Aluminum as an inducer of the mitochondrial permeability transition

Treatment of rat liver mitochondria with aluminum in the presence of Ca2+ results in large amplitude swelling accompanied by loss of endogenous Mg2+ and K+ and oxidation of endogenous pyridine nucleotides. The presence of cyclosporin A, ADP, bongkrekic acid, N-ethylmaleimide and dithioerythritol prevent these effects, indicating that binding of aluminum to the inner mitochondrial membrane, most likely at the level of adenine nucleotide translocase, correlates with the induction of the membrane permeability transition (MPT). Indeed, aluminum binding promotes such a perturbation at the level of ubiquinol-cytochrome c reductase, which favors the production of reactive oxygen species. These metabolites generate an oxidative stress involving two previously defined sites in equilibrium with the glutathione and pyridine nucleotides pools, the levels of which correlate with the increase in MPT induction. Although the above-described phenomena are typical of MPT, they are not paralleled by other events normally observed in response to treatment with inducers of MPT (e.g., phosphate), such as the collapse of the electrochemical gradient and the release of accumulated Ca2+ and oxidized pyridine nucleotides. Biochemical and ultrastructural observations demonstrate that aluminum induces a pore opening having a conformation intermediate between fully open and closed in a subpopulation of mitochondria. While inorganic phosphate enhances the MPT induced by ruthenium red plus a deenergizing agent, aluminum instead inhibits this phenomenon. This finding suggests the presence of a distinct binding site for aluminum differing from that involved in MPT induction.

BNIP3 and genetic control of necrosis-like cell death through the mitochondrial permeability transition pore

Many apoptotic signaling pathways are directed to mitochondria, where they initiate the release of apoptogenic proteins and open the proposed mitochondrial permeability transition (PT) pore that ultimately results in the activation of the caspase proteases responsible for cell disassembly. BNIP3 (formerly NIP3) is a member of the Bcl-2 family that is expressed in mitochondria and induces apoptosis without a functional BH3 domain. We report that endogenous BNIP3 is loosely associated with mitochondrial membrane in normal tissue but fully integrates into the mitochondrial outer membrane with the N terminus in the cytoplasm and the C terminus in the membrane during induction of cell death. Surprisingly, BNIP3-mediated cell death is independent of Apaf-1, caspase activation, cytochrome c release, and nuclear translocation of apoptosis-inducing factor. However, cells transfected with BNIP3 exhibit early plasma membrane permeability, mitochondrial damage, extensive cytoplasmic vacuolation, and mitochondrial autophagy, yielding a morphotype that is typical of necrosis. These changes were accompanied by rapid and profound mitochondrial dysfunction characterized by opening of the mitochondrial PT pore, proton electrochemical gradient (Deltapsim) suppression, and increased reactive oxygen species production. The PT pore inhibitors cyclosporin A and bongkrekic acid blocked mitochondrial dysregulation and cell death. We propose that BNIP3 is a gene that mediates a necrosis-like cell death through PT pore opening and mitochondrial dysfunction.

Mitochondrial amplification of death signals determines thymidine kinase/ganciclovir-triggered activation of apoptosis

Previous clinical experience shows that the efficacy of suicide gene transfer in tumor therapy is limited, resulting from inefficient gene transfer or alternatively, from intrinsic resistance of the tumor in vivo. Herpes simplex virus thymidine kinase/ganciclovir (TK/GCV), a paradigmatic suicide gene therapy system, has been described to exert its cytotoxic effect, at least in part, by inducing apoptosis in target cells. Here, we report that mitochondria amplify TK/GCV-induced apoptosis by regulating p53 accumulation and the effector phase of apoptosis. Treatment with TK/GCV led to mitochondrial perturbations including loss of the mitochondrial membrane potential and release of cytochrome c from mitochondria into the cytosol, inducing caspase activation and nuclear fragmentation. Inhibition of TK/GCV-induced mitochondrial perturbations by Bcl-2 overexpression or by the mitochondrion-specific inhibitor bongkrekic acid also strongly inhibited TK/GCV-induced activation of caspases and apoptosis. TK/GCV-induced mitochondrial perturbations depended on caspase activity possibly initiated by death receptor signaling. Perturbation of mitochondrial function mediated accumulation of wild-type p53 protein, since Bcl-2 overexpression, bongkrekic acid, or inhibition of mitochondrial protein synthesis with chloramphenicol strongly reduced TK/GCV-induced accumulation of wild-type p53 protein. These findings suggest that TK/GCV therapy may be less efficient in tumors in which the mitochondrial amplification of TK/GCV-induced apoptosis is blocked, e.g., by Bcl-2 overexpression. Given the low efficacy of currently used gene therapy systems, our data on molecular mechanisms that regulate sensitivity or resistance toward TK/GCV-induced cytotoxicity might have important implications to improve the clinical application of suicide gene therapy.

