Left: A hormone feedback loop in a female adult. (1) Follicle-Stimulating Hormone, (2) Luteinizing Hormone, (3) Progesterone, (4) Estrogen. Right: Auxin transport from leaves to roots in Arabidopsis thaliana
A hormone (from the Greek participle ὁρμῶν, "setting in motion") is any member of a class of signaling molecules in multicellular organisms, that are transported to distant organs to regulate physiology and / or behavior. Hormones are required for the correct development of animals, plants and fungi. The lax definition of a hormone (as a signalling molecule that acts distant from its site of production) means that many different classes of molecule can be defined as hormones. Among the substances that can be considered hormones, are eicosanoids (e.g. prostaglandins and thromboxanes), steroids (e.g. oestrogen and brassinosteroid), amino acid derivatives (e.g. epinephrine and auxin), protein / peptides (e.g. insulin and CLE peptides) and gases (e.g ethylene and nitrous oxide).
Hormones are used to communicate between organs and tissues. In vertebrates, hormones are responsible for the regulation of many physiological processes and behavioral activities such as digestion, metabolism, respiration, sensory perception, sleep, excretion, lactation, stress induction, growth and development, movement, reproduction, and mood manipulation. In plants, hormones modulate almost all aspects of development, from germination to senescence.
Hormones affect distant cells by binding to specific receptor proteins in the target cell, resulting in a change in cell function. When a hormone binds to the receptor, it results in the activation of a signal transduction pathway that typically activates gene transcription, resulting in increased expression of target proteins. Hormones can also act in rapid, non-genomic pathways that can be synergistic with genomic effects. Water-soluble hormones (such as peptides and amines) generally act on the surface of target cells via second messengers. Lipid soluble hormones, (such as steroids) generally pass through the plasma membranes of target cells (both cytoplasmic and nuclear) to act within their nuclei. A notable exception to this are brassinosteroids in plants, which despite being lipid soluble, still bind to their receptor at the cell surface.
In vertebrates, endocrine glands are specialized organs that secrete hormones into the endocrine signaling system. Hormone secretion occurs in response to specific biochemical signals and is often subject to negative feedback regulation. For instance, high blood sugar (serum glucose concentration) promotes insulin synthesis. Insulin then acts to reduce glucose levels and maintain homeostasis, leading to reduced insulin levels. Upon secretion water soluble hormones are readily transported through the circulatory system. Lipid-soluble hormones must bond to carrier plasma glycoproteins (e.g., thyroxine-binding globulin (TBG)) to form ligand-protein complexes. Some hormones are completely active when released into the bloodstream (as is the case for insulin and growth hormones), while others are prohormones that must be activated in specific cells through a series of activation steps that are commonly highly regulated. The endocrine system secretes hormones directly into the bloodstream, typically via fenestrated capillaries, whereas the exocrine system secretes its hormones indirectly using ducts. Hormones with paracrine function diffuse through the interstitial spaces to nearby target tissue.
Plants lack specialized organs for the secretion of hormones, although there is spacial distribution of hormone production. For example, the hormone auxin is produced mainly at the tips of young leaves and in the shoot apical meristem. The lack of specialised glands means that the main site of hormone production can change throughout the life of a plant, and the site of production is dependent on the plant's age and environment.
Introduction and overview
Hormonal signaling involves the following steps:
- Biosynthesis of a particular hormone in a particular tissue
- Storage and secretion of the hormone
- Transport of the hormone to the target cell(s)
- Recognition of the hormone by an associated cell membrane or intracellular receptor protein
- Relay and amplification of the received hormonal signal via a signal transduction process: This then leads to a cellular response. The reaction of the target cells may then be recognized by the original hormone-producing cells, leading to a downregulation in hormone production. This is an example of a homeostatic negative feedback loop.
- Breakdown of the hormone.
Hormone producing cells are typically of a specialized cell type, residing within a particular endocrine gland, such as the thyroid gland, ovaries, and testes. Hormones exit their cell of origin via exocytosis or another means of membrane transport. The hierarchical model is an oversimplification of the hormonal signaling process. Cellular recipients of a particular hormonal signal may be one of several cell types that reside within a number of different tissues, as is the case for insulin, which triggers a diverse range of systemic physiological effects. Different tissue types may also respond differently to the same hormonal signal.
