A hormone (from Greek Greek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical ancient Greek literature and the New Testament of ὁρμή - "impetus") is a chemical In chemistry, a chemical substance is a material with a specific chemical composition released by a cell in one part of the body, that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism Metabolism is the set of chemical reactions that happen in living organisms to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories. Catabolism breaks down organic matter, for example to harvest energy in cellular. It is essentially a chemical messenger that transports a signal from one cell to another. All multicellular organisms Multicellular organisms are organisms that consist of more than one cell, and have differentiated cells that perform specialized functions in the organism. Most life that can be seen with the naked eye is multicellular, as are all members of the kingdoms Planimalia produce hormones; plant Plants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants, bryophytes, ferns and fern allies. As of 2004, hormones are also called phytohormones Plant hormones are chemicals that regulate plant growth, which, in the UK, are termed 'plant growth substances'. Plant hormones are signal molecules produced within the plant, and occur in extremely low concentrations. Hormones regulate cellular processes in targeted cells locally and when moved to other locations, in other locations of the plant. Hormones in animals Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also are often transported in the blood. Cells respond to a hormone when they express Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as rRNA genes or tRNA genes, the product is a functional RNA. The process of gene expression is used by all known life - eukaryotes , prokaryotes (bacteria a specific receptor In biochemistry, a receptor is a protein molecule, embedded in either the plasma membrane or the cytoplasm of a cell, to which one or more specific kinds of signaling molecules may attach. A molecule which binds to a receptor is called a ligand, and may be a peptide (short protein) or other small molecule, such as a neurotransmitter, a hormone, a for that hormone. The hormone binds to the receptor protein Proteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues. The sequence of amino acids in a protein is defined by the sequence of a gene, which is encoded, resulting in the activation of a signal transduction In biology, signal transduction is a mechanism that converts a mechanical or chemical stimulus to a cell into a specific cellular response. Signal transduction starts with a signal to a receptor, and ends with a change in cell function mechanism that ultimately leads to cell type-specific responses.

Endocrine In animal anatomy the endocrine system is a system of glands, each of which secretes a type of hormone into the bloodstream to regulate the body. The endocrine system is an information signal system like the nervous system. Hormones regulate many functions of an organism, including mood, growth and development, tissue function, and metabolism. The hormone molecules A molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds. Molecules are distinguished from polyatomic ions in this strict sense. In organic chemistry and biochemistry, the term molecule is used less strictly and also is applied to charged organic molecules are secreted (released) directly into the bloodstream The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc. to and from cells in the body to help fight diseases and help stabilize body temperature and pH to maintain homeostasis, while exocrine hormones Exocrine glands are glands that secrete their products into ducts (duct glands) which lead directly into the external environment. They are the counterparts to endocrine glands, which secrete their products (hormones) directly into the bloodstream (ductless glands) or release hormones (paracrines) that affect only target cells nearby the release (or ectohormones) are secreted directly into a duct, and from the duct they either flow into the bloodstream or they flow from cell to cell by diffusion Diffusion describes the spread of particles through random motion from regions of higher concentration to regions of lower concentration. The time dependence of the statistical distribution in space is given by the diffusion equation. The concept of diffusion is tied to notion of mass transfer, driven by a concentration gradient, but diffusion can in a process known as paracrine signalling Paracrine signaling is a form of cell signaling in which the target cell is near the signal-releasing cell.

Hormones as a signal

Hormonal signaling involves the following:^{[citation needed]}

1. Biosynthesis Biosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step. Examples for such multi-step biosynthetic pathways are those for the of a particular hormone in a particular tissue
2. Storage and secretion Exocytosis is the durable process by which a cell directs the contents of secretory vesicles out of the cell membrane. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane of the hormone
3. Transport of the hormone to the target cell(s)
4. Recognition of the hormone by an associated cell membrane A membrane protein is a protein molecule that is attached to, or associated with the membrane of a cell or an organelle. More than half of all proteins interact with membranes or intracellular In cell biology, molecular biology and related fields, the word intracellular means "inside the cell" receptor In biochemistry, a receptor is a protein molecule, embedded in either the plasma membrane or the cytoplasm of a cell, to which one or more specific kinds of signaling molecules may attach. A molecule which binds to a receptor is called a ligand, and may be a peptide (short protein) or other small molecule, such as a neurotransmitter, a hormone, a protein.
5. Relay and amplification of the received hormonal signal via a signal transduction In biology, signal transduction is a mechanism that converts a mechanical or chemical stimulus to a cell into a specific cellular response. Signal transduction starts with a signal to a receptor, and ends with a change in cell function 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 down-regulation Downregulation is the process by which a cell decreases the quantity of a cellular component, such as RNA or protein, in response to an external variable. An increase of a cellular component is called upregulation in hormone production. This is an example of a homeostatic Homeostasis is the property of a system, either open or closed, that regulates its internal environment and tends to maintain a stable, constant condition. Typically used to refer to a living organism, the concept came from that of milieu interieur that was created by Claude Bernard and published in 1865. Multiple dynamic equilibrium adjustment negative feedback loop Negative feedback occurs when the output of a system acts to oppose changes to the input of the system; with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.
6. Degradation of the hormone.

