Activation of tyrosinase reduces the cytotoxic effects of the superoxide anion in B16 mouse melanoma cells

Knockdown of Claudin-19 in the Retinal Pigment Epithelium Is Accompanied by Slowed Phagocytosis and Increased Expression of SQSTM1

Purpose

Besides regulating paracellular diffusion, claudin-19 modulates the expression of proteins essential for the retinal pigment epithelium (RPE). This study asks how RPE responds when the expression of claudin-19 is reduced.

Methods

In stem cell-derived RPE, claudin-19 and sequestosome-1/p62 (SQSTM1) were knocked down with siRNAs. Expression was monitored by quantitative RT-PCR and western blotting. Morphology and function were monitored by immunocytochemistry and transepithelial electrical resistance (TER). Phagocytosis of photoreceptor outer segments (POSs) was followed by fluorescence-activated cell sorting and western blotting. Pharmacology was used to assess the effects of AMP-activated protein kinase (AMPK) and SQSTM1 on phagocytosis. Enzymatic activity was measured using commercial assay kits.

Results

Knockdown of claudin-19 reduced the TER without affecting the integrity of the apical junctional complex, as assessed by the distribution of zonula occludens-1 and filamentous actin. AMPK was activated without apparent effect on autophagy. Activation of AMPK alone had little effect on phagocytosis. Without affecting ingestion, knockdown reduced the rate of POS degradation and increased the steady-state levels of LC3B and SQSTM1. Proteasome inhibitors also retarded degradation, as did knockdown of SQSTM1. The expression of metallothioneins and the activity of superoxide dismutase increased.

Conclusions

Knockdown of claudin-19 slowed the degradation of internalized POSs. The study questions the role of activated AMPK in phagocytosis and suggests a role for SQSTM1. Further, knockdown was associated with a partial oxidative stress response. The study opens new avenues of experimentation to explore these essential RPE functions.

Claudins regulate gene and protein expression of the retinal pigment epithelium independent of their association with tight junctions

Claudin-19 is the major claudin in the tight junctions of the retinal pigment epithelium (RPE). Claudin-3 is also uniformly expressed albeit in lesser amounts. Besides modulating transepithelial diffusion, claudins modulate gene expression. The absence of claudin-19 and claudin-3 in the RPE cell lines, ARPE-19 and hTERT-RPE-1, provide an opportunity to examine whether exogenous claudins regulate gene expression in the absence of tight junctions. Quantitative RT-PCR was used to compare gene expression in ARPE-19 and hTERT-RPE-1 with that of highly differentiated, human fetal RPE. Claudin-19 and claudin-3 were exogenously expressed using an adenoviral vector. The transepithelial electrical resistance (TER) was measured using Endohm electrodes, and the effects of claudin on the actin cytoskeleton were determined by immunocytochemistry. The effect of claudin on gene expression was examined by quantitative RT-PCR and western blotting. Aside from claudin-19 and claudin-3, ARPE-19 and hTERT-RPE-1 expressed most junction-associated mRNAs in amounts comparable to human fetal RPE, but some RPE signature and maturation genes were under-expressed. Unlike ARPE-19, hTERT-RPE-1 failed to form tight junctions or develop a TER. Claudins exogenously expressed in hTERT-RPE-1 failed to crystalize an apical junctional complex. Actin filaments were not redistributed from stress fibers to cortical bands, and a TER was not established. In hTERT-RPE-1, claudins were found only in internal vesicular-like structures. Nonetheless, claudins increased the expression of the mRNAs for a collection of RPE-enriched proteins. Claudin-19 and claudin-3 had different effects on gene and protein expression indicating activation of overlapping, but distinct, signaling pathways. A major difference was the ability of claudin-19 to affect steady-state levels of ADAM9 and tyrosinase in ARPE-19. In conclusion, claudins can increase the barrier function of a pre-existing apical junctional complex, but on its own it cannot recruit tight junction proteins to form a complex de novo. Many effects of claudin on gene expression did not require an association with the apical junctional complex. Although claudin-19 shared many effects with claudin-3, claudin-19 exerted unique effects on the maturation of RPE.

