Purpose:The extracellular matrix (ECM) molecule osteopontin is implicated in many pathologic processes, including inflammation, cell proliferation, ECM invasion, tumor progression, and metastasis. The present study evaluated the clinical and biological importance of osteopontin in human lung cancer. Experimental Design and Results: Tissue microarrays derived from non^small cell lung cancer (NSCLC) patients were analyzed immunohistochemically. Osteopontin protein expression was observed in 64.5% (205 of 318) of primary tumors and 75.5% (108 of 143) of lymph node metastases, but in only 27.9% (12 of 43) of normal-appearing bronchial epithelial and pulmonary tissues. Osteopontin expression was associated with tumor growth, tumor staging, and lymph node invasion. In vitro osteopontin enhanced ECM invasion of NSCLC cells, and an osteopontin antibody abolished this effect. We further analyzed osteopontin levels in circulating plasma derived from 158 patients with NSCLC, 54 patients of benign pulmonary disease, and 25 healthy donors, and found that the median osteopontin levels for the three groups were 319.1, 161.6, and 17.9 ng/mL, respectively. Conclusions: Overexpression of osteopontin is common in primary NSCLC and may be important in the development and progression of the cancer. Osteopontin levels in the plasma may serve as a biomarker for diagnosing or monitoring patients with NSCLC.Lung cancer is the leading cause of cancer-related deaths in industrialized countries. It claims >150,000 lives each year in the U.S. alone, exceeding the combined mortality from breast, prostate, and colorectal cancer (1, 2). Despite recent advances in understanding lung cancer biology, the 5-year survival rate for the patients remains <15% (3). For the patients diagnosed with stage IV disease, this figure drops to a mere 1% due to local relapses and distant metastases. Predicting the metastatic behavior of the tumor and eradicating or controlling dissemination of the malignancy remain major clinical challenges to oncologists.Cancer progression depends on an accumulation of metastasis-supporting genetic modifications and physiologic alterations regulated by cell signaling molecules such as extracellular matrix (ECM) proteins. The latter contribute to interaction among cancer cells and endothelial cells, which play a critical role in the development of local invasion and distant metastasis (4, 5). One such ECM protein is osteopontin. Previous research suggests that osteopontin is up-regulated in a variety of cancers, such as breast, gastric, and colorectal cancers (6, 7). Reports also suggest that some highly metastatic cancer cell lines synthesize abundant osteopontin. For example, the metastatic cell Ca2-5-LT1 expresses osteopontin mRNA at a level nine times higher than that expressed by the nonmetastatic parental cell Rama 37 (8). These findings suggest that osteopontin is a key extracellular molecule involved in tumor development and progression. However, it has not been extensively evaluated as such in lung cancer. Evid...