Cleavage of mitogen-activated protein kinases in human neutrophils undergoing apoptosis: role in decreased responsiveness to inflammatory cytokines

TNF-α Induces Neutrophil Apoptosis Delay and Promotes Intestinal Ischemia-Reperfusion-Induced Lung Injury through Activating JNK/FoxO3a Pathway

Background

Intestinal ischemia is a common clinical critical illness. Intestinal ischemia-reperfusion (IIR) leads to acute lung injury (ALI), but the causative factors of ALI are unknown. The aim of this study was to reveal the causative factors and mechanisms of IIR-induced lung injury.

Methods

A mouse model of IIR was developed using C57BL/6 mice, followed by detection of lung injury status and plasma levels of inflammatory factors in sham-operated mice and model mice. Some model mice were treated with a tumor necrosis factor-α (TNF-α) inhibitor lenalidomide (10 mg/kg), followed by observation of lung injury status through hematoxylin and eosin staining and detection of neutrophil infiltration levels through naphthol esterase and Ly6G immunohistochemical staining. Additionally, peripheral blood polymorphonuclear neutrophils (PMNs) were cultured in vitro and then stimulated by TNF-α to mimic in vivo inflammatory stimuli; this TNF-α stimulation was also performed on PMNs after knockdown of FoxO3a or treatment with the c-Jun N-terminal kinase (JNK) inhibitor SP600125. PMN apoptosis after stimulation was detected using flow cytometry. Finally, the role of PMN apoptosis in IIR-induced lung injury was evaluated in vivo by detecting the ALI status in the model mice administered with ABT-199, a Bcl-2 inhibitor.

Results

IIR led to pulmonary histopathological injury and increased lung water content, which were accompanied by increased plasma levels of inflammatory factors, with the TNF-α plasma level showing the most pronounced increase. Inhibition of TNF-α led to effective reduction of lung tissue injury, especially that of the damaging infiltration of PMNs in the lung. In vitro knockdown of FoxO3a or inhibition of JNK activity could inhibit TNF-α-induced PMN apoptosis. Further in vivo experiments revealed that ABT-199 effectively alleviated lung injury and decreased inflammation levels by promoting PMN apoptosis during IIR-induced lung injury.

Conclusion

TNF-α activates the JNK/FoxO3a pathway to induce a delay in PMN apoptosis, which promotes IIR-induced lung injury.

Immune response during the onset of coliform mastitis in dairy cows vaccinated with STARTVAC®

The immune response during the onset of coliform mastitis in vaccinated cows was investigated by measuring lactoferrin (LF), interleukin-8 (IL-8), and interleukin-1β (IL-1β) concentrations and somatic cell counts in 28 milk samples at the onset of acute coliform mastitis (ACM) and 73 milk samples at the onset of peracute coliform mastitis (PCM). Vaccinated ACM, unvaccinated ACM, and vaccinated PCM showed significantly higher values for LF and IL-1β levels than unvaccinated PCM (p < .01). The IL-8 concentration was lower in vaccinated PCM than in unvaccinated PCM (p < .05). There was no significant difference in somatic cell counts for each parameter. There were no significant differences in the parameters between vaccinated and unvaccinated ACM cows, or vaccinated ACM and PCM cows. From the above results, it is suggested that mastitis vaccination improved the early immune response, particularly at the onset of PCM, and played a large role in host defense against the initial infection.

Resolution of inflammation: mechanisms and opportunity for drug development

Inflammation is a beneficial host reaction to tissue damage and has the essential primary purpose of restoring tissue homeostasis. Inflammation plays a major role in containing and resolving infection and may also occur under sterile conditions. The cardinal signs of inflammation dolor, calor, tumor and rubor are intrinsically associated with events including vasodilatation, edema and leukocyte trafficking into the site of inflammation. If uncontrolled or unresolved, inflammation itself can lead to further tissue damage and give rise to chronic inflammatory diseases and autoimmunity with eventual loss of organ function. It is now evident that the resolution of inflammation is an active continuous process that occurs during an acute inflammatory episode. Successful resolution requires activation of endogenous programs with switch from production of pro-inflammatory towards pro-resolving molecules, such as specific lipid mediators and annexin A1, and the non-phlogistic elimination of granulocytes by apoptosis with subsequent removal by surrounding macrophages. These processes ensure rapid restoration of tissue homeostasis. Here, we review recent advances in the understanding of resolution of inflammation, highlighting the pharmacological strategies that may interfere with the molecular pathways which control leukocyte survival and clearance. Such strategies have proved beneficial in several pre-clinical models of inflammatory diseases, suggesting that pharmacological modulation of the resolution process may be useful for the treatment of chronic inflammatory diseases in humans.

Granule swelling and cleavage of mitogen-activated protein kinases in human neutrophils undergoing apoptosis

Extracellular signal-regulated kinase and p38 have been shown to be cleaved in human neutrophils undergoing apoptosis induced by tumor necrosis factor-alpha and cycloheximide. However, the cleavage products of these molecules were undetected when apoptotic neutrophils were pretreated with phenylmethylsulfonyl fluoride or disrupted by nitrogen cavitation before preparation of cell lysates. The electron microscopy revealed that granules in apoptotic neutrophils were significantly swollen than those in control cells. These findings suggest that granule membrane may become destabilized during neutrophil apoptosis, leading to rapid proteolysis of these molecules by granule-derived serine proteases during preparation of cell lysates with the conventional lysis buffer.

