Adaptive resistance to nitric oxide in motor neurons

Role of nerve-muscle interactions and reactive oxygen species in regulation of muscle proteostasis with ageing

Skeletal muscle ageing is characterised by atrophy, a deficit in specific force generation, increased susceptibility to injury, and incomplete recovery after severe damage. The hypothesis that increased generation of reactive oxygen species (ROS) in vivo plays a key role in the ageing process has been extensively studied, but remains controversial. Skeletal muscle generates ROS at rest and during exercise. ROS can cause oxidative damage particularly to proteins. Indeed, products of oxidative damage accumulate in skeletal muscle during ageing and the ability of muscle cells to respond to increased ROS becomes defective. The aim of this review is to examine the evidence that ROS manipulation in peripheral nerves and/or muscle modifies mechanisms of proteostasis in skeletal muscle and plays a key role in initiating sarcopenia.

Role of reactive oxygen species in age-related neuromuscular deficits

Although it is now clear that reactive oxygen species (ROS) are not the key determinants of longevity, a number of studies have highlighted the key role that these species play in age-related diseases and more generally in determining individual health span. Age-related loss of skeletal muscle mass and function is a key contributor to physical frailty in older individuals and our current understanding of the key areas in which ROS contribute to age-related deficits in muscle is through defective redox signalling and key roles in maintenance of neuromuscular integrity. This topical review will describe how ROS stimulate adaptations to contractile activity in muscle that include up-regulation of short-term stress responses, an increase in mitochondrial biogenesis and an increase in some catabolic processes. These adaptations occur through stimulation of redox-regulated processes that lead to the activation of transcription factors such as NF-κB, AP-1 and HSF1 which mediate changes in gene expression. They are attenuated during ageing and this appears to occur through an age-related increase in mitochondrial hydrogen peroxide production. The potential for redox-mediated cross-talk between motor neurons and muscle is also described to illustrate how ROS released from muscle fibres during exercise may help maintain the integrity of axons and how the degenerative changes in neuromuscular structure that occur with ageing may contribute to mitochondrial ROS generation in skeletal muscle fibres.

Reactive oxygen species in sarcopenia: Should we focus on excess oxidative damage or defective redox signalling?

Physical frailty in the elderly is driven by loss of muscle mass and function and hence preventing this is the key to reduction in age-related physical frailty. Our current understanding of the key areas in which ROS contribute to age-related deficits in muscle is through increased oxidative damage to cell constituents and/or through induction of defective redox signalling. Recent data have argued against a primary role for ROS as a regulator of longevity, but studies have persistently indicated that aspects of the aging phenotype and age-related disorders may be mediated by ROS. There is increasing interest in the effects of defective redox signalling in aging and some studies now indicate that this process may be important in reducing the integrity of the aging neuromuscular system. Understanding how redox-signalling pathways are altered by aging and the causes of the defective redox homeostasis seen in aging muscle provides opportunities to identify targeted interventions with the potential to slow or prevent age-related neuromuscular decline with a consequent improvement in quality of life for older people.

Mechanisms of nitric oxide crosstalk with reactive oxygen species scavenging enzymes during abiotic stress tolerance in plants

Nitric oxide (NO) acts in a concentration and redox-dependent manner to counteract oxidative stress either by directly acting as an antioxidant through scavenging reactive oxygen species (ROS), such as superoxide anions (O(2)(-)*), to form peroxynitrite (ONOO(-)) or by acting as a signaling molecule, thereby altering gene expression. NO can interact with different metal centres in proteins, such as heme-iron, zinc-sulfur clusters, iron-sulfur clusters, and copper, resulting in the formation of a stable metal-nitrosyl complex or production of varied biochemical signals, which ultimately leads to modification of protein structure/function. The thiols (ferrous iron-thiol complex and nitrosothiols) are also involved in the metabolism and mobilization of NO. Thiols bind to NO and transport it to the site of action whereas nitrosothiols release NO after intercellular diffusion and uptake into the target cells. S-nitrosoglutathione (GSNO) also has the ability to transnitrosylate proteins. It is an NO˙ reservoir and a long-distance signaling molecule. Tyrosine nitration of proteins has been suggested as a biomarker of nitrosative stress as it can lead to either activation or inhibition of target proteins. The exact molecular mechanism(s) by which exogenous and endogenously generated NO (or reactive nitrogen species) modulate the induction of various genes affecting redox homeostasis, are being extensively investigated currently by various research groups. Present review provides an in-depth analysis of the mechanisms by which NO interacts with and modulates the activity of various ROS scavenging enzymes, particularly accompanying ROS generation in plants in response to varied abiotic stress.

Proteomic characterization of the cellular response to nitrosative stress mediated by s-nitrosoglutathione reductase inhibition

The S-nitrosoglutathione-metabolizing enzyme, GSNO reductase (GSNOR), has emerged as an important regulator of protein S-nitrosylation. GSNOR ablation is protective in models of asthma and heart failure, raising the idea that GSNOR inhibitors might hold therapeutic value. Here, we investigated the effects of a small molecule inhibitor of GSNOR (GSNORi) in mouse RAW 264.7 macrophages. We found that GSNORi increased protein S-nitrosylation in cytokine-stimulated cells, and we utilized stable isotope labeling of amino acids in cell culture (SILAC) to quantify the cellular response to this "nitrosative stress". The expression of several cytokine-inducible immunomodulators, including osteopontin, cyclooxygenase-2, and nitric oxide synthase isoform 2 (NOS2), were decreased by GSNORi. In addition, selective targets of the redox-regulated transcription factor, nuclear factor (erythroid-derived 2)-like 2 (Nrf2)-including heme oxygenase 1 (HO-1) and glutamate cysteine ligase modulatory subunit-were induced by GSNORi in a NOS2- and Nrf2-dependent manner. In cytokine-stimulated cells, Nrf2 protected from GSNORi-induced glutathione depletion and cytotoxicity and HO-1 activity was required for down-regulation of NOS2. Interestingly, GSNORi also affected a marked increase in NOS2 protein stability. Collectively, these data provide the most complete description of the global effects of GSNOR inhibition and demonstrate several important mechanisms for inducible response to GSNORi-mediated nitrosative stress.