Mitochondrial and extramitochondrial apoptotic signaling pathways in cerebrocortical neurons

In cultured cerebrocortical neurons, mild excitotoxic insults or staurosporine result in apoptosis. We show here that N-methyl-d-aspartate (NMDA) receptor-mediated, but not staurosporine-mediated, apoptosis is preceded by depolarization of the mitochondrial membrane potential (Deltapsi(m)) and ATP loss. Both insults, however, release cytochrome c (Cyt c) into the cytoplasm. What prompts mitochondria to release Cyt c and the mechanism of release are as yet unknown. We examined the effect of inhibition of the adenine nucleotide translocator (ANT), a putative component of the mitochondrial permeability transition pore. Inhibition of the mitochondrial ANT with bongkrekic acid (BA) prevented NMDA receptor-mediated apoptosis of cerebrocortical neurons. Concomitantly, BA prevented Deltapsi(m) depolarization, promoted recovery of cellular ATP content, and blocked caspase-3 activation. However, in the presence of BA, Cyt c was still released. Because BA prevented NMDA-induced caspase-3 activation and apoptosis, the presence of Cyt c in the neuronal cytoplasm is not sufficient for the induction of caspase activity or apoptosis. In contrast to these findings, BA was ineffective in preventing staurosporine-induced activation of caspases or apoptosis. Additionally, staurosporine-induced, but not NMDA-induced, apoptosis was associated with activation of caspase-8. These results indicate that, in cerebrocortical cultures, excessive NMDA receptor activation precipitates neuronal apoptosis by means of mitochondrial dysfunction, whereas staurosporine utilizes a distinct pathway.

Thiol-mediated apoptosis in prostate carcinoma cells

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

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

Tributyltin-induced apoptosis requires glycolytic adenosine trisphosphate production

The toxicity of tributyltin chloride (TBT) involves Ca(2+) overload, cytoskeletal damage, and mitochondrial failure leading to cell death by apoptosis or necrosis. Here, we examined whether the intracellular ATP level modulates the mode of cell death after exposure to TBT. When Jurkat cells were energized by the mitochondrial substrate, pyruvate, low concentrations of TBT (1-2 microM) triggered an immediate depletion of intracellular ATP followed by necrotic death. When ATP levels were maintained by the addition of glucose, the mode of cell death was typically apoptotic. Glycolytic ATP production was required for apoptosis at two distinct steps. First, maintenance of adequate ATP levels accelerated the decrease of mitochondrial membrane potential, and the release of the intermembrane proteins adenylate kinase and cytochrome c from mitochondria. A possible role of the adenine nucleotide exchanger in this first ATP-dependent step is suggested by experiments performed with the specific inhibitor, bongkrekic acid. This substance delayed cytochrome c release in a manner similar to that caused by ATP depletion. Second, caspase activation following cytochrome c release was only observed in ATP-containing cells. Bcl-2 had only a minor effect on TBT-triggered caspase activation or cell death. We conclude that intracellular ATP concentrations control the mode of cell death in TBT-treated Jurkat cells at both the mitochondrial and caspase activation levels.

