Bcl-2 targeted to the endoplasmic reticulum can inhibit apoptosis induced by Myc but not etoposide in Rat-1 fibroblasts

The C-terminal sequences of Bcl-2 family proteins mediate interactions that regulate cell death

Programmed cell death via the both intrinsic and extrinsic pathways is regulated by interactions of the Bcl-2 family protein members that determine whether the cell commits to apoptosis via mitochondrial outer membrane permeabilization (MOMP). Recently the conserved C-terminal sequences (CTSs) that mediate localization of Bcl-2 family proteins to intracellular membranes, have been shown to have additional protein-protein binding functions that contribute to the functions of these proteins in regulating MOMP. Here we review the pivotal role of CTSs in Bcl-2 family interactions including: (1) homotypic interactions between the pro-apoptotic executioner proteins that cause MOMP, (2) heterotypic interactions between pro-apoptotic and anti-apoptotic proteins that prevent MOMP, and (3) heterotypic interactions between the pro-apoptotic executioner proteins and the pro-apoptotic direct activator proteins that promote MOMP.

HIV-1 Nef counteracts autophagy restriction by enhancing the association between BECN1 and its inhibitor BCL2 in a PRKN-dependent manner

Macroautophagy/autophagy is an auto-digestive pro-survival pathway activated in response to stress to target cargo for lysosomal degradation. In recent years, autophagy has become prominent as an innate antiviral defense mechanism through multiple processes, such as targeting virions and viral components for elimination. These exciting findings have encouraged studies on the ability of autophagy to restrict HIV. However, the role of autophagy in HIV infection remains unclear. Whereas some reports indicate that autophagy is detrimental for HIV, others have claimed that HIV deliberately activates this pathway to increase its infectivity. Moreover, these contrasting findings seem to depend on the cell type investigated. Here, we show that autophagy poses a hurdle for HIV replication, significantly reducing virion production. However, HIV-1 uses its accessory protein Nef to counteract this restriction. Previous studies have indicated that Nef affects autophagy maturation by preventing the fusion between autophagosomes and lysosomes. Here, we uncover that Nef additionally blocks autophagy initiation by enhancing the association between BECN1 and its inhibitor BCL2, and this activity depends on the cellular E3 ligase PRKN. Remarkably, the ability of Nef to counteract the autophagy block is more frequently observed in pandemic HIV-1 and its simian precursor SIVcpz infecting chimpanzees than in HIV-2 and its precursor SIVsmm infecting sooty mangabeys. In summary, our findings demonstrate that HIV-1 is susceptible to autophagy restriction and define Nef as the primary autophagy antagonist of this antiviral process.Abbreviations: 3-MA: 3-methyladenine; ACTB: actin, beta; ATG16L1: autophagy related 16 like 1; BCL2: bcl2 apoptosis regulator; BECN1: beclin 1; cDNA: complementary DNA; EGFP: enhanced green fluorescence protein; ER: endoplasmic reticulum; Gag/p55: group-specific antigen; GFP: green fluorescence protein; GST: glutathione S transferase; HA: hemagglutinin; HIV: human immunodeficiency virus; IP: immunoprecipitation; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; Nef: negative factor; PRKN: parkin RBR E3 ubiquitin ligase; PtdIns3K: phosphatidylinositol 3 kinase; PtdIns3P: phosphatidylinositol 3 phosphate; PTM: post-translational modification; RT-qPCR: reverse transcription followed by quantitative PCR; RUBCN: rubicon autophagy regulator; SEM: standard error of the mean; SERINC3: serine incorporator 3; SERINC5: serine incorporator 5; SIV: simian immunodeficiency virus; SQSTM1/p62: sequestosome 1; TFEB: transcription factor EB; UVRAG: UV radiation resistance associated gene; VSV: vesicular stomatitis virus; ZFYVE1/DFCP1: zinc finger FYVE-type containing 1.

Bcl-2 regulates store-operated Ca2+ entry to modulate ER stress-induced apoptosis

Ca²⁺ plays a significant role in linking the induction of apoptosis. The key anti-apoptotic protein, Bcl-2, has been reported to regulate the movement of Ca²⁺ across the ER membrane, but the exact effect of Bcl-2 on Ca²⁺ levels remains controversial. Store-operated Ca²⁺ entry (SOCE), a major mode of Ca²⁺ uptake in non-excitable cells, is activated by depletion of Ca²⁺ in the ER. Depletion of Ca²⁺ in the ER causes translocation of the SOC channel activator, STIM1, to the plasma membrane. Thereafter, STIM1 binds to Orai1 or/and TRPC1 channels, forcing them to open and thereby allow Ca²⁺ entry. In addition, several anti-cancer drugs have been reported to induce apoptosis of cancer cells via the SOCE pathway. However, the detailed mechanism underlying the regulation of SOCE by Bcl-2 is not well understood. In this study, a three-amino acid mutation within the Bcl-2 BH1 domain was generated to verify the role of Bcl-2 in Ca²⁺ handling during ER stress. The subcellular localization of the Bcl-2 mutant (mt) is similar to that in the wild-type Bcl-2 (WT) in the ER and mitochondria. We found that mt enhanced thapsigargin and tunicamycin-induced apoptosis through ER stress-mediated apoptosis but not through the death receptor- and mitochondria-dependent apoptosis, while WT prevented thapsigargin- and tunicamycin-induced apoptosis. In addition, mt depleted Ca²⁺ in the ER lumen and also increased the expression of SOCE-related molecules. Therefore, a massive Ca²⁺ influx via SOCE contributed to caspase activation and apoptosis. Furthermore, inhibiting SOCE or chelating either extracellular or intracellular Ca²⁺ inhibited mt-mediated apoptosis. In brief, our results explored the critical role of Bcl-2 in Ca²⁺ homeostasis and the modulation of ER stress.

A systematic assessment of mitochondrial function identified novel signatures for drug-induced mitochondrial disruption in cells

Mitochondrial perturbation has been recognized as a contributing factor to various drug-induced organ toxicities. To address this issue, we developed a high-throughput flow cytometry-based mitochondrial signaling assay to systematically investigate mitochondrial/cellular parameters known to be directly impacted by mitochondrial dysfunction: mitochondrial membrane potential (MMP), mitochondrial reactive oxygen species (ROS), intracellular reduced glutathione (GSH) level, and cell viability. Modulation of these parameters by a training set of compounds, comprised of established mitochondrial poisons and 60 marketed drugs (30 nM to 1mM), was tested in HL-60 cells (a human pro-myelocytic leukemia cell line) cultured in either glucose-supplemented (GSM) or glucose-free (containing galactose/glutamine; GFM) RPMI-1640 media. Post-hoc bio-informatic analyses of IC50 or EC50 values for all parameters tested revealed that MMP depolarization in HL-60 cells cultured in GSM was the most reliable parameter for determining mitochondrial dysfunction in these cells. Disruptors of mitochondrial function depolarized MMP at concentrations lower than those that caused loss of cell viability, especially in cells cultured in GSM; cellular GSH levels correlated more closely to loss of viability in vitro. Some mitochondrial respiratory chain inhibitors increased mitochondrial ROS generation; however, measuring an increase in ROS alone was not sufficient to identify mitochondrial disruptors. Furthermore, hierarchical cluster analysis of all measured parameters provided confirmation that MMP depletion, without loss of cell viability, was the key signature for identifying mitochondrial disruptors. Subsequent classification of compounds based on ratios of IC50s of cell viability:MMP determined that this parameter is the most critical indicator of mitochondrial health in cells and provides a powerful tool to predict whether novel small molecule entities possess this liability.

