BAX-dependent transport of cytochrome c reconstituted in pure liposomes

Caspase-independent apoptosis in yeast

Apoptosis is a highly regulated cellular suicide program crucial for metazoan development. Yeast counterparts of central metazoan apoptotic regulators, such as metacaspase Yca1p, have been identified. In spite of the importance of Yca1p in yeast apoptotic process, many other factors such as Aif1p, orthologs of EndoG, AMID and cyclophilin D play important roles in caspase-independent apoptotic pathways. This review summarized recent progress about studies of various intrinsic and extrinsic apoptotic stimuli that may induce yeast cell death via caspase-independent apoptosis.

RNA suppression of ERK2 leads to collapse of mitochondrial membrane potential with acute oxidative stress in human lens epithelial cells

17beta-Estradiol (E(2)) reduces oxidative stress-induced depolarization of mitochondrial membrane potential (MMP) in cultured human lens epithelial cells (HLE-B3). The mechanism by which the nongenomic effects of E(2) contributed to the protection against mitochondrial membrane depolarization was investigated. Mitochondrial membrane integrity is regulated by phosphorylation of BAD, and it is known that phosphorylation of Ser(112) inactivates BAD and prevents its participation in the mitochondrial death pathway. We found that E(2) rapidly increased both the phosphorylation of ERK2 and Ser(112) in BAD. Ser(112) is phosphorylated by p90 ribosomal S6 kinase (RSK), a Ser/Thr kinase, which is a downstream effector of ERK1/2. Inhibition of RSK by the RSK-specific inhibitor SL0101 did not reduce the level of E(2)-induced phosphorylation of Ser(112). Silencing BAD using small interfering RNA did not alter mitochondrial membrane depolarization elicited by peroxide insult. However, under the same conditions, silencing ERK2 dramatically increased membrane depolarization compared with the control small interfering RNA. Therefore, ERK2, functioning through a BAD-independent mechanism regulates MMP in humans lens epithelial cells. We propose that estrogen-induced activation of ERK2 acts to protect cells from acute oxidative stress. Moreover, despite the fact that ERK2 plays a regulatory role in mitochondrial membrane potential, estrogen was found to block mitochondrial membrane depolarization via an ERK-independent mechanism.

Diagnosing and exploiting cancer's addiction to blocks in apoptosis

Cancer cells survive despite violating rules of normal cellular behaviour that ordinarily provoke apoptosis. The blocks in apoptosis that keep cancer cells alive are therefore attractive candidates for targeted therapies. Recent studies have significantly increased our understanding of how interactions among proteins in the BCL2 family determine cell survival or death. It is now possible to systematically determine how individual cancers escape apoptosis. Such a determination can help predict not only whether cells are likely to be killed by antagonism of BCL2, but also whether they are likely to be sensitive to chemotherapy that kills by the intrinsic apoptotic pathway.

Proapoptotic BH3-only BCL-2 family protein BIM connects death signaling from epidermal growth factor receptor inhibition to the mitochondrion

A subset of lung cancers expresses mutant forms of epidermal growth factor receptor (EGFR) that are constitutively activated. Cancers bearing activated EGFR can be effectively targeted with EGFR inhibitors such as erlotinib. However, the death-signaling pathways engaged after EGFR inhibition are poorly understood. Here, we show that death after inhibition of EGFR uses the mitochondrial, or intrinsic, pathway of cell death controlled by the BCL-2 family of proteins. BCL-2 inhibits cell death induced by erlotinib, but BCL-2-protected cells are thus rendered BCL-2-dependent and sensitive to the BCL-2 antagonist ABT-737. BH3 profiling reveals that mitochondrial BCL-2 is primed by death signals after EGFR inhibition in these cells. As this result implies, key death-signaling proteins of the BCL-2 family, including BIM, were found to be up-regulated after erlotinib treatment and intercepted by overexpressed BCL-2. BIM is induced by lung cancer cell lines that are sensitive to erlotinib but not by those resistant. Reduction of BIM by siRNA induces resistance to erlotinib. We show that EGFR activity is inhibited by erlotinib in H1650, a lung cancer cell line that bears a sensitizing EGFR mutation, but that H1650 is not killed. We identify the block in apoptosis in this cell line, and show that a novel form of erlotinib resistance is present, a block in BIM up-regulation downstream of EGFR inhibition. This finding has clear implications for overcoming resistance to erlotinib. Resistance to EGFR inhibition can be modulated by alterations in the intrinsic apoptotic pathway controlled by the BCL-2 family of proteins.

The mitochondrial permeability transition pore and its involvement in cell death and in disease pathogenesis

Current research on the mitochondrial permeability transition pore (PTP) and its role in cell death faces a paradox. Initially considered as an in vitro artifact of little pathophysiological relevance, in recent years the PTP has received considerable attention as a potential mechanism for the execution of cell death. The recent successful use of PTP desensitizers in several disease paradigms leaves little doubt about its relevance in pathophysiology; and emerging findings that link the PTP to key cellular signalling pathways are increasing the interest on the pore as a pharmacological target. Yet, recent genetic data have challenged popular views on the molecular nature of the PTP, and called into question many early conclusions about its structure. Here we review basic concepts about PTP structure, function and regulation within the framework of intracellular death signalling, and its role in disease pathogenesis.

A tale of two mitochondrial channels, MAC and PTP, in apoptosis

The crucial step in the intrinsic, or mitochondrial, apoptotic pathway is permeabilization of the mitochondrial outer membrane. Permeabilization triggers release of apoptogenic factors, such as cytochrome c, from the mitochondrial intermembrane space into the cytosol where these factors ensure propagation of the apoptotic cascade and execution of cell death. However, the mechanism(s) underlying permeabilization of the outer membrane remain controversial. Two mechanisms, involving opening of two different mitochondrial channels, have been proposed to be responsible for the permeabilization; the permeability transition pore (PTP) in the inner membrane and the mitochondrial apoptosis-induced channel (MAC) in the outer membrane. Opening of PTP would lead to matrix swelling, subsequent rupture of the outer membrane, and an unspecific release of intermembrane proteins into the cytosol. However, many believe PTP opening is a consequence of apoptosis and this channel is thought to principally play a role in necrosis, not apoptosis. Activation of MAC is exquisitely regulated by Bcl-2 family proteins, which are the sentinels of apoptosis. MAC provides specific pores in the outer membrane for the passage of intermembrane proteins, in particular cytochrome c, to the cytosol. The electrophysiological characteristics of MAC are very similar to Bax channels and depletion of Bax significantly diminishes MAC activity, suggesting that Bax is an essential constituent of MAC in some systems. The characteristics of various mitochondrial channels and Bax are compared. The involvement of MAC and PTP activities in apoptosis of disease and their pharmacology are discussed.

