Osteopontin is involved in the formation of malignant pleural effusion in lung cancer

Malignant pleural effusion (MPE) is associated with advanced-stage lung cancer and is a poor prognostic sign for these patients. Osteopontin (OPN) is a multifunctional cytokine that is involved in the tumor progression and angiogenesis of lung cancer cells. The purpose of this study is to investigate and provide evidence for the role of OPN in the formation of MPE associated with lung cancer. In this study, we established an OPN knockdown murine lung cancer cell line, 3LL cells, utilizing the small interfering RNA (siRNA) technique. To reveal the effect of OPN on the formation of MPE associated with lung cancer, we directly injected OPN knockdown 3LL cells, 3LL/OPN siRNA, or control cells, 3LL/control siRNA, into the pleural space of C57BL/6 mice. OPN knockdown significantly reduced the formation of MPE, but did not inhibit in vivo tumor growth of 3LL cells in mice. Vascular endothelial growth factor (VEGF) concentration in MPE was markedly decreased in the 3LL/OPN siRNA in comparison with that of the 3LL/control siRNA. In vitro, recombinant OPN protein enhanced VEGF secretion from human umbilical vein endothelial cell (HUVEC) or human mesothelial cell line, Met5A cells, in a concentration-dependent manner. These results suggest that OPN is positively involved in the formation of MPE of lung cancer presumably by promoting VEGF secretion from vascular endothelial cells or mesothelial cells. OPN could be an effective target molecule for reducing MPE in lung cancer patients.