Effects of aphidicolin and/or 2‘,3‘-dideoxythymidine on DNA repair induced in HeLa cells by four types of DNA-damaging agents

Biological response to ionizing radiation in mouse embryo fibroblasts with a targeted disruption of the DNA polymerase beta gene

Base excision repair (BER) is carried out by two distinct pathways in mammalian cells, one dependent on DNA polymerase beta (Polb) and the other on proliferating cell nuclear antigen (Pcna). We studied whether the Polb-dependent pathway plays an important role in BER in vivo after exposure to ionizing radiation. For this purpose, we used mouse embryo fibroblasts derived from wild-type and Polb gene knockout littermates. Both cell lines had essentially the same clonogenic cell survival and low levels of apoptosis as determined by a colony formation assay and by a change in mitochondrial membrane potential, respectively. No significant cleavage of protein kinase C delta (Pkcd) in vivo, which is a substrate for caspase 3, was detected, and intact Pkcd was retained in both cell lines for at least 72 h after irradiation. Similar significant increases in caspase 3-like activities as measured by Asp-Glu-Val-Asp (DEVD) cleaving activity in vitro were observed in both cell lines after irradiation. Radiation induced cell cycle arrest in the form of a G(2)-phase block, and G(2)/M-phase fractions reached a peak approximately 10 h after irradiation and decreased thereafter with a similar time course in both cell lines. Similar levels of chromatin-bound Pcna were observed immediately after irradiation in non-S-phase cells of both cell lines and disappeared by 4 h after irradiation. We conclude that the deficiency in Polb does not have a significant influence on the radiation responses of these cells. Together with evidence accumulated in vitro, these results strongly support the idea that the Pcna-dependent pathway predominantly acts in BER of radiation-induced DNA damage in vivo.

Cyclosporin A inhibits Ca2+-mediated upregulation of the DNA repair enzyme DNA polymerase beta in human peripheral blood mononuclear cells

Alterations in gene expression may represent an underlying cause of undesired side-effects mediated by the immunosuppressant cyclosporin A (CsA). We employed the method of differential display PCR to identify new genes whose expression is modulated by CsA. Human peripheral blood mononuclear cells (PBMCs), or subpopulations thereof, were simultaneously stimulated with the phorbol ester 4beta-phorbol 12-myristate 13-acetate (PMA) and the calcium ionophore ionomycin, in the presence or absence of therapeutic concentrations of CsA. We identify the gene encoding the DNA repair enzyme DNA polymerase beta (Pol beta) as a novel CsA-sensitive transcription unit. Our data show that transcription of pol beta mRNA is induced by Ca2+ and that CsA significantly inhibits PMA/ionomycin- and ionomycin-mediated upregulation of both pol beta mRNA and Pol beta protein. The CsA-mediated inhibition of pol beta upregulation is maintained for at least 21 h after gene activation and is exerted via the phosphatase calcineurin. FK506, another immunosuppressant that targets calcineurin, also inhibits pol beta upregulation, while rapamycin competes with FK506 action. This work identifies Ca2+ as an inducer of pol beta gene activity in primary blood cells. The demonstrated CsA sensitivity of this process suggests a novel molecular mechanism that may contribute to the increased tumor incidence in patients receiving CsA treatment.

Methods for registration of spontaneous DNA instability in mammalian cells

A phenomenon of spontaneous DNA instability displays itself as the low level of repair DNA synthesis that takes place during any cell cycle phases. However, there is a problem in detection of very low intensive repair DNA synthesis. This paper suggests two approaches to detect the spontaneous DNA instability. The first method involves a blockade of the DNA gaps sealing by a combination of inhibitors, hydroxyurea and arabinofuranosyl cytosine. An accumulation of single strand gaps leads to production of DNA double strand breaks and results to reproductive inactivation of cells. It was shown that registration of both these events by different methods (such as viscoelastometry of DNA, orthogonal pulse electrophoresis or comet assay for double strand breaks as well as effectiveness of colony growth for cell inactivation) may be used as suitable measure of the spontaneous DNA instability. The second approach bases on photolysis of bromodeoxyuridine incorporated into repair DNA patches during the spontaneous repair DNA synthesis. Long wave UV irradiation of cells containing bromodeoxyuridine labeled DNA stained with Hoechst 33342 causes their inactivation. Experimental results presented confirm that both methods actually detect the spontaneous DNA instability. It takes note of the spontaneous DNA instability varies for cells from different tissues and species and increases during aging.

