Increased cell surface exposure of phosphatidylserine on propidium iodide negative thymocytes undergoing death by necrosis.

Phosphatidylserine (PS) exposure on propidium iodide negative cells using FITC labelled annexin-V has been used to quantify apoptosis in vitro and in vivo. Detection of PS within cells undergoing necrosis is also possible if labelled annexin-V specific for PS enters the cell following early membrane damage. Necrotic or late apoptotic cells can be excluded from flow cytometric analysis using propidium iodide which enters and stains cells with compromised membrane integrity. Here we show that thymocytes undergoing death exclusively by necrosis show early exposure of PS prior to loss of membrane integrity. This early exposure of PS occurs in cells treated with agents which both raise intracellular calcium levels and are also capable of interacting with protein thiol groups. We also demonstrate that PS exposure in thymocytes induced to undergo apoptosis by three different agents does not correlate with calcium rises but correlates with and precedes DNA fragmentation.     

Novel piperazine induces apoptosis in U937 cells

The effect of 1,4-bis-(4-(1H-benzo[d]imidazol-2-yl-phenyl)) piperazine (BIPP), a newly synthesized piperazine derivative, on U937 leukemia cell viability was investigated. We show that BIPP induces dose-responsive apoptotic cell death in U937 cells by intrinsic mechanisms of apoptosis. Maximum apoptotic effect of BIPP on U937 cells was observed at 12.8μM. BIPP-induced apoptosis was evident by characteristics such as altered annexin-V binding, caspase activation, loss of mitochondrial membrane potential (MMP) and cytochrome c release. BIPP also differentially activates initiator and effector caspases combined with the loss of MMP strongly suggesting that BIPP causes an intrinsic apoptosis in U937 leukemia cells. Due to our observations that BIPP induces leukemia cell death without significantly affecting normal cells, our data suggests that it may be a potential therapeutic agent for human myeloid leukemia.     

UV-B Irradiated Cell Lines Execute Programmed Cell Death in Various Forms

We investigated changes typical for apoptosis in various cell lines after UV-B irradiation. Using established methods for detection of apoptosis we demonstrate changes of cellular morphology, phosphatidylserine (PS) exposure, ollgonucleosomal DNA fragmentation and generation of hypochrome nuclei. To isolated high-molecular-weight (hmwt) DNA fragments we engaged a new method avoiding pulse field gel electrophoresis. Most UV-B irradiated cell lines showed oligonucleosomal DNA fragmentation, hypochrome nuclei, morphological changes, annexin-V binding and positive TUNEL reaction. However, no oligonucleosomal DNA fragmentation could be detected in Raji and HaCaT cells. Whereas HaCaT cells displayed all other changes typical for apoptosis, Raji cells were TUNEL negative, formed low amounts of hmwt DNA and showed an 'atypically' low hypochrome shift. Nevertheless, UV-B irradiated Raji cells excluded propidium iodide (PI), bound annexin-V and stopped proliferation. This suggests that Raji cells underwent growth arrest with exposure of PS being the only feature of apoptosis. However, in the presence of phagocytes expressing the phosphatidylserine receptor these cells would share the removal pathway with apoptotic cells. Since UV-B induced programmed cell death differs in dependence of cells under investigation, the failure to detect oligonucleosomal DNA fragmentation or chromatin condensation is not suitable to exclude programmed (apoptotic?) cell death.     

Apoptotic rate: a new indicator for the quantification of the incidence of apoptosis in cell cultures