Death signals from the B cell antigen receptor target mitochondria, activating necrotic and apoptotic death cascades in a murine B cell line, WEHI-231

B cell antigen receptor (BCR)-mediated cell death has been proposed as a mechanism for purging the immune repertoire of anti-self specificities during B cell differentiation in bone marrow. Mitochondrial alterations and activation of caspases are required for certain aspects of apoptotic cell death, but how the mitochondria and caspases contribute to BCR-mediated cell death is not well understood. In the present study, we used the mouse WEHI-231 B cell line to demonstrate that mitochondrial alterations and activation of caspases are indeed participants in BCR-mediated cell death. The peptide inhibitor of caspases, N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (z-VAD-fmk), blocked cleavage of poly(ADP-ribose) polymerase and various manifestation of nuclear apoptosis such as nuclear fragmentation, hypodiploidy and DNA fragmentation, indicating that signals from the BCR induced the activation of caspases. In addition, z-VAD-fmk delayed apoptosis-associated changes in cellular reduction-oxidation potentials as determined by hypergeneration of superoxide anion, as well as exposure of phosphatidylserine residues in the outer plasma membrane. By contrast, although z-VAD-fmk retarded cytolysis, it was incapable of preventing disruption of the plasma membrane even under the same condition in which it completely blocked nuclear apoptosis. Mitochondrial membrane potential loss was also not blocked by z-VAD-fmk. Bongkrekic acid, a specific inhibitor of mitochondrial permeability transition pores, suppressed not only the mitochondrial membrane potential but also the change of plasma membrane permeability. Overexpression of Bcl-xL prevented mitochondrial dysfunction, nuclear apoptosis and membrane permeability cell death triggered by BCR signal transduction. These observations indicate that death signals from BCR may first cause mitochondrial alterations followed by activation of both necrotic and apoptotic cascades.

Binding of nucleotides by the mitochondrial ADP/ATP carrier as studied by 1H nuclear magnetic resonance spectroscopy

Nucleotide binding to the cytosolic binding site of the mitochondrial ADP/ATP carrier (AAC) was studied by 1H-nuclear magnetic resonance spectroscopy. Binding (as opposed to translocation) could be identified as a result of the rapid ligand on/off kinetics, using the cytosolic side specific inhibitor carboxyatractyloside (CAT) for the distinction from nonspecific interactions. The off rate constant of the nonhydrolyzable ATP analogue AMP-PCP was more than 3 orders of magnitude larger than the transport rate. The nucleotides adopt an anti conformation in the carrier binding site as shown by measurements of the transferred nuclear overhauser effect (TRNOE). A thermal transition around 14 degreesC that had been previously detected in transport studies [Klingenberg, M., Grebe, K., and Appel, M. (1982) Eur. J. Biochem. 126, 263-269] was reflected by the inhibitor sensitive line broadening, indicating that this transition also affects nucleotide binding. Nucleotide monophosphates were employed to study the relation between nucleotide structure and affinity, using selective excitation, sample spinning with digital suppression of spinning sidebands, and line shape simulation. The binding of purines depends on the distribution of the electrical potential and on the position of ring substituents, while pyrimidines are barely recognized at all by the AAC. It is also shown that the photocleavable "caged" derivatives are more tightly bound than the original nucleotides. A two step model of carrier catalysis will be discussed on the basis of these results.

Activation of mitochondria and release of mitochondrial apoptogenic factors by betulinic acid

Different classes of anticancer drugs may trigger apoptosis by acting on different subcellular targets and by activating distinct signaling pathways. Here, we report that betulinic acid (BetA) is a prototype cytotoxic agent that triggers apoptosis by a direct effect on mitochondria. In isolated mitochondria, BetA directly induces loss of transmembrane potential independent of a benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone-inhibitable caspase. This is inhibited by bongkrekic acid, an agent that stabilizes the permeability transition pore complex. Mitochondria undergoing BetA-induced permeability transition mediate cleavage of caspase-8 (FLICE/MACH/Mch5) and caspase-3 (CPP32/Yama) in a cell-free system. Soluble factors such as cytochrome c or apoptosis-inducing factor released from BetA-treated mitochondria are sufficient for cleavage of caspases and nuclear fragmentation. Addition of cytochrome c to cytosolic extracts results in cleavage of caspase-3, but not of caspase-8. However, supernatants of mitochondria, which have undergone permeability transition, and partially purified apoptosis-inducing factor activate both caspase-8 and caspase-3 in cytosolic extracts and suffice to activate recombinant caspase-8. These findings show that induction of mitochondrial permeability transition alone is sufficient to trigger the full apoptosis program and that some cytotoxic drugs such as BetA may induce apoptosis via a direct effect on mitochondria.

