Role of PI3-kinase-dependent Bad phosphorylation and altered transcription in cytokine-mediated neutrophil survival

De novo C16- and C24-ceramide generation contributes to spontaneous neutrophil apoptosis

Neutrophils rapidly undergo spontaneous apoptosis following their release from the bone marrow. Although central to leukocyte homeostasis, the mechanisms that regulate neutrophil apoptosis remain poorly understood. We show here that apoptosis of cultured neutrophils is preceded by a substantial increase in the intracellular levels of 16 and 24 carbon atom (C(16)- and C(24))-ceramides, which are lipid second messengers of apoptosis and stress signaling. Treatment of neutrophils with fumonisin B(2), a selective inhibitor of the de novo pathway of ceramide synthesis, prevented accumulation of C(16)- and C(24)-ceramides. Moreover, fumonisin B(2) significantly reduced caspase-3, -8, and -9 activation and apoptosis in these cells. Conversely, 3-O-methylsphingomyelin and fantofarone, which are specific inhibitors of neutral and acid sphingomyelinases, respectively, neither inhibited C(16)- and C(24)-ceramide production nor decreased the apoptosis rate in neutrophils, indicating that in these cells, ceramides are not generated from membrane sphingomyelin. Further experiments showed that increasing endogenous C(16)- and C(24)-ceramide levels by using DL-threo-1-phenyl-2-palmitoylamino-3-morpholino-1-propanol and (1S,2R)-D-erythro-2-(N-myristoylamino)-1-phenyl-1-propanol, two inhibitors of ceramide metabolism, enhances caspase-3, -8, and -9 activity and increases neutrophil apoptosis. Similarly, apoptosis was induced rapidly when synthetic C(16)- and/or C(24)-ceramides were added to neutrophil cultures. Finally, GM-CSF, a cytokine that delays neutrophil apoptosis, abrogated C(16)- and C(24)-ceramide accumulation totally in cultured neutrophils, whereas Fas ligation accelerated apoptosis in these cells without affecting de novo ceramide production. We conclude that de novo generation of C(16)- and C(24)-ceramides contributes to spontaneous neutrophil apoptosis via caspase activation and that GM-CSF exerts its antiapoptotic effects on neutrophils, at least partly through inhibition of ceramide accumulation.

Mitochondria in neutrophil apoptosis

Central in the regulation of the short life span of neutrophils are their mitochondria. These organelles hardly contribute to the energy status of neutrophils but play a vital role in the apoptotic process. Not only do the mitochondria contain cytotoxic proteins that are released during apoptosis and contribute to caspase activation, but they also act as sensors of the metabolic and redox state of the cell and as scavengers of free Ca2+. The balance of the expression and activity of the proapoptotic and antiapoptotic members of the Bcl-2 family of proteins determines the life span of neutrophils, because these proteins are essential for the formation of a permeability transition pore in the mitochondria and also seem to control the release of Ca2+ from the endoplasmic reticulum and thereby mitochondrial energy metabolism.

Ral is both necessary and sufficient for the inhibition of myeloid differentiation mediated by Ras

Hyperactivation of Ras is one of the most common abnormalities in acute myeloid leukemia. In experimental models, Ras inhibits myeloid differentiation, which is characteristic of leukemia; however, the mechanism through which it disrupts hematopoiesis is poorly understood. In multipotent FDCP-mix cells, Ras inhibits terminal neutrophil differentiation, thereby indefinitely extending their proliferative potential. Ras also strongly promotes the sensitivity of these cells to granulocyte-macrophage colony-stimulating factor (GM-CSF). Using this model, we have dissected the signaling elements downstream of Ras to determine their relative contribution to the dysregulation of hematopoiesis. Cells expressing Ras mutants selectively activating Raf (Ras*T35S) or phosphatidylinositol 3-kinase (Ras*Y40C) did not significantly affect differentiation or proliferative capacity, whereas Ras*E37G (which selectively activates RalGEFs) perpetuated proliferation and blocked neutrophil development in a manner similar to that of Ras. Correspondingly, expression of constitutively active versions of these effectors confirmed the overriding importance of Ral guanine nucleotide exchange factors. Cells expressing Ras demonstrated hyperactivation of Ral, which itself was able to exactly mimic the phenotype of Ras, including hypersensitivity to GM-CSF. Conversely, dominant negative Ral promoted spontaneous neutrophil development. Ral, in turn, appears to influence differentiation through multiple effectors. These data show, for the first time, the importance of Ral in regulating differentiation and self-renewal in hematopoietic cells.

Respiratory syncytial virus inhibits granulocyte apoptosis through a phosphatidylinositol 3-kinase and NF-kappaB-dependent mechanism

Respiratory syncytial virus (RSV) is a common cause of lower respiratory tract disease in children. It is associated with increased neutrophil numbers in the airway. In this study, we assessed whether this ssRNA virus can directly influence granulocyte longevity. By culturing RSV with granulocytes, it was observed that virus delays both constitutive neutrophil and eosinophil apoptosis. Using pharmacological inhibitors, the RSV-induced delay in neutrophil apoptosis was found to be dependent on both PI3K and NF-kappaB, but not p38 MAPK or MEK1/MEK2 activation. Using blocking Abs and a reporter cell line, we were able to exclude TLR4 as the receptor responsible for mediating RSV-induced delay in neutrophil apoptosis. The antiapoptotic effect was abrogated by preincubation with the lysosomotropic agent chloroquine, indicating the requirement for endolysosomal internalization. Furthermore, addition of ssRNA, a ligand for the intracellular TLR7/TLR8, also inhibited neutrophil apoptosis, suggesting that intracellular TLRs could be involved in induction of the antiapoptotic effect. Using the BioPlex cytokine detection assay (Bio-Rad), we found that IL-6 was present in supernatants from RSV-exposed neutrophils. IL-6 was found to inhibit neutrophil apoptosis, suggesting that there is an autocrine or paracrine antiapoptotic role for IL-6. Finally, RSV treatment of neutrophils resulted in increased expression of the antiapoptotic Bcl-2 protein Mcl-1. Taken together, our findings suggest involvement of multiple intracellular mechanisms responsible for RSV-induced survival of granulocytes and point toward a role for intracellular TLRs in mediating these effects.

