线粒体膜间隙蛋白在细胞凋亡中的作用

线粒体除了作为细胞内的“能量工厂”外,在控制细胞凋亡中起主导作用。细胞凋亡时,线粒体膜通透性增加,释放可溶性线粒体膜间隙蛋白质,进一步破坏细胞结构。在这些致死性蛋白质中,有些(cytc、Smac／DIABLO、Omi／HtrA2等)能够激活caspases,另一些(endo G、AIF、Omi/HtrA2等)则以非caspase依赖的方式发挥作用。多种线粒体因子参与细胞凋亡,强化了细胞器在凋亡控制中的核心作用。     

UV irradiation resistance-associated gene suppresses apoptosis by interfering with BAX activation

Ultraviolet irradiation resistance-associated gene (UVRAG) is a well-known regulator of autophagy by promoting autophagosome formation and maturation. However, little is known about the non-autophagic functions of UVRAG. Here, we present evidence that UVRAG functions as an unusual BCL2-associated X protein (Bax) suppressor to regulate apoptosis. Chemotherapy and radiation induces UVRAG expression and subsequently upregulates autophagy and apoptosis in tumour cells. Depletion of UVRAG expression by RNA interference renders tumour cells more sensitive to chemotherapy- and radiation-induced apoptosis in vitro and in vivo. Moreover, UVRAG interacts with Bax, which inhibits apoptotic stimuli-induced mitochondrial translocation of Bax, reduction of mitochondrial membrane potential, cytochrome c release and activation of caspase-9 and -3. Our findings show that UVRAG has an essential role in the intrinsic mitochondrial pathway of apoptosis by regulating the localization of Bax. This pathway represents a target for clinical intervention against tumours.     

线粒体，活性氧和细胞凋亡

在能量代谢和自由基代谢中,线粒体均占据着十分重要的地位.通过呼吸链电子漏途径,线粒体产生大量超氧阴离子,并通过链式反应形成对机体有损伤作用的活性氧.通过呼吸链电子漏,氧化磷酸化解偶联,线粒体内膜产生通透性转变孔道（PTP）及Box-和/或PTP-介导的细胞色素c向胞质的转移等种种因素,线粒体参与一般抗氧化防御及细胞凋亡等重要生理过程的调控.在与线粒体相关的细胞凋亡中,活性氧的信号作用是十分明显的.     

Smac/DIABLO and cytochrome c are released from mitochondria through a similar mechanism during UV-induced apoptosis

During apoptosis, a key event is the release of Smac/DIABLO (an inhibitor of XIAP) and cytochrome c (Cyt-c, an activator of caspase-9) from mitochondria to cytosol. It was not clear, however, whether the releasing mechanisms of these two proteins are the same. Using a combination of single living-cell analysis and immunostaining techniques, we investigated the dynamic process of Smac and Cyt-c release during UV-induced apoptosis in HeLa cells. We found that YFP-labeled Smac and GFP-labeled Cyt-c were released from mitochondria in the same time window, which coincided with the mitochondrial membrane potential depolarization. Furthermore, using immunostaining, we found that the endogenous Smac and Cyt-c were always released together within an individual cell. Finally, when cells were pre-treated with caspase inhibitor (z-VAD-fmk) to block caspase activation, the process of Smac release, like that of Cyt-c, was not affected. This was true for both YFP-labeled Smac and endogenous Smac. These results suggest that in HeLa cells, both Smac and Cyt-c are released from mitochondria during UV-induced apoptosis through the same permeability transition mechanism, which we believe is triggered by the aggregation of Bax in the outer mitochondrial membrane to form lipid-protein complex.     

