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

The Reactivity of Human and Equine Estrogen Quinones towards Purine Nucleosides

Conjugated estrogen medicines, which are produced from the urine of pregnant mares for the purpose of menopausal hormone replacement therapy (HRT), contain the sulfate conjugates of estrone, equilin, and equilenin in varying proportions. The latter three steroid sex hormones are highly similar in molecular structure as they only differ in the degree of unsaturation of the sterane ring “B”: the cyclohexene ring in estrone (which is naturally present in both humans and horses) is replaced by more symmetrical cyclohexadiene and benzene rings in the horse-specific (“equine”) hormones equilin and equilenin, respectively. Though the structure of ring “B” has only moderate influence on the estrogenic activity desired in HRT, it might still significantly affect the reactivity in potential carcinogenic pathways. In the present theoretical study, we focus on the interaction of estrogen orthoquinones, formed upon metabolic oxidation of estrogens in breast cells with purine nucleosides. This multistep process results in a purine base loss in the DNA chain (depurination) and the formation of a “depurinating adduct” from the quinone and the base. The point mutations induced in this manner are suggested to manifest in breast cancer development in the long run. We examine six reactions between deoxyadenosine and deoxyguanosine as nucleosides and estrone-3,4-quinone, equilin-3,4-quinone, and equilenin-3,4-quinone as mutagens. We performed DFT calculations to determine the reaction mechanisms and establish a structure–reactivity relationship between the degree of unsaturation of ring “B” and the expected rate of DNA depurination. As quinones might be present in the cytosol in various protonated forms, we introduce the concept of “effective barriers” to account for the different reactivity and different concentrations of quinone derivatives. According to our results, both equine estrogens have the potential to facilitate depurination as the activation barrier of one of the elementary steps (the initial Michael addition in the case of equilenin and the rearomatization step in the case of equilin) significantly decreases compared to that of estrone. We conclude that the appearance of exogenous equine estrogen quinones due to HRT might increase the risk of depurination-induced breast cancer development compared to the exposure to endogenous estrone metabolites. Still, further studies are required to identify the rate-limiting step of depurination under intracellular conditions to reveal whether the decrease in the barriers affects the overall rate of carcinogenesis.

Estrogen receptor-dependent and independent roles of benzo[a]pyrene in Ishikawa cells

Polycyclic aromatic hydrocarbons (PAHs) are environmental pollutants generated from the incomplete combustion of organic material. PAHs have been studied as genotoxicants, but some also act via non-genotoxic mechanisms in estrogen-dependent malignancies, such as breast cancer. PAHs require metabolic activation to electrophilic metabolites to exert their genotoxicity but non-genotoxic properties may also contribute to their carcinogenicity. The role of PAHs in endometrial cancer, a cancer associated with unopposed estrogen action is unknown. We assessed the metabolism of the representative PAH, benzo[a]pyrene (B[a]P), to estrogenic compounds in Ishikawa human endometrial cells in the presence and absence of cytochrome P450 induction. Using stable-isotope dilution high-performance liquid chromatography and APCI tandem mass spectrometry in the selected reaction monitoring mode, we analyzed B[a]P metabolism in Ishikawa cells. Estrogenic activity of B[a]P metabolites was determined by the endogenous estrogen inducible alkaline phosphatase reporter gene and an exogenous estrogen response element (ERE) luciferase reporter gene construct. We also assessed whether PAHs can induce a proliferative phenotype via estrogen receptor (ER)- and non-ER-regulated pathways. We demonstrate that B[a]P can be metabolized in human endometrial cells into 3-OH-B[a]P and B[a]P-7,8-dione in sufficient amounts to activate ERs. We also show that only B[a]P-7,8-dione induces endometrial cell proliferation at concentrations lower than required to activate the ER; instead non-genomic signaling by the EGF receptor (EGFR) and activation of the mitogen-activated protein kinase (MAPK) pathway was responsible. This work indicates that human endometrial cells can metabolize PAHs into estrogenic metabolites, which may induce cell proliferation through non-ER-regulated pathways.