Tin chloride enhances parvalbumin-positive interneuron survival by modulating heme metabolism in a model of cerebral ischemia

SnCl(2) has been reported to increase the expression of heme-oxygenase 1 (HO-1), a major antioxidant enzyme, and to decrease ischemic injury, in non-nervous tissues. This study examined the neuroprotective effect of SnCl(2) in the hippocampus of rats submitted to cerebral ischemia. SnCl(2) was administered 18 h before bilateral carotids obstruction. Changes in HO-1 expression and activity, heme content, inducible nitric oxide synthase (iNOS) expression and parvalbumin positive interneuron survival were studied. Thereafter both behavior and memory recovery were tested. The administration of SnCl(2) increased the expression of HO-1 protein and HO activity in the hippocampus and concomitantly decreased heme content at both mitochondrial and nuclear level. Furthermore, ischemized animals showed a strong increase in iNOS expression in the hippocampus, where a loss of parvalbumin positive interneurons also occurred. Pre-treatment with SnCl(2), decreased both iNOS expression in ischemized rats and increased cell survival. The beneficial effects of SnCl(2) were prevented by concomitant treatment with SnMP, a strong inhibitor of HO activity. SnCl(2) also caused an improvement in short term memory recovery. Our results showed that following SnCl(2) administration, HO-1 is strongly induced in the hippocampus and modulate iNOS expression, resulting in a strong neuroprotective effect.

Linking hypoxic and oxidative insults to cell death mechanisms in models of ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease that is characterized by a progressive, selective loss of motor neurons (MN) in brain and spinal cord. The mechanisms of selective and age-dependent MN degeneration in ALS have not been defined. Recent studies suggest that the elevation of intracellular oxidative toxicity contributes to death of MN, but the molecular pathways remain largely unknown. In order to study the possible molecular pathways that the oxidative toxicity induced MN death in ALS, a MN-like cell NSC34, a primary neuronal cell (PNC) of mouse prontal cortex, and a G93A-SOD1 transgenic mouse model were used. Exposure of NSC34 and PNC to cobalt chloride or chronic sustained hypoxic conditions showed a dramatic increase of cellular Hif-1α (hypoxia inducing factor-1α), HO-1 (heme oxygenases-1), and UCP4 (uncoupling protein 4) expression by Western blot analysis, accompanied with increasing cellular apoptosis by histone protein release assay. In an ALS mouse model, the caspase 3 activation, Aif (apoptosis inducing factor), cytochrome c redistribution in MN of spinal cord significantly increased at 70days of disease progression, and Hif-1α expression significantly increased at whole disease stages by an immunohistochemical positive cell counting and Western blot analysis, respectively. The data on this in vitro and in vivo study suggested that oxidative toxicity promoted multiple molecular pathways associated with MN death in ALS and at least were partially associated with the changes of Hif-1α, HO-1, UCP4 expressive increment, caspase 3 activation and Aif, cytochrome c redistribution.

Mechanisms involved in the modulation of astroglial resistance to oxidative stress induced by activated microglia: antioxidative systems, peroxide elimination, radical generation, lipid peroxidation

Microglia and astrocytes are the cellular key players in many neurological disorders associated with oxidative stress and neuroinflammation. Previously, we have shown that microglia activated by lipopolysaccharides (LPS) induce the expression of antioxidative enzymes in astrocytes and render them more resistant to hydrogen peroxide (H2O2). In this study, we examined the mechanisms involved with respect to the cellular action of different peroxides, the ability to detoxify peroxides, and the status of further antioxidative systems. Astrocytes were treated for 3 days with medium conditioned by purified quiescent (microglia-conditioned medium, MCM[-]) or LPS-activated (MCM[+]) microglia. MCM[+] reduced the cytotoxicity of the organic cumene hydroperoxide in addition to that of H2O2. Increased peroxide resistance was not accompanied by an improved ability of astrocytes to remove H2O2 or an increased expression/activity of peroxide eliminating antioxidative enzymes. Neither peroxide-induced radical generation nor lipid peroxidation were selectively affected in MCM[+] treated astrocytes. The glutathione content of peroxide resistant astrocytes, however, was increased and superoxide dismutase and heme oxygenase were found to be upregulated. These changes are likely to contribute to the higher peroxide resistance of MCM[+] treated astrocytes by improving their ability to detoxify reactive oxygen radicals and oxidation products. For C6 astroglioma cells a protective effect of microglia-derived factors could not be observed, underlining the difference of primary cells and cell lines concerning their mechanisms of oxidative stress resistance. Our results indicate the importance of microglial-astroglial cell interactions during neuroinflammatory processes.

Role of Nrf2-mediated heme oxygenase-1 upregulation in adaptive survival response to nitrosative stress

Nitrosative stress caused by reactive nitrogen species such as nitric oxide and peroxynitrite overproduced during inflammation leads to cell death and has been implicated in the pathogenesis of many human ailments. However, relatively mild nitrosative stress may fortify cellular defense capacities, rendering cells tolerant or adaptive to ongoing and subsequent cytotoxic challenges, a phenomenon known as 'preconditioning' or 'hormesis'. One of the key components of cellular stress response is heme oxygenase-1 (HO-1), the rate limiting enzyme in the process of degrading potentially toxic free heme into biliverdin, free iron and carbon monoxide. HO-1 is upregulated by a wide array of stimuli and has antioxidant, anti-inflammatory and other cytoprotective functions. This review is intended to provide readers with a welldocumented account of the research done in the area of cellular adaptive survival response against nitrosative stress with special focus on the role of HO-1 upregulation, especially through activation of the transcription factor, Nrf2.

Mitigation of peroxynitrite-mediated nitric oxide (NO) toxicity as a mechanism of induced adaptive NO resistance in the CNS

During CNS injury and diseases, nitric oxide (NO) is released at a high flux rate leading to formation of peroxynitrite (ONOO(*)) and other reactive nitrogenous species, which nitrate tyrosines of proteins to form 3-nitrotyrosine (3NY), leading to cell death. Previously, we have found that motor neurons exposed to low levels of NO become resistant to subsequent cytotoxic NO challenge; an effect dubbed induced adaptive resistance (IAR). Here, we report IAR mitigates, not only cell death, but 3NY formation in response to cytotoxic NO. Addition of an NO scavenger before NO challenge duplicates IAR, implicating reactive nitrogenous species in cell death. Addition of uric acid (a peroxynitrite scavenger) before cytotoxic NO challenge, duplicates IAR, implicating peroxynitrite, with subsequent 3NY formation, in cell death, and abrogation of this pathway as a mechanism of IAR. IAR is dependent on the heme-metabolizing enzyme, heme oxygenase-1 (HO1), as indicated by the elimination of IAR by a specific HO1 inhibitor, and by the finding that neurons isolated from HO1 null mice have increased NO sensitivity with concomitant increased 3NY formation. This data indicate that IAR is an HO1-dependent mechanism that prevents peroxynitrite-mediated NO toxicity in motor neurons, thereby elucidating therapeutic targets for the mitigation of CNS disease and injury.

