BID-dependent and BID-independent pathways for BAX insertion into mitochondria

Rapamycin attenuates PLA2R activation-mediated podocyte apoptosis via the PI3K/AKT/mTOR pathway

Membranous nephropathy (MN) is the most common cause of nephrotic syndrome in adults without diabetes. Primary MN has been associated with circulating antibodies against native podocyte antigens, including phospholipase A2 receptor (PLA2R); however, precision therapy targeting the signaling cascade of PLA2R activation is lacking. Both PLA2R and the mammalian target of rapamycin (mTOR) exist in podocytes, but the interplay between these two proteins and their roles in MN warrants further exploration. This study aimed to investigate the crosstalk between PLA2R activation and mTOR signaling in a human podocyte cell line. We demonstrated that podocyte apoptosis was induced by Group IB secretory phospholipase A2 (sPLA2IB) in a concentration- and time-dependent manner via upregulation of phosphoinositide 3-kinase (PI3K), protein kinase B (AKT), and mTOR, and inhibited by rapamycin or LY294002. Furthermore, aberrant activation of the PI3K/AKT/mTOR pathway triggers both extrinsic (caspase-8 and caspase-3) and intrinsic (Bcl-2-associated X protein [BAX], B-cell lymphoma 2 [BCL-2], cytochrome c, caspase-9, and caspase-3) apoptotic cascades in podocytes. The therapeutic implications of our findings are that strategies to reduce PLA2R activation and PI3K/AKT/mTOR pathway inhibition in PLA2R-activated podocytes help protect podocytes from apoptosis. The therapeutic potential of rapamycin shown in this study provides cellular evidence supporting the repurposing of rapamycin for MN treatment.

Aquaporin 4 and brain-related disorders: Insights into its apoptosis roles

Brain-related disorders are leading global health problems. Various internal and external factors are involved in the progression of brain-related disorders. Inflammatory pathways, oxidative stresses, apoptosis, and deregulations of various channels are critical players in brain-related disorder pathogenesis. Among these players, aquaporins (AQP) have critical roles in various physiological and pathological conditions. AQPs are water channel molecules that permit water to cross the hydrophobic lipid bilayers of cellular membranes. AQP4 is one of the important members of AQP family. AQPs are involved in controlling apoptosis pathways in brain-related disorders. In this regard, several reports have evaluated the pathological effects of AQP4 by targeting the apoptosis-related processes in brain-related disorders. Here, for the first time, we highlight the impact of AQP4 on apoptosis-related processes in brain-related disorders.

Oxidative Stress-Induced Unscheduled CDK1-Cyclin B1 Activity Impairs ER-Mitochondria-Mediated Bioenergetic Metabolism

Targeting the activities of endoplasmic reticulum (ER)-mitochondrial-dependent metabolic reprogramming is considered one of the most promising strategies for cancer treatment. Here, we present biochemical subcellular fractionation, coimmunoprecipitation, gene manipulation, and pharmacologic evidence that induction of mitochondria-localized phospho (p)-cyclin dependent kinase 1 (CDK1) (Thr 161)-cyclin B1 complexes by apigenin in nasopharyngeal carcinoma (NPC) cells impairs the ER-mitochondrial bioenergetics and redox regulation of calcium (Ca⁺⁺) homeostasis through suppressing the B cell lymphoma 2 (BCL-2)/BCL-2/B-cell lymphoma-extra large (BCL-x_L)-modulated anti-apoptotic and metabolic functions. Using a specific inducer, inhibitor, or short hairpin RNA for acid sphingomyelinase (ASM) demonstrated that enhanced lipid raft-associated ASM activity confers alteration of the lipid composition of lipid raft membranes, which leads to perturbation of protein trafficking, and induces formation of p110α free p85α-unphosphorylated phosphatase and tensin homolog deleted from chromosome 10 complexes in the lipid raft membranes, causing disruption of phosphatidylinositol 3-kinase (PI3K)-protein kinase B (Akt)-GTP-ras-related C3 botulinum toxin substrate 1 (Rac1)-mediated signaling, thus triggering the p-CDK1 (Thr 161))-cyclin B1-mediated BCL-2 (Thr 69/Ser 87)/BCL-x_L (Ser 62) phosphorylation and accompanying impairment of ER-mitochondria-regulated bioenergetic, redox, and Ca⁺⁺ homeostasis. Inhibition of apigenin-induced reactive oxygen species (ROS) generation by a ROS scavenger N-acetyl-L-cysteine blocked the lipid raft membrane localization and activation of ASM and formation of ceramide-enriched lipid raft membranes, returned PI3K-Akt-GTP-Rac1-modulated CDK1-cyclin B1 activity, and subsequently restored the BCL-2/BCL-x_L-regulated ER-mitochondrial bioenergetic activity. Thus, this study reveals a novel molecular mechanism of the pro-apoptotic activity of ASM controlled by oxidative stress to modulate the ER-mitochondrial bioenergetic metabolism, as well as suggests the disruption of CDK1-cyclin B1-mediated BCL-2/BCL-x_L oncogenic activity by triggering oxidative stress-ASM-induced PI3K-Akt-GTP-Rac1 inactivation as a therapeutic approach for NPC.

Loganin Attenuates Septic Acute Renal Injury with the Participation of AKT and Nrf2/HO-1 Signaling Pathways

Purpose

Sepsis, a destructive inflammatory response syndrome, is the principal reason to induce death in the intensive care unit. Loganin has been proved to possess the property of anti-inflammation, antioxidant, neuroprotection, and sedation. The primary aim of this study was to evaluate whether Loganin could alleviate acute kidney injury (AKI) during sepsis and investigate the latent mechanisms.

Methods

Septic AKI models were established by cecal ligation and puncture (CLP) surgery in mice and given Loganin (20, 40, 80 mg/kg) by gavage. Lipopolysaccharides (LPS)-stimulated human kidney proximal tubular (HK2) cells incubated in Loganin (5, 10, 20 μ M) were used to explore the accurate mechanisms. Survival rate, renal function (creatinine and blood urea nitrogen), and renal pathological changes were detected in septic mice. Oxidative stress markers (SOD, GSH-Px, MDA, and SOD), mitochondrial membrane potential, mitochondrial calcium overload, and nuclear factor E2-related factor 2 (Nrf2)/heme-oxygenase 1 (HO-1) pathway activation in vivo and in vitro were determined by commercial kits and Western blot. Cell apoptosis, apoptotic-related protein (cleaved caspase-3, Bcl-2, and Bax) expression and protein kinase B (AKT) phosphorylation in vivo and in vitro were measured by TUNEL staining and Western blot. Finally, AKT blockage by 10 μM LY294002 or Nrf2 inhibition by10 μ M ML385 were utilized to prove the involvement of AKT and Nrf2/HO-1 pathway in AKI during sepsis.

Results

We found Loganin treatment (20, 40, 80 mg/kg) mitigated septic AKI reflected by elevated renal function and palliative pathological changes. Oxidative stress and apoptosis in the kidney and LPS-treated HK2 cells were also inhibited by Loganin administration, which was accompanied by AKT and Nrf2/HO-1 pathway activation. Besides, the protective effects of Loganin could be diminished by AKT or Nrf2 blockage, indicating the involvement of AKT and Nrf2/HO-1 pathway.

Conclusion

The results suggested that the protective effects of Loganin on AKI during sepsis might be mediated by AKT and Nrf2/HO-1 pathway signaling activation in kidney proximal tubular cells.

