Rapid and specific efflux of reduced glutathione during apoptosis induced by anti-Fas/APO-1 antibody

Potassium Leakage During the Apoptotic Degradation Phase

The subcellular compartmentalization of ions is perturbed during the process of apoptosis. In this work, we investigated the impact of K+ on the apoptotic process in thymocytes and T cell hybridoma cells. Irrespective of the death-inducing stimulus (glucocorticoids, topoisomerase inhibition, or Fas-crosslinking), a significant K+ outflow was observed during apoptosis, as determined on the single-cell level by means of the K+-sensitive fluorochrome, benzofuran isophtalate. This loss of cytosolic K+ only occurs in cells that have completely disrupted their inner mitochondrial transmembrane potential. Inhibition of this mitochondrial transmembrane potential loss by Bcl-2 or by specific inhibitors acting on the mitochondrial permeability transition pore (bongkrekic acid, cyclosporin A) prevents K+ leakage. K+ drops at the same stage at which cells expose phosphatidylserine residues on the outer leaflet of the membrane and reduce the levels of nonoxidized glutathione, but before they hyperproduce reactive oxygen species, undergo massive Ca2+ influx, shrink, and lyse. In a cell-free system of apoptosis, isolated nuclei exposed to the supernatant of mitochondria that have undergone permeability transition only manifest chromatinolysis when the K+ concentration is lowered from physiologic to apoptotic levels. Accordingly, massive DNA fragmentation causing subdiploidy is confined to cells that have undergone K+ leakage. Together, these data point to the step-wise acquisition of membrane dysfunction in apoptosis and indicate an important role for the disruption of normal K+ homeostasis in apoptotic degradation. Derepression of endonucleases due to low K+ concentrations may be a decisive prerequisite for end-stage DNA fragmentation.

Glutathione regulation of neutral sphingomyelinase in tumor necrosis factor-alpha-induced cell death

Tumor necrosis factor-alpha (TNFalpha)-induced cell death involves a diverse array of mediators and regulators including proteases, reactive oxygen species, the sphingolipid ceramide, and Bcl-2. It is not known, however, if and how these components are connected. We have previously reported that GSH inhibits, in vitro, the neutral magnesium-dependent sphingomyelinase (N-SMase) from Molt-4 leukemia cells. In this study, GSH was found to reversibly inhibit the N-SMase from human mammary carcinoma MCF7 cells. Treatment of MCF7 cells with TNFalpha induced a marked decrease in the level of cellular GSH, which was accompanied by hydrolysis of sphingomyelin and generation of ceramide. Pretreatment of cells with GSH, GSH-methylester, or N-acetylcysteine, a precursor of GSH biosynthesis, inhibited the TNFalpha-induced sphingomyelin hydrolysis and ceramide generation as well as cell death. Furthermore, no significant changes in GSH levels were observed in MCF7 cells treated with either bacterial SMase or ceramide, and GSH did not protect cells from death induced by ceramide. Taken together, these results show that GSH depletion occurs upstream of activation of N-SMase in the TNFalpha signaling pathway. TNFalpha has been shown to activate at least two groups of caspases involved in the initiation and "execution" phases of apoptosis. Therefore, additional studies were conducted to determine the relationship of GSH and the death proteases. Evidence is provided to demonstrate that depletion of GSH is dependent on activity of interleukin-1beta-converting enzyme-like proteases but is upstream of the site of action of Bcl-2 and of the execution phase caspases. Taken together, these studies demonstrate a critical role for GSH in TNFalpha action and in connecting major components in the pathways leading to cell death.

Rescue of cells from apoptosis by inhibition of active GSH extrusion

Cells induced to apoptosis extrude glutathione in the reduced form concomitantly with (U937 cells) or before (HepG2 cells) the development of apoptosis, much earlier than plasma membrane leakage. Two specific inhibitors of carrier-mediated GSH extrusion, methionine or cystathionine, are able to decrease apoptotic GSH efflux across the intact plasma membrane, demonstrating that in these cell systems GSH extrusion occurs via a specific mechanism. While decreasing GSH efflux, cystathionine or methionine also decrease the extent of apoptosis. They fail to exert anti-apoptotic activity in cells previously deprived of GSH, indicating that the target of the protection is indeed GSH efflux. The cells rescued by methionine or cystathionine remained viable after removal of the apoptogenic inducers and were even able to replicate. This shows that a real rescue to perfect viability and not just a delay of apoptosis is achieved by forcing GSH to stay within the cells during apoptogenic treatment. All this evidence indicates that extrusion of reduced glutathione precedes and is responsible for the irreversible morphofunctional changes of apoptosis, probably by altering the intracellular redox state without intervention of reactive oxygen species, thus giving a rationale for the development of redox-dependent apoptosis under anaerobic conditions.

