Sequential activation of poly(ADP-ribose) polymerase 1, calpains, and Bax is essential in apoptosis-inducing factor-mediated programmed necrosis

Mechanistic insight into DNA damage and repair in ischemic stroke: exploiting the base excision repair pathway as a model of neuroprotection

Stroke is a common cause of death and serious long-term adult disability. Oxidative DNA damage is a severe consequence of oxidative stress associated with ischemic stroke. The accumulation of DNA lesions, including oxidative base modifications and strand breaks, triggers cell death in neurons and other vulnerable cell populations in the ischemic brain. DNA repair systems, particularly base excision repair, are endogenous defense mechanisms that combat oxidative DNA damage. The capacity for DNA repair may affect the susceptibility of neurons to ischemic stress and influence the pathological outcome of stroke. This article reviews the accumulated understanding of molecular pathways by which oxidative DNA damage is triggered and repaired in ischemic cells, and the potential impact of these pathways on ischemic neuronal cell death/survival. Genetic or pharmacological strategies that target the signaling molecules in DNA repair responses are promising for potential clinically effective treatment. Further understanding of mechanisms for oxidative DNA damage and its repair processes may lead to new avenues for stroke management.

Cell death signaling and anticancer therapy

For a long time, it was commonly believed that efficient anticancer regimens would either trigger the apoptotic demise of tumor cells or induce a permanent arrest in the G₁ phase of the cell cycle, i.e., senescence. The recent discovery that necrosis can occur in a regulated fashion and the increasingly more precise characterization of the underlying molecular mechanisms have raised great interest, as non-apoptotic pathways might be instrumental to circumvent the resistance of cancer cells to conventional, pro-apoptotic therapeutic regimens. Moreover, it has been shown that some anticancer regimens engage lethal signaling cascades that can ignite multiple oncosuppressive mechanisms, including apoptosis, necrosis, and senescence. Among these signaling pathways is mitotic catastrophe, whose role as a bona fide cell death mechanism has recently been reconsidered. Thus, anticancer regimens get ever more sophisticated, and often distinct strategies are combined to maximize efficacy and minimize side effects. In this review, we will discuss the importance of apoptosis, necrosis, and mitotic catastrophe in the response of tumor cells to the most common clinically employed and experimental anticancer agents.

Molecular and cellular pathways associated with chromosome 1p deletions during colon carcinogenesis

Chromosomal instability is a major pathway of sporadic colon carcinogenesis. Chromosome arm 1p appears to be one of the "hot spots" in the non-neoplastic mucosa that, when deleted, is associated with the initiation of carcinogenesis. Chromosome arm 1p contains genes associated with DNA repair, spindle checkpoint function, apoptosis, multiple microRNAs, the Wnt signaling pathway, tumor suppression, antioxidant activities, and defense against environmental toxins. Loss of 1p is dangerous since it would likely contribute to genomic instability leading to tumorigenesis. The 1p deletion-associated colon carcinogenesis pathways are reviewed at the molecular and cellular levels. Sporadic colon cancer is strongly linked to a high-fat/low-vegetable/low-micronutrient, Western-style diet. We also consider how selected dietary-related compounds (eg, excess hydrophobic bile acids, and low levels of folic acid, niacin, plant-derived antioxidants, and other modulatory compounds) might affect processes leading to chromosomal deletions, and to the molecular and cellular pathways specifically altered by chromosome 1p loss.

Alginate oligosaccharide protects against endoplasmic reticulum- and mitochondrial-mediated apoptotic cell death and oxidative stress

Oxidative stress is a major component of harmful cascades activated in neurodegenerative disorders. We sought to elucidate possible effects of alginate oligosaccharide (AOS) on H(2)O(2)-induced cell death and to determine the underlying molecular mechanisms in neuron-like PC12 cells. We found that AOS treatment protected PC12 cells against H(2)O(2)-induced endoplasmic reticulum (ER) and mitochondrial-dependent apoptotic cell death. AOS promoted Bcl-2 expression, while blocked Bax expression and inhibited H(2)O(2)-induced caspase-3 activation. It also blocked PARP cleavage. AOS acted on key molecules in apoptotic cell death pathway and reduced p53, p38, c-June NH2-terminal kinase phosphorylations, inhibited NFkB, and enhanced Nrf2 activation. These results suggest that treatment of PC12 cells with AOS can block H(2)O(2)-induced oxidative stress and caspase-dependent apoptotic cascades originating from both ER and mitochondria. Our in vivo experiments further confirm the neuroprotective potential of AOS against Aβ-induced neural damage. According to our data, the involvement of caspase-independent pathway in AOS-induced protection appears to be unlikely.

Helicobacter pylori VacA induces programmed necrosis in gastric epithelial cells

Helicobacter pylori is a Gram-negative bacterium that colonizes the human stomach and contributes to the development of peptic ulcer disease and gastric cancer. The secreted pore-forming toxin VacA is one of the major virulence factors of H. pylori. In the current study, we show that AZ-521 human gastric epithelial cells are highly susceptible to VacA-induced cell death. Wild-type VacA causes death of these cells, whereas mutant VacA proteins defective in membrane channel formation do not. Incubation of AZ-521 cells with wild-type VacA results in cell swelling, poly(ADP-ribose) polymerase (PARP) activation, decreased intracellular ATP concentration, and lactate dehydrogenase (LDH) release. VacA-induced death of these cells is a caspase-independent process that results in cellular release of histone-binding protein high mobility group box 1 (HMGB1), a proinflammatory protein. These features are consistent with the occurrence of cell death through a programmed necrosis pathway and suggest that VacA can be included among the growing number of bacterial pore-forming toxins that induce cell death through programmed necrosis. We propose that VacA augments H. pylori-induced mucosal inflammation in the human stomach by causing programmed necrosis of gastric epithelial cells and subsequent release of proinflammatory proteins and may thereby contribute to the pathogenesis of gastric cancer and peptic ulceration.

Mediterranean diet and cardioprotection: the role of nitrite, polyunsaturated fatty acids, and polyphenols

The continually increasing rate of myocardial infarction (MI) in the Western world at least partly can be explained by a poor diet lacking in green vegetables, fruits, and fish and enriched in food that contains saturated fat. In contrast, a number of epidemiologic studies provide strong evidence highlighting the cardioprotective benefits of the Mediterranean diet enriched in green vegetables, fruits, fish, and grape wine. Regular consumption of these products leads to an accumulation of nitrate/nitrite/NO, polyunsaturated fatty acids (PUFA), and polyphenolic compounds, such as resveratrol, in the human body. Studies have confirmed that these constituents are bioactive exogenous mediators, which induce strong protection against MI. The aim of this review is to provide a critical, in-depth analysis of the cardioprotective pathways mediated by nitrite/NO, PUFA, and phenolic compounds of grape wines discovered in the recent years, including cross-talk between different mechanisms and compounds. Overall, these findings may facilitate the design and synthesis of novel therapeutic tools for the treatment of MI.

