Effects of Dll4 expression on endothelial cell proliferation. (A) HUVECs transduced 3 to 10 days earlier with either vector only or Dll4 (70%-90% of cells expressing EGFP) were cultured for 72 hours in medium alone or medium supplemented with VEGF-A (25 ng/mL); proliferation was measured by [³H] thymidine deoxyribose uptake during the last 18 hours of culture. The results reflect the means (± SEMs) from 5 separate experiments, each performed in triplicate cultures. ^*P < .001. (B) Dose dependency of VEGF-A (3-24 ng/mL) induced HUVEC proliferation 3 to 10 days after transduction with vector only or Dll4 retrovirus. Cell proliferation was measured as described in “Materials and methods”; the results reflect the means (± SDs) of triplicate cultures (representative of 4 determinations). (C) Analysis of HUVEC proliferation in response to bFGF (3-28 ng/mL) 3 to 10 days after transduction with vector alone or Dll4 retrovirus. Cell proliferation was measured as described in “Materials and methods”; the results reflect the means (± SDs) of triplicate determinations (representative of 4 determinations).