Raf Kinase Inhibitor Protein (RKIP) has been extensively reported as an inhibitor of key signaling pathways involved in the aggressive
tumor phenotype and shows decreased expression in several types of
cancers. However, little is known about RKIP in
melanoma or regarding its function in normal cells. We examined the role of RKIP in both primary melanocytes and
malignant melanoma cells and evaluated its diagnostic and prognostic value. IHC analysis revealed a significantly higher expression of RKIP in
nevi compared with early-stage (stage I-II, AJCC 8th)
melanoma biopsies. Proliferation, wound healing, and
collagen-coated transwell assays uncovered the implication of RKIP on the motility but not on the proliferative capacity of
melanoma cells as RKIP
protein levels were inversely correlated with the migration capacity of both primary and metastatic
melanoma cells but did not alter other parameters. As shown by
RNA sequencing, endogenous RKIP knockdown in primary melanocytes triggered the deregulation of cellular differentiation-related processes, including genes (i.e., ZEB1, THY-1) closely related to the EMT. Interestingly, NANOG was identified as a putative transcriptional regulator of many of the deregulated genes, and RKIP was able to decrease the activation of the NANOG promoter. As a whole, our data support the utility of RKIP as a diagnostic marker for early-stage
melanomas. In addition, these findings indicate its participation in the maintenance of a differentiated state of melanocytic cells by modulating genes intimately linked to the cellular motility and explain the progressive decrease of RKIP often described in
tumors.