Sodium arsenite and arsenic trioxide differently affect the oxidative stress of lymphoblastoid cells: An intricate crosstalk between mitochondria, autophagy and cell death

Although the toxicity of arsenic depends on its chemical forms, few studies have taken into account the ambiguous phenomenon that sodium arsenite (NaAsO2) acts as a potent carcinogen while arsenic trioxide (ATO, As2O3) serves as an effective therapeutic agent in lymphoma, suggesting that NaAsO2 and As2O3 may act via paradoxical ways to either promote or inhibit cancer pathogenesis. Here, we compared the cellular response of the two arsenical compounds, NaAsO2 and As2O3, on the Burkitt lymphoma cell model, the Epstein Barr Virus (EBV)-positive P3HR1 cells. Using flow cytometry and biochemistry analyses, we showed that a NaAsO2 treatment induces P3HR1 cell death, combined with drastic drops in ΔΨm, NAD(P)H and ATP levels. In contrast, As2O3-treated cells resist to cell death, with a moderate reduction of ΔΨm, NAD(P)H and ATP. While both compounds block cells in G2/M and affect their protein carbonylation and lipid peroxidation, As2O3 induces a milder increase in superoxide anions and H2O2 than NaAsO2, associated to a milder inhibition of antioxidant defenses. By electron microscopy, RT-qPCR and image cytometry analyses, we showed that As2O3-treated cells display an overall autophagic response, combined with mitophagy and an unfolded protein response, characteristics that were not observed following a NaAsO2 treatment. As previous works showed that As2O3 reactivates EBV in P3HR1 cells, we treated the EBV- Ramos-1 cells and showed that autophagy was not induced in these EBV- cells upon As2O3 treatment suggesting that the boost of autophagy observed in As2O3-treated P3HR1 cells could be due to the presence of EBV in these cells. Overall, our results suggest that As2O3 is an autophagic inducer which action is enhanced when EBV is present in the cells, in contrast to NaAsO2, which induces cell death. That's why As2O3 is combined with other chemicals, as all-trans retinoic acid, to better target cancer cells in therapeutic treatments.

Tafazzin Mutation Affecting Cardiolipin Leads to Increased Mitochondrial Superoxide Anions and Mitophagy Inhibition in Barth Syndrome

Tafazzin is a phospholipid transacylase that catalyzes the remodeling of cardiolipin, a mitochondrial phospholipid required for oxidative phosphorylation. Mutations of the tafazzin gene cause Barth syndrome, which is characterized by mitochondrial dysfunction and dilated cardiomyopathy, leading to premature death. However, the molecular mechanisms underlying the cause of mitochondrial dysfunction in Barth syndrome remain poorly understood. We again highlight the fact that the tafazzin deficiency is also linked to defective oxidative phosphorylation associated with oxidative stress. All the mitochondrial events are positioned in a context where mitophagy is a key element in mitochondrial quality control. Here, we investigated the role of tafazzin in mitochondrial homeostasis dysregulation and mitophagy alteration. Using a HeLa cell model of tafazzin deficiency, we show that dysregulation of tafazzin in HeLa cells induces alteration of mitophagy. Our findings provide some additional insights into mitochondrial dysfunction associated with Barth syndrome, but also show that mitophagy inhibition is concomitant with apoptosis dysfunction through the inability of abnormal mitochondrial cardiolipin to assume its role in cytoplasmic signal transduction. Our work raises hope that pharmacological manipulation of the mitophagic pathway together with mitochondrially targeted antioxidants may provide new insights leading to promising treatment for these highly lethal conditions.

Hormetic effects of curcumin: What is the evidence?

Curcumin (diferuloylmethane), a component of the yellow powder prepared from the roots of Curcuma longa or Zingiberaceae (known as turmeric) is not only widely used to color and flavor food but also used as a pharmaceutical agent. Curcumin demonstrates anti-inflammatory, anticarcinogenic, antiaging, and antioxidant activity, as well as efficacy in wound healing. Notably, curcumin is a hormetic agent (hormetin), as it is stimulatory at low doses and inhibitory at high doses. Hormesis by curcumin could be also a particular function at low doses (i.e., antioxidant behavior) and another function at high dose (i.e., induction of autophagy and cell death). Recent findings suggest that curcumin exhibits biphasic dose-responses on cells, with low doses having stronger effects than high doses; examples being activation of the mitogen-activated protein kinase signaling pathway or antioxidant activity. This indicates that many effects induced by curcumin are dependent on dose and some effects might be greater at lower doses, indicative of a hormetic response. Despite the consistent occurrence of hormetic responses of curcumin in a wide range of biomedical models, epidemiological and clinical trials are needed to assess the nature of curcumin's dose-response in humans. Fortunately, more than one hundred clinical trials with curcumin and curcumin derivatives are ongoing. In this review, we provide the first comprehensive analysis supportive of the hormetic behavior of curcumin and curcumin derivatives.

Decreasing cytosolic translation is beneficial to yeast and human Tafazzin-deficient cells

Cardiolipin (CL) optimizes diverse mitochondrial processes, including oxidative phosphorylation (OXPHOS). To function properly, CL needs to be unsaturated, which requires the acyltransferase Tafazzin (TAZ). Loss-of-function mutations in the TAZ gene are responsible for the Barth syndrome (BTHS), a rare X-linked cardiomyopathy, presumably because of a diminished OXPHOS capacity. Herein we show that a partial inhibition of cytosolic protein synthesis, either chemically with the use of cycloheximide or by specific genetic mutations, fully restores biogenesis and the activity of the oxidative phosphorylation system in a yeast BTHS model (taz1Δ). Interestingly, the defaults in CL were not suppressed, indicating that they are not primarily responsible for the OXPHOS deficiency in taz1Δ yeast. Low concentrations of cycloheximide in the picomolar range were beneficial to TAZ-deficient HeLa cells, as evidenced by the recovery of a good proliferative capacity. These findings reveal that a diminished capacity of CL remodeling deficient cells to preserve protein homeostasis is likely an important factor contributing to the pathogenesis of BTHS. This in turn, identifies cytosolic translation as a potential therapeutic target for the treatment of this disease.

Liver X Receptor exerts a protective effect against the oxidative stress in the peripheral nerve

Reactive oxygen species (ROS) modify proteins and lipids leading to deleterious outcomes. Thus, maintaining their homeostatic levels is vital. This study highlights the endogenous role of LXRs (LXRα and β) in the regulation of oxidative stress in peripheral nerves. We report that the genetic ablation of both LXR isoforms in mice (LXRdKO) provokes significant locomotor defects correlated with enhanced anion superoxide production, lipid oxidization and protein carbonylation in the sciatic nerves despite the activation of Nrf2-dependant antioxidant response. Interestingly, the reactive oxygen species scavenger N-acetylcysteine counteracts behavioral, electrophysical, ultrastructural and biochemical alterations in LXRdKO mice. Furthermore, Schwann cells in culture pretreated with LXR agonist, TO901317, exhibit improved defenses against oxidative stress generated by tert-butyl hydroperoxide, implying that LXRs play an important role in maintaining the redox homeostasis in the peripheral nervous system. Thus, LXR activation could be a promising strategy to protect from alteration of peripheral myelin resulting from a disturbance of redox homeostasis in Schwann cell.

Paraquat Induces Peripheral Myelin Disruption and Locomotor Defects: Crosstalk with LXR and Wnt Pathways

AIMS

Paraquat (PQT), a redox-active herbicide, is a free radical-producing molecule, causing damage particularly to the nervous system; thus, it is employed as an animal model for Parkinson's disease. However, its impact on peripheral nerve demyelination is still unknown. Our aim is to decipher the influence of PQT-induced reactive oxygen species (ROS) production on peripheral myelin.

RESULTS

We report that PQT provokes severe locomotor and sensory defects in mice. PQT elicited an oxidative stress in the nerve, resulting in an increase of lipid peroxidation and protein carbonylation, despite the induction of nuclear factor erythroid 2-related factor 2 (Nrf2)-dependent antioxidant defenses. We observed a dramatic disorganization of myelin sheaths in the sciatic nerves, dysregulation of myelin gene expression, and aggregation of myelin proteins, a hallmark of demyelination. PQT altered myelin gene expression via liver X receptor (LXR) signaling, a negative regulator of peripheral myelin gene expression through its dialog with the Wnt/β-catenin pathway. PQT prevented β-catenin binding on myelin gene promoters, resulting in the inhibition of Wnt/β-catenin-dependent myelin gene expression. Wnt pathway activation by LiCl dampened the deleterious effects of PQT. LiCl blocked PQT-induced oxidative stress and reduced Schwann cell death. LiCl+PQT-treated mice had normal sensorimotor behaviors and a usual nerve structure.

