Bcl-2 family: life-or-death switch

The Bcl-2 family of proteins that consists of anti-apoptotic and pro-apoptotic members determines life-or-death of a cell by controlling the release of mitochondrial apoptogenic factors, cytochrome c and apoptosis-inducing factor (AIF), that activate downstream executional phases, including the activation of death proteases called caspases. Cytochrome c release is, thus, central to apoptotic signal transduction in mammals, making study of the mechanism for cytochrome c release a major issue. Several models for cytochrome c release have been proposed, including rupture of mitochondrial outer membrane and involvement of a specific channel. Here, we provide an overview of recent findings on the role of Bcl-2 family members in the life-or-death decision of a cell.

Proapoptotic BH3-only Bcl-2 family members induce cytochrome c release, but not mitochondrial membrane potential loss, and do not directly modulate voltage-dependent anion channel activity

Through direct interaction with the voltage-dependent anion channel (VDAC), proapoptotic Bcl-2 family members such as Bax and Bak induce apoptogenic mitochondrial cytochrome c release and membrane potential (Deltapsi) loss in isolated mitochondria. Using isolated mitochondria, we showed that Bid and Bik, BH3-only proteins from the Bcl-2 family, induced cytochrome c release but not Deltapsi loss. Unlike Bax/Bak, the cytochrome c release induced by Bid/Bik was Ca(2+)-independent, cyclosporin A-insensitive, and respiration-independent. Furthermore, in contrast to Bax/Bak, Bid/Bik neither interacted with VDAC nor directly affected the VDAC activity in liposomes. Consistently, Bid/Bik induced apoptosis without Deltapsi loss, whereas Bax induced apoptosis with Deltapsi loss. These findings indicated the involvement of a different mechanism in BH3-only, protein-induced apoptogenic cytochrome c release.

Calmodulin and cyclic ADP-ribose interaction in Ca2+ signaling related to cardiac sarcoplasmic reticulum: superoxide anion radical-triggered Ca2+ release

Reactive oxygen species (ROS) are often shown to damage cellular functions. The targets of oxidative damage depend on the nature of ROS produced and the site of generation. In contrast, ROS can also regulate signal transduction. In this case, ROS may either induce or enhance events, which lead to forward directions of cellular signaling. The consequences of regulation of signal transduction can be observed in physiological processes such as muscle contraction. Here, we discuss the concentration-dependent effects of superoxide anion radical (*O2-) on Ca2+ release from the cardiac sarcoplasmic reticulum (SR). Recent studies suggest that the ADP-ribosyl cyclase pathway, through its production of cyclic adenosine 5'-diphosphoribose (cADPR), may control Ca2+ mobilization in cardiac muscle cells. *O2- has dual effects that are concentration dependent. At low concentrations (nearly nanomolar levels), *O2- induces Ca2+ release by stimulating synthesis of cADPR, which requires calmodulin for sensitization of ryanodine-sensitive Ca2+-release channels (RyRC). At these low concentrations, *O2- is responsible for regulation of cellular signal transduction. At higher concentrations (micromolar levels), *O2- produces a loss in the function of calmodulin that is to inhibit RyRC. This results in an increase in Ca2+ release, which is linked to cell injury. The difference in the functions of low and high concentrations of *O2- may result in two distinct physiological roles in cardiac muscle Ca2+ signaling.

Cloning and secretive expression of the gene encoding the proteinaceous alpha-amylase inhibitor paim from Streptomyces corchorusii

A gene encoding the proteinaceous alpha-amylase inhibitor Paim was cloned and sequenced. Southern analysis and the amino acid sequence deduced from the cloned gene indicated that Paim isoforms were encoded in the same gene. When the gene was expressed in Escherichia coli and Streptomyces lividans, recombinant Paim inhibitors were produced in the periplasmic space and in the culture supernatant, respectively. The purified inhibitors had different N-terminal sequences from those of the authentic inhibitors.

[A case of crossed fused kidney with simple ureterocele]

A 32-year-old man consulted Osaka National hospital with chief complaints of dysuria and macrohematuria. DIP and CT revealed that the right kidney deviated to the lower pole of the left kidney and they fused together. The right ureter crossed over the supine. The calcified shadow existed in the lower end of the left ureter with cobra head image. He had no external anomalies. Under diagnosing crossed fused kidney (inverted L shaped) complicated the left ureterocele with a stone, transurethral incision of ureterocele (TUI) was performed. We made transverse incision and extracted stone, 7 mm in size (calcium oxalate 96% and calcium phosphate 4%). Three months later after the operation, IVP, CG and VCG revealed the down-sized ureterocele and no VUR. Crossed renal ectopia complicated many anomalies about 50%. Among them anomalies of the urinary tract was most frequent about 30%. But crossed renal ectopia with ureterocele wasn't reported so far in Japanese literature.

