Genome digestion is a dispensable consequence of physiological cell death mediated by cytotoxic T lymphocytes

Anchoring of FRET Sensors-A Requirement for Spatiotemporal Resolution

FRET biosensors have become a routine tool for investigating mechanisms and components of cell signaling. Strategies for improving them for particular applications are continuously sought. One important aspect to consider when designing FRET probes is the dynamic distribution and propagation of signals within living cells. We have addressed this issue by directly comparing an anchored (taFS) to a non-anchored (naFS) cleavable FRET sensor. We chose a microtubule-associated protein tau as an anchor, as microtubules are abundant throughout the cytosol of cells. We show that tau-anchored FRET sensors are concentrated at the cytoskeleton and enriched in the neurite-like processes of cells, providing high intensity of the total signal. In addition, anchoring limits the diffusion of the sensor, enabling spatiotemporally resolved monitoring of subcellular variations in enzyme activity. Thus, anchoring is an important aspect to consider when designing FRET sensors for deeper understanding of cell signaling.

Specific antibodies induce apoptosis in Trypanosoma cruzi epimastigotes

The susceptibility of Trypanosoma cruzi epimastigotes to lysis by normal or immune sera in a complement-dependent reaction has been reported. Mouse immune sera depleted complement-induced damage in epimastigotes characterized by morphological changes and death. The purpose of this work was to study the mechanism of death in epimastigotes exposed to decomplemented mouse immune serum. Epimastigotes were maintained in RPMI medium. Immune sera were prepared in mice by immunization with whole crude epimastigote extracts. Viable epimastigotes were incubated with decomplemented normal or immune sera at 37 degrees C. By electron microscopy, agglutinated parasites showed characteristic patterns of membrane fusion between two or more parasites; this fusion also produced interdigitation of the subpellicular microtubules. Apoptosis was determined by flow cytometry using terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) and annexin V assays. Nuclear features were examined by 4'-,6-diamidino-2'-phenylindole diHCI cytochemistry that demonstrated apoptotic nuclear condensation. Caspase activity was also measured. TUNEL results showed that parasites incubated with decomplemented immune sera took up 26% of specific fluorescence as compared to 1.3% in parasites incubated with decomplemented normal sera. The Annexin-V-Fluos staining kit revealed that epimastigotes incubated with decomplemented immune sera exposed phosphatidylserine on the external leaflet of the plasma membrane. The incubation of parasites with immune sera showed caspase 3 activity. We conclude that specific antibodies are able to induce agglutination and apoptosis in epimastigotes, although the pathway is not elucidated.

Schwann cell proliferation during Wallerian degeneration is not necessary for regeneration and remyelination of the peripheral nerves: axon-dependent removal of newly generated Schwann cells by apoptosis

Peripheral nerve injury is followed by a wave of Schwann cell proliferation in the distal nerve stumps. To resolve the role of Schwann cell proliferation during functional recovery of the injured nerves, we used a mouse model in which injury-induced Schwann cell mitotic response is ablated via targeted disruption of cyclin D1. In the absence of distal Schwann cell proliferation, axonal regeneration and myelination occur normally in the mutant mice and functional recovery of injured nerves is achieved. This is enabled by pre-existing Schwann cells in the distal stump that persist but do not divide. On the other hand, in the wild type littermates, newly generated Schwann cells of injured nerves are culled by apoptosis. As a result, distal Schwann cell numbers in wild type and cyclin D1 null mice converge to equivalence in regenerated nerves. Therefore, distal Schwann cell proliferation is not required for functional recovery of injured nerves.

Characterization of DNA damage in yeast apoptosis induced by hydrogen peroxide, acetic acid, and hyperosmotic shock

Saccharomyces cerevisiae has been reported to die, under certain conditions, from programmed cell death with apoptotic markers. One of the most important markers is chromosomal DNA fragmentation as indicated by terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) staining. We found TUNEL staining in S. cerevisiae to be a consequence of both single- and double-strand DNA breaks, whereas in situ ligation specifically stained double-strand DNA breaks. Cells treated with hydrogen peroxide or acetic acid staining positively for TUNEL assay stained negatively for in situ ligation, indicating that DNA damage in both cases mainly consists of single-strand DNA breaks. Pulsed field gel electrophoresis of chromosomal DNA from cells dying from hydrogen peroxide, acetic acid, or hyperosmotic shock revealed DNA breakdown into fragments of several hundred kilobases, consistent with the higher order chromatin degradation preceding DNA laddering in apoptotic mammalian cells. DNA fragmentation was associated with death by treatment with 10 mM hydrogen peroxide but not 150 mM and was absent if cells were fixed with formaldehyde to eliminate enzyme activity before hydrogen peroxide treatment. These observations are consistent with a process that, like mammalian apoptosis, is enzyme dependent, degrades chromosomal DNA, and is activated only at low intensity of death stimuli.

