tBid interaction with cardiolipin primarily orchestrates mitochondrial dysfunctions and subsequently activates Bax and Bak

Regulation of the intrinsic apoptosis pathway by reactive oxygen species

SIGNIFICANCE

The intrinsic apoptosis pathway is conserved from worms to humans and plays a critical role in the normal development and homeostatic control of adult tissues. As a result, numerous diseases from cancer to neurodegeneration are associated with either too little or too much apoptosis.

RECENT ADVANCES

B cell lymphoma-2 (BCL-2) family members regulate cell death, primarily via their effects on mitochondria. In stressed cells, proapoptotic BCL-2 family members promote mitochondrial outer membrane permeabilization (MOMP) and cytochrome c (cyt c) release into the cytoplasm, where it stimulates formation of the "apoptosome." This large, multimeric complex is composed of the adapter protein, apoptotic protease-activating factor-1, and the cysteine protease, caspase-9. Recent studies suggest that proteins involved in the processes leading up to (and including) formation of the apoptosome are subject to various forms of post-translational modification, including proteolysis, phosphorylation, and in some cases, direct oxidative modification.

CRITICAL ISSUES

Despite intense investigation of the intrinsic pathway, significant questions remain regarding how cyt c is released from mitochondria, how the apoptosome is formed and regulated, and how caspase-9 is activated within the complex.

FUTURE DIRECTIONS

Further studies on the biochemistry of MOMP and apoptosome formation are needed to understand the mechanisms that underpin these critical processes, and novel animal models will be necessary in the future to ascertain the importance of the many posttranslational modifications reported for BCL-2 family members and components of the apoptosome.

The effects of chronic muscle use and disuse on cardiolipin metabolism

Cardiolipin (CL) is a phospholipid that maintains the integrity of mitochondrial membranes. We previously demonstrated that CL content increases with chronic muscle use, and decreases with denervation-induced disuse. To investigate the underlying mechanisms, we measured the mRNA expression of 1) CL synthesis enzymes cardiolipin synthase (CLS) and CTP:PA-cytidylyltransferase-1 (CDS-1); 2) remodeling enzymes tafazzin and acyl-CoA:lysocardiolipin acyltransferase-1 (ALCAT1); and 3) outer membrane CL enzymes, mitochondrial phospholipase D and phospholipid scramblase 3 (Plscr3), during chronic contractile activity (CCA)-induced mitochondrial biogenesis and denervation. With CCA, CDS-1 expression increased by 128%, parelleling CL levels. Surprisingly, denervation also led to large increases in CDS-1 and CLS, despite a decrease in mitochondria, possibly due to a compensatory mechanism to restore lost CL. ALCAT1 decreased by 32% with CCA, but increased by 290% following denervation, indicating that both CCA and denervation alter CL remodeling. CCA and denervation also elicited 60–90% increases in Plscr3, likely to facilitate CL movement to the outer membrane. The expression of these genes was not affected by aging, but changes due to CCA and denervation were attenuated compared with young animals. The absence of PPARγ coactivator-1α in knockout animals led to a decrease in CDS-1 and an increase in ALCAT1 mRNA levels, implicating PGC-1α in regulating both CL synthesis and remodeling. These data suggest that chronic muscle use and disuse modify the expression of mRNAs encoding CL metabolism enzymes. Our data also illustrate, for the first time, that PPARγ coactivator-1α regulates the CL metabolism pathway in muscle.

Caspase-8 binding to cardiolipin in giant unilamellar vesicles provides a functional docking platform for bid

Caspase-8 is involved in death receptor-mediated apoptosis in type II cells, the proapoptotic programme of which is triggered by truncated Bid. Indeed, caspase-8 and Bid are the known intermediates of this signalling pathway. Cardiolipin has been shown to provide an anchor and an essential activating platform for caspase-8 at the mitochondrial membrane surface. Destabilisation of this platform alters receptor-mediated apoptosis in diseases such as Barth Syndrome, which is characterised by the presence of immature cardiolipin which does not allow caspase-8 binding. We used a simplified in vitro system that mimics contact sites and/or cardiolipin-enriched microdomains at the outer mitochondrial surface in which the platform consisting of caspase-8, Bid and cardiolipin was reconstituted in giant unilamellar vesicles. We analysed these vesicles by flow cytometry and confirm previous results that demonstrate the requirement for intact mature cardiolipin for caspase-8 activation and Bid binding and cleavage. We also used confocal microscopy to visualise the rupture of the vesicles and their revesiculation at smaller sizes due to alteration of the curvature following caspase-8 and Bid binding. Biophysical approaches, including Laurdan fluorescence and rupture/tension measurements, were used to determine the ability of these three components (cardiolipin, caspase-8 and Bid) to fulfil the minimal requirements for the formation and function of the platform at the mitochondrial membrane. Our results shed light on the active functional role of cardiolipin, bridging the gap between death receptors and mitochondria.

Mitochondria in traumatic brain injury and mitochondrial-targeted multipotential therapeutic strategies

Traumatic brain injury (TBI) is a major health and socioeconomic problem throughout the world. It is a complicated pathological process that consists of primary insults and a secondary insult characterized by a set of biochemical cascades. The imbalance between a higher energy demand for repair of cell damage and decreased energy production led by mitochondrial dysfunction aggravates cell damage. At the cellular level, the main cause of the secondary deleterious cascades is cell damage that is centred in the mitochondria. Excitotoxicity, Ca(2+) overload, reactive oxygen species (ROS), Bcl-2 family, caspases and apoptosis inducing factor (AIF) are the main participants in mitochondria-centred cell damage following TBI. Some preclinical and clinical results of mitochondria-targeted therapy show promise. Mitochondria- targeted multipotential therapeutic strategies offer new hope for the successful treatment of TBI and other acute brain injuries.

Cardiolipin-mediated cellular signaling

This review focuses on recent studies showing that cardiolipin (CL), a unique mitochondrial phospholipid, regulates many cellular functions and signaling pathways, both inside and outside the mitochondria. Inside the mitochondria, CL is a critical target of mitochondrial generated reactive oxygen species (ROS) and regulates signaling events related to apoptosis and aging. CL deficiency causes perturbation of signaling pathways outside the mitochondria, including the PKC-Slt2 cell integrity pathway and the high osmolarity glycerol (HOG) pathway, and is a key player in the cross-talk between the mitochondria and the vacuole. Understanding these connections may shed light on the pathology of Barth syndrome, a disorder of CL remodeling.

