Synergistic antitumor effects of Peiminine and Doxorubicin on breast cancer through enhancing DNA damage via ZEB1

Peiminine, the primary biologically active compound from Fritillaria thunbergii Miq., has demonstrated significant pharmacological activities. Doxorubicin is one of the most potent chemotherapeutic agents for breast cancer (BC). This study was designed to investigate the efficacy and underlying mechanisms of Peiminine combined with Doxorubicin in treating BC. Our results demonstrated that the combination of Peiminine and 1 mg/kg Doxorubicin exhibited more significant suppression of tumor growth compared with the monotherapy in MDA-MB-231 xenograft nude mice model, which is comparable to the effect of 3 mg/kg Doxorubicin in vivo. Notably, the 3 mg/kg Doxorubicin monotherapy resulted in organ toxicity, specifically in the liver and heart, whereas no toxicity was observed in the combination group. In vitro, this combined treatment exhibited a synergistic reduction on the viability of BC cells. Peiminine enhanced the cell cycle arrest and DNA damage induced by Doxorubicin. Furthermore, the combination treatment effectively blocked DNA repair by inhibiting the MAPKs signaling pathways. And ZEB1 knockdown attenuated the combined effect of Peiminine and Doxorubicin on cell viability and DNA damage. In conclusion, our study found that the combination of Peiminine and Doxorubicin showed synergistic inhibitory effects on BC both in vivo and in vitro through enhancing Doxorubicin-induced DNA damage. These findings support that their combination is a novel and promising therapeutic strategy for treating BC.

Prostaglandin F2α synthase promotes oxaliplatin resistance in colorectal cancer through prostaglandin F2α-dependent and F2α-independent mechanism

BACKGROUND

Oxaliplatin (Oxa) is the first-line chemotherapy drug for colorectal cancer (CRC), and Oxa resistance is crucial for treatment failure. Prostaglandin F2α synthase (PGF2α) (PGFS), an enzyme that catalyzes the production of PGF2α, is involved in the proliferation and growth of a variety of tumors. However, the role of PGFS in Oxa resistance in CRC remains unclear.

AIM

To explore the role and related mechanisms of PGFS in mediating Oxa resistance in CRC.

METHODS

The PGFS expression level was examined in 37 pairs of CRC tissues and paracancerous tissues at both the mRNA and protein levels. Overexpression or knockdown of PGFS was performed in CRC cell lines with acquired Oxa resistance (HCT116-OxR and HCT8-OxR) and their parental cell lines (HCT116 and HCT8) to assess its influence on cell proliferation, chemoresistance, apoptosis, and DNA damage. For determination of the underlying mechanisms, CRC cells were examined for platinum-DNA adducts and reactive oxygen species (ROS) levels in the presence of a PGFS inhibitor or its products.

RESULTS

Both the protein and mRNA levels of PGFS were increased in the 37 examined CRC tissues compared to the adjacent normal tissues. Oxa induced PGFS expression in the parental HCT116 and HCT8 cells in a dose-dependent manner. Furthermore, overexpression of PGFS in parental CRC cells significantly attenuated Oxa-induced proliferative suppression, apoptosis, and DNA damage. In contrast, knockdown of PGFS in Oxa-resistant HCT116 and HCT8 cells (HCT116-OxR and HCT8-OxR) accentuated the effect of Oxa treatment in vitro and in vivo. The addition of the PGFS inhibitor indomethacin enhanced the cytotoxicity caused by Oxa. Treatment with the PGFS-catalyzed product PGF2α reversed the effect of PGFS knockdown on Oxa sensitivity. Interestingly, PGFS inhibited the formation of platinum-DNA adducts in a PGF2α-independent manner. PGF2α exerts its protective effect against DNA damage by reducing ROS levels.

CONCLUSION

PGFS promotes resistance to Oxa in CRC via both PGF2α-dependent and PGF2α-independent mechanisms.

Effect of pachymaran on oxidative stress and DNA damage induced by formaldehyde

To further explore the pharmacological effect of pachymaran, this article studied the inhibition of pachymaran on oxidative stress and genetic damage induced by formaldehyde. 40 adult Kunming male mice were randomly divided into four groups with different interventions. One week later, the contents of serum SOD, GR, MDA, DNA-protein crosslink (DPC), 8-hydroxydeoxyguanosine (8-OHDG) and DNA adduct were determined by ELISA. The results showed that there were statistically significant differences in the contents of SOD, GR and MDA among the four groups (P < 0.01). The activity of SOD and GR increased along with the increase of pachymaran dosage (SOD: rs = 0.912, P < 0.01; GR: rs = 0.857, P < 0.01), while the content of MDA showing a significant negative correlation (rs = - 0.893, P < 0.01). There were statistically significant differences in the levels of DPC, 8-OHDG and DNA adduct among the four groups (DPC and DNA adduct: P < 0.01, 8-OHDG: P < 0.05), the concentration decreased along with the increase of pachymaran dosage (DPC: rs = - 0.855, P < 0.01; 8-OHDG:rs = - 0.412, P < 0.05, DNA adduct: γs = - 0.869, P < 0.01). It can be inferred that pachymaran can inhibit oxidative stress and DNA damage induced by formaldehyde with the dose-effect relationship.

Correlation Investigation between Pyrrole-DNA and Pyrrole-Protein Adducts in Male ICR Mice Exposed to Retrorsine, a Hepatotoxic Pyrrolizidine Alkaloid

Pyrrolizidine alkaloids (PAs) have been found in over 6000 plants worldwide and represent the most common hepatotoxic phytotoxins. Catalyzed by hepatic cytochrome P450 enzymes, PAs are metabolized into reactive pyrrolic metabolites, which can alkylate cellular proteins and DNA to form pyrrole-protein adducts and pyrrole-DNA adducts, leading to cytotoxicity, genotoxicity, and tumorigenicity. To date, the correlation between these PA-derived pyrrole-protein and pyrrole-DNA adducts has not been well investigated. Retrorsine is a representative hepatotoxic and carcinogenic PA. In the present study, the correlations among the PA-derived liver DNA adducts, liver protein adducts, and serum protein adducts in retrorsine-treated mice under different dosage regimens were studied. The results showed positive correlations among these adducts, in which serum pyrrole-protein adducts were more accessible and present in higher abundance, and thus could be used as a suitable surrogate biomarker for pyrrole-DNA adducts to indicate the genetic or carcinogenic risk posed by retrorsine.

Susceptibility of polar cod (Boreogadus saida) to a model carcinogen

Studies that aim to characterise the susceptibility of the ecologically relevant and non-model fish polar cod (Boreogadus saida) to model carcinogens are required. Polar cod were exposed under laboratory conditions for six months to control, 0.03 μg BaP/g fish/week and 0.3 μg BaP/g fish/week dietary benzo(a)pyrene (BaP), a reference carcinogen. The concentrations of the 3-OH-BaP bile metabolite and transcriptional responses of genes involved in DNA adduct recognition (xpc), helicase activity (xpd), DNA repair (xpf, rad51) and tumour suppression (tp53) were assessed after 0, 1, 3 and 6 months of exposure, alongside body condition indexes (gonadosomatic index, hepatosomatic index and condition factor). Micronuclei and nuclear abnormalities in blood and spleen, and liver histopathological endpoints were assessed at the end of the experiment. Fish grew steadily over the whole experiment and no mortality was recorded. The concentrations of 3-OH-BaP increased significantly after 1 month of exposure to the highest BaP concentration and after 6 months of exposure to all BaP concentrations showing the biotransformation of the mother compound. Nevertheless, no significant induction of gene transcripts involved in DNA damage repair or tumour suppression were observed at the selected sampling times. These results together with the absence of chromosomal damage in blood and spleen cells, the subtle increase in nuclear abnormalities observed in spleen cells and the low occurrence of foci of cellular alteration suggested that the exposure was below the threshold of observable effects. Taken together, the results showed that polar cod was not susceptible to carcinogenesis using the BaP exposure regime employed herein.

Terminal Deoxynucleotidyl Transferase-Catalyzed Preparation of pH-Responsive DNA Nanocarriers for Tumor-Targeted Drug Delivery and Therapy

Developing a highly efficient carrier for tumor-targeted delivery and site-specific release of anticancer drugs is a good way to overcome the side effects of traditional cancer chemotherapy. Benefiting from the nontoxic and biocompatible characteristics, DNA-based drug carriers have attracted increasing attention. Herein, we reported a novel and readily manipulated strategy to construct spherical DNA nanocarriers. In this strategy, terminal deoxynucleotidyl transferase (TdT)-catalyzed DNA extension reaction is used to prepare a thick DNA layer on a gold nanoparticle (AuNP) surface by extending long poly(C) sequences from DNA primers immobilized on AuNPs. The poly(C) extension products can then hybridize with G-rich oligonucleotides to give CG-rich DNA duplexes (for loading anticancer drug doxorubicin, Dox) and multiple AS1411 aptamers. Via synergic recognition of multiple aptamer units to nucleolin proteins, biomarker of malignant tumors, Dox-loaded DNA carrier can be efficiently internalized in cancer cells and achieve burst release of drugs in acidic organelles because of i-motif formation-induced DNA duplex destruction. An as-prepared pH-responsive drug carrier was demonstrated to be promising for highly efficient delivery of Dox and selective killing of cancer cells in both in vitro and in vivo experiments, thus showing a huge potential in anticancer therapy.

