Polyisoprenyl phosphate (PIPP) signaling regulates phospholipase D activity:: a 'stop' signaling switch for aspirin-triggered lipoxin A4

It is of wide interest to understand how opposing extracellular signals (positive or negative) are translated into intracellular signaling events. Receptor-ligand interactions initiate the generation of bioactive lipids by human neutrophils (PMN), which serve as signals to orchestrate cellular responses important in host defense and inflammation. We recently identified a novel polyisoprenyl phosphate (PIPP) signaling pathway and found that one of its components, presqualene diphosphate (PSDP), is a potent negative intracellular signal in PMN that regulates superoxide anion generation by several stimuli, including phosphatidic acid, We determined intracellular PIPP signaling by autocoids with opposing actions on PMN: leukotriene B-4 (LTB4), a potent chemoattractant, and lipoxin A(4) (IXA(4)), a 'stop signal' for recruitment, LTB4 receptor activation initiated a rapid decrease in PSDP levels concurrent with activation of PLD and cellular responses. In sharp contrast, activation of the LXA(4) receptor reversed LTB4-initiated PSDP remodeling, leading to an accumulation of PSDP and potent inhibition of both PLD and superoxide anion generation, Thus, an inverse relationship was established for PSDP levers and PLD activity with two PMN ligands that evoke opposing responses. In addition, PSDP directly inhibited both isolated human recombinant (K-i = 6 nM) and plant (K-i = 20 nM) PLD, Together, these findings link PIPP remodeling to intracellular regulation of PMN function and suggest a role for PIPPs as lipid repressors in signal transduction, a novel mechanism that may also explain aspirin's suppressive actions in vivo in cell signaling.