Molecular and cellular basis of depigmentation in vitiligo patients

Unveiling the Unseen: Exploring Cellular Dynamics in Nonlesional Vitiligo Skin

Vitiligo is a multifactorial disease involving genetic predispositions, intrinsic melanocyte abnormalities, and deregulated immune response. Recent studies show nonlesional skin involvement. Nonlesional melanocytes display mitochondrial dysfunction and increased oxidative stress affecting their adhesion and function and contribute to immune activation. Keratinocytes and fibroblasts display structural and functional abnormalities impairing melanocyte support and differentiation. They also contribute to the altered immune response by secreting proinflammatory chemokines. Despite immune cell infiltration, nonlesional skin remains clinically unaffected, suggesting that regulatory mechanisms maintain immune activation at a subclinical level. This review provides an overview of the cellular alterations in vitiligo nonlesional skin.

Segmental vitiligo: autoimmune pathogenesis, neuronal mechanisms, and somatic mosaicism

Vitiligo is a common depigmentation disorder classified into nonsegmental vitiligo (NSV) and segmental vitiligo (SV). SV accounts for 5-27.9% of patients with vitiligo. The primary pathogenesis of NSV involves the autoimmune-mediated destruction of melanocytes. Recently, an autoimmune pathogenesis of SV was identified. High levels of melanocyte antigen-specific CD8+ T cells are found in early SV lesional skin infiltrating around melanocytes along the basal layer. Mixed vitiligo suggests an overlap in pathogenesis between SV and NSV. In active SV, serum innate immune cytokines, and CD8+ T cell cytokines are increased. Oxidative stress in SV may activate autoimmune responses. SV pathogenesis is associated with a local cytotoxic response targeting epidermal melanocytes. Theories have been put forward to explain the segmental pattern in SV. The previous basis of the neurogenic theory that SV results from dermatomes is no longer accepted. However, there are still research reports supporting this theory. Evaluating the distribution pattern of SV lesions has provided clues to the mosaicism detection of suspected melanocytic defects at the site of SV lesions, supporting this theory. Evidence points to a cytotoxic response targeting mosaic melanocytes. Understanding SV's autoimmune pathogenesis prompts a reevaluation of immunosuppressive medical treatments for SV. The excellent results of autologous melanocyte transplantation in SV lesions compared with the moderate to limited results in patients with NSV support the mosaicism theory.

Disorganisation of basement membrane zone architecture impairs melanocyte residence in vitiligo

The basement membrane zone is the interface between the epidermis and dermis, and it is disrupted in several skin conditions. Here, we report the results of a comprehensive investigation into the structural and molecular factors of the basement membrane zone in vitiligo, a dermatological disorder characterised by depigmented patches on the skin. Using electron microscopy and immunofluorescence staining, we confirmed abnormal basement membrane zone morphology and disrupted basement membrane zone architecture in human vitiliginous skin. Furthermore, we identified elevated expression of matrix metalloproteinase 2 (MMP2) in human dermal fibroblasts as a key factor responsible for basement membrane zone matrix degradation. In our in vitro and ex vivo models, overexpression of MMP2 in fibroblasts led to basement membrane zone disruption and melanocyte disappearance. Importantly, we reveal that the loss of melanocytes in vitiligo is primarily linked to their weakened adhesion to the basement membrane, mediated by binding between integrin β1 and laminin and discoidin domain receptor 1 and collagen IV. Finally, inhibition of matrix metalloproteinase 2 expression reversed depigmentation in a mouse model of vitiligo. In conclusion, our research shows the importance of basement membrane zone integrity in melanocyte residence and offers new avenues for therapeutic interventions to address this challenging skin condition. © 2024 The Author(s). The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Melanocyte lineage dynamics in development, growth and disease

Melanocytes evolved to produce the melanin that gives colour to our hair, eyes and skin. The melanocyte lineage also gives rise to melanoma, the most lethal form of skin cancer. The melanocyte lineage differentiates from neural crest cells during development, and most melanocytes reside in the skin and hair, where they are replenished by melanocyte stem cells. Because the molecular mechanisms necessary for melanocyte specification, migration, proliferation and differentiation are co-opted during melanoma initiation and progression, studying melanocyte development is directly relevant to human disease. Here, through the lens of advances in cellular omic and genomic technologies, we review the latest findings in melanocyte development and differentiation, and how these developmental pathways become dysregulated in disease.

