Over expression of glutamate cysteine ligase increases cellular resistance to H2O2-induced DNA single-strand breaks

Cr(VI)-induced DNA damage is lessened by the modulation of hsp70 via increased GSH de novo synthesis in Drosophila melanogaster

Hexavalent chromium [Cr(VI)] is a genotoxic chemical, and in the chemical-exposed organism, oxidative stress is one of the leading causative mechanisms of genotoxicity. Heat shock protein-70 (Hsp70) is reported to be modulated in environmental chemical exposed organisms. Inadequate information on the protective role of Hsp70 in chemical-induced DNA lesions prompted us to investigate this possibility in a well-studied genetically tractable in vivo model Drosophila melanogaster. In the midgut cells of Cr(VI)-exposed hsp70-knockout (KO), -knockdown (KD), and -overexpression Drosophila strains, no significant change in double-strand breaks generation was observed in comparison to similarly exposed w ¹¹¹⁸ and the respective genetic control strain after 48 h. Therefore, the role of hsp70 was investigated on oxidative DNA damage induction in the exposed organisms after 24 h. Oxidized DNA lesions (particularly oxidized purine-based lesions), 8-oxo-dG level, and oxidative stress endpoints were found to be significantly elevated in hsp70-KO and -KD strains in comparison to similarly exposed w ¹¹¹⁸ and respective genetic control strain. On the contrary, in ubiquitous hsp70-overexpression strain exposed to Cr(VI), these endpoints were significantly lowered concurrently with increased GSH level through elevated gclc, and gclm expression, Gclc level, and GCL activity. The study suggests that as a consequence of hsp70 overexpression, the augmented GSH level in cells vis-a-vis GSH de novo synthesis can counteract Cr(VI)-induced oxidized DNA lesions.

Unraveling the Potential Role of Glutathione in Multiple Forms of Cell Death in Cancer Therapy

Glutathione is the principal intracellular antioxidant buffer against oxidative stress and mainly exists in the forms of reduced glutathione (GSH) and oxidized glutathione (GSSG). The processes of glutathione synthesis, transport, utilization, and metabolism are tightly controlled to maintain intracellular glutathione homeostasis and redox balance. As for cancer cells, they exhibit a greater ROS level than normal cells in order to meet the enhanced metabolism and vicious proliferation; meanwhile, they also have to develop an increased antioxidant defense system to cope with the higher oxidant state. Growing numbers of studies have implicated that altering the glutathione antioxidant system is associated with multiple forms of programmed cell death in cancer cells. In this review, we firstly focus on glutathione homeostasis from the perspectives of glutathione synthesis, distribution, transportation, and metabolism. Then, we discuss the function of glutathione in the antioxidant process. Afterwards, we also summarize the recent advance in the understanding of the mechanism by which glutathione plays a key role in multiple forms of programmed cell death, including apoptosis, necroptosis, ferroptosis, and autophagy. Finally, we highlight the glutathione-targeting therapeutic approaches toward cancers. A comprehensive review on the glutathione homeostasis and the role of glutathione depletion in programmed cell death provide insight into the redox-based research concerning cancer therapeutics.

Increased susceptibility to oxidative stress-induced toxicological evaluation by genetically modified nrf2a-deficient zebrafish

INTRODUCTION

Oxidative stress plays an important role in drug-induced toxicity. Oxidative stress-mediated toxicities can be detected using conventional animal models but their sensitivity is insufficient, and novel models to improve susceptibility to oxidative stress have been researched. In recent years, gene targeting methods in zebrafish have been developed, making it possible to generate homozygous null mutants. In this study, we established zebrafish deficient in the nuclear factor erythroid 2-related factor 2a (nrf2a), a key antioxidant-responsive gene, and its potential to detect oxidative stress-mediated toxicity was examined.

