Oxidative stress induces caspase-independent retinal apoptosis in vitro

Primary hepatocyte apoptosis is unlikely to relate to caspase-3 activity under sustained endogenous oxidative stress

We previously showed that inhibition of catalase and glutathione peroxidase activities in rat primary hepatocytes by 3-amino-1,2,4-triazole (ATZ) and mercaptosuccinic acid (MS) results in endogenous oxidative stress and apoptosis. For the present study, we determined whether this apoptosis involved activation of caspase-3, which is known to execute apoptosis in many cell types. ATZ and MS increased levels of reactive oxygen species (ROS) from 3-9 h, just before the onset of chromatin condensation (apoptosis) and decreases in protein thiols. Pretreatment with either SKF, a cytochrome P450 inhibitor, or L-ascorbic acid, an antioxidant, completely suppressed the increase in ROS levels and apoptosis, suggesting that the sustained ROS increases may cause the apoptosis. SKF also abolished the decrease in protein thiol content, further supporting the contribution of the P450 system to increased ROS levels. DEVD-CHO, a caspase-3 inhibitor, even at 1 mM had no effect on apoptosis. Caspase-3 activity remained unchanged and pro-caspase-3 processing was not detected during 18 h incubation with ATZ and MS. Moreover, the amount of unoxidized pro-caspase-3 decreased even below the level of untreated hepatocytes. These findings suggest that the sustained oxidative stress is a major cause for the hepatocyte apoptosis, which occurs independently of the caspase-3 related pathway.

Caspase-independent apoptosis in Friend's erythroleukemia cells: role of mitochondrial ATP synthesis impairment in relocation of apoptosis-inducing factor and endonuclease G

Mitochondria have emerged as the central components of both caspase-dependent and independent apoptosis signalling pathways through release of different apoptogenic proteins. We previously documented that parental and differentiated Friend's erythroleukemia cells were induced to apoptosis by oligomycin and H(2)O(2) exposure, showing that the energy impairment occurring in both cases as a consequence of a severe mitochondrial F(0)F(1)ATPsynthase inactivation was a common early feature. Here we provide evidence for AIF and Endo G mitochondrio-nuclear relocation in both cases, as a component of caspase-independent apoptosis pathways. No detectable change in mitochondrial transmembrane potential and no variation in mitochondrial levels of Bcl-2 and Bax are observed. These results point to the osmotic rupture of the mitochondrial outer membrane as occurring in response to cell exposure to the two energy-impairing treatments under conditions preserving the mitochondrial inner membrane. A critical role of the mitochondrial F(0)F(1)ATP synthase inhibition in this process is also suggested.

Aag-initiated base excision repair drives alkylation-induced retinal degeneration in mice

Vision loss affects >3 million Americans and many more people worldwide. Although predisposing genes have been identified their link to known environmental factors is unclear. In wild-type animals DNA alkylating agents induce photoreceptor apoptosis and severe retinal degeneration. Alkylation-induced retinal degeneration is totally suppressed in the absence of the DNA repair protein alkyladenine DNA glycosylase (Aag) in both differentiating and postmitotic retinas. Moreover, transgenic expression of Aag activity restores the alkylation sensitivity of photoreceptors in Aag null animals. Aag heterozygotes display an intermediate level of retinal degeneration, demonstrating haploinsufficiency and underscoring that Aag expression confers a dominant retinal degeneration phenotype.

Thioredoxin-like 6 protects retinal cell line from photooxidative damage by upregulating NF-kappaB activity

Apoptosis is the common pathway to photoreceptor cell death in many eye diseases including age-related macular degeneration which affects more than 8 million individuals in the United States alone. RdCVF, a truncated mouse thioredoxin is specifically expressed by rod photoreceptor cells and prevents the apoptosis of cone cells. However the protective mechanism of RdCVF and the implications of its human homologue, thioredoxin-like 6 (TXNL6), on the apoptosis of retinal cells remain unknown. In this study, we examined the function of TXNL6 and investigated its mechanism of protection using a cone photoreceptor cell line, 661W. We found that the photooxidative stress-induced degradation of NF-kappaB proteins is rescued by overexpression of TXNL6, which enabled the NF-kappaB transactivation activity. Furthermore, the overexpression of TXNL6 rescued the photooxidative stress-induced apoptosis of 661W cells. Interestingly, this protective effect was significantly blocked by NF-kappaB specific inhibitors demonstrating that TXNL6 exerts its protective effect against apoptosis via NF-kappaB. Taken together, our study shows that the TXNL6 probably protects retinal cells from photooxidative damage-induced apoptosis via upregulation of NF-kappaB activity. The identification of TXNL6 and the demonstration of its protective mechanism offer new insights into treatment possibilities for photoreceptor cell degradation.

Endoplasmic reticulum stress is activated in light-induced retinal degeneration

Exposure to excessive levels of light induces photoreceptor apoptosis and has previously been used as a model for the study of retinal degeneration. During the light exposure, intracellular calcium levels increase, and reactive oxygen species (ROS) are generated, which have been shown to cause endoplasmic reticulum (ER) stress. In the present study, we investigated the role of ER stress in light-induced photoreceptor apoptosis. Our study demonstrated that, after light exposure, the ER stress sensors including glucose-regulated protein-78 (GRP78/BiP), caspase-12, phospho-eukaryotic initiation factor 2 alpha (eIF2 alpha), and phospho-pancreatic ER kinase (PERK) were significantly up-regulated in a time-dependent manner. The up-regulation of these proteins coincided with or preceded the photoreceptor apoptosis indicated by TUNEL. These data showed that ER stress played an important role in light-induced photoreceptor apoptosis. Therefore, ER stress modulators could be strong candidates as therapeutic agents in the treatment of retinal degenerative diseases.

Murine cytomegalovirus infection and apoptosis in organotypic retinal cultures

PURPOSE

An organotypic retinal culture model was used to determine the pattern of murine cytomegalovirus (MCMV) infection and whether apoptosis is induced in MCMV-infected cultured retinas.

METHODS

Retinas harvested from C57BL/6 mice were individually cultured at 37 degrees C on 3-microm filter inserts placed in 24-well plates. Some retinas were infected with MCMV (5 x 10(5) PFU/well). At days 4, 7, and 11 after infection (pi), the culture medium and cultured retinas were collected for examination.

RESULTS

Replicating virus was recovered and viral early antigen (EA)- and late antigen (LA)-positive cells were observed in the MCMV-infected retinal cultures. Most MCMV-infected cells were glia and horizontal cells. Infection resulted in atrophy of the photoreceptor cells and cytomegaly. Apoptosis of uninfected bystander cells, including photoreceptor cells and horizontal cells, was observed. TNF-alpha was produced by activated microglia during MCMV infection of the retina. Mouse apoptosis microarray studies, caspase activity studies, and RT-PCR studies showed that the genes involved in both the death receptor-mediated apoptotic pathway and the mitochondrial pathway were upregulated.

CONCLUSIONS

Many aspects of MCMV infection of retinal cultures parallel those observed during MCMV retinitis in mice. Thus, this in vitro system may be used to explore the role of apoptosis of uninfected retinal cells and the contribution of cytokines and other modulators to the pathogenesis of CMV retinitis.

Radiation-induced apoptosis in retinal progenitor cells is p53-dependent with caspase-independent DNA fragmentation

Caspases are important executioners of the endogenous cell death program. However, their function is not restricted to the induction of cell death. Caspases may process cytokines and contribute to cell differentiation or lymphocyte proliferation. In addition to their pleiotropic functions we show evidence that, under certain conditions, caspases are activated during apoptosis without executing the cell death program. Following whole body irradiation, p53 and caspases were activated in both the cerebellum and eye of postnatal day 5 mice. Although p53 activation and cell death kinetics were similar in both the cerebellum and eye, the processing of caspases was protracted and reduced in the eye. In particular, retinal caspase activation appeared not to be the executioner of cell death; incubation of retinal and cerebellar explants in the presence of the pan-caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp fluoromethylketone prevented DNA fragmentation, a hallmark of apoptosis, only in cerebellar granule cells. In contrast, in retinal cells no impairment of DNA fragmentation was observed in the presence of N-benzyloxycarbonyl-Val-Ala-Asp fluoromethylketone, indicating p53-dependent but caspase-independent cell death pathways despite caspase activation.

