Animal cell DNA polymerases in DNA repair

Oxidative and antioxidant status in plasma of runners: effect of oral supplementation with natural antioxidants

Aerobic exercise increases free radical production as a consequence of enhanced oxygen consumption. If free radical formation exceeds antioxidant capacity, lipids, proteins, and DNA may be oxidized. Oxidative stress is widely recognized as a factor in many degenerative human diseases. The role of dietary antioxidants in protection against disease is a topic of continuing interest. In fact, there is epidemiological evidence correlating a higher intake of nutrients possessing antioxidant abilities with a lower incidence of various human diseases. This study was directed at investigating whether changes in plasma antioxidant capacity and oxidative stress markers occur in voluntary wheel runners, before and after oral supplementation with lycopene and isoflavones. For this purpose, plasma antioxidant capacity and oxidative stress markers were assessed in long distance runners at the end of a 60-minute run. Comparisons were made between runners before and after 60 days of supplementation with lycopene and isoflavones. DNA damage in blood cells of the same samples was also evaluated by comet assay. This investigation shows that oral supplementation with lycopene and soy-derived isoflavones significantly reduced lipid peroxidation and enhanced plasma nonproteic antioxidant defense.

DNA damage and susceptibility to oxidative damage in lymphocytes: effects of carotenoidsin vitroandin vivo

Reports on the effects of carotenoids are conflicting. The present paper examines similarities and differences from contiguous studiesin vitroandin vivo. Single-cell gel electrophoresis was used to measure the frequency of single-strand breaks (SSB) in the cell line MOLT-17 (as a model system) and human peripheral blood lymphocytes (PBL). MOLT-17 cells were supplemented with β-carotene, lutein or lycopene at a range of concentrations (0·00–8·00 μmol/l) using a liposome delivery method. Uptake was dose-dependent. β-Carotene concentration in the media had no effect on SSB in control cells, but incubation with lycopene or lutein (>2·00 μmol/l) increased the numbers of SSB in control cells. MOLT-17 DNA was less susceptible to oxidative damage (100 μmol H2O2/l, 5 min, 4 °C) following incubation with carotenoids between 0·50 and 1·00 μmol/l; at >1·00 μmol/l the effects were ambiguous. Apparently healthy male volunteers supplemented their habitual diets with lutein, β-carotene or lycopene (natural isolate capsules, 15 mg/d, 4 weeks) in three independent studies, raising plasma concentrations to different extents. Lycopene and lutein had no effect on SSB in control PBL or following oxidative challenge. However, increased plasma β-carotene was associated with more SSB in control cells whilst PBL DNA resistance to oxidative damageex vivowas unaffected. These results suggest that the carotenoids are capable of exerting two overlapping but distinct effects: antioxidant protection by scavenging DNA-damaging free radicals and modulation of DNA repair mechanisms.

Hypersensitivity phenotypes associated with genetic and synthetic inhibitor-induced base excision repair deficiency

Single-base lesions in DNA are repaired predominantly by base excision repair (BER). DNA polymerase beta (pol beta) is the polymerase of choice in the preferred single-nucleotide BER pathway. The characteristic phenotype of mouse fibroblasts with a deletion of the pol beta gene is moderate hypersensitivity to monofunctional alkylating agents, e.g., methyl methanesulfonate (MMS). Increased sensitivity to MMS is also seen in the absence of pol beta partner proteins XRCC1 and PARP-1, and under conditions where BER efficiency is reduced by synthetic inhibitors. PARP activity plays a major role in protection against MMS-induced cytotoxicity, and cells treated with a combination of non-toxic concentrations of MMS and a PARP inhibitor undergo cell cycle arrest and die by a Chk1-dependent apoptotic pathway. Since BER-deficient cells and tumors are similarly hypersensitive to the clinically used chemotherapeutic methylating agent temozolomide, modulation of DNA damage-induced cell signaling pathways, as well as BER, are attractive targets for potentiating chemotherapy.

