Induction of A.T to G.C mutations by erroneous repair of depurinated DNA following estrogen treatment of the mammary gland of ACI rats

Dysregulation of DNA repair genes in Fuchs endothelial corneal dystrophy

Fuchs Endothelial Corneal Dystrophy (FECD), a late-onset oxidative stress disorder, is the most common cause of corneal endothelial degeneration and is genetically associated with CTG repeat expansion in Transcription Factor 4 (TCF4). We previously reported accumulation of nuclear (nDNA) and mitochondrial (mtDNA) damage in FECD. Specifically, mtDNA damage was a prominent finding in development of disease in the ultraviolet-A (UVA) induced FECD mouse model. We hypothesize that an aberrant DNA repair may contribute to the increased DNA damage seen in FECD. We analyzed differential expression profiles of 84 DNA repair genes by real-time PCR arrays using Human DNA Repair RT-Profiler plates using cDNA extracted from Descemet's membrane-corneal endothelium (DM-CE) obtained from FECD patients with expanded (>40) or non-expanded (<40) intronic CTG repeats in TCF4 gene and from age-matched normal donors. Change in mRNA expression of <0.5- or >2.0-fold in FECD relative to normal was set as cutoff for down- or upregulation. Downregulated mitochondrial genes were further validated using the UVA-based mouse model of FECD. FECD specimens exhibited downregulation of 9 genes and upregulation of 8 genes belonging to the four major DNA repair pathways, namely, base excision repair (BER), nucleotide excision repair (NER), mismatch repair (MMR), and double strand break (DSB) repair, compared to normal donors. MMR gene MSH2 and BER gene POLB were preferentially upregulated in expanded FECD. BER genes LIG3 and NEIL2, DSB repair genes PARP3 and TOP3A, NER gene XPC, and unclassified pathway gene TREX1, were downregulated in both expanded and non-expanded FECD. MtDNA repair genes, Lig3, Neil2, and Top3a, were also downregulated in the UVA-based mouse model of FECD. Our findings identify impaired DNA repair pathways that may play an important role in DNA damage due to oxidative stress as well as genetic predisposition noted in FECD.

Hormonal and genotoxic estrogen-androgen carcinogenesis in the NBL rat prostate: A role for aromatase

BACKGROUND

Androgens are generally thought to cause prostate cancer, but the data from animal studies suggest that they must be aromatized to estrogen and act in concert with genotoxic estrogen metabolites. The objective of this study was to determine whether treatment with testosterone (T) combined with a nonestrogenic estrogen metabolite and a nongenotoxic estrogenic compound would all be necessary and sufficient for the induction of a high incidence of prostate cancer in the susceptible NBL rat strain.

METHODS

NBL rats were treated with low-dose testosterone via slow-release Silastic implants and with the marginally estrogenic genotoxic catechol estrogen 4-hydroxyestradiol (4OH-E2) and the nongenotoxic estrogen 2-fluoroestradiol (2F-E2) and in one experiment the aromatase inhibitor letrozole via custom-made slow-release pellets. Animals were euthanized 52 weeks after implantation and their pituitaries and prostate complexes weighed and fixed in formalin. Hematoxylin and eosin (H&E)-stained step sections were prepared and examined microscopically for proliferative lesions.

RESULTS

Animals treated with 2F-E2, with or without the other compounds, had enlarged pituitaries demonstrating its estrogenicity. Animals treated with T, with or without the other compounds, had enlarged prostates consistent with its androgenicity. Rats treated with T plus 2F-E2 and 4OH-E2 developed a high incidence of prostatic cancer (89%), while, surprisingly, rats treated with T plus only 2F-E2 also had a high incidence of prostate cancer (95%) contradicting our initial hypothesis. To test whether the formation of E2 from T by aromatase could lead to estrogen genotoxicity and prostate carcinogenesis we then rats treated with T and 2F-E2 also with letrozole and found that it reduced prostate cancer incidence by about 50%.

CONCLUSIONS

These findings indicate that long-term treatment with a nongenotoxic estrogen (2F-E2) and T as well as uninhibited prostatic aromatase activity generating genotoxic E2 are all required for induction of a high incidence of prostatic adenocarcinomas in NBL rats. These and previous data indicate that androgen receptor-mediated action, estrogen receptor mediation, and estrogen genotoxicity are all required and sufficient for hormonal carcinogenesis in the NBL rat prostate. Interference with the estrogen genotoxicity is a potential approach to prostate cancer chemoprevention.

Mechanistic Effects of Estrogens on Breast Cancer

PURPOSE

Current concepts regarding estrogen and its mechanistic effects on breast cancer in women are evolving. This article reviews studies that address estrogen-mediated breast cancer development, the prevalence of occult tumors at autopsy, and the natural history of breast cancer as predicted by a newly developed tumor kinetic model.

METHODS

This article reviews previously published studies from the authors and articles pertinent to the data presented.

RESULTS

We discuss the concepts of adaptive hypersensitivity that develops in response to long-term deprivation of estrogen and results in both increased cell proliferation and apoptosis. The effects of menopausal hormonal therapy on breast cancer in postmenopausal women are interpreted based on the tumor kinetic model. Studies of the administration of a tissue selective estrogen complex in vitro, in vivo, and in patients are described. We review the various clinical studies of breast cancer prevention with selective estrogen receptor modulators and aromatase inhibitors. Finally, the effects of the underlying risk of breast cancer on the effects of menopausal hormone therapy are outlined.

DISCUSSION

The overall intent of this review is to present data supporting recent concepts, discuss pertinent literature, and critically examine areas of controversy.

Endogenous estrogens-breast cancer and chemoprevention

Breast cancer is the most common female malignancy and the second leading cause of cancer related deaths. It is estimated that about 40% of all cancer in women is hormonally mediated. Both estrogens and androgens play critical roles in the initiation and development of breast cancer. Estrogens influence normal physiological growth, proliferation, and differentiation of breast tissues, as well as the development and progression of breast malignancy. Breast cancer is caused by numerous endo- and exogenous risk factors. The paper presents estrogen metabolism, in particular 17β-estradiol and related hormones. The mechanisms of estrogen carcinogenesis include the participation of estrogen receptors, the genotoxic effect of the estrogen metabolites, and epigenetic processes that are also presented. The role of reactive oxygen species in breast cancer has been described. It called attention to a role of numerous signaling pathways in neoplastic transformation. Chemoprotective agents, besides other phytoestrogens, classical antioxidants, synthetic compounds, and their mechanisms of action have been shown.

The 3,4-Quinones of Estrone and Estradiol Are the Initiators of Cancer whereas Resveratrol and N-acetylcysteine Are the Preventers

This article reviews evidence suggesting that a common mechanism of initiation leads to the development of many prevalent types of cancer. Endogenous estrogens, in the form of catechol estrogen-3,4-quinones, play a central role in this pathway of cancer initiation. The catechol estrogen-3,4-quinones react with specific purine bases in DNA to form depurinating estrogen-DNA adducts that generate apurinic sites. The apurinic sites can then lead to cancer-causing mutations. The process of cancer initiation has been demonstrated using results from test tube reactions, cultured mammalian cells, and human subjects. Increased amounts of estrogen-DNA adducts are found not only in people with several different types of cancer but also in women at high risk for breast cancer, indicating that the formation of adducts is on the pathway to cancer initiation. Two compounds, resveratrol, and N-acetylcysteine, are particularly good at preventing the formation of estrogen-DNA adducts in humans and are, thus, potential cancer-prevention compounds.

Interactions of the antioxidant enzymes NAD(P)H: Quinone oxidoreductase 1 (NQO1) and NRH: Quinone oxidoreductase 2 (NQO2) with pharmacological agents, endogenous biochemicals and environmental contaminants

NAD(P)H

Quinone Oxidoreductase 1 (NQO1) is an antioxidant enzyme that catalyzes the two-electron reduction of several different classes of quinone-like compounds (quinones, quinone imines, nitroaromatics, and azo dyes). One-electron reduction of quinone or quinone-like metabolites is considered to generate semiquinones to initiate redox cycling that is responsible for the generation of reactive oxygen species and oxidative stress and may contribute to the initiation of adverse drug reactions and adverse health effects. On the other hand, the two-electron reduction of quinoid compounds appears important for drug activation (bioreductive activation) via chemical rearrangement or autoxidation. Two-electron reduction decreases quinone levels and opportunities for the generation of reactive species that can deplete intracellular thiol pools. Also, studies have shown that induction or depletion (knockout) of NQO1 were associated with decreased or increased susceptibilities to oxidative stress, respectively. Moreover, another member of the quinone reductase family, NRH: Quinone Oxidoreductase 2 (NQO2), has a significant functional and structural similarity with NQO1. The activity of both antioxidant enzymes, NQO1 and NQO2, becomes critically important when other detoxification pathways are exhausted. Therefore, this article summarizes the interactions of NQO1 and NQO2 with different pharmacological agents, endogenous biochemicals, and environmental contaminants that would be useful in the development of therapeutic approaches to reduce the adverse drug reactions as well as protection against quinone-induced oxidative damage. Also, future directions and areas of further study for NQO1 and NQO2 are discussed.

