Abstract 1457: Raf kinase inhibitor protein (RKIP) negatively regulates melanoma differentiation associated gene-9/syntenin-mediated melanoma metastasis

Abstract

Abstract Melanoma differentiation associated gene-9 (mda-9/syntenin) functions as a positive regulator of melanoma progression and metastasis by physically interacting with c-Src and promoting the formation of an active FAK/c-Src signaling complex leading to NFκB and matrix metalloproteinase (MMP) activation. In contrast, Raf Kinase Inhibitor Protein (RKIP), an inhibitor of RAF-mediated activation of mitogen-activated protein/extracellular signal-regulated kinase kinase (MEKK), is strongly downregulated in metastatic melanoma cells suggesting a possible inverse relationship between expression of mda-9/syntenin and RKIP. We have now genetically modulated expression of mda-9/syntenin and RKIP in melanocytes and melanoma cells and evaluated biological consequences using invasion, anchorage-independent growth, Western blotting, immunoprecipitation and in vivo confocal imaging assays. These studies confirm an inverse correlation between mda-9/syntenin and RKIP expression. This reciprocal relationship was also documented in vivo using a chick embryo chorioallantoic membrane (CAM) model, which monitors spontaneous intravasation of cells from the upper to the lower layer by polymerase chain reaction (PCR) amplification of human Alu genomic DNA. Using tissue microarrays and a series of cell lines we confirmed a higher expression of MDA-9/syntenin and absence of RKIP in both malignant melanoma tumor sections and cell lines. We also established through gain-of-function and loss-of-function experiments that mda-9/syntenin transcriptionally downregulates RKIP, providing evidence for crosstalk between these two proteins. Moreover, transient overexpression of RKIP in metastatic melanoma cells overrides mda-9/syntenin-mediated signaling resulting in decreased cell invasion, anchorage-independent growth and spontaneous tumor cell dissemination in the CAM assay. RKIP through its RAF1 binding site (amino acids 77-108) physically interacts with MDA-9/syntenin and this interaction inversely correlates with downregulation of FAK and c-Src phosphorylation, crucial steps necessary for forming a stable FAK/c-Src complex and initiating a signaling cascade leading to an MDA-9/syntenin-mediated metastatic phenotype in melanoma cells. These provocative findings highlight a potential therapeutic role of RKIP in melanoma metastasis through its interaction with MDA-9/syntenin. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1457. doi:10.1158/1538-7445.AM2011-1457