Carcinogenicity of catechol estrogens in Syrian hamsters

Metabolism of Endogenous and Exogenous Estrogens in Women

Estrogens regulate important processes in reproductive, skeletal, cardiovascular, and central nervous systems that impact women's overall health. Understanding endogenous and exogenously administered estrogen metabolism is vital to determining therapeutic estrogen levels. The present review provides an overview of estrogen metabolites formed in non-pregnant and pregnant women, and those resulting from exogenous estrogen administration. There are four principal endogenous estrogens: estrone (E1), estradiol (E2), estriol (E3), and estetrol (E4). E4, which is produced only in pregnancy, has emerged recently as an estrogen with significant therapeutic potential. E1, E2, and E3 undergo extensive metabolism primarily through phase I (hydroxylation, oxidation, reduction) and phase II (primarily conjugation) reactions, whereas E4 undergoes only phase II reactions. Exogenous estrogens commonly used for menopausal treatment and/or contraception, including micronized E2, conjugated equine estrogens, and ethinyl estradiol, also undergo phase I and phase II reactions, but differ widely in the types of metabolites formed. The mechanisms by which estrogen metabolites are formed and their excretion in urine, bile, and feces, are still poorly understood. We highlight areas that require further research to foster a better understanding of how estrogen metabolism impacts dosing of estrogens for therapeutic use, as well as the physiological regulation of endogenous estrogens.

Gender Difference in DNA Damage Induced by the Environmental Carcinogen Dibenzo[def,p]chrysene Individually and in Combination with Mouse Papillomavirus Infection in the Mouse Oral Cavity

Tobacco smoking and human papillomavirus infection are established etiological agents in the development of head and neck squamous cell carcinoma (HNSCC). The incidence and mortality of HNSCC are higher in men than women. To provide biochemical basis for sex differences, we tested the hypothesis that carcinogen treatment using dibenzo[def,p]chrysene, which is an environmental pollutant and tobacco smoke constituent, in the absence or presence of the mouse papillomavirus infection results in significantly higher levels of DNA damage in the oral cavity in male than in female mice. However, the results of the present investigation do not support our hypothesis since we found that females were more susceptible to carcinogen-induced covalent DNA damage than males independent of the viral infection. Since DNA damage represents only a single-step in the carcinogenesis process, additional factors may contribute to sex differences in humans.

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.

Targeting Estrogens and Various Estrogen-Related Receptors against Non-Small Cell Lung Cancers: A Perspective

Non-small cell lung cancers (NSCLCs) account for ~85% of lung cancer cases worldwide. Mammalian lungs are exposed to both endogenous and exogenous estrogens. The expression of estrogen receptors (ERs) in lung cancer cells has evoked the necessity to evaluate the role of estrogens in the disease progression. Estrogens, specifically 17β-estradiol, promote maturation of several tissue types including lungs. Recent epidemiologic data indicate that women have a higher risk of lung adenocarcinoma, a type of NSCLC, when compared to men, independent of smoking status. Besides ERs, pulmonary tissues both in healthy physiology and in NSCLCs also express G-protein-coupled ERs (GPERs), epidermal growth factor receptor (EGFRs), estrogen-related receptors (ERRs) and orphan nuclear receptors. Premenopausal females between the ages of 15 and 50 years synthesize a large contingent of estrogens and are at a greater risk of developing NSCLCs. Estrogen-ER/GPER/EGFR/ERR-mediated activation of various cell signaling molecules regulates NSCLC cell proliferation, survival and apoptosis. This article sheds light on the most recent achievements in the elucidation of sequential biochemical events in estrogen-activated cell signaling pathways involved in NSCLC severity with insight into the mechanism of regulation by ERs/GPERs/EGFRs/ERRs. It further discusses the success of anti-estrogen therapies against NSCLCs.

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.

The smoking estrogens - a potential synergy between estradiol and benzo(a)pyrene

According to recent statistics, Lung Cancer (LC) is one of the most frequently diagnosed tumor types, representing nearly 12% of all global cancer cases. Moreover, in recent years, an increased mortality rate and incidence of this cancer were observed, especially among nonsmokers. Lung cancer patients are often characterized by poor prognosis and low survival rates, which encourages the scientific community to investigate the biochemical and molecular processes leading to the development of this malignancy. Furthermore, the mechanisms of LC formation and progression are not yet fully elucidated due to their high complexity, as well as a multitude of environmental, genetic, and molecular factors involved. Even though LC's association with exposure to cigarette smoke is indisputable, current research provides evidence that the development of this cancer can also be affected by the presence of estrogens and their interaction with several tobacco smoke components. Hence, the main goal of this brief review was to investigate reports of a possible synergy between 17β estradiol (E2), the most biologically active estrogen, and benzo(a)pyrene (BaP), a strongly carcinogenic compound produced as a result of incomplete tobacco combustion. The literature sources demonstrate a possible carcinogenic synergy between estrogens, especially E2, and BaP, a toxic tobacco smoke component. Therefeore, the combined effect of disturbed estrogen production in cancer cells, as well as the molecular influence exerted by BaP, could explain the increased aggressiveness and rate of LC development. Summarizing, the synergistic effect of these risk factors is an interesting area of further research.