The Siva-1 putative amphipathic helical region (SAH) is sufficient to bind to BCL-XL and sensitize cells to UV radiation induced apoptosis

The human Siva gene is localized to chromosome 14q32-33 and gives rise to the full-length predominant form, Siva-1 and a minor alternate form, Siva-2 that appears to lack the proapoptotic properties of Siva-1. Our recent work has shown that the missing region in Siva-2 encodes a unique twenty amino acid putative amphipathic helical region (SAH, residues 36-55 in Siva-1). Despite the fact that Siva-1 does not belong to the BCL-2 family, it specifically interacts with the anti-apoptotic protein BCL-XL and sensitizes MCF7 breast cancer cells expressing BCL-XL to UV radiation induced apoptosis. Deletion mutagenesis has mapped the necessary region to the SAH in Siva-1. In this paper we demonstrate that the SAH region in Siva-1 is sufficient to specifically interact with the anti-apoptotic members of the BCL2 family such as BCL-XL and BCL-2 but not its apoptotic member BAX. Using transient transfections and direct microinjection of synthetic SAH peptides, we also demonstrate that the SAH region is sufficient to inhibit the BCL-XL mediated cell survival and render MDA-MB-231 and MCF7 breast cancer cells expressing BCL-XL highly susceptible to UV radiation induced apoptosis. The underlying mechanism of action of SAH mediated inhibition of BCL-XL (and/or BCL2) cell survival appears to be due to loss of mitochondrial integrity as reflected in enhanced cytochrome c release leading to the activation of caspase 9 and finally caspase 3.     

Mitochondria are involved in apoptosis induced by ultraviolet radiation in lepidopteran Spodoptera litura cell line

Abstract Mitochondria are involved in apoptosis of mammalian cells and even single‐cell organisms, but mitochondria are not required in apoptosis in cultured Drosophila cells such as S2 and BG2 cell lines. It is not very clear whether mitochondria are involved in apoptosis in other insect cells such as lepidopteran cell lines. Thus, we determined to elucidate the role of mitochondria in apoptosis induced by ultraviolet radiation in Spodoptera litura (Lepidoptera: Noctuidae) cell line (SL‐ZSU‐1). The Western blot results suggested that cytochrome c in the ultraviolet‐treated SL‐1 cells was released from the mitochondria to cytosol as early as 4 h after the induction of ultraviolet radiation and increased in the cytosolic fractions in a time‐dependent manner. Flow cytometric analysis of mitochondrial membrane potential (ΔΨm) of SL‐ZSU‐1 cell treated with ultraviolet‐C (UV‐C) light indicated the decrease in mitochondrial membrane potential was dependent on the times of ultraviolet treatment. Both of them are different from apoptosis in cultured Drosophila melanogaster cell lines (S2 and BG2) and it appears evident mitochondria are involved in apoptosis of the studied lepidopteran cells.     

Targeting BAX ubiquitin-binding sites reveals that BAX activation is essential for its ubiquitin-dependent degradation

BAX is an important proapoptotic protein of the BCL-2 family, and its stability is essential for the regulation of the mitochondrial apoptotic pathway. A previous study revealed that BAX could undergo degradation through the ubiquitin-proteasome pathway. In this study, we identified two lysine sites, K21 and K123, that were critical ubiquitin-binding sites in BAX. Mutation of these two sites prolonged the half-life of BAX and also affected its proapoptotic ability. Intriguingly, we found that ABT-737, a BCL-2 inhibitor, significantly enhanced TRAIL-induced BAX degradation in HCT116 cells and increased TRAIL-induced apoptosis in the HCT116 only with the BAX K21R/K123R mutant, not other BAX mutants. In addition, overexpression of PARKIN, an E3 ubiquitin ligase targeting BAX, dramatically decreased BAX protein level when only treated with ABT-737 in HCT116 cells. Therefore, we speculated that BAX activation is essential for its ubiquitin-dependent degradation.     