Therapeutic Peptide Amphiphile as a Drug Carrier with ATP-Triggered Release for Synergistic Effect, Improved Therapeutic Index, and Penetration of 3D Cancer Cell Spheroids

Despite the great progress in the field of drug delivery systems for cancer treatment over the last decade, many challenges still lie ahead, such as low drug loading, deep penetration of tumors, side effects, and the development of drug resistance. A class of cationic membrane lytic peptides has shown potential as an anticancer agent by inducing cancer cell death via membrane disruption; meanwhile, their intrinsic selectivity renders them as having low cytotoxicity towards noncancerous cells. Here, we report the use of a cationic peptide amphiphile (PA), named PAH6, to load doxorubicin (Dox) that is intercalated in an ATP-binding aptamer-incorporated DNA scaffold. The PA contains a cationic lytic sequence, (KLAKLAK)₂, a polyhistidine segment for the "proton sponge" effect, and a hydrophobic alkyl tail to drive the self-assembly. Dox-loaded DNA was found to form a spherical nanocomplex (NC) with PAH6 with particle sizes below 100 nm at various ratios. Since the carrier PAH6 is also a therapeutic agent, the drug loadings of the NC reached up to ~86% within the ratios we tested, and Dox was released from the NC in an ATP-rich environment. In vitro studies indicate that the presence of PAH6 could permeabilize cell membranes and kill cells through fast membrane disruption and depolarization of mitochondrial membranes. The cytotoxicity tests were conducted using A549 nonsmall cell lung cancer cells and NIH-3T3 fibroblast cells. PAH6 showed selectivity towards A549 cells. Significantly, the Dox-DNA/PAH6 NC exhibited a synergistic effect against A549 cells, with the IC50 decreased up to ~90% for Dox and ~69% for PAH6 when compared to the IC50 values of the two components, respectively. Furthermore, the selectivity of PAH6 conferred to the complex an improved therapeutic index between A549 and NIH-3T3 cells. A 3D-cultured A549 spheroid model was adopted to test the capability of Dox-DNA/PAH6 for tumor penetration. The PAH6 or Dox-DNA/PAH6 complex was found to break the spheroids into pieces, while Dox-treated spheroids maintained their shapes. In summary, this work provides a new strategy for constructing nanomedicines using therapeutic agents to meet the features required by anticancer treatment.

Impact of tobacco-specific nitrosamine-derived DNA adducts on the efficiency and fidelity of DNA replication in human cells

The tobacco-derived nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN) are known human carcinogens. Following metabolic activation, NNK and NNN can induce a number of DNA lesions, including several 4-(3-pyridyl)-4-oxobut-1-yl (POB) adducts. However, it remains unclear to what extent these lesions affect the efficiency and accuracy of DNA replication and how their replicative bypass is influenced by translesion synthesis (TLS) DNA polymerases. In this study, we investigated the effects of three stable POB DNA adducts (O2-POB-dT, O4-POB-dT, and O6-POB-dG) on the efficiency and fidelity of DNA replication in HEK293T human cells. We found that, when situated in a double-stranded plasmid, O2-POB-dT and O4-POB-dT moderately blocked DNA replication and induced exclusively T→A (∼14.9%) and T→C (∼35.2%) mutations, respectively. On the other hand, O6-POB-dG slightly impeded DNA replication, and this lesion elicited primarily the G→A transition (∼75%) together with a low frequency of the G→T transversion (∼3%). By conducting replication studies in isogenic cells in which specific TLS DNA polymerases (Pols) were deleted by CRISPR-Cas9 genome editing, we observed that multiple TLS Pols, especially Pol η and Pol ζ, are involved in bypassing these lesions. Our findings reveal the cytotoxic and mutagenic properties of specific POB DNA adducts and unravel the roles of several TLS polymerases in the replicative bypass of these adducts in human cells. Together, these results provide important new knowledge about the biological consequences of POB adducts.

Cisplatin-DNA adduct repair of transcribed genes is controlled by two circadian programs in mouse tissues

Cisplatin is a major cancer chemotherapeutic drug. It kills cancer cells by damaging their DNA, mainly in the form of Pt-d(GpG) diadducts. However, it also has serious side effects, including nephrotoxicity and hepatotoxicity that limit its usefulness. Chronotherapy is taking circadian time into account during therapy to improve the therapeutic index, by improving efficacy and/or limiting toxicity. To this end, we tested the impact of clock time on excision repair of cisplatin-induced DNA damage at single-nucleotide resolution across the genome in mouse kidney and liver. We found that genome repair is controlled by two circadian programs. Repair of the transcribed strand (TS) of active, circadian-controlled genes is dictated by each gene's phase of transcription, which falls across the circadian cycle with prominent peaks at dawn and dusk. In contrast, repair of the nontranscribed strand (NTS) of all genes, repair of intergenic DNA, and global repair overall peaks at Zeitgeber time ZT08, as basal repair capacity, which is controlled by the circadian clock, peaks at this circadian time. Consequently, the TS and NTS of many genes are repaired out of phase. As most cancers are thought to have defective circadian rhythms, these results suggest that future research on timed dosage of cisplatin could potentially reduce damage to healthy tissue and improve its therapeutic index.

Tracking matrix effects in the analysis of DNA adducts of polycyclic aromatic hydrocarbons

LC-MS using electrospray ionization is currently the method of choice in bio-organic analysis covering a wide range of applications in a broad spectrum of biological media. The technique is noted for its high sensitivity but one major limitation that hinders achievement of its optimal sensitivity is the signal suppression due to matrix inferences introduced by the presence of co-extracted compounds during the sample preparation procedure. The analysis of DNA adducts of common environmental carcinogens is particularly sensitive to such matrix effects as sample preparation is a multistep process which involves "contamination" of the sample due to the addition of enzymes and other reagents for digestion of the DNA in order to isolate the analyte(s). This problem is further exacerbated by the need to reach low levels of quantitation (LOQ in the ppb level) while also working with limited (2-5 μg) quantities of sample. We report here on the systematic investigation of ion signal suppression contributed by each individual step involved in the sample preparation associated with the analysis of DNA adducts of polycyclic aromatic hydrocarbon (PAH) using as model analyte BaP-dG, the deoxyguanosine (dG) adduct of benzo[a]pyrene (BaP). The individual matrix contribution of each one of these sources to analyte signal was systematically addressed as were any interactive effects. The information was used to develop a validated analytical protocol for the target biomarker at levels typically encountered in vivo using as little as 2 μg of DNA and applied to a dose response study using a metabolically competent cell line.

Daunorubicin-Loaded DNA Origami Nanostructures Circumvent Drug-Resistance Mechanisms in a Leukemia Model

Many cancers show primary or acquired drug resistance due to the overexpression of efflux pumps. A novel mechanism to circumvent this is to integrate drugs, such as anthracycline antibiotics, with nanoparticle delivery vehicles that can bypass intrinsic tumor drug-resistance mechanisms. DNA nanoparticles serve as an efficient binding platform for intercalating drugs (e.g., anthracyclines doxorubicin and daunorubicin, which are widely used to treat acute leukemias) and enable precise structure design and chemical modifications, for example, for incorporating targeting capabilities. Here, DNA nanostructures are utilized to circumvent daunorubicin drug resistance at clinically relevant doses in a leukemia cell line model. The fabrication of a rod-like DNA origami drug carrier is reported that can be controllably loaded with daunorubicin. It is further directly verified that nanostructure-mediated daunorubicin delivery leads to increased drug entry and retention in cells relative to free daunorubicin at equal concentrations, which yields significantly enhanced drug efficacy. Our results indicate that DNA origami nanostructures can circumvent efflux-pump-mediated drug resistance in leukemia cells at clinically relevant drug concentrations and provide a robust DNA nanostructure design that could be implemented in a wide range of cellular applications due to its remarkably fast self-assembly (≈5 min) and excellent stability in cell culture conditions.

DNA-bound platinum is the major determinant of cisplatin sensitivity in head and neck squamous carcinoma cells

PURPOSE

The combination of systemic cisplatin with local and regional radiotherapy as primary treatment of head and neck squamous cell carcinoma (HNSCC) leads to cure in approximately half of the patients. The addition of cisplatin has significant effects on outcome, but despite extensive research the mechanism underlying cisplatin response is still not well understood.

METHODS

We examined 19 HNSCC cell lines with variable cisplatin sensitivity. We determined the TP53 mutational status of each cell line and investigated the expression levels of 11 potentially relevant genes by quantitative real-time PCR. In addition, we measured cisplatin accumulation and retention, as well as the level of platinum-DNA adducts.

RESULTS

We found that the IC50 value was significantly correlated with the platinum-DNA adduct levels that accumulated during four hours of cisplatin incubation (p = 0.002). We could not find a significant correlation between cisplatin sensitivity and any of the other parameters tested, including the expression levels of established cisplatin influx and efflux transporters. Furthermore, adduct accumulation did not correlate with mRNA expression of the investigated influx pumps (CTR1 and OCT3) nor with that of the examined DNA repair genes (ATR, ATM, BRCA1, BRCA2 and ERCC1).

CONCLUSION

Our findings suggest that the cisplatin-DNA adduct level is the most important determinant of cisplatin sensitivity in HNSCC cells. Imaging with radio-labeled cisplatin might have major associations with outcome.

Formaldehyde-releasing prodrugs in combination with adriamycin can overcome cellular drug resistance

The anticancer drug Adriamycin is widely used in cancer chemotherapy and is classified as a topoisomerase II inhibitor. However, in the presence of formaldehyde, Adriamycin also forms high levels of DNA adducts. In this study, a new series of butyric acid and formaldehyde-releasing drugs related to AN9 (pivaloyloxymethyl butyrate) was assessed for their ability to facilitate Adriamycin-DNA adduct formation in Adriamycin-sensitive and -resistant cell lines (HL60 and HL60/MX2; MES-SA and MES-SA/Dx5). Drugs that released two molar equivalents of formaldehyde per mole of prodrug were superior in their ability to enhance adduct formation compared to those that released one molar equivalent. Adduct formation (as assessed by binding of radiolabeled Adriamycin to genomic DNA) was always lower in the resistant cell lines compared to the sensitive cell lines. However, in growth inhibition experiments, prodrug combinations were able to overcome Adriamycin resistance to varying degrees, and the combination of Adriamycin with selected prodrugs that release two moles of formaldehyde totally overcame resistance in HL60/MX2 cells. These HL60-derived cells express altered levels of topoisomerase II and also express a mutant form of the enzyme. Combinations of Adriamycin with selected prodrugs that release one or two moles of formaldehyde partially overcame P-glycoprotein-mediated resistance in MES-SA/Dx5 cells. Formaldehyde-releasing prodrugs (as single agents) overcame both forms of resistance in the two resistant cell lines, demonstrating that they were not substrates of these resistance mechanisms. Collectively, these results suggest that changing the mechanism via which Adriamycin exerts its anticancer effect by dramatically increasing adduct levels (requiring coadministration of formaldehyde-releasing prodrugs) may be a useful means of cancer treatment, as well as for overcoming Adriamycin-induced resistance.

