Oxidative stress in vitiligo: photo-oxidation of pterins produces H(2)O(2) and pterin-6-carboxylic acid

Tetrahydrobiopterin as a Trigger for Vitiligo: Phototransformation during UV Irradiation

Vitiligo is a type of hypomelanosis. Tetrahydrobiopterin (H4Bip), the coenzyme of the initial stage of melanogenesis, appears to be a trigger for vitiligo. H4Bip is present in vitiligo in 3-5-fold excess and causes oxidative stress by triggering an autocatalytic cycle of excess hydrogen peroxide synthesis. Using quantum-chemical calculations, we have evaluated the possibility of H4Bip reactions occurring in the dark and under ultraviolet (UV) irradiation, including the formation of dihydropterin dimers. In order to simulate the oxidative stress, oxidative modification of human serum albumin (HSA) has been carried out in the presence of excessive H4Bip using the fluorescence method. The fraction of oxidized protein (FOP) has been calculated. It has been established that there is a strong oxidative modification of amino acids chromophores (tryptophan and tyrosine) in the protein (FOP 0.64). Under UV irradiation of the system (HSA + H4Bip), FOP is reduced to 0.39. Apparently, a part of H4Bip transforms into dihydropterin dimers and does not participate in the oxidative modification of the protein. The data on oxidative modification of HSA are consistent with dynamic light scattering: H4Bip promotes HSA aggregation with the formation of particles with a hydrodynamic radius Rh ≥ 2000 nm, which can become immunogenic.

Increased anti-oxidative action compensates for collagen tissue degeneration in vitiligo dermis

Vitiligo is a common depigmentation disorder characterized by the selective loss of melanocytes. In our daily clinic experience, we noticed that the skin tightness of hypopigmented lesions would be more evident in comparison to that of uninvolved perilesional skin in vitiligo patients. Therefore, we hypothesized that collagen homeostasis might be maintained in vitiligo lesions, irrespective of the substantial excessive oxidative stress that occurs in association with the disease. We found that the expression levels of collagen-related genes and anti-oxidative enzymes were upregulated in vitiligo-derived fibroblasts. Abundant collagenous fibers were observed in the papillary dermis of vitiligo lesions in comparison to uninvolved perilesional skin by electron microscopy. The production of matrix metalloproteinases that degraded collagen fibers was suppressed. The deposition of acrolein adduct protein, which is a product of oxidative stress, was significantly reduced in vitiligo dermis and fibroblasts. As part of the mechanism, we found upregulation of the NRF2 signaling pathway activity, which is an important defense system against oxidative stress. Taken together, we demonstrated that the anti-oxidative action and collagen production were upregulated and that the collagen degeneration was attenuated in vitiligo dermis. These new findings may provide important clues for the maintenance of antioxidant ability in vitiligo lesions.

Endogenous Photosensitizers in Human Skin

Endogenous photosensitizers play a critical role in both beneficial and harmful light-induced transformations in biological systems. Understanding their mode of action is essential for advancing fields such as photomedicine, photoredox catalysis, environmental science, and the development of sun care products. This review offers a comprehensive analysis of endogenous photosensitizers in human skin, investigating the connections between their electronic excitation and the subsequent activation or damage of organic biomolecules. We gather the physicochemical and photochemical properties of key endogenous photosensitizers and examine the relationships between their chemical reactivity, location within the skin, and the primary biochemical events following solar radiation exposure, along with their influence on skin physiology and pathology. An important take-home message of this review is that photosensitization allows visible light and UV-A radiation to have large effects on skin. The analysis presented here unveils potential causes for the continuous increase in global skin cancer cases and emphasizes the limitations of current sun protection approaches.

Insights into Molecular Structure of Pterins Suitable for Biomedical Applications

Pterins are an inseparable part of living organisms. Pterins participate in metabolic reactions mostly as tetrahydropterins. Dihydropterins are usually intermediates of these reactions, whereas oxidized pterins can be biomarkers of diseases. In this review, we analyze the available data on the quantum chemistry of unconjugated pterins as well as their photonics. This gives a comprehensive overview about the electronic structure of pterins and offers some benefits for biomedicine applications: (1) one can affect the enzymatic reactions of aromatic amino acid hydroxylases, NO synthases, and alkylglycerol monooxygenase through UV irradiation of H₄pterins since UV provokes electron donor reactions of H₄pterins; (2) the emission properties of H₂pterins and oxidized pterins can be used in fluorescence diagnostics; (3) two-photon absorption (TPA) should be used in such pterin-related infrared therapy because single-photon absorption in the UV range is inefficient and scatters in vivo; (4) one can affect pathogen organisms through TPA excitation of H₄pterin cofactors, such as the molybdenum cofactor, leading to its detachment from proteins and subsequent oxidation; (5) metal nanostructures can be used for the UV-vis, fluorescence, and Raman spectroscopy detection of pterin biomarkers. Therefore, we investigated both the biochemistry and physical chemistry of pterins and suggested some potential prospects for pterin-related biomedicine.