Higher order chromatin degradation: implications for neurodegeneration

Higher order chromatin degradation (HOCD) is a hallmark of programmed cell death. HOCD is mediated by enzymatic digestion of the DNA backbone at matrix attachment regions, and ultimately results in the excision of chromatin loops and their oligomers from chromosomes. We have recently demonstrated that hydrogen peroxide (H2O2), the major mediator of oxidative stress, rapidly induces HOCD. This demonstration allowed us to characterize several kinetic features of HOCD. Moreover, H2O2-induced HOCD provides a mechanistic link between oxidative stress and the pathology of neurodegeneration. Thus, in acute neurodegenerative conditions, which feature severe oxidative stress, H2O2-induced HOCD efficiently dismantles the genome, and thus, irreversibly commits cells to death. In chronic neurodegenerative conditions, which feature sublethal but perennial oxidative stress, cells undergo only a partial fragmentation of the genome via H2O2-induced HOCD. If unrepaired of improperly repaired, such a partial fragmentation leads to the generation and accumulation of somatic mutations that are likely to play the key role in delayed degeneration and death of neural cells.

Angiogenesis in transgenic models of multistep carcinogenesis

The histopathology and the epidemiology of human cancers, as well as studies of animal models of tumorigenesis, have led to a widely accepted notion that multiple genetic and epigenetic changes have to accumulate for progression to malignancy. Formation of new blood vessels (tumor angiogenesis) has been recognized, in addition to proliferative capabilities and to the ability to down-modulate cell death (apoptosis), as essential for the progressive growth and expansion of solid tumors beyond microscopic sizes of about 1-2 mm in diameter. Mice overexpressing activated forms of oncogenes or carrying targeted mutations in tumor suppressor genes have proven extremely useful for to linking the function of these genes with specific tumor processes; the interbreeding of these mice let us study the extent of cooperativity between different genetic lesions in disease progression, leading to a greater understanding of multi-stage nature of tumorigenesis.

Caspase-dependent Cdk activity is a requisite effector of apoptotic death events

The caspase-dependent activation of cyclin-dependent kinases (Cdks) in varied cell types in response to disparate suicidal stimuli has prompted our examination of the role of Cdks in cell death. We have tested the functional role of Cdk activity in cell death genetically, with the expression of dominant negative Cdk mutants (DN-Cdks) and Cdk inhibitory genes. Here we demonstrate that Cdk2 activity is necessary for death-associated chromatin condensation and other manifestations of apoptotic death, including cell shrinkage and the loss of adhesion to substrate. Susceptibility to the induction of the cell death pathway, including the activation of the caspase cascade, is unimpaired in cells in which Cdk2 activity is inhibited. The direct visualization of active caspase activity in these cells confirms that death-associated Cdk2 acts downstream of the caspase cascade. Cdk inhibition also does not prevent the loss of mitochondrial membrane potential and membrane phospholipid asymmetry, which may be direct consequences of caspase activity, and dissociates these events from apoptotic condensation. Our data suggest that caspase activity is necessary, but not sufficient, for the full physiological cell death program and that a requisite function of the proteolytic caspase cascade is the activation of effector Cdks.

Thioredoxin reductase mediates cell death effects of the combination of beta interferon and retinoic acid

Interferons (IFNs) and retinoids are potent biological response modifiers. By using JAK-STAT pathways, IFNs regulate the expression of genes involved in antiviral, antitumor, and immunomodulatory actions. Retinoids exert their cell growth-regulatory effects via nuclear receptors, which also function as transcription factors. Although these ligands act through distinct mechanisms, several studies have shown that the combination of IFNs and retinoids synergistically inhibits cell growth. We have previously reported that IFN-beta-all-trans-retinoic acid (RA) combination is a more potent growth suppressor of human tumor xenografts in vivo than either agent alone. Furthermore, the IFN-RA combination causes cell death in several tumor cell lines in vitro. However, the molecular basis for these growth-suppressive actions is unknown. It has been suggested that certain gene products, which mediate the antiviral actions of IFNs, are also responsible for the antitumor actions of the IFN-RA combination. However, we did not find a correlation between their activities and cell death. Therefore, we have used an antisense knockout approach to directly identify the gene products that mediate cell death and have isolated several genes associated with retinoid-IFN-induced mortality (GRIM). In this investigation, we characterized one of the GRIM cDNAs, GRIM-12. Sequence analysis suggests that the GRIM-12 product is identical to human thioredoxin reductase (TR). TR is posttranscriptionally induced by the IFN-RA combination in human breast carcinoma cells. Overexpression of GRIM-12 causes a small amount of cell death and further enhances the susceptibility of cells to IFN-RA-induced death. Dominant negative inhibitors directed against TR inhibit its cell death-inducing functions. Interference with TR enzymatic activity led to growth promotion in the presence of the IFN-RA combination. Thus, these studies identify a novel function for TR in cell growth regulation.