Role of organophosphorous pesticides and acetylcholine in breast carcinogenesis

Breast cancer is the leading cause of cancer-related death in women worldwide. Several studies have addressed the association between cancer in humans and agricultural pesticide exposure. Evidence indicates that exposure to organophosphorous pesticides such as parathion and malathion occurs as a result of occupational factors since they are extensively used to control insects. On the other hand, estrogens have been considered beneficial to the organism; however, epidemiological studies have pointed out an increased breast cancer risk in both humans and animals. Experimental female rat mammary gland cancer models were developed after exposure to parathion, malathion, eserine, an acetylcholinesterase inhibitor, and estrogen allowing the analysis of the signs of carcinogenicity as alteration of cell proliferation, receptor expression, genomic instability, and cell metabolism in vivo and in vitro. Thus, pesticides increased proliferative ducts followed by ductal carcinoma; and 17β-estradiol increased proliferative lobules followed by lobular carcinomas. The combination of both pesticides and either eserine or estrogen induced tumors with both types of structures followed by mammary gland tumors and metastasis to the lung and kidneys after 240 days of a 5-day treatment. Studies also showed that these pesticides and eserine decreased three to five times the acetylcholinesterase activity in the serum compared to controls whereas terminal end buds increased in number, being inhibited by atropine. Genomic instability was analyzed in such tissues (mp53, CYP1A2, c-myc, c-fos, ERα, M2R) and pesticides increased protein expression that was stimulated by estrogens but inhibited by atropine. Eserine also transformed the epithelium of the rat mammary gland in the presence of estrogen and increased the number of terminal end buds after treatment inducing mammary carcinomas. Then, enzymatic digestion of such structures gave rise to cells with increased DNA synthesis and induced anchorage independence. Thus, there were changes in the epithelium of the mammary gland influencing breast carcinogenesis. Furthermore, these substances and acetylcholine also showed the signs of carcinogenicity in vitro as cell proliferation, receptor expression (ERα, ErbB2, M2R), genomic instability (c-myc, mp53, ERα, M2R), and cell metabolism. A unique cellular model is also presented here based on the use of MCF-10 F, a non-tumorigenic cell line that represents a valuable clinically translatable experimental approach that identifies mechanistic links for pesticides and estrogen as suspect human carcinogenic agents.

Signs of carcinogenicity induced by parathion, malathion, and estrogen in human breast epithelial cells (Review)

Cancer development is a multistep process that may be induced by a variety of compounds. Environmental substances, such as pesticides, have been associated with different human diseases. Organophosphorus pesticides (OPs) are among the most commonly used insecticides. Despite the fact that organophosphorus has been associated with an increased risk of cancer, particularly hormone-mediated cancer, few prospective studies have examined the use of individual insecticides. Reported results have demonstrated that OPs and estrogen induce a cascade of events indicative of the transformation of human breast epithelial cells. In vitro studies analyzing an immortalized non-tumorigenic human breast epithelial cell line may provide us with an approach to analyzing cell transformation under the effects of OPs in the presence of estrogen. The results suggested hormone-mediated effects of these insecticides on the risk of cancer among women. It can be concluded that, through experimental models, the initiation of cancer can be studied by analyzing the steps that transform normal breast cells to malignant ones through certain substances, such as pesticides and estrogen. Such substances cause genomic instability, and therefore tumor formation in the animal, and signs of carcinogenesis in vitro. Cancer initiation has been associated with an increase in genomic instability, indicated by the inactivation of tumor-suppressor genes and activation of oncogenes in the presence of malathion, parathion, and estrogen. In the present study, a comprehensive summary of the impact of OPs in human and rat breast cancer, specifically their effects on the cell cycle, signaling pathways linked to epidermal growth factor, drug metabolism, and genomic instability in an MCF-10F estrogen receptor-negative breast cell line is provided.

Sulforaphane Diminishes the Formation of Mammary Tumors in Rats Exposed to 17β-Estradiol

Elevated levels of estrogen are a risk factor for breast cancer. In addition to inducing DNA damage, estrogens can enhance cell proliferation as well as modulate fatty acid metabolism that collectively contributes to mammary tumorigenesis. Sulforaphane (SFN) is an isothiocyanate derived from broccoli that is currently under evaluation in multiple clinical trials for prevention of several diseases, including cancer. Previous studies showed that SFN suppressed DNA damage and lipogenesis pathways. Therefore, we hypothesized that administering SFN to animals that are co-exposed to 17β-estradiol (E2) would prevent mammary tumor formation. In our study, 4–6 week old female August Copenhagen Irish rats were implanted with slow-release E2 pellets (3 mg x 3 times) and gavaged 3x/week with either vehicle or 100 μmol/kg SFN for 56 weeks. SFN-treated rats were protected significantly against mammary tumor formation compared to vehicle controls. Mammary glands of SFN-treated rats showed decreased DNA damage while serum free fatty acids and triglyceride species were 1.5 to 2-fold lower in SFN-treated rats. Further characterization also showed that SFN diminished expression of enzymes involved in mammary gland lipogenesis. This study indicated that SFN protects against breast cancer development through multiple potential mechanisms in a clinically relevant hormonal carcinogenesis model.

The 2-/16α-Hydroxylated Estrogen Ratio-Breast Cancer Risk Hypothesis: Insufficient Evidence for its Support

During the past 25 years or so a number of studies have been carried out to address the hypothesis that the ratio of 2-hydroxyestrone (2-hydroxy-E₁) to 16α-hydroxyestrone (16α-hydroxy-E₁) is associated with breast cancer risk. The rationale for this hypothesis is based on data from studies that suggest a tumorigenic and genotoxic effect of 16α-hydroxy-E₁ and a protective effect of 2-hydroxy-E₁ regarding breast cancer risk. The adverse effect of 16α-hydroxy-E₁ has been attributed to its potential to form covalent adducts with macromolecules. Initial studies used radiometric assays and enzyme immunoassays to test the hypothesis. However, concerns about the accuracy of these assays led to the development of a liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay that is capable of measuring 5 unconjugated and 15 conjugated endogenous estrogens, which include 2- and 16-hydroxylated estrogen metabolites, in serum or urine. The conjugated estrogens are quantified following a deconjugation (hydrolysis) step to remove the sulfate and glucuronide groups. Epidemiologic studies have been using the LC-MS/MS assay to determine whether there is an association between breast cancer risk and the ratio of the sum of the concentrations of metabolites in the 2-hydroxylated estrogen pathway and in the 16-hydroxylated estrogen pathway. However, the validity of the pathways as biomarkers was not evaluated. The 16-hydroxylated estrogen pathway includes estriol, 16-epiestriol, 17-epiestriol and 16-ketoestradiol, in addition to 16α-hydroxy-E₁. However, with the exception of 16α-hydroxy-E₁, there is no evidence that any of the other estrogens in the pathway have tumorigenic or genotoxic properties, and they do not form covalent adducts with macromolecules. Another deficiency in the epidemiological studies pertains to the accuracy of estrogen metabolite measurements obtained after the hydrolysis step in the LC-MS/MS assays. No validation was performed to demonstrate that a constant efficiency of hydrolysis is found for all the different structural forms of sulfated and glucuronidated conjugates. Other deficiencies in the assays include the need for greater sensitivity so that the very low concentrations of unconjugated 2-hydroxy-E₁, 2-hydroxy-E₂, and 16α-hydroxy-E₁ can be measured in serum. There is also a need to develop assays to measure intact forms of conjugated estrogens in both serum and urine, particularly the sulfates and glucuronides of 2-hydroxylated, 2-methoxylated, and 16α-hydroxylated E₁ and E₂. This will avoid inaccuracies that stem from hydrolysis procedures. Improvements in LC-MS/MS assay methodology to obtain accurate measurements of unconjugated and conjugated 2-hydroxylated, 2-methoxylated, and 16α-hydroxylated estrogen metabolites are needed. This should provide valuable data for testing the 2-/16α-hydroxylated estrogen-breast cancer risk hypothesis.