Methimazole-induced cell death in rat olfactory receptor neurons occurs via apoptosis triggered through mitochondrial cytochrome c-mediated caspase-3 activation pathway

The administration of methimazole is known to induce cell death in rat olfactory receptor neurons (ORNs). We investigated whether this injury occurs via apoptosis or through necrosis and whether it involves the extrinsic or intrinsic pathway. Rats were intraperitoneally injected with vehicle (control) or 300 mg/kg methimazole. The experimental animals were also administered vehicle or a caspase-3 or caspase-9 inhibitor 30 min earlier. The administration of methimazole induced cell death predominantly in the mature ORNs and partially reduced olfactory sensitivity in the rats; the injured cells were TUNEL-positive and showed a nuclear staining pattern. This insult induced cytochrome c release from the mitochondria and a significant increase in the immunoreactivity of activated caspase-3 and caspase-9 as well as that of cleaved poly-ADP-ribose-polymerase; in addition, it caused a significant increase in the fluorogenic activity of caspase-3 and caspase-9. However, it did not affect the immunoreactivity of activated caspase-8 or the fluorogenic activity of caspase-8. Pretreatment with a caspase-3 or caspase-9 inhibitor nearly completely prevented the morphologic, biochemical, and functional changes induced by methimazole. These findings suggest strongly that methimazole-induced cell death in rat ORNs is predominantly apoptosis; moreover, the majority of this apoptotic cell death is triggered through mitochondrial cytochrome c-mediated caspase-3 activation pathway, and both caspase-3 and caspase-9 inhibitors can prevent methimazole-induced cell death in the ORNs.

Telomere protection mechanisms change during neurogenesis and neuronal maturation: newly generated neurons are hypersensitive to telomere and DNA damage

Telomeres are DNA-protein complexes at the ends of eukaryotic chromosomes that play an important role in maintaining the integrity of the genome. In proliferative stem cells and cancer cells, telomere length is maintained by telomerase, and telomere structure and functions are regulated by telomere-associated proteins. We find that telomerase levels are high in embryonic cortical neural progenitor cells (NPCs) and low in newly generated neurons (NGNs) and mature neurons (MNs). In contrast, telomere repeat-binding factor 2 (TRF2) expression is undetectable in early brain development in vivo and in cultured NPCs and is expressed at progressively higher levels as NPCs cease proliferation and differentiate into postmitotic neurons. The telomere-disrupting agent telomestatin induces a DNA damage response and apoptosis in NGNs (which have low levels of TRF2 and telomerase), whereas NPCs (which have high levels of telomerase) and MNs (which have high levels of TRF2) are resistant to telomere damage. Overexpression of TRF2 in NGNs protects them against death induced by telomestatin and other DNA-damaging agents. Knockdown of TRF2 expression in MNs and knock-out of telomerase reverse transcriptase in NPCs increased their sensitivity to telomere- and DNA-damaging agents but did not affect the vulnerability of NGNs. These findings suggest that TRF2 and telomerase function as distinct telomere protection mechanisms during the processes of neurogenesis and neuronal maturation and that hypersensitivity of NGNs to telomere damage results from relative deficiencies of both telomerase and TRF2.

Significant photoreceptor rescue by treatment with a combination of antioxidants in an animal model for retinal degeneration

The purpose of this study was to investigate the presence of oxidative DNA damage in the photoreceptors of the rd1 mouse, an animal model for retinitis pigmentosa, and to determine if antioxidants could delay the progress of photoreceptor cell death. Retinas of rd1 mice and congenic wild type controls were examined for DNA oxidation and fragmentation. To study the rescue effect of antioxidants on retinal degeneration, rd1 retinas were studied in vitro and in vivo using lutein, zeaxanthin, alpha lipoic acid and reduced l-glutathione. For the in vitro studies, antioxidants were added to the culture medium. For the in vivo studies, postnatal day (PN3) pups of rd1 mice were fed antioxidants either individually or in combination and control rd1 animals received vehicle alone. Histological evaluation was performed using hematoxylin/eosin and avidin staining, as well as terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay. Many of the rd1 rod photoreceptors at PN11 displayed oxidative DNA damage and TUNEL positive reaction which co-localized in a subset of rod photoreceptors. Avidin-labeled rod photoreceptors were more abundant than the TUNEL positive photoreceptors of the rd1 mouse, indicating that oxidative DNA damage precedes fragmentation. The number of TUNEL positive and avidin positive cells was considerably decreased upon treatment with the combination of the antioxidants. Rescue of rd1 photoreceptors was significant at PN18 and PN17, respectively, in the in vitro and in vivo studies. In conclusion individual antioxidants had no significant rescue effect but the combination slowed down the rd1 rod photoreceptor degeneration, indicating an additive or synergistic effect.

Analysis of apoptotic and survival mediators in the early post-natal and mature retina

Apoptosis, a cellular process critical to retinal neurogenesis, has been implicated in several neurodegenerative disorders. As the retina matures the suppression of apoptosis occurs and the emphasis shifts towards survival. To identify the cellular changes that bring about this critical shift in the balance, we performed an expression analysis of pro- and anti-apoptotic mediators in the immature, post-natal day 6 (P6) and the post-mitotic adult P60 mouse retina. Laser capture microdissection (LCM) of the P6 and the P60 retina, followed by reverse transcriptase-polymerase chain reaction (RT-PCR) was employed to elucidate changes in the mRNA expression of Apaf-1, caspase-3 and caspase-9 in the individual retinal layers in the young and mature tissue. RT-PCR and Western blotting of whole P6 and P60 retinal preparations was carried out to determine changes in other caspases and key survival mediators at the mRNA and protein level, respectively. Our results demonstrate that each neuronal cell layer in the adult retina down-regulates the gene expression of Apaf-1 and caspase-3, and to a lesser extent, caspase-9. The protein expression levels of other executioner and initiator caspases are also reduced in the adult tissue. Interestingly, XIAP, a potent caspase inhibitor, increases in expression in the adult retina. Additionally, we demonstrate age-dependent increased expression and activation status of the components of the MAPK transduction cascade. Conversely, we observe decreased PI3-K and AKT expression and decreased activity of AKT (pAKT) in the adult retina. Furthermore, results from RNAi experiments demonstrate an additional mechanism of PI3-K regulation in photoreceptor cells. Our findings suggest that a survival strategy adopted by the post-mitotic retina involves a down-regulation of key pro-apoptotic factors concomitant with changes in expression and activation status of certain pro-survival mediators.

Docosahexaenoic acid prevents apoptosis of retina photoreceptors by activating the ERK/MAPK pathway

Identifying the trophic factors for retina photoreceptors and the intracellular pathways activated to promote cell survival is crucial for treating retina neurodegenerative diseases. Docosahexaenoic acid (DHA), the major retinal polyunsaturated fatty acid, prevents photoreceptor apoptosis during early development in vitro, and upon oxidative stress. However, the signaling mechanisms activated by DHA are still unclear. We investigated whether the extracellular signal regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) or the phosphatidylinositol-3-kinase (PI3K) pathway participated in DHA protection. 1,4-Diamino-2,3-dicyano-1,4-bis(2-aminophynyltio) butadiene (U0126), a specific MEK inhibitor, completely blocked the DHA anti-apoptotic effect. DHA rapidly increased ERK phosphorylation in photoreceptors, whereas U0126 blocked this increase. U0126 hindered DHA prevention of mitochondrial depolarization, and blocked the DHA-induced increase in opsin expression. On the contrary, PI3K inhibitors did not diminish the DHA protective effect. DHA promoted the early expression of Bcl-2, decreased Bax expression and reduced caspase-3 activation in photoreceptors. These results suggest that DHA exclusively activates the ERK/MAPK pathway to promote photoreceptor survival during early development in vitro and upon oxidative stress. This leads to the regulation of Bcl-2 and Bax expression, thus preserving mitochondrial membrane potential and inhibiting caspase activation. Hence, DHA, a lipid trophic factor, promotes photoreceptor survival and differentiation by activating the same signaling pathways triggered by peptidic trophic factors.

