Inactivation of zinc finger transcription factors provides a mechanism for a gene regulatory role of nitric oxide

S-nitrosylation attenuates pregnane X receptor hyperactivity and acetaminophen-induced liver injury

Drug-induced liver injury (DILI), especially acetaminophen overdose, is the leading cause of acute liver failure. Pregnane X receptor (PXR) is a nuclear receptor and the master regulator of drug metabolism. Aberrant activation of PXR plays a pathogenic role in the acetaminophen hepatotoxicity. Here, we aimed to examine the S-nitrosylation of PXR (SNO-PXR) in response to acetaminophen. We found that PXR was S-nitrosylated in hepatocytes and the mouse livers after exposure to acetaminophen or S-nitrosoglutathione (GSNO). Mass spectrometry and site-directed mutagenesis identified the cysteine 307 as the primary residue for S-nitrosylation (SNO) modification. In hepatocytes, SNO suppressed both agonist-induced (rifampicin and SR12813) and constitutively active PXR (VP-PXR, a human PXR fused to the minimal transactivator domain of the herpes virus transcription factor VP16) activations. Furthermore, in acetaminophen-overdosed mouse livers, PXR protein was decreased at the centrilobular regions overlapping with increased SNO. In PXR-/- mice, replenishing the livers with the SNO-deficient PXR significantly aggravated hepatic necrosis, increased HMGB1 release, and exacerbated liver injury and inflammation. Particularly, we demonstrated that S-nitrosoglutathione reductase (GSNOR) inhibitor N6022 promoted hepatoprotection by increasing the levels of SNO-PXR. In conclusion, PXR is posttranslationally modified by SNO in hepatocytes in response to acetaminophen. This modification mitigated the acetaminophen-induced PXR hyperactivity. It may serve as a target for therapeutical intervention.

Recent advances in the regulation of plant immunity by S-nitrosylation

S-nitrosylation, the addition of a nitric oxide (NO) moiety to a reactive protein cysteine (Cys) thiol, to form a protein S-nitrosothiol (SNO), is emerging as a key regulatory post-translational modification (PTM) to control the plant immune response. NO also S-nitrosylates the antioxidant tripeptide, glutathione, to form S-nitrosoglutathione (GSNO), both a storage reservoir of NO bioactivity and a natural NO donor. GSNO and, by extension, S-nitrosylation, are controlled by GSNO reductase1 (GSNOR1). The emerging data suggest that GSNOR1 itself is a target of NO-mediated S-nitrosylation, which subsequently controls its selective autophagy, regulating cellular protein SNO levels. Recent findings also suggest that S-nitrosylation may be deployed by pathogen-challenged host cells to counteract the effect of delivered microbial effector proteins that promote pathogenesis and by the pathogens themselves to augment virulence. Significantly, it also appears that S-nitrosylation may regulate plant immune functions by controlling SUMOylation, a peptide-based PTM. In this context, global SUMOylation is regulated by S-nitrosylation of SUMO conjugating enzyme 1 (SCE1) at Cys139. This redox-based PTM has also been shown to control the function of a key zinc finger transcriptional regulator during the establishment of plant immunity. Here, we provide an update of these recent advances.

Regulation of Cell Survival, Apoptosis, and Epithelial-to-Mesenchymal Transition by Nitric Oxide-Dependent Post-Translational Modifications

SIGNIFICANCE

Nitric oxide (NO) is a physiopathological messenger generating different reactive nitrogen species (RNS) according to hypoxic, acidic and redox conditions. Recent Advances: RNS and reactive oxygen species (ROS) promote relevant post-translational modifications, such as nitrosation, nitration, and oxidation, in critical components of cell proliferation and death, epithelial-to-mesenchymal transition, and metastasis.

CRITICAL ISSUES

The pro- or antitumoral properties of NO are dependent on local concentration, redox state, cellular status, duration of exposure, and compartmentalization of NO generation. The increased expression of NO synthase has been associated with cancer progression. However, the experimental strategies leading to high intratumoral NO generation have been shown to exert antitumoral properties. The effect of NO and ROS on cell signaling is critically altered by factors modulating tumor progression such as oxygen content, metabolism, and inflammatory response. The review describes the alteration of key components involved in cell survival and death, metabolism, and metastasis induced by RNS- and ROS-related post-translational modifications.

FUTURE DIRECTIONS

The identification of the molecular targets affected by nitrosation, nitration, and oxidation, as well as their interactions with other post-translational modifications, will improve the understanding on the complex signaling and cell fate decision in cancer. The therapeutic NO-based strategies have to address the complex crosstalk among NO and ROS with regard to critical components affecting tumor cell survival, metabolism, and metastasis in the progression of cancer, as well as close interaction with ionizing radiation and chemotherapy.

S-nitrosylation of the zinc finger protein SRG1 regulates plant immunity

Nitric oxide (NO) orchestrates a plethora of incongruent plant immune responses, including the reprograming of global gene expression. However, the cognate molecular mechanisms remain largely unknown. Here we show a zinc finger transcription factor (ZF-TF), SRG1, is a central target of NO bioactivity during plant immunity, where it functions as a positive regulator. NO accumulation promotes SRG1 expression and subsequently SRG1 occupies a repeated canonical sequence within target promoters. An EAR domain enables SRG1 to recruit the corepressor TOPLESS, suppressing target gene expression. Sustained NO synthesis drives SRG1 S-nitrosylation predominantly at Cys87, relieving both SRG1 DNA binding and transcriptional repression activity. Accordingly, mutation of Cys87 compromises NO-mediated control of SRG1-dependent transcriptional suppression. Thus, the SRG1-SNO formation may contribute to a negative feedback loop that attenuates the plant immune response. SRG1 Cys87 is evolutionary conserved and thus may be a target for redox regulation of ZF-TF function across phylogenetic kingdoms.

Glucocorticoid receptor control of transcription: precision and plasticity via allostery

The glucocorticoid receptor (GR) is a constitutively expressed transcriptional regulatory factor (TRF) that controls many distinct gene networks, each uniquely determined by particular cellular and physiological contexts. The precision of GR-mediated responses seems to depend on combinatorial, context-specific assembly of GR-nucleated transcription regulatory complexes at genomic response elements. In turn, evidence suggests that context-driven plasticity is conferred by the integration of multiple signals, each serving as an allosteric effector of GR conformation, a key determinant of regulatory complex composition and activity. This structural and mechanistic perspective on GR regulatory specificity is likely to extend to other eukaryotic TRFs.

