Alpha-melanocyte-stimulating hormone regulation of tyrosinase in Cloudman S-91 mouse melanoma cell cultures

Distinct cAMP Signaling Microdomains Differentially Regulate Melanosomal pH and Pigmentation

cAMP signaling is a well-established regulator of melanin synthesis. Two distinct cAMP signaling pathways-the transmembrane adenylyl cyclase pathway, activated primarily by the MC1R, and the soluble adenylyl cyclase (sAC) pathway-affect melanin synthesis. The sAC pathway affects melanin synthesis by regulating melanosomal pH, and the MC1R pathway affects melanin synthesis by regulating gene expression and post-translational modifications. However, whether MC1R genotype affects melanosomal pH is poorly understood. We now report that loss of function MC1R does not affect melanosomal pH. Thus, sAC signaling appears to be the only cAMP signaling pathway that regulates melanosomal pH. We also addressed whether MC1R genotype affects sAC-dependent regulation of melanin synthesis. Although sAC loss of function in wild-type human melanocytes stimulates melanin synthesis, sAC loss of function has no effect on melanin synthesis in MC1R nonfunctional human and mouse melanocytes or skin and hair melanin in e/e mice. Interestingly, activation of transmembrane adenylyl cyclases, which increases epidermal eumelanin synthesis in e/e mice, leads to enhanced production of eumelanin in sAC-knockout mice relative to that in sAC wild-type mice. Thus, MC1R- and sAC-dependent cAMP signaling pathways define distinct mechanisms that regulate melanosomal pH and pigmentation.

Novel Quercetin Derivative of 3,7-Dioleylquercetin Shows Less Toxicity and Highly Potent Tyrosinase Inhibition Activity

Quercetin is a well-known plant flavonol and antioxidant; however, there has been some debate regarding the efficacy and safety of native quercetin as a skin-whitening agent via tyrosinase inhibition. Several researchers have synthesized quercetin derivatives as low-toxicity antioxidants and whitening agents. However, no suitable quercetin derivatives have been reported to date. In this study, a novel quercetin derivative was synthesized by the S_N2 reaction using quercetin and oleyl bromide. The relationship between the structures and activities of quercetin derivatives as anti-melanogenic agents was assessed using in vitro enzyme kinetics, molecular docking, and quenching studies; cell line experiments; and in vivo zebrafish model studies. Novel 3,7-dioleylquercetin (OQ) exhibited a low cytotoxic concentration level at >100 µg/mL (125 µM), which is five times less toxic than native quercetin. The inhibition mechanism showed that OQ is a competitive inhibitor, similar to native quercetin. Expression of tyrosinase, tyrosinase-related protein 1 (TRP-1) and tyrosinase-related protein 2 (TRP-2), and microphthalmia-associated transcription factor was inhibited in B16F10 melanoma cell lines. mRNA transcription levels of tyrosinase, TRP-1, and TRP-2 decreased in a dose-dependent manner. Melanin formation was confirmed in the zebrafish model using quercetin derivatives. Therefore, OQ might be a valuable asset for the development of novel skin-whitening agents.

Effect of Aurea Helianthus stem extract on anti-melanogenesis

Aurea Helianthus (AH), also known as wild confederate rose or golden sunflower, is a curative herb. It has been used as a medicinal material in China due to its anti-inflammatory, immune regulatory, and anti-oxidant activities. However, its melanogenic effect on skin has not been sufficiently investigated. In this study, we tested whether AH has melanogenic inhibitory activities for the development of effective skin whitening agent. The extract showed inhibition of melanin synthesis and reduced the oxidation of 3, 4-dihydroxyphenilalanine (DOPA) to o-dopaquinone. Additionally, AH downregulated the levels of microphthalmia-associated transcription factor (MITF), tyrosinase and tyrosinase related proteins (TRPs), suggesting that AH has inhibitory effects on melanogenesis. Analysis of the components of AH showed that it contained paprazine and trans-N-feruloyltyramine (FA). We confirmed that the effect of AH resulted from paprazine and FA. Therefore, AH might have potential as an effective candidate for skin whitening.

Scopoletin Stimulates Melanogenesis via cAMP/PKA Pathway and Partially p38 Activation

Scopoletin was recently shown to stimulate melanogenesis through cAMP-response element-binding protein (CREB) phosphorylation. In this study, we investigated the molecular events of melanogenesis-induced by scopoletin. After exposure to scopoletin, the protein levels of tyrosinase and tyrosianse related protein-1 (TRP-1) were significantly increased in B16F10 cells. The mRNA levels of tyrosinase and microphthalmia-associated transcription factor (MITF) were also enhanced by scopoletin. cAMP production and phosphorylation of p38 mitogen-activated protein kinase (MAPK) were increased by scopoletin treatment. Scopoletin-mediated increase of intracellular melanin and tyrosinase expression were significantly attenuated by protein kinase A (PKA) inhibitors (H-89 and KT5720), while a protein kinase C (PKC) inhibitor (Ro-32-0432) had no effect and a p38 MAPK inhibitor (SB203580) partially blocked the scopoletin-induced intracellular melanin and tyrosinase expression. Moreover, scopoletin synergistically with cell-permeable cAMP analog (dibutyryl cAMP) significantly induced tyrosinase activity and melanin content in B16F10 cells. The silencing of p38 MAPK by small interfering RNA (siRNA) decreased the scopoletin-induced tyrosinase expression in B16F10 cells. These results suggest that scopoletin could induce melanin synthesis through the cAMP/PKA pathway and partially p38 MAPK activation in B16F10 cells.

Fermented Onions Extract Inhibits Tyrosinase and Collagenase-1 Activities as a Potential New Anti–Photoaging Agent

The bulbs of onion ( Allium cepa L.) have long been used as food and dietary supplements in the treatment of various diseases. This study aimed to investigate anti-photoaging activities of fermented onions extract rich in flavonoids such as quercetin and isoquercetin. The fermented onions extract (FOE) effectively suppressed the melanin production by inhibition of tyrosinase expression in B16F10 melanoma cells at a concentration of 100 μg/mL. Besides, the FOE exhibited down-regulation of collagenase-1 expression and up-regulation of type I collagen level in UVB-irradiated HaCaT keratinocyte cells. Hyaluronic acid production was increased to 41.11% (10 μg/mL), 107.78% (100 μg/mL), and 146.67% (200 μg/mL) compared with the UVB-irradiated control by treatment of FOE. Therapeutic attempts with FOE might be useful in preventing or treating melanin pigmentary diseases and UVB-induced wrinkle formation.

Reduction of Melanogenic Activity and Responsiveness to α-Melanocyte-Stimulating Hormone during Serial Passage of Melanoma Cells

Background:

Murine melanoma cells such as Cloudman S91 or B16 mouse melanoma cells have been used extensively to study mechanisms involved in pigmentation because these cells have tyrosinase, the key enzyme in pigmentation, and produce pigment. We have observed that serial passaged S91 cells tend to decrease their basal pigment content and to lose their responsiveness to α-melanocyte-stimulating hormone (α-MSH).

Objective:

Because this reduction of melanogenic capacity is a widely acknowledged but virtually unstudied characteristic of both human and murine melanoma cell lines in culture, we wished to document and quantify the phenomenon.

