Ethanol-induced neuronal apoptosis in vivo requires BAX in the developing mouse brain

A single episode of ethanol intoxication triggers widespread apoptotic neurodegeneration in the infant rat or mouse brain. The cell death process occurs over a 6-16 h period following ethanol administration, is accompanied by a robust display of caspase-3 enzyme activation, and meets ultrastructural criteria for apoptosis. Two apoptotic pathways (intrinsic and extrinsic) have been described, either of which may culminate in the activation of caspase-3. The intrinsic pathway is regulated by Bax and Bcl-XL and involves Bax-induced mitochondrial dysfunction and release of cytochrome c as antecedent events leading to caspase-3 activation. Activation of caspase-8 is a key event preceding caspase-3 activation in the extrinsic pathway. In the present study, following ethanol administration to infant mice, we found no change in activated caspase-8, which suggests that the extrinsic pathway is not involved in ethanol-induced apoptosis. We also found that ethanol triggers robust caspase-3 activation and apoptotic neurodegeneration in C57BL/6 wildtype mice, but induces neither phenomenon in homozygous Bax-deficient mice. Therefore, it appears that ethanol-induced neuroapoptosis is an intrinsic pathway-mediated phenomenon involving Bax-induced disruption of mitochondrial membranes and cytochrome c release as early events leading to caspase-3 activation.     

Synthetic glycosidated phospholipids induce apoptosis through activation of FADD,caspase-8 and the mitochondrial death pathway

Apoptosis is modulated by extrinsic and intrinsic signaling pathways through the formation of the death receptor-mediated death-inducing signaling complex (DISC) and the mitochondrial-derived apoptosome, respectively. Ino-C2-PAF, a novel synthetic phospholipid shows impressive antiproliferative and apoptosis-inducing activity. Little is known about the signaling pathway through which it stimulates apoptosis. Here, we show that this drug induces apoptosis through proteins of the death receptor pathway, which leads to an activation of the intrinsic apoptotic pathway. Apoptosis induced by Ino-C2-PAF and its glucosidated derivate, Glc-PAF, was dependent on the DISC components FADD and caspase-8. This can be inhibited in FADD−/− and caspase-8−/− cells, in which the breakdown of the mitochondrial membrane potential, release of cytochrome c and activation of caspase-9, -8 and -3 do not occur. In addition, the overexpression of crmA, c-Flip or dominant negative FADD as well as treatment with the caspase-8 inhibitor z-IETD-fmk protected against Ino-C2-PAF-induced apoptosis. Apoptosis proceeds in the absence of CD95/Fas-ligand expression and is independent of blockade of a putative death-ligand/receptor interaction. Furthermore, apoptosis cannot be inhibited in CD95/Fas−/− Jurkat cells. Expression of Bcl-2 in either the mitochondria or the endoplasmic reticulum (ER) strongly inhibited Ino-C2-PAF- and Glc-PAF-induced apoptosis. In conclusion, Ino-C2-PAF and Glc-PAF trigger a CD95/Fas ligand- and receptor-independent atypical DISC that relies on the intrinsic apoptotic pathway via the ER and the mitochondria.     

Angelica polymorpha Maxim Induces Apoptosis of Human SH-SY5Y Neuroblastoma Cells by Regulating an Intrinsic Caspase Pathway

Angelica polymorpha Maxim root extract (APRE) is a popular herbal medicine used for treating stomachache, abdominal pain, stomach ulcers, and rheumatism; however the effect of APRE on cancer cells has not yet been explored. Here, we examined APRE cytotoxicity seen on target neuroblastoma cells (NB) using cell viability assays, DAPI visualization of fragmented DNA, and Western blotting analysis of candidate signaling pathways involved in proliferation and apoptosis. We demonstrated that APRE reduced cell viability in NB to a greater extent than in fibroblast cells. In addition, we found that APRE could inhibit the three classes of MAPK proteins and could also down-regulate the PI3K/AKT/GSK-3β activity all being relevant for proliferation and survival. APRE could also up-regulate Bax expression and down-regulate Bcl-2 and Mcl-1. With APRE treatment, depolarization of mitochondria membrane potential and activation of caspase-3 was demonstrated in the SH-SY5Y cells. We could not found increased activity of death receptor and caspase-8 as markers of the extrinsic apoptosis pathway for the APRE treated cells. In presence of a caspase-3 siRNA and a pan-caspase inhibitor, APRE could not reduce the viability of NB cells to a significant degree. So we predicted that with APRE, the intrinsic pathway was solely responsible for inducing apoptosis as we also showed that the non-caspase autophagy pathway or ER stress-ROS mediated pathways were not involved. These findings demonstrate that an intrinsic mitochondria-mediated apoptosis pathway mediates the apoptotic effects of APRE on SH-SY5Y cells, and that APRE shows promise as a novel agent for neuroblastoma therapy.     