Targeting survivin via PI3K but not c-akt/PKB by anticancer drugs in immature neutrophils

Myelosuppression is the most common unwanted side effect associated with the administration of anticancer drugs, and infections remain a common cause of death in chemotherapy-treated patients. Several mechanisms of the cytotoxicity of these drugs have been proposed and may synergistically operate in a given cell. Survivin expression has been associated with cancer, but recent reports suggest that this molecule is also expressed in several immature and mature hematopoietic cells. Here, we provide evidence that treatment of immature neutrophils with anticancer drugs reduced endogenous survivin levels causing apoptosis. The anticancer drugs did not directly target survivin, instead they blocked the activity of phosphatidylinositol-3-OH kinase, which regulated survivin expression and apoptosis in these cells. Strikingly, and in contrast to other cells, this pathway did not involve the serine/threonine kinase c-akt/PKB. Moreover, in combination with anticancer drug therapy, rapamycin did not induce increased myelosuppression in an experimental lymphoma mouse model. These data suggest that drugs that block either c-akt/PKB or signaling molecules located distal to c-akt/PKB may preferentially induce apoptosis of cancer cells as they exhibit no cytotoxicity for immature neutrophils.

Aminopeptidase N (CD13) Regulates Tumor Necrosis Factor-α-induced Apoptosis in Human Neutrophils

Neutrophil apoptosis plays a central role in the resolution of granulocytic inflammation. We have shown previously that tumor necrosis factor-alpha (TNFalpha) enhances the rate of neutrophil apoptosis at early time points via a mechanism involving both TNF receptor (TNFR) I and TNFRII. Here we reveal a marked but consistent variation in the magnitude of the pro-apoptotic effect of TNFalpha in neutrophils isolated from healthy donors, and we show that inhibition of cell surface aminopeptidase N (APN) using actinonin, bestatin, or inhibitory peptides significantly enhanced the efficacy of TNFalpha-induced killing. Notably, an inverse correlation is shown to exist between neutrophil APN activity and the sensitivity of donor cells to TNFalpha-induced apoptosis. Inhibition of cell surface APN appears to interfere with the shedding of TNFRI, and as a consequence results in augmented TNFalpha-induced apoptosis, cell polarization, and TNFalpha-primed, formyl-methionyl-leucyl-phenylalanine-stimulated respiratory burst. Of note, actinonin and bestatin had no effect on TNFRII expression under resting or TNFalpha-stimulated conditions and did not alter CXCRI or CXCRII expression. These data suggest significant variation in the activity of APN/CD13 on the cell surface of neutrophils in normal individuals and reveal a novel mechanism whereby APN/CD13 regulates TNFalpha-induced apoptosis via inhibition of TNFRI shedding. This has therapeutic relevance for driving neutrophil apoptosis in vivo.

Granulocyte apoptosis in the pathogenesis and resolution of lung disease

Apoptosis, programmed cell death, of neutrophil and eosinophil granulocytes is a potential control point in the physiological resolution of innate immune responses. There is also increasing evidence that cellular processes of apoptosis can be dysregulated by pathogens as a mechanism of immune evasion and that delayed apoptosis, resulting in prolonged inflammatory cell survival, is important in persistence of tissue inflammation. The identification of cell-type specific pathways to apoptosis may allow the design of novel anti-inflammatory therapies or agents to augment the innate immune responses to infection. This review will explore the physiological roles of granulocyte apoptosis and their importance in infectious and non-infectious lung disease.

Regulation of Fas-mediated apoptosis in neutrophils after surgery-induced acute inflammation

BACKGROUND

Neutrophils undergo rapid Fas-mediated apoptosis during in vitro culture. The purpose of this study was to investigate the effects of surgical stress upon the Fas-mediated apoptotic response in circulating neutrophils.

MATERIALS AND METHODS

Blood samples were drawn from eight patients with a mandibular prognathism, and who had undergone a bilateral sagittal split ramus osteotomy, at 2 days before, and at 1 and 5 days after surgery. The circulating neutrophils in each blood sample were then evaluated for their susceptibility to Fas-mediated apoptosis in either the presence or the absence of autogenous plasma.