Bid is not required for Bax translocation during UV-induced apoptosis

UV irradiation triggers apoptosis through both the membrane death receptor and the intrinsic apoptotic signaling pathways. Bax, a member of the Bcl-2 family of proteins, translocates from the cytosol to the mitochondrial membrane during UV-induced apoptosis, but the regulation of Bax translocation by UV irradiation remains elusive. In this study, we show that Bax translocation, caspase-3 activation and cell death by UV irradiation are not affected by Z-IETD-fmk (caspase-8 inhibitor), but delayed by Pifithrin-alpha (p53 inhibitor), although Bid cleavage could be completely abolished by Z-IETD-fmk. Co-transfecting YFP-Bax and Bid-CFP into human lung adenocarcinoma cells, we demonstrate that translocation of YFP-Bax precedes that of Bid-CFP, there is no significant FRET (fluorescence resonance energy transfer) between them. Similar results are obtained in COS-7 cells expressing YFP-Bax and Bid-CFP. Furthermore, using acceptor photobleaching technique, we observe that there is no interaction between YFP-Bax and Bid-CFP in both healthy and apoptotic cells. Additionally, during UV-induced apoptosis there is downregulation of Bcl-x(L), an anti-apoptotic protein. Overexpression of Bcl-x(L) in cells susceptible to UV-induced apoptosis prevents Bax translocation and cell death, repression of Bid protein with siRNA (small interfering RNA) do not inhibit cell death by UV irradiation. Taken together, these data strongly suggest that Bax translocation by UV irradiation is a Bid-independent event and inhibited by overexpression of Bcl-x(L).

Targeting BCL-2 family proteins to overcome drug resistance in non-small cell lung cancer

Cytotoxic chemotherapies are standard of care for patients suffering from advanced non-small cell lung cancer (NSCLC). However, objective responses are only achieved in 20% of cases and long-term survival is rarely observed. Clinically applied anticancer drugs exert at least some of their activities by inducing apoptosis. A critical step in apoptotic signal transduction is the permeabilization of the mitochondrial outer membrane (MOM), which is regulated by the BCL-2 family of proteins. Hence, therapeutic targeting of BCL-2 proteins is a promising approach to increase the drug-sensitivity of cancers. To this end we have assessed the impact of conditional expression of the proapoptotic multidomain (BH1-2-3) protein BAK, which directly permeabilizes the MOM, and the BH3-mimetic ABT-737, which acts indirectly by derepressing BH1-2-3 proteins, on apoptosis and drug sensitivity of NSCLC cells. Conditionally expressed BAK sensitized resistant NSCLC cells to drug-induced apoptosis. In contrast, ABT-737 was ineffective in those NSCLC cells expressing high levels of the anti-apoptotic MCL-1 protein. Tissue microarray analysis of tumor samples from 84 chemotherapy-naïve NSCLC patients revealed MCL-1 expression in 56% of cases, thus supporting the relevance of this resistance factor in a clinical setting. Enforced expression of the BH3-only protein NOXA, which targets MCL-1, overcame resistance to ABT-737. Moreover, combining conditionally expressed BAK with ABT-737 enhanced apoptosis in NSCLC cells independently of their MCL-1 status. In conclusion, the heterogeneity of apoptosis defects observed in drug-resistant NSCLC demands individually tailored molecular therapies. Targeting the MOM permeabilizer BAK appears to have a broader apoptogenic activity than the BH3-only mimetic ABT-737.

Bcl-2 or Bcl-XL gene therapy reduces apoptosis and increases plasticity protein GAP-43 in PC12 cells

The anti-apoptotic gene replacement could be an option in preventing hypoxia induced neuronal loss-necrosis and/or apoptosis. This intervention is however still controversial. In this paper, we tested the bcl-2 or bcl-XL anti-apoptotic gene transfers using an adenovirus vector in PC12 cells after hypoxia and re-oxygenation. Gene delivery results in a significant increase in both Bcl-2 and Bcl-XL proteins expression. Hypoxia (1h)/re-oxygenation (4-48 h) have a detrimental effect upon cultured cells by inducing increased apoptosis by 30% compared to the controls. After hypoxia the compromised mitochondrial membrane function was detected by decreased tetramethyl-rhodamine-ethylester (TMRE) staining. Anti-apoptotic genes transferred 1h after hypoxia, prevent the cell damage; the number of apoptotic cells has been reduced significantly and the gene transfers prevent mitochondrial membrane damage. Under normoxic conditions or following hypoxia the expression of plasticity protein, growth associated protein 43 (GAP-43) increased significantly by the gene treatment. We can conclude that anti-apoptotic gene transfers are not only cytoprotective as it is already documented before but these genes activate GAP-43 as well. This link on apoptotic signals and cell plasticity is a new finding.

Bcl-xL forms two distinct homodimers at non-ionic detergents: implications in the dimerization of Bcl-2 family proteins

As the key regulator of apoptosis, Bcl-2 family protein controls the cell death by forming homo- or heterodimers among anti-apoptotic and pro-apoptotic members of this family. Here we have studied Bcl-x(L) homodimerization at different pH in the presence of various detergents and organic solvents. We found that both acidic and basic pHs are beneficial for Bcl-x(L) dimerization. High concentrations of non-ionic detergents and some organic solvents can significantly promote this event. In addition to non-covalently linked acidic-dimer as that formed at acidic pH, Bcl-x(L) formed disulphide-bonded detergent-dimer at neutral and basic pH when incubated with high concentrations of non-ionic detergents. The acidic-dimer retains the BH3 peptide binding activity, whereas the detergent-dimer does not. The formation of acidic-dimer and detergent-dimer implies that Bcl-x(L) may dimerize via two different pathways under certain conditions. The implications of these findings has been discussed with previous experimental results, which provides some new insight into the events and would help the experiment design and data interpretation when non-ionic detergents are used to study the dimerization and pore formation of Bcl-2 family proteins.

The inhibitors of apoptosis (IAPs) as cancer targets

Apoptosis has been accepted as a fundamental component in the pathogenesis of cancer, in addition to other human diseases including neurodegeneration, coronary disease and diabetes. The origin of cancer involves deregulated cellular proliferation and the suppression of apoptotic processes, ultimately leading to tumor establishment and growth. Several lines of evidence point toward the IAP family of proteins playing a role in oncogenesis, via their effective suppression of apoptosis. The central mechanisms of IAP apoptotic suppression appear to be through direct caspase and pro-caspase inhibition (primarily caspase 3 and 7) and modulation of, and by, the transcription factor NF-kappaB. Thus, when the IAPs are over-expressed or over-active, as is the case in many cancers, cells are no longer able to die in a physiologically programmed fashion and become increasingly resistant to standard chemo- and radiation therapies. To date several approaches have been taken to target and eliminate IAP function in an attempt to re-establish sensitivity, reduce toxicity, and improve efficacy of cancer treatment. In this review, we address IAP proteins as therapeutic targets for the treatment of cancer and emphasize the importance of novel therapeutic approaches for cancer therapy. Novel targets of IAP function are being identified and include gene therapy strategies and small molecule inhibitors that are based on endogenous IAP antagonists. As well, molecular mechanistic approaches, such as RNAi to deplete IAP expression, are in development.

Robustness analysis identifies the plausible model of the Bcl-2 apoptotic switch

In this paper two competing models of the B-cell lymphoma 2 (Bcl-2) apoptotic switch were contrasted by mathematical modeling and robustness analysis. Since switch-like behaviors are required for models that attempt to explain the all-or-none decisions of apoptosis, ultrasensitivity was employed as a criterion for comparison. Our results successfully exhibit that the direct activation model operates more reliably to achieve a robust switch in cellular conditions. Moreover, by investigating the robustness of other important features of the Bcl-2 apoptotic switch (including low Bax basal activation, inhibitory role of anti-apoptotic proteins and insensitivity to small perturbations) the direct activation model was further supported. In all, we identified the direct activation model as a more plausible explanation for the Bcl-2 apoptotic switch.