Targeted expression of ced-3 and Ice induces programmed cell death in Drosophila

CED-3 is a cysteine protease required for programmed cell death in the nematode, Caenorhabditis elegans, and shares a sequence similarity with mammalian ICE (interleukin-1beta converting enzyme) family proteases. Both CED-3 and ICE family proteases can induce programmed cell death in mammalian cells. Structural and functional similarities between CED-3 and ICE family proteases indicate that the mechanism of cell death is evolutionarily conserved, suggesting the presence of a similar mechanism involving CED-3/ICE-like proteases in Drosophila. Here we determined whether CED-3 or ICE functions to induce programmed cell death in Drosophila. We have generated transformant lines in which ced-3 or Ice is ectopically expressed using the GAL4-UAS system. Expression of CED-3 and ICE can elicit cell death in Drosophila and the cell death was blocked by coexpressing the p35 gene which encodes a viral inhibitor of CED-3/ICE proteases. Results support the idea that the mechanism of programmed cell death controlled by CED-3/ICE is conserved among widely divergent animal species including Drosophila, and the system described provides a tool to dissect cell death mechanism downstream of CED-3/ICE proteases.

Bis, a Bcl-2-binding protein that synergizes with Bcl-2 in preventing cell death

Bcl-2 is the best characterized inhibitor of apoptosis, although the molecular basis of this action is not fully understood. Using a protein interaction cloning procedure, we identified a human gene designated as bis (mapped to chromosome 10q25) that encoded a novel Bcl-2-interacting protein. Bis protein showed no significant homology with Bcl-2 family proteins and had no prominent functional motif. Co-immunoprecipitation analysis confirmed that Bis interacted with Bcl-2 in vivo. DNA transfection experiments indicated that Bis itself exerted only weak anti-apoptotic activity, but was synergistic with Bcl-2 in preventing Bax-induced and Fas-mediated apoptosis. These results suggest that Bis is a novel modulator of cellular anti-apoptotic activity that functions through its interaction with Bcl-2.

Leptin in peritoneal dialysate from continuous ambulatory peritoneal dialysis patients

The adipocyte-derived hormone leptin is the 16-kd product of the ob gene that regulates food intake and body weight. Plasma leptin level is elevated in patients with chronic renal failure, partly because of impaired clearance through the kidney. In this study, we examined whether leptin is cleared into peritoneal dialysate in patients with end-stage renal disease treated by continuous ambulatory peritoneal dialysis (CAPD). The subjects were 46 CAPD patients and 67 age- and gender-matched healthy subjects. Leptin concentration in peritoneal dialysate from CAPD patients was measurable by a sensitive enzyme-linked immunosorbent assay (ELISA), and the daily loss of leptin by the peritoneal route was estimated to correspond to the amount contained in approximately 2 L plasma. Dialysate leptin concentration correlated positively with plasma leptin level and with percent body fat measured by dual-energy X-ray absorptiometry. The dialysate-to-plasma (D/P) ratio of leptin concentration was twice higher than expected from its molecular weight. D/P ratios of beta2-microglobulin, albumin, and transferrin showed strong correlations with each other (r = 0.768 to 0.801), whereas the correlation between D/P ratios of leptin and beta2-microglobulin was less impressive (r = 0.378). This was also the case with the relationship between apparent peritoneal clearances of these macromolecules, suggesting that dialysate leptin had some origins other than passive transport of plasma leptin. To test the hypothesis that abdominal visceral fat may contribute to the unexpectedly raised peritoneal dialysate leptin concentration, multiple regression analysis was performed. Leptin concentration in peritoneal dialysate showed significant association with plasma leptin level and D/P ratio of beta2-microglobulin, and it also showed an independent association with abdominal visceral fat but not with subcutaneous fat assessed by ultrasonography. These results showed that peritoneal dialysate from CAPD patients contained a significant amount of leptin, which derived presumably from both plasma and local visceral fat tissue.