Dual function of mitochondrial Nm23-H4 protein in phosphotransfer and intermembrane lipid transfer: a cardiolipin-dependent switch

The nucleoside diphosphate kinase Nm23-H4/NDPK-D forms symmetrical hexameric complexes in the mitochondrial intermembrane space with phosphotransfer activity using mitochondrial ATP to regenerate nucleoside triphosphates. We demonstrate the complex formation between Nm23-H4 and mitochondrial GTPase OPA1 in rat liver, suggesting its involvement in local and direct GTP delivery. Similar to OPA1, Nm23-H4 is further known to strongly bind in vitro to anionic phospholipids, mainly cardiolipin, and in vivo to the inner mitochondrial membrane. We show here that such protein-lipid complexes inhibit nucleoside diphosphate kinase activity but are necessary for another function of Nm23-H4, selective intermembrane lipid transfer. Mitochondrial lipid distribution was analyzed by liquid chromatography-mass spectrometry using HeLa cells expressing either wild-type Nm23-H4 or a membrane binding-deficient mutant at a site predicted based on molecular modeling to be crucial for cardiolipin binding and transfer mechanism. We found that wild type, but not the mutant enzyme, selectively increased the content of cardiolipin in the outer mitochondrial membrane, but the distribution of other more abundant phospholipids (e.g. phosphatidylcholine) remained unchanged. HeLa cells expressing the wild-type enzyme showed increased accumulation of Bax in mitochondria and were sensitized to rotenone-induced apoptosis as revealed by stimulated release of cytochrome c into the cytosol, elevated caspase 3/7 activity, and increased annexin V binding. Based on these data and molecular modeling, we propose that Nm23-H4 acts as a lipid-dependent mitochondrial switch with dual function in phosphotransfer serving local GTP supply and cardiolipin transfer for apoptotic signaling and putative other functions.

Acidic pH promotes oligomerization and membrane insertion of the BclXL apoptotic repressor

Solution pH is believed to serve as an intricate regulatory switch in the induction of apoptosis central to embryonic development and cellular homeostasis. Herein, using an array of biophysical techniques, we provide evidence that acidic pH promotes the assembly of BclXL apoptotic repressor into a megadalton oligomer with a plume-like appearance and harboring structural features characteristic of a molten globule. Strikingly, our data reveal that pH tightly modulates not only oligomerization but also ligand binding and membrane insertion of BclXL in a highly subtle manner. Thus, while oligomerization and the accompanying molten globular content of BclXL is least favorable at pH 6, both of these structural features become more pronounced under acidic and alkaline conditions. However, membrane insertion of BclXL appears to be predominantly favored under acidic conditions. In a remarkable contrast, while ligand binding to BclXL optimally occurs at pH 6, it is diminished by an order of magnitude at lower and higher pH. This reciprocal relationship between BclXL oligomerization and ligand binding lends new insights into how pH modulates functional versatility of a key apoptotic regulator and strongly argues that the molten globule may serve as an intermediate primed for membrane insertion in response to apoptotic cues.

Phosphatidic acid mediates the targeting of tBid to induce lysosomal membrane permeabilization and apoptosis

Upon apoptotic stimuli, lysosomal proteases, including cathepsins and chymotrypsin, are released into cytosol due to lysosomal membrane permeabilization (LMP), where they trigger apoptosis via the lysosomal-mitochondrial pathway of apoptosis. Herein, the mechanism of LMP was investigated. We found that caspase 8-cleaved Bid (tBid) could result in LMP directly. Although Bax or Bak might modestly enhance tBid-triggered LMP, they are not necessary for LMP. To study this further, large unilamellar vesicles (LUVs), model membranes mimicking the lipid constitution of lysosomes, were used to reconstitute the membrane permeabilization process in vitro. We found that phosphatidic acid (PA), one of the major acidic phospholipids found in lysosome membrane, is essential for tBid-induced LMP. PA facilitates the insertion of tBid deeply into lipid bilayers, where it undergoes homo-oligomerization and triggers the formation of highly curved nonbilayer lipid phases. These events induce LMP via pore formation mechanisms because encapsulated fluorescein-conjugated dextran (FD)-20 was released more significantly than FD-70 or FD-250 from LUVs due to its smaller molecular size. On the basis of these data, we proposed tBid-PA interactions in the lysosomal membranes form lipidic pores and result in LMP. We further noted that chymotrypsin-cleaved Bid is more potent than tBid at binding to PA, inserting into the lipid bilayer, and promoting LMP. This amplification mechanism likely contributes to the culmination of apoptotic signaling.

Multipolar functions of BCL-2 proteins link energetics to apoptosis

Classical apoptotic cell death is now sufficiently well understood to be interrogated with mathematical modeling and manipulated with targeted drugs for clinical benefit. However, a biological black hole has emerged with the realization that apoptosis regulators are functionally multipolar. BCL-2 family proteins appear to have much greater effects on cells than can be explained by their known roles in apoptosis. Although these effects may be observable simply because the cell is not dead, the general assumption is that BCL-2 proteins have undiscovered biochemical activities. Conversely, these as yet uncharacterized day-jobs also may underlie their profound effects on cell survival, challenging current assumptions about classical apoptosis. Even their sub-mitochondrial localizations remain controversial. Here we attempt to integrate seemingly conflicting information with the prospect that BCL-2 proteins themselves may be the critical crosstalk between life and death.

BID is a critical factor controlling cell viability regulated by IFN-α

Clinical applications of human interferon (IFN)-α have met with varying degrees of success. Nevertheless, key molecules in cell viability regulated by IFN-α have not been clearly identified. Our previous study indicated that IFN (α, β, and ω) receptor (IFNAR) 1/2- and IFN regulatory factor 9-RNA interference (RNAi) completely restored cell viability after IFN-α treatment in human ovarian adenocarcinoma OVCAR3 cells sensitive to IFN-α. In this study, IFNAR1/2- and IFN regulatory factor 9-RNAi inhibited the gene expression of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), but not of Fas ligand, after IFN-α treatment. In fact, TRAIL but not Fas ligand inhibited the viability of OVCAR3 cells. IFN-α notably upregulated the levels of TRAIL protein in the supernatant and on the membrane of OVCAR3 cells. After TRAIL signaling, caspase 8 inhibitor and BH3 interacting domain death agonist (BID)-RNAi significantly restored cell viability in response to IFN-α and TRAIL in OVCAR3 cells. Furthermore, BID-RNAi prevented both IFN-α and TRAIL from collapsing the mitochondrial membrane potential (ΔΨm). Finally, we provided important evidence that BID overexpression led to significant inhibition of cell viability after IFN-α or TRAIL treatments in human lung carcinoma A549 cells resistant to IFN-α. Thus, this study suggests that BID is crucial for cell viability regulated by IFN-α which can induce mitochondria-mediated apoptosis, indicating a notable potential to be a targeted therapy for IFN-α resistant tumors.

