Translesional synthesis on DNA templates containing an estrogen quinone-derived adduct: N2-(2-hydroxyestron-6-yl)-2'-deoxyguanosine and N6-(2-hydroxyestron-6-yl)-2'-deoxyadenosine

Carcinogenic potential of fluorinated estrogens in mammary tumorigenesis

Fluorination preventing metabolic hydroxylation of 17β-estradiol (E₂) was applied to investigate the mechanisms underlying estrogen-induced carcinogenesis. Either 2-fluoro-17β-estradiol (2-FE₂) or 4-fluoro-17β-estradiol (4-FE₂) was administered subcutaneously for 52 weeks to August Copenhagen Irish (ACI) rats, the preferred animal model for human breast cancer. 4-FE₂ induced frequent mammary tumors whereas 2-FE₂ did not. The cumulative incidence of mammary tumors in rats treated with 4-FE₂ was comparable to that observed with E₂. The carcinogenic results were supported by histological examination of mammary glands of fluorinated estrogen-treated ACI rats. To evaluate the estrogenic potential of the fluorinated estrogens, 2-FE₂ or 4-FE₂ was administrated subcutaneously to ovariectomized rats. Both 4-FE₂ and 2-FE₂ showed high uterotrophic potency. Our results indicate that estrogenic potential may not be the sole factor driving mammary tumorigenesis. Since fluorination inhibits metabolic hydroxylation of E₂ at the substituted position, the carcinogenic effect may occur through the metabolic activation of 2-hydroxylated E₂, in combination with the compound's estrogenic potency.

Miscoding properties of 8-chloro-2'-deoxyguanosine, a hypochlorous acid-induced DNA adduct, catalysed by human DNA polymerases

Many chronic inflammatory conditions are associated with an increased risk of cancer development. At the site of inflammation, cellular DNA is damaged by hypochlorous acid (HOCl), a potent oxidant generated by myeloperoxidase. 8-Chloro-2'-deoxyguanosine (8-Cl-dG) is a major DNA adduct formed by HOCl and has been detected from the liver DNA and urine of rats administered lipopolysaccharide in an inflammation model. Thus, the 8-Cl-dG lesion may be associated with the carcinogenesis of inflamed tissues. In this study, we explored the miscoding properties of the 8-Cl-dG adduct generated by human DNA polymerases (pols). Site-specifically modified oligodeoxynucleotide containing a single 8-Cl-dG was prepared and used as a template in primer extension reactions catalysed by human pol α, ĸ or η. Primer extension reactions catalysed by pol α and ĸ in the presence of all four dNTPs were slightly retarded at the 8-Cl-dG site, while pol η readily bypassed the lesion. The fully extended products were analysed to quantify the miscoding frequency and specificity of 8-Cl-dG using two-phased polyacrylamide gel electrophoresis (PAGE). During the primer extension reaction in the presence of four dNTPs, pol ĸ promoted one-base deletion (6.4%), accompanied by the misincorporation of 2'-deoxyguanosine monophosphate (5.5%), dAMP (3.7%), and dTMP (3.5%) opposite the lesion. Pol α and η, on the other hand, exclusively incorporated dCMP opposite the lesion. The steady-state kinetic studies supported the results obtained from the two-phased PAGE assay. These results indicate that 8-Cl-dG is a mutagenic lesion; the miscoding frequency and specificity varies depending on the DNA polymerase used. Thus, HOCl-induced 8-Cl-dG adduct may be involved in inflammation-driven carcinogenesis.

Mutational specificities of brominated DNA adducts catalyzed by human DNA polymerases

Chronic inflammation is known to lead to an increased risk for the development of cancer. Under inflammatory condition, cellular DNA is damaged by hypobromous acid, which is generated by myeloperoxidase and eosinophil peroxidase. The reactive brominating species induced brominated DNA adducts such as 8-bromo-2'-deoxyguanosine (8-Br-dG), 8-bromo-2'-deoxyadenosine (8-Br-dA), and 5-bromo-2'-deoxycytidine (5-Br-dC). These DNA lesions may be implicated in carcinogenesis. In this study, we analyzed the miscoding properties of the brominated DNA adducts generated by human DNA polymerases (pols). Site-specifically modified oligodeoxynucleotides containing a single 8-Br-dG, 8-Br-dA, or 5-Br-dC were used as a template in primer extension reactions catalyzed by human pols α, κ, and η. When 8-Br-dG-modified template was used, pol α primarily incorporated dCMP, the correct base, opposite the lesion, along with a small amount of one-base deletion (4.8%). Pol κ also promoted one-base deletion (14.2%), accompanied by misincorporation of dGMP (9.5%), dAMP (8.0%), and dTMP (6.1%) opposite the lesion. Pol η, on the other hand, readily bypassed the 8-Br-dG lesion in an error-free manner. As for 8-Br-dA and 5-Br-dC, all the pols bypassed the lesions and no miscoding events were observed. These results indicate that only 8-Br-dG, and not 5-Br-dC and 8-Br-dA, is a mutagenic lesion; the miscoding frequency and specificity vary depending on the DNA pol used. Thus, hypobromous acid-induced 8-Br-dG adduct may increase mutagenic potential at the site of inflammation.