BACKGROUND: Late apoptotic cells divide into apoptotic bodies and are missed by current detection methods. This results in an artificially low apoptotic index (AI). METHODS: This study proposes a flow cytometry-based ratiometric method that uses an internal reference standard of microbeads combined with fluorescein-annexin V binding and 7-aminoactinomycin D to enumerate viable, necrotic, and early and late apoptotic cells within specific subsets of a heterogeneous culture. RESULTS: In the absence of cell growth, the number of apoptotic cells that undergo fragmentation into apoptotic bodies in culture can also be determined accurately by this method. This information can then be used to obtain the apoptotic rate (AR), a new indicator of apoptosis that calculates the proportion of cells that have undergone apoptosis with respect to the total number of seeded cells. The main limitation of the method is that the AR is only suitable for the study of apoptosis in noncycling cells. CONCLUSIONS: This study reveals the superiority of the proposed method over the widely used Nicoletti method and current annexin-V binding methods. The AI did not reflect the true incidence of lymphocyte apoptosis, neither in response to lectins or phorbol esters, nor to serum deprivation. AR was more sensitive than AI, detecting apoptosis at lower concentrations of cell death inducers in all the subsets studied.     

Spontaneous apoptosis and expression of cell surface heat-shock proteins in cultured EL-4 lymphoma cells

Abstract. The expression of heat-shock proteins (HSPs) is enhanced in stressed cells and can protect cells from stress-induced injury. However, existing data about the relationship between apoptosis and HSP expression is contradictory. In this paper, a mouse lymphoma cell death model system is used to detect simultaneously both the process of apoptosis and the level of HSP expression. The model was established after discovering that spontaneous apoptosis and spontaneous cell surface HSP expression occurs in EL-4 mouse lymphoma cells during normal optimal culture conditions. The data show that apoptotic EL-4 cells had higher levels of hsp25, hsp60, hsp70 and hsp90 exposed on the plasma membrane surface than viable cells. The level of surface HSPs was found to increase through several stages of early and late apoptotic death as measured by flow cytometry, with the highest levels observed during the loss of cell membrane phospholipid asymmetry. Heat shock and actinomycin D significantly increased the proportion of apoptotic cells in culture. However, hyperthermia only stimulated a weak and temporary increase in surface HSP expression, whereas actinomycin D strongly elevated the level of surface and intracellular HSPs, particularly in live cells. These results show an associative relationship between apoptosis and HSP expression. The relationship between the progression of cell death and HSP expression suggests a role for membrane HSP expression in programmed cell death.     

Exposure of phosphatidylserine on the surface of apoptotic lymphocytes triggers specific recognition and removal by macrophages.

During normal tissue remodeling, macrophages remove unwanted cells, including those that have undergone programmed cell death, or apoptosis. This widespread process extends to the deletion of thymocytes (negative selection), in which cells expressing inappropriate Ag receptors undergo apoptosis, and are phagocytosed by thymic macrophages. Although phagocytosis of effete leukocytes by macrophages has been known since the time of Metchnikoff, only recently has it been recognized that apoptosis leads to surface changes that allow recognition and removal of these cells before they are lysed. Our data suggest that macrophages specifically recognize phosphatidylserine that is exposed on the surface of lymphocytes during the development of apoptosis. Macrophage phagocytosis of apoptotic lymphocytes was inhibited, in a dose-dependent manner, by liposomes containing phosphatidyl-L-serine, but not by liposomes containing other anionic phospholipids, including phosphatidyl-D-serine. Phagocytosis of apoptotic lymphocytes was also inhibited by the L isoforms of compounds structurally related to phosphatidylserine, including glycerophosphorylserine and phosphoserine. The membranes of apoptotic lymphocytes bound increased amounts of merocyanine 540 dye relative to those of normal cells, indicating that their membrane lipids were more loosely packed, consistent with a loss of membrane phospholipid asymmetry. Apoptotic lymphocytes were shown to express phosphatidylserine (PS) externally, because PS on their surfaces was accessible to derivatization by fluorescamine, and because apoptotic cells expressed procoagulant activity. These observations suggest that apoptotic lymphocytes lose membrane phospholipid asymmetry and expose phosphatidylserine on the outer leaflet of the plasma membrane. Macrophages then phagocytose apoptotic lymphocytes after specific recognition of the exposed PS.     