Transition from caspase-dependent to caspase-independent mechanisms at the onset of apoptotic execution

We have compared cytoplasmic extracts from chicken DU249 cells at various stages along the apoptotic pathway. Extracts from morphologically normal "committed stage" cells induce apoptotic morphology and DNA cleavage in substrate nuclei but require ongoing caspase activity to do so. In contrast, extracts from frankly apoptotic cells induce apoptotic events in added nuclei in a caspase-independent manner. Biochemical fractionation of these extracts reveals that a column fraction enriched in endogenous active caspases is unable to induce DNA fragmentation or chromatin condensation in substrate nuclei, whereas a caspase-depleted fraction induces both changes. Further characterization of the "execution phase" extracts revealed the presence of an ICAD/DFF45 (inhibitor of caspase-activated DNase/DNA fragmentation factor)- inhibitable nuclease resembling CAD, plus another activity that was required for the apoptotic chromatin condensation. Despite the presence of active caspases, committed stage extracts lacked these downstream activities, suggesting that the caspases and downstream factors are segregated from one another in vivo during the latent phase. These observations not only indicate that caspases act in an executive fashion, serving to activate downstream factors that disassemble the nucleus rather than disassembling it themselves, but they also suggest that activation of the downstream factors (rather than the caspases) is the critical event that occurs at the transition from the latent to active phase of apoptosis.

Application of fluorescence microscopy to measure apoptosis in Jurkat T cells after treatment with a new investigational anticancer agent (N.C.1213)

1. Apoptosis as the mechanism of cell death induced by a new cytotoxic and anticancer agent (N.C.1213) was investigated by morphological and biochemical criteria in human Jurkat T leukemia cells. 2. The effect of N.C.1213 on the survival of Jurkat T, LV-50, H-9, and Molt-3 cells was measured. Jurkat T cells exhibited the highest response, with less than 10% of the cells remaining viable after exposure to 10 microM N.C.1213 for a 24 hr period. All other cell cultures were also affected but to a lesser extent. 3. With the use of a fluorescence microscope, several morphological features characteristic of apoptosis such as condensed chromatin and apoptotic bodies were identified in Jurkat T cells after exposure to N.C.1213 and melphalan. The results indicated that melphalan was more cytotoxic than N.C.1213 as shown by the dye exclusion test. However, N.C.1213 showed a greater apoptotic index than melphalan. The IC50 of N.C.1213 in Jurkat T cells was determined to be 3.5 microM. 4. A DNA ladder (fragmentation of DNA into multimers of approximately 200 base pairs), which is one characteristic feature of apoptosis, was not detected when Jurkat T cells were exposed to N.C.1213. Hence it is probable that the key morphological events in apoptosis observed in the present experimental conditions precede the internucleosomal cleavage of DNA.

Commitment and effector phases of the physiological cell death pathway elucidated with respect to Bcl-2 caspase, and cyclin-dependent kinase activities

Physiological cell deaths occur ubiquitously throughout biology and have common attributes, including apoptotic morphology with mitosis-like chromatin condensation and prelytic genome digestion. The fundamental question is whether a common mechanism of dying underlies these common hallmarks of death. Here we describe evidence of such a conserved mechanism in different cells induced by distinct stimuli to undergo physiological cell death. Our genetic and quantitative biochemical analyses of T- and B-cell deaths reveal a conserved pattern of requisite components. We have dissected the role of cysteine proteases (caspases) in cell death to reflect two obligate classes of cytoplasmic activities functioning in an amplifying cascade, with upstream interleukin-1beta-converting enzyme-like proteases activating downstream caspase 3-like caspases. Bcl-2 spares cells from death by punctuating this cascade, preventing the activation of downstream caspases while leaving upstream activity undisturbed. This observation permits an operational definition of the stages of the cell death process. Upstream steps, which are necessary but not themselves lethal, are modulators of the death process. Downstream steps are effectors of, and not dissociable from, actual death; the irreversible commitment to cell death reflects the initiation of this downstream phase. In addition to caspase 3-like proteases, the effector phase of death involves the activation in the nucleus of cell cycle kinases of the cyclin-dependent kinase (Cdk) family. Nuclear recruitment and activation of Cdk components is dependent on the caspase cascade, suggesting that catastrophic Cdk activity may be the actual effector of cell death. The conservation of the cell death mechanism is not reflected in the molecular identity of its individual components, however. For example, we have detected different cyclin-Cdk pairs in different instances of cell death. The ordered course of events that we have observed in distinct cases reflects essential thematic elements of a conserved sequence of modulatory and effector activities comprising a common pathway of physiological cell death.

Requirements for proteolysis during apoptosis

The key effector proteins of apoptosis are a family of cysteine proteases termed caspases. Following activation of caspases, biochemical events occur that lead to DNA degradation and the characteristic morphological changes associated with apoptosis. Here we show that cytoplasmic extracts activated in vitro by proteinase K were able to cleave the caspase substrate DEVD-7-amino-4-methylcoumarin, while neither proteinase K nor nonactivated extracts were able to do so alone. Caspase-like activity was inhibited by the specific caspase inhibitor DEVD-aldehyde and by the protease inhibitor iodoacetamide, but not by N-ethylmaleimide. When added to isolated nuclei, the activated extracts caused internucleosomal DNA degradation and morphological changes typical of apoptosis. As DNA cleavage and morphological changes could be inhibited by N-ethylmaleimide but not by iodoacetamide, we conclude that during apoptosis, caspase activation causes activation of another cytoplasmic enzyme that can be inhibited by N-ethylmaleimide. Activity of this enzyme is necessary for activation of endonucleases, DNA cleavage, and changes in nuclear morphology.

