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During the resolution of inflammatory responses, neutrophils rapidly undergo apoptosis. A direct and fast activation of caspase-8 by cathepsin D was shown to be crucial in the initial steps of neutrophil apoptosis. Nevertheless, the activation mechanism of caspase-8 remains unclear. Here, by using site-specific mutants of caspase-8, we show that both cathepsin D-mediated proteolysis and homodimerization of caspase-8 are necessary to generate an active caspase-8. At acidic pH, cathepsin D specifically cleaved caspase-8 but not the initiator caspase-9 or -10 and significantly increased caspase-8 activity in dimerizing conditions. These events were completely abolished by pepstatin A, a pharmacological inhibitor of cathepsin D. The cathepsin D intra-chain proteolysis greatly stabilized the active site of caspase-8. Moreover, the main caspase-8 fragment generated by cathepsin D cleavage could be affinity-labeled with the active site probe biotin-VAD-fluoromethyl ketone, suggesting that this fragment is enzymatically active. Importantly, in an in vitro cell-free assay, the addition of recombinant human caspase-8 protein, pre-cleaved by cathepsin D, was followed by caspase-3 activation. Our data therefore indicate that cathepsin D is able to initiate the caspase cascade by direct activation of caspase-8. As cathepsin D is ubiquitously expressed, this may represent a general mechanism to induce apoptosis in a variety of immune and nonimmune cells.

In the resolution of inflammatory responses, neutrophils rapidly undergo apoptosis. We describe a new proapoptotic pathway in which cathepsin D directly activates caspase-8. Cathepsin D is released from azurophilic granules in neutrophils in a caspase-independent but reactive oxygen species-dependent manner. Under inflammatory conditions, the translocation of cathepsin D in the cytosol is blocked. Pharmacological or genetic inhibition of cathepsin D resulted in delayed caspase activation and reduced neutrophil apoptosis. Cathepsin D deficiency or lack of its translocation in the cytosol prolongs innate immune responses in experimental bacterial infection and in septic shock. Thus, we identified a new function of azurophilic granules that is in addition to their role in bacterial defense mechanisms: to regulate the life span of neutrophils and, therefore, the duration of innate immune responses through the release of cathepsin D.

The exact molecular mechanisms leading to delayed apoptosis, a phenomenon frequently observed in eosinophil inflammatory responses, remain largely unknown. Here, we show that cultured eosinophils purified from blood of hypereosinophilic syndrome (HES) patients exhibit delayed spontaneous death and relative resistance towards ceramide- but not CD95-mediated death. The subsequent investigation of members of the inhibitor of apoptosis (IAP) family revealed that HES but not normal eosinophils expressed high levels of cellular IAP-2 (cIAP-2) and survivin. The eosinophil hematopoietins IL-3, IL-5, and GM-CSF increased the expression of cIAP-2 and survivin in normal eosinophils in vitro. In the blood of HES patients, we observed increased concentrations of IL-3 and/or IL-5, suggesting that these cytokines are, at least partially, responsible for the elevated levels of cIAP-2 and survivin in the eosinophils of these patients. Utilizing a cell-free system in which caspase-3 was activated in eosinophil cytosolic extracts by addition of cytochrome c and immunodepletion of cIAP-2 or survivin resulted in accelerated caspase activation. These data suggest that some members of the IAP family including survivin are regulated by survival cytokines and inhibit the caspase cascade in HES eosinophils. The cytokine-dependent mechanism of delayed eosinophil apoptosis described here may also apply to other eosinophilic diseases.

Background: 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., Objective: We sought to examine the expression of leptin receptors on eosinophils and the effect of recombinant leptin on proapoptotic pathways in these cells., Methods: The presence of leptin receptor was examined by means of RT-PCR and immunofluorescence analysis in freshly isolated blood eosinophils and tissue eosinophils. The effect of recombinant leptin on apoptotic pathways in eosinophils was studied by using flow cytometric, immunoblotting, and immunofluorescence techniques., Results: Human eosinophils express leptin surface receptors under in vitro and in vivo conditions, and leptin delays apoptosis of mature eosinophils in vitro. The antiapoptotic effects of leptin were concentration dependent and blocked by an anti-leptin receptor mAb. The efficacy of leptin to block eosinophil apoptosis was similar to that of GM-CSF. Leptin delayed the cleavage of Bax, as well as the mitochondrial release of cytochrome c and second mitochondria-derived activator of caspase, suggesting that it blocks proapoptotic pathways proximal to mitochondria in eosinophils. Using pharmacological inhibitors, we obtained evidence that leptin initiates a signaling cascade involving phosphatidylinositol-3-OH kinase and mitogen-activated protein kinase-dependent pathways in eosinophils., Conclusion: Leptin is a survival cytokine for human eosinophils, a finding with potential pathologic relevance in allergic and parasitic diseases.

