Nitric oxide reversibly inhibits the epidermal growth factor receptor tyrosine kinase

Multiple post-translational modifications ensure EGFR functionality: Potential therapeutic targets to overcome its drug-resistance mutations

Epidermal growth factor receptor (EGFR) mutation is the most common driver mutation in non-small cell lung cancer (NSCLC). The first-line therapy for advanced NSCLC patients with EGFR-sensitive mutation is the EGFR tyrosine kinase inhibitor (EGFR-TKI). However, most NSCLC patients with EGFR mutation will develop resistant mutations in EGFR-TKI therapy. With further studies, resistance mechanisms represented by EGFR-T790M mutations have revealed the impact of EGFR mutations in situ on EGFR-TKIs sensitivity. The third-generation EGFR-TKIs inhibit both EGFR-sensitive mutations and T790M mutations. The emergence of novel mutations such as EGFR-C797S and EGFR-L718Q may decrease efficacy. Searching for new targets to overcome EGFR-TKI resistance becomes a key challenge. Therefore, an in-depth understanding of the regulatory mechanisms of EGFR is essential to find novel targets to overcome drug-resistant mutations in EGFR-TKIs. EGFR, as a receptor-type tyrosine kinase, undergoes homo/heterodimerization and autophosphorylation upon binding to ligands, which activates multiple downstream signaling pathways. Interestingly, there is growing evidence that the kinase activity of EGFR is affected not only by phosphorylation but also by various post-translational modifications (PTMs, such as S-palmitoylation, S-nitrosylation, Methylation, etc.). In this review, we systematically review the effects of different protein PTMs on EGFR kinase activity and its functionality and suggest that influencing EGFR kinase activity by modulating multiple EGFR sites are potential targets to overcome EGFR-TKIs resistance mutations.

Increase of the Intracellular Zinc Concentration Leads to an Activation and Internalisation of the Epidermal Growth Factor Receptor in A549 Cells

(1) Background: Zinc is suggested to play a major role in epidermal growth factor (EGF)-induced cell regeneration and proliferation. To deepen the knowledge on the underlying mechanisms zinc’s effects on the epidermal growth factor receptor (EGFR) activation and its endocytosis was investigated in the alveolar carcinoma cell line A549. (2) Methods: An increase of intracellular zinc was generated by adding zinc extracellularly compared to the intracellular release of zinc from zinc-binding proteins by stimulation with a nitric oxide donor. Zinc-initiated EGFR phosphorylation was checked by Western blotting and receptor endocytosis assays were performed by using flow cytometry. (3) Results: Besides a dose-dependent EGFR phosphorylation, a dose- and time dependent significant receptor internalisation was initiated by both types of zinc increases. In addition, both increased intracellular zinc levels further promoted EGF-induced EGFR phosphorylation and internalisation. (4) Conclusion: This report confirms a transactivating effect of zinc on the EGFR for A549 cells and is the first describing an influence of zinc on the EGFR endocytosis. The transferability of the fine-tuning of EGFR-induced signalling by zinc needs to be verified in vivo, but the presented data underline that zinc might be helpful during treatment of disturbed regeneration and tissue repair.

Updates on Versatile Role of Putative Gasotransmitter Nitric Oxide: Culprit in Neurodegenerative Disease Pathology

Nitric oxide (NO) is a versatile gasotransmitter that contributes in a range of physiological and pathological mechanims depending on its cellular levels. An appropriate concentration of NO is essentially required for cellular physiology; however, its increased level triggers pathological mechanisms like altered cellular redox regulation, functional impairment of mitochondrion, and modifications in cellular proteins and DNA. Its increased levels also exhibit post-translational modifications in protein through S-nitrosylation of their thiol amino acids, which critically affect the cellular physiology. Along with such modifications, NO could also nitrosylate the endoplasmic reticulum (ER)-membrane located sensors of ER stress, which subsequently affect the cellular protein degradation capacity and lead to aggregation of misfolded/unfolded proteins. Since protein aggregation is one of the pathological hallmarks of neurodegenerative disease, NO should be taken into account during development of disease therapies. In this Review, we shed light on the diverse role of NO in both cellular physiology and pathology and discussed its involvement in various pathological events in the context of neurodegenerative diseases.

Protein kinase inhibitor-based cancer therapies: Considering the potential of nitric oxide (NO) to improve cancer treatment

The deregulation of a wide variety of protein kinases is associated with cancer cell initiation and tumor progression. Owing to their indispensable function in signaling pathways driving malignant cell features, protein kinases constitute major therapeutic targets in cancer. Over the past two decades, intense efforts in drug development have been dedicated to this field. The development of protein kinase inhibitors (PKIs) have been a real breakthrough in targeted cancer therapy. Despite obvious successes across patients with different types of cancer, the development of PKI resistance still prevails. Combination therapies are part of a comprehensive approach to address the problem of drug resistance. The therapeutic use of nitric oxide (NO) donors to bypass PKI resistance in cancer has never been tested in clinic yet but several arguments suggest that the combination of PKIs and NO donors may exert a potential anticancer effect. The present review summarized the current state of knowledge on common targets to both PKIs and NO. Herein, we attempt to provide the rationale underlying a potential combination of PKIs and NO donors for future directions and design of new combination therapies in cancer.

A tumor-targeted trimeric 4-1BB-agonistic antibody induces potent anti-tumor immunity without systemic toxicity

The costimulation of immune cells using first-generation anti-4-1BB monoclonal antibodies (mAbs) has demonstrated anti-tumor activity in human trials. Further clinical development, however, is restricted by significant off-tumor toxicities associated with FcγR interactions. Here, we have designed an Fc-free tumor-targeted 4-1BB-agonistic trimerbody, 1D8^N/CEGa1, consisting of three anti-4-1BB single-chain variable fragments and three anti-EGFR single-domain antibodies positioned in an extended hexagonal conformation around the collagen XVIII homotrimerization domain. The1D8^N/CEGa1 trimerbody demonstrated high-avidity binding to 4-1BB and EGFR and a potent in vitro costimulatory capacity in the presence of EGFR. The trimerbody rapidly accumulates in EGFR-positive tumors and exhibits anti-tumor activity similar to IgG-based 4-1BB-agonistic mAbs. Importantly, treatment with 1D8^N/CEGa1 does not induce systemic inflammatory cytokine production or hepatotoxicity associated with IgG-based 4-1BB agonists. These results implicate FcγR interactions in the 4-1BB-agonist-associated immune abnormalities, and promote the use of the non-canonical antibody presented in this work for safe and effective costimulatory strategies in cancer immunotherapy.

Cancer therapy using systemically administrated 4-1BB-targeting antibodies is often associated with severe toxicity due to the nonspecific activation of autoreactive T cells. Here, the authors have developed a trimeric antibody targeting both 4-1BB and EGFR, which activates T cells effectively and shows negligible cytotoxicity.

Toward understanding the selective anticancer capacity of cold atmospheric plasma--a model based on aquaporins (Review)

Selectively treating tumor cells is the ongoing challenge of modern cancer therapy. Recently, cold atmospheric plasma (CAP), a near room-temperature ionized gas, has been demonstrated to exhibit selective anticancer behavior. However, the mechanism governing such selectivity is still largely unknown. In this review, the authors first summarize the progress that has been made applying CAP as a selective tool for cancer treatment. Then, the key role of aquaporins in the H2O2 transmembrane diffusion is discussed. Finally, a novel model, based on the expression of aquaporins, is proposed to explain why cancer cells respond to CAP treatment with a greater rise in reactive oxygen species than homologous normal cells. Cancer cells tend to express more aquaporins on their cytoplasmic membranes, which may cause the H2O2 uptake speed in cancer cells to be faster than in normal cells. As a result, CAP treatment kills cancer cells more easily than normal cells. Our preliminary observations indicated that glioblastoma cells consumed H2O2 much faster than did astrocytes in either the CAP-treated or H2O2-rich media, which supported the selective model based on aquaporins.

