Molecular characterization of mitochondrial apoptosis-inducing factor

Reactive oxygen species regulate activation-induced T cell apoptosis

Reactive oxygen species (ROS) mediate apoptosis in a number of cell types. We studied the role that ROS play in activated T cell apoptosis by activating T cells in vivo and then culturing them for a short time. Activated T cells died independently of Fas and TNF alpha. Their death was characterized by rapid loss of mitochondrial transmembrane potential (delta psi(m)), caspase-dependent DNA fragmentation, and superoxide generation. A superoxide dismutase mimetic, Mn (III) tetrakis (5, 10, 15, 20-benzoic acid) porphyrin (MnTBAP), protected T cells from superoxide generation, caspase-dependent DNA loss, loss of delta psi(m), and cell death. These results indicate that ROS can regulate signals involved in caspase activation and apoptosis and may contribute to peripheral T cell deletion.

Spermine triggers the activation of caspase-3 in a cell-free model of apoptosis

Polyamines are ubiquitous organic cations required for cell proliferation. However, some evidence suggested that their excessive accumulation can induce apoptosis. We show here that, in a post-nuclear extract from U937 cells, the addition of spermine triggers the death program, represented by cytochrome c exit from mitochondria, the dATP-dependent processing of pro-caspase-3 and the onset of caspase activity. Spermine is more effective than spermidine, whereas putrescine has no effect. Polyamine acetylation abolishes their pro-apoptotic power. These data demonstrate a direct mechanism responsible for polyamine toxicity and also suggest that an excessive elevation of free polyamines could be involved in the transduction of a death signal.

Apoptotic nuclear morphological change without DNA fragmentation

Apoptosis is characterized morphologically by condensation and fragmentation of nuclei and cells and biochemically by fragmentation of chromosomal DNA into nucleosomal units [1]. CAD, also known as CPAN or DFF-40, is a DNase that can be activated by caspases [2] [3] [4] [5] [6]. CAD is complexed with its inhibitor, ICAD, in growing, non-apoptotic cells [2] [7]. Caspases that are activated by apoptotic stimuli [8] cleave ICAD. CAD, thus released from ICAD, digests chromosomal DNA into nucleosomal units [2] [3]. Here, we examine whether nuclear morphological changes induced by apoptotic stimuli are caused by the degradation of chromosomal DNA. Human T-cell lymphoma Jurkat cells, as well as their transformants expressing caspase-resistant ICAD, were treated with staurosporine. The chromosomal DNA in Jurkat cells underwent fragmentation into nucleosomal units, which was preceded by large-scale chromatin fragmentation (50-200 kb). The chromosomal DNA in cells expressing caspase-resistant ICAD remained intact after treatment with staurosporine but their chromatin condensed as found in parental Jurkat cells. These results indicate that large-scale chromatin fragmentation and nucleosomal DNA fragmentation are caused by an ICAD-inhibitable DNase, most probably CAD, whereas chromatin condensation during apoptosis is controlled, at least in part, independently from the degradation of chromosomal DNA.

[Apoptosis--what is it? Significance in coronary heart disease and myocardial infarct]

Apoptosis is a physiological, highly conserved program of cellular suicide, characterized by nuclear condensation with DNA-fragmentation, by alterations in the distribution of cell membrane phospholipids, and by cellular shrink-age. Apoptotic cellular remnants engulfed by cell membranes are phagocytized largely without activation of inflammatory reactions. The apoptotic program is executed by a cascade of highly specific caspases, activated by complexation of initiatorcaspases in cytosolic signalling complexes at receptors of the TNF family or at impaired mitochondria. In many forms of cellular stress with damage of nuclear DNA and mitochondria, mixed forms of cell death are triggered with regulated activation of the apoptotic program and concomitantly, with induction of catastrophic necrosis. Such a mixed form of myocyte death is observed in myocardial ischemia and reperfusion. Antiapoptotic interventions can delay ischemic myocardial damage in experiments. Therefore, those interventions appear conceivable as future strategy for acutely enhancing the available time interval for therapeutic reperfusion. However, chronic inhibition of apoptosis for ongoing prevention of myocardial ischemic damage may not become a plausible strategy because of disturbances of the immune system, because of putatively infavorable effects on arteriosclerotic lesions and because of likely disturbances in the physiologic elimination of damaged mitochondria.

