Human melanoblasts in culture: expression of BRN2 and synergistic regulation by fibroblast growth factor-2, stem cell factor, and endothelin-3

Loss of miR-101-3p in melanoma stabilizes genomic integrity, leading to cell death prevention

Malignant melanoma remains the most lethal form of skin cancer, exhibiting poor prognosis after forming distant metastasis. Owing to their potential tumor-suppressive properties by regulating oncogenes and tumor suppressor genes, microRNAs are important player in melanoma development and progression. We defined the loss of miR-101-3p expression in melanoma cells compared with melanocytes and melanoblast-related cells as an early event in tumor development and aimed to understand the tumor suppressive role of miR-101-3p and its regulation of important cellular processes. Reexpression of miR-101-3p resulted in inhibition of proliferation, increase in DNA damage, and induction of apoptosis. We further determined the nuclear structure protein Lamin B1, which influences nuclear processes and heterochromatin structure, ATRX, CASP3, and PARP as an important direct target of miR-101-3p. RNA sequencing and differential gene expression analysis after miR-101-3p reexpression supported our findings and the importance of loss of mir-101-3p for melanoma progression. The validated functional effects are related to genomic instability, as recent studies suggest miRNAs plays a key role in mediating this cellular process. Therefore, we concluded that miR-101-3p reexpression increases the genomic instability, leading to irreversible DNA damage, which leads to apoptosis induction. Our findings suggest that the loss of miR-101-3p in melanoma serves as an early event in melanoma progression by influencing the genomic integrity to maintain the increased bioenergetic demand.

The ATM Ser49Cys Variant Effects ATM Function as a Regulator of Oncogene-Induced Senescence

An apical component of the cell cycle checkpoint and DNA damage repair response is the ataxia-telangiectasia mutated (ATM) Ser/Thr protein kinase. A variant of ATM, Ser49Cys (rs1800054; minor allele frequency = 0.011), has been associated with an elevated risk of melanoma development; however, the functional consequence of this variant is not defined. ATM-dependent signalling in response to DNA damage has been assessed in a panel of patient-derived lymphoblastoid lines and primary human melanocytic cell strains heterozygous for the ATM Ser49Cys variant allele. The ATM Ser49Cys allele appears functional for acute p53-dependent signalling in response to DNA damage. Expression of the variant allele did reduce the efficacy of oncogene expression in inducing senescence. These findings demonstrate that the ATM 146C>G Ser49Cys allele has little discernible effect on the acute response to DNA damage but has reduced function observed in the chronic response to oncogene over-expression. Analysis of melanoma, naevus and skin colour genomics and GWAS analyses have demonstrated no association of this variant with any of these outcomes. The modest loss of function detected suggest that the variant may act as a modifier of other variants of ATM/p53-dependent signalling.

Prognostic significance of melanogenesis pathway and its association with the ultrastructural characterisation of melanosomes in uveal melanoma

BACKGROUND

Pigmentation could be a relevant prognostic factor in uveal melanoma (UM) development. Microphthalmia-associated transcription factor (MITF) regulates melanin synthesis by activating tyrosinase-related protein 2 (TYRP2) and silver protein (SILV) that induce the melanogenesis pathway. Although their oncogenic potential has been observed in various malignancies but has not been investigated in UM Asian population. Our aim is to study the ultrastructure of melanosomes and the prognostic significance of pigmentation markers such as TYRP2, MITF and SILV in UM.

METHODS

Transmission electron microscopy was performed to compare the ultrastructure of melanosomes in the normal choroid and UM cases. Immunoexpression of TYRP2, SILV and MITF was analysed in 82 UM samples. The mRNA expression level of all genes was measured in 70 UM cases. A statistical correlation was performed to determine the prognostic significance of all markers.

RESULTS

Premelanosomes and mature melanosomes undergoing dedifferentiation were observed in high-pigmented UM cases as compared with low-pigmented UM cases. Seventy per cent of UM cases showed high SILV expression while TYRP2 and MITF expression was present in 58% and 56% of cases, respectively. At the mRNA level, upregulation of TYRP2, SILV and MITF markers was seen in around 50% of UM cases, which was statistically significant with high pigmentation. Reduced metastatic-free survival was statistically significant with the MITF protein expression.

CONCLUSION

Our results demonstrated that ultrastructural changes in melanosomes and high expression of TYRP2, MITF and SILV could dysregulate the melanogenesis pathway and might be responsible for the aggressive behaviour of UM.

Critical Considerations for Investigating MicroRNAs during Tumorigenesis: A Case Study in Conceptual and Contextual Nuances of miR-211-5p in Melanoma

MicroRNAs are non-coding RNAs fundamental to metazoan development and disease. Although the aberrant regulation of microRNAs during mammalian tumorigenesis is well established, investigations into the contributions of individual microRNAs are wrought with conflicting observations. The underlying cause of these inconsistencies is often attributed to context-specific functions of microRNAs. We propose that consideration of both context-specific factors, as well as underappreciated fundamental concepts of microRNA biology, will permit a more harmonious interpretation of ostensibly diverging data. We discuss the theory that the biological function of microRNAs is to confer robustness to specific cell states. Through this lens, we then consider the role of miR-211-5p in melanoma progression. Using literature review and meta-analyses, we demonstrate how a deep understating of domain-specific contexts is critical for moving toward a concordant understanding of miR-211-5p and other microRNAs in cancer biology.

Therapeutic targeting of anoikis resistance in cutaneous melanoma metastasis

The acquisition of resistance to anoikis, the cell death induced by loss of adhesion to the extracellular matrix, is an absolute requirement for the survival of disseminating and circulating tumour cells (CTCs), and for the seeding of metastatic lesions. In melanoma, a range of intracellular signalling cascades have been identified as potential drivers of anoikis resistance, however a full understanding of the process is yet to be attained. Mechanisms of anoikis resistance pose an attractive target for the therapeutic treatment of disseminating and circulating melanoma cells. This review explores the range of small molecule, peptide and antibody inhibitors targeting molecules involved in anoikis resistance in melanoma, and may be repurposed to prevent metastatic melanoma prior to its initiation, potentially improving the prognosis for patients.

Targeting ferroptosis as a cell death pathway in Melanoma: From molecular mechanisms to skin cancer treatment

Melanoma, the most aggressive form of human skin cancer, has been under investigation to reach the most efficient treatment. Surgical resection for early-diagnosed primary melanoma, targeted therapies, and immune checkpoint inhibitors for advanced/metastatic melanoma is the best clinical approach. Ferroptosis, a newly identified iron-dependent cell death pathway, which is morphologically and biochemically different from apoptosis and necrosis, has been reported to be involved in several cancers. Ferroptosis inducers could provide therapeutic options in case of resistance to conventional therapies for advanced/metastatic melanoma. Recently developed ferroptosis inducers, MEK and BRAF inhibitors, miRNAs such as miR-137 and miR-9, and novel strategies for targeting major histocompatibility complex (MHC) class II in melanoma can provide new opportunities for melanoma treatment. Combining ferroptosis inducers with targeted therapies or immune checkpoint inhibitors increases patient response rates. Here we review the mechanisms of ferroptosis and its environmental triggers. We also discuss the pathogenesis and current treatments of melanoma. Moreover, we aim to elucidate the relationship between ferroptosis and melanoma and ferroptosis implications to develop new therapeutic strategies against melanoma.

