Critical role of mitochondria, but not caspases, during glucocorticosteroid-induced human eosinophil apoptosis

Glucocorticosteroids are potent anti-inflammatory drugs used in the treatment of eosinophilic disorders. These molecules directly promote eosinophil apoptosis, yet the molecular mechanisms regulating this process remain ill-defined. We show here that stimulation of human peripheral blood eosinophils with dexamethasone induced DNA fragmentation, chromatin and cytoplasm condensation, and caspase-3 activation, as assessed by the proteolysis of its zymogen form and by the increase of caspase-3-like activity in eosinophil lysates. These phenomena were accompanied by a reduced uptake of the mitochondrial potential-sensitive marker DiOC(6)(3), suggestive of mitochondrial membrane permeabilization. Eosinophil incubation with the caspase-3 inhibitor, Z-Asp-Glu-Val-Asp-fluromethylketone, or with the broad spectrum caspase inhibitor, Z-Val-Ala-Asp-fluromethylketone, inhibited caspase-3-like activity generation but failed to modify dexamethasone-mediated loss in mitochondrial transmembrane potential and eosinophil apoptosis. In contrast, bongkrekic acid, a ligand of the mitochondrial permeability transition pore component, adenine nucleotide translocator, prevented both dexamethasone-induced mitochondrial disruption and apoptosis. We conclude that the mitochondrial permeability transition pore, rather than the caspase cascade, plays a critical role in the propagation of glucocorticosteroid-mediated apoptotic signals in human eosinophils.

Bcl-2 protects against apoptosis induced by antimycin A and bongkrekic acid without restoring cellular ATP levels

Several studies indicate that mitochondrial ATP production as well as ADP/ATP exchange across mitochondrial membranes are impaired during apoptosis. We investigated whether Bcl-2 could protect against cell death under conditions in which ATP metabolism is inhibited. Inhibition of ATP production using antimycin A (AA) (complex III inhibition) combined with inhibition of ADP/ATP exchange by bongkrekic acid (BA) (adenine nucleotide translocator (ANT) inhibition) induced a sharp decrease in total cellular ATP in FL5.12 parental cells (to 35% of untreated controls after 24 h of incubation). Within 24 and 48 h, 38% and 75% of the cells had died, respectively. However, in stably transfected FL5.12 Bcl-2 subclones, no cell death occurred under these experimental conditions. Similar results were obtained with Jurkat and Bcl-2 overexpressing Jurkat cells. Total cellular ATP levels were equally affected in FL5.12 Bcl-2 overexpressing cells and FL5.12 parental cells. This indicates that Bcl-2 overexpressing cells are able to survive with very low cellular ATP content. Furthermore, Bcl-2 did not protect against cell death by restoring ATP levels. This suggests that, under these conditions, Bcl-2 acts by inhibiting the signalling cascade triggered by the inhibitors that would normally lead to apoptosis.

Close location of the first loop to the third loop of the mitochondrial ADP/ATP carrier deduced from cross-linking catalyzed by copper-o-phenanthroline of the solubilized carrier with Triton X-100

Effects of the cross-linking catalyst copper-o-phenanthroline [Cu(OP)2] on the bovine heart mitochondrial ADP/ATP carrier solubilized with Triton X-100 were studied under various conditions. Without detergent treatment, Cu(OP)2 specifically catalyzed the formation of intermolecular disulfide bridges in submitochondrial particles between two Cys56 residues in the first loop facing the matrix space of the dimeric carrier [Majima, E., Ikawa, K., Takeda, M., Hashimoto, M., Shinohara, Y., and Terada, H. (1995) J. Biol. Chem. 270, 29548-29554]. However, an intramolecular disulfide bridge between Cys56 and Cys256 in the third loop was formed in the solubilized carrier. Proteolytic digestion of the carrier with lysylendopeptidase showed that it first cleaves the Lys42-Gln43 bond and then the Lys48-Gln49 bond of the first loop in the membrane-bound carrier, but it cleaves both sites almost simultaneously in the solubilized carrier. These features were observed only with the m-state carrier; the c-state carrier was not subject to any cross-linking or proteolytic digestion. It is suggested that the protruding first loop is located close to the third loop, which could be exposed to a certain degree.

