See also Hormones, generally  See also Signal transduction

Hormones may be categorized as lipophilic (fat-soluble), which are nonpolar, or hydrophilic (water-soluble), which are polar. The lipophilic hormones include the steroid hormones and the thyroid hormones.

Hydrophilic hormones are freely soluble in blood, but cannot pass through the plasma membrane of target cells. Their receptors are transmembrane proteins that bind to the hormone outside the cell, and transmit the signal to the interio. In contrast, lipophilic hormones travel in the blood bound to soluble transport proteins. But when they reach a target cell, they can cross the plasma mebrane and bind to intracellular receptors.

Protein Hormones: 

Prolactin (PRL): is released by the anterior pituitary and sitmulates the mammary glands to produce milk in mammals. It also has diverse effects on many other targets, icnluding reulgaiton of ion and water transport across epitheila and activation of parental behaviors.

Glycoprotein hormones: 

Glycoprotein hormones are a group of evolutionary conserved hormones involved in the regulation of reproduction and metabolism. Structurally, the glycoprotein hormones are related heterodimers comprised of a common alpha subunit and a hormone specific beta subunit. This family of hormones includes the following members:

Follicle-stimulating hormone (FSH): is produced by the anterior pituitary and is required for the devleopment of ovarian folicles in feamles. In males, it is required for the development of sperm. FSH stimultes the conversion of testosterone into estrogen in females, and into dihydroxytestosterone in males. FSH adn LH below are collectively referred to as gonadotropins.

Growth Hormone (somatotropin): is released by the anterior pituitary and stimulates the growth of muscle, bond (indirectly) and other tissues, and is also essential for proper metabolic regulation.

Luteinizing hormone (LH): stimulates the production of estrogen and progesterone by the ovaries and is needed for ovulation in femal reproductive cycles. In males, it stimules the testes to produce testosterone, which is needed for sperm production and for the development of male secondary sexual characteristics. It is produced by the anterior pituitary.

Thyroid stimulating hormone (TSH or thyrotropin): TSH is a 28-30 kDa heterodimeric glycoprotein produced in the thyrtrophs of the anterior pituitary gland. TSH stimualtes the thyroid gland to produce the hormone thyroxin, which in turn regulates development and metabolism by acting on nuclear receptors. This hormone controls thyroid function by interacting with the G protein coupled TSH receptor which leads to the stimulation of pathways involving secondary messenger molecules, such as cyclic adenosine 3’5′-monophosphate (cAMP), and ultimately results in the modulation of thyroidal gene expression. Physiological roles of TSH include stimulation of differentiated thyroid functions, such as iodine uptake and the release of thyroid hormone from the gland, and promotion of thyroid growth.

Chorionic gonadotrophin (CG)

Amino Acid Derived Hormones:

Amino acid derivatives are hormones manufactured by enzymatic modificaiton of specific amino acids. They include hormones secreted by the adrenal medulla (the inner portion of the adrenal gland), thyroid and pineal glands. Those secreted by the adrenal medulla are derived form tyrosine. Known as catecholamines, they include epinephrine (adrenaline) or norepinephrine (noradrenaline). Other hormones derived from Trrosine are the thyroid hormones, created by the thyroid gland. The pineal gland secretes a different amine hormone, melatonin, derived form tryptophan.

Norepinephrine (noradrenaline): is an amino acid derived hormone (a catecholamine). Noreipnephrine is secreted into the blood by the adrenal glands, but it is also released as a neurotransmitter by sympathetic nerve endings. Norepinephrine acts as a hormone to coordinate the activity of the heart, liver, and blood vessels druing reponses to stress.

Epinephrine (adrenaline) (see below) and norepinephrine (noradrenaline) are both hormones and neurotransmitters involved in the “fight-or-flight” response, but they differ in their primary actions and effects on the body, with epinephrine primarily affecting the heart and lungs, while norepinephrine mainly focuses on blood vessels.

Peptide Hormones:

Adrenocorticotropic hormone (ACTH or corticotropin) stimulates the adrenal cortex to produce corticosteroid hormones, including cortisol (in humans) and corticosterone (in many other vertibrates). These hormones regulate glucose homeostasis and are important in teh response to stress.

