Editing Hormone (section)

==Receptors==
[[File:Steroid and Lipid Hormones.svg|thumb|400px|The left diagram shows a steroid (lipid) hormone (1) entering a cell and (2) binding to a receptor protein in the nucleus, causing (3) mRNA synthesis which is the first step of protein synthesis. The right side shows protein hormones (1) binding with receptors which (2) begins a transduction pathway. The transduction pathway ends (3) with transcription factors being activated in the nucleus, and protein synthesis beginning. In both diagrams, a is the hormone, b is the cell membrane, c is the cytoplasm, and d is the nucleus.]] Most hormones initiate a cellular response by initially binding to either [[cell surface receptor]]s or [[intracellular receptor]]s. A cell may have several different [[Receptor (biochemistry)|receptors]] that recognize the same hormone but activate different [[signal transduction]] pathways, or a cell may have several different receptors that recognize different hormones and activate the same biochemical pathway.<ref>{{Cite web|url=https://www.khanacademy.org/science/biology/cell-signaling/mechanisms-of-cell-signaling/a/intracellular-signal-transduction|title=Signal relay pathways|website=Khan Academy|access-date=2019-11-13}}</ref>

Receptors for most [[peptide hormone|peptide]] as well as many [[eicosanoid]] hormones are embedded in the [[cell membrane]] as cell surface receptors, and the majority of these belong to the [[G protein-coupled receptor]] (GPCR) class of seven [[alpha helix]] [[transmembrane]] proteins. The interaction of hormone and receptor typically triggers a cascade of secondary effects within the [[cytoplasm]] of the cell, described as [[signal transduction]], often involving [[phosphorylation]] or dephosphorylation of various other cytoplasmic proteins, changes in [[ion channel]] permeability, or increased concentrations of intracellular molecules that may act as [[second messenger|secondary messengers]] (e.g., [[cyclic AMP]]). Some [[protein hormone]]s also interact with [[intracellular]] receptors located in the [[cytoplasm]] or [[cell nucleus|nucleus]] by an [[intracrine]] mechanism.<ref>{{Cite journal| vauthors = Lodish H, Berk A, Zipursky SL, Matsudaira P, Baltimore D, Darnell J |date=2000|title=G Protein –Coupled Receptors and Their Effectors|url=https://www.ncbi.nlm.nih.gov/books/NBK21718/|journal=Molecular Cell Biology | edition = 4th }}</ref><ref>{{cite journal | vauthors = Rosenbaum DM, Rasmussen SG, Kobilka BK | title = The structure and function of G-protein-coupled receptors | journal = Nature | volume = 459 | issue = 7245 | pages = 356–63 | date = May 2009 | pmid = 19458711 | pmc = 3967846 | doi = 10.1038/nature08144 | bibcode = 2009Natur.459..356R }}</ref>

For [[steroid hormone|steroid]] or [[thyroid hormone|thyroid]] hormones, their [[steroid hormone receptor|receptors]] are located [[intracellular|inside the cell]] within the [[cytoplasm]] of the target cell. These receptors belong to the [[nuclear receptor]] family of ligand-activated [[transcription factor]]s. To bind their receptors, these hormones must first cross the cell membrane. They can do so because they are lipid-soluble. The combined hormone-receptor [[protein complex|complex]] then moves across the nuclear membrane into the nucleus of the cell, where it binds to specific [[DNA sequences]], regulating the expression of certain [[genes]], and thereby increasing the levels of the proteins encoded by these genes.<ref name="beato">{{cite journal | vauthors = Beato M, Chávez S, Truss M | title = Transcriptional regulation by steroid hormones | journal = Steroids | volume = 61 | issue = 4 | pages = 240–51 | date = April 1996 | pmid = 8733009 | doi = 10.1016/0039-128X(96)00030-X | s2cid = 20654561 }}</ref> However, it has been shown that not all steroid receptors are located inside the cell. Some are associated with the [[plasma membrane]].<ref name="hammes">{{cite journal | vauthors = Hammes SR | title = The further redefining of steroid-mediated signaling | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 5 | pages = 2168–70 | date = March 2003 | pmid = 12606724 | pmc = 151311 | doi = 10.1073/pnas.0530224100 | bibcode = 2003PNAS..100.2168H | doi-access = free }}</ref>