Differential sensitivity of oligodendrocytes and motor neurons to reactive nitrogen species: implications for multiple sclerosis

Depending on its concentration, nitric oxide (NO) has beneficial or toxic effects. In pathological conditions, NO reacts with superoxide to form peroxynitrite, which nitrates proteins forming nitrotyrosine residues (3NY), leading to loss of protein function, perturbation of signal transduction, and cell death. 3NY immunoreactivity is present in many CNS diseases, particularly multiple sclerosis. Here, using the high flux NO donor, spermine-NONOate, we report that oligodendrocytes are resistant to NO, while motor neurons are NO sensitive. Motor neuron sensitivity correlates with the NO-dependent formation of 3NY, which is significantly more pronounced in motor neurons when compared with oligodendrocytes, suggesting peroxynitrite as the toxic molecule. The heme-metabolizing enzyme, heme-oxygenase-1 (HO1), is necessary for oligodendrocyte NO resistance, as demonstrated by loss of resistance after HO1 inhibition. Resistance is reinstated by peroxynitrite scavenging with uric acid further implicating peroxynitrite as responsible for NO sensitivity. Most importantly, differential sensitivity to NO is also present in cultures of primary oligodendrocytes and motor neurons. Finally, motor neurons cocultured with oligodendrocytes, or oligodendrocyte-conditioned media, become resistant to NO toxicity. Preliminary studies suggest oligodendrocytes release a soluble factor that protects motor neurons. Our findings challenge the current paradigm that oligodendrocytes are the exclusive target of multiple sclerosis pathology.

Domoic acid induced spinal cord lesions in adult mice: evidence for the possible molecular pathways of excitatory amino acids in spinal cord lesions

Domoic acid (DA) is an excitatory amino acids (EAAs) analog which induced excitotoxicity lesion to central nervous system, but whether induced adult animal spinal cord is not known, furthermore, previous studies have shown that EAAs play an important role in spinal cord lesion, however, the molecular pathways in spinal cord lesion are not fully known. Therefore, a motor neuron-like cell culture system and a DA-induced spinal cord lesioned mice model were used to study the effect of DA on spinal cord in adult mice and the possible molecular pathways of EAAs in spinal cord lesions. Exposure of motor neuron-like cells NSC34 to DA dramatically increased reactive oxygen species (ROS) production by the DCF fluorescent oxidation assay, reduced mitochondrial function by MTT assay, cell viability by trypan blue exclusion assay, and was accompanied by an increase of cell apoptosis by histone protein release assay. In DA-induced spinal cord lesioned mice model, we showed that the decrease of proteasome activity, increase of UCP4 expression by immunohistochemistry and neural cell apoptosis by TUNEL staining, and was accompanied by an decrease of motor disturbance grade during the different stages of DA treatment. Taken together, the in vitro and in vivo data presented in the current report demonstrated that DA induces spinal cord lesions in adult mice, and the multiple molecular pathways promoted by EAAs in spinal cord lesions, at least partially was associated with ROS generation increase, mitochondrial dysfunction, proteasome activity decrease and UCP4 expression increase.

Regulation of heme oxygenase-1 mRNA deadenylation and turnover in NIH3T3 cells by nitrosative or alkylation stress

Background

Heme oxygenase-1 (HO-1) catalizes heme degradation, and is considered one of the most sensitive indicators of cellular stress. Previous work in human fibroblasts has shown that HO-1 expression is induced by NO, and that transcriptional induction is only partially responsible; instead, the HO-1 mRNA half-life is substantially increased in response to NO. The mechanism of this stabilization remains unknown.

Results

In NIH3T3 murine fibroblasts, NO exposure increased the half-life of the HO-1 transcript from ~1.6 h to 11 h, while treatments with CdCl₂, NaAsO₂ or H₂O₂ increased the half-life only up to 5 h. Although poly(A) tail shortening can be rate-limiting in mRNA degradation, the HO-1 mRNA deadenylation rate in NO-treated cells was ~65% of that in untreated controls. In untreated cells, HO-1 poly(A) removal proceeded until 30–50 nt remained, followed by rapid mRNA decay. In NO-treated cells, HO-1 deadenylation stopped with the mRNA retaining poly(A) tails 30–50 nt long. We hypothesize that NO treatment stops poly(A) tail shortening at the critical 30- to 50-nt length. This is not a general mechanism for the post-transcriptional regulation of HO-1 mRNA. Methyl methane sulfonate also stabilized HO-1 mRNA, but that was associated with an 8-fold decrease in the deadenylation rate compared to that of untreated cells. Another HO-1 inducer, CdCl₂, caused a strong increase in the mRNA level without affecting the HO-1 mRNA half-life.

Conclusion

The regulation of HO-1 mRNA levels in response to cellular stress can be induced by transcriptional and different post-transcriptional events that act independently, and vary depending on the stress inducer. While NO appears to stabilize HO-1 mRNA by preventing the final steps of deadenylation, methyl methane sulfonate achieves stabilization through the regulation of earlier stages of deadenylation.

Role of nitric oxide in chick embryonic organogenesis and dysmorphogenesis

BACKGROUND

Nitric oxide (NO), produced by the nitric oxide synthase family of enzymes, mediates multiple signaling functions, and when unchecked, NO causes pathological damage. Exposure of embryos to a variety of teratogens, including carbon monoxide (CO), has been shown to increase reactive intermediates, such as NO, and recent work showed that either the excess or absence of NO caused morphological defects. While endogenous NO is known to regulate many adult tissues, its role during embryonic organogenesis and/or in mediating responses to teratogen exposure has not been explored.

METHODS

We have examined here the presence of NO during normal chick embryonic organogenesis, and investigated the teratogenicity of NO through the application of sodium nitroprusside (SNP), which mimics NO overproduction, and NG-monomethyl-L-arginine (L-NMMA), which inhibits endogenous NOS activity.

RESULTS

Topical treatment with SNP or L-NMMA for 18 h resulted in morphological defects, specifically in the neural tube and somites, which corresponded to sites of altered apoptosis. The location of NO was histochemically correlated with the observed morphological defects. Coadministration of SNP or L-NMMA with CO showed functional coregulation and interaction between NO and CO in chick embryonic development.

