Transport of glutathione and glutathione conjugates by MRP1

The accumulation and efflux of lead partly depend on ATP-dependent efflux pump-multidrug resistance protein 1 and glutathione in testis Sertoli cells

Since lead accumulation is toxic to cells, its excretion is crucial for organisms to survive the toxicity. In this study, mouse testis sertoli (TM4) and Mrp1 lower-expression TM4-sh cells were used to explore the lead accumulation characteristics, and the role of ATP-dependent efflux pump-multidrug resistance protein 1 (Mrp1) in lead excretion. TM4 cells possess Mrp-like transport activity. The expression levels of mrp1 mRNA and Mrp1 increased after lead treatments at first and then decreased. The maximum difference of relative mRNA expression reached 10 times. In the presence of lead acetate, the amount of cumulative lead in TM4-sh was much higher than that in TM4. After the treatment with lead acetate at 10-40 μM for 12h or 24h, the differences were about 2-8 times. After with the switch to lead-free medium, the cellular lead content in TM4-sh remains higher than that in TM4 cells at 1,3, 6, and 9h time points (P<0.01). Energy inhibitor sodium azide, Mrp inhibitors MK571 and glutathione (GSH) biosynthesis inhibitor BSO could block lead efflux from TM4 cells significantly. These results indicate that lead excretion may be mediated by Mrp1 and GSH in TM4 cells. Mrp1 could be one of the important intervention points for lead detoxification.

Inflammation, DAMPs, tumor development, and progression: a vicious circle orchestrated by redox signaling

SIGNIFICANCE

Increasing evidence indicates that cancer development and progression are promoted by the joint action of redox distress and inflammation, supporting the potential impact of therapies aimed at restoring the redox homeostasis and fighting inflammation.

RECENT ADVANCES

Most of the literature of the last 40 years converges to the view that continuous oxidative stress and chronic inflammation sustain each other, leads to transformation of a normal cell to a neoplastic cell, and promotes tumor progression. Some recent findings, however, support an alternative model whereby the increased production of reactive oxygen species (ROS) is an attempt to defend more than a pathogenetic factor in cancer. Rather, tumor development and progression may be promoted by an excess of antioxidants, induced in both transformed cells and recruited inflammatory cells as an adaptive response to ROS.

CRITICAL ISSUES

Although the link among redox stress, chronic inflammation, and cancer is widely recognized, the underlying mechanisms are far to be understood. The redox unbalance of the microenvironment is likely to modulate the bioactivity of damage-associated molecular pattern molecules such as HMGB1, which are released by stressed tissues and play pleiotropic functions on tumor and inflammatory cells, but how this occur, and the relevant consequences, are still unclear.

FUTURE DIRECTIONS

In vivo measurement of cell redox status is an important challenge for future investigations. The improvement of the methodologies for ROS and antioxidant detection will allow a better understanding of the redox-related events in the tumor microenvironment with tremendous application potential in the development of rational combination therapies for cancer treatment.

Mechanism of cellular accumulation of an iridium(III) pentamethylcyclopentadienyl anticancer complex containing a C,N-chelating ligand

The effect of replacement of the N,N-chelating ligand 1,10-phenanthroline (phen) in the Ir(III) pentamethylcyclopentadienyl (Cp*) complex [(η(5)-Cp*)(Ir)(phen)Cl](+) (2) with the C,N-chelating ligand 7,8-benzoquinoline (bq) to give [(η(5)-Cp*)(Ir)(bq)Cl] (1) on the cytotoxicity of these Cp*Ir(III) complexes toward cancer cell lines was investigated. Complex 2 is inactive, similar to other Cp*Ir(III) complexes containing the N,N-chelating ligands. In contrast, a single atom change (C(-) for N) in the chelating N,N ligand resulted in potency in human ovarian carcinoma cisplatin-sensitive A2780 cells, and, strikingly, 1 is active in the cisplatin-resistant human breast cancer MCF-7 and A2780/cisR cells. Replacement of the N,N-chelating ligand with the C,N-chelating ligand gives rise to increased hydrophobicity, leading to higher cellular accumulation, higher DNA-bound iridium in cells and higher cytotoxicity. The pathways involved in cellular accumulation of 1 have been further explored and compared with conventional cisplatin. The results show that both energy-independent passive diffusion and energy-dependent transport play a role in accumulation of 1. Further results were consistent with involvement of p-glycoprotein, multidrug resistance-associated protein 1 and glutathione metabolism in the efflux of 1. In contrast, the internalization of 1 mediated by the endocytotic uptake pathway(s) seems less likely. Understanding the factors which contribute to the mechanism of cellular accumulation of this Ir(III) complex can now lead to the design of structurally similar metal complexes for antitumor chemotherapy.

Reversal of ATP-binding cassette drug transporter activity to modulate chemoresistance: why has it failed to provide clinical benefit?

Enhanced drug extrusion from cells due to the overexpression of the ATP-binding cassette (ABC) drug transporters inhibits the cytotoxic effects of structurally diverse and mechanistically unrelated anticancer agents and is a major cause of multidrug resistance (MDR) of human malignancies. Multiple compounds can suppress the activity of these efflux transporters and sensitize resistant tumor cells, but despite promising preclinical and early clinical data, they have yet to find a role in oncologic practice. Based on the knowledge of the structure, function, and distribution of MDR-related ABC transporters and the results of their preclinical and clinical evaluation, we discuss probable reasons why these inhibitors have not improved the outcome of therapy for cancer patients. We also outline new MDR-reversing strategies that directly target ABC transporters or circumvent relevant signaling pathways.

Oxidative stress in sickle cell disease: an overview of erythrocyte redox metabolism and current antioxidant therapeutic strategies

Erythrocytes have an environment of continuous pro-oxidant generation due to the presence of hemoglobin (Hb), which represents an additional and quantitatively significant source of superoxide (O2(-)) generation in biological systems. To counteract oxidative stress, erythrocytes have a self-sustaining antioxidant defense system. Thus, red blood cells uniquely function to protect Hb via a selective barrier allowing gaseous and other ligand transport as well as providing antioxidant protection not only to themselves but also to other tissues and organs in the body. Sickle hemoglobin molecules suffer repeated polymerization/depolymerization generating greater amounts of reactive oxygen species, which can lead to a cyclic cascade characterized by blood cell adhesion, hemolysis, vaso-occlusion, and ischemia-reperfusion injury. In other words, sickle cell disease is intimately linked to a pathophysiologic condition of multiple sources of pro-oxidant processes with consequent chronic and systemic oxidative stress. For this reason, newer therapeutic agents that can target oxidative stress may constitute a valuable means for preventing or delaying the development of organ complications.

