Melanocyte-stimulating properties of arachidonic acid metabolites: possible role in postinflammatory pigmentation

Responsive Mn(II) complexes for potential applications in diagnostic Magnetic Resonance Imaging

The investigation of new Mn(II)-based MRI/Molecular Imaging probes responsive to the enzyme tyrosinase for potential diagnostic applications is herein described. The expression of the enzyme tyrosinase, an oxidoreductase, is up-regulated in melanoma cancer cells. Three novel ligands (L(1), L(2) and L(3)) were designed as modified acyclic polyaminocarboxylate chelates by introducing an l-tyrosine residue in place of an aminoacetate unit. The corresponding Mn(II) complexes were fully characterised by (1)H NMR relaxometric techniques in aqueous media. The responsive activity towards the expression of tyrosinase was then assessed by monitoring the (1)H 1/T(1) relaxivity changes during incubation experiments in buffered solutions containing tyrosinase at different concentrations and in B16F10 melanoma cell homogenate. New insight on the mechanism of action of these systems was gained by measuring the magnetic field dependence of the relaxivity and ESR spectra of the incubated solutions. The systems developed showed responsive activity to tyrosinase with a relaxation enhancement spanning from 50% (MnL(1)) to 350% (MnL(3)) which augurs well for the development of diagnostic probes to detect melanoma cancer.

Management of hyperpigmentation in darker racial ethnic groups

Dyschromias, in particular hyperpigmentation, are major issues of concern for people of color. Pigmentary disorders such as melasma and postinflammatory hyperpigmentation (PIH) can cause psychological and emotional distress and can pose a negative impact on a person's health-related quality of life. The precise etiology of these conditions is unknown. Therapies for melasma and PIH target various points during the cycle of melanin production and degradation. Therapies for these conditions include topical agents and resurfacing procedures. Hydroquinone remains the gold standard of topical agents. Other efficacious agents include kojic acid, azelaic acid, mequinol, and retinoids. Cosmeceutical agents include licorice, arbutin, soy, N-acetyl glucosamine, and niacinamide. Resurfacing procedures that have been used to treat melasma and PIH include chemical peels, microdermabrasion, lasers, and intense pulsed light. These procedures are best used in combination with topical bleaching agents. Given the propensity of darker skin to hyperpigment, resurfacing procedures should be used with care and caution. Maximal results are best achieved with repetitive, superficial, resurfacing modalities. In addition, ultraviolet protective measures such as broad-spectrum sunscreens are fundamental to the successful management of these conditions.

Postinflammatory Hyperpigmentation

Background:

Postinflammatory hyperpigmentation (PIH) commonly occurs in Fitzpatrick skin types III to VI and can have a considerable impact on quality of life. The majority of cases will improve spontaneously, but this can take months or even years to resolve and in some cases can be permanent. Treatment maybe prolonged, lasting 6 to 12 months or longer for adequate restoration of normal pigmentation.

Objective:

To review the etiology, pathogenesis, and current therapy options for patients with PIH.

Methods:

This review is the outcome of a workshop that discussed literature reports of research and developments in the treatment of PIH and associated disease and current clinical practice.

Results:

Combination products containing hydroquinone and retinoids appear to be the most beneficial treatment options, although there are few evidence-based studies for PIH.

Conclusion:

More randomized controlled clinical studies in large numbers of PIH patients are needed to provide standardized measurable outcomes in this indication.

Partially purified Curcuma longa inhibits alpha-melanocyte-stimulating hormone-stimulated melanogenesis through extracellular signal-regulated kinase or Akt activation-mediated signalling in B16F10 cells

Bioassay-guided fractionation of Curcuma longa by solvent partitioning and purification with octadecylsilane open column chromatography yielded a partial purification. The active 80% methanol chromatographic fraction from the ethyl acetate layer [partial purification from C. longa (PPC)] was used to investigate the alpha-melanocyte-stimulating hormone (alpha-MSH)-stimulated melanogenesis signal pathway in B16F10 cells. In cells stimulated alpha-MSH, PPC inhibited cellular melanin contents, tyrosinase activity and expression of melanogenesis-related proteins including microphthalmia-associated transcription factor (MITF), tyrosinase and tyrosinase-related proteins (TRP). Melanogenesis-regulating signalling such as mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3-kinase (PI3K)/Akt was activated by PPC in alpha-MSH-stimulated B16F10 cells. The suppressive activity of PPC on alpha-MSH-induced melanogenesis was abrogated by selective inhibition of MEK/ERK (PD98059) and PI3K (LY294002). MEK/ERK or Akt activation by PPC may contribute to reduced melanin synthesis via MITF and its downstream signal pathway including tyrosinase and TRPs in alpha-MSH-induced melanogenesis.

