Peripheral stem cells released from bone marrow (BM) are known to be incorporated into foci of neovascularization and to contribute to solid tumor development. In the present rat Walker256 tumor model, BM suppression induced by total body irradiation resulted in poor growth of the tumor with apparently poor vascularization, and BM transplantation restored tumor growth. Endothelial progenitor cells are considered to be crucial for vasculogenesis, but they are not yet well defined, and methodology for their purification has not been established. As a model to examine the significance of endothelial progenitor cells in tumor-specific vasculogenesis, we utilized an immortalized rat BM-derived endothelial cell line named TR-BME-2. Fluorescence-labeled TR-BME-2 cells injected systemically into rats were accumulated at the tumor site 4 days later. Another rat BM-derived cell line named C2-11, which does not have an endothelial profile, did not show tumor-specific accumulation. The tumor volume in rats treated with TR-BME-2 was significantly larger than that in rats treated with C2-11. Thus, our results suggest the importance of neovascularization by bone marrow-derived endothelial cells for the promotion of tumor growth.

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