Tamoxifen Elicits Atheroprotection through Estrogen Receptor α AF-1 But Does Not Accelerate Reendothelialization

Endothelial oestrogen-myocardial cyclic guanosine monophosphate axis critically determines angiogenesis and cardiac performance during pressure overload

AIMS

Oestrogen exerts beneficial cardiovascular effects by binding to specific receptors on various cells to activate nuclear and non-nuclear actions. Oestrogen receptor α (ERα) non-nuclear signalling confers protection against heart failure remodelling, involving myocardial cyclic guanosine monophosphate (cGMP)-cGMP-dependent protein kinase G (PKG) activation; however, its tissue-specific role remains elusive. Herein, we examine the cell type-specific role of ERα non-nuclear signalling in oestrogen-conferred protection against heart failure.

METHODS AND RESULTS

We first assessed the tissue-specific impacts of ERα on the cardiac benefits derived from oestrogen, utilizing endothelial ERα deletion (ERαf/f/Tie2Cre+) and myocyte ERα deletion (ERαf/f/αMHCCre+) female mice. Female mice were ovariectomized and the effect of estradiol (E2) was assessed in hearts exposed to 3 weeks of pressure overload [transverse aortic constriction (TAC)]. E2 failed to improve cardiac function in ERαf/f/Tie2Cre+ TAC hearts but provided benefits in ERαf/f/αMHCCre+ TAC hearts, indicating that endothelial ERα is essential. We next assessed the role of non-nuclear signalling in endothelial cells (ECs), employing animals with endothelial-specific inactivation of ERα non-nuclear signalling (ERαKI/KI/Tie2Cre+). Female ovariectomized mice were supplemented with E2 and subjected to 3-week TAC. ERαKI/KI/Tie2Cre+TAC hearts revealed exacerbated cardiac dysfunction and reduced myocardial PKG activity as compared to littermate TAC hearts, which were associated with attenuated myocardial induction of vascular endothelial growth factor (VEGF) and angiogenesis as assessed by CD31-stained capillary density. This phenotype of ERαKI/KI/Tie2Cre+was rescued by myocardial PKG activation from chronic treatment with a soluble guanylate cyclase (sGC) stimulator. We performed co-culture experiments to determine endothelial-cardiomyocyte interactions. VEGF induction by E2 in cardiac myocytes required a co-existence of intact endothelial ERα signalling in a nitric oxide synthase-dependent manner. On the other hand, VEGF was induced in myocytes directly with an sGC stimulator in the absence of ECs.

CONCLUSION

An endothelial oestrogen-myocardial cGMP axis stimulates angiogenic response and improves cardiac performance during pressure overload.

Prolonged tamoxifen-enriched diet is associated with cardiomyopathy and nutritional frailty in mice

Tamoxifen (TAM) is required for gene recombination in the inducible Cre/lox system. The TAM-enriched diet is considered safe, with negligible impact on animal wellbeing. However, studies reporting the long-term effects of the TAM diet and its potential impact on experimental outcomes are scarce. We conducted a longitudinal study on mice exposed to a 4-week dietary TAM citrate supplementation. Several parameters were recorded, such as body weight, body composition, mortality, and cardiac function. The collagen1a2 (Col1a2) transgenic mouse was used to assess TAM-induced recombination in vivo in cardiac fibroblasts followed by myocardial infarction (MI). The impact of TAM on the MI outcome was also evaluated. The recombination efficiency and cytotoxic effect of the TAM active metabolite, 4-hydroxy-tamoxifen (4-OHT), were assessed in vitro. Mice exposed to a TAM diet showed body weight loss and a 10% increase in mortality (P = 0.045). The TAM diet decreased cardiac function and induced cardiac remodeling, indicated by decreased fractional shortening from 32.23% to 19.23% (P = 0.001) and left ventricular (LV) wall thinning. All measured parameters were reversed to normal when mice were returned to a normal diet. Infarcted Col1a2-CreER mice on the TAM regimen showed gene recombination in fibroblasts, but it was associated with a substantial increase in mortality post-surgery (2.5-fold) compared to the controls. In vitro, 4-OHT induced gene editing in fibroblasts; however, cell growth arrest and cytotoxicity were observed at high concentrations. In conclusion, prolonged exposure to the TAM diet can be detrimental and necessitates careful model selection and interpretation of the results.

