The Role of Estradiol in Reproduction and Health: Insights from Molecular Signaling to Hormone Replacement Strategies

Estrogen is involved in many physiological processes that include cell division, cell differentiation, cell death, and immune reactions due to a multitude of signaling pathways. These actions are through genomic as well as non-genomic effects by which the molecule elicits various cellular and systemic responses. The first one is the genomic, where estrogen binds to the nuclear receptors and gets involved in the process of gene transcription, and the second is the non-genomic, where estrogen binds to the membrane-bound receptors and initiates the intracellular signaling pathways. These signaling systems are, as such, very important in reproductive function, metabolism, the cardiovascular system, and the nervous system. Estrogen signaling disruption is closely associated with different pathological states such as hormone-sensitive cancers, bone disorders, cardiovascular diseases, and neurodegenerative diseases [32].

Description of different estrogen receptors (erα, erβ, gper) and their distribution in reproductive tissues

Despite being synthesized from the same gene, estrogen acts through three types of receptors, which include Estrogen Receptor alpha (ERa), Estrogen Receptor beta (ERβ), and G-Protein coupled Estrogen Receptor (GPER). These receptors are present in various tissues of the body, and they play various roles. ERa is most abundantly found in the uterus, breasts, liver, bones, and cardiovascular system, where its main focal areas of activity are in the commencement of reproductive tissues and reproductive organ functions. IUGR affects the growth of the uterus, development of the mammary glands, modeling of bones, lipid profile, and vascular integrity. ER is involved in the stimulation of cell division mainly in estrogen-responsive tissues, which makes it rational in conditions that are hormone-dependent, such as breast cancer and endometrial hyperplasia [33].

ERβ is mainly found in the ovaries, the prostate, the lungs, and the brain; it is involved in cell differentiation and has anti-proliferation effects. While ERa promotes cell growth in tissues, ERβ has been known to possess anti-tumor activity and is also involved in controlling the signal of estrogen regarding inflammation, immunological reaction, and neuronal signaling. Several studies have established that the activation of ERβ can have an inhibitory effect on ERa in the promotion of cell proliferation, thus providing the much-needed balance to prevent the progression of estrogen-induced carcinogenicity. Moreover, ERβ has been proven to be involved in the modulation of synaptic plasticity and age-related cognitive changes and thus might play a neuroprotective role in aging and neuropathological disorders, including Alzheimer’s disease [20].

Estrogen also binds to other nuclear receptors, but the most novel and well-known is the G-Protein-coupled Estrogen Receptor (GPER) that spans the membrane of cells and gives quick, non-genomic estrogen responses. GPER activates several pathways, such as the Mitogen-Activated Protein Kinase (MAPK) and Phosphatidylinositol-3-Kinase (PI3K/AKT) that involve various physiological aspects like metabolism, immunity, and the cardiovascular system. GPER is widely expressed in vascular endothelial cells, immune cells, and metabolic tissues and modulates vasodilation, swelling, and glucose metabolism. It has been shown that estrogen protection is mediated by GPER through an increase in NO production, a decrease in oxidative stress, and hence, the risk of atherosclerosis and hypertension [17].

The usage of ERa, ERb, and GPER in various tissues and their diverse types of functions emphasize the fact that the mechanisms of estrogen signaling are rather shades than black and white. Whereas ERa and ERb are mostly involved in the regulation of gene expressions in the target tissues, GPER mediates its rapid onset non-genomic effects on metabolism and signaling pathways [34]. This intertwined interaction makes sure that estrogen has a state and prolonged impact on numerous organ structures. Most noteworthy, ERa and ERb can regulate the biological actions of estrogen, and their ratio leads to either the promotion of cell proliferation or the inhibition and regulation of tissue homeostasis, respectively. This balance is of most significance in hormone-sensitive cancer being treated, where a selective targeting of the estrogen receptor is an approach to the management of breast and ovarian carcinomas [35].

Estrogen receptors are also involved in neuroendocrine activity, immune system function, and metabolism as well. In the brain, ERa and ERb b are implicated in processes such as synaptic plasticity, neuroprotection, and cognition, whereas GPER is involved in processes like neurotransmission and neuroinflammation [36]. Oxidative stress, neurodegeneration, and cognitive loss are the effects of low estrogen levels more evident in menopausal women, explaining why estrogenic signaling is critical in endorsing brain health. In the immune system and inflammation, estrogen interferes with cytokines and immune cells, involving both the ligand-binding domain ER and GPER, contributing to inflammation and/or immune sensitivity with autoimmune diseases. Furthermore, estrogen dependency of metabolic processes involves insulin sensitivity, adipogenesis, and energy balance through modulation of metabolic proteins and receptors, among them Peroxisome Proliferator-Activated Receptors (PPARs) and AMP-Activated Protein Kinase (AMPK) [37].

Due to the role estrogen plays in estrogen-related disorders, it is very significant for one to have a clear understanding of estrogen signaling at the molecular level in order to develop appropriate therapeutic interventions. Tamoxifen and Raloxifene are two of the most important SERMs since they are used to produce tissue-selective activation of ERa and ERb to treat certain diseases, including breast cancer and osteoporosis. Likewise, GPER agonists and antagonists are still undergoing research for their capability to treat cardiovascular diseases, metabolic diseases, and neurodegenerative diseases. It is for these reasons and the fact that the estrogen signal transduction system is one of the most complicated known to man that more research is required to establish the various molecular mechanisms that regulate the physiological effects of estrogen and their relevance to health and disease [18].

