Glutamate-cysteine Ligase Modifier Subunit Research Articles

Effect on Endothelial Cell Gene Expression of Shear Stress, Oxygen Concentration, and Low-Density Lipoprotein as Studied by a Novel Flow Cell Culture System

A new cell culture system has been developed that reflects the vascular microenvironment. By means of this system the cultured cells are exposed not only to shear stress by the circulating culture medium, but also to an oxygen concentration gradient and certain critical blood components such as low-density lipoprotein (LDL) and monocytes. DNA microarray analysis was performed for human umbilical vein endothelial cells cultured in this system in the absence and presence of laminar flow at a low shear stress, 0.2 dyn/cm 2. In addition to shear stress, either an oxygen concentration gradient, or LDL (1 mg/ml), or both were applied. Many Nrf-2-regulating genes, such as heme oxygenase 1, NAD(P)H quinone oxidoreductase 1, solute carrier family 7 No. 11, and glutamate–cysteine ligase modifier subunit, were induced by laminar flow at very low shear stress regardless of the additional conditions. Certain genes were specifically affected by exposure to the oxygen gradient and/or LDL under shear stress, but the degree was very low. These results suggest that shear stress is the most critical factor affecting gene expression in endothelial cells and that Nrf-2-regulating proteins may contribute to protecting endothelial cells against other vascular stress. This system should provide highly relevant and useful information about both vascular physiology and pathology, in the latter on such urgent matters as the specific steps involved in atherogenesis.

Binge ethanol exposure increases liver injury in obese rats

Background & Aims : The objective of this study was to address the hepatic effects of acute alcohol consumption in obesity by simulating an alcohol binge in genetically obese fa/fa rats compared with lean Fa/? rats. Methods : Ethanol 4 g/kg or saline was administered by gavage every 12 hours for 3 days. Results : Plasma alcohol levels were similar in both groups. Binge ethanol exposure caused liver injury in obese fa/fa but not in lean Fa/? rats, as assessed by alanine aminotransferase and H&E staining. Obesity impaired the antioxidant defense because basal levels of glutathione, glutamate cysteine ligase modulatory subunit, catalase, glutathione reductase, and superoxide dismutase were lower in fa/fa compared with Fa/? rats; the ethanol binge further decreased these antioxidants in fa/fa rats and also decreased glutathione peroxidase activity. Nonesterified fatty acids and lipid peroxidation were increased after ethanol treatment in fa/fa rats. Cytochrome P450 2E1 was down-regulated in fa/fa compared with Fa/? rats; however, the ethanol binge increased cytochrome P450 2E1 in both genotypes. Adenosine triphosphate decreased and uncoupling protein 2 increased in fa/fa rats treated with ethanol. 3-Nitrotyrosine protein adducts were detected only in fa/fa rats treated with ethanol, and this was accompanied by an induction of inducible nitric oxide synthase. Ethanol binge increased caspase-3 and caspase-8 activity, the expression of Fas ligand, and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling in fa/fa rats. Conclusions : These data indicate that binge drinking increases apoptosis and liver injury in obese rats more than in lean controls and suggest that the injury may involve oxidative and nitrosative damage.

The Modifier Subunit of Drosophila Glutamate-Cysteine Ligase Regulates Catalytic Activity by Covalent and Noncovalent Interactions and Influences Glutathione Homeostasis in Vivo

Glutamate-cysteine ligase (GCL) has a key influence on glutathione homeostasis. It has been proposed that mammalian GCL is regulated by the redox environment, and we show here that cysteine residues in the Drosophila melanogaster GCL modifier subunit (DmGCLM) can form covalent interactions with the catalytic subunit (DmGCLC) and modify its activity. Candidate components of intersubunit disulfides (Cys213, Cys214, and Cys267) were identified using matrix-assisted laser desorption ionization time-of-flight spectroscopy of iodoacetamide-modified DmGCLM as well as examination of the evolutionary conservation of cysteines. Mutation of the 3 cysteine residues allowed DmGCLM to associate with DmGCLC, but inhibited the formation of intersubunit disulfides. This caused a 2-fold reduction in the catalytic efficiency of Drosophila GCL, although activity remained significantly higher than the catalytic subunit alone. The cysteine mutant was also more sensitive to inhibition by glutathione than the unmodified holoenzyme. Notably, human GCLM could substitute for DmGCLM in modification of DmGCLC activity. The role of DmGCLM in vivo was examined by analysis of a Drosophila mutant (l(3)L0580) containing a P-element insertion in Gclm. We found that the P-element is not responsible for the lethal phenotype and separated the recessive lethal mutation from the P-element by recombination. This yielded two fully viable and fertile recombinants bearing the P-element insertion, which Western and Northern blotting indicated is a severely hypomorphic allele of Gclm. Glutathione levels were approximately 2-fold lower in the GclmL0580 mutants than in control strains, demonstrating the importance of DmGCLM in the regulation of glutathione homeostasis in vivo.

