Osteopontin expression in a Murine Model of Postischemic Neovascularization

he occlusion of blood vessels ultimately leads to ischemia, initiating multiple processes that promote neovasculariza-tion as a compensatory mechanism to restore blood flow and preserve tissue function. The ability to develop new collaterals is strongly associated with reduced long-term cardiac mortality in patients with acute myocardial infarction and stable coronary artery disease. 1 The formation of new collaterals is a multifactorial process that requires cytokines, the infiltration of inflammatory cells, cell proliferation, cell migration, and matrix remodeling. Several of these processes are modulated by the presence of osteopontin (OPN), 2,3 a secreted phospho-rylated matricellular protein critical for neovascularization. OPN is important for normal arterial physiology 6 and is expressed by multiple cell types including monocytes/mac-rophages, endothelial cells, and smooth muscle cells (SMCs), 7 all of which play a role in the neovascularization process. OPN mediates several processes relevant to collateral formation, including cell survival, In addition, wound healing, a process that requires angiogen-esis, is significantly impaired in OPN strongly supporting a role for OPN in angiogenesis. Moreover, our group previously demonstrated a direct role for OPN in postisch-emic neovascularization by showing dramatically impaired collateral formation in OPN −/− mice compared with wild-type (WT) controls in a murine model of hindlimb ischemia (HLI). 5 OPN is a noncollagenous, phosphorylated glycoprotein thought to mediate cellular functions by providing a link between cell-surface receptors and structural extracellular matrix molecules. 13 OPN functions in a variety of biological processes and signals by binding to cell-surface integ-rins through a conserved arginine-glycine-aspartate domain or the SVVYGLR binding domain, which is exposed when OPN is cleaved by thrombin or matrix metalloproteinases. 13 OPN is also a ligand for the CD44 receptor. 14 In addition, OPN promotes the migration of multiple cell types, including macrophages, An alternative translation start site in the OPN messenger RNA generates 2 OPN isoforms: a secreted form and an intra-cellular form. 17 Both secreted and intracellular OPN are linked to cell migration, where intracellular OPN localizes to the cell membrane and associates with CD44 18 and secreted OPN regulates cell responses through association with integrin receptors and the CD44 receptor. Reactive oxygen species (ROS), such as superoxide (O 2 • −) and hydrogen peroxide (H 2 O 2), are involved in physiological Objective—Previous findings from our laboratory demonstrated that neovascularization was impaired in osteopontin (OPN) knockout animals. However, the mechanisms responsible for the regulation of OPN expression in the setting of ischemia remain …