TBX3 deficiency accelerates apoptosis in cardiomyoblasts through regulation of P21 expression

Congenital heart disease (CHD) is the most common birth defect in newborns. There is increasing evidence that apoptosis and remodeling of the cardiomyoblasts are the major pathology of CHD. Previous research found that T-box transcription factor 3 (TBX3) was compulsory for the regulation of proliferation, cell cycle arrest and apoptosis in various cells. Hence, TBX3 might be involved in the treatment of CHD. The primary aim of this study was to study the effects of TBX3 on apoptosis in aged cardiomyoblasts and investigate the latent mechanism. In the present study, we found TBX3 knockdown induced proliferation inhibition, cell cycle arrest and apoptosis accompanied by mitochondrial dysfunction in cardiomyoblasts at passage 10 to 15. Apoptosis-inducing effects of TBX3 silence could be neutralized by silencing P21 using specific siRNA. In addition, the mRNA and protein expression levels of TBX3 in the heart tissues of sporadic type CHD donors were obviously down-regulated. In conclusion, we demonstrated that TBX3 deficiency accelerated apoptosis via directly regulating P21 expression in senescent cardiomyoblasts.

The effects of acupuncture and electroacupuncture on Parkinson's disease: Current status and future perspectives for molecular mechanisms

Among the progressive neurodegenerative disorders, Parkinson's disease (PD) is the second most common. Different factors have critical role in pathophysiology of PD such as apoptosis pathways, inflammatory cytokines, oxidative stress, and neurotransmitters and its receptors abnormalities. Acupuncture and electroacupuncture were considered as nondrug therapies for PD. Although numerous studies has been conducted for assessing the mechanism underlying electroacupuncture and acupuncture, various principal aspects of these treatment procedures remain not well-known. There have also been few investigations on the molecular mechanism of acupuncture and electroacupuncture therapy effects in PD. This review evaluates the effects of electroacupuncture and acupuncture on the molecular mechanism in PD.

Immunoproapoptotic molecule scFv-Fdt-tBid modified mesenchymal stem cells for prostate cancer dual-targeted therapy

Highly efficient target therapy is urgently needed for prostate cancer with overexpression of γ-seminoprotein (γ-SM). Recent studies indicated that mesenchymal stem cells (MSCs) are attractive candidate for cell-based, targeted therapy due to their tumor tropism. Here we designed a dual-target therapeutic system in which MSCs were engineered to produce and deliver scFv-Fdt-tBid, a novel γ-SM-targeted immunoproapoptotic molecule. Such engineered MSCs (MSC.scFv-Fdt-tBid) would home to tumor sites and release the fusion protein to induce the apoptosis of prostate cancer cells. Our data demonstrated that scFv-Fdt-tBid showed a selective, potent and dose-dependent inhibition for γ-SM-positive cells (LNCaP, C4-2, 22Rv1) rather than γ-SM-negative cells and MSCs. Importantly, MSC.scFv-Fdt-tBid caused cell death through an apoptosis-dependent manner. Further, the tropism of MSC.scFv-Fdt-tBid to prostate cancer was verified both in vitro and in vivo. Finally, the in vivo experiments demonstrated that MSC.scFv-Fdt-tBid significantly inhibited γ-SM-positive tumor growth without toxic side effects. Collectively, this study represented a novel immunoproapoptotic molecule scFv-Fdt-tBid for γ-SM-positive tumors and demonstrated the therapeutic efficiency and safety of scFv-Fdt-tBid-modified MSCs against prostate cancers.

Cellular mechanisms of the cytotoxic effects of the zearalenone metabolites α-zearalenol and β-zearalenol on RAW264.7 macrophages

Zearalenone (ZEN) and its metabolites are commonly found in many food commodities and are known to cause reproductive disorders and genotoxic effects. The major ZEN metabolites are α-zearalenol (α-ZOL) and β-zearalenol (β-ZOL). Although many studies have demonstrated the cytotoxic effects of these metabolites, the mechanisms by which α-ZOL or β-ZOL mediates their cytotoxic effects appear to differ according to cell type and the exposed toxins. We evaluated the toxicity of α-ZOL and β-ZOL on RAW264.7 macrophages and investigated the underlying mechanisms. β-ZOL not only more strongly reduced the viability of cells than did α-ZOL, but it also induced cell death mainly by apoptosis rather than necrosis. The ZEN metabolites induced loss of mitochondrial membrane potential (MMP), mitochondrial changes in Bcl-2 and Bax proteins, and cytoplasmic release of cytochrome c and apoptosis-inducing factor (AIF). Use of an inhibitor specific to c-Jun N-terminal kinase (JNK), p38 kinase or p53, but not pan-caspase or caspase-8, decreased the toxin-induced generation of reactive oxygen species (ROS) and also attenuated the α-ZOL- or β-ZOL-induced decrease of cell viability. Antioxidative enzyme or compounds such as catalase, acteoside, and (E)-1-(3,4-dihydroxyphenethyl)-3-(4-hydroxystyryl)urea suppressed the ZEN metabolite-mediated reduction of cell viability. Further, knockdown of AIF via siRNA transfection diminished the ZEN metabolite-induced cell death. Collectively, these results suggest that the activation of p53, JNK or p38 kinase by ZEN metabolites is the main upstream signal required for the mitochondrial alteration of Bcl-2/Bax signaling pathways and intracellular ROS generation, while MMP loss and nuclear translocation of AIF are the critical downstream events for ZEN metabolite-mediated apoptosis in macrophages.

Mycotoxin zearalenone induces AIF- and ROS-mediated cell death through p53- and MAPK-dependent signaling pathways in RAW264.7 macrophages

Zearalenone (ZEN) is commonly found in many food commodities and is known to cause reproductive disorders and genotoxic effects. However, the mode of ZEN-induced cell death of macrophages and the mechanisms by which ZEN causes cytotoxicity remain unclear. The present study shows that ZEN treatment reduces viability of RAW264.7 cells in a dose-dependent manner. ZEN causes predominantly necrotic and late apoptotic cell death. ZEN treatment also results in the loss of mitochondrial membrane potential (MMP), mitochondrial changes in Bcl-2 and Bax proteins, and cytoplasmic release of cytochrome c and apoptosis-inducing factor (AIF). Pre-treatment of the cells with either z-VAD-fmk or z-IETD-fmk does not attenuate ZEN-mediated cell death, whereas catalase suppresses the ZEN-induced decrease in viability in RAW264.7 cells. Treating the cells with c-Jun N-terminal kinase (JNK), p38 mitogen-activated protein kinase (MAPK), or p53 inhibitor prevented ZEN-mediated changes, such as MMP loss, cellular reactive oxygen species (ROS) increase, and cell death. JNK or p38 MAPK inhibitor inhibited mitochondrial alterations of Bcl-2 and Bax proteins with attendant decreases in cellular ROS levels. Knockdown of AIF via siRNA transfection also diminished ZEN-induced cell death. Further, adenosine triphosphate was markedly depleted in the ZEN-exposed cells. Collectively, these results suggest that ZEN induces cytotoxicity in RAW264.7 cells via AIF- and ROS-mediated signaling, in which the activations of p53 and JNK/p38 play a key role.

Combined MUC1-specific nanobody-tagged PEG-polyethylenimine polyplex targeting and transcriptional targeting of tBid transgene for directed killing of MUC1 over-expressing tumour cells

We provide evidence for combining a single domain antibody (nanobody)-based targeting approach with transcriptional targeting as a safe way to deliver lethal transgenes to MUC1 over-expressing cancer cells. From a nanobody immune library, we have isolated an anti-DF3/Mucin1 (MUC1) nanobody with high specificity for the MUC1 antigen, which is an aberrantly glycosylated glycoprotein over-expressed in tumours of epithelial origin. The anti-MUC1 nanobody was covalently linked to the distal end of poly(ethylene glycol)(3500) (PEG(3500)) in PEG(3500)-25kDa polyethylenimine (PEI) conjugates and the resultant macromolecular entity successfully condensed plasmids coding a transcriptionally targeted truncated-Bid (tBid) killer gene under the control of the cancer-specific MUC1 promoter. The engineered polyplexes exhibited favourable physicochemical characteristics for transfection and dramatically elevated the level of Bid/tBid expression in both MUC1 over-expressing caspase 3-deficient (MCF7 cells) and caspase 3-positive (T47D and SKBR3) tumour cell lines and, concomitantly, induced considerable cell death. Neither transgene expression nor cell death occurred when the MUC1 promoter was replaced with the CNS-specific synapsin I promoter. Since PEGylated PEI was only responsible for DNA compaction and played no significant role in direct transfection and cell killing, our attempts overcome previously reported PEI-mediated apoptotic and necrotic cell death, which is advantageous for future in vivo transcriptional targeting as this will minimize (or eliminate) non-targeted cell damage.

