Insulin is a peptide hormone produced by beta cells of the pancreatic islets encoded in humans by the insulin (INS) gene. It is the main anabolic hormone of the body. It regulates the metabolism of carbohydrates, fats, and protein by promoting the absorption of glucose from the blood into cells of the liver, fat, and skeletal muscles. In these tissues the ab
IdentifiersAliasesINS, IDDM, IDDM1, IDDM2, ILPR, IRDN, MODY10, insulin, PNDM4External IDsOMIM: 176730; MGI: 96573; HomoloGene: 173; GeneCards: INS; OMA:INS - orthologsMore reference expression dataBioGPSEnsemblUniProtRefSeq (mRNA)RefSeq (protein)Location (UCSC)Chr 11: 2.16 – 2.16 MbChr 7: 142.23 – 142.3 MbPubMed search[3][4]Wikidata
Insulin (/ˈɪn.sjʊ.lɪn/ ⓘ;[5][6] from Latin insula 'island') is a peptide hormone produced by beta cells of the pancreatic islets encoded in humans by the insulin (INS) gene. It is the main anabolic hormone of the body.[7] It regulates the metabolism of carbohydrates, fats, and protein by promoting the absorption of glucose from the blood into cells of the liver, fat, and skeletal muscles.[8] In these tissues the absorbed glucose is converted into either glycogen, via glycogenesis, or fats (triglycerides), via lipogenesis; in the liver, glucose is converted into both.[8] Glucose production and secretion by the liver are strongly inhibited by high concentrations of insulin in the blood.[9] Circulating insulin also affects the synthesis of proteins in a wide variety of tissues. It is thus an anabolic hormone, promoting the conversion of small molecules in the blood into large molecules in the cells. Low insulin in the blood has the opposite effect, promoting widespread catabolism, especially of reserve body fat.
Beta cells are sensitive to blood sugar levels so that they secrete insulin into the blood in response to high level of glucose, and inhibit secretion of insulin when glucose levels are low.[10] Insulin production is also regulated by glucose: high glucose promotes insulin production while low glucose levels lead to lower production.[11] Insulin enhances glucose uptake and metabolism in the cells, thereby reducing blood sugar. Their neighboring alpha cells, by taking their cues from the beta cells,[10] secrete glucagon into the blood in the opposite manner: increased secretion when blood glucose is low, and decreased secretion when glucose concentrations are high. Glucagon increases blood glucose by stimulating glycogenolysis and gluconeogenesis in the liver.[8][10] The secretion of insulin and glucagon into the blood in response to the blood glucose concentration is the primary mechanism of glucose homeostasis.[10]
Decreased or absent insulin activity results in diabetes, a condition of high blood sugar level (hyperglycaemia). There are two types of the disease. In type 1 diabetes, the beta cells are destroyed by an autoimmune reaction so that insulin can no longer be synthesized or be secreted into the blood.[12] In type 2 diabetes, the destruction of beta cells is less pronounced than in type 1, and is not due to an autoimmune process. Instead, there is an accumulation of amyloid in the pancreatic islets, which likely disrupts their anatomy and physiology.[10] The pathogenesis of type 2 diabetes is not well understood but reduced population of islet beta-cells, reduced secretory function of islet beta-cells that survive, and peripheral tissue insulin resistance are known to be involved.[7] Type 2 diabetes is characterized by increased glucagon secretion which is unaffected by, and unresponsive to the concentration of blood glucose. But insulin is still secreted into the blood in response to the blood glucose.[10] As a result, glucose accumulates in the blood.