Rapid and specific reactive oxygen species generation via NADPH oxidase activation during Fas-mediated apoptosis

To determine Fas-induced reactive oxygen species (ROS) generation in cells sensitive to Fas-mediated apoptosis, chemiluminescence, a strong indicator of ROS generation, was monitored after stimulation with agonistic anti-Fas. Fas ligation resulted in a rapid and sustained ROS generation. The generation reached a maximum within 5 min and was still observed at least 40 min after the stimulation. No significant ROS generation was observed under conditions where apoptosis was absent. Furthermore, an NADPH oxidase inhibitor, diphenylene iodonium, prevented Fas-mediated ROS generation, suggesting that Fas induces ROS generation via stimulation of the oxidase system.

Bcl-2 expression causes redistribution of glutathione to the nucleus

In this study we used HeLa cells transfected with a conditional Bcl-2 expression construct to study the effects of Bcl-2 on reduced glutathione (GSH) metabolism. Our previous work demonstrated that depletion of GSH by culturing cells in tissue culture medium lacking the amino acids cysteine and methionine, essential for GSH biosynthesis, caused cells overexpressing Bcl-2 to become sensitized to apoptotic induction. Here we report that Bcl-2 also dramatically alters GSH compartmentalization. Cellular distribution of GSH, assayed by confocal microscopy, revealed that when Bcl-2 expression was suppressed GSH was uniformly distributed primarily in the cytosol, whereas overexpression of Bcl-2 led to a relocalization of GSH into the nucleus. Isolated nuclei readily accumulated radiolabeled GSH and maintained higher nuclear GSH concentration in direct relation to Bcl-2 nuclear protein levels. Moreover, exogenous GSH blocked apoptotic changes and caspase activity in isolated nuclei exposed to the pro-apoptotic protease granzyme B. Our results indicate that one of the functions of Bcl-2 is to promote sequestration of GSH into the nucleus, thereby altering nuclear redox and blocking caspase activity as well as other nuclear alterations characteristic of apoptosis. We speculate that this mechanism contributes to the suppression of apoptosis in cells with elevated Bcl-2 levels.

Biochemical and molecular mechanisms regulating apoptosis

In eukaryotes, the regulation of tissue cell numbers is a critical homeostatic objective that is achieved through tight control of apoptosis, mitosis and differentiation. While much is known about the genetic regulation of cell growth and differentiation, the molecular basis of apoptosis is less well understood. Genes involved in both cell proliferation and apoptosis reflect the role of some stimuli in both of these processes, the cell response depending on the overall cellular milieu. Recent research has given fascinating insights into the complex genetic and molecular mechanisms regulating apoptosis. A picture is emerging of the initiation in certain cells, after an apoptotic trigger, of sequential gene expression and specific signal transduction cascades that guide cells along the cell death pathway. Changes in gene expression precede the better known biochemical and morphological changes of apoptosis. It seems possible that, as a result of increased understanding of the cellular events preceding cell death, apoptosis may become more amenable to manipulation by appropriate drug- and gene-based therapies.

Redox regulation of apoptotic cell death

Apoptosis is a special form of cell death, which can be triggered by a variety of signals and pathophysiological conditions, including oxidative stress. Activation of members of the caspase family of cysteine proteases is a crucial event in the apoptotic death program. Being cysteine proteases, the caspases are sensitive to the redox status of the cell, and their activity is blocked by excessive oxidative stress. Thus, alterations of intracellular redox status may either trigger or block the apoptotic death program, depending on the severity of the oxidative stress.

hsp27 as a switch between differentiation and apoptosis in murine embryonic stem cells