NAcM-OPT protects keratinocytes from H2O2-induced cell damage by promoting autophagy

This study aimed to investigate the protective effect of NAcM-OPT, a small molecule inhibitor of defective in cullin neddylation 1 (DCN1), on H2O2-induced oxidative damage in keratinocytes. Immortalized human keratinocytes (HaCaT cells) were treated with NAcM-OPT and exposed to oxidative stress. CCK-8 assays were used to measure cell viability. The mGFP-RFP-LC3 dual fluorescent autophagy indicator system was utilized to evaluate changes in autophagic flux. Western blotting was used to measure the expression of the autophagy-related proteins LC3 and Beclin 1. Keratinocytes were treated with the autophagy activator rapamycin, and HaCaT cell supernatant was added to PIG1 cells (immortalized human melanocytes), followed by evaluation of tyrosinase (TYR) expression via qRT-PCR. NAcM-OPT increased cell viability and cell proliferation. Furthermore, this molecule promoted autophagic flux through increased expression of autophagy-related proteins under H2O2-induced oxidative stress. Additionally, rapamycin increased the mRNA levels of TYR in PIG1 cells. Moreover, NAcM-OPT alleviated mitochondrial damage, restored mitochondrial function, and upregulated the expression of NFE2L2, HO1, NQO1, and GCLM. Importantly, NAcM-OPT also increased epidermal thickness, follicle length, and melanin synthesis under oxidative stress in vivo. These findings suggest that NAcM-OPT may be a promising small molecule antioxidant drug for the treatment of vitiligo.

Aquaporin-3 Downregulation in Vitiligo Keratinocytes Increases Oxidative Stress of Melanocytes

Oxidative stress-induced melanocyte apoptosis is linked to the immune system and plays a critical role in the pathogenesis of vitiligo. Aquaporin-3 (AQP3), which is downregulated in vitiligo keratinocytes, regulates intracellular H2O2 accumulation. However, the role of AQP3 in oxidative stress is uncertain in vitiligo. This study investigated the effect of downregulated AQP3 on oxidative stress in vitiligo using lesional and non-lesional skin specimen sets from vitiligo patients and primary cultured adult normal human epidermal keratinocytes, with or without downregulation and overexpression of AQP3 in the presence or absence of H2O2 treatment. The levels of nuclear factor E2-related factor 2 (NRF2) and/or its main target, NAD(P)H quinone dehydrogenase 1 (NQO-1), were lower in the lesional keratinocytes and cultured keratinocytes with AQP3 knockdown, but were increased in keratinocytes upon AQP3 overexpression. Ratios of NRF2 nuclear translocation and NQO-1 expression levels were further reduced in AQP3-knockdown keratinocytes following H2O2 treatment. The conditioned media from AQP3-knockdown keratinocytes treated with H2O2 contained higher concentrations of reactive oxygen species (ROS). Moreover, the number of viable melanocytes was reduced when the conditioned media were added to the culture media. Overall, AQP3 downregulation in the keratinocytes of patients with vitiligo can induce oxidative stress in neighboring melanocytes, leading to melanocyte death.

Immune dysregulation and inflammation causing hypopigmentation in post kala-azar dermal leishmaniasis: partners in crime?

Post kala-azar dermal leishmaniasis (PKDL), a heterogeneous dermal sequela of visceral leishmaniasis (VL), is challenging in terms of its etiopathogenesis. Hypopigmentation is a consistent clinical feature in PKDL, but mechanisms contributing to the loss of melanocytes remains poorly defined. Like other hypopigmentary dermatoses - for example, vitiligo, psoriasis, and leprosy - the destruction of melanocytes is likely a multifactorial phenomenon, key players being immune dysregulation and inflammation. This review focuses on immunological mechanisms responsible for the 'murder' of melanocytes, prime suspects at the lesional sites being CD8+ T cells and keratinocytes and their criminal tools being proinflammatory cytokines, for example, IFN-γ, IL-6, and TNF-α. Collectively, these may cause decreased secretion of melanocyte growth factors, loss/attenuation of cell adhesion molecules and inflammasome activation, culminating in melanocyte death.

