Regulation of aromatic oxidation of estradiol-17 beta in maternal hepatic, fetal hepatic and placental tissues: comparative effects of a series of inducing agents

Updates on Mechanisms of Cytochrome P450 Catalysis of Complex Steroid Oxidations

Cytochrome P450 (P450) enzymes dominate steroid metabolism. In general, the simple C-hydroxylation reactions are mechanistically straightforward and are generally agreed to involve a perferryl oxygen species (formally FeO3+). Several of the steroid transformations are more complex and involve C-C bond scission. We initiated mechanistic studies with several of these (i.e., 11A1, 17A1, 19A1, and 51A1) and have now established that the dominant modes of catalysis for P450s 19A1 and 51A1 involve a ferric peroxide anion (i.e., Fe3+O2¯) instead of a perferryl ion complex (FeO3+), as demonstrated with 18O incorporation studies. P450 17A1 is less clear. The indicated P450 reactions all involve sequential oxidations, and we have explored the processivity of these multi-step reactions. P450 19A1 is distributive, i.e., intermediate products dissociate and reassociate, but P450s 11A1 and 51A1 are highly processive. P450 17A1 shows intermediate processivity, as expected from the release of 17-hydroxysteroids for the biosynthesis of key molecules, and P450 19A1 is very distributive. P450 11B2 catalyzes a processive multi-step oxidation process with the complexity of a chemical closure of an intermediate to a locked lactol form.

Endocrine-disrupting chemicals: elucidating our understanding of their role in sex and gender-relevant end points

Endocrine-disrupting chemicals (EDCs) are diverse and pervasive and may have significant consequence for health, including reproductive development and expression of sex-/gender-sensitive parameters. This review chapter discusses what is known about common EDCs and their effects on reproductively relevant end points. It is proposed that one way that EDCs may exert such effects is by altering steroid levels (androgens or 17-estradiol, E₂) and/or intracellular E₂ receptors (ERs) in the hypothalamus and/or hippocampus. Basic research findings that demonstrate developmentally sensitive end points to androgens and E₂ are provided. Furthermore, an approach is suggested to examine differences in EDCs that diverge in their actions at ERs to elucidate their role in sex-/gender-sensitive parameters.

Endocrine disrupters: a review of some sources, effects, and mechanisms of actions on behaviour and neuroendocrine systems

Some environmental contaminants interact with hormones and may exert adverse consequences as a result of their actions as endocrine disrupting chemicals (EDCs). Exposure in people is typically a result of contamination of the food chain, inhalation of contaminated house dust or occupational exposure. EDCs include pesticides and herbicides (such as dichlorodiphenyl trichloroethane or its metabolites), methoxychlor, biocides, heat stabilisers and chemical catalysts (such as tributyltin), plastic contaminants (e.g. bisphenol A), pharmaceuticals (i.e. diethylstilbestrol; 17α-ethinylestradiol) or dietary components (such as phytoestrogens). The goal of this review is to address the sources, effects and actions of EDCs, with an emphasis on topics discussed at the International Congress on Steroids and the Nervous System. EDCs may alter reproductively-relevant or nonreproductive, sexually-dimorphic behaviours. In addition, EDCs may have significant effects on neurodevelopmental processes, influencing the morphology of sexually-dimorphic cerebral circuits. Exposure to EDCs is more dangerous if it occurs during specific 'critical periods' of life, such as intrauterine, perinatal, juvenile or puberty periods, when organisms are more sensitive to hormonal disruption, compared to other periods. However, exposure to EDCs in adulthood can also alter physiology. Several EDCs are xenoestrogens, which can alter serum lipid concentrations or metabolism enzymes that are necessary for converting cholesterol to steroid hormones. This can ultimately alter the production of oestradiol and/or other steroids. Finally, many EDCs may have actions via (or independent of) classic actions at cognate steroid receptors. EDCs may have effects through numerous other substrates, such as the aryl hydrocarbon receptor, the peroxisome proliferator-activated receptor and the retinoid X receptor, signal transduction pathways, calcium influx and/or neurotransmitter receptors. Thus, EDCs, from varied sources, may have organisational effects during development and/or activational effects in adulthood that influence sexually-dimorphic, reproductively-relevant processes or other functions, by mimicking, antagonising or altering steroidal actions.

