Oxidative DNA Damage inXPC-Knockout and Its Wild Mice Treated with Equine Estrogen

Differential eco-toxicological responses toward Eisenia fetida exposed to soil contaminated with naphthalene and typical metabolites

Naphthalene (NAP) was frequently detected in polycyclic aromatic hydrocarbons (PAHs)-contaminated soil, and its residues may pose an eco-toxicological threat to soil organisms. The toxic effects of NAP were closely tied to phenolic and quinone metabolites in biological metabolism. However, the present knowledge concerning the eco-toxicological impacts of NAP metabolites at the animal level is scanty. Here, we assessed the differences in the eco-toxicological responses of Eisenia fetida (E. fetida) in NAP, 1-naphthol (1-NAO) or 1,4-naphthoquinone (1,4-NQ) contaminated soils. NAP, 1-NAO, and 1,4-NQ exposure triggered the onset of oxidative stress as evidenced by the destruction of the antioxidant enzyme system. The lipid peroxidation and DNA oxidative damage levels induced by 1-NAO and 1,4-NQ were higher than those of NAP. The elevation of DNA damage varied considerably depending on differences in oxidative stress and the direct mode of action of NAP or its metabolites with DNA. All three toxicants induced different degrees of physiological damage to the body wall, but only 1, 4-NQ caused the shedding of intestinal epithelial cells. The integrated biomarker response for different exposure times illustrated that the comprehensive toxicity at the animal level was 1,4-NQ > 1-NAO > NAP, and the time-dependent trends of oxidative stress responses induced by the three toxicants were similar. At the initial stage, the antioxidant system of E. fetida responded positively to the provocation, but the ability of E. fetida to resist stimulation decreased with the prolongation of time resulting in provocation oxidative damage. This study would provide new insights into the toxicological effects and biohazard of PAHs on soil animals.

Xeroderma Pigmentosum Complementation Group C (XPC): Emerging Roles in Non-Dermatologic Malignancies

Xeroderma pigmentosum complementation group C (XPC) is a DNA damage recognition protein essential for initiation of global-genomic nucleotide excision repair (GG-NER). Humans carrying germline mutations in the XPC gene exhibit strong susceptibility to skin cancer due to defective removal via GG-NER of genotoxic, solar UV-induced dipyrimidine photoproducts. However, XPC is increasingly recognized as important for protection against non-dermatologic cancers, not only through its role in GG-NER, but also by participating in other DNA repair pathways, in the DNA damage response and in transcriptional regulation. Additionally, XPC expression levels and polymorphisms likely impact development and may serve as predictive and therapeutic biomarkers in a number of these non-dermatologic cancers. Here we review the existing literature, focusing on the role of XPC in non-dermatologic cancer development, progression, and treatment response, and highlight possible future applications of XPC as a prognostic and therapeutic biomarker.

Global DNA Methylation in Cord Blood as a Biomarker for Prenatal Lead and Antimony Exposures

DNA methylation is an epigenetic mechanism for gene expression modulation and can be used as a predictor of future disease risks. A prospective birth cohort study was performed to clarify the effects of neurotoxicants on child development, namely, the Tohoku Study of Child Development, in Japan. This study aimed to evaluate the association of prenatal exposure to five toxic metals—arsenic, cadmium, mercury, lead (Pb), antimony (Sb), and polychlorinated biphenyls (PCBs, N = 166)—with global DNA methylation in umbilical cord blood DNA. DNA methylation markers, 5-methyl-2′-deoxycytidine (mC) and 5-hydroxymethyl-2′-deoxycytidine (hmC), were determined using liquid chromatography-tandem mass spectrometry. The mC content in cord blood DNA was positively correlated with Pb and Sb levels (r = 0.435 and 0.288, respectively) but not with cord blood PCBs. We also observed significant positive correlations among Pb levels, maternal age, and hmC content (r = 0.155 and 0.243, respectively). The multiple regression analysis among the potential predictors demonstrated consistent positive associations between Pb and Sb levels and mC and hmC content. Our results suggest that global DNA methylation is a promising biomarker for prenatal exposure to Pb and Sb.

