The cytotoxicity of tumor necrosis factor depends on induction of the mitochondrial permeability transition

Ethanol induces peroxynitrite-mediated toxicity through inactivation of NADP+-dependent isocitrate dehydrogenase and superoxide dismutase

It has been reported that chronic alcohol administration increases peroxynitrite hepatotoxicity by enhancing concomitant production of nitric oxide and superoxide. Several studies have shown the importance of superoxide dismutase (SOD) in protecting cells against ethanol-induced oxidative stress. Recently, we demonstrated that the control of cytosolic and mitochondrial redox balance and the cellular defense against oxidative damage is one of the primary functions of NADP+-dependent isocitrate dehydrogenase (ICDH) through to supply NADPH for antioxidant systems. In this report, we demonstrate that ethanol induces the peroxynitrite-mediated cytotoxicity in HepG2 cells through inactivation of antioxidant enzymes such as ICDH and SOD. Upon exposure to 100mM ethanol for 3days to HepG2 cells, a significant decrease in the viability and activities of ICDH and SOD was observed. The ethanol-induced inactivation of antioxidant enzymes resulted in the cellular oxidative damage and modulation of redox status as well as mitochondrial dysfunction in HepG2 cells. The cytoxicity of ethanol and inactivation of antioxidant enzymes were effectively protected by manganeses(III) tetrakis(N-methyl-2-pyridyl) porphyrin, a manganese SOD mimetic, and N'-monomethyl-l-arginine, a nitric oxide synthase inhibitor. These results indicate that ethanol toxicity is mediated by peroxynitrite and the peroxynitrite-mediated damage to ICDH and SOD may be resulted in the perturbation of the cellular antioxidant defense systems and subsequently lead to a pro-oxidant condition.

Epigallocatechin gallate, a constituent of green tea, regulates high glucose-induced apoptosis

A high concentration of glucose has been implicated as a causal factor in initiation and progression of diabetic complications, and there is evidence to suggest that hyperglycemia increases the production of free radicals and oxidative stress. Therefore, compounds that scavenge reactive oxygen species may confer regulatory effects on high glucose-induced apoptosis. Epigallocatechin gallate (EGCG), the major polyphenolic of green tea, is reported to have an antioxidant activity. We investigated the effect of EGCG on high glucose-induced apoptosis in U937 cells. Upon exposure to 35 mM glucose for 2 days, there was a distinct difference between untreated cells and cells pre-treated with 1 microM EGCG for 2 h in regard to cellular redox status and oxidative DNA damage to cells. EGCG pre-treated cells showed significant suppression of apoptotic features such as DNA fragmentation, damage to mitochondrial function, and modulation of apoptotic marker proteins upon exposure to high glucose. This study indicates that EGCG may play an important role in regulating the apoptosis induced by high glucose presumably through scavenging of reactive oxygen species.

Low circulating levels of dehydroepiandrosterone in histologically advanced nonalcoholic fatty liver disease

The biological basis of variability in histological progression of nonalcoholic fatty liver disease (NAFLD) is unknown. Dehydroepiandrosterone (DHEA) is the most abundant steroid hormone and has been shown to influence sensitivity to oxidative stress, insulin sensitivity, and expression of peroxisome proliferator-activated receptor alpha and procollagen messenger RNA. Our aim was to determine whether more histologically advanced NAFLD is associated with low circulating levels of DHEA. Serum samples were obtained prospectively at the time of liver biopsy in 439 patients with NAFLD (78 in an initial and 361 in validation cohorts) and in controls with cholestatic liver disease (n = 44). NAFLD was characterized as mild [simple steatosis or nonalcoholic steatohepatitis (NASH) with fibrosis stage 0-2] or advanced (NASH with fibrosis stage 3-4). Serum levels of sulfated DHEA (DHEA-S) were measured by enzyme-linked immunosorbent assay. Patients with advanced NAFLD had lower plasma levels of DHEA-S than patients with mild NAFLD in both the initial (0.25 +/- 0.07 versus 1.1 +/- 0.09 microg/mL, P < 0.001) and validation cohorts (0.47 +/- 0.06 versus 0.99 +/- 0.04 microg/mL, P < 0.001). A "dose effect" of decreasing DHEA-S and incremental fibrosis stage was observed with a mean DHEA-S of 1.03 +/- 0.05, 0.96 +/- 0.07, 0.83 +/- 0.11, 0.66 +/- 0.11, and 0.35 +/- 0.06 microg/mL for fibrosis stages 0, 1, 2, 3, and 4, respectively. All patients in both cohorts in the advanced NAFLD group had low DHEA-S levels, with the majority in the hypoadrenal range. The association between DHEA-S and severity of NAFLD persisted after adjusting for age. A relationship between disease/fibrosis severity and DHEA-S levels was not seen in patients with cholestatic liver diseases.

CONCLUSION

More advanced NAFLD, as indicated by the presence of NASH with advanced fibrosis stage, is strongly associated with low circulating DHEA-S. These data provide novel evidence for relative DHEA-S deficiency in patients with histologically advanced NASH.

Silencing of mitochondrial NADP+-dependent isocitrate dehydrogenase by small interfering RNA enhances heat shock-induced apoptosis

Heat shock may increase oxidative stress due to increased production of reactive oxygen species and/or the promotion of cellular oxidation events. Mitochondrial NADP(+)-dependent isocitrate dehydrogenase (IDPm) produces NADPH, an essential reducing equivalent for the antioxidant system. In this report, we demonstrate that silencing of IDPm expression in HeLa cells greatly enhances apoptosis induced by heat shock. Transfection of HeLa cells with an IDPm small interfering RNA (siRNA) markedly decreased activity of IDPm, enhancing the susceptibility of heat shock-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 heat shock and the sensitizing effect of IDPm siRNA on the apoptotic cell death of HeLa cells offers the possibility of developing a modifier of cancer therapy.

Mitochondria-targeted antioxidants do not prevent tumour necrosis factor-induced necrosis of L929 cells

Mitochondrial production of reactive oxygen species (ROS) is widely reported as a central effector during TNF-induced necrosis. The effect of a family of mitochondria-targeted antioxidants on TNF-induced necrosis of L929 cells was studied. While the commonly used lipid-soluble antioxidant BHA effectively protected cells from TNF-induced necrosis, the mitochondria-targeted antioxidants MitoQ(3), MitoQ(5), MitoQ(10) and MitoPBN had no effect on TNF-induced necrosis. Since BHA also acts as an uncoupler of mitochondrial membrane potential, two additional uncouplers were tested. FCCP and CCCP both provided dose-dependent inhibition of TNF-induced necrosis. In conclusion, the generation of mitochondrial ROS may not be necessary for TNF-induced necrosis. Instead, these results suggest alternative mitochondrial functions, such as a respiration-dependent process, are critical for necrotic 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.

Adenine nucleotide (ADP/ATP) translocase 3 participates in the tumor necrosis factor induced apoptosis of MCF-7 cells

Mitochondrial adenine nucleotide translocase (ANT) is believed to be a component or a regulatory component of the mitochondrial permeability transition pore (mtPTP), which controls mitochondrial permeability transition during apoptosis. However, the role of ANT in apoptosis is still uncertain, because hepatocytes isolated from ANT knockout and wild-type mice are equally sensitive to TNF- and Fas-induced apoptosis. In a screen for genes required for tumor necrosis factor alpha (TNF-alpha)-induced apoptosis in MCF-7 human breast cancer cells using retrovirus insertion-mediated random mutagenesis, we discovered that the ANT3 gene is involved in TNF-alpha-induced cell death in MCF-7 cells. We further found that ANT3 is selectively required for TNF- and oxidative stress-induced cell death in MCF-7 cells, but it is dispensable for cell death induced by several other inducers. This data supplements previous data obtained from ANT knockout studies, indicating that ANT is involved in some apoptotic processes. We found that the resistance to TNF-alpha-induced apoptosis observed in ANT3 mutant (ANT3(mut)) cells is associated with a deficiency in the regulation of the mitochondrial membrane potential and cytochrome c release. It is not related to intracellular ATP levels or survival pathways, supporting a previous model in which ANT regulates mtPTP. Our study provides genetic evidence supporting a role of ANT in apoptosis and suggests that the involvement of ANT in cell death is cell type- and stimulus-dependent.

