Anti-apoptotic mechanism and reduced expression of phospholipase D in spontaneous and Fas-stimulated apoptosis of human neutrophils

Neutrophil apoptosis is a constitutive process that can be enhanced or delayed by signals triggered by various stimuli. In this work, we investigated the action mechanism of phospholipase D (PLD) and is expression in the inhibition of spontaneous and Fas-mediated apoptosis. Anti-Fas antibody-stimulated apoptosis of neutrophils was significantly blocked by the exogenous addition of bacterial PLD from Streptomyces chromofuscus (scPLD), and neutrophils cultured for 24 h in the presence of anti-Fas antibody showed lower agonist-stimulated PLD activity compared to untreated cells. The amount of PLD1a protein reduced time-dependently in cultured neutrophils, but was recovered by treating with LPS or GM-CSF. The reduction in PLD1 a protein level was blocked by caspase inhibitors. The exogenous addition of scPLD blocked the up-regulation of Fas-associated death domain expression, mitochondrial permeability, and the cleavages of procaspase-8, procaspase-3, and protein kinase C-δ. We also found that the protein level of apoptosis-inducing factor was increased in cultured neutrophils but its expression was reduced by scPLD. However, sulfasalazine-induced apoptosis and change of protein expression were not blocked by scPLD. Taken together, the activity and protein levels of PLD play a role as an anti-apoptotic factor by acting at multiple levels of the apoptotic cascade in neutrophils.