Analysis of the differential secretome of nasopharyngeal carcinoma cell lines CNE-2R and CNE-2.

Radioresistance is the major cause of poor prognosis in nasopharyngeal carcinoma (NPC). To identify and characterize the secretome associated with NPC radioresistance, we compared the conditioned serum-free medium of radioresistant CNE-2R cells with that of the parental radiosensitive CNE-2 cells using isobaric tags for relative and absolute quantitation (iTRAQ) with liquid chromatography-electrospray tandem mass spectrometry (LC-ESI-MS/MS) quantitative proteomics. Before proceeding to quantitative proteomics, we investigated the survival curves of CNE-2R and CNE-2 cells by colony formation assay, and the CNE-2R survival curves were significantly higher than those for CNE-2. In total, 3,581 proteins were identified in the quantitative proteomics experiments, and 40 proteins exhibited significant differences between the CNE-2R and CNE-2 cells. Twenty-six of the 40 proteins were secreted by classical, non-classical, or exosomal secretion pathways. To verify the reliability of iTRAQ quantitative proteomics, we applied western blotting (WB) to study the secretory protein expression of fibrillin-2, CD166, sulfhydryl oxidase 1 and cofilin-2, which are involved in cell adhesion, migration and invasion. The WB results showed that fibrillin-2 (p=0.017) and sulfhydryl oxidase 1 (p=0.000) were highly expressed in the CNE-2 cells, while CD166 (p=0.012) and cofilin-2 (p=0.003) were highly expressed in the CNE-2R cells, which was in accordance with iTRAQ quantitative proteomics. Finally, a phenotypic subset of CD166-positive NPC cells was verified by immunocytochemistry. In summary, we defined a collection of secretory proteins that may be relevant to the radioresistance in NPC cells, and we determined that CD166, which is widely used as a positive marker of cancer stem cells, is expressed in NPC cells.