Arsenic Uptake, Toxicity, Detoxification, and Speciation in Plants: Physiological, Biochemical, and Molecular Aspects

Environmental contamination with arsenic (As) is a global environmental, agricultural and health issue due to the highly toxic and carcinogenic nature of As. Exposure of plants to As, even at very low concentration, can cause many morphological, physiological, and biochemical changes. The recent research on As in the soil-plant system indicates that As toxicity to plants varies with its speciation in plants (e.g., arsenite, As(III); arsenate, As(V)), with the type of plant species, and with other soil factors controlling As accumulation in plants. Various plant species have different mechanisms of As(III) or As(V) uptake, toxicity, and detoxification. This review briefly describes the sources and global extent of As contamination and As speciation in soil. We discuss different mechanisms responsible for As(III) and As(V) uptake, toxicity, and detoxification in plants, at physiological, biochemical, and molecular levels. This review highlights the importance of the As-induced generation of reactive oxygen species (ROS), as well as their damaging impacts on plants at biochemical, genetic, and molecular levels. The role of different enzymatic (superoxide dismutase, catalase, glutathione reductase, and ascorbate peroxidase) and non-enzymatic (salicylic acid, proline, phytochelatins, glutathione, nitric oxide, and phosphorous) substances under As(III/V) stress have been delineated via conceptual models showing As translocation and toxicity pathways in plant species. Significantly, this review addresses the current, albeit partially understood, emerging aspects on (i) As-induced physiological, biochemical, and genotoxic mechanisms and responses in plants and (ii) the roles of different molecules in modulation of As-induced toxicities in plants. We also provide insight on some important research gaps that need to be filled to advance our scientific understanding in this area of research on As in soil-plant systems.

Nucleotide excision repair of oxidised genomic DNA is not a source of urinary 8-oxo-7,8-dihydro-2'-deoxyguanosine

Urinary 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) is a widely measured biomarker of oxidative stress. It has been commonly assumed to be a product of DNA repair, and therefore reflective of DNA oxidation. However, the source of urinary 8-oxodGuo is not understood, although potential confounding contributions from cell turnover and diet have been ruled out. Clearly it is critical to understand the precise biological origins of this important biomarker, so that the target molecule that is oxidised can be identified, and the significance of its excretion can be interpreted fully. In the present study we aimed to assess the contributions of nucleotide excision repair (NER), by both the global genome NER (GG-NER) and transcription-coupled NER (TC-NER) pathways, and sanitisation of the dGTP pool (e.g. via the activity of the MTH1 protein), on the production of 8-oxodGuo, using selected genetically-modified mice. In xeroderma pigmentosum A (XPA) mice, in which GG-NER and TC-NER are both defective, the urinary 8-oxodGuo data were unequivocal in ruling out a contribution from NER. In line with the XPA data, the production of urinary 8-oxodGuo was not affected in the xeroderma pigmentosum C mice, specifically excluding a role of the GG-NER pathway. The bulk of the literature supports the mechanism that the NER proteins are responsible for removing damage to the transcribed strand of DNA via TC-NER, and on this basis we also examined Cockayne Syndrome mice, which have a functional loss of TC-NER. These mice showed no difference in urinary 8-oxodGuo excretion, compared to wild type, demonstrating that TC-NER does not contribute to urinary 8-oxodGuo levels. These findings call into question whether genomic DNA is the primary source of urinary 8-oxodGuo, which would largely exclude it as a biomarker of DNA oxidation. The urinary 8-oxodGuo levels from the MTH1 mice (both knock-out and hMTH1-Tg) were not significantly different to the wild-type mice. We suggest that these findings are due to redundancy in the process, and that other enzymes substitute for the lack of MTH1, however the present study cannot determine whether or not the 2'-deoxyribonucleotide pool is the source of urinary 8-oxodGuo. On the basis of the above, urinary 8-oxodGuo is most accurately defined as a non-invasive biomarker of oxidative stress, derived from oxidatively generated damage to 2'-deoxyguanosine.