Differential functional rescue of Lys(513) and Lys(516) processing mutants of MRP1 (ABCC1) by chemical chaperones reveals different domain-domain interactions of the transporter

Multidrug resistance protein 1 (MRP1) extrudes drugs as well as pharmacologically and physiologically important organic anions across the plasma membrane in an ATP-dependent manner. We previously showed that Ala substitutions of Lys(513) and Lys(516) in the cytoplasmic loop (CL5) connecting transmembrane helix 9 (TM9) to TM10 cause misfolding of MRP1, abrogating its expression at the plasma membrane in transfected human embryonic kidney (HEK) cells. Exposure of HEK cells to the chemical chaperones glycerol, DMSO, polyethylene glycol (PEG) and 4-aminobutyric acid (4-PBA) improved levels of K513A to wild-type MRP1 levels but transport activity was only fully restored by 4-PBA or DMSO treatments. Tryptic fragmentation patterns and conformation-dependent antibody immunoreactivity of the transport-deficient PEG- and glycerol-rescued K513A proteins indicated that the second nucleotide binding domain (NBD2) had adopted a more open conformation than in wild-type MRP1. This structural change was accompanied by differences in ATP binding and hydrolysis but no changes in substrate Km. In contrast to K513A, K516A levels in HEK cells were not significantly enhanced by chemical chaperones. In more permissive insect cells, however, K516A levels were comparable to wild-type MRP1. Nevertheless, organic anion transport by K516A in insect cell membranes was reduced by >80% due to reduced substrate Km. Tryptic fragmentation patterns indicated a more open conformation of the third membrane spanning domain of MRP1. Thus, despite their close proximity to one another in CL5, Lys(513) and Lys(516) participate in different interdomain interactions crucial for the proper folding and assembly of MRP1.

MRP1 overexpression determines poor prognosis in prospectively treated patients with localized high-risk soft tissue sarcoma of limbs and trunk wall: an ISG/GEIS study

Patients with localized high-risk soft tissue sarcomas (STS) of the limbs and trunk wall still have a considerable metastatic recurrence rate of more than 50%, in spite of adjuvant chemotherapy. This drug-ceiling effect of chemotherapy in sarcoma setting could be explained, at least partially, by multidrug resistance (MDR) mechanisms. The aim of this study was to ascertain whether mRNA and protein expression of ABCB1 (P-glycoprotein), ABCC1 (MRP1), and GSTA1 (glutathione S-transferase pi) was prognostic in localized high-risk STS. Immunohistochemistry and reverse transcriptase-PCR studies were performed from biopsies at the time of diagnosis. Patients of this series were prospectively enrolled into a phase III trial that compared three versus five cycles of epirubicin plus ifosfamide. The series of 102 patients found 41 events of recurrence and 37 of death with a median follow-up of 68 months. In univariate analysis, variables with a statistically significant relationship with relapse-free survival (RFS) were: MRP1 expression (5-year RFS rate of 23% in positive cases and 63% in negative cases, P = 0.029), histology (5-year RFS rate of 74% in undifferentiated pleomorphic sarcoma and 43% in synovial sarcoma, P = 0.028), and ABCC1 expression (5-year RFS rate of 33% in overexpression and 65% in downregulation, P = 0.012). Combined ABCC1/MRP1 was the only independent prognostic factor for both RFS (HR = 2.704, P = 0.005) and overall survival (HR = 2.208, P = 0.029). ABCC1/MRP1 expression shows robust prognostic relevance in patients with localized high-risk STS treated with anthracycline-based chemotherapy, which is the standard front line treatment in STS. This finding deserves attention as it points to a new targetable protein in STS.

Identification of ABC transporter genes in gonad tissue of two Mediterranean sea urchin species: black, Arbacia lixula L., and rocky, Paracentrotus lividus L

Multixenobiotic resistance (MXR) represents an important cellular detoxification mechanism in aquatic organisms as it provides them robustness toward natural and man-made contaminants. Several ABC transporters have major roles in the MXR phenotype - P-gp/ABCB1, MRP1-3/ABCC1-3 and BCRP/ABCG2. In this study, we identified the presence of ABC transporters involved in the MXR mechanism of Arbacia lixula and Paracentrotus lividus. AlABCB1/P-gp, AlABCC3/MRP3, AlABCC9/SUR-like and AlABCG-like transcripts were identified in A. lixula; and PlABCC1/P-gp, PlABCC3/MRP3, PlABCC5/MRP5, and PlABCC9/SUR-like transcripts in P. lividus. For each of the new partial sequences, we performed detailed phylogenetic and identity analysis as a first step toward full characterization and understanding of the ecotoxicological role of these ABC transporters.

Targeting multidrug resistance protein 1 (MRP1, ABCC1): past, present, and future

The human ATP-binding cassette transporter multidrug resistance protein 1 (MRP1), encoded by ABCC1, was initially identified because of its ability to confer multidrug resistance in lung cancer cells. It is now established that MRP1 plays a role in protecting certain tissues from xenobiotic insults and that it mediates the cellular efflux of the proinflammatory cysteinyl leukotriene C4 as well as a vast array of other endo- and xenobiotic organic anions. Many of these are glutathione (GSH) or glucuronide conjugates, the products of Phase II drug metabolism. MRP1 also plays a role in the cellular efflux of the reduced and oxidized forms of GSH and thus contributes to the many physiological and pathophysiological processes influenced by these small peptides, including oxidative stress. In this review, the pharmacological and physiological aspects of MRP1 are considered in the context of the current status and future prospects of pharmacological and genetic modulation of MRP1 activity.

The heart as a target for xenobiotic toxicity: the cardiac susceptibility to oxidative stress

The heart is a target organ for oxidative stress-related injuries. Because of its very high energetic metabolic demand, the heart has the highest rate of production of reactive oxygen species, namely, hydrogen peroxide (H2O2), per gram of tissue. Additionally, the heart has lower levels of antioxidants and total activity of antioxidant enzymes when compared to other organs. Furthermore, drugs that have relevant antioxidant activity and that are used in the treatment of oxidative stress related cardiac diseases demonstrate better clinical cardiac outcomes than other drugs with similar receptor affinity but with no antioxidant activity. Several xenobiotics particularly target the heart and promote toxicity. Anticancer drugs, like anthracyclines, cyclophosphamide, mitoxantrone, and more recently tyrosine kinase targeting drugs, are well-known cardiac toxicants whose therapeutic application has been associated to a high prevalence of heart failure. High levels of catecholamines or drugs of abuse, namely, amphetamines, cocaine, and even the consumption of alcohol for long periods of time, are linked to cardiovascular abnormalities. Oxidative stress may be one common link for the cardiac toxicity elicited by these compounds. We aim to revise the mechanisms involved in cardiac lesions caused by the above-mentioned substances specially focusing in oxidative stress related pathways. Oxidative stress biomarkers can be useful in the early recognition of cardiotoxicity in patients treated with these drugs and aid to minimize the setting of cardiac irreversible events.