Identification of nuclear target proteins for S-nitrosylation in pathogen-treated Arabidopsis thaliana cell cultures

Nitric oxide (NO) is a significant signalling molecule involved in the regulation of many different physiological processes in plants. One of the most imperative regulatory modes of action of NO is protein S-nitrosylation--the covalent attachment of an NO group to the sulfur atom of cysteine residues. In this study, we focus on S-nitrosylation of Arabidopsis nuclear proteins after pathogen infection. After treatment of Arabidopsis suspension cell cultures with pathogens, nuclear proteins were extracted and treated with the S-nitrosylating agent S-nitrosoglutathione (GSNO). A biotin switch assay was performed and biotin-labelled proteins were purified by neutravidin affinity chromatography and identified by mass spectrometry. A total of 135 proteins were identified, whereas nuclear localization has been described for 122 proteins of them. 117 of these proteins contain at least one cysteine residue. Most of the S-nitrosylated candidates were involved in protein and RNA metabolism, stress response, and cell organization and division. Interestingly, two plant-specific histone deacetylases were identified suggesting that nitric oxide regulated epigenetic processes in plants. In sum, this work provides a new collection of targets for protein S-nitrosylation in Arabidopsis and gives insight into the regulatory function of NO in the nucleus during plant defense response. Moreover, our data extend the knowledge on the regulatory function of NO in events located in the nucleus.

Zinc and the prooxidant heart failure phenotype

Neurohormonal activation with attendant aldosteronism contributes to the clinical appearance of congestive heart failure (CHF). Aldosteronism is intrinsically coupled to Zn and Ca dyshomeostasis, in which consequent hypozincemia compromises Zn homeostasis and Zn-based antioxidant defenses that contribute to the CHF prooxidant phenotype. Ionized hypocalcemia leads to secondary hyperparathyroidism with parathyroid hormone-mediated Ca overloading of diverse cells, including cardiomyocytes. When mitochondrial Ca overload exceeds a threshold, myocyte necrosis follows. The reciprocal regulation involving cytosolic free [Zn]i as antioxidant and [Ca]i as prooxidant can be uncoupled in favor of Zn-based antioxidant defenses. Increased [Zn]i acts as a multifaceted antioxidant by: (1) inhibiting Ca entry through L-type channels and hence cardioprotectant from the Ca-driven mitochondriocentric signal-transducer effector pathway to nonischemic necrosis, (2) serving as catalytic regulator of Cu/Zn-superoxide dismutase, and (3) activating its cytosolic sensor, metal-responsive transcription factor that regulates the expression of relevant antioxidant defense genes. Albeit present in subnanomolar range, increased cytosolic free [Zn]i enhances antioxidant capacity that confers cardioprotection. It can be achieved exogenously by ZnSO4 supplementation or endogenously using a β3-receptor agonist (eg, nebivolol) that enhances NO generation to release inactive cytosolic Zn bound to metallothionein. By recognizing the pathophysiologic relevance of Zn dyshomeostasis in the prooxidant CHF phenotype and by exploiting the pharmacophysiologic potential of [Zn]i as antioxidant, vulnerable cardiomyocytes under assault from neurohormonal activation can be protected and the myocardium spared from adverse structural remodeling.

Nitric oxide is a ubiquitous signal for maintaining redox balance in plant cells: regulation of ascorbate peroxidase as a case study

Oxidative and nitrosative stresses and their respective antioxidant responses are common metabolic adjustments operating in all biological systems. These stresses result from an increase in reactive oxygen species (ROS) and reactive nitrogen species (RNS) and an imbalance in the antioxidant response. Plants respond to ROS and RNS accumulation by increasing the level of the antioxidant molecules glutathione and ascorbate and by activating specific antioxidant enzymes. Nitric oxide (NO) is a free radical considered to be toxic or protective depending on its concentration, combination with ROS compounds, and subcellular localization. In this review we focus on the mechanisms of NO action in combination with ROS on the regulation of the antioxidant system in plants. In particular, we describe the redox post-translational modifications of cytosolic ascorbate peroxidase and its influence on enzyme activity. The regulation of ascorbate peroxidase activity by NO as a redox sensor of acute oxidative stress or as part of a hormone-induced signalling pathway leading to lateral root development is presented and discussed.

Modulation of nuclear receptor function by cellular redox poise

Nuclear receptors (NRs) are ligand-responsive transcription factors involved in diverse cellular processes ranging from metabolism to circadian rhythms. This review focuses on NRs that contain redox-active thiol groups, a common feature within the superfamily. We will begin by describing NRs, how they regulate various cellular processes and how binding ligands, corepressors and/or coactivators modulate their activity. We will then describe the general area of redox regulation, especially as it pertains to thiol-disulfide interconversion and the cellular systems that respond to and govern this redox equilibrium. Lastly, we will discuss specific examples of NRs whose activities are regulated by redox-active thiols. Glucocorticoid, estrogen, and the heme-responsive receptor, Rev-erb, will be described in the most detail as they exhibit archetypal redox regulatory mechanisms.

Modulation of stretch-induced myocyte remodeling and gene expression by nitric oxide: a novel role for lipoma preferred partner in myofibrillogenesis

Prolonged hemodynamic load as a result of hypertension eventually leads to maladaptive cardiac adaptation and heart failure. The signaling pathways that underlie these changes are still poorly understood. The adaptive response to mechanical load is mediated by mechanosensors that convert the mechanical stimuli into a biological response. We examined the effect of cyclic mechanical stretch on myocyte adaptation using neonatal rat ventricular myocytes with 10% (adaptive) or 20% (maladaptive) maximum strain at 1 Hz for 48 h to mimic in vivo mechanical stress. Cells were also treated with and without nitro-L-arginine methyl ester (L-NAME), a general nitric oxide synthase (NOS) inhibitor to suppress NO production. Maladaptive 20% mechanical stretch led to a significant loss of intact sarcomeres that were rescued by L-NAME (P < 0.05; n ≥ 5 cultures). We hypothesized that the mechanism was through NO-induced alteration of myocyte gene expression. L-NAME upregulated the mechanosensing proteins muscle LIM protein (MLP; by 100%; P < 0.05; n = 5 cultures) and lipoma preferred partner (LPP), a novel cardiac protein (by 80%; P < 0.05; n = 4 cultures). L-NAME also significantly altered the subcellular localization of LPP and MLP in a manner that favored growth and adaptation. These findings suggest that NO participates in stretch-mediated adaptation. The use of isoform selective NOS inhibitors indicated a complex interaction between inducible NOS and neuronal NOS isoforms regulate gene expression. LPP knockdown by small intefering RNA led to formation of α-actinin aggregates and Z bodies showing that myofibrillogenesis was impaired. There was an upregulation of E3 ubiquitin ligase (MUL1) by 75% (P < 0.05; n = 5 cultures). This indicates that NO contributes to stretch-mediated adaptation via the upregulation of proteins associated with mechansensing and myofibrillogenesis, thereby presenting potential therapeutic targets during the progression of heart failure.