Reciprocal regulation of BRN2 and NOTCH1/2 signaling synergistically drives melanoma cell migration and invasion

Phenotypic plasticity drives cancer progression, impacts on treatment response and is a major driver of therapeutic resistance. In melanoma, a regulatory axis between the MITF and BRN2 transcription factors has been reported to promote tumor heterogeneity by mediating switching between proliferative and invasive phenotypes respectively. Despite strong evidence that subpopulations of cells that exhibit a BRN2^high/MITF^low expression profile switch to a predominantly invasive phenotype, the mechanisms by which this switch is propagated and promotes invasion remain poorly defined. We have found that a reciprocal relationship between BRN2 and NOTCH1/2 signaling exists in melanoma cells in vitro, within patient datasets and in vivo primary and metastatic human tumors that bolsters acquisition of invasiveness. Working through the epigenetic modulator EZH2, the BRN2-NOTCH1/2 axis is potentially a key mechanism by which the invasive phenotype is maintained. Given the emergence of agents targeting both EZH2 and NOTCH, understanding the mechanism through which BRN2 promotes heterogeneity may provide crucial biomarkers to predict treatment response to prevent metastasis.

Phenotypic plasticity of melanocytes derived from human adult skin

We previously described a novel in vitro culture technique for dedifferentiated human adult skin melanocytes. Melanocytes cultured in a defined, cholera toxin and PMA free medium became bipolar, unpigmented, and highly proliferative. Furthermore, TRP-1 and c-Kit expression disappeared and EGFR receptor and nestin expression were induced in the cells. Here, we further characterized the phenotype of these dedifferentiated cells and by comparing them to mature pigmented melanocytes we detected crucial steps in their phenotype change. Our data suggest that normal adult melanocytes easily dedifferentiate into pluripotent stem cells given the right environment. This dedifferentiation process described here for normal melanocyte is very similar to what has been described for melanoma cells, indicating that phenotype switching driven by environmental factors is a general characteristic of melanocytes that can occur independent of malignant transformation.

Learning from Embryogenesis-A Comparative Expression Analysis in Melanoblast Differentiation and Tumorigenesis Reveals miRNAs Driving Melanoma Development

Malignant melanoma is one of the most dangerous tumor types due to its high metastasis rates and a steadily increasing incidence. During tumorigenesis, the molecular processes of embryonic development, exemplified by epithelial-mesenchymal transition (EMT), are often reactivated. For melanoma development, the exact molecular differences between melanoblasts, melanocytes, and melanoma cells are not completely understood. In this study, we aimed to identify microRNAs (miRNAs) that promote melanoma tumorigenesis and progression, based on an in vitro model of normal human epidermal melanocyte (NHEM) de-differentiation into melanoblast-like cells (MBrCs). Using miRNA-sequencing and differential expression analysis, we demonstrated in this study that a majority of miRNAs have an almost equal expression level in NHEMs and MBrCs but are significantly differentially regulated in primary tumor- and metastasis-derived melanoma cell lines. Further, a target gene analysis of strongly regulated but functionally unknown miRNAs yielded the implication of those miRNAs in many important cellular pathways driving malignancy. We hypothesize that many of the miRNAs discovered in our study are key drivers of melanoma development as they account for the tumorigenic potential that differentiates melanoma cells from proliferating or migrating embryonic cells.

Melanoma mutations modify melanocyte dynamics in co-culture with keratinocytes or fibroblasts

Melanocytic cell interactions are integral to skin homeostasis, and affect the outcome of multiple diseases, including cutaneous pigmentation disorders and melanoma. By using automated-microscopy and machine-learning-assisted morphology analysis of primary human melanocytes in co-culture, we performed combinatorial interrogation of melanocyte genotypic variants and functional assessment of lentivirus-introduced mutations. Keratinocyte-induced melanocyte dendricity, an indicator of melanocyte differentiation, was reduced in the melanocortin 1 receptor (MC1R) R/R variant strain and by NRAS.Q61K and BRAF.V600E expression, while expression of CDK4.R24C and RAC1.P29S had no detectable effect. Time-lapse tracking of melanocytes in co-culture revealed dynamic interaction phenotypes and hyper-motile cell states that indicated that, in addition to the known role in activating mitogenic signalling, MEK-pathway-activating mutations may also allow melanocytes to escape keratinocyte control and increase their invasive potential. Expanding this combinatorial platform will identify other therapeutic target mutations and melanocyte genetic variants, as well as increase understanding of skin cell interactions.

DNMT1 and DNMT3B genetic polymorphisms affect the clinical course and outcome of melanoma patients

The aberrant DNA methylation plays a critical role in a number of different malignancies, including melanoma. DNA methylation is catalyzed by DNA methyltransferases (DNMTs), involved in methylation maintenance (DNMT1) and de novo DNA methylation (DNMT3A and DNMT3B). The current study investigated the association of genetic variants in the DNMT1 and DNMT3B with the clinicopathologic features and the clinical course of melanoma patients. In the present study, DNMT1 (rs2228612, rs2228611, and rs2114724) and DNMT3B (rs406193 and rs2424932) polymorphisms were examined in 123 melanoma patients. Single nucleotide polymorphisms were assessed using TaqMan SNPs Genotyping Assays according to the manufacturer's protocols. The carriers of the variant genotype of DNMT1 rs2228612 had poorer overall survival and recurrence-free survival, (P = 0.000 and 0.000, respectively), and an increased risk for adverse outcome [hazard ratio (HR) = 6.620, 95% confidence interval (CI): 2.214-19.791, P = 0.001]. DNMT1 rs2228612 was also associated with ulceration (P = 0.045), nodal status (P = 0.030), progression (P = 0. 007), and stage of disease (P = 0.003). Univariate analysis indicated that tumor-infiltrating lymphocytes could be a marker of good prognosis in melanoma patients (HR = 0.323, 95% CI: 0.127-0.855, P = 0.025), whereas the genotype distribution of the DNMT3B rs406193 polymorphism correlated significantly with the presence of tumor-infiltrating lymphocytes (P = 0.012). The multivariate analysis showed that the DNMT1 rs2228612 polymorphism (HR = 12.126, 95% CI: 2.345-62.715, P = 0.003) is an independent predictor of poor overall survival in melanoma patients. As expected, disease progression was also found to be an independent prognostic factor in melanoma patients (HR = 37.888, 95% CI: 3.615-397.062, P = 0.002). DNMT1 rs2228612 was found to be an independent predictor of poor overall survival in melanoma patients. DNMTs polymorphisms could serve as a potential target for novel therapeutic approaches.