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.

In the absence and in the resolution of inflammatory responses, neutrophils rapidly undergo spontaneous apoptosis. Here we report about a new apoptosis pathway in these cells that requires calpain-1 activation and is essential for the enzymatic activation of the critical effector caspase-3. Decreased levels of calpastatin, a highly specific intrinsic inhibitor of calpain, resulted in activation of calpain-1, but not calpain-2, in neutrophils undergoing apoptosis, a process that was blocked by a specific calpain-1 inhibitor or by intracellular delivery of a calpastatin peptide. Further support for the importance of the calpastatin-calpain system was obtained by analyzing neutrophils from patients with cystic fibrosis that exhibited delayed apoptosis, associated with markedly increased calpastatin and decreased calpain-1 protein levels compared with neutrophils from control individuals. Additional studies were designed to place calpain-1 into the hierarchy of biochemical events leading to neutrophil apoptosis. Pharmacological calpain inhibition during spontaneous and Fas receptor-induced neutrophil apoptosis prevented cleavage of Bax into an 18-kDa fragment unable to interact with Bcl-xL. Moreover, calpain blocking prevented the mitochondrial release of cytochrome c and Smac, which was indispensable for caspase-3 processing and enzymatic activation, both in the presence and absence of agonistic anti-Fas receptor antibodies. Taken together, calpastatin and calpain-1 represent critical proximal elements in a cascade of pro-apoptotic events leading to Bax, mitochondria, and caspase-3 activation, and their altered expression appears to influence the life span of neutrophils under pathologic conditions.

Macrophage migration inhibitory factor (MIF) is a proinflammatory cytokine known to activate macrophages and T cells. In this study, we demonstrate that recombinant MIF delays apoptosis of neutrophils in vitro. MIF action is dose and time dependent as well as specific since it was abolished with a neutralizing anti-MIF antibody. MIF, like G-CSF, delayed cleavage of the proapoptotic members of the Bcl-2 family Bid and Bax in neutrophils, suggesting that MIF inhibits apoptosis pathways proximal to mitochondria activation. Indeed, MIF also prevented release of cytochrome c and Smac from the mitochondria and subsequent activation of the critical effector caspase-3 in these cells. Moreover, we observed increased MIF plasma levels in patients with cystic fibrosis, a heterogeneous recessive genetic disorder associated with bacterial infections and delayed neutrophil apoptosis. In conclusion, MIF is a survival cytokine for human neutrophils, a finding with potential pathologic relevance in infectious diseases.

CED-9 blocks programmed cell death (apoptosis) in the nematode C. elegans by binding to and neutralizing CED-4, an essential activator of the aspartate-directed cysteine protease (caspase) CED-3. In mammals, the CED-9 homologs Bcl-2 and Bcl-xL also block apoptosis by interfering with the activation of CED-3-like caspases. However, it is unknown whether this occurs by binding to the CED-4 homolog Apaf-1. Whilst two groups previously detected an interaction between Bcl-xL and Apaf-1 in immunoprecipitates,1,2 another group found no interaction between Apaf-1 and any of ten individual members of the Bcl-2 family using the same experimental approach.3 In this study, we aimed to resolve this discrepancy by monitoring the binding of Apaf-1 to three Bcl-2 family members within cells. Using immunofluorescence and Western blot analysis, we show that whilst Apaf-1 is a predominantly cytoplasmic protein, Bcl-2, Bcl-xL and Bax mostly reside on nuclear/ER and mitochondrial membranes. This pattern of localization is maintained when the proteins are co-expressed in both normal and apoptotic cells, suggesting that Bcl-2, Bcl-xL or Bax do not significantly sequester cytoplasmic Apaf-1 to intracellular membranes. In addition, we confirm that Apaf-1 does not interact with Bcl-2 and Bcl-xL in immunoprecipitates. Based on these data, we propose that Apaf-1 is not a direct, physiological target of Bcl-2, Bcl-xL or Bax.

Apoptosis involves mitochondrial steps such as the release of the apoptogenic factor cytochrome c which are effectively blocked by Bcl-2. Although Bcl-2 may have a direct action on the mitochondrial membrane, it also resides and functions on the endoplasmic reticulum (ER), and there is increasing evidence for a role of the ER in apoptosis regulation as well. Here we uncover a hitherto unrecognized, apoptotic crosstalk between the ER and mitochondria that is controlled by Bcl-2. After triggering massive ER dilation due to an inhibition of secretion, the drug brefeldin A (BFA) induces the release of cytochrome c from mitochondria in a caspase-8- and Bid-independent manner. This is followed by caspase-3 activation and DNA/nuclear fragmentation. Surprisingly, cytochrome c release by BFA is not only blocked by wild-type Bcl-2 but also by a Bcl-2 variant that is exclusively targeted to the ER (Bcl-2/cb5). Similar findings were obtained with tunicamycin, an agent interfering with N-linked glycosylations in the secretory system. Thus, apoptotic agents perturbing ER functions induce a novel crosstalk between the ER and mitochondria that can be interrupted by ER-based Bcl-2.