Long intracellular retention of 4'-thio-arabinofuranosylcytosine 5'-triphosphate as a critical factor for the anti-solid tumor activity of 4'-thio-arabinofuranosylcytosine

4'-Thio-arabinofuranosylcytosine (T-araC) is a new cytosine analog, which exhibits excellent antitumor activity against various solid tumor xenografts in mice. T-araC is a structural analog of arabinofuranosylcytosine (araC), which is known to be marginally active against solid tumors. We have continued to study the biochemical pharmacology of T-araC in solid tumor cells to further characterize the mechanism of action of this new agent and to elucidate why these compounds show a profound difference in antitumor activity against solid tumors. AraC was a slightly more potent inhibitor of cell growth than T-araC when cells were continuously exposed to the drugs. However, T-araC was markedly more cytotoxic than araC when high concentrations of the compounds were given for short periods of time. Despite the fact that T-araC is a much poorer substrate, as compared to araC, for deoxycytidine kinase (the rate-limiting step in the formation of the triphosphates), similar intracellular concentrations of T-araC-5'-triphosphate (T-araCTP) and araCTP were formed in cells at these high, pharmacologically relevant concentrations due to similar Vmax's. The major difference in the metabolism of araC and T-araC was that the half-life of T-araCTP was tenfold longer than that of araCTP and much higher levels of T-araCTP were sustained in cells for long durations after exposure to T-araC. Inhibition of cytidine deaminase, deoxycytidylate deaminase, or DNA replication did not affect the half-life of either araCTP or T-araCTP. In addition, the rates of disappearance of the mono- and tri-phosphates of araC and T-araC in crude cell extracts were similar. These results indicated that these enzymes were not rate-limiting in the degradation of the respective triphosphates. However, the rate of phosphorylation of T-araC-5'-monophosphate (T-araCMP) in crude cell extracts was about tenfold greater than that of araCMP. The results of this work suggested that the longer intracellular retention of T-araCTP was responsible for the superior activity of T-araC against solid tumors in vivo, and that the greater activity of T-araCMP as a substrate of UMP/CMP kinase was responsible for the long intracellular half-life of T-araCTP.

The DNA binding properties of the Escherichia coli RecQ helicase

The RecQ helicase family is highly conserved from bacteria to men and plays a conserved role in the preservation of genome integrity. Its deficiency in human cells leads to a marked genomic instability that is associated with premature aging and cancer. To determine the thermodynamic parameters for the interaction of Escherichia coli RecQ helicase with DNA, equilibrium binding studies have been performed using the thermodynamic rigorous fluorescence titration technique. Steady-state fluorescence anisotropy measurements of fluorescein-labeled oligonucleotides revealed that RecQ helicase bound to DNA with an apparent binding stoichiometry of 1 protein monomer/10 nucleotides. This stoichiometry was not altered in the presence of AMPPNP (adenosine 5'-(beta,gamma-imido) triphosphate) or ADP. Analyses of RecQ helicase interactions with oligonucleotides of different lengths over a wide range of pH, NaCl, and nucleic acid concentrations indicate that the RecQ helicase has a single strong DNA binding site with an association constant at 25 degrees C of K=6.7 +/- 0.95 x 10(6) M(-1) and a cooperativity parameter of omega=25.5 +/- 1.2. Both single-stranded DNA and double-stranded DNA bind competitively to the same site. The intrinsic affinities are salt-dependent, and the formation of DNA-helicase complex is accompanied by a net release of 3-4 ions. Allosteric effects of nucleotide cofactors on RecQ binding to DNA were observed only for single-stranded DNA in the presence of 1.5 mM AMPPNP, whereas both AMPPNP and ADP had no detectable effect on double-stranded DNA binding over a large range of nucleotide cofactor concentrations.

Aphidicolin induces 6-thioguanine resistant mutants in human diploid fibroblasts

Cytotoxic and mutagenic effects of aphidicolin (APC), an inhibitor of DNA polymerases alpha and delta, were studied in human diploid VH-10 fibroblasts. The cells were treated (2 or 4h) with APC at concentration ranges of 10-40 microM. The effect of APC on cell survival after 4 h treatment was significantly higher than after 2 h treatment. The mutagenicity of APC was investigated at the HPRT locus, and the frequency of HPRT mutants was estimated by selection in medium containing 6-thioguanine (6-TG). Treatment of fibroblast cells with 20 microM of APC for 2 or 4 h resulted approximately in 5 or 10 times increase of 6-TG resistant mutant frequencies, respectively, compared to untreated control cells. The cell cycle analyses performed during the expression time (9-12 days) have shown that after 2 and 4h treatment with APC the cells were blocked in G2 phase during the majority of the expression period, compared to control cells. Four days after the treatment, the amount of cells in G2 phase increased about two-fold (28.6-31.8% compared to 13.5% in the untreated cells). The mode of cell death during the expression time was via necrosis, rather than apoptosis, which was demonstrated by fluorescein-diacetate (FDA)-staining and terminal dUTP nick end labeling (TUNEL)-method.

