Role of deoxynucleoside triphosphate pools in the cytotoxic and mutagenic effects of DNA alkylating agents

Potential Role of Intercellular Communication in the Rate-Limiting Step in Carcinogenesis

In order to ascertain whether there might be a scientific basis for determining practical “thresholds” for “carcinogens,” the concepts of thresholds and carcinogens were examined in the context of some current ideas on cardnogenesis. The observation that cardnogenesis seems to involve the donal expansion of a pre-malignant cell through a series of pheno-typic changes was explained by the initiation/promotion model of cardnogenesis. Unrepaired DNA lesions, acting as substrates for mutations in dividing cells, were speculated to play a role in the initiation phase of cardnogenesis (and indirectly to the promotion phase if the lesions lead to significant cell killing, forcing “compensatory hyperplasia”). Inhibition of intercellular communication, either by cell removal, cell death, growth factors or chemical promoters, was speculated to allow the donal expansion of initiated cells to reach a “critical mass.” During that donal expansion of initiated cells, additional phenotypic changes were speculated to occur during cell replication by mutational and/or epigenetic events. Therefore, it was concluded, on the basis of this model, that conditions which prevented the inhibition of intercellular communication between normal cells and the initiated cell(s) contributed to the rate limiting step of cardnogenesis.

Assuming the initiation and promotion model of cardnogenesis, the classical concepts of “thresholds” and “carcinogens” were viewed as grossly inadequate because they did not symbolically represent the known determinants of the complex carcinogenic process. Unless genetic, developmental stage, tissue, nutritional, stress, life style, as well as concurrent antagonists and/or synergists, factors are known, extrapolation about the potential carcinogenicity of a given chemical from molecular, in vitro or even in vivo experiments or epidemiological data would be extremely risky. It was concluded that, at this stage of our understanding of the mech-anism(s) of carcinogenesis, attempts to determine “thresholds” for “carcinogens” naively assume “carcinogens” are the single determinants for carcinogenesis, and that all chemicals which might influence the appearance of tumors act the same way.

Primary prevention of colorectal cancer: lifestyle, nutrition, exercise

The past two decades have provided a vast amount of literature related to the primary prevention of colorectal cancer. Large international variation in colorectal cancer incidence and mortality rates and the prominent increases in the incidence of colorectal cancer in groups that migrated from low- to high-incidence areas provided important evidence that lifestyle factors influence the development of this malignancy. Moreover, there is convincing evidence from epidemiological and experimental studies that dietary intake is an important etiological factor in colorectal neoplasia. Although the precise mechanisms have not been clarified, several lifestyle factors are likely to have a major impact on colorectal cancer development. Physical inactivity and to a lesser extent, excess body weight, are consistent risk factors for colon cancer. Exposure to tobacco products early in life is associated with a higher risk of developing colorectal neoplasia. Diet and nutritional factors are also clearly important. Diets high in red and processed meat increase risk. Excess alcohol consumption, probably in combination with a diet low in some micronutrients such as folate and methionine, appear to increase risk. There is also recent evidence supporting a protective effect of calcium and vitamin D in the etiology of colorectal neoplasia. The relationship between intake of dietary fiber and risk of colon cancer has been studied for three decades but the results are still inconclusive. However, some micronutrients or phytochemicals in fiber-rich foods may be important; folic acid is one such micronutrient that has been shown to protect against the development of colorectal neoplasia and is currently being studied in intervention trials of adenoma recurrence. The overwhelming evidence indicates that primary prevention of colon cancer is feasible. Continued focus on primary prevention of colorectal cancer, in combination with efforts aimed at screening and surveillance, will be vital in attaining the greatest possible progress against this complex, yet highly preventable disease.

Alcohol, one-carbon metabolism, and colorectal cancer: recent insights from molecular studies

A growing body of evidence implicates alcohol intake as a risk factor for colorectal cancer. Until recently, most of the data were based on epidemiologic data that examined alcohol intake in relation to risk of colorectal neoplasia, but the evidence has now been broadened by recent molecular studies on mechanisms. In particular, a number of observations strongly support a role for alcohol acting through disruptions in one-carbon metabolism. Excessive alcohol intake is a fairly consistent risk factor for colorectal neoplasia in most studies, and studies show much higher risks of colorectal neoplasia in those with high alcohol and low folate than with high alcohol or low folate individually. Interactions between high alcohol-low folate and the MTHFR677TT genotype with risk of colorectal neoplasia suggest that alcohol is acting through its effects on one-carbon metabolism because the combination of high alcohol-low folate and the MTHFR677TT genotype are related to markedly elevated serum levels of homocysteine and to DNA hypomethylation. In addition, in Japanese studies, consumers of alcohol possessing the ALDH2*2 allele, who have very elevated levels of acetaldehyde, are at high risk for colorectal cancer. The relatively high prevalence of the ALDH2*2 allele may thus account for the stronger association between alcohol and colorectal neoplasia that is frequently observed in studies in Japanese populations. Further research integrating studies with more detailed data on alcohol intake levels and patterns, folate and other related nutrient status, and relevant genotypes may help clarify unresolved questions regarding the health consequences of moderate to high alcohol drinking.

