title: Co-clustering of beta 1 integrins, cytoskeletal proteins, and tyrosine-phosphorylated substrates during integrin-mediated leukocyte aggregation.
abstract: The involvement of the VLA-4 integrin in an alternative leukocyte homotypic adhesion mechanism that is LFA-1/ICAM-1 independent, has been previously reported. We describe here the localization of beta 1 and alpha 4 integrin subunits at sites of cell-cell contact, on both beta 1- and alpha 4-induced aggregates of B lymphoblastoid Ramos cells. Moreover, the distribution of different VLA-alpha subunits was also examined on beta 1-induced cell aggregates of alpha 2- and alpha 4-transfected K-562 cells. Both alpha 2 and alpha 4 integrin subunits were mainly localized at sites of intercellular boundaries, suggesting a possible role for these integrins in leukocyte intercellular adhesion. The fibronectin receptor alpha 5 subunit was either diffuse throughout the plasma membrane, or displayed some accumulation at sites of cell-cell contact. Even though homotypic aggregation of U-937 cells was induced with the anti-alpha 5 P1D6 mAb, the alpha 5 subunit showed only partial redistribution to regions of cell-cell contact, compared with the complete redistribution of the alpha 4 subunit in the alpha 4-induced aggregates. The reorganization of the actin-cytoskeleton was observed at sites of intercellular boundaries in both the anti-beta 1- and anti-alpha 4-induced cell aggregates. Hence, F-actin and the cytoskeletal protein talin co-localized with beta 1 and alpha 4 integrin clusters at sites of cell-cell contact. Signal transduction during VLA-mediated homotypic cell adhesion has also been investigated. We found co-localization of beta 1 and alpha 4 subunits with tyrosine-phosphorylated proteins at cell-cell contact regions during cell aggregation. These data indicate that VLA integrin-mediated leukocyte aggregation results in clustering of beta 1-integrins at sites of cell-cell contact, together with co-localization of cytoskeletal proteins. These results also suggest that protein tyrosine phosphorylation is an important signal transduction mechanism when VLA integrins participate in intercellular contacts.
authors.affiliation: Sección de Inmunología, Hospital de la Princesa, Universidad Autónoma de Madrid, Spain.
authors.forename: P
authors.initials: P
authors.lastname: Sánchez-Mateos
authors.forename: M R
authors.initials: MR
authors.lastname: Campanero
authors.forename: M A
authors.initials: MA
authors.lastname: Balboa
authors.forename: F
authors.initials: F
authors.lastname: Sánchez-Madrid
mesh_terms: D000199:Actins / Q000032:analysis; D002448:Cell Adhesion; D002449:Cell Aggregation; D002450:Cell Communication; D003598:Cytoskeletal Proteins / Q000502:physiology*; D006801:Humans; D019012:Integrin beta1; D016023:Integrins / Q000502:physiology*; D007962:Leukocytes / Q000502:physiology*; D010766:Phosphorylation; D017469:Receptors, Fibronectin / Q000032:analysis; D016029:Receptors, Very Late Antigen / Q000032:analysis; D014407:Tumor Cells, Cultured; D014443:Tyrosine / Q000378:metabolism*
chemical_list: D000199:Actins; D003598:Cytoskeletal Proteins; D019012:Integrin beta1; D016023:Integrins; D017469:Receptors, Fibronectin; D016029:Receptors, Very Late Antigen; D014443:Tyrosine