RESULTS

Fas-induced apoptosis in the neutrophils of these surgically treated patients was found to be slightly accelerated at 1 day postoperatively in the presence of FBS, compared with 2 days preoperatively and 5 days postoperatively. However, we obtained different results for these experiments in the presence of autogenous plasma. The Fas-induced apoptotic response levels in the neutrophils at day 1 postsurgery following exposure to autogenous plasma were significantly suppressed compared with the levels at both 2 days preoperatively and 5 days postoperatively. The Fas expression levels on the cell surface of the neutrophils were not altered, but the levels of soluble Fas (sFas) in the plasma were reduced to almost inverse levels during the postoperative periods. The levels of granulocyte-macrophage colony-stimulating factor, interleukin-6, and interleukin-8 levels in the plasma were also markedly raised in the plasma from each of these patients at 1 day postoperatively. However, the anti-apoptotic effects of the plasma on the Fas-mediated neutrophil apoptosis were not influenced by the addition of their neutralizing antibodies for these cytokines. The suppressive effects of postoperative plasma on Fas-mediated neutrophil apoptosis were blocked by the phosphatidylinositol 3-kinase (PI 3-K) inhibitors, LY294002, and wortmannin. Additionally, these effects were also abrogated by the extracellular signal-regulated kinase (ERK) inhibitor, PD98059, but not by the p38 mitogen-activated protein kinase inhibitor, SB203580.

CONCLUSIONS

The increase in sFas levels in the plasma of patients with acute inflammation may lead to the inhibition of Fas-mediated neutrophil apoptosis. Moreover, the activation of the PI 3-K and ERK signaling-dependent pathways may, in part, also contribute to the down-regulation of the Fas-mediated apoptotic response in neutrophils.

An ultrasensitive fluorescent assay for the in vivo quantification of superoxide radical in organisms

Superoxide radical is a very important parameter of oxidative stress involved in a variety of biological phenomena; therefore, its in vivo study is of utmost significance. However, its accurate detection is a challenge due to its short lifetime and its very low physiological concentration. All current assays are qualitative and nonspecific, and at best they are performed in vitro. The current dihydroethidine-based assay overcomes all these problems and introduces the following novelties. First, it measures the in vivo superoxide production in animals, plants, and microorganisms. Second, it is ultrasensitive and very simple in that it can measure superoxide radical as low as 1.5 pmol in biological samples as low as 5 mg. Third, the very high sensitivity of the assay renders possible, for the first time, the measurement of the actual rate of formation of superoxide radical under physiological and simulated nonphysiological conditions.

Apoptotic pathways are inhibited by leptin receptor activation in neutrophils

Leptin regulates food intake as well as metabolic, endocrine, and immune functions. It exerts proliferative and antiapoptotic activities in a variety of cell types, including T cells. Leptin also stimulates macrophages and neutrophils, and its production is increased during inflammation. In this study, we demonstrate that human neutrophils express leptin surface receptors under in vitro and in vivo conditions, and that leptin delays apoptosis of mature neutrophils in vitro. The antiapoptotic effects of leptin were concentration dependent and blocked by an anti-leptin receptor mAb. The efficacy of leptin to block neutrophil apoptosis was similar to G-CSF. Using pharmacological inhibitors, we obtained evidence that leptin initiates a signaling cascade involving PI3K- and MAPK-dependent pathways in neutrophils. Moreover, leptin delayed the cleavage of Bid and Bax, the mitochondrial release of cytochrome c and second mitochondria-derived activator of caspase, as well as the activation of both caspase-8 and caspase-3 in these cells. Taken together, leptin is a survival cytokine for human neutrophils, a finding with potential pathologic relevance in inflammatory diseases.

p38 MAPK regulates phosphorylation of Bad via PP2A-dependent suppression of the MEK1/2-ERK1/2 survival pathway in TNF-alpha induced endothelial apoptosis

We recently reported that p38 MAPK regulates TNF-induced endothelial apoptosis via phosphorylation and downregulation of Bcl-xL. Here, we describe that such apoptosis includes p38 MAPK-mediated, protein phosphatase 2A (PP2A)-dependent, downregulation of the MEK-ERK pathway. Inhibition of PP2A with fostriecin or calyculin A significantly increased MEK phosphorylation, as did exposure to the p38 MAPK inhibitor SB203580. Inhibition of MEK potentiated TNF-induced caspase-3 activity and cell death, and both those events were suppressed by treatment with fostriecin or calyculin A. Immunoprecipitation experiments revealed an association between p38 MAPK, PP2A and MEK, and the results of a phosphatase assay suggested that PP2A is a downstream target of p38 MAPK. Importantly, phosphorylation of Bad at Ser-112 was found to be regulated by p38 MAPK and PP2A. In summary, the present findings indicate a novel p38 MAPK-mediated apoptosis pathway, involving activation of Bad via PP2A-dependent inhibition of the MEK-ERK pathway.

Phosphoinositide-3 kinases critically regulate the recruitment and survival of eosinophils in vivo: importance for the resolution of allergic inflammation

The phosphatidylinositol-3 kinase (PI3K) family of signaling enzymes plays a crucial role in leukocyte recruitment and activation and hence, likely regulates the induction and propagation phases of inflammation. However, little data have emerged showing a role for these processes in the resolution phase in models of in vivo inflammation. Here, we have evaluated the role of PI3K for the migration and survival of eosinophils in a model of allergic pleurisy in mice. Eosinophil accumulation in PI3Kgamma-deficient mice was inhibited at 48 h, as compared with wild-type mice but not at earlier time-points (6 and 24 h). Experiments with adoptive transfer of bone marrow showed that PI3Kgamma in eosinophils but not in non-bone marrow-derived cells was required for their accumulation. Systemic treatment with PI3K inhibitors before antigen challenge prevented the recruitment of eosinophils. This was associated with decreased Akt phosphorylation, interleukin-5 production, and eosinophil release from the bone marrow. Treatment with PI3K inhibitors 24 h after antigen challenge markedly cleared the accumulated eosinophils, an effect associated with inhibition of Akt phosphorylation and an increased number of apoptotic events. Altogether, our data demonstrate an important role of PI3Kgamma for the maintenance of eosinophilic inflammation in vivo, whereas other isoforms of PI3K may be relevant for the recruitment process.

