Human GSTA1-1 reduces c-Jun N-terminal kinase signalling and apoptosis in Caco-2 cells

Are glutathione S transferases involved in DNA damage signalling? Interactions with DNA damage and repair revealed from molecular epidemiology studies

Glutathione S-transferases (GSTs) are members of a multigene family of isoenzymes that are important in the control of oxidative stress and in phase II metabolism. Acting non-enzymically, GSTs can modulate signalling pathways of cell proliferation, cell differentiation and apoptosis. Using a molecular epidemiology approach, we have investigated a potential involvement of GSTs in DNA damage processing, specifically the modulation of DNA repair in a group of 388 healthy adult volunteers; 239 with at least 5 years of occupational exposure to asbestos, stone wool or glass fibre, and 149 reference subjects. We measured DNA damage in lymphocytes using the comet assay (alkaline single cell gel electrophoresis): strand breaks (SBs) and alkali-labile sites, oxidised pyrimidines with endonuclease III, and oxidised purines with formamidopyrimidine DNA glycosylase. We also measured GST activity in erythrocytes, and the capacity for base excision repair (BER) in a lymphocyte extract. Polymorphisms in genes encoding three GST isoenzymes were determined, namely deletion of GSTM1 and GSTT1 and single nucleotide polymorphism Ile105Val in GSTP1. Consumption of vegetables and wine correlated negatively with DNA damage and modulated BER. GST activity correlated with oxidised bases and with BER capacity, and differed depending on polymorphisms in GSTP1, GSTT1 and GSTM1. A significantly lower BER rate was associated with the homozygous GSTT1 deletion in all asbestos site subjects and in the corresponding reference group. Multifactorial analysis revealed effects of sex and exposure in GSTP1 Ile/Val heterozygotes but not in Ile/Ile homozygotes. These variants affected also SBs levels, mainly by interactions of GSTP1 genotype with exposure, with sex, and with smoking habit; and by an interaction between sex and smoking. Our results show that GST polymorphisms and GST activity can apparently influence DNA stability and repair of oxidised bases, suggesting a potential new role for these proteins in DNA damage processing via DNA damage signalling.

Regulation of signal transduction by glutathione transferases

Glutathione transferases (GST) are essentially known as enzymes that catalyse the conjugation of glutathione to various electrophilic compounds such as chemical carcinogens, environmental pollutants, and antitumor agents. However, this protein family is also involved in the metabolism of endogenous compounds which play critical roles in the regulation of signaling pathways. For example, the lipid peroxidation product 4-hydroxynonenal (4-HNE) and the prostaglandin 15-deoxy-Δ12,14-prostaglandin J(2) (15d-PGJ(2)) are metabolized by GSTs and these compounds are known to influence the activity of transcription factors and protein kinases involved in stress response, proliferation, differentiation, or apoptosis. Furthermore, several studies have demonstrated that GSTs are able to interact with different protein partners such as mitogen activated protein kinases (i.e., c-jun N-terminal kinase (JNK) and apoptosis signal-regulating kinase 1 (ASK1)) which are also involved in cell signaling. New functions of GSTs, including S-glutathionylation of proteins by GSTs and ability to be a nitric oxide (NO) carrier have also been described. Taken together, these observations strongly suggest that GST might play a crucial role during normal or cancer cells proliferation or apoptosis.

Metformin prevents endoplasmic reticulum stress-induced apoptosis through AMPK-PI3K-c-Jun NH2 pathway

Type 2 diabetes mellitus is thought to be partially associated with endoplasmic reticulum (ER) stress toxicity on pancreatic beta cells and the result of decreased insulin synthesis and secretion. In this study, we showed that a well-known insulin sensitizer, metformin, directly protects against dysfunction and death of ER stress-induced NIT-1 cells (a mouse pancreatic beta cell line) via AMP-activated protein kinase (AMPK) and phosphatidylinositol-3 (PI3) kinase activation. We also showed that exposure of NIT-1 cells to metformin (5mM) increases cellular resistance against ER stress-induced NIT-1 cell dysfunction and death. AMPK and PI3 kinase inhibitors abolished the effect of metformin on cell function and death. Metformin-mediated protective effects on ER stress-induced apoptosis were not a result of an unfolded protein response or the induced inhibitors of apoptotic proteins. In addition, we showed that exposure of ER stressed-induced NIT-1 cells to metformin decreases the phosphorylation of c-Jun NH(2) terminal kinase (JNK). These data suggest that metformin is an important determinant of ER stress-induced apoptosis in NIT-1 cells and may have implications for ER stress-mediated pancreatic beta cell destruction via regulation of the AMPK-PI3 kinase-JNK pathway.

