madman
Super Moderator
Estrogen as a key regulator of energy homeostasis and metabolic health (2022)
Fatemeh Mahboobifard, Mohammad H. Pourgholami, Masoumeh Jorjani, Leila Dargahi, Mina Amiri, Somaye Sadeghi, Fahimeh Ramezani Tehrani
ABSTRACT
Over the last two decades, it has become evident that estrogens preserve the integrity of energy homeostasis at central and peripheral levels. Estrogen deficiency, such as that caused by menopause or ovariectomy, has been linked to obesity and metabolic disorders that can be resolved or reversed by estrogen therapy. 17β-estradiol (E2), as the major estrogen in the body, primarily regulates energy balance via estrogen receptor alpha (ERα). At the central level, E2 plays its catabolic role predominantly by interacting with hypothalamic arcuate neurons and sending signals via ventromedial hypothalamic neurons to control brown adipose tissue-mediated thermogenesis. In peripheral tissues, several organs, particularly the liver, brown and white adipose tissues, and pancreatic β cells, have attracted considerable attention.
In this review, we focused on the current state of knowledge of “central and peripheral” estrogen signaling in regulating energy balance via “nuclear and extranuclear pathways” in both “females and males". In this context, according to an exploratory approach, we tried to determine the principal estrogen receptor subtype/isoform in each section, the importance of extranuclear-initiated estrogen signaling on metabolic functions, and how sex differences related to ER signaling affect the prevalence of some of the metabolic disorders. Moreover, we discussed the data from a third viewpoint, understanding the clinical significance of estrogen signaling in abnormal metabolic conditions such as obesity or being on a high-fat diet.
Collectively, this review exposes novel and important research gaps in our current understanding of dysmetabolic diseases and can facilitate finding more effective treatment options for these disorders.
1. Introduction
The control of energy homeostasis is a complex process that maintains the balance of energy intake, expenditure, and storage so that each organ has enough energy to function. Both the central, predominantly the hypothalamus, and peripheral organs, predominantly the liver, pancreas, and adipose tissue, are involved in controlling these processes [1,2]. The role of 17β-estradiol (E2) in regulating reproduction by interacting with the population of hypothalamic gonadotropin-releasing hormone (GnRH) neurons and providing feedback on the hypothalamic-pituitary-gonadal (HPG) axis is well documented. However, over the last two decades, it has become evident that estrogens play a role in almost all aspects of metabolism and preserve the integrity of energy homeostasis at central and peripheral levels. Therefore, the effects of estrogens are considered beyond reproductive function [1]. In preclinical and clinical research, states of estrogen deficiency, such as that caused by menopause or ovariectomy, have been linked to decreased energy expenditure, metabolic abnormalities, and obesity [3, 4]. Given that the average lifespan is increasing in developed countries, about half of a woman’s life is spent in an estrogen-deficient state [4]. Estrogen therapy, on the other hand, reverses these defects in humans or ovariectomized animals by reducing food intake while increasing energy expenditure [5,6]. Furthermore, evidence indicates that the fluctuations in estrogen levels depending on the status of the menstrual/estrous cycle [3,7], as well as estrogen levels during pregnancy and breastfeeding [8, 9], affect feeding quantity, body weight, and metabolic status.
This review provides a comprehensive and up-to-date overview of the matter, including novel data on "central and peripheral" estrogen signaling to regulate energy balance in both sexes. In this context, to achieve new insights on this topic, we attempted to explore from the literature the main estrogen receptor subtype/isoform, its primary signaling pathways (nuclear and extranuclear), and mechanistic links with other factors/transcription factors, as well as ER-driven sex differences in the prevalence of some of the metabolic diseases. Moreover, from a third viewpoint, we tried to explain the protective significance of ER signaling in abnormal metabolic conditions such as exposure to a high-fat diet (HFD).
These important findings could help us develop novel therapeutic strategies for several metabolic disorders such as obesity, metabolic syndrome, type 2 diabetes, non-alcoholic fatty liver disease (NAFLD), hypertension, and even, separately from metabolic disorders, some types of cancer. Moreover, having reviewed the studies on the aforementioned topics, we found some important research gaps and open questions, leaving the possibility of conducting further research to find novel biologically relevant proteins with promising therapeutic effects on dysmetabolic conditions.
2. General background of estrogen
2.1. Estrogen production in the body
17β-estradiol (E2), as the major estrogen in the body, is produced predominantly by the ovary and, to a lesser extent, is also synthesized by adrenals, testes, and adipose tissue. During pregnancy, the placenta produces high estrogen levels [10,11]. Under a two-cell model, estrogen is produced in the growing follicles of the ovary with the contribution of two cells: the theca and granulose cells. Initially, under the stimulation of the pituitary luteinizing hormone (LH), the inner theca cell layer of the follicles is responsible for the synthesis of androgens like androstenedione and testosterone from cholesterol. Theca cells, like Leydig cells in the testis, synthesize androgen in response to LH. Pituitary FSH then drives granulosa cells to express aromatase, which is responsible for converting androgens into estrogens [9].
