Posttranscriptional regulation of MRP/GS-X pump and gamma-glutamylcysteine synthetase expression by 1-(4-amino-2-methyl-5-pyrimidinyl) methyl-3-(2-chloroethyl)-3-nitrosourea and by cycloheximide in human glioma cells

Cisplatin-induced synthetic lethality to arginine-starvation therapy by transcriptional suppression of ASS1 is regulated by DEC1, HIF-1α, and c-Myc transcription network and is independent of ASS1 promoter DNA methylation

Many human tumors require extracellular arginine (Arg) for growth because the key enzyme for de novo biosynthesis of Arg, argininosuccinate synthetase 1 (ASS1), is silenced. These tumors are sensitive to Arg-starvation therapy using pegylated arginine deiminase (ADI-PEG20) which digests extracellular Arg. Many previous studies reported that ASS1 silencing is due to epigenetic inactivation of ASS1 expression by DNA methylation, and that the demethylation agent 5-aza-deoxycytidine (Aza-dC) can induce ASS1 expression. Moreover, it was reported that cisplatin suppresses ASS1 expression through ASS1 promoter methylation, leading to synthetic lethality to ADI-PEG20 treatment. We report here that cisplatin supppresses ASS1 expression is due to upregulation of HIF-1α and downregulation of c-Myc, which function as negative and positive regulators of ASS1 expression, respectively, by reciprocal bindings to the ASS1 promoter. In contrast, we found that Aza-dC induces ASS1 expression by downregulation of HIF-1α but upregulation of c-Myc. We further demonstrated that the clock protein DEC1 is the master regulator of HIF-1α and c-Myc that regulate ASS1. cDDP upregulates DEC1, whereas Aza-dC suppresses its expression. Using two proteasomal inhibitors bortezomib and carfilzomib which induce HIF-1α accumulation, we further demonstrated that HIF-1α is involved in ASS1 silencing for the maintenance of Arg auxotrophy for targeted Arg-starvation therapy.

A new aspect on glutathione-associated biological function of MRP/GS-X pump and its gene expression

The biological function as well as gene expression of the MRP/GS-X pump is closely linked with cellular GSH metabolism. This article describes two important aspects, i.e., 1) a role of the MRP/GS-X pump in the modulation of cell cycle arrest induced by anticancer prostaglandins; 2) coordinated up-regulation of gamma-glutamylcysteine synthetase gamma-GCS) and MRP1 genes. The A and J series of prostaglandins (PGs) accumulate in the nuclei to suppress the proliferation of cancer cells. Delta(7)-Prostaglandin A(1) (Delta(7)-PGA(1)) methyl ester, a synthetic anticancer PG, increased the mRNA level of the cyclin-dependent kinase inhibitor p21(Sdi1/CIP1/WAF1) in human leukemia HL-60 cells. The induction of p21(Sdi1/CIP1/WAF1) was associated with the accumulation of hypophosphorylated retinoblastoma protein (pRB) and the suppression of c-myc gene expression. Unlike HL-60 cells, cisplatin-resistant HL-60/R-CP cells were insensitive to Delta(7)-PGA(1) methyl ester. While c-myc expression was transiently suppressed, neither G1 arrest nor hypophosphorylation of pRB was observed with the anticancer PG. Plasma membrane vesicles from HL-60/R-CP cells showed an enhanced level of GS-X pump activity toward the glutathione S-conjugate of Delta(7)-PGA(1) methyl ester. GIF-0019, a potent inhibitor of the GS-X pump, dose-dependently enhanced the cellular sensitivity of HL-60/R-CP cells to Delta(7)-PGA(1) methyl ester, resulting in G1 arrest. The GS-X pump is suggested to play a pivotal role in modulating the biological action of the anticancer PG. The expression of MRP1 and gamma-GCS genes can be coordinately up-regulated by cisplatin, 1-[5-(4-amino-2-methyl)pyrimidyl]methyl-3-(2-chloroethyl)-3-nitrosourea (ACNU), and heavy metals in human cancer cells. For the up-regulation of these genes, both transcriptional and posttranscriptional regulations are considered to be involved.