BH3-only proteins: Orchestrators of apoptosis

The BH3-only proteins of Bcl-2 family are essential initiators of apoptosis that propagate extrinsic and intrinsic cell death signals. The interaction of BH3-only proteins with other Bcl-2 family members is critical for understanding the core machinery that controls commitment to apoptosis by permeabilizing the mitochondrial outer membrane. BH3-only proteins promote apoptosis by both directly activating Bax and Bak and by suppressing the anti-apoptotic proteins at the mitochondria and the endoplasmic reticulum. To prevent constitutive cell death, BH3-only proteins are regulated by a variety of mechanisms including transcription and post-translational modifications that govern specific protein-protein interactions. Furthermore, BH3-only proteins also control the initiation of autophagy, another important pathway regulating cell survival and death. Emerging evidence indicates that the interaction of BH3-only proteins with membranes regulates binding to other Bcl-2 family members, thereby specifying function. Due to the important role of BH3-only proteins in the regulation of cell death, several promising BH3-mimetic drugs that are active in pre-clinical models are currently being tested as anti-cancer agents. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.

BAX insertion, oligomerization, and outer membrane permeabilization in brain mitochondria: role of permeability transition and SH-redox regulation

BAX cooperates with truncated BID (tBID) and Ca(2+) in permeabilizing the outer mitochondrial membrane (OMM) and releasing mitochondrial apoptogenic proteins. The mechanisms of this cooperation are still unclear. Here we show that in isolated brain mitochondria, recombinant BAX readily self-integrates/oligomerizes in the OMM but produces only a minuscule release of cytochrome c, indicating that BAX insertion/oligomerization in the OMM does not always lead to massive OMM permeabilization. Ca(2+) in a mitochondrial permeability transition (mPT)-dependent and recombinant tBID in an mPT-independent manner promoted BAX insertion/ oligomerization in the OMM and augmented cytochrome c release. Neither tBID nor Ca(2+) induced BAX oligomerization in the solution without mitochondria, suggesting that BAX oligomerization required interaction with the organelles and followed rather than preceded BAX insertion in the OMM. Recombinant Bcl-xL failed to prevent BAX insertion/oligomerization in the OMM but strongly attenuated cytochrome c release. On the other hand, a reducing agent, dithiothreitol (DTT), inhibited BAX insertion/oligomerization augmented by tBID or Ca(2+) and suppressed the BAX-mediated release of cytochrome c and Smac/DIABLO but failed to inhibit Ca(2+)-induced swelling. Altogether, these data suggest that in brain mitochondria, BAX insertion/oligomerization can be dissociated from OMM permeabilization and that tBID and Ca(2+) stimulate BAX insertion/oligomerization and BAX-mediated OMM permeabilization by different mechanisms involving mPT induction and modulation of the SH-redox state.

RACK1 promotes Bax oligomerization and dissociates the interaction of Bax and Bcl-XL

Bax, a member of Bcl-2 family, plays an essential role in apoptotic pathways induced by a number of apoptotic stimulus. In a search for new potential binding partners of Bax, we identified the receptor for activated C-kinase 1 (RACK1) by a yeast two-hybrid assay. We demonstrated that RACK1 interacts with Bax through its BH3 domain both in vitro and in vivo. Using immunostaining and immunoprecipitation experiments, we found that RACK1 colocalizes with Bax oligomers and promotes Bax oligomerization both in vitro and in vivo. Furthermore, we observed that RACK1 also interacts with Bcl-XL, an anti-apoptotic protein associated with Bax. Interestingly, the Bcl-XL/Bax interaction is decreased when RACK1 is overexpressed, but is increased when RACK1 is depleted, suggesting RACK1 disrupts the association of Bax and Bcl-XL. In addition, we found that overexpression of RACK1 promotes UV-induced apoptosis, while knocking down RACK1 inhibits the effects. Together, these results indicate that RACK1 promotes apoptosis by promoting Bax oligomerization and dissociating the complex of Bax and Bcl-XL.

Cell apoptosis induced by delta-elemene in colorectal adenocarcinoma cells via a mitochondrial-mediated pathway

The chemical compound delta-elemene, isolated from the Chinese herbal medicine plant Curcuma Wenyujin, has been known to exert antitumor activity. In this study we demonstrated that apoptotic cell death induced by delta-elemene in DLD-1 cells was concentration-and time-dependent, and had little inhibition of the normal human liver cell line WRL-68. Apoptosis was further confirmed and quantified by DNA fragmentation ELISA, Annexin V (AnV) binding of externalized phosphatidylserine and the mitochondrial probe JC-1 using flow cytometry. The rapid increase in intracellular reactive oxygen species (ROS) levels was involved in the mechanism of cell death. Western blot analysis demonstrated that delta-elemene activated the caspase-signaling pathway, leading to the proteolysis conversion of pro-caspase-3 to activate caspase-3, and the subsequent cleavage of the caspase substrate PARP. In the process of the induction of apoptotic cell death, Bax translocated into mitochondria, a reduction in Deltapsim was observed and a release of cytochrome c and apoptosis inducing factor (AIF) from mitochondria into the cytosol occurred, indicating that cell death induced by delta-elemene was through a mitochondrial-mediated pathway.

Bax contains two functional mitochondrial targeting sequences and translocates to mitochondria in a conformational change- and homo-oligomerization-driven process

The apoptosis gateway protein Bax normally exists in the cytosol as a globular shaped monomer composed of nine alpha-helices. During apoptosis, Bax translocates to the mitochondria, forms homo-oligomers, and subsequently induces mitochondrial damage. The mechanism of Bax mitochondrial translocation remains unclear. Among the nine alpha-helices of Bax, helices 4, 5, 6, and 9 are capable of targeting a heterologous protein to mitochondria. However, only helices 6 and 9 can independently direct the oligomerized Bax to the mitochondria. Although Bax mitochondrial translocation can still proceed with mutations in either helix 6 or helix 9, combined mutations completely abolished mitochondrial targeting in response to activating signals. Using a proline mutagenesis scanning analysis, we demonstrated that conformational changes were sufficient to cause Bax to move from the cytosol to the mitochondria. Moreover, we found that homo-oligomerization of Bax contributed to its mitochondrial translocation. These results suggest that Bax is targeted to the mitochondria through the exposure of one or both of the two functional mitochondrial targeting sequences in a conformational change-driven and homo-oligomerization-aided process.

Silencing of HIF-1alpha suppresses tumorigenicity of renal cell carcinoma through induction of apoptosis

Hypoxia-inducible factor-1alpha (HIF-1alpha) is a main responder to intracellular hypoxia and is overexpressed in many human cancers, including renal cell carcinoma (RCC). To better understanding of the role of HIF-1alpha in the tumorigenicity of RCC, we used short-hairpin RNA (shRNA) interference to inhibit HIF-1alpha expression in the human renal cancer cell line, Caki-1 and OS-RC-2. Silencing of HIF-1alpha significantly reduced the expression of HIF-1alpha in both of renal cancer cell lines. In vitro downregulation of HIF-1alpha inhibited Caki-1 and OS-RC-2 cell growth, migration and invasion. The results further showed that HIF-1alpha silencing resulted in caspase-dependent apoptosis of Caki-1 and OS-RC-2 through regulation of PI3K/Akt pathway and Bcl-2-related proteins expression. In vivo animal studies showed that tumor growth was significantly inhibited in HIF-1alpha shRNA-transfected RCC. Intratumor gene therapy with polyethylenimine-loaded HIF-1alpha shRNA also resulted in tumor growth suppression. Thus, this study demonstrates that downregulation of HIF-1alpha could suppress tumorigenicity of RCC through induction of apoptosis, and HIF-1alpha shRNA may be a promising strategy for the treatment of RCC.