Figure 3 shows the distribution of ERa and ERb receptors in different tissues, together with their corresponding effects on these tissues. The metabolic system shows divergent reactions between ERa and ERb, whereas these receptors control food ingestion levels alongside metabolic activities such as insulin responsiveness, together with the regulation of lipid substances. This schema presents two pathways where Estrogen (E2) affects tissues: one that works directly and another that operates through the autonomic nervous system. Various organs within the body receive estrogen signals to control their metabolic functions through physiological mechanisms at the brain, liver, pancreas, adipose tissue, and muscles [38].

Figure 3: Estrogen Receptor (ERα and ERβ) distribution and its effects in reproductive.

Classical genomic and non-genomic signaling pathways of estradiol

Most of the biological functions of estradiol that have been established above occur through complex signaling processes that allow for the proper regulation of cellular processes in the microenvironment [39,40]. These activities occur through two main routes—genomic and non-genomic routes. The genomic pathway operates on a slower note and is used in maintaining long-term changes in expression of a particular gene, while the non-genomic pathway operates on a faster note and is used in eliciting intracellular signaling pathways [41]. The integration of these pathways unveils the intricate nature of estrogen and its significance to female reproductive, neurological, cardiovascular, and skeletal functions. Such knowledge of molecular changes offers key information on designing new treatments for conditions associated with estrogen, including osteoporosis, cardiovascular diseases, and neurodegenerative diseases [42].

The genomic pathway is the traditional or the first model through which estrogen elicits a response and involves intracellular nuclear estrogen receptors, Estrogen Receptor-a (ERa), and Estrogen Receptor-β (ERβ) [43]. These receptors are activated by the binding of ligands and act as transcription factors that control the activity of the genes after interaction with estradiol. It starts from the passive transport of estradiol across the plasma membrane, where it binds to ERs, which could be soluble in the cytoplasm or the nuclear forms [43]. This ligand-receptor interaction brings about an alteration of conformation, which eventually leads to dimerization of receptors. The dimerized receptor complex is then transported to the nucleus, where it binds to estrogen response elements, or EREs, that can be located in the promoter region of target genes. This leads to the formation of co-regulators such as co-activators or co-repressors, which control the chromatin fabric and the transcription of genes. The genomic pathway leads to new protein synthesis, and as such, the physiological effects resulting from the activation of this pathway take between hours and days.

Some of the genes that are directly affected by the effects of estradiol, depending on the locations and the genomic signaling, include Cyclin D1, which enables cell division in the G1/s phase of the cell cycle, thus ensuring proper tissue remodeling and repair in the body. The second target is Bcl-2, being one of the members of the family of proteins that oppose apoptosis and help to regulate cell balance. In the vascular system, Vascular Endothelial Growth Factor (VEGF) is stimulated by estradiol, playing a major role in angiogenesis and making tissues such as the reproductive organs better supplied with blood [43]. Many of the effects of the estrogen receptor also go beyond the genome binding processes; the estrogen receptor can interact with other transcription factors like AP-1 and Sp1 and regulate gene expression regardless of the presence of EREs. This transfer increases the versatility of estrogen signaling and extends its cooperation into an impact on various tissues, including the brain and heart, as well as bones [44].

In this context, while the genomic action plan takes hours to exert its effects, the non-genomic form impacts several cellular reactions that happen within seconds to minutes. It is done through the use of G protein-coupled estrogen receptors, and the membrane-bound estrogen receptors include ERa as well as ERβ. These receptors trigger intracellular signaling mechanisms once they bind to the mentioned factors involved in cell survival, growth, metabolism, and neurotrophic factors. The signaling that is initiated through non-genomic estrogen signaling involves the MAPK/ERK, which is a signaling condition that is associated with actions such as cell growth, differentiation, and survival. This pathway is particularly relevant in the case of changes in reproductive tissue, synapse formation, and the function of the cardiovascular system. The activation of ERK by estradiol augments long-term potentiation and delays long-term depression, which enhances cognitive function and has neuroprotective effects that may be beneficial in lessening the development of neurodegenerative diseases, including Alzheimer’s disease [45].

Another possible non-genomic signaling pathway is a phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) one that is important for cell survival, glycolysis, and cardioprotection. Estradiol-related signaling with PI3K/AKT enhances endothelium functionality, reduces apoptosis, and displays anti-inflammatory potentialities. Here, it plays a crucial role in the cardiovascular system, stabilizing the blood vessels, reducing the effect of oxidants, and serving as protection against atherosclerosis or the thickening of the arterial walls. Another non-genomic signaling effector by estrogen is cyclic AMP/Protein Kinase A (PKA). Estrogen binding activates adenylate cyclase and leads to a rise in intracellular cAMP levels, resulting in PKA activation. This signaling cascade affects neurotransmitters, the immune system, and metabolism, especially in the liver and the brain. Thus, it now appears that estrogen is specifically involved in these fast signal transduction processes and serves as an essential regulator of the homeostasis of entire organ systems.

Though it is important to understand that the actions of estrogen and the benefits on the body are different in the genomic and non-genomic pathways, they do not work in isolation. Rather, they are tightly tied so that estrogenic signaling can be organized and targeted to particular tissues. Non-genomic signaling can also regulate gene expression at the transcription level using essential proteins like ERK, AKT, and Nuclear Factor-kappa B (NF-κB). On the other hand, genomic signaling is capable of modulating proteins that are involved in non-genomic signaling, thereby forming negative feedback [45]. It has been evidenced in reproductive organs for both synergistically coordinated actions in regulating ovarian follicle, uterine, and mammary gland development. Estrogen also has protective effects on brain cells via two different processes: through the activation of the genes concerned with neurotrophins on a genomic level and through an influence on the neuronal plasticity on a non-genomic level. Likewise, in the cardiovascular system, estrogen has its immediate effects on NO production via direct activation of eNOS through non-genomic actions, while the genomic effects include vasomodification and lipid profile regulation in blood vessels [46].