Hepatic 3D cultures but not 2D cultures preserve specific transporter activity for acetaminophen-induced hepatotoxicity

Primary human hepatocytes (PHH) are the "gold standard" for in vitro toxicity tests. However, 2D PHH cultures have limitations that are due to a time-dependent dedifferentiation process visible by morphological changes closely connected to a decline of albumin production and CYP450 activity. The 3D in vitro culture corresponds to in vivo-like tissue architecture, which preserves functional characteristics of hepatocytes, and therefore can at least partially overcome the restrictions of 2D cultures. Consequently, several drug toxicities observed in vivo cannot be reproduced in 2D in vitro models, for example, the toxic effects of acetaminophen. The objective of this study was to identify molecular differences between 2D and 3D cultivation which explain the observed toxicity response. Our data demonstrated an increase in cell death after treatment with acetaminophen in 3D, but not in 2D cultures. Additionally, an acetaminophen concentration-dependent increase in the CYP2E1 expression level in 3D cultures was detected. However, during the treatment with 10 mM acetaminophen, the expression level of SOD gradually decreased in 3D cultures and was undetectable after 24 h. In line with these findings, we observed higher import/export rates in the membrane transport protein, multidrug resistance-associated protein-1, which is known to be specific for acetaminophen transport. The presented data demonstrate that PHH cultured in 3D preserve certain metabolic functions. Therefore, they have closer resemblance to the in vivo situation than PHH in 2D cultures. In consequence, 3D cultures will allow for a more accurate hepatotoxicity prediction in in vitro models in the future.

Formaldehyde in brain: an overlooked player in neurodegeneration?

Formaldehyde is an environmental pollutant that is also generated in substantial amounts in the human body during normal metabolism. This aldehyde is a well-established neurotoxin that affects memory, learning, and behavior. In addition, in several pathological conditions, including Alzheimer's disease, an increase in the expression of formaldehyde-generating enzymes and elevated levels of formaldehyde in brain have been reported. This article gives an overview on the current knowledge on the generation and metabolism of formaldehyde in brain cells as well as on formaldehyde-induced alterations in metabolic processes. Brain cells have the potential to generate and to dispose formaldehyde. In culture, both astrocytes and neurons efficiently oxidize formaldehyde to formate which can be exported or further oxidized. Although moderate concentrations of formaldehyde are not acutely toxic for brain cells, exposure to formaldehyde severely affects their metabolism as demonstrated by the formaldehyde-induced acceleration of glycolytic flux and by the rapid multidrug resistance protein 1-mediated export of glutathione from both astrocytes and neurons. These formaldehyde-induced alterations in the metabolism of brain cells may contribute to the impaired cognitive performance observed after formaldehyde exposure and to the neurodegeneration in diseases that are associated with increased formaldehyde levels in brain.

ABCC1 is related to the protection of the distal nephron against hyperosmolality and high sodium environment: possible implications for cancer chemotherapy

Aims

Glutathione (GSH) plays an important role in protecting cells against oxidative damage. ABCC1 protein transports GSH. Although this protein is largely studied in cancer, due to multidrug resistance phenotype, its role in the tubular cells of the kidney is unknown. The goal of this study was to find out whether ABCC1 has a role in protecting cells from the distal nephron against the stress caused by high medullar osmolality.

Main Methods

MA104 cells were treated with high concentrations of sodium chloride, urea, or both to raise the osmolality of the culture medium. Cell viability was accessed by MTT and trypan blue assays. ABCC1 expression and extrusion of carboxi-fluorescein (CF), a fluorescent ABCC1 substrate, were measured by flow cytometry.

Key Findings

Incubation of MA104 cells in a high sodium concentration medium resulted in changes in cell granularity and altered expression and activity of ABCC1. Urea did not alter ABCC1 expression or activity, but reversed the observed NaCl effects. High sodium concentrations also had a negative effect on cell viability and urea also protected cells against this effect.

Significance

Our findings demonstrate that ABCC1 plays a significant role in the protection of kidney epithelial cells against the stress caused by high sodium environment present in renal medulla.

[Detailed characterization of bile acid and glucocorticoid world by mass spectrometry]

  The Nobel Prize in Chemistry for 2002 was shared by John B. Fenn and Koichi Tanaka "for their development of soft desorption methods for mass spectrometric analyses of biological macromolecules". Indeed, electrospray ionization and soft laser desorption ionization have proved to be of great value in "omics", such as metabolomics, transcriptomics and proteomics in providing a systematic and quantitative approach to the study of biological systems and networks. Moreover, these techniques have made great contributions to metabolic studies that are used for development of new drugs, as well as to the diagnosis of diseases including cancer based on the specific and sensitive detection of molecular biomarkers. In this article, we describe our recent results on characterization of bile acid metabolism in hepatobiliary disease as well as measurement of conjugated urinary tetrahydrocorticosteroids for assessment of altered corticoid metabolism in endocrine disease and the metabolic syndrome.

Protein carbonylation, mitochondrial dysfunction, and insulin resistance

Oxidative stress has been identified as a common mechanism for cellular damage and dysfunction in a wide variety of disease states. Current understanding of the metabolic changes associated with obesity and the development of insulin resistance has focused on the role of oxidative stress and its interaction with inflammatory processes at both the tissue and organismal level. Obesity-related oxidative stress is an important contributing factor in the development of insulin resistance in the adipocyte as well as the myocyte. Moreover, oxidative stress has been linked to mitochondrial dysfunction, and this is thought to play a role in the metabolic defects associated with oxidative stress. Of the various effects of oxidative stress, protein carbonylation has been identified as a potential mechanism underlying mitochondrial dysfunction. As such, this review focuses on the relationship between protein carbonylation and mitochondrial biology and addresses those features that point to either the causal or casual relationship of lipid peroxidation-induced protein carbonylation as a determining factor in mitochondrial dysfunction.

S-alk(en)ylmercaptocysteine: chemical synthesis, biological activities, and redox-related mechanism

S-Alk(en)ylmercaptocysteine (CySSR, R = methyl, ethyl, propyl, 1-propenyl, and allyl), which are the putative metabolites of Allium thiosulfinates, were chemically synthesized. CySSR, but not the corresponding monosulfide species S-alk(en)yl cysteine (CySR), were able to induce quinone reductase (QR, a representative phase II enzyme) in Hepa 1c1c7 cells and inhibit nitric oxide (NO, an inflammatory biomarker) formation in lipopolysaccharide (LPS)-activated RAW 264.7 cells. These results indicate the importance of the disulfide bond for the biological activities of CySSR. Glutathione (GSH) and N-acetylcysteine (NAC), but not other types of cellular antioxidants, suppressed multiple biological activities of CySSR in vitro. The inhibitory effects of GSH and NAC on the biological activities of CySSR were correlated with a glutaredoxin (Grx)-dependent intracellular reduction of CySSR to generate cysteine and RSH, which were secreted into the extracellular medium.