Association of protein kinase C delta and phospholipid scramblase 3 in hippocampal mitochondria correlates with neuronal vulnerability to brain ischemia

Recent findings support the idea that mitochondrial integrity plays an important role in the propagation of excitotoxic ischemic signal and PKC is implicated in the regulation of mitochondrial membranes properties. One of the targets of PKC delta is phospholipid scramblase 3 (PLSCR3), an enzyme responsible for cardiolipin translocation from the inner to outer mitochondrial membrane. To get an insight into in vivo mechanism by which PKC delta mediates ischemia/reperfusion injury of hippocampal neurons, we examined the effects of transient brain ischemia in gerbil on association of PKC delta with mitochondria isolated from ischemia-vulnerable (CA1) and ischemia-resistant regions, and interactions between PKC delta and PLSCR3. Postischemic, biphasic and brain region-specific translocation of PKC delta to mitochondria was observed. First peak was at 30-60 min of reperfusion and the second was observed after 72-96 h following ischemia. PKC delta was translocated to mitochondria only in CA1 region. The PLSCR3 mRNA and protein was detected in brain by RT-PCR and sequence analysis, Western blotting and immunocytochemistry in electron microscopy (EM). Co-immunoprecipitation and double-labeled immuno-EM showed association of PKC delta and PLSCR3 in postischemic CA1 mitochondria. Additionally, the amount of tBid associated with mitochondria was elevated 96 h following ischemia. Our data suggest that in the postischemic brain PKC delta co-localizes with PLSCR3 in mitochondria and this event might influence the mitochondrial membranes architecture and delayed neurons degeneration.

Bcl-2 family proteins as regulators of oxidative stress

The Bcl-2 family of proteins includes pro- and anti-apoptotic factors acting at mitochondrial and microsomal membranes. An impressive body of published studies, using genetic and physical reconstitution experiments in model organisms and cell lines, supports a view of Bcl-2 proteins as the critical arbiters of apoptotic cell death decisions in most circumstances (excepting CD95 death receptor signaling in Type I cells). Evasion of apoptosis is one of the hallmarks of cancer [Hanahan D, Weinberg RA. The hallmarks of cancer. Cell 2000;100:57-70], relevant to tumorigenesis as well as resistance to cytotoxic drugs, and deregulation of Bcl-2 proteins is observed in many cancers [Manion MK, Hockenbery DM. Targeting BCL-2-related proteins in cancer therapy. Cancer Biol Ther. 2003;2:S105-14; Olejniczak ET, Van Sant C, Anderson MG, Wang G, Tahir SK, Sauter G, et al. Integrative genomic analysis of small-cell lung carcinoma reveals correlates of sensitivity to bcl-2 antagonists and uncovers novel chromosomal gains. Mol Cancer Res. 2007;5:331-9]. The rekindled interest in aerobic glycolysis as a cancer trait raises interesting questions as to how metabolic changes in cancer cells are integrated with other essential alterations in cancer, e.g. promotion of angiogenesis and unbridled growth signals. Apoptosis induced by multiple different signals involves loss of mitochondrial homeostasis, in particular, outer mitochondrial membrane integrity, releasing cytochrome c and other proteins from the intermembrane space. This integrative process, controlled by Bcl-2 family proteins, is also influenced by the metabolic state of the cell. In this review, we consider the role of reactive oxygen species, a metabolic by-product, in the mitochondrial pathway of apoptosis, and the relationships between Bcl-2 functions and oxidative stress.

Bcl-2 family proteins as regulators of oxidative stress

The Bcl-2 family of proteins includes pro- and anti-apoptotic factors acting at mitochondrial and microsomal membranes. An impressive body of published studies, using genetic and physical reconstitution experiments in model organisms and cell lines, supports a view of Bcl-2 proteins as the critical arbiters of apoptotic cell death decisions in most circumstances (excepting CD95 death receptor signaling in Type I cells). Evasion of apoptosis is one of the hallmarks of cancer [Hanahan D, Weinberg RA. The hallmarks of cancer. Cell 2000;100:57-70], relevant to tumorigenesis as well as resistance to cytotoxic drugs, and deregulation of Bcl-2 proteins is observed in many cancers [Manion MK, Hockenbery DM. Targeting BCL-2-related proteins in cancer therapy. Cancer Biol Ther. 2003;2:S105-14; Olejniczak ET, Van Sant C, Anderson MG, Wang G, Tahir SK, Sauter G, et al. Integrative genomic analysis of small-cell lung carcinoma reveals correlates of sensitivity to bcl-2 antagonists and uncovers novel chromosomal gains. Mol Cancer Res. 2007;5:331-9]. The rekindled interest in aerobic glycolysis as a cancer trait raises interesting questions as to how metabolic changes in cancer cells are integrated with other essential alterations in cancer, e.g. promotion of angiogenesis and unbridled growth signals. Apoptosis induced by multiple different signals involves loss of mitochondrial homeostasis, in particular, outer mitochondrial membrane integrity, releasing cytochrome c and other proteins from the intermembrane space. This integrative process, controlled by Bcl-2 family proteins, is also influenced by the metabolic state of the cell. In this review, we consider the role of reactive oxygen species, a metabolic by-product, in the mitochondrial pathway of apoptosis, and the relationships between Bcl-2 functions and oxidative stress.

The contributions of oestrogen receptor isoforms to the development of papillary and anaplastic thyroid carcinomas

Oestrogen (E2) is known to promote the proliferation of thyroid papillary carcinoma cells (KAT5). However, the molecular mechanism responsible is not well understood. In the study reported herein, the localization of ER alpha (ERalpha) and beta (ERbeta) in KAT5 and anaplastic carcinoma cells (FRO) was studied by immunofluorescence staining and by immunoblotting the proteins in subcellular fractions. Cell proliferation and apoptosis were also determined together with the expression of relevant proteins. The pattern of the subcellular localization of ERalpha and ERbeta differed between papillary and anaplastic cancer. Upon E2 treatment, the level of ERalpha increased in the nuclei of papillary cancer cells but ERbeta remained unchanged. The level of mitochondrial ERbeta surpassed that of ERalpha in anaplastic cancer cells. The different locations of ERalpha and ERbeta in KAT5 and FRO agreed with the finding that E2 promoted the proliferation of KAT5 but inhibited or did not affect that of FRO cells, and with the proposed functions of these two receptors. E2 inhibited the level of Bax in the mitochondria of papillary cancer, followed by a decrease of cytochrome c and/or apoptosis-inducing factor (AIF) release from the mitochondria into the cytosol. However, in anaplastic cancer, E2 promoted the expression of Bax in the mitochondria and the release of cytochrome c and/or AIF from mitochondria into the cytosol. Our results may explain the differences in epidemiology and responses to anti-tumour therapy between papillary and anaplastic cancer in terms of the subcellular localization of ER isoforms. In conclusion, the findings provide evidence to support the observation that E2 is an important factor in the development of thyroid cancer. The subcellular localization of ERalpha and ERbeta may account for the different pathogenesis of thyroid papillary and anaplastic cancers.

