Nitric oxide prevents tumor necrosis factor alpha-induced rat hepatocyte apoptosis by the interruption of mitochondrial apoptotic signaling through S-nitrosylation of caspase-8

Oxidants and antioxidants in alcohol-induced liver disease

Although there are numerous experimental data indicating that oxidative stress plays a role in the initiation and progression of alcohol-induced liver disease (ALD), this work has yet to translate into an accepted antioxidant therapy for ALD in humans. With a better understanding of the mechanisms by which oxidative stress leads to liver damage during alcohol exposure, therapies that are more targeted at the cellular/molecular level may be applied in the clinic with potentially greater success. This article discusses the general concepts of oxidative stress and how it relates to current hypotheses in alcohol-induced liver injury, as well as lists several key questions that remain to be addressed in this field: (1) Which prooxidants are involved in ALD? (2) What are the sources of prooxidants in the liver during alcohol exposure? (3) How are oxidants involved in alcohol-induced liver injury? (4) Can a rational and effective antioxidant therapy against ALD be developed?

Expression of inducible nitric oxide synthase in paired neoplastic and non-neoplastic primary prostate cell cultures and prostatectomy specimen

Nitric oxide (NO) is an important signaling molecule for ischemia, inflammation, angiogenesis, immune response, and cell growth and differentiation. It has recently been shown that increased production of NO within various human cancers may contribute to tumor angiogenesis, tumor growth and metastasis, and tumor-related immune suppression. NO can be produced by several NO synthases (NOS), including inducible synthase (iNOS), which is expressed during cell activation and produces NO in larger quantity and for a longer period of time than non-inducible NOSs. In this study, we examined the expression levels of iNOS mRNA and protein in prostate adenocarcinoma using a paired nonneoplastic and neoplastic primary prostate cell culture system and related prostatectomy specimens. Six pairs of neoplastic and nonneoplastic primary prostate cell cultures were established from radical prostatectomy specimens based on homogeneity of the originating tumor and the nonneoplastic tissue. Radioactive reverse transcriptase polymerase chain reaction and subsequent quantitative analysis of iNOS mRNA were performed on the cultures using beta-actin as an internal control. Immunohistochemical studies with an anti-iNOS monoclonal antibody were performed on the corresponding formalin-fixed paraffin-embedded prostatectomy tissue sections. We observed marked patient-to-patient variation in "normal" levels of iNOS mRNA. However, all six neoplastic cultures showed moderately to markedly higher mRNA levels than did their paired nonneoplastic cultures. In addition, iNOS protein levels were significantly higher in paraffin-embedded prostate cancer tissue sections than in adjacent nonneoplastic tissue. Overexpression of iNOS mRNA and protein levels is present in moderately differentiated prostate adenocarcinoma and may contribute to prostate cancer angiogenesis, tumor growth, and tumor-related immunosuppression.

The nitric oxide donor, V-PYRRO/NO, protects against acetaminophen-induced hepatotoxicity in mice

The liver-selective nitric oxide (NO) donor, O(2)-vinyl 1-(pyrrolidin-1-yl)diazen-1-ium-1,2-diolate (V-PYRRO/NO), is metabolized by P-450 enzymes to release NO in the liver, and is shown to protect the liver from tumor necrosis factor alpha (TNF-alpha)-induced apoptosis and D-glactosamine/endotoxin-induced hepatotoxicity. This study was undertaken to examine the effects of V-PYRRO/NO on acetaminophen-induced hepatotoxicity in mice. Mice were given V-PYRRO/NO via osmotic pumps (1.8-5.4 mg/mL, 8 microL/h) 4 to 16 hours before a hepatotoxic dose of acetaminophen (600 mg/kg, intraperitoneally [ip]). V-PYRRO/NO administration dramatically reduced acetaminophen-induced hepatotoxicity in a dose- and time-dependent manner, as evidenced by reduced serum alanine aminotransferase (ALT) activity, reduced hepatic congestion, apoptosis, and improved hepatocellular pathology. The protection afforded by V-PYRRO/NO does not appear to be caused by a decrease in the formation of toxic acetaminophen metabolites, which consumes glutathione (GSH), because V-PYRRO/NO did not alter acetaminophen-induced hepatic GSH depletion. Acetaminophen-induced lipid peroxidation, as determined by the concentrations of 4-hydroxyalkenals (4-HNE) and malondialdehyde (MDA), was reduced significantly by V-PYRRO/NO treatment. Although pretreatment was most effective, administration of V-PYRRO/NO simultaneously with acetaminophen also was able to reduce acetaminophen hepatotoxicity. Genomic analysis of the liver samples 10 hours after acetaminophen intoxication showed the enhanced expression of genes associated with stress/oxidative stress, apoptosis/cell death, and DNA damage/repair. Acetaminophen-induced alterations in gene expression were attenuated significantly by V-PYRRO/NO. Real-time reverse-transcription polymerase chain reaction (RT-PCR) and Western-blot analysis confirmed microarray results. In conclusion, V-PYRRO/NO is effective in blocking acetaminophen-induced hepatotoxicity in mice. This protection may involve the reduction of oxidative stress, the inhibition of apoptosis, and possibly the maintenance of hepatic vasculature to prevent congestion.