Resistance of actin to cleavage during apoptosis

A small number of cellular proteins present in the nucleus, cytosol, and membrane fraction are specifically cleaved by the interleukin-1beta-converting enzyme (ICE)-like family of proteases during apoptosis. Previous results have demonstrated that one of these, the cytoskeletal protein actin, is degraded in rat PC12 pheochromocytoma cells upon serum withdrawal. Extracts from etoposide-treated U937 cells are also capable of cleaving actin. It was assumed that cleavage of actin represented a general phenomenon, and a mechanism coordinating proteolytic, endonucleolytic, and morphological aspects of apoptosis was proposed. We demonstrate here that actin is resistant to degradation in several different human cells induced to undergo apoptosis in response to a variety of stimuli, including Fas ligation, serum withdrawal, cytotoxic T-cell killing, and DNA damage. On the other hand, cell-free extracts from these cells and the ICE-like protease CPP32 were capable of cleaving actin in vitro. We conclude that while actin contains cleavage sites for ICE-like proteases, it is not degraded in vivo in human cells either because of lack of access of these proteases to actin or due to the presence of other factors that prevent degradation.

Cleavage of actin by interleukin 1 beta-converting enzyme to reverse DNase I inhibition

Three of the predominant features of apoptosis are internucleosomal DNA fragmentation, plasma membrane bleb formation, and retraction of cell processes. We demonstrate that actin is a substrate for the proapoptotic cysteine protease interleukin 1beta-converting enzyme. Actin cleaved by interleukin 1beta-converting enzyme can neither inhibit DNase I nor polymerize to its filamentous form as effectively as intact actin. These findings suggest a mechanism for the coordination of the proteolytic, endonucleolytic, and morphogenetic aspects of apoptosis.

NF-M (chicken C/EBP beta) induces eosinophilic differentiation and apoptosis in a hematopoietic progenitor cell line

CAAT/enhancer binding proteins (C/EBPs) are transcriptional activators implicated in the differentiation processes of various cell lineages. We have shown earlier that NF-M, the chicken homolog of C/EBP beta, is specifically expressed in myelomonocytic and eosinophilic cells of the hematopoietic system. To investigate the role of NF-M in hematopoietic cell lineage commitment, we constructed a conditional form of the protein by fusing it to the hormone binding domain of the human estrogen receptor. This construct was stably expressed in a multipotent progenitor cell line transformed by the Myb-Ets oncoprotein. We report here that both NF-M-dependent promoter constructs and resident genes could be activated by addition of beta-estradiol to the NF-M-estrogen receptor expressing progenitors. At the same time, we observed a down-regulation of progenitor-specific surface markers and the up-regulation of differentiation markers restricted to the eosinophil and myeloid lineages. In addition to the onset of differentiation, cell death was induced with typical apoptotic features. Our results suggest that NF-M plays an important role in commitment along the eosinophil lineage and in the induction of apoptosis.

Enhancement of Ca(2+)-dependent endonuclease activity in L1210 cells during apoptosis induced by 1-beta-D-arabinofuranosylcytosine: possible involvement of activating factor(s)

Internucleosomal DNA fragmentation and morphological changes in nuclei typical of apoptosis were observed in L1210 cells incubated with 1.0 micrograms/ml of 1-beta-D-arabinofuranosylcytosine (ara-C). To investigate the mechanisms involved, we examined the activities of endogenous endonucleases in nuclei and cytoplasm. Both fractions of control cells contained Ca(2+)-dependent endonuclease which was capable of mediating internucleosomal DNA fragmentation. The assay system using two kinds of target substrates, i.e., nuclear chromatin of CCRF-CEM cells and naked DNA purified from the same cells, revealed that the activity of Ca(2+)-dependent endonuclease was enhanced in the crude nuclear extracts of cells treated with 1.0 microgram/ml of ara-C for 24 h or 48 h. The activity was extracted more easily from ara-C-treated cells than control cells without sonication of the nuclear fraction. On the other hand, in the cytoplasmic fraction of the cells, the activity towards naked DNA was unchanged, whereas that towards nuclear chromatin was clearly enhanced. These results suggest that internucleosomal DNA fragmentation induced by ara-C treatment is associated with enhancement and activation of constitutively expressed Ca(2+)-dependent endonuclease in L1210 cells.