CONCLUSIONS

Our results showed that regulation of NO is essential for normal axial development, that sites of altered NO expression correlate to those of altered apoptosis and dysmorphogenesis, and that CO coadministration resulted in a rectification of normal NO expression. Collectively, these results suggest that alteration in endogenous NO/CO signaling is responsible, at least in part, for the observed NO-induced teratogenesis.

The contribution of DNA repair and antioxidants in determining cell type-specific resistance to oxidative stress

The aims of this study were; (i) to elucidate the mechanisms involved in determining cell type-specific responses to oxidative stress and (ii) to test the hypothesis that cell types which are subjected to high oxidative burdens in vivo, have greater oxidative stress resistance. Cultures of the retinal pigment epithelium (RPE), corneal fibroblasts, alveolar type II epithelium and skin epidermal cells were studied. Cellular sensitivity to H2O2 was determined by the MTT assay. Cellular antioxidant status (CuZnSOD, MnSOD, GPX, CAT) was analyzed with enzymatic assays and the susceptibility and repair capacities of nuclear and mitochondrial genomes were assessed by QPCR. Cell type-specific responses to H2O2 were observed. The RPE had the greatest resistance to oxidative stress (P>0.05; compared to all other cell types) followed by the corneal fibroblasts (P < 0.05; compared to skin and lung cells). The oxidative tolerance of the RPE coincided with greater CuZnSOD, GPX and CAT enzymatic activity (P < 0.05; compared to other cells). The RPE and corneal fibroblasts both had up-regulated nDNA repair post-treatment (P < 0.05; compared to all other cells). In summary, variations in the synergistic interplay between enzymatic antioxidants and nDNA repair have important roles in influencing cell type-specific vulnerability to oxidative stress. Furthermore, cells located in highly oxidizing microenvironments appear to have more efficient oxidative defence and repair mechanisms.

Depletion of reduced glutathione enhances motor neuron degeneration in vitro and in vivo

The mechanism of selective and age-dependent motor neuron degeneration in human amyotrophic lateral sclerosis (ALS) has not been defined and the role of glutathione (GSH) in association with motor neuron death remains largely unknown. A motor neuron-like cell culture system and a transgenic mouse model were used to study the effect of cellular GSH alteration on motor neuron cell death. Exposure of NSC34 motor neuron-like cells to ethacrynic acid (EA) or l-buthionine sulfoximine (BSO) dramatically reduced the cellular GSH levels, and was accompanied by increased production of reactive oxygen species (ROS) measured by the dichlorofluorescin (DCF) fluorescent oxidation assay. In addition, GSH depletion enhanced oxidative stress markers, AP-1 transcriptional activation, c-Jun, c-Fos and heme oxygenase-1 (HO-1) expression in NSC34 cells analyzed by a luciferase reporter, Western blotting and quantitative PCR assays respectively. Furthermore, depletion of GSH decreased mitochondrial function, facilitated apoptosis inducing factor (AIF) translocation, cytochrome c release, and caspase 3 activation, and consequently led to motor neuron-like cell apoptosis. In an ALS-like transgenic mouse model overexpressing mutant G93A-Cu, Zn-superoxide dismutase (SOD1) gene, we showed that the reduction of GSH in the spinal cord and motor neuron cells is correlated with AIF translocation, caspase 3 activation, and motor neuron degeneration during ALS-like disease onset and progression. Taken together, the in vitro and in vivo data presented in the current report demonstrated that decreased GSH promotes multiple apoptotic pathways contributing, at least partially, to motor neuron degeneration in ALS.

Resistance to nitric oxide-induced necrosis in heme oxygenase-1 overexpressing pulmonary epithelial cells associated with decreased lipid peroxidation

Increased expression of heme oxygenase-1 (HO-1) increases NO resistance in several cell types, although the biochemical mechanism for this protection is unknown. To address this issue, we have measured different molecular markers of nitrosative stress in three stably transfected cell lines derived from the human lung epithelial line A549: two lines that overexpress rat HO-1 (L1 and A4), and a control line with the empty vector (Neo). Compared with the control Neo cells, L1 and A4 cells had, respectively, 5.8- and 3.8-fold greater HO activity accompanied by increased resistance to NO-induced necrosis. Compared with the Neo control, the HO-1-overexpressing cells also showed significantly less lipid peroxide formation and decreased perturbation of transition metal oxidation and coordination states following a cytotoxic NO exposure. These effects were blocked by the HO-1 inhibitors Zn- and Sn-protoporphyrin IX. In contrast, HO-1 overexpression did not significantly affect total reactive oxygen or nitrogen species, the levels of the nucleobase deamination products in DNA (xanthine, inosine, and uracil) following NO exposure, or NO-induced protein nitration. While increased HO-1 activity prevented NO-induced fluctuations in transition metal homeostasis, addition of an iron chelator decreased NO toxicity only slightly. Our results indicate that lipid peroxidation is a significant cause of NO-induced necrosis in human lung epithelial cells, and that the increased NO survival of L1 cells is due at least in part to decreased lipid peroxidation mediated by HO-1-generated biliverdin or bilirubin.

Nitric oxide-induced resistance to lethal photooxidative damage in a breast tumor cell line

The long-term effects of nitric oxide (NO) on cell susceptibility to photodynamic killing have been studied, using a human breast tumor line (COH-BR1). Subconfluent cells were exposed to a nonlethal dose of spermine NONOate (SPNO, 0.2 mM) and 20 h later were metabolically sensitized with protoporphyrin IX (PpIX) by incubating with 5-aminolevulinic acid. PpIX overproduced in mitochondria was allowed to diffuse to peripheral sites, including plasma membrane, after which a photooxidative challenge was imposed. Active (but not decomposed) SPNO made cells substantially more resistant to necrotic photokilling than non-SPNO-treated controls. A similar response to a tert-butyl hydroperoxide challenge was observed. Hyperresistance was detected approximately 8 h post-SPNO, maximized after approximately 20 h, and reflected diminished oxidant accumulation, as determined with 2',7'-dichlorofluorescein. Intracellular free iron determined with the fluorescent probe calcein rose to approximately 160% of the control level 6 h after SPNO, but declined to approximately 70% after 24 h. Immunoblot analyses revealed a rapid early (approximately 2 h post-NO) increase in heme oxygenase-1 level, followed by a gradual (4-20 h post-NO) increase in ferritin. Upregulation of these proteins is consistent with a cytoprotective mechanism involving mobilization of "signaling" iron. Preactivated RAW 264.7 macrophages on microporous inserts also induced a long-term photoresistance in underlying PpIX-sensitized COH-BR1 cells. This response was abolished by L-NAME, indicating that NO from induced nitric oxide synthase was involved. The NO effects described are entirely novel in the context of photooxidative stress and provide new insights into how NO might affect antitumor photodynamic therapy (PDT).