Distinct Nrf1/2-independent mechanisms mediate As 3+-induced glutamate-cysteine ligase subunit gene expression in murine hepatocytes

Trivalent arsenite (As(3+)) is a known human carcinogen that is also capable of inducing apoptotic cell death. Increased production of reactive oxygen species is thought to contribute to both the carcinogenic and the cytotoxic effects of As(3+). Glutathione (GSH) constitutes a vital cellular defense mechanism against oxidative stress. The rate-limiting enzyme in GSH biosynthesis is glutamate-cysteine ligase (GCL), a heterodimeric holoenzyme composed of a catalytic (GCLC) and a modifier (GCLM) subunit. In this study, we demonstrate that As(3+) coordinately upregulates Gclc and Gclm mRNA levels in a murine hepatocyte cell line resulting in increased GCL subunit protein expression, holoenzyme formation, and activity. As(3+) increased the rate of transcription of both the Gclm and the Gclc genes and induced the posttranscriptional stabilization of Gclm mRNA. The antioxidant N-acetylcysteine abolished As(3+)-induced Gclc expression and attenuated induction of Gclm. As(3+) induction of Gclc and Gclm was also differentially regulated by the MAPK signaling pathways and occurred independent of the Nrf1/2 transcription factors. These findings demonstrate that distinct transcriptional and posttranscriptional mechanisms mediate the coordinate induction of the Gclc and Gclm subunits of GCL in response to As(3+) and highlight the potential importance of the GSH antioxidant defense system in regulating As(3+)-induced responses in hepatocytes.

Redox regulation of multidrug resistance in cancer chemotherapy: molecular mechanisms and therapeutic opportunities

The development of multidrug resistance to cancer chemotherapy is a major obstacle to the effective treatment of human malignancies. It has been established that membrane proteins, notably multidrug resistance (MDR), multidrug resistance protein (MRP), and breast cancer resistance protein (BCRP) of the ATP binding cassette (ABC) transporter family encoding efflux pumps, play important roles in the development of multidrug resistance. Overexpression of these transporters has been observed frequently in many types of human malignancies and correlated with poor responses to chemotherapeutic agents. Evidence has accumulated showing that redox signals are activated in response to drug treatments that affect the expression and activity of these transporters by multiple mechanisms, including (a) conformational changes in the transporters, (b) regulation of the biosynthesis cofactors required for the transporter's function, (c) regulation of the expression of transporters at transcriptional, posttranscriptional, and epigenetic levels, and (d) amplification of the copy number of genes encoding these transporters. This review describes various specific factors and their relevant signaling pathways that are involved in the regulation. Finally, the roles of redox signaling in the maintenance and evolution of cancer stem cells and their implications in the development of intrinsic and acquired multidrug resistance in cancer chemotherapy are discussed.

Meta-analysis of published transcriptional and translational fold changes reveals a preference for low-fold inductions

The goals of this study were to gain a better quantitative understanding of the dynamic range of transcriptional and translational response observed in biological systems and to examine the reporting of regulatory events for trends and biases. A straightforward pattern-matching routine extracted 3,408 independent observations regarding transcriptional fold-changes and 1,125 regarding translational fold-changes from over 15 million MEDLINE abstracts. Approximately 95% of reported changes were > or =2-fold. Further, the historical trend of reporting individual fold-changes is declining in favor of high-throughput methods for transcription but not translation. Where it was possible to compare the average fold-changes in transcription and translation for the same gene/product (203 examples), approximately 53% were a < or =2-fold difference, suggesting a loose tendency for the two to be coupled in magnitude. We found also that approximately three-fourths of reported regulatory events have been at the transcriptional level. The frequency distribution appears to be normally distributed and peaks near 2-fold, suggesting that nature selects for a low-energy solution to regulatory responses. Because high-throughput technologies ordinarily sacrifice measurement quality for quantity, this also suggests that many regulatory events may not be reliably detectable by such technologies. Text mining of regulatory events and responses provides additional information incorporable into microarray analysis, such as prior fold-change observations and flagging genes that are regulated post-transcription. All extracted regulation and response patterns can be downloaded at the following website: www.ou.edu/microarray/ oumcf/Meta_analysis.xls.