S-palmitoylation represents a novel mechanism regulating the mitochondrial targeting of BAX and initiation of apoptosis

The intrinsic pathway of apoptotic cell death is mainly mediated by the BCL-2-associated X (BAX) protein through permeabilization of the mitochondrial outer membrane (MOM) and the concomitant release of cytochrome c into the cytosol. In healthy, non-apoptotic cells, BAX is predominantly localized in the cytosol and exhibits a dynamic shuttle cycle between the cytosol and the mitochondria. Thus, the initial association with mitochondria represents a critical regulatory step enabling BAX to insert into MOMs, promoting the release of cytochrome c and ultimately resulting in apoptosis. However, the molecular mode of how BAX associates with MOMs and whether a cellular regulatory mechanism governs this process is poorly understood. Here we show that in both primary tissues and cultured cells, the association with MOMs and the proapoptotic action of BAX is controlled by its S-palmitoylation at Cys-126. A lack of BAX palmitoylation reduced BAX mitochondrial translocation, BAX oligomerization, caspase activity and apoptosis. Furthermore, ectopic expression of specific palmitoyl transferases in cultured healthy cells increases BAX S-palmitoylation and accelerates apoptosis, whereas malignant tumor cells show reduced BAX S-palmitoylation consistent with their reduced BAX-mediated proapoptotic activity. Our findings suggest that S-palmitoylation of BAX at Cys126 is a key regulatory process of BAX-mediated apoptosis.     

BAX contributes to Doppel-induced apoptosis of prion-protein-deficient Purkinje cells

Research efforts to deduce the function of the prion protein (PrPc) in knock-out mouse mutants have revealed that large deletions in the PrPc genome result in the ectopic neuronal expression of the prion-like protein Doppel (Dpl). In our analysis of one such line of mutant mice, Ngsk Prnp0/0 (NP0/0), we demonstrate that the ectopic expression of Dpl in brain neurons induces significant levels of cerebellar Purkinje cell (PC) death as early as six months after birth. To investigate the involvement of the mitochondrial proapoptotic factor BAX in the Dpl-induced apoptosis of PCs, we have analyzed the progression of PC death in aging NP0/0:Bax-/- double knockout mutants. Quantitative analysis of cell numbers showed that significantly more PCs survived in NP0/0:Bax-/- double mutants than in the NP0/0:Bax+/+ mutants. However, PC numbers were not restored to wildtype levels or to the increased number of PCs observed in Bax-/- mutants. The partial rescue of NP0/0 PCs suggests that the ectopic expression of Dpl induces both BAX-dependent and BAX-independent pathways of cell death. The activation of glial cells that is shown to be associated topographically with Dpl-induced PC death in the NP0/0:Bax+/+ mutants is abolished by the loss of Bax expression in the double mutant mice, suggesting that chronic inflammation is an indirect consequence of Dpl-induced PC death.     

Bcl-2家族蛋白调控线粒体膜通透性和细胞色素C释放的新机制

Bcl-2家族蛋白在调控线粒体功能和细胞色素C释放中起重要作用。最近发现Bcl-2分子通过与其他促凋亡分子相互作用调控线粒体外膜通透性,其具体分子机制尚不完全清楚。本课题组采用化学生物学方法,在研究Bax/Bak非依赖的细胞凋亡途径中,发现了一些小分子化合物能够诱导Bim表达量急剧升高,Bim能转位到线粒体上,与Bcl-2相互作用增强,并直接促进Bcl-2构象变化。有意义的是,Bim可以诱导Bcl-2功能发生转换并能够形成大的复合体通道来介导细胞色素C释放。研究结果提示Bcl-2分子可变成促凋亡分子,参与Bax/Bak非依赖的细胞色素C释放和细胞凋亡。     

Inhibition of Mitochondrial ATP Generation by Nitric Oxide Switches Apoptosis to Necrosis