A three-helix homo-oligomerization domain containing BH3 and BH1 is responsible for the apoptotic activity of Bax

Homo-oligomerization of Bax (or Bak) has been hypothesized to be responsible for cell death through the mitochondria-dependent apoptosis pathway. However, partly due to a lack of structural information on the Bax homo-oligomerization and apoptosis inducing domain(s), this hypothesis has remained difficult to test. In this study, we identified a three-helix unit, comprised of the BH3 (helix 2) and BH1 domains (helix 4 and helix 5), as the homo-oligomerization domain of Bax. When targeted to mitochondria, this minimum oligomerization unit induced apoptosis in Bax(-/-)Bak(-/-) mouse embryonic fibroblasts (DKO). Strikingly, the central helix of Bax (helix 5), when replacing the corresponding helix (helix 5) of Bcl-xL, an anti-apoptotic Bcl-2 family protein structurally homologous to Bax, converted Bcl-xL into a Bax-like molecule capable of forming oligomers and causing apoptosis in the DKO cells. Finally, a series of systematic mutagenesis analyses revealed that homo-oligomerization is both necessary and sufficient for the apoptotic activity of Bax. These results suggest that active Bax causes mitochondrial damage through homo-oligomers of a three-helix functional unit.

Alterations in both death and survival signals for apoptosis in heart failure due to volume overload

Heart failure is known to be associated with an increase in cardiomyocyte apoptosis; however, neither its occurrence nor the mechanisms involved in hearts failing due to volume overload are completely understood. This study examined some of the signal pathways, which are known to regulate pro- or anti-apoptotic proteins, in heart failure due to volume overload induced by arteriovenous (AV) shunt in male Sprague-Dawley rats. Animals were assessed for cardiac function at 16 weeks of the operation and the left ventricle was used for studying apoptosis and associated signal transduction mechanisms. Hemodynamic and echocardiographic data indicated the presence of severe heart failure in AV shunt rats. A marked elevation in the amount of tumor necrosis factor-alpha and increased occurrence of apoptosis were detected in the volume overloaded myocardium. Western blot analysis revealed a significant increase in BAX and caspases 3/9 proteins in the failing hearts whereas the levels of phosphorylated Akt and Bcl-2 proteins were decreased. These data suggest that there is a downregulation in the Akt-dependent survival signal involving anti-apoptotic protein, Bcl-2, whereas the signals for the pro-apoptotic protein, BAX, are upregulated and these alterations may play a role in cardiomyocyte apoptosis in heart failure due to volume overload.

Interaction between caspase-8 activation and endoplasmic reticulum stress in glycochenodeoxycholic acid-induced apoptotic HepG2 cells

The accumulation of hydrophobic bile acid, such as glycochenodeoxycholic acid (GCDCA), in the liver has been thought to induce hepatocellular damage in human chronic cholestatic liver diseases. We previously reported that GCDCA-induced apoptosis was promoted by both mitochondria-mediated and endoplasmic reticulum (ER) stress-associated pathways in rat hepatocytes. In this study, we elucidated the relationship between these pathways in GCDCA-induced apoptotic HepG2 cells. HepG2 cells were treated with GCDCA (100-500microM) with or without a caspase-8 inhibitor, Z-IETD-fluoromethyl ketone (Z-IETD-FMK) (30microM) for 3-24h. We demonstrated the presence of both apoptotic pathways in these cells; that is, we showed increases in cleaved caspase-3 proteins, the release of cytochrome c from mitochondria, and the expression of ER resident molecular chaperone Bip mRNA and ER stress response-associated transcription factor Chop mRNA. On the other hand, pretreatment with Z-IETD-FMK significantly reduced the increases, compared with treatment with GCDCA alone. Immunofluorescence microscopic analysis showed that treatment with GCDCA increased the cleavage of BAP31, an integral membrane protein of ER, and pretreatment with Z-IETD-FMK suppressed the increase of caspase-8 and BAP31 cleavage. In conclusion, these results suggest that intact activated caspase-8 may promote and amplify the ER stress response by cleaving BAP31 in GCDCA-induced apoptotic cells.

The interplay between pro-death and pro-survival signaling pathways in myocardial ischemia/reperfusion injury: apoptosis meets autophagy

INTRODUCTION

Programmed cell death of cardiac myocytes occurs following a bout of ischemia/reperfusion (I/R), which results in reduced function of the heart. Numerous studies, including in vivo, have shown that cell death occurs via necrosis and apoptosis following I/R. Recently, autophagy has emerged as a powerful mediator of programmed cell death, either opposing or enhancing apoptosis, or acting as an alternative form of programmed cell death distinct from apoptosis.

AIM

Here we review the apoptotic and autophagic signaling pathways, their influences on each other, and we discuss the relevance of autophagy in the heart.

DNA-damage response network at the crossroads of cell-cycle checkpoints, cellular senescence and apoptosis

Tissue homeostasis requires a carefully-orchestrated balance between cell proliferation, cellular senescence and cell death. Cells proliferate through a cell cycle that is tightly regulated by cyclin-dependent kinase activities. Cellular senescence is a safeguard program limiting the proliferative competence of cells in living organisms. Apoptosis eliminates unwanted cells by the coordinated activity of gene products that regulate and effect cell death. The intimate link between the cell cycle, cellular senescence, apoptosis regulation, cancer development and tumor responses to cancer treatment has become eminently apparent. Extensive research on tumor suppressor genes, oncogenes, the cell cycle and apoptosis regulatory genes has revealed how the DNA damage-sensing and -signaling pathways, referred to as the DNA-damage response network, are tied to cell proliferation, cell-cycle arrest, cellular senescence and apoptosis. DNA-damage responses are complex, involving "sensor" proteins that sense the damage, and transmit signals to "transducer" proteins, which, in turn, convey the signals to numerous "effector" proteins implicated in specific cellular pathways, including DNA repair mechanisms, cell-cycle checkpoints, cellular senescence and apoptosis. The Bcl-2 family of proteins stands among the most crucial regulators of apoptosis and performs vital functions in deciding whether a cell will live or die after cancer chemotherapy and irradiation. In addition, several studies have now revealed that members of the Bcl-2 family also interface with the cell cycle, DNA repair/recombination and cellular senescence, effects that are generally distinct from their function in apoptosis. In this review, we report progress in understanding the molecular networks that regulate cell-cycle checkpoints, cellular senescence and apoptosis after DNA damage, and discuss the influence of some Bcl-2 family members on cell-cycle checkpoint regulation.

Synthetic, biologically active amphiphilic peptides

Amphiphilic peptides typically consist of a peptide portion that may be 5-25 (or more) amino acids in length. The hydrophobic portion may be a single fatty acid residue, but can also be more elaborate. The main focus of this article lies on the family of synthetic anion binders (SATs) of the general structure (R(1))(2)N-COCH(2)OCH(2)CO-(Aaa)(n)-OR(3). The most-common R(1) group is the octadecyl (C(18)H(37)) group. The most studied peptide sequence in this family is (Gly)(3)-Pro-(Gly)(3), although different sequences (and longer and shorter peptides) have been prepared as well. The C-terminal ester residue providing the most effective anion release from liposomes is heptyl (C(7)H(15)), although many others have been examined. The compound (C(18)H(37))(2)N-COCH(2)OCH(2)CO-(Gly)(3)-Pro-(Gly)(3)-OBn (Bn=benzyl) was found to mediate Cl(-) transport in mouse epithelial cells.