Free radical oxidation of cardiolipin: chemical mechanisms, detection and implication in apoptosis, mitochondrial dysfunction and human diseases

Cardiolipin (CL) is a mitochondria-specific phospholipid and is critical for maintaining the integrity of mitochondrial membrane and mitochondrial function. CL also plays an active role in mitochondria-dependent apoptosis by interacting with cytochrome c (cyt c), tBid and other important Bcl-2 proteins. The unique structure of CL with four linoleic acid side chains in the same molecule and its cellular location make it extremely susceptible to free radical oxidation by reactive oxygen species including free radicals derived from peroxidase activity of cyt c/CL complex, singlet oxygen and hydroxyl radical. The free radical oxidation products of CL have been emerged as important mediators in apoptosis. In this review, we summarize the free radical chemical mechanisms that lead to CL oxidation, recent development in detection of oxidation products of CL by mass spectrometry and the implication of CL oxidation in mitochondria-mediated apoptosis, mitochondrial dysfunction and human diseases.

Activation of protein kinase C delta following cerebral ischemia leads to release of cytochrome C from the mitochondria via bad pathway

BACKGROUND

The release of cytochrome c from the mitochondria following cerebral ischemia is a key event leading to cell death. The goal of the present study was to determine the mechanisms involved in post-ischemic activation of protein kinase c delta (δPKC) that lead to cytochrome c release.

METHODS/FINDINGS

We used a rat model of cardiac arrest as an in vivo model, and an in vitro analog, oxygen glucose deprivation (OGD) in rat hippocampal synaptosomes. Cardiac arrest triggered translocation of δPKC to the mitochondrial fraction at 1 h reperfusion. In synaptosomes, the peptide inhibitor of δPKC blocked OGD-induced translocation to the mitochondria. We tested two potential pathways by which δPKC activation could lead to cytochrome c release: phosphorylation of phospholipid scramblase-3 (PLSCR3) and/or protein phosphatase 2A (PP2A). Cardiac arrest increased levels of phosphorlyated PLSCR3; however, inhibition of δPKC translocation failed to affect the OGD-induced increase in PLSCR3 in synaptosomal mitochondria suggesting the post-ischemic phosphorylation of PLSCR3 is not mediated by δPKC. Inhibition of either δPKC or PP2A decreased cytochrome c release from synaptosomal mitochondria. Cardiac arrest results in the dephosphorylation of Bad and Bax, both downstream targets of PP2A promoting apoptosis. Inhibition of δPKC or PP2A prevented OGD-induced Bad, but not Bax, dephosphorylation. To complement these studies, we used proteomics to identify novel mitochondrial substrates of δPKC.

CONCLUSIONS

We conclude that δPKC initiates cytochrome c release via phosphorylation of PP2A and subsequent dephosphorylation of Bad and identified δPKC, PP2A and additional mitochondrial proteins as potential therapeutic targets for ischemic neuroprotection.

Bax: Addressed to kill

The pro-apoptototic protein Bax (Bcl-2 Associated protein X) plays a central role in the mitochondria-dependent apoptotic pathway. In healthy mammalian cells, Bax is essentially cytosolic and inactive. Following a death signal, the protein is translocated to the outer mitochondrial membrane, where it promotes a permeabilization that favors the release of different apoptogenic factors, such as cytochrome c. The regulation of Bax translocation is associated to conformational changes that are under the control of different factors. The evidences showing the involvement of different Bax domains in its mitochondrial localization are presented. The interactions between Bax and its different partners are described in relation to their ability to promote (or prevent) Bax conformational changes leading to mitochondrial addressing and to the acquisition of the capacity to permeabilize the outer mitochondrial membrane.

Detection of mitochondrial fission with orientation-dependent optical Fourier filters

We utilize a recently developed optical imaging method based on Fourier processing with Gabor-like filters to detect changes in light scattering resulting from alterations in mitochondrial structure in endothelial cells undergoing apoptosis. Imaging based on Gabor filters shows a significant decrease in the orientation of subcellular organelles at 60 to 100 minutes following apoptosis induction and concomitant with mitochondrial fragmentation observed by fluorescence. The optical scatter changes can be detected at low resolution at the whole cell level. At high resolution, we combine fluorescence imaging of the mitochondria with optical Fourier-based imaging to demonstrate that the dynamic decrease in organelle orientation measured by optical Gabor filtering is spatially associated with fluorescent mitochondria and remains largely absent from nonfluorescent subcellular regions. These results provide strong evidence that the optical Gabor responses track mitochondrial fission during apoptosis and can be used to provide label-free, rapid monitoring of this morphological process within single cells.

Giant unilamellar vesicles (GUVs) as a new tool for analysis of caspase-8/Bid-FL complex binding to cardiolipin and its functional activity

Cardiolipin (CL) has recently been shown to be both an anchor and an essential activating platform for caspase-8 on mitochondria. These platforms may be at the mitochondrial contact sites in which truncated Bid (tBid) has been demonstrated to be located. A possible role for CL is to anchor caspase-8 at contact sites (between inner and outer membranes), facilitating its self-activation, Bid-full length (FL) cleavage, tBid generation (and Bax/Bak activation and oligomerization), mitochondrial destabilization and apoptosis. We have developed an in vitro system that mimics the mitochondrial membrane contact site platform. This system involves reconstituting caspase-8, Bid-FL and CL complexes in giant unilamellar vesicles (GUVs). We first validated the system by flow cytometry analysis of light-scattering properties and nonyl acridine orange staining of their CL content. Then, we used flow cytometry analysis to detect the binding of active caspase-8 to CL and the subsequent truncation of bound Bid-FL. The tBid generated interacts with CL and induces GUV breakage and partial re-vesiculation at a smaller size. Our findings suggest an active role for mitochondrial membrane lipids, particularly CL, in binding active caspase-8 and providing a docking site for Bid-FL. This phenomenon was previously only poorly documented and substantially underestimated.

Mass-spectrometry based oxidative lipidomics and lipid imaging: applications in traumatic brain injury

Lipids, particularly phospholipids, are fundamental to CNS tissue architecture and function. Endogenous polyunsaturated fatty acid chains of phospholipids possess cis-double bonds each separated by one methylene group. These phospholipids are very susceptible to free-radical attack and oxidative modifications. A combination of analytical methods including different versions of chromatography and mass spectrometry allows detailed information to be obtained on the content and distribution of lipids and their oxidation products thus constituting the newly emerging field of oxidative lipidomics. It is becoming evident that specific oxidative modifications of lipids are critical to a number of cellular functions, disease states and responses to oxidative stresses. Oxidative lipidomics is beginning to provide new mechanistic insights into traumatic brain injury which may have significant translational potential for development of therapies in acute CNS insults. In particular, selective oxidation of a mitochondria-specific phospholipid, cardiolipin, has been associated with the initiation and progression of apoptosis in injured neurons thus indicating new drug discovery targets. Furthermore, imaging mass-spectrometry represents an exciting new opportunity for correlating maps of lipid profiles and their oxidation products with structure and neuropathology. This review is focused on these most recent advancements in the field of lipidomics and oxidative lipidomics based on the applications of mass spectrometry and imaging mass spectrometry as they relate to studies of phospholipids in traumatic brain injury.