Serum DNA polymerase beta as an indicator for fatal liver injury of rat induced by D-galactosamine hydrochloride and lipopolysaccharide

DNA polymerase beta (pol beta) is a nuclear enzyme that is tightly bound to chromatin. Release of the pol beta activity into serum, therefore, may indicate the occurrence of massive destruction of cell nuclei in organs or tissues. In the present study, we made a liver injury model rat by the intraperitoneal injection of D-galactosamine hydrochloride (GalN, 500 mg/kg) and lipopolysaccharide (LPS, 100 microg/kg). Serum from the GalN/LPS-treated rats showed a high level of pol beta activity up to 118 pmol/0.5 microl serum (4700 cpm) at 12 h after the treatment, while the control rat serum showed the back ground level (3.8 pmol/0. 5 microl, 150+/-70 cpm). The serum pol beta activity was sensitive to inhibition by 2',3'-dideoxyTTP and by an anti-rat pol beta antibody. Among 30 rats treated with GalN/LPS, 10 rats died within 120 h (dead group). Serum pol beta activity in the dead group was as high as 23.0+/-19.5 pmol/0.5 microl (925+/-778 cpm) at 10 h after the treatment, while in alive group (n=20), it was 3.7+/-3.2 pmol. Levels of the serum pol beta activity correlated well with the prognosis of GalN/LPS-treated rats based on an analysis of the receiver-operator characteristic curves.

An improved method of alkaline sucrose density gradient sedimentation to detect less than one lesion per 1 Mb DNA

We improved alkaline sucrose density gradient sedimentation to detect very long single-strand DNA at the megabase level (from less than 1 to about 4 Mb). Hitherto, these have not sedimented correctly due to some artifacts. One artifact was aggregation of sticky DNA and proteins formed in the gradient. Then, in some gradients, biphasic distribution was observed, the major peak of which was reasonable as a result of random scission by X-rays, but the minor, fast-sedimenting population was another artifact resulting from incomplete denaturation of the DNA. We mainly reduced the centrifugal force and used a solution for cell lysis with a high concentration of salt. By means of this procedure, DNA single-strand breaks induced by relatively low doses of X-rays and subsequent repair processes can be measured in human fibroblasts. The protocol is also applicable to the study of DNA damage accompanied by strand scission, such as by UV or dimethyl sulfate as well as their repair. The technique is sensitive enough to detect even single-strand breaks induced by 0.1 J/m2 UV and sufficiently reproducible that breaks induced by increasing UV dosages were dose dependent. Thus, this technique was proven to be very sensitive, reliable and simple to perform. Therefore, this improvement will be extremely useful to investigators studying DNA repair.

Cross-resistance to cis-diamminedichloroplatinum(II) of a multidrug-resistant lymphoma cell line associated with decreased drug accumulation and enhanced DNA repair