Relationships of diverse apoptotic death process patterns to mitochondrial membrane potential (Δψ(m)) evaluated by three-parameter flow cytometric analysis

Recently, it has been proposed that novel methodologies are needed to re-evaluate apoptotic cell death, as studies of apoptosis have shown it to be a complex process. Since mitochondria are key regulators in cell death pathways, we developed a simultaneous 3-parameter flow cytometric analysis that incorporates the change in mitochondrial membrane potential (Δψ_m) in an Annexin-V [for phosphatidyl-serine (PS)] and propidium iodide (PI) assay system (3 parameters with 4 colours), and evaluated the apoptotic process using various haematological malignant cell lines and death triggers. The present method enabled visualization of cell composition during apoptosis and captured complicated molecular events. For example, apoptotic cells that lost Δψ_m did not always externalize PS, while some late apoptotic cells had polarized Δψ_m. The findings of unchanged PS-externalization and aberrant cell death suggest that there is no relationship of PS externalization and apoptosis with an unknown apoptotic mechanism. Based on PS-externalization, sensitivity to staurosporine, and the combination of cell lines and triggers, the apoptotic process was classified into 2 types. Importantly, most of our findings could not be observed by PS–PI and Δψ_m assays when independently performed. Our method may be useful for examining mitochondrial-related apoptosis and death signalling pathways, as well as screening novel apoptosis-inducing cancer drugs.

Proximal events in 7,12-dimethylbenz[a]anthracene-induced, stromal cell-dependent bone marrow B cell apoptosis: stromal cell-B cell communication and apoptosis signaling

Intercellular communication is an essential process in stimulating lymphocyte development and in activating and shaping an immune response. B cell development requires cell-to-cell contact with and cytokine production by bone marrow stromal cells. However, this intimate relationship also may be responsible for the transfer of death-inducing molecules to the B cells. 7,12-Dimethylbenz[a]anthracene (DMBA), a prototypical polycyclic aromatic hydrocarbon, activates caspase-3 in pro/pre-B cells in a bone marrow stromal cell-dependent manner, resulting in apoptosis. These studies were designed to examine the hypothesis that an intrinsic apoptotic pathway is activated by DMBA and that the ultimate death signal is a DMBA metabolite generated by the stromal cells and transferred to the B cells. Although a loss of mitochondrial membrane potential did not occur in the DMBA/stromal cell-induced pathway, cytochrome c release was stimulated in B cells. Caspase-9 was activated, and formation of the apoptosome was required to support apoptosis, as demonstrated by the suppression of death in Apaf-1(fog) mutant pro-B cells. Investigation of signaling upstream of the mitochondria demonstrated an essential role for p53. Furthermore, DMBA-3,4-dihydrodiol-1,2-epoxide, a DNA-reactive metabolite of DMBA, was sufficient to upregulate p53, induce caspase-9 cleavage, and initiate B cell apoptosis in the absence of stromal cells, suggesting that production of this metabolite by the stromal cells and transfer to the B cells are proximal events in triggering apoptosis. Indeed, we provide evidence that metabolite transfer from bone marrow stromal cells occurs through membrane exchange, which may represent a novel communication mechanism between developing B cells and stromal cells.

Tauroursodeoxycholic acid inhibits apoptosis induced by Z alpha-1 antitrypsin via inhibition of Bad

Z alpha-1 antitrypsin (AAT) deficiency is a genetic disease associated with accumulation of misfolded AAT in the endoplasmic reticulum (ER) of hepatocytes. ZAAT-expressing cells display ER stress responses including nuclear factor kappaB activation and apoptosis. Using an in vitro model of ZAAT ER accumulation, we investigated the mechanism of ZAAT-mediated ER-induced apoptosis and evaluated methods to inhibit this process. Here we demonstrate that expression of ZAAT, but not normal MAAT, in HEK293 cells leads to cleavage and activation of caspase-4 and induces apoptosis that is characterized by activation of caspase-3 and caspase-7 and DNA fragmentation. Similar effects are also induced using the ER agonist thapsigargin. A caspase-4-specific short interfering RNA (siRNA) does not impair ZAAT-induced caspase-3/7 activation or cell death in these cells. However, inhibition studies performed using tauroursodeoxycholic acid (TUDCA) demonstrate its ability to inhibit caspase-4 and caspase-3/7 activation, mitochondrial cytochrome c release, and caspase-3 cleavage induced by ZAAT and to promote cell survival. The mechanism by which TUDCA (tauroursodeoxycholic acid) promotes cell survival in ZAAT-expressing cells involves phosphorylation and inactivation of the proapoptotic factor Bad. TUDCA is unable to rescue cells from apoptosis or phosphorylate Bad in the presence of LY294002, a selective P-I-3-kinase inhibitor.

CONCLUSION

These data show that caspase-4 is not essential for ZAAT-induced apoptosis in HEK293 cells and implicates P-I-3-kinase and Bad as potential therapeutic targets for the liver disease associated with ZAAT deficiency.

Critical Role for Mitochondrial Oxidative Phosphorylation in the Activation of Tumor Suppressors Bax and Bak

BACKGROUND

Activation of Bax and Bak, which act to permeabilize the mitochondrial membrane, is an essential step in the cell death response and therefore in the suppression of tumorigenesis. However, the mechanisms that regulate activation are poorly understood.

METHODS

Bax and Bak activation (conformational change and dimerization) was monitored in Rat-1 fibroblasts and human cancer cells subjected to endoplasmic reticulum (ER) stress, DNA damage, or tumor necrosis factor-alpha (TNF-alpha) treatment. Pharmacologic inhibitors of reactive oxygen species production, electron transport in the respiratory chain, oxidative phosphorylation, and appropriate controls were used to identify potential modes by which Bax and Bak activation and the cell death response are controlled. The oligomerization state of Bax and Bak was determined by cross-linking and subsequent immunoblot analysis; Bax conformational change was analyzed by immunoprecipitation and immunoblotting with an antibody specific for the active conformation. Cell death was evaluated by dye exclusion.

RESULTS

In both fibroblasts and human cancer cells subjected to cell death stimuli, inhibition of oxidative phosphorylation by use of antimycin A or oligomycin prevented ER stress-, DNA damage-, and TNF-alpha-induced Bax and Bak activation and cell death (UV-induced Rat-1 cell death at 15 hours: control, mean = 33.6%, 95% confidence interval [CI] = 18.8% to 48.4%; antimycin A, mean = 10.0%, 95% CI = 0% to 21.7%; oligomycin, mean = 13.1%, 95% CI = 5.7% to 20.5%; tunicamycin-induced MCF-7 cell death at 9 hours: control, mean = 29.2%, 95% CI = 21.6% to 36.8%; antimycin A, mean = 15.3%, 95% CI = 0.8% to 29.8%; oligomycin, mean = 11.5%, 95% CI = 3.9% to 19.1%; TNF-alpha-induced MCF-7 cell death at 6 hours: control, mean = 24.0%, 95% CI = 12.6% to 35.4%; antimycin A, mean = 8.9%, 95% CI = 3.9% to 13.9%; oligomycin, mean = 13.3%, 95% CI = 10.4% to 16.2%). Increasing and decreasing glycolytic adenosine triphosphate production, by adding glucose and 2-deoxy-D-glucose to the cell growth medium, respectively, neither reversed nor recapitulated, respectively, the effect of compromised oxidative phosphorylation on Bax and Bak activation.

CONCLUSION

Oxidative phosphorylation is required for the activation of Bax and Bak and cell death triggered by disparate death stimuli. The reliance of tumor cells on glycolysis in preference to oxidative phosphorylation even under normoxic conditions (Warburg effect) may therefore be a potential means by which these cells evade programmed cell death.

Bcl-XL is qualitatively different from and ten times more effective than Bcl-2 when expressed in a breast cancer cell line

Background

Bcl-2 and Bcl-XL are anti-apoptotic paralogues that inhibit apoptosis elicited by a wide variety of stimuli, and play critical roles in cancer development and resistance to treatment. Many clinical studies have indicated that expression of these anti-apoptotic proteins in tumours is associated with poor prognosis. It has therefore been assumed that in cells the essential difference between Bcl-2 and Bcl-XL involves regulation of expression and that they are otherwise functionally similar. To examine this issue, we have compared the function of the proteins and of mutants of Bcl-2 and Bcl-XL specifically targeted to different subcellular sites.

