Regulation of the gene encoding glutathione synthetase from the fission yeast

The Endophytic Fungus Piriformospora Indica-Assisted Alleviation of Cadmium in Tobacco

Increasing evidence suggests that the endophytic fungus Piriformospora indica helps plants overcome various abiotic stresses, especially heavy metals. However, the mechanism of heavy metal tolerance has not yet been elucidated. Here, the role of P. indica in alleviating cadmium (Cd) toxicities in tobacco was investigated. It was found that P. indica improved Cd tolerance to tobacco, increasing Cd accumulation in roots but decreasing Cd accumulation in leaves. The colonization of P. indica altered the subcellular repartition of Cd, increasing the Cd proportion in cell walls while reducing the Cd proportion in membrane/organelle and soluble fractions. During Cd stress, P. indica significantly enhanced the peroxidase (POD) activity and glutathione (GSH) content in tobacco. The spatial distribution of GSH was further visualized by Raman spectroscopy, showing that GSH was distributed in the cortex of P. indica-inoculated roots while in the epidermis of the control roots. A LC-MS/MS-based label-free quantitative technique evaluated the differential proteomics of P. indica treatment vs. control plants under Cd stress. The expressions of peroxidase, glutathione synthase, and photosynthesis-related proteins were significantly upregulated. This study provided extensive evidence for how P. indica enhances Cd tolerance in tobacco at physiological, cytological, and protein levels.

A GSHS-like gene from Lycium chinense maybe regulated by cadmium-induced endogenous salicylic acid and overexpression of this gene enhances tolerance to cadmium stress in Arabidopsis

A GSHS gene, LcGSHS , was cloned from L. chinense for the first time. Evidence is presented here that endogenous SA accumulation maybe important for the regulation of LcGSHS expression level. Glutathione (GSH) plays a pivotal role in heavy metal detoxification. GSH synthetase (GSHS) catalyzes the rate-limiting step of GSH synthesis in plants. Salicylic acid (SA) is one of the important plant hormones, which plays a critical role in triggering plant responses to different stresses such as cadmium (Cd) stress. Until now, little has been done to explore the relationship among the accumulation of endogenous SA, GSHS transcript levels and the GSH content in plants under Cd treatment and we will investigate this link in this study. The chlorophyll content, transcripts level of LcGSHS gene, endogenous SA accumulation, GSH accumulation and Cd concentration in the leaves of Lycium chinense were studied under different treatment conditions. Endogenous SA, LcGSHS transcript expression and GSH content can be induced by Cd treatment in L. chinense, however, reduced by co-treatment with 2-aminoindan-2-phosphonic acid (AIP), an inhibitor of SA biosynthesis. Strong staining was observed in the leaves of Arabidopsis expressing ProLcGSHS::GUS under Cd stress and the staining was reduced by co-treatment with AIP. The transgenic Arabidopsis expressing ProLcGSHS::LcGSHS also showed greater tolerance to Cd stress than wild types. Evidence was presented here that under Cd stress, GSH accumulation occurred via enhanced LcGSHS gene expression and the SA signaling cascade was involved in this accumulation. Furthermore, the overexpression of LcGSHS in transgenic Arabidopsis resulted in greater tolerance to Cd stress than wild-type lines.

A novel upregulation of glutathione peroxidase 1 by knockout of liver-regenerating protein Reg3β aggravates acetaminophen-induced hepatic protein nitration

Murine regenerating islet-derived 3β (Reg3β) represents a homologue of human hepatocarcinoma-intestine-pancreas/pancreatic-associated protein and enhances mouse susceptibility to acetaminophen (APAP)-induced hepatotoxicity. Our objective was to determine if and how knockout of Reg3β (KO) affects APAP (300 mg/kg, ip)-mediated protein nitration in mouse liver. APAP injection produced greater levels of hepatic protein nitration in the KO than in the wild-type mice. Their elevated protein nitration was alleviated by a prior injection of recombinant mouse Reg3β protein and was associated with an accelerated depletion of the peroxynitrite (ONOO(-)) scavenger glutathione by an upregulated hepatic glutathione peroxidase-1 (GPX1) activity. The enhanced GPX1 production in the KO mice was mediated by an 85% rise (p<0.05) in the activity of selenocysteine lyase (Scly), a key enzyme that mobilizes Se for selenoprotein biosynthesis. Knockout of Reg3β enhanced AP-1 protein and its binding activity to the Scly gene promoter, upregulating its gene transcription. However, knockout of Reg3β did not affect gene expression of other key factors for selenoprotein biosynthesis. In conclusion, our findings unveil a new metabolic role for Reg3β in protein nitration and a new biosynthesis control of GPX1 by a completely "unrelated" regenerating protein, Reg3β, via transcriptional activation of Scly in coping with hepatic protein nitration. Linking selenoproteins to tissue regeneration will have profound implications in understanding the mechanism of Se functions and physiological coordination of tissue regeneration with intracellular redox control.

