Relative rates of 2- and 4-hydroxyestrogen synthesis are dependent on both substrate and tissue

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.

Combined Docking and Quantum Chemical Study on CYP-Mediated Metabolism of Estrogens in Man

Long-term exposure to estrogens seriously increases the incidence of various diseases including breast cancer. Experimental studies indicate that cytochrome P450 (CYP) enzymes catalyze the bioactivation of estrogens to catechols, which can exert their harmful effects via various routes. It has been shown that the 4-hydroxylation pathway of estrogens is the most malign, while 2-hydroxylation is considered a benign pathway. It is also known experimentally that with increasing unsaturation of ring B of estrogens the prevalence of the 4-hydroxylation pathway significantly increases. In this study, we used a combination of structural analysis, docking, and quantum chemical calculations at the B3LYP/6-311+G* level to investigate the factors that influence the regioselectivity of estrogen metabolism in man. We studied the structure of human estrogen metabolizing enzymes (CYP1A1, CYP1A2, CYP1B1, and CYP3A4) in complex with estrone using docking and investigated the susceptibility of estrone, equilin, and equilenin (which only differ in the unsaturation of ring B) to undergo 2- and 4-hydroxylation using several models of CYP enzymes (Compound I, methoxy, and phenoxy radical). We found that even the simplest models could account for the experimental difference between the 2- and 4- hydroxylation pathways and thus might be used for fast screening purposes. We also show that reactivity indices, specifically in this case the radical and nucleophilic condensed Fukui functions, also correctly predict the likeliness of estrogen derivatives to undergo 2- or 4-hydroxylation.

Potential mechanisms of estrogen quinone carcinogenesis

There is a clear association between the excessive exposure to estrogens and the development of cancer in hormone-sensitive tissues (breast, endometrium). It has become clear that there are likely multiple overlapping mechanisms of estrogen carcinogenesis. One major pathway is the extensively studied hormonal pathway, by which estrogen stimulates cell proliferation through nuclear estrogen receptor (ER)-mediated signaling, thus resulting in an increased risk of genomic mutations during DNA replication. A similar "nongenomic pathway", potentially involving newly discovered membrane-associated ERs, also appears to regulate extranuclear estrogen signaling pathways. This perspective is focused on a third pathway involving the metabolism of estrogens to catechols mediated by cytochrome P450 and further oxidation of these catechols to estrogen o-quinones. Oxidative enzymes, metal ions, and in some cases molecular oxygen can catalyze o-quinone formation, so that these electrophilic/redox-active quinones can cause damage within cells by alkylation and/or oxidation of cellular proteins and DNA in many tissues. It appears that the endogenous estrogen quinones primarily form unstable N3-adenine or N7-guanine DNA adducts, ultimately resulting in mutagenic apurinic sites. In contrast, equine estrogen quinones, formed from estrogens present in popular hormone replacement therapy prescriptions, generate a variety of DNA lesions, including bulky stable adducts, apurinic sites, DNA strand cleavage, and oxidation of DNA bases. DNA damage induced by these equine quinones is significantly increased in cells containing ERs, leading us to hypothesize a mechanism involving ER binding/alkylation by the catchol/quinone, resulting in a "Trojan horse". The "Trojan horse" carries the highly redox-active catechol to estrogen -sensitive genes, where high amounts of reactive oxygen species are generated, causing selective DNA damage. Our data further suggest that other key protein targets for estrogen o-quinones could be redox-sensitive enzymes (i.e, GST P1-1, QR). These proteins are involved in stress response cascades that are known to contribute to the regulation of cell proliferation and apoptosis. Finally, it has been shown that catechol estrogens can transform breast epithelial cells into a tumorigenic phenotype and that these transformed cells had differential gene expression of several genes involved in oxidative stress. Given the direct link between excessive exposure to estrogens, metabolism of estrogens, and increased risk of breast cancer, it is crucial that factors that affect the formation, reactivity, and cellular targets of estrogen quinoids be thoroughly explored.

Equine estrogen metabolite 4-hydroxyequilenin induces DNA damage in the rat mammary tissues: formation of single-strand breaks, apurinic sites, stable adducts, and oxidized bases

Epidemiological data strongly suggest that a woman's risk of developing breast cancer is directly related to her lifetime estrogen exposure. Estrogen replacement therapy in particular has been correlated with an increased cancer risk. Previously we showed that the equine estrogens equilin and equilenin, which are major components of the estrogen replacement formulation Premarin (Wyeth-Ayerst), are metabolized to the catechol, 4-hydroxyequilenin which autoxidizes to an o-quinone causing oxidation and alkylation of DNA in vitro [Bolton, J. L., Pisha, E., Zhang, F., and Qiu, S. (1998) Chem. Res. Toxicol. 11, 1113-1227]. In the present study, we injected 4-hydroxyequilenin into the mammary fat pads of Sprague-Dawley rats. Analysis of cells isolated from the mammary tissue for DNA single-strand breaks and oxidized bases using the comet assay showed a dose-dependent increase in both types of lesions. In addition, LC-MS-MS analysis of extracted mammary tissue showed the formation of an alkylated depurinating guanine adduct. Finally, extraction of mammary tissue DNA, hydrolysis to deoxynucleosides, and analysis by LC-MS-MS showed the formation of stable cyclic deoxyguanosine and deoxyadenosine adducts as well as oxidized bases. This is the first report showing that 4-hydroxyequilenin is capable of causing DNA damage in vivo. In addition, the data showed that 4-hydroxyequilenin induced four different types of DNA damage that must be repaired by different mechanisms. This is in contrast to the endogenous estrogen 4-hydroxyestrone where only depurinating guanine adducts have been detected in vivo. These results suggest that 4-hydroxyequilenin has the potential to be a potent carcinogen through the formation of variety of DNA lesions in vivo.

Metabolism of equilenin in MCF-7 and MDA-MB-231 human breast cancer cells

Sulfate conjugates of the B-ring unsaturated estrogens, equilin, equilenin, and 8-dehydroestrone, and their 17alpha- and 17beta-dihydro analogues, constitute about 54% of Premarin (Wyeth-Ayerst), the most commonly prescribed estrogen formulation in estrogen replacement therapy. Despite the wide clinical use of Premarin, there have been very few studies on the metabolism of the B-ring unsaturated estrogens in humans and there is no information regarding the fate of these compounds in breast tissue or tumors. In this study, we investigated the metabolism of equilenin in two lines of human breast-cancer cells, MCF-7 and MDA-MB-231. MCF-7 cells respond to treatment with Ah-receptor agonists with induction of cytochromes P450 1A1 and 1B1, whereas in MDA-MB-231 cells P450 1B1 is predominantly induced. Metabolites of equilenin were identified and quantified by GC/MS utilizing a series of synthetic metabolite standards and deuterium-labeled analogues as internal standards. In the two cell lines, the same pathways of equilenin metabolism were observed. Equilenin was reduced at C-17 to the 17beta-dihydro form, with minimal production of the 17alpha-dihydro isomer. Both equilenin and 17beta-dihydroequilenin were hydroxylated at the C-4 position, and the resultant catechol metabolites were methylated to form 4-methoxyequilenin and 4-methoxy-17beta-dihydroequilenin. Rates of equilenin metabolism were markedly elevated in cultures exposed to the Ah-receptor agonists, 2,3,7,8-tetrachlorodibenzo-p-dioxin and 3,4,4',5-tetrachlorobiphenyl, implicating the activities of P450s 1A1 and 1B1 in the metabolism. The 2-hydroxylation pathways of equilenin and 17beta-dihydroequilenin metabolism were not observed. In microsomal reactions with cDNA-expressed human enzymes, both P450s 1A1 and 1B1 catalyzed the 4-hydroxylation of 17beta-dihydroequilenin, whereas with 17beta-estradiol as substrate P450 1A1 catalyzes predominantly 2-hydroxylation and P450 1B1 predominantly 4-hydroxylation. Since P450 1B1 is constitutively expressed and both P450s 1A1 and 1B1 are inducible in many extrahepatic tissues including the mammary epithelium, these results indicate the potential for 4-hydroxylation of equilenin and 17beta-dihydroequilenin in extrahepatic, estrogen-responsive tissues.

