Background: The mechanism by which tumor cells interact with endothelial cells at the primary site in metastasis remains obscure. Materials and Methods: To disclose the precise mechanisms and processes of metastasis and angiogenesis, we investigated the interactions of endothelial cells (CPAE; calf pulmonary arterial endothelial cells) with two mouse liver tumor cell lines, G-5, a highly metastatic clone, and G-1, a poorly metastatic clone, based on the difference in their angiogenic activities in vivo at a primary site. Results: G-5 cell cultured conditioned medium (CM) promoted migration of endothelial cells more than did G-1 cell CM, in migration assay using modified Boyden chambers. Anti-basic FGF antibody inhibited G-5-induced endothelial cell migration. Furthermore, endothelial cells stimulated proliferation of G-5 cells more than that of G-1 cells in coculture assay using Transwell chambers (pore size; 0.4 µm). Pretreated endothelial cells with bFGF enhanced tumor cell proliferation, suggesting the ability of activated endothelial cells to support tumor growth. In addition, incubation with endothelial cell CM also improved the proliferative activity of G-5 cells more than that of G-1 cells in a concentration-dependent fashion, indicating production and secretion of liver tumor cell growth substance by endothelial cells. Conclusion: These results provide evidence that liver tumor cells stimulate endothelial cell migration and migrated endothelial cells facilitate liver tumor cell proliferation. Tumor growth at a primary site may initially be dependent on migrated endothelial cells rather than vascularization or blood nutrient supply. This bi-directional paracrine relationship between liver tumor cells and endothelial cells may influence their metastatic ability.

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