SOX2 modulates levels of MITF in normal human melanocytes, and melanoma lines in vitro

3D Spheroid Configurations Are Possible Indictors for Evaluating the Pathophysiology of Melanoma Cell Lines

To study the molecular mechanisms responsible for inducing the spatial proliferation of malignant melanomas (MM), three-dimension (3D) spheroids were produced from several MM cell lines including SK-mel-24, MM418, A375, WM266-4, and SM2-1, and their 3D architectures and cellular metabolisms were evaluated by phase-contrast microscopy and Seahorse bio-analyzer, respectively. Several transformed horizontal configurations were observed within most of these 3D spheroids, and the degree of their deformity was increased in the order: WM266-4, SM2-1, A375, MM418, and SK-mel-24. An increased maximal respiration and a decreased glycolytic capacity were observed within the lesser deformed two MM cell lines, WM266-4 and SM2-1, as compared with the most deformed ones. Among these MM cell lines, two distinct cell lines, WM266-4 and SK-mel-24, whose 3D appearances were the closest and farthest, respectively, from being horizontally circular-shaped, were subjected to RNA sequence analyses. Bioinformatic analyses of the differentially expressed genes (DEGs) identified KRAS and SOX2 as potential master regulatory genes for inducing these diverse 3D configurations between WM266-4 and SK-mel-24. The knockdown of both factors altered the morphological and functional characteristics of the SK-mel-24 cells, and in fact, their horizontal deformity was significantly reduced. A qPCR analysis indicated that the levels of several oncogenic signaling related factors, including KRAS and SOX2, PCG1α, extracellular matrixes (ECMs), and ZO1 had fluctuated among the five MM cell lines. In addition, and quite interestingly, the dabrafenib and trametinib resistant A375 (A375DT) cells formed globe shaped 3D spheroids and showed different profiles in cellular metabolism while the mRNA expression of these molecules that were tested as above were different compared with A375 cells. These current findings suggest that 3D spheroid configuration has the potential for serving as an indicator of the pathophysiological activities associated with MM.

Culturing of Melanocytes from the Equine Hair Follicle Outer Root Sheath

Hair follicles harbor a heterogeneous regenerative cell pool and represent a putative low-to-non-invasively available source of stem cells. We previously reported a technology for culturing human melanocytes from the hair follicle outer root sheath (ORS) for autologous pigmentation of tissue engineered skin equivalents. This study translated the ORS technology to horses. We de-veloped a culture of equine melanocytes from the ORS (eMORS) from equine forelock hair follicles cultured by means of an analogue human hair follicle-based in vitro methodology. The procedure was adjusted to equine physiology by addition of equine serum to the culture medium. The hair follicles were isolated by macerating forelock skin rests, enzymatically digested and subjected to air-medium-interface cultivation method. The procedure resulted in differentiated equine melanocytes, which exhibited typical morphology, presence of melanosomes, expression of cytoskeleton proteins vimentin, α-SMA, Sox2, S100ß and tyrosinase as well as tyrosinase activity followed by production of melanin. According to all assessed parameters, eMORS could be ranked as partially melanotic melanocytes. The results of the study offer an experimental base for further insight into hair follicle biology in equine and for comparative studies of hair follicles across different species.

Stem Cell-Derived Models of Neural Crest Are Essential to Understand Melanoma Progression and Therapy Resistance

During development, neural crest (NC) cells are early precursors of several lineages including melanocytes. Along their differentiation from multipotent cells to mature melanocytes, NC cells will go through successive steps which require either proliferative or motile capacities. For example, they will undergo Epithelial to Mesenchymal Transition (EMT) in order the separate from the neural tube and migrate to their final location in the epidermis (Larribere and Utikal, 2013; Skrypek et al., 2017). The differentiated melanocytes are the cells of origin of melanoma tumors which progress through several stages such as radial growth phase, vertical growth phase, metastasis formation, and often resistance to current therapies. Interestingly, depending on the stage of the disease, melanoma tumor cells share phenotypes with NC cells (proliferative, motile, EMT). These phenotypes are tightly controlled by specific signaling pathways and transcription factors (TFs) which tend to be reactivated during the onset of melanoma. In this review, we summarize first the main TFs which control these common phenotypes. Then, we focus on the existing strategies used to generate human NCs. Finally we discuss how identification and regulation of NC-associated genes provide an additional approach to improving current melanoma targeted therapies.