The beneficial effects of tamoxifen on arteries: a key player for cardiovascular health of breast cancer patient

Breast cancer is the most common cancer in women. Over the past few decades, advances in cancer detection and treatment have significantly improved survival rate of breast cancer patients. However, due to the cardiovascular toxicity of cancer treatments (chemotherapy, anti-HER2 antibodies and radiotherapy), cardiovascular diseases (CVD) have become an increasingly important cause of long-term morbidity and mortality in breast cancer survivors. Endocrine therapies are prescribed to reduce the risk of recurrence and specific death in estrogen receptor-positive (ER+) early breast cancer patients, but their impact on CVD is a matter of debate. Whereas aromatase inhibitors and luteinizing hormone-releasing hormone (LHRH) analogs inhibit estrogen synthesis, tamoxifen acts as a selective estrogen receptor modulator (SERM), opposing estrogen action in the breast but mimicking their actions in other tissues, including arteries. This review aims to summarize the main clinical and experimental studies reporting the effects of tamoxifen on CVD. In addition, we will discuss how recent findings on the mechanisms of action of these therapies may contribute to a better understanding and anticipation of CVD risk in breast cancer patients.

Estrogen Receptor and Vascular Aging

Cardiovascular diseases remain an age-related pathology in both men and women. These pathologies are 3-fold more frequent in men than in women before menopause, although this difference progressively decreases after menopause. The vasculoprotective role of estrogens are well established before menopause, but the consequences of their abrupt decline on the cardiovascular risk at menopause remain debated. In this review, we will attempt to summarize the main clinical and experimental studies reporting the protective effects of estrogens against cardiovascular diseases, with a particular focus on atherosclerosis, and the impact of aging and estrogen deprivation on their endothelial actions. The arterial actions of estrogens, but also part of that of androgens through their aromatization into estrogens, are mediated by the estrogen receptor (ER)α and ERβ. ERs belong to the nuclear receptor family and act by transcriptional regulation in the nucleus, but also exert non-genomic/extranuclear actions. Beside the decline of estrogens at menopause, abnormalities in the expression and/or function of ERs in the tissues, and particularly in arteries, could contribute to the failure of classic estrogens to protect arteries during aging. Finally, we will discuss how recent insights in the mechanisms of action of ERα could contribute to optimize the hormonal treatment of the menopause.

A historical view of estrogen effect on arterial endothelial healing: From animal models to medical implication

Endothelial barrier integrity is required for maintaining vascular homeostasis and fluid balance between the circulation and surrounding tissues. In contrast, abnormalities of endothelial cell function and loss of the integrity of the endothelial monolayer constitute a key step in the onset of atherosclerosis. Endothelial erosion is directly responsible for thrombus formation and cardiovascular events in about one-third of the cases of acute coronary syndromes. Thus, after endothelial injury, the vascular repair process is crucial to restore endothelial junctions and rehabilitate a semipermeable barrier, preventing the development of vascular diseases. Endothelial healing can be modulated by several factors. In particular, 17β-estradiol (E2), the main estrogen, improves endothelial healing, reduces neointimal accumulation of smooth muscle cells and atherosclerosis in several animal models. The aim of this review is to highlight how various experimental models enabled the progress in the cellular and molecular mechanisms underlying the accelerative E2 effect on arterial endothelial healing through the estrogen receptor (ER) α, the main receptor mediating the physiological effects of estrogens. We first summarize the different experimental procedures used to reproduce vascular injury. We then provide an overview of how the combination of transgenic mouse models impacting ERα signalling with pharmacological tools demonstrated the pivotal role of non-genomic actions of ERα in E2-induced endothelial repair. Finally, we describe recent advances in the action of selective estrogen receptor modulators (SERMs) on this beneficial vascular effect, which surprisingly involves different cell types and activates different ERα subfunctions compared to E2.

Sex and Tamoxifen confound murine experimental studies in cardiovascular tissue engineering

Tissue engineered vascular grafts hold promise for the creation of functional blood vessels from biodegradable scaffolds. Because the precise mechanisms regulating this process are still under investigation, inducible genetic mouse models are an important and widely used research tool. However, here we describe the importance of challenging the baseline assumption that tamoxifen is inert when used as a small molecule inducer in the context of cardiovascular tissue engineering. Employing a standard inferior vena cava vascular interposition graft model in C57BL/6 mice, we discovered differences in the immunologic response between control and tamoxifen-treated animals, including occlusion rate, macrophage infiltration and phenotype, the extent of foreign body giant cell development, and collagen deposition. Further, differences were noted between untreated males and females. Our findings demonstrate that the host-response to materials commonly used in cardiovascular tissue engineering is sex-specific and critically impacted by exposure to tamoxifen, necessitating careful model selection and interpretation of results.

