The caspase-3 precursor has a cytosolic and mitochondrial distribution: implications for apoptotic signaling

Macrophages require constitutive NF-kappaB activation to maintain A1 expression and mitochondrial homeostasis

NF-kappaB is a critical mediator of macrophage inflammatory responses, but its role in regulating macrophage survival has yet to be elucidated. Here, we demonstrate that constitutive NF-kappaB activation is essential for macrophage survival. Blocking the constitutive activation of NF-kappaB with pyrrolidine dithiocarbamate or expression of IkappaBalpha induced apoptosis in macrophagelike RAW 264.7 cells and primary human macrophages. This apoptosis was independent of additional death-inducing stimuli, including Fas ligation. Suppression of NF-kappaB activation induced a time-dependent loss of mitochondrial transmembrane potential (DeltaPsi(m)) and DNA fragmentation. Examination of initiator caspases revealed the cleavage of caspase 9 but not caspase 8 or the effector caspase 3. Addition of a general caspase inhibitor, z-VAD. fmk, or a specific caspase 9 inhibitor reduced DNA fragmentation but had no effect on DeltaPsi(m) collapse, indicating this event was caspase independent. To determine the pathway leading to mitochondrial dysfunction, analysis of Bcl-2 family members established that only A1 mRNA levels were reduced prior to DeltaPsi(m) loss and that ectopic expression of A1 protected against cell death following inactivation of NF-kappaB. These data suggest that inhibition of NF-kappaB in macrophages initiates caspase 3-independent apoptosis through reduced A1 expression and mitochondrial dysfunction. Thus, constitutive NF-kappaB activation preserves macrophage viability by maintaining A1 expression and mitochondrial homeostasis.

Kinases: positive and negative regulators of apoptosis

Cells sense and respond to extracellular factors via receptors on the cell surface that trigger intracellular signaling pathways. The signals received by the receptors on hematopoietic cells often determine if the cell proliferates, survives or undergoes apoptosis. Apoptosis can be induced by almost any cytotoxic stimuli. These stimuli may be an absence of signals arising from cellular receptors, stimulation of specific ligand receptors on the cell surface, chemotherapeutic agents, and ionizing radiation or oxygen radicals, as well as a number of other factors. Cellular kinases and phosphatases participate in signaling cascades that influence this process. We review the ability of the calmodulin-dependent-kinases, I-kappaB kinases, PI3-kinases, Jakkinases, PKC, PKA, and MAP kinase signaling pathways (Erk, Jnk, and p38), to influence the apoptotic process. In addition, we discuss the cross-talk that exists between signaling cascades that are pro-apoptotic and anti-apoptotic.

Caspases disrupt the nuclear-cytoplasmic barrier

During apoptosis, caspases, a family of proteases, disassemble a cell by cleaving a set of proteins. Caspase-3 plays a major role in the dissassembly of the nucleus by processing several nuclear substrates. The question is how caspase-3 which is usually cytoplasmic, gains access to its nuclear targets. It was suggested that caspase-3 is actively transported to the nucleus through the nuclear pores. We found that caspase-9, which is activated earlier than caspase-3, directly or indirectly inactivates nuclear transport and increases the diffusion limit of the nuclear pores. This increase allows caspase-3 and other molecules that could not pass through the nuclear pores in living cells to enter or leave the nucleus during apoptosis by diffusion. Hence, caspase-9 contributes to cell disassembly by disrupting the nuclear cytoplasmic barrier.

Mechanisms of apoptosis

Programmed cell death plays critical roles in a wide variety of physiological processes during fetal development and in adult tissues. In most cases, physiological cell death occurs by apoptosis as opposed to necrosis. Defects in apoptotic cell death regulation contribute to many diseases, including disorders where cell accumulation occurs (cancer, restenosis) or where cell loss ensues (stroke, heart failure, neurodegeneration, AIDS). In recent years, the molecular machinery responsible for apoptosis has been elucidated, revealing a family of intracellular proteases, the caspases, which are responsible directly or indirectly for the morphological and biochemical changes that characterize the phenomenon of apoptosis. Diverse regulators of the caspases have also been discovered, including activators and inhibitors of these cell death proteases. Inputs from signal transduction pathways into the core of the cell death machinery have also been identified, demonstrating ways of linking environmental stimuli to cell death responses or cell survival maintenance. Knowledge of the molecular mechanisms of apoptosis is providing insights into the causes of multiple pathologies where aberrant cell death regulation occurs and is beginning to provide new approaches to the treatment of human diseases.

Mitochondrial membrane potential differentiates cells resistant to apoptosis in hybridoma cultures

Previous research has implicated mitochondrial physiology and, by extension, respiratory capacity in the initiation and progress of apoptosis of cells in culture and tissue environments. This hypothesis was tested by separating a hybridoma cell population into subpopulations of varying mitochondrial membrane potential (MMP) using Rhodamine 123 stain and fluorescence-activated cell sorter analysis and subjecting them to two apoptosis inducers, rotenone and staurosporin. Apoptotic death was characterized morphologically through the determination of apoptosis-related chromatin condensation and biochemically through the measurement of caspase-3 enzymatic activity. We found dramatic differences in the apoptotic death kinetics for the subpopulations, with the high MMP cells showing higher resistance to apoptotic death. After incubation with 30 microM rotenone, the low MMP cells exhibited one-third of the viability of the high MMP cells and a three-fold increase in the capsase-3 enzymatic activity. No changes were observed in the DNA content or the cell cycle distributions of the two cell subpopulations, which maintained their mean MMP difference after 20 generations. These results suggest that heterogeneity exists in mammalian cell populations with respect to mitochondrial physiology, which correlates with resistance to apoptotic death.

Increased mitochondrial cytochrome c levels and mitochondrial hyperpolarization precede camptothecin-induced apoptosis in Jurkat cells

Mitochondria play a central role in apoptosis through release of cytochrome c and activation of caspases. In the present study, we showed that, in Jurkat human T cells, camptothecin-induced apoptosis is preceded by (i) an increase in cytochrome c and subunit IV of cytochrome c oxidase (COX IV) levels in mitochondria; and (ii) an elevation of the mitochondrial membrane potential (Delta(Psi)m). These events are followed by cytochrome c release into the cytosol, cytochrome c and COX IV depletion from mitochondria, externalization of phosphatidylserine (PS), disruption of Delta(Psi)m, caspase activation, poly(ADP-ribose)polymerase cleavage and DNA fragmentation. The pan-caspase inhibitor z-VAD.fmk blocked camptothecin-induced PS externalization, disruption of Delta(Psi)m and DNA fragmentation, suggesting that these events are mediated by caspase activation. In contrast, z-VAD did not prevent cytochrome c release, despite preventing cytochrome c and COX IV depletion from mitochondria. Together, these data suggest that mitochondrial cytochrome c and COX IV enrichment are early events preceding the onset of apoptosis and that cytochrome c release is upstream of caspase activation and loss of Delta(Psi)m. Furthermore, prevention by z-VAD of cytochrome c and COX IV depletion in mitochondria suggests the possibility that a caspase-like activity in mitochondria is involved in the proteolytic depletion of respiratory chain proteins. Activation of this activity may play an important role in drug-induced apoptosis.