Glutathione levels modulate domoic acid induced apoptosis in mouse cerebellar granule cells

Exposure of mouse cerebellar granule neurons (CGNs) to domoic acid induced cell death, either by apoptosis or by necrosis, depending on its concentration. Necrotic damage predominated in response to domoic acid above 0.1 microM. In contrast, cell injury with apoptotic features (assessed by Hoechst staining and DNA laddering assay) was evident after exposure to lower concentrations of domoic acid (< or = 0.1 microM). The AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid)/kainate receptor antagonist 2,3-dihydroxy-6-nitro-sulfamoylbenzo [f] quinoxaline, but not the N-methyl-D-aspartate receptor antagonist MK-801, prevented domoic acid-induced apoptosis. To evaluate the role of oxidative stress in domoic acid-induced apoptosis, experiments were carried out in CGNs isolated from wild-type mice (Gclm (+/+)) and mice lacking the modifier subunit of glutamate-cysteine ligase, the first and rate-limiting step of glutathione (GSH) biosynthesis (Gclm (-/-)). CGNs from Gclm (-/-) mice have very low levels of GSH and were more sensitive to domoic acid-induced apoptosis and necrosis than Gclm (+/+) CGNs. The antioxidant melatonin (200 microM) and the membrane-permeant GSH delivery agent GSH ethyl ester (2.5 mM) prevented domoic acid-induced apoptosis. Domoic acid increased formation of reactive oxygen species but did not affect intracellular GSH levels. Domoic acid also increased cytosolic and mitochondrial calcium levels, increased oxidative stress in mitochondria, and altered mitochondrial membrane potential, which ultimately caused cytochrome c release, activation of caspase-3, and degradation of poly (ADP-ribose) polymerase. These results indicate that low concentrations of domoic acid cause apoptotic neuronal cell death mediated by oxidative stress.

N-t-Butyl hydroxylamine regulates ionizing radiation-induced apoptosis in U937 cells

Ionizing radiation induces the production of reactive oxygen species, which play an important causative role in apoptotic cell death. Therefore, compounds that scavenge reactive oxygen species may confer regulatory effects on apoptosis. Recently, it has been shown that the decomposition product of the spin-trapping agent alpha-phenyl-N-t-butylnitrone, N-t-butyl hydroxylamine (NtBHA), mimics alpha-phenyl-N-t-butylnitrone and is much more potent in delaying reactive oxygen species-associated senescence. We investigated the effects of NtBHA on ionizing radiation-induced apoptosis. Upon exposure to 2Gy of gamma-irradiation, there was a distinct difference between the control cells and the cells pre-treated with 0.1mM NtBHA for 2h in regard to apoptotic parameters, cellular redox status, mitochondria function, and oxidative damage to cells. NtBHA effectively suppressed morphological evidence of apoptosis and DNA fragmentation in U937 cells exposed to ionizing radiation. The generation of intracellular reactive oxygen species was higher and the GSH level was lower in control cells compared to NtBHA-treated cells. The ionizing radiation-induced mitochondrial damage reflected by the altered mitochondrial permeability transition, the increase in the accumulation of reactive oxygen species, and the reduction of ATP production were significantly higher in control cells compared to NtBHA-treated cells. NtBHA 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 NtBHA may play an important role in regulating the apoptosis induced by ionizing radiation presumably through scavenging of reactive oxygen species.

Ursolic acid regulates high glucose-induced apoptosis

A high concentration of glucose has been implicated as a causal factor in initiation and progression of diabetic complications and there is evidence to suggest that hyperglycemia increases the production of free radicals and oxidative stress. Therefore, compounds that scavenge reactive oxygen species (ROS) may confer regulatory effects on high glucose-induced apoptosis. Ursolic acid (UA), a pentacyclic triterpene, is reported to have an antioxidant activity. We investigated the effect of UA on high glucose-induced apoptosis in U937 cells. Upon exposure to 35 mM glucose for two days, there was a distinct difference between untreated cells and cells pre-treated with 50 nM UA for 2 h in regard to cellular redox status and oxidative DNA damage to cells. UA pre-treated cells showed significant suppression of apoptotic features such as DNA fragmentation, damage to mitochondrial function and modulation of apoptotic marker proteins upon exposure to high glucose. This study indicates that UA may play an important role in regulating the apoptosis induced by high glucose presumably through scavenging of ROS.

Role of mitochondrial permeability transition in human renal tubular epithelial cell death induced by aristolochic acid

Aristolochic acid (AA), a natural nephrotoxin and carcinogen, can induce a progressive tubulointerstitial nephropathy. However, the mechanism by which AA causes renal injury remains largely unknown. Here we reported that the mitochondrial permeability transition (MPT) plays an important role in the renal injury induced by aristolochic acid I (AAI). We found that in the presence of Ca(2+), AAI caused mitochondrial swelling, leakage of Ca(2+), membrane depolarization, and release of cytochrome c in isolated kidney mitochondria. These alterations were suppressed by cyclosporin A (CsA), an agent known to inhibit MPT. Culture of HK-2 cell, a human renal tubular epithelial cell line for 24 h with AAI caused a decrease in cellular ATP, mitochondrial membrane depolarization, cytochrome c release, and increase of caspase 3 activity. These toxic effects of AAI were attenuated by CsA and bongkrekic acid (BA), another specific MPT inhibitor. Furthermore, AAI greatly inhibited the activity of mitochondrial adenine nucleotide translocator (ANT) in isolated mitochondria. We suggested that ANT may mediate, at least in part, the AAI-induced MPT. Taken together, these results suggested that MPT plays a critical role in the pathogenesis of HK-2 cell injury induced by AAI and implied that MPT might contribute to human nephrotoxicity of aristolochic acid.

Regulation of ionizing radiation-induced apoptosis by mitochondrial NADP+-dependent isocitrate dehydrogenase

Ionizing radiation induces the production of reactive oxygen species, which play an important causative role in apoptotic cell death. By supplying NADPH for antioxidant systems, we recently demonstrated that the control of mitochondrial redox balance and the cellular defense against oxidative damage are some of the primary functions of mitochondrial NADP(+)-dependent isocitrate dehydrogenase (IDPm). In this study, we demonstrate that modulation of IDPm activity in U937 cells regulates ionizing radiation-induced apoptosis. When we examined the regulatory role of IDPm against ionizing radiation-induced apoptosis in U937 cells transfected with the cDNA for mouse IDPm in sense and antisense orientations, a clear inverse relationship was observed between the amount of IDPm expressed in target cells and their susceptibility to apoptosis. Upon exposure to 2 gray gamma-irradiation, there was a distinct difference between the IDPm transfectant cells in regard to the morphological evidence of apoptosis, DNA fragmentation, cellular redox status, oxidative damage to cells, mitochondrial function, and the modulation of apoptotic marker proteins. In addition, transfection of HeLa cells with an IDPm small interfering RNA decreased the activity of IDPm, enhancing the susceptibility of radiation-induced apoptosis. Taken together, these results indicate that IDPm may play an important role in regulating the apoptosis induced by ionizing radiation, and the effect of IDPm small interfering RNA on HeLa cells offers the possibility of developing a modifier of radiation therapy.

