title: The opsonin MFG-E8 is a ligand for the alphavbeta5 integrin and triggers DOCK180-dependent Rac1 activation for the phagocytosis of apoptotic cells.
abstract: Opsonization of apoptotic cells facilitates recognition by phagocytes for the rapid clearance of potentially inflammatory cellular material. The secreted glycoprotein Milk Fat Globule Factor-E8 (MFG-E8) is a member of this family of bridging molecules and is believed to bind phosphatidylserine (PS) on the dying cell, linking it to integrin receptors on the phagocyte. Here we report the characterization of a functional signaling module involving MFG-E8, alphavbeta5 integrin, and DOCK180 for the activation of Rac1. We show that MFG-E8 and DOCK180 are both expressed in phagocytic-competent primary immature dendritic cells (DCs) and DC2.4 cells, and are potently down-regulated upon DC maturation, consistent with their role in phagocytosis and antigen capture. Coexpression of MFG-E8 with alphavbeta5 integrin potentiated integrin-mediated Rac1 activation, which was abrogated by mutagenesis in the RGD motif in MFG-E8. Moreover, expression of antisense DOCK180 abrogated MFG-E8-alphavbeta5-mediated Rac activation and impaired the phagocytosis of apoptotic cells. These data demonstrate a biochemical link between an opsonin of apoptotic cells, the alphavbeta5 integrin, and the Crk-DOCK180-Rac1 pathway, and importantly, show that MFG-E8 and DOCK180 are expressed according to the functional status of the phagocyte.
authors.affiliation: Department of Biochemistry and Molecular Biology, UMDNJ-New Jersey Medical School, 07103, Newark, NJ, USA
authors.forename: Shin
authors.initials: S
authors.lastname: Akakura
authors.forename: Sukhwinder
authors.initials: S
authors.lastname: Singh
authors.forename: Matthew
authors.initials: M
authors.lastname: Spataro
authors.forename: Reiko
authors.initials: R
authors.lastname: Akakura
authors.forename: Jong-Il
authors.initials: JI
authors.lastname: Kim
authors.forename: Matthew L
authors.initials: ML
authors.lastname: Albert
authors.forename: Raymond B
authors.initials: RB
authors.lastname: Birge
mesh_terms: D020816:Amino Acid Motifs / Q000235:genetics; D000818:Animals; D000954:Antigens, Surface; D017209:Apoptosis / Q000502:physiology*; D002462:Cell Membrane / Q000378:metabolism; D003713:Dendritic Cells / Q000378:metabolism; D015536:Down-Regulation / Q000502:physiology; D006801:Humans; D016023:Integrins / Q000378:metabolism*; D008024:Ligands; D008562:Membrane Glycoproteins / Q000378:metabolism*; D051379:Mice; D008894:Milk Proteins; D020320:Oligoribonucleotides, Antisense / Q000494:pharmacology; D009895:Opsonin Proteins / Q000378:metabolism; D010586:Phagocytes / Q000378:metabolism*; D010587:Phagocytosis / Q000502:physiology*; D019030:Receptors, Vitronectin / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology; D020744:rac GTP-Binding Proteins / Q000378:metabolism*; D020830:rac1 GTP-Binding Protein / Q000378:metabolism*
chemical_list: D000954:Antigens, Surface; C099818:DOCK1 protein, human; D016023:Integrins; D008024:Ligands; C098942:MFGE8 protein, human; D008562:Membrane Glycoproteins; C490919:Mfge8 protein, mouse; D008894:Milk Proteins; D020320:Oligoribonucleotides, Antisense; D009895:Opsonin Proteins; D019030:Receptors, Vitronectin; C064660:integrin alphaVbeta5; D020744:rac GTP-Binding Proteins; D020830:rac1 GTP-Binding Protein