Chapter 4
The Steroid Hormone Receptors
Carolyn M. Klinge and Ch. Venkateswara Rao
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Carolyn M. Klinge, PhD
Department of Biochemistry and Molecular Biology, University of Louisville School of Medicine, Louisville, Kentucky (Vol 5, Chaps 4, 5)

Ch. Venkateswara Rao, PhD
Departments of Obstetrics and Gynecology and Biochemistry and Molecular Biology, School of Medicine, University of Louisville, Louisville, Kentucky (Vol 5, Chaps 4, 5)


Steroid hormones exert a wide variety of effects on growth, development, and differentiation, including important regulatory and behavioral functions within the reproductive system, central nervous system, and adrenal axis. These hormones act by binding to specific intracellular receptor proteins that function as signal transducers and transcription factors to modulate expression of target genes.1,2 Molecular cloning of the steroid hormone receptor genes in humans has been accomplished over the past 15 years (Table 1). Sequence comparison has revealed that steroid hormone receptors belong to a diverse family of ligand-activated gene regulators that share a highly conserved structure and common mechanisms affecting gene transcription.3 The evolutionary relationship among the steroid and nuclear receptors has been deduced by the high conservation in their DNA-binding domains (DBDs) and in their less-conserved ligand-binding domains (LBDs) and indicates that this large group of proteins arose from a common ancestral molecule.3

TABLE 1. Members of the Steroid Hormone and Nuclear Receptor Gene Superfamily in Mammalian Tissues

  Receptors with known ligands1,2,4,26,156,236,237

  TRs (α1,α2,β)
  c-erb A1
  Rev-erbAα (Rev-Erb)
  RARs (α,β,γ)237
  RXRs (α,β,γ)237
  PPARs (α,β,γ)

  Examples of orphan receptors4


  Orphan receptors with identified ligands5

  BXR (benzoate X receptor)
  CARβ (constitutive androstane receptor)
  FXR (farnesoid X receptor)
  hSXR (steroid and xenobiotic receptor)
  LXR (liver X receptor)
  PAR (pregnane receptor)

The steroid/nuclear hormone receptor superfamily (Table 1), includes more than 150 receptors for the gonadal and adrenal steroids; nonsteroidal ligands such as thyroid hormones, vitamin D, retinoic acid, and fatty acids; and numerous "orphan" receptors whose endogenous ligands, if necessary, are unknown or are just being identified.1,4,5 Until recently, only one estrogen receptor (ER) was thought to mediate the physiologic effects of estrogens. However, a second gene encoding a closely related but distinct ER, called ERβ, was identified in rat prostate6 and in humans.7 The original ER is called ERα. ERα and ERβ can form heterodimers and homodimers in vitro and in vivo.8 The superfamily also includes the ERBA1 and ERBA2 oncogene proteins that bind to DNA but lack a functional LBD9 and other orphan receptors, such as short heterodimer partner, that lack a DBD but have a LBD.10 The steroid receptors are considered class I members of the nuclear receptor superfamily, and the other receptors are class II receptors.1 A variety of mechanisms for achieving tissue-specific gene expression in response to steroid hormones have evolved to ensure diversity through the interaction of these receptors with other cellular proteins and gene elements.

In this chapter, we summarize current information on the steroid/nuclear hormone receptors, focussing primarily on receptors for the sex steroid hormones.

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To understand how steroid hormone receptors regulate gene function, it is important to know the structure of the receptor proteins and the identity and cellular function of the genes that they regulate. Members of the steroid receptor superfamily share amino acid homology and a common structure (Fig. 1). Receptors in this superfamily contain several key structural elements that enable them to bind to their respective ligands with high affinity and specificity, recognize and bind to discrete response elements within the DNA sequence of target genes with high affinity and specificity, and regulate gene transcription.2

Fig. 1. Relative lengths of several members of the steroid/ nuclear hormone receptor superfamily. Variability between members of the steroid hormone receptor family primarily results from differences in the length and amino acid sequence of the amino (N)-terminal domain. (Adapted from Wahli W, Martinez E: Superfamily of steroid nucleara receptors: Positive and negative regulators of gene expression. FASEB J 5:2243–2249, 1991.)

Molecular cloning of the complementary DNA (cDNA) for each of the major steroid receptors has greatly enhanced our understanding of the structure-function associations for these molecules. The receptor proteins have five or six domains designated A through F, from the N terminus to the C terminus, encoded by 8 to 9 exons. The receptors contain three major functional domains that have been shown experimentally to operate as independent "cassettes,"11 unrestricted according to position within the molecule. The three major functional domains (Fig. 2) of the receptor are a variable N terminus (domains A and B) that confers immunogenicity and modulates transcription in a gene and cell-specific manner through its N-terminal activation function-1 (AF-1); a central DBD (consisting of the C domain), composed of two functionally distinct zinc fingers through which the receptor physically interacts directly with the DNA helix; and the LBD (domains E and, in some receptors, F) that contains activation function-2 (AF-2). The F domain is thought to play a role in distinguishing estrogen agonists from antagonists, perhaps through interaction with cell-specific factors.12 Domain-swapping experiments in which the DBD of ERα was switched with that of the glucocorticoid receptor (GR) yielded a chimeric receptor that bound to specific DNA sequences bound by GR but upregulated of transcription of glucocorticoid-responsive target genes when treated with estrogen,13 demonstrating the specificity of the DBD in target gene regulation.

Fig. 2. Schematic representation of the common structural and functional domains of the steroid hormone receptors. The horizontal lines indicate the domains of the receptor. (Adapted from Wahli W, Martinez E: Superfamily of steroid nucleara receptors: Positive and negative regulators of gene expression. FASEB J 5:2243–2249, 1991.)

The amino-terminal domain is hypervariable (<15% homology among steroid receptors) in size and amino acid sequence, is 25 to 603 amino acids long, and constitutes the major source of size differences between receptors.14 The AF-1 domain in this region is involved in activation of gene transcription but does not depend on ligand binding. In rat GR, the AF-1 region is called tau 1 or enh2 and constitutes amino acids 108 through 317. Deletion of the C-terminal LBD of GR yields constitutive (hormone-independent) transcriptional activation, implying that the N-terminal regions harbor autonomous transcriptional activation functions.15

Some steroid receptors exist as isoforms, encoded by the same gene but differing in their N terminus. The progesterone receptor (PR) and androgen receptor (AR) exist in two distinct forms, A and B, synthesized from the same mRNA by alternate splicing. The two PR receptor isoforms differ by 128 amino acids in the N-terminal region, yielding PR-A (90 kd) and PR-B (120 kd), which have strikingly different capacities to regulate transcription.16 In contrast, AR-A and AR-B isoforms show minimal differences in activation of a reporter gene in response to androgen agonists or antagonists in transiently transfected cells.17

The central core or DBD is highly conserved and shows 60% to 95% homology among steroid receptors.1 The DBD varies in size from 66 to 70 amino acids and is hydrophilic because of its high content of basic amino acids.11 The major function of this region is to bind to specific hormone response elements (HREs) of the target gene. DNA binding is achieved through the tetrahedral coordination of zinc by four cysteine residues in each of two extensions, which form two structurally distinct zinc fingers (Fig. 3).18 Zinc fingers are common among gene regulatory proteins.18 Specificity of HRE binding is determined by the more highly conserved hydrophilic first zinc finger (C1),14 and the second zinc finger (C2) is involved in dimerization and stabilizing DNA binding by ionic interactions with the phosphate backbone of the DNA.14 The D box is involved in HRE half-site spacing recognition. The highly conserved DBD shared by AR, GR, mineralocorticoid receptor (MR), and PR enables them to bind to the same HRE, called the glucocorticoid response element (GRE). The more C-terminal part of the C2 zinc finger and amino acids in the hinge region are involved in receptor dimerization in coordination with amino acids in the LBD.

Fig. 3. Schematic diagram of type II zinc finger proteins characteristic of the DNA-binding domain structure of members of the steroid hormone receptor superfamily. Zinc fingers are common features of many transcription factors, allowing proteins to bind to DNA.18 Each circle represents one amino acid. The CI zinc finger interacts specifically with 5 base pairs of DNA and determines the DNA sequences recognized by the particular steroid receptor. The three shaded amino acids (arrows) in the knuckle of the CI zinc finger are in the P box, which allows hormone response element sequence discrimination between the glucocorticoid receptor and estrogen receptor-α. The vertically striped amino acids within the knuckle of the CII zinc finger constitute the D box, which is important for dimerization and contacts with the DNA phosphate backbone. Adapted from Gronemeyer H: Transcription activation by estrogen and progesterone receptors. Annu Rev Genet 25:89–123, 1991; and from Tsai M-J, O'Malley BW: Molecular mechanisms of action of steroid/thyroid receptor superfamily members. Annu Rev Biochem 63:451–483, 1994.)