Vitiligo and the melanocyte reservoir

Repigmentation of vitiligo depends on available melanocytes from three possible sources: from the hair follicle unit which is the main provider of pigment cells, from the border of vitiligo lesions, and from unaffected melanocytes within depigmented areas; pigment cells at these locations originate a perifollicular, border spreading and a diffuse repigmentation pattern. In order for repigmentation to take place under stimulation with diverse therapies, melanocytes should be present in appropriate numbers. Melanocyte tissue stem cells located in the niche at the bulge region of the hair follicle are the most important sources for providing immature pigment cells that undergo terminal differentiation and originate repigmentation, but cytokines, UVR and other molecules acting in melanogenesis with adequate regulation mechanisms contribute to successful recovery in vitiligo. The presence of keratinocyte stem cells in the interfollicular epidermis raises the question on the possibility of melanocyte stem cells in a similar location and the development of future strategies for therapeutic purposes.

Effects of hydroxybenzyl alcohols on melanogenesis in melanocyte-keratinocyte co-culture and monolayer culture of melanocytes

In mammalian skin, melanocyte proliferation and melanogenesis can be stimulated by keratinocytes, fibroblasts and other regulatory factors. To determine whether hydroxybenzyl alcohols (HBAs) show more inhibitory in melanocytes cultured alone or in melanocytes co-cultured with keratinocytes, we developed a murine melanocyte-keratinocyte co-culture model to investigate the pigmentation regulators in company with other melanogenic inhibitors and stimulators. It was found that the effects of HBAs and melanogenic factors were more evident in melanocytes co-cultured with keratinocytes. Keratinocytes may play a synergistic role in melanocyte melanogenesis and influence the pigment production. The tests in the co-culture model also imply that the inhibitory effects of HBAs on melanogenesis are due to the direct inhibition of melanosomal tyrosinase activity. HBAs showed a low cytotoxicity. The eventual results proved that HBAs are promising and safe agents for skin whitening in melanocyte alone and in co-culture systems. The co-culture model provides a more physiologically realistic condition to study the interaction between melanocytes and keratinocytes, which enables a reliable screening system for depigmenting compounds.

Synthesis of tyrosinase inhibitory (4-oxo-4H-pyran-2-yl)acrylic acid ester derivatives

Melanogenesis is a physiological process that results in the production of melanin pigment. However, excessive accumulations of epidermal pigmentation can cause various hyperpigmentary disorders such as, melasma and age spots. Kojic acid and hydroxylated cinnamic acid derivatives are known to inhibit tyrosinase, a key component of melanin biosynthesis. Pyronyl-acrylic acid esters 3a-i, which share structural features of kojic acid and hydroxylated cinnamic acid, were prepared and their abilities to inhibit tyrosinase and melanin production were evaluated. Of the esters synthesized, 3e and 3h, which derived from diethylene glycol moieties were found to inhibit melanin production by ca. 20% at 20 microg/ml, whereas kojic acid at 200 microg/ml inhibited melanin production by 15.8%.

Dichloromethane fraction of Cimicifuga heracleifolia decreases the level of melanin synthesis by activating the ERK or AKT signaling pathway in B16F10 cells

Cimicifuga rhizoma has long been used in traditional Korean medicine. In particular, a Cimicifuga heracleifolia extract (CHE) was reported to inhibit the formation of glutamate and the glutamate dehydrogenase activity in cultured rat islet. Glutamate activates melanogenesis by activating tyrosinase. Accordingly, it was hypothesized that a CHE might inhibit the melanogenesis-related signal pathways including the inhibition of microphthalmia-associated transcription factor (MITF)-tyrosinase signaling and/or the activation of extracellular signal-regulated kinase (ERK)-Akt signaling. The results showed that CHE inhibits the cellular melanin contents, tyrosinase activity and expression of melanogenesis-related proteins including MITF, tyrosinase and tyrosinase-related protein (TRP)s in alpha-melanocyte-stimulating hormone-stimulated B16 cells. Moreover, CHE phosphorylates MEK, ERK1/2 and Akt, which are melanogenesis inhibitory proteins. The data suggest that CHE inhibits melanogenesis signaling by both inhibiting the tyrosinase directly and activating the MEK-ERK or Akt signal pathways-mediated suppression of MITF and its downstream signal pathway, including tyrosinase and TRPs. Therefore, C. heracleifolia would be a useful therapeutic agent for treating hyperpigmentation and an effective component in whitening and/or lightening cosmetics.

Hyperpigmentation and melasma

Facial and neck pigmentations are significant cosmetic problems. They are common in middle-aged women, related to endogenous (hormones) and exogenous factors (cosmetics, perfumes, sun exposure), and often represent paramount causes of emotional distress. Although melasma is the most common cause of facial pigmentation, there are many other forms including drug-induced and postinflammatory hyperpigmentation. We review pathogenesis, clinical and histopathological data, effect on quality of life, and treatment options in facial hyperpigmentation disorders.