METHODS

Nrf2a-deficient zebrafish were generated using the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated 9 technique. The loss of nrf2a function was confirmed by the tolerability to hydrogen peroxide and hydrogen peroxide-induced gene expression profiles being related to antioxidant response element (ARE)-dependent signaling. Subsequently, vulnerability of nrf2a-deficient zebrafish to acetaminophen (APAP)- or doxorubicin (DOX)-induced toxicity was investigated.

RESULTS

Nrf2a-deficient zebrafish showed higher mortality than wild type accompanied by less induction of ARE-dependent genes with hydrogen peroxide treatment. Subsequently, this model showed increased severity and incidence of APAP-induced hepatotoxicity or DOX-induced cardiotoxicity than wild type.

DISCUSSION

Our results demonstrated that anti-oxidative response might not fully function in this model, and resulted in higher sensitivity to drug-induced oxidative stress. Our data support the usefulness of nrf2a-deficient model as a tool for evaluation of oxidative stress-related toxicity in drug discovery research.

Genetic evidence of an evolutionarily conserved role for Nrf2 in the protection against oxidative stress

Transcription factor Nrf2 is considered a master regulator of antioxidant defense in mammals. However, it is unclear whether this concept is applicable to nonmammalian vertebrates, because no animal model other than Nrf2 knockout mice has been generated to examine the effects of Nrf2 deficiency. Here, we characterized a recessive loss-of-function mutant of Nrf2 (nrf2(fh318)) in a lower vertebrate, the zebrafish (Danio rerio). In keeping with the findings in the mouse model, nrf2(fh318) mutants exhibited reduced induction of the Nrf2 target genes in response to oxidative stress and electrophiles but were viable and fertile, and their embryos developed normally. The nrf2(fh318) larvae displayed enhanced sensitivity to oxidative stress and electrophiles, especially peroxides, and pretreatment with an Nrf2-activating compound, sulforaphane, decreased peroxide-induced lethality in the wild type but not nrf2(fh318) mutants, indicating that resistance to oxidative stress is highly dependent on Nrf2 functions. These results reveal an evolutionarily conserved role of vertebrate Nrf2 in protection against oxidative stress. Interestingly, there were no significant differences between wild-type and nrf2(fh318) larvae with regard to their sensitivity to superoxide and singlet oxygen generators, suggesting that the importance of Nrf2 in oxidative stress protection varies based on the type of reactive oxygen species (ROS).

Xenobiotic-metabolizing gene variants, pesticide use, and the risk of prostate cancer

BACKGROUND

To explore associations with prostate cancer and farming, it is important to investigate the relationship between pesticide use and single nucleotide polymorphisms (SNPs) in xenobiotic metabolic enzyme (XME) genes.

OBJECTIVE

[corrected] We evaluated pesticide-SNP interactions between 45 pesticides and 1913 XME SNPs with respect to prostrate cancer among 776 cases and 1444 controls in the Agricultural Health Study.

METHODS

We used unconditional logistic regression to estimate odds ratios (ORs) and 95% confidence intervals (CIs). Multiplicative SNP-pesticide interactions were calculated using a likelihood ratio test.

RESULTS

A positive monotonic interaction was observed between petroleum oil/petroleum distillate use and rs1883633 in the oxidative stress gene glutamate cysteine ligase (GCLC; P interaction=1.0×10(-4)); men carrying at least one variant allele (minor allele) experienced an increased prostate cancer risk (OR=3.7, 95% CI: 1.9-7.3). Among men carrying the variant allele for thioredoxin reductase 2 (TXNRD2) rs4485648, microsomal epoxide hydrolase 1 (EPHX1) rs17309872, or myeloperoxidase (MPO) rs11079344, an increased prostate cancer risk was observed with high, compared with no, petroleum oil/petroleum distillate (OR=1.9, 95% CI: 1.1-3.2, P interaction=0.01; OR=2.1, 95% CI: 1.1-4.0, P interaction=0.01), or terbufos (OR=3.0, 95% CI: 1.5-6.0, P interaction=2.0×10(-3)) use, respectively. No interactions were deemed noteworthy at the false discovery rate=0.20 level; the number of observed interactions in XMEs was comparable with the number expected by chance alone.

