Selective localization of Bcl-2 to the inner mitochondrial and smooth endoplasmic reticulum membranes in mammalian cells

Mitochondrial bioenergetics as a cell fate rheostat for responsive to Bcl-2 drugs: New cues for cancer chemotherapy

Pro-survival BCL-2 proteins prevent the initiation of intrinsic apoptosis (mitochondria-dependent pathway) by inhibiting the pro-apoptotic proteins BAX and BAK, while BH3-only proteins promote apoptosis by blocking pro-survival BCL-2 proteins. Disruptions in this delicate balance contribute to cancer cell survival and chemoresistance. Recent advances in cancer therapeutics involve a new generation of drugs known as BH3-mimetics, which are small molecules designed to mimic the action of BH3-only proteins. Promising effects have been observed in patients with hematological and solid tumors undergoing treatment with these agents. However, the rapid emergence of mitochondria-dependent resistance to BH3-mimetics has been reported. This resistance involves increased mitochondrial respiration, altered mitophagy, and mitochondria with higher and tighter cristae. Conversely, mutations in isocitrate dehydrogenase 1 and 2, catalyzing R-2-hydroxyglutarate production, promote sensitivity to venetoclax. This evidence underscores the urgency for comprehensive studies on bioenergetics-based adaptive responses in both BH3 mimetics-sensitive and -resistant cancer cells. Ongoing clinical trials are evaluating BH3-mimetics in combination with standard chemotherapeutics. In this article, we discuss the role of mitochondrial bioenergetics in response to BH3-mimetics and explore potential therapeutic opportunities through metabolism-targeting strategies.

Preparation of UiO-66 loaded Letrozole nano-drug delivery system: enhanced anticancer and apoptosis activity

The use of drug delivery systems in targeting and achieving the targeting of drugs in treating diseases, especially cancer, has attracted the attention of researchers. Letrozole is one of the drugs for the treatment of breast cancer. In this study, the organic-metallic pharmaceutical porous nanostructure based on zirconium UiO-66 loaded letrozole was synthesized. Its cytotoxicity and effect on apoptosis and migration against breast cancer cell line were investigated. In this experimental study, the UiO-66 nanoparticle-loaded letrozole was synthesized using zirconium chloride (ZrCl4), dimethylformamide (DMF), and HCl. Its characteristics were determined by scanning electron microscopy, and its average size was determined by the DLS method. Also, the rate of letrozole drug release from the nanoparticle was investigated in 24, 48, and 72 h. In addition, its cytotoxicity effects were investigated using the MTT colorimetric method at concentrations of 3.125-100 µg/ml against the breast cancer cell line (MCF-7) in the periods of 48 and 72 h. Also, the expression level of apoptotic genes Bax and Bcl2 was investigated by the Real-Time PCR method. Also, the amount of cell migration was done by the migration assay method. The results showed that UiO-66 bound to letrozole had a spherical morphology and an average size of 9.2 ± 160.1. Also, the letrozole drug was loaded by 62.21 ± 1.80% in UiO-66 nanoparticles and had a slower release pattern than free letrozole in the drug release test, so within 72 h, 99.99% of free letrozole was released in If in UiO-66 containing letrozole, 57.55% of the drug has been released. Also, the cytotoxicity results showed that UiO-66 bound to letrozole has more significant cytotoxic effects than free letrozole (p < 0.05). Also, the results of Bax and Bcl2 gene expression showed that the treatment of MCF-7 cells with UiO-66 nanoparticles attached to letrozole increased the expression of Bax and Bcl2 genes compared to the reference gene Beta-actin in MCF-7 cell line, respectively. (p < 0.05) increased by 3.71 ± 0.42 and (p < 0.01) decreased by 0.636 ± 0.034 (p < 0.05). Cell migration results showed that the concentration of 50 µg/ml of UiO-66 bound to letrozole decreased the migration of MCF-7 cells. Generally, the results of this study showed that UiO-66 loaded letrozole can be used as a suitable drug carrier for cellular purposes, as it has increased the effects of cytotoxicity and the rate of apoptosis in breast cancer cell line (MCF-7), so it can be used with more studies used nanocarriers as a drug delivery system.

Co-encapsulation of hydrophilic and hydrophobic drugs into niosomal nanocarrier for enhanced breast cancer therapy: In silico and in vitro studies

Combination therapy has been considered one of the most promising approaches for improving the therapeutic effects of anticancer drugs. This is the first study that uses two different antioxidants in full-characterized niosomal formulation and thoroughly evaluates their synergistic effects on breast cancer cells. In this study, in-silico studies of hydrophilic and hydrophobic drugs (ascorbic acid: Asc and curcumin: Cur) interactions and release were investigated and validated by a set of in vitro experiments to reveal the significant improvement in breast cancer therapy using a co-delivery approach by niosomal nanocarrier. The niosomal nanoparticles containing surfactants (Span 60 and Tween 60) and cholesterol at 2:1 M ratio were prepared through the film hydration method. A systematic evaluation of nanoniosomes was carried out. The release profile demonstrated two phases (initial burst followed by sustained release) and a pH-dependent release schedule over 72 h. The optimized niosomal preparation displayed superior storage stability for up to 2 months at 4 °C, exhibiting extremely minor changes in pharmaceutical encapsulation efficiency and size. Free dual drugs (Asc + Cur) and dual-drug loaded niosomes (Niosomal (Asc + Cur)) enhanced the apoptotic activity and cytotoxicity and inhibited cell migration which confirmed the synergistic effect of co-encapsulated drugs. Also, significant up-regulation of p53 and Bax genes was observed in cells treated with Asc + Cur and Niosomal (Asc + Cur), while the anti-apoptotic Bcl-2 gene was down-regulated. These results were in correlation with the increase in the enzyme activity of SOD, CAT, and caspase, and the levels of malondialdehyde (MDA) and reactive oxygen species (ROS) upon treatment with the mentioned drugs. Furthermore, these anti-cancer effects were higher when using Niosomal (Asc + Cur) than Asc + Cur. Histopathological examination also revealed that Niosomal (Asc + Cur) had a lower mitosis index, invasion, and pleomorphism than Asc + Cur. These findings indicated that niosomal formulation for co-delivery of Asc and Cur would offer a promising delivery system for an effective breast cancer treatment.

ATP Synthase K+- and H+-fluxes Drive ATP Synthesis and Enable Mitochondrial K+-"Uniporter" Function: II. Ion and ATP Synthase Flux Regulation

We demonstrated that ATP synthase serves the functions of a primary mitochondrial K⁺ "uniporter," i.e., the primary way for K⁺ to enter mitochondria. This K⁺ entry is proportional to ATP synthesis, regulating matrix volume and energy supply-vs-demand matching. We show that ATP synthase can be upregulated by endogenous survival-related proteins via IF₁. We identified a conserved BH3-like domain of IF₁ which overlaps its "minimal inhibitory domain" that binds to the β-subunit of F₁. Bcl-xL and Mcl-1 possess a BH3-binding-groove that can engage IF₁ and exert effects, requiring this interaction, comparable to diazoxide to augment ATP synthase's H⁺ and K⁺ flux and ATP synthesis. Bcl-xL and Mcl-1, but not Bcl-2, serve as endogenous regulatory ligands of ATP synthase via interaction with IF₁ at this BH3-like domain, to increase its chemo-mechanical efficiency, enabling its function as the recruitable mitochondrial K_ATP-channel that can limit ischemia-reperfusion injury. Using Bayesian phylogenetic analysis to examine potential bacterial IF₁-progenitors, we found that IF₁ is likely an ancient (∼2 Gya) Bcl-family member that evolved from primordial bacteria resident in eukaryotes, corresponding to their putative emergence as symbiotic mitochondria, and functioning to prevent their parasitic ATP consumption inside the host cell.

