Covalent binding of estrogen metabolites to hamster liver microsomal proteins: inhibition by ascorbic acid and catechol-O-methyl transferase

Identification of Cytosolic Protein Targets of Catechol Estrogens in Breast Cancer Cells Using a Click Chemistry-Based Workflow

Catechol estrogens (CEs) are known to be toxic metabolites and the initiators of the oncogenesis of breast cancers via forming covalent adducts with DNAs. CEs shall also react with proteins, but their cellular protein targets remain unexplored. Here, we reported the identification of protein targets of CEs in the soluble cytosol of estrogen-sensitive breast cancer cells by multiple comparative proteomics using liquid chromatography-tandem mass spectrometry (LC-MS/MS) coupled with an improved click chemistry-based workflow. Multiple comparative proteomics composed of an experimental pair (probe versus solvent) and two control pairs (solvent versus solvent and probe versus solvent without enrichment) were studied using stable isotope dimethyl labeling. The use of 4-hydroxyethynylestradiol (4OHEE2) probe with an amide-free linker coupled with on-bead digestion and redigestion of the proteins cleaved from the beads was shown to greatly improve the recovery and identification of CE-adducted peptides. A total of 310 protein targets and 40 adduction sites were repeatedly (n ≥ 2) identified with D/H (probe/solvent) ratio >4 versus only one identified with D/H >4 from the two control pairs, suggesting that our workflow imposes only a very low background. Meanwhile, multiple comparative D/H ratios revealed that CEs may downregulate many target proteins involved in the metabolism or detoxification, suggesting a negative correlation between CE-induced adduction and expression of proteins acting on the alleviation of stress-induced cellular damages. The reported method and data will provide opportunities to study the progression of estrogen metabolism-derived diseases and biomarkers.

In Situ Click Reaction Coupled with Quantitative Proteomics for Identifying Protein Targets of Catechol Estrogens

Catechol estrogens (CEs) are metabolic electrophiles that actively undergo covalent interaction with cellular proteins, influencing molecular function. There is no feasible method to identify their binders in a living system. Herein, we developed a click chemistry-based approach using ethinylestradiol (EE2) as the precursor probe coupled with quantitative proteomics to identify protein targets of CEs and classify their binding strengths. Using in situ metabolic conversion and click reaction in liver microsomes, CEs-protein complex was captured by the probe, digested by trypsin, stable isotope labeled via reductive amination, and analyzed by liquid chromatography-mass spectrometry (LC-MS). A total of 334 liver proteins were repeatedly identified ( n ≥ 2); 274 identified proteins were classified as strong binders based on precursor mass mapping. The binding strength was further scaled by D/H ratio (activity probe/solvent): 259 strong binders had D/H > 5.25; 46 weak binders had 5.25 > D/H > 1; 5 nonspecific binders (keratins) had D/H < 1. These results were confirmed using spiked covalent control (strong binder) and noncovalent control (weak binder), as well as in vitro testing of cytochrome c (D/H = 5.9), which showed covalent conjugation with CEs. Many identified strong binders, such as glutathione transferase, catechol-O-methyl transferase, superoxide dismutase, catalase, glutathione peroxidase, and cytochrome c, are involved in cellular redox processes or detoxification activities. CE conjugation was shown to suppress the superoxide oxidase activity of cytochrome c, suggesting that CEs modification may alter the redox action of cellular proteins. Due to structural similarity and inert alkyne group, EE2 probe is very likely to capture protein targets of CEs in general. Thus, this strategy can be adopted to explore the biological impact of CEs modification in living systems.

Site-specific covalent modifications of human insulin by catechol estrogens: Reactivity and induced structural and functional changes

Proteins, covalently modified by catechol estrogens (CEs), were identified recently from the blood serum of diabetic patients and referred to as estrogenized proteins. Estrogenization of circulating insulin may occur and affect its molecular functioning. Here, the chemical reactivity of CEs towards specific amino acid residues of proteins and the structural and functional changes induced by the estrogenization of insulin were studied using cyclic voltammetry, liquid chromatography-mass spectrometry, circular dichroism spectroscopy, molecular modeling, and bioassays. Our results indicate that CEs, namely, 2- and 4-hydroxyl estrogens, were thermodynamically and kinetically more reactive than the catechol moiety. Upon co-incubation, intact insulin formed a substantial number of adducts with one or multiple CEs via covalent conjugation at its Cys 7 in the A or B chain, as well as at His10 or Lys29 in the B chain. Such conjugation was coupled with the cleavage of inter-chain disulfide linkages. Estrogenization on these sites may block the receptor-binding pockets of insulin. Insulin signaling and glucose uptake levels were lower in MCF-7 cells treated with modified insulin than in cells treated with native insulin. Taken together, our findings demonstrate that insulin molecules are susceptible to active estrogenization, and that such modification may alter the action of insulin.

