Christophe Fleury

Bernard Mignotte

Jean - Luc

Jean-Luc Vayssière

Mitochondrial reactive oxygen species and apoptosis

Mitochondrial reactive oxygen species and apoptosis

Introduction

The maintenance of pluricellular organisms implies that cell death can be an important part of life. First observed during amphibian metamorphosis, normal cell death was soon found to occur in many developing tissues in both invertebrates and vertebrates [START_REF] Clarke | Nineteenth century research on naturally occuring cell death and related phenomena[END_REF]. The term programmed cell death (PCD) was used to describe the cell deaths that occur in predictable places and at predictable times during development, to emphasize that the deaths are somewhat programmed into the development plan of the organism.

Apoptosis is a process whereby cells activate an intrinsic cell suicide program that is one of the potential cellular responses, such as differentiation and proliferation. It has been defined in 1972 by Kerr et al. in contrast to necrosis, which is a cell death generally due to aggressions from the external medium [START_REF] Kerr | Apoptosis: a basic biological phenomenon with wide-ranging implications in tissue kinetics[END_REF].

The apoptotic process is associated with characteristic morphological and biochemical changes, such as membrane blebbing, cell shrinkage, chromatin condensation, DNA cleavage and fragmentation of the cell into membrane-bound apoptotic bodies whose surface expresses potent triggers for phagocytosis. However, it must be kept in mind that although apoptosis is the most common form of PCD, dying cells may follow other morphological types [START_REF] Clarke | Developmental cell death: morphological diversity and multiple mechanisms[END_REF][START_REF] Schwartz | Do all programmed cell deaths occur via apoptosis?[END_REF]. Moreover, apoptotic cells do not always harbor all cardinal features of their cell death type, in particular DNA cleavage does not appear to be an absolute requirement (Schulze-Osthoff et al., 1994b). Nevertheless, the observation that most cells undergoing PCD change similarly has suggested that apoptosis reflects the operation of an intracellular death program that can be activated or inhibited by a variety of physiological or pathological environmental stimuli.

Apoptosis is then a major mechanism for the precise regulation of cell numbers, and unwanted and potentially dangerous cells removal [START_REF] Raff | Social control on cell survival and cell death[END_REF][START_REF] Vaux | An Evolutionary Perspective on Apoptosis[END_REF][START_REF] Williams | Programmed cell death : apoptosis and oncogenesis[END_REF]. In addition to the beneficial effects of apoptosis in development and tissue homeostasis, the inappropriate activation of cell death may cause or contribute to a variety of diseases [START_REF] Thompson | Apoptosis in the pathogenesis and treatment of disease[END_REF], including acquired immunodeficiency syndrome (AIDS) [START_REF] Ameisen | The relevance of apoptosis to AIDS pathogenesis[END_REF], neurodegenerative disorders, and ischemic strokes [START_REF] Martinou | Bcl-2 and neuronal cell death[END_REF][START_REF] Raff | Programmed cell death and the control of cell survival: lessons from the nervous system[END_REF]. Conversely, a defect in apoptosis activation could be responsible for some autoimmune diseases [START_REF] Tan | Autoimmunity and Apoptosis[END_REF] and is also involved in oncogenesis [START_REF] Bursch | Cell death by apoptosis and its protective role against disease[END_REF].

Cascade of events leading to apoptosis can be divided into several phases (see [START_REF] Mignotte | Roles of mitochondria in apoptosis in Mitochondrial Diseases: models and method[END_REF][START_REF] Mignotte | Mitochondrial control of apoptosis in Programmes cell death: cellular and molecular mechanisms[END_REF]). During the activation phase, multiple signaling pathways lead from the various death-triggering signals (lineage information, extracellular survival factors or signals, cell interactions, hormones, cell contacts, genotoxic and physical trauma, anoxia, oncogene expression, immune killing…), to the central control of the cell death machinery and activate it. This is followed by the execution stage, in which the activated machinery acts on multiple cellular targets, and, finally, the destruction phase in which the dead or dying cell is broken down.

Intracellular control of the cell death program

The existence of an intrinsic cell suicide program was ascertained through genetic studies in the nematode Caenorhabditis elegans that identified genes involved in the cell death program and its control [START_REF] Ellis | Genetic control of programmed cell death in the nematode C. elegans[END_REF][START_REF] Horvitz | Programmed cell death in nematode development[END_REF], and then through the finding that some of these genes were homologous to mammalian genes [START_REF] Hengartner | C. elegans cell survival gene ced-9 encodes a functional homolog of the mammalian proto-oncogene bcl-2[END_REF][START_REF] Yuan | The C. elegans cell death gene ced-3 encodes a protein similar to mammalian interleukin-1 beta-converting enzyme[END_REF]. These considerations have led to a different meaning of the term apoptosis: it now refers to any cell death that results from an intracellular death program, no matter what activates it and whatever the changes associated with the destruction of the cell. In this way PCD appears as a critical element in the repertoire of potential cellular responses, as are cell differentiation, quiescence or proliferation. Moreover, these studies demonstrated the very early occurrence of PCD in the course of metazoan evolution and the substantial conservation of its basal machinery from nematodes to humans.

Genetic studies in C. elegans were a major contribution in the understanding of the molecular nature of this death program, leading to the identification of a dozen of cell death genes (ced) [START_REF] Ellis | Genes required for the engulfment of cell corpses during programmed cell death in Caenorhabditis elegans[END_REF]. Ced-3 and ced-4 are essential for cell death, whereas ced-9 antagonises the activities of ced-3 and ced-4, and thereby protects cells that should survive from any accidental activation of the death program. Caspases (cysteinyl aspartases) are the mammalian homologues of CED-3 and form a family of proteases implicated in apoptosis [START_REF] Yuan | The C. elegans cell death gene ced-3 encodes a protein similar to mammalian interleukin-1 beta-converting enzyme[END_REF] (see table 1). The mammalian caspase family now comprises at least fourteen known members, most of which have been definitively implicated in PCD (for review see [START_REF] Cryns | Proteases to die for[END_REF]). All cleave their substrates after specific aspartic acids and are themselves activated by cleavage at specific aspartic acids [START_REF] Nicholson | Caspases: killer proteases[END_REF]. Caspases mediate apoptosis by cleaving selected intracellular proteins, including proteins of the nucleus, nuclear lamina, cytoskeleton, endoplasmic reticulum, and cytosol.

The Bcl-2 family

CED-9 protein is homologous to a family of many members termed the Bcl-2 family (Bcl-2s) in reference to the first discovered mammalian cell death regulator (for review see [START_REF] Adams | The Bcl-2 protein family: arbiters of cell survival[END_REF]). The Bcl-2 family proteins are involved in positive and negative regulations of apoptotic cell death (review in (Gross et al., 1999a); see table 1). Among the anti-apoptotic members, Bcl-2 and Bcl-XL are negative regulators of cell death, able to prevent cells from undergoing apoptosis induced by various stimuli in a wide variety of cell types [START_REF] Korsmeyer | Bcl-2 initiates a new category of oncogenes: regulators of cell death[END_REF][START_REF] Zhong | bcl-2 inhibits death of central neural cells induced by multiple agents[END_REF], whereas others, such as Bax, Bid and Egl-1 promote or accelerate cell death. CED-9, Bcl-2 [START_REF] Akao | Multiple subcellular localization of Bcl-2: detection in nuclear outer membrane, endoplasmic reticulum membrane, and mitochondrial membranes[END_REF][START_REF] Chen | The bcl-2 candidate proto-oncogene product is a 24kilodalton integral-membrane protein highly expressed in lymphoid cell lines and lymphomas carrying the t(14;18) translocation[END_REF][START_REF] De Jong | Subcellular localization of the bcl-2 protein in malignant and normal lymphoid cells[END_REF][START_REF] Hockenbery | Bcl-2 is an inner mitochondrial protein that blocks programmed cell death[END_REF][START_REF] Janiak | Assembly of Bcl-2 into microsomal and outer mitochondrial membranes[END_REF][START_REF] Krajewski | Investigation of the subcellular distribution of the bcl-2 oncoprotein: residence in the nuclear envelope, endoplasmic reticulum, and outer mitochondrial membranes[END_REF][START_REF] Nakai | The bcl-2 protein is inserted into the outer membrane but not into the inner membrane of rat liver mitochondria in vitro[END_REF][START_REF] Nguyen | Targeting of bcl-2 to the mitochondrial outer membrane by a COOH-Terminal signal anchor sequence[END_REF], Bcl-xL (Gonzalez-Garcia et al., 1994), Mcl-1 (Wang and Studzinski, 1997;[START_REF] Yang | The intracellular distribution and pattern of expression of Mcl-1 overlap with, but are not identical to, those of Bcl-2[END_REF], the BHRF1 Epstein-Barr virus protein [START_REF] Hickish | Ultrastructural localization of BHRF1: an Epstein-Barr virus gene product which has homology with bcl-2[END_REF] and probably other members of the Bcl-2 family are localized to the cytoplasmic surfaces of the nuclear envelope, the endoplasmic reticulum and the outer mitochondrial membrane. It must be yet underlined that only a fraction of Bcl-x L resides on membranes, even though it bears a C-terminal membrane anchor involved in the targeting to membranes [START_REF] Nguyen | Targeting of bcl-2 to the mitochondrial outer membrane by a COOH-Terminal signal anchor sequence[END_REF]. Moreover, this hydrophobic C-terminus is not involved in the association with the membrane in the case of Bax [START_REF] Goping | Regulated targeting of BAX to mitochondria[END_REF][START_REF] Tremblais | The Cterminus of bax is not a membrane addressing/anchoring signal[END_REF].

The membrane association of Bcl-2 is of functional significance as mutant Bcl-2 molecules lacking this membrane anchorage capacity are less effective at preventing apoptosis in some systems [START_REF] Borner | The protein bcl-2 alpha does not require membrane attachment, but two conserved domains to suppress apoptosis[END_REF][START_REF] Nguyen | Role of membrane anchor domain of Bcl-2 in suppression of apoptosis caused by E1B-defective adenovirus[END_REF][START_REF] Zhu | Bcl-2 mutants with restricted subcellular location reveal spatially distinct pathways for apoptosis in different cell types[END_REF]. Indeed, it has been reported that, in inhibiting apoptosis of MDCK cells, a mutant Bcl-2 molecule whose anchorage is targeted specifically to the mitochondria is as effective as the wild type protein, whereas mutant Bcl-2 targeted to the endoplasmic reticulum loses this capacity [START_REF] Zhu | Bcl-2 mutants with restricted subcellular location reveal spatially distinct pathways for apoptosis in different cell types[END_REF]. In contrast, Bcl-2 targeted to the endoplasmic reticulum in the Rat-1/myc fibroblasts proved to be more active than when targeted to mitochondria. Thus, Bcl-2 mutants with restricted subcellular location reveal distinct pathways for apoptosis depending on cell type. When associated to the endoplasmic reticulum membrane, Bcl-2 could be involved in maintenance of the calcium homeostasis [START_REF] Distelhorst | Bcl-2 inhibits hydrogen peroxide-induced ER Ca2+ pool depletion[END_REF][START_REF] He | Maintenance of Calcium Homeostasis in the Endoplasmic Reticulum by Bcl-2[END_REF][START_REF] Lam | Evidence that BCL-2 represses apoptosis by regulating endoplasmic reticulum-associated Ca 2+ fluxes[END_REF], while it could modulate protein subcellular trafficking through nuclear pores [START_REF] Ryan | c-myc and bcl-2 modulate p53 function by altering p53 subcellular trafficking during the cell cycle[END_REF].

