Hormone
Hormones are secreted chemical messengers that coordinate the activities of different cells in multicellular organisms. An enormous range of chemicals, including small molecules, amino acid-chains called peptides, proteins, and lipid-derived compounds, are used for this type of cell-to-cell communication.
The term hormone (from the Greek “to spur on”) was first used by biochemists William Bayliss and Ernest Starling in 1904 to describe the action of secretin. Their research generated three key concepts:
- Hormones are molecules synthesized by specific tissues (glands).
- They are secreted directly into the blood, which carries them to their sites of action.
- They specifically alter the activities of responsive cells (called target cells), which have receptors for the signaling molecules.
In vertebrates, hormones belong to the endocrine system, a control system of ductless glands and single cells. In humans, there are eight main glands that generally are considered part of the endocrine system: adrenal gland, pituitary gland, hypothalamus, pancreas, thyroid gland, pineal gland, parathyroid gland, and the reproductive glands. Other organs of the body also produce and secrete hormones, but are generally not considered part of the endocrine system; these include the heart, kidney, liver, thymus, skin, and placenta.
Broadly conceived, the role of hormones is to help maintain the homeostasis of a living organism: i.e., to regulate its internal environent. Hormonal effects vary widely and may include:
- stimulation or inhibition of growth and development
- activation or inhibition of the immune system
- regulation of metabolism (the breakdown or synthesis of biological molecules that yield chemical energy)
- preparation for a new activity in response to environmental stimuli (e.g., fighting, fleeing, mating)
- preparation for a new phase of life (e.g., puberty, caring for offspring, menopause)
- control of the reproductive cycle
Although scientific research has focused on the function of hormones in vertebrates, hormones play important roles in other multicellular organisms. The insect hormone ecdysome triggers the metamorphosis of larvae to adults, Plants produce a variety of hormones involved in processes such as cell growth and differentiation (auxins), stem elongation (gibberellins), and fruit ripening (ethylene).
[Neurohormones et al. as challenging traditional definition/schema.]
Types of signaling
In animals, there are three types of signaling by extracellular, secreted molecules—endocrine, paracrine, or autocrine—based on the distance over which the signal acts.
Hormones belong to the first type: they act on target cells distant from their site of synthesis by cells of the endocrine organs. In animals, an endocrine hormone is usually carried by the blood from its site of release to the target cell.
Paracrine signaling molecules only affect target cells in close proximity (an example is the conduction of an impulse down an xyz), while autocrine cells respond to substances that they themselves release.
However, the designations above are not so clear-cut, as some compounds can act in two or even three types of signaling. For example, certain small peptides function both as neurotransmitters (paracrine signaling) and as hormones (endocrine signaling). These neurohormones, produced by neurosecretory cells primarily in the brain, are distinguished from classical neurotransmitters in that they are able to affect cells distant from their source.
How hormones transmit signals
Hormonal signaling typically involves the following six steps:
- Biosynthesis of the hormone in a specialized tissue.
- Storage and secretion of the hormone.
- Transport of the hormone to the target cell(s), often via the bloodstream.
- 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.
- Removal of the signal, which often involves degradation of the hormone, to terminate the cellular response.
The technical term for an extracellular signal molecule (such as a hormone or a neurotransmiatter) is ligand. The ligand binds to, or “fits,” a site on the receptor protein, which is located on the surface of a target cell or in its nucleus or cytosol; binding causes a conformational change that initiates a sequence of reactions leading to a change in cellular function.
Different cells respond differently to the same ligand. In addition, different receptor-ligand complexes can induce the same biochemical response in some cell types. For example, the hormones glucagon and epinephrine both induce increased glucose breakdown in liver cells.
Some hormones bind to receptors embedded in the plasma membrane at the surface of the cell, while others are able to interact with receptors inside the cell (either in the nucleus or the cytoplasm). The former require the aid of molecules called second messengers, such as [[cyclic AMP], which convey the signal within the cell.
Major classes of vertebrate hormones and their function
Vertebrate hormones may be classified by their chemical make-up. Alternatively, they may be grouped by their solubility and mode of action (i.e., whether they bind to intracellular receptors or to receptors on the cell surface). According to this latter schema, there are three categories of vertebrate hormones:
- Small lipophilic (lipid-soluble) molecules that are able to diffuse across the plasma membrane of the target cell and interact with intracellular receptors of the cytoplasm or [[nucleus. The resulting complexes bind to transcription-control regions in DNA, affecting expression of specific genes. The steroid hormones and thyroxine are two examples of this type.
