In budding yeast, reactive oxygen species induce both RAS-dependent and RAS-independent cell cycle-specific arrest

Thymoquinone effect on the Dictyostelium discoideum model correlates with functional roles for glutathione S-transferases in eukaryotic proliferation, chemotaxis, and development

An increasing body of literature demonstrates the therapeutic relevance of polyphenols in eukaryotic cell and animal model studies. The phase II glutathione S-transferases (GST) show differential responses to thymoquinone, a major bioactive polyphenol constituent of the black seed, Nigella sativa. Beyond antioxidant defense, GSTs may act in non-enzymatic capacities to effect cell cycle, motility, and differentiation. Here, we report the impact of thymoquinone on the life cycle of the eukaryotic model Dictyostelium discoideum and accompanying profiles of its GST-alpha (DdGSTA) enzyme activity and isozyme expression. In silico molecular modeling revealed strong interaction(s) between thymoquinone and DdGSTA2 and DdGSTA3 isozymes that correlated with in vivo, dose-dependent inhibition of cell proliferation of amoebae at 24, 48, and 72hr. Similarly, cytosolic DdGST enzyme activity (CDNB activity) was also responsive to different thymoquinone concentrations. Thymoquinone generally reduced expression of DdGSTA2 and DdGSTA3 isozymes in proliferating cells, however differential expression of the isozymes occurred during starvation. Thymoquinone effectively reduced early-stage aggregation of starved amoeba, accompanied by increased reactive oxygen species and altered expression of tubulin and contact site A (gp80), which resulted in reduced morphogenesis and fruiting body formation. These observations reveal that thymoquinone can impact signaling mechanisms that regulate proliferation and development in D. discoideum.

The Hog1 MAP Kinase Promotes the Recovery from Cell Cycle Arrest Induced by Hydrogen Peroxide in Candida albicans

Eukaryotic cell cycle progression in response to environmental conditions is controlled via specific checkpoints. Signal transduction pathways mediated by MAPKs play a crucial role in sensing stress. For example, the canonical MAPKs Mkc1 (of the cell wall integrity pathway), and Hog1 (of the HOG pathway), are activated upon oxidative stress. In this work, we have analyzed the effect of oxidative stress induced by hydrogen peroxide on cell cycle progression in Candida albicans. Hydrogen peroxide was shown to induce a transient arrest at the G1 phase of the cell cycle. Specifically, a G1 arrest was observed, although phosphorylation of Mkc1 and Hog1 MAPKs can take place at all stages of the cell cycle. Interestingly, hog1 (but not mkc1) mutants required a longer time compared to wild type cells to resume growth after hydrogen peroxide challenge. Using GFP-labeled cells and mixed cultures of wild type and hog1 cells we were able to show that hog1 mutants progress faster through the cell cycle under standard growth conditions in the absence of stress (YPD at 37°C). Consequently, hog1 mutants exhibited a smaller cell size. The altered cell cycle progression correlates with altered expression of the G1 cyclins Cln3 and Pcl2 in hog1 cells compared to the wild type strain. In addition, Hgc1 (a hypha-specific G1 cyclin) as well as Cln3 displayed a different kinetics of expression in the presence of hydrogen peroxide in hog1 mutants. Collectively, these results indicate that Hog1 regulates the expression of G1 cyclins not only in response to oxidative stress, but also under standard growth conditions. Hydrogen peroxide treated cells did not show fluctuations in the mRNA levels for SOL1, which are observed in untreated cells during cell cycle progression. In addition, treatment with hydrogen peroxide prevented degradation of Sol1, an effect which was enhanced in hog1 mutants. Therefore, in C. albicans, the MAPK Hog1 mediates cell cycle progression in response to oxidative stress, and further participates in the cell size checkpoint during vegetative growth.