Methods

We generated clones of the human breast cancer line MCF-7 stably expressing known amounts of Bcl-2, or Bcl-XL as determined by quantitative immunoblotting. Clones expressing equivalent amounts of wild-type and mutants of Bcl-2 and Bcl-XL with subcellular localization restricted to the cytoplasm, endoplasmic reticulum or outer mitochondrial membrane were studied in both MCF-7 and Rat-1 fibroblasts. In MCF-7 cells we measured the functional activities of these proteins in preventing apoptosis induced by four different agents (doxorubicin, ceramide, thapsigargin, TNF-α). Etoposide and low serum were used to compare the effect of Bcl-2, Bcl-XL and mutants located at the endoplasmic reticulum on induction of apoptosis in fibroblasts.

Results

We noted both qualitative and quantitative differences in the functional activity of these two anti-apoptotic proteins in cells: Bcl-2 localized to the endoplasmic reticulum inhibits apoptosis induced by ceramide and thapsigargin but not by doxorubicin or TNFα, while Bcl-XL at the endoplasmic reticulum is active against all four drugs. In fibroblasts Bcl-2 localized to the ER did not prevent cell death due to etoposide whereas Bcl-XL in the same location did. Finally in MCF-7 cells, Bcl-XL is approximately ten times more active than Bcl-2 in repressing apoptosis induced by doxorubicin. This difference can be manifest as a large difference in clonal survival.

Conclusion

When examined in the same cellular context, Bcl-2 and Bcl-XL differ substantially in the potency with which they inhibit apoptosis, mediated in part by differences in the inhibition of specific subcellular pathways.

Bcl-2 and Ca2+ homeostasis in the endoplasmic reticulum

Recent data have revealed an unexpected role of Bcl-2 in modulating the steady-state levels and agonist-dependent fluxes of Ca(2+) ions. Direct monitoring of endoplasmic reticulum (ER) Ca(2+) concentration with recombinant probes reveals a lower state of filling in Bcl-2-overexpressing cells and a higher leak rate from the organelle. The broader set of indirect data using cytosolic probes reveals a more complex scenario, as in many cases no difference was detected in the Ca(2+) content of the intracellular pools. At the same time, Ca(2+) signals have been shown to affect important checkpoints of the apoptotic process, such as mitochondria, thus tuning the sensitivity of cells to various challenges. In this contribution, we will review (i) the data on the effect of Bcl-2 on [Ca(2+)](er), (ii) the functional significance of the Ca(2+)-signalling alteration and (iii) the current insight into the possible mechanisms of this effect.

Transcription Factors Mediating Interleukin‐3 Survival Signals

Interleukin-3 (IL-3) is one of the major hematopoietic cytokines that regulate the survival of hematopoietic cells of various lineages. Although the mechanism underlying the survival effect of IL-3 has been investigated intensively for more than a decade, our knowledge of the survival-signaling network remains incomplete. Binding of IL-3 to its cognate receptors initiates rapid tyrosine phosphorylation of Janus kinases (JAKs) and of signal transducer and activator of transcription (STAT) proteins, as well as activation of the phosphatidylinositol-3 kinase (PI-3K)/Akt and Ras/Raf/MAPK kinase (MEK)/mitogen-activated protein kinase (MAPK) pathways. These signals culminate in induction of a constellation of antiapoptotic genes and prevent cell death from occurring. Thus IL-3 signaling has substantial effects on kinase activation and gene transcription. Previous articles have summarized the roles of these kinase pathways in cell proliferation and survival. In this chapter, we will focus on the role of several newly characterized transcriptional factors, which are targets of these initial kinase cascades and bridge the gap between kinases and survival effector genes, in transducing the IL-3 survival signal. The biological significance of the existence of these multiple survival-specific transcription pathways will also be discussed.

Bcl-2 mediated inhibition of erucylphosphocholine-induced apoptosis depends on its subcellular localisation

The synthetic phospholipid derivative erucylphosphocholine (ErPC) is a potent inducer of apoptosis in human tumor cell lines. This membrane-targeted drug induces apoptosis independently from death receptor signaling through a mitochondrial pathway that is inhibited by over-expression of Bcl-2. Within the cell, Bcl-2 resides in membranes of mitochondria, endoplasmic reticulum (ER) and the nucleus. However, the importance of its subcellular localisation in distinct organelles for protection against apoptosis is not completely understood. To investigate the impact of Bcl-2 localised at defined subcellular compartments on its protective effects against ErPC-induced apoptosis, Bcl-2 expression was directed to the outer membrane of the mitochondria or the ER of Jurkat T Lymphoma cells, using Bcl-2 mutants with modified membrane anchors. The mitochondrial insertion sequence of ActA directed Bcl-2 to the mitochondria (Bcl-2/MT), the ER-specific sequence of cytochrome b5 to the ER (Bcl-2/ER). Additionally, Jurkat cells expressing wild-type Bcl-2 (Bcl-2/WT) or a transmembrane domain-lacking mutant (Bcl-2/DeltaTM) were employed. While restricted expression of Bcl-2 either at membranes of the mitochondria or the ER strongly interfered with ErPC-induced mitochondrial damage and apoptosis, cytosolic Bcl-2/DeltaTM exhibited only reduced protection. Thus, membrane localisation of Bcl-2 is a prerequisite for substantial protection against ErPC-induced apoptosis. For efficient long-term inhibition of ErPC-induced apoptosis Bcl-2 had to be present in the membranes of both compartments, the ER and the mitochondria. The finding that ER-targeted Bcl-2 interferes with ErPC-induced mitochondrial damage points to an involvement of the ER in apoptosis signaling upstream of the mitochondria and to a crosstalk between both compartments.

Molecular mechanisms of apoptosis in the cells of the immune system in human aging

Aging is associated with progressive decline in immune functions and increased frequency of infections, autoimmunity, and cancer. Among immune functions, a decline in T-cell functions during aging predominates. In this review, I discuss the molecular signaling of three distinct pathways of apoptosis, namely the death receptor pathway, the mitochondrial pathway, and the most recently described endoplasmic reticulum stress pathway, and the relative sensitivity of naive, central memory, and effector memory CD8(+) T-cell subsets to apoptosis. In addition, I review apoptosis, especially via death receptor pathway, in naive and various memory subsets of CD4(+) and CD8(+) T cells (with primary emphasis on CD8(+) naive and memory subsets) in human aging and discuss the role of apoptosis in immune senescence.

Ibuprofen-induced Walker 256 tumor cell death: cytochrome c release from functional mitochondria and enhancement by calcineurin inhibition

The participation of mitochondria in the mechanism of tumor cell death induced by non-steroid anti-inflammatory drugs is uncertain. Here we show that ibuprofen induces death of Walker 256 tumor cells independently on mitochondrial depolarization as estimated by flow cytometry using DioC(6)(3). Oligomycin increased mitochondrial transmembrane potential in both ibuprofen-treated and non-treated cells, indicating that ATP synthesis was sustained during cell death. Cyclosporin A, but not bongkrekic acid, both mitochondrial permeability transition inhibitors, increased the percentage of cell death in the presence of ibuprofen. FK506, a calcineurin inhibitor like cyclosporin A, also increased ibuprofen-induced cell death. Moreover, we showed that cytochrome c was released during ibuprofen-induced cell death. In conclusion, death of Walker 256 tumor cells induced by ibuprofen does not impair mitochondrial function, involves cytochrome c release and is accompanied by a rescue pathway via calcineurin activation.