The Role of Oxidative Stress in Vitiligo: An Update on Its Pathogenesis and Therapeutic Implications

Vitiligo is an autoimmune skin disorder caused by dysfunctional pigment-producing melanocytes which are attacked by immune cells. Oxidative stress is considered to play a crucial role in activating consequent autoimmune responses related to vitiligo. Melanin synthesis by melanocytes is the main intracellular stressor, producing reactive oxygen species (ROS). Under normal physiological conditions, the antioxidative nuclear factor erythroid 2-related factor 2 (Nrf2) pathway functions as a crucial mediator for cells to resist oxidative stress. In pathological situations, such as with antioxidant defects or under inflammation, ROS accumulate and cause cell damage. Herein, we summarize events at the cellular level under excessive ROS in vitiligo and highlight exposure to melanocyte-specific antigens that trigger immune responses. Such responses lead to functional impairment and the death of melanocytes, which sequentially increase melanocyte cytotoxicity through both innate and adaptive immunity. This report provides new perspectives and advances our understanding of interrelationships between oxidative stress and autoimmunity in the pathogenesis of vitiligo. We describe progress with targeted antioxidant therapy, with the aim of providing potential therapeutic approaches.

Protective effects of isorhamnetin against H2O2-induced oxidative damage in HaCaT cells and comprehensive analysis of key genes

Isorhamnetin (ISO) is a methylated flavonol present in the leaves, flowers, and fruits of many plants with antitumour, anti-inflammatory, antioxidant, and anti-apoptotic properties. ISO has been suggested as the active substance in Vernonia anthelmintica (L.) to treat vitiligo. However, the mechanisms underlying its effects remain unclear. In this study, human keratinocytes (HaCaT cells) were pre-treated with or without ISO and then stimulated with hydrogen peroxide (H₂O₂) to generate oxidative damage. Pre-treatment with ISO increased HaCaT cell viability, reduced malondialdehyde content, and enhanced superoxide dismutase activity, resulting in a reduction in the loss of mitochondrial membrane potential, improved cell morphological damage, and apoptosis inhibition. Furthermore, we identified 51 significantly dysregulated differentially expressed genes (DEGs) of HaCaT cells treated with ISO using RNA-sequencing. Enrichment analysis using Gene Ontology and Kyoto Encyclopedia of Genes and Genomes databases indicated that the protective effect of ISO could be related to its effects on the Wnt signalling pathway. Our study provides novel insights into key gene regulation in the progression of oxidative damage and the mechanisms of action of ISO.

Systematic Evaluation and Meta-Analysis of Randomized Controlled Trials of Fire Needle Combined Phototherapy for the Treatment of Vitiligo

This study investigated the fire needle and phototherapy combination for treating vitiligo through a meta-analysis of the published literature. Indeed, vitiligo is a common chronic skin condition characterized by the appearance of white patches. It is the most prevalent disorder, affecting approximately 1% of the global population. There is no known cure or clinical recommendation for treating vitiligo. The majority of medical guidelines suggest vitiligo treatment with the use of fire needles. Here, vitiligo was treated with a novel mechanism combining a fire needle with phototherapy. The handheld phototherapy devices include a fire needle option for disease treatment. Miniature lesions that could be used to detect and treat vitiligo at an early stage could be managed by hybrid-capable devices. The real-time study included more than 3,435 patients. The dosages were altered to control the adverse effects. Following treatment, granulation tissues developed on the injured skin, diminishing the shallow area and wound surface. The case studies demonstrate that combining phototherapy and fire needle therapy is more practical than the other methods.

MicroRNAs: Emerging players in the pathogenesis of vitiligo

Vitiligo is an autoimmune skin disease characterized by presence of pale patchy areas of depigmentation. MicroRNAs (miRNAs) are important regulators of gene expression and play significant roles in diverse biological and pathological processes. Accumulating evidence has shown that miRNAs were differentially expressed in skin lesions and peripheral blood mononuclear cells of patients with vitiligo. In particular, miRNAs are significantly correlated with the development and progression of vitiligo. The abundance of some miRNAs in serum was also correlated with the vitiligo lesion severity, indicating that miRNAs might serve as prognostic biomarkers. Importantly, the direct involvement of miRNAs in the pathogenesis of vitiligo has been demonstrated. For example, increased expression of miR-25 contributes to vitiligo through promoting the dysfunction and oxidative stress-induced destruction of melanocytes. However, there are limited studies on the function and mechanism of deregulated miRNAs in vitiligo. Further studies are required to establish clinical applications of miRNAs for vitiligo. More in-depth investigations of miRNAs are needed for the understanding of the pathogenesis of vitiligo and the development of novel therapeutic targets. This present review summarizes the current literature on the deregulation and pathogenic roles of miRNAs in vitiligo. We also highlight the potential clinical applications of miRNAs in patients with vitiligo.