DNA methylation dynamics in mouse preimplantation embryos revealed by mass spectrometry

Following fertilization in mammals, paternal genomic 5-methyl-2'-deoxycytidine (5 mC) content is thought to decrease via oxidation to 5-hydroxymethyl-2'-deoxycytidine (5 hmC). This reciprocal model of demethylation and hydroxymethylation is inferred from indirect, non-quantitative methods. We here report direct quantification of genomic 5 mC and 5 hmC in mouse embryos by small scale liquid chromatographic tandem mass spectrometry (SMM). Profiles of absolute 5 mC levels in embryos produced by in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) were almost identical. By 10 h after fertilization, 5 mC levels had declined by ~40%, consistent with active genomic DNA demethylation. Levels of 5 mC in androgenotes (containing only a paternal genome) and parthenogenotes (containing only a maternal genome) underwent active 5 mC loss in the first 6 h, showing that both parental genomes can undergo demethylation independently. We found no evidence for net loss of 5 mC 10-48 h after fertilization, implying that any passive 'demethylation' following DNA replication was balanced by active 5 mC maintenance methylation. However, levels of 5 mC declined during development after 48 h, to 1% (measured as a fraction of G-residues) in blastocysts (~96 h). 5 hmC levels were consistently low (<0.2% of G-residues) throughout development in normal diploid embryos. This work directly quantifies the dynamics of global genomic DNA modification in mouse preimplantation embryos, suggesting that SMM will be applicable to other biomedical situations with limiting sample sizes.

Thiol-mediated multiple mechanisms centered on selenodiglutathione determine selenium cytotoxicity against MCF-7 cancer cells

Selenium (Se) is an essential antioxidative micronutrient but can exert cancer-selective cytotoxicity if the nutritional levels are too high. Selenodiglutathione (GSSeSG) is a primary Se metabolite conjugated with two glutathione (GSH) moieties. GSSeSG has been suggested to be an important molecule for cytotoxicity. Here, we propose the underlying mechanisms for the potent cytotoxicity of GSSeSG: cellular intake; reductive metabolism; production of reactive oxygen species; oxidative damage to DNA; apoptosis induction. GSSeSG rather than selenite decreased cell viability and induced apoptosis accompanied by increases in intracellular Se contents. Therefore, GSSeSG-specific cytotoxicity may be ascribed to its preferable incorporation. Base oxidation and strand fragmentation in genomic DNA preceded cell death, suggesting that oxidative stress (including DNA damage) is crucial for GSSeSG cytotoxicity. Strand breaks of purified DNA were caused by the coexistence of GSSeSG and thiols (GSH, cysteine, homocysteine), but not the oxidized form or non-thiol reductants. This implies the important role of intracellular thiols in the mechanism of Se toxicity. GSH-assisted DNA strand breaks were inhibited by specific scavengers for hydrogen peroxide or hydroxyl radicals. The GSSeSG metabolite selenide induced some DNA strand breaks without GSH, whereas elemental Se did so only with GSH. These observations suggest involvement of Fenton-type reaction in the absence of transition metals and reactivation of inert elemental Se. Overall, our results suggest that chemical interactions between Se and the sulfur of thiols are crucial for the toxicity mechanisms of Se.

Slow accumulation of mutations in Xpc-/- mice upon induction of oxidative stress

XPC is one of the key DNA damage recognition proteins in the global genome repair route of the nucleotide excision repair (NER) pathway. Previously, we demonstrated that NER-deficient mouse models Xpa(-/-) and Xpc(-/-) exhibit a divergent spontaneous tumor spectrum and proposed that XPC might be functionally involved in the defense against oxidative DNA damage. Others have mechanistically dissected several functionalities of XPC to oxidative DNA damage sensitivity using in vitro studies. XPC has been linked to regulation of base excision repair (BER) activity, redox homeostasis and recruitment of ATM and ATR to damage sites, thereby possibly regulating cell cycle checkpoints and apoptosis. XPC has additionally been implicated in recognition of bulky (e.g. cyclopurines) and non-bulky DNA damage (8-oxodG). However, the ultimate contribution of the XPC functionality in vivo in the oxidative DNA damage response and subsequent mutagenesis process remains unclear. Our study indicates that Xpc(-/-) mice, in contrary to Xpa(-/-) and wild type mice, have an increased mutational load upon induction of oxidative stress and that mutations arise in a slowly accumulative fashion. The effect of non-functional XPC in vivo upon oxidative stress exposure appears to have implications in mutagenesis, which can contribute to the carcinogenesis process. The levels and rate of mutagenesis upon oxidative stress correlate with previous findings that lung tumors in Xpc(-/-) mice overall arise late in the lifespan and that the incidence of internal tumors in XP-C patients is relatively low in comparison to skin cancer incidence.

