Regulation of human skin pigmentation and responses to ultraviolet radiation

MicroRNAs in melanocyte and melanoma biology

The importance of microRNAs as key molecular components of cellular processes is now being recognized. Recent reports have shown that microRNAs regulate processes as diverse as protein expression and nuclear functions inside cells and are able to signal extracellularly, delivered via exosomes, to influence cell fate at a distance. The versatility of microRNAs as molecular tools inspires the design of novel strategies to control gene expression, protein stability, DNA repair and chromatin accessibility that may prove very useful for therapeutic approaches due to the extensive manageability of these small molecules. However, we still lack a comprehensive understanding of the microRNA network and its interactions with the other layers of regulatory elements in cellular and extracellular functions. This knowledge may be necessary before we exploit microRNA versatility in therapeutic settings. To identify rules of interactions between microRNAs and other regulatory systems, we begin by reviewing microRNA activities in a single cell type: the melanocyte, from development to disease.

Superoxide dismutase 1 inhibits alpha-melanocyte stimulating hormone and ultraviolet B-induced melanogenesis in murine skin

Background

Over the last decade, the incidence of ultraviolet B (UVB)-related skin problems has increased. Oxidative stress caused by UVB induces the secretion of melanocyte growth and activating factors from keratinocytes, which results in the formation of cutaneous hyperpigmentation. Therefore, increasing the antioxidant abilities of skin cells is thought to be a beneficial strategy for the development of sunscreen agents. Superoxide dismutase 1 (SOD1) is an antioxidant enzyme that is known to exhibit antioxidant properties.

Objective

The purpose of this study was to investigate the effect of SOD1 on alpha-melanocyte stimulating hormone (α-MSH) and UVB-induced melanogenesis in B16F10 melanoma cells and HRM-2 melanin-possessing hairless mice.

Methods

The inhibitory effect of SOD1 on tyrosinase activity was evaluated in a cell-free system. Additional experiments were performed using B16F10 melanoma cells to demonstrate the effects of SOD1 in vitro, and HRM-2 melanin-possessing hairless mice were used to evaluate the antimelanogenic effects of SOD1 in vivo.

Results

We found that SOD1 inhibited melanin production in a dose-dependent manner without causing cytotoxicity in B16F10 melanoma cells. SOD1 did not inhibit tyrosinase activity under cell-free conditions. The results indicate that SOD1 may reduce pigmentation by an indirect, nonenzymatic mechanism. We also found that SOD1 decreased UVB-induced melanogenesis in HRM-2 melanin-possessing hairless mice, as visualized through hematoxylin and eosin staining and Fontana-Masson staining.

Conclusion

Our results indicate that SOD1 has an inhibitory effect on α-MSH and UVB-induced melanogenesis, indicating that SOD1 may be a promising sunscreen agent.

Skin photoageing: mechanisms of development and particular features of clinical manifestations

The article discloses current concepts of mechanisms of development of skin photoageing caused by UV irradiation (UVA and UVB). Chronic exposure of skin to UV irradiation results in damage of genome DNA, development of DNA mutations, damage of proteins, membrane lipids, collagen and yellow fibers, degradation of the intercellular substance of the skin, development of a chronic inflammation, immunosuppression, melanogenesis disorders and increased angiogenesis. The authors described major clinical manifestations of skin photoageing and their histological characteristics.

Basis for enhanced barrier function of pigmented skin

Humans with darkly pigmented skin display superior permeability barrier function in comparison with humans with lightly pigmented skin. The reduced pH of the stratum corneum (SC) of darkly pigmented skin could account for enhanced function, because acidifying lightly pigmented human SC resets barrier function to darkly pigmented levels. In SKH1 (nonpigmented) versus SKH2/J (pigmented) hairless mice, we evaluated how a pigment-dependent reduction in pH could influence epidermal barrier function. Permeability barrier homeostasis is enhanced in SKH2/J versus SKH1 mice, correlating with a reduced pH in the lower SC that colocalizes with the extrusion of melanin granules. Darkly pigmented human epidermis also shows substantial melanin extrusion in the outer epidermis. Both acute barrier disruption and topical basic pH challenges accelerate reacidification of SKH2/J (but not SKH1) SC, while inducing melanin extrusion. SKH2/J mice also display enhanced expression of the SC acidifying enzyme, secretory phospholipase A2f (sPLA2f). Enhanced barrier function of SKH2/J mice could be attributed to enhanced activity of two acidic pH-dependent, ceramide-generating enzymes, β-glucocerebrosidase and acidic sphingomyelinase, leading to accelerated maturation of SC lamellar bilayers. Finally, organotypic cultures of darkly pigmented human keratinocytes display enhanced barrier function in comparison with lightly pigmented cultures. Together, these results suggest that the superior barrier function of pigmented epidermis can be largely attributed to the pH-lowering impact of melanin persistence/extrusion and enhanced sPLA2f expression.