Gas chromatographic determination of catecholestrogens following isolation by solid-phase extraction

A sensitive and specific assay for the determination of the catecholestrogens 2-hydroxyestradiol (2-OHE2) and 4-hydroxyestradiol (4-OHE2) using gas chromatography with electron-capture detection (GC-ECD) is described. The formation of 2- and 4-OHE2 was assessed following activation of 17beta-estradiol in the microsomal fraction of female rat livers. The analytes were isolated by solid-phase extraction, derivatized to their heptafluorobutyryl esters with heptafluorobutyric acid anhydride, and subjected to solvent exchange prior to analysis; this resulted in minimal chromatographic interference, long column life, and stable derivatized analytes. Derivatized catechols were separated and confirmed with dual column chromatography (DB-5 and DB-608) and quantitated using GC-ECD. The DB-608 column was preferred for quantitation as it provided better 4-OHE2 resolution from interference. Key validation parameters for the assay include sensitivity, intra- and inter-assay precision, and accuracy. Instrument sensitivity and limits of detection (LOD) and quantitation (LOQ) were determined statistically from fortification data approaching expected limits. For 2-OHE2 and 4-OHE2, respective values for these parameters were; instrument sensitivities of 0.4 and 0.7 pg, LODs of 0.8 and 1.3 ng/mg, and LOQs of 2.6 and 4.3 ng/mg.

Synthesis and reactivity of the catechol metabolites from the equine estrogen, 8,9-dehydroestrone

The risk factors for women developing breast and endometrial cancers are all associated with a lifetime of estrogen exposure. Estrogen replacement therapy in particular has been correlated with an increased cancer risk. Previously, we showed that the equine estrogens equilin and equilenin, which are major components of the widely prescribed estrogen replacement formulation Premarin, are metabolized to highly cytotoxic quinoids which caused oxidative stress and alkylation of DNA in vitro [Bolton, J. L., Pisha, E., Zhang, F., and Qiu, S. Chem. Res. Toxicol. 1998, 11, 1113-1127]. In this study, we have synthesized 8,9-dehydroestrone (a third equine estrogen component of Premarin) and its potential catechol metabolites, 4-hydroxy-8,9-dehydroestrone and 2-hydroxy-8,9-dehydroestrone. Both 2-hydroxy-8,9-dehydroestrone and 4-hydroxy-8,9-dehydroestrone were oxidized by tyrosinase or rat liver microsomes to o-quinones which reacted with GSH to give one mono-GSH conjugate and two di-GSH conjugates. Like endogenous estrogens, 8,9-dehydroestrone was primarily converted by rat liver microsomes to the 2-hydroxylated rather than the 4-hydroxylated o-quinone GSH conjugates; the ratio of 2-hydroxy-8,9-dehydroestrone versus 4-hydroxy-8,9-dehydroestrone was 6:1. Also in contrast to experiments with equilin, 4-hydroxyequilenin was not observed in microsomal incubations with 8,9-dehydroestrone or its catechols. The behavior of 2-hydroxy-8,9-dehydroestrone was found to be more complex than 4-hydroxy-8,9-dehydroestrone as GSH conjugates resulting from 2-hydroxy-8,9-dehydroestrone were detected even without oxidative enzyme catalysis. Under physiological conditions, 2-hydroxy-8,9-dehydroestrone isomerized to 2-hydroxyequilenin to form the very stable 2-hydroxyequilenin catechol; however, 4-hydroxy-8,9-dehydroestrone was found to be stable under similar conditions. Finally, preliminary studies conducted with the human breast tumor S-30 cell lines demonstrated that the catechol metabolites of 8,9-dehydroestrone were much less toxic than 4-hydroxyequilenin (20-40-fold). These results suggest that the catechol metabolites of 8,9-dehydroestrone may have the ability to cause cytotoxicity in vivo primarily through formation of o-quinones; however, most of the adverse effects of Premarin estrogens are likely due to formation of 4-hydroxyequilenin o-quinone from equilin and equilenin.

Evidence that a metabolite of equine estrogens, 4-hydroxyequilenin, induces cellular transformation in vitro

Estrogen replacement therapy has been correlated with an increased risk of developing hormone-dependent cancers. 4-Hydroxyequilenin (4-OHEN) is a catechol metabolite of equilenin and equilin which are components of the estrogen replacement formulation marketed under the name of Premarin (Wyeth-Ayerst). Previously, we showed that 4-OHEN autoxidizes to potent cytotoxic quinoids which can consume reducing equivalents and molecular oxygen, and cause a variety of DNA lesions, including formation of bulky stable adducts, apurinic sites, and oxidation of the phosphate-sugar backbone and purine/pyrimidine bases [Bolton, J. L., Pisha, E., Zhang, F., and Qiu, S. (1998) Chem. Res. Toxicol. 11, 1113-1127]. All of these deleterious effects could contribute to the cytotoxic/genotoxic effects of equine estrogens in vivo. In the study presented here, we studied the oxidative and carcinogenic potential of 4-OHEN and the catechol metabolite of the endogenous estrogen, 4-hydroxyestrone (4-OHE), in the JB6 clone 41 5a and C3H 10T(1/2) murine fibroblast cells. The relative ability of 4-OHEN and 4-OHE to induce oxidative stress was measured in these cells by oxidative cleavage of 2',7'-dichlorodiacylfluorosceindiacetate to dichlorofluoroscein. 4-OHEN (1 microM) displayed an increase in the level of reactive oxygen species comparable to that observed with 100 microM H(2)O(2). In contrast, 4-OHE demonstrated antioxidant capabilities in the 5-50 microM range. With both cell lines, we assessed single-strand DNA cleavage using the comet assay and the formation of oxidized DNA bases, such as 8-oxodeoxyguanosine, utilizing the Trevigen Fpg comet assay. 4-OHEN caused single-strand breaks and oxidized bases in a dose-dependent manner in both cell lines, whereas 4-OHE did not induce DNA damage. Since oxidative stress has been implicated in cellular transformation, we used the JB6 clone 41 5a anchorage independence assay to ascertain the relative ability of 4-OHEN and 4-OHE to act as tumor promoters. 4-OHEN caused a slight but significant increase in the extent of cellular transformation at the 100 nM dose; however, in the presence of NADH, which catalyzes redox cycling of 4-OHEN, the transformation ability of 4-OHEN was dramatically increased. 4-OHE did not induce transformation of the JB6 clone 41 5a in the 0.1-10 microM range. The initiation, promotion, and complete carcinogenic transformation potentials of both metabolites were measured in the C3H 10T(1/2) cells. 4-OHEN demonstrated activity in all stages of transformation at doses of 10 nM to 1 microM, whereas 4-OHE only demonstrated promotional capabilities at the 10 microM dose. These data suggest that oxidative stress could be partially responsible for the carcinogenic effects caused by 4-OHEN and that 4-OHEN is a more potent transforming agent than 4-OHE in vitro.