Regulation of singlet oxygen-induced apoptosis by cytosolic NADP+-dependent isocitrate dehydrogenase

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. Recently, we demonstrated that the control of redox balance and the cellular defense against oxidative damage are the primary functions of cytosolic NADP(+)-dependent isocitrate dehydrogenase (IDPc) through supplying NADPH for antioxidant systems. In this report, we demonstrate that modulation of IDPc activity in HL-60 cells regulates singlet oxygen-induced apoptosis. When we examined the protective role of IDPc against singlet oxygen-induced apoptosis with HL-60 cells transfected with the cDNA for mouse IDPc in sense and antisense orientations, a clear inverse relationship was observed between the amount of IDPc expressed in target cells and their susceptibility to apoptosis. The results suggest that IDPc plays an important protective role in apoptosis of HL-60 cells induced by singlet oxygen.

Survival of TNF toxicity: dependence on caspases and NO

Tumor necrosis factor (TNF) is an endogenous pro-inflammatory cytokine, implicated in pathologies such as rheumatoid arthritis and septic shock. It was originally discovered as a factor with extraordinary antitumor activity, but its shock-inducing properties still prevent its systemic use in cancer. Clinical trials revealed hypotension as the major dose-limiting factor of TNF toxicity. When administered to mice, TNF provokes a lethal shock syndrome, where cardiovascular collapse is centrally orchestrated by nitric oxide (NO). Nevertheless, NO synthase (NOS) inhibition in animal models and septic shock patients could not improve and even aggravated outcome, suggesting a bivalent role for NO. Lymphocyte and enterocyte apoptosis has been described in septic, endotoxemic, or TNF-treated animals, as well as in septic patients. In this review, we describe our recent studies on the role of NO and caspases in TNF-induced shock in mice. In summary, we have found that both NO and caspases may exert unexpected and dual functions during TNF shock. Whereas excessive NO production provokes lethal hypotension, it also has an important anti-oxidant function, protecting organs from oxidative stress and lipid peroxidation. In addition, our results also indicate that caspases may exert an important endogenous negative feedback on oxidative stress as well.

Cell death by necrosis: towards a molecular definition

Necrosis has been defined as a type of cell death that lacks the features of apoptosis and autophagy, and is usually considered to be uncontrolled. Recent research suggests, however, that its occurrence and course might be tightly regulated. After signaling- or damage-induced lesions, necrosis can include signs of controlled processes such as mitochondrial dysfunction, enhanced generation of reactive oxygen species, ATP depletion, proteolysis by calpains and cathepsins, and early plasma membrane rupture. In addition, the inhibition of specific proteins involved in regulating apoptosis or autophagy can change the type of cell death to necrosis. Because necrosis is prominent in ischemia, trauma and possibly some forms of neurodegeneration, further biochemical comprehension and molecular definition of this process could have important clinical implications.

N-t-Butyl hydroxylamine regulates heat shock-induced apoptosis in U937 cells

Heat shock may increase oxidative stress due to increased production of reactive oxygen species and/or the promotion of cellular oxidation events. Therefore, compounds that scavenge reactive oxygen species may regulate heat shock-induced cell death. Recently, it has been shown that the decomposition product of the spin-trapping agent alpha-phenyl-N-t-butylnitrone, N-t-butyl hydroxylamine (NtBHA), mimics alpha-phenyl-N-t-butylnitrone and is much more potent in delaying reactive oxygen species-associated senescence. We investigated the protective role of NtBHA against heat shock-induced apoptosis in U937 cells. Upon exposure to heat shock, there was a distinct difference between the untreated cells and the cells pre-treated with 0.1 mM NtBHA for 2 h in regard to apoptotic parameters, cellular redox status, and mitochondrial function. Upon exposure to heat shock, NtBHA pre-treated cells showed significant inhibition of apoptotic features such as activation of caspase-3, up-regulation of Bax, and down-regulation of Bcl-2 compared to untreated cells. This study indicates that NtBHA may play an important role in regulating the apoptosis induced by heat shock, presumably through scavenging of reactive oxygen species.

A wave of reactive oxygen species (ROS)-induced ROS release in a sea of excitable mitochondria

Once considered simply as the main source of ATP, mitochondria are now implicated in the control of many additional aspects of cell physiology, such as calcium signaling, and pathology, as in injury incurred on ischemia and subsequent reperfusion (I/R). Mitochondrial respiration is ordinarily accompanied by low-level ROS production, but they can respond to elevated ROS concentrations by increasing their own ROS production, a phenomenon termed ROS-induced ROS release (RIRR). Two modes of RIRR have been described. In the first mode of RIRR, enhanced ROS leads to mitochondrial depolarization via activation of the MPTP, yielding a short-lived burst of ROS originating from the mitochondrial electron transport chain (ETC). The second mode of RIRR is MPTP independent but is regulated by the mitochondrial benzodiazepine receptor (mBzR). Increased ROS in the mitochondrion triggers opening of the inner mitochondrial membrane anion channel (IMAC), resulting in a brief increase in ETC-derived ROS. Both modes of RIRR have been shown to transmit localized mitochondrial perturbations throughout the cardiac cell in the form of oscillations or waves but are kinetically distinct and may involve different ROS that serve as second messengers. In this review, we discuss the mechanisms of these different modes of RIRR.

Oxidative stress in glaucomatous neurodegeneration: mechanisms and consequences

Reactive oxygen species (ROS) are generated as by-products of cellular metabolism, primarily in the mitochondria. Although ROS are essential participants in cell signaling and regulation, when their cellular production overwhelms the intrinsic antioxidant capacity, damage to cellular macromolecules such as DNA, proteins, and lipids ensues. Such a state of "oxidative stress" is thought to contribute to the pathogenesis of a number of neurodegenerative diseases. Growing evidence supports the involvement of oxidative stress as a common component of glaucomatous neurodegeneration in different subcellular compartments of retinal ganglion cells (RGCs). Besides the evidence of direct cytotoxic consequences leading to RGC death, it also seems highly possible that ROS are involved in signaling RGC death by acting as a second messenger and/or modulating protein function by redox modifications of downstream effectors through enzymatic oxidation of specific amino acid residues. Different studies provide cumulating evidence, which supports the association of ROS with different aspects of the neurodegenerative process. Oxidative protein modifications during glaucomatous neurodegeneration increase neuronal susceptibility to damage and also lead to glial dysfunction. Oxidative stress-induced dysfunction of glial cells may contribute to spreading neuronal damage by secondary degeneration. Oxidative stress also promotes the accumulation of advanced glycation end products in glaucomatous tissues. In addition, oxidative stress takes part in the activation of immune response during glaucomatous neurodegeneration, as ROS stimulate the antigen presenting ability of glial cells and also function as co-stimulatory molecules during antigen presentation. By discussing current evidence, this review provides a broad perspective on cellular mechanisms and potential consequences of oxidative stress in glaucoma.

Mitochondrial iPLA2 Activity Modulates the Release of Cytochrome c from Mitochondria and Influences the Permeability Transition

The mitochondrial Ca(2+)-independent phospholipase A(2) is activated during energy-dependent Ca(2+) accumulation under conditions where there is a sustained depression of the membrane potential. This activation is not dependent on induction of the mitochondrial permeability transition. Bromoenol lactone, which inhibits the phospholipase, is effective as an inhibitor of the transition, and this action can be overcome by low levels of exogenous free fatty acids. Apparently, activation of the Ca(2+)-independent phospholipase is a factor in the mechanisms by which depolarization and Ca(2+) accumulation promote opening of the permeability transition pore. Sustained activity of the Ca(2+)-independent phospholipase A(2) promotes rupture of the outer mitochondrial membrane and spontaneous release of cytochrome c on a time scale similar to that of apoptosis occurring in cells. However, more swelling of the matrix space must occur to provoke release of a given cytochrome c fraction when the enzyme is active, compared with when it is inhibited. Through its effects on the permeability transition and release of intermembrane space proteins, the mitochondrial Ca(2+)-independent phospholipase A(2) may be an important factor governing cell death caused by necrosis or apoptosis.

