BMP-9 stimulation of ALK1 suppresses VEGF expression. HAECs were transfected with scrambled control siRNA or 2 concentrations of ALK1 siRNA to reduce ALK1 expression to 50% and 15% of control, respectively. The following day, the cells were treated with TGFβ1 (1 ng/mL) and BMP-9 (0-10 ng/mL) for 24 hours. ALK1 expression was determined by real-time PCR (A). VEGF expression was determined by real-time PCR, and confirmed by immunoblotting of VEGF from the media (B). Cell proliferation was determined by ³H-thymidine incorporation (C). Asterisks indicate statistically significant differences compared with control (no TGFβ1 or BMP-9). *P < .05; **P < .01; ***P < .001; Tukey test.