Adverse outcome pathways for ionizing radiation and breast cancer involve direct and indirect DNA damage, oxidative stress, inflammation, genomic instability, and interaction with hormonal regulation of the breast

Knowledge about established breast carcinogens can support improved and modernized toxicological testing methods by identifying key mechanistic events. Ionizing radiation (IR) increases the risk of breast cancer, especially for women and for exposure at younger ages, and evidence overall supports a linear dose-response relationship. We used the Adverse Outcome Pathway (AOP) framework to outline and evaluate the evidence linking ionizing radiation with breast cancer from molecular initiating events to the adverse outcome through intermediate key events, creating a qualitative AOP. We identified key events based on review articles, searched PubMed for recent literature on key events and IR, and identified additional papers using references. We manually curated publications and evaluated data quality. Ionizing radiation directly and indirectly causes DNA damage and increases production of reactive oxygen and nitrogen species (RONS). RONS lead to DNA damage and epigenetic changes leading to mutations and genomic instability (GI). Proliferation amplifies the effects of DNA damage and mutations leading to the AO of breast cancer. Separately, RONS and DNA damage also increase inflammation. Inflammation contributes to direct and indirect effects (effects in cells not directly reached by IR) via positive feedback to RONS and DNA damage, and separately increases proliferation and breast cancer through pro-carcinogenic effects on cells and tissue. For example, gene expression changes alter inflammatory mediators, resulting in improved survival and growth of cancer cells and a more hospitable tissue environment. All of these events overlap at multiple points with events characteristic of "background" induction of breast carcinogenesis, including hormone-responsive proliferation, oxidative activity, and DNA damage. These overlaps make the breast particularly susceptible to ionizing radiation and reinforce that these biological activities are important characteristics of carcinogens. Agents that increase these biological processes should be considered potential breast carcinogens, and predictive methods are needed to identify chemicals that increase these processes. Techniques are available to measure RONS, DNA damage and mutation, cell proliferation, and some inflammatory proteins or processes. Improved assays are needed to measure GI and chronic inflammation, as well as the interaction with hormonally driven development and proliferation. Several methods measure diverse epigenetic changes, but it is not clear which changes are relevant to breast cancer. In addition, most toxicological assays are not conducted in mammary tissue, and so it is a priority to evaluate if results from other tissues are generalizable to breast, or to conduct assays in breast tissue. Developing and applying these assays to identify exposures of concern will facilitate efforts to reduce subsequent breast cancer risk.

Adverse outcome pathways for ionizing radiation and breast cancer involve direct and indirect DNA damage, oxidative stress, inflammation, genomic instability, and interaction with hormonal regulation of the breast

Knowledge about established breast carcinogens can support improved and modernized toxicological testing methods by identifying key mechanistic events. Ionizing radiation (IR) increases the risk of breast cancer, especially for women and for exposure at younger ages, and evidence overall supports a linear dose-response relationship. We used the Adverse Outcome Pathway (AOP) framework to outline and evaluate the evidence linking ionizing radiation with breast cancer from molecular initiating events to the adverse outcome through intermediate key events, creating a qualitative AOP. We identified key events based on review articles, searched PubMed for recent literature on key events and IR, and identified additional papers using references. We manually curated publications and evaluated data quality. Ionizing radiation directly and indirectly causes DNA damage and increases production of reactive oxygen and nitrogen species (RONS). RONS lead to DNA damage and epigenetic changes leading to mutations and genomic instability (GI). Proliferation amplifies the effects of DNA damage and mutations leading to the AO of breast cancer. Separately, RONS and DNA damage also increase inflammation. Inflammation contributes to direct and indirect effects (effects in cells not directly reached by IR) via positive feedback to RONS and DNA damage, and separately increases proliferation and breast cancer through pro-carcinogenic effects on cells and tissue. For example, gene expression changes alter inflammatory mediators, resulting in improved survival and growth of cancer cells and a more hospitable tissue environment. All of these events overlap at multiple points with events characteristic of "background" induction of breast carcinogenesis, including hormone-responsive proliferation, oxidative activity, and DNA damage. These overlaps make the breast particularly susceptible to ionizing radiation and reinforce that these biological activities are important characteristics of carcinogens. Agents that increase these biological processes should be considered potential breast carcinogens, and predictive methods are needed to identify chemicals that increase these processes. Techniques are available to measure RONS, DNA damage and mutation, cell proliferation, and some inflammatory proteins or processes. Improved assays are needed to measure GI and chronic inflammation, as well as the interaction with hormonally driven development and proliferation. Several methods measure diverse epigenetic changes, but it is not clear which changes are relevant to breast cancer. In addition, most toxicological assays are not conducted in mammary tissue, and so it is a priority to evaluate if results from other tissues are generalizable to breast, or to conduct assays in breast tissue. Developing and applying these assays to identify exposures of concern will facilitate efforts to reduce subsequent breast cancer risk.

Dissecting the prevention of estrogen-dependent breast carcinogenesis through Nrf2-dependent and independent mechanisms

Breast cancer is the most common malignancy among women worldwide. Various studies indicate that prolonged exposure to elevated levels of estrogens is associated with development of breast cancer. Both estrogen receptor-dependent and independent mechanisms can contribute to the carcinogenic effects of estrogens. Among them, the oxidative metabolism of estrogens plays a key role in the initiation of estradiol-induced breast cancer by generation of reactive estrogen quinones as well as the associated formation of oxygen free radicals. These genotoxic metabolites can react with DNA to form unstable DNA adducts which generate mutations leading to the initiation of breast cancer. A variety of endogenous and exogenous factors can alter estrogen homeostasis and generate genotoxic metabolites. The use of specific phytochemicals and dietary supplements can inhibit the risk of breast cancer not only by the modulation of several estrogen-activating enzymes (CYP19, CYP1B1) but also through the induction of various cytoprotective enzymes (eg, SOD3, NQO1, glutathione S-transferases, OGG-1, catechol-O-methyltransferases, CYP1B1A, etc.) that reestablish the homeostatic balance of estrogen metabolism via nuclear factor erythroid 2-related factor 2 (Nrf2)-dependent and independent mechanisms.

Role of Estrogen in Androgen-Induced Prostate Carcinogenesis in NBL Rats

Androgens are thought to cause prostate cancer, but the underlying mechanisms are unclear. Data from animal studies suggest that for androgens to cause prostate cancer, they must be aromatized to estrogen and act in concert with estrogen metabolites. We tested the hypothesis that androgen-receptor and estrogen receptor-mediated effects of androgen and estrogen are necessary, as well as genotoxicity of estrogen metabolites. NBL rats were treated with androgenic and estrogenic compounds for 16-75 weeks through slow-release silastic implants or pellets. Testosterone alone induced cancer in the prostate of 37% of rats. 5α-Dihydrotestosterone, which cannot be converted to estradiol or testosterone, did not cause a significant prostate cancer incidence (4%). Addition of estradiol to 5α-dihydrotestosterone treatment did not markedly enhance prostate cancer incidence (14%), unlike adding estradiol to testosterone treatment which induced a 100% tumor incidence. Testosterone plus estradiol treatment induced a DNA adduct detectable by ³²P-postlabeling, oxidative DNA damage (8-hydroxyguanosine), and lipid peroxidation at the site within the prostate where this treatment causes cancers, preceding later cancer formation. The non-estrogenic 4-hydroxy metabolite of estradiol, when combined with testosterone, induced prostatic dysplasia within 16 weeks and, after long-term treatment, a very low incidence of prostate cancer (21%). When an estrogen that cannot be hydroxylated (2-fluoroestradiol) was added to this combined treatment with testosterone and 4-hydroxyestradiol, dysplasia frequency after 16 weeks was doubled. These results strongly support the hypothesis, but additional definitive studies are needed which may identify new targets to interfere with these mechanisms that are clinically feasible in humans.

