Nitric oxide mediates dendritic cell apoptosis by downregulating inhibitors of apoptosis proteins and upregulating effector caspase activity

Abstract

Background. Dendritic cells (DCs) play a crucial role in the amplification of the immune response by promoting antigen presentation, T-lymphocyte proliferation, and proinflammatory cytokine and nitric oxide (NO) production. We have previously shown that the exogenous NO donor, s-nitroso-N-acetyl-penicillamine, promotes DC apoptosis by disrupting the mitochondrial membrane potential, which induces cytochrome-C release and activates caspase 3. To further elucidate the signaling pathway, we examined the expression of cellular inhibitors of apoptosis proteins (cIAPs) and poly (ADP-ribose) polymerase cleavage (PARP), a terminal event in the apoptotic cascade. Methods. DC2.4 were exposed to 250 μmol/L s-nitroso-N-acetyl-penicillamine for various intervals. Apoptosis and necrosis were measured by terminal deoxynucleotidyl transferase nick-end labeling assay or flow cytometry with Annexin V and propidium iodide. DC2.4 were cultured with the pan-caspase inhibitor, ZVAD (100 μmol/L). cIAP, pro-caspases, and PARP expression or activation was measured by Western blot. Caspase enzyme activity was confirmed with the use of specific substrates. Results. NO-induced DC apoptosis correlated with the downregulation of cIAP expression. Caspase 3 and 6 were upregulated by SNAP and significantly inhibited by ZVAD. Maximal PARP cleavage occurred at 8 hours and coincided with the downregulation of cIAP and peak caspase 3 and near maximal caspase 6 activity. Conclusions. NO-induced DC apoptosis is associated with the downregulation of cIAP expression, which facilitates caspase cascade activation and subsequent PARP cleavage.