Oxidative DNA damage and nucleotide excision repair

SIGNIFICANCE

Oxidative DNA damage is repaired by multiple, overlapping DNA repair pathways. Accumulating evidence supports the hypothesis that nucleotide excision repair (NER), besides base excision repair (BER), is also involved in neutralizing oxidative DNA damage.

RECENT ADVANCES

NER includes two distinct sub-pathways: transcription-coupled NER (TC-NER) and global genome repair (GG-NER). The CSA and CSB proteins initiate the onset of TC-NER. Recent findings show that not only CSB, but also CSA is involved in the repair of oxidative DNA lesions, in the nucleus as well as in mitochondria. The XPG protein is also of importance for the removal of oxidative DNA lesions, as it may enhance the initial step of BER. Substantial evidence exists that support a role for XPC in NER and BER. XPC deficiency not only results in decreased repair of oxidative lesions, but has also been linked to disturbed redox homeostasis.

CRITICAL ISSUES

The role of NER proteins in the regulation of the cellular response to oxidative (mitochondrial and nuclear) DNA damage may be the underlying mechanism of the pathology of accelerated aging in Cockayne syndrome patients, a driving force for internal cancer development in XP-A and XP-C patients, and a contributor to the mixed exhibited phenotypes of XP-G patients.

FUTURE DIRECTIONS

Accumulating evidence indicates that DNA repair factors can be involved in multiple DNA repair pathways. However, the distinct detailed mechanism and consequences of these additional functions remain to be elucidated and can possibly shine a light on clinically related issues.

Integrative assessment of potential effects of dioxins and related compounds in wild Baikal seals (Pusa sibirica): application of microarray and biochemical analyses

We have previously indicated that accumulation of chlorinated dioxins and related compounds (DRCs) induced cytochrome P450 (CYP) 1A1, 1A2 and 1B1 isozymes in the liver of wild Baikal seals (Pusa sibirica). Here we attempt to assess the potential effects of DRCs triggered by the induction of these CYP1 isozymes in this species, using an integrative approach, combining gene expression monitoring and biochemical assays. To screen genes that may potentially respond to the exposure of DRCs, we constructed a custom cDNA oligo array that can target mRNAs in Baikal seals, and monitored hepatic mRNA expression levels in the wild population. Correlation analyses between the hepatic total 2,3,7,8-tetrachlorodibenzo-p-dioxin toxic equivalents (TEQs) and mRNA levels supported our previous findings that high accumulation of DRCs induces the transcription of CYP1A1, CYP1A2 and CYP1B1 genes. In addition, our integrative assessment indicated that the chronic exposure to DRCs may alter the hepatic transcript levels of genes related to oxidative stress, Fe ion homeostasis, and inflammatory responses. The expression levels of CYP1A2 showed significant positive correlations with levels of malondialdehyde, a biomarker of lipid peroxidation, and of etheno-dA, a DNA adduct, suggesting that the lipid peroxidation may be enhanced through the production of reactive oxygen species (ROS) triggered by CYP1A2 induction. Moreover, there was a positive correlation between heme oxygenase activities and malondialdehyde levels, suggesting the prompted heme degradation by ROS. Fetuin-A levels, which are suppressed by inflammation, showed a significant negative correlation with TEQ levels, and hepcidin levels, which are conversely increased by inflammation, had significant positive correlations with malondialdehyde and etheno-dA levels, implying the progression of inflammation by DRC-induced oxidative stress. Taken together, we propose here that wild Baikal seals may suffer from effects of chronic exposure to DRCs on the induction of CYP1 isozymes, followed by increased oxidative stress, heme degradation and inflammation.