A mechanism for the proapoptotic activity of ursodeoxycholic acid: effects on Bcl-2 conformation

Ursodeoxycholic acid (UDCA), a relatively nontoxic bile acid, enhanced the apoptotic response of tumor cells to both photosensitizers that cause photodamage to Bcl-2 and to the nonpeptidic Bcl-2/Bcl-x(L) antagonist HA14-1. The latter agent binds to the surface pocket formed by the BH1, BH2 and BH3 domains of Bcl-2 and Bcl-x(L). Fluorescence polarization studies indicated that affinity of HA14-1 for Bcl-2 was enhanced in the presence of UDCA. Moreover, Bcl-2 photodamage was promoted by UDCA using a photosensitizing agent with affinity for the endoplasmic reticulum, a site of Bcl-2 localization. Fluorescence resonance energy transfer (FRET) studies revealed that the proximity of Bcl-2 to a hydrophobic photosensitizing agent embedded in liposomes was enhanced by UDCA. Since photodamage will occur only if a protein is in close contact with a photosensitizing agent, we propose that these findings support the hypothesis that UDCA causes a conformational change in Bcl-2, promoting HA14-1 binding and enhancing affinity for certain membrane-bound photosensitizers.

Caspase-9 plays a marginal role in serum starvation-induced apoptosis

Serum withdrawal represents a potent trigger to induce caspase-dependent apoptosis in a series of cell culture models. In rat 423-cells, caspase-8 and caspase-3 were apparently sufficient to initiate and proceed apoptosis without involving the intrinsic amplification loop via caspase-9. To assess the reasons for this inactivity of an otherwise crucial initiator caspase, we examined the ability for apoptosome assembly in 423-cells. Caspase-9 and Apaf-1 were expressed and cytochrome c released from mitochondria upon serum withdrawal. Although functional apoptosomes were assembled successfully in vitro, caspase-9 processing was found essentially refrained during apoptosis in 423-cells. Cell fractionation experiments revealed that sequestration of caspase-9 to cytoskeletal structures in 423-cells contributed to the observed impairment of apoptosome formation. Altogether, these findings provide evidence that serum starvation-induced apoptosis may occur independently of the intrinsic pathway and that caspase-9 sequestration potentially represents a novel biological antiapoptotic strategy.

Role of AIF in caspase-dependent and caspase-independent cell death

The major challenge in treating cancer is that many tumor cells carry mutations in key apoptotic genes such as p53, Bcl family proteins or those affecting caspase signaling. Such defects render treatment with traditional chemotherapeutic agents ineffective. Many studies have demonstrated the importance of caspase-independent cell death pathways in injury, degenerative diseases and tumor tissue. It is now recognized that in addition to their critical role in the production of cellular energy, mitochondria are also the source of key proapoptotic molecules involved in caspase activation. More recently, it has been discovered that in response to apoptotic stimuli, mitochondria can also release caspase-independent cell death effectors such as AIF and Endonuclease G. In this review, we examine the role of Bcl family proteins and poly(ADP-ribose) polymerase-1 signaling in the regulation of these apoptotic pathways and address the ongoing controversies in this field. Continued study of the mechanisms of apoptosis including caspase-independent death processes are likely to reveal novel therapeutic targets for the treatment of diverse human pathologies including cancer, neurodegenerative diseases and acute injuries such as stroke or myocardial infarction.

Translation initiation factor 4E blocks endoplasmic reticulum-mediated apoptosis

Eukaryotic translation initiation factor 4E (eIF4E) is the mRNA cap-binding protein required for translation of cellular mRNAs utilizing the 5' cap structure. The rate-limiting factor for mRNA recruitment to ribosomes, eIF4E is a major target for regulation of translation by growth factors, hormones, and other extracellular stimuli. When overexpressed, eIF4E exerts profound effects on cell growth and survival, leading to suppression of oncogene-dependent apoptosis, causing malignant transformation and conferring tumors with multiple drug resistance. We found previously that overexpressed eIF4E interdicts the apoptotic pathway induced by growth factor withdrawal and cytotoxic drugs by selectively activating the expression of Bcl-X(L), thus preventing mitochondrial release of cytochrome c. In this study, we examined the impact of ectopic eIF4E expression on apoptosis mediated by the endoplasmic reticulum (ER). Here we show that eIF4E rescued cells from the ER stressors brefeldin A, tunicamycin, thapsigargin, and the Ca(2+) ionophore A23187. In addition, we found that cells rescued from Ca(2+) ionophore-triggered apoptosis did not release calcium from their ER nor did they translocate caspase-12 from the ER to the cytoplasm. These data lend strong support to the concept that eIF4E functions as a pleiotropic regulator of cell viability and that integration of critical organelle-mediated checkpoints for apoptosis can be controlled by the cap-dependent translation apparatus.

The coxsackievirus 2B protein suppresses apoptotic host cell responses by manipulating intracellular Ca2+ homeostasis

Enteroviruses, small cytolytic RNA viruses, confer an antiapoptotic state to infected cells in order to suppress infection-limiting apoptotic host cell responses. This antiapoptotic state also lends protection against cell death induced by metabolic inhibitors like actinomycin D and cycloheximide. The identity of the viral antiapoptotic protein and the underlying mechanism are unknown. Here, we provide evidence that the coxsackievirus 2B protein modulates apoptosis by manipulating intracellular Ca(2+) homeostasis. Using fluorescent Ca(2+) indicators and organelle-targeted aequorins, we demonstrate that ectopic expression of 2B in HeLa cells decreases the Ca(2+) content of both the endoplasmic reticulum and the Golgi, resulting in down-regulation of Ca(2+) signaling between these stores and the mitochondria, and increases the influx of extracellular Ca(2+). In our studies of the physiological importance of the 2B-induced alterations in Ca(2+) signaling, we found that the expression of 2B suppressed caspase activation and apoptotic cell death induced by various stimuli, including actinomycin D and cycloheximide. Mutants of 2B that were defective in reducing the Ca(2+) content of the stores failed to suppress apoptosis. These data implicate a functional role of the perturbation of intracellular Ca(2+) compartmentalization in the enteroviral strategy to suppress intrinsic apoptotic host cell responses. The putative down-regulation of an endoplasmic reticulum-dependent apoptotic pathway is discussed.

Critical upstream signals of cytochrome C release induced by a novel Bcl-2 inhibitor

Cytochrome c release is a central step in the apoptosis induced by many death stimuli. Bcl-2 plays a critical role in controlling this step. In this study, we investigated the upstream mechanism of cytochrome c release induced by ethyl 2-amino-6-bromo-4-(1-cyano-2-ethoxy-2-oxoethyl)-4H-chromene-3-carboxylate (HA14-1), a recently discovered small molecule inhibitor of Bcl-2. HA14-1 was found to induce cytochrome c release from the mitochondria of intact cells but not from isolated mitochondria. Cytochrome c release from isolated mitochondria requires the presence of both HA14-1 and exogenous Ca(2+). This suggests that both mitochondrial and extramitochondrial signals are important. In intact cells, treatment with HA14-1 caused Ca(2+) spike, change in mitochondrial membrane potential (Delta psi(m)) transition, Bax translocation, and reactive oxygen species (ROS) generation prior to cytochrome c release. Pretreatment with either EGTA acetoxymethyl ester or vitamin E resulted in a significant decrease in cytochrome c release and cell death induced by HA14-1. Furthermore pretreatment with RU-360, an inhibitor of the mitochondrial Ca(2+) uniporter, or with EGTA acetoxymethyl ester, but not with vitamin E, prevented the HA14-1-induced Delta psi(m) transition and Bax translocation. This suggests that ROS generation is an event that occurs after the Delta psi(m) transition and Bax translocation. Together these data demonstrate that the Ca(2+) spike, mitochondrial Bcl-2 presensitization, and subsequent Delta psi(m) transition, Bax translocation, and ROS generation are important upstream signals for cytochrome c release upon HA14-1 stimulation. The involvement of endoplasmic reticulum and mitochondrial signals suggests both organelles are crucial for HA14-1-induced apoptosis.