Regulation of tumor necrosis factor cytotoxicity by calcineurin

Cyclosporin (CsA) inhibits mitochondrial death signaling and opposes tumor necrosis factor (TNF)-induced apoptosis in vitro. However, CsA is also a potent inhibitor of calcineurin, a phosphatase that may participate in cell death. Therefore, we tested the hypothesis that calcineurin regulates TNF cytotoxicity in rat hepatoma cells (FTO2B). TNF-treated FTO2B cells appeared apoptotic by DNA fragmentation, nuclear condensation, annexin V binding, and caspase activation. We studied two calcineurin inhibitors, CsA and FK506, and found that each potently inhibited TNF cytotoxicity. Western blot demonstrated calcineurin in FTO2B homogenates. In a model of mitochondrial permeability transition (MPT), we found that CsA prevented MPT and cytochrome c release, while FK506 inhibited neither. In summary, we present evidence that calcineurin participates in an apoptotic death pathway activated by TNF. CsA may oppose programmed cell death by inhibiting calcineurin activity and/or inhibiting mitochondrial signaling.

Apoptosis control in syncytia induced by the HIV type 1-envelope glycoprotein complex: role of mitochondria and caspases

Syncytia arising from the fusion of cells expressing a lymphotropic HIV type 1-encoded envelope glycoprotein complex (Env) with cells expressing the CD4/CXC chemokine receptor 4 complex spontaneously undergo cell death. Here we show that this process is accompanied by caspase activation and signs of mitochondrial membrane permeabilization (MMP), including the release of intermembrane proteins such as cytochrome c (Cyt-c) and apoptosis-inducing factor (AIF) from mitochondria. In Env-induced syncytia, caspase inhibition did not suppress AIF- and Cyt-c translocation, yet it prevented all signs of nuclear apoptosis. Translocation of Bax to mitochondria led to MMP, which was inhibited by microinjected Bcl-2 protein or bcl-2 transfection. Bcl-2 also prevented the subsequent nuclear chromatin condensation and DNA fragmentation. The release of AIF occurred before that of Cyt-c and before caspase activation. Microinjection of AIF into syncytia sufficed to trigger rapid, caspase-independent Cyt-c release. Neutralization of endogenous AIF by injection of an antibody prevented all signs of spontaneous apoptosis occurring in syncytia, including the Cyt-c release and nuclear apoptosis. In contrast, Cyt-c neutralization only prevented nuclear apoptosis, and did not affect AIF release. Our results establish that the following molecular sequence governs apoptosis of Env-induced syncytia: Bax-mediated/Bcl-2-inhibited MMP --> AIF release --> Cyt-c release --> caspase activation --> nuclear apoptosis.

Targeting of the pro-apoptotic VDAC-like porin (PorB) of Neisseria gonorrhoeae to mitochondria of infected cells

Infection of cell cultures with Neisseria gonorrhoeae results in apoptosis that is mediated by the PorB porin. During the infection process porin translocates from the outer bacterial membrane into host cell membranes where its channel activity is regulated by nucleotide binding and voltage-dependent gating, features that are shared by the mitochondrial voltage-dependent anion channel (VDAC). Here we show that porin is selectively and efficiently transported to mitochondria of infected cells. Prevention of porin translocation also blocked the induction of apoptosis. Mitochondria of cells treated with porin both in vitro and in vivo were depleted of cytochrome c and underwent permeability transition. Overexpression of Bcl-2 blocked porin-induced apoptosis. The release of cytochrome c occurred independently of active caspases but was completely prevented by Bcl-2. Our data suggest that the Neisseria porin can, like its eukaryotic homologue, function at the mitochondrial checkpoint to mediate apoptosis.

Ganglioside GD3 and its mimetics induce cytochrome c release from mitochondria

Ganglioside GD3 induced the release of cytochrome c from isolated rat liver mitochondria. This process was completely prevented by cyclosporin A and partially prevented by a cysteine protease inhibitor, n-acetyl-leu-leu-norleucinal. Cyclosporin A is a potent inhibitor of the permeability transition pore, whereas n-acetyl-leu-leu-norleucinal has no effect on this pore. These results indicate that the release of cytochrome c from mitochondria requires both the opening of the permeability transition pore and a cysteine protease inhibitor-sensitive mechanism. Gangliosides GD1a, GD1b, GT1b, and GQ1b along with the synthetic GD3 mimetics TMS-42 and CI-22, which are glycerophospholipids carrying a disialo residue, also induced cytochrome c release. In contrast, gangliosides GM1, GM2, and GM3 did not induce cytochrome c release. These results indicate that two sialo residues must play an important role in the induction of cytochrome c release by gangliosides.

Caspase-3 is activated following axotomy of neonatal facial motoneurons and caspase-3 gene deletion delays axotomy-induced cell death in rodents

In this report, we examined the possible functions of the cell death protease, caspase-3, in the axotomy-induced apoptosis of facial motoneurons in newborn rodents. Using in situ hybridization and Western blot, we found higher levels of caspase-3 mRNA and pro-caspase-3 protein expression in motoneurons of neonatal and 2-week-old rats than adult rats. Following facial motoneuron axotomy, caspase-3 mRNA and protein expression increased in motoneurons of both neonatal and adult rats. However, using an antibody directed to the activated form of the caspase-3 protease, we found that catalytically active caspase-3 was present only in axotomized neonatal motoneurons. As motoneurons in neonatal but not adult rodents are susceptible to axotomy-induced apoptosis, we hypothesized that caspase-3 may play a role in their demise. To determine the necessity of caspase-3 activation in axotomy-induced apoptosis, we counted the number of surviving motoneurons at 4 and 7 days following axotomy in wild type mice and caspase-3 gene-deleted mice. There were nearly three times more surviving motoneurons in caspase-3 gene-deleted mice than in wild type mice at both 4 days (mean 1074 vs. 464, P<0.005) and 7 days (mean 469 vs. 190, P<0.005) following injury, indicating a slower rate of death. Examination of the dying motoneurons using TUNEL staining (for fragmented DNA) and bisbenzimide staining (for nuclear morphology) revealed incomplete nuclear condensation in caspase-3-deficient motoneurons. These results demonstrate that caspase-3 activation plays important roles in the rapid demise of axotomized neonatal motoneurons.

Mitochondria: execution central

Mitochondria play an essential function in eukaryotic life and death. They also play a central role in apoptosis regulation, reflected by the convergence of Bcl-2 family members on the mitochondrial outer membrane, and the presence of 'death factors' in the intermembrane space. Mitochondrial structure and function must be taken into consideration when evaluating mechanisms for cytochrome c release. The core machinery for caspase activation is conserved from Caenorhabditis elegans to man, and we consider parallels in the role of mitochondria in this process.

Three classes of ubiquinone analogs regulate the mitochondrial permeability transition pore through a common site

To identify the structural features required for regulation of the mitochondrial permeability transition pore (PTP) by ubiquinone analogs (Fontaine, E., Ichas, F., and Bernardi, P. (1998) J. Biol. Chem. 40, 25734-25740), we have carried out an analysis with quinone structural variants. We show that three functional classes can be defined: (i) PTP inhibitors (ubiquinone 0, decylubiquinone, ubiquinone 10, 2,3-dimethyl-6-decyl-1,4-benzoquinone, and 2,3,5-trimethyl-6-geranyl-1,4-benzoquinone); (ii) PTP inducers (2,3-dimethoxy-5-methyl-6-(10-hydroxydecyl)-1,4-benzoquinone and 2,5-dihydroxy-6-undecyl-1,4-benzoquinone); and (iii) PTP-inactive quinones that counteract the effects of both inhibitors and inducers (ubiquinone 5 and 2,3,5-trimethyl-6-(3-hydroxyisoamyl)-1,4-benzoquinone) . The structure-function correlation indicates that minor modifications in the isoprenoid side chain can turn an inhibitor into an activator, and that the methoxy groups are not essential for the effects of quinones on the PTP. Since the ubiquinone analogs used in this study have a similar midpoint potential and decrease mitochondrial production of reactive oxygen species to the same extent, these results support the hypothesis that quinones modulate the PTP through a common binding site rather than through oxidation-reduction reactions. Occupancy of this site can modulate the PTP open-closed transitions, possibly through secondary changes of the PTP Ca(2+) binding affinity.