A metabolite of equine estrogens, 4-hydroxyequilenin, induces DNA damage and apoptosis in breast cancer cell lines

Estrogen replacement therapy has been correlated with an increased risk of developing breast or endometrial cancer. 4-Hydroxyequilenin (4-OHEN) is a catechol metabolite of equilenin which is a minor component of the estrogen replacement formulation marketed under the name of Premarin (Wyeth-Ayerst). Previously, we showed that 4-OHEN autoxidizes to quinoids which can consume reducing equivalents and molecular oxygen, are potent cytotoxins, and cause a variety of damage to DNA, including formation of bulky stable adducts, apurinic sites, and oxidation of the phosphate-sugar backbone and purine/pyrimidine bases [Bolton, J. L., Pisha, E., Zhang, F., and Qiu, S. (1998) Chem. Res. Toxicol. 11, 1113-1127]. All of these deleterious effects could contribute to the cytotoxic and genotoxic effects of equilenin in vivo. In the study presented here, we examined the relative toxicity of 4-OHEN in estrogen receptor (ER) positive cells (MCF-7 and S30) compared to that in breast cancer cells without the estrogen receptor (MDA-MB-231). The data showed that 4-OHEN was 4-fold more toxic to MCF-7 cells (LC(50) = 6.0 +/- 0. 2 microM) and 6-fold more toxic to S30 cells (LC(50) = 4.0 +/- 0.1 microM) than to MDA-MB-231 cells (LC(50) = 24 +/- 0.3 microM). Using the single-cell gel electrophoresis assay (comet assay) to assess DNA damage, we found that 4-OHEN causes concentration-dependent DNA single-strand cleavage in all three cell lines, and this effect could be enhanced by agents which catalyze redox cycling (NADH) or deplete cellular GSH (diethyl maleate). In addition, the ER(+) cell lines (MCF-7 and S30) were considerably more sensitive to induction of DNA damage by 4-OHEN than the ER(-) cells (MDA-MB-231). 4-OHEN also caused a concentration-dependent increase in the amount of mutagenic lesion 8-oxo-dG in the S30 cells as determined by LC/MS-MS. Cell morphology assays showed that 4-OHEN induces apoptosis in these cell lines. As observed with the toxicity assay and the comet assay, the ER(+) cells were more sensitive to induction of apoptosis by 4-OHEN than MDA-MB-231 cells. Finally, the endogenous catechol estrogen metabolite 4-hydroxyestrone (4-OHE) was considerably less effective at inducing DNA damage and apoptosis in breast cancer cell lines than 4-OHEN. Our data suggest that the cytotoxic effects of 4-OHEN may be related to its ability to induce DNA damage and apoptosis in hormone sensitive cells in vivo, and these effects may be potentiated by the estrogen receptor.

Role of quinones in toxicology

Quinones represent a class of toxicological intermediates which can create a variety of hazardous effects in vivo, including acute cytotoxicity, immunotoxicity, and carcinogenesis. The mechanisms by which quinones cause these effects can be quite complex. Quinones are Michael acceptors, and cellular damage can occur through alkylation of crucial cellular proteins and/or DNA. Alternatively, quinones are highly redox active molecules which can redox cycle with their semiquinone radicals, leading to formation of reactive oxygen species (ROS), including superoxide, hydrogen peroxide, and ultimately the hydroxyl radical. Production of ROS can cause severe oxidative stress within cells through the formation of oxidized cellular macromolecules, including lipids, proteins, and DNA. Formation of oxidatively damaged bases such as 8-oxodeoxyguanosine has been associated with aging and carcinogenesis. Furthermore, ROS can activate a number of signaling pathways, including protein kinase C and RAS. This review explores the varied cytotoxic effects of quinones using specific examples, including quinones produced from benzene, polycyclic aromatic hydrocarbons, estrogens, and catecholamines. The evidence strongly suggests that the numerous mechanisms of quinone toxicity (i.e., alkylation vs oxidative stress) can be correlated with the known pathology of the parent compound(s).

Is estradiol a genotoxic mutagenic carcinogen?

The natural hormone 17 beta-estradiol (E2) induces tumors in various organs of rats, mice, and hamsters. In humans, slightly elevated circulating estrogen levels caused either by increased endogenous hormone production or by therapeutic doses of estrogen medications increase breast or uterine cancer risk. Several epigenetic mechanisms of tumor induction by this hormone have been proposed based on its lack of mutagenic activity in bacterial and mammalian cell test systems. More recent evidence supports a dual role of estrogen in carcinogenesis as a hormone stimulating cell proliferation and as a procarcinogen inducing genetic damage. Tumors may be initiated by metabolic conversion of E2 to 4-hydroxyestradiol catalyzed by a specific 4-hydroxylase (CYP1B1) and by further activation of this catechol to reactive semiquinone/quinone intermediates. Several types of direct and indirect free radical-mediated DNA damage are induced by E2, 4-hydroxyestradiol, or its corresponding quinone in cell-free systems, in cells in culture, and/or in vivo. E2 also induces various chromosomal and genetic lesions including aneuploidy, chromosomal aberrations, gene amplification, and microsatellite instability in cells in culture and/or in vivo and gene mutations in several cell test systems. These data suggest that E2 is a weak carcinogen and weak mutagen capable of inducing genetic lesions with low frequency. Tumors may develop by hormone receptor-mediated proliferation of such damaged cells.

The major metabolite of equilin, 4-hydroxyequilin, autoxidizes to an o-quinone which isomerizes to the potent cytotoxin 4-hydroxyequilenin-o-quinone

