Vitiligo pathogenesis: autoimmune disease, genetic defect, excessive reactive oxygen species, calcium imbalance, or what else?

Vitiligo--part 1

Vitiligo is a chronic stigmatizing disease, already known for millennia, which mainly affects melanocytes from epidermis basal layer, leading to the development of hypochromic and achromic patches. Its estimated prevalence is 0.5% worldwide. The involvement of genetic factors controlling susceptibility to vitiligo has been studied over the last decades, and results of previous studies present vitiligo as a complex, multifactorial and polygenic disease. In this context, a few genes, including DDR1, XBP1 and NLRP1 have been consistently and functionally associated with the disease. Notwithstanding, environmental factors that precipitate or maintain the disease are yet to be described. The pathogenesis of vitiligo has not been totally clarified until now and many theories have been proposed. Of these, the autoimmune hypothesis is now the most cited and studied among experts. Dysfunction in metabolic pathways, which could lead to production of toxic metabolites causing damage to melanocytes, has also been investigated. Melanocytes adhesion deficit in patients with vitiligo is mainly speculated by the appearance of Köebner phenomenon, recently, new genes and proteins involved in this deficit have been found.

Accelerating bleaching in vitiligo: balancing benefits versus risks

While the goal of available treatment in vitiligo is to regain pigmentation, some patients affected by extensive and treatment-resistant vitiligo, with a major social and emotional impact, may benefit from depigmentation therapy. However, results from such therapy may not always be satisfactory. So to achieve better, faster and complete bleaching, Webb et al. propose a synergistic approach that combines topical application of bleaching phenols which targets melanocytes and initiate local inflammation with immune adjuvants so as to obtain an enhanced immune response against remaining melanocytes. This strategy could be reliable, but should be evaluated cautiously in future studies, in terms of potential side effects and induction of undesired autoimmunity.

Vitiligo and alopecia areata: apples and oranges?

Vitiligo and alopecia areata are common autoimmune diseases of the skin. Vitiligo is caused by the destruction of melanocytes and results in the appearance of white patches on any part of the body, while alopecia areata is characterized by patchy hair loss primarily on the scalp, but may also involve other areas as well. At first glance, the two diseases appear to be quite different, targeting different cell types and managed using different treatment approaches. However, the immune cell populations and cytokines that drive each disease are similar, they are closely associated within patients and their family members, and vitiligo and alopecia areata have common genetic risk factors, suggesting that they share a similar pathogenesis. Like apples and oranges, vitiligo and alopecia areata have some obvious differences, but similarities abound. Recognizing both similarities and differences will promote research into the pathogenesis of each disease, as well as the development of new treatments.

Interferon-γ induces senescence in normal human melanocytes

BACKGROUND

Interferon-γ (IFN-γ) plays an important role in the proceedings of vitiligo through recruiting lymphocytes to the lesional skin. However, the potential effects of IFN-γ on skin melanocytes and the subsequent contribution to the vitiligo pathogenesis are still unclear.

OBJECTIVE

To investigate the effects of IFN-γ on viability and cellular functions of melanocytes.

METHODS

Primary human melanocytes were treated with IFN-γ. Cell viability, apoptosis, cell cycle melanin content and intracellular reactive oxygen species (ROS) level were measured. mRNA expression was examined by real-time PCR. The release of interleukin 6 (IL-6) and heat shock protein 70 (HSP-70) was monitored by ELISA. β-galactosidase staining was utilized to evaluate melanocyte senescence.

RESULTS

Persistent IFN-γ treatment induced viability loss, apoptosis, cell cycle arrest and senescence in melanocytes. Melanocyte senescence was characterized as the changes in pigmentation and morphology, as well as the increase of β-galactosidase activity. Increase of p21Cip1/Waf1 protein was evident in melanocytes after IFN-γ treatment. IFN-γ induction of senescence was attenuated by siRNAs against p21, Janus kinase 2 (JAK2) or signal transducer and activator of transcription 1 (STAT1), but not by JAK1 siRNA nor by p53 inhibitor pifithrin-α. IFN-γ treatment increased the accumulation of intracellular ROS in melanocytes, while ROS scavenger N-acetyl cysteine (NAC) effectively inhibited IFN-γ induced p21 expression and melanocyte senescence. IL-6 and HSP-70 release was significantly induced by IFN-γ treatment, which was largely inhibited by NAC. The increase of IL-6 and HSP-70 release could also be observed in senescent melanocytes.

CONCLUSION

IFN-γ can induce senescence in melanocytes and consequently enhance their immuno-competency, leading to a vitiligo-prone milieu.

