Microsomal activation of estrogens and binding to nucleic acids and proteins

Benign Liver Tumors and Oral Contraceptive Therapy

Clinical data obtained from 1972–1978 on 107 cases of oral-contraceptive-steroid-associated hepatic adenomas is presented in tabular form. Although a cause-and-effect relationship has not been definitively established, experimental evidence that supports an etiological role of contraceptive steroids in the production of benign liver tumors is described. Based on metabolic and toxicity studies, a mechanism is postulated whereby contraceptive steroid therapy may result in induced activities of drug metabolizing enzymes. In certain circumstances, increased levels of activated metabolites may irreversibly bind to hepatic macromolecules and produce altered cell growth. The possible promoter role of oral contraceptive steroids in tumor formation is discussed.

Reaction of thiol nucleophiles with 1,2-epoxy- and 4,5-epoxy-estrene-3-one-17 beta-ols

Four ring A steroidal epoxyenones as probable intermediate in the formation of catechol estrogens were synthesized. The isomeric 1 alpha,2 alpha-epoxy-17 beta-hydroxyestr-4-en-3-one (9) and 1 beta,2 beta-epoxy-17 beta-hydroxyestr-4-en-3-one (8) were synthesized from 17 beta-hydroxy-5 alpha-estra-3-one. The isomeric 4 alpha,5 alpha-epoxy-17 beta-hydroxyestr-1-en-3-one (11) and 4 beta,5 beta-epoxy-17 beta-hydroxyestr-1-en-3-one (10) were prepared from 19-nortestosterone. The reaction of 9 and 10 with sodium/ethanethiol resulted in the formation of three types of reactions leading to multiple products: 1,4-addition, opening of epoxide, and epoxide opening followed by dehydration. Reaction of 8 with ethanethiol gave only one compound identified as 2-ethanethio-1,4-estradien-17 beta-ol-3-one, while reaction of 9 with ethanethiol gave an unusual product identified as 4-estren-1 alpha,17 beta-diol-3-one. Unlike reaction of ethanethiol with 9 and 10, reaction with N-acetylecysteine or glutathione results in epoxide opening followed by dehydration leading to the formation of estradiol-4-thioethers.

Conversion of estradiol-17 beta to reactive embryotoxic intermediates by cytochrome P-450-dependent bioactivating systems

P-450-dependent enzyme systems added to media of cultured rat embryos markedly increased the embryotoxicity of estradiol-17 beta. Increases were markedly attenuated by omission of NADPH, omission of enzyme, substitution of female for male rat liver as enzyme source, d) replacement of N2 with CO or replacement of estradiol-17 beta with diethylstilbestrol. Embryotoxicity correlated well (r = 0.84) with catecholestrogen generating activities. Addition of a catechol-methylating system failed to modify embryotoxicity even though large quantities of methoxyestrogens were formed. The results document that endogenous estrogen can be converted by P-450 to embryotoxic intermediates and suggest that reactive proximate metabolites are precatechols, perhaps epoxyenones.

Estrogen 1,2-epoxides or estrogen quinones/semiquinones

Metabolic activation of estradiol leading to the formation of catechol estrogens is a prerequisite for its genotoxic activity. Both estrogen-o-quinones/semiquinones and estrogen 1,2-epoxides have been proposed to be responsible for this activity. Incubations of [3H]estradiol and [3H]1 alpha,2 alpha-epoxy-4-estrene-3-one-17 beta-ol (ketotautomer of estradiol 1,2-epoxide) with rat liver microsomal and cytosol preparations were carried out in the presence of SKF 525A, ascorbic acid, glutathione and cysteine. Ascorbic acid decreased binding to proteins and aqueous-soluble fraction with both [3H] estradiol and [3H]epoxyestrenolone in incubations with microsomes but no effect with cytosol fraction. Incubations of microsomes with thiols gave water-soluble metabolites which were characterized as 1(4)-thioether derivatives of 2-hydroxyestradiol and incubations of [3H]epoxyestrenolone with cytosol and thiols gave estradiol-2-thioether. Incubations with ascorbic acid and thiols resulted in decreased formation of water-soluble metabolites in microsomal incubations but not in cytosol incubations. These studies indicate that the major pathway for irreversible binding of estrogens to macromolecules involves estrogen-o-quinones/semiquinones and not estrogen 1, 2-epoxide.