HOB1/VCR, a multidrug-resistant subline of the immunoblastic B lymphoma cell line, was established by sequential selection in increasing concentrations of vincristine. The expression of the human mdr l gene, as analyzed by reverse transcription and polymerase-chain reaction (RT-PCR), revealed a 10-15-fold overexpression in this resistant cell line. A complete inhibition of vincristine resistance by verapamil was observed in the vincristine-resistant HOB1/VCR cells, which suggests that acquired resistance may be mainly due to P-glycoprotein. HOB1/VCR cells also developed a 67-fold cross-resistance to the anticancer drug cis-diamminedichloroplatinum (cisplatin). DNA repair of the resistant and the parental cell lines was investigated by in situ detection with a cisplatin-DNA adduct-specific antibody and by measurement of repair-associated host cell reactivation of damaged plasmid DNA. HOB1/VCR cells exhibited a 2-fold decrease in the level of cisplatin-DNA adducts, compared to the parental cells. The DNA repair rate following peak accumulation of cisplatin-DNA adducts (which took approximately 4 h) was also enhanced in the resistant cells. This was supported by the measurement of the cisplatin level remaining in cells by atomic absorption spectrophotometry, which showed a 2.7-fold reduction in the resistant cells. In addition, the acquired resistance and enhanced DNA repair in HOB1/VCR cells were partially reversed by nontoxic aphidicolin, a DNA polymerase-alpha and DNA repair inhibitor. Inhibition of the intracellular level of glutathione by DL-buthionine-[S,R]-sulfoximine demonstrated that cell viability was inhibited 4-fold more in the resistant cells than in the parental cells. The results suggest that the reduced formation of cisplatin-DNA adducts and the increased glutathione content of the multidrug-resistant cells play a major role in phenotypic cross-resistance to cisplatin.

DNA polymerase beta gene mutations in human bladder cancer

We examined 24 human bladder cancer tissues for possible mutations in the entire coding region of the human DNA polymerase beta gene using polymerase chain reaction analysis, single-strand conformational polymorphism analysis of RNA, and sequence analysis. DNA polymerase beta gene mutations were observed in four of the 24 cases (16.7%) and included three missense point mutations and a single base insertion. The single base insertion was also observed in our previous study of human prostate cancer, suggesting that this region may be a hot spot for mutation of the DNA polymerase beta gene. No clinical or pathological association was found among the four cases that contained the mutation. Three of the four cases with DNA polymerase beta gene mutation had mutations of the p16 or RB genes or loss of heterozygosity of the p53 and APC gene loci. The results of the study presented here suggest that DNA polymerase beta gene mutations, in combination with mutations of tumor suppressor genes, may be involved in certain cases of human bladder cancer.

Involvement of DNA polymerase delta and/or epsilon in joining UV-induced DNA single strand breaks in human fibroblasts (comparison of effects of butylphenyldeoxyguanosine with aphidicolin)

DNA polymerases involved in ultraviolet (UV)-induced DNA repair were studied in human fibroblasts using the inhibitors of DNA polymerases, aphidicolin which inhibits DNA polymerases alpha, delta and epsilon, and butylphenyldeoxyguanosine (BuPGdR) which inhibits DNA polymerase alpha strongly and weakly inhibits delta and epsilon. Both inhibitors inhibited replicative DNA synthesis in a dose dependent manner as measured by thymidine incorporation. However, BuPGdR did not accumulate single strand breaks in cells irradiated with 5 J/m2 UV-light even at the highest dosage tested, indicating that BuPGdR does not inhibit DNA repair. On the other hand, aphidicolin accumulated single strand breaks in UV-light irradiated cells. These results suggest that DNA polymerase delta and/or epsilon are mainly involved in UV-induced DNA repair.

Enhanced excision repair of DNA damage due to cis-diamminedichloroplatinum(II) in resistant cervix carcinoma HeLa cells

We have previously reported a cisplatin-resistant HeLa cell line which exhibits overproduction of nuclear proteins preferential for cisplatin-modified DNA (Chao et al., Cancer Res. 51:601-605, 1991; Biochem. J. 277: 875-878, 1991). In this study, excision repair of cisplatin-DNA adducts in a resistant and a revertant cell lines was investigated using in situ detection of cisplatin-DNA adducts by an immunoassay and the measurement of repair-associated DNA strand breaks by a sensitive alkaline elution method. The resistant cells exhibited a 2-fold decrease in the accumulation of cisplatin-DNA adducts; whereas, the revertant cells showed a similar level of cisplatin-DNA adducts as the parental cells in the parallel experiment. Immediately following cisplatin treatment, the resistant and the revertant cells accumulated respectively approximately 50% and 90% cisplatin-DNA adducts of the parental cells. However, the kinetic patterns of repair rate following peak accumulation of cisplatin-DNA adducts (which took approximately 4 h) was the same in the three cell lines. This finding was supported by the measurement of repair-associated DNA strand breaks using alkaline elution which showed 1.6- and 1.5-fold increase in the resistant and the revertant cells respectively. In addition, following transfection with plasmid DNA carrying cisplatin damage, the resistant and the revertant cells displayed a 2.4- and 1.4-fold enhancement in host cell reactivation, respectively. Furthermore, the acquired resistance in HeLa cells was partially reversed by nontoxic aphidicolin, a DNA polymerase-alpha and DNA repair inhibitor. The results strongly suggest the improved excision repair of cisplatin-DNA adducts as a mechanism of phenotypic resistance of cells to cisplatin.