Correlation of Vitamin D Levels with Pigmentation in Vitiligo Patients Treated with NBUVB Therapy

Cholecalciferol (vitamin D) might play a physiological role in photo-induced melanogenesis in human skin. We estimated the levels of 25-hydroxy vitamin D [25(OH)D] before, during, and after Narrow Band Ultraviolet B (NBUVB) radiation in patients of vitiligo and their correlation with NBUVB induced pigmentation. Thirty patients of vitiligo and equal number of age and sex matched controls were recruited for the study. Vitiligo patients were treated with NBUVB thrice weekly for 12 weeks. [25(OH)D] levels and Vitiligo Area and Severity Index (VASI) were calculated at 0 (baseline), 6, and 12 weeks. Baseline [25(OH)D] levels were measured in controls. Significant reduction in VASI score was observed after 12 weeks of therapy. Comparison and correlation between mean improvement in VASI and [25(OH)D] levels at 12 weeks showed moderate correlation, and the results were statistically insignificant. Mean reduction in VASI and increase in [25(OH)D] levels after 12 weeks of NBUVB showed moderate correlation. Thus, vitamin D might play a significant role in photo-induced melanogenesis. However, there might be additional effects of the phototherapy on melanogenesis. The complete mechanism of NBUVB induced pigmentation in vitiligo needs to be elucidated.

Innate immune mechanisms in vitiligo: danger from within

Vitiligo is an autoimmune disease of the skin in which melanocytes are destroyed by antigen-specific T cells, resulting in patchy depigmentation. Although adaptive immunity plays a clear role in disease progression, initiating factors are largely unknown. Many studies report that cellular stress pathways are dysregulated in melanocytes from vitiligo patients, suggesting that melanocyte-intrinsic defects participate in disease pathogenesis. Recent studies reveal that melanocyte stress generates damage-associated molecular patterns that activate innate immunity, thus connecting stress to organ-specific inflammation. Genetic studies in vitiligo support a role for stress, innate immunity, and adaptive mechanisms. Here, we discuss advances in the field that highlight how cellular stress, endogenous danger signals, and innate immune activation promote the onset of vitiligo.

Stimulated human melanocytes express and release interleukin-8, which is inhibited by luteolin: relevance to early vitiligo

Vitiligo is a disorder of depigmentation, for which the pathogenesis is as yet unclear. Interleukin (IL)-8 (CXCL8) is a key inflammatory chemokine. We investigated the regulation of IL-8 production in human melanocytes, and the IL-8 serum levels and skin gene expression in patients with vitiligo and in controls. Cultured melanocytes were stimulated for 24 h with tumour necrosis factor (TNF) 100 ng/mL and IL-1β 10 ng/mL, with or without pretreatment with luteolin 50 μmol/L for 30 min, and IL-8 release was measured by ELISA. Serum cytokines were measured by a microbead array. Skin biopsies were taken from healthy subjects (n = 14) as well as from marginal lesional and nonlesional skin from patients with vitiligo (n = 15). IL-8 gene expression was evaluated by quantitative real time PCR. Both TNF and IL-1β stimulated significant IL-8 release (P < 0.01) from melanocytes, whereas pretreatment with luteolin significantly inhibited this effect (P < 0.01). IL-8 gene expression was significantly increased in vitiligo compared with control skin (P < 0.05). IL-8 may be involved in vitiligo inflammation. Inhibition by luteolin of IL-8 release could be useful for vitiligo therapy.

Metabolism of melatonin and biological activity of intermediates of melatoninergic pathway in human skin cells

Indolic and kynuric pathways of skin melatonin metabolism were monitored by liquid chromatography mass spectrometry in human keratinocytes, melanocytes, dermal fibroblasts, and melanoma cells. Production of 6-hydroxymelatonin [6(OH)M], N(1)-acetyl-N(2)-formyl-5-methoxykynuramine (AFMK) and 5-methoxytryptamine (5-MT) was detected in a cell type-dependent fashion. The major metabolites, 6(OH)M and AFMK, were produced in all cells. Thus, in immortalized epidermal (HaCaT) keratinocytes, 6(OH)M was the major product with Vmax = 63.7 ng/10(6) cells and Km = 10.2 μM, with lower production of AFMK and 5-MT. Melanocytes, keratinocytes, and fibroblasts transformed melatonin primarily into 6(OH)M and AFMK. In melanoma cells, 6(OH)M and AFMK were produced endogenously, a process accelerated by exogenous melatonin in the case of AFMK. In addition, N-acetylserotonin was endogenously produced by normal and malignant melanocytes. Metabolites showed selective antiproliferative effects on human primary epidermal keratinocytes in vitro. In ex vivo human skin, both melatonin and AFMK-stimulated expression of involucrin and keratins-10 and keratins-14 in the epidermis, indicating their stimulatory role in building and maintaining the epidermal barrier. In summary, the metabolism of melatonin and its endogenous production is cell type-dependent and expressed in all three main cell populations of human skin. Furthermore, melatonin and its metabolite AFMK stimulate differentiation in human epidermis, indicating their key role in building the skin barrier.