Glucose protection from MPP+-induced apoptosis depends on mitochondrial membrane potential and ATP synthase

MPP+ inhibits mitochondrial complex I and alpha-ketoglutarate dehydrogenase causing necrosis or apoptosis of catecholaminergic neurons. Low glucose levels or glycolytic blockade has been shown to potentiate MPP+ toxicity. We found that MPP+ caused concentration-dependent apoptosis of neuronally differentiated PC12 cells and that glucose, but not pyruvate, supplementation reduced apoptosis. Oligomycin concentrations sufficient to inhibit ATP synthase blocked the decreased apoptosis afforded by glucose supplementation. Laser-scanning confocal microscope imaging of chloromethyl-tetramethylrosamine methyl ester fluorescence to estimate DeltaPsiM showed that MPP+ and atractyloside reduced DeltaPsiM, while cyclosporin A (CSA) and glucose supplementation reversed decreases in DeltaPsiM caused by MPP+. Oligomycin blocked the effect of glucose supplementation on DeltaPsiM. These findings show that (i) MPP+-induced and atractyloside-induced apoptosis are associated with reduced DeltaPsiM; (ii) CSA maintains DeltaPsiM and reduces MPP+-induced apoptosis; and (iii) glucose supplementation maintains DeltaPsiM, likely by glycolytic ATP-dependent proton pumping at ATP synthase and reduces MPP+-induced apoptosis.

Mitochondrial diseases in man and mouse

Over the past 10 years, mitochondrial defects have been implicated in a wide variety of degenerative diseases, aging, and cancer. Studies on patients with these diseases have revealed much about the complexities of mitochondrial genetics, which involves an interplay between mutations in the mitochondrial and nuclear genomes. However, the pathophysiology of mitochondrial diseases has remained perplexing. The essential role of mitochondrial oxidative phosphorylation in cellular energy production, the generation of reactive oxygen species, and the initiation of apoptosis has suggested a number of novel mechanisms for mitochondrial pathology. The importance and interrelationship of these functions are now being studied in mouse models of mitochondrial disease.

Regulation of tumor necrosis factor-induced, mitochondria- and reactive oxygen species-dependent cell death by the electron flux through the electron transport chain complex I

Tumor necrosis factor (TNF) induces a caspase-independent but mitochondria-dependent cell death process in the mouse fibrosarcoma cell line L929. Mitochondria actively participate in this TNF-induced necrotic cell death by the generation of mitochondrial reactive oxygen species (ROS). The aim of this study was to identify the mitochondrial components involved in TNF-induced production of ROS and their regulation by bioenergetic pathways. Therefore, we analyzed the bioenergetic characteristics in two metabolic L929 variants that exhibit different sensitivities to TNF. L929gln cells use glutamine as respiratory substrate and are far more susceptible to TNF-induced ROS generation and cell death as L929glc cells that use glucose as respiratory substrate. We show that the higher levels of reducing NAD(P)H equivalents, detected in the desensitized L929glc cells, do not cause diminished ROS generation. To the contrary, TNF increases the levels of NAD(P)H, probably altering complex I activity. A multiparameter analysis of electron flux through the mitochondrial electron transport chain, TNF-induced ROS levels, and cell death convincingly demonstrates a dependence of TNF signaling on complex I activity. Also, the sensitizing effect of glutamine metabolism correlates with an enhanced contribution of complex I to the overall electron flux. This participation of complex I activity in TNF-induced cell death is regulated by substrate availability rather than by a direct modification of complex I proteins. From the results presented in this paper we conclude that TNF-induced ROS generation and cell death are strongly regulated by bioenergetic pathways that define electron flux through complex I of the electron transport chain.