Altered epidermal proliferation, differentiation, and lipid composition: Novel key elements in the vitiligo puzzle

Vitiligo is an acquired skin depigmentation disease involving multiple pathogenetic mechanisms, which ultimately direct cytotoxic CD8+ cells to destroy melanocytes. Abnormalities have been described in several cells even in pigmented skin as an expression of a functional inherited defect. Keratinocytes regulate skin homeostasis by the assembly of a proper skin barrier and releasing and responding to cytokines and growth factors. Alterations in epidermal proliferation, differentiation, and lipid composition as triggers for immune response activation in vitiligo have not yet been investigated. By applying cellular and lipidomic approaches, we revealed a deregulated keratinocyte differentiation with altered lipid composition, associated with impaired energy metabolism and increased glycolytic enzyme expression. Vitiligo keratinocytes secreted inflammatory mediators, which further increased following mild mechanical stress, thus evidencing immune activation. These findings identify intrinsic alterations of the nonlesional epidermis, which can be the prime instigator of the local inflammatory milieu that stimulates immune responses targeting melanocytes.

Current Concepts of Vitiligo Immunopathogenesis

Vitiligo is an acquired immune-mediated disorder of pigmentation clinically characterized by well-defined depigmented or chalk-white macules and patches on the skin. The prevalence of vitiligo varies by geographical area, affecting 0.5% to 2% of the population. The disease imposes a significant psychological burden due to its major impact on patients’ social and emotional aspects of life. Given its autoimmune background, vitiligo is frequently associated with other autoimmune diseases or immune-mediated diseases. Vitiligo is a multifaceted disorder that involves both genetic predisposition and environmental triggers. In recent years, major predisposing genetic loci for the development of vitiligo have been discovered. The current findings emphasize the critical role of immune cells and their mediators in the immunopathogenesis of vitiligo. Oxidative-stress-mediated activation of innate immunity cells such as dendritic cells, natural killer, and ILC-1 cells is thought to be a key event in the early onset of vitiligo. Innate immunity cells serve as a bridge to adaptive immunity cells including T helper 1 cells, cytotoxic T cells and resident memory T cells. IFN-γ is the primary cytokine mediator that activates the JAK/STAT pathway, causing keratinocytes to produce the key chemokines CXCL9 and CXCL10. Complex interactions between immune and non-immune cells finally result in apoptosis of melanocytes. This paper summarizes current knowledge on the etiological and genetic factors that contribute to vitiligo, with a focus on immunopathogenesis and the key cellular and cytokine players in the disease’s inflammatory pathways.

sPmel17 Secreted by Ultraviolet B-Exposed Melanocytes Alters the Intercellular Adhesion of Keratinocytes

Repigmentation of the skin in patients with vitiligo represents an intricate process in which the depigmented epidermis is replenished by functional melanocytes (MCs) that migrate from undamaged hair follicles and/or surrounding areas. We characterized whether MCs release a secreted form of Pmel17 (sPmel17) protein after exposure to UVB, thereby weakening the cell-cell adhesions of keratinocytes (KCs), which provides MCs the opportunity to migrate to areas devoid of MCs. At first, we examined the interactions of sPmel17 and FHL2 (four-and-a-half LIM domain protein 2) in KCs treated with the conditioned media (CM) from MCs exposed to UVB. The results showed that both the protein and mRNA levels of FHL2 were significantly upregulated in KCs treated with sPmel17-enriched CM from UVB-exposed MCs. We also found that there are physical interactions between sPmel17 and FHL2 as analyzed by reciprocal coimmunoprecipitation assays and double immunofluorescence staining. The CM from UVB-exposed MCs signaled KCs to remodel the actin cytoskeleton and reduce E-cadherin expression. However, the CM from UVB-exposed and Pmel17-silenced or from UVB-unexposed MCs failed to do this. To further determine the in situ distributions of sPmel17, FHL2, and E-cadherin, we examined the expression profiles of those proteins in the skin from healthy subjects and from depigmented or repigmented vitiligo using immunofluorescence and immunohistochemical staining. The results showed that the expression of sPmel17 was positively correlated with FHL2 but not to E-cadherin. The colocalization of FHL2 and sPmel17 was also observed in UVB-exposed mouse tail skin. Together, the upregulation of FHL2 in KCs requires stimulation by sPmel17 secreted from MCs and activation of the sPmel17-FHL2-E-cadherin axis offers a potential therapeutic target to expedite the repigmentation process in patients with vitiligo.

