Melanocyte Stem Cell Activation and Translocation Initiate Cutaneous Melanoma in Response to UV Exposure

Sex-Determining Region Y Chromosome-Related High-Mobility-Group Box 10 in Cancer: A Potential Therapeutic Target

Sex-determining region Y-related high mobility group-box 10 (SOX10), a member of the SOX family, has recently been highlighted as an essential transcriptional factor involved in developmental biology. Recently, the functionality of SOX 10 has been increasingly revealed by researchers worldwide. It has been reported that SOX10 significantly regulates the proliferation, migration, and apoptosis of tumors and is closely associated with the progression of cancer. In this review, we first introduce the basic background of the SOX family and SOX10 and then discuss the pathophysiological roles of SOX10 in cancer. Besides, we enumerate the application of SOX10 in the pathological diagnosis and therapeutic potential of cancer. Eventually, we summarize the potential directions and perspectives of SOX10 in neoplastic theranostics. The information compiled herein may assist in additional studies and increase the potential of SOX10 as a therapeutic target for cancer.

Reemergence of neural crest stem cell-like states in melanoma during disease progression and treatment

Melanoma is the deadliest of all skin cancers due to its high metastatic potential. In recent years, advances in targeted therapy and immunotherapy have contributed to a remarkable progress in the treatment of metastatic disease. However, intrinsic or acquired resistance to such therapies remains a major obstacle in melanoma treatment. Melanoma disease progression, beginning from tumor initiation and growth to acquisition of invasive phenotypes and metastatic spread and acquisition of treatment resistance, has been associated with cellular dedifferentiation and the hijacking of gene regulatory networks reminiscent of the neural crest (NC)—the developmental structure which gives rise to melanocytes and hence melanoma. This review summarizes the experimental evidence for the involvement of NC stem cell (NCSC)‐like cell states during melanoma progression and addresses novel approaches to combat the emergence of stemness characteristics that have shown to be linked with aggressive disease outcome and drug resistance.

BMP signaling: at the gate between activated melanocyte stem cells and differentiation

Through recurrent bouts synchronous with the hair cycle, quiescent melanocyte stem cells (McSCs) become activated to generate proliferative progeny that differentiate into pigment-producing melanocytes. The signaling factors orchestrating these events remain incompletely understood. Here, we use single-cell RNA sequencing with comparative gene expression analysis to elucidate the transcriptional dynamics of McSCs through quiescence, activation, and melanocyte maturation. Unearthing converging signs of increased WNT and BMP signaling along this progression, we endeavored to understand how these pathways are integrated. Employing conditional lineage-specific genetic ablation studies in mice, we found that loss of BMP signaling in the lineage leads to hair graying due to a block in melanocyte maturation. We show that interestingly, BMP signaling functions downstream from activated McSCs and maintains WNT effector, transcription factor LEF1. Employing pseudotime analysis, genetics, and chromatin landscaping, we show that following WNT-mediated activation of McSCs, BMP and WNT pathways collaborate to trigger the commitment of proliferative progeny by fueling LEF1- and MITF-dependent differentiation. Our findings shed light upon the signaling interplay and timing of cues that orchestrate melanocyte lineage progression in the hair follicle and underscore a key role for BMP signaling in driving complete differentiation.

Somatic mutational landscapes of adherens junctions and their functional consequences in cutaneous melanoma development

Cell-cell interaction in skin homeostasis is tightly controlled by adherens junctions (AJs). Alterations in such regulation lead to melanoma development. However, mutations in AJs and their functional consequences are still largely unknown.

Methods: Cadherin mutations in skin cutaneous melanoma were identified using sequencing data from TCGA dataset, followed by cross-validation with data from non-TCGA cohorts. Mutations with significant occurrence were subjected to structural prediction using MODELLER and functional protein simulation using GROMACS software. Neo-antigen prediction was carried out using NetMHCpan tool. Cell-based fluorescence reporter assay was used to validate β-catenin activity in the presence of cadherin mutations. Clinical significance was analyzed using datasets from TCGA and other non-TCGA cohorts. Targeted gene exon sequencing and immunofluorescence staining on melanoma tissues were performed to confirm the in silico findings.

Results: Highly frequent mutations in type-II classical cadherins were found in melanoma with one unique recurrent mutation (S524L) in the fifth domain of CDH6, which potentially destabilizes Ca²⁺-binding and cell-cell contacts. Mutational co-occurrence and physical dynamics analyses placed CDH6 at the center of the top-four mutated cadherins (core CDHs; all type-II), suggesting altered heterophilic interactions in melanoma development. Mutations in the intracellular domains significantly disturbed CDH6/β-catenin complex formation, resulting in β-catenin translocation into cytosol or nucleus and dysregulation of canonical Wnt/β-catenin signaling. Although mutations in core CDH genes correlated with advanced cancer stages and lymph node invasion, the overall and disease-free survival times in those patients were longer in patients with wild-type. Peptide/MHC-I binding affinity predictions confirmed overall increased neo-antigen potentials of mutated cadherins, which associated with T-lymphocyte infiltration and better clinical outcomes after immunotherapy.