Pigmentation and dermal conservative effects of the astonishing algae Sargassum polycystum and Padina tenuis on guinea pigs, human epidermal melanocytes (HEM) and Chang cells

BACKGROUND

The preference for a fairer skin-tone has become a common trend among both men and women around the world. In this study, seaweeds Sargassum polycystum and Padina tenuis were investigated for their in vitro and in vivo potentials in working as skin whitening agents. Seaweed has been used as a revolutionary skin repairing agent in both traditional and modern preparations. The high antioxidant content is one of the prime reasons for its potent action. It has been employed in traditional Chinese and Japanese medicine. For centuries, most medical practitioners in the Asian cultures have known seaweed as an organic source of vitamins, minerals, fatty acids like omega-3 and omega-6 and antioxidants. The present objective of the study was to evaluate the potent dermal protective effect of the two seaweeds Sargassum polycystum and Padina tenuis on human cell lines and guinea pigs.

MATERIAL AND METHODS

Seaweeds were extracted with ethanol and further fractionated with hexane, ethyl acetate and water. The extracts were tested for mushroom tyrosinase inhibitory activity, cytotoxicity in human epidermal melanocyte (HEM), and Chang cells. Extracts with potent melanocytotoxicity were formulated into cosmetic cream and tested on guinea pigs in dermal irritation tests and de-pigmentation assessments.

RESULTS

Both Sargassum polycystum and Padina tenuis seaweeds showed significant inhibitory effect on mushroom tyrosinase in the concentration tested. SPEt showed most potent cytotoxicity on HEM (IC50 of 36µg/ml), followed by SPHF (65µg/ml), and PTHF (78.5µg/ml). SPHF and SPEt reduced melanin content in skin of guinea pigs when assessed histologically.

CONCLUSION

SPEt, SPHF and PTHF were able to inhibit HEM proliferation in vitro, with SPHF being most potent and did not cause any dermal irritation in guinea pigs. The results obtained indicate that SPHF is a promising pharmacological or cosmetic agent.

Inhibitory effect of corn silk on skin pigmentation

In this study, the inhibitory effect of corn silk on melanin production was evaluated. This study was performed to investigate the inhibitory effect of corn silk on melanin production in Melan-A cells by measuring melanin production and protein expression. The corn silk extract applied on Melan-A cells at a concentration of 100 ppm decreased melanin production by 37.2% without cytotoxicity. This was a better result than arbutin, a positive whitening agent, which exhibited a 26.8% melanin production inhibitory effect at the same concentration. The corn silk extract did not suppress tyrosinase activity but greatly reduced the expression of tyrosinase in Melan-A cells. In addition, corn silk extract was applied to the human face with hyperpigmentation, and skin color was measured to examine the degree of skin pigment reduction. The application of corn silk extract on faces with hyperpigmentation significantly reduced skin pigmentation without abnormal reactions. Based on the results above, corn silk has good prospects for use as a material for suppressing skin pigmentation.

IFN-γ signaling maintains skin pigmentation homeostasis through regulation of melanosome maturation

Cellular homeostasis is an outcome of complex interacting processes with nonlinear feedbacks that can span distinct spatial and temporal dimensions. Skin tanning is one such dynamic response that maintains genome integrity of epidermal cells. Although pathways underlying hyperpigmentation cascade are recognized, negative feedback regulatory loops that can dampen the activated melanogenesis process are not completely understood. In this study, we delineate a regulatory role of IFN-γ in skin pigmentation biology. We show that IFN-γ signaling impedes maturation of the key organelle melanosome by concerted regulation of several pigmentation genes. Withdrawal of IFN-γ signal spontaneously restores normal cellular programming. This effect in melanocytes is mediated by IFN regulatory factor-1 and is not dependent on the central regulator microphthalmia-associated transcription factor. Chronic IFN-γ signaling shows a clear hypopigmentation phenotype in both mouse and human skin. Interestingly, IFN-γ KO mice display a delayed recovery response to restore basal state of epidermal pigmentation after UV-induced tanning. Together, our studies delineate a new spatiotemporal role of the IFN-γ signaling network in skin pigmentation homeostasis, which could have implications in various cutaneous depigmentary and malignant disorders.

Inhibitory effect of liposome-encapsulated anthocyanin on melanogenesis in human melanocytes

CONTEXT

Melanin plays an important role in preventing ultraviolet (UV) light-induced skin damage. Overexposure to UV radiation can lead to the formation of free radicals and trigger inflammation and hyperpigmentation of the skin. Anthocyanin can combat excessive free radicals in the body and can reduce the occurrence of inflammation. However, anthocyanin molecules are unstable and highly susceptible to degradation.

OBJECTIVE

The present study aims to elucidate the effects of liposome-capsulated anthocyanin (LCA) from Hibiscus sabdariffa Linn. on melanogenesis in human A375 melanocytes.

MATERIALS AND METHODS

The effects of LCA with various doses (5-50 mg/mL) on cell viability, melanin content, tyrosinase activity, expression of the tyrosinase and microphthalmia-associated transcription factor (MITF) were measured.