Apoptosis-induced changes in mitochondrial lipids

Apoptosis is an active and tightly regulated form of cell death, which can also be considered a stress-induced process of cellular communication. Recent studies reveal that the lipid network within cells is involved in the regulation and propagation of death signalling. Despite the vast growth of our current knowledge on apoptosis, little is known of the specific role played by lipid molecules in the central event of apoptosis-the piercing of mitochondrial membranes. Here we review the information regarding changes in mitochondrial lipids that are associated with apoptosis and discuss whether they may be involved in the permeabilization of mitochondria to release their apoptogenic factors, or just lie downstream of this permeabilization leading to the amplification of caspase activation. We focus on the earliest changes that physiological apoptosis induces in mitochondrial membranes, which may derive from an upstream alteration of phospholipid metabolism that reverberates on the mitochondrial re-modelling of their characteristic lipid, cardiolipin. Hopefully, this review will lead to an increased understanding of the role of mitochondrial lipids in apoptosis and also help revealing new stress sensing mechanisms in cells. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.

Bioenergetics and cell death

Mitochondrial bioenergetic function is a key to cell life and death. Cells need energy not only to support their vital functions but also to die gracefully. Execution of an apoptotic program includes energy-dependent steps, including kinase signaling, formation of the apoptosome, and effector caspase activation. Under conditions of bioenergetic collapse, cells are diverted toward necrotic demise. Mitochondrial outer membrane permeabilization (MOMP) is a decisive event in the execution of apoptosis. It is also causally linked to a decline in bioenergetic function via different mechanisms, not merely due to cytochrome c dispersion. MOMP-induced bioenergetic deficiency is usually irreversible and commits cells to die, even when caspases are inactive. Here, we discuss the mechanisms by which MOMP impacts bioenergetics in different cell death paradigms.

New insights into apoptosis signaling by Apo2L/TRAIL

Apoptosis ligand 2 tumor necrosis factor (TNF)-related apoptosis-inducing ligand (Apo2L/TRAIL) belongs to a small subset of proapoptotic protein ligands in the TNF superfamily. This subset, which also includes Fas ligand and TNF-alpha, can activate the extrinsic apoptotic cell death pathway on binding to cognate death receptors at the cell surface. Over the past 10 years, Apo2L/TRAIL has emerged as a promising candidate for cancer therapy, on the basis of its unique ability to trigger apoptosis in various types of cancer cells without significant toxicity toward normal cells. Herein, we review key advances in understanding the molecular events that control apoptosis signaling by Apo2L/TRAIL, which may aid in the development of cancer therapies based on the extrinsic apoptotic pathway.

The roles of apoptotic pathways in the low recovery rate after cryopreservation of dissociated human embryonic stem cells

Human embryonic stem (hES) cells have enormous potential for clinical applications. However, one major challenge is to achieve high cell recovery rate after cryopreservation. Understanding how the conventional cryopreservation protocol fails to protect the cells is a prerequisite for developing efficient and successful cryopreservation methods for hES cell lines and banks. We investigated how the stimuli from cryopreservation result in apoptosis, which causes the low cell recovery rate after cryopreservation. The level of reactive oxygen species (ROS) is significantly increased, F-actin content and distribution is altered, and caspase-8 and caspase-9 are activated after cryopreservation. p53 is also activated and translocated into nucleus. During cryopreservation apoptosis is induced by activation of both caspase-8 through the extrinsic pathway and caspase-9 through the intrinsic pathway. However, exactly how the extrinsic pathway is activated is still unclear and deserves further investigation.

Reactive oxygen species, cellular redox systems, and apoptosis

Reactive oxygen species (ROS) are products of normal metabolism and xenobiotic exposure, and depending on their concentration, ROS can be beneficial or harmful to cells and tissues. At physiological low levels, ROS function as "redox messengers" in intracellular signaling and regulation, whereas excess ROS induce oxidative modification of cellular macromolecules, inhibit protein function, and promote cell death. Additionally, various redox systems, such as the glutathione, thioredoxin, and pyridine nucleotide redox couples, participate in cell signaling and modulation of cell function, including apoptotic cell death. Cell apoptosis is initiated by extracellular and intracellular signals via two main pathways, the death receptor- and the mitochondria-mediated pathways. Various pathologies can result from oxidative stress-induced apoptotic signaling that is consequent to ROS increases and/or antioxidant decreases, disruption of intracellular redox homeostasis, and irreversible oxidative modifications of lipid, protein, or DNA. In this review, we focus on several key aspects of ROS and redox mechanisms in apoptotic signaling and highlight the gaps in knowledge and potential avenues for further investigation. A full understanding of the redox control of apoptotic initiation and execution could underpin the development of therapeutic interventions targeted at oxidative stress-associated disorders.

Mechanistic issues of the interaction of the hairpin-forming domain of tBid with mitochondrial cardiolipin

BACKGROUND

The pro-apoptotic effector Bid induces mitochondrial apoptosis in synergy with Bax and Bak. In response to death receptors activation, Bid is cleaved by caspase-8 into its active form, tBid (truncated Bid), which then translocates to the mitochondria to trigger cytochrome c release and subsequent apoptosis. Accumulating evidence now indicate that the binding of tBid initiates an ordered sequences of events that prime mitochondria from the action of Bax and Bak: (1) tBid interacts with mitochondria via a specific binding to cardiolipin (CL) and immediately disturbs mitochondrial structure and function idependently of its BH3 domain; (2) Then, tBid activates through its BH3 domain Bax and/or Bak and induces their subsequent oligomerization in mitochondrial membranes. To date, the underlying mechanism responsible for targeting tBid to mitochondria and disrupting mitochondrial bioenergetics has yet be elucidated.

PRINCIPAL FINDINGS

The present study investigates the mechanism by which tBid interacts with mitochondria issued from mouse hepatocytes and perturbs mitochondrial function. We show here that the helix alphaH6 is responsible for targeting tBid to mitochondrial CL and disrupting mitochondrial bioenergetics. In particular, alphaH6 interacts with mitochondria through electrostatic interactions involving the lysines 157 and 158 and induces an inhibition of state-3 respiration and an uncoupling of state-4 respiration. These changes may represent a key event that primes mitochondria for the action of Bax and Bak. In addition, we also demonstrate that tBid required its helix alphaH6 to efficiently induce cytochrome c release and apoptosis.

