Identification of RKIP as an invasion suppressor protein in nasopharyngeal carcinoma by proteomic analysis.

To identify novel proteins associated with the pathogenesis of nasopharyngeal carcinoma (NPC), a proteomic approach was used to screen for differential proteins between NPC and adjacent noncancerous nasopharyngeal epithelial tissue (ANNET). As a result, 21 differential proteins were identified by two-dimensional electrophoresis and mass spectrometer. Raf kinase inhibitor protein (RKIP), one of the downregulated proteins in NPC compared to ANNET, was investigated for its role in the metastasis of NPC. Western blot analysis and immunohistochemistry were used to detect RKIP expression in 5-8F and 6-10B NPC cell lines with the different metastatic potentials, and in NNET, primary NPC and NPC metastasis. Furthermore, high metastatic 5-8F with low RKIP expression and nonmetastatic 6-10B with high RKIP expression were stably transfected with plasmids that expressed sense and antisense RKIP cDNA, respectively, or with empty vector. The effects of RKIP expression on in vitro cell invasion, and the activity of Raf-1/MEK/ERK signaling pathway were analyzed in the transfected cells. The results showed that RKIP was significantly downregulated in 5-8F compared with 6-10B, in NPC compared with NNET, and not detectable in NPC metastasis. Overexpressed RKIP in 5-8F could decrease its in vitro cell invasion, whereas downregulated RKIP in 6-10B could increase its in vitro cell invasion. RKIP negatively regulated Raf-1/MEK/ERK signaling pathway in NPC cells, and activation of this signaling pathway by RKIP downregulation increased in vitro invasion of NPC cells. Taken together, our results suggest that RKIP may be a NPC metastasis suppressor, and decreased RKIP expression is associated with the increased invasive capability of NPC cells possibly through the activation of Raf-1/MEK/ERK pathway.