Nitric oxide from inflammatory origin impairs neural stem cell proliferation by inhibiting epidermal growth factor receptor signaling

Neuroinflammation is characterized by activation of microglial cells, followed by production of nitric oxide (NO), which may have different outcomes on neurogenesis, favoring or inhibiting this process. In the present study, we investigated how the inflammatory mediator NO can affect proliferation of neural stem cells (NSCs), and explored possible mechanisms underlying this effect. We investigated which mechanisms are involved in the regulation of NSC proliferation following treatment with an inflammatory stimulus (lipopolysaccharide plus IFN-γ), using a culture system of subventricular zone (SVZ)-derived NSCs mixed with microglia cells obtained from wild-type mice (iNOS(+/+)) or from iNOS knockout mice (iNOS(-/-)). We found an impairment of NSC cell proliferation in iNOS(+/+) mixed cultures, which was not observed in iNOS(-/-) mixed cultures. Furthermore, the increased release of NO by activated iNOS(+/+) microglial cells decreased the activation of the ERK/MAPK signaling pathway, which was concomitant with an enhanced nitration of the EGF receptor. Preventing nitrogen reactive species formation with MnTBAP, a scavenger of peroxynitrite (ONOO(-)), or using the ONOO(-) degradation catalyst FeTMPyP, cell proliferation and ERK signaling were restored to basal levels in iNOS(+/+) mixed cultures. Moreover, exposure to the NO donor NOC-18 (100 μM), for 48 h, inhibited SVZ-derived NSC proliferation. Regarding the antiproliferative effect of NO, we found that NOC-18 caused the impairment of signaling through the ERK/MAPK pathway, which may be related to increased nitration of the EGF receptor in NSC. Using MnTBAP nitration was prevented, maintaining ERK signaling, rescuing NSC proliferation. We show that NO from inflammatory origin leads to a decreased function of the EGF receptor, which compromised proliferation of NSC. We also demonstrated that NO-mediated nitration of the EGF receptor caused a decrease in its phosphorylation, thus preventing regular proliferation signaling through the ERK/MAPK pathway.

Time-dependent effect of orchidectomy on vascular nitric oxide and thromboxane A2 release. Functional implications to control cell proliferation through activation of the epidermal growth factor receptor

This study analyzes whether the release of nitric oxide (NO) and thromboxane A2 (TXA2) depends on the time lapsed since gonadal function is lost, and their correlation with the proliferation of vascular smooth muscle cells (VSMC) mediated by the epidermal growth factor receptor (EGFR). For this purpose, aortic and mesenteric artery segments from control and 6-weeks or 5-months orchidectomized rats were used to measure NO and TXA2 release. The results showed that the basal and acetylcholine (ACh)-induced NO release were decreased 6 weeks post-orchidectomy both in aorta and mesenteric artery, but were recovered 5 months thereafter up to levels similar to those found in arteries from control rats. The basal and ACh-induced TXA2 release increased in aorta and mesenteric artery 6 weeks post-orchidectomy, and was maintained at high levels 5 months thereafter. Since we previously observed that orchidectomy, which decreased testosterone level, enlarged the muscular layer of mesenteric arteries, the effect of testosterone on VSMC proliferation was analyzed. The results showed that treatment of cultured VSMC with testosterone downregulated mitogenic signaling pathways initiated by the ligand-dependent activation of the EGFR. In contrast, the EGFR pathways were constitutively active in mesenteric arteries of long-term orchidectomized rats. Thus, the exposure of mesenteric arteries from control rats to epidermal growth factor (EGF) induced the activation of EGFR signaling pathways. However, the addition of EGF to arteries from orchidectomized rats failed to induce a further activation of these pathways. In conclusion, this study shows that the release of NO depends on the time lapsed since the gonadal function is lost, while the release of TXA2 is already increased after short periods post-orchidectomy. The alterations in these signaling molecules could contribute to the constitutive activation of the EGFR and its downstream signaling pathways after long period post-orchidectomy enhancing the proliferation of the vascular muscular layer.

Preclinical therapeutic potential of a nitrosylating agent in the treatment of ovarian cancer

This study examines the role of s-nitrosylation in the growth of ovarian cancer using cell culture based and in vivo approaches. Using the nitrosylating agent, S-nitrosoglutathione (GSNO), a physiological nitric oxide molecule, we show that GSNO treatment inhibited proliferation of chemoresponsive and chemoresistant ovarian cancer cell lines (A2780, C200, SKVO3, ID8, OVCAR3, OVCAR4, OVCAR5, OVCAR7, OVCAR8, OVCAR10, PE01 and PE04) in a dose dependent manner. GSNO treatment abrogated growth factor (HB-EGF) induced signal transduction including phosphorylation of Akt, p42/44 and STAT3, which are known to play critical roles in ovarian cancer growth and progression. To examine the therapeutic potential of GSNO in vivo, nude mice bearing intra-peritoneal xenografts of human A2780 ovarian carcinoma cell line (2×10⁶) were orally administered GSNO at the dose of 1 mg/kg body weight. Daily oral administration of GSNO significantly attenuated tumor mass (p<0.001) in the peritoneal cavity compared to vehicle (phosphate buffered saline) treated group at 4 weeks. GSNO also potentiated cisplatin mediated tumor toxicity in an A2780 ovarian carcinoma nude mouse model. GSNO’s nitrosylating ability was reflected in the induced nitrosylation of various known proteins including NFκB p65, Akt and EGFR. As a novel finding, we observed that GSNO also induced nitrosylation with inverse relationship at tyrosine 705 phosphorylation of STAT3, an established player in chemoresistance and cell proliferation in ovarian cancer and in cancer in general. Overall, our study underlines the significance of S-nitrosylation of key cancer promoting proteins in modulating ovarian cancer and proposes the therapeutic potential of nitrosylating agents (like GSNO) for the treatment of ovarian cancer alone or in combination with chemotherapeutic drugs.

Screening and discovery of nitro-benzoxadiazole compounds activating epidermal growth factor receptor (EGFR) in cancer cells

Peptide ligand-induced dimerization of the extracellular region of the epidermal growth factor receptor (sEGFR) is central to the signal transduction of many cellular processes. A small molecule microarray screen has been developed to search for non-peptide compounds able to bind to sEGFR. We describe the discovery of nitro-benzoxadiazole (NBD) compounds that enhance tyrosine phosphorylation of EGFR and thereby trigger downstream signaling pathways and other receptor tyrosine kinases in cancer cells. The protein phosphorylation profile in cells exposed to NBD compounds is to some extent reminiscent of the profile induced by the cognate ligand. Experimental studies indicate that the small compounds bind to the dimerization domain of sEGFR, and generate stable dimers providing allosteric activation of the receptor. Moreover, receptor phosphorylation is associated with inhibition of PTP-1B phosphatase. Our data offer a promising paradigm for investigating new aspects of signal transduction mediated by EGFR in cancer cells exposed to electrophilic NBD compounds.

Staphylococcal entertotoxins of the enterotoxin gene cluster (egcSEs) induce nitrous oxide- and cytokine dependent tumor cell apoptosis in a broad panel of human tumor cells

The egcSEs comprise five genetically linked staphylococcal enterotoxins, SEG, SEI, SElM, SElN, and SElO and two pseudotoxins which constitute an operon present in up to 80% of Staphylococcus aureus isolates. A preparation containing these proteins was recently used to treat advanced lung cancer with pleural effusion. We investigated the hypothesis that egcSEs induce nitrous oxide (NO) and associated cytokine production and that these agents may be involved in tumoricidal effects against a broad panel of clinically relevant human tumor cells. Preliminary studies showed that egcSEs and SEA activated T cells (range: 11-25%) in a concentration dependent manner. Peripheral blood mononuclear cells (PBMCs) stimulated with equimolar quantities of egcSEs expressed NO synthase and generated robust levels of nitrite (range: 200-250 μM), a breakdown product of NO; this reaction was inhibited by NG-monomethyl-L-arginine (L-NMMA) (0.3 mM), an NO synthase antagonist. Cell free supernatants (CSFs) of all egcSE-stimulated PBMCs were also equally effective in inducing concentration dependent tumor cell apoptosis in a broad panel of human tumor cells. The latter effect was due in part to the generation of NO and TNF-α since it was significantly abolished by L-NMMA, anti-TNF-α antibodies, respectively, and a combination thereof. A hierarchy of tumor cell sensitivity to these CFSs was as follows: lung carcinoma > osteogenic sarcoma > melanoma > breast carcinoma >neuroblastoma. Notably, SEG induced robust activation of NO/TNFα-dependent tumor cell apoptosis comparable to the other egcSEs and SEA despite TNF-α and IFN-γ levels that were 2 and 8 fold lower, respectively, than the other egcSEs and SEA. Thus, egcSEs produced by S. aureus induce NO synthase and the increased NO formation together with TNF-α appear to contribute to egcSE-mediated apoptosis against a broad panel of human tumor cells.