The multiple mechanisms of MCL1 in the regulation of cell fate

MCL1 (myeloid cell leukemia-1) is a widely recognized pro-survival member of the Bcl-2 (B-cell lymphoma protein 2) family and a promising target for cancer therapy. While the role MCL1 plays in apoptosis is well defined, its participation in emerging non-apoptotic signaling pathways is only beginning to be appreciated. Here, we synthesize studies characterizing MCL1s influence on cell proliferation, DNA damage response, autophagy, calcium handling, and mitochondrial quality control to highlight the broader scope that MCL1 plays in cellular homeostasis regulation. Throughout this review, we discuss which pathways are likely to be impacted by emerging MCL1 inhibitors, as well as highlight non-cancerous disease states that could deploy Bcl-2 homology 3 (BH3)-mimetics in the future.

In this review Widden and Placzek synthesize studies characterizing the influence that myeloid cell leukemia-1 (MCL1) has on cell proliferation, DNA damage response, autophagy, calcium handling, and mitochondrial quality control to highlight the broader scope that it plays in cellular homeostasis regulation. They discuss which pathways are likely to be impacted by emerging MCL1 inhibitors, as well as highlight non-cancerous disease states that could deploy BH3-mimetics in the future.

Inhibition of PLK1 by capped-dose volasertib exerts substantial efficacy in MDS and sAML while sparing healthy haematopoiesis

INTRODUCTION

Targeting the cell cycle machinery represents a rational therapeutic approach in myelodysplastic syndromes (MDS) and secondary acute myeloid leukemia (sAML). Despite substantial response rates, clinical use of the PLK inhibitor volasertib has been hampered by elevated side effects such as neutropenia and infections.

OBJECTIVES

The primary objective was to analyse whether a reduced dose of volasertib was able to limit toxic effects on the healthy haematopoiesis while retaining its therapeutic effect.

METHODS

Bone marrow mononuclear cells (BMMNCs) of patients with MDS/sAML (n = 73) and healthy controls (n = 28) were treated with volasertib (1 μM to 1 nM) or vehicle control. Short-term viability analysis was performed by flow cytometry after 72 hours. For long-term viability analysis, colony-forming capacity was assessed after 14 days. Protein expression of RIPK3 and MCL-1 was quantified via flow cytometry.

RESULTS

Reduced dose levels of volasertib retained high cell death-inducing efficacy in primary human stem and progenitor cells of MDS/sAML patients without affecting healthy haematopoiesis in vitro. Interestingly, volasertib reduced colony-forming capacity and cell survival independent of clinical stage or mutational status.

CONCLUSIONS

Volasertib offers a promising therapeutic approach in patients with adverse prognostic profile. RIPK3 and MCL-1 might be potential biomarkers for sensitivity to volasertib treatment.

A Therapeutic Role for the F1FO-ATP Synthase

Recently, the F1FO-ATP synthase, due to its dual role of life enzyme as main adenosine triphosphate (ATP) maker and of death enzyme, as ATP dissipator and putative structural component of the mitochondrial permeability transition pore (mPTP), which triggers cell death, has been increasingly considered as a drug target. Accordingly, the enzyme offers new strategies to counteract the increased antibiotic resistance. The challenge is to find or synthesize compounds able to discriminate between prokaryotic and mitochondrial F1FO-ATP synthase, exploiting subtle structural differences to kill pathogens without affecting the host. From this perspective, the eukaryotic enzyme could also be made refractory to macrolide antibiotics by chemically produced posttranslational modifications. Moreover, because the mitochondrial F1FO-ATPase activity stimulated by Ca2+ instead of by the natural modulator Mg2+ is most likely involved in mPTP formation, effectors preferentially targeting the Ca2+-activated enzyme may modulate the mPTP. If the enzyme involvement in the mPTP is confirmed, Ca2+-ATPase inhibitors may counteract conditions featured by an increased mPTP activity, such as neurodegenerative and cardiovascular diseases and physiological aging. Conversely, mPTP opening could be pharmacologically stimulated to selectively kill unwanted cells. On the basis of recent literature and promising lab findings, the action mechanism of F1 and FO inhibitors is considered. These molecules may act as enzyme modifiers and constitute new drugs to kill pathogens, improve compromised enzyme functions, and limit the deathly enzyme role in pathologies. The enzyme offers a wide spectrum of therapeutic strategies to fight at the molecular level diseases whose treatment is still insufficient or merely symptomatic.

The interaction of Atg4B and Bcl-2 plays an important role in Cd-induced crosstalk between apoptosis and autophagy through disassociation of Bcl-2-Beclin1 in A549 cells

Cadmium (Cd) is a highly ubiquitous detrimental metal in the environment. It is a well-known inducer of tumorigenesis, but the mechanism is not clear. In our previous study, we found that ROS-dependent Atg4B upregulation mediated Cd-induced autophagy and autophagy played an important role in Cd-induced proliferation and invasion in A549 cells. In this study, we found that Cd induced both apoptosis and autophagy in A549 cells, and apoptosis preceded autophagy. Z-VAD-FMK repressed Cd-induced LC3 and Beclin1, indicating that apoptosis was essential for Cd-induced autophagy. 3MA destroyed the recovery of mitochondrial membrane potential and increased Cd-induced CL-CASP9 and CL-CASP3 expression, suggesting that Cd-induced autophagy prevented A549 cells from apoptosis. Further study showed that Atg4B upregulation was mediated by mitochondrial dysfunction and conversely affected mitochondrial function by decreasing Bcl-2 protein expression and its localization in mitochondria, and played an important role in Cd-induced apoptosis. Moreover, Bcl-2 was involved in Cd-induced autophagy. Co-IP assay showed that Atg4B could directly bind to Bcl-2, and consequently promote disassociation of Bcl-2-Beclin1 and released autophagic protein Beclin1 to activate autophagic pathway. Taken together, our results demonstrated that the interaction of Atg4B and Bcl-2 might play an important role in Cd-induced crosstalk between apoptosis and autophagy through disassociation of Bcl-2-Beclin1. Cd-induced autophagy is apoptosis-dependent and prevents apoptotic cell death to ensure the growth and proliferation of A549 cells.