Conclusion: Changes in cell-cell communications by somatic mutations in AJ cadherins function as one of mechanisms to trigger melanoma development. Certain mutations in AJs may serve as potential neo-antigens which conversely benefit patients for longer survival times.

MicroRNA-21-Enriched Exosomes as Epigenetic Regulators in Melanomagenesis and Melanoma Progression: The Impact of Western Lifestyle Factors

DNA mutation-induced activation of RAS-BRAF-MEK-ERK signaling associated with intermittent or chronic ultraviolet (UV) irradiation cannot exclusively explain the excessive increase of malignant melanoma (MM) incidence since the 1950s. Malignant conversion of a melanocyte to an MM cell and metastatic MM is associated with a steady increase in microRNA-21 (miR-21). At the epigenetic level, miR-21 inhibits key tumor suppressors of the RAS-BRAF signaling pathway enhancing proliferation and MM progression. Increased MM cell levels of miR-21 either result from endogenous upregulation of melanocytic miR-21 expression or by uptake of miR-21-enriched exogenous exosomes. Based on epidemiological data and translational evidence, this review provides deeper insights into environmentally and metabolically induced exosomal miR-21 trafficking beyond UV-irradiation in melanomagenesis and MM progression. Sources of miR-21-enriched exosomes include UV-irradiated keratinocytes, adipocyte-derived exosomes in obesity, airway epithelium-derived exosomes generated by smoking and pollution, diet-related exosomes and inflammation-induced exosomes, which may synergistically increase the exosomal miR-21 burden of the melanocyte, the transformed MM cell and its tumor environment. Several therapeutic agents that suppress MM cell growth and proliferation attenuate miR-21 expression. These include miR-21 antagonists, metformin, kinase inhibitors, beta-blockers, vitamin D, and plant-derived bioactive compounds, which may represent new options for the prevention and treatment of MM.

Insufficient Sun Exposure Has Become a Real Public Health Problem

This article aims to alert the medical community and public health authorities to accumulating evidence on health benefits from sun exposure, which suggests that insufficient sun exposure is a significant public health problem. Studies in the past decade indicate that insufficient sun exposure may be responsible for 340,000 deaths in the United States and 480,000 deaths in Europe per year, and an increased incidence of breast cancer, colorectal cancer, hypertension, cardiovascular disease, metabolic syndrome, multiple sclerosis, Alzheimer's disease, autism, asthma, type 1 diabetes and myopia. Vitamin D has long been considered the principal mediator of beneficial effects of sun exposure. However, oral vitamin D supplementation has not been convincingly shown to prevent the above conditions; thus, serum 25(OH)D as an indicator of vitamin D status may be a proxy for and not a mediator of beneficial effects of sun exposure. New candidate mechanisms include the release of nitric oxide from the skin and direct effects of ultraviolet radiation (UVR) on peripheral blood cells. Collectively, this evidence indicates it would be wise for people living outside the tropics to ensure they expose their skin sufficiently to the sun. To minimize the harms of excessive sun exposure, great care must be taken to avoid sunburn, and sun exposure during high ambient UVR seasons should be obtained incrementally at not more than 5-30 min a day (depending on skin type and UV index), in season-appropriate clothing and with eyes closed or protected by sunglasses that filter UVR.

Sirt5 is dispensable for BrafV600E -mediated cutaneous melanoma development and growth in vivo

Sirt5 is known to functionally regulate mitochondrial proteins by altering posttranslational modifications, including lysine desuccinylation. While roles for Sirt5 as either a tumor promoter or suppressor, or in chemoresistance, have been implicated in other cancers, the function of Sirt5 in cutaneous melanoma has not been well examined. Therefore, to determine whether Sirt5 is necessary for Braf^V600E -mediated melanoma formation and/or disease progression, we crossed a genetically engineered murine melanoma model (Tyr^{Cre ERT 2/+} ; Braf^{LSL -V600E/+} ; Pten^flox/flox ) to Sirt5^-/- knockout animals. In addition, we tested for synergism with a selective BRAF (V600E) inhibitor in Sirt5^-/- mouse melanoma cells. Taken together, this report demonstrates that, in these models, Sirt5 is dispensable for Braf^V600E -mediated cutaneous melanoma formation and growth in vivo, and does not improve sensitivity to a selective BRAF inhibitor.