Genistein induces apoptosis of RPE-J cells by opening mitochondrial PTP

Although previous studies demonstrated that genistein-induced apoptosis of various cell types including RPE-J cells, the involvement of mitochondrial events in such types of apoptosis has not been demonstrated to date. In this investigation of genistein-induced apoptosis of RPE-J cells, genistein induced the reduction of the mitochondrial membrane potential and the release of cytochrome c to cytosol. A mitochondrial permeability transition pore (PTP) blocker bongkrekic acid prevented the reduction of the mitochondrial membrane potential and cytochrome c release, and consequently abolished caspase-3 activation, nuclear condensation, and DNA fragmentation. On the other hand, zVAD-fmk did not inhibit the mitochondrial event such as the reduction of the mitochondrial membrane potential and cytochrome c release although it prevented caspase-3 activation, nuclear condensation, and DNA fragmentation. Taken together, genistein induces apoptosis of RPE-J cells by opening the mitochondrial PTP, and the mitochondrial event in this type of apoptosis is caused independently of caspase.

Regulation of the Fas death pathway by FLICE-inhibitory protein in primary human B cells

The Fas/Fas ligand (L) system plays an important role in the maintenance of peripheral B cell tolerance and the prevention of misguided T cell help. CD40-derived signals are required to induce Fas expression on virgin B cells and to promote their susceptibility to Fas-mediated apoptosis. In the current study, we have analyzed the early biochemical events occurring upon Fas ligation in CD40L-activated primary human tonsillar B cells with respect to Fas-associated death domain protein (FADD), caspase-8/FADD-like IL-1beta-converting enzyme (FLICE), and c-FLICE inhibitory protein (FLIP). We report here that Fas-induced apoptosis in B cells does not require integrity of the mitochondrial Apaf-1 pathway and that caspase-8 is activated by association with the death-inducing signaling complex (DISC), i.e., upstream of the mitochondria. We show that both FADD and the zymogen form of caspase-8 are constitutively expressed at high levels in virgin B cells, whereas c-FLIP expression is marginal. In contrast, c-FLIP, but neither FADD nor procaspase-8, is strongly up-regulated upon ligation of CD40 or the B cell receptor on virgin B cells. Finally, we have found that c-FLIP is also recruited and cleaved at the level of the DISC in CD40L-activated virgin B cells. We propose that c-FLIP expression delays the onset of apoptosis in Fas-sensitive B cells. The transient protection afforded by c-FLIP could offer an ultimate safeguard mechanism against inappropriate cell death or allow recruitment of phagocytes to ensure efficient removal of apoptotic cells.

Role of mitochondria in neuronal apoptosis

Apoptosis is a controlled form of cell death that participates in the demise of neuronal cells during development, neurodegenerative disorders and exposure to neurotoxic agents. In recent years, the mitochondria have emerged as being pivotal in controlling apoptosis. They house a number of apoptogenic molecules that are released into the cytoplasm at the onset of apoptosis. These include cytochrome c, apoptosis-inducing factor and various caspases. Mitochondria also play an important role in intracellular Ca(2+) regulation, which is crucial to excitotoxic neurodegeneration. Alterations in energy (ATP) production by mitochondria (due to hypoxia or mutations in genes encoding mitochondrial proteins of the electron transport chain) can induce apoptosis in neurons or increase their sensitivity to apoptosis.

Nuclear apoptosis induced by isolated mitochondria

We isolated and purified mitochondria from mouse livers and spinach leaves. When added into egg extracts of Xenopus laevis, they caused nuclei of mouse liver to undergo apoptotic changes. Chromatin condensation, margination and DNA ladder were observed. After incubating isolated mitochondria in some hypotonic solutions, and centrifuging these mixtures at high speed, we got mitochondrial supernatants. It was found that in the absence of cytosolic factor, the supernatant alone was able to induce apoptotic changes in nuclei. The effective components were partly of protein. DNA fragmentation was partly inhibited by caspase inhibitors AC-DEVD-CHO and AC-YVAD-CHO. Meanwhile, caspase inhibitors fully blocked chromatin condensation. Primary characterization of the nuclear endonuclease(s) induced by mitochondrial supernatants was also conducted. It was found that this endonuclease is different from endonuclease G, cytochrome c-induced nuclease, or Ca2+-activated endonuclease.

Caspase-independent commitment phase to apoptosis in activated blood T lymphocytes: reversibility at low apoptotic insult

Little is known about the mechanisms of programmed death triggered in T lymphocytes by stimuli that can bypass caspase activation. Anti-CD2 monoclonal antibody and staurosporine are such apoptosis inducers because they operate in the presence of broad-spectrum caspase inhibitors BOC-D.fmk and Z-VAD.fmk. A system was devised, based on the isolation according to density of activated blood T cells progressively engaged in the apoptotic process. This allowed definition of a sequence of caspase-dependent and caspase-independent apoptogenic events that are triggered by anti-CD2 and staurosporine. Thus, a commitment phase to apoptosis was defined that is entirely caspase independent and that is characterized by cell volume loss, partial chromatin condensation, and release into the cytosol and the nucleus of mitochondrial “apoptosis-inducing factor ” (AIF). Committed cells were viable, displayed a high mitochondrial inner transmembrane potential (▵Ψm), and lacked large-scale and oligonucleosomal DNA fragmentation. Mitochondrial release of AIF was selective because cytochrome c was retained in mitochondria of the very same cells. Mitochondrial release of cytochrome c occurred later, at the onset of the execution phase of apoptosis, concurrently with ▵Ψm collapse, poly (ADP-ribose) polymerase cleavage, and DNA fragmentation. The apoptogenic events of this commitment phase are reversible if the strength of the stimulus is low and of short duration.

Induction of necrotic-like cell death by tumor necrosis factor alpha and caspase inhibitors: novel mechanism for killing virus-infected cells

Induction of apoptotic cell death generally requires the participation of cysteine proteases belonging to the caspase family. However, and similar to most cell types, mouse fibroblasts are normally resistant to tumor necrosis factor alpha (TNF-alpha)-induced apoptosis. Surprisingly, TNF-alpha treatment of vaccinia virus-infected mouse fibroblasts resulted in necrotic-like cell death, which was significantly reduced in cells infected with a vaccinia virus mutant lacking the caspase inhibitor B13R. Furthermore, TNF-alpha also induced necrotic-like cell death of fibroblasts in the presence of peptidyl caspase inhibitors. In both cases, necrosis was accompanied by generation of superoxide species. Caspase inhibitors also sensitized fibroblasts to killing by double-stranded RNA and gamma interferon. In all cases, cell death was efficiently blocked by antioxidants or mitochondrial respiratory chain inhibitors. These results define a new mitochondrion-dependent mechanism which may be important in the killing of cells infected with viruses encoding caspase inhibitors.