Isatis indigotica induces hepatocellular cancer cell death via caspase-independent apoptosis-inducing factor translocation apoptotic pathway in vitro and in vivo

Isatis indigotica is a biennial herbaceous cruciferous medical herb with antipyretic, antiviral, anti-inflammatory, and anti-endotoxin activity. This study explored the chemotherapeutic potential of I indigotica on human hepatoma cells and investigated the mechanism by which metabolites from I indigotica inhibit hepatoma cell growth. Antitumor activity was discovered in dried I indigotica leaf chloroform extracts (CEDLI). In nude mice xenotransplanted with human hepatoma cells, CEDLI supplementation inhibited tumor growth by ~40% compared with nonsupplemented animals without affecting body weight/food intake. CEDLI induced sub-G1 cell cycle arrest and apoptosis in hepatoma cells. Furthermore, CEDLI activates p53 and Bax, reduces Bcl-2 expression, and causes mitochondrial stress and the release of apoptosis-inducing factor into the cytosol followed by its translocation into the nucleus, resulting in hepatoma cell apoptosis. This study provides novel in vivo evidence of I indigotica's antitumor activity. The chemotherapeutic activity against human hepatoma tumorigenesis was because of a distinguished caspase-independent apoptotic pathway.

The ups and downs of tannins as inhibitors of poly(ADP-ribose)glycohydrolase

DNA damage to cells activates nuclear poly(ADP-ribose)polymerases (PARPs) and the poly(ADP-ribose) (PAR) synthesized is rapidly cleaved into ADP-ribose (ADPR) by PAR glycohydrolase (PARG) action. Naturally appearing tannin-like molecules have been implicated in specific inhibition of the PARG enzyme. This review deals with the in vitro and in vivo effects of tannins on PAR metabolism and their downstream actions in DNA damage signaling.

Control of AIF-mediated cell death by antagonistic functions of CHIP ubiquitin E3 ligase and USP2 deubiquitinating enzyme

Apoptosis inducing factor (AIF) is a mitochondrial oxidoreductase that scavenges reactive oxygen species under normal conditions. Under certain stresses, such as exposure to N-methyl-N'-nitro-N'-nitrosoguanidine (MNNG), AIF is truncated and released from the mitochondria and translocated into the nucleus, where the truncated AIF (tAIF) induces caspase-independent cell death. However, it is unknown how cells decide to kill themselves or operate ways to survive when they encounter stresses that induce the release of tAIF. Here, we demonstrated that USP2 and CHIP contribute to the control of tAIF stability. USP2 deubiquitinated and stabilized tAIF, thus promoting AIF-mediated cell death. In contrast, CHIP ubiquitinated and destabilized tAIF, thus preventing the cell death. Consistently, CHIP-deficient cells showed an increased sensitivity to MNNG. On the other hand, knockdown of USP2 attenuated MNNG-induced cell death. Moreover, exposure to MNNG caused a dramatic decrease in CHIP level, but not that of USP2, concurrent with cell shrinkage and chromatin condensation. These findings indicate that CHIP and USP2 show antagonistic functions in the control of AIF-mediated cell death, and implicate the role of the enzymes as a switch for cells to live or die under stresses that cause tAIF release.

Lipoxygenases and poly(ADP-ribose) polymerase in amyloid beta cytotoxicity

The 12/15-lipoxygenase(s) (LOX), poly(ADP-ribose) polymerase (PARP-1) activity and mitochondrial apoptosis inducing factor (AIF) protein in the amyloid β (Aβ) toxicity were investigated in PC12 cells that express either wild-type (APPwt) or double Swedish mutation (APPsw) forms of human Aβ precursor protein. Different levels of Aβ secretion and free radicals formation characterize these cells. The results demonstrated a relationship between the Aβ levels and LOX protein expression and activity. High Aβ concentration in APPsw cells correlated with a significant increase in free radicals and LOX activation, which leads to translocation of p65/NF-κB into the nucleus. An increase in AIF expression in mitochondria was observed concurrently with inhibition of PARP-1 activity in the nuclear fraction of APPsw cells. We suggested that AIF accumulation in mitochondria may be involved in adaptive/protective processes. However, inhibition of PARP-1 may be responsible for the disturbances in transcription and DNA repair as well as the degeneration of APP cells. Under conditions of increased nitrosative stress, evoked by the nitric oxide donor, sodium nitroprusside (SNP, 0.5 mM), 70-80% of all cells types died after 24 h, significantly more in APPsw cells. There was no further significant change in mitochondrial AIF level and PARP-1 activity compared to corresponding non-treated cells. Only one exception was observed in PC12 control, where SNP significantly inhibits PARP-1 activity. Moreover, SNP significantly activated gene expression for 12/15-LOX in all types of investigated cells. Inhibitors of all LOX isoforms and specific inhibitor of 12-LOX enhanced the survival of cells that were subjected to SNP. We conclude that the LOX pathways may play a role in Aβ toxicity and in nitrosative-stress-induced cell death and that inhibition of these pathways offers novel protective strategies.

Apoptosis-inducing factor mediates dopaminergic cell death in response to LPS-induced inflammatory stimulus: evidence in Parkinson's disease patients

We show that intranigral lipopolysaccharide (LPS) injection, which provokes specific degeneration of DA neurons, induced caspase-3 activation in the rat ventral mesencephalon, which was mostly associated with glial cells. In contrast, nigral DA neurons exhibited AIF nuclear translocation in response to LPS. A significant decrease of the Bcl-2/Bax ratio in nigral tissue after LPS injection was observed. We next developed an in vitro co-culture system with the microglial BV2 and the DA neuronal MN9D murine cell lines. The silencing of caspase-3 or AIF by small interfering RNAs exclusively in the DA MN9D cells demonstrated the key role of AIF in the LPS-induced death of DA cells. In vivo chemical inhibition of caspases and poly(ADP-ribose)polymerase-1, an upstream regulator of AIF release and calpain, proved the central role of the AIF-dependent pathway in LPS-induced nigral DA cell death. We also observed nuclear translocation of AIF in the ventral mesencephalon of Parkinson's disease subjects.