The Mammalian High Mobility Group Protein AT-Hook 2 (HMGA2): Biochemical and Biophysical Properties, and Its Association with Adipogenesis

The mammalian high-mobility-group protein AT-hook 2 (HMGA2) is a small DNA-binding protein and consists of three “AT-hook” DNA-binding motifs and a negatively charged C-terminal motif. It is a multifunctional nuclear protein directly linked to obesity, human height, stem cell youth, human intelligence, and tumorigenesis. Biochemical and biophysical studies showed that HMGA2 is an intrinsically disordered protein (IDP) and could form homodimers in aqueous buffer solution. The “AT-hook” DNA-binding motifs specifically bind to the minor groove of AT-rich DNA sequences and induce DNA-bending. HMGA2 plays an important role in adipogenesis most likely through stimulating the proliferative expansion of preadipocytes and also through regulating the expression of transcriptional factor Peroxisome proliferator-activated receptor γ (PPARγ) at the clonal expansion step from preadipocytes to adipocytes. Current evidence suggests that a main function of HMGA2 is to maintain stemness and renewal capacity of stem cells by which HMGA2 binds to chromosome and lock chromosome into a specific state, to allow the human embryonic stem cells to maintain their stem cell potency. Due to the importance of HMGA2 in adipogenesis and tumorigenesis, HMGA2 is considered a potential therapeutic target for anticancer and anti-obesity drugs. Efforts are taken to identify inhibitors targeting HMGA2.

Effect of hair removal on solar UV transmission into skin and implications for melanoma skin cancer development

Melanoma is the severest type of skin cancer. As distinct from many other cancer types, the incidence of melanoma has been increasing steadily over the last century. Discovering new risk factors of melanoma will not only raise public awareness but also potentially contribute to the improvement of skin cancer protection in the future. Nowadays, the tendency of shaving skin hair is becoming increasingly popular for aesthetic purposes. However, human hair serves several functions, one of which is ultraviolet (UV) protection for the skin. What is more, stem cells found in the follicles of hair could be the origin of melanoma upon exposure to ultraviolet radiation. Therefore, it is of interest to investigate the effect of shaving on solar UV transmission in the skin. To achieve that, two groups of skin models are constructed in TracePro software: one with unaltered hair and one with shaved hair. The UV transport in the models is simulated using the Monte Carlo method and the absorptions in the stem cells layer are compared. It is found that shaving will increase the UV transmission to the follicular stem cells to a certain degree. More specifically, shaving limbs will generally increase the solar UV transmission from about 5% to 20% in the UV wavelength range.

Hair(y) Matters in Melanoma Biology

Melanocyte stem cells (MeSCs), one candidate for the cellular origin of melanoma, reside in the bulge region of the hair follicle (HF), an immune-privileged tissue niche with impaired tumor immunosurveillance. Surprisingly, however, primary melanoma is only very rarely associated with HFs. Here, we explore the hypothesis that this profoundly immunoinhibitory signaling environment deprives both MeSCs and melanocytes of the anagen hair matrix of proinflammatory signals required for full oncogenic transformation. Understanding the cellular and molecular mechanisms for generating a putative antimelanoma tissue habitat, namely in the bulge, could help to recreate a similar melanoma-suppressive signaling environment in melanoma high-risk individuals. We further discuss how mimicking the bulge immune privilege may be an effective melanoma prevention strategy.

Identification of very small cancer stem cells expressing hallmarks of pluripotency in B16F10 melanoma cells and their reoccurrence in B16F10-derived clones

Melanoma is characterized by high heterogeneity and plasticity, most likely due to the presence of mutated melanocyte stem cells or immature progenitor cells in the skin that serves as precursors to melanoma. In the present study, for the first time, we identified rare cells in the murine melanoma B16F10, and human A2058 and SK-MEL-28 cell lines that express pluripotency markers, including Oct4, Nanog, Sox2 and a marker of melanoma cancer cells (ALDH1/2). These cells are very small with round morphology and they grow onto melanoma cells, thereby demonstrating feeder layer dependence similar to that of other pluripotent cells. These cells underwent self-renewal, symmetric and asymmetric division. We called these cells murine very small cancer stem cells (VSCSC). VSCSC were also found in B16F10-derived clones after 3-5 consecutive passages, where they occur as single cells or as small colonies, nevertheless, always using melanoma cells as feeders. These cells formed melanospheres enriched with Oct4-and ALDH1/2-positive cells. We also evaluated the possible effect of VSCSC that presented in the parental cell line (B16F10) and in clones based on their functional characteristics. We found that VCSCS present in the B16F10 cell line reappearing in their clones were required for continuous tumor growth and were responsible for melanoma cell heterogeneity and plasticity rather than directly affecting functional characteristics of melanoma cells. Our data, together with those of previous reports suggested the existence of melanoma-competent melanocyte stem cells, which corroborate the hypothesis of the existence of tumor-initiating cells and cancer stem cell hierarchies, at least in melanoma.

New insights into the functions of Cox-2 in skin and esophageal malignancies

Understanding the cellular and molecular mechanisms of tumor initiation and progression for each cancer type is central to making improvements in both prevention and therapy. Identifying the cancer cells of origin and the necessary and sufficient mechanisms of transformation and progression provide opportunities for improved specific clinical interventions. In the last few decades, advanced genetic manipulation techniques have facilitated rapid progress in defining the etiologies of cancers and their cells of origin. Recent studies driven by various groups have provided experimental evidence indicating the cellular origins for each type of skin and esophageal cancer and have identified underlying mechanisms that stem/progenitor cells use to initiate tumor development. Specifically, cyclooxygenase-2 (Cox-2) is associated with tumor initiation and progression in many cancer types. Recent studies provide data demonstrating the roles of Cox-2 in skin and esophageal malignancies, especially in squamous cell carcinomas (SCCs) occurring in both sites. Here, we review experimental evidence aiming to define the origins of skin and esophageal cancers and discuss how Cox-2 contributes to tumorigenesis and differentiation.

