Abstract 1009: High Mobility Group Box 1 Protein Increases Integrin-Dependent Homing of Endothelial Progenitor Cells in vitro and in vivo
Endothelial progenitor cells (EPC) are recruited to ischemic regions to improve neovascularization, and we recently demonstrated that they engage β2-integrins for their homing. However, the mechanisms of attracting EPC to ischemic tissues are not clear. High mobility group box 1 (HMGB1) is a nuclear protein, which is released extracellularly upon cell necrosis and tissue damage eliciting a pro-inflammatory response and stimulating tissue repair. Since necrosis and inflammation are hallmarks of ischemic tissues, we investigated the effects of HMGB1 on integrin-dependent functions of EPC. Here, we demonstrate that EPC express the HMGB1-receptors RAGE and TLR2. HMGB1 increased EPC migration in a RAGE-dependent manner. In addition, the HMGB1-induced migration of EPC on fibronectin and fibrinogen was significantly inhibited by antibodies against β1- and β2-integrins, respectively. Moreover, short-term prestimulation of EPC for 15 minutes with HMGB1 significantly increased EPC adhesion to mature endothelial cells by 198 ± 21 % and this effect was blocked by antibodies to β2-integrin and RAGE. Furthermore, HMGB1 increased EPC adhesion to ICAM-1 and fibronectin in a RAGE-dependent manner. HMGB1 rapidly enhanced β1- and β2-integrin affinity as assessed by FACS using integrin-antibodies recognizing activation-dependent epitopes. Moreover, HMGB1 increased the lateral motility of β2-integrins on the surface of EPC thereby inducing polarization. These in vitro data were confirmed in vivo by using intravital microscopy in a tumor model of neovascularization. Interestingly, ex vivo prestimulation of EPC with HMGB1 (200 ng/mL) for 15 minutes significantly enhanced the initial in vivo adhesion of EPC to tumor microvessels by 168 ± 18 % and the long term recruitment of EPC in the tumor tissue by 314 ± 44 %. In conclusion, these data demonstrate that HMGB1 is able to stimulate EPC migration and adhesion in a RAGE- and integrin-dependent manner by increasing integrin activity and to enhance in vivo EPC homing to sites of active neovascularization. These data represent the first link between HMGB1 and integrin function in EPC and may provide the rationale for development of novel therapeutic approaches to improve EPC homing.