Evaluation of total oxidant and antioxidant status in localized and generalized vitiligo

BACKGROUND

Vitiligo is an acquired depigmentation disorder, and oxidative stress is suggested to have a major role in its aetiopathogenesis.

AIM

To assess whether oxidative stress has a greater role in generalized than in localized vitiligo.

METHODS

We assessed 31 patients with active vitiligo (17 localized, 14 generalized) and 38 healthy controls. Serum total oxidant status (TOS), total antioxidant status (TAS) and oxidative stress index (OSI) were determined.

RESULTS

Patients with vitiligo had significantly lower TAS and higher TOS and OSI values than controls. Both localized and generalized vitiligo were associated with lower TAS and higher TOS and OSI values, compared with controls, and all three did not differ with vitiligo type.

CONCLUSIONS

A systemic oxidative stress exists in patients with vitiligo. These results indicate that the global antioxidant capacity of patients might have been exhausted through a defence mechanism against oxidative processes. The imbalance in TOS/TAS status may have an important role in the aetiopathogenesis of vitiligo, regardless of the clinical variant of the disease.

Vitiligo: a possible model of degenerative diseases

Vitiligo is characterized by the progressive disappearance of pigment cells from skin and hair follicle. Several in vitro and in vivo studies show evidence of an altered redox status, suggesting that loss of cellular redox equilibrium might be the pathogenic mechanism in vitiligo. However, despite the numerous data supporting a pathogenic role of oxidative stress, there is still no consensus explanation underlying the oxidative stress-driven disappear of melanocytes from the epidermis. In this study, in vitro characterization of melanocytes cultures from non-lesional vitiligo skin revealed at the cellular level aberrant function of signal transduction pathways common with neurodegenerative diseases including modification of lipid metabolism, hyperactivation of mitogen-activated protein kinase (MAPK) and cAMP response element-binding protein (CREB), constitutive p53-dependent stress signal transduction cascades, and enhanced sensibility to pro-apoptotic stimuli. Notably, these long-term effects of subcytotoxic oxidative stress are also biomarkers of pre-senescent cellular phenotype. Consistent with this, vitiligo cells showed a significant increase in p16 that did not correlate with the chronological age of the donor. Moreover, vitiligo melanocytes produced many biologically active proteins among the senescence-associated secretory phenotype (SAPS), such as interleukin-6 (IL-6), matrix metallo proteinase-3 (MMP3), cyclooxygenase-2 (Cox-2), insulin-like growth factor-binding protein-3 and 7 (IGFBP3, IGFBP7). Together, these data argue for a complicated pathophysiologic puzzle underlying melanocytes degeneration resembling, from the biological point of view, neurodegenerative diseases. Our results suggest new possible targets for intervention that in combination with current therapies could correct melanocytes intrinsic defects.

Network, nodes and nexus: systems approach to multitarget therapeutics

Systems biology is revealing multiple layers of regulatory networks that manifest spatiotemporal variations. Since genes and environment also influence the emergent property of a cell, the biological output requires dynamic understanding of various molecular circuitries. The metabolic networks continually adapt and evolve to cope with the changing milieu of the system, which could also include infection by another organism. Such perturbations of the functional networks can result in disease phenotypes, for instance tuberculosis and cancer. In order to develop effective therapeutics, it is important to determine the disease progression profiles of complex disorders that can reveal dynamic aspects and to develop mutitarget systemic therapies that can help overcome pathway adaptations and redundancy.

Immunohistochemical detection of P53 and Mdm2 in vitiligo

Background:

Vitiligo is a common depigmented skin disorder that is caused by selective destruction of melanocytes. It is generally accepted that the main function of melanin resides in the protection of skin cells against the deleterious effect of ultraviolet rays (UVRs). Association of vitiligo and skin cancer has been a subject of controversy. Occurrence of skin cancer in long-lasting vitiligo is rare despite multiple evidences of DNA damage in vitiliginous skin.

Aim:

To detect the expression of P53 and Mdm2 proteins in both depigmented and normally pigmented skin of vitiligo patients and to compare it to control subjects suffering from nonmelanoma skin cancer (NMSC).

Materials and Methods:

Thirty-four patients with vitiligo and 30 age and sex-matched patients with nodulo-ulcerative basal cell carcinoma (BCC) as a control group were selected. Both patients and control subjects had outdoor occupations. Skin biopsies were taken from each case and control subjects. Histopathological examination of Hematoxylin and eosin-stained sections was done. Expression of P53 and Mdm2 proteins were examined immunohistochemically.

Results:

Both P53 and Mdm2 were strongly expressed in depigmented as well as normally pigmented skin of vitiligo patients. This expression involved the epidermis, skin adnexa and blood vessels with significant differences between cases and controls.