Combined use of subclinical hydroxyurea and CHK1 inhibitor effectively controls melanoma and lung cancer progression, with reduced normal tissue toxicity compared to gemcitabine

Drugs such as gemcitabine that increase replication stress are effective chemotherapeutics in a range of cancer settings. These drugs effectively block replication and promote DNA damage, triggering a cell cycle checkpoint response through the ATR–CHK1 pathway. Inhibiting this signalling pathway sensitises cells to killing by replication stress‐inducing drugs. Here, we investigated the effect of low‐level replication stress induced by low concentrations (> 0.2 mm) of the reversible ribonucleotide reductase inhibitor hydroxyurea (HU), which slows S‐phase progression but has little effect on cell viability or proliferation. We demonstrate that HU effectively synergises with CHK1, but not ATR inhibition, in > 70% of melanoma and non‐small‐cell lung cancer cells assessed, resulting in apoptosis and complete loss of proliferative potential in vitro and in vivo. Normal fibroblasts and haemopoietic cells retain viability and proliferative potential following exposure to CHK1 inhibitor plus low doses of HU, but normal cells exposed to CHK1 inhibitor combined with submicromolar concentrations of gemcitabine exhibited complete loss of proliferative potential. The effects of gemcitabine on normal tissue correlate with irreversible ATR–CHK1 pathway activation, whereas low doses of HU reversibly activate CHK1 independently of ATR. The combined use of CHK1 inhibitor and subclinical HU also triggered an inflammatory response involving the recruitment of macrophages in vivo. These data indicate that combining CHK1 inhibitor with subclinical HU is superior to combination with gemcitabine, as it provides equal anticancer efficacy but with reduced normal tissue toxicity. These data suggest a significant proportion of melanoma and lung cancer patients could benefit from treatment with this drug combination.

MITF-the first 25 years

In this review, Goding and Arnheiter present the current understanding of MITF's role and regulation in development and disease and highlight key areas where our knowledge of MITF regulation and function is limited.

All transcription factors are equal, but some are more equal than others. In the 25 yr since the gene encoding the microphthalmia-associated transcription factor (MITF) was first isolated, MITF has emerged as a key coordinator of many aspects of melanocyte and melanoma biology. Like all transcription factors, MITF binds to specific DNA sequences and up-regulates or down-regulates its target genes. What marks MITF as being remarkable among its peers is the sheer range of biological processes that it appears to coordinate. These include cell survival, differentiation, proliferation, invasion, senescence, metabolism, and DNA damage repair. In this article we present our current understanding of MITF's role and regulation in development and disease, as well as those of the MITF-related factors TFEB and TFE3, and highlight key areas where our knowledge of MITF regulation and function is limited.

Melanocyte Chitosan/Gelatin Composite Fabrication with Human Outer Root Sheath-Derived Cells to Produce Pigment

The hair follicle serves as a melanocyte reservoir for both hair and skin pigmentation. Melanocyte stem cells (MelSCs) and melanocyte progenitors reside in the bulge/sub-bulge region of the lower permanent portion of the hair follicle and play a vital role for repigmentation in vitiligo. It would be beneficial to isolate MelSCs in order to further study their function in pigmentary disorders; however, due to the lack of specific molecular surface markers, this has not yet been successfully accomplished in human hair follicles (HuHF). One potential method for MelSCs isolation is the “side population” technique, which is frequently used to isolate hematopoietic and tumor stem cells. In the present study, we decided to isolate HuHF MelSCs using “side population” to investigate their melanotic function. By analyzing mRNA expression of TYR, SOX10, and MITF, melanosome structure, and immunofluorescence with melanocyte-specific markers, we revealed that the SP-fraction contained MelSCs with an admixture of differentiated melanocytes. Furthermore, our in vivo studies indicated that differentiated SP-fraction cells, when fabricated into a cell-chitosan/gelatin composite, could transiently repopulate immunologically compromised mice skin to regain pigmentation. In summary, the SP technique is capable of isolating HuHF MelSCs that can potentially be used to repopulate skin for pigmentation.

A PAX3/BRN2 rheostat controls the dynamics of BRAF mediated MITF regulation in MITFhigh /AXLlow melanoma

The BRAF kinase and the MAPK pathway are targets of current melanoma therapies. However, MAPK pathway inhibition results in dynamic changes of downstream targets that can counteract inhibitor-action not only in during treatment, but also in acquired resistant tumours. One such dynamic change involves the expression of the transcription factor MITF, a crucial regulator of cell survival and proliferation in untreated as well as drug-addicted acquired resistant melanoma. Tight control over MITF expression levels is required for optimal melanoma growth, and while it is well established that the MAPK pathway regulates MITF expression, the actual mechanism is insufficiently understood. We reveal here, how BRAF through action on the transcription factors BRN2 and PAX3 executes control over the regulation of MITF expression in a manner that allows for considerable plasticity. This plasticity provides robustness to the BRAF mediated MITF regulation and explains the dynamics in MITF expression that are observed in patients in response to MAPK inhibitor therapy.

Isolation and Culture of Epidermal Melanocytes

Melanocytes which represent around 5% of epidermal cells are located in the basal layer. To culture melanocytes we used trypsin digestion instead of dispase to obtain a cell suspension containing only basal keratinocytes and melanocytes. Melanocytes are cells which need a great attention. Indeed they dedifferentiate easily in culture as soon as they are in pure culture. Factors secreted by contaminating keratinocytes allow melanocytes to stay dendritic but by regulating their number avoid their growth. In order to age, phototype and other individual dependent factors regulate the behavior of melanocytes in vitro. Thus, microscopic examination of melanocytes has to be performed each day to adapt conditions of culture to each primary cell culture. This is the secret to have a nice melanocyte culture.

Targeted Therapy-Resistant Melanoma Cells Acquire Transcriptomic Similarities with Human Melanoblasts

The mechanisms of adaptive and acquired drug resistance in tumors are not completely understood. So far, gene amplifications or mutations, leading to the reactivation of the MAPK or PI3K pathways have been described. In this study, we used two different methods to generate human melanoblasts: (1) via differentiation from induced pluripotent stem cells (iPSCs) and (2) via dedifferentiation from melanocytes. The melanoblast transcriptomes were then compared to the transcriptome of MAPK inhibitor-resistant melanoma cells. We observed that the expression of genes associated with cell cycle control, DNA damage control, metabolism, and cancer was altered in both melanoblast populations and in both adaptive and acquired resistant melanoma samples, compared to drug-sensitive samples. However, genes involved in antigen presentation and cellular movement were only regulated in the melanoblast populations and in the acquired resistant melanoma samples, compared to the drug-sensitive samples. Moreover, melanocyte-derived melanoblasts and adaptive resistant melanoma samples were characterized by different expression levels of certain transcription factors or genes involved in the CDK5 pathway. In conclusion, we show here that in vitro models of human melanoblasts are very important tools to comprehend the expression profiles of drug-resistant melanoma.