Formaldehyde metabolism and formaldehyde-induced stimulation of lactate production and glutathione export in cultured neurons

Formaldehyde is endogenously produced in the human body and brain levels of this compound are elevated in neurodegenerative conditions. Although the toxic potential of an excess of formaldehyde has been studied, little is known on the molecular mechanisms underlying its neurotoxicity as well as on the ability of neurons to metabolize formaldehyde. To address these topics, we have used cerebellar granule neuron cultures as model system. These cultures express mRNAs of various enzymes that are involved in formaldehyde metabolism and were remarkably resistant toward acute formaldehyde toxicity. Cerebellar granule neurons metabolized formaldehyde with a rate of around 200 nmol/(h × mg) which was accompanied by significant increases in the cellular and extracellular concentrations of formate. In addition, formaldehyde application significantly increased glucose consumption, almost doubled the rate of lactate release from viable neurons and strongly accelerated the export of the antioxidant glutathione. The latter process was completely prevented by inhibition of the known glutathione exporter multidrug resistance protein 1. These data indicate that cerebellar granule neurons are capable of metabolizing formaldehyde and that the neuronal glycolysis and glutathione export are severely affected by the presence of formaldehyde.

Overcoming chemotherapy resistance of ovarian cancer cells by liposomal cisplatin: molecular mechanisms unveiled by gene expression profiling

Previously we reported that liposomal cisplatin (CDDP) overcomes CDDP resistance of ovarian A2780cis cancer cells (Krieger et al., Int. J. Pharm. 389, 2010, 10-17). Here we find that the cytotoxic activity of liposomal CDDP is not associated with detectable DNA platination in resistant ovarian cancer cells. This suggests that the mode of action of liposomal CDDP is different from the free drug. To gain insight into mechanisms of liposomal CDDP activity, we performed a transcriptome analysis of untreated A2780cis cells, and A2780cis cells in response to exposure with IC50 values of free or liposomal CDDP. A process network analysis of upregulated genes showed that liposomal CDDP induced a highly different gene expression profile in comparison to the free drug. p53 was identified as a key player directing transcriptional responses to free or liposomal CDDP. The free drug induced expression of essential genes of the intrinsic (mitochondrial) apoptosis pathway (BAX, BID, CASP9) most likely through p38MAPK activation. In contrast, liposomal CDDP induced expression of genes from DNA damage pathways and several genes of the extrinsic pathway of apoptosis (TNFRSF10B-DR5, CD70-TNFSF7). It thus appears that liposomal CDDP overcomes CDDP resistance by inducing DNA damage and in consequence programmed cell death by the extrinsic pathway. Predictions from gene expression data with respect to apoptosis activation were confirmed at the protein level by an apoptosis antibody array. This sheds new light on liposomal drug carrier approaches in cancer and suggests liposomal CDDP as promising strategy for the treatment of CDDP resistant ovarian carcinomas.

Glutathione export from human erythrocytes and Plasmodium falciparum malaria parasites

Glutathione export from uninfected human erythrocytes was compared with that from cells infected with the malaria parasite Plasmodium falciparum using two separate methods that distinguish between oxidized (GSSG) and reduced (GSH) glutathione. One involved enzymatic recycling with or without thiol-masking; the other involved rapid derivatization followed by HPLC. Glutathione efflux from uninfected erythrocytes under physiological conditions occurred predominantly as GSH. On exposure of the cells to oxidative challenge, efflux of GSSG exceeded that of GSH. Efflux of both species was blocked by MK571, an inhibitor of mammalian multidrug-resistance proteins. Glutathione efflux from parasitized erythrocytes was substantially greater than that from uninfected erythrocytes. Under physiological conditions, the exported species was GSH, whereas under energy-depleted conditions, GSSG efflux occurred. Glutathione export from parasitized cells was inhibited partially by MK571 and more so by furosemide, an inhibitor of the 'new permeability pathways' induced by the parasite in the host erythrocyte membrane. Efflux from isolated parasites occurred as GSH. On exposure to oxidative challenge, this GSH efflux decreased, but no GSSG export was detected. These results are consistent with the view that the parasite supplies its host erythrocyte with GSH, much of which is exported from the infected cell via parasite-induced pathways.

The chronic effects of low lead level on the expressions of Nrf2 and Mrp1 of the testes in the rats

Lead is linked to many reproductive problems. This study was to explore the chronic effects of low lead level on expressions of Nrf2 and Mrp1 in rats' testes. Maternal SD rats were administered lead acetate from 10 days before gestation to weaning at three doses respectively after randomization. From each group, 15 male offsprings were then chosen and administrated lead acetate from weaning to six months old at the doses of 0, 0.3 and 0.9g/L respectively. The dose administrations were through drinking water freely. The methods of RT-PCR, Western blotting and immunohistochemistry were used for Mrp1 and Nrf2 of the testes. Compared with control group, significant increases were observed in the expressions of Mrp1 and Nrf2 in two lead groups (P<0.05); nucleus translocation of Nrf2 was observed; both GST and GSH was decreased with increasing the lead dose. In conclusion, Mrp1 might play important roles in lead detoxification by Nrf2.

Drug Resistance Mechanisms in Non-Small Cell Lung Carcinoma

Lung cancer is the most commonly diagnosed cancer in the world. “Driver” and “passenger” mutations identified in lung cancer indicate that genetics play a major role in the development of the disease, progression, metastasis and response to therapy. Survival rates for lung cancer treatment have remained stagnant at ~15% over the past 40 years in patients with disseminated disease despite advances in surgical techniques, radiotherapy and chemotherapy. Resistance to therapy; either intrinsic or acquired has been a major hindrance to treatment leading to great interest in studies seeking to understand and overcome resistance. Genetic information gained from molecular analyses has been critical in identifying druggable targets and tumor profiles that may be predictors of therapeutic response and mediators of resistance. Mutated or overexpressed epidermal growth factor receptor (EGFR) and translocations in the echinoderm microtubule-associated protein-like 4 (EML4)-anaplastic lymphoma kinase (ALK) genes (EML4-ALK) are examples of genetic aberrations resulting in targeted therapies for both localized and metastatic disease. Positive clinical responses have been noted in patients harboring these genetic mutations when treated with targeted therapies compared to patients lacking these mutations. Resistance is nonetheless a major factor contributing to the failure of targeted agents and standard cytotoxic agents. In this review, we examine molecular mechanisms that are potential drivers of resistance in non-small cell lung carcinoma, the most frequently diagnosed form of lung cancer. The mechanisms addressed include resistance to molecular targeted therapies as well as conventional chemotherapeutics through the activity of multidrug resistance proteins.