Carbon monoxide mediates protection against nitric oxide toxicity in HeLa cells

Nitric oxide (NO) mediates cell signaling at low (nanomolar) concentrations, but can be cytotoxic at higher concentrations. Heme oxygenase-1 (HO-1), implicated in a role in NO resistance, might confer its protective effect through the direct products biliverdin and CO or the secondary product bilirubin. We have therefore tested whether biliverdin, bilirubin, or CO can provide resistance to NO toxicity. HeLa cells treated with bilirubin or biliverdin (up to 25 microM) had unchanged survival of an NO challenge (1 mM spermine-NONOate or 2 mM DEA-NO), although they displayed increased resistance to H2O2 (350 microM). In contrast, prior exposure to CO (up to 100 ppm) increased NO resistance. An interval between CO exposure and NO resistance was required for the increased NO resistance. Because the CO-activated NO resistance was also blocked by the transcription inhibitor actinomycin D, inducible gene expression seems critical for the cytoprotection elicited by CO. Experiments in the presence of HO and guanylate cyclase inhibitors indicated that HO activity and cGMP signaling are not essential for the CO-protective effect. Last, inhibition of p38 MAPK activation fully blocked the CO-protective effect, indicating the involvement of this signaling pathway(s) in the CO response.

Nitric oxide-induced resistance to hydrogen peroxide stress is a glutamate cysteine ligase activity-dependent process

Nitric oxide (*NO) is a reactive nitrogen species known to be involved in cytotoxic processes. Cells respond to cytotoxic injury by stress response induction leading to the development of cellular resistance. This report describes an *NO-induced stress response in Chinese hamster fibroblasts (HA1), which leads to glutathione synthesis-dependent resistance to H2O2-mediated oxidative stress. The development of resistance to H2O2 was completely abolished by the inhibition of glutamate cysteine ligase (GCL) during the first 8 h of recovery after *NO exposure. Altered thiol metabolism was observed immediately after *NO exposure as demonstrated by up to 75% decrease in intracellular thiol pools (glutathione, gamma-glutamylcysteine, and cysteine), which then reaccumulated during the *NO-mediated development of resistance. Immunoreactive protein and activity associated with GCL decreased immediately after exposure to *NO and then reaccumulated during the development of resistance to H2O2 challenge. Moreover, compared to N2 controls the activity levels of GCL in *NO-exposed cells increased approximately twofold 24 h after H2O2 challenge. These results demonstrate that *NO exposure is capable of inducing an adaptive response to H2O2-mediated oxidative stress in mammalian cells, which involves alterations in thiol metabolism and is dependent upon glutathione synthesis and increased GCL activity.

Antioxidant up-regulation and increased nuclear DNA protection play key roles in adaptation to oxidative stress in epithelial cells

Cells are armed with a vast repertoire of antioxidant defense mechanisms to help prevent the accumulation of oxidative damage. It is becoming increasingly apparent that the cellular adaptive response has an important antioxidant function to counteract oxidative stress. To investigate this adaptive response we assessed the effect of sublethal H2O2 on cell viability, enzymatic activity, and nuclear (nDNA) and mitochondrial DNA (mtDNA) susceptibility to damage and repair in cultured human retinal pigment epithelium (RPE) cells. This nondividing cell type exists in a highly oxidizing microenvironment in vivo. Prior exposure to sublethal H2O2 confirmed an adaptive response, resulting in a greater cellular resistance to subsequent toxic exposures compared to nonadapted RPE (p < 0.05). A greater CAT, GPX, and CuZnSOD enzymatic activity (p < 0.05) and increased nDNA protection (p < 0.05) were also observed. However, there was no adaptive benefit for mtDNA protection or repair in response to oxidative stress. This study confirms a role for the adaptive response as an important antioxidant defense for cells located in inherently oxidizing microenvironments. Furthermore, it identifies that the mitochondria are a weak link in otherwise efficient oxidative stress defenses and that this may contribute to aging and age-related disease.

A key role for heme oxygenase-1 in nitric oxide resistance in murine motor neurons and glia

Nitric oxide is utilized at low levels for intercellular signaling, and at high levels as a cytotoxic weapon during inflammation. Cellular NO resistance can be increased by prior exposure to sublethal NO levels to induce defense gene expression (adaptive NO resistance), which has been correlated with increased expression of heme oxygenase-1 (HO1) and was blocked by a heme oxygenase inhibitor. However, the possibility remained that other activities were affected by the inhibitor. To address this question, we conducted a genetic study of the HO1 role. We show here that primary cultures of spinal motor neurons and glia from homozygous HO1-null mice are strikingly more sensitive to NO cytotoxicity than are cells expressing HO1. Following an exposure to NO, the HO1-deficient cells were much more prone to apoptosis than were HO1-expressing cells with either one or two copies of a functional HO1 gene. These results confirm the in vivo role of HO1 as a front-line defense against NO toxicity in neuronal cells.

Increases in Oxidative Stress in the Progeny of X-Irradiated Cells

A number of phenotypes persist in the progeny of irradiated cells for many generations including delayed reproductive death, cell transformation, genomic instability, and mutations. It appears likely that persistent phenotypes are inherited by an epigenetic mechanism, although very little is known about the nature of such a mechanism or how it is established. One hypothesis is that radiation causes a heritable increase in oxy-radical activity. In the present study, intracellular levels of reactive oxygen species (ROS) in human lymphoblast clones derived from individually X-irradiated cells were monitored for about 55 generations after exposure. A number of clones derived from irradiated cells had an increase in dichlorofluorescein (DCF) fluorescence at various times. Cells with abrogated TP53 expression had a decreased oxidant response. Flow cytometry analysis of clones with increased fluorescence did not detect increases in the sub-G(1) fraction or decreased cell viability compared to nonirradiated clones, indicating that increased levels of apoptosis and cell death were not present. The oxidative stress response protein heme oxygenase 1 (HO1) was induced in some cultures derived from X-irradiated cells but not in cultures derived from unirradiated cells. The expression of the dual specificity mitogen-activated protein (MAP) kinase phosphatase (MPK1/CL100), which is inducible by oxidative stress and has a role in modulating ERK signaling pathways, was also increased in the progeny of some irradiated cells. Finally, there was an increase in the phosphorylated tyrosine content of a prominent protein band of about 45 kDa. These results support the hypothesis that increased oxy-radical activity is a persistent effect in X-irradiated mammalian cells and further suggest that this may lead to changes in the expression of proteins involved in signal transduction.

