Human melanocytes express functional Toll-like receptor 4

AMPs as Host-Directed Immunomodulatory Agents against Skin Infections Caused by Opportunistic Bacterial Pathogens

Skin is the primary and largest protective organ of the human body. It produces a number of highly evolved arsenal of factors to counter the continuous assault of foreign materials and pathogens from the environment. One such potent factor is the repertoire of Antimicrobial Peptides (AMPs) that not only directly destroys invading pathogens, but also optimally modulate the immune functions of the body to counter the establishment and spread of infections. The canonical direct antimicrobial functions of these AMPs have been in focus for a long time to design principles for enhanced therapeutics, especially against the multi-drug resistant pathogens. However, in recent times the immunomodulatory functions performed by these peptides at sub-microbicidal concentrations have been a point of major focus in the field of host-directed therapeutics. Such strategies have the added benefit of not having the pathogens develop resistance against the immunomodulatory pathways, since the pathogens exploit these signaling pathways to obtain and survive within the host. Thus, this review summarizes the potent immunomodulatory effect of these AMPs on, specifically, the different host immune cells with the view of providing a platform of information that might help in designing studies to exploit and formulate effective host-directed adjunct therapeutic strategies that would synergies with drug regimens to counter the current diversity of drug-resistant skin opportunistic pathogens.

Exploring the effect of butyric acid, a metabolite from periodontopathic bacteria, on primary human melanocytes: An in vitro study

Effects of butyric acid, a bacterial metabolite implicated in periodontitis progression, have never been examined on oral melanocytes. Herein, primary human epidermal melanocytes were used as a model for oral melanocytes. Results show the adverse effects of butyric acid (sodium butyrate; NaB) on them, which comprise marked cytotoxicity at higher concentrations (>1 mM) and robust differentiation at lower nontoxic concentrations. NaB did not alter MITF protein levels; however, it stimulated tyrosinase protein synthesis and inhibited tyrosinase activity, with no changes in cellular melanin. NaB did not affect oxidative stress, although it induced significant levels of the pro-inflammatory cytokine IL-6.

A systematic review of natural products for skin applications: Targeting inflammation, wound healing, and photo-aging

BACKGROUND

Every day the skin is constantly exposed to several harmful factors that induce oxidative stress. When the cells are incapable to maintain the balance between antioxidant defenses and reactive oxygen species, the skin no longer can keep its integrity and homeostasis. Chronic inflammation, premature skin aging, tissue damage, and immunosuppression are possible consequences induced by sustained exposure to environmental and endogenous reactive oxygen species. Skin immune and non-immune cells together with the microbiome are essential to efficiently trigger skin immune responses to stress. For this reason, an ever-increasing demand for novel molecules capable of modulating immune functions in the skin has risen the level of their development, particularly in the field of natural product-derived molecules.

PURPOSE

In this review, we explore different classes of molecules that showed evidence in modulate skin immune responses, as well as their target receptors and signaling pathways. Moreover, we describe the role of polyphenols, polysaccharides, fatty acids, peptides, and probiotics as possible treatments for skin conditions, including wound healing, infection, inflammation, allergies, and premature skin aging.

METHODS

Literature was searched, analyzed, and collected using databases, including PubMed, Science Direct, and Google Scholar. The search terms used included "Skin", "wound healing", "natural products", "skin microbiome", "immunomodulation", "anti-inflammatory", "antioxidant", "infection", "UV radiation", "polyphenols", "polysaccharides", "fatty acids", "plant oils", "peptides", "antimicrobial peptides", "probiotics", "atopic dermatitis", "psoriasis", "auto-immunity", "dry skin", "aging", etc., and several combinations of these keywords.

RESULTS

Natural products offer different solutions as possible treatments for several skin conditions. Significant antioxidant and anti-inflammatory activities were reported, followed by the ability to modulate immune functions in the skin. Several membrane-bound immune receptors in the skin recognize diverse types of natural-derived molecules, promoting different immune responses that can improve skin conditions.

CONCLUSION

Despite the increasing progress in drug discovery, several limiting factors need future clarification. Understanding the safety, biological activities, and precise mechanisms of action is a priority as well as the characterization of the active compounds responsible for that. This review provides directions for future studies in the development of new molecules with important pharmaceutical and cosmeceutical value.

