523 MITF-mediated extracellular matrix changes control intratumour heterogeneity in melanoma

Differential tumour cell behaviour caused by environmental conditions, termed dynamic heterogeneity, is a prime cause of drug resistance and understanding its underlying mechanisms is crucial to design effective therapies. We have demonstrated cell-cycling heterogeneity in both melanoma xenograft tumours and melanoma spheroids. This was characterized by the presence within the same tumour/spheroid of clusters of proliferating cells and clusters of G1-arrested cells. The location of the quiescent zones supported oxygen/nutrient deprivation as the cause of cell cycle arrest and the G1-arrested cells reversed to cycling when cultured under normoxia in 2D cultures. Here we demonstrate that this heterogeneity is consistently decreased in vivo and in vitro by high expression of MITF, a transcription factor strongly associated with melanoma development, progression and therapy response. This phenomenon is not associated with reduced hypoxia, indicating that high MITF expression allows a portion of hypoxic cells to maintain cell cycling despite oxygen deprivation. Conversely, modulation of MITF expression lead to changes of spheroid architecture and in extracellular matrix (ECM) and cell-ECM adhesion and crosstalk proteins. Furthermore, ECM swapping and inhibition of the Rho/ROCK signalling pathway, respectively, rescues and mimics the morphology and cell cycle effects of high MITF expression. These findings support a novel role of MITF in controlling intratumour melanoma heterogeneity through changes in cell-ECM crosstalk.