PLATELET-ACTIVATING-FACTOR STIMULATES CALCIUM-DEPENDENT ACTIVATION OFPROTEIN-TYROSINE KINASE SYK IN A HUMAN B-CELL LINE

In ASK.0 B lymphoblastoid cells, platelet activating factor (PAF) indu ced a rapid increase in Syk protein-tyrosine kinase activity which was insensitive to pertussis toxin (PTX) but was abolished by the phophol ipase C inhibitor, U731.22. In parallel, PAF-induced Ca2+ mobilization was also insensitive to PTX and was almost completely inhibited by U7 3122. Incubation of ASK.0 cells with the compounds that increase intra cellular Ca2+ (i.e., the ionophore A23187, thapsigargin which releases Ca2+ from internal store) mimicked the effect of PAF on Syk kinase ac tivity. Loading cells with the intracellular Ca2+ chelator, -aminophen oxy)-ethane-N,N,N',N'-tetraacetoxymethyl ester (BAPTA-AM), completely inhibited the activation of Syk kinase in response to PAF, thapsigargi n and ionophore. These results suggest that intracellular free Ca2+ se ems to be critical for PAF-induced activation of Syk kinase in human B lymphoblastoid cells. (C) 1997 Academic Press.