Fas/CD95-mediated apoptosis of type II cells is blocked by Toxoplasma gondii primarily via interference with the mitochondrial amplification loop

The intracellular protozoan Toxoplasma gondii induces persistent infections in various hosts and is an important opportunistic pathogen of humans with immature or deficient immune responses. The ability to survive intracellularly largely depends on the blocking of different proapoptotic signaling cascades of its host cell. Fas/CD95 triggers an apoptotic cascade that is crucial for immunity and the outcome of infectious diseases. We have determined the mechanism by which T. gondii counteracts death receptor-mediated cell death in type II cells that transduce Fas/CD95 ligation via caspase 8-mediated activation of the mitochondrial amplification loop. The results showed that infection with T. gondii significantly reduced Fas/CD95-triggered apoptosis in HeLa cells by inhibiting the activities of initiator caspases 8 and 9 and effector caspase 3/7. Parasitic infection dose dependently diminished cleavage of caspase 8, the BH3-only protein Bid, and the downstream caspases 9 and 3. Importantly, interference with Fas/CD95-triggered caspase 8 and caspase 3/7 activities after parasitic infection was largely dependent on the presence of caspase 9. Within the mitochondrial amplification loop, T. gondii significantly inhibited the Fas/CD95-triggered release of cytochrome c into the host cell cytosol. These results indicate that T. gondii inhibits Fas/CD95-mediated apoptosis in type II cells primarily by decreasing the apoptogenic function of mitochondria.

Apoptotic pathway induced by noscapine in human myelogenous leukemic cells

It has been shown that noscapine, an opium-derived phthalideisoquinoline alkaloid that is currently being used as an oral antitussive drug, induces apoptosis in myeloid leukemia cells. The molecular mechanism responsible for the anticancer effects of noscapine is poorly understood. In the current study, the apoptotic effects of noscapine on two myeloid cell lines, apoptosis-proficient HL60 cells and apoptosis-resistant K562 cells, were analyzed. An increase in the activity of caspase-2, -3, -6, -8 and -9, poly(ADP ribose) polymerase cleavage, detection of phosphatidylserine on the outer layer of the cell membrane, nucleation of chromatin, and DNA fragmentation suggested the induction of apoptosis. Noscapine increased the Bax/Bcl-2 ratio with a significant decrease of Bcl-2 expression accompanied with Bcl-2 phosphorylation. Using an inhibitory approach, the activation of the caspase cascade involved in the noscapine-induced apoptosis was analyzed. We observed no inhibitory effect of the caspase-8 inhibitor on caspase-9 activity. In view of these results and taking into consideration that K562 cells are Fas-null, we suggested that caspase-8 is activated in a Fas-independent manner downstream of caspase-9. In conclusion, noscapine can induce apoptosis in both apoptosis-proficient and apoptosis-resistant leukemic cells, and it can be a novel candidate in the treatment of hematological malignancies.

TNF pretreatment interferes with mitochondrial apoptosis in the mouse liver by A20-mediated down-regulation of Bax

Pretreatment with low doses of the proinflammatory cytokine TNF has been shown to prevent hepatocellular apoptosis and liver damage in inflammatory as well as in ischemia/reperfusion-induced liver injury. The underlying mechanisms of protection have not been elucidated so far. In this study, these mechanisms were investigated in murine hepatocyte cultures as well as in a mouse model of TNF-dependent apoptotic liver damage (galactosamine/TNF model). Our results show that pretreatment with TNF, or application of small-interfering RNA directed against the proapoptotic Bcl2 family member Bax, interfered with the onset of mitochondrial apoptosis in vivo. Knockdown of TNF-alpha-induced-protein 3 (A20) restored mitochondrial apoptosis, Bax expression, and liver damage. The underlying mechanism of protection seems to involve a cascade of events, where TNF induces the expression of A20 in hepatocytes, A20 down-modulates Bax expression by interference with transcriptional activation, and the reduced availability of Bax interferes with the onset of mitochondrial apoptosis and the ensuing apoptotic liver damage. In conclusion, we identified Bax and A20 as key players in TNF-induced protection from apoptotic liver damage. Because treatment with TNF itself might be a risk factor for patients, we propose that overexpression of A20 might represent an alternative approach for protection from inflammation related apoptotic liver damage, as well as for TNF preconditioning during transplantation.

JAB1 accelerates mitochondrial apoptosis by interaction with proapoptotic BclGs

The Bcl-2 family of proteins is the key regulators of cell apoptosis at the mitochondria level. The BH3-only pro-apoptotic member BclGs was unique among the family due to its highly specific expression in human testis and has been demonstrated to induce apoptosis dependent on the BH3 domain. However, the molecular mechanism of BclGs-induced apoptosis remains unclear. Here we show that overexpression of BclGs could induce Bax expression upregulation and translocation to mitochondria, cytochrome c release and activation of caspase-3. Moreover, we identified JAB1 as a novel BclGs-specific binding protein through a yeast two-hybrid screening in a human testis cDNA library. BclGs interacts with JAB1 both in vitro and in vivo. N-terminal region of BclGs (aa 1-67) was required for the interaction. Importantly, JAB1 and BclGs co-expression synergistically induces apoptosis. JAB1 could compete with Bcl-XL/Bcl-2 to bind to BclGs; thus, promote the apoptosis. RNAi-mediated knock-down of JAB1 results in the reduced proapoptotic activity of BclGs. Taken together, our results provided the first evidence that JAB1 is involved in the regulation of mitochondrial apoptotic pathway through specific interaction with BclGs.

Blockade of tumor necrosis factor-induced Bid cleavage by caspase-resistant Rb

Tumor necrosis factor-alpha (TNF) activates caspase-8 to cleave effector caspases or Bid, resulting in type-1 or type-2 apoptosis, respectively. We show here that TNF also induces caspase-8-dependent C-terminal cleavage of the retinoblastoma protein (Rb). Interestingly, fibroblasts from Rb(MI/MI) mice, in which the C-terminal caspase cleavage site is mutated, exhibit a defect in Bid cleavage despite caspase-8 activation. Recent results suggest that TNF receptor endocytosis is required for the activation of caspase-8. Consistent with this notion, inhibition of V-ATPase, which plays an essential role in acidification and degradation of endosomes, specifically restores Bid cleavage in Rb(MI/MI) cells. Inhibition of V-ATPase sensitizes Rb(MI/MI) but not wild-type fibroblasts to TNF-induced apoptosis and stimulates inflammation-associated colonic apoptosis in Rb(MI/MI) but not wild-type mice. These results suggest that Rb cleavage is required for Bid cleavage in TNF-induced type-2 apoptosis, and this requirement can be supplanted by the inhibition of V-ATPase.

Commitment to apoptosis in CD4(+) T lymphocytes productively infected with human immunodeficiency virus type 1 is initiated by lysosomal membrane permeabilization, itself induced by the isolated expression of the viral protein Nef

Primary CD4(+) T lymphocytes, supporting in vitro human immunodeficiency virus type 1 (HIV-1) replication, are destined to die by apoptosis. We explored the initial molecular events that act upstream from mitochondrial dysfunction in CD4(+) T lymphocytes exposed to the HIV-1(LAI) strain. We tracked by immunofluorescence the cells expressing the p24 viral antigen and used Percoll density gradients to isolate a nonapoptotic CD4(+) T-cell subset with a high inner mitochondrial transmembrane potential (DeltaPsim) but no outer mitochondrial membrane (OMM) rupture. In most p24(+) (but not bystander p24(-)) cells of this subset, the lysosomes were undergoing limited membrane permeabilization, allowing the lysosomal efflux of cathepsins (Cat) to the cytosol. This was also induced by HIV-1 isolates from infected patients. Using pepstatin A to inhibit Cat-D enzymatic activity and Cat-D small interfering RNA to silence the Cat-D gene, we demonstrate that once released into the cytosol, Cat-D induces the conformational change of Bax and its insertion into the OMM. Inhibition of Cat-D activity/expression also conferred a transient survival advantage upon productively HIV-1-infected cells, indicating that Cat-D is an early death factor. The transfection of activated CD4(+) T lymphocytes with a Nef expression vector rapidly induced the permeabilization of lysosomes and the release of Cat-D, with these two events preceding OMM rupture. These results reveal a previously undocumented mechanism in which Nef acts as an internal cytopathic factor and strongly suggest that this viral protein may behave similarly in the context of productive HIV-1 infection in CD4(+) T lymphocytes.

