title: ALK5 phosphorylation of the endoglin cytoplasmic domain regulates Smad1/5/8 signaling and endothelial cell migration.
abstract: Endoglin, an endothelial cell-specific transforming growth factor-beta (TGF-beta) superfamily coreceptor, has an essential role in angiogenesis. Endoglin-null mice have an embryonic lethal phenotype due to defects in angiogenesis and mutations in endoglin result in the vascular disease hereditary hemorrhagic telangiectasia type I. Increased endoglin expression in the proliferating endothelium of tumors has been correlated with metastasis, tumor grade and decreased survival. Although endoglin is thought to regulate TGF-beta superfamily signaling in endothelial cells through regulating the balance between two TGF-beta-responsive pathways, the activin receptor-like kinase 5 (ALK5)/Smad2/3 pathway and the activin receptor-like kinase 1 (ALK1)/Smad1/5/8 pathway, the mechanism by which endoglin regulates angiogenesis has not been defined. Here, we investigate the role of the cytoplasmic domain of endoglin and its phosphorylation by ALK5 in regulating endoglin function in endothelial cells. We demonstrate that the cytoplasmic domain of endoglin is basally phosphorylated by ALK5, primarily on serines 646 and 649, in endothelial cells. Functionally, the loss of phosphorylation at serine 646 resulted in a loss of endoglin-mediated inhibition of Smad1/5/8 signaling in response to TGF-beta and endothelial cell migration, whereas loss of phosphorylation at both serines 646 and 649 resulted in a loss of endoglin-mediated inhibition of Smad1/5/8 signaling in response to bone morphogenetic protein-9. Taken together, these results support endoglin phosphorylation by ALK5 as an important mechanism for regulating TGF-beta superfamily signaling and migration in endothelial cells.
authors.affiliation: Department of Pharmacology and Cancer Biology, Duke University Medical Center, Durham, NC 27708, USA.
authors.forename: Bridgette N
authors.initials: BN
authors.lastname: Ray
authors.forename: Nam Y
authors.initials: NY
authors.lastname: Lee
authors.forename: Tam
authors.initials: T
authors.lastname: How
authors.forename: Gerard C
authors.initials: GC
authors.lastname: Blobe
mesh_terms: D000818:Animals; D019556:COS Cells; D002465:Cell Movement; D002522:Chlorocebus aethiops; D000071063:Endoglin; D042783:Endothelial Cells / Q000378:metabolism*; D055427:Growth Differentiation Factor 2 / Q000502:physiology; D047908:Intracellular Signaling Peptides and Proteins / Q000737:chemistry / Q000172:deficiency / Q000235:genetics / Q000502:physiology*; D051379:Mice; D018345:Mice, Knockout; D008907:Mink; D010766:Phosphorylation; D010768:Phosphoserine / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D017346:Protein Serine-Threonine Kinases / Q000502:physiology*; D017434:Protein Structure, Tertiary; D000077293:Receptor, Transforming Growth Factor-beta Type I; D018125:Receptors, Transforming Growth Factor beta / Q000502:physiology*; D015398:Signal Transduction / Q000187:drug effects; D051898:Smad1 Protein / Q000502:physiology*; D051902:Smad5 Protein / Q000502:physiology*; D051904:Smad8 Protein / Q000502:physiology*; D016212:Transforming Growth Factor beta / Q000502:physiology
chemical_list: D000071063:Endoglin; C502004:Eng protein, mouse; C526108:Gdf2 protein, mouse; D055427:Growth Differentiation Factor 2; D047908:Intracellular Signaling Peptides and Proteins; D018125:Receptors, Transforming Growth Factor beta; C497797:SMAD5 protein, human; D051898:Smad1 Protein; C497401:Smad1 protein, mouse; D051902:Smad5 Protein; D051904:Smad8 Protein; C491621:Smad9 protein, mouse; D016212:Transforming Growth Factor beta; D010768:Phosphoserine; D017346:Protein Serine-Threonine Kinases; D000077293:Receptor, Transforming Growth Factor-beta Type I; C000625841:TGFBR1 protein, human; C000625842:Tgfbr1 protein, mouse