Hence, the right ratios between genomic and non-genomic estrogen signaling are vital for maintaining the body’s normal function. These pathways can be disrupted to result in several pathophysiological states; certain illnesses may be acquired due to the interruption of these pathways. For example, estrogen deficiency during the postmenopausal period is associated with the disruption of both pathways, leading to a rise in oxidative stress, inflammation, and apoptosis in different tissues. This leads to osteoporosis, cardiovascular diseases, and the decline in cognitive ability in people. Hormonal Replacement Therapy (HRT) has the objective of replacing the estrogen signaling, but it must be used according to the dosage and the time of use. SERMs are a class of drugs that help to stimulate the favorable effects of estrogen while reducing their drawback of HRT, like breast cancer and thromboembolism. The relationship between these pathways is important in the future strategy for the treatment of estrogen-related disease [47].

All physiological activities of estradiol are mediated by both the genomic and the non-genomic signaling pathways. Genomic estrogen action involves nuclear receptors and takes a longer time to change the gene expression and long-term cellular response, while the non-genomic mode of action works through membrane receptors and gives rapid responses to intracellular events [47]. They are tightly coordinated so that estrogenic regulation is precise and selective by various tissues. Thus, genomic and non-genomic actions of estrogen are crucial for ovarian function, neuronal survival, cardiovascular health, and metabolism. These disruptions can lead to various diseases and describe a need for specific therapeutic treatments. Molecular studies of these pathways to enhance the understanding of the hormonal processes will help in creating safer and more efficient statement therapies for the diseases resulting from estrogen imbalance.

Figure 4 shows two main activation routes for estrogen receptors. ER activates gene expression through DNA binding in the nucleus when using the ligand-dependent genomic pathway, and activates ER through phosphorylation with signaling interactions in the ligand-independent non-genomic pathway. The two regulatory mechanisms unite to function together for controlling gene transcription, which alters target cell operations. Membrane-bound ER plays a vital part in activating independent signaling pathways according to the figure representation [39] (Table 1).

Figure 4: Estrogen receptor signaling pathways and gene regulation.

Overview of key genes and molecular pathways regulated by estradiol in reproductive tissues

It has been proposed that estradiol is involved in the mechanisms of different molecular processes and gene interactions in reproductive tissues. These mechanisms play critical roles in proper developmental processes, cellular functions, regulation of cell death and living tissues, formation of new blood vessels, and changes in tissue organization of reproductive organs. Estrogen’s effects are exerted predominantly through Estrogen Receptors (ERα, ERβ, and G protein-coupled receptor, GPER), which, on activation, stimulate various signal transduction pathways that are vital for maintaining the health of the reproductive system. Disruptions in these pathways because of estrogen deficiency and hormonal alterations may enhance reproductive issues, for example, infertility issues, polycystic ovary syndrome, endometriosis, and ovarian failure [38].

GnRH signaling and calcium signaling pathway

As a molecular parameter the Gonadotropin-Releasing Hormone (GnRH signaling pathway is responsible for the regulation of reproductive hormone release and an appropriate working of the Hypothalamic-Pituitary-Gonadal (HPG) axis. GnRH is a decapeptide hormone produced and secreted from the neurons in the hypothalamus, and its release in burst frequency is critical for gonadotropin release from the anterior pituitary gland [48]. GnRH hence executes its function by binding to its specific receptor, the GnRH Receptor (GnRHR), which is a G Protein-Coupled Receptor (GPCR) that is located on the gonadotroph cells. This interaction leads to intracellular signaling pathways activation, particularly the Gq/11 proteins that cause the release of intracellular calcium and activation of PKC, which then activate the MAPK pathway. These intracellular pathways lead to the production and secretion of Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH), which in turn control steroidogenesis and gametogenesis of the gonads.

One of the main issues with the GnRH signal is that it is secreted in a pulsatile manner to allow for differential gonadotropin synthesis and secretion. In the secreting rate of the GnRH, there are rhythms and intensities that will lead to the selective release of LH or FSH. High frequency pulses are known to stimulate the secretion of LH, and low frequency pulses, on the other hand, stimulate the secretion of FSH. This interactive pulsatility of GnRH is very important in the process of follicular growth and ovulation. Over time and in the absence of pulsatile release, GnRH brings about down-regulation of GnRHR and GnRH secretion and forms the principle of therapy where GnRH agonists are used to treat endometriosis, PCOS, and hormonally-responsive cancers.

The calcium signaling pathway represents a core cellular signaling system that controls multiple bodily functions in reproductive tissues from the start of follicle formation until the end of implantation. The regulatory mechanism for cellular Ca2+ components relies upon tightly controlled channels, pumps, and exchangers that sit on cell membranes, endoplasmic reticulum, and mitochondria. When gonadotropins stimulate the ovary through granulosa and theca cell G protein-coupled receptors, they initiate a receptor signaling process that activates Phospholipase C (PLC). Through hydrolysis of PIP2, phospholipase C generates IP3 and DAG. IP3 reaches endoplasmic reticulum receptors to initiate the release of Ca2+storage and thereby raise cytoplasmic calcium concentrations temporarily. The elevated Ca2+ levels activate calmodulin and diverse calcium-dependent kinases and phosphatases, leading to the regulation of critical cellular processes for oocyte maturation and cumulus expansion as well as the ovulatory follicle rupture [49].