Effect of a hypoxic radiosensitizer, AK 2123 (Sanazole), on yeast Saccharomyces cerevisiae

Sanazole/DNA repair/Hypoxic radiosensitization/DNA polymerases/Saccharomyces cerevisiae Yeast Saccharomyces cerevisiae can exist in two physiological states, namely anaerobic and aerobic. They differ in their response to gamma- radiation and radiomodification. We report hereon our results concerning radiosensitization by Sanazole (AK-2123), a well-known hypoxic radio sensitizer, whose mechanism of action has been studied extensively. The results have revealed that Sanazole (1 mM) when present during irradiation could specifically sensitize wild-type anaerobic yeast cells with a DMF of 2.4. In a radiation-sensitive mutant which lacks a DNA repair pathway specific for the recovery from gamma-radiation induced DNA damage, the extent of sensitization was considerably lower and the DMF was only 1.3. Studies on the liquid holding recovery of cells of both wild- type and rad52 yeast cells exposed to radiation in presence of Sanazole revealed that sensitization by Sanazole is due to a preferential increase in the DNA damage, and not by impairing DNA repair. This system thus holds promise for screening potential hypoxic chemical radiosensitizers.

Antileishmanial activities of aphidicolin and its semisynthetic derivatives

Aphidicolin and a series of semisynthetic aphidicolan derivatives have been identified in in vitro tests as novel drugs with antiparasitic potential. All compounds have been tested against extracellular promastigotes of Leishmania donovani, L. infantum, L. enriettii, and L. major and against intracellular amastigotes of L. donovani in murine macrophages. The compounds showed antileishmanial activity at concentrations in the microgram range (50% effective concentration [EC(50)] = 0.02 to 1.83 microg/ml). The most active derivative (aphidicolin-17-glycinate hydrochloride) had EC(50)s of 0. 2 microg/ml against extracellular and 0.02 microg/ml against intracellular L. donovani parasites. To validate the pharmacological potential of tested drugs, pharmacological safety was determined by testing all compounds against two neoplastic cell lines (squamous carcinoma [KB] and melanoma [SK-Mel]) and against murine bone marrow-derived macrophages as host cells. With minor exceptions only for macrophages, tested aphidicolans did not show significant cytotoxicity (EC(50) > 25.0 microg/ml). Structure-activity relationships of these aphidicolan derivatives are discussed.

Molecular cloning and high-level expression of human polymerase beta cDNA and comparison of the purified recombinant human and rat enzymes

The cDNA encoding the human polymerase beta from HeLa cells was PCR amplified and cloned, and its nucleotide sequence determined. The DNA sequence is identical to the polymerase beta cDNA sequence from Tera-2 cells. Three expression strategies were employed that were designed to maximize translation initiation of the polymerase beta mRNA in Escherichia coli and all yielded a high level of human polymerase beta. The recombinant protein was purified and its properties were compared with those of the recombinant rat enzyme. The domain structure and kinetic parameters (k(cat) and K(m)) were nearly identical. A mouse IgG monoclonal antibody to the rat enzyme (mAb-10S) was approximately 10-fold less reactive with the human enzyme than with the rat enzyme as determined by ELISA.

The analysis of DNA adduct formation, removal and persistence in the common mussel Mytilus edulis exposed to 4-nitroquinoline 1-oxide

32P-postlabelling was used for the detailed analysis of 4-nitroquinoline 1-oxide (4-NQO) induced DNA adduct formation, removal and persistence in the marine shellfish Mytilus spp. The results had a number of important implications concerning the use of such DNA adducts as dosimeters of environmental genotoxin exposures. Our studies indicated that the maintenance of the Mytilus specimens under controlled laboratory conditions can result in the induction of 'stress-related adducts' seemingly related to the nature of the experimental set-up. The studies also indicated that the absorption and activation of genotoxins in this species appear to affect the rate of adduct formation, and that the maximum levels of adducts may not necessarily be induced immediately after the cessation of a genotoxin exposure. In addition, Mytilus specimens were shown to possess a significant capacity to remove these genotoxin-induced DNA adducts. The removal of these adducts appeared to be biphasic in nature, with the rapid removal of a large proportion of adducts occurring within 48 h of the cessation of the exposure, followed by a slow rate of adduct removal over the remaining period of the studies. Despite the relatively efficient removal of the majority of these genotoxin-induced DNA adducts, a proportion remained up to 56 days after the initial exposure. The persistence of these genotoxin-DNA adducts, combined with the information on the rates of adduct removal, indicated that under well-defined conditions, such adducts could serve as suitable biomarkers of environmental contamination.

