title: Auto-oxidation and oligomerization of protein S on the apoptotic cell surface is required for Mer tyrosine kinase-mediated phagocytosis of apoptotic cells.
abstract: Prompt phagocytosis of apoptotic cells prevents inflammatory and autoimmune responses to dying cells. We have previously shown that the blood anticoagulant factor protein S stimulates phagocytosis of apoptotic human B lymphoma cells by human monocyte-derived macrophages. In this study, we show that protein S must first undergo oxidative activation to stimulate phagocytosis. Binding of human protein S to apoptotic cells or to phosphatidylserine multilamellar vesicles promotes auto-oxidation of Cys residues in protein S, resulting in covalent, disulfide-linked dimers and oligomers that preferentially bind to and activate the human Mer tyrosine kinase (MerTK) receptor on the macrophages. The prophagocytic activity of protein S is eliminated when disulfide-mediated oligomerization is prevented, or when MerTK is blocked with neutralizing Abs. Protein S oligomerization is independent of phospholipid oxidation. The data suggest that membranes containing phosphatidylserine serve as a scaffold for protein S-protein S interactions and that the resulting auto-oxidation and oligomerization is required for the prophagocytic activity of protein S. In this way, apoptotic cells facilitate their own uptake by macrophages. The requirement for oxidative modification of protein S can explain why this abundant blood protein does not constitutively activate MerTK in circulating monocytes and tissue macrophages.
authors.affiliation: Laboratory of Biochemistry, Division of Therapeutic Proteins, Center for Drug Evaluation and Research, Food and Drug Administration, 29 Lincoln Drive, Bethesda, MD 20892.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Uehara
authors.forename: Emily
authors.initials: E
authors.lastname: Shacter
mesh_terms: D017209:Apoptosis / Q000276:immunology*; D045744:Cell Line, Tumor; D002462:Cell Membrane / Q000737:chemistry / Q000276:immunology / Q000378:metabolism; D019281:Dimerization; D004220:Disulfides / Q000737:chemistry / Q000378:metabolism; D004789:Enzyme Activation / Q000276:immunology; D006801:Humans; D015227:Lipid Peroxidation / Q000276:immunology; D008081:Liposomes; D008262:Macrophage Activation / Q000276:immunology; D010084:Oxidation-Reduction; D010587:Phagocytosis / Q000276:immunology*; D010718:Phosphatidylserines / Q000737:chemistry / Q000378:metabolism; D011485:Protein Binding / Q000276:immunology; D017293:Protein S / Q000737:chemistry* / Q000378:metabolism* / Q000502:physiology; D011518:Proto-Oncogene Proteins / Q000378:metabolism / Q000502:physiology*; D020794:Receptor Protein-Tyrosine Kinases / Q000378:metabolism / Q000502:physiology*; D044967:Serum / Q000276:immunology; D000076202:c-Mer Tyrosine Kinase
chemical_list: D004220:Disulfides; D008081:Liposomes; D010718:Phosphatidylserines; D017293:Protein S; D011518:Proto-Oncogene Proteins; C097072:MERTK protein, human; D020794:Receptor Protein-Tyrosine Kinases; D000076202:c-Mer Tyrosine Kinase