Carbohydrate-mediated phagocytic recognition of early apoptotic cells undergoing transient capping of CD43 glycoprotein

A novel mechanism of phagocytic recognition of apoptotic cells was found and characterized. Jurkat cells incubated with appropriate concentrations of etoposide or anti-Fas antibody transiently became susceptible to binding and phagocytosis by THP-1 cell-derived macrophages at 2 h. The bound Jurkat cells showed no chromatin condensation, but the binding was prevented by a caspase inhibitor, indicating that they were recognized at an early stage of apoptosis. The ligands recognized on the apoptotic cells were sialylpolylactosaminyl sugar chains because 1) the binding was inhibited by an oligosaccharide preparation of erythrocyte membrane, and its inhibitory activity was destroyed by polylactosaminoglycan-specific endo-beta-galactosidase or neuraminidase; 2) Jurkat cells pretreated with endo-beta-galactosidase or neuraminidase failed to be recognized; and 3) treatment of the apoptotic cells with polylactosaminoglycan-binding Datura stramonium agglutinin prevented recognition. The sialylpolylactosaminyl chains involved were most likely those of a major sialoglycoprotein CD43 because anti-CD43 antibody inhibited recognition. CD43 on apoptotic Jurkat cells was found to form a cap at 2 h, and the cap disappeared at 4 h. This transient capping of CD43 coincided with the transient increase in the susceptibility of the cells to macrophage recognition, suggesting that CD43 capping is responsible for generation of the carbohydrate ligands for recognition. Furthermore, microscopic observation suggested that the apoptotic cells were recognized at the CD43 cap. Taken together, we conclude that apoptotic Jurkat cells transiently undergo CD43 capping at an early stage of apoptosis and are recognized by macrophages through the cluster of sialylpolylactosaminyl chains of the capped CD43.     

Use of the monoclonal antibody DAKO-ERbeta (8D5-1) to measure oestrogen receptor beta in breast cancer cells

BACKGROUND: Following a lethal injury, two modes of cell death can be distinguished, apoptosis and primary necrosis. Cells pass through a prelethal stage characterized by a preservation of membrane integrity, in which they shrink (apoptosis) or swell (oncosis, the early phase of primary necrosis). During apoptosis, a loss of phospholipid asymmetry leads to exposure of phosphatidylserine (PS) residues on the outer leaflet of the plasma membrane. We examined whether the external PS exposure, initially supposed to be specific for apoptosis, was also observed in oncotic cells. METHODS: Human peripheral lymphocytes, Jurkat T cells, U937 cells, or HeLa cells were submitted to either apoptotic or oncotic stimuli. PS external exposure was assessed after binding of FITC-conjugated annexin V as was the loss of membrane integrity after propidium iodide (PI) uptake. Morphological examination was performed by optical or electron microscopy. RESULTS: Similarly to apoptotic cells, oncotic cells expose external PS residues while preserving membrane integrity. Consequently, oncotic cells exhibit the annexin V+ PI- phenotype, previously considered to be specific for apoptotic cells. CONCLUSIONS: This study concludes that the annexin V/PI assay does not discriminate between apoptosis and oncosis and that it can be a useful tool to study oncosis by flow cytometry.     

Early detection of staurosporine-induced apoptosis by comet and annexin V assays

Comet, TUNEL, and annexin V assays were used to identify DNA fragmentation and plasma membrane alterations occurring during staurosporine-induced apoptosis in Chinese hamster ovary cells. TUNEL assay detected apoptotic cells after 6 h treatment. The occurrence of annexin V immunofluorescence staining after 1 h treatment confirms that exposure of phosphatidylserine (PS) residues is an early biochemical feature of apoptosis. According to intensity, three annexin staining patterns were distinguished, related to different steps in the apoptotic process. The detection of highly damaged cells by the comet assay after 3 h treatment occurred earlier than the detection of DNA modifications by the TUNEL assay, but later than the exposure of PS residues. However, late apoptotic cells, otherwise characterized by plasma membrane disruption and high annexin V staining, were not detected by the comet assay. In this case, comet assay modified by omitting electrophoresis (halo assay) was more sensitive for an accurate quantification of the apoptotic fraction. Accepted: 2 June 1999     