The chicken anemia virus-derived protein apoptin requires activation of caspases for induction of apoptosis in human tumor cells

The chicken anemia virus protein Apoptin has been shown to induce apoptosis in a large number of transformed and tumor cell lines, but not in primary cells. Whereas many other apoptotic stimuli (e.g., many chemotherapeutic agents and radiation) require functional p53 and are inhibited by Bcl-2, Apoptin acts independently of p53, and its activity is enhanced by Bcl-2. Here we study the involvement of caspases, an important component of the apoptotic machinery present in mammalian cells. Using a specific antibody, active caspase-3 was detected in cells expressing Apoptin and undergoing apoptosis. Although Apoptin activity was not affected by CrmA, p35 did inhibit Apoptin-induced apoptosis, as determined by nuclear morphology. Cells expressing both Apoptin and p35 showed only a slight change in nuclear morphology. However, in most of these cells, cytochrome c is still released and the mitochondria are not stained by CMX-Ros, indicating a drop in mitochondrial membrane potential. These results imply that although the final apoptotic events are blocked by p35, parts of the upstream apoptotic pathway that affect mitochondria are already activated by Apoptin. Taken together, these data show that the viral protein Apoptin employs cellular apoptotic factors for induction of apoptosis. Although activation of upstream caspases is not required, activation of caspase-3 and possibly also other downstream caspases is essential for rapid Apoptin-induced apoptosis.

The most unkindest cut of all: on the multiple roles of mammalian caspases

The caspases, first discovered almost a decade ago, are intracellular cysteine proteases which have been shown to play an essential role in the initiation and execution phases of apoptotic cell death. Numerous strategies for the activation and inhibition of these 'killer' proteases have evolved, including the regulation of caspase expression and function at the transcriptional and post-translational level, as well as the expression of viral and cellular inhibitors of caspases. Emerging evidence in recent years has also implicated the caspases in various, nonapoptotic aspects of cellular physiology, such as cytokine processing during inflammation, differentiation of progenitor cells during erythropoiesis and lens fiber development, and proliferation of T lymphocytes, thus attesting to the pleiotropic functions of these proteases. The present review aims to discuss the multiple roles of the mammalian caspases with particular emphasis on their activation and regulation in cells of leukemic origin and the attendant possibilities of therapeutic intervention.

Caspase-independent commitment phase to apoptosis in activated blood T lymphocytes: reversibility at low apoptotic insult

Little is known about the mechanisms of programmed death triggered in T lymphocytes by stimuli that can bypass caspase activation. Anti-CD2 monoclonal antibody and staurosporine are such apoptosis inducers because they operate in the presence of broad-spectrum caspase inhibitors BOC-D.fmk and Z-VAD.fmk. A system was devised, based on the isolation according to density of activated blood T cells progressively engaged in the apoptotic process. This allowed definition of a sequence of caspase-dependent and caspase-independent apoptogenic events that are triggered by anti-CD2 and staurosporine. Thus, a commitment phase to apoptosis was defined that is entirely caspase independent and that is characterized by cell volume loss, partial chromatin condensation, and release into the cytosol and the nucleus of mitochondrial “apoptosis-inducing factor ” (AIF). Committed cells were viable, displayed a high mitochondrial inner transmembrane potential (▵Ψm), and lacked large-scale and oligonucleosomal DNA fragmentation. Mitochondrial release of AIF was selective because cytochrome c was retained in mitochondria of the very same cells. Mitochondrial release of cytochrome c occurred later, at the onset of the execution phase of apoptosis, concurrently with ▵Ψm collapse, poly (ADP-ribose) polymerase cleavage, and DNA fragmentation. The apoptogenic events of this commitment phase are reversible if the strength of the stimulus is low and of short duration.

Identification of the cytolinker plectin as a major early in vivo substrate for caspase 8 during CD95- and tumor necrosis factor receptor-mediated apoptosis

Caspase 8 plays an essential role in the execution of death receptor-mediated apoptosis. To determine the localization of endogenous caspase 8, we used a panel of subunit-specific anti-caspase 8 monoclonal antibodies in confocal immunofluorescence microscopy. In the human breast carcinoma cell line MCF7, caspase 8 predominantly colocalized with and bound to mitochondria. After induction of apoptosis through CD95 or tumor necrosis factor receptor I, active caspase 8 translocated to plectin, a major cross-linking protein of the three main cytoplasmic filament systems, whereas the caspase 8 prodomain remained bound to mitochondria. Plectin was quantitatively cleaved by caspase 8 at Asp 2395 in the center of the molecule in all cells tested. Cleavage of plectin clearly preceded that of other caspase substrates such as poly(ADP-ribose) polymerase, gelsolin, cytokeratins, or lamin B. In primary fibroblasts from plectin-deficient mice, apoptosis-induced reorganization of the actin cytoskeleton, as seen in wild-type cells, was severely impaired, suggesting that during apoptosis, plectin is required for the reorganization of the microfilament system.

Separation of cytochrome c-dependent caspase activation from thiol-disulfide redox change in cells lacking mitochondrial DNA

Release of mitochondrial cytochrome c (cyt c) is an early and common event during apoptosis. Previous studies showed that the loss of cyt c triggered superoxide production by mitochondria and contributed to the oxidation of cellular thiol-disulfide redox state. In this study, we tested whether loss of the functional electron transport chain due to depleting mitochondrial DNA (mtDNA) would affect this redox-signaling mechanism during apoptosis. Results showed that cyt c release and caspase activation in response to staurosporine treatment were preserved in cells lacking mitochondrial DNA (rho0 cells). However, unlike the case with rho+ cells, in which a dramatic oxidation of intracellular glutathione (GSH) occurred after mitochondrial cyt c release, the thiol-disulfide redox state in apoptotic rho0 cells remained largely unchanged. Thus, mitochondrial signaling of caspase activation can be separated from the bioenergetic function, and mitochondrial respiratory chain is the principal source of ROS generation in staurosporine-induced apoptosis.

Doxorubicin treatment activates a Z-VAD-sensitive caspase, which causes deltapsim loss, caspase-9 activity, and apoptosis in Jurkat cells

Doxorubicin induces caspase-3 activation and apoptosis in Jurkat cells but inhibition of this enzyme did not prevent cell death, suggesting that another caspase(s) is critically implicated. Western blot analysis of cell extracts indicated that caspases 2, 3, 4, 6, 7, 8, 9, and 10 were activated by doxorubicin. Cotreatment of cells with the caspase inhibitors Ac-DEVD-CHO, Z-VDVAD-fmk, Z-IETD-fmk, and Z-LEHD-fmk alone or in combination, or overexpression of CrmA, prevented many morphological features of apoptosis but not loss of mitochondrial membrane potential (delta(psi)m), phospatidilserine exposure, and cell death. Western blot analysis of cells treated with doxorubicin in the presence of inhibitors allowed elucidation of the sequential order of caspase activation. Z-IETD-fmk or Z-LEHD-fmk, which inhibit caspase-9 activity, blocked the activation of all caspases studied, lamin B degradation, and the development of apoptotic morphology, but not cell death. All morphological and biochemical features of apoptosis, as well as cell death, were prevented by cotreatment of cells with the general caspase inhibitor Z-VAD-fmk or by overexpression of Bcl-2. Doxorubicin cytotoxicity was also blocked by the protein synthesis inhibitor cycloheximide. Delayed addition of Z-VAD-fmk after doxorubicin treatment, but prior to the appearance of cells displaying a low delta(psi)m, prevented cell death. These results, taken together, suggest that the key mediator of doxorubicin-induced apoptosis in Jurkat cells may be an inducible, Z-VAD-sensitive caspase (caspase-X), which would cause delta(psi)m loss, release of apoptogenic factors from mitochondria, and cell death.

