IFITM3 promotes bone metastasis of prostate cancer cells by mediating activation of the TGF-β signaling pathway

Advanced-stage prostate cancer (PCa) is often diagnosed with bone metastasis, for which there are limited therapies. Transforming growth factor beta (TGF-beta) is known to induce epithelial-mesenchymal transition (EMT), and abundance of TGF-beta in the bone matrix is one of the important growth factors contributing to bone metastasis. TGF-beta is reported as a key mediator of bone metastasis, but the underlying mechanism has not been elucidated. It was found in our study that Interferon-inducible Transmembrane Protein 3 (IFITM3) played a key role in the regulation of malignant tumor cell proliferation, invasion, and bone migration by binding to Smad4, thus activating the TGF-beta-Smads Signaling Pathway. Lentivirus-mediated short hairpin RNA (shRNA) knockdown of IFITM3 inhibited cell proliferation and colony formation, induced apoptosis and inhibited migration by reversing EMT and downregulating the expression of metastasis-related molecules including FGFs and PTHrP. Microarray analysis showed that IFITM3 knockdown could alter the MAPK pathway associated with TGF-beta-Smads signaling. By knocking down and overexpressing IFITM3, we demonstrated that IFITM3 expression level had an effect on MAPK pathway activation, and this change was more pronounced upon exogenous TGF-beta stimulation. These results suggest that IFITM3 played an oncogenic role in PCa progression and bone metastasis via a novel TGF-beta-Smads-MAPK pathway.