Bcl-xL mediates RIPK3-dependent necrosis in M. tuberculosis-infected macrophages

Virulent Mycobacterium tuberculosis (Mtb) triggers necrosis in host Mϕ, which is essential for successful pathogenesis in tuberculosis. Here we demonstrate that necrosis of Mtb-infected Mϕ is dependent on the action of the cytosolic Receptor Interacting Protein Kinase 3 (RIPK3) and the mitochondrial Bcl-2 family member protein B-cell lymphoma-extra large (Bcl-xL). RIPK3-deficient Mϕ are able to better control bacterial growth in vitro and in vivo. Mechanistically, cytosolic RIPK3 translocates to the mitochondria where it promotes necrosis and blocks caspase 8-activation and apoptosis via Bcl-xL. Furthermore, necrosis is associated with stabilization of hexokinase II on the mitochondria as well as cyclophilin D-dependent mitochondrial permeability transition. Collectively, these events upregulate the level of reactive oxygen species to induce necrosis. Thus, in Mtb-infected Mϕ, mitochondria are an essential platform for induction of necrosis by activating RIPK3 function and preventing caspase 8-activation.

Non-apoptotic functions of BCL-2 family proteins

The BCL-2 family proteins are major regulators of the apoptosis process, but the mechanisms by which they regulate this process are only partially understood. It is now well documented that these proteins play additional non-apoptotic roles that are likely to be related to their apoptotic roles and to provide important clues to cracking their mechanisms of action. It seems that these non-apoptotic roles are largely related to the activation of cellular survival pathways designated to maintain or regain cellular survival, but, if unsuccessful, will switch over into a pro-apoptotic mode. These non-apoptotic roles span a wide range of processes that include the regulation of mitochondrial physiology (metabolism, electron transport chain, morphology, permeability transition), endoplasmic reticulum physiology (calcium homeostasis, unfolded protein response (UPR)), nuclear processes (cell cycle, DNA damage response (DDR)), whole-cell metabolism (glucose and lipid), and autophagy. Here we review all these different non-apoptotic roles, make an attempt to link them to the apoptotic roles, and present many open questions for future research directions in this fascinating field.

Bcl-2 Regulates Reactive Oxygen Species Signaling and a Redox-Sensitive Mitochondrial Proton Leak in Mouse Pancreatic β-Cells

In pancreatic β-cells, controlling the levels of reactive oxygen species (ROS) is critical to counter oxidative stress, dysfunction and death under nutrient excess. Moreover, the fine-tuning of ROS and redox balance is important in the regulation of normal β-cell physiology. We recently demonstrated that Bcl-2 and Bcl-xL, in addition to promoting survival, suppress β-cell glucose metabolism and insulin secretion. Here, we tested the hypothesis that the nonapoptotic roles of endogenous Bcl-2 extend to the regulation of β-cell ROS and redox balance. We exposed mouse islet cells and MIN6 cells to the Bcl-2/Bcl-xL antagonist Compound 6 and the Bcl-2-specific antagonist ABT-199 and evaluated ROS levels, Ca(2+) responses, respiratory control, superoxide dismutase activity and cell death. Both acute glucose stimulation and the inhibition of endogenous Bcl-2 progressively increased peroxides and stimulated superoxide dismutase activity in mouse islets. Importantly, conditional β-cell knockout of Bcl-2 amplified glucose-induced formation of peroxides. Bcl-2 antagonism also induced a mitochondrial proton leak that was prevented by the antioxidant N-acetyl-L-cysteine and, therefore, secondary to redox changes. We further established that the proton leak was independent of uncoupling protein 2 but partly mediated by the mitochondrial permeability transition pore. Acutely, inhibitor-induced peroxides promoted Ca(2+) influx, whereas under prolonged Bcl inhibition, the elevated ROS was required for induction of β-cell apoptosis. In conclusion, our data reveal that endogenous Bcl-2 modulates moment-to-moment ROS signaling and suppresses a redox-regulated mitochondrial proton leak in β-cells. These noncanonical roles of Bcl-2 may be important for β-cell function and survival under conditions of high metabolic demand.

Icariin displays anticancer activity against human esophageal cancer cells via regulating endoplasmic reticulum stress-mediated apoptotic signaling

In this study, we investigated the antitumor activity of icariin (ICA) in human esophageal squamous cell carcinoma (ESCC) in vitro and in vivo and explored the role of endoplasmic reticulum stress (ERS) signaling in this activity. ICA treatment resulted in a dose- and time-dependent decrease in the viability of human EC109 and TE1 ESCCs. Additionally, ICA exhibited strong antitumor activity, as evidenced by reductions in cell migration, adhesion, and intracellular glutathione (GSH) levels and by increases in the EC109 and TE1 cell apoptotic index, Caspase 9 activity, reactive oxygen species (ROS) level, and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity. Furthermore, ICA treatments upregulated the levels of ERS-related molecules (p-PERK, GRP78, ATF4, p-eIF2α, and CHOP) and a pro-apoptotic protein (PUMA) and simultaneously downregulated an anti-apoptotic protein (Bcl2) in the two ESCC cell lines. The downregulation of ERS signaling using eIF2α siRNA desensitized EC109 and TE1 cells to ICA treatment, and the upregulation of ERS signaling using thapsigargin sensitized EC109 and TE1 cells to ICA treatment. In summary, ERS activation may represent a mechanism of action for the anticancer activity of ICA in ESCCs, and the activation of ERS signaling may represent a novel therapeutic intervention for human esophageal cancer.

Activation of endoplasmic reticulum stress is involved in the activity of icariin against human lung adenocarcinoma cells

In this study, we investigated the anticancer activity of icariin (ICA) against human lung adenocarcinoma cells in vitro and in vivo and explored the role of endoplasmic reticulum (ER) stress (ERS) signaling in this process. ICA treatment resulted in a dose- and time-dependent decrease in the viability of human lung adenocarcinoma A549 cells. Additionally, ICA exhibited potent anticancer activity, as evidenced by reductions in A549 cell adhesion, migration and intracellular glutathione (GSH) levels and increases in the apoptotic index, Caspase 3 activity, and reactive oxygen species. Furthermore, ICA treatment increased the expression of ERS-related molecules (p-PERK, ATF6, GRP78, p-eIF2α, and CHOP), up-regulated the apoptosis-related protein PUMA and down-regulated the anti-apoptosis-related protein Bcl2. The down-regulation of ERS signaling using PERK siRNA desensitized lung adenocarcinoma cells to ICA treatment, whereas the up-regulation of ERS signaling using thapsigargin (THA) sensitized lung adenocarcinoma cells to ICA treatment. Additionally, ICA inhibited the growth of human lung adenocarcinoma A549 cell xenografts by increasing the expression of ERS-related molecules (p-PERK and CHOP), up-regulating PUMA, and down-regulating Bcl2. These data indicate that ICA is a potential inhibitor of lung adenocarcinoma cell growth by targeting ERS signaling and suggest that the activation of ERS signaling may represent a novel therapeutic intervention for lung adenocarcinoma.

Bcl2 inhibition of mitochondrial DNA repair

Background

Accumulation of mitochondrial DNA (mtDNA) damage could enhance the frequency of mitochondrial mutations and promote a variety of mitochondria-related diseases, including cancer. However, the mechanism(s) involved are not fully understood.

Methods

Quantitative extended length PCR was used to compare mtDNA and nDNA damage in human lung H1299 cells expressing WT Bcl2 or vector-only control. mtAPE1 endonuclease activity was analyzed by AP oligonucleotide assay. mtDNA mutation was measured by single molecule PCR. Subcellular localization of Bcl2 and APE1 was analyzed by subcellular fractionation.