INNOVATION

We reveal that PQT damages the sciatic nerve by generating an oxidative stress, dysregulating LXR and Wnt/β-catenin pathways. The activation of Wnt signaling by LiCl reduced the deleterious effects of PQT on the nerve.

CONCLUSION

We demonstrate that PQT instigates peripheral nerve demyelinating neuropathies by enhancing ROS production and deregulating LXR and Wnt pathways. Stimulating Wnt pathway could be a therapeutic strategy for neuropathy treatment. Antioxid. Redox Signal. 27, 168-183.

Synergistic cellular effects including mitochondrial destabilization, autophagy and apoptosis following low-level exposure to a mixture of lipophilic persistent organic pollutants

Humans are exposed to multiple exogenous environmental pollutants. Many of these compounds are parts of mixtures that can exacerbate harmful effects of the individual mixture components. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), is primarily produced via industrial processes including incineration and the manufacture of herbicides. Both endosulfan and TCDD are persistent organic pollutants which elicit cytotoxic effects by inducing reactive oxygen species generation. Sublethal concentrations of mixtures of TCDD and endosulfan increase oxidative stress, as well as mitochondrial homeostasis disruption, which is preceded by a calcium rise and, in fine, induce cell death. TCDD+Endosulfan elicit a complex signaling sequence involving reticulum endoplasmic destalilization which leads to Ca2+ rise, superoxide anion production, ATP drop and late NADP(H) depletion associated with a mitochondrial induced apoptosis concomitant early autophagic processes. The ROS scavenger, N-acetyl-cysteine, blocks both the mixture-induced autophagy and death. Calcium chelators act similarly and mitochondrially targeted anti-oxidants also abrogate these effects. Inhibition of the autophagic fluxes with 3-methyladenine, increases mixture-induced cell death. These findings show that subchronic doses of pollutants may act synergistically. They also reveal that the onset of autophagy might serve as a protective mechanism against ROS-triggered cytotoxic effects of a cocktail of pollutants in Caco-2 cells and increase their tumorigenicity.

Optimization of pegylated iron oxide nanoplatforms for antibody coupling and bio-targeting

PEGylation has been established as a valuable strategy to minimize nanoparticle clearance by the reticulo-endothelial system due to hydrophilicity and steric repulsion of PEG chains. In this study we functionalized superparamagnetic iron oxide nanoparticle surface with two PEG differing in their length (n = 23 and 44) and terminal functionality, COOH and CH₃. By varying the ratio of the two different PEG, we optimized the molecular architecture of the nanoplatform to obtain maximum stability and low toxicity under physiological conditions. The best nanoplatform was evaluated as MRI contrast for mouse brain vascularization imaging at 7 T. The carboxylic acid functions of the nanoplatform were used to covalently bind an antibody, Ab. This antibody, labeled with a fluorophore, targets the ET_A receptor, a G-protein-coupled receptor involved in the endothelin axis and overexpressed in various solid tumours, including ovarian, prostate, colon, breast, bladder and lung cancers. In vitro studies, performed by flow cytometry and magnetic quantification, showed the targeting efficiency of the Ab-nanoplatforms. Clearly, an imaging tracer for cancer diagnosis from a bimodal contrast agent (fluorescence and MRI) was thus obtained.

Antioxidative Theranostic Iron Oxide Nanoparticles toward Brain Tumors Imaging and ROS Production

Gliomas are the most common primary brain tumor in humans. To date, the only treatment of care consists of surgical removal of the tumor bulk, irradiation, and chemotherapy, finally resulting in a very poor prognosis due to the lack of efficiency in diagnostics. In this context, nanomedicine combining both diagnostic and magnetic resonance imaging (MRI) and therapeutic applications is a relevant strategy referred to theranostic. Magnetic nanoparticles (NP) are excellent MRI contrast agents because of their large magnetic moment, which induces high transverse relaxivity (r₂) characteristic and increased susceptibility effect (T₂*). NP can be also used for drug delivery by coating their surface with therapeutic molecules. Preliminary in vitro studies show the high potential of caffeic acid (CA), a natural polyphenol, as a promising anticancer drug due to its antioxidant, anti-inflammatory, and antimetastatic properties. In this study, the antioxidative properties of iron oxide NP functionalized with caffeic acid (γFe₂O₃@CA NP) are investigated in vitro on U87-MG brain cancer cell lines. After intravenous injection of these NP in mice bearing a U87 glioblastoma, a negative contrast enhancement was specifically observed on 11.7 T MRI images in cancerous tissue, demonstrating a passive targeting of the tumor with these nanoplatforms.

Barth Syndrome: From Mitochondrial Dysfunctions Associated with Aberrant Production of Reactive Oxygen Species to Pluripotent Stem Cell Studies

Mutations in the gene encoding the enzyme tafazzin, TAZ, cause Barth syndrome (BTHS). Individuals with this X-linked multisystem disorder present cardiomyopathy (CM) (often dilated), skeletal muscle weakness, neutropenia, growth retardation, and 3-methylglutaconic aciduria. Biopsies of the heart, liver and skeletal muscle of patients have revealed mitochondrial malformations and dysfunctions. It is the purpose of this review to summarize recent results of studies on various animal or cell models of Barth syndrome, which have characterized biochemically the strong cellular defects associated with TAZ mutations. Tafazzin is a mitochondrial phospholipidlysophospholipid transacylase that shuttles acyl groups between phospholipids and regulates the remodeling of cardiolipin (CL), a unique inner mitochondrial membrane phospholipid dimer consisting of two phosphatidyl residues linked by a glycerol bridge. After their biosynthesis, the acyl chains of CLs may be modified in remodeling processes involving up to three different enzymes. Their characteristic acyl chain composition depends on the function of tafazzin, although the enzyme itself surprisingly lacks acyl specificity. CLs are crucial for correct mitochondrial structure and function. In addition to their function in the basic mitochondrial function of ATP production, CLs play essential roles in cardiac function, apoptosis, autophagy, cell cycle regulation and Fe-S cluster biosynthesis. Recent developments in tafazzin research have provided strong insights into the link between mitochondrial dysfunction and the production of reactive oxygen species (ROS). An important tool has been the generation of BTHS-specific induced pluripotent stem cells (iPSCs) from BTHS patients. In a complementary approach, disease-specific mutations have been introduced into wild-type iPSC lines enabling direct comparison with isogenic controls. iPSC-derived cardiomyocytes were then characterized using biochemical and classical bioenergetic approaches. The cells are tested in a "heart-on-chip" assay to model the pathophysiology in vitro, to characterize the underlying mechanism of BTHS deriving from TAZ mutations, mitochondrial deficiencies and ROS production and leading to tissue defects, and to evaluate potential therapies with the use of mitochondrially targeted antioxidants.

Curcumin induces crosstalk between autophagy and apoptosis mediated by calcium release from the endoplasmic reticulum, lysosomal destabilization and mitochondrial events

Curcumin, a major active component of turmeric (Curcuma longa, L.), has anticancer effects. In vitro studies suggest that curcumin inhibits cancer cell growth by activating apoptosis, but the mechanism underlying these effects is still unclear. Here, we investigated the mechanisms leading to apoptosis in curcumin-treated cells. Curcumin induced endoplasmic reticulum stress causing calcium release, with a destabilization of the mitochondrial compartment resulting in apoptosis. These events were also associated with lysosomal membrane permeabilization and of caspase-8 activation, mediated by cathepsins and calpains, leading to Bid cleavage. Truncated tBid disrupts mitochondrial homeostasis and enhance apoptosis. We followed the induction of autophagy, marked by the formation of autophagosomes, by staining with acridine orange in cells exposed curcumin. At this concentration, only the early events of apoptosis (initial mitochondrial destabilization with any other manifestations) were detectable. Western blotting demonstrated the conversion of LC3-I to LC3-II (light chain 3), a marker of active autophagosome formation. We also found that the production of reactive oxygen species and formation of autophagosomes following curcumin treatment was almost completely blocked by N-acetylcystein, the mitochondrial specific antioxidants MitoQ10 and SKQ1, the calcium chelators, EGTA-AM or BAPTA-AM, and the mitochondrial calcium uniporter inhibitor, ruthenium red. Curcumin-induced autophagy failed to rescue all cells and most cells underwent type II cell death following the initial autophagic processes. All together, these data imply a fail-secure mechanism regulated by autophagy in the action of curcumin, suggesting a therapeutic potential for curcumin. Offering a novel and effective strategy for the treatment of malignant cells.