CONCLUSIONS

Our findings provide new insights into the mechanism of action of tBid, and particularly emphasize the importance of the interaction of the helix alphaH6 with CL for both mitochondrial targeting and pro-apoptotic activity of tBid. These support the notion that tBid acts as a bifunctional molecule: first, it binds to mitochondrial CL via its helix alphaH6 and destabilizes mitochondrial structure and function, and then it promotes through its BH3 domain the activation and oligomerization of Bax and/or Bak, leading to cytochrome c release and execution of apoptosis. Our findings also imply an active role of the membrane in modulating the interactions between Bcl-2 proteins that has so far been underestimated.

Role of membrane lipids for the activity of pore forming peptides and proteins

Bilayer lipids, far from being passive elements, have multiple roles in polypeptide-dependent pore formation. Lipids participate at all stages of the formation of pores by providing the binding site for proteins and peptides, conditioning their active structure and modulating the molecular reorganization of the membrane complex. Such general functions of lipids superimpose to other particular roles, from electrostatic and curvature effects to more specific actions in cases like cholesterol, sphingolipids or cardiolipin. Pores are natural phenomena in lipid membranes. Driven by membrane fluctuations and packing defects, transient water pores are related to spontaneous lipid flip-flop and non-assisted ion permeation. In the absence ofproteins or peptides, these are rare short living events, with properties dependent on the lipid composition of the membrane. Their frequency increases under conditions of internal membrane disturbance of the lipid packing, like in the presence of membrane-bound proteins or peptides. These latter molecules, in fact, form dynamic supramolecular assemblies together with the lipids and transmembrane pores are one of the possible structures of the complex. Active peptides and proteins can thus be considered inducers or enhancers of pores which increase their probability and lifetime by modifying the thermodynamic membrane balance. This includes destabilizing the membrane lamellar structure, lowering the activation energy for pore formation and stabilizing the open pore structure.

Dissimilar mechanisms of cytochrome c release induced by octyl glucoside-activated BAX and by BAX activated with truncated BID

In the present study, we compared alkali-resistant BAX insertion into the outer mitochondrial membrane, mitochondrial remodeling, mitochondrial membrane potential changes, and cytochrome c (Cyt c) release from isolated brain mitochondria triggered by recombinant BAX oligomerized with 1% octyl glucoside (BAX(oligo)) and by a combination of monomeric BAX (BAX(mono)) and caspase 8-cleaved C-terminal fragment of recombinant BID (truncated BID, t(c)BID). We also examined whether the effects induced by BAX(oligo) or by BAX(mono) activated with t(c)BID depended on induction of the mitochondrial permeability transition. The results obtained in this study revealed that t(c)BID plus BAX(mono) produced BAX insertion and Cyt c release without overt changes in mitochondrial morphology. On the contrary, treatment of mitochondria with BAX(oligo) resulted in BAX insertion and Cyt c release, which were accompanied by gross distortion of mitochondrial morphology. The effects of BAX(oligo) could be at least partially suppressed by mitochondrial depolarization. The effects of t(c)BID plus BAX(mono) were insensitive to depolarization. BAX(oligo) produced similar BAX insertion, mitochondrial remodeling, and Cyt c release in KCl- and in N-methyl-D-glucamine-based incubation media indicating a non-essential role for K+ influx into mitochondria in these processes. A combination of cyclosporin A and ADP, inhibitors of the mitochondrial permeability transition, attenuated Cyt c release, mitochondrial remodeling, and depolarization induced by BAX(oligo), but failed to influence the effects produced by t(c)BID plus BAX(mono). Thus, our results suggest a significant difference in the mechanisms of the outer mitochondrial membrane permeabilization and Cyt c release induced by detergent-oligomerized BAX(oligo) and by BAX activated with t(c)BID.

Regulation and pharmacology of the mitochondrial permeability transition pore

The 'mitochondrial permeability transition', characterized by a sudden induced change of the inner mitochondrial membrane permeability for water as well as for small substances ( Collapse

Key Words

• adenine nucleotide translocator
• cyclophilin d
• mitochondrial voltage-dependent anion channel
• hexokinase
• creatine kinase
• mitochondrial peripheral benzodiazepine receptor
• bcl-2
• glycogen synthase kinase-3β

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MESH Headings

• Adenine Nucleotide Translocator 1/metabolism
• Animals
• Cardiovascular Agents/pharmacology
• Creatine Kinase, Mitochondrial Form/metabolism
• Peptidyl-Prolyl Isomerase F
• Cyclophilins/metabolism
• Hexokinase/metabolism
• Humans
• Lipid Metabolism
• Membrane Potential, Mitochondrial
• Mitochondria, Heart/drug effects
• Mitochondria, Heart/metabolism
• Mitochondrial Membrane Transport Proteins/chemistry
• Mitochondrial Membrane Transport Proteins/drug effects
• Mitochondrial Membrane Transport Proteins/genetics
• Mitochondrial Membrane Transport Proteins/metabolism
• Mitochondrial Permeability Transition Pore
• Myocardium/metabolism
• Phosphates/metabolism
• Receptors, GABA-A/metabolism
• Voltage-Dependent Anion Channels/metabolism

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Grants

• Intramural NIH HHS

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Affiliation(s)

Dmitry B. Zorov
Magdalena Juhaszova
Yael Yaniv
H. Bradley Nuss
Su Wang
Steven J. Sollott Laboratory of Cardiovascular Science, Gerontology Research Center, Box 13, Intramural Research Program, National Institute on Aging, NIH, 5600 Nathan Shock Drive, Baltimore, MD 21224-6825, USA

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Cardiolipin and mitochondrial carriers

Members of the mitochondrial carrier family interact with cardiolipin (CL) as evident from a variety of functional and structural effects. CL stabilises carrier proteins on isolation with detergents, with the P(i) carrier as the prime example. CL is required for transport in reconstituted vesicles, prime examples are the P(i)- and ADP/ATP carrier (AAC). CL binds to the AAC in a graded manner; 6 CL/AAC dimer bind tightly as measured on the (31)P NMR time scale. 2 additional CL/dimer bind reversibly and a fast exchanging envelope of phospholipids includes CL as measured on the ESR time scale. In the crystal structure of the CAT-AAC complex 3 CL bind to the periphery of the AAC in a three-fold pseudo-symmetry. The binding of CL is implicated to contribute lowering the high transition energy barriers in the AAC. Para-functions of the AAC, as in the mitochondrial pore transition (MPT) and in cell death are linked to the CL binding of the AAC. Ca(++) or oxidants can sequester or destroy AAC bound CL, rendering AAC labile, allowing pore formation and degradation. Thus AAC, by being vital for energy transfer, constitutes an Achilles heel in the eukaryotic cell. AAC together with CL is also engaged in respiratory supercomplexes. Different from AAC the similarly structured uncoupling protein (UCP1) has no tightly bound CL, but CL addition lowers affinity of the inhibitory nucleotide binding that may contribute to the physiological regulation of the uncoupling activity by ATP.