Mismatch Repair Protein hMLH1, but not hMSH2, Enhances Estrogen-Induced Apoptosis of Colon Cancer Cells

BACKGROUND

Epidemiological studies suggest a protective role of estrogen against colon carcinogenesis; this effect appears to be dependent on mismatch repair (MMR) status. However, the underlying mechanism remains unclear. This study investigated the role of MMR proteins in apoptosis of colon cancer cells in the presence or absence of estrogen.

METHODS

Two major MMR proteins, human mutL homolog 1 (hMLH1) and mutS homolog 2 (hMSH2), as well as estrogen receptor-β (ERβ), were transiently expressed in either hMLH1-deficient HCT116 cells or hMSH2-deficient LoVo cells. Effects of estradiol on cell viability and apoptosis were assessed. Furthermore, we examined the apoptotic status of epithelial cells in colonic mucosa taken from previous healthy female subjects with menopausal syndrome before and after 6-month hormone replacement therapy (HRT).

RESULTS

In hMLH1-deficient HCT116 cells, re-expression of hMLH1 led to a significantly decreased cell viability and increased apoptosis, which were further enhanced by estradiol, including marked increase of activated caspase-3 and caspase-9, as well as Bax and P53. The effect of hMLH1 overexpression in LoVo cells resulted in a similar increase in apoptosis that was greatly stimulated by estradiol. The enhanced apoptosis by hMLH1 and estradiol was further validated by FACS analyses of Annexin V expression. Re-expression of hMSH2 or overexpression of ERβ in HCT116 cells also enhanced apoptosis; however, the effects were independent of estradiol. Furthermore, studies on healthy menopausal women before and after 6-month HRT demonstrated a significant HRT-mediated upregulation of the hMLH1 expression, with concomitant elevation of caspase-3 and caspase-9 activation in the colonic mucosa.

CONCLUSION

We present the first evidence that hMLH1 and hMSH2 have similar but distinct roles in the apoptosis of colon cancer cells: an increased expression of either one can promote apoptosis, while only the effect of hMLH1 but not hMSH2 is estradiol-dependent. Our data suggest that MMR status should be assessed before hormone replacement therapy or future application of estrogen-based chemoprevention.

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.

Crosstalk of ROS/RNS and autophagy in silibinin-induced apoptosis of MCF-7 human breast cancer cells in vitro

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) play important roles in regulating cell survival and death. Silibinin is a natural polyphenolic flavonoid isolated from milk thistle with anti-tumor activities, but it was found to induce cytoprotective ROS/RNS in human breast cancer MCF-7 cells. Furthermore, treatment with silibinin down-regulates ERα expression in MCF-7 cells, and inducing both autophagy and apoptosis. In this study we explored the relationship between ER-associated pathways and RNS/ROS in MCF-7 cells. We also investigated the molecular mechanisms underlying the reciprocal regulation between ROS/RNS levels and autophagy in the death signaling pathways in silibinin-treated MCF-7 cells. Silibinin (100-300 μmol/L) dose-dependently increased ROS/RNS generation in MCF-7 cells (with high expression of ERα and low expression of ERβ) and MDA-MB-231 cells (with low expression of ERα and high expression of ERβ). Scavenging ROS/RNS significantly enhanced silibinin-induced death of MCF-7 cells, but not MDA-MB231 cells. Pharmacological activation or blockade of ERα in MCF-7 cells significantly enhanced or decreased, respectively, silibinin-induced ROS/RNS generation, whereas activation or block of ERβ had no effect. In silibinin-treated MCF-7 cells, exposure to the ROS/RNS donators decreased the autophagic levels, whereas inhibition of autophagy with 3-MA significantly increased ROS/RNS levels. We further showed that increases in ROS/RNS generation, ERα activation or autophagy down-regulation had protective roles in silibinin-treated MCF-7 cells. Under a condition of ERα activation, scavenging ROS/RNS or stimulating autophagy enhanced the cytotoxicity of silibinin. These results demonstrate the existence of two conflicting pathways in silibinin-induced death of MCF-7 cells: one involves the down-regulation of ERα and thereby augmenting the pro-apoptotic autophagy downstream, leading to cell death; the other involves the up-regulation of pro-survival ROS/RNS; and that the generation of ROS/RNS and autophagy form a negative feedback loop whose balance is regulated by ERα.

