The ADP-ATP translocation in mitochondria, a membrane potential controlled transport

The role of mitochondria in protection of the heart by preconditioning

A prolonged period of ischaemia followed by reperfusion irreversibly damages the heart. Such reperfusion injury (RI) involves opening of the mitochondrial permeability transition pore (MPTP) under the conditions of calcium overload and oxidative stress that accompany reperfusion. Protection from MPTP opening and hence RI can be mediated by ischaemic preconditioning (IP) where the prolonged ischaemic period is preceded by one or more brief (2-5 min) cycles of ischaemia and reperfusion. Following a brief overview of the molecular characterisation and regulation of the MPTP, the proposed mechanisms by which IP reduces pore opening are reviewed including the potential roles for reactive oxygen species (ROS), protein kinase cascades, and mitochondrial potassium channels. It is proposed that IP-mediated inhibition of MPTP opening at reperfusion does not involve direct phosphorylation of mitochondrial proteins, but rather reflects diminished oxidative stress during prolonged ischaemia and reperfusion. This causes less oxidation of critical thiol groups on the MPTP that are known to sensitise pore opening to calcium. The mechanisms by which ROS levels are decreased in the IP hearts during prolonged ischaemia and reperfusion are not known, but appear to require activation of protein kinase Cepsilon, either by receptor-mediated events or through transient increases in ROS during the IP protocol. Other signalling pathways may show cross-talk with this primary mechanism, but we suggest that a role for mitochondrial potassium channels is unlikely. The evidence for their activity in isolated mitochondria and cardiac myocytes is reviewed and the lack of specificity of the pharmacological agents used to implicate them in IP is noted. Some K(+) channel openers uncouple mitochondria and others inhibit respiratory chain complexes, and their ability to produce ROS and precondition hearts is mimicked by bona fide uncouplers and respiratory chain inhibitors. IP may also provide continuing protection during reperfusion by preventing a cascade of MPTP-induced ROS production followed by further MPTP opening. This phase of protection may involve survival kinase pathways such as Akt and glycogen synthase kinase 3 (GSK3) either increasing ROS removal or reducing mitochondrial ROS production.

Adenosine enhances cytosolic phosphorylation potential and ventricular contractility in stunned guinea pig heart: receptor-mediated and metabolic protection

Mechanisms of adenosine (ADO) protection of reperfused myocardium are not fully understood. We tested the hypothesis that ADO (0.1 mM) alleviates ventricular stunning by ADO A(1)-receptor stimulation combined with purine metabolic enhancements. Langendorff guinea pig hearts were stunned at constant left ventricular end-diastolic pressure by low-flow ischemia. Myocardial phosphate metabolites were measured by (31)P-NMR, with phosphorylation potential {[ATP]/([ADP].[P(i)]), where brackets indicate concentration} estimated from creatine kinase equilibrium. Creatine and IMP, glycolytic intermediates, were measured enzymatically and glycolytic flux and extracellular spaces were measured by radiotracers. All treatment interventions started after a 10-min normoxic stabilization period. At 30 min reperfusion, ventricular contractility (dP/dt, left ventricular pressure) was reduced 17-26%, ventricular power (rate-pressure product) by 37%, and [ATP]/([ADP].[P(i)]) by 53%. The selective A(1) agonist 2-chloro-N(6)-cyclo-pentyladenosine marginally preserved [ATP]/([ADP].[P(i)]) and ventricular contractility but not rate-pressure product. Purine salvage precursor inosine (0.1 mM) substantially raised [ATP]/([ADP].[P(i)]) but weakly affected contractility. The ATP-sensitive potassium channel blocker glibenclamide (50 microM) abolished ADO protection of [ATP]/([ADP].[P(i)]) and contractility. ADO raised myocardial IMP and glucose-6-phosphate, demonstrating increased purine salvage and pentose phosphate pathway flux potential. Coronary hyperemia alone (papaverine) was not cardioprotective. We found that ADO protected energy metabolism and contractility in stunned myocardium more effectively than both the A(1)-receptor agonist 2-chloro-N(6)-cyclo-pentyladenosine and the purine salvage precursor inosine. Because ADO failed to stimulate glycolytic flux, the enhancement of reperfusion, [ATP]/([ADP].[P(i)]), indicates protection of mitochondrial function. Reduced ventricular dysfunction at enhanced [ATP]/([ADP].[P(i)]) argues against opening of mitochondrial ATP-sensitive potassium channel. The results establish a multifactorial mechanism of ADO antistunning, which appears to combine ADO A(1)-receptor signaling with metabolic adenylate and antioxidant enhancements.

Inositol trisphosphate receptor Ca2+ release channels

The inositol 1,4,5-trisphosphate (InsP3) receptors (InsP3Rs) are a family of Ca2+ release channels localized predominately in the endoplasmic reticulum of all cell types. They function to release Ca2+ into the cytoplasm in response to InsP3 produced by diverse stimuli, generating complex local and global Ca2+ signals that regulate numerous cell physiological processes ranging from gene transcription to secretion to learning and memory. The InsP3R is a calcium-selective cation channel whose gating is regulated not only by InsP3, but by other ligands as well, in particular cytoplasmic Ca2+. Over the last decade, detailed quantitative studies of InsP3R channel function and its regulation by ligands and interacting proteins have provided new insights into a remarkable richness of channel regulation and of the structural aspects that underlie signal transduction and permeation. Here, we focus on these developments and review and synthesize the literature regarding the structure and single-channel properties of the InsP3R.

Overexpression of adenine nucleotide translocase reduces Ca2+ signal transmission between the ER and mitochondria

The adenine nucleotide translocase (ANT), besides transferring ATP from the mitochondrial matrix to the rest of the cell, has also been proposed to be involved in mitochondrial permeability transition (MPT), and accordingly in mitochondrial Ca2+ homeostasis. In order to assess the role of ANT in Ca2+ signal transmission from the endoplasmic reticulum (ER) to mitochondria, we overexpressed the various ANT isoforms and measured the matrix [Ca2+] ([Ca2+]m) increases evoked by stimulation with IP3-dependent agonists. ANT overexpression reduced the amplitude of the [Ca2+]m peak following Ca2+ release, an effect that was markedly greater for ANT-1 and ANT-3 isoforms than for ANT-2. Three further observations might explain these findings. First, the effect was partially reversed by treating the cells with cyclosporine A, suggesting the involvement of MPT. Second, the effect was paralleled by alterations of the 3D structure of the mitochondria. Finally, ANT-1 and ANT-3 overexpression also caused a reduction of ER Ca2+ loading that caused a marginal decrease in the cytosolic Ca2+ responses. Overall, these data provide evidence for the involvement of ANT-1 and ANT-3 in the induction of MPT and indicate the relevance of this phenomenon in ER-mitochondria Ca2+ transfer.

Transport catalysis

Carrier linked solute transport through biomembranes is analysed with the viewpoint of catalysis. Different from enzymes, in carriers the unchanged substrate induces optimum fit in the transition state. The enhanced intrinsic binding energy pays for the energy required of the global conformation changes, thus decreasing the activation energy barrier. This "induced transition fit" (ITF) explains several phenomena of carrier transport, e.g., high or low affinity substrate requirements for unidirectional versus exchange, external energy requirement for "low affinity" transport, the existence of side specific inhibitors to ground states of the carrier, the requirement of external energy in active transport to supplement catalytic energy in addition to generate electrochemical gradients.