The Mitochondrial Permeability Transition Is Required for Tumor Necrosis Factor Alpha-Mediated Apoptosis and Cytochrome c Release

This study assesses the controversial role of the mitochondrial permeability transition (MPT) in apoptosis. In primary rat hepatocytes expressing an IκB superrepressor, tumor necrosis factor alpha (TNFα) induced apoptosis as shown by nuclear morphology, DNA ladder formation, and caspase 3 activation. Confocal microscopy showed that TNFα induced onset of the MPT and mitochondrial depolarization beginning 9 h after TNFα treatment. Initially, depolarization and the MPT occurred in only a subset of mitochondria; however, by 12 h after TNFα treatment, virtually all mitochondria were affected. Cyclosporin A (CsA), an inhibitor of the MPT, blocked TNFα-mediated apoptosis and cytochrome c release. Caspase 3 activation, cytochrome c release, and apoptotic nuclear morphological changes were induced after onset of the MPT and were prevented by CsA. Depolarization and onset of the MPT were blocked in hepatocytes expressing ΔFADD, a dominant negative mutant of Fas-associated protein with death domain (FADD), or crmA, a natural serpin inhibitor of caspases. In contrast, Asp-Glu-Val-Asp-cho, an inhibitor of caspase 3, did not block depolarization or onset of the MPT induced by TNFα, although it inhibited cell death completely. In conclusion, the MPT is an essential component in the signaling pathway for TNFα-induced apoptosis in hepatocytes which is required for both cytochrome c release and cell death and functions downstream of FADD and crmA but upstream of caspase 3.     

Protein Kinase RNA/FADD/Caspase-8 Pathway Mediates the Proapoptotic Activity of the RNA-binding Protein Human Antigen R (HuR)

The RNA-binding protein human antigen R (HuR) has been implicated in apoptosis in multiple ways. Several studies have shown that in response to a variety of stresses HuR promotes the expression of proapoptotic mRNAs, whereas others reported its regulatory effect on antiapoptotic messages. We recently showed that in response to severe stress, HuR is cleaved to generate two cleavage products (CPs), HuR-CP1 (24 kDa) and HuR-CP2 (8 kDa), by which it promotes apoptotic cell death. Here, we show that this cleavage event is dependent on protein kinase RNA (PKR). Surprisingly, although in response to the apoptotic inducer staurosporine PKR itself is not phosphorylated, PKR triggers the cleavage of HuR via its downstream effector FADD that in turn activates the caspase-8/caspase-3 pathway. This effect, however, does not require the phosphorylation of the eukaryotic translation initiation factor 2α. Additionally, we observed that these HuR-CPs are sufficient to trigger cell death in the absence of activation of the PKR pathway. Therefore, our results support a model whereby in response to lethal stress, PKR, without being phosphorylated, activates the FADD/caspase-8/caspase-3 pathway to trigger HuR cleavage, and the HuR-CPs are then capable of promoting apoptosis.     

Monocyte Derived Microvesicles Deliver a Cell Death Message via Encapsulated Caspase-1

Apoptosis depends upon the activation of intracellular caspases which are classically induced by either an intrinsic (mitochondrial based) or extrinsic (cytokine) pathway. However, in the process of explaining how endotoxin activated monocytes are able to induce apoptosis of vascular smooth muscle cells when co-cultured, we uncovered a transcellular apoptosis inducing pathway that utilizes caspase-1 containing microvesicles. Endotoxin stimulated monocytes induce the cell death of VSMCs but this activity is found in 100,000 g pellets of cell free supernatants of these monocytes. This activity is not a direct effect of endotoxin, and is inhibited by the caspase-1 inhibitor YVADcmk but not by inhibitors of Fas-L, IL-1β and IL-18. Importantly, the apoptosis inducing activity co-purifies with 100 nm sized microvesicles as determined by TEM of the pellets. These microvesicles contain caspase-1 and caspase-1 encapsulation is required since disruption of microvesicular integrity destroys the apoptotic activity but not the caspase-1 enzymatic activity. Thus, monocytes are capable of delivering a cell death message which depends upon the release of microvesicles containing functional caspase-1. This transcellular apoptosis induction pathway describes a novel pathway for inflammation induced programmed cell death.     