Transient focal cerebral ischemia induces long-term cognitive function deficit in an experimental ischemic stroke model

Vascular dementia ranks as the second leading cause of dementia in the United States. However, its underlying pathophysiological mechanism is not fully understood and no effective treatment is available. The purpose of the current study was to evaluate long-term cognitive deficits induced by transient middle cerebral artery occlusion (tMCAO) in rats and to investigate the underlying mechanism. Sprague-Dawley rats were subjected to tMCAO or sham surgery. Behavior tests for locomotor activity and cognitive function were conducted at 7 or 30days after stroke. Hippocampal long term potentiation (LTP) and involvement of GABAergic neurotransmission were evaluated at 30days after sham surgery or stroke. Immunohistochemistry and Western blot analyses were conducted to determine the effect of tMCAO on cell signaling in the hippocampus. Transient MCAO induced a progressive deficiency in spatial performance. At 30days after stroke, no neuron loss or synaptic marker change in the hippocampus were observed. LTP in both hippocampi was reduced at 30days after stroke. This LTP impairment was prevented by blocking GABAA receptors. In addition, ERK activity was significantly reduced in both hippocampi. In summary, we identified a progressive decline in spatial learning and memory after ischemic stroke that correlates with suppression of hippocampal LTP, elevation of GABAergic neurotransmission, and inhibition of ERK activation. Our results indicate that the attenuation of GABAergic activity or enhancement of ERK/MAPK activation in the hippocampus might be potential therapeutic approaches to prevent or attenuate cognitive impairment after ischemic stroke.

Redox regulation of protein kinases

Reactive oxygen species (ROS) have been long regarded as by-products of a cascade of reactions stemming from cellular oxygen metabolism, which, if they accumulate to toxic levels, can have detrimental effects on cellular biomolecules. However, more recently, the recognition of ROS as mediators of cellular communications has led to their classification as signalling mediators in their own right. The prototypic redox-regulated targets downstream of ROS are the protein tyrosine phosphatases, and the wealth of research that has focused on this area has come to shape our understanding of how redox-signalling contributes to and facilitates protein tyrosine phosphorylation signalling cascades. However, it is becoming increasingly apparent that there is more to this system than simply the negative regulation of protein tyrosine phosphatases. Identification of redox-sensitive kinases such as Src led to the slow emergence of a role for redox regulation of tyrosine kinases. A flow of evidence, which has increased exponentially in recent times as a result of the development of new methods for the detection of oxidative modifications, demonstrates that, by concurrent oxidative activation of tyrosine kinases, ROS fine tune the duration and amplification of the phosphorylation signal. A more thorough understanding of the complex regulatory mechanism of redox-modification will allow targeting of both the production of ROS and their downstream effectors for therapeutic purposes. The present review assesses the most relevant recent literature that demonstrates a role for kinase regulation by oxidation, highlights the most significant findings and proposes future directions for this crucial area of redox biology.

Redox regulation of epidermal growth factor receptor signaling through cysteine oxidation

Epidermal growth factor receptor (EGFR) exemplifies the family of receptor tyrosine kinases that mediate numerous cellular processes, including growth, proliferation, and differentiation. Moreover, gene amplification and EGFR mutations have been identified in a number of human malignancies, making this receptor an important target for the development of anticancer drugs. In addition to ligand-dependent activation and concomitant tyrosine phosphorylation, EGFR stimulation results in the localized generation of H(2)O(2) by NADPH-dependent oxidases. In turn, H(2)O(2) functions as a secondary messenger to regulate intracellular signaling cascades, largely through the modification of specific cysteine residues within redox-sensitive protein targets, including Cys797 in the EGFR active site. In this review, we highlight recent advances in our understanding of the mechanisms that underlie redox regulation of EGFR signaling and how these discoveries may form the basis for the development of new therapeutic strategies for targeting this and other H(2)O(2)-modulated pathways.

S-nitrosylation of EGFR and Src activates an oncogenic signaling network in human basal-like breast cancer

Increased inducible nitric oxide synthase (NOS2) expression in breast tumors is associated with decreased survival of estrogen receptor negative (ER-) breast cancer patients. We recently communicated the preliminary observation that nitric oxide (NO) signaling results in epidermal growth factor receptor (EGFR) tyrosine phosphorylation. To further define the role of NO in the pathogenesis of ER- breast cancer, we examined the mechanism of NO-induced EGFR activation in human ER- breast cancer. NO was found to activate EGFR and Src by a mechanism that includes S-nitrosylation. NO, at physiologically relevant concentrations, induced an EGFR/Src-mediated activation of oncogenic signal transduction pathways (including c-Myc, Akt, and β-catenin) and the loss of PP2A tumor suppressor activity. In addition, NO signaling increased cellular EMT, expression and activity of COX-2, and chemoresistance to adriamycin and paclitaxel. When connected into a network, these concerted events link NO to the development of a stem cell-like phenotype, resulting in the upregulation of CD44 and STAT3 phosphorylation. Our observations are also consistent with the finding that NOS2 is associated with a basal-like transcription pattern in human breast tumors. These results indicate that the inhibition of NOS2 activity or NO signaling networks may have beneficial effects in treating basal-like breast cancer patients.

Activation of the BRCA1/Chk1/p53/p21(Cip1/Waf1) pathway by nitric oxide and cell cycle arrest in human neuroblastoma NB69 cells

Nitric oxide (NO) works as a bi-modal effector of cell proliferation, inducing either the increase or decrease of cell growth when cells are exposed, respectively, to low or high NO concentrations. To get further insight into the action of NO, we tested the effect of short- and long-lived NO donors on the control of the cell cycle in human neuroblastoma NB69 cells. We demonstrated that long-time exposure of cells to NO not only decreased the expression and/or the phosphorylation of elements involved in the control of the G(1)/S transition, such as the transcriptional repressor pRb and cyclin D1, but also down-regulated systems controlling the S and G(2)/M phases, such as the phosphorylation of Cdk1(cdc2) and the expression of cyclins A and B1. Increasing concentrations of NO also induced a biphasic effect on the expression of cyclins D1, A and B1, while this effect was less pronounced for cyclin E expression, but the levels of mRNAs of those cyclins changed in a distinct and complex manner. NO also changed the phosphorylation pattern of cyclin E and decreased the levels of phospho-cyclins D1 and B1. Moreover, NO decreased the expression of the Cdk inhibitors p16(Ink4a) and p19(Ink4d), without affecting p27(Kip1). In contrast, NO induced a biphasic effect on p21(Cip1/Waf1) expression. The BRCA1/Chk1/p53 pathway mediated the upregulation of p21(Cip1/Waf1). We also demonstrated that the NO-mediated up-regulation of p21(Cip1/Waf1) was inversely correlated with the activation status of the p38MAPK pathway.

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 signaling in the retina: what have we learned in two decades?

Two decades after its first detection in the retina, nitric oxide (NO) continues to puzzle visual neuroscientists. While its liberation by photoreceptors remains controversial, recent evidence supports three subtypes of amacrine cells as main sources of NO in the inner retina. NO synthesis was shown to depend on light stimulation, and mounting evidence suggests that NO is a regulator of visual adaptation at different signal processing levels. NO modulates light responses in all retinal neuron classes, and specific ion conductances are activated by NO in rods, cones, bipolar and ganglion cells. Light-dependent gap junction coupling in the inner and outer plexiform layers is also affected by NO. The vast majority of these effects were shown to be mediated by activation of the NO receptor soluble guanylate cyclase and resultant cGMP elevation. This review analyzes the current state of knowledge on physiological NO signaling in the retina.

The role of thioredoxin in the regulation of cellular processes by S-nitrosylation

BACKGROUND

S-nitrosylation (or S-nitrosation) by Nitric Oxide (NO), i.e., the covalent attachment of a NO group to a cysteine thiol and formation of S-nitrosothiols (R-S-N=O or RSNO), has emerged as an important feature of NO biology and pathobiology. Many NO-related biological functions have been directly associated with the S-nitrosothiols and a considerable number of S-nitrosylated proteins have been identified which can positively or negatively regulate various cellular processes including signaling and metabolic pathways.