Roles Each of Snail, Yin Yang 1 and RKIP in the Regulation of Tumor Cells Chemo-immuno-resistance to Apoptosis

The current anti-cancer therapeutic armamentarium consists of surgery, chemotherapy, radiation, hormonal therapy, immunotherapy, and combinations thereof. Initial treatments usually result in objective clinical responses with prolongation of overall survival (OS) and progression-free survival (PFS) in a large subset of the treated patients. However, at the onset, there is a subset of patients who does not respond and another subset that initially responded but experiences relapses and recurrences. These latter subsets of patients develop a state of cross-resistance to a variety of unrelated therapies. Therefore, there is an urgent need to first unravel the underlying mechanisms of resistance and associated gene products that regulate the cross-resistance. Such gene products are potential therapeutic targets as well as potential prognostic/diagnostic biomarkers. In this context, we have identified three interrelated gene products involved in resistance, namely, Snail, YY1, and RKIP that are components of the dysregulated NF-κB/Snail/YY1/RKIP loop in many cancers. In this review, we will discuss the roles each of Snail, YY1 and RKIP in the regulation of tumor cell resistance to chemo and immunotherapies. Since these same gene products have also been shown to be involved in the regulation of the EMT phenotype and metastasis, we suggest that targeting any of these three gene products can simultaneously inhibit tumor cell resistance and metastasis.

Zinc and the modulation of redox homeostasis

Zinc, a redox-inactive metal, has been long viewed as a component of the antioxidant network, and growing evidence points to its involvement in redox-regulated signaling. These actions are exerted through several mechanisms based on the unique chemical and functional properties of zinc. Overall, zinc contributes to maintaining the cell redox balance through various mechanisms including: (i) the regulation of oxidant production and metal-induced oxidative damage; (ii) the dynamic association of zinc with sulfur in protein cysteine clusters, from which the metal can be released by nitric oxide, peroxides, oxidized glutathione, and other thiol oxidant species; (iii) zinc-mediated induction of the zinc-binding protein metallothionein, which releases the metal under oxidative conditions and acts per se as a scavenging oxidant; (iv) the involvement of zinc in the regulation of glutathione metabolism and of the overall protein thiol redox status; and (v) a direct or indirect regulation of redox signaling. Findings of oxidative stress, altered redox signaling, and associated cell/tissue dysfunction in cell and animal models of zinc deficiency highlight the relevant role of zinc in the preservation of cell redox homeostasis. However, although the participation of zinc in antioxidant protection, redox sensing, and redox-regulated signaling is accepted, the molecules, targets, and mechanisms involved are still partially known and the subject of active research.

Nitric oxide and cell death in liver cancer cells

Nitric oxide (NO) is a lipophillic, highly diffusible, and short-lived physiological messenger which regulates a variety of physiopathological responses. NO may exert its cellular action through cGMP-dependent and cGMP-independent pathways which includes different postranslational modifications. The effect of NO in cancer depends on the activity and localization of NOS isoforms, concentration and duration of NO exposure, cellular sensitivity, and hypoxia/re-oxygenation process. NO regulates critical factors such as the hypoxia inducible factor-1 (HIF-1) and p53 generally leading to growth arrest, apoptosis or adaptation. NO sensitizes hepatoma cells to chemotherapeutic compounds probably through increased p53 and cell death receptor expressions.

Lipoma preferred partner is a mechanosensitive protein regulated by nitric oxide in the heart

Adaptor proteins play an important role in signaling pathways by providing a platform on which many other proteins can interact. Malfunction or mislocalization of these proteins may play a role in the development of disease. Lipoma preferred partner (LPP) is a nucleocytoplasmic shuttling adaptor protein. Previous work shows that LPP plays a role in the function of smooth muscle cells and in atherosclerosis. In this study we wanted to determine whether LPP has a role in the myocardium. LPP expression increased by 56% in hearts from pressure overload aortic-banded rats (p < 0.05 n = 4), but not after myocardial infarction, suggesting hemodynamic load regulates its expression. In vitro, LPP expression was 87% higher in cardiac fibroblasts than myocytes (p < 0.05 n = 3). LPP expression was downregulated in the absence of the actin cytoskeleton but not when microtubules were disassembled. We mechanically stretched cardiac fibroblasts using the Flexcell 4000 for 48 h (1 Hz, 5% maximum strain), which decreased total LPP total expression and membrane localization in subcellular fractions (p < 0.05, n = 5). However, L-NAME, an inhibitor of nitric oxide synthase (NOS), significantly upregulated LPP expression. These findings suggest that LPP is regulated by a complex interplay between NO and mechanical cues and may play a role in heart failure induced by increased hemodynamic load.

JS-K, a glutathione/glutathione S-transferase-activated nitric oxide releasing prodrug inhibits androgen receptor and WNT-signaling in prostate cancer cells

BACKGROUND

Nitric oxide (NO) and its oxidative reaction products have been repeatedly shown to block steroid receptor function via nitrosation of zinc finger structures in the DNA-binding domain (DBD). In consequence NO-donors could be of special interest for the treatment of deregulated androgen receptor(AR)-signaling in castration resistant prostate cancer (CRPC).

METHODS

Prostate cancer (PCa) cells were treated with JS-K, a diazeniumdiolate derivate capable of generating large amounts of intracellular NO following activation by glutathione S-transferase. Generation of NO was determined indirectly by the detection of nitrate in tissue culture medium or by immunodetection of nitrotyrosine in the cytoplasm. Effects of JS-K on intracellular AR-levels were determined by western blotting. AR-dimerization was analyzed by mammalian two hybrid assay, nuclear translocation of the AR was visualized in PCa cells transfected with a green fluorescent AR-Eos fusion protein using fluorescence microscopy. Modulation of AR- and WNT-signalling by JS-K was investigated using reporter gene assays. Tumor cell proliferation following JS-K treatment was measured by MTT-Assay.

