Abstract 1014: Role of PI3Kγ for Integrin-Dependent Adhesive and Migratory Functions of Endothelial Progenitor Cells
Endothelial progenitor cells (EPC) are recruited to ischemic regions to improve neovascularization. Understanding of the molecular mechanisms of EPC homing is essential for the development of new therapeutic strategies in order to improve the efficacy of cell-based therapies in patients with ischemic disorders. We recently demonstrated a crucial role for β2-integrins for EPC homing. Chemokines (e.g. SDF1, IL-8) and their respective G-protein coupled receptors (GPCRs) are involved in the EPC homing to ischemic tissues. The phosphatidylinositol-3-kinase catalytic subunit gamma (PI3Kγ) is the PI3K isoform, which selectively transduces signals from GPCRs in contrast to the other PI3K-isoforms transducing signals from receptor tyrosine kinases. Here, we investigated the role of PI3Kγ for integrin-dependent homing functions of EPC. As assessed by western blot, EPC express the catalytic subunits PI3Kα, PI3Kβ and PI3Kγ, but not the isoform PI3Kδ. We then studied the role of PI3Kγ for EPC migration. Wortmannin, an unselective PI3K-inhibitor, significantly blocked SDF1-induced migration of EPC on fibronectin. Likewise, AS-605240 (100 nM), a selective PI3Kγ-inhibitor (Camps M, Nat. Med., 2005), significantly reduced the SDF1a-induced migration (53 ± 8 % inhibition) as well as the IL-8-induced migration (47 ± 4 % inhibition). Furthermore, the PI3Kγ-inhibitor significantly blocked the SDF1-induced transendothelial migration of EPC. Adhesion is a further essential step during EPC homing to ischemic tissues. In this regard, the PI3Kγ-inhibitor significantly reduced the SDF1-induced adhesion of EPC on HUVEC monolayers by 69 ± 5 % and on ICAM-1 (a β2-integrin ligand). However, the PI3Kγ-inhibitor did not affect the SDF1-induced adhesion of EPC on fibronectin, a β1-integrin ligand, suggesting that PI3Kγ in EPC is specifically involved in the regulation of β2-, but not of β1-integrin-dependent adhesion. In conclusion, these data represent the first link between PI3Kγ and chemokine-induced integrin-dependent homing functions in EPC. Ongoing work using the PI3Kγ-deficient mice will reveal the role of PI3Kγ for EPC homing to ischemic tissues and for vasculogenesis in vivo.