Role of pharmacogenetics in busulfan/cyclophosphamide conditioning therapy prior to hematopoietic stem cell transplantation

Hematopoietic stem cell transplantation (HSCT) is a curative treatment for several malignant and nonmalignant disorders. Busulfan (Bu) and cyclophosphamide (Cy) are the most commonly used alkylators in high-dose pretransplant conditioning for HSCT; a treatment that is correlated with drug-related toxicity and relapse. Pharmacogenetic investigations have shown that CYP450, as well as aldehyde dehydrogenase, are clearly involved with Cy metabolism and are associated with altered treatment response, Cy metabolism and the unique stem-cell sparing capacity. Moreover, glutathione-S-transferase isoenzymes have been associated with cellular outward transport of various alkylating agents, including Cy metabolites, melphalan, Bu and chlorambucil. A shift from genetic-based studies to whole-genome-based investigations of Cy- and Bu-associated markers may contribute to personalizing the conditioning therapy and enhancing the clinical outcome of HSCT.

Importance of detecting multidrug resistance proteins in acute leukemia prognosis and therapy

Multidrug resistance (MDR) is a multifactorial phenomenon and the role of these proteins in generating the MDR phenotype is controversial. With this in mind, this review compiled the current data on the role of ABCB1, ABCC1, and LRP proteins in the prognosis of hematologic neoplasms and their influence on the choice of therapy. Literature showed that the detection of these proteins, mainly ABCB1, is important in the AL prognosis. However, there is controversy regarding the methodology used for their detection. In summary, the expression and activity profiles of ABCB1, ABCC1, and LRP, proteins capable of promoting the efflux of a variety of chemotherapeutic agents from the cell cytoplasm represent one of the greatest causes of failure in AL treatment.

Glutathione efflux and cell death

SIGNIFICANCE

Glutathione (GSH) depletion is a central signaling event that regulates the activation of cell death pathways. GSH depletion is often taken as a marker of oxidative stress and thus, as a consequence of its antioxidant properties scavenging reactive species of both oxygen and nitrogen (ROS/RNS).

RECENT ADVANCES

There is increasing evidence demonstrating that GSH loss is an active phenomenon regulating the redox signaling events modulating cell death activation and progression.

CRITICAL ISSUES

In this work, we review the role of GSH depletion by its efflux, as an important event regulating alterations in the cellular redox balance during cell death independent from oxidative stress and ROS/RNS formation. We discuss the mechanisms involved in GSH efflux during cell death progression and the redox signaling events by which GSH depletion regulates the activation of the cell death machinery.

FUTURE DIRECTIONS

The evidence summarized here clearly places GSH transport as a central mechanism mediating redox signaling during cell death progression. Future studies should be directed toward identifying the molecular identity of GSH transporters mediating GSH extrusion during cell death, and addressing the lack of sensitive approaches to quantify GSH efflux.

Glutathione redox cycle dysregulation in Huntington's disease knock-in striatal cells

Huntington's disease (HD) is a CAG repeat disorder affecting the HD gene, which encodes for huntingtin (Htt) and is characterized by prominent cell death in the striatum. Oxidative stress was previously implicated in HD neurodegeneration, but the role of the major endogenous antioxidant system, the glutathione redox cycle, has been less studied following expression of full-length mutant Htt (FL-mHtt). Thus, in this work we analyzed the glutathione system in striatal cells derived from HD knock-in mice expressing mutant Htt versus wild-type cells. Mutant cells showed increased intracellular reactive oxygen species (ROS) and caspase-3 activity, which were significantly prevented following treatment with glutathione ethyl ester. Interestingly, mutant cells exhibited an increase in intracellular levels of both reduced and oxidized forms of glutathione, and enhanced activities of glutathione peroxidase (GPx) and glutathione reductase (GRed). Furthermore, glutathione-S-transferase (GST) and γ-glutamyl transpeptidase (γ-GT) activities were also increased in mutant cells. Nevertheless, glutamate-cysteine ligase (GCL) and glutathione synthetase (GS) activities and levels of GCL catalytic subunit were decreased in cells expressing FL-mHtt, highly suggesting decreased de novo synthesis of glutathione. Enhanced intracellular total glutathione, despite decreased synthesis, could be explained by decreased extracellular glutathione in mutant cells. This occurred concomitantly with decreased mRNA expression levels and activity of the multidrug resistance protein 1 (Mrp1), a transport protein that mediates cellular export of glutathione disulfide and glutathione conjugates. Additionally, inhibition of Mrp1 enhanced intracellular GSH in wild-type cells only. These data suggest that FL-mHtt affects the export of glutathione by decreasing the expression of Mrp1. Data further suggest that boosting of GSH-related antioxidant defense mechanisms induced by FL-mHtt is insufficient to counterbalance increased ROS formation and emergent apoptotic features in HD striatal cells.

Modulation of the ATPase and transport activities of broad-acting multidrug resistance factor ABCC10 (MRP7)

The cell surface molecule ABCC10 is a broad-acting transporter of xenobiotics, including cancer drugs, such as taxanes, epothilone B, and modulators of the estrogen pathway. Abcc10(-/-) mice exhibit increased tissue sensitivity and lethality resulting from paclitaxel exposure compared with wild-type counterparts, arguing ABCC10 functions as a major determinant of taxane sensitivity in mice. To better understand the mechanistic basis of ABCC10 action, we characterized the biochemical and vectorial transport properties of this protein. Using crude membranes in an ABCC10 overexpression system, we found that the ABCC10 transport substrates estrogen estradiol-glucuronide (E(2)17βG) and leukotriene C4 (LTC(4)) significantly stimulated ABCC10 beryllium fluoride (BeFx)-sensitive ATPase activity. We also defined the E(2)17βG antagonist, tamoxifen, as a novel substrate and stimulator of ABCC10. In addition, a number of cytotoxic substrates, including docetaxel, paclitaxel, and Ara-C, increased the ABCC10 basal ATPase activity. We determined that ABCC10 localizes to the basolateral cell surface, using transepithelial well assays to establish that ABCC10-overexpressing LLC-PK1 cells exported [(3)H]-docetaxel from the apical to the basolateral side. Importantly, we found that the clinically valuable multikinase inhibitor sorafenib, and a natural alkaloid, cepharanthine, inhibited ABCC10 docetaxel transport activity. Thus, concomitant use of these agents might restore the intracellular accumulation and potency of ABCC10-exported cytotoxic drugs, such as paclitaxel. Overall, our work could seed future efforts to identify inhibitors and other physiologic substrates of ABCC10.