The cardio-protecting agent and topoisomerase II catalytic inhibitor sobuzoxane enhances doxorubicin-DNA adduct mediated cytotoxicity

PURPOSE

The importance of understanding the mechanism of action of anticancer agents is sometimes overlooked in the pursuit of new and therapeutically advantageous compounds. Doxorubicin has long been identified as an inhibitor of the DNA-decatenating enzyme topoisomerase II, this being believed to be the major mechanism of action of this drug. However, the complex nature of cytotoxicity induced by doxorubicin suggests that more than one mechanism of action is responsible for cell kill. Investigation into various other cellular effects has shown that doxorubicin can, in the presence of formaldehyde, form doxorubicin-DNA adducts, resulting in enhanced cell death.

METHODS

We have used six catalytic inhibitors of topoisomerase II (aclarubicin, merbarone, suramin, staurosporine, maleimide and sobuzoxane) to investigate the role of topoisomerase II mediated cell effects in doxorubicin-DNA adduct inducing treatments. Adduct levels were determined by scintillation counting of [14C]doxorubicin-DNA lesions and DNA damage responses by Comet analysis and flow cytometry (apoptosis).

RESULTS

Here we show that sobuzoxane inhibits topoisomerase II but in the presence of doxorubicin also enhances the production of doxorubicin-DNA adducts resulting in an enhanced cytotoxic response. We show that the formation of doxorubicin-DNA adducts is mediated by formaldehyde released from sobuzoxane when it is metabolised.

CONCLUSIONS

Sobuzoxane has also been shown to decrease the normally dose limiting cardiotoxicity commonly exhibited with clinical use of doxorubicin. The potential combination of doxorubicin and sobuzoxane in cancer chemotherapy has two advantages. First, the mechanism of doxorubicin toxicity is shifted away from topoisomerase II inhibition and towards drug-DNA adduct formation which may allow for a lower drug dose to be used and circumvent some drug resistance problems. Second, the addition of a cardioprotecting agent will counteract the commonly dose limiting side effect of cardiac damage resulting from doxorubicin treatment. The importance of the potentiation of cell kill of doxorubicin and sobuzoxane provides a rationalisation of a mechanistic-based combination of anticancer drugs for an improved clinical outcome.

Tissue-specific attenuation of endogenous DNA I-compounds in rats by carcinogen azoxymethane: possible role of dietary fish oil in colon cancer prevention

I-compounds are bulky covalent DNA modifications that are derived from metabolic intermediates of nutrients. Some I-compounds may play protective roles against cancer, aging, and degenerative diseases. Many carcinogens and tumor promoters significantly reduce I-compound levels gradually during carcinogenesis. Colon cancer is the second leading cause of cancer death in the United States, whereas cancer of the small intestine is relatively rare. Here we have studied levels of I-compounds in DNA of colon and duodenum of male Sprague-Dawley rats treated with azoxymethane. The effects of dietary lipids (fish oil or corn oil) on colon and duodenal DNA I-compounds were also investigated. Rats fed a diet containing fish oil or corn oil were treated with 15 mg/kg azoxymethane. Animals were terminated 0, 6, 9, 12, or 24 hours after injection. I-compound levels were analyzed by the nuclease P1-enhanced (32)P-postlabeling assay. Rats treated with azoxymethane displayed lower levels of I-compounds in colon DNA compared with control groups (0 hour). However, I-compound levels in duodenal DNA were not diminished after azoxymethane treatment. Animals fed a fish oil diet showed higher levels of I-compounds in colonic DNA compared with corn oil groups (mean adduct levels for fish and corn oil groups were 13.35 and 10.69 in 10(9) nucleotides, respectively, P = 0.034). Taken together, these results support claims that fish oil, which contains a high level of omega-3 polyunsaturated fatty acids, may have potent chemopreventive effects on carcinogen-induced colon cancer. The fact that duodenal I-compounds were not diminished by azoxymethane treatment may have been due to the existence of tissue-specific factors protecting against carcinogenesis. In conclusion, our observations show that endogenous DNA adducts may serve not only as sensitive biomarkers in carcinogenesis and cancer prevention studies, but are also helpful to further our understanding of the chemopreventive properties of omega-3 fatty acids and mechanisms of carcinogenesis.

Platinated DNA adducts enhance poisoning of DNA topoisomerase I by camptothecin

Camptothecins constitute a novel class of chemotherapeutics that selectively target DNA topoisomerase I (Top1) by reversibly stabilizing a covalent enzyme-DNA intermediate. This cytotoxic mechanism contrasts with that of platinum drugs, such as cisplatin, which induce inter- and intrastrand DNA adducts. In vitro combination studies using platinum drugs combined with Top1 poisons, such as topotecan, showed a schedule-dependent synergistic activity, with promising results in the clinic. However, whereas the molecular mechanism of these single agents may be relatively well understood, the mode of action of these chemotherapeutic agents in combination necessitates a more complete understanding. Indeed, we recently reported that a functional homologous recombination pathway is required for cisplatin and topotecan synergy yet represses the synergistic toxicity of 1-beta-D-arabinofuranosyl cytidine in combination with topotecan (van Waardenburg, R. C., de Jong, L. A., van Delft, F., van Eijndhoven, M. A., Bohlander, M., Bjornsti, M. A., Brouwer, J., and Schellens, J. H. (2004) Mol. Cancer Ther. 3, 393-402). Here we provide direct evidence for Pt-1,3-d(GTG) poisoning of Top1 in vitro and demonstrate that persistent Pt-DNA adducts correlate with increased covalent Top1-DNA complexes in vivo. This contrasts with a lack of persistent lesions induced by the alkylating agent bis[chloroethyl]nitrosourea, which exhibits only additive activity with topotecan in a range of cell lines. In human IGROV-1 ovarian cancer cells, the synergistic activity of cisplatin with topotecan requires processive DNA polymerization, whereas overexpression of Top1 enhances yeast cell sensitivity to cisplatin. These results indicate that the cytotoxic activity of cisplatin is due, in part, to poisoning of Top1, which is exacerbated in the presence of topotecan.

Analysis of the Effect of Bulk at N2-Alkylguanine DNA Adducts on Catalytic Efficiency and Fidelity of the Processive DNA Polymerases Bacteriophage T7 Exonuclease- and HIV-1 Reverse Transcriptase

The N-2 atom of guanine (G) is susceptible to modification by various carcinogens. Oligonucleotides with increasing bulk at this position were analyzed for fidelity and catalytic efficiency with the processive DNA polymerases human immunodeficiency virus, type 1, reverse transcriptase (RT), and bacteriophage T7 exonuclease(-) (T7(-)). RT and T7(-) effectively bypassed N(2)-methyl(Me)G and readily extended primers but were strongly blocked by N(2)-ethyl(Et)G, N(2)-isobutylG, N(2)-benzylG, and N(2)-methyl(9-anthracenyl)G. Steady-state kinetics of single nucleotide incorporation by RT and T7(-) showed a decrease of 10(3) in k(cat)/K(m) for dCTP incorporation opposite N(2)-MeG and a further large decrease opposite N(2)-EtG. Misincorporation frequency was increased 10(2)-10(3)-fold by a Me group and another approximately 10(3)-fold by an Et group. dATP was preferentially incorporated opposite bulky N(2)-alkylG molecules. N(2)-MeG attenuated the pre-steady-state kinetic bursts with RT and T7(-), and N(2)-EtG eliminated the bursts. Large elemental effects with thio-dCTP(alphaS) were observed with N(2)-EtG (6- and 72-fold decreases) but were much less with N(2)-MeG, indicating that the N(2)-Et group may affect the rate of the chemistry step (phosphodiester bond formation). Similar values of K(d(dCTP)) and K(d(DNA)) and k(off) rates of DNA substrates from RT and T7(-) indicate that ground-state binding and dissociation rates are not considerably affected by the bulk. We conclude that even a Me group at the guanine N-2 atom can cause a profound interfering effect on the fidelity and efficiency; an Et or larger group causes preferential misincorporation and strong blockage of replicative polymerases, probably at and before the chemistry step, demonstrating the role of bulk in DNA lesions.

Position-specific trapping of topoisomerase II by benzo[a]pyrene diol epoxide adducts: implications for interactions with intercalating anticancer agents

DNA topoisomerase II (Top2) is the target of some of the most effective anticancer DNA intercalators. To determine the effect of intercalating ligands at defined positions relative to a known DNA cleavage site for human Top2alpha, we synthesized oligodeoxynucleotides containing single trans-opened benzo[a]pyrene 7,8-diol 9,10-epoxide (DE) deoxyadenosine (dA) adducts of known absolute configuration, placed at specific positions in a duplex sequence containing staggered Top2 cleavage sites on both strands. Because the orientations of the intercalated hydrocarbon are known from NMR solution structures of duplex oligonucleotides containing these dA adducts, a detailed analysis of the relationship between the position of intercalation and trapping of Top2 is possible. Our findings demonstrate that (i) Top2 cleavage complexes are trapped by intercalation of the hydrocarbon at either of the staggered cleavage sites or immediately adjacent to the base pairs flanking the cleavage sites within the stagger; (ii) both concerted and nonconcerted cleavage by both subunits of a Top2 homodimer were detected depending on the position of the benzo[a]pyrene DE dA adduct; and (iii) intercalation immediately outside of the staggered Top2 cleavage site, and to a lesser extent in the middle of the stagger, prevents Top2 from cleaving DNA at this site, consistent with the effect of some intercalators as suppressors of Top2-mediated DNA cleavage. These results identify specific binding sites for intercalators that result in trapping of Top2. Such poisoning of Top2 by bulky polycyclic aromatic hydrocarbon DE adducts constitutes a potential mechanism for their carcinogenic activity.

