Ultraviolet stimulated melanogenesis by human melanocytes is augmented by di-acyl glycerol but not TPA

Melanogenesis and the Targeted Therapy of Melanoma

Pigment production is a unique character of melanocytes. Numerous factors are linked with melanin production, including genetics, ultraviolet radiation (UVR) and inflammation. Understanding the mechanism of melanogenesis is crucial to identify new preventive and therapeutic strategies in the treatment of melanoma. Here, we reviewed the current available literatures on the mechanisms of melanogenesis, including the signaling pathways of UVR-induced pigment production, MC1R's central determinant roles and MITF as a master transcriptional regulator in melanogenesis. Moreover, we further highlighted the role of targeting BRAF, NRAS and MC1R in melanoma prevention and treatment. The combination therapeutics of immunotherapy and targeted kinase inhibitors are becoming the newest therapeutic option in advanced melanoma.

Modulation of Diacylglycerol-Induced Melanogenesis in Human Melanoma and Primary Melanocytes: Role of Stress Chaperone Mortalin

Skin color/pigmentation is regulated through melanogenesis process in specialized melanin-producing cells, melanocytes, involving multiple signaling pathways. It is highly influenced by intrinsic and extrinsic factors such as oxidative, ultraviolet radiations and other environmental stress conditions. Besides determining the color, it governs response and tolerance of skin to a variety of environmental stresses and pathological conditions including photodamage, hyperpigmentation, and skin cancer. Depigmenting reagents have been deemed useful not only for cosmetics but also for pigmentation-related pathologies. In the present study, we attempted modulation of 1-oleoyl-2-acetyl-glycerol- (OAG-) induced melanogenesis in human melanoma and primary melanocytes. In both cell types, OAG-induced melanogenesis was associated with increase in enhanced expression of melanin, tyrosinase, as well as stress chaperones (mortalin and HSP60) and Reactive Oxygen Species (ROS). Treatment with TXC (trans-4-(Aminomethyl) cyclohexanecarboxylic acid hexadecyl ester hydrochloride) and 5/40 natural compounds resulted in their reduction. The data proposed an important role of mortalin and oxidative stress in skin pigmentation and the use of TXC and natural extracts for modulation of pigmentation pathways in normal and pathological conditions.

Stress chaperone mortalin regulates human melanogenesis

In order to identify the cellular factors involved in human melanogenesis, we carried out shRNA-mediated loss-of-function screening in conjunction with induction of melanogenesis by 1-oleoyl-2-acetyl-glycerol (OAG) in human melanoma cells using biochemical and visual assays. Gene targets of the shRNAs (that caused loss of OAG-induced melanogenesis) and their pathways, as determined by bioinformatics, revealed involvement of proteins that regulate cell stress response, mitochondrial functions, proliferation, and apoptosis. We demonstrate, for the first time, that the mitochondrial stress chaperone mortalin is crucial for melanogenesis. Upregulation of mortalin was closely associated with melanogenesis in in vitro cell-based assays and clinical samples of keloids with hyperpigmentation. Furthermore, its knockdown resulted in compromised melanogenesis. The data proposed mortalin as an important protein that may be targeted to manipulate pigmentation for cosmetic and related disease therapeutics.

Downregulation of NFAT2 promotes melanogenesis in B16 melanoma cells

Nuclear factor of activated T-cells (NFAT) proteins are, calcium-regulated transcription factors, key regulator of stimulation-dependent gene activation. In our microarray analysis for the genes expressed in human black and white hairs, NFAT2 was significantly upregulated in the white hair, compared to the black hair. The aim of this study was to investigate functional role of NFAT2 in melanogenesis. Western blot analysis was performed to investigate the expression of NFAT2 protein in B16 melanoma cells. Our data showed that NFAT2 expression was increased in the hypopigmented B16 cells, while tyrosinase and MITF expression was decreased. To investigate the potential role of NFAT2, the recombinant adenovirus expressing microRNA specific for NFAT2 was transduced into the cultured B16 melanoma cells. Consistently, inhibition of NFAT2 enhanced tyrosinase activity and melanin content. Moreover, cyclosporine A, which is known as a calcineurin inhibitor blocking NFAT activation, enhanced tyrosinase activity and melanin content. These data suggest that NFAT2 may play an important role in regulation of melanogenesis in melanocyte.

Acetoside inhibits α-MSH-induced melanin production in B16 melanoma cells by inactivation of adenyl cyclase

Objectives

The aim of the study was to determine the mechanism of the whitening effect of acteoside.

Methods

We used tyrosinase activity and melanin production stimulated in B16 melanoma cells by α-melanocyte stimulating hormone (α-MSH) or forskolin to measure the whitening effect of acteoside.

Key findings

Acteoside did not directly inhibit mushroom tyrosinase activity, but dose-dependently inhibited tyrosinase activity and melanin production in B16 melanoma cells stimulated by 1 μmol/l α-MSH. Acteoside also reduced cyclic AMP levels in cells stimulated by 1 μmol/l α-MSH, suggesting direct inhibition of adenyl cyclase. Acteoside also inhibited productionofbothmelanin and cyclic AMP in cells stimulated by 1 μmol/l forskolin, an adenyl cyclase activator. Acteoside showed antioxidant activity in a cell-free DPPH (1-diphenyl-2-picrylhydroazyl) assay and inhibited generation of intracellular reactive oxygen species.

Conclusions

These results suggest that the whitening activity of acteoside results from inhibition of adenyl cyclase and α-MSH signalling.

Agouti protein, mahogunin, and attractin in pheomelanogenesis and melanoblast-like alteration of melanocytes: a cAMP-independent pathway

Melanocortin-1 receptor (MC1R) and its ligands, α-melanocyte stimulating hormone (αMSH) and agouti signaling protein (ASIP), regulate switching between eumelanin and pheomelanin synthesis in melanocytes. Here we investigated biological effects and signaling pathways of ASIP. Melan-a non agouti (a/a) mouse melanocytes produce mainly eumelanin, but ASIP combined with phenylthiourea and extra cysteine could induce over 200-fold increases in the pheomelanin to eumelanin ratio, and a tan-yellow color in pelletted cells. Moreover, ASIP-treated cells showed reduced proliferation and a melanoblast-like appearance, seen also in melanocyte lines from yellow (A^y/a and Mc1r^e/ Mc1r^e) mice. However ASIP-YY, a C-terminal fragment of ASIP, induced neither biological nor pigmentary changes. As, like ASIP, ASIP-YY inhibited the cAMP rise induced by αMSH analog NDP-MSH, and reduced cAMP level without added MSH, the morphological changes and depigmentation seemed independent of cAMP signaling. Melanocytes genetically null for ASIP mediators attractin or mahogunin (Atrn^mg-3J/mg-3J or Mgrn1^md-nc/md-nc) also responded to both ASIP and ASIP-YY in cAMP level, while only ASIP altered their proliferation and (in part) shape. Thus, ASIP–MC1R signaling includes a cAMP-independent pathway through attractin and mahogunin, while the known cAMP-dependent component requires neither attractin nor mahogunin.