WK175, a novel antitumor agent, decreases the intracellular nicotinamide adenine dinucleotide concentration and induces the apoptotic cascade in human leukemia cells

We recently developed a class of novel antitumor agents that elicit a potent growth-inhibitory response in many tumor cells cultured in vitro. WK175, a member of this class, was chosen as a model compound that showed strong in vitro efficacy. WK175 interferes with the intracellular steady-state level of NAD(+), resulting in a decreased cellular NAD(+) concentration. We found that WK175 induces apoptotic cell death without any DNA-damaging effect. The apoptotic death signaling pathway initiated by WK175 was examined in detail: mitochondrial membrane potential, cytochrome c release, caspase 3 activation, caspase 3 and poly(ADP-ribose) polymerase cleavage, and the appearance of a sub-G(1) cell cycle population were determined in time course studies in THP-1 (a human monocytic leukemia cell line) cells. We found activation of this cascade after 24 h of treatment with 10 nM WK175. Induction of apoptosis was prevented by bongkrekic acid, Z-Asp-Glu-Val-Asp-fluoromethylketone, and Z-Leu-Glu-His-Asp-fluoromethylketone, inhibitors of the mitochondrial permeability transition and of caspase 3 and 9, respectively, but not by Ac-Tyr-Val-Ala-Asp-CHO, a specific caspase 1 inhibitor, suggesting the involvement of the permeability transition pore, caspase 3, and caspase 9 in the WK175-induced apoptotic cascade. These results imply that decreased NAD(+) concentration initiates the apoptotic cascade, resulting in the antitumor effect of WK175.

Inhibition by R(+) or S(-) pramipexole of caspase activation and cell death induced by methylpyridinium ion or beta amyloid peptide in SH-SY5Y neuroblastoma

Cell models of neurodegenerative diseases (NDD) can involve expression of mutant nuclear genes associated with Mendelian forms of the diseases or effects of toxins believed to replicate essential disease features. Death produced by exposing neural cells to methylpyridinium ion (MPP(+)) or neurotoxic beta amyloid (BA) peptides is frequently used to study features of the sporadic, most prevalent forms of Parkinson's disease (PD) and Alzheimer's disease (AD), respectively. We examined in replicating SH-SY5Y human neuroblastoma cells the release of cytochrome C into cytoplasm, activation of caspases 9 and 3, and loss of calcein retention as markers of the "mitochondrial" pathway of cell death. Exposure to 5 mM MPP(+), which induces apoptotic cell death within 18-24 hr, released cytochrome C within 4 hr, activated caspases 9 and 3, and reduced calcein accumulation. BA 25-35 peptide produced more rapid and greater elevations of caspase 3 activity; no effects were observed with the nontoxic BA 35-25 reverse sequence. The dependence on mitochondrial transition pore (MTP) activity of MPP(+)-induced caspase activations was demonstrated by preincubation with bongkreckic acid, which blocked elevations of caspases 9 and 3. Stereoisomers of pramipexole (PPX), a free radical scavenger and inhibitor of MTP opening, inhibited caspase activation (MPP(+) and BA) and restored calcein accumulation (MPP(+)). Our results demonstrate that MPP(+) and BA can induce cell death through MTP-dependent activation of caspase cascades. PPX stereoisomers interfere with activation of these cell death pathways and may be useful clinically as neuroprotectants in PD and AD and related diseases.

Menadione-induced apoptosis: roles of cytosolic Ca2+elevations and the mitochondrial permeability transition pore

In normal pancreatic acinar cells, the oxidant menadione evokes repetitive cytosolic Ca2+ spikes, partial mitochondrial depolarisation,cytochrome c release and apoptosis. The physiological agonists acetylcholine and cholecystokinin also evoke cytosolic Ca2+ spikes but do not depolarise mitochondria and fail to induce apoptosis. Ca2+ spikes induced by low agonist concentrations are confined to the apical secretory pole of the cell by the buffering action of perigranular mitochondria. Menadione prevents mitochondrial Ca2+ uptake, which permits rapid spread of Ca2+ throughout the cell. Menadione-induced mitochondrial depolarisation is due to induction of the permeability transition pore. Blockade of the permeability transition pore with bongkrekic acid prevents activation of caspase 9 and 3. In contrast, the combination of antimycin A and acetylcholine does not cause apoptosis but elicits a global cytosolic Ca2+ rise and mitochondrial depolarisation without induction of the permeability transition pore. Increasing the cytosolic Ca2+buffering power by BAPTA prevents cytosolic Ca2+ spiking, blocks the menadione-elicited mitochondrial depolarisation and blocks menadione-induced apoptosis. These results suggest a twin-track model in which both intracellular release of Ca2+ and induction of the permeability transition pore are required for initiation of apoptosis.

