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

Estetrol: From Preclinical to Clinical Pharmacology and Advances in the Understanding of the Molecular Mechanism of Action

Estetrol (E4) is the most recently described natural estrogen. It is produced by the human fetal liver during pregnancy and its physiological function remains unclear. E4 is the estrogenic component of a recently approved combined oral contraceptive. It is also in development for use as menopausal hormone therapy. In the context of these developments, the pharmacological activity of E4, alone or in combination with a progestin, has been extensively characterized in preclinical models as well as in clinical studies in women of reproductive age and postmenopausal women. Despite the clinical benefits, the use of oral estrogens for contraception or menopause is also associated with unwanted effects, such as an increased risk of breast cancer and thromboembolic events, due to their impact on non-target tissues. Preclinical and clinical data for E4 point to a tissue-specific activity and a more selective pharmacological profile compared with other estrogens, including a low impact on the liver and hemostasis balance. This review summarizes the characterization of the pharmacological properties of E4 as well as recent advances made in the understanding of the molecular mechanisms of action driving its activity. How the unique mode of action and the different metabolism of E4 might support its favorable benefit-risk ratio is also discussed.

Genotoxicity of ortho-quinones: reactive oxygen species versus covalent modification

o-Quinones are formed metabolically from natural and synthetic estrogens as well as upon exposure to polycyclic aromatic hydrocarbons (PAH) and contribute to estrogen and PAH carcinogenesis by genotoxic mechanisms. These mechanisms include the production of reactive oxygen species to produce DNA strand breaks and oxidatively damaged nucleobases; and the formation of covalent depurinating and stable DNA adducts. Unrepaired DNA-lesions can lead to mutation in critical growth control genes and cellular transformation. The genotoxicity of the o-quinones is exacerbated by nuclear translocation of estrogen o-quinones by the estrogen receptor and by the nuclear translocation of PAH o-quinones by the aryl hydrocarbon receptor. The properties of o-quinones, their formation and detoxication mechanisms, quinone-mediated DNA lesions and their mutagenic properties support an important role in hormonal and chemical carcinogenesis.

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.

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.

Quantitative detection of 4-hydroxyequilenin-DNA adducts in mammalian cells using an immunoassay with a novel monoclonal antibody

Estrogen–DNA adducts are potential biomarkers for assessing the risk and development of estrogen-associated cancers. 4-Hydroxyequilenin (4-OHEN) and 4-hydroxyequilin (4-OHEQ), the metabolites of equine estrogens present in common hormone replacement therapy (HRT) formulations, are capable of producing bulky 4-OHEN–DNA adducts. Although the formation of 4-OHEN–DNA adducts has been reported, their quantitative detection in mammalian cells has not been done. To quantify such DNA adducts, we generated a novel monoclonal antibody (4OHEN-1) specific for 4-OHEN–DNA adducts. The primary epitope recognized is one type of stereoisomers of 4-OHEN–dA adducts and of 4-OHEN–dC adducts in DNA. An immunoassay with 4OHEN-1 revealed a linear dose–response between known amounts of 4-OHEN–DNA adducts and the antibody binding to those adducts, with a detection limit of approximately five adducts/10⁸ bases in 1 µg DNA sample. In human breast cancer cells, the quantitative immunoassay revealed that 4-OHEN produces five times more 4-OHEN–DNA adducts than does 4-OHEQ. Moreover, in a mouse model for HRT, oral administration of Premarin increased the levels of 4-OHEN–DNA adducts in various tissues, including the uterus and ovaries, in a time-dependent manner. Thus, we succeeded in establishing a novel immunoassay for quantitative detection of 4-OHEN–DNA adducts in mammalian cells.

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.