Bcl-2s proteins can dimerize with one another, with one monomer antagonizing or enhancing the function of the other. In this way, it is assumed that the ratio of activators to inhibitors in a cell could determine the propensity of the cell to undergo apoptosis [START_REF] Korsmeyer | Regulators of cell death[END_REF]. Nevertheless, it has been suggested that all the heterodimers are not formed in the cells [START_REF] Hsu | Bax in murine thymus is a soluble monomeric protein that displays differential detergent-induced conformations[END_REF]. The mechanism(s) by which proteins of the Bcl-2 family modulate apoptosis is not yet well known and several conflicting theories have been proposed. In non-stimulated cells, monomeric Bax is located in the cytosol and in peripheral association with intracellular membranes including mitochondria, but inserts into mitochondrial membranes after a death signal [START_REF] Gross | Enforced dimerization of BAX results in its translocation, mitochondrial dysfunction and apoptosis[END_REF][START_REF] Wolter | Movement of Bax from the Cytosol to Mitochondria during Apoptosis[END_REF]. A widely accepted model postulates that homodimers of Bax promote apoptosis [START_REF] Finucane | Baxinduced caspase activation and apoptosis via cytochrome c release from mitochondria is inhibitable by Bcl-xL[END_REF]Gross, et al., 1998;Gross, et al., 1999a;[START_REF] Pastorino | The overexpression of Bax produces cell death upon induction of the mitochondrial permeability transition[END_REF][START_REF] Xiang | BAX-induced cell death may not require interleukin 1converting enzyme-like proteases[END_REF], and that the functional effect of Bcl-2 related proteins is to form competing heterodimers with Bax that cannot promote apoptosis [START_REF] Oltvai | bcl-2 heterodimerizes in vivo with a conserved homolog, bax, that accelerates programmed cell death[END_REF][START_REF] Sedlak | Multiple Bcl-2 family members demonstrate selective dimerizations with Bax[END_REF]. However, in some systems, Bax binding by Bcl-2 was not sufficient to prevent apoptosis and the overexpression of Bcl-2 or Bcl-xL can repress apoptosis in the absence of Bax [START_REF] Cheng | Bax-independent inhibition of apoptosis by Bcl-x L[END_REF][START_REF] Knudson | Bcl-2 and Bax function independently to regulate cell death[END_REF]. Moreover, mechanisms other than the simple dimerization among members of the Bcl-2 family may be required for the regulation of apoptosis [START_REF] Hsu | Bax in murine thymus is a soluble monomeric protein that displays differential detergent-induced conformations[END_REF]. Thus, while an in vivo competition exists between Bax and Bcl-2, each is able to regulate apoptosis independently.

In both worm and mammalian cells, the anti-apoptotic members of the Bcl-2 family act upstream of the "execution caspases", somehow preventing their proteolytic processing into active killers [START_REF] Golstein | Controlling Cell Death[END_REF][START_REF] Shaham | An alternatively spliced C. elegans ced-4 RNA encodes a novel cell death inhibitor[END_REF]. Some proteins, such as Bap31 in the endoplasmic reticulum, may bind caspases and Bcl-2s family members, coordinating thus their activities [START_REF] Ng | p28 Bap31, a Bcl-2/Bcl-XL-and procaspase-8-associated protein in the endoplasmic reticulum[END_REF]. Furthermore, two main mechanisms of action have been proposed to connect Bcl-2s to caspases. In the first one, Bcl-2 would act by regulating the release from mitochondria of apoptogenic factors, such as the Apoptosis-Inducing-Factor (AIF), some caspases activators (e.g. cytochrome c) and pro-caspases from mitochondria to cytosol (Kluck et al., 1997a;[START_REF] Susin | Bcl-2 inhibits the mitochondrial release of an apoptogenic protease[END_REF][START_REF] Yang | Prevention of apoptosis by Bcl-2: release of cytochrome c from mitochondria blocked[END_REF]. Indeed, it was shown that 1) some Bcl-2s proteins have pore-forming properties with distinct ion-conducting properties, raising the possibility that these products can regulate the permeability of the intracellular membranes (Minn et al., 1997;[START_REF] Muchmore | X-ray and NMR structure of human Bcl-X L , an inhibitor of programmed cell death[END_REF][START_REF] Schendel | Channel formation by antiapoptotic protein Bcl-2[END_REF] ; 2) activation of "execution caspases" required the preliminary shift to the cytosol of regulatory components, such as cytochrome c or pro-caspases, previously sequestered in mitochondria (Kluck, et al., 1997a;[START_REF] Krippner | Loss of Function of Cytochrome c in Jurkat Cells Undergoing Fas-mediated Apoptosis[END_REF]Susin, et al., 1996;[START_REF] Yang | Prevention of apoptosis by Bcl-2: release of cytochrome c from mitochondria blocked[END_REF]).

An alternative model proposes that the pro-survival proteins may function downstream the release of apoptogenic factors, by directly inhibiting the ability of CED-4 like proteins to activate caspases. This model arose from the elucidation of the role of the somewhat mysterious CED-4 protein in the nematode [START_REF] Hengartner | Apoptosis. CED-4 is a stranger no more[END_REF]. Indeed, it was first shown that CED-9 interacts with CED-3 via the bridging protein CED-4 that binds simultaneously to CED-9 and CED-3. CED-9 prevents CED-4 from inducing proteolytic processing and activation of CED-3. An equivalent ternary complex was found to be present in mammalian cells involving the mammalian equivalent of CED-4, Apaf-1 (apoptosis protease-activating factor 1, [START_REF] Zou | Apaf-1, a human protein homologous to C. elegans CED-4, participates in cytochrome c-dependent activation of caspase-3[END_REF], caspase-9 and Bcl-xL, in which Bcl-xL inhibits Apaf-1-mediated maturation of caspase-9 [START_REF] Chinnaiyan | Interaction of CED-4 with CED-3 and CED-9: a molecular framework for cell death[END_REF]. In this paradigm, pro-apoptotic relatives like Bik may free CED-4/Apaf-1 from the death inhibitor. Nevertheless, Moriishi et al. have recently proposed that the prosurvival Bcl-2 homologues do not act by sequestering Apaf-1, but should constrain its activity indirectly [START_REF] Moriishi | Bcl-2 family members do not inhibit apoptosis by binding the caspase activator Apaf-1[END_REF]. Additionally, there is some evidence that binding of pro-survival proteins to the apoptosome complex alter its location in cells, pulling it from the cytosol to the intracellular membranes where Bcl-2s often reside [START_REF] Chinnaiyan | Interaction of CED-4 with CED-3 and CED-9: a molecular framework for cell death[END_REF][START_REF] Wu | Interaction and regulation of subcellular localization of CED-4 by CED-9[END_REF].

These distinct and probably independent links between Bcl-2 and caspases match with the apparent redundancy of caspases and the existence of several alternative activation pathways that seem to be the rule for mammalian cell death. The multiplicity of caspases illustrates this complexity. Although caspases appear as the key components of the execution machinery of the cell death program, they also intervene in the death signal transduction cascade in some PCD, such as those triggered by surface receptor activation [START_REF] Nagata | Apoptosis by Death Factor[END_REF]. These different possible pathways of caspase activation would be consistent with the previously described possibility for some caspases to be activated by autoprocessing and for others to require transcleavage (the proteolytic cascade). Given the complexity and diversity of caspase activation pathways, these experiments point to the mitochondrion as a unifying element of apoptosis. An irony in the story of PCD studies, for a long time the absence of mitochondrial changes was considered as a hallmark of apoptosis [START_REF] Kerr | Definition and incidence of apoptosis: an historical perspective in Apoptosis: the molecular basis of cell death[END_REF] while, conversely, mitochondria appear today as the central executioner of programmed cell death [START_REF] Golstein | Controlling Cell Death[END_REF][START_REF] Kroemer | Mitochondrial control of apoptosis[END_REF][START_REF] Reed | Double identity for proteins of the Bcl-2 family[END_REF].

Mitochondrial events

A decrease in mitochondrial membrane potential precedes DNA fragmentation during apoptosis

Several changes in mitochondrial biogenesis and function are associated with the commitment to apoptosis. A fall of the membrane potential (ΔΨm) occurs before the fragmentation of the DNA in oligonucleosomal fragments [START_REF] Kroemer | The biochemistry of programmed cell death[END_REF][START_REF] Petit | Alterations in mitochondrial structure and function are early events of dexamethasone-induced thymocytes apoptosis[END_REF][START_REF] Vayssière | Commitment to apoptosis is associated with changes in mitochondrial biogenesis and activity in cell lines conditionally immortalized with Simian Virus 40[END_REF]Zamzami et al., 1995a;Zamzami et al., 1995b). This drop of ΔΨm is responsible for a defect of maturation of mitochondrial proteins synthesized in the cytoplasm [START_REF] Mignotte | SV40 induced cellular immortalization : phenotypic changes associated with the loss of proliferative capacity in a conditionally immortalized cell line[END_REF], cessation of mitochondrial translation and an uncoupling of the oxidative phosphorylations [START_REF] Vayssière | Commitment to apoptosis is associated with changes in mitochondrial biogenesis and activity in cell lines conditionally immortalized with Simian Virus 40[END_REF]. The drop of ΔΨm is detectable whatever the cellular type, or the apoptosis induction signal and marks the point of no-return of a cell condemned to die (Kroemer, et al., 1995).

The fall of ΔΨm could be due to the permeability transition (PT), a phenomenon that is characterized by the opening of large conductance channels (mitochondrial PT pores) and by its sensitivity to very low concentration of cyclosporine A [START_REF] Zoratti | The mitochondrial permeability transition[END_REF], Zamzami, 1998#2854). These pores, permeable to compounds of molecular mass up to 1500 Da are formed under specific conditions, but their molecular composition remains elusive. The peripheral benzodiazepin receptor, that has recently been implicated in the protection against ROS [START_REF] Carayon | Involvement of peripheral benzodiazepine receptors in the protection of hematopoietic cells against oxygen radical damage[END_REF], creatine kinase and the voltage dependent anion channel (VDAC), the translocase of adenine nucleotides (ANT) and the mitochondrial matrix cyclophilin are probable components of the PT pore [START_REF] Hirsch | PK11195, a ligand of the mitochondrial benzodiazepine receptor, facilitates the induction of apoptosis and reverses bcl-2-mediated cytoprotection[END_REF]Zamzami et al., 1996a).

The opening of these PT pores in the inner membrane allows for an equilibration of ions within the matrix and intermembrane space of mitochondria, thus dissipating the H+ gradient across the inner membrane and uncoupling the respiratory chain. These events lead to the decrease of the ΔΨm, the arrest of ATP synthesis [START_REF] Bernardi | Modulation of the mitochondrial permeability transition pore. Effect of protons and divalent cations[END_REF][START_REF] Petronilli | Regulation of the permeability transition pore, a voltage-dependent mitochondrial channel inhibited by cyclosporin A[END_REF], and a volume dysregulation of mitochondria due to the hyperosmolarity of the matrix, which cause the matrix space to expand. Because the inner membrane a larger surface area than the outer membrane, this matrix volume expansion can eventually provokes outer membrane rupture, releasing intermembrane space molecules into the cytosol. Furthermore, permeability transition has properties of self-amplification: the drop of the ΔΨm that is linked to depletion of non-oxidized glutathione [START_REF] Macho | Glutathione depletion is an early and calcium elevation is a late event of thymocyte apoptosis[END_REF], and the opening of the PT pores would increase the permeability transition in a retrograde manner [START_REF] Ichas | Mitochondria are excitable organelles capable of generating and conveying electrical and calcium signals[END_REF]. We have therefore proposed that the opening of the PT pore may constitute an irreversible state of apoptosis and could account for the apparent synchronization in the drop of ΔΨm that takes place simultaneously in all the mitochondria of a same cell (Kroemer, et al., 1995). Altogether, these results sustain the hypothesis that the opening of PT pores is involved in the disruption of ΔΨm observed during apoptosis, at least during the effector phase (Kroemer, et al., 1997). For a review about PTP, see [START_REF] Bernardi | Mitochondria and cell death mechanistic aspects and methodological issues[END_REF][START_REF] Lemasters | The mitochondrial permeability transition in cell death: a common mechanism in necrosis, apoptosis and autophagy[END_REF] and references within.

Moreover, alterations of mitochondria can induce apoptosis [START_REF] Hartley | Complex I inhibitors induce dose-dependent apoptosis in PC12 cells: relevance to Parkinson's disease[END_REF][START_REF] Wolvetang | Mitochondrial respiratory chain inhibitors induce apoptosis[END_REF], and direct interventions on the mitochondrial permeability modulate apoptosis. The links between mitochondrial perturbations and nuclear alterations have been studied by means of acellular systems where purified nuclei and purified mitochondria are confronted [START_REF] Newmeyer | Cell-free apoptosis in Xenopus egg extracts: inhibition by Bcl-2 and requirement for an organelle fraction enriched in mitochondria[END_REF]. Such a system allows studying reciprocal and direct effects of one organelle on another and to characterize at the biochemical level the factors involved.