Modifiable risk factors for colon cancer

Although many mechanisms remain unclear, a large body of evidence indicates that several dietary and lifestyle factors are likely to have a major influence on the risk of colon cancer. Physical inactivity, excess body weight, and a central deposition of adiposity are consistent risk factors. Overconsumption of energy is likely to be one of the major contributors to the high rates of colon cancer in Western countries. Beyond their influence on energy balance, the independent role of specific macronutrients remain controversial. Red meat, processed meats, and perhaps refined carbohydrates contribute to risk. Recent evidence indicate that chronic hyperinsulinemia may increase risk of colon cancer. As insulin resistance and subsequent hyperinsulinemia is induced by excess energy intake and some aspects of the Western diet (e.g., saturated fats and refined carbohydrates), insulin may be a focus of factors influencing colon cancer risk. Recent evidence also points to a role of IGF-1, but our understanding of modifiable factors that influence levels of these is poor at present. Of note is that hyperinsulinemia increases free IGF-1 exposure [25]. High alcohol consumption, probably in combination with a diet low in some micronutrients such as folate and methionine, and smoking early in life are likely to increase risk of colon cancer. Recent epidemiologic studies have tended not to support a strong influence of fiber; instead, some micronutrients or phytochemicals in fiber-rich foods may be important. Folate is one such nutrient that has received attention lately and is being studied in randomized intervention trials. Agents with chemopreventive properties, such as aspirin and postmenopausal estrogens, have potential adverse effects so a careful consideration of the risk-benefit ratio is required before general recommendations can be made. Other NSAIDs with a potential for reduced toxicity, such as celecoxib, are currently being evaluated for efficacy and toxicity. The overwhelming evidence indicates that primary prevention of colon cancer is feasible. At least 70% of colon cancers may be preventable by moderate changes in diet and lifestyle [197]. Secondary prevention, through screening by sigmoidoscopy and colonoscopy, is also critically important to prevent mortality from colon cancer; however, many of the diet and lifestyle risk factors for colon cancers are the same for cardiovascular disease and for some other cancers, so focusing on the modifiable risk factors for colon cancer is likely to have many additional benefits beyond this cancer.

Nutritional factors in human cancers

A variety of external factors interacting with genetic susceptibility influence the carcinogenesis process. External factors including oxidative compounds, electrophilic agents, and chronic infections may enhance genetic damage. In addition, various hormonal factors which influence growth and differentiation are critically important in the carcinogenic process. Diet and nutrition can influence these processes directly in the gastrointestinal tract by providing bioactive compounds to specific tissues via the circulatory system, or by modulating hormone levels. Differences in certain dietary patterns among populations explain a substantial proportion of cancers of the colon, prostate and breast. These malignancies are largely influenced by a combination of factors related to diet and nutrition. Their causes are multifactorial and complex, but a major influence is the widespread availability of energy-dense, highly processed and refined foods that are also deplete in fiber. These dietary patterns in combination with physical inactivity contribute to obesity and metabolic consequences such as increased levels of IGF-1, insulin, estrogen, and possibly testosterone. These hormones tend to promote cellular growth. For prostate cancer, epidemiologic studies consistently show a positive association with high consumption of milk, dairy products, and meats. These dietary factors tend to decrease 1.25(OH)2 vitamin D, a cell differentiator, and low levels of this hormone may enhance prostate carcinogenesis. While the nutritional modulation of growth-enhancing and differentiating hormones is likely to contribute to the high prevalence of breast, colorectal, prostate, and several other cancers in the Western world, these cancers are relatively rare in less economically developed countries, where malignancies of the upper gastrointestinal tract are quite common. The major causes of upper gastrointestinal tract cancers are likely related to various food practices or preservation methods other than refrigeration, which increase mucosal exposure to irritants or carcinogens.

Sensitivity of a mutator gene in Chinese hamster ovary cell to deoxynucleoside triphosphate pool alterations

The Thy- mutants of Chinese hamster ovary cells have a 5- to 10-fold elevated pool of deoxycytidine 5'-triphosphate (dCTP) and are auxotrophic for thymidine as an apparent consequence of a single mutation. thy is also a mutator gene, elevating the spontaneous rate of mutation 5- to 200-fold for at least two genetic markers. Previous experiments suggested that this mutator activity was caused by the elevated pool of dCTP in Thy- cells. To test this, the dCTP and deoxythymidine 5'-triphosphate (dTTP) pools were manipulated by altering the external concentration of thymidine in the growth medium. The rate of mutation at one genetic locus, ouabain resistance, was directly related to cellular dCTP content. At the highest level of dCTP the rate in one Thy- strain was approximately 200 times that of wild-type cells. However, the relationship between dCTP content and the rate of mutation at the ouabain locus was different for two mutator strains and wild-type cells. The rate of mutation at a second locus, thioguanine resistance, was increased approximately 10-fold over wild type regardless of the dCTP-dTTP pools. These experiments suggest that the mutator activity of thy is clearly related to dCTP content, but the dCTP level alone does not appear to be the cause of the mutator.

International Commission for Protection Against Environmental Mutagens and Carcinogens. Deoxyribonucleoside triphosphate levels: a critical factor in the maintenance of genetic stability

DNA precursor pool imbalances can elicit a variety of genetic effects and modulate the genotoxicity of certain DNA-damaging agents. These and other observations indicate that the control of DNA precursor concentrations is essential for the maintenance of genetic stability, and suggest that factors which offset this control may contribute to environmental mutagenesis and carcinogenesis. In this article, we review the biochemical and genetic mechanisms responsible for regulating the production and relative amounts of intracellular DNA precursors, describe the many outcomes of perturbations in DNA precursor levels, and discuss implications of such imbalances for sensitivity to DNA-damaging agents, population monitoring, and human diseases.

Clustered base substitutions in CTP synthetase conferring drug resistance in Chinese hamster ovary cells

Dominantly acting mutations that eliminate the allosteric regulation of CTP synthetase confer a form of multidrug resistance and a mutator phenotype to cultured Chinese hamster ovary cells. Mutations responsible for this phenotype have been identified in 23 independent strains selected for resistance to arabinosyl cytosine and 5-fluorouracil. All these mutations were due to base substitutions at seven sites within a highly conserved region of the ctps gene. This clustering should make it feasible to assess the role of such mutations in the development of drug resistance encountered in the treatment of malignant disease.