Nature of the macromolecular binding of diethylstilbestrol to DNA and protein following oxidation by peroxidase/hydrogen peroxide

Incubation of [14C]diethylstilbestrol ([14C]DES) with horseradish peroxidase(HRP)/hydrogen peroxide in the presence of various polynucleotides and proteins led to macromolecular binding of radioactivity. Binding to DNA proved stable against ethanol precipitation, but was completely removed when the DNA was subjected to gel electrophoresis, caesium chloride density centrifugation, and mild hydrolysis. In contrast, binding to protein was stable in gel electrophoresis. The extent of binding did not differ significantly between proteins with and without thiol groups. These results imply that the products of peroxidase-mediated oxidation of DES bind to DNA in a strong but non-covalent manner, whereas binding to protein appears to be covalent and does not depend on the presence of thiol groups. The possible nature of the binding species is discussed.

Photosensitized irreversible binding of estrone to protein via a hydroperoxide intermediate: an explanation of (photo-) allergic side-effects of estrogens

After irradiation (lambda greater than 425 nm) for 15 min of a solution of [4-14C]-estrone, albumin and the photosensitizer hematoporphyrin in phosphate buffer, more than 30% of the radioactivity could not be extracted. When the protein was added after irradiation, irreversible binding also occurred. Sephadex gel filtration showed that the radiolabel was bound to albumin as well as to the photosensitizer. A 10 beta-hydroperoxide is the reactive intermediate in this binding. Inasmuch as phenolic steroids coupled to proteins have been used for the induction of estrogenic-specific antibodies, the irreversible binding observed between estrone and albumin by photosensitization might be an explanation for (photo)allergic disorders associated with estrogens.

Estrogen metabolism in rat liver microsomal and isolated hepatocyte preparations--II. Inhibition studies

The abilities of various inhibitors and metabolism modifiers to alter the metabolism of estradiol and the irreversible binding of estradiol to proteins were examined in subcellular microsomal incubations and in intact hepatocyte preparations. In studies with rat liver microsomal preparations containing estradiol and an NADPH-generating system, the irreversible binding of radiolabeled steroid metabolite(s) to the microsomal proteins was 77.59 pmol/mg/min (SD 6.1; 7.6% of total steroid). 2-Bromoestradiol and 4-bromoestradiol, inhibitors of estrogen 2-hydroxylase, effectively decreased this irreversible binding of radiolabeled estradiol metabolite(s) to microsomal proteins to 17 pmol mg-1 min-1 (2.1% of total estradiol). These haloestrogens were also effective inhibitors in the intact hepatocyte cells, decreasing the amounts of organic metabolites, aqueous-soluble conjugates, and protein-bound materials. The HPLC radiochromatograms of the organic-extracted fractions from the 2 h hepatocyte incubations demonstrate that the catechol estrogen products, i.e. 2-hydroxyestrogens and 2-methoxyestrogens, were present in lower amounts in the incubations containing the bromoestrogens than in control incubations containing no inhibitor. Ascorbic acid and cysteine, general modifiers of oxidative pathways of metabolism, also affected estradiol metabolism in microsomal and hepatocyte preparations. Both these agents were able to decrease the irreversible binding of estradiol to proteins in the microsomal assays. Ascorbic acid decreased the general metabolism of estradiol in the hepatocyte incubations but did not decrease irreversible binding to proteins. The addition of cysteine to the hepatocyte incubation resulted in an increased metabolism of estradiol and the production of more aqueous-soluble radiolabeled metabolites than the control incubations; however, cysteine did not decrease the amounts of estradiol metabolite(s) irreversibly bound to proteins. Investigations of steroid metabolism in the isolated hepatocytes thus provide an effective in vitro technique for examining the overall oxidative, reductive, and conjugative pathways that are functional in the liver and enables one to investigate the abilities of inhibitors, regulators, and modifiers to affect the metabolic processes. Also, these hepatocyte studies demonstrate that the inhibitors of estrogen 2-hydroxylase, 2-bromoestradiol and 4-bromoestradiol, can enter and act in the intact cells. Consequently, these agents may be useful pharmacological probes for examining the functions of catechol estrogens in other tissues.