The Emerging Role of Estrogens in Thyroid Redox Homeostasis and Carcinogenesis

Reactive oxygen species (ROS) are the most critical class of free radicals or reactive metabolites produced by all living organisms. ROS regulate several cellular functions through redox-dependent mechanisms, including proliferation, differentiation, hormone synthesis, and stress defense response. However, ROS overproduction or lack of appropriate detoxification is harmful to cells and can be linked to the development of several diseases, such as cancer. Oxidative damage in cellular components, especially in DNA, can promote the malignant transformation that has already been described in thyroid tissue. In thyrocyte physiology, NADPH oxidase enzymes produce large amounts of ROS that are necessary for hormone biosynthesis and might contribute to the high spontaneous mutation rate found in this tissue. Thyroid cancer is the most common endocrine malignancy, and its incidence is significantly higher in women than in men. Several lines of evidence suggest the sex hormone estrogen as a risk factor for thyroid cancer development. Estrogen in turn, besides being a potent growth factor for both normal and tumor thyroid cells, regulates different mechanisms of ROS generation. Our group demonstrated that the thyroid gland of adult female rats exhibits higher hydrogen peroxide (H₂O₂) production and lower enzymatic antioxidant defense in comparison with male glands. In this review, we discuss the possible involvement of thyroid redox homeostasis and estrogen in the development of thyroid carcinogenesis.

Genotoxic and mutagenic studies of teratogens in developing rat and mouse

In this review, genotoxic and mutagenic effects of teratogenic chemical agents in both rat and mouse have been reviewed. Of these chemicals, 97 are drugs and 33 are pesticides or belong to other groups. Large literature searches were conducted to determine the effects of chemicals on chromosome abnormalities, sister chromatid exchanges, and micronucleus formation in experimental animals such as rats and mice. In addition, studies that include unscheduled DNA synthesis, DNA adduct formations, and gene mutations, which help to determine the genotoxicity or mutagenicity of chemicals, have been reviewed. It has been estimated that 46.87% of teratogenic drugs and 48.48% of teratogenic pesticides are positive in all tests. So, all of the teratogens involved in this group have genotoxic and mutagenic effects. On the other hand, 36.45% of the drugs and 21.21% of the pesticides have been found to give negative results in at least one test, with the majority of the tests giving positive results. However, only 4.16% of the drugs and 18.18% of the pesticides were determined to give negative results in the majority of the tests. Among tests with major negative results, 12.50% of the teratogenic drugs and 12.12% of the teratogenic pesticides were negative in all conducted tests.

Unifying mechanism in the initiation of breast cancer by metabolism of estrogen (Review)

Excessive exposure to estrogen is associated with increased risk of breast cancer. The mechanisms of carcinogenesis in the breast caused by estrogen metabolism include formation of depurinating adducts which are released from DNA to generate apurinic sites, and production of reactive oxygen species (ROS). Excess ROS not only exerts genotoxicity by indirectly increasing genomic instability, but also stimulates progression of mammary carcinogenicity by inducing a redox‑associated signaling pathway. Estrogen metabolism enzymes serve an important role in estrogen metabolism. Alterations in the expression and activity of estrogen metabolism enzymes may influence estrogen metabolism homeostasis. The present review discusses the process of estrogen metabolism, the role of estrogen metabolites and ROS in breast carcinogenesis, and the effect of metabolism enzyme polymorphisms on generation of pro‑carcinogens and breast cancer susceptibility.

Etiology and prevention of prevalent types of cancer

Endogenous estrogens become carcinogens when excessive catechol estrogen quinone metabolites are formed. Specifically, the catechol estrogen-3,4-quinones can react with DNA to produce a large amount of specific depurinating estrogen-DNA adducts, formed at the N-3 of Ade and N-7 of Gua. Loss of these adducts leaves apurinic sites in the DNA, which can generate subsequent cancer-initiating mutations. Unbalanced estrogen metabolism yields excessive catechol estrogen-3,4-quinones, increasing formation of the depurinating estrogen-DNA adducts and the risk of initiating cancer. Evidence for this mechanism of cancer initiation comes from studies in vitro, in cell culture, in animal models and in human subjects. High levels of estrogen-DNA adducts have been observed in women with breast, ovarian or thyroid cancer, and in men with prostate cancer or non-Hodgkin lymphoma. Observation of high levels of depurinating estrogen-DNA adducts in high risk women before the presence of breast cancer indicates that adduct formation is a critical factor in breast cancer initiation. Two dietary supplements, N-acetylcysteine and resveratrol, complement each other in reducing formation of catechol estrogen-3,4-quinones and inhibiting formation of estrogen-DNA adducts in cultured human and mouse breast epithelial cells. They also inhibit malignant transformation of these epithelial cells. In addition, formation of adducts was reduced in women who followed a Healthy Breast Protocol that includes N-acetylcysteine and resveratrol. Blocking initiation of cancer prevents promotion, progression and development of the disease. These results suggest that reducing formation of depurinating estrogen-DNA adducts can reduce the risk of developing a variety of types of human cancer.

Critical depurinating DNA adducts: Estrogen adducts in the etiology and prevention of cancer and dopamine adducts in the etiology and prevention of Parkinson's disease

Endogenous estrogens become carcinogens when dangerous metabolites, the catechol estrogen quinones, are formed. In particular, the catechol estrogen-3,4-quinones can react with DNA to produce an excess of specific depurinating estrogen-DNA adducts. Loss of these adducts leaves apurinic sites in the DNA, generating subsequent cancer-initiating mutations. Unbalanced estrogen metabolism yields excessive catechol estrogen-3,4-quinones, increasing formation of depurinating estrogen-DNA adducts and the risk of initiating cancer. Evidence for this mechanism of cancer initiation comes from various types of studies. High levels of depurinating estrogen-DNA adducts have been observed in women with breast, ovarian or thyroid cancer, as well as in men with prostate cancer or non-Hodgkin lymphoma. Observation of high levels of depurinating estrogen-DNA adducts in high risk women before the presence of breast cancer indicates that adduct formation is a critical factor in breast cancer initiation. Formation of analogous depurinating dopamine-DNA adducts is hypothesized to initiate Parkinson's disease by affecting dopaminergic neurons. Two dietary supplements, N-acetylcysteine and resveratrol complement each other in reducing formation of catechol estrogen-3,4-quinones and inhibiting formation of estrogen-DNA adducts in cultured human and mouse breast epithelial cells. They also inhibit malignant transformation of these cells. In addition, formation of adducts was reduced in women who followed a Healthy Breast Protocol that includes N-acetylcysteine and resveratrol. When initiation of cancer is blocked, promotion, progression and development of the disease cannot occur. These results suggest that reducing formation of depurinating estrogen-DNA adducts can reduce the risk of developing a variety of types of human cancer.

Depurinating estrogen-DNA adducts, generators of cancer initiation: their minimization leads to cancer prevention

Estrogens can initiate cancer by reacting with DNA. Specific metabolites of endogenous estrogens, the catechol estrogen-3,4-quinones, react with DNA to form depurinating estrogen-DNA adducts. Loss of these adducts leaves apurinic sites in the DNA, generating mutations that can lead to the initiation of cancer. A variety of endogenous and exogenous factors can disrupt estrogen homeostasis, which is the normal balance between estrogen activating and protective enzymes. In fact, if estrogen metabolism becomes unbalanced and generates excessive catechol estrogen 3,4-quinones, formation of depurinating estrogen-DNA adducts increases and the risk of initiating cancer is greater. The levels of depurinating estrogen-DNA adducts are high in women diagnosed with breast cancer and those at high risk for the disease. High levels of depurinating estrogen-DNA adducts before the presence of breast cancer indicates that adduct formation is a critical factor in breast cancer initiation. Women with thyroid or ovarian cancer also have high levels of estrogen-DNA adducts, as do men with prostate cancer or non-Hodgkin lymphoma. Depurinating estrogen-DNA adducts are initiators of many prevalent types of human cancer. These findings and other discoveries led to the recognition that reducing the levels of estrogen-DNA adducts could prevent the initiation of human cancer. The dietary supplements N-acetylcysteine and resveratrol inhibit formation of estrogen-DNA adducts in cultured human breast cells and in women. These results suggest that the two supplements offer an approach to reducing the risk of developing various prevalent types of human cancer. Graphical abstract Major metabolic pathway in cancer initiation by estrogens.

Sustained proliferation in cancer: Mechanisms and novel therapeutic targets

Proliferation is an important part of cancer development and progression. This is manifest by altered expression and/or activity of cell cycle related proteins. Constitutive activation of many signal transduction pathways also stimulates cell growth. Early steps in tumor development are associated with a fibrogenic response and the development of a hypoxic environment which favors the survival and proliferation of cancer stem cells. Part of the survival strategy of cancer stem cells may manifested by alterations in cell metabolism. Once tumors appear, growth and metastasis may be supported by overproduction of appropriate hormones (in hormonally dependent cancers), by promoting angiogenesis, by undergoing epithelial to mesenchymal transition, by triggering autophagy, and by taking cues from surrounding stromal cells. A number of natural compounds (e.g., curcumin, resveratrol, indole-3-carbinol, brassinin, sulforaphane, epigallocatechin-3-gallate, genistein, ellagitannins, lycopene and quercetin) have been found to inhibit one or more pathways that contribute to proliferation (e.g., hypoxia inducible factor 1, nuclear factor kappa B, phosphoinositide 3 kinase/Akt, insulin-like growth factor receptor 1, Wnt, cell cycle associated proteins, as well as androgen and estrogen receptor signaling). These data, in combination with bioinformatics analyses, will be very important for identifying signaling pathways and molecular targets that may provide early diagnostic markers and/or critical targets for the development of new drugs or drug combinations that block tumor formation and progression.