The iron component of particulate matter is antiapoptotic: A clue to the development of lung cancer after exposure to atmospheric pollutants?

The classification of outdoor air pollution as carcinogenic for humans strengthens the increasing concern about particulate matter (PM). We previously demonstrated that PM exposure produces an antiapoptotic effect resulting from polycyclic aromatic hydrocarbons (PAH) and water-soluble components. In this study, we investigated transition metallic compounds, particularly iron, in order to decipher their underlying molecular mechanisms that prevent apoptosis. Human bronchial epithelial cells were exposed for 4 h to different PM samples with established antiapoptotic effect (e.g. PM-AW) or not (e.g. PM-VS) or to their metallic components (Fe, Mn, Zn and Al) before apoptosis induction by the calcium ionophore A23187 or Staurosporine. PM-AW, Fe, Mn and Al significantly reduced induced apoptosis. The antiapoptotic effect of Fe was enhanced by benzo(a)pyrene, a typical PAH compound, but was totally reversed by the iron chelator, deferiprone. Furthermore, particles and iron triggered cellular ROS generation and prevented the depletion in glutathione levels observed during A23187-induced apoptosis. In contrast to benzo(a)pyrene, PM-AW and Fe rapidly activated NRF2, subsequently upregulated several target genes (HO1, NQO1 and GPX1) and modulated some genes which control cell death (BCL2, BAX and p53). The key role of the NRF2 pathway in the antiapoptotic effect mediated by Fe and PM was demonstrated using siRNA technology. Our results suggest that the iron component participates in the antiapoptotic effect of PM by activating a NRF2-dependent antioxidant process. As resisting to cell death is one of the hallmarks of cancer cells, these findings provide additional clues for understanding the development of lung cancer after atmospheric pollution exposure.

Barth syndrome: cellular compensation of mitochondrial dysfunction and apoptosis inhibition due to changes in cardiolipin remodeling linked to tafazzin (TAZ) gene mutation

Cardiolipin is a mitochondrion-specific phospholipid that stabilizes the assembly of respiratory chain complexes, favoring full-yield operation. It also mediates key steps in apoptosis. In Barth syndrome, an X chromosome-linked cardiomyopathy caused by tafazzin mutations, cardiolipins display acyl chain modifications and are present at abnormally low concentrations, whereas monolysocardiolipin accumulates. Using immortalized lymphoblasts from Barth syndrome patients, we showed that the production of abnormal cardiolipin led to mitochondrial alterations. Indeed, the lack of normal cardiolipin led to changes in electron transport chain stability, resulting in cellular defects. We found a destabilization of the supercomplex (respirasome) I+III2+IVn but also decreased amounts of individual complexes I and IV and supercomplexes I+III and III+IV. No changes were observed in the amounts of individual complex III and complex II. We also found decreased levels of complex V. This complex is not part of the supercomplex suggesting that cardiolipin is required not only for the association/stabilization of the complexes into supercomplexes but also for the modulation of the amount of individual respiratory chain complexes. However, these alterations were compensated by an increase in mitochondrial mass, as demonstrated by electron microscopy and measurements of citrate synthase activity. We suggest that this compensatory increase in mitochondrial content prevents a decrease in mitochondrial respiration and ATP synthesis in the cells. We also show, by extensive flow cytometry analysis, that the type II apoptosis pathway was blocked at the mitochondrial level and that the mitochondria of patients with Barth syndrome cannot bind active caspase-8. Signal transduction is thus blocked before any mitochondrial event can occur. Remarkably, basal levels of superoxide anion production were slightly higher in patients' cells than in control cells as previously evidenced via an increased protein carbonylation in the taz1Δ mutant in the yeast. This may be deleterious to cells in the long term. The consequences of mitochondrial dysfunction and alterations to apoptosis signal transduction are considered in light of the potential for the development of future treatments.

COUP-TFII controls mouse pancreatic β-cell mass through GLP-1-β-catenin signaling pathways

Background

The control of the functional pancreatic β-cell mass serves the key homeostatic function of releasing the right amount of insulin to keep blood sugar in the normal range. It is not fully understood though how β-cell mass is determined.

Methodology/Principal Findings

Conditional chicken ovalbumin upstream promoter transcription factor II (COUP-TFII)-deficient mice were generated and crossed with mice expressing Cre under the control of pancreatic duodenal homeobox 1 (pdx1) gene promoter. Ablation of COUP-TFII in pancreas resulted in glucose intolerance. Beta-cell number was reduced at 1 day and 3 weeks postnatal. Together with a reduced number of insulin-containing cells in the ductal epithelium and normal β-cell proliferation and apoptosis, this suggests decreased β-cell differentiation in the neonatal period. By testing islets isolated from these mice and cultured β-cells with loss and gain of COUP-TFII function, we found that COUP-TFII induces the expression of the β-catenin gene and its target genes such as cyclin D1 and axin 2. Moreover, induction of these genes by glucagon-like peptide 1 (GLP-1) via β-catenin was impaired in absence of COUP-TFII. The expression of two other target genes of GLP-1 signaling, GLP-1R and PDX-1 was significantly lower in mutant islets compared to control islets, possibly contributing to reduced β-cell mass. Finally, we demonstrated that COUP-TFII expression was activated by the Wnt signaling-associated transcription factor TCF7L2 (T-cell factor 7-like 2) in human islets and rat β-cells providing a feedback loop.

Conclusions/Significance

Our findings show that COUP-TFII is a novel component of the GLP-1 signaling cascade that increases β-cell number during the neonatal period. COUP-TFII is required for GLP-1 activation of the β-catenin-dependent pathway and its expression is under the control of TCF7L2.

Mechanistic issues of the interaction of the hairpin-forming domain of tBid with mitochondrial cardiolipin

BACKGROUND

The pro-apoptotic effector Bid induces mitochondrial apoptosis in synergy with Bax and Bak. In response to death receptors activation, Bid is cleaved by caspase-8 into its active form, tBid (truncated Bid), which then translocates to the mitochondria to trigger cytochrome c release and subsequent apoptosis. Accumulating evidence now indicate that the binding of tBid initiates an ordered sequences of events that prime mitochondria from the action of Bax and Bak: (1) tBid interacts with mitochondria via a specific binding to cardiolipin (CL) and immediately disturbs mitochondrial structure and function idependently of its BH3 domain; (2) Then, tBid activates through its BH3 domain Bax and/or Bak and induces their subsequent oligomerization in mitochondrial membranes. To date, the underlying mechanism responsible for targeting tBid to mitochondria and disrupting mitochondrial bioenergetics has yet be elucidated.

PRINCIPAL FINDINGS

The present study investigates the mechanism by which tBid interacts with mitochondria issued from mouse hepatocytes and perturbs mitochondrial function. We show here that the helix alphaH6 is responsible for targeting tBid to mitochondrial CL and disrupting mitochondrial bioenergetics. In particular, alphaH6 interacts with mitochondria through electrostatic interactions involving the lysines 157 and 158 and induces an inhibition of state-3 respiration and an uncoupling of state-4 respiration. These changes may represent a key event that primes mitochondria for the action of Bax and Bak. In addition, we also demonstrate that tBid required its helix alphaH6 to efficiently induce cytochrome c release and apoptosis.

CONCLUSIONS

Our findings provide new insights into the mechanism of action of tBid, and particularly emphasize the importance of the interaction of the helix alphaH6 with CL for both mitochondrial targeting and pro-apoptotic activity of tBid. These support the notion that tBid acts as a bifunctional molecule: first, it binds to mitochondrial CL via its helix alphaH6 and destabilizes mitochondrial structure and function, and then it promotes through its BH3 domain the activation and oligomerization of Bax and/or Bak, leading to cytochrome c release and execution of apoptosis. Our findings also imply an active role of the membrane in modulating the interactions between Bcl-2 proteins that has so far been underestimated.

Cross-talk between oxysterols and glucocorticoids: differential regulation of secreted phopholipase A2 and impact on oligodendrocyte death

Background

Oxysterols are oxidized forms of cholesterol. They have been shown to be implicated in cholesterol turnover, inflammation and in neurodegenerative diseases such as Alzheimer's disease and multiple sclerosis. Glial cells are targets of oxysterols: they inhibit astrocyte proliferation after brain injury, and we have previously shown that 25-hydroxycholesterol (25OH) provokes oligodendrocyte apoptosis and stimulates the expression of sPLA2 type IIA (sPLA2-IIA), which has a protective effect.