Role of alpha-melanocyte stimulating hormone (α-MSH) in modulating the molecular mechanism adopted by melanocytes of Bos indicus under UVR stress

Ultraviolet radiations (UVR) are responsible for a wide variety of acute and chronic effects on the animal skin. However, the effect of UVR-induced oxidative stress and protection through paracrine factors on animal skin has received little attention. We previously demonstrated how heat stress-induced adaptation in Bos indicus melanocytes was dependent on the level of melanin and reduction of apoptosis. Therefore, in the present investigation, the survival mechanisms adopted by melanocytes under UV stress and the role of α-MSH in cell survival under in vitro conditions were studied. After the treatment of melanocyte cells with UVR (using Osram ultravitalux 300 W lamp), analysis of Gene expression using Real-Time PCR was done to study the adopted molecular pathways under stressful conditions. In addition, α-MSH was used to assess its modulating role in cell survival under stress. This study revealed the increase in the expression of genes related to melanogenesis, cell cycle, heat shock proteins, and apoptosis of the cells after UVR stress and demonstrated the role of paracrine factor (α-MSH) in elevating the protection response to stressful conditions like UVR stress by increasing the melanogenesis and decreasing the mitochondrial-mediated apoptosis. Based on the results of the present study, it can be stated that α-MSH can play a pivotal role in the protection of animal skin cells under stressful conditions in climate-changing scenario.

Differential visual system organization and susceptibility to experimental models of optic neuropathies in three commonly used mouse strains

Mouse disease models have proven indispensable in glaucoma research, yet the complexity of the vast number of models and mouse strains has also led to confusing findings. In this study, we evaluated baseline intraocular pressure, retinal histology, and retinofugal projections in three mouse strains commonly used in glaucoma research, i.e. C57Bl/6, C57Bl/6-Tyr(c), and CD-1 mice. We found that the mouse strains under study do not only display moderate variations in their intraocular pressure, retinal architecture, and retinal ganglion cell density, also the retinofugal projections to the dorsal lateral geniculate nucleus and the superior colliculus revealed striking differences, potentially underlying diverging optokinetic tracking responses and visual acuity. Next, we reviewed the success rate of three models of (glaucomatous) optic neuropathies (intravitreal N-methyl-d-aspartic acid injection, optic nerve crush, and laser photocoagulation-induced ocular hypertension), looking for differences in disease susceptibility between these mouse strains. Different genetic backgrounds and albinism led to differential susceptibility to experimentally induced retinal ganglion cell death among these three mouse strains. Overall, CD-1 mice appeared to have the highest sensitivity to retinal ganglion cell damage, while the C57Bl/6 background was more resistant in the three models used.

Proopiomelanocortin gene delivery induces apoptosis in melanoma through NADPH oxidase 4-mediated ROS generation

Hypoxia in the tumor microenvironment triggers differential signaling pathways for tumor survival. In this study, we characterize the involvement of hypoxia and reactive oxygen species (ROS) generation in the antineoplastic mechanism of proopiomelanocortin (POMC) gene delivery in a mouse B16-F10 melanoma model in vivo and in vitro. Histological analysis revealed increased TUNEL-positive cells and enhanced hypoxic activities in melanoma treated with adenovirus encoding POMC (Ad-POMC) but not control vector. Because the apoptotic cells were detected mainly in regions distant from blood vessels, it was hypothesized that POMC therapy might render melanoma cells vulnerable to hypoxic insult. Using a hypoxic chamber or cobalt chloride (CoCl2), we showed that POMC gene delivery elicited apoptosis and caspase-3 activation in cultured B16-F10 cells only under hypoxic conditions. The apoptosis induced by POMC gene delivery was associated with elevated ROS generation in vitro and in vivo. Blocking ROS generation using the antioxidant N-acetyl-l-cysteine abolished the apoptosis and caspase-3 activities induced by POMC gene delivery and hypoxia. We further showed that POMC-derived melanocortins, including α-MSH, β-MSH, and ACTH, but not γ-MSH, contributed to POMC-induced apoptosis and ROS generation during hypoxia. To elucidate the source of ROS generation, application of the NADPH oxidase inhibitor diphenyleneiodonium attenuated α-MSH-induced apoptosis and ROS generation, implicating the proapoptotic role of NADPH oxidase in POMC action. Of the NADPH oxidase isoforms, only Nox4 was expressed in B16-F10 cells, and Nox4 was also elevated in Ad-POMC-treated melanoma tissues. Silencing Nox4 gene expression with Nox4 siRNA suppressed the stimulatory effect of α-MSH-induced ROS generation and cell apoptosis during hypoxia. In summary, we demonstrate that POMC gene delivery suppressed melanoma growth by inducing apoptosis, which was at least partly dependent on Nox4 upregulation.