Cytochrome c/cardiolipin relations in mitochondria: a kiss of death

Recently, phospholipid peroxidation products gained a reputation as key regulatory molecules and participants in oxidative signaling pathways. During apoptosis, a mitochondria-specific phospholipid, cardiolipin (CL), interacts with cytochrome c (cyt c) to form a peroxidase complex that catalyzes CL oxidation; this process plays a pivotal role in the mitochondrial stage of the execution of the cell death program. This review is focused on redox mechanisms and essential structural features of cyt c's conversion into a CL-specific peroxidase that represent an interesting and maybe still unique example of a functionally significant ligand change in hemoproteins. Furthermore, specific characteristics of CL in mitochondria--its asymmetric transmembrane distribution and mechanisms of collapse, the regulation of its synthesis, remodeling, and fatty acid composition--are given significant consideration. Finally, new concepts in drug discovery based on the design of mitochondria-targeted inhibitors of cyt c/CL peroxidase and CL peroxidation with antiapoptotic effects are presented.

New insights into mitochondrial structure during cell death

Mitochondria play a pivotal role in the cascade of events associated with cell death pathways that are involved with several forms of neurodegeneration. Recent findings show that in the Bax/Bak-dependent pathway of apoptosis, the release of cytochrome c from mitochondria is a consequence of two carefully coordinated events: opening of crista junctions triggered by OPA1 oligomer disassembly and formation of outer membrane pores. Both steps are necessary for the complete release of pro-apoptotic proteins. The remodeling of mitochondrial structure accompanies this pathway, including mitochondrial fission, and cristae and crista junction alterations. Yet, there is controversy surrounding the timing of certain remodeling events and whether they are necessary early events required for the release of pro-apoptotic factors or are simply a downstream after-effect. Here, we analyze the current knowledge of mitochondrial remodeling during cell death and discuss what structural alterations occur to this organelle during neurodegeneration, focusing on the higher resolution structural correlates obtained by electron microscopy and electron tomography.

Cardiolipin acts as a mitochondrial signalling platform to launch apoptosis

Cardiolipin (CL) is a unique anionic phospholipid specific to the mitochondria. CL influences the activity of electron transport chain enzyme complexes as well as members of the Bcl-2 family. Interactions between Bcl-2 family members and other pro-apoptotic enzymes have been shown to be crucial for the transduction of the apoptotic signalling cascades during programmed cell death. Targeting of tBid to the mitochondria, which is necessary for Bax/Bak oligomerization and cristae remodelling, is dependent on the exposure of CL at contact sites between the inner and outer mitochondrial membranes. Also, the mobilization of cytochrome c, another key apoptotic event, is tightly regulated by the oxidative state of cardiolipin. Moreover, CL has been shown to be essential for translocation and autoprocessing of caspase-8 on the mitochondria after death receptor stimulation. Deficiencies in CL inhibit the formation of tBid and prevent apoptosis by removing an essential activation platform for the autoprocessing of caspase-8. It is now apparent that CL acts as a crucial signalling platform from which it orchestrates apoptosis by integrating signals from a variety of death inducing proteins.

Mitochondrial kinases and their molecular interaction with cardiolipin

Mitochondrial isoforms of creatine kinase (MtCK) and nucleoside diphosphate kinase (NDPK-D) are not phylogenetically related but share functionally important properties. They both use mitochondrially generated ATP with the ultimate goal of maintaining proper nucleotide pools, are located in the intermembrane/cristae space, have symmetrical oligomeric structures, and show high affinity binding to anionic phospholipids, in particular cardiolipin. The structural basis and functional consequences of the cardiolipin interaction have been studied and are discussed in detail in this review. They mainly result in a functional interaction of MtCK and NDPK-D with inner membrane adenylate translocator, probably by forming proteolipid complexes. These interactions allow for privileged exchange of metabolites (channeling) that ultimately regulate mitochondrial respiration. Further functions of the MtCK/membrane interaction include formation of cardiolipin membrane patches, stabilization of mitochondria and a role in apoptotic signaling, as well as in case of both kinases, a role in facilitating lipid transfer between two membranes. Finally, disturbed cardiolipin interactions of MtCK, NDPK-D and other proteins like cytochrome c and truncated Bid are discussed more generally in the context of apoptosis and necrosis.

Structural diversity of mitochondria: functional implications

Mitochondria display considerable structural diversity particularly in terms of the folding of the energy-transducing inner membrane. The hypothesis is forwarded that the topology of the mitochondrial inner membrane is not random but rather is a regulated cell parameter. Its effects on internal diffusion of metabolites and soluble proteins can influence such mitochondrial processes as ATP production and protein release during apoptosis. Progress toward understanding the factors that control mitochondrial inner-membrane curvature and dynamics (fusion and fission) is summarized, with a focus on remodeling events associated with apoptosis and oxidative stress.

Cardiolipin provides an essential activating platform for caspase-8 on mitochondria

Cardiolipin is a mitochondria-specific phospholipid known to be intimately involved with apoptosis. However, the lack of appropriate cellular models to date restricted analysis of its role in cell death. The maturation of cardiolipin requires the transacylase tafazzin, which is mutated in the human disorder Barth syndrome. Using Barth syndrome patient-derived cells and HeLa cells in which tafazzin was knocked down, we show that cardiolipin is required for apoptosis in the type II mitochondria-dependent response to Fas stimulation. Cardiolipin provides an anchor and activating platform for caspase-8 translocation to, and embedding in, the mitochondrial membrane, where it oligomerizes and is further activated, steps that are necessary for an efficient type II apoptotic response.

Cytomics and nanobioengineering

The finding that an individual's genome differs as much as by many million variants from that of the human reference assembly diminished the great enthusiasm that every disease could be predicted based on nucleotide polymorphisms. Even individual cells of an organ may be specifically equipped to perform specific tasks and that the information of individual cells in a cell system is key information to understand function or dysfunction. Therefore, cytomics received great attention during the last years as it allows to quantitatively and qualitatively analyzing great number of individual cells, cell constituents, and of their intracellular and functional interactions in a cellular system and also giving the concept of analysis of these data.Exhaustive data extraction from multiparametric assays and multiple tests are the prerequisite for prediction of drug toxicity. Cytomics, as novel approach for unsupervised data analysis give a chance to find the most predictive parameters, which describe best the toxicity of a chemical. Cytomics is intrinsically connected to drug development and drug discovery.Focused on small structures, nanobioengineering is the ideal partner of cytomics, the systems biological discipline for cell population analysis. Realizing the idea "from the molecule to the patient" develops and offers chemical compounds, proteins, and other biomolecules, cells as well as tissues as instruments and products for a wide variety of biotechnological and biomedical applications.The integrative nanobioengineering combining different disciplines of nanotechnology will promote the development of innovative therapies and diagnostic methods. It can improve the precision of the measurements with focus on single cell analysis. By nanobioengineering and whole body imaging techniques, cytomics covers the field from molecules through bacterial cells, eukaryotic tissues, and organs to small animal live analysis. Toxicological testing and medical drug development are currently strongly broadening. It harbors the promise to substantially impact on various fields of biomedicine, drug discovery, and predictive medicine.As the number of scientific data is rising exponentially, new data analysis tools and strategies like cytomics and nanobioengineering take a lead and get closer to application. Bionanoengineering may strongly support the quantitative data supply, thus strengthening the rational for cytomics approach.

