Mechanisms of peroxynitrite-induced apoptosis in HL-60 cells

Expression of tumor necrosis factor-α and inducible nitric oxide synthase in acetaminophen-induced liver injury

AIM: To investigate the role of inflammation in the pathogenesis of acetaminophen-induced liver injury.

METHODS: A model of acetaminophen-induced liver injury was developed in SD rats. The rats were killed at 3, 6, 12, and 24 h after acetaminophen injection to take blood samples for measuring serum ALT and tumor necrosis factor-α (TNF-α) levels and hepatic tissue samples for evaluating pathological changes by HE staining and detecting the expression of TNF-α and inducible nitric oxide synthase (iNOS) by immunohistochemistry and Western blot.

RESULTS: Compared to control rats, serum ALT (nKat/L) progressively increased (3 h: 1166.90 ± 151.03 vs 586.78 ± 89.35; 6 h: 2153.84 ± 254.55 vs 573.45 ± 75.18; 12 h: 4220.84 ± 928.52 vs 750.15 ± 81.68; 24 h: 13202.64 ± 1392.78 vs 780.16 ± 161.37; all P < 0.01); liver pathological damage was progressively exacerbated and peaked at 24 h; serum TNF-α (µg/L) significantly increased at 24 h after administration (5.69 ± 0.46 vs 2.64 ± 0.27, P < 0.01) and showed a significant positive correlation with serum ALT levels (r = 0.773, P < 0.01); the expression of TNF-α and iNOS in the liver was significantly increased in rats treated with acetaminophen.

CONCLUSION: Inflammation plays a key role in the development of acetaminophen-induced liver injury.

Nitric oxide in paraquat-mediated toxicity: A review

Paraquat, a cationic herbicide, produces degenerative lesions in the lung and in the nervous system after systemic administration to man and animals. Many cases of acute poisoning and death have been reported over the past few decades. Although a definitive mechanism of toxicity of paraquat has not been delineated, a cyclic single electron reduction/oxidation is a critical mechanistic event. The redox cycling of paraquat has two potentially important consequences relevant to the development of toxicity: the generation of the superoxide anion, which can lead to the formation of more toxic reactive oxygen species which are highly reactive to cellular macromolecules; and the oxidation of reducing equivalents (e.g., NADPH, reduced glutathione), which results in the disruption of important NADPH-requiring biochemical processes necessary for normal cell function. Nitric oxide is an important signaling molecule that reacts with superoxide derived from the paraquat redox cycle, to form the potent oxidant peroxynitrite, which causes serious cell damage. Although nitric oxide has been involved in the mechanism of paraquat-mediated toxicity, the role of nitric oxide has been controversial as both protective and harmful effects have been described. The present review summarizes recent findings in the field and describes new knowledge on the role of nitric oxide in the paraquat-mediated toxicity.

Involvement of peroxynitrite in capsaicin-induced apoptosis of C6 glioma cells

Capsaicin induces apoptosis in some types of cells, but its mechanism remains obscure. In this study, peroxynitrite, a powerful oxidant generated from the reaction of superoxide and nitric oxide (NO) in biological system, was demonstrated to be responsible for capsaicin-mediated apoptosis in C6 glioma cells. Capsaicin-induced apoptosis was detected by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) assay, and also identified by Annexin V staining and comet assay. Capsazepine and ruthenium red, the vanilloid receptor 1 (VR1/TPRV1) antagonists, did not inhibit capsaicin-induced apoptosis. Exposure to capsaicin not only promoted the generation of superoxide and iNOS, but also markedly suppressed the expression of SODs. Nitrite and nitrate, the NO metabolites accumulated in the medium, and the nitrotyrosine was also increased in proteins of C6 glioma cells exposed to capsaicin. Pretreatment of cells with 4 microM ebselen (a peroxynitrite scavenger) showed effective inhibitory effect on the capsaicin-induced apoptosis. These results suggest that peroxynitrite can act as a potential mediator in the capsaicin-induced apoptosis in C6 glioma cells.