Methodology/Principal Findings

As glucocorticoids are well-known for their anti-inflammatory effects, our aim was to understand their direct effects on oxysterol-induced responses in oligodendrocytes (sPLA2-IIA stimulation and apoptosis). We demonstrate that the synthetic glucocorticoid dexamethasone (Dex) abolishes the stimulation of sPLA2-IIA by 25-hydroxycholesterol (25-OH). This inhibition is mediated by the glucocorticoid receptor (GR), which decreases the expression of the oxysterol receptor Pregnane X Receptor (PXR) and interferes with oxysterol signaling by recruiting a common limiting coactivator PGC1α. Consistent with the finding that sPLA2-IIA can partially protect oligodendrocytes against oxysterol-triggered apoptosis, we demonstrate here that the inhibition of sPLA2-IIA by Dex accelerates the apoptotic phenomenon, leading to a shift towards necrosis. We have shown by atomic force microscopy and electron microscopy that 25-OH and Dex alters oligodendrocyte shape and disorganizes the cytoplasm.

Conclusions/Significance

Our results provide a new understanding of the cross-talk between oxysterol and glucocorticoid signaling pathways and their respective roles in apoptosis and oligodendrocyte functions.

25-hydroxycholesterol provokes oligodendrocyte cell line apoptosis and stimulates the secreted phospholipase A2 type IIA via LXR beta and PXR

In several neurodegenerative diseases of the CNS, oligodendrocytes are implicated in an inflammatory process associated with altered levels of oxysterols and inflammatory enzymes such as secreted phospholipase A2 (sPLA2). In view of the scarce literature related to this topic, we investigated oxysterol effects on these myelinating glial cells. Natural oxysterol 25-hydroxycholesterol (25-OH; 1 and 10 microM) altered oligodendrocyte cell line (158N) morphology and triggered apoptosis (75% of apoptosis after 72 h). These effects were mimicked by 22(S)-OH (1 and 10 microM) which does not activate liver X receptor (LXR) but not by a synthetic LXR ligand (T0901317). Therefore, oxysterol-induced apoptosis appears to be independent of LXR. Interestingly, sPLA2 type IIA (sPLA2-IIA) over-expression partially rescued 158N cells from oxysterol-induced apoptosis. In fact, 25-OH, 24(S)-OH, and T0901317 stimulated sPLA2-IIA promoter and sPLA2 activity in oligodendrocyte cell line. Accordingly, administration of T0901317 to mice enhanced sPLA2 activity in brain extracts by twofold. Short interfering RNA strategy allowed to establish that stimulation of sPLA2-IIA is mediated by pregnane X receptor (PXR) at high oxysterol concentration (10 microM) and by LXR beta at basal oxysterol concentration. Finally, GC coupled to mass spectrometry established that oligodendrocytes contain oxysterols and express their biosynthetic enzymes, suggesting that they may act through autocrine/paracrine mechanism. Our results show the diversity of oxysterol signalling in the CNS and highlight the positive effects of the LXR/PXR pathway which may open new perspectives in the treatment of demyelinating and neurodegenerative diseases.

Cardiolipin provides an essential activating platform for caspase-8 on mitochondria

Cardiolipin is a mitochondria-specific phospholipid known to be intimately involved with apoptosis. However, the lack of appropriate cellular models to date restricted analysis of its role in cell death. The maturation of cardiolipin requires the transacylase tafazzin, which is mutated in the human disorder Barth syndrome. Using Barth syndrome patient-derived cells and HeLa cells in which tafazzin was knocked down, we show that cardiolipin is required for apoptosis in the type II mitochondria-dependent response to Fas stimulation. Cardiolipin provides an anchor and activating platform for caspase-8 translocation to, and embedding in, the mitochondrial membrane, where it oligomerizes and is further activated, steps that are necessary for an efficient type II apoptotic response.

Cardiolipin deficiency releases cytochrome c from the inner mitochondrial membrane and accelerates stimuli-elicited apoptosis

Cardiolipin (CL) is a mitochondria-specific phospholipid synthesized by CL synthase (CLS). We describe here a human gene for CLS and its analysis via RNAi knockdown on apoptotic progression. Although mitochondrial membrane potential is unchanged in cells containing only 25% of the normal amount of CL, free cytochrome c (cyt. c) is detected in the intermembrane space and the mitochondria exhibit signs of reorganized cristae. However, the release of cyt. c from the mitochondria still requires apoptotic stimulation. Increased sensitivity to apoptotic signals and accelerated rates of apoptosis are observed in CL-deficient cells, followed by elevated levels of secondary necrosis. Apoptosis is thought to progress via binding of truncated Bid (tBid) to mitochondrial CL, followed by CL oxidation which results in cyt. c release. The exaggerated and accelerated apoptosis observed in CL-deficient cells is matched by an accelerated reduction in membrane potential and increased cyt. c release, but not by decreased tBid binding. This study suggests that the CL/cyt. c relationship is important in apoptotic progression and that regulating CL oxidation or/and deacylation could represent a possible therapeutic target.

Role of cardiolipin on tBid and tBid/Bax synergistic effects on yeast mitochondria

The apoptotic effector Bid regulates cell death at the level of mitochondria. Under its native state, Bid is a soluble cytosolic protein that undergoes proteolysis and yields a 15 kDa-activated form tBid (truncated Bid). tBid translocates to mitochondria and participates in cytochrome c efflux by a still unclear mechanism, some of them at least mediated by Bax. Using mitochondria isolated from wild-type and cardiolipin (CL)-synthase-less yeast strains, we observed that tBid perturbs mitochondrial bioenergetics by inhibiting state-3 respiration and ATP synthesis and that this effect was strictly dependent on the presence of CL. In a second set of experiments, heterologous coexpression of tBid and Bax in wild-type and CL-less yeast strains showed that (i) tBid binding and the subsequent alteration of mitochondrial bioenergetics increased Bax-induced cytochrome c release and (ii) the absence of CL favors Bax effects independently of the presence of t-Bid. These data support recent views suggesting a dual function of CL in mitochondria-dependent apoptosis.

tBid interaction with cardiolipin primarily orchestrates mitochondrial dysfunctions and subsequently activates Bax and Bak

TNFR1/Fas engagement results in the cleavage of cytosolic Bid to truncated Bid (tBid), which translocates to mitochondria. We demonstrate that recombinant tBid induces in vitro immediate destabilization of the mitochondrial bioenergetic homeostasis. These alterations result in mild uncoupling of mitochondrial state-4 respiration, associated with an inhibition the adenosine diphosphate (ADP)-stimulated respiration and phosphorylation rate. tBid disruption of mitochondrial homeostasis was inhibited in mitochondria overexpressing Bcl-2 and Bcl-XL. The inhibition of state-3 respiration is mediated by the reorganization of cardiolipin within the mitochondrial membranes, which indirectly affects the activity of the ADP/ATP translocator. Cardiolipin-deficient yeast mitochondria did not exhibit any respiratory inhibition by tBid, proving the absolute requirement for cardiolipin for tBid binding and activity. In contrast, the wild-type yeast mitochondria underwent a similar inhibition of ADP-stimulated respiration associated with reduced ATP synthesis. These events suggest that mitochondrial lipids rather than proteins are the key determinants of tBid-induced destabilization of mitochondrial bioenergetics.

Siva-1 and an alternative splice form lacking the death domain, Siva-2, similarly induce apoptosis in T lymphocytes via a caspase-dependent mitochondrial pathway

Siva-1 is a death domain-containing proapoptotic protein identified as an intracellular ligand of CD27 and of the glucocorticoid-induced TNFR family-related gene, which are two members of the TNFR family expressed on lymphoid cells. Although Siva-1 expression is up-regulated in multiple pathological processes, little is known about the signaling pathway underlying the Siva-induced apoptosis. In this study, we investigated the mechanism of the proapoptotic activity of Siva-1 and an alternative splice form lacking the death domain of Siva-1, Siva-2, in T lymphocytes in which Siva proteins, CD27, and glucocorticoid-induced TNFR family-related gene are primarily expressed. Overexpression of Siva proteins triggers a typical apoptotic process manifested by cell shrinkage and surface exposure of phosphatidylserine, and confirmed by ultrastructural features. Siva-induced apoptosis is related to the CD27-mediated apoptotic pathway and results in activation of both initiator and effector caspases. This pathway involves a mitochondrial step evidenced by activation of Bid and cytochrome c release, and is modulated by overexpression of Bcl-2 or Bcl-x(L). The determinants for Siva-induced apoptosis are not contained within the death domain found in the central part of Siva-1, but rather in both the N-terminal and C-terminal regions shared by both Siva proteins. The N-terminal region also participates in the translocation of both Siva proteins into the nuclear compartment. These results indicate that Siva-1 and Siva-2 mediate apoptosis in T lymphocytes via a caspase-dependent mitochondrial pathway that likely involves both cytoplasmic and nuclear events.