RESULTS

The NO-releasing compound JS-K was shown to inhibit AR-mediated reporter gene activity in 22Rv1 CRPC cells. Inhibition of AR signaling was neither due to an inhibition of nuclear import nor to a reduction in AR-dimerization. In contrast to previously tested NO-donors, JS-K was able to reduce the intracellular concentration of functional AR. This could be attributed to the generation of extremely high intracellular levels of the free radical NO as demonstrated indirectly by high levels of nitrotyrosine in JS-K treated cells. Moreover, JS-K diminished WNT-signaling in AR-positive 22Rv1 cells. In line with these observations, castration resistant 22Rv1 cells were found to be more susceptible to the growth inhibitory effects of JS-K than the androgen dependent LNCaP which do not exhibit an active WNT-signaling pathway.

CONCLUSIONS

Our results suggest that small molecules able to inhibit WNT- and AR-signaling via NO-release represent a promising platform for the development of new compounds for the treatment of CRPC.

Enhanced expression of recombinant human cyclooxygenase 1 from stably-transfected Drosophila melanogaster S2 cells by dimethyl sulfoxide is mediated by up-regulation of nitric oxide synthase and transcription factor Kr-h1

Recombinant human cyclooxygenase 1 (COX-1) was expressed from stably-transfected Drosophila melanogaster S2 (S2) cells. DMSO improved the expression of recombinant COX-1 by 180 %. DMSO increased the expression of nitric oxide synthase (NOS) at both the RNA and protein levels; NOS expression was closely correlated with the synthesis of recombinant COX-1 mRNA in stably-transfected S2 cells. DMSO also induced the gene encoding Kr-h1 which binds to the CACCC element of the metallothionein promoter to enhance the expression of recombinant COX-1. Therefore, DMSO improves the expression of recombinant COX-1 via NOS and/or the transcription factor Kr-h1.

S-Nitrosation Mediates Multiple Pathways That Lead to Tumor Progression in Estrogen Receptor-Negative Breast Cancer

Chronic inflammation within the tumor microenvironment is a major driver of tumor progression and poor prognosis. Inducible nitric oxide synthase (NOS2) is present in numerous solid tumors. Estrogen receptor-negative (ER-) patients with high expression of tumor NOS2 have a poorer outcome than patients with low expression of NOS2. Furthermore, expression of NOS2 is associated with the basal-like breast cancer phenotype. Using an in vitro model, we have found that nitrosation of critical thiols and nitration of tyrosines lead to the activation of membrane receptors such as epithelial growth factor receptor, Src, Ras, and CD63. These nitric oxide-mediated events in itiate oncogenic signaling pathways such as PI3K/Akt, Ras/ERK, β-catenin, nuclear factor-κB, and AP-1. These data suggest that NOS2 can serve as a major "nonmutatational driver" of ER- breast cancer.

Nitric oxide and cancer

Nitric oxide (NO) is a lipophilic, highly diffusible and short-lived physiological messenger which regulates a variety of important physiological responses including vasodilation, respiration, cell migration, immune response and apoptosis. NO is synthesized by three differentially gene-encoded NO synthase (NOS) in mammals: neuronal NOS (nNOS or NOS-1), inducible NOS (iNOS or NOS-2) and endothelial NOS (eNOS or NOS-3). All isoforms of NOS catalyze the reaction of L-arginine, NADPH and oxygen to NO, L-citrulline and NADP. NO may exert its cellular action by cGMP-dependent as well as by cGMP-independent pathways including postranslational modifications in cysteine (S-nitrosylation or S-nitrosation) and tyrosine (nitration) residues, mixed disulfide formation (S-nitrosoglutathione or GSNO) or promoting further oxidation protein stages which have been related to altered protein function and gene transcription, genotoxic lesions, alteration of cell-cycle check points, apoptosis and DNA repair. NO sensitizes tumor cells to chemotherapeutic compounds. The expression of NOS-2 and NOS-3 has been found to be increased in a variety of human cancers. The multiple actions of NO in the tumor environment is related to heterogeneous cell responses with particular attention in the regulation of the stress response mediated by the hypoxia inducible factor-1 and p53 generally leading to growth arrest, apoptosis or adaptation.

Nitric oxide and zinc homeostasis in pulmonary endothelium

We have shown that zinc-thiolate moieties of the metal binding protein metallothionein (MT) are critical targets for nitric oxide (NO) with resultant increases in intracellular labile zinc. Such an NO-MT-Zn signaling pathway appears to participate in important cardiovascular functions associated with biosynthesis of NO including hypoxic vasoconstriction in the lung. Although downstream effector signaling molecules and critical contractile targets remain unclear, current investigations reveal a contributory role for zinc dependent protein kinases and cytoskeletal proteins in mediating hypoxic induced constriction of pulmonary endothelial cells.

S-nitrosothiol and nitric oxide reactivity at zinc thiolates

S-Nitrosothiols undergo reversible transnitrosation reactions at tris(pyrazolyl)boratozinc thiolates (iPr2)TpZn-SR. These zinc thiolates are unreactive toward anaerobic NO but rapidly react with NO in the presence of O(2) or anaerobically with NO(2) to release the S-nitrosothiol RSNO with formation of the corresponding zinc nitrate.

NO sparks off chromatin: tales of a multifaceted epigenetic regulator

The discovery of nitric oxide (NO) revealed its ambiguous nature, which is related to its pleiotropic activities that control the homeostasis of every organism from bacteria to mammals in several physiological and pathological situations. The wide range of action of NO basically depends on two features: 1) the variety of chemical reactions depending on NO, and 2) the differential cellular responses elicited by distinct NO concentrations. Despite the increasing body of knowledge regarding its chemistry, biology and NO-dependent signaling pathways, little information is available on the nuclear actions of NO in terms of gene expression regulation. Indeed, studies of a putative role for this diatomic compound in regulating chromatin remodeling are still in their infancy. Only recently has the role of NO in epigenetics emerged, and some of its putative epigenetic properties are still only hypothetical. In the present review, we discuss the current evidence for NO-related mechanisms of epigenetic gene expression regulation. We link some of the well known NO chemical reactions and metabolic processes (e.g., S-nitrosylation of thiols, tyrosine nitration, cGMP production) to chromatin modification and address the most recent, striking hypothesis about NO and the control of chromosomes structure.