Small stress proteins are developmentally regulated and linked to cell growth and differentiation. The early phase of murine embryonic stem (ES) cell differentiation, characterized by a gradual growth arrest, is accompanied with hsp27 transient accumulation. This differentiation process also correlated with changes in hsp27 phosphorylation and oligomerization. The role of hsp27 was investigated in ES clones stably transfected with murine or human hsp27 genes, placed in sense or antisense orientation. Several clones were obtained that either underexpressed endogenous murine hsp27 or overexpressed murine or human hsp27. Maintained undifferentiated, these clones showed similar growth rates. We report here that hsp27 constitutive overexpression enhanced the differentiation-mediated decreased rate of ES cell proliferation but did not alter morphological changes. In contrast, hsp27 underexpression, which attenuated cell growth arrest, induced differentiation abortion because of an overall cell death by apoptosis. Recently, we showed that hsp27 interfered with cell death probably because of its ability to modulate intracellular glutathione. hsp27 accumulation during ES cell differentiation was also correlated with an increase in glutathione, which was attenuated by hsp27 down-expression. Hence, hsp27 transient expression seems essential for preventing differentiating ES cells from undergoing apoptosis, a switch that may be redox regulated.

Requirement for cGMP in nerve cell death caused by glutathione depletion

Glutathione depletion occurs in several forms of apoptosis and is associated with Parkinson's disease and HIV toxicity. The neurotransmitter glutamate kills immature cortical neurons and a hippocampal nerve cell line via an oxidative pathway associated with glutathione depletion. It is shown here that soluble guanylyl cyclase (sGC) activity is required for nerve cell death caused by glutathione depletion. Inhibitors of sGC block glutamate toxicity and a cGMP analogue potentiates cell death. Glutamate also induces an elevation of cGMP that occurs late in the cell death pathway. The resultant cGMP modulates the increase in intracellular calcium that precedes cell death because sGC inhibitors prevent calcium elevation and the cGMP analogue potentiates the increase in intracellular calcium. These results suggest that the final pathway of glutamate induced nerve cell death is through a cGMP-modulated calcium channel.

Tumor necrosis factor increases hepatocellular glutathione by transcriptional regulation of the heavy subunit chain of gamma-glutamylcysteine synthetase

Tumor necrosis factor (TNF) is an inflammatory cytokine that causes cell injury by generation of oxidative stress. Since glutathione (GSH) is a key cellular antioxidant that detoxifies reactive oxygen species, the purpose of our work was to examine the regulation of cellular GSH, the expression of heavy subunit chain of gamma-glutamylcysteine synthetase (gamma-GCS-HS), and control of intracellular generation of reactive oxygen species in cultured rat hepatocytes treated with TNF. Exposure of cells to TNF (10,000 units/ml) resulted in depletion of cellular GSH levels (50-70%) and overproduction of hydrogen peroxide (2-3-fold) and lipid peroxidation. However, cells treated with lower doses of TNF (250-500 units/ml) exhibited increased levels of GSH (60-80% over control). TNF treatment increased (70-100%) the levels of gamma-GCS-HS mRNA, the catalytic subunit of the regulating enzyme in GSH biosynthesis. Furthermore, intact nuclei isolated from hepatocytes treated with TNF transcribed the gamma-GCS-HS gene to a greater extent than control cells, indicating that TNF regulates gamma-GCS-HS at the transcriptional level. The capacity to synthesize GSH de novo determined in cell-free extracts incubated with GSH precursors was greater (50-70%) in hepatocytes that were treated with TNF; however, the activity of GSH synthetase remained unaltered by TNF treatment indicating that TNF selectively increased the activity of gamma-GCS. Despite activation of nuclear factor-kappaB (NF-kappaB) by TNF, this transcription factor was not required for TNF-induced transcription of gamma-GCS-HS as revealed by deletion constructs of the gamma-GCS-HS promoter subcloned in a chloramphenicol acetyltransferase reporter vector and transfected into HepG2 cells. In contrast, a construct containing AP-1 like/metal response regulatory elements increased chloramphenicol acetyltransferase activity upon exposure to TNF. Thus, TNF increases hepatocellular GSH levels by transcriptional regulation of gamma-GCS-HS gene, probably through AP-1/metal response element-like binding site(s) in its promoter, which may constitute a protective mechanism in the control of oxidative stress induced by inflammatory cytokines.