The hinge region or D domain is a 40- to 50-amino acid sequence separating the DNA-binding and LBDs that contains sequences for receptor dimerization and ligand-dependent and -independent nuclear localization sequences (NLSs).19 The hinge region interacts with nuclear corepressor proteins,20 and with L7/SPA, a 27-kd protein that increases the partial agonist activity of certain antagonist-liganded steroid hormone receptors, such as tamoxifen-liganded ERα, RU486-occupied PR, or RU486-occupied GR.21

The carboxyl-terminal domain or LBD is poorly conserved, is 218 to 264 amino acids long, and is hydrophobic. This region contains the ligand-binding site and dictates hormone binding specificity.1 Greater structural similarity between steroid hormone ligands generally indicates greater amino acid sequence homology in the LBD. Information from the x-ray crystal structures of the LBDs of the retinoic acid receptor (RAR), thyroid hormone receptor (TR), and ERα in the presence or absence of their cognate ligands has shown that the LBD has a compact structure consisting of 12 α helices with a "pocket" into which the ligand fits.22,23,24,25 Binding of the ligand within the pocket alters the conformation of the LBD with helix 12, forming a "lid" over the pocket, trapping the ligand in a hydrophobic environment, and forming a surface on the LBD with which coactivator proteins interact. Helix 12 in indispensable for AF-2 function. For ERα, 17β-estradiol (E2) and the antiestrogen or select estrogen receptor modulator (SERM), raloxifene, form different amino acid contacts within the pocket.25 This results in different positioning of helix 12 in the LBD that is thought to permit interaction with coactivators, such as SRC-1, in the presence of E2 but not raloxifene or, by inference , antiestrogens such as tamoxifen.25

Two human GR isoforms, GRα and GRβ, derived from the same gene by differential splicing at the C terminus, have been reported.26 GRβ was reported to localize in the cell nucleus in the absence of ligand and to block hGRα activity.27 Likewise, a novel isoform of ERβ, called ERβ2, containing an in-frame insertion of an exon of 54 nucleotides, resulting in an insertion of 18 amino acids in the LBD, was identified first by screening rat prostate cDNA library and is also expressed in human cell lines.28 ERβ2 binds E2 with lower affinity (Kd = 8 nM) than ERβ1 (Kd = 1 nM).

Sequences within the LBD form the binding site for heat shock protein 90 (HSP90) that blocks the DBD in the cytosolic, nonliganded GR.29 The CII and CIII regions (Fig. 2) show homology among members of the steroid/nuclear receptor superfamily and are important in forming the ligand binding pocket.30

The C-terminal AF-2 transactivation domain is highly conserved within the nuclear receptor superfamily31 and is recognized by various transcriptional coactivators.32,33 AF-2 is localized to the most C-terminal end of the E domain. A third transactivation domain called AF-2a or tau2 has been localized to the N-terminal region of the LBD of ERα31 and GR.34 Deletion experiments revealed a role for AF-2a and the DBD in targeting rat GR to the nuclear matrix,35 an interconnected ribonuclear-protein network within the nucleus that is thought to play an important roles in transcription of active genes by stabilizing the assembly of the transcriptional machinery.36

Although individual domains of steroid/nuclear receptors can be exchanged and function when spliced with nonrelated transcription factors, forming chimeric proteins, experiments on ERα37 and GR38 show that these receptors function optimally when intact. The N and C terminals of the receptor interact with each other to increase transcriptional activation.37

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The transcription of DNA to messenger RNA (mRNA) is the most important process regulated by steroid hormones. All genes share a common basic design (Fig. 4), composed of a structural region in which the DNA encodes the specific amino acids of the protein and regulatory region that interacts with various proteins to control the rate of transcription. Several key elements in the regulatory region of the target gene must be activated before mRNA synthesis can occur. These elements, called cis-acting elements because they are located on the same DNA as the gene itself, are usually located near the 5' end (beginning) of the gene and consist of four main groups: promoters, hormone-responsive enhancers, silencers, and hormone-independent enhancers.39

Fig. 4. Schematic diagram of regulatory and structural regions of a steroid hormone responsive gene. Located upstream from the transcription start site are several regulatory cis-acting elements that interact with various factors, including steroid hormone receptors that bind to hormone response elements, C/EBP that binds to CCAAT boxes,232 and Sp1 that binds to GC boxes.233 TFIID and the other general transcription factors bind to the TATA box to control transcription of the target gene.39 The structural region of the gene contains nucleotides encoding the mRNA transcript that will be translated into the protein. (Adapted from Tsai M-J, O'Malley BW: Molecular mechanisms of action of steroid/thyroid receptor superfamily members. Annu Rev Biochem 63:451–483, 1994.)

The promoter, essential for gene activation, sets the basal rate of transcription and controls the accuracy of transcription initiation.40 The promoter is located closest to the transcription start site and consists of two sub-elements: the TATA box and the upstream promoter. Located further upstream are one or more HREs, the specific DNA-binding sites to which steroid receptors bind, conferring hormone sensitivity to the gene.2 Silencers are elements that inhibit transcription of adjacent genes in the absence of hormone activation. Hormone-independent enhancers are DNA sequences bound by other transcription factors that can further increase the rate of gene expression.41 The synergistic interaction between regulatory cis-acting elements permits fine tuning of the rates of transcription of target genes in response to the local cellular and hormonal milieu.

HREs are consensus 13- to 15-bp DNA sequences derived from alignment of genes responsive to a particular steroid hormone. HREs have two "half-sites" that each bind the C1 zinc finger of one receptor monomer. Steroid hormone receptors bind DNA as homodimers with each monomer binding to adjacent major grooves on the same side of the DNA helix.18 Class II nuclear receptors may interact with a different class II nuclear receptor, forming a heterodimer and thereby creating a more stable complex with much higher affinity that is thought to enhance transcriptional activity significantly.1,4 On the basis of sequence homology and functional similarity, there are three classes of hormone response elements within the steroid hormone receptor superfamily (Table 2).

TABLE 2. Hormone Response Element Binding Sites for Steroid/Nuclear Receptors

Steroid/Nuclear Receptor

Consensus HRE

AR, GR, MR, and PR




Class II NR (i.e. PPAR, RAR, RXR, TR, and VDR)

DR: 5'-AGGTCA-3'


DR5, DR2, DR1, IR0


DR4, IR0, EvR6





Hormone response elements
(HREs) for the class II nuclear receptors are direct repeats (DR), inverted repeats (IR), or everted repeats (EvR) of the indicated half-site with the letter following the DR or IR indicating the number of nucleotides separating the half-sites, e.g., DR5 is 5'-AGGTCAnnnnnAGGTCA-3'.

The response elements for the PRs, ARs, GRs, and MRs are closely related and are collectively referred to as the GRE, consisting of a palindromic (symmetric) sequence 5'-GGTACAnnnTGTTCT-3', in which n stands for any nucleotide.42 AR, PR, GR, and MR show subtle differences in DNA base contact points to GREs.42 Examples of genes containing one or more GREs whose transcription is upregulated by glucocorticoids include the much-studied mouse mammary tumor virus (MMTV) promoter,43 tyrosine aminotransferase,44 and enzymes involved in gluconeogenesis.45 Examples of genes that are specifically inhibited by glucocorticoids through "negative GREs" include proopiomelanocortin,46 gonadotropin-releasing hormone (GnRH),47 and prolactin.48

The minimal consensus estrogen response element (ERE) sequence is also palindromic, 5'-GGTCAnnnTGACC-3',49 and differs in only 2 bp from the GRE. Extension of the length of the ERE palindrome, such as 5'-CAGGTCAnnnTGACCTG-3', and the sequences immediately flanking the ERE are important in determining the affinity with which ERα binds the ERE.50 Examples of genes whose promoters contain functional EREs include those encoding the much studied Xenopus vitellogenin A151 and human genes encoding pS2, a marker for human breast cancer diagnostics;52 FOS;53 MYC;53 and PR.54

The response elements for the various class II nuclear receptors, such as TR, RAR, retinoid X receptor (RXR), and vitamin D receptor (VDR), are composed of direct repeats of the half-site 5'-AGGTCA-3' with no space in between the half-sites (DR0) or separated by a gap of 1 to 5 nucleotides (DR1 to DR5).55 The number of nucleotides separating the half-sites determines the specificity of class II nuclear receptor binding.1,4 Many class II nuclear receptors require RXR for hormonal activation of transcription.4

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Arrival and Entry of Steroid Hormones Into Target Tissue Cells

