Proteasomal degradation of caspase-6 in 17beta-estradiol-treated neurons

In primary cultures of human neurons, 17beta-estradiol (17beta-E2) prevents caspase-6-mediated cell death and induces a caspase inhibitory factor (CIF) inhibiting active caspase-6 (Csp-6) in vitro. Here, we show that treatment of neurons with 17beta-E2 results in a proteasomal-dependent but ubiquitin-independent degradation of endogenous and exogenous active Csp-6 in live neurons and in cell free assays, respectively. We further show that the proteasomal-dependent degradation of Csp-6 is not required for its inhibition. Using several protease inhibitors, we find that leupeptin, E-64, and ALLN prevent inhibition of recombinant active Csp-6 (R-Csp-6) in 17beta-E2-treated neuronal protein extracts. Because all three protease inhibitors have the ability to inhibit cysteine proteases, we believe that a cysteinyl protease activity may be required for 17beta-E2-mediated inhibition of active Csp-6. However, we exclude caspases, calpains, and cathepsins as potential cysteinyl proteases involved in the 17beta-E2-mediated Csp-6 inhibition. The results suggest that a proteolytic activity inhibited by leupeptin, E-64, and ALLN is needed to inhibit Csp-6 and that the inhibited Csp-6 is subsequently degraded by the proteasome. The mechanism of 17beta-E2-mediated inhibition of Csp-6 is different from the ubiquitin-dependent proteasomal degradation of Csp-3 and Csp-7 by XIAP and cIAP2 but consistent with the mechanism of Baculovirus p35 inhibition of caspases.     

Cellular prion protein inhibits proapoptotic Bax conformational change in human neurons and in breast carcinoma MCF-7 cells

Prion protein (PrP) prevents Bcl-2-associated protein X (Bax)-mediated cell death, but the step at which PrP inhibits is not known. We first show that PrP is very specific for Bax and cannot prevent Bak (Bcl-2 antagonist killer 1)-, tBid-, staurosporine- or thapsigargin-mediated cell death. As Bax activation involves Bax conformational change, mitochondrial translocation, cytochrome c release and caspase activation, we investigated which of these events was inhibited by PrP. PrP inhibits Bax conformational change, cytochrome c release and cell death in human primary neurons and MCF-7 cells. Serum deprivation-induced Bax conformational change is more rapid in PrP-null cells. PrP does not prevent active caspase-mediated cell death. PrP does not colocalize with Bax in normal or apoptotic primary neurons and cannot prevent Bax-mediated cytochrome c release in a mitochondrial cell-free system. We conclude that PrP protects against Bax-mediated cell death by preventing the Bax proapoptotic conformational change that occurs initially in Bax activation.     

Protein Kinase D1 (PKD1) Phosphorylation Promotes Dopaminergic Neuronal Survival during 6-OHDA-Induced Oxidative Stress

Oxidative stress is a major pathophysiological mediator of degenerative processes in many neurodegenerative diseases including Parkinson’s disease (PD). Aberrant cell signaling governed by protein phosphorylation has been linked to oxidative damage of dopaminergic neurons in PD. Although several studies have associated activation of certain protein kinases with apoptotic cell death in PD, very little is known about protein kinase regulation of cell survival and protection against oxidative damage and degeneration in dopaminergic neurons. Here, we characterized the PKD1-mediated protective pathway against oxidative damage in cell culture models of PD. Dopaminergic neurotoxicant 6-hydroxy dopamine (6-OHDA) was used to induce oxidative stress in the N27 dopaminergic cell model and in primary mesencephalic neurons. Our results indicated that 6-OHDA induced the PKD1 activation loop (PKD1S744/S748) phosphorylation during early stages of oxidative stress and that PKD1 activation preceded cell death. We also found that 6-OHDA rapidly increased phosphorylation of the C-terminal S916 in PKD1, which is required for PKD1 activation loop (PKD1S744/748) phosphorylation. Interestingly, negative modulation of PKD1 activation by RNAi knockdown or by the pharmacological inhibition of PKD1 by kbNB-14270 augmented 6-OHDA-induced apoptosis, while positive modulation of PKD1 by the overexpression of full length PKD1 (PKD1^WT) or constitutively active PKD1 (PKD1^S744E/S748E) attenuated 6-OHDA-induced apoptosis, suggesting an anti-apoptotic role for PKD1 during oxidative neuronal injury. Collectively, our results demonstrate that PKD1 signaling plays a cell survival role during early stages of oxidative stress in dopaminergic neurons and therefore, positive modulation of the PKD1-mediated signal transduction pathway can provide a novel neuroprotective strategy against PD.     