Electrochemical and optical sensing of reactive oxygen species: pathway to an integrated intracellular and extracellular measurement platform

A comprehensive understanding of ROS (reactive oxygen species)-dependent cellular interaction requires the previously unmet ability to simultaneously monitor the intra- and extra-cellular environments. The present review assesses the potential of novel electrochemical and fluorescent-based nanosensor approaches to address the limitations of existing techniques for ROS analysis. Data generated by these new approaches have already contributed significantly to current understanding of the roles that these species play in various in vitro scenarios. However, integration of these novel approaches has the potential to offer, for the first time, the unparalleled ability to measure simultaneously and in real-time ROS flux in both the intra- and extra-cellular environments.

alpha-MSH activates immediate defense responses to UV-induced oxidative stress in human melanocytes

Exposure of cultured human melanocytes to ultraviolet radiation (UV) results in DNA damage. In melanoma, UV-signature mutations resulting from unrepaired photoproducts are rare, suggesting the possible involvement of oxidative DNA damage in melanocyte malignant transformation. Here we present data demonstrating immediate dose-dependent generation of hydrogen peroxide in UV-irradiated melanocytes, which correlated directly with a decrease in catalase activity. Pretreatment of melanocytes with alpha-melanocortin (alpha-MSH) reduced the UV-induced generation of 7,8-dihydro-8-oxyguanine (8-oxodG), a major form of oxidative DNA damage. Pretreatment with alpha-MSH also increased the protein levels of catalase and ferritin. The effect of alpha-MSH on 8-oxodG induction was mediated by activation of the melanocortin 1 receptor (MC1R), as it was absent in melanocytes expressing loss-of-function MC1R, and blocked by concomitant treatment with an analog of agouti signaling protein (ASIP), ASIP-YY. This study provides unequivocal evidence for induction of oxidative DNA damage by UV in human melanocytes and reduction of this damage by alpha-MSH. Our data unravel some mechanisms by which alpha-MSH protects melanocytes from oxidative DNA damage, which partially explain the strong association of loss-of-function MC1R with melanoma.

A wounding-induced PPO from cowpea (Vigna unguiculata) seedlings

Polyphenol oxidases (PPO) are induced in cowpea plants by wounding. The highest activity levels were detected 48h after this stimulus in both wounded and neighbor-to-wounded unifoliates of cowpea seedlings; the increase of activity was in the order of 13 to 15-fold, respectively, in comparison to control unifoliates. Multiple molecular forms of active PPO (Mrs 58, 73 and congruent with220kDa) were detected by partially denaturing SDS-PAGE. Wounding-induced cowpea PPO were extracted and purified through (NH4)2SO4 precipitation and ion-exchange chromatography. The effects of substrate specificity, pH, thermal stability and sensitivity to various inhibitors - resorcinol, EDTA, sodium azide and tropolone - of partially purified soluble PPO were investigated. Purified wounding-induced cowpea PPO (wicPPO) showed the highest activities towards 4-methylcatechol (Km=9.86mM, Vmax=24.66 EU [DeltaAmin(-1)]) and catechol (Km=3.44mM, Vmax=6.64 EU [DeltaAmin(-1)]); no activity was observed towards l-tyrosine, under the assay conditions used. The optimum pH for wound-induced cowpea PPO was 6.0 with 4-methylcatechol as substrate. The enzyme was optimally activated by 10 mM SDS and was highly stable even after 5 min at 80 degrees C. The most effective inhibitor was tropolone, whereas addition of 10mM of resorcinol, EDTA and sodium azide were able to reduce PPO activities by 40%, 15% and 100%, respectively.

Tyrosinase biosynthesis and trafficking in adult human retinal pigment epithelial cells

BACKGROUND

Tyrosinase (EC 1.14.18.1) is the key enzyme of melanin pigment formation and it is unclear whether it is synthesized in human postnatal retinal pigment epithelium (RPE). In this study, we investigated if phagocytosis of rod outer segments (ROS) can increase tyrosinase expression in vitro.

METHODS

Primary cultures of human RPE cells were fed with isolated ROS from cattle and with latex particles. After phagocytosis, RPE cells were tested for tyrosinase presence and activity with several independent methods: (1) immunocytochemistry with anti-tyrosinase antibodies and (2) ultrastructural as well as light microscopic DOPA histochemistry; (3) mRNA was isolated from human RPE before incubation with ROS and 5, 20 and 40 h after feeding with ROS. The amount of tyrosinase mRNA was determined quantitatively by real-time reverse transcription polymerase chain reaction (RT-PCR), and the tyrosinase activity was investigated by measuring tyrosine hydroxylase activity using [(3)H]tyrosine.

RESULTS

Tyrosinase was found in fed RPE cells using these methods, but was absent without feeding. Furthermore, we showed co-localization of rhodopsin and tyrosinase in the fed RPE cells. Contrary to tyrosinase activity, the mRNA for tyrosinase was clearly present in the cultured RPE cells which had not been exposed to ROS, decreased significantly from 5 h after exposure to ROS and returned to its original non-fed level 40 h after ROS feeding.