The human insulin protein is composed of 51 amino acids, and has a molecular mass of 5808 Da. It is a heterodimer of an A-chain and a B-chain, which are linked together by disulfide bonds. Insulin's structure varies slightly between species of animals. Insulin from non-human animal sources differs somewhat in effectiveness (in carbohydrate metabolism effects) from human insulin because of these variations. Porcine insulin is especially close to the human version, and was widely used to treat type 1 diabetics before human insulin could be produced in large quantities by recombinant DNA technologies.[13][14][15][16]
Insulin was the first peptide hormone discovered.[17] Frederick Banting and Charles Best, working in the laboratory of John Macleod at the University of Toronto, were the first to isolate insulin from dog pancreas in 1921. Frederick Sanger sequenced the amino acid structure in 1951, which made insulin the first protein to be fully sequenced.[18] The crystal structure of insulin in the solid state was determined by Dorothy Hodgkin in 1969. Insulin is also the first protein to be chemically synthesised and produced by DNA recombinant technology.[19] It is on the WHO Model List of Essential Medicines, the most important medications needed in a basic health system.[20]
Evolution and species distribution
Insulin may have originated more than a billion years ago.[21] The molecular origins of insulin go at least as far back as the simplest unicellular eukaryotes.[22] Apart from animals, insulin-like proteins are also known to exist in fungi and protists.[21]
Insulin is produced by beta cells of the pancreatic islets in most vertebrates and by the Brockmann body in some teleost fish.[23] Cone snails: Conus geographus and Conus tulipa, venomous sea snails that hunt small fish, use modified forms of insulin in their venom cocktails. The insulin toxin, closer in structure to fishes' than to snails' native insulin, slows down the prey fishes by lowering their blood glucose levels.[24][25]
Production
Insulin is produced exclusively in the beta cells of the pancreatic islets in mammals, and the Brockmann body in some fish. Human insulin is produced from the INS gene, located on chromosome 11.[26] Rodents have two functional insulin genes; one is the homolog of most mammalian genes (Ins2), and the other is a retroposed copy that includes promoter sequence but that is missing an intron (Ins1).[27] Transcription of the insulin gene increases in response to elevated blood glucose.[28] This is primarily controlled by transcription factors that bind enhancer sequences in the ~400 base pairs before the gene's transcription start site.[26][28]
The major transcription factors influencing insulin secretion are PDX1, NeuroD1, and MafA.[29][30][31][32]
During a low-glucose state, PDX1 (pancreatic and duodenal homeobox protein 1) is located in the nuclear periphery as a result of interaction with HDAC1 and 2,[33] which results in downregulation of insulin secretion.[34] An increase in blood glucose levels causes phosphorylation of PDX1, which leads it to undergo nuclear translocation and bind the A3 element within the insulin promoter.[35] Upon translocation it interacts with coactivators HAT p300 and SETD7. PDX1 affects the histone modifications through acetylation and deacetylation as well as methylation. It is also said to suppress glucagon.[36]
NeuroD1, also known as β2, regulates insulin exocytosis in pancreatic β cells by directly inducing the expression of genes involved in exocytosis.[37] It is localized in the cytosol, but in response to high glucose it becomes glycosylated by OGT and/or phosphorylated by ERK, which causes translocation to the nucleus. In the nucleus β2 heterodimerizes with E47, binds to the E1 element of the insulin promoter and recruits co-activator p300 which acetylates β2. It is able to interact with other transcription factors as well in activation of the insulin gene.[37]
MafA is degraded by proteasomes upon low blood glucose levels. Increased levels of glucose make an unknown protein glycosylated. This protein works as a transcription factor for MafA in an unknown manner and MafA is transported out of the cell. MafA is then translocated back into the nucleus where it binds the C1 element of the insulin promoter.[38][39]
These transcription factors work synergistically and in a complex arrangement. Increased blood glucose can after a while destroy the binding capacities of these proteins, and therefore reduce the amount of insulin secreted, causing diabetes. The decreased binding activities can be mediated by glucose induced oxidative stress and antioxidants are said to prevent the decreased insulin secretion in glucotoxic pancreatic β cells. Stress signalling molecules and reactive oxygen species inhibits the insulin gene by interfering with the cofactors binding the transcription factors and the transcription factors itself.[40]
Several regulatory sequences in the promoter region of the human insulin gene bind to transcription factors. In general, the A-boxes bind to Pdx1 factors, E-boxes bind to NeuroD, C-boxes bind to MafA, and cAMP response elements to CREB. There are also silencers that inhibit transcription.