Distinct role of c-Jun N-terminal kinase isoforms in human neutrophil apoptosis regulated by tumor necrosis factor-alpha and granulocyte-macrophage colony-stimulating factor

We studied the role of c-Jun N-terminal kinase (JNK) in human neutrophils stimulated by tumor necrosis factor-alpha (TNF-alpha) and granulocyte-macrophage colony-stimulating factor (GM-CSF). Stimulation of neutrophils with TNF-alpha and GM-CSF caused phosphorylation of p54 or p46 JNK or both. The phosphorylated p46 JNK band in TNF-alpha-stimulated neutrophils mobilized faster than that in GM-CSF-stimulated cells. The JNK isoform transcripts expressed in neutrophils were JNK1beta1, JNK1beta2, JNK2alpha1, and JNK2alpha2. The JNK isoforms phosphorylated by TNF-alpha and GM-CSF stimulation were found to be JNK1 and JNK2, respectively, on the basis of the molecular mass and the capture assay. TNF-alpha-induced JNK phosphorylation was sustained in the presence of cycloheximide, which was accompanied by accelerated neutrophil apoptosis. The JNK inhibitors (SP600125 and TAT-TI-JIP(153163)) suppressed neutrophil apoptosis induced by TNF-alpha plus cycloheximide, whereas they attenuated the GM-CSF-mediated antiapoptotic effect on neutrophils. The JNK inhibitor did not affect the levels of Mcl-1 and XIAP (antiapoptotic molecules), which were regulated by TNF-alpha plus cycloheximide and GM-CSF. The JNK inhibitor markedly suppressed TNF-alpha-induced and GM-CSF-induced superoxide release. These findings suggest that JNK1 and JNK2 are involved in TNF-alpha-induced neutrophil apoptosis and GM-CSF-mediated antiapoptotic effect on neutrophils, respectively, and both JNK isoforms are involved in TNF-alpha-induced and GM-CSF-induced superoxide release.

Calcium channel blockers suppress cytokine-induced activation of human neutrophils

BACKGROUND

Neutrophils, in concert with proinflammatory cytokines, play an important role in the progression of atherosclerosis. Calcium channel blockers are commonly used in the treatment of hypertension, and their pleiotropic effects, other than the lowering of blood pressure, have been recently recognized.

METHODS

We studied the effects of various calcium channel blockers (amlodipine, nicardipine, cilnidipine, benidipine, efonidipine, nifedipine, azelnidipine, verapamil, and diltiazem; each being used at 5 and 10 micromol/l) on superoxide (O(2)(-)) release, migration, and signaling pathways in human neutrophils stimulated by granulocyte-macrophage colony-stimulating factor (GM-CSF) or tumor necrosis factor-alpha (TNF-alpha).

RESULTS

GM-CSF-induced O(2)(-) release was suppressed by amlodipine, nicardipine, and cilnidipine, whereas TNF-alpha-induced O(2)(-) release was suppressed by amlodipine, nicardipine, cilnidipine, benidipine, efonidipine, nifedipine, and azelnidipine. TNF-alpha-induced phosphorylation of extracellular signal-regulated kinase (ERK) and Akt, but not p38 mitogen-activated protein kinase (MAPK), was attenuated by nicardipine, cilnidipine, benidipine, efonidipine, and azelnidipine. By contrast, GM-CSF-induced phosphorylation of ERK, p38, and Akt was affected by none of the blockers. GM-CSF-induced neutrophil migration was also suppressed by amlodipine and nicardipine, but not by azelnidipine, when these blockers were assessed for their effect on neutrophil migration.

CONCLUSIONS

These findings suggest that (i) some calcium channel blockers can suppress cytokine-induced neutrophil activation, leading to possible prevention of the progression of atherosclerosis; and (ii) that activation of the ERK and phosphatidylinositol 3-kinase (PI3K)/Akt pathways, induced by TNF-alpha but not by GM-CSF, is selectively affected by some blockers.

Age-related impairment of GM-CSF-induced signalling in neutrophils: role of SHP-1 and SOCS proteins

Functional activities of mature human neutrophils are strongly influenced by the pro-inflammatory cytokine granulocyte macrophage-colony stimulating factor (GM-CSF). Accordingly, a defective response to GM-CSF might have dramatic consequences for neutrophil functions and the host defence against infections. Such an event is most likely to occur in senescence. A number of studies have, in fact, reported an impairment of the GM-CSF capacity to prime and/or to activate respiratory burst, as well as to delay apoptotic events, in neutrophils from elderly individuals. In the last 2 decades many efforts have been made to explore at molecular levels the mechanism underlying these defects. Recent studies let us depict a scenario in which an increased activity of inhibitory molecules, such as Src homology domain-containing protein tyrosine phosphatase-1 (SHP-1) and suppressors of cytokine signalling (SOCS), is responsible for the age-related failure of GM-CSF to stimulate neutrophil functions via inhibition of Lyn-, phosphoinositide 3-kinase (PI3-K)/extracellular signal-regulated kinase (ERK)- and signal transducers and activators of transcription (STAT)-dependent pathways. The control of SHP-1 and/or SOCS activity might therefore be an important therapeutic target for the restoration of normal immune responses during senescence.