CONCLUSION

Our study does not present new evidence that de novo melanogenesis takes place in the adult differentiated RPE. However, in contrast to the classic hypothesis, which states that tyrosinase is only detected in embryos, we provide evidence with several independent methods that the expression of tyrosinase and its enzymatic activity are induced in cultured human adult RPE by phagocytosis of ROS.

The polymerization of melanin: a poorly understood phenomenon with egregious biological implications

Several hypotheses have explicitly implicated the role of an altered redox status of melanin in the aetiology of melanoma and macular degeneration. The balance between the intrinsic anti-oxidant and pro-oxidant properties of melanin is lost, resulting in an altered redox phenotype. We propose that such an alteration of the redox status of melanin may arise, in part, due to suboptimal conditions for the effective polymerization of melanin precursors. We suggest that a decrease in the degree of polymerization or molecular weight of the melanin polymer may cause an alteration of the redox status of the polymer towards a more pro-oxidant state. A higher propensity of smaller oligomers to complex metals, coupled with an upregulation of metallothionein expression, results in increased production of free radicals including the superoxide anion. This, in association with an increase in the rate of tyrosinase degradation, a decrease in the rate of tyrosinase activation, alterations to template protein structure or alterations in the kinetics of the oxidation of tyrosine via the Raper-Mason pathway, may result in an overcoming of the cellular anti-oxidant pool, an increased susceptibility to oxidative stress and alterations to the reaction kinetics of melanogenesis, thus setting up a cycle of increasing oxidative stress and proliferation leading to the leakage of melanin monomers outside the organelle, thereby causing cytotoxicity and necrosis.

Tyrosinase biosynthesis in adult mammalian retinal pigment epithelial cells

Tyrosinase (EC 1.14.18.1) is the rate limiting enzyme of melanogenesis and it is unclear whether it is synthesized in postnatal retinal pigment epithelium (RPE). Cultured RPE cells from cattle were fed with isolated rod outer segments (ROS). After phagocytosis, RPE cells were tested for tyrosinase presence and activity with three independent methods: (1) ultrastructural DOPA (l-3,4-dihydroxyphenylalanine) histochemistry (2) immunocytochemistry with anti-tyrosinase antibodies (3) measuring tyrosine hydroxylase activity using [(3)H]tyrosine. With all three methods tyrosinase was found in RPE cells after ROS-feeding but was absent without feeding. In contrast to the classical hypothesis, we demonstrated with three independent methods that the expression of tyrosinase and its enzymatic activity are induced in cultured adult RPE by phagocytosis of rod outer segments (ROS) in vitro.

Cytotoxicity of nanosize V(2)O(5) particles to selected fibroblast and tumor cells

Two kinds of nanosize V(2)O(5) particles were synthesized in our own laboratory and concomitantly applied to V79 and L929 fibroblasts and SCCVII, B16F10 and FsaR tumor cells. The morphologies of the cells were monitored using an inverted inverse microscope equipped with digital camera, while quantitative determination of the cytotoxicity of nanosize V(2)O(5) particles was measured using crystal violet bioassay. Twenty four hours after the addition of nanosize V(2)O(5) particles (20muM), noticeable changes in the morphology and density of fibroblast and cancer cells were observed. Reculturing in a freshly prepared medium for the next 24h showed a high recovery effect on V79, SCCVII and B16F10 cells, while FsaR and L929 cells were seriously damaged and unable to recover. At a higher concentration of nanosize V(2)O(5) particles (100muM), the cytotoxicity of V(2)O(5) prevailed against the recovery effect in all cell types. Quantitative measurements have shown that the resistance of investigated cell cultures to the cytotoxicity of nanosize V(2)O(5) particles decreases in the order V79>SCCVII>B16F10>FsaR>L929. The high cytotoxic effect found on FsaR cells suggests that nanosize V(2)O(5) particles could be regarded as poisoning material in the treatment of FsaR fibrosarcoma cells. Possible mechanisms involved in the cytotoxicity of nanosize V(2)O(5) particles were discussed.

Biphasic superoxide generation in potato tubers. A self-amplifying response to stress

Potato (Solanum tuberosum) cultivars differ quantitatively in their responses to mechanical stress including the ability to synthesize melanin pigments in tuber tissues. Investigations into the cellular events induced by mechanical stress on tuber tissues have shown that an early cellular response is a significant and rapid synthesis of superoxide radicals. This burst of radical production distinctively displays a reproducible biphasic pattern over time with peaks of generation at 2 and 5 h. A concomitant consequence of the generation of these free radicals is elevated levels of oxidatively modified tuber proteins. Both radical generation and protein modification vary between cultivars but both are directly proportional to the amount of melanin pigments produced. Cell-free extracts of mechanically stressed tissues, pectic fragments, and scission products generated from cell walls are able to induce superoxide generation in non-stressed tissues, indicating the participation of a biologically active factor that induces a further a phase of radical synthesis.