Arnold Adolph Berthold (1849)
Arnold Adolph Berthold was a German physiologist and zoologist, who, in 1849, had a question about the function of the testes. He noticed that in castrated roosters that they did not have the same sexual behaviors as roosters with their testes intact. He decided to run an experiment on male roosters to examine this phenomenon. He kept a group of roosters with their testes intact, and saw that they had normal sized wattles and combs (secondary sexual organs), a normal crow, and normal sexual and aggressive behaviors. He also had a group with their testes surgically removed, and noticed that their secondary sexual organs were decreased in size, had a weak crow, did not have sexual attraction towards females, and were not aggressive. He realized that this organ was essential for these behaviors, but he did not know how. To test this further, he removed one testis and placed it in the abdominal cavity. The roosters acted and had normal physical anatomy. He was able to see that location of the testes do not matter. He then wanted to see if it was a genetic factor that was involved in the testes that provided these functions. He transplanted a testis from another rooster to a rooster with one testis removed, and saw that they had normal behavior and physical anatomy as well. Berthold determined that the location or genetic factors of the testes do not matter in relation to sexual organs and behaviors, but that some chemical in the testes being secreted is causing this phenomenon. It was later identified that this factor was the hormone testosterone.
Charles and Francis Darwin (1880)
Although known primarily for his work on the Theory of Evolution, Charles Darwin was also keenly interested in plants. Through the 1870's, he and his son Francis studied the movement of plants towards light. They were able to show that light is perceived at the tip of a young stem (the coleoptile), whereas the bending occurs lower down the stem. They proposed that a 'transmissible substance' communicated the direction of light from the tip down to the stem. The idea of a 'transmissible substance' was initially dismissed other plant biologists, but their work later led to the discovery of the first plant hormone. In the 1920's Dutch scientist Frits Warmolt Went and Russian scientist Nikolai Cholodny (working independently of each other) conclusively showed that asymmetric accumulation of a growth hormone was responsible for this bending. In 1933 this hormone was finally isolated by Kögl, Haagen-Smit and Erxleben and christened 'auxin'.
Bayliss and Starling (1902)
William Bayliss and Ernest Starling, a physiologist and biologist, respectively, wanted to see if the nervous system had an impact on the digestive system. They knew that the pancreas was involved in the secretion of digestive fluids after the passage of food from the stomach to the intestines, which they believed to be due to the nervous system. They cut the nerves to the pancreas in an animal model and discovered that it was not nerve impulses that controlled secretion from the pancreas. It was determined that a factor secreted from the intestines into the bloodstream was stimulating the pancreas to secrete digestive fluids. This factor was named secretin: a hormone, although the term hormone was not coined until 1905 by Starling.
Types of signaling
Hormonal effects are dependent on where they are released, as they can be released in different manners. Not all hormones are released from a cell and into the blood until it binds to a receptor on a target. The major types of hormone signaling are:
Signaling Types - Hormones
||Acts on the target cells after being released into the bloodstream.
||Acts on the nearby cells and does not have to enter general circulation.
||Affects the cell types that secreted it and causes a biological effect.
||Acts intracellularly on the cells that synthesized it.
As hormones are defined functionally, not structurally, they may have diverse chemical structures. Hormones occur in multicellular organisms (plants, animals, fungi, brown algae, and red algae). These compounds occur also in unicellular organisms, and may act as signaling molecules however there is no agreement that these molecules can be called hormones.
Different types of hormones are secreted in the human body, with different biological roles and functions.
Compared with vertebrates, insects and crustaceans possess a number of structurally unusual hormones such as the juvenile hormone, a sesquiterpenoid.
Examples include abscisic acid, auxin, cytokinin, ethylene, and gibberellin.
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 membrane associated or intracellular receptors. A cell may have several different receptor types 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.
Receptors for most peptide as well as many eicosanoid hormones are embedded in the plasma membrane at the surface of the cell and the majority of these receptors 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 secondary messengers (e.g., cyclic AMP). Some protein hormones also interact with intracellular receptors located in the cytoplasm or nucleus by an intracrine mechanism.
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. The combined hormone-receptor 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. However, it has been shown that not all steroid receptors are located inside the cell. Some are associated with the plasma membrane.