Uncoupling protein 2 is upregulated in melanoma cells and contributes to the activation of Akt/mTOR and ERK signaling

The aim of the present study was to characterize the expression of uncoupling protein 2 (UCP2) in melanoma and to study the potential mechanisms underlying the involvement of UCP2 in melanomagenesis using human melanoma cell lines. The expression of UCP2 was evaluated in specimens from normal control subjects, patients with compound nevus, and patients with cutaneous and mucosal melanoma. Stable knockdown of UCP2 was achieved in human melanoma cell lines, which were used to examine whether UCP2 knockdown affects the mitochondrial membrane potential and intracellular levels of ATP, reactive oxygen species and lactate. Cell proliferation, invasion, spheroid formation and cisplatin sensitivity were also evaluated in the UCP2 knockdown cells. Finally, the effects of UCP2 knockdown on the Akt/mammalian target of rapamycin (mTOR) and extracellular signal‑regulated kinase (ERK) pathways, which are important oncogenic pathways during melanomagenesis, were evaluated. Relatively high expression of UCP2 was detected in human melanoma specimens, which was correlated with Clark level and Breslow thickness. Knockdown of UCP2 suppressed cell proliferation, invasion and spheroid formation, and increased the sensitivity of melanoma cells to cisplatin. Furthermore, the UCP2 knockdown cells exhibited inhibition of Akt/mTOR signaling and ERK activation. Therefore, human melanoma tissues exhibit relatively high UCP2 expression, which may be implicated in the mechanisms underlying tumor progression. The potential role of UCP2 in melanomagenesis may involve enhancing the Akt/mTOR and mitogen‑activated protein kinase/ERK pathways.

On-demand fully customizable drug tablets via 3D printing technology for personalized medicine

Personalized medicine should ideally be prescribed to every individual because of the unique characteristics (e.g., biological, physical, and medical) of each individual. It is, however, challenging to provide personalized medicine for the mass population of specific individuals effectively and efficiently. This manuscript describes a method of fabricating fully customizable drug tablets for personalized medicine by the 3D printing technology. This method involves the versatile fabrication of the tablets via the specifically designed 3D printed molds of different shapes and sizes, and an intuitive 1-dimensional release of drug that relates the shape of the drug-containing matrix to the release profile. The customization includes all the aspects of varying dosage, duration, release profiles, and combination of multiple drugs. In particular, it has previously been technically difficult to devise a single platform that fabricates carriers that release drug with any desired type of release profiles. This method of fabricating fully customizable tablets is simple, inexpensive, and efficient. Detailed selection and investigation of the materials ensured that the tablet and the method of fabrication are safe (e.g., biocompatible, FDA-approved ingredients used) and other desirable features (e.g., sustained release and high dosage) are achieved. These desirable characteristics of the method thus allow fully customizable drug tablets to be fabricated efficiently on the spot after the diagnosis of individual patients; at the same time, the method can be made widely accessible to the mass population. Hence, the concept of personalized medicine can truly be realized.

Particulate mediators of the bystander effect linked to suicide and interferon-β transgene expression in melanoma cells

In the context of comparative oncology, melanoma cells derived from companion animal tumors are good models for optimizing and predicting their in vivo response to therapeutic strategies. Here, we report that human, canine, and feline melanoma cells driven to death by bleomycin, interferon-β gene, or herpes simplex virus thymidine kinase/ganciclovir suicide gene (SG) treatment significantly increased their internal granularity. This fact correlated with the release of a heterogeneous collection of nano- and micro-sized granules as revealed by transmission electron microscopy. While killing lipofected cells, the expressed transgenes and their derived products were incorporated into these granules that were isolated by differential centrifugation. These particulate factors (PFs) were able to transfer, in a dose- and time-dependent manner, appreciable levels of therapeutic genes, related proteins, and drugs. Thus, when recipient cells of SG-carrying PFs were exposed to ganciclovir, this prodrug was efficiently activated, eliminating them. These PFs kept the functionality of their cargo, even after being subjected to adverse conditions, such as the presence of DNase, freezing, or heating. Since our in vitro system did not include any of the immune mechanisms that could provide additional antitumor activity, the chemo-gene treatments amplified by these delivery bags of therapeutic agents offer a great clinical potential.

