Epigenetic and pharmacological control of pigmentation via Bromodomain Protein 9 (BRD9)

Lineage-specific differentiation programs are activated by epigenetic changes in chromatin structure. Melanin-producing melanocytes maintain a gene expression program ensuring appropriate enzymatic conversion of metabolites into the pigment, melanin, and transfer to surrounding cells. During neuroectodermal development, SMARCA4 (BRG1), the catalytic subunit of SWItch/Sucrose Non-Fermentable (SWI/SNF) chromatin remodeling complexes, is essential for lineage specification. SMARCA4 is also required for development of multipotent neural crest precursors into melanoblasts, which differentiate into pigment-producing melanocytes. In addition to the catalytic domain, SMARCA4 and several SWI/SNF subunits contain bromodomains which are amenable to pharmacological inhibition. We investigated the effects of pharmacological inhibitors of SWI/SNF bromodomains on melanocyte differentiation. Strikingly, treatment of murine melanoblasts and human neonatal epidermal melanocytes with selected bromodomain inhibitors abrogated melanin synthesis and visible pigmentation. Using functional genomics, iBRD9, a small molecule selective for the bromodomain of BRD9 was found to repress pigmentation-specific gene expression. Depletion of BRD9 confirmed a requirement for expression of pigmentation genes in the differentiation program from melanoblasts into pigmented melanocytes and in melanoma cells. Chromatin immunoprecipitation assays showed that iBRD9 disrupts the occupancy of BRD9 and the catalytic subunit SMARCA4 at melanocyte-specific loci. These data indicate that BRD9 promotes melanocyte pigmentation whereas pharmacological inhibition of BRD9 is repressive.     

MicroRNA‐206 induces G1 arrest in melanoma by inhibition of CDK4 and Cyclin D

Expression profiling of microRNAs in melanoma lesional skin biopsies compared with normal donor skin biopsies, as well as melanoma cell lines compared with normal melanocytes, revealed that hsa‐miR‐206 was down‐regulated in melanoma (?75.4‐fold, P = 1.7 × 10^?4). MiR‐206 has been implicated in a large number of cancers, including breast, lung, colorectal, ovarian, and prostate cancers; however, its role in tumor development remains largely unknown, its biologic function is poorly characterized, and its targets affecting cancer cells are largely unknown. MiR‐206 reduced growth and migration/invasion of multiple melanoma cell lines. Bioinformatics identified cell cycle genes CDK2, CDK4, Cyclin C, and Cyclin D1 as strong candidate targets. Western blots and 3′UTR reporter gene assays revealed that miR‐206 inhibited translation of CDK4, Cyclin D1, and Cyclin C. Additionally, hsa‐miR‐206 transfection induced G1 arrest in multiple melanoma cell lines. These observations support hsa‐miR‐206 as a tumor suppressor in melanoma and identify Cyclin C, Cyclin D1, and CDK4 as miR‐206 targets.     

MicroRNA‐378 regulates epithelial–mesenchymal transition and metastasis of melanoma by inhibiting FOXN3 expression through the Wnt/β‐catenin pathway

MicroRNAs (miRNAs) participate in the development and progression of melanoma. However, while dysregulation of microRNA‐378 (miR‐378) has been seen in various cancer types, its clinical importance and function in melanoma are poorly elucidated. In this work, miR‐378 expression in melanoma and in adjacent non‐cancerous tissue was evaluated with a quantitative real‐time polymerase chain reaction. A series of assays (wound healing, Transwell, and nude mouse subcutaneous tumor model) were used to investigate the implications of abnormal miR‐378 regulation on melanoma cell migration and invasion in vitro, and on tumorigenicity in vivo. Prediction and conformation of the miR‐378 target gene was undertaken using bioinformatic analysis and luciferase reporter system. Expression of miR‐378 was often increased in melanoma, and shown to potentiate its migration, invasion, and tumorigenicity. miR‐378 acted, at least partially, through inhibition of the potential target FOXN3 and via Wnt/β‐catenin pathway activation. The findings indicate that miR‐378 triggers melanoma development and progression. This miRNA could be a novel diagnostic and prognostic biological marker and provide utility for targeted treatment of melanoma.     