Regulation of antiapoptotic MCL-1 function by gossypol: mechanistic insights from in vitro reconstituted systems

Small-molecule drugs that induce apoptosis in tumor cells by activation of the BCL-2-regulated mitochondrial outer membrane permeabilization (MOMP) pathway hold promise for rational anticancer therapies. Accumulating evidence indicates that the natural product gossypol and its derivatives can kill tumor cells by targeting antiapoptotic BCL-2 family members in such a manner as to trigger MOMP. However, due to the inherent complexity of the cellular apoptotic network, the precise mechanisms by which interactions between gossypol and individual BCL-2 family members lead to MOMP remain poorly understood. Here, we used simplified systems bearing physiological relevance to examine the impact of gossypol on the function of MCL-1, a key determinant for survival of various human malignancies that has become a highly attractive target for anticancer drug design. First, using a reconstituted liposomal system that recapitulates basic aspects of the BCL-2-regulated MOMP pathway, we demonstrate that MCL-1 inhibits BAX permeabilizing function via a "dual-interaction" mechanism, while submicromolar concentrations of gossypol reverse MCL-1-mediated inhibition of functional BAX activation. Solution-based studies showed that gossypol competes with BAX/BID BH3 ligands for binding to MCL-1 hydrophobic groove, thereby providing with a mechanistic explanation for how gossypol restores BAX permeabilizing function in the presence of MCL-1. By contrast, no evidence was found indicating that gossypol transforms MCL-1 into a BAX-like pore-forming molecule. Altogether, our findings validate MCL-1 as a direct target of gossypol, and highlight that making this antiapoptotic protein unable to inhibit BAX-driven MOMP may represent one important mechanism by which gossypol exerts its cytotoxic effect in selected cancer cells.

Compromised proteasome degradation elevates neuronal nitric oxide synthase levels and induces apoptotic cell death

The significance of impairment of proteasome activity in PC12 cells was examined in connection with nitrative/nitrosative stress and apoptotic cell death. Treatment of differentiated PC12 cells with MG132, a proteasome inhibitor, elicited a dose- and time-dependent increase in neuronal nitric oxide synthase (nNOS) protein levels, decreased cell viability, and increased cytotoxicity. Viability and cytotoxicity were ameliorated by L-NAME (a broad NOS inhibitor). Nitric oxide/peroxynitrite formation was increased upon treatment of PC12 cells with MG132 and decreased upon treatment with the combination of MG132 and 7-NI (a specific inhibitor of nNOS). The decreases in cell viability appeared to be effected by an activation of JNK and its effect on mitochondrial Bcl-x(L) phosphorylation. These effects are strengthened by the activation of caspase-9 along with increased caspase-3 activity upon treatment of PC12 cells with MG132. These results suggest that impairment of proteasome activity and consequent increases in nNOS levels lead to a nitrative stress that involves the coordinated response of JNK cytosolic signaling and mitochondrion-driven apoptotic pathways.

Cysteine 62 of Bax is critical for its conformational activation and its proapoptotic activity in response to H2O2-induced apoptosis

Bax is activated and translocated onto mitochondria to mediate cytochrome c release and apoptosis. The molecular mechanisms of Bax activation during apoptosis remain a subject of debate. We addressed the question of whether reactive oxygen species could directly activate Bax for its subsequent translocation and apoptosis. Using the SW480 human colon adenocarcinoma cell line stably expressing Bax fused to GFP, we showed that H2O2 induces Bax conformational change, mitochondrial translocation, and subsequent oligomerization at mitochondria. We found that H2O2-induced Bax activation is dependent on the conserved cysteine residue 62 of Bax. Mutation of cysteine 62, but not cysteine 126, to serine or alanine abolished its activation by H2O2 but not other death stimuli, both in SW480 and Bax-deficient HCT116 cells, whereas wild type Bax sensitizes these cells to apoptosis. Cysteines of Bax could chemically react with H2O2. Mutation of Bax BH3 domain in the presence of cysteine 62 also abolished Bax proapoptotic activity. We conclude that reactive oxygen species could be a direct signal for Bax activation by reacting with cysteine residues. Our results identify a critical role of cysteine 62 in oxidative stress-induced Bax activation and subsequent apoptosis.

Evaluating cytochrome c diffusion in the intermembrane spaces of mitochondria during cytochrome c release

Truncated Bid (tBid) releases cytochrome c from mitochondria by inducing Bak (and Bax) pore formation in the outer membrane. An important issue is whether a second tBid action, independent of Bak and Bax, is also required to enhance cytochrome c mobility in the intermembrane spaces. To investigate this, we developed a kinetic analysis enabling changes in the diffusibility of cytochrome c in the intermembrane spaces of isolated mitochondria to be differentiated from changes resulting from Bak activation. Cytochrome c diffusibility in the intermembrane spaces was unaffected by changes in [tBid] over the range 0.5-19.0 pmol per mg of mitochondrial protein, when tBid-dependent Bak activation was increased several-thousand fold. However, high [tBid] (100 pmol mg(-1)) did increase diffusibility by approximately twofold. This was attributable to the permeability transition. Basal cytochrome c diffusibility in the intermembrane spaces in the absence of tBid was determined to be approximately 0.2 minute(-1), which is sufficient to support cytochrome c release with a half-time of 3.4 minutes. It is concluded that tBid has a monofunctional action at low concentrations and, more generally, that the basal cytochrome c diffusibility in the intermembrane spaces is adequate for rapid and complete cytochrome c release irrespective of the mode of outer membrane permeabilisation.

Mitochondrial carriers and pores: key regulators of the mitochondrial apoptotic program?

Mitochondria play a pivotal role in the process of apoptosis. Alterations in mitochondrial structure and function during apoptosis are regulated by proteins of the BCL-2 family, however their exact mechanism of action is largely unknown. Mitochondrial carriers and pores play an essential role in maintaining the normal function of mitochondria, and BCL-2 family members were shown to interact with several mitochondrial carriers/pores and to affect their function. This review focuses on the involvement of several of these mitochondrial carriers/pores in the regulation of the mitochondrial death pathway.