Programmed necrosis from molecules to health and disease

During the past decade, cell death researchers have witnessed a gradual but deep conceptual revolution: it has been unequivocally shown that necrosis, which for long had been considered as a purely accidental cell death mode, can also be induced by finely regulated signal transduction pathways. In particular, when caspases are inhibited by pharmacological or genetic means, the ligation of death receptors such as the tumor necrosis factor receptor 1 (TNFR1) can lead to the assembly of a supramolecular complex containing the receptor-interacting protein kinases 1 and 3 (RIP1 and RIP3) that delivers a pronecrotic signal. Such complex has recently been dubbed necrosome and mediates the execution of a specific instance of regulated necrosis, necroptosis. Soon, it turned out that programmed necrosis occurs in nonmammalian model organisms and that it is implicated in human diseases including ischemia and viral infection. In this review, we first describe the historical evolution of the concept of programmed necrosis and the molecular mechanisms that underlie necroptosis initiation and execution. We then provide evidence suggesting that necroptosis represents an ancient and evolutionarily conserved cell death modality that may be targeted for drug development.

Methadone induces CAD degradation and AIF-mediated necrotic-like cell death in neuroblastoma cells

Methadone (d,l-methadone hydrochloride) is a full-opioid agonist, originally developed as a substitution for heroin or other opiates abusers. Nowadays methadone is also being applied as long-lasting analgesics in cancer, and it is proposed as a promising agent for leukemia therapy. Previously, we have demonstrated that high concentrations of methadone (0.5mM) induced necrotic-like cell death in SH-SY5Y cells. The pathway involved is caspase-independent but involves impairment of mitochondrial ATP synthesis and mitochondrial cytochrome c release. However, the downstream mitochondrial pathways remained unclear. Here, we studied the participation of apoptosis inducing factor (AIF) in methadone-induced cell death. Methadone resulted in a translocation of AIF from mitochondria to the nucleus. Translocation was inhibited by cyclosporine A, but not by lack of Bax protein. Therefore the effect seems mediated by the formation of the mitochondrial transition pore, but is apparently independent of Bax. Furthermore, methadone-treated SH-SY5Y nuclei show characteristics that are typical for stage I nuclear condensation. Methadone did not induce degradation of DNA into oligonucleosomal fragments or into high molecular weight DNA fragments. Absence of DNA fragmentation coincided with a considerable decrease in the levels of the caspase-actived endonuclase DNase and its chaperone-inhibitor ICAD. In conclusion, our results provide mechanistic insights into the molecular mechanisms that underlie methadone-induced cell death. This knowledge may prove useful to develop novel strategies to prevent toxic side-effects of methadone thereby sustaining its use as therapeutical agent against tumors.

The human cathelicidin, LL-37, induces granzyme-mediated apoptosis in cytotoxic T lymphocytes

LL-37 is a human cationic host defense peptide (antimicrobial peptide) belonging to the cathelicidin family of peptides. In this study, LL-37 was shown to kill stimulated CD8(+) T cells (Cytotoxic T lymphocytes; CTLs) via apoptosis, while having no cytotoxic effect on non-stimulated CD8(+) or CD4(+) T cells or stimulated CD4(+) T cells. Of interest, the CD8(+) cells were much more sensitive to LL-37 than many other cell types. LL-37 exposure resulted in DNA fragmentation, chromatin condensation, and the release of both granzyme A and granzyme B from intracellular granules. The importance of granzyme family members in the apoptosis of CTLs following LL-37 treatment was analyzed by using C57BL/6 lymphocytes obtained from mice that were homozygous for null mutations in the granzyme B gene, the granzyme A gene, or both granzymes A and B. Granzymes A and B were both shown to play an important role in LL-37-induced apoptosis of CTLs. Further analysis revealed that apoptosis occurred primarily through granzyme A-mediated caspase-independent apoptosis. However, caspase-dependent cell death was also observed. This suggests that LL-37 induces apoptosis in CTLs via multiple different mechanisms, initiated by the LL-37-induced leakage of granzymes from cytolytic granules. Our results imply the existence of a novel mechanism of crosstalk between the inflammatory and adaptive immune systems. Cells such as neutrophils, at the site of a tumor for example, could influence the effector, activity of CTL through the secretion of LL-37.

Role of the mitochondrion in programmed necrosis

In contrast to the “programmed” nature of apoptosis and autophagy, necrotic cell death has always been believed to be a random, uncontrolled process that leads to the “accidental” death of the cell. This dogma, however, is being challenged and the concept of necrosis also being “programmed” is gaining ground. In particular, mitochondria appear to play a pivotal role in the mediation of programmed necrosis. The purpose of this review, therefore, is to appraise the current concepts regarding the signaling mechanisms of programmed necrosis, with specific attention to the contribution of mitochondria to this process.

Receptor interacting protein kinases mediate retinal detachment-induced photoreceptor necrosis and compensate for inhibition of apoptosis

Apoptosis has been shown to be a significant form of cell loss in many diseases. Detachment of photoreceptors from the retinal pigment epithelium, as seen in various retinal disorders, causes photoreceptor loss and subsequent vision decline. Although caspase-dependent apoptotic pathways are activated after retinal detachment, caspase inhibition by the pan-caspase inhibitor Z-VAD fails to prevent photoreceptor death; thus, we investigated other pathways leading to cell loss. Here, we show that receptor interacting protein (RIP) kinase-mediated necrosis is a significant mode of photoreceptor cell loss in an experimental model of retinal detachment and when caspases are inhibited, RIP-mediated necrosis becomes the predominant form of death. RIP3 expression, a key activator of RIP1 kinase, increased more than 10-fold after retinal detachment. Morphological assessment of detached retinas treated with Z-VAD showed decreased apoptosis but significantly increased necrotic photoreceptor death. RIP1 kinase inhibitor necrostatin-1 or Rip3 deficiency substantially prevented those necrotic changes and reduced oxidative stress and mitochondrial release of apoptosis-inducing factor. Thus, RIP kinase-mediated programmed necrosis is a redundant mechanism of photoreceptor death in addition to apoptosis, and simultaneous inhibition of RIP kinases and caspases is essential for effective neuroprotection and may be a novel therapeutic strategy for treatment of retinal disorders.

Oxidative stress initiates DNA damager MNNG-induced poly(ADP-ribose)polymerase-1-dependent parthanatos cell death

The alkylating agent N-methyl-N'-nitro-N'-nitrosoguanidine (MNNG) can cause excess DNA strand breaks that lead to poly(ADP-ribose)polymerase-1 (PARP-1) overactivation and cell death (parthanatos). However, the detail mechanism of MNNG-induced parthanatos was not well-investigated. In this study, we used MNNG-treated mouse embryonic fibroblasts (MEFs) to elucidate the signaling pathways of MNNG-induced parthanatos. We found that MNNG-induced cell death accompanied by rapid PARP-1 activation, c-Jun N-terminal kinase (JNK) activation, biphasic reactive oxygen species (ROS) production and intracellular calcium increase. The early ROS production occurring at 1 min and peaking at 5-15 min after MNNG treatment partially resulted from NADPH oxidase. In contrast, the late phase of ROS production occurring at 30 min and time-dependently increasing up to 6h after MNNG treatment was generated by mitochondria. The antioxidant, NAC can abrogate all phenomena caused by MNNG. Results indicate that the calcium rise was downstream of early ROS production, and was involved in PARP-1 and JNK activation. Moreover, the PARP inhibitor was able to reduce MNNG-induced late-phase ROS production, calcium elevation, and cell death. Results further indicated the involvement of RIP1 in sustained ROS production and calcium increase. We characterized the interactive roles of ROS, calcium, JNK, and RIP1 in MNNG-induced cell death. We found that in addition to the alkylating property previously demonstrated, ROS production triggered by MNNG results in enhanced DNA damage and PARP-1 activation. Moreover, intracellular calcium elevation and ROS production have mutual amplification effects and thus contribute to PARP-1-mediated parthanatos.