Cth2 Protein Mediates Early Adaptation of Yeast Cells to Oxidative Stress Conditions

Cth2 is an mRNA-binding protein that participates in remodeling yeast cell metabolism in iron starvation conditions by promoting decay of the targeted molecules, in order to avoid excess iron consumption. This study shows that in the absence of Cth2 immediate upregulation of expression of several of the iron regulon genes (involved in high affinity iron uptake and intracellular iron redistribution) upon oxidative stress by hydroperoxide is more intense than in wild type conditions where Cth2 is present. The oxidative stress provokes a temporary increase in the levels of Cth2 (itself a member of the iron regulon). In such conditions Cth2 molecules accumulate at P bodies-like structures when the constitutive mRNA decay machinery is compromised. In addition, a null Δcth2 mutant shows defects, in comparison to CTH2 wild type cells, in exit from α factor-induced arrest at the G1 stage of the cell cycle when hydroperoxide treatment is applied. The cell cycle defects are rescued in conditions that compromise uptake of external iron into the cytosol. The observations support a role of Cth2 in modulating expression of diverse iron regulon genes, excluding those specifically involved in the reductive branch of the high-affinity transport. This would result in immediate adaptation of the yeast cells to an oxidative stress, by controlling uptake of oxidant-promoting iron cations.

The Cek1 MAPK is a short-lived protein regulated by quorum sensing in the fungal pathogen Candida albicans

Mitogen activated protein kinase (MAPK) cascades are signal transduction mechanisms present in eukaryotic cells that allow adaptation to environmental changes. MAPK activity is mainly regulated by dual phosphorylation in a TXY motif present in the kinase subdomain VIII as well as dephosphorylation by specific phosphatases. The Cek1 MAPK is involved in filamentous growth in Candida albicans and is an important determinant of virulence in this microorganism; its activation is controlled by the Sho1 adaptor protein. Here we show that Cek1 phosphorylation is regulated by quorum sensing (QS). Cek1 phosphorylation is prevented by farnesol, a compound that also regulates the dimorphic transition in this fungus. Farnesol also induced the activation of Mkc1, the MAPK of the cell integrity pathway. The role of farnesol in Cek1 phosphorylation is independent of the Chk1 histidine kinase, a putative QS sensor, as revealed by genetic analysis. In addition, Cek1, not Hog1, is degraded by proteasome, as revealed by the use of a conditional lethal protein degradation mutant. Our data therefore describe two different mechanisms (QS and protein degradation) that control a MAPK pathway that regulates virulence in a fungal pathogen.

Involvement of glutathione transferases, Gtt1and Gtt2, with oxidative stress response generated by H2O2 during growth of Saccharomyces cerevisiae

Glutathione transferases are detoxifying enzymes responsible for eliminating toxic compounds generated under a variety of stress conditions. Saccharomyces cerevisiae control cells and glutathione transferase mutant strains (gtt1 and gtt2) were used to analyze tolerance, lipid and protein oxidation as oxidative stress markers during growth in the presence of H2O2. Glucose 6-phosphate dehydrogenase (G6PD) and glutathione reductase were assayed to monitor the capacity of cells to recycle glutathione. Although a reduction in growth was observed, deletion of GTT1 showed less inhibition by H2O2 than the control strain. Cells showed a significant reduction in cellular viability during the first hours of growth, the gtt1 mutant being hypersensitive even after 24 h of H2O2 exposure. As a consequence of oxidative stress caused by exposure to H2O2, an increase in lipid peroxidation was observed, mainly in the glutathione transferase mutant strains. While protein carbonylation increased by 17% and 23%, respectively, after 2 h in the presence of H2O2 in the control and gtt2 mutant, a 40% increase was observed in the gtt1 strain after 24-h exposure. The antioxidant G6PD and glutathione reductase activities were affected in the gtt1 mutant during H2O2 exposure, which could be critical for recycling glutathione. The same was observed for the gtt2 mutant after 2-h treatment, indicating that glutathione recycling might be associated with the detoxification process. Thus, glutathione transferases, Gtt1 and Gtt2, seem to be crucial in the response to H2O2 stress.