Intensified oxidative and nitrosative stress following combined ALA-based photodynamic therapy and local hyperthermia in rat tumors

Oxidative stress-related changes in tumors upon localized hyperthermia (HT), 5-aminolevulinic acid-based photodynamic therapy (ALA-PDT) and their combination (ALA+HT) were examined after the observation that the antitumor effects of ALA-PDT could be significantly enhanced upon simultaneous application of HT. Rats bearing s.c. DS-sarcomas (0.6-1.0 ml) on the hind foot dorsum were anesthetized and underwent one of the following treatments: (i) ALA-PDT (375 mg/kg 5-ALA i.v.); (ii) localized HT, 43 degrees C for 60 min; (iii) combined ALA-PDT and HT [=ALA+HT]. Appropriate control experiments were also performed. After treatment, tumors were excised and rapidly frozen for later analysis of nitrosative stress (protein nitration), apoptotic events (TUNEL, caspase activation, DNA and RNA fragmentation), expression of heat shock proteins (hsp70 and HO-1), glutathione (GSH) levels and glutathione peroxidase (GPx) activity. Protein nitration was found to increase upon treatment, being especially pronounced in the ALA+HT group, and could partially be related to areas surrounding microvessels. The extent of nitrosative stress also correlated well with the appearance of the markers of apoptosis and the inhibition of in vivo tumor growth as seen in a previous study. GSH levels decreased upon treatment, the reduction being most prominent in the ALA-PDT and ALA+HT groups. GPx activity, however, showed a significant decrease only in the ALA-PDT group. Whereas hsp70 expression increased upon HT, ALA-PDT caused a decrease, and these opposing effects were nullified with ALA+HT. The results obtained point to a number of cellular mechanisms-including effects on cellular defense mechanisms and an abrogation of the heat shock defense mechanism-that may interact to achieve the potentiated tumor response rate seen in vivo upon combined treatment.

Effects of high glucose on NO synthesis in human keratinocyte cell line (HaCaT)

BACKGROUND

There is a possibility that alteration of nitric oxide (NO) synthesis by high glucose leads to a variety of diabetic complications.

OBJECTIVE

In this study, we examined whether NO synthesis is altered by high glucose in spontaneously immortalized human keratinocyte cell line (HaCaT) that have three isoforms of NO synthases (NOS).

METHODS

We measured NO end product nitrite in the culture medium using the Griess reagent and analyzed mRNA expression of three isoforms of NOS in HaCaT cells by RT-PCR.

RESULTS

High glucose enhanced constitutively produced NO production in HaCaT cells, which persisted for 10 days and was attenuated by an inhibitor of protein kinase C (PKC), without altering eNOS/nNOS mRNA levels. Cytokine stimulation induced iNOS mRNA in HaCaT cells. Pretreatment with high glucose for 24 h enhanced cytokine-induced NO production in HaCaT cells. However, when these cells were exposed to high glucose for 10 days, cytokine treatment did not induce iNOS mRNA and nitrite production.

CONCLUSION

These diverse alterations in NO production by high glucose may be involved in impaired host-defense and wound healing in the skin of diabetic patients.

Nitric oxide priming protects nitric oxide-mediated apoptosis via heme oxygenase-1 induction

The role of nitric oxide (NO) as a cytotoxic effector molecule of the immune system is clearly established, but recent studies demonstrate cytoprotective functions of NO at low nontoxic concentrations. However, the mechanism of cytoprotection has not been defined completely. Thus, we investigate the involvement of heme oxygenase-1 (HO-1) in the cytoprotective effects of NO. Exposure of L929 cells to sodium nitroprusside (SNP) resulted in the induction of HO-1 protein expression and heme oxygenase activity. Pretreatment of the cells with a low dose of NO (200 microM SNP) significantly inhibited a high dose of (1000 microM SNP) NO-induced apoptosis in L929 cells. Cytoprotection by a low dose of NO was abrogated in the presence of the heme oxygenase inhibitor zinc protoporphyrin IX. A cytoprotective effect comparable to a low dose of SNP was observed when the cells were transfected with HO-1 gene or preincubated with another HO-1 inducer, hemin. Additional experiments revealed the involvement of carbon monoxide in the cytoprotective effect of SNP/HO-1 in L929 cells. Our results presented here provide evidence to support the essential role of HO-1 in the cytoprotective function of NO priming.

VEGF-induced activation of the PI3-K/Akt pathway reduces mutant SOD1-mediated motor neuron cell death

The increased oxidative stress induced by mutant SOD1 is associated with motor neuron degeneration in both human ALS and transgenic mice expressing mutant SOD1. Vascular endothelial growth factor (VEGF) is neurotrophic and also protects from hypoxia-induced neuronal injury. The potential role of VEGF in preventing mutant SOD1-mediated motor neuron cell death was examined using a mouse NSC34 motor neuron-like cell culture system. Infection with adenovirus containing mutant G93A-SOD1, but not vector control or wild-type SOD1, increased cellular oxidative stress and motor neuron-like cell death. However, NSC34 cells pretreated with VEGF displayed a dose-dependent resistance to oxidative damage from hydrogen peroxide, TNF-alpha, and mutant G93A-SOD1. VEGF activated both PI3-K and MAPK activities in mouse NSC34 motor neuron-like cells. Pharmacological inhibitors and constitutively active as well as dominant negative mutants of MAPK and PI3-K revealed that the protective effects of VEGF were mediated via the PI3-K activity, and that MAPK activation was not associated with NSC34 cell survival. Furthermore, VEGF-induced downstream Akt activation promoted motor neuron-like NSC34 cell survival in the presence of mutant G93A-SOD1. Thus, VEGF protected mouse NSC34 motor neuron-like cell death from mutant G93A-SOD1 effects via PI3-K/Akt activation.