Effects in humans
Hormones have the following effects on the body:
A hormone may also regulate the production and release of other hormones. Hormone signals control the internal environment of the body through homeostasis.
The rate of hormone biosynthesis and secretion is often regulated by a homeostatic negative feedback control mechanism. Such a mechanism depends on factors that influence the metabolism and excretion of hormones. Thus, higher hormone concentration alone cannot trigger the negative feedback mechanism. Negative feedback must be triggered by overproduction of an "effect" of the hormone.
Blood glucose levels are maintained at a constant level in the body by a negative feedback mechanism. When the blood glucose level is too high, the pancreas secretes insulin and when the level is too low, the pancreas then secretes glucagon. The flat line shown represents the homeostatic set point. The sinusoidal line represents the blood glucose level.
Hormone secretion can be stimulated and inhibited by:
- Other hormones (stimulating- or releasing -hormones)
- Plasma concentrations of ions or nutrients, as well as binding globulins
- Neurons and mental activity
- Environmental changes, e.g., of light or temperature
One special group of hormones is the tropic hormones that stimulate the hormone production of other endocrine glands. For example, thyroid-stimulating hormone (TSH) causes growth and increased activity of another endocrine gland, the thyroid, which increases output of thyroid hormones.
To release active hormones quickly into the circulation, hormone biosynthetic cells may produce and store biologically inactive hormones in the form of pre- or prohormones. These can then be quickly converted into their active hormone form in response to a particular stimulus.
Eicosanoids are considered to act as local hormones. They are considered to be "local" because they possess specific effects on target cells close to their site of formation. They also have a rapid degradation cycle, making sure they do not reach distant sites within the body.
Hormones are also regulated by receptor agonists. Hormones are ligands, which are any kinds of molecules that produce a signal by binding to a receptor site on a protein. Hormone effects can be inhibited, thus regulated, by competing ligands that bind to the same target receptor as the hormone in question. When a competing ligand is bound to the receptor site, the hormone is unable to bind to that site and is unable to elicit a response from the target cell. These competing ligands are called antagonists of the hormone.
Many hormones and their structural and functional analogs are used as medication. The most commonly prescribed hormones are estrogens and progestogens (as methods of hormonal contraception and as HRT), thyroxine (as levothyroxine, for hypothyroidism) and steroids (for autoimmune diseases and several respiratory disorders). Insulin is used by many diabetics. Local preparations for use in otolaryngology often contain pharmacologic equivalents of adrenaline, while steroid and vitamin D creams are used extensively in dermatological practice.
A "pharmacologic dose" or "supraphysiological dose" of a hormone is a medical usage referring to an amount of a hormone far greater than naturally occurs in a healthy body. The effects of pharmacologic doses of hormones may be different from responses to naturally occurring amounts and may be therapeutically useful, though not without potentially adverse side effects. An example is the ability of pharmacologic doses of glucocorticoids to suppress inflammation.
At the neurological level, behavior can be inferred based on: hormone concentrations; hormone-release patterns; the numbers and locations of hormone receptors; and the efficiency of hormone receptors for those involved in gene transcription. Not only do hormones influence behavior, but also behavior and the environment influence hormones. Thus, a feedback loop is formed. For example, behavior can affect hormones, which in turn can affect behavior, which in turn can affect hormones, and so on.
Three broad stages of reasoning may be used when determining hormone-behavior interactions:
- The frequency of occurrence of a hormonally dependent behavior should correspond to that of its hormonal source
- A hormonally dependent behavior is not expected if the hormonal source (or its types of action) is non-existent
- The reintroduction of a missing behaviorally dependent hormonal source (or its types of action) is expected to bring back the absent behavior
Comparison with neurotransmitters
There are various clear distinctions between hormones and neurotransmitters:
- A hormone can perform functions over a larger spatial and temporal scale than can a neurotransmitter.
- Hormonal signals can travel virtually anywhere in the circulatory system, whereas neural signals are restricted to pre-existing nerve tracts
- Assuming the travel distance is equivalent, neural signals can be transmitted much more quickly (in the range of milliseconds) than can hormonal signals (in the range of seconds, minutes, or hours). Neural signals can be sent at speeds up to 100 meters per second.
- Neural signalling is an all-or-nothing (digital) action, whereas hormonal signalling is an action that can be continuously variable as dependent upon hormone concentration.