A genome-wide integrated analysis of lncRNA-mRNA in melanocytes from white and brown skin hair boer goats (Capra aegagrus hircus)

Long noncoding RNAs (lncRNAs) are involved in many biological processes and have been extensively researched. Nonetheless, literature focusing on the roles of lncRNA in melanocytes is limited. Melanocytes are located in the basal layer of the epidermis and determine the color of an animal's skin and hair by producing melanin. The mechanisms of melanogenesis remain unclear. Here, melanocytes from Boer goat skins were successfully isolated and verified using morphological observation, dopamine staining, silver ammonia staining, and immunohistochemical staining in vitro. Phenotypic testing revealed that melanocytes isolated from goat skins with white and brown hairs showed significant differences in proliferation, migration, and melanogenesis (^**P < 0.01). RNA sequencing was performed with the isolated melanocytes, and through bioinformatic analysis, several candidate lncRNAs and mRNAs involved in stage-specific melanogenesis were identified. Functional enrichment analysis indicated that miRNA precursors and cis-regulatory effects of lncRNAs were deeply dissected using the function prediction software. Multiple lncRNA–mRNA networks were presumed to be involved in melanocyte migration, proliferation, and melanogenesis based on the Kyoto Encyclopedia of Genes and Genomes (KEGG) annotation. This research provided novel bioinformatic insights into the roles of lncRNAs in mammalian pigmentation.

Recognition of Melanocytes in Immuno-Neuroendocrinology and Circadian Rhythms: Beyond the Conventional Melanin Synthesis

Melanocytes produce melanin to protect the skin from UV-B radiation. Notwithstanding, the spectrum of their functions extends far beyond their well-known role as melanin production factories. Melanocytes have been considered as sensory and computational cells. The neurotransmitters, neuropeptides, and other hormones produced by melanocytes make them part of the skin’s well-orchestrated and complex neuroendocrine network, counteracting environmental stressors. Melanocytes can also actively mediate the epidermal immune response. Melanocytes are equipped with ectopic sensory systems similar to the eye and nose and can sense light and odor. The ubiquitous inner circadian rhythm controls the body’s basic physiological processes. Light not only affects skin photoaging, but also regulates inner circadian rhythms and communicates with the local neuroendocrine system. Do melanocytes “see” light and play a unique role in photoentrainment of the local circadian clock system? Why, then, are melanocytes responsible for so many mysterious functions? Do these complex functional devices work to maintain homeostasis locally and throughout the body? In addition, melanocytes have also been shown to be localized in internal sites such as the inner ear, brain, and heart, locations not stimulated by sunlight. Thus, what can the observation of extracutaneous melanocytes tell us about the “secret identity” of melanocytes? While the answers to some of these intriguing questions remain to be discovered, here we summarize and weave a thread around available data to explore the established and potential roles of melanocytes in the biological communication of skin and systemic homeostasis, and elaborate on important open issues and propose ways forward.

Comparison of Transcriptional Signatures of Three Staphylococcal Superantigenic Toxins in Human Melanocytes

Staphylococcus aureus, a gram-positive bacterium, causes toxic shock through the production of superantigenic toxins (sAgs) known as Staphylococcal enterotoxins (SE), serotypes A-J (SEA, SEB, etc.), and toxic shock syndrome toxin-1 (TSST-1). The chronology of host transcriptomic events that characterizes the response to the pathogenesis of superantigenic toxicity remains uncertain. The focus of this study was to elucidate time-resolved host responses to three toxins of the superantigenic family, namely SEA, SEB, and TSST-1. Due to the evolving critical role of melanocytes in the host’s immune response against environmental harmful elements, we investigated herein the transcriptomic responses of melanocytes after treatment with 200 ng/mL of SEA, SEB, or TSST-1 for 0.5, 2, 6, 12, 24, or 48 h. Functional analysis indicated that each of these three toxins induced a specific transcriptional pattern. In particular, the time-resolved transcriptional modulations due to SEB exposure were very distinct from those induced by SEA and TSST-1. The three superantigens share some similarities in the mechanisms underlying apoptosis, innate immunity, and other biological processes. Superantigen-specific signatures were determined for the functional dynamics related to necrosis, cytokine production, and acute-phase response. These differentially regulated networks can be targeted for therapeutic intervention and marked as the distinguishing factors for the three sAgs.

Successful Repigmentation of Full-Thickness Wound Healing in Fraser’s Dolphins (Lagenodelphis hosei)

Fraser’s dolphins (Lagenodelphis hosei) exhibit the capability to restore nearly normal pigmentation after full-thickness wounding. However, the association among melanocytes, melanin and skin pigmentation during wound healing in cetaceans has yet to be addressed. Here, the number of melanocytes and the distribution of melanocytes and melanin in different-colored skin and different wound-healing stages in Fraser’s dolphins were analyzed by using Fontana–Masson staining, immunofluorescence staining and immunohistochemical staining. It was noticed that there was the highest number of melanocytes in dark skin and the lowest number of melanocytes in white skin. The appearance of functional melanocytes and full-melanized neoepidermis was observed in the early stage of wound healing in Fraser’s dolphins. Furthermore, the melanocyte number and skin pigmentation and pattern in healed wounds recovered to a similar condition of unwounded skin. This study provides fundamental knowledge of skin repigmentation in cetaceans for further research, and it will be warranted to elucidate the mechanisms of the replenishment of melanocytes and the regulation of melanocyte activity that contribute to the successful repigmentation in cetacean skin wounds.