Direct demonstration of a specific interaction between cyclophilin-D and the adenine nucleotide translocase confirms their role in the mitochondrial permeability transition

A fusion protein between cyclophilin-D (CyP-D) and glutathione S-transferase (GST) was shown to bind to purified liver inner mitochondrial membranes (IMMs) in a cyclosporin A (CsA)-sensitive manner. Binding was enhanced by diamide treatment of the IMMs. Immobilized GST-CyP-D avidly bound a single 30 kDa protein present in Triton X-100-solubilized IMMs; immunoblotting showed this to be the adenine nucleotide translocase (ANT). Binding was prevented by pretreatment of the CyP-D with CsA, but not with cyclosporin H. Purified ANT also bound specifically to GST-CyP-D, but porin did not, even in the presence of ANT.

Bax-induced cytochrome C release from mitochondria is independent of the permeability transition pore but highly dependent on Mg2+ ions

Bcl-2 family members either promote or repress programmed cell death. Bax, a death-promoting member, is a pore-forming, mitochondria-associated protein whose mechanism of action is still unknown. During apoptosis, cytochrome C is released from the mitochondria into the cytosol where it binds to APAF-1, a mammalian homologue of Ced-4, and participates in the activation of caspases. The release of cytochrome C has been postulated to be a consequence of the opening of the mitochondrial permeability transition pore (PTP). We now report that Bax is sufficient to trigger the release of cytochrome C from isolated mitochondria. This pathway is distinct from the previously described calcium-inducible, cyclosporin A-sensitive PTP. Rather, the cytochrome C release induced by Bax is facilitated by Mg2+ and cannot be blocked by PTP inhibitors. These results strongly suggest the existence of two distinct mechanisms leading to cytochrome C release: one stimulated by calcium and inhibited by cyclosporin A, the other Bax dependent, Mg2+ sensitive but cyclosporin insensitive.

Bax and adenine nucleotide translocator cooperate in the mitochondrial control of apoptosis

The proapoptotic Bax protein induces cell death by acting on mitochondria. Bax binds to the permeability transition pore complex (PTPC), a composite proteaceous channel that is involved in the regulation of mitochondrial membrane permeability. Immunodepletion of Bax from PTPC or purification of PTPC from Bax-deficient mice yielded a PTPC that could not permeabilize membranes in response to atractyloside, a proapoptotic ligand of the adenine nucleotide translocator (ANT). Bax and ANT coimmunoprecipitated and interacted in the yeast two-hybrid system. Ectopic expression of Bax induced cell death in wild-type but not in ANT-deficient yeast. Recombinant Bax and purified ANT, but neither of them alone, efficiently formed atractyloside-responsive channels in artificial membranes. Hence, the proapoptotic molecule Bax and the constitutive mitochondrial protein ANT cooperate within the PTPC to increase mitochondrial membrane permeability and to trigger cell death.

Functional F1-ATPase essential in maintaining growth and membrane potential of human mitochondrial DNA-depleted rho degrees cells