Apoptotic cytosol facilitates Bax translocation to mitochondria that involves cytosolic factor regulated by Bcl-2

Proapoptotic members of the Bcl-2 family, including Bax, Bak, and Bid, directly trigger the mitochondrial release of apoptogenic cytochrome c and apoptosis-inducing factor into the cytoplasm. One of the crucial steps before Bax can exert its proapoptotic activity is translocation from the cytoplasm to the mitochondria, but the molecular mechanism of this translocation is not understood. To investigate the mechanism of apoptosis-associated Bax translocation, we used an in vitro system comprising isolated mitochondria and cytosol. We found that both endogenous and exogenous added recombinant Bax translocated to the mitochondria more efficiently in the presence of cytosol from cells with VP16-induced apoptosis than with cytoplasm from normal cells. This apoptosis-dependent promotion of Bax translocation was not seen with cytosol that was prepared from VP16-treated cells expressing Bcl-2. Cytosol from cells with VP16-induced apoptosis, but not that from normal cells or Bcl-2-expressing cells, induced cytochrome c release from isolated mitochondria, which, as assessed by immunodepletion experiments, was mainly mediated by Bax. These results suggest that Bcl-2 exerts its antiapoptotic activity partly by inhibiting the translocation of Bax through the modification of cytosolic factors that are involved in such translocation during apoptosis.

Molecular cloning and functional analysis of the murine bax gene promoter

Bcl-2-associated X protein (Bax) is a proapoptotic protein and is suggested to have an important role in carcinogenesis. To investigate the mechanism of bax gene transcriptional regulation, we isolated and sequenced the genomic DNA fragment of the 5' flanking region of the murine bax gene, and subcloned its promoter region into a luciferase reporter construction. The murine bax promoter is TATA-less, and the sequence is only partially homologous to that of the human bax promoter. Transient transfection into NIH 3T3 cells using unidirectionally deleted promoters and mutants of Sp1 sites revealed that two Sp1 sites were partially responsible for the basal activity. The murine bax promoter was not responsive to exogenous p53, suggesting that the p53-responsive element may not exist in the region used in our current experiments.

Acinus is a caspase-3-activated protein required for apoptotic chromatin condensation

Apoptosis is defined by several unique morphological nuclear changes, such as chromatin condensation and nuclear fragmentation. These changes are triggered by the activation of a family of cysteine proteases called caspases, and caspase-activated DNase (CAD/DFF40) and lamin protease (caspase-6) have been implicated in some of these changes. CAD/DFF40 induces chromatin condensation in purified nuclei, but distinct caspase-activated factor(s) may be responsible for chromatin condensation. Here we use an in vitro system to identify a new nuclear factor, designated Acinus, which induces apoptotic chromatin condensation after cleavage by caspase-3 without inducing DNA fragmentation. Immunodepletion experiments showed that Acinus is essential for apoptotic chromatin condensation in vitro, and an antisense study revealed that Acinus is also important in the induction of apoptotic chromatin condensation in cells.

Identification of NRF2, a member of the NF-E2 family of transcription factors, as a substrate for caspase-3(-like) proteases

Apoptosis is mediated by members of the interleukin-1beta converting enzyme (ICE) family of proteases (caspases), which are activated by diverse stimuli, although the downstream molecular targets of caspases are still poorly understood. Using the modified yeast two-hybrid system, which we recently established to clone genes for caspase substrates, we identified NRF2 as a novel caspase substrate. NRF2 is a member of the NF-E2 family of basic region leucine-zipper transcription factors and has been shown to induce phase II detoxifying enzymes through anti-oxidant response elements. NRF2 was cleaved at two sites by recombinant caspase-3 in vitro as well as in HeLa cells during TNFalpha-mediated apoptosis. Overexpression of the C-terminal cleavage fragment containing the DNA binding and leucine-zipper domains induced apoptosis in HeLa cells. These observations suggest that NRF2 might have some role in the induction of apoptosis after cleavage by caspases.

