Mechanisms of AIF-mediated apoptotic DNA degradation in Caenorhabditis elegans

Functional regulation of FEN1 nuclease and its link to cancer

Flap endonuclease-1 (FEN1) is a member of the Rad2 structure-specific nuclease family. FEN1 possesses FEN, 5′-exonuclease and gap-endonuclease activities. The multiple nuclease activities of FEN1 allow it to participate in numerous DNA metabolic pathways, including Okazaki fragment maturation, stalled replication fork rescue, telomere maintenance, long-patch base excision repair and apoptotic DNA fragmentation. Here, we summarize the distinct roles of the different nuclease activities of FEN1 in these pathways. Recent biochemical and genetic studies indicate that FEN1 interacts with more than 30 proteins and undergoes post-translational modifications. We discuss how FEN1 is regulated via these mechanisms. Moreover, FEN1 interacts with five distinct groups of DNA metabolic proteins, allowing the nuclease to be recruited to a specific DNA metabolic complex, such as the DNA replication machinery for RNA primer removal or the DNA degradosome for apoptotic DNA fragmentation. Some FEN1 interaction partners also stimulate FEN1 nuclease activities to further ensure efficient action in processing of different DNA structures. Post-translational modifications, on the other hand, may be critical to regulate protein–protein interactions and cellular localizations of FEN1. Lastly, we also review the biological significance of FEN1 as a tumor suppressor, with an emphasis on studies of human mutations and mouse models.

Delayed activation of caspase-independent apoptosis during heart failure in transgenic mice overexpressing caspase inhibitor CrmA

Although caspase activation is generally thought to be necessary to induce apoptosis, recent evidence suggests that apoptosis can be activated in the setting of caspase inhibition. In this study, we tested the hypothesis that caspase-independent apoptotic pathways contribute to the development of heart failure in the absence of caspase activation. Acute cardiomyopathy was induced using a single dose of doxorubicin (Dox, 20 mg/kg) injected into male wild-type (WT) and transgenic (Tg) mice with a cardiac-specific expression of cytokine response modifier A (CrmA), a known caspase inhibitor. Early (6 day) survival was significantly better in CrmA Tg (81%) than WT (38%) mice. Twelve days after Dox injection, however, the mortality benefit had dissipated, and increased cardiac apoptosis was observed in both groups. There was, however, a significantly greater release of apoptosis-inducing factor (AIF) from mitochondria to cytosol in CrmA Tg compared with WT mice, which suggests that an enhancement of activation in caspase-independent apoptotic pathways had occurred. The administration of a poly(ADP-ribose) polymerase-1 inhibitor, 4-amino-1,8-naphthalimide (4-AN), to Dox-treated mice resulted in significantly improved cardiac function, a significant blockade of AIF released from mitochondria, and decreased cardiac apoptosis. There were also significantly improved survival in WT (18% without 4-AN vs. 89% with 4-AN) and CrmA Tg (13% without 4-AN vs. 93% with 4-AN) mice 12 days after Dox injection. In conclusion, these findings suggest that apoptosis can be induced in the heart lacking caspase activation via caspase-independent pathways and that enabling the inhibition of AIF activation may provide a significant cardiac benefit.

Protection of cisplatin cytotoxicity by an inactive cyclin-dependent kinase

Cisplatin cytotoxicity is dependent on cyclin-dependent kinase 2 (Cdk2) activity in vivo and in vitro. A Cdk2 mutant (Cdk2-F80G) was designed in which the ATP-binding pocket was altered. When expressed in mouse kidney cells, this protein was kinase inactive, did not inhibit endogenous Cdk2, but protected from cisplatin. The mutant was localized in the cytoplasm, but when coexpressed with cyclin A, it was activated, localized to the nucleus, and no longer protected from cisplatin cytotoxicity. Cells exposed to cisplatin in the presence of the activated mutant had an apoptotic phenotype, and endonuclease G was released from mitochondria similar to that mediated by endogenous Cdk2. But unlike apoptosis mediated by wild-type Cdk2, cisplatin exposure of cells expressing the activated mutant did not cause cytochrome c release or significant caspase-3 activation. We conclude that cisplatin likely activates both caspase-dependent and -independent cell death, and Cdk2 is required for both pathways. The mutant-inactive Cdk2 protected from both death pathways, but after activation by excess cyclin A, caspase-independent cell death predominated.

Endonuclease G does not play an obligatory role in poly(ADP-ribose) polymerase-dependent cell death after transient focal cerebral ischemia

Activation of poly(ADP-ribose) polymerase (PARP) and subsequent translocation of apoptosis-inducing factor contribute to caspase-independent neuronal injury from N-methyl-d-aspartate, oxygen-glucose deprivation, and ischemic stroke. Some studies have implicated endonuclease G in the DNA fragmentation associated with caspase-independent cell death. Here, we compared wild-type and endonuclease G null mice to investigate whether endonuclease G plays a role in the PARP-dependent injury that results from transient focal cerebral ischemia. Latex casts did not reveal differences in the cerebral arterial distribution territory or posterior communicating arterial diameter, and the decrease in laser-Doppler flux during middle cerebral artery occlusion was similar in wild-type and endonuclease G null mice. After 90 min of occlusion and 1 day of reperfusion, similar degrees of nuclear translocation of apoptosis-inducing factor and DNA degradation were evident in male wild-type and null mice. At 3 days of reperfusion, infarct volume and neurological deficit scores were not different between male wild-type and endonuclease G null mice or between female wild-type and endonuclease G null mice. These data demonstrate that endonuclease G is not required for the pathogenesis of transient focal ischemia in either male or female mice. Treatment with a PARP inhibitor decreased infarct volume and deficit scores equivalently in male wild-type and endonuclease G null mice, indicating that the injury in endonuclease G null mice remains dependent on PARP. Thus endonuclease G is not obligatory for executing PARP-dependent injury during ischemic stroke.