Evidence of mitochondrial involvement in the transduction of signals required for the induction of genes associated with pathogen attack and senescence

Using the mRNA differential display technique, seven cDNAs have been isolated that are rapidly induced when cultured tobacco (Nicotiana tabacum) cells are treated with the mitochondrial electron transport inhibitor antimycin A (AA). Interestingly, six of the cDNAs show distinct similarity to genes known to be induced by processes that involve programmed cell death (PCD), such as senescence and pathogen attack. All of the cDNAs as well as Aox1, a gene encoding the alternative oxidase, were found to also be strongly induced by H2O2 and salicylic acid (SA). AA, H2O2 and SA treatment of tobacco cells caused a rapid rise in intracellular ROS accumulation that, when prevented by antioxidant treatment, resulted in inhibition of gene induction. Besides AA, both H2O2 and SA were found to disrupt normal mitochondrial function resulting in decreased rates of electron transport and a lowering of cellular ATP levels. Furthermore, the pre-treatment of tobacco cells with bongkrekic acid, a known inhibitor of the mitochondrial permeability transition pore in animal cells, was found to completely block gene induction when AA, H2O2 or SA were subsequently added. These findings suggest that the mitochondrion may serve an important role in conveying intracellular stress signals to the nucleus, leading to alterations in gene expression.

Activation of caspase-8 in 3-deazaadenosine-induced apoptosis of U-937 cells occurs downstream of caspase-3 and caspase-9 without Fas receptor-ligand interaction

3-Deazaadenosine (DZA), a cellular methylation blocker was reported to induce the caspase-3-like activities-dependent apoptosis in U-937 cells. In this study, we analyzed the activation pathway of the caspase cascade involved in the DZA-induced apoptosis using specific inhibitors of caspases. In the U-937 cells treated with DZA, cytochrome c release from mitochondria and subsequent activation of caspase-9, -8 and -3 were observed before the induction of apoptosis. zDEVD-Fmk, a specific inhibitor of caspase-3, and zLEHD-Fmk, a specific inhibitor of caspase-9, prevented the activation of caspase-8 but neither caspase-3 nor caspase-9, indicating that caspase-8 is downstream of both caspase-3 and caspase-9, which are activated by independent pathways. zVAD-Fmk, a universal inhibitor of caspases, kept the caspase-3 from being activated but not caspase-9. Moreover, ZB4, an antagonistic Fas-antibody, exerted no effect on the activation of caspase-8 and induction of apoptosis by DZA. In addition, zVAD-Fmk and mitochondrial permeability transition pore (MPTP) inhibitors such as cyclosporin A (CsA) and bongkrekic acid (BA) did not block the release of cytochrome c from mitochondria. Taken together, these results suggest that in the DZA-induced apoptosis, caspase-8 may serve as an executioner caspase and be activated downstream of both caspase-3 and caspase-9, independently of Fas receptor-ligand interaction. And caspase-3 seems to be activated by other caspses including IETDase-like enzyme and caspse-9 seems to be activated by cytochrome c released from mitochondria without the involvement of caspases and CsA- and BA- inhibitory MPTP.

Zn(2+) induces permeability transition pore opening and release of pro-apoptotic peptides from neuronal mitochondria

Rapid entry of Ca(2+) or Zn(2+) kills neurons. Mitochondria are major sites of Ca(2+)-dependent toxicity. This study examines Zn(2+)-initiated mitochondrial cell death signaling. 10 nm Zn(2+) induced acute swelling of isolated mitochondria, which was much greater than that induced by higher Ca(2+) levels. Zn(2+) entry into mitochondria was dependent upon the Ca(2+) uniporter, and the consequent swelling resulted from opening of the mitochondrial permeability transition pore. Confocal imaging of intact neurons revealed entry of Zn(2+) (with Ca(2+)) to cause pronounced mitochondrial swelling, which was far greater than that induced by Ca(2+) entry alone. Further experiments compared the abilities of Zn(2+) and Ca(2+) to induce mitochondrial release of cytochrome c (Cyt-c) or apoptosis-inducing factor. In isolated mitochondria, 10 nm Zn(2+) exposures induced Cyt-c release. Induction of Zn(2+) entry into cortical neurons resulted in distinct increases in cytosolic Cyt-c immunolabeling and in cytosolic and nuclear apoptosis-inducing factor labeling within 60 min. In comparison, higher absolute [Ca(2+)](i) rises were less effective in inducing release of these factors. Addition of the mitochondrial permeability transition pore inhibitors cyclosporin A and bongkrekic acid decreased Zn(2+)-dependent release of the factors and attenuated neuronal cell death as assessed by trypan blue staining 5-6 h after the exposures.