Steroid hormones are small, hydrophobic, and lipid-soluble molecules derived from cholesterol. They circulate in blood free or bound (95%) to plasma carrier proteins.56,57,58 Sex hormone-binding globulin (SHBG), also known as testosterone-estradiol-binding globulin (TeBG), and androgen-binding protein (ABP) are encoded by the same gene.59 ABP is produced by the Sertoli cells of the testis, and SHBG is produced by the liver and is present in the circulatory system.58 SHBG binds most gonadal steroids, and corticosteroid-binding globulin (CBG or transcortin) binds glucocorticoids and progesterone with different affinities. When circulating levels of steroid hormones exceed the binding capacity of their respective binding proteins, they can then bind nonspecifically and with low affinity to albumin, from which they can readily dissociate and enter target cells.60 The unbound and loosely albumin-bound steroids are believed to be the most biologically important fractions because the steroid is free to diffuse or be actively transported through the capillary wall and lipid plasma membrane bilayer. Extracellular binding proteins may modulate hormone response by regulating the amount of steroid available to the cell.60 The binding capacity of binding globulins is influenced by endocrine status and other factors.57

SHBG binds to a specific cell membrane receptor called sex hormone-binding globulin receptor (SHBG-R) and activates adenylyl cyclase, increasing intracellular cAMP.58 Binding of SHBG to SHBG-R also transfers SHBG into the cell as a consequence of receptor-mediated endocytosis.61 The interaction of SHBG with SHBG-R is inhibited when steroids are bound to SHBG, suggesting that SHBG is an allosteric protein.61 However, if unliganded SHBG is allowed to bind to its receptor on intact cells and an appropriate steroid hormone then is introduced, adenylate cyclase is activated and intracellular cAMP increases.62 After the steroid hormone is in the cytoplasm, it is not clear whether a transport protein is required for movement of the hydrophobic steroid molecule through the aqueous cytoplasm to the receptor, regardless of whether the receptor is cytoplasmic or nuclear in location. The currently accepted model is that the steroid hormone diffuses freely in the cytoplasm.

Intracellular Localization of the Steroid/Nuclear Receptors

Early attempts to isolate steroid hormone receptors led to a major controversy regarding their intracellular localization in the unliganded state. Reports published before 1984 suggested that, before hormone treatment, steroid receptors in target tissues were located predominantly in the cytosolic fraction as large protein complexes of 300 to 400 kd. After hormone exposure, receptors were detected primarily in the nuclear fraction.63 It was initially proposed that unoccupied and untransformed receptors were located in the cytoplasm until ligand binding, which caused their translocation into the nucleus. The reported presence of ERs and PRs in cytosol is now acknowledged to be largely artifactual, the result of homogenization and centrifugation processes used to isolate the receptor protein.64

The newly synthesized unliganded receptor is highly unstable and moves into the nucleus, targeted by its NLS, or associates with the HSP90 complex of cytoplasmic proteins.65 Although newly synthesized receptor proteins would be expected to contribute a small amount to cytoplasmic levels of receptor, most steroid/nuclear receptors in the unliganded state reside in the nucleus. The exceptions are the GRs and MRs, which in the unliganded state reside in the cytoplasm in a complex containing HSP90, HSP70, and a variety of receptor-associated proteins.66 HSP90 is a general molecular chaperone involved in the folding of various proteins. The HSP90 complex of proteins has chaperonin activity that facilitates hormone binding and subsequent proper folding of the GR.67 The HSP90 dimer is thought to stabilize the receptor, protecting it from protease degradation; block the DBD and the NLS; and maintain the receptor in an inactive state until ligand binding occurs (Fig. 5).68 HSP90 is also required for ligand binding by the steroid receptors.66 In addition to HSP90, unliganded steroid receptors extracted from animal tissues or mammalian cells showed that GRs and PRs are complexed with a number of other proteins, including HSP70, FKB59, p60, p48 (Hip), and p23.66,67 These proteins are thought to be required for the assembly and maintenance of ligand-sensitive aporeceptor complexes.

Fig. 5. Mechanism of steroid hormone receptor action in a target cell. A target cell contains the steroid hormone receptors required to respond to a given steroid hormone. A. Most steroid hormones that enter the cell travel in the bloodstream in free state or loosely bound to serum albumins. Estrogens and androgens are bound to steroid hormone-binding globulin (SHBG) with high affinity. The cell membrane contains a SHBG receptor linked to adenylate cyclase. Activation of adenylate cyclase by the SHBG receptor or peptide hormones such as luteinizing hormone generate cAMP, which activates the protein kinase A (PKA) phosphorylation cascade. PKA can activate the estrogen receptor-α (ERα) and other steroid hormone receptors in the absence of ligand binding. Most receptors in the steroid/nuclear receptor superfamily reside in the nucleus in the newly synthesized or unliganded state. The exception is the glucocorticoid receptor (GR), which is complexed with the HSP90 chaperonin complex in the cytoplasm. HSP90 and its accompanying proteins are thought to stabilize the receptor until hormone binding occurs. Although receptors for progesterone, estrogens, and androgens have also been found in complexes with the primarily cytosolic HSPs, the function of these associations is unclear. In the case of estrogens, progestins, and androgens, the hormones freely enter the nucleus and bind to their cognate receptor. Binding of hormone ligand to the receptor activates the receptor by inducing alterations in the conformation of the receptor protein. This allows the receptors to form homodimers and bind to specific hormone response elements (HREs) (see Table 2) in the DNA in the regulatory region of the target gene. B. Once bound to the HRE, the liganded steroid hormone receptor induces a DNA bend and, in the presence of agonist ligand, recruits coactivator proteins. The HRE-bound receptor also interacts with other DNA-bound transcription factors, as shown for the interaction of Sp1 with ER. Certain coactivator proteins have histone acetyltransferase (HAT) activity that acetylates lysine residues in histones H3 and H4. This results in a "loosening" of chromatin structure that facilitates the binding and transcriptional activity of RNA polymerase II at the transcription start site. (Adapted from Naar Am, Beaurang PA, Robinson KM et al: Chromatin, TAFs, and a novel multiprotein coactivator are required for synergistic activation by Sp1 and SREBP-1a in vitro. Genes Dev 12:3020–3031, 1998; and from Zwijsen RML, Buckle RS, Hijmans EM et al: Ligand-independent recruitment of steroid receptor coactivators to estrogen receptor by cyclin D. Genes Dev 12:3488–3498, 1998.)

On activation by hormone binding and the release of HSP90 and the other GR-associated proteins, the hormone-receptor monomer is released from the complex, dimerizes, and translocates to the nucleus. In contrast to GR, immunohistochemical localization experiments showed that ERα,69 ERβ,6 PR,70 and AR71 are primarily nuclear in the absence of hormone treatment. All class II nuclear receptors are nuclear in the absence of ligand.1,4

Although each of the steroid receptors (GR, MR, PR, ERα, AR) has been found in association with HSP90, with the exception of GR and MR, hormone binding does not appear to be required for translocation of these receptors to the nucleus.66 The role of the HSP90 complex in ER function is not clear. Purification of ERα after chemical cross-linking in intact MCF-7 human breast cancer cells revealed one ERα monomer complexed with two molecules of HSP90 and one molecule of p59 (FKBP52) but no HSP70 or 40-kd cyclophilin.72 HSP90 is not required for ligand-dependent transcriptional activation by ERα.73 Although HSP70 was shown to be required for purified, recombinant ERα to bind EREs in vitro,74 other experiments imply that HSP70 targets inappropriately folded, artificially overexpressed nuclear proteins75 and is not required for ERα-ERE binding.76

Entry of Steroid/Nuclear Receptors Into the Nucleus

Steroid hormone receptors within living cells are dynamic. They shuttle between the cytoplasm and the nucleus.66 The hormone receptor enters the nucleus by two processes: passive diffusion through the an "ever opened" central channel of the nuclear pore or active transport that is mediated by interaction of the NLSs on the receptor proteins with the NLS receptor-HSP90 complex.77 The NLS-steroid hormone receptor-NLS receptor-HSP90 complex binds to the nuclear pore complex by means of nucleoporins in an ATP-dependent process.78 The receptor is then trapped by binding to intranuclear components.77 Steroid hormone receptor complexes have been demonstrated in association with nuclear membranes and with chromatin components including histones,79 nonhistone basic proteins, DNA, and ribonucleoproteins and with nuclear matrix.78

The TR, VDR, RAR, RXR, and other class II nuclear receptors do not form high-molecular-weight complexes but are believed to enter the nucleus directly and become tightly associated with chromatin.1,4 However, later experiments may cause rethinking of this model. The subcellular localization of human TRβ1 fused at its N terminus to green fluorescent protein (GFP) was followed in living cells.80 In the absence of the thyroid hormone T3, more GFP-TRβ1 was present in the cytoplasm, and when the cells were treated with T3, the GFP-TRβ1 was predominately localized in the nucleus.80 Because GFP-TRβ1 bound T3 and DNA in a manner identical to wild-type TRβ1 and because T3 treatment moved all of the GFP-TRβ1 into the cell nucleus, the investigators concluded that their findings reflect the behavior of endogenous TR.80 With the exception of the ERBA1 oncogene product and some of the orphan receptors that function constitutively or are activated by growth factor-mediated phosphorylation, unliganded class II nuclear receptors are not transcriptionally active until liganded, because the class II nuclear receptors, including RAR, RXR, and TR, are constitutively bound by corepressor proteins that silence transcription until the appropriate ligand is bound.32,81,82

Ligand-Dependent Activation of Steroid/Nuclear Receptors

The LBD of the receptor may act as a repressor of receptor function because deletion of the LBD from GRs and PRs causes constitutive gene activation.83,84 Ligand binding to the receptor stimulates dissociation of the receptor-HSP90 complex,2 which facilitates conformational changes in the receptor (activation) that exposes the DBD and promotes dimerization of the receptor. Binding of the hormone to the receptor may be only one of several factors that activates or transforms the receptor, enabling it to bind as a dimer to specific hormone response elements located adjacent to or sometimes at a far distance from the transcription start site of the regulated gene.