Differential regulation of glutamate-cysteine ligase subunit expression and increased holoenzyme formation in response to cysteine deprivation

GCL (glutamate-cysteine ligase) is a heterodimer of a GCLC (GCL catalytic subunit) that possesses all of the enzymatic activity and a GCLM (GCL modifier subunit) that alters the K(i) of GCLC for GSH. We hypothesized that the expression of GCLM and the association of GCLM with GCLC were responsible for the apparent increase in GCL activity state observed in the liver of rats fed low-protein diets or in hepatocytes cultured in low-sulphur amino acid-containing medium. Therefore we conducted a series of studies using rats and a human hepatoma (HepG2/C3A) cell line to assess the role of GCLM and holoenzyme formation in the regulation of GCL activity in response to sulphur amino acid intake or availability. Increases in GCL activity in rat liver, as well as in HepG2 cells, were due to the additive effects of changes in the amount of GCLC and the kcat for GCLC. The increase in the kcat for GCLC was associated with increased holoenzyme formation, which was associated with an increase in the molar ratio of GCLM to GCLC. Furthermore, our results indicate that the GCLM level in rat liver is always limiting and that up-regulation of the GCLM level results in increased holoenzyme formation and an increase in the kcat. This is the first report demonstrating that the catalytic efficiency of rat GCL is increased by holoenzyme formation and the first demonstration of differential up-regulation of the GCL subunits in response to cysteine deprivation.

Human glutamate cysteine ligase gene regulation through the electrophile response element

Glutathione (GSH) is the primary nonprotein thiol in the cell. It has many important roles in cell function, including regulating redox-dependent signal transduction pathways. The content of GSH within the cell varies with stress. In many cases, a process involving GSH synthesis results in adaptation to subsequent stressors. Sustained increases in GSH content are controlled primarily through induction of two genes, Gclc and Gclm, leading to the synthesis of the rate-limiting enzyme for GSH synthesis, glutamate cysteine ligase. Each of these genes in humans has a number of putative enhancer elements in their promoters. Overall, the most important element in both Gclc and Gclm expression is the electrophile response element. We review the evidence that has led to this conclusion and the implications for the redox-dependent regulation of this critical intracellular antioxidant.

Variable regulation of glutamate cysteine ligase subunit proteins affects glutathione biosynthesis in response to oxidative stress

Glutamate cysteine ligase (GCL), composed of a catalytic (GCLC) and modulatory (GCLM) subunit, catalyzes the first step of glutathione (GSH) biosynthesis. Using 4-hydroxy-2-nonenal (4HNE), 2,3-dimethoxy-1,4-naphthoquinone (DMNQ), and tertiary-butylhydroquinone (tBHQ) as models of oxidative stress which are known to work through different mechanisms, we measured changes in cellular GSH, GCL mRNA, and GCL protein. 4HNE and tBHQ treatments increased cellular GSH levels, while DMNQ exposure depleted GSH. Furthermore, changes in the two GCL mRNAs largely paralleled changes in the GCL proteins; however, the magnitudes differed, suggesting some form of translational control. The molar ratio of GCLC:GCLM ranged from 3:1 to 17:1 in control human bronchial epithelial (HBE1) cells and all treatments further increased this ratio. Data from several mouse tissues show molar ratios of GCLC:GCLM that range from 1:1 to 10:1 in support of these findings. These data demonstrate that alterations in cellular GSH are clearly correlated with GCLC to a greater extent than GCLM. Surprisingly, both control HBE1 cells and some mouse tissues have more GCLC than GCLM and GCLM increases to a much lesser extent than GCLC, suggesting that the regulatory role of GCLM is minimal under physiologically relevant conditions of oxidative stress.