These experiments have shown that when mitochondria are treated with substances capable to induce PT pores opening, they provoke nuclear apoptosis (condensation of the chromatin and fragmentation of the DNA) (Zamzami et al., 1996b). A correlation between induction of the PT and nuclear apoptosis has been observed by using a variety of known inductors of the PT such as atractyloside, pro-oxidants, calcium, protonophores and substances that provoke linkage of thiol groups such as diamide. These substances, that have no direct effect on nuclei in absence of mitochondria, confer pro-apoptotic properties upon mitochondria. The pro-apoptotic character (induction of nuclear apoptosis) of the mitochondria treated with atractyloside is altered by inhibitors of the PT such as bongkrekic acid, cyclosporine A and substances like monochlorobimane that block the cross-linking of the thiols. Cyclosporine A can be replaced by its non-immunosuppressor analogue, N-methyl Val-4-cyclosporine A, which shows that its inhibitory effect on PT and nuclear apoptosis is independent from its calcineurine activity. These results suggest the implication of the PT pores opening in the regulation of apoptosis induced via the mitochondria. Nuclei and mitochondria have been purified from hybridomas of T cells transfected by bcl-2 to study how Bcl-2 suppresses apoptosis in in vitro experiments (Zamzami, et al., 1996b). Upon treatment with atractyloside, in contrast to mitochondria purified from control cells, mitochondria purified from cells transfected by bcl-2 do not provoke nuclear apoptosis. On the contrary, nuclei purified from cells transfected by bcl-2 show a condensation of the chromatin and a fragmentation of the DNA when they are confronted to control mitochondria treated with atractyloside. Furthermore, bcl-2 inhibits the induction of permeability transition by agents such as atractyloside, oxidants and protonophores. These results show that, even if Bcl-2 intervenes also during latter apoptotic events [START_REF] Guénal | Bcl-2 and Hsp27 act at different levels to suppress programmed cell death[END_REF][START_REF] Marton | Apoptotic cell death induced by inhibitors of energy conservation--Bcl-2 inhibits apoptosis downstream of a fall of ATP level[END_REF] and on ΔΨm loss induced by other mechanisms [START_REF] Shimizu | Bcl-2 prevents apoptotic mitochondrial dysfunction by regulating proton flux[END_REF], at least a part of its activity is exerted by acting on the mitochondrial permeability transition [START_REF] Decaudin | Bcl-2 and Bcl-X L antagonize the mitochondrial dysfunction preceding nuclear apoptosis induced by chemotherapeutic agents[END_REF]Susin, et al., 1996). It has also been reported that the cell death caused by overexpression of Bax could be prevented by a cyclosporin treatment [START_REF] Bradham | The mitochondrial permeability transition is required for tumor necrosis factor alpha-mediated apoptosis and cytochrome c release[END_REF][START_REF] Pastorino | The overexpression of Bax produces cell death upon induction of the mitochondrial permeability transition[END_REF], suggesting that bax could act on the PT opening [START_REF] Shimizu | Bcl-2 prevents apoptotic mitochondrial dysfunction by regulating proton flux[END_REF]. However, these findings are still controversious because some studies indicate that the mitochondria effects of Bax are not countered by cyclosporin [START_REF] Eskes | Bax-induced cytochrome C release from mitochondria is independent of the permeability transition pore but highly dependent on Mg2+ ions[END_REF].

Moreover, the structure of a protein of the Bcl-2 family (Bcl-xL) has been established [START_REF] Muchmore | X-ray and NMR structure of human Bcl-X L , an inhibitor of programmed cell death[END_REF]. It recalls that of bacterial toxins, especially the diphtheria toxin, that forms a pH-sensitive transmembrane channel. Furthermore, the pro-apoptotic Bax protein can form channels [START_REF] Antonsson | Inhibition of Bax Channel-Forming Activity by Bcl-2[END_REF], as reported also for the anti-apoptotic proteins Bcl-xL (Minn, et al., 1997) and Bcl-2 [START_REF] Schendel | Channel formation by antiapoptotic protein Bcl-2[END_REF]. However, the intrinsic properties of Bax and those of Bcl-xL and Bcl-2 reveal differences. The channel forming activity of Bcl-xL and Bcl-2 is observed at highly acidic pH while Bax form channel in a wide range of pH including at pH=7, those found in cells. Furthermore, Bcl-2 can block the pore-forming activity of Bax. (Gross, et al., 1998). These results strengthen the hypothesis that these proteins are constituents of the mitochondrial PT pores. Bax might promote cell death by allowing the efflux of ions and small molecules across the mitochondrial membranes, thus triggering permeability transition, while Bcl-2 might counteract this effect.

Release of apoptogenic factors

One of the major roles of mitochondria in apoptosis is linked to the release of apoptogenic molecules from this organelle into the cytosol, such as cytochrome c and AIF (see below). Recently, the release of procaspase-2 and procaspase-9 from mitochondria was also observed during the opening of the permeability transition pores (Susin et al., 1999a). Pro-caspase-3 is found in both cytosol and mitochondria [START_REF] Mancini | The caspase-3 precursor has a cytosolic and mitochondrial distribution: implications for apoptotic signaling[END_REF], and its activation from the mitochondrial pool is regulated by Bcl-2 [START_REF] Krebs | Activation of membrane-associated procaspase-3 is regulated by Bcl-2[END_REF]. Translocation of active caspases from cytosol to mitochondria occurs in some situations [START_REF] Chandler | Different subcellular distribution of caspase-3 and caspase-7 following Fas-induced apoptosis in mouse liver[END_REF]. Pro-apoptotic Bcl-2s members translocate within the cells and undergo post-traductional modifications , such as phosphorylation/dephosphorylation or cleavage, under apoptotic stimulus [START_REF] Wang | Ca2+-induced apoptosis through calcineurin dephosphorylation of BAD[END_REF]

Cytochrome c release

Although apoptosis can occur in the absence of detectable cytochrome c release [START_REF] Chauhan | Cytochrome c-dependent and -independent induction of apoptosis in multiple myeloma cells[END_REF]Gross et al., 1999b;[START_REF] Tang | Apoptosis in the absence of cytochrome c accumulation in the cytosol[END_REF], efflux of cytochrome c from mitochondria appears to be a critical coordinating step in the killing program towards which converge the multiple signaling pathways and beyond which are initiated the entire panel of apoptotic features, procaspase 3 expressing cells being then irreversibly committed to die (Li et al., 1997a). Translocation of cytochrome c from mitochondria to cytosol has been shown to be a crucial step in the activation of the PCD machinery in various death models, including Fas-, UV-, staurosporine or etoposidetreated mammalian cells, in a cell-free system using Xenopus egg extracts or dATP-primed cytosols of growing cells (Kluck, et al., 1997a;[START_REF] Krippner | Loss of Function of Cytochrome c in Jurkat Cells Undergoing Fas-mediated Apoptosis[END_REF][START_REF] Liu | Induction of apoptotic program in cell-free extracts: requirement for dATP and cytochrome c[END_REF][START_REF] Yang | Prevention of apoptosis by Bcl-2: release of cytochrome c from mitochondria blocked[END_REF]. Furthermore, direct microinjection of cytochrome c into the cytosol can induce apoptosis in various cell types (Li, et al., 1997a;[START_REF] Zhivotovsky | Injected cytochrome c induces apoptosis[END_REF]. Cytochrome c is an essential component of the mitochondrial respiratory chain: it accepts an electron from cytochrome c reductase and passes it on to cytochrome c oxidase. It is a soluble protein that is located in the intermembrane space and is loosely attached to the surface of the inner mitochondrial membrane. Cytochrome c is translated on cytoplasmic ribosomes as apocytochrome c and follows a unique pathway into mitochondria that does not require the signal sequence, electrochemical potential and general protein translocation machinery [START_REF] Mayer | Translocation of Apocytochrome c accross the Outer membrane of Mitochondria[END_REF]. The apoprotein, on entry into the intermembrane space, gains an heme group, to become the fully folded holocytochrome c. This globular, positively charged protein can no longer pass through the outer mitochondrial membrane and is thought to become electrostatically attached to the inner membrane.

Released of cytochrome c into the cytosol induces nuclear apoptosis. Cytosolic cytochrome c forms an essential part of the vertebrate "apoptosome" which is composed of cytochrome c, Apaf-1 and procaspase-9 (Li et al., 1997b;[START_REF] Zou | Apaf-1, a human protein homologous to C. elegans CED-4, participates in cytochrome c-dependent activation of caspase-3[END_REF]. The result is the activation of Apaf-1, which activates proteolytic processing and activation of caspases to orchestrate execution of cell death.

Although efflux of cytochrome c from mitochondria appears to be a crucial step in the killing cascade initiated by a wide range of apoptogenic stimuli, it must be underlined that this event is not an universal requirement for death signal transduction. Indeed, on the one hand, apoptosis can occur in the absence of detectable cytochrome c release [START_REF] Chauhan | Cytochrome c-dependent and -independent induction of apoptosis in multiple myeloma cells[END_REF][START_REF] Tang | Apoptosis in the absence of cytochrome c accumulation in the cytosol[END_REF] and, on the other hand, under certain circumstances, cytochrome c release is not sufficient to promote cell death (Li, et al., 1997a). In contrast, recent studies indicate that cytochrome c is not released in some cell lines committed to cell death, but its conformational change is sufficient to enable Apaf-1 activation at the mitochondrial membrane [START_REF] Jemmerson | A conformational change in cytochrome c of apoptotic and necrotic cells is detected by monoclonal antibody binding and mimicked by association of the native antigen with synthetic phospholipid vesicles[END_REF][START_REF] Varkey | Altered Cytochrome c Display Precedes Apoptotic Cell Death in Drosophila[END_REF].

Moreover, it was reported that efflux of cytochrome c could constitutes only a later event of the cell death, in other words, a side effect of the terminal dismantelment of the cell [START_REF] Adachi | Lack of Release of Cytochrome c from Mitochondria into Cytosol Early in the Course of Fas-mediated Apoptosis of Jurkat Cells[END_REF][START_REF] Krippner | Loss of Function of Cytochrome c in Jurkat Cells Undergoing Fas-mediated Apoptosis[END_REF]. However, in this Fas-mediated apoptosis model, an inactivation of cytochrome c, correlated to the inhibition of mitochondrial respiration at the cytochrome c level, constitutes an very early causal event in the apoptotic program. It was suggested that inactivation of cytochrome c is associated to its release from the outer surface of the inner mitochondrial membrane where normally it functions as a shuttle connecting respiratory chain energy transducers (Skulachev, 1998a). More intriguing, the authors suggest that efflux of cytochrome c which had been associated with early events of apoptosis could be, in some instances, the result of a methodological artifact [START_REF] Adachi | Lack of Release of Cytochrome c from Mitochondria into Cytosol Early in the Course of Fas-mediated Apoptosis of Jurkat Cells[END_REF]. Indeed, most studies are based on subcellular fractionations, i.e.: after cell disruption, centrifugation allows to separate soluble cytosolic elements in the supernatant from heavy membranes, including mitochondria, included in the pellet. In this approach, the in vivo cytochrome c redistribution is inferred from its appearance into the cytosolic fraction during the in vitro procedure. In fact, this method could be more properly considered as a measure of cytochrome c extractability from mitochondria rather than as a exact view of its in situ localization. For instance, some minor alterations of the outer mitochondrial membrane c could be compatible with a mitochondrial localization of cytochrome c in the cell and its outflow from mitochondria as fractionation provoked breakdown of the fragilized outer membrane. However, these considerations refute neither the early occurrence of cytochrome c release in various apoptotic models as observed by in situ immunolocalization and nor its crucial role in the apoptotic program, ascertained by direct microinjection of cytochrome c in the cytosol. They must be correlated to the evidenced existence of alternative pathways, i.e. cytochrome c independent, in the transduction of apoptogenic stimuli [START_REF] Chauhan | Cytochrome c-dependent and -independent induction of apoptosis in multiple myeloma cells[END_REF]Li, et al., 1997a;[START_REF] Tang | Apoptosis in the absence of cytochrome c accumulation in the cytosol[END_REF].

The mechanism by which cytochrome c is released from mitochondria is largely unknown. The possible mechanisms involved in activation of this central control may be envisaged from data concerning mediators of the death signal transduction cascades.

For instance, the release of cytochrome c might result from oxidative imbalance, an upstream event in the apoptotic transduction cascade. These phenomena might lead to the alteration of some redox sensitive crucial regulatory elements of the outer mitochondrial membrane permeability, e.g. by a shift of the redox state of some sulfhydryl groups to a more inactivating oxidized state.

