S-phase arrest by reactive nitrogen species is bypassed by okadaic acid, an inhibitor of protein phosphatases PP1/PP2A

Nitric Oxide Metabolism Affects Germination in Botrytis cinerea and Is Connected to Nitrate Assimilation

Nitric oxide regulates numerous physiological processes in species from all taxonomic groups. Here, its role in the early developmental stages of the fungal necrotroph Botrytis cinerea was investigated. Pharmacological analysis demonstrated that NO modulated germination, germ tube elongation and nuclear division rate. Experimental evidence indicates that exogenous NO exerts an immediate but transitory negative effect, slowing down germination-associated processes, and that this effect is largely dependent on the flavohemoglobin BCFHG1. The fungus exhibited a “biphasic response” to NO, being more sensitive to low and high concentrations than to intermediate levels of the NO donor. Global gene expression analysis in the wild-type and ΔBcfhg1 strains indicated a situation of strong nitrosative and oxidative stress determined by exogenous NO, which was much more intense in the mutant strain, that the cells tried to alleviate by upregulating several defense mechanisms, including the simultaneous upregulation of the genes encoding the flavohemoglobin BCFHG1, a nitronate monooxygenase (NMO) and a cyanide hydratase. Genetic evidence suggests the coordinated expression of Bcfhg1 and the NMO coding gene, both adjacent and divergently arranged, in response to NO. Nitrate assimilation genes were upregulated upon exposure to NO, and BCFHG1 appeared to be the main enzymatic system involved in the generation of the signal triggering their induction. Comparative expression analysis also showed the influence of NO on other cellular processes, such as mitochondrial respiration or primary and secondary metabolism, whose response could have been mediated by NmrA-like domain proteins.

The inflammatory mediators TNFα and nitric oxide arrest spermatogonia GC-1 cell cycle

During an inflammatory process of the testis, the network of somatic, immune, and germ cell interactions is altered leading to organ dysfunction. In testicular biopsies of infertile men, spermatogenesis impairment is associated with reduced spermatogonia proliferation, increased number of immune cells, and content of pro-inflammatory cytokines. TNFα-TNFR and nitric oxide (NO)-NO synthase systems are up-regulated in models of testicular damage and in human testis with maturation arrest. The purpose of this study was to test the hypothesis that TNFα-TNFR system and NO alter the function of spermatogonia in the inflamed testis. We studied the effect of TNFα and NO on GC-1 spermatogonia cell cycle progression and death by flow cytometry. GC-1 cells expressed TNFR1 and TNFR2 (immunofluorescence). TNFα (10 and 50 ng/ml) and DETA-Nonoate (0.5 and 2 mM), a NO releaser, increased the percentage of cells in S-phase of the cell cycle and reduced the percentage in G1, inducing also cell apoptosis. TNFα effect was not mediated by oxidative stress unlike NO, since the presence of N-acetyl-l-cysteine (2.5 and 5.0 mM) prevented NO induced cell cycle arrest and death. GC-1 spermatogonia overpass NO induced cell cycle arrest but no TNFα, since after removal of NO, spermatogonia progressed through the cell cycle. We propose TNFα and NO might contribute to impairment of spermatogenesis by preventing adequate functioning of the spermatogonia population. Our results showed that TNFα and NO impaired spermatogonia cell cycle, inducing GC-1 arrest in the S phase.

Cornel Iridoid Glycoside Inhibits Tau Hyperphosphorylation via Regulating Cross-Talk Between GSK-3β and PP2A Signaling