Hyperactivation of sympathetic nerves drives depletion of melanocyte stem cells

Empirical and anecdotal evidence have associated stress with accelerated hair greying (formation of unpigmented hairs)^1,2, but the scientific evidence linking the two is scant. Here, we report that acute stress leads to hair greying through fast depletion of melanocyte stem cells (MeSCs). Combining adrenalectomy, denervation, chemogenetics^3,4, cell ablation, and MeSC-specific adrenergic receptor knockout, we found that stress-induced MeSC loss is independent of immune attack or adrenal stress hormones. Rather, hair greying results from activation of the sympathetic nerves that innervate the MeSC niche. Upon stress, sympathetic nerve activation leads to burst release of the neurotransmitter norepinephrine, which drives quiescent MeSCs into rapid proliferation, followed by differentiation, migration, and permanent depletion from the niche. Transient suppression of MeSC proliferation prevents stress-induced hair greying. Our studies demonstrate that acute stress-induced neuronal activity can drive rapid and permanent loss of somatic stem cells, and illustrate an example in which somatic stem cell maintenance is directly influenced by the overall physiological state of the organism.

Targeting Angiogenesis by Blocking the ATM-SerRS-VEGFA Pathway for UV-Induced Skin Photodamage and Melanoma Growth

Retinoic acid (RA) has been widely used to protect skin from photo damage and skin carcinomas caused by solar ultraviolet (UV) irradiation, yet the mechanism remains elusive. Here, we report that all-trans retinoic acid (tRA) can directly induce the expression of a newly identified potent anti-angiogenic factor, seryl tRNA synthetase (SerRS), whose angiostatic role can, however, be inhibited by UV-activated ataxia telangiectasia mutated (ATM) kinase. In both a human epidermal cell line, HaCaT, and a mouse melanoma B16F10 cell line, we found that tRA could activate SerRS transcription through binding with the SerRS promoter. However, UV irradiation induced activation of ATM-phosphorylated SerRS, leading to the inactivation of SerRS as a transcriptional repressor of vascular endothelial growth factor A (VEGFA), which dampened the effect of tRA. When combined with ATM inhibitor KU-55933, tRA showed a greatly enhanced efficiency in inhibiting VEGFA expression and a much better protection of mouse skin from photo damage. Also, we found the combination greatly inhibited tumor angiogenesis and growth in mouse melanoma xenograft in vivo. Taken together, tRA combined with an ATM inhibitor can greatly enhance the anti-angiogenic activity of SerRS under UV irradiation and could be a better strategy in protecting skin from angiogenesis-associated skin damage and melanoma caused by UV radiation.

A novel mouse model demonstrates that oncogenic melanocyte stem cells engender melanoma resembling human disease

Melanoma, the deadliest skin cancer, remains largely incurable at advanced stages. Currently, there is a lack of animal models that resemble human melanoma initiation and progression. Recent studies using a Tyr-CreER driven mouse model have drawn contradictory conclusions about the potential of melanocyte stem cells (McSCs) to form melanoma. Here, we employ a c-Kit-CreER-driven model that specifically targets McSCs to show that oncogenic McSCs are a bona fide source of melanoma that expand in the niche, and then establish epidermal melanomas that invade into the underlying dermis. Further, normal Wnt and Endothelin niche signals during hair anagen onset are hijacked to promote McSC malignant transformation during melanoma induction. Finally, molecular profiling reveals strong resemblance of murine McSC-derived melanoma to human melanoma in heterogeneity and gene signatures. These findings provide experimental validation of the human melanoma progression model and key insights into the transformation and heterogeneity of McSC-derived melanoma.

Currently, few mouse models exist to recapitulate human melanomagenesis. Here, the authors establish a c-Kit-CreER-driven model to target melanocyte stem cells (McSCs) and show that oncogenic McSCs give rise to epidermal melanoma that invade into the dermis, similar to human melanoma.

Contribution of Human Hair in Solar UV Transmission in Skin: Implications for Melanoma Development

Melanoma is the deadliest type of skin cancer with its prevalence on the rise. Recently, the melanocyte stem cells in hair follicles have been identified as the possible origin of melanoma upon exposure to ultraviolet radiation (UVR) through skin. It is hypothesized that colourless vellus hair (predominant in childhood) can serve as an alternative pathway in transmitting these ultraviolet (UV) photons to the stem cells. To investigate this, we have used the CRAIC microspectrophotometer to investigate the optical properties of 'vellus-like' hairs and terminal hairs of different colours using UV-VIS-NIR light sources. It was found that the average attenuation coefficient of 'vellus-like' hair is significantly lower than that of terminal hair in the UVA (p < 0.0001) and UVB (p < 0.001) wavelength ranges. Next, the optical properties of hairs are applied to simulations for examining their influence on UV transmission into the skin. The results show that the presence of vellus hair would increase the solar UV transmission to the melanocyte stem cell layer significantly. The findings explain why children are particularly vulnerable to sun exposure and the positive correlation found between the incidence of melanoma in adults' bodies and the number of vellus hairs in these areas.

