| Angiogenesis, the de novo generation of blood vessels from preexisting vessels is a complex biological process that is comprised of several distinct, often overlapping, phases. Endothelial cells and their supporting pericytes or smooth muscle cells, the two major cell types that comprise blood vessels, undergo significant alterations in adhesion, migration, proliferation, and extracellular matrix (ECM) assembly during angiogenesis. Angiopoietin-1 (Ang1) and angiopoietin-2 (Ang2), along with their cell surface receptor, Tie2, comprise one of the most widely studied families of angiogenesis regulators. Deletion of Ang1, Ang2, or Tie2 in mice results in severe defects in angiogenesis. It has been suggested that Ang1 may be a critical mediator of endothelial cell-cell or cell-ECM interactions. Currently, however, very little data is available to support this hypothesis, although structural data is suggestive of a potential function of Ang1 and Ang2 as ECM proteins. The angiopoietins are modular proteins consisting of an N-terminal coiled-coil domain and a C-terminal fibrinogen-like domain and exist as dimers, tetramers, and hexamers. Multimer formation and modularity in structure are hallmarks of ECM proteins, and it is thought that these structural features provide a platform to support their diverse biological properties such as regulation of cell adhesion, migration, and signaling. In this dissertation, the hypothesis that Ang1 and Ang2 function like ECM proteins and that these putative ECM-like activities contribute to the mechanisms whereby they regulate angiogenesis is examined. My findings demonstrate that Ang1 and Ang2 support adhesion of cells and that they differentially regulate cell spreading and migration. These cellular responses do not require Tie2 and appear to be integrin mediated. In addition, the structure-function of Ang1 and Ang2 interactions with integrins is examined in some detail. These experiments have revealed several distinct sequences capable of supporting cell adhesion and indicate that the interaction of the angiopoietins with integrins is mediated by a domain of the molecule that is separate from that involved in Tie2 binding. Collectively, these data suggest that Ang1 and Ang2 may regulate angiogenesis by interacting with two distinct classes of receptors, Tie2 and integrins. |