Metabolism of 17β-estradiol by Novosphingobium sp. ES2-1 as probed via HRMS combined with 13C3-labeling

This study investigated the biodegradation and metabolic mechanisms of 17β-estradiol (E2) by Novosphingobium sp. ES2-1 isolated from the activated sludge in a domestic sewage treatment plant (STP). It could degrade 97.1% E2 (73.5 μmol/L) in 7 d with a biodegradation half-life of 1.29 d. E2 was initially converted to estrone (E1), then to 4-hydroxyestrone (4-OH-E1), before subsequent monooxygenation reactions cleaved 4-OH-E1 into a metabolite with long-chain ketones structure (metabolite P8). However, when 4-OH-E1 was cleaved through the 4,5-seco pathway, the resulting phenol ring cleavage product could randomly condense with NH₃ to yield a pyridine derivative, accompanied by the uncertain loss of a carboxy group at C4 before the condensation. The derivative was further oxidized into the metabolites with both pyridine and long-chain ketones structure (metabolite N5) through a similar formation mechanism as for P8 performed. This research presents several novel metabolites and shows that E2 can be biodegraded into the metabolite with long-chain structure through three optional pathways, thereby reducing E2 contamination.

Toward a holistic view of multiscale breast cancer molecular biomarkers

Powered by rapid technology developments, biomarkers become increasingly diverse, including those detected at genomic, transcriptomic, proteomic, metabolomic and cellular levels. While diverse sets of biomarkers have been utilized in breast cancer predisposition, diagnosis, prognosis, treatment and management, recent additions derived from lincRNA, circular RNA, circulating DNA together with its methylated and hydroxymethylated forms and immune signatures are likely to further transform clinical practice. Here, we take breast cancer as an example of heterogeneous diseases that require many informed decisions from treatment to care to review the huge variety of biomarkers. By assessing the advantages and limitations of modern biomarkers in diverse use scenarios, this article outlines the prospects and challenges of releasing complimentary advantages by augmentation of multiscale molecular biomarkers.

CYP1B1 prevents proteasome-mediated XIAP degradation by inducing PKCε activation and phosphorylation of XIAP

Cytochrome P450 1B1 (CYP1B1) is a key enzyme that catalyzes the metabolism of 17β-estradiol (E2) into catechol estrogens, such as 2-hydroxyestradiol (2-OHE2) and 4-hydroxyestradiol (4-OHE2). CYP1B1 is related to tumor formation and is over-expressed in a variety of cancer cells. In particular, CYP1B1 is highly expressed in hormone-related cancers such as breast cancer, ovarian cancer, or prostate cancer compared to other cancers. However, the detailed mechanisms involving this protein remain unclear. In this study, we demonstrate that CYP1B1 affects X-linked inhibitor of apoptosis protein (XIAP) expression. When CYP1B1 was over-expressed in cells, there was significant increase in the XIAP protein level, whereas the XIAP mRNA level was not affected by CYP1B1 expression. Treatment with 4-OHE2, mainly formed by CYP1B1 activity, also increased XIAP protein levels, whereas treatment with 2-OHE2 did not have a significant effect. Treatment with 4-OHE2 significantly prevented proteasome-mediated XIAP degradation. In addition, phosphorylation of XIAP on serine 87, which is known to stabilize XIAP, was up-regulated by 4-OHE2, indicating that 4-OHE2 affects XIAP stability through XIAP phosphorylation. We also found that phosphorylation of protein kinase C (PKC)ε, which is required for XIAP phosphorylation, increased when cells were treated with 4-OHE2. In summary, our data show that CYP1B1 may play an important role in preventing ubiquitin-proteasome-mediated XIAP degradation through the activation of PKCε signaling in cancer cells.

Human versus non-human sex steroid use in hormone replacement therapies part 1: Preclinical data

Prior to 2002, hormone replacement therapy (HRT) was considered to be an important component of postmenopausal healthcare. This was based on a plethora of basic, epidemiological and clinical studies demonstrating the health benefits of supplementation with human sex steroids. However, adverse findings from the Women's Health Initiative (WHI) studies that examined the 2 major forms of HRT in use in the US at that time - Premarin (conjugated equine estrogens; CEE) and Prempro (CEE + medroxyprogesterone acetate; MPA), cast a shadow over the use of any form of HRT. Here we review the biochemical and physiological differences between the non-human WHI study hormones - CEE and MPA, and their respective human counterparts 17β-estradiol (E₂) and progesterone (P₄). Preclinical data from the last 30 years demonstrate clear differences between human and non-human sex steroids on numerous molecular, physiological and functional parameters in brain, heart and reproductive tissue. In contrast to CEE supplementation, which is not always detrimental although certainly not as optimal as E₂ supplementation, MPA is clearly not equivalent to P₄, having detrimental effects on cognitive, cardiac and reproductive function. Moreover, unlike P₄, MPA is clearly antagonistic of the positive effects of E₂ and CEE on tissue function. These data indicate that minor chemical changes to human sex steroids result in physiologically distinct actions that are not optimal for tissue health and functioning.

