CD69-induced monocyte apoptosis involves multiple nonredundant signaling pathways

Simultaneous stimulation of human monocytes/macrophages or THP1 cells with LPS and an antibody specific for the activation marker CD69 induces apoptosis. Here we demonstrate the involvement of multiple independent signals that are necessary for apoptosis induction. Thus, inhibitors of phospholipase A₂ and lipoxygenase prevent apoptosis induction. Similarly, the ADP-ribosylating G-protein-reactive pertussis toxin (PTX) but not a mutant toxin lacking the ADP-ribosylating moiety (mPTX) prevents apoptosis induction. Furthermore, inhibition of NO generation abrogates completely the induction of apoptosis by LPS/CD69 ligation. These three pathways can be dissociated from each other in the sense that interventions on the arachidonic acid metabolism or G proteins do not inhibit the generation of NO and that exogenous NO cannot reverse the inhibition of cell death by inhibitors of phospholipase A₂ or PTX. In addition, both PTX and mPTX affect arachidonic acid mobilization only partially, indicating that the apoptosis-inhibitory effect of PTX (which is not shared by mPTX) cannot be explained by its effect on phospholipase A₂ activation. Both LPS and anti-CDB9 are sufficient on their own to activate cells, as determined by TNF production, NO generation, or arachidonic acid metabolism, but neither LPS nor anti-CDB9 can induce apoptosis on their own. Thus, apoptosis induction in this system involves at least three independent signal transduction systems-(i) arachidonic acid metabolism, (ii) NO, and (iii) PTX-sensitive events - each of which is necessary but insufficient to induce monocyte/macrophage apoptosis. These findings underline the complex control of activation-induced apoptosis in cells of the myelomonocytic lineage.