Pterin-lysine photoadduct: a potential candidate for photoallergy

Photoallergy is a photosensitivity disorder associated with a modified ability of the skin to react to the combined effect of drugs and sunlight. It has been attributed to the covalent conjugation of proteins with a photosensitizer, yielding modified macromolecules that can act as antigen provoking the immune system response. The potential role of some endogenous compounds as photoallergens has not been fully established. It has been previously proposed that pterins, which are endogenous photosensitizers present in human skin under pathological conditions, are able to covalently bind to proteins. Here, we evaluated the capability of pterin (Ptr) to form photoadducts with free Lysine (Lys) and poly-L-lysine (poly-Lys). The findings obtained using chromatographic and spectroscopic tools, confirm the formation of photoadducts of Ptr with Lys residues. With poly-Lys the resulting adduct retains the spectroscopic properties of the photosensitizer, suggesting that the aromatic Ptr structure is conserved. On the other hand, the photoproduct formed with free Lys does not behave like Ptr, which suggests that if this product is a photoadduct, a chemical modification may have occurred during the photochemical reaction that alters the pterin moiety.

Assessing the Oxidative State of the Skin by Combining Classical Tape Stripping with ORAC Assay

The antioxidant barrier system of the skin acts as the main defence against environmental pro-oxidants. Impaired skin oxidative state is linked to unhealthy conditions such as skin autoimmune diseases and cancer. Thus, the evaluation of the overall oxidative state of the skin plays a key role in further understanding and prevention of these disorders. This study aims to present a novel ex vivo model to evaluate the skin oxidative state by the measurement of its antioxidant capacity (AOC). For this the ORAC assay was combined with classical tape stripping and infrared densitometry to evaluate the oxidative state of the stratum corneum (SC). Outcomes implied the suitability of the used model to determine the intrinsic antioxidant capacity (iAOC) of the skin. The average iAOC of untreated skin was determined as 140 ± 7.4 µM TE. Skin exposure to UV light for 1 h reduced the iAOC by about 17%, and exposure for 2 h decreased the iAOC by about 30%. Treatment with ascorbic acid (AA) increased the iAOC in a dose-dependent manner and reached an almost two-fold iAOC when 20% AA solution was applied on the skin. The application of coenzyme Q10 resulted in an increase in the iAOC at low doses but decreased the iAOC when doses > 1% were applied on the skin. The results show that the combination of classical tape stripping and ORAC assay is a cost-effective and versatile method to evaluate the skin oxidative state and the pro-oxidate and antioxidative effects of topical skin treatments on the iAOC of the skin. Therefore, the model can be considered to be a valuable tool in skin research.

Photosensitized Dimerization of Tyrosine: The Oxygen Paradox†

In electron-transfer initiated photosensitization processes, molecular oxygen (O₂ ) is not involved in the first bimolecular event, but almost always participates in subsequent steps giving rise to oxygenated products. An exception to this general behavior is the photosensitized dimerization of tyrosine (Tyr), where O₂ does not participate as a reactant in any step of the pathway yielding Tyr dimers (Tyr₂ ). In the pterin (Ptr) photosensitized oxidation of Tyr, O₂ does not directly participate in the formation of Tyr₂ and quenches the triplet excited state of Ptr, the reactive species that initiates the process. However, O₂ is necessary for the dimerization, phenomenon that we have named as the oxygen paradox. Here, we review the literature on the photosensitized formation of Tyr₂ and present results of steady-state and time resolved experiments, in search of a mechanistic model to explain the contradictory role of O₂ in this photochemical reaction system.

Properties and reactivity of the folic acid and folate photoproduct 6-formylpterin

Folates (vitamin B9) are essential components of our diet and our gut microbiota. They are omnipresent in our cells and blood. Folates are necessary for DNA synthesis, methylation, and other vital bioprocesses. Folic acid (FA), as the synthetic form of folates, is largely found in supplements and fortified foods. FA and folate drugs are also extensively used as therapeutics. Therefore, we are continuously exposed to the pterin derivatives, and their photo-degradation products, such as 6-formylpterin (6-FPT) and pterin-6-carboxylic acid. During ultraviolet radiation, these two photolytic products generate reactive oxygen species (ROS) responsible for the cellular oxidative stress. 6-FPT can exhibit variable pro/anti-oxidative roles depending on the cell type and its environment (acting as a cell protector in normal cells, or as an enhancer of drug-induced cell death in cancer cells). The ROS-modulating capacity of 6-FPT is well-known, whereas its intrinsic reactivity has been much less investigated. Here, we have reviewed the properties of 6-FPT and highlighted its capacity to form covalent adducts with the ROS-scavenging drug edaravone (used to treat stroke and amyotrophic lateral sclerosis) as well as its implication in immune surveillance. 6-FPT and its analogue acetyl-6-FPT function as small molecule antigens, recognized by the major histocompatibility complex-related class I-like molecule, MR1, for presentation to mucosal-associated invariant T (MAIT) cells. As modulators of the MR1/MAIT machinery, 6-FPT derivatives could play a significant immuno-regulatory role in different diseases. This brief review shed light on the multiple properties and cellular activities of 6-FPT, well beyond its primary ROS-generating activity.

Wavelengths and temporal effects on the response of mammalian cells to UV radiation: Limitations of action spectra illustrated by genotoxicity

All photobiological events depend on the wavelength of the incident radiation. In real-life situations and in the vast majority of laboratory experiments, exposure always involves sources with various emission spectra spreading over a wide wavelength range. Action spectra are often used to describe the efficiency of a process at different wavelengths and to predict the effects of a given light source by summation of the individual effects at each wavelength. However, a full understanding of the biological effects of complex sources requires more than considering these concomitant events at each specific wavelength. Indeed, photons of different energies may not have additive but synergistic or inhibitory effects on photochemical processes and cellular responses. The evolution of a photobiological response with post-irradiation time must also be considered. These two aspects may represent some limitations to the use of action spectra. The present review, focused on mammalian cells, illustrates the concept of action spectrum and discusses its drawbacks using theoretical considerations and examples taken from the literature. Emphasis is placed on genotoxicity for which wavelength effects have been extensively studied. Other effects of UV exposure are also mentioned.