- Lipophilic molecules that bind to cell-surface receptors, such as the eicosanoids.
- Hydrophilic (water-soluble) molecules that bind to receptors on the cell surface because they cannot diffuse across cell membrane. There are two subgroups: (a) peptide hormones, such as insulin, growth hormone, and glucagon, which range in size from a few amino acids to protein-size compounds and (b) small charged molecules, such as epinephrine and histamine, derived from amino acids, which function as both hormones and neurotransmitters.
Lipophilic molecules that diffuse across the plasma membrane
Cholesterol is an important precursor of the steroid hormones, which produce their physiological effects by binding to steroid hormone receptor proteins inside the cytoplasm of the cell. The combined hormone-receptor complex then moves into the nucleus of the cell, where it binds to specific DNA sequences, causing changes in gene transcription and cell function.
The five major classes of steroids are as follows:
- Androgens (such as testosterone) are responsible for the development of male secondary sex characteristics.
- Glucocorticoids enable animals to respond to stress. They regulate many aspects of metabolism and immune function, and are often prescribed by doctors to reduce inflammatory conditions like asthma and arthritis.
- Mineralocorticoids help maintain blood volume and control renal excretion of electrolytes.
- Estrogens and progestagens are two classes of sex steroids, a subset of the hormones that produce sex differences or support reproduction.
Thyroxine, produced by thyroid cells, also binds to internal receptors. Thyroid hormones stimulate the breakdown of glucose, fats, and proteins by increasing the levels of many enzymes that catalyze these metabolic reactions.
Lipophilic molecules that bind to cell-surface receptors
Eicosanoids are 20-carbon fatty acids derived from arachidonic acid; the group includes prostaglandins, prostacyclins, thromboxanes, and leukotrienes. Eicosanoids are considered local hormones because they are short-lived; they alter the activities in cells where they are synthesized (autocrine signaling) and in nearby cells (paracrine signaling). Prostaglandins may stimulate inflammation, regulate blood flow, control transport, and induce sleep. Aspirin, for example, works as an anti-inflammatory agent by inhibiting the synthesis of prostaglandin.
Hydrophilic molecules that bind to cell-surface receptors
- Peptide hormones consist of chains of amino acids. Examples of small peptide hormones are TRH and vasopressin. Peptides composed of scores or hundreds of amino acids are referred to as proteins. Examples of protein hormones include insulin and growth hormone. More complex protein hormones bear carbohydrate side chains and are called glycoprotein hormones. Luteinizing hormone, follicle-stimulating hormone and thyroid-stimulating hormone are glycoprotein hormones.
- Amino-acid derived hormones include the catecholamines, chemical compounds derived from the amino acid tyrosine. Catecholamines are water soluble and are 50% bound to plasma proteins, so they circulate in the bloodstream. The most abundant catecholamines are epinephrine (adrenaline), norepinephrine (noradrenaline) and dopamine. Released by the adrenal glands in situations of stress, catecholamines cause general physiological changes that prepare the body for physical activity (fight-or-flight response). Some typical effects are increases in heart rate, blood pressure, blood glucose levels, and a general reaction of the sympathetic nervous system.
Histamine, a hormone and neurotransmitter derived from the amino acid histidine, is involved in the dilation of blood vessels.
The table below provides some examples of water-soluble hormones that bind to cell-surface receptors. The size of the hormone is given in amino acids; note that the — have two polypeptide chains of varying lengths.