Bcl-2 and Bax modulate adenine nucleotide translocase activity

Bcl-2 is a prosurvival factor that reportedly prevents the nonspecific permeabilization of mitochondrial membranes, yet enhances specific ADP/ATP exchange by these organelles. Here, we show that Bcl-2 enhances the ADP/ATP exchange in proteoliposomes containing the purified adenine nucleotide translocase (ANT) in isolated mitochondria and mitoplasts, as well as in intact cells in which mitochondrial matrix ATP was monitored continuously using a specific luciferase-based assay system. Conversely, Bax, which displaces Bcl-2 from ANT in apoptotic cells, inhibits ADP/ATP exchange through a direct action on ANT. The Bax-mediated inhibition of ADP/ATP exchange can be separated from Bax-stimulated formation of nonspecific pores by ANT. Chemotherapy-induced apoptosis caused an inhibition of ANT activity, which preceded the loss of the mitochondrial transmembrane potential and could be prevented by overexpression of Bcl-2. These data are compatible with a model of mitochondrial apoptosis regulation in which ANT interacts with either Bax or Bcl-2, which both influence ANT function in opposing manners. Bcl-2 would maintain the translocase activity at high levels, whereas Bax would inhibit the translocase function of ANT.

On the evolution of programmed cell death: apoptosis of the unicellular eukaryote Leishmania major involves cysteine proteinase activation and mitochondrion permeabilization

Leishmania major is a protozoan parasite from one of the most ancient phylogenic branches of unicellular eukaryotes, and containing only one giant mitochondrion. Here we report that staurosporine, that induces apoptosis in all mammalian nucleated cells, also induces in L. major a death process with several cytoplasmic and nuclear features of apoptosis, including cell shrinkage, phosphatidyl serine exposure, maintenance of plasma membrane integrity, mitochondrial transmembrane potential (DeltaPsim) loss and cytochrome c release, nuclear chromatin condensation and fragmentation, and DNA degradation. Nuclear apoptosis-like features were prevented by cysteine proteinase inhibitors, and cell free assays using dying L. major cytoplasmic extracts indicated that the cysteine proteinases involved (i) also induced nuclear apoptosis-like features in isolated mammalian nuclei, and (ii) shared at least two nuclear substrates, but no cleavage site preference, with human effector caspases. Finally, isolated L. major mitochondria released cytochrome c and cysteine proteinases with nuclear pro-apoptotic activity when incubated with human recombinant Bax, even (although much less efficiently) when Bax was deleted of its transmembrane domain required for insertion in mitochondrial outermembranes, implying that L. major mitochondrion may express proteins able to interact with Bax. The recruitment of cysteine proteinases and mitochondria to the cell death machinery may be of very ancient evolutionary origin. Alternately, host/parasite interactions may have exerted selective pressures on the cell death phenotype of kinetoplastid parasites, resulting in the more recent emergence of an apoptotic machinery through a process of convergent evolution.

On the evolutionary conservation of the cell death pathway: mitochondrial release of an apoptosis-inducing factor during Dictyostelium discoideum cell death

Mitochondria play a pivotal role in apoptosis in multicellular organisms by releasing apoptogenic factors such as cytochrome c that activate the caspases effector pathway, and apoptosis-inducing factor (AIF) that is involved in a caspase-independent cell death pathway. Here we report that cell death in the single-celled organism Dictyostelium discoideum involves early disruption of mitochondrial transmembrane potential (DeltaPsim) that precedes the induction of several apoptosis-like features, including exposure of the phosphatidyl residues at the external surface of the plasma membrane, an intense vacuolization, a fragmentation of DNA into large fragments, an autophagy, and the release of apoptotic corpses that are engulfed by neighboring cells. We have cloned a Dictyostelium homolog of mammalian AIF that is localized into mitochondria and is translocated from the mitochondria to the cytoplasm and the nucleus after the onset of cell death. Cytoplasmic extracts from dying Dictyostelium cells trigger the breakdown of isolated mammalian and Dictyostelium nuclei in a cell-free system, and this process is inhibited by a polyclonal antibody specific for Dictyostelium discoideum apoptosis-inducing factor (DdAIF), suggesting that DdAIF is involved in DNA degradation during Dictyostelium cell death. Our findings indicate that the cell death pathway in Dictyostelium involves mitochondria and an AIF homolog, suggesting the evolutionary conservation of at least part of the cell death pathway in unicellular and multicellular organisms.

Inhibition of multicellular development switches cell death of Dictyostelium discoideum towards mammalian-like unicellular apoptosis

The multicellular development of the single celled eukaryote Dictyostelium discoideum is induced by starvation and consists of initial aggregation of the isolated amoebae, followed by their differentiation into viable spores and dead stalk cells. These stalk cells retain their structural integrity inside a stalk tube that support the spores in the fruiting body. Terminal differentiation into stalk cells has been shown to share several features with programmed cell death (Cornillon et al. (1994), J. Cell Sci. 107, 2691-2704). Here we report that, in the absence of aggregation and differentiation, D. discoideum can undergo another form of programmed cell death that closely resembles apoptosis of most mammalian cells, involves loss of mitochondrial transmembrane potential, phosphatidylserine surface exposure, and engulfment of dying cells by neighboring D. discoideum cells. This death has been studied by various techniques (light microscopy and scanning or transmission electron microscopy, flow cytometry, DNA electrophoresis), in two different conditions inhibiting D. discoideum multicellular development. The first one, corresponding to an induced unicellular cell death, was obtained by starving the cells in a "conditioned" cell-free buffer, prepared by previous starvation of another D. discoideum cell population in potassium phosphate buffer (pH 6.8). The second one, corresponding to death of D. discoideum after axenic growth in suspension, was obtained by keeping stationary cells in their culture medium. In both cases of these unicellular-specific cell deaths, microscopy revealed morphological features known as hallmarks of apoptosis for higher eukaryotic cells and apoptosis was further corroborated by flow cytometry. The occurrence in D. discoideum of programmed cell death with two different phenotypes, depending on its multicellular or unicellular status, is further discussed.

Oxidation of pyridine nucleotides during Fas- and ceramide-induced apoptosis in Jurkat cells: correlation with changes in mitochondria, glutathione depletion, intracellular acidification and caspase 3 activation

Jurkat T cells showed a major, early decrease in blue autofluorescence in response to Fas/Apo-1/CD95 cross-linking or stimulation with cell-permeant ceramide. This indicates the oxidation/depletion of NADH or NADPH before the onset of apoptosis. Kinetic studies, cytofluorimetric multiparameter analyses and cell sorting experiments indicated a close temporal relationship between NAD(P)H oxidation/depletion and the dissipation of the mitochondrial transmembrane potential (DeltaPsi(m)). In contrast, NAD(P)H depletion was detected well before several other changes associated with late apoptosis, including enhanced superoxide generation, phosphatidylserine exposure on the cell surface, loss of cytosolic K(+), decreased cytoplasmic pH, nuclear DNA fragmentation, cell shrinkage, loss of viability and the appearance of the mitochondrial antigen APO2.7. Full activation of caspase 9 and caspase 3 appeared to be correlated with the appearance of superoxide anions in the mitochondria, and followed the drop in NADPH. Overexpression of the apoptosis-inhibitory proto-oncogene Bcl-2, which encodes an inhibitor of the mitochondrial permeability transition (PT) pore, delayed both the DeltaPsi(m) disruption and the depletion of NAD(P)H. Similar effects were observed with the pharmacological PT pore inhibitors, bongkrekic acid and cyclosporin A. Thus there appears to be a close functional relationship between mitochondrial and cellular redox changes during early apoptosis; events that are inhibited by Bcl-2.