X Chromosome-linked Inhibitor of Apoptosis Regulates Cell Death Induction by Proapoptotic Receptor Agonists

Proapoptotic receptor agonists cause cellular demise through the activation of the extrinsic and intrinsic apoptotic pathways. Inhibitor of apoptosis (IAP) proteins block apoptosis induced by diverse stimuli. Here, we demonstrate that IAP antagonists in combination with Fas ligand (FasL) or the death receptor 5 (DR5) agonist antibody synergistically stimulate death in cancer cells and inhibit tumor growth. Single-agent activity of IAP antagonists relies on tumor necrosis factor-α signaling. By contrast, blockade of tumor necrosis factor-α does not affect the synergistic activity of IAP antagonists with FasL or DR5 agonist antibody. In most cancer cells, proapoptotic receptor agonist-induced cell death depends on amplifying the apoptotic signal via caspase-8-mediated activation of Bid and subsequent activation of the caspase-9-dependent mitochondrial apoptotic pathway. In the investigated cancer cell lines, induction of apoptosis by FasL or DR5 agonist antibody can be inhibited by knockdown of Bid. However, knockdown of X chromosome-linked IAP (XIAP) or antagonism of XIAP allows FasL or DR5 agonist antibody to induce activation of effector caspases efficiently without the need for mitochondrial amplification of the apoptotic signal and thus rescues the effect of Bid knockdown in these cells.     

Downregulation of doxorubicin-induced myocardial apoptosis accompanies postnatal heart maturation

Doxorubicin is a highly effective chemotherapeutic agent used for treating a wide spectrum of tumors, but its usage is limited because of its dose-dependent cardiotoxicity, especially in pediatric patients. Accumulating evidence indicates that caspase-dependent apoptosis contributes to the cardiotoxicity of doxorubicin. However, less attention has been paid to the effects of age on doxorubicin-induced apoptosis signaling in myocardium. This study focused on investigating differential apoptotic sensitivity between neonatal and adult myocardium, in particular, between neonatal and adult cardiomyocytes in vivo. Our results show that caspase-3 activity in normal mouse hearts decreased by ≥ 20-fold within the first 3 wk after birth, associated with a rapid downregulation in the expression of key proapoptotic proteins in intrinsic and extrinsic pathways. This rapid downregulation of caspase-3 activity was confirmed by immunostaining for cleaved caspase-3 and terminal deoxynucleotidyl transferase dUTP-mediated nick-end label staining. Doxorubicin treatment induced a dose-dependent increase in caspase-3 activity and apoptosis in neonatal mouse hearts, and both caspase-8 and caspase-9 activations were involved. Using transgenic mice with a nuclear localized LacZ reporter gene to label cardiomyocytes in vivo, we observed a fourfold higher level of doxorubicin-induced cardiomyocyte apoptosis in 1-wk-old mice compared with that in 3-wk-old mice. This study points to a major difference in apoptotic signaling in doxorubicin cardiotoxicity between neonatal and adult mouse hearts and reveals a critical transition from high to low susceptibility to doxorubicin-induced apoptosis during postnatal heart maturation.     

Antitumour activity on extrinsic apoptotic targets of the triterpenoid maslinic acid in p53-deficient Caco-2 adenocarcinoma cells

We report that a novel triterpenoid, (2a,3b)-2,3-dihydroxyolean-12-en-28-oic acid (maslinic acid), isolated from olive pomace from Olea europaea, triggers primarily the extrinsic and later the intrinsic apoptotic pathways in Caco-2 human colon-cancer cells. Apoptosis induced by maslinic acid was confirmed by FACS analysis using annexine-V FICT staining. This induction of apoptosis was correlated with the early activation of caspase-8 and caspase-3, the activation of caspase-8 was also correlated with higher levels of Bid cleavage and decreased Bcl-2, but with no change in Bax expression. Maslinic acid also induced a sustained activation of c-Jun N-terminal kinase (JNK). Incubation with maslinic acid also resulted in the later activation of caspase-9, which, together with the lack of any Bax activation, suggests that the mitochondrial pathway is not required for apoptosis induced by maslinic acid in this cell line. In this study we found that the mechanism of apoptotic activation in p53-deficient Caco-2 cells differs significantly from that found in HT-29 cells. Natural agents able to activate both the extrinsic and intrinsic apoptotic pathways by avoiding the mitochondrial resistance mechanisms may be useful for treatment against colon cancer regardless of its aetiology.     