Methods:

Commercially attained S91 melanoma cells were serially passaged. Basal pigmentation as well as α-MSH responsiveness and expression of protein kinase C-beta (PKC-β) were assessed.

Results:

S91 cells progressively lost their basal pigmentation under standardized conventional conditions of culture, from an initial melanin content of 20 ± 4 pg/cell content to 12 ± 5 pg/cell within 70 population doublings (16 passages) and to 4.5 ± 6 pg/cell, a level at or below the detectable level of our melanin assay, by 110 population doublings (28 passages). When responsiveness to α-MSH was assessed, a 6-day treatment with 10−6M α-MSH initially induced the pigment content four-fold from 20 ± 4 to 82 ± 2 pg/cell. In contrast, after 110 population doublings, identical treatment with α-MSH induced pigment content less than two-fold from 4.5 ± 6 to 7.5 ± 2 pg/cell. PKC-β expression was readily detected by immunofluorescence in early passage pigmented cells, but not in late passage nonpigmented cells.

Conclusion:

These results confirm that while murine melanoma cells are a useful model system for pigmentation studies, it is important to monitor changes in the cells' basal abilities to pigment and to respond to exogenous pigment-inducing factors. They further suggest that factors in the culture environment or the internal milieu of melanoma cells exposed to continuous mitogenic stimulation inhibit melanogenesis. One candidate mechanism is down regulation of PKC-β.

Recent progress in melasma pathogenesis

Melasma is a common skin pigmentation condition. Given therapeutic difficulty as one of the biggest concerns, understanding of the etiology and pathogenesis of melasma becomes essential. UV irradiation, female sex hormones, and inflammatory processes are addressed as triggering factors with genetic predisposition. The mechanism of UV-induced melanogenesis has been extensively investigated as a model system to study melasma pathogenesis. Hitherto, treatment modalities for melasma are similar to other hyperpigmentation disorders. However, individual triggering factors induce a separate pigmentation disease, whose pathogenic mechanisms and clinical phenotypes are different from the ones encountered in melasma. Fortunately, there have been ongoing updates on melasma pathogenesis with regard to major triggering factors. Presence of certain factors working independently of UV exposure and role of dermal factors and microRNAs are being identified as novel discoveries about melasma pathogenesis. In this review, the melasma pathogenesis is reviewed in association with updated and new findings.

Whitening effect of Sophora flavescens extract

CONTEXT

Sophora flavescens Ait. (Leguminosae) has been proposed as a new whitening agent for cosmetics, because it has a strong ability to inhibit tyrosinase, a key enzyme in the formation of melanin.

OBJECTIVE

We conducted a study to determine whether ethanol extract of the roots of S. flavescens has the potential for use as a whitening cosmetic agent by investigating its underlying mechanisms of action.

MATERIALS AND METHODS

To elucidate the mechanism of action of S. flavescens extract, we used DNA microarray technology. We investigated the changes in the mRNA levels of genes associated with the formation and transport of melanosomes. We also identified the formation and transport of melanosomes with immunohistochemistry and immunofluorescence analyses. Finally, the skin-whitening effect in vivo of S. flavescens extract was analyzed on human skin.

RESULTS

We found that S. flavescens extract strongly inhibited tyrosinase activity (IC50, 10.4 μg/mL). Results also showed that key proteins involved in the formation and transport of melanosomes were dramatically downregulated at both mRNA and protein level in keratinocytes exposed to S. flavescens extract. In addition, a clinical trial of a cream containing 0.05% S. flavescens extract on human skin showed it had a significant effect on skin whitening by mechanical and visual evaluation (1.14-fold).

DISCUSSION AND CONCLUSION

This study provides important clues toward understanding the effects of S. flavescens extract on the formation and transport of melanosomes. From these results, we suggest that naturally occurring S. flavescens extract might be useful as a new whitening agent in cosmetics.

The regulation of epidermal melanogenesis via cAMP and/or PKC signaling pathways: insights for the development of hypopigmenting agents

Abnormal pigmentation, particularly hyperpigmentation, is major issue of concern for people with colored skin. Several hypopigmenting agents, which exert their action by inhibiting tyrosinase activity and/or transcription, have been used for treatment. However, results have been discouraging. To manage abnormal pigmentation properly, the mechanisms of melanogenesis should be understood. Endogenous and exogenous factors affect melanogenesis via intracellular machineries. cAMP and PKC are critical factors of important transduction pathways and cross-talk between them could amplify the melanogenic effect. Here, factors involved in melanogenesis regulation via cAMP and/or PKC pathways are reviewed with their action mechanisms.

p38 regulates pigmentation via proteasomal degradation of tyrosinase

The synthesis of melanin pigments, or melanogenesis, is regulated by the balance of a variety of signal transduction pathways. Among these pathways, p38 MAPK signaling was found to be involved in stress-induced melanogenesis and to be activated by alpha-melanocyte-stimulating hormone (alpha-MSH) and ultraviolet irradiation. Previous studies have shown that alpha-MSH-stimulated melanogenesis can be inhibited by blocking p38 MAPK activity with SB203580, a pyridinyl imidazole compound. Consistent with this, we observed that pyridinyl imidazoles (SB203580 and SB202190) inhibited both basal and alpha-MSH-induced melanogenesis in B16 melanoma cells. However, SB202474, which has no ability to inhibit p38 MAPK activity and is usually used as a negative control compound in p38 MAPK studies, also suppressed melanin synthesis induction. Furthermore, the independence of the p38 kinase pathway from the repression of melanogenesis by pyridinyl imidazole compounds was also confirmed by small interfering RNA experiments. Interfering with p38 MAPK expression surprisingly stimulated melanogenesis and tyrosinase family protein expression. Although the molecular mechanism(s) by which p38 promotes the degradation of melanogenic enzymes remain to be determined, the involvement of the ubiquitin-proteasome pathway was demonstrated by co-treatment with the proteasome-specific inhibitor MG132 and the relative decrease in the ubiquitination of tyrosinase in cells transfected with p38-specific small interfering RNA.

MITF mediates cAMP-induced protein kinase C-beta expression in human melanocytes

The cAMP-dependent pathway up-regulates MITF (microphthalmia-associated transcription factor), important for key melanogenic proteins such as tyrosinase, TRP-1 (tyrosinase-related protein 1) and TRP-2. We asked whether MITF is also a key transcription factor for PKC-beta (protein kinase C-beta), required to phosphorylate otherwise inactive tyrosinase. When paired cultures of human melanocytes were treated with isobutylmethylxanthine, known to increase intracellular cAMP, both protein and mRNA levels of PKC-beta were induced by 24 h. To determine whether MITF modulates PKC-beta expression, paired cultures of human melanocytes were transfected with dn-MITF (dominant-negative MITF) or empty control vector. By immunoblotting, PKC-beta protein was reduced by 63+/-3.7% within 48 h. Co-transfection of an expression vector for MITF-M, the MITF isoform specific for pigment cells, or empty control vector with a full-length PKC-beta promoter-CAT (chloramphenicol acetyltransferase) reporter construct (PKC-beta/CAT) into Cos-7 cells showed >60-fold increase in CAT activity. Melanocytes abundantly also expressed MITF-A, as well as the MITF-B and MITF-H isoforms. However, in contrast with MITF-M, MITF-A failed to transactivate co-expressed PKC-beta/CAT or CAT constructs under the control of a full-length tyrosinase promoter. Together, these results demonstrate that MITF, specifically MITF-M, is a key transcription factor for PKC-beta, linking the PKC- and cAMP-dependent pathways in regulation of melanogenesis.

