tert-Butylhydroperoxide induces peroxynitrite-dependent mitochondrial permeability transition leading PC12 cells to necrosis

A Genome-Wide CRISPR/Cas9 Screen Reveals that Riboflavin Regulates Hydrogen Peroxide Entry into HAP1 Cells

Extracellular hydrogen peroxide can induce oxidative stress, which can cause cell death if unresolved. However, the cellular mediators of H2O2-induced cell death are unknown. We determined that H2O2-induced cytotoxicity is an iron-dependent process in HAP1 cells and conducted a CRISPR/Cas9-based survival screen that identified four genes that mediate H2O2-induced cell death: POR (encoding cytochrome P450 oxidoreductase), RETSAT (retinol saturase), KEAP1 (Kelch-like ECH-associated protein-1), and SLC52A2 (riboflavin transporter). Among these genes, only POR also mediated methyl viologen dichloride hydrate (paraquat)-induced cell death. Because the identification of SLC52A2 as a mediator of H2O2 was both novel and unexpected, we performed additional experiments to characterize the specificity and mechanism of its effect. These experiments showed that paralogs of SLC52A2 with lower riboflavin affinities could not mediate H2O2-induced cell death and that riboflavin depletion protected HAP1 cells from H2O2 toxicity through a specific process that could not be rescued by other flavin compounds. Interestingly, riboflavin mediated cell death specifically by regulating H2O2 entry into HAP1 cells, likely through an aquaporin channel. Our study results reveal the general and specific effectors of iron-dependent H2O2-induced cell death and also show for the first time that a vitamin can regulate membrane transport.IMPORTANCE Using a genetic screen, we discovered that riboflavin controls the entry of hydrogen peroxide into a white blood cell line. To our knowledge, this is the first report of a vitamin playing a role in controlling transport of a small molecule across the cell membrane.

Genetic ablation of calcium-independent phospholipase A(2)γ (iPLA(2)γ) attenuates calcium-induced opening of the mitochondrial permeability transition pore and resultant cytochrome c release

Herein, we demonstrate that calcium-independent phospholipase A(2)γ (iPLA(2)γ) is a critical mechanistic participant in the calcium-induced opening of the mitochondrial permeability transition pore (mPTP). Liver mitochondria from iPLA(2)γ(-/-) mice were markedly resistant to calcium-induced swelling in the presence or absence of phosphate in comparison with wild-type littermates. Furthermore, the iPLA(2)γ enantioselective inhibitor (R)-(E)-6-(bromomethylene)-3-(1-naphthalenyl)-2H-tetrahydropyran-2-one ((R)-BEL) was markedly more potent than (S)-BEL in inhibiting mPTP opening in mitochondria from wild-type liver in comparison with hepatic mitochondria from iPLA(2)γ(-/-) mice. Intriguingly, low micromolar concentrations of long chain fatty acyl-CoAs and the non-hydrolyzable thioether analog of palmitoyl-CoA markedly accelerated Ca(2+)-induced mPTP opening in liver mitochondria from wild-type mice. The addition of l-carnitine enabled the metabolic channeling of acyl-CoA through carnitine palmitoyltransferases (CPT-1/2) and attenuated the palmitoyl-CoA-mediated amplification of calcium-induced mPTP opening. In contrast, mitochondria from iPLA(2)γ(-/-) mice were insensitive to fatty acyl-CoA-mediated augmentation of calcium-induced mPTP opening. Moreover, mitochondria from iPLA(2)γ(-/-) mouse liver were resistant to Ca(2+)/t-butyl hydroperoxide-induced mPTP opening in comparison with wild-type littermates. In support of these findings, cytochrome c release from iPLA(2)γ(-/-) mitochondria was dramatically decreased in response to calcium in the presence or absence of either t-butyl hydroperoxide or phenylarsine oxide in comparison with wild-type littermates. Collectively, these results identify iPLA(2)γ as an important mechanistic component of the mPTP, define its downstream products as potent regulators of mPTP opening, and demonstrate the integrated roles of mitochondrial bioenergetics and lipidomic flux in modulating mPTP opening promoting the activation of necrotic and necroapoptotic pathways of cell death.

Contrasting protective effects of cannabinoids against oxidative stress and amyloid-β evoked neurotoxicity in vitro

Cannabinoids have been widely reported to have neuroprotective properties in vitro and in vivo. In this study we compared the effects of CB1 and CB2 receptor-selective ligands, the endocannabinoid anandamide and the phytocannabinoid cannabidiol, against oxidative stress and the toxic hallmark Alzheimer's protein, β-amyloid (Aβ) in neuronal cell lines. PC12 or SH-SY5Y cells were selectively exposed to either hydrogen peroxide, tert-butyl hydroperoxide or Aβ, alone or in the presence of the CB1 specific agonist arachidonyl-2'-chloroethylamide (ACEA), CB2 specific agonist JWH-015, anandamide or cannabidiol. Cannabidiol improved cell viability in response to tert-butyl hydroperoxide in PC12 and SH-SY5Y cells, while hydrogen peroxide-mediated toxicity was unaffected by cannabidiol pretreatment. Aβ exposure evoked a loss of cell viability in PC12 cells. Of the cannabinoids tested, only anandamide was able to inhibit Aβ-evoked neurotoxicity. ACEA had no effect on Aβ-evoked neurotoxicity, suggesting a CB1 receptor-independent effect of anandamide. JWH-015 pretreatment was also without protective influence on PC12 cells from either pro-oxidant or Aβ exposure. None of the cannabinoids directly inhibited or disrupted preformed Aβ fibrils and aggregates. In conclusion, the endocannabinoid anandamide protects neuronal cells from Aβ exposure via a pathway unrelated to CB1 or CB2 receptor activation. The protective effect of cannabidiol against oxidative stress does not confer protection against Aβ exposure, suggesting divergent pathways for neuroprotection of these two cannabinoids.

Peroxynitrite-induced protein nitration is responsible for renal mitochondrial damage in diabetic rat

Oxidative stress, especially mediated by peroxynitrite (ONOO-), plays a key role in diabetes. Mitochondria, as the generating source of ONOO-, may also be the major damaging target of ONOO-. Whether ONOO--induced protein nitration is responsible for renal mitochondrial damage in diabetes is not fully known. This study was aimed to clarify the relationship between nitration of entire mitochondrial proteins induced by ONOO- and the renal mitochondrial damage in diabetes. Sprague-Dawley male rats were injected ip with streptozotocin to induce diabetes. After 10 weeks, inducible nitric oxide synthase (iNOS) mRNA expression and protein content in renal cortex were detected. Distribution of nitrotyrosine (NT), a specific marker of ONOO-, in renal cortex and NT content in mitochondrial proteins were detected. The ultrastructure of glomerulus was observed. Aminoguanidine was used as a selective inhibitor of iNOS to reduce the derivation of ONOO-. In diabetic rat, increasing levels of iNOS mRNA and protein content, and NT content were observed, in accord with the pathological alterations of glomerulus. In aminoguanidine group, these alterations were attenuated significantly. In conclusion, ONOO- could induce entire mitochondrial proteins nitration, responsible for the damage of renal mitochondria in diabetes.

Comparison of uptake and neuroprotective potential of seven zinc-salts

Zinc plays an important role as an antioxidant in different cells treated with various kinds of oxidative stressors. Although intracellular Zn(2+) is important in many cellular events, little is known about the cellular uptake of this trace metal and the intracellular status that is required for its optimal function. Since previous reports usually employed only one type of zinc-salt, in this work was compared cellular uptake and antioxidative potential of seven zinc-salts in order to discriminate whether different counterions and ligands may influence its function. Oxidative stress was induced by peroxide or iron in neuronal PC12 cells. We compared uptake of zinc-salts into the labile Zn(2+) pool of PC12 cells as well as their effects on the prevention of cell death, glutathione depletion, lipid peroxidation and ROS production. Zinc-salts provided better protection against oxidative stress-induced in PC12 cultures by peroxide than by iron. Preincubations with zinc-salts displayed better neuroprotection in all cases than coincubations. Zinc-histidine complex was shown to be the most potent compound. Our results indicated that protective effect of zinc is not related to its uptake into PC12 cells, what is indicated by the rather low salt concentrations required for the cell protection and by the observation that despite a superior antioxidant effect of zinc-histidine, the uptake of this salt by PC12 cells was remarkably lower in comparison with other zinc-salts. Although zinc-sulfate exerted weak neuroprotective potential, accumulation of Zn(2+) from this salt within cells was significantly higher compared to other salts. The differences in accumulation of zinc-salts were not specific and unique to PC12 cells, since similar results were obtained in rat primary hepatocytes and endothelial HUVEC cells.

Protective effects of ellagic and chlorogenic acids against oxidative stress in PC12 cells

Following exposure of differentiated neuronal PC12 cells to either t-BHP, hydrogen peroxide (H2O2) or FeSO4 various kinds of reactive oxygen species (ROS) are generated leading to oxidative injury. The protective effects of two plant polyphenols, ellagic (EC) and chlorogenic acid (CGA), as well as of two metabolites, caffeic acid (CA) and ferulic acid (FA), were investigated in preincubation and coincubation experiments with respect to the following parameters: prevention of cell death, GSH depletion, lipid peroxidation and ROS formation. The polyphenols more efficiently suppressed cytotoxicity and loss of GSH caused by peroxides than by iron, particularly in preincubation. Lipid peroxidation which increased much stronger in response to FeSO4 was counteracted completely by the polyphenols. In case of iron, however, only coincubation was effective. EA and CGA and the metabolites CA and FA showed excellent elimination of ROS induced by all stressors. These findings suggest that two dietary antioxidants, EA and CGA, may have protective properties against oxidative stress induced in CNS.

Oxidative DNA damage protection and repair by polyphenolic compounds in PC12 cells

Biological systems are frequently exposed to excessive reactive oxygen species, causing a disturbance in the cells natural antioxidant defence systems and resulting in damage to all biomolecules, including nucleic acids. In fact, oxidative DNA damage is described as the type of damage most likely to occur in neuronal cells. In this study, three polyphenolic compounds, luteolin, quercetin and rosmarinic acid, were investigated for their protective effects against oxidative DNA damage induced in PC12 cells, a neuronal cell model. Although luteolin and quercetin prevented the formation of strand breaks to a greater extent than rosmarinic acid, this last one presented the highest capacity to repair strand breaks formation. In addition, rosmarinic acid was the only compound tested that increased the repair of oxidized nucleotidic bases induced with the photosensitizer compound [R]-1-[(10-chloro-4-oxo-3-phenyl-4H-benzo[a]quinolizin-1-yl) carbonyl]-2-pyrrolidine-methanol (Ro 19-8022). The activity of repair enzymes was indicated by the in vitro base excision repair assay, using a cell-free extract obtained from cells previously treated with the compounds to incise DNA. The protective effect of rosmarinic acid was further confirmed by the increased expression of OGG1 repair gene, observed through real time RT-PCR. The data obtained is indicative that rosmarinic acid seems to act on the intracellular mechanisms responsible for DNA repair, rather than by a direct effect on reactive oxygen species scavenging, as deducted from the effects observed for luteolin and quercetin. Therefore, these results suggest the importance of these polyphenols, and in particular rosmarinic acid, as protectors of oxidative stress-induced DNA damage that commonly occurs in several pathological conditions, such as neurodegenerative diseases.

Peroxynitrite-induced protein nitration contributes to liver mitochondrial damage in diabetic rats

Oxidative stress, especially peroxynitrite (ONOO(-))-mediated oxidative stress, plays a key role in diabetes. Mitochondria, as the generating source of ONOO(-), may also be the major damaging target of ONOO(-), which can cause a series of mitochondrial proteins nitration. Therefore, this study aimed to clarify the relationship between the nitration of entire mitochondrial proteins induced by ONOO(-) and liver mitochondrial structural damage in diabetes. Sprague-Dawley male rats were injected with streptozotocin to induce diabetes. After 10 weeks, transmission electron microscopy was used to observe the ultrastructure of liver mitochondria, and reverse transcription-polymerase chain reaction was used to detect liver inducible nitric oxide synthase (iNOS) mRNA expression. Nitrotyrosine (NT) content and distribution were detected with Western blot analysis and immunohistochemistry. In addition, some biochemical indicators were detected to represent oxidative stress and metabolic disorders. In diabetic rats, increasing levels of iNOS mRNA and NT content (P<.05) were observed, in accord with pathological alterations of the ultrastructure of liver mitochondria. Meanwhile, some alterations in biochemical indicators were observed in diabetes. Treatment with aminoguanidine could significantly attenuate these alterations (P<.01 or P<.05). In conclusion, the nitration of mitochondrial proteins induced by ONOO(-) may be responsible for structural damage to liver mitochondria, and aminoguanidine can reduce ONOO(-) generation and attenuate mitochondrial damage.

An RNAi-based genetic screen for oxidative stress resistance reveals retinol saturase as a mediator of stress resistance

Oxidative stress has been implicated in the pathogenesis of numerous late-onset diseases as well as organismal longevity. Nevertheless, the genetic components that affect cellular sensitivity to oxidative stress have not been explored extensively at the genome-wide level in mammals. Here we report an RNA interference (RNAi) screen for genes that increase resistance to an organic oxidant, tert-butylhydroperoxide (tert-BHP), in cultured fibroblasts. The loss-of-function screen allowed us to identify several short hairpin RNAs (shRNAs) that elevated the cellular resistance to tert-BHP. One of these shRNAs strongly protected cells from tert-BHP and H(2)O(2) by specifically reducing the expression of retinol saturase, an enzyme that converts all-trans-retinol (vitamin A) to all-trans-13,14-dihydroretinol. The protective effect was well correlated with the reduction in mRNA level and was observed in both primary fibroblasts and NIH3T3 cells. The results suggest a novel role for retinol saturase in regulating sensitivity to oxidative stress and demonstrate the usefulness of large-scale RNAi screening for elucidating new molecular pathways involved in stress resistance.

Apoptotic and necrotic mechanisms of stress-induced human lens epithelial cell death

Exposure to ultraviolet radiation (UVR) and reactive oxygen species (ROS) can damage the human lens and contribute to cataract formation. Recent evidence suggests that apoptosis in lens epithelial cells (LEC) is an initiating event in noncongenital cataract formation in humans and animals. The present study examines the cellular and molecular mechanisms by which environmental (ultraviolet B [UVB]) and chemical (hydrogen peroxide [H(2)O(2)], t-butyl hydroperoxide [TBHP]) stress induces cell death in an SV-40 immortalized human lens epithelial (HLE) cell line. Treatment of HLE cells with UVB, H(2)O(2), and TBHP significantly decreased cell density with LD50 values of 350 J/m(2), 500 muM, and 200 muM, respectively. Cellular morphology, DNA fragmentation, and annexin/propidium iodide staining consistent with apoptosis was observed only in UVB-treated cells, whereas lactate dehydrogenase (LDH) release was significantly higher in H(2)0(2)- and TBHP-treated cells. In addition, activation of apoptotic stress-signaling proteins, including c-Jun NH2-terminal kinase (JNK), caspase-3, and DNA fragmentation factor 45 (DFF45) was observed only in UVB-treated cells. Inhibition of JNK activity increased UVB-induced cell death, suggesting that this pathway may serve a prosurvival role in HLE cells. These findings suggest UVB predominantly induces apoptosis in HLE cells, whereas H(2)O(2) and TBHP induce necrosis.

Apoptosis and necrosis in health and disease: role of mitochondria

Mitochondria play an important role in both the life and death of cells. Mitochondria are the powerhouse of the cell, providing over 90% of adenosine triphosphate (ATP) consumed by the cell. Mitochondrial energy production, however, is disrupted in various pathological situations leading to cellular Injury. The mechanisms causing the injury are turning out to be more complex than originally expected. For instance, calcium, oxidant chemicals, ischemia/ reperfusion, and a range of other agents promote onset of the mitochondrial permeability transition in mitochondria from liver, heart, and other tissues. Often the consequence of this event is ATP depletion, ion deregulation, mitochondrial and cellular swelling, activation of degradative enzymes, plasma membrane failure, and cell lysis. This is referred to as necrotic cell death. The mitochondrial permeability transition is also involved in apoptotic cell death. In this mode of death, the role of the permeability transition is to release proapoptotic proteins from mitochondria into the cytosol where with the aid of cellular ATP they complete the apoptotic cascade. Therefore, mitochondria contribute to both apoptotic and necrotic death.

The effect of H2O2 and tertiary butyl hydroperoxide upon a murine immortal lens epithelial cell line, alphaTN4-1

The effect of comparable concentrations of H(2)O(2) and tertiary butyl hydroperoxide (TBHP) upon an immortal murine lens epithelial cell line was examined as part of an ongoing effort to delineate differences in the mechanism by which these peroxides cause cell death. Both compounds result in cell death of normal, unconditioned cells within 24hr. It was found that with similar conditions, TBHP conditioned alphaTN4-1 cells survive H(2)O(2) stress while H(2)O(2) conditioned cells are killed by TBHP. To better understand how these peroxides act, their effect upon unconditioned cells has been investigated. Both peroxides cause a rapid loss of GSH and disruption of pump activity as illustrated by (14)C-choline transport and (86)Rb uptake. While H(2)O(2) exposure resulted in extensive DNA damage, TBHP had a minimal effect. DNA damage caused by H(2)O(2) was shown to activate polyADP-ribosyl polymerase (PARP), leading to depletion of NAD and ATP. H(2)O(2) induced cell death could be delayed by addition of 3-aminobenzamide (3AB), an inhibitor of PARP. ATP levels in cells subjected to H(2)O(2) were also maintained by the presence of 3AB. H(2)O(2) stress also disrupted glycolysis and mitochondrial activity but these parameters were not affected by TBHP. TBHP induced cell death, under the relatively mild conditions used in this work, appears to be caused by membrane disruption and loss of a reducing environment.

Pivotal role of superoxides generated in the mitochondrial respiratory chain in peroxynitrite-dependent activation of phospholipase A2

Exposure of PC12 cells to reagent peroxynitrite promotes the release of arachidonic acid (AA) mediated by activation of phospholipase A(2) [Guidarelli, Palomba and Cantoni (2000) Br. J. Pharmacol. 129, 1539-1542]. We now present experimental evidence consistent with the notion that this response is not directly triggered by peroxynitrite but, rather, by reactive oxygen species generated at the level of complex III of the mitochondrial respiratory chain. In particular, superoxide (and not hydrogen peroxide) has a pivotal role in peroxynitrite-dependent activation of phospholipase A(2). This observation was confirmed by results showing that superoxide, or peroxynitrite, promotes release of AA in isolated mitochondria. Consistently, the release of AA elicited by either peroxynitrite or A23187 in intact cells was shown to be calcium-dependent and differentially affected by phospholipase A(2) inhibitors with different levels of specificity. In particular, the effects of peroxynitrite, unlike those of A23187, were both sensitive to low concentrations of two general phospholipase A(2) inhibitors and insensitive to arachidonyltrifluoromethyl ketone, which shows some selectivity towards cytosolic phospholipase A(2). In addition, peroxynitrite and A23187 synergistically enhanced the release of AA. Collectively, the above results demonstrate that peroxynitrite causes inhibition of complex III, followed by enforced formation of superoxides that stimulate the activity of a calcium-dependent PLA(2) isoform, probably localized in the mitochondria.