A novel role for the apoptosis inhibitor ARC in suppressing TNFα-induced regulated necrosis

TNFα signaling can promote apoptosis or a regulated form of necrosis. ARC (apoptosis repressor with CARD (caspase recruitment domain)) is an endogenous inhibitor of apoptosis that antagonizes both the extrinsic (death receptor) and intrinsic (mitochondrial/ER) apoptosis pathways. We discovered that ARC blocks not only apoptosis but also necrosis. TNFα-induced necrosis was abrogated by overexpression of wild-type ARC but not by a CARD mutant that is also defective for inhibition of apoptosis. Conversely, knockdown of ARC exacerbated TNFα-induced necrosis, an effect that was rescued by reconstitution with wild-type, but not CARD-defective, ARC. Similarly, depletion of ARC in vivo exacerbated necrosis caused by infection with vaccinia virus, which elicits severe tissue damage through this pathway, and sensitized mice to TNFα-induced systemic inflammatory response syndrome. The mechanism underlying these effects is an interaction of ARC with TNF receptor 1 that interferes with recruitment of RIP1, a critical mediator of TNFα-induced regulated necrosis. These findings extend the role of ARC from an apoptosis inhibitor to a regulator of the TNFα pathway and an inhibitor of TNFα-mediated regulated necrosis.     

Inducible Dimerization and Inducible Cleavage Reveal a Requirement for Both Processes in Caspase-8 Activation

Caspase-8 is a cysteine protease activated by membrane-bound receptors at the cytosolic face of the cell membrane, initiating the extrinsic pathway of apoptosis. Caspase-8 activation relies on recruitment of inactive monomeric zymogens to activated receptor complexes, where they produce a fully active enzyme composed of two catalytic domains. Although in vitro studies using drug-mediated affinity systems or kosmotropic salts to drive dimerization have indicated that uncleaved caspase-8 can be readily activated by dimerization alone, in vivo results using mouse models have reached the opposite conclusion. Furthermore, in addition to interdomain autoprocessing, caspase-8 can be cleaved by activated executioner caspases, and reports of whether this cleavage event can lead to activation of caspase-8 have been conflicting. Here, we address these questions by carrying out studies of the activation characteristics of caspase-8 mutants bearing prohibitive mutations at the interdomain cleavage sites both in vitro and in cell lines lacking endogenous caspase-8, and we find that elimination of these cleavage sites precludes caspase-8 activation by prodomain-driven dimerization. We then further explore the consequences of interdomain cleavage of caspase-8 by adapting the tobacco etch virus protease to create a system in which both the cleavage and the dimerization of caspase-8 can be independently controlled in living cells. We find that unlike the executioner caspases, which are readily activated by interdomain cleavage alone, neither dimerization nor cleavage of caspase-8 alone is sufficient to activate caspase-8 or induce apoptosis and that only the coordinated dimerization and cleavage of the zymogen produce efficient activation in vitro and apoptosis in cellular systems.     

Cathepsin B Degrades Amyloid-β in Mice Expressing Wild-type Human Amyloid Precursor Protein

Accumulation of amyloid-β (Aβ), believed to be a key trigger of Alzheimer disease (AD), could result from impaired clearance mechanisms. Previously, we showed that the cysteine protease cathepsin B (CatB) degrades Aβ, most likely by C-terminal truncation, in mice expressing human amyloid precursor protein with familial AD-linked mutations (hAPP_FAD). In addition, the Aβ-degrading activity of CatB is inhibited by its endogenous inhibitor, cystatin C (CysC). Reducing CysC expression markedly lowers Aβ levels by enhancing CatB-mediated Aβ degradation in hAPP_FAD mice. However, because a vast majority of AD patients do not carry familial mutations, we investigated how the CysC-CatB axis affects Aβ levels in mice expressing wild-type hAPP (hAPP_WT). Enhancing CatB activity by CysC deletion significantly lowered total Aβ and Aβ42 levels in hAPP_WT mice, whereas CatB deletion increased Aβ levels. To determine whether neuron-derived CatB degrades Aβ in vivo, we generated transgenic mice overexpressing CatB under the control of a neuron-specific enolase promoter. Enhancing neuronal CatB activity in hAPP_WT mice significantly lowered Aβ42 levels. The processing of hAPP_WT was unaffected by increasing or ablating CatB activity. Thus, the CysC-CatB axis affects degradation of Aβ42 derived from hAPP lacking familial mutations. These findings support the notion that enhancing CatB activity could lower Aβ, especially Aβ42, in AD patients with or without familial mutations.     