Lipid peroxidation and antioxidant capacity in selected tissues of healthy black C57BL/6J mice and B16 melanoma-bearing mice

During the process of melanogenesis free radicals are generated. The aim of this study was to investigate the effects of melanogenesis in B16 melanoma on lipid peroxidation and antioxidant capacity in selected tissues of black C57BL/6J mice. The study was conducted on 24 mice: 12 healthy controls and 12 with a transplanted B16 melanoma. Two weeks after the melanoma transplant, when the average weight of the tumours was approximately 2.0 g, blood samples were taken from the orbital venous plexus. The mice were killed by dislocation of the spinal cord, and the brain, liver and lungs were removed for analysis. The level of thiobarbituric acid-reactive substances (TBARS) and of 1,1-diphenyl-2-picrylhydrazyl (DPPH) reactive substances were determined in full liver, lung and brain homogenates and in serum. The activity of superoxide dismutase (SOD) was determined only in homogenized tissue. The concentration of TBARS and the SOD activity were statistically significantly higher in all the studied tissues from mice with B16 melanoma than in tissues from healthy mice. The antioxidant capacity, however, was lower in the tissues of melanoma-bearing mice. The results obtained demonstrate an increase in oxidative stress in the tissues of mice bearing a transplanted B16 melanoma.

Cytotoxicity of dopamine-derived tetrahydroisoquinolines on melanoma cells

Tetrahydroisoquinolines (TIQs) are endogenous alkaloid compounds detected in urine, central nervous system and some peripheral tissues in Mammalia. No data are at present available on TIQ levels in skin, although in vitro biochemical evidences indicate that they may undergo auto-oxidation with production of reactive oxygen species or may be enzymatically converted into melanin pigments. The effect of two catechol-bearing TIQs, salsolinol (SAL) and tetrahydropapaveroline (THP), on the viability of melanotic or amelanotic melanoma cell lines was investigated. Both SAL and THP were well tolerated up to roughly 30 microM and became overtly toxic at higher concentrations, with SAL being better tolerated than THP. Intracellular activity of some antioxidant enzymes, tyrosinase and alpha-ketoglutarate dehydrogenase was also evaluated to assess the cell response to oxidative and metabolic challenge of TIQs treatment. Catalase and superoxide dismutase pre-treatment only partially prevented TIQs toxicity while a complete protection was obtained with N-acetylcysteine and GSH. TIQs were able to provoke upregulation of the scavenging enzymes catalase and DT-diaphorase and to determine a decrease of the alpha-ketoglutarate dehydrogenase activity. SAL and THP enhanced tyrosinase activity and melanin production, suggesting that they were indeed tyrosinase substrates leading to melanin formation. The results support the evidence that TIQs were toxic toward melanoma cells, leading to their death by necrosis. TIQs toxicity was likely due to increased oxidative stress by generation of reactive oxygen species and oxidative metabolites. Our study represents an intent to furnish an additional contribution for the comprehension of catechol cytotoxicity.

Melanocyte function and its control by melanocortin peptides

Melanocytes are cells of neural crest origin. In the human epidermis, they form a close association with keratinocytes via their dendrites. Melanocytes are well known for their role in skin pigmentation, and their ability to produce and distribute melanin has been studied extensively. One of the factors that regulates melanocytes and skin pigmentation is the locally produced melanocortin peptide alpha-MSH. The effects of alpha-MSH on melanogenesis are mediated via the MC-1R and tyrosinase, the rate-limiting enzyme in the melanogenesis pathway. Binding of alpha-MSH to its receptor increases tyrosinase activity and eumelanin production, which accounts for the skin-darkening effect of alpha-MSH. Other alpha-MSH-related melanocortin peptides, such as ACTH1-17 and desacetylated alpha-MSH, are also agonists at the MC-1R and could regulate melanocyte function. Recent evidence shows that melanocytes have other functions in the skin in addition to their ability to produce melanin. They are able to secrete a wide range of signal molecules, including cytokines, POMC peptides, catecholamines, and NO in response to UV irradiation and other stimuli. Potential targets of these secretory products are keratinocytes, lymphocytes, fibroblasts, mast cells, and endothelial cells, all of which express receptors for these signal molecules. Melanocytes may therefore act as important local regulators of a range of skin cells. It has been shown that alpha-MSH regulates NO production from melanocytes, and it is possible that the melanocortins regulate the release of other signalling molecules from melanocytes. Therefore, the melanocortin signaling system is one of the important regulators of skin homeostasis.