The DNA polymerase activity of Saccharomyces cerevisiae Rev1 is biologically significant

A cell's ability to tolerate DNA damage is directly connected to the human development of diseases and cancer. To better understand the processes underlying mutagenesis, we studied the cell's reliance on the potentially error-prone translesion synthesis (TLS), and an error-free, template-switching pathway in Saccharomyces cerevisiae. The primary proteins mediating S. cerevisiae TLS are three DNA polymerases (Pols): Rev1, Pol ζ (Rev3/7), and Pol η (Rad30), all with human homologs. Rev1's noncatalytic role in recruiting other DNA polymerases is known to be important for TLS. However, the biological significance of Rev1's unusual conserved DNA polymerase activity, which inserts dC, is much less well understood. Here, we demonstrate that inactivating Rev1's DNA polymerase function sensitizes cells to both chronic and acute exposure to 4-nitroquinoline-1-oxide (4-NQO) but not to UV or cisplatin. Full Rev1-dependent resistance to 4-NQO, however, also requires the additional Rev1 functions. When error-free tolerance is disrupted through deletion of MMS2, Rev1's catalytic activity is more vital for 4-NQO resistance, possibly explaining why the biological significance of Rev1's catalytic activity has been elusive. In the presence or absence of Mms2-dependent error-free tolerance, the catalytic dead strain of Rev1 exhibits a lower 4-NQO-induced mutation frequency than wild type. Furthermore, Pol ζ, but not Pol η, also contributes to 4-NQO resistance. These results show that Rev1's catalytic activity is important in vivo when the cell has to cope with specific DNA lesions, such as N(2)-dG.

Bioelectronome. Integrated Approach to Receptor Chemistry, Radicals, Electrochemistry, Cell Signaling, and Physiological Effects Based on Electron Transfer

Bioelectronome refers to the host of electron transfer (ET) reactions that occur in living systems. This review presents an integrated approach to receptor chemistry based on electron transfer, radicals, electrochemistry, cell signaling, and end result. First, receptor activity is addressed from the unifying standpoint of redox transformations in which various receptors are discussed. After a listing of receptor-binding modes, receptor chemistry is treated with focus on generation of reactive oxygen species (ROS), activation by ROS, and subsequent cell signaling involving ROS. A general electrostatic mechanism is proposed for receptor-ligand action with supporting evidence. Cell-signaling processes appear to entail electron transfer, ROS, redox chains, and relays. The widespread involvement of phosphate from phosphorylation may be rationalized electrostatically by analogy with DNA phosphate. Extensive evidence supports important participation of ET functionalities in the mechanism of drugs and toxins. The integrated approach is applied to the main ET classes, namely, quinones, metal complexes, iminium species, and aromatic nitro compounds.

Miscoding properties of 2'-deoxyinosine, a nitric oxide-derived DNA Adduct, during translesion synthesis catalyzed by human DNA polymerases

Chronic inflammation involving constant generation of nitric oxide (*NO) by macrophages has been recognized as a factor related to carcinogenesis. At the site of inflammation, nitrosatively deaminated DNA adducts such as 2'-deoxyinosine (dI) and 2'-deoxyxanthosine are primarily formed by *NO and may be associated with the development of cancer. In this study, we explored the miscoding properties of the dI lesion generated by Y-family DNA polymerases (pols) using a new fluorescent method for analyzing translesion synthesis. An oligodeoxynucleotide containing a single dI lesion was used as a template in primer extension reaction catalyzed by human DNA pols to explore the miscoding potential of the dI adduct. Primer extension reaction catalyzed by pol alpha was slightly retarded prior to the dI adduct site; most of the primers were extended past the lesion. Pol eta and pol kappaDeltaC (a truncated form of pol kappa) readily bypassed the dI lesion. The fully extended products were analyzed by using two-phased PAGE to quantify the miscoding frequency and specificity occurring at the lesion site. All pols, that is, pol alpha, pol eta, and pol kappaDeltaC, promoted preferential incorporation of 2'-deoxycytidine monophosphate (dCMP), the wrong base, opposite the dI lesion. Surprisingly, no incorporation of 2'-deoxythymidine monophosphate, the correct base, was observed opposite the lesion. Steady-state kinetic studies with pol alpha, pol eta, and pol kappaDeltaC indicated that dCMP was preferentially incorporated opposite the dI lesion. These pols bypassed the lesion by incorporating dCMP opposite the lesion and extended past the lesion. These relative bypass frequencies past the dC:dI pair were at least 3 orders of magnitude higher than those for the dT:dI pair. Thus, the dI adduct is a highly miscoding lesion capable of generating A-->G transition. This ()NO-induced adduct may play an important role in initiating inflammation-driven carcinogenesis.