Fibrillar amyloid-beta peptides kill human primary neurons via NADPH oxidase-mediated activation of neutral sphingomyelinase. Implications for Alzheimer's disease

Alzheimer's disease is a major illness of dementia characterized by the presence of amyloid plaques, neurofibrillary tangles, and extensive neuronal apoptosis. However, the mechanism behind neuronal apoptosis in the Alzheimer's-diseased brain is poorly understood. This study underlines the importance of neutral sphingomyelinase in fibrillar Abeta peptide-induced apoptosis and cell death in human primary neurons. Abeta1-42 peptides induced the activation of sphingomyelinases and the production of ceramide in neurons. Interestingly, neutral (N-SMase), but not acidic (A-SMase), sphingomyelinase was involved in Abeta1-42-mediated neuronal apoptosis and cell death. Abeta1-42-induced production of ceramide was redox-sensitive, as reactive oxygen species were involved in the activation of N-SMase but not A-SMase. Abeta1-42 peptides induced the NADPH oxidase-mediated production of superoxide radicals in neurons that was involved in the activation of N-SMase, but not A-SMase, via hydrogen peroxide. Consistently, superoxide radicals generated by hypoxanthine and xanthine oxidase also induced the activation of N-SMase, but not A-SMase, through a catalase-sensitive pathway. Furthermore, antisense knockdown of p22phox, a subunit of NADPH oxidase, inhibited Abeta1-42-induced neuronal apoptosis and cell death. These studies suggest that fibrillar Abeta1-42 peptides induce neuronal apoptosis through the NADPH oxidase-superoxide-hydrogen peroxide-NS-Mase-ceramide pathway.     

p53-mediated cell death: relationship to cell cycle control.

M1 clone S6 myeloid leukemic cells do not express detectable p53 protein. When stably transfected with a temperature-sensitive mutant of p53, these cells undergo rapid cell death upon induction of wild-type (wt) p53 activity at the permissive temperature. This process has features of apoptosis. In a number of other cell systems, wt p53 activation has been shown to induce a growth arrest. Yet, wt 53 fails to induce a measurable growth arrest in M1 cells, and cell cycle progression proceeds while viability is being lost. There exists, however, a relationship between the cell cycle and p53-mediated death, and cells in G1 appear to be preferentially susceptible to the death-inducing activity of wt p53. In addition, p53-mediated M1 cell death can be inhibited by interleukin-6. The effect of the cytokine is specific to p53-mediated death, since apoptosis elicited by serum deprivation is refractory to interleukin-6. Our data imply that p53-mediated cell death is not dependent on the induction of a growth arrest but rather may result from mutually incompatible growth-regulatory signals.     

Mechanism of 6-hydroxydopamine neurotoxicity: the role of NADPH oxidase and microglial activation in 6-hydroxydopamine-induced degeneration of dopaminergic neurons

Cell death induced by 6-hydroxydopamine (6-OHDA) is thought to be caused by reactive oxygen species (ROS) derived from 6-OHDA autooxidation and by a possible direct effect of 6-OHDA on the mitochondrial respiratory chain. However, the process has not been totally clarified. In rat primary mesencephalic cultures, we observed a significant increase in dopaminergic (DA) cell loss 24 h after administration of 6-OHDA (40 micromol/L) and a significant increase in NADPH subunit expression, microglial activation and superoxide anion/superoxide-derived ROS in DA cells that were decreased by the NADPH inhibitor apocynin. Low doses of 6-OHDA (10 micromol/L) did not induce a significant loss of DA cells or a significant increase in NADPH subunit expression, microglial activation or superoxide-derived ROS. However, treatment with the NADPH complex activator angiotensin II caused a significant increase in all the latter. Forty-eight hours after intrastriatal 6-OHDA injection in rats, there was still no loss of DA neurons although there was an increase in NADPH subunit expression and NADPH oxidase activity. The results suggest that in addition to the autooxidation-derived ROS and the inhibition of the mitochondrial respiratory chain, early microglial activation and NADPH oxidase-derived ROS act synergistically with 6-OHDA and constitute a relevant and early component of the 6-OHDA-induced cell death.     