Imiquimod induced vitiligo-like lesions-A consequence of modified melanocyte function

INTRODUCTION

Imiquimod plays an important role in the management of condyloma and premalignant lesions. Successively, an increase of hypopigmented lesions following imiquimod application has been reported. However, the mechanisms of imiquimod on melanocytes remain unclear. This study was designed to assess the effect of Imiquimod on the functions of melanocytes in vitro.

METHODS

Primary cultured melanocytes were isolated from normal control skin tissue. After incubation with imiquimod for 48 h in vitro, cell viability was analyzed by cell counting kit-8 assay. Apoptosis was detected using the Annexin V-fluorescein-5-isothiocyanate flow cytometry assay. Melanin content and tyrosinase activity in melanocytes were measured by colorimetric method and the modified dopachrome method. The production of inflammatory cytokine interleukin 8 (IL-8), IL-6, and soluble ICAM-1 (soluble Intercellular Adhesion Molecule-1[sICAM-1]) in melanocytes were measured by enzyme-linked immunosorbent assay (ELISA). Toll-like receptor 7 (TLR7), toll-like receptor 9 (TLR9) protein, and autophagy-related proteins microtubule-associated protein 1A/1B-light chain 3 (LC3-II), p62, mechanistic target of rapamycin (mTOR), and Atg5 were assessed using western blot analysis.

RESULTS

Imiquimod significantly inhibited the activity of tyrosinase activity and decreased melanin content in melanocytes and significantly increased apoptosis and IL-6, IL-8, and sICAM-1 production in melanocytes. Moreover, the expression of TLR7 and TLR9 proteins were significantly increased, and the expression of mTOR, p62 protein were markedly decreased, but the expression of LC3II/I and Atg5 protein were significantly increased in melanocytes after incubating with imiquimod.

CONCLUSIONS

This study shows that imiquimod directly inhibits melanogenesis and increases melanocyte apoptosis rates. These effects combined with the upregulation of TLR7 and TLR9 together with increased autophagy activity and inflammatory cytokines production, might be the main reasons leading to hypopigmented lesions after imiquimod application.

Mucosal Melanoma: Pathological Evolution, Pathway Dependency and Targeted Therapy

Mucosal melanoma (MM) is a rare melanoma subtype that originates from melanocytes within sun-protected mucous membranes. Compared with cutaneous melanoma (CM), MM has worse prognosis and lacks effective treatment options. Moreover, the endogenous or exogenous risk factors that influence mucosal melanocyte transformation, as well as the identity of MM precursor lesions, are ambiguous. Consequently, there remains a lack of molecular markers that can be used for early diagnosis, and therefore better management, of MM. In this review, we first summarize the main functions of mucosal melanocytes. Then, using oral mucosal melanoma (OMM) as a model, we discuss the distinct pathologic stages from benign mucosal melanocytes to metastatic MM, mapping the possible evolutionary trajectories that correspond to MM initiation and progression. We highlight key areas of ambiguity during the genetic evolution of MM from its benign lesions, and the resolution of which could aid in the discovery of new biomarkers for MM detection and diagnosis. We outline the key pathways that are altered in MM, including the MAPK pathway, the PI3K/AKT pathway, cell cycle regulation, telomere maintenance, and the RNA maturation process, and discuss targeted therapy strategies for MM currently in use or under investigation.

The Cellular Composition of the Uveal Immune Environment

The uveal tract consists of the iris, the ciliary body and the choroid; these three distinct tissues form a continuous layer within the eye. Uveitis refers to inflammation of any region of the uveal tract. Despite being grouped together anatomically, the iris, ciliary body and choroid are distinct functionally, and inflammatory diseases may affect only one part and not the others. Cellular structure of tissues direct their function, and understanding the cellular basis of the immune environment of a tissue in health, the "steady state" on which the perturbations of disease are superimposed, is vital to understanding the pathogenesis of those diseases. A contemporary understanding of the immune system accepts that haematopoietic and yolk sac derived leukocytes, though vital, are not the only players of importance. An array of stromal cells, connective tissue cells such as fibroblasts and endothelial cells, may also have a role in the inflammatory reaction seen in several immune-mediated diseases. In this review we summarise what is known about the cellular composition of the uveal tract and the roles these disparate cell types have to play in immune homeostasis. We also discuss some unanswered questions surrounding the constituents of the resident leukocyte population of the different uveal tissues, and we look ahead to the new understanding that modern investigative techniques such as single cell transcriptomics, multi-omic data integration and highly-multiplexed imaging techniques may bring to the study of the uvea and uveitis, as they already have to other immune mediated inflammatory diseases.