Melanin pigmentation in mammalian skin and its hormonal regulation

Cutaneous melanin pigment plays a critical role in camouflage, mimicry, social communication, and protection against harmful effects of solar radiation. Melanogenesis is under complex regulatory control by multiple agents interacting via pathways activated by receptor-dependent and -independent mechanisms, in hormonal, auto-, para-, or intracrine fashion. Because of the multidirectional nature and heterogeneous character of the melanogenesis modifying agents, its controlling factors are not organized into simple linear sequences, but they interphase instead in a multidimensional network, with extensive functional overlapping with connections arranged both in series and in parallel. The most important positive regulator of melanogenesis is the MC1 receptor with its ligands melanocortins and ACTH, whereas among the negative regulators agouti protein stands out, determining intensity of melanogenesis and also the type of melanin synthesized. Within the context of the skin as a stress organ, melanogenic activity serves as a unique molecular sensor and transducer of noxious signals and as regulator of local homeostasis. In keeping with these multiple roles, melanogenesis is controlled by a highly structured system, active since early embryogenesis and capable of superselective functional regulation that may reach down to the cellular level represented by single melanocytes. Indeed, the significance of melanogenesis extends beyond the mere assignment of a color trait.

The relationship between Na+/H+ exchanger expression and tyrosinase activity in human melanocytes

The activity of melanosome-associated tyrosinase in human melanocytes differs based on racial skin type. In melanocytes from Black skin, tyrosinase activity is high while in White melanocytes the activity of the enzyme is low. Recent studies suggest that low tyrosinase activity in White melanocytes may be due to an acidic pH environment within the melanosome. Because sodium/hydrogen (Na(+)/H(+)) exchangers (NHEs) are known to regulate intracellular pH, melanocytes were treated with NHE inhibitors to determine what effect this inhibition might have on tyrosinase activity. Treatment of Black melanocytes with ethyl-isopropyl amiloride (EIPA) caused a rapid dose-dependent inhibition of tyrosinase activity. This inhibition was not due to either direct enzyme inhibition or to a decrease in tyrosinase abundance. In contrast, treatment of White melanocytes with EIPA, cimetidine, or clonidine resulted in little inhibition of tyrosinase activity. Reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot analysis showed that both Black and White melanocytes expressed mRNA and protein for NHE-1, NHE-3, NHE-5, NHE-6, and NHE-7. Immunohistochemical analysis showed that NHE-7 and NHE-3 co-localized with the melanosomal protein, Tyrosinase Related Protein-1 (TRP-1). In addition, the vesicular proton pump, vesicular ATPase (V-ATPase), was found to be present in both White and Black melanosomes, indicating that organelles from both racial skin types are capable of being acidified. The results suggest that one or more NHEs may help regulate melanosome pH and tyrosinase activity in human melanocytes.

Pigment-lightening effect of N,N'-dilinoleylcystamine on human melanoma cells

BACKGROUND

Cystamine and linoleic acid have been reported to reduce melanin synthesis in vitro and in vivo. N,N'-dilinoleylcystamine (DLC) is a compound of cystamine and linoleic acid connected by an ester bond.

OBJECTIVES

To investigate the effects of DLC on melanin synthesis using cultured human melanoma cells.

METHODS

Levels of total melanin, eumelanin and phaeomelanin, tyrosinase protein and tyrosinase activity in situ were measured in HM3KO melanoma cells. Changes in degree of pigmentation were quantified by image analysis and compared with absorbance values. Tyrosinase from HM3KO cells was used to measure the direct effect of DLC on DOPA and DOPAchrome production.

RESULTS

At concentrations of 1.4-14 micromol L-1, DLC reduced the pigmentation of HM3KO melanoma cells but did not affect cell growth. The visual decrease in pigmentation produced by DLC was more dramatic than the decrease in total melanin content as measured by absorbance at 500 nm. DLC treatment decreased eumelanin synthesis and increased phaeomelanin synthesis in HM3KO melanoma cells. An in situ tyrosinase assay showed that DLC inhibited tyrosinase activity, as well as the level of tyrosinase protein.

CONCLUSIONS

These results suggest that DLC has pigment-lightening effects on HM3KO melanoma cells, produced by reducing the level of eumelanin while increasing the level of phaeomelanin.

Microphthalamia-associated transcription factor: a critical regulator of pigment cell development and survival

The microphthalamia-associated transcription factor (MITF) is an integral transcriptional regulator in melanocyte, the lineage from which melanoma cells originate. This basic-helix-loop-helix-leucine-zipper (bHLHzip) protein is critical for melanocyte cell-fate choice during commitment from pluripotent precursor cells in the neural crest. Its role in differentiation pathways has been highlighted by its potent transcriptional and lineage-specific regulation of the three major pigment enzymes: tyrosinase, Tyrp1, and Dct as well as other pigmentation factors. However, the cellular functions of MITF seem to be wider than differentiation and cell-fate pathways alone, since melanocytes and melanoma cells appear to require an expression of this factor. Here, we discuss the transcriptional networks in which MITF is thought to reside and describe signaling pathways in the cell which impinge on MITF. Accumulating evidence supports the notion that MITF is involved in survival pathways during normal development as well as during neoplastic growth of melanoma.

Regulation of the human melanocortin 1 receptor expression in epidermal melanocytes by paracrine and endocrine factors and by ultraviolet radiation

The aim of this study is to investigate the regulation of the human melanocortin 1 receptor (MC1R) expression in cultured normal human melanocytes (NHM) by specific paracrine and endocrine factors, and by ultraviolet radiation (UVR). Treatment of NHM with alpha-melanotropin [alpha-melanocyte stimulating hormone (alpha-MSH)] increased MC1R mRNA level; the response was often more pronounced in NHM with a low (NHM-c) than in NHM with a high melanin content (NHM-b). Endothelin-1 increased MC1R mRNA level in NHM regardless of their melanin content. Basic fibroblast growth factor consistently up regulated MC1R mRNA level in NHM-b but not in NHM-c. Activation of protein kinase C by 12-0-tetradecanoylphorbol-13-acetate slightly increased, while stimulation of adenylate cyclase by forskolin markedly up-regulated the MC1R mRNA level. beta-Estradiol increased, and combined treatment with beta-estradiol and alpha-MSH further elevated, MC1R mRNA level in NHM-c and NHM-b. Testosterone reduced, while progesterone had no effect on, MC1R mRNA level. Agouti signaling protein reduced, and UVR down regulated dose-dependently MC1R mRNA level in NHM-b and NHM-c. This effect was reversed 24 h after irradiation with the lower doses of 7 or 14 mJ/cm2, but not after exposure to a higher, more cytotoxic dose of UVR. We conclude that the MC1R is regulated by paracrine factors, including its own ligands, by specific endocrine sex hormones, and by UVR. Differences in the responses of NHM to some of these factors suggest differential regulation of MC1R gene expression, which may contribute to the variation in constitutive and UV-induced cutaneous pigmentation in humans.