F1-ATPase assembly has been studied in human rho degrees cells devoid of mitochondrial DNA (mtDNA). Since, in these cells, oxidative phosphorylation cannot provide ATP, their growth relies on glycolysis. Despite the absence of the mtDNA-coded F0 subunits 6 and 8, rho degrees cells possessed normal levels of F1-ATPase alpha and beta subunits. This F1-ATPase was functional and azide- or aurovertin-sensitive but oligomycin-insensitive. In addition, aurovertin decreased cell growth in rho degrees cells and also reduced their mitochondrial membrane potential, as measured by rhodamine 123 fluorescence. Therefore, a functional F1-ATPase was important to maintain the mitochondrial membrane potential and the growth of these rho degrees cells. Bongkrekic acid, a specific adenine nucleotide translocator (ANT) inhibitor, also reduced rho degrees cell growth and mitochondrial membrane potential. In conclusion, rho degrees cells need both a functional F1-ATPase and a functional ANT to maintain their mitochondrial membrane potential, which is necessary for their growth. ATP hydrolysis catalyzed by F1 must provide ADP3- at a sufficient rate to maintain a rapid exchange with the glycolytic ATP4- by ANT, this electrogenic exchange inducing a mitochondrial membrane potential efficient enough to sustain cell growth. However, since the effects of bongkrekic acid and of aurovertin were additive, other electrogenic pumps should cooperate with this pathway.

Tumor necrosis factor-alpha-induced cell killing and activation of transcription factor NF-kappaB are uncoupled in L929 cells

The induction of transcription factor NF-kappaB has been shown to counteract tumor necrosis factor (TNF)-alpha-induced cell death in various cell types. In this study, we investigated the role of NF-kappaB for TNF-alpha-triggered cell death in the widely used mouse cell line L929 by various approaches. Inhibition of the mitochondrial permeability transition by bongkrekic acid impaired TNF-alpha-induced cell death without affecting the activity of NF-kappaB. The reduction of NF-kappaB-mediated gene expression by the synthetic steroid dexamethasone was associated with a decrease in TNF-alpha-mediated cell killing, suggesting that NF-kappaB does not protect L929 cells from TNF-alpha-induced cell death. This concept was reinforced by experiments employing L929 cell lines stably overexpressing a transdominant negative form of IkappaB-alpha. These cell lines were unable to activate NF-kappaB and to inducibly express the IL-6 gene, but they showed the same susceptibility toward TNF-alpha-mediated cell death as L929 wild-type cells.

Induction of apoptosis by valinomycin: mitochondrial permeability transition causes intracellular acidification

In order to determine whether disruption of mitochondrial function could trigger apoptosis in murine haematopoietic cells, we used the potassium ionophore valinomycin. Valinomycin induces apoptosis in the murine pre-B cell line BAF3, which cannot be inhibited by interleukin-3 addition or Bcl-2 over-expression. Valinomycin triggers rapid loss of mitochondrial membrane potential. This precedes cytoplasmic acidification, which leads to cysteine-active-site protease activation, DNA fragmentation and cell death. Bongkrekic acid, an inhibitor of the mitochondrial permeability transition, prevents acidification and subsequent induction of apoptosis by valinomycin.

Mutagenesis of some positive and negative residues occurring in repeat triad residues in the ADP/ATP carrier from yeast

In AAC2 from Saccharomyces cerevisiae, nine additional charged residues (six positive, three negative) were neutralized by mutagenesis following the previous mutation of six arginines. Oxidative phosphorylation (OxPhos) in cells and mitochondria, the expression level of AAC protein, and the various transport modes of AAC in the reconstituted system were measured. Mutations are: within the first helix at K38A which is exclusive for AAC; K48A, and R152A, part of a positive triad occurring in the matrix portion of each repeat; two matrix lysines, K179M and K182I, and the negative triad helix-terminating residues, E45G, D149S, D249S. Cellular ATP synthesis (OxPhos) is nearly completely inhibited in K48A, R152A, D149S, and D249S, but still amounts to 10% in K38A and between 30% and 90% in the gly+ mutants K179M, K179I + K182I, and E45G. Comparison of the AAC content measured by ELISA and the binding of [3H]CAT and [3H]BKA reveals discrepancies in K48A, D149S, and D249S mitochondria, which provide evidence that these mutations largely abolish inhibitor binding. Also these mitochondria have undetectable OxPhos. Differently in K38A, CAT and BKA binding are retained at high AAC levels but OxPhos is very low. This reveals a special functional role of K38, different from the more structural role of R152, K48, D149, and D249. Transport activity was measured with reconstituted AAC. The electroneutral ADP/ADP exchange of gly- mutants is largely or fully suppressed in K48A, D149S, and D249S. K38A and R152A are still active at 18% and 30% of wt. The other three exchange modes, ATP/ADP, ADP/ATP, and ATP/ATP, are nearly suppressed in all gly- mutants but remain high in gly+ mutants. ATP-linked modes are higher than the ADP/ADP mode in gly+ but lower in gly- mutants, resulting in an exchange mode inversion (EMI). In the competition for AAC2 transport capacity, the weak ATP exporting modes are suppressed by the much stronger unproductive ADP/ADP mode causing inhibition of OxPhos. Together with previous results all members of three charge triads are now mutagenized, revealing drastic functional rotatory asymmetries within the three repeat domains. In the intrahelical arginine triad the third (R294A), in the positive matrix triad the second (R152A), and in the helix-terminating negative triad the first (E45G) still show high activity.