Impact of drug transporter studies on drug discovery and development

Drug transporters are expressed in many tissues such as the intestine, liver, kidney, and brain, and play key roles in drug absorption, distribution, and excretion. The information on the functional characteristics of drug transporters provides important information to allow improvements in drug delivery or drug design by targeting specific transporter proteins. In this article we summarize the significant role played by drug transporters in drug disposition, focusing particularly on their potential use during the drug discovery and development process. The use of transporter function offers the possibility of delivering a drug to the target organ, avoiding distribution to other organs (thereby reducing the chance of toxic side effects), controlling the elimination process, and/or improving oral bioavailability. It is useful to select a lead compound that may or may not interact with transporters, depending on whether such an interaction is desirable. The expression system of transporters is an efficient tool for screening the activity of individual transport processes. The changes in pharmacokinetics due to genetic polymorphisms and drug-drug interactions involving transporters can often have a direct and adverse effect on the therapeutic safety and efficacy of many important drugs. To obtain detailed information about these interindividual differences, the contribution made by transporters to drug absorption, distribution, and excretion needs to be taken into account throughout the drug discovery and development process.

The importance of glutathione in human disease

Reduced glutathione (GSH) is the most prevalent non-protein thiol in animal cells. Its de novo and salvage synthesis serves to maintain a reduced cellular environment and the tripeptide is a co-factor for many cytoplasmic enzymes and may also act as an important post-translational modification in a number of cellular proteins. The cysteine thiol acts as a nucleophile in reactions with both exogenous and endogenous electrophilic species. As a consequence, reactive oxygen species (ROS) are frequently targeted by GSH in both spontaneous and catalytic reactions. Since ROS have defined roles in cell signaling events as well as in human disease pathologies, an imbalance in expression of GSH and associated enzymes has been implicated in a variety of circumstances. Cause and effect links between GSH metabolism and diseases such as cancer, neurodegenerative diseases, cystic fibrosis (CF), HIV, and aging have been shown. Polymorphic expression of enzymes involved in GSH homeostasis influences susceptibility and progression of these conditions. This review provides an overview of the biological importance of GSH at the level of the cell and organism.

Glutathione in defense and signaling: lessons from a small thiol

The mechanisms of thiol metabolism and chemistry have particular relevance to both cellular defenses against toxicant exposure and to redox signaling. Here, we will focus on glutathione (GSH), the major endogenous low- molecular-weight nonprotein thiol synthesized de novo in mammalian cells. The major pathways for GSH metabolism in defense of the cell are reduction of hydroperoxides by glutathione peroxidases (GSHPx) and some peroxiredoxins, which yield glutathione disulfide (GSSG), and conjugation reactions catalyzed by glutathione-S-transferases. GSSG can be reduced to GSH by glutathione reductase, but glutathione conjugates are excreted from cells. The exoenzyme gamma-glutamyltranspeptidase (GGT) removes the glutamate from extracellular GSH, producing cysteinyl-glycine from which a dipeptidase then generates cysteine, an amino acid often limiting for de novo GSH synthesis. Synthesis of GSH from the constituent amino acids occurs in two regulated, enzymatically catalyzed steps. The signaling pathways leading to activation of the transcription factors that regulate these genes are a current area of intense investigation. The elucidation of the signaling for GSH biosynthesis in human bronchial epithelial cells in response to 4-hydroxynonenal (4HNE), an end product of lipid peroxidation, will be used as an example. GSH also participates in redox signaling through the removal of H(2)O(2), which has the properties of a second messenger, and by reversing the formation of sulfenic acid, a moiety formed by reaction of critical cysteine residues in signaling proteins with H(2)O(2). Disruption of GSH metabolism will therefore have major a impact upon function of cells in terms of both defense and normal physiology.