Macrophage surface expression of annexins I and II in the phagocytosis of apoptotic lymphocytes

When cells undergo apoptosis, or programmed cell death, they expose phosphatidylserine (PS) on their surface. Macrophages that efficiently phagocytose apoptotic cells also express PS on their surface, although at a lower level. The PS exposed on both cells is required for phagocytosis, because uptake is inhibited by masking PS on either cell with annexin V, a PS-binding protein. The inhibition is not additive, suggesting that the exposed PS molecules on the two cells participate in a common process. We asked whether this dual requirement reflects bridging of the target cell and macrophage by bivalent, PS-binding annexins. Monoclonal antibodies (mAbs) against annexins I or II stained a variety of live phagocytes. Apoptotic Jurkat T lymphocytes and human peripheral T lymphocytes, but not apoptotic thymocytes, were stained by anti-annexin I but not II. Phagocytosis of apoptotic targets was inhibited by mAbs to annexins I or II, or by pretreatment of macrophages with the same mAbs. Pretreatment of apoptotic thymocytes had no effect, whereas pretreating Jurkat cells with anti-annexin I or removing annexin I with EGTA was inhibitory. Annexin bridging is vectorial, because annexin is bound to PS molecules on targets but not on macrophages, suggesting annexins serve as both ligand and receptor in promoting phagocytosis.     

There is substantial nuclear and cellular disintegration before detectable phosphatidylserine exposure during the camptothecin-induced apoptosis of HL-60 cells

BACKGROUND: An early sign of apoptosis in many cells is the appearance of phosphatidylserine (PS) on the outside of the plasma membrane, whilst the cells still retain the ability to exclude DNA-binding molecules such as propidium iodide and 7-aminoactinomycin D (7-AAD). The protein annexin V binds preferentially to PS and has often been used to monitor the early phase of apoptosis. There have been some conflicting results concerning whether annexin V binds to camptothecin (CAM)-treated HL-60 cells, a commonly used model for apoptosis. We investigated the effects of culturing HL-60 cells for up to 8 h with a range of CAM concentrations. METHODS: We used flow cytometry to measure cellular light scatter, annexin V-FITC binding, and 7-AAD uptake, and DNA content after fixation and permeabilization. We also used microscopy to examine the morphology of cells (both unsorted and sorted according to their light scatter) after cytocentrifugation. RESULTS: We found that CAM caused the rapid appearance of low light scatter apoptotic bodies. Even among cells with "normal" light scatter, there was widespread DNA cleavage and nuclear fragmentation by 3 h. The percentage of apoptotic bodies peaked at about 4 h and it was only afterward that annexin V binding could be detected to both intact cells and to apoptotic bodies. When they first appeared, the intact annexin V+ cells had S-phase DNA content. CONCLUSIONS: During CAM-induced apoptosis of HL-60 cells, the external exposure of PS can either precede or follow DNA cleavage, which suggests that PS exposure is not always an indicator of early apoptosis.     

Macrophage recognition of externalized phosphatidylserine and phagocytosis of apoptotic Jurkat cells--existence of a threshold

Phosphatidylserine (PS) is predominantly confined to the inner leaflet of plasma membrane in cells, but it is externalized on the cell surface during apoptosis. This externalized PS is required for effective phagocytosis of apoptotic cells by macrophages. Because PS trans-bilayer asymmetry is not absolute in different types of nonapoptotic cells, we hypothesized that the amounts of externalized PS may be critical for macrophage discrimination between apoptotic and nonapoptotic cells. We developed a sensitive electron paramagnetic resonance method to quantify the amounts of externalized PS based on specific binding of paramagnetic annexin V-microbead conjugates with PS on cell surfaces. Using this technique, we found that nonapoptotic Jurkat cells externalize 0.9 pmol of endogenous PS/10(6) Jurkat cells. For cells with different amounts of integrated exogenous PS on their surface, no phagocytic response was observed at PS levels <5 pmol/10(6) Jurkat cells; at higher PS concentrations, phagocytosis increased in a concentration-dependent manner. Apoptosis in Jurkat cells caused externalization of approximately 240 pmol PS/10(6) Jurkat cells; these amounts of externalized PS are manyfold higher than the threshold amounts of PS required for phagocytosis. Thus, macrophages have a sensitivity threshold for PS externalized on the cell surface that provides for reliable recognition and distinction between normal cells with low contents of externalized PS and apoptotic cells with remarkably elevated PS levels.     