Oxidation of tyrosine: Antioxidant mechanism of l-DOPA disclosed

Tyrosine is an amino acid related to crucial physiological events and its oxidation, that produce beneficial or detrimental effects on biological systems, has been extensively studied. Degradation of tyrosine often begins with the loss of an electron in an electron transfer reaction in the presence of a suitable electron acceptor. The reaction is facilitated by excited states of the acceptor in photosensitized processes. Several products of tyrosine oxidation have been described, the main ones being 3,4-dihydroxy-l-phenylalanine (commonly known as DOPA) and tyrosine dimers. Here, we report tyrosine recovery from tyrosyl radical, after one-electron oxidation, in the presence of DOPA. We propose that under high oxidative stress the oxidation of tyrosine may be controlled, in part, by one of its oxidation products. Also, we present strong evidence of antioxidant action of DOPA by preventing tyrosine dimerization, one of the most serious oxidative protein modifications, and the origin of structural modifications leading to the loss of protein functionality.

Theoretical determination of two-photon absorption in biologically relevant pterin derivatives

Given the prevalence of fluorescence spectroscopy in biological systems, and the prevalence of pterin derivatives throughout biological systems, presented here is an assessment of the two-photon absorption spectroscopy as it applies to a range of the most commonly studied pterin derivatives. QR-CAMB3LYP//ccpVTZ calculations suggest that the use of two-photon spectroscopic methods would enable a more capable differentiation between closely related derivatives in comparison to the one-photon spectra, which show minimal qualitative deviation. Study of short tail derivatives shows that, in most cases, two-photon accessible states solely involve the π* LUMO as the particle orbital, with biopterin, neopterin, and 6-(hydroxymethyl)pterin presenting exceptional potential for targetting. Investigation of derivatives in which the tail contains an aromatic ring resulted in the observation of a series of two-photon accessible states involving charge transfer from the tail to the pterin moiety, the cross sections of which are highly dependent on the adoption of a planar geometry. The observation of these states presents a novel method for tracking the substitution of biologically important molecules such as folic acid and 5-methenyltetrahydrofolylpolyglutamate.

Type I Photosensitized Oxidation of Methionine†

Methionine (Met) is an essential sulfur-containing amino acid, sensitive to oxidation. The oxidation of Met can occur by numerous pathways, including enzymatic modifications and oxidative stress, being able to cause relevant alterations in protein functionality. Under UV radiation, Met may be oxidized by direct absorption (below 250 nm) or by photosensitized reactions. Herein, kinetics of the reaction and identification of products during photosensitized oxidation were analyzed to elucidate the mechanism for the degradation of Met under UV-A irradiation using pterins, pterin (Ptr) and 6-methylpterin (Mep), as sensitizers. The process begins with an electron transfer from Met to the triplet-excited state of the photosensitizer (Ptr or Mep), to yield the corresponding pair of radicals, Met radical cation (Met^•+ ) and the radical anion of the sensitizer (Sens^•- ). In air-equilibrated solutions, Met^•+ incorporates one or two atoms of oxygen to yield methionine sulfoxide (MetO) and methionine sulfone (MetO₂ ), whereas Sens^•- reacts with O₂ to recover the photosensitizer and generate superoxide anion (O₂ ^•- ). In anaerobic conditions, further free-radical reactions lead to the formation of the corresponding dihydropterin derivatives (H₂ Ptr or H₂ Mep).

Photo-oxidation of lysozyme triggered by riboflavin is O2-dependent, occurs via mixed type 1 and type 2 pathways, and results in inactivation, site-specific damage and intra- and inter-molecular crosslinks

Photosensitized protein oxidation is a promising tool for medical procedures such as photochemical tissue bonding (PTB). We have recently reported that the binding of rose Bengal, a sensitizer employed in PTB, to lysozyme modulates the photooxidation and crosslinking of this protein. In this work we examined the photooxidation and crosslinking of lysozyme mediated by riboflavin (RF) an endogenous sensitizer also employed in PTB. We hypothesized that since RF does not bind strongly to proteins, the mechanism(s) and extent of enzymatic inactivation, amino acid modification and protein crosslinking would be dependent on the presence of O₂, and differ to that induced by rose Bengal. This hypothesis was tested using UV-visible spectrophotometry, isothermal titration calorimetry (ITC), SDS-PAGE gels, quantification of amino acid consumption, and LC-MS analysis of sites of modification and crosslinks. Under N₂, limited damage was detected arising from type 1 (radical) chemistry with formation of specific intra- (Tyr20-Tyr23) and inter- (Tyr23-Trp108) molecular crosslinks. In contrast, the presence of O₂ triggered extensive protein damage through mixed type 1 and type 2 (¹O₂) mechanisms leading to Trp, Met, Tyr and His oxidation, loss of enzymatic activity and protein dimerization. LC-MS analysis provided evidence for crosslinking via radical-radical recombination reactions (Trp28-Tyr53), and secondary reactions involving nucleophilic attack of the side-chain amine of Lys116 on carbonyl groups. Overall, this behavior is in marked contrast to that detected with rose Bengal indicating that the mechanisms and sites of photo-oxidative damage, and consequences for protein function, can be modulated by the choice of sensitizing dye.