The role of XPC: implications in cancer and oxidative DNA damage

The accumulation of DNA damage is a slow but hazardous phenomenon that may lead to cell death, accelerated aging features and cancer. One of the most versatile and important defense mechanisms against the accumulation of DNA damage is nucleotide excision repair (NER), in which the Xeroderma pigmentosum group C (XPC) protein plays a prominent role. NER can be divided into global genome repair (GG-NER) and transcription coupled repair (TC-NER). XPC is a key factor in GG-NER where it functions in DNA damage recognition and after which the repair machinery is recruited to eliminate the DNA damage. Defective XPC functioning has been shown to result in a cancer prone phenotype, in human as well as in mice. Mutation accumulation in XPC deficient mice is accelerated and increased, resulting in an increased tumor incidence. More recently XPC has also been linked to functions outside of NER since XPC deficient mice show a divergent tumor spectrum compared to other NER deficient mouse models. Multiple in vivo and in vitro experiments indicate that XPC appears to be involved in the initiation of several DNA damage-induced cellular responses. XPC seems to function in the removal of oxidative DNA damage, redox homeostasis and cell cycle control. We hypothesize that this combination of increased oxidative DNA damage sensitivity, disturbed redox homeostasis together with inefficient cell cycle control mechanisms are causes of the observed increased cancer susceptibility in oxygen exposed tissues. Such a phenotype is absent in other NER-deficient mice, including Xpa.

Redox cycling of catechol estrogens generating apurinic/apyrimidinic sites and 8-oxo-deoxyguanosine via reactive oxygen species differentiates equine and human estrogens

Metabolic activation of estrogens to catechols and further oxidation to highly reactive o-quinones generates DNA damage including apurinic/apyrimidinic (AP) sites. 4-Hydroxyequilenin (4-OHEN) is the major catechol metabolite of equine estrogens present in estrogen replacement formulations, known to cause DNA strand breaks, oxidized bases, and stable and depurinating adducts. However, the direct formation of AP sites by 4-OHEN has not been characterized. In the present study, the induction of AP sites in vitro by 4-OHEN and the endogenous catechol estrogen metabolite, 4-hydroxyestrone (4-OHE), was examined by an aldehyde reactive probe assay. Both 4-OHEN and 4-OHE can significantly enhance the levels of AP sites in calf thymus DNA in the presence of the redox cycling agents, copper ion and NADPH. The B-ring unsaturated catechol 4-OHEN induced AP sites without added copper, whereas 4-OHE required copper. AP sites were also generated much more rapidly by 4-OHEN. For both catechol estrogens, the levels of AP sites correlated linearly with 8-oxo-dG levels, implying that depuriniation resulted from reactive oxygen species (ROS) rather than depurination of estrogen-DNA adducts. ROS modulators such as catalase, which scavenges hydrogen peroxide and a Cu(I) chelator, blocked the formation of AP sites. In MCF-7 breast cancer cells, 4-OHEN significantly enhanced the formation of AP sites with added NADH. In contrast, no significant induction of AP sites was detected in 4-OHE-treated cells. The greater redox activity of the equine catechol estrogen produces rapid oxidative DNA damage via ROS, which is enhanced by redox cycling agents and interestingly by NADPH-dependent quinone oxidoreductase.

Absolute configurations of DNA lesions determined by comparisons of experimental ECD and ORD spectra with DFT calculations

The usefulness of modern density functional theory (DFT) methods is considered for establishing the absolute configurations of DNA lesions by comparisons of computed and experimentally measured optical rotatory dispersion (ORD) and electronic circular dichroism (ECD) spectra. Two rigid, structurally different DNA lesions (two spiroiminodihydantoin stereoisomers and four equine estrogen 4-hydoxyequilenin-DNA stereoisomeric adducts) have been investigated. In all cases, the signs and shapes of the computed ORD spectra reproduced the experimentally measured ORD spectra, although the magnitudes of the computed and experimental ORD values do not coincide exactly. The computed ECD spectra also reproduced the shapes of the experimental ECD spectra rather well, but are blue-shifted by 10-20 nm. Since the assignments of the absolute configurations of the DNA lesions studied based on computed and experimental ORD and ECD spectra are fully consistent with one another, the computational DFT method shows significant promise for determining the absolute configurations of DNA lesions. Establishing the stereochemistry of DNA lesions is highly useful for understanding their biological impact, especially when sufficient amounts of material are not available for other methods of structural characterization.

