Endoglin null endothelial cells proliferate faster and are more responsive to transforming growth factor β1 with higher affinity receptors and an activated Alk1 pathway

Endoglin is an accessory receptor for transforming growth factor beta(TGF beta) in endothelial cells, essential for vascular development. Its pivotal role in angiogenesis is underscored in Endoglin null (Eng(-/-)) murine embryos, which die at mid-gestation (E10.5) from impaired yolk sac vessel formation. Moreover, mutations in endoglin and the endothelial-specific TGF beta type I receptor, ALK1, are linked to hereditary hemorrhagic telangiectasia. To determine the role of endoglin in TGF beta pathways, we derived murine endothelial cell lines from Eng(-/-) and Eng(-/-) embryos (E9.0). Whereas Eng(+/ +) cells were only partially growth inhibited by TGF beta, Eng(-/-) cells displayed a potent anti-proliferative response. TGF beta-dependent Smad2 phosphorylation and Smad2/3 translocation were unchanged in the Eng(-/-) cells. In contrast, TGF beta treatment led to a more rapid activation of the Smad1/5 pathway in Eng null cells that was apparent at lower TGF beta concentrations. Enhanced activity of the Smad1 pathway in Eng(-/-) cells was reflected in higher expression of ALK1-dependent genes such as Id1, Smad6, and Smad7. Analysis of cell surface receptors revealed that the TGF beta type I receptor, ALK5, which is required for ALK1 function, was increased in Eng(-/-) cells. TGF beta receptor complexes were less numerous but displayed a higher binding affinity. These results suggest that endoglin modulates TGF beta signaling in endothelial cells by regulating surface TGF beta receptors and suppressing Smad1 activation. Thus an altered balance in TGF beta receptors and downstream Smad pathways may underlie defects in vascular development and homeostasis.