Pharmacodynamics SERMs are competitive partial agonists of the ER. Different tissues have different degrees of sensitivity to the activity of endogenous estrogens, so SERMs produce estrogenic or
antiestrogenic effects depending on the tissue in question, as well as the percentage of
intrinsic activity (IA) of the SERM. An example of a SERM with high IA and thus mostly estrogenic effects is
chlorotrianisene, while an example of a SERM with low IA and thus mostly antiestrogenic effects is
ethamoxytriphetol. SERMs like
clomifene and
tamoxifen are comparatively more in the middle in their IA and their balance of estrogenic and antiestrogenic activity.
Raloxifene is a SERM that is more antiestrogenic than tamoxifen; both are estrogenic in bone, but raloxifene is antiestrogenic in the
uterus while tamoxifen is estrogenic in this part of the body. Two different subtypes of ER have been identified,
ERα and
ERβ. ERα is considered the main medium where estrogen signals are
transduced at the transcriptional level and is the predominant ER in the female reproductive tract and mammary glands while ERβ is primarily in vascular
endothelial cells, bone, and male prostate tissue. Many characteristics are similar between these two types such as size (~600 and 530
amino acids) and structure. ERα and ERβ share approximately 97% of the amino-acid sequence identity in the
DNA-binding domain and about 56% in the
ligand-binding domain. The variation is greater on the N-terminus between ERα and ERβ. The DNA-binding domain consists of two subdomains. One with a proximal box that is involved in DNA recognition while the other contains a distal box responsible for DNA-dependent, DNA-binding domain
dimerization. The proximal box sequence is identical between ERα and ERβ, which indicates similar specificity and affinity between the two subgroups. DNA-binding domain's globular proteins contain eight
cysteines and allow for a tetrahedral coordination of two
zinc ions. This coordination makes the binding of ER to estrogen response elements possible. The differential positioning of the activating function 2 (AF-2) helix 12 in the ligand-binding domain by the bound ligand determines whether the ligand has an agonistic and antagonistic effect. In agonist-bound receptors, helix 12 is positioned adjacent to helices 3 and 5. Helices 3, 5, and 12 together form a binding surface for an NR box motif contained in
coactivators with the
canonical sequence LXXLL (where L represents
leucine or
isoleucine and X is any amino acid). Unliganded (apo) receptors or receptors bound to antagonist ligands turn helix 12 away from the LXXLL-binding surface that leads to preferential binding of a longer leucine-rich motif, LXXXIXXX(I/L), present on the
corepressors NCoR1 or SMRT. In addition, some
cofactors bind to ER through the terminals, the DNA-binding site or other binding sites. Thus, one compound can be an ER agonist in a tissue rich in
coactivators but an ER antagonist in tissues rich in corepressors. The structures shown here are of the ligand binding domain (LBD) of the estrogen receptor (green cartoon diagram) complexed with either the agonist
diethylstilbestrol (top, ) or antagonist
4-hydroxytamoxifen (bottom, ). The ligands are depicted as space filling spheres (white = carbon, red = oxygen). When an agonist is bound to a nuclear receptor, the C-terminal
alpha helix of the LBD (H12; light blue) is positioned such that a
coactivator protein (red) can bind to the surface of the LBD. Shown here is just a small part of the coactivator protein, the so-called NR box containing the LXXLL amino acid sequence motif. Antagonists occupy the same ligand binding cavity of the nuclear receptor. However antagonist ligands in addition have a sidechain extension which
sterically displaces H12 to occupy roughly the same position in space as coactivators bind. Hence coactivator binding to the LBD is blocked. Estrogenic compounds span a spectrum of activity, including: • Full agonists (agonistic in all tissues), such as the natural endogenous hormone
estradiol. • Mixed agonists/antagonistic (agonistic in some tissues while antagonistic in others), such as tamoxifen (a SERM). • Pure antagonists (antagonistic in all tissues), such as
fulvestrant. SERMs are known to stimulate estrogenic actions in tissues such as the liver, bone and cardiovascular system but known to block estrogen action where stimulation is not desirable, such as in the breast and the uterus. SERMs interact with receptors by diffusing into cells and their binding to ERα or ERβ subunits, which results in
