Proopiomelanocortin gene product regulation in keratinocytes

Myosin V colocalizes with melanosomes and subcortical actin bundles not associated with stress fibers in human epidermal melanocytes

Mutations of the gene encoding myosin V can produce a dilute or silvery hair color and various neurologic defects in mice and patients with Griscelli syndrome, leading to speculations that the myosin V motor protein plays a critical role in transporting melanosomes within melanocytes and neurosecretory vesicles within neurons. Therefore, we investigated the in vitro expression of myosin V in cultured normal human melanocytes, keratinocytes, and dermal fibroblasts using reverse transcriptase-polymerase chain reaction and northern blot analysis. Subcellular distribution of myosin V and proximity to actin bundles and melanosomes were determined by double indirect immunofluorescence labeling and immunogold electron microscopy. In all studied cells myosin V is expressed and treatment of melanocytes with the cyclic AMP-inducer 3-isobutyl-1-methylxanthine causes an induction of the myosin V message. In all cells myosin V colocalizes with actin bundles, concentrating in the subcortical cell zone. In the melanocyte it is closely associated with melanosomes. Quantitative analysis of myosin V labeling in melanocytes reveals a significantly higher (p < 0.005) presence of myosin V in the periphery of dendrites. These results suggest that myosin V is important in melanosome transport in human melanocytes. Possible roles in the other skin cells remain to be elucidated.

Does alpha-MSH have a role in regulating skin pigmentation in humans?

Over the years there has been much debate as to whether alpha-MSH has a role as a pigmentary hormone in humans. There are two main reasons for this. First, despite the observations in the 1960s that alpha-MSH increased skin darkening in humans, there are reports that the peptide has no effect on melanogenesis in cultured human melanocytes. Second, the human pituitary, unlike that of most mammals, secretes very little alpha-MSH and circulatory levels of the peptide in humans are extremely low. However, there is now evidence from several groups that alpha-MSH is capable of stimulating melanogenesis in cultured human melanocytes. Rather than producing an overall increase in melanin production, it appears that the peptide acts specifically to increase the synthesis of eumelanin. Such an action could well explain the previously observed skin darkening effects of alpha-MSH. It is also now known that alpha-MSH is not produced exclusively in the pituitary but has been found at numerous sites, including the skin where it is produced by several cell types. Related Proopiomelanocortin (POMC) peptides such as ACTH are also produced in human skin. The ACTH peptides act at the same receptor (MC-1) as alpha-MSH and certain of these would appear to be more potent than alpha-MSH in stimulating melanogenesis. The ACTH peptides are also present in greater amounts than alpha-MSH in human epidermis and it is likely that they play an important role in regulating pigmentary responses. These POMC peptides are released from keratinocytes in response to ultraviolet radiation (UVR) and it has been proposed that they serve as paracrine factors in mediating UV induced pigmentation. Their production by keratinocytes could therefore be critical in determining pigmentary responses and any changes in the availability of these POMC peptides might explain the variations in tanning ability seen in different individuals. However, the possibility that tanning ability is also dependent upon differences at the level of the MC-1 receptor cannot be ruled out and it has been suggested that an inability to tan may depend upon the presence of non-functional changes at the MC-1 receptor. alpha-MSH does, of course, affect human melanocytes in several ways and its stimulation of melanogenesis could be the consequence of some other fundamental action in the melanocyte. The peptide also has many other target sites in the skin and while it may have a role in regulating skin pigmentation in humans, it should not be viewed solely as a pigmentary peptide. alpha-MSH clearly has many different actions and its primary role in the skin may be to maintain homeostasis.

Effect of CRF and related peptides on calcium signaling in human and rodent melanoma cells

Corticotropin releasing factor (CRF) induces a rapid, within seconds, and dose-dependent increase in the intracellular Ca2+ in both human and hamster melanoma cells. This effect is inhibited by depletion of extracellular calcium using 3 mM EGTA and is attenuated by the CRF receptor antagonist, alpha-helical-CRF(9-41). Other peptides of the CRF superfamily, sauvagine and urocortin, also induce increases in cytoplasmic calcium concentration but at higher concentrations than CRF. We conclude that malignant melanocytes express CRF receptors, which are coupled to activation of plasma membrane calcium channels.