Glutathione s-transferases in pediatric cancer

The glutathione S-transferases (GSTs) are a family of ubiquitously expressed polymorphic enzymes important for detoxifying endogenous and exogenous compounds. In addition to their classic activity of detoxification by conjugation of compounds with glutathione, many other functions are now found to be associated with GSTs. The associations between GST polymorphisms/functions and human disease susceptibility or treatment outcome, mostly in adults, have been extensively studied and reviewed. This mini review focuses on studies related to GST epidemiology and functions related to pediatric cancer. Opportunities to exploit GST in pediatric cancer therapy are also discussed.

The role of glutathione S-transferase P in signaling pathways and S-glutathionylation in cancer

Glutathione S-transferase P is abundantly expressed in some mammalian tissues, particularly those associated with malignancies. While the enzyme can catalyze thioether bond formation between some electrophilic chemicals and GSH, novel nondetoxification functions are now ascribed to it. This review summarizes recent material that implicates GSTP in mediating S-glutathionylation of specific clusters of target proteins and in reactions that define a negative regulatory role in some kinase pathways through ligand or protein:protein interactions. It is becoming apparent that GSTP participates in the maintenance of cellular redox homeostasis through a number of convergent and divergent mechanisms. Moreover, drug platforms that have GSTP as a target have produced some interesting preclinical and clinical candidates.

Regulatory functions of glutathione S-transferase P1-1 unrelated to detoxification

Glutathione S-transferase P1-1 (GSTP) is one member of the family of GSTs and is ubiquitously expressed in human tissues. The literature is replete with reports of high levels of GSTP linked either with cancer incidence or drug resistance, and yet no entirely cogent explanation for these correlations exists. The catalytic detoxification properties of the GST isozyme family have been a primary research focus for the last four decades. However, it has become apparent that they have undergone structural and functional convergence where evolutionary selective pressures have favored the emergence of noncatalytic properties of GSTP that has imbued this isozyme with expanded biological importance. For example, GSTP has now been linked with two cell-signaling functions that are critical to survival. Through protein:protein interactions, GSTP can sequester c-jun N-terminal kinase (JNK) and act as a negative regulator of this stress kinase. Pharmacologically, this activity has been linked with the activity of GSTP inhibitors in stimulating myeloproliferation. In addition, GSTP is linked with the forward S-glutathionylation reaction, a post-translational modification that impacts the function/activity of a number of proteins. Catalytic reversal of S-glutathionylation is well characterized, but the role of GSTP in catalyzing the forward reaction contributes to the "glutathionylation cycle." Moreover, GSTP is itself susceptible to S-glutathionylation, providing an autoregulatory loop for the cycle. Because oxidative stress regulates both S-glutathionylation and JNK-signaling pathways, such links may help to explain the aberrant patterns of GSTP expression in the cancer phenotype. As such, there is an ongoing preclinical and clinical platform of drug discovery and development around GSTP.

Gene expression profiles in human HepG2 cells treated with extracts of the Tamarindus indica fruit pulp

Tamarindus indicaL. (T. indica) or locally known as asam jawa belongs to the family of Leguminosae. The fruit pulp had been reported to have antioxidant activities and possess hypolipidaemic effects. In this study, we attempted to investigate the gene expression patterns in human hepatoma HepG2 cell line in response to treatment with low concentration of the fruit pulp extracts. Microarray analysis using Affymetrix Human Genome 1.0 S.T arrays was used in the study. Microarray data were validated using semi-quantitative RT-PCR and real-time RT-PCR. Amongst the significantly up-regulated genes were those that code for the metallothioneins (MT1M, MT1F, MT1X) and glutathione S-transferases (GSTA1, GSTA2, GST02) that are involved in stress response. APOA4, APOA5, ABCG5 and MTTP genes were also significantly regulated that could be linked to hypolipidaemic activities of the T. indica fruit pulp.

The impact of redox and thiol status on the bone marrow: Pharmacological intervention strategies

Imbalances in cancer cell redox homeostasis provide a platform for new opportunities in the development of anticancer drugs. The control of severe dose-limiting toxicities associated with redox regulation, including myelosuppression and immunosuppression, remains a challenge. Recent evidence implicates a critical role for redox regulation and thiol balance in pathways that control myeloproliferation, hematopoietic progenitor cell mobilization, and immune response. Hematopoietic stem cell (HSC) self-renewal and differentiation are dependent upon levels of intracellular reactive oxygen species (ROS) and niche microenvironments. Redox status and the equilibrium of free thiol:disulfide couples are important in modulating immune response and lymphocyte activation, proliferation and differentiation. This subject matter is the focus of the present review. The potential of redox modulating chemotherapeutics as myeloproliferative and immunomodulatory agents is also covered.