Type II nuclear receptors (i.e. TR, VDR, RAR, RXR, and the orphan receptors) do not interact with HSP90. These receptors are bound to DNA in the absence of ligand and are associated with corepressor proteins, such as NCoR81,82 and SMRT.85 Corepressor proteins NCoR and SMRT are associated with a complex of proteins that have histone deacetylase activity that are believed to repress gene expression by maintaining chromatin in a more condensed conformation.86,87,88

Hormone-dependent phosphorylation of steroid hormone receptors may play an important role in binding of the receptor to its specific response element on the gene and subsequent activation of transcription. PR, GR, ERα, and VDR are all phosphorylated after binding to their respective ligands.89

Ligand-Independent Activation of Steroid Hormone Receptors

Although steroid and nuclear receptors are classically activated by ligand binding and are subsequently phosphorylated, a second mode of activation in the absence of ligand has been detected for certain receptors. Phosphorylation of certain steroid hormone/nuclear receptors in response to cell membrane-activated signaling cascades activates the receptor in the absence of the cognate ligand.89 Examples of peptide hormones and growth factors that activate steroid receptors by triggering intracellular phosphorylation cascades include dopamine, epidermal growth factor (EGF), insulin, and insulin-like growth factor.89 Activation of β2-adrenergic receptors by the antiinflammatory, antiasthmatic drugs salbutamol and salmeterol was demonstrated to activate GR, resulting in nuclear translocation and transactivation of a GRE-driven reporter gene.90 Signal transduction took place through activation of a cAMP cascade mediated by the cell membrane β2-adrenergic receptor. Likewise activation of the protein kinase A (PKA) pathway stimulated transcription by the MR in a ligand-independent manner.91 PR appears to be refractory to activation induced by phosphorylation cascades.89

Binding of Steroid/Nuclear Receptors to Hormone Response Elements and DNA Bending

The sequences specifically recognized by the various steroid/nuclear receptors are described in Table 2. When steroid/nuclear receptors bind their cognate HRE, the DNA is deformed, causing a bend in the DNA. DNA bending appears to be important in cellular processes mediated by multiprotein complexes, including transcription in prokaryotes and eukaryotes92; DNA bending is thought to facilitate interactions between components of the transcription complex bound to different sites and to promote DNA looping to allow single proteins to contact multiple DNA elements. ERα binding to an ERE results in a bend of the DNA toward the major groove.93 Other steroid receptors, including GR94 and PR95, also induce DNA bending. Similarly, class II nuclear receptors, including TR and RXR, induce DNA bending.96

That the topology of DNA is important for steroid hormone receptor recognition of HREs is reinforced by the observation that the nonhistone chromosomal protein HMG-1, which recognizes irregular DNA structure, enhances the binding of PR, ERα, AR, and GR to their respective response elements in vitro but has no effect on the binding of TR, RAR, RXR, or VDR to their target sequences.95,97 Moreover, coexpression of HMG-1 or HMG-2 increased PR-mediated transcription in transiently transfected mammalian cells by 7- to 10-fold without altering the basal promoter activity of target reporter genes.97

Direct Regulation of Gene Transcription by Steroid Hormone Receptors

Initiation of transcription is a complex event occurring through the cooperative interaction of multiple factors at the target gene promoter (Fig. 4). When bound to the specific HRE on the DNA, the hormone-receptor complex interacts with basal transcription factors and with other proteins to stabilize basal transcription factor binding and promote the assembly of the transcription initiation complex. Once the transcription initiation complex is in place, the enzyme RNA polymerase II is recruited to the transcription start site, where it begins transcribing the DNA sequence into mRNA.


For RNA polymerase II to initiate transcription, basal transcription factors TFIIA, TFIIB, TFIID, TFIIE, TRIIF, and TFIIH must assemble on the core promoter.39 TFIID consists of the TATA box binding protein (TBP) and at least eight tightly associated factors (TAFs) of 18 to 250 kd.98 Because the amount of RNA polymerase II is limited in the nucleus, genes must compete for it by assembling an appropriate set of cis-acting elements, HREs, and binding sites for sequence-specific transcription factors, such as AP-199 and Sp1.100 The net result is the synthesis of new mRNAs that move into the cytoplasm and are translated into new proteins that may alter cell function (acting in an intracrine manner) or may be modified further and secreted by the cell to act as endocrine, autocrine, or paracrine factors.

Steroid receptors interact with basal transcription factors TFIIB, TBP, and various TAFs of TFIID.101 Studies indicate that steroid and nuclear receptors use different domains for interaction with basal transcription factors.101 These interactions help stabilize the assembly of the RNA polymerase II preinitiation complex.


Steroid hormone receptors interact with multiple proteins when bound to DNA or in solution in vitro. Early experiments showed that overexpression of one type of steroid hormone receptor could inhibit or "squelch" the activation of transcription mediated by a different steroid hormone receptor, hinting that steroid receptors compete for limited amounts of a factor required for transcription.102 During the past 4 years, at least 15 different coactivators have been identified. Examples of coactivators are listed in Table 3. These proteins have also been called receptor-interacting proteins (RIPs) and receptor-associated proteins (RAPs); however, not all RIPs are coactivators. By definition, coactivators are considered to interact directly with the steroid/nuclear receptor and enhance transcription.32 The first coactivators for steroid receptors to be discovered, the yeast SWI/SNF proteins,103 may not be strictly coactivators but instead may serve as "bridging factors" that interact between coactivators and basal transcription factors. Several coactivators have general transcriptional activator function (e.g. CBP); they enhance transcription by different types of transcription factors, including steroid receptors.

TABLE 3. Coactivator Proteins That Interact With Steroid Hormone Receptors


Interacts With SN/NR

Effect of SR Ligand on Direct Interaction

Effect of Coexpression on Transcription

Other Information




Requires agonist Ligand

Stimulates E2-dependent transcription

· AIB1 expression elevated in human breast and ovarian cancers238













ARA70 (ELE1α)


Androgens and antiandrogens promote AR-ARA70 interaction--also genistein and RU486241

Stimulates AR transcription with DHT or E2

· no intrinsic HAT activity

193, 242, 243





· interacts with p/CAF that has HAT activity


  variant ELE1β




· interacts with TFIIB






· highest ELE1α and ERE1β expression in testis




Requires agonist ligand, except for AR

· stimulates transcription

· intrinsic HAT activity244

· CBP/p300 is also a cofactor for AP-1, c-myb, STAT1, E1A, p53, and Myo-D reviewed in245




Requires agonist ligand; antagonists tamoxifen and ICI 164,384 block interaction with ERα

· stimulates ERα-induced transcription

· inhibits PPAR and RXR activities

· identical to ERAP140249



AR, ERα, ERβ1, PR, GR, TR, RARβ, RXRα, PPARγ, HNR4

Requires agonist ligand; antagonist inhibits interaction

Stimulates PR, GR, and ERα induced transcription

Identical to ERAP160 and p160; intrinsic HAT activity107;interacts with p300/CBP, TBP, and TFIIB252,252






Stimulates transcription

Chromatin remodeling complex in yeast with human homologues




Requires agonist ligand

Stimulates transcription- requires agonist ligand for ERα, but for ERβ 4-OHT acts as an agonist

Interacts with heterochromatin proteins, including hSNF2b of the SWI/SNF complex and TIF1β

132, 253, 254



Requires agonist ligand

Stimulates ERα, AR, PR, TR, RAR, and RXR but not GR, or VDR

40% sequence homology to SRC-1


These proteins have been identified in yeast and mammalian two hybrid screening, during purification, in immunoprecipitation assays, and by cross-linking studies.32,41,101 Some, but not all, have been shown to stimulate transcription in cell systems.