| Type | Name | Size | Origin | Major effects |
| Peptide | Follicle-stimulating hormone (FSH) | alpha 92, beta 118 | anterior pituitary | Stimulates growth of oocytes and ovarian follices |
| Peptide | Glucagon | 29 | pancreas alpha cells | Stimulates glucose synthesis |
| Peptide | Insulin | a chain 21, b 30 | pancreas beta cells | Regulates glucose uptake; stimulates cell proliferation |
| Peptide | Luteinizing hormone (LH) | 10, beta chain 115 | anterior pituitary | maturation of oocyte; stimulates estrogen and progesterone secretion by ovarian follices |
| Growth factor | nerve growth factor (NGF) | 118 | all tissues innervated by sympathetic neurons | growth and differentiation of sympathetic neurons |
| Growth factor | Epidermal growth factor (EGF) | 53 | salivary and other glands? | Growth of epidermal and other body cells |
| Growth factor | Plactelet-derived growth factor | a: 125; b: 109 | platelets and cells in many other tissues | Proliferation of fibroblasts and other cell types; wound healing |
| Neurohormone | Oxytoxin | 9 | posterior pituitary gland | Stimulation of smooth muscle contraction |
| Neurohormone | Vasopressin | 9 | posterior pituitary gland | Stimulation of water reabsorption in the kidney |
Regulation
An important consideration, dictating the level at which cellular signal transduction pathways are activated in response to a hormonal signal is the effective concentration of hormone-receptor complexes that are formed. Hormone-receptor complex concentrations are effectively determined by three factors:
- The number of hormone molecules available for complex formation
- The number of receptor molecules available for complex formation and
- The binding affinity between hormone and receptor.
The number of hormone molecules available for complex formation is usually the key factor in determining the level at which signal transduction pathways are activated. The number of hormone molecules available being determined by the concentration of circulating hormone, which is in turn influenced by the level and rate at which they are secreted by biosynthetic cells. The number of receptors at the cell surface of the receiving cell can also be varied as can the affinity between the hormone and its receptor.
The endocrine system regulates hormone release and concentration through the negative feedback loop. Increases in hormone activity decrease the production and secretion of that hormone. Similarly, a decrease in activity of a hormone prompts an increase in the production and release of that hormone. The immune system as well as other factors contribute as control factors of hormone secretion. Together, these various mechanisms of control regulate the levels of hormones within the body.
By rate of synthesis
Organisms must be able to respond instantly to many changes in their internal or external environment; such rapid responses are mediated primarily by peptide hormones and catecholamines. Signaling cells that produce them store them in secretory vesicles just under the plasma membrane. All peptide hormones, including insulin, are synthesized as part of a longer propolypeptide, which is cleaved (split) by specific enzymes to generate the active molecule just after it is transported to a secretory vesicle. Because of their hydrophilic (water loving) nature, peptide hormones travel freely in the blood as they dissolve. Peptide hormones mediate short responses that are terminated by their own degradation.
In contrast, steroid-producing cells, like those in the adrenal cortex, store a small supply of hormone precursor; when stimulated, they are converted to active hormone, which then diffuses across the cell membrane into the blood. Because cells store little of the active hormone, release takes from hours to days. Steroids are hydrophobic (water-fearing), so they are transported by carrier proteins, and are not rapidly degraded. Thus, responses to thyroxine and steroid hormones take awhile to occur but effects last much longer (hours to days).
By feedback control
The rate of hormone biosynthesis and secretion is often regulated by a homeostatic negative feedback control mechanism. Such a mechanism depends on factors which influence the metabolism and excretion of hormones. Thus, higher hormome concentration alone cannot trigger the negative feedback mechanism. Negative feedback must be triggered by overproduction of an "effect" of the hormone.
feedback control of hormone levels: feedback circuits, in which changes in the level of one hormone affects the levels of other hormones (esp important in coordinating the complex processes of cell growth and differentiation); one ex. includes the regulation of estrogen and progesterone, steroid hormones that stimulate the growth and diff of cells in the tissue lining the interior of the uterus
By other hormones
One special group of hormones is the trophic 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.
Plant hormones
Plant hormones are internally secreted molecules that typically coordinate the responses of tissues in different areas of the plant to environmental signals, such as light or infection.
Plant hormones are traditionally divided into five major groups, although several additional plant hormones have recently been discovered:
- Auxin was the first plant hormone to be identified; early experiments leading to its discovery were conducted by Charles Darwin in the 1880s. Auxins regulate various aspects of plant development, including cell division and differentiation.
- abscisic acid (ABA): the onset of dormancy?
- cytokinins(CKs): cell division
- ethylene: fruit ripening
- gibberellins (GAs): stem elongation
Non-traditional plant hormones include the brassinolides, plant-specific ‘’steroid hormones’’ involved in developmental processes, and xyz.