Mitochondrial ATP-sensitive potassium channels attenuate matrix Ca(2+) overload during simulated ischemia and reperfusion: possible mechanism of cardioprotection

Mitochondrial ATP-sensitive potassium (mitoK(ATP)) channels play a key role in ischemic preconditioning of the heart. However, the mechanism of cardioprotection remains controversial. We measured rhod-2 fluorescence in adult rabbit ventricular cardiomyocytes as an index of mitochondrial matrix Ca(2+) concentration ([Ca(2+)](m)), using time-lapse confocal microscopy. To simulate ischemia and reperfusion (I/R), cells were exposed to metabolic inhibition (50 minutes) followed by washout with control solution. Rhod-2 fluorescence gradually increased during simulated ischemia and rose even further with reperfusion. The mitoK(ATP) channel opener diazoxide attenuated the accumulation of [Ca(2+)](m) during simulated I/R (EC(50)=18 micromol/L). These effects of diazoxide were blocked by the mitoK(ATP) channel antagonist 5-hydroxydecanoate (5HD). In contrast, inhibitors of the mitochondrial permeability transition (MPT), cyclosporin A and bongkrekic acid, did not alter [Ca(2+)](m) accumulation during ischemia, but markedly suppressed the surge in rhod-2 fluorescence during reperfusion. Measurements of mitochondrial membrane potential, DeltaPsi(m), in permeabilized myocytes revealed that diazoxide depolarized DeltaPsi(m) (by 12% at 10 micromol/L, P<0.01) in a 5HD-inhibitable manner. Our data support the hypothesis that attenuation of mitochondrial Ca(2+) overload, as a consequence of partial mitochondrial membrane depolarization by mitoK(ATP) channels, underlies cardioprotection. Furthermore, mitoK(ATP) channels and the MPT differentially affect mitochondrial calcium homeostasis: mitoK(ATP) channels suppress calcium accumulation during I/R, while the MPT comes into play only upon reperfusion.

Involvement of caspases and of mitochondria in Fas ligation-induced eosinophil apoptosis: modulation by interleukin-5 and interferon-gamma

In this study, we examined the relative importance of caspases and mitochondria in Fas-mediated eosinophil apoptosis. Stimulation of human peripheral blood eosinophils with an agonistic anti-human Fas monoclonal antibody, but not with control IgM, induced a time-dependent increase in their apoptosis, which was associated with a loss in mitochondrial transmembrane potential (DeltaPsi(m)) and with caspase-8 and caspase-3 activation. Interleukin (IL)-5 and interferon (IFN)-gamma, two cytokines known to prolong eosinophil survival, inhibited Fas-mediated apoptosis and caspase activation but poorly affected the decrease in DeltaPsi(m). Eosinophil incubation with bongkrekic acid, an inhibitor of the mitochondrial permeability transition pore (MPTP) opening, failed to modify Fas-mediated loss in DeltaPsi(m), caspase activation, and apoptosis. In contrast, caspase inhibitors markedly reduced eosinophil apoptosis without significantly affecting DeltaPsi(m) dissipation. We conclude that caspase-8 and caspase-3 activation, but not MPTP opening, mediate Fas-induced eosinophil apoptosis and are the main targets for the protective effect of IL-5 and IFN-gamma.

Characterization of loops of the yeast mitochondrial ADP/ATP carrier facing the cytosol by site-directed mutagenesis

To characterize structural features of the regions of the yeast type 2 ADP/ATP carrier (yAAC2) facing the cytosol, we prepared its Cys-less mutant, in which all four cysteine residues were replaced by alanine residues. The Cys-less mutant functioned like native yAAC2, showing that the cysteine residues are not essential. We then prepared cysteine mutants by substituting Ser(21) in the putative N-terminal region, Ala(124) and Ser(222) in the first and second loops facing cytosol, respectively, and Leu(312) in the C-terminal region of the Cys-less mutant for cysteine and examined the labeling of the substituted cysteine residues of the mutants with the membrane-impermeable SH reagent eosin-5-maleimide (EMA) from the cytosol. EMA labeled all the mutants, showing that all regions containing mutated residues faced the cytosolic side. The effects of transport inhibitors on EMA labeling were also examined. From the results, the location and conformation of the region around mutated residues were discussed.