Identification and sequencing of a putative variant of proopiomelanocortin in human epidermis and epidermal cells in culture

Proopiomelanocortin (POMC) is a precursor polypeptide for various bioactive peptides, including adrenocorticotropic hormone, alpha-, beta-, and gamma-melanotropin, beta-endorphin, and beta-lipotropin. Although the classical source of POMC is the pituitary, various studies indicate the expression of POMC in several nonpituitary tissues. In this study, in situ hybridization with anti-sense cRNA riboprobe was used to show expression of POMC mRNA in human epidermis and cultured human epidermal cells (melanocytes and keratinocytes). POMC mRNA was amplified by reverse transcriptase-polymerase chain reaction using anti-sense and sense primers designed from Exons 2 and 3 of POMC gene. A approximately 300 bp product was present in normal human skin, grafted human skin, and cultured normal human melanocytes and keratinocytes. By Southern analysis this product was hybridized specifically to the POMC cDNA. Sequence analysis of the reverse transcriptase polymerase chain reaction product from tissues or cells showed 85% homology to POMC cDNA from human, bovine, pig, and monkey sources. This suggests the existence of a putative isoform or variant of POMC mRNA in human epidermis.

Expression of mu-opiate receptor in human epidermis and keratinocytes

There is increasing evidence that neurotransmitters play a crucial role in skin physiology and pathology. The expression and production of proopiomelanocortin molecules such as beta-endorphin in human epidermis suggest that an opiate receptor is present in keratinocytes. In this paper we show that human epidermal keratinocytes express a mu-opiate receptor on both the mRNA level and the protein level. Performing polymerase chain reaction with cDNA libraries from human epidermal keratinocytes gave the polymerase chain reaction products of the expected length, which were confirmed as mu-opiate receptors by Southern blot analysis. Using in situ hybridization techniques with a specific probe for mu-opiate receptors we detected the receptor in human epidermis. There was a cytoplasmic expression in all layers of the epidermis, which was more distinct in the suprabasal layers. Immunohistochemistry using the mu-opiate receptor-specific antibody indicates that epidermis expresses protein as well, and that the protein level is more elevated in the basal layer. The correlation between the locations of both mRNA and protein expression in skin indicates that the mu-opiate receptor has not only been transcribed but also has a specific function. To prove a function of the receptor we performed a functional assay using skin organ cultures from human skin transplants. After 48 h incubation with Naloxone or beta-endorphin the expression of the mu-opiate receptor in epidermis was significantly downregulated compared with the control. These results show that a functional receptor indeed exists in human epidermis.

Post-transcriptional regulation of UV induced TNF-alpha expression

Ultraviolet (UV) irradiation exerts multiple effects on skin cells, including the induction of several cytokines involved in immunomodulation. Specifically, UV irradiation has been shown to upregulate the level of tumor necrosis factor-alpha (TNF-alpha) mRNA in keratinocytes. To determine whether the induction of TNF-alpha mRNA is regulated by transcriptional or post-transcriptional mechanisms, we examined cells of keratinocytic lineage (SCC12F) for steady state level, transcription rate, and stability of TNF-alpha mRNA after UV irradiation. Within 4 h there was a 20-40-fold induction of TNF-alpha mRNA that persisted at lower levels through 48 h. Consistently, TNF-alpha protein secretion increased at 24 and 48 h after UV irradiation. UV irradiation increased the half-life of TNF-alpha mRNA from approximately 35 min to approximately 10 h. Conversely, the transcription rate of the TNF-alpha gene increased < 2-fold at the time of peak mRNA steady state levels. Thus, post-transcriptional mechanisms play a major role in UV induced TNF-alpha transcript level.

alpha-Melanocyte stimulating hormone inhibits tumour necrosis factor-alpha stimulated intercellular adhesion molecule-1 expression in normal cutaneous human melanocytes and in melanoma cell lines

alpha-Melanocyte stimulating hormone (alpha-MSH) was found significantly to reduce tumour necrosis factor-alpha (TNF-alpha) stimulated upregulation of intercellular adhesion molecule-1 (ICAM-1) in normal adult cutaneous melanocytes. The maximum inhibitory response to alpha-MSH was obtained at around 10(-10) mol/L alpha-MSH when cells were coincubated with alpha-MSH and TNF-alpha for 24 h. alpha-MSH had little or no effect on basal ICAM-1 expression in melanocytes and the effects of alpha-MSH could be mimicked with 3-isobutyl-1-methylxanthine (IBMX). Preliminary data in three human melanoma cell lines also showed alpha-MSH and forskolin to be effective in significantly reducing TNF-alpha stimulated ICAM-1 expression over 24 h. The extent of the inhibition varied from cell line to cell line and was greatest in those cells with the highest number of alpha-MSH receptors. These data suggest that alpha-MSH has the ability to oppose the action of the pro-inflammatory cytokine TNF-alpha on melanocytes and melanoma cells.

