Background
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The estrogen receptor (ER) is a member of the nuclear hormone family of intracellular receptors which is activated by the hormone 17 beta-estradiol.The main function of the estrogen receptor is as a DNA binding transcription factor which regulates gene expression. However the estrogen receptor also has additional functions independent of DNA binding. There are two different forms of the estrogen receptor usually referred to as alpha and beta each encoded by a separate gene (ESR1 and ESR2 respectively). Hormone activated estrogen receptors form dimers, and since the two forms are coexpressed in many cell types, the receptors may form ERalpha (alphaalpha) or ERbeta (betabeta) homodimers or ERalphabeta (alphabeta) heterodimers.Estrogen receptor alpha and beta show significant overall sequence homology, and both are composed of seven domains.The ERalpha is found in endometrium, breast cancer cells, ovarian stroma cells and in the hypothalamus.The expression of the ERbeta protein has been documented in kidney, brain, bone, heart,lungs, intestinal mucosa, prostate, and endothelial cells. While they both bind estrogen as well as other agonists and antagonists, the two receptors have distinctly different localizations and concentrations within our body. Structural differences also exist between the two. The ER ligands tamoxifen, raloxifene, and ICI-164384 were activators with Erbeta as well as ERalpha, although the degree of agonism differed between cell types. These molecules are examples of SERM’s, selective estrogen receptor modulators. Thus, the role of estrogen complexed to ERbeta appears to be to turn off transcription of these genes, whereas the SERMs may override this blockade and activate gene transcription. Estrogen receptor structure-function is a vast topic and the subject of very active current research. Nuclear receptors are a large family of structurally related ligand-inducible transcription factors, including steroid receptors (SRs), thyroid/retinoids receptors (TR, RARs and RXRs), vitamin D receptors (VDR), LXR, PPARs, estrogen receptors (ERalpha and ERbeta), and orphan receptors for which no ligand has been yet identified. While having in common a modular structure, they are activated by distinct lipophilic small molecules such as glucocorticoids, progesterone, estrogens, retinoids, and fatty acid derivatives.All nuclear receptors have a hydrophobic pocket into which its specific ligand binds, with helix 12 (H12) being the key response element of NR's. When an agonist is bound to a NR, H12 is oriented anti-parallel to H11, capping the ligand binding pocket.
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Pathways
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Nuclear Receptor Transcription Pathway, EGFR Signaling Pathway, Retinoic Acid Receptor Signaling Pathway, Intracellular Steroid Hormone Receptor Signaling Pathway, Steroid Hormone Mediated Signaling Pathway, Ribonucleoprotein Complex Subunit Organization, Ribosome Assembly
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