SCOPE OF THE REVIEW

Taking account of the recent progress in the field of research, this review focuses on the regulation of cellular processes by S-nitrosylation and Trx-mediated cellular homeostasis of S-nitrosothiols.

MAJOR CONCLUSIONS

Thioredoxin (Trx) system in mammalian cells utilizes thiol and selenol groups to maintain a reducing intracellular environment to combat oxidative/nitrosative stress. Reduced glutathione (GSH) and Trx system perform the major role in denitrosylation of S-nitrosylated proteins. However, under certain conditions, oxidized form of mammalian Trx can be S-nitrosylated and then it can trans-S-nitrosylate target proteins, such as caspase 3.

GENERAL SIGNIFICANCE

Investigations on the role of thioredoxin system in relation to biologically relevant RSNOs, their functions, and the mechanisms of S-denitrosylation facilitate the development of drugs and therapies. This article is part of a Special Issue entitled Regulation of Cellular Processes.

S-nitrosylation in the regulation of gene transcription

BACKGROUND

Post-translational modification of proteins by S-nitrosylation serves as a major mode of signaling in mammalian cells and a growing body of evidence has shown that transcription factors and their activating pathways are primary targets. S-nitrosylation directly modifies a number of transcription factors, including NF-κB, HIF-1, and AP-1. In addition, S-nitrosylation can indirectly regulate gene transcription by modulating other cell signaling pathways, in particular JNK kinase and ras.

SCOPE OF REVIEW

The evolution of S-nitrosylation as a signaling mechanism in the regulation of gene transcription, physiological advantages of protein S-nitrosylation in the control of gene transcription, and discussion of the many transcriptional proteins modulated by S-nitrosylation is summarized.

MAJOR CONCLUSIONS

S-nitrosylation plays a crucial role in the control of mammalian gene transcription with numerous transcription factors regulated by this modification. Many of these proteins serve as immunomodulators, and inducible nitric oxide synthase (iNOS) is regarded as a principal mediatiator of NO-dependent S-nitrosylation. However, additional targets within the nucleus (e.g. histone deacetylases) and alternative mechanisms of S-nitrosylation (e.g. GAPDH-mediated trans-nitrosylation) are thought to play a role in NOS-dependent transcriptional regulation.

GENERAL SIGNIFICANCE

Derangement of SNO-regulated gene transcription is an important factor in a variety of pathological conditions including neoplasia and sepsis. A better understanding of protein S-nitrosylation as it relates to gene transcription and the physiological mechanisms behind this process is likely to lead to novel therapies for these disorders. This article is part of a Special Issue entitled Regulation of Cellular Processes by S-nitrosylation.

Nitric oxide changes distinct aspects of the glycophenotype of human neuroblastoma NB69 cells

It is an open question whether the presence of nitric oxide (NO) affects the cell glycophenotype. A panel of six plant lectins was used in this study to monitor distinct aspects of cell surface glycosylation under nitrosative stress. We determined that treating human neuroblastoma NB69 cells with the long-lived NO donor 2,2'-(hydroxynitrosohydrazono)bis-ethanimine (DETA/NO) and monitoring the non-apoptotic adherent cell population significantly increases the presentation of N-glycans as detected by concanavalin A. Examining fine-structural features, bisected N-glycans and branch-end tailoring including α2,6-sialylation were found to be enhanced. Confocal fluorescence microscopy and cell permeabilization experiments pointed to a major effect of NO on the extent of cell surface N-glycan presentation. We also show that NO increases the level of protein O-GlcNAcylation, a multifunctional post-translational modification. Our results thus establish the first evidence for NO as modulator of distinct aspects of cell glycosylation.

Comprehensive identification and modified-site mapping of S-nitrosylated targets in prostate epithelial cells

Background

Although overexpression of nitric oxide synthases (NOSs) has been found associated with prostate diseases, the underlying mechanisms for NOS-related prostatic diseases remain unclear. One proposed mechanism is related to the S-nitrosylation of key regulatory proteins in cell-signaling pathways due to elevated levels of NO in the prostate. Thus, our primary objective was to identify S-nitrosylated targets in an immortalized normal prostate epithelial cell line, NPrEC.

Methodology/Principal Findings

We treated NPrEC with nitroso-cysteine and used the biotin switch technique followed by gel-based separation and mass spectrometry protein identification (using the LTQ-Orbitrap) to discover S-nitrosylated (SNO) proteins in the treated cells. In parallel, we adapted a peptide pull-down methodology to locate the site(s) of S-nitrosylation on the protein SNO targets identified by the first technique. This combined approach identified 116 SNO proteins and determined the sites of modification for 82 of them. Over 60% of these proteins belong to four functional groups: cell structure/cell motility/protein trafficking, protein folding/protein response/protein assembly, mRNA splicing/processing/transcriptional regulation, and metabolism. Western blot analysis validated a subset of targets related to disease development (proliferating cell nuclear antigen, maspin, integrin β4, α-catenin, karyopherin [importin] β1, and elongation factor 1A1). We analyzed the SNO sequences for their primary and secondary structures, solvent accessibility, and three-dimensional structural context. We found that about 80% of the SNO sites that can be mapped into resolved structures are buried, of which approximately half have charged amino acids in their three-dimensional neighborhood, and the other half residing within primarily hydrophobic pockets.

Conclusions/Significance

We here identified 116 potential SNO targets and mapped their putative SNO sites in NPrEC. Elucidation of how this post-translational modification alters the function of these proteins should shed light on the role of NO in prostate pathologies. To our knowledge, this is the first report identifying SNO targets in prostate epithelial cells.

S-Nitrosylation of the epidermal growth factor receptor: a regulatory mechanism of receptor tyrosine kinase activity

Nitric oxide (NO) donors inhibit the epidermal growth factor (EGF)-dependent auto(trans)phosphorylation of the EGF receptor (EGFR) in several cell types in which NO exerts antiproliferative effects. We demonstrate in this report that NO inhibits, whereas NO synthase inhibition potentiates, the EGFR tyrosine kinase activity in NO-producing cells, indicating that physiological concentrations of NO were able to regulate the receptor activity. Depletion of intracellular glutathione enhanced the inhibitory effect of the NO donor 1,1-diethyl-2-hydroxy-2-nitrosohydrazine (DEA/NO) on EGFR tyrosine kinase activity, supporting the notion that such inhibition was a consequence of an S-nitrosylation reaction. Addition of DEA/NO to cell lysates resulted in the S-nitrosylation of a large number of proteins including the EGFR, as confirmed by the chemical detection of nitrosothiol groups in the immunoprecipitated receptor. We prepared a set of seven EGFR(C --> S) substitution mutants and demonstrated in transfected cells that the tyrosine kinase activity of the EGFR(C166S) mutant was completely resistant to NO, whereas the EGFR(C305S) mutant was partially resistant. In the presence of EGF, DEA/NO significantly inhibited Akt phosphorylation in cells transfected with wild-type EGFR, but not in those transfected with C166S or C305S mutants. We conclude that the EGFR can be posttranslationally regulated by reversible S-nitrosylation of C166 and C305 in living cells.

Brain micro-ecologies: neural stem cell niches in the adult mammalian brain

Neurogenesis persists in two germinal regions in the adult mammalian brain, the subventricular zone of the lateral ventricles and the subgranular zone in the hippocampal formation. Within these two neurogenic niches, specialized astrocytes are neural stem cells, capable of self-renewing and generating neurons and glia. Cues within the niche, from cell-cell interactions to diffusible factors, are spatially and temporally coordinated to regulate proliferation and neurogenesis, ultimately affecting stem cell fate choices. Here, we review the components of adult neural stem cell niches and how they act to regulate neurogenesis in these regions.

