Амилин: структура, биосинтез и роль в регуляции гликемии.

Peptide hormone that plays a role in glycemic regulation
the polypeptide

Amylin, or islet amyloid polypeptide (IAPP), is a 37 residue peptide hormone. It is co secreted with insulin from the pancreatic β cells in the ratio of approximately 100:1 (insulin:amylin). Amylin plays a role in glycemic regulation by slowing gastric emptying and promoting satiety, thereby preventing post prandial spikes in blood glucose levels. IAPP is processed from an 89 residue coding sequence. Proislet amyloid polypeptide (proIAPP, proamylin, proislet protein) is produced in the pancreatic beta cells (β cells) as a 67 amino acid, 7404 Dalton pro peptide and undergoes post translational modifications including protease cleavage to produce amylin.

ProIAPP consists of 67 amino acids, which follow a 22 amino acid signal peptide which is rapidly cleaved after translation of the 89 amino acid coding sequence. The human sequence (from N terminus to C terminus) is:

(MGILKLQVFLIVLSVALNHLKA) TPIESHQVEKR^ KCNTATCATQRLANFLVHSSNNFGAILSSTNVGSNTYG^ KR^ NAVEVLKREPLNYLPL. The signal peptide is removed during translation of the protein and transport into the endoplasmic reticulum. Once inside the endoplasmic reticulum, a disulfide bond is formed between cysteine residues numbers 2 and 7. Later in the secretory pathway, the precursor undergoes additional proteolysis and posttranslational modification (indicated by ^). 11 amino acids are removed from the N terminus by the enzyme proprotein convertase 2 (PC2) while 16 are removed from the C terminus of the proIAPP molecule by proprotein convertase 1/3 (PC1/3). At the C terminus Carboxypeptidase E then removes the terminal lysine and arginine residues. The terminal glycine amino acid that results from this cleavage allows the enzyme peptidylglycine alpha amidating monooxygenase (PAM) to add an amine group. After this the transformation from the precursor protein proIAPP to the biologically active IAPP is complete (IAPP sequence: KCNTATCATQRLANFLVHSSNNFGAILSSTNVGSNTY). Insulin and IAPP are regulated by similar factors since they share a common regulatory promoter motif. The IAPP promoter is also activated by stimuli which do not affect insulin, such as tumor necrosis factor alpha and fatty acids. One of the defining features of Type 2 diabetes is insulin resistance. This is a condition wherein the body is unable to utilize insulin effectively, resulting in increased insulin production; since proinsulin and proIAPP are cosecreted, this results in an increase in the production of proIAPP as well. Although little is known about IAPP regulation, its connection to insulin indicates that regulatory mechanisms that affect insulin also affect IAPP. Thus blood glucose levels play an important role in regulation of proIAPP synthesis.

Amylin functions as part of the endocrine pancreas and contributes to glycemic control. The peptide is secreted from the pancreatic islets into the blood circulation and is cleared by peptidases in the kidney. It is not found in the urine. Amylin's metabolic function is well characterized as an inhibitor of the appearance of nutrient [especially glucose] in the plasma. It thus functions as a synergistic partner to insulin, with which it is cosecreted from pancreatic beta cells in response to meals. The overall effect is to slow the rate of appearance (Ra) of glucose in the blood after eating; this is accomplished via coordinate slowing down gastric emptying, inhibition of digestive secretion [gastric acid, pancreatic enzymes, and bile ejection], and a resulting reduction in food intake. Appearance of new glucose in the blood is reduced by inhibiting secretion of the gluconeogenic hormone glucagon. These actions, which are mostly carried out via a glucose sensitive part of the brain stem, the area postrema, may be over ridden during hypoglycemia. They collectively reduce the total insulin demand. Amylin also acts in bone metabolism, along with the related peptides calcitonin and calcitonin gene related peptide. It was also demonstrated by solid state NMR spectroscopy that the fragment 20 29 of the human amylin fragments membranes. Rats and mice have six substitutions (three of which are proline substitutions at positions 25, 28 and 29) that are believed to prevent the formation of amyloid fibrils, although not completely as seen by its propensity to form amyloid fibrils in vitro. Rat IAPP is nontoxic to beta cells when overexpressed in transgenic rodents.

Before amylin deposition was associated with diabetes, already in 1901, scientists described the phenomenon of "islet hyalinization", which could be found in some cases of diabetes. A thorough study of this phenomenon was possible much later. In 1986, the isolation of an aggregate from an insulin producing tumor was successful, a protein called IAP (Insulinoma Amyloid Peptide) was characterized, and amyloids were isolated from the pancreas of a diabetic patient, but the isolated material was not sufficient for full characterization. This was achieved only a year later by two research teams whose research was a continuation of the work from 1986.

ProIAPP has been linked to Type 2 diabetes and the loss of islet β cells. Islet amyloid formation, initiated by the aggregation of proIAPP, may contribute to this progressive loss of islet β cells. It is thought that proIAPP forms the first granules that allow for IAPP to aggregate and form amyloid which may lead to amyloid induced apoptosis of β cells. IAPP is cosecreted with insulin. Insulin resistance in Type 2 diabetes produces a greater demand for insulin production which results in the secretion of proinsulin. ProIAPP is secreted simultaneously, however, the enzymes that convert these precursor molecules into insulin and IAPP, respectively, are not able to keep up with the high levels of secretion, ultimately leading to the accumulation of proIAPP. In particular, the impaired processing of proIAPP that occurs at the N terminal cleavage site is a key factor in the initiation of amyloid. The amyloid formation might be a major mediator of apoptosis, or programmed cell death, in the islet β cells. A 2008 study reported a synergistic effect for weight loss with leptin and amylin coadministration in diet induced obese rats by restoring hypothalamic sensitivity to leptin. However, in clinical trials, the study was halted at Phase 2 in 2011 when a problem involving antibody activity that might have neutralized the weight loss effect of metreleptin in two patients who took the drug in a previously completed clinical study. The study combined metreleptin, a version of the human hormone leptin, and pramlintide, which is Amylin's diabetes drug Symlin, into a single obesity therapy. A proteomics study showed that human amylin shares common toxicity targets with beta amyloid (Abeta), suggesting that type 2 diabetes and Alzheimer's disease share common toxicity mechanisms.

A synthetic analog of human amylin with proline substitutions in positions 25, 26 and 29, or pramlintide (brand name Symlin), was approved in 2005 for adult use in patients with both diabetes mellitus type 1 and diabetes mellitus type 2. Insulin and pramlintide, injected separately but both before a meal, work together to control the post prandial glucose excursion. Amylin is degraded in part by insulin degrading enzyme. Another long acting analogue of Amylin is Cagrilintide being developed by Novo Nordisk ( now in the Phase 3 trials with the proposed brand name CagriSema co formulated with Semaglutide as a once weekly subcutaneous injection ) as a measure to treat type II DM and obesity.

There appear to be at least three distinct receptor complexes that amylin binds to with high affinity. All three complexes contain the calcitonin receptor at the core, plus one of three receptor activity modifying proteins, RAMP1, RAMP2, or RAMP3.