A Perspective on Prostate Carcinogenesis and Chemoprevention

In this perspective, modifiable carcinogenic factors for the prostate are summarized. This is followed by a discussion of how current knowledge about causation of prostate cancer and chemoprevention of prostate cancer can be used to develop preventive strategies. Prostate cancer is a slowly developing cancer which offers opportunities for preventive interventions. Only a few randomized clinical trials of prostate cancer prevention have been completed. The SELECT study with selenium and vitamin E did not find protective effects, but in two trials with 5α-reductase inhibitors risk was reduced about 25%, showing that chemoprevention is possible and indicating that the androgen receptor is a suitable target. Besides smoking cessation and reduction of obesity, there are no known dietary or life style interventions that will have a major impact on prostate cancer risk. Inflammation of the prostate is an attractive target and aspirin may be a promising candidate agent, but has not been addressed yet in preclinical and clinical studies. Antioxidants other than selenium and vitamin E are unlikely to be very effective and data on several dietary supplements are not encouraging. More candidate agents need to be identified and tested in relevant and adequate preclinical models and Phase II trials that have predictive value for outcome of Phase III randomized studies. Doing this will require a systematic approach comparing preclinical and clinical study outcomes to determine their predictive value of preventive efficacy.

The molecular etiology and prevention of estrogen-initiated cancers: Ockham's Razor: Pluralitas non est ponenda sine necessitate. Plurality should not be posited without necessity

Elucidation of estrogen carcinogenesis required a few fundamental discoveries made by studying the mechanism of carcinogenesis of polycyclic aromatic hydrocarbons (PAH). The two major mechanisms of metabolic activation of PAH involve formation of radical cations and diol epoxides as ultimate carcinogenic metabolites. These intermediates react with DNA to yield two types of adducts: stable adducts that remain in DNA unless removed by repair and depurinating adducts that are lost from DNA by cleavage of the glycosyl bond between the purine base and deoxyribose. The potent carcinogenic PAH benzo[a]pyrene, dibenzo[a,l]pyrene, 7,12-dimethylbenz[a]anthracene and 3-methylcholanthrene predominantly form depurinating DNA adducts, leaving apurinic sites in the DNA that generate cancer-initiating mutations. This was discovered by correlation between the depurinating adducts formed in mouse skin by treatment with benzo[a]pyrene, dibenzo[a,l]pyrene or 7,12-dimethylbenz[a]anthracene and the site of mutations in the Harvey-ras oncogene in mouse skin papillomas initiated by one of these PAH. By applying some of these fundamental discoveries in PAH studies to estrogen carcinogenesis, the natural estrogens estrone (E1) and estradiol (E2) were found to be mutagenic and carcinogenic through formation of the depurinating estrogen-DNA adducts 4-OHE1(E2)-1-N3Ade and 4-OHE1(E2)-1-N7Gua. These adducts are generated by reaction of catechol estrogen quinones with DNA, analogously to the DNA adducts obtained from the catechol quinones of benzene, naphthalene, and the synthetic estrogens diethylstilbestrol and hexestrol. This is a weak mechanism of cancer initiation. Normally, estrogen metabolism is balanced and few estrogen-DNA adducts are formed. When estrogen metabolism becomes unbalanced, more catechol estrogen quinones are generated, resulting in higher levels of estrogen-DNA adducts, which can be used as biomarkers of unbalanced estrogen metabolism and, thus, cancer risk. The ratio of estrogen-DNA adducts to estrogen metabolites and conjugates has repeatedly been found to be significantly higher in women at high risk for breast cancer, compared to women at normal risk. These results indicate that formation of estrogen-DNA adducts is a critical factor in the etiology of breast cancer. Significantly higher adduct ratios have been observed in women with breast, thyroid or ovarian cancer. In the women with ovarian cancer, single nucleotide polymorphisms in the genes for two enzymes involved in estrogen metabolism indicate risk for ovarian cancer. When polymorphisms produce high activity cytochrome P450 1B1, an activating enzyme, and low activity catechol-O-methyltransferase, a protective enzyme, in the same woman, she is almost six times more likely to have ovarian cancer. These results indicate that formation of estrogen-DNA adducts is a critical factor in the etiology of ovarian cancer. Significantly higher ratios of estrogen-DNA adducts to estrogen metabolites and conjugates have also been observed in men with prostate cancer or non-Hodgkin lymphoma, compared to healthy men without cancer. These results also support a critical role of estrogen-DNA adducts in the initiation of cancer. Starting from the perspective that unbalanced estrogen metabolism can lead to increased formation of catechol estrogen quinones, their reaction with DNA to form adducts, and generation of cancer-initiating mutations, inhibition of estrogen-DNA adduct formation would be an effective approach to preventing a variety of human cancers. The dietary supplements resveratrol and N-acetylcysteine can act as preventing cancer agents by keeping estrogen metabolism balanced. These two compounds can reduce the formation of catechol estrogen quinones and/or their reaction with DNA. Therefore, resveratrol and N-acetylcysteine provide a widely applicable, inexpensive approach to preventing many of the prevalent types of human cancer.

BRCA1 deficiency exacerbates estrogen-induced DNA damage and genomic instability

Germline mutations in BRCA1 predispose carriers to a high incidence of breast and ovarian cancers. BRCA1 functions to maintain genomic stability through critical roles in DNA repair, cell-cycle arrest, and transcriptional control. A major question has been why BRCA1 loss or mutation leads to tumors mainly in estrogen-regulated tissues, given that BRCA1 has essential functions in all cell types. Here, we report that estrogen and estrogen metabolites can cause DNA double-strand breaks (DSB) in estrogen receptor-α-negative breast cells and that BRCA1 is required to repair these DSBs to prevent metabolite-induced genomic instability. We found that BRCA1 also regulates estrogen metabolism and metabolite-mediated DNA damage by repressing the transcription of estrogen-metabolizing enzymes, such as CYP1A1, in breast cells. Finally, we used a knock-in human cell model with a heterozygous BRCA1 pathogenic mutation to show how BRCA1 haploinsufficiency affects these processes. Our findings provide pivotal new insights into why BRCA1 mutation drives the formation of tumors in estrogen-regulated tissues, despite the general role of BRCA1 in DNA repair in all cell types.

Unbalanced estrogen metabolism in ovarian cancer

Greater exposure to estrogens is a risk factor for ovarian cancer. To investigate the role of estrogens in ovarian cancer, a spot urine sample and a saliva sample were obtained from 33 women with ovarian cancer and 34 age-matched controls. Thirty-eight estrogen metabolites, conjugates and DNA adducts were analyzed in the urine samples using ultraperformance liquid chromatography/tandem mass spectrometry, and the ratio of adducts to metabolites and conjugates was calculated for each sample. The ratio of depurinating estrogen-DNA adducts to estrogen metabolites and conjugates was significantly higher in cases compared to controls (p < 0.0001), demonstrating high specificity and sensitivity. DNA was purified from the saliva samples and analyzed for genetic polymorphisms in the genes for two estrogen-metabolizing enzymes. Women with two low-activity alleles of catechol-O-methyltransferase plus one or two high-activity alleles of cytochrome P450 1B1 had higher levels of estrogen-DNA adducts and were more likely to have ovarian cancer. These findings indicate that estrogen metabolism is unbalanced in ovarian cancer and suggest that formation of estrogen-DNA adducts plays a critical role in the initiation of ovarian cancer.