alpha-MSH and melanogenesis in normal human adult melanocytes

Normal human skin melanocytes do not pigment consistently to alpha-melanocyte stimulating hormone (alpha-MSH) in culture. The aim of this study was to establish media conditions in which to obtain a reproducible melanogenic response to alpha-MSH in these cells. Twenty-five media of varying mitogen composition were examined. As previously noted by other workers, melanocyte morphology and proliferation are greatly affected by media composition. However, under the majority of media conditions that supported melanocyte survival and proliferation, cells did not respond to alpha-MSH with any consistent increase in dopa oxidase activity or melanin content. In only one medium condition, where basic fibroblast growth factor (bFGF) was the sole mitogen present, alpha-MSH induced both an increase in dopa oxidase activity (at 48%) and in melanin content (of 283%).

Characterisation of ACTH peptides in human skin and their activation of the melanocortin-1 receptor

Alpha-Melanocyte-stimulating hormone (alpha-MSH) is a proopiomelanocortin (POMC)-derived peptide, which is produced in the pituitary and at other sites including the skin. It has numerous effects and in the skin has a pigmentary action through the activation of the melanocortin-1 (MC-1) receptor, which is expressed by melanocytes. Recent evidence suggests that the related POMC peptides such as adrenocorticotrophin (ACTH), which is the precursor of alpha-MSH, is also an agonist at the MC-1 receptor. By using immunocytochemistry, we confirmed the presence of alpha-MSH in human skin where staining was evident in keratinocytes and especially strong in melanocytes and possibly Langerhans cells. ACTH was also present and tended to show the strongest reaction in differentiated keratinocytes. Immunostaining was also observed for the prohormone convertases, PC1 and PC2, which are involved in the formation of ACTH and its cleavage to alpha-MSH, respectively. The amounts of immunoreactive ACTH exceeded those of alpha-MSH. Using HPLC we identified for the first time the presence of ACTH1-39, ACTH1-17, ACTH1-10, acetylated ACTH1-10, alpha-MSH, and desacetyl alpha-MSH in epidermis and in cultured keratinocytes. The ability of these peptides to activate the human MC-1 receptor was examined in HEK 293 cells that had been transfected with the receptor. All peptides increased adenylate cyclase in these cells with the following order of potency: ACTH1-17 > alpha-MSH > ACTH1-39 > desacetyl alpha-MSH > acetylated ACTH1-10 > ACTH1-10. ACTH1-17 also increased the dendricity and melanin content of cultured human melanocytes indicating that the peptide was able to activate MC-1 receptors when present in their normal location. However, as found with alpha-MSH, not all cultures were responsive and, as we have previously suggested, we suspect that this was the result of changes at the MC-1 receptor. Nevertheless, it would appear that ACTH peptides can serve as natural ligands of the MC-1 receptor on human melanocytes and their presence in the skin suggests that, together with alpha-MSH, they may have a role in the regulation of human melanocytes.

Identification and measurement of beta-endorphin levels in the skin during induced hair growth in mice

We describe new and effective techniques for extracting proopiomelanocortin (POMC)-derived peptides from mammaliar skin. Using this methodology (hot-acid extraction) and two independent HPLC-controlled RIA systems, we identify beta-endorphin peptide in mammalian skin and demonstrate significant hair cycle-dependent fluctuations in both the skin concentration and the in situ expression pattern of beta-endorphin (sebaceous glands) during the entire murine hair cycle. The observed anagen (growth phase) associated increase in beta-endorphin concentration and its decline during the follicle involution (catagen) or resting (telogen) phase raise the possibility of a regulatory function of this neuropeptide in cyclic changes of skin physiology.