2.2. HPG axis and estrogen feedback
The produced E2 is then transferred to the hypothalamus, where it interacts with gonadotropin-releasing hormone (GnRH) neurons in the hypothalamic preoptic area (POA) to regulate reproduction. GnRH neurons directly regulate reproduction and gonadal steroidogenesis [12]. They secrete the neurohormone GnRH in a pulsatile manner into the adenohypophysis, stimulating follicle-stimulating hormone (FSH) and LH release from the gonadotrophs. This process is dependent on GnRH neuronal depolarization and an elevation in intracellular calcium [13]. The ovarian function is cyclic during the reproductive period, with the follicular phase occurring first and the luteal phase occurring second. The follicular phase is associated with follicular growth and estrogen level elevation, with a significant rise near the end of the phase, just before ovulation, while the luteal phase is associated with corpus luteum formation and the predominant secretion of progesterone [11]. Except for the pre-ovulatory stage, when estrogen exerts positive feedback that is necessary for GnRH surge and ovulation occurrence, estrogen has a negative feedback on the HPO axis during the ovarian cycle (estrous cycle) [12]. The reason for this paradox might be due to the opposing effects of estrogen (suppressive and stimulatory) on the expression of kisspeptins, as neuropeptides with regulatory effects on GnRH neurons, in two distinct nuclei of the hypothalamus, including the arcuate nucleus (ARC), involved in the negative feedback of the E2, and the anteroventral periventricular nucleus (AVPV), involved in positive feedback [14] (Fig. 1).
2.3. Genomic and non-genomic estrogen signaling pathways
Estrogen receptors (ERs) have two primary subtypes: ERα and ERβ, which are encoded by the genes ESR1 and ESR2, and belong to the nuclear hormone receptor subclass. It is evident that ER programming is processed through genomic and non-genomic signaling. Genomic signaling is initiated by the binding of E2 to ERs in the cytoplasm and the formation of hetero- or homodimers which translocate to the nucleus [15]. This pathway takes hours to days to complete. Interaction of so-called dimers with the estrogen response element (ERE) on DNA or other transcription factors bound to DNA like specificity protein-1 (SP-1) and activator protein 1 (AP-1) triggers transcriptional activation of E2 [16]. Non-genomic signaling of E2, which is also known as rapid non-nuclear signaling, or “extranuclear- or membrane-initiated estrogen signaling”, is generated by binding ER to E2 as a monomer, which induces several kinase cascades as well as regulation of calcium flux and ion channels [17].
The rapid function of E2 was identified in early research with no knowledge about membrane estrogen receptors until the 1990 s when the membrane localization of ERα was discovered in the cultured CA1 neurons of the hippocampus and pituitary cells [18,19]. Recently, new isoforms of ERα mediating non-genomic signaling of E2, such as ERα36 and ERα46, have been identified in the brain and body [20,21]. These isoforms are produced spontaneously in the body via alternative splicing of ERα-mRNAs. Both the membrane and nuclear estrogen receptors are encoded by the same gene [21]. Membrane localization of ERα occurs through posttranslational modifications and the developing palmitoylated ERα which target caveolae as structures with cholesterol-rich plasma membranes that accelerate membrane anchoring [22]. It should be noted that membrane-initiated estrogen receptor signaling through inducing intracellular kinase cascades, including protein kinase C/protein kinase A/phosphatidylinositol 3-kinase/mitogen-activated protein kinase (PKC/PKA/PI3K/MAPK), is not only limited to protein phosphorylation and rapid alteration in the activity of ion channels but also induces some alterations in gene transcription (long-term effects), like downstream signaling pathways of many neurotransmitters [23].
2.4. G-protein-coupled estrogen receptors
In addition to ERα and ERβ, G-protein-coupled estrogen receptor (GPER) and Gαq-coupled membrane ER (Gαq-mER, Gq-mER) are two membrane estrogen receptors that do not originate from ERα or ERβ transcripts. GPER, also known as GPR30, is a G-protein-coupled estrogen receptor bound to the membrane that is implicated in the rapid nongenomic responses of estrogen via a number of pathways [24], albeit there are some contradictory reports in this area [25]. G1 is the selective agonist of GPER/GPR30.
Gαq-mER is another G protein-coupled estrogen receptor. STX (a diphenylacrylamide estrogenic compound), structurally similar to 4-OH tamoxifen, is the selective agonist of Gαq-mER with a 20-fold higher affinity for it than E2, and importantly, it does not bind to ERα or ERβ [26,27].
2.5. The predominant metabolic regulatory function of ERα rather than ERβ
Based on pharmacological evidence and genetic models, estrogens primarily regulate energy balance via ERα [3]. Specifically, according to pharmacological data, while the selective ERα agonist, propylpyrazole triol (PPT), has a strong anorectic effect, diarylpropionitrile (DPN), the selective ERβ agonist, does not [28,29]. Consistent with this data, global disruption of ERα (ERKOα) in female and male mice leads to hyperphagia, increased adiposity, hypometabolism, and insulin resistance [30,31]. Similarly, genetic deletion of the aromatase enzyme (ArKO), which is in the subset of the cytochrome P450 family responsible for converting androgens into estrogens, in male and female mice, leads to obesity. Aromatase insufficiency also develops obesity in humans [32]. In contrast, estrogen receptor beta (ERβ) deficiency has not been linked to obesity and any metabolic changes that come with it [31].
