Radiation-enhanced endostatin gene expression and effects of combination treatment

Targeting cells that support tumor growth by administering potent angiogenesis inhibitors is currently an area of intense interest. In the present study, a unique plasmid vector for the mouse endostatin gene, pXLG-mEndo, was constructed and evaluated with and without radiation using the Lewis lung carcinoma (LLC) cell line. The physical properties of the expressed endostatin protein were validated by PCR, gel electrophoresis, and Western blot. Enzyme-linked immunosorbent and immunocytochemical analyses for the therapeutic gene demonstrated that transfected LLC cells secreted the protein into the medium. Exposure of the cells to 2 gray (Gy) gamma-rays reduced the time to reach the maximum expression level of the endostatin gene and also increased the amount of secreted endostatin protein (P < 0.001). Biological activity of the endostatin was demonstrated by the inhibition of tube formation by human umbilical vein endothelial cells (HUVEC). Based on H-3-thymidine incorporation, endostatin expression significantly depressed DNA synthesis in HUVEC and LLC cells compared to controls transfected with parental vector or no vector (P < 0.005). In addition, radiation increased the efficiency of endostatin-mediated inhibition of both cell types over a 3-day period post-exposure (P < 0.05 or less). Intratumoral injection of 100 mu g pXLG-mEndo combined with 10 Gy radiation significantly delayed LLC tumor growth, especially when each modality was delivered twice (P < 0.05 or less compared to all other groups). No toxicity was observed. These findings are very promising and suggest that endostatin therapy with a plasmid vector, such as pXLG-mEndo, may enhance the efficacy of radiotherapy for lung cancer.