Disruption of redox homeostasis and induction of apoptosis by suppression of glutathione synthetase expression in a mammalian cell line

The stable HepG2 transfectants anti-sensing expression of the glutathione synthetase (GS) gene exhibited delayed cell growth and increased reactive oxygen species (ROS) level. After the treatment with hydrogen peroxide, the intracellular ROS level was much higher in the stable transfectants than in the vector control cells. However, the GSH levels decreased more significantly in the stable transfectants than in the vector control cells, in the presence of hydrogen peroxide. Hydrogen peroxide-induced apoptosis of the stable transfectants was notably higher than that of the vector control cells. The GS anti-sense RNAs rendered the HepG2 cells more sensitive to growth arrest caused by glucose deprivation. They also sensitized the HepG2 cells to cadmium chloride (Cd) and nitric oxide (NO)-generating sodium nitroprusside (SNP). In brief, the results confirm that GS plays an important role in the defense of the human hepatoma cells against oxidative stress by reducing apoptosis and maintaining redox homeostasis.

Metabolic control analysis: a tool for designing strategies to manipulate metabolic pathways

The traditional experimental approaches used for changing the flux or the concentration of a particular metabolite of a metabolic pathway have been mostly based on the inhibition or over-expression of the presumed rate-limiting step. However, the attempts to manipulate a metabolic pathway by following such approach have proved to be unsuccessful. Metabolic Control Analysis (MCA) establishes how to determine, quantitatively, the degree of control that a given enzyme exerts on flux and on the concentration of metabolites, thus substituting the intuitive, qualitative concept of rate limiting step. Moreover, MCA helps to understand (i) the underlying mechanisms by which a given enzyme exerts high or low control and (ii) why the control of the pathway is shared by several pathway enzymes and transporters. By applying MCA it is possible to identify the steps that should be modified to achieve a successful alteration of flux or metabolite concentration in pathways of biotechnological (e.g., large scale metabolite production) or clinical relevance (e.g., drug therapy). The different MCA experimental approaches developed for the determination of the flux-control distribution in several pathways are described. Full understanding of the pathway properties when is working under a variety of conditions can help to attain a successful manipulation of flux and metabolite concentration.

Gene expressions and enzyme analyses in the Schizosaccharomyces pombe Deltapap1 transcription factor mutant exposed to Cd(2+)

The objective of this study was to investigate the role of the Pap1 transcription factor in response to long-term Cd(2+) stress. The Schizosaccharomyces pombe wild-type strain and the Deltapap1 mutant, treated with 0.5 mM CdSO(4), were used in antioxidant enzyme and gene expression experiments. The Deltapap1 mutant proved to be sensitive to Cd(2+) in the spot test assay, suggesting that the Pap1 transcription factor plays an important role in the response to Cd(2+) stress. The Cd(2+) uptake was the same in both strains. Determination of the superoxide level in the wild-type strain proved that superoxide was generated, suggesting that long-term Cd(2+) treatment could trigger oxidative stress. Furthermore, the Deltapap1 mutant displayed higher amounts of superoxide. These results were supported by the significantly lower amount of peroxide generated in the reaction catalyzed by superoxide dismutase (SOD). The Deltapap1 mutant had a significantly lower glutathione S-transferase specific activity than that of the wild-type strain during long-term Cd(2+) stress, caused by the lower GSH and sulfide assimilation. We have demonstrated that GST III activity was not induced by Cd(2+) stress in the Deltapap1 mutant. The overall low GST activity was not sufficient for the cell to eliminate Cd(2+) caused damage and could result in a Cd(2+)-sensitive phenotype of the Deltapap1 mutant. The RT-PCR and Northern blot experiments proved that gst2 was not induced either by short-term or by long-term Cd(2+) treatment. The SPCC965.06 (a putative K(+) ion channel subunit) gene expression increased, while the hmt1 (an ABC-type vacuolar transporter protein) expression decreased in both strains. No detectable alteration in the mRNA levels of, gpx1, hmt2, sod1, sod, and trx1 was observed. SOD enzyme analyses revealed that the absence of Pap1 protein could result in a lower SODs activity and affect the sulfate assimilation. This is the first report on the fact that the Pap1 transcription factor could play an important role in the cellular post-transcriptional/post-translational enzyme activity induction processes of SODs that occur in response to Cd(2+).