Formation and Biological Targets of Quinones: Cytotoxic versus Cytoprotective Effects

Quinones represent a class of toxicological intermediates, which can create a variety of hazardous effects in vivo including, acute cytotoxicity, immunotoxicity, and carcinogenesis. In contrast, quinones can induce cytoprotection through the induction of detoxification enzymes, anti-inflammatory activities, and modification of redox status. The mechanisms by which quinones cause these effects can be quite complex. The various biological targets of quinones depend on their rate and site of formation and their reactivity. Quinones are formed through a variety of mechanisms from simple oxidation of catechols/hydroquinones catalyzed by a variety of oxidative enzymes and metal ions to more complex mechanisms involving initial P450-catalyzed hydroxylation reactions followed by two-electron oxidation. Quinones are Michael acceptors, and modification of cellular processes could occur through alkylation of crucial cellular proteins and/or DNA. Alternatively, quinones are highly redox active molecules which can redox cycle with their semiquinone radical anions leading to the formation of reactive oxygen species (ROS) including superoxide, hydrogen peroxide, and ultimately the hydroxyl radical. Production of ROS can alter redox balance within cells through the formation of oxidized cellular macromolecules including lipids, proteins, and DNA. This perspective explores the varied biological targets of quinones including GSH, NADPH, protein sulfhydryls [heat shock proteins, P450s, cyclooxygenase-2 (COX-2), glutathione S-transferase (GST), NAD(P)H:quinone oxidoreductase 1, (NQO1), kelch-like ECH-associated protein 1 (Keap1), IκB kinase (IKK), and arylhydrocarbon receptor (AhR)], and DNA. The evidence strongly suggests that the numerous mechanisms of quinone modulations (i.e., alkylation versus oxidative stress) can be correlated with the known pathology/cytoprotection of the parent compound(s) that is best described by an inverse U-shaped dose-response curve.

Multifactor dimensionality reduction analysis to elucidate the cross-talk between one-carbon and xenobiotic metabolic pathways in multi-disease models

Putatively functional polymorphisms of one-carbon and xenobiotic metabolic pathways influence susceptibility for wide spectrum of diseases. The current study was aimed to explore gene-gene interactions among these two metabolic pathways in four diseases i.e. breast cancer, systemic lupus erythematosus (SLE), coronary artery disease (CAD) and Parkinson's disease (PD). Multifactor dimensionality reduction analysis was carried out on four case-control datasets. Cross-talk was observed between one-carbon and xenobiotic pathways in breast cancer (RFC 80 G>A, COMT H108L and TYMS 5'-UTR 28 bp tandem repeat) and SLE (CYP1A1 m1, MTRR 66 A>G and GSTT1). Gene-gene interactions within one-carbon metabolic pathway were observed in CAD (GCPII 1561 C>T, SHMT 1420 C>T and MTHFR 677 C>T) and PD (cSHMT 1420 C>T, MTRR 66 A>G and RFC1 80 G>A). These interaction models showed good predictability of risk for PD (The area under the receiver operating characteristic curve (C) = 0.83) and SLE (C = 0.73); and moderate predictability of risk for breast cancer (C = 0.64) and CAD (C = 0.63). Cross-talk between one-carbon and xenobiotic pathways was observed in diseases with female preponderance. Gene-gene interactions within one-carbon metabolic pathway were observed in diseases with male preponderance.