Whitening activity of luteolin related to the inhibition of cAMP pathway in alpha-MSH-stimulated B16 melanoma cells

To examine the possibility of luteolin as a whitening agent, we measured antioxidant activity using DPPH assay, NBT/XO assay and intracellular ROS scavengning assay and depigmenting activity using tyrosinase assay, alpha-MSH-induced melanin production in B-16 cells. Luteolin showed dose-dependent anti-oxidant activity in DPPH, NBT/XO and intracellular ROS assay. Also, luteolin directly inhibited xanthine oxidase activity in a dose-dependent manner. Although luteolin did not directly inhibit tyrosinase activity, it dose-dependently inhibited both tyrosinase activity and melanin production in B16 melanoma cells stimulated by 1 microM alpha-MSH. Luteolin dose-dependently inhibited cAMP levels in B16 melanoma cells stimulated by 1 microM alpha-MSH and 1 microM forskolin, which suggest that luteolin directly inhibits adenyl cyclase in B16 melanoma cells. Therefore, these results suggest that whitening activity of luteolin may be due to the inhibition of adenyl cyclase involved in the signal pathway of alpha-MSH in B16 melanoma cells.

Inhibition of MITF and tyrosinase by paeonol-stimulated JNK/SAPK to reduction of phosphorylated CREB

Tyrosinase and its transcriptional regulator microphthalmia-associated transcription factor (MITF) play critical roles in regulation of melanogenesis, and are required for environmental cues or agents in modulation of melanin synthesis. Identifying the signals regulating tyrosinase and MITF is crucial to understanding how pigmentation responds to extracellular stimuli. In this report, we discovered that paeonol down-regulated melanin production via decreasing MITF expression and consequent mRNA and protein levels of tyrosinase. We also found that paeonol reduced phosphorylation of a cAMP responsive element binding protein (phospho-CREB), which binds and activates MITF. A selective inhibitor of c-jun N-terminal or stress-activated protein kinases (JNK/SAPK)-SP600125 significantly reversed paeonol-induced down-regulation of melanogenesis. Inhibition of cAMP/PKA pathway intensified the hypopigmentation response to paeonol. These results identify a mechanism in which paeonol induces the down-regulation of melanogenesis through inhibition of CREB phosphorylation, leading to the expression reduction of MITF and subsequently tyrosinase. The key kinase mediating the effects of paeonol on melanogenesis in B16F10 cells is JNK/SAPK. Additionally, the cAMP/PKA pathway may take part in this process.

Human skin pigmentation: melanocytes modulate skin color in response to stress

All organisms, from simple invertebrates to complex human beings, exist in different colors and patterns, which arise from the unique distribution of pigments throughout the body. Pigmentation is highly heritable, being regulated by genetic, environmental, and endocrine factors that modulate the amount, type, and distribution of melanins in the skin, hair, and eyes. In addition to its roles in camouflage, heat regulation, and cosmetic variation, melanin protects against UV radiation and thus is an important defense system in human skin against harmful factors. Being the largest organ of the body that is always under the influence of internal and external factors, the skin often reacts to those agents by modifying the constitutive pigmentation pattern. The focus of this review is to provide an updated overview of important physiological and biological factors that increase pigmentation and the mechanisms by which they do so. We consider endocrine factors that induce temporary (e.g., during pregnancy) or permanent (e.g., during aging) changes in skin color, environmental factors (e.g., UV), certain drugs, and chemical compounds, etc. Understanding the mechanisms by which different factors and compounds induce melanogenesis is of great interest pharmaceutically (as therapy for pigmentary diseases) and cosmeceutically (e.g., to design tanning products with potential to reduce skin cancer risk).

Proopiomelanocortin (POMC): the cutaneous roles of its melanocortin products and receptors

The precursor protein proopiomelanocortin (POMC) produces many biologically active peptides via a series of enzymatic steps in a tissue-specific manner, yielding the melanocyte-stimulating hormones (MSHs), corticotrophin (ACTH) and beta-endorphin. The gene for alpha-MSH is encoded for by the POMC gene, but alpha-MSH cannot be produced from POMC gene transcription and translation without these specific post-translational proteolytic steps taking place. The MSHs and ACTH bind to the extracellular G-protein-coupled melanocortin receptors (MCR), of which there are five subtypes. Two (MC1R and MC5R) show widespread cutaneous expression. ACTH and alpha-MSH bind to MC1R to influence both pigmentation and the immune system. MC5R regulates the sebaceous glands. Mutations in the MC1R gene lead to fair skin and red hair in humans, which is also seen with inactivating human POMC gene mutations. MC1R mutant receptor expression can also correlate with an increased incidence of the three commonest forms of skin cancer. Other mutations can occur in the POMC system or parallel interacting pathways, such as in prohormone convertase 1 and agouti signalling protein, a human homologue of murine agouti protein. However, they do not necessarily affect skin colour or function in humans, and further studies are needed to clarify these observations.

Glycyrrhizin induces melanogenesis by elevating a cAMP level in b16 melanoma cells

In mammalian melanocytes, melanin synthesis is controlled by tyrosinase, the critical enzyme in the melanogenic pathway. A recent report showed that the stimulation of melanogenesis by glycyrrhizin (GR) is because of an increased tyrosinase expression at mRNA and protein levels. But, the molecular events of melanogenesis induced by GR remain to be elucidated. In this study, using B16 melanoma cells, we showed that GR activated activator protein-1 (AP-1) and cyclic response filament "CRE" promoters, but not the nuclear factor-kappaB promoter. In addition, although GR stimulated mitogen-activated protein (MAP) kinase, p42/44(mapk), consistent with GR-induced AP-1 promoter activation, GR-induced melanogenesis was not blocked by PD98059, an MEK1 inhibitor, suggesting that MAPkinase induced by GR does not have a direct effect on the level of melanin content. But, GR-induced melanogenesis was inhibited by an inhibitor of protein kinase A (H-89). This result was further confirmed by the fact that GR induced the phosphorylation of CRE binding protein (CREB) and inhibition of glycogen synthase kinase 3beta phosphorylation as well as the production of cAMP, indicating that GR induces melanogenesis through cAMP signaling. In addition, the fact that GR-induced CRE activation was blocked by H-89 but GR-induced increase of cAMP production was not suggests that GR operates upstream of protein kinase A.