Modeling of the role of a Bax-activation switch in the mitochondrial apoptosis decision

We performed in silico modeling of the regulatory network of mitochondrial apoptosis through which we examined the role of a Bax-activation switch in governing the mitochondrial apoptosis decision. Two distinct modeling methods were used in this article. One is continuous and deterministic, comprised of a set of ordinary differential equations. The other, carried out in a discrete manner, is based on a cellular automaton, which takes stochastic fluctuations into consideration. We focused on dynamic properties of the mitochondrial apoptosis regulatory network. The roles of Bcl-2 family proteins in cellular responses to apoptotic stimuli were examined. In our simulations, a self-amplification process of Bax-activation is indicated. Further analysis suggests that the core module of Bax-activation is bistable in both deterministic and stochastic models, and this feature is robust to noise and wide ranges of parameter variation. When coupling with Bax-polymerization, it forms a one-way-switch, which governs irreversible behaviors of Bax-activation even with attenuation of apoptotic stimulus. Together with the growing biochemical evidence, we propose a novel molecular switch mechanism embedded in the mitochondrial apoptosis regulatory network and give a plausible explanation for the all-or-none, irreversible character of mitochondrial apoptosis.

Herpes simplex virus blocks apoptosis by precluding mitochondrial cytochrome c release independent of caspase activation in infected human epithelial cells

Expression of HSV-1 genes leads to the induction of apoptosis in human epithelial HEp-2 cells but the subsequent synthesis of infected cell protein prevents the process from killing the cells. Thus, viruses unable to produce appropriate prevention factors are apoptotic. We now report that the addition of either a pancaspase inhibitor or caspase-9-specific inhibitor prevented cells infected with an apoptotic HSV-1 virus from undergoing cell death. This result indicated that HSV-1-dependent apoptosis proceeds through the mitochondrial apoptotic pathway. However, the pancaspase inhibitor did not prevent the release of cytochrome c from mitochondria, implying that caspase activation is not required for this induction of cytochrome c release by HSV-1. The release of cytochrome c was first detected at 9 hpi while caspase-9, caspase-3 and PARP processing were detected at 12 hpi. Finally, Bax accumulated at mitochondria during apoptotic, but not wild type HSV-1 infection. Together, these findings indicate that HSV-1 blocks apoptosis by precluding mitochondrial cytochrome c release in a caspase-independent manner and suggest Bax as a target in infected human epithelial cells.

Preprotein Transport Machineries of Yeast Mitochondrial Outer Membrane Are not Required for Bax-induced Release of Intermembrane Space Proteins

The mitochondrial outer membrane contains protein import machineries, the translocase of the outer membrane (TOM) and the sorting and assembly machinery (SAM). It has been speculated that TOM or SAM are required for Bax-induced release of intermembrane space (IMS) proteins; however, experimental evidence has been scarce. We used isolated yeast mitochondria as a model system and report that Bax promoted an efficient release of soluble IMS proteins while preproteins were still imported, excluding an unspecific damage of mitochondria. Removal of import receptors by protease treatment did not inhibit the release of IMS proteins by Bax. Yeast mutants of each Tom receptor and the Tom40 channel were not impaired in Bax-induced protein release. We analyzed a large collection of mutants of mitochondrial outer membrane proteins, including SAM, fusion and fission components, but none of these components was required for Bax-induced protein release. The released proteins included complexes up to a size of 230 kDa. We conclude that Bax promotes efficient release of IMS proteins through the outer membrane of yeast mitochondria while the inner membrane remains intact. Inactivation of the known protein import and sorting machineries of the outer membrane does not impair the function of Bax at the mitochondria.

Mitochondrial membrane permeabilization in cell death

Irrespective of the morphological features of end-stage cell death (that may be apoptotic, necrotic, autophagic, or mitotic), mitochondrial membrane permeabilization (MMP) is frequently the decisive event that delimits the frontier between survival and death. Thus mitochondrial membranes constitute the battleground on which opposing signals combat to seal the cell's fate. Local players that determine the propensity to MMP include the pro- and antiapoptotic members of the Bcl-2 family, proteins from the mitochondrialpermeability transition pore complex, as well as a plethora of interacting partners including mitochondrial lipids. Intermediate metabolites, redox processes, sphingolipids, ion gradients, transcription factors, as well as kinases and phosphatases link lethal and vital signals emanating from distinct subcellular compartments to mitochondria. Thus mitochondria integrate a variety of proapoptotic signals. Once MMP has been induced, it causes the release of catabolic hydrolases and activators of such enzymes (including those of caspases) from mitochondria. These catabolic enzymes as well as the cessation of the bioenergetic and redox functions of mitochondria finally lead to cell death, meaning that mitochondria coordinate the late stage of cellular demise. Pathological cell death induced by ischemia/reperfusion, intoxication with xenobiotics, neurodegenerative diseases, or viral infection also relies on MMP as a critical event. The inhibition of MMP constitutes an important strategy for the pharmaceutical prevention of unwarranted cell death. Conversely, induction of MMP in tumor cells constitutes the goal of anticancer chemotherapy.

Sphingosine-induced apoptosis in rhabdomyosarcoma cell lines is dependent on pre-mitochondrial Bax activation and post-mitochondrial caspases

Sphingolipids is the collective term ascribed to components of the sphingomyelin cycle. Modulation of the cellular levels of individual sphingolipids can induce a diverse range of cellular responses including apoptosis, proliferation, and cell cycle arrest. We present data showing that rhabdomyosarcoma cell lines, independent of lineage (alveolar rhabdomyosarcoma and embryonal rhabdomyosarcoma), are particularly sensitive to the induction of apoptosis as a result of an elevation in the cellular levels of sphingosine (D-erythro-sphingosine). Sphingosine-mediated apoptosis does not require its metabolism to the related proapoptotic molecule ceramide and is stereospecific because exposure of the rhabdomyosarcoma cell line RD to the L-erythro and DL-threo isoforms of sphingosine did not induce apoptosis. Importantly, for efficient induction of apoptosis, sphingosine required Bax activation and consequential translocation to the mitochondria. This resulted in selective mitochondrial release of cytochrome c and Smac/Diablo but not other mitochondrial related factors (apoptosis-inducing factor, endonuclease G, and HtrA2/Omi). Using small interfering RNA, reduced Bax expression conferred the impaired release of mitochondrial cytochrome c to the cytoplasm following sphingosine exposure, inhibiting the induction of apoptosis. Furthermore, dissipation of the inner mitochondrial membrane potential and enhanced production of reactive oxygen species were not observed. Bax activation and cytochrome c release were independent of caspases; however, caspase-3 and caspase-9 activity distal to the mitochondria was essential for the execution of apoptosis.

