Glutathione depletion renders rat hepatocytes sensitive to nitric oxide donor-mediated toxicity

Inhibition of hepatic gluconeogenesis in type 2 diabetes by metformin: complementary role of nitric oxide

Metformin is the first-line treatment for type 2 diabetes mellitus. Type 2 diabetes mellitus is associated with decreased nitric oxide bioavailability, which has significant metabolic implications, including enhanced insulin secretion and peripheral glucose utilization. Similar to metformin, nitric oxide also inhibits hepatic glucose production, mainly by suppressing gluconeogenesis. This review explores the combined effects of metformin and nitric oxide on hepatic gluconeogenesis and proposes the potential of a hybrid metformin-nitric oxide drug for managing type 2 diabetes mellitus. Both metformin and nitric oxide inhibit gluconeogenesis through overlapping and distinct mechanisms. In hepatic gluconeogenesis, mitochondrial oxaloacetate is exported to the cytoplasm via various pathways, including the malate, direct, aspartate, and fumarate pathways. The effects of nitric oxide and metformin on the exportation of oxaloacetate are complementary; nitric oxide primarily inhibits the malate pathway, while metformin strongly inhibits the fumarate and aspartate pathways. Furthermore, metformin effectively blocks gluconeogenesis from lactate, glycerol, and glutamine, whereas nitric oxide mainly inhibits alanine-induced gluconeogenesis. Additionally, nitric oxide contributes to the adenosine monophosphate-activated protein kinase-dependent inhibition of gluconeogenesis induced by metformin. The combined use of metformin and nitric oxide offers the potential to mitigate common side effects. For example, lactic acidosis, a known side effect of metformin, is linked to nitric oxide deficiency, while the oxidative and nitrosative stress caused by nitric oxide could be counterbalanced by metformin's enhancement of glutathione. Metformin also amplifies nitric oxide -induced activation of adenosine monophosphate-activated protein kinase. In conclusion, a metformin-nitric oxide hybrid drug can benefit patients with type 2 diabetes mellitus by enhancing the inhibition of hepatic gluconeogenesis, decreasing the required dose of metformin for maintaining optimal glycemia, and lowering the incidence of metformin-associated lactic acidosis.

Pathogenic role of oxidative and nitrosative stress in primary biliary cirrhosis

Primary biliary cirrhosis is a multifactor autoimmune disease characterized by hepatic and systemic manifestations, with immune system dysregulation and abnormalities in the hepatic metabolism of bile salts, lipids, and nutrients, as well as destruction of membrane lipids and mitochondrial dysfunction. Both oxidative and nitrosative stress are associated with ongoing manifestations of the disease. In particular, abnormalities in nitric oxide metabolism and thiol oxidation already occur at early stages, thus leading to the hypothesis that these biochemical events play a pathogenic role in primary biliary cirrhosis. Moreover, the association of these metabolic abnormalities with the progression of the disease may indicate some biochemical parameters as early diagnostic markers of disease evolution, and may open up the potential for pharmacological intervention to inhibit intra- and extra-cellular stress events for resuming hepatocellular functions. The following paragraphs summarize the current knowledge by outlining molecular mechanisms of the disease related to these stress events.

Regulation of mitochondrial processes by protein S-nitrosylation

BACKGROUND

Nitric oxide (NO) exerts powerful physiological effects through guanylate cyclase (GC), a non-mitochondrial enzyme, and through the generation of protein cysteinyl-NO (SNO) adducts-a post-translational modification relevant to mitochondrial biology. A small number of SNO proteins, generated by various mechanisms, are characteristically found in mammalian mitochondria and influence the regulation of oxidative phosphorylation and other aspects of mitochondrial function.

SCOPE OF REVIEW

The principles by which mitochondrial SNO proteins are formed and their actions, independently or collectively with NO binding to heme, iron-sulfur centers, or to glutathione (GSH) are reviewed on a molecular background of SNO-based signal transduction.