Conclusions:

The overexpression of P53 and Mdm2 proteins in both normally pigmented and depigmented skin of patients with vitiligo could contribute to the decreased occurrence of actinic damage and NMSC in these patients.

Melatonin membrane receptors in peripheral tissues: distribution and functions

Many of melatonin's actions are mediated through interaction with the G-protein coupled membrane bound melatonin receptors type 1 and type 2 (MT1 and MT2, respectively) or, indirectly with nuclear orphan receptors from the RORα/RZR family. Melatonin also binds to the quinone reductase II enzyme, previously defined the MT3 receptor. Melatonin receptors are widely distributed in the body; herein we summarize their expression and actions in non-neural tissues. Several controversies still exist regarding, for example, whether melatonin binds the RORα/RZR family. Studies of the peripheral distribution of melatonin receptors are important since they are attractive targets for immunomodulation, regulation of endocrine, reproductive and cardiovascular functions, modulation of skin pigmentation, hair growth, cancerogenesis, and aging. Melatonin receptor agonists and antagonists have an exciting future since they could define multiple mechanisms by which melatonin modulates the complexity of such a wide variety of physiological and pathological processes.

Melanocyte antigen-specific antibodies cannot be used as markers for recent disease activity in patients with vitiligo

BACKGROUND

Objective parameters to assess disease activity in non-segmental vitiligo are lacking. Melanocyte antigen-specific antibodies are frequently found in the sera of patients with vitiligo and the presence of these antibodies may correlate with disease activity.

OBJECTIVE

To investigate the relationship between melanocyte antigen-specific antibodies and recent disease activity in patients with vitiligo and to evaluate the potential usefulness of this objective parameter in daily clinical practice.

METHODS

The prevalence of tyrosinase, melanoma antigen recognized by T-cells-1 (MART1), melanin-concentrating hormone receptor-1 (MCHR1), gp100 and tyrosine hydroxylase (TH) antibodies was evaluated in 21 patients with non-segmental vitiligo and in 20 healthy controls.

RESULTS

In 21 patients, nine (42.8%) showed antibody responses against tyrosinase, MART1, MCHR1, gp100 or TH. No antibody responses were found in the 20 controls. No correlation was found between the presence of antibodies and recent disease activity or other clinical characteristics such as age, gender, extension and duration of vitiligo.

CONCLUSIONS

In this study, 42.8% of the vitiligo patients showed an antibody response to melanocyte antigen-specific antigens. However, the presence of antibodies against melanocytes did not correlate with recent disease activity or other relevant disease parameters, and for the moment screening for these antibodies in individual patients does not appear to be clinically relevant.

Topical application of bleaching phenols; in-vivo studies and mechanism of action relevant to melanoma treatment

Skin depigmentation represents a well-established treatment for extensive vitiligo and may likewise be suited to prevent tumor recurrences and as a prophylactic treatment of familial melanoma, as common bleaching agents are cytotoxic to melanocytes. Effective melanoma prevention requires a bleaching agent-induced loss of exposed melanocytes supported by an immune response to distant pigment cells. Studies on human explant cultures treated with depigmenting agents such as 4-tertiary butyl phenol (4-TBP) or monobenzyl ether of hydroquinone (MBEH) showed a significant increase in the migration of Langerhans cells toward the dermis only upon treatment with MBEH, thus suggesting selective elicitation of an immune response. To assess the depigmenting potential of bleaching agents in vivo, 4-TBP and MBEH were topically applied to C57BL/6 wild type as well as k14-SCF transgenic, epidermally pigmented mice. MBEH-induced significant skin depigmentation in both strains was not observed upon treatment with 4-TBP. Cytokine expression patterns in skin treated with MBEH support activation of a Th1-mediated immune response corresponding to an influx of T cells and macrophages. Importantly, despite insensitivity of tumor cells to MBEH-induced cytotoxicity, significantly retarded tumor growth was observed in B16 challenged k14-SCF mice pretreated with MBEH, likely due to an abundance of cytotoxic T cells accompanied by an increased expression of Th1 and Th17 cytokines. These data support the use of MBEH as a prophylactic treatment for melanoma.

Monobenzone-induced depigmentation: from enzymatic blockade to autoimmunity

Autoimmune side-effects such as vitiligo regularly occur during melanoma immunotherapy. As vitiligo development is associated with a superior prognosis, the active induction of vitiligo in melanoma patients can be a useful tactic. The potent skin-depigmenting agent monobenzone can be used successfully for this purpose. However, until recently, the mechanism of action behind monobenzone-induced skin depigmentation was unclear. Lately, the mechanistic basis for the augmented immunogenicity of monobenzone-exposed pigmented cells has been unveiled, and their active role in the induction of autoimmune T-cell-mediated vitiligo has become apparent. Here, we provide an immunological framework in which we condense this knowledge to an integrated theory of the generation of monobenzone-induced vitiligo.