Differential p38 mitogen-activated protein kinase-controlled hypophosphorylation of the retinoblastoma protein induced by nitric oxide in neuroblastoma cells

In this report we show that exogenous NO added to human neuroblastoma NB69 cells inhibits cell proliferation and downregulates the epidermal growth factor receptor (EGFR) and its downstream signaling pathways. These comprise the 3-phosphoinositide-dependent kinase 1/Akt/glycogen synthase kinase-3beta pathway, the mitogen-activated protein kinase (MAPK)/extracellular-regulated kinases 1 and 2 pathway, and the phospholipase Cgamma pathway. In contrast, NO enhances the EGFR-controlled p38MAPK pathway. We also show that NO enhances the activation of the cAMP-responsive element binding protein, a transcription factor controlled by p38MAPK, as demonstrated using 4-(4-fluorophenyl)-2-(4-hydroxyphenyl)-5-(4-pyridyl)1H-imidazole (SB202190), a p38MAPK inhibitor. These processes are accompanied by the NO-mediated hypophosphorylation of the retinoblastoma protein (pRb), preferentially at Ser795 compared to Ser780 and Ser807/811, and the downregulation of p27(KIP1), p21(CIP1/WAF1), and p16(INK4a), although NO downregulated p16(INK4a) only when the p38MAPK activity was suppressed. The p38MAPK pathway controls the phosphorylation status of pRb as SB202190 enhances the hypophosphorylation of pRb. We reverted the inhibitory action of NO on EGFR and pRb phosphorylation in living cells using cell-permeable reducing agents, which suggested that reversible S-nitrosation controls these proteins. Our results support the notion that NO negatively modulates the p38MAPK-controlled phosphorylation of pRb, inducing the subsequent arrest of the cell cycle at the G1/S transition.

Chronic inhibition of nitric oxide synthesis enhances both subventricular zone neurogenesis and olfactory learning in adult mice

The ability to generate new neurons during the course of adult life is preserved in the subventricular zone of the lateral ventricles and the dentate gyrus of the hippocampus in the mammalian brain. These two regions constitute specifically regulated neurogenic niches, and provide newborn neurons involved in olfactory and spatial learning, respectively. Nitric oxide (NO) is a negative regulator of neurogenesis in the subventricular zone, whereas its role in the dentate gyrus remains controversial. Using systemic administration of NO synthase (NOS) inhibitors to chronically inhibit NO production, we increased neural precursor proliferation in the subventricular zone as well as neurogenesis in the olfactory bulb, without modifying the number of mitotic cells or the granular cell layer thickness in the dentate gyrus. The same treatment specifically improved olfactory learning performance, whereas spatial learning and memory was unchanged, thus demonstrating that olfactory memory is closely associated with the level of ongoing neurogenesis in the subventricular zone-olfactory bulb. The anatomical specificity of the NOS inhibitor actions was not due to differences in the availability of NO, as demonstrated by immunohistochemical detection of neuronal NOS and S-nitrosylated proteins in both regions. Remarkably, the distinct NO sensitivity might result from a differential expression of epidermal growth factor receptor in precursor cells in both regions, as the proliferative effect of NOS inhibitors in the subventricular zone was restricted to the cells that expressed this receptor.

Nitric oxide decreases subventricular zone stem cell proliferation by inhibition of epidermal growth factor receptor and phosphoinositide-3-kinase/Akt pathway

Nitric oxide (NO) inhibits proliferation of subventricular zone (SVZ) neural precursor cells in adult mice in vivo under physiological conditions. The mechanisms underlying this NO effect have now been investigated using SVZ-derived neural stem cells, which generate neurospheres in vitro when stimulated by epidermal growth factor (EGF). In these cultures, NO donors decreased the number of newly formed neurospheres as well as their size, which indicates that NO was acting on the neurosphere-forming neural stem cells and the daughter neural progenitors. The effect of NO was cytostatic, not proapoptotic, and did not involve cGMP synthesis. Neurosphere cells expressed the neuronal and endothelial isoforms of NO synthase (NOS) and produced NO in culture. Inhibition of NOS activity by N(omega)-nitro-L-arginine methylester (L-NAME) promoted neurosphere formation and growth, thus revealing an autocrine/paracrine action of NO on the neural precursor cells. Both exogenous and endogenous NO impaired the EGF-induced activation of the EGF receptor (EGFR) tyrosine kinase and prevented the EGF-induced Akt phosphorylation in neurosphere cells. Inhibition of the phosphoinositide-3-kinase (PI3-K)/Akt pathway by LY294002 significantly reduced the number of newly formed neurospheres, which indicates that this is an essential pathway for neural stem cell self-renewal. Chronic administration of l-NAME to adult mice enhanced phospho-Akt staining in the SVZ and reduced nuclear p27(Kip1) in the SVZ and olfactory bulb. The inhibition of EGFR and PI3-K pathway by NO explains, at least in part, its antimitotic effect on neurosphere cells and may be a mechanism involved in the physiological role of NO as a negative regulator of SVZ neurogenesis in adult mice.

Nitric oxide and cell proliferation

Nitric oxide (NO*) has been proposed to be a physiological modulator of cell proliferation, able to promote in most cases cell cycle arrest. In this review I explore the molecular basis of this mechanism of action. The modulatory action of NO* on the intracellular concentration of cGMP and the machinery directly involved in the control of cell cycle progression, including the expression and activity of diverse cyclins and cyclin-dependent kinases, their physiological inhibitors, and the master transcriptional regulator retinoblastoma protein, will be discussed. The role of NO* in proliferation mediated by tyrosine kinase receptors such as the epidermal growth factor receptor and downstream signalling pathways will also be considered. Finally, the involvement of NO* in proliferative processes relevant for normal development will be outlined.

Transcranial magnetic stimulation attenuates cell loss and oxidative damage in the striatum induced in the 3-nitropropionic model of Huntington's disease

An investigation was conducted on the effect of transcranial magnetic field stimulation (TMS) on the free radical production and neuronal cell loss produced by 3-nitropropionic acid in rats. The effects of 3-nitropropionic acid were evaluated by examining the following changes in: the quantity of hydroperoxides and total radical-trapping antioxidant potential (TRAP), lipid peroxidation products, protein carbonyl groups, reduced glutathione (GSH) content, glutathione peroxidase (GSH-Px), catalase and succinate dehydrogenase (SDH) activities; total nitrite and cell death [morphological changes, quantification of neuronal loss and lactate dehydrogenase (LDH) levels]. Our results reveal that 3-nitropropionic acid induces oxidative and nitrosative stress in the striatum, prompts cell loss and also shows that TMS prevents the harmful effects induced by the acid. In conclusion, the results show the ability of TMS to modify neuronal response to 3-nitropropionic acid.

Role of nitric oxide in subventricular zone neurogenesis

A possible role of nitric oxide (NO) in adult neurogenesis has been suggested based on anatomical findings showing that subventricular zone (SVZ) neuroblasts are located close to NO-producing cells, and on the known antiproliferative actions of NO in many cell types. Experiments have been performed in rodents with systemic and intracerebroventricular administrations of the NO synthase (NOS) inhibitor L-NAME. NOS inhibition leads to significant increases in the number of proliferating cells in the SVZ and olfactory bulb (OB). NO exerts its cytostatic action preferentially on the cell population expressing nestin but not betaIII-tubulin, which may correspond to the type C cells described in the SVZ. The negative effect of NO on SVZ cell proliferation has also been confirmed in SVZ primary cultures. An inhibition of the tyrosine kinase activity of the epidermal growth factor receptor (EGFR) is described as one of the molecular mechanisms responsible for the antiproliferative effect of NO in SVZ cells. Biochemical data supporting this conclusion has been obtained using the neuroblastoma cell line NB69, which endogenously expresses the EGFR. In these cells, the antimitotic action of NO occurs upon inhibition of the EGFR tyrosine phosphorylation, probably by a direct S-nitrosylation of the receptor. The latest published reports on NO and neurogenesis indicate that NO physiologically participates in the control of adult neurogenesis by modulating the proliferation and fate of the SVZ progenitor cells. These effects might be partially due to a direct inhibition of the EGFR by S-nitrosylation.

Nitric oxide and adult neurogenesis in health and disease

Adult neurogenesis may be functionally important as a mechanism of brain plasticity in physiological conditions and brain repair after injury. Nitric oxide (NO), a diffusible intracellular and intercellular messenger in the mammalian nervous system, has been shown to affect adult neurogenesis in different ways. In the normal brain, NO, synthesized by the neuronal isoform of NO synthase in nitrergic neurons, is a negative regulator of precursor cell proliferation. However, after brain damage, NO overproduction in different neural and nonneural cell types promotes neurogenesis. Recently reported results on the effects of NO on new neuron generation in the adult brain are reviewed, with special attention to the proposed mechanisms of action and functional consequences in health and disease.