Inhibiting effect of ethinylestradiol/levonorgestrel combination on microsomal enzymatic activities in rat liver and kidney

The aim of the study was to evaluate the effects of two therapeutic combinations of ethinylestradiol (EE) and levonorgestrel (LE), which are used in triphasic contraceptives, on the activities of drug-metabolizing enzymes in rat liver and kidney. Sexually mature female Wistar rats were given 0.03 mg EE and 0.05 mg LE, or 0.03 mg EE and 0.125 mg LE for 6 or 18 sexual cycles, i.e. for 30 or 90 days. EE/LE inhibited not only the metabolic capacity of P450, a protein which directly undergoes suicide inhibition, but also the level of rat liver cytochrome b5 (dependent on the heme pool) as well as the activities of NADPH-cytochrome P450 reductase and NADH-cytochrome b5 reductase in the liver and kidney. The majority of these effects were independent of the gestagen dose and of the duration of treatment, suggesting that estrogen is a predominant inhibiting factor in the EE/LE combination. The study has revealed differences in the enzyme activities between the liver and kidney, which may result from the fact that these organs display different sets of P450 isoforms and, therefore, their monooxygenase systems show distinct capacities to metabolize exogenous steroids.

Histone nuclear proteins are irreversibly modified by reactive metabolites of diethylstilbestrol

We demonstrate for the first time that diethylstilbestrol (DES), a synthetic estrogen, is converted by nuclei to histone-binding metabolite(s). Reaction of [3H]DES with nuclei in the presence of cumene hydroperoxide or NADPH revealed binding of [3H]DES to histone nuclear proteins. Gel electrophoresis experiments revealed that all five histones, 1, 2A, 2B, 3, and 4, were irreversibly bound to [3H]DES. Histones 1 and 3 were more susceptible to the attack by [3H]DES quinone, a metabolite of DES, than histones 2A, 2B, or 4. The kinetic constants, Km and Vmax, of this binding reaction in the presence of cumene hydroperoxide were 10 microM and 750 pmol/mg protein/30 min, respectively. This binding was significantly inhibited by cytochromes P-450 inhibitors. Low-molecular-weight thiols, such as glutathione and cysteine, or thiol modifiers, such as n-ethylmaleimide, dithionitrobenzoic acid, and hydroxymercuric benzoate, drastically inhibited binding of [3H]DES quinone to histone 3. The binding of [3H]DES metabolites to both transcriptionally active and inactive chromatin histone proteins was observed. We conclude that DES is metabolized to histone-binding metabolites, presumably by nuclear cytochrome P-450. DES quinone may be one of the histone-binding DES metabolites. These data suggest that an analogous in vivo modification in the transcriptionally active chromatin histones by DES metabolites may influence gene function.

Protective effect of estrogens and catecholestrogens against peroxidative membrane damage in vitro

The antioxidant effects of natural estrogens (estrone, E1; 17 beta-estradiol), synthetic estrogens (17 alpha-ethynylestradiol, EE2; mestranol, MES; diethylstilbestrol, DES) and catecholestrogens (2-hydroxyestradiol; 4-hydroxyestradiol, 4-OHE2) on lipid peroxidation induced by different means in rat liver microsomes were investigated. The extent of lipid peroxidation was determined by measuring thiobarbituric acid reactive substances. Prooxidants included Fe3+/ADP/reduced NADPH, Fe2+/ascorbate, tert-butyl hydroperoxide (t-BOOH) and 2,2'-azobis(2-amidinopropane) (AAPH). Estrogens and catecholestrogens decreased lipid peroxidation in all four systems tested. In the iron/ascorbate model it was shown that (i) 4-OHE2 and DES had analogous patterns of inhibition, irrespective of the presence of NADPH or the functional integrity of the microsomes, and (ii) the antioxidant activities of E1, EE2 and MES were dependent on the assay conditions with the activity being markedly higher when estrogen metabolism was favored. When peroxidation was initiated by the peroxyl radical generator AAPH, the inhibitory effects observed were least pronounced. Our data also showed that, in each of the systems, all inhibitors displayed the same order of inhibitory potency with DES and catecholestrogens being the most potent antioxidants under all experimental conditions used. The present results confirm earlier findings and point toward a link between estrogen metabolism and estrogen antioxidant activity. The data also indicate that estrogens and catecholestrogens interact with the peroxidative process at different levels with their interactions with iron or the metal-derived species being the most important modes of inhibition.