Glucagon: is a hormone that mobilizes glucose as part of the body’s mechanism to control blood glucose. This involves breaking down stored glycogen into glucose and turning on the genes that encode the enymes necessary to synthesize glucose.

Insulin: lowers blood glucose level, stimulates glycogen, fat, and protein synthesis.

Insulin is a hormone that helps regulate the body’s metabolism and blood sugar levels. It is produced by the pancrease and released into the blood when blood sugar levels rise, such as after eating. Insulin assists glucose to enter the cells, where it is used for energy. While glucagon keeps blood glucose from dropping too low, insulin is produced to keep blood glucose from rising too high.

Insulin is a peptide hormone secreted in the body by beta cells of islets of Langerhans of the pancreas and regulates blood glucose levels. Medical treatment with insulin is indicated when there is inadequate production of increased insulin demands in the body. In Type 1 or juvenile diabetes mellitus, pancreatic beta cells are destroyed by the body’s immune system or trauma or injury to the pancreas, leading to decreased or absent insulin production. Patients with type 1 diabetes always require insulin. Type 2 diabetes mellitus is the most common type of diabetes and usually occurs after 45 years. In type 2 diabetes, cells are resistant to the action of insulin. As a result, glucose starts building up in the blood. Treatment with insulin is necessary for the later stages of type 2 diabetes.

Insulin acts by directly binding to its receptors on the plasma membranes of the cells. These receptors are present on all the cells, but their density depends on the type of cells, with the maximum density on hepatic cells and adipocytes.

The role of insulin is thus to lower blood glucose, acting by binding to an RTK. The insulin receptor is a heterotetrameric glycoprotein consisting of two subunits, the alpha and beta subunits. The extracellular alpha subnunits have insulin binding sites. The beta subunits, which are transmembranous, have tyrosine kinase activity. When insulin binds to the alpha subunits, it activates the tyrosine kinase activity in the beta subunit, which causes the translocation of glucose transporters from the cytoplasm to the cell’s surface. Another protein called the insulin response protein binds to the phosphorylated receptor and is itself phosphrylated. The insulin response protein passes the signal on by binding to additional proteins that lead to the activation of the enzyme glycogen synthase, which converts glucose to glycogen, thereby lowering blood glucose.  Other proteins activated by the insulin receptor act to inhibit the synthesis of enzymes involved in making glucose, and to increase the number o glucose transporter proteins in the plasma membrane.

Epinephrine (aka, adrenaline): has diverse effects on different cell types. The distinct effects of epinephrine on different cell types depend on each cell having the same receptor for the hormone, but responding with different signal transduction pathways. In the liver, cells are stimulated to mobilized glucose, while in the heart muscle cells contract more forcefully to increase blood flow. (the fright response is due to release of epienphrine, also called adrenaline, into the bloodstream. Epinephrine receptors in the liver produce cAMP thorugh the enzyme adenylyl cyclase. The cAMP produced activates protein kinases that promote the produciton of glucose from glycogen. In smooth muscle, epinephrine receptors can activate a different G protein that activates the IP3 generating enzyme phospholipase C. As a result, epinephrine sitmulation of smooth muscle results in IP3-regulated release of intracellular calcium, causing muscle contraction.

