Key words

1 Introduction

The literature contains many reviews of estrogen receptors and their ligands [1–9], as well as the effects of estrogens on specific tissue types [4, 10–31]. The reader interested in comprehensive discussion of estrogen receptors is referred to one or more of these excellent reviews. The purpose of this discussion is to provide a brief overview of the estrogen receptors and estrogen action from various perspectives.

2 The Historical Perspective

In evolutionary terms, the estrogen receptor is an ancient protein that is expressed in all vertebrates and a few invertebrates [32]. From a historical perspective, studies to discover the mechanism by which estrogen exerted its effects on target tissues took place in the 1950s and 1960s and culminated in the determination that estrogen bound to a protein in its target cells [33, 34]. Jensen and coworkers [35] describe a fascinating historical account of the early studies that led to the discovery of the estrogen receptor. This first estrogen binding protein is the protein now known as estrogen receptor α (ERα, also known as ER1 or Esr1). Our understanding of the binding of the estrogen-receptor complex to DNA, transcription to RNA, and subsequent protein synthesis evolved over the course of the 1960s and 1970s [33, 36, 37] to the mechanism that we recognize today of ligand-activated transcription factors. This genomic mechanism of action involves transcription and translation of genes so it is characterized by the time that it takes (hours) to develop a response as well as the persistence of the response for the lifespan of the new protein. The second estrogen receptor was identified in 1986 [38] and was named estrogen receptor β (ERβ, also known as ER2 and Esr2). Like ERα, ERβ is a ligand-activated transcription factor, so they share the slow-onset, persistent response genomic mechanism of action.

As early as the 1970s, reports appeared in the literature of actions of estrogen that occurred too rapidly to be mediated by the genomic mechanism of action [39–41]. However, these reports were pushed aside by the, at the time, seemingly more compelling genomic mechanism of action. Nevertheless, reports continued to emerge of rapid, nongenomic actions of estrogen [42] as well as nongenomic actions of other steroid hormones [43]. The ability of estrogen to mediate rapid, nongenomic actions is now widely accepted. An estrogen-responsive G protein coupled receptor called GPR30 or GPER has now been well characterized and is considered by many to be the source of the rapid actions of estrogen [44–52]. However, others have reported that the genomic receptors, ERα and ERβ, can associate with the membrane and mediate rapid actions although the mechanism of association of the genomic ER with the membrane is unclear [53]. Yet others report that mutated forms of ERα are involved in the rapid nongenomic actions of estrogen [54]. The details of rapid actions of estrogen remain controversial; it is quite possible that all of these mechanisms occur, perhaps in different cell types or differentially in physiological versus pathological situations.

3 The Physiological Perspective

We can also examine the estrogen receptor from the perspective of its physiological functions. In the same time frame as the discovery of the first estrogen receptor (late 1950s–1960s), the functional perspective of estrogen was purely as a female reproductive hormone. The reproductive functions of estrogen are the material of textbooks [55, 56]. In the historical context, an estrogenic compound was defined as a substance that could stimulate uterine growth and up-regulate synthesis of the progesterone receptor, both of which serve reproductive functions. As our understanding of the estrogen receptors has evolved, so also has our understanding of the functions of estrogen. In addition to the requirement of estrogen in reproductive function, and therefore, its role in the survival of the species, we now know that estrogen is critical to many other physiological functions. Estrogenreceptors are expressed and estrogenic ligands produce specific effects in the cardiovascular system [10–13], brain [14, 15], bone [16, 17], liver [18, 19], adipose tissue [19–21], colon [22, 23], skin [24, 25, 57], prostate [26, 27], testes [28, 29], epididymis [30, 31], and salivary gland [4]. Thus estrogen receptors serve a truly pleiotropic array of functions.