When GnRH attaches to gonadotropin receptor molecules, it sparks multiple cellular signals that involve calcium regulation to control both gonadotropin protein genes and hormone production. GnRH stimulation activates the phospholipase C pathway, which yields Inositol 1,4,5-trisphosphate (IP3) and Diacylglycerol (DAG) as products. Within the cytoplasm, IP3 triggers the release of Ca2+ from endoplasmic reticulum spaces to grow calcium levels. The increase of intracellular calcium levels triggers calcium/Calmodulin-dependent protein Kinases (CaMKs) and additional downstream components that activate the transcription of Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH) subunit genes that lead to their production and secretion. The calcium signaling pathway brings extracellular hormonal signals to nuclear events that drive gene expression for regulating reproductive processes [50].

The signaling network controlling gonadotropins develops through an integration of calcium signaling with Protein Kinase C (PKC) and cyclic AMP (cAMP) pathways to achieve precise activity control. Estradiol controls GnRH pulse dynamics through feedback systems that affect calcium signals within gonadotropins, along with their hormonal production. The important function of calcium signaling involves supporting the developmental maturation of gametes, together with the growth of follicles and the process of ovulation. The normal functioning of cumulus expansion, together with meiotic resumption, depends on changing calcium concentration levels found in granulosa cells and oocytes. Impaired folliculogenesis and luteal dysfunction, together with infertility, occur because of dysregulated calcium signaling patterns, which underscore its vital role in reproductive success. The precise knowledge of GnRH and calcium signaling relationships remains essential for creating targeted methods to protect fertility, as well as hormone treatments and reproductive disorder clinical interventions.

Figure 5 shows the sequence of events that start when Gonadotropin Releasing Hormone (GnRH) binds to its receptor. Activation of GnRH triggers G protein stimulation and creates two second messengers by activating Adenylyl Cyclase (AC) and Phospholipase C (PLCβ) that result in cyclic AMP (cAMP) and Inositol Triphosphate (IP3) generation. Activated second messengers trigger signaling pathways that engage Protein Kinase A (PKA) and Protein Kinase C (PKC), which function to control transcription factors that include ERK1/2 and the calcium signaling pathway. Gonadotropin synthesis, together with its secretion from the pituitary gland, becomes possible following this activation step. Reproductive functions become regulated through this signaling process because GnRH performs an essential role in the hypothalamic-pituitary axis functions (Table 2).

Figure 5: Gonadotropin-Releasing Hormone (GnRH) signaling pathway.

Factors and cofactors that influence the estrogen response

In fact, the changes of estradiol on the human body are not only based on the presence or absence of the hormone but also depend on cofactors and epigenetic modifications that control the activity of ER. These regulatory mechanisms further modulate the action of estrogen and keep the gene activation or repression within the right bounds in the right organs and at the right time [51]. The stimulation of estradiol and related cofactors ensures an influence on such processes as cell division, cell determination and differentiation, metabolic regulation, and even occurrences of specific diseases. This way, the scientists will be able to get a better view of estrogen’s roles and come up with effective treatment for estrogen-related diseases such as infertility, osteoporosis, and hormone-dependent cancers [52].

This is because some proteins can either enhance or suppress the activity of estrogen receptors through so-called co-activator or co-repressor proteins. These co-activators include Steroid Receptor Co-activator 1 (SRC-1) and the p300 protein complex, which have been implicated in the process of mediation of the effects of estrogens by enhancing gene transcription. SRC-1 binds directly to ERα and ERβ and cooperates with them by participating in chromatin remodeling with attractive coactivators for the transcription process. All these are crucial roles for tissue repair, cell division, and the control of reproduction. Another coactivator known as CBP/p300 is also considered the HAT through altering the chromatin structure to enhance the transcription rate. These coactivators are relevant particularly in reproductive tissues, which require estrogen-promoted cell division that is essential in developing the uterus and mammary glands [53].

On the other hand, the estrogen receptor is negatively controlled by co-repressors such as the nuclear receptor co-Repressor (NcoR) and the silencing mediator of the retinoic acid and thyroid hormone receptor (SMRt). These co-Repressors also bring in Histone Deacetylases (HDACs) to add to the chromatin and reduce gene transcription [54]. Here, NCoR and SMRT participate in the specifically important task of repressing the estrogen receptor in order to prevent hormonal overstimulation due to estrogen. In reproductive tissue, these corepressors play a role in regulating endometrial cell division, thereby preventing estrogen reinforcement of cell growth by the introduction of progesterone. Abnormalities in corepressors or their activity result in conditions like endometriosis, estrogen-sensitive carcinomas, and cancers where estrogenic activity is not controlled and contributes to the development of endometriosis [55]. This way, the coactivators and corepressors play an intricate balance that defines the degree and duration of estrogen signaling that is crucial for the reproductive and overall well-being of a woman.

It is important to comprehend that the estrogen signaling pathway involves numerous cofactors and epigenetic modifications to be able to design a particular treatment for estrogen-associated diseases. Coactivator and co-Repressor regulation, as well as epigenetic markers, have direct potential for therapeutic intervention in disorders such as infertility, bone loss, and hormone-dependent cancers [56]. For instance, Tamoxifen and Raloxifene are selective estrogen receptor modulators; they bind to estrogen receptors and change the co-activator/corepressor binding destinations, which partially or fully activate estrogen in the tissues. The usage of HDACi and DNMTi is promising for the manipulation of the process, which tries to restore the normal estrogen receptor gene function in cancer treatment. Using these molecular details, it is possible to treat cases of estrogen-related disorders with enhanced effectiveness while reducing side effects [57].