Inhibitory effect of p-hydroxybenzyl alcohol on tyrosinase activity and melanogenesis

Tyrosinase is a key enzyme catalyzing the rate-limiting step for the biosynthesis pathway of melanin pigment, which is the most important determinant of the color of skin. Inhibiting tyrosinase and repressing melanocyte metabolism can reduce melanin production. Among the possible melanin reducing compounds, tyrosinase inhibitors are most promising for treating pigmentation and are used as skin-whitening agents in the cosmetic industry. In our investigation, some new tyrosinase inhibitors from plants have been identified to have high tyrosinase inhibitory activity. Specifically, p-hydroxybenzyl alcohol (4HBA) was found to inhibit the monophenolase activity of mushroom tyrosinase. When 4HBA binds with the enzyme, conformation of the enzyme is altered and its activity decreases. The inhibitory effect of 4HBA on melanogenesis has been studied using cultured mouse melanoma cells. Melanin synthesis in cell culture with 4HBA at 1.0 mM was decreased to 45% of control and below 1.0 mM there was no effect on cell growth. The inhibitory effects of 4HBA on melanogenesis are due to the direct inhibition of melanosomal tyrosinase activity, rather than to the suppression of tyrosinase gene. These results showed that 4HBA is a promising and safe agent for skin whitening.

Pigmentary Disorders in Latin America

Pigmentary disorders with hypopigmentation/depigmentation or hyperpigmentation may have special manifestations in Latin America. Most pigmentary disorders are commonly observed in all areas of Latin America, but a few are particularly seen in tropical and subtropical regions. In most pigmentary disorder ailments described, multiple factors involved in their pathogenesis are known, but etiology remains elusive. Some pigmentary disorders have peculiar clinical expressions and, in spite of being restricted to certain geographical areas, they may be observed in other world areas because of frequent traveling of affected patients. Therapy of most of these ailments is difficult or remains unknown.

Gene expression profiling analysis of solar lentigo in relation to immunohistochemical characteristics

BACKGROUND

Solar lentigo appears as dark brown spots that occur on sun-exposed areas and is considered to be a hallmark of aged skin. Although considerable knowledge about acute pigmentation has recently been accumulated, little is yet known about the mechanisms underlying chronic- and delayed-type hyperpigmentation, such as solar lentigo.

OBJECTIVES

To clarify further the mechanisms underlying the development of solar lentigo, we carried out gene expression analysis in skin biopsy specimens obtained from human solar lentigines using DNA microarray analysis.

METHODS

Two pairs of skin specimens were obtained from solar lentigo and adjacent sun-exposed normal skin, as well as normal skin on the buttocks of 16 volunteers aged 40-55 years. One set of specimens was frozen and RNA was extracted for microarray and the other set was prepared for histological sections and analysed by antibodies and probes.

RESULTS

Sixty-five genes were upregulated more than 1.8-fold in solar lentigo compared with adjacent control skin and seven melanocyte-related genes were included. Compared with sun-protected skin, many inflammation-related genes were upregulated in solar lentigo, and compared with sun-exposed control skin, upregulation of genes related to fatty-acid metabolism was apparent in solar lentigo. Moreover, we found downregulation of cornified envelope-related genes, which suggests suppression of cornification in the epidermis in solar lentigo. Immunohistochemically, larger numbers of TRP1-positive cells were found in the basal layer of solar lentigo than in normal skin. Fatty acid-related genes were highly expressed in the epidermis as detected by in situ hybridization, and they were much more prominent in the lesional skin of solar lentigo. However, cycling epidermal cells detectable with Ki67 antibody were fewer in the lesional skin of solar lentigo. Expression of filaggrin and involucrin was decreased in the lesional skin, where the number of cell layers of the stratum corneum was significantly higher than in normal skin.

CONCLUSIONS

The results of the present microarray analysis of solar lentigo, demonstrating upregulation of genes related to inflammation, fatty-acid metabolism and melanocytes and downregulation of cornified envelope-related genes, suggest that solar lentigo is induced by the mutagenic effect of repeated ultraviolet light exposures in the past, leading to the characteristic enhancement of melanin production, together with decreased proliferation and differentiation of lesional keratinocytes on the background of chronic inflammation.

PPAR-gamma agonist, ciglitazone, increases pigmentation and migration of human melanocytes

Peroxisome proliferator-activated receptors (PPARs) play an important role in cellular responses. It was reported that three subtypes of PPAR are expressed in human melanocytes. In this study, we investigated the effects of the PPAR-gamma agonist, ciglitazone, on pigmentation and migration of human melanocytes. Ciglitazone stimulated the melanin content of cells and cultured skin. This increase in pigmentation was due to the stimulation of tyrosinase activity and expression of tyrosinase and microphthalmia-associated transcription factor protein of the melanocytes. Migration was increased after ciglitazone treatment, which was observed by the Boyden chamber checkerboard analysis and a scratch motility assay. These results suggest the regulatory role of PPAR-gamma in pigmentation and migration of human melanocytes.