The gene encoding gamma-glutamyl transpeptidase II in the fission yeast is regulated by oxidative and metabolic stress

gamma-Glutamyl transpeptidase (GGT, EC 2.3.2.2.) catalyzes the transfer of the gamma-glutamyl moiety from gamma-glutamylcontaining compounds, notably glutathione (GSH), to acceptor amino acids and peptides. A second gene (GGTII) encoding GGT was previously isolated and characterized from the fission yeast Schizosaccharomyces pombe. In the present work, the GGTII-lacZ fusion gene was constructed and used to study the transcriptional regulation of the S. pombe GGTII gene. The synthesis of beta-galactosidase from the GGTII-lacZ fusion gene was significantly enhanced by NO-generating SNP and hydrogen peroxide in the wildtype yeast cells. The GGTII mRNA level was increased in the wild-type S. pombe cells treated with SNP. However, the induction by SNP was abolished in the Pap1-negative S. pombe cells, implying that the induction by SNP of GGTII is mediated by Pap1. Fermentable carbon sources, such as glucose (at low concentrations), lactose and sucrose, as a sole carbon source, enhanced the synthesis of beta-galactosidase from the GGTII-lacZ fusion gene in wildtype KP1 cells but not in Pap1-negative cells. Glycerol, a non-fermentable carbon source, was also able to induce the synthesis of beta-galactosidase from the fusion gene, but other non-fermentable carbon sources such as acetate and ethanol were not. Transcriptional induction of the GGTII gene by fermentable carbon sources was also confirmed by increased GGTII mRNA levels in the yeast cells grown with them. Nitrogen starvation was also able to induce the synthesis of beta-galactosidase from the GGTII-lacZ fusiongene in a Pap1-dependent manner. On the basis of the results, it is concluded that the S. pombe GGTII gene is regulated by oxidative and metabolic stress.

Sulfur assimilation and glutathione metabolism under cadmium stress in yeast, protists and plants

Glutathione (gamma-glu-cys-gly; GSH) is usually present at high concentrations in most living cells, being the major reservoir of non-protein reduced sulfur. Because of its unique redox and nucleophilic properties, GSH serves in bio-reductive reactions as an important line of defense against reactive oxygen species, xenobiotics and heavy metals. GSH is synthesized from its constituent amino acids by two ATP-dependent reactions catalyzed by gamma-glutamylcysteine synthetase and glutathione synthetase. In yeast, these enzymes are found in the cytosol, whereas in plants they are located in the cytosol and chloroplast. In protists, their location is not well established. In turn, the sulfur assimilation pathway, which leads to cysteine biosynthesis, involves high and low affinity sulfate transporters, and the enzymes ATP sulfurylase, APS kinase, PAPS reductase or APS reductase, sulfite reductase, serine acetyl transferase, O-acetylserine/O-acetylhomoserine sulfhydrylase and, in some organisms, also cystathionine beta-synthase and cystathionine gamma-lyase. The biochemical and genetic regulation of these pathways is affected by oxidative stress, sulfur deficiency and heavy metal exposure. Cells cope with heavy metal stress using different mechanisms, such as complexation and compartmentation. One of these mechanisms in some yeast, plants and protists is the enhanced synthesis of the heavy metal-chelating molecules GSH and phytochelatins, which are formed from GSH by phytochelatin synthase (PCS) in a heavy metal-dependent reaction; Cd(2+) is the most potent activator of PCS. In this work, we review the biochemical and genetic mechanisms involved in the regulation of sulfate assimilation-reduction and GSH metabolism when yeast, plants and protists are challenged by Cd(2+).

Control of glutathione and phytochelatin synthesis under cadmium stress. Pathway modeling for plants

Glutathione (GSH) plays several roles in cell metabolism such as redox state regulation, oxidative stress control, and protection against xenobiotics and heavy metals. GSH is synthesized in two steps catalysed by gamma-glutamylcysteine synthetase (gamma-ECS) and glutathione synthetase. gamma-ECS is feedback inhibited by GSH, which has led to the proposal that this enzyme acts as the rate-limiting step in the pathway. Thus far, the study of GSH metabolism has been confined to GSH synthesis (GSH supply), without considering the GSH-consuming enzymes (GSH demand). Several works have shown that the demand block of enzymes may have a significant control on a pathway; therefore, we hypothesize that GSH-consuming enzymes may exert some control on GSH synthesis. A kinetic model of GSH and phytochelatin synthesis in plants was constructed using the software GEPASI and the kinetic data available in the literature. The main conclusions drawn by the model concerning metabolic control analysis are (1) gamma-ECS is indeed a rate-limiting step in GSH synthesis, but only if GSH-consuming enzymes are not taken into account. (2) At low demand, GSH-consuming enzymes exert significant flux-control on GSH synthesis whereas at high demand, supply and demand blocks share the control of flux. (3) In unstressed conditions, flux to GSH is controlled mainly by demand, so that gamma-ECS determines the degree of homeostasis of the GSH concentration. Under cadmium exposure, the GSH demand increases and flux-control is re-distributed almost equally between the supply and demand blocks. (4) To enhance phytochelatins synthesis without depleting the GSH pool, at least two enzymes (gamma-ECS and PCS) should be increased and/or, alternatively, a branching flux (GSH-S-transferases) could be partially diminished.