Cardiolipin: setting the beat of apoptosis

Cardiolipin (CL) is a mitochondria-specific phospholipid which is known to be intimately linked with the mitochondrial bioenergetic machinery. Accumulating evidence now suggests that this unique lipid also has active roles in several of the mitochondria-dependent steps of apoptosis. CL is closely associated with cytochrome c at the outer leaflet of the mitochondrial inner membrane. This interaction makes the process of cytochrome c release from mitochondria more complex than previously assumed, requiring more than pore formation in the mitochondrial outer membrane. While CL peroxidation could be crucial for enabling cytochrome c dissociation from the mitochondrial inner membrane, cytochrome c itself catalyzes CL peroxidation. Moreover, peroxy-CL directly activates the release of cytochrome c and other apoptogenic factors from the mitochondria. CL is also directly involved in mitochondrial outer membrane permeabilization by enabling docking and activation of pro-apoptotic Bcl-2 proteins. It appears therefore that CL has multiple roles in apoptosis and that CL metabolism contributes to the complexity of the apoptotic process.

Anti-apoptotic Bcl-2 Family Proteins Disassemble Ceramide Channels

Early in mitochondria-mediated apoptosis, the mitochondrial outer membrane becomes permeable to proteins that, when released into the cytosol, initiate the execution phase of apoptosis. Proteins in the Bcl-2 family regulate this permeabilization, but the molecular composition of the mitochondrial outer membrane pore is under debate. We reported previously that at physiologically relevant levels, ceramides form stable channels in mitochondrial outer membranes capable of passing the largest proteins known to exit mitochondria during apoptosis (Siskind, L. J., Kolesnick, R. N., and Colombini, M. (2006) Mitochondrion 6, 118-125). Here we show that Bcl-2 proteins are not required for ceramide to form protein-permeable channels in mitochondrial outer membranes. However, both recombinant human Bcl-x(L) and CED-9, the Caenorhabditis elegans Bcl-2 homologue, disassemble ceramide channels in the mitochondrial outer membranes of isolated mitochondria from rat liver and yeast. Importantly, Bcl-x L and CED-9 disassemble ceramide channels in the defined system of solvent-free planar phospholipid membranes. Thus, ceramide channel disassembly likely results from direct interaction with these anti-apoptotic proteins. Mutants of Bcl-x L act on ceramide channels as expected from their ability to be anti-apoptotic. Thus, ceramide channels may be one mechanism for releasing pro-apoptotic proteins from mitochondria during the induction phase of apoptosis.

Analysis of HIV Protease Killing Through Caspase 8 Reveals a Novel Interaction Between Caspase 8 and Mitochondria

Human Immunodeficiency Virus (HIV) protease initiates apoptosis of HIV-infected cells by proteolytic cleavage of procaspase 8, creating a novel peptide termed casp8p41. Expression of casp8p41 alone is sufficient to initiate caspase-dependent cell death associated with mitochondrial depolarization. Since casp8p41 does not contain the catalytic cysteine at position 360, the mechanism by which casp8p41 initiates apoptosis is unclear. We demonstrate that casp8p41 directly causes mitochondrial depolarization and release of cytochrome c with downstream caspase 9 activation. Moreover, death induced by casp8p41 requires the presence of mitochondria, and in intact cells, casp8p41 colocalizes with mitochondria. These results illuminate a novel mechanism of cell death induced by a caspase 8 cleavage fragment whereby mitochondrial interaction leads to depolarization and cytochrome c release.

Silencing of prion protein sensitizes breast adriamycin-resistant carcinoma cells to TRAIL-mediated cell death

We investigated the relationship between the resistance to the proapoptotic action of tumor necrosis factor-related apoptosis inducing ligand (TRAIL) and cellular prion protein (PrPc) function, using the TRAIL-sensitive MCF-7 human breast adenocarcinoma cell line and two TRAIL-resistant sublines: 2101 and MCF-7/ADR. All of the cell lines tested expressed TRAIL-R1 and TRAIL-R2. TRAIL decoy receptors were not detected, suggesting that the resistance of 2101 and MCF-7/ADR cells, strongly expressing PrPc, to TRAIL-mediated cell death was independent from the expression of TRAIL receptors and death-inducing signaling complex formation. Down-regulation of PrPc by small interfering RNA increased the sensitivity of Adriamycin- and TRAIL-resistant cells to TRAIL, but not to epirubicin/Adriamycin. TRAIL-mediated apoptosis in PrPc knocked-down cells was associated with caspase processing, Bid cleavage, and Mcl-1 degradation. In addition, an increased sensitivity of apoptosis-resistant cells to TRAIL after PrPc silencing was not associated with the increased recruitment of receptors and intracellular signaling molecule to the death-inducing signaling complex. Bcl-2 expression was substantially decreased after PrPc knock-down but the levels of Bcl-X(L) and Mcl-1 were not affected. The down-regulation of Bcl-2 was concomitant with Bax delocalization. Our findings support the notion that silencing of PrPc facilitates the activation of proapoptotic Bax by down-regulation of Bcl-2 expression, thereby abolishing the resistance of breast cancer cells to TRAIL-induced apoptosis.

Phosphorylation of mitochondrial phospholipid scramblase 3 by protein kinase C-delta induces its activation and facilitates mitochondrial targeting of tBid

Phospholipid scramblase 3 (PLS3) is a member of the phospholipid scramblase family present in mitochondria. PLS3 plays an important role in regulation of mitochondrial morphology, respiratory function, and apoptotic responses. PLS3 is phosphorylated by PKC-delta at Thr21 and is the mitochondrial target of PKC-delta-induced apoptosis. Cells with overexpression of PLS3, but not the phosphoinhibitory mutant PLS3(T21A), are more susceptible to apoptosis induced by AD198, an extranuclear targeted anthracycline that activates PKC-delta. Here we report that the phosphomimetic mutant of PLS3(T21D) by itself can induce apoptosis in HeLa cells. Using proteoliposomes with addition of pyrene-labeled phosphatidylcholine (PC) at the outer leaflet, we measured the lipid flip-flop activity of PLS3 and its phosphorylation mutant. PLS3(T21D) is more potent than wild-type PLS3 or PLS3(T21A) to transfer pyrene-PC from the outer leaflet to the inner leaflet of liposomes. Based on our previous finding that PLS3 enhances tBid-induced mitochondrial damages, we tested the hypothesis that PLS3 enhances cardiolipin translocation to mitochondrial surface and facilitates tBid targeting. Fluorescein-labeled tBid(G94E) was used as a probe to quantify cardiolipin on the surface of mitochondria. Mitochondria from cells treated with AD198 or cells expressing PLS3(T21D) had a higher level of tBid-binding capacity than control cells or cells expressing wild-type PLS3. These findings indicate that phosphorylation of PLS3 by PKC-delta induces PLS3 activation to facilitate mitochondrial targeting of tBid and apoptosis.