Neurofibrillary pathology contributes to neuronal dysfunction and correlates with the clinical progression of Alzheimer's disease (AD). Tau phosphorylation is mainly regulated by a balance of glycogen synthase kinase-3β (GSK-3β) and protein phosphatase 2A (PP2A) activities. Cornel iridoid glycoside (CIG) is a main component extracted from Cornus officinalis. The purpose of this study was to investigate the effects of CIG on GSK-3β and PP2A, thus to explore the mechanisms of CIG to inhibit tau hyperphosphorylation. The rat model of tau hyperphosphorylation was established by intraventricular injection of wortmannin and GF-109203X (GFX) to activate GSK-3β. The results showed that intragastrical administration of CIG inhibited tau hyperphosphorylation in the brain of rats induced by wortmannin/GFX. The results in vivo and in vitro exhibited that CIG inhibited tau hyperphosphorylation and GSK-3β over-activation. In the mechanism of action, CIG's attenuating GSK-3β activity was found to be dependent on PI3K/AKT signaling pathway. PP2A catalytic C subunit (PP2Ac) siRNA abrogated the effect of CIG on PI3K/AKT/GSK-3β. Additionally and crucially, we also found that CIG inhibited the demethylation of PP2Ac at Leu309 in vivo and in vitro. It enhanced PP2A activity, decreased tau hyperphosphorylation, and protected cell morphology in okadaic acid (OA)-induced cell model in vitro. PP2Ac siRNA abated the inhibitory effect of CIG on tau hyperphosphorylation. Moreover, CIG inhibited protein phosphatase methylesterase-1 (PME-1) and demethylation of PP2Ac, enhanced PP2A activity, and decreased tau hyperphosphorylation in PME-1-transfectd cells. Taken together, CIG inhibited GSK-3β activity via promoting P13K/AKT and PP2A signaling pathways. In addition, CIG also elevated PP2A activity via inhibiting PME-1-induced PP2Ac demethylation to inhibit GSK-3β activity, thus regulated the cross-talk between GSK-3β and PP2A signaling and consequently inhibited tau hyperphosphorylation. These results suggest that CIG may be a promising agent for AD therapy.

Nitrosative stress induces a novel intra-S checkpoint pathway in Schizosaccharomyces pombe involving phosphorylation of Cdc2 by Wee1

Excess production of nitric oxide and reactive nitrogen intermediates causes nitrosative stress on cells. Schizosaccharomyces pombe was used as a model to study the cell cycle regulation under nitrosative stress response. We discovered a novel intra-S-phase checkpoint that is activated in S. pombe under nitrosative stress. The mechanism for this intra-S-phase checkpoint activation is distinctly different than previously reported for genotoxic stress in S. pombe by methyl methane sulfonate. Our flow cytometry data established the fact that Wee1 phosphorylates Cdc2 Tyr15 which leads to replication slowdown in the fission yeast under nitrosative stress. We checked the roles of Rad3, Rad17, Rad26, Swi1, Swi3, Cds1, and Chk1 under nitrosative stress but those were not involved in the activation of the DNA replication checkpoint. Rad24 was found to be involved in intra-S-phase checkpoint activation in S. pombe under nitrosative stress but that was independent of Cdc25.

Reactive nitrogen species and hydrogen sulfide as regulators of protein tyrosine phosphatase activity

SIGNIFICANCE

Redox modifications of thiols serve as a molecular code enabling precise and complex regulation of protein tyrosine phosphatases (PTPs) and other proteins. Particular gasotransmitters and even the redox modifications themselves affect each other, of which a typical example is S-nitrosylation-mediated protection against the further oxidation of protein thiols.

RECENT ADVANCES

For a long time, PTPs were considered constitutively active housekeeping enzymes. This view has changed substantially over the last two decades, and the PTP family is now recognized as a group of tightly and flexibly regulated fundamental enzymes. In addition to the conventional ways in which they are regulated, including noncovalent interactions, phosphorylation, and oxidation, the evidence that has accumulated during the past two decades suggests that many of these enzymes are also modulated by gasotransmitters, namely by nitric oxide (NO) and hydrogen sulfide (H2S).

CRITICAL ISSUES

The specificity and selectivity of the methods used to detect nitrosylation and sulfhydration remains to be corroborated, because several researchers raised the issue of false-positive results, particularly when using the most widespread biotin switch method. Further development of robust and straightforward proteomic methods is needed to further improve our knowledge of the full extent of the gasotransmitters-mediated changes in PTP activity, selectivity, and specificity. FURTHER DIRECTIONS: Results of the hitherto performed studies on gasotransmitter-mediated PTP signaling await translation into clinical medicine and pharmacotherapeutics. In addition to directly affecting the activity of particular PTPs, the use of reversible S-nitrosylation as a protective mechanism against oxidative stress should be of high interest.