RESULTS

Anthocyanin exhibits scavenging activity on DPPH radical with the inhibitory rate of 11 and 24% at 20 and 50 mg/mL concentration treatment, respectively, and inhibitory effects on melanin production by 8, 14, 23 and 30% at 5, 10, 20 and 50 mg/mL concentration treatment, respectively. However, LCA has enhanced DPPH scavenging activity (64 and 76% at 20 and 50 mg/mL concentration treatment, respectively) and inhibitory effects against melanin synthesis (23, 35, 43 and 60% at 5, 10, 20 and 50 mg/mL concentration treatment, respectively). Moreover, anthocyanin-inhibited melanin synthesis occurs through the inhibition of tyrosinase enzymatic activity and suppression of the protein expression of tyrosinase and MITF.

DISCUSSION AND CONCLUSION

Liposome encapsulation increases the stabilization of anthocyanin and the inhibition of melanogenesis. Our findings indicate that LCA may be suitable as a photoprotective agent for the skin.

Effect of serum folate status on total folate and 5-methyltetrahydrofolate in human skin

BACKGROUND

It has been suggested that human skin color adapts to balance the need for vitamin D synthesis in comparison with the protection of DNA and folate from photodegradation. However, the folate content of human skin is unknown and may affect the effectiveness of the antifolate methotrexate for the treatment of psoriasis.

OBJECTIVES

We examined whether total folate and 5-methyl-(6S)-tetrahydrofolate (5-MTHF) in human skin can be predicted by serum concentrations and whether there are differences in the proportion of 5-MTHF in dermis compared with epidermis.

DESIGN

Total folate (by using a microbiological assay) and 5-MTHF (by using high-pressure liquid chromatography) were measured in fasting serum and fresh skin obtained at surgery by using a recovery validated extraction method.

RESULTS

Total folate in human epidermis was shown to be low compared with in many other tissues, and dermal folate was an order-of-magnitude even lower. These concentrations were directly and linearly linked to serum folate status. Although the percentage of 5-MTHF of the total in the dermis was similar to that in other organs, it was especially high in the epidermis and increased to >65% as serum folate decreased.

CONCLUSIONS

The high proportion of 5-MTHF in the epidermis, which is further emphasized in subjects with a lower (10-20-nmol/L) serum folate status, points to a special role for this form of folate in skin, perhaps as a protectant from ultraviolet-induced photosensitization reactions. 5-MTHF may also maintain methylation reactions that influence the proliferative activity. These results may help to individualize the treatment of psoriasis patients with methotrexate and folate.

Effects of ultraviolet radiation, visible light, and infrared radiation on erythema and pigmentation: a review

The effects of ultraviolet radiation, visible light, and infrared radiation on cutaneous erythema, immediate pigment darkening, persistent pigment darkening, and delayed tanning are affected by a variety of factors. Some of these factors include the depth of cutaneous penetration of the specific wavelength, the individual skin type, and the absorption spectra of the different chromophores in the skin. UVB is an effective spectrum to induce erythema, which is followed by delayed tanning. UVA induces immediate pigment darkening, persistent pigment darkening, and delayed tanning. At high doses, UVA (primarily UVA2) can also induce erythema in individuals with skin types I-II. Visible light has been shown to induce erythema and a tanning response in dark skin, but not in fair skinned individuals. Infrared radiation produces erythema, which is probably a thermal effect. In this article we reviewed the available literature on the effects of ultraviolet radiation, visible light, and infrared radiation on the skin in regards to erythema and pigmentation. Much remains to be learned on the cutaneous effects of visible light and infrared radiation.

The EJC component Magoh regulates proliferation and expansion of neural crest-derived melanocytes

Melanoblasts are a population of neural crest-derived cells that generate the pigment-producing cells of our body. Defective melanoblast development and function underlies many disorders including Waardenburg syndrome and melanoma. Understanding the genetic regulation of melanoblast development will help elucidate the etiology of these and other neurocristopathies. Here we demonstrate that Magoh, a component of the exon junction complex, is required for normal melanoblast development. Magoh haploinsufficient mice are hypopigmented and exhibit robust genetic interactions with the transcription factor, Sox10. These phenotypes are caused by a marked reduction in melanoblast number beginning at mid-embryogenesis. Strikingly, while Magoh haploinsufficiency severely reduces epidermal melanoblasts, it does not significantly affect the number of dermal melanoblasts. These data indicate Magoh impacts melanoblast development by disproportionately affecting expansion of epidermal melanoblast populations. We probed the cellular basis for melanoblast reduction and discovered that Magoh mutant melanoblasts do not undergo increased apoptosis, but instead are arrested in mitosis. Mitotic arrest is evident in both Magoh haploinsufficient embryos and in Magoh siRNA treated melanoma cell lines. Together our findings indicate that Magoh-regulated proliferation of melanoblasts in the dermis may be critical for production of epidermally-bound melanoblasts. Our results point to a central role for Magoh in melanocyte development.

"Trooping the color": restoring the original donor skin color by addition of melanocytes to bioengineered skin analogs

PURPOSE

Autologous skin substitutes to cover large skin defects are used since several years. Melanocytes, although essential for solar protection and pigmentation of skin, are not yet systematically added to such substitutes. In this experimental study, we reconstructed melanocyte-containing dermo-epidermal skin substitutes from donor skins of different skin pigmentation types and studied them in an animal model. Features pertinent to skin color were analyzed and compared in both skin substitutes and original donor skin.

