Abstract Tumor cells chronically progress, involving a series of aberrant activities within distinct organs, to accomplish final metastasis. The role of fibronectin (FN) in tumor progression has been controversial, being either tumor-suppressive or metastatic-promoting. Nevertheless, little effort has been made to reconcile these discrepancies in a systemic manner. Here, we deprived metastatic tumor cells of pericellular FN (periFN) in the spontaneous metastasis assay and demonstrated a dual role of FN. We found that, while knocking down FN expression of Lewis lung carcinoma cells (FNKD-LLC) increased the tumor sizes and migratory activities, it significantly decreased the tumor metastasis in the lungs. Histological examination revealed that subcutaneous tumors of mice bearing FNKD LLC invaded the surrounding tissues to a higher extent than those of mice bearing control scramble-LLC. Immunohistochemical staining with antibodies against CD31, FN, and casepase 3, revealed that FNKD-LLC rendered more angiogenesis, tumor intravasation, and fibrous FN deposited in the surrounding inflammatory tissues, without altering the tumor apoptosis, suggesting that the FN expression in tumor cells indeed plays a role in suppressing the tumor progression in the primary tumor tissues. Conversely, the metastatic tumor nodule numbers in the lungs were significantly reduced for the FNKD-LLC cells, clearly indicative of a metastatic-promoting role of periFN. We reasoned these seemingly conflicting phenomena by hypothesizing that the FN expression of neoplastic lung carcinoma cells is initially downregulated to trigger fibrotic microenvironments (FN deposition in extracellular matrices) to which pro-inflammatory leukocytes are recruited, leading to the enhanced angiogenesis and tumor intravasation. However, the early circulating lung cancer cells fail to colonize distant organs (including lungs) until periFN is re-assembled in the later progression stages, thus facilitating the tumor cell adhesion to CD26 on endothelial cells and consequently extravasation and metastasis. This hypothesis of temporal-spatial regulations was clinically supported by comparing and statistically analyzing the FN expression levels in lung cancer tissues collected from patients hospitalized by the NCKU hospital for surgery within 2003 through 2006 with low and high survivals. This study was supported by Grant # MOST 102-2320-B-006 -031- and MOST 103-2325-B-006 -009 -. Citation Format: Hung-Chi Cheng, Ming-Min Chang, Yau-Lin Tseng. The dual role of fibronectin in lung cancer progression. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2274. doi:10.1158/1538-7445.AM2015-2274