Radical production during tyrosinase reaction, dopa-melanin formation, and photoirradiation of dopa-melanin

5G Electromagnetic Radiation Attenuates Skin Melanogenesis In Vitro by Suppressing ROS Generation

Recently, the impacts of 5G electromagnetic radiation (EMR) with 28 GHz on human health have been attracting public attention with the advent of 5G wireless communication. Here, we report that 5G (28 GHz) EMR can attenuate the skin pigmentation in murine melanoma cells (B16F10) and a 3D pigmented human epidermis model (Melanoderm™). B16 cells were exposed to 5G (28 GHz) with or without α-MSH for 4 h per day. Interestingly, 5G attenuated α-MSH-induced melanin synthesis. Fontana-Masson staining confirmed that the dendritic formation of α-MSH stimulated B16 cells was diminished by 5G exposure. To confirm the anti-melanogenic effect of 5G EMR, MelanoDerm™ was irradiated with 5G at a power intensity of 10 W/m² for 4 h a day for 16 days and melanin distribution was detected with Fontana-Masson staining, which supported the anti-melanogenic effect of 5G EMR. Consistently, 5G EMR suppressed α-MSH induced upregulation of melanogenic enzymes; tyrosinase, TRP-1, and TRP-2. Of note, 5G EMR attenuated ROS production stimulated by α-MSH and H₂O₂, suggesting that 5G EMR may dissipate ROS generation, which is pivotal for the melanin synthesis. Collectively, we demonstrated that 5G EMR can attenuate skin pigmentation by attenuating ROS generation.

Role of Melanin Chemiexcitation in Melanoma Progression and Drug Resistance

Melanoma is the deadliest type of skin cancer. Human melanomas often show hyperactivity of nitric oxide synthase (NOS) and NADPH oxidase (NOX), which, respectively, generate nitric oxide (NO ^· ) and superoxide (O₂ ^·- ). The NO ^· and O₂ ^- react instantly with each other to generate peroxynitrite (ONOO^-) which is the driver of melanin chemiexcitation. Melanoma precursors, the melanocytes, are specialized skin cells that synthesize melanin, a potent shield against sunlight's ultraviolet (UV) radiation. However, melanin chemiexcitation paradoxically demonstrates the melanomagenic properties of melanin. In a loop, the NOS activity regulates melanin synthesis, and melanin is utilized by the chemiexcitation pathway to generate carcinogenic melanin-carbonyls in an excited triplet state. These carbonyl compounds induce UV-specific DNA damage without UV. Additionally, the carbonyl compounds are highly reactive and can make melanomagenic adducts with proteins, DNA and other biomolecules. Here we review the role of the melanin chemiexcitation pathway in melanoma initiation, progression, and drug resistance. We conclude by hypothesizing a non-classical, positive loop in melanoma where melanin chemiexcitation generates carcinogenic reactive carbonyl species (RCS) and DNA damage in normal melanocytes. In parallel, NOS and NOX regulate melanin synthesis generating raw material for chemiexcitation, and the resulting RCS and reactive nitrogen species (RNS) regulate cellular proteome and transcriptome in favor of melanoma progression, metastasis, and resistance against targeted therapies.

GSK-3β-Targeting Fisetin Promotes Melanogenesis in B16F10 Melanoma Cells and Zebrafish Larvae through β-Catenin Activation

