Editing Insulin (section)

== Production ==
[[File:Insulin gene activation.png|thumb|upright=1.8|Diagram of insulin regulation upon high blood glucose]]
Insulin is produced exclusively in the beta cells of the [[pancreatic islet]]s in mammals, and the Brockmann body in some fish. Human insulin is produced from the ''INS'' [[gene]], located on chromosome 11.<ref name=Tokarz2018>{{cite journal |vauthors=Tokarz VL, MacDonald PE, Klip A |title=The cell biology of systemic insulin function |journal=J Cell Biol |volume=217 |issue=7 |pages=2273–2289 |date=July 2018 |pmid=29622564 |pmc=6028526 |doi=10.1083/jcb.201802095 }}</ref> 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'').<ref name="pmid18245324">{{cite journal | vauthors = Shiao MS, Liao BY, Long M, Yu HT | title = Adaptive evolution of the insulin two-gene system in mouse | journal = Genetics | volume = 178 | issue = 3 | pages = 1683–91 | date = March 2008 | pmid = 18245324 | doi = 10.1534/genetics.108.087023  | pmc = 2278064 }}</ref> [[Transcription (biology)|Transcription]] of the insulin gene increases in response to elevated blood glucose.<ref name=Fu2013>{{cite journal |vauthors=Fu Z, Gilbert ER, Liu D |title=Regulation of insulin synthesis and secretion and pancreatic Beta-cell dysfunction in diabetes |journal=Curr Diabetes Rev |volume=9 |issue=1 |pages=25–53 |date=January 2013 |pmid=22974359 |pmc=3934755 |doi= 10.2174/157339913804143225}}</ref> This is primarily controlled by [[transcription factor]]s that bind [[Enhancer (genetics)|enhancer sequences]] in the ~400 [[base pair]]s before the gene's transcription start site.<ref name=Tokarz2018/><ref name=Fu2013/>

The major transcription factors influencing insulin secretion are [[PDX1]], [[NeuroD1]], and [[MafA]].<ref name="Bernardo_2008">{{cite journal | vauthors = Bernardo AS, Hay CW, Docherty K | title = Pancreatic transcription factors and their role in the birth, life and survival of the pancreatic beta cell | journal = Molecular and Cellular Endocrinology | volume = 294 | issue = 1–2 | pages = 1–9 | date = November 2008 | pmid = 18687378 | doi = 10.1016/j.mce.2008.07.006 | s2cid = 28027796 | department = review | url = https://hal.archives-ouvertes.fr/hal-00532050/file/PEER_stage2_10.1016%252Fj.mce.2008.07.006.pdf }}</ref><ref name="Rutter_2015">{{cite journal | vauthors = Rutter GA, Pullen TJ, Hodson DJ, Martinez-Sanchez A | title = Pancreatic β-cell identity, glucose sensing and the control of insulin secretion | journal = The Biochemical Journal | volume = 466 | issue = 2 | pages = 203–18 | date = March 2015 | pmid = 25697093 | doi = 10.1042/BJ20141384 | s2cid = 2193329 | doi-access =  | department = review }}</ref><ref name = "Rutter_2000">{{cite journal | vauthors = Rutter GA, Tavaré JM, Palmer DG | title = Regulation of Mammalian Gene Expression by Glucose | journal = News in Physiological Sciences | volume = 15 | issue = 3| pages = 149–54 | date = June 2000 | pmid = 11390898 | doi = 10.1152/physiologyonline.2000.15.3.149 | doi-access =  | department = review }}</ref><ref name = "Poitout_2006">{{cite journal | vauthors = Poitout V, Hagman D, Stein R, Artner I, Robertson RP, Harmon JS | title = Regulation of the insulin gene by glucose and d acids | journal = The Journal of Nutrition | volume = 136 | issue = 4 | pages = 873–76 | date = April 2006 | pmid = 16549443 | pmc = 1853259 | doi = 10.1093/jn/136.4.873 | department = review }}</ref>