Differential effects of luminol, nickel, and arsenite on the rejoining of ultraviolet light and alkylation-induced DNA breaks

When Chinese hamster ovary cells were treated with ultraviolet (UV) light or methyl methanesulfonate (MMS), a large number of DNA strand breaks could be detected by alkaline elution. These strand breaks gradually disappeared if the treated cells were allowed to recover in a drug-free medium. The presence of nickel or arsenite during the recovery incubation retarded the disappearance of UV-induced strand breaks, whereas the disappearance of MMS-induced strand breaks was retarded by the presence of arsenite or of luminol, a new inhibitor for poly(ADP-ribose) synthetase. Luminol, however, had no apparent effect on the repair of UV-induced DNA strand breaks, and nickel had no effect on the repair of MMS-induced DNA strand breaks. When UV- or MMS-treated cells were incubated in cytosine arabinofuranoside (AraC) plus hydroxyurea (HU), a large amount of low molecular weight DNA was detected by alkaline sucrose sedimentation. The molecular weight of these DNAs increased if the cells were further incubated in a drug-free medium. This rejoining of breaks in cells pretreated with UV plus AraC and HU was inhibited by nickel and by arsenite, but not by luminol. The rejoining of breaks in cells pretreated with MMS plus AraC and HU was inhibited by luminol and by arsenite, but not by nickel. These results suggest that different enzymes may be used in DNA resynthesis and/or ligation during the repairing of UV- and MMS-induced DNA strand breaks, and that nickel, luminol, and arsenite may have differential inhibitory effects on these enzymes.

Effect of DNA-repair-enzyme modulators on cytotoxicity of L-phenylalanine mustard and cis-diamminedichloroplatinum (II) in mammary carcinoma cells resistant to alkylating drugs

We investigated the effect of DNA-repair-enzyme inhibitors on L-phenylalanine mustard (L-PAM) and cis-diamminedichloroplatinum (II) (CDDP) cytotoxicity in rat mammary-carcinoma MatB cells sensitive (WT) and resistant (MLNr) to bifunctional alkylating drugs. Among the modulators tested, the combination of arabinofuranosylcytosine (Ara-C) and hydroxyurea (HU) significantly increased the sensitivity of the cells to CDDP and, to a lesser extent, L-PAM as compared with cells treated with drug alone. The modulation effect of HU+Ara-C on CDDP and L-PAM cytotoxicity was more effective when intracellular glutathione (GSH) was depleted by L-buthionine-(S,R)-sulfoximine (BSO). This was also associated with a significant increase in DNA-DNA interstrand cross-links. Caffeine also sensitized both WT and MLNr cells to the cytotoxic effect of L-PAM and CDDP, and this effect was potentiated in GSH-depleted cells. No significant effect was observed with other repair modulators such as aphidicolin, 3-aminobenzamide, novobiocin, or etoposide. These results show (a) that inhibition of DNA repair by HU+Ara-C or caffeine could be a target for modulation of bifunctional alkylating-drug resistance and (b) that GSH depletion renders resistant cells more susceptible to the repair-enzyme modulators, suggesting that intracellular GSH may be involved in the regulation of some of these enzymes. Our results also indicate that a combination of a number of modulators may offer an advantage over the use of a single modulator in tumor resistance that may be associated with multifactorial mechanisms.