Under pathological conditions, the mode of cell death, apoptosis or necrosis, is relevant for the subsequent fate of the tissue. Cell demise may be shaped by endogenous mediators such as nitric oxide (NO) which interfere with subroutines of the death program. Here we show that apoptosis of Jurkat cells elicited by either staurosporine (STS) or anti-CD95 antibodies in glucose-free medium is converted to necrosis by NO donors. In the presence of NO, release of mitochondrial cytochrome c was delayed and activation of execution caspases was prevented. Stimulated cells died nonetheless. The switch in the mode of cell death was due to NO-dependent failure of mitochondrial energy production. Restoration of intracellular ATP by glucose supplementation recovered the cells' ability to activate caspases and undergo apoptosis. In this system, the apoptosis/necrosis conversion promoted by NO was not mediated by cyclic guanosine monophosphate-dependent mechanisms, poly-(ADP-ribose)-polymerase (PARP) activation, or inhibition of caspases due to S-nitrosylation and glutathione depletion. In contrast, depleting intracellular ATP with rotenone, an inhibitor of mitochondrial complex I mimicked the effect of NO. The findings presented here suggest that NO can decide the shape of cell death by lowering intracellular ATP below the level required to allow the coordinated execution of apoptosis.     

Subcellular localization and physiological consequences of introducing a mitochondrial matrix targeting signal sequence in bax and its mutants

Bax, a proapoptotic member of the Bcl-2 family of proteins, resides in the cytosol and translocates to the mitochondrial membrane upon induction of apoptosis. It has been proposed that Bax does not translocate to mitochondria under normal physiological conditions, due to interaction between amino (ART) and carboxy (TM) terminal domains. Here, we report the physiological consequences of introducing a matrix targeting mitochondrial signal sequence (Su9) at the amino terminus of Bax and its mutants lacking ART, TM, or both segments. In vitro mitochondrial protein import assays of the fusion proteins suggests localization to the mitochondrial matrix. When expressed in Cos-1 cells, Su9 could target Bax to mitochondria in the absence of an apoptotic stimulus. However, mitochondrial localization did not result in apoptosis. When ART, TM, or both segments of Bax were deleted, expression of fusion proteins containing Su9 resulted in apoptosis via cytochrome c release. Cell death was inhibited by the pan-caspase inhibitor zVAD-fmk. We thus demonstrate that an effective mitochondrial matrix targeting signal can override the inhibition of import of Bax to the organelle, presumed to arise as a result of interaction between ART and TM segments, in the absence of apoptotic stimulus. We also demonstrate the ability of truncated variants of Bax to cause apoptosis when targeted to mitochondria by cytochrome c release from an ectopic environment.     

BAX channel activity mediates lysosomal disruption linked to Parkinson disease

Lysosomal disruption is increasingly regarded as a major pathogenic event in Parkinson disease (PD). A reduced number of intraneuronal lysosomes, decreased levels of lysosomal-associated proteins and accumulation of undegraded autophagosomes (AP) are observed in PD-derived samples, including fibroblasts, induced pluripotent stem cell-derived dopaminergic neurons, and post-mortem brain tissue. Mechanistic studies in toxic and genetic rodent PD models attribute PD-related lysosomal breakdown to abnormal lysosomal membrane permeabilization (LMP). However, the molecular mechanisms underlying PD-linked LMP and subsequent lysosomal defects remain virtually unknown, thereby precluding their potential therapeutic targeting. Here we show that the pro-apoptotic protein BAX (BCL2-associated X protein), which permeabilizes mitochondrial membranes in PD models and is activated in PD patients, translocates and internalizes into lysosomal membranes early following treatment with the parkinsonian neurotoxin MPTP, both in vitro and in vivo, within a time-frame correlating with LMP, lysosomal disruption, and autophagosome accumulation and preceding mitochondrial permeabilization and dopaminergic neurodegeneration. Supporting a direct permeabilizing effect of BAX on lysosomal membranes, recombinant BAX is able to induce LMP in purified mouse brain lysosomes and the latter can be prevented by pharmacological blockade of BAX channel activity. Furthermore, pharmacological BAX channel inhibition is able to prevent LMP, restore lysosomal levels, reverse AP accumulation, and attenuate mitochondrial permeabilization and overall nigrostriatal degeneration caused by MPTP, both in vitro and in vivo. Overall, our results reveal that PD-linked lysosomal impairment relies on BAX-induced LMP, and point to small molecules able to block BAX channel activity as potentially beneficial to attenuate both lysosomal defects and neurodegeneration occurring in PD.     