A variant of DNA polymerase beta acts as a dominant negative mutant

In eukaryotic cells, DNA polymerase beta (polbeta) carries out base-excision repair (BER) required for DNA maintenance, replication, recombination, and drug resistance. A specific deletion in one allele in the coding sequence of the polbeta gene occurs in colorectal and breast carcinomas. The 87-bp deleted region encodes amino acid residues 208-236 in the catalytic domain of the enzyme. Here, we report evidence for expression of the wild-type (WT) and the truncated polbeta proteins in colorectal tumors. To elucidate the potential functional consequences of polbeta truncation, stable HeLa cell lines were established from cloned WT and variant polbetaDelta208-236. Cells expressing the variant protein exhibited substantially decreased BER activity. To test our hypothesis that truncated polbeta may disrupt the function of the WT enzyme, we stably transfected mouse embryonic fibroblast 16.3 cells with polbetaDelta208-236 cDNA. Reverse transcription-PCR and Western blot analyses showed that the new cell line, 16.3DeltaP, expresses the WT and the truncated polbeta mRNA and proteins. BER and binding activities were undetectable in these cells. Furthermore, in vivo the 16.3DeltaP cells were more sensitive to N-methyl-N'-nitro-N-nitrosoguanidine than the 16.3 cells. On adding increasing amounts of 16.3DeltaP protein extracts, the BER and DNA binding activities of extracts of the parent 16.3 cell line progressively declined. These results strongly suggest that truncated polbeta acts as a dominant negative mutant. The defective polbeta may facilitate accumulation of mutations, leading to the expression of a mutator phenotype in tumor cells.

Replication across O6-methylguanine by human DNA polymerase beta in vitro. Insights into the futile cytotoxic repair and mutagenesis of O6-methylguanine

Replication in vivo across unrepaired O6-methylguanine (m6dG) lesions by mammalian DNA polymerase beta (pol beta) during short patch repair may contribute to the cytotoxicity and mutagenesis of m6dG. We have employed in vitro steady state kinetic analysis to investigate the replication of oligonucleotide templates containing site-specific m6dG by human pol beta. Our results show that m6dG is a strong but not absolute block to replication by pol beta. pol beta exhibits mixed kinetic discrimination during overall replication across dG and m6dG. pol beta preferentially inserts dTMP rather than dCMP opposite m6dG. However, pol beta extends from the dC-m6dG base pair more efficiently than from the dT-m6dG base pair. This is in strong contrast to other polymerases such as the exonuclease-deficient Klenow fragment of Escherichia coli DNA polymerase I (exo-KF) that preferentially extends dT-m6dG by a factor of 10 over dC-m6dG. When both insertion and extension are considered, pol beta has a 15-fold overall preference for incorporation of the mutagenic substrate dTTP rather than the nonmutagenic substrate dCTP during replication across m6dG. This suggests that pol beta, in concert with the T:G-specific thymine DNA glycosylase, may be intricately involved in the futile cytotoxic repair induced by m6dG. Our results also suggest that replication across m6dG by pol beta may contribute to m6dG-induced G --> A transition mutations.

Alternative polyadenylation of the gene transcripts encoding a rat DNA polymerase beta

Rat cells produce two different transcripts of DNA polymerase beta (beta-Pol). The low-molecular-weight transcript (1.4 kb) was already sequenced. We report here the cloning and sequencing of the full-length cDNA, corresponding to the high-molecular-weight (HMW) transcript (4.0 kb) of beta-Pol. Sequence data strongly suggest that both transcripts are produced from a single gene by alternative polyadenylation. The HMW transcript contains the entire 1.4 kb transcript sequence and additional 2.2 kb on the 3' end. The 3' UTR of the HMW transcript contains some regulatory sequences which are not present in the 1.4-kb transcript. The A + U-rich fragment and (GU)21 sequence are believed to influence the stability of the mRNA. The functional significance of the A-rich region locally destabilizing double-stranded secondary structure remains unknown.

Unscheduled DNA synthesis in plant populations exposed to chronic irradiation

The intensity of gamma-ray-induced unscheduled DNA synthesis (UDS) was measured in a number of natural populations of Vicia cracca which have been growing for some decades under conditions of chronic alpha- or beta-irradiation. It was shown that the level of UDS increased in more radioresistant beta-irradiated populations as compared to control populations and this increase was dose rate-dependent. In alpha-irradiated populations, the intensity of UDS was decreased, but only at the highest dose of gamma-radiation (500 Gy) and was not changed at the lower doses. The sensitivity of UDS to inhibitors of DNA and protein synthesis was studied in control and radioresistant populations. In control plants UDS was resistant to cycloheximide (Cyc) and aphidicolin (Aph), but totally inhibited by dideoxythymidine (ddT). In radioresistant population UDS was inhibited by both Aph and ddT as well as by Cyc. I assume that in controls, UDS is mediated by beta-like DNA polymerase; however, in a radioresistant population, both DNA polymerases alpha and beta take part in this process. In the radioresistant population UDS is partially inducible.

