Synergistic Effect of Aphidicolin and 1-β-d-arabinofuranosylcytosine on the Repair of γ-ray-induced DNA Damage in Normal Human Fibroblasts

Impact of Premature Senescence on Radiosensitivity Measured by High Throughput Cell-Based Assays

In most p53 wild-type human cell types, radiosensitivity evaluated by the colony formation assay predominantly reflects stress-induced premature senescence (SIPS) and not cell death (Int. J. Mol. Sci. 2017, 18, 928). SIPS is a growth-arrested state in which the cells acquire flattened and enlarged morphology, remain viable, secrete growth-promoting factors, and can give rise to tumor-repopulating progeny. The impact of SIPS on radiosensitivity measured by short-term assays remains largely unknown. We report that in four p53 wild-type human solid tumor-derived cell lines (HCT116, SKNSH, MCF7 and A172): (i) the conventional short-term growth inhibition assay (3 days post-irradiation) generates radiosensitivity data comparable to that measured by the laborious and time-consuming colony formation assay; (ii) radiation dose-response curves obtained by multiwell plate colorimetric/fluorimetric assays are markedly skewed towards radioresistance, presumably reflecting the emergence of highly enlarged, growth-arrested and viable cells; and (iii) radiation exposure (e.g., 8 Gy) does not trigger apoptosis or loss of viability over a period of 3 days post-irradiation. Irrespective of the cell-based assay employed, caution should be exercised to avoid misinterpreting radiosensitivity data in terms of loss of viability and, hence, cell death.

Multinucleated Giant Cancer Cells Produced in Response to Ionizing Radiation Retain Viability and Replicate Their Genome

Loss of wild-type p53 function is widely accepted to be permissive for the development of multinucleated giant cells. However, whether therapy-induced multinucleation is associated with cancer cell death or survival remains controversial. Herein, we demonstrate that exposure of p53-deficient or p21^WAF1 (p21)-deficient solid tumor-derived cell lines to ionizing radiation (between 2 and 8 Gy) results in the development of multinucleated giant cells that remain adherent to the culture dish for long times post-irradiation. Somewhat surprisingly, single-cell observations revealed that virtually all multinucleated giant cells that remain adherent for the duration of the experiments (up to three weeks post-irradiation) retain viability and metabolize 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT), and the majority (>60%) exhibit DNA synthesis. We further report that treatment of multinucleated giant cells with pharmacological activators of apoptosis (e.g., sodium salicylate) triggers their demise. Our observations reinforce the notion that radiation-induced multinucleation may reflect a survival mechanism for p53/p21-deficient cancer cells. With respect to evaluating radiosensitivity, our observations underscore the importance of single-cell experimental approaches (e.g., single-cell MTT) as the creation of viable multinucleated giant cells complicates the interpretation of the experimental data obtained by commonly-used multi-well plate colorimetric assays.

Single-cell analysis of p16(INK4a) and p21(WAF1) expression suggests distinct mechanisms of senescence in normal human and Li-Fraumeni Syndrome fibroblasts

Herein we used single-cell observation methods to gain insight into the roles of p16(INK4A) and p21(WAF1) (hereafter p16 and p21) in replicative senescence and ionizing radiation-induced accelerated senescence in human [normal, ataxia telangiectasia (AT) and Li-Fraumeni syndrome (LFS)] fibroblast strains. Cultures of all strains entered a state of replicative senescence at late passages, as evident from inhibition of growth, acquisition of flattened and enlarged cell morphology, and positive staining for senescence-associated beta-galactosidase. In addition, proliferating early-passage cultures of these strains exhibited accelerated senescence in response to ionizing radiation. Immunofluorescence microscopy revealed the heterogeneous expression of p16 in normal and AT fibroblast strains, with the majority of the cells exhibiting undetectable levels of p16 irrespective of in vitro culture age. Importantly, replicative senescence as well as accelerated senescence triggered by ionizing radiation were accompanied by sustained nuclear accumulation of p21, but did not correlate with p16 expression in p53-proficient (normal and AT) fibroblasts. In p53-deficient (LFS) fibroblasts, on the other hand, replicative senescence and ionizing radiation-triggered accelerated senescence strongly correlated with expression of p16 but not of p21. Furthermore, senescence in LFS fibroblasts was associated with genomic instability encompassing polyploidy. Our findings are compatible with a model in which p16 serves as a backup regulator of senescence, triggering this response preferentially in the absence of wild-type p53 activity. The possibility that one of the tumor-suppressor functions of p16 may be associated with genomic instability, preventing the emergence of malignant progeny from polyploid giant cells, is also supported by these results.