Choline release and inhibition of phosphatidylcholine synthesis precede excitotoxic neuronal death but not neurotoxicity induced by serum deprivation

N-Methyl-d-aspartate (NMDA) receptor overactivation has been proposed to induce excitotoxic neuronal death by enhancing membrane phospholipid degradation. In previous studies, we have shown that NMDA releases choline and reduces membrane phosphatidylcholine in vivo. We now observed that glutamate and NMDA induce choline release in primary neuronal cortical cell cultures. This effect is Ca(2+)-dependent and is blocked by MK-801 ((+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate). In cortical neurons, the NMDA receptor-mediated choline release precedes excitotoxic cell death but not neuronal death induced by either osmotic lysis or serum deprivation. Glutamate, at concentrations that release arachidonic acid, does not release choline in cerebellar granule cells, unless these cells are rendered susceptible to excitotoxic death by energy deprivation. The NMDA-evoked release of choline is not mediated by phospholipases A(2) or C. Moreover, NMDA does not activate phospholipase D in cortical cells. However, NMDA inhibits incorporation of [methyl-(3)H]choline into both membrane phosphatidylcholine and sphingomyelin. These results show that the increase in extracellular choline induced by NMDA receptor activation is directly related with excitotoxic cell death and indicate that choline release is an early event of the excitotoxic process produced by inhibition of phosphatidylcholine synthesis and not by activation of membrane phospholipid degradation.     

Mechanisms of p75-mediated death of hippocampal neurons. Role of caspases

Neurotrophins support neuronal survival and differentiation via Trk receptors, yet can also induce cell death via the p75 receptor. In these studies, we investigated signaling mechanisms governing p75-mediated death of hippocampal neurons, specifically the role of caspases. Although p75 is structurally a member of the Fas/TNFR1 receptor family, caspase-8 was not required for p75-mediated death, unlike other members of this receptor family. In contrast, p75-mediated neuronal death was associated with mitochondrial loss of cytochrome c and required Apaf-1 and caspase-9, -6, and -3. In particular, caspase-6 plays a central role in mediating neurotrophin-induced death, illuminating a novel role for this caspase. Inhibition of DIABLO/Smac, which blocks inhibitor of apoptosis proteins, protected cells from death, whereas simultaneous inhibition of both DIABLO/Smac and MIAP3 allowed trophin-induced death to proceed. In vivo, pilocarpine-induced seizures, previously shown to up-regulate p75 expression and increase neurotrophin production, caused activation of caspase-6 and -3 and cleavage of poly(ADP-ribose) polymerase in p75-expressing hippocampal neurons. In p75(-/-) mice, no activated caspase-3 was detected, and there was a marked reduction in the number of dying neurons after pilocarpine treatment compared with wild type mice. Neurotrophin-induced p75-mediated death is likely to play an important role in mediating neuronal loss consequent to brain injury.     

α-Secretase-derived fragment of cellular prion, N1, protects against monomeric and oligomeric amyloid β (Aβ)-associated cell death

In physiological conditions, both β-amyloid precursor protein (βAPP) and cellular prion (PrP(c)) undergo similar disintegrin-mediated α-secretase cleavage yielding N-terminal secreted products referred to as soluble amyloid precursor protein-α (sAPPα) and N1, respectively. We recently demonstrated that N1 displays neuroprotective properties by reducing p53-dependent cell death both in vitro and in vivo. In this study, we examined the potential of N1 as a neuroprotector against amyloid β (Aβ)-mediated toxicity. We first show that both recombinant sAPPα and N1, but not its inactive parent fragment N2, reduce staurosporine-stimulated caspase-3 activation and TUNEL-positive cell death by lowering p53 promoter transactivation and activity in human cells. We demonstrate that N1 also lowers toxicity, cell death, and p53 pathway exacerbation triggered by Swedish mutated βAPP overexpression in human cells. We designed a CHO cell line overexpressing the London mutated βAPP (APP(LDN)) that yields Aβ oligomers. N1 protected primary cultured neurons against toxicity and cell death triggered by oligomer-enriched APP(LDN)-derived conditioned medium. Finally, we establish that N1 also protects neurons against oligomers extracted from Alzheimer disease-affected brain tissues. Overall, our data indicate that a cellular prion catabolite could interfere with Aβ-associated toxicity and that its production could be seen as a cellular protective mechanism aimed at compensating for an sAPPα deficit taking place at the early asymptomatic phase of Alzheimer disease.     

