Angiogenesis is an essential process for tumor growth and is regulated by tumor-derived angiogenic cytokines. Osteopontin (OPN) is one of the cytokines produced by various tumor cells and is suggested to be involved in angiogenesis by upregulating endothelial cell migration in cooperation with vascular endothelial cell growth factor (VEGF). To provide evidence of OPN involvement in a causal role in tumor angiogenesis, we generated a stable transfectant from murine neuroblastoma C1300 cells to constitutively secrete high levels of murine OPN. The OPN mRNA expression and protein secretion were confirmed by RT-PCR and ELISA, respectively. The biological activity of secreted OPN was deter- Angiogenesis, the formation of new blood vessels, is essential for tumor growth and metastasis. 1,2 Previous studies have indicated that this angiogenesis process is mediated by numerous angiogenic factors that are produced by tumor cells. 3,4 Non-vascularized tumor cells secrete angiogenic cytokines that diffuse in the direction of pre-existing blood vessels. These cytokines activate quiescent vascular endothelial cells, enabling them to proliferate and migrate toward the tumor. 5,6 In addition, cell adhesive receptors including ␣v-integrins facilitate the endothelial cell migration by interacting with adhesive proteins in the extra-cellular matrix (ECM). 7 Thus, angiogenesis process depends on the cooperation of angiogenic cytokines and cell adhesive events.Osteopontin (OPN) is a phosphoprotein that binds to ␣v integrins at the arginine-glycine-asparatic acid (RGD) motif of the central portion and exerts cell adhesion and migration activity. 8 -11 OPN is secreted from various cancer cell lines and is postulated to be related with malignant transformation. 12 When ras-transformed fibroblasts were transfected with an antisense OPN RNA, their tumorigenic and malignant growth were suppressed significantly. 13 OPN has also been revealed to be overexpressed in human cancers, 14,15 and serum levels of OPN were substantially elevated in patients with metastatic cancer. 16 Thus OPN is postulated to be related with cancer progression.OPN has been considered to be an adhesive protein due to its integrin binding activity. However, OPN has many other novel properties, suggesting that it may act as a cytokine in various situations. [17][18][19][20][21][22] Recently, much interest has been focused on the biological role of OPN in the pathogenesis of angiogenesis. For example, Senger et al. 23 reported that OPN in cooperation with VEGF promotes vascular endothelial cell migration via ␣v integrin, suggesting that OPN may be involved in angiogenesis. Shijubo et al. 24 demonstrated that coexpression of OPN and VEGF is closely associated with angiogenesis and poor prognosis in Stage I lung adenocarcinoma. Some other researchers have also suggested the possible role of OPN in angiogenesis. [25][26][27] However, there has been no studies that established the causal role for OPN in tumor angiogenesis.The purpose of our study was to determine and p...

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