Skin POMC peptides: their actions at the human MC-1 receptor and roles in the tanning response.

The melanocortin 1 (MC-1) receptor is a key control point in the regulation of skin pigmentation. Alpha-MSH is an agonist at this receptor and through its activation regulates melanocyte function. alpha-MSH is cleaved from pro-opiomelanocortin (POMC) in the pituitary, but in humans the skin is a more important source of the peptide. Skin pigmentation is therefore regulated by locally produced alpha-MSH rather than that of pituitary origin. alpha-MSH acts as a paracrine and/or autocrine mediator of UV induced pigmentation. However, the predominant alpha-MSH in human skin is desacetyl alpha-MSH and, compared to the acetylated form, is a relatively weak agonist at the human MC-1 receptor. By acting as a partial agonist desacetyl alpha-MSH may even oppose the actions of acetylated alpha-MSH and other MC-1 receptor agonists. The most abundant MC-1 receptor agonist in human epidermis is ACTH1-17. This POMC peptide, which is produced by keratinocytes, is more potent than acetylated alpha-MSH in stimulating melanogenesis in human melanocytes and, in contrast to the latter, produces a biphasic dose-response curve. This is probably a consequence of its activation of both the cAMP and IP3/DAG signalling pathways. alpha-MSH peptides, on the other hand, selectively activate the cAMP pathway. Compared with alpha-MSH, ACTH1-17 could have the more important role as a paracrine mediator of melanogenesis and other melanocytic processes. However, ACTH1-17 is not the only POMC peptide in the skin and may interact with related peptides at the MC-1 receptor. These interactions are likely to represent important determinants of melanocyte function and skin pigmentation.

alpha-melanocyte-stimulating hormone modulates nitric oxide production in melanocytes.

We have previously observed that melanocytes produce nitric oxide in response to ultraviolet radiation and lipopolysaccharide and in this study have examined how these responses are affected by alpha-melanocyte-stimulating hormone. Nitric oxide production by cultured cells was measured electrochemically in real time using an ISO-nitric oxide sensor probe. B16 mouse melanoma cells released nitric oxide in response to lipopolysaccharide and the effects were enhanced in cells that had been grown in the presence of 10-11-10-9 M alpha-melanocyte-stimulating hormone prior to stimulation. At concentrations in excess of 10-9 M alpha-melanocyte-stimulating hormone decreased nitric oxide production. Preincubation with lipopolysaccharide, a well-known inducer of inducible nitric oxide synthase, also increased nitric oxide production but this response was reduced by alpha-melanocyte-stimulating hormone. alpha-Melanocyte-stimulating hormone also increased the levels of nitric oxide produced in response to ultraviolet radiation (20-100 mJ per cm2) in B16 cells. The same effect was seen in human melanocytes and as this was inhibited by aminoguanidine would appear to involve an induction of inducible nitric oxide synthase. Reverse transcription-polymerase chain reaction showed that melanocytic cells express inducible nitric oxide synthase mRNA. Western blotting analysis and immunocytochemistry confirmed the presence of inducible nitric oxide synthase protein in B16 cells and FM55 human melanoma cells and that the levels were increased in response to alpha-melanocyte-stimulating hormone. alpha-Melanocyte-stimulating hormone, however, decreased inducible nitric oxide synthase protein expression, which occurred in response to lipopolysaccharide. These results suggest that alpha-melanocyte-stimulating hormone regulates nitric oxide production in melanocytic cells by modulating the induction of inducible nitric oxide synthase. Additional experiments showed that nitric oxide increased melanin production by B16 cells and human melanocytes. This is in keeping with a melanogenic role for nitric oxide but whether its production by melanocytes in response to alpha-melanocyte-stimulating hormone is associated with such a role or whether it has some other significance relating to melanocyte differentiation or in mediating immunomodulatory actions of alpha-melanocyte-stimulating hormone remains to be seen.

ACTH1-17 is a more potent agonist at the human MC1 receptor than alpha-MSH.

The melanocortin receptor MC1 is expressed on melanocytes and is an important control point for melanogenesis and other responses. Alpha-MSH, which is considered to be the major ligand at the human melanocortin (MC)1 receptor (hMC1R), is produced from proopiomelanocortin (POMC) in the pituitary and in the skin by melanocytes and keratinocytes. Other POMC peptides are also produced in the skin and their concentrations exceed those of alpha-MSH by several fold. One of the most abundant is ACTH1-17. We have shown that adrenocorticotrophic hormone (ACTH)1-17 is more potent than alpha-MSH in stimulating melanogenesis in human melanocytes and unlike alpha-MSH produces a biphasic dose response curve. In this study we have examined the ability of ACTH1-17 to function as a ligand at the hMC1R. Competitive binding assays with [125I]Nle4 DPhe7 alpha-MSH as labelled ligand were carried out in HEK 293 cells transfected with the hMC1R. ACTH1-17 showed high affinity for the hMC1R with a Ki value of 0.21 +/- 0.03 nM which was slightly higher than that of 0.13 +/- 0.005 nM for alpha-MSH. ACTH1-17 was, however, more potent than alpha-MSH in increasing cAMP and IP3 production in the transfected cells. Our results demonstrate that ACTH1-17 is a potent agonist at the hMC1R. It is therefore possible that ACTH1-17, which is found in the skin in greater concentrations than alpha-MSH, has an important role in the regulation of human melanocytes and other cell types that express the hMC1R.