Relationship between reactive oxygen species and autophagy in dormant mouse blastocysts during delayed implantation

Objective

Under estrogen deficiency, blastocysts cannot initiate implantation and enter dormancy. Dormant blastocysts live longer in utero than normal blastocysts, and autophagy has been suggested as a mechanism underlying the sustained survival of dormant blastocysts during delayed implantation. Autophagy is a cellular degradation pathway and a central component of the integrated stress response. Reactive oxygen species (ROS) are produced within cells during normal metabolism, but their levels increase dramatically under stressful conditions. We investigated whether heightened autophagy in dormant blastocysts is associated with the increased oxidative stress under the unfavorable condition of delayed implantation.

Methods

To visualize ROS production, day 8 (short-term dormancy) and day 20 (long-term dormancy) dormant blastocysts were loaded with 1-µM 5-(and-6)-chloromethyl-2', 7'-dichlorodihydrofluorescein diacetate, acetyl ester (CM-H₂DCFDA). To block autophagic activation, 3-methyladenine (3-MA) and wortmannin were used in vivo and in vitro, respectively.

Results

We observed that ROS production was not significantly affected by the status of dormancy; in other words, both dormant and activated blastocysts showed high levels of ROS. However, ROS production was higher in the dormant blastocysts of the long-term dormancy group than in those of the short-term group. The addition of wortmannin to dormant blastocysts in vitro and 3-MA injection in vivo significantly increased ROS production in the short-term dormant blastocysts. In the long-term dormant blastocysts, ROS levels were not significantly affected by the treatment of the autophagy inhibitor.

Conclusion

During delayed implantation, heightened autophagy in dormant blastocysts may be operative as a potential mechanism to reduce oxidative stress. Further, ROS may be one of the potential causes of compromised developmental competence of long-term dormant blastocysts after implantation.

Defining the conformation of the estrogen receptor complex that controls estrogen-induced apoptosis in breast cancer

Development of acquired antihormone resistance exposes a vulnerability in breast cancer: estrogen-induced apoptosis. Triphenylethylenes (TPEs), which are structurally similar to 4-hydroxytamoxifen (4OHT), were used for mechanistic studies of estrogen-induced apoptosis. These TPEs all stimulate growth in MCF-7 cells, but unlike the planar estrogens they block estrogen-induced apoptosis in the long-term estrogen-deprived MCF7:5C cells. To define the conformation of the TPE:estrogen receptor (ER) complex, we employed a previously validated assay using the induction of transforming growth factor α (TGFα) mRNA in situ in MDA-MB 231 cells stably transfected with wild-type ER (MC2) or D351G ER mutant (JM6). The assays discriminate ligand fit in the ER based on the extremes of published crystallography of planar estrogens or TPE antiestrogens. We classified the conformation of planar estrogens or angular TPE complexes as "estrogen-like" or "antiestrogen-like" complexes, respectively. The TPE:ER complexes did not readily recruit the coactivator steroid receptor coactivator-3 (SRC3) or ER to the PS2 promoter in MCF-7 and MCF7:5C cells, and molecular modeling showed that they prefer to bind to the ER in an antagonistic fashion, i.e., helix 12 not sealing the ligand binding domain (LBD) effectively, and therefore reduce critical SRC3 binding. The fully activated ER complex with helix 12 sealing the LBD is suggested to be the appropriate trigger to initiate rapid estrogen-induced apoptosis.

