Platelet-activating factor triggers the phosphorylation and activation of MAP-2 kinase and S6 peptide kinase activity in human B cell lines.

Platelet activating factor (PAF), a phospholipid mediator produced by a variety of cell types, has potent biological activities in many cellular systems. We report that stimulation of B cell lines with PAF induced tyrosine phosphorylation of a protein, approximately 42 kDa in size. The PAF receptor antagonist WEB 2086 as well as the structural analogue PAF antagonist CV3988 inhibited the response, demonstrating that the tyrosine phosphorylation was attributable to PAF binding to a specific receptor. Immunoblot analysis, using an antibody to microtubule-associated protein-2 kinase, demonstrated the presence of a single protein species in unstimulated cells. PAF treatment induced the appearance of an additional species with a slightly reduced mobility in the gel. This PAF-inducible band had the same mobility on SDS-PAGE gels as the protein that was tyrosine-phosphorylated after PAF stimulation. Tyrosine phosphorylation of the 42-kDa protein was rapid, detectable within 30 s, and evident over a wide range of PAF concentrations (10(-7) to 10(-10) M). Treatment of cells with phorbol myristate acetate induced tyrosine phosphorylation of a protein with the same mobility in SDS-PAGE gels as the protein induced after PAF stimulation. Lysates from PAF-stimulated lymphoblastoid cells exhibited more than twice the ability to phosphorylate myelin basic protein than lysates from untreated cells. In addition, the ribosomal S6 peptide kinase activity (S6PK) was increased after PAF stimulation of the B cells. Taken together, the results suggest that PAF stimulation of B cells induces the tyrosine phosphorylation and activation of microtubule-associated protein-2 kinase.