The mitochondrial TOM complex is required for tBid/Bax-induced cytochrome c release

Cytochrome c release from mitochondria is a key event in apoptosis signaling that is regulated by Bcl-2 family proteins. Cleavage of the BH3-only protein Bid by multiple proteases leads to the formation of truncated Bid (tBid), which, in turn, promotes the oligomerization/insertion of Bax into the mitochondrial outer membrane and the resultant release of proteins residing in the intermembrane space. Bax, a monomeric protein in the cytosol, is targeted by a yet unknown mechanism to the mitochondria. Several hypotheses have been put forward to explain this targeting specificity. Using mitochondria isolated from different mutants of the yeast Saccharomyces cerevisiae and recombinant proteins, we have now investigated components of the mitochondrial outer membrane that might be required for tBid/Bax-induced cytochrome c release. Here, we show that the protein translocase of the outer mitochondrial membrane is required for Bax insertion and cytochrome c release.

The Essential Role of the Mitochondria and Reactive Oxygen Species in Cisplatin-Mediated Enhancement of Fas Ligand-Induced Apoptosis in Malignant Pleural Mesothelioma

Cytotoxic chemotherapeutic drugs such as cisplatin (CDDP) synergistically interact with soluble Fas ligand (sFasL) to mediate profound induction of apoptosis in cancer cells, particularly those refractory to this death-inducing ligand. The goal of this study was to evaluate the roles of the mitochondria-dependent apoptotic cascade and the CDDP-generated reactive oxygen species (ROS) in mediating the supra-additive enhancement of cytotoxicity and apoptosis in combination-treated malignant pleural mesothelioma (MPM) cells. MPM cells were treated with sequential CDDP/sFasL in vitro. Cell viability and apoptosis were determined by MTT and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assays. Stable transfectants expressing high levels of Bcl2 were created by retroviral gene transfer. Specific proteolytic activity of caspases 3, 8, and 9 were measured using fluorescent substrates. Pretreating MPM cells with CDDP increased their susceptibility to sFasL by 2- to more than 20-fold. Overexpression of either Bcl-2, the selective caspase 9 inhibitor z-LEHD-fmk, or the antioxidant N-acetylcysteine significantly abrogated combination-induced cytotoxicity and apoptosis. Moreover, the robust activation of caspase 8 in combination-treated cells was completely suppressed by Bcl-2 overexpression, thus implicating a mitochondria-mediated amplification feedback loop. As an in vivo correlate, sequential intraperitoneal administration of CDDP and sFasL significantly inhibited the growth of intraperitoneal MPM human xenografts in nude mice. Our data indicate that the mitochondria-dependent feedback loop of the caspase activation cascade and the generation of ROS are both essential in mediating profound cytotoxicity and apoptosis of MPM cells treated with CDDP and sFasL. This mechanistic study establishes a the translational framework for the clinical application of sequential CDDP/sFasL in the treatment of MPM.

4-HPR-mediated leukemia cell cytotoxicity is triggered by ceramide-induced mitochondrial oxidative stress and is regulated downstream by Bcl-2

We have previously reported that, in leukemia cells, the cytotoxicity of the anticancer agent N-(4-hydroxyphenyl)retinamide (4-HPR) is mediated by mitochondria-derived reactive oxygen species (ROS) and cardiolipin peroxidation. Here, we have analyzed at greater depth the 4-HPR-triggered molecular events, demonstrating that 4-HPR induces an early (15 min) increase in ceramide levels by sphingomyelin hydrolysis and later (from 1 h) by de novo synthesis. Using specific inhibitors of both pathways, we demonstrate that ceramide accumulation is responsible for early ROS generation, which act as apoptotic signalling intermediates leading to conformational activation of Bak and Bax, loss of mitochondrial membrane potential (DeltaPsim), mitochondrial membrane permeabilization (MMP) and cell death. Enforced expression of Bcl-2 has no effect on 4-HPR-induced oxidative stress, but notably prevents mitochondrial alterations and apoptosis, indicating that Bcl-2 functions by regulating events downstream of ROS generation. In conclusion, our study delineates for the fist time the sequence and timing of the principal events induced by 4-HPR in leukemia cells and points to the potential use of modulators of ceramide metabolism as enhancers in 4-HPR-based therapies.

Enhancement of Apo2L/TRAIL-mediated cytotoxicity in esophageal cancer cells by cisplatin

Although expressing adequate levels of functional tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) receptors DR4/DR5, significant proportion of cancer cells exhibit resistance to the cytotoxic effect of this ligand. Exposure of Apo2L/TRAIL-refractory cancer cells to cytotoxic chemotherapeutic agents enhances their sensitivity to Apo2L/TRAIL cytotoxicity. This study aims to elucidate the molecular mechanism responsible for the cisplatin-mediated enhancement of Apo2L/TRAIL sensitivity in cultured esophageal cancer cells. Exposure of cancer cells to sublethal concentrations of cisplatin resulted in profound potentiation of their susceptibility to Apo2L/TRAIL cytotoxicity as indicated by 2- to >20-fold reduction in Apo2L/TRAIL IC50 values. Significant activation of caspase-8, caspase-9, and caspase-3 was observed only in cells treated with cisplatin/Apo2L/TRAIL combination and not in those exposed to either agent alone. More importantly, activation of these key caspases was significantly abrogated by overexpression of Bcl2 or by the selective caspase-9 inhibitor. This observation strongly suggested that caspase-8 activation in cells treated with the cisplatin/Apo2L/TRAIL combination was secondary to the mitochondria-mediated amplification feedback loop and activation of the executioner caspase-3 was dependent on the recruitment of the intrinsic pathway characteristic of the type II cell. Profound combination-mediated cytotoxicity and induction of apoptosis was completely suppressed either by Bcl2 overexpression or by inhibition of caspase-9 activity, which conclusively pointed to the essential role of the mitochondria-dependent death signaling cascade in this process. Cisplatin sensitizes esophageal cancer cells to Apo2L/TRAIL cytotoxicity by potentiation of the mitochondria-dependent death signaling pathway that leads to amplification of caspase activation, particularly caspase-8, by the feedback loop to efficiently induce apoptosis.

Rapid and profound potentiation of Apo2L/TRAIL-mediated cytotoxicity and apoptosis in thoracic cancer cells by the histone deacetylase inhibitor Trichostatin A: the essential role of the mitochondria-mediated caspase activation cascade

Apo2L/TRAIL is actively investigated as a novel targeted agent to directly induce apoptosis of susceptible cancer cells. Apo2L/TRAIL-refractory cells can be sensitized to the cytotoxic effect of this ligand by cytotoxic chemotherapeutics. The aim of this study was to evaluate the in vitro tumoricidal activity of the Apo2L/TRAIL + Trichostatin A in cultured thoracic cancer cells and to elucidate the molecular basis of the synergistic cytotoxicity of this combination. Concurrent exposure of cultured cancer cells to sublethal concentrations of Apo2L/TRAIL and Trichostatin A resulted in profound enhancement of Apo2L/TRAIL-mediated cytotoxicity in all cell lines regardless of their intrinsic susceptibility to this ligand. This combination was not toxic to primary normal cells. While Apo2L/TRAIL alone or Trichostatin A alone mediated < 20% cell death, 60 to 90% of cancer cells were apoptotic following treatment with TSA + Apo2L/TRAIL combinations. Complete translocation of Bax from the cytosol to the mitochondria compartment was mainly observed in combination-treated cells and this was correlated with robust elevation of caspase 9 proteolytic activity indicative of activation of the mitochondria apoptogenic effect. Profound TSA + Apo2L/TRAIL-mediated cytotoxicity and apoptosis were completely abrogated by either Bcl2 over-expression or by the selective caspase 9 inhibitor, highlighting the essential role of mitochondria-dependent apoptosis signaling cascade in this process. Moreover, increased caspase 8 activity observed in cells treated with the TSA + Apo2L/TRAIL combination was completely suppressed by Bcl-2 over-expression or by the selective caspase 9 inhibitor indicating that the elevated caspase 8 activity in combination-treated cells was secondary to a mitochondria-mediated amplification feedback loop of caspase activation. These finding form the basis for further development of HDAC inhibitors + Apo2L/TRAIL combination as novel targeted therapy for thoracic malignancies.