Glutathione participates in the modulation of starvation-induced autophagy in carcinoma cells

Glutathione (γ-L-glutamyl-L-cysteinyl-glycine, GSH) is the most abundant low molecular weight, thiol-containing compound within the cells and has a primary role in the antioxidant defense and intracellular signaling. Here we demonstrated that nutrient deprivation led to a significant decrease of intracellular GSH levels in three different carcinoma cell lines. This phenomenon was dependent on ABCC1-mediated GSH extrusion, along with GCL inhibition and, to a minor extent, the formation of GSH-protein mixed disulfides that synergistically contributed to the modulation of autophagy by shifting the intracellular redox state toward more oxidizing conditions. Modulation of intracellular GSH by inhibiting its de novo synthesis through incubation with buthionine sulfoximine, or by maintaining its levels through GSH ethyl ester, affected the oxidation of protein thiols, such as PRDXs and consequently the kinetics of autophagy activation. We also demonstrated that thiol-oxidizing or -alkylating agents, such as diamide and diethyl maleate activated autophagy, corroborating the evidence that changes in thiol redox state contributed to the occurrence of autophagy.

Gene regulation of intestinal porcine epithelial cells IPEC-J2 is dependent on the site of deoxynivalenol toxicological action

The intestinal epithelial cell layer represents the border between the luminal and systemic side of the gut. The decision between absorption and exclusion of substances is the quintessential function of the gut and varies along the gut axis. Consequently, potentially toxic substances may reach the basolateral domain of the epithelial cell layer via blood stream. The mycotoxin deoxynivalenol (DON) is a Fusarium derived secondary metabolite known to enter the blood stream and displaying a striking toxicity on the basolateral side of polarised epithelial cell layers in vitro. Here we analysed potential mechanisms of apical and basolateral DON toxicity reflected in the gene expression. We used the jejunum-derived, polarised intestinal porcine epithelial cell line IPEC-J2 as an in vitro cell culture model. Luminal and systemic DON challenge of the epithelial cell layer was mimicked by a DON application from the apical or basolateral compartment of membrane inserts for 72 h. We compared the genome-wide gene expression of untreated and DON-treated IPEC-J2 cells with the GeneChip® Porcine Genome Array of Affymetrix. Low basolateral DON (200 ng/mL) application triggered 10 times more gene transcripts in comparison to the corresponding apical application (2539 versus 267) despite the intactness of the challenged cell layer as measured by transepithelial electrical resistance. Analysis of the regulated genes by bioinformatic resource DAVID identified several groups of biochemical pathways modulated by concentration and orientation of DON application. Selected genes representing pathways of the cellular metabolism, information processing and structural design were analysed in detail by quantitative PCR. Our findings clearly show that apical and basolateral challenge of epithelial cell layers trigger different gene response profiles paralleled with a higher susceptibility towards basolateral challenge. The evaluation of toxicological potentials of mycotoxins should take this difference in gene regulation dependent on route of application into account.

Collateral sensitivity as a strategy against cancer multidrug resistance

While chemotherapy remains the most effective treatment for disseminated tumors, acquired or intrinsic drug resistance accounts for approximately 90% of treatment failure. Multidrug resistance (MDR), the simultaneous resistance to drugs that differ both structurally and mechanistically, often results from drug efflux pumps in the cell membrane that reduce intracellular drug levels to less than therapeutic concentrations. Expression of the MDR transporter P-glycoprotein (P-gp, MDR1, ABCB1) has been shown to correlate with overall poor chemotherapy response and prognosis. This review will focus on collateral sensitivity (CS), the ability of compounds to kill MDR cells selectively over the parental cells from which they were derived. Insights into CS may offer an alternative strategy for the clinical resolution of MDR, as highly selective and potent CS agents may lead to drugs that are effective at MDR cell killing and tumor resensitization. Four main mechanistic hypotheses for CS will be reviewed, followed by a discussion on quantitative and experimental evaluation of CS.

Mechanism of RPE cell death in α-crystallin deficient mice: a novel and critical role for MRP1-mediated GSH efflux

Absence of α-crystallins (αA and αB) in retinal pigment epithelial (RPE) cells renders them susceptible to oxidant-induced cell death. We tested the hypothesis that the protective effect of α-crystallin is mediated by changes in cellular glutathione (GSH) and elucidated the mechanism of GSH efflux. In α-crystallin overexpressing cells resistant to cell death, cellular GSH was >2 fold higher than vector control cells and this increase was seen particularly in mitochondria. The high GSH levels associated with α-crystallin overexpression were due to increased GSH biosynthesis. On the other hand, cellular GSH was decreased by 50% in murine retina lacking αA or αB crystallin. Multiple multidrug resistance protein (MRP) family isoforms were expressed in RPE, among which MRP1 was the most abundant. MRP1 was localized to the plasma membrane and inhibition of MRP1 markedly decreased GSH efflux. MRP1-suppressed cells were resistant to cell death and contained elevated intracellular GSH and GSSG. Increased GSH in MRP1-supressed cells resulted from a higher conversion of GSSG to GSH by glutathione reductase. In contrast, GSH efflux was significantly higher in MRP1 overexpressing RPE cells which also contained lower levels of cellular GSH and GSSG. Oxidative stress further increased GSH efflux with a decrease in cellular GSH and rendered cells apoptosis-prone. In conclusion, our data reveal for the first time that 1) MRP1 mediates GSH and GSSG efflux in RPE cells; 2) MRP1 inhibition renders RPE cells resistant to oxidative stress-induced cell death while MRP1 overexpression makes them susceptible and 3) the antiapoptotic function of α-crystallin in oxidatively stressed cells is mediated in part by GSH and MRP1. Our findings suggest that MRP1 and α crystallin are potential therapeutic targets in pathological retinal degenerative disorders linked to oxidative stress.

Impact of terminal dimethylation on the resistance profile of α-N-heterocyclic thiosemicarbazones

Triapine is an α-N-heterocyclic thiosemicarbazone with promising anticancer activity against hematologic malignancies but widely ineffective against solid tumor types in clinical trials. The anticancer activity of thiosemicarbazones can be dramatically increased by terminal dimethylation. KP1089 is a gallium compound containing two terminal dimethylated thiosemicarbazone ligands. To gain insights on the vulnerability of this highly active terminal dimethylated thiosemicarbazone to drug resistance mechanisms, a new cell model with acquired resistance against the lead compound KP1089 was established. Subsequent genomic analyses (arrayCGH and FISH) revealed amplification of the ABCC1 gene on double minute chromosomal DNA in KP1089-resistant cells as well as overexpression of ABCC1 and ABCG2 on the protein level. KP1089 was further confirmed as a substrate of ABCC1 and ABCG2 but not of ABCB1 using a panel of ABC transporter-overexpressing cell models as well as ABC transporter inhibitors. Moreover, glutathione depletion strongly enhanced KP1089 activity, although no glutathione conjugate formation by glutathione-S-transferase was observed. Thus, a co-transport of KP1089 together with glutathione is suggested. Finally, a panel of thiosemicarbazone derivatives was tested on the new KP1089-resistant cell line. Notably, KP1089-resistant cells were not cross-resistant against thiosemicarbazones lacking terminal dimethylation (e.g. Triapine) which are less active than KP1089. This suggests that terminal dimethylation of thiosemicarbazones – linked with distinctly enhanced anticancer activity – leads to altered resistance profiles compared to classical thiosemicarbazones making this compound class of interest for further (pre)clinical evaluation.