MAJOR CONCLUSIONS

Mitochondrial SNO-proteins have been demonstrated to inhibit Complex I of the electron transport chain, to modulate mitochondrial reactive oxygen species (ROS) production, influence calcium-dependent opening of the mitochondrial permeability transition pore (MPTP), promote selective importation of mitochondrial protein, and stimulate mitochondrial fission. The ease of reversibility and the affirmation of regulated S-nitros(yl)ating and denitros(yl)ating enzymatic reactions support hypotheses that SNO regulates the mitochondrion through redox mechanisms. SNO modification of mitochondrial proteins, whether homeostatic or adaptive (physiological), or pathogenic, is an area of active investigation.

GENERAL SIGNIFICANCE

Mitochondrial SNO proteins are associated with mainly protective, bur some pathological effects; the former mainly in inflammatory and ischemia/reperfusion syndromes and the latter in neurodegenerative diseases. Experimentally, mitochondrial SNO delivery is also emerging as a potential new area of therapeutics. This article is part of a Special Issue entitled: Regulation of cellular processes by S-nitrosylation.

Evidence for the role of oxidative stress in the acetylation of histone H3 by ethanol in rat hepatocytes

The relationship between ethanol-induced oxidative stress and acetylation of histone H3 at lysine 9 (H3AcK9) remains unknown and was therefore investigated in primary cultures of rat hepatocytes. Cells were treated with ethanol, and a select group of pharmacological agents and the status of H3AcK9 and reactive oxygen species (ROS) were monitored. Pretreatment of hepatocytes with N-acetyl cystein (ROS reducer), or dietary antioxidants (quercetin, reserveratrol), or NADPH (reduced nicotinamide adenine dinucleotide phosphate) oxidase inhibitor apocynin, significantly reduced ethanol (50 mM, 24 h) induced increases in ROS and H3AcK9. In contrast, l-buthionine sulfoximine (ROS inducer) and inhibitor of mitochondrial complexes I (rotenone) and III (antimycin) increased ethanol-induced H3AcK9 (P<.01). Oxidative stress also affected ethanol-induced alcohol dehydrogenase 1 mRNA expression. These results demonstrate for the first time that oxidative stress is involved in the ethanol-induced histone H3 acetylation in hepatocytes.

EPR studies of O(2)(*-), OH, and (1)O(2) scavenging and prevention of glutathione depletion in fibroblast cells by cyanidin-3-rhamnoglucoside isolated from fig (Ficus carica L.) fruits

Cyanidin-3-rhamnoglucoside (C3R) is the major anthocyanin in fresh fig fruits. In this study, the free radical scavenging potential of C3R was evaluated in vitro using several free radical generators. This naturally occurring anthocyanin was superior to other tested natural antioxidants in scavenging ABTS(*+). Electron paramagnetic resonance served to determine the scavenging properties of C3R toward superoxide radical anion (O(2)(*-)), hydroxyl radical ((*)OH), and singlet radical ((1)O(2)). The protection of NIH-3T3 fibroblast cells was then tested as the inhibition of reactive oxygen species (ROS) formation in a dose-dependent manner. It was further demonstrated that treatment with C3R elevates the reduced glutathione (GSH) concentration and the redox ratio (GSH/GSSG) in fibroblast cells in a dose-dependent manner. Moreover, C3R reduced the induction of ROS by butathionine sulfoximine (BSO) and elevated the redox ratio. Thus, it is suggested that C3R in fresh fig fruits is a potent scavenger and may influence endogenous antioxidant systems of consumers.

Intracellular glutathione mediates the denitrosylation of protein nitrosothiols in the rat spinal cord

