lines exhibiting either low (HN(R)-AT) or relatively high (H(R)-R2T) resistance to the cytotoxic effects of hydroxyurea. Both drug-resistant lines have an increased level of ribonucleotide reductase activity. EPR measurements for tyrosyl-free radical content and studies with M1-specific antibodies indicated that the elevation in enzyme activity was entirely due to an increase in the M2 component. Studies with M1 cDNA showed that both drug-resistant cell lines contained a wild-type level of M1 mRNA and a wild-type M1 gene copy number. Studies with M2 cDNA indicated that the 2 drug-resistant lines possessed elevated levels of M2 message that could explain the observed increase in M2 component. The elevation of M2 mRNA in the most resistant line, H(R)-R2T, was due to an increase in M2 gene copy number. The low resistant cell line, HN(R)-AT, exhibited a wild-type M2 gene copy number, indicating that the increase in M2 gene message occurred through a process other than gene amplification. Enzyme kinetic studies with partially purified preparations from both drug resistant lines showed reduced sensitivity to hydroxyurea and to the negative allosteric effector, dATP. In addition to hydroxyurea, H(R)-R2T cells were also resistant to several other drugs whose site of action is the M2 component. Furthermore, H(R)-R2T cells were not cross-resistant to colchicine or puromycin, suggesting that hydroxyurea-resistant cells do not share the multi-drug resistance phenotype, which is frequently associated with cross-resistance to these drugs.

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.

Variable effects of DNA-synthesis inhibitors upon DNA methylation in mammalian cells

Post-synthetic enzymatic hypermethylation of DNA was induced in hamster fibrosarcoma cells by the DNA synthesis inhibitors cytosine arabinoside, hydroxyurea and aphidicolin. This effect required direct inhibition of DNA polymerase alpha or reduction in deoxynucleotide pools and was not specific to a single cell type. At equivalently reduced levels of DNA synthesis, neither cycloheximide, actinomycin D nor serum deprivation affected DNA methylation in this way. The topoisomerase inhibitors nalidixic acid and novobiocin caused significant hypomethylation indicating that increased 5-mCyt content was not a necessary consequence of DNA synthesis inhibition. The induced hypermethylation occurred predominantly in that fraction of the DNA synthesized in the presence of inhibitor; was stable in the absence of drug; was most prominent in low molecular weight DNA representing sites of initiated but incomplete DNA synthesis; and occurred primarily within CpG dinucleotides, although other dinucleotides were overmethylated as well. Drug-induced CpG hypermethylation may be capable of silencing genes, an effect which may be relevant to the aberrantly expressed genes characteristic of neoplastic cells.

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.

Human diploid fibroblasts with alterations in ribonucleotide reductase activity, deoxyribonucleotide pools and in vitro lifespan

Three drug resistant human diploid fibroblast clones were isolated which contained elevated levels of ribonucleotide reductase activity when compared to wild type fibroblasts. The drug resistant cells do not appear to possess an enzyme with altered affinity for hydroxyurea. The increase in enzyme activity can entirely account for cellular drug resistance. In keeping with the observed changes in reductase activity in drug resistant fibroblasts, deoxyribonucleotide pools were also found to be altered. Most significantly, there was a 1.8-fold expansion of the dCTP pool size, suggesting that elevation in intracellular dCTP concentrations plays an important role in cellular resistance. Furthermore, the drug resistant fibroblasts exhibited substantial reductions in their replicative abilities, suggesting that the regulation of ribonucleotide reductase and the accompanying deoxyribonucleotide pools in human diploid cells is involved in aspects of cellular senescence.

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.

Replication of herpes simplex virus type 1 on hydroxyurea-resistant baby hamster kidney cells

Hydroxyurea-resistant (HUr) baby hamster kidney cells were isolated, subcloned, and characterized. One clonal line, which contained elevated levels of ribonucleotide reductase, lost its HU resistance during passage in the absence of the inhibitor, whereas another clonal line was stably resistant. The replication of herpes simplex virus type 1 on these cells was compared with that of the parvovirus minute virus of mice. Herpes simplex virus type 1 was found to be as sensitive to HU on both lines of HUr baby hamster kidney cells as it was on parental (HU-sensitive) cells, whereas parvovirus replication was about eight times more resistant on HUr baby hamster kidney cells compared with the parental cells. The results suggest that herpes simplex virus type 1 cannot use the cellular reductase and may code for its own.

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.

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.

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.

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.

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

A Chinese hamster ovary cell line selected for resistance to hydroxyurea was serially cultivated in the absence of a selective agent, and cells with decreased resistance to the cytotoxic effects of hydroxyurea gradually accumulated in the population. Three stable subclones with differing drug sensitivities were isolated from this mixed population and were found to contain intracellular levels of drug-sensitive ribonucleotide reductase which correlated with the degree of cellular resistance to hydroxyurea. This new class of hydroxyurea resistance was expressed in a codominant fashion in cell-cell hybridization studies. Also, the enhanced enzyme activity in the drug-resistant cells was observed only during the cell cycle S phase of synchronized cells. The properties of these drug-resistant lines indicate that they will be useful for genetic and biochemical studies.

Ribonucleotide reductase from wild type and hydroxyurea-resistant chinese hamster ovary cells

The kinetic properties of partially purified ribonucleotide reductase from Chinese hamster ovary cells have been investigated. Double reciprocal plots of velocity against substrate concentration were found to be linear for three the substrates tested, and yielded apparent Km values of 0.12 mM for CDP, 0.14 mM for ADP and 0.026 mM for GDP. Hydroxyurea, a potent inhibitor of ribonucleotide reduction, was tested against varying concentrations of ribonucleotide substrates and inhibited the enzyme activity in an uncompetitive fashion. Intercept replots were linear and exhibited Ki values for hydroxyurea of 0.08 mM for CDP reduction, 0.13 mM for ADP reduction and 0.07 mM for GDP reduction. Guanazole, another inhibitor of ribonucleotide reductase, interacted with the enzyme in a similar manner to hydroxyurea showing an uncompetitive pattern of inhibition with CDP reduction and yielding a Ki value of 0.57 mM. Partially purified ribonucleotide reductase from hydroxyurea-resistant cells was compared to enzyme activity from wild type cells. Significant differences were observed in the hydroxyurea Ki values with the three ribonucleotide substrates that were tested. Also, CDP reductase activity from the drug-resistant cells yielded a significantly higher Ki value for guanazole inhibition than the wild type activity. The properties of partially purified ribonucleotide reductase from a somatic cell hybrid constructed from wild type and hydroxyurea-resistant cells was also examined. The Ki value for hydroxyurea inhibition of CDP reductase was intermediate between the Ki values of the parental lines and indicated a codominant expression of hydroxyurea-resistance at the enzyme level. The most logical explanation for these results is that the mutant cells contain a structurally altered ribonucleotide reductase whose activity is less sensitive to inhibition by hydroxyurea or guanazole.