Pharmacology and cell biology research
Many hormones and their analogues are used as medication. The most commonly-prescribed hormones are estrogens and progestagens (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" 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. An example is the ability of pharmacologic doses of glucocorticoid to suppress inflammation.
The relation between the endocrine and nervous systems
The endocrine system works in close relation with the nervous system. It links the brain to the organs that control various aspects of the body. In addition, neurohormones are released by specialized groups of neurons in the brain. These function similarly to hormones and are often categorized into three major groups: catecholamines; hypothalamic neurohormones that monitor hormone release from the anterior pituitary; and hypothalamic neurohormones that monitor hormone release from the posterior pituitary. Neuroendocrinology is an area of medicine that focuses on the overlapping fields between the nervous and endocrine systems.
Tables
[retain table?] Spelling is not uniform for many hormones. For example, current North American and international usage is estrogen, gonadotropin, while British usage retains the Greek diphthong in oestrogen and the unvoiced aspirant h in gonadotrophin.
| Structure | Name | Abbreviation | Tissue | Cells | Mechanism |
| amine - tryptophan | Melatonin (N-acetyl-5-methoxytryptamine) | pineal gland | pinealocyte | ||
| amine - tryptophan | Serotonin | 5-HT | CNS, GI tract | enterochromaffin cell | |
| amine - tyrosine | Thyroxine (thyroid hormone) | T4 | thyroid gland | thyroid epithelial cell | direct |
| amine - tyrosine | Triiodothyronine (thyroid hormone) | T3 | thyroid gland | thyroid epithelial cell | direct |
| amine - tyrosine (cat) | Epinephrine (or adrenaline) | EPI | adrenal medulla | chromaffin cell | |
| amine - tyrosine (cat) | Norepinephrine (or noradrenaline) | NRE | adrenal medulla | chromaffin cell | |
| amine - tyrosine (cat) | Dopamine | DPM | hypothalamus | ||
| peptide | Antimullerian hormone (or mullerian inhibiting factor or hormone) | AMH | testes | Sertoli cell | |
| peptide | Adiponectin | Acrp30 | adipose tissue | ||
| peptide | Adrenocorticotropic hormone (or corticotropin) | ACTH | anterior pituitary | corticotrope | cAMP |
| peptide | Angiotensinogen and angiotensin | AGT | liver | IP3 | |
| peptide | Antidiuretic hormone (or vasopressin, arginine vasopressin) | ADH | posterior pituitary | varies | |
| peptide | Atrial-natriuretic peptide (or atriopeptin) | ANP | heart | cGMP | |
| peptide | Calcitonin | CT | thyroid gland | parafollicular cell | cAMP |
| peptide | Cholecystokinin | CCK | duodenum | ||
| peptide | Corticotropin-releasing hormone | CRH | hypothalamus | cAMP | |
| peptide | Erythropoietin | EPO | kidney | ||
| peptide | Follicle-stimulating hormone | FSH | anterior pituitary | gonadotrope | cAMP |
| peptide | Gastrin | GRP | stomach, duodenum | G cell | |
| peptide | Ghrelin | stomach | P/D1 cell | ||
| peptide | Glucagon | GCG | pancreas | alpha cells | cAMP |
| peptide | Gonadotropin-releasing hormone | GnRH | hypothalamus | IP3 | |
| peptide | Growth hormone-releasing hormone | GHRH | hypothalamus | IP3 | |
| peptide | Human chorionic gonadotropin | hCG | placenta | syncytiotrophoblast cells | cAMP |
| peptide | Human placental lactogen | HPL | placenta | ||
| peptide | Growth hormone | GH or hGH | anterior pituitary | somatotropes | |
| peptide | Inhibin | testes | Sertoli cells | ||
| peptide | Insulin | INS | pancreas | beta cells | tyrosine kinase |
| peptide | Insulin-like growth factor (or somatomedin) | IGF | liver | tyrosine kinase | |