Glutathione conjugation as a mechanism for the transport of reactive metabolites

From this and other chapters in this volume, it should be clear that GSH conjugation no longer represents a mechanism for the detoxication of xenobiotics or their metabolites. Although the majority of conjugations with GSH do facilitate the efficient excretion of xenobiotics from the body, many examples now exist where this process results in enhanced biological reactivity (Monks et al., 1990a; Monks and Lau, 1992, 1994). The number of examples in which GSH conjugation plays an important role in the generation of biologically reactive intermediates is expanding rapidly and GSH-dependent toxicity is manifested in many diverse ways. As emphasized in this chapter, GSH can act as a transport form for reactive metabolites, permitting the delivery of such metabolites to target tissues distal to the site of the initial conjugation. This type of GSH conjugate may be important in the mutagenic, carcinogenic, nephrotoxic, embryotoxic, cataractogenic, methemoglobinemic, and neurotoxic properties of a variety of redox active compounds (Monks and Lau, 1992).

In vitro characterization of estrogen induced Syrian hamster renal tumors: comparison with an immortalized cell line derived from diethylstilbestrol-treated adult hamster kidney

Primary diethylstilbestrol-induced kidney tumors from Syrian hamsters were grown in vitro and maintained in culture for 6 mo. Combined immunohistochemical studies using antibodies to intermediate filaments and ultrastructural studies of tumor cells in culture exhibited characteristics similar to tumor cells in vivo. Furthermore, the cells manifested transformed properties in culture; they grew both as multilayered colonies attached to the tissue culture substrate and as floating multicellular colonies (spheroids). When cultured cells were injected into diethylstilbestrol-treated recipient hamsters, tumors developed at the injection sites. In contrast, renal tubules or whole kidney cortex from control hamsters cultured in the same medium underwent only short-term growth, with senescence developing after approximately 1 mo. However, cell cultures of kidney cortex from animals treated in vivo for 5 mo. with diethylstilbestrol formed a cell line. This diethylstilbestrol-induced cell line has been maintained in culture for 1.5 yr and has the following characteristics: a) it is anchorage-dependent, b) it is negative in in vivo tumorigenicity tests, and c) cultured cells are histochemically and ultrastructurally similar to cultured tumor cells. This culture system should prove to be of use in studying hormonal carcinogenesis in vitro.

Metabolism of [4-14C]estrone in hamster and rat hepatic and renal microsomes: species-, sex- and age-specific differences

The metabolism of [4-14C]estrone (E1) was examined in liver and kidney microsomes of adult castrated male and ovariectomized female hamsters and rats and in neonatal and immature hamster renal microsomes. In castrated male hamster liver microsomes, E1 was metabolized extensively to six major metabolites; 15 beta-hydroxyestrone, 7 alpha-hydroxyestrone, 6 alpha-hydroxyestrone, 6 beta-hydroxyestrone, 2-hydroxyestrone, and delta(9,11)-dehydroestrone, and a nonpolar fraction. Six minor metabolites of E1 were also detected. In contrast, kidney microsomes derived from castrated male hamsters metabolized E1 to mainly 17 beta-estradiol, 2- and 4-hydroxyestrone, 6 alpha-hydroxyestrone, 6 beta-hydroxyestrone and one monohydroxyestradiol metabolite. However, 16 alpha-hydroxyestrone was not detected. A variable, but low amount of estriol was also found. Interestingly, the quantity of 2-hydroxyestrone found in kidney microsomes of the hamster represented 26% of the total amount of metabolites formed, whereas in liver microsomes, only 9% of the overall metabolism resulted in the formation of 2-hydroxyestrone. The ability of kidney microsomes of female ovariectomized hamsters and two different rat strains to metabolize E1 was 5.9- and 9.4-fold lower, respectively, compared to renal microsomes of male castrated hamsters. The onset of oxidative metabolism in newborn hamster kidneys during development was also assessed. The results indicate that the oxidative metabolism of [14C]E1 in renal microsomes of newborn hamsters was 20-fold less than in kidney microsomes of adult hamsters. While catechol E1 metabolites were essentially negligible in hamster kidneys of these ages, it was evident that the conversion of E1 to estradiol via 17 beta-hydroxysteroid dehydrogenase resembles levels seen in the adult animals. Between the age of one and two months, the male hamster kidney exhibited the capacity to metabolize E1 at levels seen in fully mature adult hamsters.