CONCLUSION

We observed several pesticide-SNP interactions in oxidative stress and phase I/II enzyme genes and risk of prostate cancer. Additional work is needed to explain the joint contribution of genetic variation in XMEs, pesticide use, and prostate cancer risk.

Unveiling the roles of the glutathione redox system in vivo by analyzing genetically modified mice

Redox status affects various cellular activities, such as proliferation, differentiation, and death. Recent studies suggest pivotal roles of reactive oxygen species not only in pathogenesis under oxidative insult but also in intracellular signal transduction. Glutathione is present in several millimolar concentrations in the cytoplasm and has multiple roles in the regulation of cellular homeostasis. Two enzymes, γ-glutamylcysteine synthetase and glutathione synthetase, constitute the de novo synthesis machinery, while glutathione reductase is involved in the recycling of oxidized glutathione. Multidrug resistant proteins and some other transporters are responsible for exporting oxidized glutathione, glutathione conjugates, and S-nitrosoglutathione. In addition to antioxidation, glutathione is more positively involved in cellular activity via its sulfhydryl moiety of a molecule. Animals in which genes responsible for glutathione metabolism are genetically modified can be used as beneficial and reliable models to elucidate roles of glutathione in vivo. This review article overviews recent progress in works related to genetically modified rodents and advances in the elucidation of glutathione-mediated reactions.

N-acetylcysteine attenuates phosgene-induced acute lung injury via up-regulation of Nrf2 expression

Previous studies indicated that oxidative stress was involved in phosgene-induced acute lung injury (ALI) and many antioxidants had been used to prevent ALI. N-acetylcysteine (NAC) had been used to protect ALI induced by various types of oxidative stress. Considering the limited information of NAC on phosgene-induced ALI, the purpose of this study was to elucidate the molecular mechanisms of phosgene-induced ALI and the protective effects of NAC. This study discovered that intraperitoneal administration of NAC significantly alleviated phosgene-induced pulmonary edema, as confirmed by decreased lung wet to dry weight ratio and oxidative stress markers. The content of l-gamma-glutamyl-l-cysteinyl-glycine (glutathione; GSH) and the ratio of the reduced and disulfide forms (GSH/GSSG), significant indicators of the antioxidative ability, were apparently inhibited by phosgene exposure. However, both indicators could be reversed by NAC administration, indicating that dysregulation of redox status of glutathione might be the cause of phosgene-induced ALI. The nuclear factor (NF)-E2-related factor 2 (Nrf2), which has been proven to up-regulate the expression of glutathione reductase (GR), was obviously decreased by phosgene exposure. However, NAC administration elevated Nrf2 expression significantly. In conclusion, these data provided the first evidences showing that it was the transcriptional factor Nrf2 that connected phosgene-induced ALI with GSH metabolism. NAC protected against oxidative stress through acting on this newly disclosed Nrf2/GR/GSH pathway, by which NAC elevated the biosynthesis of protective GSH to repair and reconstitute the defense system destroyed by phosgene.

System x(c)- and thioredoxin reductase 1 cooperatively rescue glutathione deficiency

GSH is the major antioxidant and detoxifier of xenobiotics in mammalian cells. A strong decrease of intracellular GSH has been frequently linked to pathological conditions like ischemia/reperfusion injury and degenerative diseases including diabetes, atherosclerosis, and neurodegeneration. Although GSH is essential for survival, the deleterious effects of GSH deficiency can often be compensated by thiol-containing antioxidants. Using three genetically defined cellular systems, we show here that forced expression of xCT, the substrate-specific subunit of the cystine/glutamate antiporter, in gamma-glutamylcysteine synthetase knock-out cells rescues GSH deficiency by increasing cellular cystine uptake, leading to augmented intracellular and surprisingly high extracellular cysteine levels. Moreover, we provide evidence that under GSH deprivation, the cytosolic thioredoxin/thioredoxin reductase system plays an essential role for the cells to deal with the excess amount of intracellular cystine. Our studies provide first evidence that GSH deficiency can be rescued by an intrinsic genetic mechanism to be considered when designing therapeutic rationales targeting specific redox enzymes to combat diseases linked to GSH deprivation.