Steroid receptors can interact with a number of different coactivators. Some of these coactivators, such as SRC-1, CBP, and TIF2, play a critical role in ligand-activated transcription.32,101 Coactivator proteins contain one or more copies of a nuclear receptor binding motif, also called the nuclear receptor box, consisting of the amino acids LXXLL, which physically interacts with the steroid/nuclear receptors. Steroid receptors show different affinities for the various coactivators104 and use different amino acids to contact the coactivators.105 The discovery that coactivators SRC-1, ACTF, and CREB/p300/CBP106 have histone acetyltransferase (HAT) activity107 indicates a possible mechanism for enhanced transcription. Many transcriptional regulatory proteins have intrinsic HAT activity.108 HATs acetylate lysine residues on the N-terminal tails of histones H3 and H4 in chromatin, resulting in a weaker association of histones with DNA, altering nucleosomal conformation and stability in a manner that facilitates transcriptional activation by RNA polymerase II.109 SRC-1, CBP/p300, CREB, and other coactivators are believed to form a ternary complex with liganded steroid receptors to increase the rate of hormone-responsive gene transcription.110 Several HAT activities may be tethered to hormone-activated receptors on the promoter, yielding synergistic transactivation. Not all genes are affected by histone acetylation,108 and steroid and nuclear receptors show different affinities of interaction with coactivators.111

The currently accepted model suggests that different target cells express different levels of coactivators and corepressors that, along with the amount of receptor protein and ligand, could allow fine tuning of target gene transcription in response to steroid hormones. Northern blot analysis confirmed the idea that different rat tissues112 and cell lines113 express different amounts of the mRNA for p300, CBP, SRC-1, RIP140, SMRT, and NCoR.

Indirect Regulation of Gene Transcription by Interaction of Steroid/Nuclear Receptors With Other Transcription Factors

In addition to their direct action on target gene expression mediated by direct DNA binding, steroid receptors interact physically with different transcription factors to alter target gene transcription without the steroid receptor interacting directly with DNA. The best studied example of this "transcriptional crosstalk" is the interaction of the AP-1 transcription factor with GR.114 Depending on the cell type115 and the composition of the AP-1 complex, GR synergizes with AP-1 (JUN-JUN homodimer) or suppresses the FOS-JUN heterodimer.116

Similarly, ERα liganded by the antiestrogen tamoxifen activates promoters through AP-1 sites in genes such as that encoding human collagenase.117 This contrasts with the inability of tamoxifen to activate transcription from promoters bearing classic EREs. Tamoxifen agonism at AP-1 sites is cell type specific; it occurs in cell lines of uterine but not of breast origin. The DBD of ERα is required for tamoxifen activation at AP-1 sites. Conversely, the AP-1 components JUN- and FOS-inhibited, E2-dependent, ERα-stimulated reporter gene activity in transiently transfected MCF-7 or CV-1 cells transfected with ERα.118 DNA binding experiments revealed that ERα-ERE binding was inhibited by the JUN protein and that ERαinhibited JUN-DNA binding.118

E2 signals in opposite ways through ERα and ERβ from an AP-1 site: with ERα, E2 activated transcription, whereas with ERβ, E2 inhibited transcription.119 Moreover, in contrast to ERα, the antiestrogens tamoxifen, raloxifene, and ICI 164,384 were potent transcriptional activators with ERβ at an AP-1 site. The two ERs differ in how they respond to ligand and response element, suggesting that ERα and ERβ may play different roles in gene regulation.119

Another transcription factor with which ERα interacts directly to activate gene transcription is Sp1. A number of estrogen-responsive genes are activated by ERα-Sp1 interaction, including cathepsin D,120 RARα,121 and FOS.122 ERα and Sp1 physically interact and this interaction increases Sp1-DNA binding in the presence or absence of E2. In contrast, transactivation of Sp1-driven promoter-reporter constructs is E2 dependent.123 These results indicate that transcriptional activation requires more than ERα-Sp1 interaction and increased DNA binding. A likely interpretation is that coactivators are required.

Repression of Target Gene Transcription by Steroid/Nuclear Receptors

Binding of the hormone-receptor complex can also repress transcription of target genes.32 One mechanism by which a steroid receptor represses gene transcription is by binding to the same, or an overlapping, DNA binding site as that used by a different activator protein, competitively blocking activator binding to DNA. One example of this mechanism is the mutual inhibition by GR and AP-1 on the proliferin gene promoter that contains a composite GRE-AP-1 binding site.116

Competition for limiting amounts of coactivators is another mechanism by which steroid hormones inhibit gene expression. The mutual inhibition of AP-1 and steroid receptor transactivation, including AR, ERα, GR, and PR,114 is achieved through competition for limiting amounts of the coactivators CBP/p300 in the nucleus.124 Likewise, GR and ERα block NF-κB-mediated transactivation.114 GR and other steroid receptors interact directly with NF-κB, and NF-κB inhibits GR-, ERα-, and PR-mediated transcription. Later experiments indicate that GR and NF-κB compete for limited amounts of the coactivators CBP and SRC-1.125

Another mechanism for repression by steroid receptors is by binding to the HRE and recruiting corepressor proteins that quench the activity of activators bound to the promoter. The corepressors N-CoR81,82 and SMRT85 were first identified by their binding to unliganded TR, RAR, and RXR (Table 4). Tethering of these corepressors to the DNA by their interaction with the DNA-bound TR, RAR, or RXR recruits other components of the corepressor complex that mediate the actual events of transcriptional silencing.126 Binding of TR or RAR ligands cause dissociation of NCoR, but only when the receptors are bound to DNA.82 Unlike coactivator complexes that have endogenous HAT activity, corepressors to not appear to have enzymatic activity. Corepressors NCoR and SMRT, appear to function by recruiting histone deacetylase (HDAC)-con-taining multiprotein repression complexes to the promoter.127 Deacetylation of chromatin helps maintain a condensed state of nucleosomal structure that blocks the binding of components of the RNA polymerase II transcription initiation complex.

TABLE 4. Corepressor Proteins That Interact With Steroid/Nuclear Receptors


Interacts With

Effect of Coexpression on Transcription

Other Information








Increased transcriptional activity of unliganded PR 256

NCoR levels were reduced in many breast tumors that had acquired resistance to the antiproliferative effects of tamoxifen134

21, 82




At high levels, NCoR increased RAR transcription257







Increased transcriptional activity of unliganded PR256

HDAC1 is associated with SMRT




Overexpression strongly reduced basal and 4-OHT-stimulated gene expression with no effect on E2 activity133


These proteins have been identified in yeast and mammalian two hybrid screening, during purification, in immunoprecipitation assays, and by cross-linking studies.32,41,101 In general, interaction of steroid and nuclear receptors with these proteins represses gene transcription in cell systems.

Mechanism of Repression of Target Gene Transcription Achieved by Hormone Antagonists

A variety of mechanisms are likely to be involved in hormone antagonist action. First, the antagonist activity of an antihormone may depend on the cell or tissue type. For instance, antiestrogen-liganded ERα binds EREs with high affinity,50,128 but in certain cell types, such as MCF-7 human breast cancer cells, transcription is not activated,129 whereas in endometrial cells, tamoxifen stimulates transcription.130 The proposed mechanism for these observations involves the inability of antiestrogen-liganded ERα to interact with coactivator proteins, such as SRC-1131 and TIF1a.132 Another possibility is that antiestrogen-liganded ERα may recruit corepressor proteins to the promoter, inhibiting chromatin changes mediated by HDACs that promote a "loosening" of nucleosomal structure.133 The corepressor NCoR interacts with the tamoxifen metabolite 4-hydroxytamoxifen (4-OHT)-liganded ER.134 Breast cancer cells that are resistant to tamoxifen inhibition of cell proliferation show reduced NCoR levels compared with tamoxifen-sensitive cells, suggesting a mechanism whereby cells become resistant to tamoxifen.134 Similarly, NCoR and SMRT repress the agonist activity of antiprogestin RU486-liganded PR or tamoxifen-liganded ERα.21

Nonnuclear Steroid Hormone Receptors and Nongenomic Effects of Steroid Hormones

Although most of the effects of steroid hormones are mediated through their interaction with their cognate receptors and subsequent effects on target gene transcription, certain rapid effects of steroid hormones are incompatible with a transcriptional mechanism.135 Although highly controversial, specific binding sites for estrogen, glucocorticoids, and vitamin D receptors have been reported in the plasma membrane of various target cell types.136,137

Estrogen receptors have been reported in the plasma membrane of GH3 pituitary cells.138 Exposure of GH3 pituitary cells to estradiol was reported to elicit a rapid (within 5 minute) release of prolactin in a manner that cannot be accounted for by genomic effects of estradiol mediated through nuclear ERs. Binding sites for estrogen with different biochemical properties from the classic nuclear receptor have been reported in the endoplasmic reticulum of uterine tissues.139 Estradiol appears to have receptor-mediated (genomic) and nongenomic effects in the brain.140 The cardiovascular protective effects of estradiol are thought to be mediated at least in part by nongenomic ER and involve increased intracellular cAMP,141 inhibition of Ca+ 2 influx,142 and stimulation of nitric oxide release.143 Estradiol has exerts direct beneficial effects on human oocytes during in vitro maturation, and these effects at least partly result from steroid action at the oocyte surface.144

A study compared nuclear and membrane expression of ERα and ERβ by expressing the cDNAs for each receptor type in Chinese hamster ovary (CHO) cells.145 ERα and ERβ were expressed in the nucleus and in the cell membrane. However, the membrane receptor number was only 2% of the nuclear receptors. E2 bound to the nuclear and membrane forms of each isoform with identical affinity, but E2 bound ERβ with lower affinity than ERα.145 E2 binding to CHO-ERα- or CHO-ERβ-activated G-coupled membrane receptors Gαq and Gαs and rapidly (within 5 to 10 minutes) stimulated corresponding increases in inositol phosphate and cAMP.145 Membrane ERα and ERβ showed distinct differences in their activities: JUN N-terminal kinase activity was stimulated by E2 in ERβ-expressing CHO but was inhibited in CHO-ERα cells.145 Further studies are needed to extend these results on the role of a membrane ER to human tissues.