Hormone cells are typically of a specialized cell type, residing within a particular endocrine gland In animal anatomy the endocrine system is a system of glands, each of which secretes a type of hormone into the bloodstream to regulate the body. The endocrine system is an information signal system like the nervous system. Hormones regulate many functions of an organism, including mood, growth and development, tissue function, and metabolism. The, such as thyroid gland The thyroid is one of the largest endocrine glands in the body. This gland is found in the neck, inferior to the thyroid cartilage (also known as the Adam's apple) and at approximately the same level as the cricoid cartilage. The thyroid controls how quickly the body uses energy, makes proteins, and controls how sensitive the body should be to, ovaries The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in females are homologous to testes in males, in that they are both gonads and endocrine glands, and testes. Hormones exit their cell of origin via exocytosis Exocytosis , also known as 'reverse pino-cytosis',is the durable process by which a cell directs the contents of secretory vesicles out of the cell membrane. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell or another means of membrane transport The cell membrane is one biological membrane separating the interior of a cell from the outside environment. 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 Insulin is a hormone that is central to regulate energy and glucose metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle, which triggers a diverse range of systemic physiological affects. Different tissue types may also respond differently to the same hormonal signal. Because of this, hormonal signaling is elaborate and hard to dissect.^{[citation needed]}

Interactions with receptors

Most hormones initiate a cellular response by initially combining with either a specific intracellular In cell biology, molecular biology and related fields, the word intracellular means "inside the cell" or cell membrane associated A membrane protein is a protein molecule that is attached to, or associated with the membrane of a cell or an organelle. More than half of all proteins interact with membranes receptor In biochemistry, a receptor is a protein molecule, embedded in either the plasma membrane or the cytoplasm of a cell, to which one or more specific kinds of signaling molecules may attach. A molecule which binds to a receptor is called a ligand, and may be a peptide (short protein) or other small molecule, such as a neurotransmitter, a hormone, a protein. A cell may have several different receptors that recognize the same hormone and activate different signal transduction In biology, signal transduction is a mechanism that converts a mechanical or chemical stimulus to a cell into a specific cellular response. Signal transduction starts with a signal to a receptor, and ends with a change in cell function pathways, or alternatively different hormones and their receptors may invoke the same biochemical pathway.

For many hormones, including most protein hormones, the receptor is membrane associated and embedded in the plasma membrane The cell membrane is one biological membrane separating the interior of a cell from the outside environment at the surface of the cell. The interaction of hormone and receptor typically triggers a cascade of secondary effects within the cytoplasm The cytoplasm is the part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the contents of the cell nucleus are not part of the cytoplasm and are instead called the nucleoplasm. Also in eukaryotic cells, the cytoplasm contains organelles, such as mitochondria, which are filled with liquid that is kept separate from the of the cell, often involving phosphorylation Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes, causing or preventing the mechanisms of diseases such as cancer and diabetes or dephosphorylation of various other cytoplasmic proteins, changes in ion channel Ion channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of all living cells by allowing the flow of ions down their electrochemical gradient. They are present in the membranes that surround all biological cells. The study of ion channels involves many scientific techniques such permeability, or increased concentrations of intracellular molecules that may act as secondary messengers In cell physiology, a secondary messenger system is a method of cellular signaling whereby a diffusable signaling molecule is rapidly produced/secreted, which can then go on to activate effector proteins within the cell to exert a cellular response. Secondary messengers are a component of signal transduction cascades (e.g. cyclic AMP Cyclic adenosine monophosphate is a second messenger important in many biological processes. cAMP is derived from adenosine triphosphate (ATP) and used for intracellular signal transduction in many different organisms, conveying the cAMP-dependent pathway). Some protein hormones also interact with intracellular In cell biology, molecular biology and related fields, the word intracellular means "inside the cell" receptors located in the cytoplasm The cytoplasm is the part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the contents of the cell nucleus are not part of the cytoplasm and are instead called the nucleoplasm. Also in eukaryotic cells, the cytoplasm contains organelles, such as mitochondria, which are filled with liquid that is kept separate from the or nucleus In cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes by an intracrine Intracrine refers to a hormone that acts inside a cell. Steroid hormones act through intracellular receptors and are thus considered as intracrines. In contrast, peptide or protein hormones generally act as endocrines, autocrines or paracrines by binding to their receptors present on the cell surface. Several peptide/protein hormones or their mechanism.