[A new form of cell death: Parthanatos]

Parthanatos is a form of cell death, which often occurs in many diseases such as stroke, Parkinson's disease, heart attack, diabetes, and ischemia reperfusion injury. In a cell, the activation of PARP-1 caused by excitotoxicity leads to Parthanatos, which is also known as PARP-1-dependent cell death. There are multiple cell signaling pathways involved in the process of Parthanatos, which have not been fully understood. The main focus of this article is underlying molecular mechanisms of Parthanatos.

Poly(ADP-ribose)glycohydrolase is an upstream regulator of Ca2+ fluxes in oxidative cell death

Oxidative DNA damage to cells activates poly(ADP-ribose)polymerase-1 (PARP-1) and the poly(ADP-ribose) formed is rapidly degraded to ADP-ribose by poly(ADP-ribose)glycohydrolase (PARG). Here we show that PARP-1 and PARG control extracellular Ca²⁺ fluxes through melastatin-like transient receptor potential 2 channels (TRPM2) in a cell death signaling pathway. TRPM2 activation accounts for essentially the entire Ca²⁺ influx into the cytosol, activating caspases and causing the translocation of apoptosis inducing factor (AIF) from the inner mitochondrial membrane to the nucleus followed by cell death. Abrogation of PARP-1 or PARG function disrupts these signals and reduces cell death. ADP-ribose-loading of cells induces Ca²⁺ fluxes in the absence of oxidative damage, suggesting that ADP-ribose is the key metabolite of the PARP-1/PARG system regulating TRPM2. We conclude that PARP-1/PARG control a cell death signal pathway that operates between five different cell compartments and communicates via three types of chemical messengers: a nucleotide, a cation, and proteins.

Protective functions of taurine against experimental stroke through depressing mitochondria-mediated cell death in rats

Taurine, an abundant amino acid in the nervous system, is reported to reduce ischemic brain injury in a dose-dependent manner. This study was designed to investigate whether taurine protected brain against experimental stroke through affecting mitochondria-mediated cell death pathway. Rats were subjected to 2-h ischemia by intraluminal filament, and then reperfused for 22 h. It was confirmed again that taurine (50 mg/kg) administered intravenously 1 h after ischemia markedly improved neurological function and decreased infarct volume at 22 h after reperfusion. In vehicle-treated rats, the levels of intracellular ATP and the levels of cytosolic and mitochondrial Bcl-xL in the penumbra and core were markedly reduced, while the levels of cytosolic Bax in the core and mitochondrial Bax in the penumbra and core were enhanced significantly. There was a decrease in cytochrome C in mitochondria and an increase in cytochrome C in the cytosol of the penumbra and core. These changes were reversed by taurine. Furthermore, taurine inhibited the activation of calpain and caspase-3, reduced the degradation of αII-spectrin, and attenuated the necrotic and apoptotic cell death in the penumbra and core. These data demonstrated that preserving the mitochondrial function and blocking the mitochondria-mediated cell death pathway may be one mechanism of taurine's action against brain ischemia.

Molecular mechanisms of necroptosis: an ordered cellular explosion

For a long time, apoptosis was considered the sole form of programmed cell death during development, homeostasis and disease, whereas necrosis was regarded as an unregulated and uncontrollable process. Evidence now reveals that necrosis can also occur in a regulated manner. The initiation of programmed necrosis, 'necroptosis', by death receptors (such as tumour necrosis factor receptor 1) requires the kinase activity of receptor-interacting protein 1 (RIP1; also known as RIPK1) and RIP3 (also known as RIPK3), and its execution involves the active disintegration of mitochondrial, lysosomal and plasma membranes. Necroptosis participates in the pathogenesis of diseases, including ischaemic injury, neurodegeneration and viral infection, thereby representing an attractive target for the avoidance of unwarranted cell death.

Muscle contractile activity regulates Sirt3 protein expression in rat skeletal muscles

Sirt3, a member of the sirtuin family, is known to control cellular mitochondrial function. Furthermore, because sirtuins require NAD for their deacetylase activity, nicotinamide phosphoribosyltransferase (Nampt), which is a rate-limiting enzyme in the intracellular NAD biosynthetic pathway, influences their activity. We examined the effects of exercise training and normal postural contractile activity on Sirt3 and Nampt protein expression in rat skeletal muscles. Male rats were trained by treadmill running at 20 m/min, 60 min/day, 7 days/wk for 4 wk. This treadmill training program increased the Sirt3 protein expression in the soleus and plantaris muscles by 49% and 41%, respectively ( P < 0.05). Moreover, a 4-wk voluntary wheel-running program also induced 66% and 95% increases in Sirt3 protein in the plantaris and triceps muscles of rats, respectively ( P < 0.05). Treadmill-running and voluntary running training induced no significant changes in Nampt protein expression in skeletal muscles. In resting rats, the soleus muscle, which is recruited during normal postural activity, possessed the greatest expression levels of the Sirt3 and Nampt proteins, followed by the plantaris and triceps muscles. Furthermore, the Sirt3, but not Nampt, protein level was reduced in the soleus muscles from immobilized hindlimbs compared with that shown in the contralateral control muscle. These results demonstrated that 1) Sirt3 protein expression is upregulated by exercise training in skeletal muscles and 2) local postural contractile activity plays an important role in maintaining a high level of Sirt3 protein expression in postural muscle.

Activation of B cell apoptotic pathways in the course of Francisella tularensis infection

Francisella tularensis is a facultative intracellular, gram-negative bacterium that induces apoptosis in macrophages and B cells. Here we show apoptotic pathways that are activated in the Ramos human B cell line in the course of F. tularensis infection. Live bacteria F. tularensis FSC200 activate caspases 8, 9 and 3, as well as Bid; release cytochrome c and apoptosis-inducing factor from mitochondria; and induce depolarization of mitochondrial membrane potential in the Ramos cell line, thus leading these cells to apoptosis. Unlike live bacteria, killed F. tularensis FSC200 bacteria activated only caspase 3, and did not cause apoptosis of Ramos cells as measured by annexin V. Killed bacteria also caused accumulation of anti-apoptotic protein Bclx(L) in mitochondrial membranes. Thus, live F. tularensis activates both caspase pathways (receptor-mediated and intrinsic) as well as caspase-independent mitochondrial death.