Estrogen metabolism in rat liver microsomal and isolated hepatocyte preparations--I. Metabolite formation and irreversible binding to cellular macromolecules

The metabolism of endogenous estrogens, estradiol and estrone, and the irreversible binding of estrogens to cellular macromolecules have been examined and compared in subcellular microsomal and in intact hepatocyte preparations. In studies with rat liver microsomal preparations containing estradiol, an NADPH-generating system, and denatured DNA, the irreversible binding of radiolabeled steroid metabolite(s) to the microsomal proteins was 3.26 nmoles/mg protein in 1 hr (S.D. 0.39; 7.9% of total steroid) while binding to DNA was found to be 0.288 nmole/mg DNA/mg protein (S.D. 0.025; 0.39% of total steroid). No significant difference was observed between microsomal preparations from untreated, phenobarbital-treated or 3-methylcholanthrene-treated rats. Irreversible binding to proteins was also demonstrated in the intact hepatocyte cell incubations. After 2-hr incubations of estradiol with hepatocytes, 5.9% (S.D. 1.4%) of the steroid(s) was irreversibly associated with cellular proteins (approximately 1.43 pmoles/mg/min). Analysis of the organic-soluble metabolites demonstrated the presence of the catechol estrogens and their metabolites, 2-hydroxyestradiol, 2-hydroxyestrone, 2-methoxyestradiol, and 2-methoxyestrone. Estrone and estriol were also identified. The aqueous-soluble materials isolated from hepatocyte incubations contained glucuronide, sulfate, and apparent thioether conjugates, as determined by liberation from estrogen metabolites by treatment with beta-glucuronidase, sulfatase, and Raney nickel. Thus, extensive primary and secondary metabolism of estrogens occurs in intact hepatocyte incubations. Furthermore, irreversible binding of estrogens to cellular proteins occurs in these intact cells having demonstrated conjugative pathways of metabolism.

Interaction of the photosensitization products of oestrone with DNA: comparison with the horseradish peroxidase catalyzed reaction

The interaction with DNA of [4-14C]oestrone upon photosensitization with hematoporphyrin (HP) as a photosensitizer has been investigated. By means of Sephadex LH-20 gel filtration and extraction with dichloromethane it was found that, after irradiation (lambda greater than 425 nm) of a solution of HP, DNA and [4-14C]oestrone 21% of the radiolabel was associated with DNA. If DNA was added after irradiation 23% was bound to DNA, whereas 25% of the oestrone remained after photoreaction under the conditions applied. The binding occurs via the reactive 10 beta-hydroperoxy-1,4-estradien-3,17-dione, which is the only product after photosensitization of oestrone. The hydroperoxide has a strong interaction with DNA compared with that of other steroids. By repeated precipitation with 5 M NaCl and ethanol the association can be broken. It is reported, that binding of oestrone to protein induced by both photosensitization and horseradish peroxidase (HRPO)/H2O2 is irreversible, but that the amount of binding to DNA is dependent on the method of determination. However, neither the hydroperoxide nor its reduced product, a p-quinol, is intermediate or product in the HRPO catalyzed reaction of oestrogens. The tight association of the hydroperoxide product of oestrone with DNA, which may proceed via hydrogen bonding between the -OOH group and oxygen atoms of the backbone phosphate groups or of the furanose ring, might be a cause of chemical modification of DNA and of mutagenic effects.

