Hepatic catecholestrogen synthases: differential effect of sex, inducers of cytochromes P-450 and of antibody to the glucocorticoid inducible cytochrome P-450 on NADPH-dependent estrogen-2-hydroxylase and on organic hydroperoxide-dependent estrogen-2/4-hydroxylase activity of rat hepatic microsomes

Towards a generic physiologically based kinetic model to predict in vivo uterotrophic responses in rats by reverse dosimetry of in vitro estrogenicity data

Physiologically based kinetic (PBK) modelling-based reverse dosimetry is a promising tool for the prediction of in vivo developmental toxicity using in vitro concentration–response data. In the present study, the potential of this approach to predict the dose-dependent increase of uterus weight in rats upon exposure to estrogenic chemicals was assessed. In vitro concentration–response data of 17β-estradiol (E2) and bisphenol A (BPA) obtained in the MCF-7/BOS proliferation assay, the U2OS ER-CALUX assay and the yeast estrogen screen (YES) assay, were translated into in vivo dose–response data in rat, using a PBK model with a minimum number of in vitro and in silico determined parameter values. To evaluate the predictions made, benchmark dose (BMD) analysis was performed on the predicted dose–response data and the obtained BMDL₁₀ values were compared with BMDL₁₀ values derived from data on the effects of E2 and BPA in the uterotrophic assay reported in the literature. The results show that predicted dose–response data of E2 and BPA matched with the data from in vivo studies when predictions were made based on YES assay data. The YES assay-based predictions of the BMDL₁₀ values differed 3.9-fold (E2) and 4.7- to 13.4-fold (BPA) from the BMDL₁₀ values obtained from the in vivo data. The present study provides the proof-of-principle that PBK modelling-based reverse dosimetry of YES assay data using a minimum PBK model can predict dose-dependent in vivo uterus growth caused by estrogenic chemicals. In future studies, the approach should be extended to include other estrogens.

Nuclear localization of catechol-O-methyltransferase in neoplastic and nonneoplastic mammary epithelial cells

Catechol-O-methyltransferase (COMT) plays both a regulatory and protective role in catechol homeostasis. It contributes to the regulation of tissue levels of catecholamines and catecholestrogens (CEs) and, by blocking oxidative metabolism of catechols, prevents endogenous and exogenous catechols from becoming a source of potentially mutagenic electrophiles. Evidence implicating CEs in carcinogenesis, in particular in the hamster kidney model of estrogen-induced cancer, has focused attention on the protective role of COMT in estrogen target tissues. We have previously reported that treating hamsters with estrogens causes translocation of COMT to nuclei of epithelial cells in the renal cortex, the site of CE biosynthesis and where the cancers arise. This finding suggested that nuclear COMT may be a marker of a threat to the genome by catechols, including CEs. It is postulated that CEs play a role in the genesis of breast cancer by contributing to a state of chronic oxidative stress that is presumed to underlie the high incidence of this disease in the United States. Therefore, here we used immunocytochemistry to re-examine human breast parenchyma for nuclear COMT. In addition to confirming previous reports of cytoplasmic COMT in mammary epithelial cells, we identified nuclear COMT in foci of mammary epithelial cells in histologically normal breast tissue of virtually all control (macromastia) and cancer patients and in breast cancer cells. There was no correlation between tissue histology and the numbers of cells with nuclear COMT, the size of foci containing such cells, or intensity of nuclear COMT immunostaining. The focal nature of the phenomenon suggests that nuclear COMT does not serve a housekeeping function but that it reflects a protective response to an increased local catechol load, presumably of CEs and, as such, that it may be a characteristic of the population of women studied who share the same major risk factor for developing breast cancer, that of living in the industrialized West.

Effect of combined testosterone and estradiol-17 beta treatment on the metabolism of E2 in the prostate and liver of noble rats

BACKGROUND

Long-term treatment of Noble (NBL) rats with testosterone (T) and estradiol-17 beta (E2) induces dysplasia in the dorsolateral lobe (DLP) but not in the ventral lobe (VP) of the rat prostate. The aim of this study was to determine whether metabolic conversion of E2 to catechol estrogens (CEs), which are potentially genotoxic, is a mechanism of estrogen carcinogenicity in this tissue.

METHODS

Male NBL rats were treated simultaneously with T and E2, or left untreated, for 16 weeks after which time the liver, VP, and DLP were excised for microsomal preparations. 3H-E2 metabolites generated in microsomal incubates were separated by high-performance liquid chromatography (HPLC) and identified by coelution with known E2 metabolites.