Protein S-nitrosothiols (PrSNOs) have been implicated in the pathophysiology of neuroinflammatory and neurodegenerative disorders. Although the metabolically instability of PrSNOs is well known, there is little understanding of the factors involved in the cleavage of S-NO linkage in intact cells. To address this issue, we conducted chase experiments in spinal cord slices incubated with S-nitrosoglutathione (GSNO). The results show that removal of GSNO leads to a rapid disappearance of PrSNOs (t(1/2) approximately 2 hr), which is greatly accelerated when glutathione (GSH) levels are raised with the permeable analogue GSH ethyl ester. Moreover, PrSNOs are stable in the presence of the GSH depletor diethyl maleate, indicating that GSH is critical for protein denitrosylation. Inhibition of GSH-dependent enzymes (glutathione S-transferase, glutathione peroxidase, and glutaredoxin) and enzymes that could mediate denitrosylation (alcohol dehydrogense-III, thioredoxin and protein disulfide isomerase) do not alter the rate of PrSNO decomposition. These findings and the lack of protein glutathionylation during the chase indicate that most proteins are denitrosylated via rapid transnitrosylation with GSH. The differences in the denitrosylation rate of individual proteins suggest the existence of additional structural factors in this process. This study is relevant to our recent discovery that PrSNOs accumulate in the central nervous system of patients with multiple sclerosis.

Computational insights on the competing effects of nitric oxide in regulating apoptosis

Despite the establishment of the important role of nitric oxide (NO) on apoptosis, a molecular-level understanding of the origin of its dichotomous pro- and anti-apoptotic effects has been elusive. We propose a new mathematical model for simulating the effects of nitric oxide (NO) on apoptosis. The new model integrates mitochondria-dependent apoptotic pathways with NO-related reactions, to gain insights into the regulatory effect of the reactive NO species N(2)O(3), non-heme iron nitrosyl species (FeL(n)NO), and peroxynitrite (ONOO(-)). The biochemical pathways of apoptosis coupled with NO-related reactions are described by ordinary differential equations using mass-action kinetics. In the absence of NO, the model predicts either cell survival or apoptosis (a bistable behavior) with shifts in the onset time of apoptotic response depending on the strength of extracellular stimuli. Computations demonstrate that the relative concentrations of anti- and pro-apoptotic reactive NO species, and their interplay with glutathione, determine the net anti- or pro-apoptotic effects at long time points. Interestingly, transient effects on apoptosis are also observed in these simulations, the duration of which may reach up to hours, despite the eventual convergence to an anti-apoptotic state. Our computations point to the importance of precise timing of NO production and external stimulation in determining the eventual pro- or anti-apoptotic role of NO.

A possible role of nrf2 in prevention of renal oxidative damage by ferric nitrilotriacetate

To ascertain the possible roles of nuclear erythroid 2 p45-related factor 2 (Nrf2), a key transcription factor of phase 2 drug-metabolizing enzymes, in renal cellular defense against oxidative stress, wild-type and Nrf2-knockout -/- mice were treated with ferric nitrilotriacetate (Fe-NTA) at doses of 3 or 6 mg iron/kg body weight. After Fe-NTA treatment, Nrf2 -/- mice consistently showed lower levels of glutathione (GSH) in the kidney at the low dose and the liver at the high dose than the wild-type mice. Gamma-glutamylcysteine ligase (GCL) activity in the kidney and liver of Nrf2-/- mice was also consistently lower than in wild-type mice after the Fe-NTA treatment. Histopathological examination revealed that nephrotoxicity of Fe-NTA, reflected in necrosis of renal tubule epithelial cells following nuclear damage, was more severe in the Nrf2-/- mice than in their wild-type counterparts. Overall, the data suggest that Nrf2 -/- mice are unable to compensate for depletion of renal GSH because of oxidative stress, being more susceptible to Fe-NTA-induced nephrotoxicity. In conclusion, the present study showed that Nrf2 might play an important role in protecting cells from oxidative stress in the kidney through its regulation of antioxidant enzymes.

Decreased hepatosplanchnic antioxidant uptake during hepatic ischaemia/reperfusion in patients undergoing liver resection