Neurohormones are a type of hormone that are produced by endocrine cells that receive input from neurons, or neuroendocrine cells. Both classic hormones and neurohormones are secreted by endocrine tissue; however, neurohormones are the result of a combination between endocrine reflexes and neural reflexes, creating a neuroendocrine pathway. While endocrine pathways produce chemical signals in the form of hormones, the neuroendocrine pathway involves the electrical signals of neurons. In this pathway, the result of the electrical signal produced by a neuron is the release of a chemical, which is the neurohormone. Finally, like a classic hormone, the neurohormone is released into the bloodstream to reach its target.
Hormone transport and the involvement of binding proteins is an essential aspect when considering the function of hormones. There are several benefits with the formation of a complex with a binding protein: the effective half-life of the bound hormone is increased; a reservoir of bound hormones is created, which evens the variations in concentration of unbound hormones (bound hormones will replace the unbound hormones when these are eliminated).
- ^ Shuster M (2014-03-14). Biology for a changing world, with physiology (Second ed.). New York, NY. ISBN 9781464151132. OCLC 884499940.
- ^ Neave N (2008). Hormones and behaviour: a psychological approach. Cambridge: Cambridge Univ. Press. ISBN 978-0521692014. Lay summary – Project Muse.
- ^ "Hormones". MedlinePlus. U.S. National Library of Medicine.
- ^ "Hormone - The hormones of plants". Encyclopedia Britannica. Retrieved 2021-01-05.
- ^ Ruhs S, Nolze A, Hübschmann R, Grossmann C (July 2017). "30 Years of the Mineralocorticoid Receptor: Nongenomic effects via the mineralocorticoid receptor". The Journal of Endocrinology. 234 (1): T107–T124. doi:10.1530/JOE-16-0659. PMID 28348113.
- ^ Wang ZY, Seto H, Fujioka S, Yoshida S, Chory J (March 2001). "BRI1 is a critical component of a plasma-membrane receptor for plant steroids". Nature. 410 (6826): 380–3. doi:10.1038/35066597. PMID 11268216.
- ^ "Plant Hormones/Nutrition". www2.estrellamountain.edu. Retrieved 2021-01-07.
- ^ Nussey S, Whitehead S (2001). Endocrinology: an integrated approach. Oxford: Bios Scientific Publ. ISBN 978-1-85996-252-7.
- ^ a b Belfiore A, LeRoith PE (2018). Principles of Endocrinology and Hormone Action. Cham. ISBN 9783319446752. OCLC 1021173479.
- ^ Molina PE, ed. (2018). Endocrine Physiology. McGraw-Hill Education. ISBN 9781260019353. OCLC 1034587285.
- ^ a b Whippo CW, Hangarter RP (May 2006). "Phototropism: bending towards enlightenment". The Plant Cell. 18 (5): 1110–9. doi:10.1105/tpc.105.039669. PMID 16670442.
- ^ Wieland OP, De Ropp RS, Avener J (April 1954). "Identity of auxin in normal urine". Nature. 173 (4408): 776–7. doi:10.1038/173776a0. PMID 13165644.
- ^ Holland JJ, Roberts D, Liscum E (2009-05-01). "Understanding phototropism: from Darwin to today". Journal of Experimental Botany. 60 (7): 1969–78. doi:10.1093/jxb/erp113. PMID 19357428.
- ^ Bayliss WM, Starling EH (1968). "The Mechanism of Pancreatic Secretion". In Leicester HM (ed.). Source Book in Chemistry, 1900–1950. Harvard University Press. pp. 311–313. doi:10.4159/harvard.9780674366701.c111. ISBN 9780674366701.
- ^ a b Molina PE (2018). Endocrine physiology. McGraw-Hill Education. ISBN 9781260019353. OCLC 1034587285.
- ^ Lenard J (April 1992). "Mammalian hormones in microbial cells". Trends in Biochemical Sciences. 17 (4): 147–50. doi:10.1016/0968-0004(92)90323-2. PMID 1585458.
- ^ Janssens PM (1987). "Did vertebrate signal transduction mechanisms originate in eukaryotic microbes?". Trends in Biochemical Sciences. 12: 456–459. doi:10.1016/0968-0004(87)90223-4.
- ^ Marieb E (2014). Anatomy & physiology. Glenview, IL: Pearson Education, Inc. ISBN 978-0321861580.