Melanogenesis Connection with Innate Immunity and Toll-Like Receptors

The epidermis is located in the outermost layer of the living body and is the place where external stimuli such as ultraviolet rays and microorganisms first come into contact. Melanocytes and melanin play a wide range of roles such as adsorption of metals, thermoregulation, and protection from foreign enemies by camouflage. Pigmentary disorders are observed in diseases associated with immunodeficiency such as Griscelli syndrome, indicating molecular sharing between immune systems and the machineries of pigment formation. Melanocytes express functional toll-like receptors (TLRs), and innate immune stimulation via TLRs affects melanin synthesis and melanosome transport to modulate skin pigmentation. TLR2 enhances melanogenetic gene expression to augment melanogenesis. In contrast, TLR3 increases melanosome transport to transfer to keratinocytes through Rab27A, the responsible molecule of Griscelli syndrome. TLR4 and TLR9 enhance tyrosinase expression and melanogenesis through p38 MAPK (mitogen-activated protein kinase) and NFκB signaling pathway, respectively. TLR7 suppresses microphthalmia-associated transcription factor (MITF), and MITF reduction leads to melanocyte apoptosis. Accumulating knowledge of the TLRs function of melanocytes has enlightened the link between melanogenesis and innate immune system.

Increased expression of Toll-like receptor 7 and 9 in vitiligo melanocytes: a pilot study

BACKGROUND

Toll-like receptors (TLRs) are expressed on human melanocytes, and play an important role in innate and acquired immunity. The role of TLRs in the pathogenesis of vitiligo has not been fully described.

AIM

To investigate the expression of TLRs in melanocytes from patients with vitiligo and healthy controls (HCs).

METHODS

Primary cultured vitiligo and control melanocytes were obtained from perilesional normal skin of patients with generalized vitiligo and HCs. TLRs mRNA expression in melanocytes were determined by real-time reverse transcription PCR and protein expression by western blotting. Apoptosis was analysed using an annexin V-fluorescein isothiocyanate apoptosis detection kit, and tyrosinase activity and melanin content were measured by a modified dopachrome and colorimetric method. Interleukin (IL)-6, IL-8 and soluble cell adhesion molecule (sICAM)-1 expression were measured by ELISA.

RESULTS

In vitiligo melanocytes, compared with control melanocytes, apoptosis rate, expression of TLR7 and TLR9 mRNA and protein, and production of IL-8, IL-6 and sICAM-1 were significantly increased, whereas tyrosinase activity and melanin content were significantly decreased.

CONCLUSIONS

Our results suggest that the increased expression of TLR7 and TLR9 might correlate with melanocyte dysfunction in vitiligo.

Cistanche deserticola polysaccharide induces melanogenesis in melanocytes and reduces oxidative stress via activating NRF2/HO-1 pathway

As a main part of pigmentation disorders, skin depigmentation diseases such as vitiligo and achromic naevus are very common and get more attention now. The pathogenesis of depigmentation includes melanocyte dysfunction and loss, which are possibly caused by heredity, autoimmunity and oxidative stress. Among them, oxidative stress plays a key role; however, few clinical treatments can deal with oxidative stress. As reported, Cistanche deserticola polysaccharide (CDP) is an effective antioxidant; based on that, we evaluated its role in melanocyte and further revealed the mechanisms. In this study, we found that CDP could promote melanogenesis in human epidermal melanocytes (HEMs) and mouse melanoma B16F10 cells, it also induced pigmentation in zebrafish. Furthermore, CDP could activate mitogen‐activated protein kinase (MAPK) signal pathway, then up‐regulated the expression of microphthalmia‐associated transcription factor (MITF) and downstream genes TYR, TRP1, TRP2 and RAB27A. Otherwise, we found that CDP could attenuate H₂O₂‐induced cytotoxicity and apoptosis in melanocytes. Further evidence revealed that CDP could enhance NRF2/HO‐1 antioxidant pathway and scavenge intracellular ROS. In summary, CDP can promote melanogenesis and prevent melanocytes from oxidative stress injury, suggesting that CDP helps maintain the normal status of melanocytes. Thus, CDP may be a novel drug for the treatment of depigmentation diseases.

