Glucagon Like Peptide 1: Incretin Exenatide/ Liraglutide Pharmacology: Newer antidiabetic Medication

Amylin analogs are used in the treatment of diabetes. They are stable synthetic compounds, which are administered subcutaneously before meals, and work similarly to the physiological amylin.
Amylin is a 37 amino acid polypeptide hormone that is secreted with insulin from the beta cells in the pancreas. In diabetes, as less insulin is secreted there is also a deficiency of amylin. Amylin assists insulin in postprandial glucose control. It inhibits glucagon secretion, delays gastric emptying and signals satiety, suppressing the intake of food
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.[6][7] 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.[8] 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).[9] At the C-terminus Carboxypeptidase E then removes the terminal lysine and arginine residues.[10] 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).[6]

RegulationEdit

Insulin and IAPP are regulated by similar factors since they share a common regulatory promoter motif.[11] The IAPP promoter is also activated by stimuli which do not affect insulin, such as tumor necrosis factor alpha[12] and fatty acids.[13] 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.

FunctionEdit

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.[14] 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.[15]
Amylin also acts in bone metabolism, along with the related peptides calcitonin and calcitonin gene related peptide.[14]
Rodent amylin knockouts do not have a normal reduction of appetite following food consumption.[citation needed] Because it is an amidated peptide, like many neuropeptides, it is believed to be responsible for the effect on appetite.

StructureEdit

The human form of IAPP has the amino acid sequence KCNTATCATQRLANFLVHSSNNFGAILSSTNVGSNTY, with a disulfide bridge between cysteine residues 2 and 7. Both the amidated C-terminus and the disulfide bridge are necessary for the full biological activity of amylin.[8] IAPP is capable of forming amyloid fibrils in vitro. Within the fibrillization reaction, the early prefibrillar structures are extremely toxic to beta-cell and insuloma cell cultures.[8] Later amyloid fiber structures also seem to have some cytotoxic effect on cell cultures. Studies have shown that fibrils are the end product and not necessarily the most toxic form of amyloid proteins/peptides in general. A non-fibril forming peptide (1–19 residues of human amylin) is toxic like the full-length peptide