CD36 gene transfer confers capacity for phagocytosis of cells undergoing apoptosis

Phagocyte recognition and ingestion of intact cells undergoing apoptosis are key events in this generally important program of cell death. Insufficient phagocyte capacity for apoptotic cells can result in failure to clear dying cells before membrane integrity is lost, resulting in leakage of noxious cell contents and severe tissue damage. However, no means has been available to increase phagocytic clearance of apoptotic cells. We now report that transfection of the macrophage adhesion molecule CD36 into human Bowes melanoma cells specifically conferred greatly increased capacity to ingest apoptotic neutrophils, lymphocytes, and fibroblasts, comparable to that exhibited by macrophages. Furthermore, when CD36 was transfected into another cell type with limited capacity to take up apoptotic bodies, the monkey COS-7 cell, similar effects were observed. Therefore, CD36 gene transfer can confer "professional" capacity to ingest apoptotic cells upon "amateur" phagocytes.

Synergistic roles of granzymes A and B in mediating target cell death by rat basophilic leukemia mast cell tumors also expressing cytolysin/perforin

We have studied the cytotoxic activity of rat basophilic leukemia (RBL) cells transfected with cDNAs for the cytotoxic T lymphocyte (CTL) granule components, cytolysin (perforin), granzyme A, and granzyme B. With red cell targets, cytolysin expression conferred potent hemolytic activity, which was not influenced by coexpression of granzymes. With tumor targets, RBL cells expressing cytolysin alone were weakly cytotoxic, but both cytolytic and nucleolytic activity were enhanced by coexpression of granzyme B. RBL cells expressing all three CTL granule components showed still higher cytotoxic activities, with apoptotic target death. Analysis of the cytotoxic activity of individual transfectant clones showed that cytolytic and nucleolytic activity correlated with granzyme expression but was independent of cytolysin expression within the range examined. A synergism between granzymes A and B was apparent when the triple transfectant was compared with RBL cells expressing cytolysin and one granzyme. These data implicate granzymes as the major mediators of tumor target damage by cytotoxic lymphocytes.

A null mutation in the perforin gene impairs cytolytic T lymphocyte- and natural killer cell-mediated cytotoxicity

Lymphocyte-mediated cytotoxicity has been proposed to consist of the polarized secretion of granule-stored perforin leading to target-cell lysis. Nevertheless, perforin-independent pathways were postulated to explain the cytolytic activity of apparently perforin-free lymphocytes and the DNA degradation found in dying target cells. To evaluate the role of perforin, we used gene targeting in embryonic stem cells to produce mice lacking perforin. Mice homozygous for the disrupted gene have no perforin mRNA. The mice are healthy. Activation and granzyme A secretion of perforin-free cytolytic T cells are unaltered. The killing activity of cytolytic T cells as well as natural killer (NK) cells, however, is impaired but not abolished. Approximately one-third of the killing activity remains when lysis of 3T3 fibroblast targets and the apoptotic cell death of YAC-1 NK targets are analyzed. We conclude that perforin is a crucial effector molecule in T cell- and NK cell-mediated cytolysis. However, alternative perforin-independent lytic mechanisms also exist.

Cell nucleus and DNA fragmentation are not required for apoptosis

Apoptosis is the predominant form of cell death and occurs under a variety of physiological and pathological conditions. Cells undergoing apoptotic cell death reveal a characteristic sequence of cytological alterations including membrane blebbing and nuclear and cytoplasmic condensation. Activation of an endonuclease which cleaves genomic DNA into internucleosomal DNA fragments is considered to be the hallmark of apoptosis. However, no clear evidence exists that DNA degradation plays a primary and causative role in apoptotic cell death. Here we show that cells enucleated with cytochalasin B still undergo apoptosis induced either by treatment with menadione, an oxidant quinone compound, or by triggering APO-1/Fas, a cell surface molecule involved in physiological cell death. Incubation of enucleated cells with the agonistic monoclonal anti-APO-1 antibody revealed the key morphological features of apoptosis. Moreover, in non-enucleated cells inhibitors of endonuclease blocked DNA fragmentation, but not cell death induced by anti-APO-1. These data suggest that DNA degradation and nuclear signaling are not required for induction of apoptotic cell death.

Nuclear pores in the apoptotic cell

During apoptosis, nuclear pores undergo strong modifications, which are described here in five different apoptotic models. Conventional electron microscopy, supported by freeze-fracture analysis, showed a constant migration of nuclear pores towards the diffuse chromatin areas. In contrast, dense chromatin areas appear pore-free and are frequently surrounded by strongly dilated cisternae. A possible functional significance of this pore behaviour during apoptosis is discussed.

Chromatin condensation during apoptosis is accompanied by degradation of lamin A+B, without enhanced activation of cdc2 kinase

