Activation of VEGFR-2 in Hematopoietic Progenitor Cells Leads to Accumulation of Myeloid Cells through Up-Regulation of GM-CSF.

VEGF is a secreted growth factor that mediates its biological effects by binding to two transmembrane tyrosine kinase receptors, VEGFR-1 and VEGFR-2. VEGF has previously been shown to be critical in the establishment of cells of the hematopoietic and endothelial lineages. VEGF and its receptors are expressed on subsets of hematopoietic cells, and are also expressed in some hematopoietic malignancies. However, the requirement for each of the VEGF receptors, and whether the effects of VEGF are due to autocrine or paracrine signals remains unclear. We have designed a strategy that allows us to specifically study the unique effects of VEGFR-2 signaling in hematopoietic cells in vivo. By using a VEGFR-2 cytoplasmic domain fusion protein that can be dimerized with a synthetic drug, we were able to specifically study the effects of VEGFR-2 signaling in isolation. Using mice transplanted with bone marrow transduced with the inducible VEGFR-2 fusion construct, we demonstrate an expansion of myeloid progenitors by CFC assay, as well as increases in Gr-1⁺ and CD11b⁺ cells in the bone marrow. VEGFR-2 activation did not affect the proportion of lymphoid and erythroid cells. We show, by RT-PCR and ELISA, that VEGFR-2 activation can up-regulate granulocyte/macrophage-colony stimulating factor (GM-CSF) in bone marrow cells. Using activation and blocking experiments, we show that the VEGFR-2 activity is due to autocrine effects as well paracrine effects through the induction of GM-CSF in bone marrow cells.