GRs and MRs appear to have nongenomic effects. GRs bind to cytoskeletal structures,146 and glucocorticoids stimulate the rapid onset of polymerization of actin in a nongenomic manner that involves decreased intracellular cAMP.147 GRs may activate mitochondrial gene expression in hepatic cells.148 Rapid stress-induced changes in male amphibian reproductive behavior appear to be mediated by MRs on neuronal membranes.149

One of the membrane steroid receptors that has been well characterized is the membrane PR in amphibian oocytes and in spermatids.150 This receptor differs from the nuclear PR. The novel membrane PR mediates rapid changes in Ca+ 2 conductance in human sperm plasma membrane.151 A high-affinity progesterone-binding membrane protein of 200 kd has been described in pig liver.152 In addition to membrane PR action, metabolites of progesterone and deoxycorticosterone act as positive allosteric modulators of the γ-aminobutyric acid (GABA)A receptor complex in the cortex of rat brain.153 This means that these hormones bind to an effector site (not the GABA binding site) and increase the affinity of GABA binding to the GABAA receptor. Progesterone also binds to the G-coupled oxytocin receptor and inhibits peptide-mediated signaling.154 The role for these steroid membrane receptor effects in human health is just beginning to be explored.

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Diversity of tissue responses to steroid hormone action, despite conservation of structure and function, is achieved through a variety of mechanisms. Some receptors are expressed in only a limited number of cell types (e.g. gonadal steroid receptors), and others are found in a large number of cell types (e.g. glucocorticoid and thyroid hormone receptors). Other tissue-specific gene regulatory proteins (e.g. transcription factors, coactivators, corepressors) are involved in the modulation of gene transcription by steroid/nuclear receptors. Cell-specific posttranslational modification of receptors is another mechanism to ensure tissue diversity of hormone responses.89 Multiple receptor isoforms for hormones may also account for tissue-specific gene expression.28 Receptor isoforms have been shown to arise through alternative splicing of mRNA from a single gene (PR and AR),17,155 from multiple genes (ER), or a combination of both (TR).156

Interactions with other transcription factors, coactivators, and corepressors appear to play a significant role in determining specificity. Even though receptors for several different hormones (i.e. AR, GR, MR, and PR) can bind a common HRE sequence, they may exhibit different interactions with other DNA-bound bound factors and coactivators, thereby achieving different modulation of target gene expression.32

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The half-life of steroid hormone receptors ranges from 2 to 4 hours for ERα,155 4 hours for AR,157 7 to 10 hours for PR,158 and 19 hours for GR.159 The relatively long half-life of the steroid hormone receptors strongly suggests that the receptor proteins are recycled before eventual degradation.160

Steroid hormones generally autoregulate their receptor levels.161 Desensitization or downregulation of receptor numbers, measured by decreased ligand binding capacity, occurs in response to exposure to high levels of ligand and involves the reduction in receptor mRNA levels, decreasing the number of available receptors. The receptor gene may be negatively regulated by the hormonal ligand itself through its receptor protein interacting with specific HREs in the gene.162 Upregulation or self-priming may occur in an analogous fashion.162 Steroid hormones can regulate receptor levels for other hormones (e.g. E2 increases PR levels in estrogen-responsive tissues).54 Progesterone can downregulate its own receptors, as well as ERα163 and ERβ.164 This increase or decrease in receptor levels in homologous or heterologous regulation can be caused by alterations in receptor gene transcription or decay rates for receptor mRNA or protein. Binding of the cytosolic GR complex to very long 3'-untranslated regions of its receptor mRNA has been reported to cause premature degradation.165

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Steroid and nuclear hormone receptors have been detected in virtually every major organ and tissue in the mammalian body, including the brain. Much new information on the location and function of the steroid hormone receptors has been derived from techniques that allow manipulation of a specific mouse gene in vitro to generate targeted gene disruption of the gene for a given receptor, creating homozygous "gene knockout" mice.166 The technique disrupts the linear gene by inserting an antibiotic resistance gene (e.g. neomycin) into one coding region (one exon) of the gene. The mutant DNA is inserted into genomic DNA by homologous recombination in mouse embryonic stem cells to generate transgenic mice. The mRNA for the targeted gene is truncated or nonsensical. This technique has resulted in ERα, ERβ, MR, PR, and GR knockout mice.166

Estrogen Receptor-α Knockout Mice

To the surprise of many investigators, loss of ERα expression was not lethal and had no effects on the ratio of male to female mice born.167 ERα knockout (ERKO) mice survived to adulthood and developed grossly normal external genitalia, but both sexes were infertile.168 Females have hypoplastic uteri and hyperemic ovaries with no corpora lutea.168 Serum levels of estradiol in the ERKO females are more than 10-fold higher than those in the wild type, consistent with a syndrome of hormone insensitivity.167 ERKO females have 10-fold higher circulating E2 and elevated luteinizing hormone (LH) but not follicle-stimulating hormone (FSH).169 Ovarian histology is abnormal. Mammary glands of adult ERKO female mice lacks branching and terminal end bud formation.169 Maternal behavior, as measured by retrieving of pups removed from their mothers, is reduced in ERKO females.170 In some cases, pups were killed by the ERKO females, which was not seen in wild-type animals.170 Aggression toward other females was increased, and female-typical lordosis behavior was reduced.170

Adult ERKO males exhibit a number of alterations in reproductive tract histology, including atrophied, degenerated seminiferous tubules and diluted, infertile sperm.171 The mice exhibit decreased sperm counts and significantly lower testicular weight than wild-type males.169 The reproductive capacity of sperm from ERKO males is significantly compromised in in vitro fertilization experiments. ERKO males appear to have normal mounting behavior toward wild-type females but exhibit an almost complete lack of intromission and ejaculation. ERKO males are consistently less aggressive than wild-type mice.170,172 These findings indicate that ERα gene expression during development plays a major role in the organization of male-typical aggressive and emotional behaviors in addition to simple sexual behaviors.172 E2-protected wild-type and ERKO female mice in response to carotid arterial injury, indicating that ERα may not be required for the protective actions of E2 in the vascular system.173

There has been one reported human case with an ERα mutation.174 The male patient exhibited severe osteoporosis and insufficient closure of the epiphyseal growth plates.

Estrogen Receptor-β Knockout Mice

Mice lacking ERβ (BERKO) develop normally and are indistinguishable grossly and histologically from their littermates.175 Breeding experiments with young, sexually mature females show that they are fertile and exhibit normal sexual behavior, but they have fewer and smaller litters than wild-type mice.175 Superovulation experiments indicate that this reduction in fertility is the result of reduced ovarian efficiency.175 The mutant females have normal mammary gland development and normal lactation.175 Adult male mice show no overt abnormalities and reproduced normally. Older mutant males display signs of prostate and bladder hyperplasia. The investigators concluded that ERβ is essential for normal ovulation efficiency but is not essential for female or male sexual differentiation, fertility, or lactation.175

Progesterone Receptor Knockout Mice

Mice carrying a null mutation of the PR gene (PRKO) exhibit several reproductive abnormalities, including anovulation, attenuated lordotic behavior, uterine hyperplasia, and lack of mammary gland development.176 There were no effects on the viability or sexual differentiation of homozygous PR gene-disrupted mice.176 The female homozygous for PR disruption are completely infertile, whereas males exhibit no apparent effects on fertility. Serum LH levels in PRKO mice were found to be elevated by approximately twofold over basal (metestrus) values in wild-type mice.177 In contrast, basal FSH levels were not different in PRKO and wild-type mice. Basal levels of E2 and progesterone in serum were likewise similar in the two groups, as were hypothalamic LHRH concentrations. Basal PRL levels were slightly higher in PRKO versus wild-type mice. These results confirm the essential role of PRs in the regulation of hypothalamic or pituitary processes that govern gonadotropin secretion.177