METHODS

Keratinocytes, melanocytes, and fibroblast were isolated, cultured, and expanded from skin biopsies of light- and dark-pigmented patients. For each donor, melanocytes and keratinocytes were seeded in different ratios (1:1, 1:5, 1:10) onto collagen gels previously populated with autologous fibroblasts. Skin substitutes were then transplanted onto full-thickness wounds of immuno-incompetent rats. After 8 weeks, macroscopic and microscopic analyses were conducted with regard to skin color and architecture.

RESULTS

Chromameter evaluation revealed that skin color of reconstructed light- and dark-pigmented skin was very similar to donor skin, independent of which melanocyte/keratinocyte ratio was added. Histological analyses of the skin analogs confirmed these findings.

CONCLUSION

These data suggest that adding autologous melanocytes to bioengineered dermo-epidermal skin analogs can sustainably restore the patients' native skin color.

[6]-Shogaol inhibits melanogenesis in B16 mouse melanoma cells through activation of the ERK pathway

AIM

To investigate the effect of [6]-shogaol, an active ingredient in ginger, on melanogenesis and the underlying mechanisms.

METHODS

B16F10 mouse melanoma cells were tested. Cell viability was determined with the MTT assay. Melanin content and tyrosinase activity were analyzed with a spectrophotometer. The protein expression of tyrosinase and microphthalmia associated transcription factor (MITF), as well as phosphorylated or total ERK1/2 and Akt were measured using Western blot.

RESULTS

Treatment of the cells with [6]-shogaol (1, 5, 10 μmol/L) reduced the melanin content in a concentration-dependent manner. [6]-Shogaol (5 and 10 μmol/L) significantly decreased the intracellular tyrosinase activity, and markedly suppressed the expression levels of tyrosinase and MITF proteins in the cells. Furthermore, [6]-shogaol (10 μmol/L) activated ERK, which was known to negatively regulate melanin synthesis in these cells. Pretreatment with the specific ERK pathway inhibitor PD98059 (20 μmol/L) greatly attenuated the inhibition of melanin synthesis by [6]-shogaol (10 μmol/L).

CONCLUSION

The results demonstrate that [6]-shogaol inhibits melanogenesis in B16F10 mouse melanoma cells via activating the ERK pathway.

Current principles of sunscreen use in children

PURPOSE OF REVIEW

Physicians need to be prepared to counsel patients on why and how to protect themselves from damaging ultraviolet (UV) radiation, including the proper use of sunscreens. In this article, we review the interplay between UV radiation, sunscreens and the skin, highlighting current controversies and recommendations surrounding sunscreen use.

RECENT FINDINGS

An important concept is that excessive UV exposure has long-term damaging effects on the skin beyond the immediate sunburn. Recent discoveries of the role of UVA radiation in skin cancer development have set high standards for broad-spectrum coverage to be met by sunscreens. Current evidence does not support an association between sunscreen use and melanoma, systemic toxicity or vitamin D deficiency. Although sunscreen application is the most common modality for sun protection, many people do not use it correctly. Regular sunscreen use during childhood and adolescence can significantly reduce lifetime incidence of skin cancer; therefore, targeting children in pediatric offices regarding unprotected UV exposure may be a practical approach.

SUMMARY

Sunscreens continue to be a major method of photoprotection among the public, offering numerous benefits that clearly outweigh potential risks; however, optimizing the use of sunscreens, especially among children and adolescents, remains a major challenge.

Skin melanocytes: biology and development

In the human skin, melanocytes are present in the epidermis and hair follicles. The basic features of these cells are the ability to melanin production and the origin from neural crest cells. This last element is important because there are other cells able to produce melanin but of different embryonic origin (pigmented epithelium of retina, some neurons, adipocytes). The life cycle of melanocyte consists of several steps including differentiation of melanocyte lineage/s from neural crest, migration and proliferation of melanoblasts, differentiation of melanoblasts into melanocytes, proliferation and maturation of melanocytes at the target places (activity of melanogenic enzymes, melanosome formation and transport to keratinocytes) and eventual cell death (hair melanocytes). Melanocytes of the epidermis and hair are cells sharing some common features but in general they form biologically different populations living in unique niches of the skin.

Beneficial effects of UV radiation other than via vitamin D production

Most of the positive effects of solar radiation are mediated via ultraviolet-B (UVB) induced production of vitamin D in skin. However, several other pathways may exist for the action of ultraviolet (UV) radiation on humans as focused on in this review. One is induction of cosmetic tanning (immediate pigment darkening, persistent pigment darkening and delayed tanning). UVB-induced, delayed tanning (increases melanin in skin after several days), acts as a sunscreen. Several human skin diseases, like psoriasis, vitiligo, atopic dermatitis and localized scleroderma, can be treated with solar radiation (heliotherapy) or artificial UV radiation (phototherapy). UV exposure can suppress the clinical symptoms of multiple sclerosis independently of vitamin D synthesis. Furthermore, UV generates nitric oxide (NO), which may reduce blood pressure and generally improve cardiovascular health. UVA-induced NO may also have antimicrobial effects and furthermore, act as a neurotransmitter. Finally, UV exposure may improve mood through the release of endorphins.