Nitric oxide in liver transplantation: pathobiology and clinical implications

The gaseous molecule nitric oxide is involved in a variety of liver transplant-relevant processes, including ischemia-reperfusion injury, acute cellular rejection, and circulatory changes characteristic of advanced liver disease. This review article focuses on new advances relating to the role of nitric oxide in these syndromes with an emphasis on pathobiology and potential clinical implications.

Inducible nitric oxide synthase (iNOS) and endothelial nitric oxide synthase (eNOS) expression in fulminant hepatic failure

BACKGROUND/AIMS

Inducible nitric oxide synthase (iNOS) and endothelial nitric oxide synthase (eNOS) have important functions in inflammation and vasoregulation but their role in fulminant hepatic failure (FHF) is not well understood.

METHODS

Intrahepatic in situ staining and semi-quantification of iNOS and eNOS by immunohistochemistry in 25 patients with FHF, in 40 patients with chronic liver diseases (CLD) and in ten normal controls (NC).

RESULTS

Expression patterns of iNOS and eNOS differed. While in NC only faint iNOS expression was found in some Kupffer cells/macrophages and hepatocytes, eNOS was expressed constitutively in sinusoidal and vascular endothelial cells. In CLD, iNOS expression was induced in Kupffer cells/macrophages and hepatocytes, representing the main iNOS expressing cell types. Additionally, bile ducts, vascular endothelial cells and lymphocytes also expressed iNOS (P = 0.001). In contrast, no differences were found between eNOS expression in CLD and NC (P = 0.64). The same cell types expressed eNOS and iNOS in FHF but numbers of both were significantly enhanced, exceeding the levels seen in CLD (P < 0.001, P = 0.017).

CONCLUSIONS

Our data demonstrate that iNOS and eNOS are differently regulated in physiologic conditions and in liver disease. While eNOS seems to be involved in the physiological regulation of hepatic perfusion, strong upregulation of iNOS might contribute to inflammatory processes in FHF.

Hepatocyte Fas-associating death domain protein/mediator of receptor-induced toxicity (FADD/MORT1) levels increase in response to pro-apoptotic stimuli

We examined the regulation of Fas-associating death domain (FADD) protein as an important adaptor molecule in apoptosis signaling and hypothesized that the regulation of FADD could contribute to hepatocyte death. FADD/mediator of receptor-induced toxicity (MORT1) is required for activation of several signaling pathways of cell death. In this study we report the interesting and unexpected result that actinomycin D increased the expression of FADD protein, and we demonstrate that other cellular stresses like ultraviolet irradiation or heat shock could also increase FADD levels in hepatocytes. In cells treated with actinomycin D, FADD levels were elevated homogeneously in the cytoplasm. The increase in cytoplasmic FADD protein by actinomycin D or FADD overexpression alone both correlated with cell death, and specific antisense inhibition of FADD expression consistently diminished approximately 30% of the cell death induced by actinomycin D. These data indicate that FADD protein expression can increase rapidly in hepatocytes exposed to broadly cytotoxic agents.

Role of mitochondrial inner membrane permeabilization in necrotic cell death, apoptosis, and autophagy

Inhibition of mitochondrial oxidative phosphorylation progresses to uncoupling when opening of cyclosporin A-sensitive permeability transition pores increases permeability of the mitochondrial inner membrane to small solutes. Involvement of the mitochondrial permeability transition (MPT) in necrotic and apoptotic cell death is implicated by demonstrations of protection by cyclosporin A against oxidative stress, ischemia/reperfusion, tumor necrosis factor-alpha exposure, Fas ligation, calcium overload, and a variety of toxic chemicals. Confocal microscopy directly visualizes the MPT in single mitochondria within living cells from the translocation of impermeant fluorophores, such as calcein, across the inner membrane. Simultaneously, mitochondria release potential-indicating fluorophores. Subsequently, mitochondria swell, causing outer membrane rupture and release of cytochrome c and other proapoptotic proteins from the intermembrane space. In situ a sequence of decreased NAD(P)H, increased free calcium, and increased reactive oxygen species formation within mitochondria promotes the MPT and subsequent cell death. Necrotic and apoptotic cell death after the MPT depends, in part, on ATP levels. If ATP levels fall profoundly, glycine-sensitive plasma membrane permeabilization and rupture ensue. If ATP levels are partially maintained, apoptosis follows the MPT. The MPT also signals mitochondrial autophagy, a process that may be important in removing damaged mitochondria. Cellular features of necrosis, apoptosis, and autophagy frequently occur together after death signals and toxic stresses. A new term, necrapoptosis, describes such death processes that begin with a common stress or death signal, progress by shared pathways, but culminate in either cell lysis (necrosis) or programmed cellular resorption (apoptosis), depending on modifying factors such as ATP.

