Tumor angiogenesis. Regulation of mural cell recruitment and endothelial sprouting

Abstract

The formation of new blood vessels is an inherent part of many physiological and pathological processes. Currently, there is a strong hope pathological situations like stroke, diabetic retinopathy and cancer may be affected by pharmacological regulators of blood vessel formation. This thesis work has mainly been focused on two molecules implicated in blood vessel formation, platelet-derived growth factor (PDGF)-B and vascular endothelial growth factor (VEGF)-A. In vessels, PDGF-B is expressed by endothelial cells (EC) and act as a chemotactic and mitogenic signal via the PDGF receptor (PDGFR)-b on vascular smooth muscle cells (vSMC) and pericytes (PC). Tumor vessels are often abnormal and show a sparse association of vSMC/PC. We addressed the role of PDGF-B in vSMC/PC recruitment to tumor vessels using gain- and loss-of function analyses of PDGF-B. Our results show that vSMC/PC recruitment is dependent on PDGFRb and an EC source of PDGF-B. PDGF-B binds to the extracellular matrix in the close vicinity of EC through its retention motif. This spatial distribution of PDGF-B protein is needed for proper association of the PC with vessels; lack of PDGF-B retention leads to the extension of cellular processes away from the vessel, and to partial or complete PC detachment.VEGF-A stimulates migration and proliferation by binding to VEGF receptor expressed by EC. We have addressed the mechanism by which VEGF stimulate vessel formation by analyzing postnatal retinal vascularization. Our results revealed two functions of VEGF-A; guidance of highly specialized cells, tip-cells, at the tip of sprouting vessels, and proliferation of cells in the trailing cells, stalk-cells. Both these processes are mediated by VEGF receptor (VEGFR)-2. VEGF-A is produced in isoforms of 120, 164 or 188 amino acids where VEGF188 bind to the ECM, VEGF120 is soluble and VEGF164 has intermediate properties. Their spatial distribution creates a gradient sensed by filopodial extensions from the tip-cell. We also describe the existence of tip-cells with filopodia in tumors vessels. The extent of EC filopodia varies between tumors and correlates with vSMC/PC density and VEGF-A expression. Taken together, these results suggest that PDGF-B- and VEGF-A-stimulated processes in the vasculature are analogous. Migration of vSMC and EC seem to depend on the exact spatial distribution of PDGF-B and VEGF-A respectively. Proliferation of these cells on the other hand, appears to be regulated by the concentration of PDGF-B and VEGF-A. Our work also implies that anti-angiogenic therapy targeting VEGFR2 will inhibit the tip-cell and hence guided angiogenesis. The differences in sprouting phenotype in various tumors indicate that the angiogenic process of these tumors might differ. This, and variable vSMC/PC abundance in different tumors may implicate differences in responsiveness to anti-angiogenic therapy.