Bcl-2/E1B 19 kDa-interacting protein 3-like protein (Bnip3L) interacts with bcl-2/Bcl-xL and induces apoptosis by altering mitochondrial membrane permeability

We have previously reported on cloning of the human gene encoding Bcl-2/adenovirus E1B 19 kDa-interacting protein 3-like protein (Bnip3L) and its growth inhibitory effect on cancer cells. Here we show that Bnip3L contains a motif similar to the BH3 domain which is conserved in Bcl-2 family proteins as well as containing a membrane-anchoring domain, and that Bnip3L interacts with Bcl-2 and Bcl-xL. Immunofluorescence microscopy revealed that Bnip3L was localized in the mitochondria, when in the presence of the membrane-anchoring domain. Transient expression of Bnip3L induced apoptosis of Rat-1 and HeLa cells and mutational analysis revealed that the BH3 domain and the membrane-anchoring domain were required for Bnip3L to induce cell death. Addition of recombinant Bnip3L to isolated mitochondria induced membrane potential loss and cytochrome c release both of which have been suggested to be prerequisite for apoptotic cell death. These results suggest that Bnip3L is one of the BH3-containing pro-apoptotic proteins and that it targets the mitochondria when inducing apoptosis.

Identification of the dimethylbenzyl mercapturic acid in urine of rats treated with 1,2,3-trimethylbenzene

The structure was investigated of the mercapturic acid excreted in urine of rats after the i.p. administration of 1,2,3-trimethylbenzene. Of the two regioisomeric mercapturic acids, i.e. N-acetyl-S-(2,3-dimethylbenzyl)-L-cysteine and N-acetyl-S-(2,6-dimethyl-benzyl)-L-cysteine, only the former was isolated by preparative HPLC and identified, by comparison with an authentic specimen. The excretion rate of the mercapturate was estimated to be approximately 5% of dose, not a substantial metabolic route.

Mutation in ankyrin repeats of the mouse Notch2 gene induces early embryonic lethality

Notch family genes encode transmembrane proteins involved in cell-fate determination. Using gene targeting procedures, we disrupted the mouse Notch2 gene by replacing all but one of the ankyrin repeat sequences in the cytoplasmic domain with the E. coli (beta)-galactosidase gene. The mutant Notch2 gene encodes a 380 kDa Notch2-(beta)-gal fusion protein with (beta)-galactosidase activity. Notch2 homozygous mutant mice die prior to embryonic day 11.5, whereas heterozygotes show no apparent abnormalities and are fully viable. Analysis of Notch2 expression patterns, revealed by X-gal staining, demonstrated that the Notch2 gene is expressed in a wide variety of tissues including neuroepithelia, somites, optic vesicles, otic vesicles, and branchial arches, but not heart. Histological studies, including in situ nick end labeling procedures, showed earlier onset and higher incidence of apoptosis in homozygous mutant mice than in heterozygotes or wild type mice. Dying cells were particularly evident in neural tissues, where they were seen as early as embryonic day 9.5 in Notch2-deficient mice. Cells from Notch2 mutant mice attach and grow normally in culture, demonstrating that Notch2 deficiency does not interfere with cell proliferation and that expression of the Notch2-(beta)-gal fusion protein is not toxic per se. In contrast to Notch1-deficient mice, Notch2 mutant mice did not show disorganized somitogenesis, nor did they fail to properly regulate the expression of neurogenic genes such as Hes-5 or Mash1. In situ hybridization studies show no indication of altered Notch1 expression patterns in Notch2 mutant mice. The results indicate that Notch2 plays an essential role in postimplantation development in mice, probably in some aspect of cell specification and/or differentiation, and that the ankyrin repeats are indispensable for its function.

Long-term functional assessment of encapsulated cells transfected with Tet-On system

In our previous study, xenogeneic mouse neuroblastoma cells bearing the POMC gene, the precursor of ACTH and beta-endorphin, were implanted within polymer capsules into the CSF space of rats. Although ACTH and beta-endorphin were secreted, we were not able to control the amounts or times of hormone release. A promoter that is inducible by administration of tetracycline derivatives (Tet) was linked to the POMC gene to control its gene expression (Neuro2A-Tet-On-POMC; NTP). The results showed that POMC gene expression in the implanted encapsulated NTP cells could be regulated in a dose-dependent manner by Tet administration to the hosts. However, no analysis of gene control with the Tet-On system over a long period has been performed. In this study, encapsulated NTP cells were treated in vitro with doxycycline (Dox) (1.0, 10, 100, 1000 ng/ml) continuously for a month. On day 4, the amount of ACTH secretion was dependent on the Dox dose. But in the course of the experiment, the difference of ACTH secretion among those treated with Dox 10, 100, and 1000 ng/ml was eliminated. On the other hand, NTP cells, which were treated with Dox (1000 ng/ml) just on days 7, 14, 21, and 28, secreted almost the same amount of ACTH in 24 h. From these results, for clinical use, an NTP cell line that secretes enough opiate to reduce pain sensitivity without Dox should be established, and Dox could then be administered if necessary.