Synthesis
Insulin is synthesized as an inactive precursor molecule, a 110 amino acid-long protein called preproinsulin. Preproinsulin is translated directly into the rough endoplasmic reticulum (RER), where its signal peptide is removed by signal peptidase to form proinsulin.[26] As the proinsulin folds, opposite ends of the protein, called the "A-chain" and the "B-chain", are fused together with three disulfide bonds.[26] Folded proinsulin then transits through the Golgi apparatus and is packaged into specialized secretory vesicles, or granules.[26] In the granule, proinsulin is cleaved by proprotein convertase 1/3 and proprotein convertase 2, removing the middle part of the protein, called the "C-peptide".[26] Finally, carboxypeptidase E removes two pairs of amino acids from the protein's ends, resulting in active insulin – the insulin A- and B- chains, now connected with two disulfide bonds.[26]
The resulting mature insulin is packaged inside mature granules waiting for metabolic signals (such as leucine, arginine, glucose and mannose) and vagal nerve stimulation to be exocytosed from the cell into the circulation.[41]
Insulin and its related proteins have been shown to be produced inside the brain, and reduced levels of these proteins are linked to Alzheimer's disease.[42][43][44]
Insulin release is stimulated also by beta-2 receptor stimulation and inhibited by alpha-1 receptor stimulation. In addition, cortisol, glucagon and growth hormone antagonize the actions of insulin during times of stress. Insulin also inhibits fatty acid release by hormone-sensitive lipase in adipose tissue.[8]
Structure
See also: Insulin/IGF/Relaxin family and Insulin and its analog structureContrary to an initial belief that hormones would be generally small chemical molecules, as the first peptide hormone known of its structure, insulin was found to be quite large.[17] A single protein (monomer) of human insulin is composed of 51 amino acids, and has a molecular mass of 5808 Da. The molecular formula of human insulin is C257H383N65O77S6.[45] It is a combination of two peptide chains (dimer) named an A-chain and a B-chain, which are linked together by two disulfide bonds. The A-chain is composed of 21 amino acids, while the B-chain consists of 30 residues. The linking (interchain) disulfide bonds are formed at cysteine residues between the positions A7-B7 and A20-B19. There is an additional (intrachain) disulfide bond within the A-chain between cysteine residues at positions A6 and A11. The A-chain exhibits two α-helical regions at A1-A8 and A12-A19 which are antiparallel; while the B chain has a central α -helix (covering residues B9-B19) flanked by the disulfide bond on either sides and two β-sheets (covering B7-B10 and B20-B23).[17][46]
The amino acid sequence of insulin is strongly conserved and varies only slightly between species. Bovine insulin differs from human in only three amino acid residues, and porcine insulin in one. Even insulin from some species of fish is similar enough to human to be clinically effective in humans. Insulin in some invertebrates is quite similar in sequence to human insulin, and has similar physiological effects. The strong homology seen in the insulin sequence of diverse species suggests that it has been conserved across much of animal evolutionary history. The C-peptide of proinsulin, however, differs much more among species; it is also a hormone, but a secondary one.[46]
Insulin is produced and stored in the body as a hexamer (a unit of six insulin molecules), while the active form is the monomer. The hexamer is about 36000 Da in size. The six molecules are linked together as three dimeric units to form symmetrical molecule. An important feature is the presence of zinc atoms (Zn2+) on the axis of symmetry, which are surrounded by three water molecules and three histidine residues at position B10.[17][46]
The hexamer is an inactive form with long-term stability, which serves as a way to keep the highly reactive insulin protected, yet readily available. The hexamer-monomer conversion is one of the central aspects of insulin formulations for injection. The hexamer is far more stable than the monomer, which is desirable for practical reasons; however, the monomer is a much faster-reacting drug because diffusion rate is inversely related to particle size. A fast-reacting drug means insulin injections do not have to precede mealtimes by hours, which in turn gives people with diabetes more flexibility in their daily schedules.[47] Insulin can aggregate and form fibrillar interdigitated beta-sheets. This can cause injection amyloidosis, and prevents the storage of insulin for long periods.[48]
Function
Secretion
See also: Blood glucose regulationBeta cells in the islets of Langerhans release insulin in two phases. The first-phase release is rapidly triggered in response to increased blood glucose levels, and lasts about 10 minutes. The second phase is a sustained, slow release of newly formed vesicles triggered independently of sugar, peaking in 2 to 3 hours. The two phases of the insulin release suggest that insulin granules are present in diverse stated populations or "pools". During the first phase of insulin exocytosis, most of the granules predispose for exocytosis are released after the calcium internalization. This pool is known as Readily Releasable Pool (RRP). The RRP granules represent 0.3-0.7% of the total insulin-containing granule population, and they are found immediately adjacent to the plasma membrane. During the second phase of exocytosis, insulin granules require mobilization of granules to the plasma membrane and a previous preparation to undergo their release.[49] Thus, the second phase of insulin release is governed by the rate at which granules get ready for release. This pool is known as a Reserve Pool (RP). The RP is released slower than the RRP (RRP: 18 granules/min; RP: 6 granules/min).[50] Reduced first-phase insulin release may be the earliest detectable beta cell defect predicting onset of type 2 diabetes.[51] First-phase release and insulin sensitivity are independent predictors of diabetes.[52]