Anti-oxidative vitamins prevent lipid-peroxidation and apoptosis in corneal endothelial cells

To determine the effects of vitamin supplementation on the lipid-peroxidation-mediated toxicity of iron-ions on corneal endothelial cells (CECs) leading to apoptosis, murine CECs were maintained in tissue culture medium supplemented with increasing concentrations of free iron-ions, a treatment known to lead to increased lipid-peroxidation. The concentration of anti-oxidative vitamins (ascorbic acid, tocopherol and retinoic acid) in the cell supernatant and in the cells was determined by high-pressure liquid chromatography. Apoptosis was assessed by quantification of caspase-3-like activity and by using annexin-V/propidium iodide stains for flow cytometry. Lipid-peroxidation was measured by the malondialdehyde method. Supplementation with anti-oxidative vitamins was tested for the ability to counteract the induction of apoptosis. The production of nitric oxide was assessed spectrophotometrically and the expression levels of inducible and endothelial nitric oxide synthase were determined by Western blot. Increasing levels of free iron led to a rapid loss of anti-oxidative vitamins in the supernatant and in the CECs. This was correlated with rising levels of malondialdehyde and increased apoptosis. Supplementation with ascorbic acid or alpha-tocopherol alone did not prevent lipid-peroxidation in the cells. A combination of vitamins C and E (ascorbic acid, tocopherol) or solitary supplementation with vitamin A (retinoic acid) prevented lipid-peroxidation. We thus present a novel in vitro model for testing the direct influence of pro-oxidative species on CECs. We also show that supplementation with anti-oxidative vitamins to CECs significantly prevents the generation of free-radical-induced oxidative injury and apoptosis. These findings may have important implications for the storage of human corneae prior to transplantation and for the prolongation of corneal graft survival.

The vasoactive peptide adrenomedullin is secreted by adipocytes and inhibits lipolysis through NO-mediated beta-adrenergic agonist oxidation

Adipocytes are known to secrete a number of adipokines, but many adipocyte secretions and their functional importance remain to be characterized. This work shows that human white adipocytes and 3T3-F442A-derived adipocytes produce adrenomedullin (AM) and that AM acts in an autocrine/paracrine way on lipid metabolism by extracellular inactivation of isoproterenol, a beta-adrenergic agonist. AM is described as a counter-regulatory factor involved in the control of cardiovascular homeostasis. This peptide is believed to protect the heart from several complications implicated in obesity-linked cardiomorbidity, such as arterial hypertension, cardiac fibrosis, and decreased sinusal variability. The exact source of circulating AM remains a matter of debate, although endothelial and vascular smooth muscle cells seem to be important sites of production. We show that human adipose cells and 3T3-F442A-derived adipocytes express AM receptors and secrete AM. The function of this feature was investigated in 3T3-F442A cell line at the level of lipolysis regulation. AM inhibited beta-adrenergic-stimulated lipolysis by a nitric oxide (NO)-dependent mechanism, inducing a significant decrease in pD2 value for isoproterenol (8.6 +/- 0.2 vs. 9.8 +/- 0.1, P<0.001). This effect is cGMP-independent since it occurred in the presence of the NO-sensitive guanylate cyclase inhibitor ODQ. It is apparently mediated by a novel extracellular mechanism. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) demonstrated that AM-produced NO oxidized isoproterenol to generate its aminochrome, namely isoprenochrome. Isoprenochrome amounts were increased 3.62 +/- 1.13-fold in cell culture media (P<0.05). We describe for the first time that AM down-regulates lipolysis in adipocytes through the chemical modification of a beta-agonist.

Protein S-nitrosylation: purview and parameters

S-nitrosylation, the covalent attachment of a nitrogen monoxide group to the thiol side chain of cysteine, has emerged as an important mechanism for dynamic, post-translational regulation of most or all main classes of protein. S-nitrosylation thereby conveys a large part of the ubiquitous influence of nitric oxide (NO) on cellular signal transduction, and provides a mechanism for redox-based physiological regulation.

Nitric oxide decreases IGF-1 receptor function in vitro; glutathione depletion enhances this effect in vivo

Insulin-like growth factor-1 (IGF) helps maintain healthy articular cartilage; however, arthritic cartilage becomes less responsive to the anabolic actions of IGF. We previously showed that high concentrations of nitric oxide (NO) decrease IGF receptor tyrosine phosphorylation and response to IGF in intact chondrocytes. The current studies evaluate direct effects of NO on IGF receptor kinase (IGF-RK) in vitro. NO from S-nitroso-N-acetyl-d,l-penicillamine (SNAP) or 1-hydroxy-2-oxo-3-(N-3-methyl-aminopropyl)-3-methyl-1-triazene (NOC-7) inhibits IGF-RK auto- and substrate phosphorylation in a dose and time dependent manner. There is a linear correlation between inhibition of auto- and substrate phosphorylation (r(2)=0.98). Increasing either dithiothreitol or reduced glutathione (GSH) content of the phosphorylation buffer to protect thiol groups blocks NO inhibition of IGF-RK substrate phosphorylation. Increased S-nitrosylation of cysteines in IGF-RK after exposure to SNAP suggests that NO may react with sulfhydryl groups, form S-nitrosothiols, which may result in functional modifications. NO blockade of IGF-1 stimulated proteoglycan synthesis in intact cells is enhanced when chondrocyte glutathione is depleted. The in vitro system shows that there can be direct effects of NO on IGF-RK that modify receptor function; the intact cell studies suggest that the mechanisms identified in vitro may be important in intact chondrocyte insensitivity to IGF-1 in cells exposed to NO.

Nitric oxide synthesis inhibition increases proliferation of neural precursors isolated from the postnatal mouse subventricular zone

The subventricular zone (SVZ) of rodents retains the capacity to generate new neurons throughout the entire life of the animal. Neural progenitors of the SVZ survive and proliferate in vitro in the presence of epidermal growth factor (EGF). Nitric oxide (NO) has been shown to participate in neural tissue formation during development and to have antiproliferative actions, mediated in part by inhibition of the EGF receptor. Based on these findings, we have investigated the possible effects of endogenously produced and exogenously added NO on SVZ cell proliferation and differentiation. Explants were obtained from postnatal mouse SVZ and cultured in the presence of EGF. Cells migrated out of the explants and proliferated in culture, as assessed by bromodeoxyuridine (BrdU) incorporation. After 72 h in vitro, the colonies formed around the explants were constituted by cells of neuronal or glial lineages, as well as undifferentiated progenitors. Immunoreactivity for the neuronal isoform of NO synthase was observed in neuronal cells with long varicose processes. Cultures treated with the NOS inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) showed an increase in the percentage of BrdU-immunoreactive cells, whereas treatment with the NO donor diethylenetriamine-nitric oxide adduct (DETA-NO) led to a decrease in cell proliferation, without affecting apoptosis. The differentiation pattern was also altered by L-NAME treatment resulting in an enlargement of the neuronal population. The results suggest that endogenous NO may contribute to postnatal neurogenesis by modulating the proliferation and fate of SVZ progenitor cells.

Nitric oxide is a physiological inhibitor of neurogenesis in the adult mouse subventricular zone and olfactory bulb

The subventricular zone of the rodent brain retains the capacity of generating new neurons in adulthood. The newly formed neuroblasts migrate rostrally toward the olfactory bulb, where they differentiate as granular and periglomerular interneurons. The reported presence of differentiated neurons expressing the neuronal isoform of nitric oxide synthase (NOS) in the periphery of the neurogenic region and the organization of their varicose axons as a network in which the precursors are immersed raised the hypothesis that endogenous nitric oxide (NO) may participate in the control of neurogenesis in the subventricular zone. Systemic administration of the NOS inhibitors N(omega)-nitro-L-arginine methyl ester or 7-nitroindazole to adult mice produced a dose- and time-dependent increase in the number of mitotic cells in the subventricular zone, rostral migratory stream, and olfactory bulb, but not in the dentate gyrus of the hippocampus, without affecting apoptosis. In the subventricular zone, this effect was exerted selectively on a precursor subpopulation expressing nestin but not neuronal or glial cell-specific proteins. In addition, in the olfactory bulb, analysis of maturation markers in the newly generated neurons indicated that chronic NOS inhibition caused a delay in neuronal differentiation. Postmitotic cell survival and migration were not affected when NO production was impaired. Our results suggest that NO, produced by nitrergic neurons in the adult mouse subventricular zone and olfactory bulb, exerts a negative control on the size of the undifferentiated precursor pool and promotes neuronal differentiation.