Double-stranded RNA induces melanocyte death via activation of Toll-like receptor 3

As cutaneous pigment-producing cells, melanocytes can become targets of primary and secondary immune response as can be seen in diseases like vitiligo and Vogt-Koyanagi-Harada (VKH) syndrome. Viral infections have previously been implicated as a possible precipitating factor in the destruction of melanocytes in these disorders. During viral replication, double-stranded RNA (dsRNA) is produced as an intermediate metabolite, which induces antiviral and inflammatory responses through Toll-like receptor 3 (TLR3) in cells of innate immune system. The functional responses of melanocytes to dsRNA, however, remain unclear. Herein, we demonstrated that human melanocytes expressed TLR3 at a constitutive and inducible level. Stimulation with poly(I:C), a synthetic dsRNA analogue, triggered apoptosis of melanocytes. The apoptosis-inducing effect was shown by RNA interference to be largely dependent on TLR3, but occurred independently of NF-κB activation since treatment with specific NF-κB inhibitor Bay 11-7082 failed to prevent the process. In contrast, IFN-β neutralizing Ab blocked the apoptosis-inducing effect of dsRNA, indicating the involvement of IFN-β autocrine signalling. Furthermore, studies on the intracellular signal transduction pathways revealed that dsRNA induces the activation of p38, ERK1/2 and JNK1/2 in melanocytes. Using specific inhibitors, we demonstrated that activation of p38 and ERK1/2 controlled both IFN-β secretion and IFN-β mediated cell death. Taken together, these data suggest that viral dsRNA stimulates TLR3 in human melanocytes and triggers the cellular apoptosis through autocrine of IFN-β.

A neuroendocrinological perspective on human hair follicle pigmentation

The role of neurohormones and neuropeptides in human hair follicle (HF) pigmentation extends far beyond the control of melanin synthesis by α-MSH and ACTH and includes melanoblast differentiation, reactive oxygen species scavenging, maintenance of HF immune privilege, and remodeling of the HF pigmentary unit (HFPU). It is now clear that human HFs are not only a target of multiple neuromediators, but also are a major non-classical production site for neurohormones such as CRH, proopiomelanocortin, ACTH, α-MSH, ß-endorphin, TRH, and melatonin. Moreover, human HFs have established a functional peripheral equivalent of the hypothalamic-pituitary-adrenal axis. By charting the author's own meanderings through the jungle of hair pigmentation research, the current perspectives essay utilizes four clinical observations - hair repigmentation, canities, poliosis, and 'overnight greying'- as points of entry into the enigmas and challenges of .pigmentary HF neuroendocrinology. After synthesizing key principles and defining major open questions in the field, selected research avenues are delineated that appear clinically most promising. In this context, novel neuroendocrinological strategies to retard or reverse greying and to reduce damage to the HFPU are discussed.

Heme oxygenase-1 expression protects melanocytes from stress-induced cell death: implications for vitiligo

To study protection of melanocytes from stress-induced cell death by heme oxygenases during depigmentation and repigmentation in vitiligo, expression of isoforms 1 and 2 was studied in cultured control and patient melanocytes and normal skin explants exposed to UV or bleaching agent 4-TBP. Similarly, expression of heme oxygenases was followed in skin from vitiligo patients before and after PUVA treatment. Single and double immunostainings were used in combination with light and confocal microscopic analysis and Western blotting. Melanocyte expression of heme oxygenase 1 is upregulated, whereas heme oxygenase 2 is reduced in response to UV and 4-TBP. Upregulation of inducible heme oxygenase 1 was also observed in UV-treated explant cultures, in skin of successfully PUVA-treated patients and in melanocytes cultured from vitiligo non-lesional skin. Heme oxygenase encoding genes were subsequently cloned to study consequences of either gene product on cell viability, demonstrating that HO-1 but not HO-2 overexpression offers protection from stress-induced cell death in MTT assays. HO-1 expression by melanocytes may contribute to beneficial effects of UV treatment for vitiligo patients.

In vivo and in vitro evidence for epidermal H2O2-mediated oxidative stress in piebaldism

Piebaldism is characterised by the absence of pigment in patches on the skin, usually present at birth. Mutations in the kit gene are documented. Clinically this disorder can mimic vitiligo. Here, we show for the first time the presence of oxidised pteridine-induced fluorescence in association with H2O2-mediated stress in piebald patches employing Wood's light and in vivo FT-Raman spectroscopy. In situ immunofluorescence data revealed low catalase and methionine sulphoxide reductase A (MSRA) levels whereas thioredoxin reductase and methionine sulphoxide reductase B (MSRB) are not affected. We also show low superoxide dismutase levels in these patients. The presence of thioredoxin reductase provides capacity to reduce H2O2, a mechanism which is absent in vitiligo. Importantly, this enzyme reduces biopterin back to the functioning cofactor 6-tetrahydrobiopterin. The absence of MSRA indicates deficient methionine sulphoxide repair in the cytosol, meanwhile the presence of MSRB is helpful to protect the nucleus. Taken together, we have identified H2O2-mediated stress in piebald skin with distinct differences to vitiligo.