Nitric oxide: an active nitrogen molecule that modulates cellulose synthesis in tomato roots

Nitric oxide (NO) is a bioactive molecule involved in several growth and developmental processes in plants. These processes are mostly characterized by changes in primary and secondary metabolism. Here, the effect of NO on cellulose synthesis in tomato (Solanum lycopersicum) roots was studied. The phenotype of roots, cellulose content, the incorporation of 14C-glucose into cellulosic fraction and the expression of tomato cellulose synthase (CESA) transcripts in roots treated with the NO donor sodium nitroprusside (SNP) were analysed. Nitric oxide affected cellulose content in roots in a dose dependent manner. Low concentrations of SNP (pmoles of NO) increased cellulose content in roots while higher concentrations of SNP (nmoles of NO) had the opposite effect. This result correlated with assays of 14C-glucose incorporation into cellulose in roots. The effect of NO on 14C-glucose incorporation into cellulose was transient and reversible. Microscopic analysis of roots suggested that NO affected primary cell wall cellulose synthesis. Three tomato cellulose synthase (SICESA) transcripts were identified. Reverse transcriptase polymerase chain reaction experiments were carried out and indicated that SICESA1 and SICESA3 levels were affected by high NO concentrations. Together, these results support the hypothesis that variations in NO levels influence cellulose synthesis and content in roots.

Regulation of apoptosis-related genes by nitric oxide in cancer

Nitric oxide (NO) is a simple molecule with a complex and pleiotropic biological activity. NO or related species have been implicated in the regulation of many genes that participate in many diverse biological functions including programmed cell death or apoptosis. Apoptosis is a process that may potentially be disrupted in cancer cells conferring a survival advantage. In addition, malignant tumor cells can develop an intricate system of resistance to apoptotic stimuli. NO or related species have been shown to play a dual role in the regulation of apoptosis in malignant cells either promoting cell death or protecting cells from pro-apoptotic induction. However, the specific role of NO in the regulation of apoptosis/survival-related genes expression seems to tilt the balance toward the promotion of pro-apoptotic and the suppression of anti-apoptotic genes. Herein we have reviewed the most relevant aspects involving NO and/or reactive intermediates in the regulation of apoptosis-related genes--mainly--at the transcriptional level. We described the basic apoptotic molecules that potentially are affected by NO and how NO-mediated signaling gets transmitted to the transcriptional machinery that governs the expression of these genes. In addition, we discussed some of the fundamental functional consequences of the regulation of apoptosis-related genes by NO in cancer biology and its potential therapeutic implications.

Zinc nutritional status modulates expression of ahr-responsive p450 enzymes in vascular endothelial cells

Zinc has anti-inflammatory properties and is crucial for the integrity of vascular endothelial cells, and the development and homeostasis of the cardiovascular system. The aryl hydrocarbon receptor (AhR) which is expressed in the vascular endothelium also plays an important role in responses to xenobiotic exposure and cardiovascular development. We hypothesize that cellular zinc can modulate induction of AhR responsive genes in endothelial cells. To determine if zinc deficiency can alter responses to AhR ligands, aortic endothelial cells were exposed to the AhR ligands 3,3',4,4'-tetrachlorobiphenyl (PCB77) or beta-naphthoflavone (beta-NF) alone or in combination with the membrane permeable zinc chelator TPEN, followed by measurements of the AhR responsive cytochrome P450 enzymes CYP1A1 and 1B1. Compared to vehicle treated cells, both PCB77-induced CYP1A1 activity (EROD) and mRNA expression were significantly reduced during zinc deficiency. In addition, PCB77 and beta-NF-mediated upregulation of CYP1A1 and CYP1B1 protein expression was significantly reduced in zinc-deficient endothelial cells. The inhibition of CYP1A1 and CYP1B1 protein expression caused by zinc deficiency was reversible by cellular zinc supplementation. Overall, our results strongly suggest that nutrition can modulate an environmental toxicant-induced biological outcome and that adequate levels of individual nutrients such as zinc are necessary for induction of AhR responsive genes in vascular endothelial cells.

Changes in Na+/K+-ATPase activity, unsaturated fatty acids and metallothioneins in gills of the shore crab Carcinus aestuarii after dilute seawater acclimation

We have assessed the activity of Na(+)/K(+)-ATPase, cAMP, free fatty acids (FFA) and metallothionein (MT) in the posterior gills of the brackish water shore crab Carcinus aestuarii during acclimation to 10 ppt dilute seawater (DSW). Following 3-18 days acclimation in DSW specific activity of Na(+)/K(+)-ATPase in native gill homogenates and partially purified membrane vesicles was progressively increased, from 1.7- to 3.9-fold. After short-term acclimation of crabs in DSW with added sucrose to make media isosmotic with the haemolymph the specific Na(+)/K(+)-ATPase activity in homogenates was not increased, relative to SW enzyme activity. Moreover, hyposmotic conditions led to depletion of cAMP in gills. In partially purified membrane vesicles isolated from posterior gills, fatty acids with compositions 16:0, 18:0, 18:1, 20:4 and 20:5 dominated in both SW- and DSW-acclimated Carcinus. During a year in which the metabolic activity of crabs was increased, the arachidonic/linoleic acids ratio (ARA/LA) for DSW-acclimated crabs was markedly increased relative to that in SW. Increased Na(+)+K(+)-ATPase activity under hyposmotic stress may be modulated at least partially by the changed proportion of fatty acids in the purified membranes of posterior gills. Long-term acclimation of shore crabs to DSW resulted in a 2.6-fold increase in cytosolic metallothionein (MT) content in posterior gills over those in SW crabs. Assuming an antioxidant role of MT associated with intracellular zinc partitioning, the observed MT induction in posterior gills may be considered an adaptive response of C. aestuarii to hyposmotic stress.