TMS, a chemically modified herbal derivative of resveratrol, induces cell death by targeting Bax

Breast cancer recurrence after an initial favorable response to treatment is a major concern for patients who receive hormonal therapies. Additional therapies are necessary to extend the time of response, and ideally, these therapies should exhibit minimal toxicity. Our study described herein focuses on a non-toxic pro-apoptotic agent, TMS (2,4,3',5'-tetramethoxystilbene), which belongs to the Resveratrol family of stilbenes. Prior study demonstrated that TMS was more effective than Resveratrol for inducing apoptosis. Additionally, TMS was effective for invoking death of relapsing breast cancer cells. As TMS was effective for reducing tumor burden, we sought to determine the mechanism by which it achieved its effects. Microarray analysis demonstrated that TMS treatment increased tubulin genes as well as stress response and pro-apoptotic genes. Fractionation studies uncovered that TMS treatment causes cleavage of Bax from the p21 form to a truncated p18 form which is associated with the induction of potent apoptosis. Co-localization analysis of immunofluorescent studies showed that Bax moved from the cytosol to the mitochondria. In addition, the pro-apoptotic proteins Noxa and Bim (EL, L, and S) were increased upon TMS treatment. Cell lines reduced for Bax, Bim, and Noxa are compromised for TMS-mediated cell death. Electron microscopy revealed evidence of nuclear condensation, formation of apoptotic bodies and DAPI staining showed evidence of DNA fragmentation. TMS treatment was able to induce both caspase-independent and caspase-dependent death via the intrinsic death pathway.

Poly(ADP-ribose) metabolism in brain and its role in ischemia pathology

The biological roles of poly(ADP-ribose) polymers (PAR) and poly(ADP-ribosyl)ation of proteins in the central nervous system are diverse. The homeostasis of PAR orchestrated by poly(ADP-ribose) polymerase-1 (PARP-1) and poly(ADP-ribose) glycohydrolase (PARG) is crucial for cell physiology and pathology. Both enzymes are ubiquitously distributed in neurons and glia; however, they are segregated at the subcellular level. PARP-1 serves as a "nick sensor" for single- or double-stranded breaks in DNA and is involved in long and short patch base-excision repair, while PARG breaks down PAR. The stimulation of PARP-1 and PAR formation can activate proinflammatory transcription factors, including nuclear factor kappa B. However, hyperactivation of PARP-1 can result in depletion of NAD/ATP, and in PAR-dependent mitochondrial pore formation leading to release of apoptosis inducing factor and cell death. The role of PAR as a death signaling molecule in brain ischemia-reperfusion and inflammation as well as the effect of gender and aging is presented in this review. Modulating the PAR level through pharmacological or genetic intervention on PARP-1/PARG activity and gene expression should be a valuable way for neuroprotective strategy.

Poly(ADP-ribose) polymerase (PARP)-1-independent apoptosis-inducing factor (AIF) release and cell death are induced by eleostearic acid and blocked by alpha-tocopherol and MEK inhibition

Poly(ADP-ribose)polymerase-1 (PARP-1) is thought to be required for apoptosis-inducing factor (AIF) release from mitochondria in caspase-independent apoptosis. The mechanism by which AIF is released through PARP-1 remains unclear. Here, we provide evidence that PARP-1-independent AIF release and cell death are induced by a trienoic fatty acid, alpha-eleostearic acid (alpha-ESA). Alpha-ESA induced the caspase-independent and AIF-initiated apoptotic death of neuronal cell lines, independently of PARP-1 activation. The cell death was inhibited by the MEK inhibitor U0126 and by knockdown of MEK using small interfering RNA. However, inhibitors for JNK, p38 inhibitors, calpain, phospholipase A(2), and phosphatidylinositol 3-kinase, did not block cell death. AIF was translocated to the nucleus after the induction of apoptosis by alpha-ESA in differentiated PC12 cells without activating caspase-3 and PARP-1. The alpha-ESA-mediated cell death was not inhibited by PARP inhibitor 3,4-dihydro-5-[4-(1-piperidinyl)butoxyl]-1(2H)-isoquinoline and by knockdown of PARP-1 using small interfering RNA. Unlike N-methyl-N'-nitro-N-nitrosoguanidine treatment, histone-phosphorylated histone 2AX was not phosphorylated by alpha-ESA, which suggests no DNA damage. Overexpression of Bcl-2 did not inhibit the cell death. alpha-ESA caused a small quantity of superoxide production in the mitochondria, resulting in the reduction of mitochondrial membrane potential, both of which were blocked by a trace amount of alpha-tocopherol localized in the mitochondria. Our results demonstrate that alpha-ESA induces PARP-1-independent AIF release and cell death without activating Bax, cytochrome c, and caspase-3. MEK is also a key molecule, although the link between ERK, AIF release, and cell death remains unknown. Finding molecules that regulate AIF release may be an important therapeutic target for the treatment of neuronal injury.

Physiological consequences of programmed necrosis, an alternative form of cell demise

Cell death occurs spontaneously or in response to external stimuli, and can be largely subdivided into apoptosis and necrosis by the distinct morphological and biochemical features. Unlike apoptosis, necrosis was recognized as the passive and unwanted cell demise committed in a non-regulated and disorganized manner. However, under specific conditions such as caspase intervention, necrosis has been proposed to be regulated in a well-orchestrated way as a backup mechanism of apoptosis. The term programmed necrosis has been coined to describe such an alternative cell death. Recently, at least some regulators governing programmed necrosis have been identified and demonstrated to be interconnected via a wide network of signal pathways by further extensive studies. There is growing evidence that programmed necrosis is not only associated with pathophysiological diseases, but also provides innate immune response to viral infection. Here, we will introduce recent updates on the molecular mechanism and physiological significance of programmed necrosis.

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.

Glutamate receptors, neurotoxicity and neurodegeneration

Glutamate excitotoxicity is a hypothesis that states excessive glutamate causes neuronal dysfunction and degeneration. As glutamate is a major excitatory neurotransmitter in the central nervous system (CNS), the implications of glutamate excitotoxicity are many and far-reaching. Acute CNS insults such as ischaemia and traumatic brain injury have traditionally been the focus of excitotoxicity research. However, glutamate excitotoxicity has also been linked to chronic neurodegenerative disorders such as amyotrophic lateral sclerosis, multiple sclerosis, Parkinson's disease and others. Despite the continued research into the mechanisms of excitotoxicity, there are currently no pharmacological interventions capable of providing significant neuroprotection in the clinical setting of brain ischaemia or injury. This review addresses the current state of excitotoxic research, focusing on the structure and physiology of glutamate receptors; molecular mechanisms underlying excitotoxic cell death pathways and their interactions with each other; the evidence for glutamate excitotoxicity in acute neurologic diseases; laboratory and clinical attempts at modulating excitotoxicity; and emerging targets for excitotoxicity research.