Toll-like receptor agonists stimulate human neutrophil migration via activation of mitogen-activated protein kinases

Human neutrophil migratory responses to Toll-like receptor (TLR) agonists were studied using videomicroscopy. When challenged with lipopolysaccharide (LPS, TLR4 agonist) or N-palmitoyl-S-[2,3-bis(palmitoyloxy)-(2RS)-propyl]-(R)-cysteinyl-seryl-(lysyl)(3)-lysine (P3CSK4, TLR2 agonist), neutrophils displayed enhanced motility, which was found to reflect increased random migration but not directed migration (chemotaxis). Enhanced neutrophil motility was detected within 10 min after stimulation with LPS or P3CSK4, and was sustained for more than 80 min. Stimulation of neutrophils with LPS or P3CSK4 resulted in the activation of extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK), which preceded neutrophil migration. TLR-mediated neutrophil migration was strongly suppressed by pretreatment of cells with U0126 (MAPK/ERK kinase inhibitor) but not with U0124 (an inactive analogue of U0126) or SB203580 (a p38 MAPK inhibitor), and was almost completely abolished by pretreatment of cells with U0126 and SB203580 in combination. Randomly migrating neutrophils in response to LPS or P3CSK4 displayed directed migration when further challenged with gradient concentrations of N-formyl-methionyl-leucyl-phenylalanine (FMLP) or platelet-activating factor (PAF). These findings indicate that TLR agonists stimulate human neutrophil migration via the activation of ERK and p38 MAPK, and FMLP- or PAF-induced neutrophil chemotaxis is not affected by the pre-exposure of cells to TLR agonists.

Growth factors G-CSF and GM-CSF differentially preserve chemotaxis of neutrophils aging in vitro

OBJECTIVE

The ability of human neutrophils to migrate was studied during culture in vitro.

METHODS

Neutrophils were isolated from human blood and cultured at 37 degrees C. Apoptosis was determined by Annexin-V fluorescein isothiocyanate binding. Receptor expression was measured by fluorescence in situ hybridization analysis with monoclonal antibodies. Migration was assessed with Transwell Fluoroblock inserts and calcein-stained neutrophils. Extracellular signal-regulated kinase 1/2 (ERK-1/2) activation was determined with monoclonal antibody against phosphorylated ERK-1/2.

RESULTS

Upon culture, untreated neutrophils downregulated the chemotaxin receptors FPR, CXC chemokine receptor 1, and CXC chemokine receptor 2 and lost the ability to migrate to formyl-methionyl-leucyl-phenylalanin, interleukin 8 (IL-8), and C5a. In contrast, expression of CXCR4 was induced; this receptor was able to signal (increase in intracellular free calcium ions [Ca(2+)](i), ERK-1/2 activation) but was nonfunctional (no chemotaxis to stromal cell-derived factor-1alpha). The myeloid growth factors granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF) retarded the process of functional decay during cell culture. However, while preserving chemotaxis of neutrophils toward formyl-methionyl-leucyl-phenylalanin or C5a, GM-CSF-in contrast to G-CSF-did not preserve chemotaxis toward IL-8, with a corresponding downregulation of the IL-8 receptors. The decay in neutrophil chemotaxis occurred prior to detectable phosphatidylserine (PS)-exposure. In contrast, the induction of [Ca(2+)](i) rises and ERK-1/2 activation correlated with chemotaxin receptor expression unless the cells were truly apoptotic.

CONCLUSION

Neutrophils aging in vitro lose their chemotactic capacity. Functional decay starts prior to PS exposure and can be partially prevented by G-CSF and GM-CSF, in a differential fashion. These growth factors act by increasing the number of viable neutrophils, by altering the levels of chemotaxin receptor expression, and-independently-by affecting signaling cascades.

Role of mesothelial cell-derived granulocyte colony-stimulating factor in interleukin-17-induced neutrophil accumulation in the peritoneum

Recent studies suggest that peritoneal CD4(+) T lymphocytes may control recruitment of polymorphonuclear leukocytes (PMN) during peritonitis by an interleukin-17 (IL-17)-dependent mechanism. IL-17 and granulocyte colony-stimulating factor (G-CSF) have been proposed to form an axis that regulates PMN transmigration. Here we report on the role of G-CSF released by human peritoneal mesothelial cells (HPMCs) in IL-17A-mediated peritoneal PMN accumulation. In vitro exposure of HPMCs to IL-17A resulted in a time- and dose-dependent release of G-CSF. This effect was related to the induction of G-CSF mRNA and mediated through the nuclear factor-kappaB (NF-kappaB) pathway. The novel observation was that IL-17A-stimulated NF-kappaB activation in HPMCs followed a biphasic profile, with an early induction (45 min), followed by the return to basal levels (90 min), and a delayed induction (3 h). Tumor necrosis factor alpha synergistically amplified IL-17A-induced G-CSF production by enhanced NF-kappaB activation and through stabilization of G-CSF mRNA. Intraperitoneal (i.p.) administration of IL-17A in Balb/c mice resulted in increased local levels of G-CSF and selective PMN accumulation. Administration of anti-G-CSF blocking antibody before IL-17A injection significantly reduced the IL-17A-triggered PMN infiltration. This effect occurred despite increased i.p. levels of PMN-specific chemokines KC and macrophage inflammatory protein-2 seen in animals treated with anti-G-CSF antibody. These data demonstrate that the mesothelium-derived G-CSF plays an important role in IL-17A-induced PMN recruitment into the peritoneum.

