IL-33–induced angiogenesis and vascular hyperpermeability is mediated by the ST2 receptor. (A-C) HUVECs were pretransfected with control siRNA (Con siRNA) or ST2 siRNA before IL-33 treatment. Cells were harvested for assay at 40 hours after transfection. (A) Expression levels of ST2 were determined by reverse-transcription polymerase chain reaction and Western blotting. (B) After IL-33 stimulation, chemotaxis was performed. (C) Cells were collected and replated on Matrigel-coated plates at a density of 1.5 × 10⁵ cells/well and incubated with 20 ng/mL IL-33. Microphotographs were taken after 20 hours (×200). Tube networks were quantified with ImageJ software. (D) HUVECs were stimulated with IL-33 (20 ng/mL) for 1 hour. A [¹⁴C]sucrose permeability assay was then performed. Three independent experiments were performed in duplicate. Data are means ± SDs; *P < .05 vs control cell without IL-33; #P < .05 vs control cell with IL-33.