Cyclic AMP regulation of neutrophil apoptosis occurs via a novel protein kinase A-independent signaling pathway

The second messenger molecule cyclic AMP dramatically modulates the apoptot ic program in a wide variety of cells, accelerating apoptosis in some and d elaying the rate of apoptosis in others. Human neutrophil apoptosis, a proc ess that regulates the fate and numbers of these potentially histotoxic cel ls in inflammatory sites, is profoundly delayed by the cell-permeable analo g of cyclic AMP, dibutyryl-cAMP. We have investigated the mechanisms underl ying cyclic AMP-mediated delay of neutrophil apoptosis, and we show that cy clic AMP inhibits loss of mitochondrial potential occurring during constitu tive neutrophil apoptosis. Furthermore, we demonstrate that cyclic AMP also suppresses caspase activation in these inflammatory cells. Despite increas ing protein kinase A activity, this kinase is unlikely to mediate the effec t of cyclic AMP on apoptosis because blockade of protein kinase A activatio n did not influence the survival effects of cyclic AMP. Further investigati on of the signaling mechanism demonstrated that the delay of apoptosis is i ndependent of phosphoinositide 3-kinase and MAPK activation. Our results su ggest cyclic AMP delays neutrophil apoptosis via a novel, reversible, and t ranscriptionally independent mechanism. We show that proteasome activity in the neutrophil is vitally involved in this process, and we suggest that a balance of pro-apoptotic and anti-apoptotic proteins plays a key role in th e powerful ability of cyclic AMP to delay neutrophil death.