Mechanisms of cell death in oxidative stress

Reactive oxygen or nitrogen species (ROS/RNS) generated endogenously or in response to environmental stress have long been implicated in tissue injury in the context of a variety of disease states. ROS/RNS can cause cell death by nonphysiological (necrotic) or regulated pathways (apoptotic). The mechanisms by which ROS/RNS cause or regulate apoptosis typically include receptor activation, caspase activation, Bcl-2 family proteins, and mitochondrial dysfunction. Various protein kinase activities, including mitogen-activated protein kinases, protein kinases-B/C, inhibitor-of-I-kappaB kinases, and their corresponding phosphatases modulate the apoptotic program depending on cellular context. Recently, lipid-derived mediators have emerged as potential intermediates in the apoptosis pathway triggered by oxidants. Cell death mechanisms have been studied across a broad spectrum of models of oxidative stress, including H2O2, nitric oxide and derivatives, endotoxin-induced inflammation, photodynamic therapy, ultraviolet-A and ionizing radiations, and cigarette smoke. Additionally ROS generated in the lung and other organs as the result of high oxygen therapy or ischemia/reperfusion can stimulate cell death pathways associated with tissue damage. Cells have evolved numerous survival pathways to counter proapoptotic stimuli, which include activation of stress-related protein responses. Among these, the heme oxygenase-1/carbon monoxide system has emerged as a major intracellular antiapoptotic mechanism.

Role of HPV E6 proteins in preventing UVB-induced release of pro-apoptotic factors from the mitochondria

Apoptotic elimination of UV-damaged cells from the epidermis is an important step in preventing both the emergence and expansion of cells with carcinogenic potential. A pivotal event in apoptosis is the release of apoptogenic factors from the mitochondria, although the mechanisms by which the different proteins are released are not fully understood. Here we demonstrate that UV radiation induced the mitochondrial to nuclear translocation of apoptosis inducing factor (AIF) in normal skin. The human papillomavirus (HPV) E6 protein prevented release of AIF and other apoptotic factors such as cytochrome c and Omi from mitochondria of UV-damaged primary epidermal keratinocytes and preserved mitochondrial integrity. shRNA silencing of Bak, a target for E6-mediated proteolysis, demonstrated the requirement of Bak for UV-induced AIF release and mitochondrial fragmentation. Furthermore, screening non-melanoma skin cancer biopsies revealed an inverse correlation between HPV status and AIF nuclear translocation. Our results indicate that the E6 activity towards Bak is a key factor that promotes survival of HPV-infected cells that facilitates tumor development.

How the Bcl-2 family of proteins interact to regulate apoptosis

Commitment of cells to apoptosis is governed largely by protein-protein interactions between members of the Bcl-2 protein family. Its three sub-families have distinct roles: the BH3-only proteins trigger apoptosis by binding via their BH3 domain to pro-survival relatives, while the pro-apoptotic Bax and Bak have an essential downstream role involving disruption of organellar membranes and induction of caspase activation. The BH3-only proteins act as damage sensors, held inert until their activation by stress signals. Once activated, they were thought to bind promiscuously to pro-survival protein targets but unexpected selectivity has recently emerged from analysis of their interactions. Some BH3-only proteins also bind to Bax and Bak. Whether Bax and Bak are activated directly by these BH3-only proteins, or indirectly as a consequence of BH3-only proteins neutralizing their pro-survival targets is the subject of intense debate. Regardless of this, a detailed understanding of the interactions between family members, which are often selective, has notable implications for designing anti-cancer drugs to target the Bcl-2 family.

Ischemic preconditioning negatively regulates plenty of SH3s-mixed lineage kinase 3-Rac1 complex and c-Jun N-terminal kinase 3 signaling via activation of Akt

Activation of Akt/protein kinase B has been recently reported to play an important role in ischemic tolerance. We here demonstrate that the decreased protein expression and phosphorylation of phosphatase and tensin homolog deleted from chromosome 10 (PTEN) underlie the increased Akt-Ser-473 phosphorylation in the hippocampal CA1 subfield in ischemic preconditioning (IPC). Co-immunoprecipitation analysis reveals that Akt physically interacts with Rac1, a small Rho family GTPase required for mixed lineage kinase 3 (MLK3) autophosphorylation, and both this interaction and Rac1-Ser-71 phosphorylation induced by Akt are promoted in preconditioned rats. In addition, we show that Akt activation results in the disassembly of the plenty of SH3s (POSH)-MLK3-Rac1 signaling complex and down-regulation of the activation of MLK3/c-Jun N-terminal kinase (JNK) pathway. Akt activation results in decreased serine phosphorylation of 14-3-3, a cytoplasmic anchor of Bax, and prevents ischemia-induced mitochondrial translocation of Bax, release of cytochrome c, and activation of caspase-3. The expression of Fas ligand is also decreased in the CA1 region. Akt activation protects against apoptotic neuronal death as shown in TUNEL staining following IPC. Intracerebral infusion of LY294002 before IPC reverses the increase in Akt phosphorylation and the decrease in JNK signaling activation, as well as the neuroprotective action of IPC. Our results suggest that activation of pro-apoptotic MLK3/JNK3 cascade can be suppressed through activating anti-apoptotic phosphoinositide 3-kinase/Akt pathway induced by a sublethal ischemic insult, which provides a functional link between Akt and the JNK family of stress-activated kinases in ischemic tolerance.

Mitochondria and cancer: is there a morphological connection?

Mitochondria are key players in several cellular functions including growth, division, energy metabolism, and apoptosis. The mitochondrial network undergoes constant remodelling and these morphological changes are of direct relevance for the role of this organelle in cell physiology. Mitochondrial dysfunction contributes to a number of human disorders and may aid cancer progression. Here, we summarize the recent contributions made in the field of mitochondrial dynamics and discuss their impact on our understanding of cell function and tumorigenesis.

Mitochondria as the target of the pro-apoptotic protein Bax

During apoptosis, engagement of the mitochondrial pathway involves the permeabilization of the outer mitochondrial membrane (OMM), which leads to the release of cytochrome c and other apoptogenic proteins such as Smac/DIABLO, AIF, EndoG, Omi/HtraA2 and DDP/TIMM8a. OMM permeabilization depends on activation, translocation and oligomerization of multidomain Bcl-2 family proteins such as Bax or Bak. Factors involved in Bax conformational change and the function(s) of the distinct domains controlling the addressing and the insertion of Bax into mitochondria are described in this review. We also discuss our current knowledge on Bax oligomerization and on the molecular mechanisms underlying the different models accounting for OMM permeabilization during apoptosis.

Akt inhibits MLK3/JNK3 signaling by inactivating Rac1: a protective mechanism against ischemic brain injury

The overall goal of this study was to determine the molecular basis by which mixed-lineage kinase 3 (MLK3) kinase and its signaling pathways are negatively regulated by the pro-survival Akt pathway in cerebral ischemia. We demonstrated that tyrosine phosphorylation of the phosphatase and tensin homolog deleted on chromosome 10 (PTEN) underlies the increased Akt-Ser473 phosphorylation by orthovanadate. Co-immunoprecipitation analysis revealed that endogenous Akt physically interacts with Rac1 in the hippocampal CA1 region, and this interaction is promoted on tyrosine phosphatase inhibition. The elevated Akt activation can deactivate MLK3 by phosphorylation at the Ser71 residue of Rac1, a small Rho family of guanidine triphosphatases required for MLK3 autophosphorylation. Subsequently, inhibition of c-Jun N-terminal kinase 3 (JNK3) results in decreased serine phosphorylation of 14-3-3, a cytoplasmic anchor of Bax, and prevents ischemia-induced mitochondrial translocation of Bax, release of cytochrome c and activation of caspase 3. At the same time, the expression of Fas-ligand decreases in the CA1 region after inhibition of c-Jun activation. The neuroprotective effect of Akt activation is significant in the CA1 region after global cerebral ischemia. Our results suggest that the activation of the pro-apoptotic MLK3/JNK3 cascade induced by ischemic stress can be suppressed through activation of the anti-apoptotic phosphatidylinositol 3-kinase/Akt pathway, which provides a direct link between Akt and the family of stress-activated kinases.