4-hydroxynonenal induces glutamate cysteine ligase through JNK in HBE1 cells

Glutathione is the most abundant non-protein thiol in the cell, with roles in cell cycle regulation, detoxification of xenobiotics, and maintaining the redox tone of the cell. The glutathione content is controlled at several levels, the most important being the rate of de novo synthesis, which is mediated by two enzymes, glutamate cysteine ligase (GCL), and glutathione synthetase (GS), with GCL being rate-limiting generally. The GCL holoenzyme consists of a catalytic (GCLC) and a modulatory (GCLM) subunit, which are encoded by separate genes. In the present study, the signaling mechanisms leading to de novo synthesis of GSH in response to physiologically relevant concentrations of 4-hydroxy-2-nonenal (4HNE), an endproduct of lipid peroxidation, were investigated. We demonstrated that exposure to 4HNE resulted in increased content of both Gcl mRNAs, both GCL subunits, phosphorylated JNK1 and c-Jun proteins, as well as Gcl TRE sequence-specific AP-1 binding activity. These increases were attenuated by pretreating the cells with a novel membrane-permeable JNK pathway inhibitor, while chemical inhibitors of the p38 or ERK pathways were ineffective. These data reveal that de novo GSH biosynthesis in response to 4HNE signals through the JNK pathway and suggests a major role for AP-1 driven expression of both Gcl genes in HBE1 cells.

Cellular glutathione and thiols metabolism

Low molecular weight thiol-containing compounds have an essential role in many biochemical and pharmacological reactions due to the ease with each they are oxidized, and the rapidity with which they can be regenerated. Thioredoxin and glutathione (GSH) are two of the major small molecular weight thiol-containing compounds synthesized de novo in mammalian cells that participate in those functions. Understanding the mechanisms of thiol metabolism has special relevance to understanding the cell's defense against toxicant exposure and as the focal point in redox signaling. This commentary will, however, focus on GSH consumption and synthesis, and the role of thiols in signaling. The chemical reactions of GSH, including conjugation reactions mediated by glutathione S-transferases (GST) and oxidation reactions mediated by glutathione peroxidases will be described. The regulation of GSH synthesis will be illustrated from a compilation of studies designed to understand the various levels at which enzymatic GSH biosynthesis is controlled, and the signaling pathways that mediate them. The response of the cell to 4-hydroxynonenal (4HNE), a reactive aldehyde produced physiologically in response to inflammation and various air pollutants, will be explored in detail. Finally, the direct role of thiols as signaling molecules will be addressed, with particular attention given to "redox state." It is our aim that this commentary will lead the reader to appreciate that studies investigating the signaling for and regulation of thiol metabolism must never be generalized, and that perturbations in any of step of thiol metabolism may have etiological roles in genetically, virally, and environmentally borne pathologies.

Brain-tumour drug resistance: the bare essentials

Chemotherapy lacks efficacy in most histological types of primary human brain tumours and has, for most types, failed to improve outcome for patients. The unsatisfactory results with chemotherapeutic intervention in these cancers have been chiefly attributed to tumour-cell resistance. This review summarises some of the major molecular markers that, although neither exclusive for nor specific to brain tumours, have been associated with the formation of a chemoresistant phenotype. Elucidation of the cellular mechanisms involved in resistance regulation is needed for future progress in efficient approaches to selective modulation of drug resistance in these lesions.

Expression of heavy subunit of gamma-glutamylcysteine synthetase (gamma-GCSh) in human colorectal carcinoma