Biochemical and physical parameters of the electrical currents measured with the ADP/ATP carrier by photolysis of caged ADP and ATP

The transport by the mitochondrial ADP/ATP carrier (AAC) has been shown in a preliminary communication to produce electrical capacitive currents on photolysis of caged ATP or ADP with reconstituted AAC liposomes attached to black lipid membranes [Brustovetsky, N., Becker, A., Klingenberg, M., and Bamberg, E. (1996) Proc. Natl. Acad. Sci. U.S.A. 93, 664-668]. Here we study the relation of the currents to ADP/ATP fluxes, the interaction of caged ADP and ATP with AAC and other basic facets of this method. Caged ADP and ATP are not transported by the AAC, as shown in mitochondria. Flux measurements with reconstituted AAC show that caged nucleotides are competitive inhibitors (Ki = 5 microM for caged ADP and 1 microM for caged ATP). Caged ATP competes with photolyzed ATP as shown by the dependence of the currents on the caged ATP concentration and on the light intensity. A competition of added ADP with caged ATP on the currents yields Ki = 50 microM for ADP. We conclude that caged ADP and ATP bind tighter to AAC than ADP or ATP, allowing immediate initiation of translocation by in situ photolysis. The caged compounds bind preferentially at the cytosolic side of AAC. With a regenerative hexokinase + glucose system, the currents are stabilized in repetitive flashes and can be used for applying inhibitors etc. during a flash series. The currents are completely inhibited by the combined addition of the AAC inhibitors bongkrekate (BKA) and carboxyatractylate (CAT). The partial inhibition by CAT or BKA is dependent on the number of flash cycles increasing from 60% to 90%, and by replacing chloride with gluconate from only 30% to 90%. The current are increased by a K+ diffusion potential (valinomycin + KCl) and decreased by the permeant anion TPB-. The pH dependence of the currents and of the parallel flux measurements indicates that only the fully charged ATP4- and ADP3- are transported. A strong temperature dependence of the currents with a break at 15 degrees C (EA = 95 and 28 kJ) agrees with former measurements of flux rates in mitochondria. In conclusion, the capacitive currents faithfully reflect AAC transport function and are a powerful tool for investigating the charge transfer in transport.

Conditions allowing different states of ATP- and GDP-induced permeability in mitochondria from different strains of Saccharomyces cerevisiae

The effect of ATP and other nucleotides on the respiration of Saccharomyces cerevisiae mitochondria was investigated. It was observed that ATP induced a stimulation of the respiration rate only in the presence of a salt in mitochondria from the baker's yeast Yeast Foam, whereas an ATP-induced stimulation occurred even in the absence of salt in mitochondria from three different laboratory strains. In both cases, the stimulation was related to a collapse of the transmembrane potential, suggesting the opening of ion- and/or proton-conducting pathways. Not only ATP, but also GTP and CTP, induced these pathways. Moreover, a similar stimulation was obtained with GDP and its analog GDP-beta-S. The fact that, as opposed to NTPs, GDP did not induce any non-specific anion channel, allowed us to use it to demonstrate unambiguously that a proton-conducting pathway was opened through the inner mitochondrial membrane of laboratory strains but not of Yeast Foam. Three additional aspects of this nucleotide-induced permeability were investigated. (i) The proton-conducting pathway was insensitive to Mg2+, whereas the anion-conducting pathway was fully inhibited by 4 mM Mg2-. (ii) The proton-conducting pathway of mitochondria isolated from laboratory strains was opened by the action of nucleotides outside the mitochondrion, since it was fully insensitive to (carboxy)atractyloside, and fully active in mitochondria isolated from op1 and delta anc strains. On the other hand, the cation-conducting pathway of Yeast Foam mitochondria was partly sensitive to (carboxy)atractyloside and insensitive to bongkrekic acid, suggesting a role of the conformational state of ANC in this activity. (iii) Both the proton and cation-conducting pathways were inhibited by very low concentrations of vanadate, under conditions where this oxyanion was polymerized to decavanadate: a competitor to nucleotide-binding sites on some enzymes.