Ceramide is the key mediator of oxidative stress-induced apoptosis in retinal photoreceptor cells

Nitric oxide and reactive oxygen species play a critical role in photoreceptor apoptosis. However, the exact molecular mechanisms triggered by oxidative stress in photoreceptor cell death remain undefined. Here, we demonstrate that the sphingolipid ceramide is the key mediator of oxidative stress-induced apoptosis in 661W retinal photoreceptor cells. Treatment of 661W cells with the nitric oxide donor, sodium nitroprusside, activates acid sphingomyelinase. As a result, sphingomyelin is hydrolysed, which leads to an increase in the concentration of ceramide. We also show that ceramide is responsible for the activation of the mitochondrial apoptotic pathway in 661W photoreceptor cells and subsequent activation of the caspase cascade. Furthermore, we show for the first time that ceramide is responsible for the increased Ca2+ levels in the mitochondria and cytosol that precedes activation of the calpain-mediated apoptotic pathway. Additionally, we provide evidence that ceramide also activates the endolysosomal protease cathepsin D pathway. In summary, our findings show that ceramide controls the cell death decisions in photoreceptor cells and highlight the relevance of acid sphingomyelinase as a potential therapeutic target for the treatment of retinal pathologies.

Photoreceptor cell apoptosis in the retinal degeneration of Uchl3-deficient mice

UCH-L3 belongs to the ubiquitin C-terminal hydrolase family that deubiquitinates ubiquitin-protein conjugates in the ubiquitin-proteasome system. A murine Uchl3 deletion mutant displays retinal degeneration, muscular degeneration, and mild growth retardation. To elucidate the function of UCH-L3, we investigated histopathological changes and expression of apoptosis- and oxidative stress-related proteins during retinal degeneration. In the normal retina, UCH-L3 was enriched in the photoreceptor inner segment that contains abundant mitochondria. Although the retina of Uchl3-deficient mice showed no significant morphological abnormalities during retinal development, prominent retinal degeneration became manifested after 3 weeks of age associated with photoreceptor cell apoptosis. Ultrastructurally, a decreased area of mitochondrial cristae and vacuolar changes were observed in the degenerated inner segment. Increased immunoreactivities for manganese superoxide dismutase, cytochrome c oxidase I, and apoptosis-inducing factor in the inner segment indicated mitochondrial oxidative stress. Expression of cytochrome c, caspase-1, and cleaved caspase-3 did not differ between wild-type and mutant mice; however, immunoreactivity for endonuclease G was found in the photoreceptor nuclei in the mutant retina. Hence, loss of UCH-L3 leads to mitochondrial oxidative stress-related photoreceptor cell apoptosis in a caspase-independent manner. Thus, Uchl3-deficient mice represent a model for adult-onset retinal degeneration associated with mitochondrial impairment.

Acid DNases and their interest among apoptotic endonucleases

Apoptosis is characterized by cell shrinkage, nuclear condensation and internucleosomal DNA cleavage. Besides the central role of caspases and other proteases, cell death triggers DNA degradation so that DNases have an active role in apoptotic cell death. The best-characterized apoptotic DNase is CAD, a neutral Mg-dependent endonuclease. Its activity is regulated by its inhibitor, ICAD, which is cleaved by caspases. Other neutral DNases have been shown to cleave nuclear DNA in apoptotic conditions: endonuclease G, GADD. In cells, the cytosolic pH is maintained to 7.2, mostly due to the activity of the Na(+)/H(+) exchanger. In many apoptotic conditions, a decrease of the intracellular pH has been shown. This decrease may activate different acid DNases, mostly when pH decreases below 6.5. Three acidic DNases II are so far known: DNase II alpha, DNase II beta and L-DNase II, a DNase II, derived from the serpin LEI (Leukocyte Elastase Inhibitor). Their activation during cell death is discussed in this review.

Cell death pathways in juvenile Batten disease

Apoptosis, Golgi fragmentation and elevated ceramide levels occur in Juvenile Neuronal Ceroid Lipofuscinosis (JNCL) neurons, lymphoblasts and fibroblasts. Our purpose was to examine whether apoptosis is the mechanism of cell death in JNCL. This was tested by analyzing caspase-dependent/independent pathways and autophagy, and caspase effects on ceramide and Golgi fragmentation. zVAD prevented caspase activation, but not all cell death. Inhibiting caspase-8 suppressed caspases more than inhibition of any other caspase. Inhibiting caspase-8/6 was synergistic. zVAD suppressed autophagy. 3-methyladenine suppressed caspase activation less than zVAD did. Blocking autophagy/caspase-8/or-6 was synergistic. Blocking autophagy/caspase-3/or-9 was not. Inhibiting caspase-9/3 suppressed autophagy. Golgi fragmentation was suppressed by zVAD, and blocked by CLN3. CLN3, not zVAD, prevented ceramide elevation.

IN CONCLUSION

caspase-dependent/independent apoptosis and autophagy occur caspase-dependent pathways initiate autophagy Golgi fragmentation results from apoptosis ceramide elevation is independent of caspases, and CLN3 blocks all cell death, prevents Golgi fragmentation and elevation of ceramide in JNCL.

Involvement of endonuclease G in nucleosomal DNA fragmentation under sustained endogenous oxidative stress

We have previously shown that inhibition of catalase and glutathione peroxidase activities by 3-amino-1,2,4-triazole (ATZ) and mercaptosuccinic acid (MS), respectively, in rat primary hepatocytes caused sustained endogenous oxidative stress and apoptotic cell death without caspase-3 activation. In this study, we investigated the mechanism of this apoptotic cell death in terms of nucleosomal DNA fragmentation. Treatment with ATZ+MS time-dependently increased the number of deoxynucleotidyl transferase-mediated nick end-labeling (TUNEL)-positive nuclei from 12 h, resulting in clear DNA laddering at 24 h. The deoxyribonuclease (DNase) inhibitor, aurintricarboxylic acid (ATA), completely inhibited nucleosomal DNA fragmentation but the pan-caspase inhibitor, z-VAD-fmk was without effects; furthermore, the cleavage of inhibitor of caspase-activated DNase was not detected, indicating the involvement of DNase(s) other than caspase-activated DNase. Considering that endonuclease G (EndoG) reportedly acts in a caspase-independent manner, we cloned rat EndoG cDNA for the first time. Recombinant EndoG alone digested plasmid DNA and induced nucleosomal DNA fragmentation in isolated hepatocyte nuclei. Recombinant EndoG activity was inhibited by ATA but not by hydrogen peroxide, even at 10 mm. ATZ+MS stimulation elicited decreases in mitochondrial membrane potential and EndoG translocation from mitochondria to nuclei. By applying RNA interference, the mRNA levels of EndoG were almost completely suppressed and the amount of EndoG protein was decreased to approximately half the level of untreated cells. Under these conditions, decreases in TUNEL-positive nuclei were significantly suppressed. These results indicate that EndoG is responsible, at least in part, for nucleosomal DNA fragmentation under endogenous oxidative stress conditions induced by ATZ+MS.

Multiple death pathways in retina-derived 661W cells following growth factor deprivation: crosstalk between caspases and calpains

During development of the mammalian retina, neurons that do not succeed in establishing functional synaptic connections are eliminated by apoptosis, allowing the formation of a finely tuned network. Growth factors play a crucial role in controlling the balance between apoptosis and survival signals not only at developmental stages but also in long-term preservation of retinal functions. In the present work, we explore the apoptotic mechanisms triggered by growth factor deprivation of retina-derived 661W cells. Under serum starvation conditions, these cone photoreceptors underwent cell death with participation of caspase-9, -3 and -12. Interestingly, inhibition of caspases did not prevent apoptosis but only resulted in a temporary delay. We show m-calpain activation in parallel with caspases, indicating that more than one execution pathway is available to cone photoreceptors. Moreover, crosstalk of the caspase and calpain pathways was detected, suggesting a loop that may act to amplify the apoptotic cascade.

Geldanamycin increases 4-hydroxynonenal (HNE)-induced cell death in human retinal pigment epithelial cells

Development of age-related macular degeneration (AMD) is associated with functional abnormalities and cell death in retinal pigment epithelial (RPE) cells attributable to oxidative stress. To minimize the adverse effects of oxidative stress, cells activate their defence systems, e.g., via increased expression of heat shock protein (Hsp), activation of stress sensitive AP-1 and NF-kappaB transcription factors. In this study, we examined the accumulation of Hsp70 protein, activation of AP-1 and NF-kappaB transcription factors in human ARPE-19 cells subjected to a 4-hydroxynonenal (HNE)-induced oxidative stress. In addition, the influence of Hsp90 inhibitor geldanamycin (GA) was studied in HNE-treated cells. Mitochondrial metabolic activity and apoptosis were determined to evaluate cell death in the ARPE-19 cells. The ARPE-19 cells showed increased accumulation of Hsp70 protein before of the cytotoxic hallmarks appearing in response to HNE. In contrast, increased DNA-binding activities of AP-1 or NF-kappaB transcription factors were not seen under HNE insults. Interestingly, GA significantly increased cell death in the HNE-treated cells, which was involved in caspase-3 independent apoptosis. This study reveals that the Hsps have an important role in the cytoprotection of RPE cells subjected to HNE-derived oxidative stress.