Another possibility comes from the ability of caspases to promote release intermembrane proteins, including cytochrome c, through the outer mitochondrial membrane. These observations suggested that mitochondria carry a caspase substrate that, when cleaved, promotes cytochrome c-release [START_REF] Mignotte | Mitochondria and apoptosis[END_REF]. Recent reports concerning Fas and TNF- receptor signaling could provide insight into how caspase induce cytochrome c efflux. Previously, it had been established that activation of caspases, predominantly the caspase-8, constitutes an early step in these apoptotic pathways [START_REF] Nagata | Apoptosis by Death Factor[END_REF], and that inhibition of these proteases prevents both the release of cytochrome c from mitochondria and the execution of the cell death program [START_REF] Schulze-Osthoff | Apoptosis signaling by death receptors[END_REF][START_REF] Van Der Heiden | Bcl-xL regulates the membrane potential and volume homeostasis of mitochondria[END_REF]. Recently, it has been shown that caspase-8, activated by cell surface death receptors such as TNF and Fas, cleaves Bid, a death agonist of the Bcl-2 family, which in turn transduces the apoptotic signal from the cell surface to mitochondria [START_REF] Li | Cleavage of BID by Caspase 8 Mediates the Mitochondrial Damage in th Fas Pathway of Apoptosis[END_REF][START_REF] Luo | Bid, a Bcl-2 Interacting Protein, Mediates Cytochrome c release from Mitochondria in Response to Activation of Cell surface Death receptors[END_REF]. Thus, while full-length Bid is localized in cytosol, the C-terminal part caspase-8-truncated BID translocates to mitochondria and then induces cytochrome c release, via a conformational change of Bax [START_REF] Desagher | Bid-induced conformational change of Bax is responsible for mitochondrial cytochrome c release during apoptosis[END_REF] and the downstream caspase-dependent apoptotic program. Cytosolic substrates for effector caspases, other than Bid, could be involved in the induction of cytochrome c release [START_REF] Bossy-Wetzel | Caspases induce cytochrome c release from mitochondria by activating cytosolic factors[END_REF]. Overexpression of the pro-apoptotic Bcl-2 family member Bax in cells stimulates both the release of cytochrome c and apoptosis. By using isolated mitochondria and recombinant Bax, it was shown that Bax could directly induce cytochrome c release from mitochondria.

It has been shown that Bcl-2 and Bcl-XL inhibit release of cytochrome c from the mitochondria into the cytosol (Gross, et al., 1999b;Kluck, et al., 1997a;[START_REF] Yang | Prevention of apoptosis by Bcl-2: release of cytochrome c from mitochondria blocked[END_REF]. Thus, one level of action of Bcl-2 is to control the efflux of cytochrome c from the mitochondrial intermembrane space through the outer membrane and into the cytosol where caspases are found. It remains to be determined whether this efflux is done through Bax containing channels. It has recently been proposed that the cytochrome c released be regulated by the Bcl-2s members family, through the mitochondrial voltage-dependent anion channel [START_REF] Shimizu | Bcl-2 family proteins regulate the release of apoptogenic cytochrome c by the mitochondrial channel VDAC [see comments[END_REF]. Postmitochondrial fractions from cells that overexpress Bcl-2 both prevent and reverse cytochrome c inactivation in cell free experiments [START_REF] Adachi | Bcl-2 and the outer mitochondrial membrane in the inactivation of cytochrome c during Fas-mediated apoptosis[END_REF], suggesting that Bcl-2 might even allow to transport cytochrome c back into mitochondria. Bcl-xL inhibits AraC-induced preapoptotic accumulation of cytochrome c in the cytosol [START_REF] Kim | Overexpression of Bcl-X(L) inhibits Ara-C-induced mitochondrial loss of cytochrome c and other perturbations that activate the molecular cascade of apoptosis[END_REF]. Cells that overexpress Bcl-XL fail to accumulate cytosolic cytochrome c or to undergo apoptosis in response to genotoxic stress. However, recent studies in the TNF-α death pathway indicate that, if Bcl-2 and Bcl-XL are able to block cytochrome c release, other mitochondrial dysfunction and final cell death could nonetheless occur (Gross, et al., 1999b). Co-immunoprecipitation studies have shown that cytochrome c binds to Bcl-xL and not to the proapoptotic Bcl-Xs protein, a pro-apoptotic derivative of Bcl-xL. Thus, Bcl-Xs blocks binding of cytochrome c to Bcl-xL [START_REF] Kharbanda | Role for bcl-X L as an inhibitor of cytosolic cytochrome C accumulation in DNA damage-induced apoptosis[END_REF].

Even though, altogether, these results provide evidence that Bcl-2 family members can modulate cell death process by directly controlling the availability of cytochrome c in the cytosol. It must underlined that pro-survival actors, as Bcl-2/ Bcl-xL, may also protect cells from apoptosis by inhibiting the availability of cytochrome c in the cytosol and act downstream of cytochrome c to prevent caspase activation under certain circumstances [START_REF] Rosse | Bcl-2 prolongs cell survival after Bax-induced release of cytochrome c[END_REF][START_REF] Zhivotovsky | Injected cytochrome c induces apoptosis[END_REF]. On the contrary, caspases inhibitors have no effect on this process.

These results point to a model where mitochondria might act as apoptotic amplifiers, fostering a positive feedback loop between cytochrome c-efflux and caspase activation. Any event that primes the loop will initiate the vicious "circle of death", leading to large-scale caspase activation and apoptotic cell death. However, this model does not prevail in all systems as, in numerous PCD, caspase inhibitors have no effect on the loss of cytochrome c [START_REF] Bossy-Wetzel | Mitochondrial cytochrome c release in apoptosis occurs upstream of DEVD-specific caspase activation and independently of mitochondrial transmembrane depolarization[END_REF]Kluck, et al., 1997a;Kluck et al., 1997b).

AIF

The experiments described above showed that mitochondria from apoptotic cells can effectively control nuclear apoptosis and suggest the involvement of mitochondria derived products in the apoptotic cascade. Acellular experiments have permitted to show that mitochondria contain a pre-formed approximately 50-kD protein which is released upon PT pores opening and which causes isolated nuclei to undergo apoptotic changes such as chromatin condensation and internucleosomal DNA fragmentation. This protein, named AIF (Apoptosis-inducing factor), was initially described as a caspase-activating protein which promotes nuclear apoptosis in vitro probably via the activation of procaspase-3, a major executive caspase (Susin, et al., 1996). More recently, it has been shown that AIF could directly induce nuclear apoptosis, without any caspase activation (Susin et al., 1999b).

As expected, when preventing mitochondrial permeability transition, Bcl-2 overexpressed in the outer mitochondrial membrane also impedes the release of AIF from isolated mitochondria. In contrast, Bcl-2 does not affect the formation of AIF, which is contained in comparable quantities in control mitochondria and in mitochondria from Bcl-2-hyperexpressing cells. Furthermore, the presence of Bcl-2 in the nuclear membrane does not interfere with the action of AIF on the nucleus, nor does Bcl-2 overexpression protect cells against AIF. It thus appears that Bcl-2 prevents apoptosis by favoring the retention of an apoptogenic protein in mitochondria (Susin, et al., 1996).

The above-described AIF could be a key regulator of such a cytochrome c-independent pathway inasmuch as these two mitochondrial components seem involved in distinct signaling route. Indeed, while AIF release occurs after the PT-associated mitochondrial depolarization, several reports indicates that cytochrome c release is dissociable from PT (Bossy-Wetzel, et al., 1998;[START_REF] Jurgensmeier | Bax directly induces release of cytochrome c from isolated mitochondria[END_REF]Kluck, et al., 1997a;Li, et al., 1998;[START_REF] Van Der Heiden | Bcl-xL regulates the membrane potential and volume homeostasis of mitochondria[END_REF][START_REF] Yang | Prevention of apoptosis by Bcl-2: release of cytochrome c from mitochondria blocked[END_REF]. Furthermore, cytosolic cytochrome c requires additional cytosolic factors to promote apoptotic changes, via the activation of caspase-3, whereas AIF, once released from mitochondria, directly induces nuclear apoptosis without cytosol. Beyond their differences, these two "execution caspase" activating pathways illustrate the sophistication and the apparent molecular redundancy, which characterize the mammalian cell death. They may correspond to alternative and independent links between death-triggering stimuli and the execution machinery or, in contrast, they may work together to induce complete PCD [START_REF] Golstein | Controlling Cell Death[END_REF]. Overexpression of Bcl-2/Bcl-xL in cells or addition of recombinant Bcl-2 to cell-free systems containing mitochondria prevented cytochrome c/AIF's exodus from the mitochondria that was triggered normally by a wide variety of apoptogenic stimuli [START_REF] Bossy-Wetzel | Mitochondrial cytochrome c release in apoptosis occurs upstream of DEVD-specific caspase activation and independently of mitochondrial transmembrane depolarization[END_REF]Kharbanda, et al., 1997;[START_REF] Van Der Heiden | Bcl-xL regulates the membrane potential and volume homeostasis of mitochondria[END_REF][START_REF] Yang | Prevention of apoptosis by Bcl-2: release of cytochrome c from mitochondria blocked[END_REF]. The protective effect of Bcl-2 was linked to its ability to prevent the release of these proteins.

Alternatively, opening of PT pores, which is a common event of apoptosis, might be involved in AIF and cytochrome c outflow (Skulachev, 1998a). Opening of the PT pore could be a first step of a cascade of events causing an increase in matrix volume and subsequently a mechanic disruption of the outer membrane. Recent results show that this scenario is possible at least in vitro [START_REF] Petit | Disruption of the outer mitochondrial membrane as a result of large amplitude swelling: the impact of irreversible permeability transition[END_REF][START_REF] Scarlett | Release of apoptogenic proteins from the mitochondrial intermembrane space during the mitochondrial permeability transition[END_REF][START_REF] Van Der Heiden | Bcl-xL regulates the membrane potential and volume homeostasis of mitochondria[END_REF]. Last, the pore-forming properties of Bax and other related proteins might suggest that they directly modulate the permeability of the outer mitochondrial membrane. Indeed, other studies have shown that Bax can induce cytochrome c release from mitochondria and caspase processing and activation (Jurgensmeier, et al., 1998). These events are sensitive to Bcl-xL but caspase inhibitors have no effect on release of cytochrome c from mitochondria (Gross, et al., 1998;[START_REF] Xiang | BAX-induced cell death may not require interleukin 1converting enzyme-like proteases[END_REF] although they prevent the subsequent activation of caspases in cytosolic extracts. Unlike Ca ++ , Bax did not induce PT and swelling of mitochondria in vitro in this system. These findings imply that Bax could uses an alternative mechanism for triggering release of cytochrome c from mitochondria may be through to the action of specific pores in the outer membrane.

Reactive oxygen species and apoptosis

What are ROS?

ROS, such as superoxide anion, radicals, hydrogen and organic peroxides are generated by all aerobic cells as by-products of a number of metabolite reactions and in response to various stimuli [START_REF] Fridovich | The biology of oxygen radicals[END_REF]. Mitochondria are believed to be a major site of ROS production, according to an endogenous and continuous physiological process under aerobic conditions (figure 1). However, endoplasmic reticulum and nuclear membranes also contain e _ transport chains that can lose e _ and generate superoxide radical. Some fatty acid metabolites, such as those derived from arachidonic acid by the lipoxygenase pathway, are also ROS. The physiological activity of the respiratory chain leads to the production of semi quinones, a potential source of ROS [START_REF] Papa | Reactive oxygen species, mitochondria, apoptosis and aging[END_REF].

Indeed, in aerobically respiring cells, the respiratory chain produces ROS at Complex I (NADH/Ubiquinone oxidoreductase) and Complex III (ubiquinol/cytochrome c oxido-reductase [START_REF] Boveris | Role of ubiquinone in the mitochondrial generation of hydrogen peroxide[END_REF][START_REF] Takeshige | NADH-and NADPH-dependent formation of superoxide anions by bovine heart submitochondrial particles and NADH-ubiquinone reductase preparation[END_REF][START_REF] Turrens | Ubisemiquinone is the electron donor for superoxide formation by complex III of heart mitochondria[END_REF]. The ubiquinone site in complex III appears as the major site of mitochondrial ROS production: this site catalyzes the conversion of molecular oxygen to superoxide anion radical (O2 -) by a single electron transfer to molecular oxygen (figure 1). Moreover, the inhibition of the respiratory chain, due to a lack of oxygen or to an inhibitor, such as cyanide or antimycin A, increases the ubisemiquinone free radical level in the normal catalytic mechanism of complex III [START_REF] Turrens | Ubisemiquinone is the electron donor for superoxide formation by complex III of heart mitochondria[END_REF].