The effect of alpha-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. alpha-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 gamma-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.

Reactive oxygen species production by mitochondria in endothelial cells exposed to reoxygenation after hypoxia and glucose depletion is mediated by ceramide

In endothelium, reoxygenation after hypoxia (H/R) has been shown to induce production of reactive oxygen species (ROS) by complex III of the mitochondrial respiratory chain. The purpose of the present study was to test the involvement of ceramide in this phenomenon. Human umbilical vein endothelial cells underwent 2 h of hypoxia (Po2, ∼20 mmHg) without glucose and 1 h of reoxygenation (Po2, ∼120 mmHg) with glucose. ROS production was measured by the fluorescent marker 2′,7′-dichlorodihydrofluorescein diacetate, and cell death by propidium iodide. We showed that 1) after 1 h of reoxygenation, fluorescence had risen and that ROS production was inhibited by desipramine, an inhibitor of sphingomyelinase, an enzyme responsible for ceramide production (126 ± 7% vs. 48 ± 12%, P < 0.05); 2) administration of ceramide ( N-acetylsphingosine) per se (i.e., in the absence of H/R) induced ROS production (65 ± 3%), which was inhibited by complex III inhibitor: antimycin A (24 ± 3%, P < 0.0001), or stigmatellin (31 ± 2%, P < 0.0001); 3) hypoxia/reoxygenation-induced ROS production was not affected by either ceramide-activated protein kinase inhibitor dimethyl aminopurine or mitochondrial permeability transition inhibitor cyclosporin A but was significantly inhibited by the antiapoptotic protein Bcl-2 (82 ± 8%, P < 0.05); 4) ceramide-induced ROS production was also inhibited by Bcl-2 (41 ± 4%, P < 0.0001). These results demonstrate that in endothelial cells submitted to hypoxia and glucose depletion followed by reoxygenation with glucose, the pathway implicated in mitochondrial complex III ROS production is ceramide dependent and is decreased by the antiapoptotic protein Bcl-2.

Mechanisms of ammonia-induced astrocyte swelling

Astrocyte swelling represents the major factor responsible for the brain edema associated with fulminant hepatic failure (FHF). The edema may be of such magnitude as to increase intracranial pressure leading to brain herniation and death. Of the various agents implicated in the generation of astrocyte swelling, ammonia has had the greatest amount of experimental support. This article reviews mechanisms of ammonia neurotoxicity that contribute to astrocyte swelling. These include oxidative stress and the mitochondrial permeability transition (MPT). The involvement of glutamine in the production of cell swelling will be highlighted. Evidence will be provided that glutamine induces oxidative stress as well as the MPT, and that these events are critical in the development of astrocyte swelling in hyperammonemia.

Tumor necrosis factor-related apoptosis-inducing ligand alters mitochondrial membrane lipids

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has been shown to have selective antitumor activity. TRAIL induces ubiquitous pathways of cell death in which caspase activation is mediated either directly or via the release of apoptogenic factors from mitochondria; however, the precise components of the mitochondrial signaling pathway have not been well defined. Notably, mitochondria constitute an important target in overcoming resistance to TRAIL in many types of tumors. Bid is considered to be fundamental in engaging mitochondria during death receptor-mediated apoptosis, but this action is dependent on mitochondrial lipids. Here, we report that TRAIL signaling induces an alteration in mitochondrial membrane lipids, particularly cardiolipin. This occurs independently of caspase activation and primes mitochondrial membranes to the proapoptotic action of Bid. We unveil a link between TRAIL signaling and alteration of membrane lipid homeostasis that occurs in parallel to apical caspase activation but does not take over the mode of cell death because of the concurrent activation of caspase-8. In particular, TRAIL-induced alteration of mitochondrial lipids follows an imbalance in the cellular homeostasis of phosphatidylcholine, which results in an elevation in diacylglycerol (DAG). Elevated DAG in turn activates the delta isoform of phospholipid-dependent serine/threonine protein kinase C, which then accelerates the cleavage of caspase-8. We also show that preservation of phosphatidylcholine homeostasis by inhibition of lipid-degrading enzymes almost completely impedes the activation of pro-caspase-9 while scarcely changing the activation of caspase-8.

Oxalomalate, a competitive inhibitor of NADP+-dependent isocitrate dehydrogenase, regulates heat shock-induced apoptosis

Heat shock may increase oxidative stress due to increased production of reactive oxygen species and/or the promotion of cellular oxidation events. Recently, we demonstrated that the control of cytosolic and mitochondrial redox balance and the cellular defense against oxidative damage is one of the primary functions of NADP(+)-dependent isocitrate dehydrogenase (ICDH) by supplying NADPH for antioxidant systems. The protective role of ICDH against heat shock-induced apoptosis in U937 cells was investigated in the control and the cells pre-treated with oxalomalate, a competitive inhibitor of ICDH. Upon exposure to heat shock, there was a distinct difference between the control cells and the cells pre-treated with 3mM oxalomalate for 3h in regard to apoptotic parameters, cellular redox status, and mitochondrial function. The oxalomalate pre-treated cells showed significant enhancement of apoptotic features such as activation of caspase-3, up-regulation of Bax, and down-regulation of Bcl-2 compared to the control cells upon exposure to heat shock. This study indicates that ICDH may play an important role in regulating the apoptosis induced by heat shock presumably through maintaining the cellular redox status.

TEGDMA induces mitochondrial damage and oxidative stress in human gingival fibroblasts

Free monomers including triethylene glycol dimethacrylate (TEGDMA) are released by resin composite. Recent studies in vitro have demonstrated that TEGDMA induced GSH depletion and production of radical oxygen species (ROS) in human gingival fibroblasts (HGF) but the exact mechanism of these events remains unclear. Our purpose is to investigate the origin of ROS production. TEGDMA induces a rapid (within 30 min) and drastic depletion of ATP concomitant with the GSH depletion. After 3h incubation, TEGDMA induced an increase of lipid peroxidation associated with LDH leakage. Our data also showed that TEGDMA produced damage at mitochondrial level. This is demonstrated by the collapse of mitochondrial membrane potential (MMP) in HGF treated with TEGDMA. The protective effect of carbonylcyanide m-chlorophenylhydrazone (CCCP), an uncoupler of oxidative phosphorylation on lipid peroxidation and LDH leakage suggests that mitochondria can be implicated in these events. Trolox, a soluble derivative of Tocopherol, weakly prevents ATP but not GSH depletion and totally protects the cells against lipid peroxidation, MMP collapse and cell death. Thus, the present results suggest that TEGDMA induces lipid peroxidation and mitochondrial damage, which contribute to cell death.

Regulation of ionizing radiation-induced apoptosis by a manganese porphyrin complex

Ionizing radiation induces the production of reactive oxygen species, which play an important causative role in apoptotic cell death. Therefore, compounds that scavenge reactive oxygen species may confer regulatory effects on apoptosis. Superoxide dismutase (SOD) mimetics have been shown to be protective against cell injury caused by reactive oxygen species. We investigated the effects of the manganese (III) tetrakis(N-methyl-2-pyridyl)porphyrin (MnTMPyP), a cell-permeable SOD mimetic, on ionizing radiation-induced apoptosis. Upon exposure to 2 Gy of gamma-irradiation, there was a distinct difference between the control cells and the cells pre-treated with 5 microM MnTMPyP for 2 h with regard to apoptotic parameters, cellular redox status, mitochondria function, and oxidative damage to cells. MnTMPyP 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 reactive oxygen species were higher and the [NADPH]/[NADP(+)+NADPH] ratio was lower in control cells compared to MnTMPyP-treated cells. The ionizing radiation-induced mitochondrial damage reflected by the altered mitochondrial permeability transition, the increase in the accumulation of reactive oxygen species, and the reduction of ATP production were significantly higher in control cells compared to MnTMPyP-treated cells. MnTMPyP 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 MnTMPyP may play an important role in regulating the apoptosis induced by ionizing radiation presumably through scavenging of reactive oxygen species.

