title: Early endosomal regulation of Smad-dependent signaling in endothelial cells.
abstract: Transforming growth factor beta (TGFbeta) receptors require SARA for phosphorylation of the downstream transducing Smad proteins. SARA, a FYVE finger protein, binds to membrane lipids suggesting that activated receptors may interact with downstream signaling molecules at discrete endocytic locations. In the present study, we reveal a critical role for the early endocytic compartment in regulating Smad-dependent signaling. Not only is SARA localized on early endosomes, but also its minimal FYVE finger sequence is sufficient for early endosomal targeting. Expression of a SARA mutant protein lacking the FYVE finger inhibits downstream activin A signaling in endothelial cells. Moreover, a dominant-negative mutant of Rab5, a crucial protein for early endosome dynamics, causes phosphorylation and nuclear translocation of Smads leading to constitutive (i.e. ligand independent) transcriptional activation of a Smad-dependent promoter in endothelial cells. As inhibition of endocytosis using the K44A negative mutant of dynamin and RN-tre did not lead to activation of Smad-dependent transcription, the effects of the dominant-negative Rab5 are likely to be a consequence of altered membrane trafficking of constitutively formed TGFbeta/activin type I/II receptor complexes at the level of early endosomes. The results suggest an important interconnection between early endosomal dynamics and TGFbeta/activin signal transduction pathways.
authors.affiliation: Laboratory of Biological Chemistry, University of Ioannina Medical School, 45110 Ioannina, Greece.
authors.forename: Ekaterini
authors.initials: E
authors.lastname: Panopoulou
authors.forename: David J
authors.initials: DJ
authors.lastname: Gillooly
authors.forename: Jeffrey L
authors.initials: JL
authors.lastname: Wrana
authors.forename: Marino
authors.initials: M
authors.lastname: Zerial
authors.forename: Harald
authors.initials: H
authors.lastname: Stenmark
authors.forename: Carol
authors.initials: C
authors.lastname: Murphy
authors.forename: Theodore
authors.initials: T
authors.lastname: Fotsis
mesh_terms: D000818:Animals; D002352:Carrier Proteins / Q000502:physiology; D002417:Cattle; D002478:Cells, Cultured; D004268:DNA-Binding Proteins / Q000502:physiology*; D011992:Endosomes / Q000502:physiology*; D004730:Endothelium, Vascular / Q000502:physiology*; D007700:Kinetics; D010766:Phosphorylation; D011401:Promoter Regions, Genetic; D018125:Receptors, Transforming Growth Factor beta / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology*; D051785:Smad Proteins; D015534:Trans-Activators / Q000502:physiology*; D016335:Zinc Fingers / Q000502:physiology; D020696:rab5 GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism
chemical_list: D002352:Carrier Proteins; D004268:DNA-Binding Proteins; D018125:Receptors, Transforming Growth Factor beta; D051785:Smad Proteins; D015534:Trans-Activators; D020696:rab5 GTP-Binding Proteins