3D models of human ERα and ERβ complexed with 5-androsten-3β,17β-diol

The promiscuous estrogen receptor: Evolution of physiological estrogens and response to phytochemicals and endocrine disruptors

Many actions of estradiol (E2), the principal physiological estrogen in vertebrates, are mediated by estrogen receptor-α (ERα) and ERβ. An important physiological feature of vertebrate ERs is their promiscuous response to several physiological steroids, including estradiol (E2), Δ⁵-androstenediol, 5α-androstanediol, and 27-hydroxycholesterol. A novel structural characteristic of Δ⁵-androstenediol, 5α-androstanediol, and 27-hydroxycholesterol is the presence of a C19 methyl group, which precludes the presence of an aromatic A ring with a C3 phenolic group that is a defining property of E2. The structural diversity of these estrogens can explain the response of the ER to synthetic chemicals such as bisphenol A and DDT, which disrupt estrogen physiology in vertebrates, and the estrogenic activity of a variety of plant-derived chemicals such as genistein, coumestrol, and resveratrol. Diversity in the A ring of physiological estrogens also expands potential structures of industrial chemicals that can act as endocrine disruptors. Compared to E2, synthesis of 27-hydroxycholesterol and Δ⁵-androstenediol is simpler, leading us, based on parsimony, to propose that one or both of these steroids or a related metabolite was a physiological estrogen early in the evolution of the ER, with E2 assuming this role later as the canonical estrogen. In addition to the well-studied role of the ER in reproductive physiology, the ER also is an important transcription factor in non-reproductive tissues such as the cardiovascular system, kidney, bone, and brain. Some of these ER actions in non-reproductive tissues appeared early in vertebrate evolution, long before the emergence of mammals.

Endometrium and steroids, a pathologic overview

Normal endometrial function requires of cell proliferation and differentiation; therefore, disturbances in these processes could lead to pathological entities such as hyperplasia and endometrial adenocarcinoma, where cell proliferation is increased. The development of these pathologies is highly related to alterations in the levels and/or action of sexual steroids. In the present review, it has been analyzed how steroids, particularly estrogens, androgens and progestagens are involved in the etiopathogenesis of hyperplasia and endometrial endometrioid adenocarcinoma. The emphasis is given on pathological and pharmacological conditions that are presented as risk factors for endometrial pathologies, such as obesity, polycystic ovarian syndrome and hormone replacement postmenopausal women therapy, among others. Steroids alterations may promote changes at molecular level that enhance the development of hyperplasia and endometrioid cancer. In fact, there are solid data that indicate that estrogens stimulate cell-proliferation in this tissue; meanwhile, progestagens are able to stop cell proliferation and to increase differentiation. Nevertheless, the role of androgens is less clear, since there is contradictory information. It is most likely that the major contribution of steroids to the development of cell proliferation pathologies in endometria would be in early stages, where there is a high sensitivity to these molecules. This phenomenon is present even in stages previous to the occurrence of hyperplasia, like in the condition of polycystic ovarian syndrome, where the endometria have a greater sensitivity to steroids and high expression of cell cycle molecules. These abnormalities would contribute to the pathogenesis of hyperplasia and then in the progression to endometrioid adenocarcinoma.

Age-dependent Increases in Adrenal Cytochrome b5 and Serum 5-Androstenediol-3-sulfate

CONTEXT

Adrenal production of dehydroepiandrosterone sulfate (DHEA-S) increases throughout childhood owing to expansion of the zona reticularis (ZR). ZR features cells with a steroidogenic phenotype distinct from that of the adjacent zona fasciculata, with higher expression of cytochrome b₅ type A (CYB5A) and steroid sulfotransferase type 2A1 but decreased 3β-hydroxysteroid dehydrogenase type 2 (HSD3B2). In addition to DHEA-S, three adrenal Δ5-steroid sulfates could provide additional tools to define adrenal maturation.

OBJECTIVE

This study sought to simultaneously measure serum levels of four adrenal Δ5-steroid sulfates, pregnenolone sulfate (Preg-S), 17α-hydroxypregnenolone sulfate (17OHPreg-S), DHEA-S, and 5-androstenediol-3-sulfate (Adiol-S) as a function of age and relate their production to the age-dependent adrenal localization of CYB5A.