Regulation of ornithine decarboxylase in B16 mouse melanoma cells: synergistic activation of melanogenesis by alphaMSH and ornithine decarboxylase inhibition

Ornithine decarboxylase (ODC) is the rate-limiting enzyme in the biosynthesis of polyamines, a family of cationic compounds required for optimal cell proliferation and differentiation. Within mammalian melanocytes, the expression of genes regulating cell growth and/or differentiation can be controlled by alpha-melanocyte-stimulating hormone (alphaMSH) and other melanogenesis modulating agents. In the B16 mouse melanoma model, alphaMSH stimulates melanogenesis by upmodulation of tyrosinase (tyr) activity, whereas the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) inhibits melanin synthesis. Therefore, we analyzed the regulation of ODC by these agents, as related to changes in the melanogenic pathway. Treatment of B16 cells with TPA or alphaMSH rapidly stimulated ODC activity. The effect was stronger for TPA and appeared mainly posttranslational. Irreversible inhibition of ODC with the active site-directed inhibitor alpha-difluoromethylornithine (DFMO) did not block TPA-mediated inhibition of tyr. Conversely, prolonged treatment of B16 cells with DFMO stimulated tyr activity by a posttranslational mechanism, probably requiring polyamine depletion. Combination treatment with alphaMSH and DFMO synergistically activated tyr. Therefore, ODC induction is not involved in the melanogenic response of B16 cells to alphaMSH. Rather, increased intracellular concentrations of polyamines following ODC induction might constitute a feedback mechanism to limit melanogenesis activation by alphaMSH.

Melanocyte stimulating hormone activation of tyrosinase in B16 mouse melanoma cells Evidence for a differential induction of two distinct isoenzymes

Tyrosinase induction in murine malignant melanocytes by alpha MSH is well known, but its molecular basis has not been characterized. Treatment of B16 melanoma cells with theophylline or alpha MSH mediates a larger induction of tyrosine hydroxylase than of dopa oxidase activity in total cell extracts, and in the melanosomal and microsomal fractions. No evidence for the modulation of a tyrosinase effector was found. SDS-PAGE and specific activity stain demonstrated two forms of tyrosinase, with different degrees of induction by theophylline. These results agree with the recent proposal that two tyrosinases, encoded by different genes, are present in murine melanocytes.

Tyrosinase gene expression is regulated by p53

Tyrosinase, the rate-limiting enzyme for melanin synthesis, is induced after ultraviolet irradiation as part of the tanning response, the major recognized photoprotective response of human skin. Other DNA-damaging agents and DNA fragments such as thymidine dinucleotides also induce tyrosinase gene expression. Moreover, like ultraviolet light they also activate p53. To determine whether p53 activation is required for this increased tyrosinase expression, we employed two experimental systems: (i) a human melanoma line (WM35) known to express wild-type p53 versus WM35 cells engineered to express a transcriptionally inactive dominant-negative p53 (WM35-p53DN) or the empty vector alone (WM35-pCMV7) and (ii) mice with wild-type p53 versus p53 knockout mice. In WM35-p53DN cells, the baseline p53 protein level was higher than in WM35 or WM35-pCMV7 cells, and tyrosinase transcripts were lower. After ultraviolet irradiation, in all cell lines the p53 protein level increased within the first 24 h, as expected; and at 24 h tyrosinase mRNA levels were decreased. Consistent with the literature, these data in combination suggest that increased p53 protein level downregulates tyrosinase mRNA. In WM35 and WM35-pCMV7 cells at 48 and 72 h, however, whereas p53 levels remained elevated, tyrosinase mRNA levels compared to pre-irradiation levels tripled, whereas in WM35-p53DN cells levels remained below baseline. In thymidine-dinucleotide-treated WM35 and WM35-pCMV7 cells there was a comparable upregulation of tyrosinase mRNA within 24 h that persisted through 72 h, but there was no upregulation of tyrosinase mRNA in WM35-p53DN cells any time after ultraviolet irradiation or thymidine dinucleotide treatment. In ear skin of p53 wild-type mice, topical application of thymidine dinucleotide induced a 4-5-fold increase in epidermal melanin content after 3 wk, but in p53 knockout mice thymidine dinucleotide application caused no detectable increase in melanin. Together, these data demonstrate that p53 activation increases tyrosinase mRNA level and subsequently pigmentation. The data further suggest that tanning is part of a p53-mediated adaptive response of mammalian skin to DNA damage from ultraviolet irradiation.

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.

The melanocortin 1 receptor is the principal mediator of the effects of agouti signaling protein on mammalian melanocytes

The agouti gene codes for agouti signaling protein (ASP), which is temporally expressed in wild-type mouse follicular melanocytes where it induces pheomelanin synthesis. Studies using purified full-length agouti signaling protein has shown that it competes with (α)-melanocyte stimulating hormone for binding to the melanocortin 1 receptor. We have investigated whether ASP binds exclusively to the melanocortin 1 receptor expressed on mouse melanocytes in primary culture, or additionally activates a receptor that has not been identified yet. We have compared the responses of congenic mouse melanocytes derived from C57 BL/6J-E(+)/E(+), e/e, or E(so)/E(so) mice to (alpha)-MSH and/or ASP. E(+)/E(+) melanocytes express the wild-type melanocortin 1 receptor, e/e melanocytes express a loss-of-function mutation in the melanocortin 1 receptor that results in a yellow coat color, and E(so)/E(so) is a mutation that causes constitutive activation of the melanocortin 1 receptor and renders melanocytes unresponsive to (alpha)-melanocyte stimulating hormone. Mouse E(+)/E(+) melanocytes, but not e/e or E(so)/E(so) melanocytes, respond to agouti signaling protein with decreased basal tyrosinase activity, and reduction in levels of tyrosinase and tyrosinase-related proteins 1 and 2. Only in E(+)/E(+) melanocytes does agouti signaling protein abrogate the stimulatory effects of (alpha)-melanocyte stimulating hormone on cAMP formation and tyrosinase activity. These results indicate that a functional melanocortin 1 receptor is obligatory for the response of mammalian melanocytes to agouti signaling protein.

