Thymidineless death and mutation induction in cultured mouse FM3A cell mutants deficient in thymidylate synthase

Thymidylate synthase is essential for efficient HIV-1 replication in macrophages

Thymidylate synthase (TS) is a key enzyme in nucleotide biosynthesis. A study performed by our group on human monocyte-derived macrophages (MDMs) infected with HIV-1 showed that many enzymes related to the folate cycle pathway, such as TS, are upregulated in productively infected cells. Here, we suggest that TS is essential for an effective HIV-1 infection in MDMs. Indeed, a TS specific small interfering RNA (siRNA) as well as the TS specific inhibitor Raltitrexed (RTX) caused a reduction in productively infected cells. Quantitative PCR analysis showed that this treatment decreased the efficacy of the early steps of the viral cycle. The RTX inhibitory effect was counteracted by dNTP addition. These results suggest that TS is essential for the early stages of HIV-1 infection by providing optimal dNTP concentrations in MDMs. TS and its related pathway may thus be considered as a potential therapeutic target for HIV-1 treatment.

Multisubstrate adduct inhibitors: drug design and biological tools

In drug discovery, different methods exist to create new inhibitors possessing satisfactory biological activity. The multisubstrate adduct inhibitor (MAI) approach is one of these methods, which consists of a covalent combination between analogs of the substrate and the cofactor or of the multiple substrates used by the target enzyme. Adopted as the first line of investigation for many enzymes, this method has brought insights into the enzymatic mechanism, structure, and inhibitory requirements. In this review, the MAI approach, applied to different classes of enzyme, is reported from the point of view of biological activity.

Natural antimutagenic agents

Following a brief review of recent discoveries in the field of natural antimutagenic and tumor chemopreventive agents, contemporary findings in the author's laboratories employing the direct acting mutagen, ethyl methanesulfonate, in modified Ames tests and eukaryotic murine FM3A mammary tumor cells modified to be subject to thymidine-less death are described to illustrate the underlying principles. The EMS studies are illustrated with the isolation of the novel antimutagen, plicatin B, from the medicinal plants, Psoralea juncaea and P. plicata. The FM3A studies are carried out with extracts of Styrax asiatica, a plant previously studied extensively with the EMS system. The FM3A findings closely parallel the earlier work with EMS showing that the responsible agents, cinnamic acid, cinnamoyl ricinoleate and cinnamoyl cinnamate are effective both in prokaryotic and eukaryotic tests and that the new FM3A assay system has useful properties for screening and assay of novel antimutagenic agents.

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.

The genetic toxicology of 5-fluoropyrimidines and 5-chlorouracil

The halogenated pyrimidines were synthesized in the 1950s as potential anti-tumor agents after the discovery that certain tumors preferentially incorporated uracil rather than thymine into the DNA. The fluorinated derivatives are widely recognized today as effective treatment modalities, especially with tumors of the head, neck and breast. Mechanistically, efficacy of the fluorinated pyrimidines results from the ability of these compounds to incorporate into RNA and inhibit its maturation to those forms necessary for cellular metabolism and from the inhibition of the enzyme, thymidylate synthetase, which controls the biosynthesis of thymine and DNA synthesis. The 5-fluoropyrimidines can incorporate into DNA, but the contribution of this phenomenon to the overall efficacy of this class of chemotherapeutic agents is not totally resolved. Evidence exists that this class of compounds possesses the properties to induce genotoxic effects, both in bacterial and eukaryotic cells. Most notably, these effects include the induction of cellular toxicity and the induction of chromosome aberrations. The biology and chemistry of the chlorinated pyrimidines were first explored as a possible means of sensitizing the DNA to ionizing radiation in a manner similar to the sensitization observed when DNA incorporates bromodeoxyuridine. This approach was not utilized clinically. The genetic toxicology of this compound became important with the discovery of the ribonucleoside in the effluents of sewage treatment plants. Evidence is now available that the chlorinated pyrimidines, upon conversion to deoxyribonucleosides, are effective mutagens, clastogens and toxicants, as well as extremely effective inducers of sister-chromatid exchanges.

