Recombinant Human VEGFR1/Flt-1 Fc Chimera Protein

VEGFR1 (vascular endothelial growth factor receptor 1), also called Flt-1 (Fms-like tyrosine kinase), is a 180 kDa type I transmembrane glycoprotein in the class III subfamily of receptor tyrosine kinases (RTKs) (1, 2). While family members VEGFR1, VEGFR2/KDR/Flk-1 and VEGFR3/Flt-4 are all mainly expressed on endothelial cells and play central roles in vasculogenesis, angiogenesis, and lymphangiogenesis, only VEGFR1 is expressed on macrophages, and mainly plays inhibitory roles (1-3). VEGFR1 expression is also reported on osteoblasts, placental trophoblasts, renal mesangial cells, and some hematopoietic stem cells (1, 2). Like other class III RTKs, human VEGFR1 contains a signal peptide (aa 1-22), an extracellular domain (ECD aa 27-758) with seven Ig-like repeats, a transmembrane domain (aa 759-780) and a cytoplasmic region (aa 781-1338) with a tyrosine kinase domain and several autocatalytic phosphotyrosine sites. Human VEGFR1 ECD shares 78%, 78%, 84%, 87%, and 90% aa sequence identity with mouse, rat, porcine, canine and equine VEGFR1, respectively. Soluble forms of the VEGFR1 ECD are produced by alternative splicing, and may also be shed during regulated intracellular proteolysis (4-10). Both soluble and transmembrane forms can inhibit angiogenesis by binding and sequestering its ligands, VEGF (VEGF-A), VEGF-B or PlGF (6‑11). VEGFR1 dimerizes upon ligand binding, which can include heterodimerization with VEGFR2 that modifies VEGFR2-mediated endothelial proliferation and vessel branching (8, 11, 12). VEGFR1 binds VEGF with higher affinity than does VEGFR2, but shows weaker kinase activity (9, 13). Both PlGF and VEGF induce autophosphorylation of transmembrane VEGFR1 (5, 9, 13). While deletion of mouse VEGFR1 is lethal due to overgrowth and disorganization of the vasculature, kinase-inactive mutants are viable (13, 14). VEGFR1 is up‑regulated during hypoxia, and participates in neovascularization and wound healing (1, 2, 15). VEGFR1 engagement on monocyte/macrophage lineage cells enhances their migration, and release of growth factors and cytokines (1, 3, 13, 16). Lymphangiogenesis, angiogenesis, and growth-promoting effects of VEGFR1 are thought to result from enhanced migration of macrophages from the bone marrow to tumors and tissues where they recruit endothelial progenitors (3, 16). Circulating levels of VEGFR1 increase during pregnancy and are further elevated in preeclampsia (4, 6, 17).