Synthesis of Tyrosinase Inhibitory Kojic Acid Derivative

Kojic acid derivative 2 was synthesized by joining two pyrone rings through an ethylene linkage by Horner-Emmons reaction of phosphonate 6 with aldehyde 7. The intermediates 6 and 7 were derived from kojic acid. The tyrosinase inhibitory activity of 2 was about 8 times more potent (IC(50) = 3.63 microM) than that of kojic acid (IC(50) = 30.61 microM). Compound 2 also exhibited potent melanin synthesis inhibitory activity (19.53% inhibition at 5 mug) indicating that the connection of two pyrone rings of kojic acid through a suitable linker can be an useful strategy for identification of potent tyrosinase inhibitors.

Surgical approaches for stable vitiligo

BACKGROUND

Vitiligo therapy is difficult. Depending on its clinical presentation, unilateral or bilateral vitiligo lesions respond well with different repigmentation rates, according to age, affected anatomic area, extension of lesions, time at onset, timing of depigmentation spread, and other associated factors. When stable and refractory to medical treatment, vitiligo lesions may be treated by implanting pigment cells on depigmented areas.

OBJECTIVE

To describe the main events of depigmentation and the fundamentals of surgical techniques for repigmenting vitiligo by implanting noncultured cellular or tissue grafts, in vitro cultured epidermis-bearing pigment cells, or melanocyte suspensions.

METHODS

A description of the available techniques for repigmentation of vitiligo is done, emphasizing the most important details of each procedure to obtain the best repigmentation and minimize side effects.

RESULTS

With most of these techniques, adequate repigmentation is obtained, although there are limitations when applying some methods to clinical practice.

CONCLUSIONS

Restoration of pigmentation may be accomplished with all available surgical procedures in most anatomic locations, but they are of little value for acral areas. Unilateral vitiligo responds well in a high proportion of patients, and bilateral disease may also respond when stable. Appropriate patient selection is important to achieve the best results.

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

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

Inhibitory effects of alpha-arbutin on melanin synthesis in cultured human melanoma cells and a three-dimensional human skin model

We studied the inhibitory effects of 4-hydroxyphenyl alpha-glucopyranoside (alpha-arbutin) on melanogenesis in cultured human melanoma cells, HMV-II, and in a three-dimensional cultured human skin model. alpha-Arbutin showed no inhibitory effect on HMV-II cell growth at a concentration below 1.0 mM. Melanin synthesis in cells treated with alpha-arbutin at 0.5 mM decreased to 76% of that in non-treated cells. The cellular tyrosinase activity of HMV-II cells also significantly decreased, while the expression of its mRNA was not affected. Melanin synthesis in a human skin model was also evaluated by the macro- and microscopic observation of its pigmentation as well as by quantitative measurements of melanin. Treatment of the human skin model with 250 microg of alpha-arbutin did not inhibit cell viability, while melanin synthesis was reduced to 40% of that in the control. These results indicate that alpha-arbutin is an effective and safe ingredient for skin-lightening.

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.

Novel Approach for Evaluation of Estrogenic and Anti-Estrogenic Activities of Genistein and Daidzein using B16 Melanoma Cells and Dendricity Assay

The effects of soy isoflavones, genistein and daidzein, which exhibit estrogenic, anti-estrogenic and/or tyrosine kinase inhibitory activity, on the dendritic morphology of B16 mouse melanoma cells were quantitatively evaluated and compared with those of 17 beta-estradiol (Est) and tyrphostin, a tyrosine kinase inhibitor. Dendricity was significantly stimulated in the order of Est >> genistein > daidzein = tyrphostin, but not by glycosides of genistein and daidzein. In competition experiments, Est counteracted the stimulatory activity of genistein and daidzein, but enhanced the activity of tyrphostin additively, suggesting that genistein and daidzein agonized Est. In addition, when the concentration ratios of genistein/Est and daidzein/Est were higher than 5000 and 50,000, respectively, genistein and daidzein agonized Est. In contrast, when the ratio of daidzein/Est was lower than 500, daidzein antagonized Est. Furthermore, genistein and daidzein competed with each other in stimulatory activity. These observations suggest that: 1) dendricity is stimulated by agonists (genistein and daidzein) of Est and tyrosine kinase inhibitors (genistein and tyrphostin), 2) the concentration ratio of isoflavone aglycone/Est is very important as one regulatory factor for estrogenic and/or anti-estrogenic activity, and 3) daidzein antagonizes not only Est but also genistein. It is concluded that a quantitative and simple dendricity assay using B16 mouse melanoma cells is available to evaluate estrogenic and anti-estrogenic activity in vitro.

