MEK inhibition reverses aberrant signaling in melanoma cells through reorganization of NRas and BRAF in self nano-clusters.

Hotspot mutations of the oncogenes BRAF and NRas are the most common genetic alterations in cutaneous melanoma. Still, the nanoscale organization and signal coupling of these proteins remain incompletely understood, particularly upon expression of oncogenic NRas mutants. Here we employed single-molecule localization microscopy to study the nanoscale organization of NRas and BRAF at the plasma-membrane (PM) of melanoma cells. NRas and BRAF resided in self-clusters that did not associate well in resting cells. In EGF-activated cells, NRas clusters became more diffused while overall protein levels at the PM increased; thus allowing enhanced association of NRas and BRAF and downstream signaling. In multiple melanoma cell lines, mutant NRas resided in more pronounced self-clusters relative to WT NRas yet associated more with the clustered and more abundant BRAF. In cells resistant to trametinib, a clinical MEK inhibitor (MEKi), a similar co-clustering of NRas and BRAF was observed upon EGF activation. Strikingly, treatment of cells expressing mutant NRas with trametinib reversed the effect of mutant NRas expression by restoring the non-overlapping self-clusters of NRas and BRAF and by reducing their PM levels and elevated pERK levels caused by mutant NRas. Our results indicate a new mechanism for signal regulation of NRas in melanoma through its nanoscale dynamic organization and a new mechanism for MEKi function in melanoma cells carrying NRas mutations but lacking MEK mutations.