Involvement of tyrosine kinase and phosphatidylinositol 3-kinase in phagocytosis by ascidian hemocytes

It has been proposed that protein tyrosine phosphorylation plays important roles in signal transduction in mammalian T- and B-cells and monocytes. Dur ing our investigations on the ascidian host defense system, we have shown t hat the monoclonal antibody A74 strongly inhibits both phagocytosis of shee p red blood cells (SRBCs) by hemocytes and hemocyte aggregation, and that t he A74 antigen protein has two immunoreceptor tyrosine-based activation mot ifs and several other motifs that are thought to function in signal transdu ction in mammals. In this study, we found that the A74 antibody strongly in hibited phagocytosis by ascidian hemocytes of yeast cells, as strongly as t hat of SRBCs, but not that of latex beads. We also found that herbimycin A and an erbstatin analog, tyrosine kinase inhibitors, and wortmannin, a spec ific inhibitor for phosphatidylinositol 3-kinase (PI3-kinase), inhibited th e phagocytosis of yeast cells. We investigated which hemocyte proteins were specifically tyrosine-phosphorylated during phagocytosis by ascidian hemoc ytes and found that a protein with a molecular mass of 100 kDa was specific ally tyrosine-phosphorylated upon phagocytosis; its tyrosine phosphorylatio n was inhibited by the A74 antibody. These results strongly suggest that bo th tyrosine kinase and PI3-kinase play important roles in phagocytosis by a scidian hemocytes. (C) 2000 Elsevier Science Inc. All rights reserved.