Ultraviolet light exposure triggers nuclear accumulation of p21(WAF1) and accelerated senescence in human normal and nucleotide excision repair-deficient fibroblast strains

Induction of the p21(WAF1) protein (hereafter called p21) following genotoxic stress is known to inhibit proliferating cell nuclear antigen (PCNA)-dependent DNA repair, downregulate apoptosis, and trigger a sustained growth-arrested phenotype called accelerated senescence. Studies with immortalized human and murine cell lines have revealed that exposure to ultraviolet light (UVC; 254 nm) results in the degradation of p21 to facilitate DNA repair and promote cell survival, or may lead to apoptotic cell death. The objective of the present study was to determine whether exposure of non-transformed human fibroblast strains to relatively low fluences of UVC (i.e., fluences typically used in the clonogenic survival assay) might induce sustained nuclear accumulation of p21, leading to accelerated senescence. We have evaluated the responses of normal human fibroblast (NHF) strains and nucleotide excision repair (NER)-deficient fibroblast strains representing xeroderma pigmentosum (XP) complementation groups A and G and Cockayne syndrome (CS) complementation groups A and B. We report that exposure of NHFs to < or =15 J/m(2) of UVC, and NER-deficient fibroblasts to < or =5 J/m(2) of UVC, results in sustained nuclear accumulation of p21 and growth arrest through accelerated senescence. With each fibroblast strain examined, exposure to UVC fluences that resulted in approximately 90% loss of clonogenic potential triggered significant (>60%) accelerated senescence, but only marginal (<5%) apoptosis. We conclude that nuclear accumulation of p21 accompanied by accelerated senescence may be an integral component of the response of human fibroblasts to UVC-induced DNA damage, irrespective of their DNA repair capabilities.

Detection of excision repaired DNA damage in the comet assay by using Ara-C and hydroxyurea in three different cell types

Because of its characteristics, the comet assay has been used to evaluate the ability of virtually any type of eukaryotic cell to repair different kinds of DNA damage, including double and single strand breaks and base damage. The ability to detect excision repair sites using the alkaline version can be enhanced by the inclusion of repair inhibitors, DNA synthesis inhibitors, or chain terminators. In this sense, we evaluated the ability of hydroxyurea (HU) and cytosine arabinoside (Ara-C), for detecting lesions produced by the alkylating agents ethyl methanesulfonate (EMS) and methyl methanesulfonate (MMS) in three different cell systems. Two hundred cells for experimental point were analyzed in the alkaline version of the comet assay, and the results are evidences of the utility of the assay to detect alkylation of bases in the cells lines MRC-5 and TK-6, as the treatment with HU +Ara-C significantly increases both the basal and induced frequency of DNA damage. The use of whole blood, although it detected the effects of MMS, with and without repair inhibitors, failed to detect the effect of the selected dose of EMS and does not permit detection increases in the background level.

Relationship between DNA double-strand break rejoining and cell survival after exposure to ionizing radiation in human fibroblast strains with differing ATM/p53 status: implications for evaluation of clinical radiosensitivity

PURPOSE

To better understand the impact of defects in the DNA damage-surveillance network on the various cell-based assays used for the prediction of patient radiosensitivity.

METHODS AND MATERIALS

We examined noncancerous human fibroblast strains from individuals with ataxia telangiectasia (ataxia telangiectasia mutated [ATM] deficient) or Li-Fraumeni syndrome (p53 deficient) using the neutral comet, H2AX phosphorylation, and clonogenic survival assays.

RESULTS

Using the comet assay, we found that, compared with normal fibroblasts, cells lacking either ATM or p53 function exhibited a reduced rate of double-strand break (DSB) rejoining early (< or =4 h) after exposure to 8 Gy of gamma-radiation and also exhibited high levels of unrejoined DSBs later after irradiation. ATM-deficient and p53-deficient fibroblasts also exhibited abnormally increased levels of phosphorylated H2AX (gamma-H2AX) at later intervals after irradiation. In the clonogenic assay, ATM-deficient cells exhibited marked radiosensitivity and p53-deficient cells had varying degrees of radioresistance compared with normal fibroblasts.