3. ER signaling, central perspective
It is evident that estrogen receptors are abundantly expressed in the CNS [33]. The hypothalamus is considered the most important area where estrogens have the most impact on energy balance [1]. The hypothalamus is divided into nuclei, which are physically characterized by neuronal clusters. For eating control, the hypothalamic ARC with two types of neurons, orexigenic and anorexigenic, affecting “eating willingness” is regarded as the most important component. Orexigenic neurons in the ARC express agouti-related peptide (AgRP) as well as neuropeptide Y (NPY) as eating-inducing neuropeptides, while anorexigenic neurons, namely proopiomelanocortin (POMC) neurons in the ARC, express eating-inhibiting compounds [34] (Fig. 1).
3.1. Food intake and energy expenditure
3.2. E2 functions on POMC and NPY/AgRP neurons
3.2.1. Excitatory effects of E2 on anorexigenic POMC neurons via AMPK modulation and increased synaptic plasticity
3.2.2. Rapid membrane-initiated estrogen signaling of E2 in POMC and NPY/AgRP neurons
-3.2.2.1. Mechanistic pathway for excitatory effects of E2 on anorexigenic POMC neurons via Gαq-mER.
3.2.3. The role of the mechanistic target of rapamycin (mTOR) signaling in the anorectic effects of E2
3.2.4. The protective effect of E2 against central (hypothalamic) insulin resistance
3.2.5. The role of melanocortin-4 receptor (MC4R) in mediating anorexigenic effects of POMC neurons
3.3. The impact of VMH neurons on glucose and energy consumption/ metabolism
3.3.1. BAT-mediated thermogenesis, E2 signals via AMPK modulation within the VMH
3.3.2. The connection between AMPK/GLP-1 and E2 signaling in the VMH
3.3.3. The role of ceramide metabolism and endoplasmic reticulum (ER) stress on the actions of E2 on energy metabolism
3.3.4. Thermogenesis and movement induction mediated by VMH ERαexpressing neurons
3.3.5. The crosstalk between ERα and Dnmt1 in the VMHdm region to regulate energy balance
3.4. The crosstalk between E2 and BDNF to regulate energy balance
3.5. Medial amygdala as a key node for estradiol signals on emotional and metabolic status
3.6. A bidirectional connection between metabolic and reproductive status
3.6.1. The importance of the ERE-independent pathway of ERα in connecting energy homeostasis and reproduction
4. ER signaling, peripheral perspective
4.1. Adipose tissue
4.1.1. The effect of E2 signaling on the fat distribution profile
4.1.2. BAT and ER signaling
4.1.3. The Role of ERα signaling in the browning of WAT
-4.1.3.1. The impact of tamoxifen on the browning of subcutaneous adipose tissue
4.1.4. Sex-related differences in obesity-induced hypertension and metabolic syndrome
4.2. Liver
4.2.1. Liver, as a non-reproductive target organ for the functions of sex steroids to regulate metabolic genes
4.2.2. Sex-related differences in the prevalence and severity of hepatic disorders
-4.2.2.1. ESR1/ERα-driven sex differences in the expression of metabolic genes in the liver
4.2.3. ERα signaling in males
-4.2.3.1. The importance of ERα signaling in the liver versus ERβ and GPR30
-4.2.3.2. The protective role of ERα signaling versus AR signaling in the liver
4.2.4. The protective role of ERα signaling in the liver in females
4.2.5. Androgen excess exposure accelerates liver disorders in females
4.2.6. The crosstalk between E2/ERα and liver transcriptional regulators/ proteins to regulate lipid and glucose metabolism
4.3. Pancreas
4.3.1. The effect of gonadal steroid hormones on insulin secretion
-4.3.1.1. The functional roles and biological significance of ER signaling in pancreatic β cells
-4.3.1.2. The functional roles and biological significance of AR signaling in pancreatic β cells
4.4. Skeletal muscle
4.4.1. The importance of estrogen and its time-dependent effects on insulin-driven glucose uptake in skeletal muscle
4.5. The protective signaling of ERα on metabolic function in males despite low expression levels
4.6. The protective role of membrane-initiated estrogen signaling on metabolic and vascular functions
5. The protective effects of ER signaling in abnormal metabolic conditions
5.1. Glucocorticoid-induced anabolic side effects
5.2. Diet-induced obesity and glucose clearance impairment
5.3. Transgenerational effects of maternal obesity
5.4. HFD/aging-induced hypothalamic inflammation
5.5. Harmful effects of endocrine-disrupting compounds
6. Conclusions and therapeutic opportunities
In this review, we summarized the mechanistic pathways, functional roles, and clinical significance of ER signaling in preserving the integrity of energy homeostasis and metabolic health. In addition to females, we discussed the importance of ER signaling in males and the potential biological functions of ER signaling in sex differences in the prevalence of some diseases. Estrogens primarily regulate energy balance via ERα rather than ERβ. Although males represent lower expression levels of ERα in metabolic tissues, the expression profile of ERα has been reported to be similar in males and females as WAT > liver > muscle, and ERα KO male mice display multiple features of metabolic dysfunction [30,220, 221].