Gamma-glutamylcysteine synthetase (gamma-GCS) is a heterodimer consisting of heavy (gamma-GCSh) and light (gamma-GCSl) subunits. gamma-GCS catalyzes the rate-limiting de novo biosynthesis of glutathione (GSH), an abundant physiological antioxidant that plays important roles for regulating oxidative stress. Expression of gamma-GCSh and gamma-GCSl are sensitive to oxidative stress. To investigate whether expression of gamma-GCS is correlated with tumor progression, we used immunohistochemical approaches to examine 16 human colorectal adenomas and resected 57 carcinomas from untreated patients. In adjacent normal colorectal epithelium, levels of gamma-GCSh expression were low. Strong cytoplasmic staining for gamma-GCSh was detected in 3 (18.8%) adenoma and 48 (84.2%) carcinomas. The frequency of gamma-GCSh expression in carcinoma was significantly higher than in adenoma (p<0.0001). We used RNase protation assay and Western blot to determine levels of gamma-GCSh mRNA and protein from 10 pairs of matched carcinomas with adjacent normal controls. Elevated expression of both gamma-GCSh mRNA and protein were found in 6 cases, suggesting that transcriptional and/or posttranscriptional regulation play an important role in the upregulation of gamma-GCS during colorectal carcinogenesis. We also examined the expression of another redox-regulated gene, multidrug resistance protein 1 (MRP1). Strong staining for MRP1 was detected in 1 (6.3%) adenoma and 40 (70.2%) carcinomas. The frequency of MRP1 expression in carcinoma was significantly higher than in adenoma ( p<0.0001). Nuclear p53 expression was detected in 30 (52.6%) of carcinomas. There is a significant correlation between gamma-GCSh and MRP1 expression (p=0.013) but not between gamma-GCSh and p53. Since gamma-GCS is a sensor of oxidative stress, these results are consistent with the notion that oxidative stress is associated with colorectal tumor progression.

Drug resistance in epilepsy: expression of drug resistance proteins in common causes of refractory epilepsy

Epilepsy is resistant to drug treatment in about one-third of cases, but the mechanisms underlying this drug resistance are not understood. In cancer, drug resistance has been studied extensively. Amongst the various resistance mechanisms, overexpression of drug resistance proteins, such as multi-drug resistance gene-1 P-glycoprotein (MDR1) and multidrug resistance-associated protein 1 (MRP1), has been shown to correlate with cellular resistance to anticancer drugs. Previous studies in human epilepsy have shown that MDR1 and MRP1 may also be overexpressed in brain tissue from patients with refractory epilepsy; expression has been shown in glia and neurones, which do not normally express these proteins. We examined expression of MDR1 and MRP1 in refractory epilepsy from three common causes, dysembryoplastic neuroepithelial tumours (DNTs; eight cases), focal cortical dysplasia (FCD; 14 cases) and hippocampal sclerosis (HS; eight cases). Expression was studied immunohistochemically in lesional tissue from therapeutic resections and compared with expression in histologically normal adjacent tissue. With the most sensitive antibodies, in all eight DNT cases, reactive astrocytes within tumour nodules expressed MDR1 and MRP1. In five of eight HS cases, reactive astrocytes within the gliotic hippocampus expressed MDR1 and MRP1. Of 14 cases of FCD, MDR1 and MRP1 expression was noted in reactive astrocytes in all cases. In five FCD cases, MRP1 expression was also noted in dysplastic neurones. In FCD and DNTs, accentuation of reactivity was noted around lesional vessels. Immunoreactivity was always more frequent and intense in lesional reactive astrocytes than in glial fibrillary acidic protein-positive reactive astrocytes in adjacent histologically normal tissue. MDR1 is able to transport some antiepileptic drugs (AEDs), and MRP1 may also do so. The overexpression of these drug resistance proteins in tissue from patients with refractory epilepsy suggests one possible mechanism for drug resistance in patients with these pathologies. We propose that overexpressed resistance proteins lower the interstitial concentration of AEDs in the vicinity of the epileptogenic pathology and thereby render the epilepsy caused by these pathologies resistant to treatment with AEDs.

The interplay of glutathione-related processes in antioxidant defense

This review summarizes current knowledge on glutathione (GSH) associated cellular processes that play a central role in defense against oxidative stress. GSH itself is a critical factor in maintaining the cellular redox balance and has been demonstrated to be involved in regulation of cell signalling and repair pathways. Enhanced expression of various enzymes involved in GSH metabolism, including glutathione peroxidases, γ-glutamyl cysteinyl synthetase (γ-GCS), glutathione S-transferases (GST) and membrane proteins belonging to the ATP-binding cassette family, such as the multidrug resistance associated protein, have all been demonstrated to play a prominent role in cellular resistance towards oxidative stress. This review stresses the fact that aco-ordinateinterplay between these systems is essential for efficient protection against oxidative stress.