Metabolism of stilbene estrogens and steroidal estrogens in relation to carcinogenicity

The oxidative metabolism of diethylstilbestrol (DES) and 17 alpha-ethynyl estradiol, as examples of stilbene- and steroid-type estrogens, is discussed with respect to the formation of reactive intermediates. For DES, a genotoxic potential is implied by metabolic studies and positive effects in short-term tests for genetic damage. A particularly important pathway for DES carcinogenicity appears to be peroxidase-mediated oxidation. Although data for steroidal estrogens are more ambiguous, the available evidence suggests that metabolic activation by peroxidatic oxidation may also be of importance for this class of estrogens.

Interaction of estrone and estradiol with DNA and protein of liver and kidney in rat and hamster in vivo and in vitro

[6,7-3H] Estrone (E) and [6,7-3H]estradiol-17 beta (E2) have been synthesized by reduction of 6-dehydroestrone and 6-dehydroestradiol with tritium gas. Tritiated E and E2 were administered by oral gavage to female rats and to male and female hamsters on a dose level of about 300 micrograms/kg (54 mCi/kg). After 8 h, the liver was excised from the rats; liver and kidneys were taken from the hamsters. DNA was purified either directly from an organ homogenate or via chromatin. The radioactivity in the DNA was expressed in the units of the Covalent Binding Index, CBI = (mumol chemical bound per mol DNA-P)/(mmol chemical administered per kg b.w.). Rat liver DNA isolated via chromatin exhibited the very low values of 0.08 and 0.09 for E and E2, respectively. The respective figures in hamster liver were 0.08 and 0.11 in females and 0.21 and 0.18 in the males. DNA isolated from the kidney revealed a detectable radioactivity only in the female, with values of 0.03 and 0.05 for E and E2, respectively. The values for male hamster kidney were less than 0.01 for both hormones. The minute radioactivity detectable in the DNA samples does not represent covalent binding to DNA, however, as indicated by two sets of control experiments. (A) Analysis by HPLC of the nucleosides prepared by enzyme digest of liver DNA isolated directly or via chromatin did not reveal any consistent peak which could have been attributed to a nucleoside-steroid adduct. (B) All DNA radioactivity could be due to protein contaminations, because the specific activity of chromatin protein was determined to be more than 3,000 times higher than of DNA. The high affinity of the hormone to protein was also demonstrated by in vitro incubations, where it could be shown that the specific activity of DNA and protein was essentially proportional to the concentration of radiolabelled hormone in the organ homogenate, regardless of whether the animal was treated or whether the hormone was added in vitro to the homogenate.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Influence of estrogens on peroxidase activity in the Syrian hamster liver, kidney, and renal adenocarcinoma

Only very low levels of peroxidase activity were detected in castrated male hamster kidneys [1.0 +/- 0.8(S.E.) units/g protein], and chronic estrogen administration, either diethylstilbestrol (DES) or 17 beta-estradiol, for 1-5 months did not result in any appreciable increase in this activity. In contrast, hamster liver peroxidase activity was initially 10- to 20-fold higher than kidney levels, and chronic estrogen treatment for similar periods resulted in up to a 9-fold elevation in the activity of this enzyme. Moreover, the level of liver peroxidase activity in both intact and in 5 alpha-dihydrotestosterone-treated castrate hamsters was 2-fold higher than castrate-untreated values. Pure renal carcinoma induced after 9 months of estrogen treatment exhibited peroxidase values similar to those found in hamster livers [124 +/- 27 (S.E.) units/g protein] following estrogen treatment. When administered concomitantly with DES, tamoxifen significantly reduced the elevated levels of liver peroxidase activity observed after 2 months of DES treatment alone. A high affinity (KA = 0.10 X 10(9)M-1) estrogen receptor was found in liver cytosols of DES-treated hamsters which had increased slightly from untreated castrate levels.