Human eccrine sweat gland cells turn into melanin-uptaking keratinocytes in dermo-epidermal skin substitutes

Recently, Biedermann et al. (2010) have demonstrated that human eccrine sweat gland cells can develop a multilayered epidermis. The question still remains whether these cells can fulfill exclusive and very specific functional properties of epidermal keratinocytes, such as the incorporation of melanin, a feature absent in sweat gland cells. We added human melanocytes to eccrine sweat gland cells to let them develop into an epidermal analog in vivo. The interaction between melanocytes and sweat gland-derived keratinocytes was investigated. The following results were gained: (1) macroscopically, a pigmentation of the substitutes was seen 2-3 weeks after transplantation; (2) we confirmed the development of a multilayered, stratified epidermis with melanocytes distributed evenly throughout the basal layer; (3) melanocytic dendrites projected to suprabasal layers; and (4) melanin was observed to be integrated into former eccrine sweat gland cells. These skin substitutes were similar or equal to skin substitutes cultured from human epidermal keratinocytes. The only differences observed were a delay in pigmentation and less melanin uptake. These data suggest that eccrine sweat gland cells can form a functional epidermal melanin unit, thereby providing striking evidence that they can assume one of the most characteristic keratinocyte properties.

Specification of neural crest into sensory neuron and melanocyte lineages

Elucidating the mechanisms by which multipotent cells differentiate into distinct lineages is a common theme underlying developmental biology investigations. Progress has been made in understanding some of the essential factors and pathways involved in the specification of different lineages from the neural crest. These include gene regulatory networks involving transcription factor hierarchies and input from signaling pathways mediated from environmental cues. In this review, we examine the mechanisms for two lineages that are derived from the neural crest, peripheral sensory neurons and melanocytes. Insights into the specification of these cell types may reveal common themes in the specification processes that occur throughout development.

Human skin pigmentation, migration and disease susceptibility

Human skin pigmentation evolved as a compromise between the conflicting physiological demands of protection against the deleterious effects of ultraviolet radiation (UVR) and photosynthesis of UVB-dependent vitamin D(3). Living under high UVR near the equator, ancestral Homo sapiens had skin rich in protective eumelanin. Dispersals outside of the tropics were associated with positive selection for depigmentation to maximize cutaneous biosynthesis of pre-vitamin D(3) under low and highly seasonal UVB conditions. In recent centuries, migrations and high-speed transportation have brought many people into UVR regimes different from those experienced by their ancestors and, accordingly, exposed them to new disease risks. These have been increased by urbanization and changes in diet and lifestyle. Three examples-nutritional rickets, multiple sclerosis (MS) and cutaneous malignant melanoma (CMM)-are chosen to illustrate the serious health effects of mismatches between skin pigmentation and UVR. The aetiology of MS in particular provides insight into complex and contingent interactions of genetic and environmental factors necessary to trigger lethal disease states. Low UVB levels and vitamin D deficiencies produced by changes in location and lifestyle pose some of the most serious disease risks of the twenty-first century.

Inhibition of melanogenesis by the pyridinyl imidazole class of compounds: possible involvement of the Wnt/β-catenin signaling pathway

While investigating the role of p38 MAPK in regulating melanogenesis, we found that pyridinyl imidazole inhibitors class compounds as well as the analog compound SB202474, which does not inhibit p38 MAPK, suppressed both α-MSH-induced melanogenesis and spontaneous melanin synthesis. In this study, we demonstrated that the inhibitory activity of the pyridinyl imidazoles correlates with inhibition of the canonical Wnt/β-catenin pathway activity. Imidazole-treated cells showed a reduction in the level of Tcf/Lef target genes involved in the β-catenin signaling network, including ubiquitous genes such as Axin2, Lef1, and Wisp1 as well as cell lineage-restricted genes such as microphthalmia-associated transcription factor and dopachrome tautomerase. Although over-expression of the Wnt signaling pathway effector β-catenin slightly restored the melanogenic program, the lack of complete reversion suggested that the imidazoles interfered with β-catenin-dependent transcriptional activity rather than with β-catenin expression. Accordingly, we did not observe any significant change in β-catenin protein expression. The independence of p38 MAPK activity from the repression of Wnt/β-catenin signaling pathway was confirmed by small interfering RNA knockdown of p38 MAPK expression, which by contrast, stimulated β-catenin-driven gene expression. Our data demonstrate that the small molecule pyridinyl imidazoles possess two distinct and opposite mechanisms that modulate β-catenin dependent transcription: a p38 inhibition-dependent effect that stimulates the Wnt pathway by increasing β-catenin protein expression and an off-target mechanism that inhibits the pathway by repressing β-catenin protein functionality. The p38-independent effect seems to be dominant and, at least in B16-F0 cells, results in a strong block of the Wnt/β-catenin signaling pathway.