Fisetin is found in many fruits and plants such as grapes and onions, and exerts anti-inflammatory, anti-proliferative, and anticancer activity. However, whether fisetin regulates melanogenesis has been rarely studied. Therefore, we evaluated the effects of fisetin on melanogenesis in B16F10 melanoma cell and zebrafish larvae. The current study revealed that fisetin slightly suppressed in vitro mushroom tyrosinase activity; however, molecular docking data showed that fisetin did not directly bind to mushroom tyrosinase. Unexpectedly, fisetin significantly increased intracellular and extracellular melanin production in B16F10 melanoma cells regardless of the presence or absence of α-melanocyte stimulating hormone (α-MSH). We also found that the expression of melanogenesis-related genes such as tyrosinase and microphthalmia-associated transcription factor (MITF), were highly increased 48 h after fisetin treatment. Pigmentation of zebrafish larvae by fisetin treatment also increased at the concentrations up to 200 µM and then slightly decreased at 400 µM, with no alteration in the heart rates. Molecular docking data also revealed that fisetin binds to glycogen synthase kinase-3β (GSK-3β). Therefore, we evaluated whether fisetin negatively regulated GSK-3β, which subsequently activates β-catenin, resulting in melanogenesis. As expected, fisetin increased the expression of β-catenin, which was subsequently translocated into the nucleus. In the functional assay, FH535, a Wnt/β-catenin inhibitor, significantly inhibited fisetin-mediated melanogenesis in zebrafish larvae. Our data suggested that fisetin inhibits GSK-3β, which activates β-catenin, resulting in melanogenesis through the revitalization of MITF and tyrosinase.

Functional MoS2 nanosheets inhibit melanogenesis to enhance UVB/X-ray induced damage

We produced highly dispersed MoS₂ nanosheets in water with the assistance of tryptophan (Trp) to inhibit melanogenesis by suppressing ROS production.

Aqueous Extract of Chrysanthemum morifolium ( Jú Huā) Enhances the Antimelanogenic and Antioxidative Activities of the Mixture of Soy Peptide and Collagen Peptide

The possible synergistic effect between the aqueous extract of Chrysanthemum morifolium ( Jú Huā) (AECM) and the peptide mixture (PM) containing soy peptide and collagen peptide was investigated in an ultraviolet (UV) irradiation-induced skin damage mouse model. The irradiated mice were treated with the PM or PM + AECM (containing PM and AECM), respectively. Both PM and PM + AECM groups displayed an apparent photoprotective effect on the UV-irradiated skin damage of mice. Histological evaluation demonstrated that the epidermal hyperplasia and melanocytes in the basal epidermal layer of the UV-irradiated skin in mice decreased when treated with either PM or PM + AECM. Further study showed that soy peptide, collagen peptide, and AECM also inhibited the activities of mushroom tyrosinase with IC50 values of 82.3, 28.2, and 1.6 μg/ml, respectively. Additionally, PM + AECM reduced melanogenesis by 46.2% at the concentration of 10 mg/ml in B16 mouse melanoma cells. Meanwhile, the UV-induced increase of antioxidative indicators, including glutathione peroxidase (GSH-Px), superoxide dismutase (SOD), and malondialdehyde (MDA), was reduced significantly after treatment with 1.83 g/kg/dbw of PM + AECM. This evidence supported the synergistic antioxidative effect of AECM with PM. These results demonstrated that oral intake of PM and AECM had synergistic antimelanogenic and antioxidative effects in UV-irradiated mice.

Melanocytes as instigators and victims of oxidative stress

Epidermal melanocytes are particularly vulnerable to oxidative stress owing to the pro-oxidant state generated during melanin synthesis, and to the intrinsic antioxidant defenses that are compromised in pathologic conditions. Melanoma is thought to be oxidative stress driven, and melanocyte death in vitiligo is thought to be instigated by a highly pro-oxidant state in the epidermis. We review the current knowledge about melanin and the redox state of melanocytes, how paracrine factors help counteract oxidative stress, the role of oxidative stress in melanoma initiation and progression and in melanocyte death in vitiligo, and how this knowledge can be harnessed for melanoma and vitiligo treatment.

