Phorbol 12-myristate 13-acetate inhibits binding of leukotriene B4 and platelet-activating factor and the responses they induce in neutrophils: site of action.

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Abstract
The addition of the platelet-activating factor (PAF) to neutrophils causes an increase in cytoskeletal actin, a rise in the intracellular concentration of free calcium, release of arachidonic acid, and the synthesis of PAF. The PAF synthesis in human neutrophils stimulated by PAF is greatly potentiated by the human granulocyte-macrophage colony-stimulating factor. Incubation of human neutrophils with the tumor copromoter phorbol 12-myristate 13-acetate (PMA) for 3 min prior to the addition of the stimulus inhibits all these responses produced by PAF. The inhibition is prevented when the cells are incubated with protein kinase C inhibitors such as 1-(5-isoquinolinesulfonyl)-2-methylpiperazine for 5 min prior to the addition of PMA. The rise in the intracellular concentration of free calcium in human neutrophils stimulated with leukotriene B4 is also inhibited by PMA, and this inhibition is prevented by protein kinase C inhibitors such as staurosporine. Unlike PMA, the inactive ester 4 alpha-phorbol 12,13-didecanoate has no inhibitory effect on the stimulated rise in the intracellular concentration of free calcium. The binding of either PAF or leukotriene B4 to intact cells is inhibited by PMA. The most important finding of the present studies is that PMA interferes with the binding of PAF and leukotriene B4 to their respective receptors. Whether PMA inhibits the binding of these lipid mediators by activating protein kinase C or by perturbing the membrane directly remains to be elucidated.