Wild-type p53 restores cell cycle control and inhibits gene amplification in cells with mutant p53 alleles

Loss of cell cycle control and acquisition of chromosomal rearrangements such as gene amplification often occur during tumor progression, suggesting that they may be correlated. We show here that the wild-type p53 allele is lost when fibroblasts from patients with the Li-Fraumeni syndrome (LFS) are passaged in vitro. Normal and LFS cells containing wild-type p53 arrested in G1 when challenged with the uridine biosynthesis inhibitor PALA and did not undergo PALA-selected gene amplification. The converse occurred in cells lacking wild-type p53 expression. Expression of wild-type p53 in transformants of immortal and tumor cells containing mutant p53 alleles restored G1 control and reduced the frequency of gene amplification to undetectable levels. These studies reveal that p53 contributes to a metabolically regulated G1 check-point, and they provide a model for understanding how abnormal cell cycle progression leads to the genetic rearrangements involved in tumor progression.

Thymidylate stress induces homologous recombination activity in mammalian cells

We studied whether homologous recombination activity in mammalian cells could be induced by thymidylate stress (thymidylate deprivation). In vitro recombination activity in cell extracts was measured with pSV2neo-derived plasmids. When prior to the preparation of extracts, mouse FM3A cells were grown in 5-fluorodeoxyuridine (FdUrd), an inducer of thymidylate stress, the homologous recombination activity was significantly induced, as judged from an increase in the number of neomycin-resistant bacterial colonies. Maximum induction was observed in cells treated with 1 microM FUdR for 16 h. However, 3-8 h of treatment of FM3A cells with the drug followed by an additional 8-16-h incubation in its absence was sufficient to induce the recombination activity while slightly reducing their growth rates. These results indicate that thymidylate stress induces homologous recombination activity in mammalian cells as observed in Escherichia coli and in yeast.

Thymidylate synthetase gene as a quantitative mutation marker in Chinese hamster cells

A quantitative mutation marker for cultured mammalian cells is presented which uses a selective medium containing folinic acid, aminopterin and thymidine (the 'FAT' medium) to select for mutants deficient in thymidylate synthetase (TS) activity. Optimization of FAT medium was carried out using Chinese hamster V79 cell lines having 3 levels of TS activity. By manipulating the concentration of folinic acid in FAT medium, TS-deficient mutants can be readily selected. TS mutation is inducible in a dose-dependent manner by either ethyl methanesulfonate or ultraviolet light irradiation. Expression time for TS mutation was also determined using two concentrations of ethyl methanesulfonate and found to be very short, being 1 or 2 days. This newly characterized TS mutation marker should be useful in the study of both spontaneous and induced mutagenesis.

A specialized form of chromosomal DNA degradation induced by thymidylate stress in mouse FM3A cells

Thymidylate synthase-negative mutant mouse cells starved of thymidine or their parental FM3A cells treated with 5-fluoro-2'-deoxyuridine produced DNA fragments ranging from 50 to 200 kilobase pairs with a peak at 100 kb in length as determined by pulsed-field agarose gel electrophoresis. Accumulation of the DNA fragments following such thymidylate stress was time-dependent but their size distribution did not change in either case. Regions of the chromosomal DNA breaks seemed to be restricted to those where DNA replication was in progress as shown by pulse-labeling of the DNA synthesis. Emetine, an inhibitor of protein synthesis, blocked the accumulation of the DNA fragments when present during thymidylate stress. Cell-free extracts prepared from the thymidylate-stressed cells derived by either of the above means were capable of degrading DNA in chromatins prepared from normally growing cells in vitro. The resulting DNA fragments were similar but with a somewhat broader size distribution compared to those produced in vivo. The broader distribution of the fragments produced in the in vitro reaction became closer to the pattern obtained in vivo when ATP and 4 deoxyribonucleotides were added to the reaction.