Mitochondria as therapeutic targets for cancer chemotherapy

Mitochondria are vital for cellular bioenergetics and play a central role in determining the point-of-no-return of the apoptotic process. As a consequence, mitochondria exert a dual function in carcinogenesis. Cancer-associated changes in cellular metabolism (the Warburg effect) influence mitochondrial function, and the invalidation of apoptosis is linked to an inhibition of mitochondrial outer membrane permeabilization (MOMP). On theoretical grounds, it is tempting to develop specific therapeutic interventions that target the mitochondrial Achilles' heel, rendering cancer cells metabolically unviable or subverting endogenous MOMP inhibitors. A variety of experimental therapeutic agents can directly target mitochondria, causing apoptosis induction. This applies to a heterogeneous collection of chemically unrelated compounds including positively charged alpha-helical peptides, agents designed to mimic the Bcl-2 homology domain 3 of Bcl-2-like proteins, ampholytic cations, metals and steroid-like compounds. Such MOMP inducers or facilitators can induce apoptosis by themselves (monotherapy) or facilitate apoptosis induction in combination therapies, bypassing chemoresistance against DNA-damaging agents. In addition, it is possible to design molecules that neutralize inhibitor of apoptosis proteins (IAPs) or heat shock protein 70 (HSP70). Such IAP or HSP70 inhibitors can mimic the action of mitochondrion-derived mediators (Smac/DIABLO, that is, second mitochondria-derived activator of caspases/direct inhibitor of apoptosis-binding protein with a low isoelectric point, in the case of IAPs; AIF, that is apoptosis-inducing factor, in the case of HSP70) and exert potent chemosensitizing effects.

Protein Phosphatase 2A Enhances the Proapoptotic Function of Bax through Dephosphorylation

Bax is a major proapoptotic member of the Bcl2 family that is required for apoptotic cell death. We have recently discovered that Bax phosphorylation at serine 184 induced by nicotine through activation of protein kinase AKT abolishes its proapoptotic function in human lung cancer cells. Here we found that either treatment of cells with the protein phosphatase 2A (PP2A) inhibitor okadaic acid or specific disruption of PP2A activity by expression of SV40 small tumor antigen enhanced Bax phosphorylation, whereas C(2)-ceramide, a potent PP2A activator, reduced nicotine-induced Bax phosphorylation, suggesting that PP2A may function as a physiological Bax phosphatase. PP2A co-localized and interacted with Bax. Purified, active PP2A directly dephosphorylated Bax in vitro. Overexpression of the PP2A catalytic subunit (PP2A/C) suppressed nicotine-stimulated Bax phosphorylation in association with increased apoptotic cell death. By contrast, depletion of PP2A/C by RNA interference enhanced Bax phosphorylation and prolonged cell survival. Mechanistically C(2)-ceramide-induced Bax dephosphorylation caused a conformational change by exposure of the 6A7 epitope (amino acids 13-19) that is normally hidden at its N terminus that promoted the insertion of Bax into mitochondrial membranes and formation of Bax oligomers leading to cytochrome c release and apoptosis. In addition, PP2A directly disrupted the Bcl2/Bax association to liberate Bax from the heterodimer complex. Thus, PP2A may function as a physiological Bax regulatory phosphatase that not only dephosphorylates Bax but also activates its proapoptotic function.

Neuroprotection of Tat-GluR6-9c against neuronal death induced by kainate in rat hippocampus via nuclear and non-nuclear pathways

Previous studies have suggested that glutamate receptor 6 (GluR6) subunit- and JNK-deficient mice can resist kainate-induced epileptic seizure and neuronal toxicity (Yang, D. D., Kuan, C.-Y., Whitmarsh, A. J., Rinoćn, M., Zheng, T. S., Davis, R. J., Rakic, P., and Flavell, R. A. (1997) Nature 389, 865-870; Mulle, C., Seiler, A., Perez-Otano, I., Dickinson-Anson, H., Castillo, P. E., Bureau, I., Maron, C., Gage, F. H., Mann, J. R., Bettler, B., and Heinemmann, S. F. (1998) Nature 392, 601-605). In this study, we show that kainate can enhance the assembly of the GluR6-PSD95-MLK3 module and facilitate the phosphorylation of JNK in rat hippocampal CA1 and CA3/dentate gyrus (DG) subfields. More important, a peptide containing the Tat protein transduction sequence (Tat-GluR6-9c) perturbed the assembly of the GluR6-PSD95-MLK3 signaling module and suppressed the activation of MLK3, MKK7, and JNK. As a result, the inhibition of JNK activation by Tat-GluR6-9c diminished the phosphorylation of the transcription factor c-Jun and down-regulated Fas ligand expression in hippocampal CA1 and CA3/DG regions. The inhibition of JNK activation by Tat-Glur6-9c attenuated Bax translocation, the release of cytochrome c, and the activation of caspase-3 in CA1 and CA3/DG subfields. Furthermore, kainate-induced neuronal loss in hippocampal CA1 and CA3 subregions was prevented by intracerebroventricular injection of Tat-Glur6 - 9c. Taken together, our findings strongly suggest that the GluR6-PSD95-MLK3 signaling module mediates activation of the nuclear and non-nuclear pathways of JNK, which is involved in brain injury induced by kainate. Tat-GluR6-9c, the peptide we constructed, gives new insight into seizure therapy.

Sphingosine, a product of ceramide hydrolysis, influences the formation of ceramide channels

Ceramides are known to have a regulatory function in apoptosis, including the release of cytochrome c and other proapoptotic factors from the mitochondrial intermembrane space. Ceramides can form large, stable channels in the outer mitochondrial membrane, leading to the proposal that ceramide channels are the pathway through which these proteins are released. Here, we report that sphingosine, a product of ceramide hydrolysis by ceramidase, is capable of destabilizing ceramide channels, leading to their disassembly. Sphingosine is directly responsible for the disassembly of ceramide channels in planar membrane experiments and markedly reduces the ability of ceramide to induce the release of intermembrane space proteins from mitochondria in vitro. Low concentrations of both L and D sphingosine potentiate the release of intermembrane space proteins by long-chain ceramide and channel formation in liposomes. These results provide evidence for a mechanism by which the disassembly of ceramide channels, as initiated by ceramidase, could be accelerated by the direct interaction of the hydrolysis product with the ceramide channels themselves. This mechanism therefore could form a positive feedback loop for rapid shut-down of ceramide channels. However, potentiation of ceramide channel formation is also possible and thus both effects could influence the propensity for mitochondria-mediated apoptosis.

The Bax pore in liposomes, Biophysics

The protein BAX of the Bcl-2-family is felt to be one of the two Bcl-2-family proteins that directly participate in the mitochondrial cytochrome c-translocating pore. We have studied the kinetics, stoichiometry and size of the pore formed by BAX in planar lipid bilayers and synthetic liposomes. Our data indicate that a cytochrome c-competent pore can be formed by in-membrane association of BAX monomers.