CONCLUSION

Regardless of whether ataxia telangiectasia and Li-Fraumeni syndrome fibroblasts are DSB-repair deficient per se, it is apparent that p53 and ATM defects greatly influence the cellular phenotype as evidenced by the neutral comet and gamma-H2AX assays. Our data suggest that the gamma-H2AX levels observed at later intervals after irradiation may represent a reliable measure of the overall DSB rejoining capabilities of human fibroblasts. However, it appears that using this parameter as a predictor of radiosensitivity without knowledge of the cells' p53 status could lead to incorrect conclusions.

Induction of accelerated senescence by gamma radiation in human solid tumor-derived cell lines expressing wild-type TP53

Recent studies have demonstrated that p21WAF1 (now known as CDKN1A)-dependent and -independent accelerated senescence responses are a major determinant of the sensitivity of cancer cells to chemotherapeutic agents. The objective of the present study was to determine whether human solid tumor-derived cell lines that express wild-type TP53 can exhibit levels of CDKN1A induction after exposure to ionizing radiation that are sufficient to activate the accelerated senescence program. Exposure to 60Co gamma radiation (< or =8 Gy) triggered accelerated senescence in all five TP53 wild-type tumor cell lines examined, albeit to differing degrees. Three of the TP53 wild-type tumor cell lines, HCT116, A172 and SKNSH, activated the TP53 signaling pathway similarly to normal human fibroblasts, as judged by the nuclear accumulation of TP53, magnitude and duration of induction of CDKN1A mRNA and CDKN1A protein, and propensity to undergo accelerated senescence after radiation exposure. In the clonogenic survival assay, the degree of radiosensitivity of these three tumor cell lines was also in the range displayed by normal human fibroblasts. On the other hand, two other TP53 wild-type tumor cell lines, A498 and A375, did not maintain high levels of CDKN1A mRNA and CDKN1A protein at late times postirradiation and exhibited only low levels of accelerated senescence after radiation exposure. Studies with a CDKN1A knockout cell line (HCT116CDKN1A-/-) confirmed that the radiation-triggered accelerated senescence is dependent on CDKN1A function. We conclude that (1) clinically achievable doses of ionizing radiation can trigger CDKN1A-dependent accelerated senescence in some human tumor cell lines that express wild-type TP53; and (2) as previously documented for normal human fibroblasts, some TP53 wild-type tumor cell lines (e.g. HCT116, A172 and SKNSH) may lose their clonogenic potential in response to radiation-inflicted injury primarily through undergoing accelerated senescence.

Repair of cytokine-induced DNA damage in cultured rat islets of Langerhans

Treatment of cultured rat pancreatic islets of Langerhans with the combined cytokines interleukin-1beta (IL-1beta), interferon gamma (IFN gamma) and tumour necrosis factor alpha (TNF alpha) leads to DNA damage including strand breakage. We have investigated the nature of this damage and its repairability. When islets are further incubated for 4 h in fresh medium, the level of cytokine-induced strand breakage remains constant. If the nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine (NMMA) is present during cytokine treatment, then strand breakage is prevented. If NMMA is added following, rather than during,the cytokine treatment and islets are incubated for 4 h, further nitric oxide synthesis is prevented and most cytokine-induced strand breaks are no longer seen. To investigate DNA repair following cytokine treatment, cells were transferred to fresh medium and incubated for 4 h in the presence of hydroxyurea (HU) and 1-beta-D-arabinosyl cytosine (AraC), as inhibitors of strand rejoining. In the presence of these inhibitors there was an accumulation of strand breaks that would otherwise have been repaired. However, when further nitric oxide synthesis was inhibited by NMMA, significantly less additional strand breakage was seen in the presence of HU and AraC. We interpret this, as indicating that excision repair of previously induced base damage did not contribute significantly to strand breakage. Levels of oxidised purines, as indicated by formamidopyrimidine glycosylase (Fpg) sensitive sites, were not increased in cytokine-treated islets. We conclude that in these primary insulin-secreting cells: (a) the DNA damage induced by an 18h cytokine treatment is prevented by an inhibitor of nitric oxide synthase, (b) much of the damage is in the form of apparent strand breaks rather than altered bases such as oxidised purines, (c) substantial repair is ongoing during the cytokine treatment and this repair is not inhibited in the presence of nitric oxide.