Spontaneous apoptosis in primary cultures of human and rat hepatocytes: molecular mechanisms and regulation by dexamethasone

To elucidate the biochemical pathways leading to spontaneous apoptosis in primary cultures of human and rat hepatocytes, we examined the activation of the caspase cascade, the expression of Bcl-2-related-proteins and heat shock proteins. Comparisons were made before and after dexamethasone (DEX) treatment. We show that DEX inhibited spontaneous apoptosis in a dose-dependent manner. DEX increases the expression of anti-apoptotic Bcl-2 and Bcl-x(L) proteins, decreases the expression of pro-apoptotic Bax and inhibits Bad translocation thereby preventing the release of cytochrome c, the activation of caspases, and cell death. Although, the expression of Hsp27 and Hsp70 proteins remained unchanged, the oncogenic protein c-Myc is upregulated upon DEX-treatment. These results indicate that DEX mediates its survival effect against spontaneous apoptosis by acting upstream of the mitochondrial changes. Thus, the mitochondrial apoptotic pathway plays a major role in regulating spontaneous apoptosis in these cells. Blocking this pathway therefore may assist with organ preservation for transplant, drug screening, and other purposes.

iNOS enhances rat intestinal apoptosis after ischemia-reperfusion

The aim of this study was to demonstrate (i) the role of iNOS (inducible nitric oxide synthase) on apoptosis in the rat intestinal mucosa after ischemia-reperfusion, and (ii) the effect of iNOS on the release of cytochrome c from mitochondria. The superior mesenteric artery was occluded for 60 min and was followed by a 60 min reperfusion. Rats were pretreated with an intraperitoneal injection of the following iNOS inhibitors: N-nitro-L-arginine methyl ester, aminoguanidine, and (1S,5S,6R,7R)-7- chloro-3-imino-5-methyl-2-azabicyclo [4. 1. 0] heptane hydrochloride (ONO-1714). Apoptosis was evaluated and NO(X) in the portal vein was assayed. The amount of iNOS, caspase-3, and cytochrome c were determined by a Western blot analysis. Intestinal mucosal epithelial mitochondrial dehydrogenase activity was assessed using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoilium bromide. Ischemia-reperfusion increased intestinal mucosal apoptosis, NO(X) production in the portal vein, the amount of iNOS protein, and the release of cytochrome c, but not caspase-3. Inhibitors of iNOS significantly attenuated the induction of apoptosis, increased NO(X) production, and release of cytochrome c. Mitochondrial dysfunction was induced by ischemia-reperfusion, which was ameliorated by iNOS inhibitors. Our results indicate that iNOS is related to increased mucosal apoptosis in the rat small intestine after ischemia-reperfusion, which is partly explained by the release of cytochrome c from mitochondria to cytosols following mitochondrial dysfunction.

Nitric oxide ameliorates hydrophobic bile acid-induced apoptosis in isolated rat hepatocytes by non-mitochondrial pathways

Hydrophobic bile acids are toxic to isolated rat hepatocytes by mechanisms involving mitochondrial dysfunction and oxidative stress. In the current study we examined the role of nitric oxide (NO), a potential mediator of apoptosis, during bile acid-induced apoptosis. Freshly isolated rat hepatocytes and hepatic mitochondria generated NO and peroxynitrite (ONOO(-)) in a concentration- and time-dependent manner when exposed to the toxic bile salt glycochenodeoxycholate (GCDC) (25-500 microm), which was prevented by the nitric-oxide synthase (NOS) inhibitors N(G)-monomethyl-N-arginine monoacetate (l-NMMA) and 1400W. Relationships between hepatocyte NO production and apoptosis were examined by comparing the effects of NOS inhibitors with other inhibitors of GCDC-induced apoptosis. Inhibitors of caspases 8 and 9, the mitochondrial permeability transition blocker cyclosporin A, and the antioxidant idebenone reduced NO generation and apoptosis in GCDC-treated hepatocytes. In contrast, NOS inhibitors had no effect on GCDC-induced apoptosis despite marked reduction of NO and ONOO(-). However, treatment with the NO donors S-nitroso-N-acetylpenicillamine and spermine NONOate [N-(-aminoethyl)N-(2-hydroxy-2-nitrohydrazino)-1,2-ethylenediamine) inhibited apoptosis and caspase 3 activity while significantly elevating NO levels above GCDC-stimulated levels. Neither NO donors nor NOS inhibitors affected GCDC-induced mitochondrial permeability transition or cytochrome c release from liver mitochondria or GCDC-induced mitochondrial depolarization from isolated hepatocytes, suggesting that NO inhibits bile acid-induced hepatocyte apoptosis by a non-mitochondrial-dependent pathway. In conclusion, whereas NO produced from GCDC-treated hepatocytes neither mediates nor protects against bile acid-induced apoptosis, higher levels of NO inhibit GCDC-induced hepatocyte apoptosis by caspase-dependent pathways.