Tamoxifen Accelerates Endothelial Healing by Targeting ERα in Smooth Muscle Cells

RATIONALE

Tamoxifen prevents the recurrence of breast cancer and is also beneficial against bone demineralization and arterial diseases. It acts as an ER (estrogen receptor) α antagonist in ER-positive breast cancers, whereas it mimics the protective action of 17β-estradiol in other tissues such as arteries. However, the mechanisms of these tissue-specific actions remain unclear.

OBJECTIVE

Here, we tested whether tamoxifen is able to accelerate endothelial healing and analyzed the underlying mechanisms.

METHODS AND RESULTS

Using 3 complementary mouse models of carotid artery injury, we demonstrated that both tamoxifen and estradiol accelerated endothelial healing, but only tamoxifen required the presence of the underlying medial smooth muscle cells. Chronic treatment with 17β-estradiol and tamoxifen elicited differential gene expression profiles in the carotid artery. The use of transgenic mouse models targeting either whole ERα in a cell-specific manner or ERα subfunctions (membrane/extranuclear versus genomic/transcriptional) demonstrated that 17β-estradiol-induced acceleration of endothelial healing is mediated by membrane ERα in endothelial cells, while the effect of tamoxifen is mediated by the nuclear actions of ERα in smooth muscle cells.

CONCLUSIONS

Whereas tamoxifen acts as an antiestrogen and ERα antagonist in breast cancer but also on the membrane ERα of endothelial cells, it accelerates endothelial healing through activation of nuclear ERα in smooth muscle cells, inviting to revisit the mechanisms of action of selective modulation of ERα.

The Impact of Estrogen Receptor in Arterial and Lymphatic Vascular Diseases

The lower incidence of cardiovascular diseases in pre-menopausal women compared to men is well-known documented. This protection has been largely attributed to the protective effect of estrogens, which exert many beneficial effects against arterial diseases, including vasodilatation, acceleration of healing in response to arterial injury, arterial collateral growth and atheroprotection. More recently, with the visualization of the lymphatic vessels, the impact of estrogens on lymphedema and lymphatic diseases started to be elucidated. These estrogenic effects are mediated not only by the classic nuclear/genomic actions via the specific estrogen receptor (ER) α and β, but also by rapid extra-nuclear membrane-initiated steroid signaling (MISS). The ERs are expressed by endothelial, lymphatic and smooth muscle cells in the different vessels. In this review, we will summarize the complex vascular effects of estrogens and selective estrogen receptor modulators (SERMs) that have been described using different transgenic mouse models with selective loss of ERα function and numerous animal models of vascular and lymphatic diseases.

The tissue-specific effects of different 17β-estradiol doses reveal the key sensitizing role of AF1 domain in ERα activity

17β-Estradiol (E2) action can be mediated by the full-length estrogen receptor alpha (ERα66), and also by the AF1 domain-deficient ERα (ERα46) isoform, but their respective sensitivity to E2 is essentially unknown. We first performed a dose response study using subcutaneous home-made pellets mimicking either metestrus, proestrus or a pharmacological doses of E2, which resulted in plasma concentrations around 3, 30 and 600 pM, respectively. Analysis of the uterus, vagina and bone after chronic exposure to E2 demonstrated dose-dependent effects, with a maximal response reached at the proestrus-dose in wild type mice expressing mainly ERα66. In contrast, in transgenic mice harbouring only an ERα deleted in AF1, these effects of E2 were either strongly shifted rightward (10-100-fold) and/or attenuated, depending on the tissue studied. Finally, experiments in different cell lines transfected with ERα66 or ERα46 also delineated varying profiles of ERα AF1 sensitivity to E2. Altogether, this work emphasizes the importance of dose in the tissue-specific actions of E2 and demonstrates the key sensitizing role of AF1 in ERα activity.

