Although our understanding of the molecular regulation of adult neovascularization has advanced tremendously, vascular-targeted therapies for tissue ischemia remain suboptimal. The master regulatory transcription factors of the hypoxia-inducible factor (HIF) family are attractive therapeutic targets because they coordinately up-regulate multiple genes controlling neovascularization. Here, we used an inducible model of epithelial HIF-1 activation, the TetON-HIF-1 mouse, to test the requirement for VEGF in HIF-1 mediated neovascularization. TetON-HIF-1, K14-Cre, and VEGF flox/flox alleles were combined to create TetON-HIF-1:VEGF Δ mice to activate HIF-1 and its target genes in adult basal keratinocytes in the absence of concomitant VEGF. HIF-1 induction failed to produce neovascularization in TetON-HIF-1:VEGF Δ mice despite robust up-regulation of multiple proangiogenic HIF targets, including PlGF, adrenomedullin, angiogenin, and PAI-1. In contrast, endothelial sprouting was preserved, enhanced, and more persistent, consistent with marked reduction in Dll4-Notch-1 signaling. Optical-resolution photoacoustic microscopy, which provides noninvasive, label-free, high resolution, and wide-field vascular imaging, revealed the absence of both capillary expansion and arteriovenous remodeling in serially imaged individual TetON-HIF-1:VEGF Δ mice. Impaired TetON-HIF-1:VEGF Δ neovascularization could be partially rescued by 12- O -tetradecanoylphorbol-13-acetate skin treatment. These data suggest that therapeutic angiogenesis for ischemic cardiovascular disease may require treatment with both HIF-1 and VEGF.

Endothelial cells (ECs) express fibroblast growth factor receptors (FGFRs) and are exquisitely sensitive to FGF signals. However, whether the EC or another vascular cell type requires FGF signaling during development, homeostasis, and response to injury is not known. Here, we show that Flk1-Cre or Tie2-Cre mediated deletion of FGFR1 and FGFR2 (Fgfr1/2Flk1-Cre or Fgfr1/ 2 Tie2-Cre mice), which results in deletion in endothelial and hematopoietic cells, is compatible with normal embryonic development. As adults, Fgfr1/2 Flk1-Cre mice maintain normal blood pressure and vascular reactivity and integrity under homeostatic conditions. However, neovascularization after skin or eye injury was significantly impaired in both Fgfr1/2Flk1-Cre and Fgfr1/2Tie2-Cre mice, independent of either hematopoietic cell loss of FGFR1/2 or vascular endothelial growth factor receptor 2 (Vegfr2) haploinsufficiency. Also, impaired neovascularization was associated with delayed cutaneous wound healing. These findings reveal a key requirement for cell-autonomous EC FGFR signaling in injuryinduced angiogenesis, but not for vascular homeostasis, identifying the EC FGFR signaling pathway as a target for diseases associated with aberrant vascular proliferation, such as age-related macular degeneration, and for modulating wound healing without the potential toxicity associated with direct manipulation of systemic FGF or VEGF activity.choroidal neovascularization | oxygen-induced retinopathy | retinopathy of prematurity | neoangiogenesis N eovascularization is critical for tissue repair and pathological conditions, including aberrant ocular angiogenesis and cancer (1-4). Although FGF signaling has been prominently implicated in these processes based on genetic inactivation experiments in mice and in vitro studies, the functional in vivo requirement of this pathway in the endothelial cell (EC) vs. other vascular cell types is not known (5-8).The FGF family is composed of 18 signaling ligands, which interact with four cell surface tyrosine kinase receptors. FGF receptor (FGFR) signaling regulates many biological processes, including survival, differentiation, proliferation, and angiogenesis through the activation of RAS-RAF-MAPK, PI3K, STAT, and PLC gamma pathways (6, 9). The EC response to FGF signals is well described in in vitro models of angiogenesis (10, 11). Moreover, previous gene expression analysis showed that Fgfr1 and Fgfr2 were the predominant Fgfrs in ECs (5), whereas Fgfr3 was sparsely detected (12, 13) and Fgfr4 expression was not reported (8). To this end, and given the critical role of FGFRs 1 and 2 during embryonic development, we tested the hypothesis that EC FGFR1/2 may play a key role during vascular development, homeostasis, and response to injury.Studies aimed at understanding the functional requirement of vascular FGF signaling have demonstrated a critical role in homeostasis and angiogenesis (14-16). In these studies, in vivo expression of an adenoviral-based soluble FGF trap (sFGFR) or a dominant inhibitor of all FGFRs (FGF...