Deoxyribonucleoside triphosphate pools and thymidine chemosensitization in human T-cell leukemia

Thymidine kills cells by depleting dCTP stores. The present experiments tested whether deoxycytidine, by replenishing dCTP pools, could prevent thymidine cytotoxicity and thymidine's enhancement of carboplatin killing in two human T-cell acute leukemia cell lines. MOLT3 and JM cells were exposed to combinations of thymidine, deoxycytidine, and carboplatin and then assessed for survival, the magnitude of thymidine-carboplatin chemosensitization, and changes in deoxyribonucleoside triphosphate pools. For both cell lines, deoxycytidine (up to 144.5 micrograms/ml x 24 h) completely restored dCTP pools but only partially protected against thymidine cytotoxicity (100-1000 micrograms/ml x 24 h) and thymidine-carboplatin sensitization (up to 60 micrograms carboplatin/ml during the last hour of thymidine). This contrasts with complete protection in prior studies using other cell types. Thymidine alone markedly increased dTTP and dGTP pools and decreased dCTP; dATP pools underwent a sharp decline which has not been observed before in any cell line. In subsequent studies 0.0336-137.3 micrograms deoxyadenosine/ml partially prevented cytotoxicity and carboplatin sensitization by 300 micrograms thymidine/ml. Together, deoxycytidine and deoxyadenosine completely prevented thymidine-carboplatin sensitization even though dATP and dCTP pools were not entirely returned to normal. These findings are discussed in regard to the unusual sensitivity of T-cell malignancies to thymidine toxicity, mechanisms of cytotoxicity and chemosensitization by thymidine, and the possibility of thymidine selectively sensitizing T-cell malignancies to killing by alkylating agents.

Effect of bromodeoxyuridine on the proliferation and growth of ethyl methanesulfonate-exposed P3 cells: relationship to the induction of sister-chromatid exchanges

Although sister-chromatid exchange (SCE) analysis is recognized as an indicator of exposure to DNA-damaging agents, the results of these analyses have been confounded by the use of bromodeoxyuridine (BrdUrd) to differentially label the sister chromatids. Not only does BrdUrd itself induce SCE, it also modulates the frequency of SCE induced by certain DNA-damaging agents. In order to examine this effect of BrdUrd on SCE frequency, an indirect method which lends itself to measurements both with and without BrdUrd was employed. Human teratocarcinoma-derived (P3) cells were exposed to ethyl methanesulfonate (EMS) and cultured with increasing concentrations of BrdUrd for lengths of time corresponding to one, two, and three generations of cell growth. At each time point, the distribution of nuclei among the phases of the cell-cycle and cell growth were evaluated for each concentration and chemical. A statistical model was employed which tested both for the main effects of chemicals and culture times and for interactions between these factors. Both EMS and BrdUrd significantly affected the percentages of nuclei within the cell-cycle. Exposure to EMS resulted in decreases in the percentages of nuclei in G0 + G1 and increases in the G2 + M compartment. Exposure to BrdUrd affected the size of the G0 + G1 compartment as well as the percentage of S-phase nuclei. Cell growth was reduced as a consequence of increasing EMS concentration and as a function of BrdUrd concentration; the effects of these chemicals were more readily apparent at the later time points. Most importantly, for both the cell-cycle kinetics data and the cell growth data, no evidence of an interaction between the effects of EMS and the effects of BrdUrd was detected statistically. These results may be interpreted to mean that while both EMS and BrdUrd affect the induction of SCE, under the conditions of this experiment, the effects are additive rather than interactive.

Nucleotide pools and mutagenic effects of alkylating agents in wild-type and APRT-deficient Friend erythroleukaemia cells

Wild-type Friend mouse erythroleukaemia cells (clone 707) were compared with adenine phosphoribosyltransferase (APRT)-deficient mutant subclones (707DAP8 and 707DAP10) for sensitivity to cell killing and mutagenesis by ethyl methanesulphonate (EMS) and methyl methanesulphonate (MMS). Cells were exposed to 0-300 micrograms/ml EMS and to 0-20 micrograms/ml MMS for a period of 16 h. A slight difference was found between wild-type cells and the two APRT-deficient subclones in terms of sensitivity to cell killing by both mutagens. The APRT-deficient subclones were, however, significantly more sensitive than wild-type cells to mutagenesis to 5-bromo-2-deoxyuridine resistance and 6-thioguanine resistance by EMS and MMS. The APRT-deficient subclones were found to have significantly decreased levels of dATP and dTTP nucleotides and decreased levels of all four ribonucleoside triphosphates (ATP, GTP, CTP and UTP) relative to wild-type cells. Wild-type Friend cells were found to have insignificant levels O6-methylguanine-DNA methyl transferase and it is suggested that the increased mutagen sensitivity of APRT-deficient cells may be due to imbalance of deoxyribonucleoside triphosphate pools during DNA excision-repair processes, or more probably due to deficiency of ATP for ATP-dependent DNA excision-repair enzymes.

The effects of thymidine on deoxyribonucleotide pool levels, cytotoxicity and mutation induction in Friend mouse erythroleukaemia cells

The ability of excess thymidine (10(-6)-10(-3) M) to enhance the frequency of 6-thioguanine (6-TG) resistant cell mutants and 2,6-diaminopurine (DAP) resistant cell mutants in Friend mouse erythroleukaemia cells, clone 707, was investigated. A significant increase in mutant frequency for both markers was observed at the higher (10(-4) and 10(-3) M) thymidine treatments. Measurements of deoxyribonucleoside triphosphate pool sizes in the cells revealed a dramatic elevation of the deoxythymidine triphosphate and deoxyguanosine triphosphate pools, an increase in the deoxyadenosine triphosphate pool and an almost complete disappearance of the deoxycytidine triphosphate pool at the higher thymidine treatments. This complemented the mutagenesis data. These results support the view that increases in mutant frequency may take place following perturbations in DNA precursor pools through a resultant decrease in the fidelity of DNA synthesis. Measurements of deoxyribonucleoside triphosphate pools were also carried out on clone 707 Friend cells and a thymidine kinase-deficient subclone, 707 BUF. The thymidine kinase-deficient subclone had significantly reduced deoxythymidine triphosphate and deoxyguanosine triphosphate pools relative to those observed in-clone 707 cells. The previously observed mutagen hypersensitivity in thymidine kinase-deficient Friend cells may result through pool imbalance rendering DNA excision repair error prone.