RESULTS

2- and 4-hydroxyestrogens were detected at high levels in hepatic microsomal incubates, and at extremely low levels in prostatic microsomal incubates. T + E2 treatment of rats did not increase the formation of these prostatic and hepatic metabolites.

CONCLUSIONS

These results do not support CE formation as a mediating step in estrogen-induced tumorigenesis in the rat prostate.

Chromatographic resolution of the type II estrogen binding site and a tyrosinase-like enzymatic activity from rat uterine nuclei

Nuclear extracts from estradiol-treated rat uteri which contain type II estrogen binding sites have recently been found to also contain a tyrosinase-like estradiol metabolizing activity. A recent study suggested that both the binding and enzymatic activities are significantly increased in the presence of micromolar concentrations of copper and ascorbate, display a number of common biochemical sensitivities, and share similar ligand/substrate binding affinities. Levels of both activities are significantly increased in uterus in response to hormone (estrogen) stimulation. These and other similarities indicate a possible relationship between the enzymatic and binding activities. A detailed chromatographic examination of these two activities in the present study revealed that while the type II sites and estradiol metabolizing activity exhibited virtually identical chromatographic properties on DEAE-high-performance liquid chromatography they are readily resolved on other chromatographic matrices, including phosphocellulose, DNA-cellulose, and S-Sepharose. These results demonstrate that type II binding sites are distinct from the tyrosinase-like enzyme activity previously described in rat uterine nuclear extracts.

Catecholestrogens are agonists of estrogen receptor dependent gene expression in MCF-7 cells

The catecholestrogens, namely 2-hydroxyestradiol (2-OH-E2) and 4-hydroxyestradiol (4-OH-E2) are important, naturally occurring metabolites of E2. Here we studied their role on estrogen dependent processes. Using the MCF-7 cell line as a model system we analyzed the potency of 2- and 4-OH-E2 on the synthesis of the 160 kDa secreted protein and on the transcription of the pS2 mRNA. Both processes are known to be E2 inducible and are mediated by the estrogen receptor. Control incubations using E2 and antiestrogens were performed to validate the assay procedure and to enable us to comparatively study the effects of the catecholestrogens. Stimulating MCF-7 cells for 2 days with 10(-8) M 2- or 4-OH-E2 resulted in an induction of the synthesis of the 160 kDa protein and in an increase in pS2 mRNA. Following hormonal stimulation with 2- or 4-OH-E2 [35S]methionine labeling of MCF-7 cells increased the level of newly synthesized and secreted 160 kDa protein 54 and 88% compared with the inductive potency of E2 (100%). The pS2 mRNA in MCF-7 cells was increased by a 2 day treatment with 10(-8) M 2- or 4-OH-E2 by 48 and 79%, respectively, compared to E2. Therefore, we conclude that the estrogen receptor is transcriptionally active in MCF-7 cells upon binding of catecholestrogens. The estrogen receptor in vivo may be active if the intracellular concentration of catecholestrogens generated is sufficient to allow occupation of the receptor. The possible action of these hormones in vivo is discussed.

Catecholestrogens are MCF-7 cell estrogen receptor agonists

Catecholestrogens are important metabolites of estradiol and estrone in the human. Considerable interest has focused on the catecholestrogens 2-hydroxy- and 4-hydroxyestradiol since they bind to the estrogen receptor with an affinity in the range of estradiol. Using the MCF-7 cell line, we analysed the capacity of purified catecholestrogens to transform the estrogen receptor into its high affinity nuclear binding form and to affect receptor-dependent processes such as proliferation and expression of the progesterone receptor (PR). Incubations with 2-hydroxy- and 4-hydroxyestradiol at 10(-8) M for 1 h resulted in tight nuclear binding of the estrogen receptor. During treatment of the cells with catecholestrogens we obtained a marked increase in proliferation rate of 36 and 76% for 2-hydroxy- and 4-hydroxyestradiol, respectively, relative to the inductive effect of estradiol (100%). The PR level, was slightly increased by treatment with 2-hydroxyestradiol (10%), whereas treatment with 4-hydroxyestradiol increased the PR level at 28%, compared to estradiol (100%). From these results we conclude that the 2- and 4-hydroxylated derivatives of estradiol are active hormones and are able to initiate estrogen receptor mediated processes in MCF-7 cells.