Nickel chloride inhibits the DNA repair of UV-treated but not methyl methanesulfonate-treated Chinese hamster ovary cells

Nickel, a human carcinogen, has been shown to enhance the cytotoxicity, mutagenicity, and sister-chromatid exchanges (SCE) induced by ultraviolet (UV) light but not by methyl methanesulfonate (MMS). To verify that the cocytotoxicity and cogenotoxicity of nickel are correlated with its inhibition on DNA repair, the effects of nickel on the DNA repair induced by UV and by MMS have been investigated. Our analyses of DNA repair of single-strand breaks by alkaline elution and alkaline sucrose sedimentation indicate that nickel inhibited the DNA repair in UV-treated, but not in MMS-treated cells. Therefore, the inhibition of DNA repair seems to play an important role in the cocytotoxicity and comutagenicity of nickel. However, the inhibition of DNA repair seems not to play a decisive role in enhancing SCE, because we have previously shown that arsenite inhibits the UV-induced DNA repair, but has no enhancing effect on the UV-induced SCE. Our results also show that nickel had obvious inhibitory effects on DNA ligation and postreplication repair, but had no apparent effect on nucleotide excision and DNA polymerization in the UV repair. The results of the DNA ligation inhibition by nickel in UV but not in MMS repair suggest that different ligases are used in the DNA repair of UV- and MMS-induced damages.

Enhancing effects of heterocyclic amines and beta-carbolines on the induction of chromosome aberrations in cultured mammalian cells

The effects of post-treatment with heterocyclic amines and beta-carbolines on the induction of chromosome aberrations were studied in Chinese hamster CHO K-1 cells and SV40-transformed excision repair-deficient human XP2OSSV cells. The number of chromosome aberrations induced by UV and MMC were increased by post-treatment with Trp-P-1 and Trp-P-2, in both the presence and the absence of S9 mix. A alpha C, MeA alpha C, Glu-P-1, Glu-P-2, IQ, MeIQ, harman and harmine increased chromosome aberrations only in the presence of S9 mix. Glu-P-2, IQ, MeIQ, harman, and harmine did not induce chromosome aberrations by themselves at the concentrations used in this study. Trp-P-1, Trp-P-2, A alpha C, MeA alpha C and Glu-P-1 were weak clastogens by themselves, but at much higher concentrations than those at which they increased the induction of chromosome aberrations in cells pretreated with UV or MMC. Therefore, the increases in chromosome aberrations were not considered to be additive.

Dideoxynucleoside triphosphates inhibit a late stage of SV40 DNA replication in vitro

The role of DNA polymerases in the replication of SV40 DNA was studied using a T-antigen-dependent assay supplemented with a human KB cell extract. Inhibition of DNA polymerase alpha by addition of aphidicolin or monoclonal antibodies prevented DNA synthesis, confirming the requirement for this enzyme in replication. The replication process was unaffected by ddTTP at a concentration (5 microM) inhibitory to DNA polymerases beta and gamma, however, higher concentrations of ddTTP (200 microM) caused an apparent accumulation of relaxed circular plasmid with a concomitant decrease in DNA synthesis. An analysis of this replication intermediate indicated that it was formed during the replication reaction and that the replicative cycle was nearly complete. A kinetic study of ddTTP inhibition strongly suggested DNA polymerase epsilon (PCNA-independent DNA polymerase delta) was the target of the inhibitor and that this enzyme functions during the final stages of DNA replication.

Effect of busulphan treatment and elevated temperature on the expression of the beta-pol gene in rat testis

Changes in the expression pattern of the DNA polymerase beta gene during inhibition of spermatogenesis by busulphan and by temperature (artificial cryptorchidism) have been studied. Transient arrest of spermatogenesis in two-month-old rats after injection of a single dose of busulphan (10 mg/kg) resulted in parallel but transient decrease in the 1.4 kb of beta-pol mRNA level to an undetectable value, followed by its reappearance after resumption of spermatogenesis. An artificial cryptorchidism also caused a drastic decrease of beta-pol mRNA level. Both results as well as morphological examination of testis after busulphan injection and artificial cryptorchidism revealed that spermatocytes and spermatids represent the testicular cell fraction containing the elevated amount of beta-pol mRNA. Involvement of DNA polymerase beta in meiotic recombination is discussed.