Porcine glutathione transferase Alpha 2-2 is a human GST A3-3 analogue that catalyses steroid double-bond isomerization

A primary role of GSTs (glutathione transferases) is detoxication of electrophilic compounds. In addition to this protective function, hGST (human GST) A3-3, a member of the Alpha class of soluble GSTs, has prominent steroid double-bond isomerase activity. The isomerase reaction is an obligatory step in the biosynthesis of steroid hormones, indicating a special role of hGST A3-3 in steroidogenic tissues. An analogous GST with high steroid isomerase activity has so far not been found in any other biological species. In the present study, we characterized a Sus scrofa (pig) enzyme, pGST A2-2, displaying high steroid isomerase activity. High levels of pGST A2-2 expression were found in ovary, testis and liver. In its functional properties, other than steroid isomerization, pGST A2-2 was most similar to hGST A3-3. The properties of the novel porcine enzyme lend support to the notion that particular GSTs play an important role in steroidogenesis.

Mechanisms of 4-hydroxy-2-nonenal induced pro- and anti-apoptotic signaling

In recent years, 4-hydroxy-2-nonenal (4-HNE) has emerged as an important second messenger in cell cycle signaling. Here, we demonstrate that 4-HNE induces signaling for apoptosis via both the Fas-mediated extrinsic and the p53-mediated intrinsic pathways in HepG2 cells. 4-HNE induces a Fas-mediated DISC independent apoptosis pathway by activating ASK1, JNK, and caspase-3. Parallel treatment of 4-HNE to HepG2 cells also induces apoptosis by the p53 pathway through activation of Bax, p21, JNK, and caspase-3. Exposure of HepG2 cells to 4-HNE leads to the activation of both Fas and Daxx, promotes the export of Daxx from the nucleus to cytoplasm, and facilitates Fas-Daxx binding. Depletion of Daxx by siRNA results in the potentiation of apoptosis, indicating that Fas-Daxx binding in fact is inhibitory to Fas-mediated apoptosis in cells. 4-HNE-induced translocation of Daxx is also accompanied by the activation and nuclear accumulation of HSF1 and up-regulation of heat shock protein Hsp70. All these effects of 4-HNE in cells can be attenuated by ectopic expression of hGSTA4-4, the isozyme of glutathione S-transferase with high activity for 4-HNE. Through immunoprecipitation and liquid chromatography-tandem mass spectrometry, we have demonstrated the covalent binding of 4-HNE to Daxx. We also demonstrate that 4-HNE modification induces phosphorylation of Daxx at Ser668 and Ser671 to facilitate its cytoplasmic export. These results indicate that while 4-HNE exhibits toxicity through several mechanisms, in parallel it evokes signaling for defense mechanisms to self-regulate its toxicity and can simultaneously affect multiple signaling pathways through its interactions with membrane receptors and transcription factors/repressors.

Targeting human glutathione transferase A3-3 attenuates progesterone production in human steroidogenic cells

hGSTA3-3 (human Alpha-class glutathione transferase 3-3) efficiently catalyses steroid Δ5–Δ4 double-bond isomerization in vitro, using glutathione as a cofactor. This chemical transformation is an obligatory reaction in the biosynthesis of steroid hormones and follows the oxidation of 3β-hydroxysteroids catalysed by 3β-HSD (3β-hydroxysteroid dehydrogenase). The isomerization has commonly been ascribed to a supplementary function of 3β-HSD. The present study is the first to provide evidence that hGSTA3-3 contributes to this step in steroid hormone biosynthesis in complex cellular systems. First, we find glutathione-dependent Δ5–Δ4 isomerase activity in whole-cell extracts prepared from human steroidogenic cells. Secondly, effective inhibitors of hGSTA3-3 dramatically decrease the conversion of Δ5-androstene-3,17-dione into Δ4-androstene-3,17-dione in cell lysates. Thirdly, we show that RNAi (RNA interference) targeting hGSTA3-3 expression decreases by 30% the forskolin-stimulated production of the steroid hormone progesterone in a human placental cell line. This effect is achieved at low concentrations of two small interfering RNAs directed against distinct regions of hGSTA3-3 mRNA, and is weaker in unstimulated cells, in which hGSTA3-3 expression is low. The results concordantly show that hGSTA3-3 makes a significant contribution to the double-bond isomerization necessary for steroid hormone biosynthesis and thereby complements the indispensable 3β-hydroxysteroid oxidoreductase activity of 3β-HSD. The results indicate that the lower isomerase activity of 3β-HSD is insufficient for maximal rate of cellular sex hormone production and identify hGSTA3-3 as a possible target for pharmaceutical intervention in steroid hormone-dependent diseases.