P450 enzymes of estrogen metabolism

Endogenous and exogenous estrogens undergo extensive oxidative metabolism by specific cytochrome P450 enzymes. Certain drugs and xenobiotics have been found to be potent inducers of estrogen hydroxylating enzymes with C-2 hydroxylase induction being greater than that of C-16 hydroxylase. Oxygenated estrogen metabolites have different biological activities, with C-2 metabolites having limited or no activity and C-4 and C-16 metabolites having similar potency to estradiol. Pathophysiological roles for some of the oxygenated estrogen metabolites have been proposed, e.g. 16 alpha-hydroxyestrone and 4-hydroxyestrone. These reactive estrogens are capable of damaging cellular proteins and DNA and may be carcinogenic in specific cells.

The sexually differentiated metabolism of [6,7-3H]17 beta-oestradiol in rats: male-specific 15 alpha- and male-selective 16 alpha-hydroxylation and female-selective catechol formation

The oxygenated-metabolite profiles of exogenous 17 beta-oestradiol (E2) in adult male and female Wistar rats have been characterized and major sex-dependent biotransformations observed which correlate with the regioselectivities of known sexually differentiated hepatic P450. [6,7-3H]E2 (27 micrograms/kg) was given i.v. The metabolites of E2 were rapidly and extensively excreted in bile (46 and 78% of the dose over 1 and 6 h, respectively). Female rats metabolized E2 by one major pathway: oxidation to oestrone (E1) followed by C-2 hydroxylation and O-methylation; the principal aglycones (0-1 h bile collections) were E1 (14%), 2-hydroxyE1 (2-OHE1) (42%) and 2-methoxyE1 (24%). Male rats metabolized E2 principally by two parallel composite pathways of E1 hydroxylation which yielded a complex mixture of mono- and di-oxygenated compounds: 15 alpha-OHE1 (33%), 2,15 alpha-diOHE1 (7%), and 2-methoxy-15 alpha OHE1 (14%); 16 alpha-OHE1 (13%), 2,16 alpha-diOHE1 (4%) and 2-methoxy-16 alpha-OHE1 (2%). 15 alpha-Hydroxylation was unique to males. The balance of aromatic and alkyl hydroxylation in males was dose-dependent: at 3 mg/kg, 15 alpha-hydroxylation was decreased approx. 50% in favour of 2-hydroxylation whilst 16 alpha-hydroxylation was largely unaffected. The male-specific 15 alpha-hydroxylation and male-predominant 16 alpha-hydroxylation of E1 derived from E2 in vivo may be ascribable to the male-specific isoforms P450IIC13 and P450IIC11, respectively.

Increased catechol estrogen metabolism as a risk factor for nonfamilial breast cancer

The metabolism of estrone (E1) or estradiol-17 beta (E2) to catechols seldom has been investigated in biochemical studies related to the risk of development of human breast cancer, as a result of the extreme lability and reactivity of these hormones. A method of indirect calculation was developed in which estimated catechol estrogen excretion (ECE) from urinary excretion of E1, E2, and estriol (E3) was used, based on the obligate reciprocal relation between 16 alpha-hydroxylase activity (r3) and estrogen 2/4 hydroxylase function (r2). This relationship is expressed by r2 x r3 = K, the estrogen oxidative constant. From published data relating chiefly to 2-OH estrone excretion, K = 12.4 +/- 0.8 (standard error of the mean). Urinary E1 + E2 excretion rates reflect nonprotein-bound plasma ovarian estrogen concentrations available for cell metabolism, which influence the value of K. The equation: r2 = [E1 + E2] K/[E3 + 16 alpha OH E1] = ECE gives a median correlation coefficient between actual catechol estrogen excretion and ECE in micrograms/24 hours of +0.88 (range, 0.61 to 0.97). When tested against the best product isolation analysis of catechol estrogen excretion, ECE was 95% accurate. Using this method a metaanalysis was conducted of published fractional estrogen excretion collected from 2846 healthy women worldwide aged 15 to 59 years, with a risk of breast cancer varying fivefold. Overall ECE was 78% to 97% higher in high-risk women of all ages and menstrual cycle phases (P less than 0.001, by Wilcoxon test). With increasing cancer risk (as estimated by the authors), ECE rose linearly exponentially with a slope of 0.149 (follicular phase) and 0.136 (luteal phase). The correlation coefficient (R2) between the two variables was 0.77 and 0.57, respectively (P less than 0.05). These data derived from calculations of ECE in healthy women confirmed recent analytic results of a twofold increase in the ratio of 2-OH E1/4-OH E1 in healthy Finnish women compared with recent Japanese migrants to Hawaii. In Finnish women with breast cancer, this ratio increased further (almost twofold). Metaanalysis supported the conclusion that increased rates of oxidation of estradiol 17-beta to 2-OH catechols supply the principal proximal human mammary carcinogens active after menarche.