BRN2, a POUerful driver of melanoma phenotype switching and metastasis

The POU domain family of transcription factors play a central role in embryogenesis and are highly expressed in neural crest cells and the developing brain. BRN2 is a class III POU domain protein that is a key mediator of neuroendocrine and melanocytic development and differentiation. While BRN2 is a central regulator in numerous developmental programs, it has also emerged as a major player in the biology of tumourigenesis. In melanoma, BRN2 has been implicated as one of the master regulators of the acquisition of invasive behaviour within the phenotype switching model of progression. As a mediator of melanoma cell phenotype switching, it coordinates the transition to a dedifferentiated, slow cycling and highly motile cell type. Its inverse expression relationship with MITF is believed to mediate tumour progression and metastasis within this model. Recent evidence has now outlined a potential epigenetic switching mechanism in melanoma cells driven by BRN2 expression that induces melanoma cell invasion. We summarize the role of BRN2 in tumour cell dissemination and metastasis in melanoma, while also examining it as a potential metastatic regulator in other tumour models.

Endogenous Replication Stress Marks Melanomas Sensitive to CHEK1 Inhibitors In Vivo

Purpose: Checkpoint kinase 1 inhibitors (CHEK1i) have single-agent activity in vitro and in vivo Here, we have investigated the molecular basis of this activity.Experimental Design: We have assessed a panel of melanoma cell lines for their sensitivity to the CHEK1i GNE-323 and GDC-0575 in vitro and in vivo The effects of these compounds on responses to DNA replication stress were analyzed in the hypersensitive cell lines.Results: A subset of melanoma cell lines is hypersensitive to CHEK1i-induced cell death in vitro, and the drug effectively inhibits tumor growth in vivo In the hypersensitive cell lines, GNE-323 triggers cell death without cells entering mitosis. CHEK1i treatment triggers strong RPA2 hyperphosphorylation and increased DNA damage in only hypersensitive cells. The increased replication stress was associated with a defective S-phase cell-cycle checkpoint. The number and intensity of pRPA2 Ser4/8 foci in untreated tumors appeared to be a marker of elevated replication stress correlated with sensitivity to CHEK1i.Conclusions: CHEK1i have single-agent activity in a subset of melanomas with elevated endogenous replication stress. CHEK1i treatment strongly increased this replication stress and DNA damage, and this correlated with increased cell death. The level of endogenous replication is marked by the pRPA2Ser4/8 foci in the untreated tumors, and may be a useful marker of replication stress in vivoClin Cancer Res; 24(12); 2901-12. ©2018 AACR.

Genetic variation in IRF4 expression modulates growth characteristics, tyrosinase expression and interferon-gamma response in melanocytic cells

A SNP within intron4 of the interferon regulatory factor4 (IRF4) gene, rs12203592*C/T, has been independently associated with pigmentation and age-specific effects on naevus count in European-derived populations. We have characterized the cis-regulatory activity of this intronic region and using human foreskin-derived melanoblast strains, we have explored the correlation between IRF4 rs12203592 homozygous C/C and T/T genotypes with TYR enzyme activity, supporting its association with pigmentation traits. Further, higher IRF4 protein levels directed by the rs12203592*C allele were associated with increased basal proliferation but decreased cell viability following UVR, an etiological factor in melanoma development. Since UVR, and accompanying IFNγ-mediated inflammatory response, is associated with melanomagenesis, we evaluated its effects in the context of IRF4 status. Manipulation of IRF4 levels followed by IFNγ treatment revealed a subset of chemokines and immuno-evasive molecules that are sensitive to IRF4 expression level and genotype including CTLA4 and PD-L1.

SOX10-MITF pathway activity in melanoma cells

Melanoma is one of the most dangerous and lethal skin cancers, with a considerable metastatic potential and drug resistance. It involves a malignant transformation of melanocytes. The exact course of events in which melanocytes become melanoma cells remains unclear. Nevertheless, this process is said to be dependent on the occurrence of cells with the phenotype of progenitor cells - cells characterized by expression of proteins such as nestin, CD-133 or CD-271. The development of these cells and their survival were found to be potentially dependent on the neural crest stem cell transcription factor SOX10. This is just one of the possible roles of SOX10, which contributes to melanomagenesis by regulating the SOX10-MITF pathway, but also to melanoma cell survival, proliferation and metastasis formation. The aim of this review is to describe the broad influence of the SOX10-MITF pathway on melanoma cells.

Human Adipose Mesenchymal Cells Inhibit Melanocyte Differentiation and the Pigmentation of Human Skin via Increased Expression of TGF-β1

There is accumulating evidence that interactions between epidermal melanocytes and stromal cells play an important role in the regulation of skin pigmentation. In this study we established a pigmented dermo-epidermal skin model, melDESS, of human origin to investigate the effects of distinct stromal cells on melanogenesis. melDESS is a complex, clinically relevant skin equivalent composed of an epidermis containing both melanocytes and keratinocytes. Its dermal compartment consists either of adipose tissue-derived stromal cells, dermal fibroblasts (Fbs), or a mixture of both cell types. These skin substitutes were transplanted for 5 weeks on the backs of immuno-incompetent rats and analyzed. Gene expression and Western blot analyses showed a significantly higher expression of transforming growth factor-β1 by adipose tissue-derived stromal cells compared with dermal Fbs. In addition, we showed that melanocytes responded to the increased levels of transforming growth factor-β1 by down-regulating the expression of key melanogenic enzymes such as tyrosinase. This caused decreased melanin synthesis and, consequently, greatly reduced pigmentation of melDESS. The conclusions are of utmost clinical relevance, namely that adipose tissue-derived stromal cells derived from the hypodermis fail to appropriately interact with epidermal melanocytes, thus preventing the sustainable restoration of the patient's native skin color in bioengineered skin grafts.