Oxidative stress mediates cell injury during ischaemia/reperfusion. On the other hand, experimental findings suggest that ROS (reactive oxygen species) induce processes leading to ischaemic preconditioning. The extent and source of oxidative stress and its effect on antioxidant status in the human liver during intermittent ischaemia and reperfusion remains ill-defined. Therefore the aim of the present study was to investigate the occurrence of oxidative stress in humans undergoing liver resection. Liver biopsies, and arterial and hepatic venous blood samples were taken from ten patients undergoing hepatectomy with an intermittent Pringle manoeuvre. Plasma MDA (malondialdehyde) and hepatic GSSG levels were measured as markers of oxidative stress and plasma uric acid as a marker of xanthine oxidase activity. In addition, changes in hepatosplanchnic consumption of plasma antioxidants and hepatic levels of carotenoids and glutathione (GSH) were measured. After ischaemia, hepatosplanchnic release of MDA and increased hepatic GSSG levels were found. This was accompanied by the release of uric acid, reflecting xanthine oxidase activity. During reperfusion, ongoing oxidative stress was observed by further increases in hepatic GSSG content and hepatosplanchnic MDA release. Uric acid release was minimal during reperfusion. A gradual decrease in plasma antioxidant capacity and net hepatosplanchnic antioxidant uptake was observed upon prolonged cumulative ischaemia. Oxidative stress occurs during hepatic ischaemia in man mainly due to xanthine oxidase activity. Interestingly, the gradual decline in plasma antioxidant capacity and net hepatosplanchnic antioxidant uptake during prolonged cumulative ischaemia, preserved both hydrophilic and lipophilic hepatic antioxidant levels. Decreasing plasma levels and net hepatosplanchnic uptake of plasma antioxidants may warrant antioxidant supplementation, although it should be clarified to what extent limitation of oxidative stress compromises ROS-dependent pathways of ischaemic preconditioning.

Mutual changes of thioredoxin and nitrosothiols during biliary cirrhosis: results from humans and cholestatic rats

Cholestasis is associated with changes in NO metabolism and thiol oxidation. Thioredoxin contributes to regulate vascular tone and intracellular redox status by cleaving nitrosothiols and maintaining -SH groups. This study investigated the changes in circulating thioredoxin and nitrosothiols and the relationship with protein sulfhydryls (PSH), hepatic concentrations, hyaluronate, and histology in patients with primary biliary cirrhosis (PBC) and in rats with bile duct ligation (BDL). PSH in erythrocytes were significantly decreased in stage III and IV PBC and at day 10 after BDL. Compared with controls, erythrocyte thioredoxin levels were higher in stage I through III PBC and lower in stage IV patients. Serum thioredoxin levels were significantly higher in PBC stages I and II and lower in stages III and IV. Serum nitrosothiols were higher in all PBC patients and inversely related to thioredoxin and hyaluronate. In rats, serum, hepatic, and mitochondrial thioredoxin had initially increased after BDL (day 1-3) and then decreased. After day 7 BDL, nitrosothiols were 10-fold increased in serum and liver, and even higher in mitochondria. In the liver, thioredoxin was inversely related to both nitrosothiols and PSH. In rats, the difference in time average changes from baseline among serum, hepatic, and erythrocyte thioredoxin suggests that most of circulating thioredoxin originates from the liver.

CONCLUSION

Our findings indicate that cholestasis is associated with significant mutual and interrelated changes between circulating and hepatic thioredoxin and nitrosothiols. The increase of hepatic, mitochondrial, and circulating nitrosothiols with ongoing cholestasis suggests an active participation of NO in both liver injury and extrahepatic changes.

Cytotoxicity induced by grape seed proanthocyanidins: role of nitric oxide

Grape seed proanthocyanidin extract (GPSE) at high doses has been shown to exhibit cytotoxicity that is associated with increased apoptotic cell death. Nitric oxide (NO), being a regulator of apoptosis, can be increased in production by the administration of GSPE. In a chick cardiomyocyte study, we demonstrated that high-dose (500 microg/ml) GSPE produces a significantly high level of NO that contributes to increased apoptotic cell death detected by propidium iodide and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) staining. It is also associated with the depletion of intracellular glutathione (GSH), probably due to increased consumption by NO with the formation of S-nitrosoglutathione. Co-treatment with L-NAME, a NO synthase inhibitor, results in reduction of NO and apoptotic cell death. The decline in reduced GSH/oxidized GSH (GSSG) ratio is also reversed. N-Acetylcysteine, a thiol compound that reacts directly with NO, can reduce the increased NO generation and reverse the decreased GSH/GSSG ratio, thereby attenuating the cytotoxicity induced by high-dose GSPE. Taken together, these results suggest that endogenous NO synthase (NOS) activation and excessive NO production play a key role in the pathogenesis of high-dose GSPE-induced cytotoxicity.