4 The Pharmacological Perspective

From the perspective of pharmacology, estrogen receptor agonists and estrogen receptor antagonists are both clinically relevant. The clinical uses of estrogen receptor agonists are primarily in the areas of combination hormonal contraceptives and in postmenopausal hormone replacement [58]. In combination hormonal contraceptives, an estrogen is administered with a progestin in the form of a pill, patch, or vaginal ring [59, 60]. These hormonal contraceptives are highly effective in the prevention of pregnancy. Nevertheless, the pleiotropic actions of estrogens incur some adverse effects. For example, the estrogens in contraceptives increase the risk of adverse reactions in the cardiovascular system, especially in women smokers [59, 60]. The primary use of the one pure estrogen receptor antagonist that is available, fulvestrant, is in the treatment of estrogen receptor positive (ER+) breast cancer. Fulvestrant is also sometimes called a selective estrogen receptor downregulator or degrader (SERD) because its binding to receptor leads to proteasomal degradation of the receptor [61]. In addition, an intriguing family of pharmaceutical agents called selective estrogen receptor modulators (SERMs) interacts with the estrogen receptors. The SERMs act as estrogen receptor agonists in some estrogen-sensitive tissues and as antagonists in others [59, 62]. SERMs that are on the market in the USA at this time include tamoxifen, raloxifene, bazedoxifene, and ospemifene. Tamoxifen acts as an estrogen receptor antagonist in the breast and is used in the treatment of ER+ breast cancer. The drawback to tamoxifen is that it acts as an estrogen receptor agonist in the uterus [59]. Raloxifene is an estrogen receptor agonist in bone but an antagonist in breast and uterus; it is used for postmenopausal osteoporosis [59]. Ospemifene is an estrogen receptor agonist on the vaginal epithelium, endometrium, and bone and is antiestrogenic in the breast. It is used for the treatment of dyspareunia that may occur in postmenopausal women [63]. Bazedoxifene is an estrogen receptor agonist in bone, hypothalamus, vagina, and vulva and acts as an antagonist in breast and uterus. Bazedoxifene is approved in the USA to be used in concert with conjugated estrogens for the treatment of vasomotor symptoms associated with the menopause [64]. The underlying concept is that the conjugated estrogens will alleviate the vasomotor symptoms and the bazedoxifene will prevent the adverse effects of estrogen on the breast and endometrium [64].

5 The Pathological Perspective

If we consider the estrogen receptors from the perspective of pathology, it is clear that several diseases are related to the estrogen receptor. Approximately 75 % of breast cancers express estrogen receptors [65, 66], and the presence of estrogen stimulates growth of these tumors. As mentioned above, inhibition of estrogen action in the breast is an important treatment for estrogen-sensitive breast cancer. These treatment strategies include the use of specific estrogen receptor antagonists such as fulvestrant [61], SERMs such as tamoxifen, or prevention of the synthesis of estrogen by use of aromatase inhibitors such as anastrozole, letrozole, and exemestane [59]. Similarly, endometrial cancer is also estrogen sensitive. The administration of estrogen alone for an extended period of time can cause the development of endometrial cancer in postmenopausal women [67].

The development of venous thromboembolism is another pathology associated with estrogen [58]. The risk of venous thromboembolism rises in women who take exogenous estrogens (e.g., contraceptives, postmenopausal hormone replacement therapy), as well as in situations in which endogenous estrogens are high as in women who are pregnant, or in the immediate postpartum period [68].

Autoimmune disease s occur at a higher rate in women than in men [69] and estrogen receptors play a role in the immune system [70]. Several hypotheses that been proposed to explain the discrepancy between men and women. One very intriguing idea is related to the enzyme, activation-induced deaminase (AID), which is involved in class switch recombination during antibody diversification [71]. Estrogen has been shown to activate AID [72], and may affect immune function by this mechanism. Derangement of estrogen regulation of this enzyme may be responsible for at least part of the increased risk of autoimmune disease in women.