SERMs attenuate estrogen-induced malignant transformation of human mammary epithelial cells by upregulating detoxification of oxidative metabolites

The risk of developing hormone-dependent cancers with long-term exposure to estrogens is attributed both to proliferative, hormonal actions at the estrogen receptor (ER) and to chemical carcinogenesis elicited by genotoxic, oxidative estrogen metabolites. Nontumorigenic MCF-10A human breast epithelial cells are classified as ER(-) and undergo estrogen-induced malignant transformation. Selective estrogen receptor modulators (SERM), in use for breast cancer chemoprevention and for postmenopausal osteoporosis, were observed to inhibit malignant transformation, as measured by anchorage-independent colony growth. This chemopreventive activity was observed to correlate with reduced levels of oxidative estrogen metabolites, cellular reactive oxygen species (ROS), and DNA oxidation. The ability of raloxifene, desmethylarzoxifene (DMA), and bazedoxifene to inhibit this chemical carcinogenesis pathway was not shared by 4-hydroxytamoxifen. Regulation of phase II rather than phase I metabolic enzymes was implicated mechanistically: raloxifene and DMA were observed to upregulate sulfotransferase (SULT 1E1) and glucuronidase (UGT 1A1). The results support upregulation of phase II metabolism in detoxification of catechol estrogen metabolites leading to attenuated ROS formation as a mechanism for inhibition of malignant transformation by a subset of clinically important SERMs.

Differential regulation of detoxification enzymes in hepatic and mammary tissue by hops (Humulus lupulus) in vitro and in vivo

SCOPE

Hops contain the phytoestrogen, 8-prenylnaringenin, and the cytoprotective compound, xanthohumol (XH). XH induces the detoxification enzyme, NAD(P)H-quinone oxidoreductase (NQO1) in vitro; however, the tissue distribution of XH and 8-prenylnaringenin and their tissue-specific activity have not been analyzed.

METHODS AND RESULTS

An orally administered hop extract and subcutaneously injected XH were administered to Sprague-Dawley rats over 4 days. LC-MS-MS analysis of plasma, liver, and mammary gland revealed that XH accumulated in liver and mammary glands. Compared with the low level in the original extract, 8-prenylnaringenin was enriched in the tissues. Hops and XH-induced NQO1 in the liver, while only hops reduced NQO1 activity in the mammary gland. Mechanistic studies revealed that hops modulated NQO1 through three mechanisms. In liver cells, (i) XH modified Kelch-like ECH-associated protein leading to nuclear factor (erythroid-derived 2)-like 2 (Nrf2) translocation and antioxidant response element (ARE) activation; (ii) hop-mediated ARE induction was partially mediated through phosphorylation of Nrf2 by PKC; (iii) in breast cells, 8-prenylnaringenin reduced NQO1 likely through binding to estrogen receptorα, recruiting Nrf2, and downregulating ARE-regulated genes.

CONCLUSION

XH and 8-prenylnaringenin in dietary hops are bioavailable to the target tissues. While hops and XH might be cytoprotective in the liver, 8-prenylnaringenin seems responsible for hop-mediated NQO1 reduction in the mammary gland.

Molecular mechanisms linking endometriosis under oxidative stress with ovarian tumorigenesis and therapeutic modalities

Inflammation plays a role in the pathogenesis of endometriosis. Endometriosis-associated ovarian carcinogenesis might be promoted through oxidative stress-induced increased genomic instability, aberrant methylation, and aberrant chromatin remodeling, as well as mutations of tumor suppressor genes. Aberrant expression of ARID1A, PIK3CA, and NF-kB genes has been recognized as the major target genes involved in oxidative stress-induced carcinogenesis. HNF-1beta appears to play a key role in anti-oxidative defense mechanisms. We discuss the pathophysiologic roles of oxidative stress as somatic mutations as well as highly specific agents that effectively modulate these targets.

BRCA1 requirement for the fidelity of plasmid DNA double-strand break repair in cultured breast epithelial cells