Biological properties of single chemical-DNA adducts: a twenty year perspective

The genome and its nucleotide precursor pool are under sustained attack by radiation, reactive oxygen and nitrogen species, chemical carcinogens, hydrolytic reactions, and certain drugs. As a result, a large and heterogeneous population of damaged nucleotides forms in all cells. Some of the lesions are repaired, but for those that remain, there can be serious biological consequences. For example, lesions that form in DNA can lead to altered gene expression, mutation, and death. This perspective examines systems developed over the past 20 years to study the biological properties of single DNA lesions.

Mechanism of translesion synthesis past an equine estrogen-DNA adduct by Y-family DNA polymerases

4-Hydroxyequilenin (4-OHEN)-dC is a major, potentially mutagenic DNA adduct induced by equine estrogens used for hormone replacement therapy. To study the miscoding property of 4-OHEN-dC and the involvement of Y-family human DNA polymerases (pols) eta, kappa and iota in that process, we incorporated 4-OHEN-dC into oligodeoxynucleotides and used them as templates in primer extension reactions catalyzed by pol eta, kappa and iota. Pol eta inserted dAMP opposite 4-OHEN-dC, accompanied by lesser amounts of dCMP and dTMP incorporation and base deletion. Pol kappa promoted base deletions as well as direct incorporation of dAMP and dCMP. Pol iota worked in conjunction with pol kappa, but not with pol eta, at a replication fork stalled by the adduct, resulting in increased dTMP incorporation. Our results provide a direct evidence that Y-family DNA pols can switch with one another during synthesis past the lesion. No direct incorporation of dGMP, the correct base, was observed with Y-family enzymes. The miscoding potency of 4-OHEN-dC may be associated with the development of reproductive cancers observed in women receiving hormone replacement therapy.

Translesion synthesis past tamoxifen-derived DNA adducts by human DNA polymerases eta and kappa

The long-term treatment of tamoxifen (TAM), widely used for adjuvant chemotherapy and chemoprevention for breast cancer, increases a risk of developing endometrial cancer. A high frequency of K-ras mutations has been observed in the endometrium of women treated with TAM. Human DNA polymerase (pol) eta and pol kappa are highly expressed in the reproductive organs and are associated with translesion synthesis past bulky DNA adducts. To explore the miscoding properties of alpha-(N2-deoxyguanosinyl)tamoxifen (dG-N2-TAM), a major TAM-DNA adduct, site-specifically modified oligodeoxynucleotides containing a single diastereoisomer of trans or cis forms of dG-N2-TAM were prepared by phosphoramidite chemical procedure and used as templates. The primer extension reaction catalyzed by pol kappa deltaC, a truncated form of pol kappa, extended more efficiently past the adduct than that of pol eta by incorporating dCMP, a correct base, opposite the adduct. With pol eta, all diastereoisomers of dG-N2-TAM promoted small amounts of direct incorporation of dAMP and deletions. With pol kappa deltaC, dG-N2-TAM promoted small amounts of dTMP and/or dAMP incorporations and deletions. The miscoding properties varied depending on the diastereoisomer of dG-N2-TAM adducts and the DNA pol used. Steady-state kinetic studies were also performed using either the nonspecific sequence or the K-ras gene sequence containing a single dG-N2-TAM at the second base of codon 12. With pol eta, the bypass frequency past the dA x dG-N2-TAM pair positioned in the K-ras sequence was only 2.3 times lower than that for the dC x dG-N2-TAM pair, indicating that dG-N2-TAM in the K-ras sequence has higher miscoding potential than that in the nonspecific sequence. However, with pol kappa deltaC, the bypass frequency past the dC x dG-N2-TAM pair was higher than that of the dT x dG-N2-TAM pair in both sequences. The properties of pol eta and pol kappa are consistent with the mutagenic events attributed to TAM-DNA adducts.