Generation mechanism of radical species by tyrosine-tyrosinase reaction

Alleviated melanin formation in the skin through inhibition of tyrosine-tyrosinase reaction is one of the major targets of cosmetics for whitening ability. Since melanin has a pivotal role for photoprotection, there are pros and cons of inhibition of melanin formation. This study applying electron spin resonance (ESR)-spin trapping method revealed that ^•H and ^•OH are generated through tyrosine-tyrosinase reaction. When deuterium water was used instead of H₂O, the signal of 5,5-dimethyl-1-pyrroline N-oxide (DMPO)-H (a spin adduct of DMPO and ^•H) greatly decreased, whilst DMPO-OH (a spin adduct of DMPO and ^•OH) did not. Thus, it is suggested that ^•H was derived from H₂O, and ^•OH through oxidative catalytic process of tyrosine to dopaquinone. Our study suggests that tyrosinase inhibitors might contribute to alleviate the oxidative damage of the skin by inhibiting ^•OH generation via the enzyme reaction.

An evaluation of extracts of five traditional medicinal plants from Iran on the inhibition of mushroom tyrosinase activity and scavenging of free radicals

This study aimed to evaluate the free radical scavenging and inhibition properties of five medicinal plants, including Quercus infectoria Olive., Terminalia chebula Retz., Lavendula stoechas L., Mentha longifolia L., Rheum palmatum L., toward the activity of mushroom tyrosinase using L-tyrosine and L-3,4-dihydroxyphenylalanine (L-DOPA) as the substrate.The methanol extracts of Q. infectoria and T. chebula showed strong radical scavenging effect in 2,2'-dipheny L-1-picrylhydrazyl (DPPH) assay(IC50 = 15.3 and 82.2 microg mL)1 respectively).These plants also showed inhibitory effects against the activity of mushroom tyrosinase in hydroxylation of L-tyrosine (85.9% and 82.2% inhibition,respectively). These two plants also inhibited the oxidation of l-DOPA similar to kojic acid as positive control (IC50 = 102.8 and 192.6 microg mL)1 respectively). In general Q. infectoria and T. chebula significantly inhibited tyrosinase activity and DPPH radical. Both activities were concentration dependant but not in linear manner. It is needed to study the cytotoxicity of these plant extracts in pigment cell culture before further evaluation and moving to in vivo conditions.

Regulation of tyrosinase by tetrahydropteridines and H2O2

Recently two alternative mechanisms have been put forward for the inhibition of tyrosinase by 6R-l-erythro 5,6,7,8-tetrahydrobiopterin (6BH(4)). Initially allosteric uncompetitive inhibition was demonstrated due to 1:1 binding of 10(-6)M 6BH(4) to a specific domain 28 amino acids away from the Cu(A) active site of the enzyme. Alternatively it was then shown that 10(-3)M 6BH(4) inhibit the reaction by the reduction of the product dopaquinone back to l-dopa. In the study presented herein we have used two structural analogues of 6BH(4) (i.e., 6,7-(R,S)-dimethyl tetrahydrobiopterin and 6-(R,S)-tetrahydromonapterin) confirming classical uncompetitive inhibition due to specific binding of the pyrimidine ring of the pterin moiety to the regulatory domain on tyrosinase. Under these conditions there was no reduction of l-dopaquinone back to l-dopa by both cofactor analogues. Inhibition of tyrosinase by 6BH(4) occurs in the concentration range of 10(-6)M after preactivation with l-tyrosine and this mechanism uncouples the enzyme reaction producing H(2)O(2) from O(2). Moreover, a direct oxidation of 6BH(4) to 7,8-dihydrobiopterin by tyrosinase in the absence of the substrate l-tyrosine was demonstrated. The enzyme was activated by low concentrations of H(2)O(2) (<0.3 x 10(-3)M), but deactivated at concentrations in the range 0.5-5.0 x 10(-3)M. In summary, our results confirm a major role for 6BH(4) in the regulation of human pigmentation.

Superoxide dismutase activity as a function of culture aging of B-16 mouse melanoma cells

The C3 clone of B-16 mouse melanoma was cultured for 1, 6 and 9 days and analyzed. The changes which are not directly linked to melanogenesis in the B-16 / C3 cultures during their maturation were characterized. Early (1 day) confluent (6 days) and old (9 days) cell cultures are distinguished by their leucine aminopeptidase (LAP) and ?-naphthyl acetate esterase (ANAE) isoenzyme patterns. Both quantitative and qualitative changes in LAP and ANAE isoenzyme can be observed during culture maturation. There is an increase in the activity of the enzyme copper, zinc-containing superoxide-dismutase (CuZn SOD). The increaase in the CuZn SOD enzyme activity might be related to B-16/C3 cell melanogenesis and / or to differentiation.