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.

Induction, by thymidylate stress, of genetic recombination as evidenced by deletion of a transferred genetic marker in mouse FM3A cells

Studies were made on the genetic consequences of methotrexate-directed thymidylate stress, focusing attention on a human thymidylate synthase gene that was introduced as a heterologous genetic marker into mouse thymidylate synthase-negative mutant cells. Thymidylate stress induced thymidylate synthase-negative segregants with concomitant loss of human thymidylate synthase activity with frequencies 1 to 2 orders of magnitude higher than the uninduced spontaneous level in some but not all transformant lines. Induction of the segregants was suppressed almost completely by cycloheximide and partially by caffeine. Thymidylate stress did not, however, induce mutations, as determined by measuring resistance to ouabain or 6-thioguanine. Thymidylate synthase-negative segregants were also induced by other means such as bromodeoxyuridine treatment and X-ray irradiation. In each of the synthase-negative segregants induced by thymidylate stress, a DNA segment including almost the whole coding region of the transferred human thymidylate synthase gene was deleted in a very specific manner, as shown by Southern blot analysis with a human Alu sequence and a human thymidylate synthase cDNA as probes. In the segregants that emerged spontaneously at low frequency, the entire transferred genetic marker was lost. In the segregants induced by X-ray irradiation, structural alterations of the genetic marker were random. These results show that thymidylate stress is a physiological factor that provokes the instability of this exogenously incorporated DNA in some specific manner and produces nonrandom genetic recombination in mammalian cells.

Induction, by thymidylate stress, of genetic recombination as evidenced by deletion of a transferred genetic marker in mouse FM3A cells

Studies were made on the genetic consequences of methotrexate-directed thymidylate stress, focusing attention on a human thymidylate synthase gene that was introduced as a heterologous genetic marker into mouse thymidylate synthase-negative mutant cells. Thymidylate stress induced thymidylate synthase-negative segregants with concomitant loss of human thymidylate synthase activity with frequencies 1 to 2 orders of magnitude higher than the uninduced spontaneous level in some but not all transformant lines. Induction of the segregants was suppressed almost completely by cycloheximide and partially by caffeine. Thymidylate stress did not, however, induce mutations, as determined by measuring resistance to ouabain or 6-thioguanine. Thymidylate synthase-negative segregants were also induced by other means such as bromodeoxyuridine treatment and X-ray irradiation. In each of the synthase-negative segregants induced by thymidylate stress, a DNA segment including almost the whole coding region of the transferred human thymidylate synthase gene was deleted in a very specific manner, as shown by Southern blot analysis with a human Alu sequence and a human thymidylate synthase cDNA as probes. In the segregants that emerged spontaneously at low frequency, the entire transferred genetic marker was lost. In the segregants induced by X-ray irradiation, structural alterations of the genetic marker were random. These results show that thymidylate stress is a physiological factor that provokes the instability of this exogenously incorporated DNA in some specific manner and produces nonrandom genetic recombination in mammalian cells.

Murine mammary FM3A carcinoma cells transformed with the herpes simplex virus type 1 thymidine kinase gene are highly sensitive to the growth-inhibitory properties of (E)-5-(2-bromovinyl)-2'-deoxyuridine and related compounds

Murine mammary carcinoma (FM3A TK-/HSV-1 TK+) cells, which are thymidine kinase (TK)-deficient but have been transformed with the herpes simplex virus type 1 (HSV-1) TK gene are inhibited in their growth by (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU), (E)-5-(2-iodovinyl)-2'-deoxyuridine (IVDU) and (E)-5-(2-bromovinyl)-2'-deoxycytidine (BVDC) at 0.5, 0.5 and 0.8 ng/ml, respectively; i.e., a concentration 5000 to 20 000-fold lower than that required to inhibit the growth of the corresponding wild-type FM3A/0 cells. Hence, transformation of tumor cells with the HSV-1 TK gene makes them particularly sensitive to the cytostatic action of BVDU and related compounds.