Cysteine-scanning mutagenesis and thiol modification of the Rickettsia prowazekii ATP/ADP translocase: characterization of TMs IV-VII and IX-XII and their accessibility to the aqueous translocation pathway

We have determined the accessibility of the Rickettsia prowazekii ATP/ADP translocase transmembrane domains (TMs) IV-VII and IX-XII to the putative, water-filled ATP translocation pathway. A library of 177 independent mutants, each with a single cysteine substitution, was expressed in Escherichia coli, and those with substantial ATP transport activity were assayed for inhibition by thiol-reactive, methanethiosulfonate (MTS) reagents. The MTS reagents used were MTSES (negatively charged), MTSET (positively charged), and MTSEA (amphipathic). Inhibition of ATP transport by a charged MTS reagent indicates the exposure of a TM to the water-filled ATP translocation pathway. The eight TMs characterized in this study had 32 mutants with no assayable transport activity, indicating that cysteine substitution at these positions is not tolerated. ATP transport proficient mutants in TMs IV, V, VII, X, and XI were inhibited by charged MTS reagents, indicating that these TMs are exposed to the aqueous ATP translocation pathway, which is a pattern similar to those of TMs I, II (Alexeyev, M. F. (2004) Biochemistry 43, 6995-7002), and VIII (Winkler, H. H. (2003) Biochemistry 42, 12562-12569). Conversely, ATP-transport-proficient mutants in TMs VI, IX, and XII were not inhibited by charged MTS reagents, indicating that these TMs are sequestered from the aqueous environment, which is a pattern similar to that of TM III (Alexeyev, M. F. (2004) Biochemistry 43, 6995-7002). Preexposure of several MTS-sensitive mutants in TMs V, VII, X, and XI to ATP concentrations 10 times the K(m) resulted in protection from MTS-mediated inhibition; thus, confirming exposure of these TMs to the aqueous ATP translocation pathway, a pattern of protection similar to that observed for TMs I, II, and VIII.

Nucleotide-binding sites in the voltage-dependent anion channel: characterization and localization

In this study, we addressed the presence and location of nucleotide-binding sites in the voltage-dependent anion channel (VDAC). VDAC bound to reactive red 120-agarose, from which it was eluted by ATP, less effectively by ADP and AMP, but not by NADH. The photoreactive ATP analog, benzoyl-benzoyl-ATP (BzATP), was used to identify and characterize the ATP-binding sites in VDAC. [alpha-(32)P]BzATP bound to purified VDAC at two or more binding sites with apparent high and low binding affinities. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) analysis of BzATP-labeled VDAC confirmed the binding of at least two BzATP molecules to VDAC. The VDAC BzATP-binding sites showed higher specificity for purine than for pyrimidine nucleotides and higher affinity for negatively charged nucleotide species. VDAC treatment with the lysyl residue modifying reagent, fluorescein 5'-isothiocyanate, markedly inhibited VDAC labeling with BzATP. The VDAC nucleotide-binding sites were localized using chemical and enzymatic cleavage. Digestion of [alpha-(32)P]BzATP-labeled VDAC with CNBr or V8 protease resulted in the appearance of approximately 17- and approximately 14-kDa labeled fragments. Further digestion, high performance liquid chromatography separation, and sequencing of the selected V8 peptides suggested that the labeled fragments originated from two different regions of the VDAC molecule. MALDI-TOF analysis of BzATP-labeled, tryptic VDAC fragments indicated and localized three nucleotide binding sites, two of which were at the N and C termini of VDAC. Thus, the presence of two or more nucleotide-binding sites in VDAC is suggested, and their possible function in the control of VDAC activity, and, thereby, of outer mitochondrial membrane permeability is discussed.

Mathematical modeling of mitochondrial adenine nucleotide translocase

We have developed a mathematical model of adenine nucleotide translocase (ANT) function on the basis of the structural and kinetic properties of the transporter. The model takes into account the effect of membrane potential, pH, and magnesium concentration on ATP and ADP exchange velocity. The parameters of the model have been estimated from experimental data. A satisfactory model should take into account the influence of the electric potential difference on both ternary complex formation and translocation processes. To describe the dependence of translocation constants on electric potential we have supposed that ANT molecules carry charged groups. These groups are shifted during the translocation. Using the model we have evaluated the translocator efficiency and predicted the behavior of ANT under physiological conditions.

Ligand−Protein Interaction in Biomembrane Carriers. The Induced Transition Fit of Transport Catalysis†

Carrier-linked transport through biomembranes is treated under the view of catalysis. As in enzymes, substrate-protein interaction yields catalytic energy in overcoming the activation barrier. At variance with enzymes, catalytic energy is concentrated on structural changes of the carrier rather than on the substrate destabilization for facilitating the global protein rearrangements during transport. A transition state is invoked in which the binding site assumes the best fit to the substrate, whereas in the two ground (internal and external) states, the fit is poor. The maximum binding energy released in the transition state provides catalytic energy to enable the large carrier protein transformations associated with transport. This "induced transition fit" (ITF) of carrier catalysis provides a framework of rules, concerning specificity, unidirectional versus exchange type transport, directing inhibitors to the ground state instead of the transition state, and excluding simultaneous chemical and transport catalysis (vectorial group translocation). The possible role of external energy sources (ATP and Deltapsi) in supplementing the catalytic energy is elucidated. The analysis of the structure-function relationship based on new carrier structures may be challenged to account for the workings of the ITF.

ANT2 expression under hypoxic conditions produces opposite cell-cycle behavior in 143B and HepG2 cancer cells

Under hypoxic conditions, mitochondrial ATP production ceases, leaving cells entirely dependent on their glycolytic metabolism. The cytoplasmic and intramitochondrial ATP/ADP ratios, partly controlled by the adenine nucleotide translocator (ANT), are drastically modified. In dividing and growing cells that have a predominantly glycolytic metabolism, the ANT isoform 2, which has kinetic properties allowing ATP import into mitochondria, is over-expressed in comparison to control cells. We studied the cellular metabolic and proliferative response to hypoxia in two transformed human cell lines with different metabolic backgrounds: HepG2 and 143B, and in their rho(o) derivatives, i.e., cells with no mitochondrial DNA. Transformed 143B and rho(o) cells continued their proliferation whereas HepG2 cells, with a more differentiated phenotype, arrested their cell-cycle at the G(1)/S checkpoint. Hypoxia induced an increase in glycolytic activity, correlated to an induction of VEGF and hexokinase II (HK II) expression. Thus, according to their tumorigenicity, transformed cells may adopt one of two distinct behaviors to support hypoxic stress, i.e., proliferation or quiescence. Our study links the constitutive glycolytic activity and ANT2 expression levels of transformed cells with the loss of cell-cycle control after oxygen deprivation. ATP import by ANT2 allows cells to maintain their mitochondrial integrity while acquiring insensitivity to any alterations in the proteins involved in oxidative phosphorylation. This loss of cell dependence on oxidative metabolism is an important factor in the development of tumors.

Testosterone induces cytoprotection by activating ATP-sensitive K+ channels in the cardiac mitochondrial inner membrane

BACKGROUND

Whereas in the past, androgens were mainly believed to exert adverse effects on the cardiovascular system, recent experimental data postulate a benefit of testosterone for recovery of myocardial function after ischemia/reperfusion injury. Thus, we examined whether testosterone might improve myocardial tolerance to ischemia due to activation of mitochondrial (mitoK(ATP)) and/or sarcoplasmatic (sarcK(ATP)) K(ATP) channels.

METHODS AND RESULTS

In a cellular model of ischemia, testosterone significantly decreased the rate of ischemia-induced death of cardiomyocytes that could be prevented by 5-hydroxydecainoic acid but was unaffected by the sarcK(ATP) blocker HMR1098 and the testosterone receptor antagonist flutamide. To index mitoK(ATP), mitochondrial flavoprotein fluorescence was measured. Testosterone induced a highly significant increase in mitochondrial flavoprotein fluorescence in intact myocytes and isolated mitoplasts that could be abolished by 5-hydroxydecainoic acid. Testosterone-mediated flavoprotein oxidation of mitoplasts was K+ dependent and ATP sensitive. In mitoplast-attached single-channel recordings, testosterone directly activated an ATP-sensitive K+ channel of the inner mitochondrial membrane. Addition of the K(ATP) channel opener diazoxide and pinacidil to the cytosolic solution activated the ATP-sensitive K+ current comparable to testosterone, whereas 5-hydroxydecainoic acid and glibenclamide inhibited the testosterone-induced current. Patch-clamp experiments of intact myocytes in whole-cell configuration did not demonstrate any effect of testosterone on sarcK(ATP) channels.

CONCLUSIONS

Our results provide direct evidence for the existence of cardiac mitoK(ATP) and a link between testosterone-induced cytoprotection and activation of mitoK(ATP). Endogenous testosterone might play a more important role in recovery after myocardial infarction than is currently assumed.