Enhancement of genotoxic effects in the comet assay with human blood samples by aphidicolin

The comet assay (single cell gel electrophoresis) has become increasingly used in human biomonitoring. In its standard version at pH > 13, DNA lesions such as DNA double-strand breaks (DSB), DNA single strand breaks (SSB) and alkali-labile sites (ALS) lead to increased DNA migration. Besides DNA damage, strand break formation during excision repair can also increase DNA migration. Inhibitors of DNA repair have been shown to enhance the DNA effects of mutagens and the use of repair inhibitors has been proposed for human biomonitoring studies to increase the sensitivity of the comet assay. To further evaluate the usefulness of such an approach we performed an experimental study with human blood and tested the enhancing effect of aphidicolin (APC) on DNA effects induced by different mutagens. Our results clearly show that APC enhances the genotoxic effects of benzo[a]pyrene diolepoxide (BPDE), bischloroethylnitrosurea (BCNU) and methyl methanesulfonate (MMS), but has no significant effect on gamma radiation-induced DNA effects. The enhancing effect is seen in unstimulated and PHA-stimulated blood, indicating repair activity under both conditions but the effect is stronger in stimulated blood. Our results indicate that APC can be used to increase the sensitivity of the comet assay towards a broad spectrum of induced primary DNA lesions and support the usefulness of this approach. However, for human biomonitoring, a sensitive protocol still has to be established.

Metabolic labeling of human cells with tritiated nucleosides results in activation of the ATM-dependent p53 signaling pathway and acceleration of DNA repair

We investigated the effects of metabolic labeling with [(3)H]thymidine, [(3)H]uridine, and [(14)C]thymidine on human cells in terms of cell growth, p53 signaling, and nucleotide excision repair. Labeling with [(3)H] nucleosides resulted in growth inhibition by both p53-dependent and -independent mechanisms. Tritium labeling also led to nuclear accumulation of p53 and induction of the p53-regulated gene p21(WAF1) and its encoded protein (p21). ATM-deficient cells, however, did not increase their p53 and p21 protein levels in response to radiolabeling. Thus, labeling of human cells with tritiated nucleosides activates the radiation-responsive, ATM-dependent, DNA-damage surveillance network. Labeling of normal cells with [(3)H]thymidine significantly accelerated the repair of ultraviolet (UV) light-induced cyclobutane pyrimidine dimers, as monitored by a sensitive immunofluorescence assay. Unlike [(3)H] labeling, [(14)C] labeling did not produce any impact on proliferation, p53 signaling, or DNA repair. In the light of these findings, the validity of results obtained with nucleic acid synthesis and DNA repair assays that involve [(3)H] and [(14)C] labeling is discussed. Our immunofluorescence approach detected pyrimidine dimers after exposure to UV fluences as low as 1 J/m(2) (the lowest fluence examined). This approach may prove particularly useful for monitoring DNA damage and its repair following exposure to extremely low levels of genotoxic agents.

Lack of correlation between DNA strand breakage and p53 protein levels in human fibroblast strains exposed to ultraviolet lights

The contribution of DNA strand breaks accumulating in the course of nucleotide excision repair to upregulation of the p53 tumor suppressor protein was investigated in human dermal fibroblast strains after treatment with 254 nm ultraviolet (UV) light. For this purpose, fibroblast cultures were exposed to UV and incubated for 3 h in the presence or absence of l-beta-D-arabinofuranosylcytosine (araC) and/or hydroxyurea (HU), and then assayed for DNA strand breakage and p53 protein levels. As expected from previous studies, incubation of normal and ataxia telangiectasia (AT) fibroblasts with araC and HU after UV irradiation resulted in an accumulation of DNA strand breaks. Such araC/HU-accumulated strand breaks (reflecting nonligated repair-incision events) following UV irradiation were not detected in xeroderma pigmentosum (XP) fibroblast strains belonging to complementation groups A and G. Western blot analysis revealed that normal fibroblasts exhibited little upregulation of p53 (approximately 1.2-fold) when incubated without araC after 5 J/m2 irradiation, but showed significant (three-fold) upregulation of p53 when incubated with araC after irradiation. AraC is known to inhibit nucleotide excision repair at both the damage removal and repair resynthesis steps. Therefore, the potentiation of UV-induced upregulation of p53 evoked by araC in normal cells may be a consequence of either persistent bulky DNA lesions or persistent incision-associated DNA strand breaks. To distinguish between these two possibilities, we determined p53 induction in AT fibroblasts (which do not upregulate p53 in response to DNA strand breakage) and in XP fibroblasts (which do not exhibit incision-associated breaks after UV irradiation). The p53 response after treatment with 5 J/m2 UV and incubation with araC was similar in AT, XPA, XPG and normal fibroblasts. In addition, exposure of XPA and XPG fibroblasts to UV (5, 10 or 20 J/m2) followed by incubation without araC resulted in a strong upregulation of p53. We further demonstrated that HU, an inhibitor of replicative DNA synthesis (but not of nucleotide excision repair), had no significant impact on p53 protein levels in UV irradiated and unirradiated human fibroblasts. We conclude that upregulation of p53 at early times after exposure of diploid human fibroblasts to UV light is triggered by persistent bulky DNA lesions, and that incision-associated DNA strand breaks accumulating in the course of nucleotide excision repair and breaks arising as a result of inhibition of DNA replication contribute little (if anything) to upregulation of p53.