Embryonic prostaglandin H synthase-2 (PHS-2) expression and benzo[a]pyrene teratogenicity in PHS-2 knockout mice

The developmental role of prostaglandin H synthase-2 (PHS-2), which converts xenobiotics such as benzo[a]pyrene (B[a]P) to toxic free radical intermediates, is poorly understood. In this study, we determined the embryonic expression and teratological relevance of PHS-2 in pregnant CD-1 and B6/129S7 PHS-2 knockout mice. Wild-type (+/+) B6/129S7 dams given B[a]P on gestational day (GD) 10 had three times more fetal malformations than did +/- PHS-2-deficient dams (P<0.05). GD 10-13 CD-1 embryos had high PHS-2 protein expression, and both + /+ and +/- GD 19 B6/129S7 fetuses had more B[a]P-initiated malformations and postpartum lethality than did -/- littermates (P<0.05). Thus, embryonic PHS-2 is expressed constitutively during organogenesis and contributes substantially to B[a]P teratogenicity.

Cytochrome-P450-dependent biotransformation of xenobiotics in human and rodent embryonic tissues

Profound species differences and developmental stage differences as well as a lack of solid data prevent broad, sweeping generalizations in terms of statements that can be made concerning the prenatal expression of individual P450 isoforms. It is clear, however, that several of such isoforms are expressed at levels that can be toxicologically significant. At present, the greatest interest appears to be in P450s 1A1, 1B1, 2E1, and 3A7, each of which has been reported to be expressed at toxicologically significant levels or at least at potentially toxicologically significant levels during organogenesis. Reports of the expression of other P450 isoforms at later stages of gestation also have appeared in the recent literature.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, 12-O-tetradecanoylphorbol-13-acetate and 17 beta-estradiol on estrogen receptor regulation in MCF-7 human breast cancer cells

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) exhibits remarkably potent antiestrogenic activity. To further elucidate the role of estrogen receptor (ER) regulation in this response, we examined the effects of exposure to TCDD in MCF-7 human breast cancer cells on ER mRNA levels by using an RNase protection assay, on ER accumulation by using an ER immunocytochemical essay (ER-ICA), and on ER function by competitive binding assays under conditions of saturating 17 beta-estradiol (E2). Comparative studies were conducted with E2 and 12-O-tetradecanoylphorbol-13-acetate (TPA), as both compounds are known to suppress ER expression. Our results indicate that 1 nM E2 and 100 nM TPA both suppress ER mRNA levels as early as 4 h after exposure and to 33.6% and 16.5% of control levels, respectively, after 72 h. In contrast, no significant effect on ER mRNA levels was attributed to exposure to 10 nM TCDD. A greater than 50% reduction in positive staining was observed by ER-ICA after 72 h exposure to 1 nM E2 and to 100 nM TPA, while only an 11% reduction in positive staining was observed with 10 nM TCDD. Specific binding of [3H]E2 under saturating conditions (10 nM E2) in whole cells was reduced by 50% in cultures exposed to 100 nM TPA, although no effect on binding was observed with exposure to 10 nM TCDD. In contrast, specific binding using subsaturating 1 nM [3H]E2 was depressed by 49% in MCF-7 cells exposed to 10 nM TCDD for 72 h. This depression was inhibited by a 1-h treatment with 5 microM alpha-naphthoflavone, which inhibits TCDD-induced, P450-mediated, E2 metabolism, and subsequent E2 depletion. In conclusion, while TPA and E2 effectively down-regulate ER expression, TCDD, under antiestrogenic conditions, has little if any effect on total ER levels in MCF-7 cells, and thus ER modulation is probably not necessary for the suppression of estrogenic activity in MCF-7 cells by TCDD.

