SNX18 inhibits melanoma migration by modulating the activation ratio of Cdc42 & RhoA

To identify novel proteins that control melanoma development and progression, we integrated high-throughput proteomics, mRNA and miRNAs datasets from isogenic primary (WM115) and metastatic (WM266-4) melanoma cell-lines. We focused on differentially expressed proteins that had non-differential mRNA levels and were putatively targeted by a high number of miRNAs. SNX18, a sub-family member of sorting-nexins that participates in endocytosis and autophagy was down-regulated by 6-fold in WM266-4, and predicted to be targeted by 17 miRNAs. Over-expression of SNX18 in several melanoma cell-lines significantly reduced migration rate, while silencing of SNX18 resulted in the opposite effect. Notably, progression tissue microarrays demonstrated a trend for SNX18 downregulation in early metastasis. Luciferase assays confirmed the direct regulation of SNX18 by multiple miRNAs. Indeed, over-expression of these miRNAs repressed endogenous SNX18 expression at the protein but not mRNA level. Further, protein but not mRNA expression levels correlated with better overall survival as determined in a cohort of 48 metastatic melanoma patients and in the TCGA database. Mechanistically, SNX18-mediated reduced migration coincided with RhoA activation and Cdc42 inactivation. Both play a role, as RhoA inhibitor and constitutively activated Cdc42 mutants abrogated this effect. Moreover, phosphorylation-defective SNX18 mutants (S233A,S233D) migrated faster, with S233A inactivating RhoA and activating Cdc42, while S233D activated both. The downstream phosphorylation of the p38>MAPKAPK2>HSP27 signaling pathway was concordant. Both mutants were more abundant in the plasma membrane. Taken together, phosphorylation of S233 is crucial for SNX18 activation, but de-phosphorylation dynamics, which may occur at the membrane, is required for functional effect. In this study we compared the proteome, transcriptome and miRnome of primary and metastatic melanoma isogenic cell-lines and identified SNX18 as a novel inhibitor of melanoma migration. 2 biological repeats (A & B, performed at different days) of mock vs. SNX18-V1 OX cells in 2 cell lines (WM-266-4 and 624mel).