Effects of Brn2 overexpression on eccrine sweat gland development in the mouse paw

Eccrine sweat glands regulate body temperature by secreting water and electrolytes. In humans, eccrine sweat glands are ubiquitous in the skin, except in the lips and external genitalia. In mice, eccrine sweat glands are present only in the paw pad. Brn2 is a protein belonging to a large family of transcription factors. A few studies have examined Brn2 in melanoma cells and epidermal keratinocytes. This study investigated changes in the skin in the K5-Brn2 transgenic mouse, which overexpresses Brn2 and contains the keratin 5 promotor. Interestingly, the volume of eccrine sweat glands was reduced markedly in the K5-Brn2 transgenic mouse compared with the wild-type, while the expression of aquaporin 5, important molecule in sweat secretion, was increased in each sweat gland cell, probably to compensate for the reduction in gland development. However, sweating response to a pilocarpine injection in the hind paw was significantly decreased in the K5-Brn2 transgenic mouse compared with the wild-type. The paw epidermis was thicker in the K5-Brn2 transgenic mouse compared with the wild-type. Taken together, eccrine sweat gland development and sweat secretion were suppressed markedly in the K5-Brn2 transgenic mouse. These results may be associated with dominant development of the epidermis by Brn2 overexpression in the paw skin.

Testing of viable human skin cell dilution cultures as an approach to validating microsampling

Skin biopsies are a valuable technique in the diagnosis of cutaneous inflammatory and neoplastic conditions. We were interested to test the minimal size or equivalent volume by dilution of proteolytically disassociated skin tissue required to allow the isolation and propagation of cutaneous cells, for freezing, storage and biochemical analysis. It was possible to propagate with 100% efficiency fibroblast and melanocytic cells from a 0.1 to 0.5 mm³ equivalent neonatal foreskin sample using a combination of DispaseII and CollagenaseIV. The smallest tissue dilution that allowed melanocytic cell culture was 0.01 mm³, which equated to approximately 16 cells based on the average skin density of melanocytes. However, passaging of cells to create frozen stocks was achieved routinely only from 1 mm³ skin, equating to 1560 cells. Tissue-specific antigen expression of these cultures was tested by western blot of total protein extracts. There was no pigmentation antigen expression in fibroblast cultures; however, smooth muscle actin protein expression was high in fibroblast but absent from melanocytic cell strains. Melanocytic cells expressed pigmentation antigens and E-cadherin, but these were not detected in fibroblasts. Moreover, maturation of these melanocytic cells resulted in a decrease of Dopachrome Tautomerase antigen expression and induction of Tyrosinase protein consistent with the differentiation potential seen in cell cultures derived routinely from large sections of skin tissue.

NFIB Mediates BRN2 Driven Melanoma Cell Migration and Invasion Through Regulation of EZH2 and MITF

While invasion and metastasis of tumour cells are the principle factor responsible for cancer related deaths, the mechanisms governing the process remain poorly defined. Moreover, phenotypic divergence of sub-populations of tumour cells is known to underpin alternative behaviors linked to tumour progression such as proliferation, survival and invasion. In the context of melanoma, heterogeneity between two transcription factors, BRN2 and MITF, has been associated with phenotypic switching between predominantly invasive and proliferative behaviors respectively. Epigenetic changes, in response to external cues, have been proposed to underpin this process, however the mechanism by which the phenotypic switch occurs is unclear. Here we report the identification of the NFIB transcription factor as a novel downstream effector of BRN2 function in melanoma cells linked to the migratory and invasive characteristics of these cells. Furthermore, the function of NFIB appears to drive an invasive phenotype through an epigenetic mechanism achieved via the upregulation of the polycomb group protein EZH2. A notable target of NFIB mediated up-regulation of EZH2 is decreased MITF expression, which further promotes a less proliferative, more invasive phenotype. Together our data reveal that NFIB has the ability to promote dynamic changes in the chromatin state of melanoma cells to facilitate migration, invasion and metastasis.

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NFIB mediates a slow cycling, highly invasive/migratory melanoma cell phenotype downstream of BRN2.

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NFIB increases EZH2 expression downstream of BRN2, which further decreases MITF levels.

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NFIB expression is defined by an invasive gene signature and colocalises with BRN2 in primary and metastatic human melanoma tumours.

Melanoma is a heterogeneous cancer, made up of many cellular populations that differ in their ability to induce tumour growth or invasion throughout the body (metastasis). These populations have been found to switch back and forth to drive invasion and progression. This process appears to be controlled by an inverse axis between two genes, MITF and BRN2. BRN2 drives metastatic spread, but the process by which it acts is not well characterized and cannot be targeted clinically. This study has uncovered a role for the gene NFIB in driving invasion downstream of BRN2. Importantly, it appears to drive this process through EZH2, which can be targeted therapeutically to reduce metastasis.

A Genetic Mechanism for Convergent Skin Lightening during Recent Human Evolution

Skin lightening among Eurasians is thought to have been a convergence occurring independently in Europe and East Asia as an adaptation to high latitude environments. Among Europeans, several genes responsible for such lightening have been found, but the information available for East Asians is much more limited. Here, a genome-wide comparison between dark-skinned Africans and Austro-Asiatic speaking aborigines and light-skinned northern Han Chinese identified the pigmentation gene OCA2, showing unusually deep allelic divergence between these groups. An amino acid substitution (His615Arg) of OCA2 prevalent in most East Asian populations—but absent in Africans and Europeans—was significantly associated with skin lightening among northern Han Chinese. Further transgenic and targeted gene modification analyses of zebrafish and mouse both exhibited the phenotypic effect of the OCA2 variant manifesting decreased melanin production. These results indicate that OCA2 plays an important role in the convergent skin lightening of East Asians during recent human evolution.

Understanding Melanocyte Stem Cells for Disease Modeling and Regenerative Medicine Applications

Melanocytes in the skin play an indispensable role in the pigmentation of skin and its appendages. It is well known that the embryonic origin of melanocytes is neural crest cells. In adult skin, functional melanocytes are continuously repopulated by the differentiation of melanocyte stem cells (McSCs) residing in the epidermis of the skin. Many preceding studies have led to significant discoveries regarding the cellular and molecular characteristics of this unique stem cell population. The alteration of McSCs has been also implicated in several skin abnormalities and disease conditions. To date, our knowledge of McSCs largely comes from studying the stem cell niche of mouse hair follicles. Suggested by several anatomical differences between mouse and human skin, there could be distinct features associated with mouse and human McSCs as well as their niches in the skin. Recent advances in human pluripotent stem cell (hPSC) research have provided us with useful tools to potentially acquire a substantial amount of human McSCs and functional melanocytes for research and regenerative medicine applications. This review highlights recent studies and progress involved in understanding the development of cutaneous melanocytes and the regulation of McSCs.

Long-term expression pattern of melanocyte markers in light- and dark-pigmented dermo-epidermal cultured human skin substitutes

PURPOSE

Transplantation of pigmented tissue-engineered human autologous skin substitutes represents a promising procedure to cover skin defects. We have already demonstrated that we can restore the patient's native light or dark skin color by adding melanocytes to our dermo-epidermal skin analogs. In this long-term study, we investigated if melanocytes in our skin substitutes continue to express markers as BCL2, SOX9, and MITF, known to be involved in survival, differentiation, and function of melanocytes.