Mathematical modelling of cell-fate decision in response to death receptor engagement

Cytokines such as TNF and FASL can trigger death or survival depending on cell lines and cellular conditions. The mechanistic details of how a cell chooses among these cell fates are still unclear. The understanding of these processes is important since they are altered in many diseases, including cancer and AIDS. Using a discrete modelling formalism, we present a mathematical model of cell fate decision recapitulating and integrating the most consistent facts extracted from the literature. This model provides a generic high-level view of the interplays between NFkappaB pro-survival pathway, RIP1-dependent necrosis, and the apoptosis pathway in response to death receptor-mediated signals. Wild type simulations demonstrate robust segregation of cellular responses to receptor engagement. Model simulations recapitulate documented phenotypes of protein knockdowns and enable the prediction of the effects of novel knockdowns. In silico experiments simulate the outcomes following ligand removal at different stages, and suggest experimental approaches to further validate and specialise the model for particular cell types. We also propose a reduced conceptual model implementing the logic of the decision process. This analysis gives specific predictions regarding cross-talks between the three pathways, as well as the transient role of RIP1 protein in necrosis, and confirms the phenotypes of novel perturbations. Our wild type and mutant simulations provide novel insights to restore apoptosis in defective cells. The model analysis expands our understanding of how cell fate decision is made. Moreover, our current model can be used to assess contradictory or controversial data from the literature. Ultimately, it constitutes a valuable reasoning tool to delineate novel experiments.

A systems level strategy for analyzing the cell death network: implication in exploring the apoptosis/autophagy connection

The mammalian cell death network comprises three distinct functional modules: apoptosis, autophagy and programmed necrosis. Currently, the field lacks systems level approaches to assess the extent to which the intermodular connectivity affects cell death performance. Here, we developed a platform that is based on single and double sets of RNAi-mediated perturbations targeting combinations of apoptotic and autophagic genes. The outcome of perturbations is measured both at the level of the overall cell death responses, using an unbiased quantitative reporter, and by assessing the molecular responses within the different functional modules. Epistatic analyses determine whether seemingly unrelated pairs of proteins are genetically linked. The initial running of this platform in etoposide-treated cells, using a few single and double perturbations, identified several levels of connectivity between apoptosis and autophagy. The knock down of caspase3 turned on a switch toward autophagic cell death, which requires Atg5 or Beclin-1. In addition, a reciprocal connection between these two autophagic genes and apoptosis was identified. By applying computational tools that are based on mining the protein-protein interaction database, a novel biochemical pathway connecting between Atg5 and caspase3 is suggested. Scaling up this platform into hundreds of perturbations potentially has a wide, general scope of applicability, and will provide the basis for future modeling of the cell death network.

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.

Parp1 activation in mouse embryonic fibroblasts promotes Pol beta-dependent cellular hypersensitivity to alkylation damage

Alkylating agents induce cell death in wild-type (WT) mouse embryonic fibroblasts (MEFs) by multiple mechanisms, including apoptosis, autophagy and necrosis. DNA polymerase beta (Pol beta) knockout (KO) MEFs are hypersensitive to the cytotoxic effect of alkylating agents, as compared to WT MEFs. To test the hypothesis that Parp1 is preferentially activated by methyl methanesulfonate (MMS) exposure of Pol beta KO MEFs, we have examined the relationship between Pol beta expression, Parp1 activation and cell survival following MMS exposure in a series of WT and Pol beta deficient MEF cell lines. Consistent with our hypothesis, we observed elevated Parp1 activation in Pol beta KO MEFs as compared to matched WT MEFs. Both the MMS-induced activation of Parp1 and the MMS-induced cytotoxicity of Pol beta KO MEFs are attenuated by pre-treatment with the Parp1/Parp2 inhibitor PJ34. Further, elevated Parp1 activation is observed following knockdown (KD) of endogenous Pol beta, as compared to WT cells. Pol beta KD MEFs are hypersensitive to MMS and both the MMS-induced hypersensitivity and Parp1 activation is prevented by pre-treatment with PJ34. In addition, the MMS-induced cellular sensitivity of Pol beta KO MEFs is reversed when Parp1 is also deleted (Pol beta/Parp1 double KO MEFs) and we observe no MMS sensitivity differential between Pol beta/Parp1 double KO MEFs and those that express recombinant mouse Pol beta. These studies suggest that Parp1 may function as a sensor of BER to initiate cell death when BER is aborted or fails. Parp1 may therefore function in BER as a tumor suppressor by initiating cell death and preventing the accumulation of cells with chromosomal damage due to a BER defect.

Bioenergetic metabolites regulate base excision repair-dependent cell death in response to DNA damage

Base excision repair (BER) protein expression is important for resistance to DNA damage-induced cytotoxicity. Conversely, BER imbalance [DNA polymerase beta (Polbeta) deficiency or repair inhibition] enhances cytotoxicity of radiation and chemotherapeutic DNA-damaging agents. Whereas inhibition of critical steps in the BER pathway result in the accumulation of cytotoxic DNA double-strand breaks, we report that DNA damage-induced cytotoxicity due to deficiency in the BER protein Polbeta triggers cell death dependent on poly(ADP-ribose) (PAR) polymerase activation yet independent of PAR-mediated apoptosis-inducing factor nuclear translocation or PAR glycohydrolase, suggesting that cytotoxicity is not from PAR or PAR catabolite signaling. Cell death is rescued by the NAD(+) metabolite beta-nicotinamide mononucleotide and is synergistic with inhibition of NAD(+) biosynthesis, showing that DNA damage-induced cytotoxicity mediated via BER inhibition is primarily dependent on cellular metabolite bioavailability. We offer a mechanistic justification for the elevated alkylation-induced cytotoxicity of Polbeta-deficient cells, suggesting a linkage between DNA repair, cell survival, and cellular bioenergetics.

