title: Saturation of, and competition for entry into, the apical secretory pathway.
abstract: To investigate mechanisms of apical sorting in the secretory pathway of epithelial cells, we expressed varying amounts of the 165 amino acid isoform of vascular endothelial growth factor (VEGF(165)) and transforming growth factor beta1 (TGF-beta1) via replication defective adenoviruses. Apical sorting of both proteins was efficient at low expression levels but saturated or was reversed at high expression levels. High expression levels of TGF-beta1 were effective at competing VEGF(165) out of the apical pathway; however, VEGF(165) did not compete out TGF-beta1. Tunicamycin inhibition experiments showed that the apical polarity of VEGF(165) was independent of N-glycosylation. We conclude that the apical sorting of these two molecules is a saturable, signal-mediated process, involving competition for apical sorting receptors. The sorting of the two proteins does not appear to involve N-glycans as sorting signals, or lectin sorters. The observations are particularly relevant to gene therapy because they demonstrate that overexpression of a transgene can result in undesirable missorting of the encoded protein.
authors.affiliation: Margaret M. Dyson Vision Research Institute, Department of Ophthalmology, Cornell University Medical College, 1300 York Avenue, New York, NY 10021, USA.
authors.forename: A D
authors.initials: AD
authors.lastname: Marmorstein
authors.forename: K G
authors.initials: KG
authors.lastname: Csaky
authors.forename: J
authors.initials: J
authors.lastname: Baffi
authors.forename: L
authors.initials: L
authors.lastname: Lam
authors.forename: F
authors.initials: F
authors.lastname: Rahaal
authors.forename: E
authors.initials: E
authors.lastname: Rodriguez-Boulan
mesh_terms: D000256:Adenoviridae / Q000235:genetics; D002460:Cell Line; D016228:Endothelial Growth Factors / Q000235:genetics*; D005786:Gene Expression Regulation / Q000187:drug effects / Q000235:genetics*; D005822:Genetic Vectors; D007150:Immunohistochemistry; D008222:Lymphokines / Q000235:genetics*; D014161:Transduction, Genetic; D016212:Transforming Growth Factor beta / Q000235:genetics*; D014415:Tunicamycin / Q000494:pharmacology; D042461:Vascular Endothelial Growth Factor A; D042442:Vascular Endothelial Growth Factors
chemical_list: D016228:Endothelial Growth Factors; D008222:Lymphokines; D016212:Transforming Growth Factor beta; C467484:VEGFA protein, human; D042461:Vascular Endothelial Growth Factor A; D042442:Vascular Endothelial Growth Factors; D014415:Tunicamycin