β-Carotene-induced apoptosis is mediated with loss of Ku proteins in gastric cancer AGS cells

High dietary intakes and high blood levels of β-carotene are associated with a decreased incidence of various cancers. The anticancer effect of β-carotene is related to its pro-oxidant activity. DNA repair Ku proteins, as a heterodimer of Ku70 and Ku80, play a crucial role in DNA double-strand break repair. Reductions in Ku70/80 contribute to apoptosis. Previously, we showed that reactive oxygen species (ROS) activate caspase-3 which induces degradation of Ku proteins. In the present study, we investigated the mechanism of β-carotene-induced apoptosis of gastric cancer AGS cells by determining cell viability, DNA fragmentation, apoptotic indices (increases in cytochrome c and Bax, decrease in Bcl-2), ROS levels, mitochondrial membrane potential, caspase-3 activity, Ku70/80 levels, and Ku-DNA-binding activity of the cells treated with or without antioxidant N-acetyl cysteine and caspase-3 inhibitor z-DEVED-fmk. As a result, β-carotene induced apoptosis (decrease in cell viability, increases in DNA fragmentation and apoptotic indices) and caspase-3 activation, but decreased Ku70/80 levels and Ku-DNA-binding activity. β-Carotene-induced alterations (increase in caspase-3 activity, decrease in Ku proteins) and apoptosis were inhibited by N-acetyl cysteine and z-DEVED-fmk. Increment of intracellular and mitochondrial ROS levels and loss of mitochondrial membrane potential were suppressed by N-acetyl cysteine, but not by z-DEVED-fmk in β-carotene-treated cells. Therefore, β-carotene-induced increases in ROS and caspase-3 activity may lead to reduction of Ku70/80 levels, which results in apoptosis in gastric cancer cells. Loss of Ku proteins might be the underlying mechanism for β-carotene-induced apoptosis in gastric cancer cells.     

Pretreatment with Fucoidan from Fucus vesiculosus Protected against ConA-Induced Acute Liver Injury by Inhibiting Both Intrinsic and Extrinsic Apoptosis

This study aimed to explore the effects of fucoidan from Fucus vesiculosus on concanavalin A (ConA)-induced acute liver injury in mice. Pretreatment with fucoidan protected liver function indicated by ALT, AST and histopathological changes by suppressing inflammatory cytokines, such as tumor necrosis factor alpha (TNF-α) and interferon gamma (IFN-γ). In addition, intrinsic and extrinsic apoptosis mediated by Bax, Bid, Bcl-2, Bcl-xL and Caspase 3, 8, and 9 were inhibited by fucoidan and the action was associated with the TRADD/TRAF2 and JAK2/STAT1 signal pathways. Our results demonstrated that fucoidan from Fucus vesiculosus alleviated ConA-induced acute liver injury via the inhibition of intrinsic and extrinsic apoptosis mediated by the TRADD/TRAF2 and JAK2/STAT1 pathways which were activated by TNF-α and IFN-γ. These findings could provide a potential powerful therapy for T cell-related hepatitis.     

Epirubicin induces apoptosis in osteoblasts through death-receptor and mitochondrial pathways

Epirubicin is an anthracycline and is widely used in tumor treatment, but has toxic and undesirable side effects on wide range of cells and hematopoietic stem cells (HSC). Osteoblasts play important roles in bone development and in supporting HSC differentiation and maturation. It remains unknown whether epirubicin-induced bone loss and hematological toxicity are associated with its effect on osteoblasts. In primary osteoblast cell cultures, epirubicin inhibited cell growth and decreased mineralization. Moreover, epirubicin arrested osteoblasts in the G2/M phase, and this arrest was followed by apoptosis in which both the extrinsic (death receptor-mediated) and intrinsic (mitochondrial-mediated) apoptotic pathways were evoked. The factors involved in the extrinsic apoptotic pathway were increased FasL and FADD as well as activated caspase-8. Those involved in the intrinsic apoptotic pathway were decreased Bcl-2; increased reactive oxygen species, Bax, cytochrome c; and activated caspase-9 and caspase-3. These results demonstrate that epirubicin induced osteoblast apoptosis through the extrinsic and intrinsic apoptotic pathways, leading to the destruction of osteoblasts and consequent lessening of their functions in maintaining bone density and supporting hematopoietic stem cell differentiation and maturation.     