Activation of calcium-independent phospholipase A2 (iPLA2) in brain mitochondria and release of apoptogenic factors by BAX and truncated BID

Cleaved or truncated BID (tBID) is known to oligomerize both BAK and BAX. Previously, BAK and BAX lacing the C-terminal fragment (BAXDeltaC) were shown to induce modest cytochrome c (Cyt c) release from rat brain mitochondria when activated by tBID. We now show that tBID plus monomeric full-length BAX induce extensive release of Cyt c, Smac/DIABLO, and Omi/HtrA2 (but not endonuclease G and the apoptosis inducing factor) comparable to the release induced by alamethicin. This occurs independently of the permeability transition without overt changes in mitochondrial morphology. The mechanism of the release may involve formation of reactive oxygen species (ROS) and activation of calcium-independent phospholipase A(2) (iPLA(2)). Indeed, increased ROS production and activated iPLA(2) were observed prior to massive Cyt c release. Furthermore, the extent of inhibition of Cyt c release correlated with the degree of suppression of iPLA(2) by the inhibitors propranolol, dibucaine, 4-bromophenacyl bromide, and bromenol lactone. Consistent with a requirement for iPLA(2) in Cyt c release from brain mitochondria, synthetic liposomes composed of lipids mimicking the outer mitochondrial membrane (OMM) but lacing iPLA(2) failed to release 10 kDa fluorescent dextran (FD-10) in response to tBID plus BAX. We propose that tBID plus BAX activate ROS generation, which subsequently augments iPLA(2) activity leading to changes in the OMM that allow translocation of certain mitochondrial proteins from the intermembrane space.

Response characteristics of the mitochondrial DNA genome in developmental health and disease

This review focuses on mitochondrial biology in mammalian development; specifically, the dynamics of information transfer from nucleus to mitochondrion in the regulation of mitochondrial DNA genomic expression, and the reverse signaling of mitochondrion to nucleus as an adaptive response to the environment. Data from recent studies suggest that the capacity of embryonic cells to react to oxygenation involves a tradeoff between factors that influence prenatal growth/development and postnatal growth/function. For example, mitochondrial DNA replication and metabolic set points in nematodes may be determined by mitochondrial activity early in life. The mitochondrial drug PK11195, a ligand of the peripheral benzodiazepine receptor, has antiteratogenic and antidisease action in several developmental contexts in mice. Protein malnutrition during early life in rats can program mitochondrial DNA levels in adult tissues and, in humans, epidemiological data suggest an association between impaired fetal growth and insulin resistance. Taken together, these findings raise the provocative hypothesis that environmental programming of mitochondrial status during early life may be linked with diseases that manifest during adulthood. Genetic defects that affect mitochondrial function may involve the mitochondrial DNA genome directly (maternal inheritance) or indirectly (Mendelian inheritance) through nuclear-coded mitochondrial proteins. In a growing number of cases, the depletion of, or deletion in, mitochondrial DNA is seen to be secondary to mutation of key nuclear-coded mitochondrial proteins that affect mitochondrial DNA replication, expression, or stability. These defects of intergenomic regulation may disrupt the normal cross-talk or structural compartmentation of signals that ultimately regulate mitochondrial DNA integrity and copy number, leading to depletion of mitochondrial DNA.

2n-fatty acids from phosphatidylcholine label sphingolipids—A novel role of phospholipase A2?

In order to find out whether there is a phospholipase A2 (PLA2)-mediated link between glycerophospholipids and sphingolipids, L929 cells were labeled with 1n-palmitoyl-2n-[1-14C]palmitoyl phosphatidylcholine for 16-18 h or 90 min. After labeling for 16-18 h, 14C-sphingomyelin (SM), 14C-ceramide and 14C-sphingosine were demonstrated on autoradiograms of thin layer chromatograms of untreated or mildly hydrolyzed lipid extracts in different chromatographic systems. Strong hydrolysis of labeled SM proved that both possible moieties of SM, sphingosine and acyl moiety, had been labeled. The identity of SM and its enzymatic degradation product, ceramide, was verified by mass spectrometry. The label in SM-derived ceramide was demonstrated on an autoradiogram after thin layer chromatography. The inhibitor of (dihydro)ceramide synthase fumonisin B1 suppressed the label in sphingolipids significantly during 16-18 h (ceramide and SM), as well as during 90-min labeling (SM). The presence of inhibitors of PLA2 (bromoenol lactone, aristolochic acid and quinacrine dihydrochloride) diminished the label in SM significantly during the 90-min labeling. These results demonstrate a close metabolic relationship between glycerophospholipids and sphingolipids and give evidence for a novel role of PLA2.

Redox regulation of apoptosis by members of the TNF superfamily

Tumor necrosis factor (TNF), fibroblast-associated cell surface (Fas) ligand, and TNF-related apoptosisinducing ligand (TRAIL), all members of the TNF superfamily, are arguably the most potent inducers of cell death. These cytokines induce cell death through sequential recruitment by the death receptors TNFR1- associated death domain protein (TRADD), Fas-associated death domain protein (FADD), FADD-like interleukin-1beta-converting enzyme (FLICE), and downstream caspases. Increasing evidence indicates that mitochondria play a critical role in cytokine receptor-mediated apoptosis. There is also now ample evidence that apoptosis induced by TNF and its family members is mediated through the production of reactive oxygen intermediates (also known as reactive oxygen species). Here we review the evidence linking reactive oxygen intermediates to cytokine-induced cell death mediated by TNF-alpha/beta, Fas, TRAIL, TNF-like weak inducer of apoptosis (TWEAK), and vascular endothelial cell growth inhibitor (VEGI).

Role of mitochondrial phospholipid hydroperoxide glutathione peroxidase (PHGPx) as an antiapoptotic factor

Phospholipid hydroperoxide glutathione peroxidase (PHGPx) is a unique antioxidant enzyme that markedly reduces lipid hydroperoxide generated in biomembranes. Overexpression of mitochondrial PHGPx potentially suppresses the release of cytochrome c (cyt. c) from mitochondria and apoptosis. The hydroperoxide level in mitochondria was elevated in 2-deoxyglucose (2DG)-induced apoptosis, but not in apoptosis-resistant cells in which mitochondrial PHGPx was overexpressed. From studies of the overexpression of PHGPx, the generation of hydrogen peroxide and lipid hydroperoxide in mitochondria might be important triggers of apoptosis. In particular lipid hydroperoxide could be involved in the initiation of cyt. c liberation from mitochondria in 2DG-induced apoptosis since lipid hydroperoxide is a primary substrate for PHGPx. The release of cyt. c from mitochondria is an important proapoptotic signal in the mitochondrial death pathway. Several reports demonstrated the reactive oxgen species could be involved in cyt. c liberation, although its mechanism is still unknown. Cardiolipin (CL), which exclusively locates in the innermembrane of mitochondria, shows strong affinity for cyt. c is required for the adenine nucleotide translocator (ANT) that controls the opening and closing of the permeability transition pore. Association of cyt. c with CL is lost upon peroxidation. CL hydroperoxide (CLOOH), in contrast to CL, does not bind to cyt. c. Furthermore, CLOOH can open the permeability transion pore by the inactivation of ANT. These previous results suggest that mitochondrial PHGPx inhibits the release of cyt. c from mitochondria by the scavenging CLOOH and could prevent apoptosis.