Results

Bcl2, an anti-apoptotic molecule and oncoprotein, effectively inhibits the endonuclease activity of mitochondrial APE1 (mtAPE1), leading to significant retardation of mtDNA repair and enhanced frequency of mtDNA mutations following exposure of cells to hydrogen peroxide (H₂O₂) or nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK, a carcinogen in cigarette smoke). Inversely, depletion of endogenous Bcl2 by RNA interference increases mtAPE1 endonuclease activity leading to accelerated mtDNA repair and decreased mtDNA mutation. Higher levels of mtAPE1 were observed in human lung cancer cells than in normal human bronchial epithelial cells (i.e. BEAS-2B). Bcl2 partially co-localizes with APE1 in the mitochondria of human lung cancer cells. Bcl2 directly interacts with mtAPE1 via its BH domains. Removal of any of the BH domains from Bcl2 abolishes Bcl2’s capacity to interact with mtAPE1 as well as its inhibitory effects on mtAPE1 activity and mtDNA repair.

Conclusions

Based our findings, we propose that Bcl2 suppression of mtDNA repair occurs through direct interaction with mtAPE1 and inhibition of its endonuclease activity in mitochondria, which may contribute to enhanced mtDNA mutations and carcinogenesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1594-1) contains supplementary material, which is available to authorized users.

Dynamic Modeling of the Interaction Between Autophagy and Apoptosis in Mammalian Cells

Autophagy is a conserved biological stress response in mammalian cells that is responsible for clearing damaged proteins and organelles from the cytoplasm and recycling their contents via the lysosomal pathway. In cases of mild stress, autophagy acts as a survival mechanism, while in cases of severe stress cells may switch to programmed cell death. Understanding the decision process that moves a cell from autophagy to apoptosis is important since abnormal regulation of autophagy occurs in many diseases, including cancer. To integrate existing knowledge about this decision process into a rigorous, analytical framework, we built a mathematical model of cell fate decisions mediated by autophagy. Our dynamical model is consistent with existing quantitative measurements of autophagy and apoptosis in rat kidney proximal tubular cells responding to cisplatin-induced stress.

Interaction of Bcl-2 with the autophagy-related GABAA receptor-associated protein (GABARAP): biophysical characterization and functional implications

Apoptosis and autophagy are fundamental homeostatic processes in eukaryotic organisms fulfilling essential roles in development and adaptation. Recently, the anti-apoptotic factor Bcl-2 has been reported to also inhibit autophagy, thus establishing a potential link between these pathways, but the mechanistic details are only beginning to emerge. Here we show that Bcl-2 directly binds to the phagophore-associated protein GABARAP. NMR experiments revealed that the interaction critically depends on a three-residue segment (EWD) of Bcl-2 adjacent to the BH4 region, which is anchored to one of the two hydrophobic pockets on the GABARAP molecule. This is at variance with the majority of GABARAP interaction partners identified previously, which occupy both hydrophobic pockets simultaneously. Bcl-2 affinity could also be detected for GEC1, but not for other mammalian Atg8 homologs. Finally, we provide evidence that overexpression of Bcl-2 inhibits lipidation of GABARAP, a key step in autophagosome formation, possibly via competition with the lipid conjugation machinery. These results support the regulatory role of Bcl-2 in autophagy and define GABARAP as a novel interaction partner involved in this intricate connection.

Characterization of the mitochondrial localization of the nuclear receptor SHP and regulation of its subcellular distribution by interaction with Bcl2 and HNF4α

The nuclear receptor small heterodimer partner SHP was shown recently to translocate to the mitochondria, interact with Bcl2, and induce apoptosis in liver cancer cells. However, the exact mitochondrial localization of SHP remains to be determined. In addition, the detailed interaction domains between SHP and Bcl2 have not been characterized. Using biochemistry and molecular biology approaches, we demonstrate that SHP is localized to the mitochondrial outer membrane. Interestingly, compared with the full-length SHP, the N-terminal deleted protein exhibits increased expression in the mitochondria and decreased expression in the nucleus. GST pull-down assays demonstrate that the interaction domain of SHP shows the strongest interaction with Bcl2. Furthermore, the interaction of Bcl2 with SHP is completely abolished by deletion of the Bcl2 transmembrane domain (TM), whereas deletion of the Bcl2 BH1 domain enhances the interaction. As expected, AHPN, a synthetic SHP ligand, markedly augments the direct protein-protein interaction between Bcl2 and SHP. Ectopic expression of hepatocyte nuclear factor 4 alpha (HNF4α) results in exclusive nuclear translocation of SHP proteins that contain either the full-length or the N-terminal domain, but has a minimal effect on the subcellular distribution of SHP protein containing only the interaction domain or repression domain. These results indicate that the N-terminal domain of SHP is important for itsnuclear translocation via HNF4α. Overall, this study provides novel insights into the domains of SHP that are critical for its shutting between different subcellular compartments.

Intracellular localization of the BCL-2 family member BOK and functional implications

The pro-apoptotic BCL-2 family member BOK is widely expressed and resembles the multi-BH domain proteins BAX and BAK based on its amino acid sequence. The genomic region encoding BOK was reported to be frequently deleted in human cancer and it has therefore been hypothesized that BOK functions as a tumor suppressor. However, little is known about the molecular functions of BOK. We show that enforced expression of BOK activates the intrinsic (mitochondrial) apoptotic pathway in BAX/BAK-proficient cells but fails to kill cells lacking both BAX and BAK or sensitize them to cytotoxic insults. Interestingly, major portions of endogenous BOK are localized to and partially inserted into the membranes of the Golgi apparatus as well as the endoplasmic reticulum (ER) and associated membranes. The C-terminal transmembrane domain of BOK thereby constitutes a 'tail-anchor' specific for targeting to the Golgi and ER. Overexpression of full-length BOK causes early fragmentation of ER and Golgi compartments. A role for BOK on the Golgi apparatus and the ER is supported by an abnormal response of Bok-deficient cells to the Golgi/ER stressor brefeldin A. Based on these results, we propose that major functions of BOK are exerted at the Golgi and ER membranes and that BOK induces apoptosis in a manner dependent on BAX and BAK.

Identification of a motif in BMRP required for interaction with Bcl-2 by site-directed mutagenesis studies

Bcl-2 is an anti-apoptotic protein that inhibits apoptosis elicited by multiple stimuli in a large variety of cell types. BMRP (also known as MRPL41) was identified as a Bcl-2 binding protein and shown to promote apoptosis. Previous studies indicated that the amino-terminal two-thirds of BMRP contain the domain(s) required for its interaction with Bcl-2, and that this region of the protein is responsible for the majority of the apoptosis-inducing activity of BMRP. We have performed site-directed mutagenesis analyses to further characterize the BMRP/Bcl-2 interaction and the pro-apoptotic activity of BMRP. The results obtained indicate that the 13-17 amino acid region of BMRP is necessary for its binding to Bcl-2. Further mutagenesis of this motif shows that amino acid residue aspartic acid (D) 16 of BMRP is essential for the BMRP/Bcl-2 interaction. Functional analyses conducted in mammalian cells with BMRP site-directed mutants BMRP(13Ala17) and BMRP(D16A) indicate that these mutants induce apoptosis through a caspase-mediated pathway, and that they kill cells slightly more potently than wild-type BMRP. Bcl-2 is still able to counteract BMRP(D16A)-induced cell death significantly, but not as completely as when tested against wild-type BMRP. These results suggest that the apoptosis-inducing ability of wild-type BMRP is blocked by Bcl-2 through several mechanisms.