Mutagenic events in Escherichia coli and mammalian cells generated in response to acetylaminofluorene-derived DNA adducts positioned in the Nar I restriction enzyme site

Comparative mutagenesis studies of N-(2'-deoxyguanosin-8-yl)-2-acetylaminofluorene (dG-AAF) and N-(2'-deoxyguanosin-8-yl)-2-aminofluorene (dG-AF) adducts positioned in the Nar I restriction enzyme site were performed using Escherichia coli (E. coli) and simian kidney (COS-7) cells. Oligodeoxynucleotides ((5)(')TCCTCG(1)G(2)CG(3)CCTCTC) containing a recognition sequence for the Nar I restriction enzyme were modified site-specifically with dG-AAF or dG-AF. Modified and unmodified oligomers inserted into single-stranded phagemid shuttle vectors were used to transform E. coli or to transfect COS-7 cells. Following replication in host cells, progeny plasmids were recovered and analyzed for mutations. In SOS-induced E. coli, dG-AAF primarily induced one- and two-base deletions. The mutational frequency varied, depending on the position modified in the Nar I site; 91% two-base deletions were observed at G(3), while 8.4% and 2.8% deletions were detected at G(2) and G(1), respectively. In contrast, dG-AF at any position in the Nar I site failed to produce deletions, generating primarily G --> T transversions (mutational frequency, 7.6-8.4%). In COS-7 cells, both dG-AAF and dG-AF primarily induced G --> T transversions. Mutation frequencies for dG-AAF were 9.4-24%, the highest values being at G(1) and G(3). Mutation frequencies for dG-AF were 9.3-21%, the higher value at G(2). We conclude from this study that the mutation potential of dG-AAF and dG-AF depends on the structure of the adduct, the sequence context of the lesion, and the host cell used for the experiment.

Semiquantitation of polycyclic aromatic hydrocarbon-DNA adducts in human esophagus by immunohistochemistry and the automated cellular imaging system

It has been suggested that ingestion of polycyclic aromatic hydrocarbons (PAHs) may contribute to the high incidence and mortality of esophageal cancer in Linxian, China. To explore this relationship a semiquantitative immunohistochemical staining method was developed for localization of PAH-DNA adducts. Nuclear color intensity (bright field average pink intensity per nucleus for >1000 cells) was measured using the ChromaVision Automated Cellular Imaging System (ACIS). Paraffin-embedded sections of cultured human keratinocytes exposed to increasing concentrations of 7beta,8alpha-dihydroxy-9alpha,10alpha-epoxy-7,8,9,10-tetrahydro-benzo[a]pyrene (BPDE) were incubated with BPDE-DNA antiserum and served as an internal positive control (standard curve). Values for nuclear staining intensity correlated directly with BPDE exposure concentration (r(2) = 0.99) and were reproducible. DNA adduct levels determined by BPDE-DNA chemiluminescence immunoassay in DNA from BPDE-exposed keratinocytes, correlated with BPDE exposure concentrations (r(2) = 0.99), showing that nuclear staining intensity determined by ACIS correlated directly with BPDE-DNA adduct levels determined by chemiluminescence immunoassay. The ACIS methodology was applied to 5 human samples from Linxian, and significantly positive nuclear PAH-DNA adduct staining was observed in this group when compared with esophageal tissue from 4 laboratory-housed monkey controls and 6 samples obtained at autopsy from smokers and nonsmokers in the United States. Nuclear PAH-DNA staining was absent from Linxian samples when serial sections were incubated with normal rabbit serum (negative control) and was significantly reduced on incubation with BPDE-DNA antiserum absorbed previously with the immunogen BPDE-DNA. These results appear to support the hypothesis that high PAH exposure levels may be etiologically associated with the development of esophageal cancer in Linxian.

Novel furano analogues of duocarmycin C1 and C2: design, synthesis, and biological evaluation of seco-iso-cyclopropylfurano[2,3-e]indoline (seco-iso-CFI) and seco-cyclopropyltetrahydrofurano[2,3-f]quinoline (seco-CFQ) analogues

The design, synthesis and biological evaluation of novel seco-iso-cyclopropylfurano[2,3-e]indoline (seco-iso-CFI) and the seco-cyclopropyltetrahydrofurano[2,3-f]quinoline (seco-CFQ) analogues of the duocarmycins are described. These novel analogues (4-7) were designed on the premise that the lone pair of electrons on the furano-oxygen atom could enter into conjugation with the isocyclopropylfurano[e]indolone (iso-CFI) alkylating moiety, formed from the loss of HCl in compounds 4-7. The seco-iso-CFI DNA alkylating pharmacophore was synthesized through a well precedented approach of 5-exo-trig aryl radical cyclization with a vinyl chloride. In our studies, in addition to the formation of the seco-iso-CFI product, an equal amount of an unexpected seco-CFQ product was also generated during the radical cyclization reaction. Like CC-1065 and adozelesin, using Taq DNA polymerase stop and thermal cleavage assays, the seco-iso-CFI compounds (4 and 6) and the seco-CFQ compounds (5 and 7) were shown to preferentially alkylate the adenine-N3 position within the minor groove of long stretches of A residues. A MM2 energy optimized molecular model of a 1:1 complex of compound 6 with DNA reveals that the iso-CFI compound fits snugly within the minor groove. Using a MTT based experiment, the cytotoxicity of compounds 4-7 were determined against the growth of murine leukemia (L1210), mastocytoma (P815) and melanoma (B16) cell lines. The concentrations of compounds required to inhibit the growth of these tumor cells by 50% is in the range of 10(-8)M. These compounds were also tested against a panel of human cancer cells by the National Cancer Institute, demonstrating that the compounds exhibited a high level of activity against selected solid tumors. At a concentration of 0.0084 microM (based on the IC(50) of compound 17 (seco-CBI-TMI) against the growth L1210 cells), while compounds 4 and 17 were toxic against murine bone marrow cells as judged by a colony forming study of freshly isolated murine progenitor hematopoeitic cells, compound 5, a seco-CFQ compound, was significantly less toxic. Flow cytometric analysis of P815 cells that had been incubated for 24h with compounds 4 and 5 at their cytotoxic IC(50) concentrations indicated the induction of apoptosis in a large percentage of cells, thereby suggesting that this might be the mechanism by which the iso-CFI compounds kill cells.

Repair of DNA interstrand crosslinks as a mechanism of clinical resistance to melphalan in multiple myeloma

Melphalan is widely used as a preparative agent in patients with multiple myeloma (MM) undergoing autologous stem cell transplantation (SCT). Although disease relapse is the major cause of death after a melphalan-conditioned autograft, the mechanism remains unclear. Melphalan produces a number of DNA adducts with the DNA interstrand crosslink (ICL) considered to be the critical cytotoxic lesion. By using a modification of the single-cell gel electrophoresis (Comet) assay, we have measured formation and repair of DNA ICL in plasma cells from melphalan- naive and melphalan-treated patients (ie, those who have relapsed after a melphalan-conditioned autologous SCT or oral melphalan therapy). Similar levels of dose-dependent DNA interstand crosslinking were observed in cells from both melphalan-naive and -treated patients. However, marked differences in ICL repair were observed: cells from naive patients showed no repair, whereas those from treated patients exhibited between 42% and 100% repair at 40 hours. In vitro sensitivity to melphalan in plasma cells was found to correlate with ICL repair. These findings suggest that ICL repair may be an important mechanism by which melphalan resistance emerges after autologous SCT or oral therapy. This mechanism may have implications for MM patients undergoing melphalan therapy.

Removal of benzo(a)pyrene diol epoxide (BPDE)-DNA adducts as a measure of DNA repair capacity in lymphoblastoid cell lines from sisters discordant for breast cancer

The mutagen sensitivity assay is one of the approaches used to investigate individual DNA repair capacity. This method is based on the premise that after in vitro treatment with a test mutagen, DNA from subjects with defective repair will be more damaged than DNA from those with an efficient repair system. However, very little is known about unmeasured processes that occur between cell treatment and final assessment of DNA damage. To develop a more precise assay, we modified the traditional mutagen sensitivity assay to also include measurement of DNA damage after culturing cells in the absence of mutagen. First, we treated apparently normal and xeroderma pigmentosum lymphoblastoid cell lines with various doses of benzo(a)pyrene diol epoxide (BPDE) and harvested cells at different time points. A polyclonal antiserum against BPDE-DNA was used to quantitate levels of adducts by immunoslot-blot and immunohistochemistry. Selected conditions included treatment with 10 microM BPDE, a 4-hr culture in mutagen-free medium, and immunohistochemical measurement of BPDE-DNA adducts. The method was then applied in a pilot study to 50 lymphoblastoid lines from sisters discordant for breast cancer. There was no significant difference between cases and controls in the level of BPDE-DNA adducts in lymphoblasts harvested immediately after BPDE treatment. However, after a 4-hr culture in mutagen-free medium, the level of adducts was significantly higher (P = 0.006) among cases than in controls. There was a two-fold increase in mean adduct removal in lines from nonaffected as compared to affected sisters (44% and 22% decrease, respectively). DNA repair capacity was predictive of case status (P = 0.04) in logistic regression analysis. This method, which can be easily applied to large numbers of samples, should be useful in studies to investigate the role of DNA repair in cancer risk.