Bcl-2 family proteins regulate the release of apoptogenic cytochrome c by the mitochondrial channel VDAC

During transduction of an apoptotic (death) signal into the cell, there is an alteration in the permeability of the membranes of the cell's mitochondria, which causes the translocation of the apoptogenic protein cytochrome c into the cytoplasm, which in turn activates death-driving proteolytic proteins known as caspases. The Bcl-2 family of proteins, whose members may be anti-apoptotic or pro-apoptotic, regulates cell death by controlling this mitochondrial membrane permeability during apoptosis, but how that is achieved is unclear. Here we create liposomes that carry the mitochondrial porin channel (also called the voltage-dependent anion channel, or VDAC) to show that the recombinant pro-apoptotic proteins Bax and Bak accelerate the opening of VDAC, whereas the anti-apoptotic protein Bcl-x(L) closes VDAC by binding to it directly. Bax and Bak allow cytochrome c to pass through VDAC out of liposomes, but passage is prevented by Bcl-x(L). In agreement with this, VDAC1-deficient mitochondria from a mutant yeast did not exhibit a Bax/Bak-induced loss in membrane potential and cytochrome c release, both of which were inhibited by Bcl-x(L). Our results indicate that the Bcl-2 family of proteins bind to the VDAC in order to regulate the mitochondrial membrane potential and the release of cytochrome c during apoptosis.

ATP-dependent steps in apoptotic signal transduction

Apoptotic changes of the nucleus induced by Fas (Apo1/CD95) stimulation are completely blocked by reducing intracellular ATP level. In this study, we examined the ATP-dependent step(s) of Fas-mediated apoptotic signal transduction using two cell lines. In SKW6.4 (type I) cells characterized by rapid formation of the death-inducing signaling complex on Fas treatment, the activation of caspases 8, 9, and 3, cleavage of DFF45 (ICAD), and release of cytochrome c from the mitochondria to the cytoplasm were not affected by reduction of intracellular ATP, although chromatin condensation and nuclear fragmentation were inhibited. On the other hand, in the Fas-mediated apoptosis of Jurkat (type II) cells, which is characterized by involvement of mitochondria and, thus, shares signal transduction mechanisms with apoptosis induced by other stimuli such as genotoxins, activation of the three caspases, cleavage of DFF45 (ICAD), and nuclear changes were blocked by reduction of intracellular ATP, whereas release of cytochrome c was not affected. These results suggested that the ATP-dependent step(s) of Fas-mediated apoptotic signal transduction in type I cells are only located downstream of caspase 3 activation, whereas the activation of caspase 9 by released cytochrome c is the most upstream ATP-dependent step in type II cells. These observations also confirm the existence of two pathways for Fas-mediated apoptotic signal transduction and suggest that the Apaf-1 (Ced-4 homologue) system for caspase 9 activation operates in an ATP-dependent manner in vivo.

Apoptosis and necrosis: Intracellular ATP level as a determinant for cell death modes

Apoptosis and necrosis are two distinct modes of cell death with respective morphological characteristics. However, apoptosis and some forms of necrosis must share common steps since both modes of cell death can be suppressed by the anti-apoptotic Bcl-2 protein and caspase inhibitors. Intracellular ATP levels have been implicated both in vitro and in vivo as a determinant of the cell's decision to die by apoptosis or necrosis.

Caspase-4 and caspase-5, members of the ICE/CED-3 family of cysteine proteases, are CrmA-inhibitable proteases

Proteases of the caspase family are implicated in mammalian apoptosis and constitute a protease cascade. We characterized caspase-4 (TX/ICH-2/ICErelII) and caspase-5 (ICErelIII/TY), which are most closely related to caspase-1 (ICE) among the caspase family. Although overexpression of caspase-4 and caspase-5 induced apoptosis, confirming previous observations, this apoptosis was not inhibited by a caspase-1-specific tetrapeptide inhibitor (Ac-YVAD-CHO), suggesting that caspase-4 and caspase-5 have different substrate specificities from caspase-1 and also that caspase-4- and caspase-5-induced apoptosis is not mediated by caspase-1. CrmA, a cowpox virus-derived caspase-1 inhibitor that prevents apoptosis induced by various stimuli, was cleaved by caspase-4 and caspase-5, and inhibited their proteolytic activity as assessed by cleavage of pro-caspase-3 (pro-CPP32/Yama/apopain). Thus, caspase-4 and caspase-5 are CrmA-inhibitable proteases like caspase-1 and might be involved in apoptosis.

