title: Signalling pathway involving GULP, MAPK and Rac1 for SR-BI-induced phagocytosis of apoptotic cells.
abstract: Class B scavenger receptor type I (SR-BI) is a phosphatidylserine (PS)-recognizing receptor of testicular Sertoli cells responsible for the phagocytosis of spermatogenic cells undergoing apoptosis. Here, we determined signal mediators that compose a signalling pathway for SR-BI-induced phagocytosis. Results of a yeast two-hybrid analysis and a cell-free binding assay indicated that SR-BI binds to engulfment adapter protein (GULP) using the C-terminal intracellular domain. A co-immunoprecipitation analysis showed the existence of a complex of GULP and SR-BI in cells prior to the activation of SR-BI by PS. A reduction of GULP expression in phagocytes decreased the SR-BI-mediated phagocytosis of apoptotic cells. Administration to phagocytes of PS-containing liposomes increased the levels of the GTP-bound form of Rac1 and the phosphorylated forms of mitogen-activated protein kinases (MAPK) p38 and extracellular signal-related kinase 1 and 2. Finally, lowering the expression of GULP abrogated MAPK phosphorylation, and the presence of MAPK inhibitors reduced the level of GTP-bound Rac1 in PS-activated phagocytes. These results collectively suggested the following signalling pathway for the SR-BI-induced phagocytosis: (i) PS-recognizing SR-BI activates associated GULP; (ii) activated GULP induces MAPK phosphorylation; (iii) activated MAPK increases GTP-bound Rac1; and (iv) activated Rac1 induces a rearrangement of the actin cytoskeleton.
authors.affiliation: Graduate School of Medical Science, Kanazawa University, Kanazawa, Ishikawa 920-1192, Japan.
authors.forename: Yoichi
authors.initials: Y
authors.lastname: Osada
authors.forename: Toshiro
authors.initials: T
authors.lastname: Sunatani
authors.forename: In-San
authors.initials: IS
authors.lastname: Kim
authors.forename: Yoshinobu
authors.initials: Y
authors.lastname: Nakanishi
authors.forename: Akiko
authors.initials: A
authors.lastname: Shiratsuchi
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism*; D000818:Animals; D017209:Apoptosis; D002460:Cell Line; D004789:Enzyme Activation; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D006801:Humans; D020935:MAP Kinase Signaling System; D051379:Mice; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D010587:Phagocytosis; D010766:Phosphorylation; D011485:Protein Binding; D050737:Proto-Oncogene Proteins c-crk / Q000378:metabolism; D051381:Rats; D051122:Scavenger Receptors, Class B / Q000737:chemistry / Q000378:metabolism*; D020798:Two-Hybrid System Techniques; D048051:p38 Mitogen-Activated Protein Kinases / Q000378:metabolism; D020830:rac1 GTP-Binding Protein / Q000378:metabolism*
chemical_list: D048868:Adaptor Proteins, Signal Transducing; C454119:GULP1 protein, human; D050737:Proto-Oncogene Proteins c-crk; C495125:Scarb1 protein, rat; D051122:Scavenger Receptors, Class B; D048049:Extracellular Signal-Regulated MAP Kinases; D020928:Mitogen-Activated Protein Kinases; D048051:p38 Mitogen-Activated Protein Kinases; D020830:rac1 GTP-Binding Protein