Endoglin mediates fibronectin/α5β1 integrin and TGF-β pathway crosstalk in endothelial cells

Abstract

Both the transforming growth factor b (TGF-b) and integrin signalling pathways have well-established roles in angiogenesis. However, how these pathways integrate to regulate angiogenesis is unknown. Here, we show that the extracellular matrix component, fibronectin, and its cellular receptor, a5b1 integrin, specifically increase TGF-b1-and BMP-9-induced Smad1/5/8 phosphorylation via the TGF-b superfamily receptors endoglin and activin-like kinase-1 (ALK1). Fibronectin and a5b1 integrin increase Smad1/5/8 signalling by promoting endoglin/ ALK1 cell surface complex formation. In a reciprocal manner, TGF-b1 activates a5b1 integrin and downstream signalling to focal adhesion kinase (FAK) in an endoglindependent manner. a5b1 integrin and endoglin form a complex on the cell surface and co-internalize, with their internalization regulating a5b1 integrin activation and signalling. Functionally, endoglin-mediated fibronectin/ a5b1 integrin and TGF-b pathway crosstalk alter the responses of endothelial cells to TGF-b1, switching TGF-b1 from a promoter to a suppressor of migration, inhibiting TGF-b1-mediated apoptosis to promote capillary stability, and partially mediating developmental angiogenesis in vivo. These studies provide a novel mechanism for the regulation of TGF-b superfamily signalling and endothelial function through crosstalk with integrin signalling pathways.

Conditionally immortalized human pancreatic stellate cell lines demonstrate enhanced proliferation and migration in response to IGF-I

“…The enhanced endoglin expression by TPSC may contribute to increased responsiveness to profibrotic stimuli by TGF-β and thus promote the excessive fibrotic process in the pancreatic tumor stroma, as previously shown for murine hepatic stellate cells [20]. Furthermore, endoglin contains an RGD-sequence that enables cell adhesion via interaction with integrin receptors and ECM components such as fibronectin [21]. Endothelin-1 has previously been reported to be expressed by primary rat PSC and to stimulate their contraction and migration [22].…”

The role of endoglin in post-ischemic revascularization

“…For example, TGF-β signaling inhibition decreases retinal perfusion and impairs endothelial barrier function in mice [206], while other authors have reported a TGF-β-mediated inhibition of proangiogenic molecules [207] or a proangiogenic effect of TGF-β blockade [208]. Moreover, Smad1/5/8-mediated blockade of EC migration [209], and the inhibition of tip cell differentiation -with a migratory phenotype-by BMP9 signaling through ALK1-Smad1/5/8 have been described [40]. We should take into account, as discussed above, that different TGF-β concentrations may exert opposite effects on ECs, further complicating the dissection of the precise effect of endoglin.…”

“…Human endoglin displays the prototypic RGD motif involved in integrin-based interactions within its ZP juxtamembrane domain [84,85]. In addition, a physical and functional interplay between endoglin and β1 integrins has been reported [107,196,209,234]. More recently, endothelial endoglin has been identified as a ligand for leukocyte integrins [27].…”

“…Integrin α5β1 specifically recognizes the RGD motif in extracellular matrix proteins and is expressed by VSMCs, regulating their adhesion and recruitment to blood vessels [56]. Coimmunoprecipitation of endoglin with the α5 integrin subunit demonstrated that endoglin could bind to this integrin independently of the RGD motif [209]. These authors reported that endoglin inhibits 1 integrin subunit lysosomal degradation, thus promoting fibronectin/integrin 51 complex formation, which in turn induces endoglin/ALK1 complex formation and enhances TGF-1 activation of the Smad1/5/8 pathway.…”

Inhibition of development of laser-induced choroidal neovascularization with suppression of infiltration of macrophages in Smad3-null mice

“…53 However, it was reported that ALK1-endoglin complex is formed in vascular endothelial cells cultured on fibronectin matrix and mediates Smad1/5/8 signal in the absence of ALK5. 56 In the current experiment, HUVECs were cultured on fibronectin-rich (data not shown) fibroblast feeder later and thus might not require ALK5. The effect of promoting branching of the newly formed tube-like structure was more marked with SB431542 as compared with the samples supplemented with SIS3 in TGFβ1-plus culture.…”