Peroxidative modification of a membrane protein. Conformation-dependent chemical modification of adenine nucleotide translocase in Cu2+/tert-butyl hydroperoxide treated mitochondria

Peroxidative treatment of rat heart mitochondria results in a gradual increase of the apparent molecular weight of the adenine nucleotide translocase (ANT) by up to 1.2 kDa. ANT isolated from mitochondria treated with 1 mM tert-butyl hydroperoxide and 5-40 microM Cu2+ for 1 h at 37 degrees C exhibited a progressive loss of lysine, cysteine, arginine, and valine residues compared to native ANT. N-Ethylmaleimide, dithiothreitol, and the specific inhibitor of ANT, carboxyatractyloside (CAT), inhibited the peroxidation-induced molecular weight shift without inhibiting lipid peroxidation, which is believed to be the primary cause of the observed ANT modification. Bongkrekic acid, which stabilizes ANT in a conformation different from that brought about by CAT, did not inhibit the ANT molecular weight shift. Dithiothreitol, as well as CAT, was found to protect ANT against most of the losses of amino acid residues, indicating that alteration of sulfhydryl residues is required for chemical modification of, not only cysteine, but also lysine, arginine, and valine. We conclude that the peroxidative modification of ANT is conformation-dependent and involves chemical modification of cysteine as a critical step.

Mobilization of Mg2+ from rat heart and liver mitochondria following the interaction of thyroid hormone with the adenine nucleotide translocase

The in vitro addition of thyroid hormone to isolated rat heart or liver mitochondria induces the extrusion of approximately 2-4 nmol Mg2+/mg protein from both mitochondria preparations. The mobilization of Mg2+ is not accompanied by extrusion of matrix ATP or K+, or by mitochondria swelling, thus excluding that the phenomenon occurs through the nonspecific opening of the mitochondrial permeability transition pore. Moreover, the Mg2+ extrusion is completely prevented by bongkrekic acid, a membrane-permeant inhibitor of the adenine nucleotide translocase (AdNT), and by cyclosporine, which has also been reported to inhibit AdNT in a bongkrekate-like manner, operating at the matrix site of the translocase. By contrast, atractyloside, another specific inhibitor of AdNT that operates at the cytosolic site of the AdNT, only partially affects the Mg2+ mobilization (< 30% inhibition). These findings and the binding of 125I-labeled thyroid hormone to both the dimeric and monomeric moiety of AdNT support the hypothesis that AdNT can operate as a specific receptor for thyroid hormone in the mitochondria, and suggest that thyroid hormone operates at the matrix site of the translocase. In addition, these observations may imply that some of the so called "nongenomic effects" exerted by thyroid hormone on mitochondrial metabolism could occur through changes in the matrix content of Mg2+.

