| Retinopathy of prematurity1 is a leading cause of blindness in premature infants receiving supplemental oxygen. Annexin A2 (A2), a Ca 2+-dependent cell membrane binding protein, usually forms a heterotetramer with S100A10, p11, and functions as co-receptor for both plasminogen and tissue plasminogen activator (tPA) to promote plasmin generation.;Compared to Anxa2+/+ mice, Anxa2-/- mice exhibit reduced neovascularization (NV) in oxygen-induced retinopathy (OR), a model of human ROP. In the present study, we found that A2 specifically promotes NV, but not vaso-obliteration (VO) in mouse retinas during OIR. Retinal NV and tuft formation were increased in Anxa2-/- mouse retinas after restoring A2 expression using recombinant adenovirus. Moreover, we observed that A2 protein and mRNA levels were significantly upregulated along with accumulation of hypoxia-inducible factor-1alpha (HIF-1alpha) in mouse retinas during OIR. OIR-induced A2 expression was associated with retinal neovessels.;Endothelial cells increased their expression of A2 mRNA and protein in response to hypoxia in vitro. Upregulation of A2 in hypoxic endothelial cells was HIF-1alpha-dependent. Higher levels of A2 were translocated to the cell surface, and in turn enhanced plasmin generation and directed migration of endothelial cells in response to hypoxia. The capacity for both plasmin generation and directed migration were significantly increased in Anxa2 -/- endothelial cells after A2 restoration.;Retinal NV was also increased in Anxa2-/- mice after restoring A2 expression. In addition, the HIF-1 complex bound directly and specifically to the A2 promoter, inducing its activation. In vivo studies revealed that a significantly higher level of retinal fibrin deposition was observed in Anxa2-/- versus Anxa2+/+ mice during OIR. Retinal NV was significantly increased in Anxa2-/- mice upon depletion of retinal fibrin. Conversely, retinal NV was significantly reduced in Anxa2 +/+ mice during OIR after inhibiting plasminogen activation. Moreover, there were more pericytes and macrophages, higher levels of endothelial cell proliferation, and less apoptosis, in the retinas of Anxa2+/+ as compared to Anxa2-/- mice during OIR. Taken together, we conclude that HIF-1alpha-induced upregulation of A2 facilitates OIR by enhancing plasmin, reducing fibrin deposition, and facilitating the migration of endothelial cells, and the recruitment of vascular supporting cells, thereby promoting pathologic retinal neoangiogenesis. |