Bcl-xL/Bcl-2 coordinately regulates apoptosis, cell cycle arrest and cell cycle entry

Bcl-x(L) and Bcl-2 inhibit both apoptosis and proliferation. In investigating the relationship between these two functions of Bcl-x(L) and Bcl-2, an analysis of 24 Bcl-x(L) and Bcl-2 mutant alleles, including substitutions at residue Y28 previously reported to selectively abolish the cell cycle activity, showed that cell cycle delay and anti-apoptosis co-segregated in all cases. In determining whether Bcl-2 and Bcl-x(L) act in G(0) or G(1), forward scatter and pyronin Y fluorescence measurements indicated that Bcl-2 and Bcl-x(L) cells arrested more effectively in G(0) than controls, and were delayed in G(0)-G(1) transition. The cell cycle effects of Bcl-2 and Bcl-x(L) were reversed by Bad, a molecule that counters the survival function of Bcl-2 and Bcl-x(L). When control and Bcl-x(L) cells of equivalent size and pyronin Y fluorescence were compared, the kinetics of cell cycle entry were similar, demonstrating that the ability of Bcl-x(L) and Bcl-2 cells to enhance G(0) arrest contributes significantly to cell cycle delay. Our data suggest that cell cycle effects and increased survival both result from intrinsic functions of Bcl-2 and Bcl-x(L).

Synthetic retinoids as inducers of apoptosis in ovarian carcinoma cell lines

Apoptosis is also known as programmed cell death. Apoptosis plays an essential role in maintaining normal tissue and cell physiology in multicellular organisms. Clearance of aberrant or pre-cancerous cells occurs through the induction of apoptosis. It has been reported that many tumors and tumor cell lines have dysfunctional apoptosis signaling, causing these tumors to escape immune monitoring and internal cellular control mechanisms. One potential cause of this dysfunctional apoptosis is the tumor suppressor p53, an important regulator of growth arrest and apoptosis that is mutated in over 50% of all cancers. Retinoids have great potential in the areas of cancer therapy and chemoprevention. While some tumor cells are sensitive to the growth inhibitory effects of natural retinoids such as all-trans-retinoic acid (ATRA), many ovarian tumor cells are not. 6-[3-(1-Admantyl)]-4-hydroxyphenyl]-2-naphthalene carboxylic acid (CD437) and fenretinide N-[4-hydroxyphenyl] retinamide (4-HPR) are conformationally restricted synthetic retinoids that induce growth arrest and apoptosis in both ATRA-sensitive and ATRA-resistant ovarian tumor cell lines. Recently, we have identified the molecular pathways of apoptosis induced by treatment of ovarian carcinoma cells with mutated p53 by CD437 and 4-HPR.

Calcium and apoptosis: facts and hypotheses

Although longstanding experimental evidence has associated alterations of calcium homeostasis to cell death, only in the past few years the role of calcium in the signaling of apoptosis has been extensively investigated. In this review, we will summarize the current knowledge, focusing on (i) the effect of the proteins of the Bcl-2 family on ER Ca2+ levels, (ii) the action of the proteolytic enzymes of apoptosis on the Ca2+ signaling machinery, (iii) the ensuing alterations on the signaling patterns of extracellular stimuli, and (iv) the intracellular targets of 'apoptotic' Ca2+ signals, with special emphasis on the mitochondria and cytosolic Ca2+-dependent enzymes.

Bcl-2 on the endoplasmic reticulum: protecting the mitochondria from a distance

Bcl-2 has been described both as an inhibitor of programmed cell death and as an inhibitor of mitochondrial dysfunction during apoptosis. It is still not clear what biochemical activity of Bcl-2 is responsible for its function, but increasing evidence indicates that a functional activity of Bcl-2 on the endoplasmic reticulum (ER) protects mitochondria under diverse circumstances. Indeed, an emerging hypothesis is that, during apoptosis, the Bcl-2 family regulates ER-to-mitochondrion communication by BH3-only proteins and calcium ions and thereby triggers mitochondrial dysfunction and cell death.

Cell death in irradiated prostate epithelial cells: role of apoptotic and clonogenic cell kill

Dose-escalated conformal radiotherapy is increasingly being used to radically treat prostate cancer with encouraging results and minimal long-term toxicity, yet little is known regarding the response of normal or malignant prostate cells to ionizing radiation (IR). To clarify the basis for cell killing during prostate cancer radiotherapy, we determined the IR-induced expression of several apoptotic- (bax, bcl-2, survivin and PARP) and G1-cell cycle checkpoint- (p53 and p21(WAF1/Cip1)) related proteins, in both normal (PrEC-epithelial and PrSC-stromal) and malignant (LNCaP, DU-145 and PC-3; all epithelial) prostate cells. For these experiments, we chose doses ranging from 2 to 10 Gy, to be representative of the 1.8-2 Gy daily clinical fractions given during curative radiotherapy and the 8-10 Gy single doses given in palliative radiotherapy. We observed that IR-induced bax and p21(WAF1/Cip1) protein expression were attenuated selectively in normal stromal and epithelial cell cultures, yet maintained their p53-dependency in malignant cell lines. For each cell culture, we also determined total apoptotic and overall radiation cell kill using a short-term nuclear morphologic assay and a long-term clonogenic survival assay, respectively. Clonogenic survival, as measured by the surviving fraction at 2 Gy (SF2), ranged from 0.05 (PrEC) to 0.55 (DU-145), suggesting that malignant prostate cells are more radioresistant than normal prostate cells, for this series. IR-induced apoptotic cell kill was minimal (less than 6% cell after a dose of 10 Gy at times of 24-96 h) and was not dose-dependent. Furthermore, apoptotic kill was not correlated with either molecular apoptotic response or clonogenic cell kill. Using a flow cytometric proliferation assay with the PrSC (stromal) and DU-145 (epithelial) representative cultures, we observed that a senescent-like phenotype (SLP) emerges within a sub-population of cells post-irradiation that is non-clonogenic. Terminal growth arrest was dose-responsive at 96 h following irradiation and associated with long-term expression of both p21(WAF1/Cip1) and p16(INK4a) genes. Future strategies for prostate radiotherapy prediction or novel treatments should additionally focus on terminal growth arrest as an important endpoint in prostate cancer therapy.

In vitro hypoxia and excitotoxicity in human brain induce calcineurin-Bcl-2 interactions

Although pathogenesis of neuronal ischemia is incompletely understood, evidence indicates apoptotic neuronal death after ischemia. Bcl-2, an anti-apoptotic and neuroprotective protein, interacts with calcineurin in non-neuronal tissues. Activation of calcineurin, which is abundant in the brain, may play a role in apoptosis. Using co-immunoprecipitation experiments in biopsy-derived, fresh human cortical and hippocampal slices, we examined possible interactions between calcineurin and Bcl-2. Calcineuin-Bcl-2 interactions increased after exposure in vitro to excitotoxic agents and conditions of hypoxia/aglycia. This interaction may shuttle calcineurin to substrates such as the inositol-1,4,5-tris-phosphate receptor because under these experimental conditions interactions between calcineurin and inositol-1,4,5-tris-phosphate receptor also increased. A specific calcineurin inhibitor, FK-520, attenuated insult-induced increases in calcineurin-Bcl-2 interactions and augmented caspase-3 like activity. These data suggest that Bcl-2 modulates neuroprotective effects of calcineurin and that calcineurin inhibitors increase ischemic neuronal damage.

Life span shortening of normal fibroblasts by overexpression of BCL-2: a result of potent increase in cell death

It is well known that BCL-2 protects against cell death by both apoptosis and necrosis. The culture of bcl-2-transfected normal fibroblasts showed a shorter life span by about 12 population doubling levels compared to that of vector transfectants (64 vs 76 population doubling levels, respectively). An MTT assay revealed that BCL-2-overexpressing cells (HCA2/bcl-2) showed more severe growth suppression due to hydrogen peroxide or doxorubicin treatment than vector control cells (HCA2/vector). We observed a significant number of dead cells in the HCA2/bcl-2 culture, but not in the HCA2/vector culture. Other BCL-2 family proteins with both antiapoptotic and proapoptotic activity and other apoptosis-related factors were maintained at similar levels, indicating that overexpression of BCL-2 is the major reason that normal fibroblasts are sensitized to cell death. A broad caspase inhibitor (z-Val-Ala-Asp-fmk) and inhibitors of specific caspases (acetyl-Asp-Glu-Val-Asp-CHO, acetyl-Ile-Glu-Thr-Asp-CHO, and acetyl-Leu-Glu-His-Asp-CHO) suppressed cell death of HCA2/bcl-2 effectively, suggesting involvement of caspase 3-, 8-, and 9-dependent pathways in cell death and that the form of death is apoptosis. Unexpectedly, involvement of active MEK in cell death was shown by the use of its inhibitor, suggesting that crosstalk between BCL-2 and the MAP kinase cascade regulates death as well as life span.