Reduced glutathione levels and expression of the enzymes of glutathione synthesis in cryopreserved hepatocyte monolayer cultures

Cryopreservation of monolayers of hepatocytes in a freezing medium containing 10% (v/v) dimethylsulfoxide, 90% (v/v) foetal calf serum retains cell morphology and viability, but cells lose up to 50% of their intracellular reduced glutathione. This is accompanied by a small increase in glutamate cysteine ligase expression in cryopreserved cultures, but glutathione synthetase expression is undetectable post-cryopreservation. Inclusion of ascorbic acid and alpha-tocopherol in the freezing medium improves maintenance of reduced glutathione content post-cryopreservation at 84% of the levels in non-cryopreserved monolayer cultures, but does not restore glutathione synthetase expression. The inability to synthesise reduced glutathione will mean that cryopreserved hepatocyte monolayers are more susceptible to toxic insults.

Midbrain neuronal cultures from parkin mutant mice are resistant to nitric oxide-induced toxicity

Nitric oxide (NO) is a modulator of differentiation and survival of dopamine (DA) neurons. NO may play a role in the pathogenesis of Parkinson's disease (PD) since its levels are increased in parkinsonian brains and it can nitrosylate and alter the function of key proteins involved in the pathogenesis of PD. NO producing neurons are spared in parkinsonian brains suggesting that toxicity by NO can be compensated. Furthermore, the neurotoxic or neurotrophic effects of NO on DA neurons depend on the balance between NO levels and the intracellular levels of glutathione (GSH). We have investigated the effects of NO-donating agents on midbrain neuronal cultures from parkin-deficient mice. Parkin mutations are the most common genetic deficit observed in hereditary parkinsonism. These mice have abnormal DA release and metabolism, increased production of free radicals and a compensatory elevation of GSH. Cultures from parkin knockout (PK-KO) mice were more resistant than those of wild type (WT) to the neurotoxicity by NO, and the difference of susceptibility applied equally to DA, GABA and total number of neurons, and to astrocytes. NO-induced cell death was mainly apoptotic and could be reduced by caspase inhibitors. Cultures from PK-KO had greater levels of GSH than WT and, after treatment with NO, greater levels of S-nitrosoglutathione. The differences in susceptibility disappear when the synthesis of GSH is inhibited or the GSH chelated with diethyl maleate. Our data show that, contrary to the expectations, and related to the enhanced production of GSH in parkin knockout mice, parkin-deficient dopamine neurons are less susceptible to toxicity by NO.

Inducible nitric oxide synthase up-regulation and mitochondrial glutathione depletion mediate cyanide-induced necrosis in mesencephalic cells

We have previously shown in rat primary cultured mesencephalic cells that cyanide induces a high level of oxidative stress and necrotic death. To evaluate the mechanism of the cytotoxicity, the effects of cyanide on intracellular glutathione (GSH) pools and inducible nitric oxide synthase (iNOS)-mediated reactive nitrogen species (RNS) generation were studied. Cyanide rapidly depleted intracellular GSH. Restoration of GSH blocked cell death, whereas depletion of GSH by synthesis inhibition increased the necrosis. Selective depletion of mitochondrial GSH (mtGSH) increased oxidative stress and enhanced cell death, whereas the cytoplasmic pool was not critical to cell survival. These actions were accompanied by increased iNOS expression as determined by Western blot analysis, RT-PCR and immunohistochemistry. Up-regulation of iNOS led to increased generation of NO as reflected by elevated nitrite levels (an end product of NO metabolism). It was determined by use of a selective inhibitor that up-regulation of iNOS expression was transcriptionally regulated by activation of nuclear factor-kappaB, a redox-sensitive transcription factor. It was concluded that, in cyanide-mediated neurotoxicity, mtGSH is a vital component of the cellular antioxidant defense, and its depletion can lead to oxidative stress-mediated iNOS up-regulation, thus enhancing RNS generation and necrosis.