The risk factors for women developing breast and endometrial cancers are all associated with a lifetime of estrogen exposure. Estrogen replacement therapy in particular has been correlated with a slight increased cancer risk. Previously, we showed that equilenin, a minor component of Premarin (Wyeth-Ayerst), was metabolized to highly cytotoxic quinoids which caused oxidative stress and alkylation of DNA in vitro [Bolton, J. L., Pisha, E., Zhang, F., and Qiu, S. (1998) Chem. Res. Toxicol. 11, 1113-1127]. In this study, we have compared the chemistry of the major catechol metabolite of equilin (4-hydroxyequilin), which is found in several estrogen replacement formulations, to the equilenin catechol (4-hydroxyequilenin). Unlike endogenous catechol estrogens, both equilin and equilenin were primarily converted by rat liver microsomes to 4-hydroxylated rather than 2-hydroxylated o-quinone GSH conjugates. With equilin, a small amount of 2-hydroxyequilin GSH quinoids were detected (4-hydroxyequilin:2-hydroxyequilin ratio of 6:1); however, no peaks corresponding to 2-hydroxyequilenin were observed in incubations with equilenin. These data suggest that unsaturation in the B ring alters the regiochemistry of P450-catalyzed hydroxylation from primarily 2-hydroxylation for endogenous estrogens to 4-hydroxylation for equine estrogens. 4-Hydroxyequilenin-o-quinone reacts with GSH to give two mono-GSH conjugates and one di-adduct. The behavior of 4-hydroxyequilin was found to be more complex than 4-hydroxyequilenin as conjugates resulting from 4-hydroxyequilenin were detected in addition to the 4-hydroxyequilin-GSH adducts. The mechanism of decomposition of 4-hydroxyequilin likely involves isomerization to a quinone methide which readily aromatizes to 4-hydroxyequilenin followed by autoxidation to 4-hydroxyequilenin-o-quinone. Similar results were obtained with 2-hydroxyequilin, although, in contrast to 4-hydroxyequilenin, 2-hydroxyequilenin does not autoxidize and the reaction stops at the catechol. Since 4-hydroxyequilin is converted to 4-hydroxyequilenin and 4-hydroxyequilenin-o-quinone, similar effects were observed for this equine catechol, including consumption of NAD(P)H likely by the 4-hydroxyequilenin-o-quinone, depletion of molecular oxygen by 4-hydroxyequilenin or its semiquinone radical, and alkylation of deoxynucleosides and DNA by 4-hydroxyequilenin quinoids. Finally, preliminary studies conducted with the human breast tumor cell line MCF-7 demonstrated that the cytotoxic effects of the catechol estrogens from estrone, equilin, and 2-hydroxyequilenin were similar, whereas 4-hydroxyequilenin was a much more potent cytotoxin ( approximately 30-fold). These results suggest that the catechol metabolites of equine estrogens have the ability to cause alkylation/redox damage in vivo primarily through formation of 4-hydroxyequilenin quinoids.

Synthesis of the equine estrogen metabolites 2-hydroxyequilin and 2-hydroxyequilenin

Equilin and equilenin make up approximately 20% of Premarin which is currently the most popular estrogen replacement therapy. Although there are numerous health benefits of estrogen replacement therapy, there are concerns over the link between estrogen replacement therapy and breast and endometrial cancer risk. One potential mechanism of estrogen carcinogenesis involves metabolism of estrogens to 2- and 4-hydroxylated catechols which are further oxidized to electrophilic/redox active o-quinones which have the potential to both initiate and promote the carcinogenic process. In this investigation, we have synthesized potential metabolites of equilin and equilenin, 2-hydroxyequilin and 2-hydroxyequilenin, respectively, as well as their methyl ether metabolites. These compounds were synthesized from commercially available optically pure equilin via a practical and efficient approach; five steps gave 2-methoxyequilin from which 2-hydroxyequilin was prepared by BBr3-catalyzed demethylation in one step. Similarly, treating 2-methoxyequilin with SeO2 followed by demethylation with BBr3 produced 2-hydroxyequilenin. The structures of the catechols as well as those of their methoxy ethers were unambiguously characterized by one-dimensional and two-dimensional NMR experiments, including 1H, 13C, APT, COSY, HMBC, and HMQC as well as mass spectrometry.

The equine estrogen metabolite 4-hydroxyequilenin causes DNA single-strand breaks and oxidation of DNA bases in vitro

Premarin (Wyeth-Ayerst) is the estrogen replacement treatment of choice and continues to be one of the most widely dispensed prescriptions in North America. In addition to endogenous estrogens, Premarin contains unsaturated equine estrogens, including equilenin [1,3,5(10),6,8-estrapentaen-3-ol-17-one]. In previous work, we showed that the equilenin metabolite 4-hydroxyequilenin (4-OHEN) can be autoxidized to 4-OHEN-o-quinone which readily entered into a redox couple with the semiquinone radical catalyzed by NAD(P)H, P450 reductase, or quinone reductase, resulting in generation of reactive oxygen species [Shen, L., Pisha, E., Huang, Z., Pezzuto, J. M., Krol, E., Alam, Z., van Breemen, R. B., and Bolton, J. L. (1997) Carcinogenesis 18, 1093-1101]. As oxidative damage to DNA by reactive oxygen species generated by redox active compounds has been proposed to lead to tumor formation, we investigated whether 4-OHEN could cause DNA damage. We treated lambda phage DNA with 4-OHEN and found that extensive single-strand breaks could be obtained with increasing concentrations of 4-OHEN as well as increasing incubation times. If scavengers of reactive oxygen species are included in the incubations, DNA could be completely protected from 4-OHEN-mediated damage. In contrast, NADH and CuCl2 enhanced the ability of 4-OHEN to cause DNA single-strand breaks presumably due to redox cycling between 4-OHEN and the semiquinone radical generating hydrogen peroxide and ultimately copper peroxide complexes. We also confirmed that 4-OHEN could oxidize DNA bases since hydrolysis of 4-OHEN-treated calf thymus DNA and HPLC separation with electrospray MS detection revealed oxidized deoxynucleosides, including 8-oxodeoxyguanosine and 8-oxodeoxyadenosine. Our data suggest that DNA single-strand breaks and oxidation of DNA bases by 4-OHEN could contribute to the carcinogenic mechanism(s) of equine estrogens.

Inhibition of glutathione S-transferase activity by the quinoid metabolites of equine estrogens

The risk factors for women developing breast and endometrium cancers are all associated with a lifetime of estrogen exposure. Estrogen replacement therapy (ERT) in particular has been correlated with a slight increased cancer risk, although the numerous benefits of ERT may negate this harmful side effect. Equilenin and equilin are equine estrogens which make up between 30% and 45% of the most widely prescribed estrogen replacement formulation, Premarin (Wyeth-Ayerst). In this study we have synthesized the catechol metabolites of equilenin [4-hydroxyequilenin (4-OHEN)] and equilin [4-hydroxyequilin (4-OHEQ)] and examined how changing unsaturation in the B ring affects the formation of o-quinone GSH conjugates and the ability of the o-quinones and/or GSH conjugates to inhibit glutathione S-transferase (GST). Interestingly, both 4-OHEN and 4-OHEQ autoxidized to o-quinones without the need of oxidative enzyme catalysis. 4-OHEN-o-quinone reacts with GSH to give two mono-GSH conjugates and one diadduct. The behavior of 4-OHEQ was found to be more complex than 4-OHEN as conjugates resulting from 4-OHEN were detected in addition to the 4-OHEQ GSH adducts. Both 4-OHEN and 4-OHEQ were found to be potent inhibitors of GST-catalyzed conjugation of GSH with 1-chloro-2,4-dinitrobenzene. In contrast, the endogenous catechol estrogens, 4-hydroxyestrone (4-OHE) and 2-hydroxyestrone (2-OHE), were without effect unless tyrosinase was present to convert the catechols to o-quinones. Scavengers of reactive oxygen species and metal chelators had no effect on GST inhibition by catechol estrogens with the exception of the catalase which protected GST activity. Kinetic studies showed that 4-OHEN was a potent irreversible inactivator of GST. Preincubation of the enzyme with 4-OHEN showed a time-dependent increase in inhibitory effect, and gel filtration did not restore GST activity confirming the irreversible nature of the enzyme inactivation. Analysis of the Kitz-Wilson plot gave a dissociation constant of the reversible enzyme-inhibitor complex (Ki = 620 microM) and a rate constant of conversion of the reversible enzyme-inhibitor complex to the irreversibly inhibited enzyme (k2 = 7.3 x 10(-)3 s-1). These data suggest that 4-OHEN is an irreversible inactivator with relatively low affinity for GST; however, once formed the 4-OHEN enzyme complex is rapidly converted to the irreversibly inhibited enzyme. The inhibition mechanism likely involves oxidation of the catechol estrogens to o-quinones and covalent modification and/or oxidation of critical amino acid residues on GST. In addition, hydrogen peroxide generated through redox cycling of the o-quinone and/or semiquinone radical and GSH could cause oxidative damage to GST.