Chromatin condensation paralleled by DNA fragmentation is one of the most important criteria which are used to identify apoptotic cells. However, comparable changes are also observed in interphase nuclei which have been treated with cell extracts from mitotic cells. In this respect it is known that in mitosis, the lamina structure is broken down as a result of lamin solubilization and it is possible that a similar process is happening in apoptotic cells. The experiments described in this study have used confluent cultures of an embryonic fibroblast cell line which can be induced to undergo either apoptosis at low serum conditions or mitosis. Solubilization of lamin A+B was analyzed by immunoblotting and indirect immunofluorescence. These studies showed that in mitotic cells lamina breakdown is accompanied by lamin solubilization. In apoptotic cells, a small amount of lamin is solubilized before the onset of apoptosis, thereafter, chromatin condensation is accompanied by degradation of lamin A+B to a 46-kD fragment. Analysis of cellular lysates by probing blots with anti-PSTAIR followed by anti-phosphotyrosine showed that in contrast to mitosis, dephosphorylation on tyrosine residues did not occur in apoptotic cells. At all timepoints after the onset of apoptosis there was no significant increase in the activation of p34cdc2 as determined in the histone H1 kinase assay. Coinduction of apoptosis and mitosis after release of cells from aphidicolin block showed that apoptosis could be induced in parallel with S-phase. The sudden breakdown of chromatin structure may be the result of detachment of the chromatin loops from their anchorage at the nuclear matrix, as bands of 50 kbp and corresponding multimers were detectable by field inversion gel electrophoresis (FIGE). In apoptotic cells all of the DNA was fragmented, but only 14% of the DNA was smaller than 50 kbp. DNA strand breaks were detected at the periphery of the condensed chromatin by in situ tailing (ISTAIL). Chromatin condensation during apoptosis appears to be due to a rapid proteolysis of nuclear matrix proteins which does not involve the p34cdc2 kinase.

Mechanisms of immune-mediated myocyte injury

Much progress has been made in defining the mechanisms by which altered systolic and diastolic function of the heart may be produced by components of the immune system activated during allograft rejection and myocarditis and in patients with dilated cardiomyopathy. It is clear that injury of the vascular bed can occur via both humoral and cellular mediators and probably accounts for the acute alterations in ventricular compliance that occur during allograft rejection, as well as the accelerated development of graft atherosclerosis. Altered myocyte function and lysis can be produced by CTL in vitro, but the importance of this injury process in vivo remains uncertain. Other cells present in the inflammatory infiltrate can also affect myocyte function and survival. Neutrophils may cause lysis of myocytes, and cytokines produced by infiltrating macrophages and HtL may reach a sufficient concentration in the interstitial microenvironment to decrease myocyte catecholamine responsiveness and/or directly depress myocyte contractility. Humoral antibodies to myocyte cell surface antigens may cause cell damage by an antibody-dependent cytotoxic cell mechanism or by directly binding to and altering sarcolemmal receptor and/or ion channel function. Further elucidation of the extent of involvement of these different mechanisms in specific clinical settings may provide a basis for improved therapy of immune-mediated cardiac injury and dysfunction.

Activation of cyclin A-dependent protein kinases during apoptosis

Apoptosis was induced in S-phase-arrested HeLa cells by staurosporine, caffeine, 6-dimethylaminopurine, and okadaic acid, agents that activate M-phase-promoting factor and induce premature mitosis in similarly treated hamster cell lines. Addition of these agents to asynchronously growing HeLa cells or to cells arrested in early G1 phase with lovastatin had little or no effect. S-phase arrest also promoted tumor necrosis factor alpha-induced apoptosis, eliminating the normal requirement for simultaneous cycloheximide treatment. For all of the apoptosis-inducing agents tested, the appearance of condensed chromatin was accompanied by 2- to 7-fold increases in cyclin A-associated histone H1 kinase activity, levels approximating the mitotic value. Where examined, both Cdc2 and Cdk2, the catalytic subunits known to associate with cyclin A, were activated. Stable overexpression of bcl-2 suppressed the apoptosis-inducing activity of all agents tested and reduced the amount of Cdc2 and Cdk2 in the nucleus, suggesting a possible mechanism by which bcl-2 inhibits the chromatin condensation characteristic of apoptosis. These findings suggest that at least one of the biochemical steps required for mitosis, activation of cyclin A-dependent protein kinases, is also an important event during apoptosis.

Characterization of apoptosis in cultured rat sympathetic neurons after nerve growth factor withdrawal

Sympathetic neurons depend on nerve growth factor (NGF) for their survival both in vivo and in vitro. In culture, the neurons die after NGF withdrawal by an autonomous cell death program but whether these neurons die by apoptosis is under debate. Using vital DNA stains and in situ nick translation, we show here that extensive chromatin condensation and DNA fragmentation occur before plasma membrane breakdown during the death of NGF-deprived rat sympathetic neurons in culture. Furthermore, kinetic analysis of chromatin condensation events within the cell population is consistent with a model which postulates that after NGF deprivation nearly all of the neurons die in this manner. Although the dying neurons display membrane blebbing, cell fragmentation into apoptotic bodies does not occur. Apoptotic events proceed rapidly at around the time neurons become committed to die, regardless of neuronal culture age. However the duration of NGF deprivation required to commit neurons to die, and the rate at which apoptosis occurs, increase with culture age. Thus, within the first week of culture, apoptosis is the predominant form of cell death in sympathetic neurons.