Regulation and measurement of oxidative stress in apoptosis

Cells are constantly generating reactive oxygen species (ROS) during aerobic metabolism. As a consequence, each cell is equipped with an extensive antioxidant defence system to combat excessive production of ROS. Oxidative stress occurs in cells when the generation of ROS overwhelms the cell's natural antioxidant defences. There is a growing consensus that oxidative stress and the redox state of a cell plays a pivotal role in regulating apoptosis, a tightly controlled form of cell death in which a cell partakes in its own demise. More recently, a role for reactive nitrogen species (RNI) as both positive and negative regulators of cell death has been established. This review describes the major sources of ROS and RNI in a cell, the control of cell death by these species and the role of antioxidants as regulators of oxidative stress and apoptosis. Finally, the various methods that can be employed in establishing a role for both ROS and RNI in apoptosis will be discussed with particular emphasis on their intracellular detection.

Cytokine regulation of pro- and anti-apoptotic genes in rat hepatocytes: NF-kappaB-regulated inhibitor of apoptosis protein 2 (cIAP2) prevents apoptosis

BACKGROUND/AIMS

In acute liver failure, hepatocytes are exposed to various cytokines that activate both cell survival and apoptotic pathways. NF-kappaB is a central transcription factor in these responses. Recent studies indicate that blocking NF-kappaB causes apoptosis, indicating the existence of NF-kappaB-regulated anti-apoptotic genes. In the present study the relationship between NF-kappaB activation and apoptosis has been investigated in hepatocytes.

METHODS

Primary rat hepatocytes were exposed to a cytokine mixture of tumor necrosis factor alpha, interleukin-1beta, interferon-gamma and lipopolysaccharide. Modulation of signalling pathways was performed by using dominant negative adenoviral constructs. Apoptosis and NF-kappaB activation were determined by caspase-3 activity, Hoechst staining and electrophoretic mobility shift assay, respectively. Furthermore, expression and regulation of apoptosis-related genes were investigated.

RESULTS

(1) Inhibition of NF-kappaB activation results in apoptosis. (2) Inhibitor of apoptosis protein (IAP) family members, inhibitor of apoptosis protein1 (cIAP1), and X-chromosome-linked IAP, are expressed in rat hepatocytes. cIAP2 is induced by cytokines in an NF-kappaB-dependent manner and overexpression of cIAP2 inhibits apoptosis. (3) The anti-apoptotic Bcl-2 family member A1/Bfl-1 and the pro-apoptotic members Bak and Bid are induced by cytokines and NF-kappaB-dependent. (4) Nitric oxide inhibits caspase-3 activity in hepatocytes.

CONCLUSIONS

In inflammatory conditions, hepatocyte survival is dependent on NF-kappaB activation and cIAP2 contributes significantly to this protection.

Regulation of caspases by nitric oxide

Nitric oxide can prevent or induce apoptosis depending on its concentration, cell type, and the oxidative milieu. Nitric oxide inhibits apoptosis and inflammation by S-nitrosylation of the active site cysteine of caspases, the central effector molecules of cell death as well as maturation of IL-1beta and IL-18. The ability of nitric oxide to S-nitrosylate caspases depends on multiple factors including the presence of free iron and intracellular redox potential. There are no known direct effects of nitric oxide on promoting caspase activation or activity. However, nitric oxide has been shown to promote apoptotic pathways in numerous cell types through the indirect activation of caspases. In this article we review the relationship of nitric oxide and caspase activity, modulation of this effect by iron, and clinical implications for the use of nitric oxide in regulating inflammation and apoptosis.

Mechanisms of hepatoprotection by nitric oxide

Nitric oxide (NO) exerts numerous antiapoptotic effects on hepatocytes in settings of inflammation and tissue damage. These actions of NO are modulated by a variety of mechanisms under both physiologic and pathologic conditions. Nitric oxide inhibits cell death or apoptosis by modulation of heat shock proteins, S-nitrosylation of caspases at their catalytic site cysteine residue, triggering of the cGMP pathway, and prevention of mitochondrial dysfunction. Our preliminary studies also suggest that NO can modulate apoptosis-related genes in a manner consistent with an antiapoptotic effect. This review focuses on these molecular mechanisms of cytoprotection by NO.

Chronic ethanol exposure potentiates lipopolysaccharide liver injury despite inhibiting Jun N-terminal kinase and caspase 3 activation