It is therefore important to consider that the response to estrogen, such as gene regulation, is not predetermined by the amount of hormone that is present in the body but depends on factors such as coactivators, co-Repressors, and epigenetic modifications of the estrogen receptor factor. Coactivators and co-Repressors regulate the ability of estrogen signaling, while epigenetic changes modify gene expression for physiological regulation.

There is also the challenge of external factors, which also play a role in altering the activity of the estrogen receptors by epigenetic features. Thus, a better understanding of such regulatory factors is critical for improving the treatment of hormone-associated diseases, including reproductive issues and estrogen-responsive malignancies.

Ovariectomy is a common experimental technique used for the study of estrogen deficiency and its overall impact in the body. This model is more important in the biomedical research to study the postmenopausal physiology, hormonal replacement therapy, and related pathophysiologies of postmenopausal conditions such as osteoporosis, metabolic syndrome, and cardiovascular disease. Ovariectomy in mice helps the researchers to study the initial and potential effects of estrogen deficiency and therefore gives options to investigate such aspects of the reproductive system as physiology and signaling, as well as the disease process. The surgical removal of the ovaries results in hormonal and metabolic disturbances characteristic of the postmenopausal state in female animals, making this model an indispensable tool in studies of aging and endocrinology [58].

Immediate and long-term effects of ovariectomy on the physiology and reproductive organs of mice

Ovariectomy in mice provides a broad spectrum of physiological models to study a number of effects related to estrogen deficiency. It offers a controlled environment to analyze constant changes that occur soon after ovariectomy and sequential changes that occur with time. Some of the alterations include changes in the form and function of the reproductive organs, metabolism, skeletal structure, and the heart. Ovariectomy is another model that continues to expand the knowledge concerning the multifunctional roles of estrogen; it is also constructive for the production of treatment programs for estrogen-deficient diseases [59].

Short-term effect of ovariectomy: Firstly, ovariectomy leads to a rapid reduction in the concentrations of estrogen in the circulatory system, thus significant alterations in the morphology and physiology of the reproductive organs ensue. Estrogen plays a central role in the maintenance of uterine and vaginal epithelial tissues, and their sudden depletion leads to the squamous epithelium of the uterus and vagina to wither shortly after its depletion. In the uterus, total weight loss, tubal and uterine gland atrophy, and epithelial thinning are all observed due to the absence of estrogen stimulation. It becomes thinner, gets less blood supply, and produces less mucosal secretions, which is quite similar to atrophic vaginitis in women after menopausal years [60]. These changes have an impact on fertility studies, and for investigation must be considered in relation to hormone replacement therapy as well.

Apart from the reproductive system, ovariectomy induces significant metabolic disturbances almost immediately after the procedure. Estrogen is consequently proven to be very crucial in adipose tissue distribution, insulin sensitivity, and energy balance. Ovariectomized mice lack estrogen, store more fat in the visceral depots, expend less energy, and are negatively affected in glucose handling; thus are prone to obesity and insulin resistance [61]. Some of them include changes in leptin and adiponectin signaling that occur during the disruption of estrogen’s metabolic protectiveness, leading to increases in adiposity as well as glucose intolerance. These metabolic changes are in line with the observations of metabolic changes in postmenopausal women who normally develop increased abdominal obesity and other complications, including the development of Type 2 diabetes after the onset of ovarian senescence.

Long-term effect of ovariectomy: The long-term sequestration of estrogen in ovariectomy results in the breakdown of various physiological systems in the body. One of the most documented consequences in the long run is a deterioration of the mass and density of bones due to an increased mass of bone marrow fat fraction [62]. Estrogen plays an important role in regulating bone remodeling since it inhibits osteoclast activity and provides support to the osteoblasts. Instead, bone resorption occurs at a higher rate than bone formation, which in turn results in low bone mineral density, high cortical porosity, and high risk of fractures [63]. A recent postoperative study in ovariectomized mice has revealed that there is a decrease in bone mass by 30-50% within a few months, which is almost the same as woman after menopause [64].

In addition to skeletal deterioration, ovariectomy induces significant neuroendocrine and cardiovascular changes. It results in increased FSH level, and this has been attributed to the causes of bone loss, fat deposition, and neurodegeneration. In addition, the effects of estrogen loss cause chronic stress levels that lead to suppression of the HPA axis and stress reactivity [28]. Ovariectomized mice also have altered responses to stress and increased sympathetic activity since stress causes hypertension and impairs endothelium-dependent vasodilation. Estrogens have been proven to have direct vasculoprotective effects, such as regulation of NO synthesis and anti-inflammatory activity. Its deficiency causes vascular senescence and accelerates arterial wall hardening, atherosclerosis, and cardiovascular morbidity and mortality. These results are supported by data of epidemiological nature in a postmenopausal female when estrogen deficit relates to increased risk of hypertension, dyslipidemia, and ischemic heart disease.

Some surgical models, such as the ovariectomy model, have also served to give insights into cognitive aging and neurodegenerative diseases. Estrogen is an effective anti-oxidative hormone that is involved with synaptic plasticity, neurotransmitter release, and oxidative stress. Ovariectomy leads to enhanced cognitive impairment, decreased hippocampal performance, and increased β-amyloid deposition, which are characteristics of Alzheimer’s disease [65]. It has been established that ovariectomy in mice causes the effects mentioned earlier, and estrogen replacement can prevent these by improving synaptic density, decreasing neuroinflammation, and encouraging neurogeneration. However, the time of starting estrogen therapy seems to be important, since postponing the beginning of estrogen treatment may either be ineffective for preventing the deterioration of cognitive function, or even worsen it (Table 3).