Heat shock proteins--modulators of apoptosis in tumour cells

Apoptosis is a genetically programmed, physiological method of cell destruction. A variety of genes are now recognised as positive or negative regulators of this process. Expression of inducible heat shock proteins (hsp) is known to correlate with increased resistance to apoptosis induced by a range of diverse cytotoxic agents and has been implicated in chemotherapeutic resistance of tumours and carcinogenesis. Intensive research on apoptosis over the past number of years has provided significant insights into the mechanisms and molecular events that occur during this process. The modulatory effects of hsps on apoptosis are well documented, however, the mechanisms of hsp-mediated protection against apoptosis remain to be fully defined, although several hypotheses have been proposed. Elucidation of these mechanisms should reveal novel targets for manipulating the sensitivity of leukaemic cells to therapy. This review aims to explain the currently understood process of apoptosis and the effects of hsps on this process. Several proposed mechanisms for hsp protection against apoptosis and the therapeutic implications of hsps in leukaemia are also discussed.

Protooncogenes as mediators of apoptosis

Apoptosis has been well established as a vital biological phenomenon that is important in the maintenance of cellular homeostasis. Three major protooncogene families and their encoded proteins function as mediators of apoptosis in various cell types and are the subject of this chapter. Protooncogenic proteins such as c-Myc/Max, c-Fos/c-Jun, and Bcl-2/Bax utilize a synergetic effect to enhance their roles in the pro- or antiapoptotic action. These family members activate and repress the expression of their target genes, control cell cycle progression, and execute programmed cell death. Repression or overproduction of these protooncogenic proteins induces apoptosis, which may vary as a result of either cell type specificity or the nature of the apoptotic stimuli. The proapoptotic and antiapoptotic proteins exert their effects in the membrane of cellular organelles. Here they generate cell-type-specific signals that activate the caspase family of proteases and their regulators for the execution of apoptosis.

Assessment of caspase activities in intact apoptotic thymocytes using cell-permeable fluorogenic caspase substrates

To detect caspase activities in intact apoptotic cells at the single cell level, cell-permeable fluorogenic caspase substrates were synthesized incorporating the optimal peptide recognition motifs for caspases 1, 3/7, 6, 8, and 9. Caspase activities were then assessed at various times after in vitro treatment of mouse thymocytes with dexamethasone or anti-Fas antibody. Dexamethasone induced the following order of appearance of caspase activities as judged by flow cytometry: LEHDase, WEHDase, VEIDase, IETDase, and DEVDase. Since the relative order of caspases 3 (DEVDase) and 6 (VEIDase) in the cascade has been controversial, this caspase activation order was reexamined using confocal microscopy. The VEIDase activity appeared before DEVDase in every apoptotic cell treated with dexamethasone. In contrast, anti-Fas stimulation altered this sequence: IETDase was the first measurable caspase activity and DEVDase preceded VEIDase. In an attempt to determine the intracellular target of the potent antiapoptotic agent carbobenzoxy-valyl-alanyl-aspartyl(beta-methyl ester)-fluoromethyl ketone (Z-VAD[OMe]-FMK), we examined its ability to inhibit previously activated intracellular caspases. However, no significant reductions of these activities were observed. These fluorogenic caspase substrates allow direct observation of the caspase cascade in intact apoptotic cells, showing that the order of downstream caspase activation is dependent on the apoptotic stimulus.

Apoptosis in the Retina: The Silent Death of Vision

Pathogenetic mechanisms of retinal degeneration include cell loss by apoptosis. This gene-regulated mode of single-cell death occurs in a number of widespread human diseases such as neurodegeneration. The knowledge of genes and signaling in retinal apoptosis is expanding and opens up therapeutic strategies to ameliorate blinding retinal diseases.

Granzyme B short-circuits the need for caspase 8 activity during granule-mediated cytotoxic T-lymphocyte killing by directly cleaving Bid

Cytotoxic T lymphocytes (CTL) can trigger an apoptotic signal through the Fas receptor or by the exocytosis of granzyme B and perforin. Caspase activation is an important component of both pathways. Granzyme B, a serine proteinase contained in granules, has been shown to proteolytically process and activate members of the caspase family in vitro. In order to gain an understanding of the contributions of caspases 8 and 3 during granule-induced apoptosis in intact cells, we have used target cells that either stably express the rabbitpox virus-encoded caspase inhibitor SPI-2 or are devoid of caspase 3. The overexpression of SPI-2 in target cells significantly inhibited DNA fragmentation, phosphatidylserine externalization, and mitochondrial disruption during Fas-mediated cell death. In contrast, SPI-2 expression in target cells provided no protection against granzyme-mediated apoptosis, mitochondrial collapse, or cytolysis, leading us to conclude that SPI-2-inhibited caspases are not an essential requirement for the granzyme pathway. Caspase 3-deficient MCF-7 cells were found to be resistant to CTL-mediated DNA fragmentation but not to CTL-mediated cytolysis and loss of the mitochondrial inner membrane potential. Furthermore, we demonstrate that granzyme B directly cleaves the proapoptotic molecule Bid, bypassing the need for caspase 8 activation of Bid. These results provide evidence for a two-pronged strategy for mediating target cell destruction and provide evidence of a direct link between granzyme B activity, Bid cleavage, and caspase 3 activation in whole cells.

Cell death-associated translocation of plasma membrane components induced by CTL

In the very early stages of target cell apoptosis induced by CTL, we found that fluorescence of labeling probes of the target plasma membrane, such as N-(3-triethylammoniumpropyl)-4-(p-dibutylaminostyryl)pyridin ium dibromide (FM1-43), was translocated into intracellular membrane structures including nuclear envelope and mitochondria. This translocation was associated with the execution of CTL-mediated killing, because neither the CTL-target conjugation alone nor the binding of noncytotoxic Th2 clone with target cell was sufficient to provoke the process. Although FM1-43 translocation was observed in perforin-mediated cytotoxicity, examinations with several other dyes failed to detect the evidence for membrane damages that may cause influx of the dye. Moreover, the translocation was also observed in Fas-dependent apoptosis. These data indicate that the translocation precedes the damage of plasma membrane and intracellular organella in the course of apoptotic cell death and may represent the existence of a membrane trafficking that mediates the translocation of plasma membrane components in the early onset of apoptotic cell death.

Caspase-2 is localized at the Golgi complex and cleaves golgin-160 during apoptosis

Caspases are an extended family of cysteine proteases that play critical roles in apoptosis. Animals deficient in caspases-2 or -3, which share very similar tetrapeptide cleavage specificities, exhibit very different phenotypes, suggesting that the unique features of individual caspases may account for distinct regulation and specialized functions. Recent studies demonstrate that unique apoptotic stimuli are transduced by distinct proteolytic pathways, with multiple components of the proteolytic machinery clustering at distinct subcellular sites. We demonstrate here that, in addition to its nuclear distribution, caspase-2 is localized to the Golgi complex, where it cleaves golgin-160 at a unique site not susceptible to cleavage by other caspases with very similar tetrapeptide specificities. Early cleavage at this site precedes cleavage at distal sites by other caspases. Prevention of cleavage at the unique caspase-2 site delays disintegration of the Golgi complex after delivery of a pro-apoptotic signal. We propose that the Golgi complex, like mitochondria, senses and integrates unique local conditions, and transduces pro-apoptotic signals through local caspases, which regulate local effectors.