Mutagenicity assays of estrogenic hormones in mammalian cells

In view of the extensive use of estrogenic hormones by the human population, either as therapeutic agents, or in the composition of contraceptive pills, it is important to investigate more thoroughly the adverse biological effects of synthetic hormones. Diethylstilbestrol, ethynylestradiol, estradiol-17beta and estrone were chosen for our experiments. Evidence of carcinogenicity in rodents has been reported for each of these compounds, but so far only few studies have been carried out in vitro. Because it has been shown that isolated liver cells in suspension are able efficiently to metabolize steroid hormones, we have tested these chemicals in V79 cells with a cell-mediated system using primary hepatocytes from male and female rats as the metabolic layer. The incubation in the presence of the chemical to be tested was carried out at concentrations ranging from 25 to 100 microM, for 48 h before plating the V79 cells to score for mutagenicity or toxicity. In the absence of hepatocytes, the 4 estrogenic hormones were very toxic, but not mutagenic. The co-cultivation of V79 cells with primary hepatocytes decreased the toxic effect induced by the sex hormones, except in the case of ethynylestradiol. However, no mutation, determined as 8-azaguanine- or ouabain-resistance, was induced under these conditions by any of the hormones tested. The lack of mutagenic activity of these hormones in our assay had been confirmed by the use of primary liver cells that originated from a rat treated with Aroclor, an inducer of drug-metabolizing enzymes.

Lack of oestradiol-DNA binding in mouse embryo cell cultures or following rat-liver microsomal metabolism

Oestradiol-17beta was compared to benzo[a]pyrene (BP) for its ability to bind to the DNA of mouse embryo cells in culture or to DNA added to a rat liver microsomal incubation mixture. No significant binding was found in embryo cells (at least 100-fold less than for BP). Microsomal incubation resulted in apparent oestradiol binding to a reisolated DNA-containing complex, but most of this was lost when the DNA was freed of RNA and protein.

Distribution of cytoplasmic estrogen and progesterone receptors in human endometrium

Unoccupied estrogen receptors and progesterone receptors were measured in the cytoplasm of five sections along the length of endometrium obtained from noncancerous, premenopausal hysterectomy specimens. The concentrations of the two receptors were measured with tritiated estradiol or R5020 (a synthetic progestin), the latter two having been purified by high-pressure liquid chromatography, and were found to be highest in the fundus and lowest in the cervix. Progesterone receptor levels, ranging from 50 to 3,500 fmoles of R5020 bound per milligram of protein, were generally much higher in each section of the endometrium than estrogen receptor levels, which ranged from 0 to 500 fmoles of estradiol bound per milligram of protein. Near ovulation it seemed that the distribution profiles of both receptors became very steep, with more than a tenfold difference in the receptor levels being found between the fundus and the cervix. Receptor levels measured in endometrial samples obtained by curettage or aspiration should be interpreted with caution.

Covalent binding of ethinylestradiol and estrone to rat liver DNA in vivo

The covalent binding of [6,7-3H] ethinylestradiol (EE) and [6,7-3H] estrone (E) to liver DNA of 200 g female rats was measured 8 h after the administration of 80 microgram (9.2 mCi) estrogen by gavage. The binding is 1.5 for EE and 1.1 for E, expressed as binding to DNA/dose, in units of mumol hormone/mol DNA phosphate/mmole hormone/kg body wt. It is the same order of magnitude as for benzene and about 10 000 times below the binding of typical liver carcinogens, such as aflatoxin B1 or N,N-dimethylnitrosamine.

Toxic agents resulting from the oxidative metabolism of steroid hormones and drugs

The oxidative metabolism of some exogenous compounds, and possibly some endogenous compounds as well, can lead to the formation of reactive metabolites. These intermediates react as electrophiles, and they lead in some instances to cell death or cell transformation. Three routes (other routes are also known) of toxicity are discussed. These are the epoxide/dihydrodiol pathway, the catechol/o-quinone pathway, and the alkylation pathway. The possible formation of electrophiles from diethylstilbestrol, from natural estrogens, and from ethynylestradiol is discussed in terms of protein binding. Protein binding is presumptive evidence of electrophile formation, but it does not necessarily indicate that the parent compound is highly cytotoxic, mutagenic, or carcinogenic. Mutagenic and carcinogenic activity is presumed to require reaction of an electrophile with nuclear material. There is evidence for protein binding for these estrogens (diethylstilbestrol, natural estrogens, ethnylestradiol) as a consequence of oxidative metabolism.