Mammalian multidrug-resistance proteins (MRPs)

Subfamily C of the human ABC (ATP-binding cassette) superfamily contains nine proteins that are often referred to as the MRPs (multidrug-resistance proteins). The 'short' MRP/ABCC transporters (MRP4, MRP5, MRP8 and ABCC12) have a typical ABC structure with four domains comprising two membrane-spanning domains (MSD1 and MSD2) each followed by a nucleotide-binding domain (NBD1 and NBD2). The 'long' MRP/ABCCs (MRP1, MRP2, MRP3, ABCC6 and MRP7) have five domains with the extra domain, MSD0, at the N-terminus. The proteins encoded by the ABCC6 and ABCC12 genes are not known to transport drugs and are therefore referred to as ABCC6 and ABCC12 (rather than MRP6 and MRP9) respectively. A large number of molecules are transported across the plasma membrane by the MRPs. Many are organic anions derived from exogenous sources such as conjugated drug metabolites. Others are endogenous metabolites such as the cysteinyl leukotrienes and prostaglandins which have important signalling functions in the cell. Some MRPs share a degree of overlap in substrate specificity (at least in vitro), but differences in transport kinetics are often substantial. In some cases, the in vivo substrates for some MRPs have been discovered aided by studies in gene-knockout mice. However, the molecules that are transported in vivo by others, including MRP5, MRP7, ABCC6 and ABCC12, still remain unknown. Important differences in the tissue distribution of the MRPs and their membrane localization (apical in contrast with basolateral) in polarized cells also exist. Together, these differences are responsible for the unique pharmacological and physiological functions of each of the nine ABCC transporters known as the MRPs.

Tumor malignancy is engaged to prokaryotic homolog toolbox

Cancer cells display high proliferation rates and survival provided by high glycolysis, chemoresistance and radioresistance, metabolic features that appear to be activated with malignancy, and seemed to have arisen as early in evolution as in unicellular/prokaryotic organisms. Based on these assumptions, we hypothesize that aggressive phenotypes found in malignant cells may be related to acquired unicellular behavior, launched within a tumor when viral and prokaryotic homologs are overexpressed performing likely robust functions. The ensemble of these expressed viral and prokaryotic close homologs in the proteome of a tumor tissue gives them advantage over normal cells. To assess the hypothesis validity, sequences of human proteins involved in apoptosis, energetic metabolism, cell mobility and adhesion, chemo- and radio-resistance were aligned to homologs present in other life forms, excluding all eukaryotes, using PSI-BLAST, with further corroboration from data available in the literature. The analysis revealed that selected sequences of proteins involved in apoptosis and tumor suppression (as p53 and pRB) scored non-significant (E-value>0.001) with prokaryotic homologs; on the other hand, human proteins involved in cellular chemo- and radio-resistance scored highly significant with prokaryotic and viral homologs (as catalase, E-value=zero). We inferred that such upregulated and/or functionally activated proteins in aggressive malignant cells represent a toolbox of modern human homologs evolved from a similar key set that have granted survival of ancient prokaryotes against extremely harsh environments. According to what has been discussed along this analysis, high mutation rates usually hit hotspots in important conserved protein domains, allowing uncontrolled expansion of more resistant, death-evading malignant clones. That is the case of point mutations in key viral proteins affording viruses escape to chemotherapy, and human homologs of such retroviral proteins (as Ras, Akt and EGFR) can elicit the same phenotype. Furthermore, a corollary to this hypothesis presumes that target-directed anti-cancer therapy should target human protein domains of low similarity to prokaryotic homologs for a well-succeeded anti-cancer therapy.

Mutation of Glu521 or Glu535 in cytoplasmic loop 5 causes differential misfolding in multiple domains of multidrug and organic anion transporter MRP1 (ABCC1)

The polytopic 5-domain multidrug resistance protein 1 (MRP1/ABCC1) extrudes a variety of drugs and organic anions across the plasma membrane. Four charged residues in the fifth cytoplasmic loop (CL5) connecting transmembrane helix 9 (TM9) to TM10 are critical for stable expression of MRP1 at the plasma membrane. Thus Ala substitution of Lys(513), Lys(516), Glu(521), and Glu(535) all cause misfolding of MRP1 and target the protein for proteasome-mediated degradation. Of four chemical chaperones tested, 4-phenylbutyric acid (4-PBA) was the most effective at restoring expression of MRP1 mutants K513A, K516A, E521A, and E535A. However, although 4-PBA treatment of K513A resulted in wild-type protein levels (and activity), the same treatment had little or no effect on the expression of K516A. On the other hand, 4-PBA treatment allowed both E521A and E535A to exit the endoplasmic reticulum and be stably expressed at the plasma membrane. However, the 4-PBA-rescued E535A mutant exhibited decreased transport activity associated with reduced substrate affinity and conformational changes in both halves of the transporter. By contrast, E521A exhibited reduced transport activity associated with alterations in the mutant interactions with ATP as well as a distinct conformational change in the COOH-proximal half of MRP1. These findings illustrate the critical and complex role of CL5 for stable expression of MRP1 at the plasma membrane and more specifically show the differential importance of Glu(521) and Glu(535) in interdomain interactions required for proper folding and assembly of MRP1 into a fully transport competent native structure.

Epigenetic cancer therapy: rationales, targets and drugs

The fundamental role of altered epigenetic modification patterns in tumorigenesis establishes epigenetic regulatory enzymes as important targets for cancer therapy. Over the past few years, several drugs with an epigenetic activity have received approval for the treatment of cancer patients, which has led to a detailed characterization of their modes of action. The results showed that both established drug classes, the histone deacetylase (HDAC) inhibitors and the DNA methyltransferase inhibitors, show substantial limitations in their epigenetic specificity. HDAC inhibitors are highly specific drugs, but the enzymes have a broad substrate specificity and deacetylate numerous proteins that are not associated with epigenetic regulation. Similarly, the induction of global DNA demethylation by non-specific inhibition of DNA methyltransferases shows pleiotropic effects on epigenetic regulation with no apparent tumor-specificity. Second-generation azanucleoside drugs have integrated the knowledge about the cellular uptake and metabolization pathways, but do not show any increased specificity for cancer epigenotypes. As such, the traditional rationale of epigenetic cancer therapy appears to be in need of refinement, as we move from the global inhibition of epigenetic modifications toward the identification and targeting of tumor-specific epigenetic programs. Recent studies have identified epigenetic mechanisms that promote self-renewal and developmental plasticity in cancer cells. Druggable somatic mutations in the corresponding epigenetic regulators are beginning to be identified and should facilitate the development of epigenetic therapy approaches with improved tumor specificity.

