EGFL6 Promotes Bone Metastasis of Lung Adenocarcinoma by Enhancing Osteoclast Differentiation and Stimulating Cancer Cell Metastasis

Abstract Background Lung cancer is still the most fatal cancer today with approximately 30%-40% of these patients will develop bone metastasis. Lung adenocarcinoma (LUAD) is the most common and aggressive type of lung cancer. The relationship between LUAD and bone metastasis and its underlying mechanism remains unclear. This study proved that epidermal growth factor-like domain multiple 6 (EGFL6) was highly expressed in LUAD specimens of patients, and the expression level was positively correlated with bone metastasis of LUAD. Method The expression of EGFL6 in cancer tissues was detected by IHC. CCK-8, colony formation assay, migration and invasion assay, wound healing assay, immunocytochemistry, RT-PCR, Western blotting, ELISA, bone resorption, TRAP staining and H&E staining were performed. A nude mouse model of LUAD-induced bone destruction was established by injecting A549 cells in different EGFL6 expression levels. Results EGFL6 is elevated in LUAD and is associated with bone metastasis. In vivo, implantation of human adenocarcinoma A549 cells with a higher expression of EGFL6 not only increased tumor growth rate but also bone resorption of tibias in nude mice. In vitro, the secretion of EGFL6 from A549 cells increased osteoclast differentiation but had little effect on osteogenic differentiation. To reveal the underlying mechanism, we demonstrated that EGFL6 enhanced osteoclast differentiation through activating nuclear factor-kappa B (NF-κB) and downstream c-Fos/NFATc1 signaling pathways, and in addition promoted the proliferation, migration and invasion of A549 cells through enhancing the epithelial mesenchymal transformation (EMT) and promoting Wnt/β-catenin and PI3K/AKT/mTOR signaling pathways. Conclusions We unveil EGFL6 as a predictor in bone metastasis of LUAD and underscore the relevance of EGFL6 as a therapeutic target.