Activation of PI3-kinase/PKB contributes to delay in neutrophil apoptosis after thermal injury

Neutrophil apoptosis is delayed under trauma and/or sepsis injury conditions. The molecular mechanism for the delay in apoptosis has not been well defined. We investigated whether activation of phosphatidyl inositol 3-kinase (PI3-kinase)/PKB signaling pathway contributes to the delay in neutrophil apoptosis with thermal injury. Rats were subjected to burns (30% total body surface area, 98 degrees C for 10 s), and euthanized 24 h later. Blood neutrophils were isolated with the use of Ficoll gradient centrifugation and cultured for the indicated time periods. Apoptosis was determined using annexin V and PI labeling and flow cytometry. NF-kappaB activation was examined using gel mobility shift assay and confocal microscopy. Expression levels of inhibitory apoptosis proteins (IAPs), including cellular IAP1 (cIAP1), cIAP2, X-linked IAP (XIAP), and survivin, and Bcl-2 family members such as Bcl-xl and Bad, were determined by Western blot analysis and/or RT-PCR, real-time PCR. The results showed that in culture, the decrease in apoptosis of neutrophils from thermally injured rats was prevented in the presence of PI3-kinase inhibitors wortmannin and LY-294002. There was upregulation of PKB and Bad phosphorylation and NF-kappaB activation in N-formyl-l-methionyl-l-leucyl-l-phenylalanine-stimulated neutrophils from thermally injured rats compared with the sham injured group. Increased Bad phosphorylation and NF-kappaB activation were also attenuated by wortmannin. Bcl-xl expression in neutrophils was upregulated with thermal injury and inhibited in the presence of wortmannin. However, the expression of IAP family members was neither affected by thermal injury nor inhibited by wortmannin. These data suggest that the delay in neutrophil apoptosis with thermal injury is partly caused by activation of PI3-kinase/PKB signaling and NF-kappaB, which appeared to be related to the increased Bcl-xl expression and phosphorylation of Bad, but not IAP expression.

Inhibition of neutrophil apoptosis by TLR agonists in whole blood: involvement of the phosphoinositide 3-kinase/Akt and NF-kappaB signaling pathways, leading to increased levels of Mcl-1, A1, and phosphorylated Bad

Using flow cytometry, we investigated the effect of TLR agonists on human polymorphonuclear neutrophil (PMN) apoptosis in whole blood. LPS (TLR4), peptidoglycan (TLR2), R-848 (TLR7/8), and CpG-DNA (TLR9) were equally effective at delaying spontaneous apoptosis of PMN, while PamCSK4 (TLR1/2), macrophage-activating lipopeptide-2 (TLR2/6), flagellin (TLR5), and loxoribine (TLR7) were less effective or inactive. TLR agonists found to delay apoptosis also extended the functional life span of PMN. Analysis of signaling pathways revealed that the antiapoptotic effect of TLR agonists required NF-kappaB and PI3K activation. Furthermore, analysis of intact cells by flow cytometry showed that TLR agonists delaying PMN apoptosis increased phosphorylation of Akt, a major target of PI3K. This effect was associated with a PI3K-dependent increase in heat shock protein 27 phosphorylation, which has been reported to play a key role in PMN survival. Finally, the TLR-induced delay in PMN apoptosis was associated with increased levels of Mcl-1 and A1, which are antiapoptotic members of the Bcl-2 family. These effects were reversed by PI3K and NF-kappaB inhibitors, respectively. TLR activation also led to PI3K-dependent phosphorylation of the proapoptotic protein Bad. Taken together, our results strongly suggest a role of NF-kappaB and PI3K in TLR-induced PMN survival, leading to modulation of Bcl-2 family molecules.

z-VAD-fmk augmentation of TNFα-stimulated neutrophil apoptosis is compound specific and does not involve the generation of reactive oxygen species

In most cell types constitutive and ligand-induced apoptosis is a caspase-dependent process. In neutrophils, however, the broad-spectrum caspase inhibitor z-VAD-fmk enhances tumor necrosis factor-α (TNFα)-induced cell death, and this has been interpreted as evidence for caspase-dependent and -independent cell death pathways. Our aim was to determine the specificity of the effect of z-VAD-fmk in neutrophils and define the potential mechanism of action. While confirming that z-VAD-fmk (> 100 μM) enhances TNFα-induced neutrophil apoptosis, lower concentrations (1-30 μM) completely blocked TNFα-stimulated apoptosis. Boc-D-fmk, a similar broad-spectrum caspase inhibitor, and z-IETD-fmk, a selective caspase-8 inhibitor, caused a concentration-dependent inhibition of only TNFα-stimulated apoptosis. Moreover, the caspase-9 inhibitor, Ac-LEHD-cmk, had no effect on TNFα-induced apoptosis, and z-VAD-fmk and Boc-D-fmk inhibited TNFα-stimulated reactive oxygen species (ROS) generation. These data suggest that TNFα-induced apoptosis in neutrophils is fully caspase dependent and uses a mitochondrial-independent pathway and that the proapoptotic effects of z-VAD-fmk are compound specific and ROS independent.