At the central level, E2 plays its catabolic role predominantly by stimulating anorexigenic POMC neurons and suppressing orexigenic NPY/AgRP neurons in hypothalamic arcuate population neurons, as well as sending signals via VMH SF1 neurons to enhance SNS-BAT signaling and control BAT-mediated thermogenesis [41–49]. In the liver, ERα regulates genes involved in hepatic lipogenesis and glucose metabolism/homeostasis in a cycle/circulating estrogen level-dependent manner. Sex disparity in several aspects of sex steroid metabolism in the liver might contribute to the existence of sex-related differences in the prevalence of some diseases/cancers, such as NAFLD and HCC. ER signaling protects against hepatic steatosis/fibrosis and HCC in both sexes [164, 179, 186–188]
ER signaling in pancreatic β cells, which is mediated predominantly by extranuclear ERα, induces insulin secretion and preserves cell survival by promoting proliferation and immunomodulatory pathways. According to animal studies, ER signaling might be protective against insulitis and the development of T1D after insulitis. In addition, ER signaling might be influential in the lower prevalence of T1D in females compared to males after puberty [201,204].
In terms of adipose tissue, ER signaling protects against obesity-provoked pro-inflammatory immune responses by altering the phenotype of macrophages as well as T cells. The immune response to an HFD is stronger in male mice than in female mice, suggesting sex-related differences in HFD-driven hypertension and that obese males might have a higher risk of developing hypertension than obese females [134,150, 151]. Perhaps one of the most important aspects of estrogen in metabolic disorders is its ability to reverse the trend of visceral fat storage toward subcutaneous fat accumulation [126,127,230]. Visceral adipose tissue in males represents higher fibrosis/inflammation, leading to a higher susceptibility to developing metabolic abnormalities [134]. E2/ERα signaling has been demonstrated to be protective against fibrosis/inflammation in adipose tissue [155]. In addition, extranuclear/membrane-initiated ERα signaling has been linked to the induction of adipocyte browning [136]. In contrast to WAT, BAT is considered a key regulator of energy consumption. Estrogen has a direct effect on BAT, resulting in an increase in energy expenditure due to enhanced BAT lipolysis and thermogenic activity, processes being regulated by the central and peripheral nervous systems on BAT mitochondrial biogenesis [42,43].
Having looked closely at studies in the area of estrogen and energy homeostasis, the authors speculate that several topics are highly worth further investigation in preclinical research as well as prospective population-based cohort studies or clinical trials: First, although preclinical and clinical research address beneficial effects of ER signaling in several metabolic abnormalities such as obesity and insulin resistance, it seems that these benefits of estrogen are time-dependent so that early postmenopausal women are more likely to benefit from estrogen therapy due to loss of ER protein among women in their late menopausal years [213,214]. Second, it seems that the potency of ER signaling to protect against metabolic abnormalities such as obesity and glucose intolerance in the conditions of estrogen deficiency is not the same, and it is more protective against obesity [237,238]. Third, it seems that obese women are less likely than obese men to develop metabolic abnormalities due to higher expression of ERα in the hypothalamic ARC area and its protective effect against hypothalamic inflammation [248,249]. In addition, obese women might have a lower risk of developing hypertension than obese men due to sex differences in immune cell activation [150,151]. Fourth, postmenopausal women might be more vulnerable to metabolic side effects of some drugs due to the lack of preservative effects of estradiol [226].
Recently, to achieve the superior metabolic advantages of E2 while reducing side effects, a few strategies have been used. One approach is conjugating E2 to GLP-1 to benefit from targeted ER activation in GLP-1- specific expressed tissues as well as E2 and GLP synergistic effects (section 6.3.2) [6,257]. Another strategy is using selective estrogen receptor modulators with anti-estrogenic properties in the breast and uterus, either alone or in conjugation with estrogens [258–260].
Another point of extreme physiological importance seems to be the extranuclear-initiated ERα signaling, which has been shown to be involved in the regulation of metabolic function in the liver, adipose tissue, and pancreatic beta cells [201,202,224,225]. Of note, this estrogenic pathway has been suggested to have fewer proliferative effects on mammary and reproductive tissues and might even have vascular protective effects [21,224,225]. The authors speculate that targeting newly discovered isoforms of ERα, mediating non-genomic estrogen signaling, particularly ERα36, might be a promising therapeutic approach for dysmetabolic conditions [20,21]. Moreover, in the context of control of energy balance, exploring ERα mechanistic links with other factors such as BDNF[101] and hypothalamic ceramides [80] may open new avenues of research to better understand the importance of estrogen signaling in metabolic homeostasis, health, and diseases.