Surfactant protein A (SP-A) binds to phosphatidylserine and competes with annexin V binding on late apoptotic cells

The role of surfactant protein A (SP-A) in the recognition and clearance of apoptotic cells is well established, but to date, it is still not clear which surface molecules of apoptotic cells are involved in the process. Here we present evidence that phosphatidylserine (PS) is a relevant binding molecule for human SP-A. The binding is Ca²⁺-dependent and is not inhibited by mannose, suggesting that the sugar-binding site of the carbohydrate recognition domain (CRD) of SP-A is not involved. Flow cytometry studies on apoptotic Jurkat cells revealed apparent inhibition of annexin V binding by increasing concentrations of SP-A in late apoptotic but not early apoptotic cells, and this was consistent for Jurkat cells and neutrophils. Supporting these data, confocal microscopy results show a co-localisation of annexin V and SP-A in late apoptotic but not early apoptotic cells. However, we cannot conclude that this inhibition is exclusively due to the binding of SP-A to PS on the cell surface, as annexin V is not wholly specific for PS and SP-A also interacts with other phospholipids that might become exposed on the apoptotic cell surface.     

Lactadherin detects early phosphatidylserine exposure on immortalized leukemia cells undergoing programmed cell death.

BACKGROUND: Phosphatidylserine (PS) appears on the outer membrane leaflet of cells undergoing programmed cell death and marks those cells for clearance by macrophages. Macrophages secrete lactadherin, a PS-binding protein, which tethers apoptotic cells to macrophage integrins. METHODS: We utilized fluorescein-labeled lactadherin together with the benchmark PS Probe, annexin V, to detect PS exposure by flow cytometry and confocal microscopy. Immortalized leukemia cells were treated with etoposide, and the kinetics and topology of PS exposure were followed over the course of apoptosis. RESULTS: Costaining etoposide-treated leukemoid cells with lactadherin and annexin V indicated progressive PS exposure with dim, intermediate, and bright staining. Confocal microscopy revealed localized plasma membrane staining, then diffuse dim staining by lactadherin prior to bright generalized staining with both proteins. Annexin V was primarily localized to internal cell bodies at early stages but stained the plasma membrane at the late stage. Calibration studies suggested a PS content less, less than or approximately equal to 2.5%-8% for the membrane domains that stained with lactadherin but not annexin V. CONCLUSIONS: Macrophages may utilize lactadherin to detect PS exposure prior to exposure of sufficient PS to bind annexin V. The methodology enables detection of PS exposure at earlier stages than established methodology.     

Bovine Lactadherin as a Calcium-independent Imaging Agent of Phosphatidylserine Expressed on the Surface of Apoptotic HeLa Cells

Bovine lactadherin holds a stereo-specific affinity for phosphatidylserine (PS) membrane domains and binds at PS concentrations lower than the benchmark PS probe, annexin V. Accordingly, lactadherin has recognized PS exposure on preapoptotic immortalized leukemia cells at an earlier time point than has annexin V. In the present study, the cervical cancer cell line HeLa has been employed as a model system to compare the topographic distribution of PS with the two PS binding proteins as adherent cells enter the apoptotic program. HeLa cells were cultured on glass-bottom Petri dishes, and apoptosis was induced by staurosporine. Fluorescence-labeled lactadherin and/or annexin V were used to detect PS exposure by confocal microscopy. Both lactadherin and annexin V staining revealed PS localized to plasma membrane rim and blebs. In addition, lactadherin identified PS exposure on long filopodia-like extensions, whereas annexin V internalized in granule-like structures. All in all, the data further delineate the differences in PS binding patterns of lactadherin and annexin V. (J Histochem Cytochem 57:907–914, 2009)     