Glucocorticoid Receptor Knockout Mice

Most of the mice homozygous for disruption of GR die shortly after birth because of severe lung atelectasis.178 Additional defects were found in the adrenals, liver, brain, bone marrow, and thymus and in the feedback-regulation of the hypothalamic-pituitary axis.179 However, disruption of the ability of GR to dimerize is not lethal.180

Mineralocorticoid Receptor Knockout Mice

MR–/– mice, obtained by targeted gene disruption, died between days 8 and 13 after birth after exhibiting signs of pseudohypoaldosteronism, and the pups died of dehydration because of renal sodium and water loss.181 The MR–/– mice showed severe dehydration, hyperkalemia, hyponatremia, and high plasma levels of renin, angiotensin II, and aldosterone.182 The MR knockout mice showed significant increases in the expression level of several renal angiotensin system components—renin, angiotensinogen, and angiotensin II receptor (AT1)—but no alteration in angiotensin I-converting enzyme was detected in the kidney.182

Androgen Receptor Insensitivity

No one has created an AR knockout mouse. A natural deficiency of AR occurs in the Tfm mouse,183 which has an AR mutation that results in androgen insensitivity syndrome (AIS) that is an X-linked inherited disease.166 Various mutations in the AR in humans have been shown to cause AIS.184 Male Tfm mice exhibit complete infertility.166

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Steroid hormone receptors are present in relatively low numbers in target tissues. Their instability when isolated from cells and tendency to adhere to the surfaces of test tubes require special consideration in handling under experimental and clinical conditions. The basic principles of measurement of nuclear hormone receptors are the same as for plasma membrane receptors.185 Multipoint titration and sucrose density gradient analyses are commonly used for measurement of steroid hormone receptors in tissue samples.186 Tissue homogenates containing receptors are incubated with increasing amounts of radioactively labeled steroids of high specific activity in the presence and absence of corresponding excess unlabeled steroid or incubating with a fixed concentration of labeled steroid and increasing amount of corresponding unlabeled steroid.187 Tritiated or radioiodinated steroids are most commonly used, because they are available in high specific activities. The binding in the presence of corresponding excess unlabeled steroid hormone is nonspecific binding, which is unsaturable and generally represents binding to nonreceptor sites. Nonspecific binding is subtracted from total binding (the amount bound in the absence of unlabeled steroid) to obtain saturable and high-affinity specific binding. By appropriately varying the incubation conditions, kinetic and equilibrium binding constants, specificity of binding, and other properties can be determined by titration analysis. Receptor-bound and free hormones are separated by dextran-coated charcoal187 or precipitation of steroid-receptor complex with protamine sulfate188 or absorption to hydroxyapatite189 followed by centrifugation and scintillation counting. For clinical samples, ER and PR status in breast tumor samples are quantitated by monoclonal antibody-based methods using a commercially available kit from Abbott Laboratories.190 One important clinical application of this method is the use of ERα and PR status as an indication for antiestrogen therapy.191

Scatchard analysis is the most universal method for calculating binding affinity (Kd) and number (binding capacity) of steroid hormone receptors.192 The shape of Scatchard plots can be linear or curvilinear with upward or downward concavity. The interpretation and calculation of binding constants is the same as for membrane receptors.185

The apparent dissociation constants for steroid hormones binding to their cognate receptors is in the range of 10–11 to 10–9 M, which is close to circulating steroid hormone concentrations. The affinity of nonrelated steroid hormones to the receptor is generally about 100 times lower than the appropriate steroid hormone. However, if the levels of these nonrelated hormones are greatly elevated because of some physiologic or pathologic process, a significant binding and biologic response to them can be expected.

Ligand binding specificity is also influenced by interaction with coactivators. In the presence of coactivator ARA70, E2 bound to the AR and induced AR-responsive transcriptional activity more than 30-fold in DU145 human prostate cancer cells.193 In contrast, the synthetic estrogen diethylstilbestrol (DES) had no effect. The significance of this ability of E2 to activate AR is strengthened by finding patients with Reifenstein partial androgen-insensitive syndrome with a single mutation in the AR that makes the receptor nonresponsive to E2-stimulation in the presence of ARA70. These data suggest that testosterone and dihydrotestosterone are not the only ligands for the AR.193

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Before the development of cloning techniques, steroid hormone receptor structures were analyzed by painstaking methods of classic protein chemistry. Protein purification required processing of large amounts of tissues and sequencing was laborious and slow. Ligand-binding characteristics were described by Scatchard analysis and binding displacement curves.

The advent of molecular biology has revolutionized the study of hormone receptor biology, providing a wealth of powerful techniques that are indispensable to elucidating the structure and mechanism of action of the steroid and nuclear hormone receptors. Common techniques used by molecular biologists include Southern, Northern, and Western blotting, used to detect the presence and size of specific DNA, RNA, and protein sequences, respectively.194 Molecular cloning of DNA provides a technique whereby a single segment of DNA can be isolated from a large population of genes, purified to homogeneity and amplified to produce sufficient quantities for structural and functional analysis. Receptors are routinely cloned from RNA extracted from small amounts of hormone-responsive tissues or cells. When it is possible to predict a certain degree of homology of DNA sequence of the hormone receptor with other known proteins, specific DNA segments can be amplified using the polymerase chain reaction (PCR), with the cDNA as template and highly conserved sequences from related proteins as primers.195 Alternatively, specific enzymes may be used to copy, cut, and splice together pieces of DNA that are inserted into circular plasmid DNA or bacteriophage viruses, introduced into bacteria and allowed to divide many times. The greatly amplified DNA is then isolated and purified, and the nucleotide sequence of the cloned DNA segment is determined. The cDNA clone can be transcribed into mRNA and used to confirm biologic activity in in vivo and in vitro functional expression systems, and the amino acid sequence of the encoded protein can be readily determined. The cDNA can be tagged with radioactivity or a fluorescent marker and used to detect the presence or describe the distribution of receptor mRNA in various tissues and endocrine states using solution hybridization and in situ hybridization methods.

Once the molecular structure of the receptor protein is known, the structure-function relationships of certain amino acid sequences relative to those of other receptors in the steroid hormone receptor family are investigated using domain-swapping and site-directed mutagenesis methods. In domain-swapping experiments, hybrid mRNA transcripts can be produced from a receptor cDNA in which a key functional element, such as the LBD, is replaced with the nucleotide sequence from the same region of another closely related receptor.196 With this technique, it has been possible to determine the functional significance and specificity of the various regions of the steroid receptor proteins.196 Site-directed mutagenesis has been used to determine the identity of amino acid residues that are important for DNA and ligand binding in steroid receptors.

The electrophoretic mobility shift assay (EMSA), also called gel mobility shift assay, is the most common technique to examine the binding of a given steroid/nuclear receptor to DNA. For EMSA, a preparation of a particular steroid/nuclear receptor is incubated with a short (20 to 400 bp) fragment of [32P]-labeled double-stranded DNA. The DNA may be a synthetic construct of a particular HRE or a fragment from the promoter of a particular gene that is thought to contain a HRE. The protein-DNA reaction mixture is then separated by a nondenaturing polyacrylamide gel electrophoresis. The free [32P]-labeled DNA migrates to the bottom of the gel, and the migration of the [32P]-labeled DNA that is bound by the receptor is slowed. The gel is dried and exposed to x-ray film or placed in a Phosphorimager cassette to detect where the 32P is located. The free, unbound [32P]-labeled DNA appears at the bottom and the receptor-bound DNA is located toward the middle or top of the gel. Addition of a 50- to 200-fold molar excess of cold competitor DNA is used to determine the specificity of the receptor-DNA complex. Addition of an antibody to a particular steroid/nuclear receptor is used to confirm the identity of the protein in the protein-DNA complex. The antibody may block the receptor-DNA binding, leaving an empty spot on the autoradiograph; "supershift" the receptor-DNA complex by binding stably to it and further retarding its mobility in the electrophoretic field; or have no effect, indicating that the epitope recognized by the antibody is not available under the assay conditions. EMSA is useful to compare the relative binding affinities of a given receptor for various HREs in vitro.197

To examine how a particular ligand, steroid receptor, and HRE affect transcription, transient transfection experiment are performed using cultured mammalian cells.129 For these experiments, the cell must contain the steroid receptor of interest or the receptor must be introduced into the cell by means of an expression vector encoding the cDNA of the receptor with a strong viral promoter, such as CMV or RSV, to ensure its expression. The reporter vector contains a reporter gene not expressed in normal mammalian cells. Examples of reporter genes are chloramphenicol acetyltransferase (CAT)198 and luciferase.129 The reporter and hormone receptor expression plasmids are transfected into the mammalian cells using calcium phosphate coprecipitation,199 electroporation,200 or lipid-mediated transfer (e.g. lipofectamine).129 After a recovery period, the cells are treated with hormone for 12 to 48 hours. The cells are then lysed, and reporter gene activity is measured. As a control, cells are usually co-transfected with a β-galactosidase reporter as an indication of the percent efficiency with which the cells have taken up the plasmid DNA.129 This technique allows quantitation of how different ligands and DNA sequences affect the transcriptional response of a given steroid receptor. Cotransfection of the cells with expression plasmids for coactivators or corepressors allows determination of the functional consequence of these proteins.