In aqueous solutions, O2 -is moderately reactive but can generate other potent ROS such as H2O2 by dismutation. About 1-5% of mitochondria O2 consumption leads to H2O2 production [START_REF] Chance | Hydroperoxide metabolism in mammalian organs[END_REF]. A reductive homolytic cleavage of H2O2 produces the highly oxidative and cytotoxic hydroxyl radical (OH . ) according a metal-dependent breakdown. This highly reactive hydroxyl radical, by-product of superoxide anion or hydrogen peroxide, is assumed to be directly responsible for most of the oxidative damages leading to the non-physiological necrosis [START_REF] Halliwell | Role of free radicals and catalytic metal ions in human disease: an overview[END_REF]. Alternatively, mitochondria superoxide may react with nitric oxide to produce peroxynitrite [START_REF] Groves | Peroxynitrite: reactive, invasive and enigmatic[END_REF], a potent oxidant, which causes irreversible inhibition of mitochondria respiration and damage to mitochondria components (complexes I, II, IV and V, creatine kinase, aconitase, membranes, DNA, SOD…). It induces cell death in several cell types [START_REF] Lin | Peroxynitrite induces apoptosis of HL-60 cells by activation of a caspase-3 family protease[END_REF][START_REF] Liu | Peroxyacetyl nitrate-induced apoptosis through generation of reactive oxygen species in HL-60 cells[END_REF][START_REF] O'connor | Role of peroxynitrite in the protein oxidation and apoptotic DNA fragmentation in vascular smooth muscle cells stimulated with bacterial lipopolysaccharide and interferon-gamma[END_REF][START_REF] Szabo | DNA damage induced by peroxynitrite: subsequent biological effects[END_REF]. Brown recently reviewed the relationship between nitric oxide and mitochondria respiration [START_REF] Brown | Nitric oxide and mitochondrial respiration[END_REF].

Oxidative stress

An excess of reactive oxygen free radical species (ROS) in mitochondria causes an oxidative stress, with an enhanced activity of the antioxidant defense system and mitochondrial damage. Several situations, such as dysfunctional complex I [START_REF] Robinson | Human complex I deficiency: clinical spectrum and involvement of oxygen free radicals in the pathogenicity of the defect[END_REF], chemical poisoning and ischaemia followed by reperfusion, may subject tissues and cells to oxidative stress The main targets of ROS in mitochondria are the protein components of the membranes and the polyunsaturated fatty acids. In membranes, ROS and lipids affect cysteine residues (sulfhydryl groups), causing intramolecular cross-linkings and formation of proteins aggregates. OH

. radicals can initiate lipids peroxidation and generate peroxyl-and alkoxyl radical intermediates. Oxidants increase the release of calcium from mitochondria, thus stimulating calcium-dependent enzymes, such as proteases, nucleases, phospholipases involved in apoptosis. Because of the absence of mitochondria DNA-protecting proteins, the low efficiency reparation mechanism and the proximity of the respiration chain, mtDNA is a privileged target for ROS [START_REF] Shoji | DNA damage induced by tumour necrosis factor-alpha in L929 cells is mediated by mitochondrial oxygen radical formation[END_REF].

Protection of the cell

To prevent oxidative damage, mammalian cells have developed a complex antioxidant defense system that includes non-enzymatic antioxidants (e.g. glutathione, thioredoxine) as well as enzymatic activities (e.g. catalase, superoxide dismutase (SOD)) [START_REF] Sies | Oxidative stress: from basic research to clinical application[END_REF]. Mitochondria possess an antioxidant system with the superoxide dismutase (SOD), NADH and a complete glutathione redox system, which is formed of glutathione reductase, reduced glutathione (GSH), and glutathione peroxidase (figure 1). This system allows the reduction of oxidants, such as hydroperoxide. As mitochondria are generally devoid of catalase, the hydroperoxide detoxification mainly relies on GSH peroxidase. Various chemicals are known to induce oxidative stress, either as the result of an enhanced free radical production (e.g. xenobiotics utilizing cytochrome P450 for detoxification) or by impairment of their destruction provoked by glutathione depletion. Reed has shown that irreversible injury may occur if the mitochondria pool of glutathione is depleted by about 50% by incubation conditions or any toxic agent [START_REF] Reed | Glutathione: toxicological implications[END_REF]. Mitochondrial GSH plays an important role in preserving mitochondria membrane integrity and in assuring the reduced state of the intramitochondrial protein thiol groups.

Under unfavorable conditions, such as excess of ROS, mitochondria can enlarge and form "megamitochondria" with lower membrane potential and decreased rate of oxygen consumption and ROS production [START_REF] Karbowski | Free radical-induced megamitochondria formation and apoptosis[END_REF]. Incubation of adriamycin-or hydrazin-treated rat hepatocytes with exogenous coenzyme Q10 suppresses the formation of megamitochondria and inhibits the increase of hydrogen peroxide and the drop of mitochondria potential, normally found in these treatments. [START_REF] Lenaz | Oxidative stress, antioxidant defences and aging[END_REF][START_REF] Teranishi | Effects of coenzyme Q10 on changes in the membrane potential and rate of generation of reactive oxygen species in hydrazine-and chloramphenicol-treated rat liver mitochondria[END_REF]. Thus, the antioxidant effect of quinone acts on the membrane potential and the rate of generation of ROS. Furthermore, Skulachev proposed that a mitochondrial mild uncoupling and non-coupled oxidations could safe guard cells from high oxygen level and ROS generation (Skulachev, 1996a;Skulachev, 1996b;Skulachev, 1998b).

Physiological role

ROS play a role in physiological systems: they were shown to be responsible for the inducible expression of genes associated with inflammatory and immune responses. Current evidence indicates that different stimuli use ROS as signaling messengers to activate transcription factors, such as AP-1 and NF-κB, and induce gene expression [START_REF] Pinkus | Role of Oxidants and Antioxidants in the Induction of AP-1, NF-kB, and Glutathione S-Transferase Gene Expression[END_REF]. Additionally, ROS may have divergent effects according to the cell type, as they can function as mitotic stimuli [START_REF] Clement | Reactive oxygen intermediates regulate cellular response to apoptotic stimuli: an hypothesis[END_REF], inhibitors of drug-induced tumor cell death [START_REF] Pervaiz | Superoxide anion inhibits drug-induced tumor cell death[END_REF] or as cell death mediators [START_REF] Burdon | Superoxide and hydrogen peroxide in relation to mammalian cell proliferation[END_REF][START_REF] Burdon | Control of cell proliferation by reactive oxygen species[END_REF]. Indeed, superoxide mediates or induces proliferation of cardiac [START_REF] Li | Superoxide induces apoptosis in cardiomyocytes, but proliferation and expression of transforming growth factor-beta1 in cardiac fibroblasts[END_REF], vascular smooth muscle cells [START_REF] Bhunia | Redox-regulated signaling by lactosylceramide in the proliferation of human aortic smooth muscle cells[END_REF][START_REF] Brown | Overexpression of human catalase inhibits proliferation and promotes apoptosis in vascular smooth muscle cells [In Process Citation[END_REF][START_REF] Lee | Superoxide as an intermediate signal for serotonin-induced mitogenesis[END_REF]Li et al., 1997c;[START_REF] Rao | Active oxygen species stimulate vascular smooth muscle cell growth and proto-oncogene expression[END_REF]. On the opposite, it induces cell death in cardiomyocytes (Li, et al., 1999), hippocampal neurons [START_REF] Krohn | Staurosporine-induced apoptosis of cultured rat hippocampal neurons involves caspase-1-like proteases as upstream initiators and increased production of superoxide as a main downstream effector[END_REF] or proximal tubular cells [START_REF] Lieberthal | Role of superoxide in apoptosis induced by growth factor withdrawal[END_REF] The ability of oxidative stress, which is an excessive production of ROS, to provoke necrotic cell death as a result of massive cellular damages associated to lipid peroxidation and alterations of proteins and nucleic acids, is well documented for a long time [START_REF] Halliwell | Free radicals in Biology and Medecine[END_REF]. When free radicals generation exceeds the cellular capacity to detoxify them, cellular protein and DNA damage may occur, leading to the cell death In this point of view, aerobic cells appear as being under a continual ''oxidative siege", their survival depending on a balance between ROS and antioxidants.

The possible implication of ROS as signaling molecules in more physiological deaths such as apoptosis is a more recent concept. First evidence suggesting the involvement of mitochondria in cell death arose from the study of the TNF-α-induced cytotoxicity [START_REF] Lancaster | Inhibition of target cell mitochondrial electron transfer by tumor necrosis factor[END_REF][START_REF] Schulze-Osthoff | Cytotoxic activity of tumor necrosis factor is mediated by early damage of mitochondrial functions. Evidence for the involvement of mitochondrial radical generation[END_REF]. In addition to their role during the effector phase of apoptosis, an alteration of the mitochondrial function was associated with the early phases of the cell death and was defined as a crucial step of the process. The observed inhibition of the mitochondrial respiratory chain was assumed to result in the over production of Reactive Oxygen Species (ROS) which would act as mediators of the death signaling pathway [START_REF] Schulze-Osthoff | Depletion of the mitochondrial electron transport abrogates the cytotoxic and Gene-Inductive effects of TNF[END_REF]. Thus, there is mounting evidence that these compounds may be central in the cell death transduction pathways.

ROS are important physiological reactants in mitochondria [START_REF] Richter | Oxidants in mitochondria: from physiology to diseases[END_REF] and several observations suggest that ROS might mediate PCD:

1) The addition of ROS or the depletion of endogenous antioxidants can promote cell death [START_REF] Carmody | Reactive oxygen species as mediators of photoreceptor apoptosis in vitro[END_REF][START_REF] Guénal | Bcl-2 and Hsp27 act at different levels to suppress programmed cell death[END_REF][START_REF] Kane | Bcl-2 inhibition of neural death -decreased generation of reactive oxygen species[END_REF][START_REF] Lennon | Dose-dependent induction of apoptosis in human tumour cell lines by widely diverging stimuli[END_REF][START_REF] Ratan | Oxidative stress induces apoptosis in embryonic cortical neurons[END_REF][START_REF] Sato | Thiol-Mediated Redox Regulation of Apoptosis. Possible roles of cellular thiols other than glutathione in T cell apoptosis[END_REF].

2) Some antioxidants, such as thioredoxine or N-acetylcystein, act as intracellular ROS scavengers, and can inhibit activation of caspases and subsequent steps leading to the apoptotic cell death. In these models, ROS could be considered as major mediators, and not only the source of direct damage to DNA, proteins and lipids [START_REF] Greenlund | Superoxide dismutase delays neuronal apoptosis: a role for reactive oxygen species in programmed neuronal death[END_REF][START_REF] Iwata | Adult T cell leukemia (ATL)-derived factor/human thioredoxin prevents apoptosis of lymphoid cells induced by L-cystine and glutathione depletion: possible involvement of thiol-mediated redox regulation in apoptosis caused by pro-oxidant state[END_REF][START_REF] Mayer | N-acetyl-L-cysteine is a pluripotent protector against cell death and enhancer of trophic factor-mediated cell survival in vitro[END_REF][START_REF] Mehlen | Small stress proteins as novel regulators of apoptosis[END_REF][START_REF] Sandstrom | Autocrine production of extracellular catalase prevents apoptosis of the human CEM T-cell line in serum-free medium[END_REF][START_REF] Wong | Manganous superoxide dismutase is essential for cellular resistance to cytotoxicity of tumor necrosis factor[END_REF].