Antiestrogenic action of 2,3,7,8-tetrachlorodibenzo-p-dioxin: tissue-specific regulation of estrogen receptor in CD1 mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a polychlorinated aromatic hydrocarbon with teratogenic and carcinogenic properties. Previous studies in our and other laboratories have demonstrated that TCDD has antiestrogenic properties. In order to elucidate the mechanism of action of TCDD on estrogen sensitive tissues, we studied its effects on serum estradiol and estrogen receptor (ER) levels in liver and uteri of CD1 mice. Treatment with TCDD did not result in alterations of serum estradiol levels at any of the doses tested (1.0-30 micrograms/kg). In contrast, TCDD treatment induced a dose-dependent decrease in hepatic and uterine ER protein as determined by an enzyme immunoassay and equilibrium binding assays. A decrease in cytosolic and nuclear ER levels in uteri occurred as early as 24 hr after initial treatment with 30 micrograms/kg TCDD and recovery occurred by 14 days. Hepatic cytosolic and nuclear ER also decreased at a dose of 30 micrograms/kg TCDD at 24 hr after treatment, but recovery occurred only by 21 days. Studies in ovariectomized mice indicate that the regulation of hepatic ER by TCDD is independent of ovarian factors, but ovariectomy inhibited the downregulation of uterine ER by TCDD. Furthermore, determination of TCDD-induced cytochrome P-450 levels indicates that the downregulation of uterine ER is uncoupled from induction of hepatic cytochrome P-450. This study indicates that the antiestrogenic effects of low doses of TCDD are mediated through its ability to decrease hepatic and uterine ER and are not due to alterations in serum estradiol levels. Our results on ovariectomized mice indicate that TCDD-induced downregulation of ER is tissue specific and may involve different mechanisms at transcriptional or posttranscriptional levels.

Substrate specificities and functions of the P450 cytochromes

Currently, the major recognized biochemical functions of members of the large superfamily of P450 hemoproteins (referred to commonly as the cytochromes P450) include catalyses of the monooxygenations of a wide variety of endogenous and exogenous lipophilic chemicals. Substrates that have attracted the greatest attention thus far are steroids, fatty acids, eicosanoids, retinoids, other endogenous lipids, therapeutic agents, pesticides/herbicides, chemical carcinogens, industrial chemicals and other environmental contaminants and toxic xenobiotic organics of low molecular weight. Commonly, monooxygenation of such substrates results in the generation of metabolites capable of producing biological effects that are profoundly different (qualitatively as well as quantitatively) from those elicitable by the parent chemical per se. P45OXIX-dependent conversion of testosterone to estradiol-17 beta provides a dramatic example. Thus, these hemoproteins serve as extremely important but, as yet, largely unpredictable regulators of the biological effects producible by endobiotics as well as by xenobiotics. Current focus is on the identification and acquisition of sequence information on hereto unidentified and/or uncharacterized P450 isoforms and ascertainment of the specific functions of specific, individual isoforms. The regulation of quantities and activities of such isoforms in specific species/tissues, understandably, is also of great current interest. This interest has been further intensified by recent results indicating that substrate specificity associated with one P450 may not be the same as the corresponding isoform derived from a different animal species. Recent technological advances promise to greatly hasten the acquisition of knowledge concerning the functions of these important hemoproteins.

Biotransformation, estrogenicity, and steroid structure as determinants of dysmorphogenic and generalized embryotoxic effects of steroidal and nonsteroidal estrogens

A series of nine chemicals of varying structure and estrogenicity was investigated for biochemical determinants of their relative capacities to alter normal embryonic growth and developmental patterns during organogenesis in rats. In order to circumvent the potentially confounding influences of maternal factors, the direct effects of steroidal and nonsteroidal estrogens on cultured whole embryos were compared at concentrations producing readily measurable embryotoxicity but low embryolethality (2-20%). Nonsteroidal estrogens included were diethylstilbestrol (DES), hexestrol (HES), E,E-dienestrol (alpha-DIES), and tamoxifen (TAM). Steroidal estrogens were estradiol 17 beta (E2), estrone (E1), and 17 alpha-ethinylestradiol 17 beta (EE). For comparative purposes, the effects of two essentially nonestrogenic phenols, Z,Z-dienestrol (beta-DIES) and phenol, were also studied. TAM, a weak estrogen which also exhibits antiestrogenic properties, was studied for possible interactive effects with potent estrogens. Prosencephalic hypoplasia was the abnormality most consistently observed and was elicited by each of the chemicals investigated. Embryotoxicity was neither attenuated by TAM nor related to estrogenic potency or steroidal structure, but was strongly and unpredictably influenced by biotransformational determinants. Presence of a cytochrome P450-dependent oxidizing system in the culture medium resulted in marked increases in embryotoxicity of E1, E2, and phenol, only minor increases for beta-DIES and alpha-DIES, but in strikingly decreased effects of EE, TAM, and HES. It produced no statistically significant differences in effects of DES. The results obtained were compatible with the concept that effects of these agents on growth and development during the earlier stages of organogenesis are independent of steroid structure or estrogenic activity but strongly dependent upon pathways and rates of biotransformation of some (but not all) of the parent chemicals.