The role of the mitochondria in mediating cytotoxicity of anti-cancer therapies

Optimal cytotoxic anticancer therapy, at the cellular level, requires effective and selective induction of cell death to achieve a net reduction of biomass of malignant tissues. Standard cytotoxic chemotherapeutics have been developed based on the observations that mitotically active cancer cells are more susceptible than quiescent normal cells to chromosomal, microtubular or metabolic poisons. More recent development of molecularly targeted drugs for cancer focuses on exploiting biological differentials between normal and transformed cells for selective eradication of cancers. The common thread of "standard" and "novel" cytotoxic drugs is their ability to activate the apoptosis-inducing machinery mediated by mitochondria, also known as the intrinsic death signaling cascade. The aim of this article is to provide an overview of the role of the mitochondria, an energy-generating organelle essential for life, in mediating death when properly activated by cytotoxic stresses.

Transforming growth factor (TGF)-beta1 stimulates pulmonary fibrosis and inflammation via a Bax-dependent, bid-activated pathway that involves matrix metalloproteinase-12

Fibrosis, apoptosis, and the exaggerated production of transforming growth factor (TGF)-beta(1) are juxtaposed in a variety of pulmonary diseases including the interstitial lung diseases and asthma. In these disorders, the relationships between these responses are not well defined. In addition, the apoptosis pathways that contribute to these responses and the mechanism(s) of their contribution have not been described. We hypothesized that BH3 domain-only protein-induced apoptosis plays an important role in the pathogenesis of TGF-beta(1)-induced pulmonary responses. To test this hypothesis, we characterized the effects of transgenic TGF-beta(1) in mice with wild type (WT) and null Bax loci. To investigate the mechanisms of Bax activation and its effector functions, we also compared the effects of TGF-beta(1) in mice with WT and null Bid and matrix metalloproteinase (MMP)-12 loci, respectively. These studies demonstrate that TGF-beta(1) is a potent stimulator of Bax, Bid, and MMP-12. The studies also demonstrate that Bax and Bid play key roles in the pathogenesis of TGF-beta(1)-induced inflammation, fibrosis, and apoptosis; that TGF-beta(1) stimulates MMP-12, TIMP-1, and cathepsins and inhibits MMP-9 and p21 via Bax- and Bid-dependent mechanisms; and that TGF-beta(1)-stimulated pulmonary fibrosis is ameliorated in MMP-12-deficient animals. Finally, they demonstrate that Bax, Bid, and MMP-12 play similar roles in bleomycin-induced fibrosis, thereby highlighting the importance of this Bid-activated, Bax-mediated pathway and downstream MMP-12 in a variety of fibrogenic settings.

Cell death provoked by loss of interleukin-3 signaling is independent of Bad, Bim, and PI3 kinase, but depends in part on Puma

Growth and survival of hematopoietic cells is regulated by growth factors and cytokines, such as interleukin 3 (IL-3). When cytokine is removed, cells dependent on IL-3 kill themselves by a mechanism that is inhibited by overexpression of Bcl-2 and is likely to be mediated by proapoptotic Bcl-2 family members. Bad and Bim are 2 such BH3-only Bcl-2 family members that have been implicated as key initiators in apoptosis following growth factor withdrawal, particularly in IL-3-dependent cells. To test the role of Bad, Bim, and other proapoptotic Bcl-2 family members in IL-3 withdrawal-induced apoptosis, we generated IL-3-dependent cell lines from mice lacking the genes for Bad, Bim, Puma, both Bad and Bim, and both Bax and Bak. Surprisingly, Bad was not required for cell death following IL-3 withdrawal, suggesting changes to phosphorylation of Bad play only a minor role in apoptosis in this system. Deletion of Bim also had no effect, but cells lacking Puma survived and formed colonies when IL-3 was restored. Inhibition of the PI3 kinase pathway promoted apoptosis in the presence or absence of IL-3 and did not require Bad, Bim, or Puma, suggesting IL-3 receptor survival signals and PI3 kinase survival signals are independent.

Valproic acid, an antiepileptic drug with histone deacetylase inhibitory activity, potentiates the cytotoxic effect of Apo2L/TRAIL on cultured thoracic cancer cells through mitochondria-dependent caspase activation

Inhibitors of histone deacetylases have been shown to enhance the sensitivity of cancer cells to tumor necrosis factor-related apoptosis-inducing ligand TRAIL-mediated cytotoxicity. Valproic acid (VA), a commonly used antiepileptic agent whose pharmacokinetics and toxicity profiles are well described, is a histone deacetylase inhibitor. This project aims to evaluate if VA can potentiate Apo2L/TRAIL-mediated cytotoxicity in cultured thoracic cancer cells and to elucidate the underlying molecular mechanism responsible for this effect. VA sensitized cultured thoracic cancer cells to Apo2L/TRAIL, as indicated by a 4-fold to a >20-fold reduction of Apo2L/TRAIL IC50 values in combination-treated cells. Although VA (0.5-5 mM) or Apo2L/TRAIL (20 ng/ml) induced less than 20% cell death, VA + Apo2L/TRAIL combinations caused 60% to 90% apoptosis of cancer cells. Moreover, substantial activation of caspases 8, 9, and 3, which was observed only in cells treated with the drug combination, was completely suppressed by Bcl2 overexpression or by the caspase 9 inhibitor. Both the caspase 9 inhibitor and Bcl2 completely abrogated the substantial cytotoxicity and apoptosis induced by this combination, thus highlighting the pivotal role of the type II pathway in this process. These findings provide a rationale for the development of VA and Apo2L/TRAIL combination as a novel molecular therapeutic for thoracic cancers.

The Essential Role of the Mitochondria-Dependent Death-Signaling Cascade in Chemotherapy-Induced Potentiation of Apo2L/TRAIL Cytotoxicity in Cultured Thoracic Cancer Cells

PURPOSE

Despite adequately expressing functional receptors for tumor necrosis factor receptor apoptosis-inducing ligand (TRAIL), many cultured tumor cells are refractory to the cytotoxic effect of this ligand. Cytotoxic chemotherapeutic drugs have been shown to synergize with Apo2L/TRAIL to mediate apoptosis in cancer cells. The main goal of this study was to evaluate the effect of either cisplatin or paclitaxel, two common used chemotherapeutic agents for solid tumors, on enhancing Apo2L/TRAIL cytotoxicity in a panel-cultured thoracic cancer cells and to examine the role of the mitochondria-dependent caspase activation cascade in mediating apoptosis of combination-treated cells.

METHODS

Cultured thoracic cancer cells were treated with cisplatin/Apo2L/TRAIL or paclitaxel/Apo2L/TRAIL sequential combinations in vitro. Cell viability and apoptosis were determined by 4,5-dimethylthiazo-2-yl)-2,5-diphenyl tetrazolium bromide and terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling assays. Stable transfectants expressing high levels of Bcl-2 were created by retroviral gene transfer. Specific proteolytic activity of caspases 3, 6, 8, and 9 were measured by commercially available kits using fluorescent substrates.