Anoxia-induced suspended animation in budding yeast as an experimental paradigm for studying oxygen-regulated gene expression

A lack of oxygen can force many organisms to enter into recoverable hypometabolic states. To better understand how organisms cope with oxygen deprivation, our laboratory previously had shown that when challenged with anoxia, both the nematode Caenorhabditis elegans and embryos of the zebrafish Danio rerio enter into suspended animation, in which all life processes that can be observed by light microscopy reversibly halt pending the restoration of oxygen (P. A. Padilla and M. B. Roth, Proc. Natl. Acad. Sci. USA 98:7331-7335, 2001, and P. A. Padilla, T. G. Nystul, R. A. Zager, A. C. Johnson, and M. B. Roth, Mol. Biol. Cell 13:1473-1483, 2002). Here, we show that both sporulating and vegetative cells of the budding yeast Saccharomyces cerevisiae also enter into a similar state of suspended animation when made anoxic on a nonfermentable carbon source. Transcriptional profiling using cDNA microarrays and follow-on quantitative real-time PCR analysis revealed a relative derepression of aerobic metabolism genes in carbon monoxide (CO)-induced anoxia when compared to nitrogen (N(2)) gas-induced anoxia, which is consistent with the known oxygen-mimetic effects of CO. We also found that mutants deleted for components of the mitochondrial retrograde signaling pathway can tolerate prolonged exposure to CO but not to N(2). We conclude that the cellular response to anoxia is dependent on whether the anoxic gas is an oxygen mimetic and that the mitochondrial retrograde signaling pathway is functionally important for mediating this response.

Comprehensive transcriptional analysis of the oxidative response in yeast

The oxidative stress response in Saccharomyces cerevisiae has been analyzed by parallel determination of mRNA levels and transcription rates for the entire genome. A mathematical algorithm has been adapted for a dynamic situation such as the response to stress, to calculate theoretical mRNA decay rates from the experimental data. Yeast genes have been grouped into 25 clusters according to mRNA level and transcription rate kinetics, and average mRNA decay rates have been calculated for each cluster. In most of the genes, changes in one or both experimentally determined parameters occur during the stress response. 24% of the genes are transcriptionally induced without an increase in mRNA levels. The lack of parallelism between the evolution of the mRNA amount and transcription rate predicts changes in mRNA stability during stress. Genes for ribosomal proteins and rRNA processing enzymes are abundant among those whose mRNAs are predicted to destabilize. The number of genes whose mRNAs are predicted to stabilize is lower, although some protein folding or proteasomal genes are among the latter. We have confirmed the mathematical predictions for several genes pertaining to different clusters by experimentally determining mRNA decay rates using the regulatable tetO promoter in transcriptional expression conditions not affected by the oxidative stress. This study indicates that the oxidative stress response in yeast cells is not only conditioned by gene transcription but also by the mRNA decay dynamics and that this complex response may be particularly relevant to explain the temporary down-regulation of protein synthesis occurring during stress.

The Rho5 GTPase is necessary for oxidant-induced cell death in budding yeast

In both animal and yeast cells, reactive oxygen species (ROS) are produced as byproducts of metabolism and upon exposure to diverse environmental stresses. Cellular defense systems operate to avoid molecular damage caused by ROS, but the redox balance is disturbed under excessive stress. Cells of the budding yeast Saccharomyces cerevisiae undergo apoptotic-like cell death upon exposure to hydrogen peroxide (H(2)O(2)). Here, we report that the Rho5 GTPase of budding yeast is necessary for H(2)O(2)-induced cell death, which accompanies ROS accumulation and DNA fragmentation. Unlike WT, a rho5 deletion mutant (rho5Delta) exhibits little cell death, whereas the constitutively active rho5(G12V) mutant exhibits excess ROS accumulation and increased cell death upon H(2)O(2) treatment. Consistent with a role in the oxidative stress response, Rho5 interacts with the thioredoxin reductase Trr1, a key component of the cytoplasmic thioredoxin antioxidant system, in a GTP-dependent manner. This interaction occurs on the vacuolar membrane before exposure to H(2)O(2) but also in the vacuolar lumen after H(2)O(2) treatment. Trr1 levels are elevated in rho5Delta cells but are elevated only slightly in WT and not in the rho5(G12V) cells after H(2)O(2) treatment. Taken together, these data suggest that Rho5 mediates H(2)O(2)-induced cell death by regulating the level of Trr1 or by excluding Trr1 from its cytoplasmic substrate.