Riboflavin Plays a Pivotal Role in the UVA-Induced Cytotoxicity of Fibroblasts as a Key Molecule in the Production of H2O2 by UVA Radiation in Collaboration with Amino Acids and Vitamins

To investigate environmental factors that contribute to ultraviolet A (UVA)-induced oxidative stress, which accelerates the senescence and toxicity of skin cells, we irradiated human fibroblasts cultured in commonly used essential media with UVA and evaluated their viability and production of reactive oxygen species. The viability of fibroblasts exposed to a single dose of 3.6 J/cm² UVA was not reduced when cultured in Hanks balanced salt solution, but it was significantly decreased when cultured in Dulbecco’s modified Eagle’s medium (DMEM), which contains various amino acids and vitamins. Furthermore, cell viability was not reduced when fibroblasts were cultured in DMEM and treated with a hydrogen peroxide (H₂O₂) scavenger such as glutathione or catalase added after UVA irradiation. In addition, we confirmed that the production of H₂O₂ was dramatically increased by UVA photosensitization when riboflavin (R) coexisted with amino acids such as tryptophan (T), and found that R with folic acid (F) produced high levels of H₂O₂ after UVA irradiation. Furthermore, we noticed that R and F or R and T have different photosensitization mechanisms since NaN₃, which is a singlet oxygen quencher, suppressed only R and T photosensitization. Lastly, we examined the effects of antioxidants (L-ascorbic acid, trolox, L-cysteine, and L-histidine), which are singlet oxygen or superoxide or H₂O₂ scavengers, on R and F or on R and T photosensitization, and found that 1 mM ascorbic acid, Trolox, and L-histidine were strongly photosensitized with R, and produced significant levels of H₂O₂ during UVA exposure. However, 1 mM L-cysteine dramatically suppressed H₂O₂ production by UVA photosensitization. These data suggest that a low concentration of R-derived photosensitization is elicited by different mechanisms depending on the coexisting vitamins and amino acids, and regulates cellular oxidative stress by producing H₂O₂ during UVA exposure.

Photo-induced protein oxidation: mechanisms, consequences and medical applications

Irradiation from the sun has played a crucial role in the origin and evolution of life on the earth. Due to the presence of ozone in the stratosphere most of the hazardous irradiation is absorbed, nonetheless UVB, UVA, and visible light reach the earth’s surface. The high abundance of proteins in most living organisms, and the presence of chromophores in the side chains of certain amino acids, explain why these macromolecules are principal targets when biological systems are illuminated. Light absorption triggers the formation of excited species that can initiate photo-modification of proteins. The major pathways involve modifications derived from direct irradiation and photo-sensitized reactions. In this review we explored the basic concepts behind these photochemical pathways, with special emphasis on the photosensitized mechanisms (type 1 and type 2) leading to protein oxidation, and how this affects protein structure and functions. Finally, a description of the photochemical reactions involved in some human diseases, and medical applications of protein oxidation are presented.

Sulfasalazine-induced generalized vitiligo in a patient with dermatitis herpetiformis and celiac disease

Vitiligo is an acquired idiopathic pigmentary skin disorder characterized by the development of white macules and patches due to the loss of functioning melanocytes. In this report, we describe a case of a patient with a longstanding history of dermatitis herpetiformis (DH) and celiac disease that developed rapidly progressing, biopsy-confirmed generalized vitiligo after 11 months of treatment with anti-inflammatory medication sulfasalazine, prescribed for the patient's DH. To the best of our knowledge, this is the first case report which has demonstrated the possible biochemical pathways, triggered by sulfasalazine, in the development of vitiligo.

Photo-Oxidation of Unilamellar Vesicles by a Lipophilic Pterin: Deciphering Biomembrane Photodamage

Pterins are natural products that can photosensitize the oxidation of DNA, proteins, and phospholipids. Recently, a new series of decyl-chain (i.e., lipophilic) pterins were synthesized and their photophysical properties were investigated. These decyl-pterins led to efficient intercalation in large unilamellar vesicles and produced, under UVA irradiation, singlet molecular oxygen, a highly oxidative species that react with polyunsaturated fatty acids (PUFAs) to form hydroperoxides. Here, we demonstrate that the association of 4-(decyloxy)pteridin-2-amine ( O-decyl-Ptr) to lipid membranes is key to its ability to trigger phospholipid oxidation in unilamellar vesicles of phosphatidylcholine rich in PUFAs used as model biomembranes. Our results show that O-decyl-Ptr is at least 1 order of magnitude more efficient photosensitizer of lipids than pterin (Ptr), the unsubstituted derivative of the pterin family, which is more hydrophilic and freely passes across lipid membranes. Lipid peroxidation photosensitized by O-decyl-Ptr was detected by the formation of conjugated dienes and oxidized lipids, such as hydroxy and hydroperoxide derivatives. These primary products undergo a rapid conversion into short-chain secondary products by cleavage of the fatty-acid chains, some of which are due to subsequent photosensitized reactions. As a consequence, a fast increase in membrane permeability is observed. Therefore, lipid oxidation induced by O-decyl-Ptr could promote cell photodamage due to the biomembrane integrity loss, which in turn may trigger cell death.