BCL-2 in the crosshairs: tipping the balance of life and death

The discovery of B-cell lymphoma-2 (BCL-2) over 20 years ago revealed a new paradigm in cancer biology: the development and persistence of cancer can be driven by molecular roadblocks along the natural pathway to cell death. The subsequent identification of an expansive family of BCL-2 proteins provoked an intensive investigation of the interplay among these critical regulators of cell death. What emerged was a compelling tale of guardians and executioners, each participating in a molecular choreography that dictates cell fate. Ten years into the BCL-2 era, structural details defined how certain BCL-2 family proteins interact, and molecular targeting of the BCL-2 family has since become a pharmacological quest. Although many facets of BCL-2 family death signaling remain a mechanistic mystery, small molecules and peptides that effectively target BCL-2 are eliminating the roadblock to cell death, raising hopes for a medical breakthrough in cancer and other diseases of deregulated apoptosis.

Targeting angiogenesis with integrative cancer therapies

An integrative approach for managing a patient with cancer should target the multiple biochemical and physiological pathways that support tumor development while minimizing normal tissue toxicity. Angiogenesis is a key process in the promotion of cancer. Many natural health products that inhibit angiogenesis also manifest other anticancer activities. The authors will focus on natural health products (NHPs) that have a high degree of antiangiogenic activity but also describe some of their many other interactions that can inhibit tumor progression and reduce the risk of metastasis. NHPs target various molecular pathways besides angiogenesis, including epidermal growth factor receptor (EGFR), the HER-2/neu gene, the cyclooxygenase-2 enzyme, the NF-kB transcription factor, the protein kinases, Bcl-2 protein, and coagulation pathways. The herbalist has access to hundreds of years of observational data on the anticancer activity of many herbs. Laboratory studies are confirming the knowledge that is already documented in traditional texts. The following herbs are traditionally used for anticancer treatment and are antiangiogenic through multiple interdependent processes that include effects on gene expression, signal processing, and enzyme activities: Artemisia annua (Chinese wormwood), Viscum album (European mistletoe), Curcuma longa (turmeric), Scutellaria baicalensis (Chinese skullcap), resveratrol and proanthocyanidin (grape seed extract), Magnolia officinalis (Chinese magnolia tree), Camellia sinensis (green tea), Ginkgo biloba, quercetin, Poria cocos, Zingiber officinale (ginger), Panax ginseng, Rabdosia rubescens (rabdosia), and Chinese destagnation herbs. Quality assurance of appropriate extracts is essential prior to embarking on clinical trials. More data are required on dose response, appropriate combinations, and potential toxicities. Given the multiple effects of these agents, their future use for cancer therapy probably lies in synergistic combinations. During active cancer therapy, they should generally be evaluated in combination with chemotherapy and radiation. In this role, they act as biological response modifiers and adaptogens, potentially enhancing the efficacy of the so-called conventional therapies. Their effectiveness may be increased when multiple agents are used in optimal combinations. New designs for trials to demonstrate activity in human subjects are required. Although controlled trials might be preferred, smaller studies with appropriate end points and surrogate markers for antiangiogenic response could help prioritize agents for the larger resource-intensive phase 3 trials.

Mechanism underlying mitochondrial protection of asiatic acid against hepatotoxicity in mice

Asiatic acid (AA) is one of the triterpenoid components of Terminalia catappa L., which has antioxidative, anti-inflammatory and hepatoprotective activity. This research focused on the mitochondrial protection of AA against acute liver injury induced by lipopolysaccharide (LPS) and D-galactosamine (D-GalN) in mice. It was found that pretreatment with 25, 50 or 100 mg kg(-1) AA significantly blocked the LPS + D-GalN-induced increase in both serum aspartate aminotransferase (sAST) and serum alanine aminotransferase (sALT) levels, which was confirmed by ultrastructural observation under an electron microscope, showing improved nuclear condensation, ameliorated mitochondrion proliferation and less lipid deposition. Meanwhile, different doses of AA could decrease both the transcription and the translation level of voltage-dependent anion channels (VDACs), the most important mitochondrial PTP component protein, and block the translocation of cytochrome c from mitochondria to cytosol. On the other hand, pre-incubation with 25, 50 and 100 microg mL(-1) AA inhibited the Ca(2+)-induced mitochondrial permeability transition (MPT), including mitochondrial swelling, membrane potential dissipation and releasing of matrix Ca(2+) in liver mitochondria separated from normal mice, indicating the direct role of AA on mitochondria. Collectively, the above data suggest that AA could protect liver from damage and the mechanism might be related to up-regulating mitochondrial VDACs and inhibiting the process of MPT.

Proapoptotic BCL-2 family members and mitochondrial dysfunction during ischemia/reperfusion injury, a study employing cardiac HL-1 cells and GFP biosensors

The objective of this study was to evaluate mitochondrial alterations in a cell-based model of myocardial ischemia/reperfusion (I/R) injury. Using GFP-biosensors and fluorescence deconvolution microscopy, we investigated mitochondrial morphology in relation to Bax and Bid activation in the HL-1 cardiac cell line. Mitochondria underwent extensive fragmentation during ischemia. Bax translocation from cytosol to mitochondria was initiated during ischemia and proceeded during reperfusion. However, Bax translocation was not sufficient to induce cell death or mitochondrial dysfunction. Bid processing was caspase-8 dependent, and Bid translocation to mitochondria occurred after Bax translocation and clustering, and minutes before cell death. Clustering of Bax into distinct regions on mitochondria could be prevented by CsA, an inhibitor of the mitochondrial permeability transition pore, and also by SB203580, an inhibitor of p38 MAPK. Surprisingly, mitochondrial fragmentation which occurred during ischemia and before Bax translocation could be reversed by the addition of the p38 inhibitor SB203580 at reperfusion. Taken together, these results implicate p38 MAPK in the mitochondrial remodeling response to I/R that facilitates Bax recruitment to mitochondria.

Peptides corresponding to helices 5 and 6 of Bax can independently form large lipid pores

Proteins of the B-cell lymphoma protein 2 (Bcl2) family are key regulators of the apoptotic cascade, controlling the release of apoptotic factors from the mitochondrial intermembrane space. A helical hairpin found in the core of water-soluble folds of these proteins has been reported to be the pore-forming domain. Here we show that peptides including any of the two alpha-helix fragments of the hairpin of Bcl2 associated protein X (Bax) can independently induce release of large labelled dextrans from synthetic lipid vesicles. The permeability promoted by these peptides is influenced by intrinsic monolayer curvature and accompanied by fast transbilayer redistribution of lipids, supporting a toroidal pore mechanism as in the case of the full-length protein. However, compared with the pores made by complete Bax, the pores made by the Bax peptides are smaller and do not need the concerted action of tBid. These data indicate that the sequences of both fragments of the hairpin contain the principal physicochemical requirements for pore formation, showing a parallel between the permeabilization mechanism of a complex regulated protein system, such as Bax, and the much simpler pore-forming antibiotic peptides.