Nonclassical export pathway: overexpression of NCE102 reduces protein and DNA damage and prolongs lifespan in an SGS1 deficient Saccharomyces cerevisiae

In this study, we used our recently developed screening method, Bud-Scar-based Screening (BSS), to screen a yeast cDNA expression library in an SGS1 deletion BY4742 yeast strain. One gene involved in a nonclassical export pathway, NCE102, was found to extend the life span of Deltasgs1 yeast. Deletion of NCE102 in a wild type yeast strain increased its sensitivity to oxidative stress upon diethylmaleate (DEM) treatment but did not shorten its lifespan, indicating that this gene is not essential in determining yeast lifespan. Transformation of NCE102 into either Deltance102 or Deltasgs1 strains could rescue its tolerance to DEM stress, indicating that NCE102 is protective during oxidative stress. Moreover, overexpression of NCE102 in Deltasgs1 strain leads to reduced protein damage. However, overexpression of NCE102 in wild type yeast strain BY4742 neither protected against oxidative stress due to DEM nor extended yeast lifespan compared to its parental wild type strain, indicating that nonclassical export is redundant and DNA repair is fully sufficient in the wild type strain. We therefore demonstrate that a nonclassical export pathway functions as an alternative clearance/detoxification pathway to eliminate damaged material, when the basic repair pathway is not sufficient.

In Saccharomyces cerevisiae an unbalanced level of tyrosine phosphorylation down-regulates the Ras/PKA pathway

The role of tyrosyl phosphorylation/dephosphorylation in the budding yeast Saccharomyces cerevisiae, whose genome does not encode typical tyrosine kinases, has long remained elusive. Nevertheless, several protein kinases phosphorylating poly(TyrGlu) substrates have been identified. In this work, we use the expression of the low molecular weight tyrosine phosphatase Stp1 from the distantly related yeast Schizosaccharomyces pombe, as a tool to investigate whether an unbalanced level of protein tyrosine phosphorylation affects S. cerevisiae growth and metabolism. We correlate the previously reported down-regulation of the phosphotyrosine level brought about by overexpression of Stp1 with a large number of phenotypes indicative of down-regulation of the Ras pathway. These phenotypes include reduction in both glucose- and acidification-induced GTP loading of the Ras2 protein and cAMP signaling, impaired growth on a non-fermentable carbon source, alteration of cell cycle parameters, delayed recovery from nitrogen starvation, increased heat-shock resistance, attenuated pseudohyphal and invasive growth. Genetic data suggest that Stp1 acts either at, or above, the level of Ras2, possibly on the Ira proteins. Consistently, Stp1 was found to bind to immunoprecipitated Ira2. Since a catalytically inactive mutant form of Stp1 (Stp1(C11S)) effectively binds to Ira2 without producing any effect on yeast physiology, we conclude that down-regulation of the Ras pathway by Stp1 requires its phosphatase activity. In conclusion, our data suggest a possible cross-talk between tyrosine phosphorylation and the Ras pathway in yeast.

Cells have distinct mechanisms to maintain protection against different reactive oxygen species: oxidative-stress-response genes

The complete set of viable deletion strains in Saccharomyces cerevisiae was screened for sensitivity of mutants to five oxidants to identify cell functions involved in resistance to oxidative stress. This screen identified a unique set of mainly constitutive functions providing the first line of defense against a particular oxidant; these functions are very dependent on the nature of the oxidant. Most of these functions are distinct from those involved in repair and recovery from damage, which are generally induced in response to stress, because there was little correlation between mutant sensitivity and the reported transcriptional response to oxidants of the relevant gene. The screen identified 456 mutants sensitive to at least one of five different types of oxidant, and these were ranked in order of sensitivity. Many genes identified were not previously known to have a role in resistance to reactive oxygen species. These encode functions including protein sorting, ergosterol metabolism, autophagy, and vacuolar acidification. Only two mutants were sensitive to all oxidants examined, only 12 were sensitive to at least four, and different oxidants had very different spectra of deletants that were sensitive. These findings highlight the specificity of cellular responses to different oxidants: No single oxidant is representative of general oxidative stress. Mitochondrial respiratory functions were overrepresented in mutants sensitive to H(2)O(2), and vacuolar protein-sorting mutants were enriched in mutants sensitive to diamide. Core functions required for a broad range of oxidative-stress resistance include transcription, protein trafficking, and vacuolar function.