Bcl-2 on the endoplasmic reticulum regulates Bax activity by binding to BH3-only proteins

Bcl-2 family members have been shown to be key mediators of apoptosis as either pro- or anti-apoptotic factors. It is thought that both classes of Bcl-2 family members act at the level of the mitochondria to regulate apoptosis, although the founding anti-apoptotic family member, Bcl-2 is localized to the endoplasmic reticulum (ER), mitochondrial, and nuclear membranes. In order to better understand the effect of Bcl-2 localization on its activity, we have utilized a Bcl-2 mutant that localizes only to the ER membrane, designated Bcl-2Cb5. Bcl-2Cb5 was expressed in MDA-MB-468 cells, which protected against apoptosis induced by the kinase inhibitor, staurosporine. Data presented here show that Bcl-2Cb5 inhibits this process by blocking Bax activation and cytochrome c release. Furthermore, we show that Bcl-2Cb5 can inhibit the activation of a constitutively mitochondrial mutant of Bax, indicating that an intermediate between Bcl-2 on the ER and Bax on the mitochondria must exist. We demonstrate that this intermediate is likely a BH3-only subfamily member. Data presented here show that Bcl-2Cb5 can sequester a constitutively active form of Bad (Bad3A) from the mitochondria and prevent it from activating Bax. These data suggest that Bcl-2 indirectly protects mitochondrial membranes from Bax, via BH3-only proteins.

Mitochondria: releasing power for life and unleashing the machineries of death

The mitochondrion has long been known both as a chemical powerplant and as a cellular compartment housing various biosynthetic pathways. However, studies on the function of mitochondria in apoptotic cell death have revealed a versatility and complexity of these organelles previously unsuspected. The mechanisms proposed for mitochondrial involvement in cell death are diverse and highly controversial. In one model, mitochondria are seen as passive containers that can be made to leak out cytotoxic proteins. In other scenarios, however, certain more or less familiar aspects of mitochondrial physiology, such as oxidative phosphorylation, generation of oxygen radicals, dynamic morphological rearrangements, calcium overload, and permeability transition, are proposed to play crucial roles. In this review, we examine a few promising mechanisms that have been gaining attention recently.

An atypical caspase-independent death pathway for an immunogenic cancer cell line

REGb cell line, a highly immunogenic tumor cell variant isolated from a rat colon cancer, yields regressive tumors when injected into syngeneic hosts. We previously demonstrated that REGb tumor immunogenicity was related to the capacity of releasing dead cells in vivo. Also, in vitro, REGb cell monolayers release dead cells, especially when cultured in serum-free medium. In the current study, we show that the release of dead cells results from an atypical death process associating features of necrosis and apoptosis. In spite of features considered as hallmarks of caspase-dependent apoptosis, including chromatin fragmentation and DNA oligonucleosomal cleavage, caspases are not activated and caspase inhibitors are ineffective to prevent REGb cell death. In contrast with a number of other types of cell death, the spontaneous death of REGb cells in culture depends on de novo protein synthesis as this death is blocked by low doses of the mRNA translation inhibitor cycloheximide. This unusual mode of cell death that associates necrotic and apoptotic features could provide optimal conditions for triggering a specific immune response.

The Bcl2 family: regulators of the cellular life-or-death switch

Tissue homeostasis is regulated by apoptosis, the cell-suicide programme that is executed by proteases called caspases. The Bcl2 family of intracellular proteins is the central regulator of caspase activation, and its opposing factions of anti- and pro-apoptotic members arbitrate the life-or-death decision. Apoptosis is often impaired in cancer and can limit conventional therapy. A better understanding of how the Bcl2 family controls caspase activation should result in new, more effective therapeutic approaches.

MEK kinase 1 induces mitochondrial permeability transition leading to apoptosis independent of cytochrome c release

Induction of apoptosis often converges on the mitochondria to induce permeability transition and release of apoptotic proteins into the cytoplasm resulting in the biochemical and morphological alteration of apoptosis. Activation of a serine threonine kinase MEK kinase 1 (MEKK1) is involved in the induction of apoptosis. Expression of a kinase-inactive MEKK1 blocks genotoxin-induced apoptosis. Upon apoptotic stimulation, MEKK1 is cleaved into a 91-kDa kinase fragment that further induces an apoptotic response. Mutation of a consensus caspase 3 site in MEKK1 prevents its induction of apoptosis. The mechanism of MEKK1-induced apoptosis downstream of its cleavage, however, is unknown. Herein we demonstrate that full-length and cleaved MEKK1 leads to permeability transition in the mitochondria. This permeability transition occurs through opening of the permeability transition (PT) pore. Inhibiting PT pore opening and reactive oxygen species production effectively reduced MEKK1-induced apoptosis. Overexpression of MEKK1, however, failed to release cytochrome c from the mitochondria or activate caspase 9. Since Bcl2 regulates changes in mitochondria and blocks MEKK1-induced apoptosis, we determined that Bcl2 blocks MEKK1-induced apoptosis when targeted to the mitochondria. This occurs downstream of MEKK1 cleavage, since Bcl2 fails to block cleavage of MEKK1. In mouse embryonic fibroblast cells lacking caspase 3, the cleaved but not full-length MEKK1 induces apoptosis and permeability transition in the mitochondria. Overall, this suggests that cleaved MEKK1 leads to permeability transition contributing to MEKK1-induced apoptosis independent of cytochrome c release from the mitochondria.

Bcl-2-dependent modulation of Ca(2+) homeostasis and store-operated channels in prostate cancer cells

Antiapoptotic oncoprotein Bcl-2 has extramitochondrial actions due to its localization on the endoplasmic reticulum (ER); however, the specific mechanisms of such actions remain unclear. Here we show that Bcl-2 overexpression in LNCaP prostate cancer epithelial cells results in downregulation of store-operated Ca(2+) current by decreasing the number of functional channels and inhibiting ER Ca(2+) uptake through a reduction in the expression of calreticulin and SERCA2b, two key proteins controlling ER Ca(2+) content. Furthermore, we demonstrate that Ca(2+) store depletion by itself is not sufficient to induce apoptosis in Bcl-2 overexpressing cells, and that sustained Ca(2+) entry via activated store-operated channels (SOCs) is required as well. Our data therefore suggest the pivotal role of SOCs in apoptosis and cancer progression.

A role for calcium in Bcl-2 action?

Changes in the cytosolic Ca(2+) concentration ([Ca(2+)](c)) translate a variety of extracellular signals into widely diverse intracellular effects, ranging from secretion to movement, proliferation and also cell death. As regards the last one, it has long been known that large [Ca(2+)](c) increases lead cells to death. More recently, experimental evidence has been obtained that the oncogene Bcl-2 reduces the state of filling of intracellular Ca(2+) stores and thus affects the Ca(2+) responses induced by physiological and pathological stimuli. In this contribution, we will discuss this effect and its significance for the mechanism of action of Bcl-2, an important checkpoint of the apoptotic process.