DNA single stranded gaps formed during DNA repair synthesis induced by methyl methanesulfonate are filled by sequential action of aphidicolin- and dideoxythymidine sensitive DNA polymerases in HeLa cells

DNA repair synthesis induced by methyl methanesulfonate in preconditioned HeLa cells in which DNA replicative synthesis had been highly suppressed was inhibited by aphidicolin (an inhibitor of DNA polymerases alpha and delta) and dideoxythymidine (ddThR, an inhibitor of DNA polymerase beta). Incomplete repair patches sensitive to exonuclease III were accumulated in the presence of aphidicolin while not in the presence of ddThR. These patches were comopleted by the combined action of Klenow fragment and T4 DNA ligase, indicating that the single-stranded gaps were formed during the repair synthesis. Moreover, ddThR had little effect on the repair synthesis in the presence of aphidicolin. Thus, the results suggest that the single-stranded gaps may be sealed first by aphidicolin-sensitive polymerase followed by ddThR-sensitive DNA polymerase on the same site of the repair patch.

Accumulation of polycyclic aromatic hydrocarbon-induced single strand breaks is attributed to slower rejoining processes by DNA polymerase inhibitor, cytosine arabinoside in CHO-K1 cells

We demonstrate a successful induction of DNA single strand breaks in CHO-K1 cells by cocultivation with mouse embryonic fibroblasts (MEF) during exposure to benzo(a)pyrene (BP) or 3-methylcholanthrene (MC). When compared to those induced by methyl methanesulfonate (MMS), the DNA single strand breaks induced by BP and MC were markedly accumulated by post-incubation with cytosine arabinoside (araC) and were much more delayed in their rejoining. These results suggest that the active metabolites of BP or MC produced by cocultivation with MEF or microsomal fraction (S-15) result in the formation of large DNA adducts which require an active participation of DNA polymerase alpha(delta) in the polymerization step of excision repair for their removal.

Changes in the DNA polymerase beta gene expression during development of lung, brain, and testis suggest an involvement of the enzyme in DNA recombination

Changes in the expression pattern of DNA polymerase beta gene during rat lung, brain, and testis development have been investigated. A decrease in the level of beta-pol mRNA was observed during postnatal development of lung and brain. By contrast, an almost 20-fold increase in the level of beta-pol mRNA was observed during spermatogenesis. For most adult rat tissues the abundance of beta-pol mRNA was low compared with that of beta-actin mRNA. Northern blot analysis revealed four distinct transcripts hybridizing to beta-pol probes. At least two of them, 1.4 kb and 4.0 kb, were products of a beta-polymerase gene. The changes in the expression pattern during lung and brain development, and during spermatogenesis, suggest involvement of DNA polymerase beta in gap-filling DNA synthesis during recombination.

Phenotypic reversion of cisplatin resistance in human cells accompanies reduced host cell reactivation of damaged plasmid

Revertant cell lines were established from cisplatin (CP) resistant HeLa cells. Expression of CP damaged plasmid DNA carrying bacterial chloramphenicol acetyltransferase (CAT) gene after transfection into cells was measured. Revertant cells showed reduced host cell reactivation of damaged plasmid, as compared to resistant cells. Addition of aphidicolin, an inhibitor for DNA polymerase alpha, to resistant cells effectively blocked enhanced plasmid reactivation and acquired resistance. The results are consistent with the notion that cellular ability in repair CP-damaged DNA is a mechanism for CP resistance.

Removal of 2',3'-dideoxynucleotide residues from injected DNA in Xenopus laevis oocytes

Dideoxynucleotides have proved to be potent differential inhibitors of DNA polymerases in vitro and in vivo. Used extensively in studies of DNA repair and replication, they have more recently been used as antiviral agents particularly in treating patients for acquired immunodeficiency syndrome (AIDS). Once incorporated, these sugar-modified analogues prevent the further extension of the polynucleotide chain because of the absence of a 3'-hydroxyl group. We demonstrated that, upon injection into Xenopus laevis oocytes, 2',3'-dideoxynucleotides are efficiently removed from plasmid DNA preterminated with these analogues allowing subsequent closure by ligation. The removal process is not sensitive to aphidicolin but is quantitatively inhibited by novobiocin.