Estrogen Receptor-α Non-Nuclear Signaling Confers Cardioprotection and Is Essential to cGMP-PDE5 Inhibition Efficacy

Using genetically engineered mice lacking estrogen receptor-α non-nuclear signaling, this study demonstrated that estrogen receptor-α non-nuclear signaling activated myocardial cyclic guanosine monophosphate-dependent protein kinase G and conferred protection against cardiac remodeling induced by pressure overload. This pathway was indispensable to the therapeutic efficacy of cyclic guanosine monophosphate-phosphodiesterase 5 inhibition but not to that of soluble guanylate cyclase stimulation. These results might partially explain the equivocal results of phosphodiesterase 5 inhibitor efficacy and also provide the molecular basis for the advantage of using a soluble guanylate cyclase simulator as a new therapeutic option in post-menopausal women. This study also highlighted the need for female-specific therapeutic strategies for heart failure.

Nuclear Activation Function 2 Estrogen Receptor α Attenuates Arterial and Renal Alterations Due to Aging and Hypertension in Female Mice

Background

The cardiovascular protective effects of estrogens in premenopausal women depend mainly on estrogen receptor α (ERα). ERα activates nuclear gene transcription regulation and membrane‐initiated signaling. The latter plays a key role in estrogen‐dependent activation of endothelial NO synthase. The goal of the present work was to determine the respective roles of the 2 ERα activities in endothelial function and cardiac and kidney damage in young and old female mice with hypertension, which is a major risk factor in postmenopausal women.

Methods and Results

Five‐ and 18‐month‐old female mice lacking either ERα (ERα^−/−), the nuclear activating function AF2 of ERα (AF2°), or membrane‐located ERα (C451A) were treated with angiotensin II (0.5 mg/kg per day) for 1 month. Systolic blood pressure, left ventricle weight, vascular reactivity, and kidney function were then assessed. Angiotensin II increased systolic blood pressure, ventricle weight, and vascular contractility in ERα^−/− and AF2° mice more than in wild‐type and C451A mice, independent of age. In both the aorta and mesenteric resistance arteries, angiotensin II and aging reduced endothelium‐dependent relaxation in all groups, but this effect was more pronounced in ERα^−/− and AF2° than in the wild‐type and C451A mice. Kidney inflammation and oxidative stress, as well as blood urea and creatinine levels, were also more pronounced in old hypertensive ERα^−/− and AF2° than in old hypertensive wild‐type and C451A mice.

Conclusions

The nuclear ERα‐AF2 dependent function attenuates angiotensin II–dependent hypertension and protects target organs in aging mice, whereas membrane ERα signaling does not seem to play a role.

Effects of ticagrelor, empagliflozin and tamoxifen against experimentally-induced vascular reactivity defects in rats in vivo and in vitro

BACKGROUND

In the current investigation, the effects of the P2Y12 blocker ticagrelor, the sodium/glucose transporter-2-inhibitor empagliflozin, and the selective estrogen receptor modulator tamoxifen were examined against rheumatoid arthritis (RA)/diabetes mellitus (DM)-co-morbidity-induced defects in vascular reactivity.

METHODS

After model setting, rats were allocated into a normal control, an RA/DM-co-morbidity, and three treatment groups receiving ticagrelor, empagliflozin and tamoxifen. Aorta tissue was isolated for enzyme-linked immunosorbent assay (ELISA) and western blot estimation of the pro-inflammatory molecules angiotensin-II (Ang-II) and endothelin-1 (ET-1), the adhesion molecules P-selectin and vascular cell adhesion molecule-1 (VCAM-1), the energy preserving molecule adenosine-5'-monophosphate-activated protein kinase (AMPK), and the anti-inflammatory molecule vasoactive intestinal peptide (VIP). Estimations of endothelial nitric oxide synthase (eNOS) and matrix metalloproteinase (MMP)-2 were performed immunohistochemically, together with histopathological examination using hematoxylin and eosin and Masson trichrome staining. An in vitro study on rat aortic strips was conducted to assess aorta vasorelaxation.

RESULTS

Ticagrelor, empagliflozin and tamoxifen significantly increased aorta tissue AMPK and eNOS and decreased Ang-II, ET-1, P-selectin, VCAM-1 and VIP levels compared with RA/DM-co-morbidity, coupled with improved acetylcholine vasorelaxation in vitro.

CONCLUSION

Ticagrelor, empagliflozin and tamoxifen may correct vascular reactivity defects, where modulation of vascular AMPK, eNOS, Ang-II, ET-1, P-selectin, VCAM-1 and MMP-2 underline their protective effects.