Prostaglandin production by melanocytic cells and the effect of α-melanocyte stimulating hormone

Prostaglandins are potent mediators of the inflammatory response and are also involved in cancer development. In this study, we show that human melanocytes and FM55 melanoma cells express cyclooxygenase-1 and -2 (COX-1 and -2) and thus have the capability to produce prostaglandins. The FM55 cells produced predominantly PGE2 and PGF2alpha, whereas the HaCaT keratinocyte cell line produced mainly PGE2. The anti-inflammatory peptide, alpha-melanocyte stimulating hormone (alpha-MSH), reduced prostaglandin production in FM55 and HaCaT cells and reversed the effect of the pro-inflammatory cytokine TNF-alpha in the former. These results indicate that melanocytes produce prostaglandins and that alpha-MSH, by inhibiting this response, may play an important role in regulating inflammatory responses in the skin.

Proteinase-activated receptor-2 stimulates prostaglandin production in keratinocytes: analysis of prostaglandin receptors on human melanocytes and effects of PGE2 and PGF2alpha on melanocyte dendricity

Prostaglandins (PG) are key mediators of diverse functions in the skin and several reports suggest that PG mediate post-inflammatory pigmentary changes through modulation of melanocyte dendricity and melanin synthesis. The proteinase-activated receptor 2 (PAR-2) is important for skin pigmentation because activation of keratinocyte PAR-2 stimulates uptake of melanosomes through phagocytosis in a Rho-dependent manner. In this report, we show that activation of keratinocyte PAR-2 stimulates release of PGE(2) and PGF(2alpha) and that PGE(2) and PGF(2alpha) act as paracrine factors that stimulate melanocyte dendricity. We characterized the expression of the EP and FP receptors in human melanocytes and show that human melanocytes express EP1 and EP3, and the FP receptor, but not EP2 and EP4. Treatment of melanocytes with EP1 and EP3 receptor agonists resulted in increased melanocyte dendricity, indicating that both EP1 and EP3 receptor signaling contribute to PGE(2)-mediated melanocyte dendricity. Certain EP3 receptor subtypes have been shown to increase adenosine 3',5'-cyclic monophosphate (cAMP) through coupling to Gs, whereas EP1 is known to couple to Gq to activate phospholipase C with elevation in Ca(2+). The cAMP/protein kinase A system is known to modulate melanocyte dendrite formation through modulation of Rac and Rho activity. Neither PGF(2alpha) or PGE(2) elevated cAMP in human melanocytes showing that dendricity observed in response to PGE(2) and PGF(2alpha) is cAMP-independent. Our data suggest that PAR-2 mediates cutaneous pigmentation both through increased uptake of melanosomes by keratinocytes, as well as by release of PGE(2) and PGF(2alpha) that stimulate melanocyte dendricity through EP1, EP3, and FP receptors.

Expression and function of peroxisome proliferator-activated receptors in human melanocytes

BACKGROUND

Peroxisome proliferator-activated receptors (PPARs) belong to the superfamily of nuclear receptors that heterodimerize with the retinoic X receptor. Agonists of PPAR have been known to play an important role in cellular responses including proliferation and differentiation. The expression and function of PPARs have not been investigated in human melanocytes, although they have been widely demonstrated in keratinocytes of the skin.

OBJECTIVES

To investigate the expression of PPARs in human melanocytes and the effects of PPAR activators on melanocyte growth and melanogenesis.

METHODS

We used immunocytochemistry and Western blot analysis to determine whether PPARs are expressed in melanocytes. To investigate further expression of PPAR subtypes, reverse transcriptase-polymerase chain reaction analysis was performed using PPAR subtype-specific oligonucleotides. The cell proliferation was measured using the Coulter counter. The effects on pigmentation were investigated with measurement of melanin contents, tyrosinase activity and its expression.

RESULTS

The mRNA of all three PPAR subtypes, PPAR-alpha, PPAR-beta/delta and PPAR-gamma, were expressed in melanocytes. Activators for PPAR-alpha (WY-14643) and PPAR-gamma (ciglitazone) inhibited proliferation of melanocytes in a dose-dependent manner, whereas bezafibrate, a preferential activator for PPAR-beta/delta, had no effect. This growth inhibition was accompanied by the morphological change of the melanocytes to an activated form with an increased number of dendrites and enlarged cell area compared with the control. The WY-14643 and ciglitazone also appeared to stimulate the melanin synthesis of melanocytes. This increase in pigmentation was due to stimulation of the tyrosinase activity without an increase in the expression of tyrosinase.

CONCLUSIONS

PPARs are expressed in human melanocytes and PPAR-alpha and PPAR-gamma activators inhibit melanocyte growth and stimulate melanogenesis.

Pigmentary disorders in ethnic skin

Pigmentary disorders are commonly seen in ethnic skin. They are psychologically problematic in darker skin. Treatment of many of these disorders remains difficult.