The tumor suppressor breast cancer susceptibility protein 1 (BRCA1) protects our cells from genomic instability in part by facilitating the efficient repair of DNA double-strand breaks (DSBs). BRCA1 promotes the error-free repair of DSBs through homologous recombination and is also implicated in the regulation of nonhomologous end joining (NHEJ) repair fidelity. Here, we investigate the role of BRCA1 in NHEJ repair mutagenesis following a DSB. We examined the frequency of microhomology-mediated end joining (MMEJ) and the fidelity of DSB repair relative to BRCA1 protein levels in both control and tumorigenic breast epithelial cells. In addition to altered BRCA1 protein levels, we tested the effects of cellular exposure to mirin, an inhibitor of meiotic recombination enzyme 11 (Mre11) 3'-5'-exonuclease activity. Knockdown or loss of BRCA1 protein resulted in an increased frequency of overall plasmid DNA mutagenesis and MMEJ following a DSB. Inhibition of Mre11-exonuclease activity with mirin significantly decreased the occurrence of MMEJ, but did not considerably affect the overall mutagenic frequency of plasmid DSB repair. The results suggest that BRCA1 protects DNA from mutagenesis during nonhomologous DSB repair in plasmid-based assays. The increased frequency of DSB mutagenesis and MMEJ repair in the absence of BRCA1 suggests a potential mechanism for carcinogenesis.

Raloxifene and desmethylarzoxifene block estrogen-induced malignant transformation of human breast epithelial cells

There is association between exposure to estrogens and the development and progression of hormone-dependent gynecological cancers. Chemical carcinogenesis by catechol estrogens derived from oxidative metabolism is thought to contribute to breast cancer, yet exact mechanisms remain elusive. Malignant transformation was studied in MCF-10A human mammary epithelial cells, since estrogens are not proliferative in this cell line. The human and equine estrogen components of estrogen replacement therapy (ERT) and their catechol metabolites were studied, along with the influence of co-administration of selective estrogen receptor modulators (SERMs), raloxifene and desmethyl-arzoxifene (DMA), and histone deacetylase inhibitors. Transformation was induced by human estrogens, and selectively by the 4-OH catechol metabolite, and to a lesser extent by an equine estrogen metabolite. The observed estrogen-induced upregulation of CYP450 1B1 in estrogen receptor negative MCF-10A cells, was compatible with a causal role for 4-OH catechol estrogens, as was attenuated transformation by CYP450 inhibitors. Estrogen-induced malignant transformation was blocked by SERMs correlating with a reduction in formation of nucleobase catechol estrogen (NCE) adducts and formation of 8-oxo-dG. NCE adducts can be formed consequent to DNA abasic site formation, but NCE adducts were also observed on incubation of estrogen quinones with free nucleotides. These results suggest that NCE adducts may be a biomarker for cellular electrophilic stress, which together with 8-oxo-dG as a biomarker of oxidative stress correlate with malignant transformation induced by estrogen oxidative metabolites. The observed attenuation of transformation by SERMs correlated with these biomarkers and may also be of clinical significance in breast cancer chemoprevention.

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

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

Unexpected hormonal activity of a catechol equine estrogen metabolite reveals reversible glutathione conjugation

4-Hydroxyequilenin (4-OHEN) is a major phase I metabolite of the equine estrogens present in widely prescribed hormone replacement formulations. 4-OHEN is autoxidized to an electrophilic o-quinone that has been shown to redox cycle, generating ROS, and to covalently modify proteins and DNA and thus potentially to act as a chemical carcinogen. To establish the ability of 4-OHEN to act as a hormonal carcinogen at the estrogen receptor (ER), estrogen responsive gene expression and proliferation were studied in ER(+) breast cancer cells. Recruitment by 4-OHEN of ER to estrogen responsive elements (ERE) of DNA in MCF-7 cells was also studied and observed. 4-OHEN was a potent estrogen, with additional weak activity associated with binding to the arylhydrocarbon receptor (AhR). The potency of 4-OHEN toward classical ERalpha mediated activity was unexpected given the reported rapid autoxidation and trapping of the resultant quinone by GSH. Addition of thiols to cell cultures did not attenuate the estrogenic activity of 4-OHEN, and preformed thiol conjugates added to cell incubations only marginally reduced ERE-luciferase induction. On reaction of the 4OHEN-GSH conjugate with NADPH, 4-OHEN was observed to be regenerated at a rate dependent upon NADPH concentration, indicating that intracellular nonenzymatic and enzymatic regeneration of 4-OHEN accounts for the observed estrogenic activity of 4-OHEN. 4-OHEN is therefore capable of inducing chemical and hormonal pathways that may contribute to estrogen-dependent carcinogenesis, and trapping by cellular thiols does not provide a mechanism of termination of these pathways.