Antioxidant activities of novel resveratrol analogs in breast cancer

The objective of the present study was to characterize the role of novel resveratrol (Res) analogs: 4-(E)-{(4-hydroxyphenylimino)-methylbenzene, 1, 2-diol} (HPIMBD) and 4-(E)-{(p-tolylimino)-methylbenzene-1,2-diol} (TIMBD) as potent antioxidants against breast cancer. Non-neoplastic breast epithelial cell lines MCF-10A and MCF-10F were treated with 17β-estradiol (E2), Res, HPIMBD, and TIMBD for up to 72 h. mRNA and protein levels of antioxidant genes, superoxide dismutase 3 (SOD3) and N-quinoneoxidoreductase-1 (NQO1) and transcription factors, nuclear factor erythroid 2-related factor (Nrf) 1, 2 and 3 were quantified after the above treatments. Generation of reactive oxygen species (ROS) was measured by CM-H2-DCFDA and oxidative-DNA damage was determined by measuring 8-hydroxy-2-deoxyguanosine (8-OHdG). HPIMBD and TIMBD scavenged cellular ROS production, attenuated oxidative DNA damage, increased mRNA and protein expression levels of SOD3 and NQO1 and activated Nrf signaling pathway. Our studies demonstrate that HPIMBD and TIMBD have the potential as novel antioxidants to prevent development of breast cancer.

Lack of Cell Proliferative and Tumorigenic Effects of 4-Hydroxyestradiol in the Anterior Pituitary of Rats: Role of Ultrarapid O-Methylation Catalyzed by Pituitary Membrane-Bound Catechol-O-Methyltransferase

In animal models, estrogens are complete carcinogens in certain target sites. 4-Hydroxyestradiol (4-OH-E₂), an endogenous metabolite of 17β-estradiol (E₂), is known to have prominent estrogenic activity plus potential genotoxicity and mutagenicity. We report here our finding that 4-OH-E₂ does not induce pituitary tumors in ACI female rats, whereas E₂ produces 100% pituitary tumor incidence. To probe the mechanism, we conducted a short-term animal experiment to compare the proliferative effect of 4-OH-E₂ in several organs. We found that, whereas 4-OH-E₂ had little ability to stimulate pituitary cell proliferation in ovariectomized female rats, it strongly stimulates cell proliferation in certain brain regions of these animals. Further, when we used in vitro cultured rat pituitary tumor cells as models, we found that 4-OH-E₂ has similar efficacy as E₂ in stimulating cell proliferation, but its potency is approximately 3 orders of magnitude lower than that of E₂. Moreover, we found that the pituitary tumor cells have the ability to selectively metabolize 4-OH-E₂ (but not E₂) with ultrahigh efficiency. Additional analysis revealed that the rat pituitary expresses a membrane-bound catechol-O-methyltransferase that has an ultralow K_m value (in nM range) for catechol estrogens. On the basis of these observations, it is concluded that rapid metabolic disposition of 4-OH-E₂ through enzymatic O-methylation in rat anterior pituitary cells largely contributes to its apparent lack of cell proliferative and tumorigenic effects in this target site.

CYP polymorphisms and pathological conditions related to chronic exposure to organochlorine pesticides

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Organochlorine compounds (OCs) are persistent organic pollutants acting as endocrine disruptors.

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Exposure to OCs is a risk factor for several severe pathologies.

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Specific CYP polymorphisms could affect the clinical impact of OCs exposure.

The association between genetic variations in the cytochrome P450 (CYP) family genes and pathological conditions related to long-term exposure to organochlorine compounds (OCs) deserves further elucidation. OCs are persistent organic pollutants with bioaccumulative and lipophilic characteristics. They can act as endocrine disruptors and perturb cellular mechanisms. Prolonged exposure to OCs has been associated with different pathological manifestations. CYP genes are responsible for transcribing enzymes essential in xenobiotic metabolism. Therefore, polymorphisms in these genetic sequences a. alter the metabolic pathways, b. induce false cellular responses, and c. may provoke pathological conditions. The main aim of this review is to define the interaction between parameters a, b and c at a mechanistic/molecular level, with references in clinical cases.

Biochemical Mechanisms and Catabolic Enzymes Involved in Bacterial Estrogen Degradation Pathways

Estrogens have been classified as group 1 carcinogens by the World Health Organization and represent a significant concern given that they are found in surface waters worldwide, and long-term exposure to estrogen-contaminated water can disrupt sexual development in animals. To date, the estrogen catabolic enzymes and genes remain unknown. Using a tiered functional genomics approach, we identified three estrogen catabolic gene clusters in Sphingomonas sp. strain KC8. We identified several estrone-derived compounds, including 4-hydroxyestrone, a meta-cleavage product, and pyridinestrone acid. The yeast-based estrogen assay suggested that pyridinestrone acid exhibits negligible estrogenic activity. We characterized 17β-estradiol dehydrogenase and 4-hydroxyestrone 4,5-dioxygenase, responsible for the 17-dehydrogenation and meta-cleavage of the estrogen A ring, respectively. The characteristic pyridinestrone acid was detected in estrone-spiked samples collected from two wastewater treatment plants and two suburban rivers in Taiwan. The results significantly expand our understanding of microbial degradation of aromatic steroids at molecular level.

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.

