title: TfR2 localizes in lipid raft domains and is released in exosomes to activate signal transduction along the MAPK pathway.
abstract: Transferrin receptor 2 (TfR2) possesses a YQRV motif similar to the YTRF motif of transferrin receptor 1 (TfR1) responsible for the internalization and secretion through the endosomal pathway. Raft biochemical dissection showed that TfR2 is a component of the low-density Triton-insoluble (LDTI) plasma membrane domain, able to co-immunoprecipitate with caveolin-1 and CD81, two structural raft proteins. In addition, subcellular fractionation experiments showed that TfR1, which spontaneously undergoes endocytosis and recycling, largely distributed to intracellular organelles, whereas TfR2 was mainly associated with the plasma membrane. Given the TfR2 localization in lipid rafts, we tested its capability to activate cell signalling. Interaction with an anti-TfR2 antibody or with human or bovine holotransferrin showed that it activated ERK1/ERK2 and p38 MAP kinases. Integrity of lipid rafts was required for MAPK activation. Co-localization of TfR2 with CD81, a raft tetraspanin exported through exosomes, prompted us to investigate exosomes released by HepG2 and K562 cells into culture medium. TfR2, CD81 and to a lesser extent caveolin-1, were found to be part of the exosomal budding vesicles. In conclusion, the present study indicates that TfR2 localizes in LDTI microdomains, where it promotes cell signalling, and is exported out of the cells through the exosome pathway, where it acts as an intercellular messenger.
authors.affiliation: Department of Hematology, Oncology and Molecular Medicine, Istituto Superiore di Sanità, Viale Regina Elena 299, 00161 Rome, Italy.
authors.forename: Alessia
authors.initials: A
authors.lastname: Calzolari
authors.forename: Carla
authors.initials: C
authors.lastname: Raggi
authors.forename: Silvia
authors.initials: S
authors.lastname: Deaglio
authors.forename: Nadia Maria
authors.initials: NM
authors.lastname: Sposi
authors.forename: Marit
authors.initials: M
authors.lastname: Stafsnes
authors.forename: Katia
authors.initials: K
authors.lastname: Fecchi
authors.forename: Isabella
authors.initials: I
authors.lastname: Parolini
authors.forename: Fabio
authors.initials: F
authors.lastname: Malavasi
authors.forename: Cesare
authors.initials: C
authors.lastname: Peschle
authors.forename: Massimo
authors.initials: M
authors.lastname: Sargiacomo
authors.forename: Ugo
authors.initials: U
authors.lastname: Testa
mesh_terms: D015703:Antigens, CD / Q000378:metabolism; D015153:Blotting, Western; D006528:Carcinoma, Hepatocellular / Q000378:metabolism; D051242:Caveolin 1 / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured / Q000378:metabolism; D004705:Endocytosis; D005434:Flow Cytometry; D006801:Humans; D047468:Immunoprecipitation; D020014:K562 Cells / Q000378:metabolism; D008113:Liver Neoplasms / Q000378:metabolism; D021962:Membrane Microdomains / Q000378:metabolism*; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D010766:Phosphorylation; D011990:Receptors, Transferrin / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction; D013347:Subcellular Fractions; D060225:Tetraspanin 28; D022161:Transport Vesicles / Q000378:metabolism*
chemical_list: D015703:Antigens, CD; C554798:CD81 protein, human; D051242:Caveolin 1; D011990:Receptors, Transferrin; C503640:TFR2 protein, human; D060225:Tetraspanin 28; D020928:Mitogen-Activated Protein Kinases