VEGF-R2/KDR (FLK1, Kinase insert domain receptor, CD309, VEGFR2)

Angiogenesis is a fundamental physiological process, both during the development of the organism and in adult life, requiring the well coordinated action of a variety of growth factors and adhesion molecules in endothelial cells. So far, VEGF-A (also called VEGF) and its receptors represent the best characterized signalling pathway in developmental and tumour angiogenesis. VEGF-A binds to two receptor-tyrosine kinases: VEGFR-1/Flt-1 and VEGFR-2/KDR. VEGFR-2 is the major mediator of the mitogenic, angiogenic and permeability-enhancing effects of VEGF-A. The VEGFR’s possess an approximately 750 amino acid residue extracellular domain, which is organized into seven immunoglobulin (Ig)-like folds. This extracellular domain (also called ectodomain) is followed by a single transmembrane region, a juxtamembrane domain, a split tyrosine-kinase domain interrupted by a 70-amino acid kinase insert and a C-terminal tail. Alternative splicing or proteolytic processing of VEGFR’s give rise to secreted variants of VEGFR-2, also called soluble VEGFR-2 (sVEGFR-2). Although the VEGFR’s are primarily expressed in the vascular system, sensitive methods like sandwich ELISA’s have allowed the detection of VEGFR expression in non-endothelial cells like hematopoietic stem cells. The important role of VEGFR-2 signalling during development and in neo-vascularization in physiological or pathological conditions in vivo has allowed the design of clinically beneficial therapies. A soluble form of VEGFR-2 protein can be detected in human and murine plasma. Studies confirmed that the detected soluble fragment was a truncated form of VEGFR-2, shed from mouse and human endothelial cells. Since the activation of VEGFR-2 plays an important role in tumour angiogenesis, there is broad clinical interest in monitoring plasma soluble VEGFR-2 levels in cancer patients with a focus on its potential as a surrogate biomarker for disease progression as well as monitoring marker of the efficiency of anti-angiogenesis drugs. Using mouse models with human tumours a reverse relationship could be shown between the levels of sVEGFR-2 and tumour size. Besides its putative role as a surrogate marker for tumour angiogenesis, naturally occurring sVEGFR-2 is a molecular regulator for VEGF and VEGFR signalling. Further investigations will reveal if sVEGFR-2 can arrest solid tumour angiogenesis and modulate metastasis. The sandwich ELISA to detect, measure and quantify soluble and solubilized VEGFR-2 levels will help to explain recent clinical results for anti-angiogenic therapy and will allow further understanding of VEGFR-2 as biomarker for monitoring cancer progression and its possible role in modulation of vessel growth.