Effects of PGF2alpha on human melanocytes and regulation of the FP receptor by ultraviolet radiation

Prostaglandins are potent lipid hormones that activate multiple signaling pathways resulting in regulation of cellular growth, differentiation, and apoptosis. In the skin, prostaglandins are rapidly released by keratinocytes following ultraviolet radiation and are chronically present in inflammatory skin lesions. We have shown previously that melanocytes, which provide photoprotection to keratinocytes through the production of melanin, express several receptors for prostaglandins, including the PGE2 receptors EP1 and EP3 and the PGF2alpha receptor FP, and that PGF2alpha stimulates melanocyte dendricity. We now show that PGF2alpha stimulates the activity and expression of tyrosinase, the rate-limiting enzyme in melanin synthesis. Analysis of FP receptor regulation showed that the FP receptor is regulated by ultraviolet radiation in melanocytes in vitro and in human skin in vivo. We also show that ultraviolet irradiation stimulates production of PGF2alpha by melanocytes. These results show that PGF2alpha binding to the FP receptor activates signals that stimulate a differentiated phenotype (dendricity and pigmentation) in melanocytes. The regulation of the FP receptor and the stimulation of production of PGF2alpha in melanocytes in response to ultraviolet radiation suggest that PGF2alpha could act as an autocrine factor for melanocyte differentiation.

Stimulation of melanogenesis by tetradecanoylphorbol 13-acetate (TPA) in mouse melanocytes and neural crest cells

In vitro studies have shown that the phorbol ester, 12-tetradecanoylphorbol 13-acetate (TPA) induces neural crest cell differentiation into melanocytes, and stimulates proliferation and differentiation of normal melanocytes. As TPA is not a physiological agent, its action is clearly mimicking some in vivo pathway involved in these processes. An understanding of the effect of TPA on the expression of melanogenic genes will therefore provide valuable insight into the molecular mechanisms regulating melanocyte differentiation. In this study, we utilized primary cultures of neural crest cells and an immortalized melanocyte cell line (DMEL-2) which proliferates in the absence of TPA, to explore the effects of TPA on key melanogenic effectors. In neural crest cells, TPA was found to be necessary for both microphthalmia associated transcription factor (Mitf) up-regulation and for melanin synthesis. Using northern blots, we show that in DMEL-2 cells, TPA significantly increases the messenger ribonucleic acid (mRNA) levels of the tyrosinase gene family (tyrosinase, Tyrp1 and Dct) and the expression of Mitf. Western blots demonstrate that in these TPA-treated cells there is a concomitant increase in Tyr, Tyrp1 and glycosylated Dct protein levels. Pax3, a known Mitf regulator, is unaltered by TPA treatment. This study demonstrates the utility of a novel cell line for investigating the long-term effects of TPA on melanogenesis and provides an understanding of how TPA enhances mouse melanocyte differentiation.

Topical prostaglandin analog (PGE2) in vitiligo--a preliminary study

BACKGROUND

Several therapeutic options are available for the treatment of vitiligo, but none is uniformly effective. Prostaglandin has been shown to play a role in melanocyte proliferation and melanogenesis. Topical application of prostaglandin E2 (PGE2) over mice skin has been shown to increase melanocyte density.

AIM

To evaluate the role of topical PGE2 in the treatment of vitiligo.

METHODS

We enrolled 27 patients with limited vitiligo lesions in this study. Patients were instructed to apply a translucent gel containing 0.5 mg/3 g (166.6 micro g/g) PGE2 to depigmented skin.

RESULTS

Twenty-four patients were evaluated at the end of 6 months, as three patients withdrew for reasons unrelated to the study. At the end of treatment, 15 patients showed marked to complete repigmentation and three patients showed moderate repigmentation. The remaining six patients showed none to minimal repigmentation.

CONCLUSIONS

Marginal repigmentation with hyperpigmented borders was seen in the majority of lesions. Our results are encouraging and offer a new and potentially efficacious treatment for this pigmentation disorder.

Mechanism and clinical significance of prostaglandin-induced iris pigmentation

The new glaucoma drugs latanoprost, isopropyl unoprostone, travoprost, and bimatoprost cause increased pigmentation of the iris in some patients. The purpose of the present article is to survey the available preclinical and clinical data on prostaglandin-induced iris pigmentation and to assess the phenomenon from a clinical perspective. Most of the data have been obtained with latanoprost, and it appears that there is a predisposition to latanoprost-induced iris pigmentation in individuals with hazel or heterochromic eye color. As latanoprost and travoprost are selective agonists for the prostaglandin F(2alpha) receptor, it is likely that the phenomenon is mediated by this receptor. Several studies indicate that latanoprost stimulates melanogenesis in iridial melanocytes, and transcription of the tyrosinase gene is upregulated. The safety aspects of latanoprost-induced iris pigmentation have been addressed in histopathologic studies, and no evidence of harmful consequences of the side effect has been found. Although a final assessment of the clinical significance of prostaglandin-induced iris pigmentation currently is impossible to make, it appears that the only clear-cut disadvantage is a potential heterochromia between the eyes in unilaterally treated patients because the heterochromia is likely to be permanent, or very slowly reversible.

Skin pigmentation enhancers

The highest incidences of cancer are found in the skin, but endogenous pigmentation is associated with markedly reduced risk. Agents that enhance skin pigmentation have the potential to reduce both photodamage and skin cancer incidence. The purpose of this review is to evaluate agents that have the potential to increase skin pigmentation. These include topically applied substances that simulate natural pigmentation: dihydroxyacetone and melanins; and substances that stimulate the natural pigmentation process: psoralens with UVA (PUVA), dimethylsulfoxide (DMSO), L-tyrosine, L-Dopa, lysosomotropic agents, diacylglycerols, thymidine dinucleotides, DNA fragments, melanocyte stimulating hormone (MSH) analogs, 3-isobutyl-1-methylxanthine (IBMX), nitric oxide donors, and bicyclic monoterpene (BMT) diols. These agents are compared with regards to efficacy when administered to melanoma cells, normal human epidermal melanocytes, animal skin, and human skin. In addition, mechanisms of action are reviewed since these may reveal issues related to both efficacy and safety. Both dihydroxyacetone and topically applied melanins are presently available to the consumer, and both of these have been shown to provide some photoprotection. Of the pigmentation stimulators, only PUVA and MSH analogs have been tested extensively on humans, but there are concerns about the safety and side effects of both. At least some of the remaining pigmentation stimulators under development have the potential to safely induce a photoprotective tan.

A comparative study of the effect of pigment on drug toxicity in human choroidal melanocytes and retinal pigment epithelial cells

The aim of this study was to investigate whether the presence of pigment affects the sensitivity of pigmented cells of the eye, retinal pigment epithelium (RPE) and choroidal melanocytes (CMs) to the cytotoxic effects of xenobiotic drugs. Two approaches were used to compare pigmented versus unpigmented cells: RPE cells were repigmented by phagocytosis of synthetic melanin; UVB irradiation was used to induce an increase in pigment in both RPE and CMs. Three drugs known to induce toxicity in the eye, tamoxifen, chloroquine and thioridazine, were used to assess the sensitivity of cells to xenobiotic drugs. RPE cells were more resistant than CMs to the cytotoxic effects of all three drugs by a factor of 5-fold for tamoxifen, 7-fold for thioridazine and 30-fold for chloroquine. When RPE cells were repigmented using synthetic melanin, their sensitivity to tamoxifen was unchanged, they showed a slightly improved response to thioridazine (after 3 days of incubation with this drug), but they showed greatly increased toxicity to chloroquine (after 1 and 3 days of exposure to the drug), suggesting accumulation of this latter drug on the synthetic melanin. UVB irradiation was used to achieve an increase in the pigment content of both RPE and CMs. CMs were much more sensitive to UVB than RPE cells. CMs appeared to synthesise pigment via DOPA oxidase activity; RPE cells showed an increase in fluorescent material independent of any detectable DOPA oxidase activity. Irrespective of the nature of the pigment that UVB induced in melanocytes and RPE cells, their subsequent response to thioridazine and chloroquine was unchanged by the presence of this pigment.