XPC silencing sensitizes glioma cells to arsenic trioxide via increased oxidative damage

Arsenic exerts its cytotoxicity via the generation of reactive oxygen species and inhibition of DNA repair. How arsenic disturbs oxidative DNA damage repair is, however, unclear. We found that arsenic trioxide (ATO), like ultraviolet (UV) irradiation, induced the expression of xeroderma pigmentosum group C (XPC) but not of xeroderma pigmentosum A in a human glioma cell line, U87. To explore the role of XPC in the toxic effects of ATO, small interfering RNA was used to silence XPC (siXPC) in U87 cells. siXPC cells were more susceptible to UV irradiation and ATO-induced cell death than control cells. Increased siXPC cell death induced by ATO was accompanied by increased senescence and autophagy. Because increased DNA strand breaks in siXPC cells were observed only when cells were concomitantly treated with ATO and DNA repair inhibitors, XPC silencing apparently did not interfere with repair of ATO-induced DNA damage. Although intracellular ROS levels were not significantly enhanced in siXPC cells, ATO treatment did result in increased 8-hydroxy-2'-deoxyguanosine and hyperoxidized peroxiredoxin. Enhanced superoxide production and autophagy by ATO in siXPC cells were suppressed by co-incubation with N-acetylcysteine (NAC). Furthermore, XPC silencing caused decreased glutathione levels and increased catalase and Mn-superoxide dismutase activities. Increased catalase activity in siXPC cells was suppressed by ATO treatment. XPC silencing also enhanced reporter activity of activator protein-1, whereas enhanced activity was suppressed by NAC. Taken together, our results indicate that XPC silencing causes increased ATO susceptibility by disturbing redox homeostasis rather than reducing DNA repair.

Equine estrogen-induced mammary tumors in rats

Long-term hormone replacement therapy is associated with an increased risk of breast, ovarian and endometrial cancers in women. Equine estrogens are a principal component of hormone replacement therapy; however, their tumorigenic potential toward mammary tissue and reproductive organs has not been extensively explored. A pellet containing equilin was inserted under the skin of female ACI rats and the development of mammary tumors was monitored. Histological examination revealed premalignant lesions such as apocrine metaplasia in whole-mount preparations of mammary gland from the equilin-treated rats. ACI rats given 10mg equilin developed palpable mammary tumors at 13 weeks of treatment, and 37.5% of the rats developed mammary tumors within 15 weeks. For 2.5mg equilin, palpable tumors were observed in 8.3% of the rats after 8 weeks' treatment; the frequency was lower than that (42.9%) observed with 2.5mg E(2). No tumors were observed in the untreated rats. Evidently, equilin is a mammary carcinogen, and this potential may be associated with development of breast and reproductive cancers in women receiving hormone replacement therapy.

Selective estrogen receptor modulator delivery of quinone warheads to DNA triggering apoptosis in breast cancer cells

Estrogen exposure is a risk factor for breast cancer, and estrogen oxidative metabolites have been implicated in chemical carcinogenesis. Oxidation of the catechol metabolite of estrone (4-OHE) and the beta-naphthohydroquinone metabolite of equilenin (4-OHEN) gives o-quinones that produce ROS and damage DNA by adduction and oxidation. To differentiate hormonal and chemical carcinogensis pathways in estrogen receptor positive ER(+) cells, catechol or beta-naphthohydroquinone warheads were conjugated to the selective estrogen receptor modulator (SERM) desmethylarzoxifene (DMA). ER binding was retained in the DMA conjugates; both were antiestrogens with submicromolar potency in mammary and endometrial cells. Cytotoxicity, apoptosis, and caspase-3/7 activation were compared in ER(+) and ER(-)MDA-MB-231 cells, and production of ROS was detected using a fluorescent reporter. Comparison was made to DMA, isolated warheads, and a DMA-mustard. Conjugation of warheads to DMA increased cytotoxicity accompanied by induction of apoptosis and activation of caspase-3/7. Activation of caspase-3/7, induction of apoptosis, and cytotoxicity were all increased significantly in ER(+) cells for the DMA conjugates. ROS production was localized in the nucleus for conjugates in ER(+) cells. Observations are compatible with beta-naphthohydroquinone and catechol groups being concentrated in the nucleus by ER binding, where oxidation and ROS production result, concomitant with caspase-dependent apoptosis. The results suggest that DNA damage induced by catechol estrogen metabolites can be amplified in ER(+) cells independent of hormonal activity. The novel conjugation of quinone warheads to an ER-targeting SERM gives ER-dependent, enhanced apoptosis in mammary cancer cells of potential application in cancer therapy.