Nitric oxide inhibits Fas-induced apoptosis

The Fas antigen (CD95, APO-1) is a transmembrane cell surface receptor that mediates apoptosis of many cell types when bound by Fas ligand or cross-linked by agonist antibody. The cellular factors regulating Fas-induced apoptosis have not been well defined. Here we show that basal nitric-oxide synthase (NOS) activity in human leukocytes inhibits Fas-induced apoptosis via a cGMP-independent mechanism. Further, NOS inhibits Fas-induced cleavage of poly(ADP-ribose) polymerase by members of the caspase family of cysteine proteases. These data suggest that Fas activity is under the control of the NO signaling pathway. NOS regulating the function of this member of the tumor necrosis factor receptor family suggests a new role for nitric oxide (or related molecules) in the human immune response.

Dual regulation of caspase activity by hydrogen peroxide: implications for apoptosis

The induction of apoptosis in Jurkat T-lymphocytes with 50 microM hydrogen peroxide was associated with caspase activation. Caspase activity was first detected 3 h after treatment, and the morphological features of apoptosis were apparent by 6 h. At higher concentrations of hydrogen peroxide there was no detectable caspase activity, and the cells died by necrosis. Cells treated with hydrogen peroxide were impaired in their ability to undergo Fas-mediated apoptosis. This appeared to be the result of direct inhibition of the cysteine-dependent caspases. The cells were able to recover and undergo apoptosis at later times. Therefore, hydrogen peroxide has two distinct effects. It initially inhibits the caspases and delays apoptosis. Then, depending on the degree of the initial oxidative stress, the caspases are activated and the cells die by apoptosis, or they remain inactive and necrosis occurs. We discuss the physiological implications of cells having to maintain a reducing environment during apoptosis to allow the caspases to function.

Efflux of reduced glutathione after exposure of human lung epithelial cells to crocidolite asbestos

This study investigated glutathione (GSH) homeostasis in human lung epithelial cells (A549) exposed to crocidolite. Exposure of A549 cells to 3 micrograms/cm2 crocidolite resulted in a decrease in intracellular reduced glutathione by 36% without a corresponding increase in GSH disulfide. After a 24-hr exposure to crocidolite, 75% of the intracellular GSH lost was recovered in the extracellular medium, of which 50% was in reduced form. Since the half-life of reduced GSH in culture medium was less than 1 hr, this suggests that reduced GSH was released continuously from the cells after treatment. The release of GSH did not appear to result from nonspecific membrane damage, as there was no concomitant release of lactate dehydrogenase or 14C-adenine from loaded cells after crocidolite treatment for 24 hr. Crocidolite exposure resulted in the formation of S-nitrosothiols but no increase in the level of GSH-protein mixed disulfides or GSH conjugates. Exposure of A549 cells to crocidolite for 24 hr decreased gamma glutamylcysteine synthetase (gamma-GCS) activity by 47% without changes in the activities of GSH reductase, GSH peroxidase, GSH S-transferase, or glucose-6-phosphate dehydrogenase. Treatment of cells with crocidolite pretreated with the iron chelator desferrioxamine B resulted in the same level of intracellular GSH depletion and efflux and the same decrease in gamma-GCS activity as treatment with unmodified crocidolite, which suggests that iron-catalyzed reactions were not responsible for the GSH depletion.

Inhibition of Fas-induced apoptotic cell death by osmotic cell shrinkage

Apoptosis is an active physiological mechanism permitting the elimination of cells by triggering an intracellular signalling cascade. Here, we tested whether osmotic alterations of cell volume interfere with apoptotic cell death in Jurkat T-lymphocytes. Apoptotic cell death of Jurkat cells was elicited by activation of the Fas receptor which results in sphingomyelinase stimulation, release of ceramide, activation of Ras, Rac-proteins and formation of O2. Osmotic cell shrinkage inhibited apoptotic cell death induced by the Fas receptor in Jurkat T-lymphocytes. Osmotic cell shrinkage did not interfere with Fas induced activation of the acidic sphingomyelinase or activation of Ras but impaired the formation of O2 suggesting an important function of cell volume in the synthesis of reactive oxygen intermediates upon Fas receptor ligation.

