Genetic and molecular studies suggest that activin receptor-like kinase 1 (ALK1), a transforming growth factor β (TGF-β) type I receptor and endoglin, a TGF-β co-receptor, play an essential role in... Show moreGenetic and molecular studies suggest that activin receptor-like kinase 1 (ALK1), a transforming growth factor β (TGF-β) type I receptor and endoglin, a TGF-β co-receptor, play an essential role in vascular development and pathological angiogenesis. Several agents that interfere with ALK1 and endoglin function are currently in clinical trials for anti-angiogenic activity in cancer therapy. One of these agents, PF-03446962 (anti-hALK1 antibody), shows promising results in the clinic. However, its effects on endothelial cell function and mechanism of action are unclear. Here we demonstrate that anti-hALK1 antibody selectively recognizes human ALK1. The anti hALK1-antibody interfered with bone morphogenetic protein 9 (BMP9)-induced signaling in endothelial cells. Consistent with this notion, anti-hALK1 antibody was found to compete highly efficiently with the binding of the ALK1 ligand BMP9 to ALK1. Moreover, it prevented BMP9-dependent recruitment of co-receptor endoglin into this angiogenesis-mediating signaling complex. In addition, we demonstrated that anti-hALK1 antibody inhibited endothelial cell sprouting, but did not directly interfere with vascular endothelial growth factor (VEGF) signaling, VEGF-induced proliferation and migration of endothelial cells. Finally, we demonstrated that BMP9 in serum is essential for endothelial sprouting and that anti-hALK1 antibody inhibits this potently. Our data suggest that both the VEGF-VEGFR and the BMP9-ALK1 pathways are essential for stimulating angiogenesis, and targeting both pathways simultaneously is an attractive strategy to overcome resistance to anti-angiogenesis therapy. Show less