Direct interaction of GD3 ganglioside with mitochondria generates reactive oxygen species followed by mitochondrial permeability transition, cytochrome c release, and caspase activation

Glycosphingolipids, including gangliosides, are emerging as signaling intermediates of extracellular stimuli. Because mitochondria play a key role in the orchestration of death signals, we assessed the interaction of GD3 ganglioside (GD3) with mitochondria and the subsequent cascade of events that culminate in cell death. In vitro studies with isolated mitochondria from rat liver demonstrate that GD3 elicited a burst of peroxide production within 15-30 min, which preceded the opening of the mitochondrial permeability transition, followed by cytochrome c (cyt c) release. These effects were mimicked by lactosylceramide and N-acetyl-sphingosine but not by sphinganine or sphingosine and were prevented by cyclosporin A and butylated hydroxytoluene (BHT). Reconstitution of mitochondria pre-exposed to GD3 with cytosol from rat liver in a cell-free system resulted in the proteolytic processing of procaspase 3 and subsequent caspase 3 activation. Intact hepatocytes or U937 cells selectively depleted of glutathione in mitochondria by 3-hydroxyl-4-pentenoate (HP) with the sparing of cytosol reduced glutathione (GSH) were sensitized to GD3, manifested as an apoptotic death. Inhibition of caspase 3 prevented the apoptotic phenotype of HP-treated cells caused by GD3 without affecting cell survival; in contrast, BHT fully protected HP-treated cells to GD3 treatment. Treatment of cells with tumor necrosis factor increased the level of GD3, whereas blockers of mitochondrial respiration at complex I and II protected sensitized cells to GD3 treatment. Thus, the effect of GD3 as a lipid death effector is determined by its interaction with mitochondria leading to oxidant-dependent caspase activation. Mitochondrial glutathione plays a key role in controlling cell survival through modulation of the oxidative stress induced by glycosphingolipids.

Apoptosis in systemic lupus erythematosus. Clinical implications

SLE is a heterogeneous and complex group of disorders of uncertain cause. Recent studies have suggested that abnormalities in the apoptotic cell death process may play an important role in the initiation and propagation of this spectrum of disease by altering the generation and cleavage of antigens, and through abnormalities in immunoregulation. The clustering and concentration of autoantigens in and on the surface blebs of apoptotic cells, modifications of antigen structure during certain forms of apoptotic death, and abnormalities in apoptotic cell clearance in humans with SLE and in certain animal models are reviewed and synthesized into a comprehensive model of systemic autoimmunity.

Thiol-mediated apoptosis in prostate carcinoma cells

BACKGROUND

Glutathione (GSH) maintains an optimum cellular redox potential. Chemical depletion, physical efflux from the cell, or intracellular redistribution of this thiol antioxidant is associated with the onset of apoptosis. The aim of this study was to determine the effects of a thiol-depleting agent, diethylmaleate (DEM), on androgen sensitive and insensitive prostate carcinoma cells.

METHODS

LNCaP and PC-3 cell lines were induced to undergo apoptosis by DEM and diamide. Apoptosis was quantified by annexin V binding and propidium iodide incorporation using flow cytometry and was confirmed by DNA gel electrophoresis. Intracellular GSH was quantified using a thiol quantitation kit and the generation of reactive oxygen intermediates was measured using dihydrorhodamine 123. Western blot assessed caspase-3, caspase-8, Bcl-2, and Bcl-XL protein expression. Mitochondrial permeability was measured using DiOC6 and stabilized using bongkrekic acid.

RESULTS

DEM and diamide induced apoptosis in both androgen sensitive and insensitive cells. Apoptosis was also induced in an LNCaP transfectant cell line overexpressing Bcl-2. Apoptosis was caspase-3 dependent and caspase-8 independent. Bongkrekic acid partially prevented the effects of DEM on mitochondrial permeability but was unable to prevent the induction of apoptosis. Decreased Bcl-2 and Bci-XL protein expression was observed at the time of initial caspase-3 activation.

CONCLUSIONS

This study demonstrates that thiol depletion can be used as an effective means of activating caspase-3 in both androgen sensitive and insensitive prostate carcinoma cells. Direct activation of this effector caspase may serve as a useful strategy for inducing apoptosis in prostate carcinoma cells.

Hsp27 functions as a negative regulator of cytochrome c-dependent activation of procaspase-3

The release of mitochondrial cytochrome c by genotoxic stress induces the formation of a cytosolic complex with Apaf-1 (mammalian CED4 homolog) and thereby the activation of procaspase-3 (cas-3) and procaspase-9 (cas-9). Here we demonstrate that heat-shock protein 27 (Hsp27) inhibits cytochrome c (cyt c)-dependent activation of cas-3. Hsp27 had no effect on cyt c release, Apaf-1 and cas-9 activation. By contrast, our results show that Hsp27 associates with cas-3, but not Apaf-1 or cas-9, and inhibits activation of cas-3 by cas-9-mediated proteolysis. Furthermore, the present results demonstrate that immunodepletion of Hsp27 depletes cas-3. Importantly, treatment of cells with DNA damaging agents dissociates the Hsp27/cas-3 complex and relieves inhibition of cas-3 activation. These findings define a novel function for Hsp27 and provide the first evidence that a heat shock protein represses cas-3 activation.

The mitochondrion in cell death control: certainties and incognita

Apoptosis research has recently experienced a change from a paradigm in which the nucleus determined the apoptotic process to a paradigm in which caspases and, more recently, mitochondria constitute the center of death control. Mitochondria undergo major changes in membrane integrity before classical signs of cell death become manifest. These changes concern both the inner and the outer mitochondrial membranes, leading to the dissipation of the inner transmembrane potential (DeltaPsi(m)) and/or the release of intermembrane proteins through the outer membrane. An ever-increasing number of endogenous, viral, or xenogeneic effectors directly act on mitochondria to trigger permeabilization. At least in some cases, this is achieved by a direct action on the permeability transition pore complex (PTPC), a multiprotein ensemble containing proteins from both mitochondrial membranes, which interact with pro- and antiapoptotic members of the Bcl-2 family. At present, it is elusive whether opening of the PTPC is the only physiological mechanism leading to mitochondrial membrane permeabilization. Proteins released from mitochondria during apoptosis include caspases (mainly caspases 2, 3, and 9), caspase activators (cytochrome c, hsp 10), as well as a caspase-independent death effector, AIF (apoptosis inducing factor). The functional hierarchy among these proteins and their actual impact on the decision between death and life is elusive.