Methylcobalamin Protects Melanocytes from H2O2-Induced Oxidative Stress by Activating the Nrf2/HO-1 Pathway

Purpose

Oxidative stress is considered a major determinant in the pathogenesis of vitiligo. Methylcobalamin (MeCbl) is an activated form of vitamin B12 that regulates inflammatory factors, counters oxidative stress, and reduces apoptosis in many disease models. However, the specific mechanism of MeCbl repigmentation against vitiligo is unknown. In this study, we explored the effect of MeCbl on melanocytes following hydrogen peroxide (H₂O₂)-induced oxidative stress.

Methods

We established an oxidative stress model using the immortalized human normal melanocyte cell line PIG1. We used a Cell Counting Kit-8 (CCK-8) to detect drug cytotoxicity, and we measured the melanin content of cells using the NaOH method. Intracellular oxidative damage was assessed by flow cytometry and antioxidant enzyme detection kits. In addition, we assessed the presence of apoptosis by flow cytometry and Western blots. We explored the underlying mechanisms of MeCbl during oxidative stress in melanocytes by analyzing the results of experiments based on real-time quantitative polymerase chain reaction (RT-qPCR), Western blotting, and laser scanning confocal immunofluorescence microscopy. Finally, we repeated the experiments after applying an inhibitor to block the Nrf2 pathway.

Results

We found that MeCbl treatment enhanced cell viability, increased melanin content, reduced intracellular reactive oxygen species (ROS) accumulation, increased the activities of antioxidant enzyme superoxide dismutase (SOD) and catalase (CAT), reduced melanocyte apoptosis, and up-regulated the expression of the Nrf2/HO-1 pathway. Moreover, the protective effects of MeCbl were significantly weakened after inhibiting the Nrf2/HO-1 pathway.

Conclusion

Our results indicate that MeCbl attenuated the H₂O₂-induced oxidative stress in melanocytes by activating the Nrf2/HO-1 pathway, this suggests that MeCbl may be an effective treatment against vitiligo.

Vitiligo skin T cells are prone to produce type 1- and type 2-cytokines to induce melanocyte dysfunction and epidermal inflammatory response through JAK signaling

Vitiligo is a T cell-mediated inflammatory skin disorder characterized by the loss of epidermal melanocytes. However, the contribution of melanocytes to the physiopathology of the disease in response to the T cell microenvironment remains unclear. Here, using NanoString technology and multiplex ELISA, we show that active vitiligo perilesional skin is characterized by prominent type 1 and 2 associated immune responses. The vitiligo skin T cell secretome downregulated melanocyte function and adhesion, while increasing melanocyte mitochondrial metabolism and expression of inflammatory cytokines and chemokines by epidermal cells. The JAK1/2 inhibitor ruxolitinib strongly inhibited such effects on epidermal cells. Our data highlight that vitiligo is more complex than previously thought with prominent combined activities of both Th1- and Th2-related cytokines inducing inflammatory responses of epidermal cells. Melanocytes do not appear only to be a target of T cells in vitiligo but could actively contribute to perpetuate inflammation. JAK inhibitors could prevent the impact of T cells on epidermal cells and pigmentation, highlighting their potential clinical benefit in vitiligo.

Identification of a Potentially Functional circRNA-miRNA-mRNA Regulatory Network in Melanocytes for Investigating Pathogenesis of Vitiligo

CircRNAs have been reported to play essential roles in regulating immunity and inflammation, which may be an important regulatory factor in the development of vitiligo. However, the expression profile of circRNAs and their potential biological functions in vitiligo have not been reported so far. In our study we found there are 64 dysregulated circRNAs and 14 dysregulated miRNAs in the patients with vitiligo. Through the correlation analysis, we obtained 12 dysregulated circRNAs and 5 dysregulated miRNAs, forming 48 relationships in the circRNA-miRNA-mRNA regulatory network. Gene Ontology analysis indicated dysregulated circRNAs in vitiligo is closely related to the disorder of the metabolic pathway. The KEGG pathway of dysregulation of circRNAs mainly enriched in the biological processes such as ubiquitin mediated proteolysis, endocytosis and RNA degradation, and in Jak-STAT signaling pathway. Therefore, we found the circRNA-miRNA-mRNA regulatory network are involved in the regulation of numerous melanocyte functions, and these dysregulated circRNAs may closely related to the melanocyte metabolism. Our study provides a theoretical basis for studying the vitiligo pathogenesis from the perspective of circRNA-miRNA-mRNA network.