Mutual amplification of apoptosis by statin-induced mitochondrial stress and doxorubicin toxicity in human rhabdomyosarcoma cells

Besides their cholesterol-lowering effect, 3-hydroxy-3-methyl-glutaryl-CoA reductase inhibitors (statins) show antiproliferative behaviour, which has been suggested as a promising anticancer strategy. However, the signalling cascades leading to statin-induced death of cancer cells are poorly characterized. Here we show that statins activate the mitochondrial pathway of apoptosis in rhabdomyosarcoma RD cells via translocation of Bax from the cytosol to mitochondria. The prototypical representative of statins, simvastatin, induced consecutive activation of caspase 9 and 3 in a concentration-dependent manner. The permeability transition pore inhibitor bongkrekic acid was capable of completely preventing simvastatin-induced caspase 9 and 3 activity, corroborating the mitochondrial pathway of apoptosis as the sole mechanism of statin action. Alternative pathways via death receptors, that is, caspase 8 or calpain activation, were not triggered by simvastatin. Simvastatin-treated RD cells could be completely rescued from apoptosis by the co-application of mevalonic acid, indicating that deprivation of cholesterol precursors is essential for statin-induced apoptosis. However, pretreatment with subthreshold concentrations of simvastatin was sufficient to augment doxorubicin toxicity via the mitochondrial apoptotic machinery. Moreover, the presence of doxorubicin increased the potency of simvastatin to trigger caspase activation. Taken together, these data highlight the therapeutic anticancer potential of statins and their additivity and mutual sensitization, in combination with doxorubicin in human rhabdomyosarcoma cells.

Cysteine-scanning mutagenesis and thiol modification of the Rickettsia prowazekii ATP/ADP translocase: evidence that transmembrane regions I and II, but not III, are structural components of the aqueous translocation channel

The contribution of transmembrane regions I, II, and III of the Rickettsia prowazekii ATP/ADP translocase to the structure of the putative water-filled ATP translocation channel was evaluated from the accessibility of hydrophilic, thiol-reactive, methanethiosulfonate reagents to a library of 68 independent cysteine-substitution mutants heterologously expressed in Escherichia coli. The MTS reagents used were MTSES (negatively charged) and MTSET and MTSEA (both positively charged). Mutants F036C, Y042C, and R046C (TM I), K066C and P072C (TM II), and F101C, F105C, F108C, Y113C, and P114C (TM III) had no assayable transport activity, indicating that cysteine substitution at these positions may not be tolerated. All three MTS reagents inhibit the transport of ATP in mutants of TM I (L039C, S043C, S047C, I048C) and TM II (S061C, S063C, T067C, I069C, V070C, A074C). Further, these residues appear to cluster along a single face of the transmembrane domain. Preexposure of MTS-reactive mutants S047C (TM I) and T067C (TM II) to high levels of ATP resulted in protection from MTS-mediated inhibition. This indicated that both TM I and TM II make major contributions to the structure of an aqueous ATP translocation pathway. Finally, on the basis of the lack of accessibility of charged MTS reagents to the thiol groups in mutants of TM III, it appears that TM III is not exposed to the ATP translocation channel. Cysteine substitution of residues constituting a highly conserved "phenylalanine face" in TM III resulted in ablation of ATP transport activity. Further, substituting these phenylalanine residues for either isoleucine or tyrosine also resulted in much lower transport activity, indicating that some property of phenylalanine at these positions that is not shared by cysteine, isoleucine, or tyrosine is critical to translocase activity.

Real-time flow cytometry analysis of permeability transition in isolated mitochondria

Mitochondrial membrane permeabilization (MMP) is a key event in necrotic and (intrinsic) apoptotic processes. MMP is controlled by a few major rate-limiting events, one of which is opening of the permeability transition pore (PTP). Here we develop a flow cytometry (FC)-based approach to screen and study inducers and blockers of MMP in isolated mitochondria. Fixed-time and real-time FC permits to co-evaluate and order modifications of mitochondrial size, structure and inner membrane (IM) electrochemical potential (DeltaPsi(m)) during MMP. Calcium, a major PTP opener, and alamethicin, a PTP-independent MMP inducer, trigger significant mitochondrial forward scatter (FSC) increase and side scatter (SSC) decrease, correlating with spectrophotometrically detected swelling. FC-based fluorescence detection of the DeltaPsi(m)-sensitive cationic lipophilic dye JC-1 permits to detect DeltaPsi(m) variations induced by PTP openers or specific inducers of inner MMP such as carbonylcyanide m-chlorophenylhydrazone (mClCCP). These simple, highly sensitive and quantitative FC-based methods will be pertinent to evaluate compounds for their ability to control MMP.

Permeability transition in rat liver mitochondria is modulated by the ATP-Mg/Pi carrier

Mitochondrial permeability transition, due to opening of the permeability transition pore (PTP), is triggered by Ca2+ in conjunction with an inducing agent such as phosphate. However, incubation of rat liver mitochondria in the presence of low micromolar concentrations of Ca2+ and millimolar concentrations of phosphate is known to also cause net efflux of matrix adenine nucleotides via the ATP-Mg/Pi carrier. This raises the possibility that adenine nucleotide depletion through this mechanism contributes to mitochondrial permeability transition. Results of this study show that phosphate-induced opening of the mitochondrial PTP is, at least in part, secondary to depletion of the intramitochondrial adenine nucleotide content via the ATP-Mg/Pi carrier. Delaying net adenine nucleotide efflux from mitochondria also delays the onset of phosphate-induced PTP opening. Moreover, mitochondria that are depleted of matrix adenine nucleotides via the ATP-Mg/Pi carrier show highly increased susceptibility to swelling induced by high Ca2+ concentration, atractyloside, and the prooxidant tert-butylhydroperoxide. Thus the ATPMg/Pi carrier, by regulating the matrix adenine nucleotide content, can modulate the sensitivity of rat liver mitochondria to undergo permeability transition. This has important implications for hepatocytes under cellular conditions in which the intramitochondrial adenine nucleotide pool size is depleted, such as in hypoxia or ischemia, or during reperfusion when the mitochondria are exposed to increased oxidative stress.

The evolution of cell death programs as prerequisites of multicellularity

One of the hallmarks of multicellularity is that the individual cellular fate is sacrificed for the benefit of a higher order of life-the organism. The accidental death of cells in a multicellular organism results in swelling and membrane-rupture and inevitably spills cell contents into the surrounding tissue with deleterious effects for the organism. To avoid this form of necrotic death the cells of metazoans have developed complex self-destruction mechanisms, collectively called programmed cell death, which see to an orderly removal of superfluous cells. Since evolution never invents new genes but plays variations on old themes by DNA mutations, it is not surprising, that some of the genes involved in metazoan death pathways apparently have evolved from homologues in unicellular organisms, where they originally had different functions. Interestingly some unicellular protozoans have developed a primitive form of non-necrotic cell death themselves, which could mean that the idea of an altruistic death for the benefit of genetically identical cells predated the invention of multicellularity. The cell death pathways of protozoans, however, show no homology to those in metazoans, where several death pathways seem to have evolved in parallel. Mitochondria stands at the beginning of several death pathways and also determines, whether a cell has sufficient energy to complete a death program. However, the endosymbiotic bacterial ancestors of mitochondria are unlikely to have contributed to the recent mitochondrial death machinery and therefore, these components may derive from mutated eukaryotic precursors and might have invaded the respective mitochondrial compartments. Although there is no direct evidence, it seems that the prokaryotic-eukaryotic symbiosis created the space necessary for sophisticated death mechanisms on command, which in their distinct forms are major factors for the evolution of multicellular organisms.