Severe X-linked mitochondrial encephalomyopathy associated with a mutation in apoptosis-inducing factor

We investigated two male infant patients who were given a diagnosis of progressive mitochondrial encephalomyopathy on the basis of clinical, biochemical, and morphological features. These patients were born from monozygotic twin sisters and unrelated fathers, suggesting an X-linked trait. Fibroblasts from both showed reduction of respiratory chain (RC) cIII and cIV, but not of cI activities. We found a disease-segregating mutation in the X-linked AIFM1 gene, encoding the Apoptosis-Inducing Factor (AIF) mitochondrion-associated 1 precursor that deletes arginine 201 (R201 del). Under normal conditions, mature AIF is a FAD-dependent NADH oxidase of unknown function and is targeted to the mitochondrial intermembrane space (this form is called AIF(mit)). Upon apoptogenic stimuli, a soluble form (AIF(sol)) is released by proteolytic cleavage and migrates to the nucleus, where it induces "parthanatos," i.e., caspase-independent fragmentation of chromosomal DNA. In vitro, the AIF(R201 del) mutation decreases stability of both AIF(mit) and AIF(sol) and increases the AIF(sol) DNA binding affinity, a prerequisite for nuclear apoptosis. In AIF(R201 del) fibroblasts, staurosporine-induced parthanatos was markedly increased, whereas re-expression of AIF(wt) induced recovery of RC activities. Numerous TUNEL-positive, caspase 3-negative nuclei were visualized in patient #1's muscle, again indicating markedly increased parthanatos in the AIF(R201 del) critical tissues. We conclude that AIF(R201 del) is an unstable mutant variant associated with increased parthanatos-linked cell death. Our data suggest a role for AIF in RC integrity and mtDNA maintenance, at least in some tissues. Interestingly, riboflavin supplementation was associated with prolonged improvement of patient #1's neurological conditions, as well as correction of RC defects in mutant fibroblasts, suggesting that stabilization of the FAD binding in AIF(mit) is beneficial.

Production and characterization of recombinant protein preparations of Endonuclease G-homologs from yeast, C. elegans and humans

Nuc1p, CPS-6, EndoG and EXOG are evolutionary conserved mitochondrial nucleases from yeast, Caenorhabditis elegans and humans, respectively. These enzymes play an important role in programmed cell death as well as mitochondrial DNA-repair and recombination. Whereas a significant interest has been given to the cell biology of these proteins, in particular their recruitment during caspase-independent apoptosis, determination of their biochemical properties has lagged behind. In part, biochemical as well as structural analysis of mitochondrial nucleases has been hampered by the fact that upon cloning and overexpression in Escherichia coli these enzymes can exert considerable toxicity and tend to aggregate and form inclusion bodies. We have, therefore, established a uniform E. coli expression system allowing us to obtain these four evolutionary related nucleases in active form from the soluble as well as insoluble fractions of E. coli cell lysates. Using preparations of recombinant Nuc1p, CPS-6, EndoG and EXOG we have compared biochemical properties and the substrate specificities of these related nucleases on selected substrates in parallel. Whereas Nuc1p and EXOG in addition to their endonuclease activity exert 5'-3'-exonuclease activity, CPS-6 and EndoG predominantly are endonucleases. These findings allow speculating that the mechanisms of action of these related nucleases in cell death as well as DNA-repair and recombination differ according to their enzyme activities and substrate specificities.

AIF promotes chromatinolysis and caspase-independent programmed necrosis by interacting with histone H2AX

Programmed necrosis induced by DNA alkylating agents, such as MNNG, is a caspase-independent mode of cell death mediated by apoptosis-inducing factor (AIF). After poly(ADP-ribose) polymerase 1, calpain, and Bax activation, AIF moves from the mitochondria to the nucleus where it induces chromatinolysis and cell death. The mechanisms underlying the nuclear action of AIF are, however, largely unknown. We show here that, through its C-terminal proline-rich binding domain (PBD, residues 543-559), AIF associates in the nucleus with histone H2AX. This interaction regulates chromatinolysis and programmed necrosis by generating an active DNA-degrading complex with cyclophilin A (CypA). Deletion or directed mutagenesis in the AIF C-terminal PBD abolishes AIF/H2AX interaction and AIF-mediated chromatinolysis. H2AX genetic ablation or CypA downregulation confers resistance to programmed necrosis. AIF fails to induce chromatinolysis in H2AX or CypA-deficient nuclei. We also establish that H2AX is phosphorylated at Ser139 after MNNG treatment and that this phosphorylation is critical for caspase-independent programmed necrosis. Overall, our data shed new light in the mechanisms regulating programmed necrosis, elucidate a key nuclear partner of AIF, and uncover an AIF apoptogenic motif.

Role of apoptosis-inducing factor (AIF) in programmed nuclear death during conjugation in Tetrahymena thermophila

Background

Programmed nuclear death (PND), which is also referred to as nuclear apoptosis, is a remarkable process that occurs in ciliates during sexual reproduction (conjugation). In Tetrahymena thermophila, when the new macronucleus differentiates, the parental macronucleus is selectively eliminated from the cytoplasm of the progeny, concomitant with apoptotic nuclear events. However, the molecular mechanisms underlying these events are not well understood. The parental macronucleus is engulfed by a large autophagosome, which contains numerous mitochondria that have lost their membrane potential. In animals, mitochondrial depolarization precedes apoptotic cell death, which involves DNA fragmentation and subsequent nuclear degradation.

Results

We focused on the role of mitochondrial apoptosis-inducing factor (AIF) during PND in Tetrahymena. The disruption of AIF delays the normal progression of PND, specifically, nuclear condensation and kilobase-size DNA fragmentation. AIF is localized in Tetrahymena mitochondria and is released into the macronucleus prior to nuclear condensation. In addition, AIF associates and co-operates with the mitochondrial DNase to facilitate the degradation of kilobase-size DNA, which is followed by oligonucleosome-size DNA laddering.

Conclusions

Our results suggest that Tetrahymena AIF plays an important role in the degradation of DNA at an early stage of PND, which supports the notion that the mitochondrion-initiated apoptotic DNA degradation pathway is widely conserved among eukaryotes.

Life with or without AIF

Apoptosis-inducing factor (AIF) was initially discovered as a caspase-independent death effector. AIF fulfills its lethal function after its release from mitochondria and its translocation to the nucleus of the dying cell. The contribution of AIF to programmed cell death is dependent upon the cell type and apoptotic insult. Recent in vivo data indicate that, in addition to its lethal activity, AIF plays a vital mitochondrial role in healthy cells. A segment of AIF which is dispensable for its apoptotic function carries an NADH-oxidase domain that regulates the respiratory chain complex I and is required for cell survival, proliferation and mitochondrial integrity. Mice that express reduced levels of AIF constitute a reliable model of complex I deficiency. Here we discuss recent reports on the survival-related function(s) of AIF.

The role of mitochondria in apoptosis*

Mitochondria play key roles in activating apoptosis in mammalian cells. Bcl-2 family members regulate the release of proteins from the space between the mitochondrial inner and outer membrane that, once in the cytosol, activate caspase proteases that dismantle cells and signal efficient phagocytosis of cell corpses. Here we review the extensive literature on proteins released from the intermembrane space and consider genetic evidence for and against their roles in apoptosis activation. We also compare and contrast apoptosis pathways in Caenorhabditis elegans, Drosophila melanogaster, and mammals that indicate major mysteries remaining to be solved.