3) Increases in intracellular ROS are sometimes associated with PCD (Li, et al., 1999;[START_REF] Manna | Overexpression of manganese superoxide dismutase suppresses tumor necrosis factor-induced apoptosis and activation of nuclear transcription factor-kappaB and activated protein-1[END_REF][START_REF] Martin | Ultraviolet B irradiation of human leukaemia HL-60 cells in vitro induces apoptosis[END_REF][START_REF] Quillet-Mary | Implication of mitochondrial hydrogen peroxide generation in ceramide-induced apoptosis[END_REF][START_REF] Uckun | Ionizing radiation stimulates unidentified Tyrosine-Specific protein kinases in human Lymphocyte-B precursors, triggering apoptosis and clonogenic cell death[END_REF] 4) MnSOD overexpression restores mitochondrial transmembrane potential [START_REF] Majima | Prevention of mitochondrial injury by manganese superoxide dismutase reveals a primary mechanism for alkaline-induced cell death[END_REF] and inhibits the cell death caused by an inhibition of the respiratory chain [START_REF] Kiningham | Overexpression of manganese superoxide dismutase protects against mitochondrial-initiated poly(ADP-ribose) polymerase-mediated cell death[END_REF] 5) Cu/Zn SOD overexpression delays apoptosis by scavenging of O2-in neuronal cells deprived of nerve growth factor [START_REF] Greenlund | Superoxide dismutase delays neuronal apoptosis: a role for reactive oxygen species in programmed neuronal death[END_REF] 6) Mitochondrial phospholipid hydroperoxide glutathione peroxidase overexpression inhibits increase of ROS, release of cytc, activation of caspase-3 and apoptosis [START_REF] Nomura | Mitochondrial Phospholipid Hydroperoxide Glutathione Peroxidase Suppresses Apoptosis Mediated by a Mitochondrial Death Pathway[END_REF] ROS could be considered as major mediators, and not only the source of direct damage to DNA, proteins and lipids [START_REF] Tan | The regulation of reactive oxygen species production during programmed cell death[END_REF]

ROS participate in early and late steps of the regulation of apoptosis

In addition to their role in TNF--induced killing (figure 2), the contribution of ROS to the activation of the execution machinery was extended to PCD triggered by a wide range of influences including UV light, ionizing irradiation, anthracyclines, ceramides, glucocorticoids or survivalfactor withdrawal [START_REF] Jacobson | Reactive oxygen species and programmed cell death[END_REF]. In different models, the ROS generation preceded the loss of mitochondria membrane potential, nuclear condensation and other typical apoptotic events. For example, an early and sustained production of ROS with a concomitant depletion of intra cellular glutathione was observed in an in vitro model of photoreceptor apoptosis [START_REF] Carmody | Reactive oxygen species as mediators of photoreceptor apoptosis in vitro[END_REF]. Increase in ROS levels is an early process in Fas-induced apoptosis, independent from caspases activation [START_REF] Banki | Elevation of mitochondrial transmembrane potential and reactive oxygen intermediate levels are early events and occur independently from activation of caspases in Fas signaling[END_REF]. In ischaemia-induced apoptosis, ROS are involved upstream of caspases and bax [START_REF] Maulik | Oxidative stress developed during the reperfusion of ischemic myocardium induces apoptosis[END_REF]. Tada-Oikawa et al. showed that during DNA alkylation-induced apoptosis, the hydrogen peroxide generation acted upstream the mitochondria membrane potential drop [START_REF] Tada-Oikawa | Generation of hydrogen peroxide precedes loss of mitochondrial membrane potential during DNA alkylationinduced apoptosis[END_REF].

However, in K+ deprivation induced apoptosis of cerebellar granule neurons, ROS accumulation plays an important role during the late steps. In this model, the ROS production is prevented by cycloheximide, actinomycin D and caspase inhibitor YVAD-cmk, indicating that ROS are involved downstream of gene transcription, traduction, and activation of caspases [START_REF] Schulz | Cooperative interception of neuronal apoptosis by BCL-2 and BAG-1 expression: prevention of caspase activation and reduced production of reactive oxygen species[END_REF][START_REF] Schulz | Potassium deprivation-induced apoptosis of cerebellar granule neurons: a sequential requirement for new mRNA and protein synthesis, ICE-like protease activity, and reactive oxygen species[END_REF]. Moreover some data raise the possibility that ROS are also required for the execution of the death program (Kroemer, et al., 1995). This must be cautiously considered inasmuch as, in the majority of these systems, it is difficult to ascertain that the observed ROS accumulation corresponds to a causal effect and is not a side effect of the other changes accompanying the killing process [START_REF] Cai | Superoxide in apoptosis. Mitochondrial generation triggered by cytochrome c loss[END_REF]. Moreover, in these cases, ROS increase most often arises during the later stage of the death program, i.e. during the destruction phase when the cell is broken down, and may be associated with a necrotic type terminal degradation of the cell. Exogenous sources of ROS such as hydrogen peroxide can induce PCD or necrosis depending upon the dose added [START_REF] Guénal | Bcl-2 and Hsp27 act at different levels to suppress programmed cell death[END_REF]. So a burst in ROS, in response to a dramatic perturbation of the physiology of the dying cell, could convert the late PCD steps into necrotic death. Therefore, it appears that at any moment the level of intracellular ROS can determine the fate of the cell: low levels of ROS can induce PCD while accumulation of high levels promotes necrosis or can lead PCD-committed cells toward necrotic-like destruction.

Origin of apoptosis-mediated ROS

Two opposite models have emerged concerning the source of signaling ROS, in relation with the variety of metabolic reactions and intracellular sites which can generate ROS [START_REF] Jacobson | Reactive oxygen species and programmed cell death[END_REF]. While most investigators believe that oxidants are produced by electron chain transport, some data seem to moderate this point of view. Fatty acid metabolites, such as those produced from arachidonic acid by the lipoxygenase pathway, may be better mediators of apoptosis [START_REF] O'donnell | Involvement of oxidants and oxidant-generating enzyme(s) in tumour-necrosis-factor-alpha-mediated apoptosis: role for lipoxygenase pathway but not mitochondrial respiratory chain[END_REF]. On the one hand, it is argued that these molecules harbor a more specific reactivity than superoxide anion and its by-products, this biological specificity being assumed necessary for a signaling role in apoptosis transduction pathways. On the other hand, it was shown that exogenous fatty acid metabolites can promote programmed cell death and that, in some cases, their increased production was associated with cell death. It must be underlined that such a situation is limited to systems where the death signal result is mediated by surface receptors. Nevertheless, these considerations do not refute the compelling evidence of the involvement of electron transport chainproduced ROS in cell death signaling. It appears more reasonable to consider that, depending upon the cell death stimulus and the cell model, these two types of ROS can mediate apoptosis or even both contribute to the activation of the execution machinery, as suggested by studies of TNF-induced programmed cell death.

It was established that both ROS accumulation and programmed cell death process require the presence of a functional mitochondrial respiratory chain in most ROS-dependent cell death systems [START_REF] Higuchi | Activation of CPP32-like protease in tumor necrosis factor-induced apoptosis is dependent on mitochondrial function[END_REF][START_REF] Quillet-Mary | Implication of mitochondrial hydrogen peroxide generation in ceramide-induced apoptosis[END_REF][START_REF] Schulze-Osthoff | Depletion of the mitochondrial electron transport abrogates the cytotoxic and Gene-Inductive effects of TNF[END_REF][START_REF] Sidoti-De Fraisse | TNF- activates at least two apoptotic signaling cascades[END_REF]. Indeed, it was shown that an upstream inhibition, with chemical compounds acting on complex I (Quillet- [START_REF] Quillet-Mary | Implication of mitochondrial hydrogen peroxide generation in ceramide-induced apoptosis[END_REF][START_REF] Schulze-Osthoff | Cytotoxic activity of tumor necrosis factor is mediated by early damage of mitochondrial functions. Evidence for the involvement of mitochondrial radical generation[END_REF], or an elimination of the electron transfer chain [START_REF] Higuchi | Activation of CPP32-like protease in tumor necrosis factor-induced apoptosis is dependent on mitochondrial function[END_REF][START_REF] Schulze-Osthoff | Depletion of the mitochondrial electron transport abrogates the cytotoxic and Gene-Inductive effects of TNF[END_REF][START_REF] Sidoti-De Fraisse | TNF- activates at least two apoptotic signaling cascades[END_REF], by depletion of the mtDNA, prevent ROS accumulation and consequently protect cells against PCD. Another indirect argument is provided by the scavenger role of mitochondrial glutathione in the regulation of ROS-mediated PCD [START_REF] Goossens | Direct evidence for tumor necrosis factorinduced mitochondrial reactive oxygen intermediates and their involvement in cytotoxicity[END_REF]. The ubiquinone site in complex III appears as the major site of mitochondrial ROS production as this site catalyzes the conversion of molecular oxygen to superoxide anion which can lead to the formation of other potent ROS such as hydrogen peroxide and hydrogen radicals (figure 1). The inhibition of the respiratory chain, due to a lack of oxygen or to an inhibitor, such as cyanide or antimycin A, increases the ubisemiquinone free radical level in the normal catalytic mechanism of complex III [START_REF] Turrens | Ubisemiquinone is the electron donor for superoxide formation by complex III of heart mitochondria[END_REF]. Such a model is supported by the observed potentiation of cell death processes in ROSdependent PCD when electron flow was inhibited distal to the ubiquinone pool.

Mechanisms of ROS signalling

The involvement of mitochondrial ROS in some cell death transduction pathways next leads to the fundamental questions concerning, on the one hand, the causal event of the increased ROS generation and, on the other hand, the molecular mechanisms underlying the ROS signaling. Two viewpoints must first be considered to address the question of the origin of ROS accumulation, which can indeed result from an increased production or from a reduced scavenging by the cellular detoxifying systems. Much of the available data converge to the hypothesis that ROS increases are the consequence of an impairment of the mitochondrial respiratory chain [START_REF] Cai | Superoxide in apoptosis. Mitochondrial generation triggered by cytochrome c loss[END_REF][START_REF] Esposti | Mitochondria and cells produce reactive oxygen species in virtual anaerobiosis: relevance to ceramide-induced apoptosis[END_REF][START_REF] France-Lanord | Mitochondrial free radical signal in ceramide-dependent apoptosis: a putative mechanism for neuronal death in Parkinson's disease[END_REF][START_REF] Gudz | Direct inhibition of mitochondrial respiratory cahin complex III by cell-permeable ceramide[END_REF][START_REF] Quillet-Mary | Implication of mitochondrial hydrogen peroxide generation in ceramide-induced apoptosis[END_REF][START_REF] Schulze-Osthoff | Cytotoxic activity of tumor necrosis factor is mediated by early damage of mitochondrial functions. Evidence for the involvement of mitochondrial radical generation[END_REF]. In agreement with the above considerations, the observed alterations are distal to the ubiquinone site of the complex III, but the origin of these electron flow disturbances are not clear. The only strong evidence comes from the study of ceramide-induced PCD, in which an increased H2O2 production was linked to mitochondrial Ca 2+ homeostasis perturbation as inhibition of the mitochondrial Ca 2+ uptake was shown to abolish both ROS accumulation and cell death. However, the recently observed shift of cytochrome c from mitochondria to cytosol in the early phases of many PCD (see above) could provide a clue to resolve this question (Kluck, et al., 1997a;[START_REF] Liu | Induction of apoptotic program in cell-free extracts: requirement for dATP and cytochrome c[END_REF][START_REF] Yang | Prevention of apoptosis by Bcl-2: release of cytochrome c from mitochondria blocked[END_REF]. Indeed, the release of cytochrome c must lead to a breakdown of the mitochondrial electron flow downstream of the ubiquinone site, which in turn would result in an increased generation of ROS. Such a model is supported by the described correlation between loss of cytochrome c activity and respiratory failure in a Fas-induced PCD model [START_REF] Krippner | Loss of Function of Cytochrome c in Jurkat Cells Undergoing Fas-mediated Apoptosis[END_REF] and by the study of mitochondria isolated from apoptotic cells which shows a superoxide production due to a switch from the normal 4-electron reduction of O2 to a 1-electron reduction when cytochrome c is released from mitochondria [START_REF] Cai | Superoxide in apoptosis. Mitochondrial generation triggered by cytochrome c loss[END_REF].

Beside the question of the process of mitochondrial ROS accumulation, arises the problematic concerning the targets of these compounds or more precisely: how can they mediate PCD? Two models can be proposed to approach this conflicting and not well-documented subject. The first model assumes that ROS themselves are signaling molecules, which activate some crucial components of the PCD machinery. Conversely, the alternative proposition suggests that ROS can act indirectly by modifying the cellular redox potential, which would regulate some key regulatory proteins involved in PCD. Several lines of evidence agree with an explanation based on indirectly mediated action. First, unlike fatty acid metabolites which harbor specific reactivity and are known to mediate particular signals from surface receptors, mitochondrial ROS are characterized by a lack of biological specificity or even an extreme reactivity, as for the hydroxyl radical: these are all features contrary to the requirements of a specific signaling role [START_REF] Jacobson | Reactive oxygen species and programmed cell death[END_REF]. In this way, a direct influence of ROS on PCD process would be correlated to a general damaging effect on cellular structures resulting in necrotic cell death, or perhaps to a more limited action on mitochondria, their site of production, which in turn could activate some mitochondria-dependent downstream cascades leading to PCD. Secondly, despite the compelling evidence of the role of mitochondrial ROS in PCD signaling pathways, it has been assumed that they do not represent a general mediator of cell death, as suggested by the ability of some PCD to occur in very low oxygen environments [START_REF] Jacobson | Programmed cell death and Bcl-2 protection in very low oxygen[END_REF][START_REF] Shimizu | Prevention of hypoxia-induced cell death by Bcl-2 and Bcl-X L[END_REF]. However, ROS might be produced in such conditions [START_REF] Esposti | Mitochondria and cells produce reactive oxygen species in virtual anaerobiosis: relevance to ceramide-induced apoptosis[END_REF].