Alkylation of 2'-deoxynucleosides and DNA by the Premarin metabolite 4-hydroxyequilenin semiquinone radical

Premarin (Wyeth-Ayerst) is the estrogen replacement treatment of choice and continues to be one of the most widely dispensed prescriptions in the United States. In addition to endogenous estrogens, Premarin contains unsaturated estrogens including equilenin. We synthesized the catechol metabolite of equilenin, 4-hydroxyequilenin (4-OHEN), and found that the semiquinone radical of 4-OHEN reacted with 2'-deoxynucleosides generating very unusual adducts. 2'-Deoxyguanosine (dG), 2'-deoxyadenosine (dA), or 2'-deoxycytosine (dC) all gave four isomers, but no product was observed for thymidine under similar physiological conditions. The structures of these adducts were determined by electrospray mass spectrometry and NMR experiments including 1H, 13C, DQF-COSY, ROESY, HOHAHA, HMQC, and HMBC. The spectral data show that dG forms a cyclic adduct with the 4-OHEN producing 2-N1,3-N2-deoxyguanosyl-1,3-dihydroxy-5,7,9(10)-estratriene-4,17-d ione. Similarly, reaction with dA produced 1-N6,3-C2-deoxyadenosyl-2,3-dihydroxy-5,7,9(10)-estratriene-4,17-d ione, and incubations with dC resulted in 1-N3,3-N4-deoxycytosyl-2,3-dihydroxy-5,7,9(10)-estratriene-4,17-di one. We found that care needed to be taken during the isolation of the dA adducts in particular, as any exposure to acidic environments caused hydrolysis of the sugar moiety leaving alkylated adenine. In mixtures of the deoxynucleosides treated with 4-OHEN, reaction occurred primarily with dG followed by dC and dA. With DNA significant apurinic sites were produced as 4-OHEN-adenine adducts were detected in the ethanol wash prior to hydrolysis. When the DNA was hydrolyzed to deoxynucleosides and analyzed by electrospray mass spectrometry, only one isomer of 4-OHEN-dG and one isomer of 4-OHEN-dC were observed. Our data suggest that several different types of DNA lesions could be expected from 4-OHEN including apurinic sites and bulky stable adducts, in addition to the published oxidized damage to DNA caused by 4-OHEN. The production of these semiquinone radical-derived DNA adducts could play a role in the carcinogenic effects of Premarin estrogens.

Mechanism of cytochrome P450-catalyzed aromatic hydroxylation of estrogens

The mechanism of aromatic hydroxylation of estrogens by cytochrome P450 enzymes has been examined by comparing the oxidation of estrone with that of substrates carrying additional aromaticity such as equilenin and the structural analog 2-naphthol. Hamster liver microsomes preferentially catalyzed the conversion of estrone to 2-hydroxyestrone (Km = 30 and 25 microM and Vmax = 1497 and 900 pmol (mg of protein)-1 min-1 for 2- and 4-hydroxyestrone formation, respectively). In contrast, equilenin was hydroxylated exclusively at C-4 of the steroid ring system and 2-naphthol at the corresponding C-1 position (Km = 67 and 42 microM and Vmax = 2083 and 3226 pmol (mg of protein)-1 min-1 for 4-hydroxyequilenin and 1,2-dihydroxynaphthalene formation, respectively). This shift in the specificity of hydroxylation was due to the introduction of additional aromaticity at ring B of equilenin, because hamster liver microsomes are known not to contain any estrogen-4-hydroxylase, only estrogen-2-hydroxylase activity catalyzed by cytochrome P450 3A family enzymes. The exclusive 4-hydroxylation of equilenin is proposed to be due to a preferred delocalization of the naphthoxy radical an intermediate in the hydroxylation, to C-4, whereas delocalization to C-2 requires additional activation energy and is energetically not favored. Based on these electronic considerations, a mechanism of aromatic hydroxylation of estrogens is proposed which features hydrogen abstraction from the phenolic hydroxy group, electron delocalization of the phenoxy radical to a carbon-centered radical, and subsequent formation of catechol metabolites by hydroxy radical addition at C-2 or C-4 depending on steric or electronic constraints.

Oxidative transformation of 2-hydroxyestrone. Stability and reactivity of 2,3-estrone quinone and its relationship to estrogen carcinogenicity

The carcinogenicity of estrogens in rodents and man has been attributed to either alkylation of cellular macromolecules and/or redox-cycling, generation of active radicals, and DNA damage. Metabolic activation of estradiol leading to the formation of catechol estrogens is believed to be a prerequisite for its genotoxic effects. 4-Hydroxyestradiol, although not 2-hydroxyestradiol, is a potent inducer of tumors in hamsters. Previous studies have shown that 3,4-estrone quinone can redox-cycle and is capable of inducing exclusively single strand DNA breaks in MCF-7 breast cancer cells, as well as react with various nucleophiles (thiol, imidazole, amino, phenolate, and acetoxy) to give Michael addition products. These results support the possible involvement of 3,4-catechol/quinone estrogens in estrogen's carcinogenicity. To explain the decreased carcinogenicity of 2-hydroxyestrogens, the reactions of 2,3-estrone quinone (2,3-EQ) with nucleophiles were investigated. Reactions of 4-methylimidazole with 2,3-EQ gave a complex mixture of products leadng to the formation of the catechol, C-O dimerization product, and a 1,6-Michael addition product identified as the 1-(4-methylimidazolo)-2-hydroxyestrone. Reactions of 2,3-EQ under mildly basic conditions with either ethyl phenolate or acetate gave several products which were characterized as the C-O and C-C dimers, catechol, and 3,5-dihydroxy-1(10), 3-estradiene-2, 17-dione. No Michael addition products were detected under these experimental conditions. The same products were also observed during the synthesis of 2,3-EQ, which led us to postulate that the lack of carcinogenicity of 2-hydroxyestrogens may be related to the increased reactivity and decreased stability of the quinone under physiological conditions. These results are contrasted with those obtained with 3,4-EQ which is much more stable and therefore could diffuse from the site of formation to the target tissue. These results along with rapid methylation and clearance may be very likely explanations for the decreased carcinogenicity of 2-hydroxyestrogens.

Determination of estradiol 2- and 4-hydroxylase activities by gas chromatography with electron-capture detection

A highly sensitive assay has been developed for measuring the rate of formation of 2-hydroxyestradiol and 4-hydroxyestradiol from estradiol by microsomal preparations. Catechol estrogens were converted to heptafluorobutyryl esters, which were separated by capillary column gas chromatography and quantified using electron-capture detection. 2-Hydroxyestradiol 17-acetate was used as an internal standard. The identity of catechol estrogen derivatives was verified by gas chromatography-mass spectrometry using negative-ion chemical ionization. Estrogens were identified by negative molecular ions and/or by characteristic fragments. This procedure permits quantification of catechol estrogens at the subpicogram level. The assay was validated by comparing estrogen 2- and 4-hydroxylase activities in microsomes from hamster and rat liver with values reported previously.

