Angiogenesis, the growth of new blood vessels, is important for organ development, wound healing and various pathological conditions such as tumor growth or proliferative retinopathies. Neovascularization depends on endothelial cell survival/proliferation, adhesive contacts of the endothelial cells with the extracellular matrix and with pericytes, on endothelial cell migration and sprouting, as well as on the function of growth factors. There is emerging evidence that inflammatory cells and components of the innate immunity regulate endothelial cell functions related to angiogenesis. 1) The complement system is a major component of the innate immunity. How complement may regulate angiogenesis is not clear. The participation of the complement system in hypoxia-driven retina angiogenesis was investigated by using complement components and inhibitors and genetically modified mice that lack the central complement component C3, as well as the receptors C3aR and C5aR. In particular, we found that C3-deficient mice had higher neovascularization in the model of retinopathy of prematurity (ROP) and in the in vivo Matrigel plug assay. Moreover, antibody blockade of C5, treatment with C5aR antagonist, or C5aR-deficiency in mice was associated with increased pathologic retina angiogenesis. While complement did not directly affect angiogenesis-related endothelial cell functions, we found that complement regulated the polarization of macrophages to a pro-inflammatory, anti-angiogenic macrophage phenotype that also secreted high levels of soluble VEGF receptor-1. Consistently, macrophage depletion in vivo reversed the increased neovascularization associated with C3- or C5aR-deficiency. We are currently assessing the role of complement for tumor angiogenesis. 2) Hypoxia induces replication arrest in cells, nevertheless, in hypoxia-induced angiogenesis, such as during proliferative retinopathies, endothelial cells need to actively proliferate due to the presence of hypoxia-inducible factors. To understand this we studied the DNA repair response to hypoxia in endothelial cells, including the phosphorylation of histone H2AX. In vitro and in vivo studies revealed that H2AX and the DNA repair response due to hypoxia help endothelial cells overcome the hypoxia-induced replication arrest, thus allowing for maintenance of endothelial cell proliferation under hypoxic conditions and thereby promoting hypoxia-driven angiogenesis. Consistently, H2AX-deficiency or endothelial-specific H2AX-deficiency resulted in reduced hypoxia-driven angiogenesis in the model of retinopathy of prematurity, in hind limb ischemia or in tumor angiogenesis.