Regulation of high glucose-induced apoptosis by mitochondrial NADP+-dependent isocitrate dehydrogenase

A high concentration of glucose has been implicated as a causal factor in initiation and progression of diabetic kidney complications, and there is evidence to suggest that hyperglycemia increases the production of free radicals and oxidant stress. 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) to supply NADPH for antioxidant systems. In this report, we demonstrate that modulation of IDPm activity in HEK293 cells, an embryonic kidney cell line, regulates high glucose-induced apoptosis. When we examined the protective role of IDPm against high glucose-induced apoptosis with HEK293 cells transfected with the cDNA for mouse IDPm in sense and antisense orientations, a clear inverse relationship was observed between the amount of IDPm expressed in target cells and their susceptibility to apoptosis. The results suggest that IDPm plays an important protective role in apoptosis of HEK293 cells induced by a high concentration of glucose and may contribute to various pathologies associated with the long-term complications of diabetes.

Evidence in favour of a role for peripheral-type benzodiazepine receptor ligands in amplification of neuronal apoptosis

The mitochondrial peripheral benzodiazepine receptor (PBR) is involved in a functional structure designated as the mitochondrial permeability transition (MPT) pore, which controls apoptosis. PBR expression in nervous system has been reported in glial and immune cells. We now show expression of both PBR mRNA and protein, and the appearance of binding of a synthetic ligand fluo-FGIN-1-27 in mitochondria of rat cerebellar granule cells (CGCs). Additionally, the effect of PBR ligands on colchicine-induced apoptosis was investigated. Colchicine-induced neurotoxicity in CGCs was measured at 24 h. We show that, in vitro, PBR ligands 1-(2-chlorophenyl-N-methylpropyl)-3-isoquinolinecarboxamide (PK11195), 7-chloro-5-(4-chlorophenyl)-1,3-dihydro-1-methyl-2H-1,4- benzodiazepin-2-one (Ro5-4864) and diazepam (25- 50 microM) enhanced apoptosis induced by colchicine, as demonstrated by viability experiments, flow cytometry and nuclear chromatin condensation. Enhancement of colchicine-induced apoptosis was characterized by an increase in mitochondrial release of cytochrome c and AIF proteins and an enhanced activation of caspase-3, suggesting mitochondrion dependent mechanism that is involved in apoptotic process. Our results indicate that exposure of neural cells to PBR ligands generates an amplification of apoptotic process induced by colchicine and that the MPT pore may be involved in this process.

Nonalcoholic fatty liver disease: a review of current understanding and future impact

Nonalcoholic fatty liver disease (NAFLD), already the most common form of liver disease in the United States, can be expected to increase in prevalence and severity in parallel with national epidemics of obesity and type 2 diabetes. NAFLD is frequently associated with insulin resistance. While insulin resistance, and thereby hyperinsulinemia, are, in large part, metabolic consequences of obesity, the basis of diversity in severity and progression of inflammation and fibrosis is not known. Increased susceptibility to oxidative stress is likely to play a role. Several patient characteristics have been associated with more severe histological findings in patients with NAFLD, including type 2 diabetes, hypertension, age over 40 years, and higher transaminases. Liver biopsy is, however, required to accurately grade and stage NAFLD histologically. Although the natural history of NAFLD is relatively poorly defined, NAFLD is increasingly recognized as an important cause of decompensated liver disease. Weight reduction and improved insulin sensitivity are associated with improved biochemical and histological parameters of NAFLD. There are, however, no proven safe and efficacious pharmacological treatments for NAFLD.

Dynamic movement of cytochrome c from mitochondria into cytosol and peripheral circulation in massive hepatic cell injury

BACKGROUND

In the process of apoptosis, it is known that the transition of cytochrome c from mitochondria into the cytosol occurs, and tumor necrosis factor (TNF)-alpha is one of the molecules responsible for this event. But in the state of hypercytokine induced by D-galactosamine (D-GaIN)/Lipopolysaccharide (LPS), the localization of cytochrome c is little known.

METHODS

Rats were administrated with D-GaIN(700 mg/kg)/LPS(200 microg/kg). Blood and tissue samples were collected and examined for levels of pro-inflammatory cytokines, the apoptosis of liver cells, and the localization of cytochrome c.

RESULT

Before administration of D-GaIN/LPS, cytochrome c was definitely localized in the mitochondria. At 2 h after simultaneous administration of D-GaIN/LPS, cytochrome c had accumulated in the cytosol following abrupt increases of plasma TNF-alpha. Massive cell destruction due to apoptosis proved by Terminal deoxynucleo-tidyl transferase-mediated dUTP nick end labeling staining was observed in liver tissue 4 h later and markedly increased levels of cytochrome c were detected in the plasma 12 h after D-GaIN/LPS administration.

CONCLUSION

Liver injury induced by simultaneous administration of D-GaIN/LPS was closely associated with the production of TNF-alpha, and also with the dynamic movement of cytochrome c from the mitochondria into the cytosol, and then into the systemic circulation. The detection of plasma cytochrome c levels may be a useful clinical tool for the detection of apoptosis in vivo.

Mitochondrial dysfunction is a primary event in renal cell oxalate toxicity

BACKGROUND

In cultured renal epithelial cells, exposure to oxalate, a constituent of many kidney stones, elicits a cascade of responses that often leads to cell death. Oxalate toxicity is mediated via generation of reactive oxygen species (ROS) in a process that depends at least in part upon lipid signaling molecules that are generated through membrane events that culminate in phospholipase A2 (PLA2) activation. The present studies asked whether mitochondria, a major site of ROS production, were targets of oxalate toxicity, and if so, whether mitochondrial responses to oxalate were mediated by PLA2 activation.

METHODS

Effects of oxalate and various lipids on mitochondrial membrane potential (DeltaPsim) were measured in Madin-Darby canine kidney (MDCK) cell monolayers using 5,5',6,6'-tetrachloro 1,1',3,3'-tetraethylbenzimidazolylcarbocyanine iodide (JC-1), a DeltaPsim-sensitive dye. Other studies assayed caspases, serine proteases activated during apoptosis, in response to oxalate or lipid signaling molecules. Additional studies asked whether oxalate or lipids produced by PLA2 activation promoted ROS formation in isolated renal mitochondria.

RESULTS

Oxalate exposure decreased MDCK cell DeltaPsim within 30 minutes, a response attenuated by arachidonyl trifluoromethyl ketone (AACOCF3), an inhibitor of cytosolic PLA2 (cPLA2). Exposure to arachidonic acid or to lysophosphatidylcholine (lyso-PC), lipid products of PLA2 activation, or to ceramide, another lipid signal generated in MDCK cells following oxalate exposure, also depolarized MDCK cell DeltaPsim and increased the number of caspase-positive cells. Isolated renal mitochondria responded to oxalate, arachidonic acid, lyso-PC, and ceramide by increasing their accumulation of ROS, lipid peroxides, and oxidized thiol proteins.

CONCLUSION

These studies suggest that lipid signaling molecules released after oxalate-induced PLA2 activation trigger marked, rapid changes in mitochondrial function that may mediate toxicity in renal epithelial cells.

Peripheral benzodiazepine receptor ligands: mitochondrial transmembrane potential depolarization and apoptosis induction in rat C6 glioma cells

The peripheral benzodiazepine receptor (PBR) is a component of a multiprotein complex, located at the contact site between the inner and outer mitochondrial membranes, which constitutes the mitochondrial permeability transition (MPT)-pore. The opening of the MPT-pore, leading to the transmembrane mitochondrial potential (DeltaPsi(m)) dissipation, is a critical event in the mechanism of apoptosis. In the present work, we investigated the ability of the specific PBR ligands, PK 11195 or Ro5-4864, to affect mitochondrial potential and to induce apoptotic cell death in rat C6 glioma cells. Both specific ligands inhibited cell survival in a dose- and time-dependent manner, as assessed by MTS conversion assay, whereas the non-site selective ligand Diazepam or the low-affinity benzodiazepine Clonazepam showed no significant effects. After cell exposure to PK 11195 or Ro5-4864 we evidenced typical alterations of apoptotic cell death such as DNA fragmentation and chromatin condensation assessed by flow cytometric and transmission electron microscopy (TEM) analysis, respectively. Activation of the "effector" caspase-3 confirmed the ability of specific PBR ligands to induce apoptosis. Moreover, PK 11195 and Ro5-4864 induced a decrease of DeltaPsi(m), as evidenced by JC-1 flow cytometry analysis. Our data demonstrate the pro-apoptotic effects of specific PBR ligands on rat C6 glioma cells.