Separate metabolic pathways leading to DNA fragmentation and apoptotic chromatin condensation

Apoptosis is the predominant form of cell death observed in a variety of physiological and pathological conditions such as cancer involution, insect metamorphosis, the development of the immune and nervous systems, and embryogenesis. The typical nuclear changes taking place in apoptotic cells include extensive condensation of chromatin and internucleosomal DNA fragmentation into units of 200 base pairs. However, the mechanisms responsible for both chromatin condensation and DNA fragmentation have yet to be elucidated. In this study, micrococcal nuclease and the divalent cations, Ca2+ and Mg2+, were applied to isolated nuclei in an attempt to reconstitute in vitro the digestion of genomic DNA associated with apoptosis. Micrococcal nuclease was found to induce a typical pattern of DNA fragmentation, but did not give rise to chromatin condensation, whereas Ca2+/Mg2+ induced both chromatin condensation and DNA fragmentation in isolated mouse liver nuclei. When the endonuclease inhibitor ZnCl2 was used, the DNA fragmentation induced by Ca2+/Mg2+ in nuclei could be completely inhibited, but chromatin condensation still occurred. For comparison, intact liver cells were treated with valinomycin, a potassium ionophore, which gave rise to an atypical cell death, with chromatin condensation appearing without DNA fragmentation. Our results suggest that endonuclease activation in apoptosis is neither necessary nor sufficient to induce chromatin condensation, and that DNA fragmentation and chromatin condensation may be triggered through separate pathways during apoptosis.

Target cell death triggered by cytotoxic T lymphocytes: a target cell mutant distinguishes passive pore formation and active cell suicide mechanisms

The role of the target cell in its own death mediated by cytotoxic T lymphocytes (CTL) has been controversial. The ability of the pore-forming granule components of CTL to induce target cell death directly has been taken to suggest an essentially passive role for the target. This view of CTL-mediated killing ascribes to the target the single role of providing an antigenic stimulus to the CTL; this signal results in the vectoral degranulation and secretion of pore-forming elements onto the target. On the other hand, by a number of criteria, target cell death triggered by CTL appears fundamentally different from death resulting from membrane damage and osmotic lysis. CTL-triggered target cell death involves primary internal lesions of the target cell that reflect a physiological cell death process. Orderly nuclear disintegration, including lamin phosphorylation and solubilization, chromatin condensation, and genome digestion, are among the earliest events, preceding the loss of plasma membrane integrity. We have tested directly the involvement of the target cell in its own death by examining whether we could isolate mutants of target cells that have retained the ability to be recognized by and provide an antigenic stimulus to CTL while having lost the capacity to respond by dying. Here, we describe one such mutant, BW87. We have used this CTL-resistant mutant to analyze the mechanisms of CTL-triggered target cell death under a variety of conditions. The identification of a mutable target cell element essential for the cell death response to CTL provides genetic evidence that target cell death reflects an active cell suicide process similar to other physiological cell deaths.

Target cell death triggered by cytotoxic T lymphocytes: a target cell mutant distinguishes passive pore formation and active cell suicide mechanisms

The role of the target cell in its own death mediated by cytotoxic T lymphocytes (CTL) has been controversial. The ability of the pore-forming granule components of CTL to induce target cell death directly has been taken to suggest an essentially passive role for the target. This view of CTL-mediated killing ascribes to the target the single role of providing an antigenic stimulus to the CTL; this signal results in the vectoral degranulation and secretion of pore-forming elements onto the target. On the other hand, by a number of criteria, target cell death triggered by CTL appears fundamentally different from death resulting from membrane damage and osmotic lysis. CTL-triggered target cell death involves primary internal lesions of the target cell that reflect a physiological cell death process. Orderly nuclear disintegration, including lamin phosphorylation and solubilization, chromatin condensation, and genome digestion, are among the earliest events, preceding the loss of plasma membrane integrity. We have tested directly the involvement of the target cell in its own death by examining whether we could isolate mutants of target cells that have retained the ability to be recognized by and provide an antigenic stimulus to CTL while having lost the capacity to respond by dying. Here, we describe one such mutant, BW87. We have used this CTL-resistant mutant to analyze the mechanisms of CTL-triggered target cell death under a variety of conditions. The identification of a mutable target cell element essential for the cell death response to CTL provides genetic evidence that target cell death reflects an active cell suicide process similar to other physiological cell deaths.

Neither macromolecular synthesis nor myc is required for cell death via the mechanism that can be controlled by Bcl-2

Expression of c-myc and macromolecular synthesis have been associated with physiological cell death. We have studied their requirement for the death of factor (interleukin-3)-dependent cells (FDC-P1) bearing an inducible bcl-2 expression construct. FDC-P1 cells expressing bcl-2 turned off expression of c-myc when deprived of interleukin-3 but remained viable as long as bcl-2 was maintained. A subsequent decline in Bcl-2 allowed the cells to undergo apoptosis directly from G0, in the absence of detectable c-myc expression. Thus c-myc expression may lead to apoptosis in some cases but is not directly involved in the mechanism of physiological cell death that can be controlled by Bcl-2. The macromolecular synthesis inhibitors actinomycin D and cycloheximide triggered rapid cell death of FDC-P1 cells in the presence of interleukin-3, but the cells could be protected by Bcl-2. Thus, the cell death machinery can exist in a quiescent state and can be activated by mechanisms that do not require synthesis of RNA or protein.