Although ethanol is known to sensitize hepatocytes to tumor necrosis factor (TNF) lethality, the mechanisms involved remain controversial. Recently, others have shown that adding TNFalpha to cultures of ethanol-pretreated hepatocytes provokes the mitochondrial permeability transition, cytochrome c release, procaspase 3 activation, and apoptosis. Although this demonstrates that ethanol can sensitize hepatocytes to TNF-mediated apoptosis, the hepatic inflammation and ballooning hepatocyte degeneration that typify alcohol-induced liver injury suggest that other mechanisms might predominate in vivo. To evaluate this possibility, acute responses to lipopolysaccharide (LPS), a potent inducer of TNFalpha, were compared in mice that had been fed either an ethanol-containing or control diet for 5 weeks. Despite enhanced induction of cytokines such as interleukin (IL)-10, IL-15, and IL-6 that protect hepatocytes from apoptosis, ethanol-fed mice exhibited a 4-5-fold increase in serum alanine aminotransferase after LPS, confirming increased liver injury. Six h post-LPS histology also differed notably in the two groups, with control livers demonstrating only scattered apoptotic hepatocytes, whereas ethanol-exposed livers had large foci of ballooned hepatocytes, inflammation, and scattered hemorrhage. No caspase 3 activity was noted during the initial 6 h after LPS in ethanol-fed mice, but this tripled by 1.5 h after LPS in controls. Procaspase 8 cleavage and activity of the apoptosis-associated kinase, Jun N-terminal kinase, were also greater in controls. In contrast, ethanol exposure did not inhibit activation of cytoprotective mitogen-activated protein kinases and AKT or attenuate induction of the anti-apoptotic factors NF-kappaB and inducible nitric oxide synthase. Consistent with these responses, neither cytochrome c release, an early apoptotic response, nor hepatic oligonucleosomal DNA fragmentation, the ultimate consequence of apoptosis, was increased by ethanol. Thus, ethanol exacerbates TNF-related hepatotoxicity in vivo without enhancing caspase 3-dependent apoptosis.

Nitric oxide and apoptosis during human head and neck squamous cell carcinoma development

PURPOSE

Apoptosis index (AI), Bcl-2, and Bax have shown prognostic significance in head and neck squamous cell carcinoma (HNSCCa). Other areas of research have implicated nitric oxide (NO) or its various intermediate species in both proapoptotic and antiapoptotic processes. We have previously shown that NO-generating enzymes are significantly increased during the stepwise progression to HNSCCa. The aim of this study was to explore the interrelationship of NO and a known consequence of NO-related oxidative stress, apoptosis, during this step-wise process.

MATERIALS AND METHODS

Formalin fixed-paraffin embedded tissue samples of 10 normal oral mucosa, 15 reactive/dysplastic lesions, and 17 HNSCCa lesions studied previously were subjected to the terminal deoxynucleotidyl transferase (TdT)-mediated dUTP labeling (TUNEL) assay as well as immunohistochemical staining against Bcl-2, Bax, and p53. Patient charts were reviewed and clinical data were compared. The study pathologist (G.K.H) reviewed these slides blinded to patient identifiers or clinical data. The number of immunopositive cell nuclei or staining intensity was graded, noting the pattern of immunostaining. These staining characteristics were compared with the results of immunostaining previously obtained for endothelial constitutive NO synthase (ecNOS) and nitrotyrosine.

RESULTS

Compared with normal oral mucosa, the AI, Bcl-2, Bax, Bcl-2/Bax intensity and frequency ratios, and mutant p53 intensity significantly changed in reactive/dysplastic and HNSCCa lesions (P <.001 for all). Correlations between the staining characteristics of the antigens studied are presented. Furthermore, perilesional inflammatory cells showed staining in the TUNEL assay.

CONCLUSIONS

In a set of tissue samples previously well characterized, these new findings implicate a link between NO and the induction of apoptotic cell death in HNSCCa development.

O(2)-Vinyl 1-(pyrrolidin-1-yl)diazen-1-ium-1,2-diolate protection against D-galactosamine/endotoxin-induced hepatotoxicity in mice: genomic analysis using microarrays

O(2)-Vinyl 1-(pyrrolidin-1-yl)diazen-1-ium-1,2-diolate (V-PYRRO/NO), a liver-selective nitric oxide (NO)-donating prodrug, is metabolized by hepatic enzymes to release NO within the liver. This study was undertaken to examine the effects of V-PYRRO/NO on D-galactosamine/lipopolysaccharide (GlaN/LPS)-induced liver injury in mice. Mice were given injections of V-PYRRO/NO (10 mg/kg, s.c. at 2-h intervals) before and after GlaN/LPS (700 mg/30 microg/kg, i.p.). V-PYRRO/NO administration dramatically reduced GlaN/LPS-induced hepatotoxicity, as evidenced by reduced serum alanine aminotransferase activity and improved pathology. To examine the mechanisms of the protection, cDNA microarray was performed to profile the gene expression pattern in livers of mice treated with GlaN/LPS, GlaN/LPS plus V-PYRRO/NO, or controls. V-PYRRO/NO administration greatly ameliorated GlaN/LPS-induced alterations in the expression of genes encoding the stress response, DNA damage/repair response, and drug-metabolizing enzymes in accordance with hepatoprotection. Gel shift assay and Western blot analysis supported microarray results, showing that V-PYRRO/NO suppressed GlaN/LPS-induced activation of nuclear factor-kappaB and GlaN/LPS-induced increases in caspase-1, caspase-8, tumor necrosis factor receptor 1 (TNFR1)-associated death domain, and TNF-related apoptosis-inducing ligand. Immunohistochemical analysis further revealed that GlaN/LPS-induced activation of TNFR1, caspase-3, and hepatocellular apoptosis was ameliorated by V-PYRRO/NO treatment. GlaN/LPS-induced elevation of hepatic caspase-3 activity was diminished by V-PYRRO/NO treatment. In addition, V-PYRRO/NO alone suppressed the basal expression of genes encoding inducible NO synthase and TNF-alpha-related components, as revealed by mouse 1.2 array. In summary, this study demonstrates that the liver-selective NO donor, V-PYRRO/NO, is effective in blocking GlaN/LPS-induced hepatotoxicity in mice, and that this protection appears to involve, at least in part, the suppression of the TNF-alpha-mediated cell death pathways.