Redox regulation in cancer: a double-edged sword with therapeutic potential

Oxidative stress, implicated in the etiology of cancer, results from an imbalance in the production of reactive oxygen species (ROS) and cell’s own antioxidant defenses. ROS deregulate the redox homeostasis and promote tumor formation by initiating an aberrant induction of signaling networks that cause tumorigenesis. Ultraviolet (UV) exposures, γ-radiation and other environmental carcinogens generate ROS in the cells, which can exert apoptosis in the tumors, thereby killing the malignant cells or induce the progression of the cancer growth by blocking cellular defense system. Cancer stem cells take the advantage of the aberrant redox system and spontaneously proliferate. Oxidative stress and gene-environment interactions play a significant role in the development of breast, prostate, pancreatic and colon cancer. Prolonged lifetime exposure to estrogen is associated with several kinds of DNA damage. Oxidative stress and estrogen receptor-associated proliferative changes are suggested to play important roles in estrogen-induced breast carcinogenesis. BRCA1, a tumor suppressor against hormone responsive cancers such as breast and prostate cancer, plays a significant role in inhibiting ROS and estrogen mediated DNA damage; thereby regulate the redox homeostasis of the cells. Several transcription factors and tumor suppressors are involved during stress response such as Nrf2, NFκB and BRCA1. A promising strategy for targeting redox status of the cells is to use readily available natural substances from vegetables, fruits, herbs and spices. Many of the phytochemicals have already been identified to have chemopreventive potential, capable of intervening in carcinogenesis.

Inhibition of estrogen signaling activates the NRF2 pathway in breast cancer

Exposure to higher levels of estrogen produces genotoxic metabolites that can stimulate mammary tumorigenesis. Induction of NF-E2-related factor 2 (NRF2)-dependent detoxifying enzymes (e.g., NAD(P)H-quinone oxidoreductase 1 (NQO1)) is considered an important mechanism of protection against estrogen-associated carcinogenesis because they would facilitate removal of toxic estrogens. Here, we studied the impact of estrogen-receptor (ER) signaling on NRF2-dependent gene transcription. In luciferase assay experiments using the 5-flanking region of the human NQO1 gene promoter, we observe that ERα ligand-binding domain (LBD) is required for estrogen inhibition of NQO1 promoter activity in estrogen-dependent breast cancer cells. Chromatin immunoprecipitation (ChIP) assay shows that estrogen recruits ERα and a class III histone deacetylase SIRT1 at the NQO1 promoter, leading to inhibition of NQO1 transcription. Inhibition of ERα expression by the antiestrogen shikonin reverses the inhibitory effect of estrogen on NQO1 expression. As a consequence, a chemoprevention study was undertaken to monitor the impact of shikonin on DNA lesions and tumor growth. Treatment of MCF-7 breast cancer cells with shikonin inhibits estrogen-induced 8-hydroxy-2-deoxyguanosine (8-OHdG), a marker of DNA damage. NQO1 deficiency promotes estrogen-dependent tumor formation, and shikonin inhibits estrogen-dependent tumor growth in an NQO1-dependent manner in MCF-7 xenografts. These results suggest that estrogen-receptor signaling pathway has an inhibitory effect on NRF2-dependent enzymes. Moreover, shikonin reverses the inhibitory effects of estrogen on this pathway and may contribute to breast cancer prevention.