Hypoxia-induced neutrophil survival is mediated by HIF-1alpha-dependent NF-kappaB activity

Neutrophils are key effector cells of the innate immune response and are required to migrate and function within adverse microenvironmental conditions. These inflammatory sites are characterized by low levels of oxygen and glucose and high levels of reductive metabolites. A major regulator of neutrophil functional longevity is the ability of these cells to undergo apoptosis. We examined the mechanism by which hypoxia causes an inhibition of neutrophil apoptosis in human and murine neutrophils. We show that neutrophils possess the hypoxia-inducible factor (HIF)-1α and factor inhibiting HIF (FIH) hydroxylase oxygen-sensing pathway and using HIF-1α–deficient myeloid cells demonstrate that HIF-1α is directly involved in regulating neutrophil survival in hypoxia. Gene array, TaqMan PCR, Western blotting, and oligonucleotide binding assays identify NF-κB as a novel hypoxia-regulated and HIF-dependent target, with inhibition of NF-κB by gliotoxin or parthenolide resulting in the abrogation of hypoxic survival. In addition, we identify macrophage inflammatory protein-1β as a novel hypoxia-induced neutrophil survival factor.

The effect of fever‐like temperatures on neutrophil signaling

The effect of fever on neutrophils has not been explored. We tested the hypothesis that fever-like temperature spikes affect neutrophil signaling and function. Prior 60 min, 42 degrees C heat exposure inhibited p38 MAPK, ERK, PI3-Kinase/Akt, and NF-kappaB activation in TNF-alpha-challenged suspended neutrophils. Using pharmacological inhibitors and an inhibitory peptide transduced into neutrophils by a HIV-TAT sequence, we found that p38 MAPK and NF-kappaB mediate TNF-alpha-mediated delayed apoptosis in suspended neutrophils. Heat exposure (39-42 degrees C) did not affect constitutive apoptosis but abrogated TNF-alpha-delayed apoptosis in these suspended cells. In contrast, adhesion-dependent functions were not inhibited. Furthermore, we found that heat exposure neither blocked p38 MAPK, ERK, and NF-kappaB activation in neutrophils on fibronectin nor prevented delayed apoptosis by TNF-alpha when cells interacted with fibronectin. Above and beyond apoptosis, TNF-alpha initiated NF-kappaB-dependent gene transcription. Heat exposure blocked this effect in suspended neutrophils but not in neutrophils on fibronectin. Finally, we show that beta2-integrins, which are not necessary for TNF-alpha-induced NF-kappaB activation at 37 degrees C, transduce costimulatory signals allowing NF-kappaB activation after heat exposure. The effect could protect circulating neutrophils from TNF-alpha activation, while not interfering with activation of adherent neutrophils. Fever could make neutrophils more parsimonious.

The expression and roles of Toll-like receptors in the biology of the human neutrophil

Neutrophils are amongst the first immune cells to arrive at sites of infection, where they initiate antimicrobial and proinflammatory functions, which serve to contain infection. Sensing and defeating microbial infections are daunting tasks as a result of their molecular heterogeneity; however, Toll-like receptors (TLRs) have emerged as key components of the innate-immune system, activating multiple steps in the inflammatory reaction, eliminating invading pathogens, and coordinating systemic defenses. Activated neutrophils limit infection via the phagocytosis of pathogens and by releasing antimicrobial peptides and proinflammatory cytokines and generating reactive oxygen intermediates. Through the production of chemokines, they additionally recruit and activate other immune cells to aid the clearance of the microbes and infected cells and ultimately, mount an adaptive immune response. In acute inflammation, influx of neutrophils from the circulation leads to extremely high cell numbers within tissues, which is exacerbated by their delayed, constitutive apoptosis caused by local inflammatory mediators, potentially including TLR agonists. Neutrophil apoptosis and safe removal by phagocytic cells limit tissue damage caused by release of neutrophil cytotoxic granule contents. This review addresses what is currently known about the function of TLRs in the biology of the human neutrophil, including the regulation of TLR expression, their roles in cellular recruitment and activation, and their ability to delay apoptotic cell death.

Short-term delay of Fas-stimulated apoptosis by GM-CSF as a result of temporary suppression of FADD recruitment in neutrophils: evidence implicating phosphatidylinositol 3-kinase and MEK1-ERK1/2 pathways downstream of classical protein kinase C

Granulocyte/macrophage colony-stimulating factor (GM-CSF) inhibits Fas-induced apoptosis of neutrophils. However, the exact step in the apoptotic pathway blocked by GM-CSF remained unclear. Here, we found that pretreatment of neutrophils with GM-CSF inhibits the recruitment of Fas-associated protein with death domain (FADD) to Fas, abolishing the formation of the death-inducing signaling complex required for Fas-induced apoptosis. Two-dimensional electrophoresis revealed that GM-CSF modifies the ratio of FADD subspecies. These GM-CSF-triggered changes were abrogated, and Fas-induced apoptosis was restored by an inhibitor of classical protein kinase C (PKC), Go6976, and by the combination of a phosphatidylinositol 3-kinase (PI-3K) inhibitor, LY294002, and an inhibitor of mitogen-activated protein kinase kinase (MEK)1, PD98059. Go6976 blocked GM-CSF-elicited phosphorylation of Akt/PKB and extracellular signal-regulated kinase (ERK)1/2. These results indicated that GM-CSF suppresses Fas-induced neutrophil apoptosis by inhibiting FADD binding to Fas, through redundant actions of PI-3K and MEK1-ERK1/2 pathways downstream of classical PKC.