Reduced formation of depurinating estrogen-DNA adducts by sulforaphane or KEAP1 disruption in human mammary epithelial MCF-10A cells

Sulforaphane (SFN) is a potent inducer of detoxication enzymes such as NAD(P)H:quinone oxidoreductase 1 (NQO1) and glutathione-S-transferase (GST) via the Kelch-like erythroid-derived protein with CNC homology-associated protein 1 (Keap1)-NF-E2-related factor 2 (Nrf2) signaling pathway. NQO1 reduces the carcinogenic estrogen metabolite, catechol estrogen-3,4-quinone, whereas GSTs detoxify it through conjugation with glutathione. These 3,4-quinones can react with DNA to form depurinating DNA adducts. Thus, SFN may alter estrogen metabolism and thus protect against estrogen-mediated DNA damage and carcinogenesis. Human breast epithelial MCF-10A cells were treated with either vehicle or SFN and either estradiol (E2) or its metabolite 4-hydroxyestradiol (4-OHE2). 4-Hydroxy-derived estrogen metabolites and depurinating DNA adducts formed from E2 and its interconvertable metabolite estrone (E1) were analyzed by mass spectrometry. Levels of the depurinated adducts, 4-OHE1/2-1-N3Adenine and 4-OHE1/2-1-N7Guanine, were reduced by 60% in SFN-treated cells, whereas levels of 4-OCH3E1/2 and 4-OHE1/2-glutathione conjugates increased. To constitutively enhance the expression of Nrf2-regulated genes, cells were treated with either scrambled or siKEAP1 RNA. Following E2 or 4-OHE2 treatments, levels of the adenine and guanine adducts dropped 60-70% in siKEAP1-treated cells, whereas 4-OHE1/2-glutathione conjugates increased. However, 4-OCH3E1/2 decreased 50% after siKEAP1 treatment. Thus, treatment with SFN or siKEAP1 has similar effects on reduction of depurinating estrogen-DNA adduct levels following estrogen challenge. However, these pharmacologic and genetic approaches have different effects on estrogen metabolism to O-methyl and glutathione conjugates. Activation of the Nrf2 pathway, especially elevated NQO1, may account for some but not all of the protective effects of SFN against estrogen-mediated DNA damage.

Unbalanced estrogen metabolism in thyroid cancer

Well-differentiated thyroid cancer most frequently occurs in premenopausal women. Greater exposure to estrogens may be a risk factor for thyroid cancer. To investigate the role of estrogens in thyroid cancer, a spot urine sample was obtained from 40 women with thyroid cancer and 40 age-matched controls. Thirty-eight estrogen metabolites, conjugates and DNA adducts were analyzed by using ultraperformance liquid chromatography/tandem mass spectrometry and the ratio of adducts to metabolites and conjugates was calculated for each sample. The ratio of depurinating estrogen-DNA adducts to estrogen metabolites and conjugates significantly differed between cases and controls (p < 0.0001), demonstrating high specificity and sensitivity. These findings indicate that estrogen metabolism is unbalanced in thyroid cancer and suggest that formation of estrogen-DNA adducts might play a role in the initiation of thyroid cancer.

Evaluation of serum estrogen-DNA adducts as potential biomarkers for breast cancer risk

This study was conducted to determine whether the ratio of estrogen-DNA adducts to their respective metabolites and conjugates in serum differed between women with early-onset breast cancer and those with average or high risk of developing breast cancer. Serum samples from women at average risk (n=63) or high risk (n=80) for breast cancer (using Gail model) and women newly diagnosed with early breast cancer (n=79) were analyzed using UPLC-MS/MS. Adduct ratios were statistically compared among the three groups, and the Area Under the Receiver Operating Characteristic Curve (AUC) was used to identify a diagnostic cut-off point. The median adduct ratio in the average-risk group was significantly lower than that of both the high-risk group and the breast cancer group (p values<0.0001), and provided good discrimination between those at average versus high risk of breast cancer (AUC=0.84, 95% CI 0.77-0.90). Sensitivity and specificity were maximized at an adduct ratio of 77. For women in the same age and BMI group, the odds of being at high risk for breast cancer was 8.03 (95% CI 3.46-18.7) times higher for those with a ratio of at least 77 compared to those with a ratio less than 77. The likelihood of being at high risk for breast cancer was significantly increased for those with a high adduct ratio relative to those with a low adduct ratio. These findings suggest that estrogen-DNA adducts deserve further study as potential biomarkers for risk of developing breast cancer.

A perspective on the role of estrogen in hormone-induced prostate carcinogenesis

Androgens are thought to cause prostate cancer, but the precise mechanisms by which they do so are unclear. Data, mostly from animal studies, suggest that for androgens to cause prostate cancer they must be aromatized to estrogen and act in concert with these estrogen metabolites. Androgen-receptor mediated activity of androgens and estrogen receptor-mediated effects of estrogen metabolites are likely to be necessary, but estrogen genotoxicity appears to be a probable critical factor as well. Only when all these mechanisms are active, may prostate carcinogenesis result. Convincing proof-of-concept studies are needed to definitively test this concept which, if proven, may lead to clinically feasible chemoprevention approaches interfering with these mechanisms.

Effect of tumour necrosis factor-alpha on estrogen metabolic pathways in breast cancer cells

Tumor necrosis factor-alpha (TNF-α) is a proinflammatory cytokine that has been linked to breast cancer development. Estrogen metabolic pathway is also involved in breast carcinogenesis and DNA adducts formation. In this study we investigated the effect of TNF-α on the estrogen metabolic pathway in MCF-7, a breast cancer cell line. Capillary liquid chromatography/mass spectrometry (LC/MS) and High performance liquid chromatography (HPLC) were used for analysis of estrogen metabolites and estrogen-DNA adducts levels respectively. Reporter gene assay, Real time reverse transcription polymerase chain reaction (real time RT-PCR) and Western blot were used to assess the expression of estrogen metabolizing genes and enzymes. TNF-α significantly increased the total EM and decreased the estrone (E1) / 17-β estradiol (E2) ratio. Moreover, it altered the expression of genes and enzymes involved in E2 activation and deactivation pathways e.g. Cytochrome P-450 1A1 (CYP1A1), Cytochrome P-450 1B1 (CYP1B1), Catechol-O-methyl transferase (COMT) and Nicotinamide adenine dinucleotide phosphate-quinone oxidoreductase 1 (NQO1). In addition, there were increased levels of some catechol estrogens e.g. 4-hydroxy-estrone (4-OHE1) and 2-hydroxyestradiol (2-OHE2) with decreased levels of methylated catechols e.g. 2-methoxy estradiol (2-MeOE2). DNA adducts especially 4-OHE1-[2]-1-N3 Adenine was significantly increased. TNF-α directs the estrogen metabolism into more hormonally active and carcinogenic products in MCF-7. This may implicate a new possible explanation for inflammation associated breast cancer.

Estrogen metabolism and risk of breast cancer in postmenopausal women

BACKGROUND

Estrogens are recognized causal factors in breast cancer. Interindividual variation in estrogen metabolism may also influence the risk of breast cancer and could provide clues to mechanisms of breast carcinogenesis. Long-standing hypotheses about how estrogen metabolism might influence breast cancer have not been adequately evaluated in epidemiological studies because of the lack of accurate, reproducible, and high-throughput assays for estrogen metabolites.

METHODS

We conducted a prospective case-control study nested within the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial (PLCO). Participants included 277 women who developed invasive breast cancer (case subjects) and 423 matched control subjects; at PLCO baseline, all subjects were aged 55-74 years, postmenopausal and not using hormone therapy, and provided a blood sample. Liquid chromatography-tandem mass spectrometry was used to measure serum concentrations of 15 estrogens and estrogen metabolites, in unconjugated and conjugated forms, including the parent estrogens, estrone and estradiol, and estrogen metabolites in pathways defined by irreversible hydroxylation at the C-2, C-4, or C-16 positions of the steroid ring. We calculated hazard ratios (HRs) approximating risk in highest vs lowest deciles of individual estrogens and estrogen metabolites, estrogens and estrogen metabolites grouped by metabolic pathways, and metabolic pathway ratios using multivariable Cox proportional hazards models. All statistical tests were two-sided.

RESULTS

Nearly all estrogens, estrogen metabolites, and metabolic pathway groups were associated with an increased risk of breast cancer; the serum concentration of unconjugated estradiol was strongly associated with the risk of breast cancer (HR = 2.07, 95% confidence interval [CI] = 1.19 to 3.62). No estrogen, estrogen metabolite, or metabolic pathway group remained statistically significantly associated with the risk of breast cancer after adjusting for unconjugated estradiol. The ratio of the 2-hydroxylation pathway to parent estrogens (HR = 0.66, 95% CI = 0.51 to 0.87) and the ratio of 4-hydroxylation pathway catechols to 4-hydroxylation pathway methylated catechols (HR = 1.34, 95% CI = 1.04 to 1.72) were statistically significantly associated with the risk of breast cancer and remained so after adjustment for unconjugated estradiol.

CONCLUSIONS

More extensive 2-hydroxylation of parent estrogens is associated with lower risk, and less extensive methylation of potentially genotoxic 4-hydroxylation pathway catechols is associated with higher risk of postmenopausal breast cancer.