Over-expression of Bcl-2 does not protect cells from hypericin photo-induced mitochondrial membrane depolarization, but delays subsequent events in the apoptotic pathway

Hypericin (HY) is a powerful photo-inducer of apoptosis in Jurkat cells as measured by caspase-3 activation, cell shrinkage, phosphatidylserine (PS) exposure and the appearance of hypoploid DNA. These processes are preceded by rapid Bcl-2-independent mitochondrial transmembrane depolarization and a drop in cytoplasmic pH. Pre-incubation of cells with inhibitors of the mitochondrial permeability transition pore, such as cyclosporin A or bongkrekic acid, does not protect cells from mitochondrial membrane potential (deltapsim) decrease. However, monitoring of mitochondrial entrapped calcein by confocal fluorescence imaging gives clear evidence of HY photo-induced mitochondrial permeability. This should be considered as the result of a non-specific alteration of mitochondrial membrane integrity brought about by lipid peroxidation. Nevertheless, synthesis of the anti-apoptotic protein Bcl-2 appears to delay the subsequent time course of PS exposure and to reduce caspase-3 activation and the fraction of cells which become hypoploid. We interpret this partially protective effect as the consequence of a direct interaction of Bcl-2 with cytosolic cytochrome c previously released from mitochondria upon deltapsim decrease and/or of Bcl-2 inhibition of the deleterious retro-effect of caspase-3 on the mitochondrial permeability transition pore and/or the mitochondrial membrane components.

Lonidamine triggers apoptosis via a direct, Bcl-2-inhibited effect on the mitochondrial permeability transition pore

The molecular mode of action of lonidamine, a therapeutic agent employed in cancer chemotherapy, has been elusive. Here we provide evidence that lonidamine (LND) acts on mitochondria to induce apoptosis. LND provokes a disruption of the mitochondrial transmembrane potential which precedes signs of nuclear apoptosis and cytolysis. The mitochondrial and cytocidal effects of LND are not prevented by inhibitors of caspases or of mRNA or protein synthesis. However, they are prevented by transfection-enforced overexpression of Bcl-2, an oncoprotein which inhibits apoptosis by stabilizing the mitochondrial membrane barrier function. Accordingly, the cell death-inducing effect of LND is amplified by simultaneous addition of PK11195, an isoquinoline ligand of the peripheral benzodiazepine receptor which antagonizes the cytoprotective effect of Bcl-2. When added to isolated nuclei, LND fails to provoke DNA degradation unless mitochondria are added simultaneously. In isolated mitochondria, LND causes the dissipation of the mitochondrial inner transmembrane potential and the release of apoptogenic factors capable of inducing nuclear apoptosis in vitro. Thus the mitochondrion is the subcellular target of LND. All effects of LND on isolated mitochondria are counteracted by cyclosporin A, an inhibitor of the mitochondrial PT pore. We therefore tested the effect of LND on the purified PT pore reconstituted into liposomes. LND permeabilizes liposomal membranes containing the PT pore. This effect is prevented by addition of recombinant Bcl-2 protein but not by a mutant Bcl-2 protein that has lost its apoptosis-inhibitory function. Altogether these data indicate that LND represents a novel type of anti-cancer agent which induces apoptosis via a direct effect on the mitochondrial PT pore.

Caspases disrupt mitochondrial membrane barrier function

Mitochondrial intermembrane proteins including cytochrome c are known to activate caspases. Accordingly, a disruption of the mitochondrial membrane barrier function with release of cytochrome into the cytosol has been shown to precede caspase activation in a number of different models of apoptosis. Here, we addressed the question of whether caspases themselves can affect mitochondrial membrane function. Recombinant caspases were added to purified mitochondria and were found to affect the permeability of both mitochondrial membranes. Thus, caspases cause a dissipation of the mitochondrial inner transmembrane potential. In addition, caspases cause intermembrane proteins including cytochrome c and AIF (apoptosis-inducing factor) to be released through the outer mitochondrial membrane. These observations suggest that caspases and mitochondria can engage in a circular self-amplification loop. An increase in mitochondrial membrane permeability would cause the release of caspase activators, and caspases, once activated, would in turn increase the mitochondrial membrane permeability. Such a self-amplifying system could accelerate the apoptotic process and/or coordinate the apoptotic response between different mitochondria within the same cell.

Disruption of the outer mitochondrial membrane as a result of large amplitude swelling: the impact of irreversible permeability transition

Upon induction of permeability transition with different agents (Ca2+, tert-butyl hydroperoxide, atractyloside), mouse hepatocyte mitochondria manifest a disruption of outer membrane integrity leading to the release of cytochrome c and apoptosis-inducing factor (AIF), two proteins which are involved in programmed cell death (apoptosis). Chelation of Ca2+ shortly (within 2 min) after its addition to isolated mitochondria reestablished the mitochondrial transmembrane potential (deltapsi(m)), prevented induction of large amplitude swelling and release of both cytochrome c and AIF. In contrast, late Ca2+ chelation (10 min after addition of Ca2+) failed to affect these parameters. Cytochrome c appears to be released through a mechanically damaged outer mitochondrial membrane rather than via a specific release mechanism. These findings clarify the mechanisms through which irreversible permeability transition occurs with subsequent large amplitude swelling culminating in the release of intermembrane proteins from mitochondria. Moreover, they confirm the hypothesis formulated by Skulachev [FEBS Lett. 397 (1996) 7-10 and Q. Rev. Biophys. 29 (1996) 169-2021 linking permeability transition to activation of the apoptogenic catabolic enzymes.

Low catalase activity in xeroderma pigmentosum fibroblasts and SV40-transformed human cell lines is directly related to decreased intracellular levels of the cofactor, NADPH

We have previously shown that fibroblasts from ultra-violet (UV) hypersensitive xeroderma pigmentosum patients (XP) are markedly deficient in catalase activity resulting in high intracellular levels of hydrogen peroxide (H2O2) following UV irradiation. No direct correlation between catalase activity and repair ability was found since XP variant cells which are proficient in nucleotide excision repair (NER) showed activities as low as those found in NER deficient classical XP groups A and D. However, in contrast to the skin cancer prone XP patients, another NER deficient syndrome, trichothiodystrophy (TTD), which does not exhibit any cancer predisposition, was found to present normal catalase activity. Moreover, it was found that a variety of SV40 transformed human cell lines also showed decreased catalase activities. Our previous data showed that a molecular analysis of the normal, XP, TTD or transformed human fibroblast cell lines did not reveal any differences in levels of catalase transcription or amount of catalase protein subunits. These results incited us to examine the structure/function relationship of the tetrameric active enzyme form of catalase (which is the only one able to carry out H2O2 dismutation) with its cofactor NADPH. In the present study, we have measured the effects on catalase activity after adding NADPH either to acellular extracts or during cell culture of the different cell types. The NADPH levels were also quantified directly in intact cells using flow cytometry. Our results show a clear relationship between low catalase activity and striking decrease in intracellular NADPH levels.

Implication of mitochondria in apoptosis

The induction phase of programmed cell death (PCD) or apoptosis is characterized by an extreme heterogeneity of potential PCD-triggering signal transduction pathways. During the subsequent effector phase, the numerous PCD-inducing stimuli converge into a few stereotypical pathways and cells pass a 'point of no return', thus becoming irreversibly committed to death. Evidence is accumulating that cytoplasmic structures, including mitochondria, participate in the critical effector stage and that alterations usually considered to define apoptosis, as nuclear chromatolysis and cytolysis, have to be ascribed to the late degradation phase. We and others have recently shown that nuclear features of apoptosis are preceded by alterations in mitochondrial function and structure. The importance of these alterations for the apoptotic process and also the possible link between, these observations, the permeability transition pore and the programmed cell death, are discussed.

A cytofluorometric assay of nuclear apoptosis induced in a cell-free system: application to ceramide-induced apoptosis

Purified nuclei exposed to apoptogenic factors in vitro undergo morphological and biochemical changes in chromatin organization. Most cell-free models of nuclear apoptosis are based on the quantitation of endonuclease-mediated DNA fragmentation on agarose gels or on the changes of nuclear morphology revealed by the DNA-intercalating fluorochrome 4'-6-diamidino-2-phenylindole dihydrochloride. In this work we develop a cytofluorometric system for the accurate quantitation of nuclear DNA loss. This system has been used to determine the conditions of nuclear apoptosis induced by apoptosis-inducing factor (AIF) contained in the supernatant of mitochondria induced to undergo permeability transition. AIF can provoke significant nuclear DNA loss in < or = 5 min, acts over a wide pH range (pH 6 to 9), and resists cysteine protease inhibitors such as iodoacetamide and N-ethylmaleimide. Moreover, we applied this system to the question of how the proapoptotic second messenger ceramide would induce apoptosis in vitro: via a direct effect on nuclei, a direct effect on mitochondria, or via indirect mechanisms? Our data indicate that ceramide has to activate yet unknown cytosolic effectors that, in the presence of mitochondria, can induce nuclear apoptosis in vitro.