Overview of molecular changes after ovariectomy

Some of the effects of estrogen deficiency on OVX mice are a result of multiple molecular changes throughout the physiologic signaling pathways. Estrogens are known to modulate neurogenic expression with respect to vasopressin, a neurohypophysial hormone that is also involved in the stress responses and regulation of feminine sexually selected behaviors, such as oxytocin. The OVX altered the level of AVP and oxytocin signaling, leading to enhanced anxiety, such as seen in ovariectomized females, and fine-tuning of emotion [65].

This is because estrogen deficiency at the cellular level affects multiple signaling pathways that are pertinent to maintaining neuronal viability and plasticity. This has been noted to have a significant impact on cognitive resilience and primarily affects a specific signaling pathway, which is the BDNF-TrkB signaling pathway. This impairs the expression of BDNF and downregulates the receptor TrkB due to decreased estrogen levels that cause neurotrophic and cognitive deficits. Intracellular signaling includes the estrogen modulation of the protein that is encoded by the PH domain leucine-rich repeat-containing protein phosphatase that activates the Phosphoinositide 3-Kinase (PI3K)/Akt pathway to stimulate the exonuclease-3 gene that inhibits the pro-apoptotic factors in the cell. OVX eliminates the level of estrogen and causes alteration in PI3K/Akt signaling and increased neuronal death, together with reduced synapses [66].

It has also been well studied that estrogen has its critical impact on the metabolic gene regulation that relates to glucose and lipid balances. The OVX-induced estrogen deprivation affects the insulin signaling pathway, which is associated with glomerular glucose uptake abnormalities and increased hepatic gluconeogenesis, contributing to hyperglycemia and a higher predisposition to develop the metabolic syndrome. Furthermore, estrogen plays an important role in lipid metabolism since it influences the activity of enzymes that are responsible for fatty acid elimination and storage. However, OVX mice have significantly higher levels of hepatic lipid content and dyslipidemia profile, which puts the OVX mice at a higher risk for metabolic disorders. These results indicate that estrogen plays a critical role in metabolic regulation and suggest that the OVX model may be a useful model to study metabolic dysfunction postmenopausally [67].

ERT is an important step taken to prevent and control the other systemic effects of estrogen deficiency that result from ovariectomy. Ovarian failure results in large amounts of low levels of circulating estrogen, and this deficiency triggers changes in various physiological systems in the body, such as the skeletal, cardiovascular, metabolic, and nervous systems. ERT is a way to restore the impact of estrogen, which will help some peri- or postmenopausal symptoms after ovariectomy and minimize the risk of the dangerous consequences for the woman’s health, such as osteoporosis, decreased cognition, and cardiovascular diseases. Nonetheless, the efficacy of ERT is not without limitations and complications; this means that the initiation of the therapy has to take cognizance of the patient’s status and other surrounding factors. This has brought emphasis on customized intervention plans and schedules, proper evaluation of the risks/benefits ratio in treatment, and toxicity profiles for enhancing therapeutic efficacy and minimizing harm in the patients.

Effects of exogenous estradiol administration after ovariectomy

Ovariectomy in currently used experimental animals, primarily rodents, causes the complete elimination of endogenous estrogen biosynthesis and provokes multisystem metabolic, cardiovascular, bone, and immune changes. The use of exogenous estrogen, namely estradiol, has been investigated extensively because of its effect in alleviating the above-mentioned post- ovariectomy effects [68].

Estrogen deficiency is the leading cause of metabolic change leading to insulin resistance and dyslipidemia. It is known that the administration of estradiol increases insulin effectiveness and the effectiveness of glucose uptake by the peripheral tissues, as well as reducing the rate of gluconeogenesis in the liver. Further, it also decreases ectopic lipid accumulation, especially in the liver and skeletal muscle, thereby reducing the high propensity of ailments like type 2 diabetes and non-alcoholic fatty liver diseases [37].

This is because estradiol replacement plays a very important role in the maintenance of the vascular system. Deficiency leads to endothelial dysfunction, the biomarkers of which include decreased NO production and impaired ACh-mediated relaxation. Thus, NO is a vasodilator, being involved in the regulation of vascular homeostasis, and its deficiency leads to increased vascular resistance and hypertension. Thus, estradiol seems to have cardio–protective effects by enhancing NO levels, improving endothelial function, and increasing vascular compliance [32].

Other significant effects of estrogen loss up to 6 months post-ovariectomy are related to body composition and muscle physiology. It has been reported that ovariectomy leads to changes in FFM, especially for body fat because of anabolic and catabolic hormone imbalance. These changes have, however, been observed to be prevented, among others, by estradiol replacement, which maintains the lean muscle mass and body composition of the animals to levels similar to those of non-ovariectomized animals.

The immune system is also regulated by estrogen, and estrogen depletion may have an impact on the immune system and actions involving inflammation. However, certain investigations of the impact of ERT on the immunocompetent have evidenced that exogenous estrogens do not impact neutrophil phagocytic ability or raise the risk of acquiring systemic infections. These observations indicate that although estrogen affects immunity, it is not necessarily detrimental to the innate immunity following its replacement therapy [69].

Potential side effects and limitations of estradiol replacement therapy

This is not to say that there are no drawbacks to using ERT; on the contrary, like with any clinical tool, ERT has its risks and limitations as discussed below. Another critical issue that has concerned people is the effect of HRT on cardiovascular outcomes. Essentially, estradiol has beneficial effects on the cardiovascular system, but some types of HRT, including combined oral estrogen-progestin treatment, increase the risk of thromboembolic events, which include deep vein thrombosis and stroke. Nevertheless, transdermal and bioidentical estrogen formulations have been reported to be safer, which means that patients should be treated differently depending on their cardiovascular risk factors.

