Tyrosine phosphorylation of phospholipase C concomitant with its activation by platelet-activating factor in a human...

Platelet-activating factor (PAF) is a powerful inflammatory mediator in a variety of systems. Upon binding its receptor on B lymphocytes, it activates phospholipases C and A2, thus initiating a cascade of events culminating in changes in the program of the target cell. We have now extended our previous studies of the effects of PAF on EBV-transformed human B cell lines to examine the mechanism by which phospholipase C (PLC) is activated. PAF-induced incorporation of 32P into phosphatidylinositol (PtdIns) was markedly diminished by the tyrosine kinase inhibitors, genistein, and those of the tyrphostin family, tyrphostins 25, 47, and 51. The generation of inositol phosphates induced by PAF was also significantly inhibited by these inhibitors. Correlating with this inhibition of PtdIns turnover, the elevation of intracellular calcium concentrations stimulated by PAF was observed to be inhibited in the presence of inhibitors of tyrosine kinases. In addition, the induction of expression of the proto-oncogene, c-fos, was substantially attenuated by these inhibitors. Finally, employing anti-phosphotyrosine immunoprecipitates of lysates from PAF-stimulated cells in an in vitro PLC assay, we have provided evidence for an increase in the tyrosine phosphorylation levels of PLC upon stimulation, and the inhibition of this increase by tyrosine kinase inhibitors. In summary, we have shown that in B cells tyrosine kinase activity is essential for the PtdIns turnover, the generation of inositol phosphates, and the calcium flux induced by PAF and that platelet-activating factor-induced tyrosine phosphorylation of PLC is likely to be required for the activation of this enzyme.