Extracellular and Cytoplasmic Domains of Endoglin Interact with the Transforming Growth Factor-β Receptors I and II

Abstract

Endoglin is an auxiliary component of the transforming growth factor-␤ (TGF-␤) receptor system, able to associate with the signaling receptor types I (T␤RI) and II (T␤RII) in the presence of ligand and to modulate the cellular responses to TGF-␤1. Endoglin cannot bind ligand on its own but requires the presence of the signaling receptors, supporting a critical role for the interaction between endoglin and T␤RI or T␤RII. This study shows that full-length endoglin interacts with both T␤RI and T␤RII, independently of their kinase activation state or the presence of exogenous TGF-␤1. Truncated constructs encoding either the extracellular or the cytoplasmic domains of endoglin demonstrated that the association with the signaling receptors occurs through both extracellular and cytoplasmic domains. However, a more specific mapping revealed that the endoglin/T␤RI interaction was different from that of endoglin/T␤RII. T␤RII interacts with the amino acid region 437-558 of the extracellular domain of endoglin, whereas T␤RI interacts not only with the region 437-558 but also with the protein region located between amino acid 437 and the N terminus. Both T␤RI and T␤RII interact with the cytoplasmic domain of endoglin, but T␤RI only interacts when the kinase domain is inactive, whereas T␤RII remains associated in its active and inactive forms. Upon association, T␤RI and T␤RII phosphorylate the endoglin cytoplasmic domain, and then T␤RI, but not T␤RII, kinase dissociates from the complex. Conversely, endoglin expression results in an altered phosphorylation state of T␤RII, T␤RI, and downstream Smad proteins as well as a modulation of TGF-␤ signaling, as measured by the reporter gene expression. These results suggest that by interacting through its extracellular and cytoplasmic domains with the signaling receptors, endoglin might affect TGF-␤ responses.

Characterization of murine S-endoglin isoform and its effects on tumor development

“…A protein of size identical to that of S-endoglin was detected in two murine endothelial cell lines by Western blotting, suggesting that both L-and S-endoglin isoforms could be synthesized in blood vessels. Both human L-and S-endoglin proteins form disulfide-linked homodimers at the cell surface (Gougos and Letarte, 1990;Bellon et al, 1993), at least through the Cys-582 residue present in the extracellular domain (Guerrero-Esteo et al, 2002). Furthermore, interspecies dimerization has been found between human and mouse L-endoglin (Raab et al, 1999).…”

“…Therefore, in cells in which both isoforms are cosynthesized, the presence of L-S heterodimers, in addition to L-L and S-S homodimers, would add a new level of functional complexity to the endoglin molecule. In this respect, it is tempting to speculate that the ability to modulate TGF-b responses, as a result of the interaction of the L and S cytoplasmic domains with TGF-b receptors (Guerrero-Esteo et al, 2002), or the association with cytoskeletal components (Conley et al, 2004;Sanz-Rodriguez et al, 2004) would be different for the distinct endoglin dimers. Endoglin is upregulated in angiogenic endothelial cells, and several studies suggest that it is required for angiogenesis (reviewed in Li et al, 2001;Duff et al, 2003).…”

“…The full-length cDNAs encoding human L-or Sendoglin, cloned in pcEXV vector (Lastres et al, 1996), were also subcloned into the pcDNA3 expression vector. The TMCT-L-endoglin construct, encoding a truncated human L-endoglin product, which lacks the extracellular domain, and the wild-type human L-endoglin construct, both tagged with the influenza hemagglutinin (HA) epitope, subcloned into the pDisplay vector, were previously described (Guerrero-Esteo et al, 2002). The cDNA construct encoding wild-type human S-endoglin fused to the epitope Flag was obtained by amplifying the S-endoglin cDNA, using the following primers: 5 0 -GCACAGAATTCCACGTGGACAGCATGGACCGC-3 0 (nucleotides 326-358) and 5 0 -CGGCATGGTACCTGTGGG GGCCTGGGGTACTCACG-3 0 (nucleotides 2200-2235).…”

Chronic idiopathic myelofibrosis: independent prognostic importance of bone marrow microvascular density evaluated by CD105 (endoglin) immunostaining

“…35,36 In CIMF, TGF-b is strongly expressed by megakaryocytes, 37,38 a large proportion of which express CD105.…”

Endoglin expression in breast tumor cells suppresses invasion and metastasis and correlates with improved clinical outcome

“…In endothelial cells, endoglin associates at the cell surface with the type I and type II TGF-b receptors and has a key role in modulating downstream signaling to the Smad transducer proteins (Guerrero-Esteo et al, 2002;Lebrin et al, 2004). The importance of endoglin acting as a TGF-b co-receptor in these events is evidenced by the observation that mutations in either ENG or the type I receptor ALK1 give rise to the vascular disorder hereditary hemorrhagic telangiectasia (McAllister et al, 1994;Johnson et al, 1996).…”