S-glutathionylation: indicator of cell stress and regulator of the unfolded protein response

The specific posttranslational modification of protein cysteine residues by the addition of the tripeptide glutathione is termed S-glutathionylation. This process is promoted by oxidative and nitrosative stress but also occurs in unstressed cells. Altered levels of S-glutathionylation in some proteins have been associated with numerous pathologies, many of which have been linked to redox stress in the endoplasmic reticulum (ER). Proper protein folding is dependent upon controlled redox conditions within the ER, and it seems that ER conditions can in turn affect rates of S-glutathionylation. This article seeks to bring together the ways through which these processes are interrelated and considers the implications of these interrelationships upon therapeutic approaches to disease.

Glutathione S-transferase T1 and M1: gene sequence variation and functional genomics

PURPOSE

The glutathione S-transferases (GSTs) catalyze the glutathione conjugation of reactive electrophiles, including carcinogens and many antineoplastic drugs. GSTT1 and GSTM1 are polymorphically deleted, but the full range of genetic variation in these two genes has not yet been explored. We set out to systematically identify common polymorphisms in GSTT1 and GSTM1, followed by functional genomic studies.

EXPERIMENTAL DESIGN

First, multiplex PCR was used to determine GSTT1 and GSTM1 copy number in 400 DNA samples (100 each from 4 ethnic groups). Exons, splice junctions, and 5'-flanking regions (5'-FR) were then resequenced using DNA samples that contained at least one copy of GSTT1 or GSTM1.

RESULTS

Gene deletion frequencies among ethnic groups were from 33.5% to 73.5% for GSTT1 and from 50.5% to 78.0% for GSTM1. GSTT1 deletion data correlated with the results of mRNA microarray expression studies. The 18 single nucleotide polymorphisms (SNP) observed in GSTT1 included three nonsynonymous coding SNPs (cSNPs) and one single-nucleotide deletion, whereas the 51 GSTM1 SNPs included two nonsynonymous cSNPs. Two of the GSTT1 nonsynonymous cSNPs resulted in decreases in levels of immunoreactive protein to 56% and 12% of wild type (WT), whereas those in GSTM1 resulted in modest increases in protein levels. Reporter gene assays showed that one GSTT1 5'-FR haplotype, with a frequency of 32% in African-American subjects, resulted in an increase in transcription in JEG-3 cells to 351% of that for the WT sequence, and one GSTM1 5'-FR haplotype resulted in an increase in transcription in JEG-3 cells to 129% of WT.

CONCLUSIONS

These observations suggest that functionally significant pharmacogenomic variation beyond GSTT1 and GSTM1 gene deletion may contribute to carcinogenesis or individual variation in antineoplastic drug therapy response.

Redox in redux: Emergent roles for glutathione S-transferase P (GSTP) in regulation of cell signaling and S-glutathionylation

Glutathione (GSH) provides a major source of thiol homeostasis critical to the maintenance of a reduced cellular environment that is conducive to cell survival. Mammals have accumulated a significant cadre of sulfur containing proteins, the interactive significance of which has become clear in recent times. Glutathione transferases (GST) are prevalent in eukaryotes and have been ascribed catalytic functions that involve detoxification of electrophiles through thioether bond formation with the cysteine thiol of GSH. The neutralizing impact of these reactions on products of reactive oxygen has contributed to the significant evolutionary conservation and adaptive functional redundancy of the multifaceted GSH system. Amongst the GSTs, GSTP has been implicated in tumorigenesis and in anticancer drug resistance. Emerging studies indicate that GSTP has ligand binding properties and contributes in the regulation of signaling kinases through direct protein:protein interactions. Furthermore, S-glutathionylation is a post-translational modification of low pK(a) cysteine residues in target proteins. The forward rate of the S-glutathionylation reaction can be influenced by GSTP, whereas the reverse rate is affected by a number of redox sensitive proteins including glutaredoxin, thioredoxin and sulfiredoxin. The functional importance of these reactions in governing how cells respond to oxidative or nitrosative stress exemplifies the broad importance of GSH/GST homeostasis in conditions such as cancer, ageing and neurodegenerative diseases. GSTP has also provided a platform for therapeutic drug development where some agents have completed preclinical testing and are in clinical trial for the management of cancer.