Protein kinase C activation increases binding of transcription factor PU.1 in murine tissue macrophages

PU.1 is a transcription factor found in macrophages, B cells, neutrophils, and hemopoietic stem cells. In macrophages PU.1 regulates a number of genes, including c-fms, CD11b, CD18, and FcgammaR1b. Previously, in primary macrophages PU.1 binding to the sequence GAGGAA was found to be induced by treatment with bacterial lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma). Here we investigated the role of protein kinase C (pKC) in the induction of PU.1 binding in macrophages. We report that pharmacological activation of pKC increases PU.1 binding, while inactivation of pKC inhibits the increases in PU.1 binding by agents which activate pKC in macrophages (LPS and tumor necrosis factor-alpha), but not by an agent which does not activate pKC (IFN-gamma). pKC activation may therefore be one pathway by which PU.1 binding may be increased in primary macrophages.

Human pigmentation genes and their response to solar UV radiation

Identification and characterisation of the genes involved in melanin pigment formation, together with the study of how their action is influenced by exposure to UV radiation, is providing a molecular understanding of the process of skin photoprotection through tanning. The mechanisms underlying this change in epidermal melanin involve both a transcriptional response of the pigmentation genes and post-translational control of the melanin biosynthetic pathway. UV rays are known to interact with numerous molecules within cells, and among these the photochemical reactions involving lipids and DNA are implicated in modulating melanogenesis. The combination of DNA damage, the formation of diacylglycerol, and the action of the melanocyte stimulating hormone receptor are all likely to be involved in UV-induced tanning.

Thymidine dinucleotides induce S phase cell cycle arrest in addition to increased melanogenesis in human melanocytes

Although the induction of pigmentation following exposure of melanocytes to ultraviolet light in vivo and in vitro is well documented, the intracellular mechanisms involved in this response are not yet fully understood. Exposure to UV-B radiation leads to the production of DNA damage, mainly cyclobutane pyrimidine dimers, and it was recently suggested that the thymidine dinucleotide pTpT, mimicking small DNA fragments released in the course of excision repair mechanisms, could trigger melanin synthesis. We now report that the thymidine dinucleotide pTpT induces melanogenesis both in human normal adult melanocytes and in human melanoma cells. Thus, the SOS-like response suggested by Gilchrest's work to be evolutionary conserved, based primarily on work in murine cells and guinea pigs, is also apparently present in the human. Thymidine dinucleotide is nontoxic to melanoma cells and does not induce apoptosis in these cells, but induces S phase cell cycle arrest and a proliferation slow down. Because thymidine excess in culture medium leads to the synchronization of cells in S phase, we investigated whether this phenomenon was involved in the increase in melanin synthesis. We show that melanin synthesis is specifically triggered by the dimeric form of the thymidine and not by the monomeric form pT. Thus, our data strongly support that thymidine dinucleotides pTpT mimic at least part of the effects of ultraviolet irradiation, and may hence represent an invaluable model in the study of the molecular events involved in melanogenesis induction triggered through DNA damage.

The murine misty mutation: phenotypic effects on melanocytes, platelets and brown fat

Although the recessive murine mutation misty (m) is well known, its phenotype has never been reported beyond brief descriptions of a dilution of coat color and white spotting of the belly and extremities, suggesting a developmental mutation. A report in abstract has also suggested effects on white fat and body weight. Here, we report effects of the homozygous misty mutation on an unusual combination of three cell types: melanocytes, platelets, and brown fat. Brown fat appeared to be completely absent from all expected locations in neonatal m/m mice. A prolonged bleeding time was observed; platelet count and platelet serotonin and ATP levels were normal, but the level of ADP in m/m platelets was low. Primary cultures and immortal lines of melanocytes from m/m mice showed several abnormalities. There was a marked deficiency in net proliferation, suggesting that the color dilution and spotting in vivo may result from reduced numbers of melanocytes and their precursors. m/m melanocytes were also hyperdendritic in morphology, overproduced melanin, and had deficient responses to the cAMP agonists cholera toxin and melanocyte-stimulating hormone, which normally promote melanin production. The misty gene product may be involved in adenine nucleotide metabolism or signaling.

Complementation of hypopigmentation in p-mutant (pink-eyed dilution) mouse melanocytes by normal human P cDNA, and defective complementation by OCA2 mutant sequences

Mutations in the P gene of humans and the homologous p-locus of mice, respectively, result in the homologous disorders oculocutaneous albinism type 2 (OCA2) and pink-eyed dilution. Although clearly required for melanin biosynthesis, the specific function of the P gene product, a melanosomal transmembrane protein expressed in melanocytes of the skin, hair, and eyes, is not yet known. Here we describe lines of immortal melanocytes and melanoblasts from mice of the null genotype p(cp)/p(25H). These p-null melanocytes were severely hypopigmented, although they and the melanoblasts expressed mRNAs for a number of melanosomal proteins. Proliferation of the p-null melanoblasts was normal. Both diploid and immortal p-null melanocytes grew more slowly than wild-type melanocytes, however, and were unusually susceptible to the antibiotic G418; these abnormalities were corrected by culture in high concentrations of L-tyrosine. Transfection of the p-null melanocytes with full-length normal human P cDNA resulted in complementation of deficient melanin biosynthesis and hypopigmentation. In contrast, transfection with mutant human P cDNAs containing amino acid substitutions (A481T, V443I) found in patients with OCA2 resulted in minimal or partial correction, consistent with the corresponding pigmentation phenotypes in patients with these mutations. These results demonstrate the utility of this model system for distinguishing true OCA2 mutations from nonpathologic polymorphisms and for quantitating the effect of these mutations on P function.