150-kDa oxygen-regulated protein (ORP150) suppresses hypoxia-induced apoptotic cell death

To determine the contribution of 150-kDa oxygen-regulated protein (ORP150) to cellular processes underlying adaptation to hypoxia, a cell line stably transfected to overexpress ORP150 antisense RNA was created. In human embryonic kidney (HEK) cells stably overexpressing ORP150 antisense RNA, ORP150 antigen and transcripts were suppressed to low levels in normoxia and hypoxia, whereas wild-type cells showed induction of ORP150 with oxygen deprivation. Inhibition of ORP150 in antisense transfectants was selective, as hypoxia-mediated enhancement of glucose-regulated protein (GRP) 78 and GRP94 was maintained. However, antisense ORP150 transfectants displayed reduced viability when subjected to hypoxia, compared with wild-type and sense-transfected HEK cells. In contrast, diminished levels of ORP150 had no effect on cytotoxicity induced by other stimuli, including oxygen-free radicals and sodium arsenate. Although cellular ATP content was similar in hypoxia, compared with ORP150 antisense transfectants and wild-type HEK cells, suppression of ORP150 expression was associated with accelerated apoptosis. Hypoxia-mediated cell death in antisense HEK transfectants did not cause an increase in caspase activity or in cytoplasmic cytochrome c antigen. A well recognized inducer of apoptosis in HEK cells, staurosporine, caused increased caspase activity and cytoplasmic cytochrome c levels in both wild-type and antisense cells. These data indicate that ORP150 has an important cytoprotective role in hypoxia-induced cellular perturbation and that ORP150-associated inhibition of apoptosis may involve mechanisms distinct from those triggered by other apoptotic stimuli.

Bcl-2 prevents caspase-independent cell death

Nitric oxide (NO) is implicated in apoptosis and has both cytotoxic and cytoprotective effects. Exogenous NO induced the death of PC12 and HeLa cells via a process showing features of both apoptosis and necrosis, with chromatin condensation, nuclear compaction, and mitochondrial swelling. Activation of caspases was not observed during NO-induced cell death. In addition, cell death was not inhibited by peptide caspase inhibitors or by expression of p35, a baculovirus-encoded caspase inhibitor, indicating that NO-induced cell death was independent of caspases. NO-induced cell death was enhanced by Bax expression in a caspase-independent manner and prevented by the anti-cell death protein Bcl-2. Although Bcl-2 has previously been shown to prevent cell death by inhibiting caspase activation, these results indicate that it can also prevent cell death via a caspase-independent mechanism.

Bax interacts with the permeability transition pore to induce permeability transition and cytochrome c release in isolated mitochondria

Cytochrome c release and the mitochondrial permeability transition (PT), including loss of the transmembrane potential (Deltapsi), play an important role in apoptosis. Using isolated mitochondria, we found that recombinant Bax and Bak, proapoptotic members of the Bcl-2 family, induced mitochondrial Deltapsi loss, swelling, and cytochrome c release. All of these changes were dependent on Ca2+ and were prevented by cyclosporin A (CsA) and bongkrekic acid, both of which close the PT pores (megachannels), indicating that Bax- and Bak-induced mitochondrial changes were mediated through the opening of these pores. Bax-induced mitochondrial changes were inhibited by recombinant Bcl-xL and transgene-derived Bcl-2, antiapoptotic members of the Bcl-2 family, as well as by oligomycin, suggesting a possible regulatory effect of F0F1-ATPase on Bax-induced mitochondrial changes. Proapoptotic Bax- and Bak-BH3 (Bcl-2 homology) peptides, but not a mutant BH3 peptide nor a mutant Bak lacking BH3, induced the mitochondrial changes, indicating an essential role of the BH3 region. A coimmunoprecipitation study revealed that Bax and Bak interacted with the voltage-dependent anion channel, which is a component of PT pores. Taken together, these findings suggest that proapoptotic Bcl-2 family proteins, including Bax and Bak, induce the mitochondrial PT and cytochrome c release by interacting with the PT pores.