Peroxynitrite inhibits epidermal growth factor receptor signaling in Caco-2 cells

Intestinal mucosa serves as an important barrier that may be disrupted by inflammation. A complex system of cellular and humoral factors, including epidermal growth factor (EGF), maintains the integrity of this barrier. We hypothesized that peroxynitrite, generated in inflamed intestinal epithelium, can alter the EGF receptor function by nitrating tyrosine residues and blocking ligand-activated tyrosine autophosphorylation. Caco-2 cells or A431 cell membranes were treated with peroxynitrite or its decomposed form. Cell proliferation was measured by [3H]thymidine uptake. Immunoblot and immunoprecipitation were used to assess the tyrosine phosphorylation and nitration. Binding of epidermal growth factor to its receptor was detected by affinity labeling with 125I-EGF. Peroxynitrite inhibited EGF-induced Caco-2 cell proliferation and binding of EGF to its receptor in a concentration-dependent manner. Peroxynitrite abolished EGF-stimulated receptor autophosphorylation and nitrated EGF receptor tyrosine residues. Peroxynitrite generated during inflammation may disrupt the EGF-induced signaling in intestinal epithelial cells.

Nitric oxide inhibition of IGF-1 stimulated proteoglycan synthesis: role of cGMP

Insulin-like growth factor (IGF-1) is critical for normal development and maintenance of cartilage, however arthritic cartilage responds poorly to IGF-1; part of this insensitivity is mediated by nitric oxide (NO). These studies test if cGMP is responsible for NO dependent insensitivity to IGF-1 in chondrocytes in situ in organ culture and in monolayer culture. Lapine cartilage and chondrocytes in monolayer culture and cartilage from osteoarthritic human knees were used. Tissues were exposed to NO from iNOS induced by IL-1, and proteoglycan synthesis in response to IGF-1 was evaluated in the presence and absence of cGMP dependent protein kinase (PKG) inhibitors. PKG activators inhibited IGF-1 responses in cartilage but not chondrocytes in monolayer. IL-1 stimulated cGMP synthesis in both monolayer and organ cultures. However, PKG inhibitors in cartilage slices but not in monolayer cultures restored response to IGF-1. PKG activity was detected in both fresh and monolayer chondrocytes, confirming this part of the cGMP signal cascade is intact in both of the preparations evaluated. Arthritic cartilage response to IGF-1 was restored by both N(G)-monomethyl-L-arginine inhibition of NO synthesis and PKG inhibitors. The data suggests that cGMP mediated effects are critical to NO actions on chondrocytes in situ in the cartilage matrix and supports a role for cGMP in the pathophysiologic effects of NO in osteoarthritis.

Evidence that nitroprusside stimulates glucose uptake in isolated rat cardiomyocytes via mitogen-activated protein kinase

Sodium nitroprusside (SNP), a nitric oxide (NO.) donor, stimulates glucose uptake in skeletal muscle. We investigated the stimulatory effect of SNP on glucose uptake in cardiomyocytes and the possible role of soluble guanylate cyclase, phosphatidylinositol-3-kinase (PI-3-kinase) and the mitogen-activated protein kinases (MAPKs). Cardiomyocytes were isolated from adult male Wistar rats by trypsin/collagenase perfusion and glucose uptake determined from the accumulation of 3H-2-deoxyglucose. SNP caused a dose-dependent increase in glucose uptake with 200-300% increase at 30 mM. Cytochalasin B completely prevented the SNP-induced increase in glucose uptake. 8-Br-cGMP (100 microM) and the NO. donor spermineNONOate (100 microM) were without effect on basal glucose uptake. SNP-stimulated glucose uptake was not inhibited by the guanylate cyclase inhibitor ODQ (10 microM). Sodium ferrocyanide (Na4Fe(CN)6), a compound structurally related to SNP, but without any NO. group, also stimulated glucose uptake in cardiomyocytes suggesting that the effect of SNP could be unrelated to liberation of NO. Wortmannin, an inhibitor of PI-3-kinase, inhibited insulin-stimulated glucose uptake completely but did not affect SNP-stimulated glucose uptake. SNP-stimulated glucose uptake was inhibited by 50 microM PD 098059 (inhibitor of the MAPK-kinases that activate external regulated kinase [ERK1/2]) and by 50 microM SB203580 (inhibitor of p38MAPK). In conclusion, high SNP concentrations dose-dependently stimulate glucose uptake in cardiomyocytes and our data suggest a role for MAPK signalling, but not PI-3-kinase and soluble guanylate cyclase, in stimulation of glucose uptake.

Nitric oxide-induced epidermal growth factor-dependent phosphorylations in A431 tumour cells

Nitric oxide (NO*) strongly inhibits the proliferation of human A431 tumour cells. It also inhibits tyrosine phosphorylation of a 170-kDa band corresponding to the epidermal growth factor receptor (EGFR) and induces the phosphorylation at tyrosine residue(s) of a 58-kDa protein which we have denoted NOIPP-58 (nitric oxide-induced 58-kDa phosphoprotein). The NO*-induced phosphorylation of NOIPP-58 is strictly dependent on the presence of EGF. Phosphorylation of NOIPP-58 and inhibition of the phosphorylation of the band corresponding to EGFR are both cGMP-independent processes. We also demonstrate that the p38 mitogen-activated protein kinase (p38MAPK) pathway is activated by NO* in the absence and presence of EGF, whereas the activity of the extracellular signal-regulated protein kinase 1/2 (ERK1/2) and the c-Jun N-terminal kinase 1/2 (JNK1/2) pathways are not significantly affected or are slightly decreased, respectively, on addition of this agent. Moreover, we show that the p38MAPK inhibitor, SB202190, induces rapid vanadate/peroxovanadate-sensitive dephosphorylation of prephosphorylated EGFR and NOIPP-58. We propose that the dephosphorylation of both NOIPP-58 and EGFR are mediated by a p38MAPK-controlled phosphotyrosine-protein phosphatase (PYPP). Activation of the p38MAPK pathway during nitrosative stress probably prevents the operation of this PYPP, allowing NOIPP-58, and in part EGFR, to remain phosphorylated and therefore capable of generating signalling events.

Regulation of protein tyrosine phosphatase 1B in intact cells by S-nitrosothiols

Protein tyrosine phosphatases (PTPases) contain an active site cysteine which when oxidized leads to loss of phosphatase activity and accumulation of phosphoproteins. For example, oxidants produced following EGF stimulation inhibit PTP1B and enhance EGF receptor phosphorylation. Because NO-derived species also modify reactive thiols, we postulated that NO would reversibly inhibit PTP1B. In our studies we exposed A431 or Jurkat cells to NO donors and measured PTP1B activity or used 3-maleimidylpropionylbiocytin (MPB) to measure thiol redox status. Nitrosothiols led to a rapid inhibition of PTP1B through a mechanism that was greatly enhanced by addition of cysteine to the medium. Analysis of thiol oxidation status using immunoprecipitated PTP1B showed modification consistent with loss of activity. Both enzyme inhibition and modification were reversible in intact cells or after addition of DTT to cell lysates. While DTT reversed oxidation, ascorbate did not, suggesting that formation of a mixed disulfide (possibly glutathionylation) rather than S-nitrosylation accounts for PTP1B inhibition. Importantly, PTP1B inhibition by nitrosothiols led to EGF receptor phosphorylation even in the absence of exogenously added EGF. These findings suggest an important role for NO in modulating signaling pathways since inhibition of PTPases could potentially enhance or prolong activity of phosphoproteins.