Phosphoinositide 3-OH kinase/protein kinase B inhibits apoptotic cell death induced by reactive oxygen species in Saccharomyces cerevisiae

Apoptosis is a common mode of programmed cell death in multicellular organisms. However, the recent observation of yeast cell death displaying the morphology of apoptosis has suggested the presence of an ancestral cell death machinery. Here we examined apoptotic features induced by reactive oxygen species (ROS) in yeast. Saccharomyces cerevisiae show typical apoptotic features upon exposure to ROS: membrane staining with annexin V and DNA fragmentation by the TUNEL assay. The detection of apoptotic features in yeast strongly support the existence of molecular machinery performing the basic pathways of apoptosis. The phosphoinositide 3-OH kinase (PI3K)/protein kinase B (PKB) signaling pathway has been shown to prevent apoptosis in a variety of cells. It is therefore of interest to determine whether the PI3K/PKB signaling pathway is capable of protecting yeast from apoptosis induced by ROS. We determined that PI3K/PKB is capable of significantly inhibiting ROS-evoked apoptosis in yeast. These results suggest that yeast may provide a suitable model system in which to study the apoptotic signaling pathway elicited by a variety of stimuli.     

Sequential shrinkage and swelling underlie P2X7-stimulated lymphocyte phosphatidylserine exposure and death

Patterns of change in cell volume and plasma membrane phospholipid distribution during cell death are regarded as diagnostic means of distinguishing apoptosis from necrosis, the former being associated with cell shrinkage and early phosphatidylserine (PS) exposure, whereas necrosis is associated with cell swelling and consequent lysis. We demonstrate that cell volume regulation during lymphocyte death stimulated via the purinergic receptor P2X7 is distinct from both. Within seconds of stimulation, murine lymphocytes undergo rapid shrinkage concomitant with, but also required for, PS exposure. However, within 2 min shrinkage is reversed and swelling ensues ending in cell rupture. P2X7-induced shrinkage and PS translocation depend upon K+ efflux via KCa3.1, but use a pathway of Cl- efflux distinct from that previously implicated in apoptosis. Thus, P2X7 stimulation activates a novel pathway of cell death that does not conform to those conventionally associated with apoptosis and necrosis. The mixed apoptotic/necrotic phenotype of P2X7-stimulated cells is consistent with a potential role for this death pathway in lupus disease.     

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.     

Real-time imaging of apoptotic cell-membrane changes at the single-cell level in the beating murine heart.

We report a novel real-time imaging model to visualize apoptotic membrane changes of single cardiomyocytes in the injured heart of the living mouse, using fluorescent labeled annexin-V. Annexin-V binds to externalized phosphatidylserine (PS) of cells undergoing programmed cell death. With high-magnification (x100-160) real-time imaging, we visualized the binding of annexin-V to single cardiomyocytes. Kinetic studies at the single-cell level revealed that cardiomyocytes started to bind annexin-V within minutes after reperfusion, following an ischemic period of 30 minutes. The amount of bound annexin-V increased rapidly and reached a maximum within 20-25 minutes. Caspase inhibitors decreased the number of annexin-V-positive cardiomyocytes and slowed down the rate of PS exposure of cardiomyocytes that still bound annexin-V. This technology to study cell biology in the natural environment will enhance knowledge of intracellular signaling pathways relevant for cell-death regulation and strategies to manipulate these pathways for therapeutic effect.     

Assessment of apoptosis in xenobiotic-induced immunotoxicity.

Generation of immunity is a highly complex process in which proliferation and differentiation of immune-competent cells regulated by cytokines and cell-cell interactions play a major role. Reducing the number of immune-competent cells or altering the function, selection, and differentiation of lymphocytes after xenobiotic treatment may lead to serious adverse effects. Programmed cell death, or apoptosis, is a highly regulated process by which an organism eliminates unwanted cells without eliciting an inflammatory response. However, xenobiotics are also able to trigger unwanted apoptosis or to alter the regulation of programmed cell death. Cytological characteristics of apoptosis are generally different from those seen in acute pathological cell death resulting from cell injury. The morphological characteristics of apoptosis are unique including cell shrinkage, membrane blebbing, chromatin condensation, DNA fragmentation, disruption of the nuclear lamina, nuclear fragmentation, and emergence of apoptotic bodies. It is now established that apoptosis plays a critical role in both development and homeostasis of the immune system: thymic selection, cytotoxicity, deletion of autoreactive cells, and regulation of the size of the lymphoid compartment. Assessment of apoptosis relies on the morphological and biochemical modifications of the dying cells. As a rule, and because an apoptotic cell rarely displays all of the characteristic apoptotic features, several criteria should be monitored in parallel including morphological examination. The techniques described in this paper have been divided into five categories: analysis of cell morphology by microscopy, identification of DNA fragmentation, determination of mitochondrial membrane potential, detection of plasma membrane changes, analysis of caspase activation.     

