Amylin, Calcitonin Gene-Related Peptide, Calcitonin, and Adrenomedullin: A Peptide Superfamily

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Abstract
The calcitonin gene peptide superfamily consists of calcitonin (CT), calcitonin gene-related peptide (CGRP), and amylin. CT and CGRP derive from the CT/CGRP gene, which is encoded on chromosome 11. Alternative splicing of the primary RNA transcript leads to the translation of CGRP and CT. peptides in a tissue-specific manner. CGRP (a 37-amino-acid neuropeptide) and its receptors are widely distributed in the body, and it is the most potent endogenous vasodilatory peptide discovered so far. CT (a 32-amino-acid peptide) is, however, a hormone primarily involved in protecting the skeleton during periods of ''calcium stress'' such as growth, pregnancy, and lactation. CT derives from the C cells of the thyroid gland and is the most potent peptide inhibitor of osteoclast-mediated bone resorption. Therefore, treatment with CT is highly effective for conditions associated with increased bone turnover such as Pager's disease, osteoporosis, Sudeck's atrophy, and hypercalcemia. Amylin (a 37-amino-acid peptide) is generated from a gene located on chromosome 12 (thought to be an evolutionary duplication of chromosome 11) and shares 46% amino acid sequence homology with CGRP and 20% with human CT. Amylin is predominantly located in the beta cells of the islets of the pancreas and may be involved in the pathogenesis of type IT diabetes by deposition as amyloid within the pancreas, leading to beta cell destruction. Adrenomedullin, a recently discovered 52-amino-acid vasoactive peptide from adrenal tissue, shares 24% homology with CGRP and is also a member of this superfamily of peptides. A portion of the B-chain of insulin is strongly homologous to these four peptides. Not only does adrenomedullin (13-52) show 24% amino acid homology with CGRP, it also has a biological activity profile similar to that of CGRP. CGRP, CT, and amylin are related to the insulin gene superfamily of peptides, which may all have diverged from a common ancestral gene during evolution. When the crystallographic and nuclear magnetic resonance-based molecular modeling of the three-dimensional structure of CGRP, CT, amylin, and adrenomedullin peptides and their receptors is available, it will lead to a greater understanding of the involvement of this family of peptides in pathophysiology. Together, CGRP, CT, amylin and adrenomedullin have overlapping biological effects owing to their structures and cross-reactivity between receptors. I propose that CT, CGRP, adrenomedullin, and amylin belong to a family of G-protein-coupled receptors (an ''insulin superfamily'' of peptides) and therefore share some of the characteristics of insulin, such as growth factor-like effects, and possible interaction at insulin receptor sites as an antagonist.