Roles of mitogen-activated protein kinase pathways during Escherichia coli-induced apoptosis in U937 cells

Escherichia coli (E. coli) infections play an important and growing role in the clinic. In the present study, we investigated the involvement of members of the mitogen-activated protein kinase (MAPK) superfamily, including extracellular signal-regulated kinase (ERK), c-Jun NH(2)-terminal kinase (JNK) and p38 MAPK, and caspase-3 and 9 activity in E. coli-induced apoptosis in human U937 cells. We found that E. coli induces apoptosis in U937 cell lines in a dose- and time-dependent manner, p38 MAPK and JNK were activated after 10 min of infection with E. coli. In contrast, ERK1/2 was down-regulated in a time-dependent manner. The levels of total (phosphorylation state-independent) p38 MAPK, JNK and ERK1/2 did not change in E. coli-infected U937 cells at all times examined. Moreover, exposure of U937 cells to E. coli led to caspase-3 and 9 activity. For the evaluation of the role of MAPKs, PD98059, SB203580 and SP600125 were used as MAPKs inhibitors for ERK1/2, p38 MAPK and JNK. Inhibition of ERK1/2 with PD98059 caused further enhancement in apoptosis and caspase-3 and 9 activity, while a selective p38 MAPK inhibitor, SB203580 and JNK inhibitor, SP600125 significantly inhibited E. coli-induced apoptosis and caspase-3 and 9 activity in U937 cells. The results were further confirmed by the observation that the caspase inhibitors Z-DEVD-FMK and Z-LEHD-FMK blocked E. coli-induced U937 apoptosis. Taken together, we have shown that E. coli increase p38 MAPK and JNK and decrease ERK1/2 phosphorylation and increase caspase-3 and 9 activity in U937 cells.

Neutrophil Signaling Pathways Activated by Bacterial DNA Stimulation

We have previously shown that bacterial DNA activates human neutrophils in a CpG-independent manner. In this study, we have characterized the signaling pathways involved in the activation mechanism. We found that p38 MAPK, ERK1/2, and JNK pathways, as well as the PI3K/Akt pathway, are activated by bacterial DNA. We also determined that bacterial DNA induces NF-kappaB and AP-1 activation. When analyzing the role of these pathways on neutrophil functions, we observed that up-regulation of CD11b triggered by bacterial DNA was decreased by pharmacological inhibitors of the p38 MAPK, ERK1/2, and JNK, whereas stimulation of IL-8 release was dependent on p38, ERK1/2, and NF-kappaB. Moreover, we found that IL-8 production was markedly enhanced by inhibition of JNK, suggesting that this pathway negatively modulates NF-kappaB-dependent transcription. We also observed that bacterial DNA stimulated IL-1R-associated kinase-1 kinase activity and its partial degradation. Finally, we determined that bacterial DNA stimulated CD11b up-regulation in TLR9(-/-) but not in MyD88(-/-) mouse neutrophils, supporting that bacterial DNA induces neutrophil activation through a TLR9-independent and MyD88-dependent pathway.

Fever-like temperatures affect neutrophil NF-kappaB signaling, apoptosis, and ANCA-antigen expression

The neutrophil is pivotal to ANCA vasculitis pathogenesis. Fever frequently complicates ANCA diseases. This study investigated the effects of short-term heat exposure on apoptosis in neutrophils that were treated with LPS, GM-CSF, IL-8, and dexamethasone. All compounds delayed apoptosis. Heat abrogated the apoptosis-delaying effect of LPS without affecting constitutive apoptosis or delayed apoptosis by GM-CSF, IL-8, or dexamethasone. The heat effect was dose dependent over the 39 to 42 degrees C range. NF-kappaB but not extracellular signal-regulated kinase, p38 mitogen-activated protein kinase (MAPK), or phosphatidylinositol 3-kinase/Akt controlled LPS-delayed apoptosis. Furthermore, LPS-induced IkappaBalpha degradation, DNA binding, and NF-kappaB-dependent gene transcription activation were abrogated by short-term heat. When core temperatures were raised to 40.5 degrees C for 30 min in mice, LPS-induced neutrophil NF-kappaB activation also was prevented. Short-term heat removed heat-shock protein 90 from the IkappaB kinase complex, resulting in failure of LPS-induced IkappaB kinase activation. Despite delayed apoptosis, ANCA antigen expression was increased in LPS-treated neutrophils. ANCA antigen increase was prevented by p38 MAPK inhibition and by heat exposure. Heat exposure did not inhibit LPS-induced p38 MAPK phosphorylation. Instead, apoptosis-mediated p38 MAPK degradation was accelerated, thereby decreasing the p38 MAPK that was available for LPS-mediated ANCA antigen upregulation. These data suggest that fever-like temperatures modulate neutrophil behavior in this disease.

The anti-apoptotic effect of leukotriene B4 in neutrophils: A role for phosphatidylinositol 3-kinase, extracellular signal-regulated kinase and Mcl-1

The constitutive commitment of neutrophils to apoptosis is a key process for the control and resolution of inflammation and it can be delayed by various inflammatory mediators including leukotriene B4 (LTB4). The mechanisms by which LTB4 contributes to neutrophil survival are still unclear and the present work aims at identifying intracellular pathways underlying this effect. Inhibition of human neutrophil apoptosis by LTB4 was abrogated by the phosphatidylinositol 3-kinase (PI3-K) inhibitor wortmannin and by the specific MEK inhibitor PD98059. In contrast, inhibitors of p38 MAPK, Jak2/3 and Src did not hinder the anti-apoptotic effect of LTB4. We also investigated the effects of members of the Bcl-2 family as they play a crucial role in the regulation of programmed cell death. When neutrophils were incubated with LTB4 for 1 to 6 h, the mRNA levels of the anti-apoptotic protein Mcl-1 were upregulated approximately 2-fold, while those of the pro-apoptotic protein Bax were downregulated 3- to 4-fold, as determined by real-time PCR. Accordingly, Western blot analysis revealed that the expression of Mcl-1 was upregulated in presence of LTB4, while flow cytometric analysis revealed that Bax protein was downregulated. Furthermore, the modulatory effects of LTB4 on Mcl-1 and Bax proteins were abolished in the presence of either wortmannin or PD98059. Taken together, these results demonstrate the participation of PI3-K and MEK/ERK kinases, as well as regulatory apoptotic proteins such as Mcl-1 and Bax, in the anti-apoptotic effects of LTB4 in human neutrophils.

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.

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.