B cell receptor cross-linking triggers a caspase-8-dependent apoptotic pathway that is independent of the death effector domain of Fas-associated death domain protein.

We have previously reported that B cell receptors, depending on the degree to which they are cross-linked, can promote apoptosis in various human B cell types. In this study, we show that B cell receptors can trigger two apoptotic pathways according to cross-linking and that these pathways control mitochondrial activation in human Burkitt's lymphoma cells. Whereas soluble anti-mu Ab triggers caspase-independent mitochondrial activation, cross-linked anti-mu Ab induces an apoptotic response associated with a caspase-dependent loss of mitochondrial transmembrane potential. This B cell receptor-mediated caspase-dependent mitochondrial activation is associated with caspase-8 activation. We show here that caspase-8 inhibitors strongly decrease cross-linking-dependent B cell receptor-mediated apoptosis in Burkitt's lymphoma BL41 cells. These inhibitors act upstream from the mitochondria as they prevented the loss of mitochondrial membrane potential observed in B cell receptor-treated BL41 cells. Caspase-8 activation in these cells was also evident from the detection of cleaved fragments of caspase-8 and the cleavage of specific substrates, including Bid. Our data show that cross-linked B cell receptors induced an apoptotic pathway involving sequential caspase-8 activation, loss of mitochondrial membrane potential, and the activation of caspase-9 and caspase-3. Cells expressing a dominant negative mutant of Fas-associated death domain protein were sensitive to cross-linked B cell receptor-induced caspase-8 activation and apoptosis; therefore, this caspase-8 activation was independent of the death effector domain of Fas-associated death domain protein.     

Apoptosis and eryptosis: Striking differences on biomembrane level

The cell plasma membrane plays an essential role in programmed cell death of nucleated cells (apoptosis) and erythrocytes (eryptosis), and its changes due to loss of transmembrane asymmetry are quite similar. However, nucleated cells possess the network of intracellular membranes, which are missing in erythrocytes. Providing comparative studies with series of molecular probes, we observe dramatic differences in membrane lipid order in the course of apoptosis and eryptosis. In contrast to nucleated cells, in which a significant drop of the lipid order in the plasma membrane is observed, the erythrocyte membrane retains the relatively high level of the lipid order. Observation in nucleated cells of significant differences between inner and plasma membranes and detection of apoptotic bodies with different organization suggest that the decrease in the lipid order of their plasma membrane could be at least partially explained by the phospholipid and/or cholesterol exchange between membranes. Such features are absent in erythrocytes.     

Annexin V staining due to loss of membrane asymmetry can be reversible and precede commitment to apoptotic death.

Signal-induced apoptosis is a normal phenomenon in which cells respond to changes in their environment through a cascade of intracellular biochemical changes culminating in cell death. However, it is not clear at what point in this process the cell becomes committed to die. An early biochemical change characteristic of cells undergoing apoptosis is the loss of plasma membrane asymmetry, such that high levels of phosphatidylserine become exposed on the outside cell surface. These cells can be recognized by staining with Annexin V, which binds to phosphatidylserine with high affinity. To investigate the mechanisms controlling signal-induced apoptosis we have examined the response of a B cell lymphoma to crosslinking of the membrane immunoglobulin (mIg) receptor. We have found that many of the cells that stain positive for Annexin V are viable and can resume growth and reestablish phospholipid asymmetry once the signal is removed. These results indicate that Annexin V staining, and thus loss of membrane asymmetry, precedes commitment to apoptotic death in this system.     