Cytoprotective response of A1, a Bcl-2 homologue expressed in mature human neutrophils and promyelocytic HL-60 cells, to oxidant stress-induced cell death

The ability to generate reactive oxidative intermediates is one of the quintessential properties of mature human neutrophils. Endogenously generated oxidants have been shown to be an important mechanism underlying neutrophil cell death. In acute lung inflammation, newly recruited neutrophils further encounter external oxidants, including reactive oxygen and nitrogen intermediates. In our present study, we showed that A1, a constitutive and inducible Bcl-2 homologue expressed in mature circulating human neutrophils, might confer the protection from hydrogen peroxide (H(2)O(2))- and peroxynitrite (ONOO)-induced cell death. Utilizing the myeloid precursor cell line, HL-60, we further examined the hypothesis that A1 was capable of conferring cytoprotective activity against these oxidative stresses. Whereas the control-transfected HL-60 cells expressed small amounts of A1 and were sensitive to the biologically relevant, cell death-inducing oxidants, H(2)O(2) and ONOO, the stable transfectants that overexpressed A1 were significantly more tolerant. Furthermore, there was a correlation between the level of A1 expression and the antiapoptotic activity. Thus, our results suggest a cytoprotective role of A1 in mature human neutrophils under oxidant stresses in host defense and inflammation.

Effect of a potent iNOS inhibitor (ONO-1714) on acetaminophen-induced hepatotoxicity in the rat

Overproduction of nitric oxide (NO) in the liver has been implicated as an important event in endotoxin shock and in other models of hepatic inflammation and injury. The present study was undertaken to evaluate the effect of ONO-1714, a potent and specific inhibitor of inducible NO synthase (iNOS), on acetaminophen-induced hepatotoxicity in the rats. Oral administration of ONO-1714 dose-dependently inhibited NOx (NO2- and NO3-) accumulation in rat plasma after lipopolysaccharide (LPS) treatment. Intraperitoneal acetaminophen at 1 g/kg caused damage to the centrilobular regions of the liver and increase in serum alanine and aspartate transaminase (ALT and AST, respectively) levels accompanied by elevated plasma NOx levels after 24 h. Oral administration of ONO-1714 at 10 and 100 microg/kg dose-dependently reduced the acetaminophen-induced hepatic tissue damage and the increases in serum ALT and AST levels. ONO-1714 also blocked the increase in plasma NOx concentrations. These findings demonstrate that oral ONO-1714, an iNOS inhibitor, protects against acetaminophen-evoked hepatic inflammation/injury, strongly suggesting that NO produced by iNOS plays a key role in the pathogenesis of this drug-induced hepatotoxicity.

Hyperoxia induces retinal vascular endothelial cell apoptosis through formation of peroxynitrite

Hyperoxia exposure induces capillary endothelial cell apoptosis in the developing retina, leading to vaso-obliteration followed by proliferative retinopathy. Previous in vivo studies have shown that endothelial nitric oxide synthase (NOS3) and peroxynitrite are important mediators of the vaso-obliteration. Now we have investigated the relationship between hyperoxia, NOS3, peroxynitrite, and endothelial cell apoptosis by in vitro experiments using bovine retinal endothelial cells (BREC). We found that BREC exposed to 40% oxygen (hyperoxia) for 48 h underwent apoptosis associated with activation of caspase-3 and cleavage of the caspase substrate poly(ADP-ribose) polymerase. Hyperoxia-induced apoptosis was associated with increased formation of nitric oxide, peroxynitrite, and superoxide anion and was blocked by treatment with uric acid, nitro-L-arginine methyl ester, or superoxide dismutase. Analyses of the phosphatidylinositol 3-kinase/Akt kinase survival pathway in cells directly treated with peroxynitrite revealed inhibition of VEGF- and basic FGF-induced activation of Akt kinase. These results suggest that hyperoxia-induced formation of peroxynitrite induces BREC apoptosis by crippling key survival pathways and that blocking peroxynitrite formation prevents apoptosis. These data may have important clinical implications for infants at risk of retinopathy of prematurity.