Matrix volume measurements challenge the existence of diazoxide/glibencamide-sensitive KATP channels in rat mitochondria

A mitochondrial sulphonylurea-sensitive, ATP-sensitive K+ channel (mitoKATP) that is selectively inhibited by 5-hydroxydecanoate (5-HD) and activated by diazoxide has been implicated in ischaemic preconditioning. Here we re-evaluate the evidence for the existence of this mitoKATP by measuring changes in light scattering (A520) in parallel with direct determination of mitochondrial matrix volumes using 3H2O and [14C]sucrose. Incubation of rat liver and heart mitochondria in KCl medium containing Mg2+ and inorganic phosphate caused a decrease in light scattering over 5 min, which was accompanied by a small (15-30 %) increase in matrix volume. The presence of ATP or ADP in the buffer from the start greatly inhibited the decline in A520, whilst addition after a period of incubation (1-5 min) induced a rapid increase in A520, especially in heart mitochondria. Neither response was accompanied by a change in matrix volume, as measured isotopically. However, the effects of ATP and ADP on A520 were abolished by carboxyatractyloside and bongkrekic acid, inhibitors of the adenine nucleotide translocase (ANT) that lock the transporter in two discrete conformations and cause distinct changes in A520 in their own right. These data suggest that rather than matrix volume changes, the effects of ATP and ADP on A520 reflect changes in mitochondrial shape induced by conformational changes in the ANT. Furthermore, we were unable to demonstrate either a decrease in A520 or increase in matrix volume with a range of ATP-sensitive K+ channel openers such as diazoxide. Nor did glibencamide or 5-HD cause any reduction of matrix volume, whereas the K+ ionophore valinomycin (0.2 nM), produced a 10-20 % increase in matrix volume that was readily detectable by both techniques. Our data argue against the existence of a sulphonylurea-inhibitable mitoKATP channel.

How does the mitochondrial ADP/ATP carrier distinguish transportable ATP and ADP from untransportable AMP and GTP?Dynamic modeling of the recognition/translocation process in the major substrate binding region

To understand the transport mechanism of the bovine heart mitochondrial ADP/ATP carrier at the atomic level, we studied the four-dimensional features of the interaction of various purine nucleotides with the adenine nucleotide binding region (ABR) consisting of Arg(151)-Asp(167) in the second loop facing the matrix side. After three-dimensional modeling of ABR based on the experimental results, its structural changes on interaction with purine nucleotides were examined by molecular dynamics computation at 300 K. ATP/ADP were translocated to a considerable degree from the matrix side to the inner membrane region accompanied by significant backbone conformational changes, whereas neither appreciable translocation nor a significant conformational change was observed with the untransportable nucleotides AMP/GTP. The results suggested that binding of the terminal phosphate group and the adenine ring of ATP/ADP with Arg(151) and Lys(162), respectively, and subsequent interaction of a phosphate group(s) other than the terminal phosphate with Lys(162) triggered the expansion and subsequent contraction of the backbone conformation of ABR, leading to the translocation of ATP/ADP. Based on a simplified molecular dynamic simulation, we propose a dynamic model for the initial recognition process of ATP/ADP with the carrier.

Flow cytometry of isolated mitochondria during development and under some pathological conditions

Mitochondria play an essential role in the generation of the energy needed for eukaryotic cell life and in the release of molecules involved in initiation of cell death. Here we review the changes in isolated mitochondrial fluorescent populations as distinguished by flow cytometry during postnatal development and their regulation by thyroid hormones and catecholamines. The use of flow cytometry in the study of mitochondrial changes occurring under hypothyroidism, alcohol abuse and aging is also reviewed.

Ligand-selective modulation of the permeability transition pore by arginine modification. Opposing effects of p-hydroxyphenylglyoxal and phenylglyoxal

Chemical modification of mitochondria with the arginine-specific reagents phenylglyoxal (PGO) and 2,3-butanedione (BAD) decreases the Ca(2+) sensitivity of the permeability transition pore (PTP) and stabilizes it in the closed conformation (Eriksson, O., Fontaine, E., and Bernardi, P. (1998) J. Biol. Chem. 273, 12669-12674). Unexpectedly, modification of mitochondria with the arginine-specific reagent p-hydroxyphenylglyoxal (OH-PGO) resulted instead in PTP opening. Sequential modification with OH-PGO and PGO (or BAD) revealed that the effects on the PTP depended on the order of the additions. PTP opening was observed when OH-PGO preceded, and PTP closing was observed when OH-PGO followed, the addition of PGO (or BAD). The differential effects of OH-PGO and PGO on the PTP open probability (i) were not modified by the conformation-specific ligands of the adenine nucleotide translocase bongkrekate and atractylate; and (ii) were also observed in de-energized mitochondria, indicating that the effect is exerted directly on the PTP. OH-PGO dramatically sensitized PTP opening, which was triggered by depolarization even in the presence of EGTA. These data show that arginine modification modulates the PTP conformation in a ligand-selective fashion and suggest that the effects of OH-PGO, PGO, and BAD are mediated by the same arginine residues. We analyzed the structure of the arginine adducts by matrix-assisted laser desorption ionization and time-of-flight mass spectrometry using a test peptide and N-acetylarginine. The results indicate that both OH-PGO and PGO react with arginine at a stoichiometry of 2:1 and form stable adducts that may be feasible to identify the PTP at the molecular level.

Adenine nucleotide translocator mediates the mitochondrial membrane permeabilization induced by lonidamine, arsenite and CD437

An increasing number of experimental chemotherapeutic agents induce apoptosis by directly triggering mitochondrial membrane permeabilization (MMP). Here we examined MMP induced by lonidamine, arsenite, and the retinoid derivative CD437. Cells overexpressing the cytomegalovirus-encoded protein vMIA, a protein which interacts with the adenine nucleotide translocator, were strongly protected against the MMP-inducing and apoptogenic effects of lonidamine, arsenite, and CD437. In a cell-free system, lonidamine, arsenite, and CD437 induced the permeabilization of ANT proteoliposomes, yet had no effect on protein-free liposomes. The ANT-dependent membrane permeabilization was inhibited by the two ANT ligands ATP and ADP, as well as by recombinant Bcl-2 protein. Lonidamine, arsenite, and CD437, added to synthetic planar lipid bilayers containing ANT, elicited ANT channel activities with clearly distinct conductance levels of 20+/-7, 100+/-30, and 47+/-7 pS, respectively. Altering the ATP/ADP gradient built up on the inner mitochondrial membrane by inhibition of glycolysis and/or oxidative phosphorylation differentially modulated the cytocidal potential of lonidamine, arsenite, and CD437. Inhibition of F(0)F(1)ATPase without glycolysis inhibition sensitized to lonidamine-induced cell death. In contrast, only the combined inhibition of glycolysis plus F(0)F(1)ATPase sensitized to arsenite-induced cell death. No sensitization to cell death induction by CD437 was achieved by glucose depletion and/or oligomycin addition. These results indicate that ANT is a target of lonidamine, arsenite, and CD437 and unravel an unexpected heterogeneity in the mode of action of these three compounds.

Significant expression of functional human type 1 mitochondrial ADP/ATP carrier in yeast mitochondria

As a first step to characterize the unknown functional properties of the human mitochondrial ADP/ATP carrier (AAC), we tried to express human type 1 AAC (hAAC1) in Saccharomyces cerevisiae. Expression of hAAC1 in yeast mitochondrial membrane was very low, although its transcript level was high. Its expression was improved greatly by replacement of its N-terminal region with the corresponding region of yeast type 2 AAC (yAAC2), as observed with the bovine type 1 AAC (bAAC1). This chimeric hAAC1 showed similar ADP transport activity to that of chimeric bAAC1, corresponding to the transport activity of bAAC1 in bovine heart mitochondria. These results suggested that the N-terminal region of yAAC2 is important for expression of the mammalian carriers in yeast mitochondria. Using the present expression system, studies on the functional properties of the human AAC isoforms in relation to their structures are now possible.

