Connecting the dots: Melanoma cell of origin, tumor cell plasticity, trans-differentiation, and drug resistance.

Melanoma, a lethal malignancy that arises from melanocytes, exhibits a multiplicity of clinico-pathologically distinct subtypes in sun-exposed and non-sun-exposed areas. Melanocytes are derived from multipotent neural crest cells and are present in diverse anatomical locations, including skin, eyes, and various mucosal membranes. Tissue-resident melanocyte stem cells and melanocyte precursors contribute to melanocyte renewal. Elegant studies using mouse genetic models have shown that melanoma can arise from either melanocyte stem cells or differentiated pigment-producing melanocytes depending on a combination of tissue and anatomical site of origin and activation of oncogenic mutations (or overexpression) and/or the repression in expression or inactivating mutations in tumor suppressors. This variation raises the possibility that different subtypes of human melanomas (even subsets within each subtype) may also be a manifestation of malignancies of distinct cells of origin. Melanoma is known to exhibit phenotypic plasticity and trans-differentiation (defined as a tendency to differentiate into cell lineages other than the original lineage from which the tumor arose) along vascular and neural lineages. Additionally, stem cell-like properties such as pseudo-epithelial-to-mesenchymal (EMT-like) transition and expression of stem cell-related genes have also been associated with the development of melanoma drug resistance. Recent studies that employed reprogramming melanoma cells to induced pluripotent stem cells have uncovered potential relationships between melanoma plasticity, trans-differentiation, and drug resistance and implications for cell or origin of human cutaneous melanoma. This review provides a comprehensive summary of the current state of knowledge on melanoma cell of origin and the relationship between tumor cell plasticity and drug resistance.

Role of the Skin Microenvironment in Melanomagenesis: Epidermal Keratinocytes and Dermal Fibroblasts Promote BRAF Oncogene-Induced Senescence Escape in Melanocytes.

BRAFV600E is the most common mutation driver in melanoma. This mutation is known to cause a brief burst of proliferation followed by growth arrest and senescence, which prevent an uncontrolled cell proliferation. This phenomenon is known as oncogene-induced senescence (OIS) and OIS escape is thought to lead to melanomagenesis. Much attention has been focused on the melanocyte-intrinsic mechanisms that contribute to senescence escape. Additional genetic events such as the loss of tumor suppressor PTEN and/or epigenetic changes that contribute to senescence escape have been described. However, the role of the skin microenvironment-specifically, the role of epidermal keratinocytes-on melanomagenesis is not fully understood. In this study, we employ a microfluidic platform to study the interaction between melanocytes expressing the BRAFV600E mutation as well as keratinocytes and dermal fibroblasts. We demonstrate that keratinocytes suppress senescence-related genes and promote the proliferation of transformed melanocytes. We also show that a keratinocyte-conditioned medium can alter the secretion of both pro- and anti-tumorigenic factors by transformed melanocytes. In addition, we show that melanocytes and keratinocytes from donors of white European and black African ancestry display different crosstalks; i.e., white keratinocytes appear to promote a more pro-tumorigenic phenotype compared with black keratinocytes. These data suggest that keratinocytes exert their influence on melanomagenesis both by suppressing senescence-related genes in melanocytes and by affecting the balance of the melanocyte-secreted factors that favor tumorigenesis.

Microfluidic model with air-walls reveals fibroblasts and keratinocytes modulate melanoma cell phenotype, migration, and metabolism.

Melanoma evolution is a complex process. The role epidermal keratinocytes and dermal fibroblasts play in this process and the mechanisms involved in tumor-stroma interactions remain poorly understood. Here, we used a microfluidic platform to evaluate the cross-talk between human primary melanoma cells, keratinocytes and dermal fibroblasts. The microfluidic device included multiple circular chambers separated by a series of narrow connection channels. The microdevice design allowed us to develop a new cell patterning method based on air-walls, removing the need for hydrogel barriers, porous membranes, or external equipment. Using this method, we co-cultured melanoma cells in the presence of keratinocytes and/or dermal fibroblasts. The results demonstrated that the presence of dermal fibroblasts and keratinocytes led to changes in melanoma cell morphology and growth pattern. Molecular analysis revealed changes in the chemokine secretion pattern, identifying multiple secreted factors involved in tumor progression. Finally, optical metabolic imaging showed that melanoma cells, fibroblasts, and keratinocytes exhibited different metabolic features. Additionally, the presence of stromal cells led to a metabolic shift in melanoma cells, highlighting the role the skin microenvironment on melanoma evolution.