The anticancer drug-DNA complex: femtosecond primary dynamics for anthracycline antibiotics function

The anthracycline-DNA complex, which is a potent agent for cancer chemotherapy, has a unique intercalating molecular structure with preference to the GC bases of DNA, as shown by Rich's group in studies of single-crystal x-ray diffraction. Understanding cytotoxicity and its photoenhancement requires the unraveling of the dynamics under the solution-phase, physiological condition. Here we report our first study of the primary processes of drug function. In a series of experiments involving the drug (daunomycin and adriamycin) in water, the drug-DNA complexes, the complexes with the four nucleotides (dGTP, dATP, dCTP, and dTTP), and the drug-apo riboflavin-binding protein, we show the direct involvement of molecular oxygen and DNA base-drug charge-separation-the rates for the reduction of the drug and dioxygen indicate the crucial role of drug/base/O(2) in the efficient and catalytic redox cycling. These dynamical steps, and the subsequent reactions of the superoxide product(s), can account for the photoenhanced function of the drug in cells, and potentially for the cell death.

Bacteriophage T7 RNA polymerase transcription elongation is inhibited by site-specific, stereospecific benzo[c]phenanthrene diol epoxide DNA lesions

Benzo[c]phenanthrene diol epoxide (B[c]PhDE), the ultimate carcinogenic metabolite of the environmental pollutant benzo[c]phenanthrene, reacts with DNA primarily at the exocyclic amino groups of purines, forming B[c]PhDE-DNA adducts that differ in their stereochemical configurations and their effect on biological processes such as transcription. To determine the effect of these stereoisomers on RNA synthesis, in vitro T7 RNA polymerase transcription assays were performed using DNA templates modified on the transcribed strand by either a site-specific (+)-trans- or (-)-trans-anti-B[c]PhDE-N(6)-dA lesion located within the sequence 5'-CTCTCACTTCC-3'. The results show that both (-)-trans-anti-B[c]PhDE-N(6)-dA and (+)-trans-anti-B[c]PhDE-N(6)-dA block RNA synthesis. Furthermore, both B[c]PhDE-dA stereoisomeric adducts lead to lower levels of initiation of transcription relative to that observed using an unmodified DNA template. In contrast to these results, placement of the adduct on the nontranscribed strand within the template does not impede transcription elongation. In addition to the assessment of the effect of the lesions on transcription elongation, the resulting transcripts were characterized in terms of their base composition. A high level of base misincorporation is detected at the 3'-ends of truncated transcripts, with guanosine being most frequently incorporated opposite the modified nucleotide rather than the expected uridine. This result supports the notion that translocation past a modified base in a DNA template relies in part on correct base incorporation, and suggests that stalling of RNA polymerases at damaged sites in DNA may well be dependent on both the presence of the lesion and the base which is incorporated opposite the modified nucleotide.

Mutagenic properties of estrogen quinone-derived DNA adducts in simian kidney cells

DNA damage caused by catechol estrogens has been shown to play an etiologic role in tumor formation. Catechol estrogens are reactive to DNA and form several DNA adducts via their quinone forms. To explore the mutagenic properties of 2-hydroxyestrogen-derived DNA adducts in mammalian cells, N(2)-(2-hydroxyestrogen-6-yl)-2'-deoxyguanosine and N(6)-(2-hydroxyestrogen-6-yl)-2'-deoxyadenosine adducts induced by quinones of 2-hydroxyestrone, 2-hydroxyestradiol, or 2-hydroxyestriol were incorporated site-specifically into the oligodeoxynucleotides ((5)(')TCCTCCTCXCCTCTC, where X is dG, dA, 2-OHE-N(2)-dG, or 2-OHE-N(6)-dA). The modified oligodeoxynucleotides were inserted into single-stranded phagemid vectors followed by transfection into simian kidney (COS-7) cells. Preferential incorporation of dCMP, the correct base, was observed opposite all 2-OHE-N(2)-dG adducts. Only targeted G --> T transversions were detected; the highest mutation frequency (18.2%) was observed opposite the 2-OHE(2)-N(2)-dG adduct, followed by 2-OHE(1)-N(2)-dG (4.4%) and 2-OHE(3)-N(2)-dG (1.3%). When 2-OHE-N(6)-dA adducts were used, preferential incorporation of dTMP, the correct base, was observed. Targeted mutations representing A --> T transversions were detected, accompanied by small numbers of A --> G transitions. The highest mutation frequencies were observed with 2-OHE(1)-N(6)-dA and 2-OHE(3)-N(6)-dA (14.5 and 14.1%, respectively), while 2-OHE(2)-N(6)-dA exhibited a mutation frequency of only 6.0%. No mutations were detected with vectors containing unmodified oligodeoxynucleotides. Thus, 2-OHE quinone-derived DNA adducts are mutagenic, generating primarily G --> T and A --> T mutations in mammalian cells. The mutational frequency varied depending on the nature of the 2-OHE moiety.

Pyrimido[1,2-alpha]purin-10(3H)-one: a reactive electrophile in the genome

Malondialdehyde and base propenal react with deoxyguanosine residues in DNA to form an exocyclic adduct, pyrimido[1, 2-alpha]purin-10(3H)-one (1), that has been detected at high levels in genomic DNA of healthy humans. Previous studies have shown that tris(hydroxymethyl)aminomethane adds to 1 at elevated pH, forming an enaminoimine (2), but it is uncertain whether 1 reacts directly or hydrolyzes under basic conditions to N(2)-(3-oxo-1-propenyl)deoxyguanosine (3) prior to amine addition. We report that 1 reacts at neutral pH with hydroxylamines to form oximes. The rate of reaction of 1 with hydroxylamines at pH 7 is at least 150 times faster than the rate of hydrolysis of 1 to 3. Thus, 1 is directly reactive to nucleophiles. These observations indicate that 1 is an electrophile in the human genome that may react with cellular nucleophiles to form novel cross-linked adducts.

The processing of a Benzo(a)pyrene adduct into a frameshift or a base substitution mutation requires a different set of genes in Escherichia coli

Replication through a single DNA lesion may give rise to a panel of translesion synthesis (TLS) events, which comprise error-free TLS, base substitutions and frameshift mutations. In order to determine the genetic control of the various TLS events induced by a single lesion, we have chosen the major N2-dG adduct of (+)-anti-Benzo(a)pyrene diol epoxide [(+)-anti-BPDE] adduct located within a short run of guanines as a model lesion. Within this sequence context, in addition to the major event, i.e. error-free TLS, the adduct also induces base substitutions (mostly G --> T transversions) and -1 frameshift mutations. The pathway leading to G --> T base substitution mutagenesis appears to be SOS independent, suggesting that TLS is most probably performed by the replicative Pol III holoenzyme itself. In contrast, both error-free and frameshift TLS pathways are dependent upon SOS-encoded functions that belong to the pool of inducible DNA polymerases specialized in TLS (translesional DNA polymerases), namely umuDC (Pol V) and dinB (Pol IV). It is likely that, given the diversity of conformations that can be adopted by lesion-containing replication intermediates, cells use one or several translesional DNA polymerases to achieve TLS.

Abasic DNA structure, reactivity, and recognition

Loss of a base in DNA, i.e., creation of an abasic site leaving a deoxyribose residue in the strand, is a frequent lesion that may occur spontaneously, or under the action of radiations and alkylating agents, or enzymatically as an intermediate in the repair of modified or abnormal bases. The abasic site lesion is mutagenic or lethal if not repaired. From a chemical point of view,the abasic site is an alkali-labile residue that leads to strand breakage through beta- and delta- elimination. Progress in the understanding of the chemistry and enzymology of abasic DNA largely relies upon the study of synthetic abasic duplexes. Several efficient synthetic methods have thus been developed to introduce the lesion (or a stable analogue) at defined position in the sequence. Physicochemical and spectroscopic examination of such duplexes, including calorimetry, melting temperature, high-field nmr and molecular modeling indicate that the lesion strongly destabilizes the duplex, although remaining in the canonical B-form with structural modifications strictly located at the site of the lesion. Probes have been developed to titrate the damage in DNA in vitro. Series of molecules have been devised to recognize specifically the abasic site, exhibiting a cleavage activity and mimicking the AP nucleases. Others have been prepared that bind strongly to the abasic site and show promise in potentiating the cytotoxic and antitumor activity of the clinically used nitrosourea (bis-chloroethylnitrosurea).

Cisplatin adducts inhibit 1,N(6)-ethenoadenine repair by interacting with the human 3-methyladenine DNA glycosylase

The human 3-methyladenine DNA glycosylase (AAG) is a repair enzyme that removes a number of damaged bases from DNA, including adducts formed by some chemotherapeutic agents. Cisplatin is one of the most widely used anticancer drugs. Its success in killing tumor cells results from its ability to form DNA adducts and the cellular processes triggered by the presence of those adducts in DNA. Variations in tumor response to cisplatin may result from altered expression of cellular proteins that recognize cisplatin adducts. The present study focuses on the interaction between the cisplatin intrastrand cross-links and human AAG. Using site-specifically modified oligonucleotides containing each of the cisplatin intrastrand cross-links, we found that AAG readily recognized cisplatin adducts. The apparent dissociation constants for the 1, 2-d(GpG), the 1,2-d(ApG), and the 1,3-d(GpTpG) oligonucleotides were 115 nM, 71 nM, and 144 nM, respectively. For comparison, the apparent dissociation constant for an oligonucleotide containing a single 1,N(6)-ethenoadenine (epsilonA), which is repaired efficiently by AAG, was 26 nM. Despite the affinity of AAG for cisplatin adducts, AAG was not able to release any of these adducts from DNA. Furthermore, it was demonstrated that the presence of cisplatin adducts in the reactions inhibited the excision of epsilonA by AAG. These data suggest a previously unexplored dimension to the toxicological response of cells to cisplatin. We suggest that cisplatin adducts could titrate AAG away from its natural substrates, resulting in higher mutagenesis and/or cell death because of the persistence of AAG substrates in DNA.