Fatemeh Mahboobifard, Mohammad H. Pourgholami, Masoumeh Jorjani, Leila Dargahi, Mina Amiri, Somaye Sadeghi, Fahimeh Ramezani Tehrani
ABSTRACT
Over the last two decades, it has become evident that estrogens preserve the integrity of energy homeostasis at central and peripheral levels. Estrogen deficiency, such as that caused by menopause or ovariectomy, has been linked to obesity and metabolic disorders that can be resolved or reversed by estrogen therapy. 17β-estradiol (E2), as the major estrogen in the body, primarily regulates energy balance via estrogen receptor alpha (ERα). At the central level, E2 plays its catabolic role predominantly by interacting with hypothalamic arcuate neurons and sending signals via ventromedial hypothalamic neurons to control brown adipose tissue-mediated thermogenesis. In peripheral tissues, several organs, particularly the liver, brown and white adipose tissues, and pancreatic β cells, have attracted considerable attention.
In this review, we focused on the current state of knowledge of “central and peripheral” estrogen signaling in regulating energy balance via “nuclear and extranuclear pathways” in both “females and males". In this context, according to an exploratory approach, we tried to determine the principal estrogen receptor subtype/isoform in each section, the importance of extranuclear-initiated estrogen signaling on metabolic functions, and how sex differences related to ER signaling affect the prevalence of some of the metabolic disorders. Moreover, we discussed the data from a third viewpoint, understanding the clinical significance of estrogen signaling in abnormal metabolic conditions such as obesity or being on a high-fat diet.
Collectively, this review exposes novel and important research gaps in our current understanding of dysmetabolic diseases and can facilitate finding more effective treatment options for these disorders.
1. Introduction
The control of energy homeostasis is a complex process that maintains the balance of energy intake, expenditure, and storage so that each organ has enough energy to function. Both the central, predominantly the hypothalamus, and peripheral organs, predominantly the liver, pancreas, and adipose tissue, are involved in controlling these processes [1,2]. The role of 17β-estradiol (E2) in regulating reproduction by interacting with the population of hypothalamic gonadotropin-releasing hormone (GnRH) neurons and providing feedback on the hypothalamic-pituitary-gonadal (HPG) axis is well documented. However, over the last two decades, it has become evident that estrogens play a role in almost all aspects of metabolism and preserve the integrity of energy homeostasis at central and peripheral levels. Therefore, the effects of estrogens are considered beyond reproductive function [1]. In preclinical and clinical research, states of estrogen deficiency, such as that caused by menopause or ovariectomy, have been linked to decreased energy expenditure, metabolic abnormalities, and obesity [3, 4]. Given that the average lifespan is increasing in developed countries, about half of a woman’s life is spent in an estrogen-deficient state [4]. Estrogen therapy, on the other hand, reverses these defects in humans or ovariectomized animals by reducing food intake while increasing energy expenditure [5,6]. Furthermore, evidence indicates that the fluctuations in estrogen levels depending on the status of the menstrual/estrous cycle [3,7], as well as estrogen levels during pregnancy and breastfeeding [8, 9], affect feeding quantity, body weight, and metabolic status.
This review provides a comprehensive and up-to-date overview of the matter, including novel data on "central and peripheral" estrogen signaling to regulate energy balance in both sexes. In this context, to achieve new insights on this topic, we attempted to explore from the literature the main estrogen receptor subtype/isoform, its primary signaling pathways (nuclear and extranuclear), and mechanistic links with other factors/transcription factors, as well as ER-driven sex differences in the prevalence of some of the metabolic diseases. Moreover, from a third viewpoint, we tried to explain the protective significance of ER signaling in abnormal metabolic conditions such as exposure to a high-fat diet (HFD).
These important findings could help us develop novel therapeutic strategies for several metabolic disorders such as obesity, metabolic syndrome, type 2 diabetes, non-alcoholic fatty liver disease (NAFLD), hypertension, and even, separately from metabolic disorders, some types of cancer. Moreover, having reviewed the studies on the aforementioned topics, we found some important research gaps and open questions, leaving the possibility of conducting further research to find novel biologically relevant proteins with promising therapeutic effects on dysmetabolic conditions.
2. General background of estrogen
2.1. Estrogen production in the body
17β-estradiol (E2), as the major estrogen in the body, is produced predominantly by the ovary and, to a lesser extent, is also synthesized by adrenals, testes, and adipose tissue. During pregnancy, the placenta produces high estrogen levels [10,11]. Under a two-cell model, estrogen is produced in the growing follicles of the ovary with the contribution of two cells: the theca and granulose cells. Initially, under the stimulation of the pituitary luteinizing hormone (LH), the inner theca cell layer of the follicles is responsible for the synthesis of androgens like androstenedione and testosterone from cholesterol. Theca cells, like Leydig cells in the testis, synthesize androgen in response to LH. Pituitary FSH then drives granulosa cells to express aromatase, which is responsible for converting androgens into estrogens [9].