Vitiligo, reactive oxygen species and T-cells

The acquired depigmenting disorder of vitiligo affects an estimated 1% of the world population and constitutes one of the commonest dermatoses. Although essentially asymptomatic, the psychosocial impact of vitiligo can be severe. The cause of vitiligo remains enigmatic, hampering efforts at successful therapy. The underlying pathogenesis of the pigment loss has, however, been clarified to some extent in recent years, offering the prospect of effective treatment, accurate prognosis and rational preventative strategies. Vitiligo occurs when functioning melanocytes disappear from the epidermis. A single dominant pathway is unlikely to account for all cases of melanocyte loss in vitiligo; rather, it is the result of complex interactions of biochemical, environmental and immunological events, in a permissive genetic milieu. ROS (reactive oxygen species) and H2O2 in excess can damage biological processes, and this situation has been documented in active vitiligo skin. Tyrosinase activity is impaired by excess H2O2 through oxidation of methionine residues in this key melanogenic enzyme. Mechanisms for repairing this oxidant damage are also damaged by H2O2, compounding the effect. Numerous proteins and peptides, in addition to tyrosinase, are similarly affected. It is possible that oxidant stress is the principal cause of vitiligo. However, there is also ample evidence of immunological phenomena in vitiligo, particularly in established chronic and progressive disease. Both innate and adaptive arms of the immune system are involved, with a dominant role for T-cells. Sensitized CD8+ T-cells are targeted to melanocyte differentiation antigens and destroy melanocytes either as the primary event in vitiligo 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 present review focuses on the scientific evidence linking alterations in ROS and/or T-cells to vitiligo.

The redox--biochemistry of human hair pigmentation

The biochemistry of hair pigmentation is a complex field involving a plethora of protein and peptide mechanisms. The in loco factory for melanin formation is the hair follicle melanocyte, but it is common knowledge that melanogenesis results from a fine tuned concerted interaction between the cells of the entire dermal papilla in the anagen hair follicle. The key enzyme is tyrosinase to initiate the active pigmentation machinery. Hence, an intricate understanding from transcription of mRNA to enzyme activity, including enzyme kinetics, substrate supply, optimal pH, cAMP signaling, is a must. Moreover, the role of reactive oxygen species on enzyme regulation and functionality needs to be taken into account. So far our knowledge on the entire hair cycle relies on the murine model of the C57BL/6 mouse. Whether this data can be translated into humans still needs to be shown. This article aims to focus on the effect of H(2)O(2)-redox homeostasis on hair follicle pigmentation via tyrosinase, its substrate supply and signal transduction as well as the role of methionine sulfoxide repair via methionine sulfoxide reductases A and B (MSRA and B).

Histamine effect on melanocyte proliferation and vitiliginous keratinocyte survival

Repigmention of vitiligo requires melanocyte proliferation and migration. Keratinocytes have been shown to play a role in this process. Data from this laboratory showed that bee venom (BV) stimulated melanocyte proliferation and migration as well as melanogenesis. As histamine release is associated with BV, its effect on melanocyte proliferation and migration was examined. Cultured normal human melanocytes treated with histamine were studied with and without receptor-specific antagonists or agonists. The effect of histamine on vitiliginous keratinocytes, in cultured cells treated with a PI3K inhibitor in the presence of TNF-α, was also examined. Histamine exerted a more significant effect on melanocyte proliferation than on melanogenesis. This occurred through the H2 receptor with complex signalling to ERK, CREB, and Akt activation, which stimulated melanocyte migration. Histamine and the H2 receptor agonist also increased survival of vitiliginous, but not normal, keratinocytes, with NF-κB activation. Because expression levels of the H2 receptor was significantly decreased in depigmented compared to normally pigmented epidermis, in patients with vitiligo, histamine may increase the survival of vitiliginous keratinocytes. Overall, histamine stimulated the proliferation and migration of melanocytes and the vitiliginous keratinocyte survival, providing the basis for novel therapeutic approaches to vitiligo repigmentation.

Vitiligo in an urban academic setting

BACKGROUND

Vitiligo is a depigmenting disease of unknown etiology. A more complete understanding of vitiligo and associated conditions will provide better insight into the etiology and potential treatment options for this condition. We sought to gather information regarding associated conditions and other epidemiologic data on vitiligo.