Differential involvement of p38 MAP kinase pathway and Bax translocation in the mitochondria-mediated cell death in TCR- and dexamethasone-stimulated thymocytes

Mitochondria play a central role in many apoptotic reactions. Although mitochondrial apoptotic changes and caspase activation have been demonstrated in the apoptotic thymocytes, cell death signal through mitochondria in TCR-stimulated thymocytes has not been fully understood. In this study, we show that TCR stimulation induced disruption of mitochondrial transmembrane potential (Delta Psi(m)), the cytochrome c release from mitochondira, capase-3 activation, and the cell death of thymocytes. Bongkrekic acid, an inhibitor of Delta Psi(m) disruption, blocked the cytochrome c release from mitochondria and the following caspase-3-mediated cell death. Furthermore, a pro-apoptotic Bcl-2 family protein, Bax, but not Bad or Bid, was translocated from cytosol to mitochondria in TCR-stimulated thymocytes. This translocation and the following apoptotic changes were inhibited by SB203580, a p38 kinase inhibitor, in a specific manner. These results suggest that activated p38 kinase pathway by TCR stimulation induces translocation of Bax to mitochondria, causing Delta Psi(m) disruption, and the release of cytochrome c, which finally induces caspase-3-mediated apoptosis in thymocytes.

Mitochondrial cytochrome c release is caspase-dependent and does not involve mitochondrial permeability transition in didemnin B-induced apoptosis

Permeability transition, and a subsequent drop in mitochondrial membrane potential (DeltaPsi(m)), have been suggested to be mechanisms by which cytochrome c is released from the mitochondria into the cytosol during apoptosis. Furthermore, a drop in DeltaPsi(m) has been suggested to be an obligate early step in the apoptotic pathway. Didemnin B, a branched cyclic peptolide described to have immunosuppressive, anti-tumour, and anti-viral properties, induces rapid apoptosis in a range of mammalian cell lines. Induction of apoptosis by didemnin B in cultured human pro-myeloid HL-60 cells is the fastest and most complete ever described with all cells being apoptotic after 3 h of treatment. By utilizing the system of didemnin B-induced apoptosis in HL-60 cells, and the potent inhibitors of mitochondrial permeability transition, cyclosporin A and bongkrekic acid, we show that permeability transition as determined by changes in DeltaPsi(m) and mitochondrial Ca2+ fluxing, is not a requirement for apoptosis or cytochrome c release. In this system, changes in mitochondrial membrane potential and cytochrome c release are shown to be dependent on caspase activation, and to occur concurrently with the release of caspase-9 from mitochondria, genomic DNA fragmentation and apoptotic body formation.

Serpin protein CrmA suppresses hypoxia-mediated apoptosis of ventricular myocytes

BACKGROUND

In this study, we ascertain whether caspase 8 activation and mitochondrial defects underlie apoptosis of ventricular myocytes during hypoxia. As an approach to circumvent the potential shortcomings surrounding the limited permeability and short half-life of the synthetic peptide inhibitors designed to block caspase activation, we constructed a replication-defective adenovirus encoding the serpin caspase inhibitor protein CrmA to ensure efficient and continual inhibition of caspase 8 activity during chronic hypoxia.

METHODS AND RESULTS

In contrast to normoxic cells, oxygen deprivation of postnatal ventricular myocytes for 24 hours resulted in a 9-fold increase (P<0.05) in apoptosis as determined by Hoechst 33258 staining and nucleosomal DNA laddering. Moreover, hypoxia provoked a 1.5-fold increase (P<0.01) in caspase 8-like activity. Furthermore, hypoxia provoked perturbations to mitochondria consistent with the mitochondrial death pathway, including permeability transition pore (PT) opening, loss of mitochondrial membrane potential ((m)), and cytochrome c release. Importantly, CrmA suppressed caspase 8 activity, PT pore changes, loss of (m), and apoptosis but had no effect on hypoxia-mediated cytochrome c release. Furthermore, Bongkrekic acid, an inhibitor of PT pore, prevented hypoxia-induced PT pore changes, loss of (m), and apoptosis but had no effect on hypoxia-mediated cytochrome c release.

CONCLUSIONS

To our knowledge, we provide the first direct evidence for the operation of CrmA as an antiapoptotic factor in ventricular myocytes during prolonged durations of hypoxia. Furthermore, our data suggest that perturbations to mitochondria including PT pore changes and (m) loss are caspase-regulated events that appear to be separable from cytochrome c release.