Transactivation Function-1-Mediated Partial Agonist Activity of Selective Estrogen Receptor Modulator Requires Homo-Dimerization of the Estrogen Receptor α Ligand Binding Domain

The isolation of estrogen receptor alpha (ERα) cDNA was successful around 30 years ago. The characteristics of ERα protein have been examined from various aspects, primarily through in vitro cell culture studies, but more recently using in vivo experimental models. There remains, however, some uncharacterized ERα functionalities. In particular, the mechanism of partial agonist activity of selective estrogen receptor modulators (SERMs) that involves control of the N-terminal transcription function of ERα, termed AF-1, is still an unsolved ERα functionality. We review the possible mechanism of SERM-dependent regulation of ERα AF-1-mediated transcriptional activity, which includes the role of helix 12 of ERα ligand binding domain (LBD) for SERM-dependent AF-1 regulation. In addition, we describe a specific portion of the LBD that associates with blocking AF-1 activity with an additional role of the F-domain in mediating SERM activity.

[Towards an optimization of the modulation of the estrogen receptor during menopausal hormonal therapy]

Women now live more than a third of their lives after the onset of menopause. The decline in endogenous estrogen production during this period is accompanied by functional disorders that affect quality of life. These symptoms may be relieved by menopausal hormone therapy (MHT) initially based on the administration of equine conjugated estrogens (mainly in the United States, oral route) or the natural estrogen, 17β-estradiol (in Europe, transdermal route). Estrogen receptor α (ERα), but not ERβ, mediates most of the physiological effects of estrogens. ERα belongs to the superfamily of nuclear receptors and regulates the transcription of genes via its activation functions AF1 and AF2. In addition to these classical genomic actions, estrogens can activate a subpopulation of ERα present at the cell membrane and thereby induce rapid signals. In this review, we will summarize the evolution of MHTs in last decades, as well as treatments that use various selective estrogen receptor modulators (SERMs). Next, we will describe recent advances in the understanding of the mechanisms of estrogen action, in particular the respective roles of nuclear and membrane ERα as well as the potential implications for future therapies.

Oestrogen receptor α AF-1 and AF-2 domains have cell population-specific functions in the mammary epithelium

Oestrogen receptor α (ERα) is a transcription factor with ligand-independent and ligand-dependent activation functions (AF)-1 and -2. Oestrogens control postnatal mammary gland development acting on a subset of mammary epithelial cells (MECs), termed sensor cells, which are ERα-positive by immunohistochemistry (IHC) and secrete paracrine factors, which stimulate ERα-negative responder cells. Here we show that deletion of AF-1 or AF-2 blocks pubertal ductal growth and subsequent development because both are required for expression of essential paracrine mediators. Thirty percent of the luminal cells are ERα-negative by IHC but express Esr1 transcripts. This low level ERα expression through AF-2 is essential for cell expansion during puberty and growth-inhibitory during pregnancy. Cell-intrinsic ERα is not required for cell proliferation nor for secretory differentiation but controls transcript levels of cell motility and cell adhesion genes and a stem cell and epithelial mesenchymal transition (EMT) signature identifying ERα as a key regulator of mammary epithelial cell plasticity.

Oestrogen receptors α (ERα) are expressed in a subset of mammary epithelial cells. Here, the authors identify cells with low-ERα protein levels and show that distinct cell populations have distinct requirements for the AF1 and AF2 domains of the ERα, and ERα acts in a biphasic manner dependent on developmental stage.

Novel mechanism for estrogen receptor alpha modulation of murine lupus

Female sex is a risk factor for lupus. Sex hormones, sex chromosomes and hormone receptors are implicated in the pathogenic pathways in lupus. Estrogen receptor alpha (ERα) knockout (KO) mice are used for defining hormone receptor effects in lupus. Prior studies of ERα KO in lupus have conflicting results, likely due to sex hormone levels, different lupus strains and different ERα KO constructs. Our objective was to compare a complete KO of ERα vs. the original functional KO of ERα (expressing a short ERα) on disease expression and immune phenotype, while controlling sex hormone levels. We studied female lupus prone NZM2410 WT and ERα mutant mice. All mice (n = 44) were ovariectomized (OVX) for hormonal control. Groups of each genotype were estrogen (E2)-repleted after OVX. We found that OVXed NZM mice expressing the truncated ERα (ERα short) had significantly reduced nephritis and prolonged survival compared to both wildtype and the complete ERαKO (ERα null) mice, but surprisingly only if E2-repleted. ERα null mice were not protected regardless of E2 status. We observed significant differences in splenic B cells and dendritic cells and a decrease in cDC2 (CD11b+CD8^-) dendritic cells, without a concomitant decrease in cDC1 (CD11b-CD8a+) cells comparing ERα short to ERα null or WT mice. Our data support a protective role for the ERα short protein. ERα short is similar to an endogenously expressed ERα variant (ERα46). Modulating its expression/activity represents a potential approach for treating female-predominant autoimmune diseases.