The formation and transformation of hormones in maternal, placental and fetal compartments: biological implications

The fetal endocrine system constitutes the earliest system developing in fetal life and operates during all the steps of gestation. Its regulation is in part dependent on the secretion of placental and/or maternal precursors emanating across the feto-maternal interface. Human fetal and placental compartments possess all the enzymatic systems necessary to produce steroid hormones. However, their activities are different and complementary: the fetus is very active in converting acetate into cholesterol, in transforming pregnanes to androstanes, various hydroxylases, sulfotransferases, while all these transformations are absent or very limited in the placenta. This compartment can transform cholesterol to C21-steroids, convert 5-ene to 4-ene steroids, and has a high capacity to aromatize C19 precursors and to hydrolyze sulfates. Steroid hormone receptors are present at an early stage of gestation and are functional for important physiological activities. The production rate of some steroids greatly increases with fetal evolution (e.g. estriol increases 500-1000 times in relation to non-pregnant women). Other hormones, such as glucocorticoids, in particular the stress hormone cortisol, adipokines (e.g. leptin, adiponectin), insulin-like growth factors, are also a key factor for regulating reproduction, metabolism, appetite and may be significant in programming the fetus and its growth. We can hypothesize that the fetal and placental factors controlling hormonal levels in the fetal compartment can be of capital importance in the normal development of extra-uterine life.

Exploring estrogenic activity in lung cancer

It is well known that a connection between xenobiotics inhalation, especially tobacco combustion and Lung Cancer development is strongly significant and indisputable. However, recent studies provide evidence indicating that another factors such as, estrogens are also involved in lung carcinoma biology and metabolism. Although the status of estrogen receptors (ER), in both cancerous and healthy lung tissue has been well documented, there is still inconclusive data with respect of which isoform of the receptor is present in the lungs. However according to several studies, ERβ appears to be predominant form. Apart from ERs, estrogens can work through a recently discovered G-coupled estrogen receptor. Binding with both types of the receptors causes a signal, which leads to i.e. enhanced cell proliferation. There are many published reports which suggest that estrogen can be synthesized in situ in lung cancer. Some disturbances in the activity and expression levels of enzymes involved in estrogen synthesis were proved. This suggests that increased amounts of sex-steroid hormones can affect cells biology and be the reason of the accelerated development and pathogenesis of lung cancer. There also exist phenomena which associate estrogenic metabolism and tobacco combustion and its carcinogenic influence on the lungs. Compounds present in cigarette smoke induce the activity of CYP1B1, the enzyme responsible for estrogenic metabolism and synthesis of their cateholic derivatives. These structures during their redox cycle are able to release reactive oxygen species or form DNA adduct, which generally leads to destruction of genetic material. This process may explain the synergistic effect of smoking and estrogens on estrogen-dependent lung cancer development.

Development of an In Vitro Model to Screen CYP1B1-Targeted Anticancer Prodrugs

Cytochrome P450 1B1 (CYP1B1) is an anticancer therapeutic target due to its overexpression in a number of steroid hormone–related cancers. One anticancer drug discovery strategy is to develop prodrugs specifically activated by CYP1B1 in malignant tissues to cytotoxic metabolites. Here, we aimed to develop an in vitro screening model for CYP1B1-targeted anticancer prodrugs using the KLE human endometrial carcinoma cell line. KLE cells demonstrated superior stability of CYP1B1 expression relative to transiently transfected cells and did not express any appreciable amount of cognate CYP1A1 or CYP1A2, which would have compromised the specificity of the screening assay. The effect of two CYP1B1-targeted probe prodrugs on KLE cells was evaluated in the absence and presence of a CYP1B1 inhibitor to chemically “knock out” CYP1B1 activity (CYP1B1 inhibited). Both probe prodrugs were more toxic to KLE cells than to CYP1B1-inhibited KLE cells and significantly induced G0/G1 arrest and decreased the S phase in KLE cells. They also exhibited pro-apoptotic effects in KLE cells, which were attenuated in CYP1B1-inhibited KLE cells. In summary, a KLE cell–based model has been characterized to be suitable for identifying CYP1B1-targeted anticancer prodrugs and should be further developed and employed for screening chemical libraries.

Recent Advances in chemistry and pharmacology of 2-methoxyestradiol: An anticancer investigational drug

2-Methoxyestradiol (2ME2), an estrogen hormone metabolite is a potential cancer chemotherapeutic agent. Presently, it is an investigational drug under various phases of clinical trials alone or in combination therapy. Its anticancer activity has been attributed to its antitubulin, antiangiogenic, pro-apoptotic and ROS induction properties. This anticancer drug candidate has been explored extensively in last twenty years for its detailed chemistry and pharmacology. Present review is an update of its chemistry and biological activity. It also extends an assessment of potential of 2ME2 and its analogues as possible anticancer drug in future.

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.

Perturbation of xenobiotic metabolism in Dreissena polymorpha model exposed in situ to surface water (Lake Trasimene) purified with various disinfectants

Sanitation is of crucial importance for the microbiological safety of drinking water. However, chlorination of water rich in organic material produces disinfection by-products (DBPs), many of which have been reported to be mutagenic and/or carcinogenic compounds such as haloacetic acids and trihalomethanes. Epidemiological studies have suggested a link between drinking water consumption and cancer. We previously observed that Cyprinus carpio fish exposed to DBPs, may be subject to epigenetic effects such as those referable to the up-regulation of cytochrome P450 (CYP) superfamily (ex. co-mutagenesis/co-carcinogenesis and oxidative stress) that has been associated to non-genotoxic carcinogenesis. Our goal was to study the xenobiotic metabolism in mollusks exposed in situ to surface water of Lake Trasimene (Central Italy) treated with several disinfectants such as the traditional chlorine dioxide (ClO2), sodium hypochlorite (NaClO) or the relatively new one peracetic acid (PAA). The freshwater bivalves (Dreissena polymorpha) being selected as biomarker, have the unique ability to accumulate pollutants. Freshwater bivalves were maintained in surface water containing each disinfectant individually (1-2 mg/L). Following an exposure period up to 20 days during the fall period, microsomes were collected from the mussels, then tested for various monooxygenases. Strong CYP inductions were observed. These data indicate that drinking water disinfection generates harmful DBP mixtures capable of determining a marked perturbation of CYP-supported reactions. This phenomenon, being associated to an increased pro-carcinogen bioactivation and persistent oxidative stress, could provide an explanation for the observational studies connecting the regular consumption of drinking water to increased risk of various cancers in humans.