Apoptotic cells selectively uptake minor glycoforms of vitronectin from serum

Apoptosis profoundly alters the carbohydrate layer coating the membrane of eukaryotic cells. Previously we showed that apoptotic cells became reactive with the α2,6-sialyl-specific lectin from Sambucus nigra agglutinin (SNA), regardless of their histological origin and the nature of the apoptotic stimulus. Here we reveal the basis of the phenomenon by showing that in apoptotic cancer cell lines SNA reactivity was mainly associated with a 67 kDa glycoprotein which we identified by MALDI-TOF/TOF and immunoblot analysis as bovine vitronectin (bVN). bVN was neither present in non-apoptotic cells, nor in cells induced to apoptosis in serum-free medium, indicating that its uptake from the cell culture serum occurred only during apoptosis. The bVN molecules associated with apoptotic cancer cell lines represented minor isoforms, lacking the carboxyterminal sequence and paradoxically containing a few α2,6-linked sialic acid residues. Despite their poor α2,6-sialylation, these bVN molecules were sufficient to turn apoptotic cells to SNA reactivity, which is a late apoptotic event occurring in cells positive to both annexin-V and propidium iodide. Unlike in cancer cell lines, the major bVN form taken up by apoptotic neutrophils and mononuclear cells was a 80 kDa form. In apoptotic SW948 cells we also detected the α2,6-sialylated forms of the stress-70 mitochondrial precursor (mortalin) and of tubulin-β2C. These data indicate that the acquisition of vitronectin isoforms from the environment is a general, although cell specific phenomenon, potentially playing an important role in post-apoptotic events and that the α2,6-sialylation of intracellular proteins is a new kind of posttranslational modification associated with apoptosis.     

Will imaging of apoptosis play a role in clinical care? A tale of mice and men

Programmed cell death (apoptosis) plays a role in the pathophysiology of many diseases and in the outcome of treatment. Apoptosis is the likely mechanism behind the cytoreductive effects of standard chemotherapeutic and radiation treatments, rejection of organ transplants, cellular damage in collagen vascular disorders, and delayed cell death due to hypoxic-ischemic injury in myocardial infarction and neonatal hypoxic ischemic injury. Observations about the role of apoptosis have fueled the development of novel agents and treatment strategies specifically aimed at inducing or inhibiting apoptosis.Despite these research developments there are no clinical entities where specific measures of apoptosis are used in either diagnosis or patient management. Part of the difficulty in bridging the gap between the basic science understanding of apoptosis and the clinical application of this information is the lack of a sensitive marker to monitor programmed cell death in association with disease progression or regression. Technetium-99m labeled annexin V localizes at sites of apoptosis in-vivo, due to its nanomolar affinity for membrane bound phosphatidylserine. Radiolabeled annexin V imaging permits identification of the site and extent of apoptosis in experimental animals. Annexin V has been successfully used in animal models to image organ transplant rejection, characterize successful therapy of tumors, pinpoint acute myocardial infarction, and identify hypoxic ischemic brain injury of the newborn and adult. Early studies in human subjects suggest that ^99mTc annexin imaging will be also be useful to identify rejection in transplant recipients, localize acute myocardial infarction, and characterize the effectiveness of a single treatment in patients with tumors.This review describes the imaging approaches to detect and monitor apoptosis in-vivo that are presently in early clinical trials. The preliminary data are extrapolated to identify conditions where apoptosis imaging may be valuable in clinical decision making. These conditions include: transplant rejection; hypoxic/ischemic injury of heart and brain; and determining the efficacy of therapy in cancer, heart failure and osteoporosis.     

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.     