Deoxynucleoside triphosphate pool perturbation is not a general feature in mutagen-treated mammalian cells

Deoxynucleoside triphosphate (dNTP) and ribonucleoside triphosphate (rNTP) pools were analyzed in 4 mammalian cell lines following treatment with UV-C (254 nm), UV-A (365 nm) or the carcinogen, 1-methyl-3-nitro-1-nitrosoguanidine (MNNG). No substantial alterations in dNTP pool levels were observed in any treatment group. However, the cellular conversions of exogenously added deoxycytidine and deoxyguanosine to the corresponding triphosphates were inhibited 30-97% by UV-C and MNNG treatment. In addition, the conversion of dGuo to GTP and deoxyadenosine to ATP were inhibited 25-50% in CHO cells by mutagen treatment. The data do not support the notion that modulation of specific dNTP pools is a general feature of mutagen treatment in mammalian cells, but so suggest a mutagen-sensitivity of deoxynucleoside metabolism.

Deoxyribonucleotide pools as targets for mutagenesis by N-methyl-N-nitrosourea

This paper describes a biological test of the hypothesis that one or more components of the intracellular nucleotide pool represent a significant target for the mutagenic effects of alkylating agents. In other words, we ask whether mutagenesis can occur either through alkylation of susceptible nucleotide residues in DNA, or through alkylation of a free nucleotide, followed by its incorporation into DNA. Our approach is based upon the premise that if a nucleotide pool is a mutagenic target, then transient expansion of that pool should increase the target size and enhance mutagenesis following subsequent treatment with an alkylating agent. Working either with V79 hamster lung fibroblasts or Chinese hamster embryo fibroblasts (CHEF/18), we treated cells for 30 min, under conditions that expanded one or more pools of deoxyribonucleoside triphosphates. This was followed immediately by a 30-min treatment with 0.5 mM N-methyl-N-nitrosourea. After 8 days of additional culture for recovery of cells and expression of mutations, we plated in selective media to determine the abundance of 6-thioguanine-resistant mutants in each culture. We found that conditions which expand pools of either dATP or dTTP and dGTP stimulate mutagenesis by MNU, with the degree of stimulation varying in different experiments from 2- to 6-fold. Although alternate interpretations can be entertained, the data are consistent with the hypothesis that nucleotide pools represent alkylation targets. A biochemical test of the hypothesis is warranted. During our studies we made several other noteworthy observations: (1) treatment of V79 cells with mutagen alone does not significantly affect dNTP pools; (2) deoxynucleotide pool perturbations are quite short-lived following transfer of cells to normal medium; (3) deoxyuridine is significantly more effective than thymidine in expanding dTTP pools; (4) deoxyuridine by itself is significantly mutagenic, particularly to CHEF/18 cells.

Mutagenesis and deoxyribonucleotide pool imbalance

Numerous studies have demonstrated that DNA-precursor pool imbalances are mutagenic and can modulate the lethality and mutagenicity of DNA-damaging agents. In addition, physical and chemical mutagens can induce alterations in DNA-precursor levels. Such findings suggest that regulation of intracellular concentrations of DNA precursors may be an important factor in environmental mutagenesis. In this article, results linking mutation and disturbances in DNA-precursor pools are reviewed.

Aberrant DNA repair and enhanced mutagenesis following mutagen treatment of Chinese hamster Ade-C cells in a state of purine deprivation

Ade-C is a Chinese hamster ovary cell line auxotrophic for purines because of a mutation in the de novo synthetic pathway. We now show that, in the absence of exogenous hypoxanthine, replicative DNA synthesis is rapidly shut down. Various aspects of DNA repair have been studied in purine-starved cells. Incision, the first step of excision repair of UV damage, appears normal, as do the later steps, repair synthesis (demonstrated following chemical damage as well as UV-irradiation) and ligation. However, removal of UV-induced pyrimidine dimers is not detected, and it seems that the repair that occurs is aberrant. This behaviour is associated with an increase in cell killing by UV light, and a several-fold increase in the frequency of mutations induced by UV.

Synergism between U.V. and thymidine treatments in the induction of cytogenetic damage in wild-type Friend erythroleukaemia cells

The cytogenetic aberration-inducing effect of excess thymidine and U.V. light in Friend erythroleukaemia (clone 707) cells was investigated. Three doses of U.V. were utilised, namely 2.4, 4.8 and 7.2 J/m2. Thymidine, at 1 X 10(-5) M, was present for 48 h prior to U.V. treatment and for 15 h following it. Although no significant increase in metaphase aberrations was observed following thymidine treatment alone, relative to the spontaneous frequency, clone 707 exhibited increased sensitivity to U.V.-induced cytogenetic damage, when grown in the presence of 1 X 10(-5) M thymidine. The observed synergism between U.V. and thymidine treatments may be due to thymidine-induced nucleotide pool imbalance with consequent inaccuracies in DNA repair.

Hydroxyurea: effects on deoxyribonucleotide pool sizes correlated with effects on DNA repair in mammalian cells

We have measured deoxyribonucleotide pool sizes in different cell types: normal human, transformed human (HeLa), and the permanent hamster line CHO-K1. The range of sizes of the four DNA precursor pools in CHO cells is far greater than in human cells. It is a general rule that hydroxyurea causes rapid depletion of pools (except for dTTP) until the pool present in smallest amount is exhausted; this suggests a tight coupling of the pools to DNA replication (the presumed main cause of the depletion). The effect of hydroxyurea on DNA repair after ultraviolet irradiation (namely, a relatively small accumulation of incomplete repair sites blocked at the resynthesis stage) is probably accounted for by the reduced availability of DNA precursors. However, depletion of the dCTP pool is not an adequate explanation for the observed enhancement by hydroxyurea of the inhibitory effect of cytosine arabinoside; we suggest other possible modes of action. Ultraviolet irradiation has only small effects on the levels of deoxyribonucleotides.