2.2. HPG axis and estrogen feedback
The produced E2 is then transferred to the hypothalamus, where it interacts with gonadotropin-releasing hormone (GnRH) neurons in the hypothalamic preoptic area (POA) to regulate reproduction. GnRH neurons directly regulate reproduction and gonadal steroidogenesis [12]. They secrete the neurohormone GnRH in a pulsatile manner into the adenohypophysis, stimulating follicle-stimulating hormone (FSH) and LH release from the gonadotrophs. This process is dependent on GnRH neuronal depolarization and an elevation in intracellular calcium [13]. The ovarian function is cyclic during the reproductive period, with the follicular phase occurring first and the luteal phase occurring second. The follicular phase is associated with follicular growth and estrogen level elevation, with a significant rise near the end of the phase, just before ovulation, while the luteal phase is associated with corpus luteum formation and the predominant secretion of progesterone [11]. Except for the pre-ovulatory stage, when estrogen exerts positive feedback that is necessary for GnRH surge and ovulation occurrence, estrogen has a negative feedback on the HPO axis during the ovarian cycle (estrous cycle) [12]. The reason for this paradox might be due to the opposing effects of estrogen (suppressive and stimulatory) on the expression of kisspeptins, as neuropeptides with regulatory effects on GnRH neurons, in two distinct nuclei of the hypothalamus, including the arcuate nucleus (ARC), involved in the negative feedback of the E2, and the anteroventral periventricular nucleus (AVPV), involved in positive feedback [14] (Fig. 1).
2.3. Genomic and non-genomic estrogen signaling pathways
Estrogen receptors (ERs) have two primary subtypes: ERα and ERβ, which are encoded by the genes ESR1 and ESR2, and belong to the nuclear hormone receptor subclass. It is evident that ER programming is processed through genomic and non-genomic signaling. Genomic signaling is initiated by the binding of E2 to ERs in the cytoplasm and the formation of hetero- or homodimers which translocate to the nucleus [15]. This pathway takes hours to days to complete. Interaction of so-called dimers with the estrogen response element (ERE) on DNA or other transcription factors bound to DNA like specificity protein-1 (SP-1) and activator protein 1 (AP-1) triggers transcriptional activation of E2 [16]. Non-genomic signaling of E2, which is also known as rapid non-nuclear signaling, or “extranuclear- or membrane-initiated estrogen signaling”, is generated by binding ER to E2 as a monomer, which induces several kinase cascades as well as regulation of calcium flux and ion channels [17].
The rapid function of E2 was identified in early research with no knowledge about membrane estrogen receptors until the 1990 s when the membrane localization of ERα was discovered in the cultured CA1 neurons of the hippocampus and pituitary cells [18,19]. Recently, new isoforms of ERα mediating non-genomic signaling of E2, such as ERα36 and ERα46, have been identified in the brain and body [20,21]. These isoforms are produced spontaneously in the body via alternative splicing of ERα-mRNAs. Both the membrane and nuclear estrogen receptors are encoded by the same gene [21]. Membrane localization of ERα occurs through posttranslational modifications and the developing palmitoylated ERα which target caveolae as structures with cholesterol-rich plasma membranes that accelerate membrane anchoring [22]. It should be noted that membrane-initiated estrogen receptor signaling through inducing intracellular kinase cascades, including protein kinase C/protein kinase A/phosphatidylinositol 3-kinase/mitogen-activated protein kinase (PKC/PKA/PI3K/MAPK), is not only limited to protein phosphorylation and rapid alteration in the activity of ion channels but also induces some alterations in gene transcription (long-term effects), like downstream signaling pathways of many neurotransmitters [23].
2.4. G-protein-coupled estrogen receptors
In addition to ERα and ERβ, G-protein-coupled estrogen receptor (GPER) and Gαq-coupled membrane ER (Gαq-mER, Gq-mER) are two membrane estrogen receptors that do not originate from ERα or ERβ transcripts. GPER, also known as GPR30, is a G-protein-coupled estrogen receptor bound to the membrane that is implicated in the rapid nongenomic responses of estrogen via a number of pathways [24], albeit there are some contradictory reports in this area [25]. G1 is the selective agonist of GPER/GPR30.
Gαq-mER is another G protein-coupled estrogen receptor. STX (a diphenylacrylamide estrogenic compound), structurally similar to 4-OH tamoxifen, is the selective agonist of Gαq-mER with a 20-fold higher affinity for it than E2, and importantly, it does not bind to ERα or ERβ [26,27].
2.5. The predominant metabolic regulatory function of ERα rather than ERβ
Based on pharmacological evidence and genetic models, estrogens primarily regulate energy balance via ERα [3]. Specifically, according to pharmacological data, while the selective ERα agonist, propylpyrazole triol (PPT), has a strong anorectic effect, diarylpropionitrile (DPN), the selective ERβ agonist, does not [28,29]. Consistent with this data, global disruption of ERα (ERKOα) in female and male mice leads to hyperphagia, increased adiposity, hypometabolism, and insulin resistance [30,31]. Similarly, genetic deletion of the aromatase enzyme (ArKO), which is in the subset of the cytochrome P450 family responsible for converting androgens into estrogens, in male and female mice, leads to obesity. Aromatase insufficiency also develops obesity in humans [32]. In contrast, estrogen receptor beta (ERβ) deficiency has not been linked to obesity and any metabolic changes that come with it [31].
