Abstract
It is widely believed that vascular endothelial growth factor (VEGF) induces angiogenesis by its direct mitogenic and motogenic actions on vascular endothelial cells. However, these activities are only detected when endothelial cells are cultured at very low (0.1%) serum concentrations and would not be expected to take place at the much higher serum levels found in angiogenic sites in vivo. This conundrum can be resolved by recalling VEGF's original function, that of an extremely potent vascular permeability factor (VPF). In vivo VPF/VEGF increases microvascular permeability such that whole plasma leaks into the tissues where it undergoes clotting by tissue factor that is expressed on tumor and host connective tissue cells to deposit fibrin and generate serum. By providing tissue support and by reprogramming the gene expression patterns of cells locally, fibrin and serum can together account for the formation of vascular connective tissue stroma. In sum, by increasing vascular permeability, VPF/VEGF triggers the "wound healing response," setting in motion a fundamental pathophysiological process that induces the mature stroma that is found not only in healing wounds but also in solid tumors and chronic inflammatory diseases. Once initiated by increased vascular permeability, this response may be difficult to impede, perhaps contributing to the limited success of anti-VEGF therapies in treating cancer.