A novel form of melanoma apoptosis resistance: melanogenesis up-regulation in apoptotic B16-F0 cells delays ursolic acid-triggered cell death

Melanoma is one of the most aggressive forms of cancer with a continuously growing incidence worldwide and is usually resistant to chemotherapy agents, which is due in part to a strong resistance to apoptosis. The resistance mechanisms are complex and melanoma cells may have diverse possibilities for regulating apoptosis to generate apoptotic deficiencies. In this study, we investigated the relationship between melanogenesis and resistance to apoptosis induced by ursolic acid, a natural chemopreventive agent, in B16-F0 melanoma cells. We demonstrated that cells undergoing apoptosis are able to delay their own death. It appeared that tyrosinase and TRP-1 up-regulation in apoptotic cells and the subsequent production of melanin were clearly implicated in an apoptosis resistance mechanism; while TRP-2, a well known mediator of melanoma resistance to cell death, was repressed. Our results confirm the difficulty of treating melanomas, since, even undergoing apoptosis, cells are nevertheless able to trigger a resistance mechanism to delay death.

Individual photosensitivity of human skin and UVA-induced pyrimidine dimers in DNA

Delineation of the DNA-damaging properties of UVA radiation is a major issue in understanding solar carcinogenesis. Emphasis was placed in this study on the formation of cyclobutane pyrimidine dimers (CPDs), which are now well established as the most frequent UVA-induced DNA lesions in human skin. The yield of CPDs was determined by a chromatographic assay following ex vivo UVA and UVB irradiation of biopsies taken from either phototype II or IV volunteers. A clear correlation was found between the frequency of UVB-induced CPDs and both the phototype and the minimum erythemal dose (MED). Similar results were obtained for the induction of CPDs upon exposure to UVA. Moreover, an excellent correlation was observed for each donor between the yield of DNA damage induced by either UVB or UVA. These observations show that the key parameters driving UVA-induced formation of CPDs are attenuation of radiation in the skin and the number of photons reaching skin cells rather than the cellular content in photosensitizers. In addition, the results show that both MED and phototype are good predictors of the vulnerability of DNA toward UVB and UVA in the skin. This result is of importance for the identification of individuals to be extensively protected.

5,7-dimethoxyflavone induces melanogenesis in B16F10 melanoma cells through cAMP-dependent signalling

Melanin protects the skin against ultraviolet radiation (UVR) and diverse free radicals. Agents that increase melanin synthesis in melanocytes may reduce UVR-induced skin damage and skin cancer. In the present study, we evaluated the effects of 5,7-dimethoxyflavone (5,7-DMF) on melanogenic protein expression and signalling pathways. We found that 5,7-DMF significantly increased melanin content by upregulating microphthalmia-associated transcription factor and related melanogenic proteins. Additionally, 5,7-DMF increased cAMP levels, which activates a cascade of reactions, such as cAMP responsive element-binding protein and Akt/glycogen synthase kinase-3β (GSK-3β) signalling. Thus, 5,7-DMF may be an effective pigmentation stimulator for photoprotection and hypopigmentation disorders.

Lighting a path to pigmentation: mechanisms of MITF induction by UV

While sunlight is important for life, ultraviolet radiation (UVR) can have harmful and mutagenic effects. This duality is particularly relevant to human skin, in which UVR both participates in evolutionarily important photochemical reactions yet may act as a potential carcinogen. UVR can upregulate production of melanin, the "tanning response" that serves a photoprotective function. This genetic program is centrally tuned by the transcription factor MITF, a master regulator of melanogenesis and melanocyte function. In this review, we discuss the myriad consequences of UV exposure for skin homeostasis, highlighting the diverse pathways activated by this ultraviolet radiation.

UV-ABC screens of luteolin derivatives compared to edelweiss extract

Pure luteolin is a remarkably heat (200°C/6 days) and UV stable UV-A screen, however, native luteolin enriched to 37% in an edelweiss extract lost its UV-A screen properties upon UV irradiation (∼4MJm(-2)). This contrasting behavior led to the examination of a series of purified luteolin derivatives as UV screen candidates. 3',4',5,7-Tetralipoyloxyflavones were synthesized from luteolin (3',4',5,7-tetrahydroxyflavone) and fatty acid chlorides. These acylated semi-biomolecules show a hypsochromic shift in UV-Vis spectra of about Δλ(A→B)=58nm and absorbed in the centre of the harmful UV-B band (λ(max)=295nm). Luteolin was also hydroxyethylated with Br(CH(2))(2)OH. This substitution has no effect on the λ(max)=330nm absorption of luteolin (UV-A band). Finally the natural 4'-O-β-glucosyl-3',5,7-trihydroxyflavone was extracted from edelweiss and used as a purified natural benchmark. Glycosylated and hydroxyethylated luteolin are both UV stable. Fully acylated luteolin derivatives degrade upon UV exposure to a stable UV-C screen with a hypsochroic shift Δλ(B→C)=35nm. All in all, three molecular structures based on luteolin with sunscreen properties were found, distinguishable in: UV-A, UV-B, and UV-C filters. The natural product based UV-absorbers show promise as alternatives to synthetic molecules and nanoparticles in sunscreen products.