HSP25 overexpression attenuates oxidative stress-induced apoptosis: roles of ERK1/2 signaling and manganese superoxide dismutase

HSP25 has been shown to induce resistance to radiation and oxidative stress; however, its exact mechanisms remain unclear. In the present study, a high concentration of H2O2 was found to induce DNA fragmentation in L929 mouse fibroblast cells, and HSP25 overexpression attenuated this phenomenon. To elucidate the mechanisms of H2O2-mediated cell death, ERK1/2, p38 MAPK, and JNK1/2 phosphorylation in the cells after treatment with H2O2 were examined. ERK1/2 and JNK1/2 were activated by H2O2; ERK1/2 activation was inhibited in HSP25-overexpressed cells, while JNK1/2 was indifferent. Inhibition of ERK1/2 activation by treatment of the cells with PD98059 or dominant-negative ERK2 transfection blocked H2O2-induced cell death; similarly treated HSP25-overexpressed cells were not at all affected. Moreover, inhibition of JNK1/2 by dominant-negative JNK1 or JNK2 transfection did not affect H2O2-mediated cell death in control cells. Dominant-negative Ras or Raf transfection inhibited H2O2-mediated ERK1/2 activation and cell death in control cells. On the contrary, HSP25-overexpressed cells did not show any differences. Upstream pathways of H2O2-mediated ERK1/2 activation and cell death involved both tyrosine kinase (PDGFbeta receptor and Src) and PKCdelta, while in HSP25-overexpressed cells these kinases did not respond to H2O2 treatment. Since HSP25 overexpression reduced reactive oxygen species (ROS), increased manganese superoxide dismutase (MnSOD) gene expression, and increased enzyme activity, involvement of MnSOD in HSP25-mediated attenuation of H2O2-mediated ERK1/2 activation and cell death was examined. Blockage of MnSOD with antisense oligonucleotides prevented DNA fragmentation and returned the ERK1/2 activation to the control level. Indeed, when MnSOD was overexpressed in L929 cells, similar to in HSP25-overexpressed cells, DNA fragmentation and ERK1/2 activation were reduced. From the above results, we suggest for the first time that reduced oxidative damage by HSP25 was due to MnSOD-mediated downregulation of ERK1/2.     

Cyclin D1 is an essential mediator of apoptotic neuronal cell death.

Many neurons in the developing nervous system undergo programmed cell death, or apoptosis. However, the molecular mechanism underlying this phenomenon is largely unknown. In the present report, we present evidence that the cell cycle regulator cyclin D1 is involved in the regulation of neuronal cell death. During neuronal apoptosis, cyclin D1-dependent kinase activity is stimulated, due to an increase in cyclin D1 levels. Moreover, artificial elevation of cyclin D1 levels is sufficient to induce apoptosis, even in non-neural cell types. Cyclin D1-induced apoptosis, like neuronal apoptosis, can be inhibited by 21 kDa E1B, Bcl2 and pRb, but not by 55 kDa E1B. Most importantly, however, overexpression of the cyclin D-dependent kinase inhibitor p16INK4 protects neurons from apoptotic cell death, demonstrating that activation of endogenous cyclin D1-dependent kinases is essential during neuronal apoptosis. These data support a model in which neuronal apoptosis results from an aborted attempt to activate the cell cycle in terminally differentiated neurons.     

IL-12 decreases activation-induced cell death in human naive Th cells costimulated by intercellular adhesion molecule-1. I. IL-12 alters caspase processing and inhibits enzyme function.

Th cells can receive costimulatory signals through the LFA-1/ICAM-1 accessory pathway that are sufficient to induce early Th cell proliferation, but not subsequent cell expansion and maintenance of cell viability. To investigate the regulatory role for IL-12 in ICAM-1-mediated costimulation, human naive Th cells were stimulated with coimmobilized anti-CD3 mAb and ICAM-1 Ig in the presence or absence of IL-12. The ICAM-1-mediated costimulatory signals in this model resulted in early Th cell proliferation followed by cell death that was partially mediated by Fas and involved loss of mitochondrial membrane potential, processing of procaspase-9 and -3, and activation of caspase-3. Addition of IL-12 prevented activation-induced cell death and promoted late proliferation. ICAM-1 + IL-12-costimulated Th cells were resistant to Fas-mediated cell death through a mechanism that did not appear to involve a decrease in either Fas or Fas ligand expression. IL-12 did not inhibit the loss of mitochondrial membrane potential induced by ICAM-1-mediated costimulation, and this finding was consistent with the inability of IL-12 to increase expression of the antiapoptotic Bcl-2 family members, Bcl-2 and Bcl-x(L). Interestingly, IL-12 promoted an altered processing of procaspase-9 and -3 and a decrease in the percentage of cells displaying caspase-3 catalytic function. In conclusion, we now describe a novel regulatory function for IL-12 in preventing Th cell death and, as a result, in greatly increasing Th cell viability and expansion. Together, our findings indicate that IL-12 may perform this regulatory role by preventing Fas-mediated activation-induced cell death through inhibition of caspase-3 enzyme activity.     