Regulation of the catalytic activity of preexisting tyrosinase in black and Caucasian human melanocyte cell cultures

The activity of tyrosinase, the rate-limiting enzyme for melanin synthesis, is higher in Black skin melanocytes than in melanocytes derived from Caucasian skin. This variation in enzyme activity is not due to differences in tyrosinase abundance or tyrosinase gene activity, but, rather, is due to differences in the catalytic activity of preexisting tyrosinase. In melanocytes, tyrosinase is localized to the membrane of melanosomes and in Caucasian melanocytes the melanosome-bound enzyme is largely inactive. Conversely, in melanosomes of Black melanocytes, tyrosinase has high catalytic activity. Treatment of Caucasian melanocytes with the lysosomotropic compound ammonium chloride or with the ionophores nigericin and monensin results in a rapid and pronounced increase in tyrosinase activity. This increase occurs without any change in tyrosinase abundance, indicating that these compounds are increasing the catalytic activity of preexisting enzyme. Inhibition of the vacuolar proton pump V-ATPase by treatment of Caucasian melanocytes with bafilomycin also increases tyrosinase activity. In contrast to the 10-fold increase in tyrosinase observed in Caucasian melanocytes, neither ammonium chloride, monensin, nigericin, nor bafilomycin is able to increase the already high level of tyrosinase activity present in melanosomes of melanocytes derived from Black skin. Finally, staining of Caucasian melanocytes with the fluorescent weak base acridine orange shows that melanosomes of Caucasian, but not Black, melanocytes are acidic organelles. These data support a model for racial pigmentation that is based on differences in melanosome pH in Black and Caucasian skin types. The models suggests that melanosomes of Caucasian melanocytes are acidic, while those of Black individuals are more neutral. Since tyrosinase is inactive in an acid environment, the enzyme is largely inactive in Caucasian melanosomes but fully active in Black melanosomes.

Downregulation of tyrosinase activity in human melanocyte cell cultures by yohimbine

Treatment of human melanocyte cell cultures with the alpha-2 adrenergic receptor antagonist yohimbine results in a marked down-regulation of tyrosinase activity. A 30% decrease occurs within 12 h of exposure of cells to yohimbine (100 microM), and by 48 h tyrosinase activity in treated melanocytes is less than a fifth that of control cultures. The inhibition is dose dependent and occurs in human melanocytes derived from either black or white skin types, and also in mouse melanoma cells. The yohimbine-induced decrease in tyrosinase activity is reversible, with enzyme levels returning to 90% of control values 48 h after removal of drug. Although tyrosinase activity is markedly suppressed by yohimbine, the compound has no effect on cell proliferation, cellular translation, or DNA synthesis. Treatment of melanocyte cultures with yohimbine blocks the increase in tyrosinase activity by either 3-isobutyl-1-methylxanthine, dibutyryl cAMP, or forskolin. Results of cAMP immunoassays, show that intracellular levels of the cyclic nucleotide are unaffected in cells treated with yohimbine. Tyrosinase inhibition by yohimbine does not involve a decrease in substrate availability since tyrosine uptake studies show that yohimbine has no effect on the amount of tyrosine entering the cell. Incubation of a melanosome-enriched fraction with yohimbine does not cause a lowering of tyrosinase activity, suggesting that an intact cell is required for yohimbine action. In addition, tyrosinase extracts show no reduction in activity when incubated directly with yohimbine, indicating that the drug does not act as a direct inhibitor of the enzyme. Finally, results of western immunoblotting show that yohimbine does not significantly lower the amount of tyrosinase protein in human melanocytes. These findings suggest that yohimbine acts through an as yet unidentified signaling pathway to lower the catalytic activity of pre-existing tyrosinase molecules present in melanocytes.

alpha-MSH can control the essential cofactor 6-tetrahydrobiopterin in melanogenesis

In the human epidermis both keratinocytes and melanocytes express POMC m-RNA. Immunohistochemical studies of both cell types demonstrate significantly higher levels of alpha-MSH in melanocytes than in keratinocytes. Both cell types also hold the full capacity for de novo synthesis/recycling of the essential cofactor (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin (6BH4). 6BH4 is critical for the hydroxylation of the aromatic amino acids L-phenylalanine, L-tyrosine, and L-tryptophan, for nitric oxide production and in various immune modulatory processes. Recently it was shown that tyrosinase activity is regulated by 6BH4 through a specific allosteric inhibition. The tyrosinase/6BH4 inhibition can be activated by 1:1 complex formation between 6BH4 and alpha-MSH, but an excess of alpha-MSH over 6BH4 can inhibit tyrosinase due to complex formation by tyr2 in the alpha-MSH sequence. In both melanocytes and keratinocytes 6BH4 controls the L-tyrosine supply via phenylalanine hydroxylase (PAH). Recently we were able to show that the cellular uptake of L-phenylalanine and its intracellular turnover to L-tyrosine is crucial for melanogenesis. alpha-MSH can promote the production of L-tyrosine via PAH due to activation of the PAH tetramer to the more active dimer by removing 6BH4 from the regulatory binding domain on the enzyme. In conclusion, alpha-MSH can control (1) intracellular L-tyrosine formation from L-phenylalanine in both melanocytes and keratinocytes, and (2) tyrosinase activity, directly, in melanocytes.

The melanocortin-1 receptor and human pigmentation

alpha-Melanocyte stimulating hormone (alpha-MSH) is known to be the main physiologic regulator for integumental pigmentation of various vertebrate species. However, the role of alpha-MSH and related melanocortins in the regulation of human cutaneous pigmentation is only beginning to be understood. Cloning of the melanocortin-1 receptor (MC1R), and the feasibility of establishing normal human epidermal melanocyte cultures have made it possible to demonstrate direct and specific biological effects of alpha-MSH on these cells. It is now recognized that both alpha-MSH and ACTH have similar mitogenic and melanogenic effects on human epidermal melanocytes. These effects are mediated by binding of these hormones to the specific MC1R that recognizes them both with similar affinity. Human MC1R is homologous to its mouse counterpart in that its activation leads to stimulation of eumelanin synthesis. MC1R is also the binding site for agouti signaling protein (ASP), the product of the agouti locus. Human epidermal melanocytes respond to purified recombinant mouse or human ASP, with a reduction in basal tyrosinase activity, and complete abrogation of the mitogenic and melanogenic effects of alpha-MSH. These results suggest that ASP induces pheomelanin synthesis by competing with alpha-MSH for binding to the MC1R. This receptor seems to be subject to regulation by a variety of paracrine and/or autocrine factors that are synthesized in response to exposure of the skin to ultraviolet radiation (UVR). Activation of MC1R seems to be pivotal for UV-induced melanogenesis, since stimulation of the cAMP pathway plays a key role in the melanogenic response of human epidermal melanocytes. The melanogenic response to UVR might be influenced by the presence of allelic variants of the MC1R gene. Allelic variants have been identified and shown to be associated with red hair, poor tanning ability, and possibly melanoma. The possible influence of these variants on the function of the MC1R needs to be investigated, in order to understand the physiological consequence of these mutations. Also, the interaction of alpha-MSH with other factors that are known to affect pigmentation needs to be better understood in order to define the role possible of this hormone and its receptor in acquired human cutaneous hyper- or hypopigmentation.