Multiple functions of BCL-2 family proteins

BCL-2 family proteins are the regulators of apoptosis, but also have other functions. This family of interacting partners includes inhibitors and inducers of cell death. Together they regulate and mediate the process by which mitochondria contribute to cell death known as the intrinsic apoptosis pathway. This pathway is required for normal embryonic development and for preventing cancer. However, before apoptosis is induced, BCL-2 proteins have critical roles in normal cell physiology related to neuronal activity, autophagy, calcium handling, mitochondrial dynamics and energetics, and other processes of normal healthy cells. The relative importance of these physiological functions compared to their apoptosis functions in overall organismal physiology is difficult to decipher. Apoptotic and noncanonical functions of these proteins may be intertwined to link cell growth to cell death. Disentanglement of these functions may require delineation of biochemical activities inherent to the characteristic three-dimensional shape shared by distantly related viral and cellular BCL-2 family members.

Resveratrol affects undifferentiated and differentiated PC12 cells differently, particularly with respect to possible differences in mitochondrial and autophagic functions

Since resveratrol is considered to exert a unique dual effect, protective for normal cells but toxic to tumor cells, its action on undifferentiated (original) and differentiated PC12 cells was analyzed, because undifferentiated cells are tumorigenic and differentiated ones are neuronal in nature. Compared to resveratrol-untreated cells in both undifferentiated and differentiated cell groups, cells treated with different doses of resveratrol, at dosages of 1, 10 and 100 μM, showed the following alterations. Dying/dead cells were significantly increased in a dose-dependent manner in undifferentiated cells, but they were unchanged at doses of up to 10 μM resveratrol in differentiated cells. In living cells, neurites were short in undifferentiated cells, but drastically elongated with an increased number in differentiated cells. The expression of SIRT1 was drastically reduced in undifferentiated cells, but stable in differentiated cells. SIRT3 was significantly enhanced in a dose-dependent manner at resveratrol doses of up to 10 μM in both cells, with reduction and more enhanced at a dosage of 100 μM in undifferentiated and differentiated cells, respectively. Mitochondrial number and ATP synthase β subunit expression was unaltered at doses of up to 10 μM and were significantly reduced at doses of 100 μM in undifferentiated cells, but they were significantly increased in a dose-dependent manner, with a slight reduction in the ATP synthase at doses of 100 μM, in differentiated cells. In a dose-dependent manner, the number of autophagosomes and the LC3-II/LC3-I ratio were significantly less in undifferentiated cells and greater in differentiated cells. Also, in a dose-dependent manner, the expression of phosphorylated AMP-activated kinase (AMPK) was significantly less in undifferentiated cells and greater in differentiated cells. Resveratrol-induced AMPK suppression and activation, possibly through the modulation of SIRT protein activity, may thus be related to the inhibition and promotion of mitochondrial and autophagic functions, leading to cell death and survival in undifferentiated and differentiated cells, respectively.

Rotaviral enterotoxin nonstructural protein 4 targets mitochondria for activation of apoptosis during infection

Viruses have evolved to encode multifunctional proteins to control the intricate cellular signaling pathways by using very few viral proteins. Rotavirus is known to express six nonstructural and six structural proteins. Among them, NSP4 is the enterotoxin, known to disrupt cellular Ca(2+) homeostasis by translocating to endoplasmic reticulum. In this study, we have observed translocation of NSP4 to mitochondria resulting in dissipation of mitochondrial membrane potential during virus infection and NSP4 overexpression. Furthermore, transfection of the N- and C-terminal truncated NSP4 mutants followed by analyzing NSP4 localization by immunofluorescence microscopy identified the 61-83-amino acid region as the shortest mitochondrial targeting signal. NSP4 exerts its proapoptotic effect by interacting with mitochondrial proteins adenine nucleotide translocator and voltage-dependent anion channel, resulting in dissipation of mitochondrial potential, release of cytochrome c from mitochondria, and caspase activation. During early infection, apoptosis activation by NSP4 was inhibited by the activation of cellular survival pathways (PI3K/AKT), because PI3K inhibitor results in early induction of apoptosis. However, in the presence of both PI3K inhibitor and NSP4 siRNA, apoptosis was delayed suggesting that the early apoptotic signal is initiated by NSP4 expression. This proapoptotic function of NSP4 is balanced by another virus-encoded protein, NSP1, which is implicated in PI3K/AKT activation because overexpression of both NSP4 and NSP1 in cells resulted in reduced apoptosis compared with only NSP4-expressing cells. Overall, this study reports on the mechanism by which enterotoxin NSP4 exerts cytotoxicity and the mechanism by which virus counteracts it at the early stage for efficient infection.

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.

Bcl-xL regulates mitochondrial energetics by stabilizing the inner membrane potential

To promote cell survival, the antiapoptotic factor Bcl-x_L both inhibits Bax-induced mitochondrial outer membrane permeabilization and stabilizes mitochondrial inner membrane ion flux and thus overall mitochondrial energetic capacity.

Mammalian Bcl-x_L protein localizes to the outer mitochondrial membrane, where it inhibits apoptosis by binding Bax and inhibiting Bax-induced outer membrane permeabilization. Contrary to expectation, we found by electron microscopy and biochemical approaches that endogenous Bcl-x_L also localized to inner mitochondrial cristae. Two-photon microscopy of cultured neurons revealed large fluctuations in inner mitochondrial membrane potential when Bcl-x_L was genetically deleted or pharmacologically inhibited, indicating increased total ion flux into and out of mitochondria. Computational, biochemical, and genetic evidence indicated that Bcl-x_L reduces futile ion flux across the inner mitochondrial membrane to prevent a wasteful drain on cellular resources, thereby preventing an energetic crisis during stress. Given that F₁F_O–ATP synthase directly affects mitochondrial membrane potential and having identified the mitochondrial ATP synthase β subunit in a screen for Bcl-x_L–binding partners, we tested and found that Bcl-x_L failed to protect β subunit–deficient yeast. Thus, by bolstering mitochondrial energetic capacity, Bcl-x_L may contribute importantly to cell survival independently of other Bcl-2 family proteins.

Bcl-xL regulates metabolic efficiency of neurons through interaction with the mitochondrial F1FO ATP synthase

Anti-apoptotic Bcl2 family proteins such as Bcl-x_L protect cells from death by sequestering apoptotic molecules, but also contribute to normal neuronal function. We find in hippocampal neurons that Bcl-x_L enhances the efficiency of energy metabolism. Our evidence suggests that Bcl-x_L interacts directly with the beta subunit of the F₁F_O ATP synthase, decreasing an ion leak within the F₁F_O ATPase complex and thereby increasing net transport of H⁺ by F₁F_O during F₁F_O ATPase activity. By patch clamping submitochondrial vesicles enriched in F₁F_O ATP synthase complexes, we find that, in the presence of ATP, pharmacological or genetic inhibition of Bcl-x_L increases the membrane leak conductance. In addition, recombinant Bcl-x_L protein directly increases ATPase activity of purified synthase complexes, while inhibition of endogenous Bcl-x_L decreases F₁F_O enzymatic activity. Our findings suggest that increased mitochondrial efficiency contributes to the enhanced synaptic efficacy found in Bcl-x_L expressing neurons.