Anticancer drug resistance in primary human brain tumors

The difficult clinical situation still associated with most types of primary human brain tumors has fostered significant interest in defining novel therapeutic modalities for this heterogeneous group of neoplasms. Beginning in the 1980s chemotherapy has been incorporated into the treatment protocol of a number of intractable brain tumors. However, it has predominantly failed to improve patient outcome. The unsatisfactory results with chemotherapeutic intervention have chiefly been attributed to tumor cell resistance. In recent years, there has been a literal explosion in our understanding about the mechanisms by which cancer cells become chemoresistant. During the course of their evolution (intrinsic resistance) or in response to chemotherapy (acquired resistance) these cells may follow a number of pathways of genetic alterations to possess a common (multidrug) or drug-specific (individual drug) resistant phenotype. Genomic aberrations, deregulation of membrane transporting proteins and cellular enzymes, and an altered susceptibility to commit to apoptosis are among the steps on the way that contribute to the genesis of chemotherapeutic treatment failure. Although, through the years we have come to yield information and inferences as to the roles that different molecular events may have in the resistance phenotype of cancer cells, the actual involvement of single genetic alterations in conferring drug resistance in primary brain tumors remains debatable. This uncertainty and, besides, the lack of proper drug resistance diagnostics, in a vicious circle, hinder the development of effective resistance-modulation strategies. Clinical non-responsiveness to chemotherapy remains a formidable obstacle to the successful treatment of brain tumors and one of the most serious problems to be solved in the therapy of these lesions. Future advances in the chemotherapeutic management of these neoplasms will come with an improved understanding of the significance and interrelationship of the multiple biological systems operative in promoting resistance to this treatment modality. The focus of this review is to summarize current knowledge concerning major drug resistance-related markers, to describe their functional interaction en route to chemoresistance, and to discuss their implication in rendering human brain tumor cells resistant to chemotherapy.

The human multidrug resistance-associated protein (MRP) gene family: from biological function to drug molecular design

The ATP-binding cassette transmembrane proteins play an important role in transport of drugs as well as of biologically active endogenous substances. The human multidrug resistance-associated protein (MRP) subfamily consists of at least six members, exhibiting a wide spectrum of biological functions. MRP1 operates as an ATP-dependent primary active transporter for substrates conjugated with glucuronide, sulfate or glutathione. Leukotriene C4 is an important endogenous substrate for MRP1. Glutathione serves as a cofactor in MRP1-mediated drug transport as well. Genes encoding both MRP1 and the catalytic subunit of gamma-glutamylcysteine synthetase (gamma-GCS) are coordinately regulated in cultured cancer cell lines as well as colorectal cancer tissues from colon cancer patients. The induction of MRP1 and gamma-GCS expression by oxidative stress varies among different cell lines, and p53 mutations are associated with elevated levels of induction. To modulate the transport function of MRP1, we have synthesized novel glutathione derivatives as photoreactive biochemical probes targeting the transporter protein. GIF-0019 restored the cellular sensitivity of MRP1-overexpressing drug-resistant cancer cells to anticancer prostaglandins in vitro, which was characterized by enhanced mRNA levels of the cyclin-dependent kinase inhibitor p21, suppressed c-myc expression and G1 arrest.