Cancer cell-derived long pentraxin 3 (PTX3) promotes melanoma migration through a toll-like receptor 4 (TLR4)/NF-κB signaling pathway

Cutaneous melanoma is one of the most aggressive cancers characterized by a high plasticity, a propensity for metastasis, and drug resistance. Melanomas are composed of phenotypically diverse subpopulations of tumor cells with heterogeneous molecular profiles that reflect intrinsic invasive abilities. In an attempt to identify novel factors of the melanoma invasive cell state, we previously investigated the nature of the invasive secretome by using a comparative proteomic approach. Here, we have extended this analysis to show that PTX3, an acute phase inflammatory glycoprotein, is one such factor secreted by invasive melanoma to promote tumor cell invasiveness. Elevated PTX3 production was observed in the population of MITFlow invasive cells but not in the population of MITFhigh differentiated melanoma cells. Consistently, MITF knockdown increased PTX3 expression in MITFhigh proliferative and poorly invasive cells. High levels of PTX3 were found in tissues and blood of metastatic melanoma patients, and in BRAF inhibitor-resistant melanoma cells displaying a mesenchymal invasive MITFlow phenotype. Genetic silencing of PTX3 in invasive melanoma cells dramatically impaired migration and invasion in vitro and in experimental lung extravasation assay in xenografted mice. In contrast, addition of melanoma-derived or recombinant PTX3, or expression of PTX3 enhanced motility of low migratory cells. Mechanistically, autocrine production of PTX3 by melanoma cells triggered an IKK/NFκB signaling pathway that promotes migration, invasion, and expression of the EMT factor TWIST1. Finally, we found that TLR4 and MYD88 knockdown inhibited PTX3-induced melanoma cell migration, suggesting that PTX3 functions through a TLR4-dependent pathway. Our work reveals that tumor-derived PTX3 contributes to melanoma cell invasion via targetable inflammation-related pathways. In addition to providing new insights into the biology of melanoma invasive behavior, this study underscores the notion that secreted PTX3 represents a potential biomarker and therapeutic target in a subpopulation of MITFlow invasive and/or refractory melanoma.

The Dynamics of the Skin's Immune System

The skin is a complex organ that has devised numerous strategies, such as physical, chemical, and microbiological barriers, to protect the host from external insults. In addition, the skin contains an intricate network of immune cells resident to the tissue, crucial for host defense as well as tissue homeostasis. In the event of an insult, the skin-resident immune cells are crucial not only for prevention of infection but also for tissue reconstruction. Deregulation of immune responses often leads to impaired healing and poor tissue restoration and function. In this review, we will discuss the defensive components of the skin and focus on the function of skin-resident immune cells in homeostasis and their role in wound healing.

Differential expression of inflammatory cytokines and chemokines in lipopolysaccharide-stimulated melanocytes from lightly and darkly pigmented skin

Increasing evidence suggests that human epidermal melanocytes play an important role in the skin immune system; however, a role of their pigmentation in immune and inflammatory responses is poorly examined. In the study, the expression of Toll-like receptor 4 (TLR4) and inflammatory cytokines and chemokines by cultured normal melanocytes derived from lightly and darkly pigmented skin was investigated after cell stimulation with lipopolysaccharide (LPS). The basal TLR4 mRNA level in heavily pigmented cells was higher as compared to their lightly pigmented counterparts. Melanocyte exposure to LPS upregulated the expression of TLR4 mRNA and enhanced the DNA-binding activity of NF-κB p50 and p65. We found substantial differences in the LPS-stimulated expression of numerous genes encoding inflammatory cytokines and chemokines between the cells with various melanin contents. In lightly pigmented melanocytes, the most significantly upregulated genes were nicotinamide phosphoribosyltransferase (NAMPT/visfatin), the chemokines CCL2 and CCL20, and IL6, while the genes for CXCL12, IL-16 and the chemokine receptor CCR4 were the most significantly upregulated in heavily pigmented cells. Moreover, the lightly pigmented melanocytes secreted much more NAMPT, CCL2 and IL-6. The results of our study suggest modulatory effect of melanogenesis on the immune properties of normal epidermal melanocytes.

TLR4-directed Molecular Strategies Targeting Skin Photodamage and Carcinogenesis

Background:

Exposure to solar ultraviolet (UV) radiation is a causative factor in skin photodamage and carcinogenesis, and inflammatory dysregulation is a key mechanism underlying detrimental effects of acute and chronic UV exposure. The health and economic burden of skin cancer treatment is substantial, creating an increasingly urgent need for the development of improved molecular strategies for photoprotection and photochemoprevention.