The ins and outs of phospholipid asymmetry in the plasma membrane: roles in health and disease

A common feature of all eukaryotic membranes is the non-random distribution of different lipid species in the lipid bilayer (lipid asymmetry). Lipid asymmetry provides the two sides of the plasma membrane with different biophysical properties and influences numerous cellular functions. Alteration of lipid asymmetry plays a prominent role during cell fusion, activation of the coagulation cascade, and recognition and removal of apoptotic cell corpses by macrophages (programmed cell clearance). Here we discuss the origin and maintenance of phospholipid asymmetry, based on recent studies in mammalian systems as well as in Caenhorhabditis elegans and other model organisms, along with emerging evidence for a conserved role of mitochondria in the loss of lipid asymmetry during apoptosis. The functional significance of lipid asymmetry and its disruption during health and disease is also discussed.

BH3-only proteins Mcl-1 and Bim as well as endonuclease G are targeted in spongistatin 1-induced apoptosis in breast cancer cells

Spongistatin 1, a marine experimental substance with chemotherapeutic potential, induces apoptosis and inhibits clonogenic survival of MCF-7 cells. Regarding the apoptotic signaling pathways of spongistatin 1, we present two major facts. Firstly, spongistatin 1-induced cell death, mainly caspase-independent, involves the proapoptotic proteins apoptosis-inducing factor and endonuclease G. Both proteins translocate from mitochondria to the nucleus and contribute to spongistatin 1-mediated apoptosis as shown via gene silencing. Secondly, spongistatin 1 acts as a tubulin depolymerizing agent and is able to free the proapoptotic Bcl-2 family member Bim from its sequestration both by the microtubular complex and by the antiapoptotic protein Mcl-1. Silencing of Bim by small interfering RNA leads to a diminished translocation of apoptosis-inducing factor and endonuclease G to the nucleus and subsequently reduces apoptosis rate. Thus, we identified Bim as an important factor upstream of mitochondria executing a central role in the caspase-independent apoptotic signaling pathway induced by spongistatin 1. Taken together, spongistatin 1 is both a valuable tool for the characterization of apoptotic pathways and a promising experimental anticancer drug.

C. elegans Rab GTPase activating protein TBC-2 promotes cell corpse degradation by regulating the small GTPase RAB-5

During apoptosis, dying cells are quickly internalized by neighboring cells or phagocytes, and are enclosed in phagosomes that undergo a maturation process to generate the phagoslysosome, in which cell corpses are eventually degraded. It is not well understood how apoptotic cell degradation is regulated. Here we report the identification and characterization of the C. elegans tbc-2 gene, which is required for the efficient degradation of cell corpses. tbc-2 encodes a Rab GTPase activating protein (GAP) and its loss of function affects several events of phagosome maturation, including RAB-5 release, phosphatidylinositol 3-phosphate dynamics, phagosomal acidification, RAB-7 recruitment and lysosome incorporation, which leads to many persistent cell corpses at various developmental stages. Intriguingly, the persistent cell corpse phenotype of tbc-2 mutants can be suppressed by reducing gene expression of rab-5, and overexpression of a GTP-locked RAB-5 caused similar defects in phagosome maturation and cell corpse degradation. We propose that TBC-2 functions as a GAP to cycle RAB-5 from an active GTP-bound to an inactive GDP-bound state, which is required for maintaining RAB-5 dynamics on phagosomes and serves as a switch for the progression of phagosome maturation.

HYD1-induced increase in reactive oxygen species leads to autophagy and necrotic cell death in multiple myeloma cells

HYD1 is a D-amino acid peptide that was previously shown to inhibit adhesion of prostate cancer cells to the extracellular matrix. In this study, we show that in addition to inhibiting adhesion of multiple myeloma (MM) cells to fibronectin, HYD1 induces cell death in MM cells as a single agent. HYD1-induced cell death was necrotic in nature as shown by: (a) decrease in mitochondrial membrane potential (Deltapsi(m)), (b) loss of total cellular ATP, and (c) increase in reactive oxygen species (ROS) production. Moreover, HYD1 treatment does not result in apoptotic cell death because it did not trigger the activation of caspases or the release of apoptosis-inducing factor and endonuclease G from the mitochondria, nor did it induce double-stranded DNA breaks. HYD1 did initiate autophagy in cells; however, autophagy was found to be an adaptive response contributing to cell survival rather than the cause of cell death. We were further able to show that N-acetyl-L-cysteine, a thiol-containing free radical scavenger, partially protects MM cells from HYD1-induced death. Additionally, N-acetyl-L-cysteine blocked HYD1-induced as well as basal levels of autophagy, suggesting that ROS can potentially trigger both cell death and cell survival pathways. Taken together, our data describe an important role of ROS in HYD1-induced necrotic cell death in MM cells.

Granzyme F induces a novel death pathway characterized by Bid-independent cytochrome c release without caspase activation

Granzyme F (GzmF) belongs to a unique group of granzymes in mice. Murine GzmF is highly expressed in NK3.1 cells and in lymphokine-activated killer (LAK) cells. However, the manner in which GzmF works in granule-mediated cytolysis is unknown. In this study, we first demonstrated that GzmF causes a novel cell death pathway. The death is characterized by an externalization of phosphatidylserine, by nuclear condensation, mitochondrial damage, cytochrome c (cyt c) release, caspase inactivation and single-stranded DNA nicking. GzmF-induced chromatin was incompletely condensed and segmented at the nuclear periphery. Cellular organelles were damaged and the cytoplasm showed an extensive vacuolization that is reminiscent of necroptosis. GzmF can cause rapid mitochondrial swelling, depolarization and reactive oxygen species accumulation. GzmF-induced death does not involve caspase activation, Bid cleavage or activation of DNA nickase NM23H1. GzmF-silenced LAK cells showed reduced cytotoxicity against caspase-inhibited target tumor cells. Moreover, cyt c release is independent of Bid or Bax/Bak. We further showed that GzmF impairs mitochondrial electron transport to abolish ATP generation. ATP decline may contribute to a failure of apoptosome formation, leading to caspase inactivation.

Prediction of localization and interactions of apoptotic proteins

During apoptosis several mitochondrial proteins are released. Some of them participate in caspase-independent nuclear DNA degradation, especially apoptosis-inducing factor (AIF) and endonuclease G (endoG). Another interesting protein, which was expected to act similarly as AIF due to the high sequence homology with AIF is AIF-homologous mitochondrion-associated inducer of death (AMID). We studied the structure, cellular localization, and interactions of several proteins in silico and also in cells using fluorescent microscopy. We found the AMID protein to be cytoplasmic, most probably incorporated into the cytoplasmic side of the lipid membranes. Bioinformatic predictions were conducted to analyze the interactions of the studied proteins with each other and with other possible partners. We conducted molecular modeling of proteins with unknown 3D structures. These models were then refined by MolProbity server and employed in molecular docking simulations of interactions. Our results show data acquired using a combination of modern in silico methods and image analysis to understand the localization, interactions and functions of proteins AMID, AIF, endonuclease G, and other apoptosis-related proteins.