Intracellular Bax translocation after transient cerebral ischemia: implications for a role of the mitochondrial apoptotic signaling pathway in ischemic neuronal death

Activation of terminal caspases such as caspase-3 plays an important role in the execution of neuronal cell death after transient cerebral ischemia. Although the precise mechanism by which terminal caspases are activated in ischemic neurons remains elusive, recent studies have postulated that the mitochondrial cell death-signaling pathway may participate in this process. The bcl-2 family member protein Bax is a potent proapoptotic molecule that, on translocation from cytosol to mitochondria, triggers the activation of terminal caspases by increasing mitochondrial membrane permeability and resulting in the release of apoptosis-promoting factors, including cytochrome c. In the present study, the role of intracellular Bax translocation in ischemic brain injury was investigated in a rat model of transient focal ischemia (30 minutes) and reperfusion (1 to 72 hours). Immunochemical studies revealed that transient ischemia induced a rapid translocation of Bax from cytosol to mitochondria in caudate neurons, with a temporal profile and regional distribution coinciding with the mitochondrial release of cytochrome c and caspase-9. Further, in postischemic caudate putamen in vivo and in isolated brain mitochondria in vitro, the authors found enhanced heterodimerization between Bax and the mitochondrial membrane permeabilization-related proteins adenine nucleotide translocator (ANT) and voltage-dependent anion channel. The ANT inhibitor bongkrekic acid prevented Bax and ANT interactions and inhibited Bax-triggered caspase-9 release from isolated brain mitochondria in vitro. Bongkrekic acid also offered significant neuroprotection against ischemia-induced caspase-3 and caspase-9 activation and cell death in the brain. These results strongly suggest that the Bax-mediated mitochondrial apoptotic signaling pathway may play an important role in ischemic neuronal injury.

Induction of the mitochondrial permeability transition mediates the killing of HeLa cells by staurosporine

The role of the mitochondrial permeability transition (MPT) in the killing of HeLa cells by staurosporine (STR) was assessed with the use of bongkrekic acid (BK), an inhibitor of the MPT. BK prevented cell killing as well as biochemical manifestations of the MPT: (a) the loss of the mitochondrial membrane potential (deltapsim); (b) the release of cytochrome c from the intramembranous space to the cytosol; and (c) the release of malate dehydrogenase from the mitochondrial matrix. Stable transfectants that overexpressed Akt were also resistant to cell killing and did not develop an MPT. STR inhibited the phosphorylation of Bad, whereas Bad phosphorylation was preserved in cells that overexpress Akt. In wild-type HeLa cells treated with STR, the content of Bax in the cytosol decreased as that in the mitochondria increased, a result that was again prevented by overexpression of Akt. Bid accumulation in the mitochondria with STR was not affected by overexpression of Akt. The pan-caspase inhibitor Z-Val-Ala-Val-Asp(OMe) fluoromethylketone prevented cell killing bu not induction of the MPT. The data document the central role of the MPT in the killing of HeLa cells by STR. The data are consistent with the hypothesis that induction of the MPT is a consequence of the movement of Bax to the mitochondria. Phosphorylation of Bad prevents Bax translocation. Caspases participate in the events related to cell killing that occur subsequent to induction of the MPT.

Bile acid-induced rat hepatocyte apoptosis is inhibited by antioxidants and blockers of the mitochondrial permeability transition

The accumulation of hydrophobic bile acids plays a role in the induction of apoptosis and necrosis of hepatocytes during cholestasis. The aim of this study was to determine in freshly isolated rat hepatocytes the roles of oxidant stress and the mitochondrial permeability transition (MPT) in bile acid-induced apoptosis. Hepatocytes isolated from adult male Sprague-Dawley rats were incubated for 4 hours in buffer containing the hydrophobic bile acid, glycochenodeoxycholic acid (GCDC, 0-500 micromol/L) or the hydrophilic bile acid, glycocholic acid (GCA), and either the antioxidants, alpha tocopherol, ebselen, or idebenone (a coenzyme Q analogue); or the MPT blockers, cyclosporin A, or bongkrekic acid, or a caspase-8 inhibitor. Apoptosis was assessed hourly by nuclear morphologic changes of fixed cells by DAPI fluorescence microscopy and reactive oxygen species (ROS) generation by dichlorofluorescein fluorescence of hepatocytes. The percent of cells undergoing apoptosis increased in a time- and concentration-dependent manner in cells exposed to GCDC, and to a much lesser extent to GCA. ROS generation preceded the onset of apoptosis. MPT blockers, caspase-8 inhibition, and antioxidants prevented apoptosis and reduced ROS generation by hepatocytes. Flow cytometry analysis showed that MPT occurred within 1 hour of exposure of cells to 100 micromol/L GCDC, prior to onset of significant apoptosis. In conclusion, ROS generation, MPT induction, and cytochrome c release are critical steps in the induction of apoptosis by bile acids. Antioxidants may reduce liver injury caused by low levels of bile acids by preventing the generation of oxidant stress and subsequent stimulation of the MPT and release of cytochrome c from mitochondria.