Ultraviolet B radiation acts through the nitric oxide and cGMP signal transduction pathway to stimulate melanogenesis in human melanocytes

Ultraviolet B (UVB) radiation is the main physiological stimulus for human skin pigmentation; however, the molecular mechanisms underlying this process are still unclear. Recently, nitric oxide (NO) and cGMP have been involved in mediation of skin erythema induced by UVB. Therefore, we investigated the role of NO and cGMP in UVB-induced melanogenesis. In this study, we demonstrated that UVB stimulation of melanogenesis was mimicked by exogenous NO donors. Additionally, we showed that NO stimulated cGMP synthesis and that cGMP was also a potent stimulator of melanogenesis. Furthermore, the inhibition of the melanogenic effect of NO by guanylate cyclase inhibitor demonstrated that NO mediated its effect through the activation of guanylyl cyclase. Interestingly, 1 min after UVB irradiation, we observed a significant increase in cGMP content in melanocytes. The effects of UVB on cGMP production and on melanogenesis were blocked by both guanylate cyclase and NO synthase inhibitors. Additionally, inhibition of cGMP-dependent kinase also prevented the stimulation of melanogenesis by UVB and NO. Therefore, we concluded that NO and cGMP production is required for UVB-induced melanogenesis and that cGMP mediated its melanogenic effects mainly through the activation of cGMP-dependent kinase.

Regulation of melanogenesis in B16 mouse melanoma cells by protein kinase C

Melanogenesis is regulated by a variety of environmental and hormonal factors. In this study, we showed that protein kinase C (PKC) plays a major role in regulating melanogenesis in B16 mouse melanoma cells. Chronic treatment of B16 cells with phorbol dibutyrate resulted in a concentration-dependent loss of density-dependent induction of tyrosinase activity, which correlated positively with a concentration-dependent loss of PKC enzyme activity. In contrast, B16 clones overexpressing PKC alpha had increased tyrosinase activity. Different phorbol derivatives inhibited tyrosinase activity and depleted cellular PKC alpha in a manner that reflected their reported tumor-promoting activity. Western blotting analysis showed that phorbol dibutyrate decreased the amount of the brown locus gene product (TRP-1) by 50% and lowered the amount of the albino locus gene product (tyrosinase) to undetectable levels. None of the phorbol derivatives affected the level of the slaty locus protein (TRP-2). The decrease in tyrosinase and TRP-1 protein levels was found to be due to a decrease in the mRNA encoded by these genes. In addition to inhibiting the density-dependent increase in tyrosinase activity, phorbol dibutyrate inhibited some, but not all, of the 8-bromocyclic AMP-induced increase in tyrosinase activity. This was accompanied by a decrease in the amount of tyrosinase protein induced by 8-bromocyclic AMP. Although 8-bromocyclic AMP did not change the level of TRP-1, it did reverse the decrease in the amount of this protein induced by phorbol dibutyrate. The amount of TRP-2 was not altered by any of these agents. These data suggest that PKC regulates melanogenesis primarily by controlling the constitutive expression of tyrosinase and, to a lesser extent, TRP-1.

DNA damage enhances melanogenesis

Although the ability of UV irradiation to induce pigmentation in vivo and in vitro is well documented, the intracellular signals that trigger this response are poorly understood. We have recently shown that increasing DNA repair after irradiation enhances UV-induced melanization. Moreover, addition of small DNA fragments, particularly thymine dinucleotides (pTpT), selected to mimic sequences excised during the repair of UV-induced DNA photoproducts, to unirradiated pigment cells in vitro or to guinea pig skin in vivo induces a pigment response indistinguishable from UV-induced tanning. Here we present further evidence that DNA damage and/or the repair of this damage increases melanization. (i) Treatment with the restriction enzyme Pvu II or the DNA-damaging chemical agents methyl methanesulfonate (MMS) or 4-nitroquinoline 1-oxide (4-NQO) produces a 4- to 10-fold increase in melanin content in Cloudman S91 murine melanoma cells and an up to 70% increase in normal human melanocytes, (ii) UV irradiation, MMS, and pTpT all upregulate the mRNA level for tyrosinase, the rate-limiting enzyme in melanin biosynthesis. (iii) Treatment with pTpT or MMS increases the response of S91 cells to melanocyte-stimulating hormone (MSH) and increases the binding of MSH to its cell surface receptor, as has been reported for UV irradiation. Together, these data suggest that UV-induced DNA damage and/or the repair of this damage is an important signal in the pigmentation response to UV irradiation. Because Pvu II acts exclusively on DNA and because MMS and 4-NQO, at the concentrations used, primarily interact with DNA, such a stimulus alone appears sufficient to induce melanogenesis. Of possible practical importance, the dinucleotide pTpT mimics most, if not all, of the effects of UV irradiation on pigmentation, tyrosinase mRNA regulation, and response to MSH without the requirement for antecedent DNA damage.

Cutaneous cell and extracellular matrix responses to ultraviolet-B irradiation

The present study examined fibroblasts and keratinocytes in monolayers and cultured within dermal and skin substitutes and their use in assessing the effect of UVB irradiation on cutaneous cells and extracellular matrix organization. Dermal substitutes (DS) were produced by incorporating normal fibroblasts into a collagen lattice and skin substitutes (SS) were obtained by seeding normal keratinocytes onto the DS. Keratinocyte monolayers, fibroblast monolayers, DS, and SS were exposed once a day to a UVB source (10 mJ/cm2). The irradiation protocol was stopped when the keratinocytes of the non-irradiated cultures (control groups) had reached confluence. Microscopic observations revealed that UVB radiation decreased both fibroblast and keratinocyte growth and enhanced their differentiation resulting in (1) less fibroblasts in the DS and SS, and (2) incomplete coverage of the DS by keratinocytes. Microscopic observations and histological analyses revealed major morphological changes. Both cell types became bigger and presented wide nuclei and vacuoles in the cytoplasm. No organized deep epidermal layer was observed in irradiated compared to non-irradiated SS. Irradiated DS and SS extracellular matrices showed an irregular aggregating collagen fiber organization with serious discrepancies suggesting large defects in the structural properties of the extracellular matrix. The present study demonstrated that exposure to a UVB source led to profound morphological and functional disturbances in both cutaneous cells and in the extracellular matrices of the DS and SS. The present technology would be of great interest for step-by-step studies of UVR effects on cutaneous cell morphology and functional properties, and could be an alternative to using animals for pharmacological and toxicological evaluations.

Mechanisms of ultraviolet light-induced pigmentation

Work in the past 8 years, particularly in the past 1-2 years, has greatly expanded our understanding of the mechanisms by which ultraviolet irradiation stimulates melanogenesis in the skin. A direct effect of UV photons on DNA results in up-regulation of the gene for tyrosinase, the rate-limiting enzyme in melanin synthesis, as well as an increase in cell surface expression of receptors for at least one of the several known keratinocyte-derived melanogenic factors, MSH. Direct effects of UV on melanocyte membranes, releasing DAG and arachidonic acid, may also play a role in the tanning response. Diacylglycerol may activate PKC-beta, which in turn phosphorylates and activates tyrosinase protein; the pathways by which products of other inflammatory mediator cascades may act on melanogenesis are unknown. The tanning response also relies heavily on UV-stimulated increased production and release of numerous keratinocyte-derived factors including bFGF, NGF, endothelin-1 and the POMC-derived peptides MSH, ACTH, beta-LPH and beta-endorphin. These factors variably induce melanocyte mitosis, increase melanogenesis, enhance dendricity and prevent apoptotic cell death following the UV injury. Thus, events within the epidermal melanin unit conspire to maintain or increase melanocyte number, increase melanin pigment throughout the epidermis. Overall, ultraviolet-induced melanogenesis may be one part of a eukaryotic SOS response to damaging ultraviolet irradiation that has evolved over time to provide a protective tan in skin at risk of further injury from sun exposure. These recent insights into the mechanisms underlying ultraviolet-induced melanogenesis offer the opportunity for novel therapeutic approaches to minimizing acute and chronic photodamage in human skin.