Mitochondrial permeability transition is a central coordinating event of apoptosis

In a number of experimental systems, the early stage of the apoptotic process, i.e., the stage that precedes nuclear disintegration, is characterized by the breakdown of the inner mitochondrial transmembrane potential (delta psi m). This delta psi m disruption is mediated by the opening of permeability transition (PT) pores and appears to be critical for the apoptotic cascade, since it is directly regulated by Bcl-2 and since mitochondria induced to undergo PT in vitro become capable of inducing nuclear chromatinolysis in a cell-free system of apoptosis. Here, we addressed the question of which apoptotic events are secondary to mitochondrial PT. We tested the effect of a specific inhibitor of PT, bongkrekic acid (BA), a ligand of the mitochondrial adenine nucleotide translocator, on a prototypic model of apoptosis glucocorticoid-induced thymocyte death. In addition to abolishing the apoptotic delta psi m disruption, BA prevents a number of phenomena linked to apoptosis: depletion of nonoxidized glutathione, generation of reactive oxygen species, translocation of NF kappa B, exposure of phosphatidylserine residues on the outer plasma membrane, cytoplasmic vacuolization, chromatin condensation, and oligonucleosomal DNA fragmentation. BA is also an efficient inhibitor of p53-dependent thymocyte apoptosis induced by DNA damage. These data suggest that a number of apoptotic phenomena are secondary to PT. In addition, we present data indicating that apoptotic delta psi m disruption is secondary to transcriptional events. These data connect the PT control point to the p53- and ICE/ Ced 3-regulated control points of apoptosis and place PT upstream of nuclear and plasma membrane features of PCD.

Inhibitors of permeability transition interfere with the disruption of the mitochondrial transmembrane potential during apoptosis

In a number of experimental systems, the early stage of the apoptotic process, i.e. the stage which precedes nuclear disintegration, is characterized by the breakdown of the inner mitochondrial transmembrane potential (delta psi m). Here we address the question as to whether mitochondrial permeability transition (PT) pores may account for the delta psi m dissipation in lymphocyte apoptosis. Drugs known for their PT-inhibitory potential (bongkrekic acid, cyclosporin A, and the non-immunosuppressive cyclosporin A analogue N-methyl-Val-4-cyclosporin A) are capable of preventing the apoptotic delta psi m disruption. Moreover, pharmacological modulation of PT-mediated delta psi m dissipation can prevent apoptosis. Thus, while suppressing the delta psi m disruption, bongkrekic acid also inhibits the apoptotic chromatinolysis. In conclusion, these data are compatible with the hypothesis that apoptotic delta psi m disruption is mediated by the formation of PT pores and that PT-mediated delta psi m disruption is a critical event of the apoptotic cascade.

Electrical currents associated with nucleotide transport by the reconstituted mitochondrial ADP/ATP carrier

The electrophoretic export of ATP against the import of ADP in mitochondria bridges the intra- versus extramitochondrial ATP potential gap. Here we report that the electrical nature of the ADP/ATP exchange by the mitochondrial ADP/ATP carrier (AAC) can be directly studied by measuring the electrical currents via capacitive coupling of AAC-containing vesicles on a planar lipid membrane. The currents were induced by the rapid liberation of ATP or ADP with UV flash photolysis from caged nucleotides. Six different transport modes of the AAC were studied: heteroexchange with either ADP or ATP inside the vesicles, initiated by photolysis of caged ATP or ADP; homoexchange with ADPex/ADPin or ATPex/ATPin; and caged ADP or ATP with unloaded vesicles. The heteroexchange produced the largest currents with the longest duration in line with the electrical charge difference ATP4- versus ADP3-. Surprisingly, also in the homoexchange and with unloaded vesicles, small currents were measured with shorter duration. In all three modes with caged ATP, a negative charge moved into the vesicles and with caged ADP it moved out of the vesicles. All currents were completely inhibited by a mixture of the inhibitors of the AAC, carboxyatractyloside and hongkrekate, which proves that the currents are exclusively due to AAC function. The observed charge movements in the heteroexchange system agree with the prediction from transport studies in mitochondria and reconstituted vesicles. The unexpected charge movements in the homoexchange or unloaded systems are interpreted to reveal transmembrane rearrangements of charged sites in the AAC when occupied with ADP or ATP. The results also indicate that not only ATP4- but also ADP3- contribute, albeit in opposite direction, to the electrical nature of the ADP/ATP exchange, which is at variance with former conclusions from biochemical transport studies. These measurements open up new avenues of studying the electrical interactions of ADP and ATP with the AAC.