Mitochondrial genome mutations and kidney disease

Mutations in the mitochondrial genome have been shown to be responsible for several neuromuscular diseases in humans. In this article, we discuss the molecular genetics of mitochondria, their centrality in cellular energy production, and reasons why their genome is extremely vulnerable to mutation. Mitochondrial DNA (mtDNA) mutations and their classic encephalomyopathic clinical phenotypes are briefly reviewed, and evidence presented that mtDNA mutations also present primarily as kidney diseases. Research trends in the field are discussed. Suggestions are made regarding future work, the clinical implications thereof, and potential therapeutic utility accruing from these advances.

Biochemical characterization of a mitomycin C-resistant human bladder cancer cell line

This study describes characteristics of a mitomycin C (MMC)-resistant human bladder cancer cell line, J82/MMC-2, which was established by repeated in vitro exposures of a 6-fold MMC-resistant variant (J82/MMC) to 18 nM MMC. A 9.6-fold higher concentration of MMC was required to kill 50% of the J82/MMC-2 sub-line compared with parental cells (J82/WT). NADPH cytochrome P450 reductase and DT-diaphorase activities were significantly lower in J82/MMC-2 cells compared with J82/WT, suggesting that reduced sensitivity of J82/MMC-2 cells to MMC resulted from impaired drug activation. Consistent with this hypothesis, the formation of MMC-alkylating metabolites was significantly lower in J82/MMC-2 cells compared with J82/WT. Furthermore, DT-diaphorase activity in J82/MMC-2 cells was significantly lower compared with the 6-fold MMC-resistant variant. Glutathione (GSH) levels were comparable in all 3 cell lines. Although GSH transferase (GST) activity was significantly higher in the J82/MMC-2 cells compared with J82/WT, this enzyme activity did not differ between 6- and 9.6-fold MMC-resistant variants. Whereas DNA polymerase alpha mRNA expression was comparable in these cell lines, levels of DNA ligase I mRNA were slightly lower in both MMC-resistant variants relative to J82/WT. However, the DNA polymerase beta mRNA level was markedly higher in the J82/MMC-2 cell line compared with either J82/WT or J82/MMC. Thus, emergence of a higher level of resistance to MMC in J82/MMC-2 cells compared with J82/MMC may be attributed to (i) impaired drug activation through further reduction in DT-diaphorase activity and (ii) enhanced DNA repair through over-expression of DNA polymerase beta.

DNA polymerase beta conducts the gap-filling step in uracil-initiated base excision repair in a bovine testis nuclear extract

The G:U mismatch in genomic DNA mainly arises from deamination of cytosine residues and is repaired by the base excision repair pathway. We found that a bovine testis crude nuclear extract conducts uracil-initiated base excision repair in vitro. A 51-base pair synthetic DNA substrate containing a single G:U mismatch was used, and incorporation of dCMP during repair was exclusively to replace uracil. A neutralizing polyclonal antibody against DNA polymerase beta (beta-pol) inhibited the repair reaction. ddCTP also inhibited the repair reaction, whereas aphidicolin had no significant effect, suggesting that activity of beta-pol was required. Next, the base excision repair system was reconstituted using partially purified components. Several of the enzymatic activities required were resolved, such that DNA ligase and the uracil-DNA glycosylase/apurinic/apyrimidinic endonuclease activities were separated from the DNA polymerase requirement. We found that purified beta-pol could restore full DNA repair activity to the DNA polymerase-depleted fraction, whereas purified DNA polymerases alpha, delta, and epsilon could not. These results with purified proteins corroborated results obtained with the crude extract and indicate that beta-pol is responsible for the single-nucleotide gap filling reaction involved in this in vitro base excision repair system.

The kinetics of repair of oxidative DNA damage (strand breaks and oxidised pyrimidines) in human cells

Single cell gel electrophoresis is a sensitive method for detecting DNA strand breaks. Cells embedded in agarose are converted to nucleoids by treating with detergent and high salt. DNA breaks render the nucleoid DNA susceptible to extension by electrophoresis, forming 'comets'. We find that when DNA breakage resulting from H2O2 treatment is examined, freshly isolated normal human lymphocytes are relatively resistant compared with transformed human cells. When incubated after treatment with H2O2, HeLa cells repair most strand breaks within 1 h, and a substantial fraction of the oxidised pyrimidines (detected by converting them to DNA breaks with endonuclease III) within 4 h. However, lymphocytes are less proficient at repair; during incubation for 4 h after treatment with H2O2, no detectable removal of endonuclease III-sensitive sites is seen. While the addition of deoxyribonucleosides promotes completion of repair of UV damage by lymphocytes, it has no significant effect on repair of oxidative damage.