Control of mitochondrial membrane permeabilization by adenine nucleotide translocator interacting with HIV-1 viral protein rR and Bcl-2

Viral protein R (Vpr), an apoptogenic accessory protein encoded by HIV-1, induces mitochondrial membrane permeabilization (MMP) via a specific interaction with the permeability transition pore complex, which comprises the voltage-dependent anion channel (VDAC) in the outer membrane (OM) and the adenine nucleotide translocator (ANT) in the inner membrane. Here, we demonstrate that a synthetic Vpr-derived peptide (Vpr52-96) specifically binds to the intermembrane face of the ANT with an affinity in the nanomolar range. Taking advantage of this specific interaction, we determined the role of ANT in the control of MMP. In planar lipid bilayers, Vpr52-96 and purified ANT cooperatively form large conductance channels. This cooperative channel formation relies on a direct protein-protein interaction since it is abolished by the addition of a peptide corresponding to the Vpr binding site of ANT. When added to isolated mitochondria, Vpr52-96 uncouples the respiratory chain and induces a rapid inner MMP to protons and NADH. This inner MMP precedes outer MMP to cytochrome c. Vpr52-96-induced matrix swelling and inner MMP both are prevented by preincubation of purified mitochondria with recombinant Bcl-2 protein. In contrast to König's polyanion (PA10), a specific inhibitor of the VDAC, Bcl-2 fails to prevent Vpr52-96 from crossing the mitochondrial OM. Rather, Bcl-2 reduces the ANT-Vpr interaction, as determined by affinity purification and plasmon resonance studies. Concomitantly, Bcl-2 suppresses channel formation by the ANT-Vpr complex in synthetic membranes. In conclusion, both Vpr and Bcl-2 modulate MMP through a direct interaction with ANT.

Mitochondrial membrane permeabilization during the apoptotic process

Apoptosis may be viewed as a triphasic process. During the pre-mitochondrial initiation phase, very different pro-apoptotic signal transduction or damage pathways can be activated. These pathways then converge on the mitochondrion, where they cause the permeabilization of the inner and/or outer membranes with consequent release of soluble intermembrane proteins into the cytosol. The process of mitochondrial membrane permeabilization would constitute the decision/effector phase of the apoptotic process. During the post-mitochondrial degradation phase downstream caspases and nucleases are activated and the cell acquires an apoptotic morphology. Recently, a number of different second messengers (calcium, ceramide derivatives, nitric oxide, reactive oxygen species) and pro-apoptotic proteins (Bax, Bak, Bid, and caspases) have been found to directly compromise the barrier function of mitochondrial membranes, when added to isolated mitochondria. The effects of several among these agents are mediated at least in part via the permeability transition pore complex (PTPC), a composite channel in which members of the Bcl-2 family interact with sessile transmembrane proteins such as the adenine nucleotide translocator. These findings suggest that the PTPC may constitute a pharmacological target for chemotherapy and cytoprotection.

Stimulus-specific and cell type-specific cascades: emerging principles relating to control of apoptosis in the eye

Apoptosis has a critical role in development, homeostasis, wound healing, and the pathophysiology of disease in the organs of multicellular organisms. It has been implicated in these processes in retina, lens, cornea, trabecular meshwork, optic nerve, and the central nervous system pathways that contribute to vision. Considerable interest has been focused on inhibiting apoptosis to control disease and wound healing processes in which programmed cell death is thought to have a critical role. A simplified view led to the search for effective inhibitors of 'the final common pathway for apoptosis'. Recent studies have provided important insights into the modulators that participate in and regulate the apoptosis cascades which are activated in response to cytokines, ionizing radiation, chemotherapeutic agents, growth factor deprivation, and other stimulators of cell death. These studies lead to the inescapable conclusion that the apoptosis pathways are not only stimulus-specific, but also cell-type specific. These observations have important implications related to development of pharmacological strategies for controlling apoptosis-associated disease and apoptosis-initiated wound healing.

ATP regulation of type 1 inositol 1,4,5-trisphosphate receptor channel gating by allosteric tuning of Ca(2+) activation

Inositol 1,4,5-trisphosphate (InsP(3)) mobilizes intracellular Ca(2+) by binding to its receptor (InsP(3)R), an endoplasmic reticulum-localized Ca(2+) release channel. Patch clamp electrophysiology of Xenopus oocyte nuclei was used to study the effects of cytoplasmic ATP concentration on the cytoplasmic Ca(2+) ([Ca(2+)](i)) dependence of single type 1 InsP(3)R channels in native endoplasmic reticulum membrane. Cytoplasmic ATP free-acid ([ATP](i)), but not the MgATP complex, activated gating of the InsP(3)-liganded InsP(3)R, by stabilizing open channel state(s) and destabilizing the closed state(s). Activation was associated with a reduction of the half-maximal activating [Ca(2+)](i) from 500 +/- 50 nM in 0 [ATP](i) to 29 +/- 4 nM in 9.5 mM [ATP](i), with apparent ATP affinity = 0.27 +/- 0.04 mM, similar to in vivo concentrations. In contrast, ATP was without effect on maximum open probability or the Hill coefficient for Ca(2+) activation. Thus, ATP enhances gating of the InsP(3)R by allosteric regulation of the Ca(2+) sensitivity of the Ca(2+) activation sites of the channel. By regulating the Ca(2+)-induced Ca(2+) release properties of the InsP(3)R, ATP may play an important role in shaping cytoplasmic Ca(2+) signals, possibly linking cell metabolic state to important Ca(2+)-dependent processes.

Kinetics of electrogenic transport by the ADP/ATP carrier

The electrogenic transport of ATP and ADP by the mitochondrial ADP/ATP carrier (AAC) was investigated by recording transient currents with two different techniques for performing concentration jump experiments: 1) the fast fluid injection method: AAC-containing proteoliposomes were adsorbed to a solid supported membrane (SSM), and the carrier was activated via ATP or ADP concentration jumps. 2) BLM (black lipid membrane) technique: proteoliposomes were adsorbed to a planar lipid bilayer, while the carrier was activated via the photolysis of caged ATP or caged ADP with a UV laser pulse. Two transport modes of the AAC were investigated, ATP(ex)-0(in) and ADP(ex)-0(in). Liposomes not loaded with nucleotides allowed half-cycles of the ADP/ATP exchange to be studied. Under these conditions the AAC transports ADP and ATP electrogenically. Mg(2+) inhibits the nucleotide transport, and the specific inhibitors carboxyatractylate (CAT) and bongkrekate (BKA) prevent the binding of the substrate. The evaluation of the transient currents yielded rate constants of 160 s(-1) for ATP and >/=400 s(-1) for ADP translocation. The function of the carrier is approximately symmetrical, i.e., the kinetic properties are similar in the inside-out and right-side-out orientations. The assumption from previous investigations, that the deprotonated nucleotides are exclusively transported by the AAC, is supported by further experimental evidence. In addition, caged ATP and caged ADP bind to the carrier with similar affinities as the free nucleotides. An inhibitory effect of anions (200-300 mM) was observed, which can be explained as a competitive effect at the binding site. The results are summarized in a transport model.

Characterization of a silencer element and purification of a silencer protein that negatively regulates the human adenine nucleotide translocator 2 promoter

Expression of adenine nucleotide translocator isoform 2 (ANT2) is growth regulated. In the present study, we report the presence of a silencer region in the human ANT2 promoter and the purification of a two-component factor that recognizes a specific hexanucleotide element, GTCCTG, of the silencer. Transfection of deletion constructs shows that ANT2 silencer activity extends over a region of at least 310 nts. However, mutating the GTCCTG element completely relieves silencing activity in the context of the human ANT2 promoter. The data suggest that the GTCCTG element might be required for maintaining silencer activity of the extended silencer region. The ANT2 silencer region cloned in front of the herpes simplex virus thymidine kinase promoter confers nearly complete inhibition to the heterologous promoter. However, unlike the ANT2 promoter, mutating the GTCCTG element restores only partial activity to the herpes simplex virus thymidine kinase promoter. A protein complex consisting of two major polypeptides of 37 and 49 kDa was isolated from HeLa nuclear extracts by affinity chromatography using the GTCCTG element as the affinity resin. Cross-linking studies and Southwestern analysis indicate that p37 binds DNA. p49 appears to be loosely associated with the p37/DNA complex but is necessary for strong binding of p37. Our data implicating the GTCCTG element directly in silencing of the ANT2 promoter, together with data from the literature reporting the presence of this element within the silencer region of several additional promoters, suggest a general role of the GTCCTG element in transcriptional silencing.