Free radical biology and medicine

Antioxidant defense is crucial in restoring cellular redox homeostasis. Recent findings have suggested that oxidative stress plays pivotal roles in the pathogenesis of many neurodegenerative diseases. Thus, an anti-oxidative stress remedy might be a promising means for the treatment of such disorders. In this study, we employed a novel ligand-based virtual screening system and identified a novel small molecule, N-(4-(2-pyridyl)(1,3-thiazol-2-yl))-2-(2,4,6-trimethylphenoxy) acetamide (CPN-9), which selectively suppressed oxidative stress-induced cell death in a cell-type-independent manner. CPN-9 upregulates NF-E2-related factor 2 (Nrf2), a key transcriptional regulator of the expression of phase II detoxification enzymes and antioxidant proteins, and Nrf2-regulated factors such as heme oxygenase-1 (HO-1), NAD(P)H quinone oxidoreductase 1 (NQO1), and glutamate–cysteine ligase modifier subunit (GCLM). The CPN-9-mediated upregulation of HO-1, NQO1, and GCLM was abolished by Nrf2 knockdown. Moreover, the antioxidant N-acetylcysteine reduced the protective effect of CPN-9 against oxidative stress-induced cell death with concomitant diminishing of Nrf2 nuclear translocation. These results indicate that CPN-9 exerts its activity via the reactive oxygen species-dependent activation of the Nrf2 signaling pathway in cultured cells. It is noteworthy that the postonset systemic administration of CPN-9 to a transgenic ALS mouse model carrying the H46R mutation in the human Cu/Zn superoxide dismutase (SOD1) gene sustained motor functions and delayed disease progression after onset. Collectively, CPN-9 is a novel Nrf2 activator and a neuroprotective candidate for the treatment of

Pharmacological rescue of the 14CoS/14CoS mouse: hepatocyte apoptosis is likely caused by endogenous oxidative stress.

Whereas ch/ch wild-type mice and ch/14CoS heterozygotes are viable, 14CoS/14CoS mice homozygous for a 3800 kb deletion on chromosome 7 die during the first day postpartum. Death is caused by disruption of the fumarylacetoacetate hydrolase (Fah) gene; absence of FAH, final enzyme in the tyrosine catabolism pathway, leads to accumulation of reactive electrophilic intermediates. In this study, we kept 14CoS/14CoS mice alive for 60 d with oral 2-(2-nitro-4-trifluoromethyl-benzyol)-1,3-cyclohexanedione (NTBC), an inhibitor of p-hydroxyphenylpyruvate dioxygenase, second enzyme in the tyrosine catabolic pathway. The 70% of NTBC-treated 14CoS/14CoS mice that survived 60 d showed poor growth and developed corneal opacities, compared with ch/14CoS littermates; NTBC-rescued Fah(-/-) knockout mice did not show growth retardation or ocular toxicity. NTBC-rescued 14CoS/14CoS mice also exhibited a striking oxidative stress response in liver and kidney, as measured by lower GSH levels and mRNA induction of four genes: glutamate cysteine ligase catalytic (Gclc) and modifier (Gclm) subunits, NAD(P)H:quinone oxidoreductase (Nqo1), and heme oxygenase-1 (Hmox1). Withdrawal of NTBC for 24-48 h from rescued adult 14CoS/14CoS mice resulted in severe apoptosis of the liver, detected histologically and by cytochrome c release from the mitochondria, increased caspase 3-like activity, and further decreases in GSH content. In kidney, proximal tubular epithelial cells were abnormal. Human hereditary tyrosinemia type I (HT1), caused by mutations in the FAH gene, is an autosomal recessive disorder in which the patient usually dies of liver fibrosis and cirrhosis during early childhood; NTBC treatment is known to prolong HT1 children's lives-although liver fibrosis, cirrhosis, hepatocarcinoma, and corneal opacities sometimes occur. The mouse data in the present study are consistent with the possibility that endogenous oxidative stress-induced apoptosis may be the underlying cause of liver pathology seen in NTBC-treated HT1 patients.