Annexin V-CLIO: a nanoparticle for detecting apoptosis by MRI

Annexin V, which recognizes the phosphatidylserine of apoptotic cells, was conjugated to crosslinked iron oxide (CLIO) nanoparticles, a functionalized superparamagnetic preparation developed for target-specific magnetic resonance imaging (MRI). The resulting nanoparticle had an average of 2.7 annexin V proteins linked per CLIO nanoparticle through disulfide bonds. Using camptothecin to induce apoptosis, a mixture of Jurkat T cells (69% healthy and 31% apoptotic) was incubated with annexin V-CLIO and was applied to magnetic columns. The result was an almost complete removal of the apoptotic cells (> 99%). In a phantom MRI experiment, untreated control cells (12% apoptotic cells, 88% healthy cells) and camptothecin-treated cells (65% apoptotic cells, 35% healthy cells) were incubated with either annexin V-CLIO (1.0, 0.5, and 0.1 microgram Fe/mL) or with unlabeled CLIO. A significant signal decrease of camptothecin-treated cells relative to untreated cells was observed even at the lowest concentration tested. Unmodified CLIO failed to cause a significant signal change of apoptotic cells. Hence, annexin V-CLIO allowed the identification of cell suspensions containing apoptotic cells by MRI even at very low concentrations of magnetic substrate. Conjugation of annexin V to CLIO affords a strategy for the development of a MRI imaging probe for detecting apoptosis.     

Phosphatidylserine-dependent adhesion of T cells to endothelial cells

Phosphatidylserine (PS) was exposed at the surface of human umbilical vein endothelial cells (HUVECs) and cultured cell lines by agonists that increase cytosolic Ca(2+), and factors governing the adhesion of T cells to the treated cells were investigated. Thrombin, ionophore A23187 and the Ca(2+)-ATPase inhibitor 2, 5-di-tert-butyl-1,4-benzohydroquinone each induced a PS-dependent adhesion of Jurkat T cells. A23187, which was the most effective agonist in releasing PS-bearing microvesicles, was the least effective in inducing the PS-dependent adhesion of Jurkat cells. Treatment of ECV304 and EA.hy926 cells with EGTA, followed by a return to normal medium, resulted in an influx of Ca(2+) and an increase in adhering Jurkat cells. Oxidised low-density lipoprotein induced a procoagulant response in cultured ECV304 cells and increased the number of adhering Jurkat cells, but adhesion was not inhibited by pretreating ECV304 cells with annexin V. PS was not significantly exposed on untreated Jurkat cells, as determined by flow cytometry with annexin V-FITC. However, after adhesion to thrombin-treated ECV304 cells for 10 min followed by detachment in 1 mM EDTA, there was a marked exposure of PS on the Jurkat cells. Binding of annexin V-FITC to the detached cells was inhibited by pretreating them with unlabelled annexin V. Contact with thrombin-treated ECV304 cells thus induced the exposure of PS on Jurkat cells and, as Jurkat cells were unable to adhere to thrombin-treated ECV304 cells in the presence of EGTA, the adhesion of the two cell types may involve a Ca(2+) bridge between PS on both cell surfaces. The number of T cells from normal, human peripheral blood that adhered to ECV304 cells was not increased by treating the latter with thrombin. However, findings made with several T cell lines were generally, but not completely, consistent with the possibility that adhesion to surface PS on endothelial cells may be a feature of T cells that express both CD4(+) and CD8(+) antigens. Possible implications for PS-dependent adhesion of T cells to endothelial cells in metastasis, and early in atherogenesis, are discussed.     

A novel 96-well scintillation proximity assay for the measurement of apoptosis

The translocation of phospholipids across the plasma membrane has been widely documented as one of the earliest measurable biochemical events of apoptosis. Using fluorescently labelled annexin V, which preferentially binds phosphatidylserine (PS) in the presence of Ca²⁺, the externalization of PS can be measured and apoptosis quantified using flow cytometry. Conventional detection methods utilizing annexin V, while faster than in situ DNA end-labelling or DNA laddering, require extensive sample preparation which may compromise samples and makes rapid, high volume screening prohibitive. This paper describes a novel assay for the measurement of apoptosis based upon binding of radiolabelled annexin V to apoptotic cells attached to the growth surface of a 96-well scintillating microplate (Cytostar-T®). We compared measurements of apoptosis made by flow cytometry to those obtained with the scintillating microplate in three model systems, treatment of: mouse connective tissue (L-M) cells with lymphotoxin (LT), human lung carcinoma (H460) cells with Apo-2 ligand and human umbilical vein endothelial (HUVE) cells with staurosporine. In this assay, we compare both direct and indirect labelling methods by utilizing either iodinated annexin V or biotinylated annexin V/[³⁵S] streptavidin to radiolabel apoptotic cells. The signal detected is a direct consequence of the binding of annexin V to externalized PS on apoptotic cells and the proximity of the label to the base of the plate. Using this method, separation of bound and unbound radiolabel signal occurs directly within the well resulting in a sensitive assay that requires minimal manipulation and can accomodate a large number of samples.     