Study of melanocyte density and epidermal thickness in vulvar lichen sclerosus lesions

OBJECTIVE

This study aimed to analyze changes in melanocyte density and epidermal thickness in vulvar lichen sclerosus (VLS).

METHODS

Vulvar skin tissues were collected from 15 VLS female patients in Beijing Hospital, classified into early (n = 7) and late VLS (n = 8) groups according to pathological manifestations. Melanocyte density and full epidermal and cell-layer (from the bottom of the stratum corneum to that of the basal layer) thickness were calculated using an image analysis software. The control group was normal vulvar skin tissues from 15 females after plastic surgery.

RESULTS

The early VLS (0.170 ± 0.071 µm) and late VLS (0.110 ± 0.035 µm) groups had significantly lower densities of epidermal melanocytes than the control group (0.275 ± 0.036) (F = 36.426, P < 0.001). The cell-layer thickness did not differ between the early VLS (154.603 ± 121.984 µm) and control (176.974 ± 80.296 µm) groups (P = 0.899) but significantly decreased in the late VLS group (83.455 ± 37.129 µm) compared to the control group (P = 0.003).

CONCLUSIONS

Melanocyte density decreased in early and late VLS. The full epidermal and cell-layer thickness did not significantly change in early VLS, but the cell-layer thickness decreased in late VLS.

Mechanisms of melanocyte death in vitiligo

Vitiligo is an autoimmune depigment disease results from extensive melanocytes destruction. The destruction of melanocyte is thought to be of multifactorial causation. Genome-wide associated studies have identified single-nucleotide polymorphisms in a panel of susceptible loci as risk factors in melanocyte death. But vitiligo onset can't be solely attributed to a susceptive genetic background. Oxidative stress triggered by elevated levels of reactive oxygen species accounts for melanocytic molecular and organelle dysfunction, a minority of melanocyte demise, and melanocyte-specific antigens exposure. Of note, the self-responsive immune function directly contributes to the bulk of melanocyte deaths in vitiligo. The aberrantly heightened innate immunity, type-1-skewed T helper, and incompetent regulatory T cells tip the balance toward autoreaction and CD8+ cytotoxic T lymphocytes finally execute the killing of melanocytes, possibly alarmed by resident memory T cells. In addition to the well-established apoptosis and necrosis, we discuss several death modalities like oxeiptosis, ferroptosis, and necroptosis that are probably employed in melanocyte destruction. This review focuses on the various mechanisms of melanocytic death in vitiligo pathogenesis to demonstrate a panorama of that. We hope to provide new insights into vitiligo pathogenesis and treatment strategies by the review.

An update on Vitiligo pathogenesis

Vitiligo, the most common depigmenting disorder of the skin, is undergoing a period of intense advances in both disease understanding and therapeutic possibilities leading the way to the beginning of a new era for the disorder. Its pathophysiology has gathered the attention of researchers for years, and many advances have been made in the clarification of the interaction between different factors that result in depigmented macule formation. The complex interplay between non-immunological and immunological factors in vitiligo is key for the development of the disease, and the participation of cells other than melanocytes, such as keratinocytes, fibroblasts, natural killer cells, and innate lymphoid cells, has been shown. Recent advances have also brought to the understanding of the complex part played by a specific subtype of T cells: T-resident memory cells. This review analyzes some of the most recent insights in vitiligo pathogenesis underlining the interactions between different cell types, which are the basis for the therapeutic approaches under development.

Nuclear factor erythroid 2-related factor 2 (Nrf2) as a potential therapeutic target for vitiligo

Vitiligo is an autoimmune disease of the skin which causes loss of melanocytes from the epidermis. Recently, it is demonstrated that oxidative stress (OS) plays a significant role in the immuno-pathogenesis of vitiligo. A major mechanism in the cellular defense against OS is activation of the nuclear factor erythroid2-related factor (Nrf2)-Kelch-like ECH-associated protein 1(Keap1)-antioxidant responsive element (ARE) signaling pathway. Recently it has been shown that vitiligo melanocytes have impaired Nrf2-ARE signaling. A number of drugs including those known as Nrf2 activators and those known to possess effects to activate Nrf2, have been used in treating vitiligo with certain therapeutic effects. Also, studies have shown that a number of compounds can protect melanocytes against OS via activating Nrf2. These compounds may be considered as candidates for developing new drugs for vitiligo in the future. Nrf2 can be considered as a potential therapeutic target for vitiligo.