Reactive oxygen generated by Nox1 triggers the angiogenic switch

The reactive oxygen-generating enzyme Nox1 transforms NIH 3T3 cells, rendering them highly tumorigenic and, as shown herein, also increases tumorigenicity of DU-145 prostate epithelial cells. Although Nox1 modestly stimulates cell division in both fibroblasts and epithelial cells, an increased mitogenic rate alone did not account fully for the marked tumorigenicity. Herein, we show that Nox1 is a potent trigger of the angiogenic switch, increasing the vascularity of tumors and inducing molecular markers of angiogenesis. Vascular endothelial growth factor (VEGF) mRNA becomes markedly up-regulated by Nox1 both in cultured cells and in tumors, and VEGF receptors (VEGFR1 and VEGFR2) are highly induced in vascular cells in Nox1-expressing tumors. Matrix metalloproteinase activity, another marker of the angiogenic switch, also is induced by Nox1. Nox1 induction of VEGF is eliminated by coexpression of catalase, indicating that hydrogen peroxide signals part of the switch to the angiogenic phenotype.

Retinas from albino rats are more susceptible to ischaemic damage than age-matched pigmented animals

Age- and sex-matched pigmented (Lister Hooded) and albino (Wistar) rats were used in this study. The retinas of the animals were subjected to pressure-induced ischaemia (35 min, 120 mmHg) and reperfusion (3 days) in precisely the same way. The b-wave of the electroretinogram (ERG) in the pigmented animals recovered to normal levels while those of the albino rats were reduced by more than 80%. Moreover, the choline acetyltransferase (ChAT) immunoreactivity associated with a sub-set of amacrine cells was almost completely obliterated in the retinas from the albino rats but unaffected in the retinas of the pigmented rats. Also, in certain areas of the retina from albino rats there was a suggestion that the calretinin-immunoreactivity was affected. This was never seen in the retinas of the pigmented animals. The GABA-immunoreactivity in the retina of both albino and pigmented rats appeared to be unaffected by ischaemia/reperfusion. The data presented show that retinas from albino rats are more susceptible to ischaemia/reperfusion than retinas from pigmented animals. The results also show that reduction of the b-wave of the ERG and changes in the nature of the ChAT immunoreactivity represent sensitive markers to detect the effect of ischaemia/reperfusion to the retina.

alpha-melanocyte-stimulating hormone modulates nitric oxide production in melanocytes

We have previously observed that melanocytes produce nitric oxide in response to ultraviolet radiation and lipopolysaccharide and in this study have examined how these responses are affected by alpha-melanocyte-stimulating hormone. Nitric oxide production by cultured cells was measured electrochemically in real time using an ISO-nitric oxide sensor probe. B16 mouse melanoma cells released nitric oxide in response to lipopolysaccharide and the effects were enhanced in cells that had been grown in the presence of 10-11-10-9 M alpha-melanocyte-stimulating hormone prior to stimulation. At concentrations in excess of 10-9 M alpha-melanocyte-stimulating hormone decreased nitric oxide production. Preincubation with lipopolysaccharide, a well-known inducer of inducible nitric oxide synthase, also increased nitric oxide production but this response was reduced by alpha-melanocyte-stimulating hormone. alpha-Melanocyte-stimulating hormone also increased the levels of nitric oxide produced in response to ultraviolet radiation (20-100 mJ per cm2) in B16 cells. The same effect was seen in human melanocytes and as this was inhibited by aminoguanidine would appear to involve an induction of inducible nitric oxide synthase. Reverse transcription-polymerase chain reaction showed that melanocytic cells express inducible nitric oxide synthase mRNA. Western blotting analysis and immunocytochemistry confirmed the presence of inducible nitric oxide synthase protein in B16 cells and FM55 human melanoma cells and that the levels were increased in response to alpha-melanocyte-stimulating hormone. alpha-Melanocyte-stimulating hormone, however, decreased inducible nitric oxide synthase protein expression, which occurred in response to lipopolysaccharide. These results suggest that alpha-melanocyte-stimulating hormone regulates nitric oxide production in melanocytic cells by modulating the induction of inducible nitric oxide synthase. Additional experiments showed that nitric oxide increased melanin production by B16 cells and human melanocytes. This is in keeping with a melanogenic role for nitric oxide but whether its production by melanocytes in response to alpha-melanocyte-stimulating hormone is associated with such a role or whether it has some other significance relating to melanocyte differentiation or in mediating immunomodulatory actions of alpha-melanocyte-stimulating hormone remains to be seen.