Nitrosylcobalamin potentiates the anti-neoplastic effects of chemotherapeutic agents via suppression of survival signaling

BACKGROUND

Nitrosylcobalamin (NO-Cbl) is a chemotherapeutic pro-drug derived from vitamin B12 that preferentially delivers nitric oxide (NO) to tumor cells, based upon increased receptor expression. NO-Cbl induces Apo2L/TRAIL-mediated apoptosis and inhibits survival signaling in a variety of malignant cell lines. Chemotherapeutic agents often simultaneously induce an apoptotic signal and activation of NF-kappaB, which has the undesired effect of promoting cell survival. The specific aims of this study were to 1) measure the anti-tumor effects of NO-Cbl alone and in combination with conventional chemotherapeutic agents, and to 2) examine the mechanism of action of NO-Cbl as a single agent and in combination therapy.

METHODOLOGY

Using anti-proliferative assays, electrophoretic mobility shift assay (EMSA), immunoblot analysis and kinase assays, we demonstrate an increase in the effectiveness of chemotherapeutic agents in combination with NO-Cbl as a result of suppressed NF-kappaB activation.

RESULTS

Eighteen chemotherapeutic agents were tested in combination with NO-Cbl, in thirteen malignant cell lines, resulting in a synergistic anti-proliferative effect in 78% of the combinations tested. NO-Cbl pre-treatment resulted in decreased NF-kappaB DNA binding activity, inhibition of IkappaB kinase (IKK) enzymatic activity, decreased AKT activation, increased caspase-8 and PARP cleavage, and decreased cellular XIAP protein levels.

CONCLUSION

The use of NO-Cbl to inhibit survival signaling may enhance drug efficacy by preventing concomitant activation of NF-kappaB or AKT.

Protein/DNA arrays identify nitric oxide-regulated cis-element and trans-factor activities some of which govern neuroblastoma cell viability

Toxic nitric oxide (NO) levels can regulate gene expression. Using a novel protein/DNA array, we show that toxic NO levels regulate the binding of trans-factors to various cis-elements in neuroblastoma cells, including CRE and those recognized by the transcription factors AP1, AP2, Brn-3a, EGR, E2F1 and SP1. Functionality of some of the cis-elements was confirmed by electro mobility shift and reporter assays. Interestingly, CREB, AP-1, Brn-3a, EGR and E2F1 can control mammalian cell viability. NO induced the anti-apoptotic Bcl-2 protein and its mRNA prior to the onset of death of 30-60% of the cells. Promoter analysis of the bcl-2 gene confirmed the involvement of a CRE in NO-dependent bcl-2 transcription. Neuroblastoma cells over-expressing bcl-2 became much more resistant to NO-induced apoptosis; conversely, Bcl-2 knockdown cells were rendered markedly more sensitive to NO. Together these results suggest that Bcl-2 counteracts NO-induced apoptosis in a fraction of the cell population. Thus, NO stimulates the binding of many trans-factors to their cognate cis-elements, some of which can regulate cell viability through transcriptional activation of target genes. Our results emphasize that a DNA/protein array approach can reveal novel, global transcription factor activities stimulated by cell death-regulating molecules.

Nuclear localization and DNA binding of ecdysone receptor and ultraspiracle

The Ecdysone receptor (EcR) is distributed between cytoplasm and nucleus in CHO cells. Nuclear localization is increased by the ligand Muristerone A. The most important heterodimerization partner Ultraspiracle (Usp) is localized predominantly in the nucleus. We used the diethylentriamine nitric oxide adduct DETA/NO, which releases NO and destroys the zinc-finger structure of nuclear receptors, to investigate whether nuclear EcR and Usp interact with DNA. If expressed separately, Usp and EcR in the absence of hormone do not interact with DNA. The hormone-induced increase in nuclear EcR is due to enhanced DNA binding. In the presence of Usp, EcR is shifted nearly quantitatively into the nucleus. Only a fraction (approximately 30%) of the heterodimer is sensitive to DETA/NO. Interaction of the heterodimer with DNA is mediated mainly by the C-domain of EcR. Deletion of the DNA-binding domain of Usp only slightly reduces nuclear localization of EcR/Usp, although the nuclear localization signal of Usp is not present anymore. The results indicate that EcR and Usp can enter the nucleus independently, but cotransport of both receptors mediated by dimerization via the ligand binding domains is possible even in the absence of hormone.

Nitrotyrosine and protein carbonyls are equally distributed in HT22 cells after nitrosative stress

The generation of reactive oxygen and nitrogen species is an inevitable result of cellular metabolism and environmental influence. Such oxidation processes are always combined with the formation of various protein oxidation products. Environmental oxidants might either be activated inside the cell or act by themselves. Therefore, differences in the localization of oxidant formation might change the major compartment of oxidant action. Therefore, we employed NO donors (SNOC, DETA/NO, and Spe/NO) alone or in combination with the redox-cycling bipyridinium compound paraquat, the superoxide- and NO-releasing compound SIN-1, the relatively more lipophilic oxidants tert-butyl and cumene hydroperoxide, and peroxynitrite itself to test the ability of these compounds to generate oxidized and nitrated proteins in various cellular compartments. Combined treatment with oxidants and nitrating compounds led to the formation of protein carbonyls and nitrotyrosine with a severalfold higher concentration in the cytosol, compared to the nucleus. In fluorescence microscopy studies, the resulting protein modifications show a similar distribution of oxidized proteins and nitrotyrosine with highest concentrations in the perinuclear area. Studying the time- and concentration-dependent formation and degradation of protein carbonyls and nitrated proteins large similarities could be measured. Therefore, it can be concluded that formation, localization, and kinetics of protein carbonyl and nitrotyrosine formation parallel each other depending on the stress-inducing agent.

NO* chemistry: a diversity of targets in the cell

NO(*) alone is a poorly reactive species; however, it is able to undergo secondary reactions to form highly oxidizing and nitrating species, NO(2)(*), N(2)O(3), and ONOO(-). These secondary reactive nitrogen species (RNS) are capable of modifying a diversity of biomolecular structures in the cell. The chemical properties of individual RNS will be discussed, along with their ability to react with amino acids, metal cofactors, lipids, cholesterol, and DNA bases and sugars. Many of the identified RNS-induced modifications have been observed both in vitro and in vivo. Several of these chemical modifications have been attributed with a functional role in the cell, such as the modulation of enzyme activity. Other areas in the field will be discussed, including the ability of RNS to react with metabolites, RNA, and substrates in the mitochondrion, and the cellular removal/repair of RNS-modified structures.