For hormones such as steroid Steroid hormones are steroids that act as hormones. Steroid hormones can be grouped into five groups by the receptors to which they bind: glucocorticoids, mineralocorticoids, androgens, estrogens, and progestagens. Vitamin D derivatives are a sixth closely related hormone system with homologous receptors, though technically sterols rather than or thyroid The thyroid hormones, thyroxine and triiodothyronine (T3), are tyrosine-based hormones produced by the thyroid gland primarily responsible for regulation of metabolism. An important component in the synthesis of thyroid hormones is iodine. The major form of thyroid hormone in the blood is thyroxine (T4), which has a longer half life than T3. The hormones, their receptors are located intracellularly In cell biology, molecular biology and related fields, the word intracellular means "inside the cell" within the cytoplasm The cytoplasm is the part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the contents of the cell nucleus are not part of the cytoplasm and are instead called the nucleoplasm. Also in eukaryotic cells, the cytoplasm contains organelles, such as mitochondria, which are filled with liquid that is kept separate from the of their target cell. To bind their receptors these hormones must 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, effectively amplifying or suppressing the action of certain genes, and affecting protein synthesis.^[1] However, it has been shown that not all steroid receptors are located intracellularly. some are associated with the plasma membrane.^[2]

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:

1. The number of hormone molecules available for complex formation
2. The number of receptor molecules available for complex formation and
3. 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.

Physiology of hormones

Most cells are capable of producing one or more molecules, which act as signaling molecules to other cells, altering their growth, function, or metabolism. The classical hormones produced by cells in the endocrine glands mentioned so far in this article are cellular products, specialized to serve as regulators at the overall organism level. However they may also exert their effects solely within the tissue in which they are produced and originally released.

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.

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.

A recently identified class of hormones is that of the "hunger hormones" - ghrelin, orexin and PYY 3-36 - and "satiety hormones" - e.g., leptin, obestatin, nesfatin-1.

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.

Effects of hormone

Hormones have the following effects on the body:

• stimulation or inhibition of growth
• mood swings
• induction or suppression of apoptosis (programmed cell death)
• activation or inhibition of the immune system
• regulation of metabolism
• preparation of the body for mating, fighting, fleeing, and other activity
• preparation of the body for a new phase of life, such as puberty, parenting, and menopause
• control of the reproductive cycle
• hunger cravings

A hormone may also regulate the production and release of other hormones. Hormone signals control the internal environment of the body through homeostasis.

Chemical classes of hormones

Vertebrate hormones fall into three chemical classes:

• Amine-derived hormones are derivatives of the amino acids tyrosine and tryptophan. Examples are catecholamines and thyroxine.
• 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.
• Lipid and phospholipid-derived hormones derive from lipids such as linoleic acid and arachidonic acid and phospholipids. The main classes are the steroid hormones that derive from cholesterol and the eicosanoids. Examples of steroid hormones are testosterone and cortisol. Sterol hormones such as calcitriol are a homologous system. The adrenal cortex and the gonads are primary sources of steroid hormones. Examples of eicosanoids are the widely studied prostaglandins.

Pharmacology

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.

Important human hormones

See: List of human hormones

See also

• Endocrinology
• Endocrine system
• Neuroendocrinology
• Plant hormones or plant growth regulators
• Autocrine signaling
• Paracrine signaling
• Intracrine
• Cytokine
• Growth factor
• Hormone disruptor

References

1. ^ Beato M, Chavez S and Truss M (1996). "Transcriptional regulation by steroid hormones". Steroids 61 (4): 240–251. doi:10.1016/0039-128X(96)00030-X. PMID 8733009.
2. ^ Hammes SR (2003). "The further redefining of steroid-mediated signaling". Proc Natl Acad Sci USA 100 (5): 21680–2170. doi:10.1073/pnas.0530224100. PMID 12606724.

External links

• The Hormone Foundation
• Article on hormones and their receptors
• HMRbase: A database of hormones and their receptors
• MeSH Hormones

Categories: Hormones | Physiology | Endocrinology | Cell signaling | Signal transduction | Greek loanwords

See Also:

• Adrenocorticotropic Hormone: Encyclopedia
• Corticotropin-releasing Hormone: Encyclopedia

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