Alkyl hydroperoxide reductase 1 protects Saccharomyces cerevisiae against metal ion toxicity and glutathione depletion

Alkyl hydroperoxide reductase 1 (Ahp1p) is a thioredoxin peroxidase of the peroxiredoxin family expressed by Saccharomyces cerevisiae. Recently, disruption of the AHP1 gene has shown that the gene is not essential for yeast growth on glucose medium but revealed a high sensitivity of null mutants to organic peroxides, suggesting that Ahp1p is an important enzyme implicated in oxidative stress protection in S. cerevisiae. To gain insight into antioxidant enzymatic mechanisms involved in cell protection against metal toxicity and glutathione depletion, we investigated the resistance of S. cerevisiae, in which the AHP1 gene was disrupted, against several metals and diethyl maleate, a glutathione depleting agent. We report that Ahp1p protects yeast against toxicity induced by copper, cobalt, chromium, arsenite, arsenate, mercury, zinc and diethyl maleate, suggesting that Ahp1p plays an important role in S. cerevisiae in the protection against metals possibly by reducing peroxides generated in cells by these compounds.

Identification of a Saccharomyces cerevisiae gene that is required for G1 arrest in response to the lipid oxidation product linoleic acid hydroperoxide

Reactive oxygen species cause damage to all of the major cellular constituents, including peroxidation of lipids. Previous studies have revealed that oxidative stress, including exposure to oxidation products, affects the progression of cells through the cell division cycle. This study examined the effect of linoleic acid hydroperoxide, a lipid peroxidation product, on the yeast cell cycle. Treatment with this peroxide led to accumulation of unbudded cells in asynchronous populations, together with a budding and replication delay in synchronous ones. This observed modulation of G1 progression could be distinguished from the lethal effects of the treatment and may have been due to a checkpoint mechanism, analogous to that known to be involved in effecting cell cycle arrest in response to DNA damage. By examining several mutants sensitive to linoleic acid hydroperoxide, the YNL099c open reading frame was found to be required for the arrest. This gene (designated OCA1) encodes a putative protein tyrosine phosphatase of previously unknown function. Cells lacking OCA1 did not accumulate in G1 on treatment with linoleic acid hydroperoxide, nor did they show a budding, replication, or Start delay in synchronous cultures. Although not essential for adaptation or immediate cellular survival, OCA1 was required for growth in the presence of linoleic acid hydroperoxide, thus indicating that it may function in linking growth, stress responses, and the cell cycle. Identification of OCA1 establishes cell cycle arrest as an actively regulated response to oxidative stress and will enable further elucidation of oxidative stress-responsive signaling pathways in yeast.

The Ras GDP/GTP cycle is regulated by oxidizing agents at the level of Ras regulators and effectors

Reactive oxygen species (ROS) have been found to play important roles in regulating cellular functions. Their action in vivo has been related to specific effects on signal transduction pathways, such as Ras pathway. In order to characterize which elements of Ras pathway are affected by ROS, we have analyzed the action of different oxidizing agents on the ability of GTPase activating protein GAP and nucleotide exchange factor GEF to enhance the intrinsic activities of Ras. The action of these agents on the binding between H-Ras and its effector c-Raf-1 was also investigated. No effects were observed on the intrinsic activities of H-Ras or Ras2p. On the other hand, reversible inhibitions of GEF and GAP actions on Ras were found, whose extent was dependent on the agent used. As tested with the scintillation proximity assay, these agents also inhibited the binding of c-Raf-1 to H-Ras. Our data reveal new potential targets for the action of ROS on Ras pathway and suggest that they can influence the Ras activation state indirectly via regulators and effectors.

Adaptive response of the yeast Saccharomyces cerevisiae to reactive oxygen species: defences, damage and death

The yeast Saccharomyces cerevisiae has been extensively utilised to address the mechanisms underlying the oxidative stress response. The antioxidant defences can be induced either by respiratory growth or in the presence of pro-oxidants. The cell response involves the transcriptional control of genes by protein regulators that have been recently identified and post-translational activation of pre-existing defences. The current state of the art regarding the induction of antioxidant defences during respiratory growth and by exposure to hydrogen peroxide is reviewed.