Assessment of cardiovascular apoptosis in the isolated rat heart by magnetic resonance molecular imaging

Apoptosis, an active process of cell self-destruction, is associated with myocardial ischemia. The redistribution of phosphatidylserine (PS) from the inner to the outer leaflet of the cell membrane is an early event in apoptosis. Annexin V, a protein with high specificity and tight binding to PS, was used to identify and localize apoptosis in the ischemic heart.Fluorescein-labeled annexin V has been used routinely for the assessment of apoptosis in vitro. For the detection of apoptosis in vivo, positron emission tomography and single-photon emission computed tomography have been shown to be suitable tools. In view of the relatively low spatial resolution of nuclear imaging techniques, we developed a high-resolution contrast-enhanced magnetic resonance imaging (MRI) method that allows rapid and noninvasive monitoring of apoptosis in intact organs. Instead of employing superparamagnetic iron oxide particles linked to annexin V, a new T1 contrast agent was used. To this effect, annexin V was linked to gadolinium diethylenetriamine pentaacetate (Gd-DTPA)-coated liposomes.The left coronary artery of perfused isolated rat hearts was ligated for 30 min followed by reperfusion. T(1) and T(2)* images were acquired by using an 11.7-T magnet before and after intracoronary injection of Gd-DTP-labeled annexin V to visualize apoptotic cells. A significant increase in signal intensity was visible in those regions containing cardiomyocytes in the early stage of apoptosis. Because labeling of early apoptotic cell death in intact organs by histological and immunohistochemical methods remains challenging, the use of Gd-DTPA-labeled annexin V in MRI is clearly an improvement in rapid targeting of apoptotic cells in the ischemic and reperfused myocardium.     

Phosphatidylserine externalization during bone marrow cell death after treatment of mice with WR-2721 and gamma-rays

The cell surface exposure of phosphatidylserine (PS) and the plasma membrane impairment were assessed in the bone marrow of adult male Swiss mice exposed to a single 6 Gy dose of 60 Co gamma-rays, and treated intraperitoneally with the aminothiol WR-2721 (Amifostine, S-2-/3-aminopropylamino/ethyl phosphorothioic acid), at a dose of 400 mg/kg body weight, 30 min prior to gamma-irradiation. The bone marrow cells were stained with a combination of fluoresceinated annexin V (annexin V--FITC) and propidium iodide (PI) at 3 h, 7 h, and 24 h after treatment of mice with WR-2721 and 60Co gamma-irradiation. The number of early apoptotic cells (annexin V--FITC positive/PI negative), and late apoptotic and necrotic cells (annexin V--FITC positive/PI positive), was increased at 3 h after exposure of mice to 60Co gamma-rays and thereafter declined with the frequency of apoptotic and necrotic cells remaining lower in WR-2721 pre-treated mice. Using the annexin V--FITC flow cytometric assay, the radioprotective effect of WR-2721 against induction of apoptosis and necrosis in normal cells of the haematopoietic system was shown.     