TFAP2 paralogs regulate melanocyte differentiation in parallel with MITF

Mutations in the gene encoding transcription factor TFAP2A result in pigmentation anomalies in model organisms and premature hair graying in humans. However, the pleiotropic functions of TFAP2A and its redundantly-acting paralogs have made the precise contribution of TFAP2-type activity to melanocyte differentiation unclear. Defining this contribution may help to explain why TFAP2A expression is reduced in advanced-stage melanoma compared to benign nevi. To identify genes with TFAP2A-dependent expression in melanocytes, we profile zebrafish tissue and mouse melanocytes deficient in Tfap2a, and find that expression of a small subset of genes underlying pigmentation phenotypes is TFAP2A-dependent, including Dct, Mc1r, Mlph, and Pmel. We then conduct TFAP2A ChIP-seq in mouse and human melanocytes and find that a much larger subset of pigmentation genes is associated with active regulatory elements bound by TFAP2A. These elements are also frequently bound by MITF, which is considered the “master regulator” of melanocyte development. For example, the promoter of TRPM1 is bound by both TFAP2A and MITF, and we show that the activity of a minimal TRPM1 promoter is lost upon deletion of the TFAP2A binding sites. However, the expression of Trpm1 is not TFAP2A-dependent, implying that additional TFAP2 paralogs function redundantly to drive melanocyte differentiation, which is consistent with previous results from zebrafish. Paralogs Tfap2a and Tfap2b are both expressed in mouse melanocytes, and we show that mouse embryos with Wnt1-Cre-mediated deletion of Tfap2a and Tfap2b in the neural crest almost completely lack melanocytes but retain neural crest-derived sensory ganglia. These results suggest that TFAP2 paralogs, like MITF, are also necessary for induction of the melanocyte lineage. Finally, we observe a genetic interaction between tfap2a and mitfa in zebrafish, but find that artificially elevating expression of tfap2a does not increase levels of melanin in mitfa hypomorphic or loss-of-function mutants. Collectively, these results show that TFAP2 paralogs, operating alongside lineage-specific transcription factors such as MITF, directly regulate effectors of terminal differentiation in melanocytes. In addition, they suggest that TFAP2A activity, like MITF activity, has the potential to modulate the phenotype of melanoma cells.

Identifying the elements and structure of the gene regulatory network governing melanocyte differentiation may yield insight into the mechanisms of pigmentation diseases and melanoma progression. Pigmentation is abnormal in Tfap2a mutants, but deciphering the exact role of TFAP2A in the network has been complicated by pleiotropic requirements for TFAP2A during development and the redundant function of TFAP2 paralogs in melanocytes. In this study, we find that TFAP2A directly regulates genes involved in melanocyte differentiation and melanin synthesis by binding at both promoters and enhancers associated with these genes. Furthermore, we report evidence that TFAP2A shares many targets with the melanocyte “master regulator” MITF. These findings indicate that TFAP2A drives melanocyte differentiation in parallel with MITF and affects the net pro-differentiation activity that is lost in melanoma.

Microphthalmia-associated transcription factor expression levels in melanoma cells contribute to cell invasion and proliferation

Microphthalmia-associated transcription factor (MITF) is a nodal point in melanoma transcriptional network that regulates dozens of genes with critical functions in cell differentiation, proliferation and survival. Highly variable MITF expression levels exist in tumor cell subpopulations conferring marked heterogeneity and plasticity in the tumor tissue. A model has been postulated whereby lower MITF levels favour cell invasion and suppress proliferation, whereas high levels stimulate differentiation and proliferation. Additionally, MITF is considered to be a prosurvival gene and a lineage addiction oncogene in melanoma. Herein, we review how MITF expression may affect the melanoma phenotype with consequences on the survival, invasion and metastasis of melanoma cells, and we discuss the research challenges.

Identification of Genes Expressed in Hyperpigmented Skin Using Meta-Analysis of Microarray Data Sets

More than 375 genes have been identified that are involved in regulating skin pigmentation, and those act during development, survival, differentiation and/or responses of melanocytes to the environment. Many of those genes have been cloned and disruptions of their functions are associated with various pigmentary diseases, however many remain to be identified. We have performed a series of microarray analyses of hyperpigmented compared to less pigmented skin to identify genes responsible for those differences. The rationale and goal for this study was to perform a meta-analysis on those microarray databases to identify genes that may be significantly involved in regulating skin phenotype either directly or indirectly that might not have been identified due to subtle differences by any of those individual studies alone. The meta-analysis demonstrates that 1,271 probes representing 921 genes are differentially expressed at significant levels in the 5 microarray datasets compared, which provides new insights into the variety of genes involved in determining skin phenotype. Immunohistochemistry was used to validate 2 of those markers at the protein level (TRIM63 and QPCT) and we discuss the possible functions of those genes in regulating skin physiology.

Sox2 acts as a rheostat of epithelial to mesenchymal transition during neural crest development

Precise control of self-renewal and differentiation of progenitor cells into the cranial neural crest (CNC) pool ensures proper head development, guided by signaling pathways such as BMPs, FGFs, Shh and Notch. Here, we show that murine Sox2 plays an essential role in controlling progenitor cell behavior during craniofacial development. A “Conditional by Inversion” Sox2 allele (Sox2^COIN) has been employed to generate an epiblast ablation of Sox2 function (Sox2^EpINV). Sox2^EpINV/+(H) haploinsufficient and conditional (Sox2^EpINV/mosaic) mutant embryos proceed beyond gastrulation and die around E11. These mutant embryos exhibit severe anterior malformations, with hydrocephaly and frontonasal truncations, which could be attributed to the deregulation of CNC progenitor cells during their epithelial to mesenchymal transition. This irregularity results in an exacerbated and aberrant migration of Sox10⁺ NCC in the branchial arches and frontonasal process of the Sox2 mutant embryos. These results suggest a novel role for Sox2 as a regulator of the epithelial to mesenchymal transitions (EMT) that are important for the cell flow in the developing head.