The etiology and prevention of breast cancer

Metabolism of estrogens via the catechol estrogen pathway is characterized by a balanced set of activating and protective enzymes (homeostasis). Disruption of homeostasis, with excessive production of catechol estrogen quinones, can lead to reaction of these quinones with DNA to form depurinating estrogen-DNA adducts. Some of the mutations generated by these events can lead to initiation of breast cancer. A wealth of evidence, from studies of metabolism, mutagenicity, cell transformation and carcinogenicity, demonstrates that estrogens are genotoxic. Women at high risk for breast cancer, or diagnosed with the disease, have relatively high levels of depurinating estrogen-DNA adducts compared to normal-risk women. The dietary supplements N-acetylcysteine and resveratrol can inhibit formation of catechol estrogen quinones and their reaction with DNA to form estrogen-DNA adducts, thereby preventing initiation of breast cancer.

Mechanism of DNA depurination by carcinogens in relation to cancer initiation

Depurinating DNA adducts formed by aromatic hydrocarbons and catechol estrogen quinones play a major role in cancer initiation. Most of these adducts depurinate instantaneously, but some guanine adducts depurinate from DNA with half-lives of hours. We report here, that after 10 h at 37 °C, reaction of estradiol-3,4-quinone (E(2)-3,4-Q) with ds-DNA to yield N7Gua and N3Ade adducts was complete and more efficient than with ss-DNA. When E(2)-3,4-Q reacted with t-RNA, no adducts were detected after 10 h, and the level of N3Ade and N7Gua adducts after 10 days was less than half that with ss-DNA after 10 h. Reaction of E(2)-3,4-Q and dG yielded 4-OHE(2)-1-N7dG, which spontaneously depurinated to yield 4-OHE(2)-1-N7Gua. To investigate the mechanism of depurination, E(2)-3,4-Q was reacted with carbocyclicdeoxyguanosine, in which the ring oxygen of the deoxyribose moiety is substituted with CH(2) , and depurination was observed. The results from this experiment demonstrate that the oxocarbenium ion mechanism plays the major role in depurination and provides the first experimental evidence for this mechanism. A newly discovered β-elimination mechanism also plays a minor role in depurination. Understanding why the depurinating estrogen-DNA adducts come from DNA, and not from RNA, underscores the critical role that these adducts play in initiating cancer.

Hormones and prostate carcinogenesis: Androgens and estrogens

Prostate cancer is the leading non-skin malignancy detected in US males and the second cause of death due to male cancer in the US. Androgenic hormones are generally believed to be causatively associated with prostate carcinogenesis, but human evidence, mostly epidemiological, for this is minimal. Circulating hormone levels are not associated with the risk of prostate cancer and neither are polymorphisms in various genes encoding the androgen metabolizing enzymes or androgen receptors. Evidence in support of the involvement of androgens in prostate cancer development is derived from clinical trials with 5α-reductase inhibitors, which reduced the risk by approximately 25%. Animal studies using rat models, however, provide clear evidence that testosterone can induce prostate cancer and can act as a strong tumor promoter in concert with genotoxic carcinogens. One such genotoxic factor may be 17β-estradiol, which is generated from testosterone by the aromatase enzyme. Estradiol can be converted to catecholestrogens, which through redox cycling, generate reactive metabolites that can adduct the DNA and potentially lead to mutations. Animal studies and limited human evidence suggest that estrogens can be involved in prostate carcinogenesis by such a genotoxic mechanism. However, how androgens exert their tumor-promoting effect is not clear. It is likely that hormonal and non-hormonal factors as well as genetic and non-genetic (environmental) factors interact in a highly complex and poorly understood manner to determine the risk of prostate cancer.

Formation of dopamine quinone-DNA adducts and their potential role in the etiology of Parkinson's disease

The neurotransmitter dopamine is oxidized to its quinone (DA-Q), which at neutral pH undergoes intramolecular cyclization by 1,4-Michael addition, followed by oxidation to form leukochrome, then aminochrome, and finally neuromelanin. At lower pH, the amino group of DA is partially protonated, allowing the competitive intermolecular 1,4-Michael addition with nucleophiles in DNA to form the depurinating adducts, DA-6-N3Ade and DA-6-N7Gua. Catechol estrogen-3,4-quinones react by 1,4-Michael addition to form the depurinating 4-hydroxyestrone(estradiol)-1-N3Ade [4-OHE1(E2)-1-N3Ade] and 4-OHE1(E2)-1-N7Gua adducts, which are implicated in the initiation of breast and other human cancers. The effect of pH was studied by reacting tyrosinase-activated DA with DNA and measuring the formation of depurinating adducts. The most adducts were formed at pH 4, 5, and 6, and their level was nominal at pH 7 and 8. The N3Ade adduct depurinated instantaneously, but N7Gua had a half-life of 3 H. The slow loss of the N7Gua adduct is analogous to that observed in previous studies of natural and synthetic estrogens. The antioxidants N-acetylcysteine and resveratrol efficiently blocked formation of the DA-DNA adducts. Thus, slightly acidic conditions render competitive the reaction of DA-Q with DNA to form depurinating adducts. We hypothesize that formation of these adducts could lead to mutations that initiate Parkinson's disease. If so, use of N-acetylcysteine and resveratrol as dietary supplements may prevent initiation of this disease.

Formation of diethylstilbestrol-DNA adducts in human breast epithelial cells and inhibition by resveratrol

Extensive evidence exists that the reaction of estrogen metabolites with DNA produces depurinating adducts that, in turn, induce mutations and cellular transformation. While it is clear that these estrogen metabolites result in a neoplastic phenotype in vitro, further evidence supporting the link between estrogen-DNA adduct formation and its role in neoplasia induction in vivo would strengthen the evidence for a genotoxic mechanism. Diethylstilbestrol (DES), an estrogen analogue known to increase the risk of breast cancer in women exposed in utero, is hypothesized to induce neoplasia through a similar genotoxic mechanism. Cultured MCF-10F human breast epithelial cells were treated with DES at varying concentrations and for various times to determine whether the addition of DES to MCF-10F cells resulted in the formation of depurinating adducts. This is the first demonstration of the formation of DES-DNA adducts in human breast cells. A dose-dependent increase in DES-DNA adducts was observed. Demonstrating that treatment of MCF-10F cells with DES, a known human carcinogen, yields depurinating adducts provides further support for the involvement of these adducts in the induction of breast neoplasia. Previous studies have demonstrated the ability of antioxidants such as resveratrol to prevent the formation of estrogen-DNA adducts, thus preventing a key carcinogenic event. In this study, when MCF-10F cells were treated with a combination of resveratrol and DES, a dose-dependent reduction in the level of DES-DNA adducts was also observed.

Structural determinant of chemical reactivity and potential health effects of quinones from natural products

Although many phenols and catechols found as polyphenol natural products are antioxidants and have putative disease-preventive properties, others have deleterious health effects. One possible route to toxicity is the bioactivation of the phenolic function to quinones that are electrophilic, redox-agents capable of modifying DNA and proteins. The structure-property relationships of biologically important quinones and their precursors may help understand the balance between their health benefits and risks. We describe a mass-spectrometry-based study of four quinones produced by oxidizing flavanones and flavones. Those with a C2-C3 double bond on ring C of the flavonoid stabilize by delocalization of an incipient positive charge from protonation and render the protonated quinone particularly susceptible to nucleophilic attack. We hypothesize that the absence of this double bond is one specific structural determinant that is responsible for the ability of quinones to modify biological macromolecules. Those quinones containing a C2-C3 single bond have relatively higher aqueous stability and longer half-lives than those with a double bond at the same position; the latter have short half-lives at or below ∼1 s. Quinones with a C2-C3 double bond show little ability to depurinate DNA because they are rapidly hydrated to unreactive species. Molecular-orbital calculations support that quinone hydration by a highly structure-dependent mechanism accounts for their chemical properties. The evidence taken together support a hypothesis that those flavonoids and related natural products that undergo oxidation to quinones and are then rapidly hydrated are unlikely to damage important biological macromolecules.

Unbalanced metabolism of endogenous estrogens in the etiology and prevention of human cancer

Among the numerous small molecules in the body, the very few aromatic ones include the estrogens and dopamine. In relation to cancer initiation, the estrogens should be considered as chemicals, not as hormones. Metabolism of estrogens is characterized by two major pathways. One is hydroxylation to form the 2- and 4-catechol estrogens, and the second is hydroxylation at the 16α position. In the catechol pathway, the metabolism involves further oxidation to semiquinones and quinones, including formation of the catechol estrogen-3,4-quinones, the major carcinogenic metabolites of estrogens. These electrophilic compounds react with DNA to form the depurinating adducts 4-OHE(1)(E(2))-1-N3Ade and 4-OHE(1)(E(2))-1-N7Gua. The apurinic sites obtained by this reaction generate the mutations that may lead to the initiation of cancer. Oxidation of catechol estrogens to their quinones is normally in homeostasis, which minimizes formation of the quinones and their reaction with DNA. When the homeostasis is disrupted, excessive amounts of catechol estrogen quinones are formed and the resulting increase in depurinating DNA adducts can lead to initiation of cancer. Substantial evidence demonstrates the mutagenicity of the estrogen metabolites and their ability to induce transformation of mouse and human breast epithelial cells, and tumors in laboratory animals. Furthermore, women at high risk for breast cancer or diagnosed with the disease, men with prostate cancer, and men with non-Hodgkin lymphoma all have relatively high levels of estrogen-DNA adducts, compared to matched control subjects. Specific antioxidants, such as N-acetylcysteine and resveratrol, can block the oxidation of catechol estrogens to their quinones and their reaction with DNA. As a result, the initiation of cancer can be prevented.