METHODS

Human epidermal melanocytes and keratinocytes, as well as dermal fibroblasts from light- and dark-pigmented skin biopsies were isolated and cultured. Bovine collagen hydrogels containing fibroblasts were prepared, and melanocytes and keratinocytes were seeded in a 1:5 ratio onto the gels. Pigmented dermo-epidermal skin substitutes were transplanted onto full-thickness wounds of immuno-incompetent rats and analyzed for the expression of melanocyte markers after 15 weeks.

RESULTS

Employing immunofluorescence staining techniques, we observed that our light and dark dermo-epidermal skin substitutes expressed the same typical melanocyte markers including BCL2, SOX9, and MITF 15 weeks after transplantation as normal human light and dark skin.

CONCLUSIONS

These data suggest that, even in the long run, our light and dark dermo-epidermal tissue-engineered skin substitutes contain melanocytes that display a characteristic expression pattern as seen in normal pigmented human skin. These findings have crucial clinical implications as such grafts transplanted onto patients should warrant physiological numbers, distribution, and function of melanocytes.

Isolating primary melanocyte-like cells from the mouse heart

We identified a novel population of melanocyte-like cells (also known as cardiac melanocytes) in the hearts of mice and humans that contribute to atrial arrhythmia triggers in mice. To investigate the electrical and biological properties of cardiac melanocytes we developed a procedure to isolate them from mouse hearts that we derived from those designed to isolate neonatal murine cardiomyocytes. In order to obtain healthier cardiac melanocytes suitable for more extensive patch clamp or biochemical studies, we developed a refined procedure for isolating and plating cardiac melanocytes based on those originally designed to isolate cutaneous melanocytes. The refined procedure is demonstrated in this review and produces larger numbers of healthy melanocyte-like cells that can be plated as a pure population or with cardiomyocytes.

Molecular analysis of common polymorphisms within the human Tyrosinase locus and genetic association with pigmentation traits

We have compared the melanogenic activities of cultured melanocytes carrying two common TYR alleles as homozygous 192S-402R wild-type, heterozygous and homozygous variant. This includes assays of TYR protein, DOPAoxidase activity, glycosylation and temperature sensitivity of protein and DOPAoxidase levels. Homozygous wild-type strains on average had higher levels of TYR protein and enzyme activity than other genotypes. Homozygous 402Q/Q melanocytes produced significantly less TYR protein, displayed altered trafficking and glycosylation, with reduced DOPAoxidase. However, near wild-type TYR activity levels could be recovered at lower growth temperature. In a sample population from Southeast Queensland, these two polymorphisms were present on four TYR haplotypes, designated as WT 192S-402R, 192Y-402R and 192S-402Q with a double-variant 192Y-402Q of low frequency at 1.9%. Based on cell culture findings and haplotype associations, we have used an additive model to assess the penetrance of the ten possible TYR genotypes derived from the combination of these haplotypes.

Novel method for isolating human melanoblasts from keratinocyte culture

The characterization of melanoblasts is important for understanding their in vivo development, melanoma formation, and pigment-related disorders. However, no methods have been reported for the isolation of melanoblasts from human skin. Using a 'calcium-pulse' technique, involving the differentiation of human keratinocytes with high calcium and the subsequent spontaneous separation of the epidermal sheets, we effectively isolated human melanoblasts (keratinocyte-adapted melanoblasts, KaMBs) from keratinocyte culture. The KaMBs expressed early melanogenesis-related genes, including BRN2, which is a known melanoblast marker. Moreover, the KaMBs displayed much higher proliferative and growth capacities than the primary melanocytes. Considering that keratinocytes might provide an in vivo-like environment for KaMBs during isolation and in vitro culture, the 'calcium-pulse' technique offers an unprecedented, easy, and efficient method for the isolation of human melanoblasts, retaining the original characteristics of these cells.

A polymorphism in IRF4 affects human pigmentation through a tyrosinase-dependent MITF/TFAP2A pathway

Sequence polymorphisms linked to human diseases and phenotypes in genome-wide association studies often affect noncoding regions. A SNP within an intron of the gene encoding Interferon Regulatory Factor 4 (IRF4), a transcription factor with no known role in melanocyte biology, is strongly associated with sensitivity of skin to sun exposure, freckles, blue eyes, and brown hair color. Here, we demonstrate that this SNP lies within an enhancer of IRF4 transcription in melanocytes. The allele associated with this pigmentation phenotype impairs binding of the TFAP2A transcription factor that, together with the melanocyte master regulator MITF, regulates activity of the enhancer. Assays in zebrafish and mice reveal that IRF4 cooperates with MITF to activate expression of Tyrosinase (TYR), an essential enzyme in melanin synthesis. Our findings provide a clear example of a noncoding polymorphism that affects a phenotype by modulating a developmental gene regulatory network.

De- and re-differentiation of the melanocytic lineage

Terminally differentiated cells can be reprogrammed by the transient, ectopic overexpression of different sets of genes into induced pluripotent stem cells (iPSCs). This process not only has considerable implications for regenerative medicine but is also highly relevant to multiple stages of oncogenesis, including melanoma. In other settings, the de-differentiation of normal and tumor cells is also responsible for a phenotype switch which completely changes the cell fate. Conversely, iPSCs as well as embryonic stem cells (ESCs) can be differentiated in vitro toward specific lineages, for example melanocytes, which offer useful models to investigate the genetic and epigenetic mechanisms involved in cellular differentiation. Here, we summarize recent findings regarding the reprogramming and de-differentiation of melanocytic cells as well as the latest differentiation protocols of pluripotent stem cells into the melanocyte lineage.

Comprehensive analysis of melanogenesis and proliferation potential of melanocyte lineage in solar lentigines

BACKGROUND

Solar lentigines (SLs) are characterized by hyperpigmented macules, commonly seen on sun-exposed areas of the skin. Although it has been reported that an increase in the number of melanocytes and epidermal melanin content was observed in the lesions, the following questions remain to be answered: (1) Is acceleration of melanogenesis in the epidermis caused by an increased number of melanocytes or the high melanogenic potential of each melanocyte? (2) Why does the number of melanocytes increase?

OBJECTIVE

To elucidate the pathogenic mechanism of SLs by investigating the number, melanogenic potential and proliferation status of the melanocyte lineage in healthy skin and SL lesions.

METHODS

Immunostaining for melanocyte lineage markers (tyrosinase, MART-1, MITF, and Frizzled-4) and a proliferation marker, Ki67, was performed on skin sections, and the obtained images were analyzed by image analysis software.

RESULTS

The expression level of tyrosinase to MART-1 of each melanocyte was significantly higher in SL lesions than healthy skin. The numbers of melanocytes in the epidermis, melanoblasts in the hair follicular infundibulum and melanocyte stem cells in the bulge region were increased in SL; however, no significant difference was observed in the Ki67-positive rate of these cells.