Metabolism of [3H]equilin in normal and malignant human endometrium and in endometrial adenocarcinoma transplanted into nude mice

One of the main components of conjugated equine estrogens is equilin sulfate and this estrogen in postmenopausal women is metabolized to 17 beta-dihydroequilin, 17 beta-dihydroequilenin and equilenin. To investigate the possibility that some of these estrogens may be formed directly in the target tissues, we studied the in vitro metabolism of [3H]equilin in various types of normal and malignant human endometrium, including adenocarcinoma grown in athymic nude mice. The results indicate that normal and neoplastic human endometrium can form the above three metabolites. The highest level of 17 beta-reduced products were isolated from the normal secretory endometrium. Equilenin was the most abundant metabolite isolated from both the normal and malignant endometrium. The formation of [3H]equilenin indicates the presence of a 6,8(9) steroid dehydrogenase-isomerase in the human endometrium. The formation of 17 beta-dihydroequilin in the endometrium may be of importance as this estrogen is 8 times more potent as a uterotrophic agent than equilin and estrone.

Formation, metabolism, and physiologic importance of catecholestrogens

The metabolism of natural and synthetic estrogens is governed primarily by hydroxylations, leading to polyhydroxylated derivatives of the steroid molecule. In mammals aromatic hydroxylation is most prominent quantitatively. The 2- and 4-hydroxyestrogens (catecholestrogens) formed are secreted not only in high amounts in urine but are also present in significant quantities in different organs, such as the liver, pituitary gland, and hypothalamus. This A ring hydroxylation of primary estrogens is affected by peroxidases, tyrosinases, and unspecific monooxygenases by mechanisms still not completely understood. The activity of the aromatic hydroxylases is regulated not only with respect to the overall extent but also to the relative rate of hydroxylation at C-atoms 2 and 4. The metabolism of catecholestrogens may be divided into reversible and irreversible reactions, of which the reaction with the catechol-O-methyltransferase, and thereby the interaction with catecholamines, the conjugation, and the thioether formation are the most prominent. Low- and high-affinity binding is operative in binding to plasma proteins and receptors. Finally, irreversible binding to cellular macromolecules, such as proteins and deoxyribonucleic acid, and the oncogenic potential of natural and synthetic catecholestrogens are discussed.

2,3,7,8-Tetrachlorodibenzo-p-dioxin causes an extensive alteration of 17 beta-estradiol metabolism in MCF-7 breast tumor cells

MCF-7 breast tumor cells form multicellular foci in vitro when supplemented with 17 beta-estradiol (E2). In the presence of E2 and the aryl hydrocarbon-receptor agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), MCF-7 cells grow to confluence but do not form foci. To investigate the role of E2 metabolism in this antiestrogenic effect of TCDD, analyses were performed by capillary GC/MS. The results revealed that pretreatment of MCF-7 cultures with TCDD (10 nM) rapidly depletes E2. In untreated cultures supplemented with 10 nM E2, the concentration of free E2 decreased to 4 nM in the first 12 hr, followed by a slower rate of decline. After 3 days most E2 in the medium was in conjugated form(s); 1.7 nM was present as free E2, and 2.9 nM was released by treatment with glucuronidase/sulfatase. In TCDD-treated cultures, E2 declined to 290 pM in 12 hr and after 2 days was not detected (less than 100 pM) either as free steroid or after treatment with glucuronidase/sulfatase. Intracellular E2 and estrone were likewise depleted by pretreatment with TCDD. Microsomes from TCDD-treated cells showed highly elevated aryl hydrocarbon-hydroxylase activity and catalyzed hydroxylations of E2 at C-2, C-4, C-15 alpha, and C-6 alpha with a combined rate of 0.85 nmol/min per nmol of cytochrome P-450 at saturating E2. These results suggest that depletion of E2 by enhanced metabolism accounts for the antiestrogenic activity of TCDD in MCF-7 cells.

Effect of chronic estrogen treatment of Syrian hamsters on microsomal enzymes mediating formation of catecholestrogens and their redox cycling: implications for carcinogenesis

Estrogens have previously been shown to induce DNA damage in Syrian hamster kidney, a target organ of estrogen-induced cancer. The biochemical mechanism of DNA adduction has been postulated to involve free radicals generated by redox cycling of estrogens. As part of an examination of this postulate, we measured the effect of chronic estrogen treatment of hamsters on renal microsomal enzymes mediating catechol estrogen formation and free radical generation by redox cycling of catechol estrogens. In addition, the activities of the same enzymes were assayed in liver in which tumors do not develop under these conditions. At saturating substrate concentration, 2- and 4-hydroxyestradiol were formed in approximately equal amounts (26 and 28 pmol/mg protein/min, respectively), which is 1-2 orders of magnitude higher than reported previously. Estradiol treatment for 2 months decreased 2-hydroxylase activity per mg protein by 75% and 4-hydroxylase activity by 25%. Hepatic 2- and 4-hydroxylase activities were 1256 and 250 pmol/mg protein/min, respectively. Estrogen treatment decreased both activities by 40-60%. Basal peroxidatic activity of cytochrome P-450, the enzyme which oxidizes estrogen hydroquinones to quinones in the redox cycle, was 2.5-fold higher in liver than in kidney and did not change with estrogen treatment. However, when normalized for specific content of cytochrome P-450 the enzyme activity in kidney was 2.5-fold higher than in liver and increased further by 2-3-fold with chronic estrogen treatment. The activity of cytochrome P-450 reductase, which reduces quinones to hydroquinones in the estrogen redox cycle, was 6-fold higher in liver than in kidney of both control and estrogen-treated animals. When normalized for cytochrome P-450, the activity of this enzyme was similar in liver and kidney, but over 4-fold higher in kidney than liver after estrogen treatment. Basal concentrations of superoxide, a product of redox cycling, were 2-fold higher in liver than in kidney. Estrogen treatment did not affect this parameter in liver, but increased it in kidney by 40%. These data provide evidence for a preferential preservation of enzymes involved in estrogen activation.

Effects of intrastriatal hormones on the dorsal immobility response in male rats

Previous research has shown that estradiol administered either peripherally or directly into the striatum potentiates the dorsal immobility response (DIR) in ovariectomized female rats. Male rats are even more responsive than females to intrastriatal estradiol, and furthermore respond to the effects of catecholestrogens while females do not. In order to determine whether the heightened effects of estrogens in males are due to conversion to catecholestrogens, castrated male rats were given bilateral intrastriatal implants of moxestrol, which cannot be readily converted to a catecholestrogen, and diethylstilbestrol, which can. To determine whether the effects of intrastriatal estradiol in male rats might be related to the effects of androgens on the striatum, castrated male rats were given bilateral intrastriatal implants of testosterone, which can be aromatized to estrogen, and 5 alpha-dihydrotestosterone, which cannot. The effects of each of the hormones tested were measured against those of cholesterol (an inactive control substance) and 17 beta-estradiol. In each case the DIR was measured four hours after the hormone implant. Both synthetic estrogens and 17 beta-estradiol significantly potentiated the DIR, while neither of the androgens had an effect. Thus, the effects of estradiol, synthetic estrogens and catecholestrogens on the male striatum appear to be due to the estrogenic properties of these hormones.