Mitochondria mediated cell death in diabetes

Mitochondrial dysfunction plays a role in the pathogenesis of a wide range of diseases that involve disordered cellular fuel metabolism and survival/death pathways, including neurodegenerative diseases, cancer and diabetes. Cytokine, virus recognition and cellular stress pathways converging on mitochondria cause apoptotic and/or necrotic cell death of beta-cells in type-1 diabetes. Moreover, since mitochondria generate crucial metabolic signals for glucose stimulated insulin secretion (GSIS), mitochondrial dysfunction underlies both the functional derangement of GSIS and (over-nutrition) stress-induced apoptotic/necrotic beta-cell death, hallmarks of type-2 diabetes. The apparently distinct mechanisms governing beta-cell life/death decisions during the development of diabetes provide a remarkable example where remote metabolic, immune and stress signalling meet with mitochondria mediated apoptotic/necrotic death pathways to determine the fate of the beta-cell. We summarize the main findings supporting such a pivotal role of mitochondria in beta-cell death in the context of current trends in diabetes research.

Cell death mechanisms and modulation in traumatic brain injury

Cell death after traumatic brain injury (TBI) is a major cause of neurological deficits and mortality. Understanding the mechanisms of delayed post-traumatic cell loss may lead to new therapies that improve outcome. Although TBI induces changes in multiple cell types, mechanisms of neuronal cell death have been the predominant focus. Recent work has emphasized the diversity of neuronal death phenotypes, which have generally been defined by either morphological or molecular changes. This diversity has led to confusing and at times contradictory nomenclature. Here we review the historical basis of proposed definitions of neuronal cell death, with the goal of clarifying critical research questions and implications for therapy in TBI. We believe that both morphological and molecular features must be used to clarify post-traumatic cell death and related therapeutic targets. Further, we underscore that the most effective neuroprotective strategies will need to target multiple pathways to reflect the regional and temporal changes underlying diverse neuronal cell death phenotypes.

Micro-calpain regulates caspase-dependent and apoptosis inducing factor-mediated caspase-independent apoptotic pathways in cisplatin-induced apoptosis

Cisplatin, an effective anticancer agent, can induce tumor cell apoptosis via caspase-dependent and-independent pathways. However, the precise mechanism that regulates the pathways remains unclear. In this study, we showed that micro-calpain mediated both caspase-dependent and-independent pathways during cisplatin-induced apoptosis in human lung adenocarcinoma cells. After cisplatin treatment, calpain activation, as measured by a fluorescent substrate, was an early event, taking place well before apoptosis inducing factor (AIF) release and caspase-9/-3 activation. Confocal imaging of cells transfected with AIF-GFP demonstrated that AIF release occurred about 9 hr after cisplatin treatment. The increase of micro-calpain activity proved to be a crucial event in the apoptotic machinery, as demonstrated by the significant protection of cell death in samples suppressed the endogenous micro-calpain expression level, as well as cotreated with the calpain inhibitors, calpeptin and PD150606. Inhibition of mu-calpain not only significantly reduced caspase-9/-3 activities but also completely blocked AIF redistribution. Our study also showed that endogenous mitochondrial micro-calpain could directly induce the truncation and release of AIF, while caspases and cathepsins were not necessary for this process. In conclusion, the study demonstrated that activation of micro-calpain played an essential role in regulating both caspase-dependent and AIF-mediated caspase-independent apoptotic pathways in cisplatin-induced apoptosis.

Outer mitochondrial membrane localization of apoptosis-inducing factor: mechanistic implications for release

Poly(ADP-ribose) polymerase-1-dependent cell death (known as parthanatos) plays a pivotal role in many clinically important events including ischaemia/reperfusion injury and glutamate excitotoxicity. A recent study by us has shown that uncleaved AIF (apoptosis-inducing factor), but not calpain-hydrolysed truncated-AIF, was rapidly released from the mitochondria during parthanatos, implicating a second pool of AIF that might be present in brain mitochondria contributing to the rapid release. In the present study, a novel AIF pool is revealed in brain mitochondria by multiple biochemical analyses. Approx. 30% of AIF loosely associates with the outer mitochondrial membrane on the cytosolic side, in addition to its main localization in the mitochondrial intermembrane space attached to the inner membrane. Immunogold electron microscopic analysis of mouse brain further supports AIF association with the outer, as well as the inner, mitochondrial membrane in vivo. In line with these observations, approx. 20% of uncleaved AIF rapidly translocates to the nucleus and functionally causes neuronal death upon NMDA (N-methyl-d-aspartate) treatment. In the present study we show for the first time a second pool of AIF in brain mitochondria and demonstrate that this pool does not require cleavage and that it contributes to the rapid release of AIF. Moreover, these results suggest that this outer mitochondrial pool of AIF is sufficient to cause cell death during parthanatos. Interfering with the release of this outer mitochondrial pool of AIF during cell injury paradigms that use parthanatos hold particular promise for novel therapies to treat neurological disorders.

Up-regulation of survivin by the E2A-HLF chimera is indispensable for the survival of t(17;19)-positive leukemia cells

The E2A-HLF fusion transcription factor generated by t(17;19)(q22;p13) translocation is found in a small subset of pro-B cell acute lymphoblastic leukemias (ALLs) and promotes leukemogenesis by substituting for the antiapoptotic function of cytokines. Here we show that t(17;19)+ ALL cells express Survivin at high levels and that a dominant negative mutant of E2A-HLF suppresses Survivin expression. Forced expression of E2A-HLF in t(17;19)(-) leukemia cells up-regulated Survivin expression, suggesting that Survivin is a downstream target of E2A-HLF. Analysis using a counterflow centrifugal elutriator revealed that t(17;19)+ ALL cells express Survivin throughout the cell cycle. Reporter assays revealed that E2A-HLF induces survivin expression at the transcriptional level likely through indirect down-regulation of a cell cycle-dependent cis element in the promoter region. Down-regulation of Survivin function by a dominant negative mutant of Survivin or reduction of Survivin expression induced massive apoptosis throughout the cell cycle in t(17;19)+ cells mainly through caspase-independent pathways involving translocation of apoptosis-inducing factor (AIF) from mitochondria to the nucleus. AIF knockdown conferred resistance to apoptosis caused by down-regulation of Survivin function. These data indicated that reversal of AIF translocation by Survivin, which is induced by E2A-HLF throughout the cell cycle, is one of the key mechanisms in the protection of t(17;19)+ leukemia cells from apoptosis.

In toxic demyelination oligodendroglial cell death occurs early and is FAS independent

Oligodendroglial cell death is a frequent phenomenon of many neurological diseases, e.g. in demyelinating diseases such as multiple sclerosis (MS). The underlying mechanisms are largely unknown. Here, we demonstrate that in the toxic demyelination cuprizone model, oligodendroglial cell death and downregulation of myelin genes start days after initiation of the cuprizone diet and weeks before demyelination is obvious. In early - but not in later - stages, dying oligodendrocytes express activated caspase 3, suggesting a switch from classical apoptotic pathways to caspase 3-independent mechanisms during the course of the cuprizone diet. The expression level of FAS in the corpus callosum, a cell death receptor crucial for oligodendroglial cell death in experimental autoimmune encephalomyelitis (EAE), correlates with the expression of activated caspase 3 in oligodendrocytes. However, mice lacking FAS in oligodendrocytes are not protected against cuprizone-induced oligodendroglial cell death, showing that FAS is dispensable for oligodendroglial cell death in the cuprizone model.