Lightening Effect on Ultraviolet-Induced Pigmentation of Guinea Pig Skin by Oral Administration of a Proanthocyanidin-Rich Extract from Grape Seeds

Antioxidants such as vitamins C and E have been reported to inhibit the progression of ultraviolet (UV) radiation-induced pigmentation in the skin of hairless mice. However, little is known of the lightening effect of proanthocyanidin, a powerful polyphenolic antioxidant, on UV-induced pigmentation of the skin. We investigated the lightening effect of oral administration of a proanthocyanidin-rich grape seed extract (GSE) using guinea pigs with UV-induced pigmentation. These pigmented guinea pigs were fed diets containing 1% GSE or 1% vitamin C (w/w) for 8 weeks. GSE-feeding had an apparent lightening effect on the guinea pigs' pigmented skin. Histologic evaluation demonstrated a decrease in the number of 3,4-dihydroxyphenylalanine (DOPA)-positive melanocytes as well as 8-hydroxy-2'-deoxyguanosine (8-OHdG)-positive, Ki-67-positive, proliferating cell nuclear antigen (PCNA)-positive melanin-containing cells in the basal epidermal layer of the UV-irradiated skin in GSE-fed guinea pigs. In contrast, these parameters did not change in the skin of vitamin C-fed or control guinea pigs. GSE inhibited the activity of mushroom tyrosinase and also inhibited melanogenesis without inhibiting the growth of cultured B16 mouse melanoma cells. In conclusion, we demonstrated that oral administration of GSE is effective in lightening the UV-induced pigmentation of guinea pig skin. This effect may be related to the inhibition of melanin synthesis by tyrosinase in melanocytes and the reactive oxygen species (ROS)-related proliferation of melanocytes.

New aspects on the melanocortins and their receptors

Knowledge of melanocortins and their receptors has increased tremendously over the last few years. The cloning of five melanocortin receptors, and the discovery of two endogenous antagonists for these receptors, agouti and agouti-related peptide, have sparked intense interest in the field. Here we give a comprehensive review of the pharmacology, physiology and molecular biology of the melanocortins and their receptors. In particular, we review the roles of the melanocortins in the immune system, behaviour, feeding, the cardiovascular system and melanoma. Moreover, evidence is discussed suggesting that while many of the actions of the melanocortins are mediated via melanocortin receptors, some appear to be mediated via mechanisms distinct from melanocortin receptors.

Dopamine oxidation alters mitochondrial respiration and induces permeability transition in brain mitochondria: implications for Parkinson's disease

Both reactive dopamine metabolites and mitochondrial dysfunction have been implicated in the neurodegeneration of Parkinson's disease. Dopamine metabolites, dopamine quinone and reactive oxygen species, can directly alter protein function by oxidative modifications, and several mitochondrial proteins may be targets of this oxidative damage. In this study, we examined, using isolated brain mitochondria, whether dopamine oxidation products alter mitochondrial function. We found that exposure to dopamine quinone caused a large increase in mitochondrial resting state 4 respiration. This effect was prevented by GSH but not superoxide dismutase and catalase. In contrast, exposure to dopamine and monoamine oxidase-generated hydrogen peroxide resulted in a decrease in active state 3 respiration. This inhibition was prevented by both pargyline and catalase. We also examined the effects of dopamine oxidation products on the opening of the mitochondrial permeability transition pore, which has been implicated in neuronal cell death. Dopamine oxidation to dopamine quinone caused a significant increase in swelling of brain and liver mitochondria. This was inhibited by both the pore inhibitor cyclosporin A and GSH, suggesting that swelling was due to pore opening and related to dopamine quinone formation. In contrast, dopamine and endogenous monoamine oxidase had no effect on mitochondrial swelling. These findings suggest that mitochondrial dysfunction induced by products of dopamine oxidation may be involved in neurodegenerative conditions such as Parkinson's disease and methamphetamine-induced neurotoxicity.