Resolution of oxidative stress by thioredoxin reductase: Cysteine versus selenocysteine

Thioredoxin reductase (TR) catalyzes the reduction of thioredoxin (TRX), which in turn reduces mammalian typical 2-Cys peroxiredoxins (PRXs 1-4), thiol peroxidases implicated in redox homeostasis and cell signaling. Typical 2-Cys PRXs are inactivated by hyperoxidation of the peroxidatic cysteine to cysteine-sulfinic acid, and regenerated in a two-step process involving retro-reduction by sulfiredoxin (SRX) and reduction by TRX. Here transient exposure to menadione and glucose oxidase was used to examine the dynamics of oxidative inactivation and reactivation of PRXs in mouse C10 cells expressing various isoforms of TR, including wild type cytoplasmic TR1 (Sec-TR1) and mitochondrial TR2 (Sec-TR2) that encode selenocysteine, as well as mutants of TR1 and TR2 in which the selenocysteine codon was changed to encode cysteine (Cys-TR1 or Cys-TR2). In C10 cells endogenous TR activity was insensitive to levels of hydrogen peroxide that hyperoxidize PRXs. Expression of Sec-TR1 increased TR activity, reduced the basal cytoplasmic redox state, and increased the rate of reduction of a redox-responsive cytoplasmic GFP probe (roGFP), but did not influence either the rate of inactivation or the rate of retro-reduction of PRXs. In comparison to roGFP, which was reduced within minutes once oxidants were removed reduction of 2-Cys PRXs occurred over many hours. Expression of wild type Sec-TR1 or Sec-TR2, but not Cys-TR1 or TR2, increased the rate of reduction of PRXs and improved cell survival after menadione exposure. These results indicate that expression levels of TR do not reduce the severity of initial oxidative insults, but rather govern the rate of reduction of cellular factors required for cell viability. Because Sec-TR is completely insensitive to cytotoxic levels of hydrogen peroxide, we suggest TR functions at the top of a redox pyramid that governs the oxidation state of peroxiredoxins and other protein factors, thereby dictating a hierarchy of phenotypic responses to oxidative insults.

Targeting cell cycle regulation in cancer therapy

Cell proliferation is an essential mechanism for growth, development and regeneration of eukaryotic organisms; however, it is also the cause of one of the most devastating diseases of our era: cancer. Given the relevance of the processes in which cell proliferation is involved, its regulation is of paramount importance for multicellular organisms. Cell division is orchestrated by a complex network of interactions between proteins, metabolism and microenvironment including several signaling pathways and mechanisms of control aiming to enable cell proliferation only in response to specific stimuli and under adequate conditions. Three main players have been identified in the coordinated variation of the many molecules that play a role in cell cycle: i) The cell cycle protein machinery including cyclin-dependent kinases (CDK)-cyclin complexes and related kinases, ii) The metabolic enzymes and related metabolites and iii) The reactive-oxygen species (ROS) and cellular redox status. The role of these key players and the interaction between oscillatory and non-oscillatory species have proved essential for driving the cell cycle. Moreover, cancer development has been associated to defects in all of them. Here, we provide an overview on the role of CDK-cyclin complexes, metabolic adaptations and oxidative stress in regulating progression through each cell cycle phase and transitions between them. Thus, new approaches for the design of innovative cancer therapies targeting crosstalk between cell cycle simultaneous events are proposed.

Regulation of cell cycle and stress responses under nitrosative stress in Schizosaccharomyces pombe

Nitric oxide (NO) acts as a signaling molecule in numerous physiological processes but excess production generates nitrosative stress in cells. The exact protective mechanism used by cells to combat nitrosative stress is unclear. In this study, the fission yeast Schizosaccharomyces pombe has been used as a model system to explore cell cycle regulation and stress responses under nitrosative stress. Exposure to an NO donor results in mitotic delay in cells through G2/M checkpoint activation and initiates rereplication. Western blot analysis of phosphorylated Cdc2 revealed that the G2/M block in the cell cycle was due to retention of its inactive phosphorylated form. Interestingly, nitrosative stress results in inactivation of Cdc25 through S-nitrosylation that actually leads to cell cycle delay. From differential display analysis, we identified plo1, spn4, and rga5, three cell cycle-related genes found to be differentially expressed under nitrosative stress. Exposure to nitrosative stress also results in abnormal septation and cytokinesis in S. pombe. In summary we propose a novel molecular mechanism of cell cycle control under nitrosative stress based on our experimental results and bioinformatics analysis.