Tyrosinase-like activity and estradiol binding in rat uterine nuclear extracts

Nuclear extracts from the uteri of estradiol-implanted rats contain a tyrosinase-like enzyme that has three activities: monophenolase or cresolase, diphenolase or catecholase, and estrogen binding. When [3H]estradiol was used as a substrate, 3H2O was released from the A ring in the presence of copper and ascorbic acid. The optimal concentrations of these cofactors for the cresolase activity were established. The cresolase activity was lost on attempts at further purification. Estradiol binding was observed in conjunction with the enzymatic activity and was dependent on the presence of ascorbic acid and copper. The most potent inhibitors of 3H2O release from [3H]estradiol were those with a dihydroxyphenol moiety. The reaction was also sensitive to sulfhydryl reagents. These features of the enzyme are distinctive from other oxidases capable of attacking the aromatic ring of estrogens.

The metabolism of 2,4-dibromo [6,7-3H]17 beta-oestradiol in the rat: ring-A dibromination blocks male-specific 15 alpha-hydroxylation and catechol formation

The metabolism in the rat of 2,4-dibromo-17 beta-oestradiol (2,4-DBE2), a compound of potential use for tumour imaging and assessment, has been studied. 2,4-DB[6,7-3H]E2 was synthesized by bromination of [6,7-3H]E2 with N-bromosuccinimide, and administered (40 micrograms/kg, i.v.) to anaesthetized male and female rats. Metabolites were rapidly and extensively excreted in bile (60 and 82% of the dose over 1 and 6 h, respectively). No unchanged compound was excreted. 2,4-DBE2 was almost entirely oxidized to 2,4-DB-oestrone; which was largely eliminated as its glucuronide but partly (approx. 30%) metabolized to 2,4-DB-16 alpha-hhydroxyoesterone and, to a minor extent, 2,4-DB-oestriol. No products of either oxidative or reductive debromination were detected. Neither of the two oxidative transformations of 2,4-DBE2 in the rat, in contrast with those of exogenous E2, was sex-selective, and 2,4-DB-oestrone underwent less extensive hydroxylation than oestrone formed from E2. In female rats, the substituents selectively redirected the principal site of hydroxylation from C-2 to C-16, whereas in males they had no significant effect on the existing 16 alpha-hydroxylation but did block the major pathway, 15 alpha-hydroxylation. Thus the sexual differentiation of E2 oxidative metabolism was abolished by direct blockage causing metabolic switching to a latent reaction in the female rat and long-range inhibition of the vicinal hydroxylation in the male rat.

Ontogeny of peroxidase activity in epithelium and eosinophils of the mouse uterus

Outbred CD-1 mice treated for 1 or 4 days with 1 mg/kg of diethylstilbestrol (DES) at various ages after birth were examined for histochemical localization of peroxidase in the uterine epithelium. Peroxidase activity in uterine extracts was also measured by a radiometric assay and the conversion of [3H]DES to [3H]Z,Z-diensestrol (Z,Z-DIES). While no peroxidase activity was detected by a histochemical method in uterine epithelium from untreated 5-day old mice, the enzyme was apparent in mice treated for 4 days with DES; uterine eosinophils were absent at this age. By day 9, DES-induced staining for peroxidase in uterine epithelial cells and the number of uterine eosinophils had increased significantly. In addition, at this age, the biochemical assays for uterine peroxidase were sensitive enough to show that DES is converted to Z,Z-DIES and that [3H]estradiol gives rise to 3H2O and water-soluble radioactive metabolites. The peroxidase response to DES, determined by both histochemical and biochemical methods, increased with the age of the immature mice. These data indicate that the neonatal uterus, although deficient in eosinophils, demonstrates a peroxidase response to estrogen and that this response is localized primarily in the luminal epithelium. The role of this DES-induced peroxidase activity in converting DES to activated metabolites that may cause cell damage is discussed.