Transcription of the adenine nucleotide translocase isoforms in various types of tissues in the rat

Two different isoforms of the adenine nucleotide translocase (ANT1 and ANT2) have been identified in the rat. In order to obtain enhanced knowledge of the ANT isoform expression, we analyzed the transcription pattern of both isoforms and their mRNA levels in various tissues of the rat using the PCR technique. A predominant ANT1 mRNA percentage was recorded in the skeletal muscle, heart and brain, ranging from 81 to 58%. In contrast to these tissues, the percentages of ANT2 were dominant with a range from 59 to 75% in the kidney, lung, spleen and liver. The level of total ANT mRNA varied markedly in the various organs. Tissues with a dominant ANT1 percentage simultaneously showed a high level of total ANT transcription (24-41 attomol/ng total RNA). In comparison to the latter, tissues with a prevalent ANT2 transcription were shown to have an even lower ANT transcription level (2-5 attomol/ng total RNA). The predominance of the ANT1 expression appeared to be restricted to tissues with an inability to regenerate by means of mitotic division, whereas a prevalent ANT2 transcription is found in cell types able to proliferate. The level of total ANT transcription but not the individual ANT isoform expression depends to a great extent on the energy requirements of the tissue.

Non-mitochondrial ATP transport

Exchange of organelle ATP with cytosolic ADP through the ADP/ATP carrier is a well-characterized feature of mitochondrial metabolism. Obligate intracellular bacteria, such as Rickettsia prowazekii, and higher-plant plastids possess another type of adenylate transporter, which exchanges bacterial or plastidic ADP for ATP from the eukaryotic (host cell) cytoplasm. The bacterial and plastidic transporters are similar but do not share significant sequence similarities with the mitochondrial carrier. Recent molecular and biochemical studies are providing deeper insight into the functional and evolutionary relationships between the bacterial and the plant transport proteins.

Expression of the bovine heart mitochondrial ADP/ATP carrier in yeast mitochondria: significantly enhanced expression by replacement of the N-terminal region of the bovine carrier by the corresponding regions of the yeast carriers

To characterize the transport mechanism mediated by the mammalian mitochondrial ADP/ATP carrier (AAC), we tried to express bovine heart mitochondrial AAC (bhAAC) in Saccharomyces cerevisiae. The open reading frame of the bhAAC was introduced into the haploid strain WB-12, in which intrinsic AAC genes were disrupted. Growth of the transformant was very low in glycerol medium, and a little amount of bhAAC was detected in the mitochondrial membrane. For improvement of bhAAC expression in WB-12, we introduced DNA fragments encoding chimeric bhAACs, in which the N-terminal region of the bhAAC extending into the cytosol was replaced by the corresponding regions of the type 1 and type 2 yeast AAC isoforms (yAAC1 and yAAC2). These transformants grew well, and the amounts of the chimeric bhAACs in their mitochondria were as high as that of yAAC2. The carriers expressed showed essentially the same ADP transport activities as that of AAC in bovine heart mitochondria.

Recovery of free ADP, Pi, and free energy of ATP hydrolysis in human skeletal muscle

We measured significant undershoots of the concentrations of free ADP ([ADP]) and Pi ([Pi]) and the free energy of ATP hydrolysis (DeltaGATP) below initial resting levels during recovery from severe ischemic exercise with 31P-nuclear magnetic resonance spectroscopy in 11 healthy sports students. Undershoots of the rate of oxidative phosphorylation would be predicted if the rate of oxidative phosphorylation would depend solely on free [ADP], [Pi], or DeltaGATP. However, undershoots of the rate of oxidative phosphorylation have not been reported in the literature. Furthermore, undershoots of the rate of oxidative phosphorylation are unlikely because there is evidence that a balance between ATP production and consumption cannot be achieved if an undershoot of the rate of oxidative phosphorylation actually occurs. Therefore, oxidative phosphorylation seems to depend not only on free [ADP], [Pi], or DeltaGATP. An explanation is that acidosis-related or other factors control oxidative phosphorylation additionally, at least under some conditions.

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.

Mitochondria as regulators of apoptosis: doubt no more

Scientific revolution [1] implies a transformation of the world view in which a dominant paradigm is substituted by a new one, one which furnishes an ameliorated comprehension of facts, as well as an advantage for the design of informative experiments. Apoptosis research has recently experienced a change from a paradigm in which the nucleus determined the apoptotic process to a paradigm in which mitochondria constitute the center of death control. Several pieces of evidence imply mitochondria in the process of apoptosis. Kinetic data indicate that mitochondria undergo major changes in membrane integrity before classical signs of apoptosis become manifest. These changes concern both the inner and the outer mitochondrial membranes, leading to a disruption of the inner transmembrane potential (DeltaPsim) and the release of intermembrane proteins through the outer membrane. Cell-free systems of apoptosis demonstrate that mitochondrial products are rate limiting for the activation of caspases and endonucleases in cell extracts. Functional studies indicate that drug-enforced opening or closing of the mitochondrial megachannel (also called permeability transition pore) can induce or prevent apoptosis. The anti-apoptotic oncoprotein Bcl-2 acts on mitochondria to stabilize membrane integrity and to prevent opening of the megachannel. These observations are compatible with a three-step model of apoptosis: a premitochondrial phase during which signal transduction cascades or damage pathways are activated; a mitochondrial phase, during which mitochondrial membrane function is lost; and a post-mitochondrial phase, during which proteins released from mitochondria cause the activation of catabolic proteases and nucleases. The implication of mitochondria in apoptosis has important consequences for the understanding of the normal physiology of apoptosis, its deregulation in cancer and degenerative diseases, and the development of novel cytotoxic and cytoprotective drugs.

Elucidating the molecular mechanism of the permeability transition pore and its role in reperfusion injury of the heart

First, we present a summary of the evidence for our model of the molecular mechanism of the permeability transition (MPT). Our proposal is that the MPT occurs as a result of the binding of mitochondrial cyclophilin (CyP-D) to the adenine nucleotide translocase (ANT) in the inner mitochondrial membrane. This binding is enhanced by thiol modification of the ANT caused by oxidative stress or other thiol reagents. CyP-D binding enhances the ability of the ANT to undergo a conformational change triggered by Ca2+. Binding of ADP or ATP to a matrix site of the ANT antagonises this effect of Ca2+; modification of other ANT thiol groups inhibits ADP binding and sensitises the MPT to [Ca2+]. Increased membrane potential changes the ANT conformation to enhance ATP binding and hence inhibit the MPT. Our most recent data shows that a fusion protein of CyP-D and glutathione-S-transferase immobilised to Sepharose specifically binds the ANT from Triton-solubilised inner mitochondrial membranes in a cyclosporin A (CsA) sensitive manner. Second we summarise the evidence for the MPT being a major factor in the transition from reversible to irreversible injury during reperfusion of a heart following a period of ischaemia. We describe how in the perfused heart [3H]deoxyglucose entrapment within mitochondria can be used to measure the opening of MPT pore in situ. During ischaemia pore opening does not occur, but significant opening does occur during reperfusion, and recovery of the heart is dependent on subsequent pore closure. Pore opening is inhibited by the presence in the perfusion medium of pyruvate and the anaesthetic propofol which both protect the heart from reperfusion injury. Third we discuss how the MPT may be involved in determining whether cell death occurs by necrosis (extensive pore opening and ATP depletion) or apoptosis (transient pore opening with maintenance of ATP).

Characterization of mitochondrial electron-transfer in Leishmania mexicana

Some general features of the respiratory chain and respiratory control were characterized in coupled mitochondrial preparations from Leishmania mexicana promastigotes. O2 uptake was sensitive to the electron-transfer inhibitors rotenone, flavone, malonate, 4,4,4-trifluoro-1-(2-thienyl) 1.3 butanedione (TTFA), antimycin A, 2n-nonyl-4-hydroxyquinoline-N-oxide (HQNO), myxothiazol, cyanide and azide. A high concentration of rotenone (60 microM) was required to inhibit O2 uptake effectively. Difference spectra revealed the presence of cytochromes (a + a3), b and c. Respiratory control was stimulated 2-fold by ADP with different exogenous oxidizable substrates. Calculated ADP/O ratios were consistent with the notion that ascorbate/N,N,N',N'-tetramethylphenylenediamine (TMPD)-linked and FAD-linked respiration proceeds, respectively, with one third and two thirds of the ATP producing capacity of NADH-linked respiration. State 3 was suppressed by the ATP synthase inhibitors oligomycin and aurovertin and by the adenine nucleotide translocator inhibitors atractyloside and carboxy atractyloside. The protonophore carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP) provoked state 3u respiration. The mitochondrial preparation was capable of Ca2+ uptake and Ca2+ stimulated respiration. Data obtained suggests strongly that mitochondrial complexes I, II, III and IV are present in a major pathway of electron-transfer and that oxidative phosphorylation might proceed with high bioenergetic efficiency.

