Search

The production of melanin is regulated by α-melanocyte-stimulating hormone (α-MSH), which is produced from proopiomelanocortin (POMC). Keratinocytes release POMC along with lower levels of α-MSH and ACTH. To clarify the mechanism of melanogenesis after ultraviolet (UV)-irradiation, this study focused on the expression of POMC and POMC-derived peptides after UV-irradiation. Western blot analysis and immunoassays indicated that both POMC and α-MSH-like immunoreactivity (α-MSH-LI) increased after UV-irradiation. However, other POMC-derived products were very low. In hypophysectomized mice, α-MSH-LI increased to the same level as in control mice after UV-irradiation. Structural analysis revealed that the major α-MSH-LI product was ACTH(1-8). Furthermore, ACTH(1-8) competed with [(125)I]-α-MSH for receptor binding and increased melanin production via a melanocortin-1 receptor. These results suggested that melanin was produced through ACTH(1-8) after UV-irradiation. Trypsin-like enzymatic activity, which is responsible for POMC activation, increased after UV-irradiation and was identified as tryptase. In mast cell-deficient mice, which do not produce tryptase, α-MSH-LI levels were unchanged after UV-irradiation. The present study demonstrates the production of ACTH(1-8) from POMC by tryptase, which is a novel peptide-processing mechanism in the extracellular compartment of the skin.

alpha-Melanocyte-stimulating hormone (alpha-MSH) is a tridecapeptide derived from the proopiomelanocortin by post-translational processing. In addition to its effects on melanocytes, alpha-MSH has potent anti-inflammatory effects when administered systemically or locally. The anti-inflammatory effects of alpha-MSH are mediated by direct effects on cells of the immune system as well as indirectly by affecting the function of resident non-immune cells. alpha-MSH affects several pathways implicated in regulation of inflammatory responses such as NF-kappaB activation, expression of adhesion molecules and chemokine receptors, production of pro-inflammatory cytokines and other mediators. Thus alpha-MSH may modulate inflammatory cell proliferation, activity and migration. The anti-inflammatory effects of alpha-MSH have been confirmed by means of animal models of inflammation such as irritant and allergic contact dermatitis, cutaneous vasculitis, asthma, inflammatory bowel disease, rheumatoid arthritis, ocular and brain inflammation. Most of the anti-inflammatory activities of alpha-MSH can be attributed to its C-terminal tripeptide KPV. K(D)PT, a derivative of KPV corresponding to the amino acid 193-195 of IL-1beta, is currently emerging as another tripeptide with potent anti-inflammatory effects. The anti-inflammatory potential together with the favourable physiochemical properties most likely will allow these agents to be developed for the treatment of inflammatory skin, eye and bowel diseases, allergic asthma and arthritis.

In the last years the neuropeptide a-melanocyte-stimulating hormone has emerged as a regulator of various biological processes far beyond the initially described pigment-inducing action. Expression of melanocortin-receptors (MC-Rs), mainly MC-1R, has been identified in several non-pigmentary human skin cell types. Moreover, expression of MC-5R has been detected in sebocytes and skin mast cells while MC-4R has been reported in dermal papilla cells, a specialized myofibroblast cell type regulating hair follicle activity. In accordance with early observations in the rat preputial gland alpha-melanocyte-stimulating and related peptides have lipogenic activity in the human system. The immunomodulatory actions of alpha-melanocyte-stimulating include regulation of expression and secretion of chemokines, downregulation of proinflammatory signal-induced NF-kappaB activation and adhesion molecule expression, prostaglandin E2 synthesis, as well as induction of interleukin-10. Depending on the cell type studied and the experimental conditions alpha-melanocyte-stimulating however may also have weak proinflammatory actions. In dermal fibroblasts alpha-melanocyte-stimulating was further reported to modulate collagen metabolism via upregulating interstitial collagenase as well as by attenuating the inductive effect of transforming growth factor B on 1 collagen synthesis and fibrosis, the latter pointing towards a potential role of a-melanocyte-stimulating during chronic inflammatory skin responses. The immunomudulatory effects, the established melanotropic action and the recently identified cytoprotective activity of a-melanocyte-stimulating on UVB-induced apoptosis and DNA damage may be part of the body's host defence in which this neuropeptide--typically induced by proinflammatory signals--maintains tissue homeostasis and prevents genotoxicity during inflammatory responses.