p53-dependent Fas expression is critical for Ginsenoside Rh2 triggered caspase-8 activation in HeLa cells

We have recently reported that Ginsenoside Rh2 (G-Rh2) induces the activation of two initiator caspases, caspase-8 and caspase-9 in human cancer cells. However, the molecular mechanism of its death-inducing function remains unclear. Here we show that G-Rh2 stimulated the activation of both caspase-8 and caspase-9 simultaneously in HeLa cells. Under G-Rh2 treatment, membrane death receptors Fas and TNFR1 are remarkably upregulated. However, the induced expression of Fas but not TNFR1 was contributed to the apoptosis process. Moreover, significant increases in Fas expression and caspase-8 activity temporally coincided with an increase in p53 expression in p53-nonmutated HeLa and SK-HEP-1 cells upon G-Rh2 treatment. In contrast, Fas expression and caspase-8 activity remained constant with G-Rh2 treatment in p53-mutated SW480 and PC-3 cells. In addition, siRNA-mediated knockdown of p53 diminished G-Rh2-induced Fas expression and caspase-8 activation. These results indicated that G-Rh2-triggered extrinsic apoptosis relies on p53-mediated Fas over-expression. In the intrinsic apoptotic pathway, G-Rh2 induced strong and immediate translocation of cytosolic BAK and BAX to the mitochondria, mitochondrial cytochrome c release, and subsequent caspase-9 activation both in HeLa and in SW480 cells. p53-mediated Fas expression and subsequent downstream caspase-8 activation as well as p53-independent caspase-9 activation all contribute to the activation of the downstream effector caspase-3/-7, leading to tumor cell death. Taken together, we suggest that G-Rh2 induces cancer cell apoptosis in a multi-path manner and is therefore a promising candidate for antitumor drug development.     

Critical role for NLRP3 in necrotic death triggered by Mycobacterium tuberculosis

Induction of necrotic death in macrophages is a primary virulence determinant of Mycobacterium tuberculosis. The ESX-1 secretion system and its substrate ESAT-6 are required for M. tuberculosis to induce necrosis, but host factors that mediate the ESAT-6-promoted necrosis remain unknown. Here we report that ESAT-6-promoted necrotic death in THP-1 human macrophages is dependent on the NLRP3 inflammasome, as shown by RNA interference and pharmacological inhibitions. Phagosomes containing ESAT-6-expressing M. tuberculosis recruit markers previously associated with damaged phagosomal membrane, such as galectin-3 and ubiquitinated protein aggregates. In addition, ESAT-6 promoted lysosomal permeabilization by M. tuberculosis. ESAT-6 mutants defective for ubiquitination were unable to trigger NLRP3 activation and necrotic death. Furthermore, Syk tyrosine kinase, recently implicated in NLRP3 activation during fungal and malarial infections, was necessary for mediating the ESAT-6-promoted necrosis and NLRP3 activation. Our results thus link phagosomal damage and Syk activity to NLRP3-mediated necrotic death triggered by M. tuberculosis ESAT-6 during infection.     

Moderate extracellular acidification inhibits capsaicin-induced cell death through regulating calcium mobilization, NF-κB translocation and ROS production in synoviocytes

We previously show the expression of transient receptor potential vanilloid 1 (TRPV1) in primary synoviocytes from collagen-induced arthritis (CIA) rats. Capsaicin and lowered extracellular pH from 7.4 to 5.5 induce cell death through TRPV1-mediated Ca(2+) entry and reactive oxygen species (ROS) production. However, under the pathological condition in rheumatoid arthritis, the synovial fluid is acidified to a moderate level (about pH 6.8). In the present study, we examined the effects of pH 6.8 on the TRPV1-mediated cell death. Our finding is different or even opposite from what was observed at pH 5.5. We found that the moderate extracellular acidification (from pH 7.4 to 6.8) inhibited the capsaicin-induced Ca(2+) entry through attenuating the activity of TRPV1. In the mean time, it triggered a phospholipse C (PLC)-related Ca(2+) release from intracellular stores. The nuclear translocation of NF-κB was found at pH 6.8, and this also depends on PLC activation. Moreover, the capsaicin-evoked massive ROS production and cell death were depressed at pH 6.8, both of which are dependent on the activation of PLC and NF-κB. Taken together, these results suggested that the moderate extracellular acidification inhibited the capsaicin-induced synoviocyte death through regulating Ca(2+) mobilization, activating NF-κB nuclear translocation and depressing ROS production.