Estradiol abrogates apoptosis in breast cancer cells through inactivation of BAD: Ras-dependent nongenomic pathways requiring signaling through ERK and Akt

Estrogens such as 17-beta estradiol (E(2)) play a critical role in sporadic breast cancer progression and decrease apoptosis in breast cancer cells. Our studies using estrogen receptor-positive MCF7 cells show that E(2) abrogates apoptosis possibly through phosphorylation/inactivation of the proapoptotic protein BAD, which was rapidly phosphorylated at S112 and S136. Inhibition of BAD protein expression with specific antisense oligonucleotides reduced the effectiveness of tumor necrosis factor-alpha, H(2)O(2), and serum starvation in causing apoptosis. Furthermore, the ability of E(2) to prevent tumor necrosis factor-alpha-induced apoptosis was blocked by overexpression of the BAD S112A/S136A mutant but not the wild-type BAD. BAD S112A/S136A, which lacks phosphorylation sites for p90(RSK1) and Akt, was not phosphorylated in response to E(2) in vitro(.) E(2) treatment rapidly activated phosphatidylinositol 3-kinase (PI-3K)/Akt and p90(RSK1) to an extent similar to insulin-like growth factor-1 treatment. In agreement with p90(RSK1) activation, E(2) also rapidly activated extracellular signal-regulated kinase, and this activity was down-regulated by chemical and biological inhibition of PI-3K suggestive of cross talk between signaling pathways responding to E(2). Dominant negative Ras blocked E(2)-induced BAD phosphorylation and the Raf-activator RasV12T35S induced BAD phosphorylation as well as enhanced E(2)-induced phosphorylation at S112. Chemical inhibition of PI-3K and mitogen-activated protein kinase kinase 1 inhibited E(2)-induced BAD phosphorylation at S112 and S136 and expression of dominant negative Ras-induced apoptosis in proliferating cells. Together, these data demonstrate a new nongenomic mechanism by which E(2) prevents apoptosis.

Hypoxic regulation of neutrophil apoptosis role: of reactive oxygen intermediates in constitutive and tumor necrosis factor alpha-induced cell death

Activation of the NADPH oxidase system to generate reactive oxygen species (ROS) plays a key role in bacterial killing by human neutrophils. However, the involvement of such radicals in spontaneous and TNFalpha-driven neutrophil apoptosis remains uncertain. While incubation of cells under anoxic conditions attenuated the pro-apoptotic effect of TNFalpha, full activation of the respiratory burst using PAF followed by fMLP, or the addition of physiologically relevant concentrations of H(2)O(2), had no effect on the rate of apoptosis. Furthermore, the phosphoinositide 3-kinase inhibitor, LY294002, which abolishes receptor-mediated activation of the NADPH oxidase, and five discrete anti-oxidants all failed to affect apoptotic thresholds. Thus ROS do not appear to modulate constitutive apoptosis in neutrophils or appear sufficient to mediate the pro-apoptotic effect of TNFalpha.

Selective regulation by delta-PKC and PI 3-kinase in the assembly of the antiapoptotic TNFR-1 signaling complex in neutrophils

TNF is implicated in the attenuation of neutrophil constitutive apoptosis during sepsis. Antiapoptotic signaling is mediated principally through the TNF receptor-1 (TNFR-1). In adherent neutrophils, when beta-integrin signaling is activated, TNF phosphorylates TNFR-1 and activates prosurvival and antiapoptotic signaling. Previously, we identified the delta-PKC isotype and phosphatidylinositol (PI) 3-kinase as critical regulators of TNF signaling in adherent neutrophils. Both kinases associate with TNFR-1 in response to TNF and are required for TNFR-1 serine phosphorylation, NF-kappaB activation, and inhibition of apoptosis. The purpose of this study was to examine the role of delta-PKC and PI 3-kinase in the assembly of TNFR-1 signaling complex that regulates NF-kappaB activation and antiapoptotic signaling. Coimmunoprecipitation studies established that PI 3-kinase, delta-PKC, and TNFR-1 formed a signal complex in response to TNF. delta-PKC recruitment required both delta-PKC and PI 3-kinase activity, whereas PI 3-kinase recruitment was delta-PKC independent, suggesting that PI 3-kinase acts upstream of delta-PKC. An important regulatory step in control of antiapoptotic signaling is the assembly of the TNFR-1-TNFR-1-associated death domain protein (TRADD)-TNFR-associated factor 2 (TRAF2)-receptor interacting protein (RIP) complex that controls NF-kappaB activation. Inhibition of either delta-PKC or PI 3-kinase decreased TNF-mediated recruitment of RIP and TRAF2 to TNFR-1. In contrast, TRADD recruitment was enhanced. Thus delta-PKC and PI 3-kinase are positive regulators of TNF-mediated association of TRAF2 and RIP with TNFR-1. Conversely, these kinases are negative regulators of TRADD association. These results suggest that delta-PKC and PI 3-kinase regulate TNF antiapoptotic signaling at the level of the TNFR-1 through control of assembly of a TNFR-1-TRADD-RIP-TRAF2 complex.

Granulocyte macrophage colony-stimulating factor signaling and proteasome inhibition delay neutrophil apoptosis by increasing the stability of Mcl-1

Human neutrophils normally have a very short half-life and die by apoptosis. Cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF) can delay this apoptosis via increases in the cellular levels of Mcl-1, an anti-apoptotic protein of the Bcl-2 family with a rapid turnover rate. Here we have shown that inhibition of the proteasome (a) decreases the rate of Mcl-1 turnover within neutrophils and (b) significantly delays apoptosis. This led us to determine whether GM-CSF could enhance neutrophil survival by altering the rate of Mcl-1 turnover. Addition of GM-CSF to neutrophils enhanced Mcl-1 stability and delayed apoptosis by signaling pathways requiring PI3K/Akt and p44/42 Erk/Mek, because inhibitors of these pathways completely abrogated the GM-CSF-mediated effect on both Mcl-1 stability and apoptosis delay. Conversely, induction of Mcl-1 hyperphosphorylation by the phosphatase inhibitor, okadaic acid, significantly accelerated both Mcl-1 turnover and apoptosis. Neither the calpain inhibitor, carbobenzoxy-valinyl-phenylalaninal, nor the pan caspase inhibitor, benzyloxycarbonyl-VAD-fluoromethylketone, had any effect on Mcl-1 stability under these conditions. These observations indicate that profound changes in the rate of neutrophil apoptosis following cytokine signaling occur via dynamic changes in the rate of Mcl-1 turnover via the proteasome.