Geldanamycin activates Hsp70 response and attenuates okadaic acid-induced cytotoxicity in human retinal pigment epithelial cells

Reversible protein phosphorylation regulates the biological activities of many human proteins involved in crucial cellular processes, e.g., protein-protein interactions, cell signaling, gene transcription, cell growth, and death. A malfunction of cellular homeostasis in retinal pigment epithelial (RPE) cells is involved in the age-related retinal degeneration. In this study, we examined cytotoxicity in human RPE cells subjected to the protein phosphatase inhibitor, okadaic acid (OA). Moreover, the influence of Hsp90 inhibitor geldanamycin (GA), a benzoquinone ansamycin, in cytoprotection was assessed. Hsp70 protein levels were analyzed by Western blot. Cellular viability was determined by LDH and MTT assays. To study apoptotic cell death, caspase-3 enzyme activity was measured by assaying the cleavage of a fluorescent peptide substrate and Hoechst dye was used to visualize nuclear morphology. OA treatment caused morphological changes and induced cytotoxicity by caspase-3-independent manner in the RPE cells. No evidence of nuclear fragmentation was observed in response to OA. Interestingly, GA treatment accumulated Hsp70 protein and attenuated OA-induced cytotoxicity. This study suggests that Hsp70 and Hsp90 are closely related to cytoprotection of RPE cells in response to protein phosphatase inhibition.

Caspase-independent retinal ganglion cell death after target ablation in the neonatal rat

In neonatal rats, superior colliculus (SC) ablation results in a massive and rapid increase in retinal ganglion cell (RGC) death that peaks about 24 h post-lesion (PL). Naturally occurring cell death during normal development, and RGC death after axonal injury in neonatal and adult rats, has primarily been ascribed to apoptosis. Given that normal developmental cell death is reported to involve caspase 3 activation, and blocking caspase activity in adults reduces axotomy-induced death, we examined whether blocking caspases in vivo reduces RGC death after neonatal SC lesions. Neither general nor specific caspase inhibitors increased neonatal RGC survival 6 and 24 h PL. These inhibitors were, however, effective in blocking caspases in another well-defined in vitro apoptosis model, the corpus luteum. Caspase 3 protein and mRNA levels in retinas from normal and SC-lesioned neonatal rats were assessed 3, 6 and 24 h after SC removal using immunohistochemistry, western and northern blots and quantitative real-time polymerase chain reaction. Terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling (TUNEL) was used to independently monitor retinal cell death. The polymerase chain reaction data showed a small but insignificant increase in caspase 3 mRNA in retinas 24 h PL. Western blot analysis did not reveal a significant shift to cleaved (activated) caspase 3 protein. There was a small increase in the number of cleaved caspase 3 immunolabelled cells in the ganglion cell layer 24 h PL but this represented only a fraction of the death revealed by TUNEL. Together, these data indicate that, unlike the situation in adults, most lesion-induced RGC death in neonatal rats occurs independently of caspase activation.

Hypothèses sur le rôle des facteurs nutritionnels au cours des hypertonies oculaires et du glaucome

Recent notions in connection with oxidative stress and the fat balance of long chain polyunsaturated fatty acids (PUFA) families have brought new insight to a probable role of nutritional factors in glaucoma and intraocular hypertony. The modifications of the extracellular matrix of the trabecula could be influenced by oxidative stress. On the one hand, collagen apoptosis and remodeling (associated with an increase in intraocular pressure) are mainly influenced by hydrosoluble antioxidants such as glutathione. On the other hand, elastin apoptosis and remodeling (correlated with the occurrence of optic atrophy) are particularly influenced by liposoluble antioxidants such as vitamin E. In addition, the dietary ratio of omega3/omega6PUFA intake could influence the balance of intraocular pressure. Omega-3 PUFA could influence cyclooxygenase competition. A diet with increased omega-3 and decreased omega-6 could thus favor an increase in intraocular pressure reducing synthesis of PG-F2, leading to a decrease in uveoscleral outflow. The true importance of these factors has not yet been solidly determined and studies are in progress to clarify the real implication of these nutritional factors.

Somatic cell apoptosis markers and pathways in human ejaculated sperm: potential utility as indicators of sperm quality

In this study we extended earlier work to determine whether sperm respond to somatic cell apoptotic stimuli and whether apoptotic phenotypes are significant indicators of human sperm quality. We evaluated ejaculated sperm from fertile donors and subfertile patients following purification of fractions of high and low motility. In unstimulated conditions, caspase enzymatic activity was higher in motile fractions from subfertile patients than in donors, and was higher in low motility fractions from both groups. Staurosporine, but not a Fas ligand or H2O2, significantly increased caspase activity, but only in high motility fractions. Procaspase-3, -7 and -9 and low levels of active caspase-3, -7 and -9 were identified by immunoblot analysis. Apoptosis-inducing factor (AIF) was present in all samples but poly ADP-ribose polymerase-1 (PARP-1) was not detected. Phosphatidylserine translocation was significantly increased only with H2O2 treatment. In ejaculates of both subfertile and fertile men, we demonstrated the presence and activation of several proteins that are key constituents of apoptosis-related pathways in somatic cells, which may serve as markers for sperm quality.

Oxidative Stress-induced Apoptosis in Retinal Photoreceptor Cells Is Mediated by Calpains and Caspases and Blocked by the Oxygen Radical Scavenger CR-6

A critical role for reactive oxygen species (ROS) in photoreceptor apoptosis has been established. However, the exact molecular mechanisms triggered by oxidative stress in photoreceptor cell death remain undefined. This study delineates the molecular events that occur after treatment of the photoreceptor cell line 661W with the nitric oxide donor sodium nitroprusside (SNP). Cytosolic calcium levels increased during photoreceptor apoptosis, leading to activation of the calcium-dependent proteases calpains. Furthermore, caspase activation also occurred following SNP insult. However, although treatment with the pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp fluoromethyl ketone inhibited caspase activity per se in SNP-treated 661W cells, it did not prevent apoptosis. On the other hand, CR-6 (3,4-dihydro-6-hydroxy-7-methoxy-2,2-dimethyl-1(2H)-benzopyran) acted as a scavenger of ROS and reduced 661W photoreceptor apoptosis induced by SNP by preventing the activation of a pathway in which calpains have a key role. In summary, we report for the first time that both caspases and calpains are involved in 661W photoreceptor apoptosis and that calpain activation can be prevented by the ROS scavenger CR-6.

Functional dichotomy: glutathione and vitamin E in homeostasis relevant to primary open-angle glaucoma

Primary open-angle glaucoma (POAG) is a complex chronic neurological disease that can result in blindness. The goal of understanding the aetiology of POAG is to be able to target effective treatment to individuals who will eventually go blind without it. Epidemiological studies of POAG have not specifically addressed the possibility that nutrition may play a role in the development of POAG. A handful of papers have considered that nutrition may have an impact on POAG patients. POAG is not believed to be a 'vitamin-deficiency disease'. The concept of 'vitamin-deficiency diseases' and the recommended daily allowances have not kept pace with the growing understanding of the cellular and molecular functions of vitamins and other micronutrients. The aetiology of POAG remains a mystery. Discoveries in cell physiology can be assimilated from the literature and applied to known homeostatic mechanisms of the eye. In this way the possible roles of nutritional components involved in the aetiology of POAG can be described. The mechanisms may be subject to many influences in ways that have yet to be defined. Two distinct changes in the trabecular meshwork can be identified: trabecular meshwork changes that cause intra-ocular pressure to increase and trabecular meshwork changes that are directly correlated to optic nerve atrophy. Compelling evidence suggests that collagen trabecular meshwork extracellular matrix (ECM) remodelling is correlated to increased intraocular pressure in POAG. Elastin trabecular meshwork ECM remodelling is correlated to POAG optic nerve atrophy. There appear to be two different pathways of ECM remodelling and apoptosis induction in POAG. The pathway for collagen remodelling and apoptosis induction seems to be exogenously influenced by water-soluble antioxidants, for example, glutathione. The pathway for elastin remodelling and apoptosis induction seems to be influenced by endogenous lipid-soluble antioxidants, for example, vitamin E. Roles can be defined for antioxidants in the two different pathways of ECM remodelling and apoptosis induction. This suggests that antioxidants are important in maintaining cellular homeostasis relevant to the aetiology of POAG.