Closer association of mitochondria with lipid droplets in hepatocytes and activation of Kupffer cells in resveratrol-treated senescence-accelerated mice

Resveratrol has been extensively investigated because of its beneficial effects in delaying age-related diseases, thus extending the lifespan, possibly by mimicking calorie restriction. For this study, cell biological techniques were used to examine how resveratrol influenced hepatocytes in a senescence-accelerated mouse P10 (SAMP10), treated from 35 to 55 weeks of age, with special emphasis on the relationship between mitochondria and lipid droplets. Survival ratio, body weight and food intake of SAMP10 did not differ significantly between the control and resveratrol-treated groups. Compared with the control, the treated livers were altered significantly, as follows. Lipid droplets were reduced and mitochondria were increased in number in hepatocytes. Phosphorylation of acetyl-CoA carboxylase and the expression of both the mitochondrial ATP synthase β subunit and Mn superoxide dismutase (SOD2) were increased. Mitochondria, expressing more SOD2, were more tightly associated with lipid droplets, suggesting the enhancement of lipolysis through the activation of mitochondrial functions. Cathepsin D expression was less in hepatocytes but enhanced in Kupffer cells, which were increased in number and size with more numerous lysosome-related profiles. Together, resveratrol may activate mitochondria resulting in consuming lipids, and may also activate Kupffer cells by which a beneficial milieu for hepatocytes may be created. Both might be related to improvement in the functioning of the liver, which is the organ that is central to metabolic regulation.

Hepatic gap junctions in the hepatocarcinogen-resistant DRH rat

Although the gap junction or connexin (Cx) is considered to be a tumor-suppressor, it is also required for tumor promotion. Therefore, we examined hepatic gap junctions in hepatocarcinogen-resistant (DRH) rats. Specifically, we investigated gap junction structure and Cx32 expression during normal conditions and in response to a hepatocarcinogen, 3'-methyl-4-dimethylaminoazobenzene (3'-MeDAB). On a basal diet without 3'-MeDAB, hepatic gap junctions and Cx32 protein expression were greater in DRH rats than in control Donryu rats, as evidenced by morphometry, immunohistochemistry and immunoblotting. On a diet containing 3'-MeDAB, gap junctions and expressed Cx32 were increased significantly in Donryu rats, but not in DRH rats. In this condition, Donryu rats lost weight but DRH rats increased relative liver weight. After 3'-MeDAB treatment, cathepsin D expression in hepatocytes was significantly increased only in Donryu rats, indicating that DRH rats were less susceptible to 3'-MeDAB. The abundance of mitogen-activated protein kinase, some constituent of which might be associated with the degree of Cx protein phosphorylation, was reduced to a greater extent in Donryu than in DRH rats after 3'-MeDAB treatment. The resistance of DRH rats to carcinogenesis may be due partially to their stabilized gap junctions, which could coordinate metabolic coupling to evade 3'-MeDAB toxicity.

Hepatitis B virus HBx protein localizes to mitochondria in primary rat hepatocytes and modulates mitochondrial membrane potential

Over 350 million people are chronically infected with hepatitis B virus (HBV), and a significant number of chronically infected individuals develop primary liver cancer. HBV encodes seven viral proteins, including the nonstructural X (HBx) protein. The results of studies with immortalized or transformed cells and with HBx-transgenic mice demonstrated that HBx can interact with mitochondria. However, no studies with normal hepatocytes have characterized the precise mitochondrial localization of HBx or the effect of HBx on mitochondrial physiology. We have used cultured primary rat hepatocytes as a model system to characterize the mitochondrial localization of HBx and the effect of HBx expression on mitochondrial physiology. We now show that a fraction of HBx colocalizes with density-gradient-purified mitochondria and associates with the outer mitochondrial membrane. We also demonstrate that HBx regulates mitochondrial membrane potential in hepatocytes and that this function of HBx varies depending on the status of NF-kappaB activity. In primary rat hepatocytes, HBx activation of NF-kappaB prevented mitochondrial membrane depolarization; however, when NF-kappaB activity was inhibited, HBx induced membrane depolarization through modulation of the mitochondrial permeability transition pore. Collectively, these results define potential pathways through which HBx may act in order to modulate mitochondrial physiology, thereby altering many cellular activities and ultimately contributing to the development of HBV-associated liver cancer.

Glucosamine protects neonatal cardiomyocytes from ischemia-reperfusion injury via increased protein O-GlcNAc and increased mitochondrial Bcl-2

We have previously reported that glucosamine protected neonatal rat ventricular myocytes against ischemia-reperfusion (I/R) injury, and this was associated with an increase in protein O-linked-N-acetylglucosamine (O-GlcNAc) levels. However, the protective effect of glucosamine could be mediated via pathways other that O-GlcNAc formation; thus the initial goal of the present study was to determine whether increasing O-GlcNAc transferase (OGT) expression, which catalyzes the formation of O-GlcNAc, had a protective effect similar to that of glucosamine. To better understand the potential mechanism underlying O-GlcNAc-mediated cytoprotection, we examined whether increased O-GlcNAc levels altered the expression and translocation of members of the Bcl-2 protein family. Both glucosamine (5 mM) and OGT overexpression increased basal and I/R-induced O-GlcNAc levels, significantly decreased cellular injury, and attenuated loss of cytochrome c. Both interventions also attenuated the loss of mitochondrial membrane potential induced by H2O2 and were also associated with an increase in mitochondrial Bcl-2 levels but had no effect on Bad or Bax levels. Compared with glucosamine and OGT overexpression, NButGT (100 microM), an inhibitor of O-GlcNAcase, was less protective against I/R and H2O2 and did not affect Bcl-2 expression, despite a 5- to 10-fold greater increase in overall O-GlcNAc levels. Decreased OGT expression resulted in lower basal O-GlcNAc levels, prevented the I/R-induced increase in O-GlcNAc and mitochondrial Bcl-2, and increased cellular injury. These results demonstrate that the protective effects of glucosamine are mediated via increased formation of O-GlcNAc and suggest that this is due, in part, to enhanced mitochondrial Bcl-2 translocation.