A manganese porphyrin complex is a novel radiation protector

Exposure of cells to ionizing radiation leads to formation of reactive oxygen species, which are associated with radiation-induced cytotoxicity. Therefore, compounds that scavenge reactive oxygen species may confer radioprotective effects. Superoxide dismutase (SOD) mimetics have been shown to be protective against cell injury caused by reactive oxygen species. The objective of this study was to investigate the effects of manganese(III) tetrakis(N-methyl-2-pyridyl)porphyrin (MnTMPyP), a cell-permeable SOD mimetic, on radiation-dependent toxicity. We investigated the protective role of MnTMPyP against ionizing radiation in U937 cells and mice. On exposure to ionizing radiation, there was a distinct difference between control cells and cells pretreated with MnTMPyP with respect to viability, cellular redox status, and oxidative damage to cells. Lipid peroxidation, oxidative DNA damage, and protein oxidation were significantly lower in the cells treated with MnTMPyP when the cells were exposed to ionizing radiation. The [GSSG]/[GSH + GSSG] ratio and the generation of intracellular reactive oxygen species were higher and the [NADPH]/[NADP+ + NADPH] ratio was lower in control cells compared with MnTMPyP-treated cells. Ionizing radiation-induced mitochondrial damage, as reflected by the altered mitochondrial permeability transition, increase in accumulation of reactive oxygen species, reduction of ATP production, and morphological change, was significantly higher in control cells than in MnTMPyP-treated cells. MnTMPyP administration for 14 days at a daily dosage of 5 mg/kg provided substantial protection against killing and oxidative damage in mice exposed to whole-body irradiation. These data indicate that MnTMPyP may have great application potential as a new class of in vivo, non-sulfur-containing radiation protectors.

Arachidonic Acid Released by Phospholipase A2 Activation Triggers Ca2+-dependent Apoptosis through the Mitochondrial Pathway

We studied the effects of the divalent cation ionophore A23187 on apoptotic signaling in MH1C1 cells. Addition of A23187 caused a fast rise of cytosolic Ca(2+) ([Ca(2+)](c)), which returned close to the resting level within about 40 s. The [Ca(2+)](c) rise was immediately followed by phospholipid hydrolysis, which could be inhibited by aristolochic acid or by pretreatment with thapsigargin in Ca(2+)-free medium, indicating that the Ca(2+)-dependent cytosolic phospholipase A(2) (cPLA(2)) was involved. These early events were followed by opening of the mitochondrial permeability transition pore (PTP) and by apoptosis in about 30% of the cell population. In keeping with a cause-effect relationship between addition of A23187, activation of cPLA(2), PTP opening, and cell death, all events but the [Ca(2+)](c) rise were prevented by aristolochic acid. The number of cells killed by A23187 was doubled by treatment with 0.5 microm MK886 and 5 microm indomethacin, which inhibit arachidonic acid metabolism through the 5-lipoxygenase and cyclooxygenase pathway, respectively. Consistent with the key role of free arachidonic acid, its levels increased within minutes of treatment with A23187; the increase being more pronounced in the presence of MK886 plus indomethacin. Cell death was preceded by cytochrome c release and cleavage of caspase 9 and 3, but not of caspase 8. All these events were prevented by aristolochic acid and by the PTP inhibitor cyclosporin A. Thus, A23187 triggers the apoptotic cascade through the release of arachidonic acid by cPLA(2) in a process that is amplified when transformation of arachidonic acid into prostaglandins and leukotrienes is inhibited. These findings identify arachidonic acid as the causal link between A23187-dependent perturbation of Ca(2+) homeostasis and the effector mechanisms of cell death.

Caspase activation precedes PTP opening in TNF-α-induced apoptosis in L929 cells

In this work, we studied the apoptotic pathway in murine fibrosarcoma cells L929 exposed to tumor necrosis factor alpha (TNF-alpha). DNA fragmentation, activation of caspases, cytochrome c release and poly (ADP-ribose) polymerase cleavage were demonstrated. We showed that the proapoptotic proteins Bid and Bax as well as caspase 8 are involved in the initiation of this apoptotic pathway triggered by TNF-alpha. Indeed, inhibition of caspase 8 could prevent TNF-alpha-induced DNA fragmentation. Furthermore, Bid and Bax translocation into mitochondria were already evidenced after 6 h. In contrast, permeability transition pore inhibitors did not prevent the DNA fragmentation induced by TNF-alpha. In addition, these events were not associated with changes in the mitochondrial membrane potential nor with the loss of ATP, which only occurred after 16 h. Taken together, these results underline the fact that TNF-alpha is able to induce caspase-dependent apoptosis in L929 in the absence of permeability transition pore opening.

TRAIL and Ceramide

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a clinically useful cytokine. TRAIL induces apoptosis in a wide variety of transformed cells, but does not cause toxicity to most normal cells. Recent studies show that death receptors (DR4 and DR5), decoy receptors (DcR1 and DcR2), and death inhibitors (FLIP, FAP-1, and IAP) are responsible for the differential sensitivity to TRAIL of normal and tumor cells. Several researchers have also shown that genotoxic agents, such as chemotherapeutic agents and ionizing radiation, enhance TRAIL-induced cytotoxicity by increasing DR5 gene expression or decreasing the intracellular level of FLIP, an antiapoptotic protein. Previous studies have shown that ceramide helps to regulate a cell's response to various forms of stress. Stress-induced alterations in the intracellular concentration of ceramide occur through the activation of a variety of enzymes that synthesize or catabolize ceramide. Increases in intracellular ceramide levels modulate apoptosis by acting through key proteases, phosphatases, and kinases. This review discusses the interaction between TRAIL and ceramide signaling pathways in regulating apoptotic death.

Ceramide in the antiapoptotic effect of ischemic preconditioning

Although the mechanism by which ischemic preconditioning (PC) inhibits myocardial apoptosis during ischemia-reperfusion is unclear, evidence indicates a role for the secondary messenger ceramide. We investigated in vivo whether PC may affect ceramide and sn-1,2-diacylglycerol (DAG) production, and attenuate apoptosis during ischemia. Rabbits underwent 30 min of ischemia, followed by 4 h of reperfusion. Before this, they received either no intervention (control group) or one episode of 5 min of ischemia, followed by 5 min of reperfusion (PC group), or an intravenous administration of the sphingomyelinase inhibitor D609. Myocardial content of ceramide and DAG was measured using the DAG kinase assay at different time points of the experiment. Apoptosis was detected and quantified by a sandwich enzyme immunoassay. Both AR and infarct size were measured using blue dye injection and triphenyltetrazolium chloride staining. Control hearts exhibited a peak of ceramide production at 5 min of the prolonged ischemia, with a mean value averaging 64 ± 5 ng/mg tissue ( P < 0.05 vs. 48 ± 4 ng/mg at baseline). In contrast, ischemic PC and D609 prevented ceramide increase during the prolonged ischemia. Myocardial DAG content was increased only in PC hearts at 30 min of ischemia. Preconditioned and D609 groups developed less apoptosis, as well as a limited infarct size, compared with the control group. These results suggest that the antiapoptotic effect of PC may be due to a reduced ceramide production during sustained ischemia in the rabbit heart.