PARTICIPANTS AND METHODS

Δ5-steroid sulfates were quantified by liquid chromatography-tandem mass spectrometry in sera from 247 normal children (129 males,118 females) age 1.5-18 y and 42 adults (20 males, 22 females). Immunofluorescence localized HSD3B2 and CYB5A in normal adrenal glands from subjects age 2-35 y. Finally, HAC15 adrenocortical cells were transduced with lentiviral short hairpin RNA to suppress CYB5A expression.

RESULTS

Of the Δ5-steroid sulfates quantified, DHEA-S was most abundant. Adiol-S increased in parallel with DHEA-S. Steroid ratios (17OHPreg-S/DHEA-S) suggested increases in 17,20-lyase activity during childhood. Immunofluorescence analysis showed age-related increases in ZR CYB5A immunoreactivity. Furthermore, silencing CYB5A in HAC15 adrenocortical cells significantly reduced DHEA-S and Adiol-S production.

CONCLUSION

Adiol-S shows a similar age-related increase to that of DHEA-S. This likely results from the childhood expansion of CYB5A-expressing ZR, which enhances 17,20-lyase activity and the production of DHEA-S and Adiol-S.

Use of computational modeling approaches in studying the binding interactions of compounds with human estrogen receptors

Estrogens have a whole host of physiological functions in many human organs and systems, including the reproductive, cardiovascular, and central nervous systems. Many naturally-occurring compounds with estrogenic or antiestrogenic activity are present in our environment and food sources. Synthetic estrogens and antiestrogens are also important therapeutic agents. At the molecular level, estrogen receptors (ERs) mediate most of the well-known actions of estrogens. Given recent advances in computational modeling tools, it is now highly practical to use these tools to study the interaction of human ERs with various types of ligands. There are two common categories of modeling techniques: one is the quantitative structure activity relationship (QSAR) analysis, which uses the structural information of the interacting ligands to predict the binding site properties of a macromolecule, and the other one is molecular docking-based computational analysis, which uses the 3-dimensional structural information of both the ligands and the receptor to predict the binding interaction. In this review, we discuss recent results that employed these and other related computational modeling approaches to characterize the binding interaction of various estrogens and antiestrogens with the human ERs. These examples clearly demonstrate that the computational modeling approaches, when used in combination with other experimental methods, are powerful tools that can precisely predict the binding interaction of various estrogenic ligands and their derivatives with the human ERs.

The role of androst-5-ene-3β,17β-diol (androstenediol) in cell proliferation in endometrium of women with polycystic ovary syndrome

Women with polycystic ovary syndrome (PCOS) show high prevalence of endometrial hyperplasia and adenocarcinoma. Endometrial proliferation is increased, evaluated by high levels of Ki67 (cell cycle marker) and low levels of p27 (negative regulator of cell cycle). Nevertheless, endometrial changes in cyclin D1 (positive regulator of cell cycle) in PCOS-women are not described. Androst-5-ene-3β,17β-diol (androstenediol), steroid with estrogenic activity present in endometria, could be related to increased endometrial cell proliferation. The objective of this study was to determine protein content of cyclin D1 and androstenediol levels in endometria from PCOS and control-women and to evaluate the possible mechanism favoring cell proliferation associated with hormonal characteristics of patients. Therefore, cyclin D1 protein content in PCOS-women and control-endometrial tissue were assessed by western blot and immunohistochemistry. The androstenediol levels were evaluated by ELISA. To further analyze the effect of steroids (androstenediol, 17β-estradiol, testosterone) in cell proliferation, levels of proteins cyclin D1, p27 and Ki67 were evaluated in an in vitro model of stromal endometrial cells T-HESC and St-T1b. An increase in cyclin D1 and androstenediol was observed in tissues from PCOS-women relative to control group (p<0.05). In the in vitro model, androstenediol exerted increase in cyclin D1 (p<0.05) and a decrease in p27 protein level (p<0.05), while Ki67 in St-T1b cells increased under this stimulus (p<0.05). Testosterone produces opposite effects in the levels of the above markers (p<0.05). Therefore, the hormonal imbalance associated with this syndrome could alter endometrial tissue homeostasis, promoting cell proliferation. Androstenediol is a molecule that could be involved by stimulating proliferation, whereas testosterone elicits a role of cell cycle repressor.