Hypoxia enhances S-nitrosylation-mediated NMDA receptor inhibition via a thiol oxygen sensor motif

Under ambient air conditions, NO inhibits NMDAR activity by reacting with the NR2A subunit C399 along with two additional cysteine pairs if their disulfide bonds are reduced to free thiol groups [NR1(C744,C798); NR2(C87,C320)]. Here we demonstrate that relative hypoxia enhances S-nitrosylation of NMDARs by a unique mechanism involving an "NO-reactive oxygen sensor motif" whose determinants include C744 and C798 of the NR1 subunit. Redox reactions involving these two thiol groups sensitize other NMDAR sites to S-nitrosylation and consequent receptor inhibition, while their own nitrosylation has little effect on NMDAR activity. The crystal structure of the ligand-binding domain of NR1 reveals a flexible disulfide bond (C744-C798), which may account for its susceptibility to reduction and subsequent reaction with NO that is observed with biochemical techniques. These thiols may be nitrosylated preferentially during increasing hypoxia or stroke conditions, thus preventing excessive activity associated with cytotoxicity while avoiding blockade of physiologically active NMDARs.

New perspectives on the mechanisms through which nitric oxide may affect learning and memory processes

Nitric oxide (NO) has been well established as a molecule necessary for memory consolidation. Interestingly, the majority of research has focused on only a single mechanism through which NO acts, namely the up-regulation of guanylate cyclase (GC). However, since NO and NO-derived reactive nitrogen species are capable of interacting with a broad array of enzymes, ion channels and receptors, a singular focus on GC appears short-sighted. Although NO inhibits the action of a number of molecules there are four, in addition to GC, which are up-regulated by the direct presence of NO, or NO-derived radicals, and implicated in memory processing. They are: cyclic nucleotide-gated channels; large conductance calcium-activated potassium channels; ryanodine receptor calcium release (RyR) channels; and the enzyme mono(ADP-ribosyl) transferase. This review presents evidence that not only are these four molecules worthy of investigation as GC-independent mechanisms through which NO may act, but that behavioural evidence already exists suggesting a relationship between NO and the RyR channel.

Nitric oxide suppresses preadipocyte differentiation in 3T3-L1 culture

Nitric oxide (NO) is an important chemical messenger controlling many physiological functions, involving cell proliferation, and differentiation. The purpose of this study was to investigate the effect of NO on adipocyte differentiation using a murine preadipocyte cell line, 3T3-L1. The treatment with a NO donor, 1-hydroxy-2-oxo-3,3-bis(2-aminoethyl)-1-triazene (NOC18), reduced some markers of adipocyte differentiation such as glycerol-3-phosphate dehydrogenase activity, and intracellular lipid accumulation. To examine whether these effects of NOC18 on adipocyte differentiation markers are due to its cytotoxity, lactate dehydrogenase (LDH) release from the cells were measured. NOC18 did not affect LDH release into the culture medium. Thus, the suppressive actions of NO donor were unlikely to result from its cytotoxicity. Peroxisome proliferator-activated receptor (PPAR) gamma is a critical transcription factor for adipocyte differentiation and adipocyte fatty acid binding protein (aP2) gene is one of its targets. Protein expression of PPARgamma was not diminished by NOC18 treatment, although mRNA expression of aP2 was reduced. Electrophoretic mobility shift assay showed that NOC18 interfered with the DNA binding activity of PPARgamma. Therefore, the present experiment suggest that NO suppresses adipocyte differentiation through suppressing the transcriptional activity of PPARgamma, without suppressing its expression level.

Nitric oxide-mediated inhibition of androgen receptor activity: possible implications for prostate cancer progression

Chronic inflammation increases the risk of cancer and many cancers, including prostate cancer, arise at sites of chronic inflammation. Inducible nitric oxide synthase (iNOS) is an enzyme dominantly expressed during inflammatory reactions. Although synthesis of high amounts of nitric oxide (NO) by iNOS has been demonstrated in pathophysiological processes, such as acute or chronic inflammation, autoimmune diseases or tumorigenesis, the role of iNOS activity in most of these diseases is poorly understood. Analysing prostate cancer biopsies by immunohistochemistry we found iNOS protein expression in tumor cells strongly paralleled by nitrotyrosine suggesting that iNOS is fully active. In vitro, NO inhibits androgen receptor-dependent promoter activity and prostate specific antigen production as well as DNA-binding activity of the androgen receptor (AR) in a concentration-dependent manner. Inhibition of the activity of androgen receptor-dependent reporter constructs is neither owing to diminished AR protein levels nor owing to an inhibition of its nuclear import. In addition, NO inhibits the proliferation of androgen receptor-positive prostate cancer cells significantly more efficiently than proliferation of androgen receptor-negative prostate cancer cells. In summary, our findings suggest that intratumoral iNOS activity favors development of prostate cancer cells that are able to proliferate androgen receptor-independently, thereby promoting prostate tumor progression.

Zinc coordination environments in proteins as redox sensors and signal transducers

Zinc/cysteine coordination environments in proteins are redox-active. Oxidation of the sulfur ligands mobilizes zinc, while reduction of the oxidized ligands enhances zinc binding, providing redox control over the availability of zinc ions. Some zinc proteins are redox sensors, in which zinc release is coupled to conformational changes that control varied functions such as enzymatic activity, binding interactions, and molecular chaperone activity. Whereas the released zinc ion in redox sensors has no known function, the redox signal is transduced to specific and sensitive zinc signals in redox transducers. Released zinc can bind to sites on other proteins and modulate signal transduction, generation of metabolic energy, mitochondrial function, and gene expression. The paradigm of such redox transducers is the zinc protein metallothionein, which, together with its apoprotein, thionein, functions at a central node in cellular signaling by redistributing cellular zinc, presiding over the availability of zinc, and interconverting redox and zinc signals. In this regard, the transduction of nitric oxide (NO) signals into zinc signals by metallothionein has received particular attention. It appears that redox-inert zinc has been chosen to control some aspects of cellular thiol/disulfide redox metabolism. Tight control of zinc is essential for redox homeostasis because both increases and decreases of cellular zinc elicit oxidative stress. Depending on its availability, zinc can be cytoprotective as a pro-antioxidant or cytotoxic as a pro-oxidant. Any condition with acute or chronic oxidative stress is expected to perturb zinc homeostasis.