The description of first phase release is as follows:
- Glucose enters the β-cells through the glucose transporters, GLUT 2. At low blood sugar levels little glucose enters the β-cells; at high blood glucose concentrations large quantities of glucose enter these cells.[53]
- The glucose that enters the β-cell is phosphorylated to glucose-6-phosphate (G-6-P) by glucokinase (hexokinase IV) which is not inhibited by G-6-P in the way that the hexokinases in other tissues (hexokinase I – III) are affected by this product. This means that the intracellular G-6-P concentration remains proportional to the blood sugar concentration.[10][53]
- Glucose-6-phosphate enters glycolytic pathway and then, via the pyruvate dehydrogenase reaction, into the Krebs cycle, where multiple, high-energy ATP molecules are produced by the oxidation of acetyl CoA (the Krebs cycle substrate), leading to a rise in the ATP:ADP ratio within the cell.[54]
- An increased intracellular ATP:ADP ratio closes the ATP-sensitive SUR1/Kir6.2 potassium channel (see sulfonylurea receptor). This prevents potassium ions (K+) from leaving the cell by facilitated diffusion, leading to a buildup of intracellular potassium ions. As a result, the inside of the cell becomes less negative with respect to the outside, leading to the depolarization of the cell surface membrane.
- Upon depolarization, voltage-gated calcium ion (Ca2+) channels open, allowing calcium ions to move into the cell by facilitated diffusion.
- The cytosolic calcium ion concentration can also be increased by calcium release from intracellular stores via activation of ryanodine receptors.[55]
- The calcium ion concentration in the cytosol of the beta cells can also, or additionally, be increased through the activation of phospholipase C resulting from the binding of an extracellular ligand (hormone or neurotransmitter) to a G protein-coupled membrane receptor. Phospholipase C cleaves the membrane phospholipid, phosphatidyl inositol 4,5-bisphosphate, into inositol 1,4,5-trisphosphate and diacylglycerol. Inositol 1,4,5-trisphosphate (IP3) then binds to receptor proteins in the plasma membrane of the endoplasmic reticulum (ER). This allows the release of Ca2+ ions from the ER via IP3-gated channels, which raises the cytosolic concentration of calcium ions independently of the effects of a high blood glucose concentration. Parasympathetic stimulation of the pancreatic islets operates via this pathway to increase insulin secretion into the blood.[56]
- The significantly increased amount of calcium ions in the cells' cytoplasm causes the release into the blood of previously synthesized insulin, which has been stored in intracellular secretory vesicles.
This is the primary mechanism for release of insulin. Other substances known to stimulate insulin release include the amino acids arginine and leucine, parasympathetic release of acetylcholine (acting via the phospholipase C pathway), sulfonylurea, cholecystokinin (CCK, also via phospholipase C),[57] and the gastrointestinally derived incretins, such as glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP).
Release of insulin is strongly inhibited by norepinephrine (noradrenaline), which leads to increased blood glucose levels during stress. It appears that release of catecholamines by the sympathetic nervous system has conflicting influences on insulin release by beta cells, because insulin release is inhibited by α2-adrenergic receptors[58] and stimulated by β2-adrenergic receptors.[59] The net effect of norepinephrine from sympathetic nerves and epinephrine from adrenal glands on insulin release is inhibition due to dominance of the α-adrenergic receptors.[60]
When the glucose level comes down to the usual physiologic value, insulin release from the β-cells slows or stops. If the blood glucose level drops lower than this, especially to dangerously low levels, release of hyperglycemic hormones (most prominently glucagon from islet of Langerhans alpha cells) forces release of glucose into the blood from the liver glycogen stores, supplemented by gluconeogenesis if the glycogen stores become depleted. By increasing blood glucose, the hyperglycemic hormones prevent or correct life-threatening hypoglycemia.
Evidence of impaired first-phase insulin release can be seen in the glucose tolerance test, demonstrated by a substantially elevated blood glucose level at 30 minutes after the ingestion of a glucose load (75 or 100 g of glucose), followed by a slow drop over the next 100 minutes, to remain above 120 mg/100 mL after two hours after the start of the test. In a normal person the blood glucose level is corrected (and may even be slightly over-corrected) by the end of the test. An insulin spike is a 'first response' to blood glucose increase, this response is individual and dose specific although it was always previously assumed to be food type specific only.