Monomethylarsenic diglutathione transport by the human multidrug resistance protein 1 (MRP1/ABCC1)

The ATP-binding cassette (ABC) transporter protein multidrug resistance protein 1 (MRP1; ABCC1) plays an important role in the cellular efflux of the high-priority environmental carcinogen arsenic as a triglutathione conjugate [As(GS)(3)]. Most mammalian cells can methylate arsenic to monomethylarsonous acid (MMA(III)), monomethylarsonic acid (MMA(V)), dimethylarsinous acid (DMA(III)), and dimethylarsinic acid (DMA(V)). The trivalent forms MMA(III) and DMA(III) are more reactive and toxic than their inorganic precursors, arsenite (As(III)) and arsenate (As(V)). The ability of MRP1 to transport methylated arsenicals is unknown and was the focus of the current study. HeLa cells expressing MRP1 (HeLa-MRP1) were found to confer a 2.6-fold higher level of resistance to MMA(III) than empty vector control (HeLa-vector) cells, and this resistance was dependent on GSH. In contrast, MRP1 did not confer resistance to DMA(III), MMA(V), or DMA(V). HeLa-MRP1 cells accumulated 4.5-fold less MMA(III) than HeLa-vector cells. Experiments using MRP1-enriched membrane vesicles showed that transport of MMA(III) was GSH-dependent but not supported by the nonreducing GSH analog, ophthalmic acid, suggesting that MMA(III)(GS)(2) was the transported form. MMA(III)(GS)(2) was a high-affinity, high-capacity substrate for MRP1 with apparent K(m) and V(max) values of 11 μM and 11 nmol mg(-1)min(-1), respectively. MMA(III)(GS)(2) transport was osmotically sensitive and inhibited by several MRP1 substrates, including 17β-estradiol 17-(β-D-glucuronide) (E(2)17βG). MMA(III)(GS)(2) competitively inhibited the transport of E(2)17βG with a K(i) value of 16 μM, indicating that these two substrates have overlapping binding sites. These results suggest that MRP1 is an important cellular protective pathway for the highly toxic MMA(III) and have implications for environmental and clinical exposure to arsenic.

Glyceraldehyde-3-phosphate dehydrogenase as a quinone reductase in the suppression of 1,2-naphthoquinone protein adduct formation

1,2-Naphthoquinone (1,2-NQ) is electrophilic, and forms covalent bonds with protein thiols, but its two-electron reduction product 1,2-dihydroxynaphthalene (1,2-NQH(2)) is not, so enzymes catalyzing the reduction with reduced pyridine nucleotides as cofactors could protect cells from electrophile-based chemical insults. To assess this possibility, we examined proteins isolated from the 9000g supernatant from mouse liver for 1,2-NQ reductase activity using an HPLC assay procedure for the hydroquinone of 1,2-NQ and Cibacron Blue 3GA column chromatography and Western blot analysis with specific antibody to determine 1,2-NQ-bound proteins. Among the proteins with high affinities for pyridine nucleotides that also inhibited 1,2-NQ-protein adduct formation in the presence of NADH, a 37-kDa protein was found and identified as glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Using recombinant human GAPDH, we found that this glycolytic enzyme indeed catalyzes the two-electron reduction of 1,2-NQ accompanied by extensive NADH consumption under 20% oxygen conditions. When either 1,2-NQH(2) or 1,2-NQ was incubated with GAPDH in the presence of NADH, minimal covalent bonding to the enzyme occurred compared to that in its absence. These results indicate that GAPDH can inhibit 1,2-NQ-based electrophilic protein modification by conversion to the nonelectrophilic 1,2-NQH(2) via an NADH-dependent process.

Arsenic-glutathione conjugate transport by the human multidrug resistance proteins (MRPs/ABCCs)

Millions of people world-wide are chronically exposed to inorganic forms of the environmental toxicant arsenic in drinking water. This has led to a public health crisis because arsenic is a human carcinogen, and causes a myriad of other adverse health effects. In order to prevent and treat arsenic-induced toxicity it is critical to understand the cellular handling of this metalloid. A large body of literature describes the importance of the cellular tripeptide glutathione (γ-Glu-Cys-Gly,GSH/GS) in the excretion of arsenic. The triglutathione conjugate of arsenite [As(III)(GS)(3)] and the diglutathione conjugate of monomethylarsonous acid [MMA(III)(GS)(2)] have been isolated from rat bile and mouse urine, and account for the majority of excreted arsenic, suggesting these are important transportable forms. The ATP-binding cassette (ABC) transporter proteins, multidrug resistance protein 1 (MRP1/ABCC1) and the related protein MRP2 (ABCC2), are thought to play an important role in arsenic detoxification through the cellular efflux of arsenic-GSH conjugates. Current knowledge on the cellular handling of arsenic with a special emphasis on the transport pathways of the arsenic-GSH conjugates As(III)(GS)(3), MMA(III)(GS)(2), and dimethylarsenic glutathione DMA(III)(GS), as well as, the seleno-bis(S-glutathionyl) arsinium ion [(GS)(2)AsSe](-) are reviewed.

GSH-mediated S-transarylation of a quinone glyceraldehyde-3-phosphate dehydrogenase conjugate

Many cellular proteins with reactive thiols form covalent bonds with electrophiles, thereby modifying their structures and activities. Here, we describe the recovery of a glycolytic protein, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), from such an electrophilic attack by 1,2-napthoquinone (1,2-NQ). GAPDH readily formed a covalent bond with 1,2-NQ through Cys152 at a low concentration (0.2 μM) in a cell-free system, but when human epithelial A549 cells were exposed to this quinone at 20 μM, only minimal binding was observed although extensive binding to numerous other cellular proteins occurred. Depletion of cellular glutathione (GSH) with buthionine sulfoximine (BSO) resulted in some covalent modification of cellular GAPDH by 1,2-NQ and a significant reduction of GAPDH activity in the cells. Incubation of native, but not boiled, human GAPDH that had been modified by 1,2-NQ with GSH resulted in a concentration-dependent removal of 1,2-NQ from the GAPDH conjugate, accompanied by partial recovery of lost catalytic activity and formation of a 1,2-NQ-GSH adduct (1,2-NQ-SG). While GAPDH is recognized as a multifunctional protein, our results show that GAPDH also has a unique ability to recover from electrophilic modification by 1,2-NQ through a GSH-dependent S-transarylation reaction.