Sema6a and mical-1 sustain cell growth and survival of BRAFv600e mutated melanoma

Therapeutic targeting of BRAFV600E and MEK has shown significant impact on progression-free and overall survival for advanced melanoma, but only a fraction of patients benefit from these treatments, suggesting that additional signaling pathways involved in melanoma growth need to be identified. We used whole genome microarray analysis to identify differentially expressed genes in a set of neoplastic clones, isolated from a melanoma metastasis, and characterized by mutually exclusive expression of BRAFV600E or NRASQ61R. We identified two genes, SEMA6A and MICAL1 belonging to the semaphorin-plexin signaling pathway and highly expressed, as mRNA and protein, in BRAF-mutant neoplastic clones. Real-time PCR, Western blot and immunohistochemistry confirmed preferential expression of Sema6A and Mical1 in BRAFV600E neoplastic cells from melanoma clones, primary and metastatic cell lines and tissue sections from melanoma lesions. Sema6A depletion, by specific RNA-interference, led to cytoskeletal remodeling, loss of stress fibers, generation of actin-rich protrusion, and cell death, whereas Sema6A overexpression, in NRASQ61R clones, promoted anchorage-independent growth and invasiveness. Mical1 depletion, by siRNA, in BRAFV600E melanoma did not alter the actin cytoskeleton organization but caused strong NDR phosphorylation and NDR-dependent apoptosis. Overall, these results suggest that the Sema6A and Mical1 pathways contribute to promote serviva of BRAFV600E melanomas cells.