Proteomics screening of adenosine triphosphate-interacting proteins in the liver of diazinon-treated rats

AIM

Diazinon (DZN) is one of the most important organophosphorus compounds used to control pests in agriculture in many countries. Several studies have shown that exposure to DZN may alter protein expression in the liver. In order to further investigate the mechanism of DZN toxicity, differentially expressed ATP-interacting proteins, following subacute exposure to toxin, were separated and identified in rat liver.

MAIN METHODS

Male rats were equally divided into four groups: control (corn oil) and DZN (15 mg/kg) by gavage once a day for 4 weeks. After homogenization of liver tissue, lysates were incubated ATP-sepharose beads. After several washes, ATP-interacting proteins were eluted and separated on 2-D polyacrylamide gels. Deferentially expressed proteins were cut and identified using matrix-assisted laser desorption/ionization/time-of-flight and Mascot database. Identified proteins were classified according to their biological process using protein analysis through evolutionary relationships (PANTHER) Web site.

KEY FINDING

In this work, we showed that several key proteins involved in biological processes such as antioxidant system, oxidative stress, apoptosis, and metabolism were differentially expressed after subacute exposure to DZN.

CYP1B1 promotes tumorigenesis via altered expression of CDC20 and DAPK1 genes in renal cell carcinoma

BACKGROUND

Cytochrome P450 1B1 (CYP1B1) has been shown to be up-regulated in many types of cancer including renal cell carcinoma (RCC). Several reports have shown that CYP1B1 can influence the regulation of tumor development; however, its role in RCC has not been well investigated. The aim of the present study was to determine the functional effects of CYP1B1 gene on tumorigenesis in RCC.

METHODS

Expression of CYP1B1 was determined in RCC cell lines, and tissue microarrays of 96 RCC and 25 normal tissues. To determine the biological significance of CYP1B1 in RCC progression, we silenced the gene in Caki-1 and 769-P cells by RNA interference and performed various functional analyses.

RESULTS

First, we confirmed that CYP1B1 protein expression was significantly higher in RCC cell lines compared to normal kidney tissue. This trend was also observed in RCC samples (p < 0.01). Interestingly, CYP1B1 expression was associated with tumor grade and stage. Next, we silenced the gene in Caki-1 and 769-P cells by RNA interference and performed various functional analyses to determine the biological significance of CYP1B1 in RCC progression. Inhibition of CYP1B1 expression resulted in decreased cell proliferation, migration and invasion of RCC cells. In addition, reduction of CYP1B1 induced cellular apoptosis in Caki-1. We also found that these anti-tumor effects on RCC cells caused by CYP1B1 depletion may be due to alteration of CDC20 and DAPK1 expression based on gene microarray and confirmed by real-time PCR. Interestingly, CYP1B1 expression was associated with CDC20 and DAPK1 expression in clinical samples.

CONCLUSIONS

CYP1B1 may promote RCC development by inducing CDC20 expression and inhibiting apoptosis through the down-regulation of DAPK1. Our results demonstrate that CYP1B1 can be a potential tumor biomarker and a target for anticancer therapy in RCC.

Why does infection with some helminths cause cancer?

Infections with Opisthorchis viverrini, Clonorchis sinensis and Schistosoma haematobium are classified as Group 1 biological carcinogens: definitive causes of cancer. These worms are metazoan eukaryotes, unlike the other Group 1 carcinogens including human papilloma virus, hepatitis C virus, and Helicobacter pylori. By contrast, infections with phylogenetic relatives of these helminths, also trematodes of the phylum Platyhelminthes and major human pathogens, are not carcinogenic. These inconsistencies prompt several questions, including how might these infections cause cancer? And why is infection with only a few helminth species carcinogenic? Here we present an interpretation of mechanisms contributing to the carcinogenicity of these helminth infections, including roles for catechol estrogen- and oxysterol-metabolites of parasite origin as initiators of carcinogenesis.

Identifying the Tautomeric Form of a Deoxyguanosine-Estrogen Quinone Intermediate

Mechanistic insights into the reaction of an estrogen o-quinone with deoxyguanosine has been further investigated using high level density functional calculations in addition to the use of 4-hyroxycatecholestrone (4-OHE₁) regioselectivity labeled with deuterium at the C1-position. Calculations using the M06-2X functional with large basis sets indicate the tautomeric form of an estrogen-DNA adduct present when glycosidic bonds cleavage occurs is comprised of an aromatic A ring structure. This tautomeric form was further verified by use of deuterium labelling of the catechol precursor use to form the estrogen o-quinone. Regioselective deuterium labelling at the C1-position of the estrogen A ring allows discrimination between two tautomeric forms of a reaction intermediate either of which could be present during glycosidic bond cleavage. HPLC-MS analysis indicates a reactive intermediate with a m/z of 552.22 consistent with a tautomeric form containing no deuterium. This intermediate is consistent with a reaction mechanism that involves: (1) proton assisted Michael addition; (2) re-aromatization of the estrogen A ring; and (3) glycosidic bond cleavage to form the known estrogen-DNA adduct, 4-OHE₁-1-N7Gua.

