No abstract

A line of human lymphoid cells was tested for the presence of dUMP in DNA with or without treatment with the dihydrofolate reductase inhibitor, methotrexate. Cells treated with methotrexate and labeled with [3H"dUrd contained dUMP in DNA in readily detectable amounts (t0.8 pmol of dUMP per Mmol of total DNA nucleotide), and this was increased ;3-fold if the cells were also treated with Ura at the same time. No dUMP (<1 fmol/jMmol of DNA) could be detected by these methods in DNA from cells not treated with methotrexate, regardless of whether Ura was present or absent. The presence of dUMP in DNA from cells treated with methotrexate is a result of the great increase in intracellular concentration of dUTP and the fall in dTTP that accompany inhibition of thymidylate synthetase (5,1O-methylenetetrahydrofolate:dUMP C-methyltransferase; EC 2.1.1.45) by the drug. These changes are apparently sufficient to overcome the normal mechanisms that exclude dUMP from DNA, and the enhancement by Ura reflects suppression of one of the mechanisms, Ura removal from DNA by the enzyme Ura-DNA glycosylase. (20)(21)(22)(23)(24)(25)(26)(27). No animal cell mutant is available that lacks either enzyme, but drugs that inhibit thymidylate synthetase (5,10-methylenetetrahydrofolate:dUMP C-methyltransferase; EC 2.1.1.45) offer possibilities both of studying this mechanism in vivo and of providing a new look at effects of these drugs on cells.Inhibitors of thymidylate synthetase (e.g., the dihydrofolate reductase inhibitor, methotrexate), in addition to lowering the level of thymidylate (28-S1), greatly expand the pool of dUMP (29,32,33). In studies to be reported elsewhere we have found that accompanying these changes is an increase in dUTP > 103-fold. Taken with the reduction in dTTP to -2% of the original amount, this brings the ratio of intracellular dUTP/ dTTP from <10-5 to >10-1, and implies significant incorporation of dUMP into DNA. We report here direct measurements showing the presence of Ura in mammalian cell DNA in vvo under conditions in which thymidylate synthetase is inhibited by treatment of cells with methotrexate.MATERIALS (36,37). In contrast, DNA from methotrexate-treated cells contained low or barely detectable amounts of radioactivity in the portions taken for these measurements (Fig. 1).The DNA from the Sephadex column was pooled ( Fig. 1) and digested to dNMPs, which were then chromatographed on paper. The distribution of label in dNMPs for each of the DNA pools from the two samples described in Fig.

No abstract

Previous results from this laboratory have shown that thymidylate deprivation results in dramatic elevation of intracellular dUTP and incorporation of dUMP into DNA. The goal of the present studies was to determine whether the latter changes may play a part in the associated cytotoxicity ("thymineless death"), which is ordinarily assumed to be a direct result of reduced intracellular dTTP. The approach used here was to increase intracellular dUTP without allowing dTTP to diminish and observe the effects on cell viability. dUMP pools were expanded by exposure of cells to deoxyuridine [in cell growth medium containing hypoxanthine, methotrexate, and thymidine (HAT medium)], resulting in accumulation of dUTP to levels that approached those of dTTP, which were at, or higher than, the levels in untreated cells. In conjunction with this the cells became nonviable, and newly synthesized DNA was fragmented, both of which occur with thymidylate depletion and, we assume, result from the active process of excision repair at the many uracil-containing sites in DNA. The results indicate that, although the relative importance of low dTTP remains unknown, elevated dUTP can account for the cytotoxicity caused by thymidine starvation. Most of the "dTTP" measured by the DNA polymerase assay in cells treated with methotrexate (MTX) (plus purine supplement) was, in fact, dUTP, which may explain some previous observations of only modest depression of dTTP in cells treated with MTX or similarly acting drugs.

The double-stranded replicative form (RF) DNA of the autonomous parvovirus H-1 can be isolated from infected cells in a covalent complex with protein. The protein is present on most or all of the RF DNA, including actively replicating molecules, and is associated with the 5'-terminal endonuclease Hae III fragments of both the viral and complementary strands of RF. The size of the protein is estimated to be 60,000-70,000 daltons from its effect on buoyant density of DNA. DNA with covalently bound protein has not been found in H-1 virions.Since the description of plasmid relaxation complexes (1,2) there have been reports of covalent complexes between protein and nucleic acid in several RNA and DNA viruses. The protein has been found at the 5' end of the nucleic acid molecules in virions (3-13) and, in a few cases, also associated with the intracellular replicative forms of the nucleic acids (3, 9, 11).The studies described here were carried out with the autonomous parvovirus H-1 (14), which has a linear singlestranded DNA genome of 1.6 X 106 daltons with hairpin duplexes at both the 5' and 3' ends (14-16). Replication of viral DNA in the infected cell proceeds through a double-stranded intermediate, replicative form (RF), only one strand of which is packaged into the virion. We report here the presence of protein covalently associated with intracellular H-I RF DNA. The RF was found in the form of a complex of nucleic acid with a capsid-like structure which, after treatment with strong protein-denaturing conditions, left protein covalently bound to RF DNA. The covalently bound protein was not found in virions, in contrast to the results with other viral DNA-protein complexes. Some of the features of the covalent protein-DNA complexes are described here. MATERIALS AND METHODSPreparation of Labeled H-1 Viral Intermediates. Crude stocks of unlabeled H-1 virus were prepared from infected newborn hamsters by a procedure suggested by Solon Rhode (personal communication), which included homogenization in sodium dodecyl sulfate (NaDodSO4) and centrifugation through a 30% sucrose pad. NB cells (human embryonic kidney cells transformed by simian virus 40), grown as monolayers in modified Eagle's medium with 10% calf serum, were infected with H-1 virus (15 infectious units per cell) at a cell density of 4 X 106/10-cm plate (14, 17). The cells were labeled 18-20 hr after infection with [3H]dThd at 10 ,uCi/ml or [14C]dThd at 0.5 ,uCi/ml (1 Ci = 3.7 X 10l°becquerels), and intracellular viral DNA intermediates were separated from host DNA by NaDodSO4 lysis/NaCl precipitation (18).Samples requiring Pronase treatment were kept at 60°C for 3 hr in 1% NaDodSO4/30 mM Tris-HCI/20 mM EDTA, pH 8, with three additions of Pronase (CB grade, Calbiochem) at 1 mg/ml followed by incubation for 16 hr at 370C with additional Pronase at 2 mg/ml. Where indicated, this was followed by extraction with phenol and precipitation with ethanol.Isopycnic Centrifugation. Neutral CsCl solutions for equilibrium density centrifugation contained 50 mM ...