Spontaneous mutations at aprt locus in a mammalian cell line defective in mismatch recognition

Clone B is a CHO cell line that shows a moderate mutator phenotype as a consequence of a defect in mismatch recognition. To identify the classes of mutation that accumulate spontaneously in a functional gene, we isolated and sequenced 54 clone B spontaneous mutants at the adenine phosphoribosyltransferase gene. This spectrum was compared to 42 mutants collected in the parental cells. Rates of AT-->TA transversions and frameshifts were strikingly increased in clone B (almost eight- and sixfold, respectively). Minor increases were also observed for GC-->TA transversions and GC-->AT transition rates. Frameshifts occurred in repeated sequences, and a large proportion were losses of 2 bases occurring in dinucleotide runs of a type similar to microsatellite sequences. AT-->TA transversions clustered in regions of secondary structure and their formation might be explained by slippage-mediated mechanisms. These data indicate that an important function of mismatch recognition is in repair of extrahelical bases generated by misalignment during DNA replication.

Effects of thymidine kinase and methyltransferase deficiency on mutagenesis in a human lymphoblastoid cell line

In this study the effect of thymidine kinase (TK) deficiency on mutagen sensitivity was examined in the human lymphoblastoid cell line Raji. Wild-type and TK-deficient Raji cells were treated with a range of concentrations of ethyl methanesulphonate (EMS) and a range of doses of ultraviolet (UV) light, then examined for mutagen sensitivity as measured by cell survival and mutation to HGPRT deficiency. Dose-dependent responses were observed and TK-deficient cells exhibited decreased survivals and increased mutant frequencies relative to wild-type cells. TK-deficient Raji cells are also deficient in O6-methylguanine-DNA-methyltransferase. This may partially account for their sensitivity to EMS but does not account for the results obtained with UV. It is therefore likely that an additional factor, such as alterations in supply of deoxyribonucleoside triphosphates, may affect the mutagen sensitivity of Raji cells.

A meiotic DNA polymerase from a mushroom, Agaricus bisporus

A meiotic DNA polymerase [DNA nucleotidyltransferase (DNA-directed), EC 2.7.7.7], which likely has a role in meiotic DNA repair, was isolated from a mushroom, Agaricus bisporus. The purified fraction displays three bands in SDS/PAGE, at molecular masses of 72 kDa, 65 kDa and 36 kDa. Optimal activity is at pH 7.0-8.0 in the presence of 5 mM Mg2+ and 50 mM KCl and at 28-30 degrees C, which is the temperature for meiosis. This enzyme is resistant to N-ethylmaleimide and sensitive to 2',3'-dideoxythymidine 5'-triphosphate, suggesting that it is a beta-like DNA polymerase. These characteristics are similar to those of Coprinus DNA polymerase beta [Sakaguchi and Lu (1982) Mol. Cell. Biol. 2, 752-757]. In Western-blot analysis, the antiserum against the Coprinus polymerase reacts only with the 65 kDa band, which coincides with the molecular mass of the Coprinus polymerase. Western-blot analysis also showed that the antiserum could react with crude extracts not only from the Agaricales family, to which Agaricus and Coprinus belong, but also from different mushroom families and Saccharomyces. The Agaricus polymerase activity can be found only in the meiotic-cell-rich fraction, but the enzyme is also present in the somatic cells in an inactive state.

Evaluation of the genotoxic potential of selected anti-AIDS treatment drugs at clinical doses in vivo in mice

Six anti-AIDS drugs were assessed for in vivo genotoxicity and cytotoxicity at human clinical doses with the mouse bone marrow micronucleus assay. These included four dideoxynucleosides (azidothymidine, dideoxycytidine, dideoxyadenosine, and dideoxyinosine), an anthracycline antibiotic (doxorubicin), and a chelating agent (D-penicillamine). Cytological analysis of the mouse bone marrow cells revealed: (i) The dideoxynucleosides and D-penicillamine failed to induce significant number of micronuclei, and except for one of the five doses of dideoxyinosine, none of the dideoxynucleosides were cytotoxic at the doses tested. (ii) Doxorubicin induced micronuclei in a dose-dependent manner which was statistically significant at 4-times the clinical dose but was not cytotoxic at any of the doses tested.