Topically applied diacylglycerols increase pigmentation in guinea pig skin

Exposure of human and murine melanocytes in vitro to the diacylglycerol (DAG) 1-oleoyl-2-acetyl-sn-glycerol (OAG) markedly increases melanin production within 24 h. To determine whether OAG can increase melanin production in vivo, increasing concentrations of OAG (10-60 mg/ml) in propylene glycol were applied daily for 5 d to shaved guinea pigs. Dose-dependent increased pigmentation was visible first on days 17-22 and persisted for 10-14 weeks. Peak epidermal melanin content in OAG-treated sites was more than twice that of untreated or vehicle-treated sites, as assessed by computerized image analysis of Fontana-Masson stained biopsy cross sections. In another experiment to assess the mechanism of DAG-mediated pigmentation, guinea pigs received twice daily separate applications of OAG, dipalmitoylglycerol (diC16), dioctanoylglycerol (diC8), each 50 mg/ml, 20 microliters/application, and propylene glycol vehicle alone for 5 d. Increased pigmentation was visible after 10 d in the OAG and diC8 sites but not in diC16 or vehicle sites. These results correlate with the reported ability of these compounds to activate protein kinase C in vitro. In a final experiment, guinea pigs received OAG 25 mg/ml three times daily to one test site, and once daily ultraviolet B (70 mJ/cm2, equivalent to 0.6 minimal erythemal dose) radiation to another for 10 d. The OAG and ultraviolet B test sites developed comparable pigmentation by both clinical and histologic criteria. Our data demonstrate that topically applied DAGs can produce a long-lasting increase in epidermal pigmentation, presumably through protein kinase C activation, which clinically and histologically closely resembles ultraviolet-induced tanning.

Mitogen-activated protein kinase pathway and AP-1 are activated during cAMP-induced melanogenesis in B-16 melanoma cells

In mammalian melanocytes, melanin synthesis is controlled by tyrosinase, the critical enzyme in the melanogenic pathway. We and others showed that the stimulation of melanogenesis by cAMP is due to an increased tyrosinase expression at protein and mRNA levels. However, the molecular events connecting the rise of intracellular cAMP and the increase in tyrosinase activity remain to be elucidated. In this study, using B16 melanoma cells, we showed that cAMP-elevating agents stimulated mitogen-activated protein (MAP) kinase, p44mapk. This effect was mediated by the activation of MAP kinase kinase. cAMP-elevating agents induced a translocation of p44mapk to the nucleus and an activation of the transcription factor AP-1. cAMP-induced AP-1 contained FOS-related antigen-2 in association with JunD, while after phorbol ester stimulation AP-1 complexes consist mainly of JunD/c-Fos heterodimers. In an attempt to connect these molecular events to the control of tyrosinase expression that appears to be the pivotal point of melanogenesis regulation, we hypothesized that following its activation by cAMP, p44mapk activates AP-1. Then AP-1 could stimulate tyrosinase expression through the interaction with specific DNA sequences present in the mouse tyrosinase promoter.

Intracellular calcium modulates the responses of human melanocytes to melanogenic stimuli

Ultraviolet radiation (UVR), the synthetic diacyglycerol (DAG), 1-oleoyl-2-acetylglycerol (OAG), and cyclic AMP (cAMP) stimulants, including cholera toxin (CT) have all been shown to increase melanogenesis in cultured human melanocytes. Indirect evidence suggests that an increase in intracellular free Ca2+ ([Ca2+]i) may be important in stimulated melanogenesis. Therefore, to determine whether melanogenic responses are modulated by [Ca2+]i, the Ca2+ in the culture medium of melanocytes ([Ca2+]o) was raised from 70 microM to 1 mM. This switch in [Ca2+]o was associated with a biphasic increase in [Ca2+]i, with an early transient rise, over minutes, and a delayed sustained rise in [Ca2+]i, over hours. The early increase was blocked by nickel chloride (NiCl2), but not affected by depletion of [Ca2+]i stores by thapsigargin, suggesting that this [Ca2+]i rise was due to Ca2+ entry across the plasma membrane. Melanocytes cultured in the absence of CT had a reduced basal melanin content following the switch to 1 mM [Ca2+]o, but in the presence of CT, which acts by stimulating cAMP synthesis, the basal level was increased. Raising [Ca2+]o resulted in enhanced melanogenic responses to UVR and OAG, in the presence or absence of CT, suggesting that Ca(2+)-dependent mechanisms are important. UVR also stimulated a delayed rise in [Ca2+]i, over 24 h, but OAG did not. These results indicate that while [Ca2+]i is not essential for melanogenesis, it plays an important role in modulating the responses of melanocytes to melanogenic stimuli.

Ultraviolet radiation stimulates a biphasic pattern of 1,2-diacylglycerol formation in cultured human melanocytes and keratinocytes by activation of phospholipases C and D

Ultraviolet radiation (UVR) induces melanin synthesis by human epidermal melanocytes, and phospholipid-derived 1,2-diacylglycerols (DAGs) have been implicated in mediating this response. In previous experiments, addition of the synthetic DAG 1-oleoyl-2-acetylglycerol to cultured pigment cells stimulated melanogenesis. The purpose of the present study was to analyse the effects of UVR on the endogenous generation of DAGs. It was found that in a number of cultured cell types, including human melanocytes and B16 mouse melanoma cells, but also human keratinocytes and Swiss 3T3 fibroblasts, exposure to a single dose of UVR stimulated a biphasic increase in endogenous DAG formation. An early transient rise, over seconds, was followed by a more sustained delayed rise over minutes. The early rise in DAG levels was accompanied by a transient rise in inositol trisphosphate formation, indicating activation of phosphatidylinositol-specific phospholipase C. The delayed rise was accompanied by activation of phospholipase D. This endogenous DAG formation by pigment cells is further evidence for the involvement of DAGs in UVR-induced epidermal melanin synthesis. Since DAG formation is also seen in other cells types, it is possible that DAGs may be involved in an array of UVR-induced responses.