alpha-MSH and the regulation of melanocyte function

alpha-MSH, has numerous actions in the skin and by activating the MC1 receptor (MC1-R) on melanocytes it stimulates melanogenesis. Rather than producing large increase in melanin production alpha-MSH acts specifically to stimulate eumelanin synthesis. Although this could be important in determining skin color and tanning there is debate as to the pigmentary significance of alpha-MSH in humans. Circulating levels of alpha-MSH are negligible and although it is produced in the skin by different cell types, including melanocytes, the major skin form is desacetyl alpha-MSH, and this is a weak agonist at MC1-R. Certain ACTH peptides, notably ACTH1-17, are more potent agonists at the MC1-R and, since their skin concentrations exceed those of alpha-MSH, they could serve as natural ligands at this receptor and regulate pigmentary responses in humans. Activation of MC1-R does, however, produce other responses in human melanocytes. Thus, alpha-MSH stimulates melanocyte dendricity and attachment to extracellular matrix proteins. It also protects melanocytes from the damaging effects of oxidative stress, and regulates their production of NO by modulating the induction of iNOS--as it does within macrophages. alpha-MSH clearly affects various aspects of melanocyte behavior and its melanogenic effects could be the consequence of a more fundamental role in the melanocyte. The precise nature of this role is unclear, but it could be part of a generic role that alpha-MSH and other POMC peptides have in skin homeostasis.

The depigmenting effect of alpha-tocopheryl ferulate on human melanoma cells

Oral vitamin E (alpha-tocopherol, alpha-T) supplementation has been reported to improve facial hyperpigmentation. alpha-Tocopheryl ferulate (alpha-TF) is a compound of alpha-T and ferulic acid connected by an ester bond; ferulic acid is also an antioxidant, and could scavenge free radicals induced by ultraviolet (UV) radiation, and thus maintain the long-lasting antioxidative effect of alpha-T. Our aim was to see whether alpha-TF might be useful as a whitening agent and an antioxidant to improve and prevent facial hyperpigmentation following UV exposure. In this study, the inhibitory effect of alpha-TF on melanogenesis was examined biochemically using human melanoma cells in culture. The results show that alpha-TF, solubilized in ethanol or in 0.5% lecithin, inhibited melanization significantly, as did alpha-T at a concentration of 100 microg/mL, without inhibiting cell growth. This phenotypic change was associated with inhibition of tyrosinase and 5, 6-dihydroxyindole-2-carboxylic acid polymerase activities, and the degree of inhibition was dose dependent. No significant effect on DOPAchrome tautomerase activity was observed. alpha-TF did not directly inhibit tyrosinase activity of the large granule fraction extracted from human melanoma cells, and Western blotting revealed that there were no changes in protein content or in molecular size of tyrosinase, tyrosinase-related protein (TRP)-1 or TRP-2. Therefore, the inhibition of tyrosinase activity by alpha-TF might be due to effects at the post-translational level, and possibly by a secondary molecule activated by alpha-TF. These results suggest that alpha-TF is a candidate for an efficient whitening agent which suppresses melanogenesis and inhibits biological reactions induced by reactive oxygen species.

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

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

Does alpha-MSH have a role in regulating skin pigmentation in humans?

Over the years there has been much debate as to whether alpha-MSH has a role as a pigmentary hormone in humans. There are two main reasons for this. First, despite the observations in the 1960s that alpha-MSH increased skin darkening in humans, there are reports that the peptide has no effect on melanogenesis in cultured human melanocytes. Second, the human pituitary, unlike that of most mammals, secretes very little alpha-MSH and circulatory levels of the peptide in humans are extremely low. However, there is now evidence from several groups that alpha-MSH is capable of stimulating melanogenesis in cultured human melanocytes. Rather than producing an overall increase in melanin production, it appears that the peptide acts specifically to increase the synthesis of eumelanin. Such an action could well explain the previously observed skin darkening effects of alpha-MSH. It is also now known that alpha-MSH is not produced exclusively in the pituitary but has been found at numerous sites, including the skin where it is produced by several cell types. Related Proopiomelanocortin (POMC) peptides such as ACTH are also produced in human skin. The ACTH peptides act at the same receptor (MC-1) as alpha-MSH and certain of these would appear to be more potent than alpha-MSH in stimulating melanogenesis. The ACTH peptides are also present in greater amounts than alpha-MSH in human epidermis and it is likely that they play an important role in regulating pigmentary responses. These POMC peptides are released from keratinocytes in response to ultraviolet radiation (UVR) and it has been proposed that they serve as paracrine factors in mediating UV induced pigmentation. Their production by keratinocytes could therefore be critical in determining pigmentary responses and any changes in the availability of these POMC peptides might explain the variations in tanning ability seen in different individuals. However, the possibility that tanning ability is also dependent upon differences at the level of the MC-1 receptor cannot be ruled out and it has been suggested that an inability to tan may depend upon the presence of non-functional changes at the MC-1 receptor. alpha-MSH does, of course, affect human melanocytes in several ways and its stimulation of melanogenesis could be the consequence of some other fundamental action in the melanocyte. The peptide also has many other target sites in the skin and while it may have a role in regulating skin pigmentation in humans, it should not be viewed solely as a pigmentary peptide. alpha-MSH clearly has many different actions and its primary role in the skin may be to maintain homeostasis.