L-arginine promotes DNA repair in cultured bronchial epithelial cells exposed to ozone: involvement of the ATM pathway

Ozone may lead to DNA breaks in airway epithelial cells. p-ATM (phosphorylated ataxia telangiectasia mutated) plays a pivotal role in DNA repair. Derivatives of NO (nitric oxide) are regulators of the phosphorylation, and NO is increased under oxidative stress. The present study was aimed to study the effect of NO donor L-arg (L-arginine) on DNA damage repair in human bronchial epithelial cells exposed to ozone and the potential mechanisms involved. HBECs (human bronchial epithelial cells) were cultured with or without ozone (1.5 ppm, 30 min), DNA breaks were measured with a comet assay and agarose gel electrophoresis, cell cycling was determined by flow cytometry and p-ATM was measured by immunofluorescence and Western blot. Data were analysed by ANOVA (analysis of variance). P<0.05 was considered as significant. Ozone induced marked DNA breaks, G1-phase arrest and increased expression of p-ATM in HBECs, while wortmannin reduced the levels of p-ATM induced by ozone; the NO donor, L-arg, minimized the effects of ozone-induced DNA breaks and increased the level of p-ATM, while the NO synthase inhibitor, L-NMMA [N(G)-minomethyl-L-arginine], restrained those effects of L-arg. The effect of L-arg on DNA repair is NO-mediated, and p-ATM is implicated in the processes of DNA repair.

The cell cycle is a redox cycle: linking phase-specific targets to cell fate

Reactive oxygen species (ROS) regulate the strength and duration of signaling through redox-dependent signal transduction pathways via the cyclic oxidation/reduction of cysteine residues in kinases, phosphatases, and other regulatory factors. Signaling circuits may be segregated in organelles or other subcellular domains with distinct redox states, permitting them to respond independently to changes in the oxidation state of two major thiol reductants, glutathione and thioredoxin. Studies in yeast, and in complex eukaryotes, show that oscillations in oxygen consumption, energy metabolism, and redox state are intimately integrated with cell cycle progression. Because signaling pathways play specific roles in different phases of the cell cycle and the hierarchy of redox-dependent regulatory checkpoints changes during cell cycle progression, the effects of ROS on cell fate vary during the cell cycle. In G1, ROS stimulate mitogenic pathways that control the activity of cyclin-dependent kinases (CDKs) and phosphorylation of the retinoblastoma protein (pRB), thereby regulating S-phase entry. In response to oxidative stress, Nrf2 and Foxo3a promote cell survival by inducing the expression of antioxidant enzymes and factors involved in cell cycle withdrawal, such as the cyclin-dependent kinase inhibitor (CKI) p27. In S phase, ROS induce S-phase arrest via PP2A-dependent dephosphorylation of pRB. In precancerous cells, unconstrained mitogenic signaling by activated oncogenes induces replication stress in S phase, which activates the DNA-damage response and induces cell senescence. A number of studies suggest that interactions of ROS with the G1 CDK/CKI network play a fundamental role in senescence, which is considered a barrier to tumorigenesis. Adaptive responses and loss of checkpoint proteins such as p53 and p16(INK4a) allow tumor cells to tolerate constitutive mitogenic signaling and enhanced production of ROS, leading to altered redox status in many fully transformed cells. Alterations in oxidant and energy metabolism of cancer cells have emerged as fertile ground for new therapeutic targets. The present challenge is to identify redox-dependent targets relevant to each cell cycle phase, to understand how these targets control fate decisions, and to describe the mechanisms that link metabolism to cell cycle progression.

Nitrosative stress suppresses checkpoint activation after DNA synthesis inhibition

DNA synthesis is promoted by the dephosphorylation and activation of cyclin-dependent kinase 2 (Cdk2) complexes by Cdc25A. Nitrosative stress suppresses Cdk2 dephosphorylation by Cdc25A in vitro and inhibits Cdc25A protein translation in cells, but the effects on S-phase progression remain unexamined. Herein we report that nitrosative stress catalyzed by inducible nitric oxide (*NO) synthase (iNOS) or the chemical nitrosant S-nitrosocysteine ethyl ester (SNCEE) rapidly inhibited DNA synthesis concomitant with Cdc25A loss. Surprisingly, this inhibition of DNA synthesis was refractory to ectopic expression of Cdc25A or a Cdc25-independent Cdk2 mutant. Nitrosative stress inhibited DNA synthesis without activating checkpoint signaling, thus distinguishing it from S-phase arrest mediated by other reactive *NO-derived species. The apparent lack of checkpoint activation was due to an active suppression because accumulation of pSer345-Chk1, pThr68-Chk2 and gammaH2AX was inhibited by nitrosative stress in cells exposed to DNA damage or replication inhibitors. We speculate that failure to activate the S-phase checkpoint in precancerous cells undergoing nitrosative stress may elevate the risk of transmitting damaged genomes to daughter cells upon cell cycle reentry.