Abnormal mutation frequencies in human repair-defective hybrid cell lines

Two intraspecific human cell hybrids, HD2 and HD1A, produced from fusion between HeLa cells and xeroderma pigmentosum fibroblasts, express XPD-like rates of excision repair and hypersensitivity to UV-radiation. In the present paper we describe unusual patterns of UV-induced mutation in both cell lines. Though HD2 very closely resembles XPD both phenotypically and genetically, in UV-dose response it is hypomutable at the loci for ouabain and diphtheria toxin resistance. At equitoxic dose, however, it shows normal mutability, HD1A, by contrast, is hypermutable as a function either of UV dose or in terms of equitoxicity for these genes. HD1A's mutator phenotype is a dominant characteristic and is not associated with grossly abnormal DNA precursor pool imbalance. The possibility remains that DNA polymerase infidelity underlies its hypermutability.

DNA nucleotide excision-repair synthesis is independent of perturbations of deoxynucleoside triphosphate pool size

We have investigated the effects of fluctuations in deoxynucleoside triphosphate (dNTP) pool size on DNA repair and, conversely, the effect of DNA repair on dNTP pool size. In confluent normal human skin fibroblasts, dNTP pool size was quantitated by the formation of [3H]TTP from [3H]thymidine; DNA repair was examined by repair replication in cultures irradiated with UV light. As defined by HPLC analysis, the [3H]TTP pool was formed within 30 min of the addition of [3H]thymidine and remained relatively constant for the next 6 h. Addition of 2-10 mM hydroxyurea (HU) caused a gradual 2-4-fold increase in the [3H]TTP pool as HU inhibited DNA synthesis but not TTP production. No difference was seen between the [3H]TTP pool size in cells exposed to 20 J/m2 and unirradiated controls, although DNA-repair synthesis was readily quantitated in the former. This result was observed even though the repair replication protocol caused an 8-10-fold reduction in the size of the [3H]TTP pool relative to the initial studies. In the UV excision-repair studies the presence of hydroxyurea did not alter the specific activity of [3H] thymidine 5'-monophosphate incorporated into parental DNA due to repair replication. These results suggest that fluctuations in the deoxynucleoside triphosphate pools do not limit the extent of excision-repair synthesis in human cells and demonstrate that DNA nucleotide excision-repair synthesis does not significantly diminish the size of the [3H]TTP pool.

Abilities of wild-type and thymidine kinase-deficient Friend mouse erythroleukemia cells to undergo unscheduled DNA synthesis following mutagen treatment

The abilities of wild-type and thymidine kinase-deficient Friend mouse erythroleukemia cells to perform unscheduled DNA synthesis (UDS), through the incorporation of [3H]deoxycytidine, were measured following damage with methyl methane sulfonate (MMS), ethyl methane sulfonate (EMS), and ultraviolet (UV) irradiation. For each mutagenic treatment, a positive and quantitatively similar response was observed for both wild-type and thymidine kinase-deficient cells. The extent of the response varied greatly, however, depending upon the mutagen used. The results contrast with the unscheduled incorporation of [3H]thymidine in wild-type cells following mutagen treatment, where less variation between the positive UDS responses elicited by MMS, EMS, and UV treatments was observed. Nevertheless, the results clearly indicate that thymidine kinase deficiency does not prevent excision repair (UDS) from occurring.

Analysis of bromodeoxyuridine-induced single and twin sister chromatid exchanges in tetraploid Chinese hamster ovary cells

Culture of cells in high exogenous levels (greater than 10(-4) M) of bromodeoxyuridine (BrdUrd) or thymidine will increase the baseline sister chromatid exchange (SCE) frequency. The effect is thought to be related to the balance of the DNA precursors thymidine and deoxycytidine. Exogenous addition of deoxycytidine will reverse this effect. Single and twin SCEs were analysed in Colcemid-induced tetraploid Chinese hamster ovary cells exposed to different concentrations of BrdUrd to determine at what stage SCEs are induced by high levels of BrdUrd. In cells exposed to low concentrations of BrdUrd (10(-5) M), equal numbers of SCEs were induced in each of the two cell cycles. With increasing concentrations of BrdUrd (10(-4) to 2 X 10(-4) M), SCE frequency increased in both cell cycles, but far more SCEs were induced in the second cell cycle. Deoxycytidine (2 X 10(-4) M) reduced the frequency of SCEs primarily by reducing the frequency of SCEs induced in the second cell cycle. Treatment with 3-aminobenzamide (3AB), a potent inhibitor of poly(ADP-ribose) polymerase, produced effects similar to exposure to high levels of BrdUrd including inducing SCEs in the second replication cycle. This suggests a similar mechanism of action. Deoxycytidine had no effect on 3AB-induced SCEs, however, and there was no interaction between 3AB and high exogenous levels of BrdUrd in SCE induction. Thus these two agents probably act through different mechanisms.

Quantitative and molecular analyses of ethyl methanesulfonate- and ICR 191-induced mutation in AS52 cells

A pSV2gpt-transformed Chinese hamster ovary (CHO) cell line has been used to study mutation at the molecular level. This cell line, designated AS52, was constructed from a hypoxanthine-guanine phosphoribosyl transferase (HPRT)-deficient CHO cell line, and has been previously shown to contain a single, functional copy of the E. coli xanthine-guanine phosphoribosyl transferase (XPRT) gene (gpt) stably integrated into the Chinese hamster genome. In this study, conditions for its use in the study of mammalian cell mutagenesis have been stringently defined. The spontaneous mutation rate (2 X 10(-6)/cell division) and phenotypic expression time (7 days) of the gpt locus compare favorably with those of the hprt locus in wild-type CHO-K1-BH4 cells. While both cell lines exhibit similar cytotoxic responses to ethyl methanesulfonate (EMSO and ICR 191, significant differences in mutation induction were observed. Ratios of XPRT to HPRT mutants induced per unit dose of EMS and ICR 191 are 0.70 and 1.6, respectively. Southern blot hybridization analyses revealed that most XPRT mutant cell lines which arose following treatment with EMS (20/22) or ICR 191 (20/24) exhibited no alterations of the gpt locus detectable by this technique. Similar observations were made for the hprt locus in EMS-(21/21) and ICR 191-induced (22/22) HPRT mutants. In contrast, most spontaneous gpt mutants (14/23) contained deletions, while most spontaneous hprt mutants (18/23) exhibited no detectable alterations. Results of this study indicate that the AS52 cell line promises to be useful for future study of mutation in mammalian cells at the DNA sequence level.