6-Tetrahydrobiopterin functions as a UVB-light switch for de novo melanogenesis

(6R)-L-erythro 5,6,7,8-tetrahydrobiopterin (6-BH4) and its 7-isomer (7-BH4) function as uncompetitive inhibitors of human and mushroom tyrosinases. Stoichiometry for the binding of [3H]-labeled 6-BH4 to both tyrosinases has been established as 1:1. Stable complexation of 6-BH4 to tyrosinase appears to involve a hydrophilic conserved glutamic acid (Glu131) with a pKa = 4.7. Photo-oxidation by UVB-light and O2 reverses the inhibition of tyrosinase by 6-BH4 and 7-BH4 with the 6-BH4/tyrosinase complex being four-fold more photolabile than 7-BH4/tyrosinase. The photo-oxidation of 6-BH4 by UVB-light can be assessed spectrophotometrically with this reaction yielding 7,8-dihydroxanthopterin as the final product, 7,8-Dihydroxanthopterin neither binds to nor inhibits tyrosinase. By contrast, UVA light does not catalyze the photodegradation of 6-BH4. Taken together, our results indicate that the photo-oxidation of the tetrahydrobiopterins by UVB may represent a photo-switch in the regulation of tyrosinase activity to promote de novo melanogenesis.

Pteridines in the control of pigmentation

The influence of UVB irradiation on the metabolic pathway for the production of L-tyrosine from L-phenylalanine in the human epidermis has been examined in 12 healthy volunteers with photo skin types I-VI (Fitzpatrick classification). This metabolic pathway involves the induction of GTP-cyclohydrolase 1 (GTP-CH-1), the rate-limiting enzyme for de novo synthesis of (6R)L-erythro-5,6,7,8-tetrahydrobiopterin (6-BH4). This essential cofactor controls the production of L-tyrosine from L-phenylalanine via phenylalanine hydroxylase (PAH). The de novo synthesis of 6-BH4 depends on the induction of GTP-CH-1, e.g., by tumor necrosis factor-alpha (TNF alpha). Epidermal suction blister tissues were taken before (0 h) and after (24 and 72 h) UVB exposure with a standardized dosage [1 minimal erythema dose (MED)]. In all cases, there was a significant increase in TNF alpha release, GTP-CH-1 activity, total 6-biopterin level, and PAH activity, indicative of enhanced L-tyrosine production. The response of this metabolic cascade over baseline activities was pronounced in fair photo skin types (I-III) compared to dark skin (IV-VI). Taken together, our results suggest that UVB can control the direct supply of L-tyrosine in the epidermis, and this process may represent an important factor in de novo melanogenesis.

Tyrosinase may protect human melanocytes from the cytotoxic effects of the superoxide anion

Reactive oxygen species (ROS) are potentially cytotoxic and several mechanisms have evolved to protect against their damaging effects. In melanocytes, tyrosinase may have such a rôle by utilising the superoxide anion (O2-) in the production of melanin. In the present study, we have examined the cytotoxic effects of O2- and hydrogen peroxide (H2O2) in human melanocytes both before and following the activation of tyrosinase. Xanthine oxidase (XO, 5-150 mU.ml-1) and glucose oxidase (GO, 0.1-20 mU.ml-1) were used to generate the O2- and H2O2 respectively, and the cytotoxic effects assessed by measuring cell survival using the 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyltetrazolium (MTT) assay. 3 h later, dose-related decreases in melanocyte survival were seen. Similar experiments with keratinocytes and fibroblasts showed that these cells were more resistant to the cytotoxic effects of O2- than were the melanocytes. The effect of increasing tyrosinase activity was examined by growing the melanocytes in the presence of an analogue of melanocyte-stimulating hormone (MSH) Nle4DPhe7 alpha-MSH (10(-8) M), for 48 h. This increased tyrosinase activity, melanin content, the ability to trap O2- and the resistance of the melanocytes to the cytotoxic effects of this ROS, but failed to alter their susceptibility to the damaging effects of H2O2.Nle4DPhe7 alpha-MSH had no effect on the resistance of keratinocytes and fibroblasts to either O2- or H2O2. After 3 h, XO, as opposed to GO, also increased the melanin content of human melanocytes; this effect was not accompanied by an increase in tyrosinase activity. The present results suggest that tyrosinase may utilise O2- to produce melanin and that this process may protect melanocytes from the potentially damaging effects of this ROS.