Proopiomelanocortin-Derived Peptides

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Cloning of the cDNA of POMC mRNA led to the characterization of the ACTH precursor and established the molecular links between ACTH, beta LPH and gamma LPH, and beta end. Further previously unidentified POMC-derived peptides were also unravelled, including the N-terminal fragment and the JP. POMC is a precursor polypeptide that contains the amino acid sequences of numerous small polypeptide products. It contains eight pairs of basic amino acids and one sequence of four basic amino acids, which are the sites of cleavage for the recently identified prohormone convertases (PC1 and PC2), which have different specificities. PC1 but not PC2 is found in corticotroph cells of the anterior pituitary and cleaves POMC to generate NT, JP, ACTH, beta LPH and small amounts of gamma LPH and beta end. In the melanotroph cells of the intermediate pituitary, both PC1 and PC2 are present. The proteolysis of POMC is thus more extensive, resulting in a series of smaller peptides including gamma MSHs, CLIP, alpha MSH, beta MSH, beta end, and beta end. Equimolar amounts (because they are all products of the same precursor) of the POMC peptides are released into the blood by exocytosis from corticotroph cells of the anterior pituitary. No physiological role has been definitively identified for any of these products other than ACTH. POMC peptides have been assayed in human blood by specific radioimmunoassay (RIA). In most circumstances, the concentrations of all these peptides are perfectly correlated. Therefore, for practical reasons, LPH RIA is widely used as an indicator of overall corticotroph function. In rare cases, the correlation between the POMC peptides is disrupted by a general disorder selectively modifying the metabolism of the different members of the family as in chronic renal failure, or in tumors where POMC processing is abnormal. Examples include pituitary macroadenomas, some silent corticotroph adenomas, and most frequently non-pituitary tumors responsible for ectopic ACTH syndrome. These pathologies can lead to the formation and secretion into blood of CLIP and beta MSH, or alternatively POMC may be underprocessed and secreted intact. An elegant approach, specific IRMA, has been developed to directly recognize unprocessed POMC in blood.