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

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

Development of a liquid chromatography electrospray ionization tandem mass spectrometry method for analysis of stable 4-hydroxyequilenin-DNA adducts in human breast cancer cells

Estrogen-DNA adducts are potential biomarkers for assessing cancer risk and progression in estrogen-dependent cancer. 4-Hydroxyequilenin (4-OHEN), the major catechol metabolite of equine estrogens present in hormone replacement therapy formulations, autoxidizes to a reactive o-quinone that subsequently causes DNA damage. The formation of stable stereoisomeric cyclic 4-OHEN-DNA adducts has been reported in vitro and in vivo, but their removal by DNA repair processes in cells has not been determined. Such studies have been hampered by low yields of cyclic adducts and poor reproducibility when treating cells in culture with 4-OHEN. These problems are attributed in part to the instability of 4-OHEN in aerobic, aqueous media. We show herein that low yields and reproducibility can be overcome by 4-OHEN diacetate as a novel, cell-permeable 4-OHEN precursor, in combination with a sensitive LC-MS/MS method developed for detecting adducts in human breast cancer cells. This method involves isolation of cellular DNA, DNA digestion to deoxynucleosides, followed by the addition of an isotope-labeled internal standard (4-OHEN-(15)N(5)-dG adduct) prior to analysis by LC-MS/MS. A concentration-dependent increase in adduct levels was observed in MCF-7 cells after exposure to 4-OHEN diacetate. The chemical stabilities of the adducts were also investigated to confirm that adducts were stable under assay conditions. In conclusion, this newly developed LC-MS/MS method allows detection and relative quantification of 4-OHEN-DNA adducts in human breast cancer cells, which could be adapted for adduct detection in human samples.

Aryl hydrocarbon receptor facilitates DNA strand breaks and 8-oxo-2'-deoxyguanosine formation by the aldo-keto reductase product benzo[a]pyrene-7,8-dione

Polycyclic aromatic hydrocarbon (PAH) o-quinones produced by aldo-keto reductases are ligands for the aryl hydrocarbon receptor (AhR) (Burczynski, M. E., and Penning, T. M. (2000) Cancer Res. 60, 908-915). They induce oxidative DNA lesions (reactive oxygen species-mediated DNA strand breaks and 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dGuo) formation) in human lung cells. We tested whether the AhR enhances PAH o-quinone-mediated oxidative DNA damage by translocating these ligands to the nucleus. Using the single cell gel electrophoresis (comet) assay to detect DNA strand breaks in murine hepatoma Hepa1c1c7 cells and its AhR- and aryl hydrocarbon receptor nuclear translocator-deficient variants, benzo[a]pyrene-7,8-dione (B[a]P-7,8-dione) produced fewer DNA strand breaks in AhR-deficient cells compared with aryl hydrocarbon receptor nuclear translocator-deficient and wild type Hepa1c1c7 cells. Decreased DNA strand breaks were also observed in human bronchoalveolar H358 cells in which the AhR was silenced by siRNA. The antioxidant alpha-tocopherol and the iron chelator/antioxidant desferal decreased the formation of B[a]P-7,8-dione-mediated DNA strand breaks indicating that they were reactive oxygen species-dependent. By coupling the comet assay to 8-oxoguanine glycosylase (hOGG1), which excises 8-oxo-Gua, strand breaks dependent upon this lesion were measured. hOGG1 treatment produced more DNA single strand breaks in B[a]P-7,8-dione-treated Hepa cells and H358 cells than in its absence. The levels of hOGG1-dependent DNA strand breaks mediated by B[a]P-7,8-dione were lower in AhR-deficient Hepa and AhR knockdown H358 cells. The AhR antagonist alpha-naphthoflavone also attenuated B[a]P-7,8-dione-mediated DNA strand breaks. The decrease in 8-oxo-dGuo levels in AhR-deficient Hepa cells and AhR knockdown H358 cells was validated by immunoaffinity capture stable isotope dilution ([(15)N(5)]8-oxo-dGuo) liquid chromatography-electrospray ionization/multiple reaction monitoring/mass spectrometry. We conclude that the AhR shuttles PAH o-quinone genotoxins to the nucleus and enhances oxidative DNA damage.