NMR and computational studies of stereoisomeric equine estrogen-derived DNA cytidine adducts in oligonucleotide duplexes: opposite orientations of diastereomeric forms

The equine estrogens equilin (EQ) and equilenin (EN) are the active components in the widely prescribed hormone replacement therapy formulation Premarin. Metabolic activation of EQ and EN generates the catechol 4-hydroxyequilenin (4-OHEN) that autoxidizes to the reactive o-quinone form in aerated aqueous solutions. The o-quinones react predominantly with C, and to a lesser extent with A and G, to form premutagenic cyclic covalent DNA adducts in vitro and in vivo. To obtain insights into the structural properties of these biologically important DNA lesions, we have synthesized site-specifically modified oligonucleotides containing the stereoisomeric 1'S,2'R,3'R-4-OHEN-C3 and 1'R,2'S,3'S-4-OHEN-C4 adducts derived from the reaction of 4-OHEN with the C in the oligonucleotide 5'-GGTAGCGATGG in aqueous solution. A combined NMR and computational approach was utilized to determine the conformational characteristics of the two major 4-OHEN-C3 and 4-OHEN-C4 stereoisomeric adducts formed in this oligonucleotide hybridized with its complementary strand. In both cases, the modified C adopts an anti glycosidic bond conformation; the equilenin distal ring protrudes into the minor groove while its two proximal hydroxyl groups are exposed on the major groove side of the DNA duplex. The bulky 4-OHEN-C adduct distorts the duplex within the central GC*G portion, but Watson-Crick pairing is maintained adjacent to C* in both stereoisomeric adducts. For the 4-OHEN-C3 adduct, the equilenin rings are oriented toward the 5'-end of the modified strand, while in 4-OHEN-C4 the equilenin is 3'-directed. Correspondingly, the distortions of the double-helical structures are more pronounced on the 5'- or the 3'-side of the lesion, respectively. These differences in stereoisomeric adduct conformations may play a role in the processing of these lesions in cellular environments.

Estrogen Receptor {alpha} Enhances the Rate of Oxidative DNA Damage by Targeting an Equine Estrogen Catechol Metabolite to the Nucleus

Exposure to estrogens increases the risk of breast and endometrial cancer. It is proposed that the estrogen receptor (ER) may contribute to estrogen carcinogenesis by transduction of the hormonal signal and as a "Trojan horse" concentrating genotoxic estrogen metabolites in the nucleus to complex with DNA, enhancing DNA damage. 4-Hydroxyequilenin (4-OHEN), the major catechol metabolite of equine estrogens present in estrogen replacement formulations, autoxidizes to a redox-cycling quinone that has been shown to cause DNA damage. 4-OHEN was found to be an estrogen of nanomolar potency in cell culture using a luciferase reporter assay and, using a chromatin immunoprecipitation assay, was found to activate ERalpha binding to estrogen-responsive genes in MCF-7 cells. DNA damage was measured in cells by comparing ERalpha(+) versus ERalpha(-) cells and 4-OHEN versus menadione, a reactive oxygen species (ROS)-generating, but non-estrogenic, quinone. 4-OHEN selectively induced DNA damage in ERalpha(+) cells, whereas menadione-induced damage was not dependent on cellular ER status. The rate of 4-OHEN-induced DNA damage was significantly enhanced in ERalpha(+) cells, whereas ER status had no effect on the rate of menadione-induced damage. Imaging of ROS induced by 4-OHEN showed accumulation selective for the nucleus of ERalpha(+) cells within 5 min, whereas in ERalpha(-) or menadione-treated cells, no selectivity was observed. These data support ERalpha acting as a Trojan horse concentrating 4-OHEN in the nucleus to accelerate the rate of ROS generation and thereby amplify DNA damage. The Trojan horse mechanism may be of general importance beyond estrogen genotoxins.