Assignment of human gene encoding thymidylate synthase to chromosome 18 using interspecific cell hybrids between thymidylate synthase-negative mouse mutant cells and human diploid fibroblasts

We have constructed interspecific somatic cell hybrids between a thymidine-auxotrophic mutant cell line of mouse FM3A cells that lacks thymidylate synthase and human diploid fibroblasts derived from a male patient with fragile X-linked mental retardation. Twenty primary hybrid clones were isolated independently, all of which exhibited the thymidine-prototrophic phenotype. Segregation of the hybrid cells in nonselective culture conditions gave rise to thymidine-auxotrophic hybrid clones. Both electrophoretic assay of thymidylate synthase activity and karyotype analysis of the segregants revealed a strong correlation between the expression of the human form of the enzyme and the presence of human chromosome 18. Thus, it is concluded that the functional gene for human thymidylate synthase, designated TS, is located on this chromosome.

Theoretical involvement of vitamin B6 in tumour initiation

Vitamin B6 is a co-enzyme involved in the biosynthesis of thymidine. Thymidine deficiency has been reported to increase DNA replication errors and hence increase mutagenesis. Carcinogenic stimulae increase the rate of DNA repair, and increase cell multiplication, both of which increase the requirement for thymidine. Vitamin B6 deficiency occurring at the same time as contact with carcinogens could well lead to tumour initiation and subsequent cancer development.

Thymidylate synthetase-positive and -negative murine mammary FM3A carcinoma cells as a useful system for detecting thymidylate synthetase inhibitors

The murine mammary FM3A/O and the thymidylate (dTMP) synthetase-deficient FM3A/TS- carcinoma cell lines can be considered as a novel and useful test system for the detection of nucleoside analogues which are directly aimed at the thymidylate synthetase. These compounds should be inhibitory for FM3A/O but not for FM3A/TS- cells, and their inhibitory effects on FM3A/O cell growth should be readily reversed by exogenous dThd within the concentration range of 5-20 microM.

Thymidylate synthetase-deficient mouse FM3A mammary carcinoma cell line as a tool for studying the thymidine salvage pathway and the incorporation of thymidine analogues into host cell DNA

A thymidylate (dTMP) synthetase-deficient murine mammary carcinoma cell line (FM3A/TS-), auxotrophic for thymidine (dThd), proved extremely useful for studying the dependence of cell growth on the exogenous supply of dThd, the relation between cell growth and DNA synthesis, and the ability of a series of 25 5-substituted 2'-deoxyuridines (dUrd) to substitute for dThd in sustaining cell growth. FM3A/TS-cells did not proliferate unless dThd was supplied to the cell culture medium. The 5-halogenated dUrd derivatives 5-chloro-dUrd, 5-bromo-dUrd and 5-iodo-d Urd also sustained FM3A/TS- cell growth. The extents of incorporation of [methyl-3H]dThd and 5-iodo-[6-3H]dUrd into DNA were closely correlated with their stimulatory effects on FM3A/TS- cell growth. This suggests that the stimulatory effects of the dUrd analogues on the growth rate of FM3A/TS- cells may be considered as evidence for their incorporation into host cell DNA. Based on this premise it is postulated that, in addition to 5-chloro-dUrd, 5-bromo-dUrd, 5-iodo-dUrd and dThd itself, the following dThd analogues are also incorporated into FM3A/TS- cell DNA (in order of the extent to which they are incorporated): 5-hydroxy-dUrd greater than 5-propynyloxy-dUrd greater than 5-ethyl-dUrd greater than 5-ethynyl-dUrd approximately 5-vinyl-dUrd. Thus, the dTMP synthetase-deficient FM3A/TS- cell line represents a unique system to dissociate the de novo and salvage pathways of dTMP biosynthesis and to distinguish those dUrd analogues that are incorporated into DNA from those that are not.