title: Albumin endocytosis in endothelial cells induces TGF-beta receptor II signaling.
abstract: Vascular endothelial cells undergo albumin endocytosis using a set of albumin binding proteins. This process is important for maintaining cellular homeostasis. We showed by several criteria that the previously described 73-kDa endothelial cell surface albumin binding protein is the 75-kDa transforming growth factor (TGF)-beta receptor type II (TbetaRII). Albumin coimmunoprecipitated with TbetaRII from a membrane fraction from rat lung microvascular endothelial cells. Albumin endocytosis-negative COS-7 cells became albumin endocytosis competent when transfected with wild-type TbetaRII but not when transfected with a domain-negative kinase mutant of TbetaRII. An antibody specific for TbetaRII inhibited albumin endocytosis. A mink lung epithelial cell line, which expresses both the TGF-beta receptor type I (TbetaRI) and the TbetaRII receptor, exhibited albumin binding to the cell surface and endocytosis. In contrast, mutant L-17 and DR-26 cells lacking TbetaRI or TbetaRII, respectively, each showed a dramatic reduction in binding and endocytosis. Albumin endocytosis induced Smad2 phosphorylation and Smad4 translocation as well as increased protein expression of the inhibitory Smad, Smad7. We identified regions of significant homology between amino acid sequences of albumin and TGF-beta, suggesting a structural basis for the interaction of albumin with the TGF-beta receptors and subsequent activation of TbetaRII signaling. The observed albumin-induced internalization of TbetaRII signaling may be an important mechanism in the vessel wall for controlling TGF-beta responses in endothelial cells.
authors.affiliation: Dept. of Pharmacology, College of Medicine, Univ. of Illinois, 835 South Wolcott Ave. (M/C 868 Chicago, IL 60612, USA. SSIDDIQU@UIC.EDU
authors.forename: Shahid S
authors.initials: SS
authors.lastname: Siddiqui
authors.forename: Zeba K
authors.initials: ZK
authors.lastname: Siddiqui
authors.forename: Asrar B
authors.initials: AB
authors.lastname: Malik
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001692:Biological Transport; D019556:COS Cells; D002478:Cells, Cultured; D002522:Chlorocebus aethiops; D018360:Crystallography, X-Ray; D004705:Endocytosis; D004730:Endothelium, Vascular / Q000166:cytology / Q000502:physiology*; D005822:Genetic Vectors; D006801:Humans; D008969:Molecular Sequence Data; D017346:Protein Serine-Threonine Kinases; D011652:Pulmonary Circulation / Q000502:physiology*; D051381:Rats; D000077294:Receptor, Transforming Growth Factor-beta Type II; D018125:Receptors, Transforming Growth Factor beta / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D012709:Serum Albumin / Q000493:pharmacokinetics*; D015398:Signal Transduction / Q000502:physiology*; D014162:Transfection
chemical_list: D018125:Receptors, Transforming Growth Factor beta; D011994:Recombinant Proteins; D012709:Serum Albumin; D017346:Protein Serine-Threonine Kinases; D000077294:Receptor, Transforming Growth Factor-beta Type II