Contrasting effects of estradiol-17 beta and 17 alpha-ethinyl estradiol-17 beta on cultured whole embryos

Estradiol-17 beta (E2) and 17 alpha-ethinyl estradiol-17 beta (EE) were compared in terms of their relative capacities to alter growth and developmental patterns of cultured whole embryos during the early stages of organogenesis. Embryos exhibited a notable differential susceptibility to the embryotoxic effects of parents E2 vs EE when these estrogens were added directly to the media at the onset of the culture period. At initial concentrations of 0.1 mM, E2 failed to produce statistically significant effects whereas EE elicited marked embryotoxicity. Inclusion of a P-450-dependent biotransformation system in the culture media resulted in a significant attenuation of the embryotoxic effects of parent E2 vs EE when these estrogens were added directly to the media at the onset of the culture period. At initial concentrations of 0.1 mM, E2 failed to produce statistically embryotoxicity by hepatic S9. The divergent results produced by the two steroids could not be attributed to differences in rates of catecholestrogen generation in the culture medium or by the conceptuses. The results demonstrate definitive dissimilarities between the effects of two steroidal estrogens on developmental parameters and document marked differences in the effects of biotransformation on their embryotoxic potential. The data strongly suggest that the embryotoxicity of these steroids is not mediated via interactions with estrogen receptors. Additionally, the data show that the differential capacity of these two steroids to produce embryotoxic effects is diametrically opposite to earlier reported patterns of their carcinogenic potential in the Syrian hamster kidney.

A gas chromatographic/mass spectrometric assay for catechol estrogens in microsomal incubations: comparison with a radiometric assay

A gas chromatographic/mass spectrometric assay for quantifying two catechol estrogens, 2-hydroxyestradiol and 4-hydroxyestradiol, in microsomal preparations is described. The assay employs deuterium-labeled analogs of the catechol estrogens as internal standards and permits quantification of catechol estrogens, in microsomal incubations, at low (1-2) microM concentrations. The compounds are analyzed as their trimethylsilyl derivatives following separation by capillary gas chromatography.

Estrogen-2/4-hydroxylase activities in rat brain and liver microsomes exhibit different substrate preferences and sensitivities to inhibition

NADPH-dependent estrogen-2/4-hydroxylase activities in rat brain and liver microsomes were compared with respect to the utilization of different estrogens as substrates and the inhibitory effects of alpha-naphthoflavone, metyrapone and steroids. Of 6 different estrogens used as substrates, only 17 beta- and 17 alpha-estradiol were transformed relatively effectively by brain microsomes. In contrast liver microsomes utilized these two estrogens as well as ethynyl estradiol, estrone and diethylstilbestrol effectively. Estriol was a poor substrate for estrogen-2/4-hydroxylase activity in both tissues. With 40 microM 17 beta-estradiol as substrate the estrogen-2/4-hydroxylase activities in brain and liver were inhibited by alpha-naphthoflavone, metyrapone, progesterone, 17 alpha-hydroxyprogesterone and testosterone. The brain enzyme activity appeared to be more sensitive than the liver enzyme to inhibition by alpha-naphthoflavone and metyrapone. Testosterone propionate (50-100 microM) stimulated the brain enzyme activity significantly. Progesterone and 17 alpha-hydroxyprogesterone were the most effective steroidal inhibitors of brain estrogen-2/4-hydroxylase activity. In the liver the inhibitory potencies of 3 different steroids varied, depending on the estrogen used as substrate. With 17 beta-estradiol, for example, progesterone was the most potent steroidal inhibitor, while corticosterone was the most potent inhibitor when diethylstilbestrol was used as substrate. These findings indicate that rat liver microsomes can utilize a wider range of different estrogens for catecholestrogen formation than brain microsomes and suggest that the profiles of catecholestrogen-forming P-450 isozymes in the two organs differ.