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.

Activation of human neutrophils by granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, and tumor necrosis factor alpha: role of phosphatidylinositol 3-kinase

Stimulation of human neutrophils with granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage CSF (GM-CSF), or tumor necrosis factor alpha (TNF) resulted in phosphorylation of Akt, the potency being GM-CSF > G-CSF = TNF, which was inhibited by wortmannin. The findings indicated that phosphatidylinositol 3-kinase (PI3K) is activated by these cytokines. The possible participation of PI3K in activation of neutrophil functions induced by these cytokines was explored with PI3K inhibitors (wortmannin and LY294002). Superoxide release and adherence induced by GM-CSF or TNF were inhibited by PI3K inhibitors. Actin reorganization and morphological changes induced by G-CSF or GM-CSF were also inhibited by wortmannin, whereas these responses induced by TNF were unaffected by wortmannin. These findings suggested that PI3K is differentially involved in cytokine-mediated activation of neutrophil functions depending on the cytokines used. The results also showed that activation of extracellular signal-regulated kinase, but not p38 mitogen-activated protein kinase, induced by these cytokines is partly mediated by PI3K activation.

Functional characterization of mitochondria in neutrophils: a role restricted to apoptosis

Mitochondria are known to combine life-supporting functions with participation in apoptosis by controlling caspase activity. Here, we report that in human blood neutrophils the mitochondria are different, because they preserve mainly death-mediating abilities. Neutrophil mitochondria hardly participate in ATP synthesis, and have a very low activity of the tested marker enzymes. The presence of mitochondria in neutrophils was confirmed by quantification of mitochondrial DNA copy number, by detection of mitochondrial porin, and by JC-1 measurement of Deltapsi(m). During neutrophilic differentiation, HL-60 cells demonstrated a profound cytochrome c depletion and mitochondrial shape change reminiscent of neutrophils. However, blood neutrophils containing extremely low amounts of cytochrome c displayed strong caspase-9 activation during apoptosis, which was also observed in apoptotic neutrophil-derived cytoplasts lacking any detectable cytochrome c. We suggest that other proapoptotic factors such as Smac/DIABLO and HtrA2/Omi, which are massively released from the mitochondria, have an important role in neutrophil apoptosis.

Role of defective ERK phosphorylation in the impaired GM-CSF-induced oxidative response of neutrophils in elderly humans

GM-CSF-induced oxidative responses are defective in neutrophils of elderly humans. In the present study we evaluated whether this phenomenon might be related to alterations in cytokine-dependent MAPK signalling. Neutrophils obtained from elderly humans and stimulated with GM-CSF showed a significant reduction in phosphorylated ERK1/2 levels and an even higher decrease in ERK1/2 activation with respect to baseline. No changes in GM-CSF-induced p38 MAPK phosphorylation were observed. Cell pretreatment with the MEK inhibitor PD98059 determined a marked suppression of GM-CSF-induced O2- release. Interestingly, under the above experimental condition, there was no longer any difference in O2- production observed between elderly and young subjects. Furthermore, despite the fact that the p38 MAPK pathway was activated less strongly by GM-CSF, the p38 MAPK inhibitor SB203580 reduced GM-CSF-induced O2- production in the neutrophils of the elderly to levels similar to those obtained with PD98059. TNF-alpha-triggered O2- production was not altered by ageing and in fact, a similar ERK1/2 or p38 MAPK activation was found in TNF-alpha-stimulated neutrophils from elderly and young individuals. In accordance with the different potency of TNF-alpha in activating ERK1/2 and p38 MAPK, the TNF-alpha-induced oxidative responses were more sensitive to the inhibitory effects of SB203580 than to those of PD98059 in young as well as elderly subjects. These results suggest that, along the GM-CSF-dependent ERK signalling pathway, a step proximal to MEK1/2 but distal to the connection with the p38 MAPK module likely becomes defective as a feature of age. The consequent decline in ERK1/2 activation could potentially account for the GM-CSF-dependent impairment of the neutrophil respiratory burst that occurs with ageing.

Proteolytic conversion of STAT3alpha to STAT3gamma in human neutrophils: role of granule-derived serine proteases

Four isoforms (alpha, beta, gamma, and delta) have been identified for signal transducer and activator of transcription 3 (STAT3). It has been reported that STAT3gamma, which is derived from STAT3alpha by limited proteolysis during granulocytic differentiation, is a major STAT3 isoform expressed in human neutrophils. We confirmed that STAT3gamma was a major STAT3 isoform detected in human neutrophil lysates prepared with the conventional lysis buffer. The enzymes capable of converting STAT3alpha to STAT3gamma in vitro were localized in neutrophil granule fraction and were released into the medium upon ionomycin stimulation. The enzyme activity was strongly inhibited by phenylmethylsulfonyl fluoride, CuSO(4), and ONO-5046 (a specific inhibitor of neutrophil elastase), but not by aprotinin, leupeptin, benzamidine, and EDTA. STAT3gamma was effectively generated in vitro from STAT3alpha by limited proteolysis with human neutrophil elastase or proteinase 3 but not cathepsin G. The converting activity in neutrophil lysates was reduced by immunodepletion of elastase but not proteinase 3. Unexpectedly, STAT3gamma was undetected in the lysates of neutrophil-derived cytoplasts, which lack granules, and the cytosol fraction prepared by nitrogen cavitation. The STAT3 isoform detected in these preparations was primarily STAT3alpha. STAT3gamma was also undetected in the lysates of PMSF-pretreated neutrophils and was markedly decreased in the lysates of ionomycin-pretreated neutrophils. These findings indicate that, in contrast to the previous reports, STAT3alpha, but not STAT3gamma, is primarily expressed in human neutrophils, and STAT3gamma is rapidly generated from STAT3alpha by limited proteolysis with granule-derived serine proteases during preparation of neutrophil lysates with the conventional lysis buffer.