Stress augmented ultraviolet-irradiation-induced pigmentation

It was reported that adrenocorticotropic hormone stimulates melanogenesis in cultured melanocytes. Stress (high population density and restraint stress) induced a significant increase in adrenocorticotropic hormone levels in plasma and skin compared to control. The serum obtained from HR-1 x HR/De F1 female mice subjected to stress showed significantly increased tyrosinase activity in human melanocytes compared to that from nonstressed mice. The increase in tyrosinase activity was inhibited in the presence of 10 nM corticostatin, an adrenocorticotropic hormone inhibitor. The aim of this study was to examine whether adrenocorticotropic hormone released into the circulation under stressful conditions is associated with the regulation of ultraviolet-induced pigmentation. Mice divided into three groups were housed for 22 d under the following conditions: five mice per cage (control); 10 mice per cage (high population density); restraint stress 4 h per d. The animals were exposed to ultraviolet-B irradiation (72 mJ per cm2, thrice per wk). After ultraviolet-B irradiation, delayed tanning was marked in stressed mice. The number of dihydroxyphenylalanine-positive melanocytes also significantly increased in stressed animals. Pretreatment with 100 microg of corticostatin inhibited the augmentation of the stress-induced pigmentary response and the increase in dihydroxyphenylalanine-positive melanocytes after ultraviolet irradiation. Adrenocorticotropic hormone released by stress may activate tyrosinase in melanocytes, resulting in the augmentation of ultraviolet-induced pigmentation. These results suggest that adrenocorticotropic hormone is at least partly responsible for the sensitivity of the pigmentary response after ultraviolet irradiation under stressful conditions.

Delayed ERK activation by ceramide reduces melanin synthesis in human melanocytes

Sphingolipid metabolites regulate many aspects of cell growth and differentiation. However, the effects of sphingolipids on the growth and melanogenesis of human melanocytes are not known. In the present study, we investigated the effects of sphingolipid metabolites and the possible signalling pathways involved in human melanocytes. Our data show that C(2)-ceramide inhibits cell growth in a dose-dependent manner, whereas sphingosine-1-phosphate (SPP) has no effect. Moreover, we observed that the melanin content of the cells was significantly decreased by C(2)-ceramide. The pigmentation-inhibiting effect of C(2)-ceramide at 1-10 microM was stronger than that of kojic acid, tested at 1-100 microM. The tyrosinase activity of cell extracts was reduced by C(2)-ceramide treatment. However, in the cell-free system, C(2)-ceramide could not suppress tyrosinase, whereas kojic acid directly inhibited tyrosinase. These results suggest that C(2)-ceramide decreases the pigmentation of melanocytes indirectly regulating tyrosinase. Furthermore, we found that C(2)-ceramide decreased the protein expression of microphthalmia-associated transcription factor (MITF), which is required for tyrosinase expression. To identify the signalling pathway of ceramide, we studied the ability of C(2)-ceramide to influence extracellular signal-regulated protein kinase (ERK) and Akt/protein kinase B (PKB) activation. C(2)-ceramide induced a delayed activation of ERK ( > 1 h) and a much later activation of Akt/PKB ( > 3 h) in human melanocytes. In addition, the specific inhibition of the ERK and the Akt signalling pathways by PD98059 and LY294002, respectively, increased melanin synthesis. Thus, it seems that sustained ERK and Akt activation may lead to the suppression of cell growth and melanogenesis.

Mechanism and clinical significance of prostaglandin-induced iris pigmentation

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

Putative side effects of prostaglandin analogs

Anecdotal case reports describe the occurrence of cystoid macular edema, iritis, herpes simplex keratitis, periocular skin darkening, and headaches in patients treated with prostaglandin analogs for glaucoma. The purpose of this article is to critically analyze these anecdotal case reports in light of a few well-controlled, randomized clinical studies to determine whether conclusions can be made about a causal relationship between the use of prostaglandin analogs and the occurrence of these side effects. None of these putative side effects has been proven to be causally related to latanoprost therapy using valid scientific methodology. These possible side effects occur rarely. Cystoid macular edema, iritis, and herpes simplex keratitis occur in eyes with risk factors. To scientifically establish a causal relationship between drug therapy and rare side effects, repeated rechallenging with masked controls is required. With rare exception, such methodology has not been used with any of these putative side effects. Nevertheless, even without firm establishment of a causal relationship, caution is advised with the use of prostaglandin analogs in eyes with risk factors for cystoid macular edema, iritis, and herpes simplex keratitis until properly designed, large, controlled studies provide more definitive information.