3. ER signaling, central perspective
It is evident that estrogen receptors are abundantly expressed in the CNS [33]. The hypothalamus is considered the most important area where estrogens have the most impact on energy balance [1]. The hypothalamus is divided into nuclei, which are physically characterized by neuronal clusters. For eating control, the hypothalamic ARC with two types of neurons, orexigenic and anorexigenic, affecting “eating willingness” is regarded as the most important component. Orexigenic neurons in the ARC express agouti-related peptide (AgRP) as well as neuropeptide Y (NPY) as eating-inducing neuropeptides, while anorexigenic neurons, namely proopiomelanocortin (POMC) neurons in the ARC, express eating-inhibiting compounds [34] (Fig. 1).
3.1. Food intake and energy expenditure
3.2. E2 functions on POMC and NPY/AgRP neurons
3.2.1. Excitatory effects of E2 on anorexigenic POMC neurons via AMPK modulation and increased synaptic plasticity
3.2.2. Rapid membrane-initiated estrogen signaling of E2 in POMC and NPY/AgRP neurons
-3.2.2.1. Mechanistic pathway for excitatory effects of E2 on anorexigenic POMC neurons via Gαq-mER.
3.2.3. The role of the mechanistic target of rapamycin (mTOR) signaling in the anorectic effects of E2
3.2.4. The protective effect of E2 against central (hypothalamic) insulin resistance
3.2.5. The role of melanocortin-4 receptor (MC4R) in mediating anorexigenic effects of POMC neurons
3.3. The impact of VMH neurons on glucose and energy consumption/ metabolism
3.3.1. BAT-mediated thermogenesis, E2 signals via AMPK modulation within the VMH
3.3.2. The connection between AMPK/GLP-1 and E2 signaling in the VMH
3.3.3. The role of ceramide metabolism and endoplasmic reticulum (ER) stress on the actions of E2 on energy metabolism
3.3.4. Thermogenesis and movement induction mediated by VMH ERαexpressing neurons
3.3.5. The crosstalk between ERα and Dnmt1 in the VMHdm region to regulate energy balance
3.4. The crosstalk between E2 and BDNF to regulate energy balance
3.5. Medial amygdala as a key node for estradiol signals on emotional and metabolic status
3.6. A bidirectional connection between metabolic and reproductive status
3.6.1. The importance of the ERE-independent pathway of ERα in connecting energy homeostasis and reproduction
4. ER signaling, peripheral perspective
4.1. Adipose tissue
4.1.1. The effect of E2 signaling on the fat distribution profile
4.1.2. BAT and ER signaling
4.1.3. The Role of ERα signaling in the browning of WAT
-4.1.3.1. The impact of tamoxifen on the browning of subcutaneous adipose tissue
4.1.4. Sex-related differences in obesity-induced hypertension and metabolic syndrome
4.2. Liver
4.2.1. Liver, as a non-reproductive target organ for the functions of sex steroids to regulate metabolic genes
4.2.2. Sex-related differences in the prevalence and severity of hepatic disorders
-4.2.2.1. ESR1/ERα-driven sex differences in the expression of metabolic genes in the liver
4.2.3. ERα signaling in males
-4.2.3.1. The importance of ERα signaling in the liver versus ERβ and GPR30
-4.2.3.2. The protective role of ERα signaling versus AR signaling in the liver
4.2.4. The protective role of ERα signaling in the liver in females
4.2.5. Androgen excess exposure accelerates liver disorders in females
4.2.6. The crosstalk between E2/ERα and liver transcriptional regulators/ proteins to regulate lipid and glucose metabolism
4.3. Pancreas
4.3.1. The effect of gonadal steroid hormones on insulin secretion
-4.3.1.1. The functional roles and biological significance of ER signaling in pancreatic β cells
-4.3.1.2. The functional roles and biological significance of AR signaling in pancreatic β cells
4.4. Skeletal muscle
4.4.1. The importance of estrogen and its time-dependent effects on insulin-driven glucose uptake in skeletal muscle
4.5. The protective signaling of ERα on metabolic function in males despite low expression levels
4.6. The protective role of membrane-initiated estrogen signaling on metabolic and vascular functions
5. The protective effects of ER signaling in abnormal metabolic conditions
5.1. Glucocorticoid-induced anabolic side effects
5.2. Diet-induced obesity and glucose clearance impairment
5.3. Transgenerational effects of maternal obesity
5.4. HFD/aging-induced hypothalamic inflammation
5.5. Harmful effects of endocrine-disrupting compounds
6. Conclusions and therapeutic opportunities
In this review, we summarized the mechanistic pathways, functional roles, and clinical significance of ER signaling in preserving the integrity of energy homeostasis and metabolic health. In addition to females, we discussed the importance of ER signaling in males and the potential biological functions of ER signaling in sex differences in the prevalence of some diseases. Estrogens primarily regulate energy balance via ERα rather than ERβ. Although males represent lower expression levels of ERα in metabolic tissues, the expression profile of ERα has been reported to be similar in males and females as WAT > liver > muscle, and ERα KO male mice display multiple features of metabolic dysfunction [30,220, 221].