Neural and Endocrinal Pathobiochemistry of Vitiligo: Comparative Study for a Hypothesized Mechanism

The etiology of vitiligo is still unclear. The aim is to investigate a neural and hormonal etio-pathology of vitiligo. Sixty acrofacial vitiligo patients were divided into two subgroups as active vitiligo patients group (AVPs; n = 35) and stable vitiligo patients group (SVP; n = 25). Forty healthy subjects without any systemic or dermatological disease were used as controls. Blood samples were collected, and the samples were used for measurement of free triiodothyronine (fT₃), free thyroxine (fT₄), thyroid-stimulating hormone (TSH), adrenocorticotrophic hormone (ACTH), cortisol, estrogen, testosterone, melatonin, and prolactin levels by ELISA, while norepinephrine (NE), epinephrine (Epi), dopamine (DA), homo-vanillic acid (HVA), serotonin, and 5-hydroxyindoleacetic acid (5-HIAA) by high-pressure liquid chromatography. The current results showed a significant increase in plasma levels of Epi, NE, DA, HVA, serotonin, 5-HIAA, melatonin, and in serum level of TSH and prolactin either in SVP or AVP groups than the control group and in AVP than SVP group. The serum levels of fT₃ and fT₄ were significantly decreased either in SVP or AVP groups than the control group. A significant increase in estradiol levels was observed in females within AVP than females in either SVP or control groups. There was a significant increase in serum level of cortisol in AVP than either SVP or control group. There was a significant decrease in serum level of ACTH in either AVP or SVP than control and in AVP than SVP group. In conclusion, there are some neural and endocrine markers that play a pivotal role in pathogenesis and/or consequences of vitiligo. The abnormally disturbed levels of theses markers lead to melanocyte destruction and/or depigmentation.

The catalase gene promoter and 5'-untranslated region variants lead to altered gene expression and enzyme activity in vitiligo

BACKGROUND

Oxidative stress is considered to be the initial event in the course of vitiligo. The enzyme catalase (CAT) is mainly involved in cellular defence against oxidizing agents through detoxifying H₂ O₂ .

OBJECTIVES

The aims were (i) to assess erythrocyte CAT enzyme activity and lipid peroxidation (LPO) levels as well as CAT mRNA expression in skin and blood; (ii) to investigate CAT gene promoter rs7943316, rs1001179, 5'-untranslated region rs1049982, and exon (rs17886350, rs11032709, rs17880442, rs35677492) polymorphisms; and (iii) to perform genotype/haplotype-phenotype correlation analyses in patients with vitiligo and controls from Gujarat.

METHODS

CAT activity and LPO levels were measured spectrophotometrically. CAT mRNA levels were estimated using real-time polymerase chain reaction (PCR) by the SYBR Green method. Single-nucleotide polymorphism genotyping was performed using PCR-restriction fragment length polymorphism and amplification-refractory mutation system-PCR analyses.

RESULTS

Patients with vitiligo showed significantly decreased CAT mRNA expression in lesional and nonlesional skin and in blood, with reduced CAT activity compared with that of controls. CAT -89A/T and -20T/C polymorphisms were significantly associated with patients, especially with active and generalized vitiligo, whereas no association was observed for -262G/A and exon polymorphisms. The A_-262 T_-89 C_-20 haplotype with variant alleles was found to be associated with 6·4-fold risk of vitiligo. Genotype/haplotype-phenotype correlation analyses revealed that individuals with susceptible genotypes/haplotype for CAT -89A/T and -20T/C polymorphisms showed significantly decreased CAT mRNA/activity, and only -89A/T polymorphisms showed significantly increased LPO levels compared with wild-type genotypes/haplotype.

CONCLUSIONS

The present study proposes the crucial role of CAT and its allelic variants in oxidative stress-mediated pathogenesis of vitiligo.

Photosensitization of peptides and proteins by pterin derivatives

Proteins are one of the preferential targets of the photosensitized damaging effects of ultraviolet (UV) radiation on biological system. Pterins belong to a family of heterocyclic compounds, which are widespread in living systems and participate in relevant biological functions. In pathological conditions, such as vitiligo, oxidized pterins accumulate in the white skin patches of patients suffering this depigmentation disorder. It is known that pterins are able to photosensitize damage in nucleotides and DNA by type I (electron transfer) and type II (singlet oxygen) mechanisms. Recently, it has been demonstrated that proteins and its components may also be damaged when solutions containing both proteins and pterin are exposed to UV-A radiation. Therefore, given the biological and medical relevance of the photosensitizing properties of these molecules, we present in this article an overview of the capability of different pterin derivatives to photoinduce damage in proteins present in the skin, focusing our attention on the chemical modifications of tyrosine and tryptophan residues.

Mapping architectural and transcriptional alterations in non-lesional and lesional epidermis in vitiligo

In vitiligo, chronic loss of melanocytes and consequent absence of melanin from the epidermis presents a challenge for long-term tissue maintenance. The stable vitiligo patches are known to attain an irreversible depigmented state. However, the molecular and cellular processes resulting in this remodeled tissue homeostasis is unclear. To investigate the complex interplay of inductive signals and cell intrinsic factors that support the new acquired state, we compared the matched lesional and non-lesional epidermis obtained from stable non-segmental vitiligo subjects. Hierarchical clustering of genome-wide expression of transcripts surprisingly segregated lesional and non-lesional samples in two distinct clades, despite the apparent heterogeneity in the lesions of different vitiligo subjects. Pathway enrichment showed the expected downregulation of melanogenic pathway and a significant downregulation of cornification and keratinocyte differentiation processes. These perturbations could indeed be recapitulated in the lesional epidermal tissue, including blunting of rete-ridges, thickening of stratum corneum and increase in the size of corneocytes. In addition, we identify marked increase in the putrescine levels due to the elevated expression of spermine/spermidine acetyl transferase. Our study provides insights into the intrinsic self-renewing ability of damaged lesional tissue to restore epidermal functionality in vitiligo.