Histamine is involved in ultraviolet B-induced pigmentation of guinea pig skin

We previously reported that histamine induced melanogenesis in cultured human melanocytes and that the stimulatory effect was mediated by protein kinase A activation via H2 receptors. It is well-known that ultraviolet B irradiation causes acute inflammation, known as erythema, and subsequent pigmentation, and there are several reports demonstrating an elevation of the histamine levels in ultraviolet B-irradiated skin. Thus, to evaluate the involvement of histamine in ultraviolet B-induced skin pigmentation, we examined the effect of an H2 antagonist in brownish guinea pig skin. Daily exposure to 200 mJ per cm2 ultraviolet B for 3 d evoked erythema and subsequent pigmentation in the skin samples tested. Moreover, a remarkable increase in dopa-positive melanocytes was observed in the pigmented area, which showed an increase in melanin synthesis. Topical application of famotidine, an H2 antagonist, significantly reduced pigmentation and moderated the increase of dopa-positive melanocytes in the ultraviolet B-irradiated skin. Even when the initiation of famotidine application was delayed to day 2 after irradiation, an inhibitory activity on ultraviolet B-induced pigmentation was observed; however, the ultraviolet B-induced erythema was not suppressed by topically applied famotidine. Thus, we concluded that histamine is involved in ultraviolet B-induced pigmentation and that famotidine suppressed the pigmentation by the prevention of histamine binding to H2 receptors in melanocytes but not by prevention of ultraviolet B permeability and inflammation.

Hair darkening in porphyria cutanea tarda

Repigmentation of grey hair is rare, but has been described in several clinical settings. It has most often been reported as a postinflammatory effect, but several drugs, chronic arsenic exposure and coeliac disease have also been cited in addition to darkening as a spontaneous phenomenon. We report two patients with sustained repigmentation of the hair in association with porphyria cutanea tarda. The mechanism for this repigmentation remains elusive, but presumably involves recruitment of outer root sheath melanocytes, which are then activated to form functional hair bulb melanocytes.

Effects of isopropyl unoprostone and latanoprost on melanogenesis in mouse epidermal melanocytes

PURPOSE

The authors sought to investigate the cellular mechanism of melanogenesis by prostaglandin antiglaucomatous compounds, isopropyl unoprostone (referred to as unoprostone) and latanoprost, and to quantitatively compare their effect on melanogenesis using cultured mouse epidermal melanocytes.

METHODS

M1, M2, and the acid of latanoprost, all of which are possible intraocular metabolites of unoprostone or latanoprost, were used. Tested prostaglandin-related compounds (final concentration range, 1 micromol/L-10 nmol/L) were administrated to the culture medium of purely cultured mouse melanoblasts, melan-A, once daily for 2 weeks. One micromole per liter prostaglandin F(2 alpha) solution was administered in parallel. Radioisotope assays were used to measure the total melanin synthesis and the activity of tyrosinase in converting tyrosine to L-3,4-dihydroxyphenylalanine, which is a rate-limiting reaction in melanogenesis. The effects of prostaglandin F(2 alpha), M1, or M2 on proliferation of melan-A were examined.

RESULTS

M1, M2, and acid of latanoprost but not prostaglandin F(2 alpha), significantly enhanced tyrosinase activity. M2 and acid of latanoprost more greatly enhanced tyrosinase activity than did M1. None of the tested compounds significantly altered the proliferation and total melanin synthesis of melan-A.

CONCLUSIONS

Both unoprostone and latanoprost enhanced tyrosinase activity. These prostaglandin-related compounds may influence the nature of melanin and result in pigmentation.

The ACVD task force on canine atopic dermatitis (VII): mediators of cutaneous inflammation

Controversy still exists on the role of various inflammatory mediators in the pathogenesis of canine atopic dermatitis. The objective of this article is to review the most recent information available on the inflammatory mediators that have been implicated in the pathogenesis of this disease. Studies on the role of histamine, serotonin, leukotrienes and various cytokines are presented in a comparative manner reviewing the experimental evidence for a role in the pathogenesis and the arguments against it.

Update on the role of leukotrienes in the pathogenesis of atopy: a comparative review

The pathogenesis of atopic disease (AD) is controversial in humans and dogs. In humans, leukotrienes (LT) are thought to play an important role in this disease and LT inhibitors are commonly used as treatment for AD. Leukotrienes are a heterogeneous group of biologically active compounds that mediate many aspects of inflammatory and allergic reactions. This paper will review the role of LT in atopic disease in a comparative manner. Leukotriene inhibitors and their therapeutic use in the management of atopic disease in humans and dogs are discussed.