A selective protein kinase C inhibitor (CGP 41251) positively and negatively modulates melanoma cell MSH receptors

Melanoma cells express receptors for melanocyte-stimulating hormone (MSH) in variable abundance. CGP 41251, a derivative of staurosporine with an increased selectivity for protein kinase C (PKC) inhibition, was found to modulate MSH receptors in human D10 and HBL cells and in the mouse B16 cell line. Up-regulation was observed in D10 and B16 cells at a concentration of 290 nM and 190 nM, respectively. In HBL cells, however, the PKC inhibitor induced a pronounced MSH receptor down-regulation with an EC50 of only 32 nM. In D10 and HBL cells, alpha-MSH and CGP 41251 synergistically regulated MSH receptors whereas these agents had an antagonistic effect in B16 cells. PKC stimulation by short-term treatment with phorbol ester had an opposite effect on MSH receptors as compared to CGP 41251. In B16 cells, CGP 41251 at a concentration of 100 nM increased the sensitivity to MSH-induced melanogenesis. The staurosporine derivative inhibited proliferation of HBL, B16, and D10 cells at EC50s of 180 nM, 190 nM, and 520 nM, respectively. Furthermore, CGP 41251 increased the dendricity of the cells. In a concentration range between 300 nM and 1 mu M, CGP 41251 induced a sharp increase of the mean cell diameter from 16 mu m to 19 mu m. Thus, the effects of the selective PKC inhibitor on MSH receptors are induced at lower concentrations than needed for the inhibition of proliferation or for the change in cell morphology. These results suggest that the number of MSH receptors expressed on the surface of cultured melanoma cells correlates with the level of constitutive PKC activity in individual cell lines.

Ultraviolet radiation-induced melanogenesis in human melanocytes. Effects of modulating protein kinase C

The mechanism by which ultraviolet radiation induces melanogenesis in epidermal melanocytes is unknown. Previous observations that in cultured human melanocytes 1-oleoyl-2-acetylglycerol augmented both basal and ultraviolet radiation-induced melanogenesis, suggested that the responses were mediated via protein kinase C. However, paradoxically the phorbol ester TPA was without effect. Therefore, the present study has examined the involvement of protein kinase C in melanogenesis. Analysis of the isozyme profile of human melanocytes revealed the presence of protein kinase C alpha, beta I, epsilon and zeta but not the isozyme eta. Following exposure to 500 nM TPA for 24 hours, isozymes alpha, beta I and epsilon were downregulated, but zeta was unaffected. Similar isozyme profiles were observed in S91 and SKMEL3 melanoma cells. The melanogenic responses to 1-oleoyl-2-acetylglycerol and ultraviolet radiation were unaffected by inhibition of protein kinase C with Ro31-8220, or ablation by downregulation with 500 nM TPA, in human melanocytes and melanoma cells. 1-Oleoyl-2-acetylglycerol had no effect on protein kinase C activity in human melanocytes, as measured by rapid phosphorylation of the 80 kDa protein myristoylated alanine-rich C kinase substrate (MARCKS). Ultraviolet radiation induced a small increase in MARCKS protein phosphorylation but this effect was inhibited by pretreatment for 24 hours with 500 nM TPA, which had no effect on ultraviolet-induced melanogenesis. Overall, these findings indicate that 1-oleoyl-2-acetylglycerol and ultraviolet radiation activate melanogenesis via protein kinase C-independent pathways.

Regulation of melanogenesis induced by 5-methoxypsoralen without ultraviolet light in murine melanoma cells

Melanogenesis in melanoma cells can be enhanced by psoralens in the absence of UV light. Melanin biosynthesis is regulated by a number of melanocyte-specific proteins, including tyrosinase, DOPAchrome tautomerase (DCT), and tyrosinase-related protein-1 (TRP-1, gp75). To get more insight on the molecular mechanisms involved in psoralens-induced melanogenesis, we determined tyrosinase and DCT activities as well as mRNA and protein levels of tyrosinase, DCT, and TRP-1 in S91 mouse melanoma cells treated by 5-MOP. High concentration of 5-MOP (5 x 10(-5) M) induced a time-dependent increase of tyrosinase activity and melanin content, which was correlated to an increase of both mRNA and protein levels of tyrosinase. These results demonstrate that the 5-MOP stimulation of melanogenesis is related to increased tyrosinase synthesis. In addition, 5-MOP stimulated TRP-1 synthesis and induced a dose-dependent decrease of DCT activity without any modification in the expression of the protein. We explored then the signalling pathways involved in 5-MOP-induced melanogenesis and, particularly, the role of cyclic AMP and protein kinase C (PKC). A small stimulation of cyclic AMP production was observed in presence of 5-MOP. Furthermore, 1-oleoyl-2-acetylglycerol (OAG), a PKC activator, potentiated the 5-MOP stimulation of tyrosinase activity, while calphostin, a specific PKC inhibitor, inhibited the 5-MOP induction of tyrosinase activity. Phorbol-myristate acetate (PMA), described as a strong activator of PKC, inhibited also the effect of 5-MOP when used at long term. Taken together, these results demonstrate that in murine melanoma cells 5-MOP stimulates melanogenesis by increasing activity and synthesis of tyrosinase. Tyrosinase and TRP-1 expression are coordinately regulated by 5-MOP. Furthermore, a negative correlation between melanogenesis and DCT activity was observed under 5-MOP stimulation. At least, PKA and PKC systems appear to play an important role in the melanogenic effect of 5-MOP.

The superoxide anion may mediate short- but not long-term effects of ultraviolet radiation on melanogenesis

The present study was carried out to examine the role of reactive oxygen species in mediating the melanogenic effects of UVR. B16 mouse melanoma cells responded to a single dose of UVR by showing increases in their melanin content. Although there was a small increase in melanin at 48-72 hours, which was associated with a rise in tyrosinase activity at 48 h, the greatest change occurred at 3 h and this was not associated with an increase in tyrosinase activity. This short-term response, unlike the more delayed melanogenic response, was reduced by superoxide dismutase (SOD). Xanthine oxidase (XO), which generates the superoxide anion (O2-), also increased the melanin content of B16 melanoma cells with effects at 3 h and 48 h. As with UVR, the delayed response was accompanied by an increase in tyrosinase activity but no such association was evident at 3 h. In addition, the short-term effect, like that seen with UVR, was reduced with SOD and to a lesser extent with catalase. In contrast to the effects found with XO, glucose oxidase, which generates hydrogen peroxide, had no effect on the melanin content or tyrosinase activity of the B16 cells. These results confirm previous observations that UVR is able to act directly on cells to bring about delayed increases in melanogenesis. They further demonstrate that UVR also stimulates melanogenesis through a more rapid action that is not associated with an activation of tyrosinase. This effect could be mediated by the O2- which, rather than activating tyrosinase, could act by serving as a substrate for the enzyme.