- ^ Heyland A, Hodin J, Reitzel AM (January 2005). "Hormone signaling in evolution and development: a non-model system approach". BioEssays. 27 (1): 64–75. doi:10.1002/bies.20136. PMID 15612033.
- ^ Wang YH, Irving HR (April 2011). "Developing a model of plant hormone interactions". Plant Signaling & Behavior. 6 (4): 494–500. doi:10.4161/psb.6.4.14558. PMC 3142376. PMID 21406974.
- ^ "Signal relay pathways". Khan Academy. Retrieved 2019-11-13.
- ^ Lodish H, Berk A, Zipursky SL, Matsudaira P, Baltimore D, Darnell J (2000). "G Protein –Coupled Receptors and Their Effectors". Molecular Cell Biology (4th ed.).
- ^ Rosenbaum DM, Rasmussen SG, Kobilka BK (May 2009). "The structure and function of G-protein-coupled receptors". Nature. 459 (7245): 356–63. Bibcode:2009Natur.459..356R. doi:10.1038/nature08144. PMC 3967846. PMID 19458711.
- ^ Beato M, Chávez S, Truss M (April 1996). "Transcriptional regulation by steroid hormones". Steroids. 61 (4): 240–51. doi:10.1016/0039-128X(96)00030-X. PMID 8733009. S2CID 20654561.
- ^ Hammes SR (March 2003). "The further redefining of steroid-mediated signaling". Proceedings of the National Academy of Sciences of the United States of America. 100 (5): 2168–70. Bibcode:2003PNAS..100.2168H. doi:10.1073/pnas.0530224100. PMC 151311. PMID 12606724.
- ^ Lall S (2013). Clearopathy. India: Partridge Publishing India. p. 1. ISBN 9781482815887.
- ^ Campbell M, Jialal I (2019). "Physiology, Endocrine Hormones". StatPearls. StatPearls Publishing. PMID 30860733. Retrieved 13 November 2019.
- ^ Röder PV, Wu B, Liu Y, Han W (March 2016). "Pancreatic regulation of glucose homeostasis". Experimental & Molecular Medicine. 48 (3): e219. doi:10.1038/emm.2016.6. PMC 4892884. PMID 26964835.
- ^ a b Shah SB, Saxena R (2012). Allergy-hormone links. New Delhi: Jaypee Brothers Medical Publishers (P) Ltd. ISBN 9789350250136. OCLC 761377585.
- ^ "Eicosanoids". www.rpi.edu. Retrieved 2017-02-08.
- ^ a b c d e f Silverthorn DU, Johnson BR, Ober WC, Ober CW (2016). Human physiology : an integrated approach (Seventh ed.). [San Francisco]. ISBN 9780321981226. OCLC 890107246.
- ^ "Hormone Therapy". Cleveland Clinic.
- ^ Garland T, Zhao M, Saltzman W (August 2016). "Hormones and the Evolution of Complex Traits: Insights from Artificial Selection on Behavior". Integrative and Comparative Biology. 56 (2): 207–24. doi:10.1093/icb/icw040. PMC 5964798. PMID 27252193.
- ^ Reece JB, Urry LA, Cain ML, Wasserman SA, Minorsky PV, Jackson RB, Campbell NA (2014). Campbell biology (Tenth ed.). Boston. ISBN 9780321775658. OCLC 849822337.
- ^ Siegel A, Sapru H, Hreday N, Siegel H (2006). Essential neuroscience. Philadelphia: Lippincott Williams & Wilkins. ISBN 0781750776. OCLC 60650938.
- ^ Alberts B (2002). Molecular biology of the cell. Johnson, Alexander,, Lewis, Julian,, Raff, Martin,, Roberts, Keith,, Walter, Peter (4th ed.). New York: Garland Science. ISBN 0815332181. OCLC 48122761.
- ^ Life, the science of biology. Purves, William K. (William Kirkwood), 1934- (6th ed.). Sunderland, MA: Sinauer Associates. 2001. ISBN 0716738732. OCLC 45064683.CS1 maint: others (link)
- ^ Boron WF, Boulpaep EL. Medical physiology: a cellular and molecular approach. Updated 2. Philadelphia, Pa: Saunders Elsevier; 2012.