CONCLUSION

The melanogenic potential of each melanocyte was elevated in SL lesions. It was suggested that the increased number of melanocytes in the SL epidermis might be attributed to the abnormal increase of melanocyte stem cells in the bulge.

Optimization of the method for the culture of melanocyte precursors from hair follicles and their activation by 1,25-dihydroxyvitamin D3

The melanocytes in vitiligo repigmentation are derived predominantly from melanocyte precursors (MPs) present in the outer root sheath (ORS) of hair follicles. The methods currently used for culturing MPs are unstable, and the cultured cells have the capacity to produce melanin. These factors are problematic when conducting in vitro studies to investigate the mechanism of repigmentation. Although 1,25-dihydroxyvitamin D3 (VID) has been demonstrated to be highly effective in the treatment of vitiligo in the clinic, its precise mode of action has yet to be elucidated. In the present study, the method for the culture of MPs from the ORS of hair follicles was optimized and the ability of VID to activate MPs was investigated. The results suggested that the MPs cultured using the optimized method mainly exhibited bipolar morphology. The cells proliferated well and were negative for 3,4-dihydroxy-L-phenylalanine (DOPA) staining. Transmission electron microscopy revealed that the cytoplasm of the MPs contained numerous stage I and stage II melanosomes; however, stage III and IV melanosomes were not observed. Following VID treatment, the MPs showed increased dendritic morphology, the cells stained positive for DOPA and stage III and IV melanosomes appeared in the cells. Western blotting revealed that microphthalmia-associated transcription factor (MITF), tyrosinase (TYR), tyrosinase-related protein-1 (TRP-1) and TRP-2 were expressed in the MPs and that VID increased the expression levels of MITF, TYR and TRP-1. However, the levels of MITF, TYR and TRP-1 in the MPs prior to and following VID treatment were significantly lower compared with those in cultured epidermal melanocytes, while the levels of TRP-2 in these three groups were not significantly different. Subsequent to VID treatment, the TYR activity in the MPs increased significantly, as did the corresponding melanin levels. In conclusion, the present study successfully optimized the method for MP culture. The MPs demonstrated no significant TYR activity or melanin synthesis; therefore, the MP cultures exhibited the features of MPs in vivo. In addition, VID significantly promoted the differentiation of MPs.

BRN2 is a transcriptional repressor of CDH13 (T-cadherin) in melanoma cells

T-cadherin (cadherin 13, H-cadherin, gene name CDH13) has been proposed to act as a tumor-suppressor gene as its expression is significantly diminished in several types of carcinomas, including melanomas. Allelic loss and promoter hypermethylation have been proposed as mechanisms for silencing of CDH13. However, they do not account for loss of T-cadherin expression in all carcinomas, and other genetic or epigenetic alterations can be presumed. The present study investigated transcriptional regulation of CDH13 in melanoma. Bioinformatical analysis pointed to the presence of known BRN2 (also known as POU3F2 and N-Oct-3)-binding motifs in the CDH13 promoter sequence. We found an inverse correlation between BRN2 and T-cadherin protein and transcript expression. Reporter gene analysis and electrophoretic mobility shift assays in melanoma cells demonstrated that CDH13 is a direct target of BRN2 and that BRN2 is a functional transcriptional repressor of CDH13 promoter activity. The regulatory binding element of BRN2 was located -219 bp of the CDH13 promoter proximal to the start codon and was identified as 5'-CATGCAAAA-3'. Ectopic expression of BRN2 in BRN2-negative/T-cadherin-positive melanoma cells resulted in suppression of CDH13 promoter activity, whereas BRN2 knockdown in BRN2-positive/T-cadherin-negative melanoma cells resulted in re-expression of T-cadherin transcripts and protein. Transcriptional repression of CDH13 by BRN2 may participate in malignant transformation of melanoma by increasing invasion and migration potentials of melanoma cells. The study has identified CDH13 as a novel direct BRN2 transcriptional target gene and has advanced knowledge of mechanisms underlying loss of T-cadherin expression in melanoma.

Dedifferentiation of human epidermal melanocytes into melanoblasts in vitro

Melanoblasts (MB) are also called melanocyte (MC) precursor cells. In recent years, people have successfully cultivated human and mouse MB. Previous studies have shown that EDN3 induces cultivated bird MC to re-differentiate into double potential progenitor cells of MB. However, no study has reported whether in vitro cultivated human MC can be dedifferentiated. Our research on MC that were purified and cultivated in vitro found that adding 10 nm endothelin 1 (EDN1) (ET-1) to the MC medium without phorbol 12-myristate 13-acetate (PMA) induced a few MC to dedifferentiate and become a new type of cell. This new cell type was separated, purified, cloned and identified using multiple approaches. The results show that 88.7%, 8.69% and 2.5% of this new cell type were cells in the G(0) -G(1) , G(2) -M and S stages, respectively. The new cell type did not exhibit an apparent apoptotic peak, and its apoptotic rate was 0.09%. Stage I melanosomes were observed in the cytoplasm and were negative for the DOPA reaction. The cell surface antigen expression was positive for tyrosinase-related protein 2, negative or positive for c-kit and negative for S-100 and HMB45, showing that these cells were dedifferentiated MB of MC. Our findings provided evidence for atavism of mature human MC under certain conditions.

MC1R variant allele effects on UVR-induced phosphorylation of p38, p53, and DDB2 repair protein responses in melanocytic cells in culture

Variant alleles of the human melanocortin 1 receptor (MC1R) reduce the ability of melanocytes to produce the dark pigment eumelanin, with R alleles being most deficient. Cultured melanocytes of MC1R R/R variant genotype give reduced responses to [Nle(4), D-Phe(7)]α-melanocyte-stimulating hormone (NDP-MSH) ligand stimulation and lower levels of DNA repair than MC1R wild-type strains. p38 controls xeroderma pigmentosum (XP)-C recruitment to DNA damage sites through regulating ubiquitylation of the DNA damage-binding protein 2 (DDB2) protein, and p53 is implicated in the nuclear excision repair process through its regulation of XP-C and DDB2 protein expression. We report the effects of MC1R ligand treatment and UVR exposure on phosphorylation of p38 and p53, and DDB2 protein expression in MC1R variant strains. Wild-type MC1R melanocyte strains grown together with keratinocytes in coculture, when treated with NDP-MSH and exposed to UVR, gave synergistic activation of p38 and p53 phosphorylation, and were not replicated by R/R variant melanocytes, which have lower basal levels of phosphorylated forms of p38. Minor increases in p38 phosphorylation status in R/R variant melanocyte cocultures could be attributed to the keratinocytes alone. We also found that MC1R wild-type strains regulate DDB2 protein levels through p38, but MC1R R/R variant melanocytes do not. This work confirms the important functional role that the MC1R receptor plays in UVR stress-induced DNA repair.