The role of estrogen and the estrogen receptors in several other pathologies, such as cardiovascular disease and dementia, remains controversial. The rate of most cardiovascular diseases is lower in premenopausal women than in men, but rapidly rises after the menopause [73]. This disease pattern suggests that estrogen is protective to the cardiovascular system. However, studies such as the Women’s Health Initiative (WHI) showed that replacement of estrogen in postmenopausal women could trigger cardiovascular events (heart attack and stroke) [74]. Further analysis of data from the WHI study showed that the women who experienced cardiovascular events were more likely to be many years past the menopause. These data suggested that estrogen was more likely to trigger cardiovascular events in women who had been many years without the hormone and in whom cardiovascular disease may have become well-established although still silent. Thus the concept has developed that estrogen may be protective to a healthy cardiovascular system [11] but deleterious to a cardiovascular system in which disease such as atherosclerosis has developed [73, 75]. More recent data suggest that administration of estrogen soon after the menopause prevents the development of cardiovascular disease and does not precipitate CV events [12, 58, 76, 77]. A similar controversy exists regarding the effect of estrogen on dementia in postmenopausal women [78].

6 The Structural Perspective

Another perspective from which to examine the estrogen receptors is that of structure. ERα and ERβ share the same general structure, that is, a ligand-binding domain, DNA-binding domain, and two activation function (AF) domains [79]. The two receptors share a high degree of amino acid sequence homology except in the N-terminal domain (the AF-2 domain). Proteins called coactivators (e.g., NCOA1, NCOA2, NCOA3, CREBBP, PPARBP, P68, and SRA) and corepressors (e.g., NCOR1, NRIP) can interact with ligand-bound ERα or ERβ and influence the ability of the receptor to activate or inhibit expression of a gene [80–82]. Phosphorylation of the estrogen receptor can affect its activity [83]. Ligand-bound ERα and ERβ bind to the same DNA sequence, the estrogen response element (ERE), whose sequence is defined as GGTCAnnnTGACC. For many estrogen-responsive genes, the ERE may be a significant distance upstream of the start site [84–86]. The promoters of many estrogen-responsive genes may contain only a half-site sequence of the ERE rather than the complete ERE, and ligand-bound ERα or ERβ can form protein-protein complexes with other transcription factors which then bind to their own response elements in the promoters of regulated genes [87]. Transcription factors with which ERα and ERβ can interact include Sp-1 [87, 88], Ap-1 [89, 90], and NF-κB [91]. Although they utilize the same ERE and interact with the same coregulators, ERα and ERβ exhibit differential tissue distribution and different biological effects [3–5]. Thus there remain complexities to be deciphered.

In contrast, the structure of GPER is dramatically different from that of ERα and ERβ, as one would expect of the membrane protein, and of course its mechanism of action is dramatically different as well. GPER has been reported to activate several signal transduction pathways which culminate in the phosphorylation of substrate proteins, some of which are transcription factors [46]. One of the transcription factors reported to be downstream of GPER is ERα [92], suggesting an interaction between nongenomic and genomic estrogen receptors.

7 The Ligand Perspective

Another perspective from which to examine the estrogen receptors is that of their ligands. Estrogen is a generic term that typically refers to the primary endogenous estrogenic compounds 17β-estradiol, estrone, and estriol (from most to least potent). Metabolites of these estrogens have been shown to regulate ERα and ERβ [93–95], as have other endogenous substances such as 27-hydroxycholesterol [96, 97]. Exogenous substances that can activate the estrogen receptors include phytoestrogens such as genistein, daidzein, and equol [98–101] as well as environmental substances [102–104].

8 Summary

Research on the estrogen receptors has been ongoing for over five decades; this brief overview can only provide a meager glimpse of the broad range of knowledge that is available regarding these receptors. In spite of the significant quantity of information that we have about the estrogen receptors, substantial questions remain. To name just a few: if ERα and ERβ use the same ERE and interact with the same coregulators, and are expressed in many of the same tissues, what is the mechanism by which they exert different biological effects? To what extent do ERα, ERβ, and GPER interact and do those interactions differ among estrogen-sensitive tissues? Can we delineate specific functions for each of the three receptors? Knockout animals for ERα, ERβ, and GPER have answered some of the biological questions regarding individual functions of these receptors, but not all of the models agree [4, 105, 106]. In spite of the 50+ years of investigation, the field of estrogen receptor research remains strong and vibrant, with notable questions remaining to carry the field forward into the future.