Fas/Fas ligand interaction contributes to UV-induced apoptosis in human keratinocytes

Keratinocytes in human skin undergo apoptosis during various inflammatory processes and after ultraviolet (UV) irradiation. To determine if keratinocyte apoptosis may be mediated by the Fas/APO-1 receptor (CD95), a signal transduction pathway known to initiate programmed cell death of lymphocytes, we investigated Fas expression, modulation, and function in keratinocytes. Keratinocytes constitutively expressed the 2.5- and 1.9-kb Fas transcripts, as well as the 43-kDa Fas protein. Treatment of interferon-gamma-stimulated keratinocytes with Fas agonistic antibody significantly promoted their cell death, indicating that Fas in keratinocytes is functional. UV irradiation induced Fas mRNA expression within 16 to 24 h and Fas protein within 24 h and through 48 h after irradiation. Furthermore, keratinocytes constitutively expressed Fas ligand (FasL) mRNA and protein. UV irradiation induced FasL mRNA as early as 4 h after irradiation and elevated FasL mRNA levels were maintained for at least 24 h postirradiation. Moreover, a FasL neutralizing antibody significantly reduced UV-induced apoptosis of IFN-gamma-treated keratinocytes. Our data strongly suggest that the Fas system contributes to keratinocyte apoptosis in UV-irradiated human skin.

Trypsin-induced follicular papilla apoptosis results in delayed hair growth and pigmentation

Programmed cell death is a controlled process that leads to the elimination of single cells via apoptosis. Programmed cell death is fundamental to development, morphogenesis, and homeostasis. Proteases play a major role in the death process. We have previously shown that a serine protease, secreted by a keratinocyte cell line, can induce apoptosis in numerous cell lines. Here we show that serine proteases can induce cell death in vivo as well. Using a synchronized hair growth mouse model, we show that topical trypsin treatment following depilation induces cell death at the follicular papilla. This results in delaying hair growth and pigmentation. We speculate that trypsin might affect a receptor-mediated signaling pathway that leads to follicular papilla cell death.

Transforming growth factor-beta1 inhibits basal melanogenesis in B16/F10 mouse melanoma cells by increasing the rate of degradation of tyrosinase and tyrosinase-related protein-1

Current evidence suggests that melanogenesis is controlled by epidermal paracrine modulators. We have analyzed the effects of transforming growth factor-beta1 (TGF-beta1) on the basal melanogenic activities of B16/F10 mouse melanoma cells. TGF-beta1 treatment (48 h) elicited a concentration-dependent decrease in basal tyrosine hydroxylase and 3,4-dihydroxyphenylalanine (Dopa) oxidase activities, to less than 30% of the control values but had no effect on dopachrome tautomerase activity (TRP-2). The inhibition affected to similar extents the Dopa oxidase activity associated to tyrosinase-related protein-1 (TRP-1) and tyrosinase. This inhibition was noticeable between 1 and 3 h after the addition of the cytokine, and maximal after 6 h of treatment. The decrease in the enzymatic activity was paralleled by a decrease in the abundance of the TRP-1 and tyrosinase proteins. TGF-beta1 mediated this effect by increasing the rate of degradation of tyrosinase and TRP-1. Conversely, after 48 h of treatment, the expression of the tyrosinase gene decreased only slightly, while TRP-1 and TRP-2 gene expression was not affected. An increased rate of proteolytic degradation of TRP-1 and tyrosinase seems the main mechanism accounting for the inhibitory effect of TGF-beta1 on the melanogenic activity of B16/F10 cells.

Production and release of proopiomelanocortin (POMC) derived peptides by human melanocytes and keratinocytes in culture: regulation by ultraviolet B

It is demonstrated that ultraviolet B (UVB) radiation stimulates increased expression of the proopiomelanocortin (POMC) gene which is accompanied by production and release of alpha-melanocyte stimulating hormone (alpha-MSH) and adrenocorticotropin (ACTH) by both normal and malignant human melanocytes and keratinocytes. The production and release of both peptides are also stimulated by dibutyryl cyclic adenosine monophosphate (dbcAMP) and interleukin 1 alpha (IL-1 alpha) but not by endothelin-1 (ET-1) or tumor necrosis factor-alpha (TNF-alpha). N-acetyl-cysteine (NAC), a precursor of glutathione (GSH), an intracellular free radical scavenger, abolishes the UVB-stimulated POMC peptide production and secretion. Conclusions are as follows: (1) Cultured human cells of cutaneous origin, namely keratinocytes and melanocytes, can produce and express POMC; (2) POMC expression is enhanced by exposure to UVB, possibly through a cyclic AMP-dependent pathway; and (3) The action of UVB on POMC production may involve a cellular response to oxidative stress.