Histamine induces melanogenesis and morphologic changes by protein kinase A activation via H2 receptors in human normal melanocytes

Hyperpigmentation frequently accompanies chronic or acute inflammation. A number of inflammatory mediators have been shown to stimulate melanin synthesis in human melanocytes. Although histamine is ubiquitous as an inflammatory factor, its involvement in pigmentation remains obscure. In this work, we examined the effects of histamine on cultured human melanocytes. Treatment of human melanocytes with 0.1-10 microM histamine evoked morphologic changes and increases in tyrosinase activity. The concomitant increases in melanin content of the histamine-treated melanocytes indicated an elevation of melanin synthesis by tyrosinase activation. These stimulatory effects of histamine were completely inhibited by an H2 antagonist, famotidine, whereas H1 and H3 antagonists had no inhibitory effect whatsoever. In addition, an H2 agonist, dimaprit, induced the same degree of melanogenesis as histamine at concentrations of 0.1-10 microM. We observed an increase in the intracellular cAMP contents of human melanocytes induced by histamine via the H2 receptors. We know that this cAMP accumulation and subsequent protein kinase A activation plays a critical role in histamine-induced melanogenesis, because a specific protein kinase A inhibitor, H-89, completely suppressed these stimulatory effects of histamine, and because dibutylic cAMP, a specific protein kinase A activator, stimulated human melanocytes as potently as histamine. Taken together, we show here that histamine induces melanogenesis of human cultured melanocytes by protein kinase A activation via H2 receptors.

Effects of lithium on pigmentation in the embryonic zebrafish (Brachydanio rerio)

Pigment cell precursors of the embryonic zebrafish give rise to melanophores, xanthophores and/or iridophores. Cell signaling mechanisms related to the development of pigmentation remain obscure. In order to examine the mechanisms involved in pigment cell signaling, we treated zebrafish embryos with various activators and inhibitors of signaling pathways. Among those chemicals tested, LiCl and LiCl/forskolin had a stimulatory effect on pigmentation, most notable in the melanophore population. We propose that the inositol phosphate (IP) pathway, is involved in pigment pattern formation in zebrafish through its involvement in the: (1) differentiation/proliferation of melanophores; (2) dispersion of melanosomes; and/or (3) synthesis/deposition of melanin. To discern at what level pigmentation was being effected we: (1) counted the number of melanophores in control and experimental animals 5 days after treatment; (2) measured tyrosinase activity and melanin content; and (3) employed immunoblotting techniques with anti-tyrosine-related protein-2 and anti-melanocyte-specific gene-1 as melanophore-specific markers. Although gross pigmentation increased dramatically in LiCl- and LiCl/forskolin treated embryos, the effect on pigmentation was not due to an increase in the proliferation of melanophores, but was possibly through an increase in melanin synthesis and/or deposition. Collectively, results from these studies suggest the involvement of an IP-signaling pathway in the stimulation of pigmentation in embryonic zebrafish through the synthesis/deposition of melanin within the neural crest-derived melanophores.

Topical trans-4-aminomethylcyclohexanecarboxylic acid prevents ultraviolet radiation-induced pigmentation

We have studied the effect of a plasmin inhibitor, trans-4-aminomethylcyclohexanecarboxylic acid (trans-AMCHA), on skin pigmentation induced by ultraviolet (UV) exposure in Weiser-Maples guinea pigs. When guinea pigs are exposed to UV radiation (840 mJ cm-2), skin pigmentation is clearly observed from seven days after exposure and continued to increase to 29 days. Post-exposure applications of 2 and 3% solutions of trans-AMCHA to the exposed regions prevent or inhibit the pigmentation process. When the skin is removed and stained by the Fontana-Masson method, melanin content in the basal layer of UV-exposed epidermis is significantly reduced in the regions to which 2 and 3% trans-AMCHA solutions have been applied, compared with the vehicle control. As plasmin is known to contribute to the release of arachidonic acid (AA) and the production of prostaglandins (PGs), we have examined the effects of trans-AMCHA on AA-induced pigmentation in guinea pig skin. Topical application of trans-AMCHA causes a dose-dependent decrease in AA-induced pigmentation. These results suggest that trans-AMCHA reduces melanocyte tyrosinase activity by suppressing the production of PGs, UV-induced melanogens, through the suppression of the UV-induced increase in epidermal plasmin activity.

Arbutin increases the pigmentation of cultured human melanocytes through mechanisms other than the induction of tyrosinase activity

We assessed the effects of arbutin on the pigmentation of cultured normal human melanocytes. As indicated by a cell-blotting assay, arbutin at concentrations in the range of 0.5-8 mM increased the pigmentation of the cultured melanocytes, while kojic acid at concentrations in the range of 0.5-4 mM decreased the pigmentation. The pigmentation-augmenting effect of arbutin was further confirmed by the results of a cell-pelleting assay, the traditional method of assessment. Treatment of the cells with arbutin increased the melanin content of the cells and the protein content as well. On the other hand, the tyrosinase activity in the cells was reduced by arbutin treatment. The levels of transcription of tyrosinase and tyrosinase related protein-1 genes were not affected by arbutin treatment as indicated by a semi-quantitative reverse transcription-polymerase chain reaction assay. These results demonstrate that arbutin promotes an increase in pigmentation of cultured human melanocytes that is not mediated by augmented tyrosinase activity.