The oncogene product Bcl-2 protects cells from apoptosis whereas its homolog Bax functions to kill cells. Several binding partners of Bcl-2 and Bax have been isolated, but none of them has yet provided clues as to exactly how Bcl-2 and Bax work. According to one view, Bcl-2 and Bax interact with survival and death effector molecules, respectively, and neutralize each other through heterodimerization. Alternatively, Bcl-2 requires Bax for death protection, and additional proteins bind to the heterodimer to regulate its activity. Here we used a co-immunoprecipitation strategy to distinguish between these two possibilities. We show that the Bcl-2-Bax heterodimer is maintained, and no other protein associates stably in detectable amounts with Bcl-2, Bax, or the heterodimer in anti-Bcl-2 and anti-Bax immunoprecipitates from normal cells and cells exposed to apoptotic stimuli. Analysis of cells expressing various levels of Bcl-2 and Bax, however, revealed that the degree of protection against apoptosis does not correlate with the number of Bcl-2-Bax heterodimers but the amount of Bcl-2 that is free of Bax. In addition, the survival activity of Bcl-2 is unaffected when Bax expression is ablated by an antisense strategy. Our findings suggest that the Bcl-2-Bax heterodimer is a negative regulator of death protection, and that Bcl-2 requires neither Bax nor major, stable interactions with other cellular proteins to exert its survival function. We therefore propose that Bcl-2 acts as an enzyme (capturing substrates in a transient way), as a homodi- or multimer, or through the interaction with non-proteaceous targets (lipids, ions).

Following exposure of cells to stimuli that trigger programmed cell death (apoptosis), cytochrome c is rapidly released from mitochondria into the cytoplasm where it activates proteolytic molecules known as caspases that specifically cleave the amino-acid sequence DEVD and are crucial for the execution of apoptosis. The protein Bcl-2 interferes with this activation of caspases by preventing the release of cytochrome c. Here we study these molecular interactions during apoptosis induced by the protein Bax, a pro-apoptotic homologue of Bcl-2. We show that in cells transiently transfected with bax, Bax localizes to mitochondria and induces the release of cytochrome c, activation of caspase-3, membrane blebbing, nuclear fragmentation, and cell death. Caspase inhibitors do not affect Bax-induced cytochrome c release but block caspase-3 activation and nuclear fragmentation. Unexpectedly, Bcl-2 also fails to prevent Bax-induced cytochrome c release, although it co-localizes with Bax to mitochondria. Cells overexpressing both Bcl-2 and Bax show no signs of caspase activation and survive with significant amounts of cytochrome c in the cytoplasm. These findings indicate that Bcl-2 can interfere with Bax killing downstream of and independently of cytochrome c release.

Under inflammatory conditions, neutrophil apoptosis is delayed due to survival-factor exposure, a mechanism that prevents the resolution of inflammation. One important proinflammatory cytokine involved in the regulation of neutrophil survival/activation is granulocyte–macrophage colony-stimulating factor (GM-CSF). Although GM-CSF mediates antiapoptotic effects in neutrophils, it does not prevent apoptosis, and the survival effect is both time dependent and limited. Here, we identified the proapoptotic Bcl-2 family member Bim as an important lifespan limiting molecule in neutrophils, particularly under conditions of survival factor exposure. Strikingly, GM-CSF induced Bim expression in both human and mouse neutrophils that was blocked by pharmacological inhibition of phosphatidylinositol-3 kinase (PI3K). Increased Bim expression was also seen in human immature bone marrow neutrophils as well as in blood neutrophils from septic shock patients; both cell populations are known to be exposed to GM-CSF under in vivo conditions. The functional role of Bim was investigated using Bim-deficient mouse neutrophils in the presence and absence of the survival cytokines interleukin (IL)-3 and GM-CSF. Lack of Bim expression resulted in a much higher efficacy of the survival cytokines to block neutrophil apoptosis. Taken together, these data demonstrate a functional role for Bim in the regulation of neutrophil apoptosis and suggest that GM-CSF and other neutrophil hematopoietins initiate a proapoptotic counterregulation that involves upregulation of Bim.Cell Death and Differentiation (2009) 16, 1248–1255; doi:10.1038/cdd.2009.50; published online 1 May 2009 [ABSTRACT FROM AUTHOR]