Abstract 2015: Thrombospondin-1 is a targetable marker of invasive, mesenchymal- like melanoma cells

Resistance to anti-cancer therapies arising from genetic heterogeneity is well understood. Less well characterised is the resistance to therapy proposed to occur via the plastic changes in melanoma cells generated by the impact of the tumor microenvironment. In melanoma, phenotypic-switching as means of plasticity has been well described and resembles, at least partially, an epithelial-to-mesenchymal transition (EMT). The presence of mesenchymal-like, invasive and slow-proliferating cells in melanoma, a highly aggressive cancer with intrinsic resistance to chemotherapy, is controversial and their potential contribution to therapeutic resistance has yet to be determined. Methods: We examined melanoma cell lines for the presence of mesenchymal-like cells using CM-Dil, a membrane dye evenly distributed to daughter cells with each cell division as a marker of reduced cell division. Functional assays included invasion and motility assays, in vivo mouse xenograft models as well as an in vivo chicken neural crest transplantation assay. Genome-wide gene-expression profiling was performed using Illumina HT12 arrays. Results: Here we show that mesenchymal-like cells are present in cell lines and in xenograft tumors, survive exposure to cytotoxic drugs and are invasive. Importantly, resistance was associated with a gene expression signature characterized by the expression of thrombospondin, TGFBI, genes associated with the extracellular matrix, and acquisition of increased invasiveness. This gene-expression pattern is reminiscent of an epithelial-to-mesenchymal transition in other solid cancers and a similar phenotype was detected after acquired resistance to the BRAF inhibitors PLX4720 and dabrafenib. Moreover, targeting thrombospondin in an in vivo model of melanoma cell plasticity and invasion led to the complete abrogation of the plastic and invasive potential of melanoma cells. Conclusion: The results link therapeutic resistance to the presence of mesenchymal-like and invasive cells, and identify potential targets for novel therapies directed at eradicating this key melanoma subpopulation. Citation Format: Andreas Behren, Aparna Jayachandran, Matthew Anaka, Christopher Hudson, Jonathan Cebon. Thrombospondin-1 is a targetable marker of invasive, mesenchymal- like melanoma cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2015. doi:10.1158/1538-7445.AM2014-2015