Polymerization of 5,6-dihydroxyindole-2-carboxylic acid to melanin by the pmel 17/silver locus protein

Recent advances in melanogenesis have focused on the role of dihydroxyindole-2-carboxylic acid[(HO)2IndCOOH]. For example, it has been shown that formation of (HO)2IndCOOH from dopachrome is catalyzed by dopachrome tautomerase, that the melanogenic protein tyrosinase-related protein (TRP)-1 can oxidize (HO)2IndCOOH to its indole quinone, that (HO)2IndCOOH-melanins can be synthesized chemically, that mammalian melanins are naturally rich in (HO)2IndCOOH subunits, and that (HO)2IndCOOH is incorporated into melanins of melanomas in mice. The question thus emerges as to the mechanism(s) by which (HO)2IndCOOH and other precursors become incorporated into melanins in vivo. Accordingly, an activity was partially purified that catalyzed melanin formation with (HO)2IndCOOH as a substrate. Analyses of the (HO)2IndCOOH polymerization factor from Cloudman melanoma cells revealed the following: it was proteinaceous in that it was heat labile and destroyed by proteinase K; it was a glycoprotein in that it adhered to wheat germ agglutinin and was eluted with N-acetyl glucosamine; it was located predominantly in the melanosomal fraction of cell homogenates; the activity was reduced by exposure to the metal chelators EDTA and EGTA, but not by phenylthiourea, a tyrosinase inhibitor; the (HO)2IndCOOH polymerization reaction was inhibited by superoxide dismutase. In addition, the activity was found with the mouse pmel 17/silver locus protein immunopurified from human melanoma cells, and was significantly reduced in extracts of mouse melanocytes cultured from silver (si/si) mice compared to extracts from Si/Si melanocytes. In summary, an activity has been identified in human and mouse melanoma cells that catalyzes the superoxide-dependent polymerization of (HO)2IndCOOH to melanin in vitro, and appears to be a function of the pmel 17/silver protein of the human pmel 17 gene and the mouse silver locus.

Premature avian melanocyte death due to low antioxidant levels of protection: fowl model for vitiligo

Feather melanocytes in the Barred Plymouth Rock (BPR) and White Leghorn (WL) chickens die prematurely in vivo when compared to the wild type Jungle Fowl (JF) chicken. Since these mutant melanocytes live in vitro, an environmental factor in the feather must precipitate their death. Results show that the addition of selected antioxidants, glutathione (GSH) and superoxide dismutase (SOD), can rescue these mutant melanocytes in vitro that have been placed under stress conditions that cause their premature cell death. Measurements of in vivo levels of GSH, catalase, and SOD show no significant difference in catalase activity between the JF, BPR, and WL feathers but do show a significant reduction in GSH activity in both the BPR and WL feathers to approximately 66% of the GSH concentration found in JF feathers. SOD activity in the BPR tissue is reduced significantly to approximately 50% of the JF activity and the WL SOD activity is reduced significantly to approximately 50% of the BPR SOD activity. Preliminary results of measurements of glutathione peroxidase activity indicate there is no difference in the levels of this enzyme in JF, BPR and WL feathers. A working hypothesis, based on current results, is proposed for premature cell death in BPR and WL feather melanocytes. The BPR melanocytes are genetically sensitive due to a defect in their SOD and GSH levels caused by the barring gene (B) and their death, due to reactive species of oxygen radicals, is precipitated in the poorly vascularized feather by the accumulation of oxygen radicals due to the low turnover of tissue fluids. The WL chicken carries the dominant white gene (I) in addition to the B gene. This gene directs the further reduction of the level of SOD and, when combined with the cell death mechanism already present in the BPR chicken, causes the WL feather melanocytes to die much earlier than the BPR feather melanocytes which in turn die much earlier than the wild type JF melanocytes. This same mechanistic hypothesis could apply as a cause of premature melanocyte cell death in human vitiligo wherein the vitiliginous melanocytes may have a genetic defect in their oxygen radical protection system.