Activation of cAMP-dependent protein kinase is required for optimal alpha-melanocyte-stimulating hormone-induced pigmentation

The cAMP-dependent pathway has been long presumed to play a critical role in mediating alpha-melanocyte-stimulating hormone (alpha-MSH)-induced pigmentation, but it has never been demonstrated that this pathway is obligatory. In order to determine whether the cAMP-dependent pathway is required for a alpha-MSH-induced pigmentation, we inhibited the activity of cAMP-dependent protein kinase (PKA), the main kinase mediating in this pathway, by introducing a physiologic cAMP-dependent protein kinase inhibitor (PKI) into S91 murine melanoma cells and then measuring pigment response after alpha-MSH stimulation. Cells were stably transfected either with the pMXX-PKI expression vector that encodes the active part of PKI (the amino terminal 1-31 amino acids) under a metallothionein-inducible promoter and the pSV2-Neo expression vector alone. As expected, treatment of transfected cells with 1 microM CdCl2 for 24 h induced the expression of PKI mRNA in cells transfected with both vectors, but not in cells transfected with the pSV2-Neo expression vector alone. Subsequent treatment of these transfected cells with alpha-MSH for 5-6 days in the continual presence of 1 microM CdCl2 resulted in inhibition of PKA activity by 30-40% in cells expressing PKI. Parallel measurements revealed that alpha-MSH-increased melanin content five- to six-fold in control cells transfected with pSV2-Neo alone, while there was only a two-fold increase in PKI-expressing cells, a 40-50% inhibition in alpha-MSH-induced total melanin content. alpha-MSH-induced tyrosinase activity and tyrosinase mRNA and protein levels measured in parallel were also inhibited by 40-50% in PKI-expressing cells compared to control cells transfected with pSV2-Neo alone. Together, these results demonstrate for the first time that activation of PKA through the cAMP-dependent pathway is required for optimal alpha-MSH-induced pigmentation.

Inhibition of the phosphatidylinositol 3-kinase/p70(S6)-kinase pathway induces B16 melanoma cell differentiation

alpha-Melanocyte-stimulating hormone and cAMP-elevating agents are known to induce B16 cell differentiation, characterized by increased melanin synthesis and dendrite outgrowth. In order to elucidate intracellular signaling pathways involved in this differentiation process, we focused our interest on the phosphatidylinositol 3-kinase/p70(S6)-kinase pathway. The specific inhibition of phosphatidylinositol 3-kinase by LY294002 markedly stimulated dendrite outgrowth, thus mimicking the action of cAMP-elevating agents on B16 cell morphology. In addition, LY294002 and rapamycin, a specific p70(S6)-kinase inhibitor, were found to independently stimulate tyrosinase expression, thus increasing melanin synthesis. In an attempt to better dissect the molecular mechanisms triggered by cAMP to induce melanoma cell differentiation, we examined the effects of a cAMP-elevating agent forskolin, on both phosphatidylinositol 3-kinase and p70(S6)-kinase activities. Specific kinase assays revealed that forskolin partially inhibited phosphatidylinositol 3-kinase activity and completely blocked p70(S6)-kinase activity and phosphorylation. In conclusion, our results clearly demonstrate that the inhibition of phosphatidylinositol 3-kinase and p70(S6)-kinase is involved in the regulation of B16 cell differentiation. Furthermore, we provide evidence which suggests that cAMP-induced melanogenesis and dendricity are, at least partially, mediated by the cAMP inhibition of the phosphatidylinositol 3-kinase/p70(S6)-kinase signaling pathway.

Different populations of melanocytes are present in hair follicles and epidermis

Melanocytes in human skin reside both in the epidermis and in the matrix and outer root sheath of anagen hair follicles. Comparative study of melanocytes in these different locations has been difficult as hair follicle melanocytes could not be cultured . In this study we used a recently described method of growing hair follicle melanocytes to characterize and compare hair follicle and epidermal melanocytes in the scalp of the same individual. Three morphologically and antigenically distinct types of melanocytes were observed in primary culture. These included (1) moderately pigmented and polydendritic melanocytes derived from epidermis; (2) small, bipolar, amelanotic melanocytes; and (3) large, intensely pigmented melanocytes; the latter two were derived from hair follicles. The three sub-populations of cells all reacted with melanocyte-specific monoclonal antibody. Epidermal and amelanotic hair follicle melanocytes proliferated well in culture, whereas the intensely pigmented hair follicle melanocytes did not. Amelanotic hair follicle melanocytes differed from epidermal melanocytes in being less differentiated, and they expressed less mature melanosome antigens. In addition, hair follicle melanocytes expressed some antigens associated with alopecia areata, but not antigens associated with vitiligo, whereas the reverse was true for epidermal melanocytes. Thus antigenically different populations of melanocytes are present in epidermis and hair follicle. This could account for the preferential destruction of hair follicle melanocytes in alopecia areata and of epidermal melanocytes in vitiligo.

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 tyrosinase mRNA in mouse melanoma cells by alpha-melanocyte-stimulating hormone

Cloudman S-91 mouse melanoma cells respond to alpha-melanocyte-stimulating hormone) by demonstrating a marked increase in tyrosinase activity (O-diphenol-O2 oxidoreductase, EC 1.14.18.1). This increase is the result of increased levels of tyrosinase mRNA with a subsequent increase in tyrosinase abundance. Our studies were carried out to determine the effect of melanocyte-stimulating hormone on tyrosinase gene transcription and to measure the kinetics of the hormone-induced increase in tyrosinase mRNA. When melanoma cells were exposed continuously to melanocyte-stimulating hormone for 6 d, a large but transient increase in both tyrosinase mRNA abundance and enzyme activity were observed. The maximum increase in tyrosinase mRNA occurred 60 h after melanocyte-stimulating hormone stimulation and was followed by a decline in message levels even though cells were continuously exposed to hormone. Results of nuclear run-off transcription assays showed that melanocyte-stimulating hormone caused a slow increase in the rate of transcription of the tyrosinase gene with a maximal 6-fold stimulation occurring at 48 h. In cells treated with the ribonucleic acid synthesis inhibitor, 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole, tyrosinase mRNA levels decayed with a half-life of 4-5 h. This decay rate was unaffected by treatment of cells with melanocyte-stimulating hormone, indicating that the hormone does not act to stabilize tyrosinase ribonucleic acid. Inhibition of protein synthesis by treatment with cycloheximide had no effect on the melanocyte-stimulating hormone-induced increase in tyrosinase messenger ribonucleic acid levels suggesting that ongoing protein synthesis is not required for, at least, the initial stimulation of tyrosinase gene transcription by melanocyte-stimulating hormone.

The interaction of agouti signal protein and melanocyte stimulating hormone to regulate melanin formation in mammals

Important regulatory controls of melanogenesis that operate at the subcellular level to modulate the structural and/or the functional nature of the melanins and melanin granules produced in melanocytes are reviewed. Melanocyte stimulating hormone and agouti signal protein have antagonistic roles and possibly opposing mechanisms of action in the melanocyte. In the mouse, melanocyte stimulating hormone promotes melanogenic enzyme function and elicits increases in the amount of eumelanins produced, while agouti signal protein reduces total melanin production and elicits the synthesis of pheomelanin rather than eumelanin. We are now beginning to understand the complex controls involved in regulating this switch at the molecular and biochemical levels. The quality and quantity of melanins produced by melanocytes have important physiological consequences for melanocyte function and undoubtedly play important roles in the various functions of the melanins per se, including hair and skin coloration and photoprotection.