Iron deprivation induces apoptosis via mitochondrial changes related to Bax translocation

In order to elucidate the mechanisms involved in apoptosis induction by iron deprivation, we compared cells sensitive (38C13) and resistant (EL4) to apoptosis induced by iron deprivation. Iron deprivation was achieved by incubation in a defined iron-free medium. We detected the activation of caspase-3 as well as the activation of caspase-9 in sensitive cells but not in resistant cells under iron deprivation. Iron deprivation led to the release of cytochrome c from mitochondria into the cytosol only in sensitive cells but it did not affect the cytosolic localization of Apaf-1 in both sensitive and resistant cells. The mitochondrial membrane potential (_m) was dissipated within 24 h in sensitive cells due to iron deprivation. The antiapoptotic Bcl-2 protein was found to be associated with mitochondria in both sensitive and resistant cells and the association did not change under iron deprivation. On the other hand, under iron deprivation we detected translocation of the proapoptotic Bax protein from the cytosol to mitochondria in sensitive cells but not in resistant cells. Taken together, we suggest that iron deprivation induces apoptosis via mitochondrial changes concerning proapoptotic Bax translocation to mitochondria, collapse of the mitochondrial membrane potential, release of cytochrome c from mitochondria, and activation of caspase-9 and caspase-3.     

Effect of hyperthermia on TRAIL-induced apoptotic death in human colon cancer cells: development of a novel strategy for regional therapy

Approximately 25% of patients with colorectal cancer will develop metastatic disease exclusively or largely confined to the liver, and the vast majority of these cases are not amenable to surgical resection. These unresectable cases of liver metastatic disease can be treated with isolated hepatic perfusion (IHP), which involves a method of complete vascular isolation of the liver to allow treatment of liver tumors with toxic systemic doses of chemotherapeutic agents. To improve the efficacy of IHP, hyperthermia and biological agents have been applied along with the chemotherapeutic agents. In this study, we investigated whether hyperthermia in combination with tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) enhances mortality in human colorectal carcinoma CX-1 cells. Cells were treated with various concentrations of TRAIL (0-200 ng/ml) at various temperatures (40-46 degrees C) for 1 h and further incubated at 37 degrees C in the presence of TRAIL. We observed that hyperthermia at 42-43 degrees C effectively promoted TRAIL-induced apoptosis, as indicated by cell death, poly (ADP-ribose) polymerase (PARP) cleavage, and activation of caspase-8, -9, and -3. In contrast, hyperthermia at 45-46 degrees C suppressed TRAIL-induced apoptosis. We also observed that mild hyperthermia, but not acute hyperthermia, promoted cytochrome c release during treatment with TRAIL. Our data suggest that promotion of cytochrome c release during mild hyperthermia is responsible for the enhancement of TRAIL cytotoxicity.     

Cytochrome c release from rat brain mitochondria is proportional to the mitochondrial functional deficit: implications for apoptosis and neurodegenerative disease

Apoptosis may be initiated in neurons via mitochondrial release of the respiratory protein, cytochrome c. The mechanism of cytochrome c release has been studied extensively, but little is known about its dynamics. It has been claimed that release is all-or-none, however, this is not consistent with accumulating evidence of cytosolic mechanisms for 'buffering' cytochrome c. This study has attempted to model an underlying disease pathology, rather than inducing apoptosis directly. The model adopted was diminished activity of the mitochondrial respiratory chain complex I, a recognized feature of Parkinson's disease. Titration of rat brain mitochondrial respiratory function, with the specific complex I inhibitor rotenone, caused proportional release of cytochrome c from isolated synaptic and non-synaptic mitochondria. The mechanism of release was mediated, at least in part, by the mitochondrial outer membrane component Bak and voltage-dependent anion channel rather than non-specific membrane rupture. Furthermore, preliminary data were obtained demonstrating that in primary cortical neurons, titration with rotenone induced cytochrome c release that was subthreshold for the induction of apoptosis. Implications for the therapy of neurodegenerative diseases are discussed.     