Development and in vitro assessment of psoralen and resveratrol co-loaded ultradeformable liposomes for the treatment of vitiligo

Vitiligo is a de-pigmenting skin disorder characterized by white patches on skin due to partial or complete loss of melanocytes. Psoralen in combination with ultraviolet-A (PUVA) acts by stimulation of melanin content and tyrosinase activity in melanocytes. Resveratrol, a sirtuin activator and a potential anti-oxidant reduce oxidative stress which is one of the triggering factors for initiation of vitiligo. Despite their therapeutic activity, weak percutaneous permeability of psoralen and poor solubility of resveratrol hinders their effective topical administration. The aim of present study is to formulate ultradeformable liposomes (UDL) co-loaded with psoralen and resveratrol for evaluation of PUVA and anti-oxidant combination in vitiligo treatment. For this purpose, UDL composed of DC-Chol, cholesterol and sodium deoxy cholate were prepared for their co-delivery. Liposomal carriers were characterized and evaluated for their efficacy using B16F10 cell line. Free radical scavenging potential was also determined for these carriers by in vitro anti-oxidant assays. Optimal co-loaded UDL with particle size ranging from 120 to 130nm, zeta potential of +46.2mV, entrapment efficiency of 74.09% (psoralen) and 76.91% (resveratrol) were obtained. Compared to control, co-loaded UDL showed significant stimulation of melanin and tyrosinase activity with major contribution of psoralen. Further, co-loaded UDL also exhibited potential free radical scavenging activity where resveratrol played a key role. Hence, psoralen and resveratrol co-loaded UDL acts in vitiligo through dual mechanisms of action viz., stimulation of melanin and tyrosinase activity as well as by anti-oxidant activity. These findings indicate that psoralen and resveratrol co-loaded UDL has the promising therapeutic potential for the treatment of vitiligo.

Impact of high-mobility group box 1 on melanocytic survival and its involvement in the pathogenesis of vitiligo

BACKGROUND

Vitiligo is attributable to loss of functional melanocytes and is the most common acquired depigmenting disorder. Oxidative stress and intense ultraviolet irradiation are known to aggravate this condition. The nonhistone high-mobility group box 1 (HMGB1) DNA-binding protein is a physiological activator of immune responses, cellular proliferation and cell death. Although it is implicated in the pathogenesis of autoimmune diseases and cutaneous disorders, the precise role of HMGB1 in melanocytes has yet to be studied.

OBJECTIVES

To elucidate the effect of HMGB1 on melanocytic survival and its involvement in the pathogenesis of vitiligo.

METHODS

Melanocytes were treated with recombinant HMGB1 (rHMGB1). Thereafter, apoptosis-, autophagy- and melanogenesis-related molecules were detected. Ex vivo skin organ culture was performed after rHMGB1 treatment. Also, levels of HMGB1 were examined in blood and skin specimens from patients with vitiligo.

RESULTS

In this study, rHMGB1 increased expression of cleaved caspase 3 and decreased melanin production and expression of melanogenesis-related molecules. rHMGB1-induced caspase 3 activation was confirmed through preincubation with a pan-caspase inhibitor. In ex vivo experiments for the confirmation of HMGB1-induced melanocyte apoptosis, melanocyte disappearance and increased caspase 3 activation were observed in rHMGB1-treated skin tissues. In Western blot analysis and enzyme-linked immunosorbent assay, patients with active vitiligo showed significantly higher blood levels of HMGB1 (vs. healthy controls). Also, greater expression of HMGB1 was observed in vitiliginous skin (vs. uninvolved skin).

CONCLUSIONS

External stimuli (e.g. oxidative stress and ultraviolet irradiation) may trigger HMGB1 release by keratinocytes, thereby perpetuating vitiligo through HMGB1-induced melanocytic apoptosis.

Electronic relaxation pathways of the biologically relevant pterin chromophore

Femtosecond-to-microsecond transient absorption spectroscopy is used to report the ultrafast relaxation mechanism of 2-amino-1H-pteridin-4-one (pterin) for the first time.

Metabolic engineering of folate and its precursors in Mexican common bean (Phaseolus vulgaris L.)