Morphological and biochemical signs of age-related neurodegenerative changes in klotho mutant mice

Klotho mutant mice, defective in the klotho gene, develop multiple age-related disorders with very short lifespans. Introduction of the exogenous klotho gene into these mutant mice leads to an improvement in their phenotypes, while overexpression of this gene in wild-type mice significantly extends their lifespan. These observations suggest that the klotho gene/protein has an anti-aging function. Since there have been only a few reports with some disagreement about results on the CNS of the mutant mice, we tried to clarify whether the CNS neurons generate aging-like features, even in premature stages, using biochemical and morphological approaches. Results obtained from the mutant mice, when compared with wild-type mice, were as follows. Neurofilaments (NFs) were increased significantly in axons, with the subunit proteins showing a significant enhancement in phosphorylation or expression of NF-H or NF-L, respectively. Microtubules in Purkinje cell dendrites were closer to each other, and in the CNS tissue tubulin was unaltered, but microtubule-associated protein (MAP) 2 was significantly reduced in expression. Neuronal cellular organelles were morphologically disordered. Lysosomes, cathepsin D and light chain 3 of MAP1A/B (LC3) were augmented with the appearance of putative autophagy-related structures. Antiapoptotic Bcl-xL and proapoptotic Bax were reduced and enhanced, respectively, and mitogen-activated protein kinase was reduced. Synapse-related proteins and structures were decreased. Neuronal degeneration was evident in hippocampal pyramidal cells, and possibly in Purkinje cells. Astrocytic glial filaments and glial fibrillary acidic protein were increased in density and expression, respectively. Together, the CNS neuronal alterations in klotho mutant mice were quite similar to those found in aged animals, including even premature death, so this mouse should be a more appropriate animal model for CNS aging than those previously reported.

Phosphorothioate oligodeoxynucleotides and G3139 induce apoptosis in 518A2 melanoma cells

In a previous study, we showed that G3139, an antisense phosphorothioate oligonucleotide that down-regulates the expression of Bcl-2 protein, did not cause chemosensitization of 518A2 melanoma cells. In this work, we show that G3139, and the 2-base mismatch, G4126, can initiate apoptosis in this and other melanoma cell lines as shown by increased cell surface Annexin V expression, typical nuclear phenotypic changes as assessed by 4′,6-diamidino-2-phenylindole staining, activation of caspase-3 (but not caspase-8) and Bid, appearance of DEVDase (but not IETDase) activity, and cleavage of poly(ADP-ribose)-polymerase 1. Depolarization of the mitochondrial membrane occurs as a relatively late event. All of these processes seem to be substantially, but perhaps not totally, Bcl-2 independent as shown by experiments employing an anti-Bcl-2 small interfering RNA, which as shown previously down-regulated Bcl-2 protein expression but did not produce apoptosis or chemosensitization in melanoma cells. In fact, these G3139-induced molecular events were not dramatically altered in cells that forcibly overexpressed high levels of Bcl-2 protein. Addition of irreversible caspase inhibitors (e.g., the pan-caspase inhibitor zVAD-fmk) to G3139-treated cells almost completely blocked cytotoxicity. Examination of the time course of the appearance of caspase-3 and cleaved poly(ADP-ribose)-polymerase 1 showed that this could be correlated with the release of cytochrome c from the mitochondria, an event that begins only ∼4 hours after the end of the oligonucleotide/LipofectAMINE 2000 5-hour transfection period. Thus, both G3139 and cytotoxic chemotherapy activate the intrinsic pathway of apoptosis in these cells, although Bcl-2 expression does not seem to contribute strongly to chemoresistance. These findings suggest that the attainment of G3139-induced chemosensitization in these cells will be difficult.

Physical and functional interaction between BH3-only protein Hrk and mitochondrial pore-forming protein p32

Bcl-2 homology domain (BH) 3-only proteins of the proapoptotic Bcl-2 subfamily play a key role as initiators of mitochondria-dependent apoptosis. To date, at least 10 mammalian BH3-only proteins have been identified, and it is now being realized that they have different roles and mechanisms of regulation in the transduction of apoptotic signals to mitochondria. Hrk/DP5 is one of the mammalian BH3-only proteins implicated in a variety of physiological and pathological apoptosis, yet the molecular mechanism involved in Hrk-mediated apoptosis remains poorly understood. In an attempt to identify cellular proteins participating in Hrk-mediated apoptosis, we have conducted yeast two-hybrid screening for Hrk-interacting proteins and isolated p32, a mitochondrial protein that has been shown to form a channel consisting of its homotrimer. In vitro binding, co-immunoprecipitation, as well as immunocytochemical analyses verified specific interaction and colocalization of Hrk and p32, both of which depended on the presence of the highly conserved C-terminal region of p32. Importantly, Hrk-induced apoptosis was suppressed by the expression of p32 mutants lacking the N-terminal mitochondrial signal sequence (p32(74-282)) and the conserved C-terminal region (p32 (1-221)), which are expected to inhibit binding of Hrk competitively to the endogenous p32 protein and to disrupt the channel function of p32, respectively. Furthermore, small interfering RNA-mediated knockdown of p32 conferred protection against Hrk-induced apoptosis. Altogether, these results suggest that p32 may be a key molecule that links Hrk to mitochondria and is critically involved in the regulation of Hrk-mediated apoptosis.

Bcl2-low-expressing MCF7 cells undergo necrosis rather than apoptosis upon staurosporine treatment

We present a ribozyme-based strategy for studying the effects of Bcl2 down-regulation. The anti-bcl2 hammerhead ribozyme Rz-bcl2 was stably transfected into MCF7 cancer cells and the cleavage of Bcl2 mRNA was demonstrated using a new assay for cleavage product detection, while Western blot analysis showed a concomitant depletion of Bcl2 protein. Rz-bcl2-expressing cells were more sensitive to staurosporine than control cells. Moreover, both molecular and cellular read-outs indicated that staurosporine-induced cell death was necrosis rather than apoptosis in these cells. The study of the effects of Bcl2 down-regulation was extended to the global MCF7 protein expression profile, exploiting a proteomic approach. Two reference electro-pherograms of Rz-bcl2-transfected cells, one with the ribozyme in a catalytically active form and the other with the ribozyme in a catalytically inactive form, were obtained. When comparing the two-dimensional maps, 53 differentially expressed spots were found, four of which were identified by MALDI-TOF (matrix-assisted laser-desorption ionization-time-of-flight) MS as calreticulin, nucleophosmin, phosphoglycerate kinase and pyruvate kinase. How the up-regulation of these proteins might help to explain the modification of Bcl2 activity is discussed.

Human MUC1 carcinoma-associated protein confers resistance to genotoxic anticancer agents

The MUC1 transforming protein is overexpressed by most human carcinomas. The present studies demonstrate that the MUC1 C-terminal subunit (MUC1 C-ter) localizes to mitochondria in HCT116/MUC1 colon carcinoma cells and that heregulin stimulates mitochondrial targeting of MUC1 C-ter. We also show that MUC1 attenuates cisplatin-induced (1) release of mitochondrial apoptogenic factors, (2) activation of caspase-3, and (3) induction of apoptosis. Moreover, knockdown of MUC1 expression in A549 lung and ZR-75-1 breast carcinoma cells by MUC1siRNA was associated with increased sensitivity to genotoxic drugs in vitro and in vivo. These findings indicate that MUC1 attenuates the apoptotic response to DNA damage and that this oncoprotein confers resistance to genotoxic anticancer agents.