Inhibition of rat liver microsomal estrogen 2-hydroxylase by 2-methoxyestrogens

The inhibition of estrogen 2-hydroxylase by 2-methoxyestrogens was demonstrated in screening assays and has been further investigated under initial velocity conditions. The ability of 2-methoxyestradiol and 2-methoxyestrone to block the conversion of estradiol to 2-hydroxyestradiol by male rat liver microsomal preparations was determined by measuring the release of 3H2O from [2-3H]estradiol. The apparent Kis were found to be 34.86 microM for 2-methoxyestradiol and 18.65 microM for the methoxyestrone, with the apparent Km for the substrate estradiol in these essays of 3.21 microM. Mixed inhibition studies with the methoxyestrogens and 2,4-dibromoestradiol, an effective estrogen 2-hydroxylase inhibitor, in male rat liver microsomes resulted in Dixon plots consisting of a series of parallel lines. Thus, methoxyestrogens and 2,4-dibromoestradiol are mutually exclusive inhibitors, i.e., the binding of one compound to the enzyme interferes with the binding of the other. These results indicate that the compounds are interacting at the same enzymatic site. Finally, a method utilized to measure estrogen 2-hydroxylase activity in vitro is a radioenzymatic assay involving addition of catechol o-methyltransferase (COMT) and radiolabeled S-adenosylmethionine, and the amount of catechol estrogens formed is determined by the amount of radiolabeled methoxyestrogens isolated. The results described here demonstrate inhibition of estrogen 2-hydroxylase by methoxyestrogens; however, under enzymatic conditions of low product formation, the estrogen 2-hydroxylase inhibitory effect of catechol estrogen products from the radioenzymatic assay would be insignificant. Thus, these interactions of methoxyestrogens suggest that the steroid hormonal environment be considered in the examination of estrogen 2-hydroxylase and the catechol estrogen products. off

Comparison of assays for catechol estrogen synthase activity: product isolation vs radioenzymatic catechol-O-methyltransferase-coupled procedures

Reported values for the activity of enzymes mediating catechol estrogen formation by hamster kidney and liver, measured by catechol-O-methyltransferase-coupled radioenzymatic assay, have been uniformly low and there have been marked discrepancies in values reported from different laboratories. Therefore, we examined the validity of the radioenzymatic assay used in these studies. NADPH-dependent estrogen 2- and 4-hydroxylase activity of hamster liver microsomes measured by radioenzymatic assay was comparable to that reported in the literature but at least one order of magnitude lower than that obtained with a direct product isolation assay. Several features of the radioenzymatic assay were identified which, together, contribute to the underestimation of enzyme activity. They include, incomplete protection from oxidative degradation of both the catechol estrogens generated and of the catechol-O-methyltransferase and assay conditions which are suboptimal for O-methylation of the catechol estrogens. We conclude that results obtained using the catechol-O-methyltransferase-based radioenzymatic assay can only be considered valid if a consistent stoichiometric relationship can be demonstrated between the amounts of catechol estrogens and their O-methylated products.

Novel synthesis of 2-fluoroestradiol from 19-nortestosterone: biomimetic oxidative defluorination to 2-hydroxyestradiol

A new, short synthetic route to 2-fluoroestradiol from 19-nortestosterone is described which gives the target compound in an approximately 25% overall yield. Oxidative defluorination of 2-fluoroestradiol to 2-hydroxyestradiol via treatment with Frémy's salt/iodide ion is reported. This process is regarded as biomimetic with respect to cytochrome P-450-dependent oxidative defluorination.

Binding of 2-hydroxyestradiol and 4-hydroxyestradiol to estrogen receptors from human breast cancers

The binding of catechol estrogens, epoxyenones and methoxyestrogens was evaluated using estrogen receptors in cytosol prepared from human breast cancers. The relative affinity of 2-hydroxyestradiol, a metabolite formed in vitro from estradiol-17 beta by breast cancer cells, was indistinguishable from that of estradiol-17 beta. 4-Hydroxyestradiol, which is also a metabolite of estradiol-17 beta, associated with the estrogen receptor with a relative affinity approximately 1.5-fold greater than that of estradiol-17 beta. Epoxyenones and methoxyestrogens were weak competitors compared to the binding of estradiol-17 beta, exhibiting relative affinities 3% or less than the affinity of estradiol-17 beta. Sucrose density gradient centrifugation revealed that both 2- and 4-hydroxyestradiol inhibited the binding of estradiol-17 beta to both the 4S and 8S isoforms of the estrogen receptor in a competitive manner, with a Ki = 0.94 nM for 2-hydroxyestradiol and a Ki = 0.48 nM for 4-hydroxyestradiol. It can be concluded that these data demonstrate a specific receptor-mediated estrogenic action for both of these catechol estrogens.

Enhancement of 2- and 16 alpha-estradiol hydroxylation in MCF-7 human breast cancer cells by 2,3,7,8-tetrachlorodibenzo-P-dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin exhibits antiestrogenic activity and induces cytochromes P-450 in estrogen-dependent MCF-7 human breast-cancer cells. To determine whether induction of 2- or 16 alpha-hydroxylation of 17 beta-estradiol has a role in this antiestrogenic activity, MCF-7 cells which were exposed to this xenobiotic for 72 hrs were incubated with either [2-3H] or [16 alpha-3H] 17 beta-estradiol and the extent of tritiated H2O formation, indicative of site-specific hydroxylation, was determined. 2,3,7,8-Tetrachlorodibenzo-p-dioxin-treated MCF-7 cultures showed an 8-fold increase in 2-hydroxylation and a 2-fold increase in 16 alpha-hydroxylation. These results support the suggestion that increased hydroxylation of 17 beta-estradiol may have a role in the antiestrogenic activity of 2,3,7,8-tetrachlorodibenzo-p-dioxin in MCF-7 cells.

Peroxidatic catecholestrogen production by human breast cancer tissue in vitro

The ability of breast cancer tissues from postmenopausal women to form catechol estrogens was examined by using a product isolation assay. Initial assays were carried out in the presence of either: (a) NADPH, the co-factor for monooxygenase mediated catecholestrogen (CE) formation or; (b) light-activated Tween 80 (LAT-80), a putative organic hydroperoxide co-factor for peroxidatic activity. Under monooxygenase conditions, CE formation by homogenates of 10 tumors did not exceed that obtained with heat denatured tissue. In contrast, 17 of 20 tumors incubated with LAT-80 synthesized significant amounts of CE (8.5 +/- 1.17 2-hydroxyestradiol [2-OH-E2] and 12.8 +/- 2.4 nmol/g protein/10 min 4-hydroxyestradiol [4-OH-E2]). Substitution of cumene hydroperoxide, an organic hydroperoxide, for LAT-80 enhanced estrogen 2/4 hydroxylase (E-2/4-H) activity over 200-fold, making it possible to characterize systematically the peroxidatic activity. The properties of peroxidatic E-2/4-H activity were similar to those of soluble peroxidases isolated from brain, including an acidic pH optimum, localization in the soluble fraction, an apparent Km in the range of 80 microM and an apparent Vmax in the range of 4000 nmol/g/protein/10 min for both 2- and 4-OH-E2. Under optimal assay conditions, peroxidatic E-2/4-H activity was identified in 10 of 13 tumors (2480 +/- 580 nmol/g protein/10 min for 2-OH-E2 and 2790 +/- 600 for 4-OH-E2). The level of activity detected suggests a biological relevance for CE formation by breast cancer tissue.