Survivin counteracts the therapeutic effect of microtubule de-stabilizers by stabilizing tubulin polymers

Background

Survivin is a dual function protein. It inhibits the apoptosis of cells by inhibiting caspases, and also promotes cell growth by stabilizing microtubules during mitosis. Over-expression of survivin has been demonstrated to induce drug-resistance to various chemo-therapeutic agents such as cisplatin (DNA damaging agent) and paclitaxel (microtubule stabilizer) in cancers. However, survivin-induced resistance to microtubule de-stabilizers such as Vinca alkaloids and Combretastatin A-4 (CA-4)-related compounds were seldom demonstrated in the past. Furthermore, the question remains as to whether survivin plays a dominant role in processing cytokinesis or inhibiting caspases activity in cells treated with anti-mitotic compounds. The purpose of this study is to evaluate the effect of survivin on the resistance and susceptibility of human cancer cells to microtubule de-stabilizer-induced cell death.

Results

BPR0L075 is a CA-4 analog that induces microtubule de-polymerization and subsequent caspase-dependent apoptosis. To study the relationship between the expression of survivin and the resistance to microtubule de-stabilizers, a KB-derived BPR0L075-resistant cancer cell line, KB-L30, was generated for this study. Here, we found that survivin was over-expressed in the KB-L30 cells. Down-regulation of survivin by siRNA induced hyper-sensitivity to BPR0L075 in KB cells and partially re-stored sensitivity to BPR0L075 in KB-L30 cells. Western blot analysis revealed that down-regulation of survivin induced microtubule de-stabilization in both KB and KB-L30 cells. However, the same treatment did not enhance the down-stream caspase-3/-7 activities in BPR0L075-treated KB cells. Translocation of a caspase-independent apoptosis-related molecule, apoptosis-inducing factor (AIF), from cytoplasm to the nucleus was observed in survivin-targeted KB cells under BPR0L075 treatment.

Conclusion

In this study, survivin plays an important role in the stability of microtubules, but not with caspases inhibition. Over-expression of survivin counteracts the therapeutic effect of microtubule de-stabilizer BPR0L075 probably by stabilizing tubulin polymers, instead of the inhibition of caspase activity in cancer cells. Besides microtubule-related caspase-dependent cell death, caspase-independent mitotic cell death could be initiated in survivin/BPR0L075 combination treatments. We suggest that combining microtubule de-stabilizers with a survivin inhibitor may attribute to a better clinical outcome than the use of anti-mitotic monotherapy in clinical situations.

Caenorhabditis elegans protein arginine methyltransferase PRMT-5 negatively regulates DNA damage-induced apoptosis

Arginine methylation of histone and non-histone proteins is involved in transcription regulation and many other cellular processes. Nevertheless, whether such protein modification plays a regulatory role during apoptosis remains largely unknown. Here we report that the Caenorhabditis elegans homolog of mammalian type II arginine methyltransferase PRMT5 negatively regulates DNA damage-induced apoptosis. We show that inactivation of C. elegans prmt-5 leads to excessive apoptosis in germline following ionizing irradiation, which is due to a CEP-1/p53–dependent up-regulation of the cell death initiator EGL-1. Moreover, we provide evidence that CBP-1, the worm ortholog of human p300/CBP, functions as a cofactor of CEP-1. PRMT-5 forms a complex with both CEP-1 and CBP-1 and can methylate the latter. Importantly, down-regulation of cbp-1 significantly suppresses DNA damage-induced egl-1 expression and apoptosis in prmt-5 mutant worms. These findings suggest that PRMT-5 likely represses CEP-1 transcriptional activity through CBP-1, which represents a novel regulatory mechanism of p53-dependent apoptosis.

Protein arginine methylation is an important posttranslational modification. Aberrant expression of protein arginine methyltransferases (PRMTs) are found in a wide variety of human diseases, especially in many cancers. Given that deregulation of apoptosis is usually related to tumorigenesis, it is not known whether PRMT–mediated protein arginine methylation plays a role in apoptosis. Here we employ the genetic tractable model organism C. elegans to explore the potential regulatory roles of PRMTs in apoptosis. We find that C. elegans PRMTs do not affect developmental cell deaths. However, genetic inactivation of the C. elegans homolog of the mammalian type II protein arginine methyltransferase PRMT5 causes excessive germ cell apoptosis in response to DNA damage. Our genetic analyses indicate that prmt-5–mediated apoptosis is dependent on the C. elegans p53 homolog CEP-1 and requires the core cell death pathway. We further demonstrate that loss of prmt-5 leads to a specific up-regulation of the cell death initiator EGL-1 following DNA damage. Finally, we identify CBP-1, the C. elegans homolog of human p300/CBP, as a CEP-1 cofactor in C. elegans, and we provide genetic and biochemical evidence that PRMT-5 likely functions through CBP-1 to affect CEP-1/p53 transcriptional activity, thereby negatively regulating CEP-1/p53-dependent apoptosis.

Redox-linked conformational dynamics in apoptosis-inducing factor

Apoptosis-inducing factor (AIF) is a bifunctional mitochondrial flavoprotein critical for energy metabolism and induction of caspase-independent apoptosis, whose exact role in normal mitochondria remains unknown. Upon reduction with NADH, AIF undergoes dimerization and forms tight, long-lived FADH(2)-NAD charge-transfer complexes (CTC) that are proposed to be functionally important. To obtain a deeper insight into structure/function relations and redox mechanism of this vitally important protein, we determined the X-ray structures of oxidized and NADH-reduced forms of naturally folded recombinant murine AIF. Our structures reveal that CTC with the pyridine nucleotide is stabilized by (i) pi-stacking interactions between coplanar nicotinamide, isoalloxazine, and Phe309 rings; (ii) rearrangement of multiple aromatic residues in the C-terminal domain, likely serving as an electron delocalization site; and (iii) an extensive hydrogen-bonding network involving His453, a key residue that undergoes a conformational switch to directly interact with and optimally orient the nicotinamide for charge transfer. Via the His453-containing peptide, redox changes in the active site are transmitted to the surface, promoting AIF dimerization and restricting access to a primary nuclear localization signal through which the apoptogenic form is transported to the nucleus. Structural findings agree with biochemical data and support the hypothesis that both normal and apoptogenic functions of AIF are controlled by NADH.