Glycogen Synthase Kinase-3β and Caspase-2 Mediate Ceramide- and Etoposide-Induced Apoptosis by Regulating the Lysosomal-Mitochondrial Axis

Glycogen synthase kinase-3β (GSK-3β) regulates the sequential activation of caspase-2 and caspase-8 before mitochondrial apoptosis. Here, we report the regulation of Mcl-1 destabilization and cathepsin D-regulated caspase-8 activation by GSK-3β and caspase-2. Treatment with either the ceramide analogue C₂-ceramide or the topoisomerase II inhibitor etoposide sequentially induced lysosomal membrane permeabilization (LMP), the reduction of mitochondrial transmembrane potential, and apoptosis. Following LMP, cathepsin D translocated from lysosomes to the cytoplasm, whereas inhibiting cathepsin D blocked mitochondrial apoptosis. Furthermore, cathepsin D caused the activation of caspase-8 but not caspase-2. Inhibiting GSK-3β and caspase-2 blocked Mcl-1 destabilization, LMP, cathepsin D re-localization, caspase-8 activation, and mitochondrial apoptosis. Expression of Mcl-1 was localized to the lysosomes, and forced expression of Mcl-1 prevented apoptotic signaling via the lysosomal-mitochondrial pathway. These results demonstrate the importance of GSK-3β and caspase-2 in ceramide- and etoposide-induced apoptosis through mechanisms involving Mcl-1 destabilization and the lysosomal-mitochondrial axis.     

Haplophytin-A induces caspase-8-mediated apoptosis via the formation of death-inducing signaling complex in human promyelocytic leukemia HL-60 cells

Haplophytin-A (10-methoxy-2,2-dimethyl-2,6-dihydro-pyrano[3,2-c]quinolin-5-one), a novel quinoline alkaloid, was isolated from the Haplophyllum acutifolium. In this study, we investigated the effect of haplophytin-A on the apoptotic activity and the molecular mechanism of action in human promyelocytic leukemia HL-60 cells. Treatment with haplophytin-A (50 μM) induced classical features of apoptosis, such as, DNA fragmentation, DNA ladder formation, and the externalization of annexin-V-targeted phosphatidylserine residues in HL-60 cells. In addition, haplophytin-A triggered the activations of caspase-8, -9, and -3, and the cleavage of poly (ADP-ribose) polymerase (PARP) in HL-60 cells. In addition, haplophytin-A caused the loss of mitochondrial membrane potential (ΔΨ_m) and the release of cytochrome c and Smac/DIABLO to the cytosol, and modulated the expression levels of Bcl-2 family proteins. We further demonstrated that knockdown of caspase-8 using its siRNA inhibited the mitochondrial translocation of tBid, the activations of caspase-9 and caspase-3, and subsequent DNA fragmentation by haplophytin-A. Furthermore, haplophytin-A-induced the formation of death-inducing signaling complex (DISC) and then activated caspase-8 in HL-60 cells. During haplophytin-A-induced apoptosis, caspase-8-stimulated tBid provide a link between the death receptor-mediated extrinsic pathway and the mitochondria- mediated intrinsic pathway. Taken together, these results suggest that the novel compound haplophytin-A play therapeutical role for leukemia via the potent apoptotic activity through the extrinsic pathway, involving the intrinsic pathway.     

Beta-Amyloid Oligomers Activate Apoptotic BAK Pore for Cytochrome c Release

In Alzheimer’s disease, cytochrome c-dependent apoptosis is a crucial pathway in neuronal cell death. Although beta-amyloid (Aβ) oligomers are known to be the neurotoxins responsible for neuronal cell death, the underlying mechanisms remain largely elusive. Here, we report that the oligomeric form of synthetic Aβ of 42 amino acids elicits death of HT-22 cells. But, when expression of a bcl-2 family protein BAK is suppressed by siRNA, Aβ oligomer-induced cell death was reduced. Furthermore, significant reduction of cytochrome c release was observed with mitochondria isolated from BAK siRNA-treated HT-22 cells. Our in vitro experiments demonstrate that Aβ oligomers bind to BAK on the membrane and induce apoptotic BAK pores and cytochrome c release. Thus, the results suggest that Aβ oligomers function as apoptotic ligands and hijack the intrinsic apoptotic pathway to cause unintended neuronal cell death.