Relative timing of redistribution of cytochrome c and Smac/DIABLO from mitochondria during apoptosis assessed by double immunocytochemistry on mammalian cells

Redistribution of cytochrome c and Smac/DIABLO from mitochondria occurs during apoptosis, although the relative timing of their release is not well characterized. Double immunocytochemistry was utilized here to study quantitatively the patterns of release of cytochrome c and Smac/DIABLO from mitochondria in single cells. Human osteosarcoma cells and murine embryonic cortical neurons were analyzed during apoptosis induced by staurosporine. In osteosarcoma cells treated with staurosporine for 24 h, a substantial proportion of cells (36%) released cytochrome c from the mitochondria before Smac/DIABLO. In contrast, these proteins were released mostly concordantly in neurons; only a minority of cells (< or = 15%) released cytochrome c without Smac/DIABLO (or vice versa) from mitochondria. Patterns of release in either cell type were unaltered by addition of the caspase inhibitor, zVAD-fmk. The double immunocytochemistry procedure facilitated clear definition of the temporal release of cytochrome c and Smac/DIABLO from mitochondria in intact apoptotic cells, enabling us to demonstrate for the first time that their mutual redistribution during apoptosis varies between different cell types.

Mitochondrial carrier homolog 2: a clue to cracking the BCL-2 family riddle?

BCL-2 family members are pivotal regulators of the apoptotic process. Mitochondria are a major site-of-action for these proteins. Several prominent alterations occur to mitochondria during apoptosis that seem to be part of the "mitochondrial apoptotic program." The BCL-2 family members are believed to be the major regulators of this program, however their exact mechanism of action still remains a mystery. BID, a pro-apoptotic BCL-2 family member plays an essential role in initiating this program. Recently, we have revealed that in apoptotic cells the activated/truncated form of BID, tBID, interacts with a novel, uncharacterized protein named mitochondrial carrier homolog 2 (Mtch2). Mtch2 is a conserved protein that is similar to members of the mitochondrial carrier protein (MCP) family. This review summarizes the current knowledge regarding BCL-2 family members and the mitochondrial apoptotic program and examines the possible involvement of Mtch2 in this program.

The role of the mitochondrial apoptosis induced channel MAC in cytochrome c release

Permeabilization of the mitochondrial outer membrane is a crucial event during apoptosis. It allows the release of proapoptotic factors, like cytochrome c, from the intermembrane space, and represents the commitment step in apoptosis. The mitochondrial apoptosis-induced channel, MAC, is a high-conductance channel that forms during early apoptosis and is the putative cytochrome c release channel. Unlike activation of the permeability transition pore, MAC formation occurs without loss of outer membrane integrity and depolarization. The single channel behavior and pharmacology of reconstituted MAC has been characterized with patch-clamp techniques. Furthermore, MAC's activity is compared to that detected in mitochondria inside the cells at the time cytochrome c is released. Finally, the regulation of MAC by the Bcl-2 family proteins and insights concerning its molecular composition are also discussed.

Preconditioning prevents loss in mitochondrial function and release of cytochrome c during prolonged cardiac ischemia/reperfusion

Loss in mitochondrial function and induction of mitochondrial-mediated apoptosis occur as a result of cardiac ischemia/reperfusion. Brief and repeated cycles of ischemia/reperfusion, termed ischemic preconditioning, prevent or minimize contractile dysfunction and apoptosis associated with prolonged episodes of cardiac ischemia and reperfusion. The effects of preconditioning on various indices of ischemia/reperfusion-induced alterations in mitochondrial function and structure were therefore explored. Utilizing an in vivo rat model data is provided indicating that preconditioning completely prevents cardiac ischemia/reperfusion-induced: (1) loss in the activity of the redox sensitive Krebs cycle enzyme alpha-ketoglutarate dehydrogenase; (2) declines in NADH-linked ADP-dependent mitochondrial respiration; (3) insertion of the pro-apoptotic Bcl-2 protein Bax into the mitochondrial membrane; and (4) release of cytochrome c into the cytosol. The results of the current study indicate that preconditioning prevents specific alterations in mitochondrial structure and function that are known to impact cellular viability and provide insight into the collective benefits of preconditioning.

Bid, a BH3-only multi-functional molecule, is at the cross road of life and death

Bid, BH3-interacting domain death agonist, was initially cloned based in its ability to interact with both Bcl-2 and Bax. Bid contains only the BH3 domain, which is required for its interaction with the Bcl-2 family proteins and for its pro-death activity. Bid is susceptible to proteolytic cleavage by caspases, calpains, Granzyme B and cathepsins. Bid is important to cell death mediated by these proteases and thus is the sentinel to protease-mediated death signals. Protease-cleaved Bid is able to induce multiple mitochondrial dysfunctions, including the release of the inter-membrane space proteins, cristae reorganization, depolarization, permeability transition and generation of reactive oxygen species. Thus Bid is the molecular linker bridging various peripheral death pathways to the central mitochondria pathway. Recent studies further indicate that Bid may be more than just a killer molecule. Deletion of Bid inhibits carcinogenesis in the liver, although this genetic alteration promotes tumorigenesis in the myeloid cells. This is likely related to the function of Bid to promote cell cycle progression into S phase. Bid could be also involved in the maintenance of genomic stability by engaging at mitosis checkpoint. These novel findings indicate that this BH3-only Bcl-2 family protein has a diverse array of functions that are important to both the life and death of the cell.

Myxoma virus M11L blocks apoptosis through inhibition of conformational activation of Bax at the mitochondria

Many viruses inhibit or retard apoptosis, a strategy that subverts one of the most ancient antiviral mechanisms. M11L, a myxoma virus-encoded antiapoptotic protein, has been previously shown to localize to mitochondria and block apoptosis of virus-infected cells (H. Everett, M. Barry, S. F. Lee, X. J. Sun, K. Graham, J. Stone, R. C. Bleackley, and G. McFadden, J. Exp. Med. 191:1487-1498, 2000; H. Everett, M. Barry, X. Sun, S. F. Lee, C. Frantz, L. G. Berthiaume, G. McFadden, and R. C. Bleackley, J. Exp. Med. 196:1127-1139, 2002; and G. Wang, J. W. Barrett, S. H. Nazarian, H. Everett, X. Gao, C. Bleackley, K. Colwill, M. F. Moran, and G. McFadden, J. Virol. 78:7097-7111, 2004). This protection from apoptosis involves constitutive-forming inhibitory complexes with the peripheral benzodiazepine receptor and Bak on the outer mitochondrial membrane. Here, we extend the study to investigate the interference of M11L with Bax activation during the process of apoptosis. Myxoma virus infection triggers an early apoptotic signal that induces rapid Bax translocation from cytoplasm to mitochondria, despite the existence of various viral antiapoptotic proteins. However, in the presence of M11L, the structural activation of Bax at the mitochondrial membrane, which is characterized by the occurrence of a Bax conformational change, is blocked in both M11L-expressing myxoma-infected cells and M11L-transfected cells under apoptotic stimulation. In addition, inducible binding of M11L to the mitochondrially localized Bax is detected in myxoma virus-infected cells and in M11L/Bax-cotransfected cells as measured by immunoprecipitation and tandem affinity purification analysis, respectively. Importantly, this inducible Bax/M11L interaction is independent of Bak, demonstrated by the complete block of Bax-mediated apoptosis in myxoma-infected cells that lack Bak expression. Our findings reveal that myxoma M11L modulates apoptosis by multiple independent strategies which all contribute to the blockade of apoptosis at the mitochondrial checkpoint.