A single d(GpG) cisplatin adduct on the estrogen response element decreases the binding of the estrogen receptor

Both cisplatin and the estrogen receptor (ER) are known to bend DNA. The influence of the bending of sequences by the d(GpG)cisPt adduct binding of ER to estrogen response element (ERE)-like sequences was examined. Three ERE-like oligonucleotides with different affinities for ER and which include a GG in the linker sequence were designed in order to form a single central d(GpG)cisPt adduct. Using electrophoretic mobility shift assay and Scatchard analysis, it was shown that the presence of a single d(GpG)cisPt adduct in the linker sequence decreases the ER affinity for DNA. These results do not support a critical role of a DNA bend in the initial recognition of ERE by ER. Then, the platination of DNA outside of the ERE half-sites decreases the interaction of ER with ERE.

Triple helix-forming oligonucleotides target psoralen adducts to specific chromosomal sequences in human cells

The ability to target photochemical adducts to specific genomic DNA sequences in cells is useful for studying DNA repair and mutagenesis in intact cells, and also as a potential mode of gene-specific therapy. Triple helix-forming DNA oligonucleotides linked to psoralen (psoTFOs) were designed to deliver UVA-induced psoralen photoadducts to two distinct sequences within the human interstitial collagenase gene. A primer extension assay demonstrated that the appropriate psoTFO selectively damages a collagenase cDNA target. Site-specific genomic psoTFO DNA adducts were detected by a single-strand ligation PCR assay. The adduct, formed at a single site by a psoTFO in purified genomic DNA, contrasted with the multiple sites that were damaged within the observed segment of the collagenase gene upon treatment with free psoralen and subsequent photoactivation. When treated with psoTFOs, both repair-deficient fibroblasts from xero- derma pigmentosum complementation group A and HT1080 fibrosarcoma cells exhibited site-specific DNA adducts following UVA irradiation. Addition of phorbol ester, a transcriptional activator of the collagenase gene, to xeroderma pigmentosum cells did not detectably alter the initial levels of damage produced by psoTFOs, suggesting that further stimulation of transcription neither improves accessibility of psoTFOs to their targets nor enhances removal of non-covalently bound psoTFOs.

Activity and DNA binding of new organoamidoplatinum (II) complexes

Two series of organoamidoplatinum (II) complexes were synthesized [Class 1, Pt(NRCH2)2L2 and Class 2, Pt(NRCH2CH2NR2')L(X)] and their antitumour activity examined by a range of in vitro, cellular and animal studies. All Class 1 compounds exhibited activity comparable to cisplatin in mouse leukemia L1210 cells, but were at least 8-fold more active against the cisplatin-resistant L1210/R line. The lead compound 1a (R=p-HC6F4) caused nearly complete tumour regression in the ADJ/PC6 mouse tumour model. Compound 1a exhibited similar DNA reactivity to cisplatin, resulting in virtually identical DNA sequence specificity as cisplatin, and had similar time and concentration dependency of interstrand crosslinks. Compared with cisplatin, la showed 3-fold greater cellular uptake into human ovarian carcinoma 2008 cells, and this was dramatically enhanced to 17-fold in the cisplatin-resistant 2008/R line. The activity of 1a, therefore, appears to be due at least in part to a greater cellular uptake into tumour cells, particularly cisplatin-resistant cells, and once in the cell it reacts with DNA in a similar manner to that of cisplatin. The enhanced uptake and enhanced cytotoxicity of Class 1 compounds, and 1a in particular, may be due to a greater hydrophobicity compared with cisplatin. The activity of the Class 2 compounds, especially in the cisplatin-resistant cell lines, is unusual because they have trans amine ligands, and further study of both classes of compounds is warranted.

N(2)-(1-Carboxyethyl)deoxyguanosine, a nonenzymatic glycation adduct of DNA, induces single-strand breaks and increases mutation frequencies

N(2)-(1-Carboxyethyl)deoxyguanosine (CEdG) is a major nonenzymatic glycation product of DNA. The effect of CEdG modification, which was specifically prepared by incubation with dihydroxyacetone, on plasmid DNA topology was evaluated by gel electrophoresis. A time-dependent decrease of supercoiled plasmid-DNA was observed in parallel to the increase of CEdG adducts; the half-life time of the supercoiled plasmid-DNA was estimated to be approximately 16-18 h. CEdG-modified plasmid DNA showed a 25-fold reduced transformation efficiency. When modified DNA was used to transform Escherichia coli cells, a 6-fold increase in mutation frequency was determined by measuring loss of alpha-complementation. For the mutator strain BMH71-18mutS, an 8-fold increase in mutation frequency was observed. Although the exact mechanism of DNA damage is unclear, the occurrence of spontaneous depurination is likely. These findings suggest that a defined DNA glycation reaction can lead to DNA damage in vivo.

In vitro chemopreventive effects of plant polysaccharides (Aloe barbadensis miller, Lentinus edodes, Ganoderma lucidum and Coriolus versicolor)

A plant polysaccharide, Aloe gel extract, was reported to have an inhibitory effect on benzo[a]pyrene (B[a]P)-DNA adduct formation in vitro and in vivo. Hence, chemopreventive effects of plant polysaccharides [Aloe barbadensis Miller (APS), Lentinus edodes (LPS), Ganoderma lucidum (GPS) and Coriolus versicolor (CPS)] were compared using in vitro short-term screening methods associated with both initiation and promotion processes in carcinogenesis. In B[a]P-DNA adduct formation, APS (180 micrograms/ml) was the most effective in inhibition of B[a]P binding to DNA in mouse liver cells. Oxidative DNA damage (by 8-hydroxydeoxyguanosine) was significantly decreased by APS (180 micrograms/ml) and CPS (180 micrograms/ml). In induction of glutathione S-transferase activity, GPS was found to be the most effective among plant polysaccharides. In screening anti-tumor promoting effects, APS (180 micrograms/ml) significantly inhibited phorbol myristic acetate (PMA)-induced ornithine decarboxylase activity in Balb/3T3 cells. In addition, APS significantly inhibited PMA-induced tyrosine kinase activity in human leukemic cells. APS and CPS significantly inhibited superoxide anion formation. These results suggest that some plant polysaccharides produced both anti-genotoxic and anti-tumor promoting activities in in vitro models and, therefore, might be considered as potential agents for cancer chemoprevention.

The effect of 17beta-estradiol-DNA adducts on the replication of exon # 5 of the human suppressor gene p53

Using a PCR technique, exon # 5 of the human tumor suppressor gene p53 was amplified and ligated into the pCRII vector and transformed into Escherichia coli INV alphaF' competent cells. The cloned exon # 5 was 184 bp long. Evidence is presented to show that after dimethyldioxirane epoxidation, 17beta-estradiol was able to form 17beta-estradiol-DNA adducts and to strongly inhibit the replication of the cloned exon # 5 producing smaller sizes of DNA fragments and introducing errors of incorporation at the 3'-end of the terminating DNAs. The errors occurred mainly at the clusters of the complementary 'G' and 'A' bases on the template strand DNA, presumably, the major sites where the 17beta-estradiol-DNA adducts were formed.

Association of benzo[a]pyrene-diol-epoxide-deoxyribonucleic acid (BPDE-DNA) adduct level with aging in male smokers and nonsmokers

We used our new flow cytometric method to measure benzo[a]pyrene-diolepoxide-deoxyribonucleic acid adduct levels in peripheral lymphocytes from healthy male smokers and nonsmokers. Smokers who had pack-years of 20 or more had significantly higher mean benzo[a]pyrene-diol-epoxide-deoxyribonucleic acid adduct levels than nonsmokers. In smokers, the adduct levels were correlated significantly with age, years of smoking, and pack-years, whereas daily tobacco consumption was not correlated with adduct levels. We also found a positive relationship between age and benzo[a]pyrene-diol-epoxide-deoxyribonucleic acid adduct levels in nonsmokers. Passive exposure to tobacco smoke was not associated with adduct levels. The results of our study indicate that benzo[a]pyrene-diol-epoxide-deoxyribonucleic acid adduct levels may be closely related to aging and that tobacco smoking-as well as other environmental factors-may play a role in the benzo[a]pyrene-diol-epoxide-deoxyribonucleic acid adduct formation.

Replication block by an enediyne drug-DNA deoxyribose adduct

Under anaerobic conditions neocarzinostatin chromophore, an enediyne antibiotic, forms a covalent drug-DNA adduct on the 5' carbon of deoxyribose at a specific single site in a 2-nucleotide bulge, rather than strand cleavage, by a mechanism involving general base-catalyzed intramolecular drug activation to a reactive radical species. We have taken advantage of the selectivity of this reaction to prepare a single-stranded oligonucleotide containing a single drug adduct at a T residue and to study its effect on the template properties of the oligonucleotide in replicative synthesis, as followed by 5'-32P-labeled primer extension by several DNA polymerases. With the Klenow fragment of Escherichia coli DNA polymerase I, synthesis stops at the base immediately 3' to the adduct. The same enzyme, but lacking 3' to 5' exonuclease activity, permits synthesis to proceed by one additional nucleotide. This effect is enhanced when Mn2+ is substituted for Mg2+. T4, herpes simplex virus, and cytomegalovirus DNA polymerases all act like Klenow polymerase. Sequenase (exo-minus T7 DNA polymerase) is qualitatively similar to exo-minus Klenow polymerase but is more efficient in inserting a nucleotide opposite the lesion. With the small-gap-filling human DNA polymerase beta, which lacks intrinsic exonucleolytic activity, primer extension proceeds to the nucleotide opposite the lesion. However, when a gap was created opposite the lesion, polymerase beta adds as many as two additional nucleotides 5' to the adduct site. The fidelity of base incorporation opposite the lesion was not impaired, in contrast with adducts on DNA bases.