Peroxynitrite signaling: receptor tyrosine kinases and activation of stress-responsive pathways

Peroxynitrite, generated for example in inflammatory processes, is capable of nitrating and oxidizing biomolecules, implying a considerable impact on the integrity of cellular structures. Cells respond to stressful conditions by the activation of signaling pathways, including receptor tyrosine kinase-dependent pathways such as mitogen-activated protein kinases and the phosphoinositide-3-kinase/Akt pathway. Peroxynitrite affects signaling pathways by nitration as well as by oxidation: while nitration of tyrosine residues by peroxynitrite modulates signaling processes relying on tyrosine phosphorylation and dephosphorylation, oxidation of phosphotyrosine phosphatases may lead to an alteration in the tyrosine phosphorylation/dephosphorylation balance. The flavanol (-)-epicatechin is a potent inhibitor of tyrosine nitration and may be employed as a tool to distinguish signaling effects due to tyrosine nitration from those that are due to oxidation reactions.

A novel inhibitor of inducible nitric oxide synthase (ONO-1714) prevents critical warm ischemia-reperfusion injury in the pig liver

BACKGROUND

Recently, a novel inhibitor of inducible nitric oxide synthase, ONO-1714, was developed. We evaluated the effect of ONO-1714 on a critical warm I/R model of the pig liver.

METHODS

Pigs were subjected to 180 min of hepatic warm I/R under the extracorporeal circulation. We investigated the time course of changes in the serum NO2- + NO3- (NOx), the cellular distribution of endothelial and inducible nitric oxide synthase, thrombocyte-thrombi, and nitrotyrosine by immunohistochemistry. The hepatic tissue blood flow (HTBF) was measured continuously using a laser-Doppler blood flowmeter.

RESULTS

ONO-1714 at 0.05 mg/kg improved the survival rate from 54 (control group) to 100%. The serum NOx levels in the ONO-1714 group were significantly lower than those in the control group at 1, 1.5, 2, 3, and 6 hr after reperfusion. The serum aspartate aminotransferase (AST) and lactate dehydrogenase (LDH) levels of the ONO-1714 group were significantly lower than the control group, and the HTBF of the ONO-1714 group was significantly higher than the control group. The formation of thrombocyte-thrombi and nitrotyrosine after reperfusion was significantly lower in the ONO-1714 group.

CONCLUSIONS

These results indicated that ONO-1714 improved the survival rates and attenuated I/R injury in a critical hepatic warm I/R model of the pig. ONO-1714 will be beneficial for hepatectomy or liver transplantation in the clinical field.

Oxidant-induced signaling: effects of peroxynitrite and singlet oxygen

Following the requirement for cells to cope with oxidative stress, there are cellular adaptation mechanisms at the level of gene expression. Much of what is known about oxidant-induced signaling in mammalian cells was found in experiments using hydrogen peroxide as an oxidant. However, since the biochemical reactivities of various oxidants significantly differ, 'oxidative stress' is not necessarily identical independent of the oxidant employed to bring it about. Here, the biological actions of peroxynitrite and singlet oxygen are presented, focusing on signaling effects. Peroxynitrite is generated in biological systems in the diffusion-controlled reaction of superoxide with nitrogen monoxide and is thus likely to be produced in the vicinity of activated macrophages. Singlet oxygen is generated by stimulated neutrophils in vivo and may further be generated photochemically, e.g. upon exposure of cells to ultraviolet A radiation. Exposure of cells to either of these oxidants elicits a cellular stress response, entailing the activation of signaling cascades that regulate proliferative and apoptotic responses, such as mitogen-activated protein kinase cascades or the phosphoinositide 3-kinase/Akt cascade. Two mechanisms for the oxidant-induced activation of a signaling cascade may be envisaged: (i) the indirect targeting of the cascade by interrupting negative regulation, and (ii) an activating oxidation of one of the constituting components of the cascade. Examples for both mechanisms in relation to peroxynitrite and singlet oxygen are discussed.