Manipulation of cellular GSH biosynthetic capacity via TAT-mediated protein transduction of wild-type or a dominant-negative mutant of glutamate cysteine ligase alters cell sensitivity to oxidant-induced cytotoxicity

The glutathione (GSH) antioxidant defense system plays a central role in protecting mammalian cells against oxidative injury. Glutamate cysteine ligase (GCL) is the rate-limiting enzyme in GSH biosynthesis and is a heterodimeric holoenzyme composed of catalytic (GCLC) and modifier (GCLM) subunits. As a means of assessing the cytoprotective effects of enhanced GSH biosynthetic capacity, we have developed a protein transduction approach whereby recombinant GCL protein can be rapidly and directly transferred into cells when coupled to the HIV TAT protein transduction domain. Bacterial expression vectors encoding TAT fusion proteins of both GCL subunits were generated and recombinant fusion proteins were synthesized and purified to near homogeneity. The TAT-GCL fusion proteins were capable of heterodimerization and formation of functional GCL holoenzyme in vitro. Exposure of Hepa-1c1c7 cells to the TAT-GCL fusion proteins resulted in the time- and dose-dependent transduction of both GCL subunits and increased cellular GCL activity and GSH levels. A heterodimerization-competent, enzymatically deficient GCLC-TAT mutant was also generated in an attempt to create a dominant-negative suppressor of GCL. Transduction of cells with a catalytically inactive GCLC(E103A)-TAT mutant decreased cellular GCL activity in a dose-dependent manner. TAT-mediated manipulation of cellular GCL activity was also functionally relevant as transduction with wild-type GCLC(WT)-TAT or mutant GCLC(E103A)-TAT conferred protection or enhanced sensitivity to H(2)O(2)-induced cell death, respectively. These findings demonstrate that TAT-mediated transduction of wild-type or dominant-inhibitory mutants of the GCL subunits is a viable means of manipulating cellular GCL activity to assess the effects of altered GSH biosynthetic capacity.

Overexpression of glutamate-cysteine ligase protects human COV434 granulosa tumour cells against oxidative and gamma-radiation-induced cell death

Ionizing radiation is toxic to ovarian follicles and can cause infertility. Generation of reactive oxygen species (ROS) has been implicated in the toxicity of ionizing radiation in several cell types. We have shown that depletion of the antioxidant glutathione (GSH) sensitizes follicles and granulosa cells to toxicant-induced apoptosis and that supplementation of GSH is protective. The rate-limiting reaction in GSH biosynthesis is catalysed by glutamate-cysteine ligase (GCL), which consists of a catalytic subunit (GCLC) and a regulatory subunit (GCLM). We hypothesized that overexpression of Gclc or Gclm to increase GSH synthesis would protect granulosa cells against oxidant- and radiation-induced cell death. The COV434 line of human granulosa tumour cells was stably transfected with vectors designed for the constitutive expression of Gclc, Gclm, both Gclc and Gclm or empty vector. GCL protein and enzymatic activity and total GSH levels were significantly increased in the GCL subunit-transfected cells. GCL-transfected cells were resistant to cell killing by treatment with hydrogen peroxide compared to control cells. Cell viability declined less in all the GCL subunit-transfected cell lines 1-8 h after 0.5 mM hydrogen peroxide treatment than in control cells. We next examined the effects of GCL overexpression on responses to ionizing radiation. ROS were measured using a redox-sensitive fluorogenic dye in cells irradiated with 0, 1 or 5 Gy of gamma-rays. There was a dose-dependent increase in ROS within 30 min in all cell lines, an effect that was significantly attenuated in Gcl-transfected cells. Apoptosis, assessed by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labelling and activated caspase-3 immunoblotting, was significantly decreased in irradiated Gclc-transfected cells compared to irradiated control cells. Suppression of GSH synthesis in Gclc-transfected cells reversed resistance to radiation. These findings show that overexpression of GCL in granulosa cells can augment GSH synthesis and ameliorate various sequelae associated with exposure to oxidative stress and irradiation.