Estrogen receptor subcellular localization and cardiometabolism

BACKGROUND

In addition to their crucial role in reproduction, estrogens are key regulators of energy and glucose homeostasis and they also exert several cardiovascular protective effects. These beneficial actions are mainly mediated by estrogen receptor alpha (ERα), which is widely expressed in metabolic and vascular tissues. As a member of the nuclear receptor superfamily, ERα was primarily considered as a transcription factor that controls gene expression through the activation of its two activation functions (ERαAF-1 and ERαAF-2). However, besides these nuclear actions, a pool of ERα is localized in the vicinity of the plasma membrane, where it mediates rapid signaling effects called membrane-initiated steroid signals (MISS) that have been well described in vitro, especially in endothelial cells.

SCOPE OF THE REVIEW

This review aims to summarize our current knowledge of the mechanisms of nuclear vs membrane ERα activation that contribute to the cardiometabolic protection conferred by estrogens. Indeed, new transgenic mouse models (affecting either DNA binding, activation functions or membrane localization), together with the use of novel pharmacological tools that electively activate membrane ERα effects recently allowed to begin to unravel the different modes of ERα signaling in vivo.

CONCLUSION

Altogether, available data demonstrate the prominent role of ERα nuclear effects, and, more specifically, of ERαAF-2, in the preventive effects of estrogens against obesity, diabetes, and atheroma. However, membrane ERα signaling selectively mediates some of the estrogen endothelial/vascular effects (NO release, reendothelialization) and could also contribute to the regulation of energy balance, insulin sensitivity, and glucose metabolism. Such a dissection of ERα biological functions related to its subcellular localization will help to understand the mechanism of action of "old" ER modulators and to design new ones with an optimized benefit/risk profile.

Towards optimization of estrogen receptor modulation in medicine

Women now spend more than one-third of their lives in the postmenopausal years, and the decline of endogenous estrogen production during menopause is accompanied by a series of functional disorders that affect the quality of life. These symptoms could be alleviated or even totally suppressed by menopausal hormone therapy (MHT), initially based on natural estrogens extracted from the urine of pregnant mares (mainly in the USA, using the oral route) and later from the synthesis of the natural estrogen, 17β-estradiol (mainly in Europe, in particular using the transdermal route). Estrogen receptor (ER) α is the main receptor mediating the physiological effects of estrogens. ERα belongs to the nuclear receptor superfamily and activates gene transcription in a time and tissue-specific manner through two distinct activation functions (AF), AF1 and AF2. In addition to these classical genomic actions, ERα also mediates membrane initiated signaling enabling rapid actions of estrogen, potentially along or in interaction with other receptors. Here, we provide a brief historical overview of MHT, and we then highlight recent advances in the characterization of new treatments based on the association of estrogens with selective estrogen receptor modulators (SERMs) or on the modulation of nuclear or membrane ERα.

Membrane and Nuclear Estrogen Receptor Alpha Actions: From Tissue Specificity to Medical Implications

Estrogen receptor alpha (ERα) has been recognized now for several decades as playing a key role in reproduction and exerting functions in numerous nonreproductive tissues. In this review, we attempt to summarize the in vitro studies that are the basis of our current understanding of the mechanisms of action of ERα as a nuclear receptor and the key roles played by its two activation functions (AFs) in its transcriptional activities. We then depict the consequences of the selective inactivation of these AFs in mouse models, focusing on the prominent roles played by ERα in the reproductive tract and in the vascular system. Evidence has accumulated over the two last decades that ERα is also associated with the plasma membrane and activates non-nuclear signaling from this site. These rapid/nongenomic/membrane-initiated steroid signals (MISS) have been characterized in a variety of cell lines, and in particular in endothelial cells. The development of selective pharmacological tools that specifically activate MISS and the generation of mice expressing an ERα protein impeded for membrane localization have begun to unravel the physiological role of MISS in vivo. Finally, we discuss novel perspectives for the design of tissue-selective ER modulators based on the integration of the physiological and pathophysiological roles of MISS actions of estrogens.