JNK1 is inactivated during thiamine deficiency-induced apoptosis in human neuroblastoma cells

Thiamine deficiency results in selective neuronal damage. A number of mechanisms have been proposed to account for brain damage associated with thiamine deficiency and to account for the focal nature of the loss of neurons. One proposed mechanism is programmed cell death. We found efficient induction of apoptosis in human neuroblastoma cells when the cells were deprived of thiamine. Although extensive mitochondrial damage was seen, the release of cytochrome c was not the triggering mechanism for thiamine deficiency-induced apoptosis. Instead, the activity of the cJun amino terminal kinase Jnk1 was lost, and this loss correlated temporally with induction of apoptosis. The loss was specific for Jnk1; Jnk2/3 activity remained unchanged. Loss of Jnk1 activity was not found in lymphoblasts, a cell type that did not undergo apoptosis when deprived of thiamine. These findings suggest that thiamine deficiency results in a cellular stress that brings about the loss of Jnk1 activity and the loss of its function of protecting cells from programmed cell death. We postulate that focal sensitivity to thiamine deficiency results, in part, from specific neuronal cell types being susceptible to the inactivation of Jnk1 in response to depletion of cellular thiamine.

Active caspases and cleaved cytokeratins are sequestered into cytoplasmic inclusions in TRAIL-induced apoptosis

Tumor necrosis factor-related apoptosis- inducing ligand (TRAIL) -induced apoptosis, in transformed human breast epithelial MCF-7 cells, resulted in a time-dependent activation of the initiator caspases-8 and -9 and the effector caspase-7. Cleavage of caspase-8 and its preferred substrate, Bid, preceded processing of caspases-7 and -9, indicating that caspase-8 is the apical initiator caspase in TRAIL-induced apoptosis. Using transient transfection of COOH-terminal-tagged green fluorescent protein fusion constructs, caspases-3, -7, and -8 were localized throughout the cytoplasm of MCF-7 cells. TRAIL-induced apoptosis resulted in activation of caspases-3 and -7, and the redistribution of most of their detectable catalytically active small subunits into large spheroidal cytoplasmic inclusions, which lacked a limiting membrane. These inclusions, which were also induced in untransfected cells, contained cytokeratins 8, 18, and 19, together with both a phosphorylated form and a caspase-cleavage fragment of cytokeratin 18. Similarly, in untransfected breast HBL100 and lung A549 epithelial cells, TRAIL induced the formation of cytoplasmic inclusions that contained cleaved cytokeratin 18 and colocalized with active endogenous caspase-3. We propose that effector caspase-mediated cleavage of cytokeratins, resulting in disassembly of the cytoskeleton and formation of cytoplasmic inclusions, may be a characteristic feature of epithelial cell apoptosis.

Overexpression of the human manganese superoxide dismutase (MnSOD) transgene in subclones of murine hematopoietic progenitor cell line 32D cl 3 decreases irradiation-induced apoptosis but does not alter G2/M or G1/S phase cell cycle arrest

To determine whether overexpression of the human MnSOD transgene protected 32D cl 3 hematopoietic progenitor cells from ionizing irradiation, 32D cl 3 cells were co-electroporated with the pRK5 plasmid containing the human MnSOD transgene and SV2-neo plasmid with G418-resistant colonies selected. Two clones (1F2 and 2C6) were identified to overexpress the human MnSOD transgene by nested reverse transcriptase-polymerase chain reaction (RT-PCR) and increased biochemical activity. Measurement of irradiation-induced damage was determined in cells removed from G418 for 1 week before irradiation. Irradiation survival curves, apoptosis tunnel assay, and Comet assay was performed. Cell cycle distribution was determined for each line at 0, 1, 3, 6, 24, and 48 hr after 500 cGy by fixing the cells in 70% ethanol, staining with propidium iodide, and analysis by flow cytometer. Biochemical MnSOD activity in U/mg protein was 2.6 for 32D cl 3 and significantly elevated to 8.4 and 6.6 (P < 0.001) U/mg protein for subclones 1F2 and 2C6, respectively. Irradiation survival curves demonstrated an increased shoulder on the irradiation survival curve for 1F2 and 2C6 cells with an n of 4.95 +/- 0.48 (P = 0.042) and 4.95 +/- 0.13 (P = 0.011), compared with 2.77 +/- 0.20 for 32D cl 3. A higher percent of 32D cl 3 cells demonstrated apoptosis at 24 and 48 hr after 1,000 cGy irradiation, compared with 1F2 and 2C6 cells (at 24 hr, 29.37% +/- 2.01% of 32D cl 3 cells were apoptotic compared with 5.21 +/- 2.61 (P = 0.018) and 5.27 +/- 2.58 (P = 0.004) for 1F2 and 2C6, respectively). Significantly more DNA strand breaks were detected by Comet assay in 32D cl 3 cells (Comet length at 600 cGy of 103.4 +/- 50.3 units, compared with 69.7 +/- 36.3 (P < 0.001) and 48.9 +/- 27.5 (P < 0.001) for 1F2 and 2C6, respectively). In contrast, irradiation-induced cell cycle arrest was similar between the cell lines with a G2/M phase arrest at 6 hr and a G1/S phase arrest at 24 and 48 hr after irradiation. While overexpression of MnSOD increases the shoulder on the irradiation survival curve of 32D cl 3 cells, decreases irradiation-induced apoptosis, and DNA strand breaks by Comet assay, irradiation-induced alterations in cell cycle distribution were not significantly altered. These 32D cl 3 subclonal lines overexpressing MnSOD provide a potentially valuable system with which to study the mechanism of irradiation-induced cell cycle arrest separate from irradiation-induced apoptosis.

Differential localization of ICAD-L and ICAD-S in cells due to removal of a C-terminal NLS from ICAD-L by alternative splicing

CAD/CPAN/DFF40 is an apoptotic nuclease that is associated with the regulatory subunit ICAD/DFF in healthy cells. ICAD has two forms, ICAD-L/DFF45 and ICAD-S/DFF35, which are transcribed from a single gene by alternative splicing. They differ at the C-terminus: 70 amino acids of ICAD-L are replaced by 4 different amino acids in ICAD-S. We previously showed that both transfected and endogenous ICAD-L are nuclear; however, the localization of ICAD and CAD remains controversial and an important issue to clarify. Here we present the evidence that ICAD-L is nuclear due to the presence of an autonomous nuclear localization signal located in the C-terminal 20 amino acids. This NLS is missing from ICAD-S, which is distributed throughout the cell. We also showed that a GFP:CAD fusion protein is located in the nucleus of transfected cells.

Activation of apoptosis and its inhibition

The induction of apoptosis, or controlled cell death, by various stimuli has been shown to activate a cascade of endoproteases, called caspases, that cleave numerous cellular proteins necessary for cellular homeostasis. This review discusses this family of proteases together with a variety of mammalian and viral regulatory proteins that act to control this activation.

Induction of mitochondrial manganese superoxide dismutase confers resistance to apoptosis in acute myeloblastic leukaemia cells exposed to etoposide

We investigated the possible roles of mitochondrial manganese superoxide dismutase (MnSOD) and bcl-2 in etoposide-induced cell death in acute myeloblastic leukaemia (AML) using two subclones of the OCI/AML-2 cell line, the etoposide-sensitive (ES) and the etoposide-resistant (ER), as models. Cell death after 24 h exposure to 10 micromol/l etoposide was about 60% and 70% in the ES subclone and about 20% and 25% in the ER subclone, when analysed by trypan blue and annexin V respectively. Cytochrome c efflux from mitochondria to cytosol was observed after 4 h of exposure in both subclones, whereas the activation of caspase-3 was not detectable until after 12 h of exposure in the ES subclone and 24 h of exposure in the ER subclone, using Western blotting. The decrease in mitochondrial membrane potential, when analysed by the JC-1 probe fluorocytometrically, also appeared to take place later in the ER than in the ES subclone. Both subclones showed evident basal expression of MnSOD and bcl-2 by Western blotting. Etoposide caused a potent induction of MnSOD, more than 400% at 12 h, in the ER but not in the ES subclone. No significant change in bcl-2 expression could be observed in either of the subclones during exposure to etoposide when analysed by Western blotting or flow cytometry. In conclusion, we suggest that MnSOD might have a special role in the protection of AML cells against etoposide-induced cell death. Although unable to influence the cytochrome c efflux to cytosol, MnSOD might prevent the disruption of mitochondrial membrane potential, which evidently leads to cell death by releasing various activators of apoptosis.