Inhibition of apoptotic signaling cascades causes loss of trophic factor dependence during neuronal maturation

During development, neurons are acutely dependent on target-derived trophic factors for survival. This dependence on trophic support decreases dramatically with maturation in several neuronal populations, including sympathetic neurons. Analyses of nerve growth factor deprivation in immature and mature sympathetic neurons indicate that maturation aborts the cell death pathway at a point that is mechanistically indistinguishable from Bax deletion. However, neither the mRNA nor protein level of BAX changes with neuronal maturation. Therefore, BAX must be regulated posttranslationally in mature neurons.Nerve growth factor deprivation in immature sympathetic neurons induces two parallel processes: (a) a protein synthesis-dependent, caspase-independent translocation of BAX from the cytosol to mitochondria, followed by mitochondrial membrane integration and loss of cytochrome c; and (b) the development of competence-to-die, which requires neither macromolecular synthesis nor BAX expression. Activation of both signaling pathways is required for caspase activation and apoptosis in immature sympathetic neurons. In contrast, nerve growth factor withdrawal in mature sympathetic neurons did not induce the translocation of either BAX or cytochrome c. Moreover, mature neurons did not develop competence-to-die with cytoplasmic accumulation of cytochrome c. Therefore, inhibition of both BAX-dependent cytochrome c release and the development of competence-to-die contributed to the loss of trophic factor dependence associated with neuronal maturation.     

Assessment of the downstream portion of the mitochondrial pathway of caspase activation in patients with acute myeloid leukemia

Most chemotherapeutic agents used in the treatment of acute myeloid leukemia (AML) induce apoptosis by triggering the mitochondrial pathway of caspase activation. To investigate the downstream portion of the mitochondrial pathway of caspase activation in patients with AML, cytosolic lysates were stimulated with cytochrome c and dATP and hydrolysis of Ac-DEVD-AFC by effector caspases was measured. Defects in the distal mitochondrial pathway were more common in samples from patients with AML that relapsed rapidly after induction chemotherapy compared to samples from treatment naïve patients. The incidence of blocked pathways did not differ based on response to induction chemotherapy, as even nonresponders generally had an intact pathway. When the distal mitochondrial pathway was blocked, defects were usually at the level of the effector caspases. Thus, functional defects in the distal portion of the mitochondrial pathway of caspase activation may help explain the nature of response and relapse after treatment.     

Evidence that inhibition of BAX activation by BCL-2 involves its tight and preferential interaction with the BH3 domain of BAX

Interactions between the BCL-2 family proteins determine the cell's fate to live or die. How they interact with each other to regulate apoptosis remains as an unsettled central issue. So far, the antiapoptotic BCL-2 proteins are thought to interact with BAX weakly, but the physiological significance of this interaction has been vague. Herein, we show that recombinant BCL-2 and BCL-w interact potently with a BCL-2 homology (BH) 3 domain-containing peptide derived from BAX, exhibiting the dissociation constants of 15 and 23 nM, respectively. To clarify the basis for this strong interaction, we determined the three-dimensional structure of a complex of BCL-2 with a BAX peptide spanning its BH3 domain. It revealed that their interactions extended beyond the canonical BH3 domain and involved three nonconserved charged residues of BAX. A novel BAX variant, containing the alanine substitution of these three residues, had greatly impaired affinity for BCL-2 and BCL-w, but was otherwise indistinguishable from wild-type BAX. Critically, the apoptotic activity of the BAX variant could not be restrained by BCL-2 and BCL-w, pointing that the observed tight interactions are critical for regulating BAX activation. We also comprehensively quantified the binding affinities between the three BCL-2 subfamily proteins. Collectively, the data show that due to the high affinity of BAX for BCL-2, BCL-w and A1, and of BAK for BCL-X(L), MCL-1 and A1, only a subset of BH3-only proteins, commonly including BIM, BID and PUMA, could be expected to free BAX or BAK from the antiapoptotic BCL-2 proteins to elicit apoptosis.