Melanoma Progression Inhibits Pluripotency and Differentiation of Melanoma-Derived iPSCs Produces Cells with Neural-like Mixed Dysplastic Phenotype

Melanomas are known to exhibit phenotypic plasticity. However, the role cellular plasticity plays in melanoma tumor progression and drug resistance is not fully understood. Here, we used reprogramming of melanocytes and melanoma cells to induced pluripotent stem cell (iPSCs) to investigate the relationship between cellular plasticity and melanoma progression and mitogen-activated protein kinase (MAPK) inhibitor resistance. We found that melanocyte reprogramming is prevented by the expression of oncogenic BRAF, and in melanoma cells harboring oncogenic BRAF and sensitive to MAPK inhibitors, reprogramming can be restored by inhibition of the activated oncogenic pathway. Our data also suggest that melanoma tumor progression acts as a barrier to reprogramming. Under conditions that promote melanocytic differentiation of fibroblast- and melanocyte-derived iPSCs, melanoma-derived iPSCs exhibited neural cell-like dysplasia and increased MAPK inhibitor resistance. These data suggest that iPSC-like reprogramming and drug resistance of differentiated cells can serve as a model to understand melanoma cell plasticity-dependent mechanisms in recurrence of aggressive drug-resistant melanoma.

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Metastatic melanoma exhibits less plasticity to reprogramming than primary melanoma

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Oncogenic BRAF^V600E and resistance to MAPKi inhibit reprogramming of melanoma

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Differentiation of melanoma-iPSCs produces cells with mixed dysplastic phenotype

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Melanoma-iPSC-differentiated cells exhibit acquired resistance to MAPKi

Location, Location, Location: Spatio-Temporal Cues That Define the Cell of Origin in Melanoma

It is unclear whether melanoma initiates from mature melanocytes or stem cell precursors. In this issue of Cell Stem Cell, Moon et al. (2017) and Köhler et al. (2017) use in vivo lineage tracing to demonstrate that these two possibilities may occur downstream of the same pro-tumorigenic lesions, depending on environmental factors or the anatomical location.

Bullfrog oil (Rana catesbeiana Shaw) induces apoptosis, in A2058 human melanoma cells by mitochondrial dysfunction triggered by oxidative stress

Bullfrog oil, an animal oil extracted from the adipose tissue of Rana catesbeiana Shaw, showed promising cytotoxic activity against melanoma cells and, therefore, has the potential to become a pharmaceutical active compound. However, there is a lack of information regarding the pathways involved in its pharmacological activity. Thus, the aim of this study was to investigate and elucidate the cytotoxic effect of this oil against A2058 human melanoma cells. The cytotoxic potential was evaluated by the MTT assay, the cell cycle analysis and the cell death assay. In addition, the apoptotic potential was investigated by (i) the DNA fragmentation using propidium iodide staining analysis, (ii) the evaluation of mitochondrial membrane potential and (iii) the determination of intracellular Reactive Oxygen Species (ROS) level. The results showed that the bullfrog oil was able to promote a time-dependent cytotoxic effect, decreasing cell viability to 38% after 72 h of treatment without affecting the cell cycle. Additionally, the bullfrog oil induced the apoptosis in A2058 cells, increasing up to 50 ± 13% of the intracellular ROS level, maintaining the DNA integrity and promoting an approximate decrease of 35 ± 5% in the mitochondrial membrane potential. It can be concluded that the in vitro cytotoxic effect of the bullfrog oil in A2058 human melanoma cells is mediated by oxidative stress that induces mitochondrial dysfunction, triggering the apoptosis. These unprecedented results highlight the pharmacological potential of bullfrog oil and provide important information to support studies on the development of new pharmaceutical products for complementary and alternative treatments for melanoma.

Spatial and Temporal Control of Murine Melanoma Initiation from Mutant Melanocyte Stem Cells

Cutaneous melanoma is well known as the most aggressive skin cancer. Although the risk factors and major genetic alterations continue to be documented with increasing depth, the incidence rate of cutaneous melanoma has shown a rapid and continuous increase during recent decades. In order to find effective preventative methods, it is important to understand the early steps of melanoma initiation in the skin. Previous data has demonstrated that follicular melanocyte stem cells (MCSCs) in the adult skin tissues can act as melanoma cells of origin when expressing oncogenic mutations and genetic alterations. Tumorigenesis arising from melanoma-prone MCSCs can be induced when MCSCs transition from a quiescent to active state. This transition in melanoma-prone MCSCs can be promoted by the modulation of either hair follicle stem cells' activity state or through extrinsic environmental factors such as ultraviolet-B (UV-B). These factors can be artificially manipulated in the laboratory by chemical depilation, which causes transition of hair follicle stem cells and MCSCs from a quiescent to active state, and by UV-B exposure using a benchtop light. These methods provide successful spatial and temporal control of cutaneous melanoma initiation in the murine dorsal skin. Therefore, these in vivo model systems will be valuable to define the early steps of cutaneous melanoma initiation and could be used to test potential methods for tumor prevention.