Bcl-xL overexpression attenuates glutathione depletion in FL5.12 cells following interleukin-3 withdrawal

Bcl-xL and bax are bcl-2-related genes whose protein products either inhibit or promote apoptosis. Oxidative damage, including the loss of glutathione, has been implicated in the induction of apoptosis. The ability of the Bcl proteins to affect GSH was assessed in control, bax- and bcl-xL-transfected FL5.12 cells [an interleukin (IL)-3-dependent murine prolymphocytic cell line]. Overall levels of GSH were approximately the same in control and bcl-xL transfectants during the 6 h incubation period, although levels increased in bcl-xL transfectants 24 h after replating. GSH in cells overexpressing bax was reduced by approximately 36%. There were no consistent differences between these cell lines in the activities of superoxide dismutase, catalase, glutathione peroxidase or glutathione reductase. Following IL-3 withdrawal, a condition known to cause apoptosis in these cells, a rapid loss of intracellular GSH occurred in control and bax transfectants, which preceded the onset of apoptosis. GSH depletion could not be attributed to intracellular oxidation but rather seemed to occur due to a translocation out of the cell. Cells overexpressing bcl-xL did not lose significant amounts of GSH upon withdrawal of IL-3, and no apoptosis was evident. These results suggest a possible role for GSH in the mechanism by which bcl-xL prevents cell death.

The shape of cell death

Cell death, a scheduled event during development and tissue turnover, or the ultimate consequence of toxic or pathologic insults seems to involve a relatively limited number of execution pathways. This reflects the evolution of an organized sequence of events perhaps converging onto final common pathways that are used to dispose of unwanted or injured cells. In many cases, the ordered execution of this internal death program leads to typical morphological and biochemical changes that have been termed apoptosis. Apoptosis, often equated with developmental or programmed cell death, has been opposed to unscheduled or accidental cell lysis/necrosis. However, increasing evidence suggests that the two forms of cell demise share similar characteristics, at least in the signaling and early progression phase. Recent studies have shown that, when the intensity of the insult is very high and/or when ATP generation is deficient, cells fail to execute the ordered changes ensuing in apoptosis. Then cell lysis/necrosis supervenes before the processes leading to nuclear condensation and exposure of surface molecules can be completed. Thus, apoptosis and necrosis seem to represent only different shapes of cell demise, resulting from a more or less complete execution of the internal death program.

Mechanism of dithiocarbamate inhibition of apoptosis: thiol oxidation by dithiocarbamate disulfides directly inhibits processing of the caspase-3 proenzyme

Dithiocarbamates (DCs) have been reported to be potent inhibitors of apoptosis in several different model systems, which suggests a target common to the apoptotic machinery. Without further investigation, this has been assumed to reflect an antioxidant activity of the DCs. However, we have recently shown that DCs exert prooxidant effects on T cells [Nobel et al. (1995) J. Biol. Chem. 270, 26202-26208], which are dependent on their transfer of external copper into the cells and can be inhibited by the inclusion of high-affinity external copper chelators in the medium. Investigating antiapoptotic actions of DCs, we found that inclusion of a membrane-impermeable copper chelator severely compromised the inhibitory activity of reduced DCs. Since copper can promote DC oxidation to the respective DC disulfides, the inhibitory effect on lymphocyte apoptosis might be mediated by the DC disulfides. In agreement with this we observed that DC disulfides were more potent inhibitors of T cell apoptosis than their reduced counterparts. Inhibition of apoptosis by DC disulfides correlated with the inhibition of caspase-3 proenzyme processing and activation. Similar results were obtained in a cell-free model system of caspase-3 activation. Significantly, dithiothreitol reduction of the DC disulfide abolished its inhibition of in vitro proenzyme processing, thereby demonstrating thiol-disulfide exchange between the DC disulfide and a free thiol group on an activator(s) of caspase-3. Since T cell apoptosis involves the generation of mature caspase-3 and requires caspase-3-like activity, we propose that (1) DC disulfides are the active agents behind DC inhibition of apoptosis and (2) their site of action is the proteolytic activation of this enzyme. These findings also reveal the potential for other thiol-oxidizing toxicants to inhibit apoptosis by preventing the proteolytic activation of caspases.