Cisplatin-DNA adducts inhibit ribosomal RNA synthesis by hijacking the transcription factor human upstream binding factor

Several eukaryotic cellular proteins recognize DNA modified by the anticancer drug cisplatin (cis-diamminedichloroplatinum(II) or cis-DDP); among these proteins is a class of DNA-binding molecules containing the HMG (high-mobility group) box DNA recognition motif. We have previously reported the extraordinarily high binding activity to cisplatin adducts by human upstream binding factor (hUBF), an HMG box containing transcription factor that stimulates ribosomal RNA synthesis (Treiber et al. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 5672-5676). In the present study, we discovered that (1) hUBF interacted selectively with DNA lesions formed by therapeutically effective platinum compounds [Pt(en)Cl2] and [Pt(dach)Cl2], in addition to the lesions formed by cis-DDP, suggesting a possible association with their anticancer effect; (2) multiple HMG boxes contributed additively to the hUBF-adduct interaction, providing a possible explanation for the unusually high affinity of hUBF for cis-DDP adducts as compared to the lower affinities of other HMG box proteins; and (3) ribosomal RNA transcription in a reconstituted system is specifically inhibited in the presence of cis-DDP adducts. In this third experiment, a ratio of adducts/promoter of approximately 4:1 completely abolished the transcription activated by hUBF. Taken together, these data lend support to the view that transcription factors involved in cellular growth regulation, such as ribosomal RNA transcription, may be hijacked by cis-DDP adducts resulting in functional inhibition.

The impact of an exocyclic cytosine adduct on DNA duplex properties: significant thermodynamic consequences despite modest lesion-induced structural alterations

The exocyclic base adduct 3,N4-deoxyethenocytosine (epsilonC) is a common DNA lesion that can arise from carcinogen exposure and/or as a biproduct of cellular processes. We have examined the thermal and thermodynamic impact of this lesion on DNA duplex properties, as well as the structural alterations imparted by the lesion. For these studies, we used calorimetric and spectroscopic techniques to investigate a family of 13-mer DNA duplexes of the form (5'CGCATGNGTACGC3')x(3'GCGTACNCATGCG5'), where the central NxN base pair represents the four standard Watson-Crick base pairs (corresponding to four control duplexes), and where either one of the N bases has been replaced by epsilonC, yielding eight test duplexes. Studies on these 12 duplexes permit us to assess the impact of the epsilonC lesion as a function of sequence context. Our spectroscopic and calorimetric data allow us to reach the following conclusions: (i) The epsilonC lesion imparts a large penalty on duplex stability, with sequence context only modestly modulating the extent of this lesion-induced destabilization. This result contrasts with our recent studies of duplexes with abasic sites, where sequence context was found to be the predominant determinant of thermodynamic damage. (ii) For the epsilonC-containing duplexes, sequence context effects are most often observed in the enthalpic contribution to lesion-induced duplex destabilization. However, due to compensating entropies, the free energy changes associated with this lesion-induced duplex destablization are nearly independent of sequence context. (iii) Despite significant lesion-induced changes in duplex energetics, our spectroscopic probes detect only modest lesion-induced changes in duplex structure. In fact, the overall duplex maintains a global B-form conformation, in agreement with NMR structural data. We discuss possible interpretations of the apparent disparity between the severe thermodynamic and relatively mild structural impacts of the epsilonC lesion on duplex properties. We also note and discuss the implications of empirical correlations between biophysical and biological properties of lesion-containing duplexes.

Synergistic cytotoxicity of cisplatin and topotecan or SN-38 in a panel of eight solid-tumor cell lines in vitro

The cytotoxicity of cisplatin alone and in combination with topotecan (TPT) or SN-38, two novel topoisomerase I (topo I) inhibitors, was determined in a panel of eight well-characterized human solid-tumor cell lines. Interactions between cisplatin and these topo I inhibitors were investigated using three different administration schedules: (1) simultaneous incubation (C + T and C + S), (2) cisplatin followed by TPT or SN-38 (C --> T and C --> S), and (3) TPT or SN-38 followed by cisplatin (T --> C and S --> C). Median-effect analysis revealed synergistic cytotoxicity in seven of the eight cell lines used. In addition, a significant schedule-dependent synergistic cytotoxicity was found in three of the cell lines used, with C --> T (or C --> S) being the most active schedule. The formation and repair of total cisplatin-DNA adducts in the IGROV-1 ovarian cancer cell line and its cisplatin-resistant subline IGROV(CDDP) was not significantly affected by TPT on simultaneous incubation. In contrast, the number of cisplatin-DNA interstrand cross-links detected in the IGROV-1 and IGROV(CDDP) lines at certain time points was significantly lower after coincubation of the cells with TPT. Assessment of the cell-cycle distribution revealed an accumulation of cells in the G2/M phase after exposure to cisplatin. After exposure to TPT a different pattern was observed that was cell-type-specific and dependent upon the TPT concentration. Although up to 4-fold differences in topo I activity were observed in this panel of cell lines, these differences did not appear to be related to the synergy observed between cisplatin and TPT or SN-38. The observed synergy may at least partly be explained by the increased retention of cisplatin-DNA interstrand cross-links in the presence of topo I inhibitors.

How stereochemistry affects mutagenesis by N2-deoxyguanosine adducts of 7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene: configuration of the adduct bond is more important than those of the hydroxyl groups

Previous work has shown that the major adduct from the (+)-anti diol epoxide of benzo[a]pyrene (B[a]P), which forms at N2-deoxyguanosine [(+)-trans-anti-B[a]P-N2-dG], is capable of inducing either predominantely G --> T mutations ( approximately 95%) in a 5'-TGC-3 sequence context or predominantly G --> A mutations ( approximately 80%) in a 5'-CGT-3' sequence context. This is likely to be attributable to the major adduct being in a different mutagenic conformation in each case. In the next phase of this work, the questions to be addressed are what conformation is associated with what mutation and why? To help define what aspect of adduct structure is important to mutagenesis, the work herein reports on the mutations induced in a single sequence context by four stereoisomers of B[a]P-N2-dG: (+)-trans-, (+)-cis-, (-)-trans-, and (-)-cis-. The (+)-trans- and (-)-cis-adducts show a remarkably similar mutational pattern with G --> A mutations predominating ( approximately 80%). The (-)-trans- and (+)-cis-adducts also show a similar mutational pattern with a more even mixture of G --> T, G --> A, and G --> C mutations. Each of these adducts has an adduct bond and three hydroxyl groups at four consecutive saturated carbons in the B[a]P moiety of the adduct; the stereochemistry at these four positions differs in each of the adducts. The (+)-trans- and (-)-cis-adducts are a pair sharing the S configuration for the adduct bond, although they are a mirror image vis-a-vis the hydroxyl groups. The (-)-trans- and (+)-cis-adducts share the opposite adduct bond stereochemistry (R) but differ in the stereochemistry of their hydroxyl groups. Thus, there is a correlation suggesting that anti-B[a]P-N2-dG adduct mutagenesis is more dependent on the stereochemistry of the adduct bond than on the stereochemistry of the hydroxyl groups.

Excretion of DNA adducts of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine and 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline, PhIP-dG, PhIP-DNA and DiMeIQx-DNA from the rat

The heterocyclic aromatic amines, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline (4,8-DiMeIQx) are formed during frying of meat. PhIP and 4,8-DiMeIQx have, after metabolic activation, been shown to form adducts with DNA at the C8 of guanine both in vitro and in vivo. In order to investigate possible urinary biomarkers for estimation of the genotoxic dose of PhIP and 4,8-DiMeIQx, [3H]PhIP-dG, [3H]PhIP-DNA and [14C]4,8-DiMeIQx-DNA were injected i.p. to rats and the excretion of radioactivity in urine and faeces were measured. For all three [3H]PhIP-dG, [3H]PhIP-DNA and [14C]4,8-DiMeIQx-DNA 15-20% of the dose were excreted in the urine and 80-85% of the dose were excreted in the faeces. Urinary excretion showed maximum to 24 h (90%) with a rapid decline, 10% to 48 h and 0% to 72 h. Faecal excretion also showed maximum to 24 h (60%) with a slower decline, 30% to 48 h and 10% to 72 h. HPLC analysis of samples of urine and extracts from faeces, from rats dosed with [3H]PhIP-dG, showed that approximately 90% of the radioactivity co-eluted with PhIP-dG, indicating that PhIP-dG is excreted unmetabolized. HPLC analysis of samples of urine and extracts from faeces, from rats dosed with [3H]PhIP-DNA, showed that approximately 85% of the radioactivity co-eluted with PhIP-dG, indicating that PhIP-DNA adducts is mainly excreted as nucleoside adducts. Approximately 5% of the radioactivity excreted in the urine co-eluted with PhIP-G, indicating loss of deoxyribose. HPLC analysis of samples of urine and extracts from faeces, from rats dosed with [14C]4,8-DiMeIQx-DNA, showed that approximately 90% of the radioactivity co-eluted with 4,8-DiMeIQx-dG, indicating that 4,8-DiMeIQx-DNA adducts is mainly excreted as nucleoside adducts. Man is able to eliminate compounds of a higher mol. wt in the urine than the rat, the percentage of PhIP-dG and 4,8-DiMeIQx eliminated in the urine of man would therefore be expected to be higher than in the rat. Measurement of urinary nucleoside adducts of PhIP and 4,8-DiMeIQx could therefore provide a basis for the development of a biomonitoring strategy for the genotoxic dose of these food derived HAA.