Bcl-2 family members as sentinels of cellular integrity and role of mitochondrial intermembrane space proteins in apoptotic cell death

In addition to their function as major energy-providing organelles of the cell, mitochondria accomplish a crucial role in apoptosis. The pro-apoptotic BH3-only members of the Bcl-2 family continuously sense the cellular integrity and well-being at various subcellular levels. If these sentinels are induced, released or activated, they converge on the release of mitochondrial intermembrane space proteins such as cytochrome c, the oxidoreductase AIF, endonuclease G, Smac/DIABLO and the serine protease Omi/HtrA2. We discuss how Bcl-2 family members integrate diverse survival and death signals and act as central regulators of apoptosis. Furthermore, we describe the current knowledge on the role of mitochondrial proteins in apoptotic cell death, discuss the molecular mechanisms of their release and the apoptotic role of mitochondria from a phylogenetic and immunological point of view.

C5a delays apoptosis of human neutrophils via an extracellular signal-regulated kinase and Bad-mediated signalling pathway

AIMS

We recently demonstrated that complement fragment C5a delays apoptosis of human neutrophils via induction of the phosphatidylinositol-3 kinase (PI 3-K) pathway. In the present study, we examined whether C5a modulates neutrophil survival through the extracellular signal-regulated kinase (ERK) and Bad-mediated signalling pathway.

METHODS

Human neutrophils were isolated by percoll gradient and preincubated for 1 h with or without PD98059 (20 microM), a specific ERK inhibitor, followed by incubation with C5a (1 microg mL(-1)) for 24 h. Apoptosis was quantified by flow cytometry, using propidium iodide nuclear staining. Extracellular signal-regulated kinase downstream signalling events were evaluated by measuring the expression of cytosolic total and phosphorylated p44/p42 proteins, and Bad phosphorylation using immunoblot analyses. These time-dependent analyses were performed over a brief exposure to C5a (0-30 min). Modulation of cytosolic caspase-9 and caspase-3 activity was measured by Western blot analyses.

RESULTS

C5a inhibited neutrophil apoptosis (P=0.04), which was abrogated in the presence of PD98059 (P=0.04). Time-dependent effect of C5a on p44/p42 phosphorylation was rapid, peaked at 5 min, and was abrogated by the ERK inhibitor (P=0.04). In addition, brief stimulation of neutrophils with C5a induced phosphorylation of Bad, which was inhibited by the ERK inhibitor (P=0.03). Further, C5a suppressed the proteolytic cleavage of caspase-9 and caspase-3, which was reversed by ERK inhibition. Finally, blockade of both the ERK (with PD98059) and PI 3-K (with wortmannin) pathways did not induce additive inhibition of neutrophil apoptosis by C5a.

CONCLUSION

This study demonstrates that in addition to the PI 3-K pathway, C5a also inhibits neutrophil apoptosis via an ERK-signalling pathway, resulting in phosphorylation of Bad and blockade of proteolytic cleavage of caspases. The activation of this additional survival-signalling pathway may be another important cellular mechanism that enhances neutrophil survival in inflammatory states.

Mouse bone marrow contains large numbers of functionally competent neutrophils

The mouse has become an important model for immunological studies including innate immunity. Creating transgenic mice offers unique possibilities to study gene-function relationships. However, relatively little is known about the physiology of neutrophils from wild-type mice. Do they behave like human neutrophils, or are there species-specific differences that need to be considered when extrapolating results from mice to humans? How do we isolate neutrophils from mice? For practical reasons, many studies on mouse neutrophils are done with bone marrow cells. However, human bone marrow neutrophils appear to be heterogeneous and functionally immature. We have isolated and compared neutrophils from mouse bone marrow and from peripheral blood obtained by tail bleeding. Using the same Percoll density gradient for both preparations, we have obtained morphologically mature neutrophils from bone marrow and blood. Both cell populations responded to formylmethionyl-leucyl-phenylalanine (fMLF) with primary and secondary granule release and superoxide production. Quantitative analysis of our data revealed minor differences between cells from bone marrow and blood. Superoxide production and primary granule release were stimulated at lower fMLF concentrations in blood neutrophils. However, the amplitude and the kinetics of maximal responses were similar. The principal difference was the lifespan of the two cell populations. Bone marrow cells survived significantly longer in culture, which may suggest that they are receiving antiapoptic signals that are absent in the blood. Our data suggest that mice have a large reservoir of functionally competent neutrophils in their bone marrow. This reservoir may be needed to replace circulating neutrophils rapidly during infection.