Implication of mitochondria in apoptosis

The induction phase of programmed cell death (PCD) or apoptosis is characterized by an extreme heterogeneity of potential PCD-triggering signal transduction pathways. During the subsequent effector phase, the numerous PCD-inducing stimuli converge into a few stereotypical pathways and cells pass a 'point of no return', thus becoming irreversibly committed to death. Evidence is accumulating that cytoplasmic structures, including mitochondria, participate in the critical effector stage and that alterations usually considered to define apoptosis, as nuclear chromatolysis and cytolysis, have to be ascribed to the late degradation phase. We and others have recently shown that nuclear features of apoptosis are preceded by alterations in mitochondrial function and structure. The importance of these alterations for the apoptotic process and also the possible link between, these observations, the permeability transition pore and the programmed cell death, are discussed.

Induction of mitochondrial dysfunction and apoptosis in HeLa cells by bis-pyridinium oximes, a newly synthesized family of lipophilic biscations

When tested on HeLa cells, bis-pyridinium oximes (BPO), a family of newly synthesized molecules whose charged pyridinium moieties are linked by a linear polymethylene chain of variable length (N = 3 to 12) have been shown to possess an inhibitory effect on cell growth and finally to provoke cell death. BPO-affected cells displayed reduced mitochondrial oxygen consumption and ATP stores and were blocked in the G1 phase of the cell cycle. Mitochondrial membrane potential, as assayed with the dye 3,3'-diexyloxacarbocyanine iodide [DiOC6(3)], increased in BPO-treated cells with time of exposure. Cell growth inhibition as well mitochondrial dysfunction were observed only with derivatives having a long polymethylene linking chain (N > or = 6). Furthermore, the concentration of the compound eliciting such effects was inversely related to the number of methylene groups in the linking chain. None of the BPO with N = 6 to 12 modified the mitochondrial DNA content, relative to the nuclear DNA content. In BPO (N = 8 and N = 12)-treated cells, chromatin fragmentation and internucleosomal DNA cleavage occurred massively, indicating that the death mode induced by these compounds is apoptosis. The possible pathway of action and the potential pharmacological interest of these compounds are discussed.

In vivo interference of paromomycin with mitochondrial activity of Leishmania

Paromomycin is an aminocyclitol aminoglycoside antibiotic used for the treatment of leishmaniasis. In view of the central role of mitochondria in cellular energetics and metabolism, its effect on in vivo mitochondrial activities of Leishmania donovani promastigotes-the parasite flagellate form-was investigated. The approach used flow cytometry, amperometric measure of O2 consumption, and, as a global estimate of mitochondrial dehydrogenases, thiazolyl blue reduction (MTT test); some in vitro controls were also made. When added to promastigote cultures for 24-72 h at 150-200 microM (= LC50), paromomycin doubled the generation time, inhibited respiration, and lowered its associated electric potential difference across mitochondrial membranes, as measured by rhodamine 123 fluorescence. The chemical analogue neomycin was ineffective. Furthermore, the in vivo mitochondrial dehydrogenase activities were lower, seemingly because of the shortage of respiratory substrates. Indeed, succinate addition to paromomycin-treated cultures partly restored mitochondrial membrane potential. However, no immediate effect of paromomycin on respiration was observed, neither inhibition of redox chain nor increase of membrane permeability (uncoupling). It is proposed that paromomycin acts at a metabolic level upstream of the respiratory chain itself. This would have the observed delayed consequence because the cell energy supply would progressively decline since it depends upon the proton gradient-viz., membrane potential-generated by respiration. In conclusion, paromomycin is an antibiotic affecting the cell's energetic metabolism; the respiratory dysfunction it induces may be a crucial aspect of its action against Leishmania and possibly other cells.

Mitochondrial implication in accidental and programmed cell death: apoptosis and necrosis

Both physiological cell death (apoptosis) and at least some cases of accidental cell death (necrosis) involve a two-step-process. At first level, numerous physiological or pathological stimuli can trigger mitochondrial permeability transition which constitutes a rate-limiting event and initiates the common phase of the death process. Mitochondrial permeability transition (PT) involves the formation of proteaceous, regulated pores, probably by apposition of inner and outer mitochondrial membrane proteins which cooperate to form the mitochondrial PT pore complex. Inhibition of PT by pharmacological intervention on mitochondrial structures or mitochondrial expression of the apoptosis-inhibitory oncoprotein Bcl-2 thus can prevent cell death. At a second level, the consequences of mitochondrial dysfunction (collapse of the mitochondrial transmembrane potential, uncoupling of the respiratory chain, hyperproduction of superoxide anions, disruption of mitochondrial biogenesis, outflow of matrix calcium and glutathione, and release of soluble intermembrane proteins) can entail a biogenetic catastrophe culminating in the disruption of plasma membrane integrity (necrosis) and/or the activation and action of apoptogenic proteases with secondary endonuclease activation and consequent oligonucleosomal DNA fragmentation (apoptosis). The acquisition of the biochemical and ultrastructural features of apoptosis critically relies on the liberation of apoptogenic proteases or protease activators from the mitochondrial intermembrane space. This scenario applies to very different models of cell death. The notion that mitochondrial events control cell death has major implications for the development of death-inhibitory drugs.

Mitochondria and programmed cell death: back to the future

Programmed cell death, or apoptosis, has in the past few years undoubtedly become one of the most intensively investigated biological processes. However, fundamental questions concerning the molecular and biochemical mechanisms remain to be elucidated. The central question concerns the biochemical steps shared by the numerous death induction pathways elicited by different stimuli. Heterogeneous death signals precede a common effector phase during which cells pass a threshold of 'no return' and are engaged in a degradation phase where they acquire the typical onset of late apoptosis. Alterations in mitochondrial permeability transition linked to membrane potential disruption precede nuclear and plasma membrane changes. In vitro induction of permeability transition in isolated mitochondria provokes the release of a protein factor capable of inducing nuclear chromatin condensation and fragmentation. This permeability transition is regulated by multiple endogenous effectors, including members of the bcl-2 gene family. Inhibition of these effects prevents apoptosis.

Sequential reduction of mitochondrial transmembrane potential and generation of reactive oxygen species in early programmed cell death

Programmed cell death (PCD) is a physiological process commonly defined by alterations in nuclear morphology (apoptosis) and/or characteristic stepwise degradation of chromosomal DNA occurring before cytolysis. However, determined characteristics of PCD such as loss in mitochondrial reductase activity or cytolysis can be induced in enucleated cells, indicating cytoplasmic PCD control. Here we report a sequential disregulation of mitochondrial function that precedes cell shrinkage and nuclear fragmentation. A first cyclosporin A-inhibitable step of ongoing PCD is characterized by a reduction of mitochondrial transmembrane potential, as determined by specific fluorochromes (5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolcarbocyanine++ + iodide; 3,3'dihexyloxacarbocyanine iodide). Cytofluorometrically purified cells with reduced mitochondrial transmembrane potential are initially incapable of oxidizing hydroethidine (HE) into ethidium. Upon short-term in vitro culture, such cells acquire the capacity of HE oxidation, thus revealing a second step of PCD marked by mitochondrial generation of reactive oxygen species (ROS). This step can be selectively inhibited by rotenone and ruthenium red yet is not affected by cyclosporin A. Finally, cells reduce their volume, a step that is delayed by radical scavengers, indicating the implication of ROS in the apoptotic process. This sequence of alterations accompanying early PCD is found in very different models of apoptosis induction: glucocorticoid-induced death of lymphocytes, activation-induced PCD of T cell hybridomas, and tumor necrosis factor-induced death of U937 cells. Transfection with the antiapoptotic protooncogene Bcl-2 simultaneously inhibits mitochondrial alterations and apoptotic cell death triggered by steroids or ceramide. In vivo injection of fluorochromes such as 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolcarbocyanine iodide; 3,3'dihexyloxacarbocyanine iodide; or HE allows for the detection of cells that are programmed for death but still lack nuclear DNA fragmentation. In particular, assessment of mitochondrial ROS generation provides an accurate picture of PCD-mediated lymphocyte depletion. In conclusion, alterations of mitochondrial function constitute an important feature of early PCD.