A comparison between melanotic and amelanotic retinal pigment epithelial cells in vitro concerning the effects of L-dopa and oxygen on cell cycle

Melanin precursors and free radicals, cytotoxic substances, are produced during melanin synthesis by tyrosinase. We compared these cytotoxic effects of L-dopa and oxygen on the cell cycle of melanotic retinal pigment epithelial (RPE) cells with amelanotic RPE cells because of the differences of tyrosinase activities between melanotic and amelanotic RPE cells. Flow cytometric DNA analysis of RPE cells exposed to L-dopa (100 microM and 250 microM) were conducted at several oxygen concentrations (20%, 10%, and 5%). The dose-dependent effect of L-dopa to arrest the cell cycle (the S phase) was more pronounced in melanotic than in amelanotic RPE cells, and oxygen caused arrest in the G1 phase.

The effect of oxygen on melanin precursors released from retinal pigment epithelial cells in vitro

The autoxidation of dopa to melanin in culture media causes toxicity to retinal pigment epithelial (RPE) cells and endothelial cells. The damage is specific to cell type and to the ambient oxygen concentration. To determine whether RPE cells influence the oxidation of dopa to media, we compared light absorbing dopa derivatives in the media exposed to cells with those found in the media incubated without cells. Dopa was extensively oxidized in the presence of RPE cells, and more light absorbing substances were generated with higher dopa and oxygen concentrations. However, an increase in ambient oxygen concentration decreased the quantity of several dopa derivatives which had been formed. The data provided evidence that RPE modulated dopa metabolism. Quinolic derivatives produced from a tyrosinase reaction and dopa-melanin formation moved the peak absorbance wavelength of dopa into the visible range. The spectrum between the dopa-derived compounds in the media has an absorbance at 240-275 nm and a maximum around 300 nm with a shoulder near 375 nm. Gaussian analysis (peak separation) resolved these spectra into five components: a sharp band at 248 nm, a band at 295 nm, a large band at 359 nm, and two broad bands at 459 and 585 nm.

Antioxidative activity of synthetic melanins. Cardiolipin liposome model

The inhibiting effect of melanin synthesized from dihydroxyphenylalanine (DOPA), dopamine, adrenaline and adrenolutin on the ultraviolet- or the Fe(2+)-ascorbic acid-induced peroxidation of cardiolipin liposomes has been studied. All these melanins are able to inhibit both the ultraviolet- and the Fe(2+)-ascorbic acid-induced lipid peroxidation. Antioxidative activity of melanins enhances in the order: dopamine-melanin less than melanin synthesized from dopamine in the presence of Cu(2+) less than DOPA--melanin less than melanin synthesized from adrenaline in the presence of Cu(2+) approximately equal to adrenolutin-melanin, and correlates with their ability to scavenge superoxide anion radical. The optical screening effect of the investigated melanins in the inhibition of lipid peroxidation was not higher than 15% for the most active melanins.

Properties and function of the ocular melanin--a photobiophysical view

This paper reviews the biosynthesis and physicochemical properties of the ocular melanin. Age-related changes of melanin granules and the corresponding formation of lipofuscin pigments in the retinal pigment epithelium (RPE) are also described. Adverse photoreactions of the eye and, in particular, light-induced damage to the RPE-retina are reviewed in relation to the ocular pigmentation. A hypothesis on the photoprotective role of the RPE melanin is presented that is based on the ability of the cellular melanin to bind redoxactive metal ions. Since bound-to-melanin metal ions are expected to be less damaging to the pigment cells, it is proposed that sequestration of heavy metal ions by the RPE melanin is an efficient detoxifying mechanism. It is postulated that oxidative degradation of RPE melanin may lower its metal-binding capability and decrease its anti-oxidant efficiency. Cellular and environmental factors that may contribute to possible oxidative damage of the RPE melanin are discussed in connection with the etiology of age-related macular degeneration.

