Radiotherapy is widely used in the treatment of cancer. On the other hand, endostatin is considered to be a potent inhibitor of angiogenesis. Therefore, to test whether ES combined with RT overcomes the limitations of each monotherapy the present study investigated the effects of endostatin (ES), radiotherapy (RT) or combination therapy on the growth of mouse breast cancer cells as well as on the expression of substance P in vitro. The breast cancer cell lines 4T1 and 4THMpc were treated with recombinant murine ES (0.5, 1, 2, 4 and 8 µg/ml) alone, RT (45 Gy) alone or as a combination therapy. Cell proliferation was evaluated using an MTS assay and the results were verified by the Live/Dead assay. Immunoprecipitation and Western blotting analysis were performed to determine whether the substance P levels of the two cell lines occurred due to substance P content. Results showed that ES alone resulted in a low but significant inhibition in the growth of 4T1 and 4THMpc cell lines (27.63 and 21.75%, respectively). RT alone inhibited the growth of 4T1 (30.81%) and 4THMpc (39.64%) cells. A combination of ES with RT enhanced growth inhibition in the cells (83% in 4T1 and 80% in 4THMpc). The amount of substance P, both in the conditioned media and the cell lysates, increased within 72 h after RT. This increase was inhibited when ES and RT were applied in combination. These data indicate that ES inhibits the in vitro growth of breast cancer cells and potentiates the anti-tumor effects of RT at appropriate doses via alteration of the amount of substance P and thus an increase of radioresponse.

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