Dynamics of cell proliferation and apoptosis reflect different life strategies in hydrothermal vent and cold seep vestimentiferan tubeworms

Deep-sea vestimentiferan tubeworms, which live in symbiosis with bacteria, exhibit different life strategies according to their habitat. At unstable and relatively short-lived hydrothermal vents, they grow extremely fast, whereas their close relatives at stable and long-persisting cold seeps grow slowly and live up to 300 years. Growth and age differences are thought to occur because of ecological and physiological adaptations. However, the underlying mechanisms of cell proliferation and death, which are closely linked to homeostasis, growth, and longevity, are unknown. Here, we show by immunohistochemical and ultrastructural cell cycle analyses that cell proliferation activities of the two species studied are higher than in any other characterized invertebrate, being only comparable with tumor and wound-healing processes. The slow growth in Lamellibrachia luymesi from cold seeps results from balanced activities of proliferation and apoptosis in the epidermis. In contrast, Riftia pachyptila from hydrothermal vents grows fast because apoptosis is down-regulated in this tissue. The symbiont-housing organ, the trophosome, exhibits a complex cell cycle and terminal differentiation pattern in both species, and growth is regulated by proliferation. These mechanisms have similarities to the up- and down-regulation of proliferation or apoptosis in various types of tumor, although they occur in healthy animals in this study, thus providing significant insights into the underlying mechanisms of growth and longevity.

Adenine nucleotide translocator cooperates with core cell death machinery to promote apoptosis in Caenorhabditis elegans

In Caenorhabditis elegans, the central cell-killing process is essentially controlled by the interplay of four apoptotic factors: EGL-1/BH3-only protein, CED-9/Bcl2, CED-4/Apaf1, and CED-3/caspase. In cells destined to die, EGL-1 binds to CED-9 and results in the release of CED-4 from the mitochondrion-tethered CED-9-CED-4 complex to the perinucleus, which facilitates processing of the CED-3 caspase to cause apoptosis. However, whether additional factors exist to regulate the cell-killing process remains largely unknown. We have identified here WAN-1, the C. elegans ortholog of mammalian adenine nucleotide translocator, as an important cell death regulator. Genetic inactivation of wan-1 significantly suppressed both somatic and germ line cell deaths in C. elegans. Consistently, chemical inhibition of WAN-1 activity also caused strong reduction of germ line apoptosis. WAN-1 localizes to mitochondria and can form complex with both CED-4 and CED-9. Importantly, the cell death initiator EGL-1 can disrupt the interaction between CED-9 and WAN-1. In addition, overexpression of WAN-1 induced ectopic cell killing dependently on the core cell death pathway. These findings suggest that WAN-1 is involved in the central cell-killing process and cooperates with the core cell death machinery to promote programmed cell death in C. elegans.

Curcumin induces apoptosis and inhibits growth of human Burkitt's lymphoma in xenograft mouse model

Curcumin, a natural compound extracted from rhizomes of curcuma Curcuma species, has been shown to possess potent anti-inflammatory, anti-tumor and anti-oxidative properties. However, the mechanism of action of the compound remains poorly understood. In this report, we have analyzed the effects of curcumin on the cell proliferation of Burkitt's lymphoma Raji cells. The results demonstrated that curcumin could effectively inhibit the growth of Raji cells in a dose- and time-dependent manner. Further studies indicated that curcumin treatment resulted in apoptosis of cells. Biochemical analysis showed that the expression of Bax, Bid and cytochrome C were up-regulated, while the expression of oncogene c-Myc was down regulated after curcumin treatment. Furthermore, poly (ADP-ribose) polymerase (PARP) cleavage was induced by the compound. Interestingly, the antiapoptotic Bcl-2 expression was not significantly changed in Raji cells after curcumin treatment. These results suggested that the mechanism of action of curcumin was to induce mitochondrial damage and therefore led to Raji cell apoptosis. We further investigated the in vivo effects of curcumin on the growth of xenograft tumors in nude mice. The results showed that curcumin could effectively inhibit tumor growth in the xenograft mouse model. The overall results showed that curcumin could suppress the growth of Burkitt's lymphoma cells in both in vitro and in vitro systems.

Poly(ADP-ribose) signals to mitochondrial AIF: a key event in parthanatos

Poly(ADP-ribose) polymerase-1 (PARP-1) plays a pivotal role in multiple neurologic diseases by mediating caspase-independent cell death, which has recently been designated parthanatos to distinguish it from other forms of cell death such as apoptosis, necrosis and autophagy. Mitochondrial apoptosis-inducing factor (AIF) release and translocation to the nucleus is the commitment point for parthanatos. This process involves a pathogenic role of poly(ADP-ribose) (PAR) polymer. It generates in the nucleus and translocates to the mitochondria to mediate AIF release following lethal PARP-1 activation. PAR polymer itself is toxic to cells. Thus, PAR polymer signaling to mitochondrial AIF is the key event initiating the deadly crosstalk between the nucleus and the mitochondria in parthanatos. Targeting PAR-mediated AIF release could be a potential approach for the therapy of neurologic disorders.

Modulation of stress proteins and apoptotic regulators in the anoxia tolerant turtle brain

Freshwater turtles survive prolonged anoxia and reoxygenation without overt brain damage by well-described physiological processes, but little work has been done to investigate the molecular changes associated with anoxic survival. We examined stress proteins and apoptotic regulators in the turtle during early (1 h) and long-term anoxia (4, 24 h) and reoxygenation. Western blot analyses showed changes within the first hour of anoxia; multiple stress proteins (Hsp72, Grp94, Hsp60, Hsp27, and HO-1) increased while apoptotic regulators (Bcl-2 and Bax) decreased. Levels of the ER stress protein Grp78 were unchanged. Stress proteins remained elevated in long-term anoxia while the Bcl-2/Bax ratio was unaltered. No changes in cleaved caspase 3 levels were observed during anoxia while apoptosis inducing factor increased significantly. Furthermore, we found no evidence for the anoxic translocation of Bax from the cytosol to mitochondria, nor movement of apoptosis inducing factor between the mitochondria and nucleus. Reoxygenation did not lead to further increases in stress proteins or apoptotic regulators except for HO-1. The apparent protection against cell damage was corroborated with immunohistochemistry, which indicated no overt damage in the turtle brain subjected to anoxia and reoxygenation. The results suggest that molecular adaptations enhance pro-survival mechanisms and suppress apoptotic pathways to confer anoxia tolerance in freshwater turtles.