Changes of mitochondria and relocation of the apoptosis-inducing factor during apoptosis

During apoptosis, apoptosis-inducing factor (AIF) is released from the mitochondrial intermembrane space to the cytosol and to the nucleus. We analyzed AIF in HeLa cells driven to apoptosis by either etoposide or actinomycin D, and we observed changes in the structure and function of mitochondria as well as the translocation of cytochrome c and AIF from mitochondria to the nucleus in early apoptosis. In cells with fragmented chromatin (i.e., in late apoptosis), the immunolabeling for AIF appeared to be distinct from chromatin, being mainly confined to mitochondria.

Proteasome inhibitors prevent cisplatin-induced mitochondrial release of apoptosis-inducing factor and markedly ameliorate cisplatin nephrotoxicity

We demonstrate the effect of proteasome inhibitors in mitochondrial release of apoptosis-inducing factor (AIF) in cisplatin-exposed renal tubular epithelial cells (LLC-PK(1) cells) and in a model of cisplatin nephrotoxicity. Immunofluorescence and subcellular fractionation studies revealed cisplatin-induced translocation of AIF from the mitochondria to nucleus. Mcl-1, a pro-survival member of the Bcl-2 family, is rapidly eliminated on exposure of renal cells to cisplatin. Proteasome inhibitors PS-341 and MG-132 blocked cisplatin-induced Mcl-1 depletion and markedly prevented mitochondrial release of AIF. PS-341 and MG132 also blocked cisplatin-induced activation of executioner caspases and apoptosis. These studies suggest that proteasome inhibitors prevent cisplatin-induced caspase-dependent and -independent pathways. Overexpression of Mcl-1 was effective in blocking cisplatin-induced cytochrome c and AIF release from the mitochondria. Downregulation of Mcl-1 by small interfering RNA promoted Bax activation and cytochrome c and AIF release, suggesting that cisplatin-induced Mcl-1 depletion and associated Bax activation are involved in the release of AIF. Expression of AIF protein in the mouse was highest in the kidney compared to the heart, brain, intestine, liver, lung, muscle, and spleen. In an in vivo model of cisplatin nephrotoxicity, proteasome inhibitor MG-132 prevented mitochondrial release of AIF and markedly attenuated acute kidney injury as assessed by renal function and histology. These studies provide evidence for the first time that the proteasome inhibitors prevent cisplatin-induced mitochondrial release of AIF, provide cellular protection, and markedly ameliorate cisplatin-induced acute kidney injury. Thus, AIF is an important therapeutic target in cisplatin nephrotoxicity and cisplatin-induced depletion of Mcl-1 is an important pathway involved in AIF release.

Involvement of VDAC, Bax and ceramides in the efflux of AIF from mitochondria during curcumin-induced apoptosis

Background

We previously identified curcumin as a potent inducer of fibroblast apoptosis, which could be used to treat hypertrophic scar formation. Here we investigated the underlying mechanism of this process.

Principal Findings

Curcumin-induced apoptosis could not be blocked by caspase-inhibitors and we could not detect any caspase-3/7 activity. Curcumin predominantly induced mitochondria-mediated ROS formation and stimulated the expression of the redox-sensitive pro-apoptotic factor p53. Inhibition of the pro-apoptotic signaling enzyme glycogen synthase kinase-3β (GSK-3β) blocked curcumin-induced apoptosis. Apoptosis was associated with high molecular weight DNA damage, a possible indicator of apoptosis-inducing factor (AIF) activity. Indeed, curcumin caused nuclear translocation of AIF, which could be blocked by the antioxidant N-acetyl cysteine. We next investigated how AIF is effluxed from mitochondria in more detail. The permeability transition pore complex (PTPC), of which the voltage-dependent anion channel (VDAC) is a component, could be involved since the VDAC-inhibitor DIDS (4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid) efficiently blocked AIF translocation. However, PTPC is not involved in AIF release since cyclosporine A, a specific inhibitor of the complex did not block apoptosis. Alternatively, the pro-apoptotic protein Bax could have formed mitochondrial channels and interacted with VDAC. Curcumin caused mitochondrial translocation of Bax, which was blocked by DIDS, suggesting a Bax-VDAC interaction. Interestingly, ceramide channels can also release apoptogenic factors from mitochondria and we found that addition of ceramide induced caspase-independent apoptosis. Surprisingly, this process could also be blocked by DIDS, suggesting the concerted action of Bax, VDAC and ceramide in the efflux of AIF from the mitochondrion.

Conclusions

Curcumin-induced fibroblast apoptosis is totally caspase-independent and relies on the mitochondrial formation of ROS and the subsequent nuclear translocation of AIF, which is released from a mitochondrial pore that involves VDAC, Bax and possibly ceramides. The composition of the AIF-releasing channel seems to be much more complex than previously thought.

MLH1 mediates PARP-dependent cell death in response to the methylating agent N-methyl-N-nitrosourea

Background:

Methylating agents such as N-methyl-N-nitrosourea (MNU) can cause cell cycle arrest and death either via caspase-dependent apoptosis or via a poly(ADP-ribose) polymerase (PARP)-dependent form of apoptosis. We wished to investigate the possible role of MLH1 in signalling cell death through PARP.

Methods:

Fibroblasts are particularly dependent on a PARP-mediated cell death response to methylating agents. We used hTERT-immortalised normal human fibroblasts (WT) to generate isogenic MLH1-depleted cells, confirmed by quantitative PCR and western blotting. Drug resistance was measured by clonogenic and cell viability assays and effects on the cell cycle by cell sorting. Damage signalling was additionally investigated using immunostaining.

Results:

MLH1-depleted cells were more resistant to MNU, as expected. Despite having an intact G₂/M checkpoint, the WT cells did not initially undergo cell cycle arrest but instead triggered cell death directly by PARP overactivation and nuclear translocation of apoptosis-inducing factor (AIF). The MLH1-depleted cells showed defects in this pathway, with decreased staining for phosphorylated H2AX, altered PARP activity and reduced AIF translocation. Inhibitors of PARP, but not of caspases, blocked AIF translocation and greatly decreased short-term cell death in both WT and MLH1-depleted cells. This MLH1-dependent response to MNU was not blocked by inhibitors of ATM/ATR or p53.

Conclusion:

These novel data indicate an important role for MLH1 in signalling PARP-dependent cell death in response to the methylating agent MNU.