Platelet-activating factor stimulates calcium-dependent activation of protein-tyrosine kinase Syk in a human B cell line.

In ASK.0 B lymphoblastoid cells, platelet activating factor (PAF) induced a rapid increase in Syk protein-tyrosine kinase activity which was insensitive to pertussis toxin (PTX) but was abolished by the phopholipase C inhibitor, U73122. In parallel, PAF-induced Ca2+ mobilization was also insensitive to PTX and was almost completely inhibited by U73122. Incubation of ASK.0 cells with the compounds that increase intracellular Ca2+ (i.e., the ionophore A23187, thapsigargin which releases Ca2+ from internal store) mimicked the effect of PAF on Syk kinase activity. Loading cells with the intracellular Ca2+ chelator, bis-(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetoxymethyl ester (BAPTAAM), completely inhibited the activation of Syk kinase in response to PAF, thapsigargin and ionophore. These results suggest that intracellular free Ca2+ seems to be critical for PAF-induced activation of Syk kinase in human B lymphoblastoid cells.