| peptide | Leptin | LEP | adipose tissue | ||
| peptide | Luteinizing hormone | LH | anterior pituitary | gonadotropes | cAMP |
| peptide | Melanocyte stimulating hormone | MSH or α-MSH | anterior pituitary/pars intermedia | cAMP | |
| peptide | Oxytocin | OXT | posterior pituitary | IP3 | |
| peptide | Parathyroid hormone | PTH | parathyroid gland | parathyroid chief cell | cAMP |
| peptide | Prolactin | PRL | anterior pituitary | lactotrophs | |
| peptide | Relaxin | RLN | varies | ||
| peptide | Secretin | SCT | duodenum | S cell | |
| peptide | Somatostatin | SRIF | hypothalamus, islets of Langerhans | delta cells | |
| peptide | Thrombopoietin | TPO | liver, kidney | ||
| peptide | Thyroid-stimulating hormone | TSH | anterior pituitary | thyrotropes | cAMP |
| peptide | Thyrotropin-releasing hormone | TRH | hypothalamus | IP3 | |
| steroid - glu. | Cortisol | adrenal cortex (zona fasciculata) | direct | ||
| steroid - min. | Aldosterone | adrenal cortex (zona glomerulosa) | direct | ||
| steroid - sex (and) | Testosterone | testes | Leydig cells | direct | |
| steroid - sex (and) | Dehydroepiandrosterone | DHEA | multiple | direct | |
| steroid - sex (and) | Androstenedione | adrenal glands, gonads | direct | ||
| steroid - sex (and) | Dihydrotestosterone | DHT | multiple | direct | |
| steroid - sex (est) | Estradiol | E2 | ovary | granulosa cells | direct |
| steroid - sex (est) | Estrone | ovary | granulosa cells | direct | |
| steroid - sex (est) | Estriol | placenta | syncytiotrophoblast | direct | |
| steroid - sex (pro) | Progesterone | ovary, adrenal glands, placenta | granulosa cells | direct | |
| sterol | Calcitriol (Vitamin D3) | skin/proximal tubule of kidneys | direct | ||
| eicosanoid | Prostaglandins | PG | seminal vesicle | ||
| eicosanoid | Leukotrienes | LT | white blood cells | ||
| eicosanoid | Prostacyclin | PGI2 | endothelium | ||
| eicosanoid | Thromboxane | TXA2 | platelets |
ReferencesISBN links support NWE through referral fees
- Beato, M., Chavez, S., and M. Truss. 1996. Transcriptional regulation by steroid hormones. Steroids 61(4): 240-251. PMID 8733009
- Cooper, G. M., and R. E. Hausman. 2004. The Cell: A Molecular Approach, 3rd edition. Washington, D.C.: ASM Press & Sunderland, M.A.: Sinauer Associates. ISBN 0878932143
- Hammes, S.R. 2003. The further redefining of steroid-mediated signaling. Proc Natl Acad Sci 100(5): 2168-70 PMID 12606724
- Lodish, H., D. Baltimore, A. Berk, S. L. Zipursky, P. Matsudaira, and J. Darnell. 1995. Molecular Cell Biology, 3rd edition. New York, NY: Scientific American Books. ISBN 0716723808.
- Mathews, C.K. and K.E. van Holde. 1990. Biochemistry. San Francisco: Benjamin-Cummings. ISBN 0805350152
- Stryer, L. 1995. Biochemistry, 4th edition. New York: W.H. Freeman. ISBN 0716720094
External links
- MeSH Hormones
| Hormones and endocrine glands - edit |
|---|
|
Hypothalamus: GnRH - TRH - CRH - GHRH - somatostatin - dopamine | Posterior pituitary: vasopressin - oxytocin | Anterior pituitary: GH - ACTH - TSH - LH - FSH - prolactin - MSH - endorphins - lipotropin Thyroid: T3 and T4 - calcitonin | Parathyroid: PTH | Adrenal medulla: epinephrine - norepinephrine | Adrenal cortex: aldosterone - cortisol - DHEA | Pancreas: glucagon- insulin - somatostatin | Ovary: estradiol - progesterone - inhibin - activin | Testis: testosterone - AMH - inhibin | Pineal gland: melatonin | Kidney: renin - EPO - calcitriol - prostaglandin | Heart atrium: ANP Stomach: gastrin | Duodenum: CCK - GIP - secretin - motilin - VIP | Ileum: enteroglucagon | Liver: IGF-1 Placenta: hCG - HPL - estrogen - progesterone Adipose tissue: leptin, adiponectin Target-derived NGF, BDNF, NT-3 |
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