Fibronectin stimulates human lung carcinoma cell growth by inducing cyclooxygenase‐2 (COX‐2) expression

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Abstract
Tobacco use is the most important risk factor for the development of lung carcinoma. One characteristic shared by tobacco‐related lung diseases is altered lung connective tissue content and composition. In particular, tobacco results in increased expression of fibronectin (FN), a matrix glycoprotein implicated in lung development, injury and repair and in tumor cell invasion. We hypothesized that excessive deposition of FN in lung might promote lung carcinoma cell proliferation. Consistent with this hypothesis, we found that FN stimulated human lung carcinoma cell proliferation and diminished apoptosis in vitro, and that this effect was mediated through the integrin α5β1 and associated with upregulation of cyclooxygenase‐2 (COX‐2) mRNA and protein expression, and increased prostaglandin E2 (PGE2) biosynthesis. The stimulatory effect of FN on COX‐2 was blocked by the specific COX‐2 inhibitor NS‐398 and by inhibitors of protein kinase C (PKC), Calphostin C, and extracellular signal‐regulated kinases (Erks), PD98095. Electrophoretic mobility shift assays revealed that FN increased the nuclear binding activity of cyclic AMP response element binding protein (CREB) and CCAAT/enhancer‐binding protein (C/EBP), 2 proteins known to play important roles in the regulation of COX‐2 promoter activity. Transient transfection assays with wild‐type and mutated constructs of the human COX‐2 gene promoter revealed that the stimulatory effect of FN was prevented when either the CRE or the NF‐IL6 (C/EBP) sites were mutated. Taken together, the results indicate that FN stimulates human lung carcinoma cell proliferation and diminishes apoptosis by inducing COX‐2 gene expression and PGE2 biosynthesis. Activation of PKC and Erk and DNA‐protein interactions at CRE and NF‐IL6 (C/EBP) sites in the COX‐2 gene promoter appear to play key roles in this process. This work demonstrates that signaling through specific matrix‐binding β1 integrins (i.e., α5β1) resulting from exaggerated deposition in lung of the matrix glycoprotein fibronectin might promote lung carcinoma cell growth.

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