Neither macromolecular synthesis nor myc is required for cell death via the mechanism that can be controlled by Bcl-2

Expression of c-myc and macromolecular synthesis have been associated with physiological cell death. We have studied their requirement for the death of factor (interleukin-3)-dependent cells (FDC-P1) bearing an inducible bcl-2 expression construct. FDC-P1 cells expressing bcl-2 turned off expression of c-myc when deprived of interleukin-3 but remained viable as long as bcl-2 was maintained. A subsequent decline in Bcl-2 allowed the cells to undergo apoptosis directly from G0, in the absence of detectable c-myc expression. Thus c-myc expression may lead to apoptosis in some cases but is not directly involved in the mechanism of physiological cell death that can be controlled by Bcl-2. The macromolecular synthesis inhibitors actinomycin D and cycloheximide triggered rapid cell death of FDC-P1 cells in the presence of interleukin-3, but the cells could be protected by Bcl-2. Thus, the cell death machinery can exist in a quiescent state and can be activated by mechanisms that do not require synthesis of RNA or protein.

Nuclear events of apoptosis in vitro in cell-free mitotic extracts: a model system for analysis of the active phase of apoptosis

We have developed a cell-free system that induces the morphological transformations characteristic of apoptosis in isolated nuclei. The system uses extracts prepared from mitotic chicken hepatoma cells following a sequential S phase/M phase synchronization. When nuclei are added to these extracts, the chromatin becomes highly condensed into spherical domains that ultimately extrude through the nuclear envelope, forming apoptotic bodies. The process is highly synchronous, and the structural changes are completed within 60 min. Coincident with these morphological changes, the nuclear DNA is cleaved into a nucleosomal ladder. Both processes are inhibited by Zn2+, an inhibitor of apoptosis in intact cells. Nuclear lamina disassembly accompanies these structural changes in added nuclei, and we show that lamina disassembly is a characteristic feature of apoptosis in intact cells of mouse, human and chicken. This system may provide a powerful means of dissecting the biochemical mechanisms underlying the final stages of apoptosis.

DNA fragmentation during apoptosis is caused by frequent single-strand cuts

One of the hallmarks of apoptosis is the digestion of genomic DNA by an endonuclease, generating a ladder of small fragments of double-stranded DNA. We have examined the nature of the DNA breaks produced in mouse thymocytes triggered to undergo apoptosis by steroids or by stimulation of the T cell receptor. Whereas the typical ladder pattern of oligonucleosomal fragments was observed after agarose gel electrophoresis, numerous single-strand cuts were detected after electrophoresis under denaturing conditions. Single-strand nicks were found to be very frequent in the internucleosomal regions, but also to occur in the core particle-associated DNA. An identical pattern of single-strand nicks was obtained when chromatin DNA was exposed to the single-strand cleaving deoxyribonuclease I. The nicked DNA fragments, extracted from apoptotic thymocytes, were sensitive to the action of S1-nuclease. We propose that DNA fragmentation induced during apoptosis is not due to a double-strand cutting enzyme as previously postulated, but rather is the result of single-strand breaks. This ensures the dissociation of the DNA molecule at sites where cuts are found within close proximity.

Apoptotic death in epithelial cells: cleavage of DNA to 300 and/or 50 kb fragments prior to or in the absence of internucleosomal fragmentation

To date, apoptosis has been characterized biochemically by the production of 180-200 bp internucleosomal DNA fragments resulting from the activation of an endonuclease(s). The principal morphological feature of apoptosis is the condensation of chromatin and it has been assumed that this may reflect the oligonucleosomal fragmentation pattern. We have re-examined this dogma by comparing the biochemical and morphological features of cell death in several epithelial cell types (HT-29-I1 colon adenocarcinoma, CC164 mink lung, DU-145 human prostatic carcinoma and MCF-7 human breast adenocarcinoma) and one mesenchymal cell line (H11ras-R3 ras-transformed rat fibroblasts). Cell death was induced either by serum deprivation, TGF-beta 1 or etoposide, or by leaving cells to reach confluence. Cell death was assessed with respect to detachment from monolayers, morphological changes and DNA integrity. The DNA-binding fluorophore Hoechst 33258 revealed chromatin condensation patterns consistent with apoptotic cell death in all cell types except MCF-7 cells. Using field inversion gel electrophoresis in conjunction with conventional 2% agarose gel electrophoresis, cleavage of DNA to 50 kbp fragments was observed in all cases except MCF-7 cells. This preceded the appearance of oligonucleosomal fragments in HT-29-I1, CC164 and H11ras-R3 cells. Although the DNA of DU-145 cells fragmented into 50 kbp units, and although the cells exhibited classical apoptotic morphology, no subsequent internucleosomal cleavage was observed. These results suggest that changes in the integrity of DNA indicative of the release of chromatin loop domains occur before cleavage at internucleosomal sites is initiated and that the latter is not an essential step in the apoptotic process.