Interleukin-1beta inhibits ATP-induced protein kinase B activation in renal mesangial cells by two different mechanisms: the involvement of nitric oxide and ceramide

1 Extracellular nucleotides, like ATP and UTP, have been shown to activate the protein kinase B (PKB) pathway in mesangial cells. In this study we report that the pro-inflammatory cytokine interleukin-1beta (IL-1beta) inhibits ATP-induced PKB activation. 2 Pretreatment of mesangial cells with IL-1beta leads to a time-dependent decrease of ATP-induced PKB phosphorylation. Maximal inhibition is seen after 6 h of pretreatment. Incubating cells with IL-1beta in the presence of the NO synthase inhibitor, N-monomethyl-L-arginine (L-NMMA), reversed the IL-1beta inhibition of PKB activity. A similar decrease in ATP-evoked PKB activation is obtained when cells were pretreated with the nitric oxide (NO) donor, (Z)-1-[2-Aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (Deta-NO), but not with the cell-permeable cGMP analogue, 8-bromo-cGMP. 3 The NO- and IL-1beta-mediated delayed inhibition of PKB activity is completely reversed by the phosphatase inhibitor calyculin A, but not by ocadaic acid, suggesting that NO upregulates a protein phosphatase activity, which most probably belongs to the group of protein phosphatases type 1. 4 In addition, IL-1beta also triggers a short-term and transient inhibitory effect on ATP-induced PKB activation which is maximal after 2-5 min of pre-incubation with IL-1beta. This effect occurs independently of NO generation, because no NO synthase is expressed at that time, and consequently, L-NMMA does not reverse the effect. Rather an involvement of the sphingolipid ceramide is likely, since IL-1beta triggers rapid ceramide formation and incubation of cells with the cell-permeable C6-ceramide blocked ATP-induced PKB phosphorylation. 5 In summary, our data show that IL-1beta exerts both short-term and long-term inhibitory effects on ATP-stimulated PKB activation, the short-term effect probably involves ceramide formation, whereas the long-term effect is due to inducible NO synthase induction and subsequent NO formation. These results reveal a further facet in the mechanisms of ceramide- and NO-induced cell death, i.e. by turning off the survival signal elicited by PKB.

Nitric Oxide Signalling with a Special Focus on Lipid-Derived Mediators

The ways in which cells communicate among each other concerns all aspects of biology, from developmental processes to diseases. Nitric oxide (NO) is one of the most remarkable and unusual regulatory molecules. It is a labile free radical gas that is not stored but generated on demand, and has been implicated in an extraordinarily diverse range of physiological and pathophysiological functions. The modulation of cell signalling by free radicals is an emerging area of research that provides insight into the orchestration of cell adaptation to a changing microenvironment. In a multicellular organism this serves to coordinate complex physiological responses, such as inflammation. Cell signalling is also accompanied by rapid remodelling of membrane lipids by activated lipases. The discovery that NO, which does not reversibly interact with membrane receptors like conventional hormones and growth factors, targets enzymes such as phospholipase A2, sphingomyelinases or ceramidases, has stimulated growing interest in the crosstalk between redox and lipid signalling.

Role of nitric oxide in chronic allergen-induced airway cell proliferation and inflammation

Chronic cellular inflammation and airway wall remodeling with subepithelial fibrosis and airway smooth muscle thickening are features of chronic asthma. We determined the role of nitric oxide in the pathogenesis of allergen-induced airway cell proliferation and inflammation by studying the effects of a relatively selective prodrug inhibitor of nitric-oxide synthase type 2 (NOS2), L-N6-(1-iminoethyl)-lysine-5-tetrazole amide (SC-51). Brown-Norway rats were sensitized to ovalbumin and were exposed to ovalbumin aerosol every 3rd day on six occasions and were treated orally with either vehicle or SC-51 (10 mg. kg(-1); 12 doses). We measured inflammatory cell accumulation in the airways and proliferation of cells by incorporation of bromodeoxyuridine. There was an increase in the total number of airway smooth muscle cells expressing bromodeoxyuridine from 1.3% of airway smooth muscle cells in saline exposed to 5.4% after allergen-exposure (P < 0.001) and airway epithelial cells from 3.3 cells/mm basement membrane to 9.6 after allergen-exposure (P < 0.001). SC-51 had no effect on airway smooth muscle or epithelial cell proliferation. SC-51 attenuated the allergen-induced increase in major basic protein (MBP+) eosinophil (P < 0.05) and CD4+ T-cell (P < 0.05) accumulation. We conclude that nitric oxide derived during allergic inflammation is involved in the expression of eosinophilic inflammation and not in epithelial or airway smooth muscle cell DNA synthesis induced by chronic allergen exposure.

Antiproliferative effect of nitric oxide on epidermal growth factor-responsive human neuroblastoma cells

Addition of nitric oxide (NO) donors to NB69 neuroblastoma cells produced a cGMP-independent decrease in cell proliferation, without affecting cell viability or apoptosis. The potency of short half-life NO donors was higher when cell proliferation was stimulated by epidermal growth factor (EGF), as compared with cultures exposed to fetal calf serum (FCS). Immunoprecipitation and western blot analysis of the EGF receptor (EGFR) revealed a significant reduction of its EGF-induced tyrosine phosphorylation in cells treated with the NO donor 2-(N,N-diethylamino)-diazenolate-2-oxide (DEA-NO). When total cell lysates were subjected to western blotting, we observed that DEA-NO also reduced tyrosine phosphorylation in EGF-activated phosphoproteins, but not in those proteins whose tyrosine phosphorylation was evident in the absence of EGF. The effect of NO on EGFR transphosphorylation was concentration-dependent and transient, with a total recovery observed between 1.5 and 3 h after addition of DEA-NO to the cells. When cells were incubated for 15 min with DEA-NO and then washed, the EGFR transphosphorylation returned to control levels immediately, indicating that the interaction of NO with the receptor molecule was fully reversible. NB69 cells expressed both the neuronal and the inducible isoforms of NO synthase (NOS) when cultured in the presence of FCS; under this condition, the NOS inhibitor, N(omega)-nitro-L-arginine methyl ester, produced a small but significant increase in cell proliferation. The results suggest that NO is an endogenous antimitotic agent and that its interaction with EGFR contributes to cytostasis in NB69 cells.

Evidence that peroxynitrite affects human osteoblast proliferation and differentiation

Peroxynitrite (PN), a nitric oxide (NO*)-derived anion, has been associated with NO* damage in various cell types. We examined the effects of adding PN to cultured human osteoblast-like (hOB) cells obtained after hip arthroplasty. Exposure to PN (0.1-0.4 mM) decreased both hOB proliferation and differentiation, measured by [3H]thymidine uptake and alkaline phosphatase production, respectively. Incubation with 3-morpholinosydnonimine (SIN-1; 0.25-1 mM), an NO* and O2- donor that leads to PN release, also reduced both hOB proliferation and differentiation. Coincubation with both superoxide dismutase (SOD; 100 U/ml) and catalase (CAT; 50 U/ml), rendering SIN-1 a pure NO* donor, reversed its effects on hOB proliferation and differentiation. However, SIN-1-induced NO* production, measured by nitrite release to the hOB medium, was not altered by cotreatment with SOD and CAT. Expression of nitrotyrosine by hOB, a marker of PN action, was significantly increased after SIN-1 addition, as compared with untreated cells, as revealed by Western blot analysis. Interleukin-1alpha (IL-1alpha) and interferon gamma (IFN-gamma) but not tumor necrosis factor alpha (TNF-alpha) also significantly increased nitrotyrosine expression in these cells. These data show that PN is at least partially responsible for osteoblast derangement by NO* and that cytokines released during inflammatory arthropathies can induce PN production in hOB cells.

Evidence of oxidative stress in human corneal diseases

This study localized malondialdehyde (MDA, a toxic byproduct of lipid peroxidation), nitrotyrosine [NT, a cytotoxic byproduct of nitric oxide (NO)], and nitric oxide synthase isomers (NOS) in normal and diseased human corneas. Normal corneas (n=11) and those with clinical and histopathological diagnoses of keratoconus (n=26), bullous keratopathy (n=17), and Fuchs' endothelial dystrophy (n=12) were examined with antibodies specific for MDA, NT, eNOS (constitutive NOS), and iNOS (inducible NOS). Normal corneas showed little or no staining for MDA, NT, or iNOS, whereas eNOS was detected in the epithelium and endothelium. MDA was present in all disease groups, with each group displaying a distinct pattern of staining. NT was detected in all keratoconus and approximately one half of Fuchs' dystrophy corneas. iNOS and eNOS were evident in all the diseased corneas. Keratoconus corneas showed evidence of oxidative damage from cytotoxic byproducts generated by lipid peroxidation and the NO pathway. Bullous keratopathy corneas displayed byproducts of lipid peroxidation but not peroxynitrite (MDA but not NT). Conversely, Fuchs' dystrophy corneas displayed byproducts of peroxynitrite with little lipid peroxidation (NT >> MDA). These data suggest that oxidative damage occurs within each group of diseased corneas. However, each disease exhibits a distinctive profile, with only keratoconus showing prominent staining for both nitrotyrosine and MDA. These results suggest that keratoconus corneas do not process reactive oxygen species in a normal manner, which may play a major role in the pathogenesis of this disease.