The mechanism of aphidicolin bioinactivation by rat liver in vitro systems

1. Aphidicolin is shown to undergo rapid metabolism by rat-liver microsomes resulting in its inactivation and loss of its DNA polymerase alpha/delta inhibition. Metabolism of aphidicolin was not observed with cytosolic enzymes of rat liver and was inconsistent with the involvement of microsomal 3 alpha-hydroxysteroid oxidoreductases. 2. Rates of aphidicolin inactivation as a function of microsomal enzyme induction (per nmol cytochrome P-450) followed the order: untreated microsomes greater than dexamethasone-induced greater than phenobarbital-induced greater than beta-naphthoflavone-induced greater than clofibrate-induced. 3. The principal metabolic process, constituting greater than 90% of the metabolic profile, produces 3-ketoaphidicolin 2, which exhibits approximately 10% of the activity of aphidicolin in inhibition of DNA polymerase alpha. This metabolic transformation, the oxidation of an alcohol to a ketone, is an unusual, but not unique conversion, for cytochrome P-450. 4. 3-Ketoaphidicolin 2 is an intermediate and ultimately undergoes 18-dehydroxymethylation to produce 18-noraphidicolinones 3, which are inactive in the inhibition of DNA polymerase alpha. 5. A specific constitutive cytochrome P-450 isozyme, involved in endogenous steroid regulation, was implicated as the species responsible for aphidicolin metabolism in vitro.

The roles of DNA polymerases alpha and delta in DNA replication

The identities and precise roles of the DNA polymerase(s) involved in mammalian cell DNA replication are uncertain. Circumstantial evidence suggests that DNA polymerase alpha and at least one form of DNA polymerase delta, that which is stimulated by Proliferating Cell Nuclear Antigen, catalyze mammalian cell replicative DNA synthesis. Further, the in vitro properties of polymerases alpha and delta suggest a model for their coordinate action at the replication fork. The present paper summarizes the current status of DNA polymerases alpha and delta in DNA replication, and describes newly available approaches to the study of those enzymes.

Two different mechanisms are involved for the bleomycin-induced DNA repair synthesis in permeabilized HeLa cells

Bleomycin-induced DNA repair synthesis in the permeabilized HeLa cells was sensitive to aphidicolin, an inhibitor of DNA polymerase alpha and delta, and to dideoxythymidine triphosphate (ddTTP), a specific inhibitor of DNA polymerase beta. Upon combined treatment with these inhibitors, the DNA repair synthesis was inhibited to an even higher degree. This indicated that the aphidicolin- and ddTTP-sensitive DNA repair syntheses may occur by independent mechanisms. The structure of incomplete repair patches being accumulated in the presence of these inhibitors was investigated by digestion of DNA with exonuclease III after incubation with Klenow fragment and T4 DNA ligase. The results have suggested that the patch accumulating in the presence of aphidicolin is a single-stranded gap made by excision enzyme(s), whereas that accumulating in the presence of ddTTP may be generated by strand displacement.

Levels and size complexity of DNA polymerase beta mRNA in rat regenerating liver and other organs

A cDNA probe encoding DNA polymerase beta (beta-pol) was used to study the level and size complexity of beta-pol mRNA in regenerating rat liver and other rat tissues. An almost 2-fold increase in beta-pol mRNA was observed 18-24 h after partial hepatectomy. In most adult rat tissues (liver, heart, kidney, stomach, spleen, thymus, lung and brain) the abundance of beta-pol mRNA was low. In contrast, young brain and testes exhibited beta-pol mRNA levels 5- and 15-times higher, respectively. The observed changes in the level of beta-pol mRNA in regenerating rat liver and in developing brain are correlated with reported changes in DNA polymerase beta activity. Four different (4.0, 2.5, 2.2, 1.4 kb) transcripts hybridizing to beta-pol probe were found in all tissues examined. The 4.0 kb transcript was dominant for young and adult brain, whereas the 1.4 kb transcript was dominant for testes. The significance of these transcripts is discussed.