Loss of the Nrf2 transcription factor causes a marked reduction in constitutive and inducible expression of the glutathione S-transferase Gsta1, Gsta2, Gstm1, Gstm2, Gstm3 and Gstm4 genes in the livers of male and female mice.

Mice that lack the Nrf2 basic-region leucine-zipper transcription factor are more sensitive than wild-type (WT) animals to the cytotoxic and genotoxic effects of foreign chemicals and oxidants. To determine the basis for the decrease in tolerance of the Nrf2 homozygous null mice to xenobiotics, enzyme assay, Western blotting and gene-specific real-time PCR (TaqMan) have been used to examine the extent to which hepatic expression of GSH-dependent enzymes is influenced by the transcription factor. The amounts of protein and mRNA for class Alpha, Mu and Pi glutathione S-transferases were compared between WT and Nrf2 knockout (KO) mice of both sexes under both constitutive and inducible conditions. Among the class Alpha and class Mu transferases, constitutive expression of Gsta1, Gsta2, Gstm1, Gstm2, Gstm3, Gstm4 and Gstm6 subunits was reduced in the livers of Nrf2 mutant mice to between 3% and 60% of that observed in WT mice. Induction of these subunits by butylated hydroxyanisole (BHA) was more marked in WT female mice than in WT male mice. TaqMan analyses showed the increase in transferase mRNA caused by BHA was attenuated in Nrf2(-/-) mice, with the effect being most apparent in the case of Gsta1, Gstm1 and Gstm3. Amongst class Pi transferase subunits, the constitutive hepatic level of mRNA for Gstp1 and Gstp2 was not substantially affected in the KO mice, but their induction by BHA was dependent on Nrf2; this was more obvious in female mutant mice than in male mice. Nrf2 KO mice exhibited reduced constitutive expression of the glutamate cysteine ligase catalytic subunit, and, to a lesser extent, the expression of glutamate cysteine ligase modifier subunit. Little variation was observed in the levels of glutathione synthase in the different mouse lines. Thus the increased sensitivity of Nrf2(-/-) mice to xenobiotics can be partly attributed to a loss in constitutive expression of multiple GSH-dependent enzymes, which causes a reduction in intrinsic detoxification capacity in the KO animal. These data also indicate that attenuated induction of GSH-dependent enzymes in Nrf2(-/-) mice probably accounts for their failure to adapt to chronic exposure to chemical and oxidative stress.

Polymorphism in the 5'-flanking region of human glutamate-cysteine ligase modifier subunit gene is associated with myocardial infarction.

Human glutamate-cysteine ligase (GCL) is a rate-limiting enzyme for the synthesis of glutathione that plays a crucial role in antioxidant defense mechanisms in most mammalian cells, including vascular cells. Oxidants transcriptionally upregulate GCL genes for glutathione synthesis, providing a protective mechanism against oxidative stress-induced cellular dysfunction. This study examined the hypothesis that variation in the GCL genes may be associated with coronary artery disease in which oxidative stress plays a pathogenetic role. We searched for the common variants in the 5'-flanking region of the GCL modifier subunit (GCLM) gene in patients with myocardial infarction (MI). We found a polymorphism (-588C/T) in which the T allele showed lower promoter activity (40% to 50% of C allele) in response to oxidants in the luciferase reporter gene assay. Allele frequencies were determined by polymerase chain reaction-based analysis of restriction fragment length polymorphism in 429 patients with MI and 428 control subjects (as defined by angiography) in Kumamoto Prefecture, Japan. The frequency of the T polymorphism was significantly higher in the MI group than in the control group (CT and TT genotypes: 31.5% in MI group versus 19.2% in control group; P<0.001). In multiple logistic regression analysis, the T polymorphism was a risk factor for MI independent of traditional coronary artery disease risk factors (odds ratio, 1.98; 95% confidence interval, 1.38 to 2.83; P<0.001). These findings suggest that the -588T polymorphism of the GCLM gene may suppress GCLM gene induction in response to oxidants and that it is a genetic risk factor for MI.