Methods:

A structured search of bibliographic databases for peer-reviewed research literature revealed 139 articles including our own that are presented and critically evaluated in this TLR4-directed review.

Objective:

To understand the molecular role of Toll-like receptor 4 (TLR4) as a key regulator of skin anti-microbial defense, wound healing, and cutaneous tumorigenic inflammation. The specific focus of this review is on recent published evidence suggesting that TLR4 represents a novel molecular target for skin photoprotection and cancer photochemoprevention.

Results:

Cumulative experimental evidence indicates that pharmacological and genetic antagonism of TLR4 suppresses UV-induced inflammatory signaling involving the attenuation of cutaneous NF-κB and AP-1 stress signaling observable in vitro and in vivo. TLR4-directed small molecule pharmacological antagonists [including eritoran, (+)-naloxone, ST2825, and resatorvid] have now been identified as a novel class of molecular therapeutics. TLR4 antagonists are in various stages of preclinical and clinical development for the modulation of dysregulated TLR4-dependent inflammatory signaling that may also contribute to skin photodamage and photocarcinogenesis in human populations.

Conclusion:

Future research should explore the skin photoprotective and photochemopreventive efficacy of topical TLR4 antagonism if employed in conjunction with other molecular strategies including sunscreens.

Toll-like receptors 2 and 3 enhance melanogenesis and melanosome transport in human melanocytes

Because little is known about how the innate immune response influences skin pigmentation, we examined whether Toll-like receptor (TLR) agonists participate in melanogenesis and melanosome transportation. We observed that TLR2/2 agonist HKLM and TLR3 agonist Poly(I:C) increased the amount of extracellular melanin from primary human epidermal melanocytes. HKLM, but not Poly(I:C), increased the melanogenic genes such as tyrosinase and dopachrome tautomerase. Poly(I:C) increased the expression of Rab27A, a molecule that facilitates melanosome transport to perimembranous actin filament. UVB irradiation induced Rab27A and melanosome transportation in a similar manner of Poly(I:C). SiRNA for TLR3 or Rab27A suppressed the perimembranous accumulation of Gp100-positive vesicles in melanocytes and decreased melanin transfer to neighboring keratinocytes induced by both Poly(I:C) and UVB. These results suggest that the microenvironment in the epidermis and innate immune stimuli, such as microbiome and ultraviolet represented here by TLR2 and TLR3 agonists, could affect the melanogenesis in human melanocytes.

Repeated ultraviolet irradiation induces the expression of Toll-like receptor 4, IL-6, and IL-10 in neonatal human melanocytes

BACKGROUND

Human melanocytes express Toll-like receptor 4 (TLR4), which regulates ultraviolet (UV)-induced cutaneous immunosuppression in Langerhans cells. Lipopolysaccharide (LPS) stimulation increases melanocyte pigmentation and TLR4 expression, while inducing local innate inflammatory responses.

AIMS

We investigated whether UV radiation induces TLR4 expression in neonatal human melanocytes (NHMs) and how this affects the immune system.

METHODS

We cultured NHMs with LPS treatment or with one-time or repeated UVA or UVB exposure, and investigated and compared the effects on TLR4 expression, melanin contents, and cytokine production.

RESULTS

NHMs in the resting state did not express TLR4. LPS stimulation induced TLR4 expression and increased pigmentation. TLR4 expression was not detected after single-dose UVA or UVB treatment, but pigmentation increased. Repeated UV treatment induced TLR4 expression and increased pigmentation. LPS stimulation and repeated UV treatment increased IL-6 secretion, and repeated UVB treatment increased IL-10 secretion.

CONCLUSION

These results suggest that human melanocytes may actively participate in UV-induced immune modulation.

Toll-Like Receptor Function in Acute Wounds

Significance: Inflammation is an integral part of immune response and supports optimal wound healing in adults. Inflammatory cells such as neutrophils, macrophages, dendritic cells, lymphocytes, and mast cells produce important cytokines, chemokines, and growth factors. These immune cells interact with keratinocytes, fibroblasts, and endothelial cells (ECs), as well as the extracellular matrix within a complicated network that promotes and regulates wound healing. Aberrant and persistent inflammation may result in delayed wound healing, scar formation, or chronic wounds. Targeting the molecules involved in the inflammatory response may have great potential therapeutic value. Recent Advances and Critical Issues: Toll-like receptors (TLRs) are pattern recognition receptors that recognize pathogen-associated molecular patterns from microbes or danger-associated molecular patterns from damaged cells. The discovery of TLRs sheds new light on the mechanism by which the inflammatory or innate immune response is initiated in wound healing. Convincing evidence now shows that multiple types of cells, including infiltrating or resident inflammatory cells, keratinocytes, fibroblasts, and ECs, express specific types of TLRs. Experimental reduction of certain TLRs or treatment of wounds with TLR ligands has been shown to affect wound healing. A better understanding of the involvement of TLRs in the innate immune response during skin wound healing may suggest novel strategies to improve the quality of tissue repair. Future Directions: Despite the indisputable role of TLRs in regulating the immune response in acute wound healing, the functions of TLRs that are relevant to human wound healing and chronic wounds are poorly understood.