Krt5+/Krt15+ foregut basal progenitors give rise to cyclooxygenase-2-dependent tumours in response to gastric acid stress

The effective prevention of tumor initiation, especially for potentially inoperable tumors, will be beneficial to obtain an overall higher quality of our health and life. Hence, thorough understanding of the pathophysiological mechanisms of early tumor formation arising from identifiable cellular origins is required to develop efficient preventative and early treatment options for each tumor type. Here, using genetically engineered mouse models, we provide preclinical experimental evidence for a long-standing open question regarding the pathophysiological potential of a microenvironmental and physiological stressor in tumor development, gastric acid-mediated regional microscopic injury in foregut squamous epithelia. This study demonstrates the association of gastric acid stress with Cyclooxygenase-2-dependent tumor formation originating from tumor-competent Krt5⁺/Krt15⁺ foregut basal progenitor cells. Our findings suggest that clinical management of microenvironmental stressor-mediated microscopic injury may be important in delaying tumor initiation from foregut basal progenitor cells expressing pre-existing tumorigenic mutation(s) and genetic alteration(s).

Cellular extrinsic environmental factors contribute to tumour development. Here, the authors show that gastric acid stress stimulates tumour formation from a defined tumour-competent Krt5 + /Krt15 + foregut basal progenitor cell population.

CD34 defines melanocyte stem cell subpopulations with distinct regenerative properties

Melanocyte stem cells (McSCs) are the undifferentiated melanocytic cells of the mammalian hair follicle (HF) responsible for recurrent generation of a large number of differentiated melanocytes during each HF cycle. HF McSCs reside in both the CD34+ bulge/lower permanent portion (LPP) and the CD34- secondary hair germ (SHG) regions of the HF during telogen. Using Dct-H2BGFP mice, we separate bulge/LPP and SHG McSCs using FACS with GFP and anti-CD34 to show that these two subsets of McSCs are functionally distinct. Genome-wide expression profiling results support the distinct nature of these populations, with CD34- McSCs exhibiting higher expression of melanocyte differentiation genes and with CD34+ McSCs demonstrating a profile more consistent with a neural crest stem cell. In culture and in vivo, CD34- McSCs regenerate pigmentation more efficiently whereas CD34+ McSCs selectively exhibit the ability to myelinate neurons. CD34+ McSCs, and their counterparts in human skin, may be useful for myelinating neurons in vivo, leading to new therapeutic opportunities for demyelinating diseases and traumatic nerve injury.

Simulation of UV power absorbed by follicular stem cells during sun exposure and possible implications for melanoma development

The incidence of melanoma in children is extremely rare. However, there is convincing evidence supporting a higher risk of developing melanoma in individuals who have experienced excessive sun exposure during their childhood. A possible explanation for this phenomenon is that prior to puberty, the melanocyte stem cells in the bulge region of hair follicles are much shallower in the dermis, resulting in their increased vulnerability to sun exposure. To validate this hypothesis, a Monte Carlo simulation of light transport is applied to quantify the dose of solar UV power absorbed in the stem-cell layers at different depths in both child and adult skins. The simulated results suggest that the stem cells in vellus hair follicles would absorb over 250 times higher UV photons than those in the terminal hair follicles. Due to the thinner epidermis in child skin, the stem cells in vellus hair follicles absorb about 1.9 and 3.2 times greater ultraviolet A and ultraviolet B, respectively, than those in adult skin. These findings provide a possible explanation to why children are particularly vulnerable to sun exposure.

Regulation of melanocyte stem cells in the pigmentation of skin and its appendages: Biological patterning and therapeutic potentials

Skin evolves essential appendages and indispensable types of cells that synergistically insulate the body from environmental insults. Residing in the specific regions in the skin such as epidermis, dermis and hair follicle, melanocytes perform an array of vital functions including defending the ultraviolet radiation and diversifying animal appearance. As one of the adult stem cells, melanocyte stem cells in the hair follicle bulge niche can proliferate, differentiate and keep quiescence to control and coordinate tissue homeostasis, repair and regeneration. In synchrony with hair follicle stem cells, melanocyte stem cells in the hair follicles undergo cyclic activation, degeneration and resting phases, to pigment the hairs and to preserve the stem cells. Disorder of melanocytes results in severe skin problems such as canities, vitiligo and even melanoma. Here, we compare and summarize recent discoveries about melanocyte in the skin, particularly in the hair follicle. A better understanding of the physiological and pathological regulation of melanocyte and melanocyte stem cell behaviours will help to guide the clinical applications in regenerative medicine.

Plasticity of differentiated cells in wound repair and tumorigenesis, part II: skin and intestine

Recent studies have identified and begun to characterize the roles of regenerative cellular plasticity in many organs. In Part I of our two-part Review, we discussed how cells reprogram following injury to the stomach and pancreas. We introduced the concept of a conserved cellular program, much like those governing division and death, which may allow mature cells to become regenerative. This program, paligenosis, is likely necessary to help organs repair the numerous injuries they face over the lifetime of an organism; however, we also postulated that rounds of paligenosis and redifferentiation may allow long-lived cells to accumulate and store oncogenic mutations, and could thereby contribute to tumorigenesis. We have termed the model wherein differentiated cells can store mutations and then unmask them upon cell cycle re-entry the ‘cyclical hit’ model of tumorigenesis. In the present Review (Part II), we discuss these concepts, and cell plasticity as a whole, in the skin and intestine. Although differentiation and repair are arguably more thoroughly studied in skin and intestine than in stomach and pancreas, it is less clear how mature skin and intestinal cells contribute to tumorigenesis. Moreover, we conclude our Review by discussing plasticity in all four organs, and look for conserved mechanisms and concepts that might help advance our knowledge of tumor formation and advance the development of therapies for treating or preventing cancers that might be shared across multiple organs.

