RKIP Regulates Differentiation-Related Features in Melanocytic Cells

Raf Kinase Inhibitor Protein (RKIP) has been extensively reported as an inhibitor of keysignaling pathways involved in the aggressive tumor phenotype and shows decreased expressionin several types of cancers. However, little is known about RKIP in melanoma or regarding its functionin normal cells. We examined the role of RKIP in both primary melanocytes and malignant melanomacells and evaluated its diagnostic and prognostic value. IHC analysis revealed a significantly higherexpression 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 ofRKIP on the motility but not on the proliferative capacity of melanoma cells as RKIP protein levelswere inversely correlated with the migration capacity of both primary and metastatic melanoma cellsbut did not alter other parameters. As shown by RNA sequencing, endogenous RKIP knockdownin 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 identifiedas a putative transcriptional regulator of many of the deregulated genes, and RKIP was able todecrease the activation of the NANOG promoter. As a whole, our data support the utility of RKIPas a diagnostic marker for early-stage melanomas. In addition, these findings indicate its participationin the maintenance of a differentiated state of melanocytic cells by modulating genes intimately linkedto the cellular motility and explain the progressive decrease of RKIP often described in tumors.