Melanoma cell invasiveness is regulated by miR-211 suppression of the BRN2 transcription factor

To identify microRNAs potentially involved in melanomagenesis, we compared microRNA expression profiles between melanoma cell lines and cultured melanocytes. The most differentially expressed microRNA between the normal and tumor cell lines was miR-211. We focused on this pigment-cell-enriched miRNA as it is derived from the microphthalmia-associated transcription factor (MITF)-regulated gene, TRPM1 (melastatin). We find that miR-211 expression is greatly decreased in melanoma cells and melanoblasts compared to melanocytes. Bioinformatic analysis identified a large number of potential targets of miR-211, including POU3F2 (BRN2). Inhibition of miR-211 in normal melanocytes resulted in increased BRN2 protein, indicating that endogenous miR-211 represses BRN2 in differentiated cells. Over-expression of miR-211 in melanoma cell lines changed the invasive potential of the cells in vitro through directly targeting BRN2 translation. We propose a model for the apparent non-overlapping expression levels of BRN2 and MITF in melanoma, mediated by miR-211 expression.

Inverse expression states of the BRN2 and MITF transcription factors in melanoma spheres and tumour xenografts regulate the NOTCH pathway

The use of adherent monolayer cultures have produced many insights into melanoma cell growth and differentiation, but often novel therapeutics demonstrated to act on these cells are not active in vivo. It is imperative that new methods of growing melanoma cells that reflect growth in vivo are investigated. To this end, a range of human melanoma cell lines passaged as adherent cultures or induced to form melanoma spheres (melanospheres) in stem cell media have been studied to compare cellular characteristics and protein expression. Melanoma spheres and tumours grown from cell lines as mouse xenografts had increased heterogeneity when compared with adherent cells and 3D-spheroids in agar (aggregates). Furthermore, cells within the melanoma spheres and mouse xenografts each displayed a high level of reciprocal BRN2 or MITF expression, which matched more closely the pattern seen in human melanoma tumours in situ, rather than the propensity for co-expression of these important melanocytic transcription factors seen in adherent cells and 3D-spheroids. Notably, when the levels of the BRN2 and MITF proteins were each independently repressed using siRNA treatment of adherent melanoma cells, members of the NOTCH pathway responded by decreasing or increasing expression, respectively. This links BRN2 as an activator, and conversely, MITF as a repressor of the NOTCH pathway in melanoma cells. Loss of the BRN2-MITF axis in antisense-ablated cell lines decreased the melanoma sphere-forming capability, cell adhesion during 3D-spheroid formation and invasion through a collagen matrix. Combined, this evidence suggests that the melanoma sphere-culture system induces subpopulations of cells that may more accurately portray the in vivo disease, than the growth as adherent melanoma cells.

The recycling endosome protein Rab17 regulates melanocytic filopodia formation and melanosome trafficking

Rab GTPases including Rab27a, Rab38 and Rab32 function in melanosome maturation or trafficking in melanocytes. A screen to identify additional Rabs involved in these processes revealed the localization of GFP-Rab17 on recycling endosomes (REs) and melanosomes in melanocytic cells. Rab17 mRNA expression is regulated by microphthalmia transcription factor (MITF), a characteristic of known pigmentation genes. Rab17 siRNA knockdown in melanoma cells quantitatively increased melanosome concentration at the cell periphery. Rab17 knockdown did not inhibit melanosome maturation nor movement, but it caused accumulation of melanin inside cells. Double knockdown of Rab17 and Rab27a indicated that Rab17 acts on melanosomes downstream of Rab27a. Filopodia are known to play a role in melanosome transfer, and in Rab17 knockdown cells filopodia formation was inhibited. Furthermore, we show that stimulation of melanoma cells with α-melanocyte-stimulating hormone induces filopodia formation, supporting a role for filopodia in melanosome release. Cell stimulation also caused redistribution of REs to the periphery, and knockdown of additional RE-associated Rabs 11a and 11b produced a similar accumulation of melanosomes and melanin to that seen after loss of Rab17. Our findings reveal new functions for RE and Rab17 in pigmentation through a distal step in the process of melanosome release via filopodia.

In vitro dedifferentiation of melanocytes from adult epidermis

In previous work we described a novel culture technique using a cholera toxin and PMA-free medium (Mel-mix) for obtaining pure melanocyte cultures from human adult epidermis. In Mel-mix medium the cultured melanocytes are bipolar, unpigmented and highly proliferative. Further characterization of the cultured melanocytes revealed the disappearance of c-Kit and TRP-1 and induction of nestin expression, indicating that melanocytes dedifferentiated in this in vitro culture. Cholera toxin and PMA were able to induce c-Kit and TRP-1 protein expressions in the cells, reversing dedifferentiation. TRP-1 mRNA expression was induced in dedifferentiated melanocytes by UV-B irradiated keratinocyte supernatants, however direct UV-B irradiation of the cells resulted in further decrease of TRP-1 mRNA expression. These dedifferentiated, easily accessible cultured melanocytes provide a good model for studying melanocyte differentiation and possibly transdifferentiation. Because melanocytes in Mel-mix medium can be cultured with human serum as the only supplement, this culture system is also suitable for autologous cell transplantation.

FK506 (tacrolimus) and endothelin combined treatment induces mobility of melanoblasts: new insights into follicular vitiligo repigmentation induced by topical tacrolimus on sun-exposed skin

BACKGROUND

Topical tacrolimus (FK506) has been considered as a treatment option for treating vitiligo, a dermatosis characterized by disappearance of melanocytes (MCs). Previous reports have shown that a significant portion of treated patients demonstrated follicular repigmentation, indicating that the activation of MC precursor cells residing in the outer root sheath of hair follicles played an important role during the tacrolimus-induced repigmentation process.

OBJECTIVES

To investigate the mechanisms involved in follicular pigmentation induced by topical tacrolimus.

METHODS

As stem cells of MC lineage are identified in the lower portion of mouse hair follicles throughout the hair cycle, immature mouse melanoblasts (MBs) derived from neural crest cells (NCCmelb4) were used for this study. Relevant maturation parameters were evaluated.

RESULTS

Our results revealed that FK506 stimulated the expressions of protein kinase A, protein kinase C and phosphorylated p38 mitogen-activated protein kinase. However, cell motility, a parameter associated with MB differentiation, was not enhanced by FK506 treatment. Endothelin (ET)-3, a prodifferentiation factor of MBs, also failed to promote NCCmelb4 cell locomotion. Combining ET-3 and FK506, however, stimulated cell mobility. ET B receptor, which was not present in NCCmelb4 cells, was induced after FK506 treatment.

CONCLUSIONS

In summary, we have shown that FK506 is an efficient differentiation-stimulating agent, especially for cells of neural origin. The clinical efficacy of topical tacrolimus on vitiligo may be enhanced by combination with ET-3.