Tissue-specific transcription pattern of the adenine nucleotide translocase isoforms in humans

Three adenine nucleotide translocase isoforms (ANT1, ANT2 and ANT3) are coded by different genes. The relative amounts of the three ANT isoform mRNAs were determined in detail in various human tissues. ANT isoforms were co-expressed in all tested tissues revealing tissue-specific transcription patterns. The highest ANT1 mRNA proportions were found in terminally differentiated tissues like skeletal muscle, heart and brain, whereas ANT2 was mainly expressed in tissues capable of proliferation and regeneration as in the kidneys, spleen, liver, fibroblasts and lymphocytes. The ANT3 mRNA proportion was not prominently expressed in any of the tissues tested. In conclusion, tissue-specific expression of ANT isoforms is strongly related to the state of cellular differentiation.

Human papillomavirus (HPV) 16 E6 sensitizes cells to atractyloside-induced apoptosis: role of p53, ICE-like proteases and the mitochondrial permeability transition

Infection of cervical epithelial cells with certain high risk HPV genotypes is thought to play an etiologic role in the development of cervical cancer. In particular, HPV type 16 and 18 early protein 6 (E6) is thought to contribute to epithelial transformation by binding to the tumor suppressor protein p53, targeting it for rapid proteolysis, resulting in loss of its cell cycle arrest and apoptosis-inducing activities. Recent data indicate that factors responsible for triggering apoptosis reside in the cytoplasm of cells, and not in the nucleus. In particular, the findings that mitochondria are required in certain cell-free models for induction of apoptosis and that bcl-2 is localized to mitochondria have focused attention on the role of the mitochondrial membrane permeability transition (MPT) in apoptosis. Here we present data to indicate that HPV 16 E6 expression sensitizes cells to MPT-induced apoptosis. We also report that HPV 16 E6 sensitization of cells to MPT-induced apoptosis occurs only in the presence of wildtype (wt) p53 expression. The extent of apoptosis induced by atractyloside (an inducer of the MPT) in normal, temperature-sensitive (ts) p53, and HPV-16 E6 transfected J2-3T3 cells, and the HPV expressing cervical carcinoma cell lines SiHa, Hela and CaSki was determined. C33A cells, which express mutant p53 but not HPV, were also exposed to atractyloside in the presence or absence of HPV 16 E6 expression. Dose-dependent apoptosis induced by atractyloside in normal J2-3T3 cells and cervical carcinoma cells was measured by loss of cell viability, nuclear fragmentation and DNA laddering. The sensitivity of cells to atractyloside-induced apoptosis was found to be: HPV 16 E6-J2-3T3 > CaSki > normal-J2-3T3 cells approximately ts p53-J2-3T3 approximately vector-J2-3T3 cells > Hela > SiHa > C33A approximately C33A 16 E6. Cyclosporin A (CsA), an inhibitor of the MPT, and ICE-I, a protease inhibitor, provided protection against atractyloside-induced apoptosis. These findings indicate that: 1) high risk HPV 16 E6 protein is capable of sensitizing cells to apoptosis; 2) HPV 16 E6 sensitization of cells to atractyloside-induced apoptosis occurs in a p53-dependent fashion; 3) the target of HPV 16 E6 sensitization of cells to atractyloside-induced apoptosis is the mitochondria; and 4) HPV 16 E6 sensitization of cells to atroctycoside-induced apoptosis involves an ICE-like protease-sensitive mechanism, regulating the onset of the MPT. These findings constitute the first evidence that mitochondria play a role in HPV 16 E6 modulation of apoptosis.

Cellular respiration during hypoxia. Role of cytochrome oxidase as the oxygen sensor in hepatocytes

We previously reported that hepatocytes exhibit a reversible suppression of respiration during prolonged hypoxia (PO2 = 20 torr for 3-5 h). Also, isolated bovine heart cytochrome c oxidase undergoes a reversible decrease in apparent Vmax when incubated under similar conditions. This study sought to link the hypoxia-induced changes in cytochrome oxidase to the inhibition of respiration seen in intact cells. Hepatocytes incubated at PO2 = 20 torr exhibited decreases in respiration and increases in [NAD(P)H] after 2-3 h that were reversed upon reoxygenation (PO2 = 100 torr). Respiration during hypoxia was also inhibited when N,N,N',N'-tetramethyl-p-phenylenediamine (0.5 mM) and ascorbate (5 mM) were used to reduce cytochrome c, suggesting that cytochrome oxidase was partially inhibited. Similarly, liver submitochondrial particles revealed a 44% decrease in the apparent Vmax of cytochrome oxidase after hypoxic incubation. In hepatocytes loaded with tetramethylrhodamine ethyl ester (10 nM) to quantify mitochondrial membrane potential, acute hypoxia (<30 min) produced no change in fluorescence, consistent with the absence of an acute change in respiration. However, fluorescence increased during acute reoxygenation after prolonged hypoxia, suggesting an increase in potential. The control exhibited by NADH over mitochondrial respiration was not altered during hypoxia. Thus, changes in the Vmax of cytochrome oxidase during prolonged hypoxia correlate with the changes in respiration and mitochondrial potential. This suggests that the oxidase functions as an oxygen sensor in the intact hepatocyte.

Oxidative stress, thiol reagents, and membrane potential modulate the mitochondrial permeability transition by affecting nucleotide binding to the adenine nucleotide translocase

Stimulation of the mitochondrial permeability transition (MPT) in de-energized mitochondria by phenylarsine oxide (PheArs) is greater than that by diamide and t-butylhydroperoxide (TBH), yet the increase in CyP binding to the inner mitochondrial membrane (Connern, C. P. and Halestrap, A. P. (1994) Biochem. J. 302, 321-324) is less. From a range of nucleotides tested only ADP, deoxy-ADP, and ATP inhibited the MPT. ADP inhibition involved two sites with Ki values of about 1 and 25 microM which were independent of [Ca2+] and CyP binding. Carboxyatractyloside (CAT) abolished the high affinity site. Following pretreatment of mitochondria with TBH or diamide, the Ki for ADP increased to 50-100 microM, whereas pretreatment with PheArs or eosin maleimide increased the Ki to >500 microM; only one inhibitory site was observed in both cases. Eosin maleimide is known to attack Cys159 of the adenine nucleotide translocase (ANT) in a CAT-sensitive manner (Majima, E., Shinohara, Y., Yamaguchi, N., Hong, Y. M., and Terada, H. (1994) Biochemistry 33, 9530-9536), and here we demonstrate CAT-sensitive binding of the ANT to a PheArs affinity column. In adenine nucleotide-depleted mitochondria, no stimulation of the MPT by uncoupler was observed in the presence or absence of thiol reagents, suggesting that membrane potential may inhibit the MPT by increasing adenine nucleotide binding through an effect on the ANT conformation. We conclude that CsA and ADP inhibit pore opening in distinct ways, CsA by displacing bound CyP and ADP by binding to the ANT. Both mechanisms act to decrease the Ca2+ sensitivity of the pore. Thiol reagents and oxidative stress may modify two thiol groups on the ANT and thus stimulate pore opening by both means.