Accelerated s-nitrosothiol breakdown by amyotrophic lateral sclerosis mutant copper,zinc-superoxide dismutase

Mutations in copper,zinc-superoxide dismutase (SOD) have been implicated in familial amyotrophic lateral sclerosis (FALS). We have investigated the breakdown of S-nitrosothiols by wild-type (WT) SOD and two common FALS mutants, alanine-4 valine (A4V) SOD and glycine-37 arginine (G37R) SOD. In the presence of glutathione, A4V SOD and G37R SOD catalyzed S-nitrosoglutathione breakdown three times more efficiently than WT SOD. Indeed, A4V SOD catabolized GSNO more efficiently than WT SOD throughout the physiological range of GSH concentrations. Moreover, a variety of additional S-nitrosothiols were catabolized more readily by A4V SOD than by WT SOD. Initial rate data for fully reduced WT SOD and A4V SOD, and data using ascorbic acid as the reductant, suggest that FALS mutations in SOD may influence the efficiency of reduction of the copper center by glutathione. We have identified a potentially toxic gain of function of two common FALS mutations that may contribute to neurodegeneration in FALS.

S-Nitrosylation of mitochondrial caspases

Caspase-3 is a cysteine protease located in both the cytoplasm and mitochondrial intermembrane space that is a central effector of many apoptotic pathways. In resting cells, a subset of caspase-3 zymogens is S-nitrosylated at the active site cysteine, inhibiting enzyme activity. During Fas-induced apoptosis, caspases are denitrosylated, allowing the catalytic site to function. In the current studies, we sought to identify the subpopulation of caspases that is regulated by S-nitrosylation. We report that the majority of mitochondrial, but not cytoplasmic, caspase-3 zymogens contain this inhibitory modification. In addition, the majority of mitochondrial caspase-9 is S-nitrosylated. These studies suggest that S-nitrosylation plays an important role in regulating mitochondrial caspase function and that the S-nitrosylation state of a given protein depends on its subcellular localization.

Differential protection with inhibitors of caspase-8 and caspase-3 in murine models of tumor necrosis factor and Fas receptor-mediated hepatocellular apoptosis

Excessive apoptosis has been implicated in a number of acute and chronic human diseases. The activation of caspases has been shown to be critical for the apoptotic process. The objective of this investigation was to evaluate the beneficial effects and mechanism of action of the caspase-8 inhibitor IETD-CHO and the caspase-3 inhibitor DEVD-CHO against tumor necrosis factor (TNF)-induced hepatocellular apoptosis in vivo and compare these results to effects of the same inhibitors against Fas-induced apoptosis. Treatment of C3Heb/FeJ mice with 700 mg/kg galactosamine/100 microg/kg endotoxin induced parenchymal apoptosis (indicated by caspase-3 activation and morphology) and severe liver injury (indicated by the increase in plasma alanine aminotransferase activities and histology) at 7 h. Treatment with IETD-CHO or DEVD-CHO (10 mg/kg at 3, 4.5, and 5.5 h) significantly attenuated caspase-3 activation and liver injury. Western analysis showed that DEVD-CHO had no effect while IETD-CHO substantially reduced procaspase-3 and procaspase-9 processing. On the other hand, caspase-3 activation and liver injury by the anti-Fas antibody Jo-2 was completely prevented by a single dose of DEVD-CHO and, as previously shown, by IETD-CHO at 90 min. Both inhibitors prevented procaspase-3 and procaspase-9 processing. Thus, there are fundamental differences in the efficacy of caspase inhibitors in these two models. We conclude that Fas may rely exclusively on caspase-8 activation and mitochondria to activate caspase-3, which can process more procaspase-8 and thus propagate the amplification of the apoptotic signal. TNF can activate a similar signaling pathway. However, alternative signaling mechanisms seem to exist, which can compensate if the main pathway is blocked.

Nitric oxide in gastrointestinal epithelial cell carcinogenesis: linking inflammation to oncogenesis

Chronic inflammation of gastrointestinal tissues is a well-recognized risk factor for the development of epithelial cell-derived malignancies. Although the inflammatory mediators linking chronic inflammation to carcinogenesis are numerous, current information suggests that nitric oxide (NO) contributes to carcinogenesis during chronic inflammation. Inducible nitric oxide synthase (iNOS), expressed by both macrophages and epithelial cells during inflammation, generates the bioreactive molecule NO. In addition to causing DNA lesions, NO can directly interact with proteins by nitrosylation and nitosation reactions. The consequences of protein damage by NO appear to be procarcinogenic. For example, NO inhibits DNA repair enzymes such as human 8-oxodeoxyguanosine DNA glycosylase 1 and blocks apoptosis via nitrosylation of caspases. These cellular events permit DNA damage to accumulate, which is required for the numerous mutations necessary for development of invasive cancer. NO also promotes cancer progression by functioning as an angiogenesis factor. Strategies to inhibit NO generation during chronic inflammation or to scavenge reactive nitrogen species may prove useful in decreasing the risk of cancer development in chronic inflammatory gastrointestinal diseases.