Aphidicolin inhibition of gamma-radiation-induced DNA repair in human lymphocyte subpopulations

1. DNA repair was measured in 3 Gy gamma-irradiated human peripheral lymphocyte subpopulations by means of nucleoid sedimentation. 2. The influence of aphidicolin (an inhibitor of DNA polymerase) on the repair process was investigated. 3. Repair of 40-44% of the DNA lesions induced by gamma-irradiation was blocked by aphidicolin. 4. Enriched B- and T-lymphocyte fractions were affected by aphidicolin to the same extent.

Specific inhibition of DNA biosynthesis induced by 3'-amino-2',3'-dideoxycytidine

3'-Amino-2',3'-dideoxycytidine (3'-NH2-dCyd) produced an S-phase-specific block in exponentially growing L1210 leukemia cells. The monophosphate and triphosphate forms of the drug were detected within a few hours of 3'-NH2-dCyd treatment of intact cells. No significant change in the deoxynucleoside triphosphate levels was observed during the early stages of treatment. However, by 24 h a 2-fold increase in the amount of the deoxynucleoside triphosphates was seen. The triphosphate form of the drug competitively inhibited dCTP incorporation into calf thymus DNA using highly purified DNA polymerase alpha. The Ki was determined to be 9.6 microM with respect to dCTP. Incorporation of the analogue into DNA was not detected. On the other hand, sucrose gradient analysis suggested that incorporation of the analogue into actively synthesized DNA may account for the biological activity of this compound. Treatment with 3'-NH2-dCyd induced single-strand breaks in actively synthesized DNA, but no double-strand breaks were observed in the presence of the analogue. The data indicate that 3'-amino-2',3'-dideoxycytidine specifically interferes with DNA replication at the level of DNA polymerase by inhibiting chain elongation.

DNA repair synthesis in mouse spermatogenesis involves DNA polymerase beta activity

The role of DNA polymerase alpha-DNA primase complex and DNA polymerase beta in DNA replication and ultraviolet-induced DNA repair synthesis has been analyzed in mouse spermatogenesis. Autoradiographic experiments with germ cells in culture, indicating an involvement of DNA polymerase alpha and/or delta in DNA replication, and of DNA polymerase beta in DNA repair synthesis, have been confirmed by studying partially purified enzymes. These findings support the idea that, different from other biological systems, in meiotic and post meiotic male mouse germ cells DNA polymerase beta is the main DNA polymerase form needed for DNA repair.

Involvement of DNA polymerases in the repair of DNA damage by benzo[a]pyrene in cultured Chinese hamster cells

Mechanisms for induction of single-strand scissions in DNA by S9-activated benzo[a]pyrene (B[a]P) and their repair in cultured Chinese hamster V79 cells were investigated with inhibitors of DNA-repair synthesis using alkaline sucrose gradient sedimentation analysis. The marked induction of single-strand scissions in DNA was observed following 3 h treatment of V79 cells with 5 micrograms/ml of B[a]P. These DNA lesions were repaired to the control level within 4 h after removal of B[a]P. The simultaneous addition of inhibitors of DNA-repair synthesis. 1-beta-D-arabinofuranosylcytosine (araC) plus hydroxyurea with B[a]P did not increase the formation of DNA single-strand scissions. When these inhibitors were added after removal of B[a]P, however, they significantly blocked the rejoining of DNA-strand scissions. On the other hand, when aphidicolin, a specific inhibitor of DNA polymerase alpha, was used instead of araC, a partial inhibition of the rejoining was observed, and further addition of 2',3'-dideoxythymidine, an inhibitor of DNA polymerase beta, augmented the inhibitory effect. These results indicate that B[a]P-induced single-strand scissions of DNA in V79 cells could be repaired mostly by excision repair which involved DNA polymerase alpha and a non-alpha polymerase, presumably polymerase beta.