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 [[HDAC2|2]],<ref name="Vaulont_2000">{{cite journal|vauthors=Vaulont S, Vasseur-Cognet M, Kahn A|date=October 2000|title=Glucose regulation of gene transcription|department=review|journal=The Journal of Biological Chemistry|volume=275|issue=41|pages=31555–58|doi=10.1074/jbc.R000016200|pmid=10934218|doi-access=free}}</ref> which results in downregulation of insulin secretion.<ref name="Christensen_2011">{{cite journal | vauthors = Christensen DP, Dahllöf M, Lundh M, Rasmussen DN, Nielsen MD, Billestrup N, Grunnet LG, Mandrup-Poulsen T | title = Histone deacetylase (HDAC) inhibition as a novel treatment for diabetes mellitus | journal = Molecular Medicine | volume = 17 | issue = 5–6 | pages = 378–90 | date = 2011 | pmid = 21274504 | pmc = 3105132 | doi = 10.2119/molmed.2011.00021 }}</ref> 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.<ref name="Wang_2016">{{cite journal | vauthors = Wang W, Shi Q, Guo T, Yang Z, Jia Z, Chen P, Zhou C | title = PDX1 and ISL1 differentially coordinate with epigenetic modifications to regulate insulin gene expression in varied glucose concentrations | journal = Molecular and Cellular Endocrinology | volume = 428 | pages = 38–48 | date = June 2016 | pmid = 26994512 | doi = 10.1016/j.mce.2016.03.019 | doi-access = free }}</ref> Upon translocation it interacts with coactivators [[EP300|HAT p300]] and [[SETD7]]. [[PDX1]] affects the [[histone]] modifications through [[acetylation]] and deacetylation as well as [[methylation]]. It is also said to suppress [[glucagon]].<ref>{{cite journal | vauthors = Wang X, Wei X, Pang Q, Yi F | title = Histone deacetylases and their inhibitors: molecular mechanisms and therapeutic implications in diabetes mellitus |journal=Acta Pharmaceutica Sinica B |date=August 2012 |volume=2 |issue=4 |pages=387–95 |doi=10.1016/j.apsb.2012.06.005|doi-access=free }}</ref>

[[NeuroD1]], also known as β2, regulates insulin exocytosis in pancreatic [[β cells]] by directly inducing the expression of [[genes]] involved in exocytosis.<ref name = "Andrali_2008">{{cite journal | vauthors = Andrali SS, Sampley ML, Vanderford NL, Ozcan S | title = Glucose regulation of insulin gene expression in pancreatic beta-cells | journal = The Biochemical Journal | volume = 415 | issue = 1 | pages = 1–10 | date = October 2008 | pmid = 18778246 | doi = 10.1042/BJ20081029 | doi-access =  | department = review }}</ref> It is localized in the [[cytosol]], but in response to high [[glucose]] it becomes [[glycosylated]] by [[OGT (gene)|OGT]] and/or [[phosphorylated]] by [[Extracellular signal-regulated kinases|ERK]], which causes translocation to the nucleus. In the nucleus β2 heterodimerizes with [[TCF3|E47]], binds to the E1 element of the insulin promoter and recruits co-activator [[EP300|p300]] which acetylates β2. It is able to interact with other transcription factors as well in activation of the insulin gene.<ref name = "Andrali_2008" />

[[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.<ref name="pmid19393272">{{cite journal | vauthors = Kaneto H, Matsuoka TA, Kawashima S, Yamamoto K, Kato K, Miyatsuka T, Katakami N, Matsuhisa M | title = Role of MafA in pancreatic beta-cells | journal = Advanced Drug Delivery Reviews | volume = 61 | issue = 7–8 | pages = 489–96 | date = July 2009 | pmid = 19393272 | doi = 10.1016/j.addr.2008.12.015 }}</ref><ref name="Aramata_2007">{{cite journal | vauthors = Aramata S, Han SI, Kataoka K | title = Roles and regulation of transcription factor MafA in islet beta-cells | journal = Endocrine Journal | volume = 54 | issue = 5 | pages = 659–66 | date = December 2007 | pmid = 17785922 | doi = 10.1507/endocrj.KR-101| doi-access = free }}</ref>

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.<ref name="Kaneto_2012">{{cite journal | vauthors = Kaneto H, Matsuoka TA | title = Involvement of oxidative stress in suppression of insulin biosynthesis under diabetic conditions | journal = International Journal of Molecular Sciences | volume = 13 | issue = 10 | pages = 13680–90 | date = October 2012 | pmid = 23202973 | pmc = 3497347 | doi = 10.3390/ijms131013680 | doi-access = free }}</ref>

Several [[regulatory sequence]]s in the [[Promoter (biology)|promoter]] region of the human insulin gene bind to [[transcription factor]]s.  In general, the [[A-box]]es bind to [[Pdx1]] factors, [[E-box]]es bind to [[NeuroD]], C-boxes bind to [[MafA]], and [[cAMP response element]]s to [[CREB]]. There are also [[silencer (genetics)|silencers]] that inhibit transcription.