RESULTS

All cell lines preferentially expressed high levels of DR4 and/or DR5 and low levels of DcR1/DcR2; all of which were not altered by chemotherapeutic drug treatments. Pretreatment of these cancer cells with sublethal concentrations of either cisplatin or paclitaxel increased their susceptibility to Apo2L/TRAIL by twofold to >20-fold. Profound synergistic induction of apoptosis was observed in combination-treated cells. Viability of primary normal cells was affected by neither Apo2L/TRAIL nor the combinations of chemotherapy and Apo2L/TRAIL. Overexpression of Bcl-2 or inhibition of caspase 9 activity completely abrogated combination-induced cytotoxicity and apoptosis, indicating the essential role of the mitochondria-dependent death signaling cascade in this process. Robust activation of caspase 8 in combination-treated cells was completely suppressed either by Bcl-2 overexpression or by blocking of the activity of the mitochondria-regulated caspase 9, thus identifying the amplification feedback loop as the source of elevated caspase 8 activity. Finally, mitochondria-mediated amplification of caspase 8 activity was indispensable for complete caspase activation and full execution of apoptosis, because suppression of its activity using the selective caspase 6 inhibitor (located downstream of the caspase 3 but upstream of the caspase 8 in the feedback loop) resulted in profound suppression of not only caspase 8 activity but also those of caspases 9 and 3, as well as complete protection of cancer cells from combination-induced cytotoxicity.

CONCLUSION

Cisplatin or paclitaxel synergistically interacts with Apo2L/TRAIL to mediate profound induction of apoptosis. The mitochondria-dependent caspase activation cascade and the amplification feedback loop are essential for the complete execution of the cell death program. Furthermore, our data identify mitochondria as the direct target for the development of more refined strategies to enhance the therapeutic effect of Apo2L/TRAIL as an anticancer agent.

Mitochondrial membrane dynamics, cristae remodelling and apoptosis

Mitochondria form a highly dynamic reticular network in living cells, and undergo continuous fusion/fission events and changes in ultrastructural architecture. Although significant progress has been made in elucidating the molecular events underlying these processes, their relevance to normal cell function remains largely unexplored. Emerging evidence, however, suggests an important role for mitochondrial dynamics in cellular apoptosis. The mitochondria is at the core of the intrinsic apoptosis pathway, and provides a reservoir for protein factors that induce caspase activation and chromosome fragmentation. Additionally, mitochondria modulate Ca2+ homeostasis and are a source of various metabolites, including reactive oxygen species, that have the potential to function as second messengers in response to apoptotic stimuli. One of the mitochondrial factors required for activation of caspases in most intrinsic apoptotic pathways, cytochrome c, is largely sequestered within the intracristae compartment, and must migrate into the boundary intermembrane space in order to allow passage across the outer membrane to the cytosol. Recent evidence argues that inner mitochondrial membrane dynamics regulate this process. Here, we review the contribution of mitochondrial dynamics to the intrinsic apoptosis pathway, with emphasis on the inner membrane.

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

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

NAD- and NADP-dependent mitochondrially targeted methylenetetrahydrofolate dehydrogenase-cyclohydrolases can rescue mthfd2 null fibroblasts

Mouse fibroblasts in which the mthfd2 gene encoding mitochondrial NAD-dependent methylenetetrahydrofolate dehydrogenase-cyclohydrolase (NMDMC) was previously inactivated were infected with retroviral expression constructs of dehydrogenase/cyclohydrolase cDNA. Cellular fractionation confirmed that the expressed proteins were properly targeted to the mitochondria. Expression of the NAD-dependent methylenetetrahydrofolate dehydrogenase-cyclohydrolase enzyme in mitochondria corrected the glycine auxotrophy of the null mutant cells. A construct in which the cyclohydrolase activity of NMDMC was inactivated by point mutation also rescued the glycine auxotrophy, although poorly. This suggests that the cyclohydrolase activity is also required to ensure optimal production of 10-formyltetrahydrofolate. The expression of the NADP-dependent methylenetetrahydrofolate dehydrogenase-cyclohydrolase-synthetase in the mitochondria also reversed the glycine requirement of the null cells demonstrating that the use of the NAD cofactor is not absolutely essential to maintain the flux of one-carbon metabolites. All rescued cells demonstrated a decrease in the ratio of incorporation of exogenous formate to serine in standardized radiolabeling studies. This ratio, which is approximately 2.5 for nmdmc(-/-) cells and 0.3 for the wild type cells under the conditions used, is a qualitative indicator of the ability of the mitochondria of the cells to generate formate.

Cell death induced by ent-11α-hydroxy-15-oxo-kaur-16-en-19-oic-acid in anaplastic thyroid carcinoma cells is via a mitochondrial-mediated pathway

The chemical compound ent-11alpha-hydroxy-15-oxo-kaur-16-en-19-oic-acid (5F), isolated from the Chinese herbal medicine plant Pteris semipinnata L, has been known to exert antitumor activity. However, the molecular mechanism of the action is not understood. In this study we demonstrated that apoptotic cell death induced by 5F in FRO cells was concentration- and time-dependent. The rapid increase in intracellular reactive oxygen species (ROS) levels was involved in the mechanism of cell death. c-Jun N-terminal kinase (JNK) activation and G2 block were related to cell death induced by 5F. Extracellular signal-related kinase (ERK) and p38 were also activated, but as survival signals in response to 5F treatment to counteract the induction of cell death. In the process of the induction of apoptotic cell death, Bax translocated into mitochondria, a reduction in Delta psi(m) was observed and a release of cytochrome c and apoptosis inducing factor (AIF) from mitochondria into the cytosol occurred, indicating that cell death induced by 5F was through a mitochondrial-mediated pathway.

Regulation of mitochondrial membrane permeabilization by BCL-2 family proteins and caspases

Mitochondria play an important role in the integration and transmission of cell death signals, activating caspases and other cell death execution events by releasing apoptogenic proteins from the intermembrane space. The BCL-2 family of proteins localize (or can be targeted) to mitochondria and regulate the permeability of the mitochondrial outer membrane to these apoptotic factors. Recent evidence suggests that multiple mechanisms may regulate the release of mitochondrial factors, some of which depend on the action of caspases.

Disruption of the mthfd1 gene reveals a monofunctional 10-formyltetrahydrofolate synthetase in mammalian mitochondria

The Mthfd1 gene encoding the cytoplasmic methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase-formyltetrahydrofolate synthetase enzyme (DCS) was inactivated in embryonic stem cells. The null embryonic stem cells were used to generate spontaneously immortalized fibroblast cell lines that exhibit the expected purine auxotrophy. Elimination of these cytoplasmic activities allowed for the accurate assessment of similar activities encoded by other genes in these cells. A low level of 10-formyltetrahydrofolate synthetase was detected and was shown to be localized to mitochondria. However, NADP-dependent methylenetetrahydrofolate dehydrogenase activity was not detected. Northern blot analysis suggests that a recently identified mitochondrial DCS (Prasannan, P., Pike, S., Peng, K., Shane, B., and Appling, D. R. (2003) J. Biol. Chem. 278, 43178-43187) is responsible for the synthetase activity. The lack of NADP-dependent dehydrogenase activity suggests that this RNA may encode a monofunctional synthetase. Moreover, examination of the primary structure of this novel protein revealed mutations in key residues required for dehydrogenase and cyclohydrolase activities. This monofunctional synthetase completes the pathway for the production of formate from formyltetrahydrofolate in the mitochondria in our model of mammalian one-carbon folate metabolism in embryonic and transformed cells.