Steroid signaling: ligand-binding promiscuity, molecular symmetry, and the need for gating

Steroid/sterol-binding receptors and enzymes are remarkably promiscuous in the range of ligands they can bind to and, in the case of enzymes, modify - raising the question of how specific receptor activation is achieved in vivo. Estrogen receptors (ER) are modulated by 27-hydroxycholesterol and 5α-androstane-3β,17β-diol (Adiol), in addition to estradiol (E2), and respond to diverse small molecules such as bisphenol A. Steroid-modifying enzymes are also highly promiscuous in ligand binding and metabolism. The specificity problem is compounded by the fact that the steroid core (hydrogenated cyclopentophenanthrene ring system) has several planes of symmetry. Ligand binding can be in symmetrical East-West (rotation) and North-South (inversion) orientations. Hydroxysteroid dehydrogenases (HSDs) can modify symmetrical 7 and 11, also 3 and 17/20, positions, exemplified here by yeast 3α,20β-HSD and mammalian 11β-HSD and 17β-HSD enzymes. Faced with promiscuity and symmetry, other strategies are clearly necessary to promote signaling selectivity in vivo. Gating regulates hormone access via enzymes that preferentially inactivate (or activate) a subclass of ligands, thereby governing which ligands gain receptor access - exemplified by 11β-HSD gating cortisol access to the mineralocorticoid receptor, and P450 CYP7B1 gating Adiol access to ER. Counter-intuitively, the specificity of steroid/sterol action is achieved not by intrinsic binding selectivity but by the combination of local metabolism and binding affinity.

3D models of human ERα and ERβ complexed with coumestrol

Coumestrol, a phytoestrogen found in alfalfa, clover, and beans, has nM affinity for both estrogen receptor-α [ERα] and ERβ. Recently, a novel activity of coumestrol was reported: coumestrol binding to human ERβ represses microglia-mediated inflammation, which is associated with various neurodegenerative diseases, such as multiple sclerosis. In contrast, estradiol binding to ERβ had little or no effect on repression of microglia-mediated inflammation. Coumestrol and estradiol have several structural differences, which suggest that each ligand could induce different conformations in ERβ and, thus, different transcriptional responses in brain microglia. To begin to understand how coumestrol binds to ERβ and ERα, we constructed 3D models of coumestrol with human ERβ and ERα, which were compared to the structures of these ERs with estradiol. Of four possible orientations of coumestrol in ERα and ERβ, one orientation had the most favorable contacts with both ERs. Other phytochemicals may activate ERβ and inhibit inflammation in brain microglia and be useful therapeutics for inflammatory conditions in the brain.

What are the physiological estrogens?

Estradiol (E2) is the principal physiological estrogen in mammals. E2 and its active metabolites, estrone and estriol have a characteristic phenolic A ring, unlike progesterone, testosterone, cortisol and aldosterone, which have an A ring containing a C3-ketone, a Δ(4) bond and a C19 methyl group. Crystal structures of E2 in the estrogen receptor (ER) confirm the importance of the A ring in stabilizing E2 in the ER. However, other steroids, including Δ(5)-androstenediol, 5α-androstanediol and 27-hydroxycholesterol, which have a saturated A ring containing a 3β-hydroxyl and a C19 methyl group, also mediate physiological responses through binding to estrogen receptor-α (ERα) and ERβ. Moreover, selective estrogen response modulators (SERMs) with diverse structures also regulate transcription of ERα and ERβ. Our understanding of the physiological responses mediated by these "alternative" estrogens is in its infancy. Further studies of the role of these steroids and SERMs in regulating responses mediated by ERα and ERβ a variety of tissues, during different stages of development, are likely to uncover additional estrogenic activities.