Regulation of gamma-glutamylcysteine synthetase subunit gene expression: insights into transcriptional control of antioxidant defenses

Gamma-glutamylcysteine synthetase (GCS; also referred to as glutamate-cysteine ligase, GLCL) catalyzes the rate-limiting reaction in glutathione (GSH) biosynthesis. The GCS holoenzyme is composed of a catalytic and regulatory subunit, each encoded by a unique gene. In addition to some conditions which specifically upregulate the catalytic subunit gene, expression of both genes is increased in response to many Phase II enzyme inducers including oxidants, heavy metals, phenolic antioxidants and GSH-conjugating agents. Electrophile Response Elements (EpREs), located in 5'-flanking sequences of both the GCSh and GCSl subunit genes, are hypothesized to at least partially mediate gene induction following xenobiotic exposure. Recent experiments indicate that the bZip transcription factor Nrf2 participates in EpRE-mediated GCS subunit gene activation in combination with other bZip proteins. An AP-1-like binding sequence and an NF-kappaB site have also been implicated in regulation of the catalytic subunit gene following exposure to certain pro-oxidants. Potential signaling mechanisms mediating GCS gene induction by the diverse families of Phase II enzyme inducers include thiol modification of critical regulatory sensor protein(s) and the generation of the reactive oxygen species. This review summarizes recent progress in defining the molecular mechanisms operative in transcriptional control of the genes encoding the two GCS subunits, identifying areas of agreement and controversy. The mechanisms involved in GCS regulation might also be relevant to the transcriptional control of other components of the antioxidant defense battery.

Biologic and pharmacologic regulation of mammalian glutathione synthesis

Glutathione (L-gamma-glutamyl-L-cysteinylglycine, GSH) is synthesized from its constituent amino acids by the sequential action of gamma-glutamylcysteine synthetase (gamma-GCS) and GSH synthetase. The intracellular GSH concentration, typically 1-8 mM, reflects a dynamic balance between the rate of GSH synthesis and the combined rate of GSH consumption within the cell and loss through efflux. The gamma-GCS reaction is rate limiting for GSH synthesis, and regulation of gamma-GCS expression and activity is critical for GSH homeostasis. Transcription of the gamma-GCS subunit genes is controlled by a variety of factors through mechanisms that are not yet fully elucidated. Glutathione synthesis is also modulated by the availability of gamma-GCS substrates, primarily L-cysteine, by feedback inhibition of gamma-GCS by GSH, and by covalent inhibition of gamma-GCS by phosphorylation or nitrosation. Because GSH plays a critical role in cellular defenses against electrophiles, oxidative stress and nitrosating species, pharmacologic manipulation of GSH synthesis has received much attention. Administration of L-cysteine precursors and other strategies allow GSH levels to be maintained under conditions that would otherwise result in GSH depletion and cytotoxicity. Conversely, inhibitors of gamma-GCS have been used to deplete GSH as a strategy for increasing the sensitivity of tumors and parasites to certain therapeutic interventions.

Enhancement of glucuronosyl etoposide transport by glutathione in multidrug resistance-associated protein-overexpressing cells

Multidrug resistance-associated protein (MRP) has been shown to transport glutathione (GSH) S-conjugates such as leukotriene C4 (LTC4) and S-(2,4-dinitrophenyl)-glutathione (DNP-SG). On the other hand, it has while it has been reported that MRP-overexpressing cells exhibit decreased sensitivity to drugs which do not form GSH S-conjugates. In this study, we found that GSH affects the transport of glucuronosyl etoposide as a major metabolite of etoposide in MRP-overexpressing KB/VP-4 cells. The relative resistance level of KB/VP-4 cells to etoposide was 70-fold that of wild-type KB cells. Membrane vesicles prepared from KB/VP-4 cells exhibited markedly enhanced ATP-dependent transport of glucuronosyl etoposide as well as LTC4. Transport of glucuronosyl etoposide was augmented in the presence of GSH. Treatment of KB/VP-4 cells with buthionine sulfoximine (BSO), an inhibitor of GSH synthesis, resulted in about 75% depletion of cellular GSH levels, a four-fold increase of the sensitivity to etoposide and depression of glucuronosyl etoposide efflux. These results suggest that GSH plays a role in the enhancement of MRP-mediated glucuronosyl etoposide transport.