Gene conversion can create new MHC alleles

It is possible to measure gene conversion of MHC genes with the help of a semi-nested PCR assay. Several considerations are of utmost importance when such an assay is set up. Using this assay, we have found that gene conversion occurs in MHC class II genes in mouse sperm, but not in somatic cells tested. Although this gene conversion occurs in germline cells, it is already completed in spermatogonia, and consequently is mitotic event unlinked to meiosis. The frequency of gene conversion events in MHC class II genes varies strongly from one allele to another, with the highest detected frequencies as high as 1/40,000 for an individual heterozygous for both donor and acceptor sequences. Deletions or insertions in one gene relative to the other seem to lower the efficiency of gene conversion considerably. Stretches within MHC genes amenable to gene conversion are located in CpG clusters, whereas MHC genes not involved in gene conversion have background CpG levels. DNA damage, either chemical or radiation induced, increases the frequency of gene conversion of MHC class II genes in cultured cells of the fibroblastoid lineage. The effect of chemical DNA damage seems roughly dose dependent, whereas irradiation has a maximal effect at low doses.

DNA damage caused by etoposide and gamma-irradiation induces gene conversion of the MHC in a mouse non-germline testis cell line

We have explored the effects of gamma-irradiation and etoposide on the gene conversion frequency between the endogenous major histocompatibility complex class II genes Abk and Ebd in a mouse testis cell line of non-germline origin with a polymerase chain reaction assay. Both gamma-rays and etoposide were shown to increase the gene conversion frequency with up to 15-fold compared to untreated cells. Etoposide, which is an agent that stabilise a cleavable complex between DNA and DNA topoisomerase II, shows an increased induction of gene conversion events with increased dose of etoposide. Cells treated with gamma-rays, which induce strand breaks, had an increased gene conversion frequency when they were subjected to low doses of irradiation, but increasing doses of irradiation did not lead to an increase of gene conversion events, which might reflect differences in the repair process depending on the extent and nature of the DNA damage. These results where DNA damage was shown to be able to induce gene conversion of endogenous genes in mouse testis cells suggests that the DNA repair system could be involved in the molecular genetic mechanism that results in gene conversion in higher eukaryotes like mammals.

Detection of DNA strand breaks in isolated mussel (Mytilus edulis L. ) digestive gland cells using the "Comet" assay

Isolated mussel (Mytilus edulis L.) digestive gland cells were analyzed using the single-cell gel electrophoresis or "comet" assay to assess the ability of potential aquatic contaminants to induce DNA strand breaks (SBs) and to investigate the potential application of this technique as part of an aquatic biomonitoring regime. Freshly prepared cell suspensions from digestive gland were exposed in vitro to hydrogen peroxide (H2O2, 0-200 microM), 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX, 0-200 microM), benzo[a]pyrene (BaP, 0-200 microM), 1-nitropyrene (1-NP, 0-250 microM) and nitrofurantoin (NF, 0-1000 microM) for 1 h in the dark at 15 degreesC in the presence of the DNA repair inhibitor cytosine-beta-D-arabinofuranoside (araC). DNA strand breakage was measured using the comet assay. There were significant concentration-dependent increases in the percentage of DNA in the comet tail (mean values+/-SD) for all doses compared with controls (P<0.05) with H2O2 (up to 61.4+/-5.1% at 100 microM), MX (up to 34. 3+/-2.2% at 200 microM), BaP (up to 24.7+/-5.1 at 100 microM), 1-NP (up to 54.7+/-5.0% at 200 microM), and NF (up to 68.1+/-4.5% at 500 microM). There was a decrease (P<0.05) in viability (eosin Y exclusion) of exposed compared with control cells at 200 microM H2O2 and BaP only. This study has demonstrated the potential of the comet assay to detect DNA strand breakage at subcytotoxic concentrations of a range of agents, some of which require metabolic activation. This may provide a sensitive, but nonspecific, molecular biomarker of genotoxicity.