dimerization and structural changes of the receptors. This makes it easier for the SERMs to interact with estrogen response elements which leads to the activation of estrogen-inducible genes and mediating the estrogen effects. SERMs can impact coactivator protein stability and can also regulate coactivator activity through
post-translational modifications such as
phosphorylation. Multiple growth signaling pathways, such as
HER2,
PKC,
PI3K and more, are
downregulated in response to anti-estrogen treatment. Steroid receptor coactivator 3 (SRC-3) is phosphorylated by activated
kinases that also enhance its coactivator activity, affect cell growth and ultimately contribute to drug resistance. The ratio of ERα and ERβ at a target site may be another way SERM activity is determined. High levels of cellular proliferation correlate well with a high ERα:ERβ ratio, but repression of cellular proliferation correlates to ERβ being dominant over ERα. The ratio of ERs in
neoplastic and normal breast tissue could be important when considering
chemoprevention with SERMs. When looking at the differences between ERα and ERβ, activating function 1 (AF-1) and AF-2 are important. Together they play an important part in the interaction with other co-regulatory proteins that control
gene transcription. AF-1 is located in the
amino terminus of the ER and is only 20% homologous in ERα and ERβ. On the other hand, AF-2 is very similar in ERα and ERβ, and only one amino acid is different. Studies have shown that by switching AF-1 regions in ERα and ERβ, that there are specific differences in transcription activity. Generally, SERMs can partially activate engineered genes through ERα by an estrogen receptor element, but not through ERβ. Although, raloxifene and the active form of tamoxifen can stimulate AF-1-regulated reporter genes in both ERα and ERβ. Because of the discovery that there are two ER subtypes, it has brought about the synthesis of a range of receptor specific ligands that can switch on or off a particular receptor. However, the external shape of the resulting complex is what becomes the catalyst for changing the response at a tissue target to a SERM.
X-ray crystallography of estrogens or antiestrogens has shown how ligands program the receptor complex to interact with other proteins. The ligand-binding domain of the ER demonstrates how ligands promote and prevent coactivator binding based on the shape of the estrogen or antiestrogen complex. The broad range of ligands that bind to the ER can create a spectrum of ER complexes that are fully estrogenic or antiestrogenic at a specific target site. The main result of a ligand-binding to ER is a structural rearrangement of the ligand-
binding pocket, primarily in the AF-2 of the C-terminal region. The binding of ligands to ER leads to the formation of a
hydrophobic pocket that regulates cofactors and receptor pharmacology. The correct
folding of ligand-binding domain is required for activation of transcription and for ER to interact with a number of coactivators. Coactivators are not just protein partners that connect sites together in a complex. Coactivators play an active role in modifying the activity of a complex. Post-translation modification of coactivators can result in a dynamic model of
steroid hormone action by way of multiple kinase pathways initiated by cell surface
growth factor receptors. Under the guidance of a multitude of protein remodelers to form a multiprotein coactivator complex that can interact with the phosphorylated ER at a specific gene promoter site, the core coactivator first has to recruit a specific set of cocoactivators. The proteins that the core coactivator assembles as the core coactivated complex have individual enzymatic activities to
methylate or
acetylate adjacent proteins. The ER substrates or
coenzyme A can be
polyubiquitinated by multiple cycles of the reaction or, depending on linkage proteins, they can either be activated further or degraded by the
26S proteasome. Consequently, to have an effective gene transcription that is programmed and targeted by the structure and phosphorylation status of the ER and coactivators, it is required to have a dynamic and cyclic process of remodeling capacity for transcriptional assembly, after which the transcription complex is then instantly routinely destroyed by the proteasome. == Structure and function ==