Regulation of αIIbβ3 function in human B lymphocytes

We studied the function of the platelet integrin alpha IIb beta 3 using a B lymphocyte model in which alpha II beta 3 can be induced to interact with fibrinogen using phorbol myristate acetate (PMA). To determine whether a G protein-coupled receptor could also activate alpha IIb beta 3 in lymphocytes, we coexpressed the human formyl peptide receptor (fPR) and alpha IIb beta 3, finding that the fPR agonist formyl Met-Leu-Phe (fMLP)-stimulated lymphocyte adherence to immobilized fibrinogen and binding of soluble fibrinogen to the lymphocyte surface. The response to fMLP, but not PMA, was abrogated by pertussis toxin, indicating that the fPR was coupled to the G-protein G alpha(i), whereas the protein kinase C inhibitor bisindolyl-maleimide I inhibited the response to both fMLP and PMA, indicating that signaling from the fPR included protein kinase C, On the other hand, the tyrosine kinase inhibitor genistein, the Syk inhibitor piceatannol, and the RhoA inhibitor C3 exoenzyme had no effect, implying that neither tyrosine phosphorylation nor the GTPase RhoA were involved. Furthermore, whereas micromolar concentrations of cytochalasin D inhibited the PMA-stimulated interaction of alpha IIb beta 3 with fibrinogen, nanomolar concentrations actually induced fibrinogen binding to unstimulated cells. Our studies demonstrate that alpha IIb beta 3 expressed in B lymphocytes can be activated by a physiologic agonist and outline an activating pathway that includes G alpha(i), protein kinase C, and the actin cytoskeleton.