Loss of E2F1 increases VEGF production and VEGF-dependent EC proliferation and angiogenesis. (A) Representative ribonuclease protection assay photograph (Upper) or quantification (Lower). (B) Quantitative real-time RT-PCR (n = 3 per group; ∗∗, P < 0.01) for mRNA expression of hypoxia-inducible genes in primary lung fibroblasts (WT or E2F1−/−) under conditions of normoxia (“N”) or hypoxia (“H”) for 24 h. (C) Representative immunoblotting for VEGF (Upper) and densitometric quantification of relative VEGF levels (Lower) in the ischemic limbs of mice 14 days after HLI (n = 5 per group; ∗, P < 0.05). (D) ELISA for plasma VEGF levels of mice 14 days after tumor challenge (open bar, vehicle; gray bar, LLC-1 tumor; n = 6 per group; *, P < 0.05). (E) ELISA for hypoxia-induced VEGF secretion in E2F1−/− and WT primary fibroblasts (open bar, normoxia; gray bar, hypoxia; n = 4 per group; ∗, P < 0.05). (F) 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay for proliferation of human umbilical vein endothelial cells exposed to 10% FBS/M199 medium supplemented with 20% conditioned medium collected from WT fibroblasts or from E2F1−/− fibroblasts that were cultured under either normoxic or hypoxic conditions for 24 h (open bar, no antibody; gray bar, anti-VEGF antibody at 10 μg/ml; striped bar, control IgG at 10 μg/ml; the two bars indicated by the two ends of each bracket are compared; ∗∗, P < 0.01; ∗∗∗, P < 0.001). This figure is representative of three MTT assays. The contribution of VEGF overexpression to enhanced blood flow recovery in HLI E2F1−/− mice was evaluated by siRNA-mediated VEGF gene knockdown in the ischemic limb. (G) Representative example (Upper) and quantification (Lower) of tissue VEGF levels (Western blotting) at day 7 after HLI surgery and repeated (days 1 and 3) multisite siRNA i.m. injection. (H) Serial laser Doppler perfusion imaging measurement of blood flow recovery after HLI surgery plus in vivo VEGF knockdown with siRNA (open bar, WT with control GFP siRNA; gray bar, E2F1−/− with control GFP siRNA; striped bar, WT with VEGF siRNA; filled bar, E2F1−/− with VEGF siRNA; n = 12 per group; ∗, P < 0.05).