Retinoic acid as modulator of UVB-induced melanocyte differentiation. Involvement of the melanogenic enzymes expression

Retinoic acid (RA) is a hormone-like agent involved in the control of cell differentiation. The most characteristic feature of melanocyte differentiation, melanogenesis, is stimulated by UV radiations. Excessive chronic sun exposure results in irregular skin hypermelanosis that can be partially corrected by topical RA. The basic mechanisms underlying this effect of RA are unknown. To determine whether RA can directly modulate excessive melanin synthesis, we analyzed the in vitro effect of cis- and trans-RA on UVB-induced melanogenesis in S91 mouse melanoma cells and in normal human melanocytes (NHM). In both cells types, the two RA isoforms significantly decreased the UVB-stimulated melanogenesis in term of tyrosinase activity and melanin neosynthesis. To correlate changes in melanogenesis with the expression of melanogenic enzymes, we determined the neosynthesis rate of tyrosinase, tyrosinase-related protein-1 (TRP-1/gp 75) and tyrosinase-related protein-2 (TRP-2/DOPAchrome tautomerase). Here we show that UVB-induced melanogenesis in NHM is related to an increased synthesis of tyrosinase and TRP-1 and to a dramatic decrease of TRP-2 expression. RA inhibition of UVB-induced melanogenesis acts at the post-transcriptional level leading to a decreased tyrosinase and TRP-1 synthesis. We also show that in NHM, inhibition of TRP-2 following UVB-treatment is significantly reversed by RA. This demonstrates a negative correlation between melanogenesis and TRP-2 expression.

Protein kinase C: biochemical characteristics and role in melanocyte biology

The protein kinase C (PKC) family of proteins, consisting of at least ten isoforms, has been shown to regulate major cellular functions, including the growth and differentiation in many cell types. Use of PKC activators and inhibitors in combination with molecular biology techniques, has permitted detailed exploration of their specific intracellular actions. Recently, studies have implicated PKC specifically in the regulation of growth and differentiated function in melanocytes. In particular, the beta-isoform of PKC was shown to regulate human melanogenesis through activation of tyrosinase, the rate limiting enzyme in melanogenesis. This article reviews the role of PKC in melanocyte biology.

Calcium plays a complex role in the regulation of melanogenesis in murine B16 melanoma cells

To learn more of the role of calcium in the regulation of melanogenesis, we have used direct manipulation of medium calcium and pharmacological modulation of intracellular calcium to examine the consequences on unstimulated and cyclic AMP elevated tyrosinase activity and melanin synthesis and distribution in B16 melanoma cells. In unstimulated cells, calcium is clearly inhibitory to tyrosinase activity. However, in cells stimulated with cAMP-elevating agents the requirement for extracellular calcium was changed such that cells required a minimum of 0.4-0.6 mmol medium calcium for maximum tyrosinase response to these agents. Paradoxically, pharmacologically increasing intracellular calcium in cAMP-stimulated cells with ionophore inhibited tyrosinase activity, and the calcium-lowering agent TMB8 and the calcium channel blocker verapamil both stimulated tyrosinase activity. When melanin synthesis was measured in cAMP-stimulated cells, TMB8 was found to significantly increase the sensitivity and the maximum melanogenic response to alpha-MSH, suggesting the presence of at least one level of endogenous calcium inhibitory control operative in these cells. In addition, TMB8 changed the distribution of melanin between the cell and the medium such that, in the presence of alpha-MSH and TMB8, significantly more melanin was secreted into the medium. These data suggest that calcium is required for several steps in melanogenesis, having an apparently inhibitory effect on pre-tyrosinase activity in unstimulated cells, but also showing evidence of a positive role in cyclic AMP-stimulated tyrosinase activity, as well as a further possible inhibitory role in melanin movement or secretion.

Treatment of human melanocytes and S91 melanoma cells with the DNA repair enzyme T4 endonuclease V enhances melanogenesis after ultraviolet irradiation

Tanning is a protective response of ultraviolet (UV)-irradiated skin that decreases damage from subsequent sun exposures by increasing the epidermal content of melanin, a brown-black pigment that absorbs light energy throughout the UV and visible portions of the electromagnetic spectrum. The melanin pigment is made by epidermal melanocytes and transferred to surrounding keratinocytes. The action spectrum, time course, and histologic features of tanning are well studied, but the initiating molecular events are unknown. Previous work has shown that T4 endonuclease V, a prokaryotic DNA repair enzyme that catalyzes the first and rate-limiting step in repair of UV-induced pyrimidine dimers, delivered in carrier liposomes (T4N5), enhances repair of UV-induced DNA damage in cultured human cells and protects against photocarcinogenesis in an animal model. We now report that T4N5 treatment enhances UV-induced melanogenesis, as measured by melanin content, tyrosinase activity, 14C-dopa incorporation, and visual assessment in both S91 murine melanoma cells and human melanocytes. T4N5 treatment also increases cell yields following UV irradiation. These data suggest that tanning can be stimulated through enhanced DNA repair.

Effect of the dose of ultraviolet radiation on the pigment formation by human melanocytes in vitro

Human melanocytes were cultivated under different conditions with phorbol ester (TPA), or with bovine pituitary extract (BPE). The cells altered their morphology with the different culture conditions. With TPA they were predominantly bipolar, while with BPE most of the cells had a dendritic cell shape. In order to investigate the effect of UV irradiation, the cells were irradiated with 50, 100 and 200 mJ/cm2 UVA/B. After irradiation with 200 mJ/cm2 UVA/B the cells cultured with TPA also showed a dendritic shape. We determined the tyrosinase activity, the cellular melanin content and the cell number 3 days after irradiation. In all cases the number of cells decreased depending on the UVA/B doses. In melanocytes we found a marked increase in tyrosinase activity and melanin content after irradiation with 200 mJ/cm2. The UV-induced effect on tyrosinase activity was higher in melanocytes cultured with BPE than in those cultured with TPA. The results were compared with two human melanoma cell lines. Only little pigment formation could be measured in the tested melanoma cell lines without change after UV irradiation.

Studies and perspectives of signal transduction in the skin

Tumor-promoting phorbol ester and epidermal growth factor (EGF) exert marked influences on the proliferation and differentiation of keratinocytes. These two agents bring their physiological functions into play via protein kinase C (PKC) activation (and/or down regulation) and protein tyrosine kinase, respectively. In this paper, the present situation in the studies on the signal transduction of keratinocytes centering around these two kinases is discussed. An outline of studies on signal transduction of cells other than keratinocytes in the skin is also given.

Molecular cascades in UV-induced melanogenesis: a central role for melanotropins?

When human skin is exposed to ultraviolet (UV) light, a highly complex cascade of events ensues that culminates, among other things, in increased skin melanin content. From analyses at the tissue and cellular level, it has been shown that following exposure to UV light there is an increase in the number of active melanocytes in the basal layer of the epidermis, and individual melanocytes are stimulated to produce more melanin. In addition, the rate of transfer of melanosomes from melanocytes to keratinocytes is apparently increased, although the role of UV light in this process remains to be demonstrated. Recent biochemical evidence is reviewed on factors that regulate these processes. A plausible explanation for the effects of UV on pigmentation is that there are mechanisms in the skin for the orderly, regulated reception of UV signals that are then transduced to initiate the cascade. The signals involve both melanocytes and keratinocytes, and available evidence supports a model in which melanotropins and their receptors play a central role in the process.