Mechanisms of estrogen carcinogenesis: The role of E2/E1-quinone metabolites suggests new approaches to preventive intervention--A review

Studies in hamsters, mice and rats have demonstrated that estradiol (E2), its interconvertible metabolite estrone (E1) and their catechol metabolites, in particular 4-hydroxy E2/E1, are carcinogenic in the kidney, uterus and mammary gland. Observational studies and clinical trials consistently show that sustained exposure to E2/E1 is associated with the development of sporadic breast cancer. The weight of evidence supports the contribution of two complementary pathways in the initiation, promotion and progression of breast cancer. One pathway involves activation of nuclear and cytoplasmic signaling pathways through the binding of estrogen to nuclear and membrane-bound estrogen receptors leading to increased cell proliferation. The other pathway involves the oxidative metabolism of E2/E1 to catechols and then reactive quinones that can contribute to oxidative DNA damage and form specific, mutagenic depurinating adducts with adenine and guanine which then in turn can serve as biomarkers for the occurrence of these processes. Both pathways can serve as portals to preventive intervention. Antiestrogens are used clinically to block receptor-mediated signaling to block tumor growth. Various chemopreventive agents such as sulforaphane (SFN) and resveratrol have been shown in cell culture to block oxidative metabolism of E2/E1 and thus prevent DNA damage. Pretreatment of MCF-7 and MCF-10F cells with and inhibitor of catechol-O-methyltransferase (COMT) followed by treatment with E2 or 4-OH E2 caused increased oxidative DNA damage (8-oxo-dG) and depurinating DNA adducts showing the importance of E2-catechol O-methylation by COMT as a protective pathway. E2 treatment of MCF-10A cells with E2 or 4-OH E2 caused an increase in E2-adenine and guanine adducts. Treatment with sulforaphane increased

NAD(P)H

quinone oxidoreductase 1 (NQO1) and glutathione-S-transferase A1 (GSTA1) expression without affecting expression of catechol-O-methyltransferase (COMT) or cytochrome P450 1B1. Pretreatment with SFN decreased depurinating DNA adducts while increasing levels of 4-OCH3E1/2 and 4-OHE1/2-glutathione conjugates. Treatment of MCF-10F cells with E2 or 4-OH-E2 also caused increased depurinating DNA adducts and neoplastic transformation while pretreatment with resveratrol caused a reduction in adduct levels and neoplastic transformation. Increased levels of estrogen-quinone conjugates and DNA adducts have also been detected in urine of women at increased risk for and with breast cancer. These observations support the notion that targeting the estrogen/estrone metabolism pathway may be another way to reduce breast cancer risk.

Flaxseed reduces the pro-carcinogenic micro-environment in the ovaries of normal hens by altering the PG and oestrogen pathways in a dose-dependent manner

The objective of the present study was to find the optimum dose of flaxseed that would decrease PG and alter oestrogen pathway endpoints implicated in ovarian cancer. In the study, four groups of fifty 1.5-year-old chickens were fed different amounts of flaxseed (0, 5, 10 or 15% of their total diet) for 4 months and were then killed to collect blood and tissues. Levels of flaxseed lignan metabolites, Enterolactone (EL) and Enterodiol (ED) were measured in the serum, liver and ovaries by liquid chromatography-MS/MS, and n-3 and n-6 fatty acid (FA) levels were measured by GC. The effects of the varied flaxseed doses were assessed by measuring levels of PGE2 and oestrogen metabolites (16-hydroxyestrone (16-OHE1) and 2-hydroxyestrone (2-OHE1)) as well as by analysing the expression of the oestradiol metabolising enzymes CYP3A4 (cytochrome p450, family 3, subfamily A, polypeptide 4), CYP1B1 (cytochrome p450, family 1, subfamily B, polypeptide 1) and CYP1A1 (cytochrome p450, family 1, subfamily A, polypeptide 1) and that of oestrogen receptor α (ERα) in the ovaries. The ratio of n-3:n-FA increased with an increase in flaxseed supplementation and corresponded to a dose-dependent decrease in cyclo-oxygenase-2 protein and PGE2 levels. EL and ED increased in the serum, liver and ovaries with increased concentrations of flaxseed. Flaxseed decreased the expression of ERα in the ovaries. The ratio of 2-OHE1:16-OHE1 in the serum increased significantly in the 15% flaxseed diet, and there was a corresponding increase in CYP1A1 in the liver and decrease in CYP3A4 in the ovaries. CYP1B1 mRNA also decreased with flaxseed diet in the ovaries. The 15% flaxseed-supplemented diet significantly decreased inflammatory PGE2, ERα, CYP3A4, CYP1B1 and 16-OHE1, but it increased CYP1A1 and 2-OHE1, which thus reduced the inflammatory and pro-carcinogenic micro-environment of the ovaries.

4-hydroxy estrogen induces DNA damage on codon 130/131 of PTEN in endometrial carcinoma cells

Catechol estrogens, such as 4-hydroxyestradiol (4-OHE2), are estrogen metabolites that form DNA adducts and may induce mutations and subsequent cell transformation in mammary cells; however, little is known about their roles in endometrial carcinogenesis. Furthermore, it remains unclear whether 4-OHE2 is able to induce DNA damage on specific genes involved in carcinogenesis or a 'pro'-mutation status such as microsatellite instability (MSI). Therefore, we modified terminal transferase-dependent PCR by the application of a capillary sequencer to detect DNA damage at the single base level. Using this method, we demonstrated that 4-OHE2 directly induced DNA damage on codon 130/131 in exon 5 of PTEN, which is a mutation hot spot for PTEN in endometrial carcinoma. Whereas, both estradiol and 4-OHE2 treatment did not affect MSI status in immortalized endometrial glandular cells. 4-OHE2 might contribute to endometrial carcinogenesis by inducing PTEN mutation on codon 130/131.

Role of cytochrome P450 genes in breast cancer etiology and treatment: effects on estrogen biosynthesis, metabolism, and response to endocrine therapy

PURPOSE

The cytochrome P450 (CYP) genes are oxygenases involved in estrogen biosynthesis and metabolism, generation of DNA damaging procarcinogens, and response to anti-estrogen therapies. Since lifetime estrogen exposure is an established risk factor for breast cancer, determining the role of CYP genes in breast cancer etiology may provide critical information for understanding tumorigenesis and response to treatment.

METHODS

This review summarizes literature available in PubMed published between 1993 and 2013 that focuses on studies evaluating the effects of DNA variants in CYP genes on estrogen synthesis, metabolism, and generation of procarcinogens in addition to response to anti-estrogen therapies.