The catalytic subunit of Drosophila glutamate-cysteine ligase is a nucleocytoplasmic shuttling protein

GSH concentration is considerably lower in the nucleus than in the cytoplasm; however, it is significantly elevated during active cell proliferation. The main purpose of this study was to understand the mechanism underlying these variations in nuclear/cytoplasmic distribution of GSH. The rate-limiting step in the de novo GSH biosynthesis pathway is catalyzed by glutamate cysteine ligase (GCL), a heterodimer, composed of a catalytic subunit (GCLc) and a modulatory subunit (GCLm). In Drosophila, GCLc, but not GCLm, contains a nuclear localization signal (NLS). Drosophila S2 cells, constitutively expressing regular GCLc protein or expressing GCLc protein with a mutated NLS motif, were generated by transfection. In quiescent S2 cells, GCLc is aggregated in the perinuclear cytosol and the nucleus, whereas GLCm resides solely in the cytosol. In actively proliferating S2 cells, expressing the normal NLS motif, GCLc migrates from the perinuclear cytoplasm into the nucleus, and the nuclear GSH level becomes elevated; in contrast, in proliferating cells, expressing the mutated NLS motif, neither does the GCLc migrate into the nucleus nor does the nuclear GSH amount rise. In S2 cells expressing wild type GCLc, perturbation of cellular redox state by exposure to cadmium resulted in the migration of GCLc into the nucleus but not in cells expressing GCLc with the mutated NLS motif. Overall, results indicated that GSH biosynthesis in the nucleus is associated with migration of only the GCLc subunit from the cytoplasm into the nucleus, and this migration requires the presence of an intact NLS.

Enzymatic metabolites of lycopene induce Nrf2-mediated expression of phase II detoxifying/antioxidant enzymes in human bronchial epithelial cells

Lycopene can be cleaved by carotene 9',10'-oxygenase at its 9',10' double bond to form apo-10'-lycopenoids, including apo-10'-lycopenal, -lycopenol and -lycopenoic acid. The latter has been recently shown to inhibit lung carcinogenesis both in vivo and in vitro, however, the mechanism(s) underlying this protection is not well defined. In the present study, we report that treatment with apo-10'-lycopenoic acid, in a time- and dose-dependent manner, results in the nuclear accumulation of transcription factor Nrf2 (nuclear factor E(2)-related factor 2) protein in BEAS-2B human bronchial epithelial cells. The activation of Nrf2 by apo-10'-lycopenoic acid is associated with the induction of phase II detoxifying/antioxidant enzymes including heme oxygenase-1, NAD(P)H:quinone oxidoreductase 1, glutathione S-transferases, and glutamate-cysteine ligases in BEAS-2B cells. Furthermore, apo-10'-lycopenoic acid treatment increased total intracellular glutathione levels and suppressed both endogenous reactive oxygen species generation and H(2)O(2)-induced oxidative damage in BEAS-2B cells. In addition, both apo-10'-lycopenol and apo-10'-lycopenal induced heme oxygenase-1 gene expression in BEAS-2B cells. These data strongly suggest that the anti-carcinogenic and antioxidant functions of lycopene may be mediated by apo-10'-lycopenoids via activating Nrf2 and inducing phase II detoxifying/antioxidant enzymes.