Translesion synthesis past equine estrogen-derived 2'-deoxyadenosine DNA adducts by human DNA polymerases eta and kappa

Hormone replacement therapy (HRT) increases the risk of developing breast, ovarian, and endometrial cancers. Equilin and equilenin are the major components of the widely prescribed drug used for HRT. 4-Hydroxyequilenin (4-OHEN), a major metabolite of equilin and equilenin, promotes 4-OHEN-modified dC, dA, and dG DNA adducts. These DNA adducts were detected in breast tumor and adjacent normal tissues of several patients receiving HRT. We have recently found that the 4-OHEN-dC DNA adduct is a highly miscoding lesion generating C --> T transitions and C --> G transversions. To explore the mutagenic potential of another major 4-OHEN-dA adduct, site-specifically modified oligodeoxynucleotides containing a single diastereoisomer of 4-OHEN-dA (Pk-1, Pk-2, and Pk-3) were prepared by a postsynthetic method and used as DNA templates for primer extension reactions catalyzed by human DNA polymerase (pol) eta and kappa that are highly expressed in the reproductive organs. Primer extension catalyzed by pol eta or pol kappa occurred rapidly on the unmodified template to form fully extended products. With the major 4-OHEN-dA-modified templates (Pk-2 and Pk-3), primer extension was retarded prior to the lesion and opposite the lesion; a fraction of the primers was extended past the lesion. Steady-state kinetic studies with pol eta and pol kappa indicated that dTMP, the correct base, was preferentially incorporated opposite the 4-OHEN-dA lesion. In addition, pol eta and pol kappa bypassed the lesion by incorporating dAMP and dCMP, respectively, opposite the lesion and extended past the lesion. The relative bypass frequency past the 4-OHEN-dA lesion with pol eta was at least 2 orders of magnitude higher than that observed with pol kappa. The bypass frequency past Pk-2 was more efficient than that past Pk-3. Thus, 4-OHEN-dA is a miscoding lesion generating A --> T transversions and A --> G transitions. The miscoding frequency and specificity of 4-OHEN-dA varied depending on the stereoisomer of the 4-OHEN-dA adduct and DNA polymerase used.

Translesion synthesis past 2'-deoxyxanthosine, a nitric oxide-derived DNA adduct, by mammalian DNA polymerases

Cellular DNA is damaged by nitric oxide (NO), a multifunctional bioregulator and an environmental pollutant that has been implicated in diseases associated with cancer and chronic inflammation. 2'-Deoxyxanthosine (dX) is a major NO-derived DNA lesion. To explore the mutagenic potential of dX, a 38-mer oligodeoxynucleotide ((5')CATGCTGATGAATTCCTTCXCTTCTTTCCTCTCCCTTT) modified site-specifically with dX at the X position was prepared post-synthetically and used as a DNA template in primer extension reactions catalyzed by calf thymus DNA polymerase (pol) alpha and human DNA pol beta, eta, and kappa. Primer extension reactions catalyzed by pol alpha or beta in the presence of four dNTPs were retarded at the dX lesion while pol eta and kappa readily bypassed the lesion. The fully extended products were analyzed to quantify the miscoding specificity and frequency of dX using two-phase polyacrylamide gel electrophoresis (PAGE). With pol alpha, eta and kappa, incorrect dTMP was preferentially incorporated opposite the lesion, along with lesser amounts of dCMP, the correct base. When pol beta was used, direct incorporation of correct dCMP was primarily observed, accompanied by small amounts of misincorporation of dTMP, dAMP and dGMP. Steady-state kinetic analyses supported the results obtained from the two-phase PAGE assay. dX is a miscoding lesion capable of preferentially generating G-->A mutations. The miscoding frequency varied depending on DNA polymerase used.

Structural and stereoisomer effects of model estrogen quinone-derived DNA adducts: N6-(2-hydroxyestron-6(alpha,beta)-yl)-2'-deoxyadenosine and N2-(2-hydroxyestron-6(alpha,beta)-yl)-2'-deoxyguanosine