Accurate Control of 17β-Estradiol Long-Term Release Increases Reliability and Reproducibility of Preclinical Animal Studies

Estrogens are the subject of intensive researches aiming to elucidate their mechanism of action on the various tissues they target and especially on mammary gland and breast cancer. The use of ready-to-use slow releasing devices to administer steroids, especially estrogens, to small experimental animals remains the method of choice in terms of animal well-being and of safety for both the researcher and the animal. In this study, we evaluated and compared, in vitro and in vivo, the release kinetic of estradiol (E2) over sixty days from two different slow-releasing systems: the matrix pellet (MP) and the reservoir implant (RI). We compared the impact of these systems in three E2-sensitive mouse models : mammary gland development, human MCF7 adenocarcinoma xenograft and mouse melanoma progression. The real amount of E2 that is released from both types of devices could differ from manufacturer specifications due to inadequate release for MP and initial burst effect for RI. Compared to MP, the interindividual variability was reduced with RI thanks to a superior control of the E2 release. Depending on the dose-dependent sensitivity of the physiological or pathological readout studied, this could lead to an improvement of the statistical power of in vivo experiments and thus to a reduction of the required animal number. Altogether, our data draw attention on the importance to adequately select the slow-releasing device that is the most appropriated to a specific experiment to better fulfill the 3Rs rule (Replacement, Reduction, Refinement) related to animal welfare and protection.

The Activation Function-1 of Estrogen Receptor Alpha Prevents Arterial Neointima Development Through a Direct Effect on Smooth Muscle Cells

RATIONALE

17β-Estradiol (E2) exerts numerous beneficial effects in vascular disease. It regulates gene transcription through nuclear estrogen receptor α (ERα) via 2 activation functions, AF1 and AF2, and can also activate membrane ERα. The role of E2 on the endothelium relies on membrane ERα activation, but the molecular mechanisms of its action on vascular smooth muscle cells (VSMCs) are not fully understood.

OBJECTIVE

The aim of this study was to determine which cellular target and which ERα subfunction are involved in the preventive action of E2 on neointimal hyperplasia.

METHODS AND RESULTS

To trigger neointimal hyperplasia of VSMC, we used a mouse model of femoral arterial injury. Cre-Lox models were used to distinguish between the endothelial- and the VSMC-specific actions of E2. The molecular mechanisms underlying the role of E2 were further characterized using both selective ERα agonists and transgenic mice in which the ERαAF1 function had been specifically invalidated. We found that (1) the selective inactivation of ERα in VSMC abrogates the neointimal hyperplasia protection induced by E2, whereas inactivation of endothelial and hematopoietic ERα has no effect; (2) the selective activation of membrane ERα does not prevent neointimal hyperplasia; and (3) ERαAF1 is necessary and sufficient to inhibit postinjury VSMC proliferation.

CONCLUSIONS

Altogether, ERαAF1-mediated nuclear action is both necessary and sufficient to inhibit postinjury arterial VSMC proliferation, whereas membrane ERα largely regulates the endothelial functions of E2. This highlights the exquisite cell/tissue-specific actions of the ERα subfunctions and helps to delineate the spectrum of action of selective ER modulators.

The uterine and vascular actions of estetrol delineate a distinctive profile of estrogen receptor α modulation, uncoupling nuclear and membrane activation

Estetrol (E₄) is a natural estrogen with a long half-life produced only by the human fetal liver during pregnancy. The crystal structures of the estrogen receptor α (ERα) ligand-binding domain bound to 17β-estradiol (E₂) and E₄ are very similar, as well as their capacity to activate the two activation functions AF-1 and AF-2 and to recruit the coactivator SRC3. In vivo administration of high doses of E₄ stimulated uterine gene expression, epithelial proliferation, and prevented atheroma, three recognized nuclear ERα actions. However, E₄ failed to promote endothelial NO synthase activation and acceleration of endothelial healing, two processes clearly dependent on membrane-initiated steroid signaling (MISS). Furthermore, E₄ antagonized E₂ MISS-dependent effects in endothelium but also in MCF-7 breast cancer cell line. This profile of ERα activation by E₄, uncoupling nuclear and membrane activation, characterizes E₄ as a selective ER modulator which could have medical applications that should now be considered further.