Mechanism of tert-butylhydroperoxide induced apoptosis in rat hepatocytes: involvement of mitochondria and endoplasmic reticulum

The purpose of the present work was to study the mechanisms involved in apoptosis induced by oxidative stress in rat hepatocytes. We focused on the apoptotic signaling molecules cytochrome c, Bcl-2 and Bax. Rat hepatocytes were exposed for 1 h to increasing concentrations of tert-butylhydroperoxide (t-BHP). Using lactate dehydrogenase (LDH) leakage as a biomarker for necrosis, and DNA fragmentation as a biomarker for apoptosis, we observed that a concentration of t-BHP of 0.4-0.5 mM provides a transition point below which apoptosis is favored and beyond which necrosis is favored. Malondialdehyde and 8-oxo-guanine formation indicates that t-BHP induces oxidative stress and damage. However, at 0.4 mM t-BHP, these oxidative molecular changes as well as LDH leakage no longer progress after the first hour of t-BHP exposure, suggesting the activation of some defense mechanisms. Western blot analysis of cytochrome c shows that its level increases in the cytosol while that of Bax decreases in this fraction as a result of t-BHP treatment. Moreover, there is a loss of Bcl-2 from mitochondria while, in contrast, Bax accumulates in this organelle following t-BHP treatment. However, cytochrome c appears to be relocalized to the endoplasmic reticulum as its presence in microsomes is greatly enhanced. We suggest that t-BHP triggers apoptosis through a step that involves cytochrome c release from mitochondria. This event is stimulated by Bcl-2 disappearance from mitochondria and Bax recruitment. Neutralization of excess cytosolic cytochrome c is achieved by its relocalization to the endoplasmic reticulum, hence triggering the down-regulation of apoptotic signals.

Wild-type, mitochondrial and ER-restricted Bcl-2 inhibit DNA damage-induced apoptosis but do not affect death receptor-induced apoptosis

The proto-oncogene Bcl-2 is expressed in membranes of mitochondria and endoplasmic reticulum and mediates resistance against a broad range of apoptotic stimuli. Although several mechanisms of Bcl-2 action have been proposed, its role in different cellular organelles remains elusive. Here, we analyzed the function of Bcl-2 targeted specifically to certain subcellular compartments in Jurkat cells. Bcl-2 expression was restricted to the outer mitochondrial membrane by replacing its membrane anchor with the mitochondrial insertion sequence of ActA (Bcl-2/MT) or the ER-specific sequence of cytochrome b5 (Bcl-2/ER). Additionally, cells expressing wild-type Bcl-2 (Bcl-2/WT) or a transmembrane domain-lacking mutant (Bcl-2/ΔTM) were employed. Apoptosis induced by ionizing radiation or by the death receptors for CD95L or TRAIL was analyzed by determination of the mitochondrial membrane potential (ΔΨm) and activation of different caspases.

Bcl-2/WT and Bcl-2/MT strongly inhibited radiation-induced apoptosis and caspase activation, whereas Bcl-2/ΔTM had completely lost its anti-apoptotic effect. Interestingly, Bcl-2/ER conferred protection against radiation-induced mitochondrial damage and apoptosis similarly to Bcl-2/MT. The finding that ER-targeted Bcl-2 interfered with mitochondrial ΔΨm breakdown and caspase-9 activation indicates the presence of a crosstalk between both organelles in radiation-induced apoptosis. By contrast, Bcl-2 in either subcellular position did not influence CD95- or TRAIL-mediated apoptosis.

Abeta(1-42) and aluminum induce stress in the endoplasmic reticulum in rabbit hippocampus, involving nuclear translocation of gadd 153 and NF-kappaB

Apoptosis may represent a prominent form of neuronal death in chronic neurodegenerative disorders, such as Alzheimer's disease. Although apoptosis under mitochondrial control has received considerable attention, mechanisms used within the endoplasmic reticulum (ER) and nucleus in mediating apoptotic signals are not well understood. A growing body of evidence is emerging from different studies which suggests an active role for the ER in regulating apoptosis. Disturbances of ER function have been shown to trigger two different apoptotic pathways; one involves cross-talk with mitochondria and is regulated by the antiapoptotic Bcl-2, and the second is characterized by the activation of caspase-12. Also, stress in the ER has been suggested to result in the activation of a number of proteins, such as gadd 153 and NF-kappa, and in the downregulation of the antiapoptotic protein, Bcl-2. In the present study, the intracisternal injection in aged rabbits of either the neurotoxin aluminum maltolate or of Abeta(1-42), has been found to induce nuclear translocation of gadd 153 and the inducible transcription factor, NF-kappaB. Translocation of these two proteins is accompanied by decreased levels of Bcl-2 in both the ER and the nucleus. Aluminum maltolate, but not Abeta, induces caspase-12 activation which is a mediator of ER-specific apoptosis; this is the first report of the in vivo activation of caspase-12. These findings indicate that the ER may play a role in regulating apoptosis in vivo, and could be of significance in the pathology of neurodegeneration and related disorders.

Transient expression of wild-type or mitochondrially targeted Bcl-2 induces apoptosis, whereas transient expression of endoplasmic reticulum-targeted Bcl-2 is protective against Bax-induced cell death

Bcl-2 protein family members function either to promote or inhibit programmed cell death. Bcl-2, typically an inhibitor of apoptosis, has also been demonstrated to have pro-apoptotic activity (Cheng, E. H., Kirsch, D. G., Clem, R. J., et al. (1997) Science 278, 1966-1968). The pro-apoptotic activity has been attributed to the cleavage of Bcl-2 by caspase-3, which converts Bcl-2 to a pro-apoptotic molecule. Bcl-2 is a membrane protein that is localized in the endoplasmic reticulum (ER) membrane, the outer mitochondrial membrane, and the nuclear envelope. Here, we demonstrate that transient expression of Bcl-2 at levels comparable to those found in stably transfected cells induces apoptosis in human embryonic kidney 293 cells and in the human breast cell line MDA-MB-468 cells. Furthermore, we have targeted Bcl-2 specifically to either the ER or the outer mitochondrial membrane to test whether induction of apoptosis by Bcl-2 is dependent upon its localization within either of these membranes. Our findings indicate that Bcl-2 specifically targeted to the mitochondria induces cell death, whereas Bcl-2 that is targeted to the ER does not. The expression of Bcl-2 does result in its cleavage to a 20-kDa protein; however, mutation of the caspase-3 cleavage site (D34A) does not inhibit its ability to induce cell death. Additionally, we find that transiently expressed ER-targeted Bcl-2 inhibits cell death induced by Bax overexpression. In conclusion, the ability of Bcl-2 to promote apoptosis is associated with its localization at the mitochondria. Furthermore, the ability of ER-targeted Bcl-2 to protect against Bax-induced apoptosis suggests that the ER localization of Bcl-2 may play an important role in its protective function.

Bcl-2 increases emptying of endoplasmic reticulum Ca2+ stores during photodynamic therapy-induced apoptosis

Photodynamic therapy (PDT) is clinically approved for the treatment of several types of cancer as well as age-related macular degeneration, the leading cause of blindness in the elderly. PDT using the photosensitizer verteporfin has been previously shown to induce rapid apoptosis via a mitochondrial-caspase activation pathway. The impact of PDT on other cellular organelles such as the endoplasmic reticulum (ER) is undefined. The effect of PDT on intracellular Ca2+ ([Ca2+]i) in control and Bcl-2-overexpressing HeLa cells was assessed. A greater [Ca2+]i transient was observed for Bcl-2 overexpressing cells in response to PDT. The PDT-induced Ca2+ release was due to the emptying of Ca2+ from ER and possibly mitochondrial stores and was not due to an influx of Ca2+ from the medium. For Bcl-2-transfected cells, the release of Ca2+ was incomplete as determined by a further [Ca2+]i transient produced by the addition of the Ca2+ ionophore ionomycin after PDT. Furthermore, extrusion of Ca2+ was not hindered while ER-mediated sequestration of Ca2+ was impaired after PDT. Impairment of ER-mediated sequestration of Ca2+ may be due to the immediate caspase-independent depletion of sarco/endoplasmic reticulum Ca2+ ATPase-2 (SERCA2) that occurred in response to PDT in birth HeLa/Neo and Bcl-2 overexpressed HeLa cells. In summary, PDT induced the rapid degradation of SERCA2 and release of ER and mitochondrial Ca2+ stores. Although overexpression of Bcl-2 did not protect against SERCA2 degradation, it may influence the release of Ca2+ from ER and mitochondrial stores in PDT-treated cells.

