Introduction: Melanoma therapy was revolutionized in recent years by the successful use of kinase inhibitors that target oncogenic BRAF. In most patients however success is short lived and disease progression occurs within 6-7 months. This is because melanoma develops resistance to BRAF inhibitors (BRAFi) by acquiring de novo mutations, epigenetic alterations or through activation of cell signalling in the absence of genetic mutations. Recent findings that exogenous growth factors can rescue cells from drug inhibition suggested that exosomes in the tumour extracellular environment could contribute to the development of drug resistance.
Methods: From fresh patient tumours we have established melanoma cell lines. To model acquired BRAFi resistant melanoma, cell lines were cultured under continuous BRAFi for ~3mths to simulate the persistent selective pressure that occurs during treatment in the clinic. Exosomes derived from drug sensitive and resistant cell lines were characterised and functional studies on recipient cells performed.
Results: Our study demonstrates that exosomes derived from drug resistant cells transfer drivers of resistance, such as PDGFRβ, to recipient cells. Expression of PDGFRβ permits bypass of MAPK pathway inhibition by activation of the PI3K signalling pathway and subsequent changes in biological functionality including drug resistance to previously drug-sensitive recipient cells. Preliminary miRNA analysis demonstrates that the composition of exosomes derived drug resistance melanoma cells is distinct suggesting emerging drug resistance could be identified in patient sera before treatment-failure becomes clinically apparent.
Conclusions: Melanomas are heterogeneous tumours composed of subpopulations of melanoma cells with distinct phenotypes. Our study suggests that these tumour subpopulations exchange information via exosomes to influence signalling in neighbouring melanoma cells through paracrine mechanisms and systemically to affect drug sensitivity and the development of resistance.