Pancreastatin, a chromogranin A-derived peptide, inhibits DNA and protein synthesis by producing nitric oxide in HTC rat hepatoma cells

BACKGROUND/AIMS

Pancreastatin, a chromogranin A-derived peptide, has a counter-regulatory effect on insulin action. We have previously characterized pancreastatin receptor and signalling in rat liver and HTC hepatoma cells. A G alpha(q/11)-PLC-beta pathway leads to an increase in [Ca2+]i, PKC and mitogen activated protein kinase (MAPK) activation. These data suggested that pancreastatin might have a role in growth and proliferation, similar to other calcium-mobilizing hormones.

METHODS

DNA and protein synthesis were measured as [3H]-thymidine and [3H]-leucine incorporation. Nitric oxide (NO) was determined by the Griess method and cGMP production was quantified by enzyme-linked immunoassay.

RESULTS

Contrary to the expected results, we have found that pancreastatin inhibits protein and DNA synthesis in HTC hepatoma cells. On the other hand, when the activity of NO synthase was inhibited by N-monomethyl-L-arginine (NMLA), the inhibitory effect of pancreastatin on DNA and protein synthesis was not only reverted, but a dose-dependent stimulatory effect was observed, probably due to MAPK activation, since it was prevented by PD98059. These data strongly suggested the role of NO in the inhibitory effect of pancreastatin on protein and DNA synthesis, which is overcoming the effect on MAPK activation. Moreover, pancreastatin dose-dependently increased NO production in parallel to cyclic guanosine monophosphate (cGMP). Both effects were prevented by NMLA. Finally, an indirect effect of pancreastatin through the induction of apoptosis was ruled out.

CONCLUSIONS

Therefore, the NO and the cGMP produced by the NO-activated guanylate cyclase may mediate the dose-dependent inhibitory effect of pancreastatin on growth and proliferation in HTC hepatoma cells.

Nitric oxide and the liver

In recent years, the role of nitric oxide (NO) in the pathogenesis of liver disease and its complications has been extensively studied. There remain, however, many areas of controversy. In particular, the effect of NO on vascular function in the systemic circulation and the hepatic microcirculation has received the greatest attention. It has been proposed on the one hand that increased NO synthesis is responsible for the development of the hyperdynamic circulation in cirrhosis, while decreased production of NO within the hepatic microcirculation may be important in the development of parenchymal tissue damage and the onset of portal hypertension. The purpose of this review is to examine the available data concerning the role of NO in liver disease and to discuss some of the controversies and contradictions that surround it.

Activation of pro-death Bcl-2 family proteins and mitochondria apoptosis pathway in tumor necrosis factor-alpha-induced liver injury

Tumor necrosis factor-alpha (TNFalpha)-induced cytotoxicity contributes to the pathogenesis in inflammatory and immune responses. Here, we studied the role of pro-death Bcl-2 family proteins and the mitochondria apoptosis pathway in the development of TNFalpha-induced hepatic injury during endotoxemia. After treating mice with lipopolysaccharide or TNFalpha in the presence of d-galactosamine, Bid was cleaved and translocated to mitochondria in hepatocytes. Independently, Bax was also activated by the death receptor engagement and translocated to mitochondria. However, its subsequent insertion into the mitochondrial membrane depends on Bid. Nevertheless, Bid was required, but Bax could be dispensed for the mitochondrial release of cytochrome c from mitochondria, suggesting that Bid could activate additional downstream molecules other than Bax. The lack of this Bid-dependent mitochondria activation and cytochrome c release in the bid-deficient mice was responsible for the significantly delayed effector caspase activation and hepatocyte injury upon endotoxin treatment, culminating in a prolonged survival of the bid-deficient mice. Additional genetic factor(s) could further modify the dependence of TNFalpha toxicity on the mitochondria pathway as the bid-deficient 129/SvJ mice manifested an even higher resistance than the same type of mice in C57BL/6 background. The functional significance of the mitochondria apoptosis pathway was thus elucidated in the TNFalpha-mediated pathogenesis in vivo.

Nitric oxide as a bioregulator of apoptosis

Nitric oxide (NO), synthesized from l-arginine by NO synthases, is a small, diffusible, highly reactive molecule with dichotomous regulatory roles under physiological and pathological conditions. NO can promote apoptosis (proapoptosis) in some cells, whereas it inhibits apoptosis (antiapoptosis) in other cells. This complexity is a consequence of the rate of NO production and the interaction with biological molecules such as iron, thiols, proteins, and reactive oxygen species. Long-lasting production of NO acts as a proapoptotic modulator by activating caspase family proteases through the release of mitochondrial cytochrome c into the cytosol, upregulation of p53 expression, activation of JNK/SAPK, and altering the expression of apoptosis-associated proteins including Bcl-2 family proteins. However, low or physiological concentrations of NO prevent cells from apoptosis induced by trophic factor withdrawal, Fas, TNFalpha, and lipopolysaccharide. The antiapoptotic mechanism can be understood via expression of protective genes such as heat shock proteins, Bcl-2 as well as direct inhibition of the apoptotic caspase family proteases by S-nitrosylation of the cysteine thiol. Our current understanding of the mechanisms by which NO exerts both pro- and antiapoptotic actions is discussed in this review article.