Resveratrol and N-acetylcysteine block the cancer-initiating step in MCF-10F cells

Substantial evidence suggests that catechol estrogen-3,4-quinones react with DNA to form predominantly the depurinating adducts 4-hydroxyestrone (estradiol)-1-N3Ade [4-OHE(1)(E(2))-1-N3Ade] and 4-OHE(1)(E(2))-1-N7Gua. Apurinic sites resulting from these adducts generate critical mutations that can initiate cancer. The paradigm of cancer initiation is based on an imbalance in estrogen metabolism between activating pathways that lead to estrogen-DNA adducts and deactivating pathways that lead to estrogen metabolites and conjugates. This imbalance can be improved to minimize formation of adducts by using antioxidants, such as resveratrol (Resv) and N-acetylcysteine (NAcCys). To compare the ability of Resv and NAcCys to block formation of estrogen-DNA adducts, we used the human breast epithelial cell line MCF-10F treated with 4-OHE(2). Resv and NAcCys directed the metabolism of 4-OHE(2) toward protective pathways. NAcCys reacted with the quinones and reduced the semiquinones to catechols. This pathway was also carried out by Resv. In addition, Resv induced the protective enzyme quinone reductase, which reduces E(1)(E(2))-3,4-quinones to 4-OHE(1)(E(2)). Resv was more effective at increasing the amount of 4-OCH(3)E(1)(E(2)) than NAcCys. Inhibition of estrogen-DNA adduct formation was similar at lower doses, but at higher doses Resv was about 50% more effective than NAcCys. Their combined effects were additive. Therefore, these two antioxidants provide an excellent combination to protect catechol estrogens from oxidation to catechol quinones.

Poly (ADP-ribose) polymerase as a novel therapeutic target in cancer

Cancer chemotherapy exploits limitations in repairing DNA damage in order to kill proliferating malignant cells. Recent evidence suggests that cancers within and across tissue types have specific defects in DNA repair pathways, and that these defects may predispose for sensitivity and resistance to various classes of cytotoxic agents. Poly (ADP-ribose) polymerase (PARP) and BRCA proteins are central to the repair of DNA strand breaks and, when defective, lead to the accumulation of mutations introduced by error-prone DNA repair. Breast, ovarian, and other cancers develop in the setting of BRCA deficiency, and these cancers may be more sensitive to cytotoxic agents that induce DNA strand breaks, as well as inhibitors of PARP activity. A series of recent clinical trials has tested whether PARP inhibitors can achieve synthetic lethality in BRCA-pathway-deficient tumors. Future studies must seek to identify sporadic cancers that harbor genomic instability, rendering susceptibility to agents that induce additional and lethal DNA damage.

N-acetylcysteine blocks formation of cancer-initiating estrogen-DNA adducts in cells

Catechol estrogens, especially 4-hydroxylated metabolites of 17beta-estradiol (E(2)), are responsible for estrogen-induced carcinogenesis. 4-Hydroxyestradiol (4-OHE(2)), a major metabolite of E(2) formed preferentially by cytochrome P-450 1B1, is oxidized to E(2)-3,4-quinone, which can react with DNA to yield the depurinating adducts 4-OHE(2)-1-N3Ade and 4-OHE(2)-1-N7Gua. The apurinic sites generated by the loss of these depurinating adducts induce mutations that could lead to cancer initiation. In this study, we have evaluated the effects of N-acetylcysteine (NAcCys) on the metabolism of two cell lines, MCF-10F (a normal human breast epithelial cell line) and E6 (a normal mouse mammary epithelial cell line), treated with 4-OHE(2) or its reactive metabolite, E(2)-3,4-quinone. Extensive HPLC with electrochemical detection and UPLC-MS/MS analyses of the cell media demonstrated that the presence of NAcCys very efficiently shifted the estrogen metabolism toward protective methoxylation and conjugation pathways in multiple ways, whereas formation of depurinating DNA adducts was inhibited. Protection by NAcCys seems to be similar in both cell lines, irrespective of their origin (human or mouse) or the presence of estrogen receptor-alpha. This finding suggests that NAcCys, a common dietary supplement, could be used as a potential chemopreventive agent to block the initial step in the genotoxicity caused by catechol estrogen quinones.

Depurinating estrogen-DNA adducts in the etiology and prevention of breast and other human cancers

Experiments on estrogen metabolism, formation of DNA adducts, mutagenicity, cell transformation and carcinogenicity have led to and supported the hypothesis that the reaction of specific estrogen metabolites, mostly the electrophilic catechol estrogen-3,4-quinones, with DNA can generate the critical mutations to initiate breast and other human cancers. Analysis of depurinating estrogen-DNA adducts in urine demonstrates that women at high risk of, or with breast cancer, have high levels of the adducts, indicating a critical role for adduct formation in breast cancer initiation. Men with prostate cancer or non-Hodgkin lymphoma also have high levels of estrogen-DNA adducts. This knowledge of the first step in cancer initiation suggests the use of specific antioxidants that can block formation of the adducts by chemical and biochemical mechanisms. Two antioxidants, N-acetylcysteine and resveratrol, are prime candidates to prevent breast and other human cancers because in various M in vitro and in vivo experiments, they reduce the formation of estrogen-DNA adducts.

Urinary biomarkers suggest that estrogen-DNA adducts may play a role in the aetiology of non-Hodgkin lymphoma

A variety of evidence suggests that estrogens may induce non-Hodgkin lymphoma (NHL). The reaction of catechol estrogen quinones with DNA to form depurinating estrogen-DNA adducts is hypothesized to initiate this process. These adducts are released from DNA, shed from cells into the bloodstream and excreted in urine. The aim of this study was to determine whether or not the depurinating estrogen-DNA adducts might be involved in the aetiology of human NHL. Estrogen metabolites, conjugates and depurinating DNA adducts were identified and quantified in spot urine samples from 15 men with NHL and 30 healthy control men by using ultraperformance liquid chromatography/tandem mass spectrometry. The levels of estrogen-DNA adducts were significantly higher in the men with NHL than in the healthy control men. Thus, formation of estrogen-DNA adducts may play a critical role in the aetiology of NHL, and these adducts could be potential biomarkers of NHL risk.

Benzene and dopamine catechol quinones could initiate cancer or neurogenic disease

Catechol quinones of estrogens react with DNA by 1,4-Michael addition to form depurinating N3Ade and N7Gua adducts. Loss of these adducts from DNA creates apurinic sites that can generate mutations leading to cancer initiation. We compared the reactions of the catechol quinones of the leukemogenic benzene (CAT-Q) and N-acetyldopamine (NADA-Q) with 2'-deoxyguanosine (dG) or DNA. NADA was used to prevent intramolecular cyclization of dopamine quinone. Reaction of CAT-Q or NADA-Q with dG at pH 4 afforded CAT-4-N7dG or NADA-6-N7dG, which lost deoxyribose with a half-life of 3 h to form CAT-4-N7Gua or 4 h to form NADA-6-N7Gua. When CAT-Q or NADA-Q was reacted with DNA, N3Ade adducts were formed and lost from DNA instantaneously, whereas N7Gua adducts were lost over several hours. The maximum yield of adducts in the reaction of CAT-Q or NADA-Q with DNA at pH 4 to 7 was at pH 4. When tyrosinase-activated CAT or NADA was reacted with DNA at pH 5 to 8, adduct levels were much higher (10- to 15-fold), and the highest yield was at pH 5. Reaction of catechol quinones of natural and synthetic estrogens, benzene, naphthalene, and dopamine with DNA to form depurinating adducts is a common feature that may lead to initiation of cancer or neurodegenerative disease.