Enhanced repair of a cisplatin-damaged reporter chloramphenicol-O-acetyltransferase gene and altered activities of DNA polymerases alpha and beta, and DNA ligase in cells of a human malignant glioma following in vivo cisplatin therapy

Current evidence suggest an important role for increased repair of drug-induced DNA damage as one of the major mechanisms involved in tumor cell resistance to cis-DDP. In this study, we examined the DNA repair capacity and the activities of three DNA repair related proteins, namely, DNA polymerases alpha and beta, and total DNA ligase in cells of a malignant oligodendroglioma obtained from a patient before therapy and compared it with those of a specimen of the tumor acquired after the patient had failed cis-DDP therapy. DNA repair capacity was quantitated as the extent of reactivation of the chloramphenicol-O-acetyltransferase (CAT) gene in a eukaryotic expression vector that had been damaged and inactivated by prior treatment with cis-DDP and then transfected into the tumor cells. The extent of DNA-platinum adduct formation in the expression vector was determined by flameless atomic absorption spectrometry. The level of cis-DDP resistance of cells of the two tumors was determined with the capillary tumor stem cell assay. We observed a 2.8-fold increased capacity to repair Pt-DNA adducts and reactivate the CAT gene in cells of the tumor obtained after cis-DDP therapy, compared to cells of the untreated tumor. This was associated with increases of 9.4-fold and a 2.3-fold, respectively, in DNA polymerase beta and total DNA ligase activities in cells of the treated tumor. At 5 microM cis-DDP, there was a 5.9-fold increase in the in vitro cis-DDP resistance of post-therapy tumor cells relative to cells of the untreated tumor.(ABSTRACT TRUNCATED AT 250 WORDS)

Aphidicolin potentiates apoptosis induced by arabinosyl nucleosides in human myeloid leukemia cell lines

We investigated the effect of aphidicolin, an inhibitor of DNA polymerase alpha and delta, on the induction of apoptosis by arabinosyl nucleosides in a human promyelocytic leukemia cell line, HL-60. Pretreatment of HL-60 cells with aphidicolin (2 microM) significantly increased the number of morphologically apoptotic cells induced by 1-beta-D arabinofuranosylcytosine (ara-C) during 4 hr of incubation. This is consistent with the appearance of DNA fragmentation as determined quantitatively by diphenylamine or by agarose gel electrophoresis. The inhibition of cell growth on day 3 after drug exposure was correlated with the degree of apoptosis: Such synergistic interaction between aphidicolin and ara-C has also been observed in other human myeloid leukemia cell lines, U937 and KG-1. In addition, the induction of apoptosis by 9-beta-D arabinofuranosyladenine or 9-beta-D arabinofuranosylguanine is augmented by aphidicolin.

DNA repair synthesis during base excision repair in vitro is catalyzed by DNA polymerase epsilon and is influenced by DNA polymerases alpha and delta in Saccharomyces cerevisiae

Base excision repair is an important mechanism for correcting DNA damage produced by many physical and chemical agents. We have examined the effects of the REV3 gene and the DNA polymerase genes POL1, POL2, and POL3 of Saccharomyces cerevisiae on DNA repair synthesis is nuclear extracts. Deletional inactivation of REV3 did not affect repair synthesis in the base excision repair pathway. Repair synthesis in nuclear extracts of pol1, pol2, and pol3 temperature-sensitive mutants was normal at permissive temperatures. However, repair synthesis in pol2 nuclear extracts was defective at the restrictive temperature of 37 degrees C and could be complemented by the addition of purified yeast DNA polymerase epsilon. Repair synthesis in pol1 nuclear extracts was proficient at the restrictive temperature unless DNA polymerase alpha was inactivated prior to the initiation of DNA repair. Thermal inactivation of DNA polymerase delta in pol3 nuclear extracts enhanced DNA repair synthesis approximately 2-fold, an effect which could be specifically reversed by the addition of purified yeast DNA polymerase delta to the extract. These results demonstrate that DNA repair synthesis in the yeast base excision repair pathway is catalyzed by DNA polymerase epsilon but is apparently modulated by the presence of DNA polymerases alpha and delta.

DNA repair synthesis during base excision repair in vitro is catalyzed by DNA polymerase epsilon and is influenced by DNA polymerases alpha and delta in Saccharomyces cerevisiae

Base excision repair is an important mechanism for correcting DNA damage produced by many physical and chemical agents. We have examined the effects of the REV3 gene and the DNA polymerase genes POL1, POL2, and POL3 of Saccharomyces cerevisiae on DNA repair synthesis is nuclear extracts. Deletional inactivation of REV3 did not affect repair synthesis in the base excision repair pathway. Repair synthesis in nuclear extracts of pol1, pol2, and pol3 temperature-sensitive mutants was normal at permissive temperatures. However, repair synthesis in pol2 nuclear extracts was defective at the restrictive temperature of 37 degrees C and could be complemented by the addition of purified yeast DNA polymerase epsilon. Repair synthesis in pol1 nuclear extracts was proficient at the restrictive temperature unless DNA polymerase alpha was inactivated prior to the initiation of DNA repair. Thermal inactivation of DNA polymerase delta in pol3 nuclear extracts enhanced DNA repair synthesis approximately 2-fold, an effect which could be specifically reversed by the addition of purified yeast DNA polymerase delta to the extract. These results demonstrate that DNA repair synthesis in the yeast base excision repair pathway is catalyzed by DNA polymerase epsilon but is apparently modulated by the presence of DNA polymerases alpha and delta.