The Drosophila melanogaster Gene cg4930 Encodes a High Affinity Inhibitor for Endonuclease G

Endonuclease G (EndoG) is a mitochondrial enzyme believed to be released during apoptosis to participate in the degradation of nuclear DNA. This paper describes a Drosophila protein, EndoGI, which inhibits EndoG specifically. EndoG and EndoGI associate with subpicomolar affinity, forming a 2:1 complex in which dimeric EndoG is bound by two tandemly repeated homologous domains of monomeric EndoGI. Binding appears to involve the active site of EndoG. EndoGI is present in the cell nucleus at micromolar concentrations. Upon induction of apoptosis, levels of the inhibitor appear to be reduced, and it is relocalized to the cytoplasm. EndoGI, encoded by the predicted open reading frame cg4930, is expressed throughout Drosophila development. Flies homozygous for a hypomorphic EndoGI mutation have a strongly reduced viability, which is modulated by genetic background and diet. We propose that EndoGI protects the cell against low levels of EndoG outside mitochondria.

Parthanatos, a messenger of death

Poly-ADP-ribose polymerase-1 (PARP-1)'s roles in the cell span from maintaining life to inducing death. The processes PARP-1 is involved in include DNA repair, DNA transcription, mitosis, and cell death. Of PARP-1's different cellular functions, its role in cell death is of particular interest to designing therapies for diseases. Genetic deletion of PARP-1 revealed that PARP-1 overactivation underlies cell death in models of stroke, diabetes, inflammation and neurodegeneration. Since interfering with PARP-1 mediated cell death will be clinically beneficial, great effort has been invested into understanding mechanisms downstream of PARP-1 overactivation. Recent evidence shows that poly-ADP ribose (PAR) polymer itself can act as a cell death effector downstream of PARP-1. We coined the term parthanatos after Thanatos, the personification of death in Greek mythology, to refer to PAR-mediated cell death. In this review, we will present evidence and questions raised by these recent findings, and summarize the proposed mechanisms by which PARP-1 overactivation kills. It is evident that further understanding of parthanatos opens up new avenues for therapy in ameliorating diseases related to PARP-1 overactivation.

Crystal structure of CRN-4: implications for domain function in apoptotic DNA degradation

Cell death related nuclease 4 (CRN-4) is one of the apoptotic nucleases involved in DNA degradation in Caenorhabditis elegans. To understand how CRN-4 is involved in apoptotic DNA fragmentation, we analyzed CRN-4's biochemical properties, in vivo cell functions, and the crystal structures of CRN-4 in apo-form, Mn(2+)-bound active form, and Er(3+)-bound inactive form. CRN-4 is a dimeric nuclease with the optimal enzyme activity in cleaving double-stranded DNA in apoptotic salt conditions. Both mutational studies and the structures of the Mn(2+)-bound CRN-4 revealed the geometry of the functional nuclease active site in the N-terminal DEDDh domain. The C-terminal domain, termed the Zn-domain, contains basic surface residues ideal for nucleic acid recognition and is involved in DNA binding, as confirmed by deletion assays. Cell death analysis in C. elegans further demonstrated that both the nuclease active site and the Zn-domain are required for crn-4's function in apoptosis. Combining all of the data, we suggest a structural model where chromosomal DNA is bound at the Zn-domain and cleaved at the DEDDh nuclease domain in CRN-4 when the cell is undergoing apoptosis.

Mitochondria in energy-limited states: mechanisms that blunt the signaling of cell death

Cellular conditions experienced during energy-limited states--elevated calcium, shifts in cellular adenylate status, compromised mitochondrial membrane potential--are precisely those that trigger, at least in mammals, the mitochondrion to initiate opening of the permeability transition pore, to assemble additional protein release channels, and to release pro-apoptotic factors. These pro-apototic factors in turn activate initiator and executer caspases. How is activation of mitochondria-based pathways for the signaling of apoptotic and necrotic cell death avoided under conditions of hypoxia, anoxia, diapause, estivation and anhydrobiosis? Functional trade-offs in environmental tolerance may have occurred in parallel with the evolution of diversified pathways for the signaling of cell death in eukaryotic organisms. Embryos of the brine shrimp, Artemia franciscana, survive extended periods of anoxia and diapause, and evidence indicates that opening of the mitochondrial permeability transition pore and release of cytochrome c (cyt-c) do not occur. Further, caspase activation in this crustacean is not dependent on cyt-c. Its caspases display regulation by nucleotides that is consistent with ;applying the brakes' to cell death during energy limitation. Unraveling the mechanisms by which organisms in extreme environments avoid cell death may suggest possible interventions during disease states and biostabilization of mammalian cells.

Caenorhabditis elegans drp-1 and fis-2 regulate distinct cell-death execution pathways downstream of ced-3 and independent of ced-9

The dynamin family of GTPases regulate mitochondrial fission and fusion processes and have been implicated in controlling the release of caspase activators from mitochondria during apoptosis. Here we report that profusion genes fzo-1 and eat-3 or the profission gene drp-1 are not required for apoptosis activation in C. elegans. However, minor proapoptotic roles for drp-1 and fis-2, a homolog of human Fis1, are revealed in sensitized genetic backgrounds. drp-1 and fis-2 function independent of one another and the Bcl-2 homolog CED-9 and downstream of the CED-3 caspase to promote elimination of mitochondria in dying cells, an event that could facilitate cell-death execution. Interestingly, CED-3 can cleave DRP-1, which appears to be important for DRP-1's proapoptotic function, but not its mitochondria fission function. Our findings demonstrate that mitochondria dynamics do not regulate apoptosis activation in C. elegans and reveal distinct roles for drp-1 and fis-2 as mediators of cell-death execution downstream of caspase activation.

Inhibition of CED-3 zymogen activation and apoptosis in Caenorhabditis elegans by caspase homolog CSP-3

Inhibitor of apoptosis (IAP) proteins have a crucial role in apoptosis, through negative regulation of caspases in species from fruitflies to mammals. In Caenorhabditis elegans, however, no IAP homolog or caspase inhibitor has been identified, calling into question how the cell-killing caspase CED-3 can be negatively regulated. Here we show that inactivation of the C. elegans csp-3 gene, which encodes a protein similar to the small subunit of the CED-3 caspase, causes cells that normally live to undergo apoptosis in a CED-3–dependent manner. Biochemical analysis reveals that CSP-3 associates with the large subunit of the CED-3 zymogen and inhibits zymogen autoactivation. However, CSP-3 does not block CED-3 activation induced by CED-4, nor does it inhibit the activity of the activated CED-3 protease. Therefore CSP-3 uses a previously unreported mechanism to protect cells from apoptosis.