Networks and pathways in pigmentation, health, and disease

Extensive studies of the biology of the pigment-producing cell (melanocyte) have resulted in a wealth of knowledge regarding the genetics and developmental mechanisms governing skin and hair pigmentation. The ease of identification of altered pigment phenotypes, particularly in mouse coat color mutants, facilitated early use of the pigmentary system in mammalian genetics and development. In addition to the large collection of developmental genetics data, melanocytes are of interest because their malignancy results in melanoma, a highly aggressive and frequently fatal cancer that is increasing in Caucasian populations worldwide. The genetic programs regulating melanocyte development, function, and malignancy are highly complex and only partially understood. Current research in melanocyte development and pigmentation is revealing new genes important in these processes and additional functions for previously known individual components. A detailed understanding of all the components involved in melanocyte development and function, including interactions with neighboring cells and response to environmental stimuli, will be necessary to fully comprehend this complex system. The inherent characteristics of pigmentation biology as well as the resources available to researchers in the pigment cell community make melanocytes an ideal cell type for analysis using systems biology approaches. In this review, the study of melanocyte development and pigmentation is considered as a candidate for systems biology-based analyses.

A curated online resource for SOX10 and pigment cell molecular genetic pathways

We describe the creation of a specialized web-accessible database named the Pigment Cell Gene Resource, which contains information on the genetic pathways that regulate pigment cell development and function. This manually curated database is comprised of two sections, an annotated literature section and an interactive transcriptional network diagram. Initially, this database focuses on the transcription factor SOX10, which has essential roles in pigment cell development and function, but the database has been designed with the capacity to expand in the future, allowing inclusion of many more pigmentation genes. Database URL: http://research.nhgri.nih.gov/pigment_cell/

The deceptive nature of UVA tanning versus the modest protective effects of UVB tanning on human skin

The relationship between human skin pigmentation and protection from ultraviolet (UV) radiation is an important element underlying differences in skin carcinogenesis rates. The association between UV damage and the risk of skin cancer is clear, yet a strategic balance in exposure to UV needs to be met. Dark skin is protected from UV-induced DNA damage significantly more than light skin owing to the constitutively higher pigmentation, but an as yet unresolved and important question is what photoprotective benefit, if any, is afforded by facultative pigmentation (i.e. a tan induced by UV exposure). To address that and to compare the effects of various wavelengths of UV, we repetitively exposed human skin to suberythemal doses of UVA and/or UVB over 2 weeks after which a challenge dose of UVA and UVB was given. Although visual skin pigmentation (tanning) elicited by different UV exposure protocols was similar, the melanin content and UV-protective effects against DNA damage in UVB-tanned skin (but not in UVA-tanned skin) were significantly higher. UVA-induced tans seem to result from the photooxidation of existing melanin and its precursors with some redistribution of pigment granules, while UVB stimulates melanocytes to up-regulate melanin synthesis and increases pigmentation coverage, effects that are synergistically stimulated in UVA and UVB-exposed skin. Thus, UVA tanning contributes essentially no photoprotection, although all types of UV-induced tanning result in DNA and cellular damage, which can eventually lead to photocarcinogenesis.

The fibroblast-derived paracrine factor neuregulin-1 has a novel role in regulating the constitutive color and melanocyte function in human skin

Interactions between melanocytes and neighboring cells in the skin are important in regulating skin color in humans. We recently demonstrated that the less pigmented and thicker skin on the palms and soles is regulated by underlying fibroblasts in those areas, specifically via a secreted factor (DKK1) that modulates Wnt signaling. In this study, we tested the hypothesis that dermal fibroblasts regulate the constitutive skin color of individuals ranging from very light to very dark. We used microarray analysis to compare gene expression patterns in fibroblasts derived from lighter skin types compared to darker skin types, with a focus on secreted proteins. We identified a number of genes that differ dramatically in expression and, among the expressed proteins, neuregulin-1, which is secreted by fibroblasts derived from dark skin, effectively increases the pigmentation of melanocytes in tissue culture and in an artificial skin model and regulates their growth, suggesting that it is one of the major factors determining human skin color.

Cancer stem cells versus phenotype-switching in melanoma

Tumours comprise multiple phenotypically distinct subpopulations of cells, some of which are proposed to possess stem cell-like properties, being able to self-renew, seed and maintain tumours, and provide a reservoir of therapeutically resistant cells. Here, we use melanoma as a model to explore the validity of the cancer stem cell hypothesis in the light of accumulating evidence that melanoma progression may instead be driven by phenotype-switching triggered by genetic lesions that impose an increased sensitivity to changes in the tumour microenvironment. Although at any given moment cells within a tumour may exhibit differentiated, proliferative or invasive phenotypes, an ability to switch phenotypes implies that most cells will have the potential to adopt a stem cell-like identity. Insights into the molecular events underpinning phenotype-switching in melanoma highlight the close relationship between signalling pathways that generate, maintain and activate melanocyte stem cells as well as the inverse correlation between proliferation and invasive potentials. An understanding of phenotype-switching in melanoma, and in particular the signalling events that regulate the expression of the microphthalmia-associated transcription factor Mitf, points to new therapeutic opportunities aimed at eradicating therapeutically resistant stem cell-like melanoma cells.