The metabolism of 2,4-dibromo-17 alpha-ethynyl[6,7-3H]oestradiol in the rat

1. The metabolism of 2,4-dibromoethynyloestradiol (2,4-DBEE2) in the rat was studied in order to determine the influence of ring-A substituents on the phase I biotransformations of oestrogens. 2. 2,4-Dibromo-17 alpha-ethynyl[6,7-3H]oestradiol was synthesized by the one-stage bromination of 17 alpha-ethynyl[6,7-3H]oestradiol (EE2) with N-bromoacetamide, and administered (30 micrograms/kg, i.v.) to anaesthetized male and female rats. 3. A single metabolite, identified as a glucuronide of 2,4-DBEE2, was rapidly and extensively eliminated in bile by male rats (83% of the dose over 6 h). Females excreted additional minor conjugated metabolites. Neither unchanged 2,4-DBEE2 nor EE2 was detected in bile. 4. The hepatic residues after 6 h (percentage of dose) were 2.7% and 3.4% in male and female rats, respectively, whilst less than 0.1% per organ(s) was found in kidneys, heart, spleen, lungs and brain. 5. 2,4-Dibromo substitution of EE2 effectively blocked all phase I biotransformations whilst not limiting glucuronylation in male rats, but did not entirely preclude phase I metabolism in females. The inertness of 2,4-DBEE2 to ring-A hydroxylation in male rats conforms with the insignificant debromination of 2,4-dibromoestradiol by hepatic microsomes.

Catecholestrogens inhibit proliferation and DNA synthesis of porcine granulosa cells in vitro: comparison with estradiol, 5 alpha-dihydrotestosterone, gonadotropins and catecholamines

Studies were conducted to assess the role of catecholestrogens on ovarian follicular growth using cultured porcine granulosa cells. Effects of the catecholestrogens, 2-hydroxyestradiol (2-OH-E2) and 2-methoxyestradiol (2-MeO-E2) were compared to those of estradiol (E2). Treatment with saturating concentrations of 2-OH-E2 caused a significantly greater decrease in cell numbers measured after 2 days of treatment than E2 treatment. The inhibitory effect of 2-OH-E2 was time and concentration dependent, not associated with a change in the viability of cells, and was partially reversible. The potency of 2-MeO-E2 to inhibit cell numbers was similar to or greater than that of 2-OH-E2. 2-MeO-E2 had a greater inhibitory effect on DNA synthesis, as measured by [3H]thymidine incorporation into trichloroacetic acid-precipitable macromolecules, than 2-OH-E2 or E2 in the absence or presence of insulin, epidermal growth factor or platelet-derived growth factor. Concurrent treatment with epinephrine significantly enhanced the inhibitory effect of 2-OH-E2 on granulosa cell DNA synthesis. Collectively, these studies indicate that catecholestrogens are more potent inhibitors of granulosa cell replication than E2 and 5 alpha-dihydrotestosterone (DHT), and that catecholamines may modulate the antimitotic activity of 2-OH-E2. These results support the hypothesis that catecholestrogens play a role in proliferation of granulosa cells during growth of ovarian follicles.

Covalent binding to proteins of reactive intermediates resulting from prostaglandin H synthase-catalyzed oxidation of stilbene and steroid estrogens

Prostaglandin H synthase (PSH) is known to metabolically activate a variety of xenobiotics in vitro by means of its peroxidase activity. Recently, stilbene and steroid estrogens have been found to be cooxidized by ram seminal vesical microsomes, a rich source of PHS, to nonextractable metabolites bound to microsomal protein. To investigate further the nature of this protein binding, different radiolabeled estrogens were incubated with purified PHS, holoenzyme in the presence of various amounts of albumin (BSA), and radioactivity bound to protein was determined after gel electrophoretic separation. Diethylstilbestrol (DES), its analog hexestrol, and the steroid estrogens estrone and 2-hydroxy-estrone were cooxidized by PHS in vitro to metabolites that bound covalently to PHS and to BSA. Although a preferential binding of DES to PHS was found in the presence of excess BSA, reactive intermediates derived from DES, or from the other estrogens, were sufficiently stable to react with the competing nucleophile BSA as well. With respect to the metabolic reactions catalyzed by PHS, in addition to one-electron oxidation of phenolic functions, PHS catalyzed the aromatic hydroxylation of synthetic and steroid estrogens as shown by 3H2O release from regiospecifically labeled compounds and confirmed by product identification. Although DES was extensively metabolized by PHS, its aromatic hydroxylation was minor by comparison to estradiol, a difference possibly related to the compounds' redox potentials. Thus, cooxidation of estrogens in vitro resulted in phenoxy radicals, semiquinones and quinones, reactive intermediates capable of protein binding that may contribute to the adverse effects of stilbene and steroid estrogen observed in vivo and in short-term assays.