Mechanism of DNA-dependent protein kinase inhibition by cis-diamminedichloroplatinum(II)-damaged DNA

We have determined the mechanism of DNA-dependent protein kinase (DNA-PK) inhibition by cis-diamminedichloroplatinum(II)-(cisplatin-) damaged DNA. We previously have demonstrated that Ku, the DNA binding subunit of DNA-PK, is capable of binding to DNA duplexes globally damaged with cisplatin but was unable to stimulate DNA-PKcs, the catalytic subunit [Turchi & Henkels (1996) J. Biol. Chem. 271, 2992-3000]. In this report we have assessed Ku binding and DNA-PK stimulation using a series of DNA substrates containing single, site-specific d(GpG), d(ApG), and d(GpXpG) intrastrand cisplatin adducts and a substrate with a single interstrand cisplatin adduct. Results demonstrate that Ku binding is marginally decreased by the presence of cisplatin adducts on each substrate. When assayed for the ability to stimulate DNA-PK, each cisplatin-damaged substrate resulted in significantly decreased activity compared to undamaged DNA controls. The degree of inhibition of both Ku binding and kinase activity varied depending on the specific adduct employed. The inhibition of DNA-PK activity by cisplatin-damaged DNA was observed using either a synthetic peptide or human replication protein A as a substrate. Autophosphorylation of the DNA-PKcs and Ku subunits was also inhibited in reactions performed with cisplatin-damaged DNA, demonstrating that increased autophosphorylation of DNA-PKcs does not account for the decreased kinase activity observed with cisplatin-damaged DNA. Equilibrium binding and initial velocity experiments revealed a less than 2-fold increase in the Kd of Ku and the Km of DNA-PK for DNA containing a single 1,2-d(GpG) cisplatin adduct. The mechanism of DNA-PK inhibition by cisplatin-damaged DNA can be attributed to a large decrease in the Vmax and small increase in Km.

Disruption of transcription in vitro and gene expression in vivo by DNA adducts derived from a benzo[a]pyrene diol epoxide located in heterologous sequences

Previous studies indicated a high affinity of the transcription factor Sp1 for DNA adducts derived from benzo[a]pyrene diol epoxide (BPDE) in sequences that are not normal binding sites for Sp1. We tested for functional effects of this phenomenon in three systems in which transcription is Sp1-dependent. In an in vitro, Sp1-dependent transcription system addition of heterologous plasmid DNA containing BPDE adducts abolished production of a specific run-off transcript. This inhibition was not seen with unmodified plasmid DNA, and could be overcome by addition of purified Sp1 protein. In SL2 insect cells, high-level expression of an Sp1-dependent reporter gene, which was dependent on co-transfection of an Sp1 expression vector, was inhibited >95% by co-transfection of heterologous DNA containing BPDE adducts. This inhibition could be partially overcome by increasing the amount of the Sp1 expression vector in the transfections. In human C33A cells, expression of a transfected reporter gene driven by a GC box containing fragment of the human E2F1 promoter was enhanced by co-transfection of an Sp1 expression plasmid. Expression was inhibited 3-6-fold by co-transfection of heterologous DNA containing BPDE-DNA adducts. A similar inhibition was seen in human SAOS-2 cells, which lack functional p53 protein. These data are consistent with functionally significant sequestration of the Sp1 transcription factor by BPDE-DNA adducts in all three systems. Altered availability of transcription factors such as Sp1 in carcinogen-treated cells may disrupt patterns of gene expression.

Development of monoclonal antibodies to the malondialdehyde-deoxyguanosine adduct, pyrimidopurinone

Malondialdehyde (MDA), an endogenous product of lipid peroxidation and prostaglandin biosynthesis, is mutagenic in bacterial and mammalian cells and carcinogenic in rats. In order to determine whether MDA-modified bases are formed in nucleic acids in vivo, sensitive immunoassays to detect MDA-DNA and MDA-RNA adducts are being developed in our laboratory. Murine monoclonal antibodies reactive with the MDA-deoxyguanosine adduct 3-beta-D-erythro-pentofuranosylpyrimido[1,2-alpha]purin-10(3H)-one (M1G-R) were prepared and characterized. Several MDA-modified nucleosides and deoxynucleosides and structural analogs were synthesized and characterized and were compared as competitive inhibitors in enzyme-linked immunosorbent assays (ELISAs). Less than 5 fmol of M1G in MDA-modified DNA was detected in a direct ELISA, and antibody binding to the modified DNA was competitively inhibited by free M1G-dR. DNA from Salmonella typhimurium treated with concentrations of MDA that induce reversion to histidine prototrophy was enzymatically digested, and M1G-dR was quantitated by competitive ELISA. Over a range of MDA concentrations from 10 to 40 mM, the level of M1G residues in bacterial DNA increased from 0.2 to 2.5/10(6) base pairs.

Leading versus lagging strand mutagenesis induced by 7,8-dihydro-8-oxo-2'-deoxyguanosine in Escherichia coli

We have previously shown that a single N-2-acetylaminofluorene (AAF) adduct bound to the C-8 position of a guanine residue located within plasmids containing the unidirectional ColE1 origin of replication induces a 20-fold higher mutation frequency when the adduct is located in the lagging strand as compared to the leading strand. In this study, single 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodG) lesions have been introduced in the leading and lagging strand orientation within the same sequence context as for the AAF adducts. The induced frequency of guanine to thymine transversions has been measured, using a specific PCR-based quantitative assay, in strains deficient in the repair of the oxidative lesion. The potential involvement of the UvrABC excision repair system in the removal of 8-oxodG has also been investigated and ruled out. Concerning the mutation frequency asymmetry, in contrast to AAF adducts, 8-oxodG adducts induce the same mutation frequency, irrespective of their location in the leading or lagging strands. This striking difference between 8-oxodG and dGuo-C8-AAF adducts is discussed in terms of their differential capacity to block DNA replication.

Cell cycle changes associated with formation of Pt-DNA adducts in human ovarian carcinoma cells with different cisplatin sensitivity

The possible correlation between alterations in cytokinetic response to cisplatin (CP) treatment and drug resistance in human ovarian carcinoma cell lines was examined. Using dual parameter flow cytometry, we performed detailed time-course and dose-response analysis of cell cycle modifications in the parental A2780 and resistant A2780/CP cells exposed to CP. The data suggested that drug treatment resulted in similar types of cell cycle alterations in cells with different CP sensitivity. Rapid normalization of the cytokinetic pattern in both cell lines at low doses of CP was observed. At higher drug concentrations reversible S phase delay predominated, accompanied by blocks in both G1/S and G2/M and followed by complete normalization of cytokinetic patterns in the surviving cells. CP treatment by lethal doses resulted in almost complete S phase block. The surviving cells at 72 h accumulated in G2 phase. CP-induced cell cycle perturbations, among which the most pronounced were alterations in the S phase populations, correlated with the level of DNA damage, but not with cell survival in these cell lines. However, at identical levels of DNA damage, the resistant A2780/CP cell line demonstrated decreased p53 induction and decreased apoptosis compared to the parental cell line. Thus, at equivalent levels of DNA damage, resistance in this model system correlated with a diminished p53-dependent apoptotic pathway rather than with differences in cell cycle response.

Incorrect base insertion and prematurely terminated transcripts during T7 RNA polymerase transcription elongation past benzo[a]pyrenediol epoxide-modified DNA

DNA replication and transcription are affected adversely by the presence of bulky adducts that are generated by the covalent binding of a variety of metabolically activated environmental pollutants to cellular DNA. When these lesions are not cleared by cellular repair enzymes prior to replication, mutations and ultimately tumor initiation can occur. Transcription and DNA repair appear to be intimately connected, since certain adducts are more efficiently removed from the transcribed strands of active loci than from non-transcribed strands and other quiescent domains in the genome. The mechanism by which RNA polymerases deal with bulky adducts during DNA transcription is therefore of great interest. The availability of site-specifically modified and stereochemically defined oligodeoxyribonucleotides derived from the covalent reaction of 7r, 8t-dihydroxy-9, 10t-epoxy- 7,8,9,10-tetrahydrobenzo[a]pyrene (anti-BPDE) with guanine residues prompted us to study the efficiencies of transcription past these lesions using bacteriophage T7 RNA polymerase. We show here that T7 RNA polymerase can bypass such lesions in a DNA template, providing that a cytosine residue is incorporated opposite anti-BPDE-modified guanine. However, when an incorrect base (most frequently a purine) is inserted opposite the modified site, the RNA polymerase stalls, and the complex dissociates, resulting in a truncated transcript. The ability of the T7 RNA polymerase to discriminate between a correct and an incorrect inserted base and, accordingly, to continue or terminate transcription, might constitute an important mechanism that ensures the fidelity of transcription past a modified base present on the transcribed strand of the DNA template.

Preferential repair incision of cross-links versus monoadducts in psoralen-damaged plasmid DNA by human cell-free extracts

Upon UVA irradiation psoralens covalently bind to DNA as monoadduct and interstrand crosslink. Psoralen photoadducts are processed via an excision repair reaction that has been reproduced in vitro with transcriptionnally active cell-free extracts. A derived in vitro assay that allows direct quantification of the incised sites has been set up and used to compare the efficiency of the incision reaction on monoadducts and interstrand cross-links. The incision reaction was performed with HeLa cell-free extracts on angelicin or 8-methoxypsoralen (8-MOP)-modified plasmid DNA substrates carrying known amounts of mono- and biadducts, within various relative ratios. In the case of 8-MOP modified plasmids consisting in a mixture of mono- and biadducts on the same DNA molecule, the incision signal was mainly due to the presence of interstrand cross-links. The extent of incision was linear with the number of cross-links up to about 4 cross-links per plasmid and then reached a plateau. The sensitivity of incision defined as the increase of incision by 2-fold over the background level corresponded to about 1 cross-link per plasmid molecule, and about 7% of the total cross-links were repaired under our assay conditions. The incision activity on angelicin monoadducts yielded only 27% when compared to that on 8-MOP cross-links. Furthermore, 8-MOP cross-links lowered the incision extent of angelicin monoadducts when the two photoadducts were present on distinct plasmid DNA molecules. These data are in line with the more rapid excision of psoralen interstrand cross-links vs monoadducts observed in vivo.