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.

RGD- and MLD-disintegrins, jarastatin and EC3, activate integrin-mediated signaling modulating the human neutrophils chemotaxis, apoptosis and IL-8 gene expression

The effects of jarastatin (JT), a monomeric RGD-disintegrin, were compared with those of the heterodimeric MLD-disintegrin, EC3, on human neutrophil activation and functions. Both disintegrins inhibited neutrophil chemotaxis induced by fMet-Leu-Phe and were also potent chemotactic agents. These effects were accompanied by an increase in actin polymerization, and both were inhibited by genistein, a tyrosine kinase inhibitor. While JT, but not other RGD-disintegrins, inhibited EC3-induced chemotaxis, EC3 was not able to inhibit JT effect. The chemotactic effect of JT was blocked by anti-alpha(M) antibody whereas anti-alpha(9)beta(1) inhibited EC3 effect. Both JT and EC3 induced focal adhesion kinase (FAK) and phosphoinositide 3-kinase (PI3K) activation. Accordingly, LY294002, a PI3K inhibitor, impaired their chemotactic effect on neutrophils. JT induced Erk-2 translocation to nucleus and a delay of the spontaneous apoptosis of neutrophils in vitro. In contrast, EC3 inhibited Erk-2 activation and had a proapoptotic effect. These effects were reverted by PD98059, an MEK 1/2 inhibitor and blocked by z-VAD-FMK, a caspase inhibitor. In addition, JT, but not EC3, increased the IL-8 mRNA levels in neutrophils. The data indicate that JT and EC3 directly activate an integrin-coupled signaling and modulate the MAPK pathway in different ways, leading the neutrophils to express different functional response.

Actin reorganization and morphological changes in human neutrophils stimulated by TNF, GM-CSF, and G-CSF: the role of MAP kinases

Stimulation of human neutrophils with tumor necrosis factor-alpha (TNF), granulocyte-macrophage colony-stimulating factor (GM-CSF), or granulocyte CSF (G-CSF) resulted in decreased fluorescence intensity of FITC-phalloidin (actin depolymerization) and morphological changes. Cytokine-induced actin depolymerization was dependent on the concentration of cytokines used as stimuli. The maximal changes were detected at 10 min after stimulation with TNF or GM-CSF and at 20 min after stimulation with G-CSF. Cytokine-induced actin depolymerization was sustained for at least 30 min after stimulation. In contrast, N-formyl-methionyl-leucyl-phenylalanine (FMLP) rapidly (within 45 s) induced an increase in the fluorescence intensity of FITC-phalloidin (actin polymerization) and morphological changes. TNF- and GM-CSF-induced actin depolymerization and morphological changes, but not FMLP-induced responses, were partially inhibited by either PD-98059, an inhibitor of mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase, or SB-203580, an inhibitor of p38 MAPK, and were almost completely abolished by these inhibitors in combination. G-CSF-induced responses were almost completely abolished by PD-98059 and were unaffected by SB-203580. These findings are consistent with the ability of these cytokines to activate the distinct MAPK subtype cascade in human neutrophils. Phosphorylated ERK and p38 MAPK were not colocalized with F-actin in neutrophils stimulated by cytokines or FMLP. Furthermore, FMLP-induced polarization and actin polymerization were prevented by cytokine pretreatment. These findings suggest that TNF, GM-CSF, and G-CSF induce actin depolymerization and morphological changes through activation of ERK and/or p38 MAPK and that cytokine-induced actin reorganization may be partly responsible for the inhibitory effect of these cytokines on neutrophil chemotaxis.

Serine protease inhibitors inhibit superoxide release and adherence in human neutrophils stimulated by granulocyte-macrophage colony-stimulating factor and tumor necrosis factor-alpha

Stimulation of human neutrophils with granulocyte-macrophage colony-stimulating factor (GM-CSF) or tumor necrosis factor-alpha (TNF) results in increased superoxide (O2-) release and adherence. O2- release and adherence are dependent on activation of extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK). Possible participation of serine proteases in GM-CSF- or TNF-induced activation of human neutrophils was explored with various serine protease inhibitors, including phenylmethylsulfonyl fluoride, L-1-tosylamido-2-phenylethyl-chloromethyl ketone and N-alpha-p-tosyl-L-lysine-chloromethyl ketone. GM-CSF- or TNF-induced O2- release and adherence were inhibited in parallel by pretreatment of neutrophils with these inhibitors. On the other hand, GM-CSF- or TNF-induced phosphorylation of ERK and p38 MAPK was unaffected by these inhibitors at the concentrations effective for the inhibition of O2- release and adherence. These findings suggest that serine proteases are involved in GM-CSF- and TNF-induced O2- release and adherence in human neutrophils and that serine proteases function downstream or independently of the activation of ERK and p38 MAPK.