Objective Comprehensive understanding of the mechanisms regulating angiogenesis might provide new strategies for angiogenic therapies for treating diverse physiologic and pathologic ischemic conditions. The ETS factor ETV2 (aka ER71) is essential for the formation of hematopoietic and vascular systems. Despite its indispensable function in vessel development, ETV2 role in adult angiogenesis has not yet been addressed. We have therefore investigated the role of ETV2 in vascular regeneration. Approach and Results We utilized endothelial Etv2 conditional knockout (CKO) mice and ischemic injury models to assess the role of ETV2 in vascular regeneration. While Etv2 expression was not detectable under steady state conditions, its expression was readily observed in endothelial cells following injury. Mice lacking endothelial Etv2 displayed impaired neovascularization in response to eye injury, wounding, or hindlimb ischemic injury. Lentiviral Etv2 expression in ischemic hindlimbs led to improved recovery of blood flow with enhanced vessel formation. Following injury, Flk1 expression and neovascularization were significantly upregulated by Etv2, while Flk1 expression and VEGF response were significantly blunted in Etv2 deficient endothelial cells. Conversely, enforced Etv2 expression enhanced VEGF mediated endothelial sprouting from embryoid bodies. Lentiviral Flk1 expression rescued angiogenesis defects in endothelial Etv2 CKO mice following hindlimb ischemic injury. Furthermore, Etv2+/−; Flk1+/− double heterozygous mice displayed a more severe hindlimb ischemic injury response compared to Etv2+/− or Flk1+/− heterozygous mice, revealing an epistatic interaction between ETV2 and FLK1 in vascular regeneration. Conclusion Our study demonstrates a novel obligatory role for the ETV2 in postnatal vascular repair and regeneration.

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Diabetic nephropathy is a leading cause of end-stage kidney failure. Reduced angiopoietin-TIE2 receptor tyrosine kinase signaling in the vasculature leads to increased vascular permeability, inflammation, and endothelial cell loss and is associated with the development of diabetic complications. Here, we identified a mechanism to explain how TIE2 signaling is attenuated in diabetic animals. Expression of vascular endothelial protein tyrosine phosphatase VE-PTP (also known as PTPRB), which dephosphorylates TIE2, is robustly up-regulated in the renal microvasculature of diabetic rodents, thereby reducing TIE2 activity. Increased VE-PTP expression was dependent on hypoxia-inducible factor transcriptional activity in vivo. Genetic deletion of VE-PTP restored TIE2 activity independent of ligand availability and protected kidney structure and function in a mouse model of severe diabetic nephropathy. Mechanistically, inhibition of VE-PTP activated endothelial nitric oxide synthase and led to nuclear exclusion of the FOXO1 transcription factor, reducing expression of pro-inflammatory and pro-fibrotic gene targets. In sum, we identify inhibition of VE-PTP as a promising therapeutic target to protect the kidney from diabetic injury.

Insufficient erythropoiesis due to increased demand is usually met by hypoxia-driven up-regulation of erythropoietin (Epo). Here, we uncovered vascular endothelial growth factor (VEGF) as a novel inducer of Epo capable of increasing circulating Epo under normoxic, nonanemic conditions in a previously unrecognized reservoir of Epo-producing cells (EPCs), leading to expansion of the erythroid progenitor pool and robust splenic erythropoiesis. Epo induction by VEGF occurs in kidney, liver, and spleen in a population of Gli1+SMA+PDGFRβ+ cells, a signature shared with vascular smooth muscle cells (VSMCs) derived from mesenchymal stem cell–like progenitors. Surprisingly, inhibition of PDGFRβ signaling, but not VEGF signaling, abrogated VEGF-induced Epo synthesis. We thus introduce VEGF as a new player in Epo induction and perivascular Gli1+SMA+PDGFRβ+ cells as a previously unrecognized EPC reservoir that could be harnessed for augmenting Epo synthesis in circumstances such as chronic kidney disease where production by canonical EPCs is compromised.

VEGF signaling through its tyrosine kinase receptor, VEGFR2 (FLK1), is critical for tumor angiogenesis. Previous studies have identified a critical gene dosage effect of VegfA in embryonic development and vessel homeostasis, neovascularization, and tumor growth, and potent inhibitors of VEGFR2 have been used to treat a variety of cancers. Inhibition of FGFR signaling has also been considered as an antiangiogenic approach to treat a variety of cancers. Inhibition of VEGFR2 with neutralizing antibodies or with pharmacological inhibitors of the VEGFR tyrosine kinase domain has at least short-term efficacy with some cancers; however, also affects vessel homeostasis, leading to adverse complications. We investigate gene dosage effects of Vegfr2, Fgfr1, and Fgfr2 in three independent mouse models of tumorigenesis: two-stage skin chemical carcinogenesis, and sub-cutaneous transplantation of B16F0 melanoma and Lewis Lung Carcinoma (LLC). Mice heterozygous for Vegfr2 display profound defects in supporting tumor growth and angiogenesis. Unexpectedly, additional deletion of endothelial Fgfr1 and Fgfr2 in Vegfr2 heterozygous mice shows similar tumor growth and angiogenesis as the Vegfr2 heterozygous mice. Notably, hematopoietic deletion of two alleles of Vegfr2 had minimal impact on tumor growth, with little effect on angiogenesis, reinforcing the importance of endothelial Vegfr2 heterozygosity. These studies reveal previously unrecognized Vegfr2 gene dosage effects in tumor angiogenesis and a lack of synergy between VEGFR2 and endothelial FGFR1/2 signaling during tumor growth.

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