Biol. Pharm. Bull. 27(6) 772—776 (2004)

The vascular system is the first functional organ that starts to develop in the embryo. The extraembryonic mesodermal cells, so-called hemangioblasts, aggregate in the yolk sac to form blood islands, where they differentiate into an external endothelial cell (EC) layer and an inner core of blood cells. The external ECs subsequently constitute the primary capillary plexus. Shortly thereafter, the intraembryonic mesodermal hemangioblasts and angioblasts located at the proximal lateral mesoderm differentiate and organize into the dorsal aorta. These processes are known as “vasculogenesis”. In the following process of “angiogenesis”, neo-vessels are generated from the primary capillary plexus by sprouting or intussusception and become distributed throughout the body. In the final process of vascular development, mesenchymal cells differentiate to mural cells (smooth muscle cells and pericytes), surround blood vessels, and make them mature and stabilized. In the adult, ECs on the inner surface of blood vessels are normally quiescent and play an important role in maintaining the integrity of the vessels. However, these cells in microvessels do have the ability to form neo-vessels. Angiogenesis is regulated by the balance between angiogenic factors and angiogenesis inhibitors. Many factors are reported to regulate angiogenesis. Among them, endothelium-specific factors and their receptors expressed on ECs have been extensively studied. They include vascular endothelial growth factor (VEGF) family members and their receptors, and angiopoietins and TIE receptors. VEGF receptors include VEGF receptor-1 (VEGFR-1, also known as Flt1), VEGF receptor-2 (VEGFR-2, also known as KDR/Flk-1), VEGF receptor-3 (VEGFR-3, also known as Flt-4), neuropilin-1 and neuropilin-2. VEGFR-1 and VEGFR-2 are expressed on vascular endothelium, whereas VEGFR-3 is preferentially expressed on lymphatic endothelium in the adult. Neuropilins, originally identified as regulators of axon guidance, are also expressed on ECs and act as co-factors of VEGF receptors. VEGF family members are VEGF-A, VEGF-B, VEGF-C, VEGF-D and placenta growth factor (PlGF). VEGF-A binds to VEGFR-1 and VEGFR-2, VEGFC and VEGF-D bind to VEGFR-2 and VEGFR-3, and PlGF and VEGF-B bind only to VEGFR-1. Among these VEGF family members, VEGF-A seems to be the most important factor for angiogenesis, stimulating protease synthesis in and migration and proliferation of ECs; and most of the VEGFA-mediated signals are transduced via VEGFR-2. Angiopoietins are ligands of the TIE-2 receptor. There are 4 memebers of angiopoietins, and are classified into 2 subgroups. Angiopoietin-1 (Ang-1) and angiopoietin-4 (Ang-4), comprising one subgroup, are agonistic ligands of the TIE-2 receptor; whereas angiopoietin-2 (Ang-2) and angiopoietin-3 (Ang-3), in the other subgroup, exhibit very weak activity and are thought as antagonistic ligands of the TIE-2 receptor. The most common sprouting-type angiogenesis includes following sequential steps: detachment of pre-existing pericytes for vascular destabilization, extracellular matrix (ECM) degradation by endothelial proteases, migration of ECs, proliferation of ECs, tube formation by ECs, and reattachment of pericytes for vascular stabilization. Hypoxia is one of the most important triggers of angiogenesis, which induces the expression of VEGF-A in various cell types. The resultant VEGF-A stimulates ECs and induces the expression of various genes including Ang-2. Ang-2 is an antagonistic ligand of TIE-2, and that makes the detachment of pericytes from the vascular wall for vascular destabilization. Consecutively, VEGF-A stimulates ECs to form neo-vessels by promoting protease synthesis for ECM degradation, migration, and proliferation. Ang-2 itself does not stimulate angiogenesis, but enhances the angiogenic activities of VEGF-A when combined with it. ECs of neo-vessels produce platelet-derived growth factor (PDGF), which attracts pericytes to the neovessels. The attracted pericytes produces Ang-1. When Ang-1 synthesized by pericytes dominates EC-derived Ang772 Biol. Pharm. Bull. 27(6) 772—776 (2004) Vol. 27, No. 6