Folate (vitamin B9) deficiency causes several health problems globally. However, folate biofortification of major staple crops is one alternative that can be used to improve vitamin intakes in populations at risk. We increased the folate levels in common bean by engineering the pteridine branch required for their biosynthesis. GTP cyclohydrolase I from Arabidopsis (AtGchI) was stably introduced into three common bean Pinto cultivars by particle bombardment. Seed-specific overexpression of AtGCHI caused significant increases of up to 150-fold in biosynthetic pteridines in the transformed lines. The pteridine boost enhanced folate levels in raw desiccated seeds by up to threefold (325 μg in a 100 g portion), which would represent 81% of the adult recommended daily allowance. Unexpectedly, the engineering also triggered a general increase in PABA levels, the other folate precursor. This was not observed in previous engineering studies and was probably caused by a feedforward mechanism that remains to be elucidated. Results from this work also show that common bean grains accumulate considerable amounts of oxidized pteridines that might represent products of folate degradation in desiccating seeds. Our study uncovers a probable different regulation of folate homoeostasis in these legume grains than that observed in other engineering works. Legumes are good sources of folates, and this work shows that they can be engineered to accumulate even greater amounts of folate that, when consumed, can improve folate status. Biofortification of common bean with folates and other micronutrients represents a promising strategy to improve the nutritional status of populations around the world.

Photosensitized reactions initiated by 6-carboxypterin: singlet and triplet reactivity

The photophysical and photochemical properties of 6-carboxypterin, a model compound for the pterin derivatives present at the sea surface microlayer, were studied and its singlet and triplet reactivity toward halides and selected organics demonstrated.

Vitiligo

Vitiligo is an acquired depigmenting disorder that affects 0.5% to 2% of the world population. Three different forms are classified according to the distribution of lesions; namely non-segmental, segmental and mixed vitiligo. Vitiligo is associated with polymorphisms in genes involved in the immune response and in melanogenesis. However, environmental factors are required for the development of manifest disease. In general, the diagnosis is clinical and no laboratory tests or biopsies are required. Metabolic alterations are central to current concepts in pathophysiology. They induce an increased generation of reactive oxygen species and susceptibility to mild exogenous stimuli in the epidermis. This produces a senescent phenotype of skin cells, leads to the release of innate immune molecules, which trigger autoimmunity, and ultimately causes dysfunction and death of melanocytes. Clinical management aims to halt depigmentation, and to either repigment or depigment the skin, depending on the extent of disease. New therapeutic approaches include stimulation of melanocyte differentiation and proliferation through α-melanocyte-stimulating hormone analogues and through epidermal stem cell engineering. Several questions remain unsolved, including the connection between melanocyte depletion and stem cell exhaustion, the underlying degenerative mechanisms and the biological mediators of cell death. Overall, vitiligo is an excellent model for studying degenerative and autoimmune processes and for testing novel approaches in regenerative medicine. For an illustrated summary of this Primer, visit: http://go.nature.com/vIhFSC.

Role of oxidative stress in the pathogenesis of vitiligo with special emphasis on the antioxidant action of narrowband ultraviolet B phototherapy

Objectives

To evaluate the role of oxidative stress in the pathogenesis of vitiligo and the effect of narrowband (NB) ultraviolet (UV) B phototherapy on oxidative stress markers.

Methods

Patients with vitiligo and healthy control subjects were included in the study. Patients in the vitiligo group were treated with an NB-UVB regimen (3 × weekly for 6 months). Erythrocyte superoxide dismutase activity (SOD), erythrocyte malonyldialdehyde (MDA) and erythrocyte glutathione peroxidase activity (GSH-Px) levels were assessed in all participants at baseline, and after NB-UVB phototherapy in patients with vitiligo.

Results

A total of 24 patients with vitiligo and 27 control subjects were included in the study. Before treatment, erythrocyte MDA levels were significantly higher, and SOD and GSH-Px levels were significantly lower, in patients with vitiligo compared with controls. NB-UVB phototherapy was associated with a significant reduction in MDA levels and a significant increase in GSH-Px levels, compared with baseline, in patients with vitiligo.

Conclusion

NB-UVB phototherapy may relieve oxidative stress in patients with vitiligo by reversing the oxidant–antioxidant imbalance that is considered to play a role in the pathogenesis of this disease.

Involvement of superoxide dismutase isoenzymes and their genetic variants in progression of and higher susceptibility to vitiligo

Oxidative stress has been implicated as the initial triggering event in vitiligo pathogenesis leading to melanocyte destruction. Here, we report a significant increase in oxidative stress in vitiligo patients as evidenced by high lipid peroxidation levels suggesting an imbalance in the antioxidant enzyme system as reported in our previous studies. This study examined the role of the enzymatic antioxidant SOD, which converts the pro-oxidant superoxide into H2O2, in vitiligo pathogenesis. The activity of three isoforms of SOD, i.e., SOD1, SOD2, and SOD3, was significantly higher in vitiligo patients. To identify the underlying mechanism for the increase in activities of SOD isoforms, we explored the SOD1, SOD2, and SOD3 genes for their genetic variations and transcript levels. The SOD2 Thr58Ile (rs35289490) and Leu84Phe (rs11575993) polymorphisms were significantly associated with vitiligo patients, and the Val16Ala (rs4880) polymorphism was associated with active vitiligo patients. Interestingly, SOD2 activity was contributed by these polymorphisms along with its increase in transcript levels in patients. SOD3 activity was associated with the Arg213Gly (rs8192291) polymorphism. The SOD3 transcript levels were also increased in patients, which might contribute to the increased SOD3 activity. However, we could not establish the genotype-phenotype correlation for SOD1 as we could not detect any novel or reported SNPs in SOD1. In addition, both transcript and protein levels of SOD1 were unchanged between patients and controls, though SOD1 activity was increased in patients. Activities of SOD isoforms also correlated with progression of the disease as the activity was higher in active cases of vitiligo compared to stable cases. Here, we report that SOD2 and SOD3 polymorphisms may be genetic risk factors for susceptibility and progression of vitiligo and hence the genetic makeup of an individual may form a basis for the effective treatment of the disease. Overall, our results suggest that increased activity of SOD isoforms under the influence of genetic factors may lead to accumulation of H2O2 in cytoplasmic, mitochondrial, and extracellular compartments resulting in oxidative damage to the melanocytes.