The competitive miscoding of O6-methylguanine and O6-ethylguanine and the possible importance of cellular deoxynucleoside 5'-triphosphate pool sizes in mutagenesis and carcinogenesis

Using initiated poly(dG,O6-RdG) and poly(dA,O6-RdG) polynucleotides as templates for DNA polymerase I in vitro the promutagenic potential of O6-MeG and O6-EtG has been confirmed, together with the possibility of minor miscoding pathways for O6-RG. These lead to the incorporation of dAMP and dGMP, which could give rise to some of the limited number of transversions that have been observed arising from the action of alkylating agents. The results are compatible with the current knowledge of oncogenes, explaining the changes in base sequence that have been observed. The competition for the miscoding of O6-RG which leads to the incorporation of dCMP in addition to the expected dTMP is also shown. The relative amounts of these two nucleotides incorporated depend upon the concentrations of the dCTP and dTTP in the assay. The mutagenic efficiency of O6-MeG is constant at approx. 0.4 over a wide range of dTTP and dCTP concentrations and only increases when the dCTP in the assay ceases to saturate the polymerizing enzyme, indicating that the DNA polymerase I plays a role in determining the mutagenic efficiency of a modified base. Although the mutagenic efficiency of both O6-MeG and O6-EtG depends upon the relative concentrations. of dTTP and dCTP in the assay, a reduction in the concentration of dCTP can be more effective at increasing the mutagenic efficiency than a corresponding increase in the concentration of dTTP. These results indicate the importance of cellular dNTP pools in determining the cellular response to agents.

Accuracy of UV-induced DNA repair in V79 cells with imbalance of deoxynucleotide pools

Chemical mutagens generally cause nucleotide pool imbalance. We postulated that this effect might enhance the mutagenic effect by reducing the accuracy of DNA-repair synthesis. We used an inducer of DNA repair which causes minor pool modifications, namely UV light, and imbalanced the nucleotide pools by incubating UV-irradiated V79 cells with thymidine or deoxycytidine (10(-5)-10(-2) M) during the early phases of repair. The effects on pool sizes of the incubation with deoxynucleosides were determined by directly measuring the 4 deoxynucleoside triphosphates in cell extracts. The impairment of repair accuracy was evaluated by comparing the frequency of mutations at the HGPRT locus (induction of resistance to 6TG) in irradiated cells incubated with deoxynucleosides or allowed to carry on repair synthesis in nucleoside-free medium. Despite the marked imbalance of pyrimidine nucleotide pools, an increase of mutations was observed only with the highest concentrations of thymidine and deoxycytidine. Such an increase was much lower than that reported in the case of facilitation by excess nucleosides of chemically induced mutagenesis. The results indicate that UV-induced repair is scarcely affected by precursor biases.

The effects of exogenous thymidine on endogenous deoxynucleotides and mutagenesis in mammalian cells

The intracellular deoxyribonucleoside triphosphate pools in mammalian cells affect diverse biological functions including the spontaneous or induced mutability. We have isolated from murine T-lymphosarcoma S49 cells, a mutant that is unable to convert dCMP to dUMP, contains deranged intracellular dNTP pools, and exhibits a mutator phenotype. The enzymatic defect in araC-6-1 cells is a deficiency of deoxycytidylate deaminase, which accounts for the high dCTP and low TTP intracellular pools. The addition of increasing concentrations of exogenous thymidine to araC-6-1 cells alters these dNTP pools in a predictable manner: increasing the TTP and diminishing the dCTP. Concomitant with this reversal of the dCTP:TTP ratio is a marked decrease in the mutation rate followed by an increase in the mutation rates at higher exogenous thymidine concentrations. This response of the mutation rate is in contrast to that seen in the control cell line containing normal deoxycytidylate deaminase. In the latter case, increasing thymidine concentration induces an enhanced mutation rate that parallels the later phase of the thymidine-induced mutation rate in araC-6-1 cells. The deficiency of deoxycytidylate deaminase, the endogeneous dNTP pool alterations, and the mutator phenotype of araC-6-1 cells are all recessive traits in cell-cell hybrids. These observations allow one to predict whether exogenous thymidine will be mutagenic, antimutagenic, or both for a given cell line and provide a basis for understanding conflicting reports in the literature concerning the effects of the thymidine on genomic stability.

The effect of thymidine on the induction of micronuclei by alkylating agents in V79 Chinese hamster cells

Incubation in thymidine-containing medium resulted in increased lethality and micronucleus frequency in V79 cells treated with ethyl nitrosourea (ENU), methyl nitrosourea (MNU) and ethyl methane-sulphonate (EMS) but not with methyl methanesulfonate (MMS). Thymidine had no effect in ENU treated HeLa cells. In V79 cells, the presence of thymidine during post-treatment DNA replication was necessary for the effect. It is suggested that the increase in chromosome damage was the result of an increased O6-alkylguanine-thymine mispairing in cells which are defective in the repair of O6-alkylguanine. Treatment of V79 cells with O6-ethylguanine resulted in increased production of both micronuclei and polyploid cells. These effects might be explained by spindle dysfunction caused by the alkylated guanine.

Evidence for indirect involvement of thymidine kinase in excision repair processes in mouse cell lines

Wild-type cells and thymidine kinase-deficient clones from two mouse lymphoma cell lines, P388 and L5178Y, were compared for sensitivity to killing by the mutagens, ultraviolet irradiation (UV), ethyl methane sulfonate (EMS), and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Two out of three thymidine kinase-deficient P388 clones showed significantly enhanced sensitivity to killing by all three mutagens. This increased sensitivity to killing was also reflected in increased mutagenesis by the three mutagens. In the L5178Y cell line, wild-type cells showed little difference to two thymidine kinase-deficient clones in terms of mutagen sensitivity. This indicates that thymidine kinase may be significant for DNA repair processes in P388 but not in L5178Y cells. Unscheduled DNA synthesis (UDS) experiments were carried out on P388 and L5178Y wild-type cells and wild-type Friend leukemia cells (which are mutagen-sensitive when deficient in thymidine kinase). The UDS experiments showed the L5178Y cells were low in excision repair abilities relative to the P388 cells and the Friend cell clone. This indicates that the increased mutagen sensitivity in thymidine kinase-deficient P388 and clone 707 Friend cells may be due to thymidine kinase playing an indirect role in DNA excision repair, a process which is of little significance in the L5178Y cell line.