FK506 positively regulates the migratory potential of melanocyte‐derived cells by enhancing syndecan‐2 expression

Although topical tacrolimus (FK506) is known to promote repigmentation by increasing the pigmentation and migration of melanocytes, the mechanism through which FK506 regulates cell migration remains unclear. Here, we report that FK506 treatment enhanced cell spreading on laminin‐332 and increased migration in both melanocytes and melanoma cells. Interestingly, FK506 also increased the expression of syndecan‐2, a transmembrane heparan sulfate proteoglycan through c‐jun terminal kinase activation. Moreover, siRNA‐mediated reduction of syndecan‐2 expression decreased FK506‐mediated cell spreading and migration in melanoma cells and decreased focal adhesion kinase phosphorylation in both melanocytes and melanoma cells. Consistent with these effects on syndecan‐2 expression, FK506 enhanced the membrane and melanosome localizations of PKCβII, a regulator of tyrosinase activity. This suggests that FK506 may play a dual regulatory role by affecting both melanogenesis and migration in melanocyte‐derived cells. Interestingly, however, FK506 failed to show any synergistic effect on the migration of UVB‐treated melanocyte‐derived cells. Taken together, these data indicate that FK506 regulates cell migration by enhancing syndecan‐2 expression, further suggesting that syndecan‐2 could be a potential target for the treatment of patients with vitiligo.     

Urokinase receptor and CXCR4 are regulated by common microRNAs in leukaemia cells

The urokinase‐type plasminogen activator (uPA) receptor (uPAR) focuses uPA proteolytic activity on the cell membrane, promoting localized degradation of extracellular matrix (ECM), and binds vitronectin (VN), mediating cell adhesion to the ECM. uPAR‐bound uPA and VN induce proteolysis‐independent intracellular signalling, regulating cell adhesion, migration, survival and proliferation. uPAR cross‐talks with CXCR4, the receptor for the stroma‐derived factor 1 chemokine. CXCR4 is crucial in the trafficking of hematopoietic stem cells from/to the bone marrow, which involves also uPAR. Both uPAR and CXCR4 are expressed in acute myeloid leukaemia (AML), with a lower expression in undifferentiated and myeloid subsets, and higher expression in myelomonocytic and promyelocytic subsets. We hypothesized a microRNA (miR)‐mediated co‐regulation of uPAR and CXCR4 expression, which could allow their cross‐talk at the cell surface. We identified three miRs, miR‐146a, miR‐335 and miR‐622, regulating the expression of both uPAR and CXCR4 in AML cell lines. Indeed, these miRs directly target the 3′untranslated region of both uPAR‐ and CXCR4‐mRNAs; accordingly, uPAR/CXCR4 expression is reduced by their overexpression in AML cells and increased by their specific inhibitors. Overexpression of all three miRs impairs migration, invasion and proliferation of myelomonocytic cells. Interestingly, we observed an inverse relationship between uPAR/CXCR4 expression and miR‐146a and miR‐335 levels in AML blasts, suggesting their possible role in the regulation of uPAR/CXCR4 expression also in vivo.     

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.     

MicroRNA‐137 is negatively associated with clinical outcome and regulates tumor development through EZH2 in cervical cancer

We intend to evaluate the expression, clinical relevance, and functional role of microRNA‐137 (miR‐137) in human cervical cancer (CC). MiR‐137 expressions were assessed by qPCR in CC cell lines and human CC tumors. The correlation between endogenous miR‐137 expression and CC patients’ postoperative overall survival was examined statistically. CC cell lines, Ca‐Ski, and SiHa cells were transduced with lentivirus to ectopically upregulate endogenous miR‐137 expressions. Possible inhibitory effects of miR‐137 upregulation on CC in vitro proliferation and migration, as well as in vivo transplantation were evaluated. Targeting of enhancer of zeste homolog 2 (EZH2) gene by miR‐137 in CC was assessed by dual‐luciferase activity assay and qPCR. In CC cells with upregulated miR‐137, EZH2 was overexpressed to assess its direct function in miR‐137 mediated CC proliferation and migration. MiR‐137 was downregulated in both CC cells and human CC tumors. Downregulation of endogenous miR‐137 was significantly correlated with CC patients’ short overall survival. In CC cells, miR‐137 upregulation is tumor‐suppressive by inhibiting proliferation and migration in vitro, and transplantation in vivo. EZH2 was a direct downstream target gene of miR‐137 in CC. Forced overexpression of EZH2 in miR‐137‐upregulated CC cells reversed the tumor‐suppression induced by miR‐137. MiR‐137 is lowly expressed in CC and possibly acting as a negative biomarker for CC patients’ clinical outcome. MiR‐137 upregulation may suppress CC, very likely by inversely regulating EZH2.     