Expression of the inhibitor of apoptosis (IAP) family members in human neutrophils: up-regulation of cIAP2 by granulocyte colony-stimulating factor and overexpression of cIAP2 in chronic neutrophilic leukemia

Human neutrophils were found to express members of the inhibitor of apoptosis (IAP) family, namely cellular IAP1 (cIAP1), cIAP2, and X-linked IAP. Among these members, cIAP2 expression was selectively up-regulated by stimulation with granulocyte colony-stimulating factor (G-CSF), but not with granulocyte-macrophage CSF. The increased expression of cIAP2 mRNA was detected as early as 30 minutes after in vitro stimulation with G-CSF, and the elevated level of cIAP2 protein was detected at 1 hour. The elevated level of cIAP2 protein was also detected in peripheral blood neutrophils obtained from healthy donors receiving G-CSF administration. G-CSF-induced up-regulation of cIAP2 mRNA and protein, phosphorylation of signal transducer and activator of transcription 3 (STAT3), and the antiapoptotic effects were inhibited by pretreatment of cells with AG490, a specific inhibitor of Janus kinase 2 (JAK2). Mature neutrophils from a patient with chronic neutrophilic leukemia exhibited remarkable overexpression of cIAP2 mRNA and prolongation of survival, whereas cIAP2 mRNA expression and survival in mature neutrophils from patients with chronic myelogenous leukemia were essentially similar to those in normal neutrophils. These findings suggest that cIAP2 expression is up-regulated by G-CSF through activation of the JAK2-STAT3 pathway, and increased expression of cIAP2 protein may contribute to G-CSF-mediated antiapoptosis. In addition, overexpression of cIAP2 may be partly responsible for sustained neutrophilia at least in some cases of chronic neutrophilic leukemia.

Broad-spectrum caspase inhibition paradoxically augments cell death in TNF-alpha -stimulated neutrophils

It is increasingly clear that there are caspase-dependent and -independent mechanisms for the execution of cell death and that the utilization of these mechanisms is stimulus- and cell type-dependent. Intriguingly, broad-spectrum caspase inhibition enhances death receptor agonist-induced cell death in a few transformed cell lines. Endogenously produced oxidants are causally linked to necroticlike cell death in these instances. We report here that broad-spectrum caspase inhibitors effectively attenuated apoptosis induced in human neutrophils by incubation with agonistic anti-Fas antibody or by coincubation with tumor necrosis factor-alpha (TNF-alpha) and cycloheximide ex vivo. In contrast, the same caspase inhibitors could augment cell death upon stimulation by TNF-alpha alone during the 6-hour time course examined. Caspase inhibitor-sensitized, TNF-alpha-stimulated, dying neutrophils exhibit apoptoticlike and necroticlike features. This occurred without apparent alteration in nuclear factor-kappaB (NF-kappaB) activation. Nevertheless, intracellular oxidant production was enhanced and sustained in caspase inhibitor-sensitized, TNF-alpha-stimulated neutrophils obtained from healthy subjects. However, despite reduced or absent intracellular oxidant production following TNF-alpha stimulation, cell death was also augmented in neutrophils isolated from patients with chronic granulomatous disease incubated with a caspase inhibitor and TNF-alpha. These results demonstrate that, in human neutrophils, TNF-alpha induces a caspase-independent but protein synthesis-dependent cell death signal. Furthermore, they suggest that TNF-alpha activates a caspase-dependent pathway that negatively regulates reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity.

Antiapoptotic effect of granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, and cyclic AMP on human neutrophils: protein synthesis-dependent and protein synthesis-independent mechanisms and the role of the Janus kinase-STAT pathway

Spontaneous neutrophil apoptosis during culture was delayed by granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage CSF (GM-CSF), or dibutyryl-cyclic adenosine monophosphate (cAMP), whereas apoptosis was accelerated by cycloheximide or actinomycin D. G-CSF-mediated antiapoptosis was completely abolished by cycloheximide or actinomycin D, whereas GM-CSF-mediated antiapoptosis was not completely abolished by these inhibitors. Antiapoptosis induced by dibutyryl-cAMP was highly resistant to cycloheximide, and that induced by benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone was unaffected by cycloheximide. G-CSF- and GM-CSF-mediated antiapoptosis and phosphorylation of signal transducer and activator of transcription 3 (STAT3) and STAT5 were inhibited by AG490, an inhibitor of Janus kinase. The level of Mcl-1 protein was not associated with neutrophil apoptosis. The results suggest that (a) neutrophil survival in the resting state is primarily regulated by the constitutive synthesis of antiapoptotic proteins; (b) the prevention of spontaneous apoptosis is mediated through the protein synthesis-dependent and/or protein synthesis-independent mechanisms according to the stimuli used; and (c) the Janus kinase-STAT pathway is involved in G-CSF- and GM-CSF-mediated antiapoptosis.

Mycobacterium tuberculosis promotes apoptosis in human neutrophils by activating caspase-3 and altering expression of Bax/Bcl-xL via an oxygen-dependent pathway

In addition to direct bactericidal activities, such as phagocytosis and generation of reactive oxygen species (ROS), neutrophils can regulate the inflammatory response by undergoing apoptosis. We found that infection of human neutrophils with Mycobacterium tuberculosis (Mtb) induced rapid cell death displaying the characteristic features of apoptosis such as morphologic changes, phosphatidylserine exposure, and DNA fragmentation. Both a virulent (H37Rv) and an attenuated (H37Ra) strain of Mtb were equally effective in inducing apoptosis. Pretreatment of neutrophils with antioxidants or an inhibitor of NADPH oxidase markedly blocked Mtb-induced apoptosis but did not affect spontaneous apoptosis. Activation of caspase-3 was evident in neutrophils undergoing spontaneous apoptosis, but it was markedly augmented and accelerated during Mtb-induced apoptosis. The Mtb-induced apoptosis was associated with a speedy and transient increase in expression of Bax protein, a proapoptotic member of the Bcl-2 family, and a more prominent reduction in expression of the antiapoptotic protein Bcl-x(L). Pretreatment with an inhibitor of NADPH oxidase distinctly suppressed the Mtb-stimulated activation of caspase-3 and alteration of Bax/Bcl-x(L) expression in neutrophils. These results indicate that infection with Mtb causes ROS-dependent alteration of Bax/Bcl-x(L) expression and activation of caspase-3, and thereby induces apoptosis in human neutrophils. Moreover, we found that phagocytosis of Mtb-induced apoptotic neutrophils markedly increased the production of proinflammatory cytokine TNF-alpha by human macrophages. Therefore, the ROS-dependent apoptosis in Mtb-stimulated neutrophils may represent an important host defense mechanism aimed at selective removal of infected cells at the inflamed site, which in turn aids the functional activities of local macrophages.