Cell type specific involvement of death receptor and mitochondrial pathways in drug-induced apoptosis

Apoptosis in response to cellular stress such as treatment with cytotoxic drugs is mediated by effector caspases (caspase-3) which can be activated by different initiator pathways. Here, we report on a cell type specific triggering of death receptor and/or mitochondrial pathways upon drug treatment. In type I cells (BJAB), both the receptor and the mitochondrial pathway were activated upon drug treatment, since blockade of either the receptor pathway by overexpression of dominant negative FADD (FADD-DN) or of the mitochondrial pathway by overexpression of Bcl-X(L) only partially inhibited apoptosis. Drug treatment induced formation of a FADD- and caspase-8-containing CD95 death-inducing signaling complex (DISC) in type I cells resulting in activation of caspase-8 as the most apical caspase. In contrast, in type II cells (Jurkat), apoptosis was predominantly controlled by mitochondria, since overexpression of Bcl-2 completely blocked drug-induced apoptosis, while overexpression of FADD-DN had no protective effect. In these cells, caspases including caspase-8 were activated by mitochondria-driven signaling events and no DISC was detected despite expression levels of CD95, FADD and caspase-8 proteins comparable to type I cells. Likewise, drug-induced CD95 aggregation was predominantly found in type I cells. Bid was cleaved prior to mitochondrial alterations in type I cells providing a molecular link between caspase-8 activation and mitochondrial perturbations, whereas in type II cells, Bid was cleaved downstream of mitochondria. Our findings of a cell type specific response to cytotoxic drugs have implications for the identification of molecular parameters for chemosensitivity or resistance in different tumor cells.

Tumor-induced apoptosis of T cells: amplification by a mitochondrial cascade

We have recently reported that apoptosis of T cells induced by squamous cell carcinoma of the head and neck (SCCHN) is partly Fas dependent. This tumor-induced T-cell death is mediated by the activities of caspase-8 and caspase-3 and is partially inhibited by antibodies to either Fas or Fas ligand. We report here that in contrast to apoptosis induced by agonistic anti-Fas antibody (Ab), the tumor-induced apoptotic cascade in Jurkat cells is significantly amplified by a mitochondrial loop. The involvement of mitochondria in tumor-induced apoptosis of T cells was demonstrated by changes in mitochondrial permeability transition as assessed by 3,3'-dihexiloxadicarbocyanine staining, by cleavage of cytosolic BID and its translocation to the mitochondria, by release of cytochrome c to the cytosol, and by the presence of active subunits of caspase-9 in Jurkat T cells cocultured with tumor cells. To further elucidate the significance of mitochondria in tumor-induced T-cell death, we investigated the effects of various inhibitors of the mitochondrial pathway. Specific antioxidants, as well as two inhibitors of mitochondria permeability transition, bongkrekic acid and cyclosporin A, significantly blocked the DNA degradation induced in Jurkat T cells by SCCHN cells. However, these inhibitors had no effect on cells triggered by anti-Fas Ab. Furthermore, a cell-permeable inhibitor of caspase-9, Ac-LEHD.CHO, which did not inhibit T-cell apoptosis induced by anti-Fas Ab, markedly inhibited apoptosis induced by etoposide or by coculture of Jurkat with SCCHN cells. These findings demonstrate that apoptotic cascades induced in Jurkat T lymphocytes by anti-Fas Ab or tumor cells are differentially susceptible to a panel of inhibitors of mitochondrial apoptotic events. It appears that besides the Fas-mediated pathway, additional mitochondria-dependent cascades are involved in apoptosis of tumor-associated lymphocytes. Inhibition of mitochondria-dependent cascades of caspase activation should be considered to enhance the success of immunotherapy or vaccination protocols in cancer.

