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

Abstract

Endoglin is an accessory receptor for transforming growth factor β(TGFβ) 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 TGFβ type I receptor, ALK1, are linked to hereditary hemorrhagic telangiectasia. To determine the role of endoglin in TGFβ pathways, we derived murine endothelial cell lines from Eng+/+ and Eng -/- embryos (E9.0). Whereas Eng+/+ cells were only partially growth inhibited by TGFβ, Eng-/- cells displayed a potent anti-proliferative response. TGFβ-dependent Smad2 phosphorylation and Smad2/3 translocation were unchanged in the Eng-/- cells. In contrast, TGFβ treatment led to a more rapid activation of the Smad1/5 pathway in Eng null cells that was apparent at lower TGFβ 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 TGFβ type I receptor, ALK5, which is required for ALK1 function, was increased in Eng-/- cells. TGFβ receptor complexes were less numerous but displayed a higher binding affinity. These results suggest that endoglin modulates TGFβ signaling in endothelial cells by regulating surface TGFβ receptors and suppressing Smad1 activation. Thus an altered balance in TGFβ receptors and downstream Smad pathways may underlie defects in vascular development and homeostasis. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.