At the central level, E2 plays its catabolic role predominantly by stimulating anorexigenic POMC neurons and suppressing orexigenic NPY/AgRP neurons in hypothalamic arcuate population neurons, as well as sending signals via VMH SF1 neurons to enhance SNS-BAT signaling and control BAT-mediated thermogenesis [41–49]. In the liver, ERα regulates genes involved in hepatic lipogenesis and glucose metabolism/homeostasis in a cycle/circulating estrogen level-dependent manner. Sex disparity in several aspects of sex steroid metabolism in the liver might contribute to the existence of sex-related differences in the prevalence of some diseases/cancers, such as NAFLD and HCC. ER signaling protects against hepatic steatosis/fibrosis and HCC in both sexes [164, 179, 186–188]
ER signaling in pancreatic β cells, which is mediated predominantly by extranuclear ERα, induces insulin secretion and preserves cell survival by promoting proliferation and immunomodulatory pathways. According to animal studies, ER signaling might be protective against insulitis and the development of T1D after insulitis. In addition, ER signaling might be influential in the lower prevalence of T1D in females compared to males after puberty [201,204].
In terms of adipose tissue, ER signaling protects against obesity-provoked pro-inflammatory immune responses by altering the phenotype of macrophages as well as T cells. The immune response to an HFD is stronger in male mice than in female mice, suggesting sex-related differences in HFD-driven hypertension and that obese males might have a higher risk of developing hypertension than obese females [134,150, 151]. Perhaps one of the most important aspects of estrogen in metabolic disorders is its ability to reverse the trend of visceral fat storage toward subcutaneous fat accumulation [126,127,230]. Visceral adipose tissue in males represents higher fibrosis/inflammation, leading to a higher susceptibility to developing metabolic abnormalities [134]. E2/ERα signaling has been demonstrated to be protective against fibrosis/inflammation in adipose tissue [155]. In addition, extranuclear/membrane-initiated ERα signaling has been linked to the induction of adipocyte browning [136]. In contrast to WAT, BAT is considered a key regulator of energy consumption. Estrogen has a direct effect on BAT, resulting in an increase in energy expenditure due to enhanced BAT lipolysis and thermogenic activity, processes being regulated by the central and peripheral nervous systems on BAT mitochondrial biogenesis [42,43].
Having looked closely at studies in the area of estrogen and energy homeostasis, the authors speculate that several topics are highly worth further investigation in preclinical research as well as prospective population-based cohort studies or clinical trials: First, although preclinical and clinical research address beneficial effects of ER signaling in several metabolic abnormalities such as obesity and insulin resistance, it seems that these benefits of estrogen are time-dependent so that early postmenopausal women are more likely to benefit from estrogen therapy due to loss of ER protein among women in their late menopausal years [213,214]. Second, it seems that the potency of ER signaling to protect against metabolic abnormalities such as obesity and glucose intolerance in the conditions of estrogen deficiency is not the same, and it is more protective against obesity [237,238]. Third, it seems that obese women are less likely than obese men to develop metabolic abnormalities due to higher expression of ERα in the hypothalamic ARC area and its protective effect against hypothalamic inflammation [248,249]. In addition, obese women might have a lower risk of developing hypertension than obese men due to sex differences in immune cell activation [150,151]. Fourth, postmenopausal women might be more vulnerable to metabolic side effects of some drugs due to the lack of preservative effects of estradiol [226].
Recently, to achieve the superior metabolic advantages of E2 while reducing side effects, a few strategies have been used. One approach is conjugating E2 to GLP-1 to benefit from targeted ER activation in GLP-1- specific expressed tissues as well as E2 and GLP synergistic effects (section 6.3.2) [6,257]. Another strategy is using selective estrogen receptor modulators with anti-estrogenic properties in the breast and uterus, either alone or in conjugation with estrogens [258–260].
Another point of extreme physiological importance seems to be the extranuclear-initiated ERα signaling, which has been shown to be involved in the regulation of metabolic function in the liver, adipose tissue, and pancreatic beta cells [201,202,224,225]. Of note, this estrogenic pathway has been suggested to have fewer proliferative effects on mammary and reproductive tissues and might even have vascular protective effects [21,224,225]. The authors speculate that targeting newly discovered isoforms of ERα, mediating non-genomic estrogen signaling, particularly ERα36, might be a promising therapeutic approach for dysmetabolic conditions [20,21]. Moreover, in the context of control of energy balance, exploring ERα mechanistic links with other factors such as BDNF[101] and hypothalamic ceramides [80] may open new avenues of research to better understand the importance of estrogen signaling in metabolic homeostasis, health, and diseases.
