Inhibition of the kinase activity of Alk5 impairs PGE₂-induced angiogenesis in vitro. Top panel: HUVECs were pretreated with vehicle or the Alk5 inhibitor SB431542 (5 μM) for 15 minutes and then seeded onto growth factor reduced Matrigel in the presence or absence (NS) of 1 μM PGE₂. Formation of cords was quantified after 5 hours. Data are presented as the fold induction (FI) above cord formation by untreated cells, and are the means plus or minus SD of 3 independent experiments. The absolute value corresponding to FI = 1 was 65 cords/field. All experiments were run in triplicate. *P < .001 vs control; #P < .01 vs PGE₂ alone. Bottom panel: photographs showing representative fields from experiments quantified in the top panel.