Effects of oxygen tension on the growth and pigmentation of normal human melanocytes

The effects of oxygen tension on human melanocyte growth, tyrosinase activity, and melanin production were assessed. Melanocytes, seeded at 10(4) cells/cm2, were grown in modified Eagle's medium (MEM) with 5% fetal bovine serum (FBS) and 10 ng/ml 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Flasks were equilibrated with gas mixtures containing 5% CO2 and various partial pressures of oxygen (PO2 7-620 mm Hg) and kept in incubators, which were electronically maintained at the desired oxygen tensions. Melanocytes grew best at PO2 from 6-34 mm Hg. Growth was reduced by 30% at PO2 142 mm Hg, and even more at O2 tensions greater than 230 mm Hg. A PO2 of 603 mm Hg was cytotoxic. Tyrosinase activity (assayed by the method of Pomerantz) was 300 microU/mg protein at PO2 7-34 mm Hg. At PO2 235 and 355 mm Hg tyrosinase activity decreased to about 100 microU/mg protein. The apparent Km for tyrosine was unchanged in melanocytes cultured at all experimental oxygen tensions. The Vmax, however, was decreased at the higher oxygen tensions (PO2 235 mm Hg). At PO2 6-135 mm Hg the melanin content was proportional to tyrosinase activity. At cytostatic oxygen tensions (PO2 235 and 355 mm Hg) the intracellular melanin content increased somewhat, although tyrosinase activity was decreased. Low oxygen tension is favorable for both melanocyte proliferation and tyrosinase activity.

Growth of cultured RPE and endothelial cells is inhibited by blue light but not green or red light

Blue light, but not green or red light, inhibited growth of retinal pigment epithelial (RPE) cells, aortic endothelial cells, and fibroblasts in vitro. Significant inhibition was observed in all 3 cell types exposed for 18 hr to blue light (425-500 nm) at 42 J/cm2. Damage was prevented by inclusion of superoxide dismutase (SOD) and catalase, providing evidence for a photooxidative mechanism. Dopa (100 microM) also caused oxidative damage that suppressed growth of all 3 cell types. A synergism of dopa and light effects was observed in endothelial cells and fibroblasts, but the agents caused additive effects on RPE cells. Endothelial cells were the most sensitive to dopa, light, and the two combined. Fibroblasts were the only cell type that exhibited greater sensitivity to light than to dopa. These data suggest that oxygen-mediated damage to the growing blood vessels in the retina of a premature infant may be exacerbated by exposure to blue light. A further implication is that restriction of RPE melanogenesis to the prenatal period of darkness and lower oxygen protects the retina from simultaneous oxidative challenge by light and by reactive species generated during oxidation of dopa released to the extracellular environment.

Separation of pigmented and albino melanocytes and the concomitant evaluation of endogenous peroxide content using flow cytometry

Flow cytometry (FCM) has been used extensively to analyze various biological properties of the cell. In this report, we describe a method by which FCM was used to determine the light scattering profile of a mixed population of pigmented and non-pigmented melanocytes, plus its subsequent use for the sorting and separation of the two cell types. In addition, the relative peroxide content in pigmented and non-pigmented melanocytes was compared by flow cytometry. Cultured avian melanocytes from a pigmented control and from three genetically distinct albino sources were studied. FCM analysis of forward versus side light scatter within a mixed suspension of pigmented and amelanotic melanocytes distinguished two overlapping populations of cells. Sorting of these two populations demonstrated that the population exhibiting much side and minimal forward light scatter was primarily pigmented melanocytes, while conversely the population exhibiting less side and more forward scatter was principally non-pigmented cells. These two melanocyte types also demonstrated differences in levels of endogenous peroxides. The intracellular content of peroxide in the two subpopulations of cells was measured utilizing the nonfluorescent compound, 2',7'-dichlorofluorescein diacetate (DCFH-DA), which within the cell is oxidized by intracellular peroxides to a fluorescent dichlorofluorescein (DCF). Non-pigmented albino melanocytes had the highest quantity of endogenous peroxides, while heavily pigmented cells had considerably less peroxide-related fluorescence. The amount of this DCF fluorescence could be enhanced by increasing concentrations of DCF used in the assay. These flow cytometric methods are useful for isolating and culturing subpopulations of melanocytes expressing various pigment levels and to investigate the relationship between melanin and its precursors with hydrogen and lipid peroxides in melanocytes.