Cyclosporin A-insensitive permeability transition in brain mitochondria: inhibition by 2-aminoethoxydiphenyl borate

The mitochondrial permeability transition pore (PTP) may operate as a physiological Ca2+ release mechanism and also contribute to mitochondrial deenergization and release of proapoptotic proteins after pathological stress, e.g. ischemia/reperfusion. Brain mitochondria exhibit unique PTP characteristics, including relative resistance to inhibition by cyclosporin A. In this study, we report that 2-aminoethoxydiphenyl borate blocks Ca2+-induced Ca2+ release in isolated, non-synaptosomal rat brain mitochondria in the presence of physiological concentrations of ATP and Mg2+. Ca2+ release was not mediated by the mitochondrial Na+/Ca2+ exchanger or by reversal of the uniporter responsible for energy-dependent Ca2+ uptake. Loss of mitochondrial Ca2+ was accompanied by release of cytochrome c and pyridine nucleotides, indicating an increase in permeability of both the inner and outer mitochondrial membranes. Under these conditions, Ca2+-induced opening of the PTP was not blocked by cyclosporin A, antioxidants, or inhibitors of phospholipase A2 or nitric-oxide synthase but was abolished by pretreatment with bongkrekic acid. These findings indicate that in the presence of adenine nucleotides and Mg2+,Ca2+-induced PTP in non-synaptosomal brain mitochondria exhibits a unique pattern of sensitivity to inhibitors and is particularly responsive to 2-aminoethoxydiphenyl borate.

Fas-associated death domain protein and caspase-8 are not recruited to the tumor necrosis factor receptor 1 signaling complex during tumor necrosis factor-induced apoptosis

Death receptors are a subfamily of the tumor necrosis factor (TNF) receptor subfamily. They are characterized by a death domain (DD) motif within their intracellular domain, which is required for the induction of apoptosis. Fas-associated death domain protein (FADD) is reported to be the universal adaptor used by death receptors to recruit and activate the initiator caspase-8. CD95, TNF-related apoptosis-inducing ligand (TRAIL-R1), and TRAIL-R2 bind FADD directly, whereas recruitment to TNF-R1 is indirect through another adaptor TNF receptor-associated death domain protein (TRADD). TRADD also binds two other adaptors receptor-interacting protein (RIP) and TNF-receptor-associated factor 2 (TRAF2), which are required for TNF-induced NF-kappaB and c-Jun N-terminal kinase activation, respectively. Analysis of the native TNF signaling complex revealed the recruitment of RIP, TRADD, and TRAF2 but not FADD or caspase-8. TNF failed to induce apoptosis in FADD- and caspase-8-deficient Jurkat cells, indicating that these apoptotic mediators were required for TNF-induced apoptosis. In an in vitro binding assay, the intracellular domain of TNF-R1 bound TRADD, RIP, and TRAF2 but did not bind FADD or caspase-8. Under the same conditions, the intracellular domain of both CD95 and TRAIL-R2 bound both FADD and caspase-8. Taken together these results suggest that apoptosis signaling by TNF is distinct from that induced by CD95 and TRAIL. Although caspase-8 and FADD are obligatory for TNF-mediated apoptosis, they are not recruited to a TNF-induced membrane-bound receptor signaling complex as occurs during CD95 or TRAIL signaling, but instead must be activated elsewhere within the cell.

Mitochondrial benzodiazepine receptors regulate oxygen homeostasis in the early mouse embryo

The peripheral benzodiazepine receptor (Bzrp) has been implicated in the control of several processes, including mitochondrial biogenesis and embryo development. The present study examined the impact that specific Bzrp ligands have on oxygen homeostasis in the early mouse embryo. Day 9 embryos at the 16-18 somite pair stage were exposed to standard (21% oxygen) and suboptimal (5% oxygen) oxygen tensions in whole embryo culture. Analysis of gene expression used relative PCR to monitor changes in nuclear respiratory factor-1 (Nrf1), mitochondrial 16S ribosomal RNA (16S rRNA), and genes for several glycolytic enzymes. Ocular development was highly sensitive to periods of hypoxia through a mechanism blocked with the potent Bzrp ligand PK11195. Hypoxia led to a decline of Nrf1 and 16S rRNA levels also through a mechanism blocked with PK11195. Similar activity was observed for FGIN-1-27 whereas Ro5-4864 had contradictory effects. Morpholino-based gene knockdown of Nrf1 (anti-NRF1) produced a sequence-specific decrease in 16S rRNA insensitive to PK11195. These functional relationships suggest that Bzrp-dependent signals regulate the Nrf1 --> Tfam1 --> mtDNA --> 16S rRNA pathway in response to oxygen levels. The activity of PK11195 most likely has a pharmacodynamic basis with regards to specific embryonic precursor target cell populations, transducing a mitochondrial signal to an Nrf1 response analogous to retrograde regulation in yeast for mitochondria-to-nucleus signaling.

Ammonia neurotoxicity: role of the mitochondrial permeability transition

Hepatic encephalopathy (HE) is an important cause of morbidity and mortality in patients with severe liver disease. Although the mechanisms responsible for HE remain elusive, ammonia is generally considered to be involved in its pathogenesis, and astrocytes are thought to be the principal target of ammonia neurotoxicity. Altered bioenergetics and oxidative stress are also thought to play a major role in this disorder. In this paper, we present data invoking the mitochondrial permeability transition (MPT) as a factor in the pathogenesis of HE/hyperammonemia. The MPT is a Ca2+-dependent, cyclosporin A (CsA) sensitive process due to the opening of a pore in the inner mitochondrial membrane that leads to a collapse of ionic gradients and ultimately to mitochondrial dysfunction. Many of the factors that facilitate the induction of the MPT are also known to be implicated in the mechanism of HE, including free radicals, Ca2+, nitric oxide, alkaline pH, and glutamine. We have recently shown that treatment of cultured astrocytes with 5 mM NH4Cl resulted in a dissipation of the mitochondrial membrane potential (delta(psi)m), which was sensitive to CsA. Similarly treated cultured neurons failed to show a loss of the delta(psi)m. Further support for the ammonia induction of the MPT was obtained by observing an increase in mitochondrial permeability to 2-deoxyglucose-6-phosphate, and a decrease in calcein fluorescence in astrocytes after ammonia treatment, both of which were also blocked by CsA. CsA was likewise capable of exerting a protective effect against hyperammonemia in mice. Taken together, our data suggest that the MPT represents an important component of the pathogenesis of HE and other hyperammonemic states.

The evolution of cell death programs as prerequisites of multicellularity

One of the hallmarks of multicellularity is that the individual cellular fate is sacrificed for the benefit of a higher order of life-the organism. The accidental death of cells in a multicellular organism results in swelling and membrane-rupture and inevitably spills cell contents into the surrounding tissue with deleterious effects for the organism. To avoid this form of necrotic death the cells of metazoans have developed complex self-destruction mechanisms, collectively called programmed cell death, which see to an orderly removal of superfluous cells. Since evolution never invents new genes but plays variations on old themes by DNA mutations, it is not surprising, that some of the genes involved in metazoan death pathways apparently have evolved from homologues in unicellular organisms, where they originally had different functions. Interestingly some unicellular protozoans have developed a primitive form of non-necrotic cell death themselves, which could mean that the idea of an altruistic death for the benefit of genetically identical cells predated the invention of multicellularity. The cell death pathways of protozoans, however, show no homology to those in metazoans, where several death pathways seem to have evolved in parallel. Mitochondria stands at the beginning of several death pathways and also determines, whether a cell has sufficient energy to complete a death program. However, the endosymbiotic bacterial ancestors of mitochondria are unlikely to have contributed to the recent mitochondrial death machinery and therefore, these components may derive from mutated eukaryotic precursors and might have invaded the respective mitochondrial compartments. Although there is no direct evidence, it seems that the prokaryotic-eukaryotic symbiosis created the space necessary for sophisticated death mechanisms on command, which in their distinct forms are major factors for the evolution of multicellular organisms.