METHODS

A retrospective chart review was performed of 135 patients with vitiligo seen between July 1, 2002 and June 30, 2005 at an academic medical center. Epidemiologic characteristics were recorded.

RESULTS

The patient population consisted of 80 women and 55 men with mean age of presentation of 36.8 years and average disease duration of 5.7 years. Vitiligo vulgaris was the predominant type of vitiligo and hypothyroidism was the most common co-morbidity. Anti-thyroid peroxidase and anti-thyroglobulin antibodies were found in 37% and 18% of patients, respectively. The highest proportion of thyroid abnormalities was found in age of onset category 21-30. Anti-nuclear antibodies were found in 33% of patients.

CONCLUSION

The prevalence of anti-nuclear and anti-thyroid peroxidase antibodies was higher in our vitiligo study than that reported elsewhere. In addition, autoimmune thyroid disease may be more common in adult-onset vitiligo.

[Vitiligo. What is new?]

Vitiligo occurs in Northern Europe in one of 200 people. The disease can cause significant psychological stress for the affected individual. These patients generate and accumulate massive amounts of H(2)O(2)- and peroxynitrite in the epidermal compartment. Consequently many proteins are oxidized or nitrated, leading in turn to partial or complete loss of functionality. Moreover, presence of DNA damage in the skin as well as in plasma has been shown, while apoptosis is not enhanced. Induction of DNA repair is associated with up-regulated functioning p53 protein. Considering possible genetic predisposition and /or spontaneous mutations, autoimmune reactions in the disease are put forward in the context of oxidative stress. In addition a review of recent and novel treatment modalities including the role of oxidative stress reduction and combined climatotherapy at the Dead Sea in a group are discussed.

A reactive ortho-quinone generated by tyrosinase-catalyzed oxidation of the skin depigmenting agent monobenzone: self-coupling and thiol-conjugation reactions and possible implications for melanocyte toxicity

Monobenzone (hydroquinone monobenzylether, 1) is a potent skin depigmenting agent that causes irreversible loss of epidermal melanocytes by way of a tyrosinase-dependent mechanism so far little understood. Herein, we show that 1 can be oxidized by mushroom tyrosinase to an unstable o-quinone (1-quinone) that has been characterized by comparison of its properties with those of a synthetic sample obtained by o-iodoxybenzoic acid-mediated oxidation of 1. Preparative scale oxidation of 1 with tyrosinase and catalytic l-DOPA, followed by reductive workup and acetylation, led to the isolation of two main products that were identified as the acetylated catechol derivative 4 and an unusual biphenyl-type dimer of 4, acetylated 5, arising evidently by coupling of 4 with 1-quinone. In the presence of l-cysteine or N-acetyl-l-cysteine, formation of 4 and 5 was inhibited, and the reaction led instead to monoadducts (6 or 9) and diadducts (7 and 8). A similar behavior was observed when the tyrosinase-promoted oxidation of 1 was carried out in the presence of sulfhydryl-containing peptides, such as reduced glutathione, or proteins, such as bovine serum albumin (BSA), as inferred by detection of adduct 9 by high pressure liquid chromatography-electrochemical detection (HPLC-ED) after acid hydrolysis. The generation and reaction chemistry of 1-quinone described in this article may bear relevance to the etiopathogenetic mechanisms of monobenzone-induced leukoderma as well as to the recently proposed haptenation hypothesis of vitiligo, a disabling pigmentary disorder characterized by irreversible melanocyte loss.

Membrane lipid defects are responsible for the generation of reactive oxygen species in peripheral blood mononuclear cells from vitiligo patients

The pathogenesis of vitiligo, an acquired depigmenting disease of the skin, involves oxidative stress. Based on that, the generation of reactive oxygen species (ROS) by the mitochondria may be relevant in the pathogenesis of vitiligo. Here, we evaluate the biochemical and functional alterations involved in the defective activity that has been previously described both in melanocytes and peripheral blood mononuclear cells (PBMC) from vitiligo patients. Moreover, we used a freeze-thaw test as a mild stress stimulus to disclose any latent defects in the assembly of membrane lipids that may compromise the functionality of the membrane itself. We show that the lipid constitution of the membrane is altered in vitiligo. Specifically, the cardiolipin (CL) level in the mitochondrial inner membrane is reduced and the level of cholesterol is increased. Furthermore, an increase in the expression level of 3-hydroxy-3methyl-glutaryl-CoenzymeA-reductase (HMG-CoA reductase), the rate-limiting enzyme for cholesterol biosynthesis, was also seen. Associated with that, the expression of electron transport chain (ETC) lipid-dependent subunits was also modified, and their expression was further affected by the freeze-thaw stress. The expression of CL-independent mitochondrial proteins, such as porin and Bcl2, were unaffected in vitiligo PBMC. These data confirm that ETC protein expression mainly correlates with lipid arrangement and that loss of their expression is not due to generalized or random oxidative-mediated damage. We suggest that the modification of membrane lipid components in vitiligo cells may be the biochemical basis for the mitochondrial impairment and the subsequent production of intracellular ROS following the exposure to a mild stress.