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.

Patterns and regulation of estradiol metabolism by hepatic microsomes from two species of marine teleosts

Estradiol (E2) metabolites formed in vitro by microsomes from the marine teleosts winter flounder (Pseudopleuronectes americanus) and scup (Stenotomus chrysops) included at least seven products detected by thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC). The least polar metabolite was shown to be estrone by chromatographic and mass spectrometric identity with authentic estrone. Chromatographic analyses coupled with dual-label experiments also indicated formation of the catecholestrogen 2-hydroxyestradiol (2-OH-E2), which was the most prominent metabolite determined by TLC. Analysis of microsomal E2 2-hydroxylase activity by measuring the specific release of 3H2O from [2-3H]E2 indicated that it is mediated by cytochrome P-450. E2 2-hydroxylase activity normalized to microsomal protein was lower in females than in males for microsomes from both mature scup and winter flounder. Activity normalized to liver weight or body weight in female winter flounder was also lower than that in males. However, activity normalized to cytochrome P-450 content did not show sex differences in either species. E2 2-hydroxylase activity per nanomole cytochrome P-450 was reduced in scup treated with beta-naphthoflavone, which induces the hydrocarbon hydroxylase cytochrome P-450E. Studies employing reconstituted P-450E and microsomes preincubated with polyclonal antibodies against P-450E confirmed that this isozyme does not catalyze E2 2-hydroxylase activity in microsomes. However, preliminary work with scup cytochrome P-450A suggests that it may be an E2 2-hydroxylase. The studies establish that catecholestrogen formation is prominent in fish liver and that it is sexually differentiated, but further investigation is required to define the catalysts as well as the significance and regulation of this function.

Formation of benzo[a]pyrene-DNA adducts by microsomal enzymes: comparison of maternal and fetal liver, fetal hematopoietic cells and placenta

The formation of benzo[a]pyrene (BP)-DNA adducts was studied in vitro in the presence of microsomes prepared from the isolated labyrinth zone of the rat placenta, the hematopoietic erythroblast cells of the fetal liver, the fetal liver, as well as the maternal liver. Pregnant rats received beta-naphthoflavone (beta NF; 15 mg/kg, i.p.) on day 17 gestation. One day later, placentae, fetal and maternal livers were obtained and hematopoietic erythroblast cells were separated from hepatocytes in the fetal livers. The respective microsomal fractions were incubated in the presence of calf thymus DNA, NADPH-regenerating system and [3H]BP (300 microCi) at 37 degrees C for 30 min. Following beta NF pretreatment, the levels of covalent binding (pmol/mg DNA/mg microsomal protein) for maternal liver, fetal liver, placenta and erythroblast cells were: 28.4, 2.4, 0.31 and 3.9, respectively, with the hematopoietic erythroblast cells being the most active among fetal tissue preparations. The extent of transplacental induction compared to control was greatest in the hematopoietic cells (18-fold) followed by fetal liver (16-fold) and labyrinth zone (5-fold). Further experiments characterized the BP-DNA adducts formed by induced microsomes. DNA was isolated, purified and digested sequentially with DNase I, snake venom phosphodiesterase type II and alkaline phosphatase type III. The deoxynucleoside-BP adducts were purified on a Sephadex LH-20 column and then separated on HPLC and the adducts were quantitated radiometrically. Seven distinct adducts were separated on HPLC and named A-G in order of elution. Adduct B was prominent in all preparations (22-55% total radioactivity). The adduct profile and retention time for peak B is similar to that reported for the adduct formed by microsomal activation of 9-hydroxy BP. Peak D constituted a major fraction (19%) in maternal liver profiles in comparison with the three fetal tissue preparations (8%). In subsequent experiments, peak D was shown to be derived from reaction of (+/-)7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) with DNA. Peak C was unique to erythroblast cell and labyrinth profiles, while peak G was specific for maternal liver and fetal liver profiles. These results demonstrate that fetal liver and its hematopoietic cells are significant sites of BP bioactivation which may contribute to the fetal toxicity of polyaromatic hydrocarbons.