IgH-2 cells: a reptilian model for apoptotic studies

Regulation of proper cell number in tissues depends upon a balance between cell proliferation and cell death. The process of apoptosis has thus far been studied in a variety of multicellular organisms from humans to higher plants. In order to broaden our perspective and identify another metazoan system with which to deepen our understanding of the function and evolution of the apoptotic machinery, we have characterized cell death in a reptilian cell line. We show that the death of IgH-2 Iguana (Iguana iguana) heart cells [Clark, H.F., Cohen, M.M., Karzon, D.T., 1970. Characterization of reptilian cell lines established at incubation temperatures of 23 to 36 degrees. Proc. Soc. Exp. Biol. Med. 133, 1039-1047.] is, in response to DNA damaging agents, accompanied by classic morphological changes of apoptosis including detachment from the substrate, cell shrinkage, nuclear pyknosis and externalization of the plasma membrane phospholipid phosphatidylserine. Our biochemical studies show that the death of IgH-2 cells is accompanied by internucleosomal DNA fragmentation and activation of caspases. Our studies with the pan-caspase inhibitor zVAD.fmk implicate caspases in the apoptotic process we observe. This work represents the first detailed molecular and biochemical analysis of apoptosis in cells of an organism of class Reptilia and establishes IgH-2 cells as a suitable model system with which to investigate the phenomenon of caspase dependent apoptosis and the apoptotic machinery in a reptilian model.     

A role of the mitochondrial apoptosis-inducing factor in granulysin-induced apoptosis

Granulysin is a cytolytic molecule released by CTL via granule-mediated exocytosis. In a previous study we showed that granulysin induced apoptosis using both caspase- and ceramide-dependent and -independent pathways. In the present study we further characterize the biochemical mechanism for granulysin-induced apoptosis of tumor cells. Granulysin-induced death is significantly inhibited by Bcl-2 overexpression and is associated with a rapid (1-5 h) loss of mitochondrial membrane potential, which is not mediated by ceramide generation and is not inhibited by the general caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone. Ceramide generation induced by granulysin is a slow event, only observable at longer incubation times (12 h). Apoptosis induced by exogenous natural (C(18)) ceramide is truly associated with mitochondrial membrane potential loss, but contrary to granulysin, this event is inhibited by benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone. Ceramide-induced apoptosis is also completely prevented by Bcl-2 overexpression. The nuclear morphology of cells dying after granulysin treatment in the presence of caspase inhibitors suggested the involvement of mitochondrial apoptosis-inducing factor (AIF) in granulysin-induced cell death. We demonstrate using confocal microscopy that AIF is translocated from mitochondria to the nucleus during granulysin-induced apoptosis. The majority of Bcl-2 transfectants are protected from granulysin-induced cell death, mitochondrial membrane potential loss, and AIF translocation, while a small percentage are not protected. In this small percentage the typical nuclear apoptotic morphology is delayed, being of the AIF type at 5 h time, while at longer times (12 h) the normal apoptotic morphology is predominant. These and previous results support a key role for the mitochondrial pathway of apoptosis, and especially for AIF, during granulysin-induced tumoral cell death.     

Plasma membrane phospholipid asymmetry precedes DNA fragmentation in different apoptotic cell models

 Biochemical alterations occurring in many cell types during apoptosis include the loss of plasma membrane phospholipid asymmetry and nuclear DNA fragmentation. Annexin V staining detects phosphatidylserine translocation into the outer plasma membrane layer occurring during cell death, while the in situ tailing (IST or TUNEL) reaction labels the DNA strand breaks typical of apoptosis. To compare the time course of these processes we investigated methylprednisolone-induced apoptosis of rat thymocytes, topoisomerase inhibitor-induced apoptosis in the human histiocytic lymphoma cell line U937, and serum deprivation-induced apoptosis in the rat pheochromocytoma cell line, PC12. At all time points, FACS analysis and quantitative fluorescence light microscopy showed a higher proportion of annexin V-positive than IST-positive cells, with significantly different time courses in the apoptotic cell models investigated (Anova test). Results were confirmed by confocal microscopy. Our data indicate that the exposure of phosphatidylserine, a potential phagocyte recognition signal on the cell surface of apoptotic cells in vivo, precedes DNA strand breaks during apoptosis in different cell types. Accepted: 29 June 1998