Melanocyte and melanoma cell activation by calprotectin

Calprotectin, a heterodimer of S100A8 and S100A9, is a proinflammatory cytokine released from ultraviolet radiation-exposed keratinocytes. Calprotectin binds to Toll-like receptor 4, the receptor for advanced glycation end-products, and extracellular matrix metalloproteinase inducer on target cells to stimulate migration. Melanocytes and melanoma cells produce little if any calprotectin, but they do express receptors for the cytokine. Thus, keratinocyte-derived calprotectin has the potential to activate melanocytes and melanoma cells within the epidermis in a paracrine manner. We examined the ability of calprotectin to stimulate proliferation and migration in normal human melanocytes and melanoma cells in vitro. We first showed, by immunofluorescence and quantitative RT-PCR, that the melanocytic cells employed expressed a calprotectin receptor, the receptor for advanced end-products. We then demonstrated that calprotectin significantly enhanced proliferation, migration, and Matrigel invasion in both normal human melanocytes and melanoma cells. Thus, calprotectin is one of the numerous paracrine factors released by ultraviolet radiation-exposed keratinocytes that may promote melanomagenesis and is a potential target for melanoma prevention or therapy.

Melanocytes and melanin represent a first line of innate immunity against Candida albicans

Melanocytes are dendritic cells located in the skin and mucosae that synthesize melanin. Some infections induce hypo- or hyperpigmentation, which is associated with the activation of Toll-like receptors (TLRs), especially TLR4. Candida albicans is an opportunist pathogen that can switch between blastoconidia and hyphae forms; the latter is associated with invasion. Our objectives in this study were to ascertain whether C. albicans induces pigmentation in melanocytes and whether this process is dependent on TLR activation, as well as relating this with the antifungal activity of melanin as a first line of innate immunity against fungal infections. Normal human melanocytes were stimulated with C. albicans supernatants or with crude extracts of the blastoconidia or hyphae forms, and pigmentation and TLR2/TLR4 expression were measured. Expression of the melanosomal antigens Melan-A and gp100 was examined for any correlation with increased melanin levels or antifungal activity in melanocyte lysates. Melanosomal antigens were induced earlier than cell pigmentation, and hyphae induced stronger melanization than blastoconidia. Notably, when melanocytes were stimulated with crude extracts of C. albicans, the cell surface expression of TLR2/TLR4 began at 48 h post-stimulation and peaked at 72 h. At this time, blastoconidia induced both TLR2 and TLR4 expression, whereas hyphae only induced TLR4 expression. Taken together, these results suggest that melanocytes play a key role in innate immune responses against C. albicans infections by recognizing pathogenic forms of C. albicans via TLR4, resulting in increased melanin content and inhibition of infection.

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.

Activation of Toll-like Receptors 1, 2, 4, 5, and 7 on Human Melanocytes Modulate Pigmentation

Human melanocytes are not simply pigment-producing cells. It may be part of the inflammatory response, during which the pigmentary system may produce more melanin or suppress melanization. Toll-like receptors (TLRs) have been implicated in both innate host defense against pathogens and inflammatory response. Therefore, it may be possible that activation of TLRs in melanocytes may play a role in the modulation of melanogenesis. In this study, we investigated whether normal human melanocytes expressed TLRs and analyzed pigmentation changes upon TLR stimulation. The expression of TLR1~10 mRNA in cultured human melanocyte was analyzed using RT-PCR, Western blotting and immunocytochemistry. Human melanocytes constitutively express mRNA and protein for TLR2, 3, 4, 5, 7, 9 and 10. Stimulation of TLR1/2 and 4 with Pam3CSK4 and lipopolysaccharide induced pigmentation of melanocytes. Activation of TLR5 and 7 with flagellin and imiquimod treatments reduced pigmentation of melanocytes and zebrafish. In summary, the results provided evidence for TLRs expression in normal human melanocytes. It is speculated that a response of melanocyte to TLR ligands may play a role in the pigmentary change in the skin.

Cultured human melanocytes express functional toll-like receptors 2-4, 7 and 9

BACKGROUND

Toll-like receptors (TLRs) recognize pathogen-associated molecular patterns (PAMPs), which have been evolutionarily conserved in microbes. Human melanocytes are not simply pigment-producing cells but also have the phagocytic capacity and can produce pro-inflammatory mediators. However, the mechanisms of recognition of microbes by melanocytes have not yet been fully established.