Multimodal control of Cdc25A by nitrosative stress

Cdc25A propels cell cycle progression, is overexpressed in numerous human cancers, and possesses oncogenic and antiapoptotic activities. Reactive oxygen species, such as hydrogen peroxide, regulate Cdc25A, but the physiologic and pathologic effects of nitric oxide (*NO) and *NO-derived reactive species are not well defined. Herein, we report novel independent mechanisms governing Cdc25A in response to nitrosative insult. We observed direct and rapid inhibition of Cdc25A phosphatase activity after in vitro treatment with the low molecular mass cell-permeable S-nitrosothiol S-nitrosocysteine ethyl ester (SNCEE). In addition, treatment of cancer cells with SNCEE induced nitrosative stress and decreased Cdc25A protein levels in a time-dependent and concentration-dependent manner. Similarly, iNOS-derived *NO was sufficient to suppress Cdc25A expression, consistent with its role in mediating nitrosative stress. Whereas a decrease in Cdc25A half-life was not observed in response to SNCEE, we found the translational regulator eukaryotic initiation factor 2alpha (eIF2alpha) was hyperphosphorylated and total protein translation was decreased with kinetics consistent with Cdc25A loss. Inhibition of eIF2alpha decreased Cdc25A levels, supporting the hypothesis that SNCEE suppressed Cdc25A translation through inhibition of eIF2alpha. Nitrosative stress decreased the Cdc25A-bound fraction of apoptosis signal-regulating kinase-1 (ASK-1) and sensitized cells to apoptosis induced by the ASK-1-activating chemotherapeutic cis-diaminedichloroplatinum (II), suggesting that nitrosative stress-induced suppression of Cdc25A primed cells for ASK-1-dependent apoptosis. Together these data reveal novel *NO-dependent enzymatic and translational mechanisms controlling Cdc25A, and implicate Cdc25A as a mediator of *NO-dependent apoptotic signaling.

Redox control of G(1)/S cell cycle regulators during nitric oxide-mediated cell cycle arrest

Redox regulation of cell cycle progression during nitric oxide (NO) mediated cytostasis is not well-understood. In this study, we investigated the role of the intracellular antioxidant glutathione (GSH) in regulating specific signaling events that are associated with NO-mediated cell cycle arrest. Manipulation of intracellular GSH content through pharmacological inhibition of glutamate-cysteine ligase (GCL) indicated that GSH depletion potentiated nitrosative stress, DNA damage, phosphorylation of the tumor suppressor p53 (Ser-18) and upregulation of p21(cip1/waf1) upon NO stimulation. However, we found that neither overexpression of a dominant negative p53 nor pharmacological inhibition of p53 with cyclic pifithrin-alpha (cPFT-alpha) was sufficient to reverse NO-mediated cell cycle arrest or hypophosphorylation of retinoblastoma protein (Rb). We found that the decrease in cyclin D1 levels induced by NO was GSH-sensitive implying that the redox regulation of NO-mediated cytostasis was a multifaceted process and that both p53/p21(cip1/waf1) and p53 independent cyclin D1 pathways were involved. Together, our results demonstrate that GSH serves as an important component of cellular protective mechanisms against NO-derived nitrosative stress to regulate DNA damage checkpoint.

Oxidation state governs structural transitions in peroxiredoxin II that correlate with cell cycle arrest and recovery

Inactivation of eukaryotic 2-Cys peroxiredoxins (Prxs) by hyperoxidation has been proposed to promote accumulation of hydrogen peroxide (H₂O₂) for redox-dependent signaling events. We examined the oxidation and oligomeric states of PrxI and -II in epithelial cells during mitogenic signaling and in response to fluxes of H₂O₂. During normal mitogenic signaling, hyperoxidation of PrxI and -II was not detected. In contrast, H₂O₂-dependent cell cycle arrest was correlated with hyperoxidation of PrxII, which resulted in quantitative recruitment of ∼66- and ∼140-kD PrxII complexes into large filamentous oligomers. Expression of cyclin D1 and cell proliferation did not resume until PrxII-SO₂H was reduced and native PrxII complexes were regenerated. Ectopic expression of PrxI or -II increased Prx-SO₂H levels in response to oxidant exposure and failed to protect cells from arrest. We propose a model in which Prxs function as peroxide dosimeters in subcellular processes that involve redox cycling, with hyperoxidation controlling structural transitions that alert cells of perturbations in peroxide homeostasis.