Carmen Langa

Endoglin is a transforming growth factor-beta (TGF-beta) co-receptor expressed mainly on endothelial cells and involved in cardiovascular development, angiogenesis, and vascular remodeling. This is illustrated by the fact that mutations in the endoglin gene give rise to hereditary hemorrhagic telangiectasia type 1, a dominant vascular disease with clinical manifestations that originate by a mechanism of haploinsufficiency. Thus, studies on the regulated expression of endoglin are crucial to devising therapeutic strategies for hereditary hemorrhagic telangiectasia type 1. Endoglin is highly expressed in the neovasculature associated with hypoxia such as ischemic tissues and tumors, but the molecular mechanism of this up-regulation is unknown. Here, we have investigated the possible regulation of endoglin expression by hypoxia. Surface protein, transcript, and promoter activity levels of endoglin were found to be up-regulated by hypoxia, indicating that the regulation takes place at the transcriptional level. A hypoxia-responsive element downstream of the main transcription start site of the endoglin gene was functionally characterized. Whereas hypoxia alone moderately stimulated endoglin transcription, addition of TGF-beta under hypoxic conditions resulted in transcriptional cooperation between both signaling pathways, leading to marked stimulation of endoglin expression. Because basal endoglin transcription is sustained by Sp1, and TGF-beta and hypoxia signaling pathways are mediated by Smad proteins and hypoxia-inducible factor-1 (HIF-1), respectively, the involvement of these transcription factors was analyzed. Functional and co-immunoprecipitation experiments demonstrated the existence of a multiprotein complex (Sp1.Smad3.HIF-1) on the endoglin promoter, mediating the cooperation between the hypoxia and TGF-beta pathways. Within this multiprotein complex, Smad3 appears to function not only as a coactivator factor, but also as an adaptor between HIF-1 and Sp1. We propose that basal endoglin transcription (highly dependent on Sp1) may switch from a constitutive to an inducible state through Sp1 interaction with HIF-1 and Smad transcription factors, induced by hypoxia and TGF-beta, respectively.

Endoglin is a homodimeric membrane glycoprotein which can bind the beta 1 and beta 3 isoforms of transforming growth factor-beta (TGF-beta). We reported previously that endoglin is upregulated during monocyte differentiation. We have now observed that TGF-beta itself can stimulate the expression of endoglin in cultured human monocytes and in the U-937 monocytic line. To study the functional role of endoglin, stable transfectants of U-937 cells were generated which overexpress L- or S- endoglin isoforms, differing in their cytoplasmic domain. Inhibition of cellular proliferation and downregulation of c-myc mRNA which are normally induced by TGF-beta 1 in U-937 cells were totally abrogated in L-endoglin transfectants and much reduced in the S-endoglin transfectants. Inhibition of proliferation by TGF-beta 2 was not altered in the transfectants, in agreement with the isoform specificity of endoglin. Additional responses of U-937 cells to TGF-beta 1, including stimulation of fibronectin synthesis, cellular adhesion, platelet/endothelial cell adhesion molecule 1 (PECAM-1) phosphorylation, and homotypic aggregation were also inhibited in the endoglin transfectants. However, modulation of integrin and PECAM-1 levels and stimulation of mRNA levels for TGF-beta 1 and its receptors R-I, R-II, and betaglycan occurred normally in the endoglin transfectants. No changes in total ligand binding were observed in L-endoglin transfectants relative to mock, while a 1.5-fold increase was seen in S-endoglin transfectants. The degradation rate of the ligand was the same in all transfectants. Elucidating the mechanism by which endoglin modulates several cellular responses to TGF-beta 1 without interfering with ligand binding or degradation should increase our understanding of the complex pathways which mediate the effects of this factor.

Endoglin (CD105) is the target gene for the hereditary hemorrhagic telangiectasia type I (HHT1), a dominantly inherited vascular disorder. It shares with betaglycan a limited amino acid sequence homology and being components of the membrane transforming growth factor-beta (TGF-beta) receptor complex. Using rat myoblasts as a model system, we found that overexpression of endoglin led to a decreased TGF-beta response to cellular growth inhibition and plasminogen activator inhibitor-1 synthesis, whereas overexpression of betaglycan resulted in an enhanced response to inhibition of cellular proliferation and plasminogen activator inhibitor-1 induced expression in the presence of TGF-beta. The regulation by endoglin of TGF-beta responses seems to reside on the extracellular domain, as evidenced by the functional analysis of two chimeric proteins containing different combinations of endoglin and betaglycan domains. Binding followed by cross-linking with 125I-TGF-beta1 demonstrated that betaglycan expressing cells displayed a clear increase (about 3. 5-fold), whereas endoglin expressing cells only displayed an slight increment (about 1.6-fold) in ligand binding with respect to mock transfectants. SDS-polyacrylamide gel electrophoresis analysis of radiolabeled receptors demonstrated that expression of endoglin or betaglycan is associated with an increased TGF-beta binding to the signaling receptor complex; however, while endoglin increased binding to types I and II receptors, betaglycan increased the binding to the type II receptor. Conversely, we found that TGF-beta binding to endoglin required the presence of receptor type II as evidenced by transient transfections experiments in COS cells. These findings suggest a role for endoglin in TGF-beta responses distinct from that of betaglycan.

Transforming growth factor-beta (TGF-beta) signaling in endothelial cells is able to modulate angiogenesis and vascular remodeling, although the underlying molecular mechanisms remain poorly understood. Endoglin and ALK-1 are components of the TGF-beta receptor complex, predominantly expressed in endothelial cells, and mutations in either endoglin or ALK-1 genes are responsible for the vascular dysplasia known as hereditary hemorrhagic telangiectasia. Here we find that the extracellular and cytoplasmic domains of the auxiliary TGF-beta receptor endoglin interact with ALK-1 (a type I TGF-beta receptor). In addition, endoglin potentiates TGF-beta/ALK1 signaling, with the extracellular domain of endoglin contributing to this functional cooperation between endoglin and ALK-1. By contrast, endoglin appears to interfere with TGF-beta/ALK-5 signaling. These results suggest that the functional association of endoglin with ALK-1 is critical for the endothelial responses to TGF-beta.