Redox regulation of apoptosis: impact of thiol oxidation status on mitochondrial function

The probability that a cell will undergo apoptosis is in part dictated by the cellular redox potential, which is mainly determined by the reduction and oxidation of thiol residues on glutathione and proteins. We and others have recently shown that mitochondria play a critical role in the apoptotic cascade. Here, we address the question as to whether thiol modification regulates apoptosis and in which cellular compartment apoptosis-regulatory thiols are localized. To resolve this problem, we employed the divalent thiol-reactive agent diamide, which causes thiol cross-linking and thus mimics disulfide bridge formation, and a panel of monovalent thiol-reactive compounds (which impede disulfide bridge formation due to thiol oxidation), one of which is specifically targeted to the mitochondrial matrix. Our data indicate that thymocyte apoptosis induced by diamide mimics natural apoptosis in the sense that mitochondrial transmembrane potential (delta psi(m)) disruption precedes nuclear chromatin degradation; that monovalent thiol-reactive compounds inhibit apoptosis induced by diamide, glucocorticoids, irradiation, and topoisomerase inhibition; that the critical thiols determining cell fate after exposure to diamide, glucocorticoids, or DNA damage are likely to be located in the mitochondrial matrix; and that thiol oxidation and reduction are critical for apoptosis induction by some stimuli (glucocorticoids, DNA damage), but not by Fas/CD95 cross-linking. Taken together, these findings suggest that, at least in some pathways of apoptosis, mitochondrial thiols constitute a critical sensor of the cellular redox potential.

Probing the role of positive residues in the ADP/ATP carrier from yeast. The effect of six arginine mutations on transport and the four ATP versus ADP exchange modes

Mutagenesis of three intrahelical arginines, R96, R204, or R294, and of each member of the arginine triplet R252, R253, R254 into neutral residues had resulted in a strong suppression of oxidative phosphorylation in cells and isolated mitochondria [Müller, V., Basset, G., Nelson, D. R., & Klingenberg, M. (1996) Biochemistry 35, 16132-16143]. Here we determine the transport activity of wild-type and mutant AAC in reconstituted proteoliposomes using a new rapid removal-stop method without relying on the inhibitor stop which can be compromised by mutations. The basic electroneutral ADP/ADP exchange activity is strongly or totally suppressed in six out of seven of these mutations, with the exception of R294A, which retains nearly wild-type activity. Carboxyatractylate (CAT) inhibits the ADP/ATP exchange rate only to 3-10% in wild type and R294A and up to 40% in other mutants, whereas bongkrekic acid (BKA) inhibits 50% (wild type and R294A) and 90% (other mutants). Consequently, AAC is preferentially reconstituted with the matrix surface outside. All these mutations drastically change activity distribution among the four exchange modes ADP/ADP, ADP/ATP, ATP/ADP, and ATP/ATP. Whereas in wild-type AAC the homo ATP/ATP exchange is twice as high as the ADP/ADP exchange, in mutants it is 10 to 15 times lower. Similarly, the hetero ATP/ADP exchange in wild-type AAC is higher than the ADP/ ATP exchange, but in mutants it is several times lower. Thus, these mutations afflict the ATP-linked modes, in particular those linked to external ATP. The inhibition of oxidative phosphorylation is thus explained by the suppression of ATP export versus ADP import mode. The "extra"-inhibition of oxidative phosphorylation in mutant cells is explained by the extreme shift in mutants in favour of ATP import versus ADP export. Besides structural changes, the mutant effects indicate electrostatic interactions of these arginines with the anionic substrates. The loss of one positive charge raises the translocation barrier the more negative the substrate, i.e. more for ATP4- than for ADP3-. In none of these arginine mutants was the binding of CAT or BKA abolished.

Significance of the adenine nucleotide translocator in the pathogenesis of viral heart disease

We found recently autoantibodies against the adenine nucleotide translocator (ANT), a carrier in the inner mitochondrial membrane, in sera of patients with myocarditis and dilated cardiomyopathy. To elucidate whether these antibodies are of pathophysiological importance, we investigated the function and expression of the adenine nucleotide translocator (ANT) in the heart muscle tissue of patients suffering from myocarditis and DCM. We found a markedly lowered transport capacity of the translocator accompanied by an elevation in total ANT protein content. The alteration in ANT protein amount is caused by an ANT isoform shift characterized by an increase in ANT 1 isoform protein associated with a decrease in ANT 2 isoform and an unchanged ANT 3 content. It could be shown that the isoform shift is not a progressive process during the disease period but an event in the early period of illness which becomes permanent. Simulating the effect of pathogenetic factors of autoimmunological diseases, we infected A/J mice with the enterovirus Coxsackie B3 and immunized guinea pigs with myocardial ANT protein. Both treatments led to autoimmunological responds and to a lowered myocardial transport capacity of ANT, to a disturbed energy metabolism and consequently to a depression of heart function.

Metabolic compartmentalization in neonatal swine myocytes

The transport of metabolites across the mitochondrial membrane is regulated by specific exchange and shuttle systems that are often dependent on the mitochondrial membrane potential. Thus, metabolite concentrations in the cytosol and mitochondrial compartments are largely determined by the energy state of the cardiac muscle cell. The purpose of this study was to investigate metabolic compartmentalization in ventricular myocytes isolated from newborn (< 24 h) swine hearts. Furthermore, the effect of respiratory inhibition on these distribution patterns was examined. Freshly isolated cells contained 33 nmol of ATP and 37 nmol of total adenine nucleotides (AN) per mg of myocyte protein. Rapid digitonin fractionation indicated that 95% of ATP and 86% of AN were cytosolic, whereas > 50% of the pyridine nucleotides were mitochondrial. With 11 mM added glucose, myocytes treated with the respiratory inhibitor, rotenone, maintained ATP at 88% of that of aerobic myocytes, but phosphocreatine declined by 50% over 30 min. Rotenone treatment caused the mitochondrial NAD/NADH ratio to decline from 1.2 to 0.06, whereas the cytosolic pyridine nucleotides remained > 90% oxidized. Total adenine and pyridine nucleotide content and their compartmentalization were unaffected by respiratory inhibition. Comparisons of metabolite content and respiratory activity between isolated piglet mitochondria and the mitochondrial compartment of piglet myocytes indicated that mitochondria account for approximately 30% of total myocyte protein. A similar value (29%) was obtained for the aqueous volume fraction of the in situ mitochondrial matrix using the 4000 Mr 14C-labeled polyethylene glycol-impermeable 3H2O spaces of intact and lysed myocytes. These results are comparable to literature values for myocardium from other species and age groups.

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.

Mitochondrial control of nuclear apoptosis

Anucleate cells can be induced to undergo programmed cell death (PCD), indicating the existence of a cytoplasmic PCD pathway that functions independently from the nucleus. Cytoplasmic structures including mitochondria have been shown to participate in the control of apoptotic nuclear disintegration. Before cells exhibit common signs of nuclear apoptosis (chromatin condensation and endonuclease-mediated DNA fragmentation), they undergo a reduction of the mitochondrial transmembrane potential (delta psi m) that may be due to the opening of mitochondrial permeability transition (PT) pores. Here, we present direct evidence indicating that mitochondrial PT constitutes a critical early event of the apoptotic process. In a cell-free system combining purified mitochondria and nuclei, mitochondria undergoing PT suffice to induce chromatin condensation and DNA fragmentation. Induction of PT by pharmacological agents augments the apoptosis-inducing potential of mitochondria. In contrast, prevention of PT by pharmacological agents impedes nuclear apoptosis, both in vitro and in vivo. Mitochondria from hepatocytes or lymphoid cells undergoing apoptosis, but not those from normal cells, induce disintegration of isolated Hela nuclei. A specific ligand of the mitochondrial adenine nucleotide translocator (ANT), bongkreik acid, inhibits PT and reduces apoptosis induction by mitochondria in a cell-free system. Moreover, it inhibits the induction of apoptosis in intact cells. Several pieces of evidence suggest that the proto-oncogene product Bcl-2 inhibits apoptosis by preventing mitochondrial PT. First, to inhibit nuclear apoptosis, Bcl-2 must be localized in mitochondrial but not nuclear membranes. Second, transfection-enforced hyperexpression of Bcl-2 directly abolishes the induction of mitochondrial PT in response to a protonophore, a pro-oxidant, as well as to the ANT ligand atractyloside, correlating with its apoptosis-inhibitory effect. In conclusion, mitochondrial PT appears to be a critical step of the apoptotic cascade.