Bid acts on the permeability transition pore complex to induce apoptosis

Similar to most if not all pro-apoptotic members of the Bcl-2 family, Bid (and its truncated product t-Bid) triggers cell death via mitochondrial membrane permeabilization (MMP). This effect can be monitored in intact cells, upon microinjection of recombinant Bid protein into the cytoplasm, as well as in purified mitochondria, upon addition of Bid protein. Here we show that Bid-induced MMP can be inhibited, both in cells and in the cell-free system, by three pharmacological inhibitors of the permeability transition pore complex (PTPC), namely cyclosporin A, N-methyl-4-Val-cyclosporin A, and bongkrekic acid (a ligand of the adenine nucleotide translocase, ANT, one of the PTPC components). Bid effects on synthetic membranes were studied either in proteoliposomes or in synthetic bilayers subjected to electrophysiological measurements. Full length Bid preferentially permeabilizes membranes and induces the formation of large conductance channels at neutral pH, when added to liposomes or bilayers containing both purified ANT and Bax, yet has no or little effect combined with ANT or Bax alone. t-Bid acts on membranes containing ANT alone with the same efficiency as on those containing both ANT and Bax. These results suggest that the proapoptotic effects of Bid are mediated, at least in part, by its functional interaction with ANT, one of the major components of PTPC.

Cardiomyocyte apoptosis induced by Galphaq signaling is mediated by permeability transition pore formation and activation of the mitochondrial death pathway

Expression of the wild-type alpha subunit of Gq stimulates phospholipase C and induces hypertrophy in cardiomyocytes. Addition of Gq-coupled receptor agonists additionally activates phospholipase C, as does expression of a constitutively active mutant form of Galphaq. Under these conditions, hypertrophy is rapidly succeeded by apoptotic cellular and molecular changes, including myofilament disorganization, loss of mitochondrial membrane potential, alterations in Bcl-2 family protein levels, DNA fragmentation, increased caspase activity ( approximately 4-fold), cytochrome c redistribution, and nuclear chromatin condensation in approximately 12% of the cells. We used various interventions to define the molecular relationships between these events and identify potential sites at which these features of apoptosis could be rescued. Treatment with caspase inhibitors prevented DNA fragmentation and promoted myocyte survival; however, cytochrome c release and loss of mitochondrial membrane potential still occurred. In contrast, treatment with bongkrekic acid, an inhibitor of the mitochondrial permeability transition pore, not only prevented DNA fragmentation and reduced nuclear chromatin condensation but also preserved mitochondrial membrane potential and limited cytochrome c redistribution to only approximately 2% of cells. These data demonstrate the central role of mitochondrial membrane potential in initiation of caspase activation and downstream apoptotic events and suggest that preservation of mitochondrial integrity is crucial for prolonging the life and function of cardiomyocytes exposed to pathological levels of stress.

Mechanism of nitric oxide-induced apoptosis in human neuroblastoma SH-SY5Y cells

We have attempted to elucidate the precise mechanism of nitric oxide (NO)-induced apoptotic neuronal cell death. Enzymatic cleavages of DEVD-AFC, VDVAD-AFC, and LEHD-AFC (specific substrates for caspase-3-like protease (caspase-3 and -7), caspase-2, and caspase-9, respectively) were observed by treatment with NO. Western blot analysis showed that pro-forms of caspase-2, -3, -6, and -7 are decreased during apoptosis. Interestingly, Ac-DEVD-CHO, a caspase-3-like protease inhibitor, blocked not only the decreases in caspase-2 and -7, but also the formation of p17 from p20 in caspase-3 induced by NO, suggesting that caspase-3 exists upstream of caspase-2 and -7. Bongkrekic acid, a potent inhibitor of mitochondrial permeability transition, specifically blocked both the loss of mitochondrial membrane potential and subsequent DNA fragmentation in response to NO. Thus, NO results in neuronal apoptosis through the sequential loss of mitochondrial membrane potential, caspase activation, and degradation of inhibitor of caspase-activated DNase (CAD) (CAD activation).

Bcl-2 independence of flavopiridol-induced apoptosis. Mitochondrial depolarization in the absence of cytochrome c release

The new chemotherapeutic agent, flavopiridol, presently in clinical trials, has been extensively studied yet little is known about its mechanism of action. In this study we show that the induction of apoptosis by flavopiridol is largely independent of Bcl-2. This is indicated by the observation that neither overexpression nor the antisense oligonucleotide-mediated down-regulation of Bcl-2 had any effect on flavopiridol-induced cell killing. Our results suggest that flavopiridol can induce apoptosis through different pathways of caspase activation with caspase 8 playing a pivotal role. In human lung carcinoma cells, which contain high levels of endogenous Bcl-2 and lack procaspase 8, flavopiridol treatment leads to mitochondrial depolarization in the absence of cytochrome c release, followed by the activation of caspase 3 and cell death. These results clearly differ from observations made with other anti-tumor drugs and might explain, at least in part, the unusual anti-tumor properties of flavopiridol.