Alterations in mitochondrial structure and function are early events of dexamethasone-induced thymocyte apoptosis

In this paper we used a multiparametric approach to analyze extensively the events occurring during apoptotic cell death of thymocytes, and furthermore, we asked whether alterations in mitochondrial structure and function are occurring in early stages of apoptosis. A multiparametric quantitative analysis was performed on normal or apoptotic thymocytes emerging from a few-hour culture performed in culture medium or in the presence of dexamethasone. Simultaneous detection of light scattering properties, integrity of plasma membrane (trypan blue exclusion), chromatin condensation (AO/EB staining of entire cells or PI staining of nuclei), and DNA fragmentation (in situ nick-translation in apoptotic cells) allowed a precise analysis of the preapoptotic and apoptotic stages. Moreover a thorough study of mitochondrial transmembrane potential (delta psi m) assessed following in a time course study the uptake by apoptotic cells of the cationic lipophilic dye DiOC6(3) or the J-aggregate-forming cation JC-1, indicates that a drop in delta psi m occurs very early in thymocyte apoptosis, before DNA fragmentation. This is associated with alteration in mitochondrial structure assessed by cytofluorimetric study of NAO uptake in apoptotic cells. Finally these dramatic alterations in mitochondrial structure and function occurring in early stages of apoptosis were confirmed by confocal and electron microscopy analysis.

Reduction in mitochondrial potential constitutes an early irreversible step of programmed lymphocyte death in vivo

In a number of experimental systems in which lymphocyte depletion was induced by apoptosis-inducing manipulations, no apoptotic morphology and ladder-type DNA fragmentation were detected among freshly isolated peripheral lymphocytes ex vivo. Here we report that one alteration that can be detected among splenocytes stimulated with lymphocyte-depleting doses of dexamethasone (DEX) in vivo is a reduced uptake of 3,3'dihexyloxacarbocyanine iodide (DiOC6[3]), a fluorochrome which incorporates into cells dependent upon their mitochondrial transmembrane potential (delta psi m). In contrast, ex vivo isolated splenocytes still lacked established signs of programmed cell death (PCD):DNA degradation into high or low molecular weight fragments, ultrastructural changes of chromatin arrangement and endoplasmatic reticulum, loss in viability, or accumulation of intracellular peroxides. Moreover, no changes in cell membrane potential could be detected. A reduced delta psi m has been observed in response to different agents inducing lymphoid cell depletion in vivo (superantigen and glucocorticoids [GC]), in mature T and B lymphocytes, as well as their precursors. DEX treatment in vivo, followed by cytofluorometric purification of viable delta psi mlow splenic T cells ex vivo, revealed that this fraction of cells is irreversibly committed to undergoing DNA fragmentation. Immediately after purification neither delta psi mlow, nor delta psi mhigh cells, exhibit detectable DNA fragmentation. However, after short-term culture (37 degrees C, 1 h) delta psi mlow cells show endonucleolysis, followed by cytolysis several hours later. Incubation of delta psi mlow cells in the presence of excess amount of the GC receptor antagonist RU38486 (which displaces DEX from the GC receptor), cytokines that inhibit DEX-induced cell death, or cycloheximide fails to prevent cytolysis. The antioxidant, N-acetylcysteine, as well as linomide, an agent that effectively inhibits DEX or superantigen-induced lymphocyte depletion in vivo, also stabilize the DiOC6(3) uptake. In contrast, the endonuclease inhibitor, aurintricarboxylic acid acts at later stages of apoptosis and only retards the transition from the viable delta psi mlow to the nonviable fraction. Altogether, these data suggest a sequence of PCD-associated events in which a reduction in delta psi m constitutes an obligate irreversible step of ongoing lymphocyte death, preceding other alterations of cellular physiology, and thus allowing for the ex vivo assessment of PCD.

Commitment to apoptosis is associated with changes in mitochondrial biogenesis and activity in cell lines conditionally immortalized with simian virus 40

Rodent embryo cells immortalized with temperature-sensitive mutants of simian virus 40 large tumor (T) antigen have a proliferative potential that depends on temperature. At the restrictive temperature, heat-inactivation of large T antigen causes p53 release, growth arrest, and cell death. Morphological and molecular analysis indicate that the induced cell death corresponds to apoptosis. Flow cytometric analysis using a combination of forward light scatter and side scatter allows a discrimination of cells committed to apoptosis within the whole population. These cells display a reduction in cell size and a higher cellular density, confirming the apoptotic nature of the cell death. When cells exhibiting the morphological features of apoptosis were stained with a fluorescent probe of the mitochondrial membrane potential, a decreased accumulation of the dye was recorded. Measures of cellular respiration, performed with whole-cell populations, showed that the lower mitochondrial membrane potential (delta psi m) correlates, as expected, with an uncoupling of electron transport from ATP production and is linked to the induction of apoptosis. We also show that this decrease in delta psi m is associated with a decrease in the rate of mitochondrial translation. These events are detected at early stages of the apoptotic process, when most of the cells are not irreversibly committed to death, suggesting that mitochondria could be a primary target during apoptosis.

Apoptosis is antagonized by large T antigens in the pathway to immortalization by polyomaviruses

The viability of rat embryo cells immortalized by thermosensitive mutants of SV40 or polyoma Large T antigen is impaired at the non-permissive temperature thus demonstrating that the immortal phenotype is dominantly maintained by Large T antigens. We have observed that exposing these cells to the restrictive temperature not only induces growth arrest but also causes apoptotic cell death. We present evidence supporting the model that polyomaviruses may indeed establish immortality by antagonizing the lethal effects of tumor suppressor genes via physical interactions between their products and Large T antigens. In the case of SV40-immortalized cells REtsAF, shift-up to 39.5 degrees C dissociates Large T antigen/p53 complexes releasing wild-type p53 molecules capable of inducing apoptotic cell death. In polyomavirus-immortalized cells, apoptosis may result from an alternative pathway mediated by other unidentified negatively acting molecules.

A sequence related to a DNA recognition element is essential for the inhibition by nucleotides of proton transport through the mitochondrial uncoupling protein

The uncoupling protein (UCP) is uniquely expressed in brown adipose tissue, which is a thermogenic organ of mammals. The UCP uncouples mitochondrial respiration from ATP production by introducing a proton conducting pathway through the mitochondrial inner membrane. The activity of the UCP is regulated: nucleotide binding to the UCP inhibits proton conductance whereas free fatty acids increase it. The similarities between the UCP, the ADP/ATP carrier and the DNA recognition element found in the DNA binding domain of the estrogen receptor suggested that these proteins could share common features in their respective interactions with free nucleotides or DNA, and thus defined a putative 'nucleotide recognition element' in the UCP. This article provides demonstration of the validity of this hypothesis. The putative nucleotide recognition element corresponding to the amino acids 261-269 of the UCP was gradually destroyed, and these mutant proteins were expressed in yeast. Flow cytometry, measuring the mitochondrial membrane potential in vivo, showed increased uncoupling activities of these mutant proteins, and was corroborated with studies with isolated mitochondria. The deletion of the three amino acids Phe267, Lys268 and Gly269, resulted in a mutant where proton leak could be activated by fatty acids but not inhibited by nucleotides.

Cytofluorometric analysis of chondrotoxicity of fluoroquinolone antimicrobial agents

To better understand quinolone-related arthropathy, we conceived an experimental ex vivo model using cell cultures of articular chondrocytes issued from pretreated New Zealand White rabbits (NZW). Juvenile (4- to 5-week-old) NZW were orally dosed with ofloxacin or pefloxacin (300 mg/kg of body weight for 1 day) or with pefloxacin (300 mg/kg for 7 days). Adult (5-month-old) NZW were treated with pefloxacin (300 mg/kg for 1 day). Chondrocytes were enzymatically recovered from cartilage and were analyzed by cytofluorometry using 2',7'-dichlorofluorescein diacetate (DCFH-DA) and dihydrorhodamine 123 (DHR), reflecting cellular respiratory-burst activity, and rhodamine 123 (Rh123) and 10-N-nonyl-acridine orange (NAO), specific for the mitochondrial activity and mass, respectively. A significant increase in the respiratory burst was detected by DCFH-DA and DHR in all treated groups of young animals, compared with untreated control groups. No significant increase of respiratory burst was noted in older treated rabbits. The 7-day treatment resulted in a decrease in mitochondrial uptake of Rh123 and an increase in NAO uptake. Fluoroquinolone arthrotoxicity seems to involve in its early phase the respiratory burst of immature articular chondrocytes.