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Nuclear receptors are specific to metazoans (animals) and are not found in protists, algae, fungi, or plants. [8] Amongst the early-branching animal lineages with sequenced genomes, two have been reported from the sponge Amphimedon queenslandica, two from the comb jelly Mnemiopsis leidyi [9] four from the placozoan Trichoplax adhaerens and 17 from the cnidarian Nematostella vectensis. [10]
A hormone receptor is a receptor molecule that binds to a specific hormone. Hormone receptors are a wide family of proteins made up of receptors for thyroid and steroid hormones, retinoids and Vitamin D, and a variety of other receptors for various ligands, such as fatty acids and prostaglandins. [1] Hormone receptors are of mainly two classes.
Steroid hormone receptors are found in the nucleus, cytosol, and also on the plasma membrane of target cells. They are generally intracellular receptors (typically cytoplasmic or nuclear) and initiate signal transduction for steroid hormones which lead to changes in gene expression over a time period of hours to days.
Estrogen receptors (ERs) are proteins found in cells that function as receptors for the hormone estrogen (17β-estradiol). [1] There are two main classes of ERs. The first includes the intracellular estrogen receptors, namely ERα and ERβ , which belong to the nuclear receptor family.
For steroid or thyroid hormones, their receptors are located inside the cell within the cytoplasm of the target cell. These receptors belong to the nuclear receptor family of ligand-activated transcription factors. To bind their receptors, these hormones must first cross the cell membrane. They can do so because they are lipid-soluble.
Examples are the class of nuclear receptors located in the cell nucleus and cytoplasm and the IP 3 receptor located on the endoplasmic reticulum.The ligands that bind to them are usually intracellular second messengers like inositol trisphosphate (IP 3) and extracellular lipophilic hormones like steroid hormones.
The best studied steroid hormone receptors are members of the nuclear receptor subfamily 3 (NR3) that include receptors for estrogen (group NR3A) [39] and 3-ketosteroids (group NR3C). [40] In addition to nuclear receptors, several G protein-coupled receptors and ion channels act as cell surface receptors for certain steroid hormones.
Ronald M. Evans is known for his original discoveries of nuclear hormone receptors (NR), a special class of transcriptional factor, and the elucidation of their universal mechanism of action, a process that governs how lipophilic hormones and drugs regulate virtually every developmental and metabolic pathway in animals and humans.