|, POC, proopiomelanocortin, OBAIRH, LPH, CLIP, ACTH, NPP, MSH|
Pro-opiomelanocortin (POMC) is a precursor polypeptide with 241 amino acid residues. POMC is synthesized in the pituitary from the 285-amino-acid-long polypeptide precursor pre-pro-opiomelanocortin (pre-POMC), by the removal of a 44-amino-acid-long signal peptide sequence during translation.
POMC is cut (cleaved) to give rise to multiple peptide hormones. Each of these peptides is packaged in large dense-core vesicles that are released from the cells by exocytosis in response to appropriate stimulation:
- α-MSH produced by neurons in the ventromedial nucleus has important roles in the regulation of appetite (POMC neuron stimulation results in satiety.) and sexual behavior, while α-MSH secreted from the intermediate lobe of the pituitary regulates the production of melanin.
- ACTH is a peptide hormone that regulates the secretion of glucocorticoids from the adrenal cortex.
- β-Endorphin and [Met]enkephalin are endogenous opioid peptides with widespread actions in the brain.
The POMC gene is located on chromosome 2p23.3. The POMC gene is expressed in both the anterior and intermediate lobes of the pituitary gland. This gene encodes a 285-amino acid polypeptide hormone precursor that undergoes extensive, tissue-specific, post-translational processing via cleavage by subtilisin-like enzymes known as prohormone convertases. The encoded protein is synthesized mainly in corticotroph cells of the anterior pituitary, where four cleavage sites are used; adrenocorticotrophin (ACTH), essential for normal steroidogenesis and the maintenance of normal adrenal weight, and β-lipotropin are the major end-products. However, there are at least eight potential cleavage sites within the polypeptide precursor and, depending on tissue type and the available convertases, processing may yield as many as ten biologically active peptides involved in diverse cellular functions. Cleavage sites consist of the sequences Arg-Lys, Lys-Arg, or Lys-Lys. Enzymes responsible for processing of POMC peptides include prohormone convertase 1 (PC1), prohormone convertase 2 (PC2), carboxypeptidase E (CPE), peptidyl α-amidating monooxygenase (PAM), N-acetyltransferase (N-AT), and prolylcarboxypeptidase (PRCP).
The processing of POMC involves glycosylations, acetylations, and extensive proteolytic cleavage at sites shown to contain regions of basic protein sequences. However, the proteases that recognize these cleavage sites are tissue-specific. In some tissues, including the hypothalamus, placenta, and epithelium, all cleavage sites may be used, giving rise to peptides with roles in pain and energy homeostasis, melanocyte stimulation, and immune modulation. These include several distinct melanotropins, lipotropins, and endorphins that are contained within the adrenocorticotrophin and β-lipotropin peptides.
It is synthesized by:
- Corticotrope cells of the anterior pituitary gland
- Melanotrope cells of the intermediate lobe of the pituitary gland
- Neurons in the arcuate nucleus of the hypothalamus
- Smaller populations of neurons in the dorsomedial hypothalamus and brainstem
- Melanocytes in the skin
The large molecule of POMC is the source of several important biologically active substances . POMC can be cleaved enzymatically into the following peptides:
- N-Terminal Peptide of Proopiomelanocortin (NPP, or pro-γ-MSH)
- α-Melanotropin (α-Melanocyte-Stimulating Hormone, or α-MSH)
- β-Melanotropin (β-MSH)
- γ-Melanotropin (γ-MSH)
- 𝛿-Melanocyte-Stimulating Hormone (𝛿-MSH, present in sharks)
- ε-Melanocyte-Stimulating Hormone (ε-MSH, present in some teleosts )
- Corticotropin (Adrenocorticotropic Hormone, or ACTH)
- Corticotropin-like Intermediate Peptide (CLIP)
- β-Lipotropin (β-LPH)
- Gamma Lipotropin (γ-LPH)
Although the N-terminal 5 amino acids of β-endorphin are identical to the sequence of [Met]enkephalin, it is not generally thought that β-endorphin is converted into [Met]enkephalin. Instead, [Met]enkephalin is produced from its own precursor, proenkephalin A.
The production of β-MSH occurs in humans but not in mice or rats due to the absence of the enzymatic processing site in the rodent POMC.
A study concluded that a polymorphism was associated with higher fasting insulin levels in the obese patients only. These findings support the hypothesis that the melanocortin pathway may modulate glucose metabolism in obese subjects indicating a possible gene-environment interaction. POMC variant may be involved in the natural history of polygenic obesity, contributing to the link between type 2 diabetes and obesity.
Proopiomelanocortin has been shown to interact with melanocortin 4 receptor. The endogenous agonists of melanocortin 4 receptor include α-MSH, β-MSH, γ-MSH, and ACTH. The fact that these are all cleavage products of POMC should suggest likely mechanisms of this interaction.
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