PUVA (5-methoxypsoralen plus UVA) enhances melanogenesis and modulates expression of melanogenic proteins in cultured melanocytes

PUVA (combination of psoralens and ultraviolet A radiation) is a potent inducer of melanogenesis in normal human skin. The molecular mechanisms underlying this effect are poorly characterized. This study was undertaken to investigate the action of PUVA on melanogenesis in S91 murine melanoma cells and in cultured normal human melanocytes. Tyrosinase and DOPAchrome tautomerase (DCT) activities as well as melanin neosynthesis were measured in PUVA-treated pigment cells. To determine whether a correlation exists between PUVA-induced melanogenesis and expression of melanogenic enzymes, we analyzed the levels of tyrosinase, DCT, and tyrosinase-related protein-1 (TRP-1 or gp75) by western blotting in PUVA-treated cells. We demonstrate that UVA upregulates tyrosinase activity and melanin content with 5-methoxypsoralen at 1 microM. This phenomenon depends on the energy delivered during phototreatment. In both human and mouse cells, stimulation of melanogenesis correlated with an increase of the amount of tyrosinase. In PUVA-treated S91 cells, tyrosinase mRNA was increased, but no stimulation of DCT activity occurred in these cells, in agreement with the unchanged amount of DCT protein in cell extracts. On the contrary, in melanocytes treated with PUVA, a decrease in DCT protein was observed. Finally, the amount of TRP-1 protein was not affected by PUVA in either S91 cells or melanocytes. These results show that melanogenesis induced by PUVA is related to an increase in expression of tyrosinase. In melanocytes, melanogenesis and DCT are negatively correlated, which suggests that PUVA favors the metabolic pathway of dark-eumelanins with high UV-protective properties. This study also suggests that PUVA regulates tyrosinase, DCT, and TRP-1 expression in a noncoordinate manner.

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.

Melatonin antagonizes alpha-melanocyte-stimulating hormone enhancement of melanogenesis in mouse melanoma cells by blocking the hormone-induced accumulation of the c locus tyrosinase

Melatonin was found to have a small inhibitory effect on tyrosinase activity and a slight stimulatory action on dopachrome tautomerase activity in B16 mouse melanoma cells. These effects were time and dose dependent, with the maximal response being observed after 24-48 h treatment and at concentrations of melatonin higher than the physiologic levels of the circulating hormone. Although these effects on the melanogenic activities were modest, incubation of melanocytes with melatonin prior to the addition of the melanotropin mediated a dramatic inhibition of alpha-melanocyte-stimulating-hormone-(alpha-MSH)-induced melanogenesis. This inhibitory effect was evident at melatonin concentrations as low as 10 nM. Inhibition was nearly total at 0.1 mM melatonin, even at high concentrations of alpha-MSH (1 microM). The inhibitory effect of melatonin on alpha-MSH stimulation of melanogenesis was investigated. Melatonin appeared to act at least at two stages. Pharmacological concentrations of melatonin diminished the number of alpha-MSH receptors to about 75% of the control values without an apparent effect on receptor affinity, as determined by receptor-binding studies using 125I-[N-Leu4-D-Phe7]alpha-MSH as a probe. Physiological concentrations of melatonin also appeared to interfere with the intracellular events coupling increased cAMP levels and induction of the c locus tyrosinase, since it strongly inhibited the theophylline-mediated stimulation of melanogenesis. The inhibition of tyrosinase stimulation was higher in the microsomal than in the melanosomal fractions of cells which were treated with melatonin, then exposed to either alpha-MSH (1 microM) or theophylline (1 mM), suggesting that one of the main effects of melatonin might be inhibition of the induction of tyrosinase de novo synthesis.

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.

Production and characterization of antibodies against human tyrosinase

Proteins mapping at different loci are involved in melanogenesis and share several characteristic structural features (b locus, c locus, and slaty locus products). We describe a method to produce specific antibodies against human tyrosinase, the product of the c locus. Mouse L cells transfected with a human tyrosinase cDNA were used to generate antibodies by immunization of syngeneic C3H mice. These antibodies were able to precipitate the tyrosinase glycoprotein from both melanocytic cells and transfectants expressing tyrosinase. In contrast, transfectants expressing the related but distinct b locus protein (gp75 or TRP-1) did not react with these antibodies. In most cases, tyrosinase enzymatic activity could be precipitated and recovered in immune complexes, but one antibody response blocked tyrosinase activity. Immunostaining with anti-tyrosinase antibodies revealed an intracellular granular pattern in tyrosinase transfectants and melanocytic cells, but not transfectants expressing the b locus protein. This approach provides a general method to produce specific antibodies against tyrosinase, other members of the tyrosinase family of proteins, and potentially any other differentiation antigen.

Establishment and characterization of a malignant melanoma cell line (YP-MEL) derived from a patient with neurocutaneous melanosis

A cell line, YP-MEL, was established from an intracranial malignant melanoma occurring in a neurocutaneous melanosis (NCMsis) patient. The established cell line was successfully cultured in serum-free medium with a doubling time of 41 h. The cells were refractile and small in size, with occasional pigmented giant cells. Histochemical and immunohistochemical features were compatible with common malignant melanoma and its cell line. Chromosome analysis revealed many supernumerary chromosomes and marker chromosomes including double minutes (DMs). When transplanted into nude mice, YP-MEL formed tumors histologically consistent with the original tumor. Addition of sera to the medium caused cellular spreading and elongation of cytoplasmic processes with an increase of melanin contents and tyrosinase activity. Because there was no melanoma cell line derived from a NCMsis patient, YP-MEL might be a beneficial tool for study on NCMsis.

Contribution of melanogenic proteins to the heterogeneous pigmentation of human melanocytes

Human melanocytes from individuals with different skin types, as well as from the skin of the same individual, are heterogeneous in their melanin content. This heterogeneity may be attributed to differences in the activity and expression of the three melanogenic proteins: tyrosinase and tyrosinase-related proteins 1 and 2 (gp75 and DOPAchrome tautomerase, respectively), which in turn are affected by certain regulatory factors. Established melanocyte strains that exhibited intrinsic melanogenic heterogeneity could be separated into subpopulations according to density and melanin content by Percoll density gradient centrifugation. The least melanotic subpopulation consisted of melanocytes that contained an active tyrosinase enzyme and a low amount of melanin. Tyrosinase activity and the quantities of tyrosinase enzyme, tyrosinase-related protein-1 and DOPAchrome tautomerase gradually increased with increased melanin content and Percoll density of the isolated melanocyte subpopulations. We have found a direct correlation between melanin content, tyrosinase activity and the expression of the three melanogenic proteins in melanocyte strains established from different skin types. Addition of the two epidermal cytokines, tumor necrosis factor-alpha or interleukin-1 alpha, to cultures of human melanocytes from different skin types caused decreased proliferation, tyrosinase activity and expression of tyrosinase, tyrosinase-related protein-1 and DOPAchrome tautomerase. Similar results were obtained when Percoll-derived melanocyte subpopulations were treated with tumor necrosis factor-alpha and interleukin-1 alpha. These results indicate that the variation in melanin content in human melanocytes is due to differences in the activity and expression of the melanogenic proteins, which are influenced by autocrine and paracrine factors.