Protein phosphorylation in neutrophils monitored with phosphospecific antibodies

Protein phosphorylation in neutrophils was monitored with two phosphospecific antibodies (pAbs) [termed pPKC(S1) Ab and pPKC(S2) Ab] that recognize products of protein kinase C (PKC) and other Arg/Lys-directed Ser/Thr protein kinases. The pPKC(S1) Ab bound preferentially to p-Ser/p-Thr residues with Arg or Lys in the -3 and -5 positions or the -2 and -3 positions, whereas the pPKC(S2) Ab bound preferentially to p-Ser with Arg or Lys in the -2 and +2 positions and with a hydrophobic residue at the +1 position. Phosphorylated pleckstrin, myristoylated alanine-rich C-kinase substrate (MARCKS), the 47-kDa subunit of the phagocyte oxidase (p47-phox) and numerous unidentified proteins that underwent phosphorylation during neutrophil stimulation were readily detected with these pAbs. Priming effects of tumor necrosis factor alpha (TNF-alpha) and the susceptibility of certain reactions in neutrophils to inhibitors of protein kinases could also be easily investigated with these reagents. Compared to the commonly used 32P-labeling/autoradiographic method, Western blotting with pAbs was found to be a faster, safer, more specific and in many cases more sensitive approach for monitoring protein phosphorylation events in neutrophils. These pAbs may facilitate the identification of several new phosphorylation reactions involved in neutrophil stimulation.

Inhibition of NF-kappaB by a TAT-NEMO-binding domain peptide accelerates constitutive apoptosis and abrogates LPS-delayed neutrophil apoptosis

Delivery of biologically active peptides into human polymorphonuclear neutrophils (PMNs) has implications for studying cellular functions and may be therapeutically relevant. The transcription factor nuclear factor-kappaB (NF-kappaB) regulates the expression of multiple genes controlling inflammation, proliferation, and cell survival. PMNs play a crucial role in first-line defense. Targeting NF-kappaB in these cells may promote apoptosis and therefore facilitate resolution of inflammation. We used an 11-amino acid sequence NEMO-binding domain (NBD) that selectively inhibits the IKKgamma (NEMO)/IKKbeta interaction, preventing NF-kappaB activation. An HIV-TAT sequence served as a highly effective transducing shuttle. We show that lipopolysaccharide (LPS), granulocyte-macrophage colony-stimulating factor (GM-CSF), and dexamethasone (DEX) significantly reduced apoptosis after 20 hours. LPS, but not GM-CSF or DEX, activated NF-kappaB as shown by IkappaBalpha degradation, NF-kappaB DNA binding, and transcriptional activity. The TAT-NBD blocked LPS-induced NF-kappaB activation and NF-kappaB-dependent gene expression. TAT-NBD accelerated constitutive PMN apoptosis dose dependently and abrogated LPS-delayed apoptosis. These results provide a proof of principle for peptide delivery by TAT-derived protein transduction domains to specifically inhibit NF-kappaB activity in PMNs. This strategy may help in controlling various cellular functions even in short-lived, transfection-resistant primary human cells.

Neutrophil apoptosis pathways and their modifications in inflammation

Neutrophils are constantly produced in large numbers in the bone marrow, and the same numbers of cells need to die within a defined time period in order to keep cellular homeostasis under physiologic conditions. Changing the rate of apoptosis rapidly changes cell numbers in such systems. For instance, in many bacterial and autoimmune inflammatory diseases, delayed apoptosis is one important mechanism for neutrophil accumulation. Excessive production of granulocyte/macrophage colony-stimulating factor (GM-CSF) and granulocyte colony-stimulating factor (G-CSF), two important neutrophil survival factors, is often observed in such inflammatory responses. Cytokine withdrawal, as it occurs in the resolution phase of inflammation, leads to the induction of apoptosis. Moreover, neutrophil apoptosis can be accelerated both in the presence and in the absence of survival factors by activation of distinct members of the tumor necrosis factor/nerve growth factor receptor family. This review focuses on recently published work regarding signaling pathways that regulate neutrophil apoptosis.

Selective roles for Toll-like receptor (TLR)2 and TLR4 in the regulation of neutrophil activation and life span

Neutrophil responses to commercial LPS, a dual Toll-like receptor (TLR)2 and TLR4 activator, are regulated by TLR expression, but are amplified by contaminating monocytes in routine cell preparations. Therefore, we investigated the individual roles of TLR2 and TLR4 in highly purified, monocyte-depleted neutrophil preparations, using selective ligands (TLR2, Pam(3)CysSerLys(4) and Staphylococcus aureus peptidoglycan; TLR4, purified LPS). Activation of either TLR2 or TLR4 caused changes in adhesion molecule expression, respiratory burst (alone, and synergistically with fMLP), and IL-8 generation, which was, in part, dependent upon p38 mitogen-activated protein kinase signaling. Neutrophils also responded to Pam(3)CysSerLys(4) and purified LPS with down-regulation of the chemokine receptor CXCR2 and, to a lesser extent, down-regulation of CXCR1. TLR4 was the principal regulator of neutrophil survival, and TLR2 signals showed relatively less efficacy in preventing constitutive apoptosis over short time courses. TLR4-mediated neutrophil survival depended upon signaling via NF-kappa B and mitogen-activated protein kinase cascades. Prolonged neutrophil survival required both TLR4 activation and the presence of monocytes. TLR4 activation of monocytes was associated with the release of neutrophil survival factors, which was not evident with TLR2 activation, and TLR2 activation in monocyte/neutrophil cocultures did not prevent late neutrophil apoptosis. Thus, TLRs are important regulators of neutrophil activation and survival, with distinct and separate roles for TLR2 and TLR4 in neutrophil responses. TLR4 signaling presents itself as a pharmacological target that may allow therapeutic modulation of neutrophil survival by direct and indirect mechanisms at sites of inflammation.