Oxidative damage and protection of the RPE

This review provides a model for the role of oxidative stress in the etiology of age-related macular degeneration (AMD). Epidemiological studies of diet, environmental and behavioral risk factors suggest that oxidative stress is a contributing factor of AMD. Pathological studies indicate that damage to the retinal pigment epithelium (RPE) is an early event in AMD. In vitro studies show that oxidant treated RPE cells undergo apoptosis, a possible mechanism by which RPE cells are lost during early phase of AMD. The main target of oxidative injury seems to be mitochondria, an organelle known to accumulate genomic damages in other postmitotic tissues during aging. The thiol antioxidant GSH and its amino acid precursors protect RPE cells from oxidant-induced apoptosis. Similar protection occurs with dietary enzyme inducers which increase GSH synthesis. These results indicate that therapeutic or nutritional intervention to enhance the GSH antioxidant capacity of RPE may provide an effective way to prevent or treat AMD.

Translocation of C. elegans CED-4 to nuclear membranes during programmed cell death

The Caenorhabditis elegans Bcl-2-like protein CED-9 prevents programmed cell death by antagonizing the Apaf-1-like cell-death activator CED-4. Endogenous CED-9 and CED-4 proteins localized to mitochondria in wild-type embryos, in which most cells survive. By contrast, in embryos in which cells had been induced to die, CED-4 assumed a perinuclear localization. CED-4 translocation induced by the cell-death activator EGL-1 was blocked by a gain-of-function mutation in ced-9 but was not dependent on ced-3 function, suggesting that CED-4 translocation precedes caspase activation and the execution phase of programmed cell death. Thus, a change in the subcellular localization of CED-4 may drive programmed cell death.

Target-related and intrinsic neuronal death in Lurcher mutant mice are both mediated by caspase-3 activation

The Lurcher (Lc) mutation in the delta2 glutamate receptor gene leads to the presence of a constitutive inward current in the cerebellar Purkinje cells of Lurcher heterozygous mice and to the postnatal degeneration of these neurons. In addition, cerebellar granule cells and olivary neurons of Lc/+ mice die as an indirect effect of the mutation after the loss of their target Purkinje cells. The apoptotic nature of Lc/+ Purkinje cell death remains controversial. To address this question, we studied the involvement of caspase-3, a key effector of apoptosis, in the neurodegenerative processes occurring in Lc/+ cerebellum. Several antibodies recognizing different regions of caspase-3 were used in immunoblotting and immunohistochemical experiments. We demonstrate that pro-caspase-3 is specifically upregulated in the dying Lc/+ Purkinje cells, but not in granule cells and olivary neurons, suggesting that different death-inducing signals trigger variant apoptotic pathways in the CNS. The subcellular localization of pro-caspase-3 was shown to be cytoplasmic and mitochondrial. Active caspase-3 as well as DNA fragmentation was found in numerous granule cells and some Purkinje cells of the Lc/+ cerebellum. Thus, caspase-3 activation is involved in both the direct and indirect neuronal death induced by the Lurcher mutation, strongly supporting the idea that the Lc/+ Purkinje cell dies by apoptosis.

Role of p53 family members in apoptosis

p53-mediated apoptosis involves multiple mechanisms. A number of p53-regulated apoptosis-related genes have been identified. Some of these genes encode proteins that are important in controlling the integrity of mitochondria while the others code for membrane death receptors. p53 may also induce apoptosis by interfering with the growth factor-mediated survival signals. Although the transactivation-deficient p53 can induce apoptosis, evidence suggests that both the transcription-dependent and independent functions are needed for full apoptotic activity. p73 and p63 are two other members of the p53 family that show homology to p53 in their respective transactivation, DNA-binding and oligomerization domains. Both p73 and p63 transactivate p53-regulated promoters and induce apoptosis. Evidence suggests that both p73 and p63 may mediate apoptosis via some of the same mechanisms that are utilized by p53. However, both p73 and p63 exhibit features that are different from those of p53. Hence, both p73 and p63 are predicted to mediate apoptosis via mechanisms that are completely distinct from those engaged by p53. J. Cell. Physiol. 182:171-181, 2000. Published 2000 Wiley-Liss, Inc.

Mitochondrial defects in cardiomyopathy and neuromuscular disease

Over the past 11 years, a considerable body of evidence has accumulated implicating defects in the mitochondrial energy-generating pathway, oxidative phosphorylation, in a wide variety of degenerative diseases including myopathy and cardiomyopathy. Most classes of pathogenic mitochondrial DNA mutations affect the heart, in association with a variety of other clinical manifestations that can include skeletal muscle, the central nervous system (including eye), the endocrine system, and the renal system. To better understand the pathophysiologic basis of mitochondrial diseases and their role in myopathy and cardiomyopathy, several mouse models of mitochondrial disease have been prepared. Mitochondrial DNA mutations from cultured cells have been introduced into mice; nuclear DNA genes involved in mitochondrial energy production and reactive oxygen species detoxification have been genetically inactivated, which resulted in mice with hypertrophic and dilated cardiomyopathy, respectively. Physiologic characterization of these mice has confirmed the importance of decreased mitochondrial energy production, increased mitochondrial reactive oxygen species production, and the mitochondrial initiation of apoptosis in mitochondrial disease. With these insights, new therapeutic approaches for neuromuscular and cardiac disease have been suggested.

Target-related and intrinsic neuronal death in Lurcher mutant mice are both mediated by caspase-3 activation

The Lurcher (Lc) mutation in the delta2 glutamate receptor gene leads to the presence of a constitutive inward current in the cerebellar Purkinje cells of Lurcher heterozygous mice and to the postnatal degeneration of these neurons. In addition, cerebellar granule cells and olivary neurons of Lc/+ mice die as an indirect effect of the mutation after the loss of their target Purkinje cells. The apoptotic nature of Lc/+ Purkinje cell death remains controversial. To address this question, we studied the involvement of caspase-3, a key effector of apoptosis, in the neurodegenerative processes occurring in Lc/+ cerebellum. Several antibodies recognizing different regions of caspase-3 were used in immunoblotting and immunohistochemical experiments. We demonstrate that pro-caspase-3 is specifically upregulated in the dying Lc/+ Purkinje cells, but not in granule cells and olivary neurons, suggesting that different death-inducing signals trigger variant apoptotic pathways in the CNS. The subcellular localization of pro-caspase-3 was shown to be cytoplasmic and mitochondrial. Active caspase-3 as well as DNA fragmentation was found in numerous granule cells and some Purkinje cells of the Lc/+ cerebellum. Thus, caspase-3 activation is involved in both the direct and indirect neuronal death induced by the Lurcher mutation, strongly supporting the idea that the Lc/+ Purkinje cell dies by apoptosis.