Estrogens play a fundamental role in the development of the female reproductive system, and regulate the growth and function of tissues such as the breast, uterus, and ovaries. The most potent estrogen, 17β-estradiol (E2), is present in the circulation of females from the onset of puberty to menopause, and can promote abnormal cell growth and eventually lead to the development of cancer. The effects of estrogens are largely mediated through binding to estrogen receptor (ER), which belongs to the superfamily of steroid nuclear receptors.
There are two major subtypes of estrogen receptor, ERa and ERβ. ERa is found on chromosome 6q and encodes a 595 amino acid protein, while ERβ is found on chromosome 14q and encodes a 530 amino acid protein. Estrogen receptor isoforms have distinct tissue ex
Estrogen receptors are composed of six functional regions or domains. The Nterminal (A/B) domain contains a ligand-independent and functionally minor activating function (AF)-1 domain. The DNA binding domain (DBD) or C region consists of 66-amino acid motif that forms two zinc fingers that interact with DNA. The hinge domain (D) contains the nuclear localization signal and interacts with heat shock proteins. The E region, or ligand binding domain (LBD), interacts with E2, and overlaps with the ligand-dependent, major transcriptional activation function (AF)-2 domain. The C-terminal region, or F domain, inhibits dimerization of the receptor until it is bound by ligand.
ERα (ESR1) and ERβ (ESR2) are members of the highly conserved nuclear receptor family of transcription factors. ERα and ERβ both contain six structural domains: A/B, C, D, E, and F. The central C domain is comprised of two zinc fingers responsible for DNA binding and is the most highly conserved domain with 95% sequence homology between ERα and ERβ. This allows for both receptors to bind to the same cis-acting hormone response elements (HREs), termed estrogen response elements (EREs), located within the promoter regions of target genes. While the C domains of ERα and ERβ are highly conserved, variability within the NH2-terminal, A domain allows for the recruitment of diverse co-regulatory proteins and transcription factors. This leads to distinct, non-redundant roles for the two receptors, particularly within the immune, skeletal, cardiovascular and central nervous systems. The A/B domain also contains the constitutively active activation function-1 (AF-1). A second activation function (AF-2) and the ligand-binding domain (LBD) are located within the E domain. Sixty percent sequence homology between the two receptors within the E domain results in similar binding affinities for E2.
Estrogen Receptor Signaling
Due to its small, lipophilic structure, estrogen passively crosses the cellular and nuclear membranes to bind to the predominantly nuclear localized estrogen receptors. E2 binding to the LBD induces a conformational change, leading to the release of inhibitory heat shock protein 90 (HSP90) chaperones from the D domain and the dimerization of the receptor along the LBD. ERα and ERβ are transcriptionally active in both homo- and α/β heterodimerized conformations due to homology within the DNA binding domain (DBD) and LBD of the ERs. The dimerized receptor then binds to the ERE present within the promoter sequence of the target gene and regulates transcription, either positively or negatively depending upon the recruited cofactors. As ERα and ERβ are members of the nuclear receptor family of transcription factors, the ERE contains two half-sites with the sequence RGGTCA, separated by three base pairs. The estrogen receptor dimer may also alter transcription of non-ERE containing genes through association with the AP-1 or SP-1 transcription factors and their associated binding sites. Additionally, phosphorylation by MAPK of Ser118 within the AF-1 region of ERα leads to ligand-independent transcriptional activation of estrogen-responsive genes.
Apart from the long-term genomic responses induced by transcriptional activation, ERα and ERβ also effect rapid signaling, non-genomic cellular responses, as quickly as a few seconds after ligand binding. While the genomic effects are enacted through nuclear localized estrogen receptor functioning as a transcription factor, many of the nongenomic effects occur through palmitoylated, and thus membrane associated, ERα (mERα). These rapid signaling effects include the production of phospholipase C and cAMP leading to the influx of Ca2+ and associated downstream signaling, the release of nitric oxide within the vascular endothelial cells resulting in vasodilation, and the activation of Src and thereby the PI3K and MAPK pathways.