Migration of melanoblasts into the developing murine hair follicle is accompanied by transient c-Kit expression.

Disruption of the c-Kit/stem cell factor (SCF) signaling pathway interferes with the survival, migration, and differentiation of melanocytes during generation of the hair follicle pigmentary unit. We examined c-Kit, SCF, and S100 (a marker for precursor melanocytic cells) expression, as well as melanoblast/melanocyte ultrastructure, in perinatal C57BL/6 mouse skin. Before the onset of hair bulb melanogenesis (i.e., stages 0-4 of hair follicle morphogenesis), strong c-Kit immunoreactivity (IR) was seen in selected non-melanogenic cells in the developing hair placode and hair plug. Many of these cells were S100-IR and were ultrastructurally identified as melanoblasts with migratory appearance. During the subsequent stages (5 and 6), increasingly dendritic c-Kit-IR cells successively invaded the hair bulb, while S100-IR gradually disappeared from these cells. Towards the completion of hair follicle morphogenesis (stages 7 and 8), several distinct follicular melanocytic cell populations could be defined and consisted broadly of (a) undifferentiated, non-pigmented c-Kit-negative melanoblasts in the outer root sheath and bulge and (b) highly differentiated melanocytes adjacent to the hair follicle dermal papilla above Auber's line. Widespread epithelial SCF-IR was seen throughout hair follicle morphogenesis. These findings suggest that melanoblasts express c-Kit as a prerequisite for migration into the SCF-supplying hair follicle epithelium. In addition, differentiated c-Kit-IR melanocytes target the bulb, while non-c-Kit-IR melanoblasts invade the outer root sheath and bulge in fully developed hair follicles.     

Mitochondrial respiration is restricted by miR‐2909 within human melanocytes

Most of the cancer types in general and melanoma in particular exhibit mitochondrial dysfunction leading to the Warburg effect. Our present study stemmed from the observation that human A‐375 and melanoma B16 cells displayed overexpression of a novel micro‐RNA, miR‐2909, shown in our earlier studies to be involved in aerobic glycolysis. Consequently, our study attempts to demonstrate the role of miR‐2909 in the regulation of mitochondrial function within human melanocytes. Based upon such a study, we hypothesize that mitochondrial dysfunction observed in melanomas may result from deregulated miR‐2909 expression within such cells.     

miR‐200c/Bmi1 axis and epithelial–mesenchymal transition contribute to acquired resistance to BRAF inhibitor treatment

Resistance to BRAF inhibitors (BRAFi) is one of the major challenges for targeted therapies for BRAF‐mutant melanomas. However, little is known about the role of microRNAs in conferring BRAFi resistance. Herein, we demonstrate that miR‐200c expression is significantly reduced whereas miR‐200c target genes including Bmi1, Zeb2, Tubb3, ABCG5, and MDR1 are significantly increased in melanomas that acquired BRAFi resistance compared to pretreatment tumor biopsies. Similar changes were observed in BRAFi‐resistant melanoma cell lines. Overexpression of miR‐200c or knock‐down of Bmi1 in resistant melanoma cells restores their sensitivities to BRAFi, leading to deactivation of the PI3K/AKT and MAPK signaling cascades, and acquisition of epithelial–mesenchymal transition‐like phenotypes, including upregulation of E‐cadherin, downregulation of N‐cadherin, and ABCG5 and MDR1 expression. Conversely, knock‐down of miR‐200c or overexpression of Bmi1 in BRAFi‐sensitive melanoma cells activates the PI3K/AKT and MAPK pathways, upregulates N‐cadherin, ABCG5, and MDR1 expression, and downregulates E‐cadherin expression, leading to BRAFi resistance. Together, our data identify miR‐200c as a critical signaling node in BRAFi‐resistant melanomas impacting the MAPK and PI3K/AKT pathways, suggesting miR‐200c as a potential therapeutic target for overcoming acquired BRAFi resistance.