An extensive conformational analysis has been carried out for two diastereoisomeric pairs of model estrogen quinone-derived DNA adducts, N6-(2-hydroxyestron-6(alpha,beta)-yl)-2'-deoxyadenosine (2-OHE1-6(alpha,beta)-N6-dA) and N2-(2-hydroxyestron-6(alpha,beta)-yl)-2'-deoxyguanosine (2-OHE1-6(alpha,beta)-N2-dG), in a B-DNA duplex and at a primer-template junction in a pol alpha family DNA polymerase. In vitro primer extension studies in pol alpha [Terashima, I., et al. (1998) Biochemistry 37, 13807-13815] have shown that the adenine adducts can incorporate dT, together with a small proportion of the incorrect base dC opposite the lesion, and they block less strongly than the guanine adducts. We have carried out conformational searches with energy minimization for four DNA duplexes containing 2-OHE1-6alpha-N6-dA, 2-OHE1-6beta-N6-dA, 2-OHE1-6alpha-N2-dG, or 2-OHE1-6beta-N2-dG. Our searches revealed that the four-ring nonplanar 2-hydroxyestrone (2-OHE1) moiety strongly prefers to reside in the major groove of the adenine adducts or the minor groove of the guanine adducts in a B-DNA duplex, with stereochemistry-dependent orientational differences in each case. No low energy conformations involving intercalation of the 2-OHE1 moiety were located in the searches. This stems from the largely nonplanar, nonaromatic nature of the 2-OHE1 ring system and implies that the proclivity for such bulky, nonplanar adducts to reside at the DNA helix exterior is a plausible conformational feature of other structurally similar estrogen quinone-derived DNA adducts, independent of base sequence context. In addition, the adenine adduct isomers, located in the major groove, manifest serious disturbance to the Watson-Crick base pairs at and near the lesion site, suggesting repair susceptibility. Possible structures of these adducts in a pol alpha family polymerase were also investigated through molecular modeling. The results rationalized the experimental in vitro primer extension studies. In addition, poor accommodation of the beta-stereoisomers within the polymerase was noted, suggesting that these stereoisomers would be more prone to cause blockage. Stereochemistry-dependent differences in adduct orientation could be expected to produce different biochemical effects, as has been observed in adducts derived from polycyclic aromatic hydrocarbons.

Adduction of catechol estrogens to nucleosides

We report the formation, detection, quantitation and structural characterization of products resulting from the adduction of deoxynucleosides (deoxyadenosine, deoxyguanosine, deoxycytidine and 5-methyldeoxycytidine) to the catechol estrogens (CE) of estrone, estradiol-17beta and estradiol-17 alpha. The crude products are obtained in a one-pot synthesis through oxidation of catechols to quinones and subsequent Michael-type reaction with the deoxynucleosides in acidic medium. In all experiments, adducts are detected by electrospray ionization mass spectrometry analysis after HPLC separation (LC/ESI/MS(n)). The two pyrimidines deoxycytidine and 5-methyldeoxycytidine yield only CE adducts to deoxynucleosides, which correspond to stable adducts on DNA. For purines, the results depend on the CE (2,3- or 3,4-catechols) used, the function and configuration on carbon 17 (ketone for estrone, alcohol for alpha and beta isomers of estradiol), and on the purine itself (deoxyadenosine or deoxyguanosine). Both stable adducts and deglycosylated adducts are formed, and therefore formation of stable adducts on DNA as well as the loss of purines from the DNA strands could be possible. MS(2) and MS(3) experiments prove to be relevant for further structural determinations, enabling in some cases the elucidation of the regiochemistry of adduction on the A and B rings of the steroid moiety.

Reactions of estradiol-2,3-quinone with deoxyribonucleosides: possible insights in the reactivity of estrogen quinones with DNA

Estrogen 2,3- and 3,4-quinones are reactive species toward nucleophiles and Michael acceptors. As such, they can bind to DNA and induce cellular damages. As an alkylation model, reactions of estradiol-2,3-quinone with deoxyribonucleosides were previously studied by mass spectrometry. In this work, estrogen-deoxyribonucleoside adducts were synthesized by reaction of 17beta-estradiol-2,3-quinone with deoxyguanosine or deoxyadenosine and analyzed by NMR and LC-MS(n)() in order to determine the structure and the stereochemistry of the resulting covalent adducts. Although estradiol- and estrone-2,3-quinones were previously thought to give mainly stable adducts, identification of depurinating adducts with both nucleosides, i.e., 2-OHE(2)-6(alpha,beta)-N7Gua and 2-OHE(2)-6(alpha,beta)-N7Ade, was unambiguously obtained. This is of particular interest since depurinating adducts are generated from DNA, and therefore, their amount should be correlated to the parallel formation of apurinic sites, which might play an important role in the cancer initiation process. Besides, a byproduct, i.e., 2-hydroxy-11-oxo-estradiol, corresponding to an unstable alkylation product of 2-hydroxyestradiol has been unambiguously identified and is indicative of a plausible addition process at the C9 position of catechol estrogens. The synthetic adducts will be useful as reference compounds to further elucidate the structure of adducts formed by reaction of estrogen metabolites with DNA or oligonucleotides.