Mechanism of UV-induced deoxynucleoside triphosphate pool imbalance in CHO-K1 cells

Several laboratories have reported that exposure of cells to UV radiation results in a significant imbalance in deoxynucleoside triphosphate pool concentrations. In our CHO-K1 cells, a rapid drop in dCTP is accompanied by a rapid increase in dTTP. Examination of enzyme activities associated with synthesis/degradation of these molecules suggests that UV transiently enhances a putative dCTPase, dCMP deaminase and CdR kinase activities. This results in accumulation of excess dUMP which is probably converted to dTMP, then to dTTP. The absence of dCMP deaminase in V79 cells prohibits this rapid response in those cells. Moreover, significantly different dCMP deaminase activities were observed in CHO-K1 cells obtained from other laboratories, suggesting they, too, may respond differently to irradiation.

Hypersensitivity of thymidine kinase-deficient Friend leukaemia cells to the induction of cytogenetic aberrations by mitomycin C

Clone 707 of the Friend leukaemia cell line was compared with the hypermutable thymidine kinase deficient subclone 707 BUF for sensitivity to the induction of cytogenetic aberrations by mitomycin C (MMC). Two 16-h doses of MMC were utilized, namely 0.1 and 0.15 microgram ml-1. Following removal of MMC from the cultures, metaphase spreads were prepared after 15, 29 and 43 h growth in non-selective medium. Thirteen types of aberrations were scored. The thymidine kinase deficient subclone showed considerably increased sensitivity to the induction of aberrations, with the aberrations also persisting longer. In light of these and earlier reported results, the significance of thymidine kinase for accurate DNA repair is discussed.

The effects of pyrimidine nucleotides on alkylating agent induced cytotoxicity and spontaneous and induced mutation to purine analog resistance in V79 cells

Exposure of three V79 cell lines to dT after treatment with monofunctional alkylating agents resulted in potentiation of alkylation induced cytotoxicity. The degree of potentiation achieved was dependent on the concentration and duration of exposure to dT and was reversed by equimolar concentrations of dCyd. Exposure to dT after UV or X-irradiation or treatment with HN2 or MMC did not affect the cytotoxic response. dT exposure at non-cytotoxic concentrations did not affect DNA synthesis as measured by [3H]-dT incorporation when allowance was made for reductions in specific activity of labelled thymidine. However, dT post treatment reversed the alkylation induced inhibition of DNA synthesis. Toxic concentrations of dT caused an increase in frequency of TGR colonies but this increase was shown to be due to effects of dT on cell growth rate, and differential sensitivity ot HGPRT- and HGPRT+ cells. The frequency of spontaneous and alkylation induced AZR and to a lesser extent TGR colonies was also increased by non-toxic dT concentrations. Evidence was obtained which suggests that this increase is more likely to be due to alterations in the selective efficiency of the purine analogs than alterations in coding fidelity due to altered dNTP pools.

Antipain-mediated suppression of sister chromatid exchanges induced by an inhibitor of poly (ADP-ribose) polymerase

Exposure of mammalian cells to inhibitors of poly(ADP-ribose) polymerase, such as 3-aminobenzamide (3AB) results in the induction of sister chromatid exchanges (SCEs). The mechanism for the induction of SCEs by 3AB is unknown but is thought to be related to the incorporated halogenated pyrimidine used in SCE analysis. In this characteristic, 3AB-mediated SCE induction is similar to the elevated SCE frequency found in Bloom's syndrome (BS) cells. Recently, it has been reported that certain protease inhibitors, such as antipain, will inhibit SCE induction in BS cells. We now report that antipain will also suppress 3AB-induced SCE frequency. As has been reported for BS cells, the effects of antipain on SCE induction are partial, reducing SCE frequency by 0.15 to 0.40 SCE/chromosome (5-25% of the total induced frequency), and 30 microM concentrations of antipain are saturating. Antipain has no effect on baseline SCE frequency. These effects appear to involve free-radical production because dimethylsulfoxide (DMSO), a free-radical scavenger, will mimic the effects of antipain on 3AB-induced SCEs. Both antipain and DMSO will also reduce the elevated SCE frequency found in cells exposed to high (100 microM or more) levels of bromodeoxyuridine (BrdUrd). High exogenous levels of BrdUrd produce some of the same biological effects as 3AB exposure. Thus, a minor fraction of the elevated SCE frequency seen in cells exposed to 3AB or to high levels of BrdUrd appears to be similar to that found in cultured BS cells and is probably due to some free-radical-producing process.