Staphylococcal superantigen-like protein 10 binds to phosphatidylserine and apoptotic cells

Staphylococcal superantigen-like proteins (SSLs) are a family of exoproteins that have structural similarities to staphylococcal superantigens. Although SSLs do not have superantigenic activity, some of them have been reported to bind to host immune related molecules and they have been implicated in immune evasion by S. aureus. In this study, we showed that SSL10 is capable of binding to phospholipids. SSL10 bound to phosphatidylserine (PS) containing liposome, but not to phosphatidylcholine liposome. SSL10, but not SSL7, bound to PS containing liposome, suggesting that SSL10 specifically binds to PS. Analysis of PS binding ability among recombinant truncated SSL10 fragments revealed that the β-barrel in the N-terminal oligonucleotide/oligosaccharide-binding (OB)-fold domain contributes to PS binding capacity. Fluorescein isothiocyanate labeled OB-fold of SSL10 stained hydrogen peroxide treated Jurkat cells. Annexin V is widely utilized for detection of apoptosis. Unlike annexin V, the OB-fold domain of SSL10 also bound to apoptotic cells in the presence of EDTA, suggesting that the OB-fold of SSL10 recognizes PS and apoptotic cells in a Ca(2+) independent manner. These findings suggest SSL10 and its derived peptides may be a novel detection tool for apoptotic cells.     

Magneto/optical annexin V, a multimodal protein

Multimodal proteins, or proteins labeled with both fluorescent and magnetic reporter groups, can be used in a wide range of applications including FACS or fluorescence microscopy, MRI and or near-infrared based optical imaging, or to fractionate cells by magnetic cell sorting. A problem with multimodal proteins, however, is the need to maximize bioactivity, often achieved by minimizing the number of modification points of the protein, while attaching fluorescent and magnetic labels. Here we describe the synthesis of a magneto/optical form of annexin V, achieved by reacting the amino-CLIO nanoparticle with Cy5.5 and SPDP, to produce a fluorescent, sulfhydryl reactive nanoparticle. A single reactive sulfhydryl group was added to annexin V by reaction with SATA that preserved the protein's ability to bind apoptotic Jurkat T cells. Reacting SATAylated annexin V with an SPDP activated nanoparticle yielded Anx-CLIO-Cy5.5, a magneto/optical form of annexin V. The binding of Anx-CLIO-Cy5.5 was specific for apoptotic Jurkat T cells and had an EC(50) of 3.66 nM. This was comparable to the strength of the interaction of unmodified annexin V with apoptotic cells, measured as the displacement of FITC-annexin by annexin V (2.4 nM). Our conjugation strategy preserves the strength of the interaction between annexin V and apoptotic cells, while yielding a probe, Anx-CLIO-Cy5.5, that is readily detectable by standard MR imaging or NIRF optical methods.     

Inhibition of sperm production in mice by annexin V microinjected into seminiferous tubules: possible etiology of phagocytic clearance of apoptotic spermatogenic cells and male infertility

Many differentiating spermatogenic cells die by apoptosis during the process of mammalian spermatogenesis. However, very few apoptotic spermatogenic cells are detected by histological examination of the testis, probably due to the rapid elimination of dying cells by phagocytosis. Previous in vitro studies showed that Sertoli cells selectively phagocytose dying spermatogenic cells by recognizing the membrane phospholipid phosphatidylserine (PS), which is exposed to the surface of spermatogenic cells during apoptosis. We examined here whether PS-mediated phagocytosis of apoptotic spermatogenic cells occurs in vivo. For this purpose, the PS-binding protein annexin V was microinjected into the seminiferous tubules of normal live mice, and their testes were examined. The injection of annexin V caused no histological changes in the testis, but significantly increased the number of apoptotic spermatogenic cells as assessed by the terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling assay. The number of Sertoli cells did not change in the annexin V-injected testes, and annexin V itself did not induce apoptosis in primary cultured spermatogenic cells. These results indicate that annexin V inhibited the phagocytic clearance of apoptotic spermatogenic cells and suggest that PS-mediated phagocytosis of those cells occurs in vivo. Furthermore, the injection of annexin V into the seminiferous tubules brought about a significant reduction in the number of spermatogenic cells and epididymal sperm in anticancer drug-treated mice. This suggests that the elimination of apoptotic spermatogenic cells is required for the production of sperm.     