Rapid accumulation of Akt in mitochondria following phosphatidylinositol 3-kinase activation

We describe here a new component of the phosphatidylinositol 3-kinase/Akt signaling pathway that directly impacts mitochondria. Akt (protein kinase B) was shown for the first time to be localized in mitochondria, where it was found to reside in the matrix and the inner and outer membranes, and the level of mitochondrial Akt was very dynamically regulated. Stimulation of a variety of cell types with insulin-like growth factor-1, insulin, or stress (induced by heat shock), induced translocation of Akt to the mitochondria within only several minutes of stimulation, causing increases of nearly eight- to 12-fold, and the mitochondrial Akt was in its phosphorylated, active state. Two mitochondrial proteins were identified to be phosphorylated following stimulation of mitochondrial Akt, the beta-subunit of ATP synthase and glycogen synthase kinase-3beta. The finding that mitochondrial glycogen synthase kinase-3beta was rapidly and substantially modified by Ser9 phosphorylation, which inhibits its activity, following translocation of Akt to the mitochondria is the first evidence for a regulatory mechanism affecting mitochondrial glycogen synthase kinase-3beta. These results demonstrate that signals emanating from plasma membrane receptors or generated by stress rapidly modulate Akt and glycogen synthase kinase-3beta in mitochondria.

A Bcl-2 transgene expressed in hepatocytes does not protect mice from fulminant liver destruction induced by Fas ligand

We compared the biological mechanism of cell death during hepatotoxicity induced by ligation of the Fas receptor in wild-type and liver-specific bcl-2 transgenic mice. Transgenic overexpression of Bcl-2 in mouse hepatocytes can prevent lethal hepatitis induced by agonistic anti-Fas antibodies. In contrast, Fas ligand (FasL)-induced death cannot be overcome in bcl-2 transgenic mice, indicating that anti-Fas antibodies do not reliably mimic the more physiological ligand. Different apoptotic parameters, viz. caspase activation, cytochrome c release and nuclear DNA degradation were analysed. No differences, however, could be observed between wild-type and bcl-2 transgenic mice after injection with a lethal dose of soluble FasL, indicating that apoptosis by FasL-dependent ligation is not modulated by Bcl-2 in vivo. These results demonstrate that the stimulus determines the outcome between type I mitochondria-independent apoptosis, in the case of FasL, or type II mitochondria-dependent and Bcl-2-inhibitable apoptosis, in the case of anti-Fas antibodies.

BH3-domain mimetic compound BH3I-2' induces rapid damage to the inner mitochondrial membrane prior to the cytochrome c release from mitochondria

The Bcl-2 family proteins are major regulators of cell survival and death in human leukaemia. BH3-containing peptides induce apoptosis by binding to the hydrophobic pocket of the anti-apoptotic proteins, such as Bcl-2 or Bcl-XL. A small cell-permeable compound, BH3I-2' (3-iodo-5-chloro-N-[2-chloro-5-((4-chlorophenyl)sulphonyl)phenyl]-2-hydroxybenzamide), has been recently reported to have a function similar to Bak BH3 peptide. BH3I-2' induces apoptosis by disrupting interactions mediated by the BH3 domain, between pro-apoptotic and anti-apoptotic members of the Bcl-2 family. This study found that BH3I-2' induced cytochrome c release from the mitochondrial outer membrane in a Bax-dependent manner and that this correlated with the sensitivity of leukaemic cells to apoptosis. Moreover, it also induced rapid damage to the inner mitochondrial membrane, represented by a rapid collapse of mitochondrial membrane potential (DeltaPsim), prior to the cytochrome c release. This occurred both in whole cells and isolated mitochondria, and was not associated with the sensitivity of cells to BH3I-2'-induced apoptosis. Exogenous Bcl-2 or Bcl-XL neutralized BH3I-2'in vitro and diminished its effect on the inner mitochondrial membrane. Our results indicate that BH3I-2' not only induces cytochrome c release from the outer mitochondrial membrane but also damages the inner mitochondrial membrane, probably by interacting with Bcl-2 family proteins.

Germ cell apoptosis and its molecular trigger in mouse testes

Germ cell apoptosis is very common during various stages of mammalian testicular development. However, our understanding of the mechanisms underlying male germ cell apoptosis is still limited. This review firstly covers the general features of germ cell death in normal testes of fetal, neonatal, and adult mice from electron microscopy (EM) and terminal dUTP nick-end labeling (TUNEL) staining. The issue of whether the Fas and Fas ligand (FasL) system and/or the Bax and Bcl-2 system is involved in the induction of germ cell apoptosis in normal and damaged testes will then be addressed, including a special consideration of the ischemia-reperfusion model, the endocrine disruptor-treated model, and others. Finally, this review will propose that the process of normal spermatogenesis seems skillfull in taking advantage of apoptotic processes of germ cells and that different molecular pathways may be triggered to induce male germ cell apoptosis, depending upon the physiological and pathological states of germ cells.

Enhanced resistance to salt, cold and wound stresses by overproduction of animal cell death suppressors Bcl-xL and Ced-9 in tobacco cells - their possible contribution through improved function of organella

Overproduction of animal cell death suppressors Bcl-xL and Ced-9 conferred enhanced resistance to UV-B and paraquat treatment in tobacco plants [Mitsuhara et al. (1999) CURR: Biol. 9: 775]. We report here that the progeny could germinate in 0.2 M NaCl or at 10 degrees C under light, while control plants could not. Suspension-cultured Bcl cells resisted NaCl treatment maintaining an active mitochondrial membrane potential for longer than control cells. When intracellular pH was determined by in vivo (31)P-NMR, immediate cytoplasmic acidification by 0.3 M NaCl treatment in control cells was found to be suppressed in transgenic cells. Monitoring of cytoplasmic and vacuolar pHs in control cells indicated the vacuole was disrupted 40 min after 0.5 M NaCl treatment, while the compartment between the cytoplasm and vacuole was likely to remain intact in Bcl cells for 100 min. Enhanced shoot regeneration from cut leaf pieces and more vigorous rooting from cut stem ends were found in transgenic plants. The Bcl protein was abundant in all subcellular fractions. Based on the results in transgenic plants carrying a mutant bcl-xL gene, Bcl-xL is thought to suppress cell death and enhance the viability of plants in stressful environments by contributing to the maintenance of the homeostasis of organella.

Glutathione depletion enforces the mitochondrial permeability transition and causes cell death in Bcl-2 overexpressing HL60 cells

Bcl-2, a protein that blocks apoptosis by inhibiting the mitochondrial permeability transition (MPT) and release of cytochrome c appears to affect normal mitochondrial function by altering electron flow and increasing rates of reactive oxygen species (ROS) production. In this study, we show that glutathione (GSH) depletion induces ROS production and selective toxicity in HL60 cells that overexpress Bcl-2 compared with neomycin vector control cells. Toxicity was mediated by the MPT because it was blocked with the adenine nucleotide translocator (ANT) ligand bongkrekic acid and resulted in mitochondrial cytochrome c release, caspase 3 activation, and DNA fragmentation, indicating the involvement of an apoptotic pathway. Respiratory chain inhibitors stigmatellin and antimycin A, which inhibit Qo and Qi sites of respiratory chain complex III, respectively, blocked ROS production, preserved the redox state of protein thiols, and prevented cell death. These results indicate that in the absence of GSH, endogenous ROS generated at respiratory complex III induce MPT independently of Bcl-2. The results also suggest a new model for MPT in which the central pore protein ANT is regulated by adenine nucleotide and the activity of mitochondrial respiratory complex III.