Proteomic identification of exosome-derived FAM3C as a potential biomarker for non-small cell lung cancer

Background: The discovery of biofluid-based biomarkers is urgently needed to improve early detection of lung cancer. Exosome-derived proteins are useful resources in biomarker identification. Methods: Proteomic analysis of one normal fibroblast and three NSCLC cell-derived exosomes was conducted. Exosomes were isolated by ultracentrifugation and characterized by western blot, transmission electron microscopy and Zetasizer. Human plasma and tissues samples were used for validation of FAM3C as a novel lung cancer in vivo biomarker. Written informed consent was obtained from all participants. Results: FAM3C was among the top 15 potential proteins highly expressed in cancer cell exosomes and chosen for further validation. In functional study, overexpression of FAM3C dramatically stimulated the epithelial-mesenchymal transition (EMT), migration, invasion, proliferation and colony formation of lung cancer cells while knockdown of FAM3C showed opposite effects. Further analysis showed that exosomes could serve as messengers in intercellular communication to promote metastasis in lung cancer cells. Injection of overexpressed FAM3C cells via the tail vein promoted lung metastasis in mouse models. The IHC staining indicated that FAM3C expression in lung cancer specimens was greatly increased compared to those in tumour adjacent and normal lung tissues. Moreover, granular FAM3C staining was significantly associated with improved lung cancer specific survival in squamous cell carcinoma patients. ELISA assay revealed that plasma exosome FAM3C was significantly elevated in NSCLC patients (n = 78) compared to healthy controls (n = 78) (p < 0.0001) with an AUC of 0.831, a sensitivity of 0.756 and a specificity of 0.744. Summary/conclusion: These findings demonstrate that exosomederived FAM3C is a potential biomarker which predicts lung cancer metastasis, and further large-scale clinical studies are warranted.