Nitric Oxide in Salmonella and Escherichia coli Infections

This review discusses the role that nitric oxide (NO) and its congeners play on various stages in the pathophysiology of Escherichia coli and Salmonella infections, with special emphasis on the regulatory pathways that lead to high NO synthesis, the role of reactive nitrogen species (RNS) in host resistance, and the bacterial molecular targets and defense mechanisms that protect enteric bacteria against the nitrosative stress encountered in diverse host anatomical sites. In general, NO can react directly with prosthetic groups containing transition metal centers, with other radicals, or with sulfhydryl groups in the presence of an electron acceptor. Binding to iron complexes is probably the best characterized direct reaction of NO in biological systems. The targets of RNS are numerous. RNS can facilitate oxidative modifications including lipid peroxidation, hydroxylation, and DNA base and protein oxidation. In addition, RNS can inflict nitrosative stress through the nitrosation of amines and sulfhydryls. Numerous vital bacterial molecules can be targeted by NO. It is therefore not surprising that enteropathogenic bacteria are armed with a number of sensors to coordinate the protective response to nitrosative stress, along with an assortment of antinitrosative defenses that detoxify, repair, or avoid the deleterious effects of RNS encountered within the host. NO and NO-derived RNS play important roles in innate immunity to Salmonella and E. coli. Enzymatic NO production by NO synthases can be enhanced by microbial and other inflammatory stimuli and it exerts direct antimicrobial actions as well as immunomodulatory and vasoregulatory effects.

The nitric oxide prodrug, V-PYRRO/NO, protects against cadmium toxicity and apoptosis at the cellular level

The liver is an important target tissue of cadmium. The compound O2-vinyl 1-(pyrrolidin-1-yl)diazen-1-ium-1,2 diolate (V-PYRRO/NO) is a liver-selective nitric oxide (NO) prodrug that is metabolized by hepatic P450 enzymes to release NO in hepatocytes. In vivo, V-PYRRO/NO can protect against the toxicity of various hepatotoxicants, including cadmium. Since NO is an effective vasodilator, whether this protective effect against cadmium toxicity is at the level of the hepatic vascular system or actually within the liver cells has not been defined. Thus, we studied the effects of V-PYRRO/NO pretreatment on cadmium-induced toxicity and apoptosis in cultured rat liver epithelial (TRL 1215) cells. Cells were pretreated with V-PYRRO/NO at 500 or 1000 microM for up to 24 h, then exposed to cadmium (as CdCl2) for additional 24 h and cytotoxicity was measured. Cadmium was significantly less cytotoxic in V-PYRRO/NO (1000 microM) pretreated cells (LC50=6.1+/-0.6 microM) compared to control cells (LC50=3.5+/-0.4 microM). TRL 1215 cells acted upon the prodrug to release NO, producing nitrite levels in the extracellular media after 24 h of exposure to 500 or 1000 microM V-PYRRO/NO measured at 87.0+/-4.2 and 324+/-14.8 microM, respectively, compared to basal levels of 7.70+/-0.46 microM. V-PYRRO/NO alone produced small increases in metallothionein (MT), a metal-binding protein associated with cadmium tolerance. However, V-PYRRO/NO pretreatment greatly enhanced cadmium induction of MT. V-PYRRO/NO pretreatment also markedly reduced apoptotic cell death induced by cadmium (5 microM), apparently by blocking cadmium-induced activation of the c-Jun N-terminal kinase (JNK) pathway. Thus, the prodrug, V-PYRRO/NO, protects against the adverse effects of cadmium directly within rat liver cells apparently through generation of NO and, at least in part, by facilitation of cadmium-induced MT synthesis.

Nitrosative stress and transcription

Low NO concentrations synthesized by constitutively expressed NO synthases act on several signaling pathways activating transcription factors (TF), such as NF-kappaB or AP-1, and thereby influence gene expression. In contrast, during inflammatory reactions the inducible NO synthase produces NO for prolonged periods of time. The resulting nitrosative stress directly affects redox-sensitive TF like NF-kappaB, AP-1, Oct-1, c-Myb, or zinc finger-containing TF, but also additional mechanisms have been identified. Nitrosative stress in some cases induces expression of TF (AP-1, p53), indirectly modulates activity or stability of TF (HIF-1, p53) or their inhibitors (NF-kappaB), or modulates accessibility of promoters via increased DNA methylation or histone deacetylation. Depending on the promoter the result is induced, increased, decreased or even totally inhibited expression of various target genes. In unstimulated cells nitrosative stress increases NF-kappaB- or AP-1-dependent transcription, while in activated cells nitrosative stress rather abolishes NF-kappaB- or AP-1-dependent transcription. Sometimes the oxygen concentration also is of prime importance, since under normoxic conditions nitrosative stress activates HIF-1-dependent transcription, while under hypoxic conditions nitrosative stress leads to inhibition of HIF-1-dependent transcription. This review summarizes what is known about effects of physiological NO levels as well as of nitrosative stress on transcription.

Nitric oxide-mediated regulation of gamma interferon-induced bacteriostasis: inhibition and degradation of human indoleamine 2,3-dioxygenase

Tryptophan depletion resulting from indoleamine 2,3-dioxygenase (IDO) activity within the kynurenine pathway is one of the most prominent gamma interferon (IFN-gamma)-inducible antimicrobial effector mechanisms in human cells. On the other hand, nitric oxide (NO) produced by the inducible isoform of NO synthase (iNOS) serves a more immunoregulatory role in human cells and thereby interacts with tryptophan depletion in a number of ways. We investigated the effects of NO on IDO gene transcription, protein synthesis, and enzyme activity as well as on IDO-mediated bacteriostasis in the human epithelial cell line RT4. IFN-gamma-stimulated RT4 cells were able to inhibit the growth of Staphylococcus aureus in an IDO-mediated fashion, and this bacteriostatic effect was abolished by endogenously produced NO. These findings were supported by experiments which showed that IDO activity in extracts of IFN-gamma-stimulated cells is inhibited by the chemical NO donors diethylenetriamine diazeniumdiolate, S-nitroso-L-cysteine, and S-nitroso-N-acetyl-D,L-penicillamine. Furthermore, we found that both endogenous and exogenous NO strongly reduced the level of IDO protein content in RT4 cells. This effect was not due to a decrease in IDO gene transcription or mRNA stability. By using inhibitors of proteasomal proteolytic activity, we showed that NO production led to an accelerated degradation of IDO protein in the proteasome. This is the first report, to our knowledge, that demonstrates that the IDO is degraded by the proteasome and that NO has an effect on IDO protein stability.