Differential expression pattern of the four mitochondrial adenine nucleotide transporter ant genes and their roles during the development of Caenorhabditis elegans

The adenine nucleotide transporter (ANT) mediates exchange of cytosolic ADP and mitochondrial ATP. Although most species contain more than one ANT family member, it is not known whether their roles in developmental processes are redundant or specific. Here, we show that the Caenorhabditis elegans genome encodes four candidate ant genes (ant-1.1, ant-1.2, ant-1.3, and ant-1.4). We have investigated their spatiotemporal expression patterns and discovered that, whereas ANT-1.1 is a ubiquitously expressed mitochondrial protein, the other three ANT proteins show a restricted range of cell type expression. Moreover, only the disruption of ant-1.1 function, through RNA interference (RNAi), gives a mutant phenotype. Most of the ant-1.1(RNAi) mutant embryos arrest before the morphogenesis stage. Furthermore, ant-1.1 is also required postembryonically because RNAi mutants exhibit small size and life-span extension. Our results suggest that ant-1.1 is the only ant gene strictly required for embryonic and postembryonic development in C. elegans.

A zinc-dependent nuclear endonuclease is responsible for DNA laddering during salt-induced programmed cell death in root tip cells of rice

DNA laddering is one of the biochemical processes characteristic of programmed cell death (PCD) both in animals and plants. However, the mechanism of DNA laddering varies in different species, even in different tissues of one organism. In the present study, we used root tip cells of rice, which have been induced by NaCl stress to undergo PCD, to analyze the endonuclease activities of cytoplasmic and nuclear extracts. Two endonucleases, a cytoplasmic of 20kDa (OsCyt20) and a nuclear of 37kDa (OsNuc37), were identified as PCD related. Our results indicated that OsCyt20 is a Ca(2+)/Mg(2+)-dependent nuclease, which is most active at neutral pH, and that OsNuc37 is Zn(2+)-dependent, with a pH optimum of 4.5-6. Both nucleases were induced at the early stage of PCD (2h salt treatment) and exhibited the highest activity approximately 4h after exposure to NaCl, paralleling with the occurrence of DNA laddering. In vitro assays of endonuclease activities further revealed that OsNuc37, a glycoprotein localized in the nucleus, is the executor for DNA laddering. The different effects of both endonucleases on DNA degradation during salt-induced PCD are discussed.

Denbinobin induces apoptosis by apoptosis-inducing factor releasing and DNA damage in human colorectal cancer HCT-116 cells

Denbinobin is a phenanthraquinone derivative present in the stems of Ephemerantha lonchophylla. We showed that denbinobin induces apoptosis in human colorectal cancer cells (HCT-116) in a concentration-dependent manner. The addition of a pan-caspase inhibitor (zVAD-fmk) did not suppress the denbinobin-induced apoptotic effect, and denbinobin-induced apoptosis was not accompanied by processing of procaspase-3, -6, -7, -9, and -8. However, denbinobin triggered the translocation of the apoptosis-inducing factor (AIF) from the mitochondria into the nucleus. Small interfering RNA targeting of AIF effectively protected HCT-116 cells against denbinobin-induced apoptosis. Denbinobin treatment also caused DNA damage, activation of the p53 tumor suppressor gene, and upregulation of numerous downstream effectors (p21WAF1/CIP1, Bax, PUMA, and NOXA). A HCT-116 xenograft model demonstrated the in vivo efficacy and low toxicity of denbinobin. Taken together, our findings suggest that denbinobin induces apoptosis of human colorectal cancer HCT-116 cells via DNA damage and an AIF-mediated pathway. These results indicate that denbinobin has potential as a novel anticancer agent.

Plant programmed cell death: can't live with it; can't live without it

The decision of whether a cell should live or die is fundamental for the wellbeing of all organisms. Despite intense investigation into cell growth and proliferation, only recently has the essential and equally important idea that cells control/programme their own demise for proper maintenance of cellular homeostasis gained recognition. Furthermore, even though research into programmed cell death (PCD) has been an extremely active area of research there are significant gaps in our understanding of the process in plants. In this review, we discuss PCD during plant development and pathogenesis, and compare/contrast this with mammalian apoptosis.

Living with death: the evolution of the mitochondrial pathway of apoptosis in animals

The mitochondrial pathway of cell death, in which apoptosis proceeds following mitochondrial outer membrane permeabilization, release of cytochrome c, and APAF-1 apoptosome-mediated caspase activation, represents the major pathway of physiological apoptosis in vertebrates. However, the well-characterized apoptotic pathways of the invertebrates C. elegans and D. melanogaster indicate that this apoptotic pathway is not universally conserved among animals. This review will compare the role of the mitochondria in the apoptotic programs of mammals, nematodes, and flies, and will survey our knowledge of the apoptotic pathways of other, less familiar model organisms in an effort to explore the evolutionary origins of the mitochondrial pathway of apoptosis.

C. elegansRab GTPase 2 is required for the degradation of apoptotic cells

During apoptosis, the dying cell activates an intrinsic mechanism that quickly dismantles itself. The apoptotic cell corpses are then recognized and removed by neighboring cells or professional phagocytes. How dying cells are degraded after internalization is poorly understood. Here, we report the identification and characterization of unc-108, the Caenorhabditis elegans homolog of the human Rab GTPase 2, as a novel component involved in the degradation of apoptotic cells. unc-108 is expressed and functions in the engulfing cells and is likely to affect the degradation rather than the internalization of cell corpses. Similar to other Rab GTPases, unc-108 also affects endocytosis, acting in the endosomal trafficking from early to late endosome and late endosome to lysosome. UNC-108 co-localizes with RAB-5, RAB-7 and LMP-1 to the phagosome and promotes cell corpse degradation, possibly by mediating phagosome maturation.

Caspase-independent apoptosis in yeast

Apoptosis is a highly regulated cellular suicide program crucial for metazoan development. Yeast counterparts of central metazoan apoptotic regulators, such as metacaspase Yca1p, have been identified. In spite of the importance of Yca1p in yeast apoptotic process, many other factors such as Aif1p, orthologs of EndoG, AMID and cyclophilin D play important roles in caspase-independent apoptotic pathways. This review summarized recent progress about studies of various intrinsic and extrinsic apoptotic stimuli that may induce yeast cell death via caspase-independent apoptosis.

Redox-dependent changes in molecular properties of mitochondrial apoptosis-inducing factor

Mitochondrial apoptosis-inducing factor (AIF) is a central player in the caspase-independent cell death pathway whose normal physiological function remains unclear. Our study showed that naturally folded mouse AIF very slowly reacts with NAD(P)H (k cat of 0.2-0.01 s(-1)) forming tight, dimeric, and air-stable FADH2-NAD(P) charge-transfer complexes ineffective in electron transfer. FAD reduction is accompanied by a conformational change involving AIF-specific N-terminal and regulatory 509-559 peptides and the active site His 453, and it affects susceptibility of AIF to calpain and AIF-DNA interaction, the two events critical for initiating caspase-independent apoptosis. Based on our results, we propose that formation of long lived complexes with NAD(P)H and redox reorganization may be functionally important and enable AIF to act as a redox-signaling molecule linking NAD(P)H-dependent metabolic pathways to apoptosis.