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Steroid hormones exert profound effects on mood, mental state, behavior, and memory by acting by interaction with steroid receptors in the brain. Estrogens and progestins have been shown to regulate the synthesis and release of brain neurotransmitters, including norepinephrine, dopamine, serotonin, GnRH, β-endorphin, corticotropin-releasing factor, and prolactin.201 Intracerebroventricular injection of antisense oligonucleotides to PR completely suppressed sexual behavior.202 Estrogens166 and progestins play a key role in female rodent sexual receptivity.203 Female ERKO and PRKO mice exhibited reduced170 or no lordosis,203 respectively. PR regulation of dopamine synthesis is thought to be responsible for the lack of responsiveness to male advances.203 PRKO male mice showed reduced mount frequencies compared with wild-type mice, indicating a role for progestins in regulating male sexual response as well.204 These reports indicate that steroid hormones acting through their receptors and genomic mechanisms initiate events that regulate neuronal networks involved in sexual behavior.

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Clinical manifestations of abnormal steroid hormone receptor function involve variations in receptor numbers and the ability to stimulate transcription of certain genes. Hormone resistance, the failure of tissues to respond to normal circulating hormone levels, has been shown in a variety of endocrine systems to be caused by the genetic lack or functional defect in steroid hormone receptors. Deletions in all or part of the AR gene have been shown in several individuals with complete androgen insensitivity.205,206 The lack of functional ARs results in testicular feminization.

Sex steroid hormones have been implicated in the pathogenesis of breast, uterine, ovarian, prostate, thyroid, and other cancers. Expression of mutant forms of ERα may be an important factor in some cases of breast cancer.207 Determination of steroid hormone receptor expression (i.e. ERα and PR) in breast tumors is critical to the selection of appropriate therapy. Hormone-dependent tumors tend to have elevated levels of steroid hormone receptors.208 One study found that 50% of the sixty breast tumors sampled expressed ERα and ERβ and that the ERα+ /ERβ+ phenotype was associated with node-positive status.209 Elevated levels of ERα and ERβ are highly associated with poor histologic differentiation in breast cancer and may serve as a predictor of responsiveness to endocrine therapy and a prognostic indicator of metastatic disease.208 The combined presence of ERα and PR in a breast tumor provides an additional indication of the probability of prolonged survival after antiestrogen therapy.191

Steroid hormone receptor analysis has limited value in the selection of patients with endometrial cancer for endocrine therapy, because normal and malignant uterine tissues contain significant levels of ERs and PRs.210 Although the results of different studies disagree, PR expression appears a better prognostic indicator than ERα expression in ovarian cancer.211 The relation between steroid receptor status and response to endocrine therapy in ovarian cancer remains to be established.

Glucocorticoid resistance results from an inability of glucocorticoids to exert their effects in their target tissues.212 Glucocorticoid resistance is associated with elevated ACTH and cortisol, with an attendant increase in adrenal androgens and steroids, and with salt-retaining activity. Clinical manifestations of glucocorticoid resistance vary from asymptotic to different degrees of hypertension or hypokalemic alkalosis or hyperandrogenism. In women, the excess androgens can result in acne, hirsutism, male-type baldness, menstrual irregularities, oligoanovulation, and infertility; in men, it may lead to infertility; and in children, it may cause precocious puberty. Different molecular defects in the GR gene, resulting in various defects in the receptor protein alter the functional characteristics or concentrations of GR and cause glucocorticoid resistance.212 It is postulated that acquired tissue-specific glucocorticoid resistance may play a role in the origin and pathogenesis of depression, steroid-resistant asthma, autoimmune disorders, and acquired immunodeficiency syndrome (AIDS). GR mutations have been suggested to play a role in the pathogenesis of leukemia, hereditary glucocorticoid resistance, and Nelson's syndrome.213 Corticosteroid-resistant asthma has been associated with decreased GR and increased AP-1-DNA binding in peripheral blood mononuclear cells compared with corticosteroid-sensitive asthma.214 These findings indicate that variations in the GR may play a central role in a wide variety of diseases.213 Hypertension and pseudohypoaldosteronism are associated with diminished or complete loss of high-affinity aldosterone receptors in select target tissues in some patients.215 MR induces the transcription of specific genes that regulate apical amiloride-sensitive epithelial sodium channels located in the apical membrane of epithelial cells from the distal colon and kidney collecting duct.216

Estrogens play a central role in the immune response and immune-mediated diseases, and cells involved in the immune response—thymocytes, macrophages, and endothelial cells—express ERα.217 Certain pathologic states are characterized by the production of abnormal levels of antibodies to steroid hormone receptors. Although the autoimmune disorder systemic lupus erythematosus (SLE) was postulated to result from antiestrogen receptor action,218 a later report showed that peripheral blood monocytes of SLE patients showed a similar Kd and number of specific [3H]E2 binding sites compared with normal values.219 Similarly, ERα variants do not appear to play a role in SLE.220

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Endocrine disruptors are chemicals in the environment that are thought to disrupt endogenous hormone action by acting as hormone agonists or antagonists in vivo. Endocrine disruptors have been postulated to play a role in the increased incidence of breast cancer in the United States since 1940,221 decreased sperm counts,222 and reproductive tract abnormalities in wildlife species.223

Among the chemicals thought to be xenoestrogens are various organochloride pesticides (e.g. dieldrin; polychlorinated biphenyls [PCBs], a family of 209 related compounds widely used as industrial coolants); alkylphenolic polyethoxylates (APEOs: 4-nonylphenol [NP] and 4-octylphenol [OP]), used as nonionic surfactants in detergents, paints, herbicides, and pesticides; bisphenol A, used in manufacturing plastics; and the heavy metal cadmium.222 Endocrine disruptors are also natural dietary components (e.g. the phytoestrogen coumestrol).224 Because they are lipophilic, endocrine disruptors can enter the human body by ingestion or adsorption through the skin and mucosal membranes. Certain Endocrine disruptors, such as OP and NP,225 are present in water from sewage treatment plants and bioaccumulate in aquatic species and in animal fat.221 Bisphenol A is a contaminant in canned foods226 and was found in human saliva after dental work.227 The level of human exposure to these agents and whether these levels are sufficient to cause harmful effects remain controversial.222 Long-term exposure to exogenous estrogens is thought to increase the relative risk of breast, liver, ovarian, and uterine tumors.228

Because endocrine disruptors lack structural similarity to natural estrogens (i.e. E2), it is not immediately obvious which compounds have estrogenic or antiestrogenic activity. A single endocrine disruptor may exhibit agonist activity with one steroid receptor and antagonist activity with a different receptor (e.g. DDE with ERα and AR).229 The U.S. Environmental Protection Agency (EPA) is funding considerable research effort on endocrine disruptors.

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The past 15 years have seen remarkable progress in our understanding of the mechanism of action of steroid hormones and receptor function. The development of molecular cloning techniques and targeted gene disruption has greatly facilitated the molecular understanding of steroid receptor action. One major advance that arose through the application of these techniques is the concept of a large superfamily of related receptors that share a common modular structure. The steroid receptor superfamily includes receptors for steroid hormones and thyroid hormones and a diverse set of other gene regulators, some of which are called orphan receptors and whose ligands, if necessary, are unknown. Newly identified orphan receptor ligands include metabolic intermediates, indicating an important role for these compounds in regulating lipid metabolism and steroidogenesis.5 Tremendous progress has been made in understanding the way hormone-liganded steroid receptors interact with specific binding sites in hormone-regulated genes and the role of receptor interaction with other nuclear transcription factors in the regulation of hormone target gene transcription. Another significant advance is the identification of coactivators and corepressors as proteins that interact directly with steroid/nuclear receptors, but not with DNA, and that aid receptors in modulating target gene expression. The nucleosome remodeling activity of these coactivators and corepressors and their associated proteins has revealed the importance of chromatin structure in hormone-induced gene transcription. The importance of multiple levels of crosstalk between cell membrane-bound receptors, acting through second messenger phosphorylation cascades, and nuclear hormone receptors and between different classes of transcriptional enhancer proteins indicates the overall complexity involved in specific gene regulation. Developments in the analysis of abnormal receptor structure and function have enhanced our potential for clinical diagnosis and treatment of numerous endocrine disorders.

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