Modulation of resistance to cisplatin by amphotericin B and aphidicolin in human larynx carcinoma cells

The aim of this study was to examine whether resistance to cisplatin [cis-diamminedichloroplatinum (II)] (CDDP) could be overcome by amphotericin B, cyclosporin A and aphidicolin in two sublines of human larynx carcinoma HEp2 cells. The sensitivity of parental and cisplatin-resistant CA3 and CK2 cells to amphotericin B, cyclosporin A and aphidicolin, and also the effects of these drugs (given in maximal nontoxic concentrations) on cisplatin sensitivity were determined by clonogenic survival assay. CA3 and CK2 cells were sensitive to amphotericin B, and resistant to cyclosporin A and aphidicolin, compared with their parental cells. Amphotericin B increased cisplatin toxicity 2-fold in CA3 cells and 2.7-fold in CK2 cells, while it had no effect in parental HEp2 cells. Cyclosporin A did not influence the sensitivity of examined cells to cisplatin. The sensitizing effect of aphidicolin was more obvious in cisplatin-resistant cells. Cisplatin toxicity was increased by aphidicolin: 1.5-fold in HEp2 cells, 2-fold in CA3 cells, and 1.9-fold in CK2 cells. Therefore, the resistance to cisplatin in human larynx carcinoma CA3 and CK2 cells can be partially reversed by amphotericin B and aphidicolin.

Effect of DNA polymerase inhibitors on repair of gamma ray-induced DNA damage in proliferating (intact versus permeable) human fibroblasts: evidence for differences in the modes of action of aphidicolin and 1-beta-D-arabinofuranosylcytosine

The mammalian DNA polymerase inhibitors aphidicolin and 1-beta-D-arabinofuranosylcytosine (araC), when used in combination, inhibit the repair of DNA damage induced by gamma rays or 4-nitroquinoline 1-oxide in normal human fibroblasts to an extent 2- to 4-fold greater than that seen with each inhibitor alone. Thus either aphidicolin modulates the rate of intracellular accumulation of araC 5'-triphosphate (araCTP), the presumed rate-limiting step in the genotoxic action of araC, or aphidicolin and araC inhibit repair by different mechanisms. To explore these possibilities, we compared the effects of aphidicolin, araC, araCTP, and 2',3'-dideoxythymidine triphosphate (ddTTP) on repair of DNA damage induced by 60Co gamma radiation in intact versus permeable human fibroblasts. Both aphidicolin and araC strongly inhibited repair in permeable cells, as indicated by the accumulation of DNA strand breaks in irradiated cultures that were subsequently treated with saponin (25 micrograms/ml; 10 min) and incubated for 2 h with either chemical. The extent of repair inhibition by each drug was comparable in intact and permeable cells, amounting to approximately 1.1 sites/10(8) daltons/2 h upon exposure to 150 Gy. The active metabolite of araC, araCTP, did not inhibit repair in intact cells, but did so in permeable cells to an extent within the range of that seen with araC or aphidicolin alone. The incidence of DNA strand breaks accumulating in gamma-irradiated permeable cultures as a result of incubation with araCTP plus aphidicolin, or araC plus aphidicolin, was approximately 2-fold greater than that arising in parallel cultures which had been incubated with optimal concentrations of each of the three drugs alone. Although the resolution of our assays compelled us to monitor repair events in moribund cell populations, we have reason to be confident that within the short post-irradiation period considered here, the observed drug-accumulated breaks truly represent functional repair inhibition and not merely abortive pathological responses. We thus conclude that (1) the accumulation of araCTP in intact cells is not limiting the ability of araC to inhibit DNA repair; and (2) the mode of the inhibitory action of araC/araCTP on gamma ray repair is different from that of aphidicolin. In contrast to the observations with these chemicals, ddTTP (20 microM), a potent inhibitor of DNA polymerase beta, did not produce any measurable effect on DNA repair in gamma-irradiated permeable fibroblasts, nor did it enhance the efficacy of araC, araCTP or aphidicolin to inhibit repair. These results strongly suggest that DNA polymerase beta plays no significant role in the repair of gamma radioproducts in human fibroblasts.(ABSTRACT TRUNCATED AT 400 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.