Dopa and oxygen inhibit proliferation of retinal pigment epithelial cells, fibroblasts and endothelial cells in vitro

Some quinones catalyze superoxide formation in a futile cycle involving electron transfer to molecular oxygen. If the quinolic precursors of melanin participated in the futile cycle, the high ambient oxygen surrounding postnatal RPE would make continued melanogenesis risk for the retina. To probe the possibility that arrest of melanogenesis in postnatal RPE is a protective mechanism, we assayed the growth rates of RPE cells, aortic endothelial cells and skin fibroblasts exposed to 0, 10, 50, 100 and 250 microM dopa. To assess the contribution of the futile cycle, we studied the effects of oxygen concentration and the antioxidants, superoxide dismutase and catalase. We found that all three cell types were significantly and dose-dependently inhibited by dopa and that the effects of dopa were oxygen dependent. The powerful inhibition of RPE cells by dopa was counteracted by inclusion of superoxide dismutase and catalase, but not by an inhibitor of dopa oxidase (phenylthiourea), indicating that the mechanism of growth suppression did not involve melanogenesis but, rather, dopa-dependent formation of superoxide in the media. Endothelial cells were more sensitive to the dopa-mediated oxidative damage than were RPE cells or fibroblasts. Fibroblasts were most affected by oxygen alone, and least affected by dopa. These data suggest that suppression of melanogenesis in the postnatal RPE may be an important mechanism for preventing oxidative damage to the retina and the choriocapillaris We propose that the generation of oxygen radicals by the quinone futile cycle is a viable model of the damage of cells in culture by dopa.

Modification of dopa toxicity in human tumour cells

A variety of factors were found to modify the toxicity of L-dopa in HeLa cells (D37 16 microM) and in dopa-sensitive, nonpigmented human melanoma cells (MM96) (D37 5 microM) having a similar size and doubling time. Dopa toxicity was decreased by concurrent treatment with superoxide dismutase, peroxidase or catalase, by erythrocytes, or by hypoxia. Toxicity could be increased by the enzyme inhibitors L- and D-penicillamine, sodium diethyldithiocarbamate or 3-amino-1,2,4-triazole. The two cell lines had similar levels of superoxide dismutase and peroxidase; in 6 human melanoma lines, no correlation was found between dopa killing and tyrosinase activity as determined either by formation of dopa from tyrosine or by formation of melanin from dopa. Uptake of L-dopa was similar in HeLa and MM96 cells, and the toxicity of D-dopa was the same in both lines as that of the L-isomer. Dopa decomposed within 12 hr in culture medium, the rate and products being influenced by addition of the above enzymes and by the cell density. Dopa-melanin and medium containing decomposed dopa were also selectively toxic to MM96 cells. Adenovirus 5 was used in two different ways to assess the relative importance of DNA damage and inhibition of DNA synthesis by dopa. Viral replication was found to be unaffected in cells being treated with dopa but was strongly inhibited in cells treated with the DNA polymerase inhibitor cytosine arabinoside. Secondly, the virus was itself inactivated by treatment with dopa for 24 hr (D37 1.3 mM); similar dose response curves were obtained for replication of dopa-treated virus in untreated HeLa or MM96 cells. These results show that the initial events of dopa toxicity occur outside the cell and lead to the formation of a stable, toxic product (probably melanin) which does not strongly inhibit DNA polymerase activity. Melanoma hypersensitivity was not due to differences in oxygen-metabolizing enzymes, dopa uptake, or DNA repair.