NAD(P)H:quinone oxidoreductase 1 Arg139Trp and Pro187Ser polymorphisms imbalance estrogen metabolism towards DNA adduct formation in human mammary epithelial cells

Estrogens (estrone, E(1); estradiol, E(2)) are oxidized in the breast first to catechols and then to form two ortho-quinones (E(1/2)-3,4-Q) that react with DNA to form depurinating adducts, which lead to mutations associated with breast cancer. NAD(P)H:quinone oxidoreductase 1 (NQO1) reduces these quinones back to catechols, and thus may protect against this mechanism. We examined whether the inheritance of two polymorphic variants of NQO1 (Pro187Ser or Arg139Trp) would result in poor reduction of E(1/2)-3,4-Q in normal human mammary epithelial cells (MCF-10F) and increased depurinating adduct formation. An isogenic set of stably transfected normal human breast epithelial cells (MCF-10F) that express a truncated (135Stop), the wild-type, the 139Trp variant or the 187Ser variant of human NQO1 cDNA was constructed. MCF-10F cells showed a low endogenous NQO1 activity. NQO1 expression was examined by RT-PCR and Western blotting, and catalytic activity of reducing E(2)-3,4-Q to 4-hydroxyE(1/2) and associated changes in the levels of quinone conjugates (4-methoxyE(1/2), 4-OHE(1/2)-2-glutathione, 4-OHE(1/2)-2-Cys and 4-OHE(1/2)-2-N-acetylcysteine) and depurinating DNA adducts (4-OHE(1/2)-1-N3Ade and 4-OHE(1/2)-1-N7Gua) were examined by HPLC with electrochemical detection, as well as by ultra-performance liquid chromatography with tandem mass spectrometry. The polymorphic variants transcribed comparably to the wild-type NQO1, but produced approximately 2-fold lower levels of the protein, suggesting that the variant proteins may become degraded. E(1/2)-3,4-Q toxicity to MCF-10F cells (IC50=24.74 microM) was increased (IC50=3.7 microM) by Ro41-0960 (3 microM), a catechol-O-methyltransferase inhibitor. Cells expressing polymorphic NQO1 treated with E(2)-3,4-Q with or without added Ro41-0960, showed lower ability to reduce the quinone ( approximately 50% lower levels of the free catechols and approximately 3-fold lower levels of methylated catechols) compared to the wild-type enzyme. The increased availability of the quinones in these cells did not result in greater glutathione conjugation. Instead, there was increased (2.5-fold) formation of the depurinating DNA adducts. Addition of Ro41-0960 increased the amounts of free catechols, quinone conjugates and depurinating DNA adducts. NQO1 polymorphic variants (Arg139Trp and Pro187Ser) were poor reducers of estrogen-3,4-quinones, which caused increased formation of estrogen-DNA adduct formation in MCF-10F cells. Therefore, the inheritance of these NQO1 polymorphisms may favor the estrogen genotoxic mechanism of breast cancer.

Depurinating naphthalene-DNA adducts in mouse skin related to cancer initiation

Naphthalene has been shown to be a weak carcinogen in rats. To investigate its mechanism of metabolic activation and cancer initiation, mice were topically treated with naphthalene or one of its metabolites, 1-naphthol, 1,2-dihydrodiolnaphthalene (1,2-DDN), 1,2-dihydroxynaphthalene (1,2-DHN), and 1,2-naphthoquinone (1,2-NQ). After 4 h, the mice were sacrificed, the treated skin was excised, and the depurinating and stable DNA adducts were analyzed. The depurinating adducts were identified and quantified by ultraperformance liquid chromatography/tandem mass spectrometry, whereas the stable adducts were quantified by (32)P-postlabeling. For comparison, the stable adducts formed when a mixture of the four deoxyribonucleoside monophosphates was treated with 1,2-NQ or enzyme-activated naphthalene were also analyzed. The depurinating adducts 1,2-DHN-1-N3Ade and 1,2-DHN-1-N7Gua arise from reaction of 1,2-NQ with DNA. Similarly, the major stable adducts appear to derive from the 1,2-NQ. The depurinating DNA adducts are, in general, the most abundant. Therefore, naphthalene undergoes metabolic activation to the electrophilic ortho-quinone, 1,2-NQ, which reacts with DNA to form depurinating adducts. This is the same mechanism as other weak carcinogens, such as the natural and synthetic estrogens, and benzene.

Ortho-quinones of benzene and estrogens induce hyperproliferation of human peripheral blood mononuclear cells

Benzene is a known leukemogen. It has been hypothesized that benzene and natural estrogens initiate cancer by forming ortho-quinones (catechol quinones) that react with DNA in cells. These quinones form depurinating DNA adducts that generate the mutations leading to cancer. This study examined whether the treatment of normal human peripheral blood mononuclear cells with the ortho-quinones of benzene or estradiol would form DNA adducts and elicit an alteration in the proliferation of these cells. Both estradiol-3,4-quinone and benzene ortho-quinone formed depurinating DNA adducts and significantly increased the mitogen-induced proliferation of normal blood mononuclear cells. Immunophenotyping of the estradiol-3,4-quinone-treated blood cells indicated that monocyte/macrophage, natural killer and T-cells were particularly prone to hyperproliferation. Thus, DNA damage induced by the ortho-quinones of benzene and estradiol may promote the growth of human blood mononuclear cells, including those that appear in large numbers in leukemia and lymphoma.

Mechanism of metabolic activation and DNA adduct formation by the human carcinogen diethylstilbestrol: the defining link to natural estrogens

Diethylstilbestrol (DES) is a human carcinogen, based on sufficient epidemiological evidence. DES is mainly metabolized to its catechol, 3'-hydroxyDES (3'-OH-DES), which can further oxidize to DES-3',4'-quinone (DES-3',4'-Q). Similarly to estradiol-3,4-quinone, the reaction of DES-3',4'-Q with DNA would form the depurinating 3'-OH-DES-6'-N3Ade and 3'-OH-DES-6'-N7Gua adducts. To prove this hypothesis, synthesis of DES-3',4'-Q by oxidation of 3'-OH-DES with Ag(2)O was tried; this failed due to instantaneous formation of a spiro-quinone. Oxidation of 3'-OH-DES by lactoperoxidase or tyrosinase in the presence of DNA led to the formation of 3'-OH-DES-6'-N3Ade and 3'-OH-DES-6'-N7Gua adducts. These adducts were tentatively identified by LC-MS/MS as 3'-OH-DES-6'-N3Ade, m/z = 418 [M+H](+), and 3'-OH-DES-6'-N7Gua, m/z = 434 [M+H](+). Demonstration of their structures derived from their oxidation by MnO(2) to the DES quinone adducts and subsequent tautomerization to the dienestrol (DIES) catechol adducts, which are identical to the standard 3'-OH-DIES-6'-N3Ade, m/z = 416 [M+H](+), and 3'-OH-DIES-6'-N7Gua, m/z = 432 [M+H](+), adducts. The reaction of DIES-3',4'-Q or lactoperoxidase-activated 3'-OH-DIES with DNA did not produce any depurinating adducts, due to the dienic chain being perpendicular to the phenyl planes, which impedes the intercalation of DIES into the DNA. Enzymic oxidation of 3'-OH-DES suggests that the catechol of DES intercalates into DNA and is then oxidized to its quinone to yield N3Ade and N7Gua adducts. These results suggest that the common denominator of tumor initiation by the synthetic estrogen DES and the natural estrogen estradiol is formation of their catechol quinones, which react with DNA to afford the depurinating N3Ade and N7Gua adducts.

Prevention of oxidative DNA damage by bioactive berry components

The hormone 17ss-estradiol (E(2)) causes oxidative DNA damage via redox cycling of its metabolites such as 4-hydroxy estradiol (4E(2)). In this study, ACI rats (8 wk old) were fed either AIN-93M diet or diets supplemented with 0.5% each of mixed berries (strawberry, blueberry, blackberry, and red and black raspberry), blueberry alone (BB; 2.5%), or ellagic acid (EA; 400 ppm) from 2 wk prior to and up to 12 wk of E(2) treatment. The liver DNA was analyzed for the presence of 8-oxo-7,8-dihydroguanine (8-oxodG) and other polar adducts by 32P-postlabeling. Compared to sham treatment, E(2) significantly increased the levels of both 8-oxodG and P-1 subgroup (259% and 214%, respectively; P< 0.05). EA diet significantly reduced E(2)-induced levels of 8-oxodG, P-1, P-2, and PL-1 by 79, 63, 44, and 67%, respectively (P< 0.001). BB diet also significantly reduced the levels of P-1, P-2, and PL-1 subgroups by 77, 43, and 68%, respectively (P< 0.001). Mixed berries were, however, ineffective. In addition, aqueous extracts of berries (2%) and EA (100 microM) were tested for their efficacy in diminishing oxidative DNA adducts induced by redox cycling of 4E(2) catalyzed by copper chloride in vitro. EA was the most efficacious (90%), followed by extracts of red raspberry (70%), blueberry, and strawberry (50% each; P< 0.001).