Neutrophil spontaneous death shares many features of classical
apoptosis, such as cell body shrinkage, exteriorization of phosphatidylserine
from inner to outer leaflet of plasma membrane, mitochondria
depolarization, nuclear condensation, and DNA fragmentation
). Most apoptosis is a caspase-dependent process. Caspase
activation during neutrophil death has been well documented
(4–7, 27). However, although caspase activity is essential
for ligand (e.g., TNF
)-induced apoptosis of neutrophil, the
broad-spectrum caspase inhibitor zVAD-fmk or Boc-D-fmk could
not suppress constitutive spontaneous neutrophil death (20
Pongracz et al.
) reported that Caspase 3 inhibitor DEVD-fmk
could partially inhibit neutrophil death. In their experimental
system, neutrophil apoptosis rate was much higher than the neutrophil
spontaneous death rate, reaching 70% in 24 h. Treatment with
DEVD-fmk reduced it to the spontaneous death rate (35–40%),
suggesting that the neutrophil death they investigated might
be a combination of spontaneous death and cytokine-induced death.
The molecular mechanism governing this caspase-independent neutrophilspontaneous death is still largely unknown. The susceptibilityof neutrophils to apoptosis appears to depend on the balancebetween proapoptotic and survival (antiapoptotic) signals. Celldeath can be triggered by augmenting proapoptotic signals orattenuating survival signals. In recent years, the activationof proapoptotic pathways in neutrophil spontaneous death hasbeen studied extensively, and several important players suchas BAD, reactive oxygen species, and p38MAPK have been identified(4–7). Nevertheless, the contribution of the deactivationof survival signals to neutrophil death has not yet been investigated.In this study, we reported that the deactivation of a well knownsurvival signal, PI3K-Akt pathway, plays a causal role in neutrophilspontaneous death. Augmentation of PtdIns(3,4,5)P3/Akt signalby PTEN depletion significantly prevents neutrophil spontaneousdeath, thus opening a previously undescribed avenue for interveningin neutrophil death. This study also advances our knowledgeof the molecular mechanism of cell death (particularly the caspase-independentapoptosis) in general and helps us to understand death signalingin other types of cells, such as hematopoietic stem cells, monocytes/macrophages,lymphocytes, and leukemia cells.
The upstream deactivators of Akt remain to be elucidated. Thebest established activator of Akt is PtdIns(3,4,5)P3 generatedby PI3K (12, 15). Conceivably, deactivation of PI3K is responsiblefor Akt's deactivation. PtdIns(3,4,5)P3 level also can be regulatedby the tumor suppressor PTEN and SHIP (SH2-containing inositol5'-phosphatase), which converts PtdIns(3,4,5)P3 to PtdIns(4,5)P2and PtdIns(3,4)P2, respectively (18, 30). PTEN or/and SHIP mightget activated during neutrophil spontaneous death, leading todown-regulation of Akt. Similar with what is discovered in thePTEN-null neutrophils, Gardai et al. (31) reported that thehalf-life of neutrophils depleted of SHIP also was dramaticallyincreased. Akt activation relies on its membrane translocationmediated by its specific association with PtdIns(3,4,5)P3 onthe plasma membrane. Recently, we demonstrated that two inositolphosphates, InsP7 and Ins(1,3,4,5)P4, compete for Akt-pleckstrinhomolog domain binding with PtdIns(3,4,5)P3, providing anotherlevel of regulation for Akt membrane translocation and activation(32). Thus, Akt deactivation also could be a result of elevatedInsP7 or Ins(1,3,4,5)P4 level.
Multicellular organisms defend themselves against invading pathogens(bacteria, fungi, and viruses) by mounting both innate and adaptiveimmune responses. Neutrophil accumulation at sites of infectionis essential for host defense. However, exaggerated accumulationcould be responsible for pathogenesis of many acute and chronicinflammatory diseases, such as pneumonia, asthma, multiple sclerosis,and rheumatoid arthritis. There is a consensus that the rateof neutrophil spontaneous death can determine the number ofneutrophils at the sites of inflammation and, therefore, areimportant in the regulation of inflammation. Results presentedin this paper provide a previously undescribed strategy andtarget for the treatment of a variety of infectious and inflammatorydiseases associated with delayed or enhanced neutrophil death.
Neutrophil spontaneous death also plays essential roles in neutrophilhomeostasis. We examined whether disruption of PTEN can increaseperipheral blood neutrophil count due to delayed spontaneousdeath but did not detect any alteration. Peripheral blood neutrophilcount is decided by multiple cellular processes, such as cytokine-elicitedmobilization from bone marrow, spontaneous death, transmigrationfrom blood to tissues, and clearance by phagocytic cells. Conceivably,depletion of PTEN can affect processes other than spontaneousdeath, and these effects are able to overcome the effect elicitedby delayed neutrophil death, leading to unaltered peripheralblood neutrophil count.