Synthesis of 2-methoxy and 4-methoxy equine estrogens

4-Methoxyequilin and 2-methoxyequilin were synthesized from the corresponding 4-bromoequilin and 2-iodoequilin derivatives, respectively, by nucleophilic displacement of halogen with methoxide ion in the presence of copper (II) chloride and 15-crown-5-ether. 4-Bromoequilin was prepared by reacting equilin with one equivalent of N-bromoacetamide. 2-Iodoequilin was prepared by reductive dehalogenation of 2,4-diiodoequilin, which in turn was obtained by treatment of equilin with two equivalents of iodine in methanolic ammonium hydroxide solution. 4-Methoxy-equilenin and 2-methoxyequilenin were prepared from the corresponding 4-iodo- and 2-iodo-7 epsilon, 8 epsilon-epoxyestrone derivatives, respectively. Nucleophilic displacement of iodine with methoxide ion was carried out as described earlier with simultaneous aromatization of the B ring leading to 4- and 2-methoxyequilenin derivatives. Alternatively, 4-methoxyequilenin was obtained from 4-methoxyequilin by selenium dioxide oxidation.

Substrate dependency of specific and non-specific estrogen-2/4-hydroxylase activities measured by the radio-enzymatic method in rat brain microsomes

Putative specific and non-specific estrogen-2/4-hydroxylase activities which might affect the radio-enzymatic assay were characterized in terms of their requirements for NADPH and their substrate dependency. Using rat brain microsomes and partially purified rat liver COMT three main sources of estrogen-2/4-hydroxylase activity could be distinguished; a COMT-related component and NADPH-dependent and NADPH-independent microsomal components. The COMT-related activity required NADPH and showed about equal preferences for estrone and estradiol. The NADPH-dependent component was highly specific for estradiol, the relative activities observed with estrone and estriol being 7 and 1% of that observed with estradiol. The NADPH-independent component exhibited substrate saturation, was heat-labile and could not be inhibited by alpha-naphthoflavone or metyrapone. It showed a preference for estrone over estradiol, with estriol being a very poor substrate. These findings indicate that non-enzymatic factors contribute very little to product formation in the radio-enzymatic assay. The specificity of the major NADPH-dependent microsomal component towards estradiol suggests a stereo-specific requirement for the D-ring configuration of this estrogen. The use of no-cofactor blanks in the radio-enzymatic assay may be very important when different estrogens are compared as substrates for estrogen-2/4-hydroxylases.

Androgens inhibit the formation of catechol estrogens by estrogen 2-hydroxylase present in rat liver microsomal preparations

The inhibition of estrogen 2-hydroxylase by androgens was demonstrated in screening assays and has been further investigated under initial velocity conditions. The ability of testosterone, 5 alpha-dihydrotestosterone, and dehydroepiandrosterone to block the conversion of estradiol to 2-hydroxyestradiol by male rat liver microsomal preparations was determined using two radiotracer methods--the conversion of [4-14C]estradiol to [4-14C]2-hydroxyestradiol and the release of 3H2O from [2-3H]estradiol. The apparent Ki's for the androgens ranged from 12.0 to 14.0 microM, with the apparent Km for the substrate estradiol in these assays of 2.08 microM. Multiple inhibition studies with the androgens and 2-bromoestradiol, an effective estrogen inhibitor, in male rat liver microsomes resulted in Dixon plots consisting of a series of nonparallel, intersecting lines. Thus, the androgens and 2-bromoestradiol are non-exclusive inhibitors, i.e. the binding of one compound to the enzyme does not interfere with the binding of the other. These interactions of androgens suggest that the steroid hormonal environment be considered in the examination of the physiological role(s) of the estrogen 2-hydroxylase and the catechol estrogen products.

Indirect evidence for the metabolic dehalogenation of tetrafluorodiethylstilbestrol by rat and hamster liver and kidney microsomes. Species- and organ-dependent differences

In order to assess the significance of the catechol pathway for the carcinogenic activity of diethylstilbestrol (DES), the stability of 3',5',3",5"-tetrafluoro-DES (TF-DES) against metabolic catechol formation was examined in vitro. A radioenzymatic assay was used for determining the estrogen hydroxylase activity of liver and kidney microsomes from male and female Syrian golden hamsters and from male Wistar rats for the substrates TF-DES, DES, estradiol-17 beta and 2-fluoro-estradiol-17 beta. With all microsomes tested, catechols were formed from TF-DES to an extent similar to or, in some cases, even exceeding that observed with DES and the steroidal estrogens. The estrogen hydroxylase activity measured for the various microsomes depended on the species, organ and substrate. Analysis by high performance liquid chromatography showed that four products were formed in the radioenzymatic assay with DES and TF-DES. These data demonstrate that the fluorine substitution present in TF-DES does not prevent catechol formation and imply that the catechol pathway must be taken into account as a putative pathway for the metabolic activation of DES.

Catecholestrogen binding sites in breast cancer

The binding of 2-hydroxyestrone (2OH E1), a catecholestrogen which is the main end product of the 2-hydroxylation of estrogen, was investigated in breast cancers. 2OH E1-specific bindings were found in the cytosol (Kd = 0.54 +/- 0.10 nM) and in the endoplasmic reticulum (Kd = 3.36 +/- 1.32 nM). The dissociation rate constants of complexes between [3H]2OH E1 and cytosol or membrane binding sites were 3.30 h-1 and 8.30 h-1 respectively. Qualitative analysis of [3H]2OH E1 cytosolic complexes demonstrated a specific binding component with a mol. wt of 330,000 Daltons. Specificity experiments showed that nonestrogenic hormones were unable to compete with 2OH E1 for its binding sites, whereas triphenylethylene derivatives and catecholamines were potent 2OH E1 competitors. The presence of 2OH E1 specific bindings suggests a potential role of catecholestrogen in breast cancer.

Estrogen-induced tumorigenesis in hamsters: roles for hormonal and carcinogenic activities

Based on the experimental studies presented herein, we have concluded there are hormonal and carcinogenic aspects to estrogens, both natural and synthetic, which are involved in renal tumorigenesis in the hamster. Hormonal aspects related to this tumor system are based on the presence of specific estrogen receptor in the untransformed kidney which is elevated by prolonged estrogen treatment. Moreover, antiestrogens, which inhibit estrogen receptor complex binding activity, completely block renal tumor induction by estrogens. Finally, estrogens were found to induce progesterone receptor in the hamster kidney and this induction can be inhibited by androgens and antiestrogens. Carcinogenic aspects related to renal tumorigenesis are suggested by the marked suppression of estrogen-induced kidney tumors by alpha-naphthoflavone. In addition, ethinyl estradiol, as potent an estrogen in the hamster as either DES of 17 beta-estradiol, induced only a very low renal tumor incidence. The finding that aryl hydrocarbon hydroxylase activity in the hamster kidney, but not the liver, is depressed markedly by estrogens and enhanced by androgenic hormone suggests involvement of the microsomal monooxygenase system in affecting estrogen metabolism and ultimately perhaps its carcinogenicity.