Abstract 16991: Endothelial Nitric Oxide Synthase is Essential for Hypoxia-Induced Progenitor Cell Mobilization and Homing

Abstract

Introduction: Acute myocardial infarction (AMI) is accompanied by induction of pro-angiogenic responses that lead to the recruitment of progenitor cells (PCs). Failure to induce PC mobilization, inefficient homing towards or limited survival in the ischemic microenvironment are all associated with loss in healing capacity and increased scar formation. Hypothesis: Both endothelial nitric oxide synthase (eNOS) and hypoxia-inducible factor-1 (HIF-1) have been implicated in PC biology, angiogenesis and vascular homeostasis. We investigated the effects of eNOS deficiency on HIF-1 and HIF-1 related gene expression, PC mobilization and peripheral PC homing conditions. Methods & Results: AMI was induced in wild-type (WT) (n=10) and eNOS knockout (KO) mice (n=10). One week post-AMI, bone marrow mobilization of early PCs (Lin - Sca-1 + ) was markedly attenuated under eNOS-deficient conditions (p<0.001), as assessed by flow cytometry. mRNA expression of Hif1a (p<0.001) and HIF-1-dependent chemotactic factors, such as Cxcl12 (p<0.001) and Ccl2 (p<0.05), and vascular adhesion molecules, such as Icam1 (p=0.002) and Vcam1 (p<0.001), displayed significantly lower expression in the infarction border zone in eNOS KO mice. eNOS activity was highly correlated with Hif1a expression in WT animals (r=0.876, p<0.0001), a relation that was completely lost in the absence of eNOS. Lin - Sca-1 + PC mobilization was correlated with Hif1a (p=0.01), Cxcl12 (p=0.018) and Ccl2 (p=0.008). Histologically, eNOS KO showed decreased capillary formation in the infarction border zone, which correlated with Hif1a (p=0.007) and Cxcl12 (p<0.001) expression, vascular adhesion molecules Icam1 (p=0.003) and Vcam1 (p=0.018), and with mobilization of Lin - Sca-1 + PCs (p=0.033). Conclusions: eNOS is essential for the mobilization of PCs out of the bone marrow and the expression of hypoxia-induced chemokines and vascular adhesion molecules. We provide evidence for the close relationship between eNOS and HIF-1 pathways and suggest a novel view on PC biology in which nitric oxide acts as an essential cofactor for normal HIF-1 activity, enhancing both PC mobilization and peripheral PC homing conditions.