The genetic toxicology of cobalt

Genetic and related effects of cobalt compounds are reviewed and discussed with respect to mechanisms. In prokaryotic assays, Co(II) salts generally are nonmutagenic. In Saccharomyces cerevisiae, CoCl2 is mutagenic to mitochondrial genes and weakly mutagenic or nonmutagenic to chromosomal genes. In plants, Co(II) salts induced gene mutations and chromosomal aberrations. In mammalian cells in vitro, Co(II) compounds caused DNA strand breaks, sister-chromatid exchanges and aneuploidy, but not chromosomal aberrations. In two cell lines, CoCl2 was weakly mutagenic. Interestingly, the poorly soluble compound CoS caused DNA strand breaks and morphological transformation of mammalian cell lines. In contrast to its weak clastogenic and mutagenic properties, cobalt(II) exerts pronounced antimutagenicity in bacteria and mostly comutagenic effects in mammalian cells. In Escherichia coli CoCl2 lowered the frequency of mutations induced by MNNG, uv or X rays. In Chinese hamster V79 cells, CoCl2 enhanced the mutagenicity and clastogenicity of uv light but not of gamma rays. Regarding direct genotoxic mechanisms, Co(II) induces the formation of reactive oxygen species when combined with hydrogen peroxide in cell-free systems. At high (i.e., millimolar) concentrations, Co(II) also decreases the fidelity of DNA synthesis. Regarding anti- and co-mutagenic mechanisms, evidence for the interference of Co(II) with DNA repair processes is discussed. These mechanisms are regarded as relevant for the risk assessment of human exposure to cobalt in combination with other agents.

Generation of single-nucleotide repair patches following excision of uracil residues from DNA

The extent and location of DNA repair synthesis in a double-stranded oligonucleotide containing a single dUMP residue have been determined. Gently prepared Escherichia coli and mammalian cell extracts were employed for excision repair in vitro. The size of the resynthesized patch was estimated by restriction enzyme analysis of the repaired oligonucleotide. Following enzymatic digestion and denaturing gel electrophoresis, the extent of incorporation of radioactively labeled nucleotides in the vicinity of the lesion was determined by autoradiography. Cell extracts of E. coli and of human cell lines were shown to carry out repair mainly by replacing a single nucleotide. No significant repair replication on the 5' side of the lesion was observed. The data indicate that, after cleavage of the dUMP residue by uracil-DNA glycosylase and incision of the resultant apurinic-apyrimidinic site by an apurinic-apyrimidinic endonuclease activity, the excision step is catalyzed usually by a DNA deoxyribophosphodiesterase rather than by an exonuclease. Gap-filling and ligation complete the repair reaction. Experiments with enzyme inhibitors in mammalian cell extracts suggest that the repair replication step is catalyzed by DNA polymerase beta.

Generation of single-nucleotide repair patches following excision of uracil residues from DNA

The extent and location of DNA repair synthesis in a double-stranded oligonucleotide containing a single dUMP residue have been determined. Gently prepared Escherichia coli and mammalian cell extracts were employed for excision repair in vitro. The size of the resynthesized patch was estimated by restriction enzyme analysis of the repaired oligonucleotide. Following enzymatic digestion and denaturing gel electrophoresis, the extent of incorporation of radioactively labeled nucleotides in the vicinity of the lesion was determined by autoradiography. Cell extracts of E. coli and of human cell lines were shown to carry out repair mainly by replacing a single nucleotide. No significant repair replication on the 5' side of the lesion was observed. The data indicate that, after cleavage of the dUMP residue by uracil-DNA glycosylase and incision of the resultant apurinic-apyrimidinic site by an apurinic-apyrimidinic endonuclease activity, the excision step is catalyzed usually by a DNA deoxyribophosphodiesterase rather than by an exonuclease. Gap-filling and ligation complete the repair reaction. Experiments with enzyme inhibitors in mammalian cell extracts suggest that the repair replication step is catalyzed by DNA polymerase beta.