Background: Melanoma is an aggressive skin cancer and no curative treatment exists once it has progressed into an advanced stage. We recently identified slow-cycling sub-populations within melanoma cell lines which show a gene-expression signature indicative of EMT-like processes. Mesenchymal-like melanoma cells secrete high amounts of thrombospondin-1 (TSP-1), which possibly act an as immune-evasive signal. We hypothesize that TSP-1 is a crucial player in EMT, immune evasion and invasion in melanoma.
Methods and Results: Microarray analysis of slow-cycling subpopulations within melanoma cell lines identified a gene expression signature characteristic of EMT. One of the most profoundly up-regulated genes was TSP-1. Its over-expression in mesenchymal-like cells could be confirmed by qPCR, ELISA and immunofluorescence. Blocking of TSP-1 by specific antibodies reduced the invasive properties of melanoma cells in vitro. Treatment with TGF-beta induced EMT and expression of TSP-1 in epithelial-like melanoma cells. siRNA-mediated knockdown of TSP-1 led to the up-regulation of E-cadherin and decreased melanoma cell migration within chic neural crest. Furthermore, TSP-1 was shown to reduce immune cell-mediated killing of tumour cells. High amounts of TSP-1 were inversely correlated with the expression of its receptor CD36.
Conclusion: These observations suggest that TSP-1 plays an important role as a regulator of EMT within melanoma cells, and may be responsible for the immune-escape of advanced stage melanomas and their invasive potential.