Use of multiple assay endpoints to investigate the effects of incubation time, dose of toxin, and plating density in cell-based cytotoxicity assays

Here we show the results of comparing cell viability, cytotoxicity, and apoptosis assays for measuring the time- and dose-dependent toxic effects of tamoxifen on HepG2 cells. The quantitation of adenosine 5'-triphosphate (ATP), 5-(3-carboxymethoxyphenyl)-2-(4,5- dimethylthiazolyl)-3-(4-sulfophenyl) tetrazolium, inner salt (MTS) tetrazolium reduction, and resazurin reduction methods used to estimate the number of viable cells all showed a similar trend of decreased cell viability after longer periods of tamoxifen exposure to HepG2 cells. The release of lactate dehydrogenase (LDH) as a marker for cells with a compromised membrane and the increase in caspase-3/7 activity as a marker for apoptosis were both shown to increase using the same tamoxifen exposure conditions that caused a decrease in HepG2 cell viability. The longer the duration of exposure of tamoxifen, the lower the concentration required to kill or induce apoptosis in HepG2 cells. In contrast, there was no change in LDH release from HL-60 cells using conditions of vinblastine treatment that caused an increase in caspase activity and a decrease in ATP content, suggesting a difference in the mechanism of cell death between the two model systems. Both the density of parent stock cultures used as a source of cells to prepare assay plates and the density of cells per well in the assay plates were demonstrated to be factors than can influence the apparent potency of a toxin in viability, toxicity, and apoptosis assays. These results illustrate the importance of understanding the kinetics and mechanism of cell death of each in vitro model system as prerequisites for choosing the most appropriate assay method.

Programmed cell deaths. Apoptosis and alternative deathstyles

Programmed cell death is a major component of both normal development and disease. The roles of cell death during either embryogenesis or pathogenesis, the signals that modulate this event, and the mechanisms of cell demise are the major subjects that drive research in this field. Increasing evidence obtained both in vitro and in vivo supports the hypothesis that a variety of cell death programs may be triggered in distinct circumstances. Contrary to the view that caspase-mediated apoptosis represents the standard programmed cell death, recent studies indicate that an apoptotic morphology can be produced independent of caspases, that autophagic execution pathways of cell death may be engaged without either the involvement of caspases or morphological signs of apoptosis, and that even the necrotic morphology of cell death may be consistently produced in some cases, including certain plants. Alternative cell death programs may imply novel therapeutic targets, with important consequences for attempts to treat diseases associated with disregulated programmed cell death.

Control of mitochondrial integrity by Bcl-2 family members and caspase-independent cell death

Programmed cell death (PCD) is essential for normal development and maintenance of tissue homeostasis in multicellular organisms. While it is now evident that PCD can take many different forms, apoptosis is probably the most well-defined cell death programme. The characteristic morphological and biochemical features associated with this highly regulated form of cell death have until recently been exclusively attributed to the caspase family of cysteine proteases. As a result, many investigators affiliate apoptosis with its pivotal execution system, i.e. caspase activation. However, it is becoming increasingly clear that PCD or apoptosis can also proceed in a caspase-independent manner and maintain key characteristics of apoptosis. Mitochondrial integrity is central to both caspase-dependent and-independent cell death. The release of pro-apoptotic factors from the mitochondrial intermembrane space is a key event in a cell's commitment to die and is under the tight regulation of the Bcl-2 family. However, the underlying mechanisms governing the efflux of these pro-death molecules are largely unknown. This review will focus on the regulation of mitochondrial integrity by Bcl-2 family members with particular attention to the controlled release of factors involved in caspase-independent cell death.

2-methoxyestradiol induces caspase-independent, mitochondria-centered apoptosis in DS-sarcoma cells

The anti-cancer potential of the natural estrogen metabolite 2-methoxyestradiol is associated with microtubuli interaction, anti-angiogenetic effects and inhibition of superoxide dismutase leading to apoptosis. The effectors of apoptotic signaling through 2-methoxyestradiol, however, are cell type-dependent. We investigated the effect of 2-methoxyestradiol on several events associated with apoptosis in rat DS-sarcoma cells. Translocation of the pro-apoptotic protein Bax to mitochondria was identified as an initial apoptotic event that was accompanied by a decrease in mitochondrial transmembrane potential and the formation of reactive oxygen species (ROS) followed by mitochondrial release of apoptosis inducing factor and endonuclease G. In addition, 2-methoxyestradiol treatment caused upregulation of death receptor ligands FasL and TNFalpha and induced caspase-8 activation. The pan caspase inhibitor Z-VAD-FMK did not suppress apoptotic cell death, however, indicating that the major pro-apoptotic effect of 2-methoxyestradiol is mediated by a caspase-independent mechanism. Furthermore, ROS do not seem to play a pivotal role in the toxic/apoptotic effect of 2-methoxyestradiol in DS-sarcoma cells because supplementation with various antioxidants provided only limit protection. Colony formation was not affected by antioxidants. Therefore, in DS-sarcoma cells, the breakdown of mitochondrial integrity with the subsequent release of mitochondrial nucleases is the main factor in 2-methoxyestradiol mediated cell death.

Mediating of caspase-independent apoptosis by cadmium through the mitochondria-ROS pathway in MRC-5 fibroblasts

Cadmium (Cd) is an environmental pollutant of global concern with a 10-30-year biological half-life in humans. Accumulating evidence suggests that the lung is one of the major target organs of inhaled Cd compounds. Our previous report demonstrated that 100 microM Cd induces MRC-5 cells, normal human lung fibroblasts, to undergo caspase-independent apoptosis mediated by mitochondrial membrane depolarization and translocation of apoptosis-inducing factor (AIF) from mitochondria into the nucleus. Here, using benzyloxycarbonyl-Val-Ala-Asp-(ome) fluoromethyl ketone (Z-VAD.fmk) as a tool, we further demonstrated that Cd could induce caspase-independent apoptosis at concentrations varied from 25 to 150 microM, which was modulated by reactive oxygen species (ROS) scavengers, such as N-acetylcysteine (NAC), mannitol, and tiron, indicating that ROS play a crucial role in the apoptogenic activity of Cd. Consistent with this notion, the intracellular hydrogen peroxide (H2O2) was 2.9-fold elevated after 3 h of Cd treatment and diminished rapidly within 1 h as detected by flow cytometry with 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA) staining. Using inhibitors of the mitochondrial electron transport chain (ETC) (oligomycin A and rotenone for complex I and V, respectively) and mitochondrial permeability transition pore (MPTP) (cyclosporin A and aristolochic acid), we coincidently found the ROS production, mitochondrial membrane depolarization, and apoptotic content were almost completely or partially abolished. As revealed by confocal microscopy staining with chloromethyl-X-rosamine (CMXRos) and an anti-AIF antibody, the collapse of mitochondrial membrane potential induced by Cd (3 h-treatment) was a prelude to the translocation of caspase-independent pro-apoptotic factor, AIF, into the nucleus (after 4 h of Cd treatment). In summary, this study demonstrated that, in MRC-5 fibroblasts, Cd induced caspase-independent apoptosis through a mitochondria-ROS pathway. More importantly, we provide several lines of evidence supporting a role of mitochondrial ETC and MPTP in the regulation of caspase-independent cell death triggered by Cd.

Mechanism of taxol-induced apoptosis in human SKOV3 ovarian carcinoma cells

Taxol is extensively used clinically for chemotherapy of patients with ovarian, breast, and lung cancer. Although taxol induces apoptosis of cancer cells, its exact mechanism of action is not yet known. To determine the mechanism of action of taxol in ovarian cancer, we tested the effects of the drug, on the human ovarian carcinoma cell line, SKOV3. We observed that taxol-induced apoptosis of these cells by phosphatidylserine (PS) externalization and DNA fragmentation. While treatment of cells with taxol resulted in bcl-2 phosphorylation and mitochondrial depolarization, cytochrome c was not released and pro-caspase-3 was not activated. Treatment of SKOV3 cells with taxol, however, resulted in the translocation of AIF from the mitochondria to the nucleus via the cytosol. Taken together, these findings suggest that in SKOV3 cells, taxol induces caspase-independent AIF-dependent apoptosis.