Tumor necrosis factor-related apoptosis-inducing ligand induces caspase-dependent interleukin-8 expression and apoptosis in human astroglioma cells

Among the tumor necrosis factor (TNF) family of cytokines, FasL and TNF-related apoptosis-inducing ligand (TRAIL) are known to induce cell death via caspase activation. Recently, other biological functions of these death ligands have been postulated in vitro and in vivo. It was previously shown that Fas ligation induces chemokine expression in human glioma cells. In this study, we investigated whether the TRAIL-DR5 system transduces signals similar to those induced by other TNF family ligands and receptors. To address this issue, two human glioma cell lines, CRT-MG and U87-MG, were used, and an agonistic antibody against DR5 (TRA-8) and human recombinant TRAIL were used to ligate DR5. We demonstrate that DR5 ligation by either TRAIL or TRA-8 induces two functional outcomes, apoptosis and expression of the chemokine interleukin-8 (IL-8); the nonspecific caspase inhibitor Boc-D-Fmk blocks both TRAIL-mediated cell death and IL-8 production; the caspase 3-specific inhibitor z-DEVD-Fmk suppresses TRAIL-mediated apoptosis but not IL-8 induction; caspase 1- and 8-specific inhibitors block both TRAIL-mediated cell death and IL-8 production; and DR5 ligation by TRAIL mediates AP-1 and NF-kappaB activation, which can be inhibited by caspase 1- and 8-specific inhibitors. These findings collectively indicate that DR5 ligation on human glioma cells leads to apoptosis and that the activation of AP-1 and NF-kappaB leads to the induction of IL-8 expression; these responses are dependent on caspase activation. Therefore, the TRAIL-DR5 system has a role not only as an inducer of apoptotic cell death but also as a transducer for proinflammatory and angiogenic signals in human brain tumors.

Selective activation of p38 mitogen-activated protein kinase cascade in human neutrophils stimulated by IL-1beta

We investigated activation of mitogen-activated protein kinase (MAPK) subtype cascades in human neutrophils stimulated by IL-1beta. IL-1beta induced phosphorylation and activation of p38 MAPK and phosphorylation of MAPK kinase-3/6 (MKK3/6). Maximal activation of p38 MAPK was obtained by stimulation of cells with 300 U/ml IL-1beta for 10 min. Extracellular signal-regulated kinase (ERK) was faintly phosphorylated and c-Jun N-terminal kinase (JNK) was not phosphorylated by IL-1beta. IL-1beta primed neutrophils for enhanced release of superoxide (O(2)(-)) stimulated by FMLP in parallel with increased phosphorylation of p38 MAPK. IL-1beta also induced O(2)(-) release and up-regulation of CD11b and CD15, and both responses were inhibited by SB203580 (p38 MAPK inhibitor), suggesting that p38 MAPK activation mediates IL-1beta-induced O(2)(-) release and up-regulation of CD11b and CD15. Combined stimulation of neutrophils with IL-1beta and G-CSF, a selective activator of the ERK cascade, resulted in the additive effects when the priming effect and phosphorylation of p38 MAPK and ERK were assessed. IL-1beta induced phosphorylation of ERK and JNK as well as p38 MAPK in human endothelial cells. These findings suggest that 1) in human neutrophils the MKK3/6-p38 MAPK cascade is selectively activated by IL-1beta and activation of this cascade mediates IL-1beta-induced O(2)(-) release and up-regulation of CD11b and CD15, and 2) the IL-1R-p38 MAPK pathway and the G-CSF receptor-ERK pathway work independently for activation of neutrophils.

Caspase-3 Inhibition Partially Protects Oxidant Production in Apoptotic Human Neutrophils

Apoptotic PMN lose functional activity, which emphasizes the tissue injury limiting potential of PMN apoptosis. Caspase-3 activation is the first step in the execution phase of apoptosis. We hypothesized that PMN functional activity, as evidenced by oxidant production, can be restored in apoptotic PMN by inhibition of caspase-3.

METHODS

To accelerate PMN apoptosis, PMN were UV-irradiated for 15 min as previously described. PMN were pretreated with the caspase-3 inhibitor DEVD-fmk (100 microM) for 30 min prior to UV. PMN apoptosis was quantitated by flow cytometry with CD16 staining. Oxidant production in response to 10 microM PMA was quantitated fluorometrically using the method of Hyslop and Sklar. Caspase-3 activity was quantitated fluorometrically using a commercially available assay.

RESULTS

UV-treated PMN demonstrated a 3-fold increase in caspase-3 activity. This was associated with a significant increase in apoptotic PMN and a 10-fold decrease in oxidant production compared to control PMN. DEVD-fmk blocked increases in caspase-3 activity and significantly reduced PMN apoptosis. Oxidant production was increased 5-fold compared to UV-treated PMN but was still significantly less than control PMN.

CONCLUSIONS

In UV-accelerated PMN apoptosis, inhibition of caspase-3 activity partially protects oxidant production in apoptotic PMN. This suggests that signaling events in the initiation phase of PMN apoptosis, which are proximal to caspase-3 activation, may in part be responsible for loss of oxidant production in apoptotic PMN independent of caspase-3 activity.