=== Synthesis ===
[[File:Insulin path.svg|thumb|upright=1.8|Insulin undergoes extensive posttranslational modification along the production pathway. Production and secretion are largely independent; prepared insulin is stored awaiting secretion. Both C-peptide and mature insulin are biologically active. Cell components and proteins in this image are not to scale.]]

Insulin is synthesized as an inactive [[Protein precursor|precursor]] molecule, a 110 amino acid-long protein called preproinsulin.<!-- Cite. Merged AA length from [[Preproinsulin]] --> Preproinsulin is [[translation (biology)|translated]] directly into the rough [[endoplasmic reticulum]] (RER), where its [[signal peptide]] is removed by [[signal peptidase]] to form [[proinsulin]].<ref name=Tokarz2018/> As the proinsulin [[protein folding|folds]], opposite ends of the protein, called the "A-chain" and the "B-chain", are fused together with three [[disulfide bond]]s.<ref name=Tokarz2018/> Folded proinsulin then transits through the [[Golgi apparatus]] and is packaged into specialized [[Vesicle (biology and chemistry)#Secretory vesicles|secretory vesicle]]s.<ref name=Tokarz2018/> In the granule, proinsulin is cleaved by [[Proprotein convertase 1|proprotein convertase 1/3]] and [[proprotein convertase 2]], removing the middle part of the protein, called the "[[C-peptide]]".<ref name=Tokarz2018/> 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.<ref name=Tokarz2018/><!-- the figure in this shows the two bonds but the text doesn't explicitly say it. Probably best to find a better source -->

The resulting mature insulin is packaged inside mature granules waiting for metabolic signals (such as leucine, arginine, glucose and mannose) and [[Vagus nerve stimulation|vagal nerve stimulation]] to be exocytosed from the cell into the circulation.<ref name = "Najjar_2001">{{cite book | vauthors = Najjar S | chapter = Insulin Action: Molecular Basis of Diabetes | date = 2003 | title = Encyclopedia of Life Sciences | publisher = John Wiley & Sons | doi = 10.1038/npg.els.0001402 | isbn = 978-0-470-01617-6 }}</ref>

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]].<ref name="urlResearchers discover link between insulin and Alzheimers">{{cite web | url = http://www.eurekalert.org/pub_releases/2005-03/l-rdl030205.php | title = Researchers discover link between insulin and Alzheimer's | vauthors = Gustin N | date = 7 March 2005 | work = EurekAlert! | publisher = American Association for the Advancement of Science | access-date = 1 January 2009}}</ref><ref name="pmid15750214">{{cite journal | vauthors = de la Monte SM, Wands JR |url= https://www.alzforum.org/sites/default/files/legacy/res/for/journal/delamonte/jad00401.pdf | title = Review of insulin and insulin-like growth factor expression, signaling, and malfunction in the central nervous system: relevance to Alzheimer's disease | journal = Journal of Alzheimer's Disease | volume = 7 | issue = 1 | pages = 45–61 | date = February 2005 | pmid = 15750214 | doi = 10.3233/JAD-2005-7106 }}</ref><ref name="pmid15750215">{{cite journal | vauthors = Steen E, Terry BM, Rivera EJ, Cannon JL, Neely TR, Tavares R, Xu XJ, Wands JR, de la Monte SM | title = Impaired insulin and insulin-like growth factor expression and signaling mechanisms in Alzheimer's disease—is this type 3 diabetes? | journal = Journal of Alzheimer's Disease | volume = 7 | issue = 1 | pages = 63–80 | date = February 2005 | pmid = 15750215 | doi = 10.3233/jad-2005-7107| s2cid = 28173722 | url = https://www.alzforum.org/sites/default/files/legacy/res/for/journal/delamonte/jad00400.pdf }}</ref>

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.<ref name="stryer" />