TGF-β receptor function in the endothelium

Genetic studies in mice and humans have revealed the pivotal role of transforming growth factor-beta (TGF-beta) signaling during angiogenesis. Mice deficient for various TGF-beta signaling components present an embryonic lethality due to vascular defects. In patients, mutations in the TGF-beta type I receptor ALK1 or in the accessory TGF-beta receptor endoglin are linked to an autosomal dominant disorder of vascular dysplasia termed Hereditary Haemorrhagic Telangiectasia (HHT). It has puzzled researchers for years to explain the effects of TGF-beta being a stimulator and an inhibitor of angiogenesis in vitro and in vivo. Recently, a model has been proposed in which TGFbeta by binding to the TGF-beta type II receptor can activate two distinct type I receptors in endothelial cells (ECs), i.e., the EC-restricted ALK1 and the broadly expressed ALK-5, which have opposite effects on ECs behavior. ALK1 via Smad1/5 transcription factors stimulates EC proliferation and migration, whereas ALK5 via Smad2/3 inhibits EC proliferation and migration. Here, the new findings are presented concerning the molecular mechanisms that take place in ECs to precisely regulate and even switch between TGF-beta-induced biological responses. In particular, the role of the accessory TGF-beta receptor endoglin in the regulation of EC behavior is addressed and new insights are discussed concerning the possible mechanisms that are implicated in the development of HHT. (C) 2004 European Society of Cardiology. Published by Elsevier B.V. All rights reserved.