Tryptophan oxidation photosensitized by pterin

Pterins are normal components of cells and they have been previously identified as good photosensitizers under UV-A irradiation, inducing DNA damage and oxidation of nucleotides. In this work, we have investigated the ability of pterin (Ptr), the parent compound of oxidized pterins, to photosensitize the oxidation of another class of biomolecules, amino acids, using tryptophan (Trp) as a model compound. Irradiation of Ptr in the UV-A spectral range (350 nm) in aerated aqueous solutions containing Trp led to the consumption of the latter, whereas the Ptr concentration remained unchanged. Concomitantly, hydrogen peroxide (H₂O₂) was produced. Although Ptr is a singlet oxygen ((1)O₂) sensitizer, the degradation of Trp was inhibited in O₂-saturated solutions, indicating that a (1)O₂-mediated process (type II oxidation) was not an important pathway leading to Trp oxidation. By combining different analytical techniques, we could establish that a type I photooxidation was the prevailing mechanism, initiated by an electron transfer from the Trp molecule to the Ptr triplet excited state, yielding the corresponding radical ions (Trp(·+)/Trp(-H)· and Ptr(·-)). The Trp reaction products that could be identified by UPLC-mass spectrometry are in agreement with this conclusion.

Type I photosensitization of 2'-deoxyadenosine 5'-monophosphate (5'-dAMP) by biopterin and its photoproduct formylpterin

Biopterin (Bip) and its photoproducts 6-formylpterin (Fop) and 6-carboxypterin (Cap) accumulate in the skin of patients suffering from vitiligo, a chronic depigmentation disorder where the protection against UV radiation fails because of the lack of melanin. These compounds absorb in the UV-A inducing a potential photosensitizing action that can cause damage to DNA and other biomolecules. In this work, we have investigated the capability of these pterin derivatives (Pt) to act as photosensitizers under UV-A irradiation for the degradation of 2'-deoxyadenosine 5'-monophosphate (5'-dAMP) in aqueous solutions, as model DNA target. Steady-state and time-resolved experiments were performed and the effect of pH was evaluated. The results showed that photosensitized degradation of 5'-dAMP was only observed under acidic conditions, and a mechanistic analysis revealed the participation of the triplet excited state of the pterin derivatives ((3)Pt*) by electron transfer yielding the corresponding pair of radical ions (Pt(•-) and 5'-dAMP(•+)), with successive photosensitizer recovery by electron transfer from Pt(•-) to O2. Finally, 5'-dAMP(•+) participates in subsequent reactions to yield degradation products.

Vitiligo: interplay between oxidative stress and immune system

Vitiligo is a multifactorial polygenic disorder with a complex pathogenesis, linked with both genetic and non-genetic factors. The precise modus operandi for vitiligo pathogenesis has remained elusive. Theories regarding loss of melanocytes are based on autoimmune, cytotoxic, oxidant-antioxidant and neural mechanisms. Reactive oxygen species (ROS) in excess have been documented in active vitiligo skin. Numerous proteins in addition to tyrosinase are affected. It is possible that oxidative stress is one among the main principal causes of vitiligo. However, there also exists ample evidence for altered immunological processes in vitiligo, particularly in chronic and progressive conditions. Both innate and adaptive arms of the immune system appear to be involved as a primary event or as a secondary promotive consequence. There is speculation on the interplay, if any, between ROS and the immune system in the pathogenesis of vitiligo. The article focuses on the scientific evidences linking oxidative stress and immune system to vitiligo pathogenesis giving credence to a convergent terminal pathway of oxidative stress-autoimmunity-mediated melanocyte loss.

Photosensitizing properties of biopterin and its photoproducts using 2'-deoxyguanosine 5'-monophosphate as an oxidizable target

UV-A radiation (320-400 nm) induces damage to the DNA molecule and its components through photosensitized reactions. Biopterin (Bip) and its photoproducts 6-formylpterin (Fop) and 6-carboxypterin (Cap) accumulate in the skin of human beings suffering from vitiligo, a depigmentation disorder where the protection against UV radiation fails because of the lack of melanin. This study was aimed to evaluate the photosensitizing properties of oxidized pterins present in the skin and to elucidate the mechanisms involved in the photosensitized oxidation of purine nucleotides by pterins in vitro. For this purpose, steady-state and time-resolved experiments in acidic (pH 5.0-5.8) aqueous solution were performed using Bip, Fop and Cap as photosensitizers and the nucleotide 2'-deoxyguanosine 5'-monophosphate (dGMP) as an oxidizable target. The three pterin derivatives are able to photosensitize dGMP, being Fop the most efficient sensitizer. The reactions proceed through two competing pathways: (1) electron transfer from dGMP to triplet excited-state of pterins (type I mechanism) and (2) reaction of dGMP with (1)O(2) produced by pterins (type II mechanism). Kinetic analysis revealed that the electron transfer pathway is the main mechanism and the interaction of dGMP with the triplet excited-state of pterins and the formation of the corresponding dGMP radicals were demonstrated by laser flash photolysis experiments. The biological implications of the results obtained are also discussed.