Another important factor is that long-term usage of ERT can somehow be related to cancer. Estrogens for a long time have been associated with a higher risk of cancers that are sensitive to this hormone, such as breast and endometrial cancers. However, in research, participants pointed out that the risk seems to vary with the doses and the women taking estrogen alone or along with progestins. In order to minimize this side effect, the patient should be closely monitored, as well as have the minimal amount of medication and frequency possible involved [70].

Further possible side effects of ERT include abnormal uterine bleeding, breast discomfort, emotional sensitivity, edema, and headache. These side effects are usually not serious and can be eliminated by adjustments of the dosage or by the use of other forms of administration. However, there are marked differences in the patients’ response to ERT, due to which further analysis needs to be carried out based on the age factor, initial health state, and the onset of the therapy. These studies indicate that higher benefit is achieved when ERT is started as early as possible, at the time of onset of menopause or ovariectomy, and that initiation of therapy may be useful at a later stage or may be detrimental [71].

As has already been demonstrated, estrogen—the principal subtype being 17 β-estradiol—is involved in managing reproductive health through a network of signaling mechanisms. Ligation and ovariectomy of estrogen have been found to have various direct and indirect opposing actions and interactions through the estrogen signaling pathway. These contribute to tissue-selective estrogen action by factors that include the type of estrogen receptors, availability of co-activators, the extent to which specific DNA sites are opened up for binding, and the nature of membrane signal transduction. Knowledge of these complex processes is subsequently necessary to find ways to treat reproductive health problems and estrogen-related diseases and conditions [72].

Interactions between estrogen signaling and other signaling pathways relevant to reproductive health

Estrogen, being a hormone, acts through Estrogen Receptors (ER), mostly ERα and ER(12 pt), which are intracellular proteins that function as the DNA-bound receptors only when bound to the ligand. On ligand binding, these receptors change their conformation, dimerize, and attach to particular DNA sequences referred to as Estrogen Response Elements (EREs) so as to control the processes of gene transcription [18]. Besides this classical genomic pathway, estrogen also affects several other pivotal signaling pathways that are involved in reproduction.

Another important connection of estrogen is with the growth factor signaling pathways, such as IGF-1 and EGF. Such processes can then alter the functions of the ER through phosphorylation, downstream gene expression, and further cell evaluation. This is particularly plausible in view of the finding that estrogens’ effects are regulated by the activation of the IGF-1 and EGF receptors that can phosphorylate ERs, which in turn dictate gene expression and cellular responses [46].

The estrogen signaling pathway influences inflammatory signaling components with respect to women’s fertility. Estrogen may affect the cytokine and chemokine production, so it can affect inflammation in reproductive tissues. Parturition, ovulation, and implantation processes otherwise require controlled inflammation since it plays a crucial role in these processes [33].

It’s known that the estrogen signal is regulated by epigenetic changes, that is, DNA methylation and changes in histone proteins. Estrogen receptors can bring to the chromatin/DNA complex co-regulators that possess chromatin-modifying activity, implying changes in chromatin conformation and gene regulation [33]. It plays an important role in the tissue-selective estrogenic actions and is considered crucial for the non-cancerous roles of estrogen.

How these interactions contribute to the overall response to estradiol and ovariectomy

Overall, estrogen signaling interacts with other signaling pathways to affect the consequences of estradiol treatment as well as ovariectomy.

Modulation of reproductive tissue function: In reproductive tissues, estrogen interacts with the growth factor signaling pathways to perform normal function and growth. For instance, estrogen in combination with IGF stimulates the endometrium as well as the mammary gland during female development [73]. Ovariectomy means the removal of the ovaries, and this results in a deficiency of estrogen and therefore causes shrinkage of these tissues. This shows that exogenous estradiol can effectively replenish these interactions; hence, the tissue architecture and functionality are regained (Table 4).

Influence on cellular proliferation and apoptosis: Estrogen acts in the regulation of cell growth and death of reproductive cells by interacting with growth factor signaling. This regulation is essential when it comes to processes like endometrial regeneration during the menstrual cycle. As observed, estrogen deficiency following ovariectomy can alter this balance and lead to endometrial thinning, for example. There is a possibility of replacing the proliferative signals with estradiol so that the endometrial thickness returns to normal [74].

Impact on inflammatory responses: Estrogen Signaling and inflammatory pathways are in close relation to each other to facilitate reproductive events that entail inflammation. Ovariectomy leads to estrogen deficiency and disrupts the balance of pro- and anti-inflammatory cytokines, thus having an impact on fertility and enhancing the risk of infections. These studies have shown that the administration of estradiol influences these inflammatory pathways to ensure that the tissue remains healthy and functional.

Epigenetic influences: Estrogen signaling has been shown to regulate the epigenome changes and therefore the tissue- specific effects of estradiol treatment and ovariectomy [75]. In this case, modifications of DNA molecules, such as DNA methylation and histones, affect the gene expression patterns, which in turn affect the general physiology of the body. These epigenetic modifications may have definite impacts on reproductive health and some other processes like folliculogenesis and embryo implantation [76].

Figure 6 shows multiple ways through which estrogen operates to protect vascular wellness. The combination of protective mechanisms exercised by estrogen against oxidative stress, inflammation, and termination processes promotes anti-vascular aging effects. Through its mechanism, the hormone regulates blood pressure while binding homocysteine and preventing both lipid irregularities and glycemic issues. Blood vessels stay protected through several mechanisms enabled by estrogen, which demonstrates its crucial role in cardiovascular well-being [77].

Figure 6: Protective effects of estrogen on vascular health.

