Conclusions: We could show for the first time that CD90-positive cells circulate in the peripheral blood of breast cancer patients. Ongoing studies should evaluate their suitability as diagnostic or prognostic factor. Poster No. 119 VEGFR1 Expression by Bone Marrow-Derived Myeloid Cells Mediates Tumor Metastasis via

Suppression of Anti-angiogenic Factors Jared Wels 1 , Maria Rosario Andre1, Selena Granitto1, Rosandra N. Kaplan1,2, Beth Psaila1, John Lawrence1, Stefano Rivella1, Shahin Rafii1, David Lyden1,2 1 Cell and Developmental Biology, Weill Cornell Medical College, New York, NY, USA, 2 Memorial Sloan-Kettering Cancer Center, New York, NY, USA VEGF receptor 1 (VEGFR1) expression by bone marrow-derived cell (BMDC) populations associated with CHIR98014 clinical trial primary tumors as well as the metastatic microenvironment has been reported1,2. This receptor Luminespib has been used to describe cell populations of myeloid progenitor or monocyte/macrophage lineage with pro-angiogenic and metastatic function. However, the role of VEGFR1 EGFR signaling pathway activity in these contexts remains unclear. In the present study, we tested the effect of lentiviral-mediated

knockdown of VEGFR1, specifically within BMDCs, on the development of spontaneous metastases. We report that downregulation of VEGFR1 expression in the bone marrow had a modest effect on primary B16 subcutaneous tumor growth and subsequent tumor cell seeding at early-metastatic sites, yet drastically reduced the occurrence of micro- and macro-metastatic foci. Microarray analysis of RAW 264.7 monocyte/macrophages transduced with VEGFR1 shRNA showed the Parvulin upregulation of key anti-angiogenic factors, including CXCL4 (platelet factor-4) and pigment epithelial derived factor (PEDF). Upregulation of these factors was drastically enhanced in VEGFR1-deficient RAW cells and primary bone marrow-derived myeloid cells lacking VEGFR1 when co-cultured with B16 tumor cells. Functional analyses of VEGFR1-deficient BMDCs indicate these cells inhibit endothelial cell survival in vitro. Additionally, co-injection of VEGFR1-deficient myeloid cells with B16 tumor cells suppressed subcutaneous tumor growth due to apparent

defects in functional vessel formation. These novel findings indicate that VEGFR1 expression controls the angiogenic activity of tumor-associating myeloid cells by suppressing the expression of potent angiostatic chemokines and that blocking this pathway can significantly inhibit tumor metastasis. Our results clearly demonstrate a functional role for VEGFR1 expression within BMDCs in promoting metastatic progression by mediating an angiogenic microenvironment. 1 Kaplan, R. N. et al. VEGFR1-positive haematopoietic bone marrow progenitors initiate the pre-metastatic niche. Nature 438, 820–827, (2005). 2 Lin, E. Y. et al. VEGF Restores Delayed Tumor Progression in Tumors Depleted of Macrophages. Mol Oncol 1, 288–302, (2007). Poster No.