Deoxyribonucleoside-induced selective modulation of cytotoxicity and mutagenesis

Treatment of Chinese hamster V79 cells with dThd, dCyd, or dThd plus dCyd increased MNNG-induced AGr-, TGr-, and Ouar-mutant frequencies but did not significantly increase background mutant frequencies. All the AGr colonies that were isolated possessed phenotypes characteristic of HGPRT-deficient mutants, and the deoxyribonucleosides did not selectively affect the growth of the mutants, nor the selecting efficiency of AG, and did not significantly enhance background mutagenesis. These data show that both dThd and dCyd facilitated MNNG-induced mutagenesis. This facilitation was maximal when cells were exposed to the deoxyribonucleosides throughout the first doubling time (24 h) after treatment with MNNG and for 4 more doubling times prior to mutant selection with AG. This indicates that one round of DNA replication was sufficient for mispairing of methylated bases in the DNA with the C and T provided by the deoxyribonucleosides, and that 4-6 doublings prior to mutant selection with AG were necessary to deplete pre-existing hypoxanthine: guanine phosphoribosyl transferase in newly mutated cells. The dCyd facilitated mutagenesis by FdUrd, which was not mutagenic without dCyd, indicating that increased dCTP:dTTP ratios were mutagenic. Treatment with FdUrd plus dCyd also induced FdUrdr cells, suggesting that inhibition of dCyd utilization may prevent the development of FdUrd-resistance in cancer chemotherapy. Although dCyd and dThd facilitated mutagenesis in cells treated with monofunctional alkylating agents that methylate DNA oxygens, facilitation of mutagenesis did not occur in cells treated with BCNU, which cross links DNA, nor with benzo(a)pyrene and aflatoxin B1, which are frame shift mutagens, nor with MMS, which produces barely detectable levels of O-methylation in DNA. Virtually non toxic concentrations of dThd potentiated the cytotoxicity of MNNG more than 10-fold but that of MMS was potentiated only about 2-fold showing that O-alkylation of DNA was associated not only with the facilitation of mutagenesis but also with the potentiation of cytotoxicity. The potentiation of MNNG-induced cytotoxicity was maximal in V79 and L1210 cells after only 2 h treatment with dThd, showing that not even one round of DNA replication was necessary for this potentiation. Moreover, dCyd abolished the potentiation, and, at equitoxic concentrations, MNNG induced higher mutant frequencies than did MMS. These data show that the mechanisms by which methylating agents plus dThd induce mutagenesis are fundamentally different from their mechanisms of cytotoxicity.(ABSTRACT TRUNCATED AT 400 WORDS)

Structural alterations of the aprt locus induced by deoxyribonucleoside triphosphate pool imbalances in Chinese hamster ovary cells

Mutants induced at the adenine phosphoribosyl transferase (aprt) locus by dTTP or dCTP pool imbalances were examined for alterations in genomic DNA sequences. No observable changes were detected by Southern blot analysis of most mutant DNAs, suggesting induction of base pair alterations or other events below our level of detection (approximately 30 base pairs). However, in a few strains (11 from a total collection of 125 mutant cell strains), we were able to localize these events to restriction endonuclease recognition sequences when the mutations resulted in the loss or gain of a particular site. The distribution of lost or gained sites in aprt-deficient mutants induced by the two types of pool imbalances clearly varied, with those occurring in a mutator strain with increased dCTP clustering at one end of the aprt gene. Mutants induced by dTTP also revealed novel events: multiple restriction site modifications in a small region of the aprt gene in one mutant and a small (approximately 50 base pairs) insertion or duplication of DNA sequences. As in previous studies, very few deletion or insertion mutants were detected at the aprt locus. The significance of these findings in terms of the known biochemical and genetic consequences of these pool imbalances is discussed.

Structural alterations of the aprt locus induced by deoxyribonucleoside triphosphate pool imbalances in Chinese hamster ovary cells

Mutants induced at the adenine phosphoribosyl transferase (aprt) locus by dTTP or dCTP pool imbalances were examined for alterations in genomic DNA sequences. No observable changes were detected by Southern blot analysis of most mutant DNAs, suggesting induction of base pair alterations or other events below our level of detection (approximately 30 base pairs). However, in a few strains (11 from a total collection of 125 mutant cell strains), we were able to localize these events to restriction endonuclease recognition sequences when the mutations resulted in the loss or gain of a particular site. The distribution of lost or gained sites in aprt-deficient mutants induced by the two types of pool imbalances clearly varied, with those occurring in a mutator strain with increased dCTP clustering at one end of the aprt gene. Mutants induced by dTTP also revealed novel events: multiple restriction site modifications in a small region of the aprt gene in one mutant and a small (approximately 50 base pairs) insertion or duplication of DNA sequences. As in previous studies, very few deletion or insertion mutants were detected at the aprt locus. The significance of these findings in terms of the known biochemical and genetic consequences of these pool imbalances is discussed.

Inhibition of malignant transformation in vitro by inhibitors of poly(ADP-ribose) synthesis

Malignant transformation in vitro of hamster embryo cells and mouse C3H 10T 1/2 cells by x-rays, ultraviolet light, and chemical carcinogens was inhibited by benzamide and by 3-aminobenzamide at concentrations that are specific for inhibition of poly(ADP-ribose) formation. These compounds slow the ligation stage of repair of x-ray and alkylation damage but not of ultraviolet light damage. At high concentrations they also inhibited de novo synthesis of DNA purines and DNA methylation by S-adenosylmethionine. The suppression of transformation by the benzamides is in striking contrast to their reported effectiveness in enhancing sister chromatid exchange, mutagenesis, and killing in cells exposed to alkylating agents. Our results suggest that mechanisms regulating malignant transformation are different from those regulating DNA repair, sister chromatid exchange, and mutagenesis and may be associated with changes in gene regulation and expression caused by alterations in poly(ADP-ribosyl)ation.

Deoxycytidine kinase-deficient mutants of Chinese hamster ovary cells are hypersensitive to DNA alkylating agents

Chinese hamster ovary cell strains deficient in deoxycytidine kinase activity were selected by isolating mutants resistant to high concentrations of the analogue arabinosyl cytosine. Mutants isolated were deficient in the pool of dCTP, supporting earlier a suggestion that the deoxycytidine kinase may play a role in the turnover and maintenance of the dCTP pool. Consistent with earlier observations that increased intracellular levels of dTTP relative to dCTP lead to increased sensitivity to monofunctional DNA alkylating agents, deoxycytidine kinase-deficient mutants showed a 2-5-fold increase in sensitivity to the cytotoxic and mutagenic effects of one agent, ethyl methanesulfonate (EMS). The survival of the two kinase-deficient strains after mutagen treatment was clearly related to dCTP level as the strain with lowest dCTP was most sensitive to EMS. Thus hypersensitivity to this class of DNA damaging agents can result from cellular mutations decreasing the intracellular level of dCTP.