Repeated UVB irradiations do not have the same potential to promote stimulation of melanogenesis in cultured normal human melanocytes

The major stimulus for human melanin production is ultraviolet (UV) radiation. Little is known about the mechanisms underlying this response and the eventual enzyme regulation resulting from this activation. We treated normal human melanocytes in culture with daily UVB radiations. Cumulative increases in UVB doses resulted in proportional increases in tyrosinase activity over the first few days whereas an intermittent pattern of tyrosinase activation was observed after the fifth day of irradiation. This intermittent pattern consisted of latency periods where no melanogenic response was elicited despite exposure to UVB. Tyrosinase activity in cellular extracts increased shortly after an effective irradiation, peaked at 3 hours and thereafter decreased to below basal levels. Increased tyrosinase activity was associated with increased amounts of both the newly synthesized and mature forms of the enzyme. Decreased tyrosinase activity following an activation period was correlated with decreases in both the expression of tyrosinase mRNA and the amount of the newly synthesized form of the enzyme present in the melanocytes 24 hours after six irradiations. This particular pattern of stimulation of tyrosinase was not observed in S-91 murine melanoma cells after repeated UVB irradiations. Taken together these results may suggest a photo-protective mechanism developed by irradiated normal human melanocytes.

Tyrosinase isoenzymes in mammalian melanocytes. 2. Differential activation by alpha-melanocyte-stimulating hormone

In mouse melanoma melanocytes, alpha-melanocyte-stimulating hormone (MSH) stimulates differentiation, melanin synthesis and tyrosinase activity. However, the molecular mechanisms underlying these events have not yet been characterized. We have studied the activation of tyrosinase by MSH. Treatment of B16 melanoma cells with either theophylline, MSH, or its superpotent analog [Ahx4, DPhe7]MSH promotes a larger induction of tyrosine hydroxylase than of dopa oxidase activity in whole cell extracts. This higher activation of tyrosine hydroxylation was found not only in the melanosomal but also in the microsomal fraction; it appears to be dependent on continued transcription and translation since it can be blocked by actinomycin and cycloheximide. The tyrosinase activity of control and theophylline-treated extracts displayed several kinetic differences, including different Km values for both substrates and requirements for the cofactor L-dopa. SDS/PAGE, followed by a sensitive specific activity stain, demonstrated that melanosomes of control cells contain one lower-electrophoretic-mobility form of tyrosinase, whereas melanosomes of cells treated with either theophylline or MSH display, in addition to the lower-mobility form, a faster-migrating activity band. These tyrosinase forms are not interconvertible by proteolysis or deglycosylation. Their nature is discussed as related to the properties of the previously described low- and high-electrophoretic-mobility tyrosinases (LEMT and HEMT), as well as of the proteins encoded by the c and b loci.

Differential regulation of tyrosinase activity in skin of white and black individuals in vivo by topical retinoic acid

Tyrosinase activity is a key determinant of melanin production in skin. Because retinoic acid regulates tyrosinase activity in melanoma cells, we analyzed modulation of pigmentation in vivo by retinoic acid. Black and white subjects were either not treated, or treated topically for 4 d under occlusion with vehicle, retinoic acid (0.1%), or the irritant sodium lauryl sulfate (2%). In untreated skin, tyrosinase activity and melanin content were significantly greater (2.3 times, and 3.2 times, respectively) in blacks versus whites. Four days of treatment with topical retinoic acid did not alter tyrosinase activity or melanin content in black skin. In contrast, retinoic acid treatment significantly induced (2.7 times, n = 8) tyrosinase activity, compared to vehicle treatment, in white skin. Melanin content, however, remained unchanged at 4 d. In separate experiments, tyrosinase activity in white subjects (n = 25) was increased 16% (p = 0.01) in sodium lauryl sulfate-treated skin, and 77% (p = 0.0005) in retinoic acid-treated skin, compared to vehicle-treated skin. The effect of retinoic acid on tyrosinase activity could be differentiated from non-specific irritation, because tyrosinase activity in retinoic acid-treated skin was significantly greater (52%, p = 0.004) than sodium lauryl sulfate-treated skin. Similar results were obtained with the dihydroxyphenylalanine reaction done on vehicle, sodium lauryl sulfate-, and retinoic acid-treated white skin. Northern analysis (n = 6) and semi-quantitative polymerase chain reaction (n = 6) demonstrated that retinoic acid treatment did not alter tyrosinase mRNA levels in white skin. Western analysis revealed that induction of tyrosinase activity by retinoic acid also was not associated with increased tyrosinase protein content (n = 9), indicating that regulation of tyrosinase activity by retinoic acid occurs through a post-translational mechanism. These data demonstrate that low tyrosinase activity in white skin in vivo is retinoic acid inducible and high tyrosinase activity in black skin in vivo is neither further induced nor reduced by retinoic acid.

Preparation of purified tyrosinase devoid of dopachrome tautomerase from mammalian malignant melanocytes

Although tyrosinase has been considered for a long time the only enzyme involved in mammalian melanosynthesis, it has been shown that mouse melanoma melanosomes contain high levels of dopachrome tautomerase (DCT2), an enzyme catalyzing DC tautomerization to DHICA. At least in B16 mouse melanoma, DCT is present in higher catalytic amounts than tyrosinase. Moreover, it can be anticipated that tyrosinase and DCT should be very difficult to resolve by most conventional biochemical techniques because of the structural similarity between these enzymes, as predicted from the sequence of their corresponding cDNAs. It is shown that the presence of DCT can cause serious artifacts when tyrosinase activity is determined by most of the currently available methods, such as the Dopa oxidase and melanin formation assays. We describe a simple and convenient method for the preparation of tyrosinase devoid of DCT. The method takes advantage of the different thermal stability of both enzymes. Heating of crude melanosomal extracts at 60 degrees C for 1 hr results in a complete denaturation of DCT, while tyrosinase activity is recovered almost quantitatively. The resulting tyrosinase preparation is considerably purified and the electrophoretic, immunologic and kinetic characteristics of the enzyme appear unaltered. Because if its high yield and simplicity, the method can be used for the microscale partial purification of DCT-free tyrosinase from mammalian malignant melanocytes grown in culture.

Role of tyrosinase as the determinant of pigmentation in cultured human melanocytes

Variations in human pigmentation among different racial groups are due to differences in the production and deposition of melanin in the skin. Although melanin synthesis is known to be controlled by the rate-limiting enzyme tyrosinase, the role of this enzyme as the principal determinant of skin pigmentation is unclear. Results from studies with human melanocyte cultures derived from different racial skin types reveal an excellent correlation between the melanin content of melanocyte cultures and the in situ activity of tyrosinase. Melanocytes derived from black skin have up to 10 times more tyrosinase activity and produce up to 10 times more melanin than melanocytes derived from white skin. However, the higher level of tyrosinase activity in melanocytes derived from black skin is not due to a greater abundance of tyrosinase. Results from immunotitration experiments and Western immunoblots reveal that the number of tyrosinase molecules present in white-skin melanocytes may equal the number found in highly pigmented black skin types. Moreover, approximately equivalent levels of tyrosinase mRNA are present in white and black skin cell strains. In contrast, melanocytes derived from red-haired neonates with low tyrosinase activity contain low numbers of tyrosinase molecules and low levels of tyrosinase mRNA. These results show that tyrosinase activity and melanin production in most light-skinned people is controlled primarily by a post-translational regulation of pre-existing enzyme and not by regulating tyrosinase gene activity. In contrast, melanocytes from red-haired (type I) people have low levels of tyrosinase protein and mRNA, suggesting that transcriptional activity of the tyrosinase gene is suppressed.