Osmotic stress causes a G1 cell cycle delay and downregulation of Cln3/Cdc28 activity in Saccharomyces cerevisiae

Moderate hyperosmotic stress on Saccharomyces cerevisiae cells produces a temporary delay at the G1 stage of the cell cycle. This is accompanied by transitory downregulation of CLN1, CLN2 and CLB5 transcript levels, although not of CLN3, which codes for the most upstream activator of the G1/S transition. Osmotic shock to cells synchronized in early G1, when Cln3 is the only cyclin present, causes a delay in cell cycle resumption. This points to Cln3 as being a key cell cycle target for osmotic stress. We have observed that osmotic shock causes downregulation of the kinase activity of Cln3-Cdc28 complexes. This is concomitant with a temporary accumulation of Cln3 protein as a result of increased stability. The effects of the osmotic stress in G1 are not suppressed in CLN3-1 cells with increased kinase activity, as the Cln3-Cdc28 activity in this mutant is still affected by the shock. Although Hog1 is not required for the observed cell cycle arrest in hyperosmotic conditions, it is necessary to resume the cell cycle at KCl concentrations higher than 0.4 M.

Regulation of cell cycle progression by Swe1p and Hog1p following hypertonic stress

Exposure of yeast cells to an increase in external osmolarity induces a temporary growth arrest. Recovery from this stress is mediated by the accumulation of intracellular glycerol and the transcription of several stress response genes. Increased external osmolarity causes a transient accumulation of 1N and 2N cells and a concomitant depletion of S phase cells. Hypertonic stress triggers a cell cycle delay in G2 phase cells that appears distinct from the morphogenesis checkpoint, which operates in early S phase cells. Hypertonic stress causes a decrease in CLB2 mRNA, phosphorylation of Cdc28p, and inhibition of Clb2p-Cdc28p kinase activity, whereas Clb2 protein levels are unaffected. Like the morphogenesis checkpoint, the osmotic stress-induced G2 delay is dependent upon the kinase Swe1p, but is not tightly correlated with inhibition of Clb2p-Cdc28p kinase activity. Thus, deletion of SWE1 does not prevent the hypertonic stress-induced inhibition of Clb2p-Cdc28p kinase activity. Mutation of the Swe1p phosphorylation site on Cdc28p (Y19) does not fully eliminate the Swe1p-dependent cell cycle delay, suggesting that Swe1p may have functions independent of Cdc28p phosphorylation. Conversely, deletion of the mitogen-activated protein kinase HOG1 does prevent Clb2p-Cdc28p inhibition by hypertonic stress, but does not block Cdc28p phosphorylation or alleviate the cell cycle delay. However, Hog1p does contribute to proper nuclear segregation after hypertonic stress in cells that lack Swe1p. These results suggest a hypertonic stress-induced cell cycle delay in G2 phase that is mediated in a novel way by Swe1p in cooperation with Hog1p.

Grx5 glutaredoxin plays a central role in protection against protein oxidative damage in Saccharomyces cerevisiae

Glutaredoxins are members of a superfamily of thiol disulfide oxidoreductases involved in maintaining the redox state of target proteins. In Saccharomyces cerevisiae, two glutaredoxins (Grx1 and Grx2) containing a cysteine pair at the active site had been characterized as protecting yeast cells against oxidative damage. In this work, another subfamily of yeast glutaredoxins (Grx3, Grx4, and Grx5) that differs from the first in containing a single cysteine residue at the putative active site is described. This trait is also characteristic for a number of glutaredoxins from bacteria to humans, with which the Grx3/4/5 group has extensive homology over two regions. Mutants lacking Grx5 are partially deficient in growth in rich and minimal media and also highly sensitive to oxidative damage caused by menadione and hydrogen peroxide. A significant increase in total protein carbonyl content is constitutively observed in grx5 cells, and a number of specific proteins, including transketolase, appear to be highly oxidized in this mutant. The synthetic lethality of the grx5 and grx2 mutations on one hand and of grx5 with the grx3 grx4 combination on the other points to a complex functional relationship among yeast glutaredoxins, with Grx5 playing a specially important role in protection against oxidative stress both during ordinary growth conditions and after externally induced damage. Grx5-deficient mutants are also sensitive to osmotic stress, which indicates a relationship between the two types of stress in yeast cells.