Molecular mechanisms for tissue-specific responses: It has been realized that tissue-specific response to both estradiol and ovariectomy involves receptor isoform availability, co-regulator availability, and chromatin environment.

Estrogen receptor isoforms and distribution: The fact is that the body tissue can be differentially sensitive to the action of these particular estrogen receptor subtypes, ERα and ERβ. For example, ER-α is largely associated with the uterus and the mammary gland and is involved in the proliferative processes, while ER-β is associated with the ovary and the prostate, and it usually controls differentiation and anti-proliferative activities. This distribution determines how the tissues become receptive to the signals from the estrogen hormone.

Co-regulator proteins: Experimental studies show that co-activators and co-repressors have an impact on the transcription mediated through estrogen receptors. The tissues will have an increased response to estradiol for those cells with high co-activator content and a minimal response for tissues with high co-repressor signals. This balance is essential when it comes to adjusting the quantity of a specific protein expressed by the gene in accordance with hormonal signals.

Chromatin accessibility: Chromatin structure and composition, such as the position of nucleosomes and hence the state of histone proteins, influence the binding of the estrogen receptor to DNA. This is because genes that have chromatin in the open conformation are the most likely to be affected in response to the action of estradiol. This is because ovariectomy leads to the withdrawal of estrogen that elicits chromatin remodeling, which otherwise changes the accessibility of genes. These changes can be reversed by estradiol replacement and bring back the transcriptional program [78].

Membrane-Initiated Steroid Signaling (MISS): Apart from the nuclear membrane-bound receptor model, estrogen also must bind to membrane-anchored receptors to produce non-genomic responses. These MISS pathways include cell signaling through activation of kinases and various second messengers that produce tissue-selective effects. For instance, estrogen can cause the quick activation of membrane receptors within hours, promoting vasodilation or cardioprotection [79].

Selective Estrogen Receptor Modulators (SERMs): Selective estrogen receptor modulators (SERMs) are compounds that act as estrogen receptor agonists or antagonists depending on the target tissue. This selective activity is influenced by the differential expression of ER subtypes and co-regulators in various tissues (Arnal et al., 2017) [79]. For instance, tamoxifen acts as an antagonist in breast tissue but as an agonist in bone and uterine tissues, highlighting the complexity of estrogen signaling and its tissue-specific effects. This selective modulation of estrogen receptors allows for the development of targeted therapies in conditions such as breast cancer, osteoporosis, and cardiovascular diseases, where estrogen plays a crucial regulatory role [80].

This review shows that estradiol is not limited to a reproductive hormone but acts as a system-wide regulator linking the hypothalamic–pituitary–gonadal axis with ovarian activity, uterine receptivity, bone remodeling, vascular protection, metabolic homeostasis, and neuroprotection. These effects are mediated through ERα, ERβ, and GPER via coordinated genomic and non-genomic pathways, including MAPK/ERK, PI3K/AKT, GnRH, and calcium signaling. Evidence discussed across OVX models demonstrates that estrogen loss disrupts reproductive tissues and also accelerates bone loss, metabolic imbalance, vascular dysfunction, inflammation, and neural vulnerability. Estradiol replacement can reverse several of these abnormalities, but its benefits remain dependent on dose, timing, formulation, and tissue-specific receptor responses, which explains why clinical translation requires caution. Overall, an integrated understanding of estradiol signaling is essential for improving menopause-related care and for developing safer, more selective therapies that preserve beneficial estrogenic actions while minimizing adverse effects. This synthesis also supports the continued use of OVX models as a valuable platform for mechanistic and translational endocrine research.

Despite providing a comprehensive overview of estradiol signaling, ovariectomy-induced estrogen deficiency, and estradiol replacement strategies, the available literature is highly heterogeneous with respect to animal model, species, age, experimental duration, ovariectomy timing, estradiol dose, route of administration, and measured endpoints, making direct comparison among studies difficult and limiting the ability to draw fully uniform conclusions. Although the Ovariectomy (OVX) model is widely used to mimic postmenopausal estrogen deficiency, it does not completely reproduce the gradual endocrine and physiological transition that occurs during natural menopause in women. Therefore, findings from OVX-based studies should be interpreted cautiously when considering their translational and clinical relevance. This review mainly emphasizes estradiol-centered mechanisms, whereas female reproductive physiology is regulated through a broader hormonal and molecular network that includes progesterone, gonadotropins, inflammatory mediators, metabolic regulators, and other signaling molecules. As a result, some of the biological responses discussed here cannot be attributed to estradiol alone and need further research. Greater standardization of experimental design, including OVX protocols, estradiol dosing regimens, treatment duration, and molecular endpoints, is needed to improve comparability across studies.

In future investigations, researchers should also adopt more integrative approaches, including transcriptomics, proteomics, epigenomics, and metabolomics, to better define the molecular basis of estradiol action across reproductive and extra-reproductive tissues. Such approaches may help identify novel biomarkers of estrogen deficiency, treatment response, and tissue-selective estrogen activity. Further research is needed to clarify the distinct and overlapping roles of ERα, ERβ, and GPER in reproductive tissues as well as in the brain, bone, cardiovascular system, and metabolic organs. A better understanding of receptor-specific signaling may support the development of safer and more selective therapeutic strategies, including improved hormone replacement approaches and selective estrogen receptor modulators. From a clinical perspective, future research should place greater emphasis on the timing, dose, formulation, and route of estradiol administration, particularly in relation to long-term outcomes involving fertility, cognition, bone health, cardiovascular function, and metabolism. Well-designed longitudinal studies and comparative clinical trials are essential to determine how estrogen-based interventions can be optimized for different physiological and pathological conditions.