Frontiers and controversies in the pathobiology of vitiligo: separating the wheat from the chaff

The pathogenesis of vitiligo is complex and not well understood. Genes play a role in all aspects of vitiligo pathogenesis, and studies are ongoing to identify these genes and understand their biology. There is a body of interlocking, compelling evidence supporting an autoimmune basis for most or all cases of generalized vitiligo. The development of an autoimmune disease generally involves three components; the immune system, environmental triggers and other exogenous precipitating factors, and the target tissue. In vitiligo, precipitating factors could induce melanocyte damage in genetically susceptible individuals and consequent cell death, loss of tolerance, and induction of melanocyte-directed autoimmunity. Future research will more precisely define the multiple biological events that regulate development of vitiligo.

Mapping of melanin-concentrating hormone receptor 1 B cell epitopes predicts two major binding sites for vitiligo patient autoantibodies

The melanin-concentrating hormone receptor 1 (MCHR1) has been identified as a B cell autoantigen in vitiligo with antibodies to the receptor detectable in binding and function-blocking assays. Two epitope domains (amino acids 1-138 and 139-298) have been previously identified. In this study, we aimed to further define the epitope specificity of MCHR1 antibodies using phage-display technology and to identify the epitopes recognised by receptor antibodies detected in MCHR1 function-blocking assays. Antibody reactivity to MCHR1 peptides 51-80, 85-98, 154-158 and 254-260 was identified by phage-display and subsequently confirmed in phage ELISA in 2/12, 5/12, 3/12 and 6/12 of vitiligo patients, respectively. The results suggest that major autoantibody epitopes are localised in the 85-98 and 254-260 amino acid regions of MCHR1 with minor epitopes in amino acid sequences 51-80 and 154-158. Antibodies with MCHR1 function-blocking activity were determined to recognise epitope 254-260, this being the first epitope to be reported as a target site for antibodies that block the function of the receptor.

Focus on mammalian thioredoxin reductases--important selenoproteins with versatile functions

Thioredoxin systems, involving redox active thioredoxins and thioredoxin reductases, sustain a number of important thioredoxin-dependent pathways. These redox active proteins support several processes crucial for cell function, cell proliferation, antioxidant defense and redox-regulated signaling cascades. Mammalian thioredoxin reductases are selenium-containing flavoprotein oxidoreductases, dependent upon a selenocysteine residue for reduction of the active site disulfide in thioredoxins. Their activity is required for normal thioredoxin function. The mammalian thioredoxin reductases also display surprisingly multifaceted properties and functions beyond thioredoxin reduction. Expressed from three separate genes (in human named TXNRD1, TXNRD2 and TXNRD3), the thioredoxin reductases can each reduce a number of different types of substrates in different cellular compartments. Their expression patterns involve intriguingly complex transcriptional mechanisms resulting in several splice variants, encoding a number of protein variants likely to have specialized functions in a cell- and tissue-type restricted manner. The thioredoxin reductases are also targeted by a number of drugs and compounds having an impact on cell function and promoting oxidative stress, some of which are used in treatment of rheumatoid arthritis, cancer or other diseases. However, potential specific or essential roles for different forms of human or mouse thioredoxin reductases in health or disease are still rather unclear, although it is known that at least the murine Txnrd1 and Txnrd2 genes are essential for normal development during embryogenesis. This review is a survey of current knowledge of mammalian thioredoxin reductase function and expression, with a focus on human and mouse and a discussion of the striking complexity of these proteins. Several yet open questions regarding their regulation and roles in different cells or tissues are emphasized. It is concluded that the intriguingly complex regulation and function of mammalian thioredoxin reductases within the cellular context and in intact mammals strongly suggests that their functions are highly fi ne-tuned with the many pathways involving thioredoxins and thioredoxin-related proteins. These selenoproteins furthermore propagate many functions beyond a reduction of thioredoxins. Aberrant regulation of thioredoxin reductases, or a particular dependence upon these enzymes in diseased cells, may underlie their presumed therapeutic importance as enzymatic targets using electrophilic drugs. These reductases are also likely to mediate several of the effects on health and disease that are linked to different levels of nutritional selenium intake. The thioredoxin reductases and their splice variants may be pivotal components of diverse cellular signaling pathways, having importance in several redox-related aspects of health and disease. Clearly, a detailed understanding of mammalian thioredoxin reductases is necessary for a full comprehension of the thioredoxin system and of selenium dependent processes in mammals.