The molecular archaeology of a mitochondrial death effector: AIF in Drosophila

Apoptosis-inducing factor (AIF) is a phylogenetically conserved redox-active flavoprotein that contributes to cell death and oxidative phosphorylation in Saccharomyces cerevisiae, Caenorhabditis elegans, mouse and humans. AIF has been characterized as a caspase-independent death effector that is activated by its translocation from mitochondria to the cytosol and nucleus. Here, we report the molecular characterization of AIF in Drosophila melanogaster, a species in which most cell deaths occur in a caspase-dependent manner. Interestingly, knockout of zygotic D. melanogaster AIF (DmAIF) expression using gene targeting resulted in decreased embryonic cell death and the persistence of differentiated neuronal cells at late embryonic stages. Although knockout embryos hatch, they undergo growth arrest at early larval stages, accompanied by mitochondrial respiratory dysfunction. Transgenic expression of DmAIF misdirected to the extramitochondrial compartment (DeltaN-DmAIF), but not wild-type DmAIF, triggered ectopic caspase activation and cell death. DeltaN-DmAIF-induced death was not blocked by removal of caspase activator Dark or transgenic expression of baculoviral caspase inhibitor p35, but was partially inhibited by Diap1 overexpression. Knockdown studies revealed that DeltaN-DmAIF interacts genetically with the redox protein thioredoxin-2. In conclusion, we show that Drosophila AIF is a mitochondrial effector of cell death that plays roles in developmentally regulated cell death and normal mitochondrial function.

Effect of proteasome inhibitors on proliferation and apoptosis of human cutaneous melanoma-derived cell lines

BACKGROUND

Cutaneous malignant melanoma is an aggressive type of skin cancer which causes disproportionate mortality in young and middle-aged adults. Once disseminated, melanoma can be considered an incurable disease, highly resistant to standard antineoplastic treatment, such as chemotherapy or radiation therapy. The proteasome represents a novel target for cancer therapy that can potentially be used in melanoma.

OBJECTIVES

To assess the effect of four structurally different proteasome inhibitors on human cutaneous melanoma-derived cell lines.

METHODS

Sixteen human cutaneous melanoma-derived cell lines which are original were obtained from patients who were treated by two of the authors. Cells were cultured, exposed to proteasome inhibitors (bortezomib, ALLN, MG-132 and epoxomicin) and then assayed for cell cycle and cell death analyses.

RESULTS

Proteasome inhibitors inhibited the in vitro growth of melanoma cells, and this effect was due to a reduction in cell proliferation rate and an induction of both caspase-dependent and caspase-independent cell death. Moreover, release of apoptosis-inducing factor was observed in the presence of the broad-specificity caspase inhibitor BAF (Boc-D-fmk). In addition, the four different proteasome inhibitors induced caspase 2 processing.

CONCLUSIONS

This study provides information regarding the in vitro effects of proteasome inhibitors on melanoma cell lines, and the molecular mechanisms involved. It also gives support to the future use of such inhibitors in the treatment of patients with melanoma, either administered alone or in combination with other drugs.

EXOG, a novel paralog of Endonuclease G in higher eukaryotes

Evolutionary conserved mitochondrial nucleases are involved in programmed cell death and normal cell proliferation in lower and higher eukaryotes. The endo/exonuclease Nuc1p, also termed ‘yeast Endonuclease G (EndoG)’, is a member of this class of enzymes that differs from mammalian homologs by the presence of a 5′–3′ exonuclease activity in addition to its broad spectrum endonuclease activity. However, this exonuclease activity is thought to be essential for a function of the yeast enzyme in DNA recombination and repair. Here we show that higher eukaryotes in addition to EndoG contain its paralog ‘EXOG’, a novel EndoG-like mitochondrial endo/exonuclease. We find that during metazoan evolution duplication of an ancestral nuclease gene obviously generated the paralogous EndoG- and EXOG-protein subfamilies in higher eukaryotes, thereby maintaining the full endo/exonuclease activity found in mitochondria of lower eukaryotes. We demonstrate that human EXOG is a dimeric mitochondrial enzyme that displays 5′–3′ exonuclease activity and further differs from EndoG in substrate specificity. We hypothesize that in higher eukaryotes the complementary enzymatic activities of EndoG and EXOG probably together account for both, the lethal and vital functions of conserved mitochondrial endo/exonucleases.

Therapeutic potential of AIF-mediated caspase-independent programmed cell death

Resistance to anticancer drugs is often related to deficient cell death execution pathways in cancer cells. Apoptosis, which denotes a form of cell death executed by caspases, was traditionally considered as the only physiological and programmed form of cell death. However, recent evidence indicates that programmed cell death (PCD) can occur in complete absence of caspase activation. Indeed, a large number of caspase-independent models are now defined and a key protein implicated in this type of PCD, apoptosis-inducing factor (AIF), has been identified. AIF is a mitochondrial protein with two faces looking in opposite life/death directions. Recently, the identification of five different isoforms allowed a better characterization of AIFs life/mitochondrial versus death/nuclear functions, as well as definition of its pro-apoptotic region and some of its nuclear partners. Importantly, much work on caspase-independent PCD has revealed that AIF participates in more PCD systems than initially thought. A wider molecular knowledge of AIF, and of the caspase-independent PCDs in which it is involved, are key to provide new insights into the role of PCD. There is no doubt that these insights will lead to the development of more selective and efficient drugs against cancer, degenerative diseases, and other pathological disorders implicating AIF.

Regulation of the human apoptotic DNase/RNase endonuclease G: involvement of Hsp70 and ATP

Endonuclease G (EndoG) is a mitochondrial enzyme that becomes an apoptotic nuclease when released from the mitochondrial intermembrane space. EndoG will digest either DNA or RNA, but at physiological ionic strength, RNA is a much more favorable substrate as compared to chromatin. This indicates that EndoG's major in vivo function(s) may be: (i) an apoptotic RNase, and/or (ii) an apoptotic DNase in the presence of additional co-activators. In the present study we have searched for factors that modulate the activity of human EndoG on DNA substrates. We demonstrate that EndoG forms complexes with AIF and FEN-1 but not with PCNA. Interestingly, heat shock proteins 70 interact with EndoG and are involved in the regulation of its activity. Purified Hsp70 prevented stimulation of EndoG DNase activity by other nuclear factors in the ATP-dependent manner.