Phenotypic Switch and Growth of Melanoma Spheroids in the Presence of Mast Cells: Potential Impact of Nutrient-starvation Effects

Background/Aim: Mast cells are abundant in melanoma tumors, and studies suggest that they can be either detrimental or protective for melanoma growth. However, the underlying mechanisms are not fully understood. Materials and Methods: Here, we adopted an established hanging-drop spheroid system to investigate how mast cells influence melanoma growth and phenotype in a 3-D context. To address the underlying mechanism, we conducted transcriptomic and pathway analyses. Results: In the presence of mast cells or mast cell-conditioned medium, growth of melanoma spheroids was profoundly reduced. Transcriptomic analysis revealed that mast cell-conditioned medium had extensive effects on the gene-expression patterns of melanoma. Pathway analyses revealed profound effects on the expression of genes related to amino acid and protein metabolism. The conditioned medium also induced up-regulation of cancer-related genes, including adhesion molecules implicated in metastatic spreading. In line with this, after transfer to a Matrigel extracellular matrix milieu, spheroids that had been developed in the presence of mast cell-conditioned medium displayed enhanced growth and adhesive properties. However, when assessing the possible impact of nutrient starvation, i.e., reduced nutrient content in mast cell-conditioned medium, we found that the observed effects on growth of melanoma spheroids could potentially be explained by such a scenario. Conclusion: Our findings suggest that the phenotypic alterations of melanoma spheroids grown in the presence of mast cells or mast cell-conditioned media are, at least partly, due to nutrient starvation rather than to the action of factors secreted by mast cells. Our findings may provide insight into the effects on gene-expression events that occur in melanoma tumors under nutrient stress.