Definition
Amylin or Islet amyloid polypeptide (IAPP), a 37-amino acid peptide is secreted by beta-islet cells of the pancreas and a major component of the amyloid deposits in persons with type 2 diabetes mellitus. Amylin may be referred to as insulin’s “fraternal twin” as it is constitutively expressed with insulin in response to elevations of plasma glucose.
Discovery
The knowledge of occurrence of amyloid deposits in islets of Langerhans, major pathologic feature of diabetics has been known for a century. But, the discovery of amylin as a major component of amyloid deposits was by two independent groups in 19871, 2.
Structural characteristics
The human amylin has an amino acid sequence KCNTATCATQRLANFLVHSSNNFGAILSSTNVGSNTY, with a disulfide bridge between cysteine residues 2 and 7. The features, amidated C-terminus and the disulfide bridge are necessary for the full biological activity of amylin3. Amylin amino acid sequence is 46% and 43% identical to those of the calcitonin gene-related neuropeptides CGRP-2 and CGRP-1. The (20-29) fragment of amylin is critical to the pathogenesis of islet amyloid4.
Mechanism of action
Amylin is synthesized, packaged within the golgi apparatus and secreted within the secretory granule by the islet beta cell. They have binding sites within the renal cortex in the area of the juxtaglomerular apparatus and it activates the rennin angiotensin aldosterone system. It also acts upon the circulatory system by inhibiting the secretion of the atrial natriuretic peptide (ANP)5.
Function
Amylin inhibits gastric emptying and is important in controlling and delaying the rate of meal derived glucose. It inhibits hepatic release and production of glucose in the postprandial period. They also have been shown to inhibit glucagon secretion and somatostatin. Amylin causes vasodilatation by dilating the non-striated muscles of the blood vessels. It is also known to increase thirst level which indicates it has an action within the central nervous system6.
References
1. Cooper GJ, Willis AC, Clark A, Turner RC, Sim RB, Reid KB (1987). Purification and characterization of a peptide from amyloid-rich pancreases of type 2 diabetic patients. Proc Natl Acad Sci., 84(23):8628-32.
2. Westermark P, Wernstedt C, O'Brien TD, Hayden DW, Johnson KH (1987). Islet amyloid in type 2 human diabetes mellitus and adult diabetic cats contains a novel putative polypeptide hormone. Am J Pathol, 127(3):414-417.
3. Roberts AN, Leighton B, Todd JA, Cockburn D, Schofield PN, Sutton R, Holt S, Boyd Y, Day AJ, Foot EA, et al,(1989). Molecular and functional characterization of amylin, a peptide associated with type 2 diabetes mellitus. Proc Natl Acad Sci, 86(24):9662-9666.
4. Guidobono F, Pagani F, Ticozzi C, Sibilia V, Pecile A, Netti C (1997). Protection by amylin of gastric erosions induced by indomethacin or ethanol in rats. Br J Pharmacol, 120(4):581-586.
5. Piao FL, Cao C, Han JH, Kim SZ, Cho KW, Kim SH (2004). Amylin-induced suppression of ANP secretion through receptors for CGRP1 and salmon calcitonin. Regul Pept, 117: 59-166.
6. Hayden MR, Tyagi SC (2002). Islet redox stress: the manifold toxicities of insulin resistance, metabolic syndrome and amylin derived islet amyloid in type 2 diabetes mellitus. Journal of the Pancrease, 3(4): 86-108.