Level of haem oxygenase does not obligatorily reflect the sensitivity of PC12 cells to an oxidative shock induced by glutathione depletion

In order to investigate the function of haem oxygenase in neuronal cell death or survival, we have determined in PC12 cells whether induction of haem oxygenase mRNA and protein or inhibition of haem oxygenase activity may be able to modulate the cell response to an oxidative stress. Inhibition of glutathione biosynthesis by buthionine sulfoximine (BSO) has indeed been demonstrated, in this cell line, to decrease the intracellular content of glutathione and to trigger a gradual and programmed cell death. Inhibition of haem oxygenase by zinc protoporphyrin IX, a potent inhibitor of this enzyme, or by a recently described peptidic inhibitor, induced a significant decrease in the toxicity of BSO. This protective action was not due to an alteration in the metabolism of glutathione and was still observed when the protecting agent was added several hours after BSO treatment. Induction of haem oxygenase-1 mRNA and protein by either haemin or pyrrolidine dithiocarbamate was associated with no protection or a significant reduction in the toxicity of BSO respectively. Our results indicate that induction of haem oxygenase-1 is not obligatorily associated with an improved resistance towards oxidative stress and suggest that a byproduct of haem degradation may also become detrimental.

Regulation of heme oxygenase-1 by redox signals involving nitric oxide

Heme oxygenase-1 (HO-1) is an inducible stress protein the expression of which can be markedly augmented in eukaryotes by a wide range of substances that cause a transient change in the cellular redox state. The importance of this protein in physiology and disease is underlined by the versatility of HO-1 inducers and the functional role attributed to HO-1 products (carbon monoxide and bilirubin) in conditions that are associated with moderate or severe cellular stress. An intriguing aspect is the recent evidence showing that nitric oxide, a ubiquitous signaling molecule, finely modulates the activation of HO-1 expression. As the effects of oxidative stress on the regulation of the HO-1 gene have been well established and characterized, this review will focus on the biological relevance of redox signals involving nitric oxide and reactive nitrogen species that lead to up-regulation of the HO-1 pathway, with particular emphasis on vascular tissues and the cardiovascular system.

Protection of human keratinocyte mtDNA by low-level nitric oxide

This study was designed to evaluate the DNA damaging effects of nitric oxide and to determine whether the endogenous generation of nitric oxide at low levels in the cell exerts a protective effect against this damage. Damage to mitochondrial and nuclear DNA in normal human epidermal keratinocytes (NHEK) was assessed after treatment of these cells with varying concentrations of S-nitroso-N-acetylpenicillamine, which decomposes to release nitric oxide. The results showed that mitochondrial DNA was more vulnerable to nitric oxide-induced damage than was a similarly sized fragment of the beta-globin gene. To evaluate the effects on DNA damage by pretreatment of cells with low-levels of nitric oxide, NHEK cells were treated with the prodrug V-PYRRO/NO. This agent is metabolized inside these cells and releases small quantities of nitric oxide. The cells then were exposed to damaging amounts of nitric oxide produced by S-nitroso-N-acetylpenicillamine. The results of these studies showed that pretreatment of NHEK cells with V-PYRRO/NO attenuated the mtDNA damage and loss of cell viability produced by exposure to S-nitroso-N-acetylpenicillamine.

Nitric oxide-inducible expression of heme oxygenase-1 in human cells. Translation-independent stabilization of the mRNA and evidence for direct action of nitric oxide

Expression of heme oxygenase-1 (HO-1) in mammalian cells contributes to resistance to various types of free radical damage. Nitric oxide (NO) induces HO-1 in many cell types, but the specific contribution of transcriptional or post-transcriptional effects to this induction have remained unresolved. Here we show that the extent of HO-1 mRNA expression in IMR-90 and HeLa cells depends on the rate of NO delivery, and that the induction occurs more slowly in HeLa than in human fibroblast (IMR-90) cells. We used a specific NO scavenger (2-(4-carboxylphenyl)-4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide) that completely prevented the inducible expression of HO-1 by NO, pointing to direct signaling action of NO in this induction. By inhibiting transcription during the NO exposure, we have confirmed that NO treatment activates a mechanism that stabilizes HO-1 mRNA. The increase in the HO-1 mRNA half-life in IMR-90 cells was directly correlated with increasing rates of NO release. We also show here that the stabilization of the HO-1 message does not require de novo protein synthesis. Collectively, these results show that stabilization of HO-1 mRNA can be finely tuned to the NO exposure, and that the effect in human fibroblasts is mediated by a pre-existing protein.

Oxidants, nitrosants, and the lung

The respiratory tract is subjected to a variety of environmental stresses, including oxidizing gases, particulates, and airborne microorganisms, that together, may injure structural and functional lung components and thereby jeopardize the primary lung function of gas exchange. To cope with such various environmental threats, the lung has developed elaborate defense mechanisms that include inflammatory-immune pathways as well as several antioxidant systems. These defense systems operate largely in extracellular spaces, thus protecting underlying bronchial and alveolar epithelial cells from injury, although these cells themselves are also active participants in such (inflammatory) defense mechanisms. Although potentially harmful, oxidants are increasingly recognized as pathophysiologic mediators produced primarily by inflammatory-immune cells as a host defense mechanism, but also by various other cell types as an intracellular mediator in various cell responses, thus affecting inflammatory-immune processes or inducing resistance. The molecular mechanisms and signaling pathways involved in such processes are the focus of much current investigation. Nitric oxide, a messenger molecule produced by many lung cell types, also modulates oxidant-mediated processes, thereby giving rise to a new family of reactive nitrogen species ("nitrosants") with potentially unique signaling properties. The complex role of oxidants and nitrosants in various pathophysiologic processes in the lung have confounded the design of therapeutic approaches with antioxidant substrates. This review discusses current knowledge regarding extracellular antioxidant defenses in the lung, and oxidant/nitrosant mechanisms operating under inflammatory-immune conditions and their potential contribution to common lung diseases. Finally, some recent developments in antioxidant therapeutic strategies are discussed.

Regulation of protein function by S-glutathiolation in response to oxidative and nitrosative stress

Protein S-glutathiolation, the reversible covalent addition of glutathione to cysteine residues on target proteins, is emerging as a candidate mechanism by which both changes in the intracellular redox state and the generation of reactive oxygen and nitrogen species may be transduced into a functional response. This review will provide an introduction to the concepts of oxidative and nitrosative stress and outline the molecular mechanisms of protein regulation by oxidative and nitrosative thiol-group modifications. Special attention will be paid to recently published work supporting a role for S-glutathiolation in stress signalling pathways and in the adaptive cellular response to oxidative and nitrosative stress. Finally, novel insights into the molecular mechanisms of S-glutathiolation as well as methodological problems related to the interpretation of the biological relevance of this post-translational protein modification will be discussed.