In this immunocytochemical study, we used light and electron microscopic observations in combination with morphometry to analyze the processing of pro-opiomelanocortin (POMC) in melanotrope cells of the intermediate pituitary of Xenopus laevis adapted to either a white or a black background. An antiserum was raised against a synthetic peptide including the cleavage site between ACTH and beta-lipotropic hormone in Xenopus. Western blotting revealed that this antiserum recognizes only a 38-kD protein, the POMC prohormone, from extracts of Xenopus neurointermediate pituitary. Light immunocytochemistry showed differential immunostaining for anti-POMC compared to anti-alpha-MSH. Anti-POMC was predominantly found in the perinuclear region, whereas anti-alpha-MSH yielded staining throughout the cytoplasm. Immunogold double labeling revealed that electron-dense secretory granules (DGs) show high immunoreactivity for anti-POMC and low immunoreactivity for anti-alpha-MSH. Electron-lucent granules (LGs) are immunoreactive to anti-alpha-MSH only. Moderately electron-dense granules (MGs) revealed intermediate reactivity compared to DGs and LGs. Background light intensity has significant effects on the morphology and the immunoreactivity of the secretory granules. Black-adapted animals have 4.5 times as many DGs and MGs as white-adapted animals. In addition, the MGs in black animals show 42% more anti-alpha-MSH immunogold than the MGs in white animals. Together, these findings indicate that the three granule types represent subsequent stages in granule maturation. Adaptation to a black background stimulates the formation of young immature granules, while at the same time the processing rate during granule maturation increases.

Proopiomelanocortin (POMC), the precursor for melanotropic, corticotropic, and opioid peptides such as alpha-melanocyte-stimulating hormone (alpha MSH), ACTH, and other related peptides, was originally identified as a product of the pituitary gland. However, recent evidence shows that POMC products can also be produced by nonpituitary tissues. Because keratinocytes, the major constituent of the epidermis exhibit the capacity to release a variety of proinflammatory and immunomodulatory mediators, the present study was performed to investigate whether human keratinocytes are able to produce POMC-derived peptides. Supernatants of human normal keratinocytes and an epidermal carcinoma cell line (A431) contained significant levels of immunoreactive alpha MSH and ACTH. Upon immuneprecipitation and size-exclusion chromatography, keratinocyte-derived alpha MSH exhibited a molecular mass of approximately 1 kD and was biologically active as demonstrated in a tyrosinase bioassay. Northern blot analysis revealed the expression of POMC-specific transcripts (1.3 kb) in both normal keratinocytes and A431 cells. The production of alpha MSH and ACTH could be significantly upregulated both at the protein and mRNA level upon treatment with phorbol myristate acetate, ultraviolet light, or interleukin 1. These data provide first evidence that human keratinocytes produce POMC-derived peptides such as alpha MSH and ACTH. Because POMC-derived peptides recently have been recognized as potent immunomodulatory mediators, their presence in the epidermis may have a major impact on the skin immune system.

The anatomical relationships between pro-opiomelanocortin-containing axons and serotonin neurons in the nucleus raphe dorsalis (NRD) of the rat were examined at the light microscope level with antibodies against CLIP (corticotropin-like intermediate lobe peptide), alpha-MSH (alpha-melanocyte-stimulating hormone) and serotonin. Sequential double labeling was performed with either immunofluorescence or peroxidase-antiperoxidase techniques. It was observed that the network of POMC-immunoreactive axons displayed a gradient of decreasing density from rostral to caudal levels and from dorsal to ventral parts or the NRD. The examples of close proximity between immunoreactive axons and serotonin cell bodies or dendrites were rather scarce. On the whole, the immunoreactive fibers seemed to run quasi-independently of the serotonin neurons.

This immunocytochemical study of the late postnatal development of the medio-basal hypothalamus revealed the presence of ACTH 1-39 like positivity in neurons of the arcuate nucleus form the begin of this study (day E 18-20) onwards. Alpha MSH positivity, on the contrary, is not present in cells of the same area before day P 16. No other areas in the developing medio-basal hypothalamus contain perikaryal positivity for alpha M-SH or ACTH 1-39. The pituitary contains ACTH 1-39 like positivity from the begin of this study (day E 18-20) onwards. Fibers are positive for alpha MSH during the fetal development of the medio-basal hypothalamus, demonstrating an overal reactivity without varicosities and restricted to bundles or neuropil areas. Towards P 16 the alpha MSH positivity diminishes in the whole medio-basal hypothalamus, remaining present only in large fibre systems like the fornix. ACTH 1-39 like fiber positivity is already distributed in arcuate and periventricular regions at days E 20-PO, reaching its mature extension at day P2. After P16 alpha MSH positive threads, possessing varicosities are restricted to the same areas as ACTH 1-39 like fiber positivity is.

Conjugate of horseradish peroxidase and wheat germ agglutinin (HRP-WGA conjugate) was injected into the midbrain central gray (MCG) of three adult rats. Frontal sections of the diencephalon were first treated with diaminobenzidine and hydrogen peroxide to detect the retrogradely transported conjugate. They were then stained immunohistochemically to detect pro-opiomelanocortin (POMC)-derived peptides (ACTH, beta-endorphin and alpha-MSH). The coexistence of the three POMC-derived peptides was confirmed by the immunohistochemistry of three consecutive sections stained with antiserum specific to each peptide. Some of the neuronal perikarya distributed in and around the arcuate nucleus were positive to the immunohistochemical stain for POMC-derived peptides, and, concomitantly, were labeled with HRP-WGA conjugate, which indicated that they projected to the MCG. They were mostly concentrated in the rostral three-fifths of the arcuate nucleus. The finding that some of the POMC neurons in the arcuate nucleus project to the midbrain central gray deserves interest, because the central gray is involved in analgesia induced by opioid peptides.