Glutamate–cysteine ligase modifier subunit: mouse Gclm gene structure and regulation by agents that cause oxidative stress

Glutamate–cysteine ligase is a heterodimer comprising a modifier (GCLM) and a catalytic (GCLC) subunit. In mouse Hepa-1c1c7 hepatoma cell cultures, we found that tert-butylhydroquinone (tBHQ; 50μM) induces the GCLM and GCLC mRNAs ∼10- and ∼2-fold, respectively, and that these increases primarily reflect de novo transcription. We determined that the mouse Gclm gene has seven exons, spanning 22.3kb; all exons, intron–exon junctions, and 4.7kb of 5′-flanking region were sequenced. By RNase protection analysis, we identified two major and several minor transcription start-site clusters over a 300-bp region. The Gclm 5′-flanking region is GC-rich and lacks a canonical TATA box. Transient and stable transfection studies, using luciferase reporter constructs containing incremental Gclm 5′-flanking deletions (4.7–0.5kb), showed high basal activity but only modest (∼2-fold) inducibility by tBHQ. The only candidate motif for oxidative stress regulation (in the 4.7-kb region we sequenced) is a putative inverted electrophile response element (EPRE) 9bp upstream from the 5′-most transcription start-site. Site-directed mutagenesis of this −9 EPRE demonstrated minimal (30–40%) decreases in tBHQ induction and no effect on basal activity—suggesting that this EPRE might be necessary but not sufficient. The nuclear erythroid factor-2 (NEF2)-related factor-2 (NRF2) is known to transactivate via EPRE motifs. In the presence of co-transfected NRF cDNA expression vector, however, no increase in Gclm promoter activity was observed. Thus, the endogenous Gclm gene shows robust transcriptional activation by tBHQ in the intact Hepa-1 cell, but reporter constructs containing up to 4.7kb of promoter (having only the one EPRE at −9) demonstrate a disappointing response, indicating that the major tBHQ-responsive regulatory element of the mouse Gclm gene must exist either further 5′- or 3′-ward of the 4.7-kb region studied.

RETRACTED: Cloning and Analysis of the Rat Glutamate–Cysteine Ligase Modifier Subunit Promoter

This article has been retracted by the authors.Reason: The authors have requested this retraction because of the discovery of an error in the reported sequence.

AUTHORS' REPLY

<h3>Background</h3> Glutamate-cysteine ligase modifier (GCLM) subunits, combined with catalytic (GCLC) subunits, has been known for the first rate-limiting enzyme for glutathione (GSH) synthesis. Increasing studies have shown that the non-metabolic functions of many metabolic enzymes also play a crucial role in tumor progress. However, the non-metabolic functions of GCLM remain unexplored. Hence, we wonder if there are unorthodox functions of GCLM under redox stress participating colorectal Carcinoma(CRC) tumorigenesis. <h3>Methods</h3> Western blot, real-time quantitative PCR and immunohistochemical analysis were used to detect the relative expression of GCLM in cell lines and patients‘ specimens. The MTS assay, migration and invasion assay, sphere formation assay were performed to detect the functions of GCLM <i>in vitro</i>, and the xenograft models and PDX models in nude mice <i>in vivo</i>. Cytosolic and nuclear extraction, immunofluorescent analysis were used to show the location of GCLM. Chromatin immunoprecipitation (ChIP) assay was used to screen and verify the GCLM target genes. <h3>Results</h3> We found that glucose deprivation condition upregulated the expression of GCLM rather than hypoxia or H2O2. Compared to adjacent tumor tissues, the expression of GCLM was upregulated in CRC tumor tissues. The high expression of GCLM predicted poor prognosis in CRC patients. Inhibition of GCLM results in decreased proliferation rate, migration and invasion ability and sphere formation in HCT116 and DLD1 <i>in vitro</i>. Similarly, GCLM inhibition suppressed CRC tumorigenesis and metastasis in xenograft models and PDX models<i> in vivo</i>. The cancer cell stem cell markers were also downregulated after GCLM inhibition. Interestingly, we found that glucose starvation could rescue the anti-cancer phenotypes of GCLM inhibition and led to nucleus-translocation and accumulation of GCLM. ChIP assay showed GCLM could interact with octamer-binding transcription factor 4 (<i>OCT4)</i> promoter<i>,</i> a cancer stem cell marker, to increase its expression, and the target gene of <i>OCT4</i> showed a positive correlation with GCLM expression in CRC patients. <h3>Conclusions</h3> Our data showed that GCLM displayed nucleus accumulation in the condition of glucose deprivation which increased the transcription of <i>OCT4.</i> This study revealed an unorthodox oncogenic function of GCLM in colorectal carcinoma and suggested GCLM as a potential therapeutic target in CRC.