Suppression of melanin production by expression of HSP70

Skin hyperpigmentation disorders due to abnormal melanin production induced by ultraviolet (UV) irradiation are both a clinical and cosmetic problem. UV irradiation stimulates melanin production in melanocytes by increasing intracellular cAMP. Expression of heat shock proteins (HSPs), especially HSP70, is induced by various stressors, including UV irradiation, to provide cellular resistance to such stressors. In this study we examined the effect of expression of HSP70 on melanin production both in vitro and in vivo. 3-Isobutyl-1-methylxanthine (IBMX), a cAMP-elevating agent, stimulated melanin production in cultured mouse melanoma cells, and this stimulation was suppressed in cells overexpressing HSP70. IBMX-dependent transcriptional activation of the tyrosinase gene was also suppressed in HSP70-overexpressing cells. Expression of microphthalmia-associated transcription factor (MITF), which positively regulates transcription of the tyrosinase gene, was up-regulated by IBMX; however, this up-regulation was not suppressed in HSP70-overexpressing cells. On the other hand, immunoprecipitation and immunostaining analyses revealed a physical interaction between and co-localization of MITF and HSP70, respectively. Furthermore, the transcription of tyrosinase gene in nuclear extract was inhibited by HSP70. In vivo, UV irradiation of wild-type mice increased the amount of melanin in the basal layer of the epidermis, and this increase was suppressed in transgenic mice expressing HSP70. This study provides the first evidence of an inhibitory effect of HSP70 on melanin production both in vitro and in vivo. This effect seems to be mediated by modulation of MITF activity through a direct interaction between HSP70 and MITF.

Regulation of human skin pigmentation in situ by repetitive UV exposure: molecular characterization of responses to UVA and/or UVB

UV radiation is a major environmental factor that affects pigmentation in human skin and can eventually result in various types of UV-induced skin cancers. The effects of various wavelengths of UV on melanocytes and other types of skin cells in culture have been studied, but little is known about gene expression patterns in situ following in situ exposure of human skin to different types of UV (UVA and/or UVB). Paracrine factors expressed by keratinocytes and/or fibroblasts that affect skin pigmentation might be regulated differently by UV, as might their corresponding receptors expressed on melanocytes. To test the hypothesis that different mechanisms are involved in the pigmentary responses of the skin to different types of UV, we used immunohistochemical and whole human genome microarray analyses to characterize human skin in situ to examine how melanocyte-specific proteins and paracrine melanogenic factors are regulated by repetitive exposure to different types of UV compared with unexposed skin as a control. The results show that gene expression patterns induced by UVA or UVB are distinct-UVB eliciting dramatic increases in a large number of genes involved in pigmentation as well as in other cellular functions, whereas UVA had little or no effect on these. The expression patterns characterize the distinct responses of the skin to UVA or UVB, and identify several potential previously unidentified factors involved in UV-induced responses of human skin.

Mahogunin ring finger-1 (MGRN1) E3 ubiquitin ligase inhibits signaling from melanocortin receptor by competition with Galphas

Mahogunin ring finger-1 (MGRN1) is a RING domain-containing ubiquitin ligase mutated in mahoganoid, a mouse mutation causing coat color darkening, congenital heart defects, high embryonic lethality, and spongiform neurodegeneration. The melanocortin hormones regulate pigmentation, cortisol production, food intake, and body weight by signaling through five G protein-coupled receptors positively coupled to the cAMP pathway (MC1R-MC5R). Genetic analysis has shown that mouse Mgrn1 is an accessory protein for melanocortin signaling that may inhibit MC1R and MC4R by unknown mechanisms. These melanocortin receptors (MCRs) regulate pigmentation and body weight, respectively. We show that human melanoma cells express 4 MGRN1 isoforms differing in the C-terminal exon 17 and in usage of exon 12. This exon contains nuclear localization signals. MGRN1 isoforms decreased MC1R and MC4R signaling to cAMP, without effect on beta(2)-adrenergic receptor. Inhibition was independent on receptor plasma membrane expression, ubiquitylation, internalization, or stability and occurred upstream of Galpha(s) binding to/activation of adenylyl cyclase. MGRN1 co-immunoprecipitated with MCRs, suggesting a physical interaction of the proteins. Significantly, overexpression of Galpha(s) abolished the inhibitory effect of MGRN1 and decreased co-immunoprecipitation with MCRs, suggesting competition between MGRN1 and Galpha(s) for binding to MCRs. Although all MGRN1s were located in the cytosol in the absence of MCRs, exon 12-containing isoforms accumulated in the nuclei upon co-expression with the receptors. Therefore, MGRN1 inhibits MCR signaling by a new mechanism involving displacement of Galpha(s), thus accounting for key features of the mahoganoid phenotype. Moreover, MGRN1 might provide a novel pathway for melanocortin signaling from the cell surface to the nucleus.

Short- and long-term effects of UV radiation on the pigmentation of human skin

The incidence of skin cancer, including cutaneous melanoma, has risen substantially in recent years, and epidemiological and laboratory studies show that UV radiation is a major causative factor of this increase. UV damage also underlies photoaging of the skin, and these deleterious effects of UV can be, in part, prevented in skin with higher levels of constitutive pigmentation. We review the clinical studies we have made in recent years regarding the rapid and the long-term responses of the pigmentary system in human skin to UV exposure.Journal of Investigative Dermatology Symposium Proceedings (2009) 14, 32-35; doi:10.1038/jidsymp.2009.10.