GDNF protects against aluminum-induced apoptosis in rabbits by upregulating Bcl-2 and Bcl-XL and inhibiting mitochondrial Bax translocation

Direct (intracisternal) injection of aluminum complexes into rabbit brain results in a number of similarities with the neuropathological and biochemical changes observed in Alzheimer's disease and provides the opportunity to assess early events in neurodegeneration. This mode of administration induces cytochrome c release from mitochondria, a decrease in Bcl-2 in both mitochondria and endoplasmic reticulum, Bax translocation into mitochondria, activation of caspase-3, and DNA fragmentation. Coadministration of glial cell neuronal-derived factor (GDNF) inhibits these Bcl-2 and Bax changes, upregulates Bcl-XL, and abolishes the caspase-3 activity. Furthermore, treatment with GDNF dramatically inhibits apoptosis, as assessed by the TUNEL technique for detecting DNA damage. Treatment with GDNF may represent a therapeutic strategy to reverse the neuronal death associated with Alzheimer's disease and may exert its effect on apoptosis-regulatory proteins.

Transient expression of the Bcl-2 family member, A1-a, results in nuclear localization and resistance to staurosporine-induced apoptosis

The Bcl-2 family of proteins has been characterized by either anti-apoptotic or pro-apoptotic activity. Insight into how Bcl-2 family members function has been gained by determining their intracellular localization. We have generated a monoclonal anti-A1-a antibody and used a COS-7 overexpression system to study the localization of the murine anti-apoptotic Bcl-2 family member, A1-a. A1-a overexpressed in COS-7 cells localized to the nucleus as determined by subcellular fractionation and immunofluorescent microscopy. A1-a in the COS-7 nucleus bound tightly to the nuclear matrix as evidenced by resistance to treatment with DNAse I and RNAse A and sequential extraction with 1.0% Triton X-100, 0.15 M NaCl, 0.25 M HCl, 0.5 M Tris pH 7.4 and 6 M urea. HPLC analysis of A1-a, subsequent to SDS extraction, produced fractions that gave multiple bands when analyzed by Western blot analysis suggesting a propensity to form multimers. COS-7 cells transfected with A1-a were protected from apoptotic induction by staurosporine treatment.

Endoplasmic reticulum localized Bcl-2 prevents apoptosis when redistribution of cytochrome c is a late event

The disruption of mitochondrial function is a key component of apoptosis in most cell types. Localization of Bcl-2 to the outer mitochondrial and endoplasmic reticulum membranes is consistent with a role in the inhibition of many forms of apoptosis. In Rat-1 cells, a Bcl-2 mutant targeted exclusively to the endoplasmic reticulum (Bcl-cb5) was effective at inhibiting apoptosis induced by serum starvation/myc, or ceramide but not apoptosis induced by etoposide. The former conditions cause a decrease in mitochondrial transmembrane potential (Deltapsi(m)) as an early event that precedes the release of cytochrome c from mitochondria. By contrast, when cells are exposed to etoposide, a situation in which cytochrome c release and membrane localization of the pro-apoptotic protein Bax precede loss of Deltapsi(m), wild type Bcl-2 but not Bcl-cb5 prevents apoptosis. Therefore, Bcl-2 functions in spatially distinct pathways of apoptosis distinguished by the order of cytochrome c release and loss of Deltapsi(m).

Dependence of vital cell function on endoplasmic reticulum calcium levels: implications for the mechanisms underlying neuronal cell injury in different pathological states

The endoplasmic reticulum (ER) is a subcellular compartment playing a pivotal role in the control of vital calcium-related cell functions, including calcium storage and signalling. In addition, newly synthesized membrane and secretory proteins are folded and processed in the ER, reactions which are strictly calcium dependent. The ER calcium activity is therefore high, being several orders of magnitude above that of the cytoplasm. Depletion of ER calcium stores causes an accumulation of unfolded proteins in the ER lumen, a pathological situation which induces the activation of two highly conserved stress responses, the ER overload response (EOR) and the unfolded protein response (UPR). EOR triggers activation of the transcription factor NF kappa B, which, in turn, activates the expression of target genes. UPR triggers two downstream processes: it activates the expression of genes coding for ER-resident stress proteins, and it causes a suppression of the initiation of protein synthesis. A similar stress response is activated in pathological states of the brain including cerebral ischaemia, implying common underlying mechanisms. Depending on the extent and duration of the disturbance, an isolated impairment of ER function is sufficient to induce cell injury. In this review, evidence is presented that ER function is indeed disturbed in various diseases of the brain, including acute pathological states (e.g. cerebral ischaemia) and degenerative diseases (e.g. Alzheimer's disease). A body of evidence suggests that disturbances of ER function could be a global pathomechanism underlying neuronal cell injury in various acute and chronic disorders of the central nervous system. If that is true, restoration of ER function or attenuation of secondary disturbances induced by ER dysfunction could present a highly promising new avenue for pharmacological intervention to minimize neuronal cell injury in different pathological states of the brain.

Lovastatin induces a pronounced differentiation response in acute myeloid leukemias

We recently identified HMG-CoA reductase, the rate-limiting enzyme of the mevalonate pathway, as a potential therapeutic target of various retinoic acid responsive cancers. Lovastatin, a competitive inhibitor of HMG-CoA reductase, induced a retinoic acid-like differentiation response followed by extensive apoptosis in neuroblastoma cell lines at relatively low concentrations (<20 microM) of this agent. More recently, we demonstrated that acute myeloid leukemias but not acute lymphocytic leukemias also displayed increased sensitivity to lovastatin-induced apoptosis. In this study, we examined the ability of lovastatin to induce differentiation of acute myeloid leukemic cells and to evaluate the role differentiation may hold in the anti-leukemic properties of this agent. Increased expression of the leukocyte integrins CD11b and CD18 as well as down-regulation of the anti-apoptotic gene bcl-2 are associated with late stage differentiation of the myeloid lineage and retinoic acid induced maturation of acute myeloid leukemic cells. Lovastatin exposure induced increased expression of CD11b and CD18 markers similar to retinoic acid treatment. Following 24 hrs exposure to 20 microM lovastatin, all 7 acute myeloid leukemia cell lines tested showed a decrease in bcl-2 mRNA expression while only 1/5 acute lymphocytic leukemia cell lines showed a similar response. A role for bcl-2 in the apoptotic response of acute myeloid leukemia cells to lovastatin was demonstrated as exogenous constitutive expression of bcl-2 in the AML-5 cell line inhibited apoptosis in a time and dose dependent manner. Thus, lovastatin exposure of acute myeloid leukemia cells induced a differentiation response that may contribute to the therapeutic potential of this agent in the treatment of this disease.

Bcl-2 decreases the free Ca2+ concentration within the endoplasmic reticulum

The antiapoptotic protein Bcl-2 localizes not only to mitochondria but also to the endoplasmic reticulum (ER). However, the function of Bcl-2 at the level of the ER is poorly understood. In this study, we have investigated the effects of Bcl-2 expression on Ca(2+) storage and release by the ER. The expression of Bcl-2 decreased the amount of Ca(2+) that could be released from intracellular stores, regardless of the mode of store depletion, the cell type, or the species from which Bcl-2 was derived. Bcl-2 also decreased cellular Ca(2+) store content in the presence of mitochondrial inhibitors, suggesting that its effects were not mediated through mitochondrial Ca(2+) uptake. Direct measurements with ER-targeted Ca(2+)-sensitive fluorescent "cameleon" proteins revealed that Bcl-2 decreased the free Ca(2+) concentration within the lumen of the ER, [Ca(2+)](ER). Analysis of the kinetics of Ca(2+) store depletion in response to the Ca(2+)-ATPase inhibitor thapsigargin revealed that Bcl-2 increased the permeability of the ER membrane. These results suggest that Bcl-2 decreases the free Ca(2+) concentration within the ER lumen by increasing the Ca(2+) permeability of the ER membrane. The increased ER Ca(2+) permeability conferred by Bcl-2 would be compatible with an ion channel function of Bcl-2 at the level of the ER membrane.