OBJECTIVE

We investigated the TLRs 1-10 expression profile in human epidermal melanocytes and assessed their functions after triggering by their specific ligands.

METHODS

TLRs mRNA expression was determined by RT-PCR, and the TLR protein expression was measured by flow cytometry and immunofluorescence assays. After stimulation with various TLR ligands, the production of inflammatory cytokine IL-8 and IL-6 was measured by ELISA and the mRNA for chemokine CCL2, CCL3 and CCL5 was analyzed by real-time PCR. Phosphorylation of IkappaBalpha in TLR ligands-triggered melanocytes was determined by Western blot and the nucleus translocation of NF-kappaBp65 was analyzed by immunofluorescence.

RESULTS

Human melanocytes constitutively expressed TLRs 1-4, 6, 7 and 9 mRNA. Ample amounts of TLRs 2-4, 7 and 9 were confirmed at protein level. Stimulation of melanocytes with TLRs ligands resulted in the release of cytokines (IL-8 and IL-6) and the mRNA accumulation of chemokines (CCL2, CCL3 and CCL5). Triggering of TLRs in melanocytes resulted in the up-regulation of phosphorylated IkappaBalpha and in the nucleus translocation of NF-kappaBp65.

CONCLUSION

Present study indicates human melanocytes express a panel of functional TLRs. The ligation of TLRs can turn these cells into active players of the skin innate immunity.

What are melanocytes really doing all day long...?

Everyone knows and seems to agree that melanocytes are there to generate melanin - an intriguing, but underestimated multipurpose molecule that is capable of doing far more than providing pigment and UV protection to skin (1). What about the cell that generates melanin, then? Is this dendritic, neural crest-derived cell still serving useful (or even important) functions when no-one looks at the pigmentation of our skin and its appendages and when there is essentially no UV exposure? In other words, what do epidermal and hair follicle melanocytes do in their spare time - at night, under your bedcover? How much of the full portfolio of physiological melanocyte functions in mammalian skin has really been elucidated already? Does the presence or absence of melanocytes matter for normal epidermal and/or hair follicle functions (beyond pigmentation and UV protection), and for skin immune responses? Do melanocytes even deserve as much credit for UV protection as conventional wisdom attributes to them? In which interactions do these promiscuous cells engage with their immediate epithelial environment and who is controlling whom? What lessons might be distilled from looking at lower vertebrate melanophores and at extracutaneous melanocytes in the endeavour to reveal the 'secret identity' of melanocytes? The current Controversies feature explores these far too infrequently posed, biologically and clinically important questions. Complementing a companion viewpoint essay on malignant melanocytes (2), this critical re-examination of melanocyte biology provides a cornucopia of old, but under-appreciated concepts and novel ideas on the slowly emerging complexity of physiological melanocyte functions, and delineates important, thought-provoking questions that remain to be definitively answered by future research.

Identification of the first Toll-like receptor gene in passerine birds: TLR4 orthologue in zebra finch (Taeniopygia guttata)

Toll-like receptors (TLRs) are the basic components of the vertebrate pathogen recognition system. Despite uniform general structure, remarkable variability in domain composition can be found in individual TLRs among species. Knowledge of interspecific differences is of particular importance to our understanding of selective pressures on TLRs. Currently, most TLRs are characterized only in a limited number of model species, including domestic chicken as a universal avian model. Here, we describe structure and expression pattern of TLR4 in zebra finch, a widely used passerine model species. The tgTlr4 gene consists of three exons (204, 167 and 3033-3043 bp) that are transcribed into messenger RNA with a relatively long 3'-untranslated region (788 bp). Predicted protein is composed of 842 amino acids (aas) forming extracellular domain with nine leucine-rich repeat (LRR) motives flanked at the carboxy-terminal end by leucine-rich repeat carboxy-terminal domain, transmembrane domain and cytoplasmic toll/interleukin-1 receptor domain. The overall structure is similar to other known TLR4 molecules with 32%-49% aa identity to various mammals and 74% to chicken. Although the position of most of the domains in zebra finch TLR4 resembles their position in chicken, there is one extra LRR at the aa position 207-229 in tgTLR4 and one LRR known in chTLR4 is missing. The gene is highly expressed in the bone marrow and in the spleen, intermediately in the gut and low expression was found in the liver and lungs. For the first time in birds, expression of tgTLR4 in peritoneal macrophages was found to be enhanced by the Escherichia coli lipopolysaccharide treatment.