Summary: This final installment of a two-part Review discusses how cycles of dedifferentiation and redifferentiation can promote tumorigenesis in the skin and intestine, showing how plasticity in these continuously renewing tissues might contribute to tumorigenesis.

MITF and UV responses in skin: From pigmentation to addiction

Ultraviolet radiation (UVR) has numerous effects on skin, including DNA damage, tanning, vitamin D synthesis, carcinogenesis, and immunomodulation. Keratinocytes containing damaged DNA secrete both α-melanocyte-stimulating hormone (α-MSH), which stimulates pigment production by melanocytes, and the opioid β-endorphin, which can trigger addiction-like responses to UVR. The pigmentation (tanning) response is an adaptation that provides some delayed protection against further DNA damage and carcinogenesis, while the opioid response may be an evolutionary adaptation for promoting sun-seeking behavior to prevent vitamin D deficiency. Here, we review the pigmentation response to UVR, driven by melanocytic microphthalmia-associated transcription factor (MITF), and evidence for UVR-induced melanomagenesis and addiction. We also discuss potential applications of a novel approach to generate protective pigmentation in the absence of UVR (sunless tanning) using a topical small-molecule inhibitor of the salt-inducible kinase (SIK) family.

Spatiotemporal Labeling of Melanocytes in Mice

Melanocytes are pigment producing cells in the skin that give rise to cutaneous malignant melanoma, which is a highly aggressive and the deadliest form of skin cancer. Studying melanocytes in vivo is often difficult due to their small proportion in the skin and the lack of specific cell surface markers. Several genetically-engineered mouse models (GEMMs) have been created to specifically label the melanocyte compartment. These models give both spatial and temporal control over the expression of a cellular ‘beacon’ that has an added benefit of inducible expression that can be activated on demand. Two powerful models that are discussed in this review include the melanocyte-specific, tetracycline-inducible green fluorescent protein expression system (iDct-GFP), and the fluorescent ubiquitination-based cell cycle indicator (FUCCI) model that allows for the monitoring of the cell-cycle. These two systems are powerful tools in studying melanocyte and melanoma biology. We discuss their current uses and how they could be employed to help answer unresolved questions in the fields of melanocyte and melanoma biology.

Regulation of melanocyte stem cell behavior by the niche microenvironment

Somatic stem cells are regulated by their niches to maintain tissue homeostasis and repair throughout the lifetime of an organism. An excellent example to study stem cell/niche interactions is provided by the regeneration of melanocytes during the hair cycle and in response to various types of injury. These processes are regulated by neighboring stem cells and multiple signaling pathways, including WNT/β-catenin, KITL/KIT, EDNs/EDNRB, TGF-β/TGF-βR, α-MSH/MC1R, and Notch signaling. In this review, we highlight recent studies that have advanced our understanding of the molecular crosstalk between melanocyte stem cells and their neighboring cells, which collectively form the niche microenvironment, and we focus on the question of how McSCs/niche interactions shape the responses to genotoxic damages and mechanical injury.

A path from melanocyte stem cells to cutaneous melanoma illuminated by UVB

The relationship between melanocyte stem cells (MCSCs) and melanoma has been unclear. We recently demonstrated that melanoma-prone MCSCs are able to initiate cutaneous melanoma following stem cell activation through ultraviolet-B (UVB) exposure or natural stem cell cycling. Conversely, MCSC quiescence is sufficient to suppress tumorigenesis. This provides new insight into the role of environmental factors in tumor initiation from adult stem cells.

Melanoma: Genetic Abnormalities, Tumor Progression, Clonal Evolution and Tumor Initiating Cells

Melanoma is an aggressive neoplasia issued from the malignant transformation of melanocytes, the pigment-generating cells of the skin. It is responsible for about 75% of deaths due to skin cancers. Melanoma is a phenotypically and molecularly heterogeneous disease: cutaneous, uveal, acral, and mucosal melanomas have different clinical courses, are associated with different mutational profiles, and possess distinct risk factors. The discovery of the molecular abnormalities underlying melanomas has led to the promising improvement of therapy, and further progress is expected in the near future. The study of melanoma precursor lesions has led to the suggestion that the pathway of tumor evolution implies the progression from benign naevi, to dysplastic naevi, to melanoma in situ and then to invasive and metastatic melanoma. The gene alterations characterizing melanomas tend to accumulate in these precursor lesions in a sequential order. Studies carried out in recent years have, in part, elucidated the great tumorigenic potential of melanoma tumor cells. These findings have led to speculation that the cancer stem cell model cannot be applied to melanoma because, in this malignancy, tumor cells possess an intrinsic plasticity, conferring the capacity to initiate and maintain the neoplastic process to phenotypically different tumor cells.