Type-1 cytokines regulate MMP-9 production and E-cadherin disruption to promote melanocyte loss in vitiligo

Loss of melanocytes is the pathological hallmark of vitiligo, a chronic inflammatory skin depigmenting disorder induced by exaggerated immune response, including autoreactive CD8 T cells producing high levels of type 1 cytokines. However, the interplay between this inflammatory response and melanocyte disappearance remains to be fully characterized. Here, we demonstrate that vitiligo skin contains a significant proportion of suprabasal melanocytes, associated with disruption of E-cadherin expression, a major protein involved in melanocyte adhesion. This phenomenon is also observed in lesional psoriatic skin. Importantly, apoptotic melanocytes were mainly observed once cells were detached from the basal layer of the epidermis, suggesting that additional mechanism(s) could be involved in melanocyte loss. The type 1 cytokines IFN-γ and TNF-α induce melanocyte detachment through E-cadherin disruption and the release of its soluble form, partly due to MMP-9. The levels of MMP-9 are increased in the skin and sera of patients with vitiligo, and MMP-9 is produced by keratinocytes in response to IFN-γ and TNF-α. Inhibition of MMP-9 or the JAK/STAT signaling pathway prevents melanocyte detachment in vitro and in vivo. Therefore, stabilization of melanocytes in the basal layer of the epidermis by preventing E-cadherin disruption appears promising for the prevention of depigmentation occurring in vitiligo and during chronic skin inflammation.

Current insight into the roles of microRNA in vitiligo

Vitiligo is a common chronic depigmented skin disease characterized by melanocyte loss or dysfunction in the lesion. The pathogenesis of vitiligo has not been fully clarified. Most studies have suggested that the occurrence and progression of vitiligo are due to multiple factors and gene interactions in which noncoding RNAs contribute to an individual's susceptibility to vitiligo. Noncoding RNAs, including microRNAs (miRNAs), are a hot topic in posttranscriptional regulatory mechanism research. miRNAs are noncoding RNAs with a length of approximately 22 nucleotides and play a negative regulatory role by binding to the 3'-UTR or 5'-UTR of the target mRNA to inhibit translation or initiate mRNA degradation. Previous studies have screened the differential expression profiles of miRNAs in the skin lesions, melanocytes, peripheral blood mononuclear cells (PBMCs) and sera of patients and mouse models with vitiligo. Moreover, several studies have focused on miRNA-25, miRNA-155 and other miRNAs involved in melanin metabolism, oxidative stress, and melanocyte proliferation and apoptosis. These miRNAs and regulatory processes further illuminate the pathogenesis of vitiligo and provide hope for the application of small molecules in the treatment of vitiligo. In this review, we summarize miRNA expression profiles in different tissues of vitiligo patients and the mechanisms by which key miRNAs mediate vitiligo development.

Cannabinoid Signaling in the Skin: Therapeutic Potential of the "C(ut)annabinoid" System

The endocannabinoid system (ECS) has lately been proven to be an important, multifaceted homeostatic regulator, which influences a wide-variety of physiological processes all over the body. Its members, the endocannabinoids (eCBs; e.g., anandamide), the eCB-responsive receptors (e.g., CB₁, CB₂), as well as the complex enzyme and transporter apparatus involved in the metabolism of the ligands were shown to be expressed in several tissues, including the skin. Although the best studied functions over the ECS are related to the central nervous system and to immune processes, experimental efforts over the last two decades have unambiguously confirmed that cutaneous cannabinoid ("c[ut]annabinoid") signaling is deeply involved in the maintenance of skin homeostasis, barrier formation and regeneration, and its dysregulation was implicated to contribute to several highly prevalent diseases and disorders, e.g., atopic dermatitis, psoriasis, scleroderma, acne, hair growth and pigmentation disorders, keratin diseases, various tumors, and itch. The current review aims to give an overview of the available skin-relevant endo- and phytocannabinoid literature with a special emphasis on the putative translational potential, and to highlight promising future research directions as well as existing challenges.