Prooxidant and antioxidant functions of nitric oxide in liver toxicity

In response to tissue damage and inflammation induced by a variety of xenobiotics including acetaminophen, carbon tetrachloride, ethanol, galactosamine, and endotoxin, as well as disease states such as viral hepatitis, and postischemic and regenerative injury, the liver produces large quantities of nitric oxide. Indeed, nearly all cell types in the liver including hepatocytes, Kupffer cells, stellate cells, and endothelial cells have the capacity to generate nitric oxide. Thus, these cells, as well as infiltrating leukocytes, may indirectly augment tissue injury. In many models of liver damage, nitric oxide and its oxidation products such as peroxynitrite contribute to the injury process by directly damaging the tissue or by initiating additional immunologic reactions that result in damage. In some models, nitric oxide donors or peroxynitrite can mimic the cytotoxic actions of liver toxins. Moreover, agents that prevent the generation of nitric oxide or antioxidants that bind reactive nitrogen intermediates, or knockout mice with reduced capacity to produce nitric oxide, are protected from xenobiotic-induced tissue injury. In contrast, there have been reports that blocking nitric oxide production enhances xenobiotic-induced tissue injury. This has led to the concept that nitric oxide either inactivates proteins critical for xenobiotic-induced tissue injury or acts as an antioxidant, reducing cellular levels of cytotoxic reactive oxygen intermediates. Whether or not nitric oxide or secondary oxidants generated from nitric oxide act as mediators of tissue injury or protect against toxicity is likely to depend on the precise targets of these reactive nitrogen intermediates, as well as levels of superoxide anion present and the extent to which tissue injury is mediated by reactive oxygen intermediates. In addition, as toxicity is a complex process involving a variety of cell types and many soluble mediators, the contribution of each of these factors must be taken into account when considering the role of nitric oxide as a determinant of tissue injury.

Protection by nitric oxide against liver inflammatory injury in animals carrying a nitric oxide synthase-2 transgene

The effect of pre-existent hepatic NO synthesis on liver injury induced by lipopolysaccharide was studied in animals carrying a nitric oxide synthase-2 (NOS-2) transgene under the control of the phosphoenolpyruvate carboxykinase (PEPCK) promoter. These animals expressed NOS-2 in liver cells under fasting conditions. Lipopolysaccharide-induced liver injury in D-galactosamine-conditioned mice, which enhanced notably the effect of the endotoxin on the liver, was impaired in animals expressing NOS-2. This protection against inflammatory liver damage was dependent on NO synthesis and was caused by an inhibition of nuclear factor kB (NF-kB) activity and an impairment of the synthesis of the proinflammatory cytokines tumor necrosis factor a and interleukin 1b. These data indicate that intrahepatic synthesis of NO protects liver by inhibiting the release of cascades of proinflammatory mediators and suggest a beneficial role for local delivery of NO in the control of liver injury.

The effect of nitric oxide on cell respiration: A key to understanding its role in cell survival or death

The mitochondrion is a key organelle in the control of cell death. Nitric oxide (NO) inhibits complex IV in the respiratory chain and is reported to possess both proapoptotic and antiapoptotic actions. We investigated the effects of continuous inhibition of respiration by NO on mitochondrial energy status and cell viability. Serum-deprived human T cell leukemia (Jurkat) cells were exposed to NO at a concentration that caused continuous and complete (approximately 85%) inhibition of respiration. Serum deprivation caused progressive loss of mitochondrial membrane potential (Deltapsi(m)) and apoptotic cell death. In the presence of NO, Deltapsi(m) was maintained compared to controls, and cells were protected from apoptosis. Similar results were obtained by using staurosporin as the apoptotic stimulus. As exposure of serum-deprived cells to NO progressed (>5 h), however, Deltapsi(m) fell, correlating with the appearance of early apoptotic features and a decrease in cell viability. Glucose deprivation or iodoacetate treatment of cells in the presence of NO resulted in a collapse of Deltapsi(m), demonstrating involvement of glycolytic ATP in its maintenance. Under these conditions cell viability also was decreased. Treatment with oligomycin and/or bongkrekic acid indicated that the maintenance of Deltapsi(m) during exposure to NO is caused by reversal of the ATP synthase and other electrogenic pumps. Thus, blockade of complex IV by NO initiates a protective action in the mitochondrion to maintain Deltapsi(m) this results in prevention of apoptosis. It is likely that during cellular stress involving increased generation of NO this compound will trigger a similar sequence of events, depending on its concentration and duration of release.