Heat stress downregulates FLIP and sensitizes cells to Fas receptor-mediated apoptosis

The heat shock response and death receptor-mediated apoptosis are both key physiological determinants of cell survival. We found that exposure to a mild heat stress rapidly sensitized Jurkat and HeLa cells to Fas-mediated apoptosis. We further demonstrate that Hsp70 and the mitogen-activated protein kinases, critical molecules involved in both stress-associated and apoptotic responses, are not responsible for the sensitization. Instead, heat stress on its own induced downregulation of FLIP and promoted caspase-8 cleavage without triggering cell death, which might be the cause of the observed sensitization. Since caspase-9 and -3 were not cleaved after heat shock, caspase-8 seemed to be the initial caspase activated in the process. These findings could help understanding the regulation of death receptor signaling during stress, fever, or inflammation.

Caspase-independent photoreceptor apoptosis in mouse models of retinal degeneration

Apoptosis is the mode of cell death in retinitis pigmentosa, a group of retinal degenerative disorders primarily affecting rod photoreceptors. Although caspases have been demonstrated to play a central role in many incidences of apoptosis, accumulating evidence suggests that they may not be required for all forms of apoptotic cell death. The present study examined the mechanism of cell death in two in vivo models of photoreceptor apoptosis: the retinal degeneration (rd) mouse, a naturally occurring mutant model, and N-methyl-N-nitrosourea-induced retinal degeneration. Specifically, we examined the activation status of caspase-9, -8, -7, -3, and -2 and determined the caspase requirements for cytochrome c release, DNA fragmentation, and apoptosis-associated proteolysis of specific caspase substrates. We show that apoptosis in both in vivo models is independent of caspase-9, -8, -7, -3, and -2 activation. DNA fragmentation occurs in the absence of caspase-mediated ICAD (inhibitor of caspase-activated DNase) proteolysis, suggesting that an alternative endonuclease is responsible for DNA cleavage in these models. Importantly, we show that apoptosome activation is prevented because of an absence of mitochondrial cytochrome c release. Experiments performed using a cell-free system indicate that cytochrome c-dependent proteolysis and activation of caspase-9 can be restored in a neonatal cell-free system. However, we found that cytochrome c-dependent proteolysis and activation of caspase-9 could not be restored in an adult cell-free system because of an age-related decrease in the expression of Apaf-1 in the normal developing mouse retina. In the rd mouse, however, this age-related downregulation of apoptotic proteins was not observed, highlighting a critical feature of this model and the prevention of cytochrome c release as an apical event in caspase-independent apoptosis in this system.

Mitochondria-mediated caspase-independent apoptosis induced by cadmium in normal human lung cells

Cadmium, a well-known environmental hazard, has caused serious health problems in humans and animals. Accumulating evidence suggests the cadmium toxicity is mediated by oxidative stress-induced cell death. However, the molecular signaling underlying cadmium-induced apoptosis remains unclear. In this study, we demonstrate here that cadmium induced mixed types of cell death including primary apoptosis (early apoptosis), secondary necrosis (late apoptosis), and necrosis in normal human lung cells, MRC-5, as revealed by chromatin condensation, phosphatidylserine (PS) externalization, and hypodiploid DNA content. The total apoptotic cells reached a plateau of around 40.0% after 24 h exposure of 100 microM cadmium. Pretreatment with Z-Val-Ala-Asp-fluoromethylketone (Z-VAD-fmk), a broad spectrum of caspase inhibitor, could not rescue apoptotic cells from cadmium toxicity. Coincidently, we failed to detect the activation of pro-caspase-3 and cleavage of PARP by immunoblot, which implies the apoptogenic activity of cadmium in MRC-5 cells is caspase-independent. JC-1 staining also indicated that mitochondrial depolarization is a prelude to cadmium-induced apoptosis, which was accompanied by a translocation of caspase-independent pro-apoptotic factor apoptosis-inducing factor (AIF) into the nucleus as revealed by the immunofluorescence assay. In summary, this study demonstrated for the first time that cadmium induced a caspase-independent apoptotic pathway through mitochondria-mediated AIF translocation into the nucleus.

Role of reduced glutathione efflux in apoptosis of immortalized human keratinocytes induced by UVA

We have investigated the role played by GSH efflux in apoptosis of human HaCaT keratinocytes induced by UVA irradiation. UVA irradiation of HaCaT cells caused a rapid rise in GSH efflux across the intact cell membrane, followed by an increase in apoptosis. GSH efflux was stimulated by glucose and was reduced by the addition of exogenous GSH and intracellular GSH depletion by buthionine sulfoximine, suggesting that GSH transport is active and is influenced by the GSH concentration gradient across the cell membrane. Verapamil and cyclosporin A, blockers of the multidrug resistance-associated protein, decreased UVA-induced GSH efflux. GSH efflux occurred within 2 h of UVA irradiation, suggesting that the stimulation of GSH efflux is due to an increase in the activity of pre-existing multidrug resistance-associated protein transporter carrier. Although inhibition of GSH efflux did not affect caspase activation and DNA fragmentation, it delayed the gradual increase in plasma membrane permeability and reduced phosphatidylserine translocation in HaCaT cells. It is therefore likely that upon UVA irradiation, GSH efflux increased the intracellular oxidative stress without intervention of reactive oxygen species, thus resulting in more phosphatidylserine externalization and membrane rearrangement. These provide targets for macrophage recognition and phagocytosis and thus minimize the potential to invoke inflammation or neoplastic transformation.

Changes in hydrogen peroxide homeostasis trigger an active cell death process in tobacco

In transgenic tobacco plants with reduced catalase activity, high levels of hydrogen peroxide (H2O2) can accumulate under photorespiratory conditions. Such a perturbation in H2O2 homeostasis induced cell death in clusters of palisade parenchyma cells, primarily along the veins. Ultrastructural alterations, such as chromatin condensation and disruption of mitochondrial integrity, took place before cell death. Furthermore, enhanced transcript levels of mitochondrial defense genes accompanied these mitochondrial changes. Pharmacological data indicated that the initiation and execution of cell death require de novo protein synthesis and that the signal transduction pathway leading to cell death involved changes in ion homeostasis, (de)phosphorylation events and an oxidative burst, as observed during hypersensitive responses. This oxidase-dependent oxidative burst is essential for cell death, but it is not required for the accumulation of defense proteins, suggesting a more prominent role for the oxidative burst in abiotic stress-induced cell death.

Ocular manifestations in the inherited DNA repair disorders

Deoxyribonucleic acid (DNA) repair is a fundamental process designed to keep the integrity of genomic DNA that is continuously challenged by intrinsic or environmental induced alterations. Numerous genes involved in DNA repair have been cloned and are involved in different DNA repair pathways: base excision repair, nucleotide excision repair, mismatch repair, DNA recombination. Inherited conditions due to mutations in DNA repair genes include mainly: xeroderma pigmentosum, Cockayne syndrome, Trichothiodystrophy, Bloom syndrome, Rothmund-Thomson syndrome, and Werner syndrome. Minor to major ocular manifestations occur in these syndromes. For example, eyelid skin cancers in xeroderma pigmentosum and retinal dystrophy in Cockayne syndrome are major features of these syndromes. This review focuses on the DNA repair pathways, the general and ocular features of the related syndromes, the laboratory tests useful for diagnosis, and the general processes implied with DNA repair (ultraviolet sensitivity, carcinogenesis, apoptosis, oxydative stress, and premature aging).

A caspase-3-dependent pathway is predominantly activated by the excitotoxin pregnenolone sulfate and requires early and late cytochrome c release and cell-specific caspase-2 activation in the retinal cell death

This study investigates the implication of mitochondria- and caspase-dependent pathways in the death of retinal neurones exposed to the neurosteroid pregnenolone sulfate (PS) shown to evoke apoptosis and contribute to amplification and propagation of excitotoxicity. After a brief PS challenge of intact retinas, caspase-3 and caspase-2 activation and cytochrome c release occur early and independent of changes in the oxidative state measured by superoxide dismutase activity. The temporal and spatial relationship of these events suggests that a caspase-3-dependent pathway is activated in response to cytochrome c release and requires caspase-2 activation and a late cytochrome c release in specific cellular subsets of retinal layers. The protection by caspase inhibitors indicates a predominant role of the pathway in PS-induced retinal apoptosis, although a limited use of caspase inhibitors is upheld on a conceivable shift from apoptosis toward necrosis. Conversely, 3alpha-hydroxy-5beta-pregnan-20-one sulfate and 17beta-oestradiol provide complete prevention of PS-induced retinal death.

c-Jun N-terminal kinase pathway mediates Lactacystin-induced cell death in a neuronal differentiated Neuro2a cell line

The ubiquitin-proteasome pathway is an intracellular protein degradation pathway responsible for degradation of many regulatory proteins that must be rapidly eliminated normally. Some recent studies reported that a proteasome dysfunction was involved in the pathogenesis of neurodegenerative diseases. Thus, there is now considerable interest in the possible role of proteasome in this regard. Here we show that inhibition of proteasomal function by Lactacystin-induced cell death in a neuronal differentiated Neuro2a (nN2a) cell line but not in an undifferentiated Neuro2a (N2a) cell line. Cell death was accompanied by both the activation of c-Jun N-terminal kinase, p38 and caspase-3. A pan-caspase inhibitor, Z-VAD-FMK, or SB203580, a p38 inhibitor could not inhibit cell death induced by Lactacystin, whereas nN2a cell lines with stable expression of the dominant negative mutant of c-Jun N-terminal kinase showed a remarkable suppression of cell death. Lactacystin-induced cell death is mediated through the c-Jun N-terminal kinase pathway but not the caspase-dependent pathway in a nN2a cell line. Our results shed light on the association among the proteasomal dysfunction, JNK pathway and neuronal cell death, leading to the elucidation of its possible role in the pathogenesis of neurodegenerative diseases.

Caspase-independent photoreceptor apoptosis in vivo and differential expression of apoptotic protease activating factor-1 and caspase-3 during retinal development

Apoptosis is the mode of photoreceptor cell death in many retinal dystrophies. Exposure of Balb/c mice to excessive levels of light induces photoreceptor apoptosis and represents an animal model for the study of retinal degenerations. Caspases have emerged as central regulators of apoptosis, executing this tightly controlled death pathway in many cells. Previously we have reported that light-induced photoreceptor apoptosis occurs independently of one the key executioners of apoptosis, caspase-3. This present study extends these results reporting on the lack of activation of other caspases in this model including caspases-8, -9, -7, and -1. Furthermore, photoreceptor apoptosis cannot be inhibited with the broad range caspase inhibitor zVAD-fmk indicating that light-induced retinal degeneration is caspase-independent. We demonstrate that cytochrome c does not translocate from mitochondria to the cytosol during photoreceptor apoptosis. We also show that during retinal development apoptotic protease activating factor (Apaf-1) protein levels are markedly decreased and this is associated with the inability to activate the mitochondrial caspase cascade in the mature retina. In addition, there is also a significant reduction in expression of caspases-3 and -9 during retinal maturation and these levels do not increase following light exposure. Finally, we show that the calcium-dependent proteases calpains are active during light-induced retinal degeneration and establish that the calcium channel blocker D-cis-diltiazem completely inhibits photoreceptor apoptosis.

Preferential targeting of apoptosis in tumor versus normal cells

Elimination of cancer cells by early apoptosis is preferred over other forms of cell growth inhibition. Apoptosis directly leads to tumor regression and reduces risks of selecting more aggressive and/or drug-resistant phenotypes that are often responsible for tumor regrowth and treatment failure. Although DNA damage by anticancer drugs is commonly recognized as an apoptotic stimulus, there is enormous variability in the magnitude and timing of such effects. Especially potent and rapid apoptosis seems to be a hallmark of various alkylating anticancer drugs that are regarded as DNA-reactive agents but are observed to react mainly with cellular proteins. Our studies with such dual-action drugs (irofulven, oxaliplatin) suggest that not only DNA damage, but also protein damage, contributes to apoptosis induction. DNA damage is well known to initiate death-signaling pathways leading to mitochondrial dysfunction. Protein damage, in turn, can distort cell redox homeostasis, which facilitates apoptosis execution. Such dual effects can be particularly lethal to tumor cells, which tend to function under pro-oxidative conditions. In contrast to tumor cells that are highly susceptible, normal cells show marginal apoptotic responses to the dual action drugs. This protection of normal cells might reflect their greater ability to buffer pro-oxidative changes and quickly restore redox homeostasis, despite substantial drug uptake and macromolecular binding. Importantly, by targeting the death process at multiple points, DNA- and protein-damaging drugs can be less vulnerable to various bypass mechanisms possible with single targets. The reviewed studies provide a proof of concept that differential apoptosis targeting in cancer versus normal cells can be a basis for tumor selectivity of anticancer drugs.

In-vitro effects of FSH and testosterone withdrawal on caspase activation and DNA fragmentation in different cell types of human seminiferous epithelium

BACKGROUND

Caspases are downstream elements of apoptosis-mediating pathways initiated by the Fas ligand/Fas receptor system which is supposed to play a central role in the regulation of apoptosis in the human seminiferous epithelium. However, caspase activity in different cell types of this epithelium has never been addressed.

METHODS AND RESULTS

We evaluated caspase activity and DNA integrity in Sertoli and germ cells within in-vitro cultured segments of human seminiferous tubules after induction of apoptosis by FSH or testosterone withdrawal. FSH withdrawal increased the incidence of DNA fragmentation in meiotic (primary spermatocytes) and post-meiotic (spermatids) germ cells without producing any detectable effect on caspase activity in these cells and without affecting DNA integrity or caspase activity in Sertoli cells. Testosterone withdrawal stimulated caspase activity and produced DNA fragmentation in Sertoli cells, but showed only a weak effect on DNA fragmentation in germ cells and did not alter germ cell caspase activity.

CONCLUSIONS

These findings confirm the central role of caspases in apoptosis of Sertoli cells. However, they also suggest that acute apoptosis of germ cells in the adult human testis occurs in a caspase-independent way and is controlled by Sertoli cells via an as yet undetermined mechanism.

Regulation and measurement of oxidative stress in apoptosis

Cells are constantly generating reactive oxygen species (ROS) during aerobic metabolism. As a consequence, each cell is equipped with an extensive antioxidant defence system to combat excessive production of ROS. Oxidative stress occurs in cells when the generation of ROS overwhelms the cell's natural antioxidant defences. There is a growing consensus that oxidative stress and the redox state of a cell plays a pivotal role in regulating apoptosis, a tightly controlled form of cell death in which a cell partakes in its own demise. More recently, a role for reactive nitrogen species (RNI) as both positive and negative regulators of cell death has been established. This review describes the major sources of ROS and RNI in a cell, the control of cell death by these species and the role of antioxidants as regulators of oxidative stress and apoptosis. Finally, the various methods that can be employed in establishing a role for both ROS and RNI in apoptosis will be discussed with particular emphasis on their intracellular detection.

Selective peroxidation and externalization of phosphatidylserine in normal human epidermal keratinocytes during oxidative stress induced by cumene hydroperoxide

Reactive oxygen species not only modulate important signal transduction pathways, but also induce DNA damage and cytotoxicity in keratinocytes. Hydrogen peroxide and organic peroxides are particularly important as these chemicals are widely used in dermally applied cosmetics and pharmaceuticals, and also represent endogenous metabolic intermediates. Lipid peroxidation is of fundamental interest in the cellular response to peroxides, as lipids are extremely sensitive to oxidation and lipid-based signaling systems have been implicated in a number of cellular processes, including apoptosis. Oxidation of specific phospholipid classes was measured in normal human epidermal keratinocytes exposed to cumene hydroperoxide after metabolic incorporation of the fluorescent oxidation-sensitive fatty acid, cis-parinaric acid, using a fluorescence high-performance liquid chromatography assay. In addition, lipid oxidation was correlated with changes in membrane phospholipid asymmetry and other markers of apoptosis. Although cumene hydroperoxide produced significant oxidation of cis-parinaric acid in all phospholipid classes, one phospholipid, phosphatidylserine, appeared to be preferentially oxidized above all other species. Using fluorescamine derivatization and annexin V binding it was observed that specific oxidation of phosphatidylserine was accompanied by phosphatidylserine translocation from the inner to the outer plasma membrane surface where it may serve as a recognition signal for interaction with phagocytic macrophages. These effects occurred much earlier than any detectable changes in other apoptotic markers such as caspase-3 activation, DNA fragmentation, or changes in nuclear morphology. Thus, normal human epidermal keratinocytes undergo profound lipid oxidation with preference for phosphatidylserine followed by phosphatidylserine externalization upon exposure to cumene hydroperoxide. It is therefore likely that normal human epidermal keratinocytes exposed to similar oxidative stress in vivo would under go phosphatidylserine oxidation/translocation. This would make them targets for macrophage recognition and phagocytosis, and thus limit their potential to invoke inflammation or give rise to neoplastic transformations.

Selective loss of viability of mouse NK cells in culture is associated with decreased NK cell lytic function

Cell culture methods can allow investigation of the mechanisms responsible for immunotoxicity. Unfortunately, natural killer (NK) cells in rodent splenic cultures rapidly lose their cytolytic function. It is not known if death of NK cells or loss of function in viable NK cells is primarily responsible for this loss. Flow cytometry and an assay of NK cell lytic function were used to address this issue and to determine if NK cell viability could be maintained by adding selected cytokines or a caspase inhibitor to the cultures. Total cells and NK cells in untreated 18 h cultures were 79 +/- 1% and 25 +/- 2% viable, respectively, and these cultured splenocytes caused only 4 +/- 1% specific release of (51)Cr from YAC-1 target cells. Cultures including polyinosinic:polycytidylic acid (poly I:C) or IL-2 had increased NK cell viability (43 +/- 2%, 47 +/- 1%) and function (58 +/- 2 and 43 +/- 1% specific release). IL-15 significantly increased NK cell viability, but not function. Previous studies demonstrated that treatment of mice with immunotoxicants such as ethanol or corticosterone diminishes NK cell activation in vitro in response to poly I:C. To determine if alterations in viability are responsible for this decreased NK cell activity, lytic function and NK activity were measured in cultures of splenocytes treated in vivo or in vitro with ethanol and/or corticosterone. Some treatments reduced IL-2 or poly I:C-enhanced lytic activity in vitro, but there was no clear relationship between these changes in function and changes in the percentage of viable NK cells. Thus, immunotoxicants that suppress NK cell activation can be investigated in vitro because commonly used activating stimuli also permit NK cell survival. However, no agents were identified that could maintain NK cell viability and function in culture (without activation) to allow investigation of the direct effects of immunotoxicants on basal NK activity in vitro.

Mitochondrial reactive oxygen species in cell death signaling

During apoptosis, mitochondrial membrane permeability (MMP) increases and the release into the cytosol of pro-apoptotic factors (procaspases, caspase activators and caspase-independent factors such as apoptosis-inducing factor (AIF)) leads to the apoptotic phenotype. Apart from this pivotal role of mitochondria during the execution phase of apoptosis (documented in other reviews of this issue), it appears that reactive oxygen species (ROS) produced by the mitochondria can be involved in cell death. These toxic compounds are normally detoxified by the cells, failing which oxidative stress occurs. However, ROS are not only dangerous molecules for the cell, but they also display a physiological role, as mediators in signal transduction pathways. ROS participate in early and late steps of the regulation of apoptosis, according to different possible molecular mechanisms. In agreement with this role of ROS in apoptosis signaling, inhibition of apoptosis by anti-apoptotic Bcl-2 and Bcl-x(L) is associated with a protection against ROS and/or a shift of the cellular redox potential to a more reduced state. Furthermore, the fact that active forms of cell death in yeast and plants also involve ROS suggests the existence of an ancestral redox-sensitive death signaling pathway that has been independent of caspases and Bcl-2.

Signalling apoptosis: a radical approach

Reactive oxygen species (ROS) are frequently associated with cytotoxicity, often being described as damaging, harmful or toxic. It is generally assumed that, under pathological circumstances, ROS elicit wide-spread and random acts of oxidation. This passive attack of cellular components by ROS, in conditions where oxidative stress is the initiating stimulus for apoptosis, is assumed to simply trigger cell death as a result of cumulative oxidative damage. However, accumulating evidence now suggests that ROS may act as signalling molecules for the initiation and execution of the apoptotic death programme in many, if not all, current models of apoptotic cell death. Signalling by ROS would not appear to be random, as previously assumed, but targeted at specific metabolic and signal transduction cellular components. There is also evidence that the enzymatic generation of ROS may not simply be an unwanted by-product of the primary reaction catalysed, but that ROS may be used as signalling molecules to regulate cellular processes including apoptosis. This view of ROS as signalling molecules (as opposed to toxic metabolites) has been further bolstered by the findings that cellular antioxidants such as glutathione and thioredoxin not only serve to regulate ROS levels but also act as reversible redox modifiers of enzyme function. This review will attempt to delineate the involvement of ROS in apoptosis in light of these recent discoveries and provide evidence for a crucial role for ROS in the initiation and execution of the death process.

Apoptotic morphology does not always require caspase activity in rat cerebellar granule neurons

The death of a cell via apoptosis is characterized by morphological changes including cell shrinkage and nuclear condensation. Intracellularly, proteases, including caspases, are activated. In the present article we have compared the ability of three different neurotoxic agents to induce caspase activity in cerebellar granule cells (CGC). These compounds are the microtubule-disrupting agent colchicine and the oxidative stress-inducing agents hydrogen peroxide and methylmercury (MeHg). We have previously shown that each of these agents causes nuclear changes that are consistent with apoptosis, i.e., induction of chromatin condensation and DNA cleavage into fragments of regular size (700, 300 and 50 kbp). However, only colchicine causes a large increase in caspase activity, as monitored by the ability of whole cell extracts to cleave the synthetic caspase substrate DEVD-MCA. In contrast, MeHg and hydrogen peroxide do not induce any significant increase of DEVDase activity as compared to control cells. Immunocytochemistry confirms that active caspase-3 is abundant only in colchicine-exposed cells. In agreement with these findings, the pan-caspase inhibitor, z-VAD-fmk, is efficient in protecting CGC against colchicine, but not against hydrogen peroxide or MeHg. These data suggest that in CGC the activation of caspases is not always required to induce morphological changes and pattern of DNA fragmentation consistent with apoptosis.

The proteasomal substrate Stm1 participates in apoptosis-like cell death in yeast

We have identified the yeast gene STM1 in an overexpression screen for new proteasomal substrates. Stm1 is unstable in wild-type cells and stabilized in cells with defective proteasomal activity and thus a bona fide substrate of the proteasome. It is localized in the perinuclear region and is required for growth in the presence of mutagens. Overexpression in cells with impaired proteasomal degradation leads to cell death accompanied with cytological markers of apoptosis: loss of plasma membrane asymmetry, chromatin condensation, and DNA cleavage. Cells lacking Stm1 display deficiency in the apoptosis-like cell death process induced by treatment with low concentrations of H(2)O(2). We suggest that Stm1 is involved in the control of the apoptosis-like cell death in yeast. Survival is increased when Stm1 is completely missing from the cells or when inhibition of Stm1 synthesis permits proteasomal degradation to decrease its amount in the cell. Conversely, Stm1 accumulation induces cell death. In addition we identified five other genes whose overexpression in proteasomal mutants caused similar apoptotic phenotypes.

Light-induced photoreceptor apoptosis in vivo requires neuronal nitric-oxide synthase and guanylate cyclase activity and is caspase-3-independent

Apoptosis is the mode of photoreceptor cell death in inherited and induced retinal degeneration. However, the molecular mechanisms of photoreceptor cell death in human cases and animal models of retinal dystrophies remain undefined. Exposure of Balb/c mice to excessive levels of white light results in photoreceptor apoptosis. This study delineates the molecular events occurring during and subsequent to the induction of retinal degeneration by exposure to white light in Balb/c mice. We demonstrate an early increase in intracellular calcium levels during photoreceptor apoptosis, an event that is accompanied by significant superoxide generation and mitochondrial membrane depolarization. Furthermore, we show that inhibition of neuronal nitric-oxide synthase (nNOS) by 7-nitroindazole is sufficient to prevent retinal degeneration implicating a key role for neuronal nitric oxide (NO) in this model. We demonstrate that inhibition of guanylate cyclase, a downstream effector of NO, also prevents photoreceptor apoptosis demonstrating that guanylate cyclase too plays an essential role in this model. Finally, our results demonstrate that caspase-3, frequently considered to be one of the key executioners of apoptosis, is not activated during retinal degeneration. In summary, the data presented here demonstrate that light-induced photoreceptor apoptosis in vivo is mediated by the activation of nNOS and guanylate cyclase and is caspase-3-independent.