Daocheng Zhu, Hidenori Hattori, Hakryul Jo, Yonghui Jia, Kulandayan K. Subramanian, Fabien Loison, Jian You, Yi Le, Marek Honczarenko, Leslie Silberstein, and Hongbo R. Luo*
Department of Pathology, Joint Program in Transfusion Medicine, Harvard Medical School, and Department of Laboratory Medicine, Children's Hospital Boston, Karp Family Research Building, Room 10214, Boston, MA 02115
Edited by Solomon H. Snyder, Johns Hopkins University School of Medicine, Baltimore, MD, and approved August 9, 2006 (received for review July 7, 2006)
Neutrophil spontaneous death plays essential roles in neutrophilhomeostasis and resolution of inflammation, whereas the underlyingmolecular mechanisms are still ill-defined. Neutrophils diebecause of programmed cell death or apoptosis. However, treatmentwith inhibitor of caspases, which are responsible for the majorityof apoptotic cell deaths, does not prevent the spontaneous deathof neutrophils. PKB/Akt possesses prosurvival and antiapoptoticactivities in a variety of cells. In this study, we show thatAkt activity decreases dramatically during the course of neutrophildeath. Both phosphatidylinositol 3-kinase and Akt inhibitorsenhance neutrophil death. Conditions delaying neutrophil death,such as treatment with granulocyte–macrophage colony-stimulatingfactor, granulocyte colony-stimulating factor, or IFN-, restoreAkt activity. Finally, we demonstrate that neutrophils depletedof PTEN, a phosphatidylinositol 3'-phosphatase that negativelyregulates Akt activity, live much longer than WT neutrophils.Thus, we establish Akt deactivation as a causal mediator ofneutrophil spontaneous death.
apoptosis | PTEN
PNAS | October 3, 2006 | vol. 103 | no. 40 | 14836-14841
Neutrophils are the most abundant cell type among circulatingwhite blood cells and constitute the first line of host defenseagainst invading pathogens (bacteria, fungi, viruses, etc.)(1–3). These cells are terminally differentiated and usuallyhave a very short lifespan (1–4 days in tissues). Theydie via spontaneous programmed cell death (apoptosis). The dailyturnover of human neutrophils is 0.8–1.6 x 109 cells perkg of body weight. Properly regulated death program is essentialfor neutrophil homeostasis. Augmented neutrophil death leadsto a decrease of neutrophil counts in the blood (neutropenia),which will increase the chance of contracting a bacterial orfungal infection. On the other hand, delayed neutrophil deathelevates neutrophil counts in the blood (neutrophilia), whichoften is associated with pathological conditions such as bacterialinfection, myeloid leukemia, and acute myocardial infarction.Programmed neutrophil death is also an essential cellular eventfor maintaining neutrophil numbers in the sites of infectionand inflammation. Neutrophils are recruited to the infectedtissues to engulf, kill, and digest invading microorganisms.However, the enzymes and reactive oxygen species released byneutrophils also can damage the surrounding tissues. Thus, thedeath program in neutrophils need to be well controlled to providea nice balance between their immune functions and their safeclearance. Delayed clearance of neutrophils in inflamed tissuescauses unwanted and exaggerated tissue inflammation (4–7).
PtdIns(3,4,5)P3/Akt signaling pathway possesses prosurvivaland antiapoptotic activities in a variety of cell types. PtdIns(3,4,5)P3(phosphatidylinositol 3,4,5-trisphosphate) contains two hydrophobicfatty acids and, therefore, are mainly localized on the plasmamembrane (8). PtdIns(3,4,5)P3 exerts its function by mediatingprotein translocation via binding to their pleckstrin homolog-domains(9, 10). PKB/Akt, a serine/threonine protein kinase with oncogenicand antiapoptotic activities, is one of the major downstreamfactors of PtdIns(3,4,5)P3 (11, 12). Akt contains apleckstrinhomolog domain, which specifically binds PtdIns(3,4,5)P3. ThePtdIns(3,4,5)P3-mediated membrane translocation of Akt is essentialfor its phosphorylation and activation. Activated Akt, in turn,phosphorylates a variety of proteins, including several associatedwith cell survival/death pathways such as BAD, Forkhead, ASK1,and NF-B, leading to diminished apoptotic cell death (12, 13).PtdIns(3,4,5)P3 level on the plasma membrane is regulated byphosphatidylinositol 3-kinases (PI3K) (12, 14–16) andthe tumor suppressor PTEN (Phosphatase and tensin homologuedeleted on chromosome 10), a phosphatidylinositol 3'-phosphatasethat converts PtdIns(3,4,5)P3 to PtdIns(4,5)P2 (17, 18).
Recently, we demonstrated that deactivation of PtdIns(3,4,5)P3/Aktsignal characterizes both caspase-dependent and -independentcell death (19). In the present study, we investigated the contributionof PI3K-Akt pathway in neutrophil spontaneous death. We demonstratethat PtdIns(3,4,5)P3/Akt signal is deactivated significantlyduring neutrophil death. Inhibition of PtdIns(3,4,5)P3/Akt signalfurther promotes neutrophil death. Moreover, augmentation ofPtdIns(3,4,5)P3/Akt signal by depleting PTEN prevents neutrophilspontaneous death. Thus, we establish Akt deactivation as acausal mediator in neutrophil spontaneous death.