EPO-induced tube formation is dependent on VEGF-R2. EPCs obtained from healthy control subjects were left untreated or transfected with mock shRNA or shRNA specific for VEGFR2 24 hours before plating with HUVECs on a precoated tissue culture plate and adding EPO 50 mU/mL or EPO 50 mU/mL + SU5416 1μM as indicated. (A) After 12 hours, an inverted phase-contrast microscope was used to visualize 4 randomly selected low-powered fields. (B) Tube length (mean ± SD) is shown, and the shRNA transfected and treatment with EPO or SU5416 is indicated, representative of 2 different experiments. *P < .001 compared with untreated cells.