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RECEPTOR-II SIGNALING
1 Department of Pharmacology, College of Medicine, University of Illinois, Chicago, IL, USA
* To whom correspondence should be addressed. E-mail: ssiddiqu{at}uic.edu.
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 TGF-
receptor type II (TRII). Albumin coimmunoprecipitated with TRII from a membrane fraction from rat lung microvascular
endothelial cells (RLMVEC). Albumin endocytosis-negative COS-7 cells became albumin endocytosis competent when transfected with wild type TRII; but not when
transfected with a domain-negative kinase mutant of TRII. An antibody specific for TRII inhibited albumin endocytosis. A mink lung epithelial cell line, which expresses both the TRI and the TRII receptor, exhibited albumin binding to the cell surface and endocytosis. In contrast, mutant L-17 and DR-26 cells lacking TRI or TRII, respectively, each showed a dramatic reduction in binding and endocytosis. Albumin endocytosis induced Smad2 phosphorylation and Smad2-P 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-, suggesting a structural basis for the interaction of albumin with the TGF- receptors and subsequent activation of TRII signaling. The observed albumin-induced
internalization of TRII signaling may be an important mechanism in the vessel wall for controlling TGF- responses in endothelial cells.
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