Exposure of phosphatidylserine is a general feature in the phagocytosis of apoptotic lymphocytes by macrophages

Although different macrophages exploit different cell surface receptors to recognize apoptotic lymphocytes, indirect evidence suggested that the phosphatidylserine (PS) that appears on the surface of lymphocytes undergoing apoptosis participates in specific recognition by all types of macrophages. To test this possibility directly, annexin V, a protein that specifically binds to PS, was used to mask this phospholipid on the apoptotic cell surface. Preincubation of apoptotic lymphocytes with annexin V blocked phagocytosis by elicited mouse peritoneal macrophages, macrophages of the mouse J774 cell line and mouse bone marrow macrophages. Similarly, annexin V was able to inhibit phagocytosis of lipid-symmetric erythrocytes, another target cell upon which PS is exposed. Together these results demonstrate directly that macrophages of all types depend on the PS exposed on the surface of apoptotic lymphocytes for recognition and phagocytosis.     

Caspase inhibition in apoptotic T cells triggers necrotic cell death depending on the cell type and the proapoptotic stimulus

CD95 (Fas/Apo-1) triggers apoptotic cell death via a caspase-dependent pathway. Inhibition of caspase activation blocks proapoptotic signaling and thus, prevents execution of apoptosis. Besides induction of apoptotic cell death, CD95 has been reported to trigger necrotic cell death in susceptible cells. In this study, we investigated the interplay between apoptotic and necrotic cell death signaling in T cells. Using the agonistic CD95 antibody, 7C11, we found that caspase inhibition mediated by the pancaspase inhibitor, zVAD-fmk, prevented CD95-triggered cell death in Jurkat T cells but not in A3.01 T cells, although typical hallmarks of apoptosis, such as DNA fragmentation or caspase activation were blocked. Moreover, the caspase-independent cell death in A3.01 cells exhibited typical signs of necrosis as detected by a rapid loss of cell membrane integrity and could be prevented by treatment with the radical scavenger butylated hydroxyanisole (BHA). Similar to CD95-induced cell death, apoptosis triggered by the DNA topoisomerase inhibitors, camptothecin or etoposide was shifted to necrosis when capsase activation was inhibited. In contrast to this, ZVAD was fully protective when apoptosis was triggered by the serpase inhibitor, Nalpha-tosyl-phenyl-chloromethyl ketone (TPCK). TPCK was not protective when administered to anti-CD95/ZVAD-treated A3.01 cells, indicating that TPCK does not possess anti-necrotic activity but fails to activate the necrotic death pathway. Our findings show (a) that caspase inhibition does not always protect apoptotic T cells from dying but merely activates a caspase-independent mode of cell death that results in necrosis and (b) that the caspase-inhibitor-induced shift from apoptotic to necrotic cell death is dependent on the cell type and the proapoptotic stimulus.     

Annexin V expression in apoptotic peripheral blood lymphocytes: An electron microscopic evaluation

Loss of plasma membrane asymmetry, resulting in the exposure of phosphatidylserine (PS), is considered to be an early event in apoptosis. It is generally accepted to precede nuclear condensation, independent of the apoptosis inductive agent.In the present study we focus on 2 apoptotic parameters: PS exposure in comparison with morphological alterations. Peripheral blood lymphocytes were irradiated in vitro (5 Gy Co--rays) or incubated with staurosporine (1 M, 6 hours). PS exposure was measured flow cytometrically using FITC-labelled annexin V, combined with PI. Morphological alterations were evaluated by electron microscopy (EM). Results are based on 3 independent experiments.For the irradiated lymphocytes the amount of viable cells (annexin V-/PI-) as scored by flow cytometry was comparable or slightly lower than the number of viable cells as scored by EM (75% compared to 79%). However, for the staurosporine treated lymphocytes only about 24% of the cells were designated as viable by EM, whereas by flow cytometry about 65% of the cells were annexin V-/PI-. Examination by EM showed about 40% cells with a morphology distinct from that of a normal viable cell, but without the clear-cut characteristics of apoptotic cells. Time studies revealed that these cells went into apoptosis after prolonged incubation times up to 18 hours.Application of biotinilated annexin V for EM detection with gold-conjugated anti-biotin, showed that only clear-cut apoptotic, apoptotic necrotic and oncotic cells showed the gold-label at their membranes. Cells that could be detected under the EM as non-viable but without the clear-cut characteristics of apoptotic cells, were not labelled. Data indicate that, dependent on the apoptosis inductive mechanism, morphological alterations can occur before PS exposure.