Genetic responses to free radicals. Homeostasis and gene control

Gene regulation mechanisms have evolved allowing cells to finetune the level of "endogenous" oxidative stress and to cope with increased free radicals from external sources. Levels of H2O2 are tightly controlled in E. coli by OxyR, which is activated by H2O2 to increase scavenging activities and limit H2O2 generation by the respiratory chain. Sub-micromolar levels of H2O2 are maintained in mammalian tissues, though the regulatory systems that govern this control are unknown. Excess superoxide triggers the soxRS system in E. coli, which is controlled by the oxidant-sensitive iron-sulfur centers of the SoxR protein. Nitric oxide activates SoxR by a different modification of the iron-sulfur centers. The soxRS regulon mobilizes diverse functions to scavenge free radicals and repair oxidative damage in macromolecules, and other mechanisms that exclude many environmental agents from the cell. Mammalian cells also sense and respond to sub-toxic levels of nitric oxide, activating expression of heme oxygenase 1 through stabilization of its mRNA. These inductions give rise to adaptive resistance to nitric oxide in neuronal and other cell types.

Constitutive and inducible nitric oxide synthases after dynorphin-induced spinal cord injury

It has recently been demonstrated that selective inhibition of both neuronal constitutive and inducible nitric oxide synthases (ncNOS and iNOS) is neuroprotective in a model of dynorphin (Dyn) A(1-17)-induced spinal cord injury. In the present study, various methods including the conversion of 3H-L-arginine to 3H-citrulline, immunohistochemistry and in situ hybridization are employed to determine the temporal profiles of the enzymatic activities, immunoreactivities, and mRNA expression for both ncNOS and iNOS after intrathecal injection of a neurotoxic dose (20 nmol) of Dyn A(1-17). The expression of ncNOS immunoreactivity and mRNA increased as early as 30 min after injection and persisted for 1-4 h. At 24-48 h, the number of ncNOS positive cells remained elevated while most neurons died. The cNOS enzymatic activity in the ventral spinal cord also significantly increased at 30 min 48 h, but no significant changes in the dorsal spinal cord were observed. However, iNOS mRNA expression increased later at 2 h, iNOS immunoreactivity and enzymatic activity increased later at 4 h and persisted for 24-48 h after injection of 20 nmol Dyn A(1-17). These results indicate that both ncNOS and iNOS are associated with Dyn-induced spinal cord injury, with ncNOS predominantly involved at an early stage and iNOS at a later stage.

Genetic responses against nitric oxide toxicity

The threat of free radical damage is opposed by coordinated responses that modulate expression of sets of gene products. In mammalian cells, 12 proteins are induced by exposure to nitric oxide (NO) levels that are sub-toxic but exceed the level needed to activate guanylate cyclase. Heme oxygenase 1 (HO-1) synthesis increases substantially, due to a 30- to 70-fold increase in the level of HO-1 mRNA. HO-1 induction is cGMP-independent and occurs mainly through increased mRNA stability, which therefore indicates a new NO-signaling pathway. HO-1 induction contributes to dramatically increased NO resistance and, together with the other inducible functions, constitutes an adaptive resistance pathway that also defends against oxidants such as H2O2. In E. coli, an oxidative stress response, the soxRS regulon, is activated by direct exposure of E. coli to NO, or by NO generated in murine macrophages after phagocytosis of the bacteria. This response is governed by the SoxR protein, a homodimeric transcription factor (17-kDa subunits) containing [2Fe-2S] clusters essential for its activity. SoxR responds to superoxide stress through one-electron oxidation of the iron-sulfur centers, but such oxidation is not observed in reactions of NO with SoxR. Instead, NO nitrosylates the iron-sulfur centers of SoxR both in vitro and in intact cells, which yields a form of the protein with maximal transcriptional activity. Although nitrosylated SoxR is very stable in purified form, the spectroscopic signals for the nitrosylated iron-sulfur centers disappear rapidly in vivo, indicating an active process to reverse or eliminate them.

Radical ideas: genetic responses to oxidative stress

1. Complex genetic systems counteract different types of 'oxidative stress' caused by reactive derivatives of oxygen. 2. The bacterial oxyR system responds to peroxide stress and is governed by OxyR, a transcription factor activated by the formation of an intramolecular disulphide bond in H2O2-treated cells. Activated OxyR switches on several genes encoding antioxidant functions, such as catalase. During aerobic growth, oxyR acts homeostatically to regulate cellular H2O2 levels. 3. The bacterial soxRS system responds to superoxide or nitric oxide (NO) stress and is activated in two transcriptional stages. The SoxR protein is activated by oxidation of its [2Fe-2S] centres in cells exposed to superoxide-generating agents, such as paraquat, or to No. Activated SoxR stimulates the soxS gene and SoxS protein then induces at least 15 genes encoding antioxidant functions, such as superoxide dismutase, metabolic functions, such as fumarase, and antibiotic resistance by activation of efflux pumps. The soxRS system may function in resistance to NO-generating immune cells and may contribute to clinical antibiotic resistance. 4. Human cells respond to subtoxic levels of NO by inducing 12 proteins and down-regulating others. A key induced activity is haem oxygenase 1, which is controlled post-transcriptionally. 5. Motor neurons exhibit adaptive resistance to NO, triggered by exposure to subtoxic NO levels, and providing resistance to usually cytotoxic levels of this agent or H2O2. Adaptive resistance to NO depends strongly on the inducible heam oxygenase activity.

Reactive nitrogen species in colon carcinogenesis

The role of reactive nitrogen species (RNS) in colon carcinogenesis is multifactorial and affects diverse processes, such as proliferation, apoptosis, differentiation, tumorigenesis, and metastases. This review describes the stages in colon carcinogenesis where nitric oxide (NO) and inducible NO synthase (NOS2) may influence the progression of a normal mucosa to overt metastatic cancer. Overexpression of NOS2 and an increase in the generation of NO and other RNS may lead to apoptosis resistance, DNA damage, mutation, up-regulation of COX-2, increased proliferation, an increase in oxidative stress and an increase in tumor vascularity and metastatic potential. Therefore, future goals are to establish mechanistically based biomarkers to assess individuals at risk for colon cancer and to implement chemopreventive and dietary strategies that reduce colon cancer risk. An understanding of NO signaling pathways in colon epithelial cells should provide the basis for novel biomarker development. Colon cancer prevention may be achieved effectively by chemically interfering with key components of the NO signaling pathways, changing dietary habits to reduce fat and increase antioxidant-containing vegetables, and dietary supplementation to increase DNA repair.