RESULTS

Evaluation of DNA variants in estrogen metabolism genes was largely inconclusive. Meta-analyses of data from CYP19A1 support an association between the number of (TTTA) n repeats in intron 4 and breast cancer risk, but the biological mechanism for this relationship is unknown. Associations between single nucleotide polymorphism in CYP1B1 and DNA damage caused by procarcinogenic estrogen metabolites were ambiguous. Variants in CYP2D6 are associated with altered metabolism tamoxifen; however, current data do not support widespread clinical testing. The effect of variants in CYP19A1 in response to aromatase inhibitors is also questionable.

CONCLUSION

Evaluation of DNA variants in CYP genes involved with estrogen metabolism or treatment response has been inconclusive, reflecting small samples sizes, tumor heterogeneity, and differences between populations. Better-powered studies that account for genetic backgrounds and tumor phenotypes are thus necessary.

Regioselective deuterium labeling of estrone and catechol estrogen metabolites

Increased exposure to estrogens and estrogen metabolites is linked with increased rates of breast, ovarian and other human cancers. Metabolism of estrogen can led to formation of electrophilic o-quinones capable of binding to DNA. In order to gain insight into the mechanism of estrogen-induced DNA damage, estrone and catechol estrogens derived from estrone, have been regioselectively labeled with deuterium at the 1-position. Estrone-1-d, estrone-1,2,4-d3, 4-hydroxyestrone-1-d and 2-hydroxyestrone-1-d have been synthesized with or without deuteriums at the 16-position. The key labeling step involves deuterated trifluoroacetic acid exchange catalyzed by t-butyl alcohol. This economical, straightforward labeling technique makes available a range of estrone compounds containing deuterium at the 1-position.

Defining molecular sensors to assess long-term effects of pesticides on carcinogenesis

The abundance of dioxins and dioxin-like pollutants has massively increased in the environment due to human activity. These chemicals are particularly persistent and accumulate in the food chain, which raises major concerns regarding long-term exposure to human health. Most dioxin-like pollutants activate the aryl hydrocarbon receptor (AhR) transcription factor, which regulates xenobiotic metabolism enzymes that belong to the cytochrome P450 1A family (that includes CYP1A1 and CYP1B1). Importantly, a crosstalk exists between estrogen receptor α (ERα) and AhR. More specifically, ERα represses the expression of the CYP1A1 gene, which encodes an enzyme that converts 17β-estradiol into 2-hydroxyestradiol. However, (ERα) does not repress the CYP1B1 gene, which encodes an enzyme that converts 17β-estradiol into 4-hydroxyestradiol, one of the most genotoxic estrogen metabolites. In this review, we discuss how chronic exposure to xenobiotic chemicals, such as pesticides, might affect the expression of genes regulated by the AhR–ERα crosstalk. Here, we focus on recent advances in the understanding of molecular mechanisms that mediate this crosstalk repression, and particularly on how ERα represses the AhR target gene CYP1A1, and could subsequently promote breast cancer. Finally, we propose that genes implicated in this crosstalk could constitute important biomarkers to assess long-term effects of pesticides on human health.

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.

The effects of aerobic exercise on estrogen metabolism in healthy premenopausal women

BACKGROUND

It is well accepted that exercise can decrease breast cancer risk. Limited clinical evidence suggests that this risk could be mediated through changes in estrogen metabolism in premenopausal women. Our objective was to investigate the effects of exercise on premenopausal estrogen metabolism pertinent to breast cancer risk.

METHODS

Sedentary, healthy, young eumenorrheic women were randomized into an intervention of 30 minutes of moderate-to-vigorous aerobic exercise five times a week for approximately 16 weeks (n = 212), or into a usual-lifestyle sedentary control group (n = 179). Urinary levels of estrogens [estrone [E1], estradiol, and estriol] and nine estrogen metabolites were measured at baseline and at study end by liquid chromatography/tandem mass spectrometry. The ratios of 2-hydroxyestrone to 16α-hydroxyestrone (2-OHE1/16α-OHE1) and 2-OHE1 to 4-hydroxyestrone (2- OHE1/4-OHE1) were also calculated.

RESULTS

The exercise intervention resulted in significant increases in aerobic fitness and lean body mass and a significant decrease in percent body fat. For exercisers who completed the study (n = 165), 2-OHE1/16α-OHE1 increased significantly (P = 0.043), whereas E1 decreased significantly (P = 0.030) in control participants (n = 153). The change from baseline in 2-OHE1/16α-OHE1 was significantly different between groups (P = 0.045), even after adjustment for baseline values.

CONCLUSIONS

The exercise intervention resulted in a significant increase in the 2-OHE1/16α-OHE1 ratio but no differences in other estrogen metabolites or ratios.

IMPACT

Our results suggest that changes in premenopausal estrogen metabolism may be a mechanism by which increased physical activity lowers breast cancer risk.

Estrogen receptor α can selectively repress dioxin receptor-mediated gene expression by targeting DNA methylation

Selective inhibitory crosstalk has been known to occur within the signaling pathways of the dioxin (AhR) and estrogen (ERα) receptors. More specifically, ERα represses a cytochrome P450-encoding gene (CYP1A1) that converts cellular estradiol into a metabolite that inhibits the cell cycle, while it has no effect on a P450-encoding gene (CYP1B1) that converts estrodiol into a genotoxic product. Here we show that ERα represses CYP1A1 by targeting the Dnmt3B DNA methyltransferase and concomitant DNA methylation of the promoter. We also find that histone H2A.Z can positively contribute to CYP1A1 gene expression, and its presence at that gene is inversely correlated with DNA methylation. Taken together, our results provide a framework for how ERα can repress transcription, and how that impinges on the production of an enzyme that generates genotoxic estradiol metabolites, and potential breast cancer progression. Finally, our results reveal a new mechanism for how H2A.Z can positively influence gene expression, which is by potentially competing with DNA methylation events in breast cancer cells.

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.