The sympathetic nervous system (SNS) plays a vital role in maintaining the homeostasis of the body by secreting different neurotransmitters like catecholamines, acetylcholine, glutamine, etc. Among all the neurotransmitters, categholamine has a very important and diversified role in the various organs. Catecholamines contain a categhol (3,4 dihydroxypheynl) group along with an amine group. John Jacob Abel, in 1897 first obtained a crystalline substance form the adrenal gland of sheep in an impure form that can regulated blood pressure, and he named it epinephrine. In 1900, Jokichi Takamine obtained a pure cyrstalline form of epinephrine. He patented it with the name adrenalin. British Approved Name (BAN) introduced the name “adrenaline” in the UK. US Approved Name (USAN) used the term “epinephrine” for this neurotransmitter in the USA. Adrenaline is synthesized form its precursor amino acid tyrosine; it is also synthesized form hepatic hydroxylation of another amino acid, phenylalanine. Synthesis of catecholamine beings with the rate limiting step governed by the enzyme tyrosine hydroxylase, which converts tyrosine into L-DOPA. Subsequently, L-DOPA is coverted to dopamine and then to norepinephrine by DOPA decarboxylase and dopoamine beta-hydroxylase, respectively. Dopamine beta-hydroxylase is a copper containg enzyem and requires ascorbic acid for its function. Both epinephrine and norepinephrine stimulate common receptors name adrenergic receptors. Structurally these receptors have seven hydrophobic transmembrane regions and an intracellular C-terminal domain and an extracellular N-terminalal domain, along with 3 intracellular and extracellular loops. All adrenergic receptros work through different G prtoeins like Gs, Gi,e tc. Beta2 adrenergic receptor is the most abundant and most widely studied adrengergic receptor. (lal, Role of adrenergic receptor signalling in neuroimmune communication” Current Research in Immunology, 2 (2021) 202-217).

Erythropoietin (EPO) is a hormone that belongs to the class of peptide hormones, which are molecules composed of amino acids that act as signaling molecules. It is a naturally occurring hormone which stimulates the production of red blood cells in the bone marrow through a process called erythropoiesis. The production of EPO is useful in treating blood disorders characterized by low hematocrit, which is a low ratio fo red blood cells to total blood cells. Erythropoeisis (EPO): is a process stimulated by the peptide hormone erythropoietin (EPO). Erythropoiesis is the process of red blood cell production, and EPO, produced by the kidneys, stimulates this process. 

Melanocyte-stimulating hormone (MSH) stimulates the snthesis and dispersion of melanin pigment, which darkens the epidermis of some fish, amphibians and reptibles and can control hair pigment color in mammals. It is a peptide hormone of the anterio pituitary.

Parathyroid hormone (PTH): raises blood calcium level by stimulating bone breakdown, stimualtes calcium reabsorption in kidneys and activates vitamin D. Its target tissue is the bone, kidneys and digestive tract.

Steroid Hormones:

Steroid hormones form a large class of compounds, including cortisol, estrogen, progresterone and testosterone, that share a common nonpolar structure. The nonpolar structure allows these hormones to cross the membrane and bind to intracellular receptors.

Steroids are lipids manufactured by enzymatic modifications of cholesterol. Steroid hormones can be subdivided into sex steroids, secreted by the testes, ovaries, placenta, and adrenal cortex, and corticosteroids (mineralocorticoids and cortisol), secreted only by the adrenal cortex.

Estrogens: 

The hormone estrogen has different effects in uterine tissue than in mammary tissue. This differential response is meiated by coactivators and not by the presence or absence of a receptor in the two tissues. In mammary tissue, a critical coactivtor is lacking and the hormone-receptor complex instead interacts with another protein that acts to reduce gene expression. In uterine tissue, the coactivator is present, and can bind the hormone-receptor complex to turn on the expression of genes that encode prtoeins involved in prearing the uterus for pregnancy.

Glucocorticoids are steroid hormones produced from the cortex of adrenal glands. Glcocorticoids are important modualtors of brain function and hypothalamus-pituitary-adrenal (HPA)-axis activity. To reach their central target areas they have to enter the brain by passing the blood-brain barrier (BBB), a dynamic barrier that protects the brain from periopheral influences.

–Cortisol: is a steroid hormone in the glucocorticoid class of hormones and a stress hormone. As medication, cortisol is called hydrocortisone.

Cortisol is a hormone that is produced by the adrenal glands, located on top of the kidneys. The pituitary gland, located in the brain, regulates the amount of cortisol released by the adrenal glands. Cortisol can increase levels of glucose in cells. A number of different genes involved in the synthesis of glucose have binding sties for the hormone receptor complex.

Cortisol prevents the release of substances in the body that cause inflammation. It is thus known to inhibit IL-12, IFN-gamma, IFN-alpha dn TNF-alpha by APCs and TH1 cells. It upregulates IL-4, IL-10 and IL-13 by TH2 cells. This resutls in a shift toward a Th2 immune response.

Testosterone: stimulates development of secondary sex characteristics in males and growh spurt at puberty.