Another alternative explanation would be to consider that the major effect of an increased ROS production is the subsequent imbalance of intracellular redox status, i.e. an enhancement of the cellular oxidative tonus, and that in fact the oxidative stress is the central common effector of PCD. Hence, ROS accumulation would only be one way which leads in some PCD to an oxidative status. Anti-Fas/APO-1 antibody or IL-3 withdrawal-induced PCD represent good illustrations of this model [START_REF] Bojes | Bcl-x L overexpression attenuates glutathione depletion in FL5.12 cells following interleukin-3 withdrawal[END_REF][START_REF] Van Den Dobbelsteen | Rapid and Specific Efflux of Reduced Glutathione during Apoptosis Induced by Anti-Fas/APO-1 Antibody[END_REF]. Indeed, no ROS accumulation can be measured in these two systems and anaerobic cultured cells deprived of IL-3 still undergo PCD (Schulze-Osthoff et al., 1994a;[START_REF] Shimizu | Prevention of hypoxia-induced cell death by Bcl-2 and Bcl-X L[END_REF]. However, an oxidative stress can be shown in these models as a depletion of glutathione (GSH), a non-enzymatic cellular antioxidant, as a result of a rapid and specific efflux of glutathione, an event that takes place at the very beginning of the apoptotic process [START_REF] Bojes | Bcl-x L overexpression attenuates glutathione depletion in FL5.12 cells following interleukin-3 withdrawal[END_REF][START_REF] Van Den Dobbelsteen | Rapid and Specific Efflux of Reduced Glutathione during Apoptosis Induced by Anti-Fas/APO-1 Antibody[END_REF] . Moreover, it has been shown that Bcl-2 can protect cells from PCD by shifting the cellular redox potential to a more reduced state (see below). However, the observation that oxidation of thiols other than glutathione can mediate induction of PCD suggest that the intracellular thiol redox status would be the real key factor of the cell death signaling pathways (Kane, et al., 1993;[START_REF] Marchetti | Redox regulation of apoptosis: impact of thiol oxidation status on mitochondrial function[END_REF][START_REF] Mirkovic | Resistance to radiation-induced apoptosis in Bcl-2-expressing cells is reversed by depleting cellular thiols[END_REF]Sato, et al., 1995). In this model, the redox state of glutathione or other cellular antioxidants such as thioredoxine, would be in equilibrium with that of thiols resident in some redox sensitive crucial components of the execution machinery (Kroemer, et al., 1997). Where does ROS fit this thiol hypothesis? In this putative model, an increased production of mitochondrial ROS would result, either by a direct modification of the thiols or indirectly via a depletion of the intracellular antioxidant pool, in a shift of the redox state of the sensor SH groups to a more oxidized state. The nature of the ROS and the level of the intracellular antioxidant defenses would determine in which way regulatory components are activated to commit cells to PCD. In a hypothetical model, mitochondrial ROS modify the membrane permeability leading to the release of pro-apoptotic factors and/or activate directly executing caspases.

Bcl-2 and Bcl-XL inhibits apoptosis by providing a protection against ROS and/or shifting the cellular redox potential to a more reduced state

Several lines of evidence support the idea that Bcl-2 acts in an antioxidant pathway to suppress apoptosis. Yeast mutants lacking superoxide dismutase were partially rescued by expression of Bcl-2 (Kane, et al., 1993;[START_REF] Longo | Human Bcl-2 reverses survival defects in yeast lacking superoxide dismutase and delays death of wild-type yeast[END_REF]. Oxidative stress, such as exposure to hydrogen peroxide or 13-L-hydroperoxylinoleic acid, increases the level of ROS and triggers loss of ΔΨm, apoptotic nuclear condensation and DNA fragmentation in normal PC12 cells. When transfected in these cells, Bcl-2 prevents the decrease in ΔΨm and the cell death induced by oxidative stress but not the increase in level of [START_REF] Satoh | Changes in mitochondrial membrane potential during oxidative stress-induced apoptosis in PC12 cells[END_REF]. This indicates that Bcl-2 acts downstream of ROS generation in this system. Following an apoptotic signal, overexpression of Bcl-2 suppressed lipid peroxidation completely [START_REF] Hockenbery | Bcl-2 functions in an antioxidant pathway to prevent apoptosis[END_REF] and attenuates the generation of ROS [START_REF] Bogdanov | Oxidative stress is attenuated in mice overexpressing BCL-2[END_REF]. Bcl-2 deficient mice turn gray with the second hair follicle cycle, implicating possibly a defect in redox-regulated melanin synthesis [START_REF] Veis | Bcl-2-Deficient mice demonstrate fulminant lymphoid apoptosis, polycystic kidneys, and hypopigmented hair[END_REF]. Bcl-2 can protect neural cells from delayed death resulting from chemical hypoxia and reenergization, and may do so by an antioxidant mechanism [START_REF] Myers | Bcl-2 protects neural cells from cyanide/aglycemia-induced lipid oxidation, mitochondrial injury, and loss of viability[END_REF] (reviewed in [START_REF] Korsmeyer | Reactive oxygen species and the regulation of cell death by the Bcl-2 gene family[END_REF].

It was shown that Bcl-2 might act as an antioxidant partner to block a putative ROSmediated step in the cascade of events required for apoptosis, but the way by which Bcl-2 protects from ROS remains unclear. In some systems, Bcl-2 appears to influence the generation of oxygen free radicals (Kane, et al., 1993), while in other cases it does not affect ROS production but does prevent oxidative damage to cellular constituents [START_REF] Hockenbery | Bcl-2 functions in an antioxidant pathway to prevent apoptosis[END_REF][START_REF] Satoh | Changes in mitochondrial membrane potential during oxidative stress-induced apoptosis in PC12 cells[END_REF][START_REF] Tyurina | Direct evidence for antioxidant effect of Bcl-2 in PC12 rat pheochromocytoma cells[END_REF]. Bcl-2 acts locally in mitochondria and plasma membranes, as assessed by electron paramagnetic resonance spectroscopy analysis [START_REF] Bruce-Keller | Anti-death properties of TNF against metabolic poisoning: mitochondrial stabilization by MnSOD[END_REF]. Alternatively, since superoxide is produced by mitochondria from apoptotic cells due to a switch from the normal 4-electron reduction of O2 to a 1-electron reduction, the block of cytochrome c release could provides a mechanism for the apparent antioxidant function of Bcl-2 [START_REF] Cai | Superoxide in apoptosis. Mitochondrial generation triggered by cytochrome c loss[END_REF] . It has also been proposed that it functions as a pro-oxidant and influences the levels of ROS, inducing in this way endogenous cellular antioxidants [START_REF] Steinman | The Bcl-2 oncoprotein functions as a pro-oxidant[END_REF].

However, apoptosis can proceed normally, and can be prevented by Bcl-2, under anaerobic conditions which minimize the formation of ROS [START_REF] Jacobson | Programmed cell death and Bcl-2 protection in very low oxygen[END_REF][START_REF] Shimizu | Prevention of hypoxia-induced cell death by Bcl-2 and Bcl-X L[END_REF]. This observation reinforces the view that Bcl-2 acts in more than one way either to prevent the induction of apoptosis by different stimuli (ROS dependent or not) or to control different aspects of the apoptotic effector pathway (reviewed in [START_REF] Jacobson | Reactive oxygen species and programmed cell death[END_REF]. As described above, apoptosis might be modulated by redox sensitive proteins via their sulfhydryl groups and antioxidants such as reduced glutathione or other thiols may modify the functions of these proteins.

Several authors have studied the effect of Bcl-2s on cellular redox potential. Activities of antioxidant enzymes and levels of glutathione and pyridine nucleotides have been measured in pheochromocytoma PC12 and the hypothalamic GnRH cell line GT1-7 cells transfected with bcl-2 [START_REF] Ellerby | Shift of the cellular oxidation-reduction potential in neural cells expressing Bcl-2[END_REF] Both cell lines overexpressing bcl-2 had elevated total glutathione levels when compared with control transfectants. The ratios of oxidized glutathione to total glutathione in PC12 and GT1-7 cells overexpressing bcl-2 were significantly reduced. In addition, the NAD+/NADH ratio of bcl-2-expressing PC12 and GT1-7 cells was two-to threefold less than that of control cell lines while they had approximately the same level of catalase, superoxide dismutase, glutathione peroxidase and glutathione reductase activities as control cells. These results indicate that the overexpression of bcl-2 shifts the cellular redox potential to a more reduced state, without consistently affecting the major cellular antioxidant enzymes. Furthermore, depleting cellular thiols reversed the resistance to radiation in Bcl-2 expressing lymphoma cell lines [START_REF] Mirkovic | Resistance to radiation-induced apoptosis in Bcl-2-expressing cells is reversed by depleting cellular thiols[END_REF].

The ability of bax and Bcl-xL to affect GSH was assessed in interleukin (IL)-3-dependent murine prolymphocytic FL5.12 cells [START_REF] Bojes | Bcl-x L overexpression attenuates glutathione depletion in FL5.12 cells following interleukin-3 withdrawal[END_REF] Overall levels of GSH increased in bcl-xL transfectants while, in cells overexpressing bax, GSH was reduced by approximately 36%. There were no consistent differences between these cell lines in the activities of superoxide dismutase, catalase, and glutathione peroxidase or glutathione reductase. Following IL-3 withdrawal-induced apoptosis, control cells and bax transfectants exhibit a rapid loss of intracellular GSH that 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.

Thus, both Bcl-2 and Bcl-xL can protect cells from apoptosis by shifting the cellular redox potential to a more reduced state. Assuming that mitochondrial thiols constitute a critical sensor of the cellular redox potential during apoptosis (Marchetti, et al., 1997), these effects could be at the mitochondrial level.

Yeast and apoptosis

Another exciting question to resolve is the occurrence of an apoptosis-like phenotype associated with a specific mutation in Saccharomyces cerevisiae [START_REF] Madeo | A yeast mutant showing diagnostic markers of early and late apoptosis[END_REF]. Indeed, the cdc48 S565G mutant show typical markers of apoptosis, including exposure of phosphatidylserine, chromatin condensation and fragmentation, DNA fragmentation, and formation of minicells, which look like apoptotic bodies. Saccharomyces cerevisiae and Schizosaccharomyces pombe yeast species do not contain endogenous caspases or Bcl-2s gene [START_REF] Greenhalf | Role of mitochondria and C-terminal membrane anchor Bcl2 in Bax induced growth arrest and mortality in Saccharomyces cerevisiae[END_REF][START_REF] Ink | Human Bak induces cell death in Schizosaccharomyces pombe with morphological changes similar to those with apoptosis in mammalian cells[END_REF]Zha et al., 1996b) and their cytochrome c has been shown to be ineffective to induce nuclear apoptosis in acellular assay (Kluck, et al., 1997b). Moreover, if caspases are important in the mammalian apoptotic process, some models reveal a caspase-independent pathway. For example, Bax or Bak expression, and cytosolic AIF induce apoptosis in presence of broad caspase inhibitors [START_REF] Mccarthy | Inhibition of Ced-3/ICErelated Proteases Does Not Prevent Cell Death Induced by Oncogenes, DNA Damage, or Bcl-2 Homologue Bak[END_REF][START_REF] Monney | Defects in the ubiquitin pathway induce caspase-independent apoptosis blocked by Bcl-2[END_REF]Susin, et al., 1999b;[START_REF] Xiang | BAX-induced cell death may not require interleukin 1converting enzyme-like proteases[END_REF]. The data in yeast suggest the existence of yet unknown PCD pathways, independent of Bcl-2s and caspases families, in which the place and the role of mitochondria remain to be determined

The heterologous expression of mammalian pro-apoptotic genes (ced-4, bax, bak) provokes mitochondrial dysfunction and the death of yeast cells [START_REF] Greenhalf | Role of mitochondria and C-terminal membrane anchor Bcl2 in Bax induced growth arrest and mortality in Saccharomyces cerevisiae[END_REF][START_REF] Hanada | Structure-function analysis of Bcl-2 protein. Identification of conserved domains important for homodimerization with Bcl-2 and heterodimerization with Bax[END_REF][START_REF] Ink | Human Bak induces cell death in Schizosaccharomyces pombe with morphological changes similar to those with apoptosis in mammalian cells[END_REF][START_REF] James | CED-4 induces chromatin condensation in Schizosaccharomyces pombe and is inhibited by direct physical association with CED-9[END_REF][START_REF] Ligr | Mammalian Bax triggers apoptotic changes in yeast[END_REF][START_REF] Sato | Interactions among members of the Bcl-2 protein family analyzed with a yeast two-hybrid system[END_REF]. Several authors reported that this induced-cytotoxicity was characterized by typical phenotypic markers of apoptosis [START_REF] Ink | Human Bak induces cell death in Schizosaccharomyces pombe with morphological changes similar to those with apoptosis in mammalian cells[END_REF][START_REF] Ligr | Mammalian Bax triggers apoptotic changes in yeast[END_REF], identical to those observed in the pseudo-apoptotic mutant cdc48 S565G [START_REF] Madeo | A yeast mutant showing diagnostic markers of early and late apoptosis[END_REF], and other features, such as release of cytochrome c [START_REF] Chaudhuri | Release of cytochrome c and decrease of cytochrome c oxidase in Bax-expressing yeast cells, and prevention of these effects by coexpression of Bcl-xL[END_REF]. Moreover, as observed in mammals, coexpression of Bcl-2 or Bcl-XL with Bax abolishes the pseudo-apoptotic phenotype and the cell death [START_REF] Jürgensmeier | Bax-and Bak-induced cell death in the fission yeast Schizosaccharomyces pombe[END_REF][START_REF] Ligr | Mammalian Bax triggers apoptotic changes in yeast[END_REF][START_REF] Chaudhuri | Release of cytochrome c and decrease of cytochrome c oxidase in Bax-expressing yeast cells, and prevention of these effects by coexpression of Bcl-xL[END_REF][START_REF] Matsuyama | The Mitochondrial F0F1-ATPase proton pump is required for function of the proapoptotic protein bax in yeast and mammalian cells[END_REF][START_REF] Tao | Modulation of cell death in yeast by the Bcl-2 family of proteins[END_REF]Zha et al., 1996a). Since on the one hand, Bax cytotoxicity is increased under respiratory condition [START_REF] Priault | Comparison of the effects of bax-expression in yeast under fermentative and respiratory conditions: investigation of the role of adenine nucleotides carrier and cytochrome c[END_REF], and, on the other hand, yeast mutants lacking superoxide dismutase were partially rescued by expression of Bcl-2 (Kane, et al., 1993;[START_REF] Longo | Human Bcl-2 reverses survival defects in yeast lacking superoxide dismutase and delays death of wild-type yeast[END_REF], it is tempting to speculate that a mitochondrial ROS production is involved in the bax-induced cell death process. In agreement with this hypothesis, the apoptotic-like features observed in these yeast strains is accompanied with ROS accumulation in the cells [START_REF] Madeo | Oxygen stress: a regulator of apoptosis in yeast[END_REF]. Moreover, low concentration of H2O2 treatment or glutathione depletion could induce the pseudo-apoptosis in yeast [START_REF] Madeo | Oxygen stress: a regulator of apoptosis in yeast[END_REF]. The induced-cell death is not due to a direct cell damage, but requires an active cooperation of the cell machinery. ROS accumulation in cells seem to be necessary and sufficient to lead to the pseudo-apoptotic phenotype in yeast, whatever the apoptosis inducer.

Conclusion

In conclusion, mitochondria are involved in the decision of cells to survive or not at several levels. At a first level, mitochondria are involved in the control of the activation of the cell death machinery by sequestering, via Bcl-2 family proteins, pro-caspases and caspase activators, such as AIF or cytochrome c. However, the mechanism allowing the release of AIF and cytochrome c needs further investigations. Opening of the PT pore could be just a first step of a cascade of events causing an increase in the permeability of the outer mitochondrial membrane, and Bcl-2 could inhibit apoptosis by preventing permeability transition. At present, it remains elusive whether this permeability increase is due to the action of specific pores in the outer membrane and/or to its mechanic disruption secondary to an increase in matrix volume.

This recent accumulation of data has also generated a number of new questions. Future works must address the connection between survival signals and mitochondrial functions. The study of Bad regulation, a proapoptotic Bcl-2 member that is supposed to counteract Bcl-2 has provided an example of how survival factors counteract PCD. Indeed, Bad is inactivated by phosphorylation by Akt [START_REF] Datta | Akt phosphorylation of BAD couples survival signals to the cell-intrinsic death machinery[END_REF][START_REF] Del Peso | Interleukin-3-induced phosphorylation of BAD through the protein kinase Akt[END_REF] and Raf-1 [START_REF] Wang | Bcl-2 Targets the Protein Kinase Raf-1 to Mitochondria[END_REF], two kinases involved in survival signals transduction. It has been suggested that, in the absence of phosphorylation, Bad induces cell death possibly via the formation of heterodimers with Bcl-xL (or Bcl-2 depending of the cell type) and the concomitant generation of Bax homodimers. Assuming that all mammalian cells constitutively express all the protein components required to execute the death program [START_REF] Jacobson | Programmed Cell Death and Bcl-2 Protection in the Absence of a Nucleus[END_REF], these results suggest that Bax (or an other similar pro-apoptotic member of the Bcl-2 family) are ubiquitously expressed and that survival requires their continuous inhibition. Taking into account the mitochondrial membrane localization of these proteins and their pore forming properties, it can be proposed that this kind of regulation operates at the mitochondrial level to control membrane permeability and efflux of AIF and cytochrome c.

The importance of mitochondria in apoptosis has been reinforced by studies showing the contribution of reactive oxygen species in the cell-death signaling. The mechanism of ROS signaling is still unknown, but it could be speculated that they act by modifying Bcl-2 proteins family conformation. Indeed, it was proposed that in healthy cells Bax is a cytosolic soluble monomer, whereas upon a death stimulus the Bax COOH-terminal hydrophobic domain becomes exposed, allows Bax redistribution to organelles, promoting cell death [START_REF] Hsu | Bax in murine thymus is a soluble monomeric protein that displays differential detergent-induced conformations[END_REF][START_REF] Wolter | Movement of Bax from the Cytosol to Mitochondria during Apoptosis[END_REF]. One could imagine that such a conformational change is induced by ROS. If the primer function of apoptosis was possibly to clear ROS-producing cells from tissues (Skulachev, 1996b;Skulachev, 1998a), mitochondrial ROS production are used as active mediators in the regulation of apoptosis at different levels. Moreover, the fact that pseudo-apoptosis phenotype in yeast invariably involves ROS [START_REF] Madeo | Oxygen stress: a regulator of apoptosis in yeast[END_REF] points out these compounds as major evolution-conserved regulators of cell death. ROS can be generated by various enzymes (xanthine oxidase, lipoxygenases, NADPH oxidases,…) or during the mitochondrial oxidative phosphorylation at complex I and complex III (see text). Complex I (NADH-ubiquinone oxidoreductase) catalyzes the following reaction with electron transfer : NADH + Q + nH + (matrix) NAD + + QH2 + (n-1)H + (intermembrane space) , where Q is the ubiquinone Q10. The ubiquinone site (UQ) in complex III catalyzes the conversion of molecular oxygen to superoxide anion radical (O2 .-) by a single electron transfer to molecular oxygen. O2 .-are converted to hydrogen peroxide (H2O2 ) by the superoxide dismutase (SOD).

H2O2 detoxification normally occurs according two different reactions, either by the thiolspecific antioxidant and the thioredoxine reductase (TR), which reduces thioredoxine (TX -> TX-H2), or by reacting with glutathione (GSH) by glutathione peroxidase (GPO). The latter reaction produces water and oxidized glutathione (GSSG), and GSSG is recycled to GSH by glutathione reductase (GR). Glutathione is mainly maintained in a reduced state, via NADPH generation in the reversible non-oxidative part of the pentose phosphate pathway [START_REF] Banki | Glutathione levels and sensitivity to apoptosis are regulated by changes in transaldolase expression[END_REF]. When GSH level is low, H2O2 can react with Fe 2+ to produce OH -and the harmful hydroxyl anions (OH .-). Alternatively, H2O2 reacts with O2 .-producing water, molecular oxygen and OH .-.

Superoxide may also react with nitric oxide (NO) to produce peroxynitrite (ONOO .-), a potent oxidant, which causes irreversible inhibition of mitochondria respiration and damage to mitochondria components (complexes I, II, IV and V, creatine kinase, aconitase, membranes, DNA, SOD,…) and induce cell death in several cell types.

If ROS production exceeds the cellular ability of detoxification (oxidative stress), matrix and membrane proteins, DNA and/or lipids can be damaged (

).

The complexes of the electron transfer chain in the inner mitochondrial membrane are numbered from I to IV. The electron (e-) pathway through the respiratory chain is indicated by arrows. Cyt c refers to cytochrome c.

Figure 2. Schematic model of the role of mitochondrial ROS in apoptosis

Numerous signals can lead to apoptosis and the induction pathways seem to converge to events involving mitochondria. Alternatively, activating caspases directly activate the executing caspases in a mitochondria-independent manner ( ). At a first level, mitochondria can contribute to apoptosis signaling, as shown in TNF--or ceramide-induced cell death, during which increased mitochondrial ROS production appears as an early event of the induction phase. ROS accumulate as a result of a dysfunction in the mitochondrial respiratory chain. ROS also intervene in later steps in an amplifying loop, acting on mitochondrial membrane permeability, and contributing then to the activation of execution caspase.

Proteins of the Bcl-2 family act on mitochondrial membrane permeability and regulate the release of pro-apoptotic factors from the intermembrane space to the cytosol. These apoptogenic factors activate proteins involved in the final degradation of the cell components, in executing caspases -dependent (procaspases and cytochrome c) or -independent (AIF) pathways. In return, they can act in an amplification loop upon mitochondria ( ) Table I: Programmed cell death in nematodes and mammals is controlled by homologous proteins. Mammalian caspases act either during the activation or the execution phase of PCD [START_REF] Nicholson | Caspases: killer proteases[END_REF]. A CED-4 homologue has been identified in human cells [START_REF] Hofmann | The CARD domain: a new apoptotic signalling motif[END_REF][START_REF] Zou | Apaf-1, a human protein homologous to C. elegans CED-4, participates in cytochrome c-dependent activation of caspase-3[END_REF] In mammals some members of the Bcl-2 related proteins are death antagonists while other are death agonists. For a recent review on the structure-function relations of Bcl-2 related proteins see (Gross, et al., 1999a;[START_REF] Kroemer | The proto-oncogene Bcl-2 and its role in regulating apoptosis[END_REF].

1

  Introduction ......................................................................................................................... 2 Intracellular control of the cell death program .................................................................... 3 The Bcl-2 family ................................................................................................................. 4 Mitochondrial events ........................................................................................................... 4.1 A decrease in mitochondrial membrane potential precedes DNA fragmentation during apoptosis ................................................................................................................................. 4.2 Release of apoptogeneic factors ................................................................................... 4.2.1 Cytochrome c release ............................................................................................. 4.2.2 AIF ....................................................................................................................... 5 Reactive oxygen species and apoptosis ............................................................................. 5.1 What are ROS? ........................................................................................................... 5.2 Oxidative stress ........................................................................................................... 5.3 Protection of the cell ................................................................................................... 5.4 Physiological role ....................................................................................................... 5.5 ROS participate in early and late steps of the regulation of apoptosis ....................... 5.6 Origin of apoptosis-mediated ROS ............................................................................. 5.7 Mechanisms of ROS signaling ................................................................................... 5.8 Bcl-2 and Bcl-xL inhibits apoptosis by providing a protection against ROS and/or shifting the cellular redox potential to a more reduced state ............................................................. 6 Yeast and apoptosis ........................................................................................................... 7 Conclusion ......................................................................................................................... 8 References .........................................................................................................................

Figure 1 :

 1 Figure 1: Schematic representation of the formation and the fate of superoxide ions (O2 .-) in mitochondria.