Silencing of cytosolic NADP+-dependent isocitrate dehydrogenase by small interfering RNA enhances the sensitivity of HeLa cells toward staurosporine

Staurosporine induces the production of reactive oxygen species, which play an important causative role in apoptotic cell death. Recently, it was demonstrated that the control of cellular redox balance and the defense against oxidative damage is one of the primary functions of cytosolic NADP⁺-dependent isocitrate dehydrogenase (IDPc) by supplying NADPH for antioxidant systems. The present report shows that silencing of IDPc expression in HeLa cells greatly enhances apoptosis induced by staurosporine. Transfection of HeLa cells with an IDPc small interfering RNA (siRNA) markedly decreased activity of IDPc, enhancing the susceptibility of staurosporine-induced apoptosis reflected by DNA fragmentation, cellular redox status and the modulation of apoptotic marker proteins. These results indicate that IDPc may play an important role in regulating the apoptosis induced by staurosporine and the sensitizing effect of IDPc siRNA on the apoptotic cell death of HeLa cells offers the possibility of developing a modifier of cancer chemotherapy.     

Knockdown of sensitive to apoptosis gene by small interfering RNA enhances the sensitivity of PC3 cells toward actinomycin D and etoposide

Abstract

Actinomycin D and etoposide induce the production of reactive oxygen species, which play an important causative role in apoptotic cell death. Sensitive to apoptosis gene (SAG) protein, a redox inducible zinc RING finger protein that protects mammalian cells from apoptosis by redox reagents, is a metal chelator and a potential reactive oxygen species scavenger. The present report show that knockdown of SAG expression in PC3 cells greatly enhances apoptosis induced by actinomycin D and etoposide. Transfection of human prostate cancer PC3 cells with SAG small interfering RNA (siRNA) markedly decreased the expression of SAG, enhancing the susceptibility of actinomycin D- and etoposide-induced apoptosis reflected by DNA fragmentation, cellular redox status and the modulation of apoptotic marker proteins. These results indicate that SAG may play an important role in regulating the apoptosis induced by actinomycin D and etoposide and the sensitizing effect of SAG siRNA on the apoptotic cell death of PC3 cells offers the possibility of developing a modifier of cancer chemotherapy.     

Glutathione and modulation of cell apoptosis

Apoptosis is a highly organized form of cell death that is important for tissue homeostasis, organ development and senescence. To date, the extrinsic (death receptor mediated) and intrinsic (mitochondria derived) apoptotic pathways have been characterized in mammalian cells. Reduced glutathione, is the most prevalent cellular thiol that plays an essential role in preserving a reduced intracellular environment. glutathione protection of cellular macromolecules like deoxyribose nucleic acid proteins and lipids against oxidizing, environmental and cytotoxic agents, underscores its central anti-apoptotic function. Reactive oxygen and nitrogen species can oxidize cellular glutathione or induce its extracellular export leading to the loss of intracellular redox homeostasis and activation of the apoptotic signaling cascade. Recent evidence uncovered a novel role for glutathione involvement in apoptotic signaling pathways wherein post-translational S-glutathiolation of protein redox active cysteines is implicated in the potentiation of apoptosis. In the present review we focus on the key aspects of glutathione redox mechanisms associated with apoptotic signaling that includes: (a) changes in cellular glutathione redox homeostasis through glutathione oxidation or GSH transport in relation to the initiation or propagation of the apoptotic cascade, and (b) evidence for S-glutathiolation in protein modulation and apoptotic initiation.     

Lipid hydroperoxide-induced apoptosis in human colonic CaCo-2 cells is associated with an early loss of cellular redox balance.

Apoptosis plays a critical role in maintaining homeostasis of the intestinal epithelium. Dietary oxidants like peroxidized lipids could perturb cellular redox status and disrupt mucosal turnover. The objective of this study was to delineate the role of lipid hydroperoxide (LOOH) -induced redox shifts in intestinal apoptosis using the human colonic CaCo-2 cell. We found that subtoxic concentrations of LOOH increased CaCo-2 cell apoptosis. This LOOH-induced apoptosis was associated with a significant decrease in the ratio of reduced glutathione-to-oxidized glutathione (GSH/GSSG), which preceded DNA fragmentation by 12 to 14 h, suggesting a temporal relationship between the two events. Oxidation of GSH with the thiol oxidant diamide caused significant decreases in cellular GSH and GSH/GSSG at 15 min that correlated with the activation of caspase 3 (60 min) and cleavage of PARP (120 min), confirming a temporal link between induction of cellular redox imbalance and initiation of apoptotic cell death. These kinetic studies further reveal that oxidant-mediated early redox change (within 1 h) was a primary inciting event of the apoptotic cascade. Once initiated, the recovery of redox balance did not prevent the progression of CaCo-2 cell apoptosis to its biological end point at 24 h. Collectively, the study shows that subtoxic levels of LOOH disrupt intestinal redox homeostasis, which contributes to apoptosis. These results provide insights into the mechanism of hydroperoxide-induced mucosal turnover that have important implications for understanding oxidant-mediated genesis of gut pathology.     

Sensitization of ionizing radiation-induced apoptosis by ursolic acid

Radiation therapy has been widely used for treating human cancers. However, cancer cells develop radioresistant phenotypes that decrease the efficacy of radiotherapy. Ionizing radiation (IR) induces the production of reactive oxygen species, which play an important role in apoptotic cell death. Therefore, radiation therapy combined with a sensitizer, which modulates cellular redox status, has the potential to enhance therapeutic efficacy in a variety of human cancers. Here, we investigated the radiosensitizing effects of ursolic acid (UA), a pentacyclic triterpenoid found in rosemary and holy basil. IR-induced apoptosis in cancer cell lines such as DU145, CT26 and B16F10 was significantly enhanced by UA, as reflected by DNA fragmentation, cellular redox status, mitochondrial dysfunction and modulation of apoptotic marker proteins. Additionally, UA combined with IR was also effective for inhibiting tumorigenesis in B16F10 melanoma cells implanted into mice. Taken together, these results suggest that applying UA together with IR may be an effective combination modality for treating cancer.     

Peroxiredoxins and the Regulation of Cell Death

Cell death pathways such as apoptosis can be activated in response to oxidative stress, enabling the disposal of damaged cells. In contrast, controlled intracellular redox events are proposed to be a significant event during apoptosis signaling, regardless of the initiating stimulus. In this scenario oxidants act as second messengers, mediating the post-translational modification of specific regulatory proteins. The exact mechanism of this signaling is unclear, but increased understanding offers the potential to promote or inhibit apoptosis through modulating the redox environment of cells. Peroxiredoxins are thiol peroxidases that remove hydroperoxides, and are also emerging as important players in cellular redox signaling. This review discusses the potential role of peroxiredoxins in the regulation of apoptosis, and also their ability to act as biomarkers of redox changes during the initiation and progression of cell death.     

S-Glutathiolation in life and death decisions of the cell

Abstract

Reversible S-glutathiolation of specific proteins at sensitive cysteines provides a powerful mechanism for the dynamic, post-translational regulation of many cellular processes, including apoptosis. Critical in ascribing any regulatory function to S-glutathiolation is its reversibility, mainly regulated by glutaredoxins. Apoptosis is a controlled form of cell death that plays fundamental roles during embryonic development, tissue homeostasis and some diseases. Much of what happens during the demolition phase of apoptosis is orchestrated primarily by caspases, the final executioners of cell death. Recent findings support an essential role for S-glutathiolation in apoptosis, often at the level of caspases or their inactive precursors, and several studies have demonstrated the importance of glutaredoxins in protecting against apoptosis. These observations have contributed to recent advances in apoptosis research. However, the effective relevance of protein S-glutathiolation and the precise molecular targets in apoptotic signalling remain unresolved and a key challenge for future research.     

Contribution of mitochondrial GSH transport to matrix GSH status and colonic epithelial cell apoptosis

Previously, we showed that cellular glutathione/glutathione disulfide (GSH/GSSG) play an important role in apoptotic signaling, and early studies linked mitochondrial GSH (mtGSH) loss to enhanced cytotoxicity. The current study focuses on the contribution of mitochondrial GSH transport and mitochondrial GSH/GSSG status to apoptosis initiation in a nontransformed colonic epithelial cell line, NCM460, using menadione (MQ), a quinone with redox cycling bioreactivity, as a model of oxidative challenge. Our results implicate the semiquinone radical in MQ-mediated apoptosis, which was associated with marked oxidation of the mitochondrial soluble GSH and protein-bound thiol pools, mitochondria-to-cytosol translocation of cytochrome c, and activation of caspase-9. MQ-induced apoptosis was potentiated by inhibition of mtGSH uptake in accordance with exacerbated mitochondrial GSSG (mtGSSG) and protein-SSG and compromised mitochondrial respiratory activity. Moreover, cell apoptosis was prevented by N-acetyl-L-cysteine (NAC) pretreatment, which restored cellular redox homeostasis. Importantly, mtGSH transport inhibition effectively blocked NAC-mediated protection in accordance with its failure to attenuate mtGSSG. These results support the importance of mitochondrial GSH transport and the mtGSH status in oxidative cell killing.     

Glutathione depletion and disruption of intracellular ionic homeostasis regulate lymphoid cell apoptosis

Intracellular glutathione (GSH) depletion is an important hallmark of apoptosis. We have recently shown that GSH depletion by its extrusion regulates apoptosis independently of excessive reactive oxygen species accumulation. However, the mechanisms by which GSH depletion regulates apoptosis are still unclear. Because disruption of intracellular ionic homeostasis, associated with apoptotic volume decrease (AVD), is necessary for the progression of apoptotic cell death, we sought to evaluate the relationship between GSH transport and ionic homeostasis during Fas ligand (FasL)-induced apoptosis in Jurkat cells. GSH depletion in response to FasL was paralleled by distinct degrees of AVD identified by differences in cellular forward scatter and electronic impedance analysis. Inhibition of GSH efflux prevented AVD, K+ loss, and the activation of two distinct ionic conductances, mediated by Kv1.3 and outward rectifying Cl- channels. Reciprocally, stimulation of GSH loss accelerated the loss of K+, AVD, and consequently the progression of the execution phase of apoptosis. Although high extracellular K+ inhibited FasL-induced apoptosis, GSH depletion was largely independent of K+ loss. These results suggest that deregulation of GSH and ionic homeostasis converge in the regulation of apoptosis in lymphoid cells.     

The effect of α-phenyl-N-t-butylnitrone on ionizing radiation-induced apoptosis in U937 cells

Ionizing radiation induces the production of reactive oxygen species (ROS), which play an important causative role in apoptotic cell death. α-Phenyl-N-t-butylnitrone (PBN) is one of the most widely used spin-trapping compounds for investigating the existence of free radicals in biological systems. We investigated the effects of PBN on ionizing radiation-induced apoptosis in U937 cells. Upon exposure to 2 Gy of γ-irradiation, there was a distinct difference between the control cells and the cells pre-treated with 2 mM PBN for 2 h in regard to apoptotic parameters, cellular redox status, mitochondria function and oxidative damage to cells. PBN effectively suppressed morphological evidence of apoptosis and DNA fragmentation in U937 cells exposed to ionizing radiation. The [GSSG]/[GSH+GSSG] ratio and the generation of intracellular ROS were higher and the [NADPH]/[NADP⁺+NADPH] ratio was lower in control cells compared to PBN-treated cells. The ionizing radiation-induced mitochondrial damage reflected by the altered mitochondrial permeability transition, the increase in the accumulation of ROS, and the reduction of ATP production were significantly higher in control cells compared to PBN-treated cells. PBN pre-treated cells showed significant inhibition of apoptotic features such as activation of caspase-3, up-regulation of Bax and p53, and down-regulation of Bcl-2 compared to control cells upon exposure to ionizing radiation. This study indicates that PBN may play an important role in regulating the apoptosis induced by ionizing radiation presumably through scavenging of ROS.     

Antioxidant enzyme inhibitors enhance singlet oxygen-induced cell death in HL-60 cells

Singlet oxygen is a highly reactive form of molecular oxygen that may harm living systems by oxidizing critical cellular macromolecules and it also promotes deleterious processes such as cell death. The protective role of antioxidant enzymes against singlet oxygen-induced oxidative damage in HL-60 cells was investigated in control and cells pre-treated with diethyldithiocarbamic acid, aminotriazole and oxlalomalate, specific inhibitors of superoxide dismutase, catalase and NADP⁺-dependent isocitrate dehydrogenase, respectively. Upon exposure to rose bengal (20 μM)/light (15 min), which generates singlet oxygen, to HL-60 cells, the viability was lower and the lipid peroxidation and oxidative DNA damage were higher in inhibitor-treated cells as compared to control cells. We also observed the significant increase in the endogenous production of reactive oxygen species as well as the significant decrease in the intracellular GSH level in inhibitor-treated HL-60 cells exposed to singlet oxygen. Upon exposure to rose bengal (3 μM)/light (15 min), which induced apoptotic cell death, a clear inverse relationship was observed between the control and inhibitor-treated HL-60 cells in their susceptibility to apoptosis. These results suggest that antioxidant enzymes play an important role in cellular defense against singlet oxygen-induced cell death including necrosis and apoptosis.     

Caspases: A Molecular Switch Node in the Crosstalk between Autophagy and Apoptosis

Autophagy and apoptosis are two important catabolic processes contributing to the maintenance of cellular and tissue homeostasis. Autophagy controls the turnover of protein aggregates and damaged organelles within cells, while apoptosis is the principal mechanism by which unwanted cells are dismantled and eliminated from organisms. Despite marked differences between these two pathways, they are highly interconnected in determining the fate of cells. Intriguingly, caspases, the primary drivers of apoptotic cell death, play a critical role in mediating the complex crosstalk between autophagy and apoptosis. Pro-apoptotic signals can converge to activate caspases to execute apoptotic cell death. In addition, activated caspases can degrade autophagy proteins (i.e., Beclin-1, Atg5, and Atg7) to shut down the autophagic response. Moreover, caspases can convert pro-autophagic proteins into pro-apoptotic proteints to trigger apoptotic cell death instead. It is clear that caspases are important in both apoptosis and autophagy, thus a detailed deciphering of the role of caspases in these two processes is still required to clarify the functional relationship between them. In this article, we provide a current overview of caspases in its interplay between autophagy and apoptosis. We emphasized that defining the role of caspases in autophagy-apoptosis crosstalk will provide a framework for more precise manipulation of these two processes during cell death.     

Small interfering RNA-mediated silencing of mitochondrial NADP+-dependent isocitrate dehydrogenase enhances the sensitivity of HeLa cells toward tumor necrosis factor-alpha and anticancer drugs

Tumor necrosis factor-alpha (TNF-alpha) and several anticancer drugs induce the production of reactive oxygen species, which play an important causative role in apoptotic cell death. Recently, we demonstrated that the control of mitochondrial redox balance and the cellular defense against oxidative damage is one of the primary functions of mitochondrial NADP(+)-dependent isocitrate dehydrogenase (IDPm) by supplying NADPH for antioxidant systems. In the present report, we show that silencing of IDPm expression in HeLa cells greatly enhances apoptosis induced by TNF-alpha and anticancer drugs. Transfection of HeLa cells with an IDPm small interfering RNA (siRNA) markedly decreased activity of IDPm, enhancing the susceptibility of anticancer agent-induced apoptosis reflected by morphological evidence of apoptosis, DNA fragmentation, cellular redox status, mitochondria redox status and function, and the modulation of apoptotic marker proteins. These results indicate that IDPm may play an important role in regulating the apoptosis induced by TNF-alpha and anticancer drugs and the sensitizing effect of IDPm siRNA on the apoptotic cell death of HeLa cells offers the possibility of developing a modifier of cancer chemotherapy.     

Antioxidant enzyme inhibitors enhance singlet oxygen-induced cell death in HL-60 cells

Singlet oxygen is a highly reactive form of molecular oxygen that may harm living systems by oxidizing critical cellular macromolecules and it also promotes deleterious processes such as cell death. The protective role of antioxidant enzymes against singlet oxygen-induced oxidative damage in HL-60 cells was investigated in control and cells pre-treated with diethyldithiocarbamic acid, aminotriazole and oxlalomalate, specific inhibitors of superoxide dismutase, catalase and NADP⁺-dependent isocitrate dehydrogenase, respectively. Upon exposure to rose bengal (20 μM)/light (15 min), which generates singlet oxygen, to HL-60 cells, the viability was lower and the lipid peroxidation and oxidative DNA damage were higher in inhibitor-treated cells as compared to control cells. We also observed the significant increase in the endogenous production of reactive oxygen species as well as the significant decrease in the intracellular GSH level in inhibitor-treated HL-60 cells exposed to singlet oxygen. Upon exposure to rose bengal (3 μM)/light (15 min), which induced apoptotic cell death, a clear inverse relationship was observed between the control and inhibitor-treated HL-60 cells in their susceptibility to apoptosis. These results suggest that antioxidant enzymes play an important role in cellular defense against singlet oxygen-induced cell death including necrosis and apoptosis.     

Silencing of mitochondrial NADP+-dependent isocitrate dehydrogenase gene enhances selenite-induced apoptosis

Abstract

Selenium has been shown to play a chemopreventive role in human cancer, presumably by inducing tumour cell apoptosis. Selenite is thought to induce oxidative stress by the generation of the superoxide anion and catalysing the oxidation of thiol groups. It has previously been reported that control of the mitochondrial redox balance is a primary function of mitochon-drial NADP⁺-dependent isocitrate dehydrogenase (IDPm) by supplying NADPH for antioxidant systems. When investigating whether IDPm would be a vulnerable target of selenite, the loss of enzyme activity was observed. Transfection of HeLa cells with an IDPm small interfering RNA (siRNA) markedly decreased activity of IDPm and enhanced cells’ susceptibility of selenite-induced apoptosis, as indicated by morphological evidence of apoptosis, DNA fragmentation and the modulation of mitochondrial function and apoptotic marker proteins. These results suggest that IDPm siRNA sensitizes HeLa cells to selenite-induced apoptotic cell death, presumably through the perturbation of the cellular redox status.     

Specific inhibition of hypoxia inducible factor 1 exaggerates cell injury induced by in vitro ischemia through deteriorating cellular redox environment

Hypoxia inducible factor 1 (HIF-1) has been suggested to play a critical role in the fate of cells exposed to hypoxic stress. However, the mechanism of HIF-1-regulated cell survival is still not fully understood in ischemic conditions. Redox status is critical for decisions of cell survival, death and differentiation. We investigated the effects of inhibiting HIF-1 on cellular redox status in SH-SY5Y cells exposed to hypoxia or oxygen and glucose deprivation (OGD), coupled with cell death analyses. Our results demonstrated that inhibiting HIF-1α expression by HIF-1α specific small interfering RNA (siRNA) transfection increased reactive oxygen species generation, and transformed the cells to more oxidizing environments (low GSH/GSSG ratio, low NADPH level) under either hypoxic or OGD exposure. Cell death increased dramatically in the siRNA transfected cells, compared to non-transfected cells after hypoxic/OGD exposures. In contrast, increasing HIF-1α expression by desferrioxamine, a metal chelator and hydroxylase inhibitor, induced a more reducing environment (high GSH/GSSG ratio, high NADPH level) and reduced cell death. Further studies showed that HIF-1 regulated not only glucose transporter-1 expression, but also the key enzymes of the pentose phosphate pathway such as glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. These enzymes are important in maintaining cellular redox homeostasis by generating NADPH, the primary reducing agent in cells. Moreover, catalase significantly decreased cell death in the siRNA-transfected cells induced by hypoxia and OGD. These results suggest that maintenance of cellular redox status by HIF-1 protects cells from hypoxia and ischemia mediated injuries.     

N-acetylcysteine increases apoptosis induced by H(2)O(2) and mo-antiFas triggering in a 3DO hybridoma cell line

N-Acetylcysteine (NAC) has been used as an antioxidant to prevent apoptosis triggered by different stimuli in different cell types. It is common opinion that cellular redox, which is largely determined by the ratio of oxidized and reduced glutathione (GSH), plays a significant role in the propensity of cells to undergo apoptosis. However, there are also contrasting opinions stating that intracellular GSH depletion or supplemented GSH alone are not sufficient to lead cells to apoptosis or conversely protect them. Unexpectedly, this study shows that NAC, even if it maintains the peculiar characteristics of an agent capable of reducing cell proliferation and increasing intracellular GSH content, increases apoptosis induced by H(2)O(2) treatment and mo-antiFas triggering in a 3DO cell line. We found that 24 h of NAC pre-treatment can shift cellular death from necrotic to apoptotic and determine an early expression of FasL in a 3DO cell line treated with H(2)O(2).     

Glutathione modulation of in vitro development

Summary It is well known that glutathione is a ubiquitous and multifunctional compound which is important in maintaining cellular redox homeostasis. The balance between the reduced (GSH) and oxidized (GSSG) forms of this tripeptide plays a fundamental role in basic physiological and metabolic processes in plants. Recently, a remarkable amount of evidence has suggested that the role of glutathione within the plant system extends beyond the basic metabolic functions and that it may ultimately act as a modulator of plant development and morphogenesis. Therefore